Pannexin channels and their links to human disease

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.

Inhibition of astroglial hemichannels ameliorates infrasonic noise induced short-term learning and memory impairment

As a kind of environmental noise, infrasonic noise has negative effects on various human organs. To date, research has shown that infrasound impairs cognitive function, especially the ability for learning and memory. Previously, we demonstrated that impaired learning and memory induced by infrasound was closely related with glia activation; however, the underlying mechanisms remain unclear. Connexin 43 hemichannels (Cx43 HCs), which are mainly expressed in hippocampal astrocytes, are activated under pathological conditions, lending support to the hypothesis that Cx43 HCs might function in the impaired learning and memory induced by infrasound. This study revealed that that blocking hippocampal Cx43 HCs or downregulating hippocampal Cx43 expression significantly alleviated impaired learning and memory induced by infrasound. We also observed that infrasound exposure led to the abundant release of glutamate and ATP through Cx43 HCs. In addition, the abundant release of glutamate and ATP depended on proinflammatory cytokines. Our finds suggested that the enhanced release of ATP and glutamate by astroglial Cx43 HCs may be involved in the learning and memory deficits caused by infrasound exposure.

Pharmacology of pannexin channels

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

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.

Expression of Connexins 37, 40 and 45, Pannexin 1 and Vimentin in Laryngeal Squamous Cell Carcinomas

Approximately 60% of patients with squamous cell carcinoma (LSCC) have regional occult metastatic disease/distant metastases at the time of diagnosis, putting them at higher risk for disease progression. Therefore, biomarkers are needed for early prognostic purpose. The aim of this study was to analyze the expression pattern of connexins (Cx) 37, 40 and 45, pannexin1 (Panx1) and vimentin in LSCC and correlate with tumor grade (G) and outcome.

METHODS

Thirty-four patients who underwent (hemi-)laryngectomy and regional lymphadenectomy due to LSCC from 2017 to 2018 in University Hospital Split, Croatia, were studied. Samples of tumor tissue and adjacent normal mucosa embedded in paraffin blocks were stained using the immunofluorescence method and were semi-quantitatively analyzed.

RESULTS

The expression of Cx37, Cx40, and Panx1 differed between cancer and adjacent normal mucosa and between histological grades, being the highest in well-differentiated (G1) cancer and low/absent in poorly differentiated (G3) cancer (all p < 0.05). The expression of vimentin was the highest in G3 cancer. Expression of Cx45 was generally weak/absent, with no significant difference between cancer and the controls or between grades. Lower Panx1 and higher vimentin expression were found to be prognostic factors for regional metastatic disease. Lower Cx37 and 40 expressions were present in patients with disease recurrence after the three-year follow-up period.

CONCLUSION

Cx37 and Cx40, Panx1, and vimentin have the potential to be used as prognostic biomarkers for LSCC.

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.

A Novel Defined Pyroptosis-Related Gene Signature for Predicting the Prognosis of Endometrial Cancer

Endometrial carcinoma (EC) is the second major female genital malignancy. Genetic signatures may be an improved choice to predict the prognosis of EC patients. The relationship between pyroptosis and tumours has attracted much attention in recent years. Here, we constructed a new pyroptosis-related gene (PRG) signature for predicting the prognosis of EC. In this study, gene data and clinical information of EC patients were obtained from The Cancer Genome Atlas (TCGA). Following the identification of PRGs correlated with EC prognosis, we further investigate the bioinformatics functions of these PRGs by univariate Cox regression analysis and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses. Then, we used the least absolute contraction and selection operator (LASSO) regression and multiple Cox regression analysis to construct a new PRG signature that contains seven PRGs (NFKB1, EEF2K, CTSV, MDM2, GZMB, PANX1, and PTEN) and performed the Kaplan-Meier (K-M) analysis, receiver operating characteristic curve (ROC) analysis, and principal component analysis (PCA) to evaluate the prognostic value of our novel PRG signature. Finally, we assessed the correlations between pyroptosis and immune cells/checkpoints through the CIBERSORT tool and single-sample gene set enrichment analysis (ssGSEA). The result suggested that our signature was powerful in predicting EC prognosis and may play a part in assessing response to immunotherapy in EC patients. In conclusion, our study established a novel PRG signature for EC, which can be used as an effective prognostic marker in clinical practice in the future.

Hypertensive Nephropathy: Unveiling the Possible Involvement of Hemichannels and Pannexons

Hypertension is one of the most common risk factors for developing chronic cardiovascular diseases, including hypertensive nephropathy. Within the glomerulus, hypertension causes damage and activation of mesangial cells (MCs), eliciting the production of large amounts of vasoactive and proinflammatory agents. Accordingly, the activation of AT1 receptors by the vasoactive molecule angiotensin II (AngII) contributes to the pathogenesis of renal damage, which is mediated mostly by the dysfunction of intracellular Ca²⁺ ([Ca²⁺]_i) signaling. Similarly, inflammation entails complex processes, where [Ca²⁺]_i also play crucial roles. Deregulation of this second messenger increases cell damage and promotes fibrosis, reduces renal blood flow, and impairs the glomerular filtration barrier. In vertebrates, [Ca²⁺]_i signaling depends, in part, on the activity of two families of large-pore channels: hemichannels and pannexons. Interestingly, the opening of these channels depends on [Ca²⁺]_i signaling. In this review, we propose that the opening of channels formed by connexins and/or pannexins mediated by AngII induces the ATP release to the extracellular media, with the subsequent activation of purinergic receptors. This process could elicit Ca²⁺ overload and constitute a feed-forward mechanism, leading to kidney damage.

