Retinal glial cell responses and Fas/FasL activation in rats with chronic ocular hypertension

Retinal Ganglion Cell Death in Glaucoma: Advances and Caveats

Purpose: Retinal ganglion cell death occurs during the glaucoma pathological process, and it is significant because of the poor regeneration capacity of retinal ganglion cells. With a constantly increasing understanding of retinal cell death mechanisms, we now know that simply blocking a specific mechanism of cell death might not prevent retinal ganglion cell death. This review aimed to summarize the mechanisms of retinal cell death in glaucoma models and discuss the caveats in restoring visual function in these studies.Methods: A literature search was done on PubMed using key words including glaucoma, ocular hypertension, retinal ganglion cell, cell death, apoptosis, necroptosis, pyroptosis, ferroptosis, autophagic cell death, and parthanatos. The literature was reviewed to summarize the information about the lethal pathways of retinal ganglion cell in the glaucoma-like animal models.Results: Based on the purpose, 100 studies were selected and discussed in this review.Conclusions: The damage to ganglion cells in glaucoma-like animals can occur via multiple lethal pathways and the molecular mechanisms are still incompletely understood. Further investigations on the crosstalk between different cell death pathways and the common upstream regulators could augment the development of novel targeting agents for the curative treatment of glaucoma.

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.

Neutrophils and neutrophil extracellular trap components: Emerging biomarkers and therapeutic targets for age-related eye diseases

Age-related eye diseases, including dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy, represent a major global health issue based on their increasing prevalence and disabling action. Unraveling the molecular mechanisms underlying these diseases will provide novel opportunities to reduce the burden of age-related eye diseases and improve eye health, contributing to sustainable development goals achievement. The impairment of neutrophil extracellular traps formation/degradation processes seems to be one of these mechanisms. These traps formed by a meshwork of DNA and neutrophil cytosolic granule proteins may exacerbate the inflammatory response promoting chronic inflammation, a pivotal cause of age-related diseases. In this review, we describe current findings that suggest the role of neutrophils and their traps in the pathogenesis of the above-mentioned age-related eye diseases. Furthermore, we discuss why these cells and their constituents could be biomarkers and therapeutic targets for dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy. We also examine the therapeutic potential of some neutrophil function modulators and provide several recommendations for future research in age-related eye diseases.

The FasLane to ocular pathology-metalloproteinase cleavage of membrane-bound FasL determines FasL function

Fas ligand (FasL) is best known for its ability to induce cell death in a wide range of Fas-expressing targets and to limit inflammation in immunoprivileged sites such as the eye. In addition, the ability of FasL to induce a much more extensive list of outcomes is being increasingly explored and accepted. These outcomes include the induction of proinflammatory cytokine production, T cell activation, and cell motility. However, the distinct and opposing functions of membrane-associated FasL (mFasL) and the C-terminal soluble FasL fragment (sFasL) released by metalloproteinase cleavage is less well documented and understood. Both mFasL and sFasL can form trimers that engage the trimeric Fas receptor, but only mFasL can form a multimeric complex in lipid rafts to trigger apoptosis and inflammation. By contrast, a number of reports have now documented the anti-apoptotic and anti-inflammatory activity of sFasL, pointing to a critical regulatory function of the soluble molecule. The immunomodulatory activity of FasL is particularly evident in ocular pathology where elimination of the metalloproteinase cleavage site and the ensuing increased expression of mFasL can severely exacerbate the extent of inflammation and cell death. By contrast, both homeostatic and increased expression of sFasL can limit inflammation and cell death. The mechanism(s) responsible for the protective activity of sFasL are discussed but remain controversial. Nevertheless, it will be important to consider therapeutic applications of sFasL for the treatment of ocular diseases such as glaucoma.

Oxidative Stress and the Role of NADPH Oxidase in Glaucoma

Glaucoma is characterised by loss of retinal ganglion cells, and their axons and many pathophysiological processes are postulated to be involved. It is increasingly understood that not one pathway underlies glaucoma aetiology, but rather they occur as a continuum that ultimately results in the apoptosis of retinal ganglion cells. Oxidative stress is recognised as an important mechanism of cell death in many neurodegenerative diseases, including glaucoma. NADPH oxidase (NOX) are enzymes that are widely expressed in vascular and non-vascular cells, and they are unique in that they primarily produce reactive oxygen species (ROS). There is mounting evidence that NOX are an important source of ROS and oxidative stress in glaucoma and other retinal diseases. This review aims to provide a perspective on the complex role of oxidative stress in glaucoma, in particular how NOX expression may influence glaucoma pathogenesis as illustrated by different experimental models of glaucoma and highlights potential therapeutic targets that may offer a novel treatment option to glaucoma patients.

Inflammation in Glaucoma: From the back to the front of the eye, and beyond

The pathophysiology of glaucoma is complex, multifactorial and not completely understood. Elevated intraocular pressure (IOP) and/or impaired retinal blood flow may cause initial optic nerve damage. In addition, age-related oxidative stress in the retina concurrently with chronic mechanical and vascular stress is crucial for the initiation of retinal neurodegeneration. Oxidative stress is closely related to cell senescence, mitochondrial dysfunction, excitotoxicity, and neuroinflammation, which are involved in glaucoma progression. Accumulating evidence from animal glaucoma models and from human ocular samples suggests a dysfunction of the para-inflammation in the retinal ganglion cell layer and the optic nerve head. Moreover, quite similar mechanisms in the anterior chamber could explain the trabecular meshwork dysfunction and the elevated IOP in primary open-angle glaucoma. On the other hand, ocular surface disease due to topical interventions is the most prominent and visible consequence of inflammation in glaucoma, with a negative impact on filtering surgery failure, topical treatment efficacy, and possibly on inflammation in the anterior segment. Consequently, glaucoma appears as an outstanding eye disease where inflammatory changes may be present to various extents and consequences along the eye structure, from the ocular surface to the posterior segment, and the visual pathway. Here we reviewed the inflammatory processes in all ocular structures in glaucoma from the back to the front of the eye and beyond. Our approach was to explain how para-inflammation is necessary to maintain homoeostasis, and to describe abnormal inflammatory findings observed in glaucomatous patients or in animal glaucoma models, supporting the hypothesis of a dysregulation of the inflammatory balance toward a pro-inflammatory phenotype. Possible anti-inflammatory therapeutic approaches in glaucoma are also discussed.

