Cardiac fibroblast deletion of PTBP1 modulates the profibrotic response by alternative splicing.

Dynamic cardiac fibroblast phenotypes drive pathological remodeling and fibrosis, yet post-transcriptional mechanisms controlling fibroblast activation remain poorly defined. Here, we identify the RNA-binding protein polypyrimidine tract binding protein 1 (PTBP1) as a regulator of cardiac fibroblast activation and fibrotic remodeling. Cardiac fibroblast-specific Ptbp1 deletion reduced fibrosis and preserved cardiac function after Angiotensin II infusion, transverse aortic constriction, and myocardial infarction. In vitro, PTBP1 depletion attenuated TGFβ-induced myofibroblast activation, decreasing αSMA and POSTN expression and impairing collagen gel contraction. Transcriptomic and splicing analyses revealed broad PTBP1-dependent gene expression and alternative splicing programs enriched for extracellular matrix organization, cytoskeletal remodeling, and fibroblast function. Mechanistically, PTBP1 regulated Fgfr1 alternative splicing, and restoration of the FGFR1β isoform partially rescued the contractile defect caused by PTBP1 loss. Together, these findings establish PTBP1-dependent alternative splicing as a key mechanism promoting cardiac fibroblast activation and pathological fibrosis.