Amelioration of the premature ageing-like features of Fgf-23 knockout mice by genetically restoring the systemic actions of FGF-23

Determination of the reference interval for urinary klotho to creatinine ratio of healthy dogs

For several years, alpha klotho has been considered as a candidate biomarker in chronic kidney disease (CKD), progression of CKD and CKD mineral bone disorders (CKD-MBD). The evidence on the relationship between klotho and kidney function is controversial in some areas. The aim of the study was to identify the influence of age, sex and breed on urinary alpha klotho, values in the early stages of CKD within the studied population and determine a reference interval in a group of healthy dogs. Significantly higher values were measured in older dogs over 6 years old (p = 0.026, p = 0.0007) and in the breed German Shepherd than Belgian Shepherd (p = 0.0401). On the basis of sex and in small breed dogs, no significant differences were noted. In dogs with CKD stage 2, alpha klotho values were significantly lower (p = 0.0135) than in healthy dogs. Within the studied population, a reference interval for urinary klotho to creatinine ratio (UrKl/Cr) was determined in the range of 3.94-23.55 pg/gCr. Since our findings show that alpha klotho is associated with older age, we assume that this may have influenced the results in the group of dogs with CKD stage 1 due to the presence of predominantly old dogs in this group. Future studies would be needed to consider age as a factor affecting urinary alpha klotho in dogs with CKD.

Non-Classical Effects of FGF23: Molecular and Clinical Features

This article reviews the role of fibroblast growth factor 23 (FGF23) protein in phosphate metabolism, highlighting its regulation of vitamin D, parathyroid hormone, and bone metabolism. Although it was traditionally thought that phosphate-calcium homeostasis was controlled exclusively by parathyroid hormone (PTH) and calcitriol, pathophysiological studies revealed the influence of FGF23. This protein, expressed mainly in bone, inhibits the renal reabsorption of phosphate and calcitriol formation, mediated by the α-klotho co-receptor. In addition to its role in phosphate metabolism, FGF23 exhibits pleiotropic effects in non-renal systems such as the cardiovascular, immune, and metabolic systems, including the regulation of gene expression and cardiac fibrosis. Although it has been proposed as a biomarker and therapeutic target, the inhibition of FGF23 poses challenges due to its potential side effects. However, the approval of drugs such as burosumab represents a milestone in the treatment of FGF23-related diseases.

Study of Serum Fibroblast Growth Factor 23 as a Predictor of Endothelial Dysfunction among Egyptian Patients with Diabetic Kidney Disease

Endothelial dysfunction in patients with diabetic nephropathy is caused by nontraditional factors in addition to common risk factors (e.g., hypertension) in people with normal kidney function. These nontraditional factors include factors involved in mineral bone disease in these patients. One of these factors is fibroblast growth factor 23 (FGF-23). We aimed to evaluate the relationship between flow-mediated dilatation (FMD) as a measure of endothelial dysfunction and FGF-23. This was a cross-sectional observational study that was conducted on 100 diabetic patients (Group I: 50 patients with nephropathy; Group II: 50 patients without nephropathy) and 50 healthy volunteers (Group III). Serum levels of intact FGF-23, interleukin-6, intact parathyroid hormone, and 25-hydroxyvitamin D (25-(OH)Vit D); estimated insulin resistance; and FMD were evaluated. FGF-23 was significantly higher in Group I (median: 101 pg/mL) and Group II (median: 101 pg/mL) than in Group III (median: 4 pg/mL) (P <0.001), but FGF-23 was not significantly different between Groups I and II. A significant positive correlation was found between serum levels of FGF-23 and phosphorus in Group I. A significant negative correlation was found between serum levels of FGF-23 and 25-(OH)Vit D in Group II. However, FGF-23 failed to show a significant correlation with FMD in patients with diabetic nephropathy. Our data suggest another factor that rises earlier than FGF-23 in diabetic nephropathy and causes endothelial dysfunction.

The impact of SGLT2 inhibitors on αKlotho in renal MDCK and HK-2 cells

αKlotho is a transmembrane protein predominantly expressed in the kidney serving as a co-receptor for phosphate homeostasis-regulating hormone FGF23 and has an extracellular domain that can be cleaved off and is a hormone. αKlotho deficiency results in accelerated aging and early onset of aging-associated diseases while its overexpression strongly expands the lifespan of mice. Moreover, αKlotho exerts health-beneficial anti-inflammatory, anti-neoplastic, anti-fibrotic, and anti-oxidant effects. Higher αKlotho levels are associated with better outcomes in renal and cardiovascular diseases. SGLT2 inhibitors are novel drugs in the treatment of diabetes by inhibiting renal glucose transport and have additional nephro- and cardioprotective effects. We explored whether SGLT2 inhibitors affect αKlotho gene expression and protein secretion. Experiments were performed in renal MDCK and HK-2 cells, and αKlotho transcripts were determined by qRT-PCR and Klotho protein by ELISA. SGLT2 inhibitors canagliflozin, sotagliflozin, and dapagliflozin enhanced whereas empagliflozin reduced αKlotho gene expression in MDCK cells. By the same token, canagliflozin, sotagliflozin, dapagliflozin, but not empagliflozin down-regulated p65 subunit of pro-inflammatory NFκB. In HK-2 cells, all SGLT2 inhibitors reduced αKlotho transcripts. Canagliflozin and sotagliflozin, however, increased Klotho protein concentration in the cell culture supernatant, an effect paralleled by up-regulation of ADAM17. Taken together, our investigations demonstrate complex effects of different SGLT2 inhibitors on αKlotho gene expression and protein secretion in renal MDCK and HK-2 cells.