Potential Implications of Exercise Training on Pannexin Expression and Function

Pannexins (PANX1, 2, 3) are channel-forming glycoproteins that are expressed throughout the cardiovascular and musculoskeletal system. The canonical function of these proteins is to release nucleotides that act as purinergic signalling at the cell membrane or Ca2+ channels at the endoplasmic reticulum membrane. These two forms of signalling are essential for autocrine and paracrine signalling in health, and alterations in this signalling have been implicated in the pathogenesis of many diseases. Many musculoskeletal and cardiovascular diseases are largely the result of a lack of physical activity which causes altered gene expression. Considering exercise training has been shown to alter a wide array of gene expression in musculoskeletal tissues, understanding the interaction between exercise training, gene function and expression in relevant diseases is warranted. With regards to pannexins, multiple publications have shown that exercise training can influence pannexin expression and may influence the significance of its function in certain diseases. This review further discusses the potential interaction between exercise training and pannexin biology in relevant tissues and disease models. We propose that exercise training in relevant animal and human models will provide a more comprehensive understanding of the implications of pannexin biology in disease.

ER-resident STIM1/2 couples Ca2+ entry by NMDA receptors to pannexin-1 activation

Pannexin-1 (Panx1) channels contribute to neurological disorders, including stroke and epilepsy, where their function has been linked to N-methyl D-aspartate (NMDA) receptors (NMDARs). We discovered that Ca²⁺ entry via NMDARs recruits endoplasmic reticulum–resident STIM proteins to activate Panx1 by binding to a hydrophobic region localized to the Panx1 N terminus. Using loss-of-function approaches, combined with molecular replacement and use of a STIM/Panx1 function–blocking antibody, we demonstrate that disrupting the STIM/Panx1 interaction prevents Panx1 activation by NMDARs, but not by hypotonic stimuli. Thus, our findings serve as a basis for the design of modality-specific inhibitors against STIM-dependent Panx1 activation that will aid in understanding the multimodal functions of Panx1 and their contribution to physiology and pathology.

Pannexin-1 (Panx1) is a large-pore ion and solute permeable channel highly expressed in the nervous system, where it subserves diverse processes, including neurite outgrowth, dendritic spine formation, and N-methyl D-aspartate (NMDA) receptor (NMDAR)-dependent plasticity. Moreover, Panx1 dysregulation contributes to neurological disorders, including neuropathic pain, epilepsy, and excitotoxicity. Despite progress in understanding physiological and pathological functions of Panx1, the mechanisms that regulate its activity, including its ion and solute permeability, remain poorly understood. In this study, we identify endoplasmic reticulum (ER)-resident stromal interaction molecules (STIM1/2), which are Ca²⁺ sensors that communicate events within the ER to plasma membrane channels, as binding and signaling partners of Panx1. We demonstrate that Panx1 is activated to its large-pore configuration in response to stimuli that recruit STIM1/2 and map the interaction interface to a hydrophobic region within the N terminus of Panx1. We further characterize a Panx1 N terminus–recognizing antibody as a function-blocking tool able to prevent large-pore Panx1 activation by STIM1/2. Using either the function-blocking antibody or re-expression of Panx1 deletion mutants in Panx1 knockout (KO) neurons, we show that STIM recruitment couples Ca²⁺ entry via NMDARs to Panx1 activation, thereby identifying a model of NMDAR-STIM-Panx1 signaling in neurons. Our study highlights a previously unrecognized and important role of the Panx1 N terminus in regulating channel activation and membrane localization. Considering past work demonstrating an intimate functional relation between NMDARs and Panx1, our study opens avenues for understanding activation modality and context-specific functions of Panx1, including functions linked to diverse STIM-regulated cellular responses.

Structures of human pannexin-1 in nanodiscs reveal gating mediated by dynamic movement of the N terminus and phospholipids

Pannexin (PANX) family proteins form large-pore channels that mediate purinergic signaling. We analyzed the cryo-EM structures of human PANX1 in lipid nanodiscs to elucidate the gating mechanism and its regulation by the amino terminus in phospholipids. The wild-type channel has an amino-terminal funnel in the pore, but in the presence of the inhibitor probenecid, a cytoplasmically oriented amino terminus and phospholipids obstruct the pore. Functional analysis using whole-cell patch-clamp and oocyte voltage clamp showed that PANX1 lacking the amino terminus did not open and had a dominant negative effect on channel activity, thus confirming that the amino-terminal domain played an essential role in channel opening. These observations suggest that dynamic conformational changes in the amino terminus of human PANX1 are associated with lipid movement in and out of the pore. Moreover, the data provide insight into the gating mechanism of PANX1 and, more broadly, other large-pore channels.

Connexins evolved after early chordates lost innexin diversity

Gap junction channels are formed by two unrelated protein families. Non-chordates use the primordial innexins, while chordates use connexins that superseded the gap junction function of innexins. Chordates retained innexin-homologs, but N-glycosylation prevents them from forming gap junctions. It is puzzling why chordates seem to exclusively use the new gap junction protein and why no chordates should exist that use non-glycosylated innexins to form gap junctions. Here, we identified glycosylation sites of 2388 innexins from 174 non-chordate and 276 chordate species. Among all chordates, we found not a single innexin without glycosylation sites. Surprisingly, the glycosylation motif is also widespread among non-chordate innexins indicating that glycosylated innexins are not a novelty of chordates. In addition, we discovered a loss of innexin diversity during early chordate evolution. Most importantly, lancelets, which lack connexins, exclusively possess only one highly conserved innexin with one glycosylation site. A bottleneck effect might thus explain why connexins have become the only protein used to form chordate gap junctions.

Development of a novel miR-3648-related gene signature as a prognostic biomarker in esophageal adenocarcinoma

Background

Esophageal adenocarcinoma (EA) is a typical immunogenic malignant tumor with a dismal 5-year survival rate lower than 20%. Although miRNA-3648 (miR-3648) is expressed abnormally in EA, its impact on the tumor immune microenvironment remains unknown. In this study, we sought to identify immune-related genes (IRGs) that are targeted by miR-3648 and develop an EA multigene signature.

