Interactions of pannexin 1 with NMDA and P2X7 receptors in central nervous system pathologies: Possible role on chronic pain

Functional role of P2X7 purinergic receptor in cancer and cancer-related pain

Numerous studies have revealed that the ATP-gated ion channel purinergic 2X7 receptor (P2X7R) plays an important role in tumor progression and the pathogenesis of cancer pain. P2X7R requires activation by extracellular ATP to perform its regulatory role functions. During tumor development or cancer-induced pain, ATP is released from tumor cells or other cells in the tumor microenvironment (such as tumor-associated immune cells), which activates P2X7R, opens ion channels on the cell membrane, affects intracellular molecular metabolism, and regulates the activity of tumor cells. Furthermore, peripheral organs and receptors can be damaged during tumor progression, and P2X7R expression in nerve cells (such as microglia) is significantly upregulated, enhancing sensory afferent information, sensitizing the central nervous system, and inducing or exacerbating pain. These findings reveal that the ATP-P2X7R signaling axis plays a key regulatory role in the pathogenesis of tumors and cancer pain and also has a therapeutic role. Accordingly, in this study, we explored the role of P2X7R in tumors and cancer pain, discussed the pharmacological properties of inhibiting P2X7R activity (such as the use of antagonists) or blocking its expression in the treatment of tumor and cancer pain, and provided an important evidence for the treatment of both in the future.

How is the P2X7 receptor signaling pathway involved in epileptogenesis?

Epilepsy, a condition characterized by spontaneous recurrent epileptic seizures, is among the most prevalent neurological disorders. This disorder is estimated to affect approximately 70 million people worldwide. Although antiseizure medications are considered the first-line treatments for epilepsy, most of the available antiepileptic drugs are not effective in nearly one-third of patients. This calls for the development of more effective drugs. Evidence from animal models and epilepsy patients suggests that strategies that interfere with the P2X7 receptor by binding to adenosine triphosphate (ATP) are potential treatments for this patient population. This review describes the role of the P2X7 receptor signaling pathways in epileptogenesis. We highlight the genes, purinergic signaling, Pannexin1, glutamatergic signaling, adenosine kinase, calcium signaling, and inflammatory response factors involved in the process, and conclude with a synopsis of these key connections. By unraveling the intricate interplay between P2X7 receptors and epileptogenesis, this review provides ideas for designing potent clinical therapies that will revolutionize both prevention and treatment for epileptic patients.

Involvement of microglial P2X7 receptor in pain modulation

BACKGROUND

Pain is a rapid response mechanism that compels organisms to retreat from the harmful stimuli and triggers a repair response. Nonetheless, when pain persists for extended periods, it can lead to adverse changes into in the individual's brain, negatively impacting their emotional state and overall quality of life. Microglia, the resident immune cells in the central nervous system (CNS), play a pivotal role in regulating a variety of pain-related disorders. Specifically, recent studies have shed light on the central role that microglial purinergic ligand-gated ion channel 7 receptor (P2X7R) plays in regulating pain. In this respect, the P2X7R on microglial membranes represents a potential therapeutic target.

AIMS

To expound on the intricate link between microglial P2X7R and pain, offering insights into potential avenues for future research.

METHODS

We reviewed 140 literature and summarized the important role of microglial P2X7R in regulating pain, including the structure and function of P2X7R, the relationship between P2X7R and microglial polarization, P2X7R-related signaling pathways, and the effects of P2X7R antagonists on pain regulation.

RESULTS

P2X7R activation is related to M1 polarization of microglia, while suppressing P2X7R can transfer microglia from M1 into M2 phenotype. And targeting the P2X7R-mediated signaling pathways helps to explore new therapy for pain alleviation. P2X7R antagonists also hold potential for translational and clinical applications in pain management.

CONCLUSIONS

Microglial P2X7R holds promise as a potential novel pharmacological target for clinical treatments due to its distinctive structure, function, and the development of antagonists.

Potential roles of enteric glial cells in Crohn's disease: A critical review

Enteric glial cells in the enteric nervous system are critical for the regulation of gastrointestinal homeostasis. Increasing evidence suggests two-way communication between enteric glial cells and both enteric neurons and immune cells. These interactions may be important in the pathogenesis of Crohn's disease (CD), a chronic relapsing disease characterized by a dysregulated immune response. Structural abnormalities in glial cells have been identified in CD. Furthermore, classical inflammatory pathways associated with CD (e.g., the nuclear factor kappa-B pathway) function in enteric glial cells. However, the specific mechanisms by which enteric glial cells contribute to CD have not been summarized in detail. In this review, we describe the possible roles of enteric glial cells in the pathogenesis of CD, including the roles of glia-immune interactions, neuronal modulation, neural plasticity, and barrier integrity. Additionally, the implications for the development of therapeutic strategies for CD based on enteric glial cell-mediated pathogenic processes are discussed.

Antinociceptive and neuroprotective effect of echinacoside on peripheral neuropathic pain in mice through inhibiting P2X7R/FKN/CX3CR1 pathway

Clinically, neuropathic pain treatment remains a challenging issue because the major therapy, centred around pharmacological intervention, is not satisfactory enough to patient by reason of low effectiveness and more adverse reaction. Therefore, it is still necessary to find more effective and safe therapy to ameliorate neuropathic pain. The purpose of this study was to explore the antinociceptive effect of Echinacoside (ECH), an active compound of Cistanche deserticola Ma, on peripheral neuropathic pain induced by chronic constriction injury (CCI) in mice, and to demonstrate its potential mechanism in vivo and vitro. In the present study, results showed that intraperitoneal administration of ECH (50, 100, and 200 mg/kg) could alleviate mechanical allodynia, cold allodynia and thermal hyperalgesia via behavioural test. Moreover, the structure and function of injured sciatic nerve by CCI were taken a turn for the better to a certain extent after ECH treatment using histopathological and electrophysiological test. Furthermore, ECH repressed the expression of the P2X7R and FKN and reduced the expression and release of the IL-1β, IL-6 and TNF-α. Besides, ECH could decrease Ca2+ influx and Cats efflux and inhibit phosphorylation of p38MAPK. To sum up, the present study illustrated that ECH could alleviate peripheral neuropathic pain by inhibiting microglia overactivation and inflammation through P2X7R/FKN/CX3CR1 signalling pathway in spinal cord. This study would provide a new perspective and strategy for the pharmacological treatment on neuropathic pain.

Role of cAMP in Cardiomyocyte Viability: Beneficial or Detrimental?

BACKGROUND

3', 5'-cyclic AMP (cAMP) regulates numerous cardiac functions. Various hormones and neurotransmitters elevate intracellular cAMP (i[cAMP]) in cardiomyocytes through activating GsPCRs (stimulatory-G-protein-coupled-receptors) and membrane-bound ACs (adenylyl cyclases). Increasing evidence has indicated that stimulating different GsPCRs and ACs exhibits distinct, even opposite effects, on cardiomyocyte viability. However, the underlying mechanisms are not fully understood.

METHODS

We used molecular and pharmacological approaches to investigate how different GsPCR/cAMP signaling differentially regulate cardiomyocyte viability with in vitro, ex vivo, and in vivo models.

