ATP-gated P2X receptors in health and disease

A shared mechanism for TNP-ATP recognition by members of the P2X receptor family

P2X receptors (P2X1-7) are non-selective cation channels involved in many physiological activities such as synaptic transmission, immunological modulation, and cardiovascular function. These receptors share a conserved mechanism to sense extracellular ATP. TNP-ATP is an ATP derivative acting as a nonselective competitive P2X antagonist. Understanding how it occupies the orthosteric site in the absence of agonism may help reveal the key allostery during P2X gating. However, TNP-ATP/P2X complexes (TNP-ATP/human P2X3 (hP2X3) and TNP-ATP/chicken P2X7 (ckP2X7)) with distinct conformations and different mechanisms of action have been proposed. Whether these represent species and subtype variations or experimental differences remains unclear. Here, we show that a common mechanism of TNP-ATP recognition exists for the P2X family members by combining enhanced conformation sampling, engineered disulfide bond analysis, and covalent occupancy. In this model, the polar triphosphate moiety of TNP-ATP interacts with the orthosteric site, while its TNP-moiety is deeply embedded in the head and dorsal fin (DF) interface, creating a restrictive allostery in these two domains that results in a partly enlarged yet ion-impermeable pore. Similar results were obtained from multiple P2X subtypes of different species, including ckP2X7, hP2X3, rat P2X2 (rP2X2), and human P2X1 (hP2X1). Thus, TNP-ATP uses a common mechanism for P2X recognition and modulation by restricting the movements of the head and DF domains which are essential for P2X activation. This knowledge is applicable to the development of new P2X inhibitors.

The Purinergic P2X7 Receptor as a Target for Adjunctive Treatment for Drug-Refractory Epilepsy

Epilepsy is one of the most common neurological diseases worldwide. Anti-seizure medications (ASMs) with anticonvulsants remain the mainstay of epilepsy treatment. Currently used ASMs are, however, ineffective to suppress seizures in about one third of all patients. Moreover, ASMs show no significant impact on the pathogenic mechanisms involved in epilepsy development or disease progression and may cause serious side-effects, highlighting the need for the identification of new drug targets for a more causal therapy. Compelling evidence has demonstrated a role for purinergic signalling, including the nucleotide adenosine 5'-triphosphate (ATP) during the generation of seizures and epilepsy. Consequently, drugs targeting specific ATP-gated purinergic receptors have been suggested as promising treatment options for epilepsy including the cationic P2X7 receptor (P27XR). P2X7R protein levels have been shown to be increased in the brain of experimental models of epilepsy and in the resected brain tissue of patients with epilepsy. Animal studies have provided evidence that P2X7R blocking can reduce the severity of acute seizures and the epileptic phenotype. The current review will provide a brief summary of recent key findings on P2X7R signalling during seizures and epilepsy focusing on the potential clinical use of treatments based on the P2X7R as an adjunctive therapeutic strategy for drug-refractory seizures and epilepsy.

Structural insights into the orthosteric inhibition of P2X receptors by non-ATP analog antagonists

P2X receptors are extracellular ATP-gated ion channels that form homo- or heterotrimers and consist of seven subtypes. They are expressed in various tissues, including neuronal and nonneuronal cells, and play critical roles in physiological processes such as neurotransmission, inflammation, pain, and cancer. As a result, P2X receptors have attracted considerable interest as drug targets, and various competitive inhibitors have been developed. However, although several P2X receptor structures from different subtypes have been reported, the limited structural information of P2X receptors in complex with competitive antagonists hampers the understanding of orthosteric inhibition, hindering the further design and optimization of those antagonists for drug discovery. We determined the cryogenic electron microscopy (cryo-EM) structures of the mammalian P2X7 receptor in complex with two classical competitive antagonists of pyridoxal-5'-phosphate derivatives, pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) and pyridoxal phosphate-6-azophenyl-2',5'-disulfonic acid (PPADS), and performed structure-based mutational analysis by patch-clamp recording as well as molecular dynamics (MD) simulations. Our structures revealed the orthosteric site for PPADS/PPNDS, and structural comparison with the previously reported apo- and ATP-bound structures showed how PPADS/PPNDS binding inhibits the conformational changes associated with channel activation. In addition, structure-based mutational analysis identified key residues involved in the PPNDS sensitivity of P2X1 and P2X3, which are known to have higher affinity for PPADS/PPNDS than other P2X subtypes.

