The participation of plasma membrane hemichannels to purinergic signaling

Regulation of hemichannels and gap junction channels by cytokines in antigen-presenting cells

Autocrine and paracrine signals coordinate responses of several cell types of the immune system that provide efficient protection against different challenges. Antigen-presenting cells (APCs) coordinate activation of this system via homocellular and heterocellular interactions. Cytokines constitute chemical intercellular signals among immune cells and might promote pro- or anti-inflammatory effects. During the last two decades, two membrane pathways for intercellular communication have been demonstrated in cells of the immune system. They are called hemichannels (HCs) and gap junction channels (GJCs) and provide new insights into the mechanisms of the orchestrated response of immune cells. GJCs and HCs are permeable to ions and small molecules, including signaling molecules. The direct intercellular transfer between contacting cells can be mediated by GJCs, whereas the release to or uptake from the extracellular milieu can be mediated by HCs. GJCs and HCs can be constituted by two protein families: connexins (Cxs) or pannexins (Panxs), which are present in almost all APCs, being Cx43 and Panx1 the most ubiquitous members of each protein family. In this review, we focus on the effects of different cytokines on the intercellular communication mediated by HCs and GJCs in APCs and their impact on purinergic signaling.

Extracellular gentamicin reduces the activity of connexin hemichannels and interferes with purinergic Ca(2+) signaling in HeLa cells

Gap junction channels (GJCs) and hemichannels (HCs) are composed of protein subunits termed connexins (Cxs) and are permeable to ions and small molecules. In most organs, GJCs communicate the cytoplasm of adjacent cells, while HCs communicate the intra and extracellular compartments. In this way, both channel types coordinate physiological responses of cell communities. Cx mutations explain several genetic diseases, including about 50% of autosomal recessive non-syndromic hearing loss. However, the possible involvement of Cxs in the etiology of acquired hearing loss remains virtually unknown. Factors that induce post-lingual hearing loss are diverse, exposure to gentamicin an aminoglycoside antibiotic, being the most common. Gentamicin has been proposed to block GJCs, but its effect on HCs remains unknown. In this work, the effect of gentamicin on the functional state of HCs was studied and its effect on GJCs was reevaluated in HeLa cells stably transfected with Cxs. We focused on Cx26 because it is the main Cx expressed in the cochlea of mammals where it participates in purinergic signaling pathways. We found that gentamicin applied extracellularly reduces the activity of HCs, while dye transfer across GJCs was not affected. HCs were also blocked by streptomycin, another aminoglycoside antibiotic. Gentamicin also reduced the adenosine triphosphate release and the HC-dependent oscillations of cytosolic free-Ca²⁺ signal. Moreover, gentamicin drastically reduced the Cx26 HC-mediated membrane currents in Xenopus laevis oocytes. Therefore, the extracellular gentamicin-induced inhibition of Cx HCs may adversely affect autocrine and paracrine signaling, including the purinergic one, which might partially explain its ototoxic effects.

Pannexin 1 channels: new actors in the regulation of catecholamine release from adrenal chromaffin cells

Chromaffin cells of the adrenal gland medulla synthesize and store hormones and peptides, which are released into the blood circulation in response to stress. Among them, adrenaline is critical for the fight-or-flight response. This neurosecretory process is highly regulated and depends on cytosolic [Ca²⁺]. By forming channels at the plasma membrane, pannexin-1 (Panx1) is a protein involved in many physiological and pathological processes amplifying ATP release and/or Ca²⁺ signals. Here, we show that Panx1 is expressed in the adrenal gland where it plays a role by regulating the release of catecholamines. In fact, inhibitors of Panx1 channels, such as carbenoxolone (Cbx) and probenecid, reduced the secretory activity induced with the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP, 50 μM) in whole adrenal glands. A similar inhibitory effect was observed in single chromaffin cells using Cbx or ¹⁰Panx1 peptide, another Panx1 channel inhibitors. Given that the secretory response depends on cytosolic [Ca²⁺] and Panx1 channels are permeable to Ca²⁺, we studied the possible implication of Panx1 channels in the Ca²⁺ signaling occurring during the secretory process. In support of this possibility, Panx1 channel inhibitors significantly reduced the Ca²⁺ signals evoked by DMPP in single chromaffin cells. However, the Ca²⁺ signals induced by caffeine in the absence of extracellular Ca²⁺ was not affected by Panx1 channel inhibitors, suggesting that this mechanism does not involve Ca²⁺ release from the endoplasmic reticulum. Conversely, Panx1 inhibitors significantly blocked the DMPP-induce dye uptake, supporting the idea that Panx1 forms functional channels at the plasma membrane. These findings indicate that Panx1 channels participate in the control the Ca²⁺ signal that triggers the secretory response of adrenal chromaffin cells. This mechanism could have physiological implications during the response to stress.