Methods

The gene expression data of 87 EA tumor samples and 67 normal tissue samples from The Cancer Genome Atlas (TCGA) database and the Genotype-Tissue Expression (GTEx) database were downloaded, respectively. Weighted gene co-expression network analysis (WGCNA), the CIBERSORT algorithm, and Cox regression analysis were applied to identify IRGs and to construct a prognostic signature and nomogram.

Results

MiR-3648 was expectedly highly expressed in EA tumor tissues (P=2.6e-8), and related to the infiltration of activated natural killer cells (NK cells) and activated CD4 T lymphocytes (CD4 cells). A total of 70 miR-3648-targeted genes related to immune cell infiltration were identified. Among them, 4 genes (C10orf55, DLL4, PANX2, and NKAIN1) were closely related to overall survival (OS), and were thus selected to construct a 4-gene risk score (RS). The RS had a superior capability to predict OS [area under the curve (AUC) =0.740 for 1 year; AUC =0.717 for 3 years; AUC =0.622 for 5 years]. A higher score was indicative of a poorer prognosis than a lower score [hazard ratio (HR) =2.71; 95% confidence interval (CI): 1.45-5.09; P=0.002]. Furthermore, the nomogram formed by combining the RS and the TNM classification of malignant tumors (TNM stage) improved the accuracy of survival prediction [Harrell's concordance index (C-index) =0.698].

Conclusions

MiR-3648 may play a critical role in EA pathogenesis. The novel 4-gene signature may serve as a prognostic tool to manage patients with EA.

Pannexin-1 channel opening is critical for COVID-19 pathogenesis

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapidly rampaged worldwide, causing a pandemic of coronavirus disease (COVID -19), but the biology of SARS-CoV-2 remains under investigation. We demonstrate that both SARS-CoV-2 spike protein and human coronavirus 229E (hCoV-229E) or its purified S protein, one of the main viruses responsible for the common cold, induce the transient opening of Pannexin-1 (Panx-1) channels in human lung epithelial cells. However, the Panx-1 channel opening induced by SARS-CoV-2 is greater and more prolonged than hCoV-229E/S protein, resulting in an enhanced ATP, PGE₂, and IL-1β release. Analysis of lung lavages and tissues indicate that Panx-1 mRNA expression is associated with increased ATP, PGE₂, and IL-1β levels. Panx-1 channel opening induced by SARS-CoV-2 spike protein is angiotensin-converting enzyme 2 (ACE-2), endocytosis, and furin dependent. Overall, we demonstrated that Panx-1 channel is a critical contributor to SARS-CoV-2 infection and should be considered as an alternative therapy.

Mechanisms of ATP release in pain: role of pannexin and connexin channels

Pain is a physiological response to bodily damage and serves as a warning of potential threat. Pain can also transform from an acute response to noxious stimuli to a chronic condition with notable emotional and psychological components that requires treatment. Indeed, the management of chronic pain is currently an important unmet societal need. Several reports have implicated the release of the neurotransmitter adenosine triphosphate (ATP) and subsequent activation of purinergic receptors in distinct pain etiologies. Purinergic receptors are broadly expressed in peripheral neurons and the spinal cord; thus, purinergic signaling in sensory neurons or in spinal circuits may be critical for pain processing. Nevertheless, an outstanding question remains: what are the mechanisms of ATP release that initiate nociceptive signaling? Connexin and pannexin channels are established conduits of ATP release and have been suggested to play important roles in a variety of pathologies, including several models of pain. As such, these large-pore channels represent a new and exciting putative pharmacological target for pain treatment. Herein, we will review the current evidence for a role of connexin and pannexin channels in ATP release during nociceptive signaling, such as neuropathic and inflammatory pain. Collectively, these studies provide compelling evidence for an important role of connexins and pannexins in pain processing.

PANX1 is a potential prognostic biomarker associated with immune infiltration in pancreatic adenocarcinoma: A pan-cancer analysis

Pannexin 1 (PANX1) channel is a critical ATP-releasing pathway that modulates tumor immunity, progression, and prognosis. However, the roles of PANX1 in different cancers remain unclear. We analyzed the expression of PANX1 in human pan-cancer in the Oncomine and GEPIA2.0 databases. The prognostic value of PANX1 expression was determined using Kaplan-Meier plotter and OncoLnc tools. The correlation between PANX1 and tumor-infiltrating immune cells was investigated using the TIMER 2.0. In addition, the relationship between PANX1 and immunomodulators was explored using TISIDB. Finally, gene set enrichment analysis (GSEA) was performed utilizing LinkedOmics. The results indicated that PANX1 was overexpressed in most cancers compared to normal tissues. The high expression of PANX1 was associated with poor prognosis in multiple tumors, especially in pancreatic adenocarcinoma (PAAD). In addition, PANX1 was correlated with a variety of immunomodulators, such as CD274, IL10, CD276, IL2RA, TAP1, and TAP2. PANX1 expression level was significantly related to infiltration of multiple immune cells in many cancers, including cancer associated fibroblast, macrophage, and neutrophil cells. Further analysis revealed that PANX1 was significantly associated with T cells CD8+ (rho = 0.524, P = 1.94e-13) and Myeloid dendritic cell (rho = 0.564, P = 9.45e-16). GSEA results showed that PANX1 was closely associated with leukocyte cell-cell adhesion, endoplasmic reticulum lumen, ECM-receptor interaction, and Focal adhesion pathways in PAAD. PANX1 expression was higher in pan-cancer samples than in normal tissues. The high expression of PANX1 was associated with poor outcome and immune infiltration in multiple cancers, especially in PAAD.

