FGF-23 in bone biology

Fibroblast growth factor-21 potentiates the stimulatory effects of 1,25-dihydroxyvitamin D3 on transepithelial calcium transport and TRPV6 Ca2+ channel expression

Fibroblast growth factor (FGF)-21 is a salient liver-derived endocrine regulator for metabolism of glucose and triglyceride as well as bone remodeling. Previously, certain peptides in the FGF family have been shown to modulate calcium absorption across the intestinal epithelia. Since FGF21 receptor, i.e., FGF receptor-1, is abundantly expressed in the enterocytes, there was a possibility that FGF21 might exert direct actions on the intestine. Herein, a large-scale production of recombinant FGF21 at the multi-gram level was developed in order to minimize variations among various batches. In the oral glucose tolerance test, recombinant FGF21 was found to reduce plasma glucose levels in mice fed high-fat diet. A series of experiments applying radioactive tracer 45Ca in Ussing chamber showed that FGF21 potentiated the stimulatory effect of low-dose 1,25-dihydroxyvitamin D3 [10 nM 1,25(OH)2D3] on the transepithelial calcium transport across intestinal epithelium-like Caco-2 monolayer. FGF21 + 1,25(OH)2D3 also decreased transepithelial resistance, but had no effect on epithelial potential difference or short-circuit current. Furthermore, 1,25(OH)2D3 alone upregulated the Caco-2 mRNA expression of the major apical calcium channels, i.e., transient receptor potential vanilloid subfamily member 6 (TRPV6), which was further elevated by a combination of FGF21 and 1,25(OH)2D3, consistent with the upregulated TRPV6 protein expression in enterocytes of FGF21-treated mice. However, FGF21 was without effects on the mRNA expression of voltage-gated calcium channel 1.3, calbindin-D9k, plasma membrane Ca2+-ATPase 1b, claudin-12 or claudin-15. In conclusion, FGF21 did exert a direct action on the intestinal epithelial cells by potentiating the 1,25(OH)2D3-enhanced calcium transport, presumably through the upregulation of TRPV6 expression.

Bone: A Neglected Endocrine Organ?

Bone has traditionally been viewed in the context of its structural contribution to the human body. Foremost providing necessary support for mobility, its roles in supporting calcium homeostasis and blood cell production are often afterthoughts. Recent research has further shed light on the ever-multifaceted role of bone and its importance not only for structure, but also as a complex endocrine organ producing hormones responsible for the autoregulation of bone metabolism. Osteocalcin is one of the most important substances produced in bone tissue. Osteocalcin in circulation increases insulin secretion and sensitivity, lowers blood glucose, and decreases visceral adipose tissue. In males, it has also been shown to enhance testosterone production by the testes. Neuropeptide Y is produced by various cell types including osteocytes and osteoblasts, and there is evidence suggesting that peripheral NPY is important for regulation of bone formation. Hormonal disorders are often associated with abnormal levels of bone turnover markers. These include commonly used bone formation markers (bone alkaline phosphatase, osteocalcin, and procollagen I N-propeptide) and commonly used resorption markers (serum C-telopeptides of type I collagen, urinary N-telopeptides of type I collagen, and tartrate-resistant acid phosphatase type 5b). Bone, however, is not exclusively comprised of osseous tissue. Bone marrow adipose tissue, an endocrine organ often compared to visceral adipose tissue, is found between trabecula in the bone cortex. It secretes a diverse range of hormones, lipid species, cytokines, and other factors to exert diverse local and systemic effects.

Fosl1: friend or foe?

Fos-like antigen 1 (Fosl1) is a protein that belongs to the Fos family of transcription factors. Fosl1 has an impact on (i) carcinogenesis, (ii) acute kidney injury, and (iii) fibroblast growth factor expression. Recently, the nephroprotective effect of Fosl1 by the preservation of Klotho expression was recently identified. The identification of a link between Fosl1 and Klotho expression provides an entirely new field of nephroprotection.

Pulp chamber features, prevalence of abscesses, disease severity, and PHEX mutation in X-linked hypophosphatemic rickets

INTRODUCTION

Rickets, growth failure, and recurrent periapical abscesses with fistulae are main signs in patients with X-linked hypophosphatemic rickets (XLH). Prevalence of abscesses, pulp chamber features, biochemical findings, disease severity, and PHEX gene mutation were examined.

MATERIALS AND METHODS

Pulp chambers size, shape, and morphology were assessed by orthopantomography in XLH patients (n = 24, age 5.8 ± 1.6 years) and in sex and age-matched healthy controls (n = 23, age 6.2 ± 1.4 years). XLH patients received conventional treatment (3.5 ± 1.9 years). Pulp chamber features were assessed in teeth of primary dentition and in the permanent left mandibular first molar and compared with those of controls. Rickets severity score was assessed at wrist, knee, and ankle.

RESULTS

The mean pulp chamber area/tooth area ratio, mean pulp chamber height/pulp chamber width ratio, and prominence of pulp horns into the tooth crown in primary and secondary molars were significantly higher in patients than in controls and in patients suffered abscesses than in patients without abscesses. Sixteen patients (67%) had a history of abscesses; incisors were affected more than canines and molars. Severity of rickets and mean serum parathyroid hormone (PTH) levels were significantly higher, and mean serum 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels significantly lower in patients suffered abscesses than in patients without abscesses. PHEX gene mutations were not correlated with dental phenotype and disease severity.

