Microglia Purinoceptor P2Y6: An Emerging Therapeutic Target in CNS Diseases

Modification of Neural Circuit Functions by Microglial P2Y6 Receptors in Health and Neurodegeneration

Neural circuits consisting of neurons and glial cells help to establish all functions of the CNS. Microglia, the resident immunocytes of the CNS, are endowed with UDP-sensitive P2Y6 receptors (P2Y6Rs) which regulate phagocytosis/pruning of excessive synapses during individual development and refine synapses in an activity-dependent manner during adulthood. In addition, this type of receptor plays a decisive role in primary (Alzheimer's disease, Parkinson's disease, neuropathic pain) and secondary (epilepsy, ischemic-, mechanical-, or irradiation-induced) neurodegeneration. A whole range of microglial cytokines controlled by P2Y6Rs, such as the interleukins IL-1β, IL-6, IL-8, and tumor necrosis factor-α (TNF-α), leads to neuroinflammation, resulting in neurodegeneration. Hence, small molecular antagonists of P2Y6Rs and genetic knockdown of this receptor provide feasible ways to alleviate inflammation-induced neurological disorders but might also interfere with the regulation of the synaptic circuitry. The present review aims at investigating this dual role of P2Y6Rs in microglia, both in shaping neural circuits by targeted phagocytosis and promoting neurodegenerative illnesses by fostering neuroinflammation through multiple transduction mechanisms.

P2Y6 receptor: A promising therapeutic target for atherosclerosis

Atherosclerosis is induced by lipid accumulation, inflammation, and endothelial dysfunction, and is the leading cause of death from cardiovascular disease worldwide. The P2Y6 receptor can be activated by the extracellular release of UDP. The evidence from the last decade has highlighted its critical therapeutic effect in atherosclerosis, yet with unclear mechanisms. This review introduced the P2Y6 receptor in atherosclerosis, and its mechanisms of atherosclerosis-promoting in macrophages, endothelial cells, and vascular smooth muscle cells. Finally, we discussed the development and potential of P2Y6 receptor antagonists in treating atherosclerosis.

A new perspective on the regulation of neuroinflammation in intracerebral hemorrhage: mechanisms of NLRP3 inflammasome activation and therapeutic strategies

Intracerebral hemorrhage (ICH), a specific subtype within the spectrum of stroke disorders, is characterized by its high mortality and significant risk of long-term disability. The initiation and progression of neuroinflammation play a central and critical role in the pathophysiology of ICH. The NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome, a protein complex involved in initiating inflammation, is the central focus of this article. Microglia and astrocytes play critical roles in the inflammatory damage process associated with neuroinflammation. The NLRP3 inflammasome is expressed within both types of glial cells, and its activation drives these cells toward a pro-inflammatory phenotype, which exacerbates inflammatory damage in the brain. However, the regulatory relationship between these two cell types remains to be explored. Targeting NLRP3 inflammasomes in microglia or astrocytes may provide an effective approach to mitigate neuroinflammation following ICH. This article first provides an overview of the composition and activation mechanisms of the NLRP3 inflammasome. Subsequently, it summarizes recent research findings on novel signaling pathways that regulate NLRP3 inflammasome activity. Finally, we reviewed recent progress in NLRP3 inflammasome inhibitors, highlighting the clinical translation potential of certain candidates. These inhibitors hold promise as innovative strategies for managing inflammation following ICH.

CD38 deficiency leads to a defective short-lived transcriptomic response to chronic graft-versus-host disease induction, involving purinergic signaling-related genes and distinct transcriptomic signatures associated with lupus

This study aimed to elucidate the transcriptomic signatures and dysregulated pathways associated with the autoimmune response in Cd38-/- mice compared to wild-type (WT) mice within the bm12 chronic graft-versus-host disease (cGVHD) lupus model. We conducted bulk RNA sequencing on peritoneal exudate cells (PECs) and spleen cells (SPC) at two and four weeks following adoptive cell transfer. We also analyzed cells from healthy, untreated mice. These analyses revealed a sustained upregulation of a transcriptional profile of purinergic receptors and ectonucleotidases in cGVHD WT PECs, which displayed a coordinated expression with several type I interferon-stimulated genes (ISGs) and with key molecules involved in the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling pathway, two hallmarks in the lupus pathology. A second purinergic receptor transcriptomic profile, which included P2rx7 and P2rx4, showed a coordinated gene expression of the components of the NLRP3 inflammasome with its potential activators. These processes were transcriptionally less active in cGVHD Cd38-/- PECs than in WT PECs. We have also shown evidence of a distinct enrichment in pathways signatures that define processes such as Ca2+ ion homeostasis, cell division, phagosome, autophagy, senescence, cytokine/cytokine receptor interactions, Th17 and Th1/Th2 cell differentiation in Cd38-/- versus WT samples, which reflected the milder inflammatory and autoimmune response elicited in Cd38-/- mice relative to WT counterparts in response to the allogeneic challenge. Last, we have shown an intense metabolic reprogramming toward oxidative phosphorylation in PECs and SPC from cGVHD WT mice, which may reflect an increased cellular demand for oxygen consumption, in contrast to PECs and SPC from cGVHD Cd38-/- mice, which showed a short-lived metabolic effect at the transcriptomic level. Overall, these findings support the pro-inflammatory and immunomodulatory role of CD38 during the development of the cGVHD-lupus disease.