Defective channels lead to an impaired skin barrier

Channels are integral membrane proteins that form a pore, allowing the passive movement of ions or molecules across a membrane (along a gradient), either between compartments within a cell, between intracellular and extracellular environments or between adjacent cells. The ability of cells to communicate with one another and with their environment is a crucial part of the normal physiology of a tissue that allows it to carry out its function. Cell communication is particularly important during keratinocyte differentiation and formation of the skin barrier. Keratinocytes in the skin epidermis undergo a programme of apoptosis-driven terminal differentiation, whereby proliferating keratinocytes in the basal (deepest) layer of the epidermis stop proliferating, exit the basal layer and move up through the spinous and granular layers of the epidermis to form the stratum corneum, the external barrier. Genes encoding different families of channel proteins have been found to harbour mutations linked to a variety of rare inherited monogenic skin diseases. In this Commentary, we discuss how human genetic findings in aquaporin (AQP) and transient receptor potential (TRP) channels reveal different mechanisms by which these channel proteins function to ensure the proper formation and maintenance of the skin barrier.

Shock wave treatment enhances cell proliferation and improves wound healing by ATP release-coupled extracellular signal-regulated kinase (ERK) activation

Shock wave treatment accelerates impaired wound healing in diverse clinical situations. However, the mechanisms underlying the beneficial effects of shock waves have not yet been fully revealed. Because cell proliferation is a major requirement in the wound healing cascade, we used in vitro studies and an in vivo wound healing model to study whether shock wave treatment influences proliferation by altering major extracellular factors and signaling pathways involved in cell proliferation. We identified extracellular ATP, released in an energy- and pulse number-dependent manner, as a trigger of the biological effects of shock wave treatment. Shock wave treatment induced ATP release, increased Erk1/2 and p38 MAPK activation, and enhanced proliferation in three different cell types (C3H10T1/2 murine mesenchymal progenitor cells, primary human adipose tissue-derived stem cells, and a human Jurkat T cell line) in vitro. Purinergic signaling-induced Erk1/2 activation was found to be essential for this proliferative effect, which was further confirmed by in vivo studies in a rat wound healing model where shock wave treatment induced proliferation and increased wound healing in an Erk1/2-dependent fashion. In summary, this report demonstrates that shock wave treatment triggers release of cellular ATP, which subsequently activates purinergic receptors and finally enhances proliferation in vitro and in vivo via downstream Erk1/2 signaling. In conclusion, our findings shed further light on the molecular mechanisms by which shock wave treatment exerts its beneficial effects. These findings could help to improve the clinical use of shock wave treatment for wound healing.

Possible role of hemichannels in cancer

In humans, connexins (Cxs) and pannexins (Panxs) are the building blocks of hemichannels. These proteins are frequently altered in neoplastic cells and have traditionally been considered as tumor suppressors. Alteration of Cxs and Panxs in cancer cells can be due to genetic, epigenetic and post-transcriptional/post-translational events. Activated hemichannels mediate the diffusional membrane transport of ions and small signaling molecules. In the last decade hemichannels have been shown to participate in diverse cell processes including the modulation of cell proliferation and survival. However, their possible role in tumor growth and expansion remains largely unexplored. Herein, we hypothesize about the possible role of hemichannels in carcinogenesis and tumor progression. To support this theory, we summarize the evidence regarding the involvement of hemichannels in cell proliferation and migration, as well as their possible role in the anti-tumor immune responses. In addition, we discuss the evidence linking hemichannels with cancer in diverse models and comment on the current technical limitations for their study.

The dual face of connexin-based astroglial Ca(2+) communication: a key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca(2+), as a signaling ion, largely contributes. Altered intracellular Ca(2+) levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca(2+) increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca(2+) waves, thereby recruiting a larger group of cells. Intercellular Ca(2+) wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels ('half of a gap junction channel'). This review gives an overview of the current knowledge on Cx-mediated Ca(2+) communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca(2+) communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Inhibitors of the 5-lipoxygenase arachidonic acid pathway induce ATP release and ATP-dependent organic cation transport in macrophages

We have previously described that arachidonic acid (AA)-5-lipoxygenase (5-LO) metabolism inhibitors such as NDGA and MK886, inhibit cell death by apoptosis, but not by necrosis, induced by extracellular ATP (ATPe) binding to P2X7 receptors in macrophages. ATPe binding to P2X7 also induces large cationic and anionic organic molecules uptake in these cells, a process that involves at least two distinct transport mechanisms: one for cations and another for anions. Here we show that inhibitors of the AA-5-LO pathway do not inhibit P2X7 receptors, as judged by the maintenance of the ATPe-induced uptake of fluorescent anionic dyes. In addition, we describe two new transport phenomena induced by these inhibitors in macrophages: a cation-selective uptake of fluorescent dyes and the release of ATP. The cation uptake requires secreted ATPe, but, differently from the P2X7/ATPe-induced phenomena, it is also present in macrophages derived from mice deficient in the P2X7 gene. Inhibitors of phospholipase A2 and of the AA-cyclooxygenase pathway did not induce the cation uptake. The uptake of non-organic cations was investigated by measuring the free intracellular Ca(2+) concentration ([Ca(2+)]i) by Fura-2 fluorescence. NDGA, but not MK886, induced an increase in [Ca(2+)]i. Chelating Ca(2+) ions in the extracellular medium suppressed the intracellular Ca(2+) signal without interfering in the uptake of cationic dyes. We conclude that inhibitors of the AA-5-LO pathway do not block P2X7 receptors, trigger the release of ATP, and induce an ATP-dependent uptake of organic cations by a Ca(2+)- and P2X7-independent transport mechanism in macrophages.

