The Susceptibility of Retinal Ganglion Cells to Glutamatergic Excitotoxicity Is Type-Specific

Effects of Acute High-Altitude Exposure on Morphology and Function of Retinal Ganglion Cell in Mice

Purpose

High altitude retinopathy (HAR) is a retinal functional disorder caused by inadequate adaptation after exposure to high altitude. However, the cellular and molecular mechanisms underlying retinal dysfunction remain elusive. Retinal ganglion cell (RGC) injury is the most important pathological basis for most retinal and optic nerve diseases. Studies focusing on RGC injury after high-altitude exposure (HAE) are scanty. Therefore, the present study sought to explore both functional and morphological alterations of RGCs after HAE.

Methods

A mouse model of acute hypobaric hypoxia was established by mimicking the conditions of a high altitude of 5000 m. After HAE for 2, 4, 6, 10, 24, and 72 hours, the functional and morphological alterations of RGCs were assessed using retinal hematoxylin and eosin (H&E) sections, retinal whole mounts, transmission electron microscopy (TEM), and the photopic negative response (PhNR) of the electroretinogram.

Results

Compared with the control group, the thickness of the ganglion cell layer and retinal nerve fiber layer increased significantly, RGC loss remained significant, and the amplitudes of a-wave, b-wave, and PhNR were significantly reduced after HAE. In addition, RGCs and their axons exhibited an abnormal ultrastructure after HAE, including nuclear membrane abnormalities, uneven distribution of chromatin in the nucleus, decreased cytoplasmic electron density, widening and vacuolization of the gap between axons, loosening and disorder of myelin sheath structure, widening of the gap between myelin sheath and axon membrane, decreased axoplasmic density, unclear microtubule and nerve fiber structure, and abnormal mitochondrial structure (mostly swollen, with widened membrane gaps and reduced cristae and vacuolization).

Conclusions

The study findings confirm that the morphology and function of RGCs are damaged after HAE. These findings lay the foundation for further study of the specific molecular mechanisms of HAR and promote the effective prevention.

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

A novel HDAC8 inhibitor H7E exerts retinoprotective effects against glaucomatous injury via ameliorating aberrant Müller glia activation and oxidative stress

Glaucoma is considered a neurodegenerative disease characterized by progressive visual field defects that may lead to blindness. Although controlling intraocular pressure (IOP) is the mainstay of glaucoma treatment, some glaucoma patients have unmet needs due to unclear pathogenic mechanisms. Recently, there has been growing evidence that neuroinflammation is a potential target for the development of novel antiglaucoma agents. In this study, we investigated the protective effects and cellular mechanisms of H7E, a novel small molecule inhibits HDAC8, using in vitro and in vivo glaucoma-like models. Importantly, H7E mitigated extracellular MMP-9 activity and MCP-1 levels in glutamate- or S100B-stimulated reactive Müller glia. In addition, H7E inhibited the upregulation of inflammation- and proliferation-related signaling pathways, particularly the ERK and JNK MAPK pathways. Under conditions of oxidative damage, H7E prevents retinal cell death and reduces extracellular glutamate released from stressed Müller glia. In a mouse model of NMDA-induced retinal degeneration, H7E alleviated functional and structural defects within the inner retina as assessed by electroretinography and optical coherence tomography. Our results demonstrated that the newly identified compound H7E protects against glaucoma damage by specifically targeting HDAC8 activity in the retina. This protective effect is attributed to the inhibition of Müller glial activation and the prevention of retinal cell death caused by oxidative stress.

Molecular pathways in experimental glaucoma models

Glaucoma is a complex and progressive disease that primarily affects the optic nerve axons, leading to irreversible vision loss. Although the exact molecular mechanisms underlying glaucoma pathogenesis are not fully understood, it is believed that except increased intraocular pressure, a combination of genetic and environmental factors play a role in the development of the disease. Animal models have been widely used in the study of glaucoma, allowing researchers to better understand the underlying mechanisms of the disease and test potential treatments. Several molecular pathways have been implicated in the pathogenesis of glaucoma, including oxidative stress, inflammation, and excitotoxic-induced neurodegeneration. This review summarizes the most important knowledge about molecular mechanisms involved in the glaucoma development. Although much research has been done to better understand the molecular mechanisms underlying this disease, there is still much to be learned to develop effective treatments and prevent vision loss in those affected by glaucoma.

Evaluation of the neuroprotective efficacy of the gramine derivative ITH12657 against NMDA-induced excitotoxicity in the rat retina

Purpose

The aim of this study was to investigate, the neuroprotective effects of a new Gramine derivative named: ITH12657, in a model of retinal excitotoxicity induced by intravitreal injection of NMDA.

Methods

Adult Sprague Dawley rats received an intravitreal injection of 100 mM NMDA in their left eye and were treated daily with subcutaneous injections of ITH12657 or vehicle. The best dose-response, therapeutic window study, and optimal treatment duration of ITH12657 were studied. Based on the best survival of Brn3a + RGCs obtained from the above-mentioned studies, the protective effects of ITH12657 were studied in vivo (retinal thickness and full-field Electroretinography), and ex vivo by quantifying the surviving population of Brn3a + RGCs, αRGCs and their subtypes α-ONsRGCs, α-ONtRGCs, and α-OFFRGCs.

Results

Administration of 10 mg/kg ITH12657, starting 12 h before NMDA injection and dispensed for 3 days, resulted in the best significant protection of Brn3a + RGCs against NMDA-induced excitotoxicity. In vivo, ITH12657-treated rats showed significant preservation of retinal thickness and functional protection against NMDA-induced retinal excitotoxicity. Ex vivo results showed that ITH12657 afforded a significant protection against NMDA-induced excitotoxicity for the populations of Brn3a + RGC, αRGC, and αONs-RGC, but not for the population of αOFF-RGC, while the population of α-ONtRGC was fully resistant to NMDA-induced excitotoxicity.

Conclusion

Subcutaneous administration of ITH12657 at 10 mg/kg, initiated 12 h before NMDA-induced retinal injury and continued for 3 days, resulted in the best protection of Brn3a + RGCs, αRGC, and αONs-RGC against excitotoxicity-induced RGC death. The population of αOFF-RGCs was extremely sensitive while α-ONtRGCs were fully resistant to NMDA-induced excitotoxicity.

