P2X7 receptors on osteoblasts couple to production of lysophosphatidic acid: a signaling axis promoting osteogenesis

Lysophosphatidic acid regulates implant osseointegration in murine models via YAP

BACKGROUND

Lysophosphatidic acid (LPA), a simple bioactive lysophospholipid, has been reported to regulate bone homeostasis and bone remodeling. This study aimed to elucidate the function and intrinsic mechanism of LPA in osseointegration in murine models.

METHOD

We constructed immediate implant models in murine maxillae. Micro-CT, H&E staining, and PCR assays were performed to evaluate the effects of LPA on osseointegration. Furthermore, Prx1-Cre;Yapf/f mice and Sp7-Cre;Yapf/f mice were generated to investigate the role of YAP on LPA-induced osseointegration.

RESULT

In this study, we identified that LPA might promote bone deposition on the tissue-implant interface and improve osseointegration. In addition, conditional knockout of YAP from MCSs and pre-osteoblasts blunts LPA-induced osteogenesis and osseointegration in mice.

CONCLUSION

Our data demonstrated that LPA-YAP signaling is particularly important to regulate osseointegration, which expands our understanding of LPA and provide the potential of LPA to be used in osseointegration.

Investigation of Oxidative-Stress Impact on Human Osteoblasts During Orthodontic Tooth Movement Using an In Vitro Tension Model

In recent years, there has been a growing number of adult orthodontic patients with periodontal disease. The progression of periodontal disease is well-linked to oxidative stress (OS). Nevertheless, the impact of OS on orthodontic tooth movement (OTM) is not fully clarified. Therefore, we applied an OS in vitro-model utilizing H2O2 to study its effect on tension-induced mechanotransduction in human osteoblasts (hOBs). Experimental parameters were established based on cell viability and proliferation. Apoptosis detection was based on caspase-3/7 activity. Gene expression related to bone-remodeling (RUNX2, P2RX7, TNFRSF11B/OPG), inflammation (CXCL8/IL8, IL6, PTRGS2/COX2), autophagy (MAP1LC3A/LC3, BECN1), and apoptosis (CASP3, CASP8) was analyzed by RT-qPCR. IL6 and PGE2 secretion were determined by ELISA. Tension increased the expression of PTRGS2/COX2 in all groups, especially after stimulation with higher H2O2 concentration. This corresponds also to the measured PGE2 concentrations. CXCL8/IL8 was upregulated in all groups. Cells subjected to tension alone showed a general upregulation of osteogenic differentiation-related genes; however, pre-stimulation with OS did not induce significant changes especially towards downregulation. MAP1LC3A/LC3, BECN1 and CASP8 were generally upregulated in cells without OS pre-stimulation. Our results suggest that OS might have considerable impacts on cellular behavior during OTM.

Itaconate is a metabolic regulator of bone formation in homeostasis and arthritis

OBJECTIVES

Bone remodelling is a highly dynamic process dependent on the precise coordination of osteoblasts and haematopoietic-cell derived osteoclasts. Changes in core metabolic pathways during osteoclastogenesis, however, are largely unexplored and it is unknown whether and how these processes are involved in bone homeostasis.

METHODS

We metabolically and transcriptionally profiled cells during osteoclast and osteoblast generation. Individual gene expression was characterised by quantitative PCR and western blot. Osteoblast function was assessed by Alizarin red staining. immunoresponsive gene 1 (Irg1)-deficient mice were used in various inflammatory or non-inflammatory models of bone loss. Tissue gene expression was analysed by RNA in situ hybridisation.

RESULTS

We show that during differentiation preosteoclasts rearrange their tricarboxylic acid cycle, a process crucially depending on both glucose and glutamine. This rearrangement is characterised by the induction of Irg1 and production of itaconate, which accumulates intracellularly and extracellularly. While the IRG1-itaconate axis is dispensable for osteoclast generation in vitro and in vivo, we demonstrate that itaconate stimulates osteoblasts by accelerating osteogenic differentiation in both human and murine cells. This enhanced osteogenic differentiation is accompanied by reduced proliferation and altered metabolism. Additionally, supplementation of itaconate increases bone formation by boosting osteoblast activity in mice. Conversely, Irg1-deficient mice exhibit decreased bone mass and have reduced osteoproliferative lesions in experimental arthritis.

CONCLUSION

In summary, we identify itaconate, generated as a result of the metabolic rewiring during osteoclast differentiation, as a previously unrecognised regulator of osteoblasts.

Implications of siRNA Therapy in Bone Health: Silencing Communicates

The global statistics of bone disorders, skeletal defects, and fractures are frightening. Several therapeutic strategies are being used to fix them; however, RNAi-based siRNA therapy is starting to prove to be a promising approach for the prevention of bone disorders because of its advanced capabilities to deliver siRNA or siRNA drug conjugate to the target tissue. Despite its 'bench-to-bedside' usefulness and approval by food and drug administration for five siRNA-based therapeutic medicines: Patisiran, Vutrisiran, Inclisiran, Lumasiran, and Givosiran, its use for the other diseases still remains to be resolved. By correcting the complications and complexities involved in siRNA delivery for its sustained release, better absorption, and toxicity-free activity, siRNA therapy can be harnessed as an experimental tool for the prevention of complex and undruggable diseases with a personalized medicine approach. The present review summarizes the findings of notable research to address the implications of siRNA in bone health for the restoration of bone mass, recovery of bone loss, and recuperation of bone fractures.

ATP-induced cell death: a novel hypothesis for osteoporosis

ATP-induced cell death has emerged as a captivating realm of inquiry with profound ramifications in the context of osteoporosis. This study unveils a paradigm-shifting hypothesis that illuminates the prospective involvement of ATP-induced cellular demise in the etiology of osteoporosis. Initially, we explicate the morphological attributes of ATP-induced cell death and delve into the intricacies of the molecular machinery and regulatory networks governing ATP homeostasis and ATP-induced cell death. Subsequently, our focus pivots towards the multifaceted interplay between ATP-induced cellular demise and pivotal cellular protagonists, such as bone marrow-derived mesenchymal stem cells, osteoblasts, and osteoclasts, accentuating their potential contributions to secondary osteoporosis phenotypes, encompassing diabetic osteoporosis, glucocorticoid-induced osteoporosis, and postmenopausal osteoporosis. Furthermore, we probe the captivating interplay between ATP-induced cellular demise and alternative modalities of cellular demise, encompassing apoptosis, autophagy, and necroptosis. Through an all-encompassing inquiry into the intricate nexus connecting ATP-induced cellular demise and osteoporosis, our primary goal is to deepen our comprehension of the underlying mechanisms propelling this malady and establish a theoretical bedrock to underpin the development of pioneering therapeutic strategies.

