The P2X7 receptor as a therapeutic target

C terminus of the P2X7 receptor: treasure hunting

P2X receptor (P2XR) is a family of the ATP-gated ion channel family and can permeabilize the plasma membrane to small cations such as potassium, sodium, and calcium, resulting in cellular depolarization. There are seven P2XR that have been described and cloned, with 45% identity in amino acid sequence. Each P2X receptors has two transmembrane domains that are separated by an extracellular loop and an intracellular N and C terminus. Unlike the other P2X receptors, the P2X7R has a larger C terminus with an extra 200 amino acid residues compared with the other receptors. The C terminus of the P2X7R has been implicated in regulating receptor function including signaling pathway activation, cellular localization, protein-protein interactions, and post-translational modification (PTM). In the present review, we discuss the role of the P2X7R C terminus in regards to receptor function, describe the specific domains and motifs found therein and compare the C terminus sequence with others proteins to discover predicted domains or sites of PTM.

Effect of P2X(7) receptor knockout on exocrine secretion of pancreas, salivary glands and lacrimal glands

The purinergic P2X(7) receptors are expressed in different cell types where they have varied functions, including regulation of cell survival. The P2X(7) receptors are also expressed in exocrine glands, but their integrated role in secretion is unclear. The aim of our study was to determine whether the P2X(7) receptors affect fluid secretion in pancreas, salivary glands and tear glands. We monitored gland secretions in in vivo preparations of wild-type and P2X(7)(-/-) (Pfizer) mice stimulated with pilocarpine. In cell preparations from pancreas, parotid and lacrimal glands we measured ATP release and intracellular Ca(2+) activity using Fura-2. The data showed that pancreatic secretion and salivary secretions were reduced in P2X(7)(-/-) mice, and in contrast, tear secretion was increased in P2X(7)(-/-) mice. The secretory phenotype was also dependent on the sex of the animal, such that males were more dependent on the P2X(7) receptor expression. ATP release in all cell preparations could be elicited by carbachol and other agonists, and this was independent of the P2X(7) receptor expression. ATP and carbachol increased intracellular Ca(2+) activity, but responses depended on the gland type, presence of the P2X(7) receptor and the sex of the animal. Together, these results demonstrate that cholinergic stimulation leads to release of ATP that can via P2X(7) receptors up-regulate pancreatic and salivary secretion but down-regulate tear secretion. Our data also indicate that there is an interaction between purinergic and cholinergic receptor signalling and that function of the P2X(7) receptor is suppressed in females. We conclude that the P2X(7) receptors are important in short-term physiological regulation of exocrine gland secretion.

Synthesis and biological activity of a series of tetrasubstituted-imidazoles as P2X(7) antagonists

A series of analogues of the pyrazole lead 1 were synthesized in which the heterocyclic core was replaced with an imidazole. A number of potent antagonists were identified and structure-activity relationships (SAR) were investigated both with respect to activity at the P2X(7) receptor and in vitro metabolic stability. Compound 10 was identified as a potent P2X(7) antagonist with reduced in vitro metabolism and high solubility.

Mutation of putative N-linked glycosylation sites on the human nucleotide receptor P2X7 reveals a key residue important for receptor function