Interactions between FGF23 and vitamin D

Fibroblast growth factor-23 (FGF23) controls the homeostasis of both phosphate and vitamin D. Bone-derived FGF23 can suppress the transcription of 1α-hydroxylase (1α(OH)ase) to reduce renal activation of vitamin D (1,25(OH)2D3). FGF23 can also activate the transcription of 24-hydroxylase to enhance the renal degradation process of vitamin D. There is a counter-regulation for FGF23 and vitamin D; 1,25(OH)2D3 induces the skeletal synthesis and the release of FGF23, while FGF23 can suppress the production of 1,25(OH)2D3 by inhibiting 1α(OH)ase synthesis. Genetically ablating FGF23 activities in mice resulted in higher levels of renal 1α(OH)ase, which is also reflected in an increased level of serum 1,25(OH)2D3, while genetically ablating 1α(OH)ase activities in mice reduced the serum levels of FGF23. Similar feedback control of FGF23 and vitamin D is also detected in various human diseases. Further studies are required to understand the subcellular molecular regulation of FGF23 and vitamin D in health and disease.

Salivary phosphate as a biomarker for human diseases

Phosphate is a common ingredient of the daily consumed foods and is absorbed in the intestine and is excreted in the urine through the kidney to maintain the homeostatic balance. For adults, the Recommended Dietary Allowance (RDA) for phosphorus is around 700 mg/day. The change in dietary habits resulted in far more phosphate consumption (almost double) than the RDA, contributing to increased cardiovascular diseases, kidney diseases, and tumor formation. Due to a lack of clinical appreciation for the long-term consequences of chronic phosphate burden on non-communicable disorders, it is rapidly becoming a global health concern. The possible association between dysregulated phosphate metabolism and obesity is not studied in-depth, mainly because such an association is believed to be nonexistent. However, in the animal model of obesity, serum phosphate level was higher than their non-obese controls. In a similar observation line, significantly higher salivary phosphate levels were detected in obese children compared to normal-weight children. Of clinical importance, despite the significant increase of salivary phosphate levels in obese children, the plasma phosphate levels did not change in samples collected from the same group of children. Such disparity between plasma and saliva raised the possibility that human salivary phosphate levels may be an early biomarker of childhood obesity.

miR-129 Blocks Secondary Hyperparathyroidism-Inducing Fgf23/αKlotho Signaling in Mice with Chronic Kidney Disease

BACKGROUND

Secondary hyperparathyroidism, a condition of excess parathyroid hormone (PTH, Pth) production, is often seen in chronic kidney disease (CKD) patients with elevated fibroblast growth factor 23 (FGF23, Fgf23). Elevated FGF23 levels stimulate secondary hyperparathyroidism-associated parathyroid αKlotho signaling. As overexpression of rationally selected microRNAs can suppress target gene activation, we hypothesized that microRNA-based suppression of parathyroid FGF23/αKlotho axis activity may be a potential strategy to combat secondary hyperparathyroidism.

METHODS

In vitro luciferase assays and human parathyroid adenoma cell experiments were used to determine miR-129-1-3p's effects on αKlotho expression in vitro. We also studied the effects of parathyroid-specific miR-129-1 overexpression (miR-129Ox) in CKD and non-CKD mice and parathyroid tissue cultures derived therefrom.

RESULTS

miR-129-1-3p directly targets the αKlotho mRNA strand in human parathyroid cells. miR-129Ox CKD mice and control CKD mice displayed comparable serum levels of calcium, phosphate, Fgf23, and 1,25-dihydroxyvitamin D (1,25(OH)₂D). However, miR-129Ox CKD mice displayed reduced parathyroid αKlotho expression and lower circulating Pth levels. In vitro culture of miR-129Ox CKD murine parathyroid tissue showed suppressed responses to Fgf23, with decreased Pth secretion and diminished cell proliferation after four days.

CONCLUSIONS

miR-129 negatively regulates pro-proliferative, Pth-inducing Fgf23/α​Klotho signaling in the parathyroid glands of CKD mice.

Influence of FGF23 and Klotho on male reproduction: Systemic vs direct effects

Currently, no treatment exists to improve semen quality in most infertile men. Here, we demonstrate systemic and direct effects of Fibroblast growth factor 23 (FGF23) and Klotho, which normally regulate vitamin D and mineral homeostasis, on testicular function. Direct effects are plausible because KLOTHO is expressed in both germ cells and spermatozoa and forms with FGFR1 a specific receptor for the bone-derived hormone FGF23. Treatment with FGF23 increased testicular weight in wild-type mice, while mice with global loss of either FGF23 or Klotho had low testicular weight, reduced sperm count, and sperm motility. Mice with germ cell-specific Klotho (gcKL) deficiency neither had a change in sperm count nor sperm motility. However, a tendency toward fewer pregnancies was detected, and significantly fewer Klotho heterozygous pups originated from gcKL knockdown mice than would be expected by mendelian inheritance. Moreover, gcKL mice had a molecular phenotype with higher testicular expression of Slc34a2 and Trpv5 than wild-type littermates, which suggests a regulatory role for testicular phosphate and calcium homeostasis. KLOTHO and FGFR1 were also expressed in human germ cells and spermatozoa, and FGF23 treatment augmented the calcium response to progesterone in human spermatozoa. Moreover, cross-sectional data revealed that infertile men with the highest serum Klotho levels had significantly higher serum Inhibin B and total sperm count than men with the lowest serum Klotho concentrations. In conclusion, this translational study suggests that FGF23 and Klotho influence gonadal function and testicular mineral ion homeostasis both directly and indirectly through systemic changes in vitamin D and mineral homeostasis.