The P2X7 Receptor in Autoimmunity

The P2X7 receptor (P2X7R) is an ATP-gated nonselective cationic channel that, upon intense stimulation, can progress to the opening of a pore permeable to molecules up to 900 Da. Apart from its broad expression in cells of the innate and adaptive immune systems, it is expressed in multiple cell types in different tissues. The dual gating property of P2X7R is instrumental in determining cellular responses, which depend on the expression level of the receptor, timing of stimulation, and microenvironmental cues, thus often complicating the interpretation of experimental data in comprehensive settings. Here we review the existing literature on P2X7R activity in autoimmunity, pinpointing the different functions in cells involved in the immunopathological processes that can make it difficult to model as a druggable target.

P2X7 receptor is required for the ototoxicity caused by aminoglycoside in developing cochlear hair cells

Aminoglycoside antibiotics (AGAs) are widely used in life-threatening infections, but they accumulate in cochlear hair cells (HCs) and result in hearing loss. Increases in adenosine triphosphate (ATP) concentrations and P2X7 receptor expression were observed after neomycin treatment. Here, we demonstrated that P2X7 receptor, which is a non-selective cation channel that is activated by high ATP concentrations, may participate in the process through which AGAs enter hair cells. Using transgenic knockout mice, we found that P2X7 receptor deficiency protects HCs against neomycin-induced injury in vitro and in vivo. Subsequently, we used fluorescent gentamicin-Fluor 594 to study the uptake of AGAs and found fluorescence labeling in wild-type mice but not in P2rx7-/- mice in vitro. In addition, knocking-out P2rx7 did not significantly alter the HC count and auditory signal transduction, but it did inhibit mitochondria-dependent oxidative stress and apoptosis in the cochlea after neomycin exposure. We thus conclude that the P2X7 receptor may be linked to the entry of AGAs into HCs and is likely to be a therapeutic target for auditory HC protection.

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.

Modulation of osteoblast differentiation and function by the P2X4 receptor

Bone cells are known to express multiple P2 receptor subtypes, and the functional effects of receptor activation have been described for many of these. One exception is the P2X4 receptor, which despite strong expression in osteoblasts and osteoclasts, has no defined functional activity. This study used the selective P2X4 receptor antagonists, 5-BDBD and PSB-12062, to investigate the role of this receptor in bone. Both antagonists (≥ 0.1 μM) dose-dependently decreased bone formation by 60-100%. This was accompanied by a ≤ 70% decrease in alkaline phosphatase activity, a ≤ 40% reduction in cell number, and a ≤ 80% increase in the number of adipocytes present in the culture. The analysis of gene expression showed that levels of osteoblast marker genes (e.g. Alpl, Bglap) were decreased in 5-BDBD treated cells. Conversely, expression of the adipogenic transcription factor PPARG was increased 10-fold. In osteoclasts, high doses of both antagonists were associated with a reduction in osteoclast formation and resorptive activity by ≤ 95% and ≤ 90%, respectively. Taken together, these data suggest that the P2X4 receptor plays a role in modulating bone cell function. In particular, it appears to influence osteoblast differentiation favouring the osteogenic lineage over the adipogenic lineage.

TAT&RGD Peptide-Modified Naringin-Loaded Lipid Nanoparticles Promote the Osteogenic Differentiation of Human Dental Pulp Stem Cells

BACKGROUND

Naringin is a naturally occurring flavanone that promotes osteogenesis. Owing to the high lipophilicity, poor in vivo bioavailability, and extensive metabolic alteration upon administration, the clinical efficacy of naringin is understudied. Additionally, information on the molecular mechanism by which it promotes osteogenesis is limited.

METHODS

In this study, we prepared TAT & RGD peptide-modified naringin-loaded nanoparticles (TAT-RGD-NAR-NPs), evaluated their potency on the osteogenic differentiation of human dental pulp stem cells (hDPSCs), and studied its mechanism of action through metabolomic analysis.

RESULTS

The particle size and zeta potential of TAT-RGD-NAR-NPs were 160.70±2.05 mm and -20.77±0.47mV, respectively. The result of cell uptake assay showed that TAT-RGD-NAR-NPs could effectively enter hDPSCs. TAT-RGD-NAR-NPs had a more significant effect on cell proliferation and osteogenic differentiation promotion. Furthermore, in metabolomic analysis, naringin particles showed a strong influence on the glycerophospholipid metabolism pathway of hDPSCs. Specifically, it upregulated the expression of PLA2G3 and PLA2G1B (two isozymes of phospholipase A2, PLA2), increased the biosynthesis of lysophosphatidic acid (LPA).

CONCLUSION

These results suggested that TAT-RGD-NPs might be used for transporting naringin to hDPSCs for modulating stem cell osteogenic differentiation. The metabolomic analysis was used for the first time to elucidate the mechanism by which naringin promotes hDPSCs osteogenesis by upregulating PLA2G3 and PLA2G1B.

P2X7 receptor mediated release of microglial prostanoids and miRNAs correlates with reversal of neuropathic hypersensitivity in rats

BACKGROUND

P2X7 receptor antagonists have potential for treating various CNS diseases, including neuropathic pain, although none have been approved for clinical use. Reasons may include insufficient understanding of P2X7 receptor signaling in pain and the lack of a corresponding preclinical mechanistic biomarker.

METHODS

Lu AF27139 is a highly selective and potent small molecule antagonist at rat, mouse, and human forms of the P2X7 receptor, with excellent pharmacokinetic and CNS permeability properties. In the current experiments, we probed the utility of previously characterized and novel signaling cascades exposed to Lu AF27139 using cultured microglia combined with release assays. Subsequently, we assessed the biomarker potential of identified candidate molecules in the rat chronic constriction injury (CCI) model of neuropathic pain; study design limitations precluded their assessment in spared nerve injury (SNI) rats.

RESULTS

Lu AF27139 blocked several pain-relevant pathways downstream of P2X7 receptors in-vitro. At brain and spinal cord receptor occupancy levels capable of functionally blocking P2X7 receptors, it diminished neuropathic hypersensitivity in SNI rats, and less potently in CCI rats. Although tissue levels of numerous molecules previously linked to neuropathic pain and P2X7 receptor function (e.g. IL-6, IL-1β, cathepsin-S, 2-AG) were unaffected by CCI, Lu AF27139-mediated regulation of spinal PGE2 and miRNA (e.g. rno-miR-93-5p) levels increased by CCI aligned with its ability to diminish neuropathic hypersensitivity.

CONCLUSIONS

We have identified a pain-relevant P2X7 receptor-regulated mechanism in neuropathic rats that could hold promise as a translatable biomarker and by association enhance the clinical progression of P2X7 receptor antagonists in neuropathic pain.

