Nikolaj Kulahin

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.

Role of Glial Cell Line-Derived Neurotrophic Factor (GDNF)–Neural Cell Adhesion Molecule (NCAM) Interactions in Induction of Neurite Outgrowth and Identification of a Binding Site for NCAM in the Heel Region of GDNF

The formation of appropriate neuronal circuits is an essential part of nervous system development and relies heavily on the outgrowth of axons and dendrites and their guidance to their respective targets. This process is governed by a large array of molecules, including glial cell line-derived neurotrophic factor (GDNF) and the neural cell adhesion molecule (NCAM), the interaction of which induce neurite outgrowth. In the present study the requirements for NCAM-mediated GDNF-induced neurite outgrowth were investigated in cultures of hippocampal neurons, which do not express Ret. We demonstrate that NCAM-mediated GDNF-induced signaling leading to neurite outgrowth is more complex than previously reported. It not only involves NCAM-140 and the Src family kinase Fyn but also uses NCAM-180 and the fibroblast growth factor receptor. We find that induction of neurite outgrowth by GDNF via NCAM or by trans-homophilic NCAM interactions are not mutually exclusive. However, whereas NCAM-induced neurite outgrowth primarily is mediated by NCAM-180, we demonstrate that GDNF-induced neurite outgrowth involves both NCAM-140 and NCAM-180. We also find that GDNF-induced neurite outgrowth via NCAM differs from NCAM-induced neurite outgrowth by being independent of NCAM polysialylation. Additionally, we investigated the structural basis for GDNF–NCAM interactions and find that NCAM Ig3 is necessary for GDNF binding. Furthermore, we identify within the heel region of GDNF a binding site for NCAM and demonstrate that a peptide encompassing this sequence mimics the effects of GDNF with regard to NCAM binding, activation of intracellular signaling, and induction of neurite outgrowth.

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.

Modulation of the homophilic interaction between the first and second Ig modules of neural cell adhesion molecule by heparin

The second Ig module (IgII) of the neural cell adhesion molecule (NCAM) is known to bind to the first Ig module (IgI) of NCAM (so‐called homophilic binding) and to interact with heparan sulfate and chondroitin sulfate glycoconjugates. We here show by NMR that the heparin and chondroitin sulfate‐binding sites (HBS and CBS, respectively) in IgII coincide, and that this site overlaps with the homophilic binding site. Using NMR and surface plasmon resonance (SPR) analyses we demonstrate that interaction between IgII and heparin indeed interferes with the homophilic interaction between IgI and IgII. Accordingly, we show that treatment of cerebellar granule neurons (CGNs) with heparin inhibits NCAM‐mediated outgrowth. In contrast, treatment with heparinase III or chondroitinase ABC abrogates NCAM‐mediated neurite outgrowth in CGNs emphasizing the importance of the presence of heparan/chondroitin sulfates for proper NCAM function. Finally, a peptide encompassing HBS in IgII, termed the heparin‐binding peptide (HBP), is shown to promote neurite outgrowth in CGNs. These observations indicate that neuronal differentiation induced by homophilic NCAM interaction is modulated by interactions with heparan/chondroitin sulfates.

The Neural Cell Adhesion Molecule NCAM2/OCAM/RNCAM, a Close Relative to NCAM

Cell adhesion molecules (CAMs) constitute a large class of plasma membrane-anchored proteins that mediate attachment between neighboring cells and between cells and the surrounding extracellular matrix (ECM). However, CAMs are more than simple mediators of cell adhesion. The neural cell adhesion molecule (NCAM) is a well characterized, ubiquitously expressed CAM that is highly expressed in the nervous system. In addition to mediating cell adhesion, NCAM participates in a multitude of cellular events, including survival, migration, and differentiation of cells, outgrowth of neurites, and formation and plasticity of synapses. NCAM shares an overall sequence identity of ∼44% with the neural cell adhesion molecule 2 (NCAM2), a protein also known as olfactory cell adhesion molecule (OCAM) and Rb-8 neural cell adhesion molecule (RNCAM), and the region-for-region sequence homology between the two proteins suggests that they are transcribed from paralogous genes. However, very little is known about the function of NCAM2, although it originally was described more than 20 years ago. In this review, we summarize the known properties and functions of NCAM2 and describe some of the differences and similarities between NCAM and NCAM2.