Are Osteoclasts Mechanosensitive Cells?

Skeleton metabolism is a process in which osteoclasts constantly remove old bone and osteoblasts form new osteoid and induce mineralization; disruption of this balance may cause diseases. Osteoclasts play a key role in bone metabolism, as osteoclastogenesis marks the beginning of each bone remodeling cycle. As the only cell capable of bone resorption, osteoclasts are derived from the monocyte/macrophage hematopoietic precursors that terminally adhere to mineralized extracellular matrix, and they subsequently break down the extracellular compartment. Bone is generally considered the load-burdening tissue, bone homeostasis is critically affected by mechanical conductions, and the bone cells are mechanosensitive. The functions of various bone cells under mechanical forces such as chondrocytes and osteoblasts have been reported; however, the unique bone-resorbing osteoclasts are less studied. The oversuppression of osteoclasts in mechanical studies may be because of its complicated differentiation progress and flexible structure, which increases difficulty in targeting mechanical structures. This paper will focus on recent findings regarding osteoclasts and attempt to uncover proposed candidate mechanosensing structures in osteoclasts including podosome-associated complexes, gap junctions and transient receptor potential family (ion channels). We will additionally describe possible mechanotransduction signaling pathways including GTPase ras homologue family member A (RhoA), Yes-associated protein/transcriptional co-activator with PDZ-binding motif (TAZ), Ca²⁺ signaling and non-canonical Wnt signaling. According to numerous studies, evaluating the possible influence of various physical environments on osteoclastogenesis is conducive to the study of bone homeostasis.

The role of Pannexin-1 channels and extracellular ATP in the pathogenesis of the human immunodeficiency virus

Only recently, the role of large ionic channels such as Pannexin-1 channels and Connexin hemichannels has been implicated in several physiological and pathological conditions, including HIV infection and associated comorbidities. These channels are in a closed stage in healthy conditions, but in pathological conditions including HIV, Pannexin-1 channels and Connexin hemichannels become open. Our data demonstrate that acute and chronic HIV infection induces channel opening (Pannexin and Connexin channels), ATP release into the extracellular space, and subsequent activation of purinergic receptors in immune and non-immune cells. We demonstrated that Pannexin and Connexin channels contribute to HIV infection and replication, the long-term survival of viral reservoirs, and comorbidities such as NeuroHIV. Here, we discuss the available data to support the participation of these channels in the HIV life cycle and the potential therapeutic approach to prevent HIV-associated comorbidities.

Human digital merkel cells display pannexin1 immunoreactivity

Pannexins are channel proteins displaying functional similarities to gap junctions in vertebrates and are regarded as transmembrane ATP-releasing channels. A member of this family, denominate pannexin1, has been detected in the epidermis and cutaneous adnexal structures. Here we used immunohistochemistry to investigate whether human digital Merkel cells express this protein since ATP is postulated as a neurotransmitter in the Merkel cell-axon complexes low-threshold mecahoreceptors. Pannexin1 immunoreactivity was found in cytokeratine 20-, chromogranin A- and synaptophysin-positive cells placed at the basal layer of the epidermis. Cell displaying pannexin1 immunoreactivities were thus identified as Merkel cells and showed close contact with nerve profiles. Light pannexin1 immunoreactivity in dermal blood vessels was also verified. Present results demonstrate for the first time the expression of pannexin1 in human digital Merkel cells supporting the idea that ATP can be involved directly or indirectly in the mechanotransductional process at Merkel-axon complexes.

Deacetylation as a receptor-regulated direct activation switch for pannexin channels

Activation of Pannexin 1 (PANX1) ion channels causes release of intercellular signaling molecules in a variety of (patho)physiological contexts. PANX1 can be activated by G protein-coupled receptors (GPCRs), including α1-adrenergic receptors (α1-ARs), but how receptor engagement leads to channel opening remains unclear. Here, we show that GPCR-mediated PANX1 activation can occur via channel deacetylation. We find that α1-AR-mediated activation of PANX1 channels requires Gαq but is independent of phospholipase C or intracellular calcium. Instead, α1-AR-mediated PANX1 activation involves RhoA, mammalian diaphanous (mDia)-related formin, and a cytosolic lysine deacetylase activated by mDia - histone deacetylase 6. HDAC6 associates with PANX1 and activates PANX1 channels, even in excised membrane patches, suggesting direct deacetylation of PANX1. Substitution of basally-acetylated intracellular lysine residues identified on PANX1 by mass spectrometry either prevents HDAC6-mediated activation (K140/409Q) or renders the channels constitutively active (K140R). These data define a non-canonical RhoA-mDia-HDAC6 signaling pathway for GαqPCR activation of PANX1 channels and uncover lysine acetylation-deacetylation as an ion channel silencing-activation mechanism.

Lymphatic Connexins and Pannexins in Health and Disease

This review highlights current knowledge on the expression and function of connexins and pannexins, transmembrane channel proteins that play an important role in intercellular communication, in both the developing and mature lymphatic vasculature. A particular focus is given to the involvement of these proteins in functions of the healthy lymphatic system. We describe their influence on the maintenance of extracellular fluid homeostasis, immune cell trafficking to draining lymph nodes and dietary nutrient absorption by intestinal villi. Moreover, new insights into connexin mutations in primary and secondary lymphedema as well as on the implication of lymphatic connexins and pannexins in acquired cardiovascular diseases are discussed, allowing for a better understanding of the role of these proteins in pathologies linked to dysfunctions in the lymphatic system.

Integrated Analysis of ceRNA Network Reveals Prognostic and Metastasis Associated Biomarkers in Breast Cancer

Background

Breast cancer is a malignancy and lethal tumor in women. Metastasis of breast cancer is one of the causes of poor prognosis. Increasing evidences have suggested that the competing endogenous RNAs (ceRNAs) were associated with the metastasis of breast cancer. Nonetheless, potential roles of ceRNAs in regulating the metastasis of breast cancer remain unclear.