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Versatile Endogenous Editing of GluRIIA in Drosophila melanogaster

Glutamate receptors at the postsynaptic side translate neurotransmitter release from presynapses into postsynaptic excitation. They play a role in many forms of synaptic plasticity, e.g., homeostatic scaling of the receptor field, activity-dependent synaptic plasticity and the induction of presynaptic homeostatic potentiation (PHP). The latter process has been extensively studied at Drosophila melanogaster neuromuscular junctions (NMJs). The genetic removal of the glutamate receptor subunit IIA (GluRIIA) leads to an induction of PHP at the synapse. So far, mostly imprecise knockouts of the GluRIIA gene have been utilized. Furthermore, mutated and tagged versions of GluRIIA have been examined in the past, but most of these constructs were not expressed under endogenous regulatory control or involved the mentioned imprecise GluRIIA knockouts. We performed CRISPR/Cas9-assisted gene editing at the endogenous locus of GluRIIA. This enabled the investigation of the endogenous expression pattern of GluRIIA using tagged constructs with an EGFP and an ALFA tag for super-resolution immunofluorescence imaging, including structured illumination microscopy (SIM) and direct stochastic optical reconstruction microscopy (dSTORM). All GluRIIA constructs exhibited full functionality and PHP could be induced by philanthotoxin at control levels. By applying hierarchical clustering algorithms to analyze the dSTORM data, we detected postsynaptic receptor cluster areas of ~0.15 µm2. Consequently, our constructs are suitable for ultrastructural analyses of GluRIIA.

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.

Signalling pathways and cell death mechanisms in glaucoma: Insights into the molecular pathophysiology

Glaucoma is a complex multifactorial eye disease manifesting in retinal ganglion cell (RGC) death and optic nerve degeneration, ultimately causing irreversible vision loss. Research in recent years has significantly enhanced our understanding of RGC degenerative mechanisms in glaucoma. It is evident that high intraocular pressure (IOP) is not the only contributing factor to glaucoma pathogenesis. The equilibrium of pro-survival and pro-death signalling pathways in the retina strongly influences the function and survival of RGCs and optic nerve axons in glaucoma. Molecular evidence from human retinal tissue analysis and a range of experimental models of glaucoma have significantly contributed to unravelling these mechanisms. Accumulating evidence reveals a wide range of molecular signalling pathways that can operate -either alone or via intricate networks - to induce neurodegeneration. The roles of several molecules, including neurotrophins, interplay of intracellular kinases and phosphates, caveolae and adapter proteins, serine proteases and their inhibitors, nuclear receptors, amyloid beta and tau, and how their dysfunction affects retinal neurons are discussed in this review. We further underscore how anatomical alterations in various animal models exhibiting RGC degeneration and susceptibility to glaucoma-related neuronal damage have helped to characterise molecular mechanisms in glaucoma. In addition, we also present different regulated cell death pathways that play a critical role in RGC degeneration in glaucoma.

Novel frontiers in neuroprotective therapies in glaucoma: Molecular and clinical aspects

In the last years, neuroprotective therapies have attracted the researcher interests as modern and challenging approach for the treatment of neurodegenerative diseases, aimed at protecting the nervous system from injuries. Glaucoma is a neurodegenerative disease characterized by progressive excavation of the optic nerve head, retinal axonal injury and corresponding vision loss that affects millions of people on a global scale. The molecular basis of the pathology is largely uncharacterized yet, and the therapeutic approaches available do not change the natural course of the disease. Therefore, in accordance with the therapeutic regimens proposed for other neurodegenerative diseases, a modern strategy to treat glaucoma includes prescription of drugs with neuroprotective activities. With respect to this, several preclinical and clinical investigations on a plethora of different drugs are currently ongoing. In this review, first, the conceptualization of the rationale for the adoption of neuroprotective strategies for retina is summarized. Second, the molecular aspects highlighting glaucoma as a neurodegenerative disease are reported. In conclusion, the molecular and pharmacological properties of most promising direct neuroprotective drugs used to delay glaucoma progression are examined, including: neurotrophic factors, NMDA receptor antagonists, the α2-adrenergic agonist, brimonidine, calcium channel blockers, antioxidant agents, nicotinamide and statins.

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

Large-scale survey of excitatory synapses reveals sublamina-specific and asymmetric synapse disassembly in a neurodegenerative circuit

In the nervous system, parallel circuits are organized in part by the lamina-specific compartmentalization of synaptic connections. In sensory systems such as mammalian retina, degenerating third-order neurons remodel their local presynaptic connectivity with second-order neurons. To determine whether there are sublamina-specific perturbations after injury of adult retinal ganglion cells, we comprehensively analyzed excitatory synapses across the inner plexiform layer (IPL) where bipolar cells connect to ganglion cells. Here, we show that pre- and postsynaptic component loss occurs throughout the IPL in a sublamina-dependent fashion after transient intraocular pressure elevation. Partnered synaptic components are lost as neurodegeneration progresses, while unpartnered synaptic components remain stable. Furthermore, presynaptic components are either lost first or simultaneously with the postsynaptic component. Our results demonstrate that this degenerating neural circuit exhibits differential vulnerability of excitatory synapses depending on IPL depth, highlighting the ordered disassembly of synapses that is specific to laminar compartments of the retina.

Postnatal exposure to trimethyltin chloride induces retinal developmental neurotoxicity in mice via glutamate and its transporter related changes

Exposure to toxic substances during postnatal period is one of the major factors causing retinal developmental defects. The developmental toxicity of trimethyltin chloride (TMT), a byproduct of an organotin compound widely used in agriculture and industrial fields, has been reported; however, the effect on the mammalian retina during postnatal development and the mechanism have not been elucidated to date. We exposed 0.75 and 1.5 mg/kg of TMT to neonatal ICR mice (1:1 ratio of male and female) up to postnatal day 14 and performed analysis of the retina: histopathology, apoptosis, electrophysiological function, glutamate concentration, gene expression, and fluorescence immunostaining. Exposure to TMT caused delayed eye opening, eye growth defect and thinning of retinal layer. In addition, apoptosis occurred in the retina along with b-wave and spiking activity changes in the micro-electroretinogram. These changes were accompanied by an increase in the concentration of glutamate, upregulation of astrocyte-related genes, and increased expression of glial excitatory amino acid transporter (EAAT) 1 and 2. Conversely, EAAT 3, 4, and 5, mainly located in the neurons, were decreased. Our results are the first to prove postnatal retinal developmental neurotoxicity of TMT at the mammalian model and analyze the molecular, functional as well as morphological aspects to elucidate possible mechanisms: glutamate toxicity with EAAT expression changes. These mechanisms may suggest not only a strategy to treat but also a clue to prevent postnatal retina developmental toxicity of toxic substances.

