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

Human P2X7 receptor variants Gly150Arg and Arg276His polymorphisms have differential effects on risk association and cellular functions in pancreatic cancer

BACKGROUND

The purinergic P2X7 receptor (P2X7R) plays an important role in the crosstalk between pancreatic stellate cells (PSCs) and cancer cells, thus promoting progression of pancreatic ductal adenocarcinoma (PDAC). Single nucleotide polymorphisms (SNPs) in the P2X7R have been reported for several cancers, but have not been explored in PDAC.

MATERIALS AND METHODS

Blood samples from PDAC patients and controls were genotyped for 11 non-synonymous SNPs in P2X7R and a risk analysis was performed. Relevant P2X7R-SNP GFP variants were expressed in PSCs and cancer cells and their function was assayed in the following tests. Responses in Ca2+ were studied with Fura-2 and dye uptake with YO-PRO-1. Cell migration was monitored by fluorescence microscopy. Released cytokines were measured with MSD assay.

RESULTS

Risk analysis showed that two SNPs 474G>A and 853G>A (rs28360447, rs7958316), that lead to the Gly150Arg and Arg276His variants, had a significant but opposite risk association with PDAC development, protecting against and predisposing to the disease, respectively. In vitro experiments performed on cancer cells and PSCs expressing the Gly150Arg variant showed reduced intracellular Ca2+ response, fluorescent dye uptake, and cell migration, while the Arg276His variant reduced dye uptake but displayed WT-like Ca2+ responses. As predicted, P2X7R was involved in cytokine release (IL-6, IL-1β, IL-8, TNF-α), but the P2X7R inhibitors displayed varied effects.

CONCLUSION

In conclusion, we provide evidence for the P2X7R SNPs association with PDAC and propose that they could be considered as potential biomarkers.

Silencing NTPDase3 activity rehabilitates the osteogenic commitment of post-menopausal stem cell bone progenitors

BACKGROUND

Endogenously released adenine and uracil nucleotides favour the osteogenic commitment of bone marrow-derived mesenchymal stromal cells (BM-MSCs) through the activation of ATP-sensitive P2X7 and UDP-sensitive P2Y₆ receptors. Yet, these nucleotides have their osteogenic potential compromised in post-menopausal (Pm) women due to overexpression of nucleotide metabolizing enzymes, namely NTPDase3. This prompted us to investigate whether NTPDase3 gene silencing or inhibition of its enzymatic activity could rehabilitate the osteogenic potential of Pm BM-MSCs.

METHODS

MSCs were harvested from the bone marrow of Pm women (69 ± 2 years old) and younger female controls (22 ± 4 years old). The cells were allowed to grow for 35 days in an osteogenic-inducing medium in either the absence or the presence of NTPDase3 inhibitors (PSB 06126 and hN3-B3_s antibody); pre-treatment with a lentiviral short hairpin RNA (Lenti-shRNA) was used to silence the NTPDase3 gene expression. Immunofluorescence confocal microscopy was used to monitor protein cell densities. The osteogenic commitment of BM-MSCs was assessed by increases in the alkaline phosphatase (ALP) activity. The amount of the osteogenic transcription factor Osterix and the alizarin red-stained bone nodule formation. ATP was measured with the luciferin-luciferase bioluminescence assay. The kinetics of the extracellular ATP (100 µM) and UDP (100 µM) catabolism was assessed by HPLC RESULTS: The extracellular catabolism of ATP and UDP was faster in BM-MSCs from Pm women compared to younger females. The immunoreactivity against NTPDase3 increased 5.6-fold in BM-MSCs from Pm women vs. younger females. Selective inhibition or transient NTPDase3 gene silencing increased the extracellular accumulation of adenine and uracil nucleotides in cultured Pm BM-MSCs. Downregulation of NTPDase3 expression or activity rehabilitated the osteogenic commitment of Pm BM-MSCs measured as increases in ALP activity, Osterix protein cellular content and bone nodule formation; blockage of P2X7 and P2Y₆ purinoceptors prevented this effect.

CONCLUSIONS

Data suggest that NTPDase3 overexpression in BM-MSCs may be a clinical surrogate of the osteogenic differentiation impairment in Pm women. Thus, besides P2X7 and P2Y₆ receptors activation, targeting NTPDase3 may represent a novel therapeutic strategy to increase bone mass and reduce the osteoporotic risk of fractures in Pm women.

P2X7Rs: new therapeutic targets for osteoporosis

Increasing evidence suggests that both the occurrence and progression of osteoporosis are associated with inflammation, especially in primary osteoporosis. The maintenance of skeletal homeostasis is dependent on the complex regulation of bone metabolism. Numerous evidence suggested that purinoceptor networks are essential for bone homeostasis. In this review, the relationship between inflammation and the development of osteoporosis and the role of P2X7 receptor (P2X7R) in regulating the dynamic regulation of bone reconstruction were covered. We also discussed how P2X7R regulates the balance between resorption and bone formation by osteoblasts and reviewed the relevance of P2X7R polymorphisms in skeletal physiology. Finally, we analyzed potential targets of P2X7R for osteoporosis.

Calcium-Permeable Channels Cooperation for Rheumatoid Arthritis: Therapeutic Opportunities

Rheumatoid arthritis is a common autoimmune disease that results from the deposition of antibodies–autoantigens in the joints, leading to long-lasting inflammation. The main features of RA include cartilage damage, synovial invasion and flare-ups of intra-articular inflammation, and these pathological processes significantly reduce patients’ quality of life. To date, there is still no drug target that can act in rheumatoid arthritis. Therefore, the search for novel drug targets has become urgent. Due to their unique physicochemical properties, calcium ions play an important role in all cellular activities and the body has evolved a rigorous calcium signaling system. Calcium-permeable channels, as the main operators of calcium signaling, are widely distributed in cell membranes, endoplasmic reticulum membranes and mitochondrial membranes, and mediate the efflux and entry of Ca2+. Over the last century, more and more calcium-permeable channels have been identified in human cells, and the role of this large family of calcium-permeable channels in rheumatoid arthritis has gradually become clear. In this review, we briefly introduce the major calcium-permeable channels involved in the pathogenesis of RA (e.g., acid-sensitive ion channel (ASIC), transient receptor potential (TRP) channel and P2X receptor) and explain the specific roles and mechanisms of these calcium-permeable channels in the pathogenesis of RA, providing more comprehensive ideas and targets for the treatment of RA.

