Elevated Pressure Changes the Purinergic System of Microglial Cells

Gene regulation in activated microglia by adenosine A3 receptor agonists: a transcriptomics study

Most neurodegenerative disorders, including the two most common, Alzheimer's disease (AD) and Parkinson's disease (AD), course with activation of microglia, the resident innate immune cells of the central nervous system. A3 adenosine receptor (A3R) agonists have been proposed to be neuroprotective by regulating the phenotype of activated microglia. RNAseq was performed using samples isolated from lipopolysaccharide/interferon-γ activated microglia treated with 2-Cl-IB-MECA, a selective A3R agonist. The results showed that the number of negatively regulated genes in the presence of 2-Cl-IB-MECA was greater than the number of positively regulated genes. Gene ontology enrichment analysis showed regulation of genes participating in several cell processes, including those involved in immune-related events. Analysis of known and predicted protein-protein interactions showed that Smad3 and Sp1 are transcription factors whose genes are regulated by A3R activation. Under the conditions of cell activation and agonist treatment regimen, 2-Cl-IB-MECA did not lead to any tendency to favor the expression of genes related to neuroprotective microglia (M2).

Microglia: The breakthrough to treat neovascularization and repair blood-retinal barrier in retinopathy

Microglia are the primary resident retinal macrophages that monitor neuronal activity in real-time and facilitate angiogenesis during retinal development. In certain retinal diseases, the activated microglia promote retinal angiogenesis in hypoxia stress through neurovascular coupling and guide neovascularization to avascular areas (e.g., the outer nuclear layer and macula lutea). Furthermore, continuously activated microglia secrete inflammatory factors and expedite the loss of the blood-retinal barrier which causes irreversible damage to the secondary death of neurons. In this review, we support microglia can be a potential cellular therapeutic target in retinopathy. We briefly describe the relevance of microglia to the retinal vasculature and blood-retinal barrier. Then we discuss the signaling pathway related to how microglia move to their destinations and regulate vascular regeneration. We summarize the properties of microglia in different retinal disease models and propose that reducing the number of pro-inflammatory microglial death and conversing microglial phenotypes from pro-inflammatory to anti-inflammatory are feasible for treating retinal neovascularization and the damaged blood-retinal barrier (BRB). Finally, we suppose that the unique properties of microglia may aid in the vascularization of retinal organoids.

Will Sirtuin 2 Be a Promising Target for Neuroinflammatory Disorders?

Neuroinflammatory disorder is a general term that is associated with the progressive loss of neuronal structure or function. At present, the widely studied diseases with neuroinflammatory components are mainly divided into neurodegenerative and neuropsychiatric diseases, namely, Alzheimer’s disease, Parkinson’s disease, depression, stroke, and so on. An appropriate neuroinflammatory response can promote brain homeostasis, while excessive neuroinflammation can inhibit neuronal regeneration and damage the central nervous system. Apart from the symptomatic treatment with cholinesterase inhibitors, antidepressants/anxiolytics, and neuroprotective drugs, the treatment of neuroinflammation is a promising therapeutic method. Sirtuins are a host of class III histone deacetylases, that require nicotinamide adenine dinucleotide for their lysine residue deacetylase activity. The role of sirtuin 2 (SIRT2), one of the sirtuins, in modulating senescence, myelin formation, autophagy, and inflammation has been widely studied. SIRT2 is associated with many neuroinflammatory disorders considering it has deacetylation properties, that regulate the entire immune homeostasis. The aim of this review was to summarize the latest progress in regulating the effects of SIRT2 on immune homeostasis in neuroinflammatory disorders. The overall structure and catalytic properties of SIRT2, the selective inhibitors of SIRT2, the relationship between immune homeostasis and SIRT2, and the multitasking role of SIRT2 in several diseases with neuroinflammatory components were discussed.

Expression of the Adenosine A2A-A3 Receptor Heteromer in Different Brain Regions and Marked Upregulation in the Microglia of the Transgenic APPSw,Ind Alzheimer’s Disease Model

Adenosine (Ado) receptors have been instrumental in the detection of heteromers and other higher-order receptor structures, mainly via interactions with other cell surface G-protein-coupled receptors. Apart from the first report of the A₁ Ado receptor interacting with the A_2A Ado receptor, there has been more recent data on the possibility that every Ado receptor type, A₁, A_2A, A_2B, and A₃, may interact with each other. The aim of this paper was to look for the expression and function of the A_2A/A₃ receptor heteromer (A_2AA₃Het) in neurons and microglia. In situ proximity ligation assays (PLA), performed in primary cells, showed that A_2AA₃Het expression was markedly higher in striatal than in cortical and hippocampal neurons, whereas it was similar in resting and activated microglia. Signaling assays demonstrated that the effect of the A_2AR agonist, PSB 777, was reduced in the presence of the A₃R agonist, 2-Cl-IB-MECA, whereas the effect of the A₃R agonist was potentiated by the A_2AR antagonist, SCH 58261. Interestingly, the expression of the heteromer was markedly enhanced in microglia from the APP_Sw,Ind model of Alzheimer’s disease. The functionality of the heteromer in primary microglia from APP_Sw,Ind mice was more similar to that found in resting microglia from control mice.

