Identification and characterization of a novel variant of the human P2X(7) receptor resulting in gain of function

From lead to clinic: A review of the structural design of P2X7R antagonists

P2X7R, which is a member of the purinergic P2 receptor family, is widely expressed in many immune cells, such as macrophages, lymphocytes, monocytes, and neutrophils. P2X7R is upregulated in response to proinflammatory stimulation, which is closely related to a variety of inflammatory diseases. The inhibition of P2X7 receptors has resulted in the elimination or reduction of symptoms in animal models of arthritis, depression, neuropathic pain, multiple sclerosis, and Alzheimer's disease. Therefore, the development of P2X7R antagonists is of great significance for the treatment of various inflammatory diseases. This review classifies the reported P2X7R antagonists according to their different cores, focuses on the structure-activity relationship (SAR) of the compounds, and analyzes some common substituents and strategies in the design of lead compounds, with the hope of providing valuable information for the development of new and efficient P2X7R antagonists.

P2X3-P2X7 SNPs and gene-gene and gene-environment interactions on pediatric asthma

BACKGROUND

To investigate the relationship between polymorphisms of P2X3, P2X7 genes and environment interaction with susceptibility of childhood asthma.

METHODS

We conducted a matched case-control study with 170 cases and 175 healthy controls. The rs10896611, rs2276038, rs3781899 in P2X3 and rs1718119, rs3751143 in P2X7 polymorphisms were genotyped using the technique of an improved multiplex ligation detection reaction. Gene-gene, gene-environment and haplotype-environment interactions were tested using the generalized multi-factor dimensionality reduction method.

RESULTS

There were no differences between cases and controls in allele or genotype frequencies of P2X3 and P2X7. The C/C, G/C genotypes of rs10896611, and C/C, C/T genotypes of rs2276038 and G/G, G/A genotypes of rs3781899 were associated with asthmatic cough (p > 0.05). The haplotype GCT of P2X3 reduced the risk of asthma (OR = 0.48, p = 0.048), and the haplotypes AGT (OR = 0.45, p = 0.001) and GCC (OR = 2.16, p = 0.002) were associated with asthmatic cough. The haplotype AA of P2X7 increased risk of asthma severity (p < 0.05). The three-locus model indicated a potential haplotype-environment interaction in GCT, ETS, and pet (p = 0.001).

CONCLUSIONS

The rs10896611, rs2276038 and rs3781899 of P2X3 minor alleles increased the risk of asthmatic cough. Haplotype GCT of P2X3 was a protective factor for asthma, the haplotype AGT was a protective factor and GCC was a risk factor for asthma with cough. In addition, the interactions of haplotype GCT of P2X3, ETS and pet may increase an individual's susceptibility to asthma.

A Possible Causal Involvement of Neuroinflammatory, Purinergic P2X7 Receptors in Psychiatric Disorders

P2X7 receptors (Rs) are prominent members of the P2XR family, which after binding ATP, open non-selective cationic channels, thereby allowing the transmembrane passage of Na+, Ca²⁺, and K+. Long-lasting and repetitive stimulation of the receptor by its agonist leads to the formation of large membrane pores permeable for organic cations of up to 900 Da molecular size. These pores are believed to play a role in apoptosis and inflammation. P2X7Rs are located primarily at peripheral macrophages and microglial cells, the resident macrophages of the CNS. The coactivation of toll-like receptors 4 (TLR4) by lipopolysaccharide, a constituent of the cell membrane of gram-negative bacteria, and the P2X7R by ATP leads to the generation and release of the proinflammatory cytokines interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α. Together with the microglial release of chemokines, reactive oxygen and nitrogen species, proteases, and excitotoxic glutamate, these cytokines result in neurodegeneration. P2X7Rs were found not only to amplify various neurodegenerative illnesses, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis, but also to participate in a range of psychiatric diseases, such as major depression, bipolar disorder, schizophrenia, and autism spectrum disorder. Based on the prevention/reversal of neuroinflammation, pharmacological antagonists of P2X7Rs and their genetic deletion in animal experiments counteract these deleterious psychiatric conditions. Hence, brain penetrant P2X7R antagonists are potential therapeutics for psychiatric diseases, although the available evidence still needs to be extended and validated by further clinical data.

Alternatively Spliced Isoforms of the P2X7 Receptor: Structure, Function and Disease Associations

The P2X7 receptor (P2X7R) is an ATP-gated membrane ion channel that is expressed by multiple cell types. Following activation by extracellular ATP, the P2X7R mediates a broad range of cellular responses including cytokine and chemokine release, cell survival and differentiation, the activation of transcription factors, and apoptosis. The P2X7R is made up of three P2X7 subunits that contain specific domains essential for the receptor’s varied functions. Alternative splicing produces P2X7 isoforms that exclude one or more of these domains and assemble in combinations that alter P2X7R function. The modification of the structure and function of the P2X7R may adversely affect cellular responses to carcinogens and pathogens, and alternatively spliced (AS) P2X7 isoforms have been associated with several cancers. This review summarizes recent advances in understanding the structure and function of AS P2X7 isoforms and their associations with cancer and potential role in modulating the inflammatory response.

Inherent P2X7 Receptors Regulate Macrophage Functions during Inflammatory Diseases

Macrophages are mononuclear phagocytes which derive either from blood-borne monocytes or reside as resident macrophages in peripheral (Kupffer cells of the liver, marginal zone macrophages of the spleen, alveolar macrophages of the lung) and central tissue (microglia). They occur as M1 (pro-inflammatory; classic) or M2 (anti-inflammatory; alternatively activated) phenotypes. Macrophages possess P2X7 receptors (Rs) which respond to high concentrations of extracellular ATP under pathological conditions by allowing the non-selective fluxes of cations (Na⁺, Ca²⁺, K⁺). Activation of P2X7Rs by still higher concentrations of ATP, especially after repetitive agonist application, leads to the opening of membrane pores permeable to ~900 Da molecules. For this effect an interaction of the P2X7R with a range of other membrane channels (e.g., P2X4R, transient receptor potential A1 [TRPA1], pannexin-1 hemichannel, ANO6 chloride channel) is required. Macrophage-localized P2X7Rs have to be co-activated with the lipopolysaccharide-sensitive toll-like receptor 4 (TLR4) in order to induce the formation of the inflammasome 3 (NLRP3), which then activates the pro-interleukin-1β (pro-IL-1β)-degrading caspase-1 to lead to IL-1β release. Moreover, inflammatory diseases (e.g., rheumatoid arthritis, Crohn’s disease, sepsis, etc.) are generated downstream of the P2X7R-induced upregulation of intracellular second messengers (e.g., phospholipase A2, p38 mitogen-activated kinase, and rho G proteins). In conclusion, P2X7Rs at macrophages appear to be important targets to preserve immune homeostasis with possible therapeutic consequences.

