Pannexins, a family of gap junction proteins expressed in brain

The Double-Edged Effect of Connexins and Pannexins of Glial Cells in Central and Peripheral Nervous System After Nerve Injury

Glial cells play pivotal roles in homeostatic regulation and driving reactive pathologic changes after nerve injury. Connexins (Cxs) and pannexins (Panxs) have emerged as seminal proteins implicated in cell-cell communication, exerting a profound impact on the response processes of glial cell activation, proliferation, protein synthesis and secretion, as well as apoptosis following nerve injury. These influences are mediated through various forms, including protein monomers, hemichannel (HC), and gap junction (GJ), mainly by regulating intercellular or intracellular signaling pathways. Multiple Cx and Panx isoforms have been detected in central nervous system (CNS) or peripheral nervous system (PNS). Each isoform exhibits distinct cellular and subcellular localization, and the differential regulation and functional roles of various protein isoforms are observed post-injury. The quantitative and functional alterations of the same protein isoform in different studies remain inconsistent, attributable to factors such as the predominant mode of protein polymerization, the specific injury model, and the injury site. Similarly, the same protein isoforms have different roles in regulating the response processes after nerve injury, thus exerting a double-edged sword effect. This review describes the regulatory mechanisms and bidirectional effects of Cxs and Panxs. Additionally, it surveys the current status of research and application of drugs as therapeutic targets for neuropathic injuries. We summarize comprehensive and up-to-date information on these proteins in the glial cell response to nerve injury, providing new perspectives for future mechanistic exploration and development of targeted therapeutic approaches.

Pannexin 1 channels: A bridge between synaptic plasticity and learning and memory processes

The Pannexin 1 channel is a membrane protein widely expressed in various vertebrate cell types, including microglia, astrocytes, and neurons within the central nervous system. Growing research has demonstrated the significant involvement of Panx1 in synaptic physiology, such as its contribution to long-term synaptic plasticity, with a particular focus on the hippocampus, an essential structure for learning and memory. Investigations studying the role of Panx1 in synaptic plasticity have utilized knockout animal models and channel inhibition techniques, revealing that the absence or blockade of Panx1 channels in this region promotes synaptic potentiation, dendritic arborization, and spine formation. Despite substantial progress, the precise mechanism by which Panx1 regulates synaptic plasticity remains to be determined. Nevertheless, evidence suggests that Panx1 may exert its influence by releasing signaling molecules, such as adenosine triphosphate (ATP), or through the clearance of endocannabinoids (eCBs). This review aims to comprehensively explore the current literature on the role of Panx1 in synapses. By examining relevant articles, we seek to enhance our understanding of Panx1's contribution to synaptic fundamental processes and the potential implications for cognitive function.

Can the Tumor Microenvironment Alter Ion Channels? Unraveling Their Role in Cancer

Neoplastic cells are characterized by metabolic reprogramming, known as the Warburg effect, in which glucose metabolism is predominantly directed toward aerobic glycolysis, with reduced mitochondrial oxidative phosphorylation and increased lactate production even in the presence of oxygen. This phenomenon provides cancer cells with a proliferative advantage, allowing them to rapidly produce energy (in the form of ATP) and generate metabolic intermediates necessary for the biosynthesis of macromolecules essential for cell growth. It is important to understand the role of ion channels in the tumor context since they participate in various physiological processes and in the regulation of the tumor microenvironment. These changes may contribute to the development and transformation of cancer cells, as well as affect the communication between cells and the surrounding microenvironment, including impaired or altered expression and functionality of ion channels. Therefore, the aim of this review is to elucidate the impact of the tumor microenvironment on the electrical properties of the cellular membranes in several cancers as a possible therapeutic target.

Cryo-EM structure of the human Pannexin-3 channel

Pannexin-3 (PANX3) is a member of the pannexin family of large-pore, ATP-permeable channels conserved across vertebrates. PANX3 contributes to various developmental and pathophysiological processes by permeating ATP and Ca2+ ions; however, the structural basis of PANX3 channel function remains unclear. Here, we present the cryo-EM structure of human PANX3 at 2.9-3.2 Å. The PANX3 channel is heptameric and forms a transmembrane pore along the central symmetric axis. The narrowest constriction of the pore is composed of an isoleucine ring located in the extracellular region, and its size is comparable to that of other pannexins. A structural variability analysis revealed prominent structural dynamics in intracellular regions. Our structural studies provide a foundation for understanding the detailed properties of pannexin channels.

Role of astrocytes connexins - pannexins in acute brain injury

Acute brain injuries (ABIs) encompass a broad spectrum of primary injuries such as ischemia, hypoxia, trauma, and hemorrhage that converge into secondary injury where some mechanisms show common determinants. In this regard, astroglial connexin and pannexin channels have been shown to play an important role. These channels are transmembrane proteins sharing similar topology and form gateways between adjacent cells named gap junctions (GJs) and pores into unopposed membranes named hemichannels (HCs). In astrocytes, GJs and HCs enable intercellular communication and have active participation in normal brain physiological processes, such as calcium waves, synapsis modulation, regional blood flow regulation, and homeostatic control of the extracellular environment, among others. However, after acute brain injury, astrocytes can change their phenotype and modify the activity of both channels and hemichannels, which can result in the amplification of danger signals, increased mediators of inflammation, and neuronal death, contributing to the expansion of brain damage and neurological deterioration. This is known as secondary brain damage. In this review, we discussed the main biological mechanism of secondary brain damage with a particular focus on astroglial connexin and pannexin participation during acute brain injuries.

The C-terminal activating domain promotes pannexin 1 channel opening

Pannexin 1 (Panx1) constitutes a large pore channel responsible for the release of adenosine triphosphate (ATP) from apoptotic cells. Strong evidence indicates that caspase-mediated cleavage of the C-terminus promotes the opening of the Panx1 channel by unplugging the pore. However, this simple pore-plugging mechanism alone cannot account for the observation that a Panx1 construct ending before the caspase cleavage site remains closed. Here, we show that a helical region located immediately before the caspase cleavage site, referred to as the "C-terminal activating domain (CAD)", plays a pivotal role in facilitating Panx1 activation. Electrophysiology and mutagenesis studies uncovered that two conserved leucine residues within the CAD play a pivotal role. Cryoelectron microscopy (Cryo-EM) analysis of the construct ending before reaching the CAD demonstrated that the N terminus extends into an intracellular pocket. In contrast, the construct including the CAD revealed that this domain occupies the intracellular pocket, causing the N terminus to flip upward within the pore. Analysis of electrostatic free energy landscape in the closed conformation indicated that the intracellular side of the ion permeation pore may be occupied by anions like ATP, creating an electrostatic barrier for anions attempting to permeate the pore. When the N terminus flips up, it diminishes the positively charged surface, thereby reducing the drive to accumulate anions inside the pore. This dynamic change in the electrostatic landscape likely contributes to the selection of permeant ions. Collectively, these experiments put forth a mechanism in which C-terminal cleavage liberates the CAD, causing the repositioning of the N terminus to promote Panx1 channel opening.