Unexpected link between an antibiotic, pannexin channels and apoptosis

Plasma membrane pannexin 1 channels (PANX1) release nucleotide find-me signals from apoptotic cells to attract phagocytes. In a small molecule screen, we discovered the quinolone antibiotic trovafloxacin as a novel PANX1 inhibitor. Although quinolones are widely used to treat bacterial infections, some quinolones have unexplained side effects, including deaths among children. PANX1 is a direct target of trovafloxacin at drug concentrations seen in human plasma, and its inhibition led to dysregulated fragmentation of apoptotic cells. Genetic loss of PANX1 phenocopied trovafloxacin effects, revealing a non-redundant role for pannexin channels in regulating cellular disassembly during apoptosis. Increase in drug-resistant bacteria worldwide and the dearth of new antibiotics is a major human health challenge. Comparing different quinolone antibiotics suggests that certain structural features may contribute to PANX1 blockade. These data identify a novel linkage between an antibiotic, pannexin channels, and cellular integrity, and suggest that re-engineering certain quinolones might help develop newer antibacterials.

Connexins: sensors of epidermal integrity that are therapeutic targets

Gap junction proteins (connexins) are differentially expressed throughout the multiple layers of the epidermis. A variety of skin conditions arise with aberrant connexin expression or function and suggest that maintaining the epidermal gap junction network has many important roles in preserving epidermal integrity and homeostasis. Mutations in a number of connexins lead to epidermal dysplasias giving rise to a range of dermatological disorders of differing severity. 'Gain of function' mutations reveal connexin-mediated roles in calcium signalling within the epidermis. Connexins are involved in epidermal innate immunity, inflammation control and in wound repair. The therapeutic potential of targeting connexins to improve wound healing responses is now clear. This review discusses the role of connexins in epidermal integrity, and examines the emerging evidence that connexins act as epidermal sensors to a variety of mechanical, temperature, pathogen-induced and chemical stimuli. Connexins thus act as an integral component of the skin's protective barrier.

Purinergic control of AMPK activation by ATP released through connexin 43 hemichannels - pivotal roles in hemichannel-mediated cell injury

Connexin hemichannels regulate many cell functions. However, the molecular mechanisms involved remain elusive. Hemichannel opening causes loss of ATP, we therefore speculated a potential role for AMPK in the biological actions of hemichannels. Activation of hemichannels by removal of extracellular Ca(2+) led to an efflux of ATP and a weak activation of AMPK. Unexpectedly, dysfunction of hemichannels markedly potentiated AMPK activation, which was reproduced by promotion of extracellular ATP degradation or inhibition of P2 purinoceptors but counteracted by exogenous ATP. Further analysis revealed that ATP induced a purinoceptor-dependent activation of Akt and mTOR. Suppression of Akt or mTOR augmented AMPK activation, whereas activation of Akt by transfection of cells with myristoylated Akt, a constitutively active form of Akt, abolished AMPK activation. In a pathological model of hemichannel opening triggered by Cd(2+), disclosure of hemichannels similarly enhanced AMPK activity, which protected cells from Cd(2+)-induced cell injury through suppression of mTOR. In summary, our data point to a channel-mediated mechanism for the regulation of AMPK through a purinergic signaling pathway. Furthermore, we define AMPK as a pivotal molecule that underlies the regulatory effects of hemichannels on cell survival.

Role of vesicular nucleotide transporter VNUT (SLC17A9) in release of ATP from AR42J cells and mouse pancreatic acinar cells

ATP is released from cells in response to various stimuli. Our previous studies on pancreas indicated that pancreatic acini could be major stores of secreted ATP. In the present study, our aim was to establish the role of the vesicular nucleotide transporter (VNUT), SLC17A9, in storage and release of ATP. Freshly prepared acini from mice and AR42J rat acinar cells were used in this study. We illustrate that in AR42J cells, quinacrine (an ATP store marker) and Bodipy ATP (a fluorescent ATP analog) co-localized with VNUT-mCherry to vesicles/granules. Furthermore, in acini and AR42J cells, a marker of the zymogen granule membranes, Rab3D, and VNUT co-localized. Dexamethasone treatment of AR42J cells promoted formation of acinar structures, paralleled by increased amylase and VNUT expression, and increased ATP release in response to cholinergic stimulation. Mechanical stimulus (pressure) and cell swelling also induced ATP release, but this was not influenced by dexamethasone, most likely indicating different non-zymogen-related release mechanism. In conclusion, we propose that VNUT-dependent ATP release pathway is associated with agonist-induced secretion process and downstream purinergic signalling in pancreatic ducts.

