1,25-Dihydroxyvitamin D Alone Improves Skeletal Growth, Microarchitecture, and Strength in a Murine Model of XLH, Despite Enhanced FGF23 Expression

NFATc1 Is Required for Vitamin D- and Phosphate-Mediated Regulation of Osteocyte Lacuno-Canalicular Remodeling

Osteocytes are embedded in lacunae and connected by canaliculi (lacuno-canalicular network, LCN). Bones from mice with X-linked hypophosphatemia (Hyp), which have impaired production of 1,25 dihydroxyvitamin D (1,25D) and hypophosphatemia, have abnormal LCN structure that is improved by treatment with 1,25D or an anti-FGF23 targeting antibody, supporting roles for 1,25D and phosphate in regulating LCN remodeling. Bones from mice lacking the vitamin D receptor (VDR) in osteocytes (Vdrf/f;Dmp1Cre+) and mice lacking the sodium phosphate transporter 2a (Npt2aKO), which have low serum phosphate with high serum 1,25D, have impaired LCN organization, demonstrating that osteocyte-specific actions of 1,25D and hypophosphatemia regulate LCN remodeling. In osteoclasts, nuclear factor of activated T cells cytoplasmic 1 (NFATc1) is critical for stimulating bone resorption. Since osteocytes also resorb matrix, we hypothesize that NFATc1 plays a role in 1,25D and phosphate-mediated LCN remodeling. Consistent with this, 1,25D and phosphate suppress Nfatc1 mRNA expression in IDG-SW3 osteocytes, and knockdown of Nfatc1 expression in IDG-SW3 cells blocks 1,25D- and phosphate-mediated suppression of matrix resorption gene expression and 1,25D- and phosphate-mediated suppression of RANKL-induced acidification of the osteocyte microenvironment. To determine the role of NFATc1 in 1,25D- and phosphate-mediated LCN remodeling in vivo, histomorphometric analyses of tibiae from mice lacking osteocyte-specific Nfatc1 in Vdrf/f;Dmp1Cre+ and Npt2aKO mice were performed, demonstrating that bones from these mice have decreased lacunar size and expression of matrix resorption genes, and improved canalicular structure compared to Vdrf/f;Dmp1Cre+ and Npt2aKO control. This study demonstrates that NFATc1 is necessary for 1,25D- and phosphate-mediated regulation of LCN remodeling.

A PAI-1 antagonist ameliorates hypophosphatemia in the Hyp vitamin D-resistant rickets model mouse

Congenital fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets/osteomalacia is a rare bone metabolism disorder characterized by hypophosphatemia and caused by genetic abnormalities that result in excessive secretion of FGF23. Hyp mice are a model of X-linked hypophosphatemia (XLH) caused by deletion of the PHEX gene and excessive production of FGF23. The purpose of this study was to investigate the potential of TM5614 as a therapeutic agent for the treatment of congenital FGF23-related hypophosphatemic rickets and osteomalacia in humans by administering TM5614 to Hyp mice and examining its curative effect on hypophosphatemia. After a single oral administration of TM5614 10 mg·kg-1 to female Hyp mice starting at 17 weeks of age, the serum phosphate concentration increased with a peak at 6 h after administration. ELISA confirmed that TM5614 administration decreased the intact FGF23 concentration in the blood. Expression of 25-hydroxyvitamin D-1α-hydroxylase protein encoded by Cyp27b1 mRNA in the kidney was suppressed in Hyp mice, and treatment with 10 mg·kg-1 of TM5614 normalized the expression of 25-hydroxyvitamin D-1α-hydroxylase protein and Cyp27b1 mRNA in the kidneys of these mice. Our data indicate that oral administration of TM5614 ameliorates hypophosphatemia in Hyp mice, suggesting that TM5614 may be an effective treatment for congenital FGF23-related hypophosphatemic rickets and osteomalacia.

Impaired Growth Plate Maturation in XLH Is due to Both Excess FGF23 and Decreased 1,25-Dihydroxyvitamin D Signaling

X-linked hypophosphatemia (XLH) is the most common form of hereditary hypophosphatemic rickets. The genetic basis for XLH is loss of function mutations in the phosphate-regulating endopeptidase X-linked (PHEX), which leads to increased circulating fibroblast growth factor 23 (FGF23). This increase in FGF23 impairs activation of vitamin D and attenuates renal phosphate reabsorption, leading to rickets. Previous studies have demonstrated that ablating FGF23 in the Hyp mouse model of XLH leads to hyperphosphatemia, high levels of 1,25-dihydroxyvitamin D, and is not associated with the development of rickets. Studies were undertaken to define a role for the increase in 1,25-dihydroxyvitamin D levels in the prevention of rickets in Hyp mice lacking FGF23. These mice were mated to mice lacking Cyp27b1, the enzyme responsible for activating vitamin D metabolites, to generate Hyp mice lacking both FGF23 and 1,25-dihydroxyvitamin D (FCH mice). Mice were fed a special diet to maintain normal mineral ion homeostasis. Despite normal mineral ions, Hyp mice lacking both FGF23 and Cyp27b1 developed rickets, characterized by an interrupted, expanded hypertrophic chondrocyte layer and impaired hypertrophic chondrocyte apoptosis. This phenotype was prevented when mice were treated with 1,25-dihydroxyvitamin D from day 2 until sacrifice on day 30. Interestingly, mice lacking FGF23 and Cyp27b1 without the PHEX mutation did not exhibit rickets. These findings define an essential PHEX-dependent, FGF23-independent role for 1,25-dihydroxyvitamin D in XLH and have important therapeutic implications for the treatment of this genetic disorder.

Phosphate-induced activation of VEGFR2 leads to caspase-9-mediated apoptosis of hypertrophic chondrocytes

Low circulating phosphate (Pi) leads to rickets, characterized by expansion of the hypertrophic chondrocytes (HCs) in the growth plate due to impaired HC apoptosis. Studies in HCs demonstrate that Pi activates the Raf/MEK/ERK1/2 and mitochondrial apoptotic pathways. To determine how Pi activates these pathways, a small-molecule screen was undertaken to identify inhibitors of Pi-induced ERK1/2 phosphorylation in HCs. Vascular endothelial growth factor receptor 2 (VEGFR2) was identified as a target. In vitro studies in HCs demonstrate that VEGFR2 inhibitors block Pi-induced pERK1/2 and caspase-9 cleavage. Like Pi, rhVEGF activates ERK1/2 and caspase-9 in HCs and induces phosphorylation of VEGFR2, confirming that Pi activates this signaling pathway in HCs. Chondrocyte-specific depletion of VEGFR2 leads to an increase in HCs, impaired vascular invasion, and a decrease in HC apoptosis. Thus, these studies define a role for VEGFR2 in transducing Pi signals and mediating its effects on growth plate maturation.

Impaired 1,25-dihydroxyvitamin D3 action underlies enthesopathy development in the Hyp mouse model of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by high serum fibroblast growth factor 23 (FGF23) levels, resulting in impaired 1,25-dihydroxyvitamin D3 (1,25D) production. Adults with XLH develop a painful mineralization of the tendon-bone attachment site (enthesis), called enthesopathy. Treatment of mice with XLH (Hyp) with 1,25D or an anti-FGF23 Ab, both of which increase 1,25D signaling, prevents enthesopathy. Therefore, we undertook studies to determine a role for impaired 1,25D action in enthesopathy development. Entheses from mice lacking vitamin D 1α-hydroxylase (Cyp27b1) (C-/-) had a similar enthesopathy to Hyp mice, whereas deletion of Fgf23 in Hyp mice prevented enthesopathy, and deletion of both Cyp27b1 and Fgf23 in mice resulted in enthesopathy, demonstrating that the impaired 1,25D action due to high FGF23 levels underlies XLH enthesopathy development. Like Hyp mice, enthesopathy in C-/- mice was observed by P14 and was prevented, but not reversed, with 1,25D therapy. Deletion of the vitamin D receptor in scleraxis-expressing cells resulted in enthesopathy, indicating that 1,25D acted directly on enthesis cells to regulate enthesopathy development. These results show that 1,25D signaling was necessary for normal postnatal enthesis maturation and played a role in XLH enthesopathy development. Optimizing 1,25D replacement in pediatric patients with XLH is necessary to prevent enthesopathy.

