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One of the fundamental mysteries of the human visual system is the continuous function of cone photoreceptors in bright daylight. As visual pigment is destroyed, or bleached, by light , cones require its rapid regeneration, which in turn involves rapid recycling of the pigment’s chromophore. The canonical visual cycle for rod and cone pigments involves recycling of their chromophore from all-trans retinol to 11-cis retinal in the pigment epithelium, adjacent to photoreceptors . However, shortcomings of this pathway indicate the function of a second, cone-specific, mechanism for chromophore recycling . Indeed, biochemical [3–7] and physiological  studies on lower species have described a cone-specific visual cycle additional to the long-known pigment epithelium pathway. Two important questions remain, however: what is the role of this pathway in the function of mammalian cones and is it present in higher mammals, including humans. Here we show that mouse, primate, and human neural retinas promote pigment regeneration and dark adaptation selectively in cones, but not in rods. This pathway supports rapid dark adaptation of mammalian cones and extends their dynamic range in background light independently of the pigment epithelium. This pigment-regeneration mechanism is essential for our daytime vision and appears to be evolutionarily conserved.
We measured flash sensitivity of single photoreceptors before and after exposure to bright light to determine whether the mouse retina, isolated from the pigment epithelium, is able to promote cone pigment regeneration and dark adaptation. We used a suction electrode to record membrane current from individual mouse rods and cones in retina removed from the pigment epithelium. Rod recordings were done from wild type (WT, C57BL/6) mice and cone recordings were done from rod transducin α-subunit knockout (Trα−/−) mice, as described previously .
We compared the dark current and sensitivity of photoreceptors before (dark adapted) and after 40-s exposure to bright light followed by recovery in darkness (bleached, DA). With subsequent 4-h dark incubation, exposure of rods in isolated retina to bleaching light produced significant decrease in dark current, from 15 pA to 5 pA, and 155-fold decrease in their flash sensitivity, from 3.1 ± 0.5 ×10−1 (n = 6) pA photon−1 μm2 for dark adapted cells to 2.0 ± 0.2 × 10−3 (n = 6) pA photon−1 μm2 after bleach (Figure 1A). In contrast, identical bleach followed by 2-h dark incubation of cones in Trα−/− retinas had no effect on the dark current and the sensitivity recovered to 50% of its dark adapted value, from 1.57 ± 0.35 × 10−4 (n = 5) pA photon−1 μm2 to 0.78 ± 0.25 × 10−4 (n = 5) pA photon−1 μm2 (Figure 1B). Cone recovery following a bleach took place in the absence of pigment epithelium indicating that substantial cone pigment regeneration occurred within the mouse retina. Rod pigment regeneration under the same conditions was insignificant even following longer dark incubation. These results are consistent with our recent findings from wild type mice using whole-retina electroretinogram (ERG) recordings (Wang et al., see also Fig 1C, D). Notably, cone ERG recovery could be observed after subsequent second and third exposures to the bleaching light (Figure 1D). Considering the low level of free 11-cis retinal in the mouse retina , this result indicates that cone pigment regeneration in the isolated retina was driven by recycling chromophore released from bleached visual pigment rather than by a store of retinoid. A simple explanation for the incomplete cone pigment regeneration would be that the retina was damaged during dissection, or that chromophore released from bleached cones was lost to the perfusion solution, thereby reducing the chromophore available for regeneration.
Biochemical studies in cone-dominant retinas [3, 5] as well as recordings from salamander retina  suggest that retinal Müller glia might be part of the retina visual cycle. To investigate the role of Müller cells in mammalian cone pigment regeneration, we first perturbed the physical contact between a cone and the rest of retina, including Müller cells. We achieved that by drawing the inner segment of a cone from a piece of retina into a suction electrode. We found that cone dark adaptation was blocked and could not be observed in this configuration (Figure 1B, right).
Next, we selectively disrupted the function of Müller cells by pre-incubating the retina for 2.5 h with the glial cell metabolic inhibitor α-aminoadipic acid (L-α-AAA), a well known selective gliotoxin , prior to recordings. Such treatment provides an effective block of the retina visual cycle in salamander retina . L-α-AAA pre-incubation resulted in weaker immunostaining of glial processes for Müller cell-specific glutamine synthetase (GS) and loss of Müller cell nuclei (Figure 2A). The gliotoxin did not affect the function of dark-adapted cones (Figure 2B, left) but dramatically inhibited the recovery of cone response amplitude in the isolated retina following a bleach (Figure 2B, center). Cone sensitivity also declined, from 1.75 ± 0.38 × 10−4 (n = 4) pA photon −1 μm2 in dark-adapted cells to 5.12 ± 1.41 × 10−6 (n = 4) pA photon −1 μm2 following the bleach. Thus, blocking Müller cell function, either physically or pharmacologically, inhibited the mouse retina visual cycle and blocked pigment regeneration and dark adaptation in cones.
