Pharmacological characterization of pannexin-1 currents expressed in mammalian cells

ATP and Formyl Peptides Facilitate Chemoattractant Leukotriene-B4 Synthesis and Drive Calcium Fluxes, Which May Contribute to Neutrophil Swarming at Sites of Cell Damage and Pathogens Invasion

Here, we demonstrate that human neutrophil interaction with the bacterium Salmonella typhimurium fuels leukotriene B4 synthesis induced by the chemoattractant fMLP. In this work, we found that extracellular ATP (eATP), the amount of which increases sharply during tissue damage, can effectively regulate fMLP-induced leukotriene B4 synthesis. The vector of influence strongly depends on the particular stage of sequential stimulation of neutrophils by bacteria and on the stage at which fMLP purinergic signaling occurs. Activation of 5-lipoxygenase (5-LOX), key enzyme of leukotriene biosynthesis, depends on an increase in the cytosolic concentration of Ca2+. We demonstrate that eATP treatment prior to fMLP, by markedly reducing the amplitude of the fMLP-induced Ca2+ transient jump, inhibits leukotriene synthesis. At the same time, when added with or shortly after fMLP, eATP effectively potentiates arachidonic acid metabolism, including by Ca2+ fluxes stimulation. Flufenamic acid, glibenclamide, and calmodulin antagonist R24571, all of which block calcium signaling in different ways, all suppressed 5-LOX product synthesis in our experimental model, indicating the dominance of calcium-mediated mechanisms in eATP regulatory potential. Investigation into the adhesive properties of neutrophils revealed the formation of cell clusters when adding fMLP to neutrophils exposed to the bacterium Salmonella typhimurium. eATP added simultaneously with fMLP supported neutrophil polarization and clustering. A cell-derived chemoattractant such as leukotriene B4 plays a crucial role in the recruitment of additional neutrophils to the foci of tissue damage or pathogen invasion, and eATP, through the dynamics of changes in [Ca2+]i, plays an important decisive role in fMLP-induced leukotrienes synthesis during neutrophil interactions with the bacterium Salmonella typhimurium.

NMDAR-mediated activation of pannexin1 channels contributes to the detonator properties of hippocampal mossy fiber synapses

Pannexins are large-pore ion channels expressed throughout the mammalian brain that participate in various neuropathologies; however, their physiological roles remain obscure. Here, we report that pannexin1 channels (Panx1) can be synaptically activated under physiological recording conditions in rodent acute hippocampal slices. Specifically, NMDA receptor (NMDAR)-mediated responses at the mossy fiber to CA3 pyramidal cell synapse were followed by a slow postsynaptic inward current that could activate CA3 pyramidal cells but was absent in Panx1 knockout mice. Immunoelectron microscopy revealed that Panx1 was localized near the postsynaptic density. Further, Panx1-mediated currents were potentiated by metabotropic receptors and bidirectionally modulated by burst-timing-dependent plasticity of NMDAR-mediated transmission. Lastly, Panx1 channels were preferentially recruited when NMDAR activation enters a supralinear regime, resulting in temporally delayed burst-firing. Thus, Panx1 can contribute to synaptic amplification and broadening the temporal associativity window for co-activated pyramidal cells, thereby supporting the auto-associative functions of the CA3 region.

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.

Cryo-EM structures of pannexin 1 and 3 reveal differences among pannexin isoforms

Pannexins are single-membrane large-pore channels that release ions and ATP upon activation. Three isoforms of pannexins 1, 2, and 3, perform diverse cellular roles and differ in their pore lining residues. In this study, we report the cryo-EM structure of pannexin 3 at 3.9 Å and analyze its structural differences with pannexin isoforms 1 and 2. The pannexin 3 vestibule has two distinct chambers and a wider pore radius in comparison to pannexins 1 and 2. We further report two cryo-EM structures of pannexin 1, with pore substitutions W74R/R75D that mimic the pore lining residues of pannexin 2 and a germline mutant of pannexin 1, R217H at resolutions of 3.2 Å and 3.9 Å, respectively. Substitution of cationic residues in the vestibule of pannexin 1 results in reduced ATP interaction propensities to the channel. The germline mutant R217H in transmembrane helix 3 (TM3), leads to a partially constricted pore, reduced ATP interaction and weakened voltage sensitivity. The study compares the three pannexin isoform structures, the effects of substitutions of pore and vestibule-lining residues and allosteric effects of a pathological substitution on channel structure and function thereby enhancing our understanding of this vital group of ATP-release channels.

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.

Targeting Pannexin-1 Channels: Addressing the 'Gap' in Chronic Pain

Chronic pain complicates many diseases and is notoriously difficult to treat. In search of new therapeutic targets, pannexin-1 (Panx1) channels have sparked intense interest as a key mechanism involved in a variety of chronic pain conditions. Panx1 channels are transmembrane proteins that release ions and small molecules, such as adenosine triphosphate (ATP). They are expressed along important nodes of the pain pathway, modulating activity of diverse cell types implicated in the development and progression of chronic pain caused by injury or pathology. This review highlights advances that have unlocked the core structure and machinery controlling Panx1 function with a focus on understanding and treating chronic pain.

Skin in the game: pannexin channels in healthy and cancerous skin

The skin is a highly organized tissue composed of multiple layers and cell types that require coordinated cell to cell communication to maintain tissue homeostasis. In skin cancer, this organized structure and communication is disrupted, prompting the malignant transformation of healthy cells into melanoma, basal cell carcinoma or squamous cell carcinoma tumours. One such family of channel proteins critical for cellular communication is pannexins (PANX1, PANX2, PANX3), all of which are present in the skin. These heptameric single-membrane channels act as conduits for small molecules and ions like ATP and Ca2+ but have also been shown to have channel-independent functions through their interacting partners or action in signalling pathways. Pannexins have diverse roles in the skin such as in skin development, aging, barrier function, keratinocyte differentiation, inflammation, and wound healing, which were discovered through work with pannexin knockout mice, organotypic epidermis models, primary cells, and immortalized cell lines. In the context of cutaneous cancer, PANX1 is present at high levels in melanoma tumours and functions in melanoma carcinogenesis, and both PANX1 and PANX3 expression is altered in non-melanoma skin cancer. PANX2 has thus far not been implicated in any skin cancer. This review will discuss pannexin isoforms, structure, trafficking, post-translational modifications, interactome, and channel activity. We will also outline the expression, localization, and function of pannexin channels within the diverse cell types of the epidermis, dermis, hypodermis, and adnexal structures of the skin, and how these properties are exploited or abrogated in instances of skin cancer.

Cx43 Hemichannel and Panx1 Channel Modulation by Gap19 and 10Panx1 Peptides

Cx43 hemichannels (HCs) and Panx1 channels are two genetically distant protein families. Despite the lack of sequence homology, Cx43 and Panx1 channels have been the subject of debate due to their overlapping expression and the fact that both channels present similarities in terms of their membrane topology and electrical properties. Using the mimetic peptides Gap19 and 10Panx1, this study aimed to investigate the cross-effects of these peptides on Cx43 HCs and Panx1 channels. The single-channel current activity from stably expressing HeLa-Cx43 and C6-Panx1 cells was recorded using patch-clamp experiments in whole-cell voltage-clamp mode, demonstrating 214 pS and 68 pS average unitary conductances for the respective channels. Gap19 was applied intracellularly while 10Panx1 was applied extracellularly at different concentrations (100, 200 and 500 μM) and the average nominal open probability (NPo) was determined for each testing condition. A concentration of 100 µM Gap19 more than halved the NPo of Cx43 HCs, while 200 µM 10Panx1 was necessary to obtain a half-maximal NPo reduction in the Panx1 channels. Gap19 started to significantly inhibit the Panx1 channels at 500 µM, reducing the NPo by 26% while reducing the NPo of the Cx43 HCs by 84%. In contrast 10Panx1 significantly reduced the NPo of the Cx43 HCs by 37% at 100 µM and by 83% at 200 µM, a concentration that caused the half-maximal inhibition of the Panx1 channels. These results demonstrate that 10Panx1 inhibits Cx43 HCs over the 100-500 µM concentration range while 500 µM intracellular Gap19 is necessary to observe some inhibition of Panx1 channels.