Microglia-mediated neuroinflammation in traumatic brain injury: a review

Traumatic brain injury (TBI) is a leading cause of disability worldwide, characterized by a complex interplay of primary and secondary injury mechanisms. Microglia, the resident immune cells of the central nervous system, play a crucial role in the inflammatory response following TBI. To review the current understanding of microglia-mediated neuroinflammation in TBI, exploring its dual nature as a protective and detrimental process. A comprehensive literature review was conducted using databases such as PubMed, Scopus, and Google Scholar. Relevant studies investigating the role of microglia in TBI were included. In the early stages of TBI, microglia exhibit a protective response, releasing cytokines and chemokines to promote neuronal survival and tissue repair. However, prolonged or excessive microglial activation can lead to neurotoxicity and exacerbate secondary injury. Microglia-mediated neuroinflammation involves complex signaling pathways, including Toll-like receptors, purinergic receptors, and the complement system. Microglia-mediated neuroinflammation in TBI is a double-edged sword. While acute microglial activation can promote repair, chronic or excessive inflammation contributes to neuronal damage and functional deficits. Understanding the temporal and molecular dynamics of microglial responses is crucial for developing therapeutic strategies to modulate neuroinflammation and improve outcomes after TBI.

Suppression of HIV-TAT and cocaine-induced neurotoxicity and inflammation by cell penetrable itaconate esters

HIV-associated neurological disorder (HAND) is a serious complication of HIV infection marked by neurotoxicity induced by viral proteins like Tat. Substance abuse exacerbates neurocognitive impairment in people living with HIV. There is an urgent need for therapeutic strategies to combat HAND comorbid with Cocaine Use Disorder (CUD). Our analysis of HIV and cocaine-induced transcriptomes in primary cortical cultures revealed significant overexpression of the macrophage-specific gene aconitate decarboxylase 1 (Acod1). The ACOD1 protein converts the tricarboxylic acid intermediate cis-aconitate into itaconate during the activation of inflammation. Itaconate then facilitates cytokine production and activates anti-inflammatory transcription factors, shielding macrophages from infection-induced cell death. However, the immunometabolic function of itaconate was unexplored in HIV and cocaine-exposed microglia. We assessed the potential of 4-octyl-itaconate (4OI), a cell-penetrable ester form of itaconate known for its anti-inflammatory properties. When primary cortical cultures exposed to Tat and cocaine were treated with 4OI, microglial cell number increased and the morphological altercations induced by Tat and cocaine were reversed. Microglial cells also appeared more ramified, resembling the quiescent microglia. 4OI treatment inhibited secretion of the proinflammatory cytokines IL-1α, IL-1β, IL-6, and MIP1-α induced by Tat and cocaine. Transcriptome profiling determined that Nrf2 target genes were significantly activated in Tat and 4OI treated cultures relative to Tat alone. Further, genes associated with cytoskeleton dynamics in inflammatory microglia were downregulated by 4OI treatment. Together, the results strongly suggest 4-octyl-itaconate holds promise as a potential candidate for therapeutic development to treat HAND coupled with CUD comorbidities.

P2Y6 Receptor Activation Aggravates NLRP3-dependent Microglial Pyroptosis via Downregulation of the PI3K/AKT Pathway in a Mouse Model of Intracerebral Hemorrhage

Pro-inflammatory signals generated after intracerebral hemorrhage (ICH) trigger a form of regulated cell death known as pyroptosis in microglia. White matter injury (WMI) refers to the condition where the white matter area of the brain suffers from mechanical, ischemic, metabolic, or inflammatory damage. Although the p2Y purinoceptor 6 (P2Y6R) plays a significant role in the control of inflammatory reactions in central nervous system diseases, its roles in the development of microglial pyroptosis and WMI following ICH remain unclear. In this study, we sought to clarify the role of P2Y6R in microglial pyroptosis and WMI by using an experimental mouse model of ICH. Type IV collagenase was injected into male C57BL/6 mice to induce ICH. Mice were then treated with MRS2578 and LY294002 to inhibit P2Y6R and phosphatidylinositol 3-kinase (PI3K), respectively. Bio-conductivity analysis was performed to examine PI3K/AKT pathway involvement in microglial pyroptosis. Quantitative Real-Time PCR, immunofluorescence staining, and western blot were conducted to examine microglial pyroptosis and WMI following ICH. A modified Garcia test, corner turning test, and forelimb placement test were used to assess neurobehavior. Hematoxylin-eosin staining (HE) was performed to detect cells damage around hematoma. Increases in the expression of P2Y6R, NLRP3, ASC, Caspase-1, and GSDMD were observed after ICH. P2Y6R was only expressed on microglia. MRS2578, a specific inhibitor of P2Y6R, attenuated short-term neurobehavioral deficits, brain edema and hematoma volume while improving both microglial pyroptosis and WMI. These changes were accompanied by decreases in pyroptosis-related proteins and pro-inflammatory cytokines both in vivo and vitro. Bioinformatic analysis revealed an association between the PI3K/AKT pathway and P2Y6R-mediated microglial pyroptosis. The effects of MRS2578 were partially reversed by treatment with LY294002, a specific PI3K inhibitor. P2Y6R inhibition alleviates microglial pyroptosis and WMI and ameliorates neurological deficits through the PI3K/AKT pathway after ICH. Consequently, targeting P2Y6R might be a promising approach for ICH treatment.

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.

