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

Genetic ablation of fibroblast growth factor 23 from mice (Fgf-23 ^-/-) results in a short lifespan with numerous abnormal biochemical and morphological features. Such features include kyphosis, hypogonadism and associated infertility, osteopenia, pulmonary emphysema, severe vascular and soft tissue calcifications, and generalized atrophy of various tissues. To determine whether these widespread anomalies in Fgf-23^-/- mice can be ameliorated by genetically restoring the systemic actions of FGF-23, we generated Fgf-23^-/- mice expressing the human FGF-23 transgene in osteoblasts under the control of the 2.3 kb α1(I) collagen promoter (Fgf-23^-/- /hFGF-23-Tg double mutants). This novel mouse model is completely void of all endogenous Fgf-23 activity, but produces human FGF-23 in bone cells that is subsequently released into the circulation. Our results suggest that lack of Fgf-23 activities results in extensive premature ageing-like features and early mortality of Fgf-23^-/- mice, while restoring the systemic effects of FGF-23 significantly ameliorates these phenotypes, with the resultant effect being improved growth, restored fertility, and significantly prolonged survival of double mutants. With regard to their serum biochemistry, double mutants reversed the severe hyperphosphataemia, hypercalcaemia, and hypervitaminosis D found in Fgf-23^-/- littermates; rather, double mutants show hypophosphataemia and normal serum 1,25-dihydroxyvitamin D₃ levels similar to pure FGF-23 Tg mice. These changes were associated with reduced renal expression of NaPi2a and 1α-hydroxylase, compared to Fgf-23^-/- mice. FGF-23 acts to prevent widespread abnormal features by acting systemically to regulate phosphate homeostasis and vitamin D metabolism. This novel mouse model provides us with an in vivo tool to study the systemic effects of FGF-23 in regulating mineral ion metabolism and preventing multiple abnormal phenotypes without the interference of native Fgf-23.