P2Y12 receptors in spinal microglia are required for neuropathic pain after peripheral nerve injury

P2Y12-targeted modulation of microglial phenotypes: A novel therapeutic strategy for enhanced axonal regeneration post-spinal cord injury

AIMS

Microglia activation after spinal cord injury (SCI) is a double-edged sword, modulation of the activated microglia populations toward pro-regenerative phenotypes highlights the potential therapeutic implications. P2Y12, a microglia-specific marker, remains underexplored in its capacity to polarize microglial activation populations in SCI repair. We aimed to explore the effects of modulating P2Y12 on microglia function after spinal cord injury, and further on axonal regeneration and motor recovery after spinal cord injury.

MATERIALS AND METHODS

The study employed both in vitro and in vivo models, using BV2 cells and a mouse model of SCI, respectively. Ticagrelor, a P2Y12 antagonist, was administered via a collagen scaffold to ensure stable and sustained release. Transcriptome sequencing analysis, immunofluorescence staining, and Basso Mouse Scale (BMS) scores were used to assess microglial activation, axonal regeneration, and functional recovery.

KEY FINDINGS

Herein, we observed P2Y12+ microglia localized predominantly at the lesion periphery within 3 days post injury (dpi), manifesting a pro-inflammatory phenotype, but not anti-inflammatory phenotype. In vitro investigations revealed that P2Y12 inhibition of the activated microglia curtailed pro-inflammatory differentiation while augmenting anti-inflammatory differentiation.

SIGNIFICANCE

Leveraging this insight, we engineered a collagen scaffold-based delivery system for sustained release of the P2Y12 antagonist, ticagrelor, at the injury site in a mouse complete SCI model. Notably, P2Y12 suppression markedly enhanced axonal regeneration within the injured site and ameliorated lower limb motor functions in SCI mice. Collectively, our findings illuminate P2Y12-targeted microglial modulation as a promising therapeutic approach for SCI.

Bone marrow-derived myeloid cells transiently colonize the brain during postnatal development and interact with glutamatergic synapses

Although the roles of embryonic yolk sac-derived, resident microglia in neurodevelopment were extensively studied, the possible involvement of bone marrow-derived cells remains elusive. In this work, we used a fate-mapping strategy to selectively label bone marrow-derived cells and their progeny in the brain (FLT3+IBA1+). FLT3+IBA1+ cells were confirmed to be transiently present in the healthy brain during early postnatal development. FLT3+IBA1+ cells have a distinct morphology index at postnatal day(P)0, P7, and P14 compared with neighboring microglia. FLT3+IBA1+ cells also express the microglial markers P2RY12 and TMEM119 and interact with VGLUT1 synapses at P14. Scanning electron microscopy indeed showed that FLT3+ cells contact and engulf pre-synaptic elements. Our findings suggest FLT3+IBA1+ cells might assist microglia in their physiological functions in the developing brain including synaptic pruning which is performed using their purinergic sensors. Our findings stimulate further investigation on the involvement of peripheral macrophages during homeostatic and pathological development.

TLR2 Mediates Microglial Activation and Contributes to Central Sensitization in a Recurrent Nitroglycerin-induced Chronic Migraine Model

Central sensitization is an important pathophysiological mechanism underlying chronic migraine (CM). Previous studies have shown that microglial activation and subsequent inflammation in the trigeminal nucleus caudalis (TNC) contribute to central sensitization. Toll-like receptor 2 (TLR2) is a receptor expressed on the membrane of microglia and participates in central sensitization in inflammatory and chronic pain; however, its role in CM is unclear. Therefore, this study investigated TLR2 involvement in CM in detail. Mice treated with recurrent nitroglycerin (NTG) were used as a CM model. Hyperalgesia was assessed using a 50% paw mechanical threshold and a 50% periorbital threshold on a Von Frey filament pain meter. Western blotting and immunofluorescence analyses were used to detect the expression of TLR2, microglia, c-fos and CGRP in TNC. The expression of inflammatory factors (IL-6, IL-1β、 IL-10、TNF-α and IFN-β1) was detected using quantitative real-time polymerase chain reaction (qRT-PCR). A selective TLR2 antagonist (C29) was systematically administered to observe its effect on hyperalgesia, microglia activation and the expression of c-fos, CGRP and inflammatory factors. Recurrent administration of NTG resulted in acute and chronic hypersensitivity, accompanied by upregulation of TLR2 expression and microglial activation in TNC. C29 partially inhibited pain hypersensitivity. C29 suppressed microglial activation induced by NTG administration. Inhibition of TLR2 reduced the expression of c-fos and CGRP in TNC after NTG treatment. C29 inhibited the expression of inflammatory mediators in TNC. These data showed that microglial TLR2 plays a critical role in the pathogenesis of CM by regulating microglial activation in TNC.

Electroacupuncture Alleviates Pain by Suppressing P2Y12R-Dependent Microglial Activation in Monoarthritic Rats

Electroacupuncture (EA) effectively improves arthritis-induced hyperalgesia and allodynia by repressing spinal microglial activation, which plays a crucial role in pain hypersensitivity following tissue inflammation. However, the mechanism by which EA suppresses spinal microglial activation in monoarthritis (MA) remains unclear. In the present study, a rat model of MA was established through unilateral ankle intra-articular injection of complete Freund's adjuvant (CFA). The relationship among P2Y12 receptor (P2Y12R) expression, spinal microglial activation, and EA analgesia was investigated using quantitative real-time PCR (qRT‒PCR), western blotting, immunofluorescence (IF), and behavioral testing. The results found that EA treatment at the ipsilateral "Huantiao" (GB30) and "Yanglingquan" (GB34) acupoints markedly attenuated pain and spinal microglia M1 polarization in MA rats. In particular, P2Y12R expression was significantly increased at the mRNA and protein levels in the spinal dorsal horn in MA rats, whereas EA treatment effectively repressed the MA-induced upregulation of P2Y12R. IF analysis further revealed that most P2Y12R was expressed in microglia in the spinal dorsal horn. Pharmacological inhibition of P2Y12R by its antagonist (AR-C69931MX) decreased MA-induced spinal microglial activation and subsequent proinflammatory cytokine production. Consequently, AR-C69931MX significantly intensified the anti-pain hypersensitive function of EA in MA rats. Taken together, these results demonstrate that EA alleviates MA-induced pain by suppressing P2Y12R-dependent microglial activation.

Electrochemical Monitoring of Sphingosine-1-phosphate-Induced ATP Release Using a Microsensor Based on an Entropy-Driven Bipedal DNA Walker

Neuropathic pain is a chronic and severe syndrome for which effective therapy is insufficient and the release of ATP from microglia induced by sphingosine-1-phosphate (S1P) plays a vital role in neuropathic pain. Therefore, there is an urgent demand to develop highly sensitive and selective ATP biosensors for quantitative monitoring of low-concentration ATP in the complex nervous system, which helps in understanding the mechanism involved in neuropathic pain. Herein, we developed an electrochemical microsensor based on an entropy-driven bipedal DNA walker. First, the microsensor specifically recognized ATP via ATP aptamers, initiating the entropy-driven bipedal DNA walker. Subsequently, the bipedal DNA walker autonomously traversed the microelectrode interface, introducing methylene blue to the electrode surface and achieving cascade signal amplification. This microsensor showed excellent selectivity, stability, and a low limit of detection at 1.13 nM. The S1P-induced ATP release from BV2 cells was successfully monitored, and it was observed that dicumarol could inhibit this release, suggesting dicumarol as a potential treatment for neuropathic pain. The microsensor's small size exhibited significant potential for monitoring ATP level changes in neuropathic pain in vivo, which provides a new strategy for in situ and quantitative monitoring of nonelectroactive biomolecules associated with neurological diseases.

