Nucleotides released from Aβ₁₋₄₂ -treated microglial cells increase cell migration and Aβ₁₋₄₂ uptake through P2Y₂ receptor activation

Microglial purinergic signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease. The prevalent features of AD pathogenesis are the appearance of β-amyloid (Aβ) plaques and neurofibrillary tangles, which cause microglial activation, synaptic deficiency, and neuronal loss. Microglia accompanies AD pathological processes and is also linked to cognitive deficits. Purinergic signaling has been shown to play a complex and tight interplay with the chemotaxis, phagocytosis, and production of pro-inflammatory factors in microglia, which is an important mechanism for regulating microglia activation. Here, we review recent evidence for interactions between AD, microglia, and purinergic signaling and find that the purinergic P2 receptors pertinently expressed on microglia are the ionotropic receptors P2X4 and P2X7, and the subtypes of P2YRs expressed by microglia are metabotropic receptors P2Y2, P2Y6, P2Y12, and P2Y13. The adenosine P1 receptors expressed in microglia include A1R, A2AR, and A2BR. Among them, the activation of P2X4, P2X7, and adenosine A1, A2A receptors expressed in microglia can aggravate the pathological process of AD, whereas P2Y2, P2Y6, P2Y12, and P2Y13 receptors expressed by microglia can induce neuroprotective effects. However, A1R activation also has a strong neuroprotective effect and has a significant anti-inflammatory effect in chronic neuroinflammation. These receptors regulate a variety of pathophysiological processes in AD, including APP processing, Aβ production, tau phosphorylation, neuroinflammation, synaptic dysfunction, and mitochondrial dysfunction. This review also provides key pharmacological advances in purinergic signaling receptors.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

Coupling of Alzheimer's Disease Genetic Risk Factors with Viral Susceptibility and Inflammation

Alzheimer's disease (AD) is a neurodegenerative disease characterized by persistent cognitive decline. Amyloid plaque deposition and neurofibrillary tangles are the main pathological features of AD brain, though mechanisms leading to the formation of lesions remain to be understood. Genetic efforts through genome-wide association studies (GWAS) have identified dozens of risk genes influencing the pathogenesis and progression of AD, some of which have been revealed in close association with increased viral susceptibilities and abnormal inflammatory responses in AD patients. In the present study, we try to present a list of AD candidate genes that have been shown to affect viral infection and inflammatory responses. Understanding of how AD susceptibility genes interact with the viral life cycle and potential inflammatory pathways would provide possible therapeutic targets for both AD and infectious diseases.

P2Y2 receptor mediates dying cell removal via inflammatory activated microglia

Microglial removal of dying cells plays a beneficial role in maintaining homeostasis in the CNS, whereas under some pathological conditions, inflammatory microglia can cause excessive clearance, leading to neuronal death. However, the mechanisms underlying dying cell removal by inflammatory microglia remain poorly understood. In this study, we performed live imaging to examine the purinergic regulation of dying cell removal by inflammatory activated microglia. Lipopolysaccharide (LPS) stimulation induces rapid death of primary rat microglia, and the surviving microglia actively remove dying cells. The nonselective P2 receptor antagonist, suramin, inhibited dying cell removal to the same degree as that of the selective P2Y₂ antagonist, AR-C118925. This inhibition was more potent in LPS-stimulated microglia than in non-stimulated ones. LPS stimulation elicited distribution of the P2Y₂ receptor on the leading edge of the plasma membrane and then induced drastic upregulation of P2Y₂ receptor mRNA expression in microglia. LPS stimulation caused upregulation of the dying cell-sensing inflammatory Axl phagocytic receptor, which was suppressed by blocking the P2Y₂ receptor and its downstream signaling effector, proline-rich tyrosine kinase (Pyk2). Together, these results indicate that inflammatory stimuli may activate the P2Y₂ receptor, thereby mediating dying cell removal, at least partially, through upregulating phagocytic Axl in microglia.

Action of the Purinergic and Cholinergic Anti-inflammatory Pathways on Oxidative Stress in Patients with Alzheimer's Disease in the Context of the COVID-19 Pandemic

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the etiologic agent of the 2019 coronavirus disease (COVID-19), has affected more than 20 million people in Brazil and caused a global health emergency. This virus has the potential to affect various parts of the body and compromise metabolic functions. The virus-mediated neural inflammation of the nervous system is due to a storm of cytokines and oxidative stress, which are the clinical features of Alzheimer's disease (AD). This neurodegenerative disease is aggravated in cases involving SARS-CoV-2 and its inflammatory biomarkers, accelerating accumulation of β-amyloid peptide, hyperphosphorylation of tau protein, and production of reactive oxygen species, which lead to homeostasis imbalance. The cholinergic system, through neurons and the neurotransmitter acetylcholine (ACh), modulates various physiological pathways, such as the response to stress, sleep and wakefulness, sensory information, and the cognitive system. Patients with AD have low concentrations of ACh; hence, therapeutic methods are aimed at adjusting the ACh titers available to the body for maintaining functionality. Herein, we focused on acetylcholinesterase inhibitors, responsible for the degradation of ACh in the synaptic cleft, and muscarinic and nicotinic receptor agonists of the cholinergic system owing to the therapeutic potential of the cholinergic anti-inflammatory pathway in AD associated with SARS-CoV-2 infection.

A near-infrared probe for detecting and interposing amyloid beta oligomerization in early Alzheimer's disease

BACKGROUND

The misfolding and deposition of amyloid beta (Aβ) in human brain is the main hallmark of Alzheimer's disease (AD) pathology. One of the drivers of Alzheimer´s pathogenesis is the production of soluble oligomeric Aβ, which could potentially serve as a biomarker of AD.

METHODS

Given that the diphenylalanine (FF) at the C-terminus of Aβ fragments plays a key role in inducing the AD pathology, based on the hydrophobic structure of FF, we synthesized a near-infrared BF2-dipyrrolmethane fluorescent imaging probe (NB) to detect both soluble and insoluble Aβ.

RESULTS

We found that NB not only binds Aβ, particularly oligomeric Aβ, but also interposes self-assembly of Aβ through π-π interaction between NB and FF.

CONCLUSION

This work holds great promise in the early detection of AD and may also provide an innovative approach to decelerate and even halt AD onset and progression.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

Therapeutic potential for P2Y2 receptor antagonism

G protein-coupled receptors are the target of more than 30% of all FDA-approved drug therapies. Though the purinergic P2 receptors have been an attractive target for therapeutic intervention with successes such as the P2Y₁₂ receptor antagonist, clopidogrel, P2Y₂ receptor (P2Y₂R) antagonism remains relatively unexplored as a therapeutic strategy. Due to a lack of selective antagonists to modify P2Y₂R activity, studies using primarily genetic manipulation have revealed roles for P2Y₂R in a multitude of diseases. These include inflammatory and autoimmune diseases, fibrotic diseases, renal diseases, cancer, and pathogenic infections. With the advent of AR-C118925, a selective and potent P2Y₂R antagonist that became commercially available only a few years ago, new opportunities exist to gain a more robust understanding of P2Y₂R function and assess therapeutic effects of P2Y₂R antagonism. This review discusses the characteristics of P2Y₂R that make it unique among P2 receptors, namely its involvement in five distinct signaling pathways including canonical Gα_q protein signaling. We also discuss the effects of other P2Y₂R antagonists and the pivotal development of AR-C118925. The remainder of this review concerns the mounting evidence implicating P2Y₂Rs in disease pathogenesis, focusing on those studies that have evaluated AR-C118925 in pre-clinical disease models.

