Axonin-1/TAG-1 mediates cell-cell adhesion by a cis-assisted trans-interaction

Molecular mechanism of contactin 2 homophilic interaction

Contactin 2 (CNTN2) is a cell adhesion molecule involved in axon guidance, neuronal migration, and fasciculation. The ectodomains of CNTN1-CNTN6 are composed of six Ig domains (Ig1-Ig6) and four FN domains. Here, we show that CNTN2 forms transient homophilic interactions (KD ∼200 nM). Cryo-EM structures of full-length CNTN2 and CNTN2_Ig1-Ig6 reveal a T-shaped homodimer formed by intertwined, parallel monomers. Unexpectedly, the horseshoe-shaped Ig1-Ig4 headpieces extend their Ig2-Ig3 tips outwards on either side of the homodimer, while Ig4, Ig5, Ig6, and the FN domains form a central stalk. Cross-linking mass spectrometry and cell-based binding assays confirm the 3D assembly of the CNTN2 homodimer. The interface mediating homodimer formation differs between CNTNs, as do the homophilic versus heterophilic interaction mechanisms. The CNTN family thus encodes a versatile molecular platform that supports a very diverse portfolio of protein interactions and that can be leveraged to strategically guide neural circuit development.

Contactin 2 homophilic adhesion structure and conformational plasticity

The cell-surface attached glycoprotein contactin 2 is ubiquitously expressed in the nervous system and mediates homotypic cell-cell interactions to organize cell guidance, differentiation, and adhesion. Contactin 2 consists of six Ig and four fibronectin type III domains (FnIII) of which the first four Ig domains form a horseshoe structure important for homodimerization and oligomerization. Here we report the crystal structure of the six-domain contactin 2Ig1-6 and show that the Ig5-Ig6 combination is oriented away from the horseshoe with flexion in interdomain connections. Two distinct dimer states, through Ig1-Ig2 and Ig3-Ig6 interactions, together allow formation of larger oligomers. Combined size exclusion chromatography with multiangle light scattering (SEC-MALS), small-angle X-ray scattering (SAXS) and native MS analysis indicates contactin 2Ig1-6 oligomerizes in a glycan dependent manner. SAXS and negative-stain electron microscopy reveals inherent plasticity of the contactin 2 full-ectodomain. The combination of intermolecular binding sites and ectodomain plasticity explains how contactin 2 can function as a homotypic adhesion molecule in diverse intercellular environments.

Compromised Myelin and Axonal Molecular Organization Following Adult-Onset Sulfatide Depletion

3-O-sulfogalactosylceramide, or sulfatide, is a prominent myelin glycosphingolipid reduced in the normal appearing white matter (NAWM) in Multiple Sclerosis (MS), indicating that sulfatide reduction precedes demyelination. Using a mouse model that is constitutively depleted of sulfatide, we previously demonstrated that sulfatide is essential during development for the establishment and maintenance of myelin and axonal integrity and for the stable tethering of certain myelin proteins in the sheath. Here, using an adult-onset depletion model of sulfatide, we employ a combination of ultrastructural, immunohistochemical and biochemical approaches to analyze the consequence of sulfatide depletion from the adult CNS. Our findings show a progressive loss of axonal protein domain organization, which is accompanied by axonal degeneration, with myelin sparing. Similar to our previous work, we also observe differential myelin protein anchoring stabilities that are both sulfatide dependent and independent. Most notably, stable anchoring of neurofascin155, a myelin paranodal protein that binds the axonal paranodal complex of contactin/Caspr1, requires sulfatide. Together, our findings show that adult-onset sulfatide depletion, independent of demyelination, is sufficient to trigger progressive axonal degeneration. Although the pathologic mechanism is unknown, we propose that sulfatide is required for maintaining myelin organization and subsequent myelin-axon interactions and disruptions in these interactions results in compromised axon structure and function.

Structural insights into the contactin 1 - neurofascin 155 adhesion complex

Cell-surface expressed contactin 1 and neurofascin 155 control wiring of the nervous system and interact across cells to form and maintain paranodal myelin-axon junctions. The molecular mechanism of contactin 1 - neurofascin 155 adhesion complex formation is unresolved. Crystallographic structures of complexed and individual contactin 1 and neurofascin 155 binding regions presented here, provide a rich picture of how competing and complementary interfaces, post-translational glycosylation, splice differences and structural plasticity enable formation of diverse adhesion sites. Structural, biophysical, and cell-clustering analysis reveal how conserved Ig1-2 interfaces form competing heterophilic contactin 1 - neurofascin 155 and homophilic neurofascin 155 complexes whereas contactin 1 forms low-affinity clusters through interfaces on Ig3-6. The structures explain how the heterophilic Ig1-Ig4 horseshoe's in the contactin 1 - neurofascin 155 complex define the 7.4 nm paranodal spacing and how the remaining six domains enable bridging of distinct intercellular distances.

