The P2X7 receptor and Pannexin-1 are both required for the promotion of multinucleated macrophages by the inflammatory cytokine GM-CSF

The P2×7 receptor is a master regulator of microparticle and mitochondria exchange in mouse microglia

Microparticles (MPs) are secreted by all cells where they play a key role in intercellular communication, differentiation, inflammation, and cell energy transfer. P2×7 receptor (P2×7R) activation by extracellular ATP (eATP) causes a large MP release and affects their contents in a cell-specific fashion. We investigated MP release and functional impact in microglial cells from P2×7R-WT or P2×7R-KO mice, as well as mouse microglial cell lines characterized for high (N13-P2×7RHigh) or low (N13-P2×7RLow) P2×7R expression. P2×7R stimulation promoted release of a mixed MP population enriched with naked mitochondria. Released mitochondria were taken up and incorporated into the mitochondrial network of the recipient cells in a P2×7R-dependent fashion. NLRP3 and the P2×7R itself were also delivered to the recipient cells. MP transfer increased the energy level of the recipient cells and conferred a pro-inflammatory phenotype. These data show that the P2×7R is a master regulator of intercellular organelle and MP trafficking in immune cells.

P2X7 Receptor in Dendritic Cells and Macrophages: Implications in Antigen Presentation and T Lymphocyte Activation

The P2X7 receptor, a member of the P2X purinergic receptor family, is a non-selective ion channel. Over the years, it has been associated with various biological functions, from modulating to regulating inflammation. However, its emerging role in antigen presentation has captured the scientific community's attention. This function is essential for the immune system to identify and respond to external threats, such as pathogens and tumor cells, through T lymphocytes. New studies show that the P2X7 receptor is crucial for controlling how antigens are presented and how T cells are activated. These studies focus on antigen-presenting cells, like dendritic cells and macrophages. This review examines how the P2X7 receptor interferes with effective antigen presentation and activates T cells and discusses the fundamental mechanisms that can affect the immune response. Understanding these P2X7-mediated processes in great detail opens up exciting opportunities to create new immunological therapies.

Bcl2l1 Deficiency in Osteoblasts Reduces the Trabecular Bone Due to Enhanced Osteoclastogenesis Likely through Osteoblast Apoptosis

Bcl2l1 (Bcl-XL) belongs to the Bcl-2 family, Bcl2 and Bcl2-XL are major anti-apoptotic proteins, and the apoptosis of osteoblasts is a key event for bone homeostasis. As the functions of Bcl2l1 in osteoblasts and bone homeostasis remain unclear, we generated osteoblast-specific Bcl2l1-deficient (Bcl2l1fl/flCre) mice using 2.3-kb Col1a1 Cre. Trabecular bone volume and the trabecular number were lower in Bcl2l1fl/flCre mice of both sexes than in Bcl2l1fl/fl mice. In bone histomorphometric analysis, osteoclast parameters were increased in Bcl2l1fl/flCre mice, whereas osteoblast parameters and the bone formation rate were similar to those in Bcl2l1fl/fl mice. TUNEL-positive osteoblastic cells and serum TRAP5b levels were increased in Bcl2l1fl/flCre mice. The deletion of Bcl2l1 in osteoblasts induced Tnfsf11 expression, whereas the overexpression of Bcl-XL had no effect. In a co-culture of Bcl2l1-deficient primary osteoblasts and wild-type bone-marrow-derived monocyte/macrophage lineage cells, the numbers of multinucleated TRAP-positive cells and resorption pits increased. Furthermore, serum deprivation or the deletion of Bcl2l1 in primary osteoblasts increased apoptosis and ATP levels in the medium. Therefore, the reduction in trabecular bone in Bcl2l1fl/flCre mice may be due to enhanced bone resorption through osteoblast apoptosis and the release of ATP from apoptotic osteoblasts, and Bcl2l1 may inhibit bone resorption by preventing osteoblast apoptosis.

Pannexin1 channels in the liver: an open enemy

Pannexin1 proteins form communication channels at the cell plasma membrane surface, which allow the transfer of small molecules and ions between the intracellular compartment and extracellular environment. In this way, pannexin1 channels play an important role in various cellular processes and diseases. Indeed, a plethora of human pathologies is associated with the activation of pannexin1 channels. The present paper reviews and summarizes the structure, life cycle, regulation and (patho)physiological roles of pannexin1 channels, with a particular focus on the relevance of pannexin1 channels in liver diseases.

Cell death regulation: A new way for natural products to treat osteoporosis

Osteoporosis is a common metabolic bone disease that results from the imbalance of homeostasis within the bone. Intra-bone homeostasis is dependent on a precise dynamic balance between bone resorption by osteoclasts and bone formation by mesenchymal lineage osteoblasts, which comprises a series of complex and highly standardized steps. Programmed cell death (PCD) (e.g., apoptosis, autophagy, ferroptosis, pyroptosis, and necroptosis) is a cell death process that involves a cascade of gene expression events with tight structures. These events play a certain role in regulating bone metabolism by determining the fate of bone cells. Moreover, existing research has suggested that natural products derived from a wide variety of dietary components and medicinal plants modulate the PCDs based on different mechanisms, which show great potential for the prevention and treatment of osteoporosis, thus revealing the emergence of more acceptable complementary and alternative drugs with lower costs, fewer side effects and more long-term application. Accordingly, this review summarizes the common types of PCDs in the field of osteoporosis. Moreover, from the perspective of targeting PCDs, this review also discussed the roles of currently reported natural products in the treatment of osteoporosis and the involved mechanisms. Based on this, this review provides more insights into new molecular mechanisms of osteoporosis and provides a reference for developing more natural anti-osteoporosis drugs in the future.

