ORIGINS OF ADVENTITIAL SCA1 PROGENITOR CELLS

Abstract

Adventitial Sca1 progenitor cells are abundant vascular smooth muscle progenitor cells that play a role in atherosclerosis and in the neointima during vascular injury. Currently, the origins of adventitial Sca1 progenitor cells are unknown and shown not to be neural crest or bone marrow derived. The purpose of this study was to identify a novel stem cell present in the adult murine aorta that gives rise to vascular smooth muscle cells and to identify the origins of adventitial Sca1 progenitor cells. Using a Myf5-Cre animal crossed to a floxed tdTomato reporter, which labels the somitic mesoderm, we were able to culture stem cells derived from the 8-12 week old mouse aorta in defined serum-free stem/progenitor cell medium. These stem cells displayed multilineage potential and could differentiate into Calponin+ vascular smooth muscle cells and S100β+ glial cells in vitro. In accord with their multilineage potential, these stem cells were positive for the stem cell marker Sox2 by indirect immunofluorescence. Intriguingly, a significant proportion of Sca1 progenitor cells were lineage marked in vivo in the Myf5-Cre animal suggesting a somitic origin for Sca1 progenitor cells in the aorta. In order to determine at which stage Sca1 progenitor cells were specified in vivo, we acquired a tamoxifen inducible Myf5-Cre ER mouse. We demonstrate that adventitial Sca1+ progenitors are specified during early somitogenesis starting at E8.5 which labeled 39±17% of Sca1 cells in the aorta of adult animals. Since the stem cells cultured in vitro also expressed Sox2, we wanted to determine if adult Sox2 stem cells in the adult aorta could also give rise to Sca1 adventitial progenitor cells. Using a Sox2-Cre ERT2 mouse crossed to a floxed tdTomato reporter, we demonstrated that approximately 34±5% of adventitial Sca1 progenitor cells were also derived from Sox2 stem cells in the aortas of adult animals. When we histologically examined the aorta for Sox2 expression, we observed that the majority of Sox2 staining was observed in the medial layer of the aorta. Thus, we suggest that somite-derived Sox2 stem cells reside in the medial layer where they give rise to transit amplifying Sca1 progenitor cells that reside in the adventitia. In the adult mouse, Sca1 adventitial progenitor cells are also replenished de novo by somite-derived Sox2 stem cells that reside in the medial layer of the aorta. Further studies will be performed to determine the physiological significance of Sox2 stem cells in disease.