Modulating P2X7 Receptor Signaling during Rheumatoid Arthritis: New Therapeutic Approaches for Bisphosphonates

The impact of the P2X7 receptor on the tumor immune microenvironment and its effects on tumor progression

Cancer is a significant global health concern, and finding effective methods to treat it has been a focus of scientific research. It has been discovered that the growth, invasion, and metastasis of tumors are closely related to the environment in which they exist, known as the tumor microenvironment (TME). The immune response interacting with the tumor occurring within the TME constitutes the tumor immune microenvironment, and the immune response can lead to anti-tumor and pro-tumor outcomes and has shown tremendous potential in immunotherapy. A channel called the P2X7 receptor (P2X7R) has been identified within the TME. It is an ion channel present in various immune cells and tumor cells, and its activation can lead to inflammation, immune responses, angiogenesis, immunogenic cell death, and promotion of tumor development. This article provides an overview of the structure, function, and pharmacological characteristics of P2X7R. We described the concept and components of tumor immune microenvironment and the influence immune components has on tumors. We also outlined the impact of P2X7R regulation and how it affects the development of tumors and summarized the effects of drugs targeting P2X7R on tumor progression, both past and current, assisting researchers in treating tumors using P2X7R as a target.

Effect of intra-articular injection of a hyaluronic acid-alendronate conjugate on post-traumatic osteoarthritis induced by destabilization of the medial meniscus in rats

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by pain and cartilage damage. Intra-articular (i.a) viscosupplementation with hyaluronic acid (HA) is frequently used for the management of OA. Preclinical studies have reported that bisphosphonates (BPs) may have a therapeutic potential to slow down or reverse the progression of OA. Among these, alendronate (ALN) has demonstrated chondroprotective effects in both in vitro and vivo experiments. This study evaluated the effects of a novel alendronate-hyaluronic acid (ALN-HA) conjugate on an OA in vivo model induced by medial meniscus destabilization (DMM). DMM surgery was performed on the knees of Sprague Dawley rats that received, after four weeks, one intra-articular (i.a.) injection of: (1) ALN-HA; (2) HA; (3) sodium chloride (NaCl). Sham-operated rats were used as control. Allodynia was assessed by Von Frey test. Joint degeneration was evaluated eight weeks after treatment by micro-computed tomography (micro-CT), histology, and immunohistochemistry. Collagen cross-linked C-telopeptides (CTX-I and CTX-II) serum levels were determined by ELISA. Paw withdrawal threshold increased in ALN-HA group when compared to rats treated with NaCl or HA. Micro-CT did not show differences between ALN-HA, HA and NaCl groups. ALN-HA injection produced significant improvements in articular cartilage degeneration showing an OARSI score lower than those of HA and NaCl, and reduced matrix metalloproteinase (MMP)-13, MMP-3, interleukin-6, vascular endothelial growth factor and Caspase-3 expression. CTX-I was reduced after ALN-HA treatment when compared to NaCl. Our results indicate that i.a. use of ALN after conjugation with HA limits OA development and progression in the rat DMM model, and may lead to the development of novel therapeutic strategies in OA management.

Role of the granzyme family in rheumatoid arthritis: Current Insights and future perspectives

Rheumatoid arthritis (RA) is a complex autoimmune disease characterized by chronic inflammation that affects synovial tissues of multiple joints. Granzymes (Gzms) are serine proteases that are released into the immune synapse between cytotoxic lymphocytes and target cells. They enter target cells with the help of perforin to induce programmed cell death in inflammatory and tumor cells. Gzms may have a connection with RA. First, increased levels of Gzms have been found in the serum (GzmB), plasma (GzmA, GzmB), synovial fluid (GzmB, GzmM), and synovial tissue (GzmK) of patients with RA. Moreover, Gzms may contribute to inflammation by degrading the extracellular matrix and promoting cytokine release. They are thought to be involved in RA pathogenesis and have the potential to be used as biomarkers for RA diagnosis, although their exact role is yet to be fully elucidated. The purpose of this review was to summarize the current knowledge regarding the possible role of the granzyme family in RA, with the aim of providing a reference for future research on the mechanisms of RA and the development of new therapies.