Probenecid as a pharmacotherapy for alcohol use disorder: A randomized placebo-controlled alcohol interaction trial

BACKGROUND

This study shows the first evidence for pannexin 1 channels as a new target to develop medications for alcohol use disorder (AUD). Due to its history of long-term safe clinical use and preclinical evidence of reducing excessive alcohol intake in rodents, probenecid has clinical potential for AUD.

METHODS

We conducted a Phase I/IIa randomized, double-blind, placebo-controlled, crossover trial investigating the safety, tolerability, and efficacy of an oral dose of probenecid (2 g) when administered with alcohol (0.08 g/dL) in individuals who regularly consume alcohol to the 0.08 g/dL level (N = 35) and in individuals with mild to severe AUD. Alcohol pharmacokinetics and subjective responses were evaluated to assess potential interactions between probenecid and alcohol. Alcohol craving, inflammatory biomarkers, cognitive assessments, and hemodynamics were assessed as additional alcohol research domains. All outcomes were assessed both in the ascending and descending limb of alcohol intoxication using Generalized Estimating Equation.

RESULTS

Probenecid did not exert any significant effect on alcohol pharmacokinetics and did not affect alcohol stimulation or sedation. Probenecid, compared to placebo, significantly decreased alcohol craving during the alcohol ascending limb. Inflammatory biomarkers, cognitive performance following alcohol ingestion, and hemodynamics were likewise not affected by probenecid administration. Analysis of sex as a biological variable revealed no differences of probenecid compared to placebo.

CONCLUSIONS

Taken together, our data support the potential of probenecid for treatment of AUD and suggest that pannexin 1 channels represent a novel emerging therapeutic target for the development of new pharmacotherapies for treating AUD.

Dual role for pannexin 1 at synapses: regulating functional and morphological plasticity

Pannexin 1 (PANX1) is an ion and metabolite membrane channel and scaffold protein enriched in synaptic compartments of neurons in the central nervous system. In addition to a well-established link between PANX1 and synaptic plasticity, we recently identified a role for PANX1 in the regulation of dendritic spine stability. Notably, PANX1 and its interacting proteins are linked to neurological conditions involving dendritic spine loss. Understanding the dual role of PANX1 in synaptic function and morphology may help to shed light on these links. We explore potential mechanisms, including PANX1's interactions with postsynaptic receptors and cytoskeleton regulating proteins. Finally, we contextualize PANX1's dual role within neurological diseases involving dendritic spine and synapse dysfunction.

Calcium channel signalling at neuronal endoplasmic reticulum-plasma membrane junctions

Neurons are highly specialised cells that need to relay information over long distances and integrate signals from thousands of synaptic inputs. The complexity of neuronal function is evident in the morphology of their plasma membrane (PM), by far the most intricate of all cell types. Yet, within the neuron lies an organelle whose architecture adds another level to this morphological sophistication - the endoplasmic reticulum (ER). Neuronal ER is abundant in the cell body and extends to distant axonal terminals and postsynaptic dendritic spines. It also adopts specialised structures like the spine apparatus in the postsynapse and the cisternal organelle in the axon initial segment. At membrane contact sites (MCSs) between the ER and the PM, the two membranes come in close proximity to create hubs of lipid exchange and Ca2+ signalling called ER-PM junctions. The development of electron and light microscopy techniques extended our knowledge on the physiological relevance of ER-PM MCSs. Equally important was the identification of ER and PM partners that interact in these junctions, most notably the STIM-ORAI and VAP-Kv2.1 pairs. The physiological functions of ER-PM junctions in neurons are being increasingly explored, but their molecular composition and the role in the dynamics of Ca2+ signalling are less clear. This review aims to outline the current state of research on the topic of neuronal ER-PM contacts. Specifically, we will summarise the involvement of different classes of Ca2+ channels in these junctions, discuss their role in neuronal development and neuropathology and propose directions for further research.

Pathophysiological role of connexin and pannexin hemichannels in neuromuscular disorders

A growing body of research has provided evidence that de novo expression of connexin hemichannels and upregulation of pannexin hemichannels (Cx HCs and Panx HCs, respectively) in the cytoplasmic membrane of skeletal muscle (sarcolemma) are critical steps in the pathogenesis of muscle dysfunction of many genetic and acquired muscle diseases. This review provides an overview of the current understanding of the molecular mechanisms regulating the expression of Cx and Panx HCs in skeletal muscle, as well as their roles in both muscle physiology and pathologies. Additionally, it addresses existing gaps in knowledge and outlines future challenges in the field.

Contribution of inwardly rectifying potassium channel 4.1 in orofacial neuropathic pain: Regulation of pannexin 3 via the reactive oxygen species-activated P38 MAPK signal pathway

The involvement of inwardly rectifying potassium channel 4.1 (Kir4.1) in neuropathic pain has been established. However, there is limited understanding of the downstream mechanism through which Kir4.1 contributes to orofacial neuropathic pain. The objective of this study was to examine the regulation of Kir4.1 on the expression of pannexin 3 (Panx3) in the trigeminal ganglion (TG) and the underlying mechanism in the context of orofacial neuropathic pain caused by chronic constriction injury of the infraorbital nerve (CCI-ION). The study observed a significant increase in Panx3 expression in the TG of mice with CCI-ION. Inhibition of Panx3 in the TG of CCI-ION mice resulted in alleviation of orofacial mechanical allodynia. Furthermore, conditional knockdown (CKD) of Kir4.1 in the TG of both male and female mice led to mechanical allodynia and upregulation of Panx3 expression. Conversely, overexpression of Kir4.1 decreased Panx3 levels in the TG and relieved mechanical allodynia in CCI-ION mice. In addition, silencing Kir4.1 in satellite glial cells (SGCs) decreased Panx3 expression and increased the phosphorylation of P38 MAPK. Moreover, silencing Kir4.1 in SGCs increased the levels of reactive oxygen species (ROS). The elevated phosphorylation of P38 MAPK resulting from Kir4.1 silencing was inhibited by using a superoxide scavenger known as the tempol. Silencing Panx3 in the TG in vivo attenuated the mechanical allodynia caused by Kir4.1 CKD. In conclusion, these findings suggest that the reduction of Kir4.1 promotes the expression of Panx3 by activating the ROS-P38 MAPK signalling pathway, thus contributing to the development of orofacial neuropathic pain.