The nucleotide receptor P2X(7) is an immunomodulatory cation channel and a potential therapeutic target. P2X(7) is expressed in immune cells such as monocytes and macrophages and is activated by extracellular ATP following tissue injury or infection. Ligand binding to P2X(7) can stimulate ERK1/2, the transcription factor CREB, enzymes linked to the production of reactive oxygen species and interleukin-1 isoforms, and the formation of a nonspecific pore. However, little is known about the biochemistry of P2X(7), including whether the receptor is N-linked glycosylated and if this modification affects receptor function. Here we provide evidence that P2X(7) is sensitive to the glycosidases EndoH and PNGase F and that the human receptor appears glycosylated at N187, N202, N213, N241, and N284. Mutation of N187 results in weakened P2X(7) agonist-induced phosphorylation of ERK1/2, CREB, and p90 ribosomal S6 kinase, as well as a decreased level of pore formation. In further support of a role for glycosylation in receptor function, treatment of RAW 264.7 macrophages with the N-linked glycosylation synthesis inhibitor tunicamycin attenuates P2X(7) agonist-induced, but not phorbol ester-induced, ERK1/2 phosphorylation. Interestingly, residue N187 belongs to an N-linked glycosylation consensus sequence found in six of the seven P2X family members, suggesting this site is fundamentally important to P2X receptor function. To address the mechanism whereby N187 mutation attenuates receptor activity, we developed a live cell proteinase K digestion assay that demonstrated altered cell surface expression of P2X(7) N187A. This is the first report to map human P2X(7) glycosylation sites and reveal residue N187 is critical for receptor trafficking and function.

C-terminal calmodulin-binding motif differentially controls human and rat P2X7 receptor current facilitation

P2X(7) receptors (P2X(7)R) are ATP-gated calcium-permeable cationic channels structurally unique among the P2X family by their much longer intracellular C-terminal tail. P2X(7)Rs show several unusual biophysical properties, in particular marked facilitation of currents and leftward shift in agonist affinity in response to repeated or prolonged agonist applications. We previously found the facilitation at rat P2X(7)R resulted from a Ca(2+)-calmodulin-dependent process and a distinct calcium-independent process. However, P2X(7)Rs show striking species differences; thus, this study compared the properties of ATP-evoked facilitation of currents in HEK293 cells transiently expressing the human or rat P2X(7)R as well as rat/human, human/rat chimeric, and mutated P2X(7)Rs. Facilitation at the human P2X(7)R was 5-fold slower than at the rat P2X(7)R. Facilitation did not resulting from an increase of receptor addressing the plasma membrane. We found the human P2X(7)R shows only calcium-independent facilitation with no evidence for calmodulin-dependent processes, nor does it contain the novel 1-5-16 calmodulin binding domain present in the C terminus of rat P2X(7)R. Replacement of three critical residues of this binding domain from the rat into the human P2X(7)R (T541I, C552S, and G559V) reconstituted the Ca(2+)-calmodulin-dependent facilitation, leaving the calcium-independent facilitation unaltered. The leftward shift in the ATP concentration-response curve with repeated agonist applications appears to be a property of the calcium-independent facilitation process because it was not altered in any of the chimeric or mutated P2X(7)Rs. The absence of Ca(2+)-dependent facilitation at the human P2X(7)R may represent a protective adaptation of the innate immune response in which P2X(7)R plays significant roles.

Single nucleotide polymorphisms that were identified in affective mood disorders affect ATP-activated P2X7 receptor functions

Genetic linkage studies have previously identified many single non-synonymous nucleotide polymorphisms (SNPs) in the human P2RX7 gene in individuals with affective mood disorders. The P2RX7 gene encodes the P2X(7) receptor (P2X(7)R) that operates as an ATP-activated Ca(2+)-permeable cationic channel and induces formation of a large pore, the two functional properties that are critical for the physiological and pathological roles of the receptor. The current knowledge regarding the effects of SNPs on the P2X(7)R functional properties, which is indispensable to help elucidate the disease mechanism, is limited. In this study, we introduced by site-directed mutagenesis twelve SNP mutations in the human P2X(7) receptor that were previously identified in or associated with affective mood disorders, expressed the resultant mutants in human embryonic kidney cells, and characterized their functional properties by electrophysiology. All mutations except Q460R gave rise to profound effects on the P2X(7)R function. G150R, E186K and I568N conferred complete loss of function. V76A, R117W, L191P, T357S and E496A resulted in strong impairment of, whereas H155Y and A348T caused significant increase in, both ATP-activated ion channel function and pore formation. Q521H reduced the receptor's sensitivity to extracellular Ca(2+) inhibition. An atomic structure model of the human P2X(7)R, based on the crystal structure of the zebrafish P2X(4) receptor, suggests that the SNP mutational effects may result from changes in subunit interaction, agonist binding and/or channel gating. These results provide essential knowledge for a better understanding of the relationships between human P2RX7 SNPs and associated pathologies as well as the receptor structure-function relationships.

