Distinct roles for intrinsic osteocyte abnormalities and systemic factors in regulation of FGF23 and bone mineralization in Hyp mice

Conditional deletion of Pkd1 in osteocytes disrupts skeletal mechanosensing in mice

We investigated whether polycystin-1 is a bone mechanosensor. We conditionally deleted Pkd1 in mature osteoblasts/osteocytes by crossing Dmp1-Cre with Pkd1(flox/m1Bei) mice, in which the m1Bei allele is nonfunctional. We assessed in wild-type and Pkd1-deficient mice the response to mechanical loading in vivo by ulna loading and ex vivo by measuring the response of isolated osteoblasts to fluid shear stress. We found that conditional Pkd1 heterozygotes (Dmp1-Cre;Pkd1(flox/+)) and null mice (Pkd1(Dmp1-cKO)) exhibited a ∼ 40 and ∼ 90% decrease, respectively, in functional Pkd1 transcripts in bone. Femoral bone mineral density (12 vs. 27%), trabecular bone volume (32 vs. 48%), and cortical thickness (6 vs. 17%) were reduced proportionate to the reduction of Pkd1 gene dose, as were mineral apposition rate (MAR) and expression of Runx2-II, Osteocalcin, Dmp1, and Phex. Anabolic load-induced periosteal lamellar MAR (0.58 ± 0.14; Pkd1(Dmp1-cKO) vs. 1.68 ± 0.34 μm/d; control) and increases in Cox-2, c-Jun, Wnt10b, Axin2, and Runx2-II gene expression were significantly attenuated in Pkd1(Dmp1-cKO) mice compared with controls. Application of fluid shear stress to immortalized osteoblasts from Pkd1(null/null) and Pkd1(m1Bei/m1Bei)-derived osteoblasts failed to elicit the increments in cytosolic calcium observed in wild-type controls. These data indicate that polycystin-1 is essential for the anabolic response to skeletal loading in osteoblasts/osteocytes.

Tertiary excess of fibroblast growth factor 23 and hypophosphatemia following kidney transplantation

Hypophosphatemia caused by inappropriate urinary phosphate wasting is a frequent metabolic complication of the early period following kidney transplantation. Although previously considered to be caused by tertiary hyperparathyroidism, recent evidence suggests a primary role for persistently elevated circulating levels of the phosphorus-regulating hormone, FGF23. In the setting of a healthy renal allograft, markedly increased FGF23 levels from the dialysis period induce renal phosphate wasting and inhibition of calcitriol production, which contribute to hypophosphatemia. While such tertiary FGF23 excess and resultant hypophosphatemia typically abates within the first few weeks to months post-transplant, some recipients manifest persistent renal phosphate wasting. Furthermore, increased FGF23 levels have been associated with increased risk of kidney disease progression, cardiovascular disease, and death outside of the transplant setting. Whether tertiary FGF23 excess is associated with adverse transplant outcomes is unknown. In this article, we review the physiology of FGF23, summarize its relationship with hypophosphatemia after kidney transplantation, and speculate on its potential impact on long-term outcomes of renal allograft recipients.

A Phe377del mutation in ANK leads to impaired osteoblastogenesis and osteoclastogenesis in a mouse model for craniometaphyseal dysplasia (CMD)

Craniometaphyseal dysplasia (CMD) is a rare genetic disorder with hyperostosis of craniofacial bones and widened metaphyses in long bones. Patients often suffer from neurological symptoms due to obstruction of cranial foramina. No proven treatment is available and the pathophysiology is largely unknown. A Phe377 (TTC(1130-1132)) deletion in exon 9 of the pyrophosphate (PPi) transporter ANK leads to CMD-like features in an Ank(KI/KI) mouse model. Here, we investigated the effects of CMD-mutant ANK on mineralization and bone mass at a cellular level. Ank(KI/KI) osteoblast cultures showed decreased mineral deposition. Expression of bone mineralization regulating genes Mmp13, Ocn, Osx and Phex was reduced in Ank(KI/KI) osteoblasts, while the Fgf23 mRNA level was highly elevated in Ank(KI/KI) calvarial and femoral bones. Since ANK is a known PPi transporter, we examined other regulators of Pi/PPi homeostasis Enpp1 and Tnap. Significantly increased ENPP1 activity may compensate for dysfunctional mutant ANK leading to comparable extracellular PPi levels in Ank(+/+) osteoblasts. Similar to Ank(KI/KI) bone marrow-derived macrophage cultures, peripheral blood cultures from CMD patients exhibited reduced osteoclastogenesis. Cell-autonomous effects in Ank(KI/KI) osteoclasts resulted in disrupted actin ring formation and cell fusion. In addition, Ank(KI/KI) osteoblasts failed to adequately support osteoclastogenesis. Increased bone mass could partially be rescued by bone marrow transplants supporting our hypothesis that reduced osteoclastogenesis contributes at least in part to hyperostosis. We conclude that the Phe377del mutation in ANK causes impaired osteoblastogenesis and osteoclastogenesis resulting in hypomineralization and a high bone mass phenotype.