Clonal osteoblastic cell lines with CRISPR/Cas9-mediated ablation of Pit1 or Pit2 show enhanced mineralization despite reduced osteogenic gene expression

Multiple actions of extracellular Pi on the skeletal cells are likely to be partly mediated by type III sodium/phosphate (Na⁺/Pi) cotransporters Pit1 and Pit2, although the details are not fully understood. In the current study, to determine the roles of Pit1 and Pit2 in osteoblasts, we generated Pit1-knockout (KO) and Pit2-KO osteoblastic cells by applying CRISPR/Cas9 genome editing to an osteoblastic cell line MC3T3-E1 subclone 4. The extracellular Pi level was increased in the Pit1-KO and Pit2-KO clones due to the reduced Pi uptake. Interestingly, in vitro mineralization was accelerated in the Pit1-KO and Pit2-KO clones, although the induction of the expression of osteogenic marker genes was suppressed. In the cells before mineralization, extracellular levels of pyrophosphate (PPi) and adenosine triphosphate (ATP) were increased in the Pit1-KO and Pit2-KO clones, which might be attributable to the reduced expression and activity of tissue-nonspecific alkaline phosphatase (TNSALP). A 24-h treatment with high Pi reduced the expression and activity of TNSALP, suggesting that the suppression of TNSALP in the Pit1-KO and Pit2-KO clones was caused by the increased availability of extracellular Pi. Lentiviral gene transfer of Pit1 and Pit2 restored the changes observed in Pit1-KO and Pit2-KO clones, respectively. The expressions of P2Y2 and P2X7 which encode receptors for extracellular ATP were altered in the Pit1-KO and Pit2-KO clones, suggesting an influence on purinergic signaling. In mineralized cells after long-term culture, intracellular levels of PPi and ATP were higher in the Pit1-KO and Pit2-KO clones. Taken together, ablation of Pit1 or Pit2 in this osteoblastic cell model led to accelerated mineralization, suppressed TNSALP and altered the levels of extracellular and intracellular PPi and ATP, which might be partly mediated by changes in the availability of extracellular Pi.

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

During bone resorption, the osteoclast must sustain an extraordinarily low pH environment, withstand immense ionic pressures, and coordinate nutrient and waste exchange across its membrane to sustain its unique structural and functional polarity. To achieve this, osteoclasts are equipped with an elaborate set of membrane transport proteins (pumps, transporters and channels) that serve as molecular ‘gatekeepers’ to regulate the bilateral exchange of ions, amino acids, metabolites and macromolecules across the ruffled border and basolateral domains. Whereas the importance of the vacuolar-ATPase proton pump and chloride voltage-gated channel 7 in osteoclasts has long been established, comparatively little is known about the contributions of other membrane transport proteins, including those categorized as secondary active transporters. In this Special Issue review, we provide a contemporary update on the ‘ins and outs’ of membrane transport proteins implicated in osteoclast differentiation, function and bone homeostasis and discuss their therapeutic potential for the treatment of metabolic bone diseases.

Extracellular purines and bone homeostasis

Maintenance of a healthy skeleton is highly dependent on an intricate regulation of bone metabolism, as changes in the balance between bone formation and bone resorption leads to bone loss, bone fragility and ultimately bone fractures. During the last three decades it has become increasingly evident that physiological release of purines in the extracellular space is imperative for bone homeostasis and is orchestrated via the network of purinoceptors. Adenosine derivatives are released locally in the skeleton either by the bone forming osteoblasts or the bone degrading osteoclasts actioned directly by processes like mechanical loading and indirectly by systemic hormones. Adenosine derivatives directly affect the bone cells by their action on the membranal receptors or have co-stimulatory actions with bone active hormones such as parathyroid hormone or the gut hormones. Any deviations leading to increased levels of extracellular adenosine derivatives in the bone tissue such as in pathologic situations, trigger complex pathways with opposing effects on tissue health as presented by studies involving a range of model organisms. Pathological conditions where skeletal purinergic signaling is affected are following tissue injury like microdamage and macroscopic fractures; and during inflammatory processes where nucleotides and nucleosides play an important part in the pathophysiological skeletal response. Moreover, adenosine derivatives also play an important role in the interaction between malignant cells and bone cells in several types of cancers involving the skeleton, such as but not limited to multiple myeloma and bone osteolysis. Much knowledge has been gained over the last decades. The net- resulting phenotype of adenosine derivatives in bone (including the ratio of ATP to Adenosine) is highly dependent on CD39 and CD73 enzymes together with the expression and activity of the specific receptors. Thus, each component is important in the physiological and pathophysiological processes in bone. Promising perspectives await in the future in treating skeletal disorders with medications targeting the individual components of the purinergic signaling pathway.

P2X7 receptor and the NLRP3 inflammasome: Partners in crime

Adenosine triphosphate (ATP) is a molecule that on one hand plays a central role in cellular energetics and which on the other is a ubiquitous signaling molecule when released into the extracellular media. Extracellular ATP accumulates in inflammatory environments where it acts as a damage-associated molecular pattern and activates the purinergic P2X receptor 7 (P2X7) in immune cells. P2X7 receptor activation induces the formation of the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing 3 (NLRP3) inflammasome and the activation of the inflammatory caspase-1. Caspase-1 causes an inflammatory type of cell death called pyroptosis through the release of pro-inflammatory cytokines and intracellular content. Consequently, intense research efforts have been devoted to the design of novel anti-inflammatory therapies, focusing in particular on the P2X7 receptor and the NLRP3 pathway and the introduction of new blocking molecules in early phase clinical trials.

Association between the rs3751143 polymorphism of P2RX7 gene and chronic lymphocytic leukemia: A meta-analysis

OBJECTIVES

Meta-analysis was used to determine the association between rs3751143 polymorphism of P2RX7 gene and the risk of chronic lymphocytic leukemia (CLL).

METHODS

Search for published articles about the association between the rs3751143 and CLL in PubMed, MEDINE, Web of Science, and Embase databases, with a calculated odds ratio of (OR) and 95% confidence interval (95%CI).

RESULTS

A total of 1184 cases and 1725 controls in 8 studies were pooled together for evaluation of the overall association between rs3751143 and risk of CLL. Allele model (A vs C, p = 0.16, OR = 0.85, 95%CI = 0.71-1.17), homozygous model (AA vs CC, p = 0.07; OR = 0.78, 95%CI = 0.84-1.08), and heterozygous model (AC vs CC, p = 0.76; OR = 0.85; 95%CI = 0.68-0.79) did not show decreased risk of developing CLL. Similarly, dominant model (AA + AC vs. CC: p = 0.58; OR = 1.10, 95%CI = 0.69-1.75), and recessive model (AA vs AC + CC, p = 0.21, OR = 1.18, 95%CI = 0.70-1.99) failed to show decreased risk of developing CLL. However, in familial, heterozygous model (AC vs. CC: p = 0.0006, OR = 0.64, 95%CI = 0.67-1.50) and recessive model (AA vs. AC + CC: p = 0.0017; OR = 1.02, 95%CI = 0.73-2.35) indicated the association between the inheritance of rs3751143 and the risk of developing CLL. In the overall survival prognosis, no significant association between rs3751143 and CLL was detected with relatively high heterogeneity.

CONCLUSIONS

Our pooled data indicates that there is a correlation between the inheritance of rs3751143 and the risk of CLL in familial.

Bone's Response to Mechanical Loading in Aging and Osteoporosis: Molecular Mechanisms

Mechanotransduction is pivotal in the maintenance of homeostasis in different tissues and involves multiple cell signaling pathways. In bone, mechanical stimuli regulate the balance between bone formation and resorption; osteocytes play a central role in this regulation. Dysfunctions in mechanotransduction signaling or in osteocytes response lead to an imbalance in bone homeostasis. This alteration is very relevant in some conditions such as osteoporosis and aging. Both are characterized by increased bone weakness due to different causes, for example, the increase of osteocyte apoptosis that cause an alteration of fluid space, or the alteration of molecular pathways. There are intertwined yet very different mechanisms involved among the cell-intrinsic effects of aging on bone, the cell-intrinsic and tissue-level effects of estrogen/androgen withdrawal on bone, and the effects of reduced mechanical loading on bone, which are all involved to some degree in how aged bone fails to respond properly to stress/strain compared to younger bone. This review aims at clarifying how the cellular and molecular pathways regulated and induced in bone by mechanical stimulation are altered with aging and in osteoporosis, to highlight new possible targets for antiresorptive or anabolic bone therapeutic approaches.