Discovery of 2-chloro-N-((4,4-difluoro-1-hydroxycyclohexyl)methyl)-5-(5-fluoropyrimidin-2-yl)benzamide as a potent and CNS penetrable P2X7 receptor antagonist

Focused SAR studies were carried out around 5-heteroaryl and 1-amide portions of the 2-chlorobenzamide scaffold, resulting in the discovery of a potent, metabolically stable and centrally penetrable antagonist against P2X(7) receptor.

New structure enlivens interest in P2X receptors

P2X receptors are ATP-gated membrane ion channels with multifarious roles, including afferent sensation, autocrine feedback loops, and inflammation. Their molecular operation has been less well elucidated compared with other ligand-gated channels (nicotinic acetylcholine receptors, ionotropic glutamate receptors). This will change with the recent publication of the crystal structure of a closed P2X receptor. Here we re-interpret results from 15 years of experiments using site-directed mutagenesis with a model based on the new structure. Previous predictions of receptor stoichiometry, the extracellular ATP binding site, inter-subunit contacts, and many details of the permeation pathway fall into place in three dimensions. We can therefore quickly understand how the channel operates at the molecular level. This is important not only for ion- channel aficionados, but also those engaged in developing effective antagonists at P2X receptors for potential therapeutic use.

Recent patents on novel P2X(7) receptor antagonists and their potential for reducing central nervous system inflammation

Inflammation arises in the CNS from a number of neurodegenerative and oncogenic disorders, as well as from ischemic and traumatic brain injuries. These pathologies give rise to increased levels of extracellular adenine nucleotides which, via activation of a variety of cell surface P2 purinergic receptors, influence the inflammatory activities of responding immune cells. One P2 receptor subtype in particular, the P2X(7) receptor, potentiates the release of pro-inflammatory cytokines, such as interleukin-1beta (IL-1beta) from macrophage-like cells. It is also thought to contribute to secondary brain injury by inducing neuronal cell death. Therefore, antagonism of this receptor could have significant therapeutic impact on all disorders, not just CNS, to which excessive inflammatory activities contribute. The use of currently available P2X(7) receptor antagonists for the treatment of CNS inflammation has been limited to the generally non-selective antagonists PPADS, oxidized ATP, Brilliant Blue G, suramin, calmidizolium, and KN-62. However, the recent patents and development of novel P2X(7) receptor antagonists, as discussed in this review, will provide new tools both for clinical and research purposes. Here we discuss compounds for which patents have been applied since 2006, from the following categories: benzamide inhibitors, bicycloheteroaryl compounds, acylhdranzine antagonists, biaromatic P2X(7) antagonists, heterocyclic compounds and amide derivatives, and aromatic amine antagonists.

Effects of protons on macroscopic and single-channel currents mediated by the human P2X7 receptor

Human P2X7 receptors (hP2X7Rs) belong to the P2X family, which opens an intrinsic cation channel when challenged by extracellular ATP. hP2X7Rs are expressed in cells of the inflammatory and immune system. During inflammation, ATP and protons are secreted into the interstitial fluid. Therefore, we investigated the effect of protons on the activation of hP2X7Rs. hP2X7Rs were expressed in Xenopus laevis oocytes and activated by the agonists ATP or benzoyl-benzoyl-ATP (BzATP) at different pH values. The protons reduced the hP2X7R-dependent cation current amplitude and slowed the current deactivation depending on the type and concentration of the agonist used. These effects can be explained by (i) the protonation of ATP, which reduces the effective concentration of the agonist ATP(4-) at the high- and low-affinity ATP activation site of the hP2XR, and (ii) direct allosteric inhibition of the hP2X7R channel opening that follows ATP(4-) binding to the low-affinity activation site. Due to the hampered activation via the low-affinity activation site, a low pH (as observed in inflamed tissues) leads to a relative increase in the contribution of the high-affinity activation site for hP2X7R channel opening.