Ablation of systemic phosphate-regulating gene fibroblast growth factor 23 (Fgf23) compromises the dentoalveolar complex

Fibroblast growth factor-23 (FGF23) is a hormone that modulates circulating phosphate (P(i)) levels by controlling P(i) reabsorption from the kidneys. When FGF23 levels are deficient, as in tumoral calcinosis patients, hyperphosphatemia ensues. We show here in a murine model that Fgf23 ablation disrupted morphology and protein expression within the dentoalveolar complex. Ectopic matrix formation in pulp chambers, odontoblast layer disruption, narrowing of periodontal ligament space, and alteration of cementum structure were observed in histological and electron microscopy sections. Because serum P(i) levels are dramatically elevated in Fgf23(-/-), we assayed for apoptosis and expression of members from the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family, both of which are sensitive to elevated P(i) in vitro. Unlike X-linked hypophosphatemic (Hyp) and wild-type (WT) specimens, numerous apoptotic osteocytes and osteoblasts were detected in Fgf23(-/-) specimens. Further, in comparison to Hyp and WT samples, decreased bone sialoprotein and elevated dentin matrix protein-1 protein levels were observed in cementum of Fgf23(-/-) mice. Additional dentin-associated proteins, such as dentin sialoprotein and dentin phosphoprotein, exhibited altered localization in both Fgf23(-/-) and Hyp samples. Based on these results, we propose that FGF23 and (P(i)) homeostasis play a significant role in maintenance of the dentoalveolar complex.

The osteocyte--a novel endocrine regulator of body phosphate homeostasis

Although osteocytes are the most abundant cell type in bone, much of their biology remains enigmatic. They are known to transduce mechanical stress into signals that initiate local bone remodeling, and are targets for systemic and local endocrine signals that affect bone architecture and mineral homeostasis. However, recent data reveal that osteocytes themselves act as endocrine cells that synthesize fibroblast growth factor 23 (FGF23) and several other phosphatonins, shown to underpin the systemic regulation of phosphate homeostasis. This review will synthesize the emerging discoveries concerning the osteocytic endocrine role in phosphate homeostasis through the biology and pathophysiology of these phosphatonins. We also suggest future research paths that might resolve existing uncertainties, and look ahead at how greater understanding might improve the management of clinical disorders of phosphate homeostasis.

Fibroblast growth factor 23 production in bone is directly regulated by 1{alpha},25-dihydroxyvitamin D, but not PTH

Fibroblast growth factor 23 (FGF23), which is primarily produced by osteocytes in bone, regulates renal phosphate excretion and 1α,25-dihydroxyvitamin D [1,25(OH)(2)D(3)] metabolism. Patients with chronic kidney disease (CKD) have increased levels of circulating serum FGF23, but the direct effect on circulating FGF23 levels in renal insufficiency is still unclear. To identify the major regulator of FGF23 synthesis in renal insufficiency, we compared the effect of parathyroid hormone (PTH) and 1,25(OH)(2)D(3) on FGF23 synthesis in the calvariae of normal rats with that of uremic rats in vitro. 1,25(OH)(2)D(3) treatment significantly increased the FGF23 concentration in the medium from both groups, but the degree of increase in the uremic group was markedly higher than in the control group. A significant increase in FGF23 mRNA expression occurred as early as 4 h after treatment and reached the maximum within 8 h in the uremic group, whereas in the normal group a significant increase in FGF23 mRNA expression was observed only at 8 h. In addition, the expression of vitamin D receptor (VDR) mRNA in the calvariae of uremic rats was markedly higher than in normal rats. However, in neither group did PTH treatment affect the medium FGF23 concentration or the FGF23 mRNA levels. These results suggest that FGF23 synthesis in bone is regulated by 1,25(OH)(2)D(3) directly, not by PTH, and that increased VDR mRNA expression induced the relatively swift and strong response in the uremic group.

