title: Induction of photosensitivity by heterologous expression of melanopsin.
abstract: Melanopsin has been proposed to be the photopigment of the intrinsically photosensitive retinal ganglion cells (ipRGCs); these photoreceptors of the mammalian eye drive circadian and pupillary adjustments through direct projections to the brain. Their action spectrum (lambda(max) approximately 480 nm) implicates an opsin and melanopsin is the only opsin known to exist in these cells. Melanopsin is required for ipRGC photosensitivity and for behavioural photoresponses that survive disrupted rod and cone function. Heterologously expressed melanopsin apparently binds retinaldehyde and mediates photic activation of G proteins. However, its amino-acid sequence differs from vertebrate photosensory opsins and some have suggested that melanopsin may be a photoisomerase, providing retinoid chromophore to an unidentified opsin. To determine whether melanopsin is a functional sensory photopigment, here we transiently expressed it in HEK293 cells that stably expressed TRPC3 channels. Light triggered a membrane depolarization in these cells and increased intracellular calcium. The light response resembled that of ipRGCs, with almost identical spectral sensitivity (lambda(max) approximately 479 nm). The phototransduction pathway included Gq or a related G protein, phospholipase C and TRPC3 channels. We conclude that mammalian melanopsin is a functional sensory photopigment, that it is the photopigment of ganglion-cell photoreceptors, and that these photoreceptors may use an invertebrate-like phototransduction cascade.
authors.affiliation: Department of Neuroscience, Box 1953, Brown University, Providence, Rhode Island 02912-1953, USA.
authors.forename: Xudong
authors.initials: X
authors.lastname: Qiu
authors.forename: Tida
authors.initials: T
authors.lastname: Kumbalasiri
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Carlson
authors.forename: Kwoon Y
authors.initials: KY
authors.lastname: Wong
authors.forename: Vanitha
authors.initials: V
authors.lastname: Krishna
authors.forename: Ignacio
authors.initials: I
authors.lastname: Provencio
authors.forename: David M
authors.initials: DM
authors.lastname: Berson
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000528:radiation effects; D002460:Cell Line; D004594:Electrophysiology; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D007473:Ion Channels / Q000378:metabolism; D008027:Light; D055537:Light Signal Transduction / Q000528:radiation effects*; D051379:Mice; D017299:Rod Opsins / Q000235:genetics / Q000378:metabolism*; D050052:TRPC Cation Channels; D010738:Type C Phospholipases / Q000378:metabolism; D000099210:Melanopsin
chemical_list: D002118:Calcium; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D007473:Ion Channels; D017299:Rod Opsins; D050052:TRPC Cation Channels; D010738:Type C Phospholipases; D000099210:Melanopsin; C104603:TRPC3 cation channel