Pitfalls and challenges of the purinergic signaling cascade in obesity

Obesity is a worldwide health problem which have reached pandemic proportions, now also including low and middle-income countries. Excessive or abnormal fat deposition in the abdomen especially in the visceral compartment is tightly associated with a high metabolic risk for arterial hypertension, type II diabetes, cardiovascular diseases, musculoskeletal disorders (especially articular degeneration) and some cancers. Contrariwise, accumulation of fat in the subcutaneous compartment has been associated with a neutral metabolic impact, favoring a lower risk of insulin resistance. Obesity results more often from an avoidable imbalance between food consumption and energy expenditure. There are several recommended strategies for dealing with obesity, including pharmacological therapies, but their success remains incomplete and may not compensate the associated adverse effects. Purinergic signaling operated by ATP and its metabolite, adenosine, has attracted increasing attention in obesity. The extracellular levels of purines often reflect the energy status of a given cell population. Adenine nucleotides and nucleosides fine tuning control adipogenesis and mature adipocytes function via the activation of P2 and P1 purinoceptors, respectively. These features make the purinergic signaling cascade a putative target for therapeutic intervention in obesity and related metabolic syndromes. There are, however, gaps in our knowledge regarding the role of purines in adipocyte precursors differentiation and mature adipocytes functions, as well as their impact among distinct adipose tissue deposits (e.g. white vs. brown, visceral vs. subcutaneous), which warrants further investigations before translation to clinical trials can be made.

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.

Adult mesenchymal stem cells: is there a role for purine receptors in their osteogenic differentiation?

The role played by mesenchymal stem cells (MSCs) in contributing to adult tissue homeostasis and damage repair thanks to their differentiation capabilities has raised a great interest, mainly in bone regenerative medicine. The growth/function of these undifferentiated cells of mesodermal origin, located in specialized structures (niches) of differentiated organs is influenced by substances present in this microenvironment. Among them, ancestral and ubiquitous molecules such as adenine-based purines, i.e., ATP and adenosine, may be included. Notably, extracellular purine concentrations greatly increase during tissue injury; thus, MSCs are exposed to effects mediated by these agents interacting with their own receptors when they act/migrate in vivo or are transplanted into a damaged tissue. Here, we reported that ATP modulates MSC osteogenic differentiation via different P2Y and P2X receptors, but data are often inconclusive/contradictory so that the ATP receptor importance for MSC physiology/differentiation into osteoblasts is yet undetermined. An exception is represented by P2X7 receptors, whose expression was shown at various differentiation stages of bone cells resulting essential for differentiation/survival of both osteoclasts and osteoblasts. As well, adenosine, usually derived from extracellular ATP metabolism, can promote osteogenesis, likely via A2B receptors, even though findings from human MSCs should be implemented and confirmed in preclinical models. Therefore, although many data have revealed possible effects caused by extracellular purines in bone healing/remodeling, further studies, hopefully performed in in vivo models, are necessary to identify defined roles for these compounds in favoring/increasing the pro-osteogenic properties of MSCs and thereby their usefulness in bone regenerative medicine.

Calcium Homeostasis: A Potential Vicious Cycle of Bone Metastasis in Breast Cancers

Cancers have been considered as one of the most severe health problems in the world. Efforts to elucidate the cancer progression reveal the importance of bone metastasis for tumor malignancy, one of the leading causes for high mortality rate. Multiple cancers develop bone metastasis, from which breast cancers exhibit the highest rate and have been well-recognized. Numerous cells and environmental factors have been believed to synergistically facilitate bone metastasis in breast cancers, from which breast cancer cells, osteoclasts, osteoblasts, and their produced cytokines have been well-recognized to form a vicious cycle that aggravates tumor malignancy. Except the cytokines or chemokines, calcium ions are another element largely released from bones during bone metastasis that leads to hypercalcemia, however, have not been well-characterized yet in modulation of bone metastasis. Calcium ions act as a type of unique second messenger that exhibits omnipotent functions in numerous cells, including tumor cells, osteoclasts, and osteoblasts. Calcium ions cannot be produced in the cells and are dynamically fluxed among extracellular calcium pools, intracellular calcium storages and cytosolic calcium signals, namely calcium homeostasis, raising a possibility that calcium ions released from bone during bone metastasis would further enhance bone metastasis and aggravate tumor progression via the vicious cycle due to abnormal calcium homeostasis in breast cancer cells, osteoclasts and osteoblasts. TRPs, VGCCs, SOCE, and P2Xs are four major calcium channels/routes mediating extracellular calcium entry and affect calcium homeostasis. Here we will summarize the overall functions of these four calcium channels in breast cancer cells, osteoclasts and osteoblasts, providing evidence of calcium homeostasis as a vicious cycle in modulation of bone metastasis in breast cancers.

Altered donor P2X7 activity in human leukocytes correlates with P2RX7 genotype but does not affect the development of graft-versus-host disease in humanised mice

Graft-versus-host disease (GVHD) is a life-threatening consequence of allogeneic haematopoietic stem cell transplantation, a curative therapy for haematological malignancies. The ATP-gated P2X7 receptor channel is implicated in the development of GVHD. P2X7 activity on human leukocytes can be influenced by gain-of-function (GOF) and loss-of-function (LOF) single nucleotide polymorphisms (SNPs) in the P2RX7 gene. In this study, the P2RX7 gene was sequenced in 25 human donors and the P2X7 activity on subsets of peripheral blood T cells, natural killer (NK) cells and monocytes was measured using an ATP-induced dye uptake assay. GOF and LOF SNPs representing 10 of the 17 known P2RX7 haplotypes were identified, and correlated with P2X7 activity on all leukocyte subsets investigated. Notably, invariant (i) NK T cells displayed the highest P2X7 activity amongst all cell types studied. To determine if donor P2X7 activity influenced the development of GVHD, immunodeficient NOD-SCID-IL2Rγ^null (NSG) mice were injected with human peripheral blood mononuclear cells isolated from donors of either GOF (hP2X7^GOF mice) or LOF (hP2X7^LOF mice) P2RX7 genotype. Both hP2X7^GOF and hP2X7^LOF mice demonstrated similar human leukocyte engraftment, and showed comparable weight loss, GVHD clinical score and overall survival. Donor P2X7 activity did not affect human leukocyte infiltration or GVHD-mediated tissue damage, or the relative expression of human P2X7 or human interferon-γ (hIFNγ) in tissues. Finally, hP2X7^GOF and hP2X7^LOF mice demonstrated similar concentrations of serum hIFNγ. This study demonstrates that P2X7 activity correlates with donor P2RX7 genotype on human leukocyte subsets important in GVHD development, but does not affect GVHD development in a humanised mouse model of this disease.