Microglia sense astrocyte dysfunction and prevent disease progression in an Alexander disease model

Alexander disease (AxD) is an intractable neurodegenerative disorder caused by GFAP mutations. It is a primary astrocyte disease with a pathological hallmark of Rosenthal fibres within astrocytes. AxD astrocytes show several abnormal phenotypes. Our previous study showed that AxD astrocytes in model mice exhibit aberrant Ca2+ signals that induce AxD aetiology. Here, we show that microglia have unique phenotypes with morphological and functional alterations, which are related to the pathogenesis of AxD. Immunohistochemical studies of 60TM mice (AxD model) showed that AxD microglia exhibited highly ramified morphology. Functional changes in microglia were assessed by Ca2+ imaging using hippocampal brain slices from Iba1-GCaMP6-60TM mice and two-photon microscopy. We found that AxD microglia showed aberrant Ca2+ signals, with high frequency Ca2+ signals in both the processes and cell bodies. These microglial Ca2+ signals were inhibited by pharmacological blockade or genetic knockdown of P2Y12 receptors but not by tetrodotoxin, indicating that these signals are independent of neuronal activity but dependent on extracellular ATP from non-neuronal cells. Our single-cell RNA sequencing data showed that the expression level of Entpd2, an astrocyte-specific gene encoding the ATP-degrading enzyme NTPDase2, was lower in AxD astrocytes than in wild-type astrocytes. In situ ATP imaging using the adeno-associated virus vector GfaABC1D ATP1.0 showed that exogenously applied ATP was present longer in 60TM mice than in wild-type mice. Thus, the increased ATP level caused by the decrease in its metabolizing enzyme in astrocytes could be responsible for the enhancement of microglial Ca2+ signals. To determine whether these P2Y12 receptor-mediated Ca2+ signals in AxD microglia play a significant role in the pathological mechanism, a P2Y12 receptor antagonist, clopidogrel, was administered. Clopidogrel significantly exacerbated pathological markers in AxD model mice and attenuated the morphological features of microglia, suggesting that microglia play a protective role against AxD pathology via P2Y12 receptors. Taken together, we demonstrated that microglia sense AxD astrocyte dysfunction via P2Y12 receptors as an increase in extracellular ATP and alter their morphology and Ca2+ signalling, thereby protecting against AxD pathology. Although AxD is a primary astrocyte disease, our study may facilitate understanding of the role of microglia as a disease modifier, which may contribute to the clinical diversity of AxD.

Microglia TREM1-mediated neuroinflammation contributes to central sensitization via the NF-κB pathway in a chronic migraine model

BACKGROUND

Neuroinflammation, mediated by the activation of microglia, contributes to central sensitization, which is associated with the development of chronic migraine (CM). TREM1 receptors amplify the inflammatory response. However, their relationship to CM is unclear. Thus, this study endeavoured to elucidate the exact role of TREM1 in CM.

METHODS

Nitroglycerin (NTG) was repeatedly administered intraperitoneally to establish the CM model. Mechanical and thermal sensitivities were assessed using von Frey filaments and hot plate assays. Using Western blotting, TREM1, NF-κB pathway, NLRP3 inflammasome components, and proinflammatory cytokines were all detected. Immunofluorescence was used to examine the cellular distribution of TREM1 and NLRP3, the number of microglia, immunoreactivity, and morphological changes. We examined the effects of TREM1 antagonists (LR12) and NF-κB inhibitors (PDTC) on pain behaviour, as well as the production of c-fos and CGRP. Additionally, we investigated whether LR12 and PDTC affect the activation of microglia and the NLRP3 inflammasome. We synthesized siRNA and TREM1-overexpressing plasmids to transfect BV2 cells treated with LPS and normal BV2 cells and treated TREM1-overexpressing BV2 cells with PDTC. The NF-κB pathway, NLRP3 inflammasome components, and proinflammatory cytokines were quantified using Western blotting.

RESULTS

Following NTG administration, the expression of TREM1 was significantly upregulated and exclusively localized in microglia in the TNC, and was well co-localized with NLRP3. Furthermore, activation of the classical NF-κB pathway was observed. Pre-treatment with LR12 and PDTC effectively attenuated mechanical hypersensitivity, suppressed the expression of c-fos and CGRP, and inhibited NF-κB activity in CM mice. Additionally, inhibition of TREM1 and NF-κB activity mitigated NTG-induced microglia and NLRP3 activation, as well as proinflammatory cytokines production. In vitro, knockdown of TREM1 resulted in attenuated activation of the NF-κB pathway following lipopolysaccharide (LPS) treatment and reduced expression of NLRP3 inflammasome components as well as proinflammatory cytokines. After TREM1 overexpression, the NF-κB pathway was activated, NLRP3 inflammasome components and proinflammatory cytokines were upregulated, and PDTC reversed this phenomenon.

CONCLUSIONS

Our findings suggest that TREM1 regulates microglia and NLRP3 activation via the NF-κB pathway, thereby contributing to central sensitization and implicating its involvement in chronic migraine pathogenesis.

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

P2Y12 receptor mediates apoptosis and demyelination to affect functional recovery in mice with spinal cord injury

Among diseases of the central nervous system (CNS), spinal cord injury (SCI) has a high fatality rate. It has been proven that P2Y G protein-coupled purinergic receptors have a neuroprotective role in apoptosis and regeneration inside the damaged spinal cord. The P2Y12 receptor (P2Y12R) has recently been linked to peripheral neuropathy and stroke. However, the role of P2Y12R after SCI remains unclear. Our study randomly divided C57BL/6J female mice into 3 groups: Sham+DMSO, SCI+DMSO, and SCI+MRS2395. MRS2395 as a P2Y12R inhibitor was intraperitoneally injected at a dose of 1.5 mg/kg once daily for 7 days. We showed that the P2Y12R was markedly activated after injury, and it was double labeled with the microglial and neuron. Behavioral tests were employed to assess motor function recovery. By using immunofluorescence staining, the NeuN expression level was detected. The morphology of neurons was observed by hematoxylin-eosin and Nissl staining. P2Y12R, Bax, GFAP, PCNA and calbindin expression levels were detected using Western blot. Meanwhile, mitochondria and myelin sheath were observed by transmission electron microscopy (TEM). Our findings demonstrated that MRS2395 significantly enhanced motor function induced by SCI and that was used to alleviate apoptosis and astrocyte scarring. NeuN positive cells in the SCI group were lower than in the therapy group, although Bax, GFAP, PCNA and calbindin expression levels were considerably higher. Moreover, following MRS2395 therapy, the histological damage was reversed. A notable improvement in myelin sheath and mitochondrial morphology was seen in the therapy group. Together, our findings indicate that activation of P2Y12R in damaged spinal cord may be a critical event and suggest that inhibition of P2Y12R might be a feasible therapeutic strategy for treating SCI.