Pannexin1 channels-a potential therapeutic target in inflammation

An exaggerated inflammatory response is the hallmark of a plethora of disorders. ATP is a central signaling molecule that orchestrates the initiation and resolution of the inflammatory response by enhancing activation of the inflammasome, leukocyte recruitment and activation of T cells. ATP can be released from cells through pannexin (Panx) channels, a family of glycoproteins consisting of three members, Panx1, Panx2, and Panx3. Panx1 is ubiquitously expressed and forms heptameric channels in the plasma membrane mediating paracrine and autocrine signaling. Besides their involvement in the inflammatory response, Panx1 channels have been shown to contribute to different modes of cell death (i.e., pyroptosis, necrosis and apoptosis). Both genetic ablation and pharmacological inhibition of Panx1 channels decrease inflammation in vivo and contribute to a better outcome in several animal models of inflammatory disease involving various organs, including the brain, lung, kidney and heart. Up to date, several molecules have been identified to inhibit Panx1 channels, for instance probenecid (Pbn), mefloquine (Mfq), flufenamic acid (FFA), carbenoxolone (Cbx) or mimetic peptides like 10Panx1. Unfortunately, the vast majority of these compounds lack specificity and/or serum stability, which limits their application. The recent availability of detailed structural information on the Panx1 channel from cryo-electron microscopy studies may open up innovative approaches to acquire new classes of synthetic Panx1 channel blockers with high target specificity. Selective inhibition of Panx1 channels may not only limit acute inflammatory responses but may also prove useful in chronic inflammatory diseases, thereby improving human health. Here, we reviewed the current knowledge on the role of Panx1 in the initiation and resolution of the inflammatory response, we summarized the effects of Panx1 inhibition in inflammatory pathologies and recapitulate current Panx1 channel pharmacology with an outlook towards future approaches.

Physiologic roles of P2 receptors in leukocytes

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

Osteocytes: Their Lacunocanalicular Structure and Mechanoresponses

Osteocytes connect with neighboring osteocytes and osteoblasts through their processes and form an osteocyte network. Shear stress on osteocytes, which is induced by fluid flow in the lacunae and canaliculi, has been proposed as an important mechanism for mechanoresponses. The lacunocanalicular structure is differentially developed in the compression and tension sides of femoral cortical bone and the compression side is more organized and has denser and thinner canaliculi. Mice with an impaired lacunocanalicular structure may be useful for evaluation of the relationship between lacunocanalicular structure and mechanoresponses, although their bone component cells are not normal. We show three examples of mice with an impaired lacunocanalicular structure. Ablation of osteocytes by diphtheria toxin caused massive osteocyte apoptosis, necrosis or secondary necrosis that occurred after apoptosis. Osteoblast-specific Bcl2 transgenic mice were found to have a reduced number of osteocyte processes and canaliculi, which caused massive osteocyte apoptosis and a completely interrupted lacunocanalicular network. Osteoblast-specific Sp7 transgenic mice were also revealed to have a reduced number of osteocyte processes and canaliculi, as well as an impaired, but functionally connected, lacunocanalicular network. Here, we show the phenotypes of these mice in physiological and unloaded conditions and deduce the relationship between lacunocanalicular structure and mechanoresponses.

Extracellular ATP is increased by release of ATP-loaded microparticles triggered by nutrient deprivation

Rationale: Caloric restriction improves the efficacy of anti-cancer therapy. This effect is largely dependent on the increase of the extracellular ATP concentration in the tumor microenvironment (TME). Pathways for ATP release triggered by nutrient deprivation are largely unknown.

Methods: The extracellular ATP (eATP) concentration was in vivo measured in the tumor microenvironment of B16F10-inoculated C57Bl/6 mice with the pmeLuc probe. Alternatively, the pmeLuc-TG-mouse was used. Caloric restriction was in vivo induced with hydroxycitrate (HC). B16F10 melanoma cells or CT26 colon carcinoma cells were in vitro exposed to serum starvation to mimic nutrient deprivation. Energy metabolism was monitored by Seahorse. Microparticle release was measured by ultracentrifugation and by Nanosight.

Results: Nutrient deprivation increases eATP release despite the dramatic inhibition of intracellular energy synthesis. Under these conditions oxidative phosphorylation was dramatically impaired, mitochondria fragmented and glycolysis and lactic acid release were enhanced. Nutrient deprivation stimulated a P2X7-dependent release of ATP-loaded, mitochondria-containing, microparticles as well as of naked mitochondria.

Conclusions: Nutrient deprivation promotes a striking accumulation of eATP paralleled by a large release of ATP-laden microparticles and of naked mitochondria. This is likely to be a main mechanism driving the accumulation of eATP into the TME.

ATP transporters in the joints

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

Monocyte-derived and M1 macrophages from ankylosing spondylitis patients released higher TNF-α and expressed more IL1B in response to BzATP than macrophages from healthy subjects

Macrophages participate in the pathogenesis of ankylosing spondylitis (AS) by producing inflammatory cytokines. Extracellular adenosine triphosphate (eATP), released during cell stress, acts through purinergic receptors (P2XR and P2YR) and induces inflammatory responses. We investigated the effect of 2ʹ(3ʹ)-O-(4-benzoyl benzoyl) ATP (BzATP) (a prototypic agonist of P2X7R) on the production of inflammatory cytokines in both monocyte-generated (M2-like) and M1 macrophages from patients and controls. Macrophages were differentiated from isolated periphery-monocytes (n = 14 in each group) by macrophage colony-stimulating factor (M-CSF). Using LPS and IFN-γ, macrophages were skewed toward M1 type and were treated with BzATP. Gene expression and protein release of IL-1β, IL-23, and TNF-α were evaluated by real-time PCR and ELISA methods respectively before and after treatment. BzATP significantly increased the protein release of TNF-α and the expression of TNFA and IL1B in monocyte-generated macrophages. Besides, BzATP treatment significantly upregulated IL1B expression, reduced TNFA and IL23A expression, and TNF-α release in M1 macrophages from both groups. Monocyte-generated and M1 macrophages from AS patients released higher TNF-α and expressed more IL1B in response to the same concentration of BzATP treatment respectively. Based on our results, AS macrophages were more sensitive to BzATP treatment and responded more intensively. Besides, the diverse effects of BzATP on monocyte-derived and M1 macrophages in our study may represent the differed inflammatory properties of these two groups of macrophages in response to eATP in the body.