A small peptide antagonist of the Fas receptor inhibits neuroinflammation and prevents axon degeneration and retinal ganglion cell death in an inducible mouse model of glaucoma

BACKGROUND

Glaucoma is a complex, multifactorial disease where apoptosis, microglia activation, and inflammation have been linked to the death of retinal ganglion cells (RGCs) and axon degeneration. We demonstrated previously that FasL-Fas signaling was required for axon degeneration and death of RGCs in chronic and inducible mouse models of glaucoma and that Fas activation triggered RGC apoptosis, glial activation, and inflammation. Here, we investigated whether targeting the Fas receptor with a small peptide antagonist, ONL1204, has anti-inflammatory and neuroprotective effects in a microbead-induced mouse model of glaucoma.

METHODS

Intracameral injection of microbeads was used to elevate intraocular pressure (IOP) in Fas-deficient (Faslpr) mice and WT C57BL/6J mice that received an intravitreal injection of the Fas inhibitor, ONL1204 (2 μg/1 μl) (or vehicle only), on day 0 or day 7 after microbead injection. The IOP was monitored by rebound tonometry, and at 28 days post-microbead injection, Brn3a-stained RGCs and paraphenylenediamine (PPD)-stained axons were analyzed. The effects of ONL1204 on retinal microglia activation and the expression of inflammatory genes were analyzed by immunostaining of retinal flatmounts and quantitative PCR (qPCR).

RESULTS

Rebound tonometry showed equivalent elevation of IOP in all groups of microbead-injected mice. At 28 days post-microbead injection, the RGC and axon counts from microbead-injected Faslpr mice were equivalent to saline-injected (no IOP elevation) controls. Treatment with ONL1204 also significantly reduced RGC death and loss of axons in microbead-injected WT mice when compared to vehicle-treated controls, even when administered after IOP elevation. Confocal analysis of Iba1-stained retinal flatmounts and qPCR demonstrated that ONL1204 also abrogated microglia activation and inhibited the induction of multiple genes implicated in glaucoma, including cytokines and chemokines (GFAP, Caspase-8, TNFα, IL-1β, IL-6, IL-18, MIP-1α, MIP-1β, MIP-2, MCPI, and IP10), components of the complement cascade (C3, C1Q), Toll-like receptor pathway (TLR4), and inflammasome pathway (NLRP3).

CONCLUSIONS

These results serve as proof-of-principal that the small peptide inhibitor of the Fas receptor, ONL1204, can provide robust neuroprotection in an inducible mouse model of glaucoma, even when administered after IOP elevation. Moreover, Fas signaling contributes to the pathogenesis of glaucoma through activation of both apoptotic and inflammatory pathways.

Inflammasome Activation Induces Pyroptosis in the Retina Exposed to Ocular Hypertension Injury

Mechanical stress and hypoxia during episodes of ocular hypertension (OHT) trigger glial activation and neuroinflammation in the retina. Glial activation and release of pro-inflammatory cytokines TNFα and IL-1β, complement, and other danger factors was shown to facilitate injury and loss of retinal ganglion cells (RGCs) that send visual information to the brain. However, cellular events linking neuroinflammation and neurotoxicity remain poorly characterized. Several pro-inflammatory and danger signaling pathways, including P2X7 receptors and Pannexin1 (Panx1) channels, are known to activate inflammasome caspases that proteolytically activate gasdermin D channel-formation to export IL-1 cytokines and/or induce pyroptosis. In this work, we used molecular and genetic approaches to map and characterize inflammasome complexes and detect pyroptosis in the OHT-injured retina. Acute activation of distinct inflammasome complexes containing NLRP1, NLRP3 and Aim2 sensor proteins was detected in RGCs, retinal astrocytes and Muller glia of the OHT-challenged retina. Inflammasome-mediated activation of caspases-1 and release of mature IL-1β were detected within 6 h and peaked at 12–24 h after OHT injury. These coincided with the induction of pyroptotic pore protein gasdermin D in neurons and glia in the ganglion cell layer (GCL) and inner nuclear layer (INL). The OHT-induced release of cytokines and RGC death were significantly decreased in the retinas of Casp1^−/−Casp4(11)^del, Panx1^−/− and in Wild-type (WT) mice treated with the Panx1 inhibitor probenecid. Our results showed a complex spatio-temporal pattern of innate immune responses in the retina. Furthermore, they indicate an active contribution of neuronal NLRP1/NLRP3 inflammasomes and the pro-pyroptotic gasdermin D pathway to pathophysiology of the OHT injury. These results support the feasibility of inflammasome modulation for neuroprotection in OHT-injured retinas.

The role and therapeutic potential of melatonin in age-related ocular diseases

The eye is continuously exposed to solar UV radiation and pollutants, making it prone to oxidative attacks. In fact, oxidative damage is a major cause of age-related ocular diseases including cataract, glaucoma, age-related macular degeneration, and diabetic retinopathy. As the nature of lens cells, trabecular meshwork cells, retinal ganglion cells, retinal pigment epithelial cells, and photoreceptors is postmitotic, autophagy plays a critical role in their cellular homeostasis. In age-related ocular diseases, this process is impaired, and thus, oxidative damage becomes irreversible. Other conditions such as low-grade chronic inflammation and angiogenesis also contribute to the development of retinal diseases (glaucoma, age-related macular degeneration and diabetic retinopathy). As melatonin is known to have remarkable qualities such as antioxidant/antinitridergic, mitochondrial protector, autophagy modulator, anti-inflammatory, and anti-angiogenic, it can represent a powerful tool to counteract all these diseases. The present review analyzes the role and therapeutic potential of melatonin in age-related ocular diseases, focusing on nitro-oxidative stress, autophagy, inflammation, and angiogenesis mechanisms.

