W.S.S. Jee

Prior to initiating a clinical trial in a post-menopausal osteoporosis study, it is reasonable to recommence the evaluation of treatment in the 9-month-old ovariectomized female rat. A female rat of this age has reached peak bone mass and can be manipulated to simulate clinical findings of post-menopausal osteoporosis. Ample time exists for experimental protocols that either prevent estrogen depletion osteopenia or restore bone loss after estrogen depletion. More time can be saved by acceleration of the development of the osteopenia by combining ovariectomized (OVX) plus immobilization (IM) models. Methods like serum biochemistry, histomorphometry and densitometry used in humans are applicable in rats. Like most animal models of osteopenia, the rat develops no fragility fractures, but mechanical testing of rat bones substitutes as a predictor of bone fragility. Recent studies have shown that the prevailing activity in cancellous and cortical bone of the sampling sites in rats is remodeling. The problems of dealing with a growing skeleton, the site specificity of the OVX and IM models, the lack of trabecular and Haversian remodeling and the slow developing cortical bone loss have been and can be overcome by adding beginning and pre-treatment controls and muscle mass measurements in all experimental designs, selecting cancellous bone sampling sites that are remodeling, concentrating the analysis of cortical bone loss to the peri-medullary bone and combining OVX and IM in a model to accelerate the development of both cancellous and cortical bone osteopenia. Not to be forgotten is the distal tibia site, an adult bone site with growth plate closure at 3 months and low trabecular bone turnover and architecture similar to human spongiosa. This site would be most challenging to the action of bone anabolic agents. Data about estrogen-deplete mice are encouraging, but the ovariectomized rat model suggests that developing an ovariectomized mouse model as an alternative is not urgent. Nevertheless, the mouse model has a place in drug development and skeletal research. In dealing with drug development, it could be a useful model because it is a much smaller animal requiring fewer drugs for screening. In skeletal research mice are useful in revealing genetic markers for peak bone mass and gene manipulations that affect bone mass, structure and strength. When the exciting mouse glucocorticoid-induced bone loss model of Weinstein and Manolagas is confirmed by others, it could be a significant breakthrough for that area of research. Lastly, we find that the information generated from skeletal studies of nonhuman primates has been most disappointing and recommend that these expensive skeletal studies be curtailed unless it is required by a regulatory agency for safety studies.

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,