There is growing evidence that chronic inflammation plays a role in both the development and progression of diabetic retinopathy. There is also evidence that molecules produced as a result of hyperglycemia can activate microglia. However the exact contribution of microglia, the resident immune cells of the central nervous system, to retinal tissue damage during diabetes remains unclear. Current data suggest that dysregulated microglial responses are linked to their deleterious effects in several neurological diseases associated with chronic inflammation. As inflammatory cytokines and hyperglycemia disseminate through the diabetic retina, microglia can change to an activated state, increase in number, translocate through the retina, and themselves become the producers of inflammatory and apoptotic molecules or alternatively exert anti-inflammatory effects. In addition, microglial genetic variations may account for some of the individual differences commonly seen in patient's susceptibility to diabetic retinopathy.

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.

A novel retinoid cycle has recently been identified in the cone-dominated chicken retina, and this cone cycle accumulates 11-cis-retinyl esters upon light adaptation. The purpose of this study is to investigate how 11-cis-retinyl esters are formed in the retina. Primary cultures of chicken Muller cells and cell membrane were incubated with all-trans- or 11-cis-retinol to study retinyl ester synthesis. In Muller cells, esterification of 11-cis-retinol was four times greater than esterification of all-trans-retinol. In the presence of palmitoyl-CoA and CRALBP, Muller cell membranes synthesized 11-cis-retinyl ester from 11-cis-retinol at a rate which was 20-fold higher than that of all-trans-retinyl ester. In the absence of CRALBP, 11-cis-retinyl ester synthesis was greatly reduced (by 7-fold). In the absence of palmitoyl-CoA, retinyl ester synthesis was not observed. Muller cell membranes incubated with radiolabeled palmitoyl-CoA resulted in the transfer of the labeled acyl group to retinol. This acyl transfer was greatly reduced in the presence of progesterone, a known ARAT inhibitor. 11-cis-ARAT activity remained unchanged when assayed in the presence of all-trans-retinol, suggesting a distinct catalytic activity from that of all-trans-ARAT. Apparent kinetic rates for 11-cis-ARAT were 0.135 nmol min(-)(1) mg(-)(1) (V(max)) and 11.25 microM (K(M)) and for all-trans-ARAT were 0.0065 nmol min(-)(1) mg(-)(1) (V(max)) and 28.88 microM (K(M)). Our data indicate that Muller cells in the chicken retina possess 11-cis-ARAT activity, thus providing an explanation for the accumulation of 11-cis-retinyl esters in the cone cycle.

In the classic retinoid cycle, 11-cis retinol is synthesized in the retinal pigment epithelium (RPE) by two enzymes: Isomerase I (RPE65) and lecithin:retinol acyltransferase (LRAT). The purpose of this study is to provide experimental evidence for two active isomerases in the cone-dominated chicken eye: an LRAT-dependent Isomerase I in the RPE and an ARAT (acyl CoA:retinol acyltransferase)-dependent isomerase (Isomerase II) in the retina. First, we show that whole chicken retina in vitro, removed from the RPE/choroid and sclera, produces 11-cis retinoids upon light exposure, indicating the existence of RPE-independent isomerase (Isomerase II) activity in the retina. RT-PCR studies show high levels of RPE65 expression in the RPE, low levels in the retina, and none in primary Müller cell cultures, indicating the presence of Isomerase I in the RPE and a minimal amount in the retina. Activities of the RPE and retina isomerases were then measured by enzyme assays with specific enzyme inhibitors. 2,2′-Bipyridine, a known Isomerase I inhibitor, and N-ethyl-maleimide (NEM), a known LRAT inhibitor, significantly reduced Isomerase I activity but not Isomerase II activity. Progesterone, a known ARAT inhibitor, completely blocked Isomerase II activity but not Isomerase I activity. Thus the present study reports novel results to distinguish the biochemical properties of Isomerase I from Isomerase II, as well a difference in their locations in the chicken eye. Based on these differences, the cone-dominated chicken eye must contain two retinoid cycles: a classic visual cycle for retinoid exchange between the RPE and the retina supported by Isomerase I in the RPE, and an additional visual cycle for retinoid processing in the retina supported by Isomerase II.Light sensitivity in the eye requires the regeneration of 11-cis retinaldehyde through different isomers and oxidation states of vitamin A. An intimate relationship exists between photoreceptors and the retinal pigment epithelium (RPE) to regenerate this 11-cis retinaldehyde by way of the biochemical pathway known as the visual or retinoid cycle. This classic retinoid cycle has been well established for rod photoreceptors (for review see (1-7)). Attention has recently been focused on an additional vitamin A processing pathway in the retina for the supply of visual chromophore to cone photoreceptors (6;8-13). Little is known about the pathway of this novel cone cycle (6).A key reaction in both retinoid cycles involves the isomerization of retinoids from the alltrans to the 11-cis conformation. The first (and more extensively characterized) pathway is enzymatic and driven by the protein RPE65 (Isomerase I) (14-16) where it is localized in the RPE and uses all-trans retinyl esters as the substrate for the production of 11-cis retinol (17; 18). RPE65 from the RPE of the cone-dominated chicken has been shown to possess significantly higher isomerase activity than the RPE from rod-dominated species (12).The second pathway is a photic cycle (5) driven by the retinal G prote...

SUMMARY In past decades, the role of retinoids in support of rod photopigment regeneration has been extensively characterized. In the rhodopsin cycle,retinal chromophore from bleached rod pigments is reduced to retinol and transferred to the retinal pigment epithelium (RPE) to store as all-trans retinyl ester. This ester pool is subsequently utilized for visual pigment regeneration. However, there is a lack of information on the putative cone visual cycle. In the present study, we provide experimental evidence in support of a novel retinoid cycle for cone photopigment regeneration. In the cone-rich chicken, light exposure resulted in the accumulation of 11-cis retinyl esters to the retina and all-trans retinyl esters to the RPE. Both the rate of increase and the amount of 11-cis retinyl esters in the retina far exceeded those of the all-trans retinyl esters in the RPE. In response to dark adaptation, this 11-cis retinyl ester pool in the retina depletes at a rate several times faster than the all-trans retinyl ester pool in the RPE. In vitro, isolated, dark-adapted retinas devoid of RPE show both an accumulation of 11-cis retinyl ester and a concomitant reduction of 11-cis retinal chromophore in response to light exposure. Finally, we provide experimental results to elucidate a cone visual cycle in chicken by relating the change in retinoids (retinal and retinyl ester) with time during light and dark adaptation. Our results support a new paradigm for cone photopigment regeneration in which the 11-cisretinyl ester pool in the retina serves as the primary source of visual chromophore for cone pigment regeneration.