Recent Advances in Osteoclast Biological Behavior

With the progress of the aging population, bone-related diseases such as osteoporosis and osteoarthritis have become urgent problems. Recent studies have demonstrated the importance of osteoclasts in bone homeostasis, implying these will be an important mediator in the treatment of bone-related diseases. Up to now, several reviews have been performed on part of osteoclast biological behaviors such as differentiation, function, or apoptosis. However, few reviews have shown the complete osteoclast biology and research advances in recent years. Therefore, in this review, we focus on the origin, differentiation, apoptosis, behavior changes and coupling signals with osteoblasts, providing a simple but comprehensive overview of osteoclasts for subsequent studies.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

P2X7 receptor in multifaceted cellular signalling and its relevance as a potential therapeutic target in different diseases

P2X7 receptor, a purinergic receptor family member, is abundantly expressed on many cells, including immune, muscle, bone, neuron, and glia. It acts as an ATP-activated cation channel that permits the influx of Ca²⁺, Na⁺ and efflux of K⁺ ions. The P2X7 receptor plays crucial roles in many physiological processes including cytokine and chemokine secretion, NLRP3 inflammasome activation, cellular growth and differentiation, locomotion, wound healing, transcription factors activation, cell death and T-lymphocyte survival. Past studies have demonstrated the up-regulation and direct association of this receptor in many pathophysiological conditions such as cancer, diabetics, arthritis, tuberculosis (TB) and inflammatory diseases. Hence, targeting this receptor is considered a worthwhile approach to lessen the afflictions associated with the disorders mentioned above by understanding the receptor architecture and downstream signalling processes. Here, in the present review, we have dissected the structural and functional aspects of the P2X7 receptor, emphasizing its role in various diseased conditions. This information will provide in-depth knowledge about the receptor and help to develop apt curative methodologies for the betterment of humanity in the coming years.

The effect of P2X7 on cadmium-induced osteoporosis in mice

Cadmium (Cd), an environmental pollutant, induces osteoporosis by directly destroying bone tissue, but its direct damaging effect on bone cells is not fully illustrated. Here, we treated mouse bone marrow stem cells (BMSC) and bone marrow macrophages (BMM) with Cd, and gave BALB/c mice Cd in water. Long-term Cd exposure significantly inhibited BMSC osteogenesis and osteoclast differentiation in vitro, and induced osteoporosis in vivo. Cd exposure also reduced P2X7 expression dramatically. However, P2X7 deletion significantly inhibited osteoblast and osteoclast differentiation; P2X7 overexpression obviously reduced the suppression effect of Cd on osteoblast and osteoclast differentiation. The suppression of P2X7-PI3K-AKT signaling aggravated the effect of Cd. In mice, short-term Cd exposure did not result in osteoporosis, but bone formation was inhibited, RANKL expression was increased, and osteoclasts were significantly increased in vivo. In vitro, short-term Cd exposure not only increased osteoclast numbers, but also promoted osteoclast adhesion function at late-stage osteoclast differentiation. Cd exposure also reduced P2X7 expression in vivo and in vitro. Our results demonstrate that short-term Cd exposure does not affect osteoblast and osteoclast apoptosis in vivo and in vitro, but long-term Cd exposure significantly increases bone tissue apoptosis. Overall, our results describe a novel mechanism for Cd-induced osteoporosis.

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.

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.

Cadmium exposure triggers osteoporosis in duck via P2X7/PI3K/AKT-mediated osteoblast and osteoclast differentiation

Cadmium is a common environmental pollutant that accumulates in the bone and kidneys and causes severe health and social problems. However, the effects of Cd on the occurrence of osteoporosis and its mechanism of action in this process are unclear. To test whether Cd-induced osteoporosis is mediated via P2X7/PI3K/AKT signaling, duck bone marrow mesenchymal stem cells (BMSCs) and bone marrow macrophage cells (BMMs) were treated with Cd for 5 days, and duck embryos were treated with Cd_. Micro-CT analysis indicated that Cd-induced osteoporosis occurs in vivo, and histopathology and immunohistochemical analyses also revealed that Cd induced bone damage and the downregulation of osteogenic and bone resorption-related proteins. Cd exposure significantly inhibited the differentiation of BMSCs and BMMs into osteoblasts and osteoclasts in vitro, and promoted osteoblast and osteoclast apoptosis. Cd exposure significantly downregulated the P2X7/PI3K/AKT signaling pathway in vivo and in vitro, and inhibition of this signaling pathway significantly aggravated osteoblast and osteoclast differentiation. Cd exposure also upregulated the OPG/RANKL ratio in vivo and in vitro, further inhibiting osteoclast differentiation. These results demonstrate that Cd causes osteoporosis in duck by inhibiting P2X7/PI3K/AKT signaling and increasing the OPG/RANKL ratio. These results establish a previously unknown mechanism of Cd-induced osteoporosis.

Sphingosylphosphorylcholine blocks ovariectomy-induced bone loss by suppressing Ca2+ /calmodulin-mediated osteoclast differentiation

Osteoporosis is a disease in which bone mineral density decreases due to abnormal activity of osteoclasts, and is commonly found in post‐menopausal women who have decreased levels of female hormones. Sphingosylphosphorylcholine (SPC) is an important biological lipid that can be converted to sphingosine‐1‐phosphate (S1P) by autotaxin. S1P is known to be involved in osteoclast activation by stimulating osteoblasts, but bone regulation by SPC is not well understood. In this study, we found that SPC strongly inhibits RANKL‐induced osteoclast differentiation. SPC‐induced inhibitory effects on osteoclast differentiation were not affected by several antagonists of S1P receptors or pertussis toxin, suggesting cell surface receptor independency. However, SPC inhibited RANKL‐induced calcineurin activation and subsequent NFATc1 activity, leading to decrease of the expression of Trap and Ctsk. Moreover, we found that bone loss in an experimental osteoporosis mouse model was recovered by SPC injection. SPC also blocked ovariectomy‐induced body weight increase and Nfatc1 gene expression in mice. We also found that SPC inhibits RANKL‐induced osteoclast differentiation in human macrophages. Since currently available treatments for osteoporosis, such as administration of female hormones or hormone receptor modulators, show serious side effects, SPC has potential as a new agent for osteoporosis treatment.

