Sequential shrinkage and swelling underlie P2X7-stimulated lymphocyte phosphatidylserine exposure and death

How do necrotic cells expose phosphatidylserine to attract their predators—What’s unique and what’s in common with apoptotic cells

Phosphatidylserine (PS) is a lipid component of the plasma membrane. It is asymmetrically distributed to the inner leaflet in live cells. In cells undergoing apoptosis, phosphatidylserine is exposed to the outer surfaces. The exposed phosphatidylserine acts as an evolutionarily conserved “eat-me” signal that attracts neighboring engulfing cells in metazoan organisms, including the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and mammals. During apoptosis, the exposure of phosphatidylserine to the outer surface of a cell is driven by the membrane scramblases and flippases, the activities of which are regulated by caspases. Cells undergoing necrosis, a kind of cell death frequently associated with cellular injuries and morphologically distinct from apoptosis, were initially believed to allow passive exposure of phosphatidylserine through membrane rupture. Later studies revealed that necrotic cells actively expose phosphatidylserine before any rupture occurs. A recent study in C. elegans further reported that the calcium ion (Ca2+) plays an essential role in promoting the exposure of phosphatidylserine on the surfaces of necrotic cells. These findings indicate that necrotic and apoptotic cells, which die through different molecular mechanisms, use common and unique mechanisms for promoting the exposure of the same “eat me” signal. This article will review the mechanisms regulating the exposure of phosphatidylserine on the surfaces of necrotic and apoptotic cells and highlight their similarities and differences.

Requirement of Xk and Vps13a for the P2X7-mediated phospholipid scrambling and cell lysis in mouse T cells

A high extracellular adenosine triphosphate (ATP) concentration rapidly and reversibly exposes phosphatidylserine (PtdSer) in T cells by binding to the P2X7 receptor, which ultimately leads to necrosis. Using mouse T cell transformants expressing P2X7, we herein performed CRISPR/Cas9 screening for the molecules responsible for P2X7-mediated PtdSer exposure. In addition to Eros, which is required for the localization of P2X7 to the plasma membrane, this screening identified Xk and Vps13a as essential components for this process. Xk is present at the plasma membrane, and its paralogue, Xkr8, functions as a phospholipid scramblase. Vps13a is a lipid transporter in the cytoplasm. Blue-native polyacrylamide gel electrophoresis indicated that Xk and Vps13a interacted at the membrane. A null mutation in Xk or Vps13a blocked P2X7-mediated PtdSer exposure, the internalization of phosphatidylcholine, and cytolysis. Xk and Vps13a formed a complex in mouse splenic T cells, and Xk was crucial for ATP-induced PtdSer exposure and cytolysis in CD25⁺CD4⁺ T cells. XK and VPS13A are responsible for McLeod syndrome and chorea-acanthocytosis, both characterized by a progressive movement disorder and cognitive and behavior changes. Our results suggest that the phospholipid scrambling activity mediated by XK and VPS13A is essential for maintaining homeostasis in the immune and nerve systems.

On the mechanism of anti-CD39 immune checkpoint therapy

With the coming of age of cancer immunotherapy, the search for new therapeutic targets has led to the identification of immunosuppressive adenosine as an important regulator of antitumor immunity. This resulted in the development of selective inhibitors targeting various components of the adenosinergic pathway, including small molecules antagonists targeting the high affinity A2A adenosine receptor and low affinity A2B receptor, therapeutic monoclonal antibodies (mAbs) and small molecules targeting CD73 and therapeutic mAbs targeting CD39. As each regulator of the adenosinergic pathway present non-overlapping biologic functions, a better understanding of the mechanisms of action of each targeted approach should accelerate clinical translation and improve rational design of combination treatments. In this review, we discuss the potential mechanisms-of-action of anti-CD39 cancer therapy and potential toxicities that may emerge from sustained CD39 inhibition. Caution should be taken, however, in extrapolating data from gene-targeted mice to patients treated with blocking anti-CD39 agents. As phase I clinical trials are now underway, further insights into the mechanism of action and potential adverse events associated with anti-CD39 therapy are anticipated in coming years.

High CD39 expression is associated with the non-muscle-invasive phenotype of human bladder cancer

Background: An accurate prediction of progression is critical to define the management of bladder cancer (BC). The ectonucleotidases CD39 and CD73 play strategic roles in calibrating purinergic signals via an extracellular balance between ATP and adenosine. The altered expression of these enzymes plays a potential role in tumor invasion and metastasis, therefore, has been proposed to be used for prognosis of solid tumor. In BC this is not yet clear.

Objective: This study aimed to evaluate CD39 and CD73 expression in a cohort of patients with non-muscle-invasive (NMI) and muscle-invasive (MI) BC regard to its association with clinicopathological features.

Materials and Methods: Retrospective clinical follow-up data and primary urothelial BC specimens of 162 patients were used (87 from patients who underwent transurethral resection and 75 from cystectomized patients). Tissue microarrays were constructed, and immunohistochemistry for CD39 and CD73 was performed to make associations with clinicopathological data.

Results: Overall, 96 were NMI (59.3%) and 66 MI (40.7%). CD39 immunoreactivity in BC cells was found in 72% of the cases, while CD73 was found in 97%. High CD39 expression alone was more frequent in NMI BC (p < 0.001), while CD73 expression was not powerful to predict the stage of BC. The association of both markers confirmed that only CD39 has potential in BC prognosis.

Conclusions: The altered expression of CD39 presented herein supports the idea that this ectonucleotidase may be involved in bladder tumorigenesis. High expression of CD39 in tumor cells is correlated with the early stage of BC.

P2X7 receptor in multifaceted cellular signalling and its relevance as a potential therapeutic target in different diseases

P2X7 receptor, a purinergic receptor family member, is abundantly expressed on many cells, including immune, muscle, bone, neuron, and glia. It acts as an ATP-activated cation channel that permits the influx of Ca²⁺, Na⁺ and efflux of K⁺ ions. The P2X7 receptor plays crucial roles in many physiological processes including cytokine and chemokine secretion, NLRP3 inflammasome activation, cellular growth and differentiation, locomotion, wound healing, transcription factors activation, cell death and T-lymphocyte survival. Past studies have demonstrated the up-regulation and direct association of this receptor in many pathophysiological conditions such as cancer, diabetics, arthritis, tuberculosis (TB) and inflammatory diseases. Hence, targeting this receptor is considered a worthwhile approach to lessen the afflictions associated with the disorders mentioned above by understanding the receptor architecture and downstream signalling processes. Here, in the present review, we have dissected the structural and functional aspects of the P2X7 receptor, emphasizing its role in various diseased conditions. This information will provide in-depth knowledge about the receptor and help to develop apt curative methodologies for the betterment of humanity in the coming years.

CD38: T Cell Immuno-Metabolic Modulator

Activation and subsequent differentiation of T cells following antigenic stimulation are triggered by highly coordinated signaling events that lead to instilling cells with a discrete metabolic and transcriptional feature. Compelling studies indicate that intracellular nicotinamide adenine dinucleotide (NAD⁺) levels have profound influence on diverse signaling and metabolic pathways of T cells, and hence dictate their functional fate. CD38, a major mammalian NAD⁺ glycohydrolase (NADase), expresses on T cells following activation and appears to be an essential modulator of intracellular NAD⁺ levels. The enzymatic activity of CD38 in the process of generating the second messenger cADPR utilizes intracellular NAD^+, and thus limits its availability to different NAD⁺ consuming enzymes (PARP, ART, and sirtuins) inside the cells. The present review discusses how the CD38-NAD⁺ axis affects T cell activation and differentiation through interfering with their signaling and metabolic processes. We also describe the pivotal role of the CD38-NAD⁺ axis in influencing the chromatin remodeling and rewiring T cell response. Overall, this review emphasizes the crucial contribution of the CD38^-NAD⁺ axis in altering T cell response in various pathophysiological conditions.

