When Is a Control Not a Control? Reactive Microglia Occur Throughout the Control Contralateral Pathway of Retinal Ganglion Cell Projections in Experimental Glaucoma

Integrated Bioinformatics-Based Identification and Validation of Neuroinflammation-Related Hub Genes in Primary Open-Angle Glaucoma

Glaucoma is a leading cause of permanent blindness, affecting 80 million people worldwide. Recent studies have emphasized the importance of neuroinflammation in the early stages of glaucoma, involving immune and glial cells. To investigate this further, we used the GSE27276 dataset from the GEO (Gene Expression Omnibus) database and neuroinflammation genes from the GeneCards database to identify differentially expressed neuroinflammation-related genes associated with primary open-angle glaucoma (POAG). Subsequently, these genes were submitted to Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes for pathway enrichment analyses. Hub genes were picked out through protein-protein interaction networks and further validated using the external datasets (GSE13534 and GSE9944) and real-time PCR analysis. The gene-miRNA regulatory network, receiver operating characteristic (ROC) curve, genome-wide association study (GWAS), and regional expression analysis were performed to further validate the involvement of hub genes in glaucoma. A total of 179 differentially expressed genes were identified, comprising 60 upregulated and 119 downregulated genes. Among them, 18 differentially expressed neuroinflammation-related genes were found to overlap between the differentially expressed genes and neuroinflammation-related genes, with six genes (SERPINA3, LCN2, MMP3, S100A9, IL1RN, and HP) identified as potential hub genes. These genes were related to the IL-17 signaling pathway and tyrosine metabolism. The gene-miRNA regulatory network showed that these hub genes were regulated by 118 miRNAs. Notably, GWAS data analysis successfully identified significant single nucleotide polymorphisms (SNPs) corresponding to these six hub genes. ROC curve analysis indicated that our genes showed significant accuracy in POAG. The expression of these genes was further confirmed in microglia, Müller cells, astrocytes, and retinal ganglion cells in the Spectacle database. Moreover, three hub genes, SERPINA3, IL1R1, and LCN2, were validated as potential diagnostic biomarkers for high-risk glaucoma patients, showing increased expression in the OGD/R-induced glaucoma model. This study suggests that the identified hub genes may influence the development of POAG by regulation of neuroinflammation, and it may offer novel insights into the management of POAG.

Enhanced Synaptic Inhibition in the Dorsolateral Geniculate Nucleus in a Mouse Model of Glaucoma

Elevated intraocular pressure (IOP) triggers glaucoma by damaging the output neurons of the retina called retinal ganglion cells (RGCs). This leads to the loss of RGC signaling to visual centers of the brain such as the dorsolateral geniculate nucleus (dLGN), which is critical for processing and relaying information to the cortex for conscious vision. In response to altered levels of activity or synaptic input, neurons can homeostatically modulate postsynaptic neurotransmitter receptor numbers, allowing them to scale their synaptic responses to stabilize spike output. While prior work has indicated unaltered glutamate receptor properties in the glaucomatous dLGN, it is unknown whether glaucoma impacts dLGN inhibition. Here, using DBA/2J mice, which develop elevated IOP beginning at 6-7 months of age, we tested whether the strength of inhibitory synapses on dLGN thalamocortical relay neurons is altered in response to the disease state. We found an enhancement of feedforward disynaptic inhibition arising from local interneurons along with increased amplitude of quantal inhibitory synaptic currents. A combination of immunofluorescence staining for the γ-aminobutyric acid (GABA)A-α1 receptor subunit, peak-scaled nonstationary fluctuation analysis, and measures of homeostatic synaptic scaling pointed to an ∼1.4-fold increase in GABA receptors at postsynaptic inhibitory synapses, although several pieces of evidence indicate a nonuniform scaling across inhibitory synapses within individual relay neurons. Together, these results indicate an increase in inhibitory synaptic strength in the glaucomatous dLGN, potentially pointing toward homeostatic compensation for disruptions in network and neuronal function triggered by increased IOP.

