Conditional Deletion of Fgfr1 in the Proximal and Distal Tubule Identifies Distinct Roles in Phosphate and Calcium Transport

A postnatal role of fibroblast growth factor receptor-1 (FGFR1) in the kidney is suggested by its binding to a-Klotho to form an obligate receptor for the hormone fibroblast growth factor-23 (FGF-23). FGFR1 is expressed in both the proximal and distal renal tubular segments, but its tubular specific functions are unclear. In this study, we crossed Fgfr1(flox/flox) mice with either gamma-glutamyltransferase-Cre (gamma GT-Cre) or kidney specific-Cre (Ksp-Cre) mice to selectively create proximal tubule (PT) and distal tubule (DT) Fgfr1 conditional knockout mice (designated Fgfr1(PT-cKO) and Fgfr1(DT-cKO), respectively). Fgfr1(PT-cKO) mice exhibited an increase in sodium-dependent phosphate co-transporter expression, hyperphosphatemia, and refractoriness to the phosphaturic actions of FGF-23, consistent with a direct role of FGFR1 in mediating the proximal tubular phosphate responses to FGF-23. In contrast, Fgfr1(DT-cKO) mice unexpectedly developed hypercalciuria, secondary elevations of parathyroid hormone (PTH), hypophosphatemia and enhanced urinary phosphate excretion. Fgfr1(PT-cKO) mice also developed a curly tail/spina bifida-like skeletal phenotype, whereas Fgfr1(DT-cKO) mice developed renal tubular micro-calcifications and reductions in cortical bone thickness. Thus, FGFR1 has dual functions to directly regulate proximal and distal tubule phosphate and calcium reabsorption, indicating a physiological role of FGFR1 signaling in both phosphate and calcium homeostasis.