P2X7 receptor is essential for cross-dressing of bone marrow-derived dendritic cells

T cell activation requires the processing and presentation of antigenic peptides in the context of a major histocompatibility complex (MHC complex). Cross-dressing is a non-conventional antigen presentation mechanism, involving the transfer of preformed peptide/MHC complexes from whole cells, such as apoptotic cells (ACs) to the cell membrane of professional antigen-presenting cells (APCs), such as dendritic cells (DCs). This is an essential mechanism for the induction of immune response against viral antigens, tumors, and graft rejection, which until now has not been clarified. Here we show for first time that the P2X7 receptor (P2X7R) is crucial to induce cross-dressing between ACs and Bone-Marrow DCs (BMDCs). In controlled ex vivo assays, we found that the P2X7R in both ACs and BMDCs is required to induce membrane and fully functional peptide/MHC complex transfer to BMDCs. These findings show that acquisition of ACs-derived preformed antigen/MHC-I complexes by BMDCs requires P2X7R expression.

•

Cross-dressing of antigens to Dendritic Cells (DCs) is dependent of P2X7 receptor

•

The P2X7 receptor must be present in both Dendritic Cells and antigen source

•

The transfer of antigen/MHC-I complexes to DCs is functional and activates T CD8 cells

•

The P2X7 receptor allows Cross-Dressing possibly through a membrane fusion process

Associations Between the Purinergic Receptor P2X7 and Leprosy Disease

Leprosy is an infectious disease still highly prevalent in Brazil, having been detected around 27,863 new cases in 2019. Exposure to Mycobacterium leprae may not be sufficient to trigger the disease, which seems to be influenced by host immunogenetics to determine resistance or susceptibility. The purinergic receptor P2X7 plays a crucial role in immunity, inflammation, neurological function, bone homeostasis, and neoplasia and is associated with several infectious and non-infectious diseases. Here, we first compare the P2RX7 expression in RNA-seq experiments from 16 leprosy cases and 16 healthy controls to establish the magnitude of allele-specific expression for single-nucleotide polymorphisms of the gene P2RX7 and to determine the level of gene expression in healthy and diseased skin. In addition, we also evaluated the association of two P2RX7 single-nucleotide polymorphisms (c.1513A>C/rs3751143 and c.1068A>G/rs1718119) with leprosy risk. The expression of P2RX7 was found significantly upregulated at macrophage cells from leprosy patients compared with healthy controls, mainly in macrophages from lepromatous patients. Significant risk for leprosy disease was associated with loss function of rs3751143 homozygous mutant CC [CC vs. AA: p = 0.001; odds ratio (OR) = 1.676, 95% CI = 1.251–2.247] but not with heterozygous AC (AC vs. AA: p = 0.001; OR = 1.429, 95% CI = 1.260–1.621). Contrary, the polymorphic A allele from the gain function of rs1718119 was associated with protection for the development of leprosy, as observed in the dominant model (AA + AG × GG p = 0.0028; OR = 0.03516; CI = 0.1801–0.6864). So, our results suggest that the functional P2X7 purinergic receptor may exert a key role in the Mycobacterium death inside macrophages and inflammatory response, which is necessary to control the disease.

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.

P2X7 Variants in Oncogenesis

The P2X7 receptor for extracellular ATP is a well-established mediator of tumoral development and progression both in solid cancers and hematological malignancies. The human P2X7 gene is highly polymorphic, and several splice variants of the receptor have been identified in time. P2X7 single-nucleotide polymorphisms (SNPs) have been broadly analyzed by studies relating them to pathologies as different as infectious, inflammatory, nervous, and bone diseases, among which cancer is included. Moreover, in the last years, an increasing number of reports concentrated on P2X7 splice variants’ different roles and their implications in pathological conditions, including oncogenesis. Here, we give an overview of established and recent literature demonstrating a role for human P2X7 gene products in oncological conditions, mainly focusing on current data emerging on P2X7 isoform B and nfP2X7. We explored the role of these and other genetic variants of P2X7 in cancer insurgence, dissemination, and progression, as well as the effect of chemotherapy on isoforms expression. The described literature strongly suggests that P2X7 variants are potential new biomarkers and therapeutical targets in oncological conditions and that their study in carcinogenesis deserves to be further pursued.

Cannabinoids and psychotic symptoms: A potential role for a genetic variant in the P2X purinoceptor 7 (P2RX7) gene

To investigate the biological mechanisms underlying the higher risk for psychosis in those that use cannabis, we conducted a genome-wide environment-interaction study (GWEIS). In a sample of individuals without a psychiatric disorder (N = 1262), we analyzed the interactions between regular cannabis use and genotype with psychotic-like experiences (PLE) as outcome. PLE were measured using the Community Assessment of Psychic Experiences (CAPE). The sample was enriched for those at the extremes of both cannabis use and PLE to increase power. A single nucleotide polymorphism in the P2RX7 gene (rs7958311) was associated with risk for a high level of psychotic experiences in regular cannabis users (p = 1.10 x10^-7) and in those with high levels of lifetime cannabis use (p = 4.5 × 10^-6). This interaction was replicated in individuals with high levels of lifetime cannabis use in the IMAGEN cohort (N = 1217, p = 0.020). Functional relevance of P2RX7 in cannabis users was suggested by in vitro experiments on activated monocytes. Exposure of these cells to tetrahydrocannabinol (THC) or cannabidiol (CBD) reduced the immunological response of the P2X7 receptor, which was dependent on the identified genetic variant. P2RX7 variants have been implicated in psychiatric disorders before and the P2X7 receptor is involved in pathways relevant to psychosis, such as neurotransmission, synaptic plasticity and immune regulation. We conclude that P2RX7 plays a role in vulnerability to develop psychotic symptoms when using cannabis and point to a new pathway that can potentially be targeted by newly developed P2X7 antagonists.

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.