On the formation of ordered protein assemblies in cell-cell interfaces

Crystal structures of many cell-cell adhesion receptors reveal the formation of linear "molecular zippers" comprising an ordered one-dimensional array of proteins that form both intercellular (trans) and intracellular (cis) interactions. The clustered protocadherins (cPcdhs) provide an exemplar of this phenomenon and use it as a basis of barcoding of vertebrate neurons. Here, we report both Metropolis and kinetic Monte Carlo simulations of cPcdh zipper formation using simplified models of cPcdhs that nevertheless capture essential features of their three-dimensional structure. The simulations reveal that the formation of long zippers is an implicit feature of cPcdh structure and is driven by their cis and trans interactions that have been quantitatively characterized in previous work. Moreover, in agreement with cryo-electron tomography studies, the zippers are found to organize into two-dimensional arrays even in the absence of attractive interactions between individual zippers. Our results suggest that the formation of ordered two-dimensional arrays of linear zippers of adhesion proteins is a common feature of cell-cell interfaces. From the perspective of simulations, they demonstrate the importance of a realistic depiction of adhesion protein structure and interactions if important biological phenomena are to be properly captured.

Adhesion Protein Structure, Molecular Affinities, and Principles of Cell-Cell Recognition

The ability of cells to organize into multicellular structures in precise patterns requires that they "recognize" one another with high specificity. We discuss recent progress in understanding the molecular basis of cell-cell recognition, including unique phenomena associated with neuronal interactions. We describe structures of select adhesion receptor complexes and their assembly into larger intercellular junction structures and discuss emerging principles that relate cell-cell organization to the binding specificities and energetics of adhesion receptors. Armed with these insights, advances in protein design and gene editing should pave the way for breakthroughs toward understanding the molecular basis of cell patterning in vivo.

Neuronal immunoglobulin superfamily cell adhesion molecules in epithelial morphogenesis: insights from Drosophila

In this review, we address the function of immunoglobulin superfamily cell adhesion molecules (IgCAMs) in epithelia. Work in the Drosophila model system in particular has revealed novel roles for calcium-independent adhesion molecules in the morphogenesis of epithelial tissues. We review the molecular composition of lateral junctions with a focus on their IgCAM components and reconsider the functional roles of epithelial lateral junctions. The epithelial IgCAMs discussed in this review have well-defined roles in the nervous system, particularly in the process of axon guidance, suggesting functional overlap and conservation in mechanism between that process and epithelial remodelling. We expand on the hypothesis that epithelial occluding junctions and synaptic junctions are compositionally equivalent and present a novel hypothesis that the mechanism of epithelial cell (re)integration and synaptic junction formation are shared. We highlight the importance of considering non-cadherin-based adhesion in our understanding of the mechanics of epithelial tissues and raise questions to direct future work. This article is part of the discussion meeting issue 'Contemporary morphogenesis'.

Sidekick Is a Key Component of Tricellular Adherens Junctions that Acts to Resolve Cell Rearrangements

Tricellular adherens junctions are points of high tension that are central to the rearrangement of epithelial cells. However, the molecular composition of these junctions is unknown, making it difficult to assess their role in morphogenesis. Here, we show that Sidekick, an immunoglobulin family cell adhesion protein, is highly enriched at tricellular adherens junctions in Drosophila. This localization is modulated by tension, and Sidekick is itself necessary to maintain normal levels of cell bond tension. Loss of Sidekick causes defects in cell and junctional rearrangements in actively remodeling epithelial tissues like the retina and tracheal system. The adaptor proteins Polychaetoid and Canoe are enriched at tricellular adherens junctions in a Sidekick-dependent manner; Sidekick functionally interacts with both proteins and directly binds to Polychaetoid. We suggest that Polychaetoid and Canoe link Sidekick to the actin cytoskeleton to enable tricellular adherens junctions to maintain or transmit cell bond tension during epithelial cell rearrangements.

Impact of anti-CASPR2 autoantibodies from patients with autoimmune encephalitis on CASPR2/TAG-1 interaction and Kv1 expression

Autoantibodies against CASPR2 (contactin-associated protein-like 2) have been linked to autoimmune limbic encephalitis that manifests with memory disorders and temporal lobe seizures. According to the growing number of data supporting a role for CASPR2 in neuronal excitability, CASPR2 forms a molecular complex with transient axonal glycoprotein-1 (TAG-1) and shaker-type voltage-gated potassium channels (Kv1.1 and Kv1.2) in compartments critical for neuronal activity and is required for Kv1 proper positioning. Whereas the perturbation of these functions could explain the symptoms observed in patients, the pathogenic role of anti-CASPR2 antibodies has been poorly studied. In the present study, we find that patient autoantibodies alter Caspr2 distribution at the cell membrane promoting cluster formation. We confirm in a HEK cellular model that the anti-CASPR2 antibodies impede CASPR2/TAG-1 interaction and we identify the domains of CASPR2 and TAG-1 taking part in this interaction. Moreover, introduction of CASPR2 into HEK cells induces a marked increase of the level of Kv1.2 surface expression and in cultures of hippocampal neurons Caspr2-positive inhibitory neurons appear to specifically express high levels of Kv1.2. Importantly, in both cellular models, anti-CASPR2 patient autoAb increase Kv1.2 expression. These results provide new insights into the pathogenic role of autoAb in the disease.