Pannexin1 Mediates Early-Life Seizure-Induced Social Behavior Deficits

There is a high co-morbidity between childhood epilepsy and autism spectrum disorder (ASD), with age of seizure onset being a critical determinant of behavioral outcomes. The interplay between these comorbidities has been investigated in animal models with results showing that the induction of seizures at early post-natal ages leads to learning and memory deficits and to autistic-like behavior in adulthood. Modifications of the excitation/inhibition (glutamate/GABA, ATP/adenosine) balance that follows early-life seizures (ELS) are thought to be the physiological events that underlie neuropsychiatric and neurodevelopmental disorders. Although alterations in purinergic/adenosinergic signaling have been implicated in seizures and ASD, it is unknown whether the ATP release channels, Pannexin1 (Panx1), contribute to ELS-induced behavior changes. To tackle this question, we used the ELS-kainic acid model in transgenic mice with global and cell type specific deletion of Panx1 to evaluate whether these channels were involved in behavioral deficits that occur later in life. Our studies show that ELS results in Panx1 dependent social behavior deficits and also in poor performance in a spatial memory test that does not involve Panx1. These findings provide support for a link between ELS and adult behavioral deficits. Moreover, we identify neuronal and not astrocyte Panx1 as a potential target to specifically limit astrogliosis and social behavioral deficits resultant from early-life seizures.

Ca2+ permeation through C-terminal cleaved, but not full-length human Pannexin1 hemichannels, mediates cell death

Pannexin1 hemichannels (Panx1 HCs) are found in the membrane of most mammalian cells and communicate the intracellular and extracellular spaces, enabling the passive transfer of ions and small molecules. They are involved in physiological and pathophysiological conditions. During apoptosis, the C-terminal tail of Panx1 is proteolytically cleaved, but the permeability features of hemichannels and their role in cell death remain elusive. To address these topics, HeLa cells transfected with full-length human Panx1 (fl-hPanx1) or C-terminal truncated hPanx1 (Δ371hPanx1) were exposed to alkaline extracellular saline solution, increasing the activity of Panx1 HCs. The Δ371hPanx1 HC was permeable to DAPI and Etd+, but not to propidium iodide, whereas fl-hPanx1 HC was only permeable to DAPI. Furthermore, the cytoplasmic Ca2+ signal increased only in Δ371hPanx1 cells, which was supported by bioinformatics approaches. The influx of Ca2+ through Δ371hPanx1 HCs was necessary to promote cell death up to about 95% of cells, whereas the exposure to alkaline saline solution without Ca2+ failed to induce cell death, and the Ca2+ ionophore A23187 promoted more than 80% cell death even in fl-hPanx1 transfectants. Moreover, cell death was prevented with carbenoxolone or 10Panx1 in Δ371hPanx1 cells, whereas it was undetectable in HeLa Panx1-/- cells. Pretreatment with Ferrostatin-1 and necrostatin-1 did not prevent cell death, suggesting that ferroptosis or necroptosis was not involved. In comparison, zVAD-FMK, a pancaspase inhibitor, reduced death by ~60%, suggesting the involvement of apoptosis. Therefore, alkaline pH increases the activity of Δ371hPanx1HCs, leading to a critical intracellular free-Ca2+ overload that promotes cell death.

The C-terminal activating domain promotes Panx1 channel opening

Pannexin 1 (Panx1) constitutes a large pore channel responsible for the release of ATP from apoptotic cells. Strong evidence indicates that caspase-mediated cleavage of the C-terminus promotes the opening of the Panx1 channel by unplugging the pore. However, this simple pore-plugging mechanism alone cannot account for the observation that a Panx1 construct ending before the caspase cleavage site remains closed. Here, we show that a helical region located immediately before the caspase cleavage site, referred to as the "C-terminal activating domain (CAD)," plays a pivotal role in facilitating Panx1 activation. Electrophysiology and mutagenesis studies uncovered that two conserved leucine residues within the CAD plays a pivotal role. Cryo-EM analysis of the construct ending before reaching the CAD demonstrated that the N-terminus extends into an intracellular pocket. In contrast, the construct including the CAD revealed that this domain occupies the intracellular pocket, causing the N-terminus to flip upward within the pore. Analysis of electrostatic free energy landscape in the closed conformation indicated that the intracellular side of the ion permeation pore may be occupied by anions like ATP, creating an electrostatic barrier for anions attempting to permeate the pore. When the N-terminus flips up, it diminishes the positively charged surface, thereby reducing the drive to accumulate anions inside the pore. This dynamic change in the electrostatic landscape likely contributes to the selection of permeant ions. Collectively, these experiments put forth a novel mechanism in which C-terminal cleavage liberates the CAD, causing the repositioning of the N-terminus to promote Panx1 channel opening.

Inhibition of astroglial hemichannels prevents synaptic transmission decline during spreading depression

BACKGROUND

Spreading depression (SD) is an intriguing phenomenon characterized by massive slow brain depolarizations that affect neurons and glial cells. This phenomenon is repetitive and produces a metabolic overload that increases secondary damage. However, the mechanisms associated with the initiation and propagation of SD are unknown. Multiple lines of evidence indicate that persistent and uncontrolled opening of hemichannels could participate in the pathogenesis and progression of several neurological disorders including acute brain injuries. Here, we explored the contribution of astroglial hemichannels composed of connexin-43 (Cx43) or pannexin-1 (Panx1) to SD evoked by high-K+ stimulation in brain slices.

RESULTS

Focal high-K+ stimulation rapidly evoked a wave of SD linked to increased activity of the Cx43 and Panx1 hemichannels in the brain cortex, as measured by light transmittance and dye uptake analysis, respectively. The activation of these channels occurs mainly in astrocytes but also in neurons. More importantly, the inhibition of both the Cx43 and Panx1 hemichannels completely prevented high K+-induced SD in the brain cortex. Electrophysiological recordings also revealed that Cx43 and Panx1 hemichannels critically contribute to the SD-induced decrease in synaptic transmission in the brain cortex and hippocampus.