Origin, distribution, and function of three frequent coding polymorphisms in the gene for the human P2X7 ion channel

P2X7, an ion channel gated by extracellular ATP, is widely expressed on the plasma membrane of immune cells and plays important roles in inflammation and apoptosis. Several single nucleotide polymorphisms have been identified in the human P2RX7 gene. In contrast to other members of the P2X family, non-synonymous polymorphisms in P2X7 are common. Three of these occur at overall frequencies of more than 25% and affect residues in the extracellular “head”-domain of P2X7 (155 Y/H), its “lower body” (270 R/H), and its “tail” in the second transmembrane domain (348 T/A). Comparison of the P2X7 orthologues of human and other great apes indicates that the ancestral allele is Y—R—T (at 155–270–348). Interestingly, each single amino acid variant displays lower ATP-sensitivity than the ancestral allele. The originally published reference sequence of human P2X7, often referred to as “wildtype,” differs from the ancestral allele at all three positions, i.e. H—H—A. The 1,000 Genome Project determined the sequences of both alleles of 2,500 human individuals, including roughly 500 persons from each of the five major continental regions. This rich resource shows that the ancestral alleles Y155, R270, and T348 occur in all analyzed human populations, albeit at strikingly different frequencies in various subpopulations (e.g., 25%–59% for Y155, 59%–77% for R270, and 13%–47% for T348). BLAST analyses of ancient human genome sequences uncovered several homozygous carriers of variant P2X7 alleles, possibly reflecting a high degree of inbreeding, e.g., H—R—T for a 50.000 year old Neanderthal, H—R—A for a 24.000 year old Siberian, and Y—R—A for a 7,000 year old mesolithic European. In contrast, most present-day individuals co-express two copies of P2X7 that differ in one or more amino acids at positions 155, 270, and 348. Our results improve the understanding of how P2X7 structure affects its function and suggest the importance of considering P2X7 variants of participants when designing clinical trials targeting P2X7.

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.

Association of P2X7 receptor genetic polymorphisms and expression with rheumatoid arthritis susceptibility in a sample of the Iranian population: a case-control study

OBJECTIVE

Rheumatoid arthritis (RA) is a complex inflammatory autoimmune disease with joint eruption, systemic manifestation, and numerous predisposing genetic factors. The P2X7 receptor is an essential ligand-gated channel that contributes to many physiological processes, especially inflammation. However, genetic variations can alter the P2X7 receptor function. Therefore, the present study aimed to explore the impact of P2X7 genetic polymorphisms and expression on susceptibility to RA in a sample of the Iranian population.

METHODS

We enrolled 160 (145 female, 15 male) RA patients and 160 (142 female, 18 male) healthy individuals in this study. Genotyping was performed using tetra amplification refractory mutation system-polymerase chain reaction (TARMS-PCR) for rs1718119, rs2230912, rs2393799, rs28360457, rs35933842, and allele-specific PCR for rs1653624 and rs3751143. Furthermore, 44 new cases of RA and 48 healthy controls were recruited to investigate whether P2X7 mRNA expression is associated with RA susceptibility.

RESULTS

The results revealed that the rs2393799 significantly increased the risk of RA in all genetic models (p<0.05), while rs3751143 in codominant (CC vs. AA, OR=0.49, 95% CI=0.26-0.92), dominant (AC+CC, OR=0.59, 95% CI=0.37-0.94), C allele (OR=0.63, 95% CI=0.46-0.88), and rs2230912 in codominant (AG vs. AA, OR=0.56, 95% CI=0.34-0.94), dominant (AG+GG vs. AA, OR=0.59, 95% CI=0.35-0.99), and overdominant (AG vs. AA+GG, OR=0.57, 95% CI=0.33-0.98) significantly decreased the RA risk (p<0.05). Furthermore, the rs1718119 and rs1653624 were not associated with susceptibility of RA (p>0.05), and rs28360457 and rs35933842 were not polymorphic in our study. The mRNA expression level of P2X7 in both groups revealed that the P2X7 gene was significantly upregulated in RA (3.18±0.43) compared to healthy subjects (1.47±0.15, p<0.001).

CONCLUSION

Our results suggest that rs2393799, rs3751143, and rs2230912 variants of the P2X7 gene are associated with RA's susceptibility in a sample of the Iranian population. Also, P2X7 mRNA expression was higher in our new RA patients. The P2X7 receptor has been considered as a potential pharmacologic target in RA. Key Points • P2X7 variants (rs2393799, rs2230912, rs3751143) were associated with RA susceptibility in a sample of the Iranian population. • rs2393799 increases the risk of RA, while rs2230912 and rs3751143 decrease the risk of RA. • P2X7 expression was significantly upregulated in new RA patients compared to controls.