A Land of Controversy: Fibroblast Growth Factor-23 and Uremic Cardiac Hypertrophy

Cardiac hypertrophy is a common feature in patients with CKD. Recent studies revealed that two phosphate regulators, fibroblast growth factor-23 and α-Klotho, are highly involved in the pathophysiologic process of CKD-induced cardiac hypertrophy. With decreasing renal function, elevated fibroblast growth factor-23 and decreased α-Klotho may contribute to cardiac hypertrophy by targeting the heart directly or by inducing systemic changes, such as vascular injury, hemodynamic disorders, and inflammation. However, several studies have demonstrated that disturbances in the fibroblast growth factor-23/α-Klotho axis do not lead to cardiac hypertrophy. In this review, we describe the cardiac effects of the fibroblast growth factor-23/α-Klotho axis and summarize recent progress in this field. In addition, we present not only the main controversies in this field but also provide possible directions to resolve these disputes.

FGF23 and the PTH response to paricalcitol in chronic kidney disease

BACKGROUND

The parathyroid glands are endowed both with receptors responsive to FGF23 and to 1,25 vitamin D. Vitamin D receptor (VDR) activation, besides lowering PTH, also raises serum FGF23. FGF23 has been implicated in parathyroid resistance to VDR activation but the issue has never been investigated in predialysis CKD patients.

METHODS

In the Paricalcitol and Endothelial Functio in Chronic Kidney Disease (PENNY) study (NCT01680198), a 12-week randomized trial in stage G3-4 CKD patients (placebo n = 44 and paricalcitol n = 44), we measured PTH and the active form of FGF23 with no missing value across the trial.

RESULTS

At baseline, serum FGF23 and PTH were inter-related (r = .54, P < .01). Paricalcitol reduced serum PTH (-75.1 pg/mL, 95% CI: -90.4 to -59.8; P < .001) and increased FGF23 (+107 pg/mL, 95% CI: 44-170 pg/mL, P = .001). Changes in the Ca × P product in response to paricalcitol were closely related to simultaneous FGF23 changes in an analysis adjusted for changes in serum calcium and phosphate (P < .001). Of note, baseline FGF23, appropriately adjusted for baseline PTH, was unrelated with the PTH response to paricalcitol (r = -.06, P = .72). Placebo did not change neither PTH nor FGF23.

CONCLUSION

Serum FGF23 and PTH are inter-related and changes in the Ca × P product induced by paricalcitol per se correlate with the FGF23 response to this drug. Independently of serum FGF23, the parathyroid glands of patients with moderate to severe CKD maintain an intact ability to respond to VDR activation.

Renal involvement in the pathogenesis of mineral and bone disorder in dystrophin-deficient mdx mouse

Duchenne muscular dystrophy is a severe muscular disorder, often complicated with osteoporosis, and impaired renal function has recently been featured. We aimed to clarify the involvement of renal function in the pathogenesis of mineral and bone disorder in mdx mice, a murine model of the disease. We clearly revealed renal dysfunction in adult mdx mice, in which dehydration and hypercalcemia were contributed. We also examined the effects of dietary phosphorus (P) overload on phosphate metabolism. Serum phosphate and parathyroid hormone (PTH) levels were significantly increased in mdx mice by dietary P in a dose-dependent manner; however, bone alkaline phosphatase levels were significantly lower in mdx mice. Additionally, bone mineral density in mdx mice were even worsened by increased dietary P in a dose-dependent manner. These results suggested that the uncoupling of bone formation and resorption was enhanced by skeletal resistance to PTH due to renal failure in mdx mice.

Dietary phosphate toxicity: an emerging global health concern

Phosphate is a common ingredient in many healthy foods but, it is also present in foods containing additives and preservatives. When found in foods, phosphate is absorbed in the intestines and filtered from the blood by the kidneys. Generally, any excess is excreted in the urine. In renal pathologies, however, such as chronic kidney disease, a reduced renal ability to excrete phosphate can result in excess accumulation in the body. This accumulation can be a catalyst for widespread damage to the cellular components, bones, and cardiovascular structures. This in turn can reduce mortality. Because of an incomplete understanding of the mechanism for phosphate homeostasis, and the multiple organ systems that can modulate it, treatment strategies designed to minimize phosphate burden are limited. The Recommended Dietary Allowance (RDA) for phosphorous is around 700 mg/day for adults, but the majority of healthy adult individuals consume far more phosphate (almost double) than the RDA. Studies suggest that low-income populations are particularly at risk for dietary phosphate overload because of the higher amounts of phosphate found in inexpensive, processed foods. Education in nutrition, as well as access to inexpensive healthy food options may reduce risks for excess consumption as well as a wide-range of disorders, ranging from cardiovascular diseases to kidney diseases to tumor formation. Pre-clinical and clinical studies suggest that dietary phosphate overload has toxic and prolonged adverse health effects. Improved regulations for reporting of phosphate concentrations on food labels are necessary so that people can make more informed choices about their diets and phosphate consumption. This is especially the case given the lack of treatments available to mitigate the short and long-term effects of dietary phosphate overload-related toxicity. Phosphate toxicity is quickly becoming a global health concern. Without measures in place to reduce dietary phosphate intake, the conditions associated with phosphate toxicity will likely to cause untold damage to the wellbeing of individuals around the world.

