Interactions of Pannexin1 channels with purinergic and NMDA receptor channels

Pannexins in the musculoskeletal system: new targets for development and disease progression

During cell differentiation, growth, and development, cells can respond to extracellular stimuli through communication channels. Pannexin (Panx) family and connexin (Cx) family are two important types of channel-forming proteins. Panx family contains three members (Panx1-3) and is expressed widely in bone, cartilage and muscle. Although there is no sequence homology between Panx family and Cx family, they exhibit similar configurations and functions. Similar to Cxs, the key roles of Panxs in the maintenance of physiological functions of the musculoskeletal system and disease progression were gradually revealed later. Here, we seek to elucidate the structure of Panxs and their roles in regulating processes such as osteogenesis, chondrogenesis, and muscle growth. We also focus on the comparison between Cx and Panx. As a new key target, Panxs expression imbalance and dysfunction in muscle and the therapeutic potentials of Panxs in joint diseases are also discussed.

Probenecid, an Old Drug with Potential New Uses for Central Nervous System Disorders and Neuroinflammation

Probenecid is an old uricosuric agent used in clinics to treat gout and reduce the renal excretion of antibiotics. In recent years, probenecid has gained attention due to its ability to interact with membrane proteins such as TRPV2 channels, organic anion transporters, and pannexin 1 hemichannels, which suggests new potential therapeutic utilities in medicine. Some current functions of probenecid include their use as an adjuvant to increase the bioavailability of several drugs in the Central Nervous System (CNS). Numerous studies also suggest that this drug has important neuroprotective, antiepileptic, and anti-inflammatory properties, as evidenced by their effect against neurological and neurodegenerative diseases. In these studies, the use of probenecid as a Panx1 hemichannel blocker to reduce neuroinflammation is highlighted since neuroinflammation is a major trigger for diverse CNS alterations. Although the clinical use of probenecid has declined over the years, advances in its use in preclinical research indicate that it may be useful to improve conventional therapies in the psychiatric field where the drugs used have a low bioavailability, either because of a deficient passage through the blood-brain barrier or a high efflux from the CNS or also a high urinary clearance. This review summarizes the history, pharmacological properties, and recent research uses of probenecid and discusses its future projections as a potential pharmacological strategy to intervene in neurodegeneration as an outcome of neuroinflammation.

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.

Human Pannexin 1 channel: Insight in structure-function mechanism and its potential physiological roles

Pannexins, large non-gap junction super family exists in vertebrates, play multiple roles in different cellular functions through their ATP release. Panx1-mediated adenosine 5'-triphosphate (ATP) release plays a vital role in physiological and pathophysiological conditions and is known major extracellular molecule in purinergic signaling. To modulate their function in vivo, a proper regulation of channel is necessary. Post-translational modifications are considered to be some regulating mechanisms for PANX1, while PANX2, PANX3 have been uncharacterized to date. Through their significant evidences, PANXs exclude from gap junction and conduits ATP release and other cellular molecules from cells by various mechanisms. PANX1 is most extensive characterized and implicated in ATP signaling and inflammatory processes. Despite the constant advances, much significance of PANX1 in physiological processes remains elusive. Recently, various research groups along with our group have reported the Cryo-EM structure of Panx1 channel and uncovered the hidden functions in structure-function mechanism as well as to provide the clear understanding in physiological and pathophysiological roles. These research groups reported the novel heptameric structure with contains 4 transmembrane helices (TM), two extracellular loops and one intracellular loop with N and C terminus located at the intracellular side. In addition, the structure contains a large pore of which an inhibitor CBX act as a plug that blocking the passage of substrate. In this context, this review will present current mechanistic understanding in structure and function together with significant physiological roles particularly ATP release in health and disease. As such, this review emphasizes on recent functional properties associated with novel heptameric channel and demystifies channel-mediated ATP release function.

Characterization of three connexin32 genes and their role in inflammation-induced ATP release in the Japanese flounder Paralichthys olivaceus