Structural insights into the allosteric inhibition of P2X4 receptors

P2X receptors are ATP-activated cation channels, and the P2X4 subtype plays important roles in the immune system and the central nervous system, particularly in neuropathic pain. Therefore, P2X4 receptors are of increasing interest as potential drug targets. Here, we report the cryo-EM structures of the zebrafish P2X4 receptor in complex with two P2X4 subtype-specific antagonists, BX430 and BAY-1797. Both antagonists bind to the same allosteric site located at the subunit interface at the top of the extracellular domain. Structure-based mutational analysis by electrophysiology identified the important residues for the allosteric inhibition of both zebrafish and human P2X4 receptors. Structural comparison revealed the ligand-dependent structural rearrangement of the binding pocket to stabilize the binding of allosteric modulators, which in turn would prevent the structural changes of the extracellular domain associated with channel activation. Furthermore, comparison with the previously reported P2X structures of other subtypes provided mechanistic insights into subtype-specific allosteric inhibition.

Spotlight on P2X7 Receptor PET Imaging: A Bright Target or a Failing Star?

The homotrimeric P2X7 receptor (P2X7R) is expressed by virtually all cells of the innate and adaptive immune system and plays a crucial role in various pathophysiological processes such as autoimmune and neurodegenerative diseases, inflammation, neuropathic pain and cancer. Consequently, the P2X7R is considered a promising target for therapy and diagnosis. As the development of tracers comes hand-in-hand with the development of potent and selective receptor ligands, there is a rising number of PET tracers available in preclinical and clinical studies. This review analyzes the development of P2X7R positron emission tomography (PET) tracers and their potential in various PET imaging applications.

Contribution of P2X purinergic receptor in cerebral ischemia injury

The development of cerebral ischemia involves brain damage and abnormal changes in brain function, which can cause neurosensory and motor dysfunction, and bring serious consequences to patients. P2X purinergic receptors are expressed in nerve cells and immune cells, and are mainly expressed in microglia. The P2X4 and P2X7 receptors in the P2X purinergic receptors play a significant role in regulating the activity of microglia. Moreover, ATP-P2X purine information transmission is involved in the progression of neurological diseases, including the release of pro-inflammatory factors, driving factors and cytokines after cerebral ischemia injury, inducing inflammation, and aggravating cerebral ischemia injury. P2X receptors activation can mediate the information exchange between microglia and neurons, induce neuronal apoptosis, and aggravate neurological dysfunction after cerebral ischemia. However, inhibiting the activation of P2X receptors, reducing their expression, inhibiting the activation of microglia, and has the effect of protecting nerve function. In this paper, we discussed the relationship between P2X receptors and nervous system function and the role of microglia activation inducing cerebral ischemia injury. Additionally, we explored the potential role of P2X receptors in the progression of cerebral ischemic injury and their potential pharmacological targets for the treatment of cerebral ischemic injury.

Acupuncture relieves the visceral pain of diarrhea-predominant irritable bowel syndrome rats by regulating P2X4 expression

OBJECTIVES

We researched the effect and mechanism of acupuncture treatment for visceral pain in rats with diarrhea-predominant irritable bowel syndrome (IBS-D).

METHODS

We set up a rat model of IBS-D with chemical and chronic- and acute-pressure stimulations. Then, the IBS-D rats were treated with acupuncture or 5-BDBD, and the therapeutic efficacy of acupuncture in IBS-D rats was assessed by means of the Bristol scale, diarrhea index, abdominal withdrawal reflex (AWR) score, mast cell count and histologic staining.

RESULTS

Acupuncture significantly decreased clinical symptoms in IBS-D rats after a 14 day-treatment. Furthermore, significant down-regulation of P2X4, OX42, BDNF (brain-derived neurotrophic factor) and IRF-5 (interferon regulatory factor 5) expressions were observed in the IBS-D rats, along with the decreased inflammatory factors [interleukin 1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and interleukin 6 (IL-6)], chemokines [monocyte chemoattractant protein-1 (MCP-1), regulated on activation, normal T cell expressed and secreted (RANTES), and C-X-C motif chemokine ligand 1 (CXCL1)], and neurotransmitters [substance P (SP), 5-hydroxytryptamine (5-HT), and calcitonin gene-related peptide (CGRP)]. 5-BDBD treatment had a similar effect on IBS-D rats.

CONCLUSIONS

Acupuncture can effectively alleviate abdominal pain by decreasing visceral hypersensitivity and controlling the expression of P2X4 and spinal microglial inflammation in IBS rats.

