Novel reprogramming activators and methods

Abstract

The present invention relates to methods of modulating reprogramming, comprising increasing, enhancing and / or promoting the activity of peptidylarginine deiminase 4 (PADI4) enzyme, such as wherein the reprogramming of a somatic cell to a pluripotent-like state is increased, enhanced and / or promoted. The invention further relates to activators of PADI4, in particular activators which allosterically bind to PADI4, as well as their use in methods of reprogramming somatic cells and pharmaceutical compositions comprising said activators. Also provided is the pharmaceutical composition for use in treatment, rejuvenation, regeneration and repair.

Description

[0001]BAB-C-P3557PCT NOVEL REPROGRAMMING ACTIVATORS AND METHODS FIELD OF THE INVENTION The present invention relates to methods of modulating reprogramming, comprising increasing, enhancing and / or promoting the activity of peptidylarginine deiminase 4 (PADI4) enzyme, such as wherein the reprogramming of a somatic cell to a pluripotent-like state is increased, enhanced and / or promoted. The invention further relates to activators of PADI4, in particular activators which allosterically bind to PADI4, as well as their use in methods of reprogramming somatic cells and pharmaceutical compositions comprising said activators. Also provided is the pharmaceutical composition for use in treatment, rejuvenation, regeneration and repair. BACKGROUND OF THE INVENTION The discovery that somatic cells can be reprogrammed to a pluripotent state and instructed to differentiate into variety of cell types promises to revolutionise regenerative medicine by rendering it possible to repair or replace diseased and damaged tissues. Reprogramming can be achieved via transduction of the “Yamanaka” transcription factors Oct4, Sox2, Klf4 and c- Myc, or combinations of similar factors, into the somatic cells. Using this method, a relatively small proportion of somatic cells reprogramme successfully to become induced pluripotent stem cells (iPSCs) and, while the cell-intrinsic (i.e. cell autonomous) mechanisms that mediate reprogramming have been studied extensively, we still lack rational approaches to enhance reprogramming efficiency. The majority of the cells, which do not emerge as iPSCs, are deemed to have failed reprogramming and are typically ignored. The transduction of Yamanaka factors into somatic cells to be reprogrammed leads to the genetic manipulation of said cells, and can predispose them to cancerous transformation and teratoma formation. The Yamanaka factor, c-Myc in particular has been implicated in this process. Furthermore, the transduction of Yamanaka factors in vivo to bring about iPSC reprogramming in tissue repair, regeneration and rejuvenation can be challenging. There is therefore a great need to identify other factors involved in reprogramming, including non-cell autonomous mechanisms and processes which avoid, or reduce the requirement for, the genetic manipulation of somatic cells in methods of reprogramming. Identifying the factors involved in or which promote the release / secretion of these non-cell autonomous reprogramming mechanisms / processes may allow their manipulation and regulation with the ultimate goal of achieving highly efficient reprogramming while minimising genetic manipulation of somatic cells by transduction of reprogramming factors (such as some of the BAB-C-P3557PCT Yamanaka factors), thus minimising the risk of cancerous transformation and teratoma formation. The modulation of these non-cell autonomous reprogramming mechanisms and use of the factors which modulate them / their release may then be applied towards the achievement of efficient and safe reprogramming technologies in vitro and in vivo, impacting the areas of iPSC reprogramming and tissue repair, regeneration and rejuvenation. PADI4 has been historically studied for its role in promoting various pathologies when over- expressed or over-activated and this has spurred considerable efforts to develop PADI4 inhibitors. Emerging evidence on the physiological and pathophysiological roles of PADI4 underscores the need to also generate PADI4 activators. Firstly, PADI4 activation is a feature of cell reprogramming (in the context of transcription (Yamanaka) factor-mediated somatic cell reprogramming for the generation of induced pluripotent stem cells (iPSCs) and its inhibition impairs reprogramming (Christophorou et al. (2014) Nature, 507(7490):104-108, doi: https: / / doi.org / 10.1038 / nature12942). Furthermore, unpublished data shows that PADI4 activation is a feature of in vivo reprogramming and tissue regeneration post physical or chemical injury, suggesting that PADI4 may have a role in the context of the in vivo cell fate transitions that underlie these processes. Beyond its role in reprogramming and regeneration, PADI4 has been suggested to act as a tumour suppressor in certain contexts (Indeglia et al. (2023) Cancer Discov., CD-22-1315, doi: https: / / doi.org / 10.1158 / 2159-8290.cd-22-1315; Tanikawa et al. (2009) Cancer Res., 69(22):8761-8769, doi: https: / / doi.org / 10.1158 / 0008- 5472.can-09-2280; and Tanikawa et al. (2012) Nat. Commun., 3:676, doi: https: / / doi.org / 10.1038 / ncomms1676). Therefore, specific control of PADI4 activation, as achieved by a chemical modulator, may be explored in contexts of in vitro reprogramming and interventions towards tissue regeneration and cancer therapy. The development of pharmacological activators is also likely to revolutionise understanding of PADI regulation and function. Currently, signals such as a calcium ionophore or lipopolysaccharide (LPS) are used to stimulate PADI activity. These highly pleiotropic stimuli engage a wide range of biological pathways, making it impossible to disentangle the specific mechanistic contributions of PADIs. Specific and selective PADI activators would provide powerful tools for the understanding of PADI-mediated cellular functions. Importantly, such compounds may inform a mechanistic understanding of PADI regulation, enabling the rational design of better PADI inhibitors. Although a subset of anti-PADI4 autoantibodies have been identified that increase the catalytic efficiency of PADI4 (Darrah et al., (2013) Sci Transl Med. 5(186):186ra65, doi: https: / / doi.org / 10.1126 / scitranslmed.3005370), no synthetic PADI4 activators exist currently. BAB-C-P3557PCT SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a method of modulating reprogramming of a somatic cell to be reprogrammed, comprising increasing, enhancing and / or promoting the activity of peptidylarginine deiminase 4 (PADI4) enzyme. In certain embodiments, reprogramming of the somatic cell is increased, enhanced and / or promoted, such as wherein the somatic cell is reprogrammed to a pluripotent-like state. In some embodiments, the activity of PADI4 is increased, enhanced and / or activated in a cell other than the somatic cell to be reprogrammed, such as a non-reprogramming cell (e.g. a bystander cell). In particular embodiments, the activity of PADI4 is increased, enhanced and / or activated using an activator which reduces the dependency of PADI4 activity on calcium, such as wherein the activator interacts with an allosteric binding site of PADI4. In one embodiment, the activator interacts with a calcium binding site. In one embodiment, the activator is a cyclic peptide activator. According to another aspect of the invention, there is provided the use of a PADI4 activator in a method of reprogramming a somatic cell to a pluripotent-like state. In a further aspect, there is provided a pharmaceutical composition comprising a PADI4 activator, the composition optionally further comprising one or more pharmaceutically acceptable diluents, excipients and / or carriers. In a yet further aspect, there is provided the pharmaceutical composition defined herein for use in a method of treating and / or ameliorating a degenerative disease or disorder or in a method of rejuvenating, regenerating or repairing a tissue or organ. BRIEF DESCRIPTION OF THE FIGURES Figure 1: RaPID strategy and cyclic peptides found in the selection. A) Schematic diagram of the RaPID selections used to identify PADI4 binding cyclic peptides. B) Sequence and binding affinities of the different PADI4 binding cyclic peptides determined by surface plasmon resonance. All peptides are cyclised via a thioether bond between the acetylated N- terminus and the cysteine side chain thiol. Figure 2: PADI4_11 is a PADI4 activator. A) PADI4_11 and PADI4_14 are PADI4 activators in vitro. Activation COLDER assays with PADI4 and different PADI4 binding peptides. Assays were done with different concentrations of CaCl2(10-0.003mM) and 30µM BAB-C-P3557PCT of peptide. K50is the concentration of CaCl2that yields half maximum PADI4 activity. Data was normalised against the activity of PADI4 in absence of peptide and 10mM CaCl2, and represents mean ± SEM of three different biological replicates. Each replicate was done in triplicate. B) PADI4_11 and PADI4_14 activate PADI4 at low calcium concentration. COLDER assays were performed at different concentrations of peptides (0.003-100µM) and 0.1mM CaCl2. Data represents mean ± SEM of three different biological replicates done in triplicate. C-E) PADI4_11 and PADI4_14 do not activate hPADI1, hPADI2 or hPADI3. COLDER assays were performed in presence of absence of PADI4_11 or PADI4_14 at 30 µM and different concentrations of CaCl2. K50is the concentration of CaCl2that yields half maximum PADI activity. Data represents mean ± SEM of three different biological replicates and each replicate was done in triplicate. Data was normalised against the activity of each PADI in absence of peptides and 10 mM CaCl2. F) PADI4_11 and PADI4_11_inverted can enter cells. CAPA assay with PADI4_11 and PADI4_11_inverted. Data shows mean ± SEM of three different biological experiments done in triplicate. Data is normalised with cells with no peptide treated with TMR dye (Promega) (100%) and cells with no peptide and no dye (0%). G) Representative immunofluorescence confocal micrographs of mES cells over-expressing hPADI4 treated with variable concentrations of PADI4_11. Citrullinated H3 (H3Cit) is shown in green and DAPI in blue. H) EC50 quantification. Data points represent the mean H3Cit intensity after treatment of mES cells over-expressing hPADI4 with increasing doses of PADI4_11 and PADI4_11_inverted. EC50 for PADI4_11 is calculated at 16.13 ^M (I) Quantification of calcium influx into mES cells over-expressing hPADI4 when grown in Serum / LIF, or in the presence of 25 ^ ^M PADI4_11 or PADI4_11_inverted, showing that PADI4_11 does not induce a calcium influx into the cells. Calcium ionophore treatment is presented as a positive control for calcium influx, demonstrating the robustness of the assay. J) Flow cytometry analysis of propidium iodide (PI) staining of mES cells over-expressing hPADI4 when grown in Serum / LIF, or in the presence of different amounts of PADI4_11 or PADI4_11_inverted. The lack of increase in PI staining shows that PADI4_11 does not induce cell toxicity. Hydrogen peroxide (H2O2) used as a positive control for toxicity, demonstrating the robustness of the assay. Data shown in panels (E) and (F) demonstrate that the activation of PADI4 by PADI4_11 is not due to an off- target induction of calcium influx or cell toxicity, which could result in non-specific activation of PADI4, and that PADI4_11 therefore have an on-target effect on PADI4, inducing its activation within cells. Figure 3: Mechanism of action of PADI4_11. A) Sequence of the different PADI4_11 analogues synthesised by SPPS (left panel) and in vitro activity measured by COLDER assay (right panel). Assays were performed with 50 nM PADI4 in the presence of 0.1 mM CaCl2 and 100 µM peptide. Data was normalised against the activity of PADI4 with 0.1 mM CaCl2 and BAB-C-P3557PCT 100 µM of PADI4_11. Data represents mean ± SEM of three biological repeats. B) Top panel: hPADI4 PBD structure 1WD9 (blue) with calcium ions (yellow) and calcium binding amino acids varied by site directed mutagenesis highlighted (magenta). Bottom panel: CaCl2K50of different PADI4 variants in the presence or absence of PADI4_11 and ratio between them. C) PADI4 calcium binding sites, adapted from Arita et al. (2004). D) Different PADI4 calcium binding sites mutants have different activities and PADI4_11 is not able to activate some PADI4 calcium binding mutants. Activity of each mutant was measured by COLDER assay at 10mM CaCl2and incubated for 30 minutes at room temperature. COLDER assays were performed in presence or absence of PADI4_11 at 30 µM and different concentrations of CaCl2. K50 is the concentration of CaCl2that yields half maximum PADI activity. Data represents mean ± SEM of two different biological replicates and each replicate was done in triplicate. E) Protein sequence alignment of human PADI4 and mouse PADI4. * marks human cysteine 166. F) Activity of wild-type hPADI4 and mutants hPADI4_C166F and hPADI4_C16S. Activity was measured at 10mM CaCl2after 30 minutes incubation. Data was normalised against activity of wild-type PADI4. G-H) COLDER assay with PADI4_C166F and PADI4_C166S in presence or absence of PADI4_11 peptide. Assays were done with different CaCl2 concentrations (10-0.0003mM) and 30µM of peptide. K50 is the concentration of CaCl2 that yields