Constitutively active Galpha12, Galpha13, and Galphaq induce Rho-dependent neurite retraction through different signaling pathways.

In neuronal cells, activation of a certain heterotrimeric G protein-coupled receptor causes neurite retraction and cell rounding via the small GTPase Rho. However, the specific heterotrimeric G proteins that mediate Rho-dependent neurite retraction and cell rounding have not yet been identified. Here we investigated the effects of expression of constitutively active Galpha subunits on the morphology of differentiated PC12 cells. Expression of GTPase-deficient Galpha12, Galpha13, and Galphaq, but not Galphai2, caused neurite retraction and cell rounding in differentiated PC12 cells. These morphological changes induced by Galpha12, Galpha13, and Galphaq were completely inhibited by C3 exoenzyme, which specifically ADP-ribosylates and inactivates Rho. The tyrosine kinase inhibitor tyrphostin A25 blocked the neurite retraction and cell rounding induced by Galpha13 and Galphaq. However, tyrphostin A25 failed to inhibit the Galpha12-induced neuronal morphological changes. On the other hand, inhibition of protein kinase C or elimination of extracellular Ca2+ blocked the neurite retraction and cell rounding induced by Galphaq, whereas the morphological effects of Galpha12 and Galpha13 did not require activation of protein kinase C and extracellular Ca2+. These results demonstrate that activation of Galpha12, Galpha13, and Galphaq induces Rho-dependent morphological changes in PC12 cells through different signaling pathways.