Microglia are involved in regulating histamine-dependent and non-dependent itch transmissions with distinguished signal pathways

Although itch and pain have many similarities, they are completely different in perceptual experience and behavioral response. In recent years, we have a deep understanding of the neural pathways of itch sensation transmission. However, there are few reports on the role of non-neuronal cells in itch. Microglia are known to play a key role in chronic neuropathic pain and acute inflammatory pain. It is still unknown whether microglia are also involved in regulating the transmission of itch sensation. In the present study, we used several kinds of transgenic mice to specifically deplete CX3CR1+ microglia and peripheral macrophages together (whole depletion), or selectively deplete microglia alone (central depletion). We observed that the acute itch responses to histamine, compound 48/80 and chloroquine were all significantly reduced in mice with either whole or central depletion. Spinal c-fos mRNA assay and further studies revealed that histamine and compound 48/80, but not chloroquine elicited primary itch signal transmission from DRG to spinal Npr1- and somatostatin-positive neurons relied on microglial CX3CL1-CX3CR1 pathway. Our results suggested that microglia were involved in multiple types of acute chemical itch transmission, while the underlying mechanisms for histamine-dependent and non-dependent itch transmission were different that the former required the CX3CL1-CX3CR1 signal pathway.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Global Trends and Hotspots on Microglia Associated with Pain from 2002 to 2022: A Bibliometric Analysis

Background

Researchers have made significant progress in microglia associated with pain in recent years. However, more relevant bibliometric analyses are still needed on trends and directions in this field. The aim of this study is to provide a comprehensive perspective and to predict future directions of pain-related microglia research via bibliometric tools.

Methods

English articles and reviews related with pain and microglia were extracted from the Web of Science core collection (WosCC) database between 2002 to 2022. Bibliometric tools such as VOSviewer, CiteSpace, and Bibliometrix R package were used to analyze publication characteristics, countries, authors, institutions, journals, research hotspots, and trend topics.

Results

A total of 2761 articles were included in this analysis. Research on microglia associated with pain has increased significantly over the last two decades. China (n = 1020, 36.94%) and the United States (n = 751, 27.20%) contributed the most in terms of publications and citations, respectively. Kyushu University published the most articles in this field compared to other institutions, and Professor Inoue Kazuhide (n = 54) at this university made outstanding contributions in this field. Molecular Pain (n = 113) was the journal with the most publication, while Journal of Neuroscience had the highest number of citations. According to the authors keywords analysis, the research in this area can be summarized into 7 clusters such as "microglia activation pathways", "pain treatment research", "mental symptoms of chronic pain", and so on.

Conclusion

This study provides a comprehensive analysis of pain-related microglia research in the past two decades. We identified the countries, institutions, scholars, and journals with the highest number of publications and the most influence in the field, and the research trends identified in this paper may provide new insights for future research.

Microglial P2Y12 mediates chronic stress-induced synapse loss in the prefrontal cortex and associated behavioral consequences

Chronic unpredictable stress (CUS) drives microglia-mediated neuronal remodeling and synapse loss in the prefrontal cortex (PFC), contributing to deficits in cognition and behavior. However, it remains unclear what mechanisms guide microglia-neuron interactions in stress. Evidence indicates that neuronal activity-dependent purinergic signaling directs microglial processes and synaptic engagement via P2Y12, a purinergic receptor exclusively expressed by microglia in the brain. Stress alters excitatory neurotransmission in the PFC, thus we aimed to determine if P2Y12 signaling promotes functional changes in microglia in chronic stress. Here we used genetic ablation of P2Y12 (P2ry12-/-) or pharmacological blockade (clopidogrel, ticagrelor) to examine the role of purinergic signaling in stress-induced microglia-neuron interaction. Multiple behavioral, physiological, and cytometric endpoints were analyzed. Deletion of P2Y12 led to a number of fundamental alterations in the PFC, including the heightened microglial number and increased dendritic spine density. Flow cytometry revealed that microglia in P2ry12-/- mice had shifts in surface levels of CX3CR1, CSF1R, and CD11b, suggesting changes in synaptic engagement and phagocytosis in the PFC. In line with this, pharmacological blockade of P2Y12 prevented CUS-induced increases in the proportion of microglia with neuronal inclusions, limited dendritic spine loss in the PFC, and attenuated alterations in stress coping behavior and working memory function. Overall, these findings indicate that microglial P2Y12 is a critical mediator of stress-induced synapse loss in the PFC and subsequent behavioral deficits.

Galectin-3 activates spinal microglia to induce inflammatory nociception in wild type but not in mice modelling Alzheimer's disease

Musculoskeletal chronic pain is prevalent in individuals with Alzheimer's disease (AD); however, it remains largely untreated in these patients, raising the possibility that pain mechanisms are perturbed. Here, we utilise the TASTPM transgenic mouse model of AD with the K/BxN serum transfer model of inflammatory arthritis. We show that in male and female WT mice, inflammatory allodynia is associated with a distinct spinal cord microglial response characterised by TLR4-driven transcriptional profile and upregulation of P2Y12. Dorsal horn nociceptive afferent terminals release the TLR4 ligand galectin-3 (Gal-3), and intrathecal injection of a Gal-3 inhibitor attenuates allodynia. In contrast, TASTPM mice show reduced inflammatory allodynia, which is not affected by the Gal-3 inhibitor and correlates with the emergence of a P2Y12- TLR4- microglia subset in the dorsal horn. We suggest that sensory neuron-derived Gal-3 promotes allodynia through the TLR4-regulated release of pro-nociceptive mediators by microglia, a process that is defective in TASTPM due to the absence of TLR4 in a microglia subset.

P2Y12 receptor involved in the development of chronic nociceptive pain as a sensory information mediator

Direct or indirect damage to the nervous system (such as inflammation or tumor invasion) can lead to dysfunction and pain. The generation of pain is mainly reflected in the activation of glial cells and the abnormal discharge of sensory neurons, which transmit stronger sensory information to the center. P2Y12 receptor plays important roles in physiological and pathophysiological processes including inflammation and pain. P2Y12 receptor involved in the occurrence of pain as a sensory information mediator, which enhances the activation of microglia and the synaptic plasticity of primary sensory neurons, and reaches the higher center through the ascending conduction pathway (mainly spinothalamic tract) to produce pain. While the application of P2Y12 receptor antagonists (PBS-0739, AR-C69931MX and MRS2359) have better antagonistic activity and produce analgesic pharmacological properties. Therefore, in this article, we discussed the role of the P2Y12 receptor in different chronic pains and its use as a pharmacological target for pain relief.

