Sustained elevation of extracellular ATP in aqueous humor from humans with primary chronic angle-closure glaucoma

Piezo1 and Piezo2 channels in retinal ganglion cells and the impact of Piezo1 stimulation on light-dependent neural activity

Piezo channels are associated with neuropathology in diseases like traumatic brain injury and glaucoma, but pathways linking tissue stretch to aberrant neural signaling remain unclear. The present study demonstrates that Piezo1 activation increases action potential frequency in response to light and the spontaneous dark signal from mouse retinal explants. Piezo1 stimulation was sufficient to increase cytoplasmic Ca 2+ in soma and neurites, while stretch increased spiking activity in current clamp recordings from of isolated retinal ganglion cells (RGCs). Axon-marker beta-tubulin III colocalized with both Piezo1 and Piezo2 protein in the mouse optic nerve head, while RGC nuclear marker BRN3A colocalized with Piezo channels in the soma. Piezo1 was also present on GFAP-positive regions in the optic nerve head and colocalized with glutamine synthetase in the nerve fiber layer, suggesting expression in optic nerve head astrocytes and Müller glia end feet, respectively. Human RGCs from induced pluripotent stem cells also expressed Piezo1 and Piezo2 in soma and axons, while staining patterns in rats resembled those in mice. mRNA message for Piezo1 was greatest in the RPE/choroid tissue, while Piezo2 levels were highest in the optic nerve, with both channels also expressed in the retina. Increased expression of Piezo1 and Piezo2 occurred both 1 and 10 days after a single stretch in vivo; this increase suggests a potential role in rising sensitivity to repeated nerve stretch. In summary, Piezo1 and Piezo2 were detected in the soma and axons of RGCs, and stimulation affected the light-dependent output of RGCs. The rise in RGCs excitability induced by Piezo stimulation may have parallels to the early disease progression in models of glaucoma and other retinal degenerations.

Highlights

Activation of Piezo1 excites retinal ganglion cells, paralleling the early neurodegenerative progression in glaucoma mouse models and retinal degeneration.Piezo1 and Piezo2 were expressed in axons and soma of retinal ganglion cells in mice, rats, and human iPSC-RGCs.Functional assays confirmed Piezo1 in soma and neurites of neurons. Sustained elevation of Piezo1 and Piezo2 occurred after a single transient stretch may enhance damage from repeated traumatic nerve injury.

Abstract Figure

Role of Oxysterols in Ocular Degeneration Mechanisms and Involvement of P2X7 Receptor

Ocular degeneration, including cataracts, glaucoma, macular degeneration, and diabetic retinopathy, is a major public health challenge, as it affects the quality of life of millions of people worldwide and, in its advanced stages, leads to blindness. Ocular degeneration, although it can affect different parts of the eye, shares common characteristics such as oxysterols and the P2X7 receptor. Indeed, oxysterols, which are cholesterol derivatives, are associated with ocular degeneration pathogenesis and trigger inflammation and cell death pathways. Activation of the P2X7 receptor is also linked to ocular degeneration and triggers the same pathways. In age-related macular degeneration, these two key players have been associated, but further studies are needed to extrapolate this interrelationship to other ocular degenerations.

Glial metabolic alterations during glaucoma pathogenesis

Glaucoma is the leading cause of irreversible blindness. Current treatment options are limited and often only slow disease progression. Metabolic dysfunction has recently been recognized as a key early and persistent mechanism in glaucoma pathophysiology. Several intrinsic metabolic dysfunctions have been identified and treated in retinal ganglion cells to provide neuroprotection. Growing pre-clinical and clinical evidence has confirmed that metabolic alterations in glaucoma are widespread, occurring across visual system tissues, in ocular fluids, in blood/serum, and at the level of genomic and mitochondrial DNA. This suggests that metabolic dysfunction is not constrained to retinal ganglion cells and that metabolic alterations extrinsic to retinal ganglion cells may contribute to their metabolic compromise. Retinal ganglion cells are reliant on glial metabolic support under normal physiological conditions, but the implications of metabolic dysfunction in glia are underexplored. We highlight emerging evidence that has demonstrated metabolic alterations occurring within glia in glaucoma, and how this may affect neuro-glial metabolic coupling and the metabolic vulnerability of retinal ganglion cells. In other neurodegenerative diseases which share features with glaucoma, several other glial metabolic alterations have been identified, suggesting that similar mechanisms and therapeutic targets may exist in glaucoma.

The Inflammasome-Dependent Dysfunction and Death of Retinal Ganglion Cells after Repetitive Intraocular Pressure Spikes

The dysfunction and selective loss of retinal ganglion cells (RGCs) is a known cause of vision loss in glaucoma and other neuropathies, where ocular hypertension (OHT) is the major risk factor. We investigated the impact of transient non-ischemic OHT spikes (spOHT) on RGC function and viability in vivo to identify cellular pathways linking low-grade repetitive mechanical stress to RGC pathology. We found that repetitive spOHT had an unexpectedly high impact on intraocular homeostasis and RGC viability, while exposure to steady OHT (stOHT) of a similar intensity and duration failed to induce pathology. The repetitive spOHT induced the rapid activation of the inflammasome, marked by the upregulation of NLRP1, NLRP3, AIM2, caspases -1, -3/7, -8, and Gasdermin D (GSDMD), and the release of interleukin-1β (IL-1β) and other cytokines into the vitreous. Similar effects were also detected after 5 weeks of exposure to chronic OHT in an induced glaucoma model. The onset of these immune responses in both spOHT and glaucoma models preceded a 50% deficit in pattern electroretinogram (PERG) amplitude and a significant loss of RGCs 7 days post-injury. The inactivation of inflammasome complexes in Nlrp1-/-, Casp1-/-, and GsdmD-/- knockout animals significantly suppressed the spOHT-induced inflammatory response and protected RGCs. Our results demonstrate that mechanical stress produced by acute repetitive spOHT or chronic OHT is mechanistically linked to inflammasome activation, which leads to RGC dysfunction and death.

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

ATP-P2X7R-mediated microglia senescence aggravates retinal ganglion cell injury in chronic ocular hypertension

BACKGROUND

Dysfunction of microglia during aging affects normal neuronal function and results in the occurrence of neurodegenerative diseases. Retinal microglial senescence attributes to retinal ganglion cell (RGC) death in glaucoma. This study aims to examine the role of ATP-P2X₇R in the mediation of microglia senescence and glaucoma progression.

METHODS

Forty-eight participants were enrolled, including 24 patients with primary open-angle glaucoma (POAG) and age-related cataract (ARC) and 24 patients with ARC only. We used ARC as the inclusion criteria because of the availability of aqueous humor (AH) before phacoemulsification. AH was collected and the adenosine triphosphate (ATP) concentration was measured by ATP Assay Kit. The chronic ocular hypertension (COH) mouse model was established by microbead occlusion. Microglia were ablated by feeding PLX5622 orally. Mouse bone marrow cells (BMCs) were prepared and infused into mice through the tail vein for the restoration of microglia function. Western blotting, qPCR and ELISA were performed to analyze protein and mRNA expression in the ocular tissue, respectively. Microglial phenotype and RGC survival were assessed by immunofluorescence. The mitochondrial membrane potential was measured using a JC-1 assay kit by flow cytometry.

