Intrinsic mineralization defect inHyp mouse osteoblasts

Abstract

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 ⋅ μg 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.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

“…There is overwhelming evidence implicating the osteoblast as the cell intrinsically defective [13,18,[20][21][22]30,37,50,58,59,67,69,80,101,102] and the HYP osteoblast directly secretes factors (phosphatonins) that impact adversely on renal phosphate uptake and mineralization in vivo and in vitro. FGF23 activating mutations are responsible for the changes in phosphate, vitamin D metabolism and mineralization in ADHR [4,82,97,98] and overexpression of wild-type FGF23 in some but not all OHO tumors is directly or indirectly responsible for the changes in some tumor-induced osteomalacias [15,81,97,99].…”

“…Evidence suggests, however, that other factors may also play a role in this complex bone-renal pathway. The mineralization and phosphate-inhibiting modalities of conditioned media derived from Hyp osteoblasts and normal osteoblast cell lines stably transfected with PHEX [17,51,59,80,101], the confirmation of intrinsic Hyp osteoblast mineralization defects [18,[20][21][22][23][67][68][69]102,103], in osteoblasts/bone indicates that there may be osteoblastic phosphatonins (OB-PTN) and extra-osseous phosphatonins (EO-PTN) molecules and pathways. Both OB-PTN and EO-PTN pathways may be contiguous, overlapping or separate.…”

“…However, extensive studies in Hyp mice indicate that the defect in mineralization and renal function is mediated by circulating factor(s) derived from Phex defective osteoblasts. These investigations include parabiosis experiments [48,49], Hyp/normal kidney cross-transplantations [57], Hyp/normal osteoblast intra-muscular cross-transplantation and mineralization studies [18,[20][21][22]102], Hyp osteoblast conditioned media phosphate-uptake and mineralization studies [58,59,101], PHEX antisense studies in normal osteoblasts [80], demonstration of intrinsic abnormalities in the Hyp osteoblast [13,30,50,67,69] and direct analysis of mineralization inhibition and phosphate-uptake inhibition activity of Hyp serum [37]. Thus, in Hyp, the evidence strongly implicates a circulating humoral factor or factors [phosphatonin(s)] that are secreted by the Hyp osteoblast and impact on mineralization and renal phosphate handling.…”

1,25-Dihydroxyvitamin D3 Down-regulation of PHEX Gene Expression Is Mediated by Apparent Repression of a 110 kDa Transfactor That Binds to a Polyadenine Element in the Promoter

“…This hypothesis is supported by the observation that osteoblasts from Hyp mice (the murine homologue of XLH, deficient in PHEX), fail to mineralize in media supporting mineralization of normal osteoblasts. Further, when co-cultured with Hyp osteoblasts, normal osteoblasts exhibit a significant decrease in mineralization (14). Finally, targeted expression of PHEX in Hyp mice osteoblasts is not sufficient to rescue the Hyp phenotype (15,16).…”

“…PHEX normally inactivates phosphatonin, limiting P i excretion; in XLH, the mutant PHEX fails to inactivate phosphatonin, resulting in phosphaturia and hypophosphatemia (10). The defects in bone associated with XLH are not fully attributable to hypophosphatemia, and there is evidence for an intrinsic abnormality in the osteoblast (14). This hypothesis is supported by the observation that osteoblasts from Hyp mice (the murine homologue of XLH, deficient in PHEX), fail to mineralize in media supporting mineralization of normal osteoblasts.…”