CONCLUSIONS

Targeting Cx43 and Panx1 hemichannels could serve as a new therapeutic strategy to prevent the initiation and propagation of SD in several acute brain injuries.

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.

Pannexin1 Channel-Mediated Inflammation in Acute Ischemic Stroke

Emerging evidence suggests that inflammation mediated by the pannexin1 channel contributes significantly to acute ischemic stroke. It is believed that the pannexin1 channel is key in initiating central system inflammation during the early stages of acute ischemic stroke. Moreover, the pannexin1 channel is involved in the inflammatory cascade to maintain the inflammation levels. Specifically, the interaction of pannexin1 channels with ATP-sensitive P2X7 purinoceptors or promotion of potassium efflux mediates the activation of the NLRP3 inflammasome, triggering the release of pro-inflammatory factors such as IL-1 and IL-18, exacerbating and sustaining inflammation of brain. Also, increased release of ATP induced by cerebrovascular injury activates pannexin1 in vascular endothelial cells. This signal directs peripheral leukocytes to migrate into ischemic brain tissue, leading to an expansion of the inflammatory zone. Intervention strategies targeting pannexin1 channels may greatly alleviate inflammation after acute ischemic stroke to improve this patient population's clinical outcomes. In this review, we sought to summarize relevant studies on inflammation mediated by the pannexin1 channel in acute ischemic stroke and discussed the possibility of using brain organoid-on-a-chip technology to screen miRNAs that exclusively target the pannexin1 channel to provide new therapeutic measures for targeted regulation of pannexin1 channel to reduce inflammation in acute ischemic stroke.

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.

Activation of Pannexin-1 channels causes cell dysfunction and damage in mesangial cells derived from angiotensin II-exposed mice

Chronic kidney disease (CKD) is a prevalent health concern associated with various pathological conditions, including hypertensive nephropathy. Mesangial cells are crucial in maintaining glomerular function, yet their involvement in CKD pathogenesis remains poorly understood. Recent evidence indicates that overactivation of Pannexin-1 (Panx1) channels could contribute to the pathogenesis and progression of various diseases. Although Panx1 is expressed in the kidney, its contribution to the dysfunction of renal cells during pathological conditions remains to be elucidated. This study aimed to investigate the impact of Panx1 channels on mesangial cell function in the context of hypertensive nephropathy. Using an Ang II-infused mouse model and primary mesangial cell cultures, we demonstrated that in vivo exposure to Ang II sensitizes cultured mesangial cells to show increased alterations when they are subjected to subsequent in vitro exposure to Ang II. Particularly, mesangial cell cultures treated with Ang II showed elevated activity of Panx1 channels and increased release of ATP. The latter was associated with enhanced basal intracellular Ca2+ ([Ca2+]i) and increased ATP-mediated [Ca2+]i responses. These effects were accompanied by increased lipid peroxidation and reduced cell viability. Crucially, all the adverse impacts evoked by Ang II were prevented by the blockade of Panx1 channels, underscoring their critical role in mediating cellular dysfunction in mesangial cells. By elucidating the mechanisms by which Ang II negatively impacts mesangial cell function, this study provides valuable insights into the pathogenesis of renal damage in hypertensive nephropathy.

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.

Cx43 hemichannels and panx1 channels contribute to ethanol-induced astrocyte dysfunction and damage

BACKGROUND

Alcohol, a widely abused drug, significantly diminishes life quality, causing chronic diseases and psychiatric issues, with severe health, societal, and economic repercussions. Previously, we demonstrated that non-voluntary alcohol consumption increases the opening of Cx43 hemichannels and Panx1 channels in astrocytes from adolescent rats. However, whether ethanol directly affects astroglial hemichannels and, if so, how this impacts the function and survival of astrocytes remains to be elucidated.

RESULTS

Clinically relevant concentrations of ethanol boost the opening of Cx43 hemichannels and Panx1 channels in mouse cortical astrocytes, resulting in the release of ATP and glutamate. The activation of these large-pore channels is dependent on Toll-like receptor 4, P2X7 receptors, IL-1β and TNF-α signaling, p38 mitogen-activated protein kinase, and inducible nitric oxide (NO) synthase. Notably, the ethanol-induced opening of Cx43 hemichannels and Panx1 channels leads to alterations in cytokine secretion, NO production, gliotransmitter release, and astrocyte reactivity, ultimately impacting survival.

CONCLUSION

Our study reveals a new mechanism by which ethanol impairs astrocyte function, involving the sequential stimulation of inflammatory pathways that further increase the opening of Cx43 hemichannels and Panx1 channels. We hypothesize that targeting astroglial hemichannels could be a promising pharmacological approach to preserve astrocyte function and synaptic plasticity during the progression of various alcohol use disorders.

Cell Death, by Any Other Name…

Studies trying to understand cell death, this ultimate biological process, can be traced back to a century ago. Yet, unlike many other fashionable research interests, research on cell death is more alive than ever. New modes of cell death are discovered in specific contexts, as are new molecular pathways. But what is "cell death", really? This question has not found a definitive answer yet. Nevertheless, part of the answer is irreversibility, whereby cells can no longer recover from stress or injury. Here, we identify the most distinctive features of different modes of cell death, focusing on the executive final stages. In addition to the final stages, these modes can differ in their triggering stimulus, thus referring to the initial stages. Within this framework, we use a few illustrative examples to examine how intercellular communication factors in the demise of cells. First, we discuss the interplay between cell-cell communication and cell death during a few steps in the early development of multicellular organisms. Next, we will discuss this interplay in a fully developed and functional tissue, the gut, which is among the most rapidly renewing tissues in the body and, therefore, makes extensive use of cell death. Furthermore, we will discuss how the balance between cell death and communication is modified during a pathological condition, i.e., colon tumorigenesis, and how it could shed light on resistance to cancer therapy. Finally, we briefly review data on the role of cell-cell communication modes in the propagation of cell death signals and how this has been considered as a potential therapeutic approach. Far from vainly trying to provide a comprehensive review, we launch an invitation to ponder over the significance of cell death diversity and how it provides multiple opportunities for the contribution of various modes of intercellular communication.

