Secretion of intracellular IL-1 receptor antagonist (type 1) is dependent on P2X7 receptor activation

Inflammatory mechanisms are critical in the arterial response to injury. Both IL-1 and the naturally occurring inhibitor of IL-1, IL-1R antagonist (IL-1ra), are expressed in the arterial wall, and in particular in the endothelium. Previous studies suggest that endothelial cells only make the intracellular type I isoform of IL-1ra (icIL-1ra1), an isoform known to lack a secretory signal peptide. It is unclear how icIL-1ra is released from the endothelial cell to act as an antagonist on cell surface IL-1 type I receptors. IL-1beta, which also lacks a secretory signal peptide, may be released by ATP stimulation of the P2X(7)R. Therefore, we examined whether icIL-1ra1 release occurs in an analogous manner, using both the mouse macrophage cell line RAW264.7 and HUVECs. P2X(7)R activation caused icIL-1ra1 release from LPS-primed RAW264.7 macrophages and from HUVECs. This release was inhibited in the absence of extracellular calcium, and attenuated by preincubation with oxidized ATP, KN62, and apyrase. Endogenous ATP release, which also facilitated release of icIL-1ra1, was detected during LPS treatment of both RAW264.7 macrophages and HUVECs. Annexin V assays showed that ATP stimulation resulted in a rapid phosphatidylserine (PS) exposure on the cell surface of RAW264.7 macrophages, and that PS-exposed microvesicles contained icIL-1ra1. However, PS flip and microvesicle shedding was not apparent in ATP-treated HUVECs. These data support a general role for the P2X(7)R in the release of leaderless cytokines into the extracellular medium, and indicate how icIL-1ra1 may act upon its extracellular target, the IL-1R.     

Mechanisms of caspase-1 activation by P2X7 receptor-mediated K+ release

The mechanisms underlying caspase-1 activation and IL-1 processing during inflammatory activation of monocytes and macrophages are not well defined. Here, we describe an in vitro proteolytic processing assay that allows for comparison of caspase-1 regulatory components in a cell-free system separately from the confounding issue of IL-1 secretion. Analysis of in vitro IL-1 and caspase-1 processing in lysates from unstimulated Bac1 murine macrophages indicated a slow rate of basal caspase-1 activation and proteolytic maturation of IL-1. In contrast, brief (5 min) treatment of intact macrophages with extracellular ATP (as an activator of the P2X₇ receptor) or nigericin before cell lysis markedly accelerated the in vitro processing of caspase-1 and IL-1. This acceleration of in vitro processing was strictly dependent on loss of intracellular K⁺ from the intact cells. The induction of in vitro caspase-1 activation by lysis per se or by K⁺ loss before lysis was sensitive to pretreatment of intact macrophages with the tyrphostin AG-126 or bromoenol lactone, an inhibitor of Ca²⁺-independent phospholipase A₂. Caspase-1 activation and IL-1 processing in lysates from unstimulated macrophages were also accelerated by addition of recombinant ASC, a previously identified adapter protein that directly associates with caspase-1. These data indicate that increased K⁺ efflux via P2X₇ nucleotide receptor stimulation activates AG-126- and bromoenol lactone-sensitive signaling pathways in murine macrophages that result in stably maintained signals for caspase-1 regulation in cell-free assays. AG-126; ASC; bromoenol lactone; IL-1; inflammation     

Inhibitory effects of chloride on the activation of caspase-1, IL-1beta secretion, and cytolysis by the P2X7 receptor