The Pannexin 1 Channel and the P2X7 Receptor Are in Complex Interplay to Regulate the Release of Soluble Ectonucleotidases in the Murine Bladder Lamina Propria

The bladder urothelium releases ATP into the lamina propria (LP) during filling, which can activate P2X receptors on afferent neurons and trigger the micturition reflex. Effective ATP concentrations are largely dependent on metabolism by membrane-bound and soluble ectonucleotidases (s-ENTDs), and the latter are released in the LP in a mechanosensitive manner. Pannexin 1 (PANX1) channel and P2X7 receptor (P2X7R) participate in urothelial ATP release and are physically and functionally coupled, hence we investigated whether they modulate s-ENTDs release. Using ultrasensitive HPLC-FLD, we evaluated the degradation of 1,N6-etheno-ATP (eATP, substrate) to eADP, eAMP, and e-adenosine (e-ADO) in extraluminal solutions that were in contact with the LP of mouse detrusor-free bladders during filling prior to substrate addition, as an indirect measure of s-ENDTS release. Deletion of Panx1 increased the distention-induced, but not the spontaneous, release of s-ENTDs, whereas activation of P2X7R by BzATP or high concentration of ATP in WT bladders increased both. In Panx1-/- bladders or WT bladders treated with the PANX1 inhibitory peptide 10Panx, however, BzATP had no effect on s-ENTDS release, suggesting that P2X7R activity depends on PANX1 channel opening. We concluded, therefore, that P2X7R and PANX1 are in complex interaction to regulate s-ENTDs release and maintain suitable ATP concentrations in the LP. Thus, while stretch-activated PANX1 hinders s-ENTDS release possibly to preserve effective ATP concentration at the end of bladder filling, P2X7R activation, presumably in cystitis, would facilitate s-ENTDs-mediated ATP degradation to counteract excessive bladder excitability.

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.

Probenecid slows disease progression in a murine model of autosomal dominant polycystic kidney disease

Development of autosomal dominant polycystic kidney disease (ADPKD) involves renal epithelial cell abnormalities. Cystic fluid contains a high level of ATP that, among other effects, leads to a reduced reabsorption of electrolytes in cyst-lining cells, and thus results in cystic fluid accumulation. Earlier, we demonstrated that Pkd1RC/RC mice, a hypomorphic model of ADPKD, exhibit increased expression of pannexin-1, a membrane channel capable of ATP release. In the current study, we found that human ADPKD cystic epithelia have higher pannexin-1 abundance than normal collecting ducts. We hypothesized that inhibition of pannexin-1 function with probenecid can be used to attenuate ADPKD development. Renal function in male and female Pkd1RC/RC and control mice was monitored between 9 and 20 months of age. To test the therapeutic effects of probenecid (a uricosuric agent and a pannexin-1 blocker), osmotic minipumps were implanted in male and female Pkd1RC/RC mice, and probenecid or vehicle was administered for 42 days until 1 year of age. Probenecid treatment improved glomerular filtration rates and slowed renal cyst formation in male mice (as shown in histopathology). The mechanistic effects of probenecid on sodium reabsorption and fluid transport were tested on polarized mpkCCDcl4 cells subjected to short-circuit current measurements, and in 3D cysts grown in Matrigel. In the mpkCCDcl4 epithelial cell line, probenecid elicited higher ENaC currents and attenuated in vitro cyst formation, indicating lower sodium and less fluid retention in the cysts. Our studies open new avenues of research into targeting pannexin-1 in ADPKD pathology.

Pharmacology of pannexin channels

Pannexin channels play fundamental roles in regulating inflammation and have been implicated in many diseases including hypertension, stroke, and neuropathic pain. Thus, the ability to pharmacologically block these channels is a vital component of several therapeutic approaches. Pharmacologic interrogation of model systems also provides a means to discover new roles for pannexins in cell physiology. Here, we review the state of the art for agents that can be used to block pannexin channels, with a focus on chemical pharmaceuticals and peptide mimetics that act on pannexin 1. Guidance on interpreting results obtained with pannexin pharmacologics in experimental systems is discussed, as well as strengths and caveats of different agents, including specificity and feasibility of clinical application.

Connexin hemichannels and pannexin channels in toxicity: Recent advances and mechanistic insights

Connexin hemichannels and pannexin channels are two types of transmembrane channels that allow autocrine/paracrine signalling through the exchange of ions and molecules between the intra- and extracellular compartments. However, owing to the poor selectivity of permeable ions and metabolites, the massive opening of these plasma membrane channels can lead to an excessive influx of toxic substances and an outflux of essential metabolites, such as adenosine triphosphate, glutathione, glutamate and ions, resulting in unbalanced cell homeostasis and impaired cell function. It is becoming increasingly clear that these channels can be activated in response to external stimuli and are involved in toxicity, yet their concrete mechanistic roles in the toxic effects induced by stress and various environmental changes remain poorly defined. This review provides an updated understanding of connexin hemichannels and pannexin channels in response to multiple extrinsic stressors and how these activated channels and their permeable messengers participate in toxicological pathways and processes, including inflammation, oxidative damage, intracellular calcium imbalance, bystander DNA damage and excitotoxicity.

Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are competitive antagonists of the human P2X3 receptor

Introduction: The P2X3 receptor (P2X3R), an ATP-gated non-selective cation channel of the P2X receptor family, is expressed in sensory neurons and involved in nociception. P2X3R inhibition was shown to reduce chronic and neuropathic pain. In a previous screening of 2000 approved drugs, natural products, and bioactive substances, various non-steroidal anti-inflammatory drugs (NSAIDs) were found to inhibit P2X3R-mediated currents.

Methods: To investigate whether the inhibition of P2X receptors contributes to the analgesic effect of NSAIDs, we characterized the potency and selectivity of various NSAIDs at P2X3R and other P2XR subtypes using two-electrode voltage clamp electrophysiology.

Results: We identified diclofenac as a hP2X3R and hP2X2/3R antagonist with micromolar potency (with IC50 values of 138.2 and 76.7 µM, respectively). A weaker inhibition of hP2X1R, hP2X4R, and hP2X7R by diclofenac was determined. Flufenamic acid (FFA) inhibited hP2X3R, rP2X3R, and hP2X7R (IC50 values of 221 µM, 264.1 µM, and ∼900 µM, respectively), calling into question its use as a non-selective ion channel blocker, when P2XR-mediated currents are under study. Inhibition of hP2X3R or hP2X2/3R by diclofenac could be overcome by prolonged ATP application or increasing concentrations of the agonist α,β-meATP, respectively, indicating competition of diclofenac and the agonists. Molecular dynamics simulation showed that diclofenac largely overlaps with ATP bound to the open state of the hP2X3R. Our results suggest a competitive antagonism through which diclofenac, by interacting with residues of the ATP-binding site, left flipper, and dorsal fin domains, inhibits the gating of P2X3R by conformational fixation of the left flipper and dorsal fin domains. In summary, we demonstrate the inhibition of the human P2X3 receptor by various NSAIDs. Diclofenac proved to be the most effective antagonist with a strong inhibition of hP2X3R and hP2X2/3R and a weaker inhibition of hP2X1R, hP2X4R, and hP2X7R.