Identification of novel NRF2-dependent genes as regulators of lead and arsenic toxicity in neural progenitor cells

Lead (Pb) and arsenic (As) are prevalent metal contaminants in the environment. Exposures to these metals are associated with impaired neuronal functions and adverse effects on neurodevelopment in children. However, the molecular mechanisms by which Pb and As impair neuronal functions remain poorly understood. Here, we identified F2RL2, TRIM16L, and PANX2 as novel targets of Nuclear factor erythroid 2-related factor 2 (NRF2)-the master transcriptional factor for the oxidative stress response-that are commonly upregulated with both Pb and As in human neural progenitor cells (NPCs). Using a ChIP (Chromatin immunoprecipitation)-qPCR assay, we showed that NRF2 directly binds to the promoter region of F2RL2, TRIM16L, and PANX2 to regulate expression of these genes. We demonstrated that F2RL2, PANX2, and TRIM16L have differential effects on cell death, proliferation, and differentiation of NPCs in both the presence and absence of metal exposures, highlighting their roles in regulating NPC function. Furthermore, the analyses of the transcriptomic data on NPCs derived from autism spectrum disorder (ASD) patients revealed that dysregulation of F2RL2, TRIM16L, and PANX2 was associated with ASD genetic backgrounds and ASD risk genes. Our findings revealed that Pb and As induce a shared NRF2-dependent transcriptional response in NPCs and identified novel genes regulating NPC function. While further in vivo studies are warranted, this study provides a novel mechanism linking metal exposures to NPC function and identifies potential genes of interest in the context of neurodevelopment.

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.

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.

Progress of research into microglial mediation of central post-stroke pain

Central post-stroke pain (CPSP) is a chronic neuropathic pain syndrome that commonly occurs after cerebral stroke, and it severely impairs the daily activities of stroke patients. A number of fundamental and clinical studies support the theory that CPSP is mainly caused by ischemic and hemorrhagic injury of the spinal-thalamic-cortical neural pathway. However, the underlying reasons of CPSP genesis and development are far from clear. In recent years, the majority of research focused on microglia, the main resident immune cells of the central nervous system, which highlighted its critical role in the regulation of CPSP. The present article concentrated on exciting discoveries of microglia in mediating CPSP from the perspectives of their bioactive factors, cellular receptors, and signaling pathways, in order to offer a convenient and easy-to-digest overview. In addition, the potential and challenges of several agents in clinical translation of CPSP treatment was discussed based on recent preclinical studies.

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.

The Inflammasome-Dependent Dysfunction and Death of Retinal Ganglion Cells after Repetitive Intraocular Pressure Spikes

The dysfunction and selective loss of retinal ganglion cells (RGCs) is a known cause of vision loss in glaucoma and other neuropathies, where ocular hypertension (OHT) is the major risk factor. We investigated the impact of transient non-ischemic OHT spikes (spOHT) on RGC function and viability in vivo to identify cellular pathways linking low-grade repetitive mechanical stress to RGC pathology. We found that repetitive spOHT had an unexpectedly high impact on intraocular homeostasis and RGC viability, while exposure to steady OHT (stOHT) of a similar intensity and duration failed to induce pathology. The repetitive spOHT induced the rapid activation of the inflammasome, marked by the upregulation of NLRP1, NLRP3, AIM2, caspases -1, -3/7, -8, and Gasdermin D (GSDMD), and the release of interleukin-1β (IL-1β) and other cytokines into the vitreous. Similar effects were also detected after 5 weeks of exposure to chronic OHT in an induced glaucoma model. The onset of these immune responses in both spOHT and glaucoma models preceded a 50% deficit in pattern electroretinogram (PERG) amplitude and a significant loss of RGCs 7 days post-injury. The inactivation of inflammasome complexes in Nlrp1-/-, Casp1-/-, and GsdmD-/- knockout animals significantly suppressed the spOHT-induced inflammatory response and protected RGCs. Our results demonstrate that mechanical stress produced by acute repetitive spOHT or chronic OHT is mechanistically linked to inflammasome activation, which leads to RGC dysfunction and death.

Multi-omics analysis of expression profile and prognostic values of connexin family in LUAD

PURPOSE

Our study first explored the expression differences and prognostic significance of Cx genes in pan-cancer and then focused on LUAD. Our objectives were to conducted a comprehensive analysis of the expression profile, prognostic significance, genetic alterations, potential biological functions and drug sensitivity of the Connexin gene family in LUAD.

METHODS

We developed a comprehensive prognostic model for LUAD by combining risk scores with clinical features and created a nomogram to predict 1-, 3-, and 5-year overall survival. Using single-cell sequencing, we examined the expression and biological functions of the identified prognostic markers.

RESULTS

Our risk model revealed that GJB2-5 play a critical role in the prognosis of LUAD patients, associated with many biological processes such as cell cycle, DNA damage, EMT, hypoxia, invasion, and metastasis. Furthermore, the connexin gene family is linked to transcriptional mechanisms such as the extracellular matrix (ECM), migration, mobility, angiogenesis, and the epithelial-mesenchymal transition (EMT) genetic program.

CONCLUSION

The risk model can be used as a potential prognostic factor for LUAD patients and may provide new insights into cancer treatment from perspective of the expression of Cx genes.

Role of perivascular cells in kidney homeostasis, inflammation, repair and fibrosis

Perivascular niches in the kidney comprise heterogeneous cell populations, including pericytes and fibroblasts, with distinct functions. These perivascular cells have crucial roles in preserving kidney homeostasis as they maintain microvascular networks by stabilizing the vasculature and regulating capillary constriction. A subset of kidney perivascular cells can also produce and secrete erythropoietin; this ability can be enhanced with hypoxia-inducible factor-prolyl hydroxylase inhibitors, which are used to treat anaemia in chronic kidney disease. In the pathophysiological state, kidney perivascular cells contribute to the progression of kidney fibrosis, partly via transdifferentiation into myofibroblasts. Moreover, perivascular cells are now recognized as major innate immune sentinels in the kidney that produce pro-inflammatory cytokines and chemokines following injury. These mediators promote immune cell infiltration, leading to persistent inflammation and progression of kidney fibrosis. The crosstalk between perivascular cells and tubular epithelial, immune and endothelial cells is therefore a key process in physiological and pathophysiological states. Here, we examine the multiple roles of kidney perivascular cells in health and disease, focusing on the latest advances in this field of research.

Conjunctival transcriptomics in ocular mucous membrane pemphigoid

PURPOSE

Ocular Mucous Membrane Pemphigoid (OcMMP) is an orphan disease characterized by chronic autoimmune-driven conjunctival inflammation leading to progressive scarring, debilitating symptoms, and blinding sequelae. This feasibility study aims to demonstrate conjunctival genetic transcriptomic analyses as a putative tool for interrogation of pathogenic signaling pathways in OcMMP.