Microglia target synaptic sites early during excitatory circuit disassembly in neurodegeneration

During development, microglia prune excess synapses to refine neuronal circuits. In neurodegeneration, the role of microglia-mediated synaptic pruning in circuit remodeling and dysfunction is important for developing therapies aimed at modulating microglial function. Here we analyzed the role of microglia in the synapse disassembly of degenerating postsynaptic neurons in the inner retina. After inducing transient intraocular pressure elevation to injure retinal ganglion cells, microglia increase in number, shift to ameboid morphology, and exhibit greater process movement. Furthermore, due to the greater number of microglia, there is increased colocalization of microglia with synaptic components throughout the inner plexiform layer and with excitatory synaptic sites along individual ganglion cell dendrites. Microglia depletion partially restores ganglion cell function, suggesting that microglia activation may be neurotoxic in early neurodegeneration. Our results demonstrate the important role of microglia in synapse disassembly in degenerating circuits, highlighting their recruitment to synaptic sites early after neuronal injury.

Nonuniform scaling of synaptic inhibition in the dorsolateral geniculate nucleus in a mouse model of glaucoma

Elevated intraocular pressure (IOP) triggers glaucoma by damaging the output neurons of the retina called retinal ganglion cells (RGCs). This leads to the loss of RGC signaling to visual centers of the brain such as the dorsolateral geniculate nucleus (dLGN), which is critical for processing and relaying information to the cortex for conscious vision. In response to altered levels of activity or synaptic input, neurons can homeostatically modulate postsynaptic neurotransmitter receptor numbers, allowing them to scale their synaptic responses to stabilize spike output. While prior work has indicated unaltered glutamate receptor properties in the glaucomatous dLGN, it is unknown whether glaucoma impacts dLGN inhibition. Here, using DBA/2J mice, which develop elevated IOP beginning at 6-7 months of age, we tested whether the strength of inhibitory synapses on dLGN thalamocortical relay neurons is altered in response to the disease state. We found an enhancement of feed-forward disynaptic inhibition arising from local interneurons along with increased amplitude of quantal inhibitory synaptic currents. A combination of immunofluorescence staining for the GABA A -α1 receptor subunit, peak-scaled nonstationary fluctuation analysis, and measures of homeostatic synaptic scaling indicated this was the result of an approximately 1.4-fold increase in GABA receptor number at post-synaptic inhibitory synapses, although several pieces of evidence strongly indicate a non-uniform scaling across inhibitory synapses within individual relay neurons. Together, these results indicate an increase in inhibitory synaptic strength in the glaucomatous dLGN, potentially pointing toward homeostatic compensation for disruptions in network and neuronal function triggered by increased IOP.

Significance Statement

Elevated eye pressure in glaucoma leads to loss of retinal outputs to the dorsolateral geniculate nucleus (dLGN), which is critical for relaying information to the cortex for conscious vision. Alterations in neuronal activity, as could arise from excitatory synapse loss, can trigger homeostatic adaptations to synaptic function that attempt to maintain activity within a meaningful dynamic range, although whether this occurs uniformly at all synapses within a given neuron or is a non-uniform process is debated. Here, using a mouse model of glaucoma, we show that dLGN inhibitory synapses undergo non-uniform upregulation due to addition of post-synaptic GABA receptors. This is likely to be a neuronal adaptation to glaucomatous pathology in an important sub-cortical visual center.

Prophylactic nicotinamide treatment protects from rotenone-induced neurodegeneration by increasing mitochondrial content and volume

Leber's hereditary optic neuropathy (LHON) is driven by mtDNA mutations affecting Complex I presenting as progressive retinal ganglion cell dysfunction usually in the absence of extra-ophthalmic symptoms. There are no long-term neuroprotective agents for LHON. Oral nicotinamide provides a robust neuroprotective effect against mitochondrial and metabolic dysfunction in other retinal injuries. We explored the potential for nicotinamide to protect mitochondria in LHON by modelling the disease in mice through intravitreal injection of the Complex I inhibitor rotenone. Using MitoV mice expressing a mitochondrial-tagged YFP in retinal ganglion cells we assessed mitochondrial morphology through super-resolution imaging and digital reconstruction. Rotenone induced Complex I inhibition resulted in retinal ganglion cell wide mitochondrial loss and fragmentation. This was prevented by oral nicotinamide treatment. Mitochondrial ultrastructure was quantified by transition electron microscopy, demonstrating a loss of cristae density following rotenone injection, which was also prevented by nicotinamide treatment. These results demonstrate that nicotinamide protects mitochondria during Complex I dysfunction. Nicotinamide has the potential to be a useful treatment strategy for LHON to limit retinal ganglion cell degeneration.