Discussion: Considering their involvement in nociception, inhibition of hP2X3R and hP2X2/3R by micromolar concentrations of diclofenac, which are rarely reached in the therapeutic range, may play a minor role in analgesia compared to the high-potency cyclooxygenase inhibition but may explain the known side effect of taste disturbances caused by diclofenac.

Pharmacologic inhibition by spironolactone attenuates experimental abdominal aortic aneurysms

Background

Abdominal aortic aneurysms (AAA) are characterized by vascular inflammation and remodeling that can lead to aortic rupture resulting in significant mortality. Pannexin-1 channels on endothelial cells (ECs) can modulate ATP secretion to regulate the pathogenesis of AAA formation. Our hypothesis focused on potential of spironolactone to inhibit EC-mediated ATP release for the mitigation of AAA formation.

Methods

A topical elastase AAA model was used initially in C57BL/6 (wild-type; WT) male mice. Mice were administered either a vehicle control (saline) or spironolactone and analyzed on day 14. In a second chronic AAA model, mice were subjected to elastase and β-aminopropionitrile (BAPN) treatment with/without administration of spironolactone to pre-formed aneurysms starting on day 14 and analyzed on day 28. Aortic diameter was evaluated by video micrometry and aortic tissue was analyzed for cytokine expression and histology. ATP measurement and matrix metalloproteinase (MMP2) activity was evaluated in aortic tissue on days 14 or -28. In vitro studies were performed to evaluate the crosstalk between aortic ECs with macrophages or smooth muscle cells.

Results

In the elastase AAA model, spironolactone treatment displayed a significant decrease in aortic diameter compared to elastase-treated controls on day 14. A significant increase in smooth muscle α-actin expression as well as decrease in elastic fiber disruption and immune cell (macrophages and neutrophils) infiltration was observed in mice treated with spironolactone compared to saline-treated controls. Spironolactone treatment also significantly mitigated pro-inflammatory cytokine expression, MMP2 activity and ATP content in aortic tissue compared to controls. Moreover, in the chronic AAA model, spironolactone treatment of pre-formed aneurysms significantly attenuated vascular inflammation and remodeling to attenuate the progression of AAAs compared to controls. Mechanistically, in vitro data demonstrated that spironolactone treatment attenuates extracellular ATP release from endothelial cells to mitigate macrophage activation (IL-1β and HMGB1 expression) and smooth muscle cell-dependent vascular remodeling (MMP2 activity).

Conclusion

These results demonstrate that spironolactone can mitigate aortic inflammation and remodeling to attenuate AAA formation as well as decrease growth of pre-formed aneurysms via inhibition of EC-dependent ATP release. Therefore, this study implicates a therapeutic application of spironolactone in the treatment of AAAs.

Astrocyte and neuronal Panx1 support long-term reference memory in mice

Pannexin 1 (Panx1) are ubiquitously expressed proteins that form plasma membrane channels permeable to anions and moderate sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels have been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.) but knowledge of extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the 8-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral - CA1 synapses without alterations basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

Astrocyte and Neuronal Panx1 Support Long-Term Reference Memory in Mice

Pannexin 1 (Panx1) is an ubiquitously expressed protein that forms plasma membrane channels permeable to anions and moderate-sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels has been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.), but knowledge of the extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the eight-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral-CA1 synapses without alterations of basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

Pannexin1 channels-a potential therapeutic target in inflammation

An exaggerated inflammatory response is the hallmark of a plethora of disorders. ATP is a central signaling molecule that orchestrates the initiation and resolution of the inflammatory response by enhancing activation of the inflammasome, leukocyte recruitment and activation of T cells. ATP can be released from cells through pannexin (Panx) channels, a family of glycoproteins consisting of three members, Panx1, Panx2, and Panx3. Panx1 is ubiquitously expressed and forms heptameric channels in the plasma membrane mediating paracrine and autocrine signaling. Besides their involvement in the inflammatory response, Panx1 channels have been shown to contribute to different modes of cell death (i.e., pyroptosis, necrosis and apoptosis). Both genetic ablation and pharmacological inhibition of Panx1 channels decrease inflammation in vivo and contribute to a better outcome in several animal models of inflammatory disease involving various organs, including the brain, lung, kidney and heart. Up to date, several molecules have been identified to inhibit Panx1 channels, for instance probenecid (Pbn), mefloquine (Mfq), flufenamic acid (FFA), carbenoxolone (Cbx) or mimetic peptides like 10Panx1. Unfortunately, the vast majority of these compounds lack specificity and/or serum stability, which limits their application. The recent availability of detailed structural information on the Panx1 channel from cryo-electron microscopy studies may open up innovative approaches to acquire new classes of synthetic Panx1 channel blockers with high target specificity. Selective inhibition of Panx1 channels may not only limit acute inflammatory responses but may also prove useful in chronic inflammatory diseases, thereby improving human health. Here, we reviewed the current knowledge on the role of Panx1 in the initiation and resolution of the inflammatory response, we summarized the effects of Panx1 inhibition in inflammatory pathologies and recapitulate current Panx1 channel pharmacology with an outlook towards future approaches.

The Role of Pannexin-1 Channels in HIV and NeuroHIV Pathogenesis

The human immunodeficiency virus-1 (HIV) enters the brain shortly after infection, leading to long-term neurological complications in half of the HIV-infected population, even in the current anti-retroviral therapy (ART) era. Despite decades of research, no biomarkers can objectively measure and, more importantly, predict the onset of HIV-associated neurocognitive disorders. Several biomarkers have been proposed; however, most of them only reflect late events of neuronal damage. Our laboratory recently identified that ATP and PGE₂, inflammatory molecules released through Pannexin-1 channels, are elevated in the serum of HIV-infected individuals compared to uninfected individuals and other inflammatory diseases. More importantly, high circulating ATP levels, but not PGE₂, can predict a decline in cognition, suggesting that HIV-infected individuals have impaired ATP metabolism and associated signaling. We identified that Pannexin-1 channel opening contributes to the high serological ATP levels, and ATP in the circulation could be used as a biomarker of HIV-associated cognitive impairment. In addition, we believe that ATP is a major contributor to chronic inflammation in the HIV-infected population, even in the anti-retroviral era. Here, we discuss the mechanisms associated with Pannexin-1 channel opening within the circulation, as well as within the resident viral reservoirs, ATP dysregulation, and cognitive disease observed in the HIV-infected population.

Effects of Drugs Formerly Suggested for COVID-19 Repurposing on Pannexin1 Channels

Although many efforts have been made to elucidate the pathogenesis of COVID-19, the underlying mechanisms are yet to be fully uncovered. However, it is known that a dysfunctional immune response and the accompanying uncontrollable inflammation lead to troublesome outcomes in COVID-19 patients. Pannexin1 channels are put forward as interesting drug targets for the treatment of COVID-19 due to their key role in inflammation and their link to other viral infections. In the present study, we selected a panel of drugs previously tested in clinical trials as potential candidates for the treatment of COVID-19 early on in the pandemic, including hydroxychloroquine, chloroquine, azithromycin, dexamethasone, ribavirin, remdesivir, favipiravir, lopinavir, and ritonavir. The effect of the drugs on pannexin1 channels was assessed at a functional level by means of measurement of extracellular ATP release. Immunoblot analysis and real-time quantitative reversetranscription polymerase chain reaction analysis were used to study the potential of the drugs to alter pannexin1 protein and mRNA expression levels, respectively. Favipiravir, hydroxychloroquine, lopinavir, and the combination of lopinavir with ritonavir were found to inhibit pannexin1 channel activity without affecting pannexin1 protein or mRNA levels. Thusthree new inhibitors of pannexin1 channels were identified that, though currently not being used anymore for the treatment of COVID-19 patients, could be potential drug candidates for other pannexin1-related diseases.

