ABSTRACT
Parathyroid‐hormone‐type 1 receptor (PTH1R) is extensively expressed in key regulatory organs for systemic mineral ion homeostasis, including kidney and bone. We investigated the ...bone‐specific functions of PTH1R in modulating mineral ion homeostasis by generating a novel mouse model in which PTH1R is ablated in the limb mesenchyme using Prx1Cre transgenic mice. Such ablation decreased FGF23 protein and serum levels by 50%, despite normal Fgf23 mRNA levels in long bones. Circulating calcium and PTH levels were unchanged, but inorganic phosphate and 1,25 (OH)2D3 levels were significantly decreased and accompanied by elevated urinary calcium and phosphate wasting. Key renal genes for balancing mineral ion homeostasis, calbindinD28k, Klotho, and Napi2a were suppressed by 30‐40%. Intermittent hPTH(1‐34) injections increased Fgf23 mRNA (7.3‐fold), Nurr1 mRNA (3.1‐fold), and serum intact‐FGF23 (1.6‐fold) in controls, but failed to induce Fgf23, Nurr1 mRNA, or intact FGF23 production in mutants. Moreover, a significant elevation in serum C‐terminal‐FGF23 levels (4‐fold) was detected in both genotypes. PTH markedly down‐regulated Galnt3 expression (2.7‐fold) in controls but not in mutants. These results demonstrate the pivotal role of PTH1R in long bones to regulate systemic mineral ion homeostasis and the direct induction of FGF23 by PTH1R signaling.—Fan, Y., Bi, R., Densmore, M. J., Sato, T., Kobayashi, T., Yuan, Q., Zhou, X., Erben, R. G., Lanske, B. Parathyroid hormone 1 receptor is essential to induce FGF23 production and maintain systemic mineral ion homeostasis. FASEB J. 30, 428‐440 (2016). www.fasebj.org
Klotho-knockout mice (klotho(-/-)) have increased renal expression of sodium/phosphate cotransporters (NaPi2a), associated with severe hyperphosphatemia. Such serum biochemical changes in klotho(-/-) ...mice lead to extensive soft-tissue anomalies and vascular calcification. To determine the significance of increased renal expression of the NaPi2a protein and concomitant hyperphosphatemia and vascular calcification in klotho(-/-) mice, we generated klotho and NaPi2a double-knockout (klotho(-/-)/NaPi2a(-/-)) mice.
Genetic inactivation of NaPi2a activity from klotho(-/-) mice reversed the severe hyperphosphatemia to mild hypophosphatemia or normophosphatemia. Importantly, despite significantly higher serum calcium and 1,25-dihydroxyvitamin D levels in klotho(-/-)/NaPi2a(-/-) mice, the vascular and soft-tissue calcifications were reduced. Extensive soft-tissue anomalies and cardiovascular calcification were consistently noted in klotho(-/-) mice by 6 weeks of age; however, these vascular and soft-tissue abnormalities were absent even in 12-week-old double-knockout mice. Klotho(-/-)/NaPi2a(-/-) mice also regained body weight and did not develop the generalized tissue atrophy often noted in klotho(-/-) single-knockout mice.
Our in vivo genetic manipulation studies have provided compelling evidence for a pathological role of increased NaPi2a activities in regulating abnormal mineral ion metabolism and soft-tissue anomalies in klotho(-/-) mice. Notably, our results suggest that serum phosphate levels are the important in vivo determinant of calcification and that lowering serum phosphate levels can reduce or eliminate soft-tissue and vascular calcification, even in presence of extremely high serum calcium and 1,25-dihydroxyvitamin D levels. These in vivo observations have significant clinical importance and therapeutic implications for patients with chronic kidney disease with cardiovascular calcification.
Vitamin D (Vit D) deficiency is a common condition in chronic kidney disease (CKD) patients that negatively affects bone regeneration and fracture healing. Previous study has shown that timely ...healing of titanium implants is impaired in CKD. This study aimed to investigate the effect of Vit D supplementation on implant osseointegration in CKD mice. Uremia was induced by 5/6 nephrectomy in C57BL mice. Eight weeks after the second renal surgery, animals were given 1,25(OH)2D3 three times a week intraperitoneally for four weeks. Experimental titanium implants were inserted into the distal end of femurs two weeks later. Serum measurements confirmed decreased 1,25(OH)2D levels in CKD mice, which could be successfully corrected by Vit D injections. Moreover, the hyperparathyroidism observed in CKD mice was also corrected. X-ray examination and histological sections showed successful osseointegration in these mice. Histomorphometrical analysis revealed that the bone-implant contact (BIC) ratio and bone volume (BV/TV) around the implant were significantly increased in the Vit D-supplementation group. In addition, resistance of the implant, as measured by a push-in method, was significantly improved compared to that in the vehicle group. These results demonstrate that Vit D supplementation is an effective approach to improve the fixation of titanium implants in CKD.