Insights into the Molecular and Hormonal Regulation of Complications of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by mutations in the PHEX gene, leading to elevated serum levels of FGF23, decreased production of 1,25 dihydroxyvitamin D3 (1,25D), and hypophosphatemia. Those affected with XLH manifest impaired growth and skeletal and dentoalveolar mineralization as well as increased mineralization of the tendon–bone attachment site (enthesopathy), all of which lead to decreased quality of life. Many molecular and murine studies have detailed the role of mineral ions and hormones in regulating complications of XLH, including how they modulate growth and growth plate maturation, bone mineralization and structure, osteocyte-mediated mineral matrix resorption and canalicular organization, and enthesopathy development. While these studies have provided insight into the molecular underpinnings of these skeletal processes, current therapies available for XLH do not fully prevent or treat these complications. Therefore, further investigations are needed to determine the molecular pathophysiology underlying the complications of XLH.

Biochanin A protects against iron overload associated knee osteoarthritis via regulating iron levels and NRF2/System xc-/GPX4 axis

BACKGROUND

Iron homeostasis plays a positive role in articular cartilage health. Excessive iron or iron overload can induce oxidative stress damage in chondrocytes and ferroptosis cell death, advancing knee osteoarthritis (KOA). However, up to date, few effective agents treat iron overload-induced KOA (IOKOA). Chinese herbal medicine (CHM) provides abundant resources for drug selection to manage bone metabolic conditions, including osteoporosis. Biochanin A (BCA) is a novel bioactive multifunctional natural compound isolated from Huangqi, which has protective effects on bone loss. Nevertheless, the function and mechanism of BCA in treating IOKOA are still elusive.

PURPOSE

This study seeks to uncover the potential therapeutic targets and mechanisms of BCA in the management of KOA with iron accumulation.

METHODS

Iron dextrin (500 mg/kg) was intraperitoneally injected into mice to establish the iron overloaded mice model. OA was induced through surgery, and the progression was evaluated eight weeks following surgery. OA severity was evaluated with micro-CT and Safranin-O/Fast green staining in vivo. Iron deposition in the knee joint and synovium was assessed using Perl's Prussian blue staining. Ferric ammonium citrate (FAC) was then administered to primary chondrocytes to evaluate iron regulators mediated iron homeostasis. Toluidine blue staining was utilized to identify chondrocytes in vitro. The vitality of the cells was assessed using the CCK-8 test. The apoptosis rate of cells was measured using Annexin V-FITC/PI assay. The intracellular iron level was detected utilizing the calcein-AM test. Reactive oxygen species (ROS), lipid-ROS, and mitochondrial membrane potentiality were reflected via fluorescence density. Utilizing RT-qPCR and western blotting, the expression level was determined.

RESULTS

Micro-CT and histological staining of knee joints showed greater cartilage degradation and higher iron buildup detected in iron-overloaded mice. BCA can reduce iron deposition and the severity of KOA. Toluidine blue staining and the CCK-8 assay indicated that BCA could rescue chondrocytes killed by iron. Cell apoptosis rates were increased due to iron overload but improved by BCA. Further, the intracellular content of iron, ROS, and lipid-ROS was increased with ferric ammonium citrate (FAC) treatment but restored after treatment with different concentrations of BCA. JC-1 staining revealed that BCA could reduce mitochondrial damage induced by iron overload.

CONCLUSION

Iron overload was shown to promote chondrocyte ferroptosis in vivo and in vitro. Moreover, iron overload suppressed the expression of collagen II and induced MMP expression by catalyzing ROS generation with mitochondrial dysfunction. Our results showed that BCA could directly reduce intracellular iron concentration by inhibiting TfR1 and promoting FPN but also target the Nrf2/system xc-/GPX4 signaling pathway to scavenge free radicals and prevent lipid peroxidation. The results of this research indicate that BCA regulates iron homeostasis during the progression of osteoarthritis, which can open a new field of treatment for KOA.

Contributions of SGK3 to transporter-related diseases

Serum- and glucocorticoid-induced kinase 3 (SGK3), which is ubiquitously expressed in mammals, is regulated by estrogens and androgens. SGK3 is activated by insulin and growth factors through signaling pathways involving phosphatidylinositol-3-kinase (PI3K), 3-phosphoinositide-dependent kinase-1 (PDK-1), and mammalian target of rapamycin complex 2 (mTORC2). Activated SGK3 can activate ion channels (TRPV5/6, SOC, Kv1.3, Kv1.5, Kv7.1, BKCa, Kir2.1, Kir2.2, ENaC, Nav1.5, ClC-2, and ClC Ka), carriers and receptors (Npt2a, Npt2b, NHE3, GluR1, GluR6, SN1, EAAT1, EAAT2, EAAT4, EAAT5, SGLT1, SLC1A5, SLC6A19, SLC6A8, and NaDC1), and Na+/K+-ATPase, promoting the transportation of calcium, phosphorus, sodium, glucose, and neutral amino acids in the kidney and intestine, the absorption of potassium and neutral amino acids in the renal tubules, the transportation of glutamate and glutamine in the nervous system, and the transportation of creatine. SGK3-sensitive transporters contribute to a variety of physiological and pathophysiological processes, such as maintaining calcium and phosphorus homeostasis, hydro-salinity balance and acid-base balance, cell proliferation, muscle action potential, cardiac and neural electrophysiological disturbances, bone density, intestinal nutrition absorption, immune function, and multiple substance metabolism. These processes are related to kidney stones, hypophosphorous rickets, multiple syndromes, arrhythmia, hypertension, heart failure, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, glaucoma, ataxia idiopathic deafness, and other diseases.

The efficacy and safety of different doses of calcitriol combined with neutral phosphate in X-linked hypophosphatemia: a prospective study

The present study was the first prospective cohort evaluated the efficacy and safety of different doses of calcitriol in XLH children. The results suggested that a dose of 40 ng/kg/day calcitriol, compared with 20 ng/kg/day, was more effective in relieving the rickets, with similar safety outcomes. Further investigations were expected to set more dose groups.

INTRODUCTION

Dose recommended for calcitriol in X-linked hypophosphatemia (XLH) varies in different studies. Therefore, we aimed to compare the efficacy as well as the safety of 20 ng/kg/d and 40 ng/kg/d calcitriol in Chinese XLH pediatrics population.

METHODS

A 2-year, randomized, open-label, prospective study recruited 68 XLH children, which were randomized to receive either 40 ng/kg/day or 20 ng/kg/day calcitriol. Efficacy endpoints were the total Thacher ricket severity score (RSS) change from baseline to month 12 and 24, the difference in serum TALP level, fasting serum phosphate level, body height Z-score, and frequency of dental abscess. Safety assessments were done using renal ultrasound nephrocalcinosis grades (0-4), fasting serum and 24 h urine calcium level, and the occurrence of hyperparathyroidism.

RESULTS

The decrease in the total RSS from baseline was more significant in the high-dose group at 12 (difference 0.87, p = 0.049) and 24 month (difference 1.23, p = 0.011). The serum TALP level was significantly lower in the high-dose group at 6 months. Pi level, height Z-score change, frequency of dental abscess and ratio of de novo nephrocalcinosis were comparable. A lower incidence of secondary hyperparathyroidism was seen in the high-dose group (p < 0.0001).

CONCLUSION

For the first time in this prospective cohort, 40 ng/kg/d calcitriol was shown to be the more effective therapy in XLH children than the 20 ng/kg/d. Moreover, 40 ng/kg/d calcitriol was not associated with increasing adverse events.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT 03,820,518.

Drug discovery of sclerostin inhibitors

Sclerostin, a protein secreted from osteocytes, negatively regulates the WNT signaling pathway by binding to the LRP5/6 co-receptors and further inhibits bone formation and promotes bone resorption. Sclerostin contributes to musculoskeletal system-related diseases, making it a promising therapeutic target for the treatment of WNT-related bone diseases. Additionally, emerging evidence indicates that sclerostin contributes to the development of cancers, obesity, and diabetes, suggesting that it may be a promising therapeutic target for these diseases. Notably, cardiovascular diseases are related to the protective role of sclerostin. In this review, we summarize three distinct types of inhibitors targeting sclerostin, monoclonal antibodies, aptamers, and small-molecule inhibitors, from which monoclonal antibodies have been developed. As the first-in-class sclerostin inhibitor approved by the U.S. FDA, the monoclonal antibody romosozumab has demonstrated excellent effectiveness in the treatment of postmenopausal osteoporosis; however, it conferred high cardiovascular risk in clinical trials. Furthermore, romosozumab could only be administered by injection, which may cause compliance issues for patients who prefer oral therapy. Considering these above safety and compliance concerns, we therefore present relevant discussion and offer perspectives on the development of next-generation sclerostin inhibitors by following several ways, such as concomitant medication, artificial intelligence-based strategy, druggable modification, and bispecific inhibitors strategy.