Architecture of cell-cell adhesion mediated by sidekicks

Cell-cell adhesion is important for cell growth, tissue development, and neural network formation. Structures of cell adhesion molecules have been widely studied by crystallography, revealing the molecular details of adhesion interfaces. However, due to technical limitations, the overall structure and organization of adhesion molecules at cell adhesion interfaces has not been fully investigated. Here, we combine electron microscopy and other biophysical methods to characterize the structure of cell-cell adhesion mediated by the cell adhesion molecule Sidekick (Sidekick-1 and Sidekick-2) and obtain 3D views of the Sidekick-mediated adhesion interfaces as well as the organization of Sidekick molecules between cell membranes by electron tomography. The results suggest that the Ig-like domains and the fibronectin III (FnIII) domains of Sidekicks play different roles in cell adhesion. The Ig-like domains mediate the homophilic transinteractions bridging adjacent cells, while the FnIII domains interact with membranes, resulting in a tight adhesion interface between cells that may contribute to the specificity and plasticity of cell-cell contacts during cell growth and neural development.

Contactin-associated protein-like 2, a protein of the neurexin family involved in several human diseases

Contactin-associated protein-like 2 (CASPR2) is a cell adhesion protein of the neurexin family. Proteins of this family have been shown to play a role in the development of the nervous system, in synaptic functions, and in neurological diseases. Over recent years, CASPR2 function has gained an increasing interest as demonstrated by the growing number of publications. Here, we gather published data to comprehensively review CASPR2 functions within the nervous system in relation to CASPR2-related diseases in humans. On the one hand, studies on Cntnap2 (coding for CASPR2) knockout mice revealed its role during development, especially, in setting-up the inhibitory network. Consistent with this result, mutations in the CNTNAP2 gene coding for CASPR2 in human have been identified in neurodevelopmental disorders such as autism, intellectual disability, and epilepsy. On the other hand, CASPR2 was shown to play a role beyond development, in the localization of voltage-gated potassium channel (VGKC) complex that is composed of TAG-1, Kv1.1, and Kv1.2. This complex was found in several subcellular compartments essential for action potential propagation: the node of Ranvier, the axon initial segment, and the synapse. In line with a role of CASPR2 in the mature nervous system, neurological autoimmune diseases have been described in patients without neurodevelopmental disorders but with antibodies directed against CASPR2. These autoimmune diseases were of two types: central with memory disorders and temporal lobe seizures, or peripheral with muscular hyperactivity. Overall, we review the up-to-date knowledge on CASPR2 function and pinpoint confused or lacking information that will need further investigation.

Understanding axon guidance: are we nearly there yet?

During nervous system development, neurons extend axons to reach their targets and form functional circuits. The faulty assembly or disintegration of such circuits results in disorders of the nervous system. Thus, understanding the molecular mechanisms that guide axons and lead to neural circuit formation is of interest not only to developmental neuroscientists but also for a better comprehension of neural disorders. Recent studies have demonstrated how crosstalk between different families of guidance receptors can regulate axonal navigation at choice points, and how changes in growth cone behaviour at intermediate targets require changes in the surface expression of receptors. These changes can be achieved by a variety of mechanisms, including transcription, translation, protein-protein interactions, and the specific trafficking of proteins and mRNAs. Here, I review these axon guidance mechanisms, highlighting the most recent advances in the field that challenge the textbook model of axon guidance.

αvβ5 integrin mediates the effect of vitronectin on the initial stage of differentiation in mouse cerebellar granule cell precursors

Vitronectin (VN), one of the extracellular matrix proteins, controls the maturation of cerebellar granule cells (CGCs) through the promotion of the initial differentiation stage progress. However, the receptors of VN in the initial differentiation stage of CGC precursors (CGCPs) have not been clarified. In this study, we characterized the receptor candidates for VN in CGCPs. First, we confirmed that αvβ3 and αvβ5 integrins, which are receptor candidates for VN, were co-localized with VN in the developing cerebellum and primary cultured CGCPs. Next, the knockdown (KD) of αv, β3, and β5 integrins with small interference RNA (siRNA) for each integrin reduced the ratio of Tuj1, a final differentiation marker, -positive CGCPs. We further studied whether αvβ3 and αvβ5 integrins control the initial differentiation stage. The KD of αv and β5, but not β3, integrins significantly increased the ratio of transient axonal glycoprotein 1 (TAG1), an initial differentiation marker, -positive CGCPs, whereas the KD of αv and β3 integrins, not β5 integrin, stimulated the proliferation of CGCPs. Overexpression of β5 integrin stimulated the progress of the initial differentiation stage as well. To confirm the interaction between αvβ5 integrin and VN, VN was added to β5 integrin-KD CGCPs. The promotion of the progress of initial differentiation by VN was abrogated by β5 integrin KD using small hairpin RNA (shRNA). Taken together, our results indicated that αvβ5 integrin, as the very receptor of VN, is responsible for the progress of the initial differentiation stage in mouse CGCPs.