A critical role for pannexin-1 in activation of innate immune cells of the choroid plexus

Epiplexus cells are a population of innate immune cells in the choroid plexus of the brain ventricles. They are thought to contribute to the immune component of the blood-cerebrospinal-fluid-barrier (BCSFB). Here we have developed a novel technique for studying epiplexus cells in acutely isolated, live and intact choroid plexus. We show that epiplexus cells are potently activated by exogenous ATP, increasing their motility within the tissue. This ATP-induced chemokinesis required activation of pannexin-1 channels, which are expressed by the epithelial cells of the choroid plexus and not the epiplexus cells themselves. Furthermore, ATP acts at least in part through the P2X4 ionotropic purinergic receptor. Thus, the resident immune cells of the choroid plexus appear to be in communication with the epithelial cells through pannexin-1 channels.

Astrocytes and microglia in acute cerebral injury underlying cerebral palsy associated with preterm birth

Cerebral palsy is one of the most devastating consequences of brain injury around the time of birth, and nearly a third of cases are now associated with premature birth. Compared with term babies, preterm babies have an increased incidence of complications that may increase the risk of disability, such as intraventricular hemorrhage, periventricular leukomalacia, sepsis, and necrotizing enterocolitis. The response to injury is highly dependent on brain maturity, and although cellular vulnerability is well documented, there is now evidence that premyelinating axons are also particularly sensitive to ischemic injury. In this review, we will explore recent evidence highlighting a central role for glia in mediating increased risk of disability in premature infants, including excessive activation of microglia and opening of astrocytic gap junction hemichannels in spreading injury after brain ischemia, in part likely involving release of adenosine triphosphate (ATP) and overactivation of purinergic receptors, particularly in white matter. We propose the hypothesis that inflammation-induced opening of connexin hemichannels is a key regulating event that initiates a vicious circle of excessive ATP release, which in turn propagates activation of purinergic receptors on microglia and astrocytes. This suggests that developing effective neuroprotective strategies for preterm infants requires a detailed understanding of glial responses.

Bradykinin-induced Ca2+ signaling in human subcutaneous fibroblasts involves ATP release via hemichannels leading to P2Y12 receptors activation

Background

Chronic musculoskeletal pain involves connective tissue remodeling triggered by inflammatory mediators, such as bradykinin. Fibroblast cells signaling involve changes in intracellular Ca²⁺ ([Ca²⁺]_i). ATP has been related to connective tissue mechanotransduction, remodeling and chronic inflammatory pain, via P2 purinoceptors activation. Here, we investigated the involvement of ATP in bradykinin-induced Ca²⁺ signals in human subcutaneous fibroblasts.

Results

Bradykinin, via B₂ receptors, caused an abrupt rise in [Ca²⁺]_i to a peak that declined to a plateau, which concentration remained constant until washout. The plateau phase was absent in Ca²⁺-free medium; [Ca²⁺]_i signal was substantially reduced after depleting intracellular Ca²⁺ stores with thapsigargin. Extracellular ATP inactivation with apyrase decreased the [Ca²⁺]_i plateau. Human subcutaneous fibroblasts respond to bradykinin by releasing ATP via connexin and pannexin hemichannels, since blockade of connexins, with 2-octanol or carbenoxolone, and pannexin-1, with ¹⁰Panx, attenuated bradykinin-induced [Ca²⁺]_i plateau, whereas inhibitors of vesicular exocytosis, such as brefeldin A and bafilomycin A1, were inactive. The kinetics of extracellular ATP catabolism favors ADP accumulation in human fibroblast cultures. Inhibition of ectonucleotidase activity and, thus, ADP formation from released ATP with POM-1 or by Mg²⁺ removal from media reduced bradykinin-induced [Ca²⁺]_i plateau. Selective blockade of the ADP-sensitive P2Y₁₂ receptor with AR-C66096 attenuated bradykinin [Ca²⁺]_i plateau, whereas the P2Y₁ and P2Y₁₃ receptor antagonists, respectively MRS 2179 and MRS 2211, were inactive. Human fibroblasts exhibited immunoreactivity against connexin-43, pannexin-1 and P2Y₁₂ receptor.

Conclusions

Bradykinin induces ATP release from human subcutaneous fibroblasts via connexin and pannexin-1-containing hemichannels leading to [Ca²⁺]_i mobilization through the cooperation of B₂ and P2Y₁₂ receptors.

