Project Summary/Abstract: Investigating the molecular etiology of disorders caused by disturbed mineral
metabolism has been instrumental in identifying new circulating regulators of phosphate homeostasis. We
identified Fibroblast growth factor-23 (FGF23) in a positional cloning approach to isolate the gene
responsible for autosomal dominant hypophosphatemic rickets (ADHR), characterized by
hypophosphatemia secondary to renal phosphate wasting, rickets/osteomalacia, and fracture. The FGF23
co-receptor alpha-Klotho (KL), acting in a heteromeric complex with a canonical FGF receptor (FGFR), is
required for normal phosphate metabolism. This is emphasized by the fact that KL loss of function
mutations lead to end-organ FGF23 resistance, and cause the phenotypic reciprocal disorder to ADHR,
hyperphosphatemic familial tumoral calcinosis (hfTC). In a similar manner, patients with chronic kidney
disease (CKD) demonstrate impaired FGF23-responsiveness due to a loss of functional kidney mass and
reduced Klotho expression, leading to increased serum phosphate concentrations and further increases in
FGF23 production. KL is expressed as a membrane-bound protein (`mKL') that mediates FGF23-dependent
signaling in target tissues, as well as a major circulating species that originates from the proteolytic
cleavage of mKL within its juxta-extracellular membrane domain to derive a soluble form or `sKL'. Although
the recent solving of the FGF23-sKL-FGFR1 triple crystal structure revealed insight into static sKL-FGF23
interactions, the complete scope of mKL versus sKL biological functions in the control of FGF23 and mineral
metabolism remains unclear due to a lack of appropriate in vivo models. Our initial studies in mice with
genetically reduced sKL expression showed aberrant FGF23 production in response to dietary phosphate
challenges. Unlike global KL-KO mice, this model has the advantage of a lifespan that allows extended
studies, providing new opportunities to gain critical insight into the regulation of FGF23 bioactivity in chronic
conditions. Collectively, our results support mechanistic aims to identify specific sKL-FGF23 interactions in
the control of phosphate metabolism, as well as to test sKL as a translational target in the treatment of
human disorders. The central hypothesis to be tested in this proposal is: the sKL form of Klotho is required
for normal FGF23-mediated phosphate handling and is protective during renal disease. We expect our
studies using dovetailed, cutting-edge in vivo and in vitro techniques to provide novel, translational insight
into the basic biology of phosphate metabolism, as well as into both rare and common syndromes of altered
Klotho and FGF23 expression.
