Inner retinal photoreception independent of the visual retinoid cycle

Abstract

Mice lacking the visual cycle enzymes RPE65 or lecithin-retinol acyl transferase (Lrat) have pupillary light responses (PLR) that are less sensitive than those of mice with outer retinal degeneration (rd͞rd or rdta). Inner retinal photoresponses are mediated by melanopsinexpressing, intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting that the melanopsin-dependent photocycle utilizes RPE65 and Lrat. To test this hypothesis, we generated rpe65 ؊/؊ ; rdta and lrat ؊/؊ ; rd͞rd mutant mice. Unexpectedly, both rpe65 ؊/؊ ; rdta and lrat ؊/؊ ; rd͞rd mice demonstrate paradoxically increased PLR photosensitivity compared with mice mutant in visual cycle enzymes alone. Acute pharmacologic inhibition of the visual cycle of melanopsin-deficient mice with all-trans-retinylamine results in a near-total loss of PLR sensitivity, whereas treatment of rd͞rd mice has no effect, demonstrating that the inner retina does not require the visual cycle. Treatment of rpe65 ؊/؊ ; rdta with 9-cis-retinal partially restores PLR sensitivity. Photic sensitivity in P8 rpe65 ؊/؊ and lrat ؊/؊ ipRGCs is intact as measured by ex vivo multielectrode array recording. These results demonstrate that the melanopsin-dependent ipRGC photocycle is independent of the visual retinoid cycle.melanopsin ͉ pupillary light response ͉ retinal degeneration ͉ retinal ganglion cell ͉ visual photocycle M ice blind from outer retinal degeneration retain the ability to entrain their circadian rhythms to external light-dark cycles, have active pupillary light responses (PLRs), and exhibit photically induced melatonin suppression (1-4). These responses depend on the opsin family member melanopsin (5), which is expressed exclusively in a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) (6-8). Mice with nonfunctional or degenerated rods and cones that also lack melanopsin show no photically influenced behavior or physiology (9, 10), and intrinsic photosensitivity of retinal ganglion cells is lost in the absence of melanopsin (9,11,12). Melanopsin forms a functional photopigment when heterologously expressed in COS cells (13), Xenopus oocytes (14), Neuro-2A cells (15), or HEK-293 cells (16). Melanopsin is thus necessary for ipRGC photoreception, and is sufficient to confer photosensitivity on intrinsically insensitive cell types. Taken together, these results strongly suggest that melanopsin is the photopigment of inner retinal photoreception.All opsin photopigments use a retinoid chromophore that undergoes isomerization upon absorption of an appropriate wavelength photon. In rod photoreceptors, 11-cis-retinal chromophore is photoisomerized to all-trans-retinal which dissociates from the opsin protein (17). This all-trans-retinal photoproduct is recycled to the active (11-cis) form by enzymatic conversion in the adjacent retinal pigment epithelium (RPE) (18). The chromophore used by melanopsin in situ is not presently known. ipRGCs are physically distant from the enzymatic chromophore regeneration machinery of the RPE. It is present...

Light-Evoked Calcium Responses of Isolated Melanopsin-Expressing Retinal Ganglion Cells

“…Chromophore regeneration occurs in invertebrate photoreceptors and there is indirect evidence that this process exists in mammalian ipRGCs Tu et al, 2006). However, rather than chromophore bleaching, the pattern of response decay may instead reflect an active adaptation to the constantintensity light stimulus.…”

Phototransduction in ganglion-cell photoreceptors

“…This result appeared to suggest that melanopsin depended at least in part on an extrinsic enzymatic retinoid cascade for its normal function. However, more recent evidence indicates that this effect is secondary to the devastating effects of the knockout on rod and cone function ( [59]; see also [13,34]). In mice with degenerative loss of rods and cones, neither RPE65 deletion nor acute pharmacological disruption of the retinoid cycle has any significant impact on ipRGC sensitivity [59].…”

“…However, more recent evidence indicates that this effect is secondary to the devastating effects of the knockout on rod and cone function ( [59]; see also [13,34]). In mice with degenerative loss of rods and cones, neither RPE65 deletion nor acute pharmacological disruption of the retinoid cycle has any significant impact on ipRGC sensitivity [59]. These results do not entirely exclude a retinoid cycle for ipRGCs localized within the inner retina and involving proteins other than melanopsin itself, but at present, there seems no strong evidence favoring such a mechanism.…”

Chromophore regeneration: Melanopsin does its own thing

“…This observation reveals that the reduced ipRGC activity of animals lacking visual cycle is largely attributable to an inhibitory influence of the inactive outer retina and suggests that ipRGCs are in fact resistant to the loss of Rpe65 or Lrat. Importantly, Tu et al (10) continue to confirm this by using all-trans-retinylamine to block the visual cycle acutely. ipRGC activity is not impaired by this treatment.…”

“…However, undoubtedly the most important implication of these articles (9,10) relates to the nature of melanopsin's local chromophore supply. If melanopsin does not use the visual cycle to obtain cis-retinaldehyde, what does it use?…”