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

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 system in psychiatric diseases

Psychiatric disorders are debilitating diseases, affecting >80 million people worldwide. There are no causal cures for psychiatric disorders and available therapies only treat the symptoms. The etiology of psychiatric disorders is unknown, although it has been speculated to be a combination of environmental, stress and genetic factors. One of the neurotransmitter systems implicated in the biology of psychiatric disorders is the purinergic system. In this review, we performed a comprehensive search of the literature about the role and function of the purinergic system in the development and predisposition to psychiatric disorders, with a focus on depression, schizophrenia, bipolar disorder, autism, anxiety and attention deficit/hyperactivity disorder. We also describe how therapeutics used for psychiatric disorders act on the purinergic system.

Microglial dynamics after axotomy-induced retinal ganglion cell death

BACKGROUND

Microglial cells (MCs) are the sentries of the central nervous system. In health, they are known as surveying MCs because they examine the tissue to maintain the homeostasis. In disease, they activate and, among other functions, become phagocytic to clean the cellular debris. In this work, we have studied the behavior of rat retinal MCs in two models of unilateral complete intraorbital optic nerve axotomy which elicit a different time course of retinal ganglion cell (RGC) loss.

METHODS

Albino Sprague-Dawley rats were divided into these groups: (a) intact (no surgery), (b) fluorogold (FG) tracing from the superior colliculi, and (c) FG tracing + crush or transection of the left optic nerve. The retinas were dissected from 2 days to 2 months after the lesions (n = 4-12 group/lesion and time point) and then were subjected to Brn3a and Iba1 double immunodetection. In each intact retina, the total number of Brn3a+RGCs and Iba+MCs was quantified. In each traced retina (b and c groups), FG-traced RGCs and phagocytic microglial cells (PMCs, FG+Iba+) were also quantified. Topographical distribution was assessed by neighbor maps.

RESULTS

In intact retinas, surveying MCs are homogenously distributed in the ganglion cell layer and the inner plexiform layer. Independently of the axotomy model, RGC death occurs in two phases, one quick and one protracted, and there is a lineal and topographical correlation between the appearance of PMCs and the loss of traced RGCs. Furthermore, the clearance of FG+RGCs by PMCs occurs 3 days after the actual loss of Brn3a expression that marks RGC death. In addition, almost 50% of MCs from the inner plexiform layer migrate to the ganglion cell layer during the quick phase of RGC loss, returning to the inner plexiform layer during the slow degeneration phase. Finally, in contrast to what happens in mice, in rats, there is no microglial phagocytosis in the contralateral uninjured retina.

CONCLUSIONS

Axotomy-induced RGC death occurs earlier than RGC clearance and there is an inverse correlation between RGC loss and PMC appearance, both numerically and topographically, suggesting that phagocytosis occurs as a direct response to RGC death rather than to axonal damage.

Treatment with A2A receptor antagonist KW6002 and caffeine intake regulate microglia reactivity and protect retina against transient ischemic damage

Transient retinal ischemia is a major complication of retinal degenerative diseases and contributes to visual impairment and blindness. Evidences indicate that microglia-mediated neuroinflammation has a key role in the neurodegenerative process, prompting the hypothesis that the control of microglia reactivity may afford neuroprotection to the retina against the damage induced by ischemia–reperfusion (I–R). The available therapeutic strategies for retinal degenerative diseases have limited potential, but the blockade of adenosine A_2A receptor (A_2AR) emerges as candidate strategy. Therefore, we evaluated the therapeutic potential of a selective A_2AR antagonist (KW6002) against the damage elicited by I–R. The administration of KW6002 after I–R injury reduced microglia reactivity and inflammatory response and afforded protection to the retina. Moreover, we tested the ability of caffeine, an adenosine receptor antagonist, in mediating protection to the retina in the I–R injury model. We demonstrated that caffeine administration dually regulated microglia reactivity and cell death in the transient retinal ischemic model, depending on the reperfusion time. At 24 h of reperfusion, caffeine increased microglial reactivity, inflammatory response and cell death elicited by I–R. However, at 7 days of reperfusion, caffeine administration decreased microglia reactivity and reduced the levels of proinflammatory cytokines and cell death. Together, these results provide a novel evidence for the use of adenosine A_2AR antagonists as potential therapy for retinal ischemic diseases and demonstrate the effect of caffeine on the regulation of microglia-mediated neuroinflammation in the transient ischemic model.

