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

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.

The Influence of Dietary Interventions on Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD)

Chronic kidney disease is a health problem whose prevalence is increasing worldwide. The kidney plays an important role in the metabolism of minerals and bone health and therefore, even at the early stages of CKD, disturbances in bone metabolism are observed. In the course of CKD, various bone turnover or mineralization disturbances can develop including adynamic hyperparathyroid, mixed renal bone disease, osteomalacia. The increased risk of fragility fractures is present at any age in these patients. Nutritional treatment of patients with advanced stages of CKD is aiming at prevention or correction of signs, symptoms of renal failure, avoidance of protein-energy wasting (PEW), delaying or prevention of the occurrence of mineral/bone disturbances, and delaying the start of dialysis. The results of studies suggest that progressive protein restriction is beneficial with the progression of renal insufficiency; however, other aspects of dietary management of CKD patients, including changes in sodium, phosphorus, and energy intake, as well as the source of protein and lipids (animal or plant origin) should also be considered carefully. Energy intake must cover patients' energy requirement, in order to enable correct metabolic adaptation in the course of protein-restricted regimens and prevent negative nitrogen balance and protein-energy wasting.

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.

Fibroblast growth factor 21 (FGF21) is a sensitive marker of osteoporosis in haemodialysis patients: a cross-sectional observational study

Introduction

Osteoporosis is one of the important bone abnormalities in chronic kidney disease-mineral and bone disorder (CKD-MBD) and still lacks a sensitive biomarker to diagnose. Fibroblast growth factor 21 (FGF21) can stimulate bone loss in patients with diabetes and increase in CKD patients. In this study, we investigated whether FGF21 could serve as a biomarker to predict osteoporosis in a haemodialysis cohort.

Methods

We recorded demographic information, biochemical data, and serum FGF21 and FGF23 levels and measured the CT attenuation values of 339 haemodialysis patients from two large medical centres. We assessed the correlation of CT attenuation values with serum FGF21 and FGF23 levels and tested whether they were independent factors for osteoporosis. ROC curves were constructed to compare the prognostic value of FGF21 and FGF23 for osteoporosis.

Results

Based on the CT attenuation value, serum FGF21 levels were higher in our osteoporosis group (median 640.86 pg/ml vs. 245.46 pg/ml, P ˂ 0.01). Meanwhile, FGF21 (r = -0.136, P < 0.05) and FGF23 (r = -0.151, P < 0.05) were both negatively associated with osteoporosis. Moreover, FGF21 (β = -0.067, P < 0.05) was an independent factor for osteoporosis. Furthermore, FGF21 combined with age yielded a marked specificity (90.5 %) and sensitivity (61.8 %) in predicting osteoporosis of haemodialysis patients with less residual renal function.

Conclusions

FGF21 has a positive relationship with the incidence of osteoporosis in patients on haemodialysis. FGF21 combined with age is a good predictive biomarker for osteoporosis in patients on haemodialysis, especially those with less residual renal function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02393-z.

The bone is the major source of high circulating intact fibroblast growth factor-23 in acute murine polymicrobial sepsis induced by cecum ligation puncture

Fibroblast growth factor-23 (FGF23), a bone-produced hormone, plays a critical role in mineral homeostasis. Human diseases associated with excessive intact circulating FGF23 (iFGF23) result in hypophosphatemia and low vitamin D hormone in patients with normal kidney function. In addition, there is accumulating evidence linking FGF23 with inflammation. Based on these studies and the frequent observation of hypophosphatemia among septic patients, we sought to elucidate further the relationship between FGF23 and mineral homeostasis in a clinically relevant murine polymicrobial sepsis model. Medium-severity sepsis was induced by cecum ligation puncture (CLP) in adult CD-1 mice of both sexes. Healthy CD-1 mice (without CLP) were used as controls. Forty-eight hours post-CLP, spontaneous urine was collected, and serum, organs and bones were sampled at necropsy. Serum iFGF23 increased ~20-fold in CLP compared to control mice. FGF23 protein concentration was increased in the bones, but not in spleen or liver of CLP mice. Despite the ~20-fold iFGF23 increase, we did not observe any significant changes in mineral homeostasis or parathyroid hormone levels in the blood of CLP animals. Urinary excretion of phosphate, calcium, and sodium remained unchanged in male CLP mice, whereas female CLP mice exhibited lower urinary calcium excretion, relative to healthy controls. In line with renal FGF23 resistance, expression of phosphate-, calcium- and sodium-transporting proteins did not show consistent changes in the kidneys of male and female CLP mice. Renal expression of the co-receptor αKlotho was downregulated in female, but not in male CLP mice. In conclusion, our data demonstrate that the dramatic, sex-independent rise in serum iFGF23 post-CLP was mainly caused by an upregulation of FGF23 secretion in the bone. Surprisingly, the upsurge in circulating iFGF23 did not alter humoral mineral homeostasis in the acutely septic mice. Hence, the biological function of elevated FGF23 in sepsis remains unclear and warrants further studies.

The roles of osteocytes in alveolar bone destruction in periodontitis

Periodontitis, a bacterium-induced inflammatory disease that is characterized by alveolar bone loss, is highly prevalent worldwide. Elucidating the underlying mechanisms of alveolar bone loss in periodontitis is crucial for understanding its pathogenesis. Classically, bone cells, such as osteoclasts, osteoblasts and bone marrow stromal cells, are thought to dominate the development of bone destruction in periodontitis. Recently, osteocytes, the cells embedded in the mineral matrix, have gained attention. This review demonstrates the key contributing role of osteocytes in periodontitis, especially in alveolar bone loss. Osteocytes not only initiate physiological bone remodeling but also assist in inflammation-related changes in bone remodeling. The latest evidence suggests that osteocytes are involved in regulating bone anabolism and catabolism in the progression of periodontitis. The altered secretion of receptor activator of NF-κB ligand (RANKL), sclerostin and Dickkopf-related protein 1 (DKK1) by osteocytes affects the balance of bone resorption and formation and promotes bone loss. In addition, the accumulation of prematurely senescent and apoptotic osteocytes observed in alveolar bone may exacerbate local destruction. Based on their communication with the bloodstream, it is noteworthy that osteocytes may participate in the interaction between local periodontitis lesions and systemic diseases. Overall, further investigations of osteocytes may provide vital insights that improve our understanding of the pathophysiology of periodontitis.

Pro-inflammatory Cytokines: Cellular and Molecular Drug Targets for Glucocorticoid-induced-osteoporosis via Osteocyte

Glucocorticoids are widely used to treat varieties of allergic and autoimmune diseases, however, long-term application results in glucocorticoid-induced osteoporosis (GIOP). Inflammatory cytokines: tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) play important regulatory roles in bone metabolism, but their roles in GIOP remain largely unknown. Osteocytes can modulate the formation and function of both osteoblasts and osteoclasts, directly via gap junctions, or indirectly by transferring molecule signaling. Apoptotic osteocytes release RANKL, HMGB1 and pro-inflammatory cytokines to stimulate osteoclastogenesis. Moreover, osteocytes can secrete FGF23 to regulate bone metabolism. Exposure to high levels of GCs can drive osteocyte apoptosis and influence gap junctions, leading to bone loss. GCs treatment is regarded to produce more FGF23 to inhibit bone mineralization. GCs also disrupt the vascular to decrease osteocyte feasibility and mineral appositional rate, resulting in a decline in bone strength. Apoptotic bodies from osteocytes induced by GCs treatment can enhance production of TNF-α and IL-6. On the other hand, TNF-α and IL-6 show synergistic effects by altering osteocytes signaling towards osteoclasts and osteoblasts. In addition, TNF-α can induce osteocyte apoptosis and attribute to a worsened bone quality in GCs. IL-6 and osteocytes may interact with each other. Therefore, we hypothesize that GCs regulate osteocyteogenesis through TNF-α and IL-6, which are highly expressed around osteocyte undergoing apoptosis. In the present review, we summarized the roles of osteocytes in regulating osteoblasts and osteoclasts. Furthermore, the mechanism of GCs altered relationship between osteocytes and osteoblasts/osteoclasts. In addition, we discussed the roles of TNF-α and IL-6 in GIOP by modulating osteocytes. Lastly, we discussed the possibility of using pro-inflammatory signaling pathway as therapeutic targets to develop drugs for GIOP.