Cellular and Molecular Alterations Underlying Abnormal Bone Growth in X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets, is caused by inactivating mutations of the phosphate-regulating endopeptidase gene (PHEX). XLH is mainly characterized by short stature, bone deformities and rickets, while in hypophosphatemia, normal or low vitamin D levels and low renal phosphate reabsorption are the principal biochemical aspects. The cause of growth impairment in patients with XLH is not completely understood yet, thus making the study of the growth plate (GP) alterations necessary. New treatment strategies targeting FGF23 have shown promising results in normalizing the growth velocity and improving the skeletal effects of XLH patients. However, further studies are necessary to evaluate how this treatment affects the GP as well as its long-term effects and the impact on adult height.

Fibroblast Growth Factor 2 High Molecular Weight Isoforms in Dentoalveolar Mineralization

Transgenic mice overexpressing human high molecular weight fibroblast growth factor 2 (HMWFGF2) isoforms in osteoblast and odontoblast lineages (HMWTg) exhibit decreased dentin and alveolar bone mineralization, enlarged pulp chamber, and increased fibroblast growth factor 23 (FGF23). We examined if the alveolar bone and dentin mineralization defects in HMWTg mice resulted from increased FGF23 expression and whether an FGF23 neutralizing antibody could rescue the hypomineralization phenotype. HMWTg and VectorTg control mice were given subcutaneous injections of FGF23 neutralizing antibody twice/week starting at postnatal day 21 for 6 weeks. Since Calcitriol (1,25D) have direct effects in promoting bone mineralization, we also determined if 1,25D protects against the defective dentin and alveolar bone mineralization. Therefore, HMWTg mice were given subcutaneous injections of 1,25D daily or concomitantly with FGF23 neutralizing antibody for 6 weeks. Our results showed that HMWTg mice displayed thickened predentin, alveolar bone hypomineralization, and enlarged pulp chambers. FGF23 neutralizing antibody and 1,25D monotherapy partially rescued the dentin mineralization defects and the enlarged pulp chamber phenotype in HMWTg mice. 1,25D alone was not sufficient to rescue the alveolar bone hypomineralization. Interestingly, HMWTg mice treated with both FGF23 neutralizing antibody and 1.25D further rescued the enlarged pulp chamber size, and dentin and alveolar bone mineralization defects. We conclude that the dentin and alveolar bone mineralization defects in HMWTg mice might result from increased FGF23 expression. Our results show a novel role of HMWFGF2 on dentoalveolar mineralization.

Prevalence of Enthesopathies in Adults With X-linked Hypophosphatemia: Analysis of Risk Factors

CONTEXT

Enthesopathies are the determinant of a poor quality of life in adults with X-linked hypophosphatemia (XLH).

OBJECTIVE

To describe the prevalence of patients with enthesopathies and to identify the risk factors of having enthesopathies.

METHODS

Retrospective study in the French Reference Center for Rare Diseases of the Calcium and Phosphate Metabolism between June 2011 and December 2020. Adult XLH patients with full body X-rays performed using the EOS® low-dose radiation system and clinical data collected from medical records. The main outcome measures were demographics, PHEX mutation, conventional treatment, and dental disease with the presence of enthesopathies.

RESULTS

Of the 114 patients included (68% women, mean age 42.2 ± 14.3 years), PHEX mutation was found in 105 patients (94.6%), 86 (77.5%) had been treated during childhood. Enthesopathies (spine and/or pelvis) were present in 67% of the patients (n = 76). Patients with enthesopathies were significantly older (P = .001) and more frequently reported dental disease collected from medical records (P = .03). There was no correlation between the PHEX mutations and the presence of enthesopathies. Sixty-two patients had a radiographic dental examination in a reference center. Severe dental disease (number of missing teeth, number of teeth endodontically treated, alveolar bone loss, and proportion of patients with 5 abscesses or more) was significantly higher in patients with enthesopathies.

CONCLUSION

Adult XLH patients have a high prevalence of enthesopathies in symptomatic adults patients with XLH seen in a reference center. Age and severe dental disease were significantly associated with the presence of enthesopathies.

Conductive Hearing Loss in the Hyp Mouse Model of X-Linked Hypophosphatemia Is Accompanied by Hypomineralization of the Auditory Ossicles

X-linked hypophosphatemia (XLH) is a hereditary musculoskeletal disorder caused by loss-of-function mutations in the PHEX gene. In XLH, increased circulating fibroblast growth factor 23 (FGF23) levels cause renal phosphate wasting and low concentrations of 1,25-dihydroxyvitamin D, leading to an early clinical manifestation of rickets. Importantly, hearing loss is commonly observed in XLH patients. We present here data from two XLH patients with marked conductive hearing loss. To decipher the underlying pathophysiology of hearing loss in XLH, we utilized the Hyp mouse model of XLH and measured auditory brain stem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) to functionally assess hearing. As evidenced by the increased ABR/DPOAE threshold shifts in the mid-frequency range, these measurements indicated a predominantly conductive hearing loss in Hyp mice compared to wild-type (WT) mice. Therefore, we carried out an in-depth histomorphometric and scanning electron microscopic analysis of the auditory ossicles. Quantitative backscattered electron imaging (qBEI) indicated a severe hypomineralization of the ossicles in Hyp mice, evidenced by lower calcium content (CaMean) and higher void volume (ie, porosity) compared to WT mice. Histologically, voids correlated with unmineralized bone (ie, osteoid), and the osteoid volume per bone volume (OV/BV) was markedly higher in Hyp mice than WT mice. The density of osteocyte lacunae was lower in Hyp mice than in WT mice, whereas osteocyte lacunae were enlarged. Taken together, our findings highlight the importance of ossicular mineralization for hearing conduction and point toward the potential benefit of improving mineralization to prevent hearing loss in XLH. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Adult rheumatologic features, treatment and complications of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare genetic phosphate disorder caused mainly by PHEX mutations. Unlike for children, knowledge of the disease's manifestations in adults is limited. Musculoskeletal symptoms are the main feature of the disease in young adults associated with a heavy burden on patients' life. They include fractures and pseudofractures, pain, joint stiffness, osteoarthritis, enthesopathies, and muscle weakness, eventually leading to impaired quality of life. Conventional treatment with phosphate supplements and vitamin D analogs is indicated in symptomatic patients. Appropriate rehabilitation is also a key to the management of the disease to improve physical function and decrease pain, stiffness, and fatigue. Regarding the incidence and consequences of musculoskeletal features in XLH, all patients should be assessed by a bone disease specialist and, if necessary, managed by a multidisciplinary team.

Impaired 1,25 dihydroxyvitamin D3 action and hypophosphatemia underlie the altered lacuno-canalicular remodeling observed in the Hyp mouse model of XLH

Osteocytes remodel the perilacunar matrix and canaliculi. X-linked hypophosphatemia (XLH) is characterized by elevated serum levels of fibroblast growth factor 23 (FGF23), leading to decreased 1,25 dihydroxyvitamin D₃ (1,25D) production and hypophosphatemia. Bones from mice with XLH (Hyp) have enlarged osteocyte lacunae, enhanced osteocyte expression of genes of bone remodeling, and impaired canalicular structure. The altered lacuno-canalicular (LCN) phenotype is improved with 1,25D or anti-FGF23 antibody treatment, pointing to roles for 1,25D and/or phosphate in regulating this process. To address whether impaired 1,25D action results in LCN alterations, the LCN phenotype was characterized in mice lacking the vitamin D receptor (VDR) in osteocytes (VDR^f/f;DMP1Cre+). Mice lacking the sodium phosphate transporter NPT2a (NPT2aKO) have hypophosphatemia and high serum 1,25D levels, therefore the LCN phenotype was characterized in these mice to determine if increased 1,25D compensates for hypophosphatemia in regulating LCN remodeling. Unlike Hyp mice, neither VDR^f/f;DMP1Cre+ nor NPT2aKO mice have dramatic alterations in cortical microarchitecture, allowing for dissecting 1,25D and phosphate specific effects on LCN remodeling in tibial cortices. Histomorphometric analyses demonstrate that, like Hyp mice, tibiae and calvariae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enlarged osteocyte lacunae (tibiae: 0.15±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.19±0.02μm²(VDR^f/f;DMP1Cre+), 0.12±0.02μm²(WT) vs 0.18±0.0μm²(NPT2aKO), calvariae: 0.09±0.02μm²(VDR^f/f;DMP1Cre-) vs 0.11±0.02μm²(VDR^f/f;DMP1Cre+), 0.08±0.02μm²(WT) vs 0.13±0.02μm²(NPT2aKO), p<0.05 all comparisons) and increased immunoreactivity of bone resorption marker Cathepsin K (Ctsk). The osteocyte enriched RNA isolated from tibiae in VDR^f/f;DMP1Cre+ and NPT2aKO mice have enhanced expression of matrix resorption genes that are classically expressed by osteoclasts (Ctsk, Acp5, Atp6v0d2, Nhedc2). Treatment of Ocy454 osteocytes with 1,25D or phosphate inhibits the expression of these genes. Like Hyp mice, VDR^f/f;DMP1Cre+ and NPT2aKO mice have impaired canalicular organization in tibia and calvaria. These studies demonstrate that hypophosphatemia and osteocyte-specific 1,25D actions regulate LCN remodeling. Impaired 1,25D action and low phosphate levels contribute to the abnormal LCN phenotype observed in XLH.