RESULTS

For prodeath GsPCRs, we explored β1AR (beta1-adrenergic receptor) and H2R (histamine-H2-receptor). We found that their prodeath effects were similarly dependent on AC5 activation, ATP release to the extracellular space via PANX1 (pannexin-1) channel, and extracellular ATP (e[ATP])-mediated signaling involving in P2X7R (P2X purinoceptor 7) and CaMKII (Ca2+/calmodulin-dependent protein kinase II). PANX1 phosphorylation at Serine 206 by cAMP-dependent-PKA (protein-kinase-A) promoted PANX1 activation, which was critical in β1AR- or H2R-induced cardiomyocyte death in vitro and in vivo. β1AR or H2R was localized proximately to PANX1, which permits ATP release. For prosurvival GsPCRs, we explored adenosine-A2-receptor (A2R), CGRPR (calcitonin-gene-related-peptide-receptor), and RXFP1 (relaxin-family peptide-receptor 1). Their prosurvival effects were dependent on AC6 activation, cAMP efflux via MRP4 (multidrug resistance protein 4), extracellular cAMP metabolism to adenosine (e[cAMP]-to-e[ADO]), and e[ADO]-mediated signaling. A2R, CGRPR, or RXFP1 was localized proximately to MRP4, which enables cAMP efflux. Interestingly, exogenously increasing e[cAMP] levels by membrane-impermeable cAMP protected against cardiomyocyte death in vitro and in ex vivo and in vivo mouse hearts with ischemia-reperfusion injuries.

CONCLUSIONS

Our findings indicate that the functional diversity of different GsPCRs in cardiomyocyte viability could be achieved by their ability to form unique signaling complexes (signalosomes) that determine the fate of cAMP: either stimulate ATP release by activating PKA or directly efflux to be e[cAMP].

Cross-species gene expression patterns of purinergic signaling in the human and mouse trigeminal ganglion

Purinergic signaling system plays a pivotal role in the trigeminal ganglion (TG) which is a primary sensory tissue in vertebrate nervous systems involving orofacial nociception and peripheral sensitization. Despite previous efforts to reveal the expression patterns of purinergic components in the mouse TG, it is still unknown the interspecies differences between human and mouse. In this study, we provide a comprehensive transcriptome profile of the purinergic signaling system across diverse cell types and neuronal subpopulations within the human TG, systematically comparing it with mouse TG. In addition, the evolutionary conservation and species-specific expression patterns of the purinergic components are also discussed. We propose that the data can improve our understanding of purinergic signaling in the peripheral nervous system and facilitate the identification of novel therapeutic targets.

Design, Synthesis, and Biological Evaluation of Novel P2X7 Receptor Antagonists for the Treatment of Septic Acute Kidney Injury

Sepsis-associated acute kidney injury (AKI) is a serious clinical problem, without effective drugs. Abnormal activation of the purinergic P2X7 receptor (P2X7R) in septic kidneys makes its antagonist a promising therapeutic approach. Herein, a series of novel P2X7R antagonists were designed, synthesized, and structurally optimized. Based on in vitro potency in human/mouse P2X7R using HEK293 cells, hepatic microsomal stability, and pharmacokinetic and preliminary in vivo assessments, compound 14a was identified by respective human and mouse P2X7R IC50 values of 64.7 and 10.1 nM, together with favorable pharmacokinetic properties. Importantly, 14a dose-dependently alleviated kidney dysfunction and pathological injury in both lipopolysaccharide (LPS)- and cecal ligation/perforation (CLP)-induced septic AKI mice with a good safety profile. Mechanistically, 14a could suppress NLRP3 inflammasome activation to inhibit the expression of cleaved caspase-1, gasdermin D, IL-1β, and IL-18 in the injured kidneys of septic mice. Collectively, these results highlighted that P2X7R antagonist 14a exerted a therapeutic potential against septic AKI.

Increased serum pannexin-1 concentrations reflect illness severity and predict a poor prognosis after acute supratentorial intracerebral hemorrhage: A prospective longitudinal cohort study

BACKGROUND

Pannexin-1 is a nonselective, large pore and voltage gated channel protein, whose activation may aggravate acute brain injury. We ascertained the clinical significance of serum pannexin-1 as a prognostic biomarker of acute intracerebral hemorrhage (ICH).

METHODS

In this prospective, observational study of 124 acute supratentorial ICH patients and 124 healthy controls, serum pannexin-1 concentrations were determined. Admission National Institutes of Health Stroke Scale (NIHSS) score and hematoma volume were used for assessment of hemorrhagic severity, post-stroke 6-month modified Rankin scale (mRS) score was registered to reflect clinical outcome and early neurologic deterioration (END) and 6-month poor outcome (mRS score of 3-6) were regarded as the 2 prognostic parameters. Their associations with serum pannexin-1 concentrations were investigated using multivariate analysis. The predictive performance was evaluated in terms of area under receiver operating characteristic curve (AUC).

RESULTS

In comparison to controls, significantly increased serum pannexin-1 concentrations after ICH (median, 6.8 vs. 2.7 mg/ml) were independently correlative with NIHSS score (β, 0.193; 95% CI: 0.086-0.300), hematoma volume (β, 0.641; 95% CI: 0.423-0.859) and mRS score (β, 0.199; 95% CI: 0.065-0.174), were independently predictive of END (OR, 1.176; 95% CI: 1.081-1.280) and poor outcome (odds ratio, 1.218; 95% CI: 1.059-1.400), as well as were efficiently discriminative of END (AUC, 0.764; 95% CI: 0.663-0.864) and poor 6-month outcome (AUC, 0.790; 95% CI: 0.711-0.870). Serum pannexin-1 combined with NIHSS score and hematoma volume (AUC, 0.908; 95% CI: 0.857-0.960) displayed significantly higher predictive ability for poor 6-month outcome than NIHSS score and hematoma volume alone (both P < 0.05).

CONCLUSION

Rising serum pannexin-1 concentrations following ICH, in strong correlation with hemorrhagic severity, independently distinguish the risk of END and 90-day poor outcome. Assumably, serum pannexin-1 may represent a valuable prognostic biomarker of ICH.

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED₅₀ = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

A Single Episode of Cortical Spreading Depolarization Increases mRNA Levels of Proinflammatory Cytokines, Calcitonin Gene-Related Peptide and Pannexin-1 Channels in the Cerebral Cortex

Cortical spreading depolarization (CSD) is the neuronal correlate of migraine aura and the reliable consequence of acute brain injury. The role of CSD in triggering headaches that follow migraine aura and brain injury remains to be uncertain. We examined whether a single CSD occurring in awake animals modified the expression of proinflammatory cytokines (Il1b, TNF, and Il6) and endogenous mediators of nociception/neuroinflammation-pannexin 1 (Panx1) channel and calcitonin gene-related peptide (CGRP), transforming growth factor beta (TGFb) in the cortex. Unilateral microinjury of the somatosensory cortex triggering a single CSD was produced in awake Wistar rats. Three hours later, tissue samples from the lesioned cortex, intact ipsilesional cortex invaded by CSD, and homologous areas of the contralateral sham-treated cortex were harvested and analyzed using qPCR. Three hours post-injury, intact CSD-exposed cortexes increased TNF, Il1b, Panx1, and CGRP mRNA levels. The strongest upregulation of proinflammatory cytokines was observed at the injury site, while CGRP and Panx1 were upregulated more strongly in the intact cortexes invaded by CSD. A single CSD is sufficient to produce low-grade parenchymal neuroinflammation with simultaneous overexpression of Panx1 and CGRP. The CSD-induced molecular changes may contribute to pathogenic mechanisms of migraine pain and post-injury headache.