Osteocytic FGF23 and Its Kidney Function

Osteocytes, which represent up to 95% of adult skeletal cells, are deeply embedded in bone. These cells exhibit important interactive abilities with other bone cells such as osteoblasts and osteoclasts to control skeletal formation and resorption. Beyond this local role, osteocytes can also influence the function of distant organs due to the presence of their sophisticated lacunocanalicular system, which connects osteocyte dendrites directly to the vasculature. Through these networks, osteocytes sense changes in circulating metabolites and respond by producing endocrine factors to control homeostasis. One critical function of osteocytes is to respond to increased blood phosphate and 1,25(OH)₂ vitamin D (1,25D) by producing fibroblast growth factor-23 (FGF23). FGF23 acts on the kidneys through partner fibroblast growth factor receptors (FGFRs) and the co-receptor Klotho to promote phosphaturia via a downregulation of phosphate transporters, as well as the control of vitamin D metabolizing enzymes to reduce blood 1,25D. In the first part of this review, we will explore the signals involved in the positive and negative regulation of FGF23 in osteocytes. In the second portion, we will bridge bone responses with the review of current knowledge on FGF23 endocrine functions in the kidneys.

Molecular Control of Phosphorus Homeostasis and Precision Treatment of Hypophosphatemic Disorders

Purpose of Review

Serum phosphorus is maintained in a narrow range by balancing dietary phosphate absorption, influx and efflux of phosphorus from bone and intracellular stores, and renal reabsorption of filtered phosphate. Acute hypophosphatemia, typically caused by transient increases in cellular uptake, can lead to severe complications such as cardiopulmonary dysfunction and rhabdomyolysis that can warrant parenteral phosphate repletion. Chronic hypophosphatemia, however, generally represents true phosphate deficiency and may result in long-term metabolic and skeletal complications, particularly in children due to the critical importance of phosphorus to skeletal mineralization and longitudinal growth.

Recent Findings

In addition to the well characterized roles of vitamin D and parathyroid hormone (PTH), a new bone-kidney axis has been discovered that regulates phosphate homeostasis through the bone-derived hormone Fibroblast Growth Factor 23 (FGF23) and its phosphaturic actions that are mediated by activation of fibroblast growth factor receptors (FGFRs) complexed with α-Klotho in renal tubules. Chronic hypophosphatemia can now be classified as FGF23 dependent or independent.

Summary

In cases of FGF23 dependent hypophosphatemia, traditional non-specific treatments with elemental phosphorus and 1,25(OH)₂ vitamin D (calcitriol) can now be replaced with a targeted approach by using an FGF-23 blocking antibody (Burosumab).

Marked alterations in the structure, dynamics and maturation of growth plate likely explain growth retardation and bone deformities of young Hyp mice

Mechanisms underlying growth impairment and bone deformities in X-linked hypophosphatemia are not fully understood. We here describe marked alterations in the structure, dynamics and maturation of growth plate in growth-retarded young Hyp mice, in comparison with wild type mice. Hyp mice exhibited reduced proliferation and apoptosis rates of chondrocytes as well as severe disturbance in the process of chondrocyte hypertrophy disclosed by abnormal expression of proteins likely involved in cell enlargement, irregular chondro-osseous junction and disordered bone trabecular pattern and vascular invasion in the primary spongiosa. (Hyp mice had elevated circulating FGF23 levels and over activation of ERK in the growth plate.) All these findings provide a basis to explain growth impairment and metaphyseal deformities in XLH. Hyp mice were compared with wild type mice serum parameters, nutritional status and growth impairment by evaluation of growth cartilage and bone structures. Hyp mice presented hyphosphatemia with high FGF23 levels. Weight gain and longitudinal growth resulted reduced in them with numerous skeletal abnormalities at cortical bone. It was also observed aberrant trabecular organization at primary spongiosa and atypical growth plate organization with abnormal proliferation and hypertrophy of chondrocytes and diminished apoptosis and vascular invasion processes. The present results show for the first time the abnormalities present in the growth plate of young Hyp mice and suggest that both cartilage and bone alterations may be involved in the growth impairment and the long bone deformities of XLH.

Analysis of 2 novel mutations of PHEX gene inducing X-linked dominant hypophosphatemia rickets in 2 families: Two case reports

RATIONALE

X-linked dominant hypophosphatemia rickets (XLH, OMIM 307800) is the most common hereditary hypophosphatemic rickets and characterized by growth retardation, skeletal malformations, dental dysplasia, spontaneous fractures and osteomalacia. PHEX gene was identified for XLH and novel mutations were consistent with loss of function.

PATIENT CONCERNS

Case1: the proband 1 III3 in family 1 was a fourteen-year-old boy with bowing of bilateral legs, obviously enlarged joints, tooth absence and difficulty in walking; X-rays showed bilateral femoral multiple fractures with sclerosis at the fracture edge. Case 2: the proband 2 III2, a five-year-old boy in family 2, showed growth retardation, dental dysplasia, gingiva abscess and bilateral legs malformations; X-rays showed low bone density, delayed bone age, bowing of legs, frayed and widened metaphyses of the distal femurs and proximal tibias. Both of their mothers suffered from skeletal malformations, tooth absence and were performed with osteotomy due to fractures of lower limb. Their biochemical parameters showed hypophosphatemia, elevated alkaline phosphatase.

DIAGNOSES

X-linked dominant hypophosphatemia rickets (XLH).

INTERVENTIONS AND OUTCOMES

Treatment with high doses of phosphate and 1,25-dihydroxyvitamin D3, the height of proband 2 increased 10 cm and femoral Multiple fracture of proband1 almost healed after treatment for 6 months and the patients's PHEX gene was investigated.

LESSONS

Two novel pathogenic PHEX mutations were found: c.497delG in family 1 and c.388G> T in family 2, both of which caused early termination of translation and produced truncated protein. Serum FGF23 concentration in our XLH patients were obviously higher than the normal and may be related to age to some extent. Early initiation of treatment produces better effect.

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.

