Differential expression of genes encoding α, β and γ retinoic acid receptors and CRABP in the developing limbs of the mouse

Abstract

RETINOIC acid has profound effects on vertebrate limb morphogenesis (refs 1–6, reviewed in refs 7–9), including in the mouse, where it can act as a teratogen generating phocomelia and bone defects^10–12. A retinoic acid gradient^13–15, possibly amplified by a graded distribution of cellular retinoic acid-binding protein (CRABP) ¹⁶, could provide positional information across the antero-posterior axis of the chick limb bud. The discovery of nuclear retinoic acid receptors (RARs) ^17–22 acting as retinoic acid-inducible enhancer factors (reviewed in refs 23,24) provided a basis for understanding how retinoic acid signals could be transduced at the level of gene expression²⁵. We have now used in situ hybridization to study the distribution of messenger RNA transcripts of the three murine receptors (mRARs) and CRABP during mouse limb development. Both mRARoα and mRARγ transcripts, but not those for mRARβ, are present and uniformly distributed in the limb bud at day 10 post-coitum, whereas CRABP transcripts have a graded proximo-distal distribution, indicating that differential expression of CRABP, but not of mRARα or mRARγ, could participate in the establishment of the morphogenetic field. At later stages, mRARγ transcripts become specific to the cartilage cell lineage and to the differentiating skin and mRARβ transcripts are mostly restricted to the interdigital mesenchyme. CRABP transcripts, however, are excluded from regions expressing mRARγ; and mRARβ. These results indicate that all three RARs and CRABP have specific functions during morphogenesis and differentiation of the mouse limb.