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.

Pyrroloquinoline quinone drives ATP synthesis in vitro and in vivo and provides retinal ganglion cell neuroprotection

Retinal ganglion cells are highly metabolically active requiring strictly regulated metabolism and functional mitochondria to keep ATP levels in physiological range. Imbalances in metabolism and mitochondrial mechanisms can be sufficient to induce a depletion of ATP, thus altering retinal ganglion cell viability and increasing cell susceptibility to death under stress. Altered metabolism and mitochondrial abnormalities have been demonstrated early in many optic neuropathies, including glaucoma, autosomal dominant optic atrophy, and Leber hereditary optic neuropathy. Pyrroloquinoline quinone (PQQ) is a quinone cofactor and is reported to have numerous effects on cellular and mitochondrial metabolism. However, the reported effects are highly context-dependent, indicating the need to study the mechanism of PQQ in specific systems. We investigated whether PQQ had a neuroprotective effect under different retinal ganglion cell stresses and assessed the effect of PQQ on metabolic and mitochondrial processes in cortical neuron and retinal ganglion cell specific contexts. We demonstrated that PQQ is neuroprotective in two models of retinal ganglion cell degeneration. We identified an increased ATP content in healthy retinal ganglion cell-related contexts both in in vitro and in vivo models. Although PQQ administration resulted in a moderate effect on mitochondrial biogenesis and content, a metabolic variation in non-diseased retinal ganglion cell-related tissues was identified after PQQ treatment. These results suggest the potential of PQQ as a novel neuroprotectant against retinal ganglion cell death.

Neuroprotection in glaucoma: Mechanisms beyond intraocular pressure lowering

Glaucoma is a common, complex, multifactorial neurodegenerative disease characterized by progressive dysfunction and then loss of retinal ganglion cells, the output neurons of the retina. Glaucoma is the most common cause of irreversible blindness and affects ∼80 million people worldwide with many more undiagnosed. The major risk factors for glaucoma are genetics, age, and elevated intraocular pressure. Current strategies only target intraocular pressure management and do not directly target the neurodegenerative processes occurring at the level of the retinal ganglion cell. Despite strategies to manage intraocular pressure, as many as 40% of glaucoma patients progress to blindness in at least one eye during their lifetime. As such, neuroprotective strategies that target the retinal ganglion cell and these neurodegenerative processes directly are of great therapeutic need. This review will cover the recent advances from basic biology to on-going clinical trials for neuroprotection in glaucoma covering degenerative mechanisms, metabolism, insulin signaling, mTOR, axon transport, apoptosis, autophagy, and neuroinflammation. With an increased understanding of both the basic and clinical mechanisms of the disease, we are closer than ever to a neuroprotective strategy for glaucoma.

MorphoMacro for in vivo and ex vivo quantitative morphometric analysis of microglia

Microglia cells dynamically survey the central nervous system microenvironment and, in response to tissue damage inflicted by radiation therapy, disease or infection, undergo morphological and functional changes that culminate in microglia activation. Cell shape transformation can be assessed descriptively or, alternatively, it can be quantified as a continuous variable for parameters including total cell size as well as protrusion length, ramification and complexity. The purpose of the MorphoMacro method is to quantitatively profile multiple and single microglia cells using the available ImageJ platform. This method outlines the required steps and ImageJ plugins to convert fluorescence and bright-field photomicrographs into representative binary and skeletonized images and to analyze them using the MorphoMacro software plugin for multiparametric and multilevel description of microglia cell morphology in vivo and ex vivo. Overall, the protocol provides a quantitative and comprehensive tool that can be used to identify, stratify, and monitor diverse microglia morphologies in homeostatic, different disease conditions and subsequent therapeutic monitoring.

