Evidence that Shh cooperates with a retinoic acid inducible co-factor to establish ZPA-like activity

Patterning across the anteroposterior axis of the vertebrate limb bud involves a signal from the polarizing region, a small group of cells at the posterior margin of the bud. Retinoic acid and Sonic hedgehog have been independently postulated as such signals because they can mimic the mirror image digit duplication obtained after grafting polarizing cells to the anterior of limb buds. Here we show that a embryonal carcinoma cell line, P19, transfected with a Shh expression vector shows low polarizing activity, but when cultured with retinoic acid, duplications like those induced by the polarizing region (ZPA) arise. Complete duplications are also obtained by cotransfecting P19 Shh cells with a constitutively active human retinoic acid receptor (VP16-hRARα). These data suggest that Shh and RA cooperate in generating ZPA activity and that Shh, while essential, may not act alone in this process.