Pannexin 1 channels mediate ‘find-me’ signal release and membrane permeability during apoptosis. Nature. 2010;467:863-867. [PMID: 20944749 DOI: 10.1038/nature09413]

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Hemichannels: permeants and their effect on development, physiology and death

Hemichannels, which are one half of the gap junction channels, have independent physiological roles. Although hemichannels consisting of connexins are more widely documented, hemichannels of pannexins, proteins homologous to invertebrate gap junction proteins also have been studied. There are at least 21 different connexin and three pannexin isotypes. This variety in isotypes results in tissue-specific hemichannels, which have been implicated in varied events ranging from development, cell survival, to cell death. Hemichannel function varies with its spatio-temporal opening, thus demanding a refined degree of regulation. This review discusses the activity of hemichannels and the molecules released in different physiological states and their impact on tissue functioning.

Caspase-1-induced pyroptotic cell death

Programmed cell death is a necessary part of development and tissue homeostasis enabling the removal of unwanted cells. In the setting of infectious disease, cells that have been commandeered by microbial pathogens become detrimental to the host. When macrophages and dendritic cells are compromised in this way, they can be lysed by pyroptosis, a cell death mechanism that is distinct from apoptosis and oncosis/necrosis. Pyroptosis is triggered by Caspase-1 after its activation by various inflammasomes and results in lysis of the affected cell. Both pyroptosis and apoptosis are programmed cell death mechanisms but are dependent on different caspases, unlike oncosis. Similar to oncosis and unlike apoptosis, pyroptosis results in cellular lysis and release of the cytosolic contents to the extracellular space. This event is predicted to be inherently inflammatory and coincides with interleukin-1β (IL-1β) and IL-18 secretion. We discuss the role of distinct inflammasomes, including NLRC4, NLRP3, and AIM2, as well as the role of the ASC focus in Caspase-1 signaling. We further review the importance of pyroptosis in vivo as a potent mechanism to clear intracellular pathogens.

Beginnings of a good apoptotic meal: the find-me and eat-me signaling pathways

Prompt and efficient clearance of apoptotic cells is necessary to prevent secondary necrosis of dying cells and to avoid immune responses to autoantigens. Recent studies have shed light on how apoptotic cells through soluble "find-me" signals advertise their presence to phagocytes at the earliest stages of cell death. Phagocytes sense the find-me signal gradient, and in turn the presence of dying cells, and migrate to their vicinity. The apoptotic cells also expose specific "eat-me" signals on their surface that are recognized by phagocytes through specific engulfment receptors. This review covers the recent progress in the areas of find-me and eat-me signals and how these relate to prompt and immunologically silent clearance of apoptotic cells.

Helicobacter pylori relies primarily on the purine salvage pathway for purine nucleotide biosynthesis

Helicobacter pylori is a chronic colonizer of the gastric epithelium and plays a major role in the development of gastritis, peptic ulcer disease, and gastric cancer. In its coevolution with humans, the streamlining of the H. pylori genome has resulted in a significant reduction in metabolic pathways, one being purine nucleotide biosynthesis. Bioinformatic analysis has revealed that H. pylori lacks the enzymatic machinery for de novo production of IMP, the first purine nucleotide formed during GTP and ATP biosynthesis. This suggests that H. pylori must rely heavily on salvage of purines from the environment. In this study, we deleted several genes putatively involved in purine salvage and processing. The growth and survival of these mutants were analyzed in both nutrient-rich and minimal media, and the results confirmed the presence of a robust purine salvage pathway in H. pylori. Of the two phosphoribosyltransferase genes found in the H. pylori genome, only gpt appears to be essential, and an Δapt mutant strain was still capable of growth on adenine, suggesting that adenine processing via Apt is not essential. Deletion of the putative nucleoside phosphorylase gene deoD resulted in an inability of H. pylori to grow on purine nucleosides or the purine base adenine. Our results suggest a purine requirement for growth of H. pylori in standard media, indicating that H. pylori possesses the ability to utilize purines and nucleosides from the environment in the absence of a de novo purine nucleotide biosynthesis pathway.

Programmed cell removal: a new obstacle in the road to developing cancer

The development of cancer involves mechanisms by which aberrant cells overcome normal regulatory pathways that limit their numbers and their migration. The evasion of programmed cell death is one of several key early events that need to be overcome in the progression from normal cellular homeostasis to malignant transformation. Recently, we provided evidence in mouse and human cancers that successful cancer clones must also overcome programmed cell removal. In this Opinion article, we explore the role of programmed cell removal in both normal and neoplastic cells, and we place this pathway in the context of the initiation of programmed cell death.

