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

Hypercalcemia produced by parathyroid hormone suppresses experimental autoimmune encephalomyelitis in female but not male mice

Besides its role in regulating serum levels of calcium and phosphorus, 1alpha, 25-dihydroxyvitamin D3 (1,25-(OH)2D3) has potent effects on the immune system and suppresses disease in several animal models of autoimmune disorders including experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. While the amount of 1,25-(OH)2D3 needed to prevent EAE is dependent on the gender of the mouse and amount of calcium available in the diet, the minimum levels of 1,25-(OH)2D3 sufficient to prevent disease cause hypercalcemia. To test if hypercalcemia independent of high levels of 1,25-(OH)2D3 can suppress EAE, we used a 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-hydroxylase) knockout mouse strain. Because these 1alpha-hydroxylase knockout mice lack the parathyroid hormone (PTH)-regulated enzyme that synthesizes 1,25-(OH)2D3, hypercalcemia from increased bone turnover was created by continuous administration of PTH without changing the circulating levels of 1,25-(OH)2D3. This PTH-mediated hypercalcemia generated after EAE induction prevented disease in female mice but not male mice. When hypercalcemia was prevented by diet manipulation, PTH administration no longer prevented EAE. We conclude that hypercalcemia is able to prevent EAE after disease induction in female mice.

Cloning of the bone Gla protein gene from the teleost fish Sparus aurata. Evidence for overall conservation in gene organization and bone-specific expression from fish to man

Bone Gla protein (BGP, Osteocalcin) is a bone-specific vitamin K-dependent protein which has been intensively studied in mammals. Although BGP is the most abundant non-collagenous protein of bone, its mode of action at the molecular level remains unclear. From an evolutionary point of view, the appearance of BGP seems to parallel the appearance of hydroxyapatite-containing bone structures since it has never been found in elasmobranchs, whose skeleton is composed of calcified cartilage. Accordingly, recent work indicates that, in mammalian bone, BGP is required for adequate maturation of the hydroxyapatite crystal. Taken together, these data suggest that teleost fishes, presumably the first vertebrates to develop a BGP-containing skeleton, may be a useful model to further investigate BGP function. In addition, fish offer several advantages over mammalian models, due to a large progeny, external embryonic development and transparency of larvae. In the present work, the BGP cDNA and gene were cloned from a teleost fish, Sparus aurata, and its tissue distribution, pattern of developmental expression and evolutionary pathways analyzed. The molecular organization of the Sparus BGP (spBGP) gene is similar to mammalian BGP genes, and its expression throughout development follows the onset of calcification. The spBGP gene encodes a pre-propeptide of 97 amino acid residues, expressed only in bone and showing extensive homology to its mammalian homologs. Phylogenetic analysis of the available BGP sequences supports the hypothesis that all BGPs have a single origin and share a common ancestor with a related vitamin K-dependent protein (Matrix Gla protein).

Intrinsic mineralization defect in Hyp mouse osteoblasts

X-linked hypophosphatemia (XLH) is caused by inactivating mutations of PEX, an endopeptidase of uncertain function. This defect is shared by Hyp mice, the murine homologue of the human disease, in which a 3' Pex deletion has been documented. In the present study, we report that immortalized osteoblasts derived from the simian virus 40 (SV40) transgenic Hyp mouse (TMOb-Hyp) have an impaired capacity to mineralize extracellular matrix in vitro. Compared with immortalized osteoblasts from the SV40 transgenic normal mouse (TMOb-Nl), osteoblast cultures from the SV40 Hyp mouse exhibit diminished 45Ca accumulation into extracellular matrix (37 +/- 6 vs. 1,484 +/- 68 counts . min-1 . microgram protein-1) and reduced formation of mineralization nodules. Moreover, in coculture experiments, we found evidence that osteoblasts from the SV40 Hyp mouse produce a diffusible factor that blocks mineralization of extracellular matrix in normal osteoblasts. Our findings indicate that abnormal PEX in osteoblasts is associated with the accumulation of a factor(s) that inhibits mineralization of extracellular matrix in vitro.

Osteocalcin production in primary osteoblast cultures derived from normal and Hyp mice

Rickets and osteomalacia are characteristic features of the Hyp mouse model of human X-linked hypophosphatemia. Hyp mice demonstrate elevated circulating osteocalcin levels, as well as altered regulation of osteocalcin by 1,25(OH)2D3. Whether this osteocalcin abnormality is intrinsic to the osteoblast, or mediated by the in vivo milieu, has not been established. We therefore characterized osteocalcin production and its regulation by 1,25(OH)2D3 in primary cultures of murine osteoblasts and examined osteocalcin and its messenger RNA in response to 1,25(OH)2D3 in cultures of Hyp mouse-derived osteoblasts. Cell viability and osteocalcin production are optimal when murine cells are harvested within 36 h of age. Murine primary osteoblast cultures mineralize and produce osteocalcin in a maturation-dependent fashion (as demonstrated in other species), and continuous exposure to 1,25(OH)2D3, beginning at day 9 of culture, inhibits osteoblast differentiation and osteocalcin production and prevents mineralization of the culture. However, in contrast to other species, exposure to 1,25(OH)2D3, added later (days 17-25) in culture, does not stimulate osteocalcin but arrests osteocalcin production at current levels. Ambient media levels of osteocalcin were no different in cultures from Hyp mice and their normal litter mates, and the down-regulatory response to 1,25(OH)2D3 was comparable in cultures from normal and Hyp mice. Furthermore, expression of osteocalcin messenger RNA in murine cultures is reduced with exposure to 1,25(OH)2D3, and there is no difference between normal and Hyp cultures in this response. Thus, primary murine osteoblasts manifest a species-specific effect of 1,25(OH)2D3 on osteocalcin production. Furthermore, the increased serum osteocalcin production seen in intact Hyp mice, and the altered response to 1,25(OH)2D3 in Hyp mice, are not observed in osteoblast cultures derived from the mutant strain. These data indicate that abnormalities of osteocalcin described in intact Hyp mice require factors other than those present in cultured cells.