JUN is important for ocular hypertension-induced retinal ganglion cell degeneration

Ocular hypertension, a major risk factor for glaucoma, is thought to trigger glaucomatous neurodegeneration through injury to retinal ganglion cell (RGC) axons. The molecular signaling pathway leading from ocular hypertension to RGC degeneration, however, is not well defined. JNK signaling, a component of the mitogen-activated protein kinase (MAPK) family, and its canonical target, the transcription factor JUN, have been shown to regulate neurodegeneration in many different systems. JUN is expressed after glaucoma-relevant injuries and Jun deficiency protects RGCs after mechanical injury to the optic nerve. Here, we tested the importance of JNK–JUN signaling for RGC death after ocular hypertensive axonal injury in an age-related, mouse model of ocular hypertension. Immunohistochemistry was performed to evaluate JUN expression in ocular hypertensive DBA/2J mice. JUN was expressed in a temporal and spatial pattern consistent with a role in glaucomatous injury. To determine the importance of JUN in ocular hypertension-induced RGC death, a floxed allele of Jun and a retinal expressed cre recombinase (Six3-cre) were backcrossed onto the DBA/2J background. Intraocular pressure (IOP) and gross morphology of the retina and optic nerve head were assessed to determine whether removing Jun from the developing retina altered IOP elevation or retinal development. Jun deficiency in the retina did not alter DBA/2J IOP elevation or retinal development. Optic nerves and retinas were assessed at ages known to have glaucomatous damage in DBA/2J mice. Jun deficiency protected RGC somas from ocular hypertensive injury, but did not protect RGC axons from glaucomatous neurodegeneration. Jun is a major regulator of RGC somal degeneration after glaucomatous ocular hypertensive injury. These results suggest in glaucomatous neurodegeneration, JNK–JUN signaling has a major role as a pro-death signaling pathway between axonal injury and somal degeneration.

Overexpression of Soluble Fas Ligand following Adeno-Associated Virus Gene Therapy Prevents Retinal Ganglion Cell Death in Chronic and Acute Murine Models of Glaucoma

Glaucoma is a multifactorial disease resulting in the death of retinal ganglion cells (RGCs) and irreversible blindness. Glaucoma-associated RGC death depends on the proapoptotic and proinflammatory activity of membrane-bound Fas ligand (mFasL). In contrast to mFasL, the natural cleavage product, soluble Fas ligand (sFasL) inhibits mFasL-mediated apoptosis and inflammation and, therefore, is an mFasL antagonist. DBA/2J mice spontaneously develop glaucoma and, predictably, RGC destruction is exacerbated by expression of a mutated membrane-only FasL gene that lacks the extracellular cleavage site. Remarkably, one-time intraocular adeno-associated virus-mediated gene delivery of sFasL provides complete and sustained neuroprotection in the chronic DBA/2J and acute microbead-induced models of glaucoma, even in the presence of elevated intraocular pressure. This protection correlated with inhibition of glial activation, reduced production of TNF-α, and decreased apoptosis of RGCs and loss of axons. These data indicate that cleavage of FasL under homeostatic conditions, and the ensuing release of sFasL, normally limits the neurodestructive activity of FasL. The data further support the notion that sFasL, and not mFasL, contributes to the immune-privileged status of the eye.

Retinal and Optic Nerve Damage is Associated with Early Glial Responses in an Experimental Autoimmune Glaucoma Model

It is well established that the immunization with ocular antigens causes a retinal ganglion cell (RGC) decline, which is accompanied by glia alterations. In this study, the degenerative effects of the immunization with an optic nerve homogenate (ONA) and its purified compound S100 were analyzed on retinas and optic nerves. Since a participation of glia cells in cell death mechanisms is currently discussed, rats were immunized with S100 or ONA. At 14 and 28 days, immune-histological and Western blot analyses were performed to investigate the optic nerve structure (SMI-32), retinal ganglion cells (Brn-3a), apoptosis (cleaved caspase 3, FasL), and glial profile (Iba1, ED1, GFAP, vimentin). Neurofilament dissolution in S100 animals was evident at 14 days (p = 0.047) and increased at 28 days (p = 0.01). ONA optic nerves remained intact at early stages and degenerated later on (p = 0.002). In both groups, RGC loss was detected via immune-histology and Western blot at 28 days (ONA: p = 0.02; S100: p = 0.005). Additionally, more Iba1(+) retinal microglia could be detected at early stages (ONA: p = 0.006; S100: p = 0.028). A slight astrocyte response was detected on Western blots only on ONA retinas (p = 0.01). Hence, the RGC and optic nerve decline was partly antigen dependent, while neuronal loss is paralleled by an early microglial response.

HMGB1 release triggered by the interaction of live retinal cells and uveitogenic T cells is Fas/FasL activation-dependent

Background

It is not clear how invading autoreactive T cells initiate the pathogenic process inside the diseased organ in T cell-mediated organ-specific autoimmune disease. In experimental autoimmune uveitis (EAU) induced by adoptive transfer of interphotoreceptor retinoid-binding protein (IRBP)-specific T cells in mice, we have previously reported that intraocular inflammation was initiated by infiltrating IRBP-specific T cells that directly interacted with retinal cells and resulted in the active release of high mobility group box 1 (HMGB1), an important member of damage associate molecular patterns (DAMPs). Furthermore, blockade of HMGB1 in our murine model reduced intraocular inflammation via suppression of IRBP-specific T cell functions. These results have demonstrated that HMGB1 is an early and critical mediator of induction of intraocular inflammation. The present study identified the cell surface molecule that triggers HMGB1 secretion.

Methods

Retinal explants from Fas-deficient (Fas^lpr) and wild-type (Wt) C57BL/6 (B6) mice were cultured with activated IRBP 1–20 peptide-specific T cells or with a Fas-activating antibody (Jo2), and then the level of HMGB1 in culture supernatants were detected by ELISA. In addition, released HMGB1 was examined in the eye of Fas^lpr and Wt mice after IRBP-specific T cell transfer. Uveitis was evaluated in the IRBP-specific T cell transferred Fas^lpr mice after recombinant HMGB1 was restored within the eye and in the IRBP-specific T cell transferred Wt mice after they were treated with a Fas antagonist (Met12).

Results

In contrast to retinal explants from Wt mice, those from Fas^lpr mice did not release HMGB1 after exposure to IRBP-specific T cells or to Jo2. The release of HMGB1 by Wt retinal explants was suppressed by Met 12. Moreover, after IRBP-specific T cell injection, Fas^lpr mice did not release HMGB1 in the eye or develop EAU, but intravitreous injection of HMGB1 resulted in intraocular inflammation. Finally, tEAU in Wt mice was attenuated by local treatment with Met 12. Unlike HMGB1, Fas-induced IL-1 and IL-18 were not essential for tEAU induction.

Conclusion

Our results show that interaction of retinal cells with infiltrating uveitogenic T cells leads to rapid release of HMGB1 via the Fas/FasL inflammatory signaling pathway.

