The osteocyte, a terminally differentiated cell comprising 90%-95% of all bone cells 1,2 , may have multiple functions, including acting as a mechanosensor in bone (re)modeling 3 . Dentin matrix protein 1 (encoded by DMP1) is highly expressed in osteocytes 4 and, when deleted in mice, results in a hypomineralized bone phenotype 5 . We investigated the potential for this gene not only to direct skeletal mineralization but also to regulate phosphate (P i ) homeostasis. Both Dmp1-null mice and individuals with a newly identified disorder, autosomal recessive hypophosphatemic rickets, manifest rickets and osteomalacia with isolated renal phosphate-wasting associated with elevated fibroblast growth factor 23 (FGF23) levels and normocalciuria. Mutational analyses showed that autosomal recessive hypophosphatemic rickets family carried a mutation affecting the DMP1 start codon, and a second family carried a 7-bp deletion disrupting the highly conserved DMP1 C terminus. Mechanistic studies using Dmp1-null mice demonstrated that absence of DMP1 results in defective osteocyte maturation and increased FGF23 expression, leading to pathological changes in bone mineralization. Our findings suggest a bone-renal axis that is central to guiding proper mineral metabolism.Human disorders of phosphate (P i ) handling and skeletal mineralization can result from mutations in PHEX 6 , which cause X-linked hypophosphatemic rickets (XLH). A similar phenotype is also observed in Hyp mice, which have mutant Phex 7 and show increased osteocyte expression of the phosphaturic factor FGF23 (ref. 8 Individuals F1-1 and F1-3 presented with rickets and progressive lower limb deformity in late infancy, whereas sister F1-2 had rachitic changes on a chest X-ray at age 7 months. In contrast, F2-1 presented with a mild genu valgum at 8 years of age. The pre-or off-treatment age-related metabolic profiles for both kindreds were similar, characterized by hypophosphatemia owing to renal phosphate-wasting (serum P i : 0.7-0.9 mmol/l, normal: 1.2-1.8; threshold maximum for renal tubular phosphate reabsorption/glomerular filtration rate (TmP/GFR): 0.61-0.81 mmol/l, lower limit of normal: ≥1.0), high normal to moderately elevated alkaline phosphatase, normal intact parathyroid hormone (PTH) levels (4.6-6.9 pmol/l, normal: 1.6-6.9), normocalcemia (ionized calcium: 1. Resolution of rickets and normalization of alkaline phosphatase were observed during treatment with phosphate supplementation and calcitriol; however, the TmP/GFR remained low. Linear growth trajectories were heterogeneous among the affected individuals: affected individuals in F1 had a mid-parental height of 154.5 cm (5 th -10 th percentiles), with F1-1 and F1-2 measuring 153 cm (5 th percentile) and 136.5 cm (<5 th percentile) at final adult height, respectively. F1-3 had a height of 153.5 cm at 10 months post-menarche, well within the genetic target. The affected individual in F-2 had a final adult height of 172 cm (90 th -95 th percentile), 3 cm above the upper limit of her genetic ta...

The regulation of the phosphaturic factor fibroblast growth factor 23 (FGF23) is not well understood. It was found that administration of 1,25-dihydroxyvitamin D 3 (1,25[OH] 2 D 3 ) to mice rapidly increased serum FGF23 concentrations from a basal level of 90.6 ؎ 8.1 to 213.8 ؎ 14.6 pg/ml at 8 h (mean ؎ SEM; P < 0.01) and resulted in a four-fold increase in FGF23 transcripts in bone, the predominate site of FGF23 expression. In the Hyp-mouse homologue of X-linked hypophosphatemic rickets, administration of 1,25(OH) 2 D 3 further increased circulating FGF23 levels. In Gcm2 null mice, low 1,25(OH) 2 D 3 levels were associated with a three-fold reduction in FGF23 levels that were increased by administration of 1,25 ( FGF23 also suppresses 1␣ hydroxylase activity in the proximal renal tubule, leading to reduced circulating levels of 1,25(OH) 2 D 3 (2,10,14,15). The significance of FGF23 regulation of 1,25(OH) 2 D 3 production is not clear, but the findings that FGF23 is produced predominantly by osteoblasts in bone and that FGF23 regulates phosphate reabsorption and 1,25(OH) 2 D 3 production by the kidney raise the possibility that FGF23 may be involved in a bone-kidney axis that controls phosphate and vitamin D homeostasis (16,17). How FGF23 is integrated with the vitamin D-parathyroid hormone (PTH) axis, which plays a central role in calcium homeostasis, skeletal development, and mineralization (18), however, is not clear. Understanding the effects of 1,25(OH) 2 D 3 on FGF23 production is important, because vitamin D therapy often is used to treat FGF23-mediated hypophosphatemic disorders, such as XLH (19).In an effort to understand more fully the regulation of FGF23 expression in osteoblasts and bone, we assessed the effect of 1,25(OH) 2 D 3 administration on circulating levels of FGF23 in wild-type Gcm2 null (20) and Hyp mice (21), as well as the effects of 1,25(OH) 2 D 3 on the FGF23 transcripts in bone. In addition, we investigated the ability of 1,25(OH) 2 D 3 to regulate endogenous FGF23 transcripts and the activity of a transfected murine FGF23 promoter luciferase reporter in osteoblasts. Our findings demonstrate the importance of bone as a target for vitamin D-mediated increments in FGF23 production and suggest that FGF23 production serves as a counterregulatory hormone to enhance renal phosphate clearance in response to vitamin D-mediated increments in gastrointestinal phosphate absorption and decrements in the phosphaturic hormone PTH. Materials and Methods 1,25(OH) 2 D 3 and PTH AdministrationBoth Hyp mice (21) and C57BL/6J mice were purchased from Jackson Laboratory (Bar Harbor, ME). Male and female Gcm2 ϩ/Ϫ mice were mated to generate homozygous Gcm2 null mice that lacked parathyroid glands (22). All mice were maintained and used in accordance with recommendations in

Inactivating mutations of the PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) endopeptidase, the disease-causing gene in X-linked hypophosphatemia (XLH), results in increased circulating levels of fibroblastic growth factor-23 (FGF23), a bone-derived phosphaturic factor. To determine the causal role of FGF23 in XLH, we generated a combined Fgf23-deficient enhanced green fluorescent protein (eGFP) reporter and Phex-deficient Hyp mouse model (Fgf23(+/-)/Hyp). eGFP expression was expressed in osteocytes embedded in bone that exhibited marked upregulation of eGFP in response to Phex deficiency and in CD31-positive cells in bone marrow venules that expressed low eGFP levels independently of Phex. In bone marrow stromal cells (BMSCs) derived from Fgf23(-/-)/Hyp mice, eGFP expression was also selectively increased in osteocyte-like cells within mineralization nodules and detected in low levels in CD31-positive cells. Surprisingly, eGFP expression was not increased in cell surface osteoblasts, indicating that Phex deficiency is necessary but not sufficient for increased Fgf23 expression in the osteoblast lineage. Additional factors, associated with either osteocyte differentiation and/or extracellular matrix, are necessary for Phex deficiency to stimulate Fgf23 gene transcription in bone. Regardless, the deletion of Fgf23 from Hyp mice reversed the hypophosphatemia, abnormal 1,25(OH)(2)D(3) levels, rickets, and osteomalacia associated with Phex deficiency. These results suggest that Fgf23 acts downstream of Phex to cause both the renal and bone phenotypes in Hyp mice.