Effects of Active Vitamin D or FGF23 Antibody on Hyp Mice Dentoalveolar Tissues

Mutations in the PHEX gene lead to X-linked hypophosphatemia (XLH), a form of inherited rickets featuring elevated fibroblast growth factor 23 (FGF23), reduced 1,25-dihydroxyvitamin D (1,25D), and hypophosphatemia. Hyp mutant mice replicate the XLH phenotype, including dentin, alveolar bone, and cementum defects. We aimed to compare effects of 1,25D versus FGF23-neutralizing antibody (FGF23Ab) monotherapies on Hyp mouse dentoalveolar mineralization. Male Hyp mice, either injected subcutaneously with daily 1,25D or thrice weekly with FGF23 blocking antibody from 2 to 35 d postnatal, were compared to wild-type (WT) controls and untreated Hyp mice. Mandibles were analyzed by high-resolution micro-computed tomography (micro-CT), histology, and immunohistochemistry. Both interventions maintained normocalcemia, increased serum phosphate levels, and improved dentoalveolar mineralization in treated versus untreated Hyp mice. 1,25D increased crown dentin volume and thickness and root dentin/cementum volume, whereas FGF23Ab effects were limited to crown dentin volume. 1,25D increased bone volume fraction, bone mineral density, and tissue mineral density in Hyp mice, whereas FGF23Ab failed to significantly affect these alveolar bone parameters. Neither treatment fully attenuated dentin and bone defects to WT levels, and pulp volumes remained elevated regardless of treatment. Both treatments reduced predentin thickness and improved periodontal ligament organization, while 1,25D promoted a more profound improvement in acellular cementum thickness. Altered cell densities and lacunocanalicular properties of alveolar and mandibular bone osteocytes and cementocytes in Hyp mice were partially corrected by either treatment. Neither treatment normalized the altered distributions of bone sialoprotein and osteopontin in Hyp mouse alveolar bone. Moderate improvements from both 1,25D and FGF23Ab treatment regimens support further studies and collection of oral health data from subjects receiving a newly approved anti-FGF23 therapy. The inability of either treatment to fully correct Hyp mouse dentin and bone prompts further experiments into underlying pathological mechanisms to identify new therapeutic approaches.

1ɑ,25-Dihydroxyvitamin D3 promotes osteogenesis by down-regulating FGF23 in diabetic mice

1ɑ,25‐dihydroxyvitamin D3 (1,25D) and fibroblast growth factor 23 (FGF23) play important roles in bone metabolism through mutual regulation. However, the underlying mechanism between 1,25D and FGF23 in diabetes‐induced bone metabolism disorders has not yet been elucidated. In this study, we investigated the effect of 1,25D on FGF23 under diabetic condition both in vitro and in vivo. The results showed that 1,25D down‐regulated the expression of FGF23 in osteoblast significantly though a dose‐dependent manner in vitro within high glucose environment. Western blot and immunofluorescence analysis indicated that 1,25D activated PI3K/Akt signalling through binding to vitamin D receptor (VDR), which inhibited the phosphorylation of the transcription factor Forkhead Box O1 (FOXO1). Decreased phosphorylation of FOXO1 down‐regulated the expression Dickkopf‐1 (DKK1), a well‐known inhibitor of Wnt signalling. In addition, we observed that 1,25D remarkably ameliorated osteogenic phenotypic markers such as Ocn and Runx2 and rescued diabetes‐induced bone loss in vivo. Our results suggested that 1,25D could promote osteogenesis though down‐regulating FOXO1/FGF23 in diabetes.

Vitamin D supplementation improves bone mineralisation independent of dietary phosphate in male X-linked hypophosphatemic (Hyp) mice

The disorder of X-linked hypophosphatemia (XLH), results in the supressed renal production of active 1α,25-dihydroxyvitamin D (1,25(OH)₂D) due to elevated fibroblast growth factor-23 (FGF23) levels. While adequate 25(OH)D levels are generally associated with improved mineralisation of the skeleton independent of circulating 1,25(OH)₂D levels, it is unclear whether raising 25(OH)D to sufficiently high levels through dietary vitamin D₃ administration contributes to improving bone mineralisation in the murine homolog for XLH, Hyp mice. Three-week-old male Hyp mice were fed one of four diets containing either 1000 IU (C) or 20,000 IU (D) vitamin D3/kg diet with either 0.35% phosphate or 1.25% phosphate (P) until 12 weeks of age (n = 12/group). When compared to C-fed mice, D-fed mice significantly elevated serum 25(OH)D levels to 72.8 ± 4.9 nmol/L (2-fold, p < 0.001) and increased both cortical bone mineral density (15%, p < 0.01), and vertebral trabecular BV/TV% (80%, p < 0.001), despite persistent hypophosphatemia and normocalcemia. The increase in bone volume was associated with improved Tb.Th (12%, p < 0.01) and Tb.N (63%, p < 0.001). Unlike with D-diet, P-fed mice resulted in increased femoral (15%, p < 0.001) and vertebral (12%, p < 0.001) length, and a 34% increase in vertebral trabecular BV/TV% when compared to control fed animals (p < 0.001). However, the addition of the high P diet to the high D diet did not result in additive effects on bone mineralisation when compared to the effects of D diet alone, despite serum 25(OH)D levels elevated to 118.8 ± 8.6 nmol/L. In D-fed mice, the increase in bone mineral density and volume was associated with reduced osteoid volume, reduced ObS/BS, and a trend for reduced serum PTH levels, suggesting reduced bone turnover in these animals. Thus, elevating serum 25(OH)D levels independently improves bone mineralisation in Hyp mice without causing hypercalcemia, suggesting that further studies are required in XLH patients to establish the role of increasing 25(OH)D levels in improving bone mineralisation.

Probing the Scope and Mechanisms of Calcitriol Actions Using Genetically Modified Mouse Models

Genetically modified mice have provided novel insights into the mechanisms of activation and inactivation of vitamin D, and in the process have provided phenocopies of acquired human disease such as rickets and osteomalacia and inherited diseases such as pseudovitamin D deficiency rickets, hereditary vitamin D resistant rickets, and idiopathic infantile hypercalcemia. Both global and tissue-specific deletion studies leading to decreases of the active form of vitamin D, calcitriol [1,25(OH)2D], and/or of the vitamin D receptor (VDR), have demonstrated the primary role of calcitriol and VDR in bone, cartilage and tooth development and in the regulation of mineral metabolism and of parathyroid hormone (PTH) and FGF23, which modulate calcium and phosphate fluxes. They have also, however, extended the spectrum of actions of calcitriol and the VDR to include, among others: modulation, jointly and independently, of skin metabolism; joint regulation of adipose tissue metabolism; cardiovascular function; and immune function. Genetic studies in older mice have also shed light on the molecular mechanisms underlying the important role of the calcitriol/VDR pathway in diseases of aging such as osteoporosis and cancer. In the course of these studies in diverse tissues, important upstream and downstream, often tissue-selective, pathways have been illuminated, and intracrine, as well as endocrine actions have been described. Human studies to date have focused on acquired or genetic deficiencies of the prohormone vitamin D or the (generally inactive) precursor metabolite 25-hyrodxyvitamin D, but have yet to probe the pleiotropic aspects of deficiency of the active form of vitamin D, calcitriol, in human disease. © 2020 American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium

X-linked hypophosphatemia (XLH) is the most common genetic form of hypophosphatemic rickets and osteomalacia. In this disease, mutations in the PHEX gene lead to elevated levels of the hormone fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and impaired skeletal and dental mineralization. Recently, international guidelines for the diagnosis and treatment of this condition have been published. However, more specific recommendations are needed to provide guidance at the national level, considering resource availability and health economic aspects. A national multidisciplinary group of Belgian experts convened to discuss translation of international best available evidence into locally feasible consensus recommendations. Patients with XLH may present to a wide array of primary, secondary and tertiary care physicians, among whom awareness of the disease should be raised. XLH has a very broad differential-diagnosis for which clinical features, biochemical and genetic testing in centers of expertise are recommended. Optimal care requires a multidisciplinary approach, guided by an expert in metabolic bone diseases and involving (according to the individual patient's needs) pediatric and adult medical specialties and paramedical caregivers, including but not limited to general practitioners, dentists, radiologists and orthopedic surgeons. In children with severe or refractory symptoms, FGF23 inhibition using burosumab may provide superior outcomes compared to conventional medical therapy with phosphate supplements and active vitamin D analogues. Burosumab has also demonstrated promising results in adults on certain clinical outcomes such as pseudofractures. In summary, this work outlines recommendations for clinicians and policymakers, with a vision for improving the diagnostic and therapeutic landscape for XLH patients in Belgium.

