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Seeing the Light

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Seeing the Light RESEARCH HIGHLIGHTS File name: NRN0305_RJ2_HL.doc Word count: 540 Accompanying picture: YES/no SENSORY TRANSDUCTION File name of picture: Seeing the light Three studies have shown that ity, three groups expressed the pig- opsins themselves can carry out the melanopsin — a pigment that is ment in different types of cell — photoisomerase activity that is found in the type of retinal ganglion Xenopus oocytes, human embryonic needed to regenerate the chro- cell that allows light to entrain the cir- kidney (HEK293) cells and a mouse mophore. Both Melyan et al. and cadian clock — can function as a neuronal cell line called neuro-2a. In Panda et al. provide evidence that photopigment in other types of cell. each case, the expression of melanopsin resembles invertebrate As well as confirming that melanopsin caused the cells to opsins in that it has an intrinsic pho- melanopsin is photosensitive, the become photosensitive. toisomerase activity that can convert studies reveal that it is closer in some The three groups also investigated all-trans-retinaldehyde into 11-cis- ways to invertebrate photopigments the signalling pathways that mediated retinaldehyde. than to other photopigments in ver- phototransduction in the transfected Although further studies are tebrates. cells. Molyan et al.found that, in needed to pin down the exact mecha- Circadian entrainment in mam- neuro-2a cells, melanopsin signals nism by which melanopsin mediates mals relies on a set of intrinsically through a G-protein signalling path- phototransduction in ipRGCs, these photoreceptive retinal ganglion cells way to regulate the opening of an three studies provide proof that (ipRGCs). Although these contain intrinsic ion channel. In Xenopus melanopsin can function as a pho- melanopsin, and lose their photore- oocytes and HEK293 cells, according topigment, and also point towards an ceptive properties if melanopsin is to Panda et al. and Qiu et al., the acti- invertebrate-like signalling mecha- removed, it has not previously been vation of melanopsin by light can nism. In a fourth study that investi- shown that melanopsin itself is the trigger the opening of TRPC3 cal- gated the melanopsin-driven disper- photopigment in these cells. To show cium channels — a mammalian sal of melanosomes in cultured that melanopsin is not only necessary, homologue of the TRP and TRPL Xenopus melanophores, Isoldi et al. but also sufficient, for photosensitiv- channels, which are involved in pho- also found evidence for a signalling totransduction in Drosophila.The pathway that resembled those in activation of TRPC3 channels in invertebrate photoreceptors. This these cells also involves signalling similarity between ipRGCs and inver- through a G-protein pathway, and the tebrate photoreceptors could give signalling pathway is similar to that valuable insights into the biology and found in invertebrate photoreceptors. evolution of the circadian light- Photosensitive opsins, such as entrainment system in vertebrates. melanopsin, use 11-cis-retinaldehyde Rachel Jones as a chromophore. When light con- References and links verts 11-cis-retinaldehyde to all-trans- ORIGINAL RESEARCH PAPERS Melyan, Z. et al. Addition of human melanopsin renders retinaldehyde, it creates a conforma- mammalian cells photoresponsive. Nature 26 tional change in the opsin that January 2005 (10.1038/nature03344) | Qiu, X. et al. Induction of photosensitivity by heterologous triggers G-protein activation. In ver- expression of melanopsin. Nature 26 January tebrate photoreceptors, the chro- 2005 (10.1038/nature03345) | Panda, S. et al. mophore is converted back to 11-cis- Illumination of the melanopsin signaling pathway. Science 307, 600–604 (2005) | Isoldi, M. C. et al. retinaldehyde through a complex Rhabdomeric phototransduction initiated by the pathway in the retinal pigment vertebrate photopigment melanopsin. Proc. Natl epithelium, but in invertebrates the Acad. Sci. USA 102, 1217–1221 (2004) 2 | MARCH 2005 | VOLUME 6 www.nature.com/reviews/neuro © 2005 Nature Publishing Group .
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