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

X-ray structure and function of fibronectin domains two and three of the neural cell adhesion molecule L1

The cell adhesion molecule L1 (L1CAM, L1 in short) plays crucial roles during neural development, regeneration after injury, synapse formation, synaptic plasticity and tumor cell migration. L1 belongs to the immunoglobulin superfamily and comprises in its extracellular part six immunoglobulin (Ig)-like domains and five fibronectin type III homologous repeats (FNs). The second Ig-like domain has been validated for self- (so-called homophilic) binding between cells. Antibodies against this domain inhibit neuronal migration in vitro and in vivo. The fibronectin type III homologous repeats FN2 and FN3 bind small molecule agonistic L1 mimetics and contribute to signal transduction. FN3 has a stretch of 25 amino acids that can be triggered with a monoclonal antibody, or the L1 mimetics, to enhance neurite outgrowth and neuronal cell migration in vitro and in vivo. To correlate the structural features of these FNs with function, we determined a high-resolution crystal structure of a FN2FN3 fragment, which is functionally active in cerebellar granule cells and binds several mimetics. The structure illustrates that both domains are connected by a short linker sequence allowing a flexible and largely independent organization of both domains. This becomes further evident by comparing the X-ray crystal structure with models derived from Small-Angle X-ray Scattering (SAXS) data for FN2FN3 in solution. Based on the X-ray crystal structure, we identified five glycosylation sites which we believe are crucial for folding and stability of these domains. Our study signifies an advance in the understanding of structure-functional relationships of L1.

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

The 5-HT4 receptor interacts with adhesion molecule L1 to modulate morphogenic signaling in neurons

Morphological remodeling of dendritic spines is critically involved in memory formation and depends on adhesion molecules. Serotonin receptors are also implicated in this remodeling, though the underlying mechanisms remain enigmatic. Here, we uncovered a signaling pathway involving the adhesion molecule L1CAM (L1) and serotonin receptor 5-HT₄ (5-HT₄R, encoded by HTR4). Using Förster resonance energy transfer (FRET) imaging, we demonstrated a physical interaction between 5-HT₄R and L1, and found that 5-HT₄R-L1 heterodimerization facilitates mitogen-activated protein kinase activation in a G_s-dependent manner. We also found that 5-HT₄R-L1-mediated signaling is involved in G₁₃-dependent modulation of cofilin-1 activity. In hippocampal neurons in vitro, the 5-HT₄R-L1 pathway triggers maturation of dendritic spines. Thus, the 5-HT₄R-L1 signaling module represents a previously unknown molecular pathway regulating synaptic remodeling.

Different Shades of L1CAM in the Pathophysiology of Cancer Stem Cells

L1 cell adhesion molecule (L1CAM) is aberrantly expressed in several tumor types where it is causally linked to malignancy and therapy resistance, acting also as a poor prognosis factor. Accordingly, several approaches have been developed to interfere with L1CAM function or to deliver cytotoxic agents to L1CAM-expressing tumors. Metastatic dissemination, tumor relapse and drug resistance can be fueled by a subpopulation of neoplastic cells endowed with peculiar biological properties that include self-renewal, efficient DNA repair, drug efflux machineries, quiescence, and immune evasion. These cells, known as cancer stem cells (CSC) or tumor-initiating cells, represent, therefore, an ideal target for tumor eradication. However, the molecular and functional traits of CSC have been unveiled only to a limited extent. In this context, it appears that L1CAM is expressed in the CSC compartment of certain tumors, where it plays a causal role in stemness itself and/or in biological processes intimately associated with CSC (e.g., epithelial-mesenchymal transition (EMT) and chemoresistance). This review summarizes the role of L1CAM in cancer focusing on its functional contribution to CSC pathophysiology. We also discuss the clinical usefulness of therapeutic strategies aimed at targeting L1CAM in the context of anti-CSC treatments.