The role of extracellular matrix phosphorylation on energy dissipation in bone

Protein phosphorylation, critical for cellular regulatory mechanisms, is implicated in various diseases. However, it remains unknown whether heterogeneity in phosphorylation of key structural proteins alters tissue integrity and organ function. Here, osteopontin phosphorylation level declined in hypo- and hyper- phosphatemia mouse models exhibiting skeletal deformities. Phosphorylation increased cohesion between osteopontin polymers, and adhesion of osteopontin to hydroxyapatite, enhancing energy dissipation. Fracture toughness, a measure of bone’s mechanical competence, increased with ex-vivo phosphorylation of wildtype mouse bones and declined with ex-vivo dephosphorylation. In osteopontin-deficient mice, global matrix phosphorylation level was not associated with toughness. Our findings suggest that phosphorylated osteopontin promotes fracture toughness in a dose-dependent manner through increased interfacial bond formation. In the absence of osteopontin, phosphorylation increases electrostatic repulsion, and likely protein alignment and interfilament distance leading to decreased fracture resistance. These mechanisms may be of importance in other connective tissues, and the key to unraveling cell–matrix interactions in diseases.

X-Linked Hypophosphatemia: A New Era in Management

X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive musculoskeletal disease that often causes pain and short stature, as well as decreased physical function, mobility, and quality of life. Hypophosphatemia in XLH is caused by loss of function mutations in the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene, resulting in excess levels of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23), which leads to renal phosphate wasting and decreased serum 1,25-dihydroxyvitamin D production. Historically, treatment options were limited to oral phosphate and active vitamin D analogues (conventional management) dosed several times daily in an attempt to improve skeletal mineralization by increasing serum phosphorus. The recent approval of burosumab, a fully human monoclonal antibody to FGF23, has provided a new, targeted treatment option for patients with XLH. This review summarizes our current understanding of XLH, the safety and efficacy of conventional management and burosumab, existing recommendations for managing patients, and unanswered questions in the field.

Mineralized tissues in hypophosphatemic rickets

Hypophosphatemic rickets is caused by renal phosphate wasting that is most commonly due to X-linked dominant mutations in PHEX. PHEX mutations cause hypophosphatemia indirectly, through the increased expression of fibroblast growth factor 23 (FGF23) by osteocytes. FGF23 decreases renal phosphate reabsorption and thereby increases phosphate excretion. The lack of phosphate leads to a mineralization defect at the level of growth plates (rickets), bone tissue (osteomalacia), and teeth, where the defect facilitates the formation of abscesses. The bone tissue immediately adjacent to osteocytes often remains unmineralized ("periosteocytic lesions"), highlighting the osteocyte defect in this disorder. Common clinical features of XLH include deformities of the lower extremities, short stature, enthesopathies, dental abscesses, as well as skull abnormalities such as craniosynostosis and Chiari I malformation. For the past four decades, XLH has been treated by oral phosphate supplementation and calcitriol, which improves rickets and osteomalacia and the dental manifestations, but often does not resolve all aspects of the mineralization defects. A newer treatment approach using inactivating FGF23 antibodies leads to more stable control of serum inorganic phosphorus levels and seems to heal rickets more reliably. However, the long-term benefits of FGF23 antibody treatment remain to be elucidated.

Development of Enthesopathies and Joint Structural Damage in a Murine Model of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia, caused by inactivating mutations in the Phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. With aging, adult patients develop paradoxical heterotopic calcifications of tendons and ligaments at their insertion sites (enthesophytes), and joint alterations. Understanding the progression of this structural damage that severely affects patients’ quality of life will help to improve the management of XLH. Here, we characterized the occurrence of enthesophytes and joint alterations through a 12 month in vivo micro-CT follow-up in the Hyp mouse, a murine model of XLH (n = 5 mice per group). Similar to adult patients with XLH, Hyp mice developed calcaneal enthesophytes, hip joint alterations, erosions of the sacroiliac joints and periarticular calcifications. These lesions were already present at month 3 and gradually worsened over time. In sharp contrast, no abnormalities were observed in control mice at early time points. Histological analyses confirmed the presence of bone erosions, calcifications and expansion of mineralizing enthesis fibrocartilage in Hyp mice and their absence in controls and suggested that new bone formation is driven by altered mechanical strain. Interestingly, despite a strong deformation of the curvature, none of the Hyp mice displayed enthesophyte at the spine. Peripheral enthesophytes and joint alterations develop at the early stages of the disease and gradually worsen overtime. Overall, our findings highlight the relevance of this preclinical model to test new therapies aiming to prevent bone and joint complications in XLH.

Alterations of bone material properties in adult patients with X-linked hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) caused by PHEX mutations results in elevated serum FGF23 levels, renal phosphate wasting and low 1,25-dihydroxyvitamin D. The glycophosphoprotein osteopontin, a potent inhibitor of mineralization normally degraded by PHEX, accumulates within the bone matrix. Conventional therapy consisting of supplementation with phosphate and vitamin D analogs is burdensome and the effects on bone material poorly characterized. We analyzed transiliac bone biopsies from four adult patients, two of them severely affected due to no diagnosis and no treatment until adulthood. We used light microscopy, qBEI and FTIRI to study histology, histomorphometry, bone mineralization density distribution, properties of the organic matrix and size of hypomineralized periosteocytic lesions. Non-treatment resulted in severe osteomalacia, twice the amount of mineralized trabecular volume, multiple osteon-like perforations, continuity of lamellae from mineralized to unmineralized areas and distinctive patches of woven bone. Periosteocytic lesions were larger than in treated patients. The latter had nearly normal osteoid thicknesses, although surface was still elevated. The median calcium content of the matrix was always within normal range, although the percentage of lowly mineralized bone areas was highly increased in non-treated patients, resulting in a marked heterogeneity in mineralization. Divalent collagen cross-links were evident independently of the mineral content of the matrix. Broad osteoid seams lacked measurable pyridinoline, a mature trivalent cross-link and exhibited considerable acidic lipid content, typically found in matrix vesicles. Based on our results, we propose a model that possibly integrates the relationship between the observed mineralization disturbances, FGF23 secretion and the known osteopontin accumulation in XLH.

Weaker bones and white skin as adaptions to improve anthropological "fitness" for northern environments

The vitamin D paradox relates to the lower risk of osteoporosis in people of sub-Saharan African ancestry (Blacks) compared with people of European ancestry (Whites). The paradox implies that for bone health, Blacks require less vitamin D and calcium than Whites do. Why should populations that migrated northward out of Africa have ended up needing more vitamin D than tropical Blacks? Human skin color became lighter away from the tropics to permit greater skin penetration of the UVB light that generates vitamin D. Lack of vitamin D impairs intestinal calcium absorption and limits the amount of calcium that can deposit into the protein matrix of bone, causing rickets or osteomalacia. These can cause cephalopelvic disproportion and death in childbirth. Whiter skin was more fit for reproduction in UV-light restricted environments, but natural selection was also driven by the phenotype of bone per se. Bone formation starts with the deposition of bone-matrix proteins. Mineralization of the matrix happens more slowly, and it stiffens bone. If vitamin D and/or calcium supplies are marginal, larger bones will not be as fully mineralized as smaller bones. For the same amount of mineral, unmineralized or partially mineralized bone is more easily deformed than fully mineralized bone. The evidence leads to the hypothesis that to minimize the soft bone that causes pelvic deformation, a decrease in amount of bone, along with more rapid mineralization of osteoid improved reproductive fitness in Whites. Adaptation of bone biology for reproductive fitness in response to the environmental stress of limited availability of vitamin D and calcium came at the cost of greater risk of osteoporosis later in life.