Effect of uremic toxin-indoxyl sulfate on the skeletal system

Chronic kidney disease-mineral bone disorders (CKD-MBD) exhibit abnormalities in the circulating mineral levels, vitamin D metabolism, and parathyroid function that contribute to the formation of a bone lesion. The uremic toxin, indoxyl sulfate (IS), accumulates in the blood in cases of renal failure and leads to bone loss. The bone and renal responses to the action of the parathyroid hormone (PTH) are progressively decreased in CKD in spite of increasing PTH levels, a condition commonly called PTH resistance. There is a high prevalence of low bone turnover or adynamic bone disease in the early stages of CKD. This could be due to the inhibition of bone turnover, such as in PTH resistance, reduced active vitamin D levels, diabetes, aluminum, and, increased IS. With an increase in IS, there is a decrease in the osteoblast Wnt/b-catenin signaling and increase in the expression of Wnt signaling inhibitors, such as sclerostin and Dickkopf-1 (DKK1). Thus, a majority of early CKD patients exhibit deterioration of bone quality owing to the action of IS, this scenario could be termed uremic osteoporosis. However, this mechanism is complicated and not fully understood. With progressive deterioration in the renal function, IS accumulates along with persistent PTH secretion, potentially leading to high-turnover bone disease because high serum PTH levels have the ability of overriding peripheral PTH resistance and other inhibitory factors of bone formation. Finally, it leads to deterioration in bone quantity with prominent bone resorption in end stage renal disease. Uremic toxins adsorbents may decelerate oxidative stress and improve bone health in CKD patients. This review article focuses on IS and bone loss in CKD patients.

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.

Erythropoietin induces bone marrow and plasma fibroblast growth factor 23 during acute kidney injury

It is accepted that osteoblasts/osteocytes are the major source for circulating fibroblast growth factor 23 (FGF23). However, erythropoietic cells of bone marrow also express FGF23. The modulation of FGF23 expression in bone marrow and potential contribution to circulating FGF23 has not been well studied. Moreover, recent studies show that plasma FGF23 may increase early during acute kidney injury (AKI). Erythropoietin, a kidney-derived hormone that targets erythropoietic cells, increases in AKI. Here we tested whether an acute increase of plasma erythropoietin induces FGF23 expression in erythropoietic cells of bone marrow thereby contributing to the increase of circulating FGF23 in AKI. We found that erythroid progenitor cells of bone marrow express FGF23. Erythropoietin increased FGF23 expression in vivo and in bone marrow cell cultures via the homodimeric erythropoietin receptor. In experimental AKI secondary to hemorrhagic shock or sepsis in rodents, there was a rapid increase of plasma erythropoietin, and an induction of bone marrow FGF23 expression together with a rapid increase of circulating FGF23. Blockade of the erythropoietin receptor fully prevented the induction of bone marrow FGF23 and partially suppressed the increase of circulating FGF23. Finally, there was an early increase of both circulating FGF23 and erythropoietin in a cohort of patients with severe sepsis who developed AKI within 48 hours of admission. Thus, increases in plasma erythropoietin and erythropoietin receptor activation are mechanisms implicated in the increase of plasma FGF23 in AKI.

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.

The chronic kidney disease - Mineral bone disorder (CKD-MBD): Advances in pathophysiology

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. New advances in the causes of the CKD-MBD are discussed in this review. They demonstrate that repair and disease processes in the kidneys release factors to the circulation that cause the systemic complications of CKD. The discovery of WNT inhibitors, especially Dickkopf 1 (Dkk1), produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. This lead to the discovery that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, wherein it stimulates fibrosis, and decreases tubular klotho expression. Activin A binds to the type 2 activin A receptor, ActRIIA, which is variably affected by CKD in the vasculature. In diabetic/atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC), ActRIIA signaling is inhibited and contributes to CKD induced VSMC dedifferentiation, osteogenic transition and neointimal atherosclerotic calcification. In nondiabetic/nonatherosclerotic aortas, CKD increases VSMC ActRIIA signaling, and vascular fibroblast signaling causing the latter to undergo osteogenic transition and stimulate vascular calcification. In both vascular situations, a ligand trap for ActRIIA prevented vascular calcification. In the skeleton, activin A is responsible for CKD stimulation of osteoclastogenesis and bone remodeling increasing bone turnover. These studies demonstrate that circulating renal repair and injury factors are causal of the CKD-MBD and CKD associated cardiovascular disease.

FGF23-Klotho signaling axis in the kidney

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone protecting against the potentially deleterious effects of hyperphosphatemia by suppression of phosphate reabsorption and of active vitamin D hormone synthesis in the kidney. The kidney is one of the main target organs of FGF23 signaling. The purpose of this review is to highlight the recent advances in the area of FGF23-Klotho signaling in the kidney. During recent years, it has become clear that FGF23 acts independently on proximal and distal tubular epithelium. In proximal renal tubules, FGF23 suppresses phosphate reabsorption by a Klotho dependent activation of extracellular signal-regulated kinase-1/2 (ERK1/2) and of serum/glucocorticoid-regulated kinase-1 (SGK1), leading to phosphorylation of the scaffolding protein Na⁺/H⁺ exchange regulatory cofactor (NHERF)-1 and subsequent internalization and degradation of sodium-phosphate cotransporters. In distal renal tubules, FGF23 augments calcium and sodium reabsorption by increasing the apical membrane expression of the epithelial calcium channel TRPV5 and of the sodium-chloride cotransporter NCC through a Klotho dependent activation of with-no-lysine kinase-4 (WNK4). In proximal and distal renal tubules, FGF receptor-1 is probably the dominant FGF receptor mediating the effects of FGF23 by forming a complex with membrane-bound Klotho in the basolateral membrane. The newly described sodium- and calcium-conserving functions of FGF23 may have major implications for the pathophysiology of diseases characterized by chronically increased circulating FGF23 concentrations such as chronic kidney disease.

The Role of the Osteocyte in Bone and Nonbone Disease

When normal physiologic functions go awry, disorders and disease occur. This is universal; even for the osteocyte, a cell embedded within the mineralized matrix of bone. It was once thought that this cell was simply a placeholder in bone. Within the last decade, the number of studies of osteocytes has increased dramatically, leading to the discovery of novel functions of these cells. With the discovery of novel physiologic functions came the discoveries of how these cells can also be responsible for not only bone diseases and disorders, but also those of the kidney, heart, and potentially muscle.

X-linked hypophosphatemia and growth

X-Linked hypophosphatemia (XLH) is the most common form of hereditary rickets caused by loss-of function mutations in the PHEX gene. XLH is characterized by hypophosphatemia secondary to renal phosphate wasting, inappropriately low concentrations of 1,25 dihydroxyvitamin D and high circulating levels of fibroblast growth factor 23 (FGF23). Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients. The degree of growth impairment is not dependent on the magnitude of hypophosphatemia or the extent of legs´ bowing and height is not normalized by chronic administration of phosphate supplements and 1α hydroxyvitamin D derivatives. Treatment with growth hormone accelerates longitudinal growth rate but there is still controversy regarding the potential risk of increasing bone deformities and body disproportion. Treatments aimed at blocking FGF23 action are promising, but information is lacking on the consequences of counteracting FGF23 during the growing period. This review summarizes current knowledge on phosphorus metabolism in XLH, presents updated information on XLH and growth, including the effects of FGF23 on epiphyseal growth plate of the Hyp mouse, an animal model of the disease, and discusses growth hormone and novel FGF23 related therapies.