Europium-Doped Cerium Oxide Nanoparticles for Microglial Amyloid Beta Clearance and Homeostasis

Alzheimer's disease (AD) is the most common neurodegenerative disorder. Pathologically, the disease is characterized by the deposition of amyloid beta (Aβ) plaques and the presence of neurofibrillary tangles. These drive microglia neuroinflammation and consequent neurodegeneration. While the means to affect Aβ plaque accumulation pharmacologically was achieved, how it affects disease outcomes remains uncertain. Cerium oxide (CeO₂) reduces Aβ plaques, oxidative stress, inflammation, and AD signs and symptoms. In particular, CeO₂ nanoparticles (CeO₂NPs) induce free-radical-scavenging and cell protective intracellular signaling. This can ameliorate the pathobiology of an AD-affected brain. To investigate whether CeO₂NPs affect microglia neurotoxic responses, a novel formulation of europium-doped CeO₂NPs (EuCeO₂NPs) was synthesized. We then tested EuCeO₂NPs for its ability to generate cellular immune homeostasis in AD models. EuCeO₂NPs attenuated microglia BV2 inflammatory activities after Aβ_1-42 exposure by increasing the cells' phagocytic and Aβ degradation activities. These were associated with increases in the expression of the CD36 scavenger receptor. EuCeO₂NPs facilitated Aβ endolysosomal trafficking and abrogated microglial inflammatory responses. We posit that EuCeO₂NPs may be developed as an AD immunomodulator.

Circulating Naturally Occurring Antibodies to P2RY2 Are Decreased in Alzheimer's Disease

BACKGROUND

The G protein-coupled receptor P2RY2 protein of the purinergic receptor family is involved in the pathogenesis of Alzheimer's disease (AD). Naturally occurring antibodies against P2RY2 (NAbs-P2RY2) are present in human plasma, with their clinical relevance in AD patients unknown.

OBJECTIVE

To explore the alteration of NAbs-P2RY2 in AD patients and its correlations with biomarkers and cognition of AD.

METHODS

The levels of naturally occurring antibodies against the four extracellular domains of P2RY2 (NAbs-P2RY2-1, NAbs-P2RY2-2, NAbs-P2RY2-3, and NAbs-P2RY2-4) were measured in the plasma of 55 AD patients, 28 non-AD dementia patients, and 70 cognitively normal participants. The correlations of autoantibody levels with cognitive scale scores, AD plasma biomarkers, and brain amyloid burden were examined.

RESULTS

NAbs-P2RY2-1, NAbs-P2RY2-3, and NAbs-P2RY2-4 were reduced in AD patients. Plasma levels of NAbs-P2RY2-2 and NAbs-P2RY2-3 levels were positively associated with cognitive and functional performances. Among these antibodies, plasma NAbs-P2RY2-2 levels were positively associated with plasma amyloid-β 42 levels. Plasma, while plasma NAbs-P2RY2-3 levels were negatively associated with brain amyloid burden in AD patients.

CONCLUSION

These findings indicate an alteration of humoral immunity against P2RY2 in AD patients. Further mechanistical investigations are needed to reveal the role of NAbs-P2RY2 in the pathogenesis of AD.

Sleep Disturbance and Alzheimer's Disease: The Glial Connection

Poor quality and quantity of sleep are very common in elderly people throughout the world. Growing evidence has suggested that sleep disturbances could accelerate the process of neurodegeneration. Recent reports have shown a positive correlation between sleep deprivation and amyloid-β (Aβ)/tau aggregation in the brain of Alzheimer's patients. Glial cells have long been implicated in the progression of Alzheimer's disease (AD) and recent findings have also suggested their role in regulating sleep homeostasis. However, how glial cells control the sleep-wake balance and exactly how disturbed sleep may act as a trigger for Alzheimer's or other neurological disorders have recently gotten attention. In an attempt to connect the dots, the present review has highlighted the role of glia-derived sleep regulatory molecules in AD pathogenesis. Role of glia in sleep disturbance and Alzheimer's progression.

Probable Reasons for Neuron Copper Deficiency in the Brain of Patients with Alzheimer’s Disease: The Complex Role of Amyloid

Alzheimer’s disease is a progressive neurodegenerative disorder that eventually leads the affected patients to die. The appearance of senile plaques in the brains of Alzheimer’s patients is known as a main symptom of this disease. The plaques consist of different components, and according to numerous reports, their main components include beta-amyloid peptide and transition metals such as copper. In this disease, metal dyshomeostasis leads the number of copper ions to simultaneously increase in the plaques and decrease in neurons. Copper ions are essential for proper brain functioning, and one of the possible mechanisms of neuronal death in Alzheimer’s disease is the copper depletion of neurons. However, the reason for the copper depletion is as yet unknown. Based on the available evidence, we suggest two possible reasons: the first is copper released from neurons (along with beta-amyloid peptides), which is deposited outside the neurons, and the second is the uptake of copper ions by activated microglia.

Role of G Protein-Coupled Receptors in Microglial Activation: Implication in Parkinson's Disease

Parkinson’s disease (PD) is one of the prevalent neurodegenerative diseases associated with preferential loss of dopaminergic (DA) neurons in the substantia nigra compacta (SNc) and accumulation of α-synuclein in DA neurons. Even though the precise pathogenesis of PD is not clear, a large number of studies have shown that microglia-mediated neuroinflammation plays a vital role in the process of PD development. G protein-coupled receptors (GPCRs) are widely expressed in microglia and several of them act as regulators of microglial activation upon corresponding ligands stimulations. Upon α-synuclein insults, microglia would become excessively activated through some innate immune receptors. Presently, as lack of ideal drugs for treating PD, certain GPCR which is highly expressed in microglia of PD brain and mediates neuroinflammation effectively could be a prospective source for PD therapeutic intervention. Here, six kinds of GPCRs and two types of innate immune receptors were introduced, containing adenosine receptors, purinergic receptors, metabotropic glutamate receptors, adrenergic receptors, cannabinoid receptors, and melatonin receptors and their roles in neuroinflammation; we highlighted the relationship between these six GPCRs and microglial activation in PD. Based on the existing findings, we tried to expound the implication of microglial GPCRs-regulated neuroinflammation to the pathophysiology of PD and their potential to become a new expectation for clinical therapeutics.

