The Calcium Sensing Receptor Mediates Bone Turnover Induced By Dietary Calcium And Parathyroid Hormone

We employed a genetic approach to determine whether the calcium sensing receptor (CaSR) mediates bone turnover associated with dietary calcium (Ca) and parathyroid hormone (PTH). Wild-type (WT) pups and pups with targeted deletion of the PTH (PTH^-/-) gene or of both PTH and CaSR (PTH^-/-CaSR^-/-) genes were nursed by WT dames: 1) on a normal Ca (1.0% Ca and 0.67% Pi) diet or 2) a high Ca (2% Ca and 0.4% Pi) diet. Pups nursed by dames on a normal Ca diet received daily injections of vehicle or of PTH1-34 (80μg/kg) for 2 weeks starting from 1 week of age. Pups fed by dames on a high Ca diet received vehicle only. These regimens induced different bone phenotypic changes, thereby disclosing selective modulation by Ca and PTH mediated through CaSR.Milk Ca content was raised, but milk PTH related protein (PTHrP and 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) levels were decreased in PTH^+/-CaSR^+/- dames fed on a high Ca diet compared to PTH^+/-CaSR^+/- dames fed on a normal Ca diet. Serum calcium was decreased in 3-week-old PTH^-/- pups, and was decreased more dramatically in 3-week-old PTH^-/-CaSR^-/- pups, whatever they were fed by dams on either the normal diet or the high Ca diet or were administrated with the exogenous PTH, despite it was raised paralleled in the genotype-matched pups fed by dams on the high Ca diet or administrated with exogenous PTH compared to pups fed by dams on the normal diet. Both mutant animals were hyperphosphatemia, whatever they were fed by dams on either the normal diet or the high Ca diet or were administrated with the exogenous PTH, despite it was reduced paralleled in the genotype-matched pups fed by dams on the high Ca diet compared to pups fed by dams on the normal diet. Serum PTH levels were decreased in WT pups fed by dams on the high Ca diet and were raised in WT pups administrated with PTH compared to vehicle treated WT pups fed by dams on the normal diet, and were undetectable in PTH^-/- and PTH^-/-CaSR^-/- pups fed by dams on any diet. With PTH treatment, serum PTH levels were detectable, but was lower significantly in PTH ^-/- and PTH^-/-CaSR^-/- pups compared to their WT littermates. Serum 1,25(OH)2D3 was decreased in 3-week-old PTH^-/- pups, and was decreased more dramatically in 3-week-old PTH^-/-CaSR^-/- pups, whatever they were fed by dams on either the normal diet or the high Ca diet or were administrated with the exogenous PTH, despite it was reduced paralleled in the genotype-matched pups fed by dams on the high Ca diet or it was raised paralleled in the genotype-matched pups administrated with exogenous PTH compared to pups fed by dams on the normal diet.In 3-week-old pups receiving vehicle and fed by dames either on a normal or a high Ca diet, bone mineral density (BMD), trabecular volume (BV/TV), the number of osteoblasts, the type I collagen (Col I) positive areas, mineral apposition rate( MAR), the gene expression levels of Cbfα-1, ALP, Col I, OCN genes and the protein expression of PTHR and IGF-1 were all reduced significantly in PTH^-/- pups compared to WT pups and were decreased more dramatically in PTH^-/-CaSR^-/- pups even compared to PTH^-/- pups. These bone formation parameters were increased in WT and PTH^-/- pups, but not in PTH^-/-CaSR^-/- pups fed by dames on a high Ca diet compared to the pups fed by dames on a normal Ca diet. These bone formation parameters were increased in 3-week-old WT, PTH^-/- and PTH^-/-CaSR^-/- pups following exogenous PTH treatment for 2 weeks, however, the percentage increase was less in PTH^-/-CaSR^-/- pups than in WT and PTH^-/- pups. The percentage increase in these bone formation parameters was higher in PTH-treated pups than in pups fed by dames on a high Ca diet in both WT and PTH^-/- pups.In vehicle treated 3-week-old pups fed by dames on either a normal or a high Ca diet, and in PTH-treated pups fed by dames on a normal Ca diet, the number and surface of TRAP positive osteoclasts were reduced significantly in PTH^-/- pups compared to WT pups and were decreased even more dramatically in PTH^-/-CaR^-/- pups than in PTH^-/- pups. However, these parameters were decreased significantly in pups fed by dames on a high Ca diet and increased significantly in PTH-treated pups compared to genotype-matched pups fed by dames on a normal diet. There results indicate that CaSR mediates bone turnover mediated by dietary Ca and can impact on the increased bone turnover elicited by PTH.There results indicate that CaSR mediates bone turnover mediated by dietary Ca and can impact on the increased bone turnover elicited by PTH, suggest that cooperative roles of PTH and calcium on bone formation and imply that dietary calcium and phosphorous in an appropriate proportion play as benefits on neonatal skeletal development and growth. This study provides an experimental and theoretical evidence for the combination of PTH with calcium or CaSR agonist to treat osteoporosis.