RESULTS

ATP concentrations in the AH were increased in older adults and patients with POAG. The expression of P2X₇R was upregulated in the retinal tissues of mice with glaucoma, and functional enrichment analysis showed that P2X₇R was closely related to cell aging. Through in vivo and in vitro approaches, we showed that pathological activation of ATP-P2X₇R induced accelerated microglial senescence through impairing PTEN-induced kinase 1 (PINK1)-mediated mitophagy, which led to RGC damage. Additionally, we found that replacement of senescent microglia in COH model of old mice with BMCs from young mice reversed RGC damage.

CONCLUSION

ATP-P2X₇R induces microglia senescence by inhibiting PINK1-mediated mitophagy pathway. Specific inhibition of ATP-P2X₇R may be a fundamental approach for targeted therapy of RGC injury in microglial aging-related glaucoma.

Molecular signaling from microglia impacts macroglia autophagy and neurons survival in glaucoma

Interactions between microglia and macroglia play important roles in the neurodegeneration of the central nervous system and so is the situation between microglia and Müller cells in retina neurodegenerations like glaucoma. This study focuses on the roles of microglia-derived osteopontin (OPN) in impacting Müller cells and retinal ganglion cells (RGCs). Rat model and cell pressurization culture were used to simulate glaucoma scenarios. Animals were differently treated with anti-OPN, suppressors of OPN receptors (Itgαvβ3/CD44) or microglia inhibitor minocycline, while isolated retinal Müller cells were accordingly treated with conditioned media from microglia culture pretreated with pressuring, overexpression-OPN, SiR-OPN, or minocycline. SB203580 was introduced to explore the role of p38 MAPK signaling pathway. Results revealed microglia may secret OPN to impact Müller cells' autophagy and RGCs survival via binding to Itgαvβ3/CD44 receptors in glaucomatous neurodegeneration with involvement of p38 MAPK pathway. This discovery may benefit understanding neurodegenerative disorders and exploring therapeutics.

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

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

Glaucoma and microglia-induced neuroinflammation

Glaucoma is a multifactorial neurodegenerative disease characterized by a progressive optic neuropathy resulting in visual field defects. Elevated intraocular pressure (IOP) is the greatest risk factor for the development of glaucoma, and IOP reduction therapy is the only treatment currently available. However, there are many cases in which retinal degeneration progresses despite sufficient control of IOP. Therefore, it is important to elucidate the pathophysiology of glaucoma that is resistant to current IOP lowering therapies. Experiments using animal glaucoma models show the relationships between microglial neuroinflammatory responses and damage of retinal ganglion cells (RGCs). Inhibition of neuroinflammatory pathways associated with microglial activation appears to be neuroprotective, indicating that microglia may be an important therapeutic target for RGC protection. In this review, we will focus on microglia-induced neuroinflammation in the pathogenesis of glaucoma to offer new insights into the possibility of developing novel neuroprotective therapies targeting microglia.

Ocular P2 receptors and glaucoma

Adenosine triphosphate (ATP), an energy source currency in cells, is released or leaked to the extracellular space under both physiological and pathological conditions. Extracellular ATP functions as an intercellular signaling molecule through activation of purinergic P2 receptors. Ocular tissue and cells release ATP in response to physiological stimuli such as intraocular pressure (IOP), and P2 receptor activation regulates IOP elevation or reduction. Dysregulated purinergic signaling may cause abnormally elevated IOP, which is one of the major risk factors for glaucoma. Glaucoma, a leading cause of blindness worldwide, is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs), which are essential retinal neurons that transduce visual information to the brain. An elevation in IOP may stress RGCs and increase the risk for glaucoma pathogenesis. In the aqueous humor of human patients with glaucoma, the ATP level is significantly elevated. Such excess amount of ATP may directly cause RGC death via a specific subtype of P2 receptors. Dysregulated purinergic signaling may also trigger inflammation, oxidative stress, and excitotoxicity via activating non-neuronal cell types such as glial cells. In this review, we discussed the physiological roles of extracellular nucleotides in the ocular tissue and their potential role in the pathogenesis of glaucoma. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Detection of phosphates in water utilizing a Eu3+-mediated relay mechanism

Fluorescent carboxyamidoquinolines form ensembles with Eu3+ that can be successfully leveraged in sensing of phosphates showing off–on fluorescence signaling.

Rapid morphologic changes to microglial cells and upregulation of mixed microglial activation state markers induced by P2X7 receptor stimulation and increased intraocular pressure

BACKGROUND

The identification of endogenous signals that lead to microglial activation is a key step in understanding neuroinflammatory cascades. As ATP release accompanies mechanical strain to neural tissue, and as the P2X7 receptor for ATP is expressed on microglial cells, we examined the morphological and molecular consequences of P2X7 receptor stimulation in vivo and in vitro and investigated the contribution of the P2X7 receptor in a model of increased intraocular pressure (IOP).

METHODS

In vivo experiments involved intravitreal injections and both transient and sustained elevation of IOP. In vitro experiments were performed on isolated mouse retinal and brain microglial cells. Morphological changes were quantified in vivo using Sholl analysis. Expression of mRNA for M1- and M2-like genes was determined with qPCR. The luciferin/luciferase assay quantified retinal ATP release while fura-2 indicated cytoplasmic calcium. Microglial migration was monitored with a Boyden chamber.

RESULTS

Sholl analysis of Iba1-stained cells showed retraction of microglial ramifications 1 day after injection of P2X7 receptor agonist BzATP into mouse retinae. Mean branch length of ramifications also decreased, while cell body size and expression of Nos2, Tnfa, Arg1, and Chil3 mRNA increased. BzATP induced similar morphological changes in ex vivo tissue isolated from Cx3CR1+/GFP mice, suggesting recruitment of external cells was unnecessary. Immunohistochemistry suggested primary microglial cultures expressed the P2X7 receptor, while functional expression was demonstrated with Ca2+ elevation by BzATP and block by specific antagonist A839977. BzATP induced process retraction and cell body enlargement within minutes in isolated microglial cells and increased Nos2 and Arg1. While ATP increased microglial migration, this required the P2Y12 receptor and not P2X7 receptor. Transient elevation of IOP led to microglial process retraction, cell body enlargement, and gene upregulation paralleling changes observed with BzATP injection, in addition to retinal ATP release. Pressure-dependent changes were reduced in P2X7-/- mice. Death of retinal ganglion cells accompanied increased IOP in C57Bl/6J, but not P2X7-/- mice, and neuronal loss showed some association with microglial activation.

CONCLUSIONS

P2X7 receptor stimulation induced rapid morphological activation of microglial cells, including process retraction and cell body enlargement, and upregulation of markers linked to both M1- and M2-type activation. Parallel responses accompanied IOP elevation, suggesting ATP release and P2X7 receptor stimulation influence the early microglial response to increased pressure.

Effects of a New Combination of Natural Extracts on Glaucoma-Related Retinal Degeneration

BACKGROUND

Glaucoma is currently the leading cause of irreversible blindness; it is a neuropathy characterized by structural alterations of the optic nerve, leading to visual impairments. The aim of this work is to develop a new oral formulation able to counteract the early changes connected to glaucomatous degeneration. The composition is based on gastrodin and vitamin D3 combined with vitamin C, blackcurrant, and lycopene.

METHODS

Cells and tissues of the retina were used to study biological mechanisms involved in glaucoma, to slow down the progression of the disease. Experiments mimicking the conditions of glaucoma were carried out to examine the etiology of retinal degeneration.