Neuroprotective Properties of Chlorogenic Acid and 4,5-Caffeoylquinic Acid from Brazilian arnica (Lychnophora ericoides) after Acute Retinal Ischemia

Lychnophora is a genus of South American flowering plants in the daisy family, popularly known as "Brazilian arnica". It is used in traditional medicine as an anti-inflammatory and analgesic agent, whose active components are derived from chlorogenic acid (CGA) and C-flavonoids. Since the drugs currently used are ineffective to treat glaucoma, agents with antioxidant and anti-inflammatory properties may represent new alternatives in preventing cellular lesions in retinal ischemia. In this study, we report the neuroprotective effects of CGA and 4,5-di-O-[E]-caffeoylquinic (CQA) acid, isolated from Lychnophora plants, in a rodent glaucoma model. Wistar rats were administered intravitreally with 10 µg CGA or CGA, and then subjected to acute retinal ischemia (ISC) by increasing intraocular pressure (IPO) for 45 minutes followed (or not) by 15 minutes of reperfusion (I/R). Qualitative and quantitative analyses of neurodegeneration were performed using hematoxylin-eosin or Fluoro-Jade C staining protocols. All retinas submitted to ISC or I/R exhibited matrix disorganization, pyknotic nuclei, and pronounced vacuolization of the cytoplasm in the ganglion cell layer (GCL) and inner nuclear layer (INL). Pretreatment with CGA or CQA resulted in the protection of the retinal layers against matrix disorganization and a reduction in the number of vacuolized cells and pyknotic nuclei. Also, pretreatment with CGA or CQA resulted in a significant reduction in neuronal death in the GCL, the INL, and the outer nuclear layer (ONL) after ischemic insult. Our study demonstrated that CGA and CQA exhibit neuroprotective activities in retinas subjected to ISC and I/R induced by IPO in Wistar rats.

Longitudinal in vivo Ca2+ imaging reveals dynamic activity changes of diseased retinal ganglion cells at the single-cell level

Retinal ganglion cells (RGCs) are heterogeneous projection neurons that convey distinct visual features from the retina to brain. Here, we present a high-throughput in vivo RGC activity assay in response to light stimulation using noninvasive Ca2+ imaging of thousands of RGCs simultaneously in living mice. Population and single-cell analyses of longitudinal RGC Ca2+ imaging reveal distinct functional responses of RGCs and unprecedented individual RGC activity conversions during traumatic and glaucomatous degeneration. This study establishes a foundation for future in vivo RGC function classifications and longitudinal activity evaluations using more advanced imaging techniques and visual stimuli under normal, disease, and neural repair conditions. These analyses can be performed at both the population and single-cell levels using temporal and spatial information, which will be invaluable for understanding RGC pathophysiology and identifying functional biomarkers for diverse optic neuropathies.

Echium amoenum L. Ethanol Extract Protects Retinal Ganglion Cell after Glutamate and Optic Nerve Crush Injury

The development of low-cost and effective natural products for treating neuron degenerative diseases have proven to be safe and potentially effective. Echium amoenum L. (Boraginaceae) is an annual herb that grows wildly in Europe and western Asia. The aim of this study was to evaluate the neuroprotective properties of an ethanol extract of E. amoenum L. The effects of E. amoenum L. extract on oxidative stress were measured in the rat R28 retinal precursor cell line. Furthermore, the protective role of the extract on the glutamate-induced and optic nerve crush (ONC) injury-induced cell death were evaluated in vitro and in vivo, respectively. Our results showed that the ethanol extract of E. amoenum L. prevented the glutamate-induced decrease in cell viability and increase in cell death in R28 cells and suppressed the overproduction of ROS induced by glutamate. Moreover, the extract significantly inhibited microglial activation and optic nerve damage induced by ONC injury in mice. In addition, the mechanism was attributed to the ability of the extract to decrease NF-κB pathway activation and its downstream inflammatory cytokine production. In conclusion, E. amoenum L. ethanol extract had a potent neuroprotective effect against glutamate-induced and ONC-induced cell death. This is likely due to its antioxidant and anti-inflammatory properties.

Subtype-specific survival and regeneration of retinal ganglion cells in response to injury

Retinal ganglion cells (RGCs) are a heterogeneous population of neurons that function synchronously to convey visual information through the optic nerve to retinorecipient target areas in the brain. Injury or disease to the optic nerve results in RGC degeneration and loss of visual function, as few RGCs survive, and even fewer can be provoked to regenerate their axons. Despite causative insults being broadly shared, regeneration studies demonstrate that RGC types exhibit differential resilience to injury and undergo selective survival and regeneration of their axons. While most early studies have identified these RGC types based their morphological and physiological characteristics, recent advances in transgenic and gene sequencing technologies have further enabled type identification based on unique molecular features. In this review, we provide an overview of the well characterized RGC types and identify those shown to preferentially survive and regenerate in various regeneration models. Furthermore, we discuss cellular characteristics of both the resilient and susceptible RGC types including the combinatorial expression of different molecular markers that identify these specific populations. Lastly, we discuss potential molecular mechanisms and genes found to be selectively expressed by specific types that may contribute to their reparative capacity. Together, we describe the studies that lay the important groundwork for identifying factors that promote neural regeneration and help advance the development of targeted therapy for the treatment of RGC degeneration as well as neurodegenerative diseases in general.

Protective Effect of Curcuma Extract in an Ex Vivo Model of Retinal Degeneration via Antioxidant Activity and Targeting the SUMOylation

Retinal degeneration is the major and principal cause behind many incurable blindness diseases. Several studies indicated the neuroprotective effect of Curcuma longa in eye pathologies, specifically retinopathy. However, the molecular mechanism behind its effect has not been completely elucidated. Using an ex vivo model of retinal degeneration obtained from an ex vivo optic nerve cut (ONC), we demonstrated that Curcuma extract (Cur) exerted a neuroprotective effect. Importantly, Cur was able to modulate apoptosis and MAPK signaling pathway activation and prevent retinal ganglion cell (RGC) loss. Other well-known neuroprotective pharmacological tools, including memantine (Mem), citicoline (Cit), and ginkgolic acid (GA), were used to compare the potential mechanisms of Cur. The antioxidant activity of retinas treated with Cur following optic nerve cut was significantly higher than control, but Cur failed to change the retina glutamate content. Considering the antioxidant effect of Cur and taking advantage of our recent findings on the crosstalk between oxidative stress and post-translational protein modifiers, in particular, small ubiquitin-related modifier (SUMO), we were interested in exploring the effect of Cur on SUMOylation. We found that Cur significantly prevented the increase of protein SUMOylation, confirming our previous in vitro data indicating the cytoprotective effect of curcumin through modulating the oxidative stress and SUMO-JNK axis. Altogether, these results suggest that Curcuma protects the retina from degeneration via antioxidant activity and targets SUMOylation. Therefore, it might be considered for the combination therapy with other neuroprotective agents with different mechanisms in preclinical studies on retinal degeneration.