P2X7 receptor acts as an efficient drug target in regulating bone metabolism system

Skeletal system is a highly dynamic system going through continuous resorption and reconstruction to maintain homeostasis, which is influenced by numerous factors. Once the balance is disrupted, various kinds of bone diseases may occur such as osteoporosis. It has been well known that ATP (adenosine triphosphate), an important signaling molecule, is important in maintaining the dynamic balance of bone matrix. ATP mainly functions through P2X receptors, a kind of ATP receptors expressed by various kinds of bone cells to regulate the whole network of skeleton system. Among P2X receptors, P2X7 plays a crucial role in bone since P2X7 is widely expressed by bone cells and the mutation of P2X7 receptor is associated with kinds of bone diseases. It's acknowledged that P2X7 acts as a potential therapeutic target for clinical treatment of bone-related diseases but further investigations are needed for the practical application. However, since P2X7 has a complicated effect in many aspects, the exact role of P2X7 in skeleton system is ambiguous. This review discusses the function of P2X7 in bone and other cells and their general effect on skeleton system, especially focusing on the possible clinical application for bone diseases.

Pathological ATPergic Signaling in Major Depression and Bipolar Disorder

The mood disorders, major depression (MD) and bipolar disorder (BD), have a high lifetime prevalence in the human population and accordingly generate huge costs for health care. Efficient, rapidly acting, and side-effect-free pharmaceuticals are hitherto not available, and therefore, the identification of new therapeutic targets is an imperative task for (pre)clinical research. Such a target may be the purinergic P2X7 receptor (P2X7R), which is localized in the central nervous system (CNS) at microglial and neuroglial cells mediating neuroinflammation. MD and BD are due to neuroinflammation caused in the first line by the release of the pro-inflammatory cytokine interleukin-1β (IL-1β) from the microglia. IL-1β in turn induces the secretion of corticotropin-releasing hormone (CRH) and in consequence the secretion of adrenocorticotropic hormone (ACTH) and cortisol, which together with a plethora of further cytokines/chemokines lead to mood disorders. A number of biochemical/molecular biological measurements including the use of P2X7R- or IL-1β-deficient mice confirmed this chain of events. More recent studies showed that a decrease in the astrocytic release of ATP in the prefrontal cortex and hippocampus is a major cause of mood disorders. It is an attractive hypothesis that compensatory increases in P2X7Rs in these areas of the brain are the immediate actuators of MD and BD. Hence, blood-brain barrier-permeable P2X7R antagonists may be promising therapeutic tools to improve depressive disorders in humans.

A P2RX7 single nucleotide polymorphism haplotype promotes exon 7 and 8 skipping and disrupts receptor function

P2X7 is an ATP-gated membrane ion channel that is expressed by multiple cell types. Brief exposure to ATP induces the opening of a nonselective cation channel; while repeated or prolonged exposure induces formation of a transmembrane pore. This process may be partially regulated by alternative splicing of full-length P2RX7A pre-mRNA, producing isoforms that delete or retain functional domains. Here, we report cloning and expression of a novel P2RX7 splice variant, P2RX7L, that is, characterized by skipping of exons 7 and 8. In HEK 293 cells, expression of P2RX7L produces a protein isoform, P2X7L, that forms a heteromer with P2X7A. A haplotype defined by six single nucleotide polymorphisms (SNPs) (rs208307, rs208306, rs36144485, rs208308, rs208309, and rs373655596) promotes allele-specific alternative splicing, increasing mRNA levels of P2RX7L and another isoform, P2RX7E, which in addition has a truncated C-terminus. Skipping of exons 7 and 8 is predicted to delete critical amino acids in the ATP-binding site. P2X7L-transfected HEK 293 cells have phagocytic but not channel, pore, or membrane-blebbing function, and double-transfected P2X7L and P2X7A cells have reduced pore function. Heteromeric receptor complexes of P2X7A and P2X7L are predicted to have reduced numbers of ATP-binding sites, which potentially alters receptor function compared to homomeric P2X7A complexes.

Association of P2RX7 functional variants with localized aggressive periodontitis

OBJECTIVE

The purpose of this study was to investigate involvement of the P2X7 receptor in the rare condition, localized aggressive periodontitis.

MATERIAL AND METHODS

Peripheral blood from 220 African Americans (103 with localized aggressive periodontitis and 117 healthy unrelated controls) was stimulated with lipopolysaccharide from E coli and Porphyromonas gingivalis. P2RX7 single nucleotide polymorphisms rs208294 (H155Y), rs1718119 (T348A), rs2230911 (T357S) and rs3751143 (E496A) were genotyped in 103 localized aggressive periodontitis patients and 117 healthy unrelated subjects. We examined genetic association between four P2RX7 single nucleotide polymorphisms and localized aggressive periodontitis, and tested for correlations between the single nucleotide polymorphisms and inflammatory response to lipopolysaccharide in blood samples from these patients.

RESULTS

A significant association with localized aggressive periodontitis was observed with rs1718119 A (Thr) allele (P = 0.0063, odds ratio = 1.904) and with a haplotype containing this allele (P = 0.0075). Additionally, significant correlations with these data were found: the rs1718119 G allele correlated with greater production of IL-6, IL-2 and GM-CSF; the C (His) allele of rs208294 correlated with lower levels of IL-12p40; and the C (Thr) allele of rs2230911 correlated with greater levels of G-CSF.

CONCLUSION

The data from these analyses support a possible biological relationship between P2RX7 genetic variants and inflammatory response in localized aggressive periodontitis patients.

Ion Channels of the Islets in Type 2 Diabetes

Ca²⁺ is an essential signal for pancreatic β-cell function. Ca²⁺ plays critical roles in numerous β-cell pathways such as insulin secretion, transcription, metabolism, endoplasmic reticulum function, and the stress response. Therefore, β-cell Ca²⁺ handling is tightly controlled. At the plasma membrane, Ca²⁺ entry primarily occurs through voltage-dependent Ca²⁺ channels. Voltage-dependent Ca²⁺ channel activity is dependent on orchestrated fluctuations in the plasma membrane potential or voltage, which are mediated via the activity of many ion channels. During the pathogenesis of type 2 diabetes the β-cell is exposed to stressful conditions, which result in alterations of Ca²⁺ handling. Some of the changes in β-cell Ca²⁺ handling that occur under stress result from perturbations in ion channel activity, expression or localization. Defective Ca²⁺ signaling in the diabetic β-cell alters function, limits insulin secretion and exacerbates hyperglycemia. In this review, we focus on the β-cell ion channels that control Ca²⁺ handling and how they impact β-cell dysfunction in type 2 diabetes.