Inhibition of Schwann cell pannexin 1 attenuates neuropathic pain through the suppression of inflammatory responses

BACKGROUND

Neuropathic pain is still a challenge for clinical treatment as a result of the comprehensive pathogenesis. Although emerging evidence demonstrates the pivotal role of glial cells in regulating neuropathic pain, the role of Schwann cells and their underlying mechanisms still need to be uncovered. Pannexin 1 (Panx 1), an important membrane channel for the release of ATP and inflammatory cytokines, as well as its activation in central glial cells, contributes to pain development. Here, we hypothesized that Schwann cell Panx 1 participates in the regulation of neuroinflammation and contributes to neuropathic pain.

METHODS

A mouse model of chronic constriction injury (CCI) in CD1 adult mice or P0-Cre transgenic mice, and in vitro cultured Schwann cells were used. Intrasciatic injection with Panx 1 blockers or the desired virus was used to knock down the expression of Panx 1. Mechanical and thermal sensitivity was assessed using Von Frey and a hot plate assay. The expression of Panx 1 was measured using qPCR, western blotting, and immunofluorescence. The production of cytokines was monitored through qPCR and enzyme-linked immunosorbent assay (ELISA). Panx1 channel activity was detected by ethidium bromide (EB) uptake.

RESULTS

CCI induced persistent neuroinflammatory responses and upregulation of Panx 1 in Schwann cells. Intrasciatic injection of Panx 1 blockers, carbenoxolone (CBX), probenecid, and Panx 1 mimetic peptide (¹⁰Panx) effectively reduced mechanical and heat hyperalgesia. Probenecid treatment of CCI-induced mice significantly reduced Panx 1 expression in Schwann cells, but not in dorsal root ganglion (DRG). In addition, Panx 1 knockdown in Schwann cells with Panx 1 shRNA-AAV in P0-Cre mice significantly reduced CCI-induced neuropathic pain. To determine whether Schwann cell Panx 1 participates in the regulation of neuroinflammation and contributes to neuropathic pain, we evaluated its effect in LPS-treated Schwann cells. We found that inhibition of Panx 1 via CBX and Panx 1-siRNA effectively attenuated the production of selective cytokines, as well as its mechanism of action being dependent on both Panx 1 channel activity and its expression.

CONCLUSION

In this study, we found that CCI-related neuroinflammation correlates with Panx 1 activation in Schwann cells, indicating that inhibition of Panx 1 channels in Schwann cells reduces neuropathic pain through the suppression of neuroinflammatory responses.

NMDA and P2X7 Receptors Require Pannexin 1 Activation to Initiate and Maintain Nociceptive Signaling in the Spinal Cord of Neuropathic Rats

Pannexin 1 (Panx1) is involved in the spinal central sensitization process in rats with neuropathic pain, but its interaction with well-known, pain-related, ligand-dependent receptors, such as NMDA receptors (NMDAR) and P2X7 purinoceptors (P2X7R), remains largely unexplored. Here, we studied whether NMDAR- and P2X7R-dependent nociceptive signaling in neuropathic rats require the activation of Panx1 channels to generate spinal central sensitization, as assessed by behavioral (mechanical hyperalgesia) and electrophysiological (C-reflex wind-up potentiation) indexes. Administration of either a selective NMDAR agonist i.t. (NMDA, 2 mM) or a P2X7R agonist (BzATP, 150 μM) significantly increased both the mechanical hyperalgesia and the C-reflex wind-up potentiation, effects that were rapidly reversed (minutes) by i.t. administration of a selective pannexin 1 antagonist (10panx peptide, 300 μM), with the scores even reaching values of rats without neuropathy. Accordingly, 300 μM 10panx completely prevented the effects of NMDA and BzATP administered 1 h later, on mechanical hyperalgesia and C-reflex wind-up potentiation. Confocal immunofluorescence imaging revealed coexpression of Panx1 with NeuN protein in intrinsic dorsal horn neurons of neuropathic rats. The results indicate that both NMDAR- and P2X7R-mediated increases in mechanical hyperalgesia and C-reflex wind-up potentiation require neuronal Panx1 channel activation to initiate and maintain nociceptive signaling in neuropathic rats.

H2O2 gel bleaching induces cytotoxicity and pain conduction in dental pulp stem cells via intracellular reactive oxygen species on enamel/dentin disc

Background

Bleaching is widely accepted for improving the appearance of discolored teeth; however, patient compliance is affected by bleaching-related complications, especially bleaching sensitivity. This study aimed to investigate the role of reactive oxygen species (ROS) in cytotoxicity and pain conduction activated by experimental tooth bleaching.

Methods

Dental pulp stem cells with or without N-acetyl-L-cysteine (NAC), an ROS scavenger, were cultured on the dentin side of the enamel/dentin disc. Subsequently, 15% (90 min) and 40% (30 min) bleaching gels were painted on the enamel surface. Cell viability, intracellular ROS, Ca²⁺, adenosine triphosphate (ATP), and extracellular ATP levels were evaluated using the Cell Counting Kit-8 assay, 2’,7’-dichlorodihydrofluorescein diacetate, CellROX, fura-3AM fluorescence assay, and ATP measurement kit. The rat incisor model was used to evaluate in vivo effects after 0, 1, 3, 7, and 30 days of bleaching. Changes in gene and protein expression of interleukin 6 (IL-6), tumor necrosis factor-alpha (TNFα), transient receptor potential ankyrin 1 (TRPA1), and Pannexin1 (PANX1) in dental pulp stem cells and pulp tissue were detected through RT-PCR, western blotting, and immunofluorescence.

Results

The bleaching gel suppressed dental pulp stem cell viability and extracellular ATP levels and increased intracellular ROS, Ca²⁺, and intracellular ATP levels. The mRNA and protein expression of IL-6, TNFα, TRPA1, and PANX1 were up-regulated in vitro and in vivo. Furthermore, the 40% gel had a stronger effect than the 15% gel, and NAC ameliorated the gel effects.

Conclusions

Our findings suggest that bleaching gels induce cytotoxicity and pain conduction in dental pulp stem cells via intracellular ROS, which may provide a potential therapeutic target for alleviating tooth bleaching nociception.

Pannexin 1 channels and ATP release in epilepsy: two sides of the same coin : The contribution of pannexin-1, connexins, and CALHM ATP-release channels to purinergic signaling

Purinergic signaling mediated by ATP and its metabolites contributes to various brain physiological processes as well as to several pathological conditions, including neurodegenerative and neurological disorders, such as epilepsy. Among the different ATP release pathways, pannexin 1 channels represent one of the major conduits being primarily activated in pathological contexts. Investigations on in vitro and in vivo models of epileptiform activity and seizures in mice and human tissues revealed pannexin 1 involvement in aberrant network activity and epilepsy, and highlighted that pannexin 1 exerts a complex role. Pannexin 1 can indeed either sustain seizures through release of ATP that can directly activate purinergic receptors, or tune down epileptic activity via ATP-derived adenosine that decreases neuronal excitability. Interestingly, in-depth analysis of the literature unveils that this dichotomy is only apparent, as it depends on the model of seizure induction and the type of evoked epileptiform activity, two factors that can differentially activate pannexin 1 channels and trigger distinct intracellular signaling cascades. Here, we review the general properties and ATP permeability of pannexin 1 channels, and discuss their impact on acute epileptiform activity and chronic epilepsy according to the regime of activity and disease state. These data pave the way for the development of new antiepileptic strategies selectively targeting pannexin 1 channels in a context-dependent manner.