Microglial diversity in neuropathic pain

Microglia play pivotal roles in controlling CNS functions in diverse physiological and pathological contexts, including neuropathic pain, a chronic pain condition caused by lesions or diseases of the somatosensory nervous system. In this review article, we summarize evidence primarily from basic research on the role of microglia in the development and remission of neuropathic pain. The identification of a subset of microglia that emerged after pain development and that was necessary for remission of neuropathic pain highlights the highly divergent and dynamic nature of microglia in the course of neuropathic pain. Understanding microglial diversity in terms of gene expression, physiological states, and functional roles could lead to new strategies that aid in the diagnosis and management of neuropathic pain, and that may not have been anticipated from the viewpoint of targeting all microglia uniformly.

G protein-coupled P2Y12 receptor is involved in the progression of neuropathic pain

The pathological mechanism of neuropathic pain is complex, which seriously affects the physical and mental health of patients, and its treatment is also difficult. The role of G protein-coupled P2Y12 receptor in pain has been widely recognized and affirmed. After nerve injury, stimulated cells can release large amounts of nucleotides into the extracellular matrix, act on P2Y12 receptor. Activated P2Y12 receptor activates intracellular signal transduction and is involved in the development of pain. P2Y12 receptor activation can sensitize primary sensory neurons and receive sensory information. By transmitting the integrated information through the dorsal root of the spinal cord to the secondary neurons of the posterior horn of the spinal cord. The integrated information is then transmitted to the higher center through the ascending conduction tract to produce pain. Moreover, activation of P2Y12 receptor can mediate immune cells to release pro-inflammatory factors, increase damage to nerve cells, and aggravate pain. While inhibits the activation of P2Y12 receptor can effectively relieve pain. Therefore, in this article, we described P2Y12 receptor antagonists and their pharmacological properties. In addition, we explored the potential link between P2Y12 receptor and the nervous system, discussed the intrinsic link of P2Y12 receptor and neuropathic pain and as a potential pharmacological target for pain suppression.

ATP-releasing SWELL1 channel in spinal microglia contributes to neuropathic pain

Following peripheral nerve injury, extracellular adenosine 5'-triphosphate (ATP)-mediated purinergic signaling is crucial for spinal cord microglia activation and neuropathic pain. However, the mechanisms of ATP release remain poorly understood. Here, we show that volume-regulated anion channel (VRAC) is an ATP-releasing channel and is activated by inflammatory mediator sphingosine-1-phosphate (S1P) in microglia. Mice with microglia-specific deletion of Swell1 (also known as Lrrc8a), a VRAC essential subunit, had reduced peripheral nerve injury-induced increase in extracellular ATP in spinal cord. The mutant mice also exhibited decreased spinal microgliosis, dorsal horn neuronal hyperactivity, and both evoked and spontaneous neuropathic pain-like behaviors. We further performed high-throughput screens and identified an FDA-approved drug dicumarol as a novel and potent VRAC inhibitor. Intrathecal administration of dicumarol alleviated nerve injury-induced mechanical allodynia in mice. Our findings suggest that ATP-releasing VRAC in microglia is a key spinal cord determinant of neuropathic pain and a potential therapeutic target for this debilitating disease.

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".

Purinergic signaling: A gatekeeper of blood-brain barrier permeation

This review outlined evidence that purinergic signaling is involved in the modulation of blood-brain barrier (BBB) permeability. The functional and structural integrity of the BBB is critical for maintaining the homeostasis of the brain microenvironment. BBB integrity is maintained primarily by endothelial cells and basement membrane but also be regulated by pericytes, neurons, astrocytes, microglia and oligodendrocytes. In this review, we summarized the purinergic receptors and nucleotidases expressed on BBB cells and focused on the regulation of BBB permeability by purinergic signaling. The permeability of BBB is regulated by a series of purinergic receptors classified as P2Y₁, P2Y₄, P2Y₁₂, P2X4, P2X7, A₁, A_2A, A_2B, and A₃, which serve as targets for endogenous ATP, ADP, or adenosine. P2Y₁ and P2Y₄ antagonists could attenuate BBB damage. In contrast, P2Y₁₂-mediated chemotaxis of microglial cell processes is necessary for rapid closure of the BBB after BBB breakdown. Antagonists of P2X4 and P2X7 inhibit the activation of these receptors, reduce the release of interleukin-1 beta (IL-1β), and promote the function of BBB closure. In addition, the CD39/CD73 nucleotidase axis participates in extracellular adenosine metabolism and promotes BBB permeability through A₁ and A_2A on BBB cells. Furthermore, A_2B and A₃ receptor agonists protect BBB integrity. Thus, the regulation of the BBB by purinergic signaling is complex and affects the opening and closing of the BBB through different pathways. Appropriate selective agonists/antagonists of purinergic receptors and corresponding enzyme inhibitors could modulate the permeability of the BBB, effectively delivering therapeutic drugs/cells to the central nervous system (CNS) or limiting the entry of inflammatory immune cells into the brain and re-establishing CNS homeostasis.

Regulation of neuropathic pain by microglial Orai1 channels

Microglia are important mediators of neuroinflammation, which underlies neuropathic pain. However, the molecular checkpoints controlling microglial reactivity are not well-understood. Here, we investigated the role of Orai1 channels for microglia-mediated neuroinflammation following nerve injury and find that deletion of Orai1 in microglia attenuates Ca2+ signaling and the production of inflammatory cytokines by proalgesic agonists. Conditional deletion of Orai1 attenuated microglial proliferation in the dorsal horn, spinal cytokine levels, and potentiation of excitatory neurotransmission following peripheral nerve injury. These cellular effects were accompanied by mitigation of pain hyperalgesia in microglial Orai1 knockout mice. A small-molecule Orai1 inhibitor, CM4620, similarly mitigated allodynia in male mice. Unexpectedly, these protective effects were not seen in female mice, revealing sexual dimorphism in Orai1 regulation of microglial reactivity and hyperalgesia. Together, these findings indicate that Orai1 channels are key regulators of the sexually dimorphic role of microglia for the neuroinflammation that underlies neuropathic pain.

Microglial P2Y12 Signaling Contributes to Cisplatin-induced Pain Hypersensitivity via IL-18-mediated Central Sensitization in the Spinal Cord

Administration of cisplatin and other chemotherapy drugs is crucial for treating tumors. However, cisplatin-induced pain hypersensitivity is still a critical clinical issue, and the underlying molecular mechanisms have remained unresolved to date. In this study, we found that repeated cisplatin treatments remarkedly upregulated the P2Y12 expression in the spinal cord. Expression of P2Y12 was predominant in the microglia. Pharmacological inhibition of P2Y12 expression markedly attenuated the cisplatin-induced pain hypersensitivity. Meanwhile, blocking the P2Y12 signal also suppressed cisplatin-induced microglia hyperactivity. Furthermore, the microglia Src family kinase/p38 pathway is required for P2Y12-mediated cisplatin-induced pain hypersensitivity via the proinflammatory cytokine IL-18 production in the spinal cord. Blocking the P2Y12/IL-18 signaling pathway reversed cisplatin-induced pain hypersensitivity, as well as activation of N-methyl-D-aspartate receptor and subsequent Ca²⁺-dependent signals. Collectively, our data suggest that microglia P2Y12-SFK-p38 signaling contributes to cisplatin-induced pain hypersensitivity via IL-18-mediated central sensitization in the spinal, and P2Y12 could be a potential target for intervention to prevent chemotherapy-induced pain hypersensitivity. PERSPECTIVE: Our work identified that P2Y12/IL-18 played a critical role in cisplatin-induced pain hypersensitivity. This work suggests that P2Y12/IL-18 signaling may be a useful strategy for the treatment of chemotherapy-induced pain hypersensitivity.