ADAM10 sheddase activation is controlled by cell membrane asymmetry

Dysregulation of the disintegrin-metalloproteinase ADAM10 may contribute to the development of diseases including tumorigenesis and Alzheimer's disease. The mechanisms underlying ADAM10 sheddase activation are incompletely understood. Here, we show that transient exposure of the negatively charged phospholipid phosphatidylserine (PS) is necessarily required. The soluble PS headgroup was found to act as competitive inhibitor of substrate cleavage. Overexpression of the Ca2+-dependent phospholipid scramblase Anoctamin-6 (ANO6) led to increased PS externalization and substrate release. Transfection with a constitutively active form of ANO6 resulted in maximum sheddase activity in the absence of any stimulus. Calcium-dependent ADAM10 activation could not be induced in lymphocytes of patients with Scott syndrome harbouring a missense mutation in ANO6. A putative PS-binding motif was identified in the conserved stalk region. Replacement of this motif resulted in strong reduction of sheddase activity. In conjunction with the recently described 3D structure of the ADAM10 extracellular domain, a model is advanced to explain how surface-exposed PS triggers ADAM10 sheddase function.

P2X7 Receptor at the Crossroads of T Cell Fate

The P2X7 receptor is a ligand-gated, cation-selective channel whose main physiological ligand is ATP. P2X7 receptor activation may also be triggered by ARTC2.2-dependent ADP ribosylation in the presence of extracellular NAD. Upon activation, this receptor induces several responses, including the influx of calcium and sodium ions, phosphatidylserine externalization, the formation of a non-selective membrane pore, and ultimately cell death. P2X7 receptor activation depends on the availability of extracellular nucleotides, whose concentrations are regulated by the action of extracellular nucleotidases such as CD39 and CD38. The P2X7 receptor has been extensively studied in the context of the immune response, and it has been reported to be involved in inflammasome activation, cytokine production, and the migration of different innate immune cells in response to ATP. In adaptive immune responses, the P2X7 receptor has been linked to T cell activation, differentiation, and apoptosis induction. In this review, we will discuss the evidence of the role of the P2X7 receptor on T cell differentiation and in the control of T cell responses in inflammatory conditions.

P2X7 in Cancer: From Molecular Mechanisms to Therapeutics

P2X7 is a transmembrane receptor expressed in multiple cell types including neurons, dendritic cells, macrophages, monocytes, B and T cells where it can drive a wide range of physiological responses from pain transduction to immune response. Upon activation by its main ligand, extracellular ATP, P2X7 can form a nonselective channel for cations to enter the cell. Prolonged activation of P2X7, via high levels of extracellular ATP over an extended time period can lead to the formation of a macropore, leading to depolarization of the plasma membrane and ultimately to cell death. Thus, dependent on its activation state, P2X7 can either drive cell survival and proliferation, or induce cell death. In cancer, P2X7 has been shown to have a broad range of functions, including playing key roles in the development and spread of tumor cells. It is therefore unsurprising that P2X7 has been reported to be upregulated in several malignancies. Critically, ATP is present at high extracellular concentrations in the tumor microenvironment (TME) compared to levels observed in normal tissues. These high levels of ATP should present a survival challenge for cancer cells, potentially leading to constitutive receptor activation, prolonged macropore formation and ultimately to cell death. Therefore, to deliver the proven advantages for P2X7 in driving tumor survival and metastatic potential, the P2X7 macropore must be tightly controlled while retaining other functions. Studies have shown that commonly expressed P2X7 splice variants, distinct SNPs and post-translational receptor modifications can impair the capacity of P2X7 to open the macropore. These receptor modifications and potentially others may ultimately protect cancer cells from the negative consequences associated with constitutive activation of P2X7. Significantly, the effects of both P2X7 agonists and antagonists in preclinical tumor models of cancer demonstrate the potential for agents modifying P2X7 function, to provide innovative cancer therapies. This review summarizes recent advances in understanding of the structure and functions of P2X7 and how these impact P2X7 roles in cancer progression. We also review potential therapeutic approaches directed against P2X7.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Ca2+ flux through splice variants of the ATP-gated ionotropic receptor P2X7 is regulated by its cytoplasmic N terminus

Activation of ionotropic P2X receptors increases free intracellular Ca²⁺ ([Ca²⁺] _i ) by initiating a transmembrane cation flux. We studied the "a" and "k" splice variants of the rat purinergic P2X7 receptor (rP2X7aR and rP2X7kR) to exhibit a significant difference in Ca²⁺ flux through this channel. This difference is surprising because the variants share absolute sequence identity in the area of the pore that defines ionic selectivity. Here, we used patch-clamp fluorometry and chimeric receptors to show that the fraction of the total current carried by Ca²⁺ is a function of the primary sequence of the cytoplasmic N terminus. Using scanning mutagenesis, we identified five sites within the N terminus that respond to mutagenesis with a decrease in fractional calcium current and an increase in permeability to the polyatomic cation, N-methyl-d-glucamine (NMDG⁺), relative to Na⁺ (P _NMDG/P _Na). We tested the hypothesis that these sites line the permeation pathway by measuring the ability of thiol-reactive MTSET⁺ to alter the current of cysteine-substituted variants, but we detected no effect. Finally, we studied the homologous sites of the rat P2X2 receptor (rP2X2R) and observed that substitutions at Glu¹⁷ significantly reduced the fractional calcium current. Taken together, our results suggest that a change in the structure of the N terminus alters the ability of an intra-pore Ca²⁺ selectivity filter to discriminate among permeating cations. These results are noteworthy for two reasons: they identify a previously unknown outcome of mutagenesis of the N-terminal domain, and they suggest caution when assigning structure to function for truncated P2X receptors that lack a part of the N terminus.

P2X7 receptor restrains pathogenic Tfh cell generation in systemic lupus erythematosus

T follicular helper cells promote the generation of protective antibodies, but can also foster pathogenic antibodies. The ATP-gated P2X7 receptor selectively limits the expansion of Tfh cells that amplify self-reactive antibodies in systemic lupus erythematosus.

Altered control of T follicular helper (Tfh) cells can lead to generation of autoantibodies and autoimmune manifestations. Signaling pathways that selectively limit pathogenic responses without affecting the protective function of Tfh cells are unknown. Here we show that the ATP-gated ionotropic P2X7 receptor restricts the expansion of aberrant Tfh cells and the generation of self-reactive antibodies in experimental murine lupus, but its activity is dispensable for the expansion of antigen-specific Tfh cells during vaccination. P2X7 stimulation promotes caspase-mediated pyroptosis of Tfh cells and controls the development of pathogenic ICOS⁺ IFN-γ–secreting cells. Circulating Tfh cells from patients with systemic lupus erythematosus (SLE) but not primary antiphospholipid syndrome (PAPS), a nonlupus systemic autoimmune disease, were hyporesponsive to P2X7 stimulation and resistant to P2X7-mediated inhibition of cytokine-driven expansion. These data point to the P2X7 receptor as a checkpoint regulator of Tfh cells; thus, restoring P2X7 activity in SLE patients could selectively limit the progressive amplification of pathogenic autoantibodies, which deteriorate patients’ conditions.