De novo expression of connexin hemichannels in denervated fast skeletal muscles leads to atrophy

Denervation of skeletal muscles induces atrophy, preceded by changes in sarcolemma permeability of causes not yet completely understood. Here, we show that denervation-induced Evans blue dye uptake in vivo of fast, but not slow, myofibers was acutely inhibited by connexin (Cx) hemichannel/pannexin1 (Panx1) channel and purinergic ionotropic P2X7 receptor (P2X7R) blockers. Denervated myofibers showed up-regulation of Panx1 and de novo expression of Cx39, Cx43, and Cx45 hemichannels as well as P2X7Rs and transient receptor potential subfamily V, member 2, channels, all of which are permeable to small molecules. The sarcolemma of freshly isolated WT myofibers from denervated muscles also showed high hemichannel-mediated permeability that was slightly reduced by blockade of Panx1 channels or the lack of Panx1 expression, but was completely inhibited by Cx hemichannel or P2X7R blockers, as well as by degradation of extracellular ATP. However, inhibition of transient receptor potential subfamily V, member 2, channels had no significant effect on membrane permeability. Moreover, activation of the transcription factor NFκB and higher mRNA levels of proinflammatory cytokines (TNF-α and IL-1β) were found in denervated WT but not Cx43/Cx45-deficient muscles. The atrophy observed after 7 d of denervation was drastically reduced in Cx43/Cx45-deficient but not Panx1-deficient muscles. Therefore, expression of Cx hemichannels and P2X7R promotes a feed-forward mechanism activated by extracellular ATP, most likely released through hemichannels, that activates the inflammasome. Consequently, Cx hemichannels are potential targets for new therapeutic agents to prevent or reduce muscle atrophy induced by denervation of diverse etiologies.

Disruption in connexin-based communication is associated with intracellular Ca²⁺ signal alterations in astrocytes from Niemann-Pick type C mice

Reduced astrocytic gap junctional communication and enhanced hemichannel activity were recently shown to increase astroglial and neuronal vulnerability to neuroinflammation. Moreover, increasing evidence suggests that neuroinflammation plays a pivotal role in the development of Niemann-Pick type C (NPC) disease, an autosomal lethal neurodegenerative disorder that is mainly caused by mutations in the NPC1 gene. Therefore, we investigated whether the lack of NPC1 expression in murine astrocytes affects the functional state of gap junction channels and hemichannels. Cultured cortical astrocytes of NPC1 knock-out mice (Npc1^−/−) showed reduced intercellular communication via gap junctions and increased hemichannel activity. Similarly, astrocytes of newborn Npc1^−/− hippocampal slices presented high hemichannel activity, which was completely abrogated by connexin 43 hemichannel blockers and was resistant to inhibitors of pannexin 1 hemichannels. Npc1^−/− astrocytes also showed more intracellular Ca²⁺ signal oscillations mediated by functional connexin 43 hemichannels and P2Y₁ receptors. Therefore, Npc1^−/− astrocytes present features of connexin based channels compatible with those of reactive astrocytes and hemichannels might be a novel therapeutic target to reduce neuroinflammation in NPC disease.

Communication between neuronal somata and satellite glial cells in sensory ganglia

Studies of the structural organization and functions of the cell body of a neuron (soma) and its surrounding satellite glial cells (SGCs) in sensory ganglia have led to the realization that SGCs actively participate in the information processing of sensory signals from afferent terminals to the spinal cord. SGCs use a variety ways to communicate with each other and with their enwrapped soma. Changes in this communication under injurious conditions often lead to abnormal pain conditions. "What are the mechanisms underlying the neuronal soma and SGC communication in sensory ganglia?" and "how do tissue or nerve injuries affect the communication?" are the main questions addressed in this review.

Carbenoxolone blocks endotoxin-induced protein kinase R (PKR) activation and high mobility group box 1 (HMGB1) release

The pathogen- and damage-associated molecular patterns (for example, bacterial endotoxin and adenosine 5'-triphosphate [ATP]) activate the double-stranded RNA-activated protein kinase R (PKR) to trigger the inflammasome-dependent high mobility group box 1 (HMGB1) release. Extracellular ATP contributes to the inflammasome activation through binding to the plasma membrane purinergic P2X7 receptor (P2X7R), triggering the opening of P2X7R channels and the pannexin-1 (panx-1) hemichannels permeable for larger molecules up to 900 daltons. It was previously unknown whether panx-1 channel blockers can abrogate lipopolysaccharide (LPS)-induced PKR activation and HMGB1 release in innate immune cells. Here we demonstrated that a major gancao (licorice) component (glycyrrhizin, or glycyrrhizic acid) derivative, carbenoxolone (CBX), dose dependently abrogated LPS-induced HMGB1 release in macrophage cultures with an estimated IC50 ≈ 5 μmol/L. In an animal model of polymicrobial sepsis (induced by cecal ligation and puncture [CLP]), repetitive CBX administration beginning 24 h after CLP led to a significant reduction of circulating and peritoneal HMGB1 levels, and promoted a significant increase in animal survival rates. As did P2X7R antagonists (for example, oxidized ATP, oATP), CBX also effectively attenuated LPS-induced P2X7R/panx-1 channel activation (as judged by Lucifer Yellow dye uptake) and PKR phosphorylation in primary peritoneal macrophages. Collectively, these results suggested that CBX blocks LPS-induced HMGB1 release possibly through impairing PKR activation, supporting the involvement of PKR in the regulation of HMGB1 release.

