Cardiomyocyte contractile impairment in heart failure results from reduced BAG3-mediated sarcomeric protein turnover

Abstract

The association between reduced myofilament force-generating capacity (Fmax) and heart failure (HF) is clear, however the underlying molecular mechanisms are poorly understood. Here, we show impaired Fmax arises from reduced BAG3-mediated sarcomere turnover. Myofilament BAG3 expression decreases in human HF and positively correlates with Fmax. We confirm this relationship using BAG3 haploinsufficient mice, which display reduced Fmax and increased myofilament ubiquitination, suggesting impaired protein turnover. We show cardiac BAG3 operates via chaperone-assisted selective autophagy (CASA), conserved from skeletal muscle, and confirm sarcomeric CASA complex localization is BAG3/proteotoxic stress-dependent. Using mass spectrometry, we characterize the myofilament CASA interactome in the human heart and identify eight clients of BAG3-mediated turnover. To determine if increasing BAG3 expression in HF can restore sarcomere proteostasis/Fmax, HF mice were treated with rAAV9-BAG3. Gene therapy fully rescued Fmax and CASA protein turnover after four weeks. Our findings indicate BAG3-mediated sarcomere turnover is fundamental for myofilament functional maintenance.

BAG3 expression and sarcomere localization in the human heart are linked to HSF-1 and are differentially affected by sex and disease

“…Preclinical models of heart failure provide evidence that altered BAG3 protein expression is also involved in the progression of heart failure and show BAG3 levels decrease in heart failure secondary to pressure overload and myocardial infarction (8,14,15). Additionally, diminished BAG3 expression was found in LV tissue from patients with end-stage heart failure compared with nonfailing donor hearts (16), and we recently showed the myofilament-specific pool of BAG3 also decreases in DCM (4). However, these studies were not powered to examine potential sex differences in cardiac BAG3 expression and myofilament localization, such as have been observed with BAG3 mutation penetrance.…”

“…Through various protein interactions BAG3 mediates numerous cellular processes, including cell survival, protein quality control, cytoskeleton maintenance, and development (1). Notably, BAG3 protein expression is highest in cancer cells, where it enhances tumor pathogenesis by inhibiting apoptosis/activating autophagy (2), and in striated muscle, where it mediates turnover of sarcomere proteins via selective macroautophagy (3)(4)(5). In cardiac muscle, BAG3 mutations and decreased BAG3 protein expression are associated with increased apoptosis and sarcomere structural disarray (6,7).…”

Maintaining proteostasis under mechanical stress

“…However, BAG3‐deficient animals and patients carrying BAG3 mutant alleles display a profound pathology in skeletal and cardiac muscle, characterized by a progressive deterioration of sarcomeric structures under mechanical stress (Homma et al , 2006 ; Selcen et al , 2009 ; Arndt et al , 2010 ; Arimura et al , 2011 ; Claeys et al , 2013 ; Ruparelia et al , 2014 ; Konersman et al , 2015 ; Quintana et al , 2016 ; Fang et al , 2017 ; Meister‐Broekema et al , 2018 ; Schänzer et al , 2018 ; Kimura et al , 2021 ). Moreover, a decline in BAG3‐mediated sarcomere turnover contributes to the development of heart failure, the leading cause of mortality in the industrialized world (Martin et al , 2021 ). Ultimately, the mechanosensitive actin‐crosslinking protein filamin was identified as a target of CASA (Arndt et al , 2010 ; Ulbricht et al , 2013 ).…”

“…BAG3 and its chaperone partners HSP70 and HSPB8 bind to the mechanosensitive region of filamin comprising Ig domains 19–21 (see above) and direct mechanically unfolded and damaged forms onto the CASA pathway for disposal (Fig 3C ) (Arndt et al , 2010 ; Klimek et al , 2017 ). Indeed, filamin forms aggregates in skeletal and cardiac muscle when the CASA pathway is compromised under pathological conditions (Arimura et al , 2011 ; Ruparelia et al , 2016 ; Fang et al , 2017 ; Kimura et al , 2021 ; Martin et al , 2021 ), illustrating the relevance of CASA for myofilament homeostasis in contracting muscle.…”

“…In a recent study, Martin et al ( 2021 ) monitored the BAG3‐mediated release of proteins from the cytoskeleton of cardiomyocytes by mass spectrometry and identified several sarcomere proteins as potential new clients of the CASA pathway, including the mechanosensitive protein spectrin. Quality control functions of BAG3 under mechanical stress thus seem to extend beyond filamin homeostasis.…”