Vladislav V. Kiselyov

Structural Basis for a Direct Interaction between FGFR1 and NCAM and Evidence for a Regulatory Role of ATP

The neural cell adhesion molecule (NCAM) promotes axonal outgrowth, presumably through an interaction with the fibroblast growth factor receptor (FGFR). NCAM also has a little-understood ATPase activity. We here demonstrate for the first time a direct interaction between NCAM (fibronectin type III [F3] modules 1 and 2) and FGFR1 (Ig modules 2 and 3) by surface plasmon resonance (SPR) analysis. The structure of the NCAM F3 module 2 was determined by NMR and the module was shown by NMR to interact with the FGFR1 Ig module 3 and ATP. The NCAM sites binding to FGFR and ATP were found to overlap and ATP was shown by SPR to inhibit the NCAM-FGFR binding, indicating that ATP probably regulates the NCAM-FGFR interaction. Furthermore, we demonstrate that the NCAM module was able to induce activation (phosphorylation) of FGFR and to stimulate neurite outgrowth. In contrast, ATP inhibited neurite outgrowth induced by the module.

Structural biology of NCAM homophilic binding and activation of FGFR

In this review, we analyse the structural basis of the homophilic interactions of the neural cell adhesion molecule (NCAM) and the NCAM‐mediated activation of the fibroblast growth factor receptor (FGFR). Recent structural evidence suggests that NCAM molecules form cis‐dimers in the cell membrane through a high affinity interaction. These cis‐dimers, in turn, mediate low affinity trans‐interactions between cells via formation of either one‐ or two‐dimensional ‘zippers’. We provide evidence that FGFR is probably activated by NCAM very differently from the way by which it is activated by FGFs, reflecting the different conditions for NCAM–FGFR and FGF–FGFR interactions. The affinity of FGF for FGFR is approximately 106 times higher than that of NCAM for FGFR. Moreover, in the brain NCAM is constantly present on the cell surface in a concentration of about 50 µm, whereas FGFs only appear transiently in the extracellular environment and in concentrations in the nanomolar range. We discuss the structural basis for the regulation of NCAM–FGFR interactions by two molecular ‘switches’, polysialic acid (PSA) and adenosine triphosphate (ATP), which determine whether NCAM acts as a signalling or an adhesion molecule.

An NCAM-derived FGF-receptor agonist, the FGL-peptide, induces neurite outgrowth and neuronal survival in primary rat neurons.

The Neural Cell Adhesion Molecule (NCAM) plays a crucial role in development of the central nervous system regulating cell migration, differentiation and synaptogenesis. NCAM mediates cell–cell adhesion through homophilic NCAM binding, subsequently resulting in activation of the fibroblast growth factor receptor (FGFR). NCAM‐mediated adhesion leads to activation of various intracellular signal transduction pathways, including the Ras‐mitogen activated protein kinase (MAPK) and the phosphatidylinositol‐3‐kinase (PI3K)‐Akt pathways. A synthetic peptide derived from the second fibronectin type III module of NCAM, the FGL peptide, binds to and induces phosphorylation of FGFR without prior homophilic NCAM binding. We here present evidence that this peptide is able to mimic NCAM heterophilic binding to the FGFR by inducing neuronal differentiation as reflected by neurite outgrowth through a direct interaction with FGFR in primary cultures of three different neuronal cell types all expressing FGFR subtype 1: dopaminergic, hippocampal and cerebellar granule neurons. Moreover, we show that the FGL peptide promotes neuronal survival upon induction of cell death in the same three cell types. The effects of the FGL peptide are shown to depend on activation of FGFR and the MAPK and PI3K intracellular signalling pathways, all three kinases being necessary for the effects of FGL on neurite outgrowth and neuronal survival.

Identification of a neuritogenic ligand of the neural cell adhesion molecule using a combinatorial library of synthetic peptides

The neural cell adhesion molecule (NCAM) plays a key role in neural development, regeneration, and learning. In this study, we identified a synthetic peptide-ligand of the NCAM Ig1 module by combinatorial chemistry and showed it could modulate NCAM-mediated cell adhesion and signal transduction with high potency. In cultures of dissociated neurons, this peptide, termed C3, stimulated neurite outgrowth by activating a signaling pathway identical to that activated by homophilic NCAM binding. A similar effect was shown for the NCAM Ig2 module, the endogenous ligand of NCAM Ig1. By nuclear magnetic resonance spectroscopy, the C3 binding site in the NCAM Ig1 module was mapped and shown to be different from the binding site of the NCAM Ig2 module. The C3 peptide may prove useful as a lead in development of therapies for neurodegenerative disorders, and the C3 binding site of NCAM Ig1 may represent a target for discovery of nonpeptide drugs.

Harmonic oscillator model of the insulin and IGF1 receptors’ allosteric binding and activation

The insulin and insulin‐like growth factor 1 receptors activate overlapping signalling pathways that are critical for growth, metabolism, survival and longevity. Their mechanism of ligand binding and activation displays complex allosteric properties, which no mathematical model has been able to account for. Modelling these receptors’ binding and activation in terms of interactions between the molecular components is problematical due to many unknown biochemical and structural details. Moreover, substantial combinatorial complexity originating from multivalent ligand binding further complicates the problem. On the basis of the available structural and biochemical information, we develop a physically plausible model of the receptor binding and activation, which is based on the concept of a harmonic oscillator. Modelling a network of interactions among all possible receptor intermediaries arising in the context of the model (35, for the insulin receptor) accurately reproduces for the first time all the kinetic properties of the receptor, and provides unique and robust estimates of the kinetic parameters. The harmonic oscillator model may be adaptable for many other dimeric/dimerizing receptor tyrosine kinases, cytokine receptors and G‐protein‐coupled receptors where ligand crosslinking occurs.