ATP release and Ca2+ signalling by human bronchial epithelial cells following Alternaria aeroallergen exposure

  Exposure of human bronchial epithelial (HBE) cells from normal and asthmatic subjects to extracts from Alternaria alternata evoked a rapid and sustained release of ATP with greater efficacy observed in epithelial cells from asthmatic patients. Previously, Alternaria allergens were shown to produce a sustained increase in intracellular Ca2+ concentration ([Ca2+]i) that was dependent on the coordinated activation of specific purinergic receptor (P2Y2 and P2X7) subtypes. In the present study, pretreatment with a cell-permeable Ca2+-chelating compound (BAPTA-AM) significantly inhibited ATP release, indicating dependency on [Ca2+]i. Alternaria-evoked ATP release exhibited a greater peak response and a slightly lower EC50 value in cells obtained from asthmatic donors compared to normal control cells. Furthermore, the maximum increase in [Ca2+]i resulting from Alternaria treatment was greater in cells from asthmatic patients compared to normal subjects. The vesicle transport inhibitor brefeldin A and BAPTA-AM significantly blocked Alternaria-stimulated incorporation of fluorescent lipid (FM1-43)-labelled vesicles into the plasma membrane and ATP release. In addition, inhibiting uptake of ATP into exocytotic vesicles with bafilomycin also reduced ATP release comparable to the effects of brefeldin A and BAPTA-AM. These results indicate that an important mechanism for Alternaria-induced ATP release is Ca2+ dependent and involves exocytosis of ATP. Serine and cysteine protease inhibitors also reduced Alternaria-induced ATP release; however, the sustained increase in [Ca2+]i typically observed following Alternaria exposure appeared to be independent of protease-activated receptor (PAR2) stimulation.

Connexin hemichannel blockade is neuroprotective after, but not during, global cerebral ischemia in near-term fetal sheep

There is increasing evidence that connexin hemichannels, the half gap junctions that sit unopposed in the cell membrane, can open during ischemia and that blockade of connexin43 hemichannels after cerebral ischemia can improve neural outcomes. However, it is unclear whether connexin blockade during ischemia is protective. In the present study global cerebral ischemia was induced by 30 min of bilateral carotid artery occlusion in near-term (128 ± 1 day gestation age) fetal sheep. A specific mimetic peptide that blocks connexin43 hemichannels was infused into the lateral ventricle for either 1h before and during ischemia (intra-ischemia group, n=6) or for 25 h starting 90 min after the end of ischemia (post-ischemia group, n=7). The vehicle was infused in the ischemia-vehicle group (n=6) and sham-controls received sham occlusion plus vehicle (n=10). The post-ischemia group showed enhanced recovery of EEG power from day five until the end of the experiment (-5 ± 1.6 dB) compared to ischemia-vehicle (-13 ± 1.9 dB, p<0.05) and intra-ischemia infusion (-14.4 ± 3.6 dB, p<0.05). Post-ischemic infusion was associated with higher neuronal counts compared to ischemia-vehicle and intra-ischemia in the cortex (p<0.05) but not the CA1 and CA3 regions of the hippocampus. Oligodendrocyte cell counts in the intragyral and periventricular white matter were significantly higher in the post-ischemia group compared to ischemia-vehicle and intra-ischemia infusion (p<0.05). These large animal data support the hypothesis that connexin hemichannel opening after, but not during, ischemia contributes to the spread of white and gray matter injury of the developing brain.

Assessing a potential role of host Pannexin 1 during Chlamydia trachomatis infection

Pannexin 1 (Panx1) is a plasma membrane channel glycoprotein that plays a role in innate immune response through association with the inflammasome complex. Probenecid, a classic pharmacological agent for gout, has also been used historically in combination therapy with antibiotics to prevent cellular drug efflux and has been reported to inhibit Panx1. As the inflammasome has been implicated in the progression of Chlamydia infections, and with chlamydial infections at record levels in the US, we therefore investigated whether probenecid would have a direct effect on Chlamydia trachomatis development through inhibition of Panx1. We found chlamydial development to be inhibited in a dose-dependent, yet reversible manner in the presence of probenecid. Drug treatment induced an aberrant chlamydial morphology consistent with persistent bodies. Although Panx1 was shown to localize to the chlamydial inclusion, no difference was seen in chlamydial development during infection of cells derived from wild-type and Panx1 knockout mice. Therefore, probenecid may inhibit C. trachomatis growth by an as yet unresolved mechanism.