Associations of P2RX7 Functional Diplotypes with Localized Aggressive Periodontitis

AIM

The purpose of this study was to test for the role of the P2X7 receptor in localized aggressive periodontitis (LAP).

METHODS

Peripheral blood was obtained from 95 subjects with LAP and 76 healthy unrelated controls (HUCs). Three P2RX7 single-nucleotide polymorphisms (rs1718119, rs2230911, and rs3751143) were genotyped from these subjects, and their peripheral blood samples were stimulated with lipopolysaccharide (LPS) from Escherichia coli and tested for inflammatory markers. The 3 P2RX7 single-nucleotide polymorphisms were in found to be in perfect linkage disequilibrium, and a total of 4 haplotypes and 9 diplotypes were identified among all subjects. For both subject populations, the 9 diplotypes were grouped into 4 functional groups and tested for association with subject inflammatory response. To specifically study the effects of extrinsic activation of the P2X7 receptor in LAP, peripheral blood samples from were stimulated under 3 treatments: LPS, LPS + ATP, and LPS +ATP+ P2X7 selective inhibitor. The effects of these treatments on P2X7 receptor activity were measured through Luminex protein assay. Last, to test whether receptor stimulation was related to P2RX7 expression, relative mRNA levels of P2RX7 were quantified with real-time quantitative polymerase chain reaction.

RESULTS

Several associations between the P2RX7 diplotypes and LPS-stimulated blood chemokine/cytokine levels were found between the LAP and HUC populations (P < 0.05). P2X7 activation resulted in statistically significant differences in IL-1β and IL-12p40 concentrations for both subject populations. The relative P2RX7 mRNA levels increased significantly after addition of its inhibitor for both LAP and HUC populations.

CONCLUSIONS

This study detected an association between P2RX7 functional diplotypes and in vitro immune response of whole blood from subjects with LAP. In addition, we found that inhibition of the activated P2X7 receptor leads to increased P2RX7 mRNA levels, suggesting a feedback loop ( ClinicalTrials.gov NCT01330719).

KNOWLEDGE TRANSFER STATEMENT

The results of this study suggest that P2RX7 functional diplotypes are associated with LAP and their in vitro immune response to bacteria. Ongoing studies to uncover the mechanistic link between P2RX7 and LAP phenotypes could lead to the development of preventive approaches for susceptible subjects.

Genetic variation in P2RX7 and pain tolerance

P2X7 is a nonselective cation channel activated by extracellular ATP. P2X7 activation contributes to the proinflammatory response to injury or bacterial invasion and mediates apoptosis. Recently, P2X7 function has been linked to chronic inflammatory and neuropathic pain. P2X7 may contribute to pain modulation both by effects on peripheral tissue injury underlying clinical pain states, and through alterations in central nervous system processing, as suggested by animal models. To further test its role in pain sensitivity, we examined whether variation within the P2RX7 gene, which encodes the P2X7 receptor, was associated with experimentally induced pain in human patients. Experimental pain was assessed in Tromsø 6, a longitudinal and cross-sectional population-based study (N = 3016), and the BrePainGen cohort, consisting of patients who underwent breast cancer surgery (N = 831). For both cohorts, experimental pain intensity and tolerance were assessed with the cold-pressor test. In addition, multisite chronic pain was assessed in Tromsø 6 and pain intensity 1 week after surgery was assessed in BrePainGen. We tested whether the single-nucleotide polymorphism rs7958311, previously implicated in clinical pain, was associated with experimental and clinical pain phenotypes. In addition, we examined effects of single-nucleotide polymorphisms rs208294 and rs208296, for which previous results have been equivocal. Rs7958311 was associated with experimental pain intensity in the meta-analysis of both cohorts. Significant associations were also found for multisite pain and postoperative pain. Our results strengthen the existing evidence and suggest that P2X7 and genetic variation in the P2RX7-gene may be involved in the modulation of human pain sensitivity.

ATP-activated P2X7 receptor in the pathophysiology of mood disorders and as an emerging target for the development of novel antidepressant therapeutics

Mood disorders are a group of psychiatric conditions that represent leading global disease burdens. Increasing evidence from clinical and preclinical studies supports that innate immune system dysfunction plays an important part in the pathophysiology of mood disorders. P2X7 receptor, belonging to the ligand-gated ion channel P2X subfamily of purinergic P2 receptors for extracellular ATP, is highly expressed in immune cells including microglia in the central nervous system (CNS) and has a vital role in mediating innate immune response. The P2X7 receptor is also important in neuron-glia signalling in the CNS. The gene encoding human P2X7 receptor is located in a locus of susceptibility to mood disorders. In this review, we will discuss the recent progress in understanding the role of the P2X7 receptor in the pathogenesis and development of mood disorders and in discovering CNS-penetrable P2X7 antagonists for potential uses in in vivo imaging to monitor brain inflammation and antidepressant therapeutics.

SNPs in the vicinity of P2X7R, RANK/RANKL/OPG and Wnt signalling pathways and their association with bone phenotypes in academy footballers

CONTEXT

Genotype plays an important role in influencing bone phenotypes, such as bone mineral density, but the role of genotype in determining responses of bone to exercise has yet to be elucidated.

OBJECTIVE

To determine whether 10 SNPs associated with genes in the vicinity of P2X7R, RANK/RANKL/OPG and Wnt Signalling Pathways are associated with bone phenotypes in elite academy footballers (Soccer players) and to determine whether these genotypes are associated with training induced changes in bone. Design, participants, and methods: 99 elite academy footballers volunteered to participate. Peripheral computed tomography of the tibia (4%, 14%, 38% and 66% sites) was performed immediately before and 12 weeks after an increase in football training volume. Genotypes were determined using proprietary fluorescence-based competitive allele-specific PCR assays.

RESULTS

No significant genotype by time interactions were shown for any of the SNPs analysed (P > .05). A main effect of genotype was shown. SOST SNP rs1877632 (trabecular density), P2X7R SNPs rs1718119 (cortical thickness and CSA), rs3751143 (SSI, CSA, cortical CSA and periosteal circumference) RANK/RANKL/OPG SNPs rs9594738 (periosteal circumference), rs1021188 (cortical thickness and CSA) and rs9594759 (cortical density) were associated with bone phenotypes (P < .05).