P2Y2 deficiency impacts adult neurogenesis and related forebrain functions

Adult neurogenesis occurs particularly in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle. This continuous addition of neurons to pre-existing neuronal networks is essential for intact cognitive and olfactory functions, respectively. Purinergic signaling modulates adult neurogenesis, however, the role of individual purinergic receptor subtypes in this dynamic process and related cognitive performance is poorly understood. In this study, we analyzed the role of P2Y₂ receptor in the neurogenic niches and in related forebrain functions such as spatial working memory and olfaction using mice with a targeted deletion of the P2Y₂ receptor (P2Y2^-/- ). Proliferation, migration, differentiation, and survival of neuronal precursor cells (NPCs) were analyzed by BrdU assay and immunohistochemistry; signal transduction pathway components were analyzed by immunoblot. In P2Y2^-/- mice, proliferation of NPCs in the SGZ and the SVZ was reduced. However, migration, neuronal fate decision, and survival were not affected. Moreover, p-Akt expression was decreased in P2Y2^-/- mice. P2Y2^-/- mice showed an impaired performance in the Y-maze and a higher latency in the hidden food test. These data indicate that the P2Y₂ receptor plays an important role in NPC proliferation as well as in hippocampus-dependent working memory and olfactory function.

Targeting Neuroinflammation via Purinergic P2 Receptors for Disease Modification in Drug-Refractory Epilepsy

Treatment of epilepsy remains a clinical challenge, with >30% of patients not responding to current antiseizure drugs (ASDs). Moreover, currently available ASDs are merely symptomatic without altering significantly the progression of the disease. Inflammation is increasingly recognized as playing an important role during the generation of hyperexcitable networks in the brain. Accordingly, the suppression of chronic inflammation has been suggested as a promising therapeutic strategy to prevent epileptogenesis and to treat drug-refractory epilepsy. As a consequence, a strong focus of ongoing research is identification of the mechanisms that contribute to sustained inflammation in the brain during epilepsy and whether these can be targeted. ATP is released in response to several pathological stimuli, including increased neuronal activity within the central nervous system, where it functions as a neuro- and gliotransmitter. Once released, ATP activates purinergic P2 receptors, which are divided into metabotropic P2Y and ionotropic P2X receptors, driving inflammatory processes. Evidence from experimental models and patients demonstrates widespread expression changes of both P2Y and P2X receptors during epilepsy, and critically, drugs targeting both receptor subtypes, in particular the P2Y₁ and P2X₇ subtypes, have been shown to possess both anticonvulsive and antiepileptic potential. This review provides a detailed summary of the current evidence suggesting ATP-gated receptors as novel drug targets for epilepsy and discusses how P2 receptor–driven inflammation may contribute to the generation of seizures and the development of epilepsy.

Purinergic Receptors of the Central Nervous System: Biology, PET Ligands, and Their Applications

Purinergic receptors play important roles in central nervous system (CNS). These receptors are involved in cellular neuroinflammatory responses that regulate functions of neurons, microglial and astrocytes. Based on their endogenous ligands, purinergic receptors are classified into P1 or adenosine, P2X and P2Y receptors. During brain injury or under pathological conditions, rapid diffusion of extracellular adenosine triphosphate (ATP) or uridine triphosphate (UTP) from the damaged cells, promote microglial activation that result in the changes in expression of several of these receptors in the brain. Imaging of the purinergic receptors with selective Positron Emission Tomography (PET) radioligands has advanced our understanding of the functional roles of some of these receptors in healthy and diseased brains. In this review, we have accumulated a list of currently available PET radioligands of the purinergic receptors that are used to elucidate the receptor functions and participations in CNS disorders. We have also reviewed receptors lacking radiotracer, laying the foundation for future discoveries of novel PET radioligands to reveal these receptors roles in CNS disorders.

Alzheimer and Purinergic Signaling: Just a Matter of Inflammation?

Alzheimer's disease (AD) is a widespread neurodegenerative pathology responsible for about 70% of all cases of dementia. Adenosine is an endogenous nucleoside that affects neurodegeneration by activating four membrane G protein-coupled receptor subtypes, namely P1 receptors. One of them, the A_2A subtype, is particularly expressed in the brain at the striatal and hippocampal levels and appears as the most promising target to counteract neurological damage and adenosine-dependent neuroinflammation. Extracellular nucleotides (ATP, ADP, UTP, UDP, etc.) are also released from the cell or are synthesized extracellularly. They activate P2X and P2Y membrane receptors, eliciting a variety of physiological but also pathological responses. Among the latter, the chronic inflammation underlying AD is mainly caused by the P2X7 receptor subtype. In this review we offer an overview of the scientific evidence linking P1 and P2 mediated purinergic signaling to AD development. We will also discuss potential strategies to exploit this knowledge for drug development.

Glial Purinergic Signaling in Neurodegeneration

Purinergic signaling regulates neuronal and glial cell functions in the healthy CNS. In neurodegenerative diseases, purinergic signaling becomes dysregulated and can affect disease-associated phenotypes of glial cells. In this review, we discuss how cell-specific expression patterns of purinergic signaling components change in neurodegeneration and how dysregulated glial purinergic signaling and crosstalk may contribute to disease pathophysiology, thus bearing promising potential for the development of new therapeutical options for neurodegenerative diseases.

Control of Macrophage Inflammation by P2Y Purinergic Receptors

Macrophages comprise a phenotypically and functionally diverse group of hematopoietic cells. Versatile macrophage subsets engage to ensure maintenance of tissue integrity. To perform tissue stress surveillance, macrophages express many different stress-sensing receptors, including purinergic P2X and P2Y receptors that respond to extracellular nucleotides and their sugar derivatives. Activation of G protein-coupled P2Y receptors can be both pro- and anti-inflammatory. Current examples include the observation that P2Y14 receptor promotes STAT1-mediated inflammation in pro-inflammatory M1 macrophages as well as the demonstration that P2Y11 receptor suppresses the secretion of tumor necrosis factor (TNF)-α and concomitantly promotes the release of soluble TNF receptors from anti-inflammatory M2 macrophages. Here, we review macrophage regulation by P2Y purinergic receptors, both in physiological and disease-associated inflammation. Therapeutic targeting of anti-inflammatory P2Y receptor signaling is desirable to attenuate excessive inflammation in infectious diseases such as COVID-19. Conversely, anti-inflammatory P2Y receptor signaling must be suppressed during cancer therapy to preserve its efficacy.

Microglia Do Not Take Up Soluble Amyloid-beta Peptides, But Partially Degrade Them by Secreting Insulin-degrading Enzyme

Microglia play important roles in the pathogenesis of Alzheimer's disease (AD), in part, by affecting the clearance of amyloid-β (Aβ) peptides. Most studies, however, used synthetic soluble Aβ (sAβ) at higher concentrations. The exact mechanisms underlying microglia-mediated clearance of physiological sAβ at very low concentrations remain unclear. Here we reported that there were much more Iba-1- and CD68-positive microglia and significantly less sAβ left in the brain of adult mice 5 days after the surgery of sAβ microinjection compared to 2 h after the surgery (p < 0.05). However, very few Iba-1- and CD68-positive microglia co-localized with microinjected fluorescently labeled sAβ (FLsAβ₄₂) 5 days after the surgery. Also, there was no co-localization of FLsAβ₄₂ with a lysosomal marker (LAMP-1) 5 days after the surgery. There was no significant difference in the percentage of Aβ⁺/PE-CD11b⁺/APC-CD45^low microglia between the control group and the group microinjected with TBS-soluble Aβ extracted from the brains of AD patients (p > 0.05). The degradation of physiological sAβ was prevented by a highly selective insulin-degrading enzyme inhibitor (Ii1) but not by a phagocytosis inhibitor (polyinosinic acid) or pinocytosis inhibitor (cytochalasin B) in vitro. Furthermore, the reduction of synthetic and physiological sAβ in the brain was partially prevented by the co-injection of Ii1 in vivo (p < 0.05). Our results demonstrate that microglia do not take up synthetic or physiological sAβ, but partially degrade it via the secretion of insulin-degrading enzyme, which will be beneficial for understanding how sAβ is removed from the brain by microglia.

