Pannexin1 and Pannexin2 channels show quaternary similarities to connexons and different oligomerization numbers from each other

P2X7 receptors and pannexin1 hemichannels shape presynaptic transmission

Over the last decades, since the discovery of ATP as a transmitter, accumulating evidence has been reported about the role of this nucleotide and purinergic receptors, in particular P2X7 receptors, in the modulation of synaptic strength and plasticity. Purinergic signaling has emerged as a crucial player in orchestrating the molecular interaction between the components of the tripartite synapse, and much progress has been made in how this neuron-glia interaction impacts neuronal physiology under basal and pathological conditions. On the other hand, pannexin1 hemichannels, which are functionally linked to P2X7 receptors, have appeared more recently as important modulators of excitatory synaptic function and plasticity under diverse contexts. In this review, we will discuss the contribution of ATP, P2X7 receptors, and pannexin hemichannels to the modulation of presynaptic strength and its impact on motor function, sensory processing, synaptic plasticity, and neuroglial communication, with special focus on the P2X7 receptor/pannexin hemichannel interplay. We also address major hypotheses about the role of this interaction in physiological and pathological circumstances.

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.

Pannexin-3 stabilizes the transcription factor Bcl6 in a channel-independent manner to protect against vascular oxidative stress

Targeted degradation regulates the activity of the transcriptional repressor Bcl6 and its ability to suppress oxidative stress and inflammation. Here, we report that abundance of endothelial Bcl6 is determined by its interaction with Golgi-localized pannexin 3 (Panx3) and that Bcl6 transcriptional activity protects against vascular oxidative stress. Consistent with data from obese, hypertensive humans, mice with an endothelial cell-specific deficiency in Panx3 had spontaneous systemic hypertension without obvious changes in channel function, as assessed by Ca2+ handling, ATP amounts, or Golgi luminal pH. Panx3 bound to Bcl6, and its absence reduced Bcl6 protein abundance, suggesting that the interaction with Panx3 stabilized Bcl6 by preventing its degradation. Panx3 deficiency was associated with increased expression of the gene encoding the H2O2-producing enzyme Nox4, which is normally repressed by Bcl6, resulting in H2O2-induced oxidative damage in the vasculature. Catalase rescued impaired vasodilation in mice lacking endothelial Panx3. Administration of a newly developed peptide to inhibit the Panx3-Bcl6 interaction recapitulated the increase in Nox4 expression and in blood pressure seen in mice with endothelial Panx3 deficiency. Panx3-Bcl6-Nox4 dysregulation occurred in obesity-related hypertension, but not when hypertension was induced in the absence of obesity. Our findings provide insight into a channel-independent role of Panx3 wherein its interaction with Bcl6 determines vascular oxidative state, particularly under the adverse conditions of obesity.

Age-Dependent Activation of Pannexin1 Function Contributes to the Development of Epileptogenesis in Autosomal Dominant Sleep-related Hypermotor Epilepsy Model Rats

To explore the processes of epileptogenesis/ictogenesis, this study determined the age-dependent development of the functional abnormalities in astroglial transmission associated with pannexin1-hemichannel using a genetic rat model of autosomal dominant sleep-related hypermotor epilepsy (ADSHE) named 'S286L-TG'. Pannexin1 expression in the plasma membrane of primary cultured cortical astrocytes and the orbitofrontal cortex (OFC), which is an ADSHE focus region, were determined using capillary immunoblotting. Astroglial D-serine releases induced by artificial high-frequency oscillation (HFO)-evoked stimulation, the removal of extracellular Ca2+, and the P2X7 receptor agonist (BzATP) were determined using ultra-high performance liquid chromatography (UHPLC). The expressions of pannexin1 in the plasma membrane fraction of the OFC in S286L-TG at four weeks old were almost equivalent when compared to the wild type. The pannexin1 expression in the OFC of the wild type non-statistically decreased age-dependently, whereas that in S286L-TG significantly increased age-dependently, resulting in relatively increasing pannexin1 expression from the 7- (at the onset of interictal discharge) and 10-week-old (after the ADSHE seizure onset) S286L-TG compared to the wild type. However, no functional abnormalities of astroglial pannexin1 expression or D-serine release through the pannexin1-hemichannels from the cultured astrocytes of S286L-TG could be detected. Acutely HFO-evoked stimulation, such as physiological ripple burst (200 Hz) and epileptogenic fast ripple burst (500 Hz), frequency-dependently increased both pannexin1 expression in the astroglial plasma membrane and astroglial D-serine release. Neither the selective inhibitors of pannexin1-hemichannel (10PANX) nor connexin43-hemichannel (Gap19) affected astroglial D-serine release during the resting stage, whereas HFO-evoked D-serine release was suppressed by both inhibitors. The inhibitory effect of 10PANX on the ripple burst-evoked D-serine release was more predominant than that of Gap19, whereas fast ripple burst-evoked D-serine release was predominantly suppressed by Gap19 rather than 10PANX. Astroglial D-serine release induced by acute exposure to BzATP was suppressed by 10PANX but not by Gap19. These results suggest that physiological ripple burst during the sleep spindle plays important roles in the organization of some components of cognition in healthy individuals, but conversely, it contributes to the initial development of epileptogenesis/ictogenesis in individuals who have ADSHE vulnerability via activation of the astroglial excitatory transmission associated with pannexin1-hemichannels.

