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.
Pathophysiologic disturbances in phosphate homeostasis involving the hormone FGF23, and its co-receptor Klotho, such as those in rare Mendelian disorders autosomal dominant hypophosphatemic rickets (ADHR) and hyperphosphatemic familial tumoral calcinosis (hfTC), or common disorders such as chronic kidney disease (CKD), lead to severe mineral disease. These disorders currently have inadequate treatments. We expect that our proposed studies will reveal new mechanisms involved in phosphate homeostasis, providing novel therapeutic targets.