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.

Adenosine receptors as promising targets for the management of ocular diseases

The ocular drug discovery arena has undergone a significant improvement in the last few years culminating in the FDA approvals of 8 new drugs. However, despite a large number of drugs, generics, and combination products available, it remains an urgent need to find breakthrough strategies and therapies for tackling ocular diseases. Targeting the adenosinergic system may represent an innovative strategy for discovering new ocular therapeutics. This review focused on the recent advance in the field and described the numerous nucleoside and non-nucleoside modulators of the four adenosine receptors (ARs) used as potential tools or clinical drug candidates.

Activation of Adenosine A3 Receptor Inhibits Microglia Reactivity Elicited by Elevated Pressure

Glaucoma is a progressive chronic retinal degenerative disease and a leading cause of global irreversible blindness, characterized by optic nerve damage and retinal ganglion cell (RGC) death. Elevated intraocular pressure (IOP) is a main risk factor of glaucoma. Neuroinflammation plays an important role in glaucoma. We have been demonstrating that elevated pressure triggers microglia reactivity that contribute to the loss of RGCs. Adenosine, acting on adenosine receptors, is a crucial modulator of microglia phenotype. Microglia express all adenosine receptors. Previously, we demonstrated that the activation of adenosine A₃ receptor (A₃R) affords protection to the retina, including RGCs, unveiling the possibility for a new strategy for glaucoma treatment. Since microglial cells express A₃R, we now studied the ability of a selective A₃R agonist (2-Cl-IB-MECA) in controlling microglia reactivity induced by elevated hydrostatic pressure (EHP), used to mimic elevated IOP. The activation of A₃R reduced EHP-induced inducible nitric oxide synthase (iNOS) expression, microglia migration and phagocytosis in BV-2 cells. In retinal microglia, proliferation and phagocytosis elicited by EHP were also decreased by A₃R activation. This work demonstrates that 2-Cl-IB-MECA, the selective agonist of A₃R, is able to hinder microglia reactivity, suggesting that A₃R agonists could afford protection against glaucomatous degeneration through the control of neuroinflammation.

Exosomes derived from microglia exposed to elevated pressure amplify the neuroinflammatory response in retinal cells

Glaucoma is a degenerative disease that causes irreversible loss of vision and is characterized by retinal ganglion cell (RGC) loss. Others and we have demonstrated that chronic neuroinflammation mediated by reactive microglial cells plays a role in glaucomatous pathology. Exosomes are extracellular vesicles released by most cells, including microglia, that mediate intercellular communication. The role of microglial exosomes in glaucomatous degeneration remains unknown. Taking the prominent role of microglial exosomes in brain neurodegenerative diseases, we studied the contribution of microglial-derived exosomes to the inflammatory response in experimental glaucoma. Microglial cells were exposed to elevated hydrostatic pressure (EHP), to mimic elevated intraocular pressure, the main risk factor for glaucoma. Naïve microglia (BV-2 cells or retinal microglia) were exposed to exosomes derived from BV-2 cells under EHP conditions (BV-Exo-EHP) or cultured in control pressure (BV-Exo-Control). We found that BV-Exo-EHP increased the production of pro-inflammatory cytokines, promoted retinal microglia motility, phagocytic efficiency, and proliferation. Furthermore, the incubation of primary retinal neural cell cultures with BV-Exo-EHP increased cell death and the production of reactive oxygen species. Exosomes derived from retinal microglia (MG-Exo-Control or MG-Exo-EHP) were injected in the vitreous of C57BL/6J mice. MG-Exo-EHP sustained activation of retinal microglia, mediated cell death, and impacted RGC number. Herein, we show that exosomes derived from retinal microglia have an autocrine function and propagate the inflammatory signal in conditions of elevated pressure, contributing to retinal degeneration in glaucomatous conditions.

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

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

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.

