1.A.108. The Fibroblast Growth Factor 2 (FGF2) Family
Unconventional secretory proteins lack signal peptides and their export from cells is not affected by brefeldin A, an inhibitor of protein transport along the classical secretory pathway (Nickel 2010). Export can be mediated by direct translocation across plasma membranes of cytoplasmic proteins such as fibroblast growth factor 2 which can be modified by phosphorylation involving the Tec protein kinase (La Venuta et al. 2016) so that it inserts into membranes, creating oligomeric pores that are intermediates in the secretion process (see Nickel and Seedorf 2008 and TC# 9.A.48). Tyrosyl phosphorylation of FGF2 followed by association with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) triggers formation of a lipidic membrane pore, essential for FGF2 secretion (Steringer et al. 2012). Formation of disulfide bridges is also essential for this process (Müller et al. 2015). The cytoplasmic domain of the Na+/K+ ATPase, ATP1A1 (α-subunit) may function as a direct interaction receptor for the process (Zacherl et al. 2015). Extracellular FGF2 is trapped by cell surface proteoglycans, and it then plays a role in tumor-indiced angiogenesis (La Venuta et al. 2015). Proteoglycans may also play a role in the secretion of the cytoskeletal Alzheimer's disease-associated tau protein (P10636; Pompa et al. 2017). Tau secretion correlated with tau phosphorylation and aggregation (Hannan et al. 2016).
Alpha-Klotho (α-Klotho) is a member of the Klotho family consisting of this protein and two other single-pass transmembrane proteins: betaKlotho (β-Klotho) and gammaKlotho (γ-Klotho); α-Klotho circulates in the blood. FGF23 is a member of the FGF superfamily of 22 genes/proteins. α-Klotho serves as a co-receptor with FGF receptors (FGFRs) to provide a receptacle for physiological FGF23 signaling, including regulation of phosphate metabolism (Hu et al. 2018). The extracellular domain of transmembrane α-Klotho is shed by secretases and released into blood circulation (soluble α-Klotho). Soluble alphaKlotho has both FGF23-independent and FGF23-dependent roles in phosphate homeostasis by modulating intestinal phosphate absorption, urinary phosphate excretion, and phosphate distribution into bone in concerted interactions with other calciophosphotropic hormones such as PTH and 1,25-(OH)2D. The direct role of alphaKlotho and FGF23 in the maintenance of phosphate homeostasis is partly mediated by modulation of type II Na+-dependent phosphate co-transporters in target organs. alphaKlotho and FGF23 are principal phosphotropic hormones, and manipulation of the alphaKlotho-FGF23 axis provides a therapeutic strategy for genetic and acquired phosphate disorders and for conditions with FGF23 excess or alphaKlotho deficiency such as chronic kidney disease (Hu et al. 2018).