Microglia depletion exacerbates retinal ganglion cell loss in a mouse model of glaucoma

To test whether depletion of microglia in the optic nerve head has a beneficial effect on retinal ganglion cell numbers and function, we depleted microglia by oral administration of the CSF1R antagonist PLX5622. Then, ocular hypertension was induced by unilateral injection of magnetic microbeads into the anterior chamber. Visual function was assessed with pattern electroretinography and measurement of the optomotor reflex. Retinal ganglion cell bodies and axons were counted and gene expression patterns in optic nerve head astrocytes were tested on freshly dissociated astrocytes. PLX5622 efficiently depleted microglia in the retina and the optic nerve head, but about 20% of microglia persisted in the myelinated optic nerve proper even after prolonged exposure to the drug. PLX5622 did not affect ganglion cell function by itself. Elevation of the IOP for four weeks led to the expected decrease in visual acuity and pattern ERG amplitude. Microglia ablation did not affect these parameters. Ganglion cell and axon numbers were counted histologically post mortem. Mice in the microglia depletion group showed a moderate but significantly greater loss of ganglion cells than the control group. At four weeks post microbead injection, gene expression patterns in optic nerve head astrocytes are consistent with an A2 (or neuroprotective) pattern. Microglia depletion blunted the up-regulation of A2 genes in astrocytes. In conclusion, microglia depletion is unlikely to protect retinal ganglion cells in early glaucoma.

Optic Nerve Head Myelin-Related Protein, GFAP, and Iba1 Alterations in Non-Human Primates With Early to Moderate Experimental Glaucoma

Purpose

The purpose of this study was to test if optic nerve head (ONH) myelin basic protein (MBP), 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNPase), glial fibrillary acidic protein (GFAP), and ionized calcium binding adaptor molecule 1 (Iba1) proteins are altered in non-human primate (NHP) early/moderate experimental glaucoma (EG).

Methods

Following paraformaldehyde perfusion, control and EG eye ONH tissues from four NHPs were paraffin embedded and serially (5 µm) vertically sectioned. Anti-MBP, CNPase, GFAP, Iba1, and nuclear dye-stained sections were imaged using sub-saturating light intensities. Whole-section images were segmented creating anatomically consistent laminar (L) and retrolaminar (RL) regions/sub-regions. EG versus control eye intensity/pixel-cluster density data within L and two RL regions (RL1 [1-250 µm]/RL2 [251-500 µm] from L) were compared using random effects models within the statistical program “R.”

Results

EG eye retinal nerve fiber loss ranged from 0% to 20%. EG eyes’ MBP and CNPase intensity were decreased within the RL1 (MBP = 31.4%, P < 0.001; CNPase =62.3%, P < 0.001) and RL2 (MBP = 19.6%, P < 0.001; CNPase = 56.1%, P = 0.0004) regions. EG eye GFAP intensity was decreased in the L (41.6%, P < 0.001) and RL regions (26.7% for RL1, and 28.4% for RL2, both P < 0.001). Iba1+ and NucBlue pixel-cluster density were increased in the laminar (28.2%, P = 0.03 and 16.6%, P = 0.008) and both RL regions (RL1 = 37.3%, P = 0.01 and 23.7%, P = 0.0002; RL2 = 53.7%, P = 0.002 and 33.2%, P < 0.001).

Conclusions

Retrolaminar myelin disruption occurs early in NHP EG and may be accompanied by laminar and retrolaminar decreases in astrocyte process labeling and increases in microglial/ macrophage density. The mechanistic and therapeutic implications of these findings warrant further study.

