BAG3^V468M impairs proteasomal protein clearance and induces dilated cardiomyopathy in vivo

Idiopathic dilated cardiomyopathy (DCM) is one of the major causes of heart failure, characterized by left ventricular dilation and systolic dysfunction in the absence of an identifiable cause, and is associated with reduced life expectancy. Genetic studies, including genome-wide association studies, have identified variants in BAG3, a key regulator of protein quality control (PQC), as contributors to both familial and sporadic forms of DCM. Impaired PQC and the accumulation of misfolded proteins (proteinopathy) have emerged as potential pathogenic mechanisms. Here, we investigated the molecular consequences of a recently identified BAG3 missense variant (V468 M) associated with familial DCM. To assess the in vivo effects of the variant, human BAG3^V468M was ectopically expressed in wild-type zebrafish embryos. Overexpression of BAG3^V468M resulted in a DCM-like phenotype characterized by ventricular dilation, reduced heart rate, and impaired contractility. Transmission electron microscopy revealed marked disruption of myocardial ultrastructure and sarcomeric organization. To explore the impact on proteostasis, markers of autophagy (LC3-I/II and p62) were analyzed and showed no significant differences between BAG3^V468M and control embryos under basal conditions. In contrast, analysis of the ubiquitin-proteasome system demonstrated a significant accumulation of ubiquitinated proteins in BAG3^V468M-expressing embryos, suggesting impaired proteasomal protein clearance or increased proteotoxic stress. Expression of BAG3^V468M induces a DCM-like phenotype in vivo associated with disrupted myocardial architecture and altered proteostasis. While canonical autophagy markers remain unchanged, the accumulation of ubiquitinated proteins points toward a disturbance in ubiquitin-mediated protein turnover. These findings implicate mutation-specific alterations in proteostasis as a potential mechanism contributing to BAG3-associated cardiomyopathy.