The homophilic binding site of the neural cell adhesion molecule NCAM is directly involved in promoting neurite outgrowth from cultured neural retinal cells

CD56/NCAM mediates cell migration of human NK cells by promoting integrin-mediated adhesion turnover

Natural killer (NK) cells patrol tissue to mediate lysis of virally infected and tumorigenic cells. Human NK cells are typically identified by their expression of neural cell adhesion molecule (NCAM, CD56), yet despite its ubiquitous expression on NK cells, CD56 remains a poorly understood protein on immune cells. CD56 has been previously demonstrated to play roles in NK cell cytotoxic function and cell migration. Specifically, CD56-deficient NK cells have impaired cell migration on stromal cells and CD56 is localized to the uropod of NK cells migrating on stroma. Here, we show that CD56 is required for NK cell migration on ICAM-1 and is required for the establishment of persistent cell polarity and unidirectional actin flow. The intracellular domain of CD56 (NCAM-140) is required for its function and the loss of CD56 leads to enlarged actin foci and sequestration of phosphorylated Pyk2 accompanied by increased size and frequency of activated LFA-1 clusters. Together, these data identify a role for CD56 in regulating human NK cell migration through modulation of actin dynamics and integrin turnover.

CD56/NCAM mediates cell migration of human NK cells by promoting integrin-mediated adhesion turnover

Natural killer (NK) cells patrol tissue to mediate lysis of virally infected and tumorigenic cells. Human NK cells are typically identified by their expression of neural cell adhesion molecule (NCAM, CD56), yet, despite its ubiquitous expression on NK cells, CD56 remains a poorly understand protein on immune cells. CD56 has been previously demonstrated to play roles in NK cell cytotoxic function and cell migration. Specifically, CD56-deficient NK cells have impaired cell migration on stromal cells and CD56 is localized to the uropod of NK cells migrating on stroma. Here, we show that CD56 is required for NK cell migration on ICAM-1 and is required for the establishment of persistent cell polarity and unidirectional actin flow. The intracellular domain of CD56 (NCAM-140) is required for its function, and the loss of CD56 leads to enlarged actin foci and sequestration of phosphorylated Pyk2, accompanied by increased size and frequency of activated LFA-1 clusters. Together, these data identify a role for CD56 in regulating human NK cell migration through modulation of actin dynamics and integrin turnover.

MicroRNA-125a-3p is involved in early behavioral disorders in stroke-afflicted rats through the regulation of Cadm2

Ischemic strokes carry a significant risk of mortality and recurrent vascular events. Recent studies suggest that changes in microRNAs (miRNAs or miRs) may affect the development of the stroke. However, few studies have investigated the role of miRNAs in behavioral disorder in early stroke. In the present study, animal models of middle cerebral artery occlusion (MCAO) are used, as well as a cell model of neurite outgrowth to further investigate the role of miRNAs in targeting synapse-associated proteins expression in early stroke. The authors used miRNA expression microarrays on RNA extracted from the cortex tissue samples from the rats of MCAO and control rats. Reverse transcription-quantitative polymerase chain reaction was conducted to verify the candidate miRNAs discovered by microarray analysis. Data indicated that miR-125a was significantly increased in the cortex of the model of MCAO, which were concomitant with that rats of MCAO at the same age displayed significant behavioral deficits. Bioinformatics analysis predicted the cell adhesion molecule 2 (Cadm2, mRNA) neurite outgrowth-associated protein is targeted by miR-125a. Overexpression of miR-125a reduced the level of Cadm2 expression in PC12 cell injury induced by free-serum. In contrast, inhibition of miR-125a using miR-125a inhibitors significantly resulted in higher levels of Cadm2 expression. In conclusion, miR-125a is involved in the behavioral disorder of animal models of MCAO by regulation of Cadm2.

Physical and biological regulation of neuron regenerative growth and network formation on recombinant dragline silks

Recombinant spider silks produced in transgenic goat milk were studied as cell culture matrices for neuronal growth. Major ampullate spidroin 1 (MaSp1) supported neuronal growth, axon extension and network connectivity, with cell morphology comparable to the gold standard poly-lysine. In addition, neurons growing on MaSp1 films had increased neural cell adhesion molecule (NCAM) expression at both mRNA and protein levels. The results indicate that MaSp1 films present useful surface charge and substrate stiffness to support the growth of primary rat cortical neurons. Moreover, a putative neuron-specific surface binding sequence GRGGL within MaSp1 may contribute to the biological regulation of neuron growth. These findings indicate that MaSp1 could regulate neuron growth through its physical and biological features. This dual regulation mode of MaSp1 could provide an alternative strategy for generating functional silk materials for neural tissue engineering.