Gap junctional communication promotes apoptosis in a connexin-type-dependent manner

Gap junctions (GJs) have been described to modulate cell death and survival. It still remains unclear whether this effect requires functional GJ channels or depends on channel-independent effects of connexins (Cx), the constituents of GJs. Therefore, we analysed the apoptotic response to streptonigrin (SN, intrinsic apoptotic pathway) or to α-Fas (extrinsic apoptotic pathway) in HeLa cells expressing Cx43 as compared with empty vector-transfected (CTL) cells. Apoptosis assessed by annexin V-fluorescein isothiocyanate/propidium iodide staining was significantly higher in HeLa-Cx43 compared with HeLa-CTL cells. Moreover, the cleavage of caspase-7 or Parp occurred earlier in HeLa-Cx43 than in HeLa-CTL cells. Comparative analysis of the effect of two further (endothelial) Cx (Cx37 and Cx40) on apoptosis revealed that apoptosis was highest in HeLa-Cx43 and lowest in HeLa-Cx37 cells, and correlated with the GJ permeability (assessed by spreading of a GJ-permeable dye and locally induced Ca(2+) signals). Pharmacologic inhibition of GJ formation in HeLa-Cx43 cells reduced apoptosis significantly. The role of GJ communication was further analysed by the expression of truncated Cx43 proteins with and without channel-forming capacity. Activation of caspases was higher in cells expressing the channel-building part (HeLa-Cx43NT-GFP) than in cells expressing the channel-incompetent C-terminal part of Cx43 (HeLa-Cx43CT-GFP) only. A hemichannel-dependent release and, hence, paracrine effect of proapoptotic signals could be excluded since the addition of a peptide (Pep)-blocking Cx43-dependent hemichannels (but not GJs) did not reduce apoptosis in HeLa-Cx43 cells. Treatment with SN resulted in a significant higher increase of the intracellular free Ca(2+) concentration in HeLa-Cx43 and HeLa-Cx43NT-GFP cells compared with HeLa-CTL or HeLa-Cx43CT-GFP cells, suggesting that Ca(2+) or a Ca(2+)-releasing agent could play a signalling role. Blocking of inositol triphosphate receptors reduced the SN-induced Ca(2+) increase as well as the increase in apoptosis. Our observations suggest that Cx43 and Cx40 but not Cx37 promote apoptosis via gap junctional transfer of pro-apoptotic signals between cells.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

The role of transcription-independent damage signals in the initiation of epithelial wound healing

Wound healing is an essential biological process that comprises sequential steps aimed at restoring the architecture and function of damaged cells and tissues. This process begins with conserved damage signals, such as Ca(2+), hydrogen peroxide (H2O2) and ATP, that diffuse through epithelial tissues and initiate immediate gene transcription-independent cellular effects, including cell shape changes, the formation of functional actomyosin structures and the recruitment of immune cells. These events integrate the ensuing transcription of specific wound response genes that further advance the wound healing response. The immediate importance of transcription-independent damage signals illustrates that healing a wound begins as soon as damage occurs.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Involvement of connexin43 hemichannel in ATP release after γ-irradiation

Ionizing radiation induces biological effects not only in irradiated cells but also in non-irradiated cells, which is called the bystander effect. Recently, in vivo and in vitro experiments have suggested that both gap junction hemichannel connexin43 (Cx43) and extracellular adenosine triphosphate (ATP) released from cells play a role in the bystander effect. We have reported that γ-irradiation induces ATP release from B16 melanoma cells, which is dependent on the P2X(7) receptor. However, the mechanism of ATP release caused by irradiation remains unclear. We here show the involvement of Cx43 in P2X(7) receptor-dependent ATP release after 0.5 Gy γ-irradiation. Inhibitors of gap junction hemichannels and an inhibitory peptide for Cx43 (gap26), but not an inhibitory peptide for pannexin1 (Panx1), significantly blocked γ-irradiation-induced ATP release from B16 melanoma cells. We confirmed high expression of Cx43 mRNA in B16 melanoma cells. These results suggest involvement of Cx43 in radiation-induced ATP release. We found that after 0.5 Gy γ-irradiation tyrosine phosphorylation was significantly blocked by P2X(7) receptor antagonist, but not gap26, suggesting that tyrosine phosphorylation is a downstream event from the P2X(7) receptor. Since tyrosine kinase inhibitor significantly suppressed radiation-induced ATP release, tyrosine phosphorylation appears to play an important role in the Cx43-mediated ATP release downstream of the P2X(7) receptor. In conclusion, the Cx43 hemichannel, which lies downstream of the P2X(7) receptor, is involved in ATP release in response to radiation. Our results suggest a novel mechanism for radiation-induced biological effects mediated by both ATP and Cx43.