Microglial P2Y6 calcium signaling promotes phagocytosis and shapes neuroimmune responses in epileptogenesis

Microglial calcium signaling is rare in a baseline state but strongly engaged during early epilepsy development. The mechanism(s) governing microglial calcium signaling are not known. By developing an in vivo uridine diphosphate (UDP) fluorescent sensor, GRABUDP1.0, we discovered that UDP release is a conserved response to seizures and excitotoxicity across brain regions. UDP can signal through the microglial-enriched P2Y6 receptor to increase calcium activity during epileptogenesis. P2Y6 calcium activity is associated with lysosome biogenesis and enhanced production of NF-κB-related cytokines. In the hippocampus, knockout of the P2Y6 receptor prevents microglia from fully engulfing neurons. Attenuating microglial calcium signaling through calcium extruder ("CalEx") expression recapitulates multiple features of P2Y6 knockout, including reduced lysosome biogenesis and phagocytic interactions. Ultimately, P2Y6 knockout mice retain more CA3 neurons and better cognitive task performance during epileptogenesis. Our results demonstrate that P2Y6 signaling impacts multiple aspects of myeloid cell immune function during epileptogenesis.

Surface-Tailored Nanoplatform for the Diagnosis and Management of Stroke: Current Strategies and Future Outlook

Stroke accounts for one of the top leading reasons for neurological mortality and morbidity around the globe. Both ischemic and hemorrhagic strokes lead to local hypoxia and are brought about by the occlusion or rupturing of the blood vessels. The events taking place after the onset of a stroke include membrane ion pump failure, calcium and glutamate-mediated excitotoxicity, increased ROS production causing DNA damage, mitochondrial dysfunction, oxidative stress, development of brain edema, and microvascular dysfunction. To date, tissue plasminogen activator (tPA) therapy and mechanical removal of blood clots are the only clinically available stroke therapies, approved by Food and Drug Administration (FDA). But because of the narrow therapeutic window of around 4.5 h for tPA therapy and complications like systemic bleeding and anaphylaxis, more clinical trials are ongoing in the same field. Therefore, using nanocarriers with diverse physicochemical properties is a promising strategy in treating and diagnosing stroke as they can efficiently bypass the tight blood-brain barrier (BBB) through mechanisms like receptor-mediated transcytosis and help achieve controlled and targeted drug delivery. In this review, we will mainly focus on the pathophysiology of stroke, BBB alterations following stroke, strategies to target BBB for stroke therapies, different types of nanocarriers currently being used for therapeutic intervention of stroke, and biomarkers as well as imaging techniques used for the detection and diagnosis of stroke.

Paradigm Shift: Multiple Potential Pathways to Neurodegenerative Dementia

Neurodegenerative dementia can result from multiple underlying abnormalities, including neurotransmitter imbalances, protein aggregation, and other neurotoxic events. A major complication in identifying effective treatment targets is the frequent co-occurrence of multiple neurodegenerative processes, occurring either in parallel or sequentially. The path towards developing effective treatments for Alzheimer's disease (AD) and other dementias has been relatively slow and until recently has focused on disease symptoms. Aducanumab and lecanemab, recently approved by the FDA, are meant to target disease structures but have only modest benefit on symptom progression and remain unproven in reversing or preventing dementia. A third, donanemab, appears more promising but awaits FDA approval. Ongoing trials include potential cognition enhancers, new combinations of known drugs for synergistic effects, prodrugs with less toxicity, and increasing interest in drugs targeting neuroinflammation or microbiome. Scientific and technological advances offer the opportunity to move in new therapy directions, such as modifying microglia to prevent or suppress underlying disease. A major challenge, however, is that underlying comorbidities likely influence the effectiveness of therapies. Indeed, the full range of comorbidity, today only definitively identified postmortem, likely contributes to failed clinical trials and overmedication of older adults, since it is difficult to exclude (during life) people unlikely to respond. Our current knowledge thus signals that a paradigm shift towards individualized and multimodal treatments is necessary to effectively advance the field of dementia therapeutics.

A Distinct Microglial Cell Population Expressing Both CD86 and CD206 Constitutes a Dominant Type and Executes Phagocytosis in Two Mouse Models of Retinal Degeneration

Microglial cells are the key regulators of inflammation during retinal degeneration (RD) and are conventionally classified as M1 or M2. However, whether the M1/M2 classification exactly reflects the functional classification of microglial cells in the retina remains debatable. We examined the spatiotemporal changes of microglial cells in the blue-LED and NaIO3-induced RD mice models using M1/M2 markers and functional genes. TUNEL assay was performed to detect photoreceptor cell death, and microglial cells were labeled with anti-IBA1, P2RY12, CD86, and CD206 antibodies. FACS was used to isolate microglial cells with anti-CD206 and CD86 antibodies, and qRT-PCR was performed to evaluate Il-10, Il-6, Trem-2, Apoe, and Lyz2 expression. TUNEL-positive cells were detected in the outer nuclear layer (ONL) from 24 h to 72 h post-RD induction. At 24 h, P2RY12 was decreased and CD86 was increased, and CD86/CD206 double-labeled cells occupied the dominant population at 72 h. And CD86/CD206 double-labeled cells showed a significant increase in Apoe, Trem2, and Lyz2 levels but not in those of Il-6 and Il-10. Our results demonstrate that microglial cells in active RD cannot be classified as M1 or M2, and the majority of microglia express both CD86 and CD206, which are involved in phagocytosis rather than inflammation.

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.