P2X7, a critical regulator and potential target for bone and joint diseases

Abundant evidence indicted that P2X7 receptor show a essential role in human health and some human diseases including hypertension, atherosclerosis, pulmonary inflammation, tuberculosis infection, psychiatric disorders, and cancer. P2X7 receptor also has an important role in some central nervous system diseases such as neurodegenerative disorders. Recently, more research suggested that P2X7 receptor also plays a crucial role in bone and joint diseases. But the effect of P2X7 receptor on skeletal and joint diseases has not been systematically reviewed. In this article, the role of P2X7 receptor in skeletal and joint diseases is elaborated. The activation of P2X7 receptor can ameliorate osteoporosis by inducing a fine balance between osteoclastic resorption and osteoblastic bone formation. The activation of P2X7 receptor can relieve the stress fracture injury by increasing the response to mechanical loading and inducing osteogenesis. But the activation of P2X7 receptor mediates the cell growth and cell proliferation in bone cancer. In addition, the activation of P2X7 receptor can aggravate the process of some joint diseases such as osteoarthritis, rheumatoid arthritis, and acute gouty arthritis. The inhibition of P2X7 receptor can alleviate the pathological process of joint disease to some extent. In conclusion, P2X7 receptor may be a critical regulator and therapeutic target for bone and joint diseases.

P2X7 Receptor: A Potential Therapeutic Target for Depression?

Depression is a prime contributor to global disease burden with 300 million affected patients worldwide. The persistent lack of progress with regards to pharmacotherapy stands in stark contrast to the pandemic magnitude of the disease. Alterations of inflammatory pathways in depressed patients, including altered circulating pro-inflammatory cytokines, have been put forward as a potential pathophysiological mechanism. The P2X7 receptor (P2X7R) plays an important role regulating the release of interleukin-1β and other cytokines. Comprehensive investigation of the P2X7R Gln460Arg missense mutation (rs2230912), which has been associated with major depression and bipolar disorder, has substantially contributed to validate P2X7R as a potential genetic risk factor. We propose that P2X7R is a putative target with good prospects for therapeutic intervention in depressive disorders.

Genetic interaction of purinergic P2X7 receptor and ER-α polymorphisms in susceptibility to osteoporosis in Chinese postmenopausal women

Osteoporosis (OP) is an increasing public health problem worldwide. Genetic factors are considered to be major contributors to the pathogenesis of OP. The aim of this study was to investigate the association of the purinergic P2X7 receptor (P2X7R) and estrogen receptor-α (ER-α) genes with OP risk, and the effect of the possible interaction between the two genes on predisposition to OP in Chinese postmenopausal women. A total of 596 subjects, including 350 OP patients and 246 controls, were recruited in this case-control study. Five functional single-nucleotide polymorphisms (SNPs) in the P2X7R gene (rs2393799, rs7958311, rs1718119, rs2230911, rs3751143) and two ER-α PvuII and XbaI polymorphisms were genotyped and analyzed. Single-gene variant analysis showed that the carriers of the CC genotype of P2X7R rs3751143 revealed an increased OP risk. Haplotype rs1718119G-rs2230911G-rs3751143C also appeared to be a significant 'risk' haplotype with OP. For the ER-α gene, no evidence of significant association of PvuII or XbaI polymorphism with OP risk was found. Moreover, there was a significant gene-gene interaction between P2X7R rs3751143 and ER-α PvuII; the cross-validation consistency was 10/10 and the testing accuracy was 0.5818 (P = 0.0107). A 1.67-fold-increased risk for OP was detected in individuals carrying the genotypes of AC or CC of rs3751143 and Pp or PP of PvuII compared to subjects with AA of rs3751143 and pp of PvuII. Our findings suggest an important association of the P2X7R rs3751143CC genotype and the rs1718119G-rs2230911G-rs3751143C haplotype with an increased OP risk. Also, the P2X7R rs3751143 and ER-α PvuII two-locus interaction confers a significantly high susceptibility to OP in Chinese postmenopausal women.

The P2X7 receptor and pannexin-1 are involved in glucose-induced autocrine regulation in β-cells

Extracellular ATP is an important short-range signaling molecule that promotes various physiological responses virtually in all cell types, including pancreatic β-cells. It is well documented that pancreatic β-cells release ATP through exocytosis of insulin granules upon glucose stimulation. We hypothesized that glucose might stimulate ATP release through other non-vesicular mechanisms. Several purinergic receptors are found in β-cells and there is increasing evidence that purinergic signaling regulates β-cell functions and survival. One of the receptors that may be relevant is the P2X7 receptor, but its detailed role in β-cell physiology is unclear. In this study we investigated roles of the P2X7 receptor and pannexin-1 in ATP release, intracellular ATP, Ca²⁺ signals, insulin release and cell proliferation/survival in β-cells. Results show that glucose induces rapid release of ATP and significant fraction of release involves the P2X7 receptor and pannexin-1, both expressed in INS-1E cells, rat and mouse β-cells. Furthermore, we provide pharmacological evidence that extracellular ATP, via P2X7 receptor, stimulates Ca²⁺ transients and cell proliferation in INS-1E cells and insulin secretion in INS-1E cells and rat islets. These data indicate that the P2X7 receptor and pannexin-1 have important functions in β-cell physiology, and should be considered in understanding and treatment of diabetes.

Pore forming channels as a drug delivery system for photodynamic therapy in cancer associated with nanoscintillators

According to the World Health Organization (WHO), cancer is one of main causes of death worldwide, with 8.2 million people dying from this disease in 2012. Because of this, new forms of treatments or improvement of current treatments are crucial. In this regard, Photodynamic therapy (PDT) has been used to successfully treat cancers that can be easily accessed externally or by fibre-optic endoscopes, such as skin, bladder and esophagus cancers. In addition, this therapy can used alongside radiotherapy and chemotherapy in order to kill cancer cells. The main problem in implementing PDT is penetration of visible light deeper than 10 mm in tissues, due to scattering and absorption by tissue chromophores. Unfortunately, this excludes several internal organs affected by cancer. Another issue in this regard is the use of a selective cancer cell-photosensitizing compound. Nevertheless, several groups have recently developed scintillation nanoparticles, which can be stimulated by X-rays, thereby making this a possible solution for light production in deeper tissues. Alternative approaches have also been developed, such as photosensitizer structure modifications and cell membrane permeabilizing agents. In this context, certain channels lead to transitory plasma membrane permeability changes, such as pannexin, connexin hemmichannels, TRPV1-4 and P2×7, which allow for the non-selective passage of molecules up to 1,000 Da. Herein, we discuss the particular case of the P2×7 receptor-associated pore as a drug delivery system for hydrophilic substances to be applied in PDT, which could also be carried out with other channels. Methylene blue (MB) is a low cost dye used as a prototype photosensitizer, approved for clinical use in several other clinical conditions, as well as photodynamic therapy for fungi infections.

The purinergic P2X7 ion channel receptor-a 'repair' receptor in bone

A strong skeleton relies on adaptation to varying physical demands and on maintenance of the bone tissue in order to avoid accumulation of micro-damage. In bone, the purinergic P2X7 ion channel receptor is expressed on both cells of the stromal lineage such as the bone forming osteoblasts and the mechano-sensing osteocytes and on cells belonging to the immune-related monocyte-macrophage lineage, the bone resorbing osteoclasts. Recent studies have demonstrated that the receptor plays important roles in the anabolic responses to mechanical loading on bone and, together with the pannexin1 hemi-channel, in the process of initiating bone remodeling in response to micro-damage. Thus, the receptor is crucial in skeletal mechano-transduction and in the continuous repair process. However, under pathophysiological conditions such as diabetes with high glucose concentrations or glucocorticoid-treatment the receptor is downregulated thus blunting the important effects it has for maintaining a healthy and strong skeleton.