Extracellular Nucleotides Regulate Arterial Calcification by Activating Both Independent and Dependent Purinergic Receptor Signaling Pathways

Arterial calcification, the deposition of calcium-phosphate crystals in the extracellular matrix, resembles physiological bone mineralization. It is well-known that extracellular nucleotides regulate bone homeostasis raising an emerging interest in the role of these molecules on arterial calcification. The purinergic independent pathway involves the enzymes ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-nucleoside triphosphate diphosphohydrolases (NTPDases), 5′-nucleotidase and alkaline phosphatase. These regulate the production and breakdown of the calcification inhibitor—pyrophosphate and the calcification stimulator—inorganic phosphate, from extracellular nucleotides. Maintaining ecto-nucleotidase activities in a well-defined range is indispensable as enzymatic hyper- and hypo-expression has been linked to arterial calcification. The purinergic signaling dependent pathway focusses on the activation of purinergic receptors (P1, P2X and P2Y) by extracellular nucleotides. These receptors influence arterial calcification by interfering with the key molecular mechanisms underlying this pathology, including the osteogenic switch and apoptosis of vascular cells and possibly, by favoring the phenotypic switch of vascular cells towards an adipogenic phenotype, a recent, novel hypothesis explaining the systemic prevention of arterial calcification. Selective compounds influencing the activity of ecto-nucleotidases and purinergic receptors, have recently been developed to treat arterial calcification. However, adverse side-effects on bone mineralization are possible as these compounds reasonably could interfere with physiological bone mineralization.

Involvement of P2X7 Receptors in the Osteogenic Differentiation of Mesenchymal Stromal/Stem Cells Derived from Human Subcutaneous Adipose Tissue

The ionotropic P2X7 receptor (P2X7R) is involved in bone homeostasis but its role in osteogenesis is controversial. Thus, we investigated the expression of P2X7R and the effects exerted by its modulation in mesenchymal stromal cells from human subcutaneous adipose tissue (S-ASCs), which have potential therapeutic application in bone regenerative medicine. We found that undifferentiated S-ASCs expressed P2X7R and its functional splice variants P2X7AR and P2X7BR. Cell stimulation by P2X7R agonist BzATP (100 μM) neither modified proliferation nor caused membrane pore opening while increasing intracellular Ca²⁺ levels and migration. The P2X7R antagonist A438079 reversed these effects. However, 25-100 μM BzATP, administered to S-ASCs undergoing osteogenic differentiation, dose-dependently decreased extracellular matrix mineralization and expression of osteogenic transcription factors Runx2, alkaline phosphatase and osteopontin. These effects were not coupled to cell proliferation reduction or to cell death increase, but were associated to decrease in P2X7AR and P2X7BR expression. In contrast, expression of P2X7R, especially P2X7BR isoform, significantly increased during the osteogenic process. Noteworthy, the antagonist A438079, administered alone, at first restrained cell differentiation, enhancing it later. Accordingly, A438079 reversed BzATP effects only in the second phase of S-ASCs osteogenic differentiation. Apyrase, a diphosphohydrolase converting ATP/ADP into AMP, showed a similar behavior. Altogether, findings related to A438079 or apyrase effects suggest an earlier and prevailing pro-osteogenic activity by endogenous ATP and a later one by adenosine derived from endogenous ATP metabolism. Conversely, P2X7R pharmacological stimulation by BzATP, mimicking the effects of high ATP levels occurring during tissue injuries, depressed receptor expression/activity impairing MSC osteogenic differentiation.

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.

To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect the individual roles of P2X receptors and the pharmaceutical industry has been involved in pushing forward clinical development of several lead compounds. During the discovery phase, a number of positive allosteric modulators have been described for P2X receptors and these have been useful in assigning physiological roles to receptors. This review will consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement of P2X receptor activity would offer any therapeutic benefit. We will review what is known about identified compounds acting as positive allosteric modulators and the recent identification of drug binding pockets for such modulators.

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

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

Signaling of the Purinergic System in the Joint

The joint is a complex anatomical structure consisting of different tissues, each with a particular feature, playing together to give mobility and stability at the body. All the joints have a similar composition including cartilage for reducing the friction of the movement and protecting the underlying bone, a synovial membrane that produces synovial fluid to lubricate the joint, ligaments to limit joint movement, and tendons for the interaction with muscles. Direct or indirect damage of one or more of the tissues forming the joint is the foundation of different pathological conditions. Many molecular mechanisms are involved in maintaining the joint homeostasis as well as in triggering disease development. The molecular pathway activated by the purinergic system is one of them.The purinergic signaling defines a group of receptors and intermembrane channels activated by adenosine, adenosine diphosphate, adenosine 5’-triphosphate, uridine triphosphate, and uridine diphosphate. It has been largely described as a modulator of many physiological and pathological conditions including rheumatic diseases. Here we will give an overview of the purinergic system in the joint describing its expression and function in the synovium, cartilage, ligament, tendon, and bone with a therapeutic perspective.

Autotaxin Implication in Cancer Metastasis and Autoimunne Disorders: Functional Implication of Binding Autotaxin to the Cell Surface

Autotaxin (ATX) is an exoenzyme which, due to its unique lysophospholipase D activity, is responsible for the synthesis of lysophosphatidic acid (LPA). ATX activity is responsible for the concentration of LPA in the blood. ATX expression is increased in various types of cancers, including breast cancer, where it promotes metastasis. The expression of ATX is also remarkably increased under inflammatory conditions, particularly in the osteoarticular compartment, where it controls bone erosion. Biological actions of ATX are mediated by LPA. However, the phosphate head group of LPA is highly sensitive to degradation by the action of lipid phosphate phosphatases, resulting in LPA inactivation. This suggests that for efficient action, LPA requires protection, which is potentially achieved through docking to a carrier protein. Interestingly, recent reports suggest that ATX might act as a docking molecule for LPA and also support the concept that binding of ATX to the cell surface through its interaction with adhesive molecules (integrins, heparan sulfate proteoglycans) could facilitate a rapid route of delivering active LPA to its cell surface receptors. This new mechanism offers a new vision of how ATX/LPA works in cancer metastasis and inflammatory bone diseases, paving the way for new therapeutic developments.

The Number of Platelets in Patient's Blood Influences the Mechanical and Morphological Properties of PRP-Clot and Lysophosphatidic Acid Quantity in PRP

The objectives of this study were to compare platelet-rich plasma (PRP) from patients with different concentrations of platelets and to assess the influence of these PRP preparations on human osteoblast (hOB) activity. In the literature, growth factors released by activated platelets have been considered responsible for the active role of PRP on bone regeneration but no specific role has been attributed to lysophosphatidic acid (LPA) as a possible effector of biological responses. In this study, patients were grouped into either group A (poor in platelets) or group B (rich in platelets). Clots from PRP fraction 2 (F2-clots), obtained with CaCl2 activation of PRP from the two groups, were compared macroscopically and microscopically and for their mechanical properties before testing their activity on the proliferation and migration of hOB. LPA was quantified before and after PRP fractioning and activation. The fibrin network of F2-clots from patients with a lower platelet concentration had an organized structure with large and distinct fibers while F2-clots from patients in group B revealed a similar structure to those in group A but with a slight increase in density. ELISA results showed a significantly higher plasma level of LPA in patients with a higher platelet concentration (group B) in comparison to those in group A (p < 0.05). This different concentration was evidenced in PRP but not in the clots. Depending on the number of platelets in patient's blood, a PRP-clot with higher or lower mechanical properties can be obtained. The higher level of LPA in PRP from patients richer in platelets should be considered as responsible for the higher hOB activity in bone regeneration.

