N-Methyl-D-Aspartate Receptors in the Retina

P-aminobenzoic acid promotes retinal regeneration through activation of Ascl1a in zebrafish

JOURNAL/nrgr/04.03/01300535-202408000-00040/figure1/v/2023-12-16T180322Z/r/image-tiff The retina of zebrafish can regenerate completely after injury. Multiple studies have demonstrated that metabolic alterations occur during retinal damage; however to date no study has identified a link between metabolites and retinal regeneration of zebrafish. Here, we performed an unbiased metabolome sequencing in the N-methyl-D-aspartic acid-damaged retinas of zebrafish to demonstrate the metabolomic mechanism of retinal regeneration. Among the differentially-expressed metabolites, we found a significant decrease in p-aminobenzoic acid in the N-methyl-D-aspartic acid-damaged retinas of zebrafish. Then, we investigated the role of p-aminobenzoic acid in retinal regeneration in adult zebrafish. Importantly, p-aminobenzoic acid activated Achaetescute complex-like 1a expression, thereby promoting Müller glia reprogramming and division, as well as Müller glia-derived progenitor cell proliferation. Finally, we eliminated folic acid and inflammation as downstream effectors of PABA and demonstrated that PABA had little effect on Müller glia distribution. Taken together, these findings show that PABA contributes to retinal regeneration through activation of Achaetescute complex-like 1a expression in the N-methyl-D-aspartic acid-damaged retinas of zebrafish.

Methanol Induced Optic Neuropathy: Molecular Mysteries, Public Health Perspective, Clinical Insights and Treatment Strategies

Methanol-induced optic neuropathy (MION) represents a critical public health issue, particularly prevalent in lower socioeconomic populations and regions with restricted alcohol access. MION, characterized by irreversible visual impairment, arises from the toxic metabolization of methanol into formaldehyde and formic acid, leading to mitochondrial oxidative phosphorylation inhibition, oxidative stress, and subsequent neurotoxicity. The pathogenesis involves axonal and glial cell degeneration within the optic nerve and potential retinal damage. Despite advancements in therapeutic interventions, a significant proportion of affected individuals endure persistent visual sequelae. The study comprehensively investigates the pathophysiology of MION, encompassing the absorption and metabolism of methanol, subsequent systemic effects, and ocular impacts. Histopathological changes, including alterations in retinal layers and proteins, Müller cell dysfunction, and visual symptoms, are meticulously examined to provide insights into the disease mechanism. Furthermore, preventive measures and public health perspectives are discussed to highlight the importance of awareness and intervention strategies. Therapeutic approaches, such as decontamination procedures, ethanol and fomepizole administration, hemodialysis, intravenous fluids, electrolyte balance management, nutritional therapy, corticosteroid therapy, and erythropoietin (EPO) treatment, are evaluated for their efficacy in managing MION. This comprehensive review underscores the need for increased awareness, improved diagnostic strategies, and more effective treatments to mitigate the impact of MION on global health.

Environmental exposures to cadmium and lead as potential causes of eye diseases

Humans are exposed to cadmium and lead in various regions of the world daily due to industrial development and climate change. Increasing numbers of preclinical and clinical studies indicate that heavy metals, such as cadmium and lead, play a role in the pathogenesis of eye diseases. Excessive exposure to heavy metals such as cadmium and lead can increase the risk of impaired vision. Therefore, it is essential to better characterize the role of these non-essential metals in disease etiology and progression. This article discusses the potential role of cadmium and lead in the development of age-related eye diseases, including age-related macular degeneration, cataracts, and glaucoma. Furthermore, we discuss how cadmium and lead affect ocular cells and provide an overview of putative pathological mechanisms associated with their propensity to damage the eye.

Early inner plexiform layer thinning and retinal nerve fiber layer thickening in excitotoxic retinal injury using deep learning-assisted optical coherence tomography

Excitotoxicity from the impairment of glutamate uptake constitutes an important mechanism in neurodegenerative diseases such as Alzheimer's, multiple sclerosis, and Parkinson's disease. Within the eye, excitotoxicity is thought to play a critical role in retinal ganglion cell death in glaucoma, diabetic retinopathy, retinal ischemia, and optic nerve injury, yet how excitotoxic injury impacts different retinal layers is not well understood. Here, we investigated the longitudinal effects of N-methyl-D-aspartate (NMDA)-induced excitotoxic retinal injury in a rat model using deep learning-assisted retinal layer thickness estimation. Before and after unilateral intravitreal NMDA injection in nine adult Long Evans rats, spectral-domain optical coherence tomography (OCT) was used to acquire volumetric retinal images in both eyes over 4 weeks. Ten retinal layers were automatically segmented from the OCT data using our deep learning-based algorithm. Retinal degeneration was evaluated using layer-specific retinal thickness changes at each time point (before, and at 3, 7, and 28 days after NMDA injection). Within the inner retina, our OCT results showed that retinal thinning occurred first in the inner plexiform layer at 3 days after NMDA injection, followed by the inner nuclear layer at 7 days post-injury. In contrast, the retinal nerve fiber layer exhibited an initial thickening 3 days after NMDA injection, followed by normalization and thinning up to 4 weeks post-injury. Our results demonstrated the pathological cascades of NMDA-induced neurotoxicity across different layers of the retina. The early inner plexiform layer thinning suggests early dendritic shrinkage, whereas the initial retinal nerve fiber layer thickening before subsequent normalization and thinning indicates early inflammation before axonal loss and cell death. These findings implicate the inner plexiform layer as an early imaging biomarker of excitotoxic retinal degeneration, whereas caution is warranted when interpreting the ganglion cell complex combining retinal nerve fiber layer, ganglion cell layer, and inner plexiform layer thicknesses in conventional OCT measures. Deep learning-assisted retinal layer segmentation and longitudinal OCT monitoring can help evaluate the different phases of retinal layer damage upon excitotoxicity.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

GSK872 and necrostatin-1 protect retinal ganglion cells against necroptosis through inhibition of RIP1/RIP3/MLKL pathway in glutamate-induced retinal excitotoxic model of glaucoma

Background

Glaucoma, the major cause of irreversible blindness worldwide, is characterized by progressive degeneration of retinal ganglion cells (RGCs). Current treatments for glaucoma only slow or partially prevent the disease progression, failing to prevent RGCs death and visual field defects completely. Glutamate excitotoxicity via N-methyl-d-aspartic acid (NMDA) receptors plays a vital role in RGCs death in glaucoma, which is often accompanied by oxidative stress and NLRP3 inflammasome activation. However, the exact mechanisms remain unclear.

Methods

The glutamate-induced R28 cell excitotoxicity model and NMDA-induced mouse glaucoma model were established in this study. Cell counting kit-8, Hoechst 33342/PI dual staining and lactate dehydrogenase release assay were performed to evaluate cell viability. Annexin V-FITC/PI double staining was used to detect apoptosis and necrosis rate. Reactive oxygen species (ROS) and glutathione (GSH) were used to detect oxidative stress in R28 cells. Levels of proinflammatory cytokines were measured by qRT-PCR. Transmission electron microscopy (TEM) was used to detect necroptotic morphological changes in RGCs. Retinal RGCs numbers were detected by immunofluorescence. Hematoxylin and eosin staining was used to detect retinal morphological changes. The expression levels of RIP1, RIP3, MLKL and NLRP3 inflammasome-related proteins were measured by immunofluorescence and western blotting.

Results

We found that glutamate excitotoxicity induced necroptosis in RGCs through activation of the RIP1/RIP3/MLKL pathway in vivo and in vitro. Administration of the RIP3 inhibitor GSK872 and RIP1 inhibitor necrostatin-1 (Nec-1) prevented glutamate-induced RGCs loss, retinal damage, neuroinflammation, overproduction of ROS and a decrease in GSH. Furthermore, after suppression of the RIP1/RIP3/MLKL pathway by GSK872 and Nec-1, glutamate-induced upregulation of key proteins involved in NLRP3 inflammasome activation, including NLRP3, pro-caspase-1, cleaved-caspase-1, and interleukin-1β (IL-1β), was markedly inhibited.