Piperazine squaric acid diamides, a novel class of allosteric P2X7 receptor antagonists

The P2X7 receptor (P2X7R) stands out among the purinergic receptors due to its strong involvement in the regulation of tumor growth and metastasis formation as well as in innate immune responses and afferent signal transmission. Numerous studies have pointed out the beneficial effects of P2X7R antagonism for the treatment of a variety of cancer types, inflammatory diseases, and chronic pain. Herein we describe the development of novel P2X7R antagonists, incorporating piperazine squaric diamides as a central element. Besides improving the antagonists' potency from pIC₅₀ values of 5.7-7.6, ADME properties (logD_7.4 value, plasma protein binding, in vitro metabolic stability) of the generated compounds were investigated and optimized to provide novel P2X7R antagonists with drug-like properties. Furthermore, docking studies revealed the antagonists binding to the allosteric binding pocket in two distinct binding poses, depending on the substitution of the central piperazine moiety.

Synthesis, characterization and biological evaluation of thiadiazole amide derivatives as nucleoside triphosphate diphosphohydrolases (NTPDases) inhibitors

Importance of extracellular nucleotides is widely understood. These nucleotides act as ligand for P2X and P2Y receptors and modulate a variety of biological functions. However, their extracellular concentration is maintained by a chain of enzymes termed as ecto-nucleotidases. Amongst them, nucleoside triphosphate diphosphohydrolases (NTPDases) is an important enzyme family responsible for the dephosphorylation of these nucleotides. Overexpression of NTPDases leads to many pathological conditions such as cancer and thrombosis. So far, only a few NTPDase inhibitors have been reported. Considering this scarcity of (NTPDase) inhibitors, a number of thiadiazole amide derivatives were synthesized and screened against human (h)-NTPDases. Several compounds showed promising inhibitory activity; compound 5a (IC₅₀ (µM); 0.05 ± 0.008) and 5g (IC₅₀ (µM); 0.04 ± 0.006) appeared to be the most distinguished molecules corresponding to h-NTPDase1 and -2. However, h-NTPDase3 was the least susceptible isozyme and only three compounds (5d, 5e, 5j) strongly inhibited h-NTPDase3. Interestingly, compound 5e was recognized as the most active compound that showed dual inhibition against h-NTPDase3 as well as against h-NTPDase8. For better comprehension of binding mode of these inhibitors, most potent inhibitors were docked with their respective isozyme.

Chem-inspired hollow ceria nanozymes with lysosome-targeting for tumor synergistic phototherapy

The precise operation of the hypoxic tumor microenvironment presents a promising way to improve treatment efficacy, in particular in tumor synergistic phototherapy. This work reports an innovative approach to build adenosine triphosphate-modified hollow ceria nanozymes (ATP-HCNPs@Ce6) that manipulate tumor hypoxia to effectively achieve drug delivery. Hollow ceria nanoparticles (HCNPs) exhibit a controllable hollow structure through varying nitric acid concentrations in the nanocomposites. Specifically, ATP modification makes HCNPs exceptionally biocompatible and stable and acts as a regulator of HCNP enzymatic activity. In the stage of drug loading, newly prepared ATP-HCNPs@Ce6 serves as an in situ oxygen-generating agent because of its ability to simulate catalase. Therefore, ATP-HCNPs@Ce6 has adjustable enzymatic properties that act like a "switch" to selectively supply oxygen in response to high levels of hydrogen peroxide expression and the slightly acidic lysosomal environment of the tumor to enhance lysosome-targeted photodynamic therapy. Moreover, the obvious anticancer effects of ATP-HCNPs@Ce6 are demonstrated in vitro and in vivo. Overall, a simple and rapid self-assembly strategy to form and modify multifunctional HCNPs is reported, which may further propel their application in the field of precision tumor treatment.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Meningeal Mast Cells Contribute to ATP-Induced Nociceptive Firing in Trigeminal Nerve Terminals: Direct and Indirect Purinergic Mechanisms Triggering Migraine Pain

Peripheral mechanisms of primary headaches such as a migraine remain unclear. Meningeal afferents surrounded by multiple mast cells have been suggested as a major source of migraine pain. Extracellular ATP released during migraine attacks is a likely candidate for activating meningeal afferents via neuronal P2X receptors. Recently, we showed that ATP also increased degranulation of resident meningeal mast cells (Nurkhametova et al., 2019). However, the contribution of ATP-induced mast cell degranulation in aggravating the migraine pain remains unknown. Here we explored the role of meningeal mast cells in the pro-nociceptive effects of extracellular ATP. The impact of mast cells on ATP mediated activation of peripheral branches of trigeminal nerves was measured electrophysiologically in the dura mater of adult wild type (WT) or mast cell deficient mice. We found that a spontaneous spiking activity in the meningeal afferents, at baseline level, did not differ in two groups. However, in WT mice, meningeal application of ATP dramatically (24.6-fold) increased nociceptive firing, peaking at frequencies around 10 Hz. In contrast, in mast cell deficient animals, ATP-induced excitation was significantly weaker (3.5-fold). Application of serotonin to meninges in WT induced strong spiking. Moreover, in WT mice, the 5-HT3 antagonist MDL-7222 inhibited not only serotonin but also the ATP induced nociceptive firing. Our data suggest that extracellular ATP activates nociceptive firing in meningeal trigeminal afferents via amplified degranulation of resident mast cells in addition to direct excitatory action on the nerve terminals. This highlights the importance of mast cell degranulation via extracellular ATP, in aggravating the migraine pain.