Oriented, multimeric biointerfaces of the L1 cell adhesion molecule: an approach to enhance neuronal and neural stem cell functions on 2-D and 3-D polymer substrates

This article focuses on elucidating the key presentation features of neurotrophic ligands at polymer interfaces. Different biointerfacial configurations of the human neural cell adhesion molecule L1 were established on two-dimensional films and three-dimensional fibrous scaffolds of synthetic tyrosine-derived polycarbonate polymers and probed for surface concentrations, microscale organization, and effects on cultured primary neurons and neural stem cells. Underlying polymer substrates were modified with varying combinations of protein A and poly-D-lysine to modulate the immobilization and presentation of the Fc fusion fragment of the extracellular domain of L1 (L1-Fc). When presented as an oriented and multimeric configuration from protein A-pretreated polymers, L1-Fc significantly increased neurite outgrowth of rodent spinal cord neurons and cerebellar neurons as early as 24 h compared to the traditional presentation via adsorption onto surfaces treated with poly-D-lysine. Cultures of human neural progenitor cells screened on the L1-Fc/polymer biointerfaces showed significantly enhanced neuronal differentiation and neuritogenesis on all protein A oriented substrates. Notably, the highest degree of βIII-tubulin expression for cells in 3-D fibrous scaffolds were observed in protein A oriented substrates with PDL pretreatment, suggesting combined effects of cell attachment to polycationic charged substrates with subcellular topography along with L1-mediated adhesion mediating neuronal differentiation. Together, these findings highlight the promise of displays of multimeric neural adhesion ligands via biointerfacially engineered substrates to "cooperatively" enhance neuronal phenotypes on polymers of relevance to tissue engineering.

Homophilic adhesion mechanism of neurofascin, a member of the L1 family of neural cell adhesion molecules

The L1 family neural cell adhesion molecules play key roles in specifying the formation and remodeling of the neural network, but their homophilic interaction that mediates adhesion is not well understood. We report two crystal structures of a dimeric form of the headpiece of neurofascin, an L1 family member. The four N-terminal Ig-like domains of neurofascin form a horseshoe shape, akin to several other immunoglobulin superfamily cell adhesion molecules such as hemolin, axonin, and Dscam. The neurofascin dimer, captured in two crystal forms with independent packing patterns, reveals a pair of horseshoes in trans-synaptic adhesion mode. The adhesion interaction is mediated mostly by the second Ig-like domain, which features an intermolecular β-sheet formed by the joining of two individual GFC β-sheets and a large but loosely packed hydrophobic cluster. Mutagenesis combined with gel filtration assays suggested that the side chain hydrogen bonds at the intermolecular β-sheet are essential for the homophilic interaction and that the residues at the hydrophobic cluster play supplementary roles. Our structures reveal a conserved homophilic adhesion mode for the L1 family and also shed light on how the pathological mutations of L1 affect its structure and function.

Inside-out regulation of L1 conformation, integrin binding, proteolysis, and concomitant cell migration

The ectodomain structure and function of the neural cell adhesion molecule L1 is shown to be regulated by the intracellular phosphorylation of a novel threonine, T1172. In pancreatic cancer cells, T1172 exhibits steady-state saturated phosphorylation, an event regulated by CKII and PKC, and which further regulates cell migration.

Previous reports on the expression of the cell adhesion molecule L1 in pancreatic ductal adenocarcinoma (PDAC) cells range from absent to high. Our data demonstrate that L1 is expressed in poorly differentiated PDAC cells in situ and that threonine-1172 (T1172) in the L1 cytoplasmic domain exhibits steady-state saturated phosphorylation in PDAC cells in vitro and in situ. In vitro studies support roles for casein kinase II and PKC in this modification, consistent with our prior studies using recombinant proteins. Importantly, T1172 phosphorylation drives, or is associated with, a change in the extracellular structure of L1, consistent with a potential role in regulating the shift between the closed conformation and the open, multimerized conformation of L1. We further demonstrate that these distinct conformations exhibit differential binding to integrins αvβ3 and αvβ5 and that T1172 regulates cell migration in a matrix-specific manner and is required for a disintegrin and metalloproteinase-mediated shedding of the L1 ectodomain that has been shown to regulate cell migration. These data define a specific role for T1172 of L1 in regulating aspects of pancreatic adenocarcinoma cell phenotype and suggest the need for further studies to elucidate the specific ramifications of L1 expression and T1172 phosphorylation in the pathobiology of pancreatic cancer.