Conclusions

Our findings suggest that the RIP1/RIP3/MLKL pathway mediates necroptosis of RGCs and regulates NLRP3 inflammasome activation induced by glutamate excitotoxicity. Moreover, GSK872 and Nec-1 can protect RGCs from necroptosis and suppress NLRP3 inflammasome activation through inhibition of RIP1/RIP3/MLKL pathway, conferring a novel neuroprotective treatment for glaucoma.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02626-4.

Visual system assessment for predicting a transition to psychosis

The field of psychiatry is far from perfect in predicting which individuals will transition to a psychotic disorder. Here, we argue that visual system assessment can help in this regard. Such assessments have generated medium-to-large group differences with individuals prior to or near the first psychotic episode or have shown little influence of illness duration in larger samples of more chronic patients. For example, self-reported visual perceptual distortions-so-called visual basic symptoms-occur in up to 2/3rds of those with non-affective psychosis and have already longitudinally predicted an impending onset of schizophrenia. Possibly predictive psychophysical markers include enhanced contrast sensitivity, prolonged backward masking, muted collinear facilitation, reduced stereoscopic depth perception, impaired contour and shape integration, and spatially restricted exploratory eye movements. Promising brain-based markers include visual thalamo-cortical hyperconnectivity, decreased occipital gamma band power during visual detection (MEG), and reduced visually evoked occipital P1 amplitudes (EEG). Potentially predictive retinal markers include diminished cone a- and b-wave amplitudes and an attenuated photopic flicker response during electroretinography. The foregoing assessments are often well-described mechanistically, implying that their findings could readily shed light on the underlying pathophysiological changes that precede or accompany a transition to psychosis. The retinal and psychophysical assessments in particular are inexpensive, well-tolerated, easy to administer, and brief, with few inclusion/exclusion criteria. Therefore, across all major levels of analysis-from phenomenology to behavior to brain and retinal functioning-visual system assessment could complement and improve upon existing methods for predicting which individuals go on to develop a psychotic disorder.

Implications of Diabetes-Induced Altered Metabolites on Retinal Neurodegeneration

Diabetic retinopathy (DR) is one of the major complications of diabetic eye diseases, causing vision loss and blindness worldwide. The concept of diabetic retinopathy has evolved from microvascular disease into more complex neurovascular disorders. Early in the disease progression of diabetes, the neuronal and glial cells are compromised before any microvascular abnormalities clinically detected by the ophthalmoscopic examination. This implies understanding the pathophysiological mechanisms at the early stage of disease progression especially due to diabetes-induced metabolic alterations to damage the neural retina so that early intervention and treatments options can be identified to prevent and inhibit the progression of DR. Hyperglycemia has been widely considered the major contributor to the progression of the retinal damage, even though tight control of glucose does not seem to have a bigger effect on the incidence or progression of retinal damage that leads to DR. Emerging evidence suggests that besides diabetes-induced hyperglycemia, dyslipidemia and amino acid defects might be a major contributor to the progression of early neurovascular retinal damage. In this review, we have discussed recent advances in the alterations of key metabolites of carbohydrate, lipid, and amino acids and their implications for neurovascular damage in DR.

Pharmacological inhibition of Na+/K+-ATPase induces neurovascular degeneration and glial cell alteration in the rat retina

Na⁺/K⁺-ATPase (NKA) plays an important role in ion homeostasis and neurotransmitter uptake. In the retina, multidirectional communications among neurons, glia, and blood vessels (that is, neuro-glio-vascular interaction) are crucial for maintaining tissue homeostasis. We investigated the role of NKA in the elements of neuro-glio-vascular unit in neonatal and adult rat retinas. Male Sprague-Dawley rats (1- and 8-week-old) were injected intravitreally with ouabain (20 nmol/eye), an inhibitor of NKA. Morphological changes in retinal neurons, glia, and blood vessels were examined. The intravitreal injection of ouabain decreased the number of cells in the ganglion cell layer, as well as the thicknesses of the inner plexiform and inner nuclear layers in neonatal and adult rats compared to age-matched controls. The ouabain-induced neuronal cell damage was partially prevented by D-(-)-2-amino-5-phosphonopentanoic acid, an antagonist of N-methyl-D-aspartic acid receptors. In the deep retinal vascular plexus of the ouabain-injected eyes, angiogenesis was delayed in neonatal rats, whereas capillary degeneration occurred in adult rats. The immunoreactivity of glutamine synthetase and vascular endothelial growth factor (VEGF) decreased in the retinas of neonatal and adult rats injected intravitreally with ouabain. The immunoreactivity of glial fibrillary acidic protein was enhanced in the retinas of ouabain-injected adult eyes. After the ouabain injection, CD45-positive leukocytes and Iba1-positive microglia increased in the inner retinal layer of neonatal rats, whereas they increased in the middle retinal layer of adult rats. These results suggest that the inhibition of NKA induces the degeneration of neuronal and vascular cells and alteration of glial cells in both neonatal and adult retinas. In addition to the direct effects of NKA inhibition, the disturbance of retinal glutamate metabolism and decreased VEGF expression may contribute to neurovascular degeneration. The activity of NKA is crucial for maintaining elements of neuro-glio-vascular unit in the retina.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Methanol-induced optic neuropathy: a still-present problem

Methanol-induced optic neuropathy (Me-ION) is a serious condition that may result in long-term or irreversible visual impairment or even blindness secondary to damage and loss of function of the optic nerve and retina. Me-ION shows a tendency to occur as mass poisonings around the world with a clear predilection for poor societies in developing countries. The main mechanism underlying the molecular basis of Me-ION is the inhibition of the mitochondrial oxidative phosphorylation process through the binding of the toxic metabolite of methanol—formic acid—with the key enzyme of this process—cytochrome c oxidase. However, other mechanisms, including damage to the eye tissues by oxidative stress causing the intensification of the oxidative peroxidation process with the formation of cytotoxic compounds, as well as an increase in the synthesis of pro-inflammatory cytokines and influence on the expression of key proteins responsible for maintaining cell homeostasis, also play an important role in the pathogenesis of Me-ION. Histopathological changes in the eye tissues are mainly manifested as the degeneration of axons and glial cells of the optic nerve, often with accompanying damage of the retina that may involve all its layers. Despite the development of therapeutic approaches, persistent visual sequelae are seen in 30–40% of survivors. Thus, Me-ION continues to be an important problem for healthcare systems worldwide.

The neuroprotective effect of melatonin in glutamate excitotoxicity of R28 cells and mouse retinal ganglion cells

Glaucoma is the leading cause of irreversible blindness. The progressive degeneration of retinal ganglion cells (RGCs) is the major characteristic of glaucoma. Even though the control of intraocular pressure could delay the loss of RGCs, current clinical treatments cannot protect them directly. The overactivation of N-methyl-D-aspartic acid (NMDA) receptors by excess glutamate (Glu) is among the important mechanisms of RGC death in glaucoma progression. Melatonin (MT) is an indole neuroendocrine hormone mainly secreted by the pineal gland. This study aimed to investigate the therapeutic effect of MT on glutamate excitotoxicity of mouse RGCs and R28 cells. The Glu-induced R28 cell excitotoxicity model and NMDA-induced retinal injury model were established. MT was applied to R28 cells and the vitreous cavity of mice by intravitreal injection. Cell counting kit-8 assay and propidium iodide/Hoechst were performed to evaluate cell viability. Reactive oxygen species and glutathione synthesis assays were used to detect the oxidative stress state of R28 cells. Retina immunofluorescence and hematoxylin and eosin staining were applied to assess RGC counts and retinal structure. Flash visual-evoked potential was performed to evaluate visual function in mice. RNA sequencing of the retina was performed to explore the underlying mechanisms of MT protection. Our results found that MT treatment could successfully protect R28 cells from Glu excitotoxicity and decrease reactive oxygen species. Also, MT rescued RGCs from NMDA-induced injury and protected visual function in mice. This study enriches the indications of MT in the treatment of glaucoma, providing practical research ideas for its comprehensive prevention and treatment.