ATP-Gated P2X7 Receptors Require Chloride Channels To Promote Inflammation in Human Macrophages

Immune cells of myeloid origin show robust expression of ATP-gated P2X7 receptors, two-transmembrane ion channels permeable to Na⁺, K⁺, and Ca²⁺ Receptor activation promotes inflammasome activation and release of the proinflammatory cytokines IL-1β and IL-18. In this study, we show that ATP generates facilitating cationic currents in monocyte-derived human macrophages and permeabilizes the plasma membrane to polyatomic cationic dyes. We find that antagonists of PLA₂ and Cl^- channels abolish P2X7 receptor-mediated current facilitation, membrane permeabilization, blebbing, phospholipid scrambling, inflammasome activation, and IL-1β release. Our data demonstrate significant differences in the actions of ATP in murine and human macrophages and suggest that PLA₂ and Cl^- channels mediate innate immunity downstream of P2X7 receptors in human macrophages.

Ion channels as therapeutic antibody targets

It is now well established that antibodies have numerous potential benefits when developed as therapeutics. Here, we evaluate the technical challenges of raising antibodies to membrane-spanning proteins together with enabling technologies that may facilitate the discovery of antibody therapeutics to ion channels. Additionally, we discuss the potential targeting opportunities in the anti-ion channel antibody landscape, along with a number of case studies where functional antibodies that target ion channels have been reported. Antibodies currently in development and progressing towards the clinic are highlighted.

Human P2Y11 Expression Level Affects Human P2X7 Receptor-Mediated Cell Death

Adenosine triphosphate (ATP) is known to induce cell death in T lymphocytes at high extracellular concentrations. CD4⁺ and CD8⁺ T lymphocytes have a differential response to ATP, which in mice is due to differences in the P2X7 receptor expression levels. By contrast, we observed that the difference in human CD4⁺ and CD8⁺ T lymphocyte response toward the synthetic ATP-analog BzATP is not explained by a difference in human P2X7 receptor expression. Rather, the BzATP-induced human P2X7 receptor response in naïve and immune-activated lymphocyte subtypes correlated with the expression of another ATP-binding receptor: the human P2Y₁₁ receptor. In a recombinant expression system, the coexpression of the human P2Y₁₁ receptor counteracted BzATP-induced human P2X7 receptor-driven lactate dehydrogenase release (a marker of cell death) and pore formation independent of calcium signaling. A mutated non-signaling human P2Y₁₁ receptor had a similar human P2X7 receptor-inhibitory effect on pore formation, thus demonstrating that the human P2X7 receptor interference was not caused by human P2Y₁₁ receptor signaling. In conclusion, we demonstrate an important species difference in the ATP-mediated cell death between mice and human cells and show that in human T lymphocytes, the expression of the human P2Y₁₁ receptor correlates with human P2X7 receptor-driven cell death following BzATP stimulation.

P2X7 ionotropic receptor is functionally expressed in rabbit articular chondrocytes and mediates extracellular ATP cytotoxicity

Extracellular ATP regulates various cellular functions by engaging multiple subtypes of P2 purinergic receptors. In many cell types, the ionotropic P2X7 receptor mediates pathological events such as inflammation and cell death. However, the importance of this receptor in chondrocytes remains largely unexplored. Here, we report the functional identification of P2X7 receptor in articular chondrocytes and investigate the involvement of P2X7 receptors in ATP-induced cytotoxicity. Chondrocytes were isolated from rabbit articular cartilage, and P2X7 receptor currents were examined using the whole-cell patch-clamp technique. ATP-induced cytotoxicity was evaluated by measuring caspase-3/7 activity, lactate dehydrogenase (LDH) leakage, and prostagrandin E₂ (PGE₂) release using microscopic and fluorimetric/colorimetric evaluation. Extracellular ATP readily evoked a cationic current without obvious desensitization. This ATP-activated current was dose related, but required millimolar concentrations. A more potent P2X7 receptor agonist, BzATP, also activated this current but at 100-fold lower concentrations. ATP-induced currents were largely abolished by selective P2X7 antagonists, suggesting a predominant role for the P2X7 receptor. RT-PCR confirmed the presence of P2X7 in chondrocytes. Heterologous expression of a rabbit P2X7 clone successfully reproduced the ATP-induced current. Exposure of chondrocytes to ATP increased caspase-3/7 activities, an effect that was totally abrogated by P2X7 receptor antagonists. Extracellular ATP also enhanced LDH release, which was partially attenuated by the P2X7 inhibitor. The P2X7 receptor-mediated elevation in apoptotic caspase signaling was accompanied by increased PGE₂ release and was attenuated by inhibition of either phospholipase A₂ or cyclooxygenase-2. This study provides direct evidence for the presence of functional P2X7 receptors in articular chondrocytes. Our results suggest that the P2X7 receptor is a potential therapeutic target in chondrocyte death associated with cartilage injury and disorders including osteoarthritis.

Modulation of innate and adaptive immunity by P2X ion channels

Extracellular ATP is a major component of the inflammatory microenvironment where it accumulates following cell and tissue injury but also as a consequence of non-lytic release from activated inflammatory cells. In the inflammatory microenvironment ATP binds and activates nucleotide receptors of the P2Y and P2X subfamilies expressed by immune cells. P2Y receptors are G-protein-coupled, while P2X receptors are cation-selective channels. Changes in the intracellular ion homeostasis triggered by P2X receptor stimulation trigger multiple key responses crucial for initiation, propagation, and resolution of inflammation. In the P2X receptor family, the P2X7 subtype has an important role in the activation of lymphocyte, granulocyte, macrophage and dendritic cell responses. Although clinical studies have been so far rather inconclusive, it is believed that P2X7 receptor targeting might offer novel perspectives for anti-inflammatory therapy.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

The P2X7 Receptor in Infection and Inflammation

Adenosine triphosphate (ATP) accumulates at sites of tissue injury and inflammation. Effects of extracellular ATP are mediated by plasma membrane receptors named P2 receptors (P2Rs). The P2R most involved in inflammation and immunity is the P2X7 receptor (P2X7R), expressed by virtually all cells of innate and adaptive immunity. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. Ten human P2RX7 gene splice variants and several SNPs that produce complex haplotypes are known. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. However, an in-depth knowledge of its structure and of the associated signal transduction mechanisms is needed for an effective therapeutic development.