Increased Extracellular ATP in Plasma of Rheumatoid Arthritis Patients Activates CD8+T Cells

BACKGROUND

Rheumatoid arthritis (RA) is an autoimmune disorder with genetic and environmental causes often linked with the disease etiology. A disrupted metabolism has often been a characteristic of RA and an altered metabolic state of immune cells has been associated with their phenotypic and functional changes. The energy in the form of ATP produced by the metabolically active cells may thus initiate a cascade of immune responses there by influencing the disease pathogenesis or progression.

AIM OF THE STUDY

Through this study we have focused on determining the role of ATP in etiology of RA and aberrant cellular functions.

METHODS

Blood samples of 80 healthy controls (HC) and 95 RA patients were screened for extracellular ATP concentration, transcriptome analyses, an inflammatory mediator and the results were statistically analysed.

RESULTS

In this study, ATP is shown to be excessive in the plasma of RA patients (453.5 ± 16.09% in RA vs. 233.9 ± 10.07% in HC, p <0.0001) and significantly increases with the disease severity. The abundant extracellular ATP could activate circulating cytotoxic CD8⁺T cells in RA patients to produce Granzyme B.

CONCLUSION

Plasma ATP is thus identified to have a significant potential in progression and prognosis of RA and may thus be studied further to design better therapeutic approaches for the disease.

Elevated circulating level of P2X7 receptor is related to severity of coronary artery stenosis and prognosis of acute myocardial infarction

BACKGROUND

Acute myocardial infarction (AMI) is a severely life-threatening cardiovascular disease. Previous research has identified an association between the P2X7 receptor (P2X7R) and the development of atherosclerosis. However, the correlation of its expression with the clinical prognosis of patients with AMI remains unclear. The present study aimed to investigate the potential role of P2X7R in Chinese patients with AMI.

METHODS

Seventy-nine patients with AMI and 48 controls were consecutively enrolled in this prospective observational study. Circulating P2X7R mRNA expression levels and other clinical variables were determined upon admission to the hospital. Patients were followed up for 360 days, and the end-point was considered as the occurrence of major adverse cardiovascular events (MACE).

RESULTS

Circulating P2X7R mRNA expression level in peripheral blood mononuclear cells of patients with AMI were significantly higher than those in controls and had promising diagnostic ability of AMI with an area under the curve of 0.928. Furthermore, P2X7R was demonstrated to be correlated positively with the severity of coronary artery stenosis. Additionally, this is the first study to indicate that higher P2X7R mRNA expression is associated with a higher rate of MACE within 360 days after AMI.

CONCLUSIONS

The present study showed that the circulating level of P2X7R was elevated in AMI patients and was closely associated with the severity of coronary artery stenosis and prognosis of AMI.

Pharmacology of bisphosphonates in pain

The treatment of pain, in particular, chronic pain, remains a clinical challenge. This is particularly true for pain associated with severe or rare conditions, such as bone cancer pain, vulvodynia, or complex regional pain syndrome. Over the recent years, there is an increasing interest in the potential of bisphosphonates in the treatment of pain, although there are few papers describing antinociceptive and anti-hypersensitizing effects of bisphosphonates in various animal models of pain. There is also increasing evidence for clinical efficacy of bisphosphonates in chronic pain states, although the number of well-controlled studies is still limited. However, the mechanisms underlying the analgesic effects of bisphosphonates are still largely elusive. This review provides an overview of preclinical and clinical studies of bisphosphonates in pain and discusses various pharmacological mechanisms that have been postulated to explain their analgesic effects. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

ATP and adenosine: Role in the immunopathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a classic inflammatory autoimmune disease. Local joint destruction and extra-articular manifestations of RA deeply compromise the life quality of the affected patients. RA immunopathogenesis depends on continuous immunogenic activation in which the purinergic system participates. The purinergic system comprises the signaling and metabolism of purines such as adenosine triphosphate (ATP) and adenosine. ATP signaling is involved in the activation and maintenance of the inflammatory state of RA through the activation of P2X7 and the production of cytokines, which orchestrate the pathogenesis of RA. The breakdown of ATP through the CD39/CD73 axis produces adenosine, which mostly inhibits the inflammatory process through activation of specific P1 receptors. Adenosine is hydrolyzed by adenosine deaminase (ADA) that interacts with other molecules playing additional roles in this disease. This review explores the release, metabolism, and the effects of binding of ATP and adenosine to their respective receptors in the context of RA, as well as their potential use as biomarkers and therapeutic targets.