Alzheimer's disease: microglia targets and their modulation to promote amyloid phagocytosis and mitigate neuroinflammation

Introduction: Despite the revolutionary progress in neurodegenerative disease research, there is no cure for Alzheimer's disease (AD). This is a chronic progressive neurodegenerative disease affecting aged people and is associated with chronic neuroinflammation and amyloid-beta (Aβ) deposition in the brain parenchyma. Microglia, the resident myeloid cells in the central nervous system, are critically involved in the pathogenesis of AD and have emerged as a potential therapeutic target for treating or preventing AD. The failure of microglia to keep up with persistent amyloid-beta development along with secretion of inflammatory cytokines is detrimental to neurons and favors Aβ accumulation.Areas covered: This review illuminates the latest research that is focused on molecules and their intracellular targets that promote microglial phagocytosis and /or its polarization to an anti-inflammatory state.Expert opinion: A robust inflammatory response of microglia is not necessary to improve their efficiency of Aβ clearance. The challenge is to master inflammatory/anti-inflammatory phenotypes depending on the stage of AD and to maintain efficient responses to remove Aβ. Therefore, promoting microglia phagocytosis without a persistent excessive inflammatory response could be a potential therapeutic strategy.

Targeting Purinergic Signaling and Cell Therapy in Cardiovascular and Neurodegenerative Diseases

Extracellular purines exert several functions in physiological and pathophysiological mechanisms. ATP acts through P2 receptors as a neurotransmitter and neuromodulator and modulates heart contractility, while adenosine participates in neurotransmission, blood pressure, and many other mechanisms. Because of their capability to differentiate into mature cell types, they provide a unique therapeutic strategy for regenerating damaged tissue, such as in cardiovascular and neurodegenerative diseases. Purinergic signaling is pivotal for controlling stem cell differentiation and phenotype determination. Proliferation, differentiation, and apoptosis of stem cells of various origins are regulated by purinergic receptors. In this chapter, we selected neurodegenerative and cardiovascular diseases with clinical trials using cell therapy and purinergic receptor targeting. We discuss these approaches as therapeutic alternatives to neurodegenerative and cardiovascular diseases. For instance, promising results were demonstrated in the utilization of mesenchymal stem cells and bone marrow mononuclear cells in vascular regeneration. Regarding neurodegenerative diseases, in general, P2X7 and A_2A receptors mostly worsen the degenerative state. Stem cell-based therapy, mainly through mesenchymal and hematopoietic stem cells, showed promising results in improving symptoms caused by neurodegeneration. We propose that purinergic receptor activity regulation combined with stem cells could enhance proliferative and differentiation rates as well as cell engraftment.

Microglia in Alzheimer Disease: Well-Known Targets and New Opportunities

Microglia are the resident macrophages of the central nervous system. They play key roles in brain development, and physiology during life and aging. Equipped with a variety of molecular sensors and through the various functions they can fulfill, they are critically involved in maintaining the brain’s homeostasis. In Alzheimer disease (AD), microglia reaction was initially thought to be incidental and triggered by amyloid deposits and dystrophic neurites. However, recent genome-wide association studies have established that the majority of AD risk loci are found in or near genes that are highly and sometimes uniquely expressed in microglia. This leads to the concept of microglia being critically involved in the early steps of the disease and identified them as important potential therapeutic targets. Whether microglia reaction is beneficial, detrimental or both to AD progression is still unclear and the subject of intense debate. In this review, we are presenting a state-of-knowledge report intended to highlight the variety of microglial functions and pathways shown to be critically involved in AD progression. We first address both the acquisition of new functions and the alteration of their homeostatic roles by reactive microglia. Second, we propose a summary of new important parameters currently emerging in the field that need to be considered to identify relevant microglial targets. Finally, we discuss the many obstacles in designing efficient therapeutic strategies for AD and present innovative technologies that may foster our understanding of microglia roles in the pathology. Ultimately, this work aims to fly over various microglial functions to make a general and reliable report of the current knowledge regarding microglia’s involvement in AD and of the new research opportunities in the field.

Modulation of Innate Immunity by Amyloidogenic Peptides

Amyloid formation contributes to the development of progressive metabolic and neurodegenerative diseases, while also serving functional roles in host defense. Emerging evidence suggests that as amyloidogenic peptides populate distinct aggregation states, they interact with different combinations of pattern recognition receptors (PRRs) to direct the phenotype and function of tissue-resident and infiltrating innate immune cells. We review recent evidence of innate immunomodulation by distinct forms of amyloidogenic peptides produced by mammals (humans, non-human primates), bacteria, and fungi, as well as the corresponding cell-surface and intracellular PRRs in these interactions, in human and mouse models. Our emerging understanding of peptide aggregate-innate immune cell interactions, and the factors regulating the balance between amyloid function and pathogenicity, might aid the development of anti-amyloid and immunomodulating therapies.

Characterizing the Molecular Architecture of Cortical Regions Associated with High Educational Attainment in Older Individuals

Neuroimaging investigations have revealed interindividual variations in anatomy, metabolism, activity, and connectivity of specific cortical association areas through which years of education (YoE), as a common proxy of cognitive reserve, may operate in the face of age- or pathology-associated brain changes. However, the associated molecular properties of YoE-related brain regions and the biological pathways involved remain poorly understood. In the present study we first identified brain areas that showed an association between cortical thickness and YoE among 122 cognitively healthy older human individuals (87 female). We subsequently characterized molecular properties of these regions by studying brain-wide microarray measurements of regional gene expression. In accordance with previous studies, we observed that YoE were associated with higher cortical thickness in medial prefrontal, anterior cingulate, and orbitofrontal areas. Compared with the rest of the cortex, these regions exhibited a distinct gene expression profile characterized by relative upregulation of gene sets implicated in ionotropic and metabotropic neurotransmission as well as activation of immune response. Our genome-wide expression profile analysis of YoE-related brain regions points to distinct molecular pathways that may underlie a higher capacity for plastic changes in response to lifetime intellectual enrichment and potentially also a higher resilience to age-related pathologic brain changes.SIGNIFICANCE STATEMENT We combined a neuroimaging-based analysis with a transcriptome-wide gene expression approach to investigate the molecular-functional properties of cortical regions associated with educational attainment, as a commonly used proxy for cognitive reserve, in older individuals. The strongest association with education was observed in specific areas of the medial prefrontal cortex, and these areas exhibited a distinct gene expression profile characterized by relative upregulation of gene sets implicated in neurotransmission and immune responses. These findings complement previous neuroimaging studies in the field and point to novel biological pathways that may mediate the beneficial effects of high educational attainment on adaptability to cope with, or prevent, age-related brain changes. The identified genes and pathways now warrant further exploration in mechanistic studies.