half maximum PADI4 activity. Data was normalised against the activity of PADI4_C166F (G) or PADI_C166S (H) in absence of peptide and 10mM CaCl2, and represents mean ± SEM of three different biological replicates. Each replicate was done in triplicate. Figure 4: PADI4_11 in Reprogramming. A) Schematic of experimental plan for reprogramming of human adult fibroblasts. Human adult fibroblasts were seeded and transduced with viral vectors expressing the Yamanaka factors Oct4, Sox2, Klf4 and c-Myc. PADI4_11 was added on Day 7, at the time of seeding on vitronectin matrix. Colony formation assessment and immunostaining were performed on Day 12. B) Assessment of colony formation during reprogramming of human adult fibroblasts. Brightfield microscopy images of human adult fibroblast reprogramming cultures. Cells were reprogrammed using transduction of Yamanaka factors Oct4, Sox2, Klf4 and c-Myc, as described in slide 1. PADI4_11 was added at doses 50 ^M or 100 ^M on Day 7 and replenished with each daily medium change. Colony formation was assessed on Day 12, an early time point in reprogramming. Scale bar = 500 ^m. Magnification = 4X. C) Assessment of epithelial and mesenchymal markers during reprogramming. qRT-PCR analysis of human adult fibroblasts reprogramming cultures. Cells were reprogrammed using transduction of Yamanaka factors Oct4, Sox2, Klf4 and c-Myc, as described in slide 1. PADI4_11 (100 ^M) or the PADI4 inhibitor Cl-amidine (100 ^M) were added from Day 7 and replenished with each daily medium change. The initial fibroblast cells (Day 0, D0) or reprogramming cells at Day 19 (D19) of reprogramming were harvested and BAB-C-P3557PCT analysed for expression of epithelial marker E-cadherin (Cdh1), which is expected to increase with reprogramming, or expression of mesenchymal marker Vimentin, which is expected to decrease with reprogramming. Expression normalized to Gapdh. D) Assessment of colony formation during reprogramming of human adult fibroblasts. Immunocytochemistry analysis of human adult fibroblasts reprogramming cultures. Cells were reprogrammed using transduction of Yamanaka factors Oct4, Sox2, Klf4 and c-Myc, as described in slide 1. PADI4_11 was added at doses 50 ^M or 100 ^M on Day 7 and replenished with each daily medium change. Cells were stained on Day 12, an early time point in reprogramming. Vimentin, a mesenchymal marker known to be downregulated during reprogramming, presented in green. Citrullinated histone H3 (H3Cit), a mark of PADI4 activation is presented in red. DAPI, a DNA marker, is presented in blue. Scale bar = 200 ^m. Magnification = 20X. DETAILED DESCRIPTION OF THE INVENTION The present invention is based in part on the previous discovery that non-iPSCs within reprogramming cultures (i.e. non-reprogramming cells / cells other than the somatic cell to be reprogrammed) play a role in supporting iPSC reprogramming through the release / secretion of pluripotency-promoting factors, referred to herein and elsewhere as “non-cell autonomous reprogramming factors”, i.e. that they are ‘active bystanders’. This previous discovery is detailed in GB2215629.3 and GB2309737.1, the contents of which are hereby incorporated by reference in their entirety. Therefore, without being bound by theory it is believed that the release / secretion of said non-cell autonomous reprogramming factors, and thus the reprogramming of a somatic cell, can be modulated by increasing, enhancing and / or promoting the activity of the peptidylarginine deiminase enzyme, peptidylarginine deiminase 4 (PADI4), in particular the activity of PADI4 in non-reprogramming or bystander cells. Thus, according to a first aspect of the invention there is provided a method of modulating reprogramming of a somatic cell to be reprogrammed, comprising increasing, enhancing and / or promoting the activity of peptidylarginine deiminase 4 (PADI4) enzyme. During reprogramming, somatic cells are converted or de-differentiated into pluripotent stem cells, i.e. they are induced to become pluripotent. The resulting reprogrammed cells are therefore known as induced pluripotent stem cells (iPSCs). Such iPSCs are similar to natural pluripotent stem cells (e.g. embryonic stem (ES) cells) in many respects, including in their ability to differentiate into multiple cell types and lineages, such as all types of cell found in an organism. Reprogramming may be “complete” or “full” whereby somatic cells are converted or de-differentiated into iPSCs which are similar to natural pluripotent stem cells (e.g. ES cells) BAB-C-P3557PCT in many respects, including in their ability to differentiate into multiple cell types, and also whereby their DNA methylation age is reset to zero years old regardless of the age of the donor tissue from which the somatic cell was obtained. As such, “complete” or “full” reprogramming resets the epigenetic signature of the somatic cell to an embryonic-like state and causes loss of somatic cell lineage identity. Alternatively, reprogramming may be “partial” or “incomplete” whereby the epigenetic signature of the reprogrammed cell is not completely reset and, for example, somatic lineage identity may be retained. Thus, references herein to “modulating reprogramming” may include any increase, enhancement and / or promotion of the reprogramming of a somatic cell to a pluripotent or pluripotent-like state, or may include the reduction and / or inhibition of reprogramming. Preferably, reprogramming is increased, enhanced and / or promoted. Thus in one embodiment, reprogramming of the somatic cell is increased, enhanced and / or promoted. As described hereinbefore, said reprogramming may be complete / full or may be partial / incomplete. In one embodiment, the somatic cell is fully reprogrammed, such as is fully reprogrammed to a pluripotent state. In another embodiment, the somatic cell is partially reprogrammed, such as partially reprogrammed to a pluripotent-like state. Thus, in some embodiments the somatic cell is reprogrammed to a pluripotent-like state as described herein. iPSCs are forced to express genes and factors important for inducing and maintaining an ES cell-like state during reprogramming, and these derive from the reprogramming cell itself, often expressed by the reprogramming cell from the endogenous genes or from transfected / transduced genetic material encoding said factors, i.e. they are cell autonomous reprogramming factors. They include the Yamanaka factors OCT3 / 4, SOX2, KLF4 and c-MYC. NANOG and LIN28 may also be used together with the Yamanaka factors and can increase the induction of pluripotency. References herein to one or more “cell autonomous reprogramming factors” or “autonomous reprogramming factors” include the Yamanaka factors, which include one or more of: OCT4, KLF4, c-MYC and SOX2. Thus in one embodiment, the one or more cell autonomous reprogramming factors are one or more Yamanaka factors. In a further embodiment, said one or more Yamanaka factors may additionally comprise LIN28 and NANOG. In an alternative embodiment, the one or more Yamanaka factors may be selected from one or more, two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, or all of: OCT4, KLF4, c-MYC, SOX2, LIN28, NANOG, ESSRRB, NR5A2 and / or C / EBPα. In a further embodiment, the one or more Yamanaka factors are selected from: OCT4, KLF4, c-MYC and / or SOX2. In a yet further embodiment, the one or more Yamanaka factors are BAB-C-P3557PCT selected from: OCT4, KLF4, c-MYC, SOX2, LIN28 and / or NANOG. In still further embodiments, such as wherein the non-cell autonomous reprogramming factor supplements or replaces one or more autonomous reprogramming factor, the one or more Yamanaka factors do not include c-MYC. In certain embodiments, one or more cell autonomous reprogramming factors (e.g. Yamanaka factors) are provided which derive from the somatic cell to be reprogrammed. In a further embodiment, the one or more cell autonomous reprogramming factors derive from the reprogramming cell (e.g. the somatic cell to be reprogrammed). Thus in a yet further embodiment, the effect of the one or more cell autonomous reprogramming factors on the reprogramming cell (e.g. the somatic cell to be reprogrammed) is enhanced by the non-cell autonomous reprogramming factor described herein. In another embodiment, the non-cell autonomous reprogramming factor enhances the reprogramming effect of the one or more cell autonomous reprogramming factors (e.g. Yamanaka factors). In a further embodiment, the non-cell autonomous reprogramming factor supplements or replaces one or more of the cell autonomous reprogramming factors. For example, the non-cell autonomous programming factor may replace the requirement for the cell autonomous reprogramming factor c-MYC, a potent oncogene, in the reprogramming cell (e.g. somatic cell to be reprogrammed). In a particular embodiment, the method is performed in a reprogramming culture comprising one or more cell autonomous reprogramming factors as defined herein (e.g. Yamanaka factors). In a yet further embodiment, the reprogramming culture comprising one or more cell autonomous reprogramming factors is in vitro. In one embodiment, the one or more cell autonomous reprogramming factors are expressed by the reprogramming cell (e.g. the somatic cell to be reprogrammed). In a further embodiment, the one or more cell autonomous reprogramming factors are expressed from a nucleic acid sequence encoding said cell autonomous factors in the reprogramming cell, in particular an exogenous nucleic acid sequence. In some embodiments, the nucleic acid encoding the one or more cell autonomous reprogramming factors is transfected or transduced into the reprogramming cell. In certain embodiments, the one or more cell autonomous reprogramming factors are Yamanaka factors as described hereinbefore. In a further embodiment, the Yamanaka factors are selected from one or more of: OCT4, KLF4, c-MYC, SOX2, LIN28, NANOG, ESSRRB, NR5A2 and / or C / EBPα, in particular one or more of: OCT4, KLF4, c-MYC, SOX2. References herein to “somatic” refer to any type of cell that makes up the body of an organism, excluding germ cells and undifferentiated stem cells. Somatic cells therefore include, for example, fibroblasts, skin, heart, muscle, gut, eye, bone or blood cells and their partly- differentiated tissue stem cells. In one embodiment, the methods of modulating BAB-C-P3557PCT reprogramming in a somatic cell defined herein comprise providing a somatic cell to be reprogrammed, such as to a pluripotent state (e.g. to an iPSC) or a pluripotent-like state as described herein. Non-Cell Autonomous Modulation of Reprogramming References herein to “non-cell autonomous” refer to effects and processes that derive from cells other than those being manipulated or provided with a cell autonomous reprogramming factor (e.g. Yamanaka factors) described herein. For example, non-cell autonomous modulation as described herein comprises a reprogramming factor which effects / brings about reprogramming in a somatic cell but derives from a cell other than said somatic cell to be reprogrammed, i.e. the reprogramming factor is non-cell autonomous. In other words, a non- cell autonomous process is a process which occurs in one cell and brings about a change in another, distinct cell, while a non-cell autonomous factor is a factor released by one cell and which brings about a change in another, distinct cell. Thus references herein to a non-cell autonomous reprogramming factor that “derives from” a non-reprogramming / bystander cell include said factors that are “produced by”, “released from / by” and / or “secreted from / by” non- reprogramming / bystander cells, and the terms “derives from”, “produced by”, “released from / by” and “secreted from / by” may be used interchangeably herein. References herein to such non-cell autonomous reprogramming factors that “derive from” a non-reprogramming / bystander cell may also include analogues thereof (e.g. synthetic analogues) which perform the same function and can be considered as derivatives of said non-cell autonomous reprogramming factor. A further example of a non-cell autonomous process is where the differentiation state (e.g. the state of reprogramming) of a cell causes another cell of a different differentiation state to display an altered phenotype, such as because of cell-cell interactions or the release of signalling molecules. Thus, references herein to “non-cell autonomous reprogramming factor” refer to a factor which brings about reprogramming in a somatic cell but which derives from cells other than said somatic cell to be reprogrammed, such as non- reprogramming or bystander cells in vivo or in an in vitro culture. In certain embodiments, the non-cell autonomous reprogramming factor does not derive from the somatic cell to be reprogrammed. In a further embodiment, the non-cell autonomous reprogramming factor does not derive from the reprogramming cell. Therefore in one embodiment, the increased, enhanced and / or promoted PADI4 activity described herein leads to the release or secretion of a non-cell autonomous reprogramming factor. Said non-cell autonomous reprogramming factor brings about or effects the reprogramming of a somatic cell but may derive from a cell other than said somatic cell to be reprogrammed, such as wherein the non-cell autonomous reprogramming factor is a BAB-C-P3557PCT component of (e.g. is within