Electromagnetic field treatment increases purinergic receptor P2X7 expression and activates its downstream Akt/GSK3β/β-catenin axis in mesenchymal stem cells under osteogenic induction

Background

Imbalance in bone formation and resorption is a crucial component of the pathological process leading to osteoporosis. Electromagnetic fields (EMFs) have been reported to be beneficial to osteogenesis, although the exact mechanism has not been fully clarified. Purinergic receptor P2X7 is expressed in osteoblasts and is reported to participate in the regulation of bone metabolism.

Objectives

To elucidate the link between EMFs and P2X7 expression and investigate its potential as a novel therapeutic target in osteoporosis.

Method

We investigated the effect of EMFs on P2X7 expression and downstream signaling in human bone marrow mesenchymal stem cells (h-MSCs). We also established an ovariectomized (OVX) osteoporosis rat model to evaluate the therapeutic efficacy of combining EMFs with P2X7 agonists.

Results

EMF treatment increased P2X7 expression in h-MSCs under conditions of osteogenic induction but not under regular culture conditions. P2X7 or PI3K/Akt inhibition partially inhibited the pro-osteogenic effect of EMF and lowered the EMF-stimulated activity of the Akt/GSK3β/β-catenin axis. No additive effect of this suppression was observed following simultaneous inhibition of P2X7 and PI3K/Akt. EMF treatment in the presence of a P2X7 agonist had a greater effect in increasing osteogenic marker expression than that of EMF treatment alone. In the OVX osteoporosis model, the therapeutic efficacy of combining EMFs with P2X7 agonists was superior to that of EMF treatment alone.

Conclusions

EMF treatment increases P2X7 expression by h-MSCs during osteogenic differentiation, leading to activation of the Akt/GSK3β/β-catenin axis, which promotes the osteogenesis. Our findings also indicate that combined EMF and P2X7 agonist treatment may be an effective novel strategy for osteoporosis therapy.

Regulation of intracellular Ca2+/CaMKII signaling by TRPV4 membrane translocation during osteoblastic differentiation

Bone constantly remodels between resorption by osteoclasts and formation by osteoblasts; therefore the functions of osteoblasts are pivotal for maintaining homeostasis of bone mass. Transient receptor potential vanilloid 4 (TRPV4), a type of mechanosensitive channel, has been reported to be a key regulator in bone remodeling. However, the relationship between TRPV4 and osteoblast function remains largely elusive. Only little is known about the spatial distribution change of TRPV4 during osteoblastic differentiation and related signal events. Based on three-dimensional super-resolution microscopy, our results clearly showed a different distribution of TRPV4 in undifferentiated and differentiated osteoblasts, which reflected the plasma membrane translocation of TRPV4 along with prolonged differentiation. GSK1016790A (GSK101), the most potent agonist of TRPV4, triggered rapid calcium entry and calmodulin-dependent protein kinase II (CaMKII) phosphorylation via TRPV4 activation in a differentiation-dependent manner, indicating that the abundance of TRPV4 at the cell surface resulting from differentiation may be related to the modulation of Ca2+ response and CaMKII activity. These data provide compelling evidences for the plasma membrane translocation of TRPV4 during osteoblastic differentiation as well as demonstrate the regulation of downstream Ca2+/CaMKII signaling.

Osteoclast-Derived Autotaxin, a Distinguishing Factor for Inflammatory Bone Loss

OBJECTIVE

The severity of rheumatoid arthritis (RA) correlates directly with bone erosions arising from osteoclast (OC) hyperactivity. Despite the fact that inflammation may be controlled in patients with RA, those in a state of sustained clinical remission or low disease activity may continue to accrue erosions, which supports the need for treatments that would be suitable for long-lasting inhibition of OC activity without altering the physiologic function of OCs in bone remodeling. Autotaxin (ATX) contributes to inflammation, but its role in bone erosion is unknown.

METHODS

ATX was targeted by inhibitory treatment with pharmacologic drugs and also by conditional inactivation of the ATX gene Ennp2 in murine OCs (ΔATX^{C tsk} ). Arthritic and erosive diseases were studied in human tumor necrosis factor-transgenic (hTNF^+/- ) mice and mice with K/BxN serum transfer-induced arthritis. Systemic bone loss was also analyzed in mice with lipopolysaccharide (LPS)-induced inflammation and estrogen deprivation. Joint inflammation and bone erosion were assessed by histology and micro-computed tomography. The role of ATX in RA was also examined in OC differentiation and activity assays.

RESULTS

OCs present at sites of inflammation overexpressed ATX. Pharmacologic inhibition of ATX in hTNF^+/- mice, as compared to vehicle-treated controls, significantly mitigated focal bone erosion (36% decrease; P < 0.05) and systemic bone loss (43% decrease; P < 0.05), without affecting synovial inflammation. OC-derived ATX was revealed to be instrumental in OC bone resorptive activity and was up-regulated by the inflammation elicited in the presence of TNF or LPS. Specific loss of ATX in OCs from mice subjected to ovariectomy significantly protected against the systemic bone loss and erosion that had been induced with LPS and K/BxN serum treatments (30% reversal of systemic bone loss [P < 0.01]; 55% reversal of erosion [P < 0.001]), without conferring bone-protective properties.

CONCLUSION

Our results identify ATX as a novel OC factor that specifically controls inflammation-induced bone erosions and systemic bone loss. Therefore, ATX inhibition offers a novel therapeutic approach for potentially preventing bone erosion in patients with RA.

Lysophosphatidic acid: Its role in bone cell biology and potential for use in bone regeneration

Lysophosphatidic acid (LPA) is a simple phospholipid that exerts pleiotropic effects on numerous cell types by activating its family of cognate G protein-coupled receptors (GPCRs) and participates in many biological processes, including organismal development, wound healing, and carcinogenesis. Bone cells, such as bone marrow mesenchymal stromal (stem) cells (BMSCs), osteoblasts, osteocytes and osteoclasts play essential roles in bone homeostasis and repair. Previous studies have identified the presence of specific LPA receptors in these bone cells. In recent years, an increasing number of cellular effects of LPA, such as the induction of cell proliferation, survival, migration, differentiation and cytokine secretion, have been found in different bone cells. Moreover, some biomaterials containing LPA have shown the ability to enhance osteogenesis. This review will focus on findings associated with LPA functions in these bone cells and present current studies related to the application of LPA in bone regenerative medicine. Further understanding this information will help us develop better strategies for bone healing.