Neuroprotective and antioxidant effects of ghrelin in an experimental glaucoma model

Damage to retinal ganglion cells due to elevation of intraocular pressure (IOP) is responsible for vision loss in glaucoma. Given that loss of these cells is irreversible, neuroprotection is crucial in the treatment of glaucoma. In this study, we investigated the possible antioxidant and neuroprotective effects of ghrelin on the retina in an experimental glaucoma model. Twenty-one Sprague–Dawley rats were randomly assigned to three groups comprising seven rats each. The rats in the control group were not operated on and did not receive any treatment. In all rats in the other groups, IOP was increased by cauterization of the limbal veins. After creation of the IOP increase, saline 1 mL/kg or ghrelin 40 μg/kg was administered intraperitoneally every day for 14 days in the vehicle control group and ghrelin groups, respectively. On day 14 of the study, the eyes were enucleated. Levels of malondialdehyde (MDA), nitric oxide (NO), and nitric oxide synthase-2 (NOS2) in anterior chamber fluid were measured. The retinas were subjected to immunohistochemistry staining for glial fibrillary acidic protein (GFAP), S-100, and vimentin expression. Mean levels of MDA, NO, and NOS2 in the aqueous humor were higher in the vehicle control group than in the control group (P<0.05). Mean levels of MDA, NO, and NOS2 in the ghrelin group did not show a significant increase compared with levels in the control group (P>0.05). Retinal TUNEL and immunohistochemistry staining in the vehicle control group showed an increase in apoptosis and expression of GFAP, S-100, and vimentin compared with the control group (P<0.05). In the ghrelin group, apoptosis and expression of GFAP, S-100, and vimentin was significantly lower than in the vehicle control group (P<0.05). This study suggests that ghrelin has antioxidant and neuroprotective effects on the retina in an experimental glaucoma model.

Purinergic signaling in retinal degeneration and regeneration

Purinergic signaling is centrally involved in mediating the degeneration of the injured and diseased retina, the induction of retinal gliosis, and the protection of the retinal tissue from degeneration. Dysregulated calcium signaling triggered by overactivation of P2X7 receptors is a crucial step in the induction of neuronal and microvascular cell death under pathogenic conditions like ischemia-hypoxia, elevated intraocular pressure, and diabetes, respectively. Overactivation of P2X7 plays also a pathogenic role in inherited and age-related photoreceptor cell death and in the age-related dysfunction and degeneration of the retinal pigment epithelium. Gliosis of micro- and macroglial cells, which is induced and/or modulated by purinergic signaling and associated with an impaired homeostatic support to neurons, and the ATP-mediated propagation of retinal gliosis from a focal injury into the surrounding noninjured tissue are involved in inducing secondary cell death in the retina. On the other hand, alterations in the glial metabolism of extracellular nucleotides, resulting in a decreased level of ATP and an increased level of adenosine, may be neuroprotective in the diseased retina. Purinergic signals stimulate the proliferation of retinal glial cells which contributes to glial scarring which has protective effects on retinal degeneration and adverse effects on retinal regeneration. Pharmacological modulation of purinergic receptors, e.g., inhibition of P2X and activation of adenosine receptors, may have clinical importance for the prevention of photoreceptor, neuronal, and microvascular cell death in diabetic retinopathy, retinitis pigmentosa, age-related macular degeneration, and glaucoma, respectively, for the clearance of retinal edema, and the inhibition of dysregulated cell proliferation in proliferative retinopathies. This article is part of a Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Retinal dendritic cell recruitment, but not function, was inhibited in MyD88 and TRIF deficient mice

BACKGROUND

Immune system cells are known to affect loss of neurons due to injury or disease. Recruitment of immune cells following retinal/CNS injury has been shown to affect the health and survival of neurons in several models. We detected close, physical contact between dendritic cells and retinal ganglion cells following an optic nerve crush, and sought to understand the underlying mechanisms.

METHODS

CD11c-DTR/GFP mice producing a chimeric protein of diphtheria toxin receptor (DTR) and GFP from a transgenic CD11c promoter were used in conjunction with mice deficient in MyD88 and/or TRIF. Retinal ganglion cell injury was induced by an optic nerve crush, and the resulting interactions of the GFPhi cells and retinal ganglion cells were examined.

RESULTS

Recruitment of GFPhi dendritic cells to the retina was significantly compromised in MyD88 and TRIF knockout mice. GFPhi dendritic cells played a significant role in clearing fluorescent-labeled retinal ganglion cells post-injury in the CD11c-DTR/GFP mice. In the TRIF and MyD88 deficient mice, the resting level of GFPhi dendritic cells was lower, and their influx was reduced following the optic nerve crush injury. The reduction in GFPhi dendritic cell numbers led to their replacement in the uptake of fluorescent-labeled debris by GFPlo microglia/macrophages. Depletion of GFPhi dendritic cells by treatment with diphtheria toxin also led to their displacement by GFPlo microglia/macrophages, which then assumed close contact with the injured neurons.

CONCLUSIONS

The contribution of recruited cells to the injury response was substantial, and regulated by MyD88 and TRIF. However, the presence of these adaptor proteins was not required for interaction with neurons, or the phagocytosis of debris. The data suggested a two-niche model in which resident microglia were maintained at a constant level post-optic nerve crush, while the injury-stimulated recruitment of dendritic cells and macrophages led to their transient appearance in numbers equivalent to or greater than the resident microglia.

Inhibitor of apoptosis-stimulating protein of p53 (iASPP) is required for neuronal survival after axonal injury

The transcription factor p53 mediates the apoptosis of post-mitotic neurons exposed to a wide range of stress stimuli. The apoptotic activity of p53 is tightly regulated by the apoptosis-stimulating proteins of p53 (ASPP) family members: ASPP1, ASPP2 and iASPP. We previously showed that the pro-apoptotic members ASPP1 and ASPP2 contribute to p53-dependent death of retinal ganglion cells (RGCs). However, the role of the p53 inhibitor iASPP in the central nervous system (CNS) remains to be elucidated. To address this, we asked whether iASPP contributes to the survival of RGCs in an in vivo model of acute optic nerve damage. We demonstrate that iASPP is expressed by injured RGCs and that iASPP phosphorylation at serine residues, which increase iASPP affinity towards p53, is significantly reduced following axotomy. We show that short interference RNA (siRNA)-induced iASPP knockdown exacerbates RGC death, whereas adeno-associated virus (AAV)-mediated iASPP expression promotes RGC survival. Importantly, our data also demonstrate that increasing iASPP expression in RGCs downregulates p53 activity and blocks the expression of pro-apoptotic targets PUMA and Fas/CD95. This study demonstrates a novel role for iASPP in the survival of RGCs, and provides further evidence of the importance of the ASPP family in the regulation of neuronal loss after axonal injury.