Thrombin-induced ATP release from human umbilical vein endothelial cells

ATP and its degradation products play an important role as signaling molecules in the vascular system, and endothelial cells are considered to be an important source of nucleotide release. To investigate the mechanism and physiological significance of endothelial ATP release, we compared different pharmacological stimuli for their ability to evoke ATP release from first passage cultivated human umbilical vein endothelial cells (HUVECs). Agonists known to increase intracellular Ca(2+) levels (A23187, histamine, thrombin) induced a stable, non-lytic ATP release. Since thrombin proved to be the most robust and reproducible stimulus, the molecular mechanism of thrombin-mediated ATP release from HUVECs was further investigated. ATP rapidly increased with thrombin (1 U/ml) and reached a steady-state level after 4 min. Loading the cells with BAPTA-AM to capture intracellular calcium suppressed ATP release. The thrombin-specific, protease-activated receptor 1 (PAR-1)-specific agonist peptide TFLLRN (10 μM) fully mimicked thrombin action on ATP release. To identify the nature of the ATP-permeable pathway, we tested various inhibitors of potential ATP channels for their ability to inhibit the thrombin response. Carbenoxolone, an inhibitor of connexin hemichannels and pannexin channels, as well as Gd(3+) were highly effective in blocking the thrombin-mediated ATP release. Specifically targeting connexin43 (Cx43) and pannexin1 (Panx1) revealed that reducing Panx1 expression significantly reduced ATP release, while downregulating Cx43 was ineffective. Our study demonstrates that thrombin at physiological concentrations is a potent stimulus of endothelial ATP release involving PAR-1 receptor activation and intracellular calcium mobilization. ATP is released by a carbenoxolone- and Gd(3+)- sensitive pathway, most likely involving Panx1 channels.

Pannexins in ischemia-induced neurodegeneration

Pannexin 1 (Px1, Panx1) and pannexin 2 (Px2, Panx2) form large-pore nonselective channels in the plasma membrane of cells and were suggested to play a role in the pathophysiology of cerebral ischemia. To directly test a potential contribution of pannexins in ischemia-related mechanisms, we performed experiments in Px1(-/-), Px2(-/-), and Px1(-/-)Px2(-/-) knockout mice. IL-1β release, channel function in astrocytes, and cortical spreading depolarization were not altered in Px1(-/-)Px2(-/-) mice, indicating that, in contrast to previous concepts, these processes occur normally in the absence of pannexin channels. However, ischemia-induced dye release from cortical neurons was lower, indicating that channel function in Px1(-/-)Px2(-/-) neurons was impaired. Furthermore, Px1(-/-)Px2(-/-) mice had a better functional outcome and smaller infarcts than wild-type mice when subjected to ischemic stroke. In conclusion, our data demonstrate that Px1 and Px2 underlie channel function in neurons and contribute to ischemic brain damage.

Adenosine and hypoxia-inducible factor signaling in intestinal injury and recovery

The gastrointestinal mucosa has proven to be an interesting tissue in which to investigate disease-related metabolism. In this review, we outline some of the evidence that implicates hypoxia-mediated adenosine signaling as an important signature within both healthy and diseased mucosa. Studies derived from cultured cell systems, animal models, and human patients have revealed that hypoxia is a significant component of the inflammatory microenvironment. These studies have revealed a prominent role for hypoxia-induced factor (HIF) and hypoxia signaling at several steps along the adenine nucleotide metabolism and adenosine receptor signaling pathways. Likewise, studies to date in animal models of intestinal inflammation have demonstrated an almost uniformly beneficial influence of HIF stabilization on disease outcomes. Ongoing studies to define potential similarities with and differences between innate and adaptive immune responses will continue to teach us important lessons about the complexity of the gastrointestinal tract. Such information has provided new insights into disease pathogenesis and, importantly, will provide insights into new therapeutic targets.

Ischemia and reperfusion--from mechanism to translation

Ischemia and reperfusion-elicited tissue injury contributes to morbidity and mortality in a wide range of pathologies, including myocardial infarction, ischemic stroke, acute kidney injury, trauma, circulatory arrest, sickle cell disease and sleep apnea. Ischemia-reperfusion injury is also a major challenge during organ transplantation and cardiothoracic, vascular and general surgery. An imbalance in metabolic supply and demand within the ischemic organ results in profound tissue hypoxia and microvascular dysfunction. Subsequent reperfusion further enhances the activation of innate and adaptive immune responses and cell death programs. Recent advances in understanding the molecular and immunological consequences of ischemia and reperfusion may lead to innovative therapeutic strategies for treating patients with ischemia and reperfusion-associated tissue inflammation and organ dysfunction.