Identification of NCAM-binding peptides promoting neurite outgrowth via a heterotrimeric G-protein-coupled pathway

A combinatorial library of undecapeptides was produced and utilized for the isolation of peptide binding to the fibronectin type 3 modules (F3I-F3II) of the neural cell adhesion molecule (NCAM). The isolated peptides were sequenced and produced as dendrimers. Two of the peptides (denoted ENFIN2 and ENFIN11) were confirmed to bind to F3I-F3II of NCAM by surface plasmon resonance. The peptides induced neurite outgrowth in primary cerebellar neurons and PC12E2 cells, but had no apparent neuroprotective properties. NCAM is known to activate different intracellular pathways, including signaling through the fibroblast growth factor receptor, the Src-related non-receptor tyrosine kinase Fyn, and heterotrimeric G-proteins. Interestingly, neurite outgrowth stimulated by ENFIN2 and ENFIN11 was independent of signaling through fibroblast growth factor receptor and Fyn, but could be inhibited with pertussis toxin, an inhibitor of certain heterotrimeric G-proteins. Neurite outgrowth induced by trans-homophilic NCAM was unaffected by the peptides, whereas knockdown of NCAM completely abrogated ENFIN2- and ENFIN11-induced neuritogenesis. These observations suggest that ENFIN2 and ENFIN11 induce neurite outgrowth in an NCAM-dependent manner through G-protein-coupled signal transduction pathways. Thus, ENFIN2 and ENFIN11 may be valuable for exploring this particular type of NCAM-mediated signaling.

Plexin B3 promotes neurite outgrowth, interacts homophilically, and interacts with Rin

Background

Plexins, known to date as receptors of semaphorins, are implicated in semaphorin-mediated axon repulsion and growth cone collapse. However, subtype-specific functions of the majority of the nine members of the mammalian plexin family are largely unknown. In order to investigate functional properties of B-plexins, we analyzed the expression of human and murine plexin B3 and expressed full-length human plexins B2 (B2) and B3 (B3) in NIH-3T3 cells.

Results

Unexpectedly, B3 strongly and B2 moderately stimulate neurite outgrowth of primary murine cerebellar neurons. Both plexins mediate Ca²⁺/Mg²⁺-dependent cell aggregation due to homophilic trans-interaction, which is strong in the case of B3 and moderate for B2. Using different deletion constructs we show that the sema domain of B3 is essential for homophilic interaction. Using yeast two-hybrid analysis, we identified the neuron-specific and calmodulin-binding Ras-related GTPase Rin as an interaction partner of the intracellular part of B3, but not of B2. Rin, also known for its neurite outgrowth-inducing characteristics, co-localizes and co-immunoprecipitates with B3 in co-transfected COS-7 cells.

Conclusion

Our data suggest an involvement of homophilic interaction of B3 in semaphorin-independent signaling mechanisms positively influencing neuronal morphogenesis or function. Furthermore the neuron-specific small GTPase Rin is involved in downstream signaling of plexin B3.

Backbone Dynamics of the First, Second, and Third Immunoglobulin Modules of the Neural Cell Adhesion Molecule (NCAM)

The neural cell adhesion molecule (NCAM) is a cell surface multimodular protein, which plays an important role in cell-cell adhesion by homophilic (NCAM-NCAM) and heterophilic (NCAM-non-NCAM molecules) binding. In the present study, the backbone dynamics of the first three immunoglobulin-like (Ig) modules of NCAM have been investigated by NMR spectroscopy. Ig1, Ig2, and Ig3 share low sequence identity but possess the same fold and have very similar three-dimensional structures. (15)N longitudinal and transverse relaxation rates and heteronuclear NOEs have been measured and subsequently analyzed by the axial symmetric Lipari-Szabo modelfree formalism to characterize fast (pico- to nanosecond) and slow (micro- to millisecond) motions in the three protein modules. We found that backbone motions of residues located in the beta-strand regions are generally restricted, while increased flexibility is observed in turns and loops. In all three modules, residues located in the segments connecting the C- and D-strand plus residues located in the segment connecting the E- and F-strand show significant chemical exchange on the micro- to millisecond time scale. In addition, a number of residues with small chemical exchange contribution seem to form contiguous regions in the beta sheets, suggesting that these motions might be correlated. Only few residues in the homophilic binding sites in the NCAM Ig1 and Ig2 modules show increased flexibility, indicating that the Ig1-Ig2-mediated NCAM homophilic binding does not depend on the local backbone mobility of the interacting modules.

