The chicken retinoid-X-receptor-α (RXR-α) gene and its expression in the developing limb

Antero-posterior (a-p) patterning of the vertebrate limb bud is controlled by signals from the polarizing region, a group of cells in the posterior mesenchyme of the bud. Application of retinoic acid to the anterior margin of the chick wing bud induces polarizing region activity in anterior mesenchyme cells, resulting in digit duplications. Retinoic acid acts by binding to nuclear retinoid receptors, and so regulating expression of target genes. Retinoid receptors of the RXR class are essential for this activity. We have previously described a chicken RXR-γ cDNA clone (Rowe et al. 1991a). In this paper we report the isolation and characterization of a chicken RXR-α cDNA clone, and show by northern blotting that an RXR-α mRNA of approximately 5 kilobases is present in a range of tissues in embryonic and adult chickens. In situ hybridization experiments showed that RXR-α transcripts were present throughout the epithelium and mesenchyme of the chick wing bud at stages when retinoic acid can affect a-p patterning. In contrast, RXR-γ transcripts were undetectable in these cells, being restricted to peripheral nervous tissue in the bud. These data suggest that RXR-α, but not RXR-γ, could mediate the effects of locally applied retinoic acid on a-p patterning in the chick wing bud.