Proteomics analysis of N-methyl-d-aspartate-induced cell death in retinal and optic nerves

This study aimed to comprehensively understand the proteomic characteristics and modulation of the neural microenvironment with N-methyl-d-aspartate (NMDA)-induced neuronal degeneration in the retina and optic nerve at 12 h after intravitreal injection of 40 nmol NMDA. Male Sprague-Dawley rats were sacrificed at 12 h after intravitreal injection of 40 nmol NMDA. PBS-injected eyes served as controls. The key cell death-linked proteins from the retina and optic nerve tissues were assessed by a mass spectrometry-based label-free approach. In proteomics analysis, we identified 3532 proteins in retinal tissues and 2593 proteins in optic nerve tissues. The ACSL3 (Q63151) and Prnp (P13852) proteins were upregulated in the NMDA-damaged retina and connected with ferroptosis. The Gabarapl2 (P60522) protein was upregulated in NMDA-damaged optic nerves and connected with autophagy. We performed parallel reaction monitoring (PRM) to validate the liquid chromatography-tandem mass spectrometry (LC-MS/MS) results. Data are available ProteomeXchange with identifiers PXD022466 (label-free quantification) and PXD022729 (PRM validation). SIGNIFICANCE: Excitotoxicity is one of the pathogeneses of various retinal disorders, including glaucoma, retinal ischemia-reperfusion and traumatic optic neuropathy. This study indicated that ferroptosis may be linked to pathological cell death in the retina with NMDA insult. Autophagy may be induced by NMDA overstimulation in both the optic nerve and retina. Regulating these types of death simultaneously may provide the maximum benefit for retinal disease therapy.

Multimodal imaging and functional analysis of the chick NMDA retinal damage model

Objectives

The chick is rapidly becoming a standardized preclinical model in vision research to study mechanisms of ocular disease. We seek to comprehensively evaluate the N-methyl-D-aspartate (NMDA) model of excitotoxic retinal damage using multimodal imaging, functional, and histologic approaches in NMDA-damaged, vehicle-treated, and undamaged chicks.

Methods

Chicks were either left undamaged in both eyes or were injected with NMDA in the left eye and saline (vehicle) in the right eye. TUNEL assay was performed on chicks to assess levels of retinal cell death one day post-injection of NMDA or saline and on age-matched untreated chicks. Spectral domain optical coherence tomography (SD-OCT) was performed weekly on chicks and age-matched controls day 1 (D1) up to D28 post-injection. Light adapted electroretinograms (ERG) were performed alongside SD-OCT measurements on post-injection chicks along with age-matched untreated controls.

Results

Untreated and vehicle-treated eyes had no TUNEL positive cells while NMDA-treated eyes accumulated large numbers of TUNEL positive cells in the Inner Nuclear Layer (INL), but not other layers, at D1 post injection. Significant inner retina swelling or edema was found on SD-OCT imaging at D1 post-injection which resolved at subsequent timepoints. Both the INL and the inner plexiform layer significantly thinned by one-week post-injection and did not recover for the duration of the measurements. On ERG, NMDA-treated eyes had significantly reduced amplitudes of all parameters at D1 with all metrics improving over time. The b-wave, oscillatory potentials, and ON/OFF bipolar responses were the most affected with at least 70% reduction immediately after damage compared to the fellow eye control.

Conclusion

This study establishes a normative baseline on the retinal health and gross functional ability as well as intraocular pressures of undamaged, vehicle-treated, and NMDA-damaged chicks to provide a standard for comparing therapeutic treatment studies in this important animal model.

NMDA Receptor Expression by Retinal Ganglion Cells Is Not Required for Retinofugal Map Formation nor Eye-Specific Segregation in the Mouse

Retinal ganglion cells (RGCs) project topographically to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Spontaneous activity plays a critical role in retinotopic mapping in both regions; however, the molecular mechanisms underlying activity-dependent refinement remain unclear. Previous pharmacologic studies implicate NMDA receptors (NMDARs) in the establishment of retinotopy. In other brain regions, NMDARs are expressed on both the presynaptic and postsynaptic side of the synapse, and recent work suggests that presynaptic and postsynaptic NMDARs play distinct roles in retinotectal developmental dynamics. To directly test the role of NMDARs expressed by RGCs in retinofugal map formation, we took a conditional genetic knock-out approach to delete the obligate GluN1 subunit of NMDARs in RGCs. Here, we demonstrate reduced GluN1 expression in the retina of Chrnb3-Cre;GluN1^flox/flox (pre-cKO) mice without altered expression in the SC. Anatomical tracing experiments revealed no significant changes in termination zone size in the SC and dLGN of pre-cKO mice, suggesting NMDAR function in RGCs is not an absolute requirement for topographic refinement. Further, we observed no change in the eye-specific organization of retinal inputs to the SC nor dLGN. To verify that NMDA induces activity in RGC terminals, we restricted GCaMP5 expression to RGCs and confirmed induction of calcium transients in RGC terminals. Together, these findings demonstrate that NMDARs expressed by RGCs are not required for retinofugal topographic map formation nor eye-specific segregation in the mouse.

NEUROTRANSMISSION IN VISUAL ANALYZER AND BIONIC EYE. A REVIEW

AIMS

The aim of the work is to point out the transmission of electrical voltage changes in the visual analyser and thus the efficiency of the bionic eye.

MATERIAL AND METHODS

The review deals with the question of the transmission of electrical changes in visual path voltage under physiological and pathological conditions. In particular, it points to feedback autoregulatory damage not only of primarily altered cellular structures, but of all other, both horizontally and vertically localized. Based on the results of functional magnetic resonance imaging and electrophysiological methods, it shows the pathology of the entire visual pathway in three eye diseases: retinitis pigmentosa, age-related macular degeneration and glaucoma.

RESULTS

The thesis also provides an overview of possible systems that are used to replace lost vision, from epiretinal, subretinal, suprachoroidal implants, through stimulation of the optic nerve, corpus geniculatum laterale to the visual cortex.

CONCLUSION

Due to the pathology of neurotransmission, bionic eye systems cannot be expected to be restored after stabilization of binocular functions.

Retinal Damage in Amyotrophic Lateral Sclerosis: Underlying Mechanisms

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease resulting in a gradual loss of motor neuron function. Although ophthalmic complaints are not presently considered a classic symptom of ALS, retinal changes such as thinning, axonal degeneration and inclusion bodies have been found in many patients. Retinal abnormalities observed in postmortem human tissues and animal models are similar to spinal cord changes in ALS. These findings are not dramatically unexpected because retina shares an ontogenetic relationship with the brain, and many genes are associated both with neurodegeneration and retinal diseases. Experimental studies have demonstrated that ALS affects many “vulnerable points” of the retina. Aggregate deposition, impaired nuclear protein import, endoplasmic reticulum stress, glutamate excitotoxicity, vascular regression, and mitochondrial dysfunction are factors suspected as being the main cause of motor neuron damage in ALS. Herein, we show that all of these pathways can affect retinal cells in the same way as motor neurons. Furthermore, we suppose that understanding the patterns of neuro-ophthalmic interaction in ALS can help in the diagnosis and treatment of this disease.

Pharmacological depletion of retinal neurons prevents vertical angiogenic sprouting without affecting the superficial vascular plexus

BACKGROUND

In mice, a tri-layered (superficial, intermediate, and deep) vascular structure is formed in the retina during the third postnatal week. Short-term treatment of newborn mice with vascular endothelial growth factor (VEGF) receptor inhibitors delays the formation of superficial vascular plexus and this allows us to investigate the developmental process of superficial and deep vascular plexuses at the same time. Using this model, we examined the effect of pharmacological depletion of retinal neurons on the formation of superficial and deep vascular plexuses.

