Osmotic shock-induced neurite extension via activation of p38 mitogen-activated protein kinase and CREB

Although it is known that sustained activation of classical mitogen-induced protein kinase (MAPK, also known as ERK) induced by nerve growth factor (NGF) plays an important role in the induction of neurite outgrowth, the role of p38 MAPK in neural cell function is still not clear. We developed two neuronal cell lines from PC12 cells, PC12m3 and PC12m32, in which NGF-induced neurite outgrowth is impaired and that show neurite outgrowth in response to hyperosmotic shock. The frequencies of neurite outgrowth of PC12m3 and PC12m32 cells induced by osmotic shock were approximately 10- and 12-fold greater, respectively, than that in PC12 parental cells. The p38 MAPK pathway inhibitor SB203580 but not the ERK pathway blocker U0126 inhibited the ability of PC12m3 and PC12m32 cells to induce neurite outgrowth in response to osmotic shock. Furthermore, expression of a nonactivable form of p38 but not that of wild-type p38 significantly blocked neurite outgrowth induced by osmotic shock. The extent of phosphorylation of p38 MAPK induced by osmotic shock in PC12m32 cells was much greater than that in PC12 parental cells. The upstream kinases MKK3 and MKK6, which phosphorylate and activate p38 MAPK, also showed higher levels in PC12m32 cells than in PC12 parental cells when treated with osmotic shock. Inhibition of p38 MAPK by SB203580 resulted in inhibition of the activity of the transcription factor CREB, which is activated by osmotic shock. These findings indicate that activation of CREB mediated by a p38 pathway distinct from the NGF signaling pathway may be required for neurite outgrowth.