The P2Y13 receptor regulates extracellular ATP metabolism and the osteogenic response to mechanical loading

Abstract

ATP release and subsequent activation of purinergic receptors has been suggested to be one of the key transduction pathways activated by mechanical stimulation of bone. The P2Y 13 receptor, recently found to be expressed by osteoblasts, has been suggested to provide a negative feedback pathway for ATP release in different cell types. Therefore, we hypothesized that the P2Y 13 receptor may contribute to the mediation of osteogenic responses to mechanical stimulation by regulating ATP metabolism by osteoblasts. To test this hypothesis, wild-type (WT) and P2Y 13 receptor knockout (P2Y 13 R À/À ) mice were subject to non-invasive axial mechanical loading of the left tibiae to induce an osteogenic response. Micro-computed tomography analysis showed mechanical loading induced an osteogenic response in both strains of mice in terms of increased total bone volume and cortical bone volume, with the P2Y 13 R À/À mice having a significantly greater response. The extent of the increased osteogenic response was defined by dynamic histomorphometry data showing dramatically increased bone formation and mineral apposition rates in P2Y 13 R À/À mice compared with controls. In vitro, primary P2Y 13 R À/À osteoblasts had an accumulation of mechanically induced extracellular ATP and reduced levels of hydrolysis. In addition, P2Y 13 R À/À osteoblasts also had a reduction in their maximal alkaline phosphatase (ALP) activity, one of the main ecto-enzymes expressed by osteoblasts, which hydrolyzes extracellular ATP. In conclusion, deletion of the P2Y 13 receptor leads to an enhanced osteogenic response to mechanical loading in vivo, possibly because of the reduced extracellular ATP degradation by ALP. The augmented osteogenic response to mechanical stimulation, combined with suppressed bone remodeling activities and protection from OVXinduced bone loss after P2Y 13 receptor depletion as previously described, suggests a potential role for P2Y 13 receptor antagonist-based therapy, possibly in combination with mechanical loading, for the treatment of osteoporosis.

P2Y2 receptors regulate osteoblast mechanosensitivity during fluid flow

“…We, and others, have shown that the P2X 7 receptor enhances proliferation, ERK1/2 phosphorylation, and COX-2 production in osteoblasts (24,31), and that knockout of this receptor significantly attenuates load-induced bone formation in vivo (29). In contrast, P2Y 2 R and P2Y 13 R knockout mice exhibit excessive bone formation in response to mechanical loading (40,56), demonstrating that these receptors inhibit the anabolic response of bone. Based on our results, the increased bone formation in the P2Y 2 R knockout mice under dynamic loading (40) may be explained by the increased mechanosensitivity due to a lack of ASFF to downregulate the osteoblasts sensitivity to mechanical stimuli.…”

“…P2 receptors are classified as G protein-coupled receptors P2Y (P2Y 1,2,4,6,11,12,13,14 ) or nonselective, ligand-gated ion channels P2X (P2X 1-7 ) (39). The P2Y 2 R and P2Y 13 R knockout mice have both been shown to increase trabecular and cortical bone formation to dynamic loading, which suggests a negative regulation by the P2Y receptor in bone formation (40,56). The negative feedback associated with the P2Y 13 R is thought to be mediated by excessive ATP release (56).…”

“…The P2Y 2 R and P2Y 13 R knockout mice have both been shown to increase trabecular and cortical bone formation to dynamic loading, which suggests a negative regulation by the P2Y receptor in bone formation (40,56). The negative feedback associated with the P2Y 13 R is thought to be mediated by excessive ATP release (56). In contrast, the mechanism through which P2Y 2 R downregulates the mechanosensitivity of bone is unknown, but we predict this receptor has a crucial function in regulating mechanotransduction.…”

Role of the P2X7 receptor in the osteogenic differentiation of mesenchymal cells and in osteoclast fusion

“…Other receptors are also emerging to play a role in the balance of MSC differentiation to bone, including the P2Y 13 receptor [7]. Interestingly, the P2Y 13 receptor could possibly play a role in the mechanism of release of ATP from MSCs as it has been shown to also regulate ATP release in other cell types, including bone [8,9]. The observation that shockwaves or ATP increased P2X7 receptor expression led the authors to suggest a "feed forward mechanism to control bone homeostasis."…”

Efficacy of anti-sclerostin monoclonal antibody BPS804 in adult patients with hypophosphatasia

“…This might result in impaired bone formation, as the phenotype of P2Y2 (ATP-binding) receptor-KO mice showed increased bone mass, whereas P2Y13 (ADP-binding) receptor-and adenosine receptor-KO mice showed low bone mass with altered osteoblast differentiation and osteoclast activation (13). If these complex phenotypes are considered together, with a focus on ALP activity, it could be hypothesized that substrate accumulation, notably PPi, inhibits bone mineralization and that ADP/adenosine deficiency further contributes to compromised bone formation with a low-turnover bone phenotype (13)(14)(15)(16)(17).…”