Caffeine Prevents Blood Retinal Barrier Damage in a Model, In Vitro, of Diabetic Macular Edema

Diabetic macular edema (DME) is the major cause of vision loss in patients affected by diabetic retinopathy. Hyperglycemia and hypoxia represent the key elements in the progression of these pathologies, leading to breakdown of the blood-retinal barrier (BRB). Caffeine, a psychoactive substance largely consumed in the world, is a nonselective antagonist of adenosine receptors (AR) and it possesses a protective effect in various diseases, including eye pathologies. Here, we have investigated the effect of this substance on BRB integrity following exposure to hyperglycemic/hypoxic insult. Retinal pigmented epithelial cells, ARPE-19, have been grown on semi-permeable supports mimicking an experimental model, in vitro, of outer BRB. Caffeine treatment has reduced cell monolayer permeability after exposure to high glucose and desferoxamine as shown by TEER and FITC-dextran permeability assays. This effect is also mediated through the restoration of membrane's tight junction expression, ZO-1. Moreover, we have demonstrated that caffeine is able to prevent outer BRB damage by inhibiting apoptotic cell death induced by hyperglycemic/hypoxic insult since it downregulates the proapoptotic Bax and upregulates the anti-apoptotic Bcl-2 genes. Although further studies are needed to better comprise the beneficial effect of caffeine, we can speculate that it might be used as an innovative drug for DME treatment. J. Cell. Biochem. 118: 2371-2379, 2017. © 2017 Wiley Periodicals, Inc.

Having a Coffee Break: The Impact of Caffeine Consumption on Microglia-Mediated Inflammation in Neurodegenerative Diseases

Caffeine is the major component of coffee and the most consumed psychostimulant in the world and at nontoxic doses acts as a nonselective adenosine receptor antagonist. Epidemiological evidence suggests that caffeine consumption reduces the risk of several neurological and neurodegenerative diseases. However, despite the beneficial effects of caffeine consumption in human health and behaviour, the mechanisms by which it impacts the pathophysiology of neurodegenerative diseases still remain to be clarified. A promising hypothesis is that caffeine controls microglia-mediated neuroinflammatory response associated with the majority of neurodegenerative conditions. Accordingly, it has been already described that the modulation of adenosine receptors, namely, the A2A receptor, affords neuroprotection through the control of microglia reactivity and neuroinflammation. In this review, we will summarize the main effects of caffeine in the modulation of neuroinflammation in neurodegenerative diseases.

Translational Pharmacology in Glaucoma Neuroprotection

Glaucoma is both the most common optic neuropathy worldwide and the most common cause of irreversible blindness in the world. The only proven treatment for glaucomatous optic neuropathy is lowering the intraocular pressure, achieved with a variety of pharmacological, laser, and surgical approaches. Over the past 2 decades there has been much basic and clinical research into achieving treatment of the underlying optic nerve damage with neuroprotective approaches. However, none has resulted in regulatory approval based on successful phase 3 studies. This chapter discusses the reasons for this "lost in translation" aspect of glaucoma neuroprotection, and outlines issues at the laboratory and clinical trial level that need to be addressed for successful development of neuroprotective therapies.

Mimicking the ocular environment for the study of inflammatory posterior eye disorders

The common inflammatory posterior eye disorders, age-related degeneration and glaucoma often lead to irreversible vision loss. Current treatments do not target early stages or prevent disease progression. Consequently, the identification of biomarkers or early disease models that can accurately mimic the pathological processes involved is essential. Although none of the existing models can recapitulate all pathological aspects of these disorders, these models have revealed new therapeutic targets. Efforts to accurately phenotype eye disorders at various disease stages are warranted to generate a 'super' model that can replicate the microenvironment of the eye and associated pathological hallmarks effectively.