SynCAMs - From axon guidance to neurodevelopmental disorders

Many cell adhesion molecules are located at synapses but only few of them can be considered synaptic cell adhesion molecules in the strict sense. Besides the Neurexins and Neuroligins, the LRRTMs (leucine rich repeat transmembrane proteins) and the SynCAMs/CADMs can induce synapse formation when expressed in non-neuronal cells and therefore are true synaptic cell adhesion molecules. SynCAMs (synaptic cell adhesion molecules) are a subfamily of the immunoglobulin superfamily of cell adhesion molecules. As suggested by their name, they were first identified as cell adhesion molecules at the synapse which were sufficient to trigger synapse formation. They also contribute to myelination by mediating axon-glia cell contacts. More recently, their role in earlier stages of neural circuit formation was demonstrated, as they also guide axons both in the peripheral and in the central nervous system. Mutations in SynCAM genes were found in patients diagnosed with autism spectrum disorders. The diverse functions of SynCAMs during development suggest that neurodevelopmental disorders are not only due to defects in synaptic plasticity. Rather, early steps of neural circuit formation are likely to contribute.

Elucidating the general principles of cell adhesion with a coarse-grained simulation model

Coarse-grained simulation of interplay between cell adhesion and cell signaling.

Pioneering axons regulate neuronal polarization in the developing cerebral cortex

The polarization of neurons, which mainly includes the differentiation of axons and dendrites, is regulated by cell-autonomous and non-cell-autonomous factors. In the developing central nervous system, neuronal development occurs in a heterogeneous environment that also comprises extracellular matrices, radial glial cells, and neurons. Although many cell-autonomous factors that affect neuronal polarization have been identified, the microenvironmental cues involved in neuronal polarization remain largely unknown. Here, we show that neuronal polarization occurs in a microenvironment in the lower intermediate zone, where the cell adhesion molecule transient axonal glycoprotein-1 (TAG-1) is expressed in cortical efferent axons. The immature neurites of multipolar cells closely contact TAG-1-positive axons and generate axons. Inhibition of TAG-1-mediated cell-to-cell interaction or its downstream kinase Lyn impairs neuronal polarization. These results show that the TAG-1-mediated cell-to-cell interaction between the unpolarized multipolar cells and the pioneering axons regulates the polarization of multipolar cells partly through Lyn kinase and Rac1.

Analysis of cell-cell contact mediated by Ig superfamily cell adhesion molecules

Cell-cell adhesion is a fundamental requirement for all multicellular organisms. The calcium-independent cell adhesion molecules of the immunoglobulin superfamily (IgSF-CAMs) represent a major subgroup. They consist of immunoglobulin folds alone or in combination with other protein modules, often fibronectin type-III folds. More than 100 IgSF-CAMs have been identified in vertebrates and invertebrates. Most of the IgSF-CAMs are cell surface molecules that are membrane-anchored either by a single transmembrane segment or by a glycosylphosphatidylinositol (GPI) anchor. Some of the IgSF-CAMs also occur in soluble form, e.g., in the cerebrospinal fluid or in the vitreous fluid of the eye, due to naturally occurring cleavage of the GPI anchor or the membrane-proximal peptide segment. Some IgSF-CAMs, such as NCAM, occur in various forms that are generated by alternative splicing. This unit contains a series of protocols that have been used to study the function of IgSF-CAMs in vitro and in vivo.

Contactins in the neurobiology of autism

Autism is a disease of brain plasticity. Inspiring work of Willem Hendrik Gispen on neuronal plasticity has stimulated us to investigate gene defects in autism and the consequences for brain development. The central process in the pathogenesis of autism is local dendritic mRNA translation which is dependent on axodendritic communication. Hence, most autism-related gene products (i) are part of the protein synthesis machinery itself, (ii) are components of the mTOR signal transduction pathway, or (iii) shape synaptic activity and plasticity. Accordingly, prototype drugs have been recognized that interfere with these pathways. The contactin (CNTN) family of Ig cell adhesion molecules (IgCAMs) harbours at least three members that have genetically been implicated in autism: CNTN4, CNTN5, and CNTN6. In this chapter we review the genetic and neurobiological data underpinning their role in normal and abnormal development of brain systems, and the consequences for behavior. Although data on each of these CNTNs are far from complete, we tentatively conclude that these three contactins play roles in brain development in a critical phase of establishing brain systems and their plasticity. They modulate neuronal activities, such as neurite outgrowth, synaptogenesis, survival, guidance of projections and terminal branching of axons in forming neural circuits. Current research on these CNTNs concentrate on the neurobiological mechanism of their developmental functions. A future task will be to establish if proposed pharmacological strategies to counteract ASD-related symptomes can also be applied to reversal of phenotypes caused by genetic defects in these CNTN genes.