RESULTS

Neuronal cell loss induced by an intravitreal injection of N-methyl-d-aspartic acid on postnatal day (P) 8 delayed vascular development in the deep layer but not in the superficial layer in mice treated with KRN633, a VEGF receptor inhibitor, on P0 and P1. In KRN633-treated mice, neuronal cell loss decreased the number of vertical sprouts originating from the superficial plexus without affecting the number of angiogenic sprouts growing in front. Neuronal cell loss did not impair networks of fibronectin and astrocytes in the superficial layer.

CONCLUSIONS

Our results suggest that inner retinal neurons play a crucial role in forming the deep vascular plexus by directing the sprouts from the superficial blood vessels to the deep layer.

Neuroprotection: A versatile approach to combat glaucoma

In most retinal diseases, neuronal loss is the main cause of vision loss. Neuroprotection is the alteration of neurons and/or their environment to encourage the survival and function of the neurons, especially in environments that are deleterious to the neuronal health. The area of neuroprotection progresses with a therapeutically-based hope of improving vision and clinical outcomes for patients through the developments in neurotrophic therapy, antioxidative therapy, anti-excitotoxic, anti-ischemic, anti-inflammatory, and anti-apoptotic care. In this review, we summarize the various neuroprotection strategies for the treatment of glaucoma, genetics of glaucoma and the role of various nanoplatforms in the treatment of glaucoma.

Electoretinographic evidence of retinal ganglion cell-dependent function in schizophrenia

Schizophrenia is a complex disorder that is diagnosed mainly with clinical observation and evaluation. Recent studies suggest that many people with schizophrenia have abnormalities in the function of the N-methyl-d-aspartate receptor (NMDAR). The retina is part of the central nervous system and expresses the NMDAR, raising the possibility of the early detection of NMDAR-related schizophrenia by detecting differences in retinal function. As a first-step, we used two non-invasive outpatient tests of retinal function, the photopic negative response (PhNR) of the light-adapted flash-electroretinogram (PhNR-fERG) and the pattern ERG (PERG), to test individuals with schizophrenia and controls to determine if there were measurable differences between the two populations. The PhNR-fERG showed that males with schizophrenia had a significant increase in the variability of the overall response, which was not seen in the females with schizophrenia. Additionally at the brightest flash strength, there were significant increases in the PhNR amplitude in people with schizophrenia that were maximal in controls. Our results show measurable dysfunction of retinal ganglion cells (RGCs) in schizophrenia using the PhNR-fERG, with a good deal of variability in the retinal responses of people with schizophrenia. The PhNR-fERG holds promise as a method to identify individuals more at risk for developing schizophrenia, and may help understand heterogeneity in etiology and response to treatment.

Neuroprotection in glaucoma-electrophysiology

Hypertensive glaucoma is defined as a group of diseases with progressive loss of the neuroretinal margin of the optic disc that causes characteristic degenerative optic neuropathy. The present study provided an updated summary of the physiology and pathology of neurotransmission in the visual path, with the focus on glaucoma. The results of positron emission tomography, functional magnetic resonance imaging and mainly electrophysiological methods demonstrated pathogenesis of nerve cell damage in the visual pathway. Based on these conclusions, neuroprotection in glaucoma was proposed. This consists mainly of the reduction of the intraocular pressure. It is followed by a decrease of glutamate in the synaptic cleft and blockade of its binding to the NMDA receptors. The supply of energy substrates to altered nerve cells is also indispensable. Therapy should be systemic due to impairment of the complete visual path.

Retinal correlates of psychiatric disorders

Diagnosis and monitoring of psychiatric disorders rely heavily on subjective self-reports of clinical symptoms, which are complicated by the varying consistency of accounts reported by patients with an impaired mental state. Hence, more objective and quantifiable measures have been sought to provide clinicians with more robust methods to evaluate symptomology and track progression of disease in response to treatments. Owing to the shared origins of the retina and the brain, it has been suggested that changes in the retina may correlate with structural and functional changes in the brain. Vast improvements in retinal imaging, namely optical coherence tomography (OCT) and electrodiagnostic technology, have made it possible to investigate the eye at a microscopic level, allowing for the investigation of potential biomarkers in vivo. This review provides a summary of retinal biomarkers associated with schizophrenia, bipolar disorder and major depression, demonstrating how retinal biomarkers may be used to complement existing methods and provide structural markers of pathophysiological mechanisms that underpin brain dysfunction in psychiatric disorders.

Norrin Protects Retinal Ganglion Cells from Excitotoxic Damage via the Induction of Leukemia Inhibitory Factor

PURPOSE

To investigate whether and how leukemia inhibitory factor (Lif) is involved in mediating the neuroprotective effects of Norrin on retinal ganglion cells (RGC) following excitotoxic damage. Norrin is a secreted protein that protects RGC from N-methyl-d-aspartate (NMDA)-mediated excitotoxic damage, which is accompanied by increased expression of protective factors such as Lif, Edn2 and Fgf2.

METHODS

Lif-deficient mice were injected with NMDA in one eye and NMDA plus Norrin into the other eye. RGC damage was investigated and quantified by TUNEL labeling 24 h after injection. Retinal mRNA expression was analyzed by quantitative real-time polymerase chain reaction following retinal treatment.

RESULTS

After intravitreal injection of NMDA and Norrin in wild-type mice approximately 50% less TUNEL positive cells were observed in the RGC layer when compared to NMDA-treated littermates, an effect which was lost in Lif-deficient mice. The mRNA expression for Gfap, a marker for Müller cell gliosis, as well as Edn2 and Fgf2 was induced in wild-type mice following NMDA/Norrin treatment but substantially blocked in Lif-deficient mice.

CONCLUSIONS

Norrin mediates its protective properties on RGC via Lif, which is required to enhance Müller cell gliosis and to induce protective factors such as Edn2 or Fgf2.

VEGFD Protects Retinal Ganglion Cells and, consequently, Capillaries against Excitotoxic Injury

In the central nervous system, neurons and the vasculature influence each other. While it is well described that a functional vascular system is trophic to neurons and that vascular damage contributes to neurodegeneration, the opposite scenario in which neural damage might impact the microvasculature is less defined. In this study, using an in vivo excitotoxic approach in adult mice as a tool to cause specific damage to retinal ganglion cells, we detected subsequent damage to endothelial cells in retinal capillaries. Furthermore, we detected decreased expression of vascular endothelial growth factor D (VEGFD) in retinal ganglion cells. In vivo VEGFD supplementation via neuronal-specific viral-mediated expression or acute intravitreal delivery of the mature protein preserved the structural and functional integrity of retinal ganglion cells against excitotoxicity and, additionally, spared endothelial cells from degeneration. Viral-mediated suppression of expression of the VEGFD-binding receptor VEGFR3 in retinal ganglion cells revealed that VEGFD exerts its protective capacity directly on retinal ganglion cells, while protection of endothelial cells is the result of upheld neuronal integrity. These findings suggest that VEGFD supplementation might be a novel, clinically applicable approach for neuronal and vascular protection.

