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

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 P2Y₁₃ 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 P2Y₁₃ 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 P2Y₁₃ receptor knockout (P2Y₁₃R^-/-) 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 P2Y₁₃R^-/- 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 P2Y₁₃R ^-/- mice compared with controls. In vitro, primary P2Y ₁₃R^-/- osteoblasts had an accumulation of mechanically induced extracellular ATP and reduced levels of hydrolysis. In addition, P2Y₁₃R^-/- 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 P2Y₁₃ 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 OVX-induced bone loss after P2Y₁₃ receptor depletion as previously described, suggests a potential role for P2Y₁₃ receptor antagonist-based therapy, possibly in combination with mechanical loading, for the treatment of osteoporosis.