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

Abstract

The P2X 7 nucleotide receptor is an ATP-gated ion channel expressed widely in cells of hematopoietic origin. Our purpose was to explore the involvement of the P2X 7 receptor in bone development and remodeling by characterizing the phenotype of mice genetically modified to disrupt the P2X 7 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 P2X 7 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 con-firmed expression of functional P2X 7 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 P2X 7 receptors were also found in osteoblasts from WT but not KO mice, suggesting a direct role in bone formation. P2X 7 receptor KO mice demonstrate a unique skeletal phenotype that involves deficient periosteal bone formation together with excessive trabecular bone resorption. Thus, the P2X 7 receptor represents a novel therapeutic target for the management of skeletal disorders such as osteoporosis. (Molecular Endocrinology 17: 1356-1367, 2003) Abbreviations: BFR/BS, Bone formation rate/bone surface referent (m 2 /m/d 100); BFR/BV, bone formation rate/ bone volume referent (%/yr); BFR/TV, bone formation rate/ tissue volume referent (%/yr); BV/TV, trabecular bone volume/total tissue volume (%); BzATP, 2-and 3-O-(4benzoylbenzoyl)-ATP; E-ES/BS, endocortical eroded surface/ endocortical bone surface (%); KO, knockout; MAR, mineral apposition rate (m/d); MS/BS, mineralizing surface/bone surface (%); OCN/BS, osteoclast number/mm bone surface (no./mm); OCS/BS, osteoclast surface/total bone surface (%); pQCT,