P2X7 receptor drives Th1 cell differentiation and controls the follicular helper T cell population to protect against Plasmodium chabaudi malaria

A complete understanding of the mechanisms underlying the acquisition of protective immunity is crucial to improve vaccine strategies to eradicate malaria. However, it is still unclear whether recognition of damage signals influences the immune response to Plasmodium infection. Adenosine triphosphate (ATP) accumulates in infected erythrocytes and is released into the extracellular milieu through ion channels in the erythrocyte membrane or upon erythrocyte rupture. The P2X7 receptor senses extracellular ATP and induces CD4 T cell activation and death. Here we show that P2X7 receptor promotes T helper 1 (Th1) cell differentiation to the detriment of follicular T helper (Tfh) cells during blood-stage Plasmodium chabaudi malaria. The P2X7 receptor was activated in CD4 T cells following the rupture of infected erythrocytes and these cells became highly responsive to ATP during acute infection. Moreover, mice lacking the P2X7 receptor had increased susceptibility to infection, which correlated with impaired Th1 cell differentiation. Accordingly, IL-2 and IFNγ secretion, as well as T-bet expression, critically depended on P2X7 signaling in CD4 T cells. Additionally, P2X7 receptor controlled the splenic Tfh cell population in infected mice by promoting apoptotic-like cell death. Finally, the P2X7 receptor was required to generate a balanced Th1/Tfh cell population with an improved ability to transfer parasite protection to CD4-deficient mice. This study provides a new insight into malaria immunology by showing the importance of P2X7 receptor in controlling the fine-tuning between Th1 and Tfh cell differentiation during P. chabaudi infection and thus in disease outcome.

Malaria still causes the death of approximately half a million people yearly despite efforts to develop vaccines. The ability of Plasmodium parasites to survive the immune effector mechanisms indicates how suitable the immune response must be to eliminate the infection. CD4 T cells have a dual role in protection against blood-stage malaria by producing IFNγ and helping B cells to secrete antibodies. Infected erythrocytes release adenosine triphosphate (ATP), a damage signal that can be recognized by purinergic receptors. Among them, the P2X7 receptor senses extracellular ATP and induces CD4 T cell activation and death. Here, we evaluated the role of P2X7 receptor in the CD4 T cell response during blood-stage Plasmodium chabaudi malaria. We observed that the selective expression of P2X7 receptor in CD4 T cells was required for T helper 1 (Th1) cell differentiation, contributing to IFNγ production and parasite control. In contrast, we found an increase in follicular T helper (Tfh) cell population, germinal center reaction and anti-parasite antibody production in the absence of the P2X7 receptor. Our findings provide mechanistic insights into malaria pathogenesis by demonstrating the importance of damage signals for the fine-tuning between Th1 and Tfh cell populations and thus for the outcome of the disease.

P2Y1 Receptor Signaling Contributes to High Salt-Induced Priming of the NLRP3 Inflammasome in Retinal Pigment Epithelial Cells

Background

Systemic hypertension is a risk factor of age-related macular degeneration (AMD), a chronic inflammatory disease. Acute hypertension is caused by increased extracellular osmolarity after intake of dietary salt (NaCl). We determined in cultured human retinal pigment epithelial (RPE) cells whether high extracellular NaCl alters the gene expression of inflammasome-associated proteins, and whether autocrine/paracrine purinergic (P2) receptor signaling contributes to the NaCl-induced NLRP3 gene expression.

Methodology/Principal Findings

Hyperosmolarity was induced by the addition of 100 mM NaCl or sucrose to the culture medium. Gene and protein expression levels were determined with real-time RT-PCR and Western blot analysis, respectively. IL-1β and IL-18 levels were evaluated with ELISA. Nuclear factor of activated T cell 5 (NFAT5) expression was knocked down with siRNA. High extracellular NaCl induced NLRP3 and pro-IL-1β gene expression, while the gene expression of further inflammasome-associated proteins (NLRP1, NLRP2, NLRP6, NLRP7, NLRP12, NLRC4, AIM2, ASC, procaspase-1, pro-IL-18) was not altered or below the detection threshold. The NaCl-induced NLRP3 gene expression was partially dependent on the activities of phospholipase C, IP₃ receptors, protein kinase C, the serum and glucocorticoid-regulated kinase, p38 MAPK, ERK1/2, JNK, PI3K, and the transcription factors HIF-1 and NFAT5. Pannexin-dependent ATP release and P2Y₁ receptor activation is required for the full induction of NLRP3 gene expression. High NaCl induced a transient increase of the NLRP3 protein level and a moderate NLRP3 inflammasome activation, as indicated by the transient increase of the cytosolic level of mature IL-1β. High NaCl also induced secretion of IL-18.

Conclusion

High extracellular NaCl induces priming of the NLRP3 inflammasome in RPE cells, in part via P2Y₁ receptor signaling. The inflammasome priming effect of NaCl suggests that high intake of dietary salt may promote local retinal inflammation implicated in the development of AMD.

Exposure of phosphatidylserine on the cell surface

Phosphatidylserine (PtdSer) is a phospholipid that is abundant in eukaryotic plasma membranes. An ATP-dependent enzyme called flippase normally keeps PtdSer inside the cell, but PtdSer is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation. Once exposed to the cell surface, PtdSer acts as an 'eat me' signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets. The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers. Indeed, their identity as well as the mechanism of the PtdSer exposure to the cell surface has only recently been revealed. Here, we describe how PtdSer is exposed in apoptotic cells and in activated platelets, and discuss PtdSer exposure in other biological processes.

P2X7 receptor antagonist activity of the anti-allergic agent oxatomide

Activation of the P2X7 receptor by extracellular ATP is associated with various immune responses including allergic inflammation. Anti-allergic agents, such as H1-antihistamines, are known to inhibit the effects of different chemical mediators such as acetylcholine and platelet-activating factor. Therefore, we hypothesized that some anti-allergic agents might affect P2X7 receptor function. Using N18TG2 and J774 cells, which express functional P2X7 receptors, the effects of several anti-allergic agents on P2X7 receptor function were investigated by monitoring the ATP-induced increase in intracellular Ca(2+) concentrations ([Ca(2+)]i). Among the various agents tested, oxatomide significantly inhibited P2X7 receptor-mediated [Ca(2+)]i elevation in a concentration-dependent manner without affecting the P2Y2 receptor-mediated response in both N18TG2 and J774 cells. Consistently, oxatomide inhibited P2X7 receptor-mediated membrane current and downstream responses such as mitogen-activated protein kinase activation, inflammation-related gene induction, and cell death. In addition, oxatomide inhibited P2X7 receptor-mediated degranulation in mouse bone marrow-derived mast cells. Whole cell patch clamp analyses in HEK293 cells expressing human, mouse, and rat P2X7 receptors revealed that the inhibitory effect of oxatomide on ATP-induced current was most prominent for the human P2X7 receptor and almost non-existent for the rat P2X7 receptor. The potent inhibitory effects of oxatomide on human P2X7 receptor-mediated function were confirmed in RPMI8226 human B cell-like myeloma cells, which endogenously express the P2X7 receptor. Our results demonstrated that the antihistamine oxatomide also acts as a P2X7 receptor antagonist. Future studies should thus evaluate whether P2X7 receptor antagonism contributes to the anti-allergic effects of oxatomide.

Beyond apoptosis: the mechanism and function of phosphatidylserine asymmetry in the membrane of activating mast cells

Loss of plasma membrane asymmetry is a hallmark of apoptosis, but lipid bilayer asymmetry and loss of asymmetry can contribute to numerous cellular functions and responses that are independent of programmed cell death. Exofacial exposure of phosphatidylserine occurs in lymphocytes and mast cells after antigenic stimulation and in the absence of apoptosis, suggesting that there is a functional requirement for phosphatidylserine exposure in immunocytes. In this review we examine current ideas as to the nature of this functional role in mast cell activation. Mechanistically, there is controversy as to the candidate proteins responsible for phosphatidylserine translocation from the internal to external leaflet, and here we review the candidacies of mast cell PLSCR1 and TMEM16F. Finally we examine the potential relationship between functionally important mast cell membrane perturbations and phosphatidylserine exposure during activation.