Metallothionein and a peptide modeled after metallothionein, EmtinB, induce neuronal differentiation and survival through binding to receptors of the low-density lipoprotein receptor family

Accumulating evidence suggests that metallothionein (MT)‐I and ‐II promote neuronal survival and regeneration in vivo. The present study investigated the molecular mechanisms underlying the differentiation and survival‐promoting effects of MT and a peptide modeled after MT, EmtinB. Both MT and EmtinB directly stimulated neurite outgrowth and promoted survival in vitro using primary cultures of cerebellar granule neurons. In addition, expression and surface localization of megalin, a known MT receptor, and the related lipoprotein receptor‐related protein‐1 (LRP) are demonstrated in cerebellar granule neurons. By means of surface plasmon resonance MT and EmtinB were found to bind to both megalin and LRP. The bindings were abrogated in the presence of receptor‐associated protein‐1, an antagonist of the low‐density lipoprotein receptor family, which also inhibited MT‐ and EmtinB‐induced neurite outgrowth and survival. MT‐mediated neurite outgrowth was furthermore inhibited by an anti‐megalin serum. EmtinB‐mediated inhibition of apoptosis occurred without a reduction of caspase‐3 activity, but was associated with reduced expression of the pro‐apoptotic B‐cell leukemia/lymphoma‐2 interacting member of cell death (BimS). Finally, evidence is provided that MT and EmtinB activate extracellular signal‐regulated kinase, protein kinase B, and cAMP response element binding protein. Altogether, these results strongly suggest that MT and EmtinB induce their neuronal effects through direct binding to surface receptors belonging to the low‐density lipoprotein receptor family, such as megalin and LRP, thereby activating signal transduction pathways resulting in neurite outgrowth and survival.

The neural cell adhesion molecule binds to fibroblast growth factor receptor 2.

The neural cell adhesion molecule (NCAM) can bind to and activate fibroblast growth factor receptor 1 (FGFR1). However, there are four major FGFR isoforms (FGFR1–FGFR4), and it is not known whether NCAM also interacts directly with the other three FGFR isoforms. In this study, we show by surface plasmon resonance analysis that NCAM can bind to FGFR2 with an affinity similar to that for the NCAM–FGFR1 interaction. However, the kinetic parameters for the NCAM–FGFR2 binding are different from those of the NCAM–FGFR1 binding. Both receptors were shown to cycle relatively fast between the NCAM bound and unbound states, although FGFR2 cycling was clearly faster (13 times) than the FGFR1 cycling. Moreover, ATP was more effective in inhibiting the binding of NCAM to FGFR1 than to FGFR2, indicating that the binding sites in NCAM for the two receptors are similar, but not identical.

Structural and Biological Properties of the Drosophila Insulin-Like Peptide 5 Show Evolutionary Conservation.

We report the crystal structure of two variants of Drosophila melanogaster insulin-like peptide 5 (DILP5) at a resolution of 1.85 Å. DILP5 shares the basic fold of the insulin peptide family (T conformation) but with a disordered B-chain C terminus. DILP5 dimerizes in the crystal and in solution. The dimer interface is not similar to that observed in vertebrates, i.e. through an anti-parallel β-sheet involving the B-chain C termini but, in contrast, is formed through an anti-parallel β-sheet involving the B-chain N termini. DILP5 binds to and activates the human insulin receptor and lowers blood glucose in rats. It also lowers trehalose levels in Drosophila. Reciprocally, human insulin binds to the Drosophila insulin receptor and induces negative cooperativity as in the human receptor. DILP5 also binds to insect insulin-binding proteins. These results show high evolutionary conservation of the insulin receptor binding properties despite divergent insulin dimerization mechanisms.

A Peptide Motif from the Second Fibronectin Module of the Neural Cell Adhesion Molecule, NCAM, NLIKQDDGGSPIRHY, is a Binding Site for the FGF Receptor

The mechanism of fibroblast growth factor receptor (FGFR) activation by the neural cell adhesion molecule (NCAM) is not well understood. A motif in the second NCAM fibronectin type III (FN3) module, termed FGL, has by means of nuclear magnetic resonance (NMR) and surface plasmon resonance (SPR) analyses been demonstrated to be involved in NCAM–FGFR interactions. An FGFR activation motif (FRM) in the first NCAM FN3 module also has been suggested to take part in NCAM interactions with FGFR. Here, we show for the first time that a peptide motif in the second NCAM FN3 module, different from the previously described FGL motif (NLIKQDDGGSPIRHY; termed BCL) binds and activates FGFR and induces FGFR-dependent neurite outgrowth in cultures of cerebellar granule neurons. Our results provide evidence that the BCL motif is one of the multiple FGFR binding sites in NCAM.

Structural basis of allosteric ligand-receptor interactions in the insulin/relaxin peptide family: implications for other receptor tyrosine kinases and G-protein-coupled receptors.

The insulin/relaxin superfamily of peptide hormones comprises 10 members in humans. The three members of the insulin‐related subgroup bind to receptor tyrosine kinases (RTKs), while four of the seven members of the relaxin‐like subgroup are now known to bind to G‐protein‐coupled receptors (GPCRs), the so‐called relaxin family peptide receptors (RXFPs). Both systems have a long evolutionary history and play a critical role in fundamental biological processes, such as metabolism, growth, survival and longevity, and reproduction. The structural biology and ligand‐binding kinetics of the insulin and insulin‐like growth factor I receptors have been studied in great detail, culminating in the recent crystal structure of the insulin receptor extracellular domain. Some of the fundamental properties of these receptors, including constitutive dimerization and negative cooperativity, have recently been shown to extend to other RTKs and GPCRs, including RXFPs, confirming kinetic observations made over 30 years ago.