Effects of kynurenate and other excitatory amino acid antagonists as blockers of light- and kainate-induced retinal rod photoreceptor disc shedding

Glycative stress as a cause of macular degeneration

People are living longer and rates of age-related diseases such as age-related macular degeneration (AMD) are accelerating, placing enormous burdens on patients and health care systems. The quality of carbohydrate foods consumed by an individual impacts health. The glycemic index (GI) is a kinetic measure of the rate at which glucose arrives in the blood stream after consuming various carbohydrates. Consuming diets that favor slowly digested carbohydrates releases sugar into the bloodstream gradually after consuming a meal (low glycemic index). This is associated with reduced risk for major age-related diseases including AMD, cardiovascular disease, and diabetes. In comparison, consuming the same amounts of different carbohydrates in higher GI diets, releases glucose into the blood rapidly, causing glycative stress as well as accumulation of advanced glycation end products (AGEs). Such AGEs are cytotoxic by virtue of their forming abnormal proteins and protein aggregates, as well as inhibiting proteolytic and other protective pathways that might otherwise selectively recognize and remove toxic species. Using in vitro and animal models of glycative stress, we observed that consuming higher GI diets perturbs metabolism and the microbiome, resulting in a shift to more lipid-rich metabolomic profiles. Interactions between aging, diet, eye phenotypes and physiology were observed. A large body of laboratory animal and human clinical epidemiologic data indicates that consuming lower GI diets, or lower glycemia diets, is protective against features of early AMD (AMDf) in mice and AMD prevalence or AMD progression in humans. Drugs may be optimized to diminish the ravages of higher glycemic diets. Human trials are indicated to determine if AMD progression can be retarded using lower GI diets. Here we summarized the current knowledge regarding the pathological role of glycative stress in retinal dysfunction and how dietary strategies might diminish retinal disease.

Circadian Vision in Zebrafish: From Molecule to Cell and from Neural Network to Behavior

Most visual system functions, such as opsin gene expression, retinal neural transmission, light perception, and visual sensitivity, display robust day-night rhythms. The rhythms persist in constant lighting conditions, suggesting the involvement of endogenous circadian clocks. While the circadian pacemakers that control the rhythms of animal behaviors are mostly found in the forebrain and midbrain, self-sustained circadian oscillators are also present in the neural retina, where they play important roles in the regulation of circadian vision. This review highlights some of the correlative studies of the circadian control of visual system functions in zebrafish. Because zebrafish maintain a high evolutionary proximity to mammals, the findings from zebrafish research may provide insights for a better understanding of the mechanisms of circadian vision in other vertebrate species including humans.

Involvement of a gut-retina axis in protection against dietary glycemia-induced age-related macular degeneration

Age-related macular degeneration (AMD) is the major cause of blindness in developed nations. AMD is characterized by retinal pigmented epithelial (RPE) cell dysfunction and loss of photoreceptor cells. Epidemiologic studies indicate important contributions of dietary patterns to the risk for AMD, but the mechanisms relating diet to disease remain unclear. Here we investigate the effect on AMD of isocaloric diets that differ only in the type of dietary carbohydrate in a wild-type aged-mouse model. The consumption of a high-glycemia (HG) diet resulted in many AMD features (AMDf), including RPE hypopigmentation and atrophy, lipofuscin accumulation, and photoreceptor degeneration, whereas consumption of the lower-glycemia (LG) diet did not. Critically, switching from the HG to the LG diet late in life arrested or reversed AMDf. LG diets limited the accumulation of advanced glycation end products, long-chain polyunsaturated lipids, and their peroxidation end-products and increased C3-carnitine in retina, plasma, or urine. Untargeted metabolomics revealed microbial cometabolites, particularly serotonin, as protective against AMDf. Gut microbiota were responsive to diet, and we identified microbiota in the Clostridiales order as being associated with AMDf and the HG diet, whereas protection from AMDf was associated with the Bacteroidales order and the LG diet. Network analysis revealed a nexus of metabolites and microbiota that appear to act within a gut-retina axis to protect against diet- and age-induced AMDf. The findings indicate a functional interaction between dietary carbohydrates, the metabolome, including microbial cometabolites, and AMDf. Our studies suggest a simple dietary intervention that may be useful in patients to arrest AMD.

Circadian rhythms of behavioral cone sensitivity and long wavelength opsin mRNA expression: a correlation study in zebrafish

Using a behavioral assay based on visually mediated escape responses, we measured long-wavelength-sensitive red cone (LC) sensitivities in zebrafish. In a 24 h period, the zebrafish were least sensitive to red light in the early morning and most sensitive in the late afternoon. To investigate if the fluctuation of behavioral cone sensitivity correlates with opsin gene expression, we measured LC opsin mRNA expression at different times in the day and night under different lighting conditions. Under a normal light-dark cycle, the expression of LC opsin mRNA determined by real-time RT-PCR was low in the early morning and high in the late afternoon, similar to the fluctuation of behavioral cone sensitivity. This rhythm of LC opsin mRNA expression, however, dampened out gradually in constant conditions. After 24 h of constant light (LL), the expression of LC opsin mRNA dropped to levels similar to those determined in the early morning in control animals. By contrast, when the zebrafish were kept in constant dark (DD), the expression of LC opsin mRNA increased, to levels about 30-fold higher than the expression in the early morning in control animals. This day-night fluctuation in LC opsin mRNA expression was correlated to changes in opsin density in the outer segment of cone photoreceptor cells. Microspectrophotometry (MSP) measurements found significant differences in red cone outer segment optical density with a rhythm following the behavioral sensitivity. Furthermore, dopamine modulated the circadian rhythms in expression of LC opsin mRNA. Administration of dopamine increased LC opsin mRNA expression, but only in the early morning.

