Human FGFR4/α-Klotho Reporter Assay Kit

Target Background

The family of Fibroblast Growth Factors (FGFs) comprise approximately 23 members that are related by core sequence and structure conservation, with the majority of FGFs being secreted signaling proteins. Secreted FGFs are predominantly autocrine and paracrine factors, with only three members evolved to function as endocrine factors. FGFs bind and activate FGF Receptors (FGFRs) which, themselves, are members of the family of high-affinity tyrosine kinase receptors.

Paracrine FGFs show high affinity towards the extracellular matrix (ECM) component heparin sulfate (HS) and are thus retained in the ECM and function locally. In contrast, the atypical endocrine subfamily of FGFs, that comprise FGF-19, FGF-21, and FGF-23, have reduced affinity for HS and can therefore escape from the ECM into the circulation to reach target distant organs. However, this subfamily typically requires association with members of the Klotho family of proteins as cofactors for efficient binding to their cognate receptor(s).

FGF-23 is a principal regulator in the maintenance of serum phosphorus concentration by inhibiting renal tubular phosphate reabsorption. Unfortunately, various FGF-23-mediated disorders exist. The most common of these is X-linked hypophosphoatemia (XLH). Additional disorders of FGF-23 signaling include autosomal dominant and recessive hypophosphophatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Consequently, FGF-23 and FGFR4/α-Klotho command considerable interest as therapeutic targets in drug development and drug safety screening.

FGFR4 and α-Klotho are both single-pass transmembrane proteins. FGFR4 has an extracellular ligand-binding domain, transmembrane domain, and intracellular tyrosine kinase domain. It has been established that FGFR4 association with the co-receptor α-Klotho generates a scaffold that is essential for endocrine growth factor binding interactions, such as those with FGF-23. Following growth factor binding, the activated tyrosine kinase activities of the FGFR initiate intracellular signaling cascades that may include RAS-MAPK, PI3-AKT, PLCγ and/or STAT pathways. For example, activation of the PLCγ pathway leads to an increase of intracellular calcium. One prominent outcome of the FGF/FGFR > PLCγ pathway is that calcineurin, a calcium-dependent phosphatase, dephosphorylates and activates the transcription factor NFAT. It is FGFR4/α-Klotho signal transduction via the Ca+2∙calcineurin / NFAT cascade that is exploited by the reporter cells provided in this kit.