CONCLUSIONS

No association was shown between P2X7R, RANK/RANKL/OPG and Wnt Signalling SNPs and a change in bone phenotypes following 12 weeks of increased training volume in elite academy footballers. However, SNPs were associated with bone phenotypes pre training. These data highlight the complexity of the interaction between SNPs in the vicinity of the RANK/RANKL/OPG, P2X7R and Wnt metabolic regulatory pathways and bone phenotypes in elite academy footballers.

Pharmacological Evaluation of Novel Bioisosteres of an Adamantanyl Benzamide P2X7 Receptor Antagonist

Adamantanyl benzamide 1 was identified as a potent P2X₇R antagonist but failed to progress further due to poor metabolic stability. We describe the synthesis and SAR of a series of bioisosteres of benzamide 1 to explore improvements in the pharmacological properties of this lead. Initial efforts investigated a series of heteroaromatic bioisosteres, which demonstrated improved physicochemical properties but reduced P2X₇R antagonism. Installation of bioisosteric fluorine on the adamantane bridgeheads was well tolerated and led to a series of bioisosteres with improved physicochemical properties and metabolic stability. Trifluorinated benzamide 34 demonstrated optimal physicochemical parameters, superior metabolic stability (ten times longer than lead benzamide 1), and an improved physicokinetic profile and proved effective in the presence of several known P2X₇R polymorphisms.

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.

Searching Novel Therapeutic Targets for Scleroderma: P2X7-Receptor Is Up-regulated and Promotes a Fibrogenic Phenotype in Systemic Sclerosis Fibroblasts

Objectives: Systemic sclerosis (SSc) is a connective tissue disorder presenting fibrosis of the skin and internal organs, for which no effective treatments are currently available. Increasing evidence indicates that the P2X7 receptor (P2X7R), a nucleotide-gated ionotropic channel primarily involved in the inflammatory response, may also have a key role in the development of tissue fibrosis in different body districts. This study was aimed at investigating P2X7R expression and function in promoting a fibrogenic phenotype in dermal fibroblasts from SSc patients, also analyzing putative underlying mechanistic pathways. Methods: Fibroblasts were isolated by skin biopsy from 9 SSc patients and 8 healthy controls. P2X7R expression, and function (cytosolic free Ca²⁺ fluxes, α-smooth muscle actin [α-SMA] expression, cell migration, and collagen release) were studied. Moreover, the role of cytokine (interleukin-1β, interleukin-6) and connective tissue growth factor (CTGF) production, and extracellular signal-regulated kinases (ERK) activation in mediating P2X7R-dependent pro-fibrotic effects in SSc fibroblasts was evaluated. Results: P2X7R expression and Ca²⁺ permeability induced by the selective P2X7R agonist 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) were markedly higher in SSc than control fibroblasts. Moreover, increased αSMA expression, cell migration, CTGF, and collagen release were observed in lipopolysaccharides-primed SSc fibroblasts after BzATP stimulation. While P2X7-induced cytokine changes did not affect collagen production, it was completely abrogated by inhibition of the ERK pathway. Conclusion: In SSc fibroblasts, P2X7R is overexpressed and its stimulation induces Ca²⁺-signaling activation and a fibrogenic phenotype characterized by increased migration and collagen production. These data point to the P2X7R as a potential, novel therapeutic target for controlling exaggerated collagen deposition and tissue fibrosis in patients with SSc.

The P2X7 Receptor

The P2X7 receptor is a trimeric ion channel gated by extracellular adenosine 5'-triphosphate. The receptor is present on an increasing number of different cells types including stem, blood, glial, neural, ocular, bone, dental, exocrine, endothelial, muscle, renal and skin cells. The P2X7 receptor induces various downstream events in a cell-specific manner, including inflammatory molecule release, cell proliferation and death, metabolic events, and phagocytosis. As such this receptor plays important roles in heath and disease. Increasing knowledge about the P2X7 receptor has been gained from studies of, but not limited to, protein chemistry including cloning, site-directed mutagenesis, crystal structures and atomic modeling, as well as from studies of primary tissues and transgenic mice. This chapter focuses on the P2X7 receptor itself. This includes the P2RX7 gene and its products including splice and polymorphic variants. This chapter also reviews modulators of P2X7 receptor activation and inhibition, as well as the transcriptional regulation of the P2RX7 gene via its promoter and enhancer regions, and by microRNA and long-coding RNA. Furthermore, this chapter discusses the post-translational modification of the P2X7 receptor by N-linked glycosylation, adenosine 5'-diphosphate ribosylation and palmitoylation. Finally, this chapter reviews interaction partners of the P2X7 receptor, and its cellular localisation and trafficking within cells.

Genetic determinants and postorthodontic external apical root resorption in Czech children

OBJECTIVE

Genes, involved in the modulation of inflammatory response and bone remodeling, play a role in the development of postorthodontic external apical root resorption (EARR). The aim of our study was to analyze possible associations between seven single nucleotide polymorphisms (SNPs) in interleukin-17A (IL-17), osteopontin (SPP1), purinoreceptor P2X7 (P2RX7), and tumor necrosis factor receptor superfamily member 11B (TNFRSF11B) genes and EARR in children after orthodontic treatment.

SUBJECTS AND METHODS

This case-control study comprised 99 orthodontically treated patients (69 controls and 30 subjects with EARR). Genotype determinations of rs2275913, rs11730582, rs9138, rs208294, rs1718119, rs3102735, and rs2073618 were based on polymerase chain reaction using 5' nuclease TaqMan^® assays.

RESULTS

While no significant differences were observed in allele or genotype frequencies of all seven studied SNPs, specific haplotype of P2RX7 (rs208294 and rs1718119) modified the risk of EARR development (P < 0.05). In addition, the length of treatment with a fixed orthodontic appliance positively correlated with the presence of EARR (P < 0.05).

CONCLUSIONS

Although the effect of individual SNPs studied on the EARR development was not confirmed in the Czech population, complex analysis suggested that variability in the P2RX7 gene and the length of orthodontic treatment may be important factors contributing to the etiopathogenesis of postorthodontic EARR.

Gain and loss of function of P2X7 receptors: mechanisms, pharmacology and relevance to diabetic neuropathic pain

Background

Genetic causes of exaggerated or reduced pain sensitivity in humans are well known. Recently, single nucleotide polymorphisms (SNPs) in the gene P2RX7, coding for the ATP-gated ion channel P2X_7, have been described that cause gain-of-function (GOF) and loss-of-function (LOF), respectively of this channel. Importantly, P2RX7 SNPs have been associated with more or less severe pain scores in patient suffering of post-mastectomy pain and osteoarthritis.