Identification and molecular characterization of numerous Histomonas meleagridis proteins using a cDNA library

SUMMARYHistomonas meleagridis is a protozoan parasite of various galliform birds causing a type of enterohepatitis termed histomonosis or 'blackhead disease'. Due to the ban of chemotherapeutic substances and an increase in free-range poultry production, histomonosis is currently a re-emerging disease. So far limited molecular knowledge is available. In the present work, mRNAs coding for antigenic proteins of H. meleagridis were identified. For this purpose, a cDNA expression library was constructed from a mono-eukaryotic culture of H. meleagridis. The library was screened with polyclonal rabbit serum raised against purified H. meleagridis trophozoites. Polyclonal rabbit serum specifically recognized the same major H. meleagridis antigens as chicken and turkey sera originating from animal trials, but displayed a significantly lower bacteria-dependent background signal. After 2 rounds of screening, a total of 95 positive clones were sequenced. Bioinformatics analyses were performed on nucleotide and deduced amino acid sequences, identifying 37 unique clones. Based on the homology to other protozoan parasites, mostly Trichomonas vaginalis, the clones were grouped according to functional aspects: structural proteins, possible surface proteins, oxygen reducing proteins, ribosomal proteins, protein kinases and various other intracellular proteins.

Kinetic analysis of L1 homophilic interaction: role of the first four immunoglobulin domains and implications on binding mechanism

L1 is a cell adhesion molecule of the immunoglobulin (Ig) superfamily, critical for central nervous system development, and involved in several neuronal biological events. It is a type I membrane glycoprotein. The L1 ectodomain, composed of six Ig-like and five fibronectin (Fn) type-III domains, is involved in homophilic binding. Here, co-immunoprecipitation studies between recombinant truncated forms of human L1 expressed and purified from insect Spodoptera frugiperda Sf9 cells, and endogenous full-length L1 from human NT2N neurons, showed that the L1 ectodomain (L1/ECD) and L1/Ig1-4 interacted homophilically in trans, contrary to mutants L1/Ig1-3 and L1/Ig2-Fn5. All mutants were correctly folded as evaluated by combination of far-UV CD and fluorescence spectroscopy. Surface plasmon resonance analysis showed comparable dissociation constants of 116 +/- 2 and 130 +/- 6 nm for L1/ECD-L1/ECD and L1/ECD-L1/Ig1-4, respectively, whereas deletion mutants for Ig1 or Ig4 did not interact. Accordingly, in vivo, Sf9 cells stably expressing L1 were found to adhere only to L1/ECD- and L1/Ig1-4-coated surfaces. Furthermore, only these mutants bound to HEK293 cells overexpressing L1 at the cell surface. Enhancement of neurite outgrowth, which is the consequence of signaling events caused by L1 homophilic binding, was comparable between L1/ECD and L1/Ig1-4. Altogether, these results showed that domains Ig1 to Ig4 are necessary and sufficient for L1 homophilic binding in trans, and that the rest of the molecule does not contribute to the affinity under the conditions of the current study. Furthermore, they are compatible with a cooperative interaction between modules Ig1-Ig4 in a horseshoe conformation.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

Production and purification of functional truncated soluble forms of human recombinant L1 cell adhesion glycoprotein from Spodoptera frugiperda Sf9 cells

L1 is a human cell adhesion glycoprotein involved in the development of the central nervous system that comprises six immunoglobulin-like domains (Ig1-Ig6), five fibronectin-type III (FN1-FN5) domains, a single transmembrane region and a cytoplasmic domain. It contains 20 potential N-glycosylation sites and is heavily glycosylated in a variety of cell types. In this work, seven truncated soluble forms including L1 ectodomain (L1/ECD) and Ig domains 5-6 (L1/Ig5-6) have been constructed by PCR and have been cloned, as well as the full-length form (L1), in the stable expression vector for insect cells pMIB/V5-His-TOPO. Spodoptera frugiperda Sf9 cell lines expressing the truncated forms have been obtained, and all proteins were successfully secreted. L1/ECD and L1/Ig5-6 were produced in shake flasks with productions of 3 and 32 mg/L on the third and fourth day of culture, respectively. When L1/Ig5-6 was produced for four days in 2L bioreactor 200 mg/L protein were recovered from the supernatants on the fourth day of culture. Affinity-purified L1/ECD and L1/Ig5-6 were immobilized on poly-d-lysine coated coverslips, and were shown to be active in inducing neurite outgrowth from human NT2N neurons. Therefore, correctly folded and functional truncated forms of human L1 have been produced in high amounts from insect cells using a stable expression system.

