Summary The hair follicle bulge in the epidermis associates with the arrector pili muscle (APM) that is responsible for piloerection (commonly called "goosebumps"). We show that deposition of nephronectin into the basement membrane by bulge stem cells mediates selective adhesion of α8β1 integrin-expressing mesenchymal cells, including APM progenitors. Nephronectin induces α8 integrin-positive cells to upregulate smooth muscle markers. In nephronectin-null skin, the number of APM is reduced and those that form insert above the bulge, where there is compensatory upregulation of the nephronectin family member EGFL6. Deletion of α8 integrin also abolishes the selectivity of APM anchorage to the bulge. Nephronectin is a Wnt target gene; epidermal β-catenin activation upregulates epidermal nephronectin and dermal α8 integrin expression. We conclude that by expressing nephronectin bulge stem cells provide a smooth muscle cell niche and act as tendon cells for the APM. The results also reveal a functional role for basement membrane heterogeneity in tissue patterning.

Mammalian skin comprises a multi-layered epithelium, the epidermis, and an underlying connective tissue, the dermis. The epidermal extracellular matrix is a basement membrane, whereas the dermal ECM comprises fibrillar collagens and associated proteins. There is considerable heterogeneity in ECM composition within both epidermis and dermis. The functional significance of this extends beyond cell adhesion to a range of cell autonomous and nonautonomous processes, including control of epidermal stem cell fate. In skin, cell-ECM interactions influence normal homeostasis, aging, wound healing, and disease. Disturbed integrin and ECM signaling contributes to both tumor formation and fibrosis. Strategies for manipulating cell-ECM interactions to repair skin defects and intervene in a variety of skin diseases hold promise for the future.

It has long been recognized that the hair follicle growth cycle and oscillation in the thickness of the underlying adipocyte layer are synchronized. Although factors secreted by adipocytes are known to regulate the hair growth cycle, it is unclear whether the epidermis can regulate adipogenesis. We show that inhibition of epidermal Wnt/β-catenin signaling reduced adipocyte differentiation in developing and adult mouse dermis. Conversely, ectopic activation of epidermal Wnt signaling promoted adipocyte differentiation and hair growth. When the Wnt pathway was activated in the embryonic epidermis, there was a dramatic and premature increase in adipocytes in the absence of hair follicle formation, demonstrating that Wnt activation, rather than mature hair follicles, is required for adipocyte generation. Epidermal and dermal gene expression profiling identified keratinocyte-derived adipogenic factors that are induced by β-catenin activation. Wnt/β-catenin signaling-dependent secreted factors from keratinocytes promoted adipocyte differentiation in vitro, and we identified ligands for the bone morphogenetic protein and insulin pathways as proadipogenic factors. Our results indicate epidermal Wnt/β-catenin as a critical initiator of a signaling cascade that induces adipogenesis and highlight the role of epidermal Wnt signaling in synchronizing adipocyte differentiation with the hair growth cycle.skin | niche cross-talk | stem cells M ammalian skin is a complex organ composed of a variety of cell and tissue types, including interfollicular epidermis, hair follicles (HFs), melanocytes, nerves, blood vessels, muscles, fibroblasts, and adipocytes. The development and patterning of these cells and tissues are governed by intercellular communication (1-3).One well-known example of this communication is the link between the HF growth cycle and the oscillation in thickness of the dermal adipocyte layer (4-6). When HFs grow deep into the dermal adipocyte layer in the anagen (growth) phase of the cycle, the adipocyte layer dramatically increases in thickness. This event reflects both increased adipogenesis and hypertrophy of individual adipocytes (7). When HFs regress (catagen phase) and enter the resting (telogen) phase, the adipocyte layer becomes thinner. The growth cycle of rodent HFs is coordinated to form waves of hair growth that traverse the body and the thickness of the skin adipocyte layer oscillates in synchrony with these waves (3).The synchronized patterns of HF growth and expansion of dermal fat correlate with the activation of the canonical Wnt pathway, which is well established to positively regulate anagen (3,8). Expression of bone morphogenetic protein 2 (Bmp2) in the mature dermal adipocyte layer is inversely correlated with Wnt activity and inhibits HF growth (3). There is also evidence that immature dermal adipocytes activate HF stem cells to initiate the hair growth cycle (7). These reports suggest that the adipocyte differentiation process is a natural on-off cycling switch for the regulation of hair grow...

S U M M A R YRecent studies suggest important functions for laminin-8 (Ln-8; ␣ 4 ␤ 1 ␥ 1) in vascular and blood cell biology, but its distribution in human tissues has remained elusive. We have raised a monoclonal antibody (MAb) FC10, and by enzyme-linked immunoassay (EIA) and Western blotting techniques we show that it recognizes the human Ln ␣ 4-chain. Immunoreactivity for the Ln ␣ 4-chain was localized in tissues of mesodermal origin, such as basement membranes (BMs) of endothelia, adipocytes, and skeletal, smooth, and cardiac muscle cells. In addition, the Ln ␣ 4-chain was found in regions of some epithelial BMs, including epidermis, salivary glands, pancreas, esophageal and gastric glands, intestinal crypts, and some renal medullary tubules. Developmental differences in the distribution of Ln ␣ 4-chain were detected in skeletal muscle, walls of vessels, and intestinal crypts. Ln ␣ 4-and Ln ␣ 2-chains co-localized in BMs of fetal skeletal muscle cells and in some epithelial BMs, e.g., in gastric glands and acini of pancreas. Cultured human pulmonary artery endothelial (HPAE) cells produced Ln ␣ 4-chain as M r 180,000 and 200,000 doublet and rapidly deposited it to the growth substratum. In cell-free extracellular matrices of human kidney and lung, Ln ␣ 4-chain was found as M r 180,000 protein.