Rhein inhibits ATP-triggered inflammatory responses in rheumatoid rat fibroblast-like synoviocytes

Rheumatoid arthritis (RA) is a chronic and systemic inflammatory disorder, which may lead to joint disabilities. So far the pathogenesis of RA remains largely undetermined, and there are still no potent drugs for clinical treatment. Rhein, a natural bioactive anthraquinone derivative, exhibited significant anti-inflammatory activities demonstrated by previous studies. Here we aimed to investigate the effects of rhein on ATP-induced inflammation responses in fibroblast-like synoviocytes isolated from a rat model of collagen induced arthritis (CIA). Our results showed that ATP triggered rapid cytosolic calcium concentration ([Ca²⁺]_c) increase depending on extracellular Ca²⁺ entry. Given the major P2 subtypes expressed in rat synoviocytes were P2X₄ and P2Y₂ receptors, ATP-elicited calcium entry should be mainly resulted from activating P2X₄. Interestingly, rhein could effectively block the ATP-induced [Ca²⁺]_c increases in a dose-dependent manner. Besides, rhein also suppressed the production of intracellular reactive oxygen species (ROS) induced by ATP in synoviocytes that was resulted from P2X₄-mediated Ca²⁺ entry. Brilliant blue G (BBG), which can block P2X₄ receptor at high concentration, showed similar suppressive effects on above responses. Furthermore, in lipopolysaccharide-primed cells, application of ATP synergistically promoted the gene expression of cyclooxygenase-2, interleukin-6 and matrix metalloproteinase-9. Both rhein and BBG attenuated these inflammatory gene expressions enhanced by ATP. Above data together suggested a potential anti-arthritic role of rhein by inhibiting ATP-induced [Ca²⁺]_c increase, ROS production and inflammatory gene expression targeting P2X₄ in CIA rat synoviocytes, which will provide a novel insight in the therapy of RA.

P2X7 receptor: A potential therapeutic target for autoimmune diseases

P2X7 receptor (P2X7R), a distinct ligand-gated ion channel, is a member of purinergic type 2 receptor family with ubiquitous expression in human body. Previous studies have revealed a pivotal role of P2X7R in innate and adaptive immunity. Once activated, it will meditate some vital cascaded responses including the assembly of nucleotide-binding domain (NOD) like receptor protein 3 (NLRP3) inflammasome, non-classical secretion of IL-1β, modulation of cytokine-independent pathways in inflammation such as P2X7R- transglutaminase-2 (TG2) and P2X7R-cathepsin pathway, activation and regulation of T cells, etc. In fact, above responses have been identified to be involved in the development of autoimmunity, specifically, the NLRP3 inflammasome could promote inflammation in massive autoimmune diseases and TG2, as well as cathepsin may contribute to joint destruction and degeneration in inflammatory arthritis. Recently, numerous evidences further suggested the significance of P2X7R in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), inflammatory bowel disease (IBD), multiple sclerosis (MS), etc. In this review, we will succinctly discuss the biological characteristics and summarize the recent progress of the involvement of P2X7R in the development and pathogenesis of autoimmune diseases, as well as its clinical implications and therapeutic potential.

The role of the P2X7 receptor in myeloid-derived suppressor cells and immunosuppression

Myeloid derived suppressor cells (MDSC) are a heterogeneous population of immature myeloid cells expanded and recruited from the bone marrow to the periphery or to a specific site of inflammation/infection. MDSC have been described in different pathological conditions including cancer, infections, autoimmunity and obesity. The main function of MDSC is immunosuppression occurring through different mechanisms such as induction of immunosuppressive cells, impairment of lymphocyte homing, free radical production, depletion of amino acids critical for T cell functions, upregulation of ectoenzymes involved in adenosine production and activation of immune regulatory molecules responsible of T cell anergy. A novel immunosuppressive mechanism MDSC-mediated involves the ATP/P2X7 receptor axis that induces the release of immunosuppressive chemokines/cytokines upon triggering with ATP.

[Diagnostic value of P2X7 receptor and its role in inflammatory reaction in rheumatoid arthritis]

OBJECTIVE

To study the diagnostic value of P2X7 receptor for rheumatoid arthritis (RA) and its role in the inflammatory response.

METHODS

With the synovial tissues from 25 patients with bone and joint replacement as the control,the synovial tissues of 25 RA patients were examined for the relative expression of P2X7 receptor mRNA using qRT-PCR.In an immortalized RA synovial cell line (MH7A),the effect of P2X7 receptor knockdown via a small interfering RNA were examined on the productions of the inflammatory cytokines including interleukin-1β(IL-1β),IL-6,and IL-8 using ELISA.