Interrelated role of Klotho and calcium-sensing receptor in parathyroid hormone synthesis and parathyroid hyperplasia

The pathogenesis of parathyroid gland hyperplasia is poorly understood, and a better understanding is essential if there is to be improvement over the current strategies for prevention and treatment of secondary hyperparathyroidism. Here we investigate the specific role of Klotho expressed in the parathyroid glands (PTGs) in mediating parathyroid hormone (PTH) and serum calcium homeostasis, as well as the potential interaction between calcium-sensing receptor (CaSR) and Klotho. We generated mouse strains with PTG-specific deletion of Klotho and CaSR and dual deletion of both genes. We show that ablating CaSR in the PTGs increases PTH synthesis, that Klotho has a pivotal role in suppressing PTH in the absence of CaSR, and that CaSR together with Klotho regulates PTH biosynthesis and PTG growth. We utilized the tdTomato gene in our mice to visualize and collect PTGs to reveal an inhibitory function of Klotho on PTG cell proliferation. Chronic hypocalcemia and ex vivo PTG culture demonstrated an independent role for Klotho in mediating PTH secretion. Moreover, we identify an interaction between PTG-expressed CaSR and Klotho. These findings reveal essential and interrelated functions for CaSR and Klotho during parathyroid hyperplasia.

Serum Phosphorus and Risk of Cardiovascular Disease, All-Cause Mortality, or Graft Failure in Kidney Transplant Recipients: An Ancillary Study of the FAVORIT Trial Cohort

BACKGROUND

Mild hyperphosphatemia is a putative risk factor for cardiovascular disease [CVD], loss of kidney function, and mortality. Very limited data are available from sizable multicenter kidney transplant recipient (KTR) cohorts assessing the potential relationships between serum phosphorus levels and the development of CVD outcomes, transplant failure, or all-cause mortality.

STUDY DESIGN

Cohort study.

SETTING & PARTICIPANTS

The Folic Acid for Vascular Outcome Reduction in Transplantation (FAVORIT) Trial, a large, multicenter, multiethnic, controlled clinical trial that provided definitive evidence that high-dose vitamin B-based lowering of plasma homocysteine levels did not reduce CVD events, transplant failure, or total mortality in stable KTRs.

PREDICTOR

Serum phosphorus levels were determined in 3,138 FAVORIT trial participants at randomization.

RESULTS

During a median follow-up of 4.0 years, the cohort had 436 CVD events, 238 transplant failures, and 348 deaths. Proportional hazards modeling revealed that each 1-mg/dL higher serum phosphorus level was not associated with a significant increase in CVD risk (HR, 1.06; 95% CI, 0.92-1.22), but increased transplant failure (HR, 1.36; 95% CI, 1.15-1.62) and total mortality risk associations (HR, 1.21; 95% CI, 1.04-1.40) when adjusted for treatment allocation, traditional CVD risk factors, kidney measures, type of kidney transplant, transplant vintage, and use of calcineurin inhibitors, steroids, or lipid-lowering drugs. These associations were strengthened in models without kidney measures: CVD (HR, 1.14; 95% CI, 1.00-1.31), transplant failure (HR, 1.72; 95% CI, 1.46-2.01), and mortality (HR, 1.34; 95% CI, 1.15-1.54).

LIMITATIONS

We lacked data for concentrations of parathyroid hormone, fibroblast growth factor 23, or vitamin D metabolites.

CONCLUSIONS

Serum phosphorus level is marginally associated with CVD and more strongly associated with transplant failure and total mortality in long-term KTRs. A randomized controlled clinical trial in KTRs that assesses the potential impact of phosphorus-lowering therapy on these hard outcomes may be warranted.

Persistent fibroblast growth factor 23 signalling in the parathyroid glands for secondary hyperparathyroidism in mice with chronic kidney disease

Secondary hyperparathyroidism, in which parathyroid hormone (PTH) is excessively secreted in response to factors such as hyperphosphataemia, hypocalcaemia, and low 1,25-dihydroxyvitamin D (1,25(OH)₂D) levels, is commonly observed in patients with chronic kidney disease (CKD), and is accompanied by high levels of fibroblast growth factor 23 (FGF23). However, the effect of FGF23 on the parathyroid glands (PG) remains controversial. To bind to FGF receptors, FGF23 requires αKlotho, which is highly expressed in the PG. Here, we examined the effects of Fgfr1–3, αKlotho, or Fgfr1–4 ablation specifically in the PG (conditional knockout, cKO). When mice with early to mid-stage CKD with and without cKO were compared, plasma concentrations of calcium, phosphate, FGF23, and 1,25(OH)₂D did not change significantly. In contrast, plasma PTH levels, which were elevated in CKD mice, were significantly decreased in cKO mice. PG from CKD mice showed augmentation of cell proliferation, which was significantly suppressed by cKO. Parathyroid tissue cultured for 4 days showed upregulation of PTH secretion and cell proliferation in response to FGF23. Both these effects were inhibited by cKO. These findings suggest that FGF23 is a long-term inducer of parathyroid cell proliferation and PTH secretion, and is one cause of secondary hyperparathyroidism in CKD.