Alanine substitution scanning of pannexin1 reveals amino acid residues mediating ATP sensitivity

Pannexin1 is a prime candidate to represent an ATP release channel. The pannexin1 channel can be activated by extracellular ATP through purinergic receptors P2X7 or P2Y. Recent studies have shown that the Pannexin1 channel is inhibited by its own permeant ion, ATP, and also by P2X7 receptor agonists and antagonists. However, the dose dependence of this inhibition indicated that significant inhibition was prominent at ATP concentrations higher than required for activation of purinergic receptors, including P2X7 and P2Y2. The inhibitory effect of ATP is largely decreased when R75 in the first extracellular loop of Pannexin1 is mutated to alanine, indicating that ATP regulates this channel presumably through binding. To further investigate the structural property of the putative ATP binding site, we performed alanine-scanning mutagenesis of the extracellular loops of pannexin1. Mutations on W74, S237, S240, I247 and L266 in the extracellular loops 1 and 2 severely impaired the inhibitory effect of BzATP, indicating that they might be the essential amino acids in the putative binding site. Mutations on R75, S82, S93, L94, D241, S249, P259 and I267 moderately (≥50%) decreased BzATP sensitivity, suggesting their supporting roles in the binding. Mutations of other residues did not change the BzATP potency compared to wild-type except for some nonfunctional mutants. These data demonstrate that several amino acid residues on the extracellular loops of Pannexin1 mediate ATP sensitivity. Cells expressing mutant Panx1W74A exhibited an enhanced release of ATP, consistent with the removal of a negative feedback loop controlling ATP release.

Red blood cell dynamics: from cell deformation to ATP release

The mechanisms of red blood cell (RBC) deformation under both static and dynamic, i.e., flow, conditions have been studied extensively since the mid 1960s. Deformation-induced biochemical reactions and possible signaling in RBCs, however, were proposed only fifteen years ago. Therefore, the fundamental relationship between RBC deformation and cellular signaling dynamics i.e., mechanotransduction, remains incompletely understood. Quantitative understanding of the mechanotransductive pathways in RBCs requires integrative studies of physical models of RBC deformation and cellular biochemical reactions. In this article we review the physical models of RBC deformation, spanning from continuum membrane mechanics to cellular skeleton dynamics under both static and flow conditions, and elaborate the mechanistic links involved in deformation-induced ATP release.

Single cysteines in the extracellular and transmembrane regions modulate pannexin 1 channel function

Pannexins form high-conductance ion channels in the membranes of many vertebrate cells. Functionally, they have been associated with multiple functional pathways like the propagation of calcium waves, ATP release, responses to ischemic conditions and apoptosis. In contrast to accumulating details which uncovered their functions, the molecular mechanisms for pannexin channel regulation and activation are hardly understood. To further elucidate regulatory mechanisms, we substituted cysteine residues, expected key elements for channel function, in extracellular and transmembrane regions of Pannexin 1 (Panx1). Most apparently, substitution of the transmembrane cysteine C40 resulted in constitutively open channels with profoundly increased activity. Hence, Xenopus laevis oocytes injected with corresponding cRNA showed strongly impaired viability, anomalous dye uptake and greatly increased whole-cell conductivity. All changes induced by C40 substitution were significantly reduced by the Panx1 channel blocker carbenoxolone, indicating that channel activity of the mutated Panx1 had been affected. In contrast, no changes occurred after substitution of the two other transmembrane cysteines, C215 and C227, in terms of channel conductivity. Finally, substitution of any of the four extracellular cysteines resulted in complete loss of channel function in both X. laevis oocytes and transfected N2A cells. From this, we conclude that cysteine residues of Panx1 reveal differential functional profiles for channel activation and drug sensitivity.

P2X7 receptor-pannexin 1 hemichannel association: effect of extracellular calcium on membrane permeabilization