Application of exogenous 11-cis retinol reversed the cone desensitization in the bleached gliotoxin-treated mouse retina and promoted recovery of cone sensitivity to dark-adapted levels, 1.52 ± 0.04 × 10−4 (n = 5) pA photon−1 μm2 (Figure 2B, right; see also Figure 2C). This result indicates that the gliotoxin did not affect the ability of cones to oxidize 11-cis retinol or to regenerate their pigment but rather blocked the supply of recycled chromophore, most likely 11-cis retinol, from Müller cells to cones. In contrast, 11-cis retinol was not able to promote rod pigment regeneration and had no effect on the sensitivity of bleached rods (data not shown).
To study the recovery of cone sensitivity in real time and determine the kinetics of chromophore recycling by the cone-specific retina visual cycle, we recorded rod and cone ERG photoresponses from isolated mouse retina. The Trα−/− retina restored cone sensitivity to half of its dark-adapted level within 5 min following a bleach (Figure 3A). Consistent with our single-cell results, this recovery was inhibited when the function of Müller cells was blocked by L-α-AAA (Figure 3A). Thus, the Müller cell-mediated retina visual cycle was rapid and could promote the dark adaptation of mouse cones at a rate identical to the one measured in vivo .
To establish whether the rapid cone pigment regeneration by the retina visual cycle affects the dynamic range of cones in background light, we compared light adaptation of cones in isolated retina with and without functional retina visual cycle. Blocking the retina visual cycle using L-α-AAA resulted in a substantial shift to the left in the cone background adaptation curve (Figure 3B). The background light required to reduce sensitivity to half of its dark adapted level decreased 9.2-fold, from 25,165 photons μm−2 s−1 (n = 7) in control retina to 2,747 photons μm−2 s−1 (n = 8) in L-α-AAA-treated retina. Applying exogenous 11-cis retinal reversed the effect of L-α-AAA on background adaptation in low intensity backgrounds (Figure 3B). The efficiency of 11-cis retinal in rescuing background adaptation declined with increasing light intensity, most likely because the limited supply of exogenous chromophore could not keep up with the high rate of pigment bleaching in brighter backgrounds. These results indicate that in background light conditions the retina visual cycle provides chromophore for cones to broaden their functional range.
To investigate whether a visual cycle is present in the primate and human retinas, we studied their cones using whole-retina ERG recordings. As primate eye enucleation was performed in bright light, both rods and cones were initially bleached. Following 3-h dark incubation of primate retina attached to the pigment epithelium in eyecup, we observed prominent rod (slow, high sensitivity) and cone (fast, low sensitivity) components in the ERG response (Figure 4A, left) consistent with a dark adaptation of both rods and cones. However, after this retina was isolated from the eyecup, bleached, and then incubated in darkness for 3 h, the rod component was abolished whereas the cone component recovered essentially completely (Figure 4A, right). Similarly, a light-adapted primate retina removed from the eyecup immediately after enucleation and incubated for 1 h in darkness exhibited a robust cone response, but no rod response (Figure 4B). Following a subsequent bleach and 1-h dark incubation, the cone response amplitude recovered essentially completely and cone sensitivity recovered to 30% (n = 8) of the pre-bleach level (Figure 4B).
Finally, we performed recordings from freshly harvested human retina. As enucleation and retina removal were performed in bright light, both rods and cones were initially bleached. Following incubation in darkness for 3 h, we observed robust cone responses but no rod responses (Figure 4C, left). The ability of the isolated human retina to promote cone dark adaptation was further demonstrated by the recovery of cone sensitivity following a 40-s bleach and 1-h dark incubation (Figure 4C, right). Similarly to the case of mouse, inhibiting the function of Müller cells with gliotoxin in isolated human retina blocked the recovery of cone sensitivity (Figure 4D). Subsequent treatment with exogenous 11-cis retinol reversed the effect of the bleach and promoted cone-specific dark adaptation (Figure 4D). In contrast, exogenous 11-cis retinal induced dark adaptation and response recovery in both rods and cones of gliotoxin-treated bleached human retina (data not shown). Together, these results demonstrate that, similarly to salamander and mouse, the primate and human retinas promote cone-selective pigment regeneration and dark adaptation independently of the pigment epithelium.