Quantitative evaluation of cellular internalization of polymeric nanoparticles within laryngeal cancer cells and immune cells for enhanced drug delivery

Clinical translation of poly (lactic-co-glycolic acid) (PLGA)-based nanomedicine is limited, partly because of the poor delivery efficiency resulting from non-specific phagocytosis by phagocytes. Understanding the nanoparticle interplay between cancer cells and immune cells remains largely elusive. In this study, a quantitative investigation on cellular internalization of fluorescent PLGA particles (100 nm, 500 nm, and 1 µm) against laryngeal carcinoma cells with or without monocytes/macrophages in monoculture or co-culture systems was first performed. PLGA particles at concentrations of 5-20 µg/mL show superior biocompatibility except for 500 nm and 1 µm PLGA particles at 20 µg/mL slightly reduce cell viability. Microscopic observation has discovered all three sizes of particles are effectively ingested by both cancer cells and macrophages; however, quantitative fluorescence examination has disclosed that the uptake index of cancer cells (mean intracellular particle fluorescence per cancer cell normalized to that of per macrophage) is substantially declined for all PLGA particles in co-cultures compared to that in monocultures (1.35-1.05, 1.50-0.59, and 1.4-0.47 for 100 nm, 500 nm, and 1 µm particles, respectively). Quantitative analysis using flow cytometry further confirmed the reduced uptake index of cancer cells in co-cultures, but higher particle counts per macrophage. It has also been found that the formation of multinucleated giant cells via the fusion of macrophages increased after PLGA treatment, which could be further exploited as a potential approach for tumor drug delivery. Overall, these findings provide new insights into the interaction of nanoparticle-immune-cancer cells, which may facilitate the application of PLGA-based nanocarriers for the treatment of laryngeal carcinoma.

Does Cholinergic Stimulation Affect the P2X7 Receptor-Mediated Dye Uptake in Mast Cells and Macrophages?

Background: Extracellular ATP is a powerful trigger of neuroinflammation by activating immune cells via P2X7 receptors. Acetylcholine and nicotinic agonists inhibit ATP-triggered proinflammatory cytokines via the so-called “cholinergic anti-inflammatory pathway” (CAP). However, it remains unclear as to what stage of ATP-induced signaling cholinergic agents provide this anti-inflammatory effect. Using the specific property of P2X7 receptor to open a pathway permeable to large molecules, associated with activation of inflammasome, we studied the action of cholinergic agents on this key event in CAP activation.

Methods: Freshly isolated mouse peritoneal mast cells and primary human macrophages were used. To assess P2X7 channel opening, the permeability to the fluorescent dye YO-PRO1 or ethidium bromide (EtBr) was measured by flow cytometry. Expression of nicotinic receptors was probed in macrophages with the fluorescently labeled α-bungarotoxin or with patch-clamp recordings.

Results: ATP opened P2X7 ion channels in mast cells and macrophages permeable to YO-PRO1 or EtBr, respectively. This stimulatory effect in mast cells was inhibited by the specific P2X7 antagonist A839977 confirming that YO-PRO1 uptake was mediated via ATP-gated P2X7 ion channels. Cholinergic agents also slightly induced dye uptake to mast cells but not in macrophages, which expressed functional α7 nicotinic receptors. However, both in mast cells and in macrophages, acetylcholine and nicotine failed to inhibit the stimulatory effect of ATP on dye uptake.

Conclusion: These data suggest that in immune cells, cholinergic agents do not act on P2X7 receptor-coupled large pore formation but can mediate the anti-inflammatory effect underlying CAP downstream of ATP-driven signaling.

GM-CSF and IL-33 Orchestrate Polynucleation and Polyploidy of Resident Murine Alveolar Macrophages in a Murine Model of Allergic Asthma

Allergic asthma is a chronical pulmonary disease with high prevalence. It manifests as a maladaptive immune response to common airborne allergens and is characterized by airway hyperresponsiveness, eosinophilia, type 2 cytokine-associated inflammation, and mucus overproduction. Alveolar macrophages (AMs), although contributing to lung homeostasis and tolerance to allergens at steady state, have attracted less attention compared to professional antigen-presenting and adaptive immune cells in their contributions. Using an acute model of house dust mite-driven allergic asthma in mice, we showed that a fraction of resident tissue-associated AMs, while polarizing to the alternatively activated M2 phenotype, exhibited signs of polynucleation and polyploidy. Mechanistically, in vitro assays showed that only Granulocyte-Macrophage Colony Stimulating Factor and interleukins IL-13 and IL-33, but not IL-4 or IL-5, participate in the establishment of this phenotype, which resulted from division defects and not cell-cell fusion as shown by microscopy. Intriguingly, mRNA analysis of AMs isolated from allergic asthmatic lungs failed to show changes in the expression of genes involved in DNA damage control except for MafB. Altogether, our data support the idea that upon allergic inflammation, AMs undergo DNA damage-induced stresses, which may provide new unconventional therapeutical approaches to treat allergic asthma.

