Bag3 is a multifunctional
protein expressed predominantly in the heart, the
skeletal muscle, the
central nervous system and in many cancers. Although BAG3 was cloned only a decade ago, studies have shown that
genetic variants, particularly those that result in haplo-insufficiency, can lead to severe left ventricular dysfunction; however, the full mechanisms responsible have remained obscure. To obviate the influence of
heart failure itself on the
biology of Bag3, transgenic mice harboring a single
allele knock-out were studied between 8 and 10 weeks of age before any obvious signs of
heart failure were evident. The results were surprising and informative. First, it was found that despite a normal
phenotype, young Bag3+ / − had marked changes in the
proteome that were characterized by changes in proteins associated with
metabolism and
apoptosis. Consistent with this finding, a decrease in the levels of critical proteins charged with maintaining the mitochondrial
membrane potential was observed. It was also found that young mice shifted from a balance between the extrinsic and intrinsic pathways of
apoptosis. However, in the presence of stress and the absence of Bag3 there was a shift from a balanced to an extrinsic dominant
system (cleaved
caspase 8). The diverse array of critical pathways regulated by Bag3 suggests a more important role especially during stress and that this role might include serving as an
intracellular glue that holds proteins where they can be most effective rather than having them meet accidentally.