ATP-releasing SWELL1 channel in spinal microglia contributes to neuropathic pain

Following peripheral nerve injury, extracellular ATP-mediated purinergic signaling is crucial for spinal cord microglia activation and neuropathic pain. However, the mechanisms of ATP release remain poorly understood. Here, we show that volume-regulated anion channel (VRAC) is an ATP-releasing channel and is activated by inflammatory mediator sphingosine-1-phosphate (S1P) in microglia. Mice with microglia-specific deletion of Swell1 (also known as Lrrc8a), a VRAC essential subunit, had reduced peripheral nerve injury-induced increase in extracellular ATP in spinal cord. The mutant mice also exhibited decreased spinal microgliosis, dorsal horn neuronal hyperactivity, and both evoked and spontaneous neuropathic pain-like behaviors. We further performed high-throughput screens and identified an FDA-approved drug dicumarol as a novel and potent VRAC inhibitor. Intrathecal administration of dicumarol alleviated nerve injury-induced mechanical allodynia in mice. Our findings suggest that ATP-releasing VRAC in microglia is a key spinal cord determinant of neuropathic pain and a potential therapeutic target for this debilitating disease.

The role and pharmacological properties of P2Y12 receptor in cancer and cancer pain

The G protein-coupled P2Y12 receptor (P2Y12R) was cloned in platelets and found to play a key role in maintaining platelet function in hemostasis and thrombosis, and these effects could be mediated by the P2Y12R. However, it has recently been found that P2Y12R-mediated the progression of tumor through interactions between platelets and tumor and stromal cells, as well as through products secreted by platelets. During tumor progression, tumor cells or other cells in the tumor microenvironment (such as immune cells) can secrete large amounts of ATP into the extracellular matrix, and extracellular ATP can be hydrolyzed into ADP. ADP is a P2Y12R activator and plays an important regulatory role in the proliferation and metastasis of tumor cells. P2Y12R is involved in platelet-cancer cell crosstalk and become a potential target for anticancer therapy. Moreover, tumor progression can induce pain, which seriously affects the quality of life of patients. P2Y12R is expressed in microglia and mediates the activities of microglial and participates in the occurrence of cancer pain. Conversely, inhibiting P2Y12R activation and down-regulating its expression has the effect of inhibiting tumor progression and pain. Therefore, P2Y12R can be a common therapeutic target for both. In this article, we explored the potential link between P2Y12R and cancer, discussed the intrinsic link of P2Y12R in cancer pain and the pharmacological properties of P2Y12R antagonists in the treatment of both.

Sex differences in the inflammatory response of the mouse DRG and its connection to pain in experimental autoimmune encephalomyelitis

Multiple Sclerosis (MS) is an autoimmune disease with notable sex differences. Women are not only more likely to develop MS but are also more likely than men to experience neuropathic pain in the disease. It has been postulated that neuropathic pain in MS can originate in the peripheral nervous system at the level of the dorsal root ganglia (DRG), which houses primary pain sensing neurons (nociceptors). These nociceptors become hyperexcitable in response to inflammation, leading to peripheral sensitization and eventually central sensitization, which maintains pain long-term. The mouse model experimental autoimmune encephalomyelitis (EAE) is a good model for human MS as it replicates classic MS symptoms including pain. Using EAE mice as well as naïve primary mouse DRG neurons cultured in vitro, we sought to characterize sex differences, specifically in peripheral sensory neurons. We found sex differences in the inflammatory profile of the EAE DRG, and in the TNFα downstream signaling pathways activated intracellularly in cultured nociceptors. We also found increased cell death with TNFα treatment. Given that TNFα signaling has been shown to initiate intrinsic apoptosis through mitochondrial disruption, this led us to investigate sex differences in the mitochondria's response to TNFα. Our results demonstrate that male sensory neurons are more sensitive to mitochondrial stress, making them prone to neuronal injury. In contrast, female sensory neurons appear to be more resistant to mitochondrial stress and exhibit an inflammatory and regenerative phenotype that may underlie greater nociceptor hyperexcitability and pain. Understanding these sex differences at the level of the primary sensory neuron is an important first step in our eventual goal of developing sex-specific treatments to halt pain development in the periphery before central sensitization is established.

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.

Spinal microglia contribute to sustained inflammatory pain via amplifying neuronal activity

Microglia are highly dynamic immune cells of the central nervous system (CNS). Microglial processes interact with neuronal elements constantly on the order of minutes. The functional significance of this acute microglia-neuron interaction and its potential role in the context of pain is still largely unknown. Here, we found that spinal microglia increased their process motility and electrophysiological reactivity within an hour after the insult in a mouse model of formalin-induced acute, sustained, inflammatory pain. Using an ablation strategy to specifically deplete resident microglia in the CNS, we demonstrate that microglia participate in formalin-induced acute sustained pain behaviors by amplifying neuronal activity in the spinal dorsal horn. Moreover, we identified that the P2Y12 receptor, which is specifically expressed in microglia in the CNS, was required for microglial function in formalin-induced pain. Taken together, our study provides a novel insight into the contribution of microglia and the P2Y12 receptor in inflammatory pain that could be used for potential therapeutic strategies.

Chemogenetic and Optogenetic Manipulations of Microglia in Chronic Pain

Chronic pain relief remains an unmet medical need. Current research points to a substantial contribution of glia-neuron interaction in its pathogenesis. Particularly, microglia play a crucial role in the development of chronic pain. To better understand the microglial contribution to chronic pain, specific regional and temporal manipulations of microglia are necessary. Recently, two new approaches have emerged that meet these demands. Chemogenetic tools allow the expression of designer receptors exclusively activated by designer drugs (DREADDs) specifically in microglia. Similarly, optogenetic tools allow for microglial manipulation via the activation of artificially expressed, light-sensitive proteins. Chemo- and optogenetic manipulations of microglia in vivo are powerful in interrogating microglial function in chronic pain. This review summarizes these emerging tools in studying the role of microglia in chronic pain and highlights their potential applications in microglia-related neurological disorders.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

Role of Microglia and Astrocytes in Spinal Cord Injury Induced Neuropathic Pain

Background:

Spinal cord injuries incite varying degrees of symptoms in patients, ranging from weakness and incoordination to paralysis. Common amongst spinal cord injury (SCI) patients, neuropathic pain (NP) is a debilitating medical condition. Unfortunately, there remain many clinical impediments in treating NP because there is a lack of understanding regarding the mechanisms behind SCI-induced NP (SCINP). Given that more than 450,000 people in the United States alone suffer from SCI, it is unsatisfactory that current treatments yield poor results in alleviating and treating NP.

