Frizzled genes encode a family of Wnt ligand receptors, which have a conserved cysteine-rich Wnt binding domain and include both transmembrane and secreted forms. Work by others has shown that experimental perturbation of Wnt signaling results in aberrant hair formation, hair growth and hair structure. However, to date there is no information on the contribution of individual Frizzled proteins to hair development. We now report that Frizzled-3 (Fzd3) expression in skin is restricted to the epidermis and to the developing hair follicle. Northern analysis on total mouse skin mRNA revealed a single Fzd3 transcript of 3.7 kb. RT-PCR and in situ hybridization analysis revealed Fzd3 expression in epidermal and hair follicle keratinocytes. Fzd3 transcripts are first detected in discrete foci in the developing epidermis of 13 day embryos and later in the hair follicle placodes of 15 day embryos, suggesting a role for this Frizzled isoform in follicle development. In 17 day embryos and 1 day old newborn mice Fzd3 expression is limited to suprabasal keratinocytes and is not seen in pelage follicles until 3 days post partum. In 7 day old neonatal skin, Fzd3 is expressed throughout the epidermis and in the outer cell layers of hair follicles. We have also identified the mRNA encoding human Frizzled-3 (FZD3) in epidermal keratinocytes and in the HaCaT keratinocyte cell line. FZD3 mRNA encodes a 666 amino-acid protein with 97.8% identity to the mouse protein. The FZD3 gene was mapped using a radiation-hybrid cell line panel to the short arm of chromosome 8 (8p21) between the markers WI-1172 and WI-8496 near the loci for the Hypotrichosis of Marie Unna and Hairless genes.
The exon-intron organization and sequences of the exon-intron boundaries of the mouse Frizzled-3 (mFzd3) transmembrane receptor gene have been determined. The mFzd3 gene comprises 7 exons and shows organizational conservation with the human homolog (hFZD3) within the coding region but differs in the numbers of 5' non-coding exon. Interestingly, the 5'-untranslated region (5'-UTR) of both mouse and human transcripts contain several upstream ORFs that are conserved in sequence and position. This study has also identified several alternative spliced transcripts, which are predicted to encode soluble variants that lack the transmembrane and C-terminal domain of Frizzled-3 protein (Fzd3). The mFzd3α isoform which is structurally similar to sFRP/Frzb proteins may function to prevent formation of the Frizzled-Wnt complex and inhibit signaling. The mFzd3δ and mFzd3ε isoforms which are structurally similar to FZD4S (Sagara et al., 2001) may heretodimerize with mFzd3β, suggesting that Frizzled activity requires receptor oligomerization.