Vitamin D, phosphate, and vasculotoxicity

Vascular calcification is a complex process that results in the ectopic deposition of calcium-phosphate hydroxyapatite. Medial and intimal vascular calcification is frequently present in patients with diabetes mellitus and chronic kidney disease (CKD), and markedly increases the morbidity and mortality of these patients. Increased serum levels of calcium and phosphate, along with the use of active vitamin D metabolites, are commonly implicated in the evolvement of vascular wall mineralization in CKD patients. Because CKD patients have lower serum levels of vitamin D, they are routinely prescribed vitamin D supplements that exert a dualistic role that is both healthful and harmful in these patients, perhaps protecting bone health, but at the expense of promoting vascular pathology. This review briefly explains how reducing the phosphate burden in CKD patients could minimize vitamin-D-associated vascular wall calcification.

Dysregulation of phosphate metabolism and conditions associated with phosphate toxicity

Phosphate homeostasis is coordinated and regulated by complex cross-organ talk through delicate hormonal networks. Parathyroid hormone (PTH), secreted in response to low serum calcium, has an important role in maintaining phosphate homeostasis by influencing renal synthesis of 1,25-dihydroxyvitamin D, thereby increasing intestinal phosphate absorption. Moreover, PTH can increase phosphate efflux from bone and contribute to renal phosphate homeostasis through phosphaturic effects. In addition, PTH can induce skeletal synthesis of another potent phosphaturic hormone, fibroblast growth factor 23 (FGF23), which is able to inhibit renal tubular phosphate reabsorption, thereby increasing urinary phosphate excretion. FGF23 can also fine-tune vitamin D homeostasis by suppressing renal expression of 1-alpha hydroxylase (1α(OH)ase). This review briefly discusses how FGF23, by forming a bone-kidney axis, regulates phosphate homeostasis, and how its dysregulation can lead to phosphate toxicity that induces widespread tissue injury. We also provide evidence to explain how phosphate toxicity related to dietary phosphorus overload may facilitate incidence of noncommunicable diseases including kidney disease, cardiovascular disease, cancers and skeletal disorders.

FGF23 affects the lineage fate determination of mesenchymal stem cells

FGF23 is a bone-derived hormone that regulates mineral metabolism by inhibiting renal tubular phosphate reabsorption and suppressing circulating 1,25(OH)2D and PTH levels. These effects are mediated by FGF-receptor binding and activation in the presence of its coreceptor Klotho, which is expressed in the distal tubules of the kidney. Recently, expression of Klotho in skeletal tissues has been reported, indicating a direct, yet unclear, extrarenal effect of FGF23 on cells involved with bone development and remodeling. In the present study, we found that bone marrow stromal cells harvested from Klotho null mice developed fewer osteoblastic but more adipocytic colonies than cells from wild-type mice. The underlying mechanism was explored by experiments on mouse C3H10T1/2 cells. We found that Klotho was weakly expressed and that FGF23 dose-dependently affected the lineage fate determination. The effects of FGF23 on cell differentiation can be diminished by SU 5402, a specific tyrosine kinase inhibitor for FGF receptors. Our results indicate that FGF23 directly affects the differentiation of bone marrow stromal cells.

Fibroblast growth factor 23: associations with cardiovascular disease and mortality in chronic kidney disease

Fibroblast growth factor-23 (FGF-23) has emerged as an important hormone involved in phosphorus and vitamin D homeostasis. Chronic kidney disease (CKD) is the most common clinical condition in which FGF-23 levels are persistently and markedly elevated. Abnormal phosphate homeostasis and high circulating levels of FGF-23 are early complications of CKD. Although increases in FGF-23 levels may help maintain serum phosphate levels in the normal range in CKD, the long-term effects of its sustained elevated levels are unclear. Patients with CKD have high risks of developing end-stage renal disease (ESRD), cardiovascular disease, and premature death. Recent prospective studies in populations with predialysis CKD, ESRD on hemodialysis, and kidney transplant recipients demonstrate that elevated FGF-23 levels are independently associated with cardiovascular events and mortality. It was originally thought that FGF-23 was only a biomarker of disturbed phosphate balance; however, recent studies have shown that FGF-23 can have a direct effect on the heart, inducing left ventricular hypertrophy. This suggests that elevated FGF-23 levels may be a novel mechanism that explains the poor cardiovascular outcomes in CKD patients. Interventional studies are required in order to clarify the relation of causality between FGF-23 and cardiovascular mortality in this population.

FGF23, klotho and vitamin D interactions: What have we learned from in vivo mouse genetics studies?

The molecular interactions of fibroblast growth factor 23 (FGF23), klotho and vitamin D coordinate to regulate the delicate phosphate levels of the body. Vitamin D can induce both FGF23 and klotho synthesis to influence renal phosphate balance. In the presence of klotho, FGF23 protein gains bioactivity to influence systemic phosphate homeostasis. Experimental studies have convincingly shown that in the absence of klotho, FGF23 is unable to regulate in vivo phosphate homeostasis. Furthermore, genetic inactivation of FGF23, klotho or both of the genes have resulted in markedly increased renal expression of 1-alpha hydroxylase [1α(OH)ase] and concomitant elevated serum levels of 1,25, dihydroxyvitamin D [1,25(OH)(2)D] in the mutant mice. Vitamin D can induce the expression of both FGF23 and klotho while, FGF23 can suppress renal expression of 1α(OH)ase to reduce 1,25(OH)(2)D activity. In this brief chapter, I will summarize the possible in vivo interactions of FGF23, klotho and vitamin D, in the light of recent mouse genetics studies.

