There are four forms of the FGF receptor, encoded by different genes, and designated FGFR1–FGFR4. Each has the common structural elements shown in Figure 1A. The extracellular portion of the receptor is composed of three immunoglobulin-like domains of which the two nearest the membrane (D2 and D3) are required for FGF binding. The purple section in loop D3 is subject to alternative splicing in FGFRs 1–3, giving rise to the subtypes FGFR1b (epithelial cells) and FGFR1c (mesenchymal cells), etc., shown in Figure 2. Table 1 gives a partial list of the presence of FGF receptors in various cells and tissues. FGFR has a single pass transmembrane region, a juxtamembrane region followed by a split tyrosine kinase domain, i.e., one in which the catalytic parts of the enzyme are separated by a run of noncatalytic amino acids.
Fig1. FGF receptor (FGFR) structure. A. FGFR monomer. This diagram represents the common structural features of each of the four separately encoded FGF receptors, FGFR1–FGFR4. The extracellular domain of the FGF monomer consists of three immunoglobulin (Ig)-like loops, D1, D2, and D3. Loops D2 and D3 comprise the ligand binding domain and are separated from loop D1 by a stretch of acidic amino acids referred to as the acid box. Alternative splicing of mRNA in loop D3 (purple) generates the –b and –c forms of FGFRS 1, 2, and 3 listed in Figure 2B (there is an –a form which is a truncated secreted version of the receptor, not shown). In order to form a stable dimer upon ligand binding (panel B) interaction between heparan sulfate and its binding site on the FGFR (red) is required. The FGF receptor has a single pass transmembrane domain (dark green). The intracellular portion consists of a juxtamembrane region followed by a split tyrosine kinase (TK) in which the TK catalytic domain is interrupted by a nonfunctional amino acid sequence. B. FGF-HS-mediated dimerization of FGFR (paracrine FGF subfamilies 1,4,7,8,9). In this diagram, The 2:2:2 complex of FGF, FGFR, and heparan sulfate (HS) is shown. HS is attached to membrane proteoglycan (PG), ensuring its abundance in the extracellular membrane environment, and is required for high-affinity binding of FGF to its receptor and stabilization of the complex shown. C. FGF-klotho-mediated activation of FGFR (endocrine) subfamily 19. FGFs that do not have a heparan sulfate (HS) binding site are released into the circulation and react with distant target cells that have both an FGFR and a membrane bound protein called klotho. The details (for example, dimerization) of klotho-FGFR interactions are not as well understood as those with heparan sulfate, but at least one molecule each of the ligand (FGF, gold), receptor, FGFR (green), and the co-receptor, klotho (blue), are required for activation of the receptor.
Fig2. Fibroblast growth factor (FGF) family. A. Structural schematic of FGF structure. The 22-member family of human fibroblast growth factors, each encoded by a separate gene, range in length from 155 to 267 amino acids. All share a common conserved globular core composed of 12 antiparallel β-sheets (blue). The degree of conservation of this core as well as some functional considerations have led to the grouping of the 22 FGFs into seven subfamilies as listed in parts A and B of the figure. These in turn can be placed into three groups based on differences in the N- and C-termini of the proteins which lead to distinct functional differences. Four of the five paracrine subfamilies that act on nearby cells have a signal sequence (SS) which allows them to be secreted from the cell and a heparin-binding sequence (HB) which ensures their accumulation and binding to receptors on nearby cells. FGFs 1 and 2 lack the N-terminal signal sequence and are released by damaged cells or by a non-Golgi exocytotic mechanism. FGFs 11-14, sometimes referred to as FGF homologous factors (FHF), comprise the intracrine family because, having no signal sequence to bring about their secretion, they work in the cells in which they are produced. The members of endocrine subfamily, FGFs-19, -21, and -23, lack a functional heparin binding site and thus are not concentrated in the vicinity of the cell but are released into the circulation to reach distant target cells. B. Binding of FGFs to the FGF receptor. The binding of each FGF in a mitogenic bioassay to the seven forms of the FGF receptor (see legend to Figure 17-5) is shown by a closed circle. All measurements are relative to FGF1 and, for the purposes of this table, 15% relative binding was the arbitrary cutoff for binding activity. Data is from Zhang et al. (2006). J. Biol. Chem 281:15694–15700 and Ornitz et al. (1996) J. Biol. Chem. 271: 15292–15297.
Table1. Distribution of FGF Receptors
Heparan sulfate (HS) is a chain of alternating sul fated glucuronic acid and N-acetyl glucosamine units and is attached to proteoglycan which is anchored to the cell membrane. The majority (the paracrine FGFs, subfamilies 1,4,7,8,9; Figure 2) of FGFs require HS binding to specific sites on both ligand and receptor for receptor activation. The HS binding site on the FGFR contains basic amino acids and is on the face of D2. The acid box, a stretch of about 20 amino acids with a predominance of aspartates, glutamates, and serines, along with the N-terminal loop, D1, interferes with the basic HS-binding site, and thus acts as a restraining influence on FGF activation. This phenomenon is called autoinhibition and it is the removal of this restraint by ligand binding that contributes to the activation of the receptor.
Lacking functional HS binding sites, members of the endocrine subfamily 19, FGFs 19, 21, and 23, are not retained in the HS-rich vicinity of the cells in which they are produced but rather are released into the circulation to be transported to distant target organs. In order to be responsive to one of these FGFs, a cell must have not only an FGF receptor but a membrane protein called klotho, which acts as a co-receptor (Figure 1C).
Klotho was originally discovered as an anti-aging protein in mice in 1997. Mice lacking the gene for this protein age rapidly and when it is overexpressed, aging is slowed. A few years later, disturbances in phosphate metabolism similar to some hyperphosphatemic diseases in humans (see Chapter 9) implicated klotho in FGF23/ FGFR binding in the kidney. A second form of klotho, termed β-klotho (while α was added to the name of the original protein), was recognized as a FGFR co-receptor for binding of FGFs 19 and 21. Both forms of klotho are single pass membrane proteins with a short intracellular domain. The extracellular domain consists of two tan dem β-glucuronidase-like segments. Figure 1C shows a schematic diagram of klotho in the context of one model for its interaction with the FGF receptor.
