Light induces Fos expression via extracellular signal-regulated kinases 1/2 in melanopsin-expressing PC12 cells

The photopigment melanopsin is expressed in a subtype of mammalian ganglion cells in the retina that project to the circadian clock in the hypothalamic suprachiasmatic nucleus to mediate non-visual light information. Melanopsin renders these retinal ganglion cells intrinsically photosensitive and the cells respond to light by a membrane depolarization and induction of the immediate early response gene Fos. Previous studies showed that the light activated melanopsin-induced signaling, the phototransduction, leading to depolarization of the membrane resembles the invertebrate opsins, which involves a Galpha(q/11) coupled phospholipase C activation. However, the signaling proteins mediating melanopsin-induced Fos expression are unresolved. In this study, we examined the phototransduction leading to Fos expression in melanopsin-transfected PC12 cells. A pivotal role of the extracellular signal-regulated protein kinase 1/2 (ERK1/2) was found as pharmacological blockage of this kinase suppressed the light-induced Fos expression. Illumination increased the inositol phosphate turnover and induced phosphorylation of ERK1/2 and p38 but not the c-Jun N-terminal kinase. The Galpha(q/11) protein inhibitor YM254890 attenuated these intracellular light responses. Our data strongly indicate that Galpha(q/11)-mediated ERK1/2 activation is essential for expression of Fos upon illumination of melanopsin-expressing PC12 cells.