or on the cell surface of) the non-reprogramming or bystander cell. In a particular embodiment, the non-cell autonomous reprogramming factor is released by or from the cell other than the somatic cell to be reprogrammed, i.e. a non-reprogramming or bystander cell. In a further embodiment, the non-cell autonomous reprogramming factor is secreted by or from the non-reprogramming or bystander cell. Thus in certain embodiments, the somatic cell to be reprogrammed is distinct from the cell other than said somatic cell or non-reprogramming / bystander cell. In a particular embodiment, the increased, enhanced and / or promoted PADI4 activity leads to the release or secretion of a non-cell autonomous reprogramming factor from the cell other than the somatic cell to be reprogrammed, i.e. a non- reprogramming and / or bystander cell. Alternatively or additionally, the non-cell autonomous reprogramming factor may derive from a somatic cell to be reprogrammed but act on another, distinct somatic cell to be reprogrammed, i.e. a reprogramming cell. Thus in one embodiment, the non-cell autonomous reprogramming factor may derive from any cell in a reprogramming culture, including the reprogramming cell and / or a non-reprogramming cell. In one embodiment, the non-cell autonomous reprogramming factor derives from a non- reprogramming cell. In a further embodiment, the non-cell autonomous reprogramming factor is released by or from a non-reprogramming cell. Such non-reprogramming cells include those in a reprogramming culture which appear to have failed to reprogramme. However, contrary to this apparent ‘failure’ to reprogramme, the inventors have previously shown (GB2215629.3, the non-cell autonomous reprogramming methods of which are hereby incorporated by reference) that these non-reprogramming cells (i.e. the bystander cells or cells other than the somatic cell to be reprogrammed) provide active support to reprogramming cells in culture in the form of a non-cell autonomous factor and that they may therefore be considered ‘active bystanders’. In a further embodiment, the non-cell autonomous reprogramming factor derives from a bystander cell. In a yet further embodiment, the non-cell autonomous reprogramming factor is released by or from a bystander cell. Thus, the terms “cell other than the somatic cell to be reprogrammed”, “non-reprogramming cell” and “bystander cell” may be used interchangeably herein and refer to any cell in vivo or in vitro (e.g. in culture) from which the non-cell autonomous reprogramming factor may derive, such as is released / secreted by or from. Alternatively or additionally, the non-cell autonomous reprogramming factor may derive from a somatic cell to be reprogrammed as described hereinbefore. Thus in one embodiment, the non-cell autonomous reprogramming factor is released or secreted from any cell in a reprogramming culture, including the reprogramming cell and / or a non-reprogramming cell. As will be appreciated from the disclosures herein, the inventors have surprisingly found that PADI4 activity is correlated with non-cell autonomous reprogramming factor release / secretion BAB-C-P3557PCT and thus reprogramming, i.e. that wherein PADI4 activity is increased, so too is the release / secretion of non-cell autonomous reprogramming factors (in particular extracellular chromatin) and reprogramming. Therefore in one embodiment, the release / secretion of the non-cell autonomous reprogramming factor is increased, enhanced and / or promoted. In a particular embodiment, the release / secretion of the non-cell autonomous reprogramming factor by the non-reprogramming / bystander cell is increased, enhanced and / or promoted by the increased, enhanced and / or promoted PADI4 activity in said cell. In another embodiment, the release / secretion of the non-cell autonomous reprogramming factor by a reprogramming cell is increased, enhanced and / or promoted by the increased, enhanced and / or promoted PADI4 activity in said reprogramming cell, i.e. a somatic cell to be reprogrammed. In a yet further embodiment, the release / secretion of the non-cell autonomous reprogramming factor by a reprogramming cell and a non-reprogramming cell is increased, enhanced and / or promoted by the increased, enhanced and / or promoted PADI4 activity in said cells, i.e. in both the reprogramming cell and the non-reprogramming / bystander cell. Thus in certain embodiments, the activity of PADI4 is increased, enhanced and / or promoted in a cell other than the somatic cell to be reprogrammed. Such cells include non- reprogramming cells, such as bystander (e.g. ‘active bystander’) cells as described hereinbefore. In one embodiment, the cell other than the somatic cell to be reprogrammed is a non-reprogramming cell. In a further embodiment, the cell other than the somatic cell to be reprogrammed is a bystander cell. In another embodiment, the activity of PADI4 is increased, enhanced and / or promoted in a reprogramming cell, i.e. in a somatic cell to be reprogrammed. In a yet further embodiment, the activity of PADI4 is increased, enhanced and / or promoted in a reprogramming cell and a non-reprogramming / bystander cell, i.e. in both the reprogramming cell and the non-reprogramming / bystander cell. References herein to “culturing” include the addition of cells (e.g. both the somatic cells to be reprogrammed and non-reprogramming / bystander cells), to media comprising growth factors and / or essential nutrients. It will be appreciated that such culture conditions may be adapted as appropriate for the reprogramming of a somatic cell to an iPSC or iPSC-like state. Thus in one embodiment, the method defined herein is performed in a culture. In a further embodiment, the method is performed in vitro. In a particular embodiment, the method is performed in an in vitro culture. In a further particular embodiment, the culture is a reprogramming culture comprising the somatic cell to be reprogrammed and one or more cells other than the somatic cell to be reprogrammed as described herein. Optionally, a non-cell autonomous reprogramming factor is provided to the reprogramming culture. Thus in a further BAB-C-P3557PCT embodiment, the culture is a reprogramming culture comprising the somatic cell to be reprogrammed and the non-cell autonomous reprogramming factor is provided to the reprogramming culture. In a yet further embodiment, the method is performed in vitro and the non-cell autonomous reprogramming factor is provided to a reprogramming culture. In another embodiment, the method is performed in a reprogramming culture (e.g. in vitro) and the non- cell autonomous reprogramming factor is provided to said culture comprising the somatic cell to be reprogrammed and one or more cell other than the somatic cell to be reprogrammed. In yet other embodiment, the method is performed in a reprogramming culture (e.g. in vitro) and the non-cell autonomous reprogramming factor is provided to said culture comprising the somatic cell to be reprogrammed. In alternative embodiments, the culture is a reprogramming culture comprising the somatic cell to be reprogrammed and the non-cell autonomous reprograming factor is released / secreted by one or more cell other than the somatic cell to be reprogrammed, wherein said other cell is comprised in the reprogramming culture, i.e. the non-cell autonomous reprogramming factor is not provided to the reprogramming culture. In a further embodiment, the method is performed in an in vitro reprogramming culture and the non-cell autonomous reprogramming factor is released or secreted into the culture medium. Release or secretion of the non-cell autonomous reprogramming factor may be by or from the non-reprogramming or bystander cell, i.e. by / from the cell other than the somatic cell to be reprogrammed. In a yet further embodiment, the non-reprogramming / bystander cell releases or secretes the non- cell autonomous reprogramming factor into the in vitro reprogramming culture, such as into the cell culture medium. Non-Cell Autonomous Reprogramming Factor In certain embodiments of the present invention, the non-cell autonomous reprogramming factor is released from the non-reprogramming cell and / or a bystander cell as described herein. In a particular embodiment, the non-cell autonomous reprogramming factor is released / secreted from the non-reprogramming cell and / or a bystander cell. In alternative embodiments, the non-cell autonomous reprogramming factor is released from the reprogramming cell, i.e. from the somatic cell to be reprogrammed, such as wherein the non- cell autonomous reprogramming factor is released / secreted from the reprogramming cell. As described hereinbefore, the non-cell autonomous reprogramming factor released / secreted from the non-reprogramming / bystander cell in a reprogramming culture is in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI 4 activity in said non- reprogramming / bystander cell. The released or secreted non-cell autonomous BAB-C-P3557PCT reprogramming factor in turn acts on the somatic cell to be reprogrammed to bring about or effect reprogramming or the enhancement / promotion of reprogramming. Alternatively or additionally, the non-cell autonomous reprogramming factor released / secreted from the reprogramming cell is in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI4 activity in said reprogramming cell. Said non-cell autonomous reprogramming factor therefore acts on the somatic cell to be reprogrammed together with the one or more cell autonomous reprogramming factors (e.g. Yamanaka factors) described hereinbefore. Thus in one embodiment, wherein the method is performed in an in vitro reprogramming culture, the non-cell autonomous reprogramming factor is released into the culture medium. Alternatively, the non-cell autonomous reprogramming factor released from the non-reprogramming / bystander cell and / or from the reprogramming cell in a reprogramming culture may be in response to cell autonomous reprogramming factors acting on said cell, such as on both cells. In one embodiment, the non-cell autonomous reprogramming factor released from the non-reprogramming / bystander cell and / or from the reprogramming cell in a reprogramming culture is in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI4 activity in said non-reprogramming / bystander and / or reprogramming cell, and in response to cell autonomous reprogramming factors acting on said cell, such as on both cells. In a further embodiment, the release / secretion of the non- cell autonomous reprogramming factor is in response to cell autonomous reprogramming factors acting on and PADI4 activity in the non-reprogramming / bystander cell and / or in the reprogramming cell. In one embodiment, the culture medium comprising the non-cell autonomous reprogramming factor released by the non-reprogramming / bystander cell and / or the reprogramming cell may be added to a somatic cell to be reprogrammed (e.g. a distinct somatic cell to be reprogrammed), optionally together with one or more cell autonomous reprogramming factors (e.g. Yamanaka factors) which derive from said somatic cell directly. Thus, provided herein is a culture medium comprising the non-cell autonomous reprogramming factor released or secreted by the non-reprogramming or bystander cell and / or by the reprogramming cell. In one embodiment, the non-cell autonomous reprogramming factor is released into the culture medium by the non-reprogramming / bystander and / or reprogramming cell in response to one or more cell autonomous reprogramming factors acting on said cell. In another embodiment, the non-cell autonomous reprogramming factor is released / secreted into the culture medium by the non-reprogramming / bystander and / or reprogramming cell in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI 4 activity in said cell, such as in both the non-reprogramming / bystander cell and the reprogramming cell. In a further embodiment, the non-cell autonomous reprogramming factor is released / secreted in response to one or more cell autonomous BAB-C-P3557PCT reprogramming factors acting on the non-reprogramming / bystander cell and / or reprogramming cell and in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI4 activity in said cell. In a yet further embodiment, the cell culture medium optionally additionally comprises one or more cell autonomous reprogramming factors as described herein. As demonstrated previously, medium removed from reprogramming cultures in which cell autonomous reprogramming factors have been used (in particular the Yamanaka factors OCT4, KLF4, c-MYC and SOX2), i.e. a conditioned medium, is able to promote / enhance the reprogramming of somatic cells in a separate reprogramming culture. In one embodiment, the separate reprogramming culture to which the conditioned medium is added is at an earlier stage of reprogramming (e.g. is at an earlier timepoint of reprogramming) than the culture from which the medium is removed. Further provided herein is a use of said culture medium (i.e. the conditioned culture medium) in a reprogramming culture. In some embodiments, said culture medium further comprises one or more cell autonomous reprogramming factors (e.g. Yamanaka factors) which derive directly from a somatic cell to be reprogrammed as described herein. Also provided is a kit for reprogramming a somatic cell, said kit comprising a culture