Loss-of-function mutations in co-chaperone BAG3 destabilize small HSPs and cause cardiomyopathy
Xi Fang, … , Sylvia M. Evans, Ju Chen
Xi Fang, … , Sylvia M. Evans, Ju Chen
Published July 24, 2017
Citation Information: J Clin Invest. 2017;127(8):3189-3200. https://doi.org/10.1172/JCI94310.
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Loss-of-function mutations in co-chaperone BAG3 destabilize small HSPs and cause cardiomyopathy

Abstract

Defective protein quality control (PQC) systems are implicated in multiple diseases. Molecular chaperones and co-chaperones play a central role in functioning PQC. Constant mechanical and metabolic stress in cardiomyocytes places great demand on the PQC system. Mutation and downregulation of the co-chaperone protein BCL-2–associated athanogene 3 (BAG3) are associated with cardiac myopathy and heart failure, and a BAG3 E455K mutation leads to dilated cardiomyopathy (DCM). However, the role of BAG3 in the heart and the mechanisms by which the E455K mutation leads to DCM remain obscure. Here, we found that cardiac-specific Bag3-KO and E455K-knockin mice developed DCM. Comparable phenotypes in the 2 mutants demonstrated that the E455K mutation resulted in loss of function. Further experiments revealed that the E455K mutation disrupted the interaction between BAG3 and HSP70. In both mutants, decreased levels of small heat shock proteins (sHSPs) were observed, and a subset of proteins required for cardiomyocyte function was enriched in the insoluble fraction. Together, these observations suggest that interaction between BAG3 and HSP70 is essential for BAG3 to stabilize sHSPs and maintain cardiomyocyte protein homeostasis. Our results provide insight into heart failure caused by defects in BAG3 pathways and suggest that increasing BAG3 protein levels may be of therapeutic benefit in heart failure.

Authors

Xi Fang, Julius Bogomolovas, Tongbin Wu, Wei Zhang, Canzhao Liu, Jennifer Veevers, Matthew J. Stroud, Zhiyuan Zhang, Xiaolong Ma, Yongxin Mu, Dieu-Hung Lao, Nancy D. Dalton, Yusu Gu, Celine Wang, Michael Wang, Yan Liang, Stephan Lange, Kunfu Ouyang, Kirk L. Peterson, Sylvia M. Evans, Ju Chen

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Figure 6

Amino acid E455 is critical for BAG3 function.

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Amino acid E455 is critical for BAG3 function. (A) Kaplan-Meier survival...
(A) Kaplan-Meier survival curves for BAG3 E455K-mutant homozygous (m/m), heterozygous (m/+), and WT control (+/+) mice. n = 10 mice per group. (B) Representative image illustrating the significant growth retardation in BAG3 E455K-mutant homozygous mice (m/m) compared with littermate control (+/+) mice at 20 days of age. Scale bar: 1 cm. (C) Growth curves of BAG3 E455K-mutant homozygous (m/m), heterozygous (m/+), and control (+/+) mice. n = 10 mice per group. (D) Representative microscopic views of whole mouse hearts and cross-sectional views of H&E-stained hearts isolated from cardiac-specific cMUT and control mice at 6 months of age. n = 3 mice per group. Scale bars: 1 mm. (E and F) HW to BW ratio (E) and HW to TL ratio (F) for control (n = 7) versus cMUT (n = 6) mice at 5 months of age. (G and H) qRT-PCR analysis of cardiac fetal gene markers (G) and profibrotic genes (H) in control (n = 4) and cMUT (n = 4) mouse hearts at 5 months of age. Data were normalized to corresponding 18S levels, and cMUT is expressed as the fold-change versus control. (I) Silver staining of total and insoluble proteins in control and cMUT mice. n = 4. (J) Western blot analysis of proteins in the insoluble fraction of control and cMUT mouse hearts. n = 4 mice per group. Data are represented as the mean ± SEM. *P < 0.05, by 2-tailed Student’s t test.