Identification of a novel dentin matrix protein-1 (DMP-1) mutation and dental anomalies in a kindred with autosomal recessive hypophosphatemia

An autosomal recessive form of hypophosphatemia (ARHP) was recently shown to be caused by homozygous mutations in DMP1, the gene encoding dentin matrix protein-1 (DMP-1), a non-collagenous bone matrix protein with an important role in the development and mineralization of bone and teeth. Here, we describe a previously not reported consanguineous ARHP kindred in which the three affected individuals carry a novel homozygous DMP-1 mutation. The index case presented at the age of 3 years with bowing of his legs and showed hypophosphatemia due to insufficient renal phosphate retention. Serum alkaline phosphatase activity was elevated, with initially normal PTH. FGF23 was inappropriately normal at an older age while being treated with oral phosphate and 1,25(OH)(2)D. Similar clinical and biochemical findings, except for elevated FGF23 levels, were present in his 16-month-old brother and his 12.5-year-old female cousin; the parents of the three affected children are first-degree cousins. Nucleotide sequence analysis was performed on PCR-amplified exons encoding DMP-1 and flanking intronic regions. A novel homozygous frame-shift mutation (c.485Tdel; p.Glu163ArgfsX53) in exon 6 resulting in a premature stop codon was identified in all effected individuals. The parents and available unaffected siblings were heterozygous for c.485Tdel. Tooth growth and shape were normal for the index case, his affected brother and cousin, but their permanent and deciduous teeth displayed enlarged pulp chambers. The identified genetic mutation underscores the importance of DMP-1 mutations in the pathogenesis of ARHP. Furthermore, DMP-1 mutations appear to contribute, through yet unknown mechanisms, to tooth development.

Post-renal transplantation hypophosphatemia

An understanding of the pathophysiologic mechanisms of post-renal transplant (PRT) bone disease is of important clinical significance. Although bone disease occurs after all solid organ transplantation, the cumulative skeletal fracture rate remains high in PRT subjects while reaching a plateau with other transplantations. One major difference in the pathophysiology of PRT bone disease is, perhaps, due to persistent renal phosphorus (Pi) wasting. Novel phosphaturic agents have recently been suggested to participate in the development of bone disease in PRT subjects. However, it is unclear as of yet whether these factors alone or in conjunction with excess parathyroid hormone (PTH) secretion play a key role in the development of negative Pi balance and consequent bone disease in this population. In this review, I present a natural history of PRT hypophosphatemia and persistent renal Pi leak, provide pathophysiologic insight into these developments, and discuss the difficulty in diagnosing these phenotypes in both adult and pediatric populations.

Nuclear isoforms of fibroblast growth factor 2 are novel inducers of hypophosphatemia via modulation of FGF23 and KLOTHO

FGF2 transgenic mice were developed in which type I collagen regulatory sequences drive the nuclear high molecular weight FGF2 isoforms in osteoblasts (TgHMW). The phenotype of TgHMW mice included dwarfism, decreased bone mineral density (BMD), osteomalacia, and decreased serum phosphate (P(i)). When TgHMW mice were fed a high P(i) diet, BMD was increased, and dwarfism was partially reversed. The TgHMW phenotype was similar to mice overexpressing FGF23. Serum FGF23 was increased in TgHMW mice. Fgf23 mRNA in bones and fibroblast growth factor receptors 1c and 3c and Klotho mRNAs in kidneys were increased in TgHMW mice, whereas the renal Na(+)/P(i) co-transporter Npt2a mRNA was decreased. Immunohistochemistry and Western blot analyses of TgHMW kidneys showed increased KLOTHO and decreased NPT2a protein. The results suggest that overexpression of HMW FGF2 increases FGF23/FGFR/KLOTHO signaling to down-regulate NPT2a, causing P(i) wasting, osteomalacia, and decreased BMD. We assessed whether HMW FGF2 expression was altered in the Hyp mouse, a mouse homolog of the human disease X-linked hypophosphatemic rickets/osteomalacia. Fgf2 mRNA was increased in bones, and Western blots showed increased FGF2 protein in nuclear fractions from osteoblasts of Hyp mice. In addition, immunohistochemistry demonstrated co-localization of FGF23 and HMW FGF2 protein in osteoblasts and osteocytes from Hyp mice. This study reveals a novel mechanism of regulation of the FGF23-P(i) homeostatic axis.