Axon hyperexcitability in the contralateral projection following unilateral optic nerve crush in mice

Optic neuropathies are characterized by degeneration of retinal ganglion cell axonal projections to the brain, including acute conditions like optic nerve trauma and progressive conditions such as glaucoma. Despite different aetiologies, retinal ganglion cell axon degeneration in traumatic optic neuropathy and glaucoma share common pathological signatures. We compared how early pathogenesis of optic nerve trauma and glaucoma influence axon function in the mouse optic projection. We assessed pathology by measuring anterograde axonal transport from retina to superior colliculus, current-evoked optic nerve compound action potential and retinal ganglion cell density 1 week following unilateral optic nerve crush or intraocular pressure elevation. Nerve crush reduced axon transport, compound axon potential and retinal ganglion cell density, which were unaffected by intraocular pressure elevation. Surprisingly, optic nerves contralateral to crush demonstrated 5-fold enhanced excitability in compound action potential compared with naïve nerves. Enhanced excitability in contralateral sham nerves is not due to increased accumulation of voltage-gated sodium channel 1.6, or ectopic voltage-gated sodium channel 1.2 expression within nodes of Ranvier. Our results indicate hyperexcitability is driven by intrinsic responses of αON-sustained retinal ganglion cells. We found αON-sustained retinal ganglion cells in contralateral, sham and eyes demonstrated increased responses to depolarizing currents compared with those from naïve eyes, while light-driven responses remained intact. Dendritic arbours of αON-sustained retinal ganglion cells of the sham eye were like naïve, but soma area and non-phosphorylated neurofilament H increased. Current- and light-evoked responses of sham αOFF-sustained retinal ganglion cells remained stable along with somato-dendritic morphologies. In retinas directly affected by crush, light responses of αON- and αOFF-sustained retinal ganglion cells diminished compared with naïve cells along with decreased dendritic field area or branch points. Like light responses, αOFF-sustained retinal ganglion cell current-evoked responses diminished, but surprisingly, αON-sustained retinal ganglion cell responses were similar to those from naïve retinas. Optic nerve crush reduced dendritic length and area in αON-sustained retinal ganglion cells in eyes ipsilateral to injury, while crush significantly reduced dendritic branching in αOFF-sustained retinal ganglion cells. Interestingly, 1 week of intraocular pressure elevation only affected αOFF-sustained retinal ganglion cell physiology, depolarizing resting membrane potential in cells of affected eyes and blunting current-evoked responses in cells of saline-injected eyes. Collectively, our results suggest that neither saline nor sham surgery provide a true control, chronic versus acute optic neuropathies differentially affect retinal ganglion cells composing the ON and OFF pathways, and acute stress can have near-term effects on the contralateral projection.

Apolipoprotein E4 impairs the response of neurodegenerative retinal microglia and prevents neuronal loss in glaucoma

The apolipoprotein E4 (APOE4) allele is associated with an increased risk of Alzheimer disease and a decreased risk of glaucoma, but the underlying mechanisms remain poorly understood. Here, we found that in two mouse glaucoma models, microglia transitioned to a neurodegenerative phenotype characterized by upregulation of Apoe and Lgals3 (Galectin-3), which were also upregulated in human glaucomatous retinas. Mice with targeted deletion of Apoe in microglia or carrying the human APOE4 allele were protected from retinal ganglion cell (RGC) loss, despite elevated intraocular pressure (IOP). Similarly to Apoe-/- retinal microglia, APOE4-expressing microglia did not upregulate neurodegeneration-associated genes, including Lgals3, following IOP elevation. Genetic and pharmacologic targeting of Galectin-3 ameliorated RGC degeneration, and Galectin-3 expression was attenuated in human APOE4 glaucoma samples. These results demonstrate that impaired activation of APOE4 microglia is protective in glaucoma and that the APOE-Galectin-3 signaling can be targeted to treat this blinding disease.