Rhein antagonizes P2X7 receptor in rat peritoneal macrophages

P2X7 receptor plays important roles in inflammation and immunity, and thereby it serves as a potential therapeutic target for inflammatory diseases. Rhein, an anthraquinone derivative, exhibits significant anti-inflammatory and immunosuppressive activities in therapy. However, the underlying mechanisms are largely unclear. Here, we aimed to investigate the effects of rhein on P2X7 receptor-mediated responses in vitro. In HEK293 cells expressing rat P2X7 receptor, we first found that rhein concentration-dependently blocked ATP-induced cytosolic calcium concentration ([Ca(2+)]c) elevation and pore formation of the plasma membrane, two hallmarks of the P2X7 receptor activation. These two inhibitory effects of rhein were also observed in rat peritoneal macrophages. Furthermore, rhein counteracted macrophage phagocytosis attenuation and suppressed reactive oxygen species (ROS) production triggered by ATP/BzATP. Meanwhile, rhein reduced ATP/BzATP-induced IL-1β release in lipopolysaccharide-activated macrophages. Prolonged application of ATP caused macrophage apoptosis, while the presence of rhein suppressed this cell cytotoxicity. Such ATP/BzATP-induced cellular reactions were also inhibited by a well-known rat P2X7 receptor antagonist, brilliant blue G, in a similar way to rhein. Together, our results demonstrate that rhein inhibit ATP/BzATP-induced [Ca(2+)]c increase, pore formation, ROS production, phagocytosis attenuation, IL-1β release and cell apoptosis by antagonizing the P2X7 receptor in rat peritoneal macrophages.

Nucleotide-Induced Membrane-Proximal Proteolysis Controls the Substrate Specificity of T Cell Ecto-ADP-Ribosyltransferase ARTC2.2

ARTC2.2 is a toxin-related, GPI-anchored ADP-ribosyltransferase expressed by murine T cells. In response to NAD(+) released from damaged cells during inflammation, ARTC2.2 ADP-ribosylates and thereby gates the P2X7 ion channel. This induces ectodomain shedding of metalloprotease-sensitive cell surface proteins. In this study, we show that ARTC2.2 itself is a target for P2X7-triggered ectodomain shedding. We identify the metalloprotease cleavage site 3 aa upstream of the predicted GPI anchor attachment site of ARTC2.2. Intravenous injection of NAD(+) increased the level of enzymatically active ARTC2.2 in serum, indicating that this mechanism is operative also under inflammatory conditions in vivo. Radio-ADP-ribosylation assays reveal that shedding refocuses the target specificity of ARTC2.2 from membrane proteins to secretory proteins. Our results uncover nucleotide-induced membrane-proximal proteolysis as a regulatory mechanism to control the substrate specificity of ARTC2.2.

P2X7 on Mouse T Cells: One Channel, Many Functions

The P2X7 receptor is an adenosine triphosphate (ATP)-gated cation channel that is expressed by several cells of the immune system. P2X7 is best known for its proinflammatory role in promoting inflammasome formation and release of mature interleukin (IL)-1β by innate immune cells. Mounting evidence indicates that P2X7 is also an important regulatory receptor of murine and human T cell functions. Murine T cells express a sensitive splice variant of P2X7 that can be activated either by non-covalent binding of ATP or, in the presence of nicotinamide adenine dinucleotide, by its covalent ADP-ribosylation catalyzed by the ecto-ADP-ribosyltransferase ARTC2.2. Prolonged activation of P2X7 by either one of these pathways triggers the induction of T cell death. Conversely, lower concentrations of ATP can activate P2X7 to enhance T cell proliferation and production of IL-2. In this review, we will highlight the molecular and cellular consequences of P2X7 activation on mouse T cells and its versatile role in T cell homeostasis and activation. Further, we will discuss important differences in the function of P2X7 on human and murine T cells.

Anoctamin 6 mediates effects essential for innate immunity downstream of P2X7 receptors in macrophages

Purinergic P2X7 receptors (P2X7R) are fundamental to innate immune response. In macrophages, transient stimulation of P2X7R activates several transport mechanisms and induces the scrambling of phospholipids with subsequent membrane blebbing and apoptosis. These processes support phagocytosis and subsequent killing of phagocytosed bacteria. Here we demonstrate that the stimulation of P2X7 receptors activates anoctamin 6 (ANO6, TMEM16F), a protein that functions as Ca(2+) dependent phospholipid scramblase and Ca(2+)-activated Cl(-) channel. Inhibition or knockdown of ANO6 attenuates ATP-induced cell shrinkage, cell migration and phospholipid scrambling. In mouse macrophages, Ano6 produces large ion currents by stimulation of P2X7 receptors and contributes to ATP-induced membrane blebbing and apoptosis, which is largely reduced in macrophages from Ano6-/- mice. ANO6 supports bacterial phagocytosis and killing by mouse and human THP-1 macrophages. Our data demonstrate that anoctamin 6 is an essential component of the immune defense by macrophages.

ATP-gated ionotropic P2X7 receptor controls follicular T helper cell numbers in Peyer's patches to promote host-microbiota mutualism

Microbial colonization of the gut induces the development of gut-associated lymphoid tissue (GALT). The molecular mechanisms that regulate GALT function and result in gut-commensal homeostasis are poorly defined. T follicular helper (Tfh) cells in Peyer's patches (PPs) promote high-affinity IgA responses. Here we found that the ATP-gated ionotropic P2X7 receptor controls Tfh cell numbers in PPs. Lack of P2X7 in Tfh cells enhanced germinal center reactions and high-affinity IgA secretion and binding to commensals. The ensuing depletion of mucosal bacteria resulted in reduced systemic translocation of microbial components, lowering B1 cell stimulation and serum IgM concentrations. Mice lacking P2X7 had increased susceptibility to polymicrobial sepsis, which was rescued by Tfh cell depletion or administration of purified IgM. Thus, regulation of Tfh cells by P2X7 activity is important for mucosal colonization, which in turn results in IgM serum concentrations necessary to protect the host from bacteremia.

Role of the potassium channel KCa3.1 in diabetic nephropathy

There is an urgent need to identify novel interventions for mitigating the progression of diabetic nephropathy. Diabetic nephropathy is characterized by progressive renal fibrosis, in which tubulointerstitial fibrosis has been shown to be the final common pathway of all forms of chronic progressive renal disease, including diabetic nephropathy. Therefore targeting the possible mechanisms that drive this process may provide novel therapeutics which allow the prevention and potentially retardation of the functional decline in diabetic nephropathy. Recently, the Ca2+-activated K+ channel KCa3.1 (KCa3.1) has been suggested as a potential therapeutic target for nephropathy, based on its ability to regulate Ca2+ entry into cells and modulate Ca2+-signalling processes. In the present review, we focus on the physiological role of KCa3.1 in those cells involved in the tubulointerstitial fibrosis, including proximal tubular cells, fibroblasts, inflammatory cells (T-cells and macrophages) and endothelial cells. Collectively these studies support further investigation into KCa3.1 as a therapeutic target in diabetic nephropathy.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

Is the inflammasome a potential therapeutic target in renal disease?