medium comprising the non-cell autonomous reprogramming factor released or secreted by the non-reprogramming or bystander cell and / or by the reprogramming cell, optionally wherein said non-cell autonomous reprogramming factor has been released in response to one or more cell autonomous reprogramming factors. Alternatively or in addition, said non-cell autonomous reprogramming factor may be released or secreted by the non-reprogramming / bystander cell and / or by the reprogramming cell in response to PADI4 activity, in particular increased, enhanced and / or promoted PADI4 activity in said cell. In one embodiment, the kit further comprises and / or the cell medium of the kit further comprises one or more cell autonomous reprogramming factors (e.g. Yamanaka factors) which derive directly from the somatic cell to be reprogrammed as described herein, such as wherein said cell autonomous reprogramming factors are comprised in the form of nucleic acid sequences encoding said cell autonomous reprogramming factors. In a further embodiment, the kit further comprises and / or the cell medium of the kit further comprises an activator of PADI4 activity as described herein. In one embodiment, the non-cell autonomous reprogramming factor is chromatin or a component thereof. Thus in certain embodiments, the non-cell autonomous reprogramming factor that acts non-autonomously on the somatic cell to be reprogrammed and which effects reprogramming or promotes / enhances reprogramming of said somatic cell is extracellular chromatin. In a further embodiment, the non-cell autonomous reprogramming factor is an extracellular chromatin-associated moiety, such as an extracellular chromatin-associated nuclear protein. For example, in response to one or more cell autonomous reprogramming BAB-C-P3557PCT factors (e.g. Yamanaka factors) and / or PADI4 activity (in particular increased, enhanced and / or promoted PADI4 activity in said cells), the non-reprogramming / bystander cells release or secrete chromatin (and / or a chromatin-associated moiety, such as an associated nuclear protein), such as into the culture medium. Alternatively or additionally, the reprogramming cells may release or secrete chromatin (and / or a chromatin-associated moiety, e.g. into the culture medium) in response to one or more cell autonomous reprogramming factors and / or PADI4 activity. In another example, the non-reprogramming / bystander and / or reprogramming cells release or secrete chromatin in response to a known chromatin-release factor or as part of a known chromatin-release process, such as PADI4 activity. Thus in a particular embodiment, the non-reprogramming / bystander cells release or secrete chromatin in response to PADI4 activity, such as increased, enhanced and / or promoted PADI4 activity in said cells. In a further embodiment, the non-reprogramming / bystander cells and / or the reprogramming cells release or secrete chromatin in response to PADI4 activity. Components of chromatin include, without limitation, DNA, histones and / or nucleosomes containing histones. Thus in one embodiment, the non-cell autonomous reprogramming factor released by the non-reprogramming / bystander cell and / or by the reprogramming cell is extracellular DNA. In a further embodiment, the non-cell autonomous reprogramming factor is an extracellular nucleosome. In a yet further embodiment, the non-cell autonomous reprogramming factor released by the non-reprogramming / bystander cell and / or by the reprogramming cells is a histone as described hereinbefore. In a still further embodiment, the non-cell autonomous reprogramming factor is histone H3. In some embodiments, the chromatin, such as the histone, is modified such as post-translationally modified. Modifications include phosphorylation, ubiquitination, SUMOylation, citrullination and ADP- ribosylation, in particular citrullination. Thus in a further embodiment, the signal or signalling agent is a citrullinated histone. In a particular embodiment, the signal or signalling agent is citrullinated histone H3 (H3Cit). As has been previously demonstrated, extracellular chromatin, in particular containing citrullinated histone H3, can be detected in the culture medium of reprogramming cultures. Those cells within the reprogramming culture which can be identified by staining for citrullinated histone H3 are distinct from those undergoing reprogramming (as identified using OCT4 and NANOG expression), i.e. cells which release / secrete the non-cell autonomous reprogramming factor and thus stain positive for citrullinated histone H3 are the non-reprogramming / bystander cells. As also demonstrated previously, the blocking of chromatin sensing pathways in the somatic cell to be reprogrammed (in particular the cGAS / STING and TLR pathways) reduces the enhancing / promoting effect on reprogramming in a reprogramming culture when a conditioned medium as described herein is added. BAB-C-P3557PCT Thus, also provided herein is a use of an agonist or activator of a chromatin sensing pathway in an in vitro method of reprogramming a somatic cell. Further provided is an agonist or activator of a chromatin sensing pathway for use in a method of treating and / or ameliorating a degenerative disease or disorder or for use in the rejuvenation, repair or regeneration of a tissue or organ, wherein said method comprises reprogramming a somatic cell in vivo. Also provided is an agonist or activator of a chromatin sensing pathway for use in a method of rejuvenating, repairing or regenerating a tissue or organ. Further provided is an agonist or activator of a chromatin sensing pathway for use in a method of rejuvenating, repairing or regenerating a tissue or organ, wherein said method comprises reprogramming a somatic cell in vivo. There is also provided a method of rejuvenating, repairing or regenerating a tissue or organ, said method comprising the method of non-cell autonomous modulation of reprogramming defined herein, and said method further comprising administering to a subject an agonist or activator of a chromatin sensing pathway. In a still further aspect, there is provided a method of rejuvenating a tissue or organ, said method comprising administering to a subject an agonist or activator of a chromatin sensing pathway, and said method comprises reprogramming a somatic cell in vivo. In some embodiments, the chromatin sensing pathway agonist / activator is comprised in a pharmaceutical composition, optionally further comprising one or more pharmaceutically acceptable carriers, diluents and / or excipients. Thus in a yet further aspect of the invention, there is provided a pharmaceutical composition comprising an agonist or activator of a chromatin sensing pathway for use in a method of treating and / or ameliorating a degenerative disease or disorder, or for use in the rejuvenation, repair or regeneration of a tissue or organ. Histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes, which are in turn are wrapped into 30nm fibres that form tightly packed chromatin. Histones play important roles in gene regulation and DNA replication. The five families of histones are designated H1 / H5 (linker histones), H2, H3 and H4 (core histones). The nucleosome core is formed of two H2A-H2B dimers and two H3-H4 dimers. The tight wrapping of DNA around histones is to a large degree a result of electrostatic attraction between the positively charged histones and negatively charged phosphate backbone of DNA. The core histones (H2A, H2B, H3 and H4) are relatively similar in structure and are highly conserved through evolution, all featuring a 'helix turn helix turn helix' motif. They also share the feature of long 'tails' on one end of the amino acid structure – this being the location of post-translational modification. Histones may be chemically modified through the action of enzymes to regulate gene transcription. The most common modifications are the methylation of arginine or lysine residues or the acetylation of lysine. Methylation can affect how other BAB-C-P3557PCT proteins such as transcription factors interact with the nucleosomes, while lysine acetylation eliminates a positive charge on lysine thereby weakening the electrostatic attraction between histone and DNA, and resulting in partial unwinding of the DNA making it more accessible for gene expression. Further modifications include modifications of the tail include phosphorylation, ubiquitination, SUMOylation, citrullination and ADP-ribosylation. Citrullination (also known as peptidylarginine deimination) is the conversion of the amino acid arginine to citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code and is instead the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a by- product of enzymes of the nitric oxide synthase family. Citrullination is catalysed by enzymes called arginine deiminases (ADIs) which catalyse the deimination of free arginine, while protein-arginine deiminases or peptidylarginine deiminases (PADIs or PADs) replace the primary ketimine group (>C=NH) by a ketone group (>C=O). Arginine is positively charged at a neutral pH, whereas citrulline has no net charge. This increases the hydrophobicity of the protein, which can lead to changes in protein folding, affecting the structure and function. The immune system can attack citrullinated self-proteins, leading to autoimmune diseases such as rheumatoid arthritis (RA) and multiple sclerosis (MS). The release of chromatin from cells is well known in the context of immune cells and inflammation, in particular in neutrophils which release chromatin / nucleic material (NETs; neutrophil extracellular traps) in a process called NETosis. One PAD enzyme, PADI4 has been shown to be involved in NETosis in immune cells, but this function in other cell types has not been previously described. Thus, the novel and surprising findings presented previously which demonstrate that reprogramming can be reduced / inhibited by PADI4 inhibition / knock- down, suggest that a non-cell autonomous reprogramming factor as described herein may be chromatin, in particular extracellular chromatin comprising citrullinated histone H3. The additional findings previously provided that the inhibition / blocking of chromatin / DNA sensing pathways also reduces / inhibits reprogramming further supports this suggestion. Increased, Enhanced and / or Promoted PADI4 Activity Thus in particular embodiments of the present invention, the activity of PADI4 is increased, enhanced and / or promoted in a cell. The terms “increase(d)”, “enhance(d)”, “promote(d)” and the like are used interchangeably herein and refer to an elevated level of activity compared to in a cell in which there is no such increase, enhancement or promotion of activity. For example, said PADI4 activity may be increased, enhanced and / or promoted in a cell subjected to a PADI4 activator as described herein, wherein said PADI4 activity is compared to a cell not subjected to a PADI4 activator. Alternatively, the activity of PADI4 may be increased, BAB-C-P3557PCT enhanced and / or promoted in a cell subjected to a PADI4 activator as described herein, compared to the activity of PADI4 prior to the cell being subjected to the activator. In one embodiment, the activity of PADI4 is increased, enhanced and / or promoted using an activator. Whilst inhibitors of PADI4 are known in the art (e.g. Cl-amidine, GSK484 and PADI4- targeting shRNA), activators have never before been described. However, examples of activators include those which act directly on PADI4 and those which may affect PADI4 indirectly, i.e. which activate an upstream modulator of PADI4 activity or a co-factor of PADI4 required for its activity. Such indirect PADI4 activators may include moieties considered as “inhibitors” if their method of action is to inhibit or reduce the activity of an upstream negative modulator or a co-factor of PADI4. While the mechanisms through which PADIs are regulated within cells are surprisingly poorly understood, it is well-established that PADIs require calcium for catalysis (Arita et al. (2004) Nat. Struct. Mol. Biol., 11(8):777-783, doi: https: / / doi.org / 10.1038 / nsmb799; and Slade et al. (2015) ACS Chem. Biol., 10(4):1043-1053, doi: https: / / doi.org / 10.1021 / cb500933j) and crystallographic studies have demonstrated that the binding of calcium ions results in a large structural rearrangement and the formation of the active site cleft (Arita et al. (2004)). As demonstrated by the data presented herein, the PADI4 activators identified by the inventors herein reduced the requirement of PADI4 for calcium compared to in the absence of activators at non-saturating calcium concentrations, and in the presence of saturating calcium concentrations increased PADI4 activity. Without being bound by theory it is believed that the identified PADI4 activators affect the binding of calcium and thus the structural / conformational change associated with PADI4 activation. Thus in one embodiment, the activity of PADI4 is increased, enhanced and / or activated using an activator which reduces the dependency of PADI4 activity on calcium. In a further embodiment, the activator elicits a conformational change of PADI4. In a certain embodiment, said conformational change is to the active conformation of PADI4, i.e. the calcium-bound conformation. In a yet further embodiment, the activator promotes and / or stabilises