Factors influencing multinucleated giant cell formation in vitro

Macrophages fuse together to form multinucleated giant cells (MGC) in granulomas associated with various pathological conditions. Improved in vitro methods are required to better enable investigations of MGC biology and potential contribution to disease. There is a need for standardization of MGC quantification, purification of MGC populations, and characterization of how cell culture variables influence MGC formation. This study examined solutions to address these needs while providing context with other current and alternative methods. Primary mouse bone marrow-derived macrophages were treated with interleukin-4, a cytokine known to induce fusion into MGC. This model was used to systematically assess the influence of cell stimulant timing, cell seeding density, colony stimulating factors, and culture vessel type. Results indicated that MGC formation is greatly impacted by alterations in certain culture variables. An assessment of previously published research showed that these culture conditions varied widely between different laboratories, which may explain inconsistencies in the literature. A particularly novel and unexpected observation was that MGC formation appears to be greatly increased by silicone, which is a component of a chamber slide system commonly used for MGC studies. The most successful quantification method was fluorescent staining with semi-automated morphological evaluation. The most successful enrichment method was microfiltration. Overall, this study takes steps toward standardizing in vitro methods, enhancing replicability, and guiding investigators attempting to culture, quantify, and enrich MGC.

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

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

The P2X7 receptor modulates immune cells infiltration, ectonucleotidases expression and extracellular ATP levels in the tumor microenvironment

In the tumor microenvironment (TME) ATP and its receptor P2X7 exert a pivotal influence on cancer growth and tumor-host interactions. Here we analyzed the different effect of P2X7 genetic deficiency versus its antagonism on response against P2X7-expressing implanted tumors. We focused on immune cell expression of ATP degrading enzymes CD39 and CD73 and in vivo measured TME's ATP. The immune infiltrate of tumors growing in P2X7 null mice shows a decrease in CD8+ cells and an increased number of Tregs, overexpressing the fitness markers OX40, PD-1, and CD73. A similar Treg phenotype is also present in the spleen of tumor-bearing P2X7 null mice and it is paralleled by a decrease in proinflammatory cytokines and an increase in TGF-β. Differently, systemic administration of the P2X7 blocker A740003 in wild-type mice left unaltered the number of tumor-infiltrating CD8+ and Treg lymphocytes but increased CD4+ effector cells and decreased their expression of CD39 and CD73. P2X7 blockade did not affect spleen immune cell composition or ectonucleotidase expression but increased circulating INF-γ. Augmented CD73 in P2X7 null mice was mirrored by a decrease in TME ATP concentration and nucleotide reduced secretion from immune cells. On the contrary, TME ATP levels remained unaltered upon P2X7 antagonism, owing to release of ATP from cancerous cells and diminished ectonucleotidase expression by CD4+ and dendritic cells. These data point at P2X7 receptor as a key determinant of TME composition due to its combined action on immune cell infiltrate, ectonucleotidases, and ATP release.

Cell Death in Chondrocytes, Osteoblasts, and Osteocytes

Cell death in skeletal component cells, including chondrocytes, osteoblasts, and osteocytes, plays roles in skeletal development, maintenance, and repair as well as in the pathogenesis of osteoarthritis and osteoporosis. Chondrocyte proliferation, differentiation, and apoptosis are important steps for endochondral ossification. Although the inactivation of P53 and RB is involved in the pathogenesis of osteosarcomas, the deletion of p53 and inactivation of Rb are insufficient to enhance chondrocyte proliferation, indicating the presence of multiple inhibitory mechanisms against sarcomagenesis in chondrocytes. The inflammatory processes induced by mechanical injury and chondrocyte death through the release of danger-associated molecular patterns (DAMPs) are involved in the pathogenesis of posttraumatic osteoarthritis. The overexpression of BCLXL increases bone volume with a normal structure and maintains bone during aging by inhibiting osteoblast apoptosis. p53 inhibits osteoblast proliferation and enhances osteoblast apoptosis, thereby reducing bone formation, but also exerts positive effects on osteoblast differentiation through the Akt–FoxOs pathway. Apoptotic osteocytes release ATP, which induces the receptor activator of nuclear factor κ-B ligand (Rankl) expression and osteoclastogenesis, from pannexin 1 channels. Osteocyte death ultimately results in necrosis; DAMPs are released to the bone surface and promote the production of proinflammatory cytokines, which induce Rankl expression, and osteoclastogenesis is further enhanced.

Purinergic signalling in autoimmunity: A role for the P2X7R in systemic lupus erythematosus?

Purinergic signalling plays a crucial role in immunity and autoimmunity. Among purinergic receptors, the P2X7 receptor (P2X7R) has an undisputed role as it is expressed to high level by immune cells, triggers cytokine release and modulates immune cell differentiation. In this review, we focus on evidence supporting a possible role of the P2X7R in the pathogenesis of systemic lupus erythematosus (SLE).

Pannexin1 as mediator of inflammation and cell death

Pannexins form channels at the plasma membrane surface that establish a pathway for communication between the cytosol of individual cells and their extracellular environment. By doing so, pannexin signaling dictates several physiological functions, but equally underlies a number of pathological processes. Indeed, pannexin channels drive inflammation by assisting in the activation of inflammasomes, the release of pro-inflammatory cytokines, and the activation and migration of leukocytes. Furthermore, these cellular pores facilitate cell death, including apoptosis, pyroptosis and autophagy. The present paper reviews the roles of pannexin channels in inflammation and cell death. In a first part, a state-of-the-art overview of pannexin channel structure, regulation and function is provided. In a second part, the mechanisms behind their involvement in inflammation and cell death are discussed.

Disassembly of the Dying: Mechanisms and Functions

The disassembly of an apoptotic cell into subcellular fragments, termed apoptotic bodies (ApoBDs), is a hallmark of apoptosis. Although the generation of ApoBDs is generally understood as being stochastic, it is becoming increasingly clear that ApoBD formation is a highly regulated process involving distinct morphological steps and molecular factors. Functionally, ApoBDs could facilitate the efficient clearance of apoptotic material by surrounding phagocytes as well as mediate the transfer of biomolecules including microRNAs and proteins between cells to aid in intercellular communications. Therefore, the formation of ApoBDs is an important process downstream from apoptotic cell death. We discuss here the mechanisms and functions of apoptotic cell disassembly.