Histone Deacetylases Contribute to Excitotoxicity-Triggered Degeneration of Retinal Ganglion Cells In Vivo

Excitotoxicity is known to modulate the nuclear accumulation, and thus activity state, of histone deacetylases (HDACs) in pyramidal neurons. In the retina, deregulation in activity and expression of different HDACs has been linked to pathological conditions such as retinitis pigmentosa, retinal ischemia, glaucoma, and acute optic nerve injury. Up to now, however, the effects of in vivo excitotoxicity on the different HDACs in retinal ganglion cells (RGCs) have not been thoroughly investigated. Here, we injected adult mice intravitreally with N-methyl-D-aspartate (NMDA) as a mean to trigger excitotoxicity-mediated RGC degeneration and we detected time-dependent loss of RGCs at 1 and 7 days after the insult. Further, we characterized the subcellular localization of HDACs belonging to class I (HDAC1, HDAC3), IIa (HDAC4, HDAC5, HDAC7, HDAC9), IIb (HDAC6, HDAC10), and IV (HDAC11) in RGCs. Our analyses revealed a differential pattern of HDACs nuclear distribution in RGCs following excitotoxicity. After 1 day, HDAC3, HDAC5, HDAC6, HDAC7, and HDAC11 showed altered subcellular localization in RGCs while 7 days after the excitotoxic insult, HDAC4 and HDAC9 were the only HDACs displaying changes in their subcellular distribution. Moreover, we found that in vivo selective inhibition of HDAC1/3 or HDAC4/5 via MS-275 (entinostat) or LMK-235, respectively, could prevent ongoing RGC degeneration. In conclusion, our results point towards a role of HDACs in RGC degeneration and identify HDAC1/3 and HDAC4/5 as potential therapeutic targets to treat degenerative retinal diseases.

Molecular and Cellular Mechanisms Underlying Somatostatin-Based Signaling in Two Model Neural Networks, the Retina and the Hippocampus

Neural inhibition plays a key role in determining the specific computational tasks of different brain circuitries. This functional "braking" activity is provided by inhibitory interneurons that use different neurochemicals for signaling. One of these substances, somatostatin, is found in several neural networks, raising questions about the significance of its widespread occurrence and usage. Here, we address this issue by analyzing the somatostatinergic system in two regions of the central nervous system: the retina and the hippocampus. By comparing the available information on these structures, we identify common motifs in the action of somatostatin that may explain its involvement in such diverse circuitries. The emerging concept is that somatostatin-based signaling, through conserved molecular and cellular mechanisms, allows neural networks to operate correctly.

Decreased d-Serine Levels Prevent Retinal Ganglion Cell Apoptosis in a Glaucomatous Animal Model

Purpose

The purpose of this study was to determine d-Serine and d-Serine synthetase serine racemase (SR) expression and whether decreased d-Serine expression has protective effects on retinal ganglion cells (RGCs) in a glaucomatous animal model.

Methods

The rat chronic intraocular hypertension (COH) model was generated as a glaucomatous animal model by cauterizing three episcleral veins. Quantitative analysis of RGC survival was determined by the counting of retrograde FluoroGold-labeled RGCs. The level of d-Serine in the retinas and aqueous humor was determined by Ultra High Performance Liquid Chromatography coupled to triple-quadrupole Mass Spectrometry (UHPLC-MS/MS). Retinal expression of serine racemase (SR) protein was determined by immunohistochemistry and Western blot analysis. The TUNEL assay was used to detect cell apoptosis.

Results

The content of d-Serine increased significantly in the glaucomatous retina of the COH model 2 weeks after surgery compared with the control retina. d-Serine synthetase SR expression in the right glaucomatous eye increased slightly after surgery compared with that in the left control eye and remained at this high level for 6 weeks after surgery. SR-positive cells were located mainly in the ganglion cell layer (GCL) of the retina. d-Amino acid oxidase (DAAO) treatment significantly increased RGC survival in the glaucomatous eyes, and the TUNEL assay was used to confirm that DAAO reduced the number of TUNEL-positive cells in glaucomatous eyes. However, excess d-Serine could not exacerbate RGC loss in the COH model.

Conclusions

Increased d-Serine and SR expressions in the retina of the COH model were detected. DAAO treatment significantly increased RGC survival in the glaucomatous eyes. These results suggest that decreased d-Serine expression has protective effects on RGCs.

NLRP3 inflammasome in NMDA-induced retinal excitotoxicity

N-methyl-D-aspartate (NMDA)-induced excitotoxicity is an acute form of experimental retinal injury as a result of overactivation of glutamate receptors. NLRP3 (nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain containing-3) inflammasome, one of the most studied sensors of innate immunity, has been reported to play a critical role in retinal neurodegeneration with controversial implications regarding neuroprotection and cell death. Thus far, it has not been elucidated whether NMDA-mediated excitotoxicity can trigger NLRP3 inflammasome in vivo. Moreover, it is unknown if NLRP3 is beneficial or detrimental to NMDA-mediated retinal cell death. Here, we employed a murine model of NMDA-induced retinal excitotoxicity by administering 100 nmoles of NMDA intravitreally, which resulted in massive TUNEL⁺ (TdT-dUTP terminal nick-end labelling) cell death in all retinal layers and especially in retinal ganglion cells (RGCs) 24 h post injection. NMDA insult in the retina potentiates macrophage/microglia cell infiltration, primes the NLRP3 inflammasome in a transcription-dependent manner and induces the expression of interleukin-1β (IL-1β). However, despite NLRP3 inflammasome upregulation, systemic deletion of Nlrp3 or Casp1 (caspase-1) did not significantly alter the NMDA-induced, excitotoxicity-mediated TUNEL⁺ retinal cell death at 24 h (acute phase). Similarly, the deletion of the two aforementioned genes did not alter the survival of the Brn3a⁺ (brain-specific homeobox/POU domain protein 3A) RGCs in a significant way at 3- or 7-days post injection (long-term phase). Our results indicate that NMDA-mediated retinal excitotoxicity induces immune cell recruitment and NLRP3 inflammasome activity even though inflammasome-mediated neuroinflammation is not a leading contributing factor to cell death in this type of retinal injury.

Targeting Polyamine Oxidase to Prevent Excitotoxicity-Induced Retinal Neurodegeneration

Dysfunction of retinal neurons is a major cause of vision impairment in blinding diseases that affect children and adults worldwide. Cellular damage resulting from polyamine catabolism has been demonstrated to be a major player in many neurodegenerative conditions. We have previously shown that inhibition of polyamine oxidase (PAO) using MDL 72527 significantly reduced retinal neurodegeneration and cell death signaling pathways in hyperoxia-mediated retinopathy. In the present study, we investigated the impact of PAO inhibition in limiting retinal neurodegeneration in a model of NMDA (N-Methyl-D-aspartate)-induced excitotoxicity. Adult mice (8–10 weeks old) were given intravitreal injections (20 nmoles) of NMDA or NMLA (N-Methyl-L-aspartate, control). Intraperitoneal injection of MDL 72527 (40 mg/kg body weight/day) or vehicle (normal saline) was given 24 h before NMDA or NMLA treatment and continued until the animals were sacrificed (varied from 1 to 7 days). Analyses of retinal ganglion cell (RGC) layer cell survival was performed on retinal flatmounts. Retinal cryostat sections were prepared for immunostaining, TUNEL assay and retinal thickness measurements. Fresh frozen retinal samples were used for Western blotting analysis. A marked decrease in the neuronal survival in the RGC layer was observed in NMDA treated retinas compared to their NMLA treated controls, as studied by NeuN immunostaining of retinal flatmounts. Treatment with MDL 72527 significantly improved survival of NeuN positive cells in the NMDA treated retinas. Excitotoxicity induced neurodegeneration was also demonstrated by reduced levels of synaptophysin and degeneration of inner retinal neurons in NMDA treated retinas compared to controls. TUNEL labeling studies showed increased cell death in the NMDA treated retinas. However, treatment with MDL 72527 markedly reduced these changes. Analysis of signaling pathways during excitotoxic injury revealed the downregulation of pro-survival signaling molecules p-ERK and p-Akt, and the upregulation of a pro-apoptotic molecule BID, which were normalized with PAO inhibition. Our data demonstrate that inhibition of polyamine oxidase blocks NMDA-induced retinal neurodegeneration and promotes cell survival, thus offering a new therapeutic target for retinal neurodegenerative disease conditions.

