Abnormal modulation of serum osteocalcin by dietary phosphate and 1,25-dihydroxyvitamin D3 in the hypophosphatemic mouse.

We evaluated in normal and hypophosphatemic (Hyp) mice whether changes in serum levels of osteocalcin in response to dietary phosphate supplementation, parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration were related to perturbations in calcium phosphate homeostasis. In normal mice, serum osteocalcin levels were not altered by phosphate supplementation. In contrast, phosphate supplementation in Hyp mice led to a 2-fold decrease in serum osteocalcin to normal levels after 3 days and to an increase in osteocalcin levels after 14 days. The decrease in osteocalcin was associated with normophosphatemia, severe hypocalcemia, and marked increases in circulating 1,25(OH)2D3 levels, whereas the increase in osteocalcin levels was associated with normophosphatemia and no change in serum calcium and 1,25(OH)2D3. Administration of PTH decreased serum osteocalcin in both genotypes. Infusion of 1,25(OH)2D3 for 3 days elicited increases in serum osteocalcin and calcium levels in normal mice, whereas in Hyp mice it produced significant decreases in osteocalcin levels and no change in serum calcium. However, with a more prolonged infusion of 1,25(OH)2D3, hypercalcemia and increases in serum osteocalcin were induced in mutant mice. Our results suggest that the abnormal osteocalcin response of Hyp mice is not directly attributable to an osteoblast dysfunction but is secondary, at least in part, to perturbations in factors that modulate the osteoblast activity, especially serum calcium and/or PTH.

cDNA cloning of the murine Pex gene implicated in X-linked hypophosphatemia and evidence for expression in bone.

The recently identified human PEX gene apparently encodes for a neutral endopeptidase that is mutated in patients with X-linked hypophosphatemia. The 3' and 5' ends of the coding region of PEX have not been cloned, nor has the tissue expression of the gene been identified. Here we report the isolation and characterization of the complete open reading frame of the mouse Pex gene and the demonstration of its expression in bone. Mouse Pex cDNA is predicted to encode a protein of 749 amino acids with 95% identity to the available human PEX sequence and significant homology to members of the membrane-bound metalloendopeptidase family. Northern blot analysis revealed a 6.6-kb transcript in bone and in cultured osteoblasts from normal mice that was not detectable in samples from the Hyp mouse, the murine homolog of human X-linked hypophosphatemia. Pex transcripts were, however, detectable in Hyp bone by RT-PCR amplification. Of particular interest, a cDNA clone from rat incisor shows 93% sequence identity to the 5' end of Pex cDNA, suggesting that Pex may be expressed in another calcified tissue, the tooth. The association of impaired mineralization of bone and teeth and disturbed renal phosphate reabsorption with altered expression of Pex suggests that the Pex gene product may play a critical role in these processes.