CONCLUSION

Enlarged pulp chambers with altered shape and morphology affected the majority of XLH patients predisposing to recurrent periapical abscesses with fistulae. Dental phenotype was associated with severity of rickets, high serum PTH, and low serum 1,25(OH)₂D levels.

Investigating the causal effect of fibroblast growth factor 23 on osteoporosis and cardiometabolic disorders: A Mendelian randomization study

Pathological excess of fibroblast growth factor 23 (FGF23) causes mineral and bone disorders. However, the causality of FGF23 in the development of osteoporosis remains unknown. Whether FGF23 has systemic effects on cardiometabolic disorders beyond regulating mineral metabolism is also controversial. In this study, we investigated the causal effect of FGF23 on osteoporosis and cardiometabolic disorders using Mendelian randomization (MR) analysis. Summary statistics for single-nucleotide polymorphisms with traits of interest were obtained from the relevant genome-wide association studies. As a result, FGF23 was found to be inversely associated with femoral neck-BMD (odds ratio [OR] 0.682, 95% confidence interval [CI] 0.546-0.853, p = 8e-04) and heel estimated BMD (eBMD) (OR 0.898, 95%CI 0.820-0.985, p = 0.022) in the inverse-variance-weighted analysis, but not lumbar spine-BMD and fractures. The results were supported by the weighted-median analysis, and there was no evidence of pleiotropy in the MR-Egger analysis. FGF23 was associated with FN-BMD and eBMD after adjustment for estimated glomerular filtration rate, height, and body mass index in multivariable MR analysis. On the other hand, there was no association between FGF23 and cardiometabolic traits including cardio artery disease, brachial-ankle pulse wave velocity, intima-media thickness of carotid arteries, systolic and diastolic blood pressure, fasting glucose, high and low-density lipoprotein cholesterol, and triglycerides. Therefore, this MR study established that FGF23 was involved in bone loss and, in contrast, was not involved in cardiometabolic disorders. Our findings provide important insights into the role of FGF23 in the pathogenesis of osteoporosis and cardiometabolic disorders.

Low bone mineral density due to secondary hyperparathyroidism in the GlatmTg(CAG-A4GALT) mouse model of Fabry disease

Low bone mineral density (BMD)-diagnosed as osteoporosis or osteopenia-has been reported as a new characteristic feature of Fabry disease; however, the mechanism underlying the development of low BMD is unknown. We previously revealed that a mouse model of Fabry disease [GlatmTg(CAG-A4GALT)] exhibits impaired functioning of medullary thick ascending limb (mTAL), leading to insufficient Ca2+ reabsorption and hypercalciuria. Here, we investigated bone metabolism in GlatmTg(CAG-A4GALT) mice without marked glomerular or proximal tubular damage. Low BMD was detected by 20 weeks of age via micro-X-ray-computed tomography. Bone histomorphometry revealed that low BMD results by accelerated bone resorption and osteomalacia. Plasma parathyroid hormone levels increased in response to low blood Ca2+-not plasma fibroblast growth factor 23 (FGF-23) elevation-by 5 weeks of age and showed progressively increased phosphaturic action. Secondary hyperparathyroidism developed by 20 weeks of age and caused hyperphosphatemia, which increased plasma FGF-23 levels with phosphaturic action. The expression of 1α-hydroxylase [synthesis of 1α,25(OH)2D3] in the kidney did not decrease, but that of 24-hydroxylase [degradation of 1α,25(OH)2D3] decreased. Vitamin D deficiency was ruled out as the cause of osteomalacia, as plasma 1α,25(OH)2D3 and 25(OH)D3 levels were maintained. Results demonstrate that secondary hyperparathyroidism due to mTAL impairment causes accelerated bone resorption and osteomalacia due to hyperphosphaturia and hypercalciuria, leading to low BMD in Fabry model mice.

Osteoimmunology: A Current Update of the Interplay Between Bone and the Immune System

Immunology, already a discipline in its own right, has become a major part of many different medical fields. However, its relationship to orthopedics and trauma surgery has unfortunately, and perhaps unjustly, been developing rather slowly. Discoveries in recent years have emphasized the immense breadth of communication and connection between both systems and, importantly, the highly promising therapeutic opportunities. Recent discoveries of factors originally assigned to the immune system have now also been shown to have a significant impact on bone health and disease, which has greatly changed how we approach treatment of bone pathologies. In case of bone fracture, immune cells, especially macrophages, are present throughout the whole healing process, assure defense against pathogens and discharge a complex variety of effectors to regulate bone modeling. In rheumatoid arthritis and osteoporosis, the immune system contributes to the formation of the pathological and chronic conditions. Fascinatingly, prosthesis failure is not at all solely a mechanical problem of improper strain but works in conjunction with an active contribution of the immune system as a reaction to irritant debris from material wear. Unraveling conjoined mechanisms of the immune and osseous systems heralds therapeutic possibilities for ailments of both. Contemplation of the bone as merely an unchanging support pillar is outdated and obsolete. Instead it is mandatory that this highly diverse network be incorporated in our understanding of the immune system and hematopoiesis.