RESULTS

Our results show a significant ability to restore glaucoma-induced damage, by counteracting ROS production and promoting cell survival by inhibiting apoptosis. These effects were confirmed by the intracellular mechanism that was activated following administration of the compound, either before or after the glaucoma induction. In particular, the main results were obtained as a preventive action of glaucoma, showing a beneficial action on all selected markers, both on cells and on eyecup preparations. It is therefore possible to hypothesize both the preventive and therapeutic use of this formulation, in the presence of risk factors, and due to its ability to inhibit the apoptotic cycle and to stimulate cell survival mechanisms, respectively.

CONCLUSION

This formulation has exhibited an active role in the prevention or restoration of glaucoma damage for the first time.

Immune Responses in the Glaucomatous Retina: Regulation and Dynamics

Glaucoma is a multifactorial disease resulting in progressive vision loss due to retinal ganglion cell (RGC) dysfunction and death. Early events in the pathobiology of the disease include oxidative, metabolic, or mechanical stress that acts upon RGC, causing these to rapidly release danger signals, including extracellular ATP, resulting in micro- and macroglial activation and neuroinflammation. Danger signaling also leads to the formation of inflammasomes in the retina that enable maturation of proinflammatory cytokines such IL-1β and IL-18. Chronic neuroinflammation can have directly damaging effects on RGC, but it also creates a proinflammatory environment and compromises the immune privilege of the retina. In particular, continuous synthesis of proinflammatory mediators such as TNFα, IL-1β, and anaphylatoxins weakens the blood–retina barrier and recruits or activates T-cells. Recent data have demonstrated that adaptive immune responses strongly exacerbate RGC loss in animal models of the disease as T-cells appear to target heat shock proteins displayed on the surface of stressed RGC to cause their apoptotic death. It is possible that dysregulation of these immune responses contributes to the continued loss of RGC in some patients.

Loss of P2Y1 receptors triggers glaucoma-like pathology in mice

BACKGROUND AND PURPOSE

Glaucoma, the leading cause of blindness, damages the retinal ganglion cells. Elevated intraocular pressure (IOP) is a high-risk factor for glaucoma, so topical hypotensive drugs are usually used for treatment. Because not all patients do not respond adequately to current treatments, there is a need to identify a new molecular target to reduce IOP. Here, we have assessed the role of P2Y1 receptors in mediating elevated IOP.

EXPERIMENTAL APPROACH

P2Y₁ receptor agonist was instilled into the eyes of mice, and the IOP changes were measured by a rebound-type tonometer. Expression of P2Y₁ receptors was estimated by immunohistochemistry. Ocular function was measured by a multifocal electroretinogram.

KEY RESULTS

A single dose of the P2Y₁ receptor agonist transiently reduced IOP and such effects were absent in P2Y₁ receptor-deficient (P2Y₁ KO) mice. P2Y₁ receptors were functionally expressed in the ciliary body, trabecular meshwork and Schlemm's canal. Activation of P2Y₁ receptors negatively regulated aquaporin 4 (AQP4) function but up-regulated endothelial NOS (eNOS). P2Y₁ KO mice showed chronic ocular hypertension regardless of age. P2Y₁ KO mice at 3 months old showed no damage to retinal ganglion cells, whereas 12-month-old mice showed a significant loss of these cells and impairment of ocular functions. Damage to retinal ganglion cells was attenuated by chronic administration of an IOP-reducing agent.

CONCLUSION AND IMPLICATIONS

Activation of P2Y₁ receptors reduced IOP via dual pathways including AQP4 and eNOS. Loss of P2Y₁ receptors resulted in glaucomatous optic neuropathy, suggesting that P2Y₁ receptors might provide an effective target in the treatment of glaucoma.

Crosstalk Between Dysfunctional Mitochondria and Inflammation in Glaucomatous Neurodegeneration

Mitochondrial dysfunction and excessive inflammatory responses are both sufficient to induce pathology in age-dependent neurodegenerations. However, emerging evidence indicates crosstalk between damaged mitochondrial and inflammatory signaling can exacerbate issues in chronic neurodegenerations. This review discusses evidence for the interaction between mitochondrial damage and inflammation, with a focus on glaucomatous neurodegeneration, and proposes that positive feedback resulting from this crosstalk drives pathology. Mitochondrial dysfunction exacerbates inflammatory signaling in multiple ways. Damaged mitochondrial DNA is a damage-associated molecular pattern, which activates the NLRP3 inflammasome; priming and activation of the NLRP3 inflammasome, and the resulting liberation of IL-1β and IL-18 via the gasdermin D pore, is a major pathway to enhance inflammatory responses. The rise in reactive oxygen species induced by mitochondrial damage also activates inflammatory pathways, while blockage of Complex enzymes is sufficient to increase inflammatory signaling. Impaired mitophagy contributes to inflammation as the inability to turnover mitochondria in a timely manner increases levels of ROS and damaged mtDNA, with the latter likely to stimulate the cGAS-STING pathway to increase interferon signaling. Mitochondrial associated ER membrane contacts and the mitochondria-associated adaptor molecule MAVS can activate NLRP3 inflammasome signaling. In addition to dysfunctional mitochondria increasing inflammation, the corollary also occurs, with inflammation reducing mitochondrial function and ATP production; the resulting downward spiral accelerates degeneration. Evidence from several preclinical models including the DBA/2J mouse, microbead injection and transient elevation of IOP, in addition to patient data, implicates both mitochondrial damage and inflammation in glaucomatous neurodegeneration. The pressure-dependent hypoxia and the resulting metabolic vulnerability is associated with mitochondrial damage and IL-1β release. Links between mitochondrial dysfunction and inflammation can occur in retinal ganglion cells, microglia cells and astrocytes. In summary, crosstalk between damaged mitochondria and increased inflammatory signaling enhances pathology in glaucomatous neurodegeneration, with implications for other complex age-dependent neurodegenerations like Alzheimer's and Parkinson's disease.

Adenosine: The common target between cancer immunotherapy and glaucoma in the eye

Adenosine, an endogenous purine nucleoside, is a well-known actor of the immune system and the inflammatory response both in physiologic and pathologic conditions. By acting upon particular, G-protein coupled adenosine receptors, i.e., A1, A2- a & b, and A3 receptors mediate a variety of intracellular and immunomodulatory actions. Several studies have elucidated Adenosine's effect and its up-and downstream molecules and enzymes on the anti-tumor response against several types of cancers. We have also targeted a couple of molecules to manipulate this pathway and get the immune system's desired response in our previous experiences. Besides, the outgrowth of the studies on ocular Adenosine in recent years has significantly enhanced the knowledge about Adenosine and its role in ocular immunology and the inflammatory response of the eye. Glaucoma is the second leading cause of blindness globally, and the recent application of Adenosine and its derivatives has shown the critical role of the adenosine pathway in its pathophysiology. However, despite a very promising background, the phase III clinical trial of Trabodenoson failed to achieve the non-inferiority goals of the study. In this review, we discuss different aspects of the abovementioned pathway in ophthalmology and ocular immunology; following a brief evaluation of the current immunotherapeutic strategies, we try to elucidate the links between cancer immunotherapy and glaucoma in order to introduce novel therapeutic targets for glaucoma.