RESULTS

The RA patients showed significantly higher levels of P2X7 receptor mRNA expression in the synovial tissue than the control patients.P2X7 receptor had a good diagnostic value for RA.The expression levels of IL-1β,IL-6,and IL-8 were positively correlated with the levels of P2X7 receptor in the synovial tissues of RA patients (P&lt;0.001).In MH7A cells,P2X7 receptor knockdown obviously reduced the secretion of IL-1β and IL-6.

CONCLUSIONS

RA patients show elevated P2X7 receptor level in the synovial tissue, which has a good diagnostic value for RA.Blocking P2X7 receptor can inhibit inflammatory factor secretion and suppress inflammatory reactions.

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.

A protocol for a multicentre, randomised, double-blind, placebo-controlled trial to compare the effect of annual infusions of zoledronic acid to placebo on knee structural change and knee pain over 24 months in knee osteoarthritis patients - ZAP2

BACKGROUND

Bisphosphonates are a class of drugs that slow bone loss and are a promising candidate to treat knee osteoarthritis (OA) patients. In a pilot study, we demonstrated that zoledronic acid reduced knee pain and size of subchondral bone marrow lesions (BMLs) over 6 months in knee OA patients with significant knee pain and BMLs. A longer, larger study is required to assess whether decreases in BML size will translate to reductions in cartilage loss over time. We are currently conducting a multicentre, randomised, double-blind, placebo-controlled trial over 24 months that aims to compare the effect of annual infusions of zoledronic acid to placebo on knee structural change (assessed using magnetic resonance imaging (MRI)) and knee pain in knee OA patients.

METHODS

Two hundred sixty-four patients with clinical knee OA, significant knee pain and subchondral BMLs present on MRI will be recruited in Hobart, Melbourne, Sydney and Adelaide. They will be randomly allocated to the two arms of the study, receiving an annual identical intravenous infusion of either 100 mL of fluid containing zoledronic acid (5 mg/100 mL) or placebo (0.9% NaCl 100 mL), at baseline and 1 year later. MRI of the study knee will be performed at screening, month 6 and 24. Knee structure, symptoms and function will be assessed using validated methods. The primary outcome is absolute change in tibiofemoral cartilage volume (mm3) over 24 months. Secondary outcomes include improvement in knee pain over 3, 6, 12, 18, and 24 months and reductions in BML size over 6 and 24 months. The primary analyses will be intention-to-treat analyses of primary and secondary outcomes. Per protocol analyses will be performed as the secondary analyses.

DISCUSSION

This study will provide high-quality evidence to assess whether zoledronic acid has a novel disease modifying effect in OA by slowing cartilage loss and reducing pain. If zoledronic acid proves effective, it suggests great potential for cost savings through a delay or reduced need for joint replacement surgery, and potential for great improvements in quality of life for OA suffers.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry: ACTRN12613000039785 , registered on 14 January 2013.

P2X7 receptor regulates EMMPRIN and MMP‑9 expression through AMPK/MAPK signaling in PMA‑induced macrophages

The rupture of atherosclerotic plaques may result in the formation of thrombi, which may induce subsequent cardiac events such as acute myocardial infarction. Overproduction of matrix metalloproteinases (MMPs) and extracellular matrix metalloproteinase inducers (EMMPRINs) by monocytes and macrophages may lead to rupture of atherosclerotic plaques as a result of the degradation of the extracellular matrix. The purinergic 2X7 receptor (P2X7R) is expressed in macrophages that are assembled in atherosclerotic lesions of human carotid arteries. P2X7R may serve a crucial role in the development of atherosclerosis; therefore, the present study aimed to determine whether P2X7R regulated the expression of EMMPRIN and MMP‑9 in phorbol 12‑myristate 13‑acetate (PMA)‑induced macrophages. In addition, the potential molecular mechanisms involved in this process were investigated. THP‑1 human monocytic cells were pretreated with A‑438079 (a specific inhibitor of P2X7R) for 1 h and subsequently incubated with or without PMA for 48 h. Exposure to A‑438079 significantly decreased the expression of MMP‑9 and EMMPRIN in the PMA‑induced macrophages and attenuated the activation (phosphorylation) of mitogen‑activated protein kinase (MAPK) signaling, including c‑Jun N‑terminal kinase, p38 and extracellular signal‑regulated kinase. The present study also demonstrated that 5'‑AMP‑activated protein kinase (AMPK) was activated by PMA exposure during differentiation from monocytes to macrophages. This activation was reversed by A‑438079 treatment through the inhibition of P2X7R expression. These results suggested that the inhibition of P2X7R may be able to suppress the AMPK/MAPK signaling pathway and consequently downregulate both EMMPRIN and MMP‑9 expression in PMA‑induced macrophages.