Methods

The RNA expression (3 levels) and follow-up data of breast cancer and noncancerous tissue samples were downloaded from the Cancer Genome Atlas (TCGA). Differentially expressed and metastasis associated RNAs were identified for functional analysis and constructing the metastasis associated ceRNA network by comprehensively bioinformatic analysis. The Kaplan-Meier (K-M) survival curve was utilized to screen the prognostic RNAs in metastasis associated ceRNA network. Moreover, we further identified the metastasis associated biomarkers with operating characteristic (ROC) curve. Ultimately, the data of Cancer Cell Line Encyclopedia (CCLE, https://portals.broadinstitute.org/ccle) website were selected to obtained the reliable metastasis associated biomarkers.

Results

1005 mRNAs, 22 miRNAs and 164 lncRNAs were screened as differentially expressed and metastasis associated RNAs. The results of GO function and KEGG pathway enrichment analysis showed that these RNAs are mainly associated with the metabolic processes and stress responses. Next, a metastasis associated ceRNA (including 104 mRNAs, 19 miRNAs, and 16 lncRNAs) network was established, and 12 RNAs were found to be related to the overall survival (OS) of patients. In addition, 3 RNAs (hsa-miR-105-5p, BCAR1, and PANX2) were identified to serve as reliable metastasis associated biomarkers. Eventually, the results of mechanism analysis suggested that BCAR1 might promote the metastasis of breast cancer by facilitating Rap 1 signaling pathway.

Conclusion

In the present research, we identified 3 RNAs (hsa-miR-105-5p, BCAR1 and PANX2) might associated with prognosis and metastasis of breast cancer, which might be provide a new perspective for metastasis of breast cancer and contributed to the treatment of breast cancer.

Pannexin 1 Influences Lineage Specification of Human iPSCs

Every single cell in the body communicates with nearby cells to locally organize activities with their neighbors and dysfunctional cell-cell communication can be detrimental during cell lineage commitment, tissue patterning and organ development. Pannexin channels (PANX1, PANX2, and PANX3) facilitate purinergic paracrine signaling through the passage of messenger molecules out of cells. PANX1 is widely expressed throughout the body and has recently been identified in human oocytes as well as 2 and 4-cell stage human embryos. Given its abundance across multiple adult tissues and its expression at the earliest stages of human development, we sought to understand whether PANX1 impacts human induced pluripotent stem cells (iPSCs) or plays a role in cell fate decisions. Western blot, immunofluorescence and flow cytometry reveal that PANX1 is expressed in iPSCs as well as all three germ lineages derived from these cells: ectoderm, endoderm, and mesoderm. PANX1 demonstrates differential glycosylation patterns and subcellular localization across the germ lineages. Using CRISPR-Cas9 gene ablation, we find that loss of PANX1 has no obvious impact on iPSC morphology, survival, or pluripotency gene expression. However, PANX1 gene knockout iPSCs exhibit apparent lineage specification bias under 3-dimensional spontaneous differentiation into the three germ lineages. Indeed, loss of PANX1 increases representation of endodermal and mesodermal populations in PANX1 knockout cells. Importantly, PANX1 knockout iPSCs are fully capable of differentiating toward each specific lineage when exposed to the appropriate external signaling pressures, suggesting that although PANX1 influences germ lineage specification, it is not essential to this process.

PANX2 and brain lower grade glioma genesis: A bioinformatic analysis

PANX2 forms large-pore channels mediating ATP release in response to physiological and pathological stimuli. Although PANX2 shows involvements in glioma genesis, the underlying mechanism remains unclear. PANX2 mRNA expression was analyzed via Oncomine and was confirmed via Gene Expression Profiling Interactive Analysis (GEPIA). The influence of PANX2 on overall survival (OS) of glioma was evaluated using LinkedOmics and further assessed through Cox regression analysis. The correlated genes with PANX2 acquired from LinkedOmics were validated through GEPIA and cBioPortal. Protein-protein interaction (PPI) of these genes was then obtained using Search Tool for the Retrieval of Interacting Genes (STRING) and Cytoscape with MCODE plug-in. All the PANX2-related genes underwent Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. The correlation between PANX2 and cancer immune infiltrates was evaluated via Tumor Immune Estimation Resource (TIMER). A higher expression of PANX2 only revealed a better OS in brain low grade glioma (LGG). PANX2-related genes in LGG functionally enriched in neuroactive ligand-receptor interaction, synaptic vesicle cycle, and calcium signaling. The hub genes from highest module of PPI were mainly linked to chemical synaptic transmission, plasma membrane, neuropeptide, and the pathway of neuroactive ligand-receptor interaction. Besides, PANX2 expression was negatively associated with infiltrating levels of macrophage, dendritic cells, and CD4+ T cells. This study demonstrated that PANX2 likely participated in LGG pathogenesis by affecting multiple molecular pathways and immune-related processes. PANX2 was associated with LGG prognosis and might become a promising therapeutic target of LGG.

Mutational effects of Pannexin 1 R217H depend on the carboxyl-terminus

Pannexin 1 (Panx1) has been implicated in a plethora of physiological and pathophysiological processes. It is one of the major ATP release channels in many cell types. Extracellular ATP, activates purinergic P2X and P2Y receptors, triggering several signaling cascades. A disease-associated mutation, Arg-217-His (R217H) in the 3rd transmembrane domain of Panx1 attenuates channel functions through an unknown mechanism. Since carboxyl terminus (CT) gates the channel, we hypothesized that R217 interacts with the CT, and this interaction is required for optimum channel activities. R217H mutation though reduced the currents in the full-length channel, did not affect CT-truncated Panx1-Δ386. Also, compared to the wild-type, Panx1-R217H expressing cells showed lesser cell death when activated through P2X7 receptor. However, cell death in Panx1-R217H-Δ386 and Panx1-Δ386 expressing cells were similar. The mutation is ineffective unless the channel has an intact CT. Based on our results we propose that R217H mutation perturbs the conformational flexibility of CT, leading to channel dysfunction.