Enhanced Methodologies for Investigating the Electrophysiological Characteristics of Endogenous Pannexin 1 Intercellular Cell-Cell Channels

Gap junctions, pivotal intercellular conduits, serve as communication channels between adjacent cells, playing a critical role in modulating membrane potential distribution across cellular networks. The family of Pannexin (Panx) proteins, in particular Pannexin1 (Panx1), are widely expressed in vertebrate cells and exhibit sequence homology with innexins, the invertebrate gap junction channel constituents. Despite being ubiquitously expressed, detailed functional and pharmacological properties of Panx1 intercellular cell-cell channels require further investigation. In this chapter, we introduce optimized cell culture methodologies and electrophysiology protocols to expedite the exploration of endogenous Panx1 cell-cell channels in TC620 cells, a human oligodendroglioma cell line that naturally expresses Panx1. We anticipate these refined protocols will significantly contribute to future characterizations of Panx1-based intercellular cell-cell channels across diverse cell types and offer valuable insights into both normal cellular physiology and pathophysiology.

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.

ATP-release pannexin channels are gated by lysophospholipids

Adenosine triphosphate (ATP) serves as an extracellular messenger that mediates diverse cell-to-cell communication. Compelling evidence supports that ATP is released from cells through pannexins, a family of heptameric large pore-forming channels. However, the activation mechanisms that trigger ATP release by pannexins remain poorly understood. Here, we discover lysophospholipids as endogenous pannexin activators, using activity-guided fractionation of mouse tissue extracts combined with untargeted metabolomics and electrophysiology. We show that lysophospholipids directly and reversibly activate pannexins in the absence of other proteins. Molecular docking, mutagenesis, and single-particle cryo-EM reconstructions suggest that lysophospholipids open pannexin channels by altering the conformation of the N-terminal domain. Our results provide a connection between lipid metabolism and ATP signaling, both of which play major roles in inflammation and neurotransmission.

One-Sentence Summary

Untargeted metabolomics discovers a class of messenger lipids as endogenous activators of membrane channels important for inflammation and neurotransmission.

Structural and functional analysis of human pannexin 2 channel

The pannexin 2 channel (PANX2) participates in multiple physiological processes including skin homeostasis, neuronal development, and ischemia-induced brain injury. However, the molecular basis of PANX2 channel function remains largely unknown. Here, we present a cryo-electron microscopy structure of human PANX2, which reveals pore properties contrasting with those of the intensely studied paralog PANX1. The extracellular selectivity filter, defined by a ring of basic residues, more closely resembles that of the distantly related volume-regulated anion channel (VRAC) LRRC8A, rather than PANX1. Furthermore, we show that PANX2 displays a similar anion permeability sequence as VRAC, and that PANX2 channel activity is inhibited by a commonly used VRAC inhibitor, DCPIB. Thus, the shared channel properties between PANX2 and VRAC may complicate dissection of their cellular functions through pharmacological manipulation. Collectively, our structural and functional analysis provides a framework for development of PANX2-specific reagents that are needed for better understanding of channel physiology and pathophysiology.

Gap Junctions and Connexins in Microglia-Related Oxidative Stress and Neuroinflammation: Perspectives for Drug Discovery

Microglia represent the immune system of the brain. Their role is central in two phenomena, neuroinflammation and oxidative stress, which are at the roots of different pathologies related to the central nervous system (CNS). In order to maintain the homeostasis of the brain and re-establish the equilibrium after a threatening imbalance, microglia communicate with each other and other cells within the CNS by receiving specific signals through membrane-bound receptors and then releasing neurotrophic factors into either the extracellular milieu or directly into the cytoplasm of nearby cells, such as astrocytes and neurons. These last two mechanisms rely on the activity of protein structures that enable the formation of channels in the membrane, namely, connexins and pannexins, that group and form gap junctions, hemichannels, and pannexons. These channels allow the release of gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, together with calcium ion (Ca2+), that seem to play a pivotal role in inter-cellular communication. The aim of the present review is focused on the physiology of channel protein complexes and their contribution to neuroinflammatory and oxidative stress-related phenomena, which play a central role in neurodegenerative disorders. We will then discuss how pharmacological modulation of these channels can impact neuroinflammatory phenomena and hypothesize that currently available nutraceuticals, such as carnosine and N-acetylcysteine, can modulate the activity of connexins and pannexins in microglial cells and reduce oxidative stress in neurodegenerative disorders.

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.

New Panx-1 Blockers: Synthesis, Biological Evaluation and Molecular Dynamic Studies

The channel protein Panx-1 is involved in some pathologies, such as epilepsy, ischemic stroke, cancer and Parkinson's disease, as well as in neuropathic pain. These observations make Panx-1 an interesting biological target. We previously published some potent indole derivatives as Panx-1 blockers, and as continuation of the research in this field we report here the studies on additional chemical scaffolds, naphthalene and pyrazole, appropriately substituted with those functions that gave the best results as in our indole series (sulphonamide functions and one/two carboxylic groups) and in Panx-1 blockers reported in the literature (sulphonic acid). Compounds 4 and 13, the latter being an analogue of the drug Probenecid, are the most potent Panx-1 blockers obtained in this study, with I = 97% and I = 93.7% at 50 µM, respectively. Both compounds, tested in a mouse model of oxaliplatin-induced neuropathic pain, showed a similar anti-hypersensitivity profile and are able to significantly increase the mouse pain threshold 45 min after the injection of the doses of 1 nmol and 3 nmol. Finally, the molecular dynamic studies and the PCA analysis have made it possible to identify a discriminating factor able to separate active compounds from inactive ones.

Mechanisms of Pannexin 1 (PANX1) Channel Mechanosensitivity and Its Pathological Roles

Pannexins (PANX) were cloned based on their sequence homology to innexins (Inx), invertebrate gap junction proteins. Although there is no sequence homology between PANX and connexins (Cx), these proteins exhibit similar configurations. The PANX family has three members, PANX1, PANX2 and PANX3. Among them, PANX1 has been the most extensively studied. The PANX1 channels are activated by many factors, including high extracellular K⁺ ([K⁺]_e), high intracellular Ca²⁺ ([Ca²⁺]_i), Src family kinase (SFK)-mediated phosphorylation, caspase cleavage and mechanical stimuli. However, the mechanisms mediating this mechanosensitivity of PANX1 remain unknown. Both force-from-lipids and force-from-filaments models are proposed to explain the gating mechanisms of PANX1 channel mechanosensitivity. Finally, both the physiological and pathological roles of mechanosensitive PANX1 are discussed.