Role of hesperidin in P2X3 receptor-mediated neuropathic pain in the dorsal root ganglia

Aim: This study investigated whether the neuronal P2X3 receptor in rat dorsal root ganglia (DRG) mediated the effects of hesperidin on neuropathic pain. Materials and methods: The chronic constriction injury (CCI) model was used as a model of neuropathic pain. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were measured. The mRNA and protein expression levels were assayed by real-time RT-PCR and Western blotting. Results: The results showed that mechanical and thermal hyperalgesia in the CCI rats were increased as compared to those in the sham group. The expression levels of P2X3 mRNA and protein in CCI rats were higher than those in the sham group. Dual-labelling immunofluorescence showed that the elevated P2X3 receptor was co-expressed with the neuronal marker NeuN in the DRG of CCI rats. Hesperidin treatment decreased both the mechanical and thermal hyperalgesia, and upregulated P2X3 expression in the CCI rats. Hesperidin treatment also reduced the ERK1/2 phosphorylation in the DRG of CCI rats. Moreover, hesperidin inhibited the P2X3 agonist ATP-induced currents in HEK293 cells transfected with the P2X3 plasmid. Therefore, hesperidin treatment could reverse the elevated expression of neuronal P2X3 receptor and reduce the activation of ERK1/2 in the DRG of CCI rats. Conclusions: Our findings suggested that hesperidin inhibited the nociceptive transmission mediated by the P2X3 receptor in neurons of DRG, and thus, relieved the mechanical and thermal hyperalgesia in CCI rats.

Molecular mechanism underlying the subtype-selectivity of competitive inhibitor NF110 and its distinct potencies in human and rat P2X3 receptors

P2X receptors are a family of extracellular ATP-gated trimeric cation channels that is widely distributed in human tissues. Quite some drug candidates targeting P2X receptors have entered into preclinical or main phases of clinical trials, but many of them failed due to low subtype-selectivity or species differences in pharmacological activities between human and experimental animals. Here, we identified the distinct inhibitory efficacies of NF110, a competitive inhibitor, between the rat (rP2X3) and human (hP2X3) P2X3 receptors. We demonstrated that this difference is determined by two amino acids located in the dorsal fin (DF) domain of P2X3 receptors. As revealed by mutagenesis, metadynamics, and covalent modification, NF110-mediated rP2X3 inhibition may be through a filling in the cavity formed by the DF, left flipper (LF) and lower body (LB) to partially, rather than fully, occupy the ATP-binding pocket. Moreover, substitution of residues located in the DF and/or LF domains of the rP2X2 receptor, a NF110-insensitive subtype, with the equivalent amino acids of rP2X3, bestowed the sensitivity of rP2X2 to NF110. The critical roles of the DF and LF domains in channel gating of P2X and low conservativity in residue sequences of those two domains raise the possibility that small molecules differentially interacting with the residues of the DF and LF domains of different P2X receptors may modulate channel's activity in a subtype-selective manner. However, the possible species-specificity of P2X inhibitors/modulators makes it more complex when interpreting the preclinical data into clinical researches. Nevertheless, our data provide new insights into the subtype-selectivity of competitive inhibitors and their distinct potencies in the human and experimental animals, both of which are extremely important in the drug discovery of P2X receptors.

ATP-Gated P2X Receptor Channels: Molecular Insights into Functional Roles

In the nervous system, ATP is co-stored in vesicles with classical transmitters and released in a regulated manner. ATP from the intracellular compartment can also exit the cell through hemichannels and following shear stress or membrane damage. In the past 30 years, the action of ATP as an extracellular transmitter at cell-surface receptors has evolved from somewhat of a novelty that was treated with skepticism to purinergic transmission being accepted as having widespread important functional roles mediated by ATP-gated ionotropic P2X receptors (P2XRs). This review focuses on work published in the last five years and provides an overview of ( a) structural studies, ( b) the molecular basis of channel properties and regulation of P2XRs, and ( c) the physiological and pathophysiological roles of ATP acting at defined P2XR subtypes.