Endogenous pannexin1 channels form functional intercellular cell-cell channels with characteristic voltage-dependent properties

Pannexin1 is a glycoprotein that has been shown to form functional plasma membrane channels and mediate many cellular signaling pathways. However, the formation and function of pannexin1-based intercellular cell–cell channels in mammalian cells and vertebrate tissue is a question of substantial debate. This work provides robust electrophysiological evidence to demonstrate that endogenously expressed human pannexin1 forms cell–cell channels and lays the groundwork for studying a potential new type of electrical synapses between many mammalian cell types that endogenously express pannexin1.

The occurrence of intercellular channels formed by pannexin1 has been challenged for more than a decade. Here, we provide an electrophysiological characterization of exogenous human pannexin1 (hPanx1) cell–cell channels expressed in HeLa cells knocked out for connexin45. The observed hPanx1 cell–cell channels show two phenotypes: O-state and S-state. The former displayed low transjunctional voltage (V_j) sensitivity and single-channel conductance of ∼175 pS, with a substate of ∼35 pS; the latter showed a peculiar dynamic asymmetry in V_j dependence and single-channel conductance identical to the substate conductance of the O-state. S-state hPanx1 cell–cell channels were also identified between TC620 cells, a human oligodendroglioma cell line that endogenously expresses hPanx1. In these cells, dye and electrical coupling increased with temperature and were strongly reduced after hPanx1 expression was knocked down by small interfering RNA or inhibited with Panx1 mimetic inhibitory peptide. Moreover, cell–cell coupling was augmented when hPanx1 levels were increased with a doxycycline-inducible expression system. Application of octanol, a connexin gap junction (GJ) channel inhibitor, was not sufficient to block electrical coupling between HeLa KO Cx45-hPanx1 or TC620 cell pairs. In silico studies suggest that several arginine residues inside the channel pore may be neutralized by hydrophobic interactions, allowing the passage of DAPI, consistent with dye coupling observed between TC620 cells. These findings demonstrate that endogenously expressed hPanx1 forms intercellular cell–cell channels and their unique properties resemble those described in innexin-based GJ channels. Since Panx1 is ubiquitously expressed, finding conditions to recognize Panx1 cell–cell channels in different cell types might require special attention.

Chloride channels with ClC-1-like properties differentially regulate the excitability of dopamine receptor D1- and D2-expressing striatal medium spiny neurons

Dynamic chloride (Cl^-) regulation is critical for synaptic inhibition. In mature neurons, Cl^- influx and extrusion are primarily controlled by ligand-gated anion channels (GABA_A and glycine receptors) and the potassium chloride cotransporter K⁺-Cl^- cotransporter 2 (KCC2), respectively. Here, we report for the first time, to our knowledge, a presence of a new source of Cl^- influx in striatal neurons with properties similar to chloride voltage-gated channel 1 (ClC-1). Using whole cell patch-clamp recordings, we detected an outwardly rectifying voltage-dependent current that was impermeable to the large anion methanesulfonate (MsO^-). The anionic current was sensitive to the ClC-1 inhibitor 9-anthracenecarboxylic acid (9-AC) and the nonspecific blocker phloretin. The mean fractions of anionic current inhibition by MsO^-, 9-AC, and phloretin were not significantly different, indicating that anionic current was caused by active ClC-1-like channels. In addition, we found that Cl^- current was not sensitive to the transmembrane protein 16A (TMEM16A; Ano1) inhibitor Ani9 and that the outward Cl^- rectification was preserved even at a very high intracellular Ca²⁺ concentration (2 mM), indicating that TMEM16B (Ano2) did not contribute to the total current. Western blotting and immunohistochemical analyses confirmed the presence of ClC-1 channels in the striatum mainly localized to the somata of striatal neurons. Finally, we found that 9-AC decreased action potential firing frequencies and increased excitability in medium spiny neurons (MSNs) expressing dopamine type 1 (D1) and type 2 (D2) receptors in the brain slices, respectively. We conclude that ClC-1-like channels are preferentially located at the somata of MSNs, are functional, and can modulate neuronal excitability.

Mitochondria regulate TRPV4-mediated release of ATP

BACKGROUND AND PURPOSE

Ca2+ influx via TRPV4 channels triggers Ca2+ release from the IP3 -sensitive internal store to generate repetitive oscillations. Although mitochondria are acknowledged regulators of IP3 -mediated Ca2+ release, how TRPV4-mediated Ca2+ signals are regulated by mitochondria is unknown. We show that depolarised mitochondria switch TRPV4 signalling from relying on Ca2+ -induced Ca2+ release at IP3 receptors to being independent of Ca2+ influx and instead mediated by ATP release via pannexins.

EXPERIMENTAL APPROACH

TRPV4-evoked Ca2+ signals were individually examined in hundreds of cells in the endothelium of rat mesenteric resistance arteries using the indicator Cal520.

KEY RESULTS

TRPV4 activation with GSK1016790A (GSK) generated repetitive Ca2+ oscillations that required Ca2+ influx. However, when the mitochondrial membrane potential was depolarised, by the uncoupler CCCP or complex I inhibitor rotenone, TRPV4 activation generated large propagating, multicellular, Ca2+ waves in the absence of external Ca2+ . The ATP synthase inhibitor oligomycin did not potentiate TRPV4-mediated Ca2+ signals. GSK-evoked Ca2+ waves, when mitochondria were depolarised, were blocked by the TRPV4 channel blocker HC067047, the SERCA inhibitor cyclopiazonic acid, the PLC blocker U73122 and the inositol trisphosphate receptor blocker caffeine. The Ca2+ waves were also inhibited by the extracellular ATP blockers suramin and apyrase and the pannexin blocker probenecid.

CONCLUSION AND IMPLICATIONS

These results highlight a previously unknown role of mitochondria in shaping TRPV4-mediated Ca2+ signalling by facilitating ATP release. When mitochondria are depolarised, TRPV4-mediated release of ATP via pannexin channels activates plasma membrane purinergic receptors to trigger IP3 -evoked Ca2+ release.

Endogenous Regulation and Pharmacological Modulation of Sepsis-Induced HMGB1 Release and Action: An Updated Review

Sepsis remains a common cause of death in intensive care units, accounting for approximately 20% of total deaths worldwide. Its pathogenesis is partly attributable to dysregulated inflammatory responses to bacterial endotoxins (such as lipopolysaccharide, LPS), which stimulate innate immune cells to sequentially release early cytokines (such as tumor necrosis factor (TNF) and interferons (IFNs)) and late mediators (such as high-mobility group box 1, HMGB1). Despite difficulties in translating mechanistic insights into effective therapies, an improved understanding of the complex mechanisms underlying the pathogenesis of sepsis is still urgently needed. Here, we review recent progress in elucidating the intricate mechanisms underlying the regulation of HMGB1 release and action, and propose a few potential therapeutic candidates for future clinical investigations.