Purinergic Signaling in Gut Inflammation: The Role of Connexins and Pannexins

Purinergic receptors play an important role in inflammation, and can be activated by ATP released via pannexin channels and/or connexin hemichannels. The purinergic P2X7 receptor (P2X7R) is of interest since it is involved in apoptosis when activated. Most studies focus on the influence of pannexin-1 (Panx1) and connexin 43 (Cx43) on ATP release and how it affects P2X7R function during inflammation. Inflammatory bowel disease (IBD) is characterized by uncontrolled inflammation within the gastrointestinal system. At present, the pathophysiology of this disease remains largely unknown but it may involve the interplay between P2X7R, Panx1, and Cx43. There are two main types of IBD, ulcerative colitis and Crohn's disease, that are classified by their location and frequency of inflammation. Current research suggests that alterations to normal functioning of innate and adaptive immunity may be a factor in disease progression. The involvement of purinergic receptors, connexins, and pannexins in IBD is a relatively novel notion in the context of gastrointestinal inflammation, and has been explored by various research groups. Thus, the present review focuses on the current research involving connexins, pannexins, and purinergic receptors within the gut and enteric nervous system, and will examine their involvement in inflammation and the pathophysiology of IBD.

Pannexin-1 and P2X7-Receptor Are Required for Apoptotic Osteocytes in Fatigued Bone to Trigger RANKL Production in Neighboring Bystander Osteocytes

Osteocyte apoptosis is required to induce intracortical bone remodeling after microdamage in animal models, but how apoptotic osteocytes signal neighboring "bystander" cells to initiate the remodeling process is unknown. Apoptosis has been shown to open pannexin-1 (Panx1) channels to release adenosine diphosphate (ATP) as a "find-me" signal for phagocytic cells. To address whether apoptotic osteocytes use this signaling mechanism, we adapted the rat ulnar fatigue-loading model to reproducibly introduce microdamage into mouse cortical bone and measured subsequent changes in osteocyte apoptosis, receptor activator of NF-κB ligand (RANKL) expression and osteoclastic bone resorption in wild-type (WT; C57Bl/6) mice and in mice genetically deficient in Panx1 (Panx1KO). Mouse ulnar loading produced linear microcracks comparable in number and location to the rat model. WT mice showed increased osteocyte apoptosis and RANKL expression at microdamage sites at 3 days after loading and increased intracortical remodeling and endocortical tunneling at day 14. With fatigue, Panx1KO mice exhibited levels of microdamage and osteocyte apoptosis identical to WT mice. However, they did not upregulate RANKL in bystander osteocytes or initiate resorption. Panx1 interacts with P2X7 R in ATP release; thus, we examined P2X7 R-deficient mice and WT mice treated with P2X7 R antagonist Brilliant Blue G (BBG) to test the possible role of ATP as a find-me signal. P2X7 RKO mice failed to upregulate RANKL in osteocytes or induce resorption despite normally elevated osteocyte apoptosis after fatigue loading. Similarly, treatment of fatigued C57Bl/6 mice with BBG mimicked behavior of both Panx1KO and P2X7 RKO mice; BBG had no effect on osteocyte apoptosis in fatigued bone but completely prevented increases in bystander osteocyte RANKL expression and attenuated activation of resorption by more than 50%. These results indicate that activation of Panx1 and P2X7 R are required for apoptotic osteocytes in fatigued bone to trigger RANKL production in neighboring bystander osteocytes and implicate ATP as an essential signal mediating this process.

Enhancement of Muscle T Regulatory Cells and Improvement of Muscular Dystrophic Process in mdx Mice by Blockade of Extracellular ATP/P2X Axis

Infiltration of immune cells and chronic inflammation substantially affect skeletal and cardiac muscle degeneration in Duchenne muscular dystrophy. In the immune system, extracellular adenosine triphosphate (ATP) released by dying cells is sensed as a danger associated molecular pattern through P2 purinergic receptors. Specifically, the P2X7 subtype has a prominent role in regulating immune system physiology and contributes to inflammasome activation also in muscle cells. Here, we show that in vivo blockade of the extracellular ATP/P2X purinergic signaling pathway by periodate-oxidized ATP delayed the progression of the dystrophic phenotype and dampened the local inflammatory response in mdx mice, a spontaneous mouse model of dystrophin deficiency. Reduced infiltration of leukocytes and macrophages and decreased expression of IL-6 were revealed in the muscles of periodate-oxidized ATP-treated mdx mice. Concomitantly, an increase in Foxp3(+) immunosuppressive regulatory T cells was observed and correlated with enhanced myofiber regeneration. Moreover, we detected reduced concentrations of profibrotic cytokines, including transforming growth factor-β and connective tissue growth factor, in muscles of periodate-oxidized ATP-treated mdx mice. The improvement of inflammatory features was associated with increased strength and reduced necrosis, thus suggesting that pharmacologic purinergic antagonism altering the adaptive immune component in the muscle infiltrates might represent a promising therapeutic approach in Duchenne muscular dystrophy.

