Fibroblast Growth Factor 23 Expression Is Increased in Multiple Organs in Mice With Folic Acid-Induced Acute Kidney Injury

Fibroblast growth factor 23 (FGF23) regulates phosphate homeostasis and vitamin D metabolism. In patients with acute kidney injury (AKI), FGF23 levels rise rapidly after onset of AKI and are associated with AKI progression and increased mortality. In mouse models of AKI, excessive rise in FGF23 levels is accompanied by a moderate increase in FGF23 expression in bone. We examined the folic acid-induced AKI (FA-AKI) mouse model to determine whether other organs contribute to the increase in plasma FGF23 and assessed the vitamin D axis as a possible trigger for increased Fgf23 gene expression. Twenty-four hours after initiation of FA-AKI, plasma intact FGF23 and 1,25(OH)2D were increased and kidney function declined. FA-treated mice developed renal inflammation as shown by increased Tnf and Tgfb mRNA expression. Fgf23 mRNA expression was 5- to 15-fold upregulated in thymus, spleen and heart of FA-treated mice, respectively, but only 2-fold in bone. Ectopic renal Fgf23 mRNA expression was also detected in FA-AKI mice. Plasma FGF23 and Fgf23 mRNA expression in thymus, spleen, heart, and bone strongly correlated with renal Tnf mRNA expression. Furthermore, Vdr mRNA expression was upregulated in spleen, thymus and heart and strongly correlated with Fgf23 mRNA expression in the same organ. In conclusion, the rapid rise in plasma FGF23 in FA-AKI mice is accompanied by increased Fgf23 mRNA expression in multiple organs and increased Vdr expression in extra osseous tissues together with increased plasma 1,25(OH)2D and inflammation may trigger the rise in FGF23 in FA-AKI

A novel method for automated crystal visualization and quantification in murine folic acid-induced acute kidney injury

Here, we describe a novel method for the visualization and quantification of renal folic acid (FA) crystals in the rodent FA-induced acute kidney injury (FA-AKI) model. The protocol involves a straightforward histological approach followed by fully automated imaging and quantification steps. Applicability was confirmed by showing that the FA-AKI model is sex-dependent. The method can serve as a tool to aid in characterizing FA-AKI and to control for studies investigating prophylactic therapeutic avenues using FA-AKI

Autoimmune hyperphosphatemic tumoral calcinosis in a patient with FGF23 autoantibodies

Hyperphosphatemic familial tumoral calcinosis (HFTC)/hyperostosis-hyperphosphatemia syndrome (HHS) is an autosomal recessive disorder of ectopic calcification due to deficiency of or resistance to intact fibroblast growth factor 23 (iFGF23). Inactivating mutations in FGF23, N-acetylgalactosaminyltransferase 3 (GALNT3), or KLOTHO (KL) have been reported as causing HFTC/HHS. We present what we believe is the first identified case of autoimmune hyperphosphatemic tumoral calcinosis in an 8-year-old boy. In addition to the classical clinical and biochemical features of hyperphosphatemic tumoral calcinosis, the patient exhibited markedly elevated intact and C-terminal FGF23 levels, suggestive of FGF23 resistance. However, no mutations in FGF23, KL, or FGF receptor 1 (FGFR1) were identified. He subsequently developed type 1 diabetes mellitus, which raised the possibility of an autoimmune cause for hyperphosphatemic tumoral calcinosis. Luciferase immunoprecipitation systems revealed markedly elevated FGF23 autoantibodies without detectable FGFR1 or Klotho autoantibodies. Using an in vitro FGF23 functional assay, we found that the FGF23 autoantibodies in the patient's plasma blocked downstream signaling via the MAPK/ERK signaling pathway in a dose-dependent manner. Thus, this report describes the first case, to our knowledge, of autoimmune hyperphosphatemic tumoral calcinosis with pathogenic autoantibodies targeting FGF23. Identification of this pathophysiology extends the etiologic spectrum of hyperphosphatemic tumoral calcinosis and suggests that immunomodulatory therapy may be an effective treatment

Secreted Klotho and FGF23 in chronic kidney disease Stage 1 to 5: a sequence suggested from a cross-sectional study

