Discovery Of A Cardioprotective Mechanism Regulated By Hspb7

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Small heat shock proteins are chaperones with variable mechanisms of action. The function of cardiac family member Hspb7 is unknown, despite being identified through GWAS as a potential cardiomyopathy risk gene. I discovered that zebrafish hspb7 mutants develop cardiomegaly with mild focal cardiac fibrosis and sarcomeric abnormalities. Strikingly, significant mortality was observed in hspb7 mutants subjected to exercise stress, demonstrating a genetic and environmental interaction that determines disease outcome. I identified large sarcomeric proteins FilaminC and Titin as Hspb7 binding partners. In the absence of Hspb7, FilaminC was prone to aggregate formation, which arises following protein damage and is detrimental to cardiac function. Damaged FilaminC undergoes autophagic processing to maintain sarcomeric homeostasis. I discovered that loss of Hspb7 in zebrafish or human cardiomyocytes stimulated autophagic pathways and expression of the sister gene encoding Hspb5. Finally, inhibiting autophagy caused FilaminC aggregation in HSPB7 mutant human cardiomyocytes and developmental cardiomyopathy in hspb7 mutant zebrafish embryos, demonstrating enhanced reliance on autophagy in the absence of Hspb7. These studies highlight the importance of damage-processing networks in cardiomyocytes, and a previously unrecognized role in this context for Hspb7. Additionally, hspb7 mutant zebrafish are a model system of cardiomyopathy in which additional insults precipitate a severe phenotype, mimicking human cardiac disease. Thus, it provides a novel tool for finding modifier genes or pathways that impact severity of cardiomyopathy. Previous work suggested that hspb7 is regulated by the Gata4 transcription factor. In a separate but complementary project, I generated novel zebrafish lines with mutations in gata4, gata5, and gata6. I characterized defects in cardiac development that arise in embryos null for gata6, including a previously undiscovered alteration in ventricular myocyte identity. Additionally, I observed failure in pancreas and liver development. Surprisingly, I discovered that gata4 is dispensable for zebrafish development. However, gata4 mutants have a developmental delay, suggesting a role for Gata4 in regulating early epiboly. Finally, I documented compensation between gata4 and gata6 in the developing zebrafish, including a novel role in the establishment of left-right asymmetry. This work provides tools for further elucidation of the unique and overlapping roles of GATA factors during zebrafish development.
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Autophagy; Heart Development; HSPB7; Proteostasis; Zebrafish
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Doctor of Philosophy
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Government Document
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Attribution-NonCommercial-NoDerivatives 4.0 International
dissertation or thesis
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