A BRET assay for monitoring insulin receptor interactions and ligand pharmacology

The insulin receptor (IR) belongs to the receptor tyrosine kinase super family and plays an important role in glucose homeostasis. The receptor interacts with several large docking proteins that mediate signaling from the receptor, including the insulin receptor substrate (IRS) family and Src homology-2-containing proteins (Src). Here, we applied the bioluminescence resonance energy transfer 2 (BRET2) technique to study the IR signaling pathways. The interaction between the IR and the substrates IRS1, IRS4 and Shc was examined in response to ligands with different signaling properties. The association between IR and the interacting partners could successfully be monitored when co-expressing green fluorescent protein 2 (GFP2) tagged substrates with Renilla reniformis luciferase 8 (Rluc8) tagged IR. Through additional optimization steps, we developed a stable and flexible BRET2 assay for monitoring the interactions between the IR and its substrates. Furthermore, the insulin analogue X10 was characterized in the BRET2 assay and was found to be 10 times more potent with respect to IRS1, IRS4 and Shc recruitment compared to human insulin. This study demonstrates that the BRET2 technique can be applied to study IR signaling pathways, and that this assay can be used as a platform for screening and characterization of IR ligands.

Direct demonstration of NCAM cis‐dimerization and inhibitory effect of palmitoylation using the BRET2 technique

MINT‐8071483: NCAM140 (uniprotkb:P13591‐1) physically interacts (MI:0915) with NCAM140 (uniprotkb:P13591‐1) by competition binding (MI:0405)

Identification of neural cell adhesion molecule L1‐derived neuritogenic ligands of the fibroblast growth factor receptor

The neural cell adhesion molecule L1 plays an important role in axon growth, neuronal survival, and synaptic plasticity. We recently demonstrated that the L1 fibronectin type III (FN3) modules interact directly with the fibroblast growth factor (FGF) receptor (FGFR). Sequence alignment of individual L1 FN3 modules with various FGFs suggested that four sequence motifs located in the third and fifth L1 FN3 modules might be involved in interactions with FGFR. The present study found that corresponding synthetic peptides, termed elcamins 1, 2, 3, and 4, bind and activate FGFR in the absence of FGF1. Conversely, in the presence of FGF1, elcamins inhibited receptor phosphorylation, indicating that the peptides are FGFR partial agonists. Elcamins 1, 3, and 4 dose dependently induced neurite outgrowth in cultured primary cerebellar neurons. The neuritogenic effect of elcamins was dependent on FGFR activation, insofar as the effect was abolished by the receptor inhibition. Thus, the identified peptides act as L1 mimetics with regard to activation of FGFR and induction of neurite outgrowth.

Crystal Structure of the Ig1 Domain of the Neural Cell Adhesion Molecule NCAM2 Displays Domain Swapping

The crystal structure of the first immunoglobulin (Ig1) domain of neural cell adhesion molecule 2 (NCAM2/OCAM/RNCAM) is presented at a resolution of 2.7 A. NCAM2 is a member of the immunoglobulin superfamily of cell adhesion molecules (IgCAMs). In the structure, two Ig domains interact by domain swapping, as the two N-terminal beta-strands are interchanged. beta-Strand swapping at the terminal domain is the accepted mechanism of homophilic interactions amongst the cadherins, another class of CAMs, but it has not been observed within the IgCAM superfamily. Gel-filtration chromatography demonstrated the ability of NCAM2 Ig1 to form dimers in solution. Taken together, these observations suggest that beta-strand swapping could have a role in the molecular mechanism of homophilic binding for NCAM2.

Expression, crystallization and preliminary X-ray analysis of extracellular modules of the neural cell-adhesion molecules NCAM and L1.

Recombinant proteins consisting of either the four or five amino-terminal immunoglobulin (Ig) modules of the rat neural cell-adhesion molecule NCAM or the whole extracellular part [six Ig and five fibronectin type III (F3) modules] of mouse L1 have been expressed in Drosophila S2 cells. The proteins have been purified and crystallized. The crystals of the recombinant protein containing the four amino-terminal Ig modules of NCAM diffract X-rays to approximately 4 A resolution and belong to space group P622 or P6(3)22, with unit-cell parameters a = b = 258.7, c = 182.4 A. No diffraction was observed for the other two protein constructs. This is a step towards determining the structure of multimodular constructs of cell-adhesion molecules that exhibit high structural flexibility.