Sclerostin Promotes Bone Remodeling in the Process of Tooth Movement

Tooth movement is a biological process of bone remodeling induced by mechanical force. Sclerostin secreted by osteocytes is mechanosensory and important in bone remodeling. However, little is known regarding the role of sclerostin in tooth movement. In this study, models of experimental tooth movement were established in rats and mice. Sclerostin expression was investigated with immunohistochemistry staining, and osteoclastic activity was analyzed with tartrate-resistant acid phosphatase (TRAP) staining. MLO-Y4 osteocyte-like cells underwent uniaxial compression and tension stress or were cultured in hypoxia conditions. Expression of sclerostin was assessed by RT-qPCR and ELISA. MLO-Y4 cells were cultured with recombinant human sclerostin (rhSCL) interference and then co-cultured with RAW264.7 osteoclast precursor cells. Expressions of RANKL and OPG were analyzed by RT-qPCR, and osteoclastic activity was assessed by TRAP staining. During tooth movement, sclerostin was expressed differently in compression and tension sites. In SOST knock-out mice, there were significantly fewer TRAP-positive cells than in WT mice during tooth movement in compression sites. In-vitro studies showed that the expression of sclerostin in MLO-Y4 osteocyte-like cells was not different under a uniaxial compression and tension force, whereas hypoxia conditions significantly increased sclerostin expression in MLO-Y4 cells. rhSCL interference increased the expression of RANKL and the RANKL/OPG ratio in MLO-Y4 cells and the osteoclastic induction ability of MLO-Y4 cells in experimental osteocyte-osteoclast co-culture. These data suggest that sclerostin plays an important role in the bone remodeling of tooth movement.

Conditional Deletion of Murine Fgf23: Interruption of the Normal Skeletal Responses to Phosphate Challenge and Rescue of Genetic Hypophosphatemia

The transgenic and knockout (KO) animals involving Fgf23 have been highly informative in defining novel aspects of mineral metabolism, but are limited by shortened lifespan, inability of spatial/temporal FGF23 control, and infertility of the global KO. To more finely test the role of systemic and genetic influences in FGF23 production, a mouse was developed that carried a floxed ("f")-Fgf23 allele (exon 2 floxed) which demonstrated in vivo recombination when bred to global-Cre transgenic mice (eIIa-cre). Mice homozygous for the recombined allele ("Δ") had undetectable serum intact FGF23, elevated serum phosphate (p < 0.05), and increased kidney Cyp27b1 mRNA (p < 0.05), similar to global Fgf23-KO mice. To isolate cellular FGF23 responses during phosphate challenge, Fgf23(Δ/f) mice were mated with early osteoblast type Iα1 collagen 2.3-kb promoter-cre mice (Col2.3-cre) and the late osteoblast/early osteocyte Dentin matrix protein-1-cre (Dmp1-cre). Fgf23(Δ/f) /Col2.3-cre(+) and Fgf23(Δ/f) /Dmp1-cre(+) exhibited reduced baseline serum intact FGF23 versus controls. After challenge with high-phosphate diet Cre(-) mice had 2.1-fold to 2.5-fold increased serum FGF23 (p < 0.01), but Col2.3-cre(+) mice had no significant increase, and Dmp1-cre(+) mice had only a 37% increase (p < 0.01) despite prevailing hyperphosphatemia in both models. The Fgf23(Δ/f) /Col2.3-cre was bred onto the Hyp (murine X-linked hypophosphatemia [XLH] model) genetic background to test the contribution of osteoblasts and osteocytes to elevated FGF23 and Hyp disease phenotypes. Whereas Hyp mice maintained inappropriately elevated FGF23 considering their marked hypophosphatemia, Hyp/Fgf23(Δ/f) /Col2.3-cre(+) mice had serum FGF23 <4% of Hyp (p < 0.01), and this targeted restriction normalized serum phosphorus and ricketic bone disease. In summary, deleting FGF23 within early osteoblasts and osteocytes demonstrated that both cell types contribute to baseline circulating FGF23 concentrations, and that targeting osteoblasts/osteocytes for FGF23 production can modify systemic responses to changes in serum phosphate concentrations and rescue the Hyp genetic syndrome. © 2016 American Society for Bone and Mineral Research.

Skeletal Muscle, but not Cardiovascular Function, Is Altered in a Mouse Model of Autosomal Recessive Hypophosphatemic Rickets

Autosomal recessive hypophosphatemic rickets (ARHR) is a heritable disorder characterized by hypophosphatemia, osteomalacia, and poor bone development. ARHR results from inactivating mutations in the DMP1 gene with the human phenotype being recapitulated in the Dmp1 null mouse model which displays elevated plasma fibroblast growth factor 23. While the bone phenotype has been well-characterized, it is not known what effects ARHR may also have on skeletal, cardiac, or vascular smooth muscle function, which is critical to understand in order to treat patients suffering from this condition. In this study, the extensor digitorum longus (EDL-fast-twitch muscle), soleus (SOL–slow-twitch muscle), heart, and aorta were removed from Dmp1 null mice and ex-vivo functional tests were simultaneously performed in collaboration by three different laboratories. Dmp1 null EDL and SOL muscles produced less force than wildtype muscles after normalization for physiological cross sectional area of the muscles. Both EDL and SOL muscles from Dmp1 null mice also produced less force after the addition of caffeine (which releases calcium from the sarcoplasmic reticulum) which may indicate problems in excitation contraction coupling in these mice. While the body weights of the Dmp1 null were smaller than wildtype, the heart weight to body weight ratio was higher. However, there were no differences in pathological hypertrophic gene expression compared to wildtype and maximal force of contraction was not different indicating that there may not be cardiac pathology under the tested conditions. We did observe a decrease in the rate of force development generated by cardiac muscle in the Dmp1 null which may be related to some of the deficits observed in skeletal muscle. There were no differences observed in aortic contractions induced by PGF_2α or 5-HT or in endothelium-mediated acetylcholine-induced relaxations or endothelium-independent sodium nitroprusside-induced relaxations. In summary, these results indicate that there are deficiencies in both fast twitch and slow twitch muscle fiber type contractions in this model of ARHR, while there was less of a phenotype observed in cardiac muscle, and no differences observed in aortic function. These results may help explain skeletal muscle weakness reported by some patients with osteomalacia and need to be further investigated.

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

X-linked hypophosphatemia (XLH) is characterized by impaired renal tubular reabsorption of phosphate owing to increased circulating FGF23 levels, resulting in rickets in growing children and impaired bone mineralization. Increased FGF23 decreases renal brush border membrane sodium-dependent phosphate transporter IIa (Npt2a) causing renal phosphate wasting, impairs 1-α hydroxylation of 25-hydroxyvitamin D, and induces the vitamin D 24-hydroxylase, leading to inappropriately low circulating levels of 1,25-dihydroxyvitamin D (1,25D). The goal of therapy is prevention of rickets and improvement of growth in children by phosphate and 1,25D supplementation. However, this therapy is often complicated by hypercalcemia and nephrocalcinosis and does not always prevent hyperparathyroidism. To determine if 1,25D or blocking FGF23 action can improve the skeletal phenotype without phosphate supplementation, mice with XLH (Hyp) were treated with daily 1,25D repletion, FGF23 antibodies (FGF23Ab), or biweekly high-dose 1,25D from d2 to d75 without supplemental phosphate. All treatments maintained normocalcemia, increased serum phosphate, and normalized parathyroid hormone levels. They also prevented the loss of Npt2a, α-Klotho, and pERK1/2 immunoreactivity observed in the kidneys of untreated Hyp mice. Daily treatment with 1,25D decreased urine phosphate losses despite a marked increase in bone FGF23 mRNA and in circulating FGF23 levels. Daily 1,25D was more effective than other treatments in normalizing the growth plate and metaphyseal organization. In addition to being the only therapy that normalized lumbar vertebral height and body weight, daily 1,25D therapy normalized bone geometry and was more effective than FGF23Ab in improving trabecular bone structure. Daily 1,25D and FGF23Ab improved cortical microarchitecture and whole-bone biomechanical properties more so than biweekly 1,25D. Thus, monotherapy with 1,25D improves growth, skeletal microarchitecture, and bone strength in the absence of phosphate supplementation despite enhancing FGF23 expression, demonstrating that 1,25D has direct beneficial effects on the skeleton in XLH, independent of its role in phosphate homeostasis. © 2016 American Society for Bone and Mineral Research.

