Far from the inert structural component that was the prevailing view for many years, the arterial adventitia is a region of active signaling that harbors Sca1+ vascular progenitor cells as well as many other cell types. We have shown that a sonic hedgehog (Shh) signaling domain is restricted to the adventitial layer of artery wall beginning at embryonic day 15.5. Hedgehog (Hh)-responsive cells, colocalize with a circumferential ring of Shh protein concentrated between the media and adventitia. Furthermore, Sca1+ progenitor cells (AdvSca1 cells) reside within the Hh signaling domain and are reduced in number in Shh-/- mice. As a population, AdvSca1 cells express transcription factors thought to be required for smooth muscle cell (SMC) differentiation, including serum response factor (SRF) and myocardin family members, yet the cells do not express SMC marker proteins in vivo. However, upon removal of AdvSca1 cells from the adventitial environment, they readily differentiate to SMC-like cells in vitro. Repression of SRF-dependent transcription may be mediated by SRF co-repressors such as Klf4, Msx1, and FoxO4, all of which are expressed by AdvSca1 cells in vivo. Knockdown of Klf4 in AdvSca1 cells in vitro results in extensive down-regulation of Sca1 and defects in Sca1+ cell proliferation. In vitro and in vivo experiments have shown that AdvSca1 cells possess the potential for SMC, pericyte/mural cell, adipogenic, ostogenic, endothelial, and macrophage differentiation. Thus, AdvSca1 cells are regulated, in part, by Hh signaling and the transcription factor Klf4 and function as progenitors for multiple cell types with physiological and pathological relevance to the artery wall