Targeted Metabolomics Shows That the Level of Glutamine, Kynurenine, Acyl-Carnitines and Lysophosphatidylcholines Is Significantly Increased in the Aqueous Humor of Glaucoma Patients

The composition of the aqueous humor of patients with glaucoma is relevant to understand the underlying causes of the pathology. Information on the concentration of metabolites and small molecules in the aqueous humor of healthy subjects is limited. Among the causes of the limitations is the lack of healthy controls since, until recently, they were not surgically intervened; therefore, the aqueous humor of patients operated for cataract was used as a reference. Sixteen aqueous humor samples from healthy subjects undergoing refractive surgery and eight samples from glaucoma patients were used to assess the concentration of 188 compounds using chromatography and mass spectrometry. The concentration of 80 of the 188 was found to be reliable, allowing comparison of data from the two groups (glaucoma and control). The pattern found in the controls is similar to, but not the same as, that reported using samples from “controls” undergoing cataract surgery. Comparing data from glaucoma patients and healthy subjects, 57 of the 80 compounds were significantly (p < 0.05) altered in the aqueous humor. Kynurenine and glutamine, but not glutamate, were significantly increased in the glaucoma samples. Furthermore, 10 compounds were selected considering a statistical score of p < 0.0001 and the degree of change of more than double or less than half. The level of C10 (decanoyl)-carnitine decreased, while the concentration of spermidine and various acyl-carnitines and lysophosphatidylcholines increased in glaucoma. Principal component analysis showed complete segregation of controls and cases using the data for the 10 selected compounds. The receiver operating characteristic curve these 10 compounds and for glutamine allowed finding cut-off values and significant sensitivity and specificity scores. The concentration of small metabolites in the aqueous humor of glaucoma patients is altered even when they take medication and are well controlled. The imbalance affects membrane components, especially those of the mitochondria, suggesting that mitochondrial abnormalities are a cause or consequence of glaucoma. The increase in glutamine in glaucoma is also relevant because it could be a means of keeping the concentration of glutamate under control, thus avoiding its potential to induce the death of neurons and retinal cells. Equally notable was the increase in kynurenine, which is essential in the metabolism of nicotine adenine dinucleotides.

The Effect of Aging on Retinal Function and Retinal Ganglion Cell Morphology Following Intraocular Pressure Elevation

Aging and elevated intraocular pressure (IOP) are two major risk factors for glaucomatous optic neuropathy; a condition characterized by the selective, progressive injury, and subsequent loss of retinal ganglion cells (RGCs). We examined how age modified the capacity for RGCs to functionally recover following a reproducible IOP elevation (50 mmHg for 30 min). We found that RGC functional recovery (measured using electroretinography) was complete by 7 days in 3-month-old mice but was delayed in 12-month-old mice until 14 days. At the 7-day recovery endpoint when RGC function had recovered in young but not older eyes, we examined RGC structural responses to IOP-related stress by analyzing RGC dendritic morphology. ON-RGC cell volume was attenuated following IOP elevation in both young and older mice. We also found that following IOP elevation OFF-RGC dendritic morphology became less complex per cell volume in young mice, an effect that was not observed in older eyes. Our data suggest that adaptations in OFF-RGCs in young eyes were associated with better functional recovery 7 days after IOP elevation. Loss of RGC cellular adaptations may account for delayed functional recovery in older eyes.

The Intertwined Roles of Oxidative Stress and Endoplasmic Reticulum Stress in Glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide, and the burden of the disease continues to grow as the global population ages. Currently, the only treatment option is to lower intraocular pressure. A better understanding of glaucoma pathogenesis will help us to develop novel therapeutic options. Oxidative stress has been implicated in the pathogenesis of many diseases. Oxidative stress occurs when there is an imbalance in redox homeostasis, with reactive oxygen species producing processes overcoming anti-oxidant defensive processes. Oxidative stress works in a synergistic fashion with endoplasmic reticulum stress, to drive glaucomatous damage to trabecular meshwork, retinal ganglion cells and the optic nerve head. We discuss the oxidative stress and endoplasmic reticulum stress pathways and their connections including their key intermediary, calcium. We highlight therapeutic options aimed at disrupting these pathways and discuss their potential role in glaucoma treatment.

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.

Differential susceptibility of retinal ganglion cell subtypes against neurodegenerative diseases

Retinal ganglion cells (RGCs) are essential to propagate external visual information from the retina to the brain. Death of RGCs is speculated to be closely correlated with blinding retinal diseases, such as glaucoma and traumatic optic neuropathy (TON). Emerging innovative technologies have helped refine and standardize the classification of RGCs; at present, they are classified into more than 40 subpopulations in mammals. These RGC subtypes are identified by a combination of anatomical morphologies, electrophysiological functions, and genetic profiles. Increasing evidence suggests that neurodegenerative diseases do not collectively affect the RGCs. In fact, which RGC subtype exhibits the strongest or weakest susceptibility is hotly debated. Although a consensus has not yet been reached, it is certain that assorted RGCs display differential susceptibility against irreversible degeneration. Interestingly, a single RGC subtype can exhibit various vulnerabilities to optic nerve damage in diverse injury models. Thus, elucidating how susceptible RGC subtypes are to various injuries can protect vulnerable RGCs from damage and improve the possibility of preventing and treating visual impairment caused by neurodegenerative diseases. In this review, we summarize in detail the progress and status quo of research on the type-specific susceptibility of RGCs and point out current limitations and the possible directions for future research in this field.

Effects of lighting environment on the degeneration of retinal ganglion cells in glutamate/aspartate transporter deficient mice, a mouse model of normal tension glaucoma

Lighting conditions may affect the development of retinal degenerative diseases such as macular degeneration. In this study, to determine whether the lighting environment affects the progression of degeneration of retinal ganglion cells (RGCs), we examined glutamate/aspartate transporter (GLAST) heterozygous (GLAST^+/-) mice, a mouse model of normal tension glaucoma. GLAST^+/- mice were reared under a 12-h light-dark cycle (Light/Dark) or complete darkness (Dark/Dark) condition after birth. The total RGC number in the Dark/Dark group was significantly decreased compared with the Light/Dark group at 3 weeks old, while the number of osteopontin-positive αRGCs were similar in both groups. At 6 and 12 weeks old, the total RGC number were not significantly different in both conditions. In addition, the retinal function examined by multifocal electroretinogram were similar at 12 weeks old. These results suggest that lighting conditions may regulate the progression of RGC degeneration in some types of glaucoma.