ATP transporters in the joints

Extracellular adenosine triphosphate (ATP) plays a central role in a wide variety of joint diseases. ATP is generated intracellularly, and the concentration of the extracellular ATP pool is determined by the regulation of its transport out of the cell. A variety of ATP transporters have been described, with connexins and pannexins the most commonly cited. Both form intercellular channels, known as gap junctions, that facilitate the transport of various small molecules between cells and mediate cell-cell communication. Connexins and pannexins also form pores, or hemichannels, that are permeable to certain molecules, including ATP. All joint tissues express one or more connexins and pannexins, and their expression is altered in some pathological conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA), indicating that they may be involved in the onset and progression of these pathologies. The aging of the global population, along with increases in the prevalence of obesity and metabolic dysfunction, is associated with a rising frequency of joint diseases along with the increased costs and burden of related illness. The modulation of connexins and pannexins represents an attractive therapeutic target in joint disease, but their complex regulation, their combination of gap-junction-dependent and -independent functions, and their interplay between gap junction and hemichannel formation are not yet fully elucidated. In this review, we try to shed light on the regulation of these proteins and their roles in ATP transport to the extracellular space in the context of joint disease, and specifically OA and RA.

Discovering the Ultimate Limits of Protein Secondary Structure Prediction

Secondary structure prediction (SSP) of proteins is an important structural biology technique with many applications. There have been ~300 algorithms published in the past seven decades with fierce competition in accuracy. In the first 60 years, the accuracy of three-state SSP rose from ~56% to 81%; after that, it has long stayed at 81–86%. In the 1990s, the theoretical limit of three-state SSP accuracy had been estimated to be 88%. Thus, SSP is now generally considered not challenging or too challenging to improve. However, we found that the limit of three-state SSP might be underestimated. Besides, there is still much room for improving segment-based and eight-state SSPs, but the limits of these emerging topics have not been determined. This work performs large-scale sequence and structural analyses to estimate SSP accuracy limits and assess state-of-the-art SSP methods. The limit of three-state SSP is re-estimated to be ~92%, 4–5% higher than previously expected, indicating that SSP is still challenging. The estimated limit of eight-state SSP is 84–87%. Several proposals for improving future SSP algorithms are made based on our results. We hope that these findings will help move forward the development of SSP and all its applications.

Pannexin 1-a novel regulator of acute hypoxic pulmonary vasoconstriction

AIMS

Hypoxic pulmonary vasoconstriction (HPV) is a physiological response to alveolar hypoxia that diverts blood flow from poorly ventilated to better aerated lung areas to optimize ventilation-perfusion matching. Yet, the exact sensory and signaling mechanisms by which hypoxia triggers pulmonary vasoconstriction remain incompletely understood. Recently, ATP release via pannexin 1 (Panx1) and subsequent signaling via purinergic P2Y receptors has been identified as regulator of vasoconstriction in systemic arterioles. Here, we probed for the role of Panx1-mediated ATP release in HPV and chronic hypoxic pulmonary hypertension (PH).

METHODS AND RESULTS

Pharmacological inhibition of Panx1 by probenecid, spironolactone, the Panx1 specific inhibitory peptide (10Panx1) and genetic deletion of Panx1 specifically in smooth muscle attenuated HPV in isolated perfused mouse lungs. In pulmonary artery smooth muscle cells (PASMC), both spironolactone and 10Panx1 attenuated the increase in intracellular Ca2+ concentration ([Ca2+]i) in response to hypoxia. Yet, genetic deletion of Panx1 in either endothelial or smooth muscle cells did not prevent the development of PH in mice. Unexpectedly, ATP release in response to hypoxia was not detectable in PASMC, and inhibition of purinergic receptors or ATP degradation by ATPase failed to attenuate HPV. Rather, transient receptor potential vanilloid 4 (TRPV4) antagonism and Panx1 inhibition inhibited the hypoxia-induced [Ca2+]i increase in PASMC in an additive manner, suggesting that Panx1 regulates [Ca2+]i independently of the ATP-P2Y-TRPV4 pathway. In line with this notion, Panx1 overexpression increased the [Ca2+]i response to hypoxia in HeLa cells.

CONCLUSION

In the present study we identify Panx1 as novel regulator of HPV. Yet, the role of Panx1 in HPV was not attributable to ATP release and downstream signaling via P2Y receptors or TRPV4 activation, but relates to a role of Panx1 as direct or indirect modulator of the PASMC Ca2+ response to hypoxia. Panx1 did not affect the development of chronic hypoxic PH.

TRANSLATIONAL PERSPECTIVE

Hypoxic pulmonary vasoconstriction (HPV) optimizes lung ventilation-perfusion matching, but also contributes to pulmonary pathologies including high altitude pulmonary edema (HAPE) or chronic hypoxic pulmonary hypertension. Here, we demonstrate that pharmaceutical inhibition as well as genetic deletion of the hemichannel pannexin-1 (Panx1) in pulmonary artery smooth muscle cells attenuates the physiological HPV response. Panx1 deficiency did, however, not prevent the development of chronic hypoxic pulmonary hypertension in mice. Panx1 inhibitors such as the mineralocorticoid receptor antagonist spironolactone may thus present a putative strategy for the prevention or treatment of HAPE, yet not for chronic hypoxic lung disease.

