Abstract 50: GATA4-Dependent Control of Myocyte Protein Quality Mediated by HSPB7
The transcription factor GATA4 plays essential roles in heart development, including regulation of cardiomyocyte stress response. GATA4 depletion leads to defective cardiogenesis while GATA4 overexpression is protective in stress models including myocardial ischemia and cardiotoxin administration. However, the molecular basis for GATA4 function remains poorly understood. We discovered that expression of small heat shock protein beta 7 (HSPB7) is regulated by GATA4 and that HSPB7 depletion causes defects in cardiomorphogenesis. The goal of our study is to determine the cellular and molecular mechanism of action of HSPB7 in cardiomyogenesis. Through proteomic mass spectrometry and co-immunoprecipitation, we identified Filamin C (FLNC), a large sarcomeric protein, as an HSPB7 binding partner. FLNC mutations lead to pathological aggregate formation and progressive myopathy. Previous work suggests HSPB7 may prevent damaged protein aggregation through autophagic pathways, and our studies in zebrafish embryos suggest HSPB7 depletion alters the morphology and cytoskeletal architecture of ventricular cardiomyocytes, in particular the trabeculae. We hypothesize that HSPB7 facilitates myocyte function by processing damaged FLNC to prevent aggregation. Preliminary data suggest that BAG3 is also a part of this chaperone complex, linking HSPB7 to the process of Chaperone Assisted Selective Autophagy. Interestingly, disturbing autophagic pathways in the chick embryo precipitates dextrocardia, a phenotype observed in HPSB7 morphant embryos. We also generated a spectrum of defined HSPB7 mutant alleles using TALEN technology. Four such alleles have been bred to homozygosity including a suspected N-terminal truncation that deletes a putative regulatory polyserine stretch. Although homozygous mutants do not exhibit an overt cardiac phenotype, we hypothesize that loss of this polyserine domain may result in a hypomorphic HSPB7 protein. Exposure of mutant embryos to various stressors, including increased temperature (32°C) and the autophagy inducer rapamycin (1μM), led to an increase in phenotypes we have observed in HSPB7 morphants, including dextrocardia and the incidence of defects in cardiac morphology.
Author Disclosures: E.J. Mercer: None. T. Evans: 2. Research Grant; Significant; NIH R01 HL111400.
- © 2015 by American Heart Association, Inc.