Neuroprotection Against NMDA-Induced Retinal Damage by Philanthotoxin-343 Involves Reduced Nitrosative Stress

N-methyl-D-aspartate receptor (NMDAR) overstimulation is known to mediate neurodegeneration, and hence represents a relevant therapeutic target for neurodegenerative disorders including glaucoma. This study examined the neuroprotective effects of philanthotoxin (PhTX)-343 against NMDA-induced retinal injury in rats. Male Sprague Dawley rats were divided into three groups; group 1 received phosphate buffer saline as the negative control, group 2 was injected with NMDA (160 nM) to induce retinal excitotoxic injury, and group 3 was pre-treated with PhTX-343 (160 nM) 24 h before NMDA exposure. All treatments were given intravitreally and bilaterally. Seven days post-treatment, rats were subjected to visual behaviour assessments using open field and colour recognition tests. Rats were then euthanized, and the retinas were harvested and subjected to haematoxylin and eosin (H&E) staining for morphometric analysis and 3-nitrotyrosine (3-NT) ELISA protocol as the nitrosative stress biomarker. PhTX-343 treatment prior to NMDA exposure improved the ability of rats to recognize visual cues and preserved visual functions (i.e., recognition of objects with different colours). Morphological examination of retinal tissues showed that the fractional ganglion cell layer thickness within the inner retina (IR) in the PhTX-343 treated group was greater by 1.28-fold as compared to NMDA-treated rats (p < 0.05) and was comparable to control rats (p > 0.05). Additionally, the number of retinal cell nuclei/100 μm² in IR for the PhTX-343-treated group was greater by 1.82-fold compared to NMDA-treated rats (p < 0.05) and was comparable to control group (p > 0.05). PhTX-343 also reduced the retinal 3-NT levels by 1.74-fold compared to NMDA-treated rats (p < 0.05). In conclusion, PhTX-343 pretreatment protects against NMDA-induced retinal morphological changes and visual impairment by suppressing nitrosative stress as reflected by the reduced retinal 3-NT level.

Proteomic Analysis of Retinal Tissue in an S100B Autoimmune Glaucoma Model

Glaucoma is a neurodegenerative disease that leads to damage of retinal ganglion cells and the optic nerve. Patients display altered antibody profiles and increased antibody titer, e.g., against S100B. To identify the meaning of these antibodies, animals were immunized with S100B. Retinal ganglion cell loss, optic nerve degeneration, and increased glial cell activity were noted. Here, we aimed to gain more insights into the pathophysiology from a proteomic point of view. Hence, rats were immunized with S100B, while controls received sodium chloride. After 7 and 14 days, retinae were analyzed through mass spectrometry and immunohistology. Using data-independent acquisition-based mass spectrometry, we identified more than 1700 proteins on a high confidence level for both study groups, respectively. Of these 1700, 43 proteins were significantly altered in retinae after 7 days and 67 proteins revealed significant alterations at 14 days. For example, α2-macroglobulin was found significantly increased not only by mass spectrometry analysis, but also with immunohistological staining in S100B retinae at 7 and 14 days. All in all, the identified proteins are often associated with the immune system, such as heat shock protein 60. Once more, these data underline the important role of immunological factors in glaucoma pathogenesis.

Light-Sheet Microscopy of the Optic Nerve Reveals Axonal Degeneration and Microglial Activation in NMDA-Induced Retinal Injury

PURPOSE

Optic nerve degeneration is a feature of neurodegenerative eye diseases and causes irreversible vision loss. Therefore, understanding the degenerating patterns of the optic nerve is critical to find the potential therapeutic target for optic neuropathy. However, the traditional method of optic nerve degeneration has the limitations of losing spatiotemporal tissue information. Light sheet fluorescence microscopy (LSFM) is a fluorescence microscopy technique that allows capturing 3D images rapidly with a high spatial optical resolution. In this study, we evaluated the availability of LSFM on the optic nerve with NMDA injected Thy1-CFP mice.

METHODS

NMDA injected to both eyes of Thy1-CFP mice. After 7 days from the injection, the retina and optic nerve were collected and immunostained with anti-Iba1 antibody. NMDA excitotoxicity induced RGC, and its axon loss and microglial activation in the retina were observed using confocal microscopy. The immunostained optic nerve was completed the optical clearing process with TDE and mounted for LSFM imaging.

RESULTS

We found that retinal flatmounts confirmed significant loss of CFP-expressing RGC and axon degradation and loss in Thy1-CFP mice at 7 days after NMDA injection. Together with these data verifying that NMDA induces RGC and its axon loss, we confirmed that NMDA excitotoxicity induced microglia activation and leukostasis, such as increased microglia number, transform its morphology to ameboid or round, and increase in attached leukocytes in vessels. Using LSFM, we observed that CFP expressing nerve fiber was well organized and arranged parallel in vehicle treated optic nerve, whileas NMDA injected optic nerve showed axon swelling and fragmentation and loss of axon density from the anterior to the posterior regions. Furthermore, LSFM enabled the observation of microglia phenotype transformation in the entire optic nerve. Unlike microglia in vehicle injected optic nerve, microglia in NMDA injected optic nerve displayed larger soma and short process with high Iba1 expression through the entire optic nerve from the anterior to posterior.

CONCLUSIONS

In summary, we examined the applicability of the modified optic clearing protocol for the optic nerve and verified it enabled to acquiring of the 3D images of the optic nerve successfully revealing the complex spatial relationships between the axons, microglia and vasculature throughout the entire organ with single acquisitions. With these optimized techniques, we successfully obtained the high-resolution 3D images of NMDA-induced optic neuropathy, including the clues for optic nerve degeneration such as axon swelling, axonal fragmentation, and microglia activation. Overall, we believe that our current study could help understand the pathology of the optic nerve in neurodegenerative diseases, and it will be the basis for translational research.

Dpp/TGFβ-superfamily play a dual conserved role in mediating the damage response in the retina

Retinal homeostasis relies on intricate coordination of cell death and survival in response to stress and damage. Signaling mechanisms that coordinate this process in the adult retina remain poorly understood. Here we identify Decapentaplegic (Dpp) signaling in Drosophila and its mammalian homologue Transforming Growth Factor-beta (TGFβ) superfamily, that includes TGFβ and Bone Morphogenetic Protein (BMP) signaling arms, as central mediators of retinal neuronal death and tissue survival following acute damage. Using a Drosophila model for UV-induced retinal damage, we show that Dpp released from immune cells promotes tissue loss after UV-induced retinal damage. Interestingly, we find a dynamic response of retinal cells to this signal: in an early phase, Dpp-mediated stimulation of Saxophone/Smox signaling promotes apoptosis, while at a later stage, stimulation of the Thickveins/Mad axis promotes tissue repair and survival. This dual role is conserved in the mammalian retina through the TGFβ/BMP signaling, as supplementation of BMP4 or inhibition of TGFβ using small molecules promotes retinal cell survival, while inhibition of BMP negatively affects cell survival after light-induced photoreceptor damage and NMDA induced inner retinal neuronal damage. Our data identify key evolutionarily conserved mechanisms by which retinal homeostasis is maintained.

