Primed smooth muscle cells acting as first responder cells in disease

Vascular smooth muscle cell (VSMC) dysregulation is a hallmark of vascular disease, including atherosclerosis. In particular, the majority of cells within atherosclerotic lesions are generated from pre-existing VSMCs and a clonal nature has been documented for VSMC-derived cells in multiple disease models. However, the mechanisms underlying the generation of oligoclonal lesions and the phenotype of proliferating VSMCs are unknown.Here we analyse clonal dynamics in multi-color lineage-traced animals over time after vessel injury to understand the cellular mechanisms underlying clonal VSMC expansion in disease.We demonstrate that VSMC proliferation is initiated in a small fraction of VSMCs that initially expand clonally in the medial layer and then migrate to form the oligoclonal neointima. Selective activation of VSMC proliferation also occurs in vitro, suggesting that this is a cell-autonomous feature. Mapping of VSMC trajectories using single-cell RNA-sequencing reveals a continuum of cellular states after injury and suggests that VSMC proliferation initiates in cells that have downregulated the contractile phenotype and show evidence of pronounced phenotypic switching. We show that proliferation is associated with induced expression of stem cell antigen 1 (SCA1) and the expression signature previously identified in SCA1+ VSMCs in healthy arteries. A remarkably increased proliferation of SCA1+ VSMCs, directly validated in functional assays, indicates that SCA1+ VSMCs act as "first responders" in vascular injury. Early atherosclerotic lesions also had clonal VSMC contribution and we show that the proliferation-associated injury response is conserved in plaque VSMCs, extending these findings to atherosclerosis. Finally, we identify VSMCs in healthy human arteries that correspond to the SCA1+ state in mouse VSMCs and show that genes identified as differentially expressed in this human VSMC subpopulation are enriched for genes showing genetic association with cardiovascular disease. We show that cell-intrinsic, selective VSMC activation drives clonal proliferation after injury and in atherosclerosis. Our study suggests that healthy mouse and human arteries contain VSMCs characterised by expression of disease-associated genes that are predisposed for proliferation. Targeting such "first responder" cells in patients undergoing vascular surgery could effectively prevent injury-associated VSMC activation and neoatherosclerosis. Overall design: This entry contains data from the following analyses: (1) 10X single-cell RNA-seq analysis of VSMC-lineage labeled cells isolated by flow-assisted cell sorting (FACS) from ligated left carotid arteries of Myh11-EYFP-Ki67/RFP animals 5 days after surgery. (2) 10X single-cell RNA-seq analysis of human medial cells isolated from a healthy aorta (65-year-old male). (3) 10X single-cell RNA-seq analysis of VSMC-lineage labeled cells isolated by flow-assisted cell sorting (FACS) from ligated left carotid arteries of Myh11-EYFP-Ki67/RFP animals 7 days after surgery. (4) Single-cell RNA-seq of index-sorted cells from Myh11-EYFP-Ki67/RFP animals 7 days after surgery and control unligated animals. (5) 10X single-cell RNA-seq analysis of VSMC-lineage labeled cells and other arterial cells generated 5 days after surgery.