A circadian clock regulates the process of ERG b- and d-wave dominance transition in dark-adapted zebrafish

In zebrafish, during dark adaptation following bright light adaptation, the dominance of electroretinogram (ERG) b- and d-waves switches. In the early dark adaptation, when visual sensitivity is cone-dominant, both the b- and d-waves are readily recorded. In the late dark adaptation, along with the increase of rod sensitivity, the b-wave becomes dominant whereas the d-wave is gradually lost. The time for the ERG b- and d-wave dominance transition varies between the day and night. The transition requires a longer amount of time in the night and early morning than in the afternoon. This pattern of timing for ERG b- and d-wave dominance transition persists in constant light and can be reversed after exposure to a reversed light-dark cycle. The data suggest that the transition of the dominance of ERG b- and d-waves is regulated by an endogenous circadian clock.

A model of retinal cell differentiation in the chick embryo

This article presents an overview of retinal cell differentiation in the chick embryo, in the context of a hypothetical model based on information generated during the last several years. The model proposes that: (1) most (if not all) proliferating neuroepithelial cells have the potential to give rise to a progeny comprising two or more different cell types; (2) the time at which cells undergo their terminal mitosis does not determine their differentiated fate; (3) many postmitotic precursor cells remain plastic (i.e., uncommitted) for some time after terminal mitosis, during which they encounter position-dependent signals as they migrate toward their definitive laminar position within the retina; (4) as a consequence of these inductive stimuli, precursor cells that migrate to different retinal layers express different transcriptional regulators; (5) morphologically undifferentiated precursor cells are committed to cell type-specific, complex patterns of differentiation, which they can express even when isolated from their normal microenvironment, and (6) even after precursor cells become committed to a specific identity, additional inductive signals are necessary for the cells to complete the development of a fully mature phenotype. The article presents a summary of the supportive evidence, as well as a critical evaluation of the model, and concludes with an overview of unanswered questions regarding retinal cell differentiation and a brief evaluation of the prospects for further progress in this field.

Specific interaction of glutamate with membranes from cultured retinal pigment epithelium

Excitatory amino acids (EAA) have been shown to induce phagocytosis in retinal pigment epithelial (RPE) cells. In order to explore if this action is receptor-mediated, we have identified and characterized receptors for L-glutamate through the binding of [3H]L-glutamate to membranes from chick RPE cells in primary culture. Specific binding was found saturable, with KB = 333nM and Bmax = 3.2 pmol/mg protein in frozen/thawed membranes. Na(+)-independent binding was present in cultures of 16 and 25 days in vitro, and was not affected by temperature. Pharmacological profile of analogues of EAA at different receptor types suggests the presence of a metabotropic type receptor (L-glutamate > S-2-amino-3-phosphonopropionate > 2-amino-4-phosphonobutyrate = trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylate > quisqualate). Excitatory amino acid analogues acting at the NMDA-receptor also displaced bound L-glutamate, and a noticeable stimulation of specific binding of this ligand by glycine was shown; this effect was mimicked by D-serine and 1-hydroxy-3-aminopyrrolidone-2 (HA-966) but not by 7-chlorokynurenate, and was not inhibited by strychnine. Since taurine and GABA also increased specific binding, it is likely that modulation of EAA receptors in RPE differs from that in neurons.

Cytochalasin D inhibits L-glutamate-induced disc shedding without altering L-glutamate-induced increase in adhesiveness

Excitatory amino acid-stimulated disc shedding is correlated with the appearance of microfilament-rich ensheathing processes of the retinal pigment epithelium (RPE) and increased apparent adhesiveness between photoreceptors and RPE in explanted eyecups of Xenopus laevis. We have compared the time course of disc shedding and increased retinal adherence during L-glutamate treatment. Increased adherence was measured on the basis of the tendency of the apical RPE domains to partition with isolated neural retinas. In medium supplemented with L-glutamate (12 mM) or kainate (100 microM), a glutamate analog, the time course of increased partitioning of melanin pigment-rich cell fragments which contain ensheathing processes differs, even though the kinetics of induced disc shedding is the same in either case. Co-treatment with cytochalasin D (5 microM) completely blocks L-glutamate-induced disc shedding, as well as formation of microfilament-rich ensheathing processes, even though it has little effect upon apparent adhesiveness. The virtually complete dissociation of the effects of L-glutamate on disc shedding from that on increased adhesiveness of photoreceptors to RPE suggests that increased retinal adherence and pseudopod formation may be unrelated causally.

Excitatory amino acids and rod photoreceptor disc shedding: analysis using specific agonists

L-Glutamate and L-aspartate stimulate photoreceptor disc shedding. In order to evaluate the possible involvement of a receptor, we examined the effects of specific excitatory amino acid agonists on rod photoreceptor disc shedding and neural retinal toxicity. Using eyecups from both Xenopus laevis and Rana pipiens, we found that kainate, quisqualate, and N-methyl-D-aspartate (NMDA) were all neurotoxic, but that kainate caused a more extensive inner retinal lesion. Kainate also caused disc shedding at concentrations as low as 10 microM; dihydrokainate, a structural analogue, was at least 100-fold less potent. In contrast, quisqualate induced disc shedding only at concentrations above 5.0 mM, and NMDA had no effect on disc shedding at any concentration examined. Our results suggest that excitatory amino acids act via a receptor of the kainate type to effect disc shedding. The mechanism in the retina or photoreceptor-pigment epithelial complex by which an excitatory amino acid receptor system influences disc shedding remains to be identified.