FGF-23 in bone biology

Recent studies have demonstrated that levels of fibroblast growth factor 23 (FGF-23), a key regulator of phosphorus and vitamin D metabolism, rise dramatically as renal function declines and may play a key initiating role in disordered mineral and bone metabolism in patients with chronic kidney disease (CKD). The physiologic importance of FGF-23 in mineral metabolism was first identified in human genetic and acquired rachitic diseases and further characterized in animal models. FGF-23 and its regulators, including phosphate regulating endopeptidase homolog, dentin matrix 1 (DMP1), and matrix extracellular phosphoglycoprotein, are made primarily in bone, specifically in osteocytes. Dysregulation of these proteins results in osteomalacia, implicating the osteocyte in the regulation of skeletal mineralization. Studies in pediatric patients with CKD, the majority of whom have altered skeletal mineralization in early stages of CKD, have demonstrated that skeletal expression of both FGF-23 and its regulator, DMP1, are increased in early stages of CKD and that expression of these proteins is associated with alterations in skeletal mineralization. Thus, dysregulation of osteocytic proteins occur very early in the course of CKD and appear to be central to altered bone and mineral metabolism in this patient population.

The role of bone in phosphate metabolism

Fibroblast growth factor 23 (FGF23) is a humoral factor that is produced by osteocytes and reduces serum phosphate and 1,25-dihydroxyvitamin D levels by acting on kidney through some FGF receptor and Klotho. Excessive action of FGF23 results in several hypophosphatemic diseases characterized by impaired renal tubular phosphate reabsorption. In contrast, deficient action of FGF23 causes familial hyperphosphatemic tumoral calcinosis with enhanced renal tubular phosphate reabsorption. In addition, FGF23 null mice also show hyperphosphatemia. The production and circulatory level of FGF23 seem to be tightly regulated while the detailed mechanism of this regulation remains to be clarified. These results indicate that FGF23 is a physiological factor working as a hormone produced by bone. The discovery of FGF23 has revealed the possibility that bone produces several humoral factors to communicate with other organs.

Novel regulators of Fgf23 expression and mineralization in Hyp bone

We used gene array analysis of cortical bone to identify Phex-dependent gene transcripts associated with abnormal Fgf23 production and mineralization in Hyp mice. We found evidence that elevation of Fgf23 expression in osteocytes is associated with increments in Fgf1, Fgf7, and Egr2 and decrements in Sost, an inhibitor in the Wnt-signaling pathway, were observed in Hyp bone. beta-Catenin levels were increased in Hyp cortical bone, and TOPflash luciferase reporter assay showed increased transcriptional activity in Hyp-derived osteoblasts, consistent with Wnt activation. Moreover, activation of Fgf and Wnt-signaling stimulated Fgf23 promoter activity in osteoblasts. We also observed reductions in Bmp1, a metalloproteinase that metabolizes the extracellular matrix protein Dmp1. Alterations were also found in enzymes regulating the posttranslational processing and stability of Fgf23, including decrements in the glycosyltransferase Galnt3 and the proprotein convertase Pcsk5. In addition, we found that the Pcsk5 and the glycosyltransferase Galnt3 were decreased in Hyp bone, suggesting that reduced posttranslational processing of FGF23 may also contribute to increased Fgf23 levels in Hyp mice. With regard to mineralization, we identified additional candidates to explain the intrinsic mineralization defect in Hyp osteoblasts, including increases in the mineralization inhibitors Mgp and Thbs4, as well as increases in local pH-altering factors, carbonic anhydrase 12 (Car12) and 3 (Car3) and the sodium-dependent citrate transporter (Slc13a5). These studies demonstrate the complexity of gene expression alterations in bone that accompanies inactivating Phex mutations and identify novel pathways that may coordinate Fgf23 expression and mineralization of extracellular matrix in Hyp bone.

The PHEX transgene corrects mineralization defects in 9-month-old hypophosphatemic mice

Hypophosphatemia is an X-linked dominant disorder resulting from a mutation in the PHEX gene. While osteoblast-specific expression of the PHEX transgene has been reported to decrease the phosphate wasting associated with the disease in male hypophosphatemic (HYP) mice, there are reports that the mineralization defect is only partially corrected in young animals. To test the hypothesis that osteoblast-specific expression of the PHEX gene for a longer time would correct the mineralization defect, this study examined the bones of 9-month-old male and female HYP mice and their wild-type controls with or without expression of the transgene under a collagen type I promoter. Serum phosphate levels, alkaline phosphatase activity, and FGF23 levels were also measured. Mineral analyses based on wide-angle X-ray diffraction, Fourier transform-infrared (FT-IR) spectroscopy, and FT-IR imaging confirmed the decreased mineral content and increased mineral crystal size in male HYP humerii compared to wild-type males and females with or without the transgene and in female HYP mice with or without the transgene. There was a significant increase in mineral content and a decrease in crystallinity in the HYP males' bones with the transgene, compared to those without. Of interest, expression of the transgene in wild-type animals significantly increased the mineral content in both males and females without having a detectable effect on crystallinity or carbonate content. In contrast to the bones, based on micro-computed tomography and FT-IR imaging, at 9 months there were no significant differences between the HYP and the WT teeth, precluding analysis of the effect of the transgene.