Microglial P2Y6 calcium signaling promotes phagocytosis and shapes neuroimmune responses in epileptogenesis

Microglial calcium signaling is rare in a baseline state but shows strong engagement during early epilepsy development. The mechanism and purpose behind microglial calcium signaling is not known. By developing an in vivo UDP fluorescent sensor, GRABUDP1.0, we discovered that UDP release is a conserved response to seizures and excitotoxicity across brain regions. UDP signals to the microglial P2Y6 receptor for broad increases in calcium signaling during epileptogenesis. UDP-P2Y6 signaling is necessary for lysosome upregulation across limbic brain regions and enhances production of pro-inflammatory cytokines-TNFα and IL-1β. Failures in lysosome upregulation, observed in P2Y6 KO mice, can also be phenocopied by attenuating microglial calcium signaling in Calcium Extruder ("CalEx") mice. In the hippocampus, only microglia with P2Y6 expression can perform full neuronal engulfment, which substantially reduces CA3 neuron survival and impairs cognition. Our results demonstrate that calcium activity, driven by UDP-P2Y6 signaling, is a signature of phagocytic and pro-inflammatory function in microglia during epileptogenesis.

Synthesis, structure-activity relationships and biological evaluation of benzimidazole derived sulfonylurea analogues as a new class of antagonists of P2Y1 receptor

The P2Y receptors are responsible for the regulation of various physiological processes including neurotransmission and inflammatory responses. These receptors are also considered as novel potential therapeutic targets for prevention and treatment of thrombosis, neurological disorders, pain, cardiac diseases and cancer. Previously, number of P2Y receptor antagonists has been investigated but they are less potent and non-selective with poor solubility profile. Herein, we present the synthesis of new class of benzimidazole derived sulfonylureas (1a-y) as potent antagonists of P2Y receptors, with the specific aim to explore selective antagonists of P2Y1 receptors. The efficacy and selectivity of the synthesized derivatives 1) against four P2Y receptors i.e., t-P2Y1, h-P2Y2, h-P2Y4, and r-P2Y6Rs was carried out by calcium mobilization assay. The results revealed that except 1b, 1d, 1l, 1m, 1o, 1u, 1v, 1w, and 1y, rest of the synthesized derivatives exhibited moderate to excellent inhibitory potential against P2Y1 receptors. Among the potent antagonists, derivative 1h depicted the maximum inhibition of P2Y1 receptor in calcium signalling assay, with an IC₅₀ value of 0.19 ± 0.04 µM. The potential of inhibition was validated by computational investigations where bonding and non-bonding interactions between ligand and targeted receptor further strengthen the study. The best identified derivative 1h revealed the same binding mechanism as that of already reported selective antagonist of P2Y1 receptor i.e (1-(2- (2-tert-butyl-phenoxy) pyridin-3-yl)-3-4-(trifluoromethoxy) phenylurea but the newly synthesized derivative exhibited better solubility profile. Hence, this derivative can be used as lead candidate for the synthesis of more potential antagonist with much better solubility profile and medicinal importance.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

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.

Molecular Regulation Mechanism of Microglial Autophagy in the Pathology of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the progressive accumulation of abnormal protein aggregates, neuronal loss, synaptic dysfunction, and neuroinflammation. Microglia are resident macrophages of the central nervous system (CNS). Evidence has shown that impaired microglial autophagy exerts considerable detrimental impact on the CNS, thus contributing to AD pathogenesis. This review highlights the association between microglial autophagy and AD pathology, with a focus on the inflammatory response, defective clearance, and propagation of Aβ and Tau, and synaptic dysfunction. Mechanistically, several lines of research support the roles of microglial receptors in autophagy regulation during AD. In light of accumulating evidence, a strategy for inducing microglial autophagy has great potential in AD drug development.

Drug-induced microglial phagocytosis in multiple sclerosis and experimental autoimmune encephalomyelitis and the underlying mechanisms

Microglia are resident macrophages of the central nervous system (CNS). It plays a significant role in immune surveillance under physiological conditions. On stimulation by pathogens, microglia change their phenotypes, phagocytize toxic molecules, secrete pro-inflammatory/anti-inflammatory factors, promotes tissue repair, and maintain the homeostasis in CNS. Accumulation of myelin debris in multiple sclerosis (MS)/experimental autoimmune encephalomyelitis (EAE) inhibits remyelination by decreasing the phagocytosis by microglia and prevent the recovery of MS/EAE. Drug induced microglia phagocytosis could be a novel therapeutic intervention for the treatment of MS/EAE. But the abnormal phagocytosis of neurons and synapses by activated microglia will lead to neuronal damage and degeneration. It indicates that the phagocytosis of microglia has many beneficial and harmful effects in central neurodegenerative diseases. Therefore, simply promoting or inhibiting the phagocytic activity of microglia may not achieve ideal therapeutic results. However, limited reports are available to elucidate the microglia mediated phagocytosis and its underlying molecular mechanisms. On this basis, the present review describes microglia-mediated phagocytosis, drug-induced microglia phagocytosis, molecular mechanism, and novel approach for MS/EAE treatment.

P2X7 Receptor and Purinergic Signaling: Orchestrating Mitochondrial Dysfunction in Neurodegenerative Diseases

Mitochondrial dysfunction is one of the basic hallmarks of cellular pathology in neurodegenerative diseases. Since the metabolic activity of neurons is highly dependent on energy supply, nerve cells are especially vulnerable to impaired mitochondrial function. Besides providing oxidative phosphorylation, mitochondria are also involved in controlling levels of second messengers such as Ca2+ ions and reactive oxygen species (ROS). Interestingly, the critical role of mitochondria as producers of ROS is closely related to P2XR purinergic receptors, the activity of which is modulated by free radicals. Here, we review the relationships between the purinergic signaling system and affected mitochondrial function. Purinergic signaling regulates numerous vital biological processes in the CNS. The two main purines, ATP and adenosine, act as excitatory and inhibitory neurotransmitters, respectively. Current evidence suggests that purinergic signaling best explains how neuronal activity is related to neuronal electrical activity and energy homeostasis, especially in the development of Alzheimer's and Parkinson's diseases. In this review, we focus on the mechanisms underlying the involvement of the P2RX7 purinoreceptor in triggering mitochondrial dysfunction during the development of neurodegenerative disorders. We also summarize various avenues by which the purine signaling pathway may trigger metabolic dysfunction contributing to neuronal death and the inflammatory activation of glial cells. Finally, we discuss the potential role of the purinergic system in the search for new therapeutic approaches to treat neurodegenerative diseases.