Accelerated tumor progression in mice lacking the ATP receptor P2X7

The ATP receptor P2X7 (P2X7R or P2RX7) has a key role in inflammation and immunity, but its possible roles in cancer are not firmly established. In the present study, we investigated the effect of host genetic deletion of P2X7R in the mouse on the growth of B16 melanoma or CT26 colon carcinoma cells. Tumor size and metastatic dissemination were assessed by in vivo calliper and luciferase luminescence emission measurements along with postmortem examination. In P2X7R-deficient mice, tumor growth and metastatic spreading were accelerated strongly, compared with wild-type (wt) mice. Intratumoral IL-1β and VEGF release were drastically reduced, and inflammatory cell infiltration was abrogated nearly completely. Similarly, tumor growth was also greatly accelerated in wt chimeric mice implanted with P2X7R-deficient bone marrow cells, defining hematopoietic cells as a sufficient site of P2X7R action. Finally, dendritic cells from P2X7R-deficient mice were unresponsive to stimulation with tumor cells, and chemotaxis of P2X7R-less cells was impaired. Overall, our results showed that host P2X7R expression was critical to support an antitumor immune response, and to restrict tumor growth and metastatic diffusion.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Role of transient receptor potential and pannexin channels in cigarette smoke-triggered ATP release in the lung

BACKGROUND

COPD is an inflammatory disease usually associated with cigarette smoking (CS) with an increasing global prevalence and no effective medication. Extracellular ATP is increased in the COPD affected lung and may play a key role in driving CS-induced airway inflammation, but the mechanism involved in ATP release has eluded researchers. Recently, the transient receptor potential (TRP) and pannexin-1 channels have been suggested to play a role in other experimental paradigms. Thus, the aim of this work is to investigate if these channels are involved in CS-induced ATP release in the lung.

METHODS

Primary human cells were exposed to CS and extracellular ATP levels measured. Mice were exposed to mainstream CS and airway inflammation assessed. TRPV1/4 mRNA expression was assessed in human lung parenchyma.

RESULTS

CS exposure caused a dose-related increase in ATP from primary airway bronchial epithelial cells. This was attenuated by blockers of TRPV1, TRPV4 and pannexin-1 channels. Parallel data was obtained using murine acute CS-driven model systems. Finally, TRPV1/4 mRNA expression was increased in lung tissue samples from patients with COPD.

CONCLUSIONS

Extracellular ATP is increased in the COPD affected lung and may play a key role in driving disease pathophysiology. These experiments uncover a novel mechanism which may be responsible for CS-induced ATP release. These findings highlight novel targets that could lead to the development of medicine to treat this devastating disease.

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.

A P2X7 receptor antagonist attenuates experimental autoimmune myocarditis via suppressed myocardial CD4+ T and macrophage infiltration and NADPH oxidase 2/4 expression in mice

Myocarditis is a clinically serious disease; however, no effective treatment has been elucidated. The P2X7 receptor is related to the pathophysiology of inflammation in many cardiovascular diseases. The P2X7 receptor antagonist is promising as an immunosuppressive treatment; however, its role in myocarditis is still to be established. To clarify the role of the P2X7 receptor, we used a murine experimental autoimmune myocarditis (EAM) model. Mice were immunized on day 0 and 7 with synthetic cardiac myosin peptide to establish EAM. The mice with induced EAM were treated with A740003, the P2X7 receptor antagonist (n = 10), or not treated (n = 11); hearts were harvested on day 21. The P2X7 receptor antagonist improved myocardial contraction of the EAM hearts via suppressed infiltration of CD4+ T cells and macrophages. Similarly, mRNA expression of interleukin 1 beta, the P2X7 receptor and NADPH oxidase 2/4 was lower in the heart of the P2X7 receptor antagonist-treated group compared to the non-treat group. The P2X7 receptor antagonist suppressed EAM development; thus, this inhibition is promising for treating clinical myocarditis.

The role of pannexin1 in the induction and resolution of inflammation

Extracellular ATP is an important signaling molecule throughout the inflammatory cascade, serving as a danger signal that causes activation of the inflammasome, enhancement of immune cell infiltration, and fine-tuning of several signaling cascades including those important for the resolution of inflammation. Recent studies demonstrated that ATP can be released from cells in a controlled manner through pannexin (Panx) channels. Panx1-mediated ATP release is involved in inflammasome activation and neutrophil/macrophage chemotaxis, activation of T cells, and a role for Panx1 in inducing and propagating inflammation has been demonstrated in various organs, including lung and the central and peripheral nervous system. The recognition and clearance of dying cells and debris from focal points of inflammation is critical in the resolution of inflammation, and Panx1-mediated ATP release from dying cells has been shown to recruit phagocytes. Moreover, extracellular ATP can be broken down by ectonucleotidases into ADP, AMP, and adenosine, which is critical in the resolution of inflammation. Together, Panx1, ATP, purinergic receptors, and ectonucleotidases contribute to important feedback loops during the inflammatory response, and thus represent promising candidates for new therapies.

Pannexin1 channels act downstream of P2X 7 receptors in ATP-induced murine T-cell death

Death of murine T cells induced by extracellular ATP is mainly triggered by activation of purinergic P2X 7 receptors (P2X 7Rs). However, a link between P2X 7Rs and pannexin1 (Panx1) channels, which are non-selective, has been recently demonstrated in other cell types. In this work, we characterized the expression and cellular distribution of pannexin family members (Panxs 1, 2 and 3) in isolated T cells. Panx1 was the main pannexin family member clearly detected in both helper (CD4+) and cytotoxic (CD8+) T cells, whereas low levels of Panx2 were found in both T-cell subsets. Using pharmacological and genetic approaches, Panx1 channels were found to mediate most ATP-induced ethidium uptake since this was drastically reduced by Panx1 channel blockers (10Panx1, Probenecid and low carbenoxolone concentration) and absent in T cells derived from Panx1-/- mice. Moreover, electrophysiological measurements in wild-type CD4+ cells treated with ATP unitary current events and pharmacological sensitivity compatible with Panx1 channels were found. In addition, ATP release from T cells treated with 4Br-A23187, a calcium ionophore, was completely blocked with inhibitors of both connexin hemichannels and Panx1 channels. Panx1 channel blockers drastically reduced the ATP-induced T-cell mortality, indicating that Panx1 channels mediate the ATP-induced T-cell death. However, mortality was not reduced in T cells of Panx1-/- mice, in which levels of P2X 7Rs and ATP-induced intracellular free Ca2+ responses were enhanced suggesting that P2X 7Rs take over Panx1 channels lose-function in mediating the onset of cell death induced by extracellular ATP.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.