The P2X7 receptor (P2X7R) is an ATP-gated cation channel that activates caspase-1 leading to the maturation and secretion of IL-1beta. Because previous studies indicated that extracellular Cl- exerts a negative allosteric effect on ATP-gating of P2X7R channels, we tested whether Cl- attenuates the P2X7R-->caspase-1-->IL-1beta signaling cascade in murine and human macrophages. In Bac1 murine macrophages, substitution of extracellular Cl- with gluconate produced a 10-fold increase in the rate and extent of ATP-induced IL-1beta processing and secretion, while reducing the EC50 for ATP by 5-fold. Replacement of Cl- with gluconate also increased the potency of ATP as an inducer of mature IL-1beta secretion in primary mouse bone marrow-derived macrophages and in THP-1 human monocytes/macrophages. Our observations were consistent with actions of Cl- at three levels: 1) a negative allosteric effect of Cl-, which limits the ability of ATP to gate the P2X7R-mediated cation fluxes that trigger caspase-1 activation; 2) an intracellular accumulation of Cl- via nonselective pores induced by P2X7R with consequential repression of caspase-1-mediated processing of IL-1beta; and 3) a facilitative effect of Cl- substitution on the cytolytic release of unprocessed pro-IL-1beta that occurs with sustained activation of P2X7R. This cytolysis was repressed by the cytoprotectant glycine, permitting dissociation of P2X7R-regulated secretion of mature IL-1beta from the lytic release of pro-IL-1beta. These results suggest that under physiological conditions P2X7R are maintained in a conformationally restrained state that limits channel gating and coupling of the receptor to signaling pathways that regulate caspase-1.     

Differential requirement of P2X7 receptor and intracellular K+ for caspase-1 activation induced by intracellular and extracellular bacteria

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases. The maturation and secretion of IL-1beta are mediated by caspase-1, a protease that processes pro-IL-1beta into biologically active IL-1beta. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration of K(+) potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and intracellular K(+) in IL-1beta secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists such as lipopolysaccharide or infection with extracellular bacteria Staphylococcus aureus and Escherichia coli, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through depletion of intracellular K(+) caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K(+). Consistently, caspase-1 activation induced by intracellular Salmonella or Listeria was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally in the absence of P2X7 receptor-mediated cytoplasmic K(+) perturbations.     

Microglia: Proliferation and activation driven by the P2X7 receptor

Microglial activation is associated with the pathogenesis and progression of conditions such as Alzheimer's disease (AD), Parkinsons’ disease, prion disease, multiple sclerosis, and ischemic and traumatic brain injury. The molecular mechanism of microglial activation is largely unknown. The expression of the purinergic, P2X7 receptor (P2X7R), is known to be enhanced in many brain pathologies where presence of activated microglia is a concurrent feature. This review focuses on the links between P2X7R expression and microglial activation and proliferation. The P2X7R is identified as a key player in the process of microgliosis, where by driving microglial activation, it can potentially lead to a deleterious cycle of neuroinflammation and neurodegeneration.     

ATP-induced apoptosis of thymocytes is mediated by activation of P2 X 7 receptor and involves de novo ceramide synthesis and mitochondria

Thymocytes were reported to undergo apoptosis in the presence of extracellular ATP through the activation of the purinergic receptors P2 X 1R, P2 X 7R or both. We investigated the identity of the P2 X R and the signaling pathways involved in ATP-mediated apoptosis. Apoptosis elicited by ATP was prevented by inhibition of P2 X 7R, or in thymocytes bearing a mutated P2 X 7R, and reproduced with a P2 X 7R agonist, but not with a P2 X 1R agonist. Stimulation of thymocytes with either ATP or a P2 X 7R agonist was found to stimulate a late de novo ceramide synthesis and mitochondrial alterations. Inhibition of either processes attenuated apoptosis. Interestingly, stimulation with either ATP or a P2 X 1R agonist induced an early ceramide accumulation and a weak caspases-3/7 activation that did not lead to apoptosis. In conclusion, de novo ceramide generation and mitochondrial alterations, both resulting from P2 X 7R activation, were implicated in ATP-induced thymocyte apoptosis.     

