title: 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.
authors.affiliation: Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, United States of America.
authors.forename: Despina
authors.initials: D
authors.lastname: Sitara
authors.forename: Somi
authors.initials: S
authors.lastname: Kim
authors.forename: Mohammed S
authors.initials: MS
authors.lastname: Razzaque
authors.forename: Clemens
authors.initials: C
authors.lastname: Bergwitz
authors.forename: Takashi
authors.initials: T
authors.lastname: Taguchi
authors.forename: Christiane
authors.initials: C
authors.lastname: Schüler
authors.forename: Reinhold G
authors.initials: RG
authors.lastname: Erben
authors.forename: Beate
authors.initials: B
authors.lastname: Lanske
mesh_terms: D000818:Animals; D015519:Bone Density; D001842:Bone and Bones / Q000378:metabolism* / Q000503:physiopathology; D002113:Calcification, Physiologic; D002478:Cells, Cultured; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D017674:Hypophosphatemia / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018482:Muscle, Skeletal / Q000378:metabolism; D010006:Osteoblasts / Q000378:metabolism; D010641:Phenotype; D010710:Phosphates / Q000378:metabolism*; D044967:Serum / Q000737:chemistry; D012886:Skull / Q000378:metabolism; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000235:genetics / Q000378:metabolism; D014556:Urine / Q000737:chemistry
chemical_list: C000717428:Fgf23 protein, mouse; D010710:Phosphates; C493555:Slc34a1 protein, mouse; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23