Involvement of P2X7 receptor signaling on regulating the differentiation of Th17 cells and type II collagen-induced arthritis in mice

Interleukin (IL)-17 producing T helper (Th17) cells are major effector cells in the pathogenesis of rheumatoid arthritis (RA). The P2X7 receptor (P2X7R) has emerged as a potential site in the regulation of inflammation in RA but little is known of its functional role on the differentiation of Th17 cells. This study investigates the in vitro and in vivo effects of P2X7R on Th17 cell differentiation during type II collagen (CII) induced experimental arthritis model. In CII-treated dendritic cells (DCs) and DC/CD4⁺ T coculture system, pretreatment with pharmacological antagonists of P2X7R (Suramin and A-438079) caused strong inhibition of production of Th17-promoting cytokines (IL-1β, TGF-β1, IL-23p19 and IL-6). Exposure to CII induced the elevation of mRNAs encoding retinoic acid receptor-related orphan receptor α and γt, which were abolished by pretreatment with P2X7R antagonists. Furthermore, blocking P2X7R signaling abolished the CII-mediated increase in IL-17A. Blockade of P2X7R remarkably inhibited hind paw swelling and ameliorated pathological changes in ankle joint of the collagen-induced arthritis mice. Thus, we demonstrated a novel function for P2X7R signaling in regulating CII-induced differentiation of Th17 cells. P2X7R signaling facilitates the development of the sophisticated network of DC-derived cytokines that favors a Th17 phenotype.

Alarmins firing arthritis: Helpful diagnostic tools and promising therapeutic targets

Alarmins are endogenous molecules with homeostatic roles that have reached the focus of research in inflammatory arthritis in the last two decades, mostly due to their ability to indicate tissue related damage after active or passive release from injured cells. From HMGB1, S100A8/A9 and S100A12 proteins, over heat-shock proteins (HSPs) and purine metabolites (e.g. uric acid, ATP) to altered matrix proteins and interleukin-33 (IL-33), a number of alarmins have been determined until now as having a role in rheumatoid arthritis, psoriatic and juvenile idiopathic arthritis, as well as spondyloarthritis and gout. Although formerly being linked to initiation and chronification of inflammatory arthritis, driving auto- and paracrine inflammatory loops, more recent research has also unraveled the alarmins' role in the crosstalk between innate and adaptive immunity and in resolution of inflammation. Providing a state-of-the-art overview of known alarmins, this review lists the known modes of action and pathologic contribution of alarmins to inflammatory arthritis, as well as biomarker potential of alarmins in the clinical setting for tracking disease severity. Based upon research on animal experimental models (CIA, AIA) and clinical trials, a look is made into potentially viable strategies for modifying alarmin secretion and their target receptor (e.g. TLR, RAGE) interaction with the purpose of attenuating arthritic disease.

Fluorescent dyes as a reliable tool in P2X7 receptor-associated pore studies

Since the nineteenth century, a great amount of different biological structures and processes have been assessed by fluorescent dyes. Along with the uses of these compounds as vital and histological dyes, some fluorescent dyes have become valuable tools for the study of the pore phenomenon in plasma membranes. Some ion channels capable of forming large conductance channels, such as P2X7, TRPV1, VDAC-1 and the maxi-anion channels transiently alter the plasma membrane permeability, producing pores, which permit the passage of molecules of up to 1,000 Da. In this review, we discuss the uses of the fluorescent dyes chosen in diverse studies of this topic up to now.

