1123. Adenovirus-Mediated Expression of Sonic Hedgehog Increases the Number of CD34+ Cells in the Epidermal Stem Cell Niche in Hair Follicles

Abstract

The hair follicle stem cell niche, also known as the |[ldquo]|bulge,|[rdquo]| contains adult stem cells that give rise to the follicle keratinocytes that generate hair shafts . While epidermal stem cells are utilized cyclically for hair shaft regrowth, epidermal stem cells also contribute progenitor cells of interfollicular epidermis as part of wound repair. Prior work from this laboratory (Sato et al. J Clin Invest 1999; 104:855) showed that exogenous, adenovirus-mediated expression of Sonic hedgehog (Shh) in skin with resting hair follicles (telogen) activated the growth phase of the hair cycle (anagen) leading to the hypothesis that Shh expression would affect the physiology of the cells in the bulge. To test this hypothesis, analysis of the bulge was conducted under normal hair growth conditions and following adenovirus-mediated local delivery of genes to accelerate hair growth. The mouse epidermal stem cell niche was visualized by immunohistochemical staining using anti-CD34+ primary antibodies. In naive mice, the morphology of the stem cell niche was constant throughout the hair follicle cycle (anagen vs telogen) and over a broad range of mouse ages (2 to 24 months). Mouse skin in telogen was examined without treatment (naive) or following intradermal administration of AdNull (control vector) or AdShh, and 2 wk following vector administration, the CD34+ bulge region was evaluated. Naive mouse skin and AdNull-infected mouse skin remained in telogen and did not show evidence of a change in bulge morphology. In contrast, AdShh-treated skin entered anagen and showed increases in CD34+ area per follicle (67 |[plusmn]| 18% increase). In naive and AdNull-treated skin, the area of the CD34+ staining in the bulge did not change as the skin entered anagen, indicating that the increase in the area of CD34+ cells was observed only following AdShh administration. Three-dimensional reconstructions made from serial stained sections of murine skin, showed that the increase in CD34+ area in single sections corresponded with an increase in CD34+ volume per follicle (140 |[plusmn]| 26% increase) as well as the absolute number of CD34+ cells per hair follicle. Similarly, AdShh-treated skin showed an increase in the number of CD34+ cells per follicle bulge (116 |[plusmn]| 33% increase). To determine whether the changes in bulge morphology were specifically related to expression of the Shh transgene, murine skin was treated with an Ad vector expressing the cDNA for Noggin, another secreted protein that initiates the onset of anagen in hair follicles. While AdNoggin, like AdShh, induced early onset of anagen in murine skin, administration of AdNoggin did not result in an increase in CD34+ area, volume, or cell number, indicating that the effect observed using AdShh was a specific property of Shh expression. Taken together, the data suggest that Shh expression modulates the physiology of adult epidermal stem cells in the hair follicle stem cell niche, creating a potential means for therapeutic manipulation of these cells.