Associations of FGF23 With Change in Bone Mineral Density and Fracture Risk in Older Individuals

Elevated levels of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23) have been linked to greater risk of fractures in some studies, especially among individuals with chronic kidney disease (CKD). We evaluated FGF23 as a risk factor for bone loss and fractures in the Health, Aging, and Body Composition (Health ABC) study, which is a prospective biracial cohort of well-functioning adults aged 70 to 79 years recruited at two clinical centers in the United States. The sample for the bone mineral density (BMD) analyses consisted of 2234 participants who had at least two serial total hip areal BMD measures. The fracture analyses included 2786 participants, 567 of whom sustained a fracture during a median follow up of 4.95 years. Linear mixed-effects models were used for longitudinal measurements of total hip areal BMD and the proportional subdistribution hazard regression model subject to competing risks of death was used for risk of fracture. The median FGF23 was 46.7 (interquartile range [IQR] 36.7 to 60.2) pg/mL. The mean annualized percent change in total hip areal BMD did not vary significantly according to FGF23 quartile in all participants (p for trend = 0.70), but the effect was modified by CKD status (adjusted p for interaction <0.001). Among participants with CKD, the unadjusted mean annualized percent change in total hip areal BMD was greater with higher levels of FGF23 (unadjusted p for trend = 0.02), but the trend was attenuated with adjustment for estimated glomerular filtration rate and parathyroid hormone (adjusted p for trend = 0.30). FGF23 was not significantly associated with fracture risk in crude (hazard ratio [HR] per doubling of FGF23, 0.97; 95% CI, 0.85 to 1.12) or adjusted models (HR per doubling of FGF23, 1.02; 95% CI, 0.86 to 1.22), and these findings were not modified by gender or CKD status. FGF23 levels are not associated with bone loss or fracture risk in older adults with low prevalence of CKD.

Excessive Osteocytic Fgf23 Secretion Contributes to Pyrophosphate Accumulation and Mineralization Defect in Hyp Mice

X-linked hypophosphatemia (XLH) is the most frequent form of inherited rickets in humans caused by mutations in the phosphate-regulating gene with homologies to endopeptidases on the X-chromosome (PHEX). Hyp mice, a murine homologue of XLH, are characterized by hypophosphatemia, inappropriately low serum vitamin D levels, increased serum fibroblast growth factor-23 (Fgf23), and osteomalacia. Although Fgf23 is known to be responsible for hypophosphatemia and reduced vitamin D hormone levels in Hyp mice, its putative role as an auto-/paracrine osteomalacia-causing factor has not been explored. We recently reported that Fgf23 is a suppressor of tissue nonspecific alkaline phosphatase (Tnap) transcription via FGF receptor-3 (FGFR3) signaling, leading to inhibition of mineralization through accumulation of the TNAP substrate pyrophosphate. Here, we report that the pyrophosphate concentration is increased in Hyp bones, and that Tnap expression is decreased in Hyp-derived osteocyte-like cells but not in Hyp-derived osteoblasts ex vivo and in vitro. In situ mRNA expression profiling in bone cryosections revealed a ~70-fold up-regulation of Fgfr3 mRNA in osteocytes versus osteoblasts of Hyp mice. In addition, we show that blocking of increased Fgf23-FGFR3 signaling with anti-Fgf23 antibodies or an FGFR3 inhibitor partially restored the suppression of Tnap expression, phosphate production, and mineralization, and decreased pyrophosphate concentration in Hyp-derived osteocyte-like cells in vitro. In vivo, bone-specific deletion of Fgf23 in Hyp mice rescued the suppressed TNAP activity in osteocytes of Hyp mice. Moreover, treatment of wild-type osteoblasts or mice with recombinant FGF23 suppressed Tnap mRNA expression and increased pyrophosphate concentrations in the culture medium and in bone, respectively. In conclusion, we found that the cell autonomous increase in Fgf23 secretion in Hyp osteocytes drives the accumulation of pyrophosphate through auto-/paracrine suppression of TNAP. Hence, we have identified a novel mechanism contributing to the mineralization defect in Hyp mice.

A novel mechanism involving autocrine and paracrine actions of fibroblast growth factor-23 contributes to the mineralization defect observed in Hyp, a mouse model for X-linked hypophosphatemia.

X-linked hypophosphatemia (XLH) is the most frequent form of inherited rickets in humans. A mouse model of XLH, known as Hyp, is characterized by exceptionally low serum phosphate and vitamin D levels, increased serum levels of the hormone fibroblast growth factor-23 (Fgf23), and impaired bone mineralization. Fgf23 is secreted from two classes of bone cells known as osteoblasts and osteocytes. Fgf23 increases urinary phosphate excretion and suppresses vitamin D hormone production in the kidney. Although Fgf23 is known to be responsible for lower blood phosphate and vitamin D hormone levels in Hyp mice, its putative role as a signaling factor causing impaired mineralization has not been explored. We recently reported that Fgf23 is a suppressor of tissue nonspecific alkaline phosphatase (Tnap) gene expression via FGF receptor-3 (FGFR3) signaling in osteoblasts, leading to inhibition of mineralization through accumulation of the TNAP substrate pyrophosphate. Pyrophosphate is a potent inhibitor of mineralization. Using a combination of cell culture and animal models, we report that the increase in osteocyte Fgf23 secretion of Hyp mice leads to FGFR3-mediated suppression of TNAP with subsequent accumulation of pyrophosphate. Hence, we have identified a novel signaling mechanism by which excessive osteocytic secretion of Fgf23 contributes to the mineralization defect in Hyp mice.

Pathophysiology of the chronic kidney disease-mineral bone disorder

PURPOSE OF REVIEW

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. The causes of the CKD-MBD are not well known and they will be discussed in this review

RECENT FINDINGS

The discovery of WNT (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair and participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical, leading to the finding that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, where it stimulates fibrosis, and decreases tubular klotho expression. The type 2 activin A receptor, ActRIIA, is decreased by CKD in atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC). Inhibition of activin signaling by a ligand trap inhibited CKD induced VSMC dedifferentiation, osteogenic transition and atherosclerotic calcification. Inhibition of activin signaling in the kidney decreased renal fibrosis and proteinuria.

SUMMARY

These studies demonstrate that circulating renal repair factors are causal for the CKD-MBD and CKD associated cardiovascular disease, and identify ActRIIA signaling as a therapeutic target in CKD that links progression of renal disease and vascular disease.

Regulation of hormone-sensitive renal phosphate transport

Phosphate is essential for growth and maintenance of the skeleton and for generating high-energy phosphate compounds. Evolutionary adaptation to high dietary phosphorous in humans and other terrestrial vertebrates involves regulated mechanisms assuring the efficient renal elimination of excess phosphate. These mechanisms prominently include PTH, FGF23, and Vitamin D, which directly and indirectly regulate phosphate transport. Disordered phosphate homeostasis is associated with pathologies ranging from kidney stones to kidney failure. Chronic kidney disease results in hyperphosphatemia, an elevated calcium×phosphate product with considerable morbidity and mortality, mostly associated with adverse cardiovascular events. This chapter highlights recent findings and insights regarding the hormonal regulation of renal phosphate transport along with imbalances of phosphate balance due to acquired or inherited diseases states.