Structure and interactions of NCAM Ig1-2-3 suggest a novel zipper mechanism for homophilic adhesion

The neural cell adhesion molecule, NCAM, mediates Ca(2+)-independent cell-cell and cell-substratum adhesion via homophilic (NCAM-NCAM) and heterophilic (NCAM-non-NCAM molecules) binding. NCAM plays a key role in neural development, regeneration, and synaptic plasticity, including learning and memory consolidation. The crystal structure of a fragment comprising the three N-terminal Ig modules of rat NCAM has been determined to 2.0 A resolution. Based on crystallographic data and biological experiments we present a novel model for NCAM homophilic binding. The Ig1 and Ig2 modules mediate dimerization of NCAM molecules situated on the same cell surface (cis interactions), whereas the Ig3 module mediates interactions between NCAM molecules expressed on the surface of opposing cells (trans interactions) through simultaneous binding to the Ig1 and Ig2 modules. This arrangement results in two perpendicular zippers forming a double zipper-like NCAM adhesion complex.

Increased NCAM-180 Immunoreactivity and Maintenance of L1 Immunoreactivity in Injured Optic Fibers of Adult Mice

The injury related expression of two axon-growth promoting cell adhesion molecules (CAMs), NCAM-180 which is developmentally downregulated and L1 which is regionally restricted, were compared in optic fibers in the adult mouse. The neuron-specific isoform of NCAM (NCAM-180) is present at very low levels in unlesioned adult optic axons. At 7 days after nerve crush, immunoreactivity was strongly and uniformly increased in optic axons within the nerve and throughout retina. Reactivity in surviving axons had returned to control levels at 4 weeks. To induce regrowth of adult retinal ganglion cell axons retinal explants were placed in culture. Strong NCAM-180 staining was observed on these regenerating optic axons. The neuronal cell adhesion molecule L1 is restricted to retina and to the unmyelinated segment of the optic nerve near the optic nerve head in unlesioned adult animals. Following nerve crush, L1 immunoreactivity was retained within retina and proximal nerve and novel staining was detected in the more distal segment of the optic nerve up to the lesion site where it persisted for at least eight months. The capacity of optic fibers to show increased NCAM-180 immunoreactivity and maintain L1 expression after a lesion may explain why these fibers exhibit relatively good potential for regeneration.

PC12 cells utilize the homophilic binding site of L1 for cell-cell adhesion but L1-alphavbeta3 interaction for neurite outgrowth

Treatment of PC12 cells with nerve growth factor induces their differentiation into sympathetic neuron-like cells and the concomitant expression of the neural cell adhesion molecule L1, a member of the Ig superfamily. To investigate the mechanism of L1-stimulated neurite outgrowth in PC12 cells, substrate-immobilized fusion proteins containing different extracellular domains of L1 were assayed for their neuritogenic activity. Surprisingly, domain Ig2 of L1, which was previously found to contain both homophilic binding and neuritogenic activities, failed to promote neurite outgrowth. In contrast, L1-Ig6 stimulated neurite outgrowth from PC12 cells. Despite this, homotypic binding of PC12 cells was significantly inhibited by antibodies against L1-Ig2, indicating that L1-L1 binding contributed to the intercellular adhesiveness of PC12 cells, but L1-stimulated neurite outgrowth depends on heterophilic interactions. Thus, PC12 cells provide a valuable model for the study of these two distinct functions of L1. Mutagenesis of L1-Ig6 highlighted the importance of the Arg-Gly-Asp motif in this domain for neuritogenesis. Inhibition studies using cyclic Arg-Gly-Asp-containing peptide and anti-integrin antibodies suggested the involvement of alphavbeta3 integrin. Furthermore, neurite outgrowth stimulated by L1-Ig6 was inhibited by lavendustin A and the MEK inhibitor PD98059, suggesting a signaling pathway that involves tyrosine kinase activation and the mitogen-activated protein kinase cascade.