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.

Pannexin 1 binds β-catenin to modulate melanoma cell growth and metabolism

Melanoma is the most aggressive skin malignancy with increasing incidence worldwide. Pannexin1 (PANX1), a member of the pannexin family of channel-forming glycoproteins, regulates cellular processes in melanoma cells including proliferation, migration, and invasion/metastasis. However, the mechanisms responsible for coordinating and regulating PANX1 function remain unclear. Here, we demonstrated a direct interaction between the C-terminal region of PANX1 and the N-terminal portion of β-catenin, a key transcription factor in the Wnt pathway. At the protein level, β-catenin was significantly decreased when PANX1 was either knocked down or inhibited by two PANX1 blockers, Probenecid and Spironolactone. Immunofluorescence imaging showed a disrupted pattern of β-catenin localization at the cell membrane in PANX1-deficient cells, and transcription of several Wnt target genes, including MITF, was suppressed. In addition, a mitochondrial stress test revealed that the metabolism of PANX1-deficient cells was impaired, indicating a role for PANX1 in the regulation of the melanoma cell metabolic profile. Taken together, our data show that PANX1 directly interacts with β-catenin to modulate growth and metabolism in melanoma cells. These findings provide mechanistic insight into PANX1-mediated melanoma progression and may be applicable to other contexts where PANX1 and β-catenin interact as a potential new component of the Wnt signaling pathway.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

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

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

HIV gp120 Protein Increases the Function of Connexin 43 Hemichannels and Pannexin-1 Channels in Astrocytes: Repercussions on Astroglial Function

At least half of human immunodeficiency virus (HIV)-infected individuals suffer from a wide range of cognitive, behavioral and motor deficits, collectively known as HIV-associated neurocognitive disorders (HAND). The molecular mechanisms that amplify damage within the brain of HIV-infected individuals are unknown. Recently, we described that HIV augments the opening of connexin-43 (Cx43) hemichannels in cultured human astrocytes, which result in the collapse of neuronal processes. Whether HIV soluble viral proteins such as gp120, can regulate hemichannel opening in astrocytes is still ignored. These channels communicate the cytosol with the extracellular space during pathological conditions. We found that gp120 enhances the function of both Cx43 hemichannels and pannexin-1 channels in mouse cortical astrocytes. These effects depended on the activation of IL-1β/TNF-α, p38 MAP kinase, iNOS, cytoplasmic Ca²⁺ and purinergic signaling. The gp120-induced channel opening resulted in alterations in Ca²⁺ dynamics, nitric oxide production and ATP release. Although the channel opening evoked by gp120 in astrocytes was reproduced in ex vivo brain preparations, these responses were heterogeneous depending on the CA1 region analyzed. We speculate that soluble gp120-induced activation of astroglial Cx43 hemichannels and pannexin-1 channels could be crucial for the pathogenesis of HAND.

A Genetic Polymorphism in the Pannexin1 Gene Predisposes for The Development of Endothelial Dysfunction with Increasing BMI

Endothelial dysfunction worsens when body mass index (BMI) increases. Pannexin1 (Panx1) ATP release channels regulate endothelial function and lipid homeostasis in mice. We investigated whether the Panx1-400A>C single nucleotide polymorphism (SNP), encoding for a gain-of-function channel, associates with endothelial dysfunction in non-obese and obese individuals. Myocardial blood flow (MBF) was measured by ¹³N-ammonia positron emission/computed tomography at rest, during cold pressor test (CPT) or dipyridamole-induced hyperemia. Myocardial flow reserve (MFR) and endothelial function were compared in 43 non-obese (BMI < 30 kg/m²) vs. 29 obese (BMI 30 kg/m²) participants and genotyping for the Panx1-400A>C SNP was performed. Groups comprised subjects homozygous for the C allele (n = 40) vs. subjects with at least one A allele (n = 32). MBF (during CPT or hyperemia), MFR and endothelial function correlated negatively with BMI in the full cohort. BMI correlated negatively with MFR and endothelial function in non-obese Panx1-400C subjects, but not in Panx1-400A individuals nor in obese groups. BMI correlated positively with serum triglycerides, insulin or HOMA. MFR correlated negatively with these factors in non-obese Panx1-400C but not in Panx1-400A individuals. Here, we demonstrated that Panx1-400C SNP predisposes to BMI-dependent endothelial dysfunction in non-obese subjects. This effect may be masked by excessive dysregulation of metabolic factors in obese individuals.

The Caenorhabditis elegans INX-4/Innexin is required for the fine-tuning of temperature orientation in thermotaxis behavior

Innexins in invertebrates are considered to play roles similar to those of connexins and pannexins in vertebrates. However, it remains poorly understood how innexins function in biological phenomena including their function in the nervous systems. Here, we identified inx‐4, a member of the innexin family in C. elegans, by a forward screening of thermotaxis‐defective mutants. The inx‐4 mutants exhibited abnormal migration to a temperature slightly higher than the cultivation temperature, called mild thermophilic behavior. Rescue experiments revealed that INX‐4 acts in the major thermosensory neuron AFD to regulate thermotaxis behavior. INX‐4::GFP fusion protein localized exclusively along axons in AFD neurons. In addition, over‐expression of INX‐4 in AFD neurons induced a cryophilic behavior, which is opposite to inx-4 mutants. Our findings suggest that INX‐4/Innexin in AFD may fine‐tune the execution of thermotaxis behavior when moving to desired temperatures.