Elevated histamine levels in aqueous humor of patients with glaucoma

PURPOSE

Neurotransmitters (NTs) are the key mediators of essential ocular functions, such as processing the visual functions of the retina, maintaining homeostasis of aqueous humor, and regulating ocular blood flow. This study aims to determine variations in the levels of L-glutamate and γ-aminobutyric acid (GABA), histaminergic, adrenergic, cholinergic, and serotonergic NTs in patients with primary glaucoma versus patients with cataract.

METHODS

This case-control study involved three age-matched groups of patients with primary open angle glaucoma (POAG, n = 14), primary angle closure glaucoma (PACG, n = 21), and cataract (control, n = 19). Patients' aqueous humor and plasma were collected, snap frozen at -80 °C, and subjected to ultrasensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis for quantification of NTs.

RESULTS

Baseline intraocular pressure and the cup-to-disc ratio were found to be statistically significantly elevated in the POAG and PACG groups compared to the cataract control group. In aqueous humor, histamine was found to be statistically significantly elevated (5-fold, p<0.0001), whereas 1-methyl histamine was statistically significantly decreased (p<0.05) in POAG compared to the control group. A statistically significant increase in L-glutamate and GABA was observed among both patient groups with glaucoma compared to the cataract control group. Adrenaline was found to be elevated only in the PACG group (2.7-fold, p<0.05). No statistically significant difference was observed among the plasma NT levels between the groups.

CONCLUSIONS

This study demonstrated the prominent role of the histaminergic system apart from autonomic mechanisms in the progression of glaucoma. Elevated L-glutamate and GABA could be due to retinal ganglionic cell death. Further studies are required to evaluate the effects of histamine on Müller cell dysfunction.

The Role of HIF1α-PFKFB3 Pathway in Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of blindness for adults in developed countries. Both microvasculopathy and neurodegeneration are implicated in mechanisms of DR development, with neuronal impairment preceding microvascular abnormalities, which is often underappreciated in the clinic. Most current therapeutic strategies, including anti-vascular endothelial growth factor (anti-VEGF)-antibodies, aim at treating the advanced stages (diabetic macular edema and proliferative diabetic retinopathy) and fail to target the neuronal deterioration. Hence, new therapeutic approach(es) intended to address both vascular and neuronal impairment are urgently needed. The hypoxia-inducible factor 1α (HIF1α)-6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) pathway is critically implicated in the islet pathology of diabetes. Recent evidence highlighted the pathway relevance for pathologic angiogenesis and neurodegeneration, two key aspects in DR. PFKFB3 is key to the sprouting angiogenesis, along with VEGF, by determining the endothelial tip-cell competition. Also, PFKFB3-driven glycolysis compromises the antioxidative capacity of neurons leading to neuronal loss and reactive gliosis. Therefore, the HIF1α-PFKFB3 signaling pathway is unique as being a pervasive pathological component across multiple cell types in the retina in the early as well as late stages of DR. A metabolic point-of-intervention based on HIF1α-PFKFB3 targeting thus deserves further consideration in DR.

Systemic treatment with 7,8-Dihydroxiflavone activates TtkB and affords protection of two different retinal ganglion cell populations against axotomy in adult rats

PURPOSE

To analyze responses of different RGC populations to left intraorbital optic nerve transection (IONT) and intraperitoneal (i.p.) treatment with 7,8-Dihydroxyflavone (DHF), a potent selective TrkB agonist.

METHODS

Adult albino Sprague-Dawley rats received, following IONT, daily i.p. injections of vehicle (1%DMSO in 0.9%NaCl) or DHF. Group-1 (n = 58) assessed at 7days (d) the optimal DHF amount (1-25 mg/kg). Group-2, using freshly dissected naïve or treated retinas (n = 28), investigated if DHF treatment was associated with TrkB activation using Western-blotting at 1, 3 or 7d. Group-3 (n = 98) explored persistence of protection and was analyzed at survival intervals from 7 to 60d after IONT. Groups 2-3 received daily i.p. vehicle or DHF (5 mg/kg). Retinal wholemounts were immunolabelled for Brn3a and melanopsin to identify Brn3a⁺RGCs and m⁺RGCs, respectively.

RESULTS

Optimal neuroprotection was achieved with 5 mg/kg DHF and resulted in TrkB phosphorylation. The percentage of surviving Brn3a⁺RGCs in vehicle treated rats was 60, 28, 18, 13, 12 or 8% of the original value at 7, 10, 14, 21, 30 or 60d, respectively, while in DHF treated retinas was 94, 70, 64, 17, 10 or 9% at the same time intervals. The percentages of m⁺RGCs diminished by 7d-13%, and recovered by 14d-38% in vehicle-treated and to 48% in DHF-treated retinas, without further variations.

CONCLUSIONS

DHF neuroprotects Brn3a ⁺ RGCs and m ⁺ RGCs; its protective effects for Brn3a⁺RGCs are maximal at 7 days but still significant at 21d, whereas for m⁺RGCs neuroprotection was significant at 14d and permanent.

Visual Disfunction due to the Selective Effect of Glutamate Agonists on Retinal Cells

One of the causes of nervous system degeneration is an excess of glutamate released upon several diseases. Glutamate analogs, like N-methyl-DL-aspartate (NMDA) and kainic acid (KA), have been shown to induce experimental retinal neurotoxicity. Previous results have shown that NMDA/KA neurotoxicity induces significant changes in the full field electroretinogram response, a thinning on the inner retinal layers, and retinal ganglion cell death. However, not all types of retinal neurons experience the same degree of injury in response to the excitotoxic stimulus. The goal of the present work is to address the effect of intraocular injection of different doses of NMDA/KA on the structure and function of several types of retinal cells and their functionality. To globally analyze the effect of glutamate receptor activation in the retina after the intraocular injection of excitotoxic agents, a combination of histological, electrophysiological, and functional tools has been employed to assess the changes in the retinal structure and function. Retinal excitotoxicity caused by the intraocular injection of a mixture of NMDA/KA causes a harmful effect characterized by a great loss of bipolar, amacrine, and retinal ganglion cells, as well as the degeneration of the inner retina. This process leads to a loss of retinal cell functionality characterized by an impairment of light sensitivity and visual acuity, with a strong effect on the retinal OFF pathway. The structural and functional injury suffered by the retina suggests the importance of the glutamate receptors expressed by different types of retinal cells. The effect of glutamate agonists on the OFF pathway represents one of the main findings of the study, as the evaluation of the retinal lesions caused by excitotoxicity could be specifically explored using tests that evaluate the OFF pathway.