Anomalous bone changes in ovariectomized type 2 diabetic rats: inappropriately low bone turnover with bone loss in an estrogen-deficient condition

Estrogen deprivation accelerates bone resorption, leading to imbalance of bone remodeling and osteoporosis in postmenopausal women. In the elderly, type 2 diabetes mellitus (T2DM) frequently coexists as an independent factor of bone loss. However, little is known about the skeletal changes in a combined condition of estrogen deficiency and T2DM. Herein, we performed ovariectomy (OVX) in nonobese Goto-Kakizaki (GK) T2DM rats to examine changes associated with calcium and phosphate metabolism and bone microstructures and strength. As expected, wild-type (WT) rats subjected to ovariectomy (OVX-WT) had low trabecular bone volume and serum calcium with increased dynamic histomorphometric and serum bone markers, consistent with the high turnover state. T2DM in GK rats also led to low trabecular volume and serum calcium. However, the dynamic histomorphometric markers of bone remodeling were unaffected in these GK rats, indicating the distinct mechanism of T2DM-induced bone loss. Interestingly, OVX-GK rats were found to have anomalous and unique changes in bone turnover-related parameters, i.e., decreased osteoblast and osteoclast surfaces with lower COOH-terminal telopeptide of type I collagen levels compared with OVX-WT rats. Furthermore, the levels of calciotropic hormones, i.e., parathyroid hormone and 1,25(OH)₂D₃, were significantly decreased in OVX-GK rats. Although the OVX-induced bone loss did not further worsen in GK rats, a three-point bending test indicated that OVX-GK bones exhibited a decrease in bone elasticity. In conclusion, T2DM and estrogen deficiency both led to microstructural bone loss, the appearance of which did not differ from each factor alone. Nevertheless, the combination worsened the integrity and suppressed the turnover, which might eventually result in adynamic bone disease.

Assessment of the Concentration of Bone Metabolism Markers: Sclerostin and FGF-23 in Children with Idiopathic Nephrotic Syndrome Treated with Glucocorticosteroids

Recurring nature of idiopathic nephrotic syndrome (INS) and steroid dependence imply a long-term treatment with glucocorticosteroids (GCSs), which increases the risk of bone metabolism disorders. The search for new markers of that process is essential. The aims of this study were to assess the concentrations of sclerostin (Scl) and fibroblast growth factor-23 (FGF-23) in the plasma of children with INS and compare Scl and FGF-23 to existing markers of bone metabolism, mainly parathyroid hormone (PTH). The study involved 70 children, 50 with INS and 20 healthy children. Patients with INS were divided into 4 groups depending on the number of relapses and applied therapy. Significantly higher concentrations of FGF-23 and Scl were found in all patient groups with INS compared to the control group, and increase in the concentrations of examined parameters depending on the number of NS relapses was showed. In patients from the group with numerous relapses, higher concentrations of FGF-23 and Scl in the relapse phase than those in the remission phase were found. We observed positive correlation in these proteins with parathyroid hormone. Positive correlation of FGF-23 and Scl in the examined group was noted. Children having relapsing INS treated with steroids have higher levels of Scl and FGF-23 that can indicate the bone metabolism disorders. The significance of these observations requires further research.

Acute ketamine administration corrects abnormal inflammatory bone markers in major depressive disorder

Patients with major depressive disorder (MDD) have clinically relevant, significant decreases in bone mineral density (BMD). We sought to determine if predictive markers of bone inflammation-the osteoprotegerin (OPG)-RANK-RANKL system or osteopontin (OPN)-play a role in the bone abnormalities associated with MDD and, if so, whether ketamine treatment corrected the abnormalities. The OPG-RANK-RANKL system plays the principal role in determining the balance between bone resorption and bone formation. RANKL is the osteoclast differentiating factor and diminishes BMD. OPG is a decoy receptor for RANKL, thereby increasing BMD. OPN is the bone glue that acts as a scaffold between bone tissues matrix composition to bind them together and is an important component of bone strength and fracture resistance. Twenty-eight medication-free inpatients with treatment-resistant MDD and 16 healthy controls (HCs) participated in the study. Peripheral bone marker levels and their responses to IV ketamine infusion in MDD patients and HCs were measured at four time points: at baseline, and post-infusion at 230 min, Day 1, and Day 3. Patients with MDD had significant decreases in baseline OPG/RANKL ratio and in plasma OPN levels. Ketamine significantly increased both the OPG/RANKL ratio and plasma OPN levels, and significantly decreased RANKL levels. Bone marker levels in HCs remained unaltered. We conclude that the OPG-RANK-RANKL system and the OPN system play important roles in the serious bone abnormalities associated with MDD. These data suggest that, in addition to its antidepressant effects, ketamine also has a salutary effect on a major medical complication of depressive illness.

The Biological Activities of Vitamin D and Its Receptor in Relation to Calcium and Bone Homeostasis, Cancer, Immune and Cardiovascular Systems, Skin Biology, and Oral Health

Vitamin D plays an important role in calcium homeostasis and bone metabolism, with the capacity to modulate innate and adaptive immune function, cardiovascular function, and proliferation and differentiation of both normal and malignant keratinocytes. 1,25(OH)2D, the biologically active form of vitamin D, exerts most of its functions through the almost universally distributed nuclear vitamin D receptor (VDR). Upon stimulation by 1,25(OH)2D, VDR forms a heterodimer with the retinoid X receptor (RXR). In turn, VDR/RXR binds to DNA sequences termed vitamin D response elements in target genes, regulating gene transcription. In order to exert its biological effects, VDR signalling interacts with other intracellular signalling pathways. In some cases 1,25(OH)2D exerts its biological effects without regulating either gene expression or protein synthesis. Although the regulatory role of vitamin D in many biological processes is well documented, there is not enough evidence to support the therapeutic use of vitamin D supplementation in the prevention or treatment of infectious, immunoinflammatory, or hyperproliferative disorders. In this review we highlight the effects of 1,25(OH)2D on bone and calcium homeostasis, on cancer, and refer to its effects on the cardiovascular and immune systems.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

Markers of bone metabolism are affected by renal function and growth hormone therapy in children with chronic kidney disease

Objectives

The extent and relevance of altered bone metabolism for statural growth in children with chronic kidney disease is controversial. We analyzed the impact of renal dysfunction and recombinant growth hormone therapy on a panel of serum markers of bone metabolism in a large pediatric chronic kidney disease cohort.