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.

P2X7 receptor regulates osteoclast function and bone loss in a mouse model of osteoporosis

Post-menopausal osteoporosis is a condition that affects millions worldwide and places a huge socio-economic burden on society. Previous research has shown an association of loss of function SNPs in the gene for the purinergic receptor P2X7R with low bone mineral density, increased rates of bone loss and vertebral fractures in post-menopausal women. In this study we use a mouse model of oestrogen deficiency-induced bone loss and the BALB/cJ P2X7R^-/- to show that absence of the P2X7R resulted in increased bone loss. Osteoclast precursors were isolated from both BALB/cJ P2X7R^-/- and BALB/cJ P2X7R^+/+ mice and then cultured in vitro to form mature resorbing osteoclasts. The BALB/cJ P2X7R^-/- derived precursors generated slightly more osteoclasts but with a significant reduction in the amount of resorption per osteoclast. Furthermore, when using modified culture conditions osteoclast activity was additionally increased in the absence of the P2X7R suggest that P2X7R may regulate the lifespan and activity of osteoclasts. Finally using mechanical loading as an anabolic stimulus for bone formation, we demonstrated that the increased oestrogen-deficient bone loss could be rescued, even in the absence of P2X7R. This study paves the way for clinical intervention for women with post-menopausal osteoporosis and P2XR7 loss of function 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.

Osteoblast migration in vertebrate bone

Bone formation, for example during bone remodelling or fracture repair, requires mature osteoblasts to deposit bone with remarkable spatial precision. As osteoblast precursors derive either from circulation or resident stem cell pools, they and their progeny are required to migrate within the three-dimensional bone space and to navigate to their destination, i.e. to the site of bone formation. An understanding of this process is emerging based on in vitro and in vivo studies of several vertebrate species. Receptors on the osteoblast surface mediate cell adhesion and polarization, which induces osteoblast migration. Osteoblast migration is then facilitated along gradients of chemoattractants. The latter are secreted or released proteolytically by several cell types interacting with osteoblasts, including osteoclasts and vascular endothelial cells. The positions of these cellular sources of chemoattractants in relation to the position of the osteoblasts provide the migrating osteoblasts with tracks to their destination, and osteoblasts possess the means to follow a track marked by multiple chemoattractant gradients. In addition to chemotactic cues, osteoblasts sense other classes of signals and utilize them as landmarks for navigation. The composition of the osseous surface guides adhesion and hence migration efficiency and can also provide steering through haptotaxis. Further, it is likely that signals received from surface interactions modulate chemotaxis. Besides the nature of the surface, mechanical signals such as fluid flow may also serve as navigation signals for osteoblasts. Alterations in osteoblast migration and navigation might play a role in metabolic bone diseases such as osteoporosis.

Purinergic P2X7 receptor functional genetic polymorphisms are associated with the susceptibility to osteoporosis in Chinese postmenopausal women

Osteoporosis (OP) is a major public health problem worldwide. Genetic factors are considered to be major contributors to the pathogenesis of OP. The purinergic P2X7 receptor (P2X7R) has been shown to play a role in the regulation of osteoblast and osteoclast activity and has been considered as an important candidate gene for OP. A case-control study was performed to investigate the associations of functional single nucleotide polymorphisms (SNPs) in the P2X7R gene (rs2393799, rs7958311, rs1718119, rs2230911, and rs3751143) with susceptibility to OP in 400 Chinese OP patients and 400 controls. Results showed that rs3751143 was associated with OP; in particular, carriers of the C allele and CC/(AC + CC) genotypes were at a higher risk of OP, but no significant association of rs2230911, rs7958311, rs1718119, and rs2393799 with OP risk was observed. Analysis of the haplotypes revealed one haplotype (rs1718119G-rs2230911G-rs3751143C) that appeared to be a significant "risk" haplotype with OP. The rs3751143 polymorphism was associated with osteoclast apoptosis; ATP-induced caspase-1 activity of osteoclasts with AC and CC genotypes is lower than that of osteoclasts with AA genotype in vitro. The findings suggest that the P2X7R rs3751143 functional polymorphism might contribute to OP susceptibility in Chinese postmenopausal women.

P2X7Rs are involved in cell death, growth and cellular signaling in primary human osteoblasts

The ionotropic ATP-gated P2X7 receptor (P2X7R) is involved in the regulation of many physiological functions including bone metabolism. Several studies on osteoblasts from rodents and human osteoblast-like cell lines have addressed the expression and function of P2X7R on these bone-forming cells however; its role in human primary osteoblasts has not yet been reported. The aim of this study was to assess the expression of the P2X7R in bone marrow-derived stromal cells and in primary human trabecular osteoblasts and to determine the function in bone formation and cell signaling. We report that osteoblasts derived from human trabecular explants express a functional P2X7R capable of agonist-induced increase in intracellular calcium concentration and a positive permeability to fluorescent dyes. These osteoblasts are fully differentiated cells with alkaline phosphatase activity and the ability to form mineralized nodules. We show that the transcriptional regulation of osteoblastic markers can be modulated by P2X7R activity or blockade thereby influencing the differentiation, proliferation and bone matrix formation by these primary human osteoblasts. Finally, we demonstrate that the P2X7R is involved in propagation of mechanically-induced intercellular signaling in addition to the known mechanisms involving calcium signaling via P2Y2 receptors and gap junction.

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.

Association Between P2RX7 Gene and Hepatocellular Carcinoma Susceptibility: A Case-Control Study in a Chinese Han Population

Background

Hepatocellular carcinoma (HCC) is one of the most common types of liver cancer. It is hypothesized that P2RX7 genetic polymorphisms have strong association with HCC susceptibility. Therefore, a case-control study was designed and performed to verify the association between P2RX7 gene polymorphisms and HCC susceptibility.

Material/Methods

A total of 646 subjects were recruited in our study, including 323 HCC patients and 323 healthy controls. Five gene polymorphisms, −762C>T (rs2393799), 946G>A (rs28360457), 1513A>C (rs3751143), 1068G>A (rs1718119), and 1096C>G (rs2230911), were selected. Odds ratio (ORs) and 95% confidence interval (CI) were used to quantify the association between P2RX7 gene polymorphisms and the susceptibility to HCC. All tests were performed using SPSS 20 and a 2-sided P value of less than 0.05 was considered to be statistically significant.

Results

Our results suggest that allelic frequencies of these 5 SNPs all conformed to Hardy-Weinberg equilibrium (HWE). There was no significant difference in genotype and allele distributions of −762C>T and 1096C>G between the case group and the control group. However, an increased risk of HCC was associated with 946G>A (A vs. G: OR=1.48, 95%CI=1.09–2.01, P=0.013; GA+AA vs. GG: OR=1.46, 95%CI=1.03–2.07, P=0.033). A similar increased risk was associated with 1513A>C polymorphism (C vs. A: OR=1.37, 95%CI=1.05–1.79, P=0.021; AC+CC vs. AA: OR=1.40, 95%CI=1.01–1.93, P=0.041). On the other hand, a decreased risk of HCC was associated with gene polymorphism of 1068G>A (A vs. G: OR=0.68, 95%CI=0.51–0.91, P=0.010; GA+AA vs. GG: OR=0.68, 95%CI=0.49–0.96, P=0.027; AA vs. GG: OR=0.42, 95%CI=0.18–0.99, P=0.048).