Ocular Purine Receptors as Drug Targets in the Eye

Agonists and antagonists of various subtypes of G protein coupled adenosine receptors (ARs), P2Y receptors (P2YRs), and ATP-gated P2X receptor ion channels (P2XRs) are under consideration as agents for the treatment of ocular diseases, including glaucoma and dry eye. Numerous nucleoside and nonnucleoside modulators of the receptors are available as research tools and potential therapeutic molecules. Three of the 4 subtypes of ARs have been exploited with clinical candidate molecules for treatment of the eye: A1, A2A, and A3. An A1AR agonist is in clinical trials for glaucoma, A2AAR reduces neuroinflammation, A3AR protects retinal ganglion cells from apoptosis, and both A3AR agonists and antagonists had been reported to lower intraocular pressure (IOP). Extracellular concentrations of endogenous nucleotides, including dinucleoside polyphosphates, are increased in pathological states, activating P2Y and P2XRs throughout the eye. P2YR agonists, including P2Y2 and P2Y6, lower IOP. Antagonists of the P2X7R prevent the ATP-induced neuronal apoptosis in the retina. Thus, modulators of the purinome in the eye might be a source of new therapies for ocular diseases.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Caffeine exposure alters adenosine system and neurochemical markers during retinal development

Evidence points to beneficial properties of caffeine in the adult central nervous system, but teratogenic effects have also been reported. Caffeine exerts most of its effects by antagonizing adenosine receptors, especially A1 and A2A subtypes. In this study, we evaluated the role of caffeine on the expression of components of the adenosinergic system in the developing avian retina and the impact of caffeine exposure upon specific markers for classical neurotransmitter systems. Caffeine exposure (5-30 mg/kg by in ovo injection) to 14-day-old chick embryos increased the expression of A1 receptors and concomitantly decreased A2A adenosine receptors expression after 48 h. Accordingly, caffeine (30 mg/kg) increased [(3) H]-8-cyclopentyl-1,3-dipropylxanthine (A1 antagonist) binding and reduced [(3) H]-ZM241385 (A2A antagonist) binding. The caffeine time-response curve demonstrated a reduction in A1 receptors 6 h after injection, but an increase after 18 and 24 h. In contrast, caffeine exposure increased the expression of A2A receptors from 18 and 24 h. Kinetic assays of [(3) H]-S-(4-nitrobenzyl)-6-thioinosine binding to the equilibrative adenosine transporter ENT1 revealed an increase in Bmax with no changes in Kd , an effect accompanied by an increase in adenosine uptake. Immunohistochemical analysis showed a decrease in retinal content of tyrosine hydroxylase, calbindin and choline acetyltransferase, but not Brn3a, after 48 h of caffeine injection. Furthermore, retinas exposed to caffeine had increased levels of phosphorylated extracellular signal-regulated kinase and cAMP-response element binding protein. Overall, we show an in vivo regulation of the adenosine system, extracellular signal-regulated kinase and cAMP-response element binding protein function and protein expression of specific neurotransmitter systems by caffeine in the developing retina. The beneficial or maleficent effects of caffeine have been demonstrated by the work of different studies. It is known that during animal development, caffeine can exert harmful effects, impairing the correct formation of CNS structures. In this study, we demonstrated cellular and tissue effects of caffeine's administration on developing chick embryo retinas. Those effects include modulation of adenosine receptors (A1 , A2 ) content, increasing in cAMP response element-binding protein (pCREB) and extracellular signal-regulated kinase phosphorylation (pERK), augment of adenosine equilibrative transporter content/activity, and a reduction of some specific cell subpopulations. ENT1, Equilibrative nucleoside transporter 1.