Extracellular ATP (eATP) is a potent singling molecule in activation of fish innate immunity while the molecular determinants for eATP release in fish were not completely understood. Connexin32 (Cx32) is a member of gap junction protein family that plays important immunological functions in mammals. However, the immune relevance of Cx32 and its role in ATP release in fish has not been investigated. Here, we identified, characterized three Cx32 isoform genes (Cx32.2, Cx32.2x and Cx32.7) from the Japanese flounder Paralichthys olivaceus, and investigated their role in inflammation-induced ATP release in fish. Expression analysis revealed that even though all the three Cx32 genes are constitutively expressed in all examined Japanese flounder tissues, Cx32.2 and Cx32.2x are dominantly expressed in liver, and Cx32.7 is highly expressed in intestine and head kidney macrophages. In addition, we showed that gene expression of all the three Cx32 isoforms was modulated by cAMP stimulation and inflammatory challenges. Furthermore, we revealed that Cx32 expression was upregulated in TNF-alpha overexpressed Japanese flounder FG-9307 cells. Moreover, overexpression of the three Cx32 isoforms significantly reduced the gene expression level of LPS-induced pro-inflammatory cytokine IL-8 and TNF-alpha, indicating that Cx32 is involved in modulating inflammatory response in fish. Finally, we showed that inflammation-induced ATP release was significantly increased in Cx32-overexpressed Japanese flounder FG-9307 cells, and this increased ATP release could be attenuated by pre-incubation with gap junction protein blocker carbenoxolone. Taken together, we for the first time reported the involvement of Cx32 in fish immunity. Our findings suggested that in addition to Cx43 and pannexin1 channels, Cx32 also plays a role in inflammation-induced ATP release in fish.

Molecular Drivers of Platelet Activation: Unraveling Novel Targets for Anti-Thrombotic and Anti-Thrombo-Inflammatory Therapy

Cardiovascular diseases (CVDs) are the leading cause of death globally-partly a consequence of increased population size and ageing-and are major contributors to reduced quality of life. Platelets play a major role in hemostasis and thrombosis. While platelet activation and aggregation are essential for hemostasis at sites of vascular injury, uncontrolled platelet activation leads to pathological thrombus formation and provokes thrombosis leading to myocardial infarction or stroke. Platelet activation and thrombus formation is a multistage process with different signaling pathways involved to trigger platelet shape change, integrin activation, stable platelet adhesion, aggregation, and degranulation. Apart from thrombotic events, thrombo-inflammation contributes to organ damage and dysfunction in CVDs and is mediated by platelets and inflammatory cells. Therefore, in the past, many efforts have been made to investigate specific signaling pathways in platelets to identify innovative and promising approaches for novel antithrombotic and anti-thrombo-inflammatory strategies that do not interfere with hemostasis. In this review, we focus on some of the most recent data reported on different platelet receptors, including GPIb-vWF interactions, GPVI activation, platelet chemokine receptors, regulation of integrin signaling, and channel homeostasis of NMDAR and PANX1.

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.

Functional characterization of purinergic receptor P2Y14 in the Japanese flounder (Paralichthys olivaceus) head kidney macrophages

Extracellular nucleotides and nucleotide sugars are important danger-associated signaling molecules that play critical roles in regulation of immune responses in mammals through activation of purinergic receptors located on the cell surface. However, the immunological role of extracellular UDP-glucose-activated P2Y₁₄ receptor (P2Y₁₄R) in fish still remains unknown. In this study, we identified and characterized a P2Y₁₄R paralog in the Japanese flounder (Paralichthys olivaceus). The mRNA transcripts of P2Y₁₄R are detected in all examined Japanese flounder tissues. Compared with the UDP-activated P2Y₆ receptor, however, P2Y₁₄R gene is highly expressed in Japanese flounder head kidney macrophages (HKMs). In addition, P2Y₁₄R is significantly upregulated following inflammatory stimulation with LPS and poly (I:C) in the HKMs, suggesting a role of P2Y₁₄R in response to inflammation in fish. Furthermore, activation of P2Y₁₄ receptor with its potent and selective agonist MRS 2905 resulted in a decreased expression of LPS-induced pro-inflammatory cytokine IL-1beta gene in the HKMs. In contrast, inhibition of P2Y₁₄ receptor activity or down-regulation of the endogenous expression of P2Y₁₄R by small interfering RNA significantly upregulates the LPS-induced pro-inflammatory cytokine IL-1beta gene expression in the HKMs, demonstrating that P2Y₁₄R is involved in inflammation regulation in fish. Moreover, stimulation of the Japanese flounder HKMs with UDP-glucose evoked a rapid increase of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in a dose- and time-dependent manner, indicating the involvement of P2Y₁₄R in activation of ERK1/2 signaling in fish immune cells. Taken together, we demonstrated that the inducible P2Y₁₄R plays an important role in regulation of fish innate immunity.

P2X7R and PANX-1 channel relevance in a zebrafish larvae copper-induced inflammation model

Copper is a metal that participates in several essential reactions in living organisms, and it has been used as an inflammatory inducing agent in zebrafish larvae. In this study, we evaluated the effect P2X7 receptor and/or pannexin channel 1 (PANX-1) blockage in this inflammation model. To perform the experiments, 7 dpf larvae were exposed to 10 μM of copper and treated with 100 μM probenecid, PANX-1 inhibitor, and/or 300 nM A740003, a P2X7R selective antagonist. Larvae survival was assessed up to 24 h after treatments. The evaluation of larvae behavior was evaluated after acute (4 h) and chronic (24 h) exposure. The parameters of locomotor activity measured were: mobile time, average speed, distance and turn angle. We analyzed the gene expression of the P2X7 receptor, PANX1a and PANX1b channels and interleukins IL-10 and IL-1b after 24 h of treatment. Treatments did not decrease larval survival in the time interval studied. Changes in larvae locomotion were observed after the longest time of exposure to copper and the treatment with probenecid was able to reverse part of the effects caused by copper. No significant difference was observed in the oxidative stress assays and probenecid and copper treatment decrease partially PANX1a gene expression groups. The data presented herein shows the relevance of the blockage of P2X7-PANX-1 in copper-induced inflammation.

