Abstract 15700: ALK1-Mediated Smad1 Activation in a Myofibroblast Subset Protects the Infarcted Heart From Adverse Remodeling and Excessive Fibrosis

Abstract

Cardiac fibroblasts acquire inflammatory, proangiogenic, or matrix-synthetic phenotypes following MI and play a pivotal role in repair and remodeling of the infarcted heart. Members of the Transforming Growth Factor (TGF)β superfamily (including TGFβs and BMPs) are activated following MI and regulate fibroblast phenotype and cardiac fibrosis. Canonical TGFβ signaling is mediated through the receptor-activated Smads: TGF-βs activate Smad2/3, whereas BMPs predominantly stimulate Smad1/5 cascade. Smad3 signaling plays an important role in activation of infarct myofibroblasts; however, the role of the Smad1 pathway remains unknown. We hypothesized that fibroblast-specific actions of Smad1 regulate repair and remodeling in the infarcted myocardium. In a mouse model of non-reperfused infarction, Smad1 was activated in a subset of cardiac myofibroblasts not only by BMPs, but also by all three TGF-β isoforms. TGF-β activation of Smad1 was dependent on both ALK1 and ALK5 signaling. Analysis of single cell RNA-seq data suggested that ALK1 is a marker of a myofibroblast subset exhibiting anti-fibrotic properties. In order to examine the role of fibroblast Smad1 in vivo, we generated mice with myofibroblast-specific Smad1 loss (MFS1KO). Echocardiography showed that MFS1KO mice have accentuated systolic dysfunction and LV dilation 7-28 days after MI (p<0.01, N=13-49) . Worse adverse remodeling in infarcted MFS1KO hearts was associated with scar expansion, higher myofibroblast density and accentuated fibrosis. Smad1 knockdown in cardiac fibroblasts increased mRNA levels of structural and matricellular genes while reducing matrix metalloproteinase synthesis. RNA-seq showed that Smad1 loss activates pathways involved in matrix regulation and collagen biosynthesis, and suggested that Smad1 may exert anti-fibrotic actions through inhibition of TGF-β/Smad3 axis, p53, and IL4/STAT6 pro-fibrotic cascades. In conclusion, Smad1 activation in a subset of anti-fibrotic infarct myofibroblasts restrains cardiac fibrosis and protects from adverse remodeling by inhibiting key pro-fibrotic pathways. These findings suggest that Smad1 activation in fibroblasts may be a promising strategy to restrain cardiac fibrosis in infarcted and failing hearts.