Synthesis and structure-activity relationships of pyrazolodiazepine derivatives as human P2X7 receptor antagonists

Screening of library compounds has yielded pyrazolodiazepine derivatives with P2X7 receptor antagonist activity. To explore the structure-activity relationships (SAR) of these pyrazolodiazepines as human P2X7 receptor antagonists, derivatives were synthesized by substitutions at positions R2 and R3 of the pyrazolodiazepine skeleton. Using a 2'(3')-O-(4-benzoylbenzoyl)ATP (BzATP)-induced fluorescent ethidium uptake assay, the activities of these derivatives were tested in HEK-293 cells stably expressing human P2X7 receptors. Moreover, the effect of these derivatives was assessed by measuring their effect on IL-1beta release induced by BzATP-induced activation of differentiated THP-1 cells. A 2-phenethyl pyrazolodiazepine derivative with a 1-methyl-1H-3-indolyl group at position R2 had fivefold greater activity than the derivative with a 5-isoquinolinyl at R2. Moreover, a benzyl moiety at R3 had fivefold greater activity than a bicyclic moiety. The stereochemical effect at C-6 showed a preference for the (R)-isomer. Among the series of active derivatives, compound 23b, with a phenethyl group at R1, a 3-methyl indole at R2, and a benzyl at R3, exhibited activity similar to that of the positive control, KN-62, as shown by the inhibitory effects of IL-1beta release.

Ca2+/calmodulin-dependent kinase II signalling cascade mediates P2X7 receptor-dependent inhibition of neuritogenesis in neuroblastoma cells

ATP, via purinergic P2X receptors, acts as a neurotransmitter and modulator in both the central and peripheral nervous systems, and is also involved in many biological processes, including cell proliferation, differentiation and apoptosis. Previously, we have reported that P2X7 receptor inhibition promotes axonal growth and branching in cultured hippocampal neurons. In this article, we demonstrate that the P2X7 receptor negatively regulates neurite formation in mouse Neuro-2a neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. Using both molecular and immunocytochemical techniques, we characterized the presence of endogenous P2X1, P2X3, P2X4 and P2X7 subunits in these cells. Of these, the P2X7 receptor was the only functional receptor, as its activation induced intracellular calcium increments similar to those observed in primary neuronal cultures, exhibiting pharmacological properties characteristic of homomeric P2X7 receptors. Patch-clamp experiments were also conducted to fully demonstrate that ionotropic P2X7 receptors mediate nonselective cation currents in this cell line. Pharmacological inhibition of the P2X7 receptor and its knockdown by small hairpin RNA interference resulted in increased neuritogenesis in cells cultured in low serum-containing medium, whereas P2X7 overexpression significantly reduced the formation of neurites. Interestingly, P2X7 receptor inhibition also modified the phosphorylation state of focal adhesion kinase, Akt and glycogen synthase kinase 3, protein kinases that participate in the Ca2+/calmodulin-dependent kinase II signalling cascade and that have been related to neuronal differentiation and axonal growth. Taken together, our results provide the first mechanistic insight into P2X7 receptor-triggered signalling pathways that regulate neurite formation in neuroblastoma cells.