Activation of P2X(7) receptor (P2X(7)R) and pannexin have been implicated in membrane permeabilization associated with ischemic cell death and many other inflammatory processes. P2X(7)R has a unique property of forming large pore upon repeated or prolonged application of agonist like ATP or 2', 3'-(4-benzoyl) benzoyl ATP. It has been proposed that pannexin 1 (panx1) hemichannel associates with P2X(7)R to form large pore, though the actual mechanism is not yet understood. Calcium concentration in extracellular milieu drops in many patho-physiological conditions, e.g. ischemia, when P2X(7)R/pannexin is also known to be activated. Therefore, we hypothesize that extracellular calcium ([Ca(2+)](o)) plays an important role in the coupling of P2X(7)R-panx1 and subsequent membrane permeabilization. In this study we show that membrane permeability of the P2X(7)R and panx1 expressing N2A cell increases in ([Ca(2+)](o))-free solution. In [Ca(2+)](o)-free solution, fluorescent dye calcein trapped cells exhibited time-dependent dye leakage resulting in about 50% decrease of fluorescence intensity in 30 min. Control cells in 2 mM [Ca(2+)](o) did not show such leakage. Like N2A cells, mixed culture of neuron and glia, derived from hippocampal progenitor cells showed similar dye leakage. Dye leakage was blocked either by pannexin-specific blocker, carbenoxolone or P2X(7)R antagonists, Brilliant Blue G, and oxidized ATP. Furthermore P2X(7)R and panx1 were co-immunoprecipitated. The amount of P2X(7)R protein pulled-down with panx1, increased by twofold when cells were incubated 30 min in [Ca(2+)](o)-free buffer. Taken together, the results of this study demonstrate the activation and association of P2X(7)R-panx1, triggered by the removal of [Ca(2+)](o).

HIV-1 Tat-induced microgliosis and synaptic damage via interactions between peripheral and central myeloid cells

Despite the ability of combination antiretroviral treatment (cART) to reduce viral burden to nearly undetectable levels in cerebrospinal fluid and serum, HIV-1 associated neurocognitive disorders (HAND) continue to persist in as many as half the patients living with this disease. There is growing consensus that the actual substrate for HAND is destruction of normal synaptic architecture but the sequence of cellular events that leads to this outcome has never been resolved. To address whether central vs. peripheral myeloid lineage cells contribute to synaptic damage during acute neuroinflammation we injected a single dose of the HIV-1 transactivator of transcription protein (Tat) or control vehicle into hippocampus of wild-type or chimeric C57Bl/6 mice genetically marked to distinguish infiltrating and resident immune cells. Between 8–24 hr after injection of Tat, invading CD11b⁺ and/or myeloperoxidase-positive leukocytes with granulocyte characteristics were found to engulf both microglia and synaptic structures, and microglia reciprocally engulfed invading leukocytes. By 24 hr, microglial processes were also seen ensheathing dendrites, followed by inclusion of synaptic elements in microglia 7 d after Tat injection, with a durable microgliosis lasting at least 28 d. Thus, central nervous system (CNS) exposure to Tat induces early activation of peripheral myeloid lineage cells with phagocytosis of synaptic elements and reciprocal microglial engulfment of peripheral leukocytes, and enduring microgliosis. Our data suggest that a single exposure to a foreign antigen such as HIV-1 Tat can lead to long-lasting disruption of normal neuroimmune homeostasis with deleterious consequences for synaptic architecture, and further suggest a possible mechanism for enduring neuroinflammation in the absence of productive viral replication in the CNS.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Connexin and pannexin as modulators of myocardial injury

Multicellular organisms have developed a variety of mechanisms that allow communication between their cells. Whereas some of these systems, as neurotransmission or hormones, make possible communication between remote areas, direct cell-to-cell communication through specific membrane channels keep in contact neighboring cells. Direct communication between the cytoplasm of adjacent cells is achieved in vertebrates by membrane channels formed by connexins. However, in addition to allowing exchange of ions and small metabolites between the cytoplasms of adjacent cells, connexin channels also communicate the cytosol with the extracellular space, thus enabling a completely different communication system, involving activation of extracellular receptors. Recently, the demonstration of connexin at the inner mitochondrial membrane of cardiomyocytes, probably forming hemichannels, has enlarged the list of actions of connexins. Some of these mechanisms are also shared by a different family of proteins, termed pannexins. Importantly, these systems allow not only communication between healthy cells, but also play an important role during different types of injury. The aim of this review is to discuss the role played by both connexin hemichannels and pannexin channels in cell communication and injury. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Gap junction-mediated intercellular communication in the adrenal medulla: an additional ingredient of stimulus-secretion coupling regulation

The traditional understanding of stimulus-secretion coupling in adrenal neuroendocrine chromaffin cells states that catecholamines are released upon trans-synaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. Although this statement remains largely true, it deserves to be tempered. In addition to its neurogenic control, catecholamine secretion also depends on a local gap junction-mediated communication between chromaffin cells. We review here the insights gained since the first description of gap junctions in the adrenal medullary tissue. Adrenal stimulus-secretion coupling now appears far more intricate than was previously envisioned and its deciphering represents a challenge for neurobiologists engaged in the study of the regulation of neuroendocrine secretion. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Interaction between nitric oxide signaling and gap junctions: effects on vascular function