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Neural circuit formation in the cerebellum is controlled by cell adhesion molecules of the Contactin family

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) have been implicated in neural circuit formation in both the peripheral and the central nervous system. Several recent studies highlight a role of the Contactin group of IgSF CAMs in cerebellar development, in particular in the development of granule cells. Granule cells are the most numerous type of neurons in the nervous system and by forming a secondary proliferative zone in the cerebellum they provide an exception to the rule that neuronal precursors proliferate in the ventricular zone. Granule cells express Contactin-2, Contactin-1, and Contactin-6 in a sequential manner. Contactins are required for axon guidance, fasciculation, and synaptogenesis, and thus affect multiple steps in neural circuit formation in the developing cerebellum.

Contactins: structural aspects in relation to developmental functions in brain disease

The contactins are members of a protein subfamily of neural immunoglobulin (Ig) domain-containing cell adhesion molecules. Their architecture is based on six N-terminal Ig domains, four fibronectin type III domains, and a C-terminal glycophosphatidylinositol (GPI)-anchor to the extracellular part of the cell membrane. Genetics of neuropsychiatric disorders, particularly autism spectrum disorders, have pinpointed contactin-4, -5, and -6 (CNTN4, -5, and -6) as potential disease genes in neurodevelopmental disorders and suggested that they participate in pathways important for appropriate brain development. These contactins have distinct but overlapping patterns of brain expression, and null-mutation causes subtle morphological and functional defects in the brain. The molecular basis of their neurodevelopmental functions is likely conferred by heterophilic protein interactions. Cntn4, -5, and -6 interact with protein tyrosine phosphatase receptor gamma (Ptptg) using a shared binding site that spans their second and third Ig repeats. Interactions with amyloid precursor protein (APP), Notch, and other IgCAMs have also been indicated. The present data indicate that Cntn4, -5, and -6 proteins may be part of heteromeric receptor complexes as well as serve as ligands themselves.

Axo-glial antigens as targets in multiple sclerosis: implications for axonal and grey matter injury

Multiple sclerosis is thought to be an autoimmune-mediated disease of the central nervous system. For many years, T-cells were regarded as the key players in the pathogenesis, and myelin of white matter was considered as the main victim. However, research during recent years showed a more complex picture. Besides T-cells, also B-cells, antibodies and the innate immunity contribute to the tissue damage. Modern imaging techniques and neuropathological examinations showed that not only myelin but also axons, cortical neurons and nodes of Ranvier are damaged. The autoimmune targets of this widespread injury are so far not known. The identification of the axo-glial proteins contactin-2 and neurofascin provides excellent examples how antibodies can induce axonal injury at the node of Ranvier and how T-cells can destruct cortical integrity. This review will discuss the pathogenic implications of an autoimmune response against these newly discovered antigens.

Identification of gene transcripts expressed by postsynaptic neurons during synapse formation encoding cell surface proteins with presumptive synaptogenic activity

Synapse formation is a well-programmed developmental process involving a variety of cell-cell interactions carried out by distinct groups of molecules. Various molecules that contribute to the assembly of synaptic contacts have been characterized; however, the repertoire of identified proteins expressed by postsynaptic neurons capable of inducing presynaptic differentiation is quite limited. To identify gene transcripts encoding cell surface proteins expressed by postsynaptic cells with molecular features suggestive of synaptogenic activity, this study carried out a genome-wide expression analysis in the chick ciliary ganglion during the different phases of synapse formation. It was found that from the 21,493 gene-probes detected throughout development, 302 protein-coding transcripts were upregulated during the initiation of synapse formation. Analysis of this pool of transcripts showed that 51 of them encoded cell surface proteins (27 membrane-bound and 24 secreted) with protein-protein interacting domains. This includes twelve cell adhesion molecules, six ligand-receptors, six proteins with ligand-like domains, three membrane bound enzymes, eight components of the extracellular matrix, three neuropeptides, three cytokines and growth factors, five extracellular modulators of cell signaling, and five unrelated secreted proteins. Furthermore, the role of synaptic transmission during the initiation of synapse formation was evaluated by assessing the effect of synaptic activity blockade with d-tubocurarine on the expression levels of the pool of 51 transcripts encoding cell surface proteins. Treatment with d-tubocurarine reduced the expression levels of 22% of the selected genes, while the expression levels of 78% of the genes was not affected or was enhanced.