Kinesin Nanomotors Mediated Trafficking of NMDA-Loaded Cargo as A Novel Target in Chronic Pain

Chronic pain is among the most prevalent burdensome disorders worldwide. The N-methyl-d-aspartate (NMDA) receptor system plays a critical role in central sensitization, a primary feature of chronic pain. Despite the proven efficacy of exogenous ligands to this receptor system in preclinical studies, evidence for the clinical efficacy of NMDA antagonists for the treatment of chronic pain is weak. Researchers are studying alternate approaches, rather than direct inhibition of the NMDA receptors in pain processing neurons. This indirect approach utilizes the modulation of molecular switches that regulates the synthesis, maturation, and transport of receptors from cellular organelles to the synaptic membrane. Kinesins are nanomotors that anterogradely transport the cargo using microtubule tracks across the neurons. Various members of the kinesin family, including KIF17, KIF11, KIF5b, and KIF21a, regulate the intracellular transport of NMDA receptors. Pharmacological targeting of these ATP-driven nanomotors could be a useful tool for manipulating the NMDAR functioning. It could provide the potential for the development of a novel strategy for the management of chronic pain.

NMDAR1-Src-Pannexin1 Signal Pathway in the Trigeminal Ganglion Contributed to Orofacial Ectopic Pain Following Inferior Alveolar Nerve Transection

The N-methyl-d-aspartate receptor (NMDAR) is a glutamate-gated receptor channel that plays a role in peripheral neuropathic pain. Src, a protein tyrosine kinase, can regulate the activation of NMDARs in chronic pain conditions. Pannexin 1 (Panx1), a plasma membrane channel, plays an important role in neuropathic pain and functionally interacts with NMDARs in the pathological condition of epilepsy. In this study, the roles of NMDAR1 (NR1), Src, and Panx1 and their interactions in the trigeminal ganglion (TG) in orofacial ectopic pain attributed to inferior alveolar nerve transection (IANX) were investigated. IANX induced mechanical allodynia in the whisker pad with increased expression levels of NR1, Src phosphorylation (p-Src), and Panx1 in the TG. Double immunostaining revealed that NR1, Src, and Panx1 all colocalized with glutamine synthetase (GS) and neuronal nuclei (NeuN), and they overlapped in the TG, suggesting that they might be structurally connected to one another. In addition, trigeminal injection of memantine, PP2, or ¹⁰Panx attenuated IANX-induced mechanical allodynia in the whisker pad. Continuous intraganglionic administration of memantine (an antagonist of NMDAR) decreased IANX-induced upregulated expression of p-Src and Panx1. Similarly, PP2 (an inhibitor of Src) also decreased Panx1 protein expression but had no effect on NR1. In addition, intraganglionic injection of ¹⁰Panx (a blocker of Panx1) decreased NR1 protein expression but did not affect Src. In general, our findings demonstrated that NR1, Src, and Panx1 all contributed to orofacial ectopic pain following IANX and that they composed a signalling pathway in the TG involved in mechanical allodynia.

Reduction of prefrontal purinergic signaling is necessary for the analgesic effect of morphine

Morphine is commonly used to relieve moderate to severe pain, but repeated doses cause opioid tolerance. Here, we used ATP sensor and fiber photometry to detect prefrontal ATP level. It showed that prefrontal ATP level decreased after morphine injection and the event amplitude tended to decrease with continuous morphine exposure. Morphine had little effect on prefrontal ATP due to its tolerance. Therefore, we hypothesized that the analgesic effect of morphine might be related to ATP in the medial prefrontal cortex (mPFC). Moreover, local infusion of ATP partially antagonized morphine analgesia. Then we found that inhibiting P2X7R in the mPFC mimicked morphine analgesia. In morphine-tolerant mice, pretreatment with P2X4R or P2X7R antagonists in the mPFC enhanced analgesic effect. Our findings suggest that reduction of prefrontal purinergic signaling is necessary for the morphine analgesia, which help elucidate the mechanism of morphine analgesia and may lead to the development of new clinical treatments for neuropathic pain.

Purinergic signaling in nervous system health and disease: Focus on pannexin 1

Purinergic signaling encompasses the cycle of adenosine 5' triphosphate (ATP) release and its metabolism into nucleotide and nucleoside derivatives, the direct release of nucleosides, and subsequent receptor-triggered downstream intracellular pathways. Since the discovery of nerve terminal and glial ATP release into the neuropil, purinergic signaling has been implicated in the modulation of nervous system development, function, and disease. In this review, we detail our current understanding of the roles of the pannexin 1 (PANX1) ATP-release channel in neuronal development and plasticity, glial signaling, and neuron-glial-immune interactions. We additionally provide an overview of PANX1 structure, activation, and permeability to orientate readers and highlight recent research developments. We identify areas of convergence between PANX1 and purinergic receptor actions. Additional highlights include data on PANX1's participation in the pathophysiology of nervous system developmental, degenerative, and inflammatory disorders. Our aim in combining this knowledge is to facilitate the movement of our current understanding of PANX1 in the context of other nervous system purinergic signaling mechanisms one step closer to clinical translation.

Immune Actions on the Peripheral Nervous System in Pain

Pain can be induced by tissue injuries, diseases and infections. The interactions between the peripheral nervous system (PNS) and immune system are primary actions in pain sensitizations. In response to stimuli, nociceptors release various mediators from their terminals that potently activate and recruit immune cells, whereas infiltrated immune cells further promote sensitization of nociceptors and the transition from acute to chronic pain by producing cytokines, chemokines, lipid mediators and growth factors. Immune cells not only play roles in pain production but also contribute to PNS repair and pain resolution by secreting anti-inflammatory or analgesic effectors. Here, we discuss the distinct roles of four major types of immune cells (monocyte/macrophage, neutrophil, mast cell, and T cell) acting on the PNS during pain process. Integration of this current knowledge will enhance our understanding of cellular changes and molecular mechanisms underlying pain pathogenies, providing insights for developing new therapeutic strategies.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Purinergic System Signaling in Metainflammation-Associated Osteoarthritis

Inflammation triggered by metabolic imbalance, also called metainflammation, is low-grade inflammation caused by the components involved in metabolic syndrome (MetS), including central obesity and impaired glucose tolerance. This phenomenon is mainly due to excess nutrients and energy, and it contributes to the pathogenesis of osteoarthritis (OA). OA is characterized by the progressive degeneration of articular cartilage, which suffers erosion and progressively becomes thinner. Purinergic signaling is involved in several physiological and pathological processes, such as cell proliferation in development and tissue regeneration, neurotransmission and inflammation. Adenosine and ATP receptors, and other members of the signaling pathway, such as AMP-activated protein kinase (AMPK), are involved in obesity, type 2 diabetes (T2D) and OA progression. In this review, we focus on purinergic regulation in osteoarthritic cartilage and how different components of MetS, such as obesity and T2D, modulate the purinergic system in OA. In that regard, we describe the critical role in this disease of receptors, such as adenosine A2A receptor (A2AR) and ATP P2X7 receptor. Finally, we also assess how nucleotides regulate the inflammasome in OA.

