Genetic Evidence of Serum Phosphate-Independent Functions of FGF-23 on Bone

Abstract

Maintenance of physiologic phosphate balance is of crucial biological importance, as it is fundamental to cellular function, energy metabolism, and skeletal mineralization. Fibroblast growth factor-23 (FGF-23) is a master regulator of phosphate homeostasis, but the molecular mechanism of such regulation is not yet completely understood. Targeted disruption of the Fgf-23 gene in mice (Fgf-23−/−) elicits hyperphosphatemia, and an increase in renal sodium/phosphate co-transporter 2a (NaPi2a) protein abundance. To elucidate the pathophysiological role of augmented renal proximal tubular expression of NaPi2a in Fgf-23−/− mice and to examine serum phosphate–independent functions of Fgf23 in bone, we generated a new mouse line deficient in both Fgf-23 and NaPi2a genes, and determined the effect of genomic ablation of NaPi2a from Fgf-23−/− mice on phosphate homeostasis and skeletal mineralization. Fgf-23−/−/NaPi2a−/− double mutant mice are viable and exhibit normal physical activities when compared to Fgf-23−/− animals. Biochemical analyses show that ablation of NaPi2a from Fgf-23−/− mice reversed hyperphosphatemia to hypophosphatemia by 6 weeks of age. Surprisingly, despite the complete reversal of serum phosphate levels in Fgf-23−/−/NaPi2a−/−, their skeletal phenotype still resembles the one of Fgf23−/− animals. The results of this study provide the first genetic evidence of an in vivo pathologic role of NaPi2a in regulating abnormal phosphate homeostasis in Fgf-23−/− mice by deletion of both NaPi2a and Fgf-23 genes in the same animal. The persistence of the skeletal anomalies in double mutants suggests that Fgf-23 affects bone mineralization independently of systemic phosphate homeostasis. Finally, our data support (1) that regulation of phosphate homeostasis is a systemic effect of Fgf-23, while (2) skeletal mineralization and chondrocyte differentiation appear to be effects of Fgf-23 that are independent of phosphate homeostasis.

FGF23 Suppresses Chondrocyte Proliferation in the Presence of Soluble α-Klotho both in Vitro and in Vivo

“…ple, FGF23 has been recently shown to be responsible for the development of left ventricular hypertrophy through activating calcineurin-NFAT signaling pathway in mice (26). Non-canonical activity of FGF23 could be operative as well in chondrocytes as evidenced by the previous study in which Fgf23 and Slc34a1 genes were deleted in mice (10). The lack of Slc34a1 in Fgf23-deficient mice did not correct the decreased number of hypertrophic chondrocytes in Fgf23-deficient mice despite of the correction of serum phosphate levels, suggesting the presence of phosphate-independent action of FGF23 in chondrocytes; however, the precise mechanisms of phosphate-independent function of FGF23 in chondrocyte biology remain to be elucidated.…”

“…Contrary to a widely accepted tenet that membrane-bound ␣-Klotho is mandatory for FGF23 to activate its downstream signaling pathways in physiological conditions (7,8), accumulating evidence highlights the possibility that FGF23 may stimulate its downstream signaling pathways in cells that lack or have little expression of membranebound ␣-Klotho. Although membrane-bound ␣-Klotho is not expressed in the skeleton, FGF23 may be operative in skeletal cells (10,11). Sitara et al generated a mouse model where both Fgf23 and Slc34a1, encoding for the type IIa sodium-phosphate (Na ϩ /Pi) co-transporter, were deleted to reverse the hyperphosphatemia noted in Fgf23-null mice (10).…”

“…Although membrane-bound ␣-Klotho is not expressed in the skeleton, FGF23 may be operative in skeletal cells (10,11). Sitara et al generated a mouse model where both Fgf23 and Slc34a1, encoding for the type IIa sodium-phosphate (Na ϩ /Pi) co-transporter, were deleted to reverse the hyperphosphatemia noted in Fgf23-null mice (10). These double mutants exhibited similar skeletal phenotypes to Fgf23-null mice, despite a correction in serum phosphate levels, suggesting the possibility of a phosphate-independent action of FGF23 in the skeleton.…”

Entpd5 is essential for skeletal mineralization and regulates phosphate homeostasis in zebrafish

“…These mice are viable (31) but appear smaller than littermates, a phenotype often found in hypophosphatemic mice (4,28). Furthermore, the mice were shown to have increased serum alkaline phosphatase (31).…”

“…Indeed, Fgf23 is significantly down-regulated, and the sodium/phosphate cotransporter npt2a (NaPi2a) is significantly up-regulated in nob mutants. Fgf23 is known as a key regulator of phosphate homeostasis (28), and changes in FGF23 activity lead to human disorders associated with either phosphate wasting or retention (29). Fgf23 is a circulating hormone produced in the bone that mainly targets the kidneys to control the activity of Npt2a and Npt2c (30).…”