GPI-anchored proteins at the node of Ranvier

Contactin and TAG-1 are glycan phosphatidyl inositol (GPI)-anchored cell adhesion molecules that play a crucial role in the organization of axonal subdomains at the node of Ranvier of myelinating fibers. Contactin and TAG-1 mediate axo-glial selective interactions in association with Caspr-family molecules at paranodes and juxtaparanodes, respectively. How membrane proteins can be confined in these neighbouring domains along the axon has been the subject of intense investigations. This review will specifically examine the properties conferred by the lipid microenvironment to regulate trafficking and selective association of these axo-glial complexes. Increasing evidences from genetic and neuropathological models point to a role of lipid rafts in the formation or stabilization of the paranodal junctions.

Cryo-electron tomography of homophilic adhesion mediated by the neural cell adhesion molecule L1

The neural cell adhesion molecule L1 participates in homophilic interactions important for axon guidance and neuronal development. The structural details of homophilic adhesion mediated by L1 and other immunoglobulin superfamily members containing an N-terminal horseshoe arrangement of four immunoglobulin-like domains are unknown. Here we used cryo-electron tomography to study liposomes to which intact or truncated forms of the L1 ectodomain were attached. Tomographic reconstructions revealed an adhesion interface with a regular and repeating pattern consistent with interactions between paired horseshoes contributed by L1 proteins from neighboring liposomes. The characteristics of the pattern changed when N-linked carbohydrates were altered by removing sialic acids or converting from complex to high mannose or oligomannose glycans, suggesting a regulatory role for carbohydrates in L1-mediated homophilic adhesion. Using the results from tomograms and crystal structures of L1-related molecules, we present a structural model for L1-mediated homophilic adhesion that depends on protein-protein, protein-carbohydrate, and carbohydrate-carbohydrate interactions.

The cell adhesion molecule Tag1, transmembrane protein Stbm/Vangl2, and Lamininalpha1 exhibit genetic interactions during migration of facial branchiomotor neurons in zebrafish

Interactions between a neuron and its environment play a major role in neuronal migration. We show here that the cell adhesion molecule Transient Axonal Glycoprotein (Tag1) is necessary for the migration of the facial branchiomotor neurons (FBMNs) in the zebrafish hindbrain. In tag1 morphant embryos, FBMN migration is specifically blocked, with no effect on organization or patterning of other hindbrain neurons. Furthermore, using suboptimal morpholino doses and genetic mutants, we found that tag1, lamininalpha1 (lama1) and stbm, which encodes a transmembrane protein Vangl2, exhibit pairwise genetic interactions for FBMN migration. Using time-lapse analyses, we found that FBMNs are affected similarly in all three single morphant embryos, with an inability to extend protrusions in a specific direction, and resulting in the failure of caudal migration. These data suggest that tag1, lama1 and vangl2 participate in a common mechanism that integrates signaling between the FBMN and its environment to regulate migration.

Structural requirement of TAG-1 for retinal ganglion cell axons and myelin in the mouse optic nerve

White matter axons organize into fascicles that grow over long distances and traverse very diverse environments. The molecular mechanisms preserving this structure of white matter axonal tracts are not well known. Here, we used the optic nerve as a model and investigated the role of TAG-1, a cell adhesion molecule expressed by retinal axons. TAG-1 was first expressed in the embryonic retinal ganglion cells (RGCs) and later in the postnatal myelin-forming cells in the optic nerve. We describe the consequences of genetic loss of Tag-1 on the developing and adult retinogeniculate tract. Tag-1-null embryos display anomalies in the caliber of RGC axons, associated with an abnormal organization of the astroglial network in the optic nerve. The contralateral projections in the lateral geniculate nucleus are expanded postnatally. In the adult, Tag-1-null mice show a loss of RGC axons, with persistent abnormalities of axonal caliber and additional cytoskeleton and myelination defects. Therefore, TAG-1 is an essential regulator of the structure of RGC axons and their surrounding glial cells in the optic nerve.

Axonin-1/TAG-1 is required for pathfinding of granule cell axons in the developing cerebellum

BACKGROUND

Neural development consists of a series of steps, including neurogenesis, patterning, cell migration, axon guidance, and finally, synaptogenesis. Because all these steps proceed in a constantly changing environment, functional gene analyses during development have to take time into account. This is quite challenging, however, as loss-of-function approaches based on classic genetic tools do not allow for the precise temporal control that is required for developmental studies. Gene silencing by RNA interference (RNAi) in combination with the chicken embryo or with cultured embryos opens new possibilities for functional gene analysis in vivo. Axonin-1/TAG-1 is a cell adhesion molecule of the immunoglobulin superfamily with a well defined temporal and spatial expression pattern in the developing vertebrate nervous system. Axonin-1/TAG-1 was shown to promote neurite outgrowth in vitro and to be required for commissural and sensory axon pathfinding in vivo.

