The decoupling of Smoothened from Gαi proteins has little effect on Gli3 protein processing and Hedgehog-regulated chick neural tube patterning

The Hedgehog (Hh) signal is transmitted by two receptor molecules, Patched (Ptc) and Smoothened (Smo). Ptc Suppresses Smo activity, while Hh binds Ptc and alleviates the Suppression, which results in activation of Hh targets. Smo is a seven-transmembrane protein with a long carboxyl terminal tail. Vertebrate Sino has been previously shown to be coupled to G alpha(i) proteins, but the biological significance of the coupling in Hh signal transduction is not clear. Here we show that although inhibition of G alpha(i) protein activity appears to significantly reduce Hh pathway activity in Ptc(-/-) mouse embryonic fibroblasts and the NIH3T3-based Shh-light cells, it fails to derepress Shh- or a Smo-agonist-induced inhibition of Gli3 protein processing, a known in vivo indicator of Hh signaling activity. The inhibition of Gai protein activity also cannot block the Sonic Hedgehog (Shh)-dependent specification of neural progenitor cells in the neural tube. Consistent with these results, overexpression of a constitutively active G alpha(i) protein, G alpha(i2)QL, cannot ectopically specify the neural cell types in the spinal cord, whereas an active Smo, SmoM2, can. Thus, our results indicate that the Smo-induced G alpha(i) activity plays an insignificant role in the regulation of GO processing and Shh-regulated neural tube patterning. (C) 2008 Elsevier Inc. All rights reserved.