Pannexin biology and emerging linkages to cancer

Pannexins are a family of glycoproteins that comprises three members, PANX1, PANX2, and PANX3. The widely expressed and interrogated PANX1 forms heptameric membrane channels that primarily serve to connect the cytoplasm to the extracellular milieu by being selectively permeable to small signaling molecules when activated. Apart from notable exceptions, PANX1 in many tumor cells appears to facilitate tumor growth and metastasis, suggesting that pannexin-blocking therapeutics may have utility in cancer. Attenuation of PANX1 function must also consider the fact that PANX1 is found in stromal cells of the tumor microenvironment (TME), including immune cells. This review highlights the key discoveries of the past 5 years that suggest pannexins facilitate, or in some cases inhibit, tumor cell growth and metastasis via direct protein interactions and through the regulated efflux of signaling molecules.

A secondary structure-based position-specific scoring matrix applied to the improvement in protein secondary structure prediction

Protein secondary structure prediction (SSP) has a variety of applications; however, there has been relatively limited improvement in accuracy for years. With a vision of moving forward all related fields, we aimed to make a fundamental advance in SSP. There have been many admirable efforts made to improve the machine learning algorithm for SSP. This work thus took a step back by manipulating the input features. A secondary structure element-based position-specific scoring matrix (SSE-PSSM) is proposed, based on which a new set of machine learning features can be established. The feasibility of this new PSSM was evaluated by rigid independent tests with training and testing datasets sharing <25% sequence identities. In all experiments, the proposed PSSM outperformed the traditional amino acid PSSM. This new PSSM can be easily combined with the amino acid PSSM, and the improvement in accuracy was remarkable. Preliminary tests made by combining the SSE-PSSM and well-known SSP methods showed 2.0% and 5.2% average improvements in three- and eight-state SSP accuracies, respectively. If this PSSM can be integrated into state-of-the-art SSP methods, the overall accuracy of SSP may break the current restriction and eventually bring benefit to all research and applications where secondary structure prediction plays a vital role during development. To facilitate the application and integration of the SSE-PSSM with modern SSP methods, we have established a web server and standalone programs for generating SSE-PSSM available at http://10.life.nctu.edu.tw/SSE-PSSM.

Pannexin 1 as a driver of inflammation and ischemia-reperfusion injury

Pannexin 1 (Panx1) is a ubiquitously expressed protein forming large conductance channels that are central to many distinct inflammation and injury responses. There is accumulating evidence showing ATP released from Panx1 channels, as well as metabolites, provide effective paracrine and autocrine signaling molecules that regulate different elements of the injury response. As channels with a broad range of permselectivity, Panx1 channels mediate the secretion and uptake of multiple solutes, ranging from calcium to bacterial derived molecules. In this review, we describe how Panx1 functions in response to different pro-inflammatory stimuli, focusing mainly on signaling coordinated by the vasculature. How Panx1 mediates ATP release by injured cells is also discussed. The ability of Panx1 to serve as a central component of many diverse physiologic responses has proven to be critically dependent on the context of expression, post-translational modification, interacting partners, and the mode of stimulation.

Connexin therapeutics: blocking connexin hemichannel pores is distinct from blocking pannexin channels or gap junctions

Compounds that block the function of connexin and pannexin protein channels have been suggested to be valuable therapeutics for a range of diseases. Some of these compounds are now in clinical trials, but for many of them, the literature is inconclusive about the molecular effect on the tissue, despite evidence of functional recovery. Blocking the different channel types has distinct physiological and pathological implications and this review describes current knowledge of connexin and pannexin protein channels, their function as channels and possible mechanisms of the channel block effect for the latest therapeutic compounds. We summarize the evidence implicating pannexins and connexins in disease, considering their homeostatic versus pathological roles, their contribution to excesive ATP release linked to disease onset and progression.

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.

The Multifaceted Role of Astrocyte Connexin 43 in Ischemic Stroke Through Forming Hemichannels and Gap Junctions

Ischemic stroke is a multi-factorial cerebrovascular disease with high worldwide morbidity and mortality. In the past few years, multiple studies have revealed the underlying mechanism of ischemia/reperfusion injury, including calcium overload, amino acid toxicity, oxidative stress, and inflammation. Connexin 43 (Cx43), the predominant connexin protein in astrocytes, has been recently proven to display non-substitutable roles in the pathology of ischemic stroke development and progression through forming gap junctions and hemichannels. Under normal conditions, astrocytic Cx43 could be found in hemichannels or in the coupling with other hemichannels on astrocytes, neurons, or oligodendrocytes to form the neuro-glial syncytium, which is involved in metabolites exchange between communicated cells, thus maintaining the homeostasis of the CNS environment. In ischemic stroke, the phosphorylation of Cx43 might cause the degradation of gap junctions and the opening of hemichannels, contributing to the release of inflammatory mediators. However, the remaining gap junctions could facilitate the exchange of protective and harmful metabolites between healthy and injured cells, protecting the injured cells to some extent or damaging the healthy cells depending on the balance of the exchange of protective and harmful metabolites. In this study, we review the changes in astrocytic Cx43 expression and distribution as well as the influence of these changes on the function of astrocytes and other cells in the CNS, providing new insight into the pathology of ischemic stroke injury; we also discuss the potential of astrocytic Cx43 as a target for the treatment of ischemic stroke.

