Fibroblast-specific Smad6 Restrains Adverse Remodeling And Dysfunction Following Myocardial Infarction

Signaling induced by the Transforming Growth Factor (TGF)-β superfamily members in fibroblasts plays a central role in repair and remodeling after myocardial infarction (MI). However, TGF-b signals need to be tightly regulated, as excessive or sustained fibroblast activation after MI can accentuate fibrosis, precipitating heart failure. The inhibitory Smads, Smad6 and Smad7, function as endogenous negative regulators that restrain TGF-β and BMP signaling. We have recently demonstrated that fibroblast-specific Smad7 restrains fibrosis after MI acting through TGF-dependent and -independent pathways. However, the role of Smad6 in tissue repair and fibrosis following injury is not known. We hypothesized that Smad6 expression in infarct fibroblasts may play an important role in repair and remodeling of the heart, regulating TGF-b and BMP-mediated responses. In a mouse model of non-reperfused MI, Smad6 expression was increased in infarct fibroblasts and myofibroblasts at 7d post-infarction. To investigate the role of Smad6 in injury and repair, we used CRISPR/Cas9 to generate Smad6 loxp mice. These animals were bred with Col1a2CreER mice to generate fibroblast-specific conditional Smad6 KOs (FS6KO). 7d after MI, FS6KO mice had accentuated systolic and diastolic dysfunction (p<0.01, N=10) associated with scar expansion, accentuated fibrosis, and higher myofibroblast density in the infarct area and border zone. Fibroblast-specific Smad6 loss increased collagen denaturation in the infarct and border zone. In vitro, siRNA knockdown experiments showed that Smad6 suppresses collagen and fibronectin expression, and modulates protease and matricellular gene synthesis. Reactome analysis of RNA-Seq data suggested effects of Smad6 on pathways involved in assembly and crosslinking of collagen fibrils, collagen biosynthesis and degradation, and VEGF-dependent angiogenesis. Furthermore, Ingenuity Pathway Analysis identified signaling by Integrins, Ephrin receptors and Receptor Tyrosine Kinases as key regulators of the effects of Smad6 loss on the fibroblast transcriptome. In conclusion, fibroblast-specific Smad6 restrains adverse remodeling after MI. Ongoing studies explore the molecular mechanisms for the Smad6 effects.