The P2X7 receptor: a key player in immune-mediated bone loss?

Inflammatory diseases are often multiorganic diseases with manifestations not related directly to the primary affected organ. They are often complicated by a generalized bone loss that subsequently leads to osteoporosis and bone fractures. The exact mechanism for the accompanying bone loss is not understood in full detail, but factors such as glucocorticoid treatment, immobilization, malnutrition, and insufficient intake of vitamin D play a role. However, it has become evident that the inflammatory process itself is involved and the resulting bone loss is termed immune-mediated bone loss. It stems from an increase in bone resorption and the pro-inflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta and has been shown to not only mediate the inflammatory response but also to strongly stimulate bone degradation. The purinergic P2X7 receptor is central in the processing of these two cytokines and in the initiation of the inflammatory response, and it is a key molecule in the regulation of both bone formation and bone resorption. The aim of this review is therefore to provide evidence-based novel hypotheses of the role of ATP-mediated purinergic signalling via the P2X7 receptor in immune-mediated bone loss and -osteoporosis.

Deletion of CD73 promotes dyslipidemia and intramyocellular lipid accumulation in muscle of mice

CONTEXT

CD73 converts extracellular AMP to adenosine which is well known to inhibit lipolysis. It is unknown, however, whether adenosine formed directly by CD73 is functionally relevant in this process.

OBJECTIVE

We therefore explored the effect of CD73-derived adenosine on body fat of aged mice.

RESULTS

In lean mice, extracellular adenosine formation by adipocytes is dependent on CD73. High fat diet down-regulates the expression of CD73 in wildtype mice similar to ob/ob mice. Transgenic mice chronically lacking CD73 (CD73(-/-)) gain significantly less body weight and show decreased superficial white fat content as well as increased serum free fatty acids and triglycerides. In addition, intramyocellular lipid levels are significantly increased. This phenotype is accompanied by an increase in blood glucose and serum insulin levels although insulin secretion and the level of insulin degrading enzyme are unaltered. Additionally, insulin-induced Akt phosphorylation is reduced in skeletal muscle of CD73(-/-) mice.

CONCLUSION

CD73-derived adenosine is functionally involved in body fat homeostasis.

Selective impairment of P2Y signaling by prostaglandin E2 in macrophages: implications for Ca2+-dependent responses

Extracellular nucleotides have been recognized as important modulators of inflammation via their action on specific pyrimidine receptors (P2). This regulation coexists with the temporal framework of proinflammatory and proresolution mediators released by the cells involved in the inflammatory response, including macrophages. Under proinflammatory conditions, the expression of cyclooxygenase-2 leads to the release of large amounts of PGs, such as PGE2, that exert their effects through EP receptors and other intracellular targets. The effect of these PGs on P2 receptors expressed in murine and human macrophages was investigated. In thioglycollate-elicited and alternatively activated macrophages, PGE2 selectively impairs P2Y but not P2X7 Ca(2+) mobilization. This effect is absent in LPS-activated cells and is specific for PGE2 because it cannot be reproduced by other PGs with cyclopentenone structure. The inhibition of P2Y responses by PGE2 involves the activation of nPKCs (PKCε) and PKD that can be abrogated by selective inhibitors or by expression of dominant-negative forms of PKD. The inhibition of P2Y signaling by PGE2 has an impact on the cell migration elicited by P2Y agonists in thioglycollate-elicited and alternatively activated macrophages, which provide new clues to understand the resolution phase of inflammation, when accumulation of PGE2, anti-inflammatory and proresolving mediators occurs.

Genetic basis of cell-cell fusion mechanisms

Cell-cell fusion in sexually reproducing organisms is a mechanism to merge gamete genomes and, in multicellular organisms, it is a strategy to sculpt organs, such as muscle, bone, and placenta. Moreover, this mechanism has been implicated in pathological conditions, such as infection and cancer. Studies of genetic model organisms have uncovered a unifying principle: cell fusion is a genetically programmed process. This process can be divided in three stages: competence (cell induction and differentiation); commitment (cell determination, migration, and adhesion); and cell fusion (membrane merging and cytoplasmic mixing). Recent work has led to the discovery of fusogens, which are cell fusion proteins that are necessary and sufficient to fuse cell membranes. Two unrelated families of fusogens have been discovered, one in mouse placenta and one in Caenorhabditis elegans (syncytins and F proteins, respectively). Current research aims to identify new fusogens and determine the mechanisms by which they merge membranes.

P2X7 receptor activation induces reactive oxygen species formation and cell death in murine EOC13 microglia

The P2X7 purinergic receptor is a ligand-gated cation channel expressed on leukocytes including microglia. This study aimed to determine if P2X7 activation induces the uptake of organic cations, reactive oxygen species (ROS) formation, and death in the murine microglial EOC13 cell line. Using the murine macrophage J774 cell line as a positive control, RT-PCR, immunoblotting, and immunolabelling established the presence of P2X7 in EOC13 cells. A cytofluorometric assay demonstrated that the P2X7 agonists adenosine-5′-triphosphate (ATP) and 2′(3′)-O-(4-benzoylbenzoyl) ATP induced ethidium⁺ or YO-PRO-1²⁺ uptake into both cell lines. ATP induced ethidium⁺ uptake into EOC13 cells in a concentration-dependent manner, with an EC₅₀ of ~130 μM. The P2X7 antagonists Brilliant Blue G, A438079, AZ10606120, and AZ11645373 inhibited ATP-induced cation uptake into EOC13 cells by 75–100%. A cytofluorometric assay demonstrated that P2X7 activation induced ROS formation in EOC13 cells, via a mechanism independent of Ca²⁺ influx and K⁺ efflux. Cytofluorometric measurements of Annexin-V binding and 7AAD uptake demonstrated that P2X7 activation induced EOC13 cell death. The ROS scavenger N-acetyl-L-cysteine impaired both P2X7-induced EOC13 ROS formation and cell death, suggesting that ROS mediate P2X7-induced EOC13 death. In conclusion, P2X7 activation induces the uptake of organic cations, ROS formation, and death in EOC13 microglia.