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

Bone mineralization is regulated by signaling cross talk between molecular factors of local and systemic origin: the role of fibroblast growth factor 23

Body phosphate homeostasis is regulated by a hormonal counter-balanced intestine-bone-kidney axis. The major systemic hormones involved in this axis are parathyroid hormone (PTH), 1,25-dihydroxyvitamin-D, and fibroblast growth factor-23 (FGF23). FGF23, produced almost exclusively by the osteocytes, is a phosphaturic hormone that plays a major role in regulation of the bone remodeling process. Remodeling composite components, bone mineralization and resorption cycles create a continuous influx-efflux loop of the inorganic phosphate (Pi) through the skeleton. This "bone Pi loop," which is formed, is controlled by local and systemic factors according to phosphate homeostasis demands. Although FGF23 systemic actions in the kidney, and for the production of PTH and 1,25-dihydroxyvitamin-D are well established, its direct involvement in bone metabolism is currently poorly understood. This review presents the latest available evidence suggesting two aspects of FGF23 bone local activity: (a) Regulation of FGF23 production by both local and systemic factors. The suggested local factors include extracellular levels of Pi and pyrophosphate (PPi), (the Pi/PPi ratio), and another osteocyte-derived protein, sclerostin. In addition, 1,25-dihydroxyvitamin-D, synthesized locally by bone cells, may contribute to regulation of FGF23 production. The systemic control is achieved via PTH and 1,25-dihydroxyvitamin-D endocrine functions. (b) FGF23 acts as a local agent, directly affecting bone mineralization. We support the assumption that under balanced physiological conditions, sclerostin, by para- autocrine signaling, upregulates FGF23 production by the osteocyte. FGF23, in turn, acts as a mineralization inhibitor, by stimulating the generation of the major mineralization antagonist-PPi.

Asfotase-α improves bone growth, mineralization and strength in mouse models of neurofibromatosis type-1

Mineralization of the skeleton depends on the balance between levels of pyrophosphate (PPi), an inhibitor of hydroxyapatite formation, and phosphate generated from PPi breakdown by alkaline phosphatase (ALP). We report here that ablation of Nf1, encoding the RAS/GTPase–activating protein neurofibromin, in bone–forming cells leads to supraphysiologic PPi accumulation, caused by a chronic ERK–dependent increase in genes promoting PPi synthesis and extracellular transport, namely Enpp1 and Ank. It also prevents BMP2–induced osteoprogenitor differentiation and, consequently, expression of ALP and PPi breakdown, further contributing to PPi accumulation. The short stature, impaired bone mineralization and strength in mice lacking Nf1 in osteochondroprogenitors or osteoblasts could be corrected by enzyme therapy aimed at reducing PPi concentration. These results establish neurofibromin as an essential regulator of bone mineralization, suggest that altered PPi homeostasis contributes to the skeletal dysplasiae associated with neurofibromatosis type-1 (NF1), and that some of the NF1 skeletal conditions might be preventable pharmacologically.

Dysregulated gene expression in the primary osteoblasts and osteocytes isolated from hypophosphatemic Hyp mice

Osteocytes express multiple genes involved in mineral metabolism including PHEX, FGF23, DMP1 and FAM20C. In Hyp mice, a murine model for X-linked hypophosphatemia (XLH), Phex deficiency results in the overproduction of FGF23 in osteocytes, which leads to hypophosphatemia and impaired vitamin D metabolism. In this study, to further clarify the abnormality in osteocytes of Hyp mice, we obtained detailed gene expression profiles in osteoblasts and osteocytes isolated from the long bones of 20-week-old Hyp mice and wild-type (WT) control mice. The expression of Fgf23, Dmp1, and Fam20c was higher in osteocytic cells than in osteoblastic cells in both genotypes, and was up-regulated in Hyp cells. Interestingly, the up-regulation of these genes in Hyp bones began before birth. On the other hand, the expression of Slc20a1 encoding the sodium/phosphate (Na⁺/Pi) co-transporter Pit1 was increased in osteoblasts and osteocytes from adult Hyp mice, but not in Hyp fetal bones. The direct effects of extracellular Pi and 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] on isolated osteoblastic and osteocytic cells were also investigated. Twenty-four-hour treatment with 10⁻⁸ M 1,25(OH)₂D₃ increased the expression of Fgf23 in WT osteoblastic cells but not in osteocytic cells. Dmp1 expression in osteocytic cells was increased due to the 24-hour treatment with 10 mM Pi and was suppressed by 10⁻⁸ M 1,25(OH)₂D₃ in WT osteocytic cells. We also found the up-regulation of the genes for FGF1, FGF2, their receptors, and Egr-1 which is a target of FGF signaling, in Hyp osteocytic cells, suggesting the activation of FGF/FGFR signaling. These results implicate the complex gene dysregulation in osteoblasts and osteocytes of Hyp mice, which might contribute to the pathogenesis.

[Fibroblast growth factor 23 mediates the phosphaturic actions of cadmium]

Phosphaturia has been documented following cadmium (Cd) exposure in both humans and experimental animals. Fibroblast growth factor 23 (FGF23) serves as an essential phosphate homeostasis pathway in the bone-kidney axis. In the present study, we investigated the effects of Cd on phosphate (Pi) homeostasis in mice. Following Cd injection into C57BL/6J mice, plasma FGF23 concentration significantly increased. The urinary Pi excretion level was significantly higher in the Cd-injected C57BL/6J mice than in the control group. Plasma Pi concentration decreased only slightly in the Cd-injected mice compared with the control group. No changes were observed in the concentration of the plasma parathyroid hormone and 1,25-dihydroxy vitamin D(3) in both groups of mice. We observed a decrease in phosphate transport activity and also a decrease in the expression level of renal phosphate transporter Npt2c, but not that of Npt2a. Furthermore, we examined the effect of Cd on Npt2c in Npt2a-knockout (KO) mice, which expresses Npt2c as a major NaPi cotransporter. Injecting Cd to Npt2aKO mice induced a significant increase in plasma FGF23 concentration and urinary Pi excretion level. Furthermore, we observed decreases in phosphate transport activity and renal Npt2c expression level in the Cd-injected Npt2a KO mice. The present study suggests that hypophosphatemia induced by Cd may be closely associated with FGF23.

Increased Col10a1 expression is not causative for the phenotype of Phex-deficient Hyp mice

X-linked hypophosphatemic rickets (XLHR) is a severe disorder of phosphate homeostasis and skeletal mineralization caused by mutations of PHEX, encoding a bone-specific endopeptidase. Phex-deficient Hyp mice have been extensively studied to understand the molecular bases of XLHR, and here it was found that Fgf23, encoding a major phosphaturic hormone, was transcriptionally activated in bone-forming osteoblasts. We and others could additionally show that Col10a1 expression is increased in Hyp osteoblasts and bones, thereby raising the possibility that ectopic production of type X collagen could contribute to the impaired mineralization of the Hyp bone matrix. Here we show that an additional deficiency of the Col10a1 gene does not overtly affect the skeletal phenotype of Hyp mice. More specifically, Col10a1-deficient Hyp mice displayed severe disturbances of skeletal growth, bone mass acquisition and bone matrix mineralization, and they were essentially indistinguishable from Hyp littermates. This was confirmed by non-decalcified histology and bone-specific histomorphometry quantifying all relevant parameters of growth plate maturation, trabecular bone architecture and osteoid accumulation. Taken together, our results show that increased Col10a1 expression in Phex-deficient osteoblasts is not a major cause of the XLHR phenotype, which was an important issue to address based on the previous findings.