Widespread retina and optic nerve neuroinflammation in enucleated eyes from glaucoma patients

Neuroinflammation is recognized as a key component of neurodegenerative disease. In glaucoma, a common neurodegenerative disease and the leading cause of irreversible blindness, the evidence for neuroinflammation in patients is lacking. Animal models have demonstrated significant pro-inflammatory activation of resident glia in the retina, as well as influx of blood-derived monocytes and pro-inflammatory factors. Confirmation of this in human donor tissue has been challenging due to a lack of well-preserved and well-characterized post-mortem tissue. To address this we utilize archived, wax embedded eyes fixed immediately following enucleation from living glaucoma patients. We compared glaucoma to control eyes (enucleated for uveal melanoma where the tumor did not impact the central retina or optic nerve). We performed immunolabelling for neurodegenerative and glial markers (CD45, CD163, IBA1, GFAP, Vimentin) which were quantified by high-resolution light microscopy and image analysis in FIJI. Glaucoma eyes demonstrated significant neural loss consistent with advanced neurodegeneration. IBA1 and GFAP were significantly increased in the retina and optic nerve head of the glaucomatous eyes indicating that significant neuroinflammation had occurred which support findings in animal models. Inflammation is a treatable symptom of many diseases and as such, identification of earlier inflammatory processes in glaucoma could be important for potential future treatment options.

Valproic Acid Reduces Neuroinflammation to Provide Retinal Ganglion Cell Neuroprotection in the Retina Axotomy Model

Neuroinflammation is a critical and targetable pathogenic component of neurodegenerative diseases, including glaucoma, the leading cause of irreversible blindness. Valproic acid has previously been demonstrated to reduce neuroinflammation and is neuroprotective in a number of experimental settings. To determine whether valproic acid can limit retinal neuroinflammation and protect retinal neurons we used an ex vivo retina explant (axotomy) model to isolate resident glial responses from blood-derived monocytes. Neuroinflammatory status was defined using high resolution confocal imaging with 3D morphological reconstruction and cytokine protein arrays. Valproic acid significantly reduced microglia and astrocyte morphological changes, consistent with a reduction in pro-inflammatory phenotypes. Cytokine profiling demonstrated that valproic acid significantly attenuated or prevented expression of pro-inflammatory cytokines in injured retina. This identifies that the retinal explant model as a useful tool to explore resident neuroinflammation in a rapid timescale whilst maintaining a complex system of cell interactions and valproic acid as a useful drug to further explore anti-neuroinflammatory strategies in retinal disease.

Molecular regulation of neuroinflammation in glaucoma: Current knowledge and the ongoing search for new treatment targets

Neuroinflammation relying on the inflammatory responses of glial cells has emerged as an impactful component of the multifactorial etiology of neurodegeneration in glaucoma. It has become increasingly evident that despite early adaptive and reparative features of glial responses, prolonged reactivity of the resident glia, along with the peripheral immune cells, create widespread toxicity to retinal ganglion cell (RGC) axons, somas, and synapses. As much as the synchronized responses of astrocytes and microglia to glaucoma-related stress or neuron injury, their bi-directional interactions are critical to build and amplify neuroinflammation and to dictate the neurodegenerative outcome. Although distinct molecular programs regulate somatic and axonal degeneration in glaucoma, inhibition of neurodegenerative inflammation can provide a broadly beneficial treatment strategy to rescue RGC integrity and function. Since inflammatory toxicity and mitochondrial dysfunction are converging etiological paths that can boost each other and feed into a vicious cycle, anti-inflammatory treatments may also offer a multi-target potential. This review presents an overview of the current knowledge on neuroinflammation in glaucoma with particular emphasis on the cell-intrinsic and cell-extrinsic factors involved in the reciprocal regulation of glial responses, the interdependence between inflammatory and mitochondrial routes of neurodegeneration, and the research aspects inspiring for prospective immunomodulatory treatments. With the advent of powerful technologies, ongoing research on molecular and functional characteristics of glial responses is expected to accumulate more comprehensive and complementary information and to rapidly move the field forward to safe and effective modulation of the glial pro-inflammatory activities, while restoring or augmenting the glial immune-regulatory and neurosupport functions.