METHODS

Conjunctival RNA profiling using the NanoString nCounter Human Fibrosis panel was undertaken on RNA extracted from conjunctival swabs obtained from 6 MMP patients (8 eyes; 4 M/2F; median age 78 [range 64-84] years); and 8 age-matched control participants (15 eyes; 3 M/5F; median age 69.5 [range 69-88] years). Data from 770 genes were analyzed with ROSALIND HyperScale architecture and stratified according to the level of clinically visible bulbar conjunctival inflammation. Normalization, fold-changes (≥+1.5-fold or ≤ -1.5-fold) and p-values adjustment (<0.05) using the Benjamini-Hochberg method were calculated.

RESULTS

93 differentially expressed genes (DEGs) were observed between OcMMP versus controls of which 48 were upregulated, and 45 downregulated. The top 4 upregulated DEGs represented fibrosis (COL3A1, COL1A1, FN1 and THBS1) while the key under-expressed genes (SCIN, HMGS2, XCL1/2) were indicative of ocular surface failure (goblet cell loss, keratinization, vulnerability to secondary infections). Forty-four pathways had a global significance score ≥2, the most significant being those related to extracellular matrix (ECM) remodeling, synthesis, and degradation. These pathways were accentuated in eyes with visible inflammation.

CONCLUSIONS

NanoString methodology acquired via a simple conjunctival swab identifies profibrotic genes in OcMMP group and differentiates inflamed eyes. Longitudinal sampling and following investigative intervention will further mechanistic insight and development of novel biomarkers to monitor disease progression.

Determination of structural features that underpin the pannexin1 channel inhibitory activity of the peptide 10Panx1

Pannexin1 channels facilitate paracrine communication and are involved in a broad spectrum of diseases. Attempts to find appropriate pannexin1 channel inhibitors that showcase target-selective properties and in vivo applicability remain nonetheless scarce. However, a promising lead candidate, the ten amino acid long peptide mimetic 10Panx1 (H-Trp1-Arg2-Gln3-Ala4-Ala5-Phe6-Val7-Asp8-Ser9-Tyr10-OH), has shown potential as a pannexin1 channel inhibitor in both in vitro and in vivo studies. Nonetheless, structural optimization is critical for clinical use. One of the main hurdles to overcome along the optimization process consists of subduing the low biological stability (10Panx1 t1/2 = 2.27 ± 0.11 min). To tackle this issue, identification of important structural features within the decapeptide structure is warranted. For this reason, a structure-activity relationship study was performed to proteolytically stabilize the sequence. Through an Alanine scan, this study demonstrated that the side chains of Gln3 and Asp8 are crucial for 10Panx1's channel inhibitory capacity. Guided by plasma stability experiments, scissile amide bonds were identified and stabilized, while extracellular adenosine triphosphate release experiments, indicative of pannexin1 channel functionality, allowed to enhance the in vitro inhibitory capacity of 10Panx1.

Recent advances in the structure and activation mechanisms of metabolite-releasing Pannexin 1 channels

Pannexin 1 (PANX1) is a widely expressed large-pore ion channel located in the plasma membrane of almost all vertebrate cells. It possesses a unique ability to act as a conduit for both inorganic ions (e.g. potassium or chloride) and bioactive metabolites (e.g. ATP or glutamate), thereby activating varying signaling pathways in an autocrine or paracrine manner. Given its crucial role in cell-cell interactions, the activity of PANX1 has been implicated in maintaining homeostasis of cardiovascular, immune, and nervous systems. Dysregulation of PANX1 has also been linked to numerous diseases, such as ischemic stroke, seizure, and inflammatory disorders. Therefore, the mechanisms underlying different modes of PANX1 activation and its context-specific channel properties have gathered significant attention. In this review, we summarize the roles of PANX1 in various physiological processes and diseases, and analyze the accumulated lines of evidence supporting diverse molecular mechanisms associated with different PANX1 activation modalities. We focus on examining recent discoveries regarding PANX1 regulations by reversible post-translational modifications, elevated intracellular calcium concentration, and protein-protein interactions, as well as by irreversible cleavage of its C-terminal tail. Additionally, we delve into the caveats in the proposed PANX1 gating mechanisms and channel open-closed configurations by critically analyzing the structural insights derived from cryo-EM studies and the unitary properties of PANX1 channels. By doing so, we aim to identify potential research directions for a better understanding of the functions and regulations of PANX1 channels.

Antiproliferative effect of boldine on neural progenitor cells and on glioblastoma cells

Introduction

The subventricular zone (SVZ) is a brain region that contains neural stem cells and progenitor cells (NSCs/NPCs) from which new neurons and glial cells are formed during adulthood in mammals. Recent data indicate that SVZ NSCs are the cell type that acquires the initial tumorigenic mutation in glioblastoma (GBM), the most aggressive form of malignant glioma. NSCs/NPCs of the SVZ present hemichannel activity whose function has not yet been fully elucidated. In this work, we aimed to analyze whether hemichannel-mediated communication affects proliferation of SVZ NPCs and GBM cells.

Methods and Results

For that purpose, we used boldine, an alkaloid derived from the boldo tree (Peumus boldus), that inhibits connexin and pannexin hemichannels, but without affecting gap junctional communication. Boldine treatment (50 μM) of rat SVZ NPCs grown as neurospheres effectively inhibited dye uptake through hemichannels and induced a significant reduction in neurosphere diameter and in bromodeoxyuridine (BrdU) incorporation. However, the differentiation pattern was not modified by the treatment. Experiments with specific blockers for hemichannels formed by connexin subunits (D4) or pannexin 1 (probenecid) revealed that probenecid, but not D4, produced a decrease in BrdU incorporation similar to that obtained with boldine. These results suggest that inhibition of pannexin 1 hemichannels could be partially responsible for the antiproliferative effect of boldine on SVZ NPCs. Analysis of the effect of boldine (25-600 μM) on different types of primary human GBM cells (GBM59, GBM96, and U87-MG) showed a concentration-dependent decrease in GBM cell growth. Boldine treatment also induced a significant inhibition of hemichannel activity in GBM cells.

Discussion

Altogether, we provide evidence of an antimitotic action of boldine in SVZ NPCs and in GBM cells which may be due, at least in part, to its hemichannel blocking function. These results could be of relevance for future possible strategies in GBM aimed to suppress the proliferation of mutated NSCs or glioma stem cells that might remain in the brain after tumor resection.