the active conformation of PADI4 observed when bound to calcium. In another embodiment, the activator allosterically modulates PADI4 activity, in particular increased, enhanced and / or promoted PADI4 activity. Also shown herein is the dependency of the exemplified activators for the calcium-binding sites of PADI4. Thus in certain embodiments, the activator interacts with an allosteric binding site of PADI4, altering PADI4 calcium binding. In a further embodiment, the activator interacts with one or more calcium binding site of PADI4, such as two calcium binding sites. PADI4 comprises five calcium binding sites (Arita et al. (2004)), and it is known that substitution of BAB-C-P3557PCT single amino acid residues within these can completely abrogate or at least significantly reduce PADI4 activity. For example, the substitution of either E353 or N373 of human PADI4 for alanine completely abolishes activity and the D388A mutation reduces activity by around 60% (Arita et al. (2004)). However, as demonstrated herein, the novel substitution of aspartic acids 165 and 168 of human PADI4, which are involved in the coordination of Ca3 and Ca5, for alanine also partially reduces PADI4 activity when saturated with calcium. While the amount of calcium required to achieve K50(Ca+)for the D168A and D388A PADI4 mutants was increased compared to wild-type and was near-rescued to wild-type levels by the activators exemplified herein, the increased requirement of the D165A PADI4 mutant for calcium was not altered by the presence of activators. Thus in a yet further embodiment, the activator interacts with an allosteric binding site of PADI4, altering calcium binding at calcium binding site 3 and / or 5 (Ca3 and / or Ca5) of PADI4. In a still further embodiment, the activator binds to PADI4 within the hinge region, such that the activator binds to one or more amino acid residues within the ranges S162-D169, K377-M385, and / or T393-T398. The sequence of human PADI4 (hPADI4) is known and is available as Uniprot accession number: Q9UM07 (last updated: 2010-05-18 v2; checksum: 79D7AF7B3D307A3B; accessed: June 2023). Of particular note is residue C166 in hPADI4 which is not found in the mouse sequence, in which instead there is a phenylalanine at this position (which has a much bulkier side chain compared to cysteine which is relatively small). The sequence of mouse PADI4 (mPADI4) is available as Uniprot accession number: Q9Z183 (last updated: 2011-07-27 v3; checksum: F25B7A7B69285C32; accessed: June 2023). In certain embodiments, the activator is a cyclic peptide activator. Thus in some embodiments, the activity of PADI4 is increased, enhanced and / or promoted using a cyclic peptide activator. In one embodiment, the cyclic peptide activator is an activator of PADI4, in particular of PADI4 activity. As described hereinbefore, the cyclic peptide activator may bind or interact with one or more calcium binding site of PADI4, such as Ca3 and / or Ca5. As demonstrated by the data provided herein, binding of the exemplified cyclic peptide activators to human PADI4 was seen but not to mouse PADI4 or to hPADI4 with a phenylalanine substitution at C166 (i.e. a hPADI4_C166F mutant). Thus in one embodiment, the cyclic peptide activator binds to the hinge region of hPADI4, such that the activator binds to one or more amino acid residues within the ranges S162-D169, K377-M385, and / or T393-T398. Thus, provided herein are novel cyclic peptides which are shown to be able to activate PADI4. In one embodiment, the cyclic peptide activator comprises the amino acid sequence: XEXCRYRQXXXL (SEQ ID NO: 3), BAB-C-P3557PCT wherein X represents any amino acid, and wherein the peptide is cyclised via a thioether bond between the acetylated N-terminus and the cysteine side chain thiol. In a further embodiment, the cyclic peptide activator comprises the amino acid sequence: YESCRYRQVLQL (SEQ ID NO: 1; referred to herein as peptide 11 and PADI4_11), wherein the peptide is cyclised via a thioether bond between the acetylated N-terminus and the cysteine side chain thiol as shown in the following structure: . In a further embodiment, the cyclic peptide activator consists of the amino acid sequence: YESCRYRQVLQL (SEQ ID NO: 1). In a further embodiment, the cyclic peptide of SEQ ID NO: 1 or SEQ ID NO: 2 may comprise one or more, e.g., two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, ten or more or eleven or more amino acid substitutions. In one embodiment, the one or amino acid substitutions is a substitution at an amino acid position selected from tyrosine 1 (Y1), serine 3 (S3), valine 9 (V9), leucine 10 (L10), and glutamine 11 (Q11). In another embodiment, the cyclic peptide activator comprises the amino acid sequence: YEPCRFREILDL (SEQ ID NO: 2; referred to herein as peptide 14 and PADI4_14), wherein the peptide is cyclised via a thioether bond between the acetylated N-terminus and the cysteine side chain thiol as shown in the following structure: BAB-C-P3557PCT . In a yet further embodiment, the cyclic peptide activator consists of the amino acid sequence: YEPCRFREILDL (SEQ ID NO: 2). Thus in one aspect of the invention, there is provided a method of modulating reprogramming of a somatic cell to be reprogrammed, comprising increasing, enhancing and / or promoting PADI4 activity using a cyclic peptide activator. In one embodiment, the cyclic peptide activator comprises or consists of the amino acid sequence: YESCRYRQVLQL (SEQ ID NO: 1). In a further embodiment, the cyclic peptide activator comprises or consists of the amino acid sequence: YEPCRFREILDL (SEQ ID NO: 2). In a further aspect, there is provided a method of reprogramming a somatic cell to be reprogrammed, comprising increasing, enhancing and / or promoting PADI4 activity using a cyclic peptide activator, wherein the PADI4 activity leads to the release or secretion of a non- cell autonomous reprogramming factor, optionally wherein said PADI4 activity is increased, enhanced and / or promoted in a cell other than the somatic cell to be reprogrammed, such as a non-reprogramming or a bystander cell, and optionally wherein the non-cell autonomous reprogramming factor is released / secreted by said cell other than the somatic cell to be reprogrammed. In Vitro, In Vivo and Therapeutic Uses In one aspect of the invention, there is provided the use of a PADI4 activator in an in vitro method of reprogramming a somatic cell to a pluripotent-like state. As will be appreciated from the disclosures herein of novel first-in-class PADI4 activators and that no such activators of PADI4 have previously been described, this aspect of the present invention is not limited to the specific activators exemplified herein. However, in certain embodiments the PADI4 activator for use in methods of reprogramming may be an activator as described herein, in BAB-C-P3557PCT particular the cyclic peptide activators described herein. Thus in some embodiments, the PADI4 activator for use in methods of reprogramming reduces the dependency of PADI4 on calcium. In other embodiments, the PADI4 activator interacts with a calcium binding site of PADI4, such as Ca3 and / or Ca5 as described herein. In yet further embodiments, the PADI4 activator binds to D165 and / or C166 of hPADI4. In certain embodiments, the PADI4 activator for use in methods of reprogramming is a cyclic peptide activator comprising or consisting of the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO: 2. As described hereinbefore, the use of a PADI4 activator may lead to the release / secretion of a non-cell autonomous reprogramming factor in the method of reprogramming a somatic cell to a pluripotent-like state, optionally wherein the non-cell autonomous reprogramming factor is released / secreted from a cell other than the somatic cell to be reprogrammed. Thus in a further aspect, there is provided the use of a PADI4 activator for the release and / or secretion of a non-cell autonomous reprogramming factor in vitro. In one embodiment, the non-cell autonomous reprogramming factor is released / secreted from a non-reprogramming or bystander cell (i.e. from a cell other than the somatic cell to be reprogrammed). In one embodiment, the non-cell autonomous reprogramming factor is isolated, such as purified. In another embodiment, the non-cell autonomous reprogramming factor may be synthetic. In a further embodiment, the non-cell autonomous reprogramming factor has been generated in vitro, such as wherein the non-cell autonomous reprogramming factor is a histone, in particular histone H3, which has been citrullinated using a PAD enzyme in vitro. In another embodiment, in vitro generation of the non-cell autonomous reprogramming factor comprises in vitro synthesis, such as in vitro synthesis of a histone, in particular histone H3, containing one or more citrulline residues in place of one or more arginine residues. In an alternative embodiment, in vitro generation comprises the use of a PADI4 activator to cause the release / secretion of the non-cell autonomous reprogramming factor. In a yet further embodiment, the non-cell autonomous reprogramming factor is a functional analogue of the non-cell autonomous reprogramming factor defined herein, such as a functional analogue of chromatin, extracellular DNA or a histone, such as of histone H3, in particular of citrullinated histone H3. Such analogues will be readily appreciated to include those which share significant structural similarity with the signal or signalling agent. However, analogues which do not share structural similarity but which perform similar functions, e.g. activate the same or similar (such as redundant) signalling pathways, are also within the scope of the term “analogues” herein. BAB-C-P3557PCT Thus as defined hereinbefore, in one embodiment the non-cell autonomous reprogramming factor is chromatin or a component thereof, such as extracellular chromatin. In a further embodiment, the non-cell autonomous reprogramming factor is a nucleosome. In another embodiment, the non-cell autonomous reprogramming factor is extracellular DNA. In a further embodiment, the non-cell autonomous reprogramming factor is a histone. In a yet further embodiment, the non-cell autonomous reprogramming factor is histone H3. In a particular embodiment, the non-cell autonomous reprogramming factor is citrullinated histone H3. In a further embodiment, the in vitro method of reprogramming a somatic cell is a reprogramming culture as defined hereinbefore. In further embodiments, the non-cell autonomous reprogramming factor is an agonist of a chromatin sensing pathway. In a further embodiment, the non-cell autonomous reprogramming factor is a cGAS / STING pathway agonist. In another embodiment, the non- cell autonomous reprogramming factor is a TLR agonist. In certain embodiments, the non- cell autonomous reprogramming factor is an agonist of the TLR2, TLR3 and / or TLR4 pathways, in particular a TLR2 agonist as demonstrated herein, an agonist of the TLR3 pathway which has been shown to sense double stranded DNA, or an agonist of the TLR4 pathway which senses citrullinated histones. In a yet further embodiment, the non-cell autonomous reprogramming factor is an agonist of the extracellular chromatin receptor, CCDC25 (as described in Yang et al. (2020) Nature, 583:133-138, doi: In some embodiments, the methods described herein may be performed in vivo. Thus in a further aspect of the invention, there is provided a PADI4 activator as defined herein for use in an in vivo method of reprogramming. In another aspect, there is provided an in vivo method of reprogramming, said method comprising administering the PADI4 activator defined herein to a subject. Thus in certain embodiments, the in vivo methods of reprogramming comprise administering the PADI4 activator to a subject. Such administration may be systemically, e.g. enteral or parenteral, such as via intravenous infusion, or locally, such as directly into the tissue or organ to be treated / rejuvenated, e.g. by topical administration. In other embodiments, the PADI4 activator may be administered singly or in combination with one or more reprogramming factors, such as cell autonomous reprogramming factors and / or non-cell autonomous reprogramming factors. Such combined administration may be concurrently or sequentially as suitably determined by the skilled practitioner. In a further aspect, there is provided the PADI4 activator defined herein for use in a method of treating and / or ameliorating a degenerative disease or disorder. In another aspect, there is BAB-C-P3557PCT provided the PADI4 activator defined herein for use in a method of treating and / or ameliorating a degenerative disease or disorder, wherein said method comprises reprogramming a somatic cell in vivo. Thus in certain embodiments, the subject is suffering or is at risk of suffering from a degenerative disease or disorder. In a further aspect, there is provided a method of treating and / or ameliorating a degenerative disease or disorder, said method comprising the method of modulation of reprogramming defined herein, and said method further comprising administering to a subject the PADI4 activator defined herein. In a yet further aspect, there is provided a method of treating and / or ameliorating a degenerative disease or disorder, said method comprising administering to a subject the PADI4 activator defined herein, and said