Controlled release and enhanced biological activity of chitosan-fabricated carbenoxolone nanoparticles

Nanotechnology based antimicrobial drugs are developed to enhance their properties to combat multidrug resistant microbes. Carbenoxolone (CBX) is a semi-synthetic derivate of a natural substance from the licorice plant, with anti- (inflammatory, fungal, viral, microbial, fibrotic and cancer) properties. Though used to treat gastric ulcers, its low aqueous stability, low bioavailability and toxicity limited the drug's utility. To enhance its antimicrobial activity and reduce cytotoxicity, a controlled release nanoformulation was developed using natural biodegradable polymer chitosan (CS) as a carrier which is biocompatible, nontoxic with placid antimicrobial property. UV-visible spectroscopy, electron microscopy, and Fourier transform infrared spectroscopy were used for characterization of the resultant CS-CBX nanoparticles (NPs). They were spherical with uniform dispersion, ~200 nm in size with surface charge of +18.6 mV and drug encapsulation of >80%. Drug release kinetics exhibited a controlled release of 86% over 36 h following zero order kinetics. The anti-microbial activity against common pathogenic Gram -ve and +ve bacteria and yeast increased ~2-fold with a concomitant 4-fold reduction in cytotoxicity assessed using human lung adeno carcinoma (A549) cells. This study demonstrates the affirmative aspects of CS-CBX NPs as a promising antibacterial agent and may facilitate repositioning of the drug for diverse applications.

Role of an Aromatic-Aromatic Interaction in the Assembly and Trafficking of the Zebrafish Panx1a Membrane Channel

Pannexin 1 (Panx1) is a ubiquitously expressed hexameric integral membrane protein known to function as an adenosine triphosphate (ATP) release channel. Panx1 proteins exist in unglycosylated core form (Gly0). They undergo critical post-translational modifications forming the high mannose glycosylation state (Gly1) in the endoplasmic reticulum (ER) and the complex glycosylation state (Gly2) in the Golgi apparatus. The regulation of transition from the ER to the cell membrane is not fully understood. Using site-specific mutagenesis, dye uptake assays, and interaction testing, we identified two conserved aromatic residues, Trp123 and Tyr205, in the transmembrane domains 2 and 3 of the zebrafish panx1a protein. Results suggest that both residues primarily govern the assembly of panx1a subunits into channels, with mutant proteins failing to interact. The results provide insight into a mechanism enabling regulation of Panx1 oligomerization, glycosylation, and trafficking.

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.

A novel motif in the proximal C-terminus of Pannexin 1 regulates cell surface localization

The Pannexin 1 (Panx1) ion and metabolite channel is expressed in a wide variety of cells where it regulates a number of cell behaviours including proliferation and differentiation. Panx1 is expressed on the cell surface as well as intracellular membranes. Previous work suggests that a region within the proximal Panx1 C-terminus (Panx1CT) regulates cell surface localization. Here we report the discovery of a putative leucine-rich repeat (LRR) motif in the proximal Panx1CT necessary for Panx1 cell surface expression in HEK293T cells. Deletion of the putative LRR motif results in significant loss of Panx1 cell surface distribution. Outcomes of complementary cell surface oligomerization and glycosylation state analyses were consistent with reduced cell surface expression of Panx1 LRR deletion mutants. Of note, the oligomerization analysis revealed the presence of putative dimers and trimers of Panx1 at the cell surface. Expression of Panx1 increased HEK293T cell growth and reduced doubling time, while expression of a Panx1 LRR deletion mutant (highly conserved segment) did not reproduce this effect. In summary, here we discovered the presence of a putative LRR motif in the Panx1CT that impacts on Panx1 cell surface localization. Overall these findings provide new insights into the molecular mechanisms underlying C-terminal regulation of Panx1 trafficking and raise potential new lines of investigation with respect to Panx1 oligomerization and glycosylation.

Destination and consequences of Panx1 and mutant expression in polarized MDCK cells

The channel-forming membrane glycoprotein pannexin 1 (Panx1) is best characterized as an ATP release channel. To investigate the trafficking and sorting of Panx1, we used polarized MDCK cells and non-polarized BICR-M1R_k cells to track the fate of GFP-tagged Panx1. In non-polarized cells, Panx1 was found throughout the plasma membrane, including the lamellipodia of the tumor cells and the cell surface-targeting domain was mapped to residues 307-379. Panx1 was preferentially enriched at the apical membrane domain of polarized MDCK cells grown as monolayer sheets or as spheroids. Residual Panx1 localized within basolateral membranes of polarized MDCK cells was independent of a putative dileucine sorting motif LL365/6 found within the C-terminal of Panx1. Unexpectedly, stable expression of a Panx1 mutant, where a putative tyrosine-based basolateral sorting motif (YxxØ) was mutated (Y308F), or a truncated Δ379 Panx1 mutant, caused MDCK cells to lose cell-cell contacts and their ability to polarize as they underwent a switch to a more fibroblast-like phenotype. We conclude that Panx1 is preferentially delivered to the apical domain of polarized epithelial cells, and Panx1 mutants drive phenotypic changes to MDCK cells preventing their polarization.

Facets of Communication: Gap Junction Ultrastructure and Function in Cancer Stem Cells and Tumor Cells

Gap junction proteins are expressed in cancer stem cells and non-stem cancer cells of many tumors. As the morphology and assembly of gap junction channels are crucial for their function in intercellular communication, one focus of our review is to outline the data on gap junction plaque morphology available for cancer cells. Electron microscopic studies and freeze-fracture analyses on gap junction ultrastructure in cancer are summarized. As the presence of gap junctions is relevant in solid tumors, we exemplarily outline their role in glioblastomas and in breast cancer. These were also shown to contain cancer stem cells, which are an essential cause of tumor onset and of tumor transmission into metastases. For these processes, gap junctional communication was shown to be important and thus we summarize, how the expression of gap junction proteins and the resulting communication between cancer stem cells and their surrounding cells contributes to the dissemination of cancer stem cells via blood or lymphatic vessels. Based on their importance for tumors and metastases, future cancer-specific therapies are expected to address gap junction proteins. In turn, gap junctions also seem to contribute to the unattainability of cancer stem cells by certain treatments and might thus contribute to therapeutic resistance.

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.