The osteocyte: an endocrine cell ... and more

Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.

Glucocorticoids and osteocyte autophagy

Glucocorticoids are used for the treatment of inflammatory and autoimmune diseases. While they are effective therapy, bone loss and incident fracture risk are high. While previous studies have found GC effects on both osteoclasts and osteoblasts, our work has focused on the effects of GCs on osteocytes. Osteocytes exposed to low dose GCs undergo autophagy while osteocytes exposed to high doses of GCs or for a prolonged period of time undergo apoptosis.

Retinol deprivation partially rescues the skeletal mineralization defects of Phex-deficient Hyp mice

X-linked hypophosphatemic rickets (XLH) is a genetic disorder caused by mutational inactivation of the PHEX gene, encoding a transmembrane endopeptidase expressed in osteoblasts. Since several experiments involving Phex-deficient Hyp mice have demonstrated that an increased expression of Fgf23 in osteoblasts is causative for the renal phosphate loss characteristic of XLH, we performed genome-wide expression analysis to compare differentiated osteoblasts from wildtype and Hyp mice. Here we did not only observe the expected increase of Fgf23 expression in the latter ones, but also a differential expression of genes that are either induced by or involved in retinoic acid signaling, which led us to analyze whether dietary retinol deprivation would influence the phenotype of Hyp mice. Unexpectedly, feeding a retinol-free diet resulted in a partial rescue of the growth plate and bone mineralization defects in 6 weeks old Hyp mice. When we fed the same diet for 24 weeks the amount of non-mineralized bone matrix (osteoid) was reduced by more than 70%, although phosphate homeostasis was unaffected. In contrast, a dietary normalization of serum phosphate levels in Hyp mice reduced the osteoid amount by less than 30%, thereby demonstrating a previously unknown impact of retinol on the cell-autonomous mineralization defect of Phex-deficient osteoblasts.

Hypophosphatemic rickets: unraveling the role of FGF23

The classification of the various forms of hypophosphatemic rickets has been rationalized by the discovery of the central role that fibroblast growth factor 23 (FGF23) plays in the pathogenesis of a number of genetic and acquired forms of the disease. Although the details of the interaction of FGF23 with other osteoblast/osteocyte-derived proteins remain unclear at present, the measurement of circulating levels of FGF23 appears to be a useful biochemical test in determining the various causes of hypophosphatemic rickets. Furthermore, animal studies suggest that agents interfering in the action of FGF23 might play important roles in the future management of the FGF23-mediated forms of rickets. Phase 1 and phase 2 trials in humans with X-linked hypophosphatemic rickets are currently under way.

Overexpression of the DMP1 C-terminal fragment stimulates FGF23 and exacerbates the hypophosphatemic rickets phenotype in Hyp mice

Dentin matrix protein-1 (DMP1) or phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) inactivation results in elevation of the phosphaturic hormone fibroblast growth factor (FGF)-23, leading to hypophosphatemia, aberrant vitamin D metabolism, and rickets/osteomalacia. Compound mutant Phex-deficient Hyp and Dmp1(ko) mice exhibit nonadditive phenotypes, suggesting that DMP1 and PHEX may have interdependent effects to regulate FGF23 and bone mineralization. To determine the relative importance of DMP1 and PHEX in regulating FGF23 and mineralization, we tested whether the transgenic expression of full-length [Dmp1(Tg(full-length))] or C-terminal Dmp1 [Dmp1(Tg(57kDa))] could rescue the phenotype of Hyp mice. We found that Dmp1(ko) and Hyp mice have similar phenotypes characterized by decreased cortical bone mineral density (-35% vs. wild type, P < 0.05) and increased serum FGF23 levels (~12-fold vs. wild type, P < 0.05). This was significantly corrected by the overexpression of either the full-length or the C-terminal transgene in Dmp1(ko) mice. However, neither of the transgenes rescued the Hyp mice phenotype. Hyp/Dmp1(Tg(full-length)) and Hyp mice were similar, but Hyp/Dmp1(Tg(57 kDa)) mice exhibited worsening of osteomalacia (-20% cortical bone mineral density) in association with increased serum FGF23 levels (+2-fold) compared with Hyp mice. Bone FGF23 mRNA expression was decreased and a 2-fold increase in the ratio of the full-length/degraded circulating FGF23 was observed, indicating that degradation of FGF23 was impaired in Hyp/Dmp1(Tg(57 kDa)) mice. The paradoxical effects of the C-terminal Dmp1 transgene were observed in Hyp/Dmp1(Tg(57 kDa)) but not in Dmp1(Tg(57 kDa)) mice expressing a functional PHEX. These findings indicate a functional interaction between PHEX and DMP1 to regulate bone mineralization and circulating FGF23 levels and for the first time demonstrate effects of the C-terminal DMP1 to regulate FGF23 degradation.

Amelogenesis imperfecta and other biomineralization defects in Fam20a and Fam20c null mice

The FAM20 family of secreted proteins consists of three members (FAM20A, FAM20B, and FAM20C) recently linked to developmental disorders suggesting roles for FAM20 proteins in modulating biomineralization processes. The authors report here findings in knockout mice having null mutations affecting each of the three FAM20 proteins. Both Fam20a and Fam20c null mice survived to adulthood and showed biomineralization defects. Fam20b (-/-) embryos showed severe stunting and increased mortality at E13.5, although early lethality precluded detailed investigations. Physiologic calcification or biomineralization of extracellular matrices is a normal process in the development and functioning of various tissues (eg, bones and teeth). The lesions that developed in teeth, bones, or blood vessels after functional deletion of either Fam20a or Fam20c support a significant role for their encoded proteins in modulating biomineralization processes. Severe amelogenesis imperfecta (AI) was present in both Fam20a and Fam20c null mice. In addition, Fam20a (-/-) mice developed disseminated calcifications of muscular arteries and intrapulmonary calcifications, similar to those of fetuin-A deficient mice, although they were normocalcemic and normophosphatemic, with normal dentin and bone. Fam20a gene expression was detected in ameloblasts, odontoblasts, and the parathyroid gland, with local and systemic effects suggesting both local and/or systemic effects for FAM20A. In contrast, Fam20c (-/-) mice lacked ectopic calcifications but were severely hypophosphatemic and developed notable lesions in both dentin and bone to accompany the AI. The bone and dentin lesions, plus the marked hypophosphatemia and elevated serum alkaline phosphatase and FGF23 levels, are indicative of autosomal recessive hypophosphatemic rickets/osteomalacia in Fam20c (-/-) mice.