Expression analysis of Pannexin1 channel gene in response to immune challenges and its role in infection-induced ATP release in tilapia (Oreochromis niloticus)

ATP released from immune cells plays an important role in activation of host innate immunity. However, the molecular mechanisms for pathogen infection-induced ATP release in fish remains unclear. Pannexin1 (Panx1) is a recently identified ATP release channel important for controlling immune responses. The immune relevance of Panx1 in fish, however, is still poorly understood. In this study, we characterized a Panx1 gene homologue (termed tPanx1) from Nile tilapia (Oreochromis niloticus) and analyzed its expression in response to different immune challenges. We also investigated the role of tPanx1 channel in bacterial infection-induced ATP release. Real-time quantitative PCR analysis revealed that tPanx1 gene is expressed in all tested tissues with predominant expression in intestine. Immune challenges with lipopolysaccharide, polyinosinic-polycytidylic acid and zymosan led to increased gene expression of tPanx1 in tilapia head kidney cells and peripheral blood leucocytes. In addition, tPanx1 gene was up-regulated in hepatopancreas, muscle, spleen, gill, head kidney and blood after Aeromonas hydrophila infection. Furthermore, pharmacological inhibition of tPanx1 channel activity with Panx1 channel inhibitor, carbenoxolone, significantly attenuated A. hydrophila infection-induced ATP release in tilapia head kidney cells. Taken together, our findings suggested that tPanx1 is an important immune response gene involved in bacterial infection-induced ATP release in tilapia O. niloticus.

Functional characterization of purinergic P2Y2 and P2Y12 receptors involved in Japanese flounder (Paralichthys olivaceus) innate immune responses

G-protein-coupled P2Y receptors activated by extracellular nucleotides play important roles under different physiological and pathophysiological conditions in mammals. To investigate the immunological relevance of P2Y receptors in fish, we identified and characterized the P2Y₂ and P2Y₁₂ receptors in Japanese flounder Paralichthys olivaceus. The P. olivaceus P2Y₂ and P2Y₁₂ receptors harbor seven transmembrane domains but share only 24% sequence identity. Real-time PCR analysis revealed the constitutive but unequal mRNA expression pattern of P2Y₂R and P2Y₁₂R in normal Japanese flounder tissues with the dominant expression of P2Y₂R in head kidney and blood and P2Y₁₂R in hepatopancreas. In addition, the expression of P2Y₂ and P2Y₁₂ receptors was markedly modulated by PAMPs stimulation and Edwardsiella tarda infection. Furthermore, blockage of P2Y₁₂R potently increased ADP-activated pro-inflammatory cytokine IL-1beta gene expression in the head kidney macrophages (HKMs). Moreover, inhibition of P2Y₂ and P2Y₁₂ receptor activity with their respective potent antagonists significantly altered some of the LPS-induced pro-inflammatory cytokine gene expression in the HKMs. However, blockade of P2Y₁₂R did not affect the poly(I:C)-induced pro-inflammatory cytokine gene expression examined in the HKMs. Collectively, we have for the first time reported the role of purinergic P2Y₂ and P2Y₁₂ receptors in fish innate immunity. Our findings have also addressed the importance of extracellular ATP and its metabolites in fish innate immune responses.

Sensory Processing and Integration at the Carotid Body Tripartite Synapse: Neurotransmitter Functions and Effects of Chronic Hypoxia