Can features of phosphate toxicity appear in normophosphatemia?

Phosphate is an indispensable nutrient for the formation of nucleic acids and the cell membrane. Adequate phosphate balance is a prerequisite for basic cellular functions ranging from energy metabolism to cell signaling. More than 85% of body phosphate is present in the bones and teeth. The remaining phosphate is distributed in various soft tissues, including skeletal muscle. A tiny amount, around 1% of total body phosphate, is distributed both in the extracellular fluids and within the cells. Impaired phosphate balance can affect the functionality of almost all human systems, including muscular, skeletal, and vascular systems, leading to an increase in morbidity and mortality of the involved patients. Currently, measuring serum phosphate level is the gold standard to estimate the overall phosphate status of the body. Despite the biological and clinical significance of maintaining delicate phosphate balance, serum levels do not always reflect the amount of phosphate uptake and its distribution. This article briefly discusses the potential that some of the early consequences of phosphate toxicity might not be evident from serum phosphate levels.

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.

PTH ablation ameliorates the anomalies of Fgf23-deficient mice by suppressing the elevated vitamin D and calcium levels

Fibroblast growth factor 23 (FGF23) is a key regulator of mineral ion homeostasis. Genetic ablation of Fgf23 in mice leads to severe biochemical disorders including elevated serum 1,25-dihydroxyvitamin D [1,25(OH)2D], hypercalcemia, hyperphosphatemia, and marked decreased PTH levels. Because PTH stimulates 1,25(OH)2D production and increases serum calcium levels, we hypothesized that ablation of PTH from the Fgf23 knockout (Fgf23-/-) mice could suppress these affects, thus ameliorating the soft tissue and skeletal anomalies in these animals. In this study, we generated a genetic mouse model with dual ablation of the Fgf23/PTH genes. The data show that deletion of PTH does suppress the markedly higher serum 1,25(OH)2D and calcium levels observed in Fgf23-/- mice and results in much larger, heavier, and more active double-knockout mice with improved soft tissue and skeletal phenotypes. On the contrary, when we infused PTH (1-34) peptide into Fgf23-/- mice using osmotic minipumps, serum 1,25(OH)2D and calcium levels were increased even further, leading to marked reduction in trabecular bone. These results indicate that PTH is able to modulate the anomalies of Fgf23-/- mice by controlling serum 1,25(OH)2D and calcium levels.

Parathyroid hormone receptor signaling in osteocytes increases the expression of fibroblast growth factor-23 in vitro and in vivo

Mice with constitutive activation of parathyroid hormone (PTH) receptor signaling in osteocytes (DMP1-caPTHR1 transgenic mice) exhibit increased bone mass and remodeling, two of the recognized skeletal actions of PTH. Moreover, similar to PTH administration, DMP1-caPTHR1 mice exhibit decreased expression of the osteocyte-derived Wnt antagonist Sost/sclerostin. We now report that PTH receptor activation also regulates in vivo and in vitro the expression of fibroblast growth factor 23 (FGF23), an osteocyte product involved in inorganic phosphate (Pi) homeostasis and bone mineralization. Whole bones and osteocytes, but not osteoblasts, from DMP1-caPTHR1 mice exhibit elevated FGF23 expression, which is corrected in double transgenic mice overexpressing Sost in osteocytes. PTH, PTH related protein (PTHrP), or a cAMP stable analog, increase FGF23 transcripts in a time- and dose-dependent manner in osteocyte-containing calvarial cell cultures. Circulating FGF23 is also elevated in DMP1-caPTHR1 mice; however, plasma Pi or renal Pi reabsorption is not altered. Furthermore, the FGF23 receptor complex comprising FGFR1 and KLOTHO is expressed in osteoblastic cells; and FGFR1, GALNT3, as well as downstream targets of FGF23 signaling, are increased in osteocytes but not in osteoblasts from DMP1-caPTHR1 mice. Thus, PTH receptor signaling has the potential to modulate the endocrine and auto/paracrine functions of osteocytes by regulating FGF23 through cAMP- and Wnt-dependent mechanisms.

Molecular regulation of phosphate metabolism by fibroblast growth factor-23-klotho system

Phosphorus is an essential nutrient and is routinely assimilated through consumption of food. The body's need of phosphate is usually fulfilled by intestinal absorption of this element from the consumed food, whereas its serum level is tightly regulated by renal excretion or reabsorption. Sodium-dependent phosphate transporters, located in the luminal side of the proximal tubular epithelial cells, have a molecular control on renal phosphate excretion and reabsorption. The systemic regulation of phosphate metabolism is a complex multiorgan process, and the identification of fibroblast growth factor-23 (FGF23)-Klotho system as a potent phosphatonin has provided new mechanistic insights into the homeostatic control of phosphate. Hypophosphatemia as a result of an increase in urinary phosphate wasting after activation of the FGF23-Klotho system is a common phenomenon, observed in both animal and human studies, whereas suppression of the FGF23-Klotho system leads to the development of hyperphosphatemia. This article will briefly summarize how delicate interactions of the FGF23-klotho system can regulate systemic phosphate homeostasis.