Protective Role of Vitamin D in Renal Tubulopathies

Vitamin D is tightly linked with renal tubular homeostasis: the mitochondria of proximal convoluted tubule cells are the production site of 1α,25-dihydroxyvitamin D3. Patients with renal impairment or tubular injury often suffer from chronic inflammation. This alteration comes from oxidative stress, acidosis, decreased clearance of inflammatory cytokines and stimulation of inflammatory factors. The challenge is to find the right formula for each patient to correctly modulate the landscape of treatment and preserve the essential functions of the organism without perturbating its homeostasis. The complexity of the counter-regulation mechanisms and the different axis involved in the Vitamin D equilibrium pose a major issue on Vitamin D as a potential effective anti-inflammatory drug. The therapeutic use of this compound should be able to inhibit the development of inflammation without interfering with normal homeostasis. Megalin-Cubilin-Amnionless and the FGF23-Klotho axis represent two Vitamin D-linked mechanisms that could modulate and ameliorate the damage response at the renal tubular level, balancing Vitamin D therapy with an effect potent enough to contrast the inflammatory cascades, but which avoids potential severe side effects.

Sclerostin Antibody Treatment Increases Bone Mass and Normalizes Circulating Phosphate Levels in Growing Hyp Mice

X-linked hypophosphatemia (XLH), caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX), is the most common form of vitamin D-resistant rickets. Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23) levels, impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily in osteocytes, suppresses bone formation by antagonizing Wnt signaling and is reported to be elevated in XLH patients. This study used the Hyp mouse model to investigate sclerostin's role in the pathophysiology of XLH by evaluating the use of a monoclonal antibody to sclerostin in a mouse model of XLH, the Hyp mouse. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or sclerostin antibody (Scl-Ab) twice weekly, beginning at 4 weeks of age and euthanized at 8 weeks of age. Scl-Ab treatment increased serum phosphate levels and suppressed circulating levels of intact FGF23 in treated wild-type and Hyp mice of both sexes. Cortical area, trabecular bone volume fraction (BV/TV), metaphyseal apparent density, and the peak load increased with Scl-Ab treatment in both sexes. This short-term treatment study suggests that Scl-Ab treatment can effectively improve some of the pathologies associated with XLH, including normalization of phosphate, and that sclerostin may play a role in regulating FGF23 and phosphate metabolism in XLH. © 2019 American Society for Bone and Mineral Research.

An Inverse Agonist Ligand of the PTH Receptor Partially Rescues Skeletal Defects in a Mouse Model of Jansen's Metaphyseal Chondrodysplasia

Jansen's metaphyseal chondrodysplasia (JMC) is a rare disease of bone and mineral ion physiology that is caused by activating mutations in PTHR1. Ligand-independent signaling by the mutant receptors in cells of bone and kidney results in abnormal skeletal growth, excessive bone turnover, and chronic hypercalcemia and hyperphosphaturia. Clinical features further include short stature, limb deformities, nephrocalcinosis, and progressive losses in kidney function. There is no effective treatment option available for JMC. In previous cell-based assays, we found that certain N-terminally truncated PTH and PTHrP antagonist peptides function as inverse agonists and thus can reduce the high rates of basal cAMP signaling exhibited by the mutant PTHR1s of JMC in vitro. Here we explored whether one such inverse agonist ligand, [Leu¹¹ ,dTrp¹² ,Trp²³ ,Tyr³⁶ ]-PTHrP(7-36)NH₂ (IA), can be effective in vivo and thus ameliorate the skeletal abnormalities that occur in transgenic mice expressing the PTHR1-H223R allele of JMC in osteoblastic cells via the collagen-1α1 promoter (C1HR mice). We observed that after 2 weeks of twice-daily injection and relative to vehicle controls, the IA analog resulted in significant improvements in key skeletal parameters that characterize the C1HR mice, because it reduced the excess trabecular bone mass, bone marrow fibrosis, and levels of bone turnover markers in blood and urine. The overall findings provide proof-of-concept support for the notion that inverse agonist ligands targeted to the mutant PTHR1 variants of JMC can have efficacy in vivo. Further studies of such PTHR1 ligand analogs could help open paths toward the first treatment option for this debilitating skeletal disorder. © 2019 American Society for Bone and Mineral Research.

1,25-Dihydroxyvitamin D Maintains Brush Border Membrane NaPi2a and Attenuates Phosphaturia in Hyp Mice

Phosphate homeostasis is critical for many cellular processes and is tightly regulated. The sodium-dependent phosphate cotransporter, NaPi2a, is the major regulator of urinary phosphate reabsorption in the renal proximal tubule. Its activity is dependent upon its brush border localization that is regulated by fibroblast growth factor 23 (FGF23) and PTH. High levels of FGF23, as are seen in the Hyp mouse model of human X-linked hypophosphatemia, lead to renal phosphate wasting. Long-term treatment of Hyp mice with 1,25-dihydroxyvitamin D (1,25D) or 1,25D analogues has been shown to improve renal phosphate wasting in the setting of increased FGF23 mRNA expression. Studies were undertaken to define the cellular and molecular basis for this apparent FGF23 resistance. 1,25D increased FGF23 protein levels in the cortical bone and circulation of Hyp mice but did not impair FGF23 cleavage. 1,25D attenuated urinary phosphate wasting as early as one hour postadministration, without suppressing FGF23 receptor/coreceptor expression. Although 1,25D treatment induced expression of early growth response 1, an early FGF23 responsive gene required for its phosphaturic effects, it paradoxically enhanced renal phosphate reabsorption and NaPi2a protein expression in renal brush border membranes (BBMs) within one hour. The Na-H+ exchange regulatory factor 1 (NHERF1) is a scaffolding protein thought to anchor NaPi2a to the BBM. Although 1,25D did not alter NHERF1 protein levels acutely, it enhanced NHERF1-NaPi2a interactions in Hyp mice. 1,25D also prevented the decrease in NHERF1/NaPi2a interactions in PTH-treated wild-type mice. Thus, these investigations identify a novel role for 1,25D in the hormonal regulation of renal phosphate handling.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

Clinical and translational pharmacological aspects of the management of fibrous dysplasia of bone

Fibrous dysplasia (FD) is a genetic, noninheritable rare bone disease caused by a postzygotic activating mutation of the α subunit of the stimulatory G‐protein causing increased abnormal bone formation leading to pain, deformity and fractures. To date, no cure has been identified for FD/McCune–Albright syndrome (MAS) and treatment is symptomatic and aimed at decreasing pain and/or local bone turnover. Various drugs have been used to achieve clinical improvement in FD/MAS patients including bisphosphonates and denosumab, however further translational studies are also warranted to address unresolved pathophysiological issues and explore novel pharmacological targets for the management of FD/MAS.

In this article, we review literature on the medical treatment of FD/MAS, discuss the unresolved pathophysiological issues and explore novel pharmacological targets for the management of FD/MAS.

Fibroblast growth factor-23 may serve as a novel biomarker for renal osteodystrophy progression

The purpose of the present study was to determine whether fibroblast growth factor (FGF)‑23 could serve as a novel biomarker for renal osteodystrophy (ROD) progression. A rat model of ROD was induced by left nephrectomy plus intravenous injection of Adriamycin. Serum FGF‑23 was determined using an enzyme‑linked immunosorbent assay. Serum level and bone expression of FGF‑23 were both significantly elevated in the ROD group at 24 h post‑surgery. Serum FGF‑23 was negatively correlated with calcium, phosphate, 25‑hydroxyvitamin D, conventional bone biomarkers and bone collagen X. More importantly, serum FGF‑23 was significantly associated with abnormalities in bone formation rate, osteoblasts, osteoclasts, trabecular volume thickness and osteoid volume. Therefore, FGF‑23 may serve as a novel biomarker for ROD.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

Molecular analysis of enthesopathy in a mouse model of hypophosphatemic rickets

The bone tendon attachment site known as the enthesis comprises a transitional zone between bone and tendon, and plays an important role in enabling movement at this site. X-linked hypophosphatemia (XLH) is characterized by impaired activation of vitamin D, elevated serum FGF23 levels and low serum phosphate levels, which impair bone mineralization. Paradoxically, an important complication of XLH is mineralization of the enthesis (enthesopathy). Studies were undertaken to identify the cellular and molecular pathways important for normal post-natal enthesis maturation and to examine their role during the development of enthesopathy in mice with XLH (Hyp). The Achilles tendon entheses of Hyp mice demonstrate an expansion of hypertrophic-appearing chondrogenic cells by P14. Post-natally, cells in wild-type and Hyp entheses similarly descend from scleraxis- and Sox9-expressing progenitors; however, Hyp entheses exhibit an expansion of Sox9-expressing cells, and enhanced BMP and IHH signaling. These results support a role for enhanced BMP and IHH signaling in the development of enthesopathy in XLH.