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

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

Retinal ganglion cell dysfunction in mice following acute intraocular pressure is exacerbated by P2X7 receptor knockout

There is increasing evidence for the vulnerability of specific retinal ganglion cell (RGC) types in those with glaucoma and in animal models. In addition, the P2X7-receptor (P2X7-R) has been suggested to contribute to RGC death following stimulation and elevated IOP, though its role in RGC dysfunction prior to death has not been examined. Therefore, we examined the effect of an acute, non-ischemic intraocular pressure (IOP) insult (50 mmHg for 30 min) on RGC function in wildtype mice and P2X7-R knockout (P2X7-KO) mice. We examined retinal function using electroretinogram recordings and individual RGC responses using multielectrode arrays, 3 days following acute IOP elevation. Immunohistochemistry was used to examine RGC cell death and P2X7-R expression in several RGC types. Acute intraocular pressure elevation produced pronounced dysfunction in RGCs; whilst other retinal neuronal responses showed lesser changes. Dysfunction at 3 days post-injury was not associated with RGC loss or changes in receptive field size. However, in wildtype animals, OFF-RGCs showed reduced spontaneous and light-elicited activity. In the P2X7-KO, both ON- and OFF-RGC light-elicited responses were reduced. Expression of P2X7-R in wildtype ON-RGC dendrites was higher than in other RGC types. In conclusion, OFF-RGCs were vulnerable to acute IOP elevation and their dysfunction was not rescued by genetic ablation of P2X7-R. Indeed, knockout of P2X7-R also caused ON-RGC dysfunction. These findings aid our understanding of how pressure affects RGC function and suggest treatments targeting the P2X7-R need to be carefully considered.

Potential roles of astrocytes and Müller cells in the pathogenesis of glaucoma

Glaucoma, a progressive optic neuropathy and the leading cause of blindness, is characterized by impairment or degeneration of retinal ganglion cells (RGCs), which transmit visual information to the brain. Currently, 70 million people worldwide are affected by glaucoma. Elevated intraocular pressure (IOP), a major risk factor of glaucoma, directly damages RGCs. However, a substantial proportion of glaucoma patients have a normal IOP level. In particular, over 90% of Japanese glaucoma patients are reported to have normal IOP levels. Thus, a new focus for glaucoma pathology has emerged. Glial cells contribute to tissue homeostasis. Under pathological conditions, glial cells become reactive, lose their homeostatic functions, and gain neurotoxic functions, which trigger neurodegeneration in several diseases including glaucoma. Reactive glial cells have been identified in the eyes of glaucoma patients. In a glaucoma animal model, reactive glial cells are observed at early stages of the disease when RGCs are intact, indicating the possible role of glial cells in the pathogenesis of glaucoma. In this review, we introduce potential roles of glial cells in the pathogenesis of glaucoma. We focus on the roles of the ocular macroglial cells such as astrocytes and Müller cells, and discuss their roles in the pathogenesis of glaucoma.

[Pathogenic roles of retinal glia in glaucoma]

Glaucoma, progressive optic neuropathy, is the first cause of blindness in Japan. Blindness in this disease is induced by damages or degeneration of retinal ganglion cells (RGCs), retinal neurons transmit visual information to brain. An elevated intraocular pressure (IOP) is widely recognized as one of the most important risk factors and that IOP directly damages RGCs by mechanical stress, however, accumulating evidences have shown that a majority of Japanese patients for primary open angle glaucoma shows normal level of IOP. Thus, new target for glaucoma pathology is emerged. In this issue, we introduce potential roles of glial cells for pathogenesis of glaucoma. In the CNS, reactive gliosis has been recognized in a variety of neurodegenerative diseases. Such glial activation is also found in retinae of human glaucoma patients and animal models. Importantly, glial activation precedes RGS degeneration, indicating the possibility that reactive glial cells actively contribute to pathogenesis of glaucoma. In this issue, we will focus on macroglial cells such as Muller cells and astrocytes, and discuss their roles in glaucoma.

Keep an eye on adenosine: Its role in retinal inflammation

Adenosine is an endogenous purine nucleoside ubiquitously distributed throughout the body that interacts with G protein-coupled receptors, classified in four subtypes: A₁R, A_2AR, A_2BR and A₃R. Among the plethora of functions of adenosine, it has been increasingly recognized as a key mediator of the immune response. Neuroinflammation is a feature of chronic neurodegenerative diseases and contributes to the pathophysiology of several retinal degenerative diseases. Animal models of retinal diseases are helping to elucidate the regulatory roles of adenosine receptors in the development and progression of those diseases. Mounting evidence demonstrates that the adenosinergic system is altered in the retina during pathological conditions, compromising retinal physiology. This review focuses on the roles played by adenosine and the elements of the adenosinergic system (receptors, enzymes, transporters) in the neuroinflammatory processes occurring in the retina. An improved understanding of the molecular and cellular mechanisms of the signalling pathways mediated by adenosine underlying the onset and progression of retinal diseases will pave the way towards the identification of new therapeutic approaches.

Glaucoma: A Degenerative Optic Neuropathy Related to Neuroinflammation?

Glaucoma is one of the leading causes of irreversible blindness in the world and remains a major public health problem. To date, incomplete knowledge of this disease’s pathophysiology has resulted in current therapies (pharmaceutical or surgical) unfortunately having only a slowing effect on disease progression. Recent research suggests that glaucomatous optic neuropathy is a disease that shares common neuroinflammatory mechanisms with “classical” neurodegenerative pathologies. In addition to the death of retinal ganglion cells (RGCs), neuroinflammation appears to be a key element in the progression and spread of this disease. Indeed, early reactivity of glial cells has been observed in the retina, but also in the central visual pathways of glaucoma patients and in preclinical models of ocular hypertension. Moreover, neuronal lesions are not limited to retinal structure, but also occur in central visual pathways. This review summarizes and puts into perspective the experimental and clinical data obtained to date to highlight the need to develop neuroprotective and immunomodulatory therapies to prevent blindness in glaucoma patients.

Potential role of P2X7 receptor in neurodegenerative processes in a murine model of glaucoma

Glaucoma is a common cause of visual impairment and blindness, characterized by retinal ganglion cell (RGC) death. The mechanisms that trigger the development of glaucoma remain unknown and have gained significant relevance in the study of this neurodegenerative disease. P2X7 purinergic receptors (P2X7R) could be involved in the regulation of the synaptic transmission and neuronal death in the retina through different pathways. The aim of this study was to characterize the molecular signals underlying glaucomatous retinal injury. The time-course of functional, morphological, and molecular changes in the glaucomatous retina of the DBA/2J mice were investigated. The expression and localization of P2X7R was analysed in relation with retinal markers. Caspase-3, JNK, and p38 were evaluated in control and glaucomatous mice by immunohistochemical and western-blot analysis. Furthermore, electroretinogram recordings (ERG) were performed to assess inner retina dysfunction. Glaucomatous mice exhibited changes in P2X7R expression as long as the pathology progressed. There was P2X7R overexpression in RGCs, the primary injured neurons, which correlated with the loss of function through ERG measurements. All analyzed MAPK and caspase-3 proteins were upregulated in the DBA/2J retinas suggesting a pro-apoptotic cell death. The increase in P2X7Rs presence may contribute, together with other factors, to the changes in retinal functionality and the concomitant death of RGCs. These findings provide evidence of possible intracellular pathways responsible for apoptosis regulation during glaucomatous degeneration.