RESULTS

To knock down axonin-1 in a temporally and spatially controlled manner during development of the nervous system, we have combined RNAi with the accessibility of the chicken embryo even at late stages of development. Using ex ovo RNAi, we analyzed the function of axonin-1/TAG-1 in cerebellar development. Axonin-1 is expressed in postmitotic granule cells while they extend their processes, the parallel fibers. In the absence of axonin-1 these processes still extend but no longer in a parallel manner to each other or to the pial surface of the cerebellum.

CONCLUSION

Axonin-1/TAG-1 is required for the navigation, but not for the elongation, of granule cell processes in the developing cerebellum in vivo.

The crystal structure of the ligand-binding module of human TAG-1 suggests a new mode of homophilic interaction

Human TAG-1 is a neural cell adhesion molecule that is crucial for the development of the nervous system during embryogenesis. It consists of six immunoglobulin-like and four fibronectin III-like domains and is anchored to the membrane by glycosylphosphatidylinositol. Herein we present the crystal structure of the four N-terminal immunoglobulin-like domains of TAG-1 (TAG-1(Ig1-4)), known to be important in heterophilic and homophilic macromolecular interactions. The contacts of neighboring molecules within the crystal were investigated. A comparison with the structure of the chicken ortholog resulted in an alternative mode for the molecular mechanism of homophilic TAG-1 interaction. This mode of TAG-1 homophilic interaction is based on dimer formation rather than formation of a molecular zipper as proposed for the chicken ortholog.

Molecular analysis of receptor protein tyrosine phosphatase mu-mediated cell adhesion

Type IIB receptor protein tyrosine phosphatases (RPTPs) are bi-functional cell surface molecules. Their ectodomains mediate stable, homophilic, cell-adhesive interactions, whereas the intracellular catalytic regions can modulate the phosphorylation state of cadherin/catenin complexes. We describe a systematic investigation of the cell-adhesive properties of the extracellular region of RPTPmu, a prototypical type IIB RPTP. The crystal structure of a construct comprising its N-terminal MAM (meprin/A5/mu) and Ig domains was determined at 2.7 A resolution; this assigns the MAM fold to the jelly-roll family and reveals extensive interactions between the two domains, which form a rigid structural unit. Structure-based site-directed mutagenesis, serial domain deletions and cell-adhesion assays allowed us to identify the four N-terminal domains (MAM, Ig, fibronectin type III (FNIII)-1 and FNIII-2) as a minimal functional unit. Biophysical characterization revealed at least two independent types of homophilic interaction which, taken together, suggest that there is the potential for formation of a complex and possibly ordered array of receptor molecules at cell contact sites.

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 10(6) 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 microm, 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.

Novel CD47-dependent intercellular adhesion modulates cell migration

CD47 is a ubiquitously expressed plasma membrane protein, also known as Integrin Associated Protein, that modulates cell adhesion both through alteration of the avidity of integrin binding and through interaction with its own ligands, the extracellular matrix protein thrombospondin (TSP) and the plasma membrane response regulator SIRPalpha1. We now show that CD47 expression on fibroblasts can induce intercellular adhesion resulting in cell aggregation in the absence of active integrins, SIRPalpha1 binding, and detectable TSP. CD47-expressing cells preferentially bind to other CD47-expressing cells, and intercellular adhesion requires stimulation by serum or a CD47-binding peptide from TSP. Cell-cell adhesion is inhibited by pertussis toxin and C. difficile toxin B, and both adherent and aggregating CD47-expressing fibroblasts have more rac in the GTP bound state than CD47-deficient cells. Spontaneous migration of Jurkat lymphocytes through a fibroblast monolayer is decreased by fibroblast expression of CD47, consistent with an increased barrier function of the CD47 expressing cells. The lymphocyte chemoattractant SDF-1alpha stimulates migration of Jurkat cells through this monolayer only if both the lymphocytes and fibroblasts express CD47, and the inhibition of migration by a CD47-interacting peptide from TSP similarly requires CD47 expression on both cell types. Thus, signaling dependent on both heterotrimeric and rho family GTPases can induce CD47 to participate in cell-cell interactions independent of known ligands that enhance intercellular adhesion and modulate cell migration.

Diglons are heterodimeric proteins composed of IgLON subunits, and Diglon-CO inhibits neurite outgrowth from cerebellar granule cells

IgLONs are a family of four cell adhesion molecules belonging to the Ig superfamily that are thought to play a role in cell-cell recognition and growth-cone migration. One member of the family, opioid-binding cell-adhesion molecule (OBCAM), might act as a tumour suppressor. Previous work has shown that limbic-system-associated protein (LAMP), CEPU-1/Neurotrimin and OBCAM interact homophilically and heterophilically within the family. Here, we show that, based on their relative affinities, CEPU-1 might be both a homo- and a heterophilic cell adhesion molecule, whereas LAMP and OBCAM act only as heterophilic cell adhesion molecules. A binding assay using recombinant IgLONs fused to human Fc showed that IgLONs are organized in the plane of the membrane as heterodimers, and we propose that IgLONs function predominantly as subunits of heterodimeric proteins (Diglons). Thus, the four IgLONs can form six Diglons. Furthermore, although singly transfected cell lines have little effect on neurite outgrowth, CHO cell lines expressing both CEPU-1 and OBCAM (Diglon-CO) inhibit neurite outgrowth from cerebellar granule cells.