Maintenance of homeostasis in the respiratory and cardiovascular systems depends on reflexes that are initiated at specialized peripheral chemoreceptors that sense changes in the chemical composition of arterial blood. In mammals, the bilaterally-paired carotid bodies (CBs) are the main peripheral chemoreceptor organs that are richly vascularized and are strategically located at the carotid bifurcation. The CBs contribute to the maintenance of O₂, CO₂/H⁺, and glucose homeostasis and have attracted much clinical interest because hyperactivity in these organs is associated with several pathophysiological conditions including sleep apnea, obstructive lung disease, heart failure, hypertension, and diabetes. In response to a decrease in O₂ availability (hypoxia) and elevated CO₂/H⁺ (acid hypercapnia), CB receptor type I (glomus) cells depolarize and release neurotransmitters that stimulate apposed chemoafferent nerve fibers. The central projections of those fibers in turn activate cardiorespiratory centers in the brainstem, leading to an increase in ventilation and sympathetic drive that helps restore blood PO₂ and protect vital organs, e.g., the brain. Significant progress has been made in understanding how neurochemicals released from type I cells such as ATP, adenosine, dopamine, 5-HT, ACh, and angiotensin II help shape the CB afferent discharge during both normal and pathophysiological conditions. However, type I cells typically occur in clusters and in addition to their sensory innervation are ensheathed by the processes of neighboring glial-like, sustentacular type II cells. This morphological arrangement is reminiscent of a "tripartite synapse" and emerging evidence suggests that paracrine stimulation of type II cells by a variety of CB neurochemicals may trigger the release of "gliotransmitters" such as ATP via pannexin-1 channels. Further, recent data suggest novel mechanisms by which dopamine, acting via D2 receptors (D2R), may inhibit action potential firing at petrosal nerve endings. This review will update current ideas concerning the presynaptic and postsynaptic mechanisms that underlie chemosensory processing in the CB. Paracrine signaling pathways will be highlighted, and particularly those that allow the glial-like type II cells to participate in the integrated sensory response during exposures to chemostimuli, including acute and chronic hypoxia.

Role of glial-like type II cells as paracrine modulators of carotid body chemoreception

Mammalian carotid bodies (CB) are chemosensory organs that mediate compensatory cardiorespiratory reflexes in response to low blood PO₂ (hypoxemia) and elevated CO₂/H⁺ (acid hypercapnia). The chemoreceptors are glomus or type I cells that occur in clusters enveloped by neighboring glial-like type II cells. During chemoexcitation type I cells depolarize, leading to Ca²⁺-dependent release of several neurotransmitters, some excitatory and others inhibitory, that help shape the afferent carotid sinus nerve (CSN) discharge. Among the predominantly excitatory neurotransmitters are the purines ATP and adenosine, whereas dopamine (DA) is inhibitory in most species. There is a consensus that ATP and adenosine, acting via postsynaptic ionotropic P2X2/3 receptors and pre- and/or postsynaptic A2 receptors respectively, are major contributors to the increased CSN discharge during chemoexcitation. However, it has been proposed that the CB sensory output is also tuned by paracrine signaling pathways, involving glial-like type II cells. Indeed, type II cells express functional receptors for several excitatory neurochemicals released by type I cells including ATP, 5-HT, ACh, angiotensin II, and endothelin-1. Stimulation of the corresponding G protein-coupled receptors increases intracellular Ca²⁺, leading to the further release of ATP through pannexin-1 channels. Recent evidence suggests that other CB neurochemicals, e.g., histamine and DA, may actually inhibit Ca²⁺ signaling in subpopulations of type II cells. Here, we review evidence supporting neurotransmitter-mediated crosstalk between type I and type II cells of the rat CB. We also consider the potential contribution of paracrine signaling and purinergic catabolic pathways to the integrated sensory output of the CB during chemotransduction.

Ion Channels of Pituitary Gonadotrophs and Their Roles in Signaling and Secretion

Gonadotrophs are basophilic cells of the anterior pituitary gland specialized to secrete gonadotropins in response to elevation in intracellular calcium concentration. These cells fire action potentials (APs) spontaneously, coupled with voltage-gated calcium influx of insufficient amplitude to trigger gonadotropin release. The spontaneous excitability of gonadotrophs reflects the expression of voltage-gated sodium, calcium, potassium, non-selective cation-conducting, and chloride channels at their plasma membrane (PM). These cells also express the hyperpolarization-activated and cyclic nucleotide-gated cation channels at the PM, as well as GABA_A, nicotinic, and purinergic P2X channels gated by γ-aminobutyric acid (GABA), acetylcholine (ACh), and ATP, respectively. Activation of these channels leads to initiation or amplification of the pacemaking activity, facilitation of calcium influx, and activation of the exocytic pathway. Gonadotrophs also express calcium-conducting channels at the endoplasmic reticulum membranes gated by inositol trisphosphate and intracellular calcium. These channels are activated potently by hypothalamic gonadotropin-releasing hormone (GnRH) and less potently by several paracrine calcium-mobilizing agonists, including pituitary adenylate cyclase-activating peptides, endothelins, ACh, vasopressin, and oxytocin. Activation of these channels causes oscillatory calcium release and a rapid gonadotropin release, accompanied with a shift from tonic firing of single APs to periodic bursting type of electrical activity, which accounts for a sustained calcium signaling and gonadotropin secretion. This review summarizes our current understanding of ion channels as signaling molecules in gonadotrophs, the role of GnRH and paracrine agonists in their gating, and the cross talk among channels.