Caffeine administration prevents retinal neuroinflammation and loss of retinal ganglion cells in an animal model of glaucoma

Glaucoma is the second leading cause of blindness worldwide, being characterized by progressive optic nerve damage and loss of retinal ganglion cells (RGCs), accompanied by increased inflammatory response involving retinal microglial cells. The etiology of glaucoma is still unknown, and despite elevated intraocular pressure (IOP) being a major risk factor, the exact mechanisms responsible for RGC degeneration remain unknown. Caffeine, which is an antagonist of adenosine receptors, is the most widely consumed psychoactive drug in the world. Several evidences suggest that caffeine can attenuate the neuroinflammatory responses and afford protection upon central nervous system (CNS) injury. We took advantage of a well characterized animal model of glaucoma to investigate whether caffeine administration controls neuroinflammation and elicits neuroprotection. Caffeine or water were administered ad libitum and ocular hypertension (OHT) was induced by laser photocoagulation of the limbal veins in Sprague Dawley rats. Herein, we show that caffeine is able to partially decrease the IOP in ocular hypertensive animals. More importantly, we found that drinking caffeine prevented retinal microglia-mediated neuroinflammatory response and attenuated the loss of RGCs in animals with ocular hypertension (OHT). This study opens the possibility that caffeine or adenosine receptor antagonists might be a therapeutic option to manage RGC loss in glaucoma.

Neurodegeneration and Neuroprotection in Glaucoma

Glaucoma is the principal cause of irreversible blindness in the world. The disease leads to progressive optic nerve degeneration with a gradual loss of retinal ganglion cells. Neurodegeneration in glaucoma extends beyond the eye into the lateral geniculate nucleus and visual cortex, and the disease even shares some characteristics with other central nervous system degenerative disorders. Glaucoma destroys neurons through oxidative stress, impairment in axonal transport, neuroinflammation, and excitotoxicity. Autophagy may promote or inhibit disease progression. Currently, lowering intraocular pressure is the only way proven to delay glaucoma advancement. However, many new therapies are being developed, including antioxidants, adenosine receptor antagonists, Rho-pathway inhibitors, stem cell therapy, and neurotrophic factors. These therapies focus on neuroprotection, and they may eventually halt glaucoma progression or reverse the process of the disease itself.

Selective A2A receptor antagonist prevents microglia-mediated neuroinflammation and protects retinal ganglion cells from high intraocular pressure-induced transient ischemic injury

Glaucoma is a leading cause of vision loss and blindness worldwide, characterized by chronic and progressive neuronal loss. Reactive microglial cells have been recognized as a neuropathologic feature, contributing to local inflammation and retinal neurodegeneration. In a recent in vitro work (organotypic cultures), we demonstrated that blockade of adenosine A2A receptor (A2AR) prevents the neuroinflammatory response and affords protection to retinal ganglion cells (RGCs) against exposure to elevated hydrostatic pressure (EHP), to mimic elevated intraocular pressure (IOP), the main risk factor for glaucoma development. Herein, we investigated whether a selective A2AR antagonist (SCH 58261) could modulate retinal microglia reactivity and their inflammatory response. Furthermore, we took advantage of the high IOP-induced transient ischemia (ischemia-reperfusion, I-R) animal model to evaluate the protective role of A2AR blockade in the control of retinal neuroinflammation and neurodegeneration. Primary microglial cell cultures were challenged either with lipopolysaccharide or with EHP, in the presence or absence of A2AR antagonist SCH 58261 (50 nM). In addition, I-R injury was induced in adult Wistar rats after intravitreal administration of SCH 58261 (100 nM, 5 μL). Our results showed that SCH 58261 attenuated microglia reactivity and the increased expression and release of proinflammatory cytokines. Moreover, intravitreal administration of SCH 58261 prevented I-R-induced cell death and RGC loss, by controlling microglial-mediated neuroinflammatory response. These results prompt the proposal that A2AR blockade may have great potential in the management of retinal neurodegenerative diseases characterized by microglia reactivity and RGC death, such as glaucoma and ischemic diseases.