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.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Inhibition of pannexin-1 channel activity by adiponectin in podocytes: Role of acid ceramidase activation

The pannexin-1 (Panx1) channel has been reported to mediate the release of ATP that is involved in local tissue inflammation, obesity, and many chronic degenerative diseases. It remains unknown whether Panx1 is present in podocytes and whether this channel in podocytes mediates ATP release leading to glomerular inflammation or fibrosis. To answer these questions, we first characterized the expression of Panx channels in podocytes. Among the three known pannexins, Panx1 was the most enriched in podocytes, either cultured or native in mouse glomeruli. Using a Port-a-Patch planar patch-clamp system, we recorded a large voltage-gated outward current through podocyte membrane under the Cs⁺in/Na⁺out gradient. Substitution of gluconate or aspartate for chloride in the bath solution blocked voltage-gated outward currents and shifted the reversal potential of Panx1 currents to the right, indicating the anion permeability of this channel. Pharmacologically, the recorded voltage-gated outward currents were substantially attenuated by specific Panx1 channel inhibitors. Given the anti-inflammatory and intracellular ATP restorative effects of adiponectin, we tested whether this adipokine inhibits Panx1 channel activity to block ATP release. Adiponectin blocked Panx1 channel activity in podocytes. Mechanistically, inhibition of acid ceramidase (AC) remarkably enhanced Panx1 channel activity under control conditions and prevented the inhibition of Panx1 channel by adiponectin. Correspondingly, intracellular addition of AC products, sphingosine or sphingosine-1-phosphate (S1P), blocked Panx1 channel activity, while elevation of intracellular ceramide had no effect on Panx1 channel activity. These results suggest that adiponectin inhibits Panx1 channel activity in podocytes through activation of AC and associated elevation of intracellular S1P.

N-Glycosylation Regulates Pannexin 2 Localization but Is Not Required for Interacting with Pannexin 1

Pannexins (Panx1, 2, 3) are channel-forming glycoproteins expressed in mammalian tissues. We previously reported that N-glycosylation acts as a regulator of the localization and intermixing of Panx1 and Panx3, but its effects on Panx2 are currently unknown. Panx1 and Panx2 intermixing can regulate channel properties, and both pannexins have been implicated in neuronal cell death after ischemia. Our objectives were to validate the predicted N-glycosylation site of Panx2 and to study the effects of Panx2 glycosylation on localization and its capacity to interact with Panx1. We used site-directed mutagenesis, enzymatic de-glycosylation, cell-surface biotinylation, co-immunoprecipitation, and confocal microscopy. Our results showed that N86 is the only N-glycosylation site of Panx2. Panx2 and the N86Q mutant are predominantly localized to the endoplasmic reticulum (ER) and cis-Golgi matrix with limited cell surface localization was seen only in the presence of Panx1. The Panx2 N86Q mutant is glycosylation-deficient and tends to aggregate in the ER reducing its cell surface trafficking but it can still interact with Panx1. Our study indicates that N-glycosylation may be important for folding and trafficking of Panx2. We found that the un-glycosylated forms of Panx1 and 2 can readily interact, regulating their localization and potentially their channel function in cells where they are co-expressed.

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

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

Intracellular and extracellular loops of LRRC8 are essential for volume-regulated anion channel function

The volume-regulated anion channel (VRAC) is expressed ubiquitously in vertebrate cells and mediates swelling-induced release of Cl^- and organic solutes. Recent studies by several laboratories have demonstrated conclusively that VRAC is encoded by members of the leucine-rich repeat containing 8 (Lrrc8) gene family, which comprises five members, termed Lrrc8a-e. Numerous observations indicate that VRAC is a heteromeric channel comprising the essential subunit LRRC8A and one or more of the other LRRC8 paralogs. Here we demonstrate that the intracellular loop (IL) connecting transmembrane domains 2 and 3 of LRRC8A and the first extracellular loop (EL1) connecting transmembrane domains 1 and 2 of LRRC8C, LRRC8D, or LRRC8E are both essential for VRAC activity. We generate homomeric VRACs by replacing EL1 of LRRC8A with that of LRRC8C and demonstrate normal regulation by cell swelling and shrinkage. We also observe normal volume-dependent regulation in VRAC homomers in which the IL of LRRC8C, LRRC8D, or LRRC8E is replaced with the LRRC8A IL. A 25-amino acid sequence unique to the LRRC8A IL is sufficient to generate homomeric VRAC activity when inserted into the corresponding region of LRRC8C and LRRC8E. LRRC8 chimeras containing these partial LRRC8A IL sequences exhibit altered anion permeability, rectification, and voltage sensitivity, suggesting that the LRRC8A IL plays a role in VRAC pore structure and function. Our studies provide important new insights into the structure/function roles of the LRRC8 EL1 and IL. Homomeric LRRC8 channels will simplify future studies aimed at understanding channel structure and the longstanding and vexing problem of how VRAC is regulated by cell volume changes.

Pannexin-1 in Human Lymphatic Endothelial Cells Regulates Lymphangiogenesis

The molecular mechanisms governing the formation of lymphatic vasculature are not yet well understood. Pannexins are transmembrane proteins that form channels which allow for diffusion of ions and small molecules (<1 kDa) between the extracellular space and the cytosol. The expression and function of pannexins in blood vessels have been studied in the last few decades. Meanwhile, no studies have been conducted to evaluate the role of pannexins during human lymphatic vessel formation. Here we show, using primary human dermal lymphatic endothelial cells (HDLECs), pharmacological tools (probenecid, Brilliant Blue FCF, mimetic peptides [¹⁰Panx]) and siRNA-mediated knockdown that Pannexin-1 is necessary for capillary tube formation on Matrigel and for VEGF-C-induced invasion. These results newly identify Pannexin-1 as a protein highly expressed in HDLECs and its requirement during in vitro lymphangiogenesis.