Nitric oxide signaling, through eNOS (or possibly nNOS), and gap junction communication are essential for normal vascular function. While each component controls specific aspects of vascular function, there is substantial evidence for cross-talk between nitric oxide signaling and the gap junction proteins (connexins), and more recently, protein-protein association between eNOS and connexins. This review will examine the evidence for interaction between these pathways in normal and diseased arteries, highlight the questions that remain about the mechanisms of their interaction, and explore the possible interaction between nitric oxide signaling and the newly discovered pannexin channels. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Sensing sterile injury: opportunities for pharmacological control

Sterile injury can trigger an acute inflammatory response, which might be responsible for the pathogenesis of several diseases, including rheumatoid arthritis, lung fibrosis and acute liver failure. A key event for the pathogenesis of these diseases is the recruitment of leukocytes to necrotic areas. Much is known about the mechanisms of recruitment to sites of infection. However, only now is it becoming clear how leukocytes, especially neutrophils, are recruited to areas of tissue damage and necrosis in the absence of infection. Here, we review and discuss mechanisms responsible for sensing and driving the influx of leukocytes, specifically neutrophils, into sites of sterile injury. This knowledge clearly opens new opportunities for therapeutic intervention.

Leukocyte migratory responses to apoptosis: the attraction and the distraction

An expanding body of evidence demonstrates that cells undergoing apoptosis send out a selection of molecular navigational signals including proteins, lipids and nucleotides that serve to recruit phagocytes to the dying targets which are subsequently engulfed and removed. This homeostatic process is essentially non-phlogistic, contrasting markedly with the acute inflammatory responses elicited in phagocytes by damaging or infectious agents. The "professional" scavengers of apoptotic cells are mononuclear phagocytes-the macrophages-and sites of high-rate apoptosis are clearly characterized by macrophages associated with the apoptotic cells. By contrast, members of the other class of professional phagocytes-the granulocytes-are not recruited to sites of apoptosis as a direct consequence of the cell-death program. Indeed, recent work indicates that apoptotic cells release a mixture of migratory cues to leukocytes in order to selectively attract mononuclear phagocytes but not granulocytes through functional balancing of positive and negative signals. Here we discuss these molecular mechanisms which not only serve as migratory cues but also may activate responding phagocytes to engulf apoptotic cells effectively. Finally, we speculate upon new therapeutic opportunities these mechanisms offer for a range of pathological conditions, including inflammatory disorders and cancer.

Anthrax toxin induces macrophage death by p38 MAPK inhibition but leads to inflammasome activation via ATP leakage

Detection of microbial constituents by membrane associated and cytoplasmic pattern recognition receptors is the essence of innate immunity, leading to activation of protective host responses. However, it is still unclear how immune cells specifically respond to pathogenic bacteria. Using virulent and nonvirulent strains of Bacillus anthracis, we have shown that secretion of ATP by infected macrophages and the sequential activation of the P2X7 purinergic receptor and nucleotide binding oligomerization domain (NOD)-like receptors are critical for IL-1-dependent host protection from virulent B. anthracis. Importantly, lethal toxin produced by virulent B. anthracis blocked activation of protein kinases, p38 MAPK and AKT, resulting in opening of a connexin ATP release channel and induction of macrophage death. Prevention of cell death or ATP release through constitutive p38 or AKT activation interfered with inflammasome activation and IL-1β production, thereby compromising antimicrobial immunity.

Pathways regulating the trafficking and turnover of pannexin1 protein and the role of the C-terminal domain

Pannexin1 (Panx1) is an integral membrane protein comprised of three species as follows: an unglycosylated core-Gly0, a high mannose-Gly1, and a complex glycosylated Gly2 species. Although Panx1 channels mediate several cellular responses, the domain regulating its oligomerization and cell surface trafficking and the mechanisms governing its internalization and degradation have not been identified. This study characterizes the role of the Panx1 C-tail domain by truncating the polypeptide at residue 307 and expressing the mutant in BICR-M1R(k) and HEK-293T cells. Enzymatic digestion and immunolabeling assays revealed that the Panx1(T307)-RFP was glycosylated primarily to the high mannose species consistent with its retention in the endoplasmic reticulum. Co-expression of Panx1(T307)-RFP with Panx1 followed by co-immunoprecipitation assays revealed that the mutant and Panx1 could interact, whereas biotinylation assays showed that this interaction inhibited Panx1 from maturing into the Gly2 species and reaching the cell surface. Additional inhibitor studies indicated that the degradation of the mutant was via proteasomes, whereas Panx1 was degraded by lysosomes. Analysis of the pathways important in Panx1 internalization revealed partial co-distribution of Panx1 with many molecular constituents of the endocytic machinery that include clathrin, AP2, dynamin II, caveolin-1, and caveolin-2. However, co-immunoprecipitation assays together with the disruption of lipid rafts by methyl-β-cyclodextrin suggest that Panx1 does not engage this endocytic machinery. Furthermore, dominant-negative and pharmacological studies revealed that Panx1 internalization was dynamin II-independent. Collectively, these results indicate that the oligomerization and trafficking of Panx1 are regulated by the C-terminal domain, whereas internalization of long lived Panx1 channels occurs in a manner that is distinct from classical endocytic pathways.