Transient P2X7 receptor activation triggers macrophage death independent of Toll-like receptors 2 and 4, caspase-1, and pannexin-1 proteins

The function of P2X(7) receptors (ATP-gated ion channels) in innate immune cells is unclear. In the setting of Toll-like receptor (TLR) stimulation, secondary activation of P2X(7) ion channels has been linked to pro-caspase-1 cleavage and cell death. Here we show that cell death is a surprisingly early triggered event. We show using live-cell imaging that transient (1-4 min) stimulation of mouse macrophages with high extracellular ATP ([ATP]e) triggers delayed (hours) cell death, indexed as DEVDase (caspase-3 and caspase-7) activity. Continuous or transient high [ATP]e did not induce cell death in P2X(7)-deficient (P2X(7)(-/-)) macrophages or neutrophils (in which P2X(7) could not be detected). Blocking sustained Ca(2+) influx, a signature of P2X(7) ligation, was highly protective, whereas no protection was conferred in macrophages lacking caspase-1 or TLR2 and TLR4. Furthermore, pannexin-1 (Panx1) deficiency had no effect on transient ATP-induced delayed cell death or ATP-induced Yo-Pro-1 uptake (an index of large pore pathway formation). Thus, "transient" P2X(7) receptor activation and Ca(2+) overload act as a death trigger for native mouse macrophages independent of Panx1 and pro-inflammatory caspase-1 and TLR signaling.     

Stress-activated protein kinase/JNK activation and apoptotic induction by the macrophage P2X7 nucleotide receptor

In human and rodent macrophages, activation of the P2X7 nucleotide receptor stimulates interleukin-1beta processing and release, apoptosis, and killing of intracellular Mycobacterium tuberculosis. Signaling pathways downstream of this ionotropic ATP receptor are poorly understood. Here we describe the rapid activation of the stress-activated protein kinase (SAPK)/JNK pathway in BAC1 murine macrophages stimulated by extracellular ATP. Brief exposure of the cells to ATP (10-30 min) was sufficient to trigger a rapid accumulation of activated SAPK that was then sustained for >120 min. Several observations indicated that the P2X7 receptor mediated this effect. 1) ATP and 3'-O-(4-benzoyl)benzoyl-ATP were the only agonistic nucleotides. 2) The effect was inhibited by oxidized ATP and the isoquinoline KN-62, two known P2X7 receptor antagonists. 3) ATP-induced SAPK activation could be recapitulated in P2X7 receptor-transfected HEK293 cells, but not in wild-type HEK293 cells. Because P2X7 receptor stimulation can rapidly activate caspase family proteases that have been implicated in the induction of the SAPK pathway, we investigated whether ATP-dependent SAPK activation involved such proteases. Brief exposure of BAC1 macrophages to extracellular ATP induced DNA fragmentation, alpha-fodrin breakdown, and elevated levels of caspase-3-type activity. Asp-Glu-Val-Asp-cho, a caspase-3 inhibitor, inhibited ATP-induced DNA fragmentation and alpha-fodrin proteolysis, but had no effect on ATP-induced SAPK activation. Tyr-Val-Ala-Asp-chloromethyl ketone, a caspase-1 inhibitor, prevented ATP-induced release of processed interleukin-1beta, but not ATP-dependent SAPK activity. We conclude that activation of ionotropic P2X7 nucleotide receptors triggers a strong activation of SAPK via a pathway independent of caspase-1- or caspase-3-like proteases.     

Probenecid directly impairs activation of the canine P2X7 receptor

The current study aimed to determine if probenecid could directly impair the canine P2X7 receptor, a ligand-gated cation channel activated by extracellular adenosine 5′-triphosphate (ATP). Patch clamp measurements demonstrated that probenecid impairs ATP-induced inward currents in HEK-293 cells expressing canine P2X7. Flow cytometric measurements of ethidium⁺ uptake into HEK-293 cells expressing canine P2X7 showed that probenecid impairs ATP-induced pore formation in a concentration-dependent manner, with a half maximal inhibitory concentration of 158 µM. Finally, ELISA measurements revealed that probenecid impairs ATP-induced interleukin-1β release in dog blood. In conclusion, this study reveals that probenecid can directly impair canine P2X7 activation.     