Potential mechanism of action of Ixeris sonchifolia extract injection against cardiovascular diseases revealed by combination of HPLC-Q-TOF-MS, virtual screening and systems pharmacology approach

Ixeris sonchifolia extract injection, a Chinese medicine preparation named as Kudiezi injection (KDZI) in China, has been widely used for the treatment of cardiovascular diseases (CVDs) in recent years. Owing to the component complexity of the preparation, the study on the effect mechanism of the herbal medicine against CVDs is a big challenge. In this research, HPLC-Q-TOF-MS was used to analyze the constituents of the preparation, disclosing that the KDZI mainly consists of 10 ingredients, namely 3-caffeoylquinic acid (KDZI-1), 4-caffeoylquinic acid (KDZI-2), 5-caffeoylquinic acid (KDZI-3), apigenin-7-O-β-d-glucuronide (KDZI-4), caffeic acid (KDZI-5), chicoric acid (KDZI-6), caftaric acid (KDZI-7), luteolin-7-O-β-d-gentiobioside (KDZI-8), luteolin-7-O-β-d-glucopyranoside (KDZI-9) and luteolin-7-O-β-d-glucuronide (KDZI-10). Afterwards, target fishing and an integrated systems pharmacology approach combined with molecular docking (Sybyl 1.3 and AutoDock Vina) were adopted to predict the potential targets and pathways for the main ingredients in KDZI. As results, 39 protein targets and 9 KEGG pathways, possessing high relevance to the therapeutic effects of the ingredients of KDZI against CVDs, were screened out reasonably. The integrated pharmacology analysis suggested that KDZI could exert its therapeutic effects against CVDs possibly via multi-targets including EGFR, MAPK10, and SRC and multi-pathways referring to MAPK, focal adhesion, complement and coagulation cascades, etc. This research provides insights into understanding the comprehensive therapeutic effect and mechanism of the KDZI on CVDs.

Ixeris sonchifolia extract injection, a Chinese medicine preparation named as Kudiezi injection (KDZI) in China, has been widely used for the treatment of cardiovascular diseases (CVDs) in recent years.

Potentiation of cannabinoid signaling in microglia by adenosine A2A receptor antagonists

Neuroprotective M2-skewed microglia appear as promising to alter the course of neurodegenerative diseases and G protein-coupled receptors (GPCRs) are potential targets to achieve such microglial polarization. A common feature of adenosine A_2A (A_2A R) and cannabinoid CB₂ (CB₂ R) GPCRs in microglia is that their expression is upregulated in Alzheimer's disease (AD). On the one hand, CB₂ R seems a target for neuroprotection, delaying neurodegenerative processes like those associated to AD or Parkinson's diseases. A_2A R antagonists reduce amyloid burden and improve cognitive performance and memory in AD animal models. We here show a close interrelationship between these two receptors in microglia; they are able to physically interact and affect the signaling of each other, likely due to conformational changes within the A_2A -CB₂ receptor heteromer (A_2A -CB₂ Het). Particularly relevant is the upregulation of A_2A -CB₂ Het expression in samples from the APP_Sw ,_Ind AD transgenic mice model. The most relevant finding, confirmed in both heterologous cells and in primary cultures of microglia, was that blockade of A_2A receptors results in increased CB₂ R-mediated signaling. This heteromer-specific feature suggests that A_2A R antagonists would potentiate, via microglia, the neuroprotective action of endocannabinoids with implications for AD therapy.

Blockade of microglial adenosine A2A receptor suppresses elevated pressure-induced inflammation, oxidative stress, and cell death in retinal cells

Glaucoma is a retinal degenerative disease characterized by the loss of retinal ganglion cells and damage of the optic nerve. Recently, we demonstrated that antagonists of adenosine A_2A receptor (A_2A R) control retinal inflammation and afford protection to rat retinal cells in glaucoma models. However, the precise contribution of microglia to retinal injury was not addressed, as well as the effect of A_2A R blockade directly in microglia. Here we show that blocking microglial A_2A R prevents microglial cell response to elevated pressure and it is sufficient to protect retinal cells from elevated pressure-induced death. The A_2A R antagonist SCH 58261 or the knockdown of A_2A R expression with siRNA in microglial cells prevented the increase in microglia response to elevated hydrostatic pressure. Furthermore, in retinal neural cell cultures, the A_2A R antagonist decreased microglia proliferation, as well as the expression and release of pro-inflammatory mediators. Microglia ablation prevented neural cell death triggered by elevated pressure. The A_2A R blockade recapitulated the effects of microglia depletion, suggesting that blocking A_2A R in microglia is able to control neurodegeneration in glaucoma-like conditions. Importantly, in human organotypic retinal cultures, A_2A R blockade prevented the increase in reactive oxygen species and the morphological alterations in microglia triggered by elevated pressure. These findings place microglia as the main contributors for retinal cell death during elevated pressure and identify microglial A_2A R as a therapeutic target to control retinal neuroinflammation and prevent neural apoptosis elicited by elevated pressure.