Summary:

In this review, we briefly discussed the models of SCINP along with the mechanisms of NP progression. Further, current treatment modalities are herein explored for SCINP involving pharmacological interventions targeting glia cells and astrocytes.

Key message:

The studies presented in this review provide insight for new directions regarding SCINP alleviation. Given the severity and incapacitating effects of SCINP, it is imperative to study the pathways involved and find new therapeutic targets in coordination with stem cell research, and to develop a new gold-standard in SCINP treatment.

The Role of Microglial Purinergic Receptors in Pain Signaling

Pain is an essential modality of sensation in the body. Purinergic signaling plays an important role in nociceptive pain transmission, under both physiological and pathophysiological conditions, and is important for communication between both neuronal and non-neuronal cells. Microglia and astrocytes express a variety of purinergic effectors, and a variety of receptors play critical roles in the pathogenesis of neuropathic pain. In this review, we discuss our current knowledge of purinergic signaling and of the compounds that modulate purinergic transmission, with the aim of highlighting the importance of purinergic pathways as targets for the treatment of persistent pain.

Single-nucleotide polymorphisms of the SLC17A9 and P2RY12 genes are significantly associated with phantom tooth pain

Phantom tooth pain (PTP) is a rare and specific neuropathic pain that occurs after pulpectomy and tooth extraction, but its cause is not understood. We hypothesized that there is a genetic contribution to PTP. We focused on solute carrier family 17 member 9 (SLC17A9)/vesicular nucleotide transporter (VNUT) and purinergic receptor P2Y12 (P2RY12), both of which have been associated with neuropathic pain and pain transduction signaling in the trigeminal ganglion in rodents. We sought to corroborate these associations in humans. We investigated gene polymorphisms that contribute to PTP. We statistically examined the association between genetic polymorphisms and PTP vulnerability in 150 patients with orofacial pain, including PTP, and 500 healthy subjects. We found that the rs735055 polymorphism of the SLC17A9 gene and rs3732759 polymorphism of the P2RY12 gene were associated with the development of PTP. Carriers of the minor allele of rs735055 and individuals who were homozygous for the major allele of rs3732759 had a higher rate of PTP. Carriers of the minor allele of rs735055 reportedly had high SLC17A9 mRNA expression in the spinal cord, which may increase the storage and release of adenosine triphosphate. Individuals who were homozygous for the major allele of rs3732759 may have higher P2RY12 expression that is more active in microglia. Therefore, these carriers may be more susceptible to PTP. These results suggest that specific genetic polymorphisms of the SLC17A9 and P2RY12 genes are involved in PTP. This is the first report on genes that are associated with PTP in humans.

Pharmacological interactions between intrathecal pregabalin plus tianeptine or clopidogrel in a rat model of neuropathic pain

Background

There is still unmet need in treating neuropathic pain and increasing awareness regarding the use of drug combinations to increase the effectiveness of treatment and reduce adverse effects in patients with neuropathic pain.

Methods

This study was performed to determine the individual and combined effects of pregabalin, tianeptine, and clopidogrel in a rat model of neuropathic pain. The model was created by ligation of the L5-L6 spinal nerve in male Sprague-Dawley rats; mechanical allodynia was confirmed using von Frey filaments. Drugs were administered to the intrathecal space and mechanical allodynia was assessed; drug interactions were estimated by isobolographic or fixed-dose analyses.

Results

Intrathecal pregabalin and tianeptine increased the mechanical withdrawal threshold in a dose-dependent manner, but intrathecal clopidogrel had little effect on the mechanical withdrawal threshold. An additive effect was noted between pregabalin and tianeptine, but not between pregabalin and clopidogrel.

Conclusions

These findings suggest that intrathecal coadministration of pregabalin and tianeptine effectively attenuated mechanical allodynia in the rat model of neuropathic pain. Thus, pregabalin plus tianeptine may be a valid option to enhance the efficacy of neuropathic pain treatment.

Gut-innervating TRPV1+ Neurons Drive Chronic Visceral Pain via Microglial P2Y12 Receptor

BACKGROUND & AIMS

Chronic abdominal pain is a common symptom of inflammatory bowel diseases (IBDs). Peripheral and central mechanisms contribute to the transition from acute to chronic pain during active disease and clinical remission. Lower mechanical threshold and hyperexcitability of visceral afferents induce gliosis in central pain circuits, leading to persistent visceral hypersensitivity (VHS). In the spinal cord, microglia, the immune sentinels of the central nervous system, undergo activation in multiple models of VHS. Here, we investigated the mechanisms of microglia activation to identify centrally acting analgesics for chronic IBD pain.

METHODS

Using Designer Receptors Exclusively Activated by Designer Drugs (DREADD) expressed in transient receptor potential vanilloid member 1-expressing visceral neurons that sense colonic inflammation, we tested whether neuronal activity was indispensable to control microglia activation and VHS. We then investigated the neuron-microglia signaling system involved in visceral pain chronification.

RESULTS

We found that chemogenetic inhibition of transient receptor potential vanilloid member 1⁺ visceral afferents prevents microglial activation in the spinal cord and subsequent VHS in colitis mice. In contrast, chemogenetic activation, in the absence of colitis, enhanced microglial activation associated with VHS. We identified a purinergic signaling mechanism mediated by neuronal adenosine triphosphate (ATP) and microglial P2Y12 receptor, triggering VHS in colitis. Inhibition of P2RY12 prevented microglial reactivity and chronic VHS post-colitis.

CONCLUSIONS

Overall, these data provide novel insights into the central mechanisms of chronic visceral pain and suggest that targeting microglial P2RY12 signaling could be harnessed to relieve pain in patients with IBD who are in remission.

Mechanisms of ATP release in pain: role of pannexin and connexin channels

Pain is a physiological response to bodily damage and serves as a warning of potential threat. Pain can also transform from an acute response to noxious stimuli to a chronic condition with notable emotional and psychological components that requires treatment. Indeed, the management of chronic pain is currently an important unmet societal need. Several reports have implicated the release of the neurotransmitter adenosine triphosphate (ATP) and subsequent activation of purinergic receptors in distinct pain etiologies. Purinergic receptors are broadly expressed in peripheral neurons and the spinal cord; thus, purinergic signaling in sensory neurons or in spinal circuits may be critical for pain processing. Nevertheless, an outstanding question remains: what are the mechanisms of ATP release that initiate nociceptive signaling? Connexin and pannexin channels are established conduits of ATP release and have been suggested to play important roles in a variety of pathologies, including several models of pain. As such, these large-pore channels represent a new and exciting putative pharmacological target for pain treatment. Herein, we will review the current evidence for a role of connexin and pannexin channels in ATP release during nociceptive signaling, such as neuropathic and inflammatory pain. Collectively, these studies provide compelling evidence for an important role of connexins and pannexins in pain processing.