Microglia mediate non-cell-autonomous cell death of retinal ganglion cells

Excitotoxicity is well known in the neuronal death in the brain and is also linked to neuronal damages in the retina. Recent accumulating evidence show that microglia greatly affect excitotoxicity in the brain, but their roles in retina have received only limited attention. Here, we report that retinal excitotoxicity is mediated by microglia. To this end, we employed three discrete methods, that is, pharmacological inhibition of microglia by minocycline, pharmacological ablation by an antagonist for colony stimulating factor 1 receptor (PLX5622), and genetic ablation of microglia using Iba1-tTA::DTA^tetO/tetO mice. Intravitreal injection of NMDA increased the number of apoptotic retinal ganglion cells (RGCs) followed by reduction in the number of RGCs. Although microglia did not respond to NMDA directly, they became reactive earlier than RGC damages. Inhibition or ablation of microglia protected RGCs against NMDA. We found up-regulation of proinflammatory cytokine genes including Il1b, Il6 and Tnfa, among which Tnfa was selectively blocked by minocycline. PLX5622 also suppressed Tnfa expression. Tumor necrosis factor α (TNFα) signals were restricted in microglia at very early followed by spreading into other cell types. TNFα up-regulation in microglia and other cells were significantly attenuated by minocycline and PLX5622, suggesting a central role of microglia for TNFα induction. Both inhibition of TNFα and knockdown of TNF receptor type 1 by siRNA protected RGCs against NMDA. Taken together, our data demonstrate that a phenotypic change of microglia into a neurotoxic one is a critical event for the NMDA-induced degeneration of RGCs, suggesting an importance of non-cell-autonomous mechanism in the retinal neuronal excitotoxicity.

Retinal Layer Abnormalities as Biomarkers of Schizophrenia

OBJECTIVE

Schizophrenia is associated with several brain deficits, as well as visual processing deficits, but clinically useful biomarkers are elusive. We hypothesized that retinal layer changes, noninvasively visualized using spectral-domain optical coherence tomography (SD-OCT), may represent a possible "window" to these abnormalities.

METHODS

A Leica EnvisuTM SD-OCT device was used to obtain high-resolution central foveal B-scans in both eyes of 35 patients with schizophrenia and 50 demographically matched controls. Manual retinal layer segmentation was performed to acquire individual and combined layer thickness measurements in 3 macular regions. Contrast sensitivity was measured at 3 spatial frequencies in a subgroup of each cohort. Differences were compared using adjusted linear models and significantly different layer measures in patients underwent Spearman Rank correlations with contrast sensitivity, quantified symptoms severity, disease duration, and antipsychotic medication dose.

RESULTS

Total retinal and photoreceptor complex thickness was reduced in all regions in patients (P < .0001). Segmentation revealed consistent thinning of the outer nuclear layer (P < .001) and inner segment layer (P < .05), as well as a pattern of parafoveal ganglion cell changes. Low spatial frequency contrast sensitivity was reduced in patients (P = .002) and correlated with temporal parafoveal ganglion cell complex thinning (R = .48, P = .01). Negative symptom severity was inversely correlated with foveal photoreceptor complex thickness (R = -.54, P = .001) and outer nuclear layer thickness (R = -.47, P = .005).

CONCLUSIONS

Our novel findings demonstrate considerable retinal layer abnormalities in schizophrenia that are related to clinical features and visual function. With time, SD-OCT could provide easily-measurable biomarkers to facilitate clinical assessment and further our understanding of the disease.

G-quartet oligonucleotide mediated delivery of functional X-linked inhibitor of apoptosis protein into retinal cells following intravitreal injection

There is currently no efficient method available for the delivery of full length functional proteins into the cytoplasm of retinal cells in vivo. Historically, the most successful approach for the treatment of retinal diseases has been intravitreal injection of antibodies or recombinant proteins, but this approach is not yet utilized for the delivery of proteins that require intracellular access for a therapeutic effect. Here we describe a platform for the delivery of functional proteins into ganglion cells, photoreceptors and retinal pigment epithelium via intravitreal injection. A nucleolin binding aptamer, AS1411, was biotinylated and complexed with traptavidin and utilized as a platform for the delivery of GFP or X-linked inhibitor of apoptosis (XIAP) proteins by intravitreal injection in BALB/c mice. Retinal sections were analyzed for uptake of proteins in the retina. Apoptosis was induced by intravitreal injection of N-methyl-D-aspartate (NMDA). Retinas were harvested for analysis of TUNEL and caspase 3/7 activity. Intravitreal injection of AS1411-directed GFP or XIAP complexes enabled delivery of these proteins into ganglion cells, photoreceptors and retinal pigment epithelium in vivo. AS1411-XIAP complexes conferred significant protection to cells in the outer and inner nuclear layers following NMDA induced apoptosis. A concomitant decrease in activity of Caspase 3/7 was observed in eyes injected with the AS1411-XIAP complex. In conclusion, AS1411 can be used as a platform for the delivery of therapeutic proteins into retinal cells. This approach can potentially be utilized to introduce a large variety of therapeutically relevant proteins that are previously well characterized to maintain the structural integrity and function of retina, thus, preventing vision loss due to ocular trauma or inherited retinal degeneration.

Regulation of Excitatory Amino Acid Transmission in the Retina: Studies on Neuroprotection

Excitotoxicity occurs in neurons due to the accumulation of excitatory amino acids such as glutamate in the synaptic and extrasynaptic locations. In the retina, excessive glutamate concentrations trigger a neurotoxic cascade involving several mechanisms, including the elevation of intracellular calcium (Ca²⁺⁾ and the activation of α-amino-3-hydroxy 5-methyl-4-iso-xazole-propionic acid/kainate (AMPA/KA) and N-methyl-d-aspartate (NMDA) receptors leading to retinal degeneration. Both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are present in the mammalian retina. Indeed, due to the abundant expression of GluRs, the mammalian retina is highly susceptible to excitotoxic neurodegeneration. Excitotoxicity has been postulated to present a common downstream mechanism for several stimuli, including hypoglycemia, hypoxia, ischemia, and chronic neurodegenerative diseases. Experimental approaches to the study of neuroprotection in the retina have utilized insults that trigger hypoxia, hypoglycemia, or excitotoxicity. Using these experimental approaches, the neuroprotective potential of GluR agents, including the NMDA receptor modulators (MK801, ifenprodil, memantine); AMPA/KA receptor antagonist (CNQX); Group II and III mGluR agonists (LY354740, quisqualate); and Ca²⁺-channel blockers (diltiazem, lomerizine, verapamil, ω-conotoxin), and others (pituitary adenylate cyclase activating polypeptide, neuropeptide Y, acetylcholine receptor agonists) have been elucidated. In addition to corroborating the exocytotic role of excitatory amino acids in retinal degeneration, these studies affirm that multiple mechanism/s contribute to the prevention of damage caused by excitotoxicity in the retina. Therefore, it is feasible that several pathways are involved in protecting the retina from toxic insults in ocular neurodegenerative conditions such as glaucoma and retinal ischemia. Furthermore, these experimental models are viable tools for evaluating therapeutic candidates in ocular neuropathies.

Definition, Classification, and Pathophysiology of Canine Glaucoma

Glaucoma is a common ocular condition in humans and dogs leading to optic nerve degeneration and irreversible blindness. Primary glaucoma is a group of spontaneous heterogeneous diseases. Multiple factors are involved in its pathogenesis and these factors vary across human ethnic groups and canine breeds, so the clinical phenotypes are numerous and their classification can be challenging and remain superficial. Aging and oxidative stress are major triggers for the manifestation of disease. Multiple, intertwined inflammatory and biochemical cascades eventually alter cellular and extracellular physiology in the optic nerve and trabecular meshwork and lead to vision loss.