Targeting P2X7 receptors as a means for treating retinal disease

Age-related macular degeneration and glaucoma are the commonest causes of irreversible vision loss in industrialized countries. The purine ATP is known to regulate a range of cellular functions in the retina via its action on P2 receptors, especially the P2X7 receptor. Although agents that attenuate P2X7 receptor function have been in development for many years, no compound is currently approved for the treatment of eye disease. However, newer compounds that cross the blood-brain barrier could have potential to reduce vision loss. This review will outline recent information relating to the role of P2X7 in age-related macular degeneration and glaucoma and, subsequently, we will discuss recent developments for attenuating P2X7 receptor function.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

The two faces of pannexins: new roles in inflammation and repair

Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin "protective" and "harmful" functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin's dual roles, related to extracellular ATP and K⁺ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets.

Increased Ap4A levels and ecto-nucleotidase activity in glaucomatous mice retina

The pathogenesis of glaucoma involves numerous intracellular mechanisms including the purinergic system contribution. Furthermore, the presence and release of nucleotides and dinucleotides during the glaucomatous damage and the maintenance of degradation machinery through ecto-nucleotidase activity are participating in the modulation of the suitable extracellular complex balance. The aim of this study was to investigate the levels of diadenosine tetraphosphate (Ap₄A) and the pattern of ecto-nucleotidase activity expression in glaucomatous retinas during the progress the pathology. Ap₄A levels were analyzed by HPLC in glaucomatous retinas from the DBA/2J mice at 3, 9, 15, and 23 months of age. For that, retinas were dissected as flattened whole-mounts and stimulated in Ringer buffer with or without 59 mM KCl. NPP1 expression was analyzed by RT-PCR and western blot and its distribution was assessed by immunohistochemistry studies examined under confocal microscopy. Glaucomatous mice exhibited Ap₄A values, which changed in stimulated retinas as long as the pathology progressed varying from 0.73 ± 0.04 (3 months) to 0.170 ± 0.05 pmol/mg retina (23 months). Concomitantly, NPP1 expression was significantly increased (82.15%) in the DBA/2J mice at 15 months. Furthermore, immunohistochemical studies showed that NPP1 labeling was stronger in OPL and IPL labeling tangentially in the vitreal part of the retina and was upregulated at 15 months of age. Our findings demonstrate that Ap₄A decreased levels may be related with exacerbated activity of NPP1 protein in glaucomatous degeneration and in this way contributing to elucidate different mechanisms involved in retinal impairment in glaucomatous degeneration.

Pannexin 1 sustains the electrophysiological responsiveness of retinal ganglion cells

Pannexin 1 (Panx1) forms ATP-permeable membrane channels that play a key role in purinergic signaling in the nervous system in both normal and pathological conditions. In the retina, particularly high levels of Panx1 are found in retinal ganglion cells (RGCs), but the normal physiological function in these cells remains unclear. In this study, we used patch clamp recordings in the intact inner retina to show that evoked currents characteristic of Panx1 channel activity were detected only in RGCs, particularly in the OFF-type cells. The analysis of pattern electroretinogram (PERG) recordings indicated that Panx1 contributes to the electrical output of the retina. Consistently, PERG amplitudes were significantly impaired in the eyes with targeted ablation of the Panx1 gene in RGCs. Under ocular hypertension and ischemic conditions, however, high Panx1 activity permeated cell membranes and facilitated the selective loss of RGCs or stably transfected Neuro2A cells. Our results show that high expression of the Panx1 channel in RGCs is essential for visual function in the inner retina but makes these cells highly sensitive to mechanical and ischemic stresses. These findings are relevant to the pathophysiology of retinal disorders induced by increased intraocular pressure, such as glaucoma.

Elevated Pressure Changes the Purinergic System of Microglial Cells

Glaucoma is the second cause of blindness worldwide and is characterized by the degeneration of retinal ganglion cells (RGCs) and optic nerve atrophy. Increased microglia reactivity is an early event in glaucoma that may precede the loss of RGCs, suggesting that microglia and neuroinflammation are involved in the pathophysiology of this disease. Although global changes of the purinergic system have been reported in experimental and human glaucoma, it is not known if this is due to alterations of the purinergic system of microglial cells, the resident immune cells of the central nervous system. We now studied if elevated hydrostatic pressure (EHP), mimicking ocular hypertension, changed the extracellular levels of ATP and adenosine and the expression, density and activity of enzymes, transporters and receptors defining the purinergic system. The exposure of the murine microglial BV-2 cell line to EHP increased the extracellular levels of ATP and adenosine, increased the density of ecto-nucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1, CD39) and decreased the density of the equilibrative nucleotide transporter 2 as well as the activity of adenosine deaminase. The expression of adenosine A₁ receptor also decreased, but the adenosine A₃ receptor was not affected. Notably, ATP and adenosine selectively control migration rather than phagocytosis, both bolstered by EHP. The results show that the purinergic system is altered in microglia in conditions of elevated pressure. Understanding the impact of elevated pressure on the purinergic system will help to unravel the mechanisms underlying inflammation and neurodegeneration associated with glaucoma.

Purinergic dysregulation causes hypertensive glaucoma-like optic neuropathy

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs) and visual loss. Although one of the highest risk factors for glaucoma is elevated intraocular pressure (IOP) and reduction in IOP is the only proven treatment, the mechanism of IOP regulation is poorly understood. We report that the P2Y6 receptor is critical for lowering IOP and that ablation of the P2Y6 gene in mice (P2Y6KO) results in hypertensive glaucoma-like optic neuropathy. Topically applied uridine diphosphate, an endogenous selective agonist for the P2Y6 receptor, decreases IOP. The P2Y6 receptor was expressed in nonpigmented epithelial cells of the ciliary body and controlled aqueous humor dynamics. P2Y6KO mice exhibited sustained elevation of IOP, age-dependent damage to the optic nerve, thinning of ganglion cell plus inner plexiform layers, and a reduction of RGC numbers. These changes in P2Y6KO mice were attenuated by an IOP lowering agent. Consistent with RGC damage, visual functions were impaired in middle-aged P2Y6KO mice. We also found that expression and function of P2Y6 receptors in WT mice were significantly reduced by aging, another important risk factor for glaucoma. In summary, our data show that dysfunctional purinergic signaling causes IOP dysregulation, resulting in glaucomatous optic neuropathy.