Emerging Roles of Microglia in Neuro-vascular Unit: Implications of Microglia-Neurons Interactions

Microglia, which serve as the defensive interface of the nervous system, are activated in many neurological diseases. Their role as immune responding cells has been extensively studied in the past few years. Recent studies have demonstrated that neuronal feedback can be shaped by the molecular signals received and sent by microglia. Altered neuronal activity or synaptic plasticity leads to the release of various communication messages from neurons, which in turn exert effects on microglia. Research on microglia-neuron communication has thus expanded from focusing only on neurons to the neurovascular unit (NVU). This approach can be used to explore the potential mechanism of neurovascular coupling across sophisticated receptor systems and signaling cascades in health and disease. However, it remains unclear how microglia-neuron communication happens in the brain. Here, we discuss the functional contribution of microglia to synapses, neuroimmune communication, and neuronal activity. Moreover, the current state of knowledge of bidirectional control mechanisms regarding interactions between neurons and microglia are reviewed, with a focus on purinergic regulatory systems including ATP-P₂RY₁₂R signaling, ATP-adenosine-A₁Rs/A_2ARs, and the ATP-pannexin 1 hemichannel. This review aims to organize recent studies to highlight the multifunctional roles of microglia within the neural communication network in health and disease.

Involvement of P2Y12 receptors in an NTG-induced model of migraine in male mice

BACKGROUND AND PURPOSE

P2Y₁₂ receptors (P2Y₁₂ Rs) are known to regulate different forms of pain and inflammation. In this study we investigated the participation of P2Y₁₂ Rs in an animal model of migraine.

EXPERIMENTAL APPROACH

We tested the effect of the centrally administered selective P2Y₁₂ R antagonist PSB-0739, and P2Y₁₂ R gene deficiency in acute nitroglycerin (NTG)-treated mice. Additionally, platelet depletion was used to investigate the role of platelet P2Y₁₂ Rs during migraine-like pain.

KEY RESULTS

NTG induced sensory hypersensitivity of C57BL/6 wild-type (P2ry12^+/+ ) mice, accompanied by an increase in c-fos and CGRP expression in the upper cervical spinal cord (C1-C2) and trigeminal nucleus caudalis (TNC). Similar changes were also observed in P2Y₁₂ R gene-deficient (P2ry12^-/- ) mice. Prophylactic intrathecal application of PSB-0739 reversed thermal hyperalgesia and head grooming time in wild-type mice but had no effect in P2ry12^-/- mice; furthermore, it was also effective when applied as a post-treatment. PSB-0739 administration suppressed the expression of c-fos in C1-C2 and TNC, and decrease C1-C2 levels of dopamine and serotonin in wild-type mice. NTG treatment itself did not change adenosine diphosphate (ADP)-induced platelet activation measured by CD62P upregulation in wild-type mice. Platelet depletion by anti-mouse CD41 antibody and clopidogrel attenuated NTG-induced thermal hypersensitivity and head grooming time in mice.

CONCLUSION AND IMPLICATIONS

Taken together, our findings show that acute inhibition of P2Y₁₂ Rs alleviates migraine-like pain in mice, by modulating the expression of c-fos, and platelet P2Y₁₂ Rs might contribute to this effect. Hence, it is suggested that the blockade of P2Y₁₂ Rs may have therapeutic potential against migraine.

Microglial heterogeneity in chronic pain

In this review, we report existing preclinical evidence on how the CNS compartment as well as sex affect microglia functions in health. We highlight that recent advances in transcriptomics analyses have led to thorough characterization of disease-associated microglial states in mice and humans. We then consider the specific scenario of peripheral nerve or tissue injury which induce expression of a specific subset of genes in microglia in the dorsal horn of the spinal cord. We suggest the intriguing possibility that future studies may disclose the existence of a unique microglia transcriptional profile that is associated with chronic pain conditions. We also collect evidence that microglial activation in pain-related areas of the brain can be observed in models of neuropathic pain in agreement with recent neuroimaging studies in chronic pain patients. Based on the evidence discussed here, we predict that future studies on the neuroimmune interactions in chronic pain should complement our current understanding of microglia functions, but also adventure in using novel approaches such as scRNA-seq, spatial transcriptomics, CYTOF and transmission electron microscopy to provide a more complete characterization of the function, transcriptome and structure of microglia in chronic pain.

Antinociceptive Effect of Magnolol in a Neuropathic Pain Model of Mouse

BACKGROUND AND OBJECTIVE

Neuropathic pain remains a clinical challenge with limited effective treatments. Previous studies have found that magnolol (Mag), an ingredient existing in some herbs, showed neuroprotective effect. However, it remains unclear whether Mag can alleviate neuropathic pain.

METHODS

Chronic constriction injury (CCI) is used as the neuropathic pain model. Mice were randomly divided into 5 groups: Sham, CCI, CCI + 5, 10, 30 mg/kg Mag groups. Thermal and mechanical paw withdrawal threshold were performed at baseline and on the 3rd, 5th, 7th, 14th days post-surgery. Lumbar spinal cord and blood samples were collected on the 14th day. Blood lipid profile, kidney and liver functions, as well as the activation of microglia were evaluated, along with the related signal pathway examined using multiple methods including immunohistochemistry, RT-PCR and Western blot.

RESULTS

Mag alleviated thermal and mechanical hypersensitivity in CCI mice. CCI activated microglia and upregulated the expression of P2Y12, while Mag inhibited microglial activation, and downregulated the expression of P2Y12. Mag also blocked the activation of p38 mitogen-activated protein kinase (MAPK) and other pain-related cytokines such as IL-6, TNF-α and IL-1β.

CONCLUSION

The findings indicate that Mag has antinociceptive effect on neuropathic pain, probably mediated through P2Y12 receptors and p38 MAPK mediated pathways. With its relatively safe profile, Mag may be a potential therapeutic agent for neuropathic pain.

Treatment of chronic neuropathic pain: purine receptor modulation

Extracellular nucleosides and nucleotides have widespread functions in responding to physiological stress. The "purinome" encompasses 4 G-protein-coupled receptors (GPCRs) for adenosine, 8 GPCRs activated by nucleotides, 7 adenosine 5'-triphosphate-gated P2X ion channels, as well as the associated enzymes and transporters that regulate native agonist levels. Purinergic signaling modulators, such as receptor agonists and antagonists, have potential for treating chronic pain. Adenosine and its analogues potently suppress nociception in preclinical models by activating A1 and/or A3 adenosine receptors (ARs), but safely harnessing this pathway to clinically treat pain has not been achieved. Both A2AAR agonists and antagonists are efficacious in pain models. Highly selective A3AR agonists offer a novel approach to treat chronic pain. We have explored the structure activity relationship of nucleoside derivatives at this subtype using a computational structure-based approach. Novel A3AR agonists for pain control containing a bicyclic ring system (bicyclo [3.1.0] hexane) in place of ribose were designed and screened using an in vivo phenotypic model, which reflected both pharmacokinetic and pharmacodynamic parameters. High specificity (>10,000-fold selective for A3AR) was achieved with the aid of receptor homology models based on related GPCR structures. These A3AR agonists are well tolerated in vivo and highly efficacious in models of chronic neuropathic pain. Furthermore, signaling molecules acting at P2X3, P2X4, P2X7, and P2Y12Rs play critical roles in maladaptive pain neuroplasticity, and their antagonists reduce chronic or inflammatory pain, and, therefore, purine receptor modulation is a promising approach for future pain therapeutics. Structurally novel antagonists for these nucleotide receptors were discovered recently.

