Smad3Regulates Smooth Muscle Cell Fate and Governs Adverse Remodeling and Calcification of Atherosclerotic Plaque

Atherosclerotic plaques consist mostly of smooth muscle cells (SMC), and genes that influence SMC biology can modulate coronary artery disease (CAD) risk. Allelic variation at 15q22.33 has been identified by genome-wide association studies to modify the risk of CAD, and is associated with expression ofSMAD3in SMC, but the mechanism by which this gene modifies CAD risk remains poorly understood. SMC-specific deletion ofSmad3in a murine atherosclerosis model resulted in greater plaque burden, positive remodeling, and increased vascular calcification. Single-cell transcriptomic analyses revealed that loss ofSmad3altered SMC progeny phenotype toward the previously described chondromyocyte fate, but importantly also promoted transition to a novel cell-state that governs remodeling and recruitment of inflammatory cells. This new remodeling population was marked by uniquely highMmp3andCxcl12expression, and its appearance correlated with higher-risk plaque features such as increased positive remodeling and macrophage content. Further, investigation of transcriptional mechanisms by which Smad3 alters SMC cell-fate revealed novel roles for Hox and Sox transcription factors whose direct interaction with Smad3 regulate an extensive transcriptional program balancing remodeling and vascular ECM with significant implications for human Mendelian aortic aneurysmal diseases. Together, these data suggest thatSmad3expression in SMC inhibits the emergence of specific SMC phenotypic transition cells that mediate adverse plaque features, including positive remodeling, monocyte recruitment, and vascular calcification.