The Role of NMDA Receptors in the Effect of Purinergic P2X7 Receptor on Spontaneous Seizure Activity in WAG/Rij Rats With Genetic Absence Epilepsy

P2X7 receptors (P2X7Rs) are ATP sensitive cation channels and have been shown to be effective in various epilepsy models. Absence epilepsy is a type of idiopathic, generalized, non-convulsive epilepsy. Limited data exist on the role of P2X7Rs and no data has been reported regarding the interaction between P2X7Rs and glutamate receptor NMDA in absence epilepsy. Thus, this study was designed to investigate the role of P2X7 and NMDA receptors and their possible interaction in WAG/Rij rats with absence epilepsy. Permanent cannula and electrodes were placed on the skulls of the animals. After the healing period of the electrode and cannula implantation, ECoG recordings were obtained during 180 min before and after drug injections. P2X7R agonist BzATP, at doses of 50 μg and 100 μg (intracerebroventricular; i.c.v.) and antagonist A-438079, at doses of 20 μg and 40 μg (i.c.v.) were administered alone or prior to memantine (5 mg/kg, intraperitoneal; i.p.) injection. The total number (in every 20 min), the mean duration, and the amplitude of spike-wave discharges (SWDs) were calculated and compared. Rats were decapitated and the right and left hemisphere, cerebellum, and brainstem were separated for the measurements of the advanced oxidation protein product (AOPP), malondialdehyde (MDA), superoxide dismutase (SOD), glutathione (GSH), catalase (CAT), glutathione peroxide (GPx), and glutathione reductase (GR). BzATP and A-438079 did not alter measured SWDs parameters, whereas memantine reduced them, which is considered anticonvulsant. BzATP did not alter the anticonvulsant effect of memantine, while A-438079 decreased the effect of memantine. Administration of BzATP increased the levels of SOD and GR in cerebrum hemispheres. A-438079 did not alter any of the biochemical parameters. Memantine reduced the levels of MDA, GSH, and GR while increased the level of CAT in the cerebrum. Administration of BzATP before memantine abolished the effect of memantine on MDA levels. The evidence from this study suggests that P2X7Rs does not directly play a role in the formation of absence seizures. P2X7Rs agonist, reduced the antioxidant activity of memantine whereas agonist of P2X7Rs reduced the anticonvulsant action of memantine, suggesting a partial interaction between P2X7 and NMDA receptors in absence epilepsy model.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Pannexin-1 Channel Regulates ATP Release in Epilepsy

With the deepening of research on epilepsy in recent decades, great progress has been made in the diagnosis and treatment of the disease. However, the clinical outcome remains unsatisfactory due to the confounding symptoms and complications, as well as complex intrinsic pathogenesis. A better understanding of the pathogenesis of epilepsy should be able to hinder the progress of the disease and improve the therapeutic effectiveness. Since the discovery of pannexin (Panx), unremitting efforts on the study of this gap junction protein family member have revealed its role in participating in the expression of various physiopathological processes. Among them, the activation or inhibition of Panx channel has been shown to regulate the release of adenosine triphosphate (ATP) and other signals, which is very important for the onset and control of nervous system diseases including epilepsy. In this article, we summarize the factors influencing the regulation of Panx channel opening, hoping to find a way to interfere with the activation or inhibition of Panx channel that regulates the signal transduction of ATP and other factors so as to control the progression of epilepsy and improve the quality of life of epileptic patients who fail to respond to the existing medical therapies and those at risk of surgical treatment.

Sarcoma Family Kinase-Dependent Pannexin-1 Activation after Cortical Spreading Depression is Mediated by NR2A-Containing Receptors

Cortical spreading depression (CSD) is a propagating wave of depolarization followed by depression of cortical activity. CSD triggers neuroinflammation via the pannexin-1 (Panx1) channel opening, which may eventually cause migraine headaches. However, the regulatory mechanism of Panx1 is unknown. This study investigates whether sarcoma family kinases (SFK) are involved in transmitting CSD-induced Panx1 activation, which is mediated by the NR2A-containing N-methyl-D-aspartate receptor. CSD was induced by topical application of K⁺ to cerebral cortices of rats and mouse brain slices. SFK inhibitor, PP2, or NR2A–receptor antagonist, NVP–AAM077, was perfused into contralateral cerebral ventricles (i.c.v.) of rats prior to CSD induction. Co-immunoprecipitation and Western blot were used for detecting protein interactions, and histofluorescence for addressing Panx1 activation. The results demonstrated that PP2 attenuated CSD-induced Panx1 activation in rat ipsilateral cortices. Cortical susceptibility to CSD was reduced by PP2 in rats and by TAT-Panx308 that disrupts SFK–Panx1 interaction in mouse brain slices. Furthermore, CSD promoted activated SFK coupling with Panx1 in rat ipsilateral cortices. Moreover, inhibition of NR2A by NVP–AAM077 reduced elevation of ipsilateral SFK–Panx1 interaction, Panx1 activation induced by CSD and cortical susceptibility to CSD in rats. These data suggest NR2A-regulated, SFK-dependent Panx1 activity plays an important role in migraine aura pathogenesis.

The role and pharmacological properties of the P2X7 receptor in neuropathic pain

Neuropathic Pain (NPP) is caused by direct or indirect damage to the nervous system and is a common symptom of many diseases. Clinically, drugs are usually used to suppress pain, such as (lidocaine, morphine, etc.), but the effect is short-lived, poor analgesia, and there are certain dependence and side effects. Therefore, the investigation of the treatment of NPP has become an urgent problem in medical, attracting a lot of research attention. P2X7 is dependent on Adenosine triphosphate (ATP) ion channel receptors and has dual functions for the development of nerve damage and pain. In this review, we explored the link between the P2X7 receptor (P2X7R) and NPP, providing insight into the P2X7R and NPP, discussing the pathological mechanism of P2 X7R in NPP and the biological characteristics of P2X7R antagonist inhibiting its over-expression for the targeted therapy of NPP.

NF-κB Phosphorylation Inhibition Prevents Articular Cartilage Degradation in Osteoarthritis Rats via 2-Aminoquinoline

Background

Osteoarthritis is a chronic degenerative disease of the joints that is common in older people worldwide. The characteristic features of osteoarthritis include cartilage degradation, synovitis, and remodelling of subchondral bone. The present study investigated the effect of 2-aminoquinoline on knee articular cartilage degradation in an osteoarthritis rat model.

Material/Methods

The rat model of osteoarthritis was established in Wistar rats by intra-articular injection of monosodium iodoacetate. The rats were randomly divided into 6 groups of 10 rats each: a normal control group, an untreated group, and 4 (5, 10, 15 and 20 mg/kg) treatment groups. The rats in treatment groups received 5, 10, 15, or 20 mg/kg doses of 2-aminoquinoline on day 2 of monosodium iodoacetate injection.

Results

The 2-aminoquinoline treatment of monosodium iodoacetate-injected rats markedly decreased weight-bearing asymmetry, inhibited edema formation, and improved paw withdrawal thresholds. The expression of inflammatory cytokines was markedly higher in the osteoarthritis rats. Treatment with 2-aminoquinoline led to a significant reduction in inflammatory cytokine expression in osteoarthritis rats in a dose-dependent manner. In osteoarthritis rats, the expressions of prostaglandin E2 (PGE2), matrix metalloproteinase-13 (MMP-13), and substance P were also higher in comparison to the control group. The 2-aminoquinoline treatment supressed PGE2, MMP-13, and substance P levels in osteoarthritis rats. Moreover, the expression of phosphorylated nuclear factor kappaB (p-NF-κB) was markedly higher in the untreated rats. However, activation of NF-κB was downregulated in the osteoarthritis rats by treatment with 2-aminoquinoline.