Conclusions

Our results suggest that 3 of the 5 polymorphisms of P2RX7 described above (1513A>C, 946G>A, and 1068G>A) are significantly associated with HCC susceptibility in a Chinese Han population. Studies with larger sample sizes are recommended to confirm whether our results will be applicable to different ethnic populations in China.

Bone-associated gene evolution and the origin of flight in birds

BACKGROUND

Bones have been subjected to considerable selective pressure throughout vertebrate evolution, such as occurred during the adaptations associated with the development of powered flight. Powered flight evolved independently in two extant clades of vertebrates, birds and bats. While this trait provided advantages such as in aerial foraging habits, escape from predators or long-distance travels, it also imposed great challenges, namely in the bone structure.

RESULTS

We performed comparative genomic analyses of 89 bone-associated genes from 47 avian genomes (including 45 new), 39 mammalian, and 20 reptilian genomes, and demonstrate that birds, after correcting for multiple testing, have an almost two-fold increase in the number of bone-associated genes with evidence of positive selection (~52.8 %) compared with mammals (~30.3 %). Most of the positive-selected genes in birds are linked with bone regulation and remodeling and thirteen have been linked with functional pathways relevant to powered flight, including bone metabolism, bone fusion, muscle development and hyperglycemia levels. Genes encoding proteins involved in bone resorption, such as TPP1, had a high number of sites under Darwinian selection in birds.

CONCLUSIONS

Patterns of positive selection observed in bird ossification genes suggest that there was a period of intense selective pressure to improve flight efficiency that was closely linked with constraints on body size.

Adenosine Triphosphate stimulates differentiation and mineralization in human osteoblast-like Saos-2 cells

In the last years adenosine triphosphate (ATP) and subsequent purinergic system activation through P2 receptors were investigated highlighting their pivotal role in bone tissue biology. In osteoblasts ATP can regulate several activities like cell proliferation, cell death, cell differentiation and matrix mineralization. Since controversial results exist, in this study we analyzed the ATP effects on differentiation and mineralization in human osteoblast-like Saos-2 cells. We showed for the first time the altered functional activity of ATP receptors. Despite that, we found that ATP can reduce cell proliferation and stimulate osteogenic differentiation mainly in the early stages of in vitro maturation as evidenced by the enhanced expression of alkaline phosphatase (ALP), Runt-related transcription factor 2 (Runx2) and Osteocalcin (OC) genes and by the increased ALP activity. Moreover, we found that ATP can affect mineralization in a biphasic manner, at low concentrations ATP always increases mineral deposition while at high concentrations it always reduces mineral deposition. In conclusion, we show the osteogenic effect of ATP on both early and late stage activities like differentiation and mineralization, for the first time in human osteoblastic cells.

Mesenchymal Stem Cells Ageing: Targeting the "Purinome" to Promote Osteogenic Differentiation and Bone Repair

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into bone forming cells. Such ability is compromised in elderly individuals resulting in bone disorders such as osteoporosis, also limiting their clinical usage for cell transplantation and bone tissue engineering strategies. In bone marrow niches, adenine and uracil nucleotides are important local regulators of osteogenic differentiation of MSCs. Nucleotides can be released to the extracellular milieu under both physiological and pathological conditions via (1) membrane cell damage, (2) vesicle exocytosis, (3) ATP-binding cassette transporters, and/or (4) facilitated diffusion through maxi-anion channels, hemichannels or ligand-gated receptor pores. Nucleotides and their derivatives act via adenosine P1 (A1 , A2A , A2B , and A3 ) and nucleotide-sensitive P2 purinoceptors comprising ionotropic P2X and G-protein-coupled P2Y receptors. Purinoceptors activation is terminated by membrane-bound ecto-nucleotidases and other ecto-phosphatases, which rapidly hydrolyse extracellular nucleotides to their respective nucleoside 5'-di- and mono-phosphates, nucleosides and free phosphates, or pyrophosphates. Current knowledge suggests that different players of the "purinome" cascade, namely nucleotide release sites, ecto-nucleotidases and purinoceptors, orchestrate to fine-tuning regulate the activity of MSCs in the bone microenvironment. Increasing studies, using osteoprogenitor cell lines, animal models and, more recently, non-modified MSCs from postmenopausal women, raised the possibility to target chief components of the purinergic signaling pathway to regenerate the ability of aged MSCs to differentiate into functional osteoblasts. This review summarizes the main findings of those studies, prompting for novel therapeutic strategies to control ageing disorders where bone destruction exceeds bone formation, like osteoporosis, rheumatoid arthritis, and fracture mal-union. J. Cell. Physiol. 231: 1852-1861, 2016. © 2016 Wiley Periodicals, Inc.

Functional polymorphisms in the P2X7 receptor gene are associated with stress fracture injury

Military recruits and elite athletes are susceptible to stress fracture injuries. Genetic predisposition has been postulated to have a role in their development. The P2X7 receptor (P2X7R) gene, a key regulator of bone remodelling, is a genetic candidate that may contribute to stress fracture predisposition. The aim of this study is to evaluate the putative contribution of P2X7R to stress fracture injury in two separate cohorts, military personnel and elite athletes. In 210 Israeli Defense Forces (IDF) military conscripts, stress fracture injury was diagnosed (n = 43) based on symptoms and a positive bone scan. In a separate cohort of 518 elite athletes, self-reported medical imaging scan-certified stress fracture injuries were recorded (n = 125). Non-stress fracture controls were identified from these cohorts who had a normal bone scan or no history or symptoms of stress fracture injury. Study participants were genotyped for functional SNPs within the P2X7R gene using proprietary fluorescence-based competitive allele-specific PCR assay. Pearson's chi-squared (χ (2)) tests, corrected for multiple comparisons, were used to assess associations in genotype frequencies. The variant allele of P2X7R SNP rs3751143 (Glu496Ala-loss of function) was associated with stress fracture injury, whilst the variant allele of rs1718119 (Ala348Thr-gain of function) was associated with a reduced occurrence of stress fracture injury in military conscripts (P < 0.05). The association of the variant allele of rs3751143 with stress fractures was replicated in elite athletes (P < 0.05), whereas the variant allele of rs1718119 was also associated with reduced multiple stress fracture cases in elite athletes (P < 0.05). The association between independent P2X7R polymorphisms with stress fracture prevalence supports the role of a genetic predisposition in the development of stress fracture injury.

P2X7 Receptor as a Therapeutic Target

P2X7 receptor is an ATP-gated cation channel that upon agonist interaction leads to cellular influx of Na(+) and Ca(2+) and efflux of K(+). P2X7 is expressed by a wide variety of cells and its activation mediates a large number of biological processes like inflammation, neuromodulation, cell death or cell proliferation and it has been associated to related pathological conditions including infectious, inflammatory, autoimmune, neurological, and musculoskeletal disorders and, in the last years, to cancer. This chapter describes structural features of P2X7, chemical properties of its agonist, antagonist, and allosteric modulators and summarizes recent advances on P2X7 receptor as therapeutic target in the aforementioned diseases. We also give an overview on recent literature suggesting that P2X7 single-nucleotide polymorphisms could be exploited as diagnostic biomarkers for the development of tailored therapies.