P2Y6 receptor-mediated signaling amplifies TLR-induced pro-inflammatory responses in microglia

TLR-induced signaling initiates inflammatory responses in cells of the innate immune system. These responses are amongst others characterized by the secretion of high levels of pro-inflammatory cytokines, which are tightly regulated and adapted to the microenvironment. Purinergic receptors are powerful modulators of TLR-induced responses, and we here characterized the effects of P2Y6 receptor (P2RY6)-mediated signaling on TLR responses of rhesus macaque primary bone marrow-derived macrophages (BMDM) and microglia, using the selective P2RY6 antagonist MRS2578. We demonstrate that P2RY6-mediated signaling enhances the levels of TLR-induced pro-inflammatory cytokines in microglia in particular. TLR1, 2, 4, 5 and 8-induced responses were all enhanced in microglia, whereas such effects were much less pronounced in BMDM from the same donors. Transcriptome analysis revealed that the overall contribution of P2RY6-mediated signaling to TLR-induced responses in microglia leads to an amplification of pro-inflammatory responses. Detailed target gene analysis predicts that P2RY6-mediated signaling regulates the expression of these genes via modulation of the activity of transcription factors NFAT, IRF and NF-κB. Interestingly, we found that the expression levels of heat shock proteins were strongly induced by inhibition of P2RY6-mediated signaling, both under homeostatic conditions as well as after TLR engagement. Together, our results shed new lights on the specific pro-inflammatory contribution of P2RY6-mediated signaling in neuroinflammation, which might open novel avenues to control brain inflammatory responses.

Inhibition of P2Y6 receptor expression in Kupffer cells alleviates alcoholic steatohepatitis in mice

Inflammation plays an important role in the progression of alcohol-related liver disease (ALD). UDP-P2Y₆ signaling is involved in many human diseases. The purinergic P2Y₆ receptor, an important regulator of inflammation and phagocytosis, has attracted attention, but its role in alcoholic steatohepatitis remains unclear. Here, we found that P2Y₆ levels were significantly elevated in Kupffer cells in the livers of mice with alcoholic steatohepatitis and ethanol (EtOH)-induced RAW264.7 cells. In this study, mice with alcoholic steatohepatitis were intraperitoneally injected with MRS2578, a specific inhibitor of the P2Y₆ receptor, and P2Y₆ was silenced in EtOH-induced RAW264.7 cells. We found a marked improvement in steatosis and inflammation in the livers of mice with alcoholic steatohepatitis and EtOH-induced RAW264.7 cells. However, P2Y₆ activation in vivo and overexpression in vitro showed contrasting results. In addition, the expression of phospho-p38 mitogen-activated protein kinase (p-p38 MAPK), a phosphorylated protein in the p38 MAPK signaling pathway, was significantly altered after P2Y₆ silencing or overexpression in vitro. P2Y₆ can induce the activation of the p38 MAPK signaling pathway by mediating the calcium influx, whereas inhibition of the expression of P2Y₆ can block the inflammatory process to some extent and thus improve the inflammatory response. The results of this study suggested that targeting P2Y₆ signaling may be a potentially effective strategy for the treatment of alcoholic steatohepatitis.

Microglia Phenotypes in Aging and Neurodegenerative Diseases

Neuroinflammation is a hallmark of many neurodegenerative diseases (NDs) and plays a fundamental role in mediating the onset and progression of disease. Microglia, which function as first-line immune guardians of the central nervous system (CNS), are the central drivers of neuroinflammation. Numerous human postmortem studies and in vivo imaging analyses have shown chronically activated microglia in patients with various acute and chronic neuropathological diseases. While microglial activation is a common feature of many NDs, the exact role of microglia in various pathological states is complex and often contradictory. However, there is a consensus that microglia play a biphasic role in pathological conditions, with detrimental and protective phenotypes, and the overall response of microglia and the activation of different phenotypes depends on the nature and duration of the inflammatory insult, as well as the stage of disease development. This review provides a comprehensive overview of current research on the various microglia phenotypes and inflammatory responses in health, aging, and NDs, with a special emphasis on the heterogeneous phenotypic response of microglia in acute and chronic diseases such as hemorrhagic stroke (HS), Alzheimer's disease (AD), and Parkinson's disease (PD). The primary focus is translational research in preclinical animal models and bulk/single-cell transcriptome studies in human postmortem samples. Additionally, this review covers key microglial receptors and signaling pathways that are potential therapeutic targets to regulate microglial inflammatory responses during aging and in NDs. Additionally, age-, sex-, and species-specific microglial differences will be briefly reviewed.

Overexpression of GRK6 alleviates chronic visceral hypersensitivity through downregulation of P2Y6 receptors in anterior cingulate cortex of rats with prenatal maternal stress

Aims

Visceral hypersensitivity is a major clinic symptom in patients with irritable bowel syndrome (IBS). Anterior cingulate cortex (ACC) is involved in processing the information of pain. Both G protein‐coupled receptor kinase 6 (GRK6) and P2Y purinoceptor 6 (P2Y6) are associated with neuroinflammation and pathological pain. The aim of this study was to investigate the interaction between GRK6 and P2Y6 in ACC in the development of visceral hypersensitivity of adult offspring rats with prenatal maternal stress (PMS).