Blocking Pannexin-1 Channels Alleviates Thalamic Hemorrhage-Induced Pain and Inflammatory Depolarization of Microglia in Mice

Central post-stroke pain (CPSP) is a neuropathic pain syndrome that frequently occurs following cerebral stroke. The pathogenesis of CPSP is mainly due to thalamic injury caused by ischemia and hemorrhage. However, its underlying mechanism is far from clear. In the present study, a thalamic hemorrhage (TH) model was established in young male mice by microinjection of 0.075 U of type IV collagenase into the unilateral ventral posterior lateral nucleus and ventral posterior medial nucleus of the thalamus. We found that TH led to microglial pannexin (Panx)-1, a large-pore ion channel, opening within the thalamus accompanied with thalamic tissue injury, pain sensitivities, and neurological deficit, which were significantly prevented by either intraperitoneal injection of the Panx1 blocker carbenoxolone or intracerebroventricular perfusion of the inhibitory mimetic peptide 10Panx. However, inhibition of Panx1 has no additive effect on pain sensitivities upon pharmacological depletion of microglia. Mechanistically, we found that carbenoxolone alleviated TH-induced proinflammatory factors transcription, neuronal apoptosis, and neurite disassembly within the thalamus. In summary, we conclude that blocking of microglial Panx1 channels alleviates CPSP and neurological deficit through, at least in part, reducing neural damage mediated by the inflammatory response of thalamic microglia after TH. Targeting Panx1 might be a potential strategy in the treatment of CPSP.

Pannexin-1 Modulates Inhibitory Transmission and Hippocampal Synaptic Plasticity

Pannexin-1 (Panx1) hemichannel is a non-selective transmembrane channel that may play important roles in intercellular signaling by allowing the permeation of ions and metabolites, such as ATP. Although recent evidence shows that the Panx1 hemichannel is involved in controlling excitatory synaptic transmission, the role of Panx1 in inhibitory transmission remains unknown. Here, we studied the contribution of Panx1 to the GABAergic synaptic efficacy onto CA1 pyramidal neurons (PyNs) by using patch-clamp recordings and pharmacological approaches in wild-type and Panx1 knock-out (Panx1-KO) mice. We reported that blockage of the Panx1 hemichannel with the mimetic peptide ¹⁰Panx1 increases the synaptic level of endocannabinoids (eCB) and the activation of cannabinoid receptors type 1 (CB1Rs), which results in a decrease in hippocampal GABAergic efficacy, shifting excitation/inhibition (E/I) balance toward excitation and facilitating the induction of long-term potentiation. Our finding provides important insight unveiling that Panx1 can strongly influence the overall neuronal excitability and play a key role in shaping synaptic changes affecting the amplitude and direction of plasticity, as well as learning and memory processes.

Channel-mediated ATP release in the nervous system

ATP is well established as a transmitter and modulator in the peripheral and central nervous system. While conventional exocytotic release of ATP at synapses occurs, this transmitter is unusual in also being released into the extracellular space via large-pored plasma membrane channels. This review considers the channels that are known to be permeable to ATP and some of the functions of channel-mediated ATP release. While the possibility of ATP release via channels mediating volume transmission has been known for some time, localised ATP release via channels at specialised synapses made by taste cells to the afferent nerve has recently been documented in taste buds. This raises the prospect that "channel synapses" may occur in other contexts. However, volume transmission and channel synapses are not necessarily mutually exclusive. We suggest that certain glial cells in the brain stem and hypothalamus, which possess long processes and are known to release ATP, may be candidates for both modes of ATP release -channel-mediated volume transmission in the region of their somata and more localised transmission possibly via either conventional or channel synapses from their processes at distal targets. Finally, we consider the different characteristics of vesicular and channel synapses and suggest that channel synapses may be advantageous in requiring less energy than their conventional vesicular counterparts. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

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.

Cryo-EM structure of human heptameric pannexin 2 channel

Pannexin 2 (Panx2) is a large-pore ATP-permeable channel with critical roles in various physiological processes, such as the inflammatory response, energy production and apoptosis. Its dysfunction is related to numerous pathological conditions including ischemic brain injury, glioma and glioblastoma multiforme. However, the working mechanism of Panx2 remains unclear. Here, we present the cryo-electron microscopy structure of human Panx2 at a resolution of 3.4 Å. Panx2 structure assembles as a heptamer, forming an exceptionally wide channel pore across the transmembrane and intracellular domains, which is compatible with ATP permeation. Comparing Panx2 with Panx1 structures in different states reveals that the Panx2 structure corresponds to an open channel state. A ring of seven arginine residues located at the extracellular entrance forms the narrowest site of the channel, which serves as the critical molecular filter controlling the permeation of substrate molecules. This is further verified by molecular dynamics simulations and ATP release assays. Our studies reveal the architecture of the Panx2 channel and provide insights into the molecular mechanism of its channel gating.

Functional Roles of Connexins and Gap Junctions in Osteo-Chondral Cellular Components

Gap junctions (GJs) formed by connexins (Cxs) play an important role in the intercellular communication within most body tissues. In this paper, we focus on GJs and Cxs present in skeletal tissues. Cx43 is the most expressed connexin, participating in the formation of both GJs for intercellular communication and hemichannels (HCs) for communication with the external environment. Through GJs in long dendritic-like cytoplasmic processes, osteocytes embedded in deep lacunae are able to form a functional syncytium not only with neighboring osteocytes but also with bone cells located at the bone surface, despite the surrounding mineralized matrix. The functional syncytium allows a coordinated cell activity through the wide propagation of calcium waves, nutrients and anabolic and/or catabolic factors. Acting as mechanosensors, osteocytes are able to transduce mechanical stimuli into biological signals that spread through the syncytium to orchestrate bone remodeling. The fundamental role of Cxs and GJs is confirmed by a plethora of investigations that have highlighted how up- and downregulation of Cxs and GJs critically influence skeletal development and cartilage functions. A better knowledge of GJ and Cx mechanisms in physiological and pathological conditions might help in developing therapeutic approaches aimed at the treatment of human skeletal system disorders.

Design, Synthesis and Pharmacological Evaluation of New Quinoline-Based Panx-1 Channel Blockers

Pannexins are an interesting new target in medicinal chemistry, as they are involved in many pathologies such as epilepsy, ischemic stroke, cancer and Parkinson's disease, as well as in neuropathic pain. They are a family of membrane channel proteins consisting of three members, Panx-1, Panx-2 and Panx-3, and are expressed in vertebrates. In the present study, as a continuation of our research in this field, we report the design, synthesis and pharmacological evaluation of new quinoline-based Panx-1 blockers. The most relevant compounds 6f and 6g show an IC₅₀ = 3 and 1.5 µM, respectively, and are selective Panx-1 blockers. Finally, chemical stability, molecular modelling and X-ray crystallography studies have been performed providing useful information for the realization of the project.