Methods

Bone alkaline phosphatase (BAP), tartrate-resistant acid phosphatase 5b (TRAP5b), sclerostin and C-terminal FGF-23 (cFGF23) normalized for age and sex were analyzed in 556 children aged 6–18 years with an estimated glomerular filtration rate (eGFR) of 10–60 ml/min/1.73m². 41 children receiving recombinant growth hormone therapy were compared to an untreated matched control group.

Results

Standardized levels of BAP, TRAP5b and cFGF-23 were increased whereas sclerostin was reduced. BAP was correlated positively and cFGF-23 inversely with eGFR. Intact serum parathormone was an independent positive predictor of BAP and TRAP5b and negatively associated with sclerostin. BAP and TRAP5B were negatively affected by increased C-reactive protein levels. In children receiving recombinant growth hormone, BAP was higher and TRAP5b lower than in untreated controls. Sclerostin levels were in the normal range and higher than in untreated controls. Serum sclerostin and cFGF-23 independently predicted height standard deviation score, and BAP and TRAP5b the prospective change in height standard deviation score.

Conclusion

Markers of bone metabolism indicate a high-bone turnover state in children with chronic kidney disease. Growth hormone induces an osteoanabolic pattern and normalizes osteocyte activity. The osteocyte markers cFGF23 and sclerostin are associated with standardized height, and the markers of bone turnover predict height velocity.

The role of vitamin D in human fracture healing: a systematic review of the literature

INTRODUCTION

Vitamin D is essential for bone mineralization and for the subsequent maintenance of bone quality. Mineralization is part of hard callus formation and bone remodelling, processes, which are part of fracture healing. We provide a comprehensive review of the literature to summarize and clarify if possible, the cellular effects of vitamin D and its clinical involvement in the process of fracture healing in human.

MATERIAL AND METHODS

We conducted a literature search in PubMed, Embase (OVID version), and Web of Science.

RESULTS

A total of 75 in vitro and 30 in vivo studies were found with inconsistent results about the cellular effect of vitamin D on fracture involved inflammatory cells, cytokines, growth factors, osteoblasts, osteoclasts and on the process of mineralization. With only five in vitro studies performed on material derived from a fracture site and one in vivo study in fracture patients, the exact cellular role remains unclear. Seven studies investigated the circulating vitamin D metabolites in fracture healing. Although it appears that 25(OH)D and 24,25(OH)2D3 are not affected by the occurrence of a fracture, this might not be the case with serum concentrations of 1,25(OH)2D3. The potential clinical effect of vitamin D deficiency is only described in one case series and three case controlled studies, where the results tend to show no effect of a vitamin D deficiency. No clinical studies were found investigating solely vitamin D supplementation. Two clinical studies found a positive effect of vitamin D supplementation and calcium, of increased bone mineral density or respectively increased fracture callus area at the fracture site. One study found indirect evidence that vitamin D and calcium promoted fracture healing.

CONCLUSION

Despite these results, and the presumed beneficial effect of vitamin D supplementation in deficient patients, clinical studies that address the effects of vitamin D deficiency or supplementation on fracture healing are scarce and remain inconclusive. We conclude that vitamin D has a role in fracture healing, but the available data are too inconsistent to elucidate how and in what manner.

Hypophosphatemic rickets due to perturbations in renal tubular function

The common denominator for all types of rickets is hypophosphatemia, leading to inadequate supply of the mineral to the growing bone. Hypophosphatemia can result from insufficient uptake of the mineral from the gut or its disproportionate losses in the kidney, the latter being caused by either tubular abnormalities per se or the effect on the tubule of circulating factors like fibroblast growth factor-23 and parathyroid hormone (PTH). High serum levels of the latter result in most cases from abnormalities in vitamin D metabolism which lead to decreased calcium absorption in the gut and hypocalcemia, triggering PTH secretion. Rickets is a disorder of the growth plate and hence pediatric by definition. However, it is important to recognize that the effect of hypophosphatemia on other parts of the skeleton results in osteomalacia in both children and adults. This review addresses the etiology, pathophysiologic mechanisms, clinical manifestations and treatment of entities associated with hypophosphatemic rickets due to perturbations in renal tubular function.

Fibroblast growth factor 23

There is growing interest in the role of fibroblast growth factor 23 (FGF23) in various diseases of disordered mineral metabolism. In chronic kidney disease (CKD), where biochemical evidence of mineral disturbances is especially common, FGF23 measurement has been advocated as an early and sensitive marker for CKD-related bone disease. In this setting, FGF23 analysis may also improve the discrimination of risk of adverse renal and cardiovascular outcomes and aid targeting of those patients that are likely to benefit from interventions. Nonetheless, while the physiological relevance of FGF23 in the control of mineral metabolism is now firmly established, relatively little attention has been paid to important preanalytical and analytical aspects of FGF23 measurement that may impact on its clinical utility. Here we review these issues and discuss the suitability of FGF23 testing strategies for routine clinical practice. The current ‘state-of-the-art’ enzyme-linked immunosorbent assay methods for FGF23 measurement show poor agreement due to differences in FGF23 fragment detection, antibody specificity and calibration. Such analytical variability does not permit direct comparison of FGF23 measurements made with different assays and is likely to at least in part account for some of the inconsistencies noted between observational studies. From a clinical perspective, the lack of concordance has implications for the development of standardized reference intervals and clinical decision limits. Finally, the inherent assay-dependent biological variability of plasma FGF23 concentration can further complicate the interpretation of results and the design of FGF23-based testing protocols. Currently, it would be premature to consider incorporating FGF23 measurements into standard testing repertoires.