Methods

Visceral hypersensitivity was quantified by abdominal withdrawal reflex threshold to colorectal distension (CRD). The expression and cellular distribution of GRK6 and P2Y6 were determined by Western blotting, qPCR, and fluorescence immunohistochemistry. Co‐immunoprecipitation was used to evaluate the interaction between GRK6 and P2Y6.

Results

The mRNA and protein levels of GRK6 were significantly decreased in ACC of PMS rats. The injection of GRK6 overexpression virus significantly attenuated visceral hypersensitivity of PMS rats. P2Y6’s mRNA level, protein level, and ratio of membrane protein over total protein expression was markedly increased in PMS rats. P2Y6 antagonist MRS2578 microinjection reversed visceral hypersensitivity of PMS rats. GRK6 overexpression significantly reduced P2Y6’s expression in membrane proteins and P2Y6’s ratio of membrane protein over total protein expression.

Conclusions

These results indicate that decreased GRK6 leads to the accumulation of P2Y6 at neuron membrane in ACC, thereby contributing to visceral hypersensitivity of PMS rats.

Changes in P2Y6 receptor-mediated vasoreactivity following focal and global ischemia

Ischemia, both in the form of focal thromboembolic stroke and following subarachnoid hemorrhage (SAH), causes upregulation of vasoconstrictive receptor systems within the cerebral vasculature. Descriptions regarding changes in purinergic signaling following ischemia are lacking, especially when the importance of purinergic signaling in regulating vascular tone is taken into consideration. This prompted us to evaluate changes in P2Y₆ -mediated vasomotor reactivity in two different stroke models in rat. We used wire myography to measure changes in cerebral vasoreactivity to the P2Y₆ agonist UDP-β-S following either experimental SAH or transient middle cerebral artery occlusion. Changes in receptor localization or receptor expression were evaluated using immunohistochemistry and quantitative flow cytometry. Transient middle cerebral artery occlusion caused an increase in Emax when compared to sham (233.6 [206.1-258.5]% vs. 161.1 [147.1-242.6]%, p = 0.0365). No such change was seen following SAH. Both stroke models were associated with increased levels of P2Y₆ receptor expression in the vascular smooth muscle cells (90.94 [86.99-99.15]% and 93.79 [89.96-96.39]% vs. 80.31 [70.80-80.86]%, p = 0.021) and p = 0.039 respectively. There was no change in receptor localization in either of the stroke models. Based on these findings, we conclude that focal ischemic stroke increases vascular sensitivity to UDP-β-S by upregulating P2Y₆ receptors on vascular smooth muscle cells while experimental SAH did not induce changes in vasoreactivity in spite of increased P2Y₆ receptor expression.

Knockout of the P2Y6 Receptor Prevents Peri-Infarct Neuronal Loss after Transient, Focal Ischemia in Mouse Brain

After stroke, there is a delayed neuronal loss in brain areas surrounding the infarct, which may in part be mediated by microglial phagocytosis of stressed neurons. Microglial phagocytosis of stressed or damaged neurons can be mediated by UDP released from stressed neurons activating the P2Y6 receptor on microglia, inducing microglial phagocytosis of such neurons. We show evidence here from a small trial that the knockout of the P2Y6 receptor, required for microglial phagocytosis of neurons, prevents the delayed neuronal loss after transient, focal brain ischemia induced by endothelin-1 injection in mice. Wild-type mice had neuronal loss and neuronal nuclear material within microglia in peri-infarct areas. P2Y6 receptor knockout mice had no significant neuronal loss in peri-infarct brain areas seven days after brain ischemia. Thus, delayed neuronal loss after stroke may in part be mediated by microglial phagocytosis of stressed neurons, and the P2Y6 receptor is a potential treatment target to prevent peri-infarct neuronal loss.

Homozygous ALS-linked FUS P525L mutations cell- autonomously perturb transcriptome profile and chemoreceptor signaling in human iPSC microglia

Amyotrophic lateral sclerosis is a fatal disease pathologically typified by motor and cortical neurodegeneration as well as microgliosis. The FUS P525L mutation is highly penetrant and causes ALS cases with earlier disease onset and more aggressive progression. To date, how P525L mutations may affect microglia during ALS pathogenesis had not been explored. In this study, we engineered isogenic control and P525L mutant FUS in independent human iPSC lines and differentiated them into microglia-like cells. We report that the P525L mutation causes FUS protein to mislocalize from the nucleus to cytoplasm. Homozygous P525L mutations perturb the transcriptome profile in which many differentially expressed genes are associated with microglial functions. Specifically, the dysregulation of several chemoreceptor genes leads to altered chemoreceptor-activated calcium signaling. However, other microglial functions such as phagocytosis and cytokine release are not significantly affected. Our study underscores the cell-autonomous effects of the ALS-linked FUS P525L mutation in a human microglia model.