Caspase-1-mediated pathway promotes generation of thromboinflammatory microparticles

Extracellular ATP is a signal of tissue damage and induces macrophage responses that amplify inflammation and coagulation. Here we demonstrate that ATP signaling through macrophage P2X7 receptors uncouples the thioredoxin (TRX)/TRX reductase (TRXR) system and activates the inflammasome through endosome-generated ROS. TRXR and inflammasome activity promoted filopodia formation, cellular release of reduced TRX, and generation of extracellular thiol pathway-dependent, procoagulant microparticles (MPs). Additionally, inflammasome-induced activation of an intracellular caspase-1/calpain cysteine protease cascade degraded filamin, thereby severing bonds between the cytoskeleton and tissue factor (TF), the cell surface receptor responsible for coagulation activation. This cascade enabled TF trafficking from rafts to filopodia and ultimately onto phosphatidylserine-positive, highly procoagulant MPs. Furthermore, caspase-1 specifically facilitated cell surface actin exposure, which was required for the final release of highly procoagulant MPs from filopodia. Together, the results of this study delineate a thromboinflammatory pathway and suggest that components of this pathway have potential as pharmacological targets to simultaneously attenuate inflammation and innate immune cell-induced thrombosis.

Targeting P2 receptors--current progress in treating musculoskeletal diseases

It is widely recognized that purinergic signalling, extracellular nucleotides acting at purinergic receptors, is the most primitive and ubiquitous signalling system participating in numerous biological processes in almost all tissue types. The P2 receptors, including P2X and P2Y purinoceptor subtypes, have been proposed to play important roles in the musculoskeletal systems since the early 1990s. During the past five years, significant progress in this field has been made; this review will summarize these most recent developments and highlight the pharmaceutical potential from these findings.

P2X7 receptor antagonism inhibits p38 mitogen-activated protein kinase activation and ameliorates neuronal apoptosis after subarachnoid hemorrhage in rats

OBJECTIVES

Brilliant blue G, a selective P2X7 receptor antagonist, exhibits neuroprotective properties. This study examined whether brilliant blue G treatment ameliorates early brain injury after experimental subarachnoid hemorrhage, specifically via inhibiting p38 mitogen-activated protein kinase-related proapoptotic pathways.

DESIGN

Controlled in vivo laboratory study.

SETTING

Animal research laboratory.

SUBJECTS

One hundred fifty-four adult male Sprague-Dawley rats weighing 280-320 g.

INTERVENTIONS

Subarachnoid hemorrhage was induced in rats by endovascular perforation. Experiment 1 implemented sham-operated rats (sham) and subarachnoid hemorrhage animals, which received vehicle (subarachnoid hemorrhage + vehicle), brilliant blue G (subarachnoid hemorrhage + brilliant blue G), or brilliant blue G plus 2'(3')-O-(4-Benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) (subarachnoid hemorrhage + brilliant blue G + BzATP). The animals were intraperitoneally treated with brilliant blue G (30 mg/kg) at 30 minutes after subarachnoid hemorrhage. BzATP (50 μg/rat), a P2X7 receptor agonist, was intracerebroventricularly administered. Experiment 2 implemented sham-operated rats (sham) and subarachnoid hemorrhage animals, which received vehicle (subarachnoid hemorrhage + vehicle), scramble small interfering RNA (subarachnoid hemorrhage + scramble small interfering RNA), or P2X7 receptor small interfering RNA (subarachnoid hemorrhage + P2X7 receptor small interfering RNA). Subarachnoid hemorrhage grading, neurobehavioral score, and brain edema were evaluated at 24 and 72 hours after surgery. The expression of phosphorylated p38 mitogen-activated protein kinase, phosphorylated extracellular signal-regulated kinases, phosphorylated c-Jun N-terminal kinases, P2X7 receptor, Bcl-2, and cleaved caspase-3 in the left cerebral hemisphere were determined by Western blot. Neuronal apoptosis was examined by double immunofluorescence staining using P2X7 receptor, terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end-labeling, and neuronal nuclei.

MEASUREMENTS AND MAIN RESULTS

Brilliant blue G significantly improved neurobehavioral function and ameliorated brain water content at 24 and 72 hours after subarachnoid hemorrhage. BzATP reversed these treatment effects. Brilliant blue G attenuated neuronal apoptosis in the subcortex, which was associated with decreased expression of phosphorylated p38 mitogen-activated protein kinase and cleaved caspase-3 and an increased expression of Bcl-2 in the left cerebral hemisphere. The beneficial effects of P2X7 receptor small interfering RNA were also mediated by a p38 mitogen-activated protein kinase pathway.

CONCLUSIONS

Inhibition of P2X7 receptor by brilliant blue G or P2X7 receptor small interfering RNA can prevent early brain injury via p38 mitogen-activated protein kinase after subarachnoid hemorrhage.