The P2X7 carboxyl tail is a regulatory module of P2X7 receptor channel activity

P2X(7) receptors are ATP-gated cation channels composed of three identical subunits, each having intracellular amino and carboxyl termini and two transmembrane segments connected by a large ectodomain. Within the P2X family, P2X(7) subunits are unique in possessing an extended carboxyl tail. We expressed the human P2X(7) subunit as two complementary fragments, a carboxyl tail-truncated receptor channel core (residues 1-436 or 1-505) and a tail extension (residues 434-595) in Xenopus laevis oocytes. P2X(7) channel core subunits efficiently assembled as homotrimers that appeared abundantly at the oocyte surface, yet produced only approximately 5% of the full-length P2X(7) receptor current. Co-assembly of channel core subunits with full-length P2X(7) subunits inhibited channel current, indicating that the lack of a single carboxyl tail domain is dominant-negative for P2X(7) receptor activity. Co-expression of the tail extension as a discrete protein increased ATP-gated current amplitudes of P2X(7) channel cores 10-20-fold, fully reconstituting the wild type electrophysiological phenotype of the P2X(7) receptor. Chemical cross-linking revealed that the discrete tail extension bound with unity stoichiometry to the carboxyl tail of the P2X(7) channel core. We conclude that a non-covalent association of crucial functional importance exists between the carboxyl tail of the channel core and the tail extension. Using a slightly shorter P2X(7) subunit core and subfragments of the tail extension, this association could be narrowed down to include residues 409-436 and 434-494 of the split receptor. Together, these results identify the tail extension as a regulatory gating module, potentially making P2X(7) channel gating sensitive to intracellular regulation.     

Nonclassical IL-1 beta secretion stimulated by P2X7 receptors is dependent on inflammasome activation and correlated with exosome release in murine macrophages

Several mechanistically distinct models of nonclassical secretion, including exocytosis of secretory lysosomes, shedding of plasma membrane microvesicles, and direct efflux through plasma membrane transporters, have been proposed to explain the rapid export of caspase-1-processed IL-1 beta from monocytes/macrophages in response to activation of P2X7 receptors (P2X7R) by extracellular ATP. We compared the contribution of these mechanisms to P2X7R-stimulated IL-1 beta secretion in primary bone marrow-derived macrophages isolated from wild-type, P2X7R knockout, or apoptosis-associated speck-like protein containing a C-terminal CARD knockout mice. Our experiments revealed the following: 1) a novel correlation between IL-1 beta secretion and the release of the MHC-II membrane protein, which is a marker of plasma membranes, recycling endosomes, multivesicular bodies, and released exosomes; 2) a common and absolute requirement for inflammasome assembly and active caspase-1 in triggering the cotemporal export of IL-1 beta and MHC-II; and 3) mechanistic dissociation of IL-1 beta export from either secretory lysosome exocytosis or plasma membrane microvesicle shedding on the basis of different requirements for extracellular Ca(2+) and differential sensitivity to pharmacological agents that block activation of caspase-1 inflammasomes. Thus, neither secretory lysosome exocytosis nor microvesicle shedding models constitute the major pathways for nonclassical IL-1 beta secretion from ATP-stimulated murine macrophages. Our findings suggest an alternative model of IL-1 beta release that may involve the P2X7R-induced formation of multivesicular bodies that contain exosomes with entrapped IL-1 beta, caspase-1, and other inflammasome components.     