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FUS P525L mutation causes FUS protein mislocalization in human microglia-like cells

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Homozygous P525L mutations perturb transcriptome profile of microglia-like cells

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Dysregulated chemoreceptor genes lead to altered chemoreceptor calcium signaling

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Effects of homozygous P525L occur cell-autonomously in this human microglia model

Histamine and Microglia

Microglia, a category of glial cells in the central nervous system (CNS), have attracted much attention because of their important role in neuroinflammation. Many translational studies are currently ongoing to discover novel drugs targeting microglia for the treatment of various CNS disorders, such as Alzheimer's disease, Parkinson's disease (PD), and depression. Recent studies have shown that brain histamine, a neurotransmitter essential for the regulation of diverse brain functions, controls glial cells and neurons. In vitro studies using primary microglia and microglial cell lines have reported that histamine receptors are expressed in microglia and control microglial functions, including chemotaxis, migration, cytokine secretion, and autophagy. In vivo studies have demonstrated that histamine-related reagents could ameliorate abnormal symptoms in animal models of human diseases, such as amyotrophic lateral sclerosis (ALS), PD, and brain ischemia. Several human studies have revealed alterations in histamine receptor levels in ALS and PD, emphasizing the importance of the CNS histamine system, including histamine-dependent microglial modulation, as a therapeutic target for these disorders. In this review article, we summarize histamine-related research focusing on microglial functions.

The Influence of Mitochondrial-DNA-Driven Inflammation Pathways on Macrophage Polarization: A New Perspective for Targeted Immunometabolic Therapy in Cerebral Ischemia-Reperfusion Injury

Cerebral ischemia-reperfusion injury is related to inflammation driven by free mitochondrial DNA. At the same time, the pro-inflammatory activation of macrophages, that is, polarization in the M1 direction, aggravates the cycle of inflammatory damage. They promote each other and eventually transform macrophages/microglia into neurotoxic macrophages by improving macrophage glycolysis, transforming arginine metabolism, and controlling fatty acid synthesis. Therefore, we propose targeting the mtDNA-driven inflammatory response while controlling the metabolic state of macrophages in brain tissue to reduce the possibility of cerebral ischemia-reperfusion injury.

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.

Inflammatory neuronal loss in the substantia nigra induced by systemic lipopolysaccharide is prevented by knockout of the P2Y6 receptor in mice

Inflammation may contribute to multiple brain pathologies. One cause of inflammation is lipopolysaccharide/endotoxin (LPS), the levels of which are elevated in blood and/or brain during bacterial infections, gut dysfunction and neurodegenerative diseases, such as Parkinson’s disease. How inflammation causes neuronal loss is unclear, but one potential mechanism is microglial phagocytosis of neurons, which is dependent on the microglial P2Y₆ receptor. We investigated here whether the P2Y₆ receptor was required for inflammatory neuronal loss. Intraperitoneal injection of LPS on 4 successive days resulted in specific loss of dopaminergic neurons (measured as cells staining with tyrosine hydroxylase or NeuN) in the substantia nigra of wild-type mice, but no neuronal loss in cortex or hippocampus. This supports the hypothesis that neuronal loss in Parkinson’s disease may be driven by peripheral LPS. By contrast, there was no LPS-induced neuronal loss in P2Y₆ receptor knockout mice. In vitro, LPS-induced microglial phagocytosis of cells was prevented by inhibition of the P2Y₆ receptor, and LPS-induced neuronal loss was reduced in mixed glial–neuronal cultures from P2Y₆ receptor knockout mice. This supports the hypothesis that microglial phagocytosis contributes to inflammatory neuronal loss, and can be prevented by blocking the P2Y₆ receptor, suggesting that P2Y₆ receptor antagonists might be used to prevent inflammatory neuronal loss in Parkinson’s disease and other brain pathologies involving inflammatory neuronal loss.

Structure activity relationship of 3-nitro-2-(trifluoromethyl)-2H-chromene derivatives as P2Y6 receptor antagonists

Various 6-alkynyl analogues of a known 3-nitro-2-(trifluoromethyl)-2H-chromene antagonist 3 of the Gq-coupled P2Y6 receptor (P2Y6R) were synthesized using a Sonogashira reaction to replace a 6-iodo group. The analogues were tested in a functional assay consisting of inhibition of calcium mobilization in P2Y6R-expressing astrocytoma cells elicited by native P2Y6R agonist UDP. 6-Ethynyl and 6-cyano groups were installed, and the alkynes were extended through both alkyl and aryl spacers. The most potent antagonists, with IC50 of ~1 µM, were found to be trialkylsilyl-ethynyl 7 and 8 (3-5 fold greater affinity than reference 3), t-butyl prop-2-yn-1-ylcarbamate 14 and p-carboxyphenyl-ethynyl 16 derivatives, and 3 and 8 displayed surmountable antagonism of UDP-induced production of inositol phosphates. Other chain-extended terminal carboxylate derivatives were less potent than the corresponding methyl ester derivatives. Thus, the 6 position in this chromene series is suitable for derivatization with flexibility of substitution, even with sterically extended chains, without losing P2Y6R affinity. However, a 3-carboxylic acid or 3-ester substitution did not serve as a nitro bioisostere, as the affinity was eliminated. These compounds provide additional ligand tools for the underexplored P2Y6R, which is a target for inflammatory, neurodegenerative and metabolic diseases.

Microglia in Neurodegenerative Events-An Initiator or a Significant Other?

A change in microglia structure, signaling, or function is commonly associated with neurodegeneration. This is evident in the patient population, animal models, and targeted in vitro assays. While there is a clear association, it is not evident that microglia serve as an initiator of neurodegeneration. Rather, the dynamics imply a close interaction between the various cell types and structures in the brain that orchestrate the injury and repair responses. Communication between microglia and neurons contributes to the physiological phenotype of microglia maintaining cells in a surveillance state and allows the cells to respond to events occurring in their environment. Interactions between microglia and astrocytes is not as well characterized, nor are interactions with other members of the neurovascular unit; however, given the influence of systemic factors on neuroinflammation and disease progression, such interactions likely represent significant contributes to any neurodegenerative process. In addition, they offer multiple target sites/processes by which environmental exposures could contribute to neurodegenerative disease. Thus, microglia at least play a role as a significant other with an equal partnership; however, claiming a role as an initiator of neurodegeneration remains somewhat controversial.