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.

P2 receptors mRNA expression profiles in macrophages from ankylosing spondylitis patients and healthy individuals

BACKGROUND

Ankylosing spondylitis (AS) is a multifactorial rheumatic disease which mainly involves the axial skeleton. Macrophages and extracellular nucleotides have been shown to contribute to the inflammation process in autoimmune diseases. Membrane-bound purinergic P2 receptors might be involved in the modulation of immune cells in AS. Therefore, we aimed to analyze the messenger RNA (mRNA) expression of P2 receptors in the macrophages of AS patients and healthy controls.

METHODS

Twenty-three AS patients and 23 age- and sex-matched healthy individuals were included in our study. Whole blood-separated monocytes of study participants were stimulated by macrophage colony-stimulating factor for 7 days and differentiated to macrophages. Monocyte and macrophage markers were analyzed by flow cytometry. SYBR green real-time polymerase chain reaction was used to measure the relative expression levels of P2RX₁ , P2RX₂ , P2RX₃ , P2RX₄ , P2RX₅ , P2RX₆ , P2RX₇ , P2RY₁ , P2RY₂ , P2RY₄ , P2RY₆ , P2RY₁₁ , P2RY₁₂ , P2RY₁₃ , P2RY₁₄ , and PANX1 genes.

RESULTS

P2RY₁₃ and P2RY₆ genes had the highest expression levels in macrophages among P2RY genes. P2RY₁ mRNA expression was significantly down-regulated (-1.75 fold) and P2RY₁₄ was up-regulated (2.6 fold) in macrophages of AS patients compared to healthy individuals. P2RX₄ gene had the highest expression in monocyte-derived macrophages, followed by P2RX₇ and P2RX₁  genes. There was no significant difference in P2X receptor mRNA expression level between macrophages of AS patients and healthy individuals.

CONCLUSIONS

Our results indicate that AS patients show altered expression levels of P2 receptor genes. Moreover, these changes might be associated with disease activity and patients' status.

Pannexin 3 ER Ca2+ channel gating is regulated by phosphorylation at the Serine 68 residue in osteoblast differentiation

Pannexin 3 (Panx3) is a regulator of bone formation. Panx3 forms three distinct functional channels: hemichannels, gap junctions, and endoplasmic reticulum (ER) Ca²⁺ channels. However, the gating mechanisms of the Panx3 channels remain unclear. Here, we show that the Panx3 ER Ca²⁺ channel is modulated by phosphorylation of the serine 68 residue (Ser68) to promote osteoblast differentiation. Among the 17 candidate phosphorylation sites identified, the mutation of Ser68 to Ala (Ser68Ala) was sufficient to inhibit Panx3-mediated osteoblast differentiation via reduction of Osterix and ALP expression. Using a Ser68 phospho-specific antibody (P-Panx3) revealed Panx3 was phosphorylated in prehypertrophic, hypertrophic chondrocytes, and bone areas of the newborn growth plate. In osteogenic C2C12 cells, P-Panx3 was located on the ER membranes. Importantly, the Ser68Ala mutation only affected Panx3 ER Ca²⁺ channel function. Ser68 on Panx3 was phosphorylated by ATP stimulation and PI3K/Akt signaling. Finally, real-time FRET imaging and ratio analysis revealed that the Panx3 channel conformation was sensitive to ATP. Together, the phosphorylation of Panx3 at Ser68 is an essential step controlling the gating of the Panx3 ER Ca²⁺ channel to promote osteogenesis.

Selective inhibition of Panx1 channels decreases hemostasis and thrombosis in vivo

BACKGROUND

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

OBJECTIVE

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

RESULTS

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

CONCLUSIONS

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

Age- and sex-dependent role of osteocytic pannexin1 on bone and muscle mass and strength

Pannexins (Panxs), glycoproteins that oligomerize to form hemichannels on the cell membrane, are topologically similar to connexins, but do not form cell-to-cell gap junction channels. There are 3 members of the family, 1-3, with Panx1 being the most abundant. All Panxs are expressed in bone, but their role in bone cell biology is not completely understood. We now report that osteocytic Panx1 deletion (Panx1Δot) alters bone mass and strength in female mice. Bone mineral density after reaching skeletal maturity is higher in female Panx1Δot mice than in control Panx1fl/fl mice. Further, osteocytic Panx1 deletion partially prevented aging effects on cortical bone structure and mechanical properties. Young 4-month-old female Panx1Δot mice exhibited increased lean body mass, even though pannexin levels in skeletal muscle were not affected; whereas no difference in lean body mass was detected in male mice. Furthermore, female Panx1-deficient mice exhibited increased muscle mass without changes in strength, whereas Panx1Δot males showed unchanged muscle mass and decreased in vivo maximum plantarflexion torque, indicating reduced muscle strength. Our results suggest that osteocytic Panx1 deletion increases bone mass in young and old female mice and muscle mass in young female mice, but has deleterious effects on muscle strength only in males.

In vitro effect of Pannexin 1 channel on the invasion and migration of I-10 testicular cancer cells via ERK1/2 signaling pathway

Pannexin (Panx) plays a crucial role in several cellular processes such as immune cell death, cell proliferation, invasion, and migration, apoptosis, and autophagy. However, the role of Panx in regulating cell migration and invasion in testicular cancer remains to be elucidated. In the present study, we determined the correlation between Panx-1 channel function and migration and invasion in I-10 testicular cancer cells. Transwell and wound healing assays showed that inhibition of Panx-1 by carbenoxolone (CBX) and probenecid (PBN) attenuated the migration and invasion of testicular cancer cells in vitro. Moreover, knockdown of Panx-1 with short hairpin RNA (shRNA) remarkably decreased the migration and invasion ability of I-10 cells. In shRNA-transfected cells, extracellular ATP (released through Panx channel) was also found to be decreased. Similarly, overexpression of Panx-1 with mPanx-1 increased the migration and invasion ability of I-10 cells. Moreover, we found that in mPanx-1-transfected cells treated with U0126 (inhibitor of p-ERK1/2), the migration and invasion of I-10 cells were remarkably attenuated. Overall, increased Panx-1 promotes migration and invasion in testicular cancer cells, and the effect is probably be related with ERK1/2 kinase activity. Thus, Panx-1 can serve as a potential therapeutic target for the treatment of testicular cancer.

