We have demonstrated that circulating phosphate is a critical determinant of normal growth platematuration. Hypophosphatemia impairs appptosis of hypertrophic chondrocytes, both in vivo and invitro, leading to the development of rickets in growing animals. Interestingly, while hypophosphatemicVitamin D Receptor (VDR) null mice, mice with diet-induced hypercalcemia/ hypophosphatemia and hypmice all develop progressive expansion of late hypertrophic chondrocytes due to impaired apoptosis ofthese cells, Npt2a null mice have not been reported to develop rickets. Our preliminary datademonstrate that, while deletion of this renal Na-dependent phosphate transporter leads to the samedegree of hypophosphatemia as is observed in the other models, the growth plate phenotype in thesemice is transient. An expansion of the late hypertrophic chondrocyte layer, accompanied by impairedapoptosis of these cells is observed at 16 days of age, whereas by 35 days of age, there is resolution ofthis abnormality.The studies in this proposal will address the hypothesis that a specific transport mechanism forphosphate is present in hypertrophic chondrocytes and mediates apoptosis of these cells. They willexamine whether there is an intrinsic difference in phosphate transport in cultures of hypertrophicchondrocytes from hyp mice, Npt2a null mice and wildtype mice. They will also determine whetherintrinsic cellular factors modify the susceptibility of chondrocytes, isolated from these animals, tophosphate-mediated apoptosis.Investigations will be performed to address the hypothesis that a difference in circulating hormonelevels contributes to the difference in the growth plate phenotype of the Npt2a and hyp mice. Primarychondrocytes, isolated from these mice will be treated with PTH, 1,25-dihydroxyvitamin D and FGF23 toaddress the hypothesis that these agents modulate phosphate transport and/or susceptibility tophosphate-mediated apoptosis. To address the hypothesis that enhanced 1,25-dihydroxyvitamin Daction is responsible for normalization of the growth plate of Npt2a null mice, 1,25-dihydroxyvitamin Daction will be blocked by mating them with VDR null mice. Hyp mice will be treated with 1,25-dihydroxyvitamin D to address the hypothesis that impaired 1,25-dihydroxyvitamin D action and /orincreases in PTH contribute to the development of rickets in this disorder.These investigations are expected to reconcile the difference in the growth plate phenotypeobserved in hypophosphatemic mouse models and to identify a role for circulating hormones inmodulating the susceptibility of hypertrophic chondrocytes to phosphate-mediated apoptosis.
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