A physiologic rise in cytoplasmic calcium ion signal increases pannexin1 channel activity via a C-terminus phosphorylation by CaMKII

Pannexin1 (Panx1) channels are ubiquitously expressed in vertebrate cells and are widely accepted as adenosine triphosphate (ATP)-releasing membrane channels. Activation of Panx1 has been associated with phosphorylation in a specific tyrosine residue or cleavage of its C-terminal domains. In the present work, we identified a residue (S394) as a putative phosphorylation site by Ca²⁺/calmodulin-dependent kinase II (CaMKII). In HeLa cells transfected with rat Panx1 (rPanx1), membrane stretch (MS)-induced activation-measured by changes in DAPI uptake rate-was drastically reduced by either knockdown of Piezo1 or pharmacological inhibition of calmodulin or CaMKII. By site-directed mutagenesis we generated rPanx1S394A-EGFP (enhanced green fluorescent protein), which lost its sensitivity to MS, and rPanx1S394D-EGFP, mimicking phosphorylation, which shows high DAPI uptake rate without MS stimulation or cleavage of the C terminus. Using whole-cell patch-clamp and outside-out excised patch configurations, we found that rPanx1-EGFP and rPanx1S394D-EGFP channels showed current at all voltages between ±100 mV, similar single channel currents with outward rectification, and unitary conductance (∼30 to 70 pS). However, using cell-attached configuration we found that rPanx1S394D-EGFP channels show increased spontaneous unitary events independent of MS stimulation. In silico studies revealed that phosphorylation of S394 caused conformational changes in the selectivity filter and increased the average volume of lateral tunnels, allowing ATP to be released via these conduits and DAPI uptake directly from the channel mouth to the cytoplasmic space. These results could explain one possible mechanism for activation of rPanx1 upon increase in cytoplasmic Ca²⁺ signal elicited by diverse physiological conditions in which the C-terminal domain is not cleaved.

Pleiotropic effects of probenecid on three-dimensional cultures of prostate cancer cells

AIMS

Chemoresistance remains a persistent challenge in advanced prostate cancer therapy. Probenecid reportedly inhibits multiple drug-efflux transporters; hence, it can be employed as a potential sensitizer for chemotherapy. In the present study, we evaluated the effects of probenecid on three-dimensional (3D)-cultures of prostate cancer cells.

MAIN METHODS

Prostate cancer cell lines, 22Rv1 and PC-3 were cultured as multicellular tumor spheroids. The effects of probenecid were evaluated using the MTT assay for viability, microscopy for spheroid size, and soft agar colony formation assay for anchorage-independent growth.

KEY FINDINGS

The 3D-cultured 22Rv1 cells were less sensitive to cisplatin and doxorubicin than two-dimensional (2D) cell culture. Co-administration of probenecid at a low (100 or 300 μM), but not high (500 μM), concentration increased the sensitivity to cisplatin or doxorubicin in 22Rv1 spheroids. Probenecid increased the expression of ABCG2, a multidrug resistance transporter, in a dose-dependent manner. Furthermore, treatment with probenecid alone reduced the growth of 22Rv1 spheroids. Conversely, probenecid inhibited spheroid compaction rather than growth inhibition in 3D-cultured PC-3 cells. Moreover, probenecid inhibited colony formation of 22Rv1 and PC-3 cells in soft agar, as well as downregulated focal adhesion kinase (FAK), a crucial factor in anchorage-independent growth.

SIGNIFICANCE

In 3D-cultured prostate cancer cells, probenecid demonstrated pleiotropic effects such as chemosensitization, growth suppression, inhibition of spheroid compaction, and suppression of anchorage-independent growth. Elucidating the detailed mechanism underlying these probenecid actions could result in the identification of novel therapeutic targets toward the advanced prostate cancer.

Structure versus function: Are new conformations of pannexin 1 yet to be resolved?

Pannexin 1 (Panx1) plays a decisive role in multiple physiological and pathological settings, including oxygen delivery to tissues, mucociliary clearance in airways, sepsis, neuropathic pain, and epilepsy. It is widely accepted that Panx1 exerts its role in the context of purinergic signaling by providing a transmembrane pathway for ATP. However, under certain conditions, Panx1 can also act as a highly selective membrane channel for chloride ions without ATP permeability. A recent flurry of publications has provided structural information about the Panx1 channel. However, while these structures are consistent with a chloride selective channel, none show a conformation with strong support for the ATP release function of Panx1. In this Viewpoint, we critically assess the existing evidence for the function and structure of the Panx1 channel and conclude that the structure corresponding to the ATP permeation pathway is yet to be determined. We also list a set of additional topics needing attention and propose ways to attain the large-pore, ATP-permeable conformation of the Panx1 channel.

Mind the gap: connexins and pannexins in platelet function

Connexins are a family of gap junction forming proteins widely expressed by mammalian cells. They assemble into hexameric hemichannels, which can either function independently or dock with opposing hemichannels on apposite cells, forming a gap junction. Pannexins are structurally related to the connexins but extensive glycosylation of these channels prevents docking to form gap junctions and they function as membrane channels. Platelets express pannexin-1 and several connexin family members (Cx37, Cx40 and Cx62). These channels are permeable to molecules up to 1,000 Daltons in molecular mass and functional studies demonstrate their role in non-vesicular ATP release. Channel activation is regulated by (patho)physiological stimuli, such as mechanical stimulation, making them attractive potential drug targets for the management of arterial thrombosis. This review explores the structure and function of platelet pannexin-1 and connexins, the mechanisms by which they are gated and their therapeutic potential.

Identification of pannexin 1-regulated genes, interactome, and pathways in rhabdomyosarcoma and its tumor inhibitory interaction with AHNAK

Rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in children, is an aggressive cancer with a poor prognosis. Despite current management, the 5-year survival rate for patients with metastatic RMS is ∼30%; underscoring the need to develop better treatment strategies. We have recently reported that pannexin 1 (PANX1) levels are downregulated in RMS and that restoring its expression inhibits RMS progression. Here, we have surveyed and characterized the molecular changes induced by PANX1 re-expression in RMS. We cataloged transcriptomic changes in this context by RNA sequencing. At the protein level, we unveiled PANX1 interactors using BioID, complemented by co-immunoprecipitation coupled to high-performance liquid chromatography/electrospray ionization tandem mass spectrometry performed in PANX1-enriched fractions. Using these data, we generated searchable public databases for the PANX1 interactome and changes to the RMS transcriptome occurring when PANX1 expression is restored. STRING network analyses revealed a PANX1 interactome involving plasma membrane and cytoskeleton-associated proteins including the previously undescribed interactor AHNAK. Indeed, AHNAK knockdown abrogated the PANX1-mediated reduction in RMS cell viability and migration. Using these unbiased approaches, we bring insight to the mechanisms by which PANX1 inhibits RMS progression, identifying the cell migration protein AHNAK as a key modifier of PANX1-mediated changes in RMS malignant properties.

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.

Structural and Functional Basis for Understanding the Biological Significance of P2X7 Receptor

The P2X7 receptor (P2X7R) possesses a unique structure associated to an as yet not fully understood mechanism of action that facilitates cell permeability to large ionic molecules through the receptor itself and/or nearby membrane proteins. High extracellular adenosine triphosphate (ATP) levels—inexistent in physiological conditions—are required for the receptor to be triggered and contribute to its role in cell damage signaling. The inconsistent data on its activation pathways and the few studies performed in natively expressed human P2X7R have led us to review the structure, activation pathways, and specific cellular location of P2X7R in order to analyze its biological relevance. The ATP-gated P2X7R is a homo-trimeric receptor channel that is occasionally hetero-trimeric and highly polymorphic, with at least nine human splice variants. It is localized predominantly in the cellular membrane and has a characteristic plasticity due to an extended C-termini, which confers it the capacity of interacting with membrane structural compounds and/or intracellular signaling messengers to mediate flexible transduction pathways. Diverse drugs and a few endogenous molecules have been highlighted as extracellular allosteric modulators of P2X7R. Therefore, studies in human cells that constitutively express P2X7R need to investigate the precise endogenous mediator located nearby the activation/modulation domains of the receptor. Such research could help us understand the possible physiological ATP-mediated P2X7R homeostasis signaling.

