Frizzled6 deficiency disrupts the differentiation process of nail development

Nails protect the soft tissue of the tips of digits. The molecular mechanism of nail (and claw) development is largely unknown, but we have recently identified a Wnt receptor gene, Frizzled6 (Fzd6), that is mutated in a human autosomal-recessive nail dysplasia. To investigate the action of Fzd6 in claw development at the molecular level, we compared gene expression profiles of digit tips of wild-type and Fzd6(-/-) mice, and showed that Fzd6 regulates the transcription of a striking number of epidermal differentiation-related genes. Sixty-three genes encoding keratins (Krts), keratin-associated proteins, and transglutaminases (Tgms) and their substrates were significantly downregulated in the knockout mice. Among them, four hard Krts, Krt86, Krt81, Krt34, and Krt31; two epithelial Krts, Krt6a and Krt6b; and Tgm 1 were already known to be involved in nail abnormalities when dysregulated. Immunohistochemical studies revealed decreased expression of Krt86, Krt6b, and involucrin in the epidermal portion of the claw field in the knockout embryos. We further showed that Dkk4, a Wnt antagonist, was significantly downregulated in Fzd6(-/-) mice along with Wnt, Bmp, and Hh family genes; and Dkk4 transgenic mice showed a subtly but appreciably modified claw phenotype. Thus, Fzd6-mediated Wnt signaling likely regulates the overall differentiation process of nail/claw formation.