Structural and functional plasticity in the dorsolateral geniculate nucleus of mice following bilateral enucleation

Within the nervous system, plasticity mechanisms attempt to stabilize network activity following disruption by injury, disease, or degeneration. Optic nerve injury and age-related diseases can induce homeostatic-like responses in adulthood. We tested this possibility in the thalamocortical (TC) neurons in the dorsolateral geniculate nucleus (dLGN) using patch-clamp electrophysiology, optogenetics, immunostaining, and single-cell dendritic analysis following loss of visual input via bilateral enucleation. We observed progressive loss of vGlut2-positive retinal terminals in the dLGN indicating degeneration post-enucleation that was coincident with changes in microglial morphology indicative of microglial activation. Consistent with the decline of vGlut2 puncta, we also observed loss of retinogeniculate (RG) synaptic function assessed using optogenetic activation of RG axons while performing whole-cell voltage clamp recordings from TC neurons in brain slices. Surprisingly, we did not detect any significant changes in the frequency of miniature post-synaptic currents (mEPSCs) or corticothalamic feedback synapses. Analysis of TC neuron dendritic structure from single-cell dye fills revealed a gradual loss of dendrites proximal to the soma, where TC neurons receive the bulk of RG inputs. Finally, analysis of action potential firing demonstrated that TC neurons have increased excitability following enucleation, firing more action potentials in response to depolarizing current injections. Our findings show that degeneration of the retinal axons/optic nerve and loss of RG synaptic inputs induces structural and functional changes in TC neurons, consistent with neuronal attempts at compensatory plasticity in the dLGN.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

Inhibition of the cysteinyl leukotriene pathways increases survival of RGCs and reduces microglial activation in ocular hypertension

Glaucoma is the second leading cause of blindness worldwide. This multifactorial, neurodegenerative group of diseases is characterized by the progressive loss of retinal ganglion cells (RGCs) and their axons, leading to irreversible visual impairment and blindness. There is a huge unmet and urging need for the development of new and translatable strategies and treatment options to prevent this progressive loss of RGC. Accumulating evidence points towards a critical role of neuroinflammation, in particular microglial cells, in the pathogenesis of glaucoma. Leukotrienes are mediators of neuroinflammation and are involved in many neurodegenerative diseases. Therefore, we tested the leukotriene receptors CysLT1R/GPR17-selective antagonist Montelukast (MTK) for its efficacy to modulate the reactive state of microglia in order to ameliorate RGCs loss in experimental glaucoma. Ocular hypertension (OHT) was induced unilaterally by injection of 8 μm magnetic microbead (MB) into the anterior chamber of female Brown Norway rats. The contralateral, untreated eye served as control. Successful induction of OHT was verified by daily IOP measurement using a TonoLab rebound tonometer. Simultaneously to OHT induction, one group received daily MTK treatment and the control group vehicle solution by oral gavage. Animals were sacrificed 13-15 days after MB injection. Retina and optic nerves (ON) of OHT and contralateral eyes were analyzed by immunofluorescence with specific markers for RGCs (Brn3a), microglial cells/macrophages (Iba1 and CD68), and cysteinyl leukotriene pathway receptors (CysLT1R and GPR17). Protein labeling was documented by confocal microscopy and analyzed with ImageJ plugins. Further, mRNA expression of genes of the inflammatory and leukotriene pathway was analyzed in retinal tissue. MTK treatment resulted in a short-term IOP reduction at day 2, which dissipated by day 5 of OHT induction in MTK treated animals. Furthermore, MTK treatment resulted in a decreased activation of Iba1⁺ microglial cells in the retina and ON, and in a significantly increased RGC survival in OHT eyes. Within the retina, GPR17 and CysLT1R expression was demonstrated in single RCGs and in microglial cells respectively. Further, increased mRNA expression of pro-inflammatory genes was detected in OHT induced retinas. In the ON, OHT induction increased the number of GPR17⁺ cells, showing a trend of reduction following MTK treatment. This study shows for the first time a significantly increased RGC survival in an acute OHT model following treatment with the leukotriene receptor antagonist MTK. These results strongly suggest a neuroprotective effect of MTK and a potential new therapeutic strategy for glaucoma treatment.