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.

Single channel properties of pannexin-1 and connexin-43 hemichannels and P2X7 receptors in astrocytes cultured from rodent spinal cords

Astrocytes express surface channels involved in purinergic signaling. Among these channels, pannexin-1 (Px1) and connexin-43 (Cx43) hemichannels (HCs) release ATP that acts directly, or through its derivatives, on neurons and glia via purinergic receptors. Although HCs are functional, that is, open and close under physiological and pathological conditions, single channel properties of Px1 HCs in astrocytes have not been defined. Here, we developed a dual voltage clamp technique in HeLa cells expressing human Px1-YFP, and then applied this system to rodent spinal astrocytes to compare their single channel properties with other surface channels, that is, Cx43 HCs and P2X7 receptors (P2X7Rs). Channels were recorded in cell attached patches and evoked with ramp cycles applied through another pipette in whole cell voltage clamp. The mean unitary conductances of Px1 HCs were comparable in HeLa Px1-YFP cells and spinal astrocytes, ~42 and ~48 pS, respectively. Based on their unitary conductance, voltage-dependence, and unitary activity after pharmacological and gene silencing, Px1 HCs in astrocytes could be distinguished from Cx43 HCs and P2X7Rs. Channel activity of Px1 HCs and P2X7Rs was greater than that of Cx43 HCs in control astrocytes during ramps. Unitary activity of Px1 HCs was decreased and that of Cx43 HCs and P2X7Rs increased in astrocytes treated with fibroblast growth factor 1 (FGF-1). In summary, we resolved single channel properties of three different surface channels involved in purinergic signaling in spinal astrocytes, which were differentially modulated by FGF-1, a growth factor involved in neurodevelopment, inflammation and repair.

The Long-Term Pannexin 1 Ablation Produces Structural and Functional Modifications in Hippocampal Neurons

Enhanced activity and overexpression of Pannexin 1 (Panx1) channels contribute to neuronal pathologies such as epilepsy and Alzheimer's disease (AD). The Panx1 channel ablation alters the hippocampus's glutamatergic neurotransmission, synaptic plasticity, and memory flexibility. Nevertheless, Panx1-knockout (Panx1-KO) mice still retain the ability to learn, suggesting that compensatory mechanisms stabilize their neuronal activity. Here, we show that the absence of Panx1 in the adult brain promotes a series of structural and functional modifications in the Panx1-KO hippocampal synapses, preserving spontaneous activity. Compared to the wild-type (WT) condition, the adult hippocampal neurons of Panx1-KO mice exhibit enhanced excitability, a more complex dendritic branching, enhanced spine maturation, and an increased proportion of multiple synaptic contacts. These modifications seem to rely on the actin-cytoskeleton dynamics as an increase in the actin polymerization and an imbalance between the Rac1 and the RhoA GTPase activities were observed in Panx1-KO brain tissues. Our findings highlight a novel interaction between Panx1 channels, actin, and Rho GTPases, which appear to be relevant for synapse stability.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

Activity and Stability of Panx1 Channels in Astrocytes and Neuroblastoma Cells Are Enhanced by Cholesterol Depletion

Pannexin1 (Panx1) is expressed in both neurons and glia where it forms ATP-permeable channels that are activated under pathological conditions such as epilepsy, migraine, inflammation, and ischemia. Membrane lipid composition affects proper distribution and function of receptors and ion channels, and defects in cholesterol metabolism are associated with neurological diseases. In order to understand the impact of membrane cholesterol on the distribution and function of Panx1 in neural cells, we used fluorescence recovery after photobleaching (FRAP) to evaluate its mobility and electrophysiology and dye uptake to assess channel function. We observed that cholesterol extraction (using methyl-β-cyclodextrin) and inhibition of its synthesis (lovastatin) decreased the lateral diffusion of Panx1 in the plasma membrane. Panx1 channel activity (dye uptake, ATP release and ionic current) was enhanced in cholesterol-depleted Panx1 transfected cells and in wild-type astrocytes compared to non-depleted or Panx1 null cells. Manipulation of cholesterol levels may, therefore, offer a novel strategy by which Panx1 channel activation might modulate various pathological conditions.

A new path to mental disorders: Through gap junction channels and hemichannels

Behavioral disturbances related to emotional regulation, reward processing, cognition, sleep-wake regulation and activity/movement represent core symptoms of most common mental disorders. Increasing empirical and theoretical evidence suggests that normal functioning of these behavioral domains relies on fine graded coordination of neural and glial networks which are maintained and modulated by intercellular gap junction channels and unapposed pannexin or connexin hemichannels. Dysfunctions in these networks might contribute to the development and maintenance of psychopathological and neurobiological features associated with mental disorders. Here we review and discuss the evidence indicating a prominent role of gap junction channel and hemichannel dysfunction in core symptoms of mental disorders. We further discuss how the increasing knowledge on intercellular gap junction channels and unapposed pannexin or connexin hemichannels in the brain might lead to deeper mechanistic insight in common mental disorders and to the development of novel treatment approaches. We further attempt to exemplify what type of future research on this topic could be integrated into multidimensional approaches to understand and cure mental disorders.

Expression of connexins and pannexins in diseased human liver

Connexin proteins can form hexameric hemichannels and gap junctions that mediate paracrine and direct intercellular communication, respectively. Gap junction activity is crucial for the maintenance of hepatic homeostasis, while connexin hemichannels become particularly active in liver disease, such as hepatitis, fibrosis, cholestasis or even hepatocellular carcinoma. Channels consisting of connexin-like proteins named pannexins have been directly linked to liver inflammation and cell death. The goal of the present study was to characterize the expression and subcellular localization of connexins and pannexins in liver of patients suffering from various chronic and neoplastic liver diseases. Specifically, real-time quantitative reverse transcription polymerase chain reaction, immunoblotting and immunohistochemistry analyses were performed on human liver biopsies. It was found that pannexin1 and pannexin2 gene expression are correlated to a certain degree, as is pannexin1 protein expression with connexin32 and connexin43 protein expression. Furthermore, this study is the first to detect pannexin3 in human patient liver biopsies via both immunoblot and immunohistochemistry.