The role of the P2X7 receptor on bone loss in a mouse model of inflammation-mediated osteoporosis

In inflammatory autoimmune diseases, bone loss is frequent. In most cases, secondary osteoporosis is caused by treatment with systemic glucocorticoid. However, the pathogenesis behind the bone loss is presumed multifactorial. We aimed to elucidate the role of the P2X7 receptor on bone mineral density (BMD), microarchitecture, and bone strength in a standardized mouse model of inflammation-mediated osteoporosis (IMO). In total 146 mice completed our protocol, 70 wild type (WT) mice and 76 P2X7 -/- (knockout, KO). BMD at the femur and spine decreased significantly from baseline to day 20 in the WT IMO mice (p < 0.01). In the WT vehicle, KO vehicle and KO IMO, no significant BMD changes were found. Bone strength showed a lower mid-shaft max strength (p = 0.038) and also a non-significant trend towards lower strength at the femoral neck of the WT IMO group. Trabecular bone volume fraction (BV/TV) and connectivity density (CD) after 20 days were significantly decreased in the WT IMO group (p = 0.001). In contrast, the WT vehicle and KO vehicle, BV/TV and CD did no change at 20 days. Cortical bone revealed no significant microarchitectural changes after 20 days in the WT IMO group, whereas the total cortical area increased significantly in WT vehicle and KO IMO after 20 days (5.2% and 8.8%, respectively). In conclusion, the P2X7 receptor KO mice did not respond to inflammation with loss of BMD whereas the WT mice had a significant loss of BMD, bone strength and trabecular microarchitecture, demonstrating a role for the P2X7 receptor in inflammatory bone loss.

Control of bone development by P2X and P2Y receptors expressed in mesenchymal and hematopoietic cells

Bone development and homeostasis require the interplay between several cell types, including mesenchymal osteoblasts and osteocytes, as well as hematopoietic osteoclasts. Recent evidence suggests that cell proliferation, differentiation and apoptosis of both mesenchymal and hematopoietic stem cells, which are fundamental for tissue regeneration and treatment of degenerative diseases, are controlled by P2 receptors (i.e., P2X and P2Y receptors). Both types of P2 receptors are versatile transducers of diverse signals activated by extracellular nucleotides like ATP that are released in response to tissue injury, infection or shear stress. The P2X family of receptors has been shown to mediate multiple signaling events including the influx of calcium, activation of mitogen activated protein kinases (MAPKs) and induction of AP-1 family members known to regulate bone development. Support for the significance of P2X7 in regulating bone development and homeostasis has been provided by several studies focusing on animal models and single nucleotide polymorphisms. P2 receptors are functionally expressed in both bone forming osteoblasts and bone resorbing osteoclasts, while recent findings also suggest that these receptors translate mechanical stimuli in osteocytes. Their ability to respond to external nucleotide analogs renders these cell surface proteins excellent targets for skeletal regenerative therapies. This overview summarizes mechanisms by which nucleotide receptors control skeletal cells and contribute to bone tissue development remodeling and repair.

P2X7 receptors: role in bone cell formation and function

The role of the P2X7 receptor (P2X7R) is being explored with intensive interest in the context of normal bone physiology, bone-related diseases and, to an extent, bone cancer. In this review, we cover the current understanding of P2X7R regulation of bone cell formation, function and survival. We will discuss how the P2X7R drives lineage commitment of undifferentiated bone cell progenitors, the vital role of P2X7R activation in bone mineralisation and its relatively unexplored role in osteocyte function. We also review how P2X7R activation is imperative for osteoclast formation and its role in bone resorption via orchestrating osteoclast apoptosis. Variations in the gene for the P2X7R (P2RX7) have implications for P2X7R-mediated processes and we review the relevance of these genetic variations in bone physiology. Finally, we highlight how targeting P2X7R may have therapeutic potential in bone disease and cancer.

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

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

The primary cilium as sensor of fluid flow: new building blocks to the model. A review in the theme: cell signaling: proteins, pathways and mechanisms

The primary cilium is an extraordinary organelle. For many years, it had the full attention of only a few dedicated scientists fascinated by its uniqueness. Unexpectedly, after decades of obscurity, it has moved very quickly into the limelight with the increasing evidence of its central role in the many genetic variations that lead to what are now known as ciliopathies. These studies implicated unique biological functions of the primary cilium, which are not completely straightforward. In parallel, and initially completely unrelated to the ciliopathies, the primary cilium was characterized functionally as an organelle that makes cells more susceptible to changes in fluid flow. Thus the primary cilium was suggested to function as a flow-sensing device. This characterization has been substantiated for many epithelial cell types over the years. Nevertheless, part of the central mechanism of signal transduction has not been explained, largely because of the substantial technical challenges of working with this delicate organelle. The current review considers the recent advances that allow us to fill some of the holes in the model of signal transduction in cilium-mediated responses to fluid flow and to pursue the physiological implications of this peculiar organelle.

Muscle strength, power and cardiorespiratory fitness are associated with bone mineral density in men aged 31-60 years

BACKGROUND

Osteoporotic fractures represent a growing economical burden to society, not only because of fractures in women, but also because of an increasing number of fractures in men.

AIMS

In this cross-sectional study we aimed to investigate the association of muscular and cardio-respiratory fitness with BMD at the spine and hip in men.

RESULTS

The association between independent variables maximal aerobic capacity (VO(2max)), leg power and hand grip strength, and dependent variables BMD at the spine and total hip was explored in a series of linear regression models successively adjusted for age, weight and height, smoking, alcohol intake and leisure time physical activity level. In the fully adjusted model we found a significant association of VO(2max) with BMD at the lumbar spine, p<0.0089. Furthermore, we observed significant associations of VO(2max) (p<0.0022) and leg power (p<0.011) with BMD at total hip.

CONCLUSIONS

We found that cardiorespiratory fitness was associated with BMD in men. Furthermore, hand grip strength and leg power were associated with increasing BMD at the lumbar spine and total hip in men, respectively. Further prospective studies are needed to further investigate the association between physical activity and BMD in men.