Our results establish the function of a mechanism by which the mammalian neural retina recycles chromophore and enables pigment regeneration independently of the pigment epithelium. This visual cycle promotes the rapid pigment regeneration and dark adaptation selectively in cones, but not in rods. The recycling of chromophore released from bleached cone pigment involves the retinal Müller glia, where all-trans retinol is converted most likely to 11-cis retinol. We demonstrate the robust function of this pathway in the retinas of mice, primates, and humans. Together with our previous salamander results  and recent biochemical studies from zebrafish , chicken , and ground squirrel , our findings indicate that the retina visual cycle is evolutionally conserved. The presence of a retina visual cycle in a wide range of species, from amphibian to human, also points to its critical role in cone photoreceptor function.
The neural retina visual cycle plays an important role for mammalian daytime vision. We find that, following an essentially complete bleach, the mammalian retina promotes cone dark adaptation at rates comparable to the rates of recovery of cone sensitivity in vivo[12, 14]. Thus, the neural retina visual cycle helps explain the substantially faster pigment regeneration and dark adaptation of mammalian cones compared to rods. In addition, we show that this rapid pigment regeneration enables mammalian cones to maintain sufficient pigment levels to function in steady bright light, that bleaches their pigment at a high rate, and in that way extends the functional range of cones. As the canonical pathway for recycling chromophore through the pigment epithelium is too slow to keep up with the high rate of pigment bleaching in steady bright light , the neural retina visual cycle should be critical for maintaining adequate levels of pigment and the continuous function of cones during the day. Together, our results demonstrate that the mammalian retina visual cycle is essential for maintaining our daytime vision, mediated by the cones, in bright and rapidly changing light conditions during the day.
We find that the presumptive product of the retina visual cycle, 11-cis retinol, promotes pigment regeneration and dark adaptation only in mammalian cones but not in rods. The exclusive ability of mammalian cones to utilize 11-cis retinol for pigment regeneration grants the cone-selectivity of the retina visual cycle. The enzyme responsible for converting 11-cis retinol to 11-cis retinal in cones remains unknown. Previous biochemical studies from mouse retina have failed to detect enzymatic activities consistent with a retina visual cycle  most likely due to the small number of cones (3%) in the mouse retina. In addition, two studies of all-cone Nrl−/− mice lacking RPE65, a key component of the pigment epithelium visual cycle, also could not find evidence for chromophore recycling in the mouse retina, further raising doubts about the function of a cone-specific visual cycle in mammals with rod-dominant retinas [15, 16]. However, our results clearly demonstrate the presence of a visual cycle in mouse, primate, and human retinas indicating the function of an 11-cis retinol dehydrogenase in their cones.
The characterization of a cone-specific visual cycle in the human retina calls attention to its possible clinical implication for cone visual disorders. Deficits in the canonical visual cycle have been implicated in multiple visual disorders and the mechanisms by which chromophore deficiency affects rod function have been well characterized . However, understanding of the mechanisms by which such disorders affect cones is lagging behind. Our characterization of a human cone-specific visual cycle indicates that cones might be affected differently from rods in disorders of the pigment epithelium visual cycle. One example of such disorder is Leber congenital amaurosis (LCA) associated with chromophore deficiency due to mutations in RPE65. Notably, in LCA patients cone sensitivity is less severely affected than rod sensitivity  indicating a possible role for the retina visual cycle in maintaining higher chromophore level in cones compared to rods. In addition, possible deficiencies in the retina visual cycle are likely to selectively disturb the function of cones. Finally, it would be important to consider how the function of the retina visual cycle is affected with age and whether gradual deterioration of that pathway is linked to age-related cone disorders, most notably age-related macular degeneration.
We thank Janis Lem for the Trα−/− mice, Rosalie Crouch for the gift of 11-cis retinal and 11-cis retinol, Milam Brantley for the gift of α-GS antibody, Dan Moran, Dana Abendschein, Chad Faulkner, and Mary-Kay Harmon for the donation of primate eyes, and William Harbour and Lori Worley for the donation of human retina. We also thank King-Wai Yau, Carter Cornwall, Peter Lukasiewicz, and Rosalie Crouch for comments on the manuscript. Supported by Career Development Award from Research to Prevent Blindness, NIH grant EY 019312, and unrestricted grant from Research to Prevent Blindness and EY 02687 (Department of Ophthalmology & Visual Sciences at Washington University).
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