Calcium and connexin-based intercellular communication, a deadly catch?

Ca(2+) is known as a universal messenger mediating a wide variety of cellular processes, including cell death. In fact, this ion has been proposed as the 'cell death master', not only at the intracellular but also at the intercellular level. The most direct form of intercellular spread of cell death is mediated by gap junction channels. These channels have been shown to propagate cell death as well as cell survival signals between the cytoplasm of neighbouring cells, reflecting the dual role of Ca(2+) signals, i.e. cell death versus survival. Its precursor, the unopposed hemichannel (half of a gap junction channel), has recently joined in as a toxic pore connecting the intracellular with the extracellular environment and allowing the passage of a range of substances. The biochemical nature of the so-called intercellular cell death molecule, transferred through gap junctions or released/taken up via hemichannels, remains elusive but several studies pinpoint Ca(2+) itself or its messenger inositol trisphosphate as the responsible masters in crime. Although direct evidence is still lacking, indirect data including Ca(2+) involvement in intercellular communication and cell death, and effects of intercellular communication on intracellular Ca(2+) homeostasis, support this hypothesis. In addition, hemichannels and their molecular building blocks, connexin or pannexin proteins, may exert their effects on Ca(2+)-dependent cell death at the intracellular level, independently from their channel functions. This review provides a cutting edge overview of the current knowledge and underscores the intimate connection between intercellular communication, Ca(2+) signalling and cell death.

Regulation of purinergic signaling in biliary epithelial cells by exocytosis of SLC17A9-dependent ATP-enriched vesicles

ATP in bile is a potent secretogogue, stimulating biliary epithelial cell (BEC) secretion through binding apical purinergic receptors. In response to mechanosensitive stimuli, BECs release ATP into bile, although the cellular basis of ATP release is unknown. The aims of this study in human and mouse BECs were to determine whether ATP release occurs via exocytosis of ATP-enriched vesicles and to elucidate the potential role of the vesicular nucleotide transporter SLC17A9 in purinergic signaling. Dynamic, multiscale, live cell imaging (confocal and total internal reflection fluorescence microscopy and a luminescence detection system with a high sensitivity charge-coupled device camera) was utilized to detect vesicular ATP release from cell populations, single cells, and the submembrane space of a single cell. In response to increases in cell volume, BECs release ATP, which was dependent on intact microtubules and vesicular trafficking pathways. ATP release occurred as stochastic point source bursts of luminescence consistent with exocytic events. Parallel studies identified ATP-enriched vesicles ranging in size from 0.4 to 1 μm that underwent fusion and release in response to increases in cell volume in a protein kinase C-dependent manner. Present in all models, SLC17A9 contributed to ATP vesicle formation and regulated ATP release. The findings are consistent with the existence of an SLC17A9-dependent ATP-enriched vesicular pool in biliary epithelium that undergoes regulated exocytosis to initiate purinergic signaling.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Pannexin1 regulates α1-adrenergic receptor- mediated vasoconstriction

RATIONALE

The coordination of vascular smooth muscle cell constriction plays an important role in vascular function, such as regulation of blood pressure; however, the mechanism responsible for vascular smooth muscle cell communication is not clear in the resistance vasculature. Pannexins (Panx) are purine-releasing channels permeable to the vasoconstrictor ATP and thus may play a role in the coordination of vascular smooth muscle cell constriction.

OBJECTIVE

We investigated the role of pannexins in phenylephrine- and KCl-mediated constriction of resistance arteries.

