Activation of the P2Y2 receptor regulates bone cell function by enhancing ATP release

Bone cells constitutively release ATP into the extracellular environment where it acts locally via P2 receptors to regulate bone cell function. Whilst P2Y₂ receptor stimulation regulates bone mineralisation the functional effects of this receptor in osteoclasts remain unknown. This investigation used the P2Y₂ receptor knockout (P2Y₂R^-/-) mouse model to investigate the role of this receptor in bone. MicroCT analysis of P2Y₂R^-/- mice demonstrated age-related increases in trabecular bone volume (≤48%), number (≤30%) and thickness (≤17%). In vitro P2Y₂R^-/- osteoblasts displayed a 3-fold increase in bone formation and alkaline phosphatase activity whilst P2Y₂R^-/- osteoclasts exhibited a 65% reduction in resorptive activity. Serum cross-linked c-telopeptide levels (CTX, bone resorption marker) were also decreased (≤35%). The resorption defect in P2Y₂R^-/- osteoclasts was rescued by the addition of exogenous ATP, suggesting that an ATP deficit could be a key factor in the reduced function of these cells. In agreement, we found that basal ATP release was reduced up to 53% in P2Y₂R^-/- osteoclasts. The P2Y₂ receptor agonists, UTP and 2-thioUTP, increased osteoclast activity and ATP release in wildtype but not P2Y₂R^-/-cells. This indicates that the P2Y₂ receptor may regulate osteoclast function indirectly by promoting ATP release. UTP and 2-thioUTP also stimulate ATP release from osteoblasts suggesting that the P2Y₂ receptor exerts a similar function in these cells. Taken together, our findings are consistent with the notion that the primary action of P2Y₂ receptor signalling in bone is to regulate extracellular ATP levels.

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