Spatiotemporal changes in NFATc4 expression of retinal ganglion cells after light-induced damage

Nuclear factor of activated T cells, cytoplasmic 4 (NFATc4) is one of the four members of the NFAT family, which were described first as essential components of T cells activation and lately as important regulators for the initiation and coordination of the immune response, including B cells and natural killer cells. Accumulating evidence has demonstrated that NFATc4 exerted a pro-apoptotic effect in the pathogenesis of various experimental central nervous system diseases by upregulating Fas ligand (FasL) levels. However, the function of NFATc4 in the retina is still with limited acquaintance. To investigate whether NFATc4 is involved in retinal neuron apoptosis, we performed a light-induced retinal damage model in adult rats. A significant upregulation of NFATc4 was detected in the retina after light-induced damage by using Western blotting and reverse transcriptase PCR (RT-PCR). Besides this, NFATc4 was observed to be localized mainly in the retinal ganglion cells (RGCs). In addition, the expression patterns of active caspase-3, active caspase-8, and FasL were parallel with that of NFATc4. We also found the co-localization of NFATc4 with active caspase-3 and FasL in RGCs after light exposure. Collectively, we hypothesized that NFATc4 might participate in RGCs apoptosis by upregulating FasL levels.

Unexpected low-dose toxicity of the universal solvent DMSO

Dimethyl sulfoxide (DMSO) is an important aprotic solvent that can solubilize a wide variety of otherwise poorly soluble polar and nonpolar molecules. This, coupled with its apparent low toxicity at concentrations <10%, has led to its ubiquitous use and widespread application. Here, we demonstrate that DMSO induces retinal apoptosis in vivo at low concentrations (5 μl intravitreally dosed DMSO in rat from a stock concentration of 1, 2, 4, and 8% v/v). Toxicity was confirmed in vitro in a retinal neuronal cell line, at DMSO concentrations >1% (v/v), using annexin V, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and AlamarBlue cell viability assays. DMSO concentrations >10% (v/v) have recently been reported to cause cellular toxicity through plasma membrane pore formation. Here, we show the mechanism by which low concentrations (2-4% DMSO) induce caspase-3 independent neuronal death that involves apoptosis-inducing factor (AIF) translocation from mitochondria to the nucleus and poly-(ADP-ribose)-polymerase (PARP) activation. These results highlight safety concerns of using low concentrations of DMSO as a solvent for in vivo administration and in biological assays. We recommend that methods other than DMSO are employed for solubilizing drugs but, where no alternative exists, researchers compute absolute DMSO final concentrations and include an untreated control group in addition to DMSO vehicle control to check for solvent toxicity.

Increased resistance to Staphylococcus aureus endophthalmitis in BALB/c mice: Fas ligand is required for resolution of inflammation but not for bacterial clearance

FasL was recently shown be required for bacterial clearance in C57BL/6 mice that express the FasL.1 allotype. The FasL.2 allotype is expressed in BALB/c mice and exhibits increased binding affinity to and increased cytotoxic activity against Fas(+) target cells. Therefore, we hypothesized that BALB/c mice would be more resistant to Staphylococcus aureus-induced endophthalmitis. To test this hypothesis, C57BL/6, BALB/c, and BALB(gld) mice received intravitreal injections of 2,500 CFU of S. aureus (RN6390). Clinical examinations, electroretinography (ERG), histology, and bacterial quantification were performed at 24, 48, 72, and 96 h postinjection. The myeloperoxidase (MPO) assay was used to quantitate neutrophil infiltration. At 96 h postinfection, 86% of C57BL/6 mice presented with complete destruction of the eye, compared to only 29% of BALB/c mice with complete destruction. To our surprise, in the absence of Fas ligand, BALB(gld) mice showed no difference in bacterial clearance compared to BALB/c mice. However, histology and ERG analysis revealed increased retinal damage and significant loss of retinal function. MPO analysis revealed equal numbers of neutrophils in BALB(gld) and BALB/c mice at 24 h postinfection. However, at 48 h, the neutrophil numbers remained significantly elevated in BALB(gld) mice, correlating with the increased retinal damage observed in BALB(gld) mice. We conclude that the increased resistance to S. aureus induced endophthalmitis in BALB/c mice is not dependent upon the FasL. However, in contrast to C57BL/6 mice, FasL is required for resolution of inflammation and protecting host tissue from nonspecific damage in BALB/c mice.

ASPP1/2 regulate p53-dependent death of retinal ganglion cells through PUMA and Fas/CD95 activation in vivo

The transcription factor p53 mediates neuronal death in a variety of stress-related and neurodegenerative conditions. The proapoptotic activity of p53 is tightly regulated by the apoptosis-stimulating proteins of p53 (ASPP) family members: ASPP1 and ASPP2. However, whether ASPP1/2 play a role in the regulation of p53-dependent neuronal death in the CNS is currently unknown. To address this, we asked whether ASPP1/2 contribute to the death of retinal ganglion cells (RGCs) using in vivo models of acute optic nerve damage in mice and rats. Here, we show that p53 is activated in RGCs soon after injury and that axotomy-induced RGC death is attenuated in p53 heterozygote and null mice. We demonstrate that ASPP1/2 proteins are abundantly expressed by injured RGCs, and that short interfering (si)RNA-based ASPP1 or ASPP2 knockdown promotes robust RGC survival. Comparative gene expression analysis revealed that siASPP-mediated downregulation of p53-upregulated-modulator-of-apoptosis (PUMA), Fas/CD95, and Noxa depends on p53 transcriptional activity. Furthermore, siRNA against PUMA or Fas/CD95 confers neuroprotection, demonstrating a functional role for these p53 targets in RGC death. Our study demonstrates a novel role for ASPP1 and ASPP2 in the death of RGCs and provides evidence that blockade of the ASPP-p53 pathway is beneficial for central neuron survival after axonal injury.

