Fibroblast growth factor receptor-alpha-Klotho complex (FGFR-alpha-Klotho)

The Fibroblast growth factor receptor-alpha-Klotho complex is a specialized signaling assembly required for the systemic activity of the bone-derived hormone FGF23 (Chen et al., 2018). While canonical FGFs signal through FGFRs in the presence of heparin/heparan sulfate, endocrine FGFs like FGF23 require the co-receptor alpha-Klotho to bind their cognate receptors (primarily FGFR1c, FGFR3c, and FGFR4) with high affinity (Kuro-o, 2019). This complex is primarily localized in the renal proximal and distal tubules and the parathyroid glands, where it serves as the master regulator of phosphate reabsorption and vitamin D metabolism (Urakawa et al., 2006). In the kidney, activation of the complex leads to the downregulation of sodium-phosphate cotransporters (NaPi-2a and NaPi-2c) and the suppression of 1-alpha-hydroxylase, thereby promoting phosphaturia and reducing active vitamin D levels (Sawada et al., 2020). Dysregulation of the FGFR-alpha-Klotho axis is a hallmark of several pathological states, most notably chronic kidney disease (CKD), where a decline in alpha-Klotho expression and a compensatory rise in FGF23 contribute to mineral bone disorders and cardiovascular complications (AHA, 2020). It is also the primary driver of hereditary hypophosphatemic conditions such as X-linked hypophosphatemia (XLH). Therapeutic interventions include the monoclonal antibody burosumab, which neutralizes excess FGF23 to restore phosphate balance, and small-molecule FGFR inhibitors used in oncology (FDA, 2018; FDA, 2019). Notably, the inhibition of FGFRs often results in hyperphosphatemia as a significant side effect, directly resulting from the blockade of FGF23 signaling through the FGFR-alpha-Klotho complex in the kidney (NIH, 2020).

Other names

FGF23-FGFR-Klotho complexKlotho-FGFR complexalpha-Klotho-FGFR complexFGFR-KL complexFibroblast growth factor receptor-alpha-Klotho ternary complex

Burosumab acts as an FGF23 antagonist, preventing its binding to the FGFR-alpha-Klotho complex; FGFR inhibitors block the intracellular tyrosine kinase activity of the receptor component; Recombinant alpha-Klotho serves as a co-receptor to restore signaling in deficiency states.