The inflammasome is a large, multiprotein complex that drives proinflammatory cytokine production in response to infection and tissue injury. Pattern recognition receptors that are either membrane bound or cytoplasmic trigger inflammasome assembly. These receptors sense danger signals including damage-associated molecular patterns and pathogen-associated molecular patterns (DAMPS and PAMPS respectively). The best-characterized inflammasome is the NLRP3 inflammasome. On assembly of the NLRP3 inflammasome, post-translational processing and secretion of pro-inflammatory cytokines IL-1β and IL-18 occurs; in addition, cell death may be mediated via caspase-1. Intrinsic renal cells express components of the inflammasome pathway. This is most prominent in tubular epithelial cells and, to a lesser degree, in glomeruli. Several primary renal diseases and systemic diseases affecting the kidney are associated with NLRP3 inflammasome/IL-1β/IL-18 axis activation. Most of the disorders studied have been acute inflammatory diseases. The disease spectrum includes ureteric obstruction, ischaemia reperfusion injury, glomerulonephritis, sepsis, hypoxia, glycerol-induced renal failure, and crystal nephropathy. In addition to mediating renal disease, the IL-1/ IL-18 axis may also be responsible for development of CKD itself and its related complications, including vascular calcification and sepsis. Experimental models using genetic deletions and/or receptor antagonists/antiserum against the NLRP3 inflammasome pathway have shown decreased severity of disease. As such, the inflammasome is an attractive potential therapeutic target in a variety of renal diseases.

Pharmacological characterization of the P2 receptors profile in the podocytes of the freshly isolated rat glomeruli

Calcium flux in the podocytes is critical for normal and pathophysiological regulation of these types of cells, and excessive calcium signaling results in podocytes damage and improper glomeruli function. Purinergic activation of P2 receptors is a powerful and rapid signaling process; however, the exact physiological identity of P2 receptors subtypes in podocytes remains essentially unknown. The goal of this study was to determine the P2 receptor profile in podocytes of the intact Sprague-Dawley rat glomeruli using available pharmacological tools. Glomeruli were isolated by differential sieving and loaded with Fluo-4/Fura Red cell permeable calcium indicators, and the purinergic response in the podocytes was analyzed with ratiometric confocal fluorescence measurements. Various P2 receptors activators were tested and compared with the effect of ATP, specifically, UDP, MRS 2365, bzATP, αβ-methylene, 2-meSADP, MRS 4062, and MRS 2768, were analyzed. Antagonists (MRS 2500, 5-BDBD, A438079, and NF 449) were tested when 10 μM ATP was applied as the EC50 for ATP activation of the calcium influx in the podocytes was determined to be 10.7 ± 1.5 μM. Several agonists including MRS 2365 and 2-meSADP caused calcium flux. Importantly, only the P2Y1-specific antagonist MRS 2500 (1 nM) precluded the effects of ATP concentrations of the physiological range. Immunohistochemical analysis confirmed that P2Y1 receptors are highly expressed in the podocytes. We conclude that P2Y1 receptor signaling is the predominant P2Y purinergic pathway in the glomeruli podocytes and P2Y1 might be involved in the pathogenesis of glomerular injury and could be a target for treatment of kidney diseases.

Epistasis with HLA DR3 implicates the P2X7 receptor in the pathogenesis of primary Sjögren's syndrome

INTRODUCTION

The aim of this study was to examine the association between functional polymorphisms in the pro-inflammatory P2X7 receptor and the Ro/La autoantibody response in primary Sjögren's syndrome (pSS).

METHODS

Twelve functional P2RX7 polymorphisms were genotyped in 114 pSS patients fulfilling the Revised American-European Consensus Criteria for pSS, and 136 controls. Genotyping of the A1405G (rs2230912) polymorphism was performed on a replication cohort consisting of 281 pSS patients and 534 controls. P2X7 receptor function in lymphocytes and monocytes was assessed by measurement of ATP-induced ethidium+ uptake. Serum IL-18 levels were determined by ELISA.

RESULTS

The minor allele of P2RX7 A1405G is a tag for a common haplotype associated with gain in receptor function, as assessed by ATP-induced ethidium+ uptake. A positive association between 1405G and anti-Ro±La seropositive pSS patients was observed in Cohort 1. Although not replicated in Cohort 2, there was a consistent, significant, negative epistatic interaction effect with HLA-DR3 in seropositive pSS patients from both cohorts, thereby implicating this gain of function variant in the pathogenesis of pSS. Serum IL-18 was elevated in seropositive pSS patients, but was not influenced by P2RX7 A1405G.

CONCLUSIONS

The P2RX7 1405G gain-of-function haplotype may be a risk factor for seropositive pSS in a subset of subjects who do not carry HLA risk alleles, but has no effect in subjects who do (epistasis). Potential mechanisms relate to autoantigen exposure and inflammatory cytokine expression. The observed elevation of IL-18 levels is consistent with P2X7 receptor activation in seropositive pSS patients. Collectively these findings implicate P2X7 receptor function in the pathogenesis of pSS.

Calcium-activated and apoptotic phospholipid scrambling induced by Ano6 can occur independently of Ano6 ion currents

Immune cells and platelets maintain plasma membrane phospholipid asymmetry. Upon activation, this asymmetry is disrupted by phospholipid scrambling (PS), which is a major step during activation of immune cells, hemostasis and apoptosis. Anoctamin 6 (Ano6; TMEM16F) causes chloride (Cl(-)) and cation currents and is required for Ca(2+)-dependent PS. It is defective in blood cells from patients with Scott syndrome, a rare bleeding disorder. We examined if Cl(-) currents and PS are related, whether both processes are Ca(2+) dependent, and whether Ca(2+)-independent scrambling during intrinsic and extrinsic apoptosis is controlled by Ano6. Ca(2+) increase by ionomycin activated Ano6 Cl(-) currents and PS in normal lymphocytes, but not in B-lymphocytes from two different patients with Scott syndrome. Fas ligand (FasL) did not increase intracellular Ca(2+), but activated Cl(-) currents in normal but not in Scott lymphocytes. Whole-cell currents were inhibited by Cl(-) channel blockers and by siRNA knockdown of Ano6. In contrast, intrinsic mitochondrial apoptosis by ABT-737 did not induce Cl(-) currents in lymphocytes. PS was not inhibited by blockers of Ano6 or removal of Cl(-) ions. Remarkably, Ca(2+)-independent scrambling due to extrinsic (FasL) or intrinsic (ABT-737) apoptosis was unchanged in Scott cells. We conclude that: (i) Ano6 Cl(-) currents are activated by increase in cytosolic Ca(2+), or Ca(2+) independent by stimulation of Fas receptors; (ii) Ca(2+)-dependent PS induced by Ano6 does not require Cl(-) currents; (iii) Ca(2+)-independent PS does not require Ano6; (iv) Ano6 is necessary for Ca(2+)-dependent PS, but not by increasing intracellular Ca(2+).