Cationic control of Panx1 channel function

The sequence and predicted membrane topology of pannexin1 (Panx1) places it in the family of gap junction proteins. However, rather than forming gap junction channels, Panx1 forms channels in the nonjunctional membrane. Panx1 operates in two distinct open states, depending on the mode of stimulation. The exclusively voltage-gated channel has a small conductance (<100 pS) and is highly selective for the flux of chloride ions. The Panx1 channel activated by various physiological stimuli or by increased concentrations of extracellular potassium ions has a large conductance (~500 pS, however, with multiple, long-lasting subconductance states) and is nonselectively permeable to small molecules, including ATP. To test whether the two open conformations also differ pharmacologically, the effects of di-and trivalent cations on the two Panx1 channel conformations were investigated. The rationale for this venture was that, under certain experimental conditions, ATP release from cells can be inhibited by multivalent cations, yet the literature indicates that the ATP release channel Panx1 is not affected by these ions. Consistent with previous reports, the Panx1 channel was not activated by removal of extracellular Ca²⁺ and the currents through the voltage-activated channel were not altered by Ca²⁺, Zn²⁺, Ba²⁺, or Gd³⁺. In contrast, the Panx1 channel activated to the large channel conformation by extracellular K⁺, osmotic stress, or low oxygen was inhibited by the multivalent cations in a dose-dependent way. Thus, monovalent cations activated the Panx1 channel from the closed state to the "large" conformation, while di- and trivalent cations exclusively inhibited this large channel conformation.

The Neuroglial Dialog Between Cannabinoids and Hemichannels

The formation of gap junctions was initially thought to be the central role of connexins, however, recent evidence had brought to light the high relevance of unopposed hemichannels as an independent mechanism for the selective release of biomolecules during physiological and pathological conditions. In the healthy brain, the physiological opening of astrocyte hemichannels modulates basal excitatory synaptic transmission. At the other end, the release of potentially neurotoxic compounds through astroglial hemichannels and pannexons has been insinuated as one of the functional alterations that negatively affect the progression of multiple brain diseases. Recent insights in this matter have suggested encannabinoids (eCBs) as molecules that could regulate the opening of these channels during diverse conditions. In this review, we discuss and hypothesize the possible interplay between the eCB system and the hemichannel/pannexon-mediated signaling in the inflamed brain and during event of synaptic plasticity. Most findings indicate that eCBs seem to counteract the activation of major neuroinflammatory pathways that lead to glia-mediated production of TNF-α and IL-1β, both well-known triggers of astroglial hemichannel opening. In contrast to the latter, in the normal brain, eCBs apparently elicit the Ca²⁺-activation of astrocyte hemichannels, which could have significant consequences on eCB-dependent synaptic plasticity.

Interactions of Pannexin1 channels with purinergic and NMDA receptor channels

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

Synchronized roles of pannexin and connexin in nasal mucosal epithelia

BACKGROUND

Nasal mucosal epithelial cells express connexins, the prototypical gap junction proteins, and pannexins, a new family of channel proteins homologous to the invertebrate gap junction proteins. The physiological and pathophysiological roles of these transmembrane proteins in nasal mucosa are largely still unknown.

PURPOSE

Pannexins participate in ATP release into the extracellular space in various tissues, and ATP plays important roles in mucociliary clearance, especially by regulating ciliary beat activity. Therefore, we focused on the functional relationship between connexins, pannexin-1, ATP release, and mucociliary clearance in nasal epithelia.

RESULTS AND CONCLUSIONS

Connexins participate in the generation of intercellular calcium waves, in which calcium-mediated signaling responses spread to contiguous cells through the gap junction formed by connexins to transmit calcium signaling throughout the airway epithelium. Pannexins in the nasal mucosa may contribute to not only ciliary beat modulation via ATP release, but also regulation of mucus blanket components via H₂O efflux. The synchronized roles of pannexin and connexin may provide a new insight into effective mucociliary clearance systems in nasal mucosa.

Revisiting multimodal activation and channel properties of Pannexin 1

Pannexin 1 (Panx1) forms plasma membrane ion channels that are widely expressed throughout the body. Panx1 activation results in the release of nucleotides such as adenosine triphosphate and uridine triphosphate. Thus, these channels have been implicated in diverse physiological and pathological functions associated with purinergic signaling, such as apoptotic cell clearance, blood pressure regulation, neuropathic pain, and excitotoxicity. In light of this, substantial attention has been directed to understanding the mechanisms that regulate Panx1 channel expression and activation. Here we review accumulated evidence for the various activation mechanisms described for Panx1 channels and, where possible, the unitary channel properties associated with those forms of activation. We also emphasize current limitations in studying Panx1 channel function and propose potential directions to clarify the exciting and expanding roles of Panx1 channels.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Investigation of LRRC8-Mediated Volume-Regulated Anion Currents in Xenopus Oocytes

Volume-regulated anion channels (VRACs) play an important role in controlling cell volume by opening upon cell swelling. Recent work has shown that heteromers of LRRC8A with other LRRC8 members (B, C, D, and E) form the VRAC. Here, we used Xenopus oocytes as a simple system to study LRRC8 proteins. We discovered that adding fluorescent proteins to the C-terminus resulted in constitutive anion channel activity. Using these constructs, we reproduced previous findings indicating that LRRC8 heteromers mediate anion and osmolyte flux with subunit-dependent kinetics and selectivity. Additionally, we found that LRRC8 heteromers mediate glutamate and ATP flux and that the inhibitor carbenoxolone acts from the extracellular side, binding to probably more than one site. Our results also suggest that the stoichiometry of LRRC8 heteromers is variable, with a number of subunits ≥6, and that the heteromer composition depends on the relative expression of different subunits. The system described here enables easy structure-function analysis of LRRC8 proteins.