method comprises reprogramming a somatic cell in vivo. In one embodiment, the subject is suffering or is at risk of suffering from a degenerative disease or disorder of the skin. In an alternative embodiment, the subject is suffering or is at risk of suffering from a degenerative disease or disorder of the pancreas, such as type 2 diabetes. In a further embodiment, the subject is suffering or is at risk of suffering from a neurodegenerative disorder. In a yet further embodiment, the subject is suffering or is at risk of suffering from a disease or disorder of the blood and / or bone marrow. In a still further embodiment, the subject is suffering or is at risk of suffering from a disease or disorder of the heart. Thus, in one embodiment, the disease or disorder is cardiovascular disease. In a further embodiment, the disease or disorder is a cardiomyopathy. In another embodiment, the disease or disorder is ischaemic heart disease. In a yet further embodiment, the disease or disorder is cardiac arrhythmia. In another embodiment, the disease or disorder is heart failure. In another embodiment, the subject is suffering or is at risk of suffering from a disease or disorder of the gut. In a yet further embodiment, the subject is suffering or is at risk of suffering from a disease or disorder of the eye. In another embodiment, the methods described herein are for the rejuvenation of a tissue or organ. Rejuvenation is useful in the reversal of the effects of ageing on said tissue or organ. Thus in some embodiments, the tissue or organ is aged, such as is obtained from an aged subject or is in an aged subject. Thus in another aspect of the invention, there is provided the PADI4 activator defined herein for use in a method of rejuvenating a tissue or organ. In a further aspect, there is provided the PADI4 activator defined herein for use in a method of rejuvenating a tissue or organ, wherein said method comprises reprogramming a somatic cell in vivo. In a yet further aspect, there is provided a method of rejuvenating a tissue or organ, said method comprising the method of modulation of reprogramming defined herein, and said method further comprising administering to a subject the PADI4 activator defined herein. In a still further aspect, there is provided a method of rejuvenating a tissue or organ, said method comprising administering to a subject the PADI4 activator defined herein, and said method BAB-C-P3557PCT comprises reprogramming a somatic cell in vivo. In certain embodiments, the method of rejuvenating a tissue or organ comprises reprogramming a somatic cell according to the methods defined herein and providing said reprogrammed somatic cell to a subject in need thereof. In a further embodiment, the somatic cell to be reprogrammed may be derived from the subject in need of treatment and / or amelioration of a degenerative disease or disorder, or from the subject in need of tissue or organ rejuvenation. Thus in some embodiments, the methods described herein are performed ex vivo. In another embodiment, the methods described herein are for the regeneration or repair of a tissue or organ. Regeneration and / or repair of tissue or organ may be required in response or following damage, such as damage due to an acute injury or disease, or due to a chronic disease or disorder. Damage may also occur in aged tissues and organs. Thus in some embodiments, the tissue or organ is damaged, such as damaged as a result of an acute or chronic disease or disorder. In other embodiments, the damaged tissue or organ is aged, such as is obtained from an aged subject or is in an aged subject. Thus in another aspect of the invention, there is provided the PADI4 activator defined herein for use in a method of regenerating or repairing a tissue or organ. In a further aspect, there is provided the PADI4 activator defined herein for use in a method of regenerating or repairing a tissue or organ, wherein said method comprises reprogramming a somatic cell in vivo. In a yet further aspect, there is provided a method of regenerating or repairing a tissue or organ, said method comprising the method of modulation of reprogramming defined herein, and said method further comprising administering to a subject the PADI4 activator defined herein. In a still further aspect, there is provided a method of regenerating or repairing a tissue or organ, said method comprising administering to a subject the PADI4 activator defined herein, and said method comprises reprogramming a somatic cell in vivo. In certain embodiments, the method of regenerating or repairing a tissue or organ comprises reprogramming a somatic cell according to the methods defined herein and providing said reprogrammed somatic cell to a subject in need thereof. In a further embodiment, the somatic cell to be reprogrammed may be derived from the subject in need of treatment and / or amelioration of a degenerative disease or disorder, or from the subject in need of tissue or organ regeneration or repair. Thus in some embodiments, the methods of non-cell autonomous modulation of reprogramming described herein are performed ex vivo. In one embodiment the use of the PADI4 activator defined herein additionally comprises one or more cell autonomous reprogramming factors and / or one or more non-cell autonomous reprogramming factors as described herein. In another embodiment, the in vitro, ex vivo or in vivo method of modulation of reprogramming additionally comprises one or more cell BAB-C-P3557PCT autonomous reprogramming factors and / or one or more non-cell autonomous reprogramming factors as described herein. In a further embodiment, the one or more cell autonomous reprogramming factors comprise one or more Yamanaka factors as described herein. In a yet further embodiment, the Yamanaka factors are selected from one or more of: OCT4, KLF4, c-MYC, SOX2, LIN28, NANOG, ESSRRB, NR5A2 and / or C / EBPα, in particular one or more of: OCT4, KLF4, c-MYC, SOX2. In another embodiment, the one or more Yamanaka factors do not include c-MYC. In a still further embodiment, the non-cell autonomous reprogramming factor is chromatin or a component thereof, or a functional analogue thereof as described herein. In some embodiments, the PADI4 activator is comprised in a pharmaceutical composition, optionally further comprising one or more pharmaceutically acceptable carriers, diluents and / or excipients. Thus in a yet further aspect of the invention, there is provided a pharmaceutical composition comprising the PADI4 activator for use in a method of treating and / or ameliorating a degenerative disease or disorder, or for use in the rejuvenation, regeneration or repair of a tissue or organ. In further aspects, there is provided a method of treating and / or ameliorating a degenerative disease or disorder or a method of rejuvenating, regenerating or repairing a tissue or organ, said method comprising administering to a subject the pharmaceutical composition comprising the PADI4 activator defined herein. In a further aspect, there is provided a reprogrammed somatic cell obtainable by the method of modulation of reprogramming defined herein. In one aspect, there is provided a pharmaceutical composition comprising said reprogrammed somatic cell. In one embodiment, the pharmaceutical composition optionally further comprises one or more pharmaceutically acceptable carriers, diluents and / or excipients. Thus in another aspect, there is provided the reprogrammed somatic cell obtained by the methods described herein or the pharmaceutical composition comprising said reprogrammed somatic cell for use in a method of treating and / or ameliorating a degenerative disease or disorder, or for use in a method of rejuvenating, regenerating or repairing a tissue or organ. In a further aspect, there is provided a method of treating and / or ameliorating a degenerative disease or disorder or a method of rejuvenating, regenerating or repairing a tissue or organ, said method comprising administering to a subject the reprogrammed somatic cell obtained by the methods described herein or the pharmaceutical composition comprising said reprogrammed somatic cell. BAB-C-P3557PCT In one embodiment, the somatic cell is fully reprogrammed, such as to an iPSC or an iPSC- like state. In an alternative embodiment, the somatic cell is partially / incompletely reprogrammed, such as to a pluripotent-like state. It will be appreciated that references herein to a patient or subject relate equally to animals and humans and that the invention finds particular utility in veterinary treatment of any of the above mentioned diseases, disorders and conditions which are also present in said animals. It will also be appreciated that references herein to “treatment” and “amelioration” include such terms as “prevention”, “reversal” and “suppression”. Furthermore, such references include administration of the reprogrammed somatic cell or composition comprising the reprogrammed somatic cell as defined herein prior to the onset of the disease or disorder, e.g. wherein the subject is at risk of the disease or disorder. Administration of the reprogrammed somatic cell or composition as defined herein may also be anticipated after the induction event of the injury, damage, disease or disorder, either before clinical presentation of said disease or disorder, or after symptoms manifest. Such references further include performing the method of non-cell autonomous modulation of reprogramming as defined herein in vivo either prior to the onset of the disease or disorder, or after the induction event of the disease or disorder. Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which this invention belongs. As used herein, the term “about” when used herein includes up to and including 10% greater and up to and including 10% lower than the value specified, suitably up to and including 5% greater and up to and including 5% lower than the value specified, especially the value specified. The term “between” as used herein includes the values of the specified boundaries. Throughout the specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations thereof such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer, step, group of integers or group of steps but not to the exclusion of any other integer, step, group of integers or group of steps. In addition, as used herein and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example reference to “a reprogrammed somatic cell” includes two or more such cells, or reference to “a non-reprogramming / bystander cell” includes two or more such non-reprogramming or BAB-C-P3557PCT bystander cells, i.e. two or more cells other than the somatic cell(s) to be reprogrammed and the like. It will be understood that all embodiments described herein may be applied to all aspects of the invention and vice versa, and such combinations would be readily apparent from the description provided herein and to those skilled in the art. Other features and advantages of the present invention will be apparent from the description provided herein. It should be understood, however, that the description and the specific examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications will become apparent to those skilled in the art. The invention will now be described using the following, non-limiting examples: EXAMPLES Example 1: A strategy for Identifying Potent and Conformation-Selective PADI4 Binders Random non-standard Peptide Integrated Discovery (RaPID) selections (discussed in Goto and Suga (2021) Chem. Res. 54(18):3604–3617, doi: https: / / doi.org / 10.1021 / acs.accounts.1c00391) were performed against recombinant human PADI4 (hPADI4) under three different conditions to identify cyclic peptides that bind to different conformations of PADI4 and thus have diverse effects on its activity. To identify peptides that bound preferentially to either the calcium-free ‘inactive’ form or the calcium-bound ‘active’ form of the enzyme, selections were carried out using a calcium-free buffer (Selection 1) or buffer containing concentrations of calcium (10mM), known to achieve full PADI4 activity in vitro (Selection 2; Figure 1A). For the final selection the active, substrate-bound form of PADI4 was mimicked to select for peptides that stabilised this protein conformation and consequently activate PADI4 at lower calcium concentrations. This selection was therefore performed in the presence of 10mM CaCl2 and with the active site occupied by the covalent pan-PADI inhibitor Cl-amidine (200μM Cl-amidine; Selection 3). These parallel selections were carried out using biotinylated PADI4 as the bait and libraries of peptides consisting of a random region of 6-12 residues initiated either with N-chloroacetyl-D-tyrosine (Selection 1 and 2) or N-chloroacetyl- L-tyrosine (Selection 3) to permit peptide cyclisation with a cysteine in the encoded peptide sequence. Sheared salmon sperm DNA was included in the selection buffers to ameliorate high levels of non-specific DNA binding to PADI4, which were observed during initial optimisations. Eight rounds of selection were performed and library DNA recovered from later rounds sequenced to identify the most enriched sequences. Following sequence deconvolution, twelve of the most enriched peptide sequences across the three selections BAB-C-P3557PCT were selected for solid-phase peptide synthesis (SPPS) and further characterisation (Figure 1B). To