Lysosomal exocytosis of ATP is coupled to P2Y2 receptor in marginal cells in the stria vascular in neonatal rats

Adenosine triphosphate (ATP) is stored as lysosomal vesicles in marginal cells of the stria vascular in neonatal rats, but the mechanisms of ATP release are unclear. Primary cultures of marginal cells from 1-day-old Sprague-Dawley rats were established. P2Y₂ receptor and inositol 1,4,5-trisphosphate (IP3) receptor were immunolabelled in marginal cells of the stria vascular. We found that 30 μM ATP and 30 μM uridine triphosphate (UTP) evoked comparable significant increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the absence of extracellular Ca²⁺, whereas the response was suppressed by 100 μM suramin, 10 μM 1-(6-(17β-3-methoxyester-1,3,5(10)-trien-17-yl)amino)-hexyl)-1H-pyrrole-2,5-dione(U-73122), 100 μM 2-aminoethoxydiphenyl borate (2-APB) and 5 μM thapsigargin (TG), thus indicating that ATP coupled with the P2Y₂R-PLC-IP3 pathway to evoke Ca²⁺ release from the endoplasmic reticulum (ER). Incubation with 200 μM Gly-Phe-β-naphthylamide (GPN) selectively disrupted lysosomes and caused significant increases in [Ca²⁺]_I; this effect was partly inhibited by P2Y₂R-PLC-IP3 pathway antagonists. After pre-treatment with 5 μM TG, [Ca²⁺]_i was significantly lower than that after treatment with P2Y₂R-PLC-IP3 pathway antagonists under the same conditions, thus indicating that lysosomal Ca²⁺ triggers Ca²⁺ release from ER Ca²⁺ stores. Baseline [Ca²⁺]_i declined after treatment with the Ca²⁺ chelator 50 μM bis-(aminophenolxy) ethane-N,N,N',N'-tetra-acetic acid acetoxyme-thyl ester (BAPTA-AM) and 4 IU/ml apyrase. 30 μM ATP decrease of the number of quinacrine-positive vesicles via lysosome exocytosis, whereas the number of lysosomes did not change. However, lysosome exocytosis was significantly suppressed by pre-treatment with 5 μM vacuolin-1. Release of ATP and β-hexosaminidase both increased after treatment with 200 μM GPN and 5 μM TG, but decreased after incubation with 50 μM BAPTA-AM, 4 IU/ml apyrase and 5 μM vacuolin-1. We suggest that ATP triggers Ca²⁺ release from the ER, thereby contributing to secretion of lysosomal ATP via lysosomal exocytosis. Lysosomal stored Ca²⁺ triggers Ca²⁺ release from the ER directly though the IP3 receptors, and lysosomal ATP evokes Ca²⁺ signals indirectly via the P2Y₂R-PLC-IP3 pathway.

CD39 and CD73 in the aortic valve-biochemical and immunohistochemical analysis in valve cell populations and its changes in valve mineralization

BACKGROUND

The calcific aortic valve disease (CAVD) is a common heart pathology that involves inflammation, fibrosis, and calcification of aortic valve leaflets. All these processes could be affected by changes in the extracellular purinergic signaling that depend on the activity of ectonucleotidases, mainly ectonucleoside triphosphate diphosphohydrolase 1 (CD39, eNTPD1) and ecto-5'nucleotidase (CD73, e5NT).

OBJECTIVE AND METHODS

We investigated the localization of CD39 and CD73 proteins in human noncalcified and calcified aortic valves using immunohistochemistry together with analysis of NTPDases and e5NT activities in aortic valve homogenates by analysis of substrate into product conversion by high-performance liquid chromatography. We also measured the rates of extracellular nucleotide catabolism on the surface of isolated cultured aortic valve endothelial (hAVECs) and interstitial cells (hAVICs) as well as characterized cellular CD39 and CD73 distribution.

RESULTS

In noncalcified valves, CD39 and CD73 were expressed in both endothelial and interstitial cells, while in calcified valves, the expressions of CD39 and CD73 were significantly down-regulated with the exception of calcified regions where the expression of CD73 was maintained. This correlated with activities in valve homogenates. NTPDase was reduced by 35% and e5NT activity by 50% in calcified vs. noncalcified valve. CD39 and CD73 were present mainly in the cell membrane of hAVECs, but in hAVICs, these proteins were also present intracellularly. The rates of extracellular adenosine triphosphate and adenosine monophosphate hydrolysis in isolated hAVECs and hAVICs were comparable.

