Microarray analysis and functional Genomics identify novel components of melanopsin signaling

Background: Within the mammalian retina, there exists a third photoreceptive system based upon a population of melanopsin (Opn4) expressing photosensitive retinal ganglion cells (pRGCs; also termed ipRGCs or intrinsically photosensitive RGCs). Here, we use a microarray-based approach, which we term transcriptional recalibration, coupled with functional genomics to identify downstream targets of melanopsin signaling. Results: In a mouse with genetically ablated rods and cones (rd/rd cl, approximately 30% of the ocular transcriptome is transiently regulated in response to nocturnal light exposure (3112 genes). A total of 163 of these genes were associated with the intracellular signaling gene ontology term. On the basis of their similarity to invertebrate phototransduction genes, 14 were selected for further study. Laser capture microdissection demonstrated that eight of these genes (Gnas, Gnb2l1, Gnaq, Prkcz, Pik3r1, Inadl, Slc9a3r1, and Drd1a) colocalized with melanopsin. The impact of genetic ablation of one of these genes, protein kinase C zeta (Prkcz), was assessed. Prkcz(-/-) animals show attenuated phase-shifting responses to light, reduced period lengthening under constant light, and attenuated pupillary responses at high irradiances, as well as impaired light-induced gene expression in the suprachiasmatic nuclei (SCN). These attenuated responses are indistinguishable from the deficits observed in melanopsin knockout mice. Conclusions: Here, we show that (1) Prkcz plays an as yet unidentified role in melanopsin signaling, (2) the proteins of seven further light-regulated genes emerge as strong candidates in melanopsin signaling, and (3) transcriptional recalibration may provide a powerful new approach for dissecting unmapped signaling pathways.