Purinergic receptors and the inflammatory response mediated by lipids

The inflammatory response is regulated by the production of different extracellular mediators, including lipids and extracellular nucleotides. In the extracellular environment, intermediate lipids activate specific G-protein-coupled receptors (GPCRs) in target cells and promote cell recruitment and activation. Extracellular nucleotides activate two types of receptors, the ionotropic purinergic P2X and the metabotropic purinergic P2Y receptors, inducing the release of cytokines and promoting cell recruitment. Several P2X receptors are associated with an increase in the production of immunoactive lipids mediators, which in turn are able to interfere with the activation of different P2Y receptors, establishing a tight signalling link between purinergic receptors and lipid mediators. In this review, we summarise recent studies indicating signalling crosstalk between purinergic P2X and P2Y receptor activation and lipid mediators with a focus on inflammatory diseases. Novel concepts arising from this crosstalk would result in the development of combinatorial therapies targeting lipid synthesis together with individual P2 receptors for the management of inflammatory diseases.

A Mechanism-Based Approach to P2X7 Receptor Action

The ligand-gated ion channel P2X7 receptor attracts special attention due to its widespread presence as well as its unusual responses. Besides relatively well-understood mechanisms such as intracellular Ca2+ increase and K+ depletion, the P2X7 receptor activates other peculiar responses whose mechanisms are not fully understood. The best known among these is the permeabilization of the cell membrane to large molecules. This permeabilization has been explained by the activation of a nonselective permeation pathway by the P2X7 receptor, a phenomenon called "pore formation." However, with the emergence of new data, it became apparent that large molecules enter the cell directly through the pore of the ion channel, similar to the smaller ions. This explanation seems to be true for cationic large molecules. On the other hand, there is still convincing evidence indicating that the P2X7 receptor activates a separate pathway that permeates anionic large molecules in some cell types. Furthermore, there exist functional data suggesting that the P2X7 receptor may also activate other intracellular signaling molecules or other ion channels. Interestingly and contrary to what is expected from a ligand-gated channel, these activations occur in a seemingly direct manner. Somewhat overshadowed by the pore formation hypothesis, these action mechanisms may lead to a better understanding of not only the P2X7 receptor itself but also some important physiologic functions such as the release of anionic autocoids/neurotransmitters in the central nervous system. This review discusses, assesses, and draws attention to the data concerning these neglected but potentially important points in the P2X7 receptor field.

Role of the P2X7 receptor in inflammation-mediated changes in the osteogenesis of periodontal ligament stem cells

Accumulating evidence indicates that the pluripotency of periodontal ligament stem cells (PDLSCs) is compromised under inflammatory conditions; however, the underlying mechanisms remain largely unexplored. In this study, we hypothesize that the P2X7 receptor (P2X7R) is a key molecule linked to inflammation-associated impairment of PDLSCs. We first investigated P2X7R expression in PDLSCs under normal and inflammatory conditions and then determined the effect of a P2X7R agonist (BzATP) or antagonist (BBG) on PDLSC osteogenesis under various conditions. Gene-modified PDLSCs were used to further examine the role of P2X7R and the signaling pathway underlying P2X7R-enhanced osteogenesis. We found that inflammatory conditions decreased P2X7R expression in PDLSCs and reduced osteogenesis in these cells. In addition, activation of P2X7R by BzATP or overexpression of P2X7R via gene transduction reversed the inflammation-mediated decrease in PDLSC osteogenic differentiation. When selected osteogenesis-related signaling molecules were screened, the PI3K-AKT-mTOR pathway was identified as potentially involved in P2X7R-enhanced PDLSC osteogenesis. Our data reveal a crucial role for P2X7R in PDLSC osteogenesis under inflammatory conditions, suggesting a new therapeutic target to reverse or rescue inflammation-mediated changes in PDLSCs for future mainstream therapeutic uses.

The interactions of physical activity, exercise and genetics and their associations with bone mineral density: implications for injury risk in elite athletes

Low bone mineral density (BMD) is established as a primary predictor of osteoporotic risk and can also have substantial implications for athlete health and injury risk in the elite sporting environment. BMD is a highly multi-factorial phenotype influenced by diet, hormonal characteristics and physical activity. The interrelationships between such factors, and a strong genetic component, suggested to be around 50-85% at various anatomical sites, determine skeletal health throughout life. Genome-wide association studies and case-control designs have revealed many loci associated with variation in BMD. However, a number of the candidate genes identified at these loci have no known associated biological function or have yet to be replicated in subsequent investigations. Furthermore, few investigations have considered gene-environment interactions-in particular, whether specific genes may be sensitive to mechanical loading from physical activity and the outcome of such an interaction for BMD and potential injury risk. Therefore, this review considers the importance of physical activity on BMD, genetic associations with BMD and how subsequent investigation requires consideration of the interaction between these determinants. Future research using well-defined independent cohorts such as elite athletes, who experience much greater mechanical stress than most, to study such phenotypes, can provide a greater understanding of these factors as well as the biological underpinnings of such a physiologically "extreme" population. Subsequently, modification of training, exercise or rehabilitation programmes based on genetic characteristics could have substantial implications in both the sporting and public health domains once the fundamental research has been conducted successfully.

TNF-α suppresses osteogenic differentiation of MSCs by accelerating P2Y2 receptor in estrogen-deficiency induced osteoporosis

Tumor Necrosis Factor-α (TNF-α)-inhibited osteogenic differentiation of mesenchymal stem cells (MSCs) contributes to impaired bone formation, which plays a central role in the pathogenesis of postmenopausal osteoporosis. However, the exact mechanisms of TNF-α-inhibited osteoblast differentiation have not been fully elucidated. Multiple P2 purinoceptor subtypes are expressed in several species of osteoblasts and are confirmed to regulate bone metabolism. The purpose of this study is to investigate whether P2 purinoceptors are involved in TNF-α-inhibited osteoblast differentiation. This study shows TNF-α increased P2Y₂ receptor expression in the differentiation of MSCs into osteoblasts in a noticeable manner. Overexpressing or silencing of the P2Y₂ receptor either impaired or promoted osteogenic differentiation of MSCs significantly. Silencing of the P2Y₂ receptor attenuated the inhibitory effects of TNF-α on osteoblastic differentiation of MSCs. In addition, silencing of the P2Y₂ receptor evidently alleviated TNF-α-inhibited MSC proliferation. P2Y₂ receptor expression was mechanistically upregulated by TNF-α mainly through extracellular regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) signaling pathways. Overall, our results revealed a novel function of the P2Y₂ receptor and suggested suppressing the P2Y₂ receptor may be an effective strategy to promote bone formation in estrogen deficiency-induced osteoporosis.