CONCLUSION

The presence of ectonucleotidases in valves and especially in aortic valve interstitial cells highlights important local role of purinergic signaling and metabolism. Changes in the local expression and hence the activity of CD39 and CD73 in calcified valves suggest their potential role in CAVD.

ATP is stored in lamellar bodies to activate vesicular P2X4 in an autocrine fashion upon exocytosis

P2X₄ receptor activation facilitates secretion of pulmonary surfactant from secretory vesicles called lamellar bodies in alveolar epithelial cells. Fois et al. reveal that P2X₄ receptors on the lamellar body membranes are activated by ATP stored within the vesicles themselves upon vesicle exocytosis.

Vesicular P2X₄ receptors are known to facilitate secretion and activation of pulmonary surfactant in the alveoli of the lungs. P2X₄ receptors are expressed in the membrane of lamellar bodies (LBs), large secretory lysosomes that store lung surfactant in alveolar type II epithelial cells, and become inserted into the plasma membrane after exocytosis. Subsequent activation of P2X₄ receptors by adenosine triphosphate (ATP) results in local fusion-activated cation entry (FACE), facilitating fusion pore dilation, surfactant secretion, and surfactant activation. Despite the importance of ATP in the alveoli, and hence lung function, the origin of ATP in the alveoli is still elusive. In this study, we demonstrate that ATP is stored within LBs themselves at a concentration of ∼1.9 mM. ATP is loaded into LBs by the vesicular nucleotide transporter but does not activate P2X₄ receptors because of the low intraluminal pH (5.5). However, the rise in intravesicular pH after opening of the exocytic fusion pore results in immediate activation of vesicular P2X₄ by vesicular ATP. Our data suggest a new model in which agonist (ATP) and receptor (P2X₄) are located in the same intracellular compartment (LB), protected from premature degradation (ATP) and activation (P2X₄), and ideally placed to ensure coordinated and timely receptor activation as soon as fusion occurs to facilitate surfactant secretion.

Structural insights into the competitive inhibition of the ATP-gated P2X receptor channel

P2X receptors are non-selective cation channels gated by extracellular ATP, and the P2X7 receptor subtype plays a crucial role in the immune and nervous systems. Altered expression and dysfunctions of P2X7 receptors caused by genetic deletions, mutations, and polymorphic variations have been linked to various diseases, such as rheumatoid arthritis and hypertension. Despite the availability of crystal structures of P2X receptors, the mechanism of competitive antagonist action for P2X receptors remains controversial. Here, we determine the crystal structure of the chicken P2X7 receptor in complex with the competitive P2X antagonist, TNP-ATP. The structure reveals an expanded, incompletely activated conformation of the channel, and identified the unique recognition manner of TNP-ATP, which is distinct from that observed in the previously determined human P2X3 receptor structure. A structure-based computational analysis furnishes mechanistic insights into the TNP-ATP-dependent inhibition. Our work provides structural insights into the functional mechanism of the P2X competitive antagonist.

P2X receptors are nonselective cation channels that are gated by extracellular ATP. Here the authors present the crystal structure of chicken P2X7 with its bound competitive antagonist TNP-ATP and give mechanistic insights into TNP-ATP dependent inhibition through further computational analysis and electrophysiology measurements.

Reconstructed Serine 288 in the Left Flipper Region of the Rat P2X7 Receptor Stabilizes Nonsensitized States

Serine 275, a conserved residue of the left flipper region of ATP-gated P2X3 receptors, plays a key role in both agonist binding and receptor desensitization. It is conserved in most of the P2X receptors except P2X7 and P2X6. By combining experimental patch-clamp and modeling approaches, we explored the role of the corresponding residue in the rat P2X7 receptor (rP2X7) by replacing the phenylalanine at position 288 with serine and characterizing the membrane currents generated by either the wild-type (WT) or the mutated rP2X7 receptor. F288S, an rP2X7 mutation, slowed the deactivation subsequent to 2 and 20 s applications of 1 mM ATP. F288S also prevented sensitization (a progressive current growth) observed with the WT in response to a 20 s application of 1 mM ATP. Increasing the ATP concentration to 5 mM promoted sensitization also in the mutated rP2X7 receptor, accelerating the deactivation rate to typical WT values. YO-PRO1 uptake in cells expressing either the WT or the F288S P2X7 receptor was consistent with recorded membrane current data. Interestingly, in the human P2X7 (hP2X7) receptor, substitution Y288S did not change the deactivation rate, while the Y288F mutant generated a "rat-like" phenotype with a fast deactivation rate. Our combined experimental, kinetic, and molecular modeling data suggest that the rat F288S novel phenotype is due to a slower rate of ATP binding and/or unbinding and stabilization of nonsensitized receptor states.