[Mineral and bone disorders in renal transplantation]

The deregulation of bone and mineral metabolism during chronic kidney disease (CKD) is a daily challenge for physicians, its management aiming at decreasing the risk of both fractures and vascular calcifications. Renal transplantation in the context of CKD, with pre-existing renal osteodystrophy as well as nutritional impairment, chronic inflammation, hypogonadism and corticosteroids exposure, represents a major risk factor for bone impairment in the post-transplant period. The aim of this review is therefore to provide an update on the pathophysiology of mineral and bone disorders after renal transplantation.

Update on tenofovir toxicity in the kidney

Tenofovir (TFV) is a widely used and effective treatment for HIV infection. Numerous studies have shown that TFV exposure is associated with small but significant declines in estimated glomerular filtration rate (eGFR). However, TFV toxicity is targeted mainly at the proximal tubule (PT), and in severe cases can cause the renal Fanconi syndrome or acute kidney injury. Severe toxicity occurs in a minority of patients, but milder PT dysfunction is more common; the long-term significance of this on kidney and bone health is uncertain. Recent work suggests that changes in eGFR on TFV therapy might be explained by inhibition of PT creatinine secretion rather than actual alterations in glomerular function. Risk factors for nephrotoxicity include pre-existing kidney disease, increased age, and low body mass. Mitochondria in the PT are the targets of TFV toxicity, but the exact mechanisms remain unclear. Substantial improvement of renal function occurs in many patients with TFV toxicity upon stopping therapy, but function does not always return to baseline. In recent years, TFV usage has been extended to new clinical spheres, including pediatrics, resource-poor settings and treatment of hepatitis B infection; theoretical reasons exist as to why some of these patients might be at higher or lower risk of TFV toxicity. Finally, strategies have been proposed to prevent TFV toxicity or enhance recovery.

Recent advances in the noninvasive diagnosis of renal osteodystrophy

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is the term used to describe a constellation of biochemical abnormalities, bone disturbances that may lead to fractures, and extraskeletal calcification in soft tissues and arteries seen in CKD. This review focuses on the noninvasive diagnosis of renal osteodystrophy, the term used exclusively to define the bone pathology associated with CKD. Transiliac bone biopsy and histomorphometry with double-labeled tetracycline or its derivatives remains the gold standard for diagnosis of renal osteodystrophy. However, histomorphometry provides a 'window' into bone only at a single point in time, and is not clinically practical for studying continuous changes in bone morphology. Furthermore, the etiology of fractures in CKD is multifactorial and not fully explained by histomorphometry findings alone. The propensity of a bone to fracture is determined by bone strength, which is affected by bone mass and bone quality; the latter is a term used to describe the structure and composition of bone. Bone quantity is traditionally assessed by dual X-ray absorptiometry (DXA) and CT-based methods. Bone quality is more difficult to assess noninvasively, but newer techniques are emerging and are described in this review. Ultimately, the optimal diagnostic strategy for renal osteodystrophy may be a combination of multiple imaging techniques and biomarkers that are specific to each gender and race in CKD, with a goal of predicting fracture risk and optimizing therapy.

Inactivation of Lrp5 in osteocytes reduces young's modulus and responsiveness to the mechanical loading

Low-density-lipoprotein receptor-related protein 5 (Lrp5) is a co-receptor in Wnt signaling, which plays a critical role in development and maintenance of bone. Osteoporosis-pseudoglioma syndrome, for instance, arises from loss-of-function mutations in Lrp5, and global deletion of Lrp5 in mice results in significantly lower bone mineral density. Since osteocytes are proposed to act as a mechanosensor in the bone, we addressed a question whether a conditional loss-of-function mutation of Lrp5 selective to osteocytes (Dmp1-Cre;Lrp5(f/f)) would alter responses to ulna loading. Loading was applied to the right ulna for 3 min (360 cycles at 2Hz) at a peak force of 2.65 N for 3 consecutive days, and the contralateral ulna was used as a non-loaded control. Young's modulus was determined using a midshaft section of the femur. The results showed that compared to age-matched littermate controls, mice lacking Lrp5 in osteocytes exhibited smaller skeletal size with reduced bone mineral density and content. Compared to controls, Lrp5 deletion in osteocytes also led to a 4.6-fold reduction in Young's modulus. In response to ulna loading, mineralizing surface, mineral apposition rate, and bone formation rate were diminished in mice lacking Lrp5 in osteocytes by 52%, 85%, and 69%, respectively. Collectively, the results support the notion that the loss-of-function mutation of Lrp5 in osteocytes causes suppression of mechanoresponsiveness and reduces bone mass and Young's modulus. In summary, Lrp5-mediated Wnt signaling significantly contributes to maintenance of mechanical properties and bone mass.

