Deletion of the P2X₇Nucleotide Receptor Reveals Its Regulatory Roles in Bone Formation and Resorption

Abstract

The P2X7 nucleotide receptor is an ATP-gated ion channel expressed widely in cells of hematopoietic origin. Our purpose was to explore the involvement of the P2X7 receptor in bone development and remodeling by characterizing the phenotype of mice genetically modified to disrupt the P2X7 receptor [knockout (KO)]. Femoral length did not differ between KO and wild-type (WT) littermates at 2 or 9 months of age, indicating that the P2X7 receptor does not regulate longitudinal bone growth. However, KO mice displayed significant reduction in total and cortical bone content and periosteal circumference in femurs, and reduced periosteal bone formation and increased trabecular bone resorption in tibias. Patch clamp recording confirmed expression of functional P2X7 receptors in osteoclasts from WT but not KO mice. Osteoclasts were present in vivo and formed in cultures of bone marrow from KO mice, indicating that this receptor is not essential for fusion of osteoclast precursors. Functional P2X7 receptors were also found in osteoblasts from WT but not KO mice, suggesting a direct role in bone formation. P2X7 receptor KO mice demonstrate a unique skeletal phenotype that involves deficient periosteal bone formation together with excessive trabecular bone resorption. Thus, the P2X7 receptor represents a novel therapeutic target for the management of skeletal disorders such as osteoporosis.

Purinergic signalling in the musculoskeletal system

“…This study also demonstrated large variations in the relative expression of the P2 receptors in osteoblasts, with mRNA for the P2X4 receptor being the most abundant [167]. Expression of the P2X7 receptor was first demonstrated in a subpopulation of human differentiated osteoblasts [169]; later studies also demonstrated expression in rat [158,167] and mouse osteoblasts [137]. Interestingly, a recent study claimed that P2X7 receptors are localised in caveolae in the plasma membranes of osteoblasts [170].…”

“…The initial study by Ke et al [137] reported a decreased bone mineral content and periosteal circumference accompanied by decreased bone formation parameters and increased bone resorption parameters [137]. A separate study by Gartland et al [136] reported no differences in bone mineral density of trabecular bone, but an increase in cortical bone was observed.…”

“…Analogues of the P2X7 receptor antagonist, KN-62, have also been shown to induce osteoclast apoptosis [268]. In contrast, Ke et al demonstrated that P2X7 receptor knockout mice possessed functional osteoclasts in vivo [137]. In addition, using knockout precursor cells, osteoclasts were generated in vitro, suggesting that the P2X7 receptor is not required for cell fusion [137].…”

“…In contrast, Ke et al demonstrated that P2X7 receptor knockout mice possessed functional osteoclasts in vivo [137]. In addition, using knockout precursor cells, osteoclasts were generated in vitro, suggesting that the P2X7 receptor is not required for cell fusion [137]. Other studies have also shown that P2X7 receptor knockout mice retain the ability to form multinucleated cells both in vitro and in vivo [114,116].…”

P2X7 nucleotide receptor signaling potentiates the Wnt/β-catenin pathway in cells of the osteoblast lineage

“…Mice in which P2X7 receptor function is disrupted (knockout) exhibit diminished periosteal bone formation and increased trabecular bone resorption [10] and impaired anabolic responses of the skeleton to mechanical load [11]. In cells of the osteoblast lineage, stimulation of the P2X7 receptor promotes differentiation and matrix mineralization [12].…”