Pharmacology of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). The P2Y receptors are expressed in various cell types and play important roles in physiology and pathophysiology including inflammatory responses and neuropathic pain. The antagonism of P2Y₁₂ receptors is used in pharmacotherapy for the prevention and therapy of cardiovascular events. The nucleoside analogue ticagrelor and active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel inhibit platelet P2Y₁₂ receptors and reduce thereby platelet aggregation. The P2Y₂ receptor agonist diquafosol is used for the treatment of the dry eye syndrome. The P2Y receptor subtypes differ in their amino acid sequences, their pharmacological profiles and their signaling transduction pathways. Recently, selective receptor ligands have been developed for all subtypes. The published crystal structures of the human P2Y₁ and P2Y₁₂ receptors as well as receptor models will facilitate the development of novel drugs for pharmacotherapy.

Purinergic signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by three major histopathological markers: amyloid-β (Aβ) plaques, neurofibrillary tangles and gliosis in the central nervous system (CNS). It is now accepted that neuroinflammatory events in the CNS play a crucial role in the development of AD. This review focuses on neuroinflammatory signaling mediated by purinergic receptors (P1 adenosine receptors, P2X ATP-gated ion channels and G protein-coupled P2Y nucleotide receptors) and how therapeutic modulation of purinergic signaling influences disease progression in AD patients and animal models of AD.

Role of purinergic receptors in the Alzheimer's disease

Etiology of the Alzheimer’s disease (AD) is not fully understood. Different pathological processes are considered, such as amyloid deposition, tau protein phosphorylation, oxidative stress (OS), metal ion disregulation, or chronic neuroinflammation. Purinergic signaling is involved in all these processes, suggesting the importance of nucleotide receptors (P2X and P2Y) and adenosine receptors (A1, A2A, A2B, A3) present on the CNS cells. Ecto-purines, ecto-pyrimidines, and enzymes participating in their metabolism are present in the inter-cellular spaces. Accumulation of amyloid-β (Aβ) in brain induces the ATP release into the extra-cellular space, which in turn stimulates the P2X7 receptors. Activation of P2X7 results in the increased synthesis and release of many pro-inflammatory mediators such as cytokines and chemokines. Furthermore, activation of P2X7 leads to the decreased activity of α-secretase, while activation of P2Y2 receptor has an opposite effect. Simultaneous inhibition of P2X7 and stimulation of P2Y2 would therefore be the efficient way of the α-secretase activation. Activation of P2Y2 receptors present in neurons, glia cells, and endothelial cells may have a positive neuroprotective effect in AD. The OS may also be counteracted via the purinergic signaling. ADP and its non-hydrolysable analogs activate P2Y13 receptors, leading to the increased activity of heme oxygenase, which has a cytoprotective activity. Adenosine, via A1 and A2A receptors, affects the dopaminergic and glutaminergic signaling, the brain-derived neurotrophic factor (BNDF), and also changes the synaptic plasticity (e.g., causing a prolonged excitation or inhibition) in brain regions responsible for learning and memory. Such activity may be advantageous in the Alzheimer’s disease.

Importance of GPCR-Mediated Microglial Activation in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder associated with impairment of cognition, memory deficits and behavioral abnormalities. Accumulation of amyloid beta (Aβ) is a characteristic hallmark of AD. Microglia express several GPCRs, which, upon activation by modulators, mediate microglial activation and polarization phenotype. This GPCR-mediated microglial activation has both protective and detrimental effects. Microglial GPCRs are involved in amyloid precursor protein (APP) cleavage and Aβ generation. In addition, microglial GPCRs are featured in the regulation of Aβ degradation and clearance through microglial phagocytosis and chemotaxis. Moreover, in response to Aβ binding on microglial Aβ receptors, they can trigger multiple inflammatory pathways. However, there is still a lack of insight into the mechanistic link between GPCR-mediated microglial activation and its pathological consequences in AD. Currently, the available drugs for the treatment of AD are mostly symptomatic and dominated by acetylcholinesterase inhibitors (AchEI). The selection of a specific microglial GPCR that is highly expressed in the AD brain and capable of modulating AD progression through Aβ generation, degradation and clearance will be a potential source of therapeutic intervention. Here, we have highlighted the expression and distribution of various GPCRs connected to microglial activation in the AD brain and their potential to serve as therapeutic targets of AD.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Amyloid-beta modulates microglial responses by binding to the triggering receptor expressed on myeloid cells 2 (TREM2)

BACKGROUND

TREM2 is an innate immune receptor specifically expressed in microglia. Coding variations in TREM2 have been reported to increase the risk for Alzheimer's disease (AD) and other neurodegenerative diseases. While multiple studies support a role for TREM2 in microglial recruitment to amyloid plaques, the chemoattractant factor modulating TREM2-dependent microglial responses has not been defined.

METHODS

Potential binding of oligomeric amyloid-β 1-42 (oAβ1-42) to TREM2 was tested by complementary approaches including solid phase binding, surface plasmon resonance and immunoprecipitation assays. The ability of oAβ1-42 to activate TREM2 signaling pathways was examined by analyzing the phosphorylation of Syk and Akt in primary microglia as well as TREM2-mediated signaling in a reporter cell system. Lastly, the functional outcome of oAβ1-42-TREM2 interaction was tested by examining impacts on microglial migration in vitro and clustering around oAβ1-42-bearing brain areas in vivo.

RESULTS

We found that oAβ1-42 bound to TREM2 with high affinity and activated TREM2-dependent signaling pathway. Neither monomeric nor scrambled Aβ bound to TREM2 supporting a specific interaction between oAβ and TREM2. The disease-associated mutations of TREM2 reduced its binding affinity to oAβ1-42. Furthermore, we identified several positively charged amino acids within residues 31-91 of TREM2 that were crucial for its interaction with oAβ1-42. Importantly, oAβ1-42 promoted microglial migration in vitro and clustering in vivo in a TREM2-dependent manner.

CONCLUSIONS

Our data establish a critical link between oAβ1-42, a major pathological component of AD, and TREM2, a strong genetic risk factor for AD expressed in microglia, and suggest that such interaction contributes to the pathogenic events in AD by modulating microglial responses.

Pannexin 1 Modulates Axonal Growth in Mouse Peripheral Nerves

The pannexin family of channels consists of three members—pannexin-1 (Panx1), pannexin-2 (Panx2), and pannexin-3 (Panx3) that enable the exchange of metabolites and signaling molecules between intracellular and extracellular compartments. Pannexin-mediated release of intracellular ATP into the extracellular space has been tied to a number of cellular activities, primarily through the activity of type P2 purinergic receptors. Previous work indicates that the opening of Panx1 channels and activation of purinergic receptors by extracellular ATP may cause inflammation and apoptosis. In the CNS (central nervous system) and PNS (peripheral nervous system), coupled pannexin, and P2 functions have been linked to peripheral sensitization (pain) pathways. Purinergic pathways are also essential for other critical processes in the PNS, including myelination and neurite outgrowth. However, whether such pathways are pannexin-dependent remains to be determined. In this study, we use a Panx1 knockout mouse model and pharmacological inhibitors of the Panx1 and the ATP-mediated signaling pathway to fill gaps in our understanding of Panx1 localization in peripheral nerves, roles for Panx1 in axonal outgrowth and myelination, and neurite extension. Our data show that Panx1 is localized to axonal, myelin, and vascular compartments of the peripheral nerves. Knockout of Panx1 gene significantly increased axonal caliber in vivo and axonal growth rate in cultured dorsal root ganglia (DRG) neurons. Furthermore, genetic knockout of Panx1 or inhibition of components of purinergic signaling, by treatment with probenecid and apyrase, resulted in denser axonal outgrowth from cultured DRG explants compared to untreated wild-types. Our findings suggest that Panx1 regulates axonal growth in the peripheral nervous system.