Immune regulation toward immunomodulation for neuroprotection in glaucoma

Although the immune system functions to preserve and restore tissue homeostasis, accumulating risk factors, prolonged glial activation, and sustained release of pro-inflammatory mediators in glaucoma may lead to a failure in the regulation of stress-induced immune response, and innate immune cells, autoreactive T cells, autoantibodies, and excess complement attack may exhibit potent stimuli that harm retinal ganglion cell somas, axons, and synapses. Identification of the cellular and molecular components of immune response pathways can provide immunomodulatory treatment strategies to attenuate neuroinflammation, protect neural tissue from collateral injury, and enhance endogenous recovery processes. This review highlights the current knowledge of molecular mechanisms regulating neuroinflammation in glaucoma.

Etanercept, a widely used inhibitor of tumor necrosis factor-α (TNF-α), prevents retinal ganglion cell loss in a rat model of glaucoma

BACKGROUND

Visual loss in glaucoma is associated with pathological changes in retinal ganglion cell (RGC) axons and a slow decline in the RGC population. Age and elevated intraocular pressure (IOP) are the main risk factors for glaucomatous loss of vision. Several studies have implicated the proinflammatory cytokine tumor necrosis factor-α (TNF-α) as a link between elevated IOP and RGC death, but the cellular source of TNF-α and its causative role in RGC death remain uncertain. Here, using a rat model of glaucoma, we investigated the source of elevated TNF-α and examined whether Etanercept, a TNF-α blocker that is in common clinical use for other indications, is protective against RGC death.

METHODOLOGY/PRINCIPAL FINDINGS

Episcleral vein cauterization (EVC) caused intraocular pressure (IOP) to be elevated for at least 28 days. IOP elevation resulted in a dramatic increase in TNF-α levels within a few days, axonal degeneration, and a 38% loss of RGCs by 4 weeks. Immunostaining coupled with confocal microscopy showed that OHT induced robust induction of TNF-α in Iba-1-positive microglia around the optic nerve head (ONH). Despite persistent elevation of IOP, Etanercept reduced microglial activation, TNF-α levels, axon degeneration in the optic nerve, and the loss of RGCs.

CONCLUSIONS/SIGNIFICANCE

Ocular hypertension (OHT) triggers an inflammatory response characterized by the appearance of activated microglia around the ONH that express TNF-α. Blocking TNF-α activity with a clinically approved agent inhibits this microglial response and prevents axonal degeneration and loss of RGCs. These findings suggest a new treatment strategy for glaucoma using TNF-α antagonists or suppressors of inflammation.

ER stress and apoptosis: a new mechanism for retinal cell death

The endoplasmic reticulum (ER) is the primary subcellular organelle where proteins are synthesized and folded. When the homeostasis of the ER is disturbed, unfolded or misfolded proteins accumulate in the ER lumen, resulting in ER stress. In response to ER stress, cells activate a set of tightly controlled regulatory programs, known as the unfolded protein response (UPR), to restore the normal function of the ER. However, if ER stress is sustained and the adaptive UPR fails to eliminate unfolded/misfolded proteins, apoptosis will occur to remove the stressed cells. In recent years, a large body of studies has shown that ER stress-induced apoptosis is implicated in numerous human diseases, such as diabetes and neurogenerative diseases. Moreover, emerging evidence supports a role of ER stress in retinal apoptosis and cell death in blinding disorders such as age-related macular degeneration and diabetic retinopathy. In the present review, we summarize recent progress on ER stress and apoptosis in retinal diseases, focusing on various proapoptotic and antiapoptotic pathways that are activated by the UPR, and discuss how these pathways contribute to ER stress-induced apoptosis in retinal cells.

WITHDRAWN: Reprint of: Mechanisms of retinal ganglion cell injury and defense in glaucoma

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.exer.2010.04.002. The duplicate article has therefore been withdrawn.

Opposing roles for membrane bound and soluble Fas ligand in glaucoma-associated retinal ganglion cell death

Glaucoma, the most frequent optic neuropathy, is a leading cause of blindness worldwide. Death of retinal ganglion cells (RGCs) occurs in all forms of glaucoma and accounts for the loss of vision, however the molecular mechanisms that cause RGC loss remain unclear. The pro-apoptotic molecule, Fas ligand, is a transmembrane protein that can be cleaved from the cell surface by metalloproteinases to release a soluble protein with antagonistic activity. Previous studies documented that constitutive ocular expression of FasL maintained immune privilege and prevented neoangeogenesis. We now show that FasL also plays a major role in retinal neurotoxicity. Importantly, in both TNFα triggered RGC death and a spontaneous model of glaucoma, gene-targeted mice that express only full-length FasL exhibit accelerated RGC death. By contrast, FasL-deficiency, or administration of soluble FasL, protected RGCs from cell death. These data identify membrane-bound FasL as a critical effector molecule and potential therapeutic target in glaucoma.

Involvement of the CD200 receptor complex in microglia activation in experimental glaucoma

The interaction of the myeloid restricted molecule CD200R with its widely expressed ligand CD200 is involved in the down-regulation of microglia activation. In the present study, we examined the involvement of CD200R in microglia activation in experimental ocular hypertension to determine the role of microglia activation in retinal ganglion cell (RGC) death, the key pathological event in glaucoma. Experimental glaucoma was induced in adult Brown Norway rats by sclerosis of the episcleral veins with the injection of hypertonic saline. Immunohistochemical methods were used to determine the involvement of microglia using GFAP, CD45, OX42 and OX41 and the involvement of CD200 and CD200R in the optic nerve head. Our data demonstrate the increased presence of microglia within the optic nerve head during ocular hypertension, identified by positive staining with OX42 and OX41. The peak of microglia correlates with peak in RGC death at days 20-27 (T3) post OHT induction. In addition, CD200 and CD200R positive cells were increased in ocular hypertensive eyes. Increased expression of CD200 was detected in the early phase (days 1-7; T1) of OHT and decreased over time, whilst the expression of CD200R was detected in the middle phase (days 20-27; T3) of OHT, correlating with the increase in microglia markers. Changes in the expression of CD200R/CD200 occur early in experimental glaucoma and precede the peak in microglia infiltration and RGC death, suggesting that CD200R-positive microglia play an important role in the initiation of RGC death during OHT, indicating a potential area for therapeutic intervention in treating glaucoma.