The P2X7 Receptor Primes IL-1β and the NLRP3 Inflammasome in Astrocytes Exposed to Mechanical Strain

Inflammatory responses play a key role in many neural pathologies, with localized signaling from the non-immune cells making critical contributions. The NLRP3 inflammasome is an important component of innate immune signaling and can link neural insult to chronic inflammation. The NLRP3 inflammasome requires two stages to contribute: priming and activation. The priming stage involves upregulation of inflammasome components while the activation stage results in the assembly and activation of the inflammasome complex. The priming step can be rate limiting and can connect insult to chronic inflammation, but our knowledge of the signals that regulate NLRP3 inflammasome priming in sterile inflammation is limited. This study examined the link between mechanical strain and inflammasome priming in neural systems. Transient non-ischemic elevation of intraocular pressure increased mRNA for inflammasome components IL-1β, NLRP3, ASC, and CASP1 in rat and mouse retinas. The elevation was greater 1 day after the insult, with the rise in IL-1β most pronounced. The P2X7 receptor was implicated in the mechanosensitive priming of IL-1β mRNA in vivo, as the antagonist Brilliant Blue G (BBG) blocked the increased expression, the agonist BzATP mimicked the pressure-dependent rise in IL-1β, and the rise was absent in P2X7 knockout mice. In vitro measurements from optic nerve head astrocytes demonstrated an increased expression of IL-1β following stretch or swelling. This increase in IL-1β was eliminated by degradation of extracellular ATP with apyrase, or by the block of pannexin hemichannels with carbenoxolone, probenecid, or 10panx1 peptide. The rise in IL-1β expression was also blocked by P2X7 receptor antagonists BBG, A839977 or A740003. The rise in IL-1β was prevented by blocking transcription factor NFκB with Bay 11-7082, while the swelling-dependent fall in NFκB inhibitor IκB-α was reduced by A839977 and in P2X7 knockout mice. In summary, mechanical trauma to the retina primed NLRP3 inflammasome components, but only if there was ATP release through pannexin hemichannels, and autostimulation of the P2X7 receptor. As the P2X7 receptor can also trigger stage two of inflammasome assembly and activation, the P2X7 receptor may have a central role in linking mechanical strain to neuroinflammation.

The P2X7 receptor links mechanical strain to cytokine IL-6 up-regulation and release in neurons and astrocytes

Mechanical strain in neural tissues can lead to the up-regulation and release of multiple cytokines including interleukin 6 (IL-6). In the retina, the mechanosensitive release of ATP can autostimulate P2X7 receptors on both retinal ganglion cell neurons and optic nerve head astrocytes. Here, we asked whether the purinergic signaling contributed to the IL-6 response to increased intraocular pressure (IOP) in vivo, and stretch or swelling in vitro. Rat and mouse eyes were exposed to non-ischemic elevations in IOP to 50-60 mmHg for 4 h. A PCR array was used to screen cytokine changes, with quantitative (q)PCR used to confirm mRNA elevations and immunoblots used for protein levels. P2X7 antagonist Brilliant Blue G (BBG) and agonist (4-benzoyl-benzoyl)-ATP (BzATP) were injected intravitreally. ELISA was used to quantify IL-6 release from optic nerve head astrocytes or retinal ganglion cells. Receptor identity was confirmed pharmacologically and in P2X7^-/- mice, acute elevation of IOP altered retinal expression of multiple cytokine genes. Elevation of IL-6 was greatest, with expression of IL1rn, IL24, Tnf, Csf1, and Lif also increased more than twofold, while expression of Tnfsf11, Gdf9, and Tnfsf4 were reduced. qPCR confirmed the rise in IL-6 and extracellular ATP marker ENTPD1, but not pro-apoptotic genes. Intravitreal injection of P2X7 receptor antagonist BBG prevented the pressure-dependent rise in IL-6 mRNA and protein in the rat retina, while injection of P2X7 receptor agonist BzATP was sufficient to elevate IL-6 expression. IOP elevation increased IL-6 in wild-type but not P2X7R knockout mice. Application of mechanical strain to isolated optic nerve head astrocytes increased IL-6 levels. This response was mimicked by agonist BzATP, but blocked by antagonists BBG and A839977. Stretch or BzATP led to IL-6 release from both astrocytes and isolated retinal ganglion cells. The mechanosensitive up-regulation and release of cytokine IL-6 from the retina involves the P2X7 receptor, with both astrocytes and neurons contributing to the response.

Targeting Schlemm's Canal in the Medical Therapy of Glaucoma: Current and Future Considerations

Schlemm’s canal (SC) is a unique, complex vascular structure responsible for maintaining fluid homeostasis within the anterior segment of the eye by draining the excess of aqueous humour. In glaucoma, a heterogeneous group of eye disorders afflicting approximately 60 million individuals worldwide, the normal outflow of aqueous humour into SC is progressively hindered, leading to a gradual increase in outflow resistance, which gradually results in elevated intraocular pressure (IOP). By and large available antiglaucoma therapies do not target the site of the pathology (SC), but rather aim to decrease IOP by other mechanisms, either reducing aqueous production or by diverting aqueous flow through the unconventional outflow system. The present review first outlines our current understanding on the functional anatomy of SC. It then summarizes existing research on SC cell properties; first in the context of their role in glaucoma development/progression and then as a target of novel and emerging antiglaucoma therapies. Evidence from ongoing research efforts to develop effective antiglaucoma therapies targeting SC suggests that this could become a promising site of future therapeutic interventions.

Diadenosine tetraphosphate as a potential therapeutic nucleotide to treat glaucoma

Glaucoma is a neurodegenerative disease that produces blindness. The main factor associated with this disease is an abnormally elevated intraocular pressure (IOP). To date, some attempts have been made to demonstrate the role of nucleotides modulating IOP, but never in a model of glaucoma. The DBA/2J mouse is an animal that develops the pathology spontaneously, starting from the typical rise in IOP at 9 months of age. Using this animal model, together with a control mouse, C57BL/6J, it has been possible to monitor the elevation in IOP in the glaucomatous mice and to check the ability of the dinucleotide diadenosine tetraphosphate AKA Ap₄A to reduce IOP. The topical application of Ap₄A when IOP is maximal (9-12 months) reduced IOP 30.6 ± 6.6% in the DBA/2J and 17.9 ± 4.0% in the C57BL/6J mice. Concentration response curves in both animal strains produced similar pD2 values; these being 4.9 ± 0.5 and 5.1 ± 0.4 for the normotensive C57BL/6J and the glaucomatous DBA/2J respectively. Antagonist studies showed differences between the control and the glaucomatous animals. In particular, the main receptor reducing IOP in the control animal was the P2Y₁ receptor and in the glaucomatous model the P2Y₆, although the participation of other P2 receptors cannot be ruled out. The long-term effect of Ap₄A applied three times a week for 3 months showed a clear stop in the elevation of IOP in the glaucomatous model, thus indicating the possibility of using Ap₄A as an effective compound for the treatment of glaucoma.

Retinal glial responses to optic nerve crush are attenuated in Bax-deficient mice and modulated by purinergic signaling pathways

Background

Retinal ganglion cell (RGC) soma death is a consequence of optic nerve damage, including in optic neuropathies like glaucoma. The activation of the innate immune network in the retina after nerve damage has been linked to RGC pathology. Since the eye is immune privileged, innate immune functions are the responsibility of the glia, specifically the microglia, astrocytes, and Müller cells that populate the retina. Glial activation, leading to the production of inflammatory cytokines, is a hallmark feature of retinal injury resulting from optic nerve damage and purported to elicit secondary degeneration of RGC somas.

Methods

A mouse model of optic nerve crush (ONC) was used to study retinal glial activation responses. RGC apoptosis was blocked using Bax-deficient mice. Glial activation responses were monitored by quantitative PCR and immunofluorescent labeling in retinal sections of activation markers. ATP signaling pathways were interrogated using P2X receptor agonists and antagonists and Pannexin 1 (Panx1)-deficient mice with RGC-specific deletion.

Results

ONC induced activation of both macroglia and microglia in the retina, and both these responses were dramatically muted if RGC death was blocked by deletion of the Bax gene. Macroglial, but not microglial, activation was modulated by purinergic receptor activation. Release of ATP after optic nerve damage was not mediated by PANX1 channels in RGCs.