P2X receptor overexpression induced by soluble oligomers of amyloid beta peptide potentiates synaptic failure and neuronal dyshomeostasis in cellular models of Alzheimer's disease

The most common cause of dementia is Alzheimer's disease. The etiology of the disease is unknown, although considerable evidence suggests a critical role for the soluble oligomers of amyloid beta peptide (Aβ). Because Aβ increases the expression of purinergic receptors (P2XRs) in vitro and in vivo, we studied the functional correlation between long-term exposure to Aβ and the ability of P2XRs to modulate network synaptic tone. We used electrophysiological recordings and Ca²⁺ microfluorimetry to assess the effects of chronic exposure (24 h) to Aβ oligomers (0.5 μM) together with known inhibitors of P2XRs, such as PPADS and apyrase on synaptic function. Changes in the expression of P2XR were quantified using RT-qPCR. We observed changes in the expression of P2X1R, P2X7R and an increase in P2X2R; and also in protein levels in PC12 cells (143%) and hippocampal neurons (120%) with Aβ. In parallel, the reduction on the frequency and amplitude of mEPSCs (72% and 35%, respectively) were prevented by P2XR inhibition using a low PPADS concentration. Additionally, the current amplitude and intracellular Ca²⁺ signals evoked by extracellular ATP were increased (70% and 75%, respectively), suggesting an over activation of purinergic neurotransmission in cells pre-treated with Aβ. Taken together, our findings suggest that Aβ disrupts the main components of synaptic transmission at both pre- and post-synaptic sites, and induces changes in the expression of key P2XRs, especially P2X2R; changing the neuromodulator function of the purinergic tone that could involve the P2X2R as a key factor for cytotoxic mechanisms. These results identify novel targets for the treatment of dementia and other diseases characterized by increased purinergic transmission.

Key Aging-Associated Alterations in Primary Microglia Response to Beta-Amyloid Stimulation

Alzheimer's disease (AD) is characterized by a progressive cognitive decline and believed to be driven by the self-aggregation of amyloid-β (Aβ) peptide into oligomers and fibrils that accumulate as senile plaques. It is widely accepted that microglia-mediated inflammation is a significant contributor to disease pathogenesis; however, different microglia phenotypes were identified along AD progression and excessive Aβ production was shown to dysregulate cell function. As so, the contribution of microglia to AD pathogenesis remains to be elucidated. In this study, we wondered if isolated microglia cultured for 16 days in vitro (DIV) would react differentially from the 2 DIV cells upon treatment with 1000 nM Aβ_1-42 for 24 h. No changes in cell viability were observed and morphometric alterations associated to microglia activation, such as volume increase and process shortening, were obvious in 2 DIV microglia, but less evident in 16 DIV cells. These cells showed lower phagocytic, migration and autophagic properties after Aβ treatment than the 2 DIV cultured microglia. Reduced phagocytosis may derive from increased CD33 expression, reduced triggering receptor expressed on myeloid cells 2 (TREM2) and milk fat globule-EGF factor 8 protein (MFG-E8) levels, which were mainly observed in 16 DIV cells. Activation of inflammatory mediators, such as high mobility group box 1 (HMGB1) and pro-inflammatory cytokines, as well as increased expression of Toll-like receptor 2 (TLR2), TLR4 and fractalkine/CX3C chemokine receptor 1 (CX3CR1) cell surface receptors were prominent in 2 DIV microglia, while elevation of matrix metalloproteinase 9 (MMP9) was marked in 16 DIV cells. Increased senescence-associated β-galactosidase (SA-β-gal) and upregulated miR-146a expression that were observed in 16 DIV cells showed to increase by Aβ in 2 DIV microglia. Additionally, Aβ downregulated miR-155 and miR-124, and reduced the CD11b+ subpopulation in 2 DIV microglia, while increased the number of CD86+ cells in 16 DIV microglia. Simultaneous M1 and M2 markers were found after Aβ treatment, but at lower expression in the in vitro aged microglia. Data show key-aging associated responses by microglia when incubated with Aβ, with a loss of reactivity from the 2 DIV to the 16 DIV cells, which course with a reduced phagocytosis, migration and lower expression of inflammatory miRNAs. These findings help to improve our understanding on the heterogeneous responses that microglia can have along the progression of AD disease and imply that therapeutic approaches may differ from early to late stages.

Changes in phagocytosis and potassium channel activity in microglia of 5xFAD mice indicate alterations in purinergic signaling in a mouse model of Alzheimer's disease

As the immunocompetent cells of the central nervous system, microglia accumulate at amyloid beta plaques in Alzheimer's disease (AD) and acquire a morphological phenotype of activated microglia. Recent functional studies, however, indicate that in mouse models of amyloidosis and AD, these cells are rather dysfunctional indicated by a reduced phagocytic activity. Here, we report that this reduction in phagocytic activity is associated with perturbed purinergic receptor signaling, since phagocytosis could be stimulated by P2Y₆ receptor activation in control, but not in 5xFAD transgenic animals, an animal model of amyloid deposition. Impaired phagocytosis is not innate, and develops only at later stages of amyloidosis. Furthermore, we show that membrane currents induced by uridine diphosphate, a ligand activating P2Y₆ receptors, are altered in response rate and amplitude in microglia in close vicinity to plaques, but not in plaque-free areas of 5xFAD animals. These changes were accompanied by changes in membrane properties and potassium channel activity of plaque-associated microglia in early and late stages of amyloidosis. As a conclusion, the physiological properties of plaque-associated microglia are altered with a strong impact on purinergic signaling.

Altered motility of plaque-associated microglia in a model of Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia in the elderly, is characterized by the presence of extracellular plaques composed of amyloid β (Aβ) peptides and intracellular tau aggregates. The plaques are surrounded by microglia, the brain's resident immune cells, which likely participate in the clearance of Aβ by phagocytosis. The microglia that are associated with plaques display an abnormal ameboid morphology and do not respond to tissue damage, in contrast to microglia in healthy brains. Here, we used time lapse confocal microscopy to perform a detailed real-time examination of microglial motility in acute hippocampal brain slices from the 5xFAD mouse model of AD, which was crossed to Cx3cr1(GFP/GFP) mice to achieve microglia-specific GFP expression for visualization. During baseline conditions, microglia around plaques appeared hypermotile, moving the processes that were pointing away from plaques at higher speed than microglia not associated with plaques. Yet, neither plaque-associated, nor plaque-free microglia were able to extend processes toward sites of modest mechanical damage. Application of the selective adenosine A2A receptor antagonist preladenant, which restores microglial response to cellular damage in a mouse model of Parkinson's disease, reduced the hypermotility of plaque-associated microglia, but did not restore motility toward damaged cells in slices from 5xFAD mice. Our results suggest that process hypermotility and resistance to A2A antagonism during response to tissue damage may represent unique functional phenotypes of plaque-associated microglia that impair their ability to function properly in the AD brain.