Background Klotho and fibroblast growth factor 23 (FGF23) are key regulators of mineral metabolism in renal insufficiency. FGF23 levels have been shown to increase early in chronic kidney disease (CKD); however, the corresponding soluble Klotho levels at the different CKD stages are not known. Methods Soluble Klotho, FGF23, parathyroid hormone (PTH), 1,25-dihydroxy vitamin D3 (1,25D) and other parameters of mineral metabolism were measured in an observational cross-sectional study in 87 patients. Locally weighted scatter plot smoothing function of these parameters were plotted versus estimated glomerular filtration rate (eGFR) to illustrate the pattern of the relationship. Linear and non-linear regression analyses were performed to estimate changes in mineral metabolism parameters per 1mL/min/1.73 m2 decline. Results In CKD 1-5, Klotho and 1,25D linearly decreased, whereas both FGF23 and PTH showed a baseline at early CKD stages and then a curvilinear increase. Crude mean Klotho level declined by 4.8 pg/mL (95% CI 3.5-6.2 pg/mL, P < 0.0001) and 1,25D levels by 0.30 ng/L (95% CI 0.18-0.41 ng/L, P < 0.0001) as GFR declined by 1 mL/min/1.73 m2. After adjustment for age, gender, serum 25-hydroxyvitamin D levels and concomitant medications (calcium, supplemental vitamin D and calcitriol), we estimated that the mean Klotho change was 3.2 pg/mL (95% CI 1.2-5.2 pg/mL, P = 0.0019) for each 1 mL/min/1.73 m2 GFR change. FGF23 departed from the baseline at an eGFR of 47 mL/min/1.73 m2 (95% CI 39-56 mL/min/1.73 m2), whereas PTH departed at an eGFR of 34 mL/min/1.73 m2 (95% CI 19-50 mL/min/1.73 m2). Conclusions Soluble Klotho and 1,25D levels decrease and FGF23 levels increase at early CKD stages, whereas PTH levels increase at more advanced CKD stage

Phosphate Restriction Prevents Metabolic Acidosis and Curbs Rise in FGF23 and Mortality in Murine Folic Acid–Induced AKI

Background: In AKI, plasma FGF23 and P$_{i}$ rise rapidly and are independently associated with disease severity and outcome. Methods: The effects of normal (NP) and low (LP) dietary P$_{i}$ were investigated in mice with FA-AKI after 3, 24, and 48 hours and 14 days. Results: After 24 hours of AKI, the LP diet curbed the rise in plasma FGF23 and prevented that of parathyroid hormone and calcitriol as well as of osseous but not splenic or thymic Fgf23 mRNA expression. The absence of Pth prevented the rise in calcitriol and reduced the elevation of FGF23 in FA-AKI with the NP diet. Furthermore, the LP diet attenuated the rise in renal and plasma IL-6 and mitigated the decline in renal α-Klotho. After 48 hours, the LP diet further dampened renal IL-6 expression and resulted in lower urinary neutrophil gelatinase-associated lipocalin. In addition, the LP diet prevented the increased formation of CPPs. Fourteen days after AKI induction, the LP diet group maintained less elevated plasma FGF23 levels and had greater survival than the NP diet group. This was associated with prevention of metabolic acidosis, hypocalcemia, hyperkalemia, and cardiac electrical disturbances. Conclusions: This study reveals P$_{i}$-sensitive FGF23 expression in the bone but not in the thymus or spleen in FA-AKI and demonstrates that P$_{i}$ restriction mitigates CPP formation, inflammation, acidosis, and mortality in this model. These results suggest that dietary P$_{i}$ restriction could have prophylactic potential in patients at risk for AKI

Inflammation: a putative link between phosphate metabolism and cardiovascular disease

Dietary habits in the western world lead to increasing phosphate intake. Under physiological conditions, extraosseous precipitation of phosphate with calcium is prevented by a mineral buffering system composed of calcification inhibitors and tight control of serum phosphate levels. The coordinated hormonal regulation of serum phosphate involves fibroblast growth factor 23 (FGF23), αKlotho, parathyroid hormone (PTH) and calcitriol. A severe derangement of phosphate homeostasis is observed in patients with chronic kidney disease (CKD), a patient collective with extremely high risk of cardiovascular morbidity and mortality. Higher phosphate levels in serum have been associated with increased risk for cardiovascular disease (CVD) in CKD patients, but also in the general population. The causal connections between phosphate and CVD are currently incompletely understood. An assumed link between phosphate and cardiovascular risk is the development of medial vascular calcification, a process actively promoted and regulated by a complex mechanistic interplay involving activation of pro-inflammatory signalling. Emerging evidence indicates a link between disturbances in phosphate homeostasis and inflammation. The present review focuses on critical interactions of phosphate homeostasis, inflammation, vascular calcification and CVD. Especially, pro-inflammatory responses mediating hyperphosphatemia-related development of vascular calcification as well as FGF23 as a critical factor in the interplay between inflammation and cardiovascular alterations, beyond its phosphaturic effects, are addressed