Characterization of Wnt gene expression in murine skin: Possible involvement of epidermis-derived Wnt-4 in cutaneous epithelial-mesenchymal interactions

Wnt glycoproteins mediate short range intracellular communication that facilitates morphogenesis and, in some settings, promotes tumor formation. Although the involvement of the Drosophila homolog wingless in ectodermal patterning is well established, the role that Wnt genes play in mammalian skin biology is not defined. We detected Wnt-4 and Wnt-10b mRNA in adult murine epidermis using degenerate primers and reverse transcriptase PCR, and confirmed expression by RNase protection. Normal murine keratinocytes and a melanocyte cell line (melan-A) propagated in vitro also contained Wnt-4 mRNA, whereas dermal fibroblasts and Langerhans cell-like dendritic cells did not. Because Wnt-4 mRNA was more abundant than Wnt-10b mRNA in epidermis and Wnt- 10b trancripts were not detected in cells propagated in vitro, additional studies emphasized Wnt-4 exclusively. Wnt-4 mRNA levels were increased in cultured keratinocytes as they approached confluence and were strikingly downregulated by mitogenic growth factors. Although Wnt-4 mRNA levels were not modulated during calcium-induced keratinocyte differentiation in vitro, assessment of Wnt-4 transcripts in keratinocyte cell lines suggested that loss of Wnt-4 gene expression was associated with a less differentiated, more malignant, phenotype. Despite this, epidermal abnormalities were not identified in newborn Wnt-4 null (-/-) skin, or in full-thickness -/- skin that was engrafted to nude or athymic mice and allowed to mature for as long as 3 months. However, histologic examination of newborn Wnt-4 null skin did reveal fibroplasia involving the dermis with increased accumulation of type I collagen fibrils. These results indicate that several Wnt genes are expressed in adult murine epidermis and suggest that Wnt-4 proteins may be involved in epidermal-dermal interactions in mammalian skin.