Indian hedgehog (Ihh) is essential for chondrocyte and osteoblast proliferation/differentiation during prenatal endochondral bone formation. The early lethality of various Ihh-ablated mutant mice, ...however, prevented further analysis of its role in postnatal bone growth and development. In this study, we describe the generation and characterization of a mouse model in which the Ihh gene was successfully ablated from postnatal chondrocytes in a temporal/spatial-specific manner; postnatal deletion of Ihh resulted in loss of columnar structure, premature vascular invasion, and formation of ectopic hypertrophic chondrocytes in the growth plate. Furthermore, destruction of the articular surface in long bones and premature fusion of growth plates of various endochondral bones was evident, resulting in dwarfism in mutant mice. More importantly, these mutant mice exhibited continuous loss of trabecular bone over time, which was accompanied by reduced Wnt signaling in the osteoblastic cells. These results demonstrate, for the first time, that postnatal chondrocyte-derived Ihh is essential for maintaining the growth plate and articular surface and is required for sustaining trabecular bone and skeletal growth.
Maintenance of physiologic phosphate balance is of crucial biological importance, as it is fundamental to cellular function, energy metabolism, and skeletal mineralization. Fibroblast growth ...factor-23 (FGF-23) is a master regulator of phosphate homeostasis, but the molecular mechanism of such regulation is not yet completely understood. Targeted disruption of the Fgf-23 gene in mice (Fgf-23-/-) elicits hyperphosphatemia, and an increase in renal sodium/phosphate co-transporter 2a (NaPi2a) protein abundance. To elucidate the pathophysiological role of augmented renal proximal tubular expression of NaPi2a in Fgf-23-/- mice and to examine serum phosphate-independent functions of Fgf23 in bone, we generated a new mouse line deficient in both Fgf-23 and NaPi2a genes, and determined the effect of genomic ablation of NaPi2a from Fgf-23-/- mice on phosphate homeostasis and skeletal mineralization. Fgf-23-/-/NaPi2a-/- double mutant mice are viable and exhibit normal physical activities when compared to Fgf-23-/- animals. Biochemical analyses show that ablation of NaPi2a from Fgf-23-/- mice reversed hyperphosphatemia to hypophosphatemia by 6 weeks of age. Surprisingly, despite the complete reversal of serum phosphate levels in Fgf-23-/-/NaPi2a-/-, their skeletal phenotype still resembles the one of Fgf23-/- animals. The results of this study provide the first genetic evidence of an in vivo pathologic role of NaPi2a in regulating abnormal phosphate homeostasis in Fgf-23-/- mice by deletion of both NaPi2a and Fgf-23 genes in the same animal. The persistence of the skeletal anomalies in double mutants suggests that Fgf-23 affects bone mineralization independently of systemic phosphate homeostasis. Finally, our data support (1) that regulation of phosphate homeostasis is a systemic effect of Fgf-23, while (2) skeletal mineralization and chondrocyte differentiation appear to be effects of Fgf-23 that are independent of phosphate homeostasis.
Fibroblast growth factor-23 (FGF-23), a recently identified molecule that is mutated in patients with autosomal dominant hypophosphatemic rickets (ADHR), appears to be involved in the regulation of ...phosphate homeostasis. Although increased levels of circulating FGF-23 were detected in patients with different phosphate-wasting disorders such as oncogenic osteomalacia (OOM) and X-linked hypophosphatemia (XLH), it is not yet clear whether FGF-23 is directly responsible for the abnormal regulation of mineral ion homeostasis and consequently bone development. To address some of these unresolved questions, we generated a mouse model, in which the entire
Fgf-23 gene was replaced with the
lacZ gene.
Fgf-23 null (
Fgf-23
+IhI-/+IhI) mice showed signs of growth retardation by day 17, developed severe hyperphosphatemia with elevated serum 1,25(OH)
2D
3 levels, and died by 13 weeks of age. Hyperphosphatemia in
Fgf-23
+IhI-/+IhI mice was accompanied by skeletal abnormalities, as demonstrated by histological, molecular, and various other morphometric analyses.