Conclusions

The present study demonstrated that 2-aminoquinoline prevents articular cartilage damage in osteoarthritis rats through inhibition of inflammatory factors and downregulation of NF-κB activation, suggesting that 2-aminoquinoline would be effective in treatment of osteoarthritis.

Autophagy dysfunction in neuropathic pain

Autophagy is a lysosomal degradation pathway that maintains tissue homeostasis by recycling damaged and aged cellular components, which plays important roles in development of the nervous system, as well as in neuronal function and survival. In addition, autophagy dysfunction underlies neuropathic pain. Thus, the modulation of autophagy can alleviate neuropathic pain. Here, we describe the definition, mechanisms of autophagy and neuropathic pain. On this basis, we further discuss the role of autophagy dysfunction in neuropathic pain. This review updates our knowledge on autophagy mechanisms which propose potential therapeutic targets for the treatment of neuropathic pain.

The interaction between P2X7Rs and T-type calcium ion channels in penicillin-induced epileptiform activity

Limited information exists on the link between purinergic class P2X7 receptors (P2X7Rs) and calcium ion channels in epilepsy; no data has been reported regarding the interaction between P2X7Rs and T-type calcium ion channels in epilepsy. Thus, this study is an evaluation of the role that T-type calcium ion channels play in the effect of P2X7Rs on penicillin-induced epileptiform activity. In the first set of experiments, P2X7R agonist BzATP (at 25-, 50-, 100- and 200-μg doses), P2X7R antagonist A-438079 (at 5-, 10-, 20- and 40-μg doses) and T-type calcium ion channel antagonist, NNC-550396 were administered for electrophysiological analyses 30 min after penicillin injection (2.5 μl, 500 IU). In the second set of experiments, the effective doses of these substances were used for biochemical analyses. Malondialdehyde (MDA), advanced oxidation protein product (AOPP), glutathione (GSH), glutathione reductase (GR), glutathione peroxide (GPx), catalase (CAT) and superoxide dismutase (SOD) levels were measured in the cerebrum, cerebellum and brainstem of rats. BzATP (100 μg, icv) increased the mean frequency of epileptiform activity, whereas A-438079 (40 μg, icv) and NNC-550396 (30 μg, ic) reduced it. Both A-438079 and NNC-550396 reversed BzATP's proconvulsant action. BzATP increased lipid peroxidation and protein oxidation; it also altered other antioxidant enzymes measured in this study, which were all then reversed via A-438079 and NNC-550396, at least in the cerebrum. The electrophysiological and biochemical analysis of present study suggest that P2X7Rs and its interaction with T-type calcium ion channels play an important role in the experimental model of epilepsy.

The Effect of Alpha-Tocopherol on Morphine Tolerance-induced Expression of c-fos Proto-oncogene from a Biotechnological Perspective

BACKGROUND

The increase of oxidant compounds is the most well-known reasons for the tolerance to the analgesic properties of Morphine. Additionally, the production of proxy-nitrite impairs receptors, proteins and enzymes involved in the signaling pathways of analgesia, apoptosis and necrosis. Also, we revised all patents relating to opioid tolerance control methods.

OBJECTIVE

The aim of this study was to assess the effects of Alpha-tocopherol as an anti-oxidant agent to reduce Morphine tolerance.

METHOD

Forty male rats randomly divided into four groups. 10 mg/kg of morphine was injected subcutaneously to create the desired level of tolerance. After modeling, 70 mg/kg Alpha- Tocopherol was injected intraperitoneal. Also, the hot plate recorded pain threshold alterations was used to evaluate the behavioral test. All tissue samples were extracted from the spinal cord, thalamus and frontal cortex for molecular and gene expression evaluations. Also, the effect of Alpha- Tocopherol on the apoptosis and necrosis parameters was analyzed using nissl staining and tunel test.

RESULTS

The time latency results showed that there were no significant differences in the different days in groups treated with Morphine plus Alpha-Tocopherol. However, our data highlighted that the pain threshold and their time latency in respond to it had substantially increased in comparison with the control group. Furthermore, we found that the Alpha-Tocopherol obviously decreased c-fos gene expression, especially in the spinal cord.

CONCLUSION

Thus, co-administration of Alpha-Tocopherol with Morphine can decrease the adverse effects of nitrite proxy, which is released due to repeated injections of Morphine.

Evaluation of serum pannexin-1 as a prognostic biomarker for traumatic brain injury

BACKGROUND

Pannexin-1 is a type of hexameric plasma membrane channel-forming proteins, and plays a significant role in brain injury. We investigated the potential prognostic value of pannexin-1 in traumatic brain injury.

METHODS

A single peripheral blood sample in 112 patients with severe traumatic brain injury and 112 controls was prospectively collected for subsequent measurement of serum pannexin-1. Clinical follow-up was performed at 6 months. An unfavorable outcome was defined as Glasgow Outcome Scale score of 1-3.

RESULTS

The patients showed markedly higher serum pannexin-1 concentrations than the controls. Among the patients, pannexin-1 concentrations were significantly and negatively correlated with Glasgow coma scale scores. On receiver operating characteristic curve analysis, the predictive value in terms of area under the curve was substantially high for serum pannexin-1 as a predictor for both 6-month mortality and unfavorable outcome. Regression analyses confirmed that there was an increased risk of either 6-month mortality, overall survival or unfavorable outcome associated with serum pannexin-1 concentrations after adjusting for possible confounders.

CONCLUSIONS

Serum pannexin-1 may represent a potential prognostic biomarker for head trauma.

The two faces of pannexins: new roles in inflammation and repair

Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin "protective" and "harmful" functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin's dual roles, related to extracellular ATP and K⁺ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets.

P2X7R: independent modulation of aquaporin 5 expression in CdCl2-injured alveolar epithelial cells

The expression of aquaporin 5 in alveolar epithelial type I cells under conditions of cadmium-induced injury has not yet been discovered. We investigated the effect of the P2X7R agonist BzATP under this condition, since P2X7R is involved in altered regulation of aquaporin 5 in pulmonary fibrosis. CdCl₂/TGF-β1 treatment of lung epithelial MLE-12 cells was leading to increasing P2X7R, and aquaporin 5 protein levels. The aquaporin 5 expression was P2X7R-independent in MLE-12 cells under cadmium, as was shown in blocking experiments with oxATP. Further, the expression of both proteins increased after 24 h CdCl₂/TGF-β1 treatment of precision-cut lung slices, but decreased after 72 h. Using immunohistochemistry, the activation of the P2X7R with the agonist BzATP modulated the aquaporin 5 immunoreactivity in the alveolar epithelium of precision-cut lung slices from wild-type but not from P2X7R knockout mice. Similarly, aquaporin 5 protein was reduced in BzATP-treated immortal lung epithelial E10 cells. Surprisingly, untreated alveolar epithelial type II cells of P2X7R knockouts exhibited a pronounced apical immunoreactivity in addition to the remaining alveolar epithelial type I cells. BzATP exposure did not alter this distribution pattern, but increased the number of apoptotic alveolar epithelial type II cells in wild-type lung slices.