The role of microglia in chronic pain and depression: innocent bystander or culprit?

Clinical evidence shows that chronic pain and depression often accompany each other, but the underlying pathogenesis of comorbid chronic pain and depression remains mostly undetermined. Biotechnology is gradually revealing the phenotype and function of microglia, with great progress regarding microglia's role in neurodegeneration, depression, chronic pain, and other conditions. This article summarizes the role of microglia in chronic pain, depression, and comorbidities, which is conducive to finding new targets to treat chronic pain and depression.

Presence and activation of pro-inflammatory macrophages are associated with CRYAB expression in vitro and after peripheral nerve injury

BACKGROUND

Inflammation constitutes both positive and negative aspects to recovery following peripheral nerve injury. Following damage to the peripheral nervous system (PNS), immune cells such as macrophages play a beneficial role in creating a supportive environment for regrowing axons by phagocytosing myelin and axonal debris. However, a prolonged inflammatory response after peripheral nerve injury has been implicated in the pathogenesis of negative symptoms like neuropathic pain. Therefore, the post-injury inflammation must be carefully controlled to prevent secondary damage while allowing for regeneration. CRYAB (also known as alphaB-crystallin/HSPB5) is a small heat shock protein that has many protective functions including an immunomodulatory role in mouse models of multiple sclerosis, spinal cord injury, and stroke. Because its expression wanes and rebounds in the early and late periods respectively after PNS damage, and CRYAB null mice with sciatic nerve crush injury display symptoms of pain, we investigated whether CRYAB is involved in the immune response following PNS injury.

METHODS

Sciatic nerve crush injuries were performed in age-matched Cryab knockout (Cryab-/-) and wildtype (WT) female mice. Nerve segments distal to the injury site were processed by immunohistochemistry for macrophages and myelin while protein lysates of the nerves were analyzed for cytokines and chemokines using Luminex and enzyme-linked immunosorbent assay (ELISA). Peritoneal macrophages from the two genotypes were also cultured and polarized into pro-inflammatory or anti-inflammatory phenotypes where their supernatants were analyzed for cytokines and chemokines by ELISA and protein lysates for macrophage antigen presenting markers using western blotting.

RESULTS

We report that (1) more pro-inflammatory CD16/32+ macrophages are present in the nerves of Cryab-/- mice at days 14 and 21 after sciatic nerve crush-injury compared to WT counterparts, and (2) CRYAB has an immunosuppressive effect on cytokine secretion [interleukin (IL)-β, IL-6, IL-12p40, tumor necrosis factor (TNF)-α] from pro-inflammatory macrophages in vitro.

CONCLUSIONS

CRYAB may play a role in curbing the potentially detrimental pro-inflammatory macrophage response during the late stages of peripheral nerve regeneration.

Perineural high-mobility group box 1 induces mechanical hypersensitivity through activation of spinal microglia: Involvement of glutamate-NMDA receptor dependent mechanism in spinal dorsal horn

High mobility box 1 (HMGB1), a damage-associated molecular pattern, has crucial roles in induction of neuropathic pain. Upregulation of HMGB1 around the injured sciatic nerve contributes to mechanical hypersensitivity following partial sciatic nerve ligation (PSNL) of mice. However, central mechanisms mediating perineural HMGB1-induced nociceptive hypersensitivity, especially within the spinal dorsal horn, have not been determined. The current study shows that perineural treatment of naïve mice with recombinant HMGB1, which mimics increased HMGB1 around the injured sciatic nerve of PSNL mice, significantly induced activation of microglia, but not astrocytes, in the spinal dorsal horn. Intraperitoneal injection of minocycline, a microglial inhibitor, ameliorated perineural rHMGB1-induced mechanical hypersensitivity. In addition, blockade of spinal N-methyl-D-aspartate (NMDA) receptors significantly prevented perineural rHMGB1-induced mechanical hypersensitivity and microglial activation. In contrast, non-NMDA receptors, neurokinin 1 receptor, colony-stimulating factor 1 receptor and P2Y₁₂ receptor were not involved in perineural rHMGB1-induced mechanical hypersensitivity. Furthermore, repeated perineural treatment with an anti-HMGB1 antibody blocked activation of spinal microglia in PSNL mice. Collectively, the current findings demonstrate that increased HMGB1 around injured sciatic nerve might induce nociceptive hypersensitivity through activation of spinal microglia. Thus, HMGB1-dependent mechanisms between the injured sciatic nerve and spinal dorsal horn could be crucial in induction of neuropathic pain.

Loss of P2Y12 Has Behavioral Effects in the Adult Mouse

While microglia have been established as critical mediators of synaptic plasticity, the molecular signals underlying this process are still being uncovered. Increasing evidence suggests that microglia utilize these signals in a temporally and regionally heterogeneous manner. Subsequently, it is necessary to understand the conditions under which different molecular signals are employed by microglia to mediate the physiological process of synaptic remodeling in development and adulthood. While the microglial purinergic receptor P2Y12 is required for ocular dominance plasticity, an adolescent form of experience-dependent plasticity, it remains unknown whether P2Y12 functions in other forms of plasticity at different developmental time points or in different brain regions. Using a combination of ex vivo characterization and behavioral testing, we examined how the loss of P2Y12 affects developmental processes and behavioral performance in adulthood in mice. We found P2Y12 was not required for an early form of plasticity in the developing visual thalamus and did not affect microglial migration into barrels in the developing somatosensory cortex. In adult mice, however, the loss of P2Y12 resulted in alterations in recognition and social memory, as well as anxiety-like behaviors, suggesting that while P2Y12 is not a universal regulator of synaptic plasticity, the loss of P2Y12 is sufficient to cause functional defects.

Regulating quantal size of neurotransmitter release through a GPCR voltage sensor

Current models emphasize that membrane voltage (Vm) depolarization-induced Ca²⁺ influx triggers the fusion of vesicles to the plasma membrane. In sympathetic adrenal chromaffin cells, activation of a variety of G protein coupled receptors (GPCRs) can inhibit quantal size (QS) through the direct interaction of G protein Giβγ subunits with exocytosis fusion proteins. Here we report that, independently from Ca²⁺, Vm (action potential) per se regulates the amount of catecholamine released from each vesicle, the QS. The Vm regulation of QS was through ATP-activated GPCR-P2Y₁₂ receptors. D76 and D127 in P2Y₁₂ were the voltage-sensing sites. Finally, we revealed the relevance of the Vm dependence of QS for tuning autoinhibition and target cell functions. Together, membrane voltage per se increases the quantal size of dense-core vesicle release of catecholamine via Vm → P2Y₁₂(D76/D127) → Giβγ → QS → myocyte contractility, offering a universal Vm-GPCR signaling pathway for its functions in the nervous system and other systems containing GPCRs.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.