Myeloid cells in retinal and brain degeneration

Retinal inflammation underlies multiple prevalent ocular and neurological diseases. Similar inflammatory processes are observed in glaucomatous optic neuropathy, age-related macular degeneration, retinitis pigmentosa, posterior uveitis, Alzheimer's disease, and Parkinson's disease. In particular, human and animal studies have demonstrated the important role microglia/macrophages play in initiating and maintaining a pro-inflammatory environment in degenerative processes impacting vision. On the other hand, microglia have also been shown to have a protective role in multiple central nervous system diseases. Identifying the mechanisms underlying cell dysfunction and death is the first step toward developing novel therapeutics for these diseases impacting the central nervous system. In addition to reviewing recent key studies defining important mediators of retinal inflammation, with an emphasis on translational studies that bridge this research from bench to bedside, we also highlight a promising therapeutic class of medications, the glucagon-like peptide-1 receptor agonists. Finally, we propose areas where additional research is necessary to identify mechanisms that can be modulated to shift the balance from a neurotoxic to a neuroprotective retinal environment.

Multifactorial Pathogenic Processes of Retinal Ganglion Cell Degeneration in Glaucoma towards Multi-Target Strategies for Broader Treatment Effects

Glaucoma is a chronic neurodegenerative disease characterized by apoptosis of retinal ganglion cell (RGC) somas, degeneration of axons, and loss of synapses at dendrites and axon terminals. Glaucomatous neurodegeneration encompasses multiple triggers, multiple cell types, and multiple molecular pathways through the etiological paths with biomechanical, vascular, metabolic, oxidative, and inflammatory components. As much as intrinsic responses of RGCs themselves, divergent responses and intricate interactions of the surrounding glia also play decisive roles for the cell fate. Seen from a broad perspective, multitarget treatment strategies have a compelling pathophysiological basis to more efficiently manipulate multiple pathogenic processes at multiple injury sites in such a multifactorial neurodegenerative disease. Despite distinct molecular programs for somatic and axonal degeneration, mitochondrial dysfunction and glia-driven neuroinflammation present interdependent processes with widespread impacts in the glaucomatous retina and optic nerve. Since dysfunctional mitochondria stimulate inflammatory responses and proinflammatory mediators impair mitochondria, mitochondrial restoration may be immunomodulatory, while anti-inflammatory treatments protect mitochondria. Manipulation of these converging routes may thus allow a unified treatment strategy to protect RGC axons, somas, and synapses. This review presents an overview of recent research advancements with emphasis on potential treatment targets to achieve the best treatment efficacy to preserve visual function in glaucoma.

Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction

Nicotinamide adenine dinucleotide (NAD) is a REDOX cofactor and metabolite essential for neuronal survival. Glaucoma is a common neurodegenerative disease in which neuronal levels of NAD decline. We assess the effects of nicotinamide (a precursor to NAD) on retinal ganglion cells (the affected neuron in glaucoma) in normal physiological conditions and across a range of glaucoma relevant insults including mitochondrial stress and axon degenerative insults. We demonstrate retinal ganglion cell somal, axonal, and dendritic neuroprotection by nicotinamide in rodent models which represent isolated ocular hypertensive, axon degenerative, and mitochondrial degenerative insults. We performed metabolomics enriched for small molecular weight metabolites for the retina, optic nerve, and superior colliculus which demonstrates that ocular hypertension induces widespread metabolic disruption, including consistent changes to α-ketoglutaric acid, creatine/creatinine, homocysteine, and glycerophosphocholine. This metabolic disruption is prevented by nicotinamide. Nicotinamide provides further neuroprotective effects by increasing oxidative phosphorylation, buffering and preventing metabolic stress, and increasing mitochondrial size and motility whilst simultaneously dampening action potential firing frequency. These data support continued determination of the utility of long-term nicotinamide treatment as a neuroprotective therapy for human glaucoma.

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Nicotinamide is neuroprotective in cell and animal models that recapitulate isolated features of glaucoma.

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Systemic nicotinamide administration has limited molecular side-effects on visual system tissue under basal conditions.

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Nicotinamide provides a robust reversal in the disease metabolic profile of glaucomatous animals.

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Nicotinamide increases oxidative phosphorylation, buffers and prevents metabolic stress, and increases mitochondrial size.