Phosphate toxicity: new insights into an old problem

Phosphorus is an essential nutrient required for critical biological reactions that maintain the normal homoeostatic control of the cell. This element is an important component of different cellular structures, including nucleic acids and cell membranes. Adequate phosphorus balance is vital for maintaining basic cellular functions, ranging from energy metabolism to cell signalling. In addition, many intracellular pathways utilize phosphate ions for important cellular reactions; therefore, homoeostatic control of phosphate is one of the most delicate biological regulations. Impaired phosphorus balance can affect the functionality of almost every human system, including musculoskeletal and cardiovascular systems, ultimately leading to an increase in morbidity and mortality of the affected patients. Human and experimental studies have found that delicate balance among circulating factors, like vitamin D, PTH (parathyroid hormone) and FGF23 (fibroblast growth factor 23), are essential for regulation of physiological phosphate balance. Dysregulation of these factors, either alone or in combination, can induce phosphorus imbalance. Recent studies have shown that suppression of the FGF23-klotho system can lead to hyperphosphataemia with extensive tissue damage caused by phosphate toxicity. The cause and consequences of phosphate toxicity will be briefly summarized in the present review.

Vitamin D deficiency and toxicity in chronic kidney disease: in search of the therapeutic window

Both vitamin D deficiency and vitamin D toxicity are associated with cardiovascular complications in chronic kidney disease (CKD). Clinical and experiment data indicate that the association of vitamin D levels with cardiovascular disease is best illustrated as a biphasic, or U-shaped, curve. Children and adolescents with CKD need vitamin D due to the demands of a growing skeleton, to prevent renal rickets. However, this therapy carries the risk of severe side effects and chronic toxicity. Observational studies show that vitamin D deficiency and toxicity are frequently present in patients with CKD. In view of the importance of cardiovascular complications for the long-term survival of young patients, these findings demand a judicious use of vitamin D preparations. In clinical practice, the therapeutic window is rather small, presenting a therapeutic challenge to avoid both vitamin D deficiency and toxicity.

FGF-23 and vascular dysfunction in patients with stage 3 and 4 chronic kidney disease

Studies in animals show that fibroblast growth factor (FGF)-23 interferes with vascular reactivity induced by the nitric oxide (NO) system. To investigate the relationship between circulating FGF-23 levels and the response of forearm blood flow to ischemia (flow-mediated vasodilatation, FMD) and nitroglycerin, we tested 183 patients with stage 3-4 chronic kidney disease (CKD). None of them had cardiovascular complications or were taking drugs interfering with vascular function. Patients with FGF-23 levels above the median had significantly lower glomerular filtration rate, FMD, and fetuin-A levels (an anti-inflammatory molecule and potent inhibitor of calcification). They also had higher proteinuria and phosphate levels when compared to patients whose FGF-23 levels were below the median. The response to nitroglycerin was not different between the two groups. Multiple regression analysis showed that the relationship between FGF-23 and FMD was only modestly sensitive to adjustment for classical risk factors, biomarkers of bone mineral metabolism, high-sensitivity C-reactive protein, and homeostatic model assessment index. Adjustment for asymmetrical dimethyl arginine (ADMA) weakened the strength of this link; however, it remained highly significant. There was no independent association between FGF-23 and nitroglycerin. Thus, attenuation of FMD by ADMA suggests that this endogenous inhibitor of NO synthase may, in part, mediate the vascular effects of FGF-23 in patients with CKD.

Vitamin D, nervous system and aging

This is a mini-review of vitamin D(3), its active metabolites and their functioning in the central nervous system (CNS), especially in relation to nervous system pathologies and aging. The vitamin D(3) endocrine system consists of 3 active calcipherol hormones: calcidiol (25OHD(3)), 1alpha-calcitriol (1alpha,25(OH)2D(3)) and 24-calcitriol (24,25(OH)2D(3)). The impact of the calcipherol hormone system on aging, health and disease is discussed. Low serum calcidiol concentrations are associated with an increased risk of several chronic diseases including osteoporosis, cancer, diabetes, autoimmune disorders, hypertension, atherosclerosis and muscle weakness all of which can be considered aging-related diseases. The relationship of many of these diseases and aging-related changes in physiology show a U-shaped response curve to serum calcidiol concentrations. Clinical data suggest that vitamin D(3) insufficiency is associated with an increased risk of several CNS diseases, including multiple sclerosis, Alzheimer's and Parkinson's disease, seasonal affective disorder and schizophrenia. In line with this, recent animal and human studies suggest that vitamin D insufficiency is associated with abnormal development and functioning of the CNS. Overall, imbalances in the calcipherol system appear to cause abnormal function, including premature aging, of the CNS.

The FGF23-Klotho axis: endocrine regulation of phosphate homeostasis

Appropriate levels of phosphate in the body are maintained by the coordinated regulation of the bone-derived growth factor FGF23 and the membrane-bound protein Klotho. The endocrine actions of FGF23, in association with parathyroid hormone and vitamin D, mobilize sodium-phosphate cotransporters that control renal phosphate transport in proximal tubular epithelial cells. The availability of an adequate amount of Klotho is essential for FGF23 to exert its phosphaturic effects in the kidney. In the presence of Klotho, FGF23 activates downstream signaling components that influence the homeostasis of phosphate, whereas in the absence of this membrane protein, it is unable to exert such regulatory effects, as demonstrated convincingly in animal models. Several factors, including phosphate and vitamin D, can regulate the production of both FGF23 and Klotho and influence their functions. In various acquired and genetic human diseases, dysregulation of FGF23 and Klotho is associated with vascular and skeletal anomalies owing to altered phosphate turnover. In this Review, I summarize how the endocrine effects of bone-derived FGF23, in coordination with Klotho, can regulate systemic phosphate homeostasis, and how an inadequate balance of these molecules can lead to complications that are caused by abnormal mineral ion metabolism.