Interactions of Pannexin1 channels with purinergic and NMDA receptor channels

Pannexins are a three-member family of vertebrate plasma membrane spanning molecules that have homology to the invertebrate gap junction forming proteins, the innexins. However, pannexins do not form gap junctions but operate as plasma membrane channels. The best-characterized member of these proteins, Pannexin1 (Panx1) was suggested to be functionally associated with purinergic P2X and N-methyl-D-aspartate (NMDA) receptor channels. Activation of these receptor channels by their endogenous ligands leads to cross-activation of Panx1 channels. This in turn potentiates P2X and NMDA receptor channel signaling. Two potentiation concepts have been suggested: enhancement of the current responses and/or sustained receptor channel activation by ATP released through Panx1 pore and adenosine generated by ectonucleotidase-dependent dephosphorylation of ATP. Here we summarize the current knowledge and hypotheses about interactions of Panx1 channels with P2X and NMDA receptor channels. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Sarcoma family kinase activity is required for cortical spreading depression

Objectives Sarcoma family kinase activity is associated with multiple diseases including ischemia and cancer; however, its role in the mechanism of migraine aura has been less well characterised. This study aims to investigate whether sarcoma family kinase is required for cortical spreading depression. Methods Cortical spreading depression was induced by topical application of K⁺ to the cerebral cortex and was monitored using electrophysiology in rats, and intrinsic optical signal in mouse brain slices. Drugs were perfused into the contralateral cerebral ventricle for pharmacological manipulations in rats. Western blot analysis was used for detecting the level of phosphorylated, and total, sarcoma family kinase in the ipsilateral cortex of rats. Key results The data demonstrate that a single cortical spreading depression in rats induced ipsilateral cortical sarcoma family kinase phosphorylation at the Y416 site. Deactivation of sarcoma family kinase by its inhibitor (3-(4-chlorophenyl) 1-(1,1-dimethylethyl)-1 H-pyrazolo[3,4- dpyrimidin-4-amine) suppressed the elevated enzyme activity and cortical susceptibility to cortical spreading depression. Interestingly, the inhibitory effect of the N-methyl-D-aspartate receptor antagonist NVP-AAM077 on cortical spreading depression was reversed by the sarcoma family kinase activator pYEEI (EPQY(PO₃H₂)EEEIPIYL), suggesting a link between this enzyme and N-methyl-D-aspartate receptors. Similarly, after deactivation of sarcoma family kinase, a reduction of sarcoma family kinase phosphorylation and cortical susceptibility to cortical spreading depression was observed with NVP-AAM077. Conclusions We conclude that activation of sarcoma family kinase is required for cortical spreading depression, and this process is regulated by recruiting N-methyl-D-aspartate receptors. This study provides novel insight for sarcoma family kinase function in the mechanism of migraine aura.

Pannexin 1 Modulates Axonal Growth in Mouse Peripheral Nerves

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

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Regulation of Pannexin 1 Surface Expression by Extracellular ATP: Potential Implications for Nervous System Function in Health and Disease

Pannexin 1 (Panx1) channels are widely recognized for their role in ATP release, and as follows, their function is closely tied to that of ATP-activated P2X7 purinergic receptors (P2X7Rs). Our recent work has shown that extracellular ATP induces clustering of Panx1 with P2X7Rs and their subsequent internalization through a non-canonical cholesterol-dependent mechanism. In other words, we have demonstrated that extracellular ATP levels can regulate the cell surface expression of Panx1. Here we discuss two situations in which we hypothesize that ATP modulation of Panx1 surface expression could be relevant for central nervous system function. The first scenario involves the development of new neurons in the ventricular zone. We propose that ATP-induced Panx1 endocytosis could play an important role in regulating the balance of cell proliferation, survival, and differentiation within this neurogenic niche in the healthy brain. The second scenario relates to the spinal cord, in which we posit that an impairment of ATP-induced Panx1 endocytosis could contribute to pathological neuroplasticity. Together, the discussion of these hypotheses serves to highlight important outstanding questions regarding the interplay between extracellular ATP, Panx1, and P2X7Rs in the nervous system in health and disease.

Mechanisms of pannexin1 channel gating and regulation

Pannexins are a family of integral membrane proteins with distinct post-translational modifications, sub-cellular localization and tissue distribution. Panx1 is the most studied and best-characterized isoform of this gene family. The ubiquitous expression, as well as its function as a major ATP release and nucleotide permeation channel, makes Panx1 a primary candidate for participating in the pathophysiology of CNS disorders. While many investigations revolve around Panx1 functions in health and disease, more recently, details started emerging about mechanisms that control Panx1 channel activity. These advancements in Panx1 biology have revealed that beyond its classical role as an unopposed plasma membrane channel, it participates in alternative pathways involving multiple intracellular compartments, protein complexes and a myriad of extracellular participants. Here, we review recent progress in our understanding of Panx1 at the center of these pathways, highlighting its modulation in a context specific manner. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

P2X7 receptor cross-talk regulates ATP-induced pannexin 1 internalization

In the nervous system, extracellular ATP levels transiently increase in physiological and pathophysiological circumstances, effecting key signalling pathways in plasticity and inflammation through purinergic receptors. Pannexin 1 (Panx1) forms ion- and metabolite-permeable channels that mediate ATP release and are particularly enriched in the nervous system. Our recent study demonstrated that elevation of extracellular ATP triggers Panx1 internalization in a concentration- and time-dependent manner. Notably, this effect was sensitive to inhibition of ionotropic P2X7 purinergic receptors (P2X7Rs). Here, we report our novel findings from the detailed investigation of the mechanism underlying P2X7R-Panx1 cross-talk in ATP-stimulated internalization. We demonstrate that extracellular ATP triggers and is required for the clustering of P2X7Rs and Panx1 on Neuro2a cells through an extracellular physical interaction with the Panx1 first extracellular loop (EL1). Importantly, disruption of P2X7R-Panx1 clustering by mutation of tryptophan 74 within the Panx1 EL1 inhibits Panx1 internalization. Notably, P2X7R-Panx1 clustering and internalization are independent of P2X7R-associated intracellular signalling pathways (Ca²⁺ influx and Src activation). Further analysis revealed that cholesterol is required for ATP-stimulated P2X7R-Panx1 clustering at the cell periphery. Taken together, our data suggest that extracellular ATP induces and is required for Panx1 EL1-mediated, cholesterol-dependent P2X7R-Panx1 clustering and endocytosis. These findings have important implications for understanding the role of Panx1 in the nervous system and provide important new insights into Panx1-P2X7R cross-talk.

Roles of Pannexin-1 Channels in Inflammatory Response through the TLRs/NF-Kappa B Signaling Pathway Following Experimental Subarachnoid Hemorrhage in Rats

Background: Accumulating evidence suggests that neuroinflammation plays a critical role in early brain injury after subarachnoid hemorrhage (SAH). Pannexin-1 channels, as a member of gap junction proteins located on the plasma membrane, releases ATP, ions, second messengers, neurotransmitters, and molecules up to 1 kD into the extracellular space, when activated. Previous studies identified that the opening of Pannexin-1 channels is essential for cellular migration, apoptosis and especially inflammation, but its effects on inflammatory response in SAH model have not been explored yet.