Supplementation of exogenous adenosine 5'-triphosphate enhances mechanical properties of 3D cell-agarose constructs for cartilage tissue engineering

Formation of tissue-engineered cartilage is greatly enhanced by mechanical stimulation. However, direct mechanical stimulation is not always a suitable method, and the utilization of mechanisms underlying mechanotransduction might allow for a highly effective and less aggressive alternate means of stimulation. In particular, the purinergic, adenosine 5'-triphosphate (ATP)-mediated signaling pathway is strongly implicated in mechanotransduction within the articular cartilage. We investigated the effects of transient and continuous exogenous ATP supplementation on mechanical properties of cartilaginous constructs engineered using bovine chondrocytes and human mesenchymal stem cells (hMSCs) encapsulated in an agarose hydrogel. For both cell types, we have observed significant increases in equilibrium and dynamic compressive moduli after transient ATP treatment applied in the fourth week of cultivation. Continuous ATP treatment over 4 weeks of culture only slightly improved the mechanical properties of the constructs, without major changes in the total glycosaminoglycan (GAG) and collagen content. Structure-function analyses showed that transiently ATP-treated constructs, and in particular those based on hMSCs, had the highest level of correlation between compositional and mechanical properties. Transiently treated groups showed intense staining of the territorial matrix for GAGs and collagen type II. These results indicate that transient ATP treatment can improve functional mechanical properties of cartilaginous constructs based on chondrogenic cells and agarose hydrogels, possibly by improving the structural organization of the bulk phase and territorial extracellular matrix (ECM), that is, by increasing correlation slopes between the content of the ECM components (GAG, collagen) and mechanical properties of the construct.

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.

Effect of bisphosphonate use in patients with symptomatic and radiographic knee osteoarthritis: data from the Osteoarthritis Initiative

OBJECTIVES

Bisphosphonates have some reported beneficial effects in treating osteoarthritis (OA). This study examined the effects of bisphosphonate use on symptoms and structural progression of knee OA in participants from the NIH Osteoarthritis Initiative cohort.

METHODS

People with typical OA trial entry criteria (KL2/3, minimum joint space width 2.5-5.0 mm and pain ≥4 on a numeric rating scale) were classified as bisphosphonate users (≥3 of the 5 years; n=55) or non-users (no use in the preceding 5 years or during follow-up; n=268). Annual data over 4 years were analysed using linear mixed modelling and generalised estimating equations.

RESULTS

Bisphosphonate compliance was 85% at year 1, reducing to 76% by year 4. Numeric rating scale pain scores were significantly reduced among bisphosphonate users at years 2 and 3 (year 3, -0.9 vs -2.2, p=0.004), though not year 4, after adjustment for baseline pain and analgesic use. Differences in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain and disability scores did not reach statistical significance at any time point. There was a trend to less joint space narrowing in bisphosphonate users over time (year 4, 0.51 vs 0.29 mm; p=0.06).

CONCLUSIONS

Significant reduction in numeric rating scale pain was observed in the first 3 years with bisphosphonate use; diminution of effects by year 4 may reflect reduced compliance. Differences in results obtained using numeric rating scale and WOMAC may reflect different constructs measured by these tools. The beneficial trend on structural progression should be considered in terms of the sample size.

Activation of calcium-insensitive phospholipase A(2) (iPLA(2)) by P2X(7) receptors in murine peritoneal macrophages

Free fatty acid releases are triggered by PLA2 activation and are substrates for many enzymes such as cyclooxygenases. These reactions are responsible for the production of many prostaglandins implicated in the inflammation yet many purinergic receptors have been implicated in diseases characterised by chronic inflammation. The role of P2X receptors was evaluated in LPS-primed murine peritoneal macrophages which were labelled with either [(3)H]-oleic acid or [(3)H]-arachidonic acid. Ten μmolar thapsigargin and 1mM ATP stimulated the release of both unsaturated acids. ATP had no effect at 10 μM and ivermectin had no effect on the response to ATP. The response to ATP was inhibited by magnesium and was not observed with cells from P2X(7)(-/-) mice. The response to ATP was not affected by the removal of extracellular calcium and was inhibited by arachidonyltrifluoromethyl ketone and bromoenol lactone but not by pyrrophenone. The release of the [(3)H]-fatty acids by ATP and thapsigargin was diminished by PD-98058, an inhibitor of MEK-1. It was concluded that in LPS-primed macrophages, P2X(7) receptors, not P2X(4) receptors, activated an iPLA(2) and promoted the release of unsaturated fatty acids secondary to the activation of a kinase. This response might contribute to the inflammation provoked by extracellular ATP.