Elevated dimethylarginine, ATP, cytokines, metabolic remodeling involving tryptophan metabolism and potential microglial inflammation characterize primary open angle glaucoma

Glaucoma of which primary open angle glaucoma (POAG) constitutes 75%, is the second leading cause of blindness. Elevated intra ocular pressure and Nitric oxide synthase (NOS) dysfunction are hallmarks of POAG. We analyzed clinical data, cytokine profile, ATP level, metabolomics and GEO datasets to identify features unique to POAG. N9 microglial cells are used to gain mechanistic insights. Our POAG cohort showed elevated ATP in aqueous humor and cytokines in plasma. Metabolomic analysis showed changes in 21 metabolites including Dimethylarginine (DMAG) and activation of tryptophan metabolism in POAG. Analysis of GEO data sets and previously published proteomic data sets bins genes into signaling and metabolic pathways. Pathways from reanalyzed metabolomic data from literature significantly overlapped with those from our POAG data. DMAG modulated purinergic signaling, ATP secretion and cytokine expression were inhibited by N-Ethylmaleimide, NO donors, BAPTA and purinergic receptor inhibitors. ATP induced elevated intracellular calcium level and cytokines expression were inhibited by BAPTA. Metabolomics of cell culture supernatant from ATP treated sets showed metabolic deregulation and activation of tryptophan metabolism. DMAG and ATP induced IDO1/2 and TDO2 were inhibited by N-Ethylmaleimide, sodium nitroprusside and BAPTA. Our data obtained from clinical samples and cell culture studies reveal a strong association of elevated DMAG, ATP, cytokines and activation of tryptophan metabolism with POAG. DMAG mediated ATP signaling, inflammation and metabolic remodeling in microglia might have implications in management of POAG.

Lepidium sativum as candidate against excitotoxicity in retinal ganglion cells

Glutamate excitotoxicity is considered one of the major causes of retinal ganglion cell death in many retinal diseases. Retinal ganglion cell degeneration causes severe blindness since visual signals from the eye to the brain are conducted only through retinal ganglion cells.

OBJECTIVE

We aimed to explore the potential ameliorative effects of L. sativum against glutamate excitotoxicity-induced retinal ganglion cell damage.

METHODS

Pure retinal ganglion cells were divided into a control group (untreated); L. sativum-treated groups in which retinal ganglion cells were treated with 5, 10, 50, or 100 µg/mL L. sativum seed extract for 2 h; glutamate-treated groups in which cells were treated with 5, 10, 50, or 100 µM glutamate for 48 h; and L. sativum/glutamate groups [pretreatment with L. sativum for 2 h (50 or 100 µg/mL) before glutamate treatment at 100 µM for 48 h]. Cell damage was assessed by comet assay and cell viability was by MTT test.

RESULTS

Tailed DNA, tail length, and tail moment of the 50 and 100 mM glutamate-treated groups were significantly greater than those of the blank control group, while the L. sativum-treated groups demonstrated nonsignificantly different tailed DNA, tail length, and tail moment compared with the blank control group, but significantly lower values compared with the glutamate-treated groups.

CONCLUSION

L. sativum ameliorated the cell viability in retinal ganglion cells after high-concentration glutamate exposure. L. sativum seed extracts were efficient anti-excitotoxic and antioxidant agent that might improve the clinical presentation of many neurological disorders.

Probing ON and OFF Retinal Pathways in Glaucoma Using Electroretinography

Glaucoma is a progressive neurodegenerative disease involving damage and eventually death of retinal ganglion cells (RGCs) that comprise the optic nerve. This review summarizes current understanding of specific RGC type vulnerability in glaucoma and how electroretinography (ERG) may provide an objective measure of these functional perturbations. There is building evidence to suggest that ON RGCs, which respond to light increments, may be more resilient to elevated intraocular pressure and glaucoma, whereas OFF RGCs, which respond to light decrements, may be more susceptible. ERG experiments in nonhuman primates and mice have also shown that the ON- and OFF-pathways can be separated using a variety of techniques such as pattern ERG and the photopic negative response. Another ERG paradigm of interest to separate the ON and OFF responses is a flicker stimulus at varying temporal frequencies. Response to lower temporal frequencies is associated with the ON-pathway, and ERG response to higher frequencies is associated with the OFF-pathway. In mice, experimental glaucoma models have shown greater decreases in ERG response at higher frequencies, suggesting that the OFF-pathway is more susceptible. We also summarize current clinical ERG protocols used for glaucoma and discuss innovations for developing new types of stimuli that can further separate the ON- and OFF-pathways. Applying these novel paradigms that distinguish ON- and OFF-pathways may ultimately improve glaucoma diagnostics and monitoring of glaucoma progression.

Tyro3 Contributes to Retinal Ganglion Cell Function, Survival and Dendritic Density in the Mouse Retina

Retinal ganglion cells (RGCs) are the only output neurons of the vertebrate retina, integrating signals from other retinal neurons and transmitting information to the visual centers of the brain. The death of RGCs is a common outcome in many optic neuropathies, such as glaucoma, demyelinating optic neuritis and ischemic optic neuropathy, resulting in visual defects and blindness. There are currently no therapies in clinical use which can prevent RGC death in optic neuropathies; therefore, the identification of new targets for supporting RGC survival is crucial in the development of novel treatments for eye diseases. In this study we identify that the receptor tyrosine kinase, Tyro3, is critical for normal neuronal function in the adult mouse retina. The loss of Tyro3 results in a reduction in photoreceptor and RGC function as measured using electroretinography. The reduction in RGC function was associated with a thinner retinal nerve fiber layer and fewer RGCs. In the central retina, independent of the loss of RGCs, Tyro3 deficiency resulted in a dramatic reduction in the number of RGC dendrites in the inner plexiform layer. Our results show that Tyro3 has a novel, previously unidentified role in retinal function, RGC survival and RGC morphology. The Tyro3 pathway could therefore provide an alternative, targetable pathway for RGC protective therapeutics.