A nuclear fluorescent dye identifies pericytes at the neurovascular unit

Perivascular pericytes are key regulators of the blood-brain barrier, vascular development, and cerebral blood flow. Deciphering pericyte roles in health and disease requires cellular tracking; yet, pericyte identification remains challenging. A previous study reported that the far-red fluorophore TO-PRO-3 (642/661), usually employed as a nuclear dye in fixed tissue, was selectively captured by live pericytes from the subventricular zone. Herein, we validated TO-PRO-3 as a specific pericyte tracer in the nervous system (NS). Living pericytes from ex vivo murine hippocampus, cortex, spinal cord, and retina robustly incorporated TO-PRO-3. Classical pericyte immunomarkers such as chondroitin sulphate proteoglycan neuron-glial antigen 2 (NG2) and platelet-derived growth factor receptor beta antigen (PDGFrβ) and the new pericyte dye NeuroTrace 500/525 confirmed cellular specificity of dye uptake. The TO-PRO-3 signal enabled quantification of pericytes density and morphometry; likewise, TO-PRO-3 labeling allowed visualization of pericytes associated with other components of the neurovascular unit. A subset of TO-PRO-3 stained cells expressed the contractile protein α-SMA, indicative of their ability to control the capillary diameter. Uptake of TO-PRO-3 was independent of connexin/pannexin channels but was highly sensitive to temperature and showed saturation, suggesting that a yet unidentified protein-mediated active transport sustained dye incorporation. We conclude that TO-PRO-3 labeling provides a reliable and simple tool for the bioimaging of pericytes in the murine NS microvasculature.

The interplay between α7 nicotinic acetylcholine receptors, pannexin-1 channels and P2X7 receptors elicit exocytosis in chromaffin cells

Pannexin-1 (Panx1) forms plasma membrane channels that allow the exchange of small molecules between the intracellular and extracellular compartments, and are involved in diverse physiological and pathological responses in the nervous system. However, the signaling mechanisms that induce their opening still remain elusive. Here, we propose a new mechanism for Panx1 channel activation through a functional crosstalk with the highly Ca²⁺ permeable α7 nicotinic acetylcholine receptor (nAChR). Consistent with this hypothesis, we found that activation of α7 nAChRs induces Panx1-mediated dye uptake and ATP release in the neuroblastoma cell line SH-SY5Y-α7. Using membrane permeant Ca²⁺ chelators, total internal reflection fluorescence microscopy in SH-SY5Y-α7 cells expressing a membrane-tethered GCAMP3, and Src kinase inhibitors, we further demonstrated that Panx1 channel opening depends on Ca²⁺ signals localized in submembrane areas, as well as on Src kinases. In turn, Panx1 channels amplify cytosolic Ca²⁺ signals induced by the activation of α7 nAChRs, by a mechanism that seems to involve ATP release and P2X7 receptor activation, as hydrolysis of extracellular ATP with apyrase or blockage of P2X7 receptors with oxidized ATP significantly reduces the α7 nAChR-Ca²⁺ signal. The physiological relevance of this crosstalk was also demonstrated in neuroendocrine chromaffin cells, wherein Panx1 channels and P2X7 receptors contribute to the exocytotic release of catecholamines triggered by α7 nAChRs, as measured by amperometry. Together these findings point to a functional coupling between α7 nAChRs, Panx1 channels and P2X7 receptors with physiological relevance in neurosecretion.

Release of ATP by TRPV4 activation is dependent upon the expression of AQP2 in renal cells

Increasing evidence indicates that aquaporins (AQPs) exert an influence in cell signaling by the interplay with the transient receptor potential vanilloid 4 (TRPV4) channel. We previously found that TRPV4 physically and functionally interacts with AQP2 in cortical collecting ducts (CCD) cells, favoring cell volume regulation and cell migration. Because TRPV4 was implicated in ATP release in several tissues, we investigated the possibility that TRPV4/AQP2 interaction influences ATP release in CCD cells. Using two CCD cell lines expressing or not AQP2, we measured extracellular ATP (ATPe) under TRPV4 activation and intracellular Ca²⁺ under ATP addition. We found that AQP2 is critical for the release of ATP induced by TRPV4 activation. This ATP release occurs by an exocytic and a conductive route. ATPe, in turn, stimulates purinergic receptors leading to ATPe-induced ATP release by a Ca²⁺ -dependent mechanism. We propose that AQP2 by modulating Ca²⁺ and ATP differently could explain AQP2-increased cell migration.

Pannexin 1 Channels as a Therapeutic Target: Structure, Inhibition, and Outlook

Pannexin 1 (Panx1) channels are transmembrane proteins that release adenosine triphosphate and play an important role in intercellular communication. They are widely expressed in somatic and nervous system tissues, and their activity has been associated with many pathologies such as stroke, epilepsy, inflammation, and chronic pain. While there are a variety of small molecules known to inhibit Panx1, currently little is known about the mechanism of channel inhibition, and there is a dearth of sufficiently potent and selective drugs targeting Panx1. Herein we provide a review of the current literature on Panx1 structural biology and known pharmacological agents that will help provide a basis for rational development of Panx1 chemical modulators.

Glial ATP and Large Pore Channels Modulate Synaptic Strength in Response to Chronic Inactivity

A key feature of neurotransmission is its ability to adapt to changes in neuronal environment, which is essential for many brain functions. Homeostatic synaptic plasticity (HSP) emerges as a compensatory mechanism used by neurons to adjust their excitability in response to changes in synaptic activity. Recently, glial cells emerged as modulators for neurotransmission by releasing gliotransmitters into the synaptic cleft through pathways that include P2X₇ receptors (P2X₇R), connexons, and pannexons. However, the role of gliotransmission in the activity-dependent adjustment of presynaptic strength is still an open question. Here, we investigated whether glial cells participate in HSP upon chronic inactivity and the role of adenosine triphosphate (ATP), connexin43 hemichannels (Cx43HCs), and pannexin1 (Panx1) channels in this process. We used immunocytochemistry against vesicular glutamate transporter 1 (vGlut1) to estimate changes in synaptic strength in hippocampal dissociated cultures. Pharmacological manipulations indicate that glial-derived ATP and P2X₇R are required for HSP. In addition, inhibition of Cx43 and Panx1 channels reveals a pivotal role for these channels in the compensatory adjustment of synaptic strength, emerging as new pathways for ATP release upon inactivity. The involvement of Panx1 channels was confirmed by using Panx1-deficient animals. Lacking Panx1 in neurons is sufficient to prevent the P2X₇R-dependent upregulation of presynaptic strength; however, the P2X₇R-dependent compensatory adjustment of synapse density requires both neuronal and glial Panx1. Together, our data supports an essential role for glial ATP signaling and Cx43HCs and Panx1 channels in the homeostatic adjustment of synaptic strength in hippocampal cultures upon chronic inactivity.

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.

Pharmacological and genetic characterisation of the canine P2X4 receptor

BACKGROUND AND PURPOSE

P2X4 receptors are emerging therapeutic targets for treating chronic pain and cardiovascular disease. Dogs are well-recognised natural models of human disease, but information regarding P2X4 receptors in dogs is lacking. To aid the development and validation of P2X4 receptor ligands, we have characterised and compared canine and human P2X4 receptors.

EXPERIMENTAL APPROACH

Genomic DNA was extracted from whole blood samples from 101 randomly selected dogs and sequenced across the P2RX4 gene to identify potential missense variants. Recombinant canine and human P2X4 receptors tagged with Emerald GFP were expressed in 1321N1 and HEK293 cells and analysed by immunoblotting and confocal microscopy. In these cells, receptor pharmacology was characterised using nucleotide-induced Fura-2 AM measurements of intracellular Ca²⁺ and known P2X4 receptor antagonists. P2X4 receptor-mediated inward currents in HEK293 cells were assessed by automated patch clamp.