Effect of memantine: A NMDA receptor blocker, on ethambutol-induced retinal injury

Ethambutol (ETM)-induced retinal injury is associated with a deterioration in visual function caused by a mechanism similar to many other retinal injuries; i.e. glutamate-induced N-methyl-d-aspartate (NMDA) receptor hyperexitability. Therefore, the current study was carried out to investigate the effect of memantine (MEM), NMDA receptor blocker, on ETM-induced retinal injury. A total of 36 rats were divided equally into: group I, control, group II (ETM administration, 100mg/kg/d, orally for 4 weeks) and group III (administration of ETM+MEM, 100 and 5mg/kg/d, respectively, orally for 4 weeks). Specimens of the retina were prepared for histological study by haematoxylin, eosin (H&E) as well as for immunohistochemical study by Bcl-2, cleaved caspase-3 and glial fibrillary acidic protein (GFAP). In the ETM group, the neural retina became significantly thinner (p<0.05) and the ganglion cell layer (GCL) was the main layer affected in the form of a significant decrease (p<0.001) in its cellularity, along with an obvious increase in Bcl-2 and GFAP expression as well as caspase-3 and oxidative stress markers level compared with other groups. On the other hand, on combining MEM with ETM, the retinal thickness, NFL appearance and GCL cellularity returned to amounts nearly similar to the control group coupled with a significant decrease (p<0.05) in the detected caspase-3, Bcl-2 levels and minimal GFAP expression. Therefore, memantine could be an effective neuroprotective agent in ETM-induced retinal injury by a mechanism that may involve correction of the pro/anti-apoptotic pathways and normalization of the oxidative and Müller cell stress responses.

Activation of Neuropeptide Y Receptors Modulates Retinal Ganglion Cell Physiology and Exerts Neuroprotective Actions In Vitro

Neuropeptide Y (NPY) is expressed in mammalian retina but the location and potential modulatory effects of NPY receptor activation remain largely unknown. Retinal ganglion cell (RGC) death is a hallmark of several retinal degenerative diseases, particularly glaucoma. Using purified RGCs and ex vivo rat retinal preparations, we have measured RGC intracellular free calcium concentration ([Ca²⁺]_i) and RGC spiking activity, respectively. We found that NPY attenuated the increase in the [Ca²⁺]_i triggered by glutamate mainly via Y₁ receptor activation. Moreover, (Leu³¹, Pro³⁴)−NPY, a Y₁/Y₅ receptor agonist, increased the initial burst response of OFF-type RGCs, although no effect was observed on RGC spontaneous spiking activity. The Y₁ receptor activation was also able to directly modulate RGC responses by attenuating the NMDA-induced increase in RGC spiking activity. These results suggest that Y₁ receptor activation, at the level of inner or outer plexiform layers, leads to modulation of RGC receptive field properties. Using in vitro cultures of rat retinal explants exposed to NMDA, we found that NPY pretreatment prevented NMDA-induced cell death. However, in an animal model of retinal ischemia-reperfusion injury, pretreatment with NPY or (Leu³¹, Pro³⁴)−NPY was not able to prevent apoptosis or rescue RGCs. In conclusion, we found modulatory effects of NPY application that for the first time were detected at the level of RGCs. However, further studies are needed to evaluate whether NPY neuroprotective actions detected in retinal explants can be translated into animal models of retinal degenerative diseases.

Adenosine transporters and receptors: key elements for retinal function and neuroprotection

Adenosine is an important neuroactive substance in the central nervous system, including in the retina where subclasses of adenosine receptors and transporters are expressed since early stages of development. Here, we review some evidence showing that adenosine plays important functions in the mature as well as in the developing tissue. Adenosine transporters are divided into equilibrative and concentrative, and the major transporter subtype present in the retina is the ENT1. This transporter is responsible for a bidirectional transport of adenosine and the uptake or release of this nucleoside appears to be regulated by different signaling pathways that are also controlled by activation of adenosine receptors. Adenosine receptors are also key players in retina physiology regulating a variety of functions in the mature and developing tissue. Regulation of excitatory neurotransmitter release and neuroprotection are the main functions played be adenosine in the mature tissue, while regulation of cell survival and neurogenesis are some of the functions played by adenosine in developing retina. Since adenosine is neuroprotective against excitotoxic and metabolic dysfunctions observed in neurological and ocular diseases, the search for adenosine-related drugs regulating adenosine transporters and receptors can be important for advancement of therapeutic strategies against these diseases.

Cell surface translocation of annexin A2 facilitates glutamate-induced extracellular proteolysis

Glutamate-induced elevation in intracellular Ca(2+) has been implicated in excitotoxic cell death. Neurons respond to increased glutamate levels by activating an extracellular proteolytic cascade involving the components of the plasmin-plasminogen system. AnxA2 is a Ca(2+)-dependent phospholipid binding protein and serves as an extracellular proteolytic center by recruiting the tissue plasminogen activator and plasminogen and mediating the localized generation of plasmin. Ratiometric Ca(2+) imaging and time-lapse confocal microscopy demonstrated glutamate-induced Ca(2+) influx. We showed that glutamate translocated both endogenous and AnxA2-GFP to the cell surface in a process dependent on the activity of the NMDA receptor. Glutamate-induced translocation of AnxA2 is dependent on the phosphorylation of tyrosine 23 at the N terminus, and mutation of tyrosine 23 to a non-phosphomimetic variant inhibits the translocation process. The cell surface-translocated AnxA2 forms an active plasmin-generating complex, and this activity can be neutralized by a hexapeptide directed against the N terminus. These results suggest an involvement of AnxA2 in potentiating glutamate-induced cell death processes.

HIV-1 Tat-mediated apoptosis in human blood-retinal barrier-associated cells

HIV-1-associated ocular complications, such as microvasculopathies, can lead to the loss of vision in HIV-1-infected patients. Even in patients under highly active antiretroviral therapy, ocular lesions are unavoidable. Ocular complications have been demonstrated to be closely related to the breakdown of the blood-retinal-barrier (BRB); however, the underlying mechanism is not clear. The data from this study indicated that the HIV-1 Tat protein induced the apoptosis of human retinal microvascular endothelial cells (HRMECs) and retinal pigmen epithelium (RPE) cells, which compose the inner BRB and the outer BRB, respectively. In addition, this study found that the activation of N-methyl-D-aspartate receptors (NMDARs) was involved in the apoptosis of RPE cells, but it caused no changes in HRMECs. Furthermore, both cell types exhibited enhanced expression of Bak, Bax and Cytochrome c. The inhibition of Tat activity protected against the apoptosis induced by NMDAR activation and prevented the dysregulation of Bak, Bax and Cytochrome c, revealing an important role for the mitochondrial pathway in HIV-1 Tat-induced apoptosis. Together, these findings suggest a possible mechanism and may identify a potential therapeutic strategy for HIV-1-associated ocular complications.

Nitric oxide production and the expression of two nitric oxide synthases in the avian retina

Nitric oxide (NO) is known to exert multiple effects on the function of many retinal neurons and their synapses. Therefore, it is equally important to understand the potential sources of NO within the retina. To explore this, we employ a combination of 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) based NO detection and immunohistochemistry for the NO synthetic enzymes, neuronal and endothelial nitric oxide synthase (nNOS and eNOS). We find DAF signals in photoreceptors, horizontal cells, amacrine cells, efferent synapses, Müller cells, and cells in the ganglion cell layer (GCL). nNOS immunoreactivity was consistent with the DAF signal with the exception that horizontal cells and Müller cells were not clearly labeled. eNOS-like immunoreactivity (eNOS-LI) was more widespread with photoreceptors, horizontal cells, occasional bipolar cells, amacrine cells, Müller cells, and cells in the GCL all showing labeling. Double labeling with antibodies raised against calretinin, syntaxin, and glutamine synthetase confirmed that horizontal cells, amacrine cells, and Müller cells (respectively) were expressing eNOS-LI. Although little or no nNOS labeling is observed in horizontal cells or Müller cells, the expression of eNOS-LI is consistent with the ability of these cells to produce NO. Together these results suggest that the capability to produce NO is widespread in the chicken retina. We propose that multiple forms of regulation for nNOS and eNOS play a role in the patterning of NO production in the chicken retina.