Expression of vesicular glutamate transporter 2 and 3 and glutamate receptor 1 and 2 mRNAs in the retina of adult laughing doves (Streptopelia senegalensis): An in situ hybridization study

The retina possesses few types of neurons so; it is considered an excellent model for understanding the neural mechanisms underlying basic neural information processing in the brain. Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system and retina. The present study was carried out to characterize the expression pattern of vesicular glutamate transporter 2 (Vglut2) and 3 (Vglut3) and glutamate receptor 1 (GluR1) and 2 (GluR2) mRNAs in the retina of adult laughing dove (Streptopelia senegalensis) by RT-PCR and in situ hybridization histochemistry. The cerebellum of adult laughing dove was used as a positive control in this study. Vglut2 mRNA was highly expressed only in the granular layer of the cerebellum while Vglut3 mRNA was weakly expressed only in the Purkinje cells layer. In the retina, Vglut2 mRNA was highly expressed in the ganglion cell layer and moderately expressed in the inner nuclear layer while Vglut3 mRNA was moderately expressed only in the inner nuclear layer. GluR1 mRNA was intensely expressed in the Purkinje cells layer while GluR2 mRNA signals were highly detectable in both granular and Purkinje cells layers of the cerebellum. In the retina, moderate expression of GluR1 and intense expression of GluR2 was found in both ganglion cell layer and the internal half of inner nuclear layer mostly amacrine cells. These results suggest that some retinal neuronal cells in the adult laughing dove are glutamatergic. Therefore, GluR1 and 2 are suggested as useful markers for glutamatergic retinal neuronal cells in the adult laughing doves.

Neuroprotective Strategies for Retinal Ganglion Cell Degeneration: Current Status and Challenges Ahead

The retinal ganglion cells (RGCs) are the output cells of the retina into the brain. In mammals, these cells are not able to regenerate their axons after optic nerve injury, leaving the patients with optic neuropathies with permanent visual loss. An effective RGCs-directed therapy could provide a beneficial effect to prevent the progression of the disease. Axonal injury leads to the functional loss of RGCs and subsequently induces neuronal death, and axonal regeneration would be essential to restore the neuronal connectivity, and to reestablish the function of the visual system. The manipulation of several intrinsic and extrinsic factors has been proposed in order to stimulate axonal regeneration and functional repairing of axonal connections in the visual pathway. However, there is a missing point in the process since, until now, there is no therapeutic strategy directed to promote axonal regeneration of RGCs as a therapeutic approach for optic neuropathies.

The Susceptibility of Retinal Ganglion Cells to Optic Nerve Injury is Type Specific

Retinal ganglion cell (RGC) death occurs in many eye diseases, such as glaucoma and traumatic optic neuropathy (TON). Increasing evidence suggests that the susceptibility of RGCs varies to different diseases in an RGC type-dependent manner. We previously showed that the susceptibility of several genetically identified RGC types to N-methyl-D-aspartate (NMDA) excitotoxicity differs significantly. In this study, we characterize the susceptibility of the same RGC types to optic nerve crush (ONC). We show that the susceptibility of these RGC types to ONC varies significantly, in which BD-RGCs are the most resistant RGC type while W3-RGCs are the most sensitive cells to ONC. We also show that the survival rates of BD-RGCs and J-RGCs after ONC are significantly higher than their survival rates after NMDA excitotoxicity. These results are consistent with the conclusion that the susceptibility of RGCs to ONC varies in an RGC type-dependent manner. Further, the susceptibilities of the same types of RGCs to ONC and NMDA excitotoxicity are significantly different. These are valuable insights for understanding of the selective susceptibility of RGCs to various pathological insults and the development of a strategy to protect RGCs from death in disease conditions.

Deleterious Effect of NMDA Plus Kainate on the Inner Retinal Cells and Ganglion Cell Projection of the Mouse

Combined administration of N-Methyl-D-Aspartate (NMDA) and kainic acid (KA) on the inner retina was studied as a model of excitotoxicity. The right eye of C57BL6J mice was injected with 1 µL of PBS containing NMDA 30 mM and KA 10 mM. Only PBS was injected in the left eye. One week after intraocular injection, electroretinogram recordings and immunohistochemistry were performed on both eyes. Retinal ganglion cell (RGC) projections were studied by fluorescent-cholerotoxin anterograde labeling. A clear decrease of the retinal "b" wave amplitude, both in scotopic and photopic conditions, was observed in the eyes injected with NMDA/KA. No significant effect on the "a" wave amplitude was observed, indicating the preservation of photoreceptors. Immunocytochemical labeling showed no effects on the outer nuclear layer, but a significant thinning on the inner retinal layers, thus indicating that NMDA and KA induce a deleterious effect on bipolar, amacrine and ganglion cells. Anterograde tracing of the visual pathway after NMDA and KA injection showed the absence of RGC projections to the contralateral superior colliculus and lateral geniculate nucleus. We conclude that glutamate receptor agonists, NMDA and KA, induce a deleterious effect of the inner retina when injected together into the vitreous chamber.

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.

Melanopsin+RGCs Are fully Resistant to NMDA-Induced Excitotoxicity

We studied short- and long-term effects of intravitreal injection of N-methyl-d-aspartate (NMDA) on melanopsin-containing (m⁺) and non-melanopsin-containing (Brn3a⁺) retinal ganglion cells (RGCs). In adult SD-rats, the left eye received a single intravitreal injection of 5µL of 100nM NMDA. At 3 and 15 months, retinal thickness was measured in vivo using Spectral Domain-Optical Coherence Tomography (SD-OCT). Ex vivo analyses were done at 3, 7, or 14 days or 15 months after damage. Whole-mounted retinas were immunolabelled for brain-specific homeobox/POU domain protein 3A (Brn3a) and melanopsin (m), the total number of Brn3a⁺RGCs and m⁺RGCs were quantified, and their topography represented. In control retinas, the mean total numbers of Brn3a⁺RGCs and m⁺RGCs were 78,903 ± 3572 and 2358 ± 144 (mean ± SD; n = 10), respectively. In the NMDA injected retinas, Brn3a⁺RGCs numbers diminished to 49%, 28%, 24%, and 19%, at 3, 7, 14 days, and 15 months, respectively. There was no further loss between 7 days and 15 months. The number of immunoidentified m⁺RGCs decreased significantly at 3 days, recovered between 3 and 7 days, and were back to normal thereafter. OCT measurements revealed a significant thinning of the left retinas at 3 and 15 months. Intravitreal injections of NMDA induced within a week a rapid loss of 72% of Brn3a⁺RGCs, a transient downregulation of melanopsin expression (but not m⁺RGC death), and a thinning of the inner retinal layers.