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.

Lysophosphatidic Acid Analogue rather than Lysophosphatidic Acid Promoted the Bone Formation In Vivo

Lysophosphatidic acid (LPA), a bioactive lipid molecule, has recently emerged as physiological and pathophysiological regulator in skeletal biology. Here we evaluate the effects of LPA on bone formation in vivo in murine femoral critical defect model. Primary femoral osteoblasts were isolated and treated with osteogenic induction conditional media supplemented with 20 μM LPA or LPA analogue. Mineralized nodules were visualized by Alizarin Red S staining. Forty-five C57BL/6 mice underwent unilateral osteotomy. The femoral osteotomy gap was filled with porous scaffolds of degradable chitosan/beta-tricalcium phosphate containing PBS, LPA, or LPA analogue. 2, 5, and 10 weeks after surgery, mice were sacrificed and femurs were harvested and prepared for Micro-Computed Tomography (Micro-CT) and histological analysis. Alizarin Red S staining showed that LPA and LPA analogue significantly enhanced the mineral deposition in osteoblasts. Micro-CT 3D reconstruction images and HE staining revealed that significantly more newly formed bone in osteotomy was treated with LPA analogue when compared to control and LPA group, which was verified by histological analysis and biomechanical characterization testing. In summary, our study demonstrated that although LPA promotes mineralized matrix formation in vitro, the locally administrated LPA was not effective in promoting bone formation in vivo. And bone formation was enhanced by LPA analogue, administrated locally in vivo. LPA analogue was a potent stimulating factor for bone formation in vivo due to its excellent stability.

P2X7 ionotropic receptor is functionally expressed in rabbit articular chondrocytes and mediates extracellular ATP cytotoxicity

Extracellular ATP regulates various cellular functions by engaging multiple subtypes of P2 purinergic receptors. In many cell types, the ionotropic P2X7 receptor mediates pathological events such as inflammation and cell death. However, the importance of this receptor in chondrocytes remains largely unexplored. Here, we report the functional identification of P2X7 receptor in articular chondrocytes and investigate the involvement of P2X7 receptors in ATP-induced cytotoxicity. Chondrocytes were isolated from rabbit articular cartilage, and P2X7 receptor currents were examined using the whole-cell patch-clamp technique. ATP-induced cytotoxicity was evaluated by measuring caspase-3/7 activity, lactate dehydrogenase (LDH) leakage, and prostagrandin E₂ (PGE₂) release using microscopic and fluorimetric/colorimetric evaluation. Extracellular ATP readily evoked a cationic current without obvious desensitization. This ATP-activated current was dose related, but required millimolar concentrations. A more potent P2X7 receptor agonist, BzATP, also activated this current but at 100-fold lower concentrations. ATP-induced currents were largely abolished by selective P2X7 antagonists, suggesting a predominant role for the P2X7 receptor. RT-PCR confirmed the presence of P2X7 in chondrocytes. Heterologous expression of a rabbit P2X7 clone successfully reproduced the ATP-induced current. Exposure of chondrocytes to ATP increased caspase-3/7 activities, an effect that was totally abrogated by P2X7 receptor antagonists. Extracellular ATP also enhanced LDH release, which was partially attenuated by the P2X7 inhibitor. The P2X7 receptor-mediated elevation in apoptotic caspase signaling was accompanied by increased PGE₂ release and was attenuated by inhibition of either phospholipase A₂ or cyclooxygenase-2. This study provides direct evidence for the presence of functional P2X7 receptors in articular chondrocytes. Our results suggest that the P2X7 receptor is a potential therapeutic target in chondrocyte death associated with cartilage injury and disorders including osteoarthritis.

Effect of Icariin on Tibial Dyschondroplasia Incidence and Tibial Characteristics by Regulating P2RX7 in Chickens

Tibial dyschondroplasia (TD) is a disease of rapid growing chickens that occurs in many avian species; it is characterized by nonvascular and nonmineralized growth plates, along with tibia bone deformation and lameness. Icariin is widely used to treat bone diseases in humans, but no report is available regarding the effectiveness of icariin against avian TD. Therefore, this study was designed to determine its effect against TD. For this purpose, a total of 180 broiler chicks were distributed into three groups including control, TD, and icariin group. Control group was given a standard normal diet, while TD and icariin groups received normal standard diet containing 50 mg/kg thiram to induce TD from days 3 to 7 after hatch. After the induction of TD, the chicks of icariin group were fed with standard normal diet by adding 10 mg/kg icariin in water. Then morphological and production parameters analysis of tibial bone indicators, physiological index changes, and gene expression were examined. The results showed that icariin administration not only decreased the mortality but also mitigated the lameness and promoted the angiogenesis, which diminished the TD lesion and significantly increased the expression of P2RX7 (P < 0.05) in TD affected thiram induced chicks. In conclusion, present findings suggest that icariin has a significant role in promoting the recovery of chicken growth plates affected by TD via regulating the P2RX7. Our findings reveal a new target for clinical treatment and prevention of TD in broiler chickens.

Osteoblast-osteoclast interactions

Bone homeostasis depends on the resorption of bones by osteoclasts and formation of bones by the osteoblasts. Imbalance of this tightly coupled process can cause diseases such as osteoporosis. Thus, the mechanisms that regulate communication between osteoclasts and osteoblasts are critical to bone cell biology. It has been shown that osteoblasts and osteoclasts can communicate with each other through direct cell-cell contact, cytokines, and extracellular matrix interaction. Osteoblasts can affect osteoclast formation, differentiation, or apoptosis through several pathways, such as OPG/RANKL/RANK, RANKL/LGR4/RANK, Ephrin2/ephB4, and Fas/FasL pathways. Conversely, osteoclasts also influence formation of bones by osteoblasts via the d2 isoform of the vacuolar (H+) ATPase (v-ATPase) V0 domain (Atp6v0d2), complement component 3a, semaphorin 4D or microRNAs. In addition, cytokines released from the resorbed bone matrix, such as TGF-β and IGF-1, also affect the activity of osteoblasts. Drugs could be developed by enhancing or restricting some of these interactions. Several reviews have been performed on the osteoblast-osteoclast communication. However, few reviews have shown the research advances in the recent years. In this review, we summarized the current knowledge on osteoblast-osteoclast communication.