Chronic inhibition of ERK1/2 signaling improves disordered bone and mineral metabolism in hypophosphatemic (Hyp) mice

The X-linked hypophosphatemic (Hyp) mouse carries a loss-of-function mutation in the phex gene and is characterized by hypophosphatemia due to renal phosphate (Pi) wasting, inappropriately suppressed 1,25-dihydroxyvitamin D [1,25(OH)₂D] production, and rachitic bone disease. Increased serum fibroblast growth factor-23 concentration is responsible for the disordered metabolism of Pi and 1,25(OH)₂D. In the present study, we tested the hypothesis that chronic inhibition of fibroblast growth factor-23-induced activation of MAPK signaling in Hyp mice can reverse their metabolic derangements and rachitic bone disease. Hyp mice were administered the MAPK inhibitor, PD0325901 orally for 4 wk. PD0325901 induced a 15-fold and 2-fold increase in renal 1α-hydroxylase mRNA and protein abundance, respectively, and thereby higher serum 1,25(OH)₂D concentrations (115 ± 13 vs. 70 ± 16 pg/ml, P < 0.05), compared with values in vehicle-treated Hyp mice. With PD0325901, serum Pi levels were higher (5.1 ± 0.5 vs. 3 ± 0.2 mg/dl, P < 0.05), and the protein abundance of sodium-dependent phosphate cotransporter Npt2a, was greater than in vehicle-treated mice. The rachitic bone disease in Hyp mice is characterized by abundant unmineralized osteoid bone volume, widened epiphyses, and disorganized growth plates. In PD0325901-treated Hyp mice, mineralization of cortical and trabecular bone increased significantly, accompanied by a decrease in unmineralized osteoid volume and thickness, as determined by histomorphometric analysis. The improvement in mineralization in PD0325901-treated Hyp mice was confirmed by microcomputed tomography analysis, which showed an increase in cortical bone volume and thickness. These findings provide evidence that in Hyp mice, chronic MAPK inhibition improves disordered Pi and 1,25(OH)₂D metabolism and bone mineralization.

Skeletal secretion of FGF-23 regulates phosphate and vitamin D metabolism

The discovery of fibroblast growth factor 23 (FGF-23) has expanded our understanding of phosphate and vitamin D homeostasis and provided new insights into the pathogenesis of hereditary hypophosphatemic and hyperphosphatemic disorders, as well as acquired disorders of phosphate metabolism, such as chronic kidney disease. FGF-23 is secreted by osteoblasts and osteocytes in bone and principally targets the kidney to regulate the reabsorption of phosphate, the production and catabolism of 1,25-dihydroxyvitamin D and the expression of α-Klotho, an anti-ageing hormone. Secreted FGF-23 plays a central role in complex endocrine networks involving local bone-derived factors that regulate mineralization of extracellular matrix and systemic hormones involved in mineral metabolism. Inactivating mutations of PHEX, DMP1 and ENPP1, which cause hereditary hypophosphatemic disorders and primary defects in bone mineralization, stimulate FGF23 gene transcription in osteoblasts and osteocytes, at least in part, through canonical and intracrine FGF receptor pathways. These FGF-23 regulatory pathways may enable systemic phosphate and vitamin D homeostasis to be coordinated with bone mineralization. FGF-23 also functions as a counter-regulatory hormone for 1,25-dihydroxyvitamin D in a bone-kidney endocrine loop. FGF-23, through regulation of additional genes in the kidney and extrarenal tissues, probably has broader physiological functions beyond regulation of mineral metabolism that account for the association between FGF-23 and increased mortality and morbidity in chronic kidney disease.

Regulation and function of the FGF23/klotho endocrine pathways

Calcium (Ca(2+)) and phosphate (PO(4)(3-)) homeostasis are coordinated by systemic and local factors that regulate intestinal absorption, influx and efflux from bone, and kidney excretion and reabsorption of these ions through a complex hormonal network. Traditionally, the parathyroid hormone (PTH)/vitamin D axis provided the conceptual framework to understand mineral metabolism. PTH secreted by the parathyroid gland in response to hypocalcemia functions to maintain serum Ca(2+) levels by increasing Ca(2+) reabsorption and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] production by the kidney, enhancing Ca(2+) and PO(4)(3-) intestinal absorption and increasing Ca(2+) and PO(4)(3-) efflux from bone, while maintaining neutral phosphate balance through phosphaturic effects. FGF23 is a recently discovered hormone, predominately produced by osteoblasts/osteocytes, whose major functions are to inhibit renal tubular phosphate reabsorption and suppress circulating 1,25(OH)(2)D levels by decreasing Cyp27b1-mediated formation and stimulating Cyp24-mediated catabolism of 1,25(OH)(2)D. FGF23 participates in a new bone/kidney axis that protects the organism from excess vitamin D and coordinates renal PO(4)(3-) handling with bone mineralization/turnover. Abnormalities of FGF23 production underlie many inherited and acquired disorders of phosphate homeostasis. This review discusses the known and emerging functions of FGF23, its regulation in response to systemic and local signals, as well as the implications of FGF23 in different pathological and physiological contexts.

Bone proteins PHEX and DMP1 regulate fibroblastic growth factor Fgf23 expression in osteocytes through a common pathway involving FGF receptor (FGFR) signaling

Fibroblastic growth factor 23 (FGF23) is a circulating phosphaturic hormone. Inactivating mutations of the endopeptidase PHEX or the SIBLING protein DMP1 result in equivalent intrinsic bone mineralization defects and increased Fgf23 expression in osteocytes. The mechanisms whereby PHEX and DMP1 regulate Fgf23 expression are unknown. We examined the possibility that PHEX and DMP1 regulate Fgf23 through a common pathway by analyzing the phenotype of compound Phex and Dmp1 mutant mice (Hyp/Dmp1(-/-)). Compared to single-mutant littermates, compound-mutant Hyp/Dmp1(-/-) mice displayed nonadditive elevations of serum FGF23 (1912 ± 183, 1715 ± 178, and 1799 ± 181 pg/ml), hypophosphatemia (P(i): 6.0 ± 0.3, 5.8 ± 0.2, and 5.4 ± 0.1 mg/dl), and severity of rickets/osteomalacia (bone mineral density: -36, -36, and -30%). Microarray analysis of long bones identified gene expression profiles implicating common activation of the FGFR pathway in all the mutant groups. Furthermore, inhibiting FGFR signaling using SU5402 in Hyp- and Dmp1(-/-)-derived bone marrow stromal cells prevented the increase in Fgf23 mRNA expression (129- and 124-fold increase in Hyp and Dmp1(-/-) vs. 1.3-fold in Hyp+SU5402 and 2.5-fold in Dmp1(-/-)+SU5402, P<0.05). For all analyses, samples collected from nonmutant wild-type littermates served as controls. These findings indicate that PHEX and DMP1 control a common pathway regulating bone mineralization and FGF23 production, the latter involving activation of the FGFR signaling in osteocytes.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

Clinical practice. Fibroblast growth factor (FGF)23: a new hormone

Until a decade ago, two main hormones were recognized as directly affecting phosphate homeostasis and, with that, bone metabolism: parathyroid hormone and 1,25(OH)(2) vitamin D (calcitriol). It was only a decade ago that the third major player hormone was found, linking gut, bone, and kidney. The physiologic role of fibrinogen growth factor (FGF)23 is to maintain serum phosphate concentration within a narrow range. Secreted from osteocytes, it modulates kidney handling of phosphate reabsorption and calcitriol production. Genetic and acquired abnormalities in FGF23 structure and metabolism cause conditions of either hyper-FGF23-manifested by hypophosphatemia, low serum calcitriol, and rickets/osteomalacia-or hypo-FGF23, expressed by hyperphosphatemia, high serum calcitriol, and extra-skeletal calcifications. In patients with chronic renal failure, FGF23 levels increase as kidney functions deteriorate and are under investigation to learn if the hormone actually participates in the pathophysiology of the deranged bone and mineral metabolism typical for these patients and, if so, whether it might serve as a therapeutic target. This review addresses the physiology and pathophysiology of FGF23 and its clinical applications.