Sclerostin alters serum vitamin D metabolite and fibroblast growth factor 23 concentrations and the urinary excretion of calcium

Inactivating mutations of the SOST (sclerostin) gene are associated with overgrowth and sclerosis of the skeleton. To determine mechanisms by which increased amounts of calcium and phosphorus are accreted to enable enhanced bone mineralization in the absence of sclerostin, we measured concentrations of calciotropic and phosphaturic hormones, and urine and serum calcium and inorganic phosphorus in mice in which the sclerostin (sost) gene was replaced by the β-D-galactosidase (lacZ) gene in the germ line. Knockout (KO) (sost(-/-)) mice had increased bone mineral density and content, increased cortical and trabecular bone thickness, and greater net bone formation as a result of increased osteoblast and decreased osteoclast surfaces compared with wild-type (WT) mice. β-Galactosidase activity was detected in osteocytes of sost KO mice but was undetectable in WT mice. Eight-week-old, male sost KO mice had increased serum 1α,25-dihydroxyvitamin D, decreased 24,25-dihydroxyvitamin D, decreased intact fibroblast growth factor 23, and elevated inorganic phosphorus concentrations compared with age-matched WT mice. 25-Hydroxyvitamin D 1α-hydroxylase cytochrome P450 (cyp27B1) mRNA was increased in kidneys of sost KO mice compared with WT mice. Treatment of cultured proximal tubule cells with mouse recombinant sclerostin decreased cyp27B1 mRNA transcripts. Urinary calcium and renal fractional excretion of calcium were decreased in sost KO mice compared with WT mice. Sost KO and WT mice had similar serum calcium and parathyroid hormone concentrations. The data show that sclerostin not only alters bone mineralization, but also influences mineral metabolism by altering concentrations of hormones that regulate mineral accretion.

Renal phosphate handling in Gitelman syndrome--the results of a case-control study

BACKGROUND

Patients with Gitelman syndrome, a hereditary salt-wasting tubulopathy, have loss-of-function mutations in the SLC12A3 gene coding for the thiazide-sensitive sodium chloride co-transporter in the distal convoluted tubule. Since the bulk of filtered phosphate is reabsorbed in the proximal tubule, renal phosphate wasting is considered exceptional in Gitelman syndrome.

METHODS

We investigated the renal handling of inorganic phosphate in 12 unselected Italian patients affected with Gitelman syndrome (5 females and 7 males, aged 6.0-18 years, median age 12 years) and in 12 healthy subjects matched for gender and age (controls). The diagnosis of Gitelman syndrome among the patients had been made clinically and confirmed by molecular biology studies.

RESULTS

The biochemical hallmarks of Gitelman syndrome, namely hypochloremia, hypokalemia, hypomagnesemia, increased urinary excretion of sodium, chloride, potassium and magnesium and reduced urinary excretion of calcium, were present in the 12 patients. In addition, both the plasma inorganic phosphate concentration (median and interquartile range: 1.28 [1.12-1.36] vs. 1.61 [1.51-1.66)] mmol/L) and the maximal tubular reabsorption of inorganic phosphate (1.08 [0.99-1.22] vs. 1.41 [1.38-1.47] mmol/L) were significantly lower (P < 0.001) in Gitelman patients than in control subjects. Circulating levels of 25-hydroxyvitamin D, intact parathyroid hormone and osteocalcin were similar in patients and controls.

CONCLUSIONS

The results of our case-control study disclose a hitherto unrecognized tendency towards renal phosphate wasting with mild to moderate hypophosphatemia in Gitelman syndrome.

Phosphate homeostasis and its role in bone health

Phosphate is one of the most abundant minerals in the body, and its serum levels are regulated by a complex set of processes occurring in the intestine, skeleton, and kidneys. The currently known main regulators of phosphate homeostasis include parathyroid hormone (PTH), calcitriol, and a number of peptides collectively known as the "phosphatonins" of which fibroblast growth factor-23 (FGF-23) has been best defined. Maintenance of extracellular and intracellular phosphate levels within a narrow range is important for many biological processes, including energy metabolism, cell signaling, regulation of protein synthesis, skeletal development, and bone integrity. The presence of adequate amounts of phosphate is critical for the process of apoptosis of mature chondrocytes in the growth plate. Without the presence of this mineral in high enough quantities, chondrocytes will not go into apoptosis, and the normal physiological chain of events that includes invasion of blood vessels and the generation of new bone will be blocked, resulting in rickets and delayed growth. In the rest of the skeleton, hypophosphatemia will result in osteomalacia due to an insufficient formation of hydroxyapatite. This review will address phosphate metabolism and its role in bone health.

Vitamin D status in children with chronic kidney disease

BACKGROUND

The role of vitamin D status in patients with renal insufficiency and its relation to dietary intake and parathyroid hormone (PTH) secretion is of utmost interest given the morbidity and mortality associated with the disordered mineral metabolism seen in chronic kidney disease (CKD).

METHODS

We conducted a cross-sectional study of 100 pediatric patients with a diagnosis of CKD stage 1-5 at Children's Hospital Boston, measuring blood levels of 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D], and parathyroid hormone and obtaining data on nutrient intake and other variables related to vitamin D status.

RESULTS

Subjects ranged in age from 6 months to 18 years, and 60 were male, 40 female. Of the 100 patients, 16 % were deficient in 25(OH)D (≤ 20 ng/mL) and another 24 % were insufficient (≤ 30 ng/mL), with 40 % in the suboptimal range. Serum 25(OH)D and dietary vitamin D intake were not correlated.