Fgf-23
+IhI-/+IhI mice had increased total-body bone mineral content (BMC) but decreased bone mineral density (BMD) of the limbs. Overall,
Fgf-23
+IhI-/+IhI mice exhibited increased mineralization, but also accumulation of unmineralized osteoid leading to marked limb deformities. Moreover,
Fgf-23
+IhI-/+IhI mice showed excessive mineralization in soft tissues, including heart and kidney. To further expand our understanding regarding the role of
Fgf-23 in phosphate homeostasis and skeletal mineralization, we crossed
Fgf-23
+IhI-/+IhI animals with
Hyp mice, the murine equivalent of XLH. Interestingly,
Hyp males lacking both
Fgf-23 alleles were indistinguishable from
Fgf-23
+IhI-/+IhI mice, both in terms of serum phosphate levels and skeletal changes, suggesting that
Fgf-23 is upstream of the phosphate regulating gene with homologies to endopeptidases on the X chromosome (Phex) and that the increased plasma Fgf-23 levels in
Hyp mice (and in XLH patients) may be at least partially responsible for the phosphate imbalance in this disorder.
The motor protein Kif3a and primary cilia regulate important developmental processes, but their roles in skeletogenesis remain ill-defined. Here we created mice deficient in Kif3a in cartilage and ...focused on the cranial base and synchondroses. Kif3a deficiency caused cranial base growth retardation and dysmorphogenesis, which were evident in neonatal animals by anatomical and micro-computed tomography (microCT) inspection. Kif3a deficiency also changed synchondrosis growth plate organization and function, and the severity of these changes increased over time. By postnatal day (P)7, mutant growth plates lacked typical zones of chondrocyte proliferation and hypertrophy, and were instead composed of chondrocytes with an unusual phenotype characterized by strong collagen II (Col2a1) gene expression but barely detectable expression of Indian hedgehog (Ihh), collagen X (Col10a1), Vegf (Vegfa), MMP-13 (Mmp13) and osterix (Sp7). Concurrently, unexpected developmental events occurred in perichondrial tissues, including excessive intramembranous ossification all along the perichondrial border and the formation of ectopic cartilage masses. Looking for possible culprits for these latter processes, we analyzed hedgehog signalling topography and intensity by monitoring the expression of the hedgehog effectors Patched 1 and Gli1, and of the hedgehog-binding cell-surface component syndecan 3. Compared with controls, hedgehog signaling was quite feeble within mutant growth plates as early as P0, but was actually higher and was widespread all along mutant perichondrial tissues. Lastly, we studied postnatal mice deficient in Ihh in cartilage; their cranial base defects only minimally resembled those in Kif3a-deficient mice. In summary, Kif3a and primary cilia make unique contributions to cranial base development and synchondrosis growth plate function. Their deficiency causes abnormal topography of hedgehog signaling, growth plate dysfunction, and un-physiologic responses and processes in perichondrial tissues, including ectopic cartilage formation and excessive intramembranous ossification.
Fibroblast growth factor-23 (FGF23) controls serum phosphorus largely through actions on the kidneys to excrete phosphorus and reduce calcitriol. Although these actions are well established in adults ...and children, the role that FGF23 plays in regulating fetal phosphorus metabolism has not been previously studied. We used several mouse models to study the effect of endogenous deficiency or excess of FGF23 on fetal phosphorus metabolism. We found that intact FGF23 does not cross the placenta from mother to fetus, but wild-type fetuses normally have intact FGF23 levels that approximately equal the maternal level. Deletion of Fgf23 or 7.8-fold higher serum FGF23 levels did not disturb any parameter of fetal mineral homeostasis, including serum and amniotic fluid phosphorus, skeletal morphology, skeletal mineral content, and placental phosphorus transport. Placentas and fetal kidneys abundantly express FGF23 target genes. Cyp24a1 was significantly reduced in Fgf23 null kidneys and was significantly increased in Phex null placentas and fetal kidneys. Phex null kidneys also showed reduced expression of Klotho. However, these changes in gene expression did not disturb any physiological parameter related to phosphorus. A 50% reduction in FGF23 also failed to affect renal phosphorus excretion into amniotic fluid when either PTH or the vitamin D receptor were absent. In conclusion, FGF23 is not an important regulator of fetal phosphorous metabolism. The active delivery of phosphorus across the placenta does not require FGF23, and that process overrides any effects that absence or excess of FGF23 might otherwise have on phosphate handling by the fetal kidneys.
PDF