KEY RESULTS

No P2RX4 missense variants were identified in any canine samples. Canine and human P2X4 receptors were localised primarily to lysosomal compartments. ATP was the primary agonist of canine P2X4 receptors with near identical efficacy and potency at human receptors. 2'(3')-O-(4-benzoylbenzoyl)-ATP, but not ADP, was a partial agonist with reduced potency for canine P2X4 receptors compared to the human orthologues. Five antagonists inhibited canine P2X4 receptors, with 1-(2,6-dibromo-4-isopropyl-phenyl)-3-(3-pyridyl)urea displaying reduced sensitivity and potency at canine P2X4 receptors.

CONCLUSION AND IMPLICATIONS

P2X4 receptors are highly conserved across dog pedigrees and display expression patterns and pharmacological profiles similar to human receptors, supporting validation and use of therapeutic agents for P2X4 receptor-related disease onset and management in dogs and humans.

Pannexin 1 activation and inhibition is permeant-selective

KEY POINTS

The large-pore channel pannexin 1 (Panx1) is expressed in many cell types and can open upon different, yet not fully established, stimuli. Panx1 permeability is often inferred from channel permeability to fluorescent dyes, but it is currently unknown whether dye permeability translates to permeability to other molecules. Cell shrinkage and C-terminal cleavage led to a Panx1 open-state with increased permeability to atomic ions (current), but did not alter ethidium uptake. Panx1 inhibitors affected Panx1-mediated ion conduction differently from ethidium permeability, and inhibitor efficiency towards a given molecule therefore cannot be extrapolated to its effects on the permeability of another. We conclude that ethidium permeability does not reflect equal permeation of other molecules and thus is no measure of general Panx1 activity.

ABSTRACT

Pannexin 1 (Panx1) is a large-pore membrane channel connecting the extracellular milieu with the cell interior. While several activation regimes activate Panx1 in a variety of cell types, the selective permeability of an open Panx1 channel remains unresolved: does a given activation paradigm increase Panx1's permeability towards all permeants equally and does fluorescent dye flux serve as a proxy for biological permeation through an open channel? To explore permeant-selectivity of Panx1 activation and inhibition, we employed Panx1-expressing Xenopus laevis oocytes and HEK293T cells. We report that different mechanisms of activation of Panx1 differentially affected ethidium and atomic ion permeation. Most notably, C-terminal truncation or cell shrinkage elevated Panx1-mediated ion conductance, but had no effect on ethidium permeability. In contrast, extracellular pH changes predominantly affected ethidium permeability but not ionic conductance. High [K⁺ ]_o did not increase the flux of either of the two permeants. Once open, Panx1 demonstrated preference for anionic permeants, such as Cl^- , lactate and glutamate, while not supporting osmotic water flow. Panx1 inhibitors displayed enhanced potency towards Panx1-mediated currents compared to that of ethidium uptake. We conclude that activation or inhibition of Panx1 display permeant-selectivity and that permeation of ethidium does not necessarily reflect an equal permeation of smaller biological molecules and atomic ions.

Pannexin1 Is Associated with Enhanced Epithelial-To-Mesenchymal Transition in Human Patient Breast Cancer Tissues and in Breast Cancer Cell Lines

Loss of connexin-mediated cell-cell communication is a hallmark of breast cancer progression. Pannexin1 (PANX1), a glycoprotein that shares structural and functional features with connexins and engages in cell communication with its environment, is highly expressed in breast cancer metastatic foci; however, PANX1 contribution to metastatic progression is still obscure. Here we report elevated expression of PANX1 in different breast cancer (BRCA) subtypes using RNA-seq data from The Cancer Genome Atlas (TCGA). The elevated PANX1 expression correlated with poorer outcomes in TCGA BRCA patients. In addition, gene set enrichment analysis (GSEA) revealed that epithelial-to-mesenchymal transition (EMT) pathway genes correlated positively with PANX1 expression. Pharmacological inhibition of PANX1, in MDA-MB-231 and MCF-7 breast cancer cells, or genetic ablation of PANX1, in MDA-MB-231 cells, reverted the EMT phenotype, as evidenced by decreased expression of EMT markers. In addition, PANX1 inhibition or genetic ablation decreased the invasiveness of MDA-MB-231 cells. Our results suggest PANX1 overexpression in breast cancer is associated with a shift towards an EMT phenotype, in silico and in vitro, attributing to it a tumor-promoting effect, with poorer clinical outcomes in breast cancer patients. This association offers a novel target for breast cancer therapy.

Selective inhibition of Panx1 channels decreases hemostasis and thrombosis in vivo

BACKGROUND

Hemostasis is a tightly regulated physiological process to rapidly induce hemostatic plugs at sites of vascular injury. Inappropriate activation of this process may lead to thrombosis, i.e. pathological blood clot formation in uninjured vessels or on atherosclerotic lesions. ATP release through Pannexin1 (Panx1) membrane channels contributes to collagen-induced platelet aggregation in vitro.

OBJECTIVE

To investigate the effects of genetic and pharmacological inhibition of Panx1 on hemostasis and thrombosis in vivo.

RESULTS

Bleeding time after tail clipping was increased by 2.5-fold in Panx1^-/- mice compared to wild-type controls, suggesting that Panx1 deficiency impairs primary hemostasis. Wire myography on mesenteric arteries revealed diminished vasoconstriction in response to phenylephrine or U446619 in Panx1^-/- mice. Mice with platelet-specific deletion of Panx1 (Panx1^PDel) displayed 2-fold longer tail bleeding times than Panx1^fl/fl controls. Moreover, venous thromboembolism (VTE) after injection of collagen/epinephrine in the jugular vein was reduced in Panx1^-/- and Panx1^PDel mice. Panx1^PDel mice also showed reduced FeCl₃-induced thrombosis in mesenteric arteries. BrilliantBlue-FCF, a Panx1 channel inhibitor, decreased collagen-induced platelet aggregation in vitro, increased tail bleeding time and reduced VTE in wild-type mice. Furthermore, we developed a specific Panx1 blocking antibody targeting a Panx1 extracellular loop, which reduced ATP release from platelets in vitro. Treating wild-type mice with this antibody increased tail bleeding time and decreased VTE compared to control antibody.

CONCLUSIONS

Panx1 channel deletion or inhibition diminishes clot formation during hemostasis and thrombosis in vivo. Blocking Panx1 channels may be an attractive strategy for modulating platelet aggregation in thrombotic disease.

Pannexin-1 channels contribute to seizure generation in human epileptic brain tissue and in a mouse model of epilepsy

Epilepsies are characterized by recurrent seizures, which disrupt normal brain function. Alterations in neuronal excitability and excitation-inhibition balance have been shown to promote seizure generation, yet molecular determinants of such alterations remain to be identified. Pannexin channels are nonselective, large-pore channels mediating extracellular exchange of neuroactive molecules. Recent data suggest that these channels are activated under pathological conditions and regulate neuronal excitability. However, whether pannexin channels sustain or counteract chronic epilepsy in human patients remains unknown. We studied the impact of pannexin-1 channel activation in postoperative human tissue samples from patients with epilepsy displaying epileptic activity ex vivo. These samples were obtained from surgical resection of epileptogenic zones in patients suffering from lesional or drug-resistant epilepsy. We found that pannexin-1 channel activation promoted seizure generation and maintenance through adenosine triphosphate signaling via purinergic 2 receptors. Pharmacological inhibition of pannexin-1 channels with probenecid or mefloquine-two medications currently used for treating gout and malaria, respectively-blocked ictal discharges in human cortical brain tissue slices. Genetic deletion of pannexin-1 channels in mice had anticonvulsant effects when the mice were exposed to kainic acid, a model of temporal lobe epilepsy. Our data suggest a proepileptic role of pannexin-1 channels in chronic epilepsy in human patients and that pannexin-1 channel inhibition might represent an alternative therapeutic strategy for treating lesional and drug-resistant epilepsies.

