Investigating the molecular etiology of disorders caused by disturbed mineral metabolism has been instrumental in identifying new circulating regulators of phosphate homeostasis, collectively referred to as 'phosphatonins.'We identified the phosphatonin Fibroblast growth factor-23 (FGF23) in a positional cloning approach to isolate the gene for autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to renal phosphate wasting, rickets/osteomalacia and fracture. The FGF23 co-receptor ?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, familial hyperphosphatemic tumoral calcinosis (TC). ?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 'cKL'. The biological activity of the cKL species is unknown. The novel preliminary studies presented herein make important, mechanistic connections regarding the regulation of Fgf23 production, and significantly modify the current models explaining phosphate homeostasis. In this regard, cKL delivery to wild type mice potently stimulates bone Fgf23 mRNA production, leading to highly elevated serum Fgf23. The treated animals manifest severe hypophosphatemia, alterations in renal 1,25(OH)2 vitamin D production pathways and hyperparathyroidism, with reduced bone mineral content and fractures. Further, blood cKL concentrations in vivo are correlated with changes in phosphate metabolism, and cKL demonstrates FGFR-dependent activity. Of significance, the mice treated with cKL are biological phenocopies of a patient with an ?Klotho gene translocation (t9:13), that resulted in markedly increased serum cKL concentrations. Collectively, these new findings demonstrate that the molecular mechanisms dictating Fgf23 production, as well as the control of Fgf23 bioactivity and expression via the ?KL isoforms remain to be defined. Thus, the central hypothesis to be tested is: the cKL form of ?KL controls an endocrine homeostatic axis between FGF23 target tissues and the skeleton by stimulating Fgf23 production. We expect our studies will provide novel, translational insight into rare, and common syndromes of altered FGF23 expression such as CKD-MBD, and into the basic biology of phosphate metabolism.

Public Health Relevance

The regulation of serum phosphate concentrations is critical for normal skeletal formation and cellular function. Pathophysiologic disturbances in phosphate homeostasis, such as those in autosomal dominant hypophosphatemic rickets (ADHR) and hyperphosphatemic tumoral calcinosis (TC), or common disorders such as chronic kidney disease-mineral bone disorder (CKD-MBD), lead to severe hormonal and skeletal disease. These disorders currently have inadequate treatments, and we expect that our proposed studies will reveal new mechanisms involved in phosphate homeostasis to aid in providing novel therapeutic targets.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Research Project (R01)
Project #
Application #
Study Section
Skeletal Biology Development and Disease Study Section (SBDD)
Program Officer
Kimmel, Paul
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Indiana University-Purdue University at Indianapolis
Schools of Medicine
United States
Zip Code
Hum, Julia M; O'Bryan, Linda; Smith, Rosamund C et al. (2017) Novel functions of circulating Klotho. Bone 100:36-40
Clinkenbeard, Erica L; White, Kenneth E (2017) Heritable and acquired disorders of phosphate metabolism: Etiologies involving FGF23 and current therapeutics. Bone 102:31-39
Hum, Julia M; O'Bryan, Linda M; Tatiparthi, Arun K et al. (2017) Chronic Hyperphosphatemia and Vascular Calcification Are Reduced by Stable Delivery of Soluble Klotho. J Am Soc Nephrol 28:1162-1174
Clinkenbeard, Erica L; Hanudel, Mark R; Stayrook, Keith R et al. (2017) Erythropoietin stimulates murine and human fibroblast growth factor-23, revealing novel roles for bone and bone marrow. Haematologica 102:e427-e430
Hum, Julia M; Clinkenbeard, Erica L; Ip, Colin et al. (2017) The metabolic bone disease associated with theHypmutation is independent of osteoblastic HIF1? expression. Bone Rep 6:38-43
Clinkenbeard, Erica L; Cass, Taryn A; Ni, Pu et al. (2016) Conditional Deletion of Murine Fgf23: Interruption of the Normal Skeletal Responses to Phosphate Challenge and Rescue of Genetic Hypophosphatemia. J Bone Miner Res 31:1247-57
Fleet, James C; Replogle, Rebecca A; Reyes-Fernandez, Perla et al. (2016) Gene-by-Diet Interactions Affect Serum 1,25-Dihydroxyvitamin D Levels in Male BXD Recombinant Inbred Mice. Endocrinology 157:470-81
Murali, Sathish K; Andrukhova, Olena; Clinkenbeard, Erica L et al. (2016) Excessive Osteocytic Fgf23 Secretion Contributes to Pyrophosphate Accumulation and Mineralization Defect in Hyp Mice. PLoS Biol 14:e1002427
Clinkenbeard, Erica L; White, Kenneth E (2016) Systemic Control of Bone Homeostasis by FGF23 Signaling. Curr Mol Biol Rep 2:62-71
Ichikawa, Shoji; Gray, Amie K; Padgett, Leah R et al. (2014) Genetic rescue of glycosylation-deficient Fgf23 in the Galnt3 knockout mouse. Endocrinology 155:3891-8

Showing the most recent 10 out of 14 publications