Defective bone formation by hyp mouse bone cells transplanted into normal mice: Evidence in favor of an intrinsic osteoblast defect

Abstract

The hypophosphatemic (Hyp) mouse is an animal model for human hypophosphatemic vitamin D-resistant rickets. We have reported that bone cells isolated from Hyp mice born to homozygous mutant females produce abnormal bone when transplanted into normal mice. To test whether an environmentally acquired defect of the mutant cells contributed to the impaired bone formation observed in transplants, periostea and osteoblasts from normal and Hyp littermates were transplanted intramuscularly into normal animals. To test more specifically for an hypophosphatemia-induced cell alteration before transplantation, bone cells isolated from phosphate-depleted normal mice were transplanted into normal animals. The bone nodules formed in 2 week transplants were characterized by measuring their osteoid thickness and volume. Impaired bone formation was evidenced in Hyp transplants compared to normal littermate transplants by increased osteoid thickness and volume. In contrast to cells from mutant mice, cells isolated from normal mice with comparable hypophosphatemia produced normal bone. These results indicate that the inability of Hyp osteoblasts to produce normal bone when placed in a normal environment is not the consequence of prior exposure to an altered environmental but likely of an intrinsic cellular abnormality. These observations add further support to the concept that the osteoblast is an important target for the Hyp mutation.

Aberrant Phex function in osteoblasts and osteocytes alone underlies murine X-linked hypophosphatemia

“…Hence, the osteoblast and osteocyte production of biological products with activity that can modulate such functions as bone mineralization and renal Pi transport would be anticipated. In this regard, several reports indicate that cultured hyp-mouse osteoblasts secrete a factor(s) that not only inhibits renal Pi transport (27,28) but impairs bone mineralization (9,29). Thus, these data support the possibility that the osteoblast and/or the osteocyte are the relevant site(s) of Phex mutation, a conclusion reaffirmed and established by our studies.…”

“…Since PHEX/Phex codes for a membrane-bound enzyme, several groups have postulated that PHEX/Phex activates or degrades a putative Pi-and bone mineralization-regulating factor(s), phosphatonin or minhibin, which are involved in the regulation of Pi and mineralization processes (1,(8)(9)(10)(11). Studies in patients with XLH and the hyp-mouse, as well as in a wide variety of other Pi wasting disorders, have identified several candidate proteins as the phosphatonin(s) or minhibin(s), including FGF-23 (12,13), sFRP4 (14,15), MEPE (16,17), and FGF-7 (18).…”

Proteolytic processing of osteopontin by PHEX and accumulation of osteopontin fragments in Hyp mouse bone, the murine model of X-linked hypophosphatemia

“…(44) Thus, it has been proposed that the accumulation of a secreted, circulating substrate of PHEX, present in the absence of this enzyme, contributes to the XLH/Hyp phenotype. (81)(82)(83)(84)(85)(86) In addition to this systemic, phosphaturic change initiated by the loss of PHEX activity, it is also reasonable to consider that more local, direct changes in the extracellular matrix of bone might occur that could affect mineralization. Indeed, previous in vivo studies have shown partial correction of the osteomalacic phenotype when Phex/PHEX is overexpressed in osteoblasts, (83,84) and local, microenvironmental effects of PHEX within the extracellular matrix have been explicitly proposed.…”

MEPE-ASARM Peptides Control Extracellular Matrix Mineralization by Binding to Hydroxyapatite: An Inhibition Regulated by PHEX Cleavage of ASARM

“…Previous studies have shown that the osteomalacic phenotype of Hyp mice is not entirely a result of the hypophosphatemia and that Hyp osteoblasts have an intrinsic mineralization defect. Hyp osteoblast cultures fail to mineralize in vitro, and Hyp osteoblasts transplanted into normal mice form abnormal, hypomineralized bone having increased osteoid thickness. Furthermore, correction of the hypophosphatemia in Hyp mice by dietary phosphate supplementation fails to correct the bone mineralization defect, whereas osteoblast‐targeted PHEX expression in transgenic mice partially corrects it without normalizing the hypophosphatemia.…”