Results

The functional consequences of some P2RX7 SNPs (rs208294 (His155Tyr), rs1718119 (Ala348Thr) and rs3751143 (Glu496Ala)) were studied in recombinant cells in vitro. Our findings suggest a correlation between GOF and LOF of P2X₇ and actual channel protein expression. Both channel and pore function for these mutant P2X₇ receptors changed in parallel to protein levels. On the other hand, the mutant receptors did not differ in their sensitivity to known P2X₇ agonists and antagonists. We further demonstrated that in patients with diabetic peripheral neuropathic pain (DPNP), the presence of the GOF SNPs rs208294 (His155Tyr) and rs1718119 (Ala348Thr) is associated, in females, with higher pain intensity scores.

Conclusions

Our present results confirm the physiological relevance of some of the SNPs in the P2RX7 gene and show that the presence of these genetic variants correlates with pain sensitivity also in a diabetic neuropathic pain patient population.

Two open states of P2X receptor channels

The occupancy of the orthosteric ligand binding sites of P2X receptor (P2XR) channels causes the rapid opening of a small cation-permeable pore, followed by a gradual dilation that renders the pore permeable to large organic cations. Electrophysiologically, this phenomenon was shown using whole-cell current recording on P2X2R-, P2X2/X5R-, P2X4R- and P2X7R-expressing cells that were bathed in N-methyl-D-glucamine (NMDG⁺)-containing buffers in the presence and/or absence of small monovalent and divalent cations. The pore dilation of P2X4R and P2X7R caused a secondary current growth, whereas that of P2X2R showed a sustained kinetic coupling of dilation and desensitization, leading to receptor channel closure. The pore size of the P2X7R open and dilated states was estimated to be approximately 0.85 nm and greater than 1 nm, respectively. The P2XR pore dilation was also observed in intact cells by measurement of fluorescent dye uptake/release, application of polyethylene glycols of different sizes, and atomic force microscopy. However, pore dilation was not observed at the single channel level. Structural data describing the dilated state are not available, and the relevance of orthosteric and allosteric ligand interactions to pore dilation was not studied.

The purinergic P2×7 receptor is expressed on monocytes in Behçet's disease and is modulated by TNF-α

The P2×7 receptor (P2×7r) is expressed in innate immune cells (e.g. monocyte/macrophages), playing a key role in IL-1β release. Since innate immune activation and IL-1β release seem to be implicated in Behçet's disease (BD), a systemic immune-inflammatory disorder of unknown origin, we hypothesized that P2×7r is involved in the pathogenesis of the disease. Monocytes were isolated from 18 BD patients and 17 healthy matched controls. In BD monocytes, an increased P2×7r expression and Ca(2+) permeability induced by the selective P2×7r agonist 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) was observed. Moreover, IL-1β release from LPS-primed monocytes stimulated with BzATP was markedly higher in BD patients than in controls. TNF-α-incubated monocytes from healthy subjects almost reproduced the findings observed in BD patients, as demonstrated by the increase in P2×7r expression and BzATP-induced Ca(2+) intake. Our results provide evidence that in BD monocytes both the expression and function of the P2×7r are increased compared with healthy controls, as the possible result, at least in part, of a positive modulating effect of TNF-α on the receptor. These data indicate P2×7r as a new potential therapeutic target for the control of BD, further supporting the rationale for the use of anti-TNF-α drugs in the treatment of the disease.

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

The mammalian P2X7 receptors (P2X7Rs), a member of the ionotropic P2X receptor family with distinctive functional properties, play an important part in mediating extracellular ATP signaling in health and disease. A clear delineation of the molecular mechanisms underlying the key receptor properties, such as ATP-binding, ion permeation, and large pore formation of the mammalian P2X7Rs, is still lacking, but such knowledge is crucial for a better understanding of their physiological functions and contributions in diseases and for development of therapeutics. The recent breakthroughs in determining the atomic structures of the zebrafish P2X4.1R in the closed and ATP-bound open states have provided the long-awaited structural information. The human P2RX7 gene is abundant with non-synonymous single nucleotide polymorphisms (NS-SNPs), which generate a repertoire of human P2X7Rs with point mutations. Characterizations of the NS-SNPs identified in patients of various disease conditions and the resulting mutations have informed previously unknown molecular mechanisms determining the mammalian P2X7R functions and diseases. In this review, we will discuss the new insights into such mechanisms provided by structural modeling and recent functional and genetic linkage studies of NS-SNPs.

The second transmembrane domain of P2X7 contributes to dilated pore formation

Activation of the purinergic receptor P2X7 leads to the cellular permeability of low molecular weight cations. To determine which domains of P2X7 are necessary for this permeability, we exchanged either the C-terminus or portions of the second transmembrane domain (TM2) with those in P2X1 or P2X4. Replacement of the C-terminus of P2X7 with either P2X1 or P2X4 prevented surface expression of the chimeric receptor. Similarly, chimeric P2X7 containing TM2 from P2X1 or P2X4 had reduced surface expression and no permeability to cationic dyes. Exchanging the N-terminal 10 residues or C-terminal 14 residues of the P2X7 TM2 with the corresponding region of P2X1 TM2 partially restored surface expression and limited pore permeability. To further probe TM2 structure, we replaced single residues in P2X7 TM2 with those in P2X1 or P2X4. We identified multiple substitutions that drastically changed pore permeability without altering surface expression. Three substitutions (Q332P, Y336T, and Y343L) individually reduced pore formation as indicated by decreased dye uptake and also reduced membrane blebbing in response to ATP exposure. Three others substitutions, V335T, S342G, and S342A each enhanced dye uptake, membrane blebbing and cell death. Our results demonstrate a critical role for the TM2 domain of P2X7 in receptor function, and provide a structural basis for differences between purinergic receptors.

Association of P2X7 receptor polymorphisms with bone mineral density and osteoporosis risk in a cohort of Dutch fracture patients

The P2X7 receptor is thought to be involved in bone physiology in a pro-osteogenic manner. Therefore, we examined associations between genetic variations in the P2X7 receptor gene and bone mineral density (BMD). We found an association between four non-synonymous polymorphism of the human P2X7 receptor and the risk of osteoporosis.

INTRODUCTION

The purpose of this study was to determine whether genetic variation in the P2X7 receptor gene (P2RX7) is associated with decreased BMD and risk of osteoporosis in fracture patients.