Conclusions

RGC death in response to ONC plays a principal stimulatory role in the retinal glial activation response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0558-y) contains supplementary material, which is available to authorized users.

Müller cell-mediated neurite outgrowth of the retinal ganglion cells via P2Y6 receptor signals

Müller cells, the primary macroglia of the retina, support various functions of retinal ganglion cells (RGCs). Here, we demonstrate a nucleotide-mediated communication between these two types of cells, by which Müller cells control neurite outgrowth of RGCs by activation of P2 receptors such as P2Y₆ . Cultured mouse RGCs had significantly enhanced neurite outgrowth when cultured with either cultured mouse Müller cells or conditioned medium derived from Müller cells, and this was completely inhibited by the nucleotide-degrading enzyme, apyrase. This increase in outgrowth was mimicked by exogenously applied nucleotides such as ATP, uridine triphosphate, and uridine diphosphate. Pharmacological and genetic analysis revealed that P2Y₆ receptor in RGCs was responsible for the increased neurite outgrowth. P2Y₆ receptor was expressed in the ganglion cell layer of the retina and in RGC primary cultures. High performance liquid chromatography has revealed that Müller cells constitutively release uridine triphosphate, which is immediately metabolized into uridine diphosphate, an endogenous agonist for P2Y₆ receptor. In the in vitro ocular hypertension model (i.e., glaucoma model), neurite outgrowth in RGCs was significantly reduced, which was associated with a decrease in P2Y₆ receptors. Taken together, Müller cells control neurite outgrowth of RGCs by activating P2 receptors such as P2Y₆ receptor, and the receptor expression level might be down-regulated in glaucoma. Müller cells support various functions of retina including those of retinal ganglion cells (RGCs). Here, we report an importance of nucleotide-mediated communication between these two types of cells. Müller cells were found to release uridine diphosphate (UTD), uridine triphosphate (UTP), and activate P2Y6 receptors in RGCs, which was essential for neurite outgrowth of RGCs. In addition, P2Y₆ receptors in RGCs were reduced in a glaucoma model in vitro, suggesting an involvement of their dysfunction in the pathogenesis of glaucoma.

The Roles of Mitochondrial Damage-Associated Molecular Patterns in Diseases

SIGNIFICANCE

Mitochondria, vital cellular power plants to generate energy, are involved in immune responses. Mitochondrial damage-associated molecular patterns (DAMPs) are molecules that are released from mitochondria to extracellular space during cell death and include not only proteins but also DNA or lipids. Mitochondrial DAMPs induce inflammatory responses and are critically involved in the pathogenesis of various diseases.

RECENT ADVANCES

Recent studies elucidate the molecular mechanisms by which mitochondrial DAMPs are released and initiate immune responses by use of genetically modulated cells or animals. Importantly, the levels of mitochondrial DAMPs in patients are often associated with severity and prognosis of human diseases, such as infection, asthma, ischemic heart disease, and cancer.

CRITICAL ISSUES

Although mitochondrial DAMPs can represent proinflammatory molecules in various experimental models, their roles in human diseases may be multifunctional and complex. It remains unclear where and how mitochondrial DAMPs are liberated into extracellular spaces and exert their biological functions particularly in vivo. In addition, while mitochondria can secrete several types of DAMPs during cell death, the interaction of each mitochondrial DAMP (e.g., synergistic effects) remains unclear.

FUTURE DIRECTIONS

Regulation of mitochondrial DAMP-mediated immune responses may be important to alter the progression of human diseases. In addition, measuring mitochondrial DAMPs in patients may be clinically useful as biomarkers to predict prognosis or response to therapies. Further studies of the mechanisms by which mitochondrial DAMPs impact the initiation and progression of diseases may lead to the development of therapeutics specifically targeting this pathway. Antioxid.

Rat, mouse, and primate models of chronic glaucoma show sustained elevation of extracellular ATP and altered purinergic signaling in the posterior eye

PURPOSE

The cellular mechanisms linking elevated IOP with glaucomatous damage remain unresolved. Mechanical strains and short-term increases in IOP can trigger ATP release from retinal neurons and astrocytes, but the response to chronic IOP elevation is unknown. As excess extracellular ATP can increase inflammation and damage neurons, we asked if sustained IOP elevation was associated with a sustained increase in extracellular ATP in the posterior eye.

METHODS

No ideal animal model of chronic glaucoma exists, so three different models were used. Tg-Myoc(Y437H) mice were examined at 40 weeks, while IOP was elevated in rats following injection of hypertonic saline into episcleral veins and in cynomolgus monkeys by laser photocoagulation of the trabecular meshwork. The ATP levels were measured using the luciferin-luciferase assay while levels of NTPDase1 were assessed using qPCR, immunoblots, and immunohistochemistry.

RESULTS

The ATP levels were elevated in the vitreal humor of rats, mice, and primates after a sustained period of IOP elevation. The ecto-ATPase NTPDase1 was elevated in optic nerve head astrocytes exposed to extracellular ATP for an extended period. NTPDase1 was also elevated in the retinal tissue of rats, mice, and primates, and in the optic nerve of rats, with chronic elevation in IOP.

CONCLUSIONS

A sustained elevation in extracellular ATP, and upregulation of NTPDase1, occurs in the posterior eye of rat, mouse, and primate models of chronic glaucoma. This suggests the elevation in extracellular ATP may be sustained in chronic glaucoma, and implies a role for altered purinergic signaling in the disease.

Increased levels of extracellular ATP in glaucomatous retinas: Possible role of the vesicular nucleotide transporter during the development of the pathology

PURPOSE

To study retinal extracellular ATP levels and to assess the changes in the vesicular nucleotide transporter (VNUT) expression in a murine model of glaucoma during the development of the disease.

METHODS

Retinas were obtained from glaucomatous DBA/2J mice at 3, 9, 15, and 22 months together with C57BL/6J mice used as age-matched controls. To study retinal nucleotide release, the retinas were dissected and prepared as flattened whole mounts and stimulated in Ringer buffer with or without 59 mM KCl. To investigate VNUT expression, sections of the mouse retinas were evaluated with immunohistochemistry and western blot analysis using newly developed antibodies against VNUT. All images were examined and photographed under confocal microscopy. Electroretinogram (ERG) recordings were performed on the C57BL/6J and DBA/2J mice to analyze the changes in the electrophysiological response; a decrease in the scotopic threshold response was observed in the 15-month-old DBA/2J mice.

RESULTS

In the 15-month-old control and glaucomatous mice, electrophysiological changes of 42% were observed. In addition, 50% increases in the intraocular pressure (IOP) were observed when the pathology was fully established. The responses in the retinal ATP net release as the pathology progressed varied from 0.32±0.04 pmol/retina (3 months) to 1.10±0.06 pmol/retina (15 months; threefold increase). Concomitantly, VNUT expression was significantly increased during glaucoma progression in the DBA/2J mice (58%) according to the immunohistochemical and western blot analysis.

CONCLUSIONS

These results may indicate a possible correlation between retinal dysfunction and increased levels of extracellular ATP and nucleotide transporter. These data support an excitotoxicity role for ATP via P2X7R in glaucoma. This modified cellular environment could contribute to explaining the functional and biochemical alterations observed during the development of the pathology.