Ectopic vesicular glutamate release at the optic nerve head and axon loss in mouse experimental glaucoma

Although clinical and experimental observations indicate that the optic nerve head (ONH) is a major site of axon degeneration in glaucoma, the mechanisms by which local retinal ganglion cell (RGC) axons are injured and damage spreads among axons remain poorly defined. Using a laser-induced ocular hypertension (LIOH) mouse model of glaucoma, we found that within 48 h of intraocular pressure elevation, RGC axon segments within the ONH exhibited ectopic accumulation and colocalization of multiple components of the glutamatergic presynaptic machinery including the vesicular glutamate transporter VGLUT2, several synaptic vesicle marker proteins, glutamate, the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex and active zone cytomatrix components, as well as ultrastructurally identified, synaptophysin-containing vesicles. Ectopic vesicle exocytosis and glutamate release were detected in acute preparations of the LIOH ONH. Immunolocalization and analysis using the ionotropic receptor channel-permeant cation agmatine indicated that ONH axon segments and glia expressed glutamate receptors, and these receptors were more active after LIOH compared with controls. Pharmacological antagonism of glutamate receptors and neuronal activity resulted in increased RGC axon sparing in vivo. Furthermore, in vivo RGC-specific genetic disruption of the vesicular glutamate transporter VGLUT2 or the obligatory NMDA receptor subunit NR1 promoted axon survival in experimental glaucoma. As the inhibition of ectopic glutamate vesicular release or glutamate receptivity can independently modify the severity of RGC axon loss, synaptic release mechanisms may provide useful therapeutic entry points into glaucomatous axon degeneration.

Expression of NMDA receptor subunit 1 in the rat retina

N-methyl-D-aspartate receptors (NMDARs) belong to the ionotropic glutamate receptors, which play key roles in neuronal communication in the retina. NMDA receptors are tetrameric protein complexes usually comprising two obligatory NMDA receptor 1 (NR1) subunits and modulatory NMDA receptor 2/3 (NR2/3) subunits. Although the expression patterns of different NMDA receptor subunits have been extensively studied, in this study we focused on NR1 protein expression in the rat retina by immunofluorescence double labeling. We show that NR1 labeling is diffusely distributed in the outer plexiform layer (OPL) and throughout the whole inner plexiform layer (IPL). The NR1-immunoreactivity (IR) was displayed in a variety of cells in the inner nuclear layer (INL) and the ganglion cell layer (GCL). Interestingly, NR1 was expressed in both rod and cone bipolar cells identified by specific bipolar cell markers Chx10, protein kinase C (PKC) and recoverin. All the amacrine cells that we studied, including cholinergic, dopaminergic, GABAergic and glycinergic amacrine cells, were NR1-IR positive. In the ganglion cell layer, NR1-IR was expressed in all cells that were positive for the ganglion cell marker Brn3a. Our study suggests that the NR1 subunit is expressed more widely than was previously appreciated.

N-methyl-D-aspartate (NMDA)-mediated excitotoxic damage: a mouse model of acute retinal ganglion cell damage

The animal model of N-methyl-D-aspartate (NMDA)-induced excitotoxic damage of retinal ganglion cells (RGC) is widely used to study the molecular mechanisms of RGC apoptosis and/or its prevention by neuroprotective agents. This chapter provides protocols for applying NMDA-induced excitotoxic damage to RGC of mouse eyes and for subsequent measuring of the extent of the resulting damage.

NR2C and NR2D subunits of NMDA receptors in frog and turtle retina

Glutamate NMDA (N-methyl-D-aspartate) receptors are widely distributed in the central nervous system where they are involved in cognitive processes, motor control and many other functions. They are also well studied in the retina, which may be regarded as a biological model of the nervous system. However, little is known about NR2C and NR2D subunits of NMDA receptors, which have some specific features as compared to other subunits. Consequently the aim of the present study was to investigate their distribution in frog (Rana ridibunda) and turtle (Emys orbicularis) retinas which possess mixed and cone types of retina respectively. The experiments were performed using an indirect immunofluorescence method. Four antibodies directed to NR2C and NR2D subunits of NMDA receptor, as well as three antibodies directed to different splice variants of NR1 subunit, which is known to be obligatory for proper functioning of the receptor, were applied. All antibodies caused well expressed labeling in frog and turtle retinas. The NR2C and NR2D subunits were localized in glial Müller cells, while the NR1 subunit had both neuronal and glial localization. Our results show that glial NMDA receptors differ from neuronal ones in their subunit composition. The functional significance of the NMDA receptors and their NR2C and NR2D subunits, in particular for the neuron-glia interactions, is discussed.

Involvement of calpain/p35-p25/Cdk5/NMDAR signaling pathway in glutamate-induced neurotoxicity in cultured rat retinal neurons

We investigated possible involvement of a calpain/p35-p25/cyclin-dependent kinase 5 (Cdk5) signaling pathway in modifying NMDA receptors (NMDARs) in glutamate-induced injury of cultured rat retinal neurons. Glutamate treatment decreased cell viability and induced cell apoptosis, which was accompanied by an increase in Cdk5 and p-Cdk5^T15 protein levels. The Cdk5 inhibitor roscovitine rescued the cell viability and inhibited the cell apoptosis. In addition, the protein levels of both calpain 2 and calpain-specific alpha-spectrin breakdown products (SBDPs), which are both Ca²⁺-dependent, were elevated in glutamate-induced cell injury. The protein levels of Cdk5, p-Cdk5^T15, calpain 2 and SBDPs tended to decline with glutamate treatments of more than 9 h. Furthermore, the elevation of SBDPs was attenuated by either D-APV, a NMDAR antagonist, or CNQX, a non-NMDAR antagonist, but was hardly changed by the inhibitors of intracellular calcium stores dantrolene and xestospongin. Moreover, the Cdk5 co-activator p35 was significantly up-regulated, whereas its cleaved product p25 expression showed a transient increase. Glutamate treatment for less than 9 h also considerably enhanced the ratio of the Cdk5-phosphorylated NMDAR subunit NR2A at Ser1232 site (p-NR2A^S1232) and NR2A (p-NR2A^S1232/NR2A), and caused a translocation of p-NR2A^S1232 from the cytosol to the plasma membrane. The enhanced p-NR2A^S1232 was inhibited by roscovitine, but augmented by over-expression of Cdk5. Calcium imaging experiments further showed that intracellular Ca²⁺ concentrations ([Ca²⁺]_i) of retinal cells were steadily increased following glutamate treatments of 2 h, 6 h and 9 h. All these results suggest that the activation of the calpain/p35-p25/Cdk5 signaling pathway may contribute to glutamate neurotoxicity in the retina by up-regulating p-NR2A^S1232 expression.

Protecting the retinal neurons from glaucoma: lowering ocular pressure is not enough

The retina is theater of a number of biochemical reactions allowing, within its layers, the conversion of light impulses into electrical signals. The axons of the last neuronal elements, the ganglion cells, form the optic nerve and transfer the signals to the brain. Therefore, an appropriate cellular communication, not only within the different retinal cells, but also between the retina itself and the other brain structures, is fundamental. One of the most diffuse pathologies affecting retinal function and communication, which thus reverberates in the whole visual system, is glaucoma. This insidious disease is characterized by a progressive optic nerve degeneration and sight loss which may finally lead to irreversible blindness. Nevertheless, the progressive nature of this pathology offers an opportunity for therapeutic intervention. To better understand the cellular processes implicated in the development of glaucoma useful to envision a targeted pharmacological strategy, this manuscript first examines the complex cellular and functional organization of the retina and subsequently identifies the targets sensitive to neurodegeneration. Within this context, high ocular pressure represents a key risk factor. However, recent literature findings highlight the concept that lowering ocular pressure is not enough to prevent/slow down glaucomatous damage, suggesting the importance of combining the hypotensive treatment with other pharmacological approaches, such as the use of neuroprotectants. Therefore, this important and more novel aspect is extensively considered in this review, also emphasizing the idea that the neuroprotective strategy should be extended to the entire visual system and not restricted to the retina.