Trophic activity of human P2X7 receptor isoforms A and B in osteosarcoma

The P2X7 receptor (P2X7R) is attracting increasing attention for its involvement in cancer. Several recent studies have shown a crucial role of P2X7R in tumour cell growth, angiogenesis and invasiveness. In this study, we investigated the role of the two known human P2X7R functional splice variants, the full length P2X7RA and the truncated P2X7RB, in osteosarcoma cell growth. Immunohistochemical analysis of a tissue array of human osteosarcomas showed that forty-four, of a total fifty-four tumours (81.4%), stained positive for both P2X7RA and B, thirty-one (57.4%) were positive using an anti-P2X7RA antibody, whereas fifteen of the total number (27.7%) expressed only P2X7RB. P2X7RB positive tumours showed increased cell density, at the expense of extracellular matrix. The human osteosarcoma cell line Te85, which lacks endogenous P2X7R expression, was stably transfected with either P2X7RA, P2X7RB, or both. Receptor expression was a powerful stimulus for cell growth, the most efficient growth-promoting isoform being P2X7RB alone. Growth stimulation was matched by increased Ca²⁺ mobilization and enhanced NFATc1 activity. Te85 P2X7RA+B cells presented pore formation as well as spontaneous extracellular ATP release. The ATP release was sustained in all clones by P2X7R agonist (BzATP) and reduced following P2X7R antagonist (A740003) application. BzATP also increased cell growth and activated NFATc1 levels. On the other hand cyclosporin A (CSA) affected both NFATc1 activation and cell growth, definitively linking P2X7R stimulation to NFATc1 and cell proliferation. All transfected clones also showed reduced RANK-L expression, and an overall decreased RANK-L/OPG ratio. Mineralization was increased in Te85 P2X7RA+B cells while it was significantly diminished in Te85 P2X7RB clones, in agreement with immunohistochemical results. In summary, our data show that the majority of human osteosarcomas express P2X7RA and B and suggest that expression of either isoform is differently coupled to cell growth or activity.

P2X7-induced zeiosis promotes osteogenic differentiation and mineralization of postmenopausal bone marrow-derived mesenchymal stem cells

Polymorphisms of the P2X7 receptor have been associated with increased risk of fractures in postmenopausal women. Although both osteoblasts and osteoclasts express P2X7 receptors, their function in osteogenesis remains controversial. Here, we investigated the role of the P2X7 receptor on osteogenic differentiation and mineralization of bone marrow mesenchymal stem cell (BMSC) cultures from postmenopausal women (age 71±3 yr, n=18). We focused on the mechanisms related to intracellular [Ca(2+)]i oscillations and plasma membrane-dynamics. ATP, and the P2X7 agonist BzATP (100 μM), increased [Ca(2+)]i in parallel to the formation of membrane pores permeable to TO-PRO-3 dye uptake. ATP and BzATP elicited reversible membrane blebs (zeiosis) in 38 ± 1 and 70 ± 1% of the cells, respectively. P2X7-induced zeiosis was Ca(2+) independent, but involved phospholipase C, protein kinase C, and Rho-kinase activation. BzATP (100 μM) progressively increased the expression of Runx-2 and Osterix transcription factors by 452 and 226% (at d 21), respectively, alkaline phosphatase activity by 88% (at d 28), and mineralization by 329% (at d 43) of BMSC cultures in a Rho-kinase-dependent manner. In summary, reversible plasma membrane zeiosis involving cytoskeleton rearrangements due to activation of the P2X7-Rho-kinase axis promotes osteogenic differentiation and mineralization of BMSCs, thus providing new therapeutic targets for postmenopausal bone loss.

Non-synonymous single nucleotide polymorphisms in the P2X receptor genes: association with diseases, impact on receptor functions and potential use as diagnosis biomarkers

P2X receptors are Ca²⁺-permeable cationic channels in the cell membranes, where they play an important role in mediating a diversity of physiological and pathophysiological functions of extracellular ATP. Mammalian cells express seven P2X receptor genes. Single nucleotide polymorphisms (SNPs) are widespread in the P2RX genes encoding the human P2X receptors, particularly the human P2X7 receptor. This article will provide an overview of the non-synonymous SNPs (NS-SNPs) that have been associated with or implicated in altering the susceptibility to pathologies or disease conditions, and discuss the consequences of the mutations resulting from such NS-SNPs on the receptor functions. Disease-associated NS-SNPs in the P2RX genes have been valuable in understanding the disease etiology and the receptor function, and are promising as biomarkers to be used for the diagnosis and development of stratified therapeutics.

P2X-selective purinergic antagonists are strong inhibitors of HIV-1 fusion during both cell-to-cell and cell-free infection

Human immunodeficiency virus type 1 (HIV-1) infection is chronic and presently still incurable. Antiretroviral drugs effectively suppress replication; however, persistent activation of inflammatory pathways remains a key cause of morbidity. Recent studies proposed that purinergic signaling is required for HIV-1 infection. Purinergic receptors are distributed throughout a wide variety of tissue types and detect extracellular ATP as a danger signal released from dying cells. We have explored how these pathways are involved in the transmission of HIV-1 from cell to cell through virological synapses. Infection of CD4+ T lymphocytes with HIV-1 in the presence of an inhibitor of P2X receptors effectively inhibited HIV-1 infection through both cell-free and cell-to-cell contact in a dose-dependent manner. Inhibition of direct cell-to-cell infection did not affect the formation of virological synapses or the subsequent cell-to-cell transfer of HIV-1. During both cell-free and cell-to-cell CD4+ T lymphocyte infection, purinergic antagonists blocked infection at the level of viral membrane fusion. During cell-to-cell transmission, we observed CXCR4 colocalization with the newly internalized virus particles within target lymphocytes and found that the purinergic antagonists did not impair the recruitment of the coreceptor CXCR4 to the site of Gag internalization in the target cell. In a screen of a library of purinergic antagonists, we found that the most potent inhibitors of HIV-1 fusion were those that target P2X receptors, while P2Y-selective receptor antagonists or adenosine receptor antagonists were ineffective. Our results suggest that P2X receptors may provide a therapeutic target and that purinergic antagonists may have potent activity against viral infection of CD4+ T lymphocytes by both cell-free and cell-to-cell transmission.

IMPORTANCE

This study identifies purinergic antagonists to be potent inhibitors of HIV-1 cell-free and cell-to-cell-mediated infection and provides a stepwise determination of when these compounds inhibit HIV-1 infection. These data provide a rationale for the development of novel antiretroviral therapies that have a dual role in both direct antiviral activity and the reduction of HIV-associated inflammation. Purinergic antagonists are shown here to have equivalent efficacy in inhibiting HIV infection via cell-free and cell-to-cell infection, and it is shown that purinergic receptors could provide an attractive therapeutic anti-HIV target that might avoid resistance by targeting a host signaling pathway that potently regulates HIV infection. The high-throughput screen of HIV-1 fusion inhibitors further defines P2X-selective compounds among the purinergic compounds as being the most potent HIV entry inhibitors. Clinical studies on these drugs for other inflammatory indications suggest that they are safe, and thus, if developed for use as anti-HIV agents, they could reduce both HIV replication and HIV-related inflammation.

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.

The P2X7 receptor: a key player in immune-mediated bone loss?

Inflammatory diseases are often multiorganic diseases with manifestations not related directly to the primary affected organ. They are often complicated by a generalized bone loss that subsequently leads to osteoporosis and bone fractures. The exact mechanism for the accompanying bone loss is not understood in full detail, but factors such as glucocorticoid treatment, immobilization, malnutrition, and insufficient intake of vitamin D play a role. However, it has become evident that the inflammatory process itself is involved and the resulting bone loss is termed immune-mediated bone loss. It stems from an increase in bone resorption and the pro-inflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta and has been shown to not only mediate the inflammatory response but also to strongly stimulate bone degradation. The purinergic P2X7 receptor is central in the processing of these two cytokines and in the initiation of the inflammatory response, and it is a key molecule in the regulation of both bone formation and bone resorption. The aim of this review is therefore to provide evidence-based novel hypotheses of the role of ATP-mediated purinergic signalling via the P2X7 receptor in immune-mediated bone loss and -osteoporosis.

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.