Neuroinflammation in Glaucoma and Optic Nerve Damage

Glaucoma is a group of optic neuropathies characterized by the degeneration of retinal ganglion cell axons and somas, ultimately preventing light signals in the retina from reaching the brain. Glaucoma is a leading cause of blindness in the world, and treatment options for patients remain limited and minimally efficacious. A number of mechanisms have been linked to glaucomatous pathophysiology. A leading role is now attributed to neuroinflammatory conditions generated by the resident innate immune cells in the optic nerve and retina. Since the eye is immune privileged, the adaptation of these innate immune cells, termed glia, is crucial following trauma. In this chapter, we discuss the mechanisms associated with normal glial function in a healthy eye, and how changes in glial activation can contribute to the process of glaucomatous neurodegeneration in both the optic nerve and retina.

Adenosine A2AR blockade prevents neuroinflammation-induced death of retinal ganglion cells caused by elevated pressure

BACKGROUND

Elevated intraocular pressure (IOP) is a major risk factor for glaucoma, a degenerative disease characterized by the loss of retinal ganglion cells (RGCs). There is clinical and experimental evidence that neuroinflammation is involved in the pathogenesis of glaucoma. Since the blockade of adenosine A2A receptor (A2AR) confers robust neuroprotection and controls microglia reactivity in the brain, we now investigated the ability of A2AR blockade to control the reactivity of microglia and neuroinflammation as well as RGC loss in retinal organotypic cultures exposed to elevated hydrostatic pressure (EHP) or lipopolysaccharide (LPS).

METHODS

Retinal organotypic cultures were either incubated with LPS (3 μg/mL), to elicit a pro-inflammatory response, or exposed to EHP (+70 mmHg), to mimic increased IOP, for 4 or 24 h, in the presence or absence of the A2AR antagonist SCH 58261 (50 nM). A2AR expression, microglial reactivity and neuroinflammatory response were evaluated by immunohistochemistry, quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). RGC loss was assessed by immunohistochemistry. In order to investigate the contribution of pro-inflammatory mediators to RGC loss, the organotypic retinal cultures were incubated with rabbit anti-tumour necrosis factor (TNF) (2 μg/mL) and goat anti-interleukin-1β (IL-1β) (1 μg/mL) antibodies.

RESULTS

We report that the A2AR antagonist (SCH 58261) prevented microglia reactivity, increase in pro-inflammatory mediators as well as RGC loss upon exposure to either LPS or EHP. Additionally, neutralization of TNF and IL-1β prevented RGC loss induced by LPS or EHP.

CONCLUSIONS

This work demonstrates that A2AR blockade confers neuroprotection to RGCs by controlling microglia-mediated retinal neuroinflammation and prompts the hypothesis that A2AR antagonists may be a novel therapeutic option to manage glaucomatous disorders.

Emerging functions of pannexin 1 in the eye

Pannexin 1 (Panx1) is a high-conductance, voltage-gated channel protein found in vertebrates. Panx1 is widely expressed in many organs and tissues, including sensory systems. In the eye, Panx1 is expressed in major divisions including the retina, lens and cornea. Panx1 is found in different neuronal and non-neuronal cell types. The channel is mechanosensitive and responds to changes in extracellular ATP, intracellular calcium, pH, or ROS/nitric oxide. Since Panx1 channels operate at the crossroad of major signaling pathways, physiological functions in important autocrine and paracrine feedback signaling mechanisms were hypothesized. This review starts with describing in depth the initial Panx1 expression and localization studies fostering functional studies that uncovered distinct roles in processing visual information in subsets of neurons in the rodent and fish retina. Panx1 is expressed along the entire anatomical axis from optical nerve to retina and cornea in glia, epithelial and endothelial cells as well as in neurons. The expression and diverse localizations throughout the eye points towards versatile functions of Panx1 in neuronal and non-neuronal cells, implicating Panx1 in the crosstalk between immune and neural cells, pressure related pathological conditions like glaucoma, wound repair or neuronal cell death caused by ischemia. Summarizing the literature on Panx1 in the eye highlights the diversity of emerging Panx1 channel functions in health and disease.

Mechanosensitive release of adenosine 5'-triphosphate through pannexin channels and mechanosensitive upregulation of pannexin channels in optic nerve head astrocytes: a mechanism for purinergic involvement in chronic strain

As adenosine 5'-triphosphate (ATP) released from astrocytes can modulate many neural signaling systems, the triggers and pathways for this ATP release are important. Here, the ability of mechanical strain to trigger ATP release through pannexin channels and the effects of sustained strain on pannexin expression were examined in rat optic nerve head astrocytes. Astrocytes released ATP when subjected to 5% of equibiaxial strain or to hypotonic swelling. Although astrocytes expressed mRNA for pannexins 1-3, connexin 43, and VNUT, pharmacological analysis suggested a predominant role for pannexins in mechanosensitive ATP release, with Rho kinase contribution. Astrocytes from panx1(-/-) mice had reduced baseline and stimulated levels of extracellular ATP, confirming the role for pannexins. Swelling astrocytes triggered a regulatory volume decrease that was inhibited by apyrase or probenecid. The swelling-induced rise in calcium was inhibited by P2X7 receptor antagonists A438079 and AZ10606120, in addition to apyrase and carbenoxolone. Extended stretch of astrocytes in vitro upregulated expression of panx1 and panx2 mRNA. A similar upregulation was observed in vivo in optic nerve head tissue from the Tg-MYOC(Y437H) mouse model of chronic glaucoma; genes for panx1, panx2, and panx3 were increased, whereas immunohistochemistry confirmed increased expression of pannexin 1 protein. In summary, astrocytes released ATP in response to mechanical strain, with pannexin 1 the predominant efflux pathway. Sustained strain upregulated pannexins in vitro and in vivo. Together, these findings provide a mechanism by which extracellular ATP remains elevated under chronic mechanical strain, as found in the optic nerve head of patients with glaucoma.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Increased expression of the transient receptor potential cation channel 6 gene in patients with primary open-angle glaucoma

BACKGROUND

The result of primary open-angle glaucoma is the loss of retinal ganglion cells. Transient receptor potential cation channel 6 is a pressure-related channel that may function in the survival of retinal ganglion cells. The purpose of this study was to evaluate the expression levels of the transient receptor potential cation channel 6 gene in patients with primary open-angle glaucoma.

DESIGN

Randomization study at Zhongshan Ophthalmic Center in China.

PARTICIPANTS

80 primary open-angle glaucoma patients and 75 cataract patients recruited from Zhongshan Ophthalmic Center.

METHODS

Total RNA was extracted from the leukocytes of the peripheral blood collected. The levels of transient receptor potential cation channel 6-messenger RNA were determined by real-time polymerase chain reaction. Related factors including age, intraocular pressure, optic cup-to-disc ratio and visual field defect were analysed accordingly.

MAIN OUTCOME MEASURES

Clinical examination and the messenger RNA level.

RESULTS

The expression level of the transient receptor potential cation channel 6 gene in the leukocytes of primary open-angle glaucoma patients was two times higher when compared with control cataract patients. The gene expression level was also correlated with intraocular pressure and cup-to-disc ratio. Treatment with different anti-glaucoma drugs did not affect the gene expression.

CONCLUSIONS

Increasing expression levels of the transient receptor potential cation channel 6 gene in the blood accompanies chronic elevation of intraocular pressure in primary open-angle glaucoma and may serve as a genetic biomarker for primary open-angle glaucoma.