METHODS

Six hundred ninety women and 231 men aged≥50 years were genotyped for 15 non-synonymous P2RX7 SNPs. BMD was measured at the total hip, lumbar spine and femoral neck.

RESULTS

Four non-synonymous SNPs were associated with BMD. The Ala348Thr gain-of-function polymorphism was associated with increased BMD values at the lumbar spine (p=0.012). Decreased hip BMD values were associated with two loss-of-function SNPs in the P2RX7, i.e., in subjects homozygous for the Glu496Ala polymorphism as well as in subjects carrying at least one variant allele of the Gly150Arg polymorphism (p=0.018 and p=0.011; respectively). In men, we showed that subjects either heterozygous or homozygous for the Gln460Arg gain-of-function polymorphism in the P2RX7 had a significantly 40% decrease in risk of a lower T-score value (OR=0.58 [95%CI, 0.33-1.00]).

CONCLUSION

Thus, genetic aberrations of P2X7R function are associated with lower BMD and increased osteoporosis risk. Therefore, detection of non-synonymous SNPs within the P2RX7 might be useful for osteoporosis risk estimation at an early stage, potentially enabling better osteoporosis prevention and treatment.

Functional polymorphisms in the P2X7 receptor gene are associated with osteoporosis

The P2X(7) receptor is an ATP-gated cation channel. We investigated the effect of both loss-of-function and gain-of-function polymorphisms in the P2X(7) receptor gene on BMD and risk of vertebral fractures and found that five polymorphisms and haplotypes containing three of these polymorphisms were associated with BMD and fracture risk.

INTRODUCTION

The P2X(7) receptor is an ATP-gated cation channel. P2X(7) receptor knockout mice have reduced total bone mineral content, and because several functional polymorphisms have been identified in the human P2X(7) receptor gene, we wanted to investigate the effect of these polymorphisms on BMD and risk of vertebral fractures in a case-control study including 798 individuals.

METHODS

Genotyping was carried out using TaqMan assays. BMD was measured using dual energy X-ray absorptiometry, and vertebral fractures were assessed by lateral spinal X-rays.

RESULTS

The rare allele of a splice site polymorphism, 151 + 1: G-T, was associated with increased fracture risk and reduced BMD in women. Two other loss-of-function polymorphisms, Glu496Ala and Gly150Arg, were also associated with BMD. The Glu496Ala variant allele was associated with decreased lumbar spine BMD in women and decreased total hip BMD in men. The 150Arg allele was associated with decreased total hip BMD in women and men combined. The minor allele of the gain-of-function polymorphism, Ala348Thr, was associated with reduced fracture risk and increased BMD at all sites in men. The Gln460Arg variant allele, which has been associated with increased receptor function in monocytes, was associated with increased total hip BMD in women. With the exception of His155Tyr for which we found conflicting results in men and women, our results are consistent with the phenotype of the knockout mouse. Analysis of a haplotype containing Ala348Thr, Gln460Arg, and Glu496Ala showed that the effects of the haplotypes on BMD and fracture were driven by Ala348Thr in men and by Gln460Arg and Glu496Ala in women.

CONCLUSION

In conclusion, we found that functional polymorphisms in the P2X(7) receptor gene and haplotypes containing three of these polymorphisms are associated with osteoporosis.

The P2X7 receptor as a route for non-exocytotic glutamate release: dependence on the carboxyl tail

P2X7 receptors trigger Ca(2+) -dependent exocytotic glutamate release, but also function as a route for non-exocytotic glutamate release from neurons or astrocytes. To gain an insight into the mechanisms involving the P2X7 receptor as a direct pathway for glutamate release, we compared the behavior of a full-length rat P2X7 receptor, a truncated rat P2X7 receptor in which the carboxyl tail had been deleted, a rat P2X7 receptor with the 18-amino acid cysteine-rich motif of the carboxyl tail deleted, and a rat P2X2 receptor, all of which are expressed in HEK293 cells. We found that the P2X7 receptor function as a route for glutamate release was antagonized in a non-competitive way by extracellular Mg(2+) , did not require the recruitment of pore-forming molecules, and was dependent on the carboxyl tail. Indeed, the truncated P2X7 receptor and the P2X7 receptor with the deleted cysteine-rich motif both lost their function as a pathway for glutamate release, while still evoking intracellular Ca(2+) elevation. No glutamate efflux was observed through the P2X2 receptor. Notably, HEK293 cells (lacking the machinery for Ca(2+) -dependent exocytosis), when transfected with P2X7 receptors, appear to be a suitable model for investigating the P2X7 receptor as a route for non-exocytotic glutamate efflux.

The specificity protein factor Sp1 mediates transcriptional regulation of P2X7 receptors in the nervous system

P2X7 receptors are involved not only in physiological functions but also in pathological brain processes. Although an increasing number of findings indicate that altered receptor expression has a causative role in neurodegenerative diseases and cancer, little is known about how expression of P2rx7 gene is controlled. Here we reported the first molecular and functional evidence that Specificity protein 1 (Sp1) transcription factor plays a pivotal role in the transcriptional regulation of P2X7 receptor. We delimited a minimal region in the murine P2rx7 promoter containing four SP1 sites, two of them being highly conserved in mammals. The functionality of these SP1 sites was confirmed by site-directed mutagenesis and Sp1 overexpression/down-regulation in neuroblastoma cells. Inhibition of Sp1-mediated transcriptional activation by mithramycin A reduced endogenous P2X7 receptor levels in primary cultures of cortical neurons and astrocytes. Using P2rx7-EGFP transgenic mice that express enhanced green fluorescent protein under the control of P2rx7 promoter, we found a high correlation between reporter expression and Sp1 levels in the brain, demonstrating that Sp1 is a key element in the transcriptional regulation of P2X7 receptor in the nervous system. Finally, we found that Sp1 mediates P2X7 receptor up-regulation in neuroblastoma cells cultured in the absence of serum, a condition that enhances chromatin accessibility and facilitates the exposure of SP1 binding sites.