Retracted: Downregulation of microRNA-367 promotes osteoblasts growth and proliferation of mice during fracture by activating the PANX3-mediated Wnt/β-catenin pathway

A majority of people suffering from bone fractures fail to heal and develop a nonunion, which is a challenging orthopedic complication requiring complex and expensive treatment. Previous data showed the inhibition of some microRNAs (miRNAs or miRs) can enhance fracture healing. The objective of the present study is to explore effects of miR-367 on the osteoblasts growth and proliferation of mouse during fracture via the Wnt/β-catenin pathway by targeting PANX3. Primarily, the femur fracture model was successfully established in 66 (C57BL/6) 6-week-old male mice. To verify whether miR-367 target PANX3, we used the target prediction program and performed luciferase activity determination. Subsequently, to figure out the underlying regulatory roles of miR-367 in fracture, osteoblasts were elucidated by treatment with miR-367 mimic, miR-367 inhibitor, or siRNA against PANX3 to determine the expression of miR-367, siPANX3, β-catenin, and Wnt5b as well as cell proliferation and apoptosis. The results demonstrated that PANX3 was verified as a target gene of miR-367. MiR-367 was found to highly expressed but PANX3, β-catenin, and Wnt5b were observed poorly expressed in fracture mice. downregulated miR-367 increased the mRNA and protein expression of PANX3, β-catenin, and Wnt5b, increased cell growth, proliferation, and migration, while decreased cell apoptosis in osteoblasts. Altogether, our study demonstrates that the downregulation of miR-367 may promote osteoblasts growth and proliferation in fracture through the activation of the PANX3-dependent Wnt/β-catenin pathway.

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

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

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

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

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

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

Pannexin 1 regulates adipose stromal cell differentiation and fat accumulation

Pannexin 1 (Panx1) is a channel-forming glycoprotein important in paracrine signaling and cellular development. In this study, we discovered that mice globally lacking Panx1 (KO) have significantly greater total fat mass and reduced lean mass compared to wild type (WT) mice under a normal diet. Despite having higher fat content, Panx1 KO mice on a high fat diet exhibited no differences in weight gain and blood markers of obesity as compared to WT controls, except for an increase in glucose and insulin levels. However, metabolic cage data revealed that these Panx1 KO mice display significantly increased activity levels, higher ambulatory activity, and reduced sleep duration relative to their WT littermates on a high-fat diet. To uncover the cellular mechanism responsible for the increased fat content in the KO, we isolated primary cultures of adipose-derived stromal cells (ASCs) from WT and KO fat pads. In WT ASCs we observed that Panx1 protein levels increase upon induction into an adipogenic lineage. ASCs isolated from Panx1 KO mice proliferate less but demonstrate enhanced adipogenic differentiation with increased intracellular lipid accumulation, glycerol-3-phosphate dehydrogenase (GPDH) enzyme activity, and adipokine secretion, as compared to WT ASCs. This was consistent with the increased adipocyte size and decreased adipocyte numbers observed in subcutaneous fat of the Panx1 KO mice compared to WT. We concluded that Panx1 plays a key role in adipose stromal cells during the early stages of adipogenic proliferation and differentiation, regulating fat accumulation in vivo.

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

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

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

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.

Reversal of loss of bone mass in old mice treated with mefloquine

Aging is accompanied by imbalanced bone remodeling, elevated osteocyte apoptosis, and decreased bone mass and mechanical properties; and improved pharmacologic approaches to counteract bone deterioration with aging are needed. We examined herein the effect of mefloquine, a drug used to treat malaria and systemic lupus erythematosus and shown to ameliorate bone loss in glucocorticoid-treated patients, on bone mass and mechanical properties in young and old mice. Young 3.5-month-old and old 21-month-old female C57BL/6 mice received daily injections of 5 mg/kg/day mefloquine for 14 days. Aging resulted in the expected changes in bone volume and mechanical properties. In old mice mefloquine administration reversed the lower vertebral cancellous bone volume and bone formation; and had modest effects on cortical bone volume, thickness, and moment of inertia. Mefloquine administration did not change the levels of the circulating bone formation markers P1NP or alkaline phosphatase, whereas levels of the resorption marker CTX showed trends towards increase with mefloquine treatment. In addition, and as expected, aging bones exhibited an accumulation of active caspase3-expressing osteocytes and higher expression of apoptosis-related genes compared to young mice, which were not altered by mefloquine administration at either age. In young animals, mefloquine induced higher periosteal bone formation, but lower endocortical bone formation. Further, osteoclast numbers were higher on the endocortical bone surface and circulating CTX levels were increased, in mefloquine- compared to vehicle-treated young mice. Consistent with this, addition of mefloquine to bone marrow cells isolated from young mice led to increased osteoclastic gene expression and a tendency towards increased osteoclast numbers in vitro. Taken together our findings identify the age and bone-site specific skeletal effects of mefloquine. Further, our results highlight a beneficial effect of mefloquine administration on vertebral cancellous bone mass in old animals, raising the possibility of using this pharmacologic inhibitor to preserve skeletal health with aging.

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.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.