CONCLUSIONS

We found a high prevalence of hyperparathyroidism in early-stage CKD and a significant relationship between 25(OH)D and PTH regardless of calcitriol level. Our study results support the suggestion that optimization of vitamin D levels may provide additional benefit in preventing or improving hyperparathyroidism in patients with early CKD and likely remains important as an adjunctive therapy in children with advanced CKD.

FGF-23 in children with CKD: a new player in the development of CKD-mineral and bone disorder

Disturbances in mineral and bone metabolism in children with chronic kidney disease (CKD) lead to specific abnormalities of skeletal homeostasis called CKD-mineral and bone disorder (CKD-MBD). These disturbances should be diagnosed and managed appropriately to prevent bone deformities and disturbed growth. Changes in the vitamin D and parathyroid hormone (PTH), and the subsequent alterations in calcium (Ca) and phosphate (P) homeostasis are considered responsible for the development of CKD-MBD. Recently, a phosphaturic hormone, the fibroblast growth factor-23 (FGF-23), has been reported as a key regulator of P and vitamin D metabolism. A number of recent studies in paediatric populations have documented that the FGF-23 levels are increased early in CKD, before any abnormalities in serum Ca, P or PTH are apparent. The elevated FGF-23 levels result in a negative P balance to maintain P homeostasis, inducing phosphaturia, independently of PTH, and suppressing vitamin D synthesis. Therefore, the bone-kidney-parathyroid endocrine axis mediated by FGF-23 should be a novel therapeutic target in clinical practice, even in early stages of CKD in children.

Fibroblast growth factor-23 helps explain the biphasic cardiovascular effects of vitamin D in chronic kidney disease

Hypovitaminosis D is highly prevalent in chronic kidney disease (CKD). Recently, vitamin D has sparked widespread interest because of its potential favorable benefits on cardiovascular disease (CVD). Evidence from clinical studies and animal models supports the existence of biphasic cardiovascular effects of vitamin D, in which lower doses suppress CVD and higher doses stimulate CVD. However, the mechanism for the different effects remains unclear. Fibroblast growth factor-23 (FGF-23) is a recently identified member of the FGF family, and thought to be actively involved in renal phosphate and vitamin D homeostasis. More specifically, Vitamin D stimulates FGF-23 secretion and is inhibited by increased FGF-23. Given this background, we hypothesize that FGF-23 may provide a unique tool to explain the biphasic cardiovascular effects of vitamin D in CKD. The data presented in this review support the hypothesis that FGF-23 may be linked with the high cardiovascular risk in CKD through accelerating the onset of vascular calcification, secondary hyperparathyroidism, left ventricular hypertrophy and endothelial dysfunction. Therefore, modulation of FGF-23 may become a potential therapeutic target to lowing cardiovascular risk in CKD. Several clinical interventions, including decreased phosphate intake, phosphate binders, cinacalcet plus concurrent low-dose vitamin D, C-terminal tail of FGF-23 and renal transplantation, have been employed to manipulate FGF-23.

Fibroblast growth factor-23 abolishes 1,25-dihydroxyvitamin D₃-enhanced duodenal calcium transport in male mice

Despite being widely recognized as the important bone-derived phosphaturic hormone, whether fibroblast growth factor (FGF)-23 modulated intestinal calcium absorption remained elusive. Since FGF-23 could reduce the circulating level of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], FGF-23 probably compromised the 1,25(OH)₂D₃-induced intestinal calcium absorption. FGF-23 may also exert an inhibitory action directly through FGF receptors (FGFR) in the intestinal cells. Herein, we demonstrated by Ussing chamber technique that male mice administered 1 μg/kg 1,25(OH)₂D₃ sc daily for 3 days exhibited increased duodenal calcium absorption, which was abolished by concurrent intravenous injection of recombinant mouse FGF-23. This FGF-23 administration had no effect on the background epithelial electrical properties, i.e., short-circuit current, transepithelial potential difference, and resistance. Immunohistochemical evidence of protein expressions of FGFR isoforms 1-4 in mouse duodenal epithelial cells suggested a possible direct effect of FGF-23 on the intestine. This was supported by the findings that FGF-23 directly added to the serosal compartment of the Ussing chamber and completely abolished the 1,25(OH)₂D₃-induced calcium absorption in the duodenal tissues taken from the 1,25(OH)₂D₃-treated mice. However, direct FGF-23 exposure did not decrease the duodenal calcium absorption without 1,25(OH)₂D₃ preinjection. The observed FGF-23 action was mediated by MAPK/ERK, p38 MAPK, and PKC. Quantitative real-time PCR further showed that FGF-23 diminished the 1,25(OH)₂D₃-induced upregulation of TRPV5, TRPV6, and calbindin-D(9k), but not PMCA(1b) expression in the duodenal epithelial cells. In conclusion, besides being a phosphatonin, FGF-23 was shown to be a novel calcium-regulating hormone that acted directly on the mouse intestine, thereby compromising the 1,25(OH)₂D₃-induced calcium absorption.

Chronic Kidney Disease-Mineral Bone Disorder in Diabetes Mellitus Patients

Diabetes mellitus (DM) and chronic kidney disease (CKD) are two diseases with increasing prevalence and adverse outcomes that represent an international health problem. Chronic kidney disease- mineral and bone disorder (CKD-MBD) is defined as a systemic disorder of mineral and bone metabolism due to CKD manifested by either one or a combination of the following: abnormalities of calcium, phosphorus, PTH, or vitamin D metabolism; abnormalities in bone turnover, mineralization, volume, linear growth, or strength and vascular or other soft-tissue calcification. Disturbances in mineral and bone metabolism are prevalent in CKD and are an important cause of decreased quality of life, cardiovascular morbidity and mortality; these disturbances settle in earlier and have a more severe evolution in DM patients.