METHODS AND RESULTS

Western blot, immunohistochemistry, and immunogold labeling coupled to scanning and transmission electron microscopy revealed the presence of Panx1 but not Panx2 or Panx3 in thoracodorsal resistance arteries. Functionally, the contractile response of pressurized thoracodorsal resistance arteries to phenylephrine was decreased significantly by multiple Panx inhibitors (mefloquine, probenecid, and (10)Panx1), ectonucleotidase (apyrase), and purinergic receptor inhibitors (suramin and reactive blue-2). Electroporation of thoracodorsal resistance arteries with either Panx1-green fluorescent protein or Panx1 small interfering RNA showed enhanced and decreased constriction, respectively, in response to phenylephrine. Lastly, the Panx inhibitors did not alter constriction in response to KCl. This result is consistent with coimmunoprecipitation experiments from thoracodorsal resistance arteries, which suggested an association between Panx1 and α1D-adrenergic receptor.

CONCLUSIONS

Our data demonstrate for the first time a key role for Panx1 in resistance arteries by contributing to the coordination of vascular smooth muscle cell constriction and possibly to the regulation of blood pressure.

Activity-dependent targeting of TRPV1 with a pore-permeating capsaicin analog

The capsaicin receptor TRPV1 is the principal transduction channel for nociception. Excessive TRPV1 activation causes pathological pain. Ideal pain mangement requires selective inhibition of hyperactive pain-sensing neurons, but sparing normal nociception. We sought to determine whether it is possible to use activity-dependent TRPV1 agonists to identify nerves with excessive TRPV1 activity, as well as exploit the TRPV1 pore to deliver charged anesthetics for neuronal silencing. We synthesized a series of permanently charged capsaicinoids and found that one, cap-ET, efficaciously evoked TRPV1-dependent entry of Ca(2+) or the large cationic dye YO-PRO-1 comparably to capsaicin, but far smaller electrical currents. Cap-ET-induced YO-PRO-1 transport required permeation of both the agonist and the dye through the TRPV1 pore and could be enhanced by kinase activation or oxidative covalent modification. Moreover, cap-ET reduced capsaicin-induced currents by a voltage-dependent block of the pore. A low dose of cap-ET elicited entry of permanently charged Na(+) channel blockers to effectively suppress Na(+) currents in sensory neurons presensitized with oxidative chemicals. These results implicate therapeutic potential of these unique TRPV1 agonists exhibiting activity-dependent ion transport but of minimal pain-producing risks.

Pannexin channels are not gap junction hemichannels

Pannexins, a class of membrane channels, bear significant sequence homology with the invertebrate gap junction proteins, innexins and more distant similarities in their membrane topologies and pharmacological sensitivities with the gap junction proteins, connexins. However, the functional role for the pannexin oligomers, or pannexons, is different from connexin oligomers, the connexons. Many pannexin publications have used the term "hemichannels" to describe pannexin oligomers while others use the term "channels" instead. This has led to confusion within the literature about the function of pannexins that promotes the idea that pannexons serve as gap junction hemichannels and thus have an assembly and functional state as gap junctional intercellular channels. Here we present the case that unlike the connexin gap junction intercellular channels, so far, pannexin oligomers have repeatedly been shown to be channels that are functional in single membranes, but not as intercellular channel in appositional membranes. Hence, they should be referred to as channels and not hemichannels. Thus, we advocate that in the absence of firm evidence that pannexins form gap junctions, the use of the term "hemichannel" be discontinued within the pannexin literature.

Cell death assays for drug discovery

Cell death has an important role in many human diseases, and strategies aimed at modulating the associated pathways have been successfully applied to treat various disorders. Indeed, several clinically promising cytotoxic and cytoprotective agents with potential applications in cancer, ischaemic and neurodegenerative diseases have recently been identified by high-throughput screening (HTS), based on appropriate cell death assays. Given that different cell death modalities may be dysregulated in different diseases, it is becoming increasingly clear that such assays need to not only quantify the extent of cell death, but they must also be able to distinguish between the various pathways. Here, we systematically describe approaches to accurately quantify distinct cell death pathways, discuss their advantages and pitfalls, and focus on those techniques that are amenable to HTS.

Pannexin-1 is required for ATP release during apoptosis but not for inflammasome activation

Apoptotic cell death is important for embryonic development, immune cell homeostasis, and pathogen elimination. Innate immune cells also undergo a very rapid form of cell death termed pyroptosis after activating the protease caspase-1. The hemichannel pannexin-1 has been implicated in both processes. In this study, we describe the characterization of pannexin-1-deficient mice. LPS-primed bone marrow-derived macrophages lacking pannexin-1 activated caspase-1 and secreted its substrates IL-1β and IL-18 normally after stimulation with ATP, nigericin, alum, silica, flagellin, or cytoplasmic DNA, indicating that pannexin-1 is dispensable for assembly of caspase-1-activating inflammasome complexes. Instead, thymocytes lacking pannexin-1, but not the P2X7R purinergic receptor, were defective in their uptake of the nucleic acid dye YO-PRO-1 during early apoptosis. Cell death was not delayed but, unlike their wild-type counterparts, Panx1(-/-) thymocytes failed to recruit wild-type peritoneal macrophages in a Transwell migration assay. These data are consistent with pannexin-1 liberating ATP and other yet to be defined "find me" signals necessary for macrophage recruitment to apoptotic cells.