P2Y Receptors in Synaptic Transmission and Plasticity: Therapeutic Potential in Cognitive Dysfunction

ATP released from neurons and astrocytes during neuronal activity or under pathophysiological circumstances is able to influence information flow in neuronal circuits by activation of ionotropic P2X and metabotropic P2Y receptors and subsequent modulation of cellular excitability, synaptic strength, and plasticity. In the present paper we review cellular and network effects of P2Y receptors in the brain. We show that P2Y receptors inhibit the release of neurotransmitters, modulate voltage- and ligand-gated ion channels, and differentially influence the induction of synaptic plasticity in the prefrontal cortex, hippocampus, and cerebellum. The findings discussed here may explain how P2Y₁ receptor activation during brain injury, hypoxia, inflammation, schizophrenia, or Alzheimer's disease leads to an impairment of cognitive processes. Hence, it is suggested that the blockade of P2Y₁ receptors may have therapeutic potential against cognitive disturbances in these states.

NADPH oxidases in oxidant production by microglia: activating receptors, pharmacology and association with disease

Microglia are the resident immune cells of the CNS and constitute a self-sustaining population of CNS-adapted tissue macrophages. As mononuclear phagocytic cells, they express high levels of superoxide-producing NADPH oxidases (NOX). The sole function of the members of the NOX family is to generate reactive oxygen species (ROS) that are believed to be important in CNS host defence and in the redox signalling circuits that shape the different activation phenotypes of microglia. NOX are also important in pathological conditions, where over-generation of ROS contributes to neuronal loss via direct oxidative tissue damage or disruption of redox signalling circuits. In this review, we assess the evidence for involvement of NOX in CNS physiopathology, with particular emphasis on the most important surface receptors that lead to generation of NOX-derived ROS. We evaluate the potential significance of the subcellular distribution of NOX isoforms for redox signalling or release of ROS to the extracellular medium. Inhibitory mechanisms that have been reported to restrain NOX activity in microglia and macrophages in vivo are also discussed. We provide a critical appraisal of frequently used and recently developed NOX inhibitors. Finally, we review the recent literature on NOX and other sources of ROS that are involved in activation of the inflammasome and discuss the potential influence of microglia-derived oxidants on neurogenesis, neural differentiation and culling of surplus progenitor cells. The degree to which excessive, badly timed or misplaced NOX activation in microglia may affect neuronal homeostasis in physiological or pathological conditions certainly merits further investigation.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Microglial P2 Purinergic Receptor and Immunomodulatory Gene Transcripts Vary By Region, Sex, and Age in the Healthy Mouse CNS

Inflammatory damage in many neurodegenerative diseases is restricted to certain regions of the CNS, and while microglia have long been implicated in the pathology of many of these disorders, information comparing their gene expression in different CNS regions is lacking. Here we tested the hypothesis that the expression of purinergic receptors, estrogen receptors and other neuroprotective and pro-inflammatory genes differed among CNS regions in healthy mice. Because neurodegenerative diseases vary in incidence by sex and age, we also examined the regional distribution of these genes in male and female mice of four different ages between 21 days and 12 months. We postulated that pro-inflammatory gene expression would be higher in older animals, and lower in young adult females. We found that microglial gene expression differed across the CNS. Estrogen receptor alpha (Esr1) mRNA levels were often lower in microglia from the brainstem/spinal cord than from the cortex, whereas tumor necrosis factor alpha (Tnfα) expression was several times higher. In addition, the regional pattern of gene expression often changed with animal age; for example, no regional differences in P2X7 mRNA levels were detected in 21 day-old animals, but at 7 weeks and older, expression was highest in cerebellar microglia. Lastly, the expression of some genes was sexually dimorphic. In microglia from 12 month-old animals, mRNA levels of inducible nitric oxide synthase, but not Tnfα, were higher in females than males. These data suggest that microglial gene expression is not uniformly more pro-inflammatory in males or older animals. Moreover, microglia from CNS regions in which neuronal damage predominates in neurodegenerative disease do not generally express more pro-inflammatory genes than microglia from regions less frequently affected. This study provides an in-depth assessment of regional-, sex- and age-dependent differences in key microglial transcripts from the healthy mouse CNS.

Purinergic receptors as potential therapeutic targets in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of memory and cognitive ability and is a serious cause of mortality. Many of the pathological characteristics associated with AD are revealed post-mortem, including amyloid-β plaque deposition, neurofibrillary tangles containing hyperphosphorylated tau proteins and neuronal loss in the hippocampus and cortex. Although several genetic mutations and risk factors have been associated with the disease, the causes remain poorly understood. Study of disease-initiating mechanisms and AD progression in humans is inherently difficult as most available tissue specimens are from late-stages of disease. Therefore, AD researchers rely on in vitro studies and the use of AD animal models where neuroinflammation has been shown to be a major characteristic of AD. Purinergic receptors are a diverse family of proteins consisting of P1 adenosine receptors and P2 nucleotide receptors for ATP, UTP and their metabolites. This family of receptors has been shown to regulate a wide range of physiological and pathophysiological processes, including neuroinflammation, and may contribute to the pathogenesis of neurodegenerative diseases like Parkinson's disease, multiple sclerosis and AD. Experimental evidence from human AD tissue has suggested that purinergic receptors may play a role in AD progression and studies using selective purinergic receptor agonists and antagonists in vitro and in AD animal models have demonstrated that purinergic receptors represent novel therapeutic targets for the treatment of AD. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Pharmacology and structure of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y receptors are widely expressed and play important roles in physiology and pathophysiology. One important example is the ADP-induced platelet aggregation mediated by P2Y1 and P2Y12 receptors. Active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel as well as the nucleoside analogue ticagrelor block P2Y12 receptors and thereby platelet aggregation. These drugs are used for the prevention and therapy of cardiovascular events. Moreover, P2Y receptors play important roles in the nervous system. Adenine nucleotides modulate neuronal activity and neuronal fibre outgrowth by activation of P2Y1 receptors and control migration of microglia by P2Y12 receptors. UDP stimulates microglial phagocytosis through activation of P2Y6 receptors. There is evidence for a role for P2Y2 receptors in Alzheimer's disease pathology. The P2Y receptor subtypes are highly diverse in both their amino acid sequences and their pharmacological profiles. Selective receptor ligands have been developed for the pharmacological characterization of the receptor subtypes. The recently published three-dimensional crystal structures of the human P2Y1 and P2Y12 receptors will facilitate the development of therapeutic agents that selectively target P2Y receptors. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Supportive or detrimental roles of P2Y receptors in brain pathology?--The two faces of P2Y receptors in stroke and neurodegeneration detected in neural cell and in animal model studies

