Rhodopsin photoproducts and the visual response of vertebrate rods

Retinoid uptake, processing, and secretion in human iPS-RPE support the visual cycle

PURPOSE

Retinal pigmented epithelium derived from human induced pluripotent stem (iPS) cells (iPS-RPE) may be a source of cells for transplantation. For this reason, it is essential to determine the functional competence of iPS-RPE. One key role of the RPE is uptake and processing of retinoids via the visual cycle. The purpose of this study is to investigate the expression of visual cycle proteins and the functional ability of the visual cycle in iPS-RPE.

METHODS

iPS-RPE was derived from human iPS cells. Immunocytochemistry, RT-PCR, and Western blot analysis were used to detect expression of RPE genes lecithin-retinol acyl transferase (LRAT), RPE65, cellular retinaldehyde-binding protein (CRALBP), and pigment epithelium-derived factor (PEDF). All-trans retinol was delivered to cultured cells or whole cell homogenate to assess the ability of the iPS-RPE to process retinoids.

RESULTS

Cultured iPS-RPE expresses visual cycle genes LRAT, CRALBP, and RPE65. After incubation with all-trans retinol, iPS-RPE synthesized up to 2942 ± 551 pmol/mg protein all-trans retinyl esters. Inhibition of LRAT with N-ethylmaleimide (NEM) prevented retinyl ester synthesis. Significantly, after incubation with all-trans retinol, iPS-RPE released 188 ± 88 pmol/mg protein 11-cis retinaldehyde into the culture media.

CONCLUSIONS

iPS-RPE develops classic RPE characteristics and maintains expression of visual cycle proteins. The results of this study confirm that iPS-RPE possesses the machinery to process retinoids for support of visual pigment regeneration. Inhibition of all-trans retinyl ester accumulation by NEM confirms LRAT is active in iPS-RPE. Finally, the detection of 11-cis retinaldehyde in the culture medium demonstrates the cells' ability to process retinoids through the visual cycle. This study demonstrates expression of key visual cycle machinery and complete visual cycle activity in iPS-RPE.

The cone-specific visual cycle

Cone photoreceptors mediate our daytime vision and function under bright and rapidly-changing light conditions. As their visual pigment is destroyed in the process of photoactivation, the continuous function of cones imposes the need for rapid recycling of their chromophore and regeneration of their pigment. The canonical retinoid visual cycle through the retinal pigment epithelium cells recycles chromophore and supplies it to both rods and cones. However, shortcomings of this pathway, including its slow rate and competition with rods for chromophore, have led to the suggestion that cones might use a separate mechanism for recycling of chromophore. In the past four decades biochemical studies have identified enzymatic activities consistent with recycling chromophore in the retinas of cone-dominant animals, such as chicken and ground squirrel. These studies have led to the hypothesis of a cone-specific retina visual cycle. The physiological relevance of these studies was controversial for a long time and evidence for the function of this visual cycle emerged only in very recent studies and will be the focus of this review. The retina visual cycle supplies chromophore and promotes pigment regeneration only in cones but not in rods. This pathway is independent of the pigment epithelium and instead involves the Müller cells in the retina, where chromophore is recycled and supplied selectively to cones. The rapid supply of chromophore through the retina visual cycle is critical for extending the dynamic range of cones to bright light and for their rapid dark adaptation following exposure to light. The importance of the retina visual cycle is emphasized also by its preservation through evolution as its function has now been demonstrated in species ranging from salamander to zebrafish, mouse, primate, and human.

A novel cone visual cycle in the cone-dominated retina

The visual processing of humans is primarily reliant upon the sensitivity of cone photoreceptors to light during daylight conditions. This underscores the importance of understanding how cone photoreceptors maintain the ability to detect light. The vertebrate retina consists of a combination of both rod and cone photoreceptors. Subsequent to light exposure, both rod and cone photoreceptors are dependent upon the recycling of vitamin A to regenerate photopigments, the proteins responsible for detecting light. Metabolic processing of vitamin A in support of rod photopigment renewal, the so-called "rod visual cycle", is well established. However, the metabolic processing of vitamin A in support of cone photopigment renewal remains a challenge for characterization in the recently discovered "cone visual cycle". In this review we summarize the research that has defined the rod visual cycle and our current concept of the novel cone visual cycle. Here, we highlight the research that supports the existence of a functional cone-specific visual cycle: the identification of novel enzymatic activities that contribute to retinoid recycling, the observation of vitamin A recycling in cone-dominated retinas, and the localization of some of these activities to the Müller cell. In the opinions of the authors, additional research on the possible interactions between these two visual cycles in the duplex retina is needed to understand visual detection in the human retina.