LP533401 restores bone health in 5/6 nephrectomized rats by a decrease of gut-derived serotonin and regulation of serum phosphate through the inhibition of phosphate co-transporters expression in the kidneys

LP533401 is an orally bioavailable small molecule that inhibits tryptophan hydroxylase-1, an enzyme responsible for the synthesis of gut-derived serotonin (GDS). Recently, we showed that increased GDS in rats with chronic kidney disease (CKD) affected bone strength and metabolism. We tested the hypothesis that treatment with LP533401 could reverse CKD-induced bone loss in uremia. Sixteen weeks after 5/6 nephrectomy, rats were randomized into untreated (CKD), treated with vehicle (VEH) and LP533401 at a dose of 30 or 100 mg/kg daily for 8 weeks. Treatment with LP533401 decreased serotonin turnover and restored bone mineral status, microarchitecture, and strength in CKD rats to the values observed in the controls. In parallel with the reduction of serotonin, serum phosphate levels also decreased, particularly in the LP533401, 100 mg/kg group. The mechanism underlying this phenomenon resulted from decreased expression of the renal VDR/FGF1R/Klotho/Npt2a/Npt2c axis, leading to elevated phosphate excretion in the kidneys. The elevated urinary phosphate excretion resulted in improved bone mineral status and strength in LP533401-treated rats. Unexpectedly, the standard VEH used in this model was able to reduce renal VDR/FGF1R/Klotho/Npt2a expression, leading to a compensatory increase in Npt2c mRNA levels, secondary disturbances in phosphate-regulated hormones and partial improvement in the mineral status of the trabecular bone. The decrease of serotonin synthesis together with the simultaneous reduction of renal Npt2a and Npt2c expression in rats treated with LP533401, 100 mg/kg led to an increase in 1,25(OH)₂D₃ levels; this mechanism seems to be particularly beneficial in relation to the mineral status of cortical bone.

Eldecalcitol Causes FGF23 Resistance for Pi Reabsorption and Improves Rachitic Bone Phenotypes in the Male Hyp Mouse

X-linked hypophosphatemia (XLH), the most common form of inheritable rickets, is caused by inactivation of phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and leads to fibroblast growth factor (FGF) 23-dependent renal inorganic phosphate (Pi) wasting. In the present study, we investigated whether maintaining Pi homeostasis with a potent vitamin D3 analog, eldecalcitol [1α,25-dihydroxy-2β-(3-hydroxypropyloxy) vitamin D3; ED71], could improve hypophosphatemic rickets in a murine model of XLH, the Hyp mouse. Vehicle, ED71, or 1,25-dihydroxyvitamin D was subcutaneously injected five times weekly in wild-type (WT) and Hyp mice for 4 weeks, from 4 to 8 weeks of age. Injection of ED71 into WT mice suppressed the synthesis of renal 1,25-dihydroxyvitamin D and promoted phosphaturic activity. In contrast, administration of ED71 to Hyp mice completely restored renal Pi transport and NaPi-2a protein levels, although the plasma-intact FGF23 levels were further increased. In addition, ED71 markedly increased the levels of the scaffold proteins, renal sodium-hydrogen exchanger regulatory factor 1, and ezrin in the Hyp mouse kidney. Treatment with ED71 increased the body weight and improved hypophosphatemia, the bone volume/total volume, bone mineral content, and growth plate structure in Hyp mice. Thus, ED71 causes FGF23 resistance for phosphate reabsorption and improves rachitic bone phenotypes in Hyp mice. In conclusion, ED71 has opposite effects on phosphate homeostasis in WT and Hyp mice. Analysis of Hyp mice treated with ED71 could result in an additional model for elucidating PHEX abnormalities.

FGF23 Neutralizing Antibody Partially Improves Bone Mineralization Defect of HMWFGF2 Isoforms in Transgenic Female Mice

Mice overexpressing high molecular weight FGF2 isoforms (HMWTg) in osteoblast lineage phenocopy human X-linked hypophosphatemic rickets (XLH) and a Hyp murine model of XLH demonstrating increased FGF23/FGF receptor signaling and hypophosphatemic rickets/osteomalacia. Because HMWFGF2 was upregulated in bones of Hyp mice and abnormal FGF23 signaling is important in XLH, HMWTg mice were used to examine the effect of the FGF23 neutralizing antibody (FGF23Ab). Eight-week-old female Vector control mice and HMWTg mice were treated with FGF23Ab or control IgG. A single injection of FGF23Ab rescued abnormal hypophosphatemia in HMWTg. The decreased type II sodium-dependent phosphate co-transporter (Npt2a) was rescued by FGF23Ab treatment. Inappropriately low serum 1,25(OH)2 D in HMWTg mice was normalized by FGF23Ab treatment, which is accompanied by increased anabolic vitamin D hydroxylase Cyp27b1 and decreased catabolic vitamin D hydroxylase Cyp24 mRNA in kidney. Long-term treatment with FGF23Ab normalized femur length and significantly increased vertebrae BMD and BMC, and femur BMC in HMWTg mice compared to IgG-treated HMWTg mice. Micro-computed tomography (μCT) revealed increased cortical porosity and decreased cortical apparent density in the HMWTg-IgG group compared with the Vector-IgG group; however, FGF23Ab treatment rescued defective cortical mineralization, decreased porosity, and increased apparent density in HMWTg mice. Bone histomorphometry analysis showed FGF23Ab treatment decreased osteoid volume, increased intra-label thickness, mineralization apposition rate, and bone formation rate in HMWTg mice. FGF23Ab improved disorganized double labeling in femurs from HMWTg mice. Quantitative real-time PCR analysis of tibia shafts showed FGF23Ab treatment normalized the osteocalcin (Ocn) mRNA expression in HMWTg mice, but further increased expression of SIBLING protein-related and pyrophosphate-related genes that are important in matrix mineralization, suggesting that HMWFGF2 modulates these genes independent of FGF23. We conclude that FGF23Ab partially rescued hypophosphatemic osteomalacia in HMWTg. However, long-term treatment with FGF23Ab further increased SIBLING protein-related genes and pyrophosphate-related genes in bone that could contribute to incomplete rescue of the mineralization defect. © 2018 American Society for Bone and Mineral Research.

Increased Circulating FGF23 Does Not Lead to Cardiac Hypertrophy in the Male Hyp Mouse Model of XLH

Serum levels of fibroblast growth factor 23 (FGF23) markedly increase with renal impairment, with FGF23 levels correlating with the presence of left ventricular hypertrophy (LVH) and mortality in patients with chronic kidney disease (CKD). FGF23 activates calcineurin/nuclear factor of activated T cell (NFAT) signaling and induces hypertrophy in murine cardiomyocytes. X-linked hypophosphatemia (XLH) is characterized by high circulating levels of FGF23 but, in contrast to CKD, is associated with hypophosphatemia. The cardiac effects of high circulating levels of FGF23 in XLH are not well defined. Thus, studies were undertaken to define the cardiac phenotype in the mouse model of XLH (Hyp mice). Echocardiographic and histological analyses demonstrated that Hyp left ventricles (LVs) are smaller than those of wild-type mice. Messenger RNA expression of cardiac hypertrophy markers was not altered in the LV or right ventricle of Hyp mice. However, the Hyp LVs had increased expression of the NFAT target genes NFATc1 and RCAN1. To determine whether phosphate alone can induce markers of hypertrophy, differentiated C2C12 myocytes were treated with phosphate. Phosphate treatment increased expression of cardiac hypertrophy markers, supporting a primary role for phosphate in inducing LVH. Although previous studies showed that increased circulating FGF23 and phosphate levels are associated with LVH, our results demonstrated that in XLH, high circulating levels of FGF23 in the setting of hypophosphatemia do not induce cardiac hypertrophy.

