Retinoic acid signaling regulates embryonic clock hairy2 gene expression in the developing chick limb

Embryo development proceeds under strict temporal control and an embryonic molecular clock (EC), evidenced by cyclic gene expression, is operating during somite formation and limb development, providing temporal information to precursor cells. In somite precursor cells, EC gene expression and periodicity depends on Retinoic acid (RA) signaling and this morphogen is also essential for limb initiation, outgrowth and patterning. Since the limb EC gene hairy2 is differentially expressed along the proximal-distal axis as growth proceeds, concomitant with changes in flank-derived RA activity in the mesenchyme, we have interrogated the role of RA signaling on limb hairy2 expression regulation. We describe RA as a positive regulator of limb hairy2 expression. Ectopic supplementation of RA induced hairy2 in a short time period, with simultaneous transient activation of Erk/MAPK, Akt/PI3K and Gli3 intracellular pathways. We further found that FGF8, an inducer of Erk/MAPK, Akt/PI3K pathways, was not sufficient for ectopic hairy2 induction. However, joint treatment with both RA and FGF8 induced hairy2, indicating that RA is creating a permissive condition for p-Erk/p-Akt action on hairy2, most likely by enhancing Gli3-A/Gli3-R levels. Finally, we observed an inhibitory action of BMP4 on hairy2 and propose a model whereby RA shapes limb hairy2 expression during limb development, by activating its expression and counteracting the inhibitory action of BMP4 on hairy2. Overall, our work reports a novel role for RA in the regulation of limb clock hairy2 gene expression and elucidates the temporal response of multiple intracellular pathways to RA signaling in limb development.