P2X7 receptor activation mediates organic cation uptake into human myeloid leukaemic KG-1 cells

The P2X7 purinergic receptor is an ATP-gated cation channel with an emerging role in neoplasia. In this study we demonstrate that the human KG-1 cell line, a model of acute myelogenous leukaemia, expresses functional P2X7. RT-PCR and immunochemical techniques demonstrated the presence of P2X7 mRNA and protein respectively in KG-l cells, as well as in positive control multiple myeloma RPMI 8226 cells. Flow cytometric measurements demonstrated that ATP induced ethidium(+) uptake into KG-l cells suspended in sucrose medium (EC(50) of ≈ 3 μM), but not into cells in NaCl medium. In contrast, ATP induced ethidium(+) uptake into RPMI 8226 cells suspended in either sucrose or NaCl medium (EC(50) of ≈ 3 or ≈ 99 μM, respectively), as well as into RPMI 8226 cells in KCl medium (EC(50) of ≈ 18 μM). BzATP and to a lesser extent ATPγS and αβ-methylene ATP, but not ADP or UTP, also induced ethidium(+) uptake into KG-1 cells. ATP-induced ethidium(+) uptake was completely impaired by the P2X7 antagonists, AZ10606120 and A-438079. ATP-induced ethidium(+) uptake was also impaired by probenecid but not by carbenoxolone, both pannexin-1 antagonists. ATP induced YO-PRO-1(2+) and propidium(2+) uptake into KG-1 cells. Finally, sequencing of full-length P2X7 cDNA identified several single nucleotide polymorphisms (SNPs) in KG-1 cells including H155Y, A348T, T357S and Q460R. RPMI 8226 cells contained A348T, A433V and H521Q SNPs. In conclusion, the KG-1 cell line expresses functional P2X7. This cell line may help elucidate the signalling pathways involved in P2X7-induced survival and invasiveness of myeloid leukaemic cells.

Single-nucleotide polymorphisms in the P2X7 receptor gene are associated with post-menopausal bone loss and vertebral fractures

The purinergic P2X7 receptor has a major role in the regulation of osteoblast and osteoclast activity and changes in receptor function may therefore affect bone mass in vivo. The aim of this study was to determine the association of non-synonymous single-nucleotide polymorphisms in the P2RX7 gene to bone mass and fracture incidence in post-menopausal women. A total of 1694 women (aged 45-58) participating in the Danish Osteoporosis Prevention Study were genotyped for 12 functional P2X7 receptor variants. Bone mineral density was determined at baseline and after 10 years. In addition, vertebral fracture incidence was documented at 10 years. We found that the rate of bone loss was clearly associated with the Arg307Gln amino acid substitution such that individuals heterozygous for this polymorphism had a 40% increased rate of bone loss. Furthermore, individuals carrying the Ile568Asn variant allele had increased bone loss. In contrast, the Gln460Arg polymorphism was associated with protection against bone loss. The Ala348Thr polymorphism was associated with a lower vertebral fracture incidence 10 years after menopause. Finally, we developed a risk model, which integrated P2RX7 genotypes. Using this model, we found a clear association between the low-risk (high-P2X7 function) alleles and low rate of bone loss. Conversely, high-risk (reduced P2X7 function) alleles were associated with a high rate of bone loss. In conclusion, an association was demonstrated between variants that reduce P2X7 receptor function and increased rate of bone loss. These data support that the P2X7 receptor is important in regulation of bone mass.

Residues 155 and 348 contribute to the determination of P2X7 receptor function via distinct mechanisms revealed by single-nucleotide polymorphisms

P2X(7) receptors are important in mediating the physiological functions of extracellular ATP, and altered receptor expression and function have a causative role in the disease pathogenesis. Here, we investigated the mechanisms determining the P2X(7) receptor function by following two human single-nucleotide polymorphism (SNP) mutations that replace His-155 and Ala-348 in the human (h) P2X(7) receptor with the corresponding residues, Tyr-155 and Thr-348, in the rat (r) P2X(7) receptor. H155Y and A348T mutations in the hP2X(7) receptor increased ATP-induced currents, whereas the reciprocal mutations, Y155H and T348A, in the rP2X(7) receptor caused the opposite effects. Such a functional switch is a compelling indication that these residues are critical for P2X(7) receptor function. Additional mutations of His-155 and Ala-348 in the hP2X(7) receptor to residues with diverse side chains revealed a different dependence on the side chain properties, supporting the specificity of these two residues. Substitutions of the residues surrounding His-155 and Ala-348 in the hP2X(7) receptor with the equivalent ones in the rP2X(7) receptor also affected ATP-induced currents but were not fully reminiscent of the H155Y and A348T effects. Immunofluorescence imaging and biotin labeling assays showed that H155Y in the hP2X(7) receptor increased and Y155H in the rP2X(7) receptor decreased cell-surface expression. Such contrasting effects were not obvious with the reciprocal mutations of residue 348. Taken together, our results suggest that residues at positions 155 and 348 contribute to P2X(7) receptor function via determining the surface expression and the single-channel function, respectively. Such interpretations are consistent with the locations of the residues in the structural model of the hP2X(7) receptor.

Two haplotypes of the P2X(7) receptor containing the Ala-348 to Thr polymorphism exhibit a gain-of-function effect and enhanced interleukin-1beta secretion

The P2X(7) receptor is an ATP-gated cation channel expressed in immune cells and plays a role in proinflammatory cytokine release from monocytes and macrophages. This study investigated the coinheritance of 12 functionally relevant single nucleotide polymorphisms (SNPs) in the human P2X(7) gene (P2RX7), and the functional effect of each singly and in combination was assessed by measurements of ATP-induced currents and ethidium(+) uptake. Genotyping of 3430 Caucasian subjects identified 4 common haplotypes in addition to the common (wild-type) P2X(7)-1. Two haplotypes (denoted P2X(7)-2 and P2X(7)-4) contained various combinations of gain-of-function SNPs. P2X(7)-4 was identified uniquely by the Gln-460 to Arg polymorphism (rs2230912). When expressed in HEK-293 cells, recombinant P2X(7)-2, and P2X(7)-4 haplotypes displayed a 3-fold and 5-fold increase, respectively, in receptor function compared to the wild-type P2X(7)-1. Both P2X(7) haplotypes contained the Ala-348>Thr polymorphism (rs1718119), and this mutation was critical for the gain-of-function effect. Peripheral blood monocytes and erythrocytes from subjects homozygous for gain-of-function P2X(7) haplotypes exhibited increased ATP-induced ethidium(+) uptake and (86)Rb(+) efflux, respectively, and this correlated with increased IL-1beta secretion from LPS-primed monocytes. Inheritance of these P2X(7) haplotypes predisposing to increased proinflammatory cytokine secretion may be important in genetic association studies of inflammatory, infectious, and psychiatric disorders.