Autocrine regulation of γ-irradiation-induced DNA damage response via extracellular nucleotides-mediated activation of P2Y6 and P2Y12 receptors

A key component of the response to DNA damage caused by ionizing radiation is DNA repair. Release of extracellular nucleotides, such as ATP, from cells plays a role in signaling via P2 receptors. We show here that release of ATP, followed by activation of P2Y receptors, is involved in the response to γ-irradiation-induced DNA damage. Formation of phosphorylated histone variant H2AX (γH2AX) foci, which are induced in nuclei by DNA damage and contribute to accumulation of DNA-repair factors, was increased at 1-3h after γ-ray irradiation (2.0Gy) of human lung cancer A549 cells. Focus formation was suppressed by pre-treatment with the ecto-nucleotidase apyrase. Pre-treatment with ecto-nucleotidase inhibitor ARL67156 or post-treatment with ATP or UTP facilitated induction of γH2AX, indicating that extracellular nucleotides play a role in induction of γH2AX foci. Next, we examined the effect of P2 receptor inhibitors on activation of ataxia telangiectasia mutated (ATM; a protein kinase) and accumulation of 53BP1 (a DNA repair factor), both of which are important for DNA repair, at DNA damage sites. P2Y6 receptor antagonist MRS2578, P2Y12 receptor antagonist clopidogrel, and P2X7 receptor antagonists A438079 and oxATP significantly inhibited these processes. Release of ATP was detected within 2.5min after irradiation, but was blocked by A438079. Activation of ATM and accumulation of 53BP1 were decreased in P2Y6 or P2Y12 receptor-knockdown cells. We conclude that autocrine/paracrine signaling through P2X7-dependent ATP release and activation of P2Y6 and P2Y12 receptors serves to amplify the cellular response to DNA damage caused by γ-irradiation.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Neurotransmitters and integration in neuronal-astroglial networks

Two major neural cell types, glia, astrocytes in particular, and neurones can release chemical transmitters that act as soluble signalling compounds for intercellular communication. Exocytosis, a process which depends on an increase in cytosolic Ca(2+) levels, represents a common denominator for release of neurotransmitters, stored in secretory vesicles, from these neural cells. While neurones rely predominately on the immediate entry of Ca(2+) from the extracellular space to the cytosol in this process, astrocytes support their cytosolic Ca(2+) increases by appropriating this ion from the intracellular endoplasmic reticulum store and extracellular space. Additionally, astrocytes can release neurotransmitters using a variety of non-vesicular pathways which are mediated by an assortment of plasmalemmal channels and transporters. Once a neuronal and/or astrocytic neurotransmitter is released into the extracellular space, it can activate plasma membrane neurotransmitter receptors on neural cells, causing autocrine and/or paracrine signalling. Moreover, chemical transmission is essential not only for homocellular, but also for heterocellular bi-directional communication in the brain. Further detailed understanding of chemical transmission will aid our comprehension of the brain (dys)function in heath and disease.

The P2X7 Receptor is an Important Regulator of Extracellular ATP Levels

Controlled ATP release has been demonstrated from many neuronal and non-neuronal cell types. Once released, extracellular ATP acts on cells in a paracrine manner via purinergic receptors. Considerable evidence now suggests that extracellular nucleotides, signaling via P2 receptors, play important roles in bone homeostasis modulating both osteoblast and osteoclast function. In this study, we demonstrate that mouse osteoclasts and their precursors constitutively release ATP into their extracellular environment. Levels were highest at day 2 (precursor cells), possibly reflecting the high number of red blood cells and accessory cells present. Mature osteoclasts constitutively released ATP in the range 0.05-0.5 pmol/ml/cell. Both osteoclasts and osteoblasts express mRNA and protein for the P2X7 receptor. We found that in osteoclasts, expression levels are fourfold higher in mature cells relative to precursors, whilst in osteoblasts expression remains relatively constant during differentiation. Selective antagonists (0.1-100 μM AZ10606120, A438079, and KN-62) were used to determine whether this release was mediated via P2X7 receptors. AZ10606120, A438079, and KN-62, at 0.1-10 μM, decreased ATP release by mature osteoclasts by up to 70, 60, and 80%, respectively. No differences in cell viability were observed. ATP release also occurs via vesicular exocytosis; inhibitors of this process (1-100 μM NEM or brefeldin A) had no effect on ATP release from osteoclasts. P2X7 receptor antagonists (0.1-10 μM) also decreased ATP release from primary rat osteoblasts by up to 80%. These data show that ATP release via the P2X7 receptor contributes to extracellular ATP levels in osteoclast and osteoblast cultures, suggesting an important additional role for this receptor in autocrine/paracrine purinergic signaling in bone.