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.

Sea urchin tube feet are photosensory organs that express a rhabdomeric-like opsin and PAX6

“…The rhabdomeric photoreceptor cells may have transformed into ganglion cells expressing melanopsin, acquiring a new role in light detection and signal transduction [49]. First described in Xenopus, melanopsin is now known to confer photosensitivity on non-photosensitive cells types [50,51]. It has been hypothesized that melanopsin sets the circadian clock through non-ocular light detection [52], and is also expressed in sea urchins [30].…”

Phototransduction and retinal degeneration in Drosophila

“…The cascade in the ipRGCs is initiated through a receptor, melanopsin, which has greater sequence and biophysical similarities to the Drosophila rhodopsins [12][13][14][15][16][17][18] than the light receptors in rods and cones. Furthermore, pharmacological and electrophysiological studies suggest that the ipRGC cascade functions through a PLC and opening of channels that display features reminiscent of TRP channels [19][20][21]. Although, most of the specific signaling proteins in the ipRGCs remain to be identified, it appears that the Drosophila and mammalian ipRGC phototransduction cascades share common origins.…”

Phototransduction in Drosophila

“…higher similarities in sequence and biophysical properties to fly rhodopsins than the light receptors in rods and cones [10][11][12][13][14][15][16]. Some studies have demonstrated that ipRGC cascade functions through the activation of PLC and the opening of channels, which displays the features of TRP channels [10,17,18].…”

“…Some studies have demonstrated that ipRGC cascade functions through the activation of PLC and the opening of channels, which displays the features of TRP channels [10,17,18].…”