Mitochondrial superoxide generation enhances P2X7R-mediated loss of cell surface CD62L on naive human CD4+ T lymphocytes

Migration of naive CD4(+) T lymphocytes into lymphoid tissue is essential for their activation and subsequent roles in adaptive immunity. The adhesion molecule L-selectin (CD62L), critical for this process, is highly expressed on naive CD4(+) T lymphocytes and is downregulated upon T lymphocyte activation. We demonstrate protein expression of P2X7R on naive CD4(+) T lymphocytes and show functional channel activity in whole-cell patch clamp recordings. CD62L downregulation occurs rapidly in response to extracellular ATP, a process that is blocked by selective antagonists of P2X7R. This loss of surface CD62L expression was not associated with externalization of phosphatidylserine. While investigating the mechanisms for this process, we revealed that pharmacological modulation of mitochondrial complex I or III, but not inhibition of NADPH oxidase, enhanced P2X7R-dependent CD62L downregulation by increasing ATP potency. Enhanced superoxide generation in the mitochondria of rotenone- and antimycin A-treated cells was observed and may contribute to the enhanced sensitivity of P2X7R to ATP. P2X7R-dependent exposure of phosphatidylserine was also revealed by preincubation with mitochondrial uncouplers prior to ATP treatment. This may present a novel mechanism whereby P2X7R-dependent phosphatidylserine exposure occurs only when cells have enhanced mitochondrial reactive oxygen species generation. The clearance of apoptotic cells may therefore be enhanced by this mechanism which requires functional P2X7R expression.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Cell volume regulation modulates NLRP3 inflammasome activation

Cell volume regulation is a primitive response to alterations in environmental osmolarity. The NLRP3 inflammasome is a multiprotein complex that senses pathogen- and danger-associated signals. Here, we report that, from fish to mammals, the basic mechanisms of cell swelling and regulatory volume decrease (RVD) are sensed via the NLRP3 inflammasome. We found that a decrease in extracellular osmolarity induced a K(+)-dependent conformational change of the preassembled NLRP3-inactive inflammasome during cell swelling, followed by activation of the NLRP3 inflammasome and caspase-1, which was controlled by transient receptor potential channels during RVD. Both mechanisms were necessary for interleukin-1β processing. Increased extracellular osmolarity prevented caspase-1 activation by different known NLRP3 activators. Collectively, our data identify cell volume regulation as a basic conserved homeostatic mechanism associated with the formation of the NLRP3 inflammasome and reveal a mechanism for NLRP3 inflammasome activation.

P2X7 receptor activation induces inflammatory responses in salivary gland epithelium

Inflammation of the salivary gland is a well-documented aspect of salivary gland dysfunction that occurs in Sjogren's syndrome (SS), an autoimmune disease, and in γ-radiation-induced injury during treatment of head and neck cancers. Extracellular nucleotides have gained recognition as key modulators of inflammation through activation of cell surface ionotropic and metabotropic receptors, although the contribution of extracellular nucleotides to salivary gland inflammation is not well understood. In vitro studies using submandibular gland (SMG) cell aggregates isolated from wild-type C57BL/6 mice indicate that treatment with ATP or the high affinity P2X7R agonist 3'-O-(4-benzoyl)benzoyl-ATP (BzATP) induces membrane blebbing and enhances caspase activity, responses that were absent in SMG cell aggregates isolated from mice lacking the P2X7R (P2X7R(-/-)). Additional studies with SMG cell aggregates indicate that activation of the P2X7R with ATP or BzATP stimulates the cleavage and release of α-fodrin, a cytoskeletal protein thought to act as an autoantigen in the development of SS. In vivo administration of BzATP to ligated SMG excretory ducts enhances immune cell infiltration into the gland and initiates apoptosis of salivary epithelial cells in wild-type, but not P2X7R(-/-), mice. These findings indicate that activation of the P2X7R contributes to salivary gland inflammation in vivo, suggesting that the P2X7R may represent a novel target for the treatment of salivary gland dysfunction.

Swelling rather than shrinkage precedes apoptosis in serum-deprived vascular smooth muscle cells

Contrasting cell volume behaviours (swelling vs. shrinkage) are considered as criteria to distinguish necrosis from apoptosis. In this study, we employed a time-lapse, dual-image surface reconstruction technique to assess the volume of single vascular smooth muscle cells transfected with E1A-adenoviral protein (E1A-VSMC) and undergoing rapid apoptosis in the absence of growth factors or in the presence of staurosporine. After 30- to 60-min lag-phase, serum-deprived E1A-VSMC volume was increased by ~40%, which preceded maximal increments of caspase-3 activity and chromatin cleavage. Swollen cells underwent rapid apoptotic collapse, documented by plasma membrane budding, and terminated in 10-15 min by the formation of numerous apoptotic bodies. Suppression of apoptosis by inhibition of Na(+),K(+)-ATPase and activation of cAMP signalling with ouabain and forskolin, respectively, completely abolished the swelling of serum-deprived E1A-VSMC. In contrast to serum deprivation, apoptotic collapse of staurosporine-treated E1A-VSMC preceded attenuation of their volume by ~30%. Neither transient hyposmotic swelling nor isosmtotic shrinkage triggered apoptosis. Our results show that cell shrinkage can not be considered as ubiquitous hallmark of apoptosis. The involvement of stimulus-specific cell volume perturbations in initiation and progression of apoptosis in vascular smooth muscle cells should be examined further.

Overexpression of CD39 in mouse airways promotes bacteria-induced inflammation

In airways, the ecto-nucleoside triphosphate diphosphohydrolase CD39 plays a central role in the regulation of physiological mucosal nucleotide concentrations and likely contributes to the control of inflammation because accelerated ATP metabolism occurs in chronic inflammatory lung diseases. We sought to determine whether constant elevated CD39 activity in lung epithelia is sufficient to cause inflammation and whether this affects the response to acute LPS or Pseudomonas aeruginosa exposure. We generated transgenic mice overexpressing human CD39 under the control of the airway-specific Clara cell 10-kDa protein gene promoter. Transgenic mice did not develop any spontaneous lung inflammation. However, intratracheal instillation of LPS resulted in accelerated recruitment of neutrophils to the airways of transgenic mice. Macrophage clearance was delayed, and the amounts of CD8(+) T and B cells were augmented. Increased levels of keratinocyte chemoattractant, IL-6, and RANTES were produced in transgenic lungs. Similarly, higher numbers of neutrophils and macrophages were found in the lungs of transgenic mice infected with P. aeruginosa, which correlated with improved bacteria clearance. The transgenic phenotype was partially and differentially restored by coinstillation of P2X(1) or P2X(7) receptor antagonists or of caffeine with LPS. Thus, a chronic increase of epithelial CD39 expression and activity promotes airway inflammation in response to bacterial challenge by enhancing P1 and P2 receptor activation.

Splice variants of the P2X7 receptor reveal differential agonist dependence and functional coupling with pannexin-1

P2X7 receptors function as ATP-gated cation channels but also interact with other proteins as part of a larger signalling complex to mediate a variety of downstream responses that are dependent upon the cell type in which they are expressed. Receptor-mediated membrane permeabilization to large molecules precedes the induction of cell death, but remains poorly understood. The mechanisms that underlie differential sensitivity to NAD are also unknown. By studying alternative variants of the mouse P2X7 receptor we show that sensitivity to NAD is mediated through the P2X7k variant, which has a much more restricted distribution than the P2X7a receptor, but is expressed in T lymphocytes. The altered N-terminus and TM1 of the P2X7k receptor enhances the stability of the active state of this variant compared with P2X7a, thereby increasing the efficacy of NAD-dependent ADP ribosylation as measured by ethidium uptake, a rise in intracellular Ca(2+) and the activation of inward currents. Co-expression of P2X7k and P2X7a receptors reduced NAD sensitivity. P2X7k-receptor-mediated ethidium uptake was also triggered by much lower BzATP concentrations and was insensitive to the P451L single nucleotide polymorphism. P2X7k-receptor-mediated ethidium uptake occurred independently of pannexin-1 suggesting a pathway intrinsic to the receptor. Only for the P2X7aL451 receptor could we resolve a component of dye uptake dependent upon pannexin-1. Signalling occurred downstream of the activation of caspases rather than involving direct cross talk between the channels. However, an in situ proximity assay showed close association between P2X7 receptors and pannexin-1, which would facilitate ATP efflux through pannexin-1 acting in an autocrine manner.