Calmidazolium selectively inhibits exocytotic glutamate release evoked by P2X7 receptor activation

We previously observed that activation of presynaptic P2X7 receptors located on rat cerebrocortical nerve terminals induced the release of glutamate through different modes: the channel conformation allowing Ca(2+) entry triggered exocytotic release, while the receptor itself functioned as a permeation pathway for the non-exocytotic glutamate release. Considering that exocytotic and non-exocytotic glutamate release evoked by the activation of P2X7 receptors might play a role in the control of glutamatergic synapses, we investigated whether calmidazolium (which has been found to inhibit small cation currents through recombinant P2X7 receptors, but not organic molecule permeation) could distinguish between P2X7-related exocytotic and non-exocytotic modes of glutamate release. We found that calmidazolium inhibited the intrasynaptosomal Ca(2+) response to P2X7 receptor activation and the Ca(2+)-dependent exocytotic glutamate release from rat cerebrocortical nerve terminals, but was ineffective against the Ca(2+)-independent glutamate release. The P2X7 competitive antagonist A-438079 eliminated both exocytotic and non-exocytotic P2X7 receptor-evoked glutamate release. Selective inhibition of exocytotic glutamate release indicates that calmidazolium inhibits events dependent on the function of native rat P2X7 receptors as Ca(2+) channels, and suggests that it can be used as a tool to dissociate P2X7-evoked exocytotic from non-exocytotic glutamate release.     

P2X7 receptor and macrophage function

Macrophages are unique innate immune cells that play an integral role in the defense of the host by virtue of their ability to recognize, engulf, and kill pathogens while sending out danger signals via cytokines to recruit and activate inflammatory cells. It is becoming increasingly clear that purinergic signaling events are essential components of the macrophage response to pathogen challenges and disorders such as sepsis may be, at least in part, regulated by these important sensors. The activation of the P2X₇ receptor is a powerful event in the regulation of the caspase-1 inflammasome. We provide evidence that the inflammasome activation requires “priming” of macrophages prior to ATP activation of the P2X₇R. Inhibition of the inflammasome activation by the tyrosine kinase inhibitor, AG126, suggests regulation by phosphorylation. Finally, the P2X₇R may also be activated by other elements of the host response such as the antimicrobial peptide LL-37, which adds a new, physiologically relevant agonist to the P2X₇R pathway. Therapeutic approaches to inflammation and sepsis will certainly be enhanced by an increased understanding of how purinergic receptors modulate the inflammasomes.     

Potentiation of ATP-induced currents due to the activation of P2X receptors by ubiquitin carboxy-terminal hydrolase L1

Mammalian neuronal cells abundantly express a de-ubiquitinating isozyme, ubiquitin carboxy-terminal hydrolase L1 (UCH L1). Loss of UCH L1 function causes dying-back type of axonal degeneration. However, the function of UCH L1 in neuronal cells remains elusive. Here we show that overexpression of UCH L1 potentiated ATP-induced currents due to the activation of P2X receptors that are widely distributed in the brain and involved in various biological activities including neurosecretion. ATP-induced inward currents were measured in mock-, wild-type or mutant (C90S)-UCH L1-transfected PC12 cells under the conventional whole-cell patch clamp configuration. The amplitude of ATP-induced currents was significantly greater in both wild-type and C90S UCH L1-transfected cells, suggesting that hydrolase activity was not involved but increased level of mono-ubiquitin might play an important role. The increased currents were dependent on cAMP-dependent protein kinase (PKA) and Ca2+ and calmodulin-dependent protein kinase (CaMKII) but not protein kinase C. In addition, ATP-induced currents were likely to be modified via dopamine and cyclic AMP-regulated phosphoprotein (DARPP-32) that is regulated by PKA and phosphatases. Our finding shows the first evidence that there is a relationship between UCH L1 and neurotransmitter receptor, suggesting that UCH L1 may play an important role in synaptic activity.     

P2X7受体与炎性细胞因子

P2X7受体是嘌呤受体中功能独特的一个亚型,为ATP控制的离子通道,在单核细胞、巨噬细胞、中性粒细胞中高表达,被ATP激活后导致K＋外流和Ca^2＋内流、非选择性膜孔形成,启动一系列信号途径如炎症小体NALP3的活化,丝裂原蛋白激酶途径激活NF-κB增强炎性细胞因子转录,ROS和氮介质的产生,介导IL-1β、IL-6、IL-18、TNF-α、MIP-2、CCL2、HMGB1等多种炎性细胞因子的释放,参与炎症的发生发展,与真菌感染及阿尔茨海默病、类风湿性关节炎、哮喘等炎症性疾病密切相关.     