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

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

Gap junction gene and protein families: Connexins, innexins, and pannexins

Gap junction channels facilitate the intercellular exchange of ions and small molecules. While this process is critical to all multicellular organisms, the proteins that form gap junction channels are not conserved. Vertebrate gap junctions are formed by connexins, while invertebrate gap junctions are formed by innexins. Interestingly, vertebrates and lower chordates contain innexin homologs, the pannexins, which also form channels, but rarely (if ever) make intercellular channels. While the connexin and the innexin/pannexin polypeptides do not share significant sequence similarity, all three of these protein families share a similar membrane topology and some similarities in quaternary structure. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Exciting and not so exciting roles of pannexins

It is the current view that purinergic signaling regulates many physiological functions. Pannexin1 (Panx1), a member of the gap junction family of proteins is an ATP releasing channel that plays important physio-pathological roles in various tissues, including the CNS. Upon binding to purinergic receptors expressed in neural cells, ATP triggers cellular responses including increased cell proliferation, cell morphology changes, release of cytokines, and regulation of neuronal excitability via release of glutamate, GABA and ATP itself. Under pathological conditions such as ischemia, trauma, inflammation, and epilepsy, extracellular ATP concentrations increases drastically but the consequences of this surge is still difficult to characterize due to its rapid metabolism in ADP and adenosine, the latter having inhibitory action on neuronal activity. For seizures, for instance, the excitatory effect of ATP on neuronal activity is mainly related to its action of P2X receptors, while the inhibitory effects are related to activation of P1, adenosine receptors. Here we provide a mini review on the properties of pannexins with a main focus on Panx1 and its involvement in seizure activity. Although there are only few studies implicating Panx1 in seizures, they are illustrative of the dual role that Panx1 has on neuronal excitability.

Transcriptional and post-translational regulation of pannexins

Pannexins are a 3-membered family of proteins that form large pore ion and metabolite channels in vertebrates. The impact of pannexins on vertebrate biology is intricately tied to where and when they are expressed, and how they are modified, once produced. The purpose of this review is therefore to outline our current understanding of transcriptional and post-translational regulation of pannexins. First, we briefly summarize their discovery and characteristics. Next, we describe several aspects of transcriptional regulation, including cell and tissue-specific expression, dynamic expression over development and disease, as well as new insights into the underlying molecular machinery involved. Following this, we delve into the role of post-translational modifications in the regulation of trafficking and channel properties, highlighting important work on glycosylation, phosphorylation, S-nitrosylation and proteolytic cleavage. Embedded throughout, we also highlight important knowledge gaps and avenues of future research. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Gap junction proteins and the wiring (Rewiring) of neuronal circuits

The unique morphology and pattern of synaptic connections made by a neuron during development arise in part by an extended period of growth in which cell-cell interactions help to sculpt the arbor into its final shape, size, and participation in different synaptic networks. Recent experiments highlight a guiding role played by gap junction proteins in controlling this process. Ectopic and overexpression studies in invertebrates have revealed that the selective expression of distinct gap junction genes in neurons and glial cells is sufficient to establish selective new connections in the central nervous systems of the leech (Firme et al. [2012]: J Neurosci 32:14265-14270), the nematode (Rabinowitch et al. [2014]: Nat Commun 5:4442), and the fruit fly (Pézier et al., 2016: PLoS One 11:e0152211). We present here an overview of this work and suggest that gap junction proteins, in addition to their synaptic/communicative functions, have an instructive role as recognition and adhesion factors. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 575-586, 2017.

Regulation of Connexin-Based Channels by Fatty Acids

In this mini-review, we briefly summarize the current knowledge about the effects of fatty acids (FAs) on connexin-based channels, as well as discuss the limited information about the impact FAs may have on pannexins (Panxs). FAs regulate diverse cellular functions, some of which are explained by changes in the activity of channels constituted by connexins (Cxs) or Panxs, which are known to play critical roles in maintaining the functional integrity of diverse organs and tissues. Cxs are transmembrane proteins that oligomerize into hexamers to form hemichannels (HCs), which in turn can assemble into dodecamers to form gap junction channels (GJCs). While GJCs communicate the cytoplasm of contacting cells, HCs serve as pathways for the exchange of ions and small molecules between the intra and extracellular milieu. Panxs, as well as Cx HCs, form channels at the plasma membrane that enable the interchange of molecules between the intra and extracellular spaces. Both Cx- and Panx-based channels are controlled by several post-translational modifications. However, the mechanism of action of FAs on these channels has not been described in detail. It has been shown however that FAs frequently decrease GJC-mediated cell-cell communication. The opposite effect also has been described for HC or Panx-dependent intercellular communication, where, the acute FA effect can be reversed upon washout. Additionally, changes in GJCs mediated by FAs have been associated with post-translational modifications (e.g., phosphorylation), and seem to be directly related to chemical properties of FAs (e.g., length of carbon chain and/or degree of saturation), but this possible link remains poorly understood.

Pannexin channel and connexin hemichannel expression in vascular function and inflammation

Control of blood flow distribution and tissue homeostasis depend on the tight regulation of and coordination between the microvascular network and circulating blood cells. Channels formed by connexins or pannexins that connect the intra- and extracellular compartments allow the release of paracrine signals, such as ATP and prostaglandins, and thus play a central role in achieving fine regulation and coordination of vascular function. This review focuses on vascular connexin hemichannels and pannexin channels. We review their expression pattern within the arterial and venous system with a special emphasis on how post-translational modifications by phosphorylation and S-nitrosylation of these channels modulate their function and contribute to vascular homeostasis. Furthermore, we highlight the contribution of these channels in smooth muscle cells and endothelial cells in the regulation of vasomotor tone as well as how these channels in endothelial cells regulate inflammatory responses such as during ischemic and hypoxic conditions. In addition, this review will touch on recent evidence implicating a role for these proteins in regulating red blood cell and platelet function.