validate the enriched sequences, the binding affinities to PADI4 were measured using surface plasmon resonance (SPR). Affinities were measured both in the presence and absence of calcium (Figure 1B and Table 1). Peptides were identified that bound tightly both in the presence and absence of calcium (PADI4_1 and PADI4_7). Four others bound tightly to the calcium-bound form of the enzyme but were unable to bind in the absence of calcium (PADI4_2, PADI4_3, PADI4_11 and PADI4_14, KD> 5µM). Peptides that did not show binding to PADI4 by SPR were removed from subsequent analyses. The conditions used in Selection 3 were carried out to interrogate the mode of binding of the identified peptides (10mM Ca2+, PADI4 blocked with Cl-amidine). The two peptides identified from Selection 3 (PADI4_11 and PADI4_14) were unaffected by Cl-amidine blocking the active site (Figure 1B and Table 1), indicating they may bind elsewhere on PADI4. Table 1: SPR data of the macrocyclic peptides enriched in the different RaPID selections Example 2: A Novel High Content Imaging Method for Quantification of PADI4 Activity in Cells To enable comparative and quantitative measurements of peptide activity in cells, a high content microscopy method was set up based upon the quantification of cellular citrullinated histone H3 (H3Cit) levels at single cell resolution by immunofluorescence, as a read-out of cellular PADI4 activity. To allow for tractable measurements, a non-transformed, adherent cell type was identified where PADI4 activation does not depend upon the use of calcium ionophores, which can be damaging to cells. It was discovered that PADI4 is activated in pluripotent mouse embryonic stem (mES) cells when transitioned from primed pluripotency to naïve pluripotency (Marks et al. (2012) Cell, 149(3):590-604, doi: https: / / doi.org / 10.1016 / j.cell.2012.03.026; data not shown). This is achieved by switching from ground pluripotency BAB-C-P3557PCT culture conditions (Serum / LIF), to naïve pluripotency culture confitions (2i / LIF), which involves the withdrawal of serum and provision of knock-out serum replacement and inhibitors to kinases MEK and GSK3 ^ (2i). Using mES cells stably expressing human PADI4 it can be seen from the mean H3Cit intensity in these cells that transition from Serum / LIF to 2i / LIF for 3 hours is sufficient to induce a measurable induction of cellular PADI4 activity. This method is quantitative and treatment with Cl-amidine can achieve dose response read-outs. Example 3: PADI4_11 is a Novel First-in-Class Cell Permeable PADI4 Activator It was hypothesised that the strong binders that bind outside the active site may activate PADI4. To test this, a modified Colour Developing Reagent (COLDER) assay (an otherwise established colorimetric activity assay with the model substrate N^-benzoyl-L-arginine ethyl ester; Knipp & Vasák (2000) Anal. Biochem., 286(2):257-264, doi: https: / / doi.org / 10.1006 / abio.2000.4805) was performed, where the calcium concentration was titrated at a constant concentration of cyclic peptide (30µM). In the absence of peptide, PADI4 required 225 ± 10µM calcium to achieve 50% of maximal activity under saturating calcium concentrations (K50(Ca2+)). In the presence of PAD4_11 and PAD4_14, the two peptides identified from selection 3, this calcium requirement was reduced 4-fold (Figure 2A). Additionally, even under saturating calcium concentrations, the presence of PADI4_11 or PADI4_14 increased PADI4 activity 36% or 12% over the DMSO control respectively, suggesting a hyper-active conformation of PADI4 in the presence of either of these two peptides. This activation was dependent on the peptide concentration, requiring 1.6 ± 0.6µM PAD4_11 and 6.5 ± 0.1µM PAD4_14, to achieve 50% activation (AC50) at 0.1mM calcium (Figure 2B). The isozyme specificity of both PADI4_11 and PADI4_14 were also confirmed using COLDER activity assays with all the catalytically active human PADIs (Figures 2C-E). PADI4_11 was taken forward as the most potent peptide identified. A negative control, PADI4_11_inverted, was synthesised which consists of the parental sequence with the second and third amino acids reversed. To test whether PADI4_11 can activate PADI4 within cells, its cell permeability was assessed using CAPA assays, an established cell penetration assay (Peraro et al. (2018) J. Am. Chem Soc., 140(36):11360-11369, doi: https: / / doi.org / 10.1021 / jacs.8b06144). This demonstrated that both PADI4_11 and PADI4_11inv are cell permeable, with CP50s of 3.08 ± 1.18µM and 1.97 ± 0.49µM respectively (Figure 2F). Using the above-described high content imaging method for quantification of cellular H3Cit, it was observed that PADI4_11 can induce PADI4 activation in a dose-dependent manner in hPADI4-stable mES cells (Figures 2G-H), while PADI4_11_inverted does not have the same effect (Figure 2H). Since calcium influx resulting from a compromise to cell membrane integrity may be sufficient to induce intracellular PADI4 BAB-C-P3557PCT activation, it was tested whether PADI4_11 results in increased calcium influx, using live cell calcium imaging. As shown in Figure 2I, neither PADI4_11, nor PADI4_11_inverted were responsible for increased intracellular calcium levels. Similarly, no evidence of increased permeability of the PADI4_11-treated cells to external factors was observed, as demonstrated by the fact that peptide treatment does not make the cells permeable to propidium iodine (Figure 2J). These results also indicate that PADI4_11 is not toxic to the cells, and this was confirmed using live cell imaging of cell death over a period of 48 hours after treatment (data not shown). Therefore, the activating effect of PADI4_11 is specific and on-target. Example 4: The Mechanism of Action of PADI4_11 Provides Proof-of-Principle for Allosteric Regulation of PADI4 One possibility for the activation of PADI4 at physiological calcium concentrations, which however has not been demonstrated to date, is that a mode of allosteric regulation, such as a post-translational modification or binding to an interacting protein, elicits a conformational change that allows activation of the enzyme at lowers calcium levels. Given that PADI4_11 can induce activation of PADI4 at 4-fold lower calcium, the underlying mechanism was investigated. Direct calcium binding by the peptide itself was first excluded through isothermal titration calorimetry between PAD4_11 and calcium (data not shown). We then performed an alanine scan of PAD4_11, which revealed that residues E2, R5 and R7 were particularly important for activation (Figure 3A). Substitution of R5 and R7 with alanine almost abrogated their binding to and activation of PADI4, which was restored by changing either residue to a lysine. This highlighted the necessity of a positively charged amino acid in these positions, potentially to interact with negatively charged amino acids in PADI4. By contrast, substitution of E2 by alanine, aspartic acid or glutamine all resulted in loss of activity, suggesting the importance of the appropriate side chain length as well as negative charge in position 2 of PADI4_11. Although no individual amino acid in the linear region of PADI4_11 (residues 8-12) appeared critical individually, a truncated 6mer peptide PADI4_11b, was inactive. In addition, it was confirmed that cyclisation is necessary for binding, as substitution of C4 to alanine abrogated binding and activity of the peptide. A biochemical approach was first taken to identify the region of PADI4 to which peptide 11 binds. It was hypothesised that, given that peptide binding reduced calcium dependency, it was likely that the peptide interacted with one of the 5 calcium binding sites previously characterised by Arita et al. and others. Therefore, 5 PADI4 mutants were produced, each with a single amino acid substitution that was either known, or predicted based on structure, to alter calcium binding at each of the five sites (Figures 3B-C). As previously reported (Arita et al. (2004)), substitution of either E353 or N373 for alanine completely abolished PADI4 BAB-C-P3557PCT activity, whilst D388A retained 40% of wild-type activity. Aspartic acids 165 and 168 are involved in the coordination of Ca3 and Ca5 and the effect of their substitution had not been reported before. Substitution of either of these residues for alanine did not substantially affect the activity of the protein when saturated with calcium (Figure 3D). To investigate whether PADI4_11 was able to activate any of these variants the K50(Ca2+)for each protein variant was determined in the presence or absence of PADI4_11 (30µM) using COLDER assays. Incubation of the peptide with the catalytically inactive variants (E353A and N373A) did not rescue their activity. Variants D168A and D388A required more calcium to activate than the wild-type protein with a K50(Ca2+)of 3.0mM and 1.5mM respectively, but their calcium requirement was decreased by about 3-fold when incubated with peptide PADI4_11 (Figures 3B-D), almost to the same degree as with the wild-type protein, suggesting that the activation mechanism had not been disrupted. However, whilst PADI4_D165A also required more calcium than the wild-type protein for activation (K50(Ca2+)= 2.7 ± 0.5mM), this requirement changed very little when incubated with PADI4_11. This suggests that PADI4_11 may bind near aspartic acid 165. SPR analysis of PADI4_D165A binding to PADI4_11 confirmed that affinity between the protein and peptide was more than 30 times reduced in the D165A mutant, compared to the wild-type protein (KD = 6 ± 2µM). Based on the knowledge that mouse PADI4 is also not activated by PADI4_11, it was investigated whether there were sequence differences between human and mouse PADI4 around hPADI4_D165 that could explain this lack of activation. The calcium binding residues themselves are highly conserved between human and mouse PADI4. Nonetheless, sequence alignment of the region around human residue 165 revealed a striking change at position 166, where a substitution of a cysteine to phenylalanine is found. If PADI4_11 was binding in this region, it could be predicted that substitution of the small cysteine side chain with the bulky side chain of phenylalanine would have a significant impact on the binding between the protein and the peptide (Figure 3E). To test this hypothesis, hPADI4_C166F was generated. This variant has similar activity to the wild-type protein (Figure 3F), suggesting that the variant is correctly folded. It was then investigated whether PADI4_11 was able to activate PADI4_C166F in vitro. As with hPADI4_D165A, hPADI4_C166F had almost the same calcium K50(Ca2+) in the presence (K50(Ca2+) = 0.44 ± 0.27nM) or absence (K50(Ca2+) = 0.38 ± 0.25mM) of PADI4_11 (Figure 3G), suggesting that C166 is necessary for PADI4_11 activation of hPADI4. By contrast, a more conservative substitution of serine for cysteine at position 166 weakened, but did not abolish, peptide binding and reduced the calcium required for activation of PADI4_C166S by 2-fold (Figure 3H) suggesting that the cysteine thiol itself was not critical for binding, but that hydrogen bonding ability may be important. BAB-C-P3557PCT Example 5: PADI4_11 Promotes the Reprogramming of Human Fibroblasts The transition to induced pluripotency during cell reprogramming is a multi-step process that involves several cellular changes. One of these changes is the mesenchymal to epithelial transition (MET), which involves up-regulation of epithelial markers such as E-cadherin and down-regulation of mesenchymal markers like vimentin (Li et al., Cell Stem Cell, 2010: doi: https: / / doi.org / 10.1016 / j.stem.2010.04.014; Samavarchi-Tehrani et al., Cell Stem Cell, 2010: doi: https: / / doi.org / 10.1016 / j.stem.2010.04.015; Polo & Hochedlinger, Cell Stem Cell, 2010: doi: https: / / doi.org / 10.1016 / j.stem.2010.05.018). Comparative levels of these markers were therefore used to track the reprogramming process and progress of cells towards pluripotency. Human adult fibroblasts were seeded and transduced with viral vectors expressing the Yamanaka factors Oct4, Sox2, Klf4 and c-Myc. PADI4_11 was added on Day 7, at the time of seeding on vitronectin matrix. Colony formation assessment and immunostaining were performed on Day 12 (Figure 4A). As shown in Figure 4B, the appearance of colonies at day 12 in the PADI4_11 treated cultures is suggestive of PADI4_11 treatment leading to increased and / or faster reprogramming. Further, treatment with PADI4_11 is shown to enhance the reprogramming-associated changes (up-regulation of E-cadherin, down-regulation of Vimentin) and have the opposite effect of PADI4 inhibition by Cl-amidine (Figure 4C). Immunocytochemistry analysis of human adult fibroblasts reprogramming cultures demonstrated that treatment with PADI4_11 leads to activation of PADI4 (H3Cit) and reduction of Vimentin expression, a change associated with reprogramming. Typically, H3Cit activation is not seen in reprogramming cultures until Day 17, therefore these results again suggest that PADI4_11 promotes or expedites the reprogramming process. In summary, the experiments presented with PADI4_11 indicate that PADI4_11 promotes MET- related changes in reprogramming cultures, while the well-established PADI4 inhibitor Cl- amidine has the opposite effect. Funding Acknowledgement The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no.6657292.

