Cardiac fibroblast BAG3 regulates TGFBR2 signaling and fibrosis in dilated cardiomyopathy

Loss of Bcl2-associated anthanogene 3 (BAG3) is associated with dilated cardiomyopathy (DCM). Recent studies show that BAG3 regulates sarcomere protein turnover in cardiomyocytes (CMs). However, the function of BAG3 in other cell types of the heart is unknown. In this study, we used an isogenic pair of BAG3 knockout and wild-type human induced pluripotent stem cells (hiPSC) to interrogate the function of BAG3 in hiPSC-derived cardiac fibroblasts (CFs). Generating a series of conditional knockout (cKO) engineered heart tissues, we determined the contribution of CF BAG3 to engineered heart tissue function. Despite normal CM genotype, the loss of fibroblast-specific BAG3 (FBKO) caused a reduction in contractile force and an increase in cardiac fibrosis in the engineered heart tissues, recapitulating the phenotype of DCM. Using culture substrates with defined stiffness, we decoupled the mechanical activation of fibroblasts from their ligand response. In BAG3-/- CFs, we observed an increased sensitivity to transforming growth factor ß (TGFß) signaling and activation of a fibrogenic response when cultured at physiological stiffness (8kPa). Mechanistically, loss of BAG3 increased transforming growth factor receptor 2 (TGFBR2) levels. BAG3 binds TGFBR2 and mediates its ubiquitination and proteasomal degradation. To further validate these results, we performed single-nuclei sequencing of cardiac tissue from DCM patients carrying pathogenic BAG3 mutations. Mutations in BAG3 increased fibrotic gene expression in cardiac fibroblasts. Together, these results extend our understanding of the roles of BAG3 in heart disease beyond the cardiomyocyte-centric view. This study also highlights the ability of hiPSC-based models and tissue engineering to answer questions about the cell-type specific aspects of cardiac disease. Overall design: Control, BAG3 heterozygous knock-out, and BAG3 homozygous knockout hiPSCs cells were differentiated into cardiac fibroblasts. Subsequently, they were grown on 8kPa substrate for 5 days and frozen down. Frozen cell pellets were barcoded using the CellPlex Kit from 10X Genomics. Nuclei from the barcoded cells were extracted, pooled, and sequenced using the 10X Genomics platform. Reads mapping to the barcodes were used for demultiplexing.