Adenosine A2A receptor activation prevents wear particle-induced osteolysis

Prosthesis loosening, associated with wear particle-induced inflammation and osteoclast-mediated bone destruction, is a common cause for joint implant failure, leading to revision surgery. Adenosine A(2A) receptors (A(2A)Rs) mediate potent anti-inflammatory effects in many tissues and prevent osteoclast differentiation. We tested the hypothesis that an A(2A)R agonist could reduce osteoclast-mediated bone resorption in a murine calvaria model of wear particle-induced bone resorption. C57BL/6 and A(2A)R knockout (A(2A)R KO) mice received ultrahigh-molecular weight polyethylene particles and were treated daily with either saline or the A(2A)R agonist CGS21680. After 2 weeks, micro-computed tomography of calvaria demonstrated that CGS21680 reduced particle-induced bone pitting and porosity in a dose-dependent manner, increasing cortical bone and bone volume compared to control mice. Histological examination demonstrated diminished inflammation after treatment with CGS21680. In A(2A)R KO mice, CGS21680 did not affect osteoclast-mediated bone resorption or inflammation. Levels of bone resorption markers receptor activator of nuclear factor κB (RANK), RANK ligand, cathepsin K, CD163, and osteopontin were reduced after CGS21680 treatment, together with a reduction in osteoclasts. Secretion of interleukin-1β (IL-1β) and tumor necrosis factor-α was significantly decreased, whereas IL-10 was markedly increased in bone by CGS21680. These results in mice suggest that site-specific delivery of an adenosine A(2A)R agonist could enhance implant survival, delaying or eliminating the need for revision arthroplastic surgery.

Purinergic signalling in bone

Purinergic signaling in bone was first proposed in the early 1990s with the observation that extracellular ATP could modulate events crucial to the normal functioning of bone cells. Since then the expression of nearly all the P2Y and P2X receptors by osteoblasts and osteoclasts has been reported, mediating multiple processes including cell proliferation, differentiation, function, and death. This review will highlight the most recent developments in the field of purinergic signaling in bone, with a special emphasis on recent work resulting from the European Framework 7 funded collaboration ATPBone, as well as Arthritis Research UK and Bone Research Society supported projects.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

P2X7 receptor activation leads to increased cell death in a radiosensitive human glioma cell line

Gliomas are the most lethal tumors of central nervous system. ATP is an important signaling molecule in CNS and it is a selective P2X7 purinergic receptor ligand at high concentrations. Herein, we investigated whether the activation of P2X7R might be implicated in death of a radiosensitive human glioma lineage. The effects of P2X7R agonists (ATP and BzATP) and irradiation (2 Gy) on glioma cells were analyzed by MTT assay and annexin-V/PI determination, whereas mRNA and protein P2X7R expression was assessed by qRT-PCR and flow cytometry, respectively. P2X7R pore formation was functionality examined by analyzing ethidium bromide uptake. The human glioma cells U-138 MG and U-251 MG were resistant to death when treated with either ATP (5 mM) or BzATP (100 μM), but the radiosensitive M059J glioma cells displayed a significant decrease of cell viability (32.4 ± 4.1 % and 25.6 ± 3.3 %, respectively). The M059J lineage expresses significantly higher mRNA P2X7R levels when compared to the U-138 MG and U-251 cell lines (0.40 ± 0.00; 0.28 ± 0.01, and 0.31 ± 0.01, respectively), and irradiation upregulated P2X7R expression (0.55 ± 0.08) in this lineage. Noteworthy, P2X7R protein doubled after irradiation on M059J lineage, and increased in 50 % and 42.6 % when comparing M059J-irradiated to irradiated U-138 MG and U-251 MG cells, respectively. Ethidium bromide uptake was significantly increased in 104 % and 77.8 % when comparing M059J to U-138 MG and U-251MG, respectively. Finally, the selective P2X7R antagonist A740003 significantly decreased the cell death caused by irradiation. We provide novel evidence indicating that M059J human glioma cell line is ATP-P2X7R sensitive, pointing out the relevance of the purinergic P2X7R on glioma radiosensitivity.

Purinergic receptors are required for HIV-1 infection of primary human macrophages

Macrophages play a significant role in HIV infection, viral rebound, and the development of AIDS. However, the function of host proteins in viral replication is incompletely characterized in macrophages. Purinergic receptors P2X and P2Y are major components of the macrophage immune response to pathogens, inflammation, and cellular damage. We demonstrate that these receptors are necessary for HIV infection of primary human macrophages. Inhibition of purinergic receptors results in a significant reduction in HIV replication in macrophages. This inhibition is independent of viral strain and is dose dependent. We also identify that P2X(1), P2X(7), and P2Y(1) receptors are involved in viral replication. We show that P2X(1), but not P2X(7) or P2Y(1), is necessary for HIV entry into macrophages. We demonstrate that interaction of the HIV surface protein gp120 with macrophages stimulates an increase in ATP release. Thus, we propose that HIV's binding to macrophages triggers a local release of ATP that stimulates purinergic receptors and facilitates HIV entry and subsequent stages of viral replication. Our data implicate a novel role for a family of host proteins in HIV replication in macrophages and suggest new therapeutic targets to reduce the devastating consequences of HIV infection and AIDS.

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.