Claims

BAB-C-P3557PCT CLAIMS 1. A method of modulating reprogramming of a somatic cell to be reprogrammed, comprising increasing, enhancing and / or promoting the activity of peptidylarginine deiminase 4 (PADI4) enzyme.

2. The method of claim 1, wherein reprogramming of the somatic cell is increased, enhanced and / or promoted, optionally wherein the somatic cell is reprogrammed to a pluripotent-like state, such as is fully reprogrammed or partially reprogrammed to a pluripotent-like state.

3. The method of claim 1 or claim 2, wherein the activity of PADI4 is increased, enhanced and / or activated in a cell other than the somatic cell to be reprogrammed.

4. The method of claim 3, wherein the cell other than the somatic cell to be reprogrammed is a non-reprogramming cell, such as a bystander cell.

5. The method of any one of claims 1 to 4, wherein the increased, enhanced and / or promoted PADI4 activity leads to the release or secretion of a non-cell autonomous reprogramming factor, optionally wherein the release or secretion is from the cell other than the somatic cell to be reprogrammed, such as from a non-reprogramming cell and / or a bystander cell.

6. The method of claim 5, wherein the release of the non-cell autonomous reprogramming factor is increased, enhanced and / or promoted.

7. The method of claim 5 or claim 6, wherein the method is performed in an in vitro reprogramming culture and the non-cell autonomous reprogramming factor is released or secreted into the culture medium.

8. The method of any one of claims 5 to 7, wherein the non-cell autonomous reprogramming factor is chromatin or a component thereof, or a functional analogue thereof.

9. The method of claim 8, wherein the non-cell autonomous reprogramming factor is an extracellular nucleosome or a component thereof, or a functional analogue thereof.BAB-C-P3557PCT 10. The method of claim 8 or claim 9, wherein the non-cell autonomous reprogramming factor is extracellular DNA, or a functional analogue thereof.

11. The method of any one of claims 8 to 10, wherein the chromatin comprises histone H3 or a functional analogue thereof, such as wherein the non-cell autonomous reprogramming factor is histone H3.

12. The method of any one of claims 8 to 11, wherein the chromatin or component thereof is modified, such as post-translationally modified.

13. The method of claim 12, wherein the chromatin or component thereof is citrullinated.

14. The method of claim 12 or claim 13, wherein the chromatin or component thereof comprises a citrullinated histone.

15. The method of claim 14, wherein the citrullinated histone is citrullinated histone H3.

16. The method of any one of claims 5 to 15, wherein the non-cell autonomous reprogramming factor is an agonist of a chromatin sensing pathway.

17. The method of any one of claims 5 to 16, wherein the non-cell autonomous reprogramming factor enhances or promotes the reprogramming effect of one or more cell autonomous reprogramming factors, such as one or more Yamanaka factors which derive from the somatic cell to be reprogrammed, in particular which are expressed by the somatic cell to be reprogrammed.

18. The method of any one of claims 1 to 17, wherein the activity of PADI4 is increased, enhanced and / or activated using an activator which reduces the dependency of PADI4 activity on calcium, elicits a conformational change of PADI4, promotes and / or stabilises the active conformation of PADI4 observed when bound to calcium, and / or allosterically modulates PADI4 activity.

19. The method of claim 18, wherein the activator interacts with an allosteric binding site of PADI4.BAB-C-P3557PCT 20. The method of claim 19, wherein the activator binds to PADI4 within the hinge region, such that the activator binds to one or more amino acid residues within the ranges S162-D169, K377-M385, and / or T393-T398.

21. The method of any one of claims 1 to 20, wherein the activity of PADI4 is increased, enhanced and / or activated using a cyclic peptide activator.

22. The method of claim 21, wherein the cyclic peptide activator comprises an amino acid sequence selected from: XEXCRYRQXXXL (SEQ ID NO: 3) wherein X represents any amino acid, such as wherein the cyclic peptide activator comprises an amino acid sequence YESCRYRQVLQL (SEQ ID NO: 1; referred to herein as peptide 11 and PADI4_11); or YEPCRFREILDL (SEQ ID NO: 2; referred to herein as peptide 14 and PADI4_14), wherein the peptide is cyclised via a thioether bond between the acetylated N-terminus and the cysteine side chain thiol.

23. Use of a PADI4 activator in an in vitro method of reprogramming a somatic cell to a pluripotent-like state, optionally wherein the activator is an activator as defined in any one of claims 18 to 22.

24. A pharmaceutical composition comprising a PADI4 activator, the composition optionally further comprising one or more pharmaceutically acceptable diluents, excipients and / or carriers, optionally wherein the activator is an activator as defined in any one of claims 18 to 22.

25. The pharmaceutical composition of claim 24 for use in a method of treating and / or ameliorating a degenerative disease or disorder or in a method of rejuvenating, regenerating or repairing a tissue or organ.

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