FGF23-mediated regulation of systemic phosphate homeostasis: is Klotho an essential player?

Understanding the physiological regulation of mineral ion metabolism is essential for determining the pathomechanisms of skeletal, vascular, and renal diseases associated with an abnormal regulation of calcium and phosphate homeostasis. Normal calcium and phosphate balance is delicately maintained by endocrine factors that coordinate to influence the functions of the intestine, bone, parathyroid gland, and kidney. Under physiological conditions, the kidneys play an important role in maintaining normal mineral ion balance by fine-tuning the amount of urinary excretion of calcium and phosphate according to the body's needs. Fibroblast growth factor (FGF)23 regulates urinary phosphate excretion to maintain systemic phosphate homeostasis. The exact mode of action of the phosphaturic effects of FGF23 is not fully understood and is an intense area of research. Studies suggest, however, that FGF23, by interacting with FGF receptors, can initiate downstream signaling events and that Klotho, a transmembrane protein, facilitates the interaction of FGF23 with its receptor. FGF23 can inhibit the activities of 1-alpha-hydroxylase and sodium-phosphate cotransporter in the kidney to influence the overall systemic phosphate balance. This article briefly summarizes how FGF23 might coordinately regulate systemic phosphate homeostasis and how Klotho is involved in such regulation.

Reversal of mineral ion homeostasis and soft-tissue calcification of klotho knockout mice by deletion of vitamin D 1alpha-hydroxylase

Changes in the expression of klotho, a beta-glucuronidase, contribute to the development of features that resemble those of premature aging, as well as chronic renal failure. Klotho knockout mice have increased expression of the sodium/phosphate cotransporter (NaPi2a) and 1alpha-hydroxylase in their kidneys, along with increased serum levels of phosphate and 1,25-dihydroxyvitamin D. These changes are associated with widespread soft-tissue calcifications, generalized tissue atrophy, and a shorter lifespan in the knockout mice. To determine the role of the increased vitamin D activities in klotho knockout animals, we generated klotho and 1alpha-hydroxylase double-knockout mice. These double mutants regained body weight and developed hypophosphatemia with a complete elimination of the soft-tissue and vascular calcifications that were routinely found in klotho knockout mice. The markedly increased serum fibroblast growth factor 23 and the abnormally low serum parathyroid hormone levels, typical of klotho knockout mice, were significantly reversed in the double-knockout animals. These in vivo studies suggest that vitamin D has a pathologic role in regulating abnormal mineral ion metabolism and soft-tissue anomalies of klotho-deficient mice.

Klotho ablation converts the biochemical and skeletal alterations in FGF23 (R176Q) transgenic mice to a Klotho-deficient phenotype

Transgenic mice overexpressing fibroblast growth factor (FGF23) (R176Q) (F(Tg)) exhibit biochemical {hypophosphatemia, phosphaturia, abnormal 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] metabolism} and skeletal (rickets and osteomalacia) abnormalities attributable to FGF23 action. In vitro studies now implicate the aging-related factor Klotho in the signaling mechanism of FGF23. In this study, we used a mouse genetic approach to validate in vivo the pivotal role of Klotho in the metabolic and skeletal derangements associated with FGF23 (R176Q) overexpression. To this end, we crossed mice heterozygous for the hypomorphic Klotho allele (Kl(+/-)) to F(Tg) mice and obtained F(Tg) transgenic mice homozygous for the Kl-hypomorphic allele (F(Tg)/Kl(-/-)). Mice were killed on postnatal day 50, and serum and tissues were procured for analysis and comparison with F(Tg), wild-type, and Kl(-/-) controls. From 4 wk onward, F(Tg)/Kl(-/-) mice were clearly distinguishable from F(Tg) mice and exhibited a striking phenotypic resemblance to the Kl(-/-) controls. Serum analysis for calcium, phosphorus, parathyroid hormone, 1,25(OH)(2)D(3), and alkaline phosphatase activity confirmed the biochemical similarity between the F(Tg)/Kl(-/-) and Kl(-/-) mice and their distinctness from the F(Tg) controls. The characteristic skeletal changes associated with FGF23 (R176Q) overexpression were also dramatically reversed by the absence of Klotho. Hence the wide, unmineralized growth plates and the osteomalacic abnormalities apparent in trabecular and cortical bone were completely reversed in the F(Tg)/Kl(-/-) mice. Nevertheless, independent actions of Klotho on bone were suggested as manifested by alterations in mineralized bone, and in cortical bone volume which were observed in both the Kl(-/-) and F(Tr)/Kl(-/-) mutants. In summary, our findings substantiate in vivo the essential role of Klotho in the mechanism of action of FGF23 in view of the fact that Klotho ablation converts the biochemical and skeletal manifestations resulting from FGF23 overexpression to a phenotype consistent with Klotho deficiency.