Role of NCAM, cadherins, and microfilaments in cell-cell contact formation in TM4 immature mouse sertoli cells

To determine events that lead to the formation of intercellular contacts, we examined the spatial and temporal distribution of NCAM, cadherins, and F-actin in TM4 cells by immunofluorescence and laser scanning confocal microscopy. TM4 cells exhibited epithelioid characteristics and formed large overlapping lamella-like cell-cell contacts that contained a high concentration of NCAM. NCAM-rich lamellae formed from smaller NCAM patches at the ends of filopodia-like contacts between adjacent cells. Cadherins, as visualized by a pan-cadherin antibody, were present in a pattern distinctly different from that of NCAM. Although in filopodia-like contacts, both cadherins and NCAM were often concentrated at filopodial tips, in the larger lamella-like contacts that developed later, cadherins were located in an irregular punctate pattern only at the distal and more apical margins of the slanted NCAM-rich contact regions. Patterns of NCAM and microfilament (MF) bundle distribution were distinctly different, suggesting that the ends of these MF bundles were not physically linked to NCAM. By contrast, cadherins were concentrated at the ends of MF bundles at all stages of contact formation examined. Interestingly, this association of cadherins with MF bundles was mostly seen at the edge of the overlapping processes. In the lower cell process, MF bundles at the contact site were often arranged in random fashion, indicating an asymmetric distribution of MF in the junctional region. However, N-cadherin was enriched only at sites where MF bundles from both the upper and lower cell processes were aligned and terminated at the junctional membrane. Thus the organization of the actin cytoskeleton at cell-cell contact sites is influenced by the differential localization of different cadherins. These data also suggest that different mechanisms are involved in the accumulation of NCAM and cadherins in cell-cell contact regions.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

The Arg-Gly-Asp motif in the cell adhesion molecule L1 promotes neurite outgrowth via interaction with the alphavbeta3 integrin

The cell adhesion molecule L1 is a potent inducer of neurite outgrowth and it has been implicated in X-linked hydrocephalus and related neurological disorders. To investigate the mechanisms of neurite outgrowth stimulated by L1, attempts were made to identify the neuritogenic sites in L1. Fusion proteins containing different segments of the extracellular region of L1 were prepared and different neuronal cells were assayed on substrate-coated fusion proteins. Interestingly, both immunoglobulin (Ig)-like domains 2 and 6 (Ig2, Ig6) promoted neurite outgrowth from dorsal root ganglion cells, whereas neural retinal cells responded only to Ig2. L1 Ig2 contains a previously identified homophilic binding site, whereas L1 Ig6 contains an Arg-Gly-Asp (RGD) sequence. The neuritogenic activity of Ig6 was abrogated by mutations in the RGD site. The addition of RGD-containing peptides also inhibited the promotion of neurite outgrowth from dorsal root ganglion cells by glutathione S-transferase-Ig6, implicating the involvement of an integrin. The monoclonal antibody LM609 against alphavbeta3 integrin, but not an anti-beta1 antibody, inhibited the neuritogenic effects of Ig6. These data thus provide the first evidence that the RGD motif in L1 Ig6 is capable of promoting neurite outgrowth via interaction with the alphavbeta3 integrin on neuronal cells.

Monoclonal antibody interaction with the third immunoglobulin-like domain of N-CAM is sufficient to cause cell migration

Cellular adhesion molecules can influence a variety of biological mechanisms in the nervous system. These range from the processes of normal development and maintenance to neural plasticity and recovery following injury. The elucidation of the intricate contributions of these molecules will require the correlation of functional assays with specific molecules and the specific binding domains of such molecules with multiple signaling pathways. The data presented in this paper show that the monoclonal antibody anti-NCAM16, directed against the third immunoglobulin-like domain of the neural cell adhesion molecule N-CAM, is capable of stimulating the complex biological process of cell migration in primary embryonic motor neurons and human neuronal cell lines.

Structure/function relationships of axon-associated adhesion receptors of the immunoglobulin superfamily

Evidence is accumulating that axonal members of the Ig superfamily (IgSF) interact in a complex manner with other axonal Ig-like proteins and with proteins of the extracellular matrix. Studies investigating the structure/function relationships of these proteins have highlighted the importance of Ig-like domains near the amino terminus (N-proximal) as both necessary and sufficient for homophilic and heterophilic binding. Although efforts have been made in the past year to correlate the structure and neurite-outgrowth-promoting ability of axonal IgSF members, this work is still at an early stage.

