Fibroblast Growth Factor Receptors 1 and 2 Interact Differently with Heparin/Heparan Sulfate

Abstract

Heparan sulfate (HS) regulates the kinetics of fibroblast growth factor 2 (FGF2)-stimulated intracellular signaling and differentially activates cell proliferation of cells expressing different FGF receptors (FGFRs). Evidence suggests that HS interacts with both FGFs andFGFRs to form active ternary signaling complexes. Here we compare the interactions of two FGFRs with HS. We show that the ectodomains of FGFR1 IIIc and FGFR2 IIIc exhibit specific interactions with different characteristics for both heparin and porcine mucosal HS. These glycans are both known to activate FGF signaling via these receptors. FGFR2 interacts with a higher apparent affinity than FGFR1 despite both involving 6-O-, 2-O-, and N-sulfates. FGFR1 and FGFR2 bind heparin with mean association rate constants of 1.9 ؋ 10 5 and 2.1 ؋ 10 6 M ؊1 s ؊1 , respectively, and dissociation rate constants of 1.2 ؋ 10 ؊2 and 2.7 ؋ 10 ؊2 s ؊1 , respectively. These produced calculated affinities of 63 and 13 nM, respectively. Hence, FGFR1 and FGFR2 bind to heparin chains with markedly different kinetics and affinities. We propose a mechanistic model where the kinetic parameters of the HS/FGFR interaction are a key element regulating the formation of ternary complexes and the resulting FGF signaling outcomes. Fibroblast growth factors (FGFs)1 utilize a co-receptor system consisting of tyrosine kinase receptors (FGFRs) and heparan sulfate proteoglycans (HSPGs) (1, 2). The FGFRs belong to a family of five genes (FGFR1-5), from which alternative splicing generates diverse isoforms (3, 4). Heparan sulfate (HS) is a family of linear polysaccharides located at the surface of cells and in the extracellular matrix. HS chains are attached to core proteins, forming a class of glycoproteins called proteoglycans (5). HS consists of a backbone disaccharide repeat of alternating glucosamine and hexuronate monosaccharides on which are superimposed highly variable patterns of N-and O-linked sulfate groups and uronate epimerization; that is, iduronate (IdoUA) or glucuronate residues. This creates diverse molecular motifs, which present unique displays of sulfate, carboxyl, and hydroxyl groups (6). The modifications are clustered, producing the domain structure of HS; that is, stretches of sulfated saccharides (termed NS domains or S domains) separated by N-acetyl-rich saccharides (termed NA domains). The NS domains are rich in IdoUA and the NA domains in glucuronate. Heparin can be regarded as a special class of HS expressed by mast cells and also oligodendrocyte type-2 astrocyte progenitor cells (7). Heparin is highly sulfated, contains a high abundance of IdoUA, and lacks the ordered domain structure seen in HS (8). The sequences within heparin may in many aspects resemble those of highly sulfated NS domains of HS.It has been shown that heparin and HS bind to proteins and modulate their activities via specific sulfated sequences within the chains (commonly within the NS domains) (6,8). This is particularly apparent for FGFs, which recognize different structural motifs in HS...

Interactions of Multiple Heparin Binding Growth Factors with Neuropilin-1 and Potentiation of the Activity of Fibroblast Growth Factor-2

“…Thus, these growth factors recognize binding sites that would be found in the classic S-domain of the polysaccharide (50,51). FGFR1 probably also recognizes a similar structural motif (52). In marked contrast antithrombin III and CyPB, which bind Npn-1 poorly or not at all, have additional structural requirements in the polysaccharide.…”

DefectiveN-sulfation of heparan sulfate proteoglycans limits PDGF-BB binding and pericyte recruitment in vascular development

“…An alternative mechanistic model could involve establishment of a ternary complex in which, similar to FGFR signaling, both the receptor and the ligand directly interact with HS (Powell et al 2002). Such an interaction could function to enhance receptor signaling in the presence of ligand, but also help keep the receptor silent when ligand is not bound.…”

Cooperative Dimerization of Fibroblast Growth Factor 1 (FGF1) upon a Single Heparin Saccharide May Drive the Formation of 2:2:1 FGF1·FGFR2c·Heparin Ternary Complexes

“…The D2 domain of the FGFRs contains a short stretch of basic amino acids referred to as the K18K loop (12). This region mediates interaction with heparin and HS (13) and is essential for receptor function. The FGFs are also heparin-binding proteins, and it is now appreciated that signal transduction requires the formation of a ternary complex at the cell surface formed between FGF, FGFR, and heparan sulfate (HS).…”