Pathogenic role of Fgf23 in Dmp1-null mice - PubMed (original) (raw)
Pathogenic role of Fgf23 in Dmp1-null mice
Shiguang Liu et al. Am J Physiol Endocrinol Metab. 2008 Aug.
Abstract
Autosomal recessive hypophosphatemic rickets (ARHR), which is characterized by renal phosphate wasting, aberrant regulation of 1alpha-hydroxylase activity, and rickets/osteomalacia, is caused by inactivating mutations of dentin matrix protein 1 (DMP1). ARHR resembles autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH), hereditary disorders respectively caused by cleavage-resistant mutations of the phosphaturic factor FGF23 and inactivating mutations of PHEX that lead to increased production of FGF23 by osteocytes in bone. Circulating levels of FGF23 are increased in ARHR and its Dmp1-null mouse homologue. To determine the causal role of FGF23 in ARHR, we transferred Fgf23 deficient/enhanced green fluorescent protein (eGFP) reporter mice onto Dmp1-null mice to create mice lacking both Fgf23 and Dmp1. Dmp1(-/-) mice displayed decreased serum phosphate concentrations, inappropriately normal 1,25(OH)(2)D levels, severe rickets, and a diffuse form of osteomalacia in association with elevated Fgf23 serum levels and expression in osteocytes. In contrast, Fgf23(-/-) mice had undetectable serum Fgf23 and elevated serum phosphate and 1,25(OH)(2)D levels along with severe growth retardation and focal form of osteomalacia. In combined Dmp1(-/-)/Fgf23(-/-), circulating Fgf23 levels were also undetectable, and the serum levels of phosphate and 1,25(OH)(2)D levels were identical to Fgf23(-/-) mice. Rickets and diffuse osteomalacia in Dmp1-null mice were transformed to severe growth retardation and focal osteomalacia characteristic of Fgf23-null mice. These data suggest that the regulation of extracellular matrix mineralization by DMP1 is coupled to renal phosphate handling and vitamin D metabolism through a DMP1-dependent regulation of FGF23 production by osteocytes.
Figures
Fig. 1.
Generation of combined fibroblast growth factor 23 (Fgf23)- and dentin matrix protein 1 (Dmp1)-deficient mice. A: genotyping of mice by PCR. Representative PCR analysis of genomic DNA for the Dmp1 gene, specific DNA fragment in _Dmp1_-knockout (KO) allele, Fgf23 gene, and specific DNA fragment in _Fgf23_-knockout allele [enhanced green fluroescent protein (eGFP)] in wild-type (WT), Dmp1-null (_Dmp1_−/−), _Fgf23_-null (_Fgf23_−/−), and combined _Dmp1_−/−/_Fgf23_−/− mice. B: gross appearance of WT, _Dmp1_-null (_Dmp1_−/−), _Fgf23_-null (_Fgf23_−/−), and combined _Dmp1_−/−/ _Fgf23_−/− mice at 5 wk of age. C: body weight of WT, _Dmp1_−/−, _Fgf23_−/−, and combined _Dmp1_−/−/_Fgf23_−/− mice at 5 wk of age. * and #P < 0.05 by 1-way ANOVA compared with WT and Hyp mice, respectively.
Fig. 2.
Expression of eGFP driven by endogenous Fgf23 promoter in bone and bone marrow stromal cell (BMSC) cultures. A: longitudinal frozen sections from femurs of WT (used as control for autofluorescent background), Dmp1+/+/Fgf23+/−, and _Dmp1_−/−/Fgf23+/− mice viewed under fluorescent microscope showing eGFP expression (top) and 4,6-diamidino-2-phenylindole (DAPI)-stained nuclei (bottom) (×200). Very few osteocytes express eGFP in Dmp1+/+/Fgf23+/− mice with intact Dmp1 expression; however, eGFP is expressed in the majority of osteocytes in _Dmp1_−/−/Fgf23+/− mice. The faint surface fluorescence observed on bone surfaces at top is due to background autofluorescence, which is not different from WT background control Dmp1+/+/Fgf23+/+. B: BMSCs were grown in ascorbic acid and β-glycerol phosphate for 16 days to induce osteoblast differentiation. Top: the high eGFP expression is shown only in the subset of cells embedded in mineralization nodules in BMSCs derived from _Dmp1_−/−/Fgf23+/− mice, but minimal eGFP expression in mineralization nodule BMSC cultures from Dmp1+/−/Fgf23+/−, consistent with Dmp1 deficiency stimulating Fgf23 promoter activity in osteocytes. Bottom: images from the same fields viewed under bright-field light microscope showing mineralization nodule formed by palisading and clustering cells (×200). CB, cortical bone; BM, bone marrow.
Fig. 3.
Effects of superimposed Dmp1 and Fgf23 deficiency on radiological features of bones. A: X-ray of hind legs from 5-wk-old WT and mutant mice. B: microcomputed tomography coronal section images of femurs from 5-wk-old WT and mutant mice.
Fig. 4.
Effects of superimposed Dmp1 and Fgf23 deficiency on histological features of cartilage and bone. A: growth plate of proximal tibias of 5-wk-old mice (Goldner stain, ×200). B and C: CB of tibias of 5-wk-old mice (Goldner stain, ×200). Mineralized bone was stained in green, and unmineralized osteoid was stained in red. D: unstained cross-section of tibias viewed under fluorescent light (×200). Red fluorescent label was from alizarin complexone, and green fluorescent label was from calcein.
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