Synapses in the spotlight with synthetic optogenetics

Membrane receptors and ion channels respond to various stimuli and relay that information across the plasma membrane by triggering specific and timed processes. These include activation of second messengers, allowing ion permeation, and changing cellular excitability, to name a few. Gaining control over equivalent processes is essential to understand neuronal physiology and pathophysiology. Recently, new optical techniques have emerged proffering new remote means to control various functions of defined neuronal populations by light, dubbed optogenetics. Still, optogenetic tools do not typically address the activity of receptors and channels native to neurons (or of neuronal origin), nor gain access to their signaling mechanisms. A related method-synthetic optogenetics-bridges this gap by endowing light sensitivity to endogenous neuronal receptors and channels by the appending of synthetic, light-receptive molecules, or photoswitches. This provides the means to photoregulate neuronal receptors and channels and tap into their native signaling mechanisms in select regions of the neurons, such as the synapse. This review discusses the development of synthetic optogenetics as a means to study neuronal receptors and channels remotely, in their natural environment, with unprecedented spatial and temporal precision, and provides an overview of tool design, mode of action, potential clinical applications and insights and achievements gained.

Intersubunit physical couplings fostered by the left flipper domain facilitate channel opening of P2X4 receptors

P2X receptors are ATP-gated trimeric channels with important roles in diverse pathophysiological functions. A detailed understanding of the mechanism underlying the gating process of these receptors is thus fundamentally important and may open new therapeutic avenues. The left flipper (LF) domain of the P2X receptors is a flexible loop structure, and its coordinated motions together with the dorsal fin (DF) domain are crucial for the channel gating of the P2X receptors. However, the mechanism underlying the crucial role of the LF domain in the channel gating remains obscure. Here, we propose that the ATP-induced allosteric changes of the LF domain enable it to foster intersubunit physical couplings among the DF and two lower body domains, which are pivotal for the channel gating of P2X4 receptors. Metadynamics analysis indicated that these newly established intersubunit couplings correlate well with the ATP-bound open state of the receptors. Moreover, weakening or strengthening these physical interactions with engineered intersubunit metal bridges remarkably decreased or increased the open probability of the receptors, respectively. Further disulfide cross-linking and covalent modification confirmed that the intersubunit physical couplings among the DF and two lower body domains fostered by the LF domain at the open state act as an integrated structural element that is stringently required for the channel gating of P2X4 receptors. Our observations provide new mechanistic insights into P2X receptor activation and will stimulate development of new allosteric modulators of P2X receptors.

Impaired Aortic Contractility to Uridine Adenosine Tetraphosphate in Angiotensin II-Induced Hypertensive Mice: Receptor Desensitization?

OBJECTIVE

We previously showed that uridine adenosine tetraphosphate (Up4A)-mediated aortic contraction is partly mediated through purinergic P2X1 receptors (P2X1R). It has been reported that the plasma level of Up4A is elevated in hypertensive patients, implying a potential role for Up4A-P2X1R signaling in hypertension. This study investigated the vasoactive effect of Up4A in aortas isolated from angiotensin (Ang) II-infused (21 days) hypertensive mice.

METHODS

Blood pressure was measured by tail cuff plethysmography. Aortas were isolated for isometric tension measurements, and protein expression was analyzed by western blot.

RESULTS

Mean and systolic arterial pressures were elevated by ~50% in Ang II-infused mice. Protein levels of both AT1R and P2X1R were upregulated in Ang II-infused aortas. Surprisingly, Up4A (10-9-10-5 M)-induced concentration-dependent contraction was significantly impaired in Ang II-infused mice. Studies in control mice revealed that both P2X1R (MRS2159) and AT1R (losartan) antagonists significantly attenuated Up4A-induced aortic contraction. In addition, desensitization of AT1R by prior Ang II (100 nM) exposure had no effect on Up4A-induced aortic contraction. However, subsequent serial exposure responses to Up4A-induced aortic contraction were markedly reduced, suggesting a desensitization of purinergic receptors. This desensitization was further confirmed in control mice by prior exposure of aortas to the P2X1R desensitizer α, β-methylene ATP (10 μM).