Relationship between plasma fibroblast growth factor-23 concentration and bone mineralization in children with renal failure on peritoneal dialysis

CONTEXT

Fibroblast growth factor (FGF)-23 is produced in bone, and circulating levels are markedly elevated in patients with end-stage kidney disease, but the relationship between plasma levels of FGF-23 and bone histology in dialysis patients with secondary hyperparathyroidism is unknown.

OBJECTIVE

The aim of the study was to evaluate the correlation between plasma levels of FGF-23 and bone histology in pediatric patients with end-stage kidney disease who display biochemical evidence of secondary hyperparathyroidism.

DESIGN

We performed a cross-sectional analysis of the relationship between plasma FGF-23 levels and bone histomorphometry.

SETTING

The study was conducted in a referral center.

STUDY PARTICIPANTS

Participants consisted of forty-nine pediatric patients who were treated with maintenance peritoneal dialysis and who had serum PTH levels (1st generation Nichols assay) greater than 400 pg/ml.

INTERVENTION

There were no interventions.

MAIN OUTCOME MEASURE

Plasma FGF-23 levels and bone histomorphometry were measured.

RESULTS

No correlation existed between values of PTH and FGF-23. Bone formation rates correlated with PTH (r = 0.44; P < 0.01), but not with FGF-23. Higher FGF-23 concentrations were associated with decreased osteoid thickness (r = -0.49; P < 0.01) and shorter osteoid maturation time (r = -0.48; P < 0.01).

CONCLUSIONS

High levels of FGF-23 are associated with improved indices of skeletal mineralization in dialyzed pediatric patients with high turnover renal osteodystrophy. Together with other biomarkers, FGF-23 measurements may indicate skeletal mineralization status in this patient population.

Endocrine functions of bone in mineral metabolism regulation

Given the dramatic increase in skeletal size during growth, the need to preserve skeletal mass during adulthood, and the large capacity of bone to store calcium and phosphate, juxtaposed with the essential role of phosphate in energy metabolism and the adverse effects of hyperphosphatemia, it is not surprising that a complex systems biology has evolved that permits cross-talk between bone and other organs to adjust phosphate balance and bone mineralization in response to changing physiological requirements. This review examines the newly discovered signaling pathways involved in the endocrine functions of bone, such as those mediated by the phosphaturic and 1,25(OH)2D-regulating hormone FGF23, and the broader systemic effects associated with abnormalities of calcium and phosphate homeostasis.

Pathogenic role of Fgf23 in Dmp1-null mice

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.

Phosphatonins: new hormones that control phosphorus homeostasis

Phosphorus (Pi) plays an important role in nucleic acid synthesis, energy metabolism, bone mineralization and cell signaling, and is also present in sugars, phospholipids and phosphoproteins. Phosphate homeostasis is controlled by processes that regulate the intestinal absorption and renal excretion of Pi, and bone turnover. These processes are influenced by peptide and sterol hormones, such as parathyroid hormone and 1α,25-dihydroxyvitamin D (1α,25[OH]₂D₃). Recently, a new class of phosphate-regulating peptides has been discovered: phosphatonins. These factors, such as FGF-23, secreted frizzled-related protein-4, matrix extracellular phosphoglycoprotein and FGF-7, are circulating peptides with potent phosphaturic activity. These peptides inhibit Na/Pi transporters in renal epithelial cells and, therefore, increase renal Pi excretion. In addition, FGF-23 and secreted frizzled-related protein-4 inhibit 25-hydroxyvitamin D 1α-hydroxylase activity, reducing 1α,25(OH)2D3 synthesis and, thus, intestinal Pi absorption. Phosphatonins have been associated with hypophosphatemic diseases, such as tumor-induced osteomalacia, X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets and hyperphosphatemic disease (e.g., tumoral calcinosis). The aim of this article is to review the role of phosphatonins in Pi metabolism in normal and pathologic conditions and also to investigate the correlations among the various phosphatonins.