This review describing the role of P2Y receptors in neuropathological conditions focuses on obvious differences between results demonstrating either a role in neuroprotection or in neurodegeneration, depending on in vitro and in vivo models. Such critical juxtaposition puts special emphasis on discussions of beneficial and detrimental effects of P2Y receptor agonists and antagonists in these models. The mechanisms reported to underlie the protection in vitro include increased expression of oxidoreductase genes, like carbonyl reductase and thioredoxin reductase; increased expression of inhibitor of apoptosis protein-2; extracellular signal-regulated kinase- and Akt-mediated antiapoptotic signaling; increased expression of Bcl-2 proteins, neurotrophins, neuropeptides, and growth factors; decreased Bax expression; non-amyloidogenic APP shedding; and increased neurite outgrowth in neuronal cells. Animal studies investigating the influence of P2Y receptors in middle cerebral artery occlusion (MCAO) models for stroke prove beneficial effects of P2Y receptor antagonists. In MCAO mice and rats, the application of broad-range P2 receptor antagonists decreased the infarct volume and improved neurological outcome. Moreover, antagonists of the P2Y1 receptor, one of the most abundant P2Y receptor subtypes in brain tissue, decreased neuronal loss and improved spatial memory in rats after traumatic brain injury (TBI). Currently available data show a discrepancy between in vitro and in vivo models concerning the benefits of P2Y receptor activation in pathological conditions. In vitro models demonstrate protection by P2Y receptor agonists, but in vivo P2Y receptor activation deteriorates the outcome after MCAO and controlled cortical impact brain injury, a TBI model. To broaden the scope of the review, we additionally discuss publications that demonstrate detrimental effects of P2Y receptor agonists in vitro and publications showing protective effects of agonists in vivo. All these studies help to better understand the significant role of P2Y receptors especially in stroke models and to develop pharmacological strategies for the treatment of stroke.

Purinergic mechanisms in neuroinflammation: An update from molecules to behavior

The principle functions of neuroinflammation are to limit tissue damage and promote tissue repair in response to pathogens or injury. While neuroinflammation has utility, pathophysiological inflammatory responses, to some extent, underlie almost all neuropathology. Understanding the mechanisms that control the three stages of inflammation (initiation, propagation and resolution) is therefore of critical importance for developing treatments for diseases of the central nervous system. The purinergic signaling system, involving adenosine, ATP and other purines, plus a host of P1 and P2 receptor subtypes, controls inflammatory responses in complex ways. Activation of the inflammasome, leading to release of pro-inflammatory cytokines, activation and migration of microglia and altered astroglial function are key regulators of the neuroinflammatory response. Here, we review the role of P1 and P2 receptors in mediating these processes and examine their contribution to disorders of the nervous system. Firstly, we give an overview of the concept of neuroinflammation. We then discuss the contribution of P2X, P2Y and P1 receptors to the underlying processes, including a discussion of cross-talk between these different pathways. Finally, we give an overview of the current understanding of purinergic contributions to neuroinflammation in the context of specific disorders of the central nervous system, with special emphasis on neuropsychiatric disorders, characterized by chronic low grade inflammation or maternal inflammation. An understanding of the important purinergic contribution to neuroinflammation underlying neuropathology is likely to be a necessary step towards the development of effective interventions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

The P2Y₂ receptor promotes Wnt3a- and EGF-induced epithelial tubular formation by IEC6 cells by binding to integrins

Epithelial tubular structures are essential units in various organs. Here, we used rat intestinal epithelial IEC6 cells to investigate tubulogenesis and we found that tubular formation was induced by a combination of Wnt3a and EGF signaling during three-dimensional culture. Wnt3a and EGF induced the expression of the P2Y2 receptor (P2Y2R, also known as P2RY2), and knockdown of P2Y2R suppressed tubular formation. A P2Y2R mutant that lacks nucleotide responsiveness rescued the phenotypes resulting from P2Y2R knockdown, suggesting that nucleotide-dependent responses are not required for P2Y2R functions in tubular formation. The Arg-Gly-Asp (RGD) sequence of P2Y2R has been shown to interact with integrins, and a P2Y2R mutant lacking integrin-binding activity was unable to induce tubular formation. P2Y2R expression inhibited the interaction between integrins and fibronectin, and induced cell morphological changes and proliferation. Inhibition of integrin and fibronectin binding by treatment with the cyclic RGD peptide and fibronectin knockdown induced tubular formation in the presence of EGF alone, but a fibronectin coat suppressed Wnt3a- and EGF-induced tubular formation. These results suggest that Wnt3a- and EGF-induced P2Y2R expression causes tubular formation by preventing the binding of integrins and fibronectin rather than by mediating nucleotide responses.

P2X and P2Y receptors—role in the pathophysiology of the nervous system

Purinergic signalling plays a crucial role in proper functioning of the nervous system. Mechanisms depending on extracellular nucleotides and their P2 receptors also underlie a number of nervous system dysfunctions. This review aims to present the role of purinergic signalling, with particular focus devoted to role of P2 family receptors, in epilepsy, depression, neuropathic pain, nervous system neoplasms, such as glioma and neuroblastoma, neurodegenerative diseases like Parkinson’s disease, Alzheimer’s disease and multiple sclerosis. The above-mentioned conditions are associated with changes in expression of extracellular ectonucleotidases, P2X and P2Y receptors in neurons and glial cells, as well as releasing considerable amounts of nucleotides from activated or damaged nervous tissue cells into the extracellular space, which contributes to disturbance in purinergic signalling. The numerous studies indicate a potential possibility of using synthetic agonists/antagonists of P2 receptors in treatment of selected nervous system diseases. This is of particular significance, since numerous available agents reveal a low effectiveness and often produce side effects.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

P2Y6 receptor-mediated proinflammatory signaling in human bronchial epithelia

P2Y receptors are expressed in virtually all epithelia and are responsible for the control of fluid and electrolyte transport. In asthmatic inflammation, the bronchial epithelia are damaged by eosinophil-derived, highly toxic cationic proteins, such as major basic protein (MBP). Consequently, extracellular nucleotides are released into the extracellular space from airway epithelial cells, and act in an autocrine or paracrine fashion to regulate immune functions. Our data show damage to the human bronchial epithelial cell line, 16HBE14o-, by poly-L-arginine-induced UDP release into the extracellular medium. Activation of P2Y6 receptor by its natural ligand, UDP, or its specific agonist, MRS 2693, led to the production of two proinflammatory cytokines, interleukin (IL)-6 and IL-8. This may have resulted from increased IL-6 and IL-8 mRNA expression, and activation of p38 and ERK1/2 MAPK, and NF-κB pathways. Our previous study demonstrated that UDP stimulated transepithelial Cl- secretion via both Ca2+- and cAMP-dependent pathways in 16HBE14o- epithelia. This was further confirmed in this study by simultaneous imaging of Ca2+ and cAMP levels in single cells using the Fura-2 fluorescence technique and a FRET-based approach, respectively. Moreover, the P2Y6 receptor-mediated production of IL-6 and IL-8 was found to be dependent on Ca2+, but not the cAMP/PKA pathway. Together, these studies show that nucleotides released during the airway inflammatory processes will activate P2Y6 receptors, which will lead to further release of inflammatory cytokines. The secretion of cytokines and the formation of such "cytokine networks" play an important role in sustaining the airway inflammatory disease.