Regulators of neurite outgrowth: role of cell adhesion molecules

Neuronal differentiation is a fundamental event in the development of the nervous system as well as in the regeneration of damaged nervous tissue. The initiation and guidance of a neurite are accomplished by positive (permissive or attractive), negative (inhibitory or repulsive), or guiding (affecting the advance of the growth cone) signals from the extracellular space. The signals may arise from either the extracellular matrix (ECM) or the surface of other cells, or be diffusible secreted factors. Based on this classification, we briefly describe selected positive, negative, and guiding signaling cues focusing on the role of cell adhesion molecules (CAMs). CAMs not only regulate cell-cell and cell-ECM adhesion "mechanically," they also trigger intracellular signaling cascades launching neurite outgrowth. Here, we describe the structure, function, and signaling of three key CAMs found in the nervous system: N-cadherin and two Ig-CAMs, L1 and the neural cell adhesion molecule NCAM.

Brain development in mice lacking L1-L1 homophilic adhesion

A new mouse line has been produced in which the sixth Ig domain of the L1 cell adhesion molecule has been deleted. Despite the rather large deletion, L1 expression is preserved at normal levels. In vitro experiments showed that L1–L1 homophilic binding was lost, along with L1-α5β1 integrin binding. However, L1–neurocan and L1–neuropilin binding were preserved and sema3a responses were intact. Surprisingly, many of the axon guidance defects present in the L1 knockout mice, such as abnormal corticospinal tract and corpus callosum, were not observed. Nonetheless, when backcrossed on the C57BL/6 strain, a severe hydrocephalus was observed and after several generations, became an embryonic lethal. These results imply that L1 binding to L1, TAG-1, or F3, and L1-α5β1 integrin binding are not essential for normal development of a variety of axon pathways, and suggest that L1–L1 homophilic binding is important in the production of X-linked hydrocephalus.

Distinct domains of the limbic system-associated membrane protein (LAMP) mediate discrete effects on neurite outgrowth

The limbic system-associated membrane protein (LAMP) is a glycosylphosphatidylinositol-anchored glycoprotein with three immunoglobulin (Ig) domains that can either enhance or inhibit neurite outgrowth depending upon the neuronal population examined. In the present study, we investigate the domains responsible for these activities. Domain deletion revealed that the N-terminal IgI domain is necessary and sufficient for the neurite-promoting activity observed in hippocampal neurons. In contrast, inhibition of neurite outgrowth in SCG neurons, which is mediated by heterophilic interactions, requires full-length LAMP, although selective inhibition of the second Ig domain, but not the first or third domains, prevented the inhibitory effect. This indicates that the IgII domain of LAMP harbors the neurite-inhibiting activity, but only in the context of the full-length configuration. Covasphere-binding analyses demonstrate IgI/IgI interactions, but no interaction between IgII and any other domain, consistent with the biological activities that each domain mediates. The data suggest that LAMP may serve as a bifunctional guidance molecule, with distinct structural domains contributing to the promotion and inhibition of neurite outgrowth.

Contact-dependent signaling during the late events of platelet activation

Signaling events downstream from collagen receptors and G protein-coupled receptors are responsible for the initiation and extension of platelet plug formation. This creates the platelet plug and hopefully results in the cessation of bleeding. It is not, however, all that is required for hemostasis, and growing evidence is emerging that the perpetuation of a stable hemostatic plug requires additional intracellular signaling. At least part of this process is made possible by the persistent close contacts between platelets that can only occur after the onset of aggregation. This review discusses several examples of such signaling mechanisms that help to perpetuate the platelet plug in a contact-dependent manner, including outside-in signaling through integrins, signaling though Eph kinases and ephrins, and the role of CD40L.

Alterations in hippocampal GAP-43, BDNF, and L1 following sustained cerebral ischemia

Alterations in factors involved in the regeneration of the neuronal network in the hippocampus of rats with microsphere embolism (ME) were examined. Nine hundred microspheres (48 microm in diameter) were injected into the right hemisphere, and immunochemical and immunohistochemical studies on the hippocampus were performed on the seventh day thereafter. Hematoxylin-eosin staining showed progressive and severe degeneration of the hippocampus after ME. The protein levels of brain-derived neurotrophic factor (BDNF), 43-kDa growth-associated protein (GAP-43), and adhesion protein L1 (L1) in the ipsilateral hippocampus of the ME animal, determined by Western blot analysis or enzyme immunoassay, were increased, unaltered, and decreased, respectively. In contrast, the immunohistochemical study showed increases in a marker of axonal sprouting GAP-43, and a neurotrophic factor BDNF, and a decrease in an adhesion molecule L1 in some areas of the hippocampal ischemic penumbra of such animals. These results suggest that some factors for regeneration of the neuronal network in the ischemic penumbra responded to sustained cerebral ischemia for a certain period, although functional network of the nerve cells in the microsphere-injected hemisphere would be unlikely established after ME.