Serum osteoprotegerin, RANKL and fibroblast growth factor-23 in children with chronic kidney disease

Osteoprotegerin (OPG), receptor activator of the nuclear factor κB ligand (RANKL) and fibroblast growth factor-23 (FGF-23) play a central role in renal osteodystrophy. We evaluated OPG/RANKL and FGF-23 levels in 51 children with chronic kidney disease (CKD) [n = 26 stage 3 or 4 (CKD3-4) and n = 25 stage 5 (CKD5)] and 61 controls. Any possible association with intact parathyroid hormone (iPTH) and bone turnover markers was also investigated. The OPG levels were lower in the CKD3-4 group (p < 0.001) and higher in the CKD5 group (p < 0.01) than in the controls, while RANKL levels did not differ. The FGF-23 levels were higher in both patient groups (p < 0.0001), while the levels of phosphate and iPTH were higher only in the CKD5 group (p < 0.0001). There were independent positive correlations between OPG and RANKL (β = 0.297, p < 0.01) and FGF-23 (β = 0.352, p < 0.05) and a negative correlation with the bone resorption marker TRAP5b (β = -0.519, p < 0.001). OPG was positively correlated with iPTH (R = 0.391, p < 0.01). An independent positive correlation between FGF-23 and phosphate (β = 0.368, p < 0.05) or iPTH (β = 0.812, p < 0.0001) was noted. In conclusion, we found that higher OPG levels in patients with CKD stage 5 correlated with the levels of RANKL, FGF-23, iPTH, and TRAP5b. These findings may reflect a compensatory mechanism to the negative balance of bone turnover. High FGF-23 levels in early CKD stages may indicate the need for intervention to manage serum phosphate (Pi) levels.

[FGF23 and Klotho: the new cornerstones of phosphate/calcium metabolism]

Since its first description as a phosphaturic agent in the early 2000s, fibroblast growth factor 23 (FGF23) has rapidly become the third key player of phosphate/calcium metabolism after PTH and vitamin D. FGF23 is a protein synthesized by osteocytes that acts mainly as a phosphaturic factor and a suppressor of 1α hydroxylase activity in the kidney. It inhibits the expression of type IIa and IIc sodium-phosphate cotransporters on the apical membrane of proximal tubular cells, thus leading to inhibition of phosphate reabsorption. Moreover, it also inhibits 1α hydroxylase activity. These two renal pathways account together for the hypophosphatemic effect of FGF23, but FGF23 has also been recently described as an inhibiting factor for PTH synthesis. Its exact role in bone remains to be defined. A transmembrane protein, Klotho, is an essential cofactor for FGF23 biological activity, but it can also act by itself for calcium and PTH regulation. This paper gives an overview of these recent data of phosphate/calcium physiology, as well as a description of clinical conditions associated with FGF23 deregulation (genetic diseases and chronic kidney disease). As a conclusion, future therapeutic consequences of the FGF23/Klotho axis are discussed.

Parathyroid hormone and growth in chronic kidney disease

Growth failure is common in children with chronic kidney disease, and successful treatment is a major challenge in the management of these children. The aetiology is multi-factorial with "chronic kidney disease-metabolic bone disorder" being a key component that is particularly difficult to manage. Parathyroid hormone is at the centre of this mineral imbalance, consequent skeletal disease and, ultimately, growth failure. When other aetiologies are treated, good growth can be achieved throughout the course of the disease when parathyroid hormone (PTH) levels are in the normal range or slightly elevated. A direct correlation between PTH levels and growth has not been convincingly established, and the direct effect of PTH on growth has not been adequately described; furthermore, direct actions of PTH on the growth plate are unproven. The effects of PTH on growth stem from the pivotal role that PTH plays in the development of renal osteodystrophy. In severe secondary hyperparathyroidism, the growth plate is altered and growth is affected. At the other end of the spectrum, with an over-suppressed parathyroid gland, the rate of bone turnover and remodelling is markedly diminished, and some data suggest this is associated with poor growth. Most of the data available suggests that avoiding the development of significant bone disease through the strict control of PTH levels permits good growth. Absolute optimal ranges for PTH that maximise growth or minimise growth failure are not yet established.

Elevated fibroblast growth factor 23 levels in a newborn with secondary hypoparathyroidism

Fibroblast growth factor 23 (FGF-23) is a recently identified hormone that is of prime importance for phosphate homeostasis in humans. FGF-23 is secreted by osteocytes in response to phosphate-loading. It stimulates renal phosphate excretion and suppresses the formation of 1.25-dihydroxy-vitamin D by inhibiting renal 1α-hydroxylase activity. Knowledge about FGF-23 in early infancy is limited. We report here the case of a newborn with transient secondary hypoparathyroidism caused by maternal primary hyperparathyroidism during pregnancy. FGF-23 levels at birth were extremely high in the child (15.850 kilo-Relative Units per liter, kRU/L) (ie, ∼45 times higher than in the mother) and ∼7 times higher than in healthy newborns. The child's FGF-23 levels declined gradually and reached the normal adult range after ∼7 months. We discuss the potential physiologic significance of FGF-23 in newborns.