miR-373 regulates inflammatory cytokine-mediated chondrocyte proliferation in osteoarthritis by targeting the P2X7 receptor

Inflammatory cytokines commonly initiate extreme changes in the synovium and cartilage microenvironment of osteoarthritis (OA) patients, which subsequently cause cellular dysfunction, especially in chondrocytes. It has been reported that induction of the purinergic P2X7 receptor (P2X7R) can regulate the expression of a variety of inflammatory factors, including interleukin (IL)-6 and -8, leading to OA pathogenesis. However, knowledge of the mechanism of upregulation of P2X7R in OA is still incomplete, and its role in chondrocyte proliferation is also not clear. It was reported previously that the expression of P2X7R was controlled by certain microRNAs, and so we tested the expression of several microRNAs and found that microRNA-373 (miR-373) was downregulated in the chondrocytes from OA patients. Regarding the mechanism of action, miR-373 inhibited chondrocyte proliferation by suppressing the expression of P2X7R, as well as inflammatory factors such as IL-6 and IL-8. Furthermore, the proliferative and pro-inflammatory effects of miR-373 on the chondrocytes could be suppressed by a P2X7R antagonist, further suggesting that miR-373 mediates chondrocyte proliferation and inflammation by targeting P2X7R. Generally, our results suggest a novel method for OA treatment by targeting the miR-373-P2X7R pathway.

PYK2 mediates BzATP-induced extracellular matrix proteins synthesis

Mechanical stimuli such as fluid shear and cyclic tension force induced extracellular adenosine triphosphate (ATP) release in osteoblasts. In particular, cyclic tension force-induced ATP enhances bone formation through P2X7 activation. Proline-rich tyrosine kinase 2 (PYK2) mediate osteoblasts differentiation is induced by mechanical stimuli. Furthermore, activation of PYK2 also was a response to integrin by mechanical stimuli. Extracellular matrix protein (ECMP)s, which are important factors for bone formation are expressed by osteoblasts. However, the effect of the interaction of 2'(3)-Ο-(4-Benzoylbenzoyl) adenosine-5'-triphosphate (BzATP), which is the agonist of the mechanosensitive receptor P2X7, with PYK2 on ECMP production is poorly understood. Thus, our purpose was to investigate the effects of PYK2 on BzATP-induced ECMP production in osteoblasts. BzATP increased phospho-PYK2 protein expression on days 3 and 7 of culture. Furthermore, the PYK2 inhibitor PF431394 inhibited the stimulatory effect of BzATP on the expression of type I collagen, bone sialoprotein and osteocalcin expression. PF431396 did not inhibit the stimulatory effect of BzATP on osteopontin (OPN) mRNA expression. These results suggest that mechanical stimuli activate P2X7 might induce ECMPs expression through PYK2 except in the case of OPN expression. Altogether, mechanical stimuli-induced ECMPs production might be implicated by extracellular ATP secretion or integrin via PYK2 activation.

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.

Purinergic 2X7 receptor activation regulates WNT signaling in human mandibular-derived osteoblasts

OBJECTIVE

Purinergic 2X7 receptor (P2X7R) activation modulates in vitro mineralization by primary rat and human osteoblasts. However, the detailed mechanism of how P2X7R activation affects primary human osteoblasts remains unclear. The aim of this study was to investigate the effect of P2X7R activation on human mandibular-derived osteoblast (hMOB) differentiation.

DESIGN

Primary human osteoblasts were obtained from non-pathologic mandibular bone from healthy patients. The hMOBs were cultured in osteogenic medium with or without 0.5-5μM 2'(3')-O-(4-benzoyl) benzoyl-ATP (BzATP), a selective P2X7R agonist. The mRNA expression of osteogenic differentiation markers and WNT-signaling molecules was investigated by quantitative real time polymerase chain reaction. In vitro mineral deposition was determined by Alizarin Red S staining. Transfection of small interfering RNA was performed to confirm the effect of P2X7R activation. WNT/β-catenin signaling was detected by immunofluorescence staining for β-catenin.

RESULTS

BzATP inhibited osteogenic medium-induced RUNX2 and OSX mRNA expression in hMOBs. Moreover, BzATP significantly retarded in vitro mineralization. These findings indicated that BzATP/P2X7R activation inhibited hMOB differentiation. Interestingly, reduced WNT3A mRNA expression and blockage of osteogenic medium-induced β-catenin nuclear translocation were also found. These data suggested that WNT signaling might be a target of P2X7R-regulated osteogenic differentiation. Furthermore, when recombinant human WNT3A was added to the BzATP-treated group, it rescued the reduced RUNX2 and OSX expression, and in vitro mineralization.

CONCLUSION

Our results demonstrate that P2X7R activation by BzATP inhibits hMOB differentiation. This inhibitory effect was associated with inhibition of the WNT/β-catenin signaling pathway.

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.

Ticagrelor regulates osteoblast and osteoclast function and promotes bone formation in vivo via an adenosine-dependent mechanism

As many as 10% of bone fractures heal poorly, and large bone defects resulting from trauma, tumor, or infection may not heal without surgical intervention. Activation of adenosine A2A receptors (A2ARs) stimulates bone formation. Ticagrelor and dipyridamole inhibit platelet function by inhibiting P2Y12 receptors and platelet phosphodiesterase, respectively, but share the capacity to inhibit cellular uptake of adenosine and thereby increase extracellular adenosine levels. Because dipyridamole promotes bone regeneration by an A2AR-mediated mechanism we determined whether ticagrelor could regulate the cells involved in bone homeostasis and regeneration in a murine model and whether inhibition of P2Y12 or indirect A2AR activation via adenosine was involved. Ticagrelor, dipyridamole and the active metabolite of clopidogrel (CAM), an alternative P2Y12 antagonist, inhibited osteoclast differentiation and promoted osteoblast differentiation in vitro. A2AR blockade abrogated the effects of ticagrelor and dipyridamole on osteoclast and osteoblast differentiation whereas A2BR blockade abrogated the effects of CAM. Ticagrelor and CAM, when applied to a 3-dimentional printed resorbable calcium-triphosphate/hydroxyapatite scaffold implanted in a calvarial bone defect, promoted significantly more bone regeneration than the scaffold alone and as much bone regeneration as BMP-2, a growth factor currently used to promote bone regeneration. These results suggest novel approaches to targeting adenosine receptors in the promotion of bone regeneration.-Mediero, A., Wilder, T., Reddy, V. S. R., Cheng, Q., Tovar, N., Coelho, P. G., Witek, L., Whatling, C., Cronstein, B. N. Ticagrelor regulates osteoblast and osteoclast function and promotes bone formation in vivo via an adenosine-dependent mechanism.