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 show that Fzd6 regulates the transcription of a striking number of epidermal differentiation-related genes. Sixty-three genes encoding keratins, keratin associated proteins, and transglutaminases and their substrates were significantly down-regulated in the knockout mice. Among them, four hard keratins, Krt86, Krt81, Krt34 and Krt31; two epithelial keratins, Krt6a and Krt6b; and transglutaminase1 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 down-regulated 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.