PROJECT SUMMARY/ABSTRACT Atherosclerosis is a complex inflammatory disease and a major cause of morbidity and mortality worldwide, but current therapies fail to adequately meet clinical needs. Until recently, most atherosclerosis research has focused on endothelial activation, immune cell recruitment, and lipid retention within the intima, but emerging evidence implicates the vessel adventitia in the pathogenesis of atherosclerosis. Specifically, it has been suggested that expansion of adventitial microvessels, the vasa vasorum (VV), drives atherosclerosis progression by facilitating inflammatory cell infiltration to the vessel wall and developing atheroma. This project focuses on the role that smooth muscle-derived vascular progenitor cells, termed AdvSca1-SM cells, play in vasa vasorum expansion and plaque neovascularization. Our lab previously identified AdvSca1-SM cells as a subset of resident vascular progenitor cells reprogrammed from mature smooth muscle cells in situ in a Klf4 depended process. These cells are enriched for genes in the hedgehog/Wnt/β-catenin pathways, particularly Gli1, and have multilineage differentiation potential. In the setting of acute vascular injury, AdvSca1-SM cells expand robustly to contribute to fibrotic remodeling, but it is unclear what role they play in chronic injuries such as atherosclerosis. Work from several groups has suggested that adventitial progenitor cells can contribute to VV expansion, and our group has previously demonstrated that AdvSca1-SM cells are capable of de novo vessel formation via endothelial or smooth muscle cell differentiation in Matrigel plug assays. Additionally, we have identified AdvSca1-SM-like cells in the microvessels surrounding human coronary arteries, suggesting a potential role in the VV. The aim of this project is to determine the contribution of AdvSca1-SM cells to VV expansion, plaque neovascularization, and atherosclerotic progression. We hypothesize that AdvSca1-SM cells are major contributors to atherosclerosis, and that modulation of these cells could alter disease progression. Specific Aim 1 will define the functional contribution of AdvSca1-SM cells to VV expansion and/or plaque progression using a highly specific AdvSca1- SM genetic lineage tracing mouse model; AdvSca1-SM contribution will be assessed using immunofluorescent microscopy and flow cytometry. Specific Aim 2 will define the mechanisms controlling AdvSca1-SM cells in the setting of atherosclerosis, with an emphasis on the Gli/Wnt/β-catenin/Klf4 signaling axis previously identified as a regulator of AdvSca1-SM cells in the setting of acute vascular injury. We will modulate this signaling axis using AdvSca1-SM-specific Gli1 overexpression or Klf4 knockout; the effects of these modifications on AdvSca1-SM maintenance or differentiation will be assessed using immunofluorescent microscopy, flow cytometry, and single cell RNA-seq. Ultimately, the work proposed here represents a novel avenue of research tha...