Methods: Adult male Sprague-Dawley rats were divided into six groups: sham group (n = 20), SAH group (n = 20), SAH + LV-Scramble-ShRNA group (n = 20), SAH + LV-ShRNA-Panx1 group (n = 20), SAH + LV-NC group (n = 20), and SAH + LV-Panx1-EGFP group (n = 20). The rat SAH model was induced by injection of 0.3 ml fresh arterial, non-heparinized blood into the prechiasmatic cistern in 20 s. In SAH + LV-ShRNA-Panx1 group and SAH + LV-Panx1-EGFP group, lentivirus was administered via intracerebroventricular injection (i.c.v.) at 72 h before the induction of SAH. The Quantitative real-time polymerase chain reaction, electrophoretic mobility shift assay, enzyme-linked immunosorbent assay, immunofluorescence staining, and western blotting were performed to explore the potential interactive mechanism between Pannexin-1 channels and TLR2/TLR4/NF-κB-mediated signaling pathway. Cognitive and memory changes were investigated by the Morris water maze test.

Results: Administration with LV-ShRNA-Panx1 markedly decreased the expression levels of TLR2/4/NF-κB pathway-related agents in the brain cortex and significantly ameliorated neurological cognitive and memory deficits in this SAH model. On the contrary, administration of LV-Panx1-EGFP elevated the expressions of TLR2/4/NF-κB pathway-related agents, which correlated with augmented neuronal apoptosis.

Conclusion: Pannexin-1 channels may contribute to inflammatory response and neurobehavioral dysfunction through the TLR2/TLR4/NF-κB-mediated pathway signaling after SAH, suggesting a potential role of Pannexin-1 channels could be a potential therapeutic target for the treatment of SAH.

Connexins and pannexins: At the junction of neuro-glial homeostasis & disease

In the central nervous system (CNS), connexin (Cx)s and pannexin (Panx)s are an integral component of homeostatic neuronal excitability and synaptic plasticity. Neuronal Cx gap junctions form electrical synapses across biochemically similar GABAergic networks, allowing rapid and extensive inhibition in response to principle neuron excitation. Glial Cx gap junctions link astrocytes and oligodendrocytes in the pan-glial network that is responsible for removing excitotoxic ions and metabolites. In addition, glial gap junctions help constrain excessive excitatory activity in neurons and facilitate astrocyte Ca²⁺ slow wave propagation. Panxs do not form gap junctions in vivo, but Panx hemichannels participate in autocrine and paracrine gliotransmission, alongside Cx hemichannels. ATP and other gliotransmitters released by Cx and Panx hemichannels maintain physiologic glutamatergic tone by strengthening synapses and mitigating aberrant high frequency bursting. Under pathological depolarizing and inflammatory conditions, gap junctions and hemichannels become dysregulated, resulting in excessive neuronal firing and seizure. In this review, we present known contributions of Cxs and Panxs to physiologic neuronal excitation and explore how the disruption of gap junctions and hemichannels lead to abnormal glutamatergic transmission, purinergic signaling, and seizures.

Hematopoietic pannexin 1 function is critical for neuropathic pain

Neuropathic pain symptoms respond poorly to available therapeutics, with most treated patients reporting unrelieved pain and significant impairment in daily life. Here, we show that Pannexin 1 (Panx1) in hematopoietic cells is required for pain-like responses following nerve injury in mice, and a potential therapeutic target. Panx1 knockout mice (Panx1^-/-) were protected from hypersensitivity in two sciatic nerve injury models. Bone marrow transplantation studies show that expression of functional Panx1 in hematopoietic cells is necessary for mechanical hypersensitivity following nerve injury. Reconstitution of irradiated Panx1 knockout mice with hematopoietic Panx1^-/- cells engineered to re-express Panx1 was sufficient to recover hypersensitivity after nerve injury; this rescue required expression of a Panx1 variant that can be activated by G protein-coupled receptors (GPCRs). Finally, chemically distinct Panx1 inhibitors blocked development of nerve injury-induced hypersensitivity and partially relieved this hypersensitivity after it was established. These studies indicate that Panx1 expressed in immune cells is critical for pain-like effects following nerve injury in mice, perhaps via a GPCR-mediated activation mechanism, and suggest that inhibition of Panx1 may be useful in treating neuropathic pain.

P2X7 receptor antagonists: a patent review (2010-2015)

INTRODUCTION

The P2X7 receptor (P2X7R) is a unique subtype among the family of seven purinergic P2X receptors, which are ATP-gated non-selective cation channels. P2X7R has been reported to have pathological roles in various diseases, including autoimmune diseases such as arthritis and inflammatory bowel disease, neurodegenerative diseases, chronic pain, mood disorders and cancers. Therefore, many pharmaceutical companies have endeavored to develop a clinical candidate targeting P2X7R. Areas covered: This review provides a summary of various patents on chemicals and biologics and their clinical use published between 2010 and 2015. The reader will gain information on structural claims, representative structures and biological activities of recently developed P2X7R antagonists. Expert opinion: P2X7R is a fascinating therapeutic target and potential biomarker for inflammation, pain disorders and cancers. Research on the development of P2X7R antagonists has continually increased despite the failure of AstraZeneca and Merck's compounds in phase II clinical trials. Various scaffolds have been disclosed by several pharmaceutical industries, and some compounds are currently under investigation in clinical trials.

Blocking of the P2X7 receptor inhibits the activation of the MMP-13 and NF-κB pathways in the cartilage tissue of rats with osteoarthritis

P2X purinoceptor 7 (P2X7) receptor (P2X7R) is known to play a significant role in inflammation and pain-causing diseases, including osteoarthritis (OA). However, the mechanisms of action of P2X7R and its role in OA remain unclear. The articular cartilage is the crucial region in which pathological changes occur in OA, involving the dysregulation of degradation and maintenance mechanisms. In this study, we aimed to reveal the molecular mechanisms of action of P2X7R in articular cartilage in OA-induced pain and inflammation by using AZD9056, an antagonist of P2X7R. We created an animal model of OA by using Wistar rats administered (by intra-articular injection) monosodium iodoacetate (MIA), and the rats with OA were then treated with the P2X7R antagonist, AZD9056. We found that treatment with AZD9056 exerted pain-relieving and anti-inflammatory effects. Importantly, we found that the upregulated expression of interleukin (IL)-1β, IL-6, tumor necrosis factor-α (TNF-α), matrix metalloproteinase-13 (MMP-13), substance P (SP) and prostaglandin E2 (PGE2) which was induced by MIA in cartilage tissues was reversed by AZD9056. Western blot analysis was used to examine the expression of inhibitor of nuclear factor-κB (NF-κB) kinase (IKK)α, IKKβ, inhibitor of NF-κB (IκB)α, NF-κB p65 and their phosphorylation forms; they were found to be significantly increased in the knee cartilage tissues from rats with OA; however, opposite effects were observed by the injection of AZD9056. These results implied that P2X7R was associated with the activation of the NF-κB pathway in the development of OA. Our results also revealed that helenalin, an NF-κB pathway inhibitor, decreased the expression of P2X7R, IL-1β, IL-6, TNF-α, SP, PGE2 and MMP-13, which was induced by MIA, in the knee cartilage tissues of rats with OA. On the whole, our findings suggest that P2X7R regulates the MMP-13 and NF-κB pathways in cartilage tissue and mediate OA-induced pain and inflammation.