Calcium-dependent block of P2X7 receptor channel function is allosteric

Among purinergic P2X receptor (P2XR) channels, the P2X7R exhibits the most complex gating kinetics; the binding of orthosteric agonists at the ectodomain induces a conformational change in the receptor complex that favors a gating transition from closed to open and dilated states. Bath Ca(2+) affects P2X7R gating through a still uncharacterized mechanism: it could act by reducing the adenosine triphosphate(4-) (ATP(4-)) concentration (a form proposed to be the P2X7R orthosteric agonist), as an allosteric modulator, and/or by directly altering the selectivity of pore to cations. In this study, we combined biophysical and mathematical approaches to clarify the role of calcium in P2X7R gating. In naive receptors, bath calcium affected the activation permeability dynamics indirectly by decreasing the potency of orthosteric agonists in a concentration-dependent manner and independently of the concentrations of the free acid form of agonists and status of pannexin-1 (Panx1) channels. Bath calcium also facilitated the rates of receptor deactivation in a concentration-dependent manner but did not affect a progressive delay in receptor deactivation caused by repetitive agonist application. The effects of calcium on the kinetics of receptor deactivation were rapid and reversible. A438079, a potent orthosteric competitive antagonist, protected the rebinding effect of 2'(3')-O-4-benzoylbenzoyl)ATP on the kinetics of current decay during the washout period, but in the presence of A438079, calcium also increased the rate of receptor deactivation. The corresponding kinetic (Markov state) model indicated that the decrease in binding affinity leads to a decrease in current amplitudes and facilitation of receptor deactivation, both in an extracellular calcium concentration-dependent manner expressed as a Hill function. The results indicate that calcium in physiological concentrations acts as a negative allosteric modulator of P2X7R by decreasing the affinity of receptors for orthosteric ligand agonists, but not antagonists, and not by affecting the permeability dynamics directly or indirectly through Panx1 channels. We expect these results to generalize to other P2XRs.     

Extracellular ATP increases cation fluxes in human erythrocytes by activation of the P2X7 receptor

Canine erythrocytes are known to undergo a reversible increase in cation permeability when incubated with extracellular ATP. We have examined the expression and function of P2X receptors on human erythrocytes using confocal microscopy and a panel of anti-P2X(1-7) antibodies and have measured monovalent cation fluxes in the presence of various nucleotide agonists. Human erythrocytes expressed P2X7 receptors on all cells examined from eight of eight subjects, as well as P2X2 at a far lower staining intensity in six of eight subjects. ATP stimulated the efflux of 86Rb+ (K+) from human erythrocytes in a dose-dependent fashion with an EC50 of approximately 95 microM. Other nucleotides also induced an efflux of 86Rb+ from erythrocytes with an order of agonist potency of 2'- and 3'-O(4-benzoylbenzoyl) ATP (BzATP) > ATP > 2-methylthio-ATP (2MeSATP) > adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), whereas ADP or UTP had no effect. ATP-induced efflux of 86Rb+ from erythrocytes was inhibited by extracellular Na+ and oxidized ATP, as well as by KN-62, an antagonist specific for the human P2X7 receptor. When erythrocytes were incubated in isotonic KCl medium, the addition of ATP stimulated an 86Rb+ influx approximately equal in magnitude to ATP-stimulated 86Rb+ efflux from the same cells. BzATP also stimulated the influx of 22Na+ into erythrocytes incubated in isotonic NaCl medium. Both ATP-induced efflux and influx of 86Rb+ and 22Na+ were impaired in erythrocytes from subjects who had inherited loss-of-function polymorphisms in the P2X7 receptor. These results suggest that the reversible permeabilization of erythrocytes by extracellular ATP is mediated by the P2X7 receptor.