Inflammatory mechanisms of neurodegeneration in toxin-based models of Parkinson's disease

Parkinson's disease (PD) has been associated with exposure to a variety of environmental agents, including pesticides, heavy metals, and organic pollutants; and inflammatory processes appear to constitute a common mechanistic link among these insults. Indeed, toxin exposure has been repeatedly demonstrated to induce the release of oxidative and inflammatory factors from immunocompetent microglia, leading to damage and death of midbrain dopamine (DA) neurons. In particular, proinflammatory cytokines such as tumor necrosis factor-α and interferon-γ, which are produced locally within the brain by microglia, have been implicated in the loss of DA neurons in toxin-based models of PD; and mounting evidence suggests a contributory role of the inflammatory enzyme, cyclooxygenase-2. Likewise, immune-activating bacterial and viral agents were reported to have neurodegenerative effects themselves and to augment the deleterious impact of chemical toxins upon DA neurons. The present paper will focus upon the evidence linking microglia and their inflammatory processes to the death of DA neurons following toxin exposure. Particular attention will be devoted to the possibility that environmental toxins can activate microglia, resulting in these cells adopting a “sensitized” state that favors the production of proinflammatory cytokines and damaging oxidative radicals.

The immune response in glaucoma: a perspective on the roles of oxidative stress

Neurodegenerative insults and glial activation during glaucomatous neurodegeneration initiate an immune response to restore tissue homeostasis and facilitate tissue cleaning and healing. However, increasing risk factors over a chronic and cumulative period may lead to a failure in the regulation of innate and adaptive immune response pathways and represent a route for conversion of the beneficial immunity into a neuroinflammatory degenerative process contributing to disease progression. Oxidative stress developing through the pathogenic cellular processes of glaucoma, along with the aging-related component of oxidative stress, likely plays a critical role in shifting the physiological equilibrium. This review aims to provide a perspective on the complex interplay of cellular events during glaucomatous neurodegeneration by proposing a unifying scheme that integrates oxidative stress-related risk factors with the altered regulation of immune response in glaucoma.

Mechanisms of retinal ganglion cell injury and defense in glaucoma

Glaucoma is a disease in which retinal ganglion cells (RGCs) die leading ultimately to blindness. Over the past decade and a half, information has begun to emerge regarding specific molecular responses of the retina to conditions of elevated intraocular pressure (IOP). It is now clear that the state of the RGC in glaucoma depends on a balance of pro-survival and pro-death pathways in the retina and details of these responses are still being worked out. In this review, we will discuss the evidence supporting the involvement of specific apoptotic cascades as well as the insults that trigger RGC apoptosis. In addition, we will present evidence supporting the existence of endogenous protective mechanisms as well as exogenous neuroprotective strategies.

A critical discussion of the rates of progression and causes of optic nerve damage in glaucoma: International Glaucoma Think Tank II: July 25-26, 2008, Florence, Italy

The International Glaucoma Think Tank II brought together glaucoma clinicians and researchers from all over the world to discuss current practices in glaucoma diagnosis and management, and the neurobiology of glaucoma. The meeting focused on several themes, including rates of deterioration in glaucoma patients, mechanisms of optic nerve damage, and implications for treatment. Issues such as how to measure and integrate progression information into clinical practice, screening protocols, or trials were discussed, as were promising new technologies and limitations of currently available measurement tools. Clinical applications for genetic testing were considered. Study of the neurobiology of glaucoma continues to inform our understanding of underlying degenerative processes, as well as to introduce possibilities for early detection or prevention. Many questions regarding glaucoma pathophysiology and best treatment practices remain unanswered, but with continued research and discussion, we will advance our understanding of this disease and ensure that patients receive optimal care.

Axonal/glial upregulation of EphB/ephrin-B signaling in mouse experimental ocular hypertension

PURPOSE

To use a laser-induced ocular hypertension (LIOH) mouse model to examine the optic nerve head (ONH) expression of EphB/ephrin-B, previously shown to be upregulated in glaucomatous DBA/2J mice. To relate ephrin-B reverse signaling with states of axonal response to disease.

METHODS

LIOH was induced unilaterally in CD-1 mice by laser photocoagulation of limbal and episcleral veins. Intraocular pressure (IOP) was measured with a tonometer. EphB/ephrin-B mRNA expression was assessed by in situ hybridization on eyecup cryosections and real-time PCR. Cell specific markers were used to identify the cellular origin of EphB/ephrin-B expression. Activation of ephrin-B signaling was investigated with a phosphospecific antibody on cryosections and retinal whole-mounts.

RESULTS

Upregulation of EphB/ephrin-B expression occurred early within a day of IOP elevation. A transient increase of phosphorylation-dependent ephrin-B (pEB) reverse signaling was observed in ONH axons, microglia, and some astrocytes. Morphologically unaffected retinal ganglion cell (RGC) axons differed from axons with reactive aberrant trajectories by exhibiting increased pEB activation, whereas pEB levels in morphologically affected axons were comparable to those of controls.

CONCLUSIONS

An Eph-ephrin signaling network is activated at the ONH after LIOH in CD-1 mice, either before or coincident with the initial morphologic signs of RGC axon damage reported previously. Of note, ephrin-B reverse signaling was transiently upregulated in RGC axons at the ONH early in their response to IOP elevation but was downregulated in axons that had been damaged by glaucomatous injury and exhibited aberrant trajectories. Ephrin-B reverse signaling may mark RGC axons for damage or confer a protective advantage against injury.

Friend or foe? Resolving the impact of glial responses in glaucoma

Glaucomatous vision loss results from the progressive degeneration of optic nerve axons and the death of retinal ganglion cells. This process is accompanied by dramatic alterations in the functional properties and distribution of glial cells in both the retina and the optic nerve head in a reaction commonly referred to as glial activation. The recent availability of rodent and cell culture glaucoma models has substantially contributed to our knowledge of glial activation under glaucomatous conditions. Conclusions drawn from these studies have led to the refinement of existing hypotheses and the generation of new ones. Because these hypotheses encompass both protective and injurious roles for glia, the impact of specific aspects of glial activation are current topics of intensive research, speculation, and debate in the field. With these unresolved issues in mind, this review will summarize recent progress in our understanding of the process of glial activation in the glaucomatous optic nerve head and retina.