Relationship between membrane permeability and specificity of human secretory phospholipase A(2) isoforms during cell death

During apoptosis, a number of physical changes occur in the cell membrane including a gradual increase in permeability to vital stains such as propidium iodide. This study explored the possibility that one consequence of membrane changes concurrent with early modest permeability is vulnerability to degradation by secretory phospholipase A(2). The activity of this hydrolytic enzyme toward mammalian cells depends on the health of the cell; healthy cells are resistant, but they become susceptible early during programmed death. Populations of S49 lymphoma cells during programmed death were classified by flow cytometry based on permeability to propidium iodide and susceptibility to secretory phospholipase A(2). The apoptotic inducers thapsigargin and dexamethasone caused modest permeability to propidium iodide and increased staining by merocyanine 540, a dye sensitive to membrane perturbations. Various secretory phospholipase A(2) isozymes (human groups IIa, V, X, and snake venom) preferentially hydrolyzed the membranes of cells that displayed enhanced permeability. In contrast, cells exposed briefly to a calcium ionophore showed the increase in cell staining intensity by merocyanine 540 without accompanying uptake of propidium iodide. Under that condition, only the snake venom and human group X enzymes hydrolyzed cells that were dying. These results suggested that cells showing modest permeability to propidium iodide during the early phase of apoptosis are substrates for secretory phospholipase A(2) and that specificity among isoforms of the enzyme depends on the degree to which the membrane has been perturbed during the death process. This susceptibility to hydrolysis may be important as part of the signal to attract macrophages toward apoptotic cells.

Connexins and the gap in context

Gap junctions (GJ) can no longer be thought of as simple channel forming structures that mediate intercellular communication. Hemi-channel and channel-independent functions of connexins (Cxs) have been described and numerous Cx interacting partners have been uncovered ranging from enzymes to structural and scaffolding molecules to transcription factors. With the growing number of Cx partners and functions, including well-documented roles for Cxs as conditional tumor suppressors, it has become essential to understand how Cxs are regulated in a context-dependent manner to mediate distinct functions. In this review we will shed light on the tissue and context-dependent regulation and function of Cxs and on the importance of Cx-interactions in modulating tissue-specific function. We will emphasize how the context-dependent functions of Cxs can help in understanding the impact of Cx mis-expression on cancer development and, ultimately, explore whether Cxs can be used as potential therapeutic targets in cancer treatment. In the end, we will address the need for developing relevant assays for studying Cx and GJ functions and will highlight how advances in bioengineering tools and the design of 3D biological platforms can help studying gap junction function in real time in a non-intrusive manner.

Apoptosis: opening PANdora's BoX

Extracellular nucleotides have been reported to act as a 'find-me' signal in the context of phagocyte recruitment by apoptotically dying cells. A new study now examines the mechanisms of nucleotide release during apoptosis and describes the hemichannel-forming protein pannexin 1 as a crucial player in this scenario.

Emerging role of damage-associated molecular patterns derived from mitochondria in inflammation

Cell death and injury often lead to release or exposure of intracellular molecules called damage-associated molecular patterns (DAMPs) or cell death-associated molecules. These molecules are recognized by the innate immune system by pattern recognition receptors - the same receptors that detect pathogen-associated molecular patterns, thus revealing similarities between pathogen-induced and non-infectious inflammatory responses. Many DAMPs are derived from the plasma membrane, nucleus, endoplasmic reticulum and cytosol. Recently, mitochondria have emerged as other organelles that function as a source of DAMPs. Here, we highlight the significance of mitochondrial DAMPs and discuss their contribution to inflammation and development of human pathologies.

Immune cell regulation by autocrine purinergic signalling

Stimulation of almost all mammalian cell types leads to the release of cellular ATP and autocrine feedback through a diverse array of purinergic receptors. Depending on the types of purinergic receptors that are involved, autocrine signalling can promote or inhibit cell activation and fine-tune functional responses. Recent work has shown that autocrine signalling is an important checkpoint in immune cell activation and allows immune cells to adjust their functional responses based on the extracellular cues provided by their environment. This Review focuses on the roles of autocrine purinergic signalling in the regulation of both innate and adaptive immune responses and discusses the potential of targeting purinergic receptors for treating immune-mediated disease.