Effect of bleached rhodopsin on signal amplification in rod visual receptors

Bleaching of rhodopsin markedly desensitizes the vertebrate visual system during a subsequent period of dark adaptation. Previous studies have indicated an origin of bleaching desensitization in the visual pigment itself, but have not identified the mechanism of action. A candidate for the site at which densensitization is initially expressed is the activation of transducin (formation of T*) on the rod disk membranes; this reaction directly involves rhodopsin in its photoactivated (R*) form and mediates initial amplification of the visual signal (reviewed in refs 7-9). We have analysed the effect of bleaching on the sensitivity of a flash-induced light-scattering signal known to monitor the disk-based amplifier, and which has been established as specifically monitoring transducin activation. We have recorded this signal from functioning retinal rods in situ ('ATR' signal) and find that bleaches inducing a pronounced, sustained loss in rod electrophysiological sensitivity do not alter the sensitivity of the ATR response after correction for reduced quantum catch. Our results indicate that the biochemical gain of the R*----T* transduction stage remains unchanged in the presence of bleached pigment and implicate a subsequent reaction as the first to show a sustained, bleaching-dependent gain reduction.

Interaction of visual pigment with G-protein: effects of bleaching in native and reconstituted ROS preparations

Native and reconstituted preparations of bullfrog rod outer segment (ROS) membranes were analysed for the binding of the alpha and beta subunits of G-protein. Following incubation at 22-28 degrees C under varying conditions of illumination and chemical treatment, preparations were chilled and extracted with hypotonic medium lacking-vs.-containing GTP (H and HG extracts, respectively). The extent of binding of G alpha,beta ('percentage G bound') was determined by measurement of the relative abundance of G alpha, beta in the H and HG extracts. The addition of G to unbleached, G-depleted membranes (D-ROS) yielded relatively little binding of the G (14 +/- 13%). G reintroduced at congruent to 1 min vs. congruent to 90 min after major bleaching yielded, respectively, levels of binding of 88 +/- 8% and 24 +/- 8%. Native G, extracted at congruent to 90 min post-bleach and added to freshly bleached D-ROS, exhibited binding exceeding that in the parent (bleached +90 min) sample. Supplementation of reconstituted suspensions with all-trans retinal, and subsequent incubation in darkness, yielded 30-35% G-binding; similar treatment with 11-cis retinal yielded binding in the range of 0-32%. Upon irradiation, suspensions previously supplemented with either isomer exhibited approximately 60% binding. Incubation with 11-cis retinal had no substantial effect on the condition of tight binding observed shortly after bleaching. The data are discussed in relation to previous studies of visual pigment bleaching and regeneration.

Photoreceptor processes in visual adaptation

In this paper we have stressed two experimental results in need of explanation: (i) the reduced efficacy with which (remaining, abundant) rhodopsin in the light-adapted receptor mediates the flash response; and (ii) the disparity in conditions of irradiation (weak background vs. extensive bleaching) leading to equivalent conditions of threshold. The model discussed above suggests, in molecular terms, a possible basis for both properties of receptor adaptation. On the view developed here, property (i) derives from the ability of photoactivated or bleached pigment (R or B) to restrict dramatically the availability of a substance required for phototransduction. Property (ii) derives in large part from the pronounced disparity in the effectiveness of R (during illumination) and B (remaining after illumination) in reducing the availability of this substance. On this view, the "equivalence" of threshold elevation in states of light- vs. dark-adaptation derives from an overall equality of a product of factors (Q, Etot/Es, and J of equation 2). Under all but extreme conditions, this aggregate of factors is dominated by the term Etot/Es, reflecting the functional state of E.