Integrity of the homophilic binding site is required for the preferential localization of NCAM in intercellular contacts

The neural cell adhesion molecule NCAM is a member of the immunoglobulin (Ig) superfamily. NCAM can undergo homophilic binding and heterophilic interactions with cell surface components and is often concentrated at sites of intercellular contact. To investigate the molecular basis of this biased surface distribution, we examined L cell transfectants expressing wild-type or mutant forms of chick NCAM-140 by laser scanning confocal microscopy. Mutant NCAMs that lacked Ig-like domains 1, 2, 4, or 5 were preferentially localized in contact regions. However, the relative concentration of these mutant NCAMs in contact sites was substantially reduced compared with wild-type NCAM. In contrast, NCAM redistribution to intercellular contacts was abolished in cells expressing mutant NCAMs that either lacked Ig-like domain 3 or contained mutations in the homophilic binding site in this domain. In heterotypic contacts between PC12 cells and L cell transfectants, colocalization of rat NCAM and chick NCAM was again dependent on the integrity of the homophilic binding site of the NCAM expressed on L cells. These results provide evidence that homophilic binding is the main mechanism by which NCAM becomes redistributed to intercellular contacts. They also implicate a role for other Ig-like domains in the accumulation of NCAM at cell-cell contacts.

Differential effects of two hydrocephalus/MASA syndrome-related mutations on the homophilic binding and neuritogenic activities of the cell adhesion molecule L1

The cell adhesion molecule L1 plays an important role in neural development. We have previously demonstrated that the second immunoglobulin-like domain (Ig2) of L1 contains both homophilic binding and neuritogenic activities (Zhao, X., and Siu, C.-H. (1995) J. Biol. Chem. 270, 29413-29421). Recently, two mutations (R184Q and H210Q) within the Ig2 region of the human L1 gene have been shown to be responsible for X-linked hydrocephalus and the related MASA (mental retardation, aphasia, shuffling gait, and adducted thumbs) syndrome. Glutathione S-transferase-Ig2 fusion proteins containing these mutations were used to evaluate their effects on L1. The homophilic binding activity of fusion proteins and their ability to promote neurite outgrowth from retinal cells were examined. The R184Q mutation led to a complete loss of both homophilic binding and neuritogenic activities, while the H210Q mutation resulted only in a partial loss. These results provide, for the first time, direct demonstration of the deleterious effects of hydrocephalus/MASA mutations on two intrinsic properties of L1.

Colocalization of the homophilic binding site and the neuritogenic activity of the cell adhesion molecule L1 to its second Ig-like domain

The cell adhesion molecule L1 has been implicated in mediating cell-cell adhesion and in promoting neurite outgrowth. The extracellular region of L1 contains six immunoglobulin (Ig)-like domains in the amino-terminal region, followed by five fibronectin type III-like repeats. L1 is capable of undergoing homophilic binding as well as heterophilic interactions. To map the homophilic binding domain in L1, three glutathione S-transferase (GST) fusion proteins (GST-Ig1-2-3, GST-Ig4-5-6, and GST-Fn) were prepared and coupled to Covaspheres and their homophilic binding activity was determined using the Covasphere-to-substratum binding assay. Only GST-Ig1-2-3 was capable of homophilic binding. Next, His-tagged recombinant Ig-domain proteins (His-Ig1-2, His-Ig1, and His-Ig2) were expressed and subjected to similar assays. Only His-Ig1-2 and His-Ig2 were capable of homophilic interactions. Binding of His-Ig2-conjugated Covaspheres to substrate-coated His-Ig2 was inhibited by anti-Ig1-2-3 Fab and soluble His-Ig2. These results indicate that the L1 homophilic binding site resides within Ig2. To examine effects of these L1 recombinant proteins on neurite outgrowth, neural retinal cells were cultured on different substrate-coated fusion proteins. Both GST-Ig1-2-3 and His-Ig2 were potent inducers of neurite extension. These results thus indicate that the L1 Ig-like domain 2 alone is sufficient to mediate L1-L1 interaction and promote neurite outgrowth from retinal cells.