Microglial phagocytosis of neurons in neurodegeneration, and its regulation

There is growing evidence that excessive microglial phagocytosis of neurons and synapses contributes to multiple brain pathologies. RNA-seq and genome-wide association (GWAS) studies have linked multiple phagocytic genes to neurodegenerative diseases, and knock-out of phagocytic genes has been found to protect against neurodegeneration in animal models, suggesting that excessive microglial phagocytosis contributes to neurodegeneration. Here, we review recent evidence that microglial phagocytosis of live neurons and synapses causes neurodegeneration in animal models of Alzheimer's disease and other tauopathies, Parkinson's disease, frontotemporal dementias, multiple sclerosis, retinal degeneration and neurodegeneration induced by ischaemia, infection or ageing. We also review factors regulating microglial phagocytosis of neurons, including: nucleotides, frackalkine, phosphatidylserine, calreticulin, UDP, CD47, sialylation, complement, galectin-3, Apolipoprotein E, phagocytic receptors, Siglec receptors, cytokines, microglial epigenetics and expression profile. Some of these factors may be potential treatment targets to prevent neurodegeneration mediated by excessive microglial phagocytosis of live neurons and synapses.

Receptors, Channel Proteins, and Enzymes Involved in Microglia-mediated Neuroinflammation and Treatments by Targeting Microglia in Ischemic Stroke

Stroke is the largest contributor to global neurological disability-adjusted life-years, posing a huge economic and social burden to the world. Though pharmacological recanalization with recombinant tissue plasminogen activator and mechanical thrombectomy have greatly improved the prognosis of patients with ischemic stroke, clinically, there is still no effective treatment for the secondary injury caused by cerebral ischemia. In recent years, more and more evidences show that neuroinflammation plays a pivotal role in the pathogenesis and progression of ischemic cerebral injury. Microglia are brain resident innate immune cells and act the role peripheral macrophages. They play critical roles in mediating neuroinflammation after ischemic stroke. Microglia-mediated neuroinflammation is not an isolated process and has complex relationships with other pathophysiological processes as oxidative/nitrative stress, excitotoxicity, necrosis, apoptosis, pyroptosis, autophagy, and adaptive immune response. Upon activation, microglia differentially express various receptors, channel proteins, and enzymes involved in promoting or inhibiting the inflammatory processes, making them the targets of intervention for ischemic stroke. To inhibit microglia-related neuroinflammation and promote neurological recovery after ischemic stroke, numerous biochemical agents, cellular therapies, and physical methods have been demonstrated to have therapeutic potentials. Though accumulating experimental evidences have demonstrated that targeting microglia is a promising approach in the treatment of ischemic stroke, the clinical progress is slow. Till now, no clinical study could provide convincing evidence that any biochemical or physical therapies could exert neuroprotective effect by specifically targeting microglia following ischemic stroke.

From astrocytes to satellite glial cells and back: A 25 year-long journey through the purinergic modulation of glial functions in pain and more

Fundamental progresses have been made in pain research with a comprehensive understanding of the neuronal pathways which convey painful sensations from the periphery and viscera to the central nervous system and of the descending modulating pathways. Nevertheless, many patients still suffer from various painful conditions, which are often associated to other primary pathologies, and get no or poor relief from available painkillers. Thus, the interest of many researchers has concentrated on new and promising cellular targets and biochemical pathways. This is the case of glia cells, both in the peripheral and in the central nervous system, and of purinergic receptors. Starting from many intuitions and hypotheses raised by Prof. Geoffrey Burnstock, data have accumulated which clearly highlight the fundamental role exerted by several nucleotide and nucleoside receptors in the modulation of glial cell reaction to pain triggers and of their cross-talk with sensory neurons which significantly contributes to the transition from acute to chronic pain. The purinergic system has therefore become an appealing pharmacological target in pain research, also based on the quite unexpected discovery that purines are involved in ancient analgesic techniques such as acupuncture. A more in-depth understanding of the complex and intricated purine-orchestrated scenario in pain conditions will hopefully lead to the identification and clinical development of new and effective analgesics.

Tribute to Prof. Geoffrey Burnstock: transition of purinergicsignaling to drug discovery

Geoffrey Burnstock made a chance observation early in his research career that did not fit the conventional scientific dogma-non-noradrenergic, non-cholinergic (NANC) nerves. Instead of rejecting these as an artifact, he followed their logical course to characterize the actions of extracellular ATP on nerves and muscles, eventually founding a large branch of pharmacology around purinergic signaling. The solid proof that validated his concept and dismissed many detractors was the cloning of seven ionotropic P2X receptors and eight metabotropic P2Y receptors, which are expressed in some combination in every tissue and organ. Given the broad importance of this signaling system in biology, medicinal chemists, inspired by Burnstock, began creating synthetic agonists and antagonists for these purinergic receptors. Various ligands have advanced to clinical trials, for disorders of the immune, nervous, cardiovascular, and other systems, and a few are already approved. Thus, medically important approaches have been derived from Burnstock's original pharmacological concepts and his constant guiding of the course of the field. The therapeutic potential of modulators of purinergic signaling is vast.