Differential expression of two glutamate transporters, GLAST and GLT-1, in an experimental rat model of glaucoma

Glutamate is the major excitatory neurotransmitter of the mammalian retina, and excessive glutamate has been implicated in the pathogenesis of glaucoma. It is well known that glutamate transport, mainly via GLAST and GLT-1, is cardinal mechanism for maintaining glutamate homeostasis in normal and pathological conditions, including ischemia in the brain. In an effort to understand the role of glutamate and the glutamate regulation system of the retina in the pathogenesis of glaucoma, we examined changes in the expression of two glutamate transporters, GLAST and GLT-1, by Western blot analysis and immunocytochemistry in a rat glaucoma model. GLT-1 was expressed in cone photoreceptors and some cone bipolar cells and the levels of expression were significantly increased in the cauterized eyes throughout the entire experimental period. In contrast, GLAST expression, which occurred in Müller cells, the main retinal glial cells, remained stable during the experimental period. These results suggest that GLT-1 may be a prerequisite for the maintenance of glutamate homeostasis in the retina undergoing glaucoma.

Neurotrophin roles in retinal ganglion cell survival: lessons from rat glaucoma models

The neurotrophin (NT) hypothesis proposes that the obstruction of retrograde transport at the optic nerve head results in the deprivation of neurotrophic support to retinal ganglion cells (RGC) leading to apoptotic cell death in glaucoma. An important corollary to this concept is the implication that appropriate enhancement of neurotrophic support will prolong the survival of injured RGC indefinitely. This hypothesis is, perhaps, the most widely recognized theory to explain RGC loss resulting from exposure of the eye to elevated intraocular pressure (IOP). Recent studies of NT signaling using rat glaucoma models, have examined the endogenous responses of the retina to pressure exposure as well as studies designed to augment NT signaling in order to rescue RGC from apoptosis following pressure-induced injury. The examination of these studies in this review reveals a number of consistent observations and provides direction for further investigations of this hypothesis.

Induced autoimmunity to heat shock proteins elicits glaucomatous loss of retinal ganglion cell neurons via activated T-cell-derived fas-ligand

Glaucomatous optic neuropathy causes blindness through the degeneration of retinal ganglion cells (RGCs) and their axons, which comprise the optic nerve. Glaucoma traditionally is associated with elevated intraocular pressure, but often occurs or may progress with intraocular pressure in the normal range. Like other diseases of the CNS, a subset of glaucoma has been proposed to involve an autoimmune component to help explain the loss of RGCs in the absence of elevated intraocular pressure. One hypothesis involves heat shock proteins (HSPs), because increased serum levels of HSP autoantibodies are prominent in some glaucoma patients with normal pressures. In the first direct support of this hypothesis, we found that HSP27 and HSP60 immunization in the Lewis rat induced RGC degeneration and axon loss 1-4 months later in vivo in a pattern with similarities to human glaucoma, including topographic specificity of cell loss. Infiltration of increased numbers of T-cells in the retina occurred much earlier, 14-21 d after HSP immunization, and appeared to be transient. In vitro studies found that T-cells activated by HSP immunization induced RGC apoptosis via the release of the inflammatory cytokine FasL, whereas HSP immunization induced activation of microglia cells and upregulation of the FasL receptor in RGCs. In summary, our results suggest that RGC degeneration in glaucoma for selected individuals likely involves failed immunoregulation of the T-cell-RGC axis and is thus a disturbance of both proapoptotic and protective pathways.

Contribution of TRPV1 to microglia-derived IL-6 and NFkappaB translocation with elevated hydrostatic pressure

PURPOSE

The authors investigated the contributions of the transient receptor potential vanilloid-1 receptor (TRPV1) and Ca(2+) to microglial IL-6 and nuclear factor kappa B (NFkappaB) translocation with elevated hydrostatic pressure.

METHODS

The authors first examined IL-6 colocalization with the microglia marker Iba-1 in the DBA/2 mouse model of glaucoma to establish relevance. They isolated microglia from rat retina and maintained them at ambient or elevated (+70 mm Hg) hydrostatic pressure in vitro and used ELISA and immunocytochemistry to measure changes in the IL-6 concentration and NFkappaB translocation induced by the Ca(2+) chelator EGTA, the broad-spectrum Ca(2+) channel inhibitor ruthenium red, and the TRPV1 antagonist iodo-resiniferatoxin (I-RTX). They applied the Ca(2+) dye Fluo-4 AM to measure changes in intracellular Ca(2+) at elevated pressure induced by I-RTX and confirmed TRPV1 expression in microglia using PCR and immunocytochemistry.

RESULTS

In DBA/2 retina, elevated intraocular pressure increased microglial IL-6 in the ganglion cell layer. Elevated hydrostatic pressure (24 hours) increased microglial IL-6 release, cytosolic NFkappaB, and NFkappaB translocation in vitro. These effects were reduced substantially by EGTA and ruthenium red. Antagonism of TRPV1 in microglia partially inhibited pressure-induced increases in IL-6 release and NFkappaB translocation. Brief elevated pressure (1 hour) induced a significant increase in microglial intracellular Ca(2+) that was partially attenuated by TRPV1 antagonism.

CONCLUSIONS

Elevated pressure induces an influx of extracellular Ca(2+) in retinal microglia that precedes the activation of NFkappaB and the subsequent production and release of IL-6 and is at least partially dependent on the activation of TRPV1 and other ruthenium red-sensitive channels.

Changes in visual fields and lateral geniculate nucleus in monkey laser-induced high intraocular pressure model

Monkey eyes are useful for ophthalmologic research into eye diseases because their histological and functional properties are very similar to those of humans. The monkey laser-induced high intraocular pressure (IOP) model is a common model for ophthalmologic research, especially into glaucoma. Although several studies using this model have focused on changes in visual field, retinal ganglion cells (RGC), and lateral geniculate nucleus (LGN), clear relationships among these changes in one and the same monkey have not been established. We therefore examined visual field changes, RGC and LGN numbers, and glial fibrous acidic protein (GFAP) immunohistochemistry in the LGN in each of two monkeys. Visual field sensitivity, RGC number, and neuronal density of LGN were all decreased by high IOP. The relationship between loss of RGC and decrease in visual field sensitivity depended on the eccentricity from the fovea. Moreover, LGN immunohistochemistry revealed greater increases in GFAP expression in the layers receiving a neuronal input from the high IOP eye than in those receiving a neuronal input from the contralateral untreated eye. From these results, we suggest that glaucoma may lead to changes in glial function not only in the retina, but also in the visual pathway, and that such central nervous system changes may be a hallmark of neuropathy in glaucoma, as in other neurodegenerative diseases.