CONCLUSION

Despite upregulation of AT1R and P2X1R in hypertension, Up4A-mediated aortic contraction was impaired in Ang II-infused mice, likely through the desensitization of P2X1R but not AT1R. This implies that vascular P2X1R activity, rather than plasma Up4A level, may determine the role of Up4A in hypertension.

Conductance of P2X4 purinergic receptor is determined by conformational equilibrium in the transmembrane region

Ligand-gated ion channels are partially activated by their ligands, resulting in currents lower than the currents evoked by the physiological full agonists. In the case of P2X purinergic receptors, a cation-selective pore in the transmembrane region expands upon ATP binding to the extracellular ATP-binding site, and the currents evoked by α,β-methylene ATP are lower than the currents evoked by ATP. However, the mechanism underlying the partial activation of the P2X receptors is unknown although the crystal structures of zebrafish P2X4 receptor in the apo and ATP-bound states are available. Here, we observed the NMR signals from M339 and M351, which were introduced in the transmembrane region, and the endogenous alanine and methionine residues of the zebrafish P2X4 purinergic receptor in the apo, ATP-bound, and α,β-methylene ATP-bound states. Our NMR analyses revealed that, in the α,β-methylene ATP-bound state, M339, M351, and the residues that connect the ATP-binding site and the transmembrane region, M325 and A330, exist in conformational equilibrium between closed and open conformations, with slower exchange rates than the chemical shift difference (<100 s(-1)), suggesting that the small population of the open conformation causes the partial activation in this state. Our NMR analyses also revealed that the transmembrane region adopts the open conformation in the state bound to the inhibitor trinitrophenyl-ATP, and thus the antagonism is due to the closure of ion pathways, except for the pore in the transmembrane region: i.e., the lateral cation access in the extracellular region.

Effect of electroacupuncture on P2X3 receptor regulation in the peripheral and central nervous systems of rats with visceral pain caused by irritable bowel syndrome

The aim of this study is to investigate the role of the purinergic receptor P2X3 in the peripheral and central nervous systems during acupuncture treatment for the visceral pain of irritable bowel syndrome (IBS). A total of 24 8-day-old Sprague-Dawley (SD) neonatal male rats (SPF grade) were stimulated using colorectal distention (CRD) when the rats were awake. The modeling lasted for 2 weeks with one stimulation per day. After 6 weeks, the rats were randomly divided into three groups of eight each: (1) the normal group (NG, n = 8); (2) the model group (MG, n = 8); and (3) the model + electroacupuncture group (EA, n = 8) that received electroacupuncture at a needling depth of 5 mm at the Shangjuxu (ST37, bilateral) and Tianshu (ST25, bilateral) acupoints. The parameters of the Han's acupoint nerve stimulator (HANS) were as follows: sparse-dense wave with a frequency of 2/100 Hz, current of 2 mA, 20 min/stimulation, and one stimulation per day; the treatment was provided for seven consecutive days. At the sixth week after the treatment, the abdominal withdrawal reflex (AWR) score was determined; immunofluorescence and immunohistochemistry were used to measure the expression of the P2X3 receptor in myenteric plexus neurons, prefrontal cortex, and anterior cingulate cortex; and, a real-time PCR assay was performed to measure the expression of P2X3 messenger RNA (mRNA) in the dorsal root ganglion (DRG) and spinal cord. After stimulation with CRD, the expression levels of the P2X3 receptor in the inter-colonic myenteric plexus, DRG, spinal cord, prefrontal cortex, and anterior cingulate cortex were upregulated, and the sensitivity of the rats to IBS visceral pain was increased. Electroacupuncture (EA) could downregulate the expression of the P2X3 receptor and ease the sensitivity to visceral pain. The P2X3 receptor plays an important role in IBS visceral pain. The different levels of P2X3 in the peripheral enteric nervous system and central nervous system mediate the effects of the EA treatment of the visceral hyperalgesia of IBS.

Expression of the ATP-gated P2X7 Receptor on M Cells and Its Modulating Role in the Mucosal Immune Environment

Interactions between microbes and epithelial cells in the gastrointestinal tract are closely associated with regulation of intestinal mucosal immune responses. Recent studies have highlighted the modulation of mucosal immunity by microbe-derived molecules such as ATP and short-chain fatty acids. In this study, we undertook to characterize the expression of the ATP-gated P2X₇ receptor (P2X₇R) on M cells and its role in gastrointestinal mucosal immune regulation because it was poorly characterized in Peyer's patches, although purinergic signaling via P2X₇R and luminal ATP have been considered to play an important role in the gastrointestinal tract. Here, we present the first report on the expression of P2X₇R on M cells and characterize the role of P2X₇R in immune enhancement by ATP or LL-37.