Structural determinants underlying permeant discrimination of the Cx43 hemichannel

Connexin (Cx) gap junction channels comprise two hemichannels in neighboring cells, and their permeability is well-described, but permeabilities of the single Cx hemichannel remain largely unresolved. Moreover, determination of isoform-specific Cx hemichannel permeability is challenging because of concurrent expression of other channels with similar permeability profiles and inhibitor sensitivities. The mammalian Cx hemichannels Cx30 and Cx43 are gated by extracellular divalent cations, removal of which promotes fluorescent dye uptake in both channels but atomic ion conductance only through Cx30. To determine the molecular determinants of this difference, here we employed chimeras and mutagenesis of predicted pore-lining residues in Cx43. We expressed the mutated channels in Xenopus laevis oocytes to avoid background activity of alternative channels. Oocytes expressing a Cx43 hemichannel chimera containing the N terminus or the first extracellular loop from Cx30 displayed ethidium uptake and, unlike WT Cx43, ion conduction, an observation further supported by molecular dynamics simulations. Additional C-terminal truncation of the chimeric Cx43 hemichannel elicited an even greater ion conductance with a magnitude closer to that of Cx30. The inhibitory profile for the connexin hemichannels depended on the permeant, with conventional connexin hemichannel inhibitors having a higher potency toward the ion conductance pathway than toward fluorescent dye uptake. Our results demonstrate a permeant-dependent, isoform-specific inhibition of connexin hemichannels. They further reveal that the outer segments of the pore-lining region, including the N terminus and the first extracellular loop, together with the C terminus preclude ion conductance of the open Cx43 hemichannel.

The interaction of SiO2 nanoparticles with the neuronal cell membrane: activation of ionic channels and calcium influx

AIM

To clarify the mechanisms of interaction between SiO₂ nanoparticles (NPs) and the plasma membrane of GT1-7 neuroendocrine cells, with focus on the activation of calcium-permeable channels, responsible for the long lasting calcium influx and modulation of the electrical activity in these cells.

MATERIALS & METHODS

Nontoxic doses of SiO₂ NPs were administered to the cells. Calcium imaging and patch clamp techniques were combined with a pharmacological approach.

RESULTS

TRPV4, Cx and Panx-like channels are the major components of the NP-induced inward currents. Preincubation with the antioxidant N-acetyl-L-cysteine strongly reduced the [Ca²⁺]_i increase.

CONCLUSION

These findings suggest that SiO₂ NPs directly activate a complex set of calcium-permeable channels, possibly by catalyzing free radical production.

Constitutive SRC-mediated phosphorylation of pannexin 1 at tyrosine 198 occurs at the plasma membrane

Pannexin 1 (PANX1)-mediated ATP release in vascular smooth muscle coordinates α1-adrenergic receptor (α1-AR) vasoconstriction and blood pressure homeostasis. We recently identified amino acids 198-200 (YLK) on the PANX1 intracellular loop that are critical for α1-AR-mediated vasoconstriction and PANX1 channel function. We report herein that the YLK motif is contained within an SRC homology 2 domain and is directly phosphorylated by SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) at Tyr¹⁹⁸ We demonstrate that PANX1-mediated ATP release occurs independently of intracellular calcium but is sensitive to SRC family kinase (SFK) inhibition, suggestive of channel regulation by tyrosine phosphorylation. Using a PANX1 Tyr¹⁹⁸-specific antibody, SFK inhibitors, SRC knockdown, temperature-dependent SRC cells, and kinase assays, we found that PANX1-mediated ATP release and vasoconstriction involves constitutive phosphorylation of PANX1 Tyr¹⁹⁸ by SRC. We specifically detected SRC-mediated Tyr¹⁹⁸ phosphorylation at the plasma membrane and observed that it is not enhanced or induced by α1-AR activation. Last, we show that PANX1 immunostaining is enriched in the smooth muscle layer of arteries from hypertensive humans and that Tyr¹⁹⁸ phosphorylation is detectable in these samples, indicative of a role for membrane-associated PANX1 in small arteries of hypertensive humans. Our discovery adds insight into the regulation of PANX1 by post-translational modifications and connects a significant purinergic vasoconstriction pathway with a previously identified, yet unexplored, tyrosine kinase-based α1-AR constriction mechanism. This work implicates SRC-mediated PANX1 function in normal vascular hemodynamics and suggests that Tyr¹⁹⁸-phosphorylated PANX1 is involved in hypertensive vascular pathology.

Inhibition of Pannexin 1 Reduces the Tumorigenic Properties of Human Melanoma Cells

Pannexin 1 (PANX1) is a channel-forming glycoprotein expressed in many tissues including the skin. PANX1 channels allow the passage of ions and molecules up to 1 kDa, including ATP and other metabolites. In this study, we show that PANX1 is highly expressed in human melanoma tumors at all stages of disease progression, as well as in patient-derived cells and established melanoma cell lines. Reducing PANX1 protein levels using shRNA or inhibiting channel function with the channel blockers, carbenoxolone (CBX) and probenecid (PBN), significantly decreased cell growth and migration, and increased melanin production in A375-P and A375-MA2 cell lines. Further, treatment of A375-MA2 tumors in chicken embryo xenografts with CBX or PBN significantly reduced melanoma tumor weight and invasiveness. Blocking PANX1 channels with PBN reduced ATP release in A375-P cells, suggesting a potential role for PANX1 in purinergic signaling of melanoma cells. In addition, cell-surface biotinylation assays indicate that there is an intracellular pool of PANX1 in melanoma cells. PANX1 likely modulates signaling through the Wnt/β-catenin pathway, because β-catenin levels were significantly decreased upon PANX1 silencing. Collectively, our findings identify a role for PANX1 in controlling growth and tumorigenic properties of melanoma cells contributing to signaling pathways that modulate melanoma progression.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

The weak voltage dependence of pannexin 1 channels can be tuned by N-terminal modifications

Voltage stimulation is commonly used to study pannexin 1 (Panx1). However, whether Panx1 is a voltage-gated channel remains controversial. Michalski et al. demonstrate that Panx1 is a channel with weak voltage dependence, whose activity can be tuned by N-terminal modifications.

Pannexins are a family of ATP release channels important for physiological and pathological processes like blood pressure regulation, epilepsy, and neuropathic pain. To study these important channels in vitro, voltage stimulation is the most common and convenient tool, particularly for pannexin 1 (Panx1). However, whether Panx1 is a voltage-gated channel remains controversial. Here, we carefully examine the effect of N-terminal modification on voltage-dependent Panx1 channel activity. Using a whole-cell patch-clamp recording technique, we demonstrate that both human and mouse Panx1, with their nativeN termini, give rise to voltage-dependent currents, but only at membrane potentials larger than +100 mV. This weak voltage-dependent channel activity profoundly increases when a glycine–serine (GS) motif is inserted immediately after the first methionine. Single-channel recordings reveal that the addition of GS increases the channel open probability as well as the number of unitary conductance classes. We also find that insertions of other amino acid(s) at the same position mimics the effect of GS. On the other hand, tagging the N terminus with GFP abolishes voltage-dependent channel activity. Our results suggest that Panx1 is a channel with weak voltage dependence whose activity can be tuned by N-terminal modifications.