The immunoglobulin superfamily in Drosophila melanogaster and Caenorhabditis elegans and the evolution of complexity

Drosophila melanogaster is an arthropod with a much more complex anatomy and physiology than the nematode Caenorhabditis elegans. We investigated one of the protein superfamilies in the two organisms that plays a major role in development and function of cell-cell communication: the immunoglobulin superfamily (IgSF). Using hidden Markov models, we identified 142 IgSF proteins in Drosophila and 80 in C. elegans. Of these, 58 and 22, respectively, have been previously identified by experiments. On the basis of homology and the structural characterisation of the proteins, we can suggest probable types of function for most of the novel proteins. Though overall Drosophila has fewer genes than C. elegans, it has many more IgSF cell-surface and secreted proteins. Half the IgSF proteins in C. elegans and three quarters of those in Drosophila have evolved subsequent to the divergence of the two organisms. These results suggest that the expansion of this protein superfamily is one of the factors that have contributed to the formation of the more complex physiological features that are found in Drosophila.

A locus for complicated hereditary spastic paraplegia maps to chromosome 1q24-q32

We updated the clinical features of a consanguineous Arab Israeli family, in which four of seven children were affected by spastic paraplegia complicated by skin pigmentary abnormalities. A genomewide linkage screen performed for the family identified a new locus (SPG23) for this form of hereditary spastic paraplegia, in an approximately 25cM region of chromosome 1q24-q32, with a peak logarithm of odds score of 3.05.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

Signaling at the growth cone: ligand-receptor complexes and the control of axon growth and guidance

The guidance of axons during the establishment of the nervous system is mediated by a variety of extracellular cues that govern cytoskeletal dynamics in axonal growth cones. A large number of these guidance cues and their cell-surface receptors have now been identified, and the intracellular signaling pathways by which these cues induce cytoskeletal rearrangements are becoming defined. This review summarizes our current understanding of the major families of axon guidance cues and their receptors, with a particular emphasis on receptor signaling mechanisms. We also discuss recent advances in understanding receptor cross talk and how the activities of guidance cues and their receptors are modulated during neural development.

Close homolog of L1 is an enhancer of integrin-mediated cell migration

Close homolog of L1 (CHL1) is a member of the L1 family of cell adhesion molecules expressed by subpopulations of neurons and glia in the central and peripheral nervous system. It promotes neurite outgrowth and neuronal survival in vitro. This study describes a novel function for CHL1 in potentiating integrin-dependent cell migration toward extracellular matrix proteins. Expression of CHL1 in HEK293 cells stimulated their haptotactic migration toward collagen I, fibronectin, laminin, and vitronectin substrates in Transwell assays. CHL1-potentiated cell migration to collagen I was dependent on alpha1beta1 and alpha2beta1 integrins, as shown with function blocking antibodies. Potentiated migration relied on the early integrin signaling intermediates c-Src, phosphatidylinositol 3-kinase, and mitogen-activated protein kinase. Enhancement of migration was disrupted by mutation of a potential integrin interaction motif Asp-Gly-Glu-Ala (DGEA) in the sixth immunoglobulin domain of CHL1, suggesting that CHL1 functionally interacts with beta1 integrins through this domain. CHL1 was shown to associate with beta1 integrins on the cell surface by antibody-induced co-capping. Through a cytoplasmic domain sequence containing a conserved tyrosine residue (Phe-Ile-Gly-Ala-Tyr), CHL1 recruited the actin cytoskeletal adapter protein ankyrin to the plasma membrane, and this sequence was necessary for promoting integrin-dependent migration to extracellular matrix proteins. These results support a role for CHL1 in integrin-dependent cell migration that may be physiologically important in regulating cell migration in nerve regeneration and cortical development.

Reciprocal raft-receptor interactions and the assembly of adhesion complexes

Cell adhesion complexes are critical for the physical coordination of cell-cell interactions and the morphogenesis of tissues and organs. Many adhesion receptors are anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) moiety and are thereby partitioned into membrane rafts. In this review, we focus on reciprocal interactions between rafts and adhesion molecules, leading to receptor clustering and raft expansion and stability. A model for a three-stage adhesion complex assembly process is also proposed. First, GPI-anchored adhesion molecules are recruited into rafts, which in turn promote receptor cis-oligomerization and thereby produce precursory complexes primed for avid trans-interactions. Second, trans-interactions of the receptors cross-link and stabilize large amalgams of rafts at sites of adhesion complex assembly. Finally, the enlarged and stabilized rafts acquire enhanced abilities to recruit the cytoskeleton and induce signaling. This process exemplifies how the domain structure of the plasma membrane can impact the function of its receptors.