The Role of the Co-Chaperone Protein Bcl-2-Associated Athanogene 3 (BAG3) at the Cardiomyocyte Sarcomere
Date of Award
Doctor of Philosophy (PhD)
The co-chaperone protein Bcl-2-associated athanogene-3 (BAG3) is a central mediator of cellular protein quality control through autophagy. In the heart, decreased BAG3 activity through mutation or decreased expression are linked to dilated cardiomyopathy (DCM). Unfortunately, though clinical data indicates BAG3 mutations are a definitive cause of DCM, few mechanistic studies had been performed to decipher the fundamental role of BAG3 in cardiomyocytes. However, several studies suggested BAG3 was involved in maintenance of the sarcomere, the molecular contractile structure in muscle cells. The goals of this dissertation were to: 1. Determine the functional significance of BAG3 for the sarcomere, 2. Identify the mechanism(s) of sarcomere maintenance by BAG3, and 3. Assess the impact of proposed BAG3-targeting heart failure and cancer therapies on cardiomyocytes. Using biophysical assays to assess sarcomere function in cardiomyocytes from human DCM and mouse models of BAG3 deficiency/mutation, we identified BAG3 was required for maximal sarcomere contractile strength. To determine the mechanism of BAG3-dependent sarcomere maintenance, we employed a combination of molecular biology, cell culture, super-resolution imaging, and proteomics approaches and found that BAG3 maintained sarcomere function through chaperone-assisted selective autophagy (CASA) with HSP70 and HSPB8. We further found this CASA complex localized to the Z-disc in cardiomyocytes and mediated turnover of sarcomere proteins. In a study of human myocardial samples, we found sarcomeric BAG3 decreased in male DCM patients, indicating a potential sex-dependent mislocalization in disease. Importantly, in male mice with heart failure secondary to myocardial infarction, BAG3 gene therapy restored CASA protein clearance and rescued sarcomere function. However, exposure of cardiomyocytes to proposed small molecule cancer therapeutics that disrupt the CASA complex resulted in reduced autophagy flux, increased apoptosis, and sarcomere structural disintegration, suggesting such treatments will be cardiotoxic. Sarcomere protein quality control mechanisms are poorly understood. Herein, we have characterized a mechanism of sarcomere protein turnover in the heart. We hope others will build on these findings to further advance our collective understanding of this complex process. Finally, we hope our findings will help to inform future therapies for both genetic and non-genetic cases of heart failure where reduced BAG3 activity is implicated.
Martin, Thomas Gwynn, "The Role of the Co-Chaperone Protein Bcl-2-Associated Athanogene 3 (BAG3) at the Cardiomyocyte Sarcomere" (2021). Dissertations. 3895.
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