A novel role for P2X7 receptor signalling in the survival of mouse embryonic stem cells

The growth of a pluripotent embryonic stem (ES) cell population is dependent on cell survival, proliferation and self-renewal. The nucleotide ATP represents an important extracellular signalling molecule that regulates the survival of differentiated cells, however, its role is largely undefined in embryonic stem cells. Here we report a role for ATP-gated P2X7 receptors in ES cell survival. The functional expression of P2X7 receptors in undifferentiated mouse ES cells is demonstrated using a selective P2X7 antagonist and small interfering RNA knockdown of these receptors. Our data illustrate a key role for the P2X7 receptor as an essential pro-survival signal required for optimal ES cell colony growth in the presence of leukemia inhibitor factor (LIF). However, chronic exposure to exogenous ATP leads to rapid P2X7-dependent cell death via necrosis. Together, these data demonstrate a novel role for P2X7 receptors in regulation of ES cell behaviour where they can mediate either a pro-survival or pro-death signal depending on the mode of activation.

Therapeutic potential of KCa3.1 blockers: recent advances and promising trends

The Ca(2+)-activated K(+) channel K(Ca)3.1 regulates membrane potential and calcium signaling in erythrocytes, activated T and B cells, macrophages, microglia, vascular endothelium, epithelia, and proliferating vascular smooth muscle cells and fibroblasts. K(Ca)3.1 has therefore been suggested as a potential therapeutic target for diseases such as sickle cell anemia, asthma, coronary restenosis after angioplasty, atherosclerosis, kidney fibrosis and autoimmunity, where activation and excessive proliferation of one or more of these cell types is involved in the pathology. This article will review the physiology and pharmacology of K(Ca)3.1 and critically examine the available preclinical and clinical data validating K(Ca)3.1 as a therapeutic target.

A potential therapeutic role for P2X7 receptor (P2X7R) antagonists in the treatment of inflammatory diseases

INTRODUCTION

The P2X7 receptor (P2X7R) has an important role in inflammation and immunity, but until recently, clinical application has been limited by a lack of specific antagonists. Recent studies using P2X7R knockout mice and specific receptor antagonists have shown that the P2X7R is an important therapeutic target in inflammatory diseases.

AREAS COVERED

We have reviewed the current literature on the role of the P2X7R in inflammatory diseases, focusing on potential therapeutic applications of selective P2X7R antagonists as anti-inflammatory agents. Particular emphasis has been placed on the potential role of P2X7R in common inflammatory diseases. The latest developments in Phase I and II clinical trials of P2X7R antagonists are covered.

EXPERT OPINION

Recent studies using gene knockout mice and selective P2X7R antagonists suggest that P2X7R is a viable therapeutic target for inflammatory diseases. However, efficacious P2X7R antagonists for use in clinical studies are still at an early stage of development. Future challenges include: identifying potential toxicity and side effects of treatment, timing of treatment initiation and its duration in chronic inflammatory conditions, optimum dosage and development of a functional assay for P2X7R that would help to guide treatment.

Common variants in P2RY11 are associated with narcolepsy

Growing evidence supports the hypothesis that narcolepsy with cataplexy is an autoimmune disease. We here report genome-wide association analyses for narcolepsy with replication and fine mapping across three ethnic groups (3,406 individuals of European ancestry, 2,414 Asians and 302 African Americans). We identify a SNP in the 3' untranslated region of P2RY11, the purinergic receptor subtype P2Y₁₁ gene, which is associated with narcolepsy (rs2305795, combined P = 6.1 × 10⁻¹⁰, odds ratio = 1.28, 95% CI 1.19-1.39, n = 5689). The disease-associated allele is correlated with reduced expression of P2RY11 in CD8(+) T lymphocytes (339% reduced, P = 0.003) and natural killer (NK) cells (P = 0.031), but not in other peripheral blood mononuclear cell types. The low expression variant is also associated with reduced P2RY11-mediated resistance to ATP-induced cell death in T lymphocytes (P = 0.0007) and natural killer cells (P = 0.001). These results identify P2RY11 as an important regulator of immune-cell survival, with possible implications in narcolepsy and other autoimmune diseases.

Characterization of ATP-induced cell death in the GL261 mouse glioma

Gliomas have one of the worst prognosis among cancers. Their resistance to cell death induced by endogenous neurotoxic agents, such as extracellular ATP, seems to play an important role in their pathobiology since alterations in the degradation rate of extracellular ATP drastically affects glioma growth in rats. In the present work we characterized the mechanisms of cell death induced by extracellular ATP in a murine glioma cell line, GL261. ATP and BzATP, a P2X7 agonist, induced cell death at concentrations that are described to activate the P2X7 receptor in mouse. oATP, an antagonist of P2X7, blocked the ATP-induced cell death. Agonists of purinergic receptors expressed in GL261 such as adenosine, ADP, UTP did not cause any cell death, even at mM concentrations. A sub-population of cells more sensitive to ATP expressed more P2X7 when compared to a less sensitive subpopulation. Accordingly, RNA interference of the P2X7 receptor drastically reduced ATP-induced cell death, suggesting that this receptor is necessary for this effect. The mechanism of ATP-induced cell death is predominantly necrotic, since cells presented shrinkage accompanied by membrane permeabilization, but not apoptotic, since no phosphatidylserine externalization or caspase activity was observed. These data show the importance of P2X7 in ATP-induced cell death and shed light on the importance of ATP-induced cell death in glioma development.

Inhibition of ATP-induced macrophage death by emodin via antagonizing P2X7 receptor

Emodin (1,3,8-trihydroxy-6-methylanthraquinone), an anthraquinone derivative from Rheum officinale Baill, exhibits anti-inflammatory and immunosuppressive activities, however, the underlying mechanisms are not fully understood. This study examined the effects of emodin on ATP-evoked responses in rat peritoneal macrophages and in human embryonic kidney 293 cells (HEK293) heterologously expressing the cloned rat P2X7 receptor. Emodin reduced macrophage death induced by millimolar ATP in a concentration-dependent manner with the half of maximal inhibition values (IC50) of 0.2 microM. It also strongly inhibited ATP-induced dye uptake or pore formation, a hallmark property associated with P2X7 receptor activation, and 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP) induced increases in intracellular Ca2+ concentrations in macrophages with an IC50 of 0.5 microM. Furthermore, emodin significantly suppressed BzATP-evoked currents in P2X7 receptor expressing HEK293 cells with an IC50 of 3.4 microM. Taken together, these results provide compelling evidence for a novel action of emodin as a P2X7 receptor antagonist, which may underlie its anti-inflammatory and immunosuppressive activities.