Cyclic GMP-dependent neurite outgrowth by genipin and nerve growth factor in PC12h cells

We have demonstrated previously that a natural iridoid compound, genipin, induced neuritogenesis through activation of nitric oxide synthase (NOS) and mitogen-activated protein kinase (MAPK) in PC12h cells. In this paper, we investigated whether cyclic GMP (cGMP) and cGMP-dependent protein kinase (PKG) are involved in the neuritogenesis as a result of NOS activation. Furthermore, we also investigated the relationship between cGMP and MAPK activation in the signaling pathway. The genipin-induced neuritogenesis accompanied by induction of neurofilament was significantly inhibited by 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ) and KT5823, inhibitors of soluble guanylate cyclase and PKG, respectively. Genipin-induced MAPK phosphorylation was also abolished by ODQ. These inhibitory effects of ODQ were similar to those observed for nerve growth factor (NGF)-induced neurite outgrowth and MAPK phosphorylation. The membrane-permeable cGMP analog, 8-Bromo-cGMP, had prominent neuritogenic activity, which was completely inhibited by a MAPK kinase inhibitor, PD98059. These results suggest that the soluble guanylate cyclase-PKG signaling pathway is important for MAPK activation by genipin as well as NGF during neuritogenesis in PC12h cells.