Osteoblasts remotely supply lung tumors with cancer-promoting SiglecFhigh neutrophils

Bone marrow-derived myeloid cells can accumulate within tumors and foster cancer outgrowth. Local immune-neoplastic interactions have been intensively investigated, but the contribution of the systemic host environment to tumor growth remains poorly understood. Here, we show in mice and cancer patients (n = 70) that lung adenocarcinomas increase bone stromal activity in the absence of bone metastasis. Animal studies reveal that the cancer-induced bone phenotype involves bone-resident osteocalcin-expressing (Ocn⁺) osteoblastic cells. These cells promote cancer by remotely supplying a distinct subset of tumor-infiltrating SiglecF^high neutrophils, which exhibit cancer-promoting properties. Experimentally reducing Ocn⁺ cell numbers suppresses the neutrophil response and lung tumor outgrowth. These observations posit osteoblasts as remote regulators of lung cancer and identify SiglecF^high neutrophils as myeloid cell effectors of the osteoblast-driven protumoral response.

Hormonal Regulation of Osteocyte Perilacunar and Canalicular Remodeling in the Hyp Mouse Model of X-Linked Hypophosphatemia

Osteocytes remodel their surrounding perilacunar matrix and canalicular network to maintain skeletal homeostasis. Perilacunar/canalicular remodeling is also thought to play a role in determining bone quality. X-linked hypophosphatemia (XLH) is characterized by elevated serum fibroblast growth factor 23 (FGF23) levels, resulting in hypophosphatemia and decreased production of 1,25 dihydroxyvitamin D (1,25D). In addition to rickets and osteomalacia, long bones from mice with XLH (Hyp) have impaired whole-bone biomechanical integrity accompanied by increased osteocyte apoptosis. To address whether perilacunar/canalicular remodeling is altered in Hyp mice, histomorphometric analyses of tibia and 3D intravital microscopic analyses of calvaria were performed. These studies demonstrate that Hyp mice have larger osteocyte lacunae in both the tibia and calvaria, accompanied by enhanced osteocyte mRNA and protein expression of matrix metalloproteinase 13 (MMP13) and genes classically used by osteoclasts to resorb bone, such as cathepsin K (CTSK). Hyp mice also exhibit impaired canalicular organization, with a decrease in number and branching of canaliculi extending from tibial and calvarial lacunae. To determine whether improving mineral ion and hormone homeostasis attenuates the lacunocanalicular phenotype, Hyp mice were treated with 1,25D or FGF23 blocking antibody (FGF23Ab). Both therapies were shown to decrease osteocyte lacunar size and to improve canalicular organization in tibia and calvaria. 1,25D treatment of Hyp mice normalizes osteocyte expression of MMP13 and classic osteoclast markers, while FGF23Ab decreases expression of MMP13 and selected osteoclast markers. Taken together, these studies point to regulation of perilacunar/canalicular remodeling by physiologic stimuli including hypophosphatemia and 1,25D. © 2017 American Society for Bone and Mineral Research.

Comparison of calcimimetic R568 and calcitriol in mineral homeostasis in the Hyp mouse, a murine homolog of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) caused by mutations in the Phex gene is the most common human inherited phosphate wasting disorder characterized by enhanced synthesis of fibroblast growth factor 23 (FGF23) in bone, renal phosphate wasting, 1,25(OH)₂D₃ (1,25D) deficiency, rickets and osteomalacia. Here we studied the effects of calcimimetic R568 and calcitriol treatment in the Hyp mouse, a murine homolog of XLH. We hypothesized that mineral homeostasis is differentially affected by R568 and 1,25D with respect to the PTH-vitamin D-FGF23-Klotho axis and bone health. Four-week-old male Hyp mice received R568 in different doses, 1,25D or vehicle for 28days. Vehicle-treated wild-type mice served as controls. Both R568 and 1,25D reduced PTH levels, yet only 1,25D raised serum phosphate levels in Hyp mice. 1,25D increased calciuria and further enhanced FGF23 synthesis in bone and circulating FGF23 levels. By contrast, R568 reduced bone FGF23 expression and serum total but not intact FGF23 concentrations. Renal 1,25D metabolism was further impaired by 1,25D and improved although not normalized by R568. Hyp mice showed reduced renal Klotho levels, which were increased by 1,25D and high dose R568. 1,25D, but not R568, significantly improved femur growth, and weight gain, and partially restored growth plate morphology and bone mineralization. Although a significant improvement of trabecular bone was noted by μCT, compared to 1,25D the effects of R568 on bone histomophometric parameters were marginal. Our data indicate that monotherapy with R568 reduced PTH and FGF23 synthesis in bone, but failed to restore vitamin D and phosphate metabolism and skeletal abnormalities in Hyp mice. By contrast, 1,25D improved body growth, and defective mineralization despite further enhancement of skeletal FGF23 synthesis thereby highlighting the importance of vitamin D in bone mineralization in Hyp mice.

Tissue-specific mineralization defects in the periodontium of the Hyp mouse model of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a dento-osseous disorder caused by inactivating mutations in the PHEX gene, leading to renal phosphate wasting and hypophosphatemia, and impaired mineralization of bones and teeth. In the oral cavity, recent reports suggest a higher susceptibility of XLH patients to periodontitis, where patients present with impaired tooth cementum - a bone-like tissue involved in tooth attachment to the jaw bones and post-eruption tooth positioning - and a higher frequency of intrabony defects. In the present study, the pathobiology of alveolar bone and tooth cementum was investigated in the Hyp mouse, the murine analog of XLH. PHEX deficiency in XLH/Hyp dramatically alters the periodontal phenotype, with hypoplasia of tooth root cementum associated with a lack of periodontal ligament attachment and the presence of an immature apatitic mineral phase of all periodontal mineralized tissues. Challenging the Hyp periodontium in two surgical experimental models - ligature-induced periodontal breakdown and repair, and a model of tooth movement adaptation inducing cementum formation - we show that bone and cementum formation, and their healing, are altered. Bone and cementum mineralization appear similarly disturbed, where hypomineralized pericellular matrix surrounds cells, and where the protein osteopontin (OPN, a mineralization inhibitor) accumulates in a tissue-specific manner, most notably in the perilacunar matrix surrounding osteocytes. Although the pathobiology is different between XLH/Hyp bone and cementum, our results show a major XLH phenotype in oral mineralized tissues consistent with variations in patient susceptibility to periodontal disorders.

Heritable and acquired disorders of phosphate metabolism: Etiologies involving FGF23 and current therapeutics

Phosphate is critical for many cellular processes and structural functions, including as a key molecule for nucleic acid synthesis and energy metabolism, as well as hydroxyapatite formation in bone. Therefore it is critical to maintain tight regulation of systemic phosphate levels. Based upon its broad biological importance, disruption of normal phosphate homeostasis has detrimental effects on skeletal integrity and overall health. Investigating heritable diseases of altered phosphate metabolism has led to key discoveries underlying the regulation and systemic actions of the phosphaturic hormone Fibroblast growth factor-23 (FGF23). Both molecular and clinical studies have revealed novel targets for the development and optimization of therapies for disorders of phosphate handling. This review will focus upon the bridge between genetic discoveries involving disorders of altered FGF23 bioactivity, as well as describe how these findings have translated into pharmacologic application.

Effects of klotho deletion from bone during chronic kidney disease

Klotho is a type I transmembrane protein that acts as a permissive co-receptor for FGF23 and helps to maintain proper mineral metabolism. Mice carrying a loss-of-function mutation in either the Klotho or Fgf23 gene develop many similar phenotypes including osteoporosis. Based on these observations it was hypothesized that the bone phenotypes in Klotho- and Fgf23-null mice may be mediated through a common signaling pathway. Recent improvements in antibody specificity have shown that osteoblasts and osteocytes, which produce FGF23, also express low amount of membrane Klotho. But, the role of Klotho in bone is still largely unclear. In this review we summarize the literature and show that Klotho has an FGF23 dependent and independent effect in bone.

Renal Osteodystrophy-Time for Common Nomenclature

PURPOSE OF REVIEW

The term renal osteodystrophy has been used to describe a wide variety of bone problems facing patients with chronic kidney disease (CKD). Here, we review the history of the use of this term.

RECENT FINDINGS

Bone disease resulting from CKD was first noticed in 1890. The term "renal osteodystrophy" was used to define the bone disease in 1942. Since then, important discoveries have increased our knowledge of the complexities of bone physiology in these patients. At the same time, secular changes in the disease have occurred. The terms used to describe the bone histological findings have changed as well, reflecting new understanding of the physiological processes. However, since different investigators used the terms in different ways, the need to standardize the nomenclature has become increasingly important. Ongoing international collaboration about nosography will allow more optimal communication among scientists and clinicians as we continue to make new discoveries.