Impaired interleukin-1beta and c-Fos expression in the hippocampus is associated with a spatial memory deficit in P2X(7) receptor-deficient mice

Recent evidence suggests that interleukin-1beta (IL-1beta), which was originally identified as a proinflammatory cytokine, is also required in the brain for memory processes. We have previously shown that IL-1beta synthesis in the hippocampus is dependent on P2X(7) receptor (P2X(7)R), which is an ionotropic receptor of ATP. To substantiate the role of P2X(7)R in both brain IL-1beta expression and memory processes, we examined the induction of IL-1beta mRNA expression in the hippocampus of wild-type (WT) and homozygous P2X(7) receptor knockout mice (P2X(7)R(-/-)) following a spatial memory task. The spatial recognition task induced both IL-1beta mRNA expression and c-Fos protein activation in the hippocampus of WT but not of P2X(7)R(-/-) mice. Remarkably, P2X(7)R(-/-) mice displayed spatial memory impairment in a hippocampal-dependant task, while their performances in an object recognition task were unaltered. Taken together, our results show that P2X(7)R plays a critical role in spatial memory processes and the associated hippocampal IL-1beta mRNA synthesis and c-Fos activation.

Purinergic P2X(7) receptor antagonists: Chemistry and fundamentals of biological screening

The purinergic P2X(7) receptor is a unique member of the ATP-gated P2X family. This receptor has been implicated in numerous diseases and many structurally diverse ligands have been discovered via high throughput screening. This perspective will attempt to highlight some of the most recent key findings in both the biology and chemistry.

The role of P2X7 receptor in ATP-mediated human leukemia cell death: calcium influx-independent

Activation of the P2X7 receptor leads to a rapid, bidirectional flux of cations, causing broad range of biological responses including cytotoxicity. However, the mechanism of P2X7-mediated cytotoxicity remains largely unexplored. In our previous study, the lack of P2X7-mediated calcium response under normal conditions was found in P2X7(+) hematopoietic cell lines. In this study, the P2X7-mediated cytotoxicity in different type of cells (P2X7(-), P2X7(+) with calcium response, and P2X7(+) without calcium response) was investigated. Our results showed that P2X7 agonists, adenosine 5'-triphosphate (ATP) or 2',3'-O-(4 benzoylbenzoyl)-ATP, dose-dependently reduced the cell viability in all P2X7(+) cells tested, including J6-1, LCL, and Namalva cells which are negative for P2X7-mediated calcium response, although these effects were lower than those observed in KG1a cells which has normal P2X7 functions. The cytotoxic effect could be blocked by P2X7 antagonists, oxidized ATP and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine. In addition, externalization of phosphatidylserine could be detected in a time-dependent manner and apoptotic morphological changes could be observed after the activation of P2X7 receptor in J6-1 cells. Furthermore, P2X7-mediated pore formation could be detected in KG1a and J6-1 cells under low-ionic conditions, but not under low-divalent conditions. These effects could not be observed in P2X7(-) Ramos cells. These results suggested that P2X7 receptor-mediated cytotoxic effects may occur independent of calcium response.

ATP-gated P2X cation-channels

P2X receptors are ATP-gated cation channels with important roles in diverse pathophysiological processes. Substantial progress has been made in the last few years with the discovery of both subunit selective antagonists and modulators. The purpose of this brief review is to summarize the advances in the pharmacology of P2X receptors, with key properties presented in an easy to access format. Ligand-gated ion channels consist of three families in mammals; the ionotropic glutamate receptors, the Cys-loop receptors (for GABA, ACh, glycine and serotonin) and the P2X receptors for ATP. The first two of these are considered in articles accompanying this Special Issue. Here we consider the pharmacological properties of P2X receptors. We do not present a detailed discussion of P2X receptor physiological roles or structure-function studies. Moreover, the pharmacological basis for discriminating between the main subtypes of P2X receptor and their nomenclature has been published by the Nomenclature Committee of the International Union of Pharmacology (NC-IUPHAR) P2X Receptor Subcommittee, and so these aspects are not revisited here. Instead in this brief article we seek to present a summary of the pharmacology of recombinant homomeric and heteromeric P2X receptors, with particular emphasis on new antagonists. In this article we have tried to present as much information as possible in two tables in the hope this will be useful as a day-to-day resource, and also because an excellent and detailed review has recently been published.