Applications of novel affinity cassette methods: use of peptide fusion handles for the purification of recombinant proteins

In this article, recent progress related to the use of different types of polypeptide fusion handles or 'tags' for the purification of recombinant proteins are critically discussed. In addition, novel aspects of the molecular cassette concept are elaborated, together with areas of potential application of these fundamental principles in molecular recognition. As evident from this review, the use of these concepts provides a powerful strategy for the high throughput isolation and purification of recombinant proteins and their derived domains, generated from functional genomic or zeomic studies, as part of the bioprocess technology leading to their commercial development, and in the study of molecular recognition phenomena per se. In addition, similar concepts can be exploited for high sensitivity analysis and detection, for the characterisation of protein bait/prey interactions at the molecular level, and for the immobilisation and directed orientation of proteins for use as biocatalysts/biosensors.

Involvement of integrin alpha(v)beta(3) and cell adhesion molecule L1 in transendothelial migration of melanoma cells

Tumor metastasis involves many stage-specific adhesive interactions. The expression of several cell adhesion molecules, notably the integrin alpha(v)beta(3), has been associated with the metastatic potential of tumor cells. In this study, we used a novel in vitro assay to examine the role of alpha(v)beta(3) in the transmigration of melanoma cells through a monolayer of human lung microvascular endothelial cells. Confocal microscopy revealed the presence of the integrin alpha(v)beta(3) on melanoma membrane protrusions and pseudopods penetrating the endothelial junction. alpha(v)beta(3) was also enriched in heterotypic contacts between endothelial cells and melanoma cells. Transendothelial migration of melanoma cells was inhibited by either a cyclic Arg-Gly-Asp peptide or the anti-alpha(v)beta(3) monoclonal antibody LM609. Although both platelet endothelial cell adhesion molecule-1 and L1 are known to bind integrin alpha(v)beta(3), only L1 serves as a potential ligand for alpha(v)beta(3) during melanoma transendothelial migration. Also, polyclonal antibodies against L1 partially inhibited the transendothelial migration of melanoma cells. However, addition of both L1 and alpha(v)beta(3) antibodies did not show additive effects, suggesting that they are components of the same adhesion system. Together, the data suggest that interactions between the integrin alpha(v)beta(3) on melanoma cells and L1 on endothelial cells play an important role in the transendothelial migration of melanoma cells.

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.

Intercellular adhesion molecule-5 induces dendritic outgrowth by homophilic adhesion

Intercellular adhesion molecule-5 (ICAM-5) is a dendritically polarized membrane glycoprotein in telencephalic neurons, which shows heterophilic binding to leukocyte beta(2)-integrins. Here, we show that the human ICAM-5 protein interacts in a homophilic manner through the binding of the immunoglobulin domain 1 to domains 4-5. Surface coated ICAM-5-Fc promoted dendritic outgrowth and arborization of ICAM- 5-expressing hippocampal neurons. During dendritogenesis in developing rat brain, ICAM-5 was in monomer form, whereas in mature neurons it migrated as a high molecular weight complex. The findings indicate that its homophilic binding activity was regulated by nonmonomer/monomer transition. Thus, ICAM-5 displays two types of adhesion activity, homophilic binding between neurons and heterophilic binding between neurons and leukocytes.

Trimerization of cell adhesion molecule L1 mimics clustered L1 expression on the cell surface: influence on L1-ligand interactions and on promotion of neurite outgrowth

Several studies indicate that cell adhesion molecules have to be clustered on the cell surface to engage in adhesive functions. We investigated adhesive functions of clustered versus monomeric L1 extracellular parts in vitro to distinguish how clustering affects ligand binding and promotion of neurite outgrowth. Trimeric L1 was recombinantly expressed and covalently assembled by the cartilage matrix protein's coiled-coil domain. Trimeric L1 has an apparent molecular mass of approximately 380 kDa in the nonreduced form and approximately 130 kDa in the reduced form. Rotary shadowing electron micrographs of trimeric L1 revealed a rod-like shape terminating in three globular domains. Monomeric L1 assumes a horseshoe shape of domains Ig I-IV followed by a rod-like structure consisting of Ig V and VI and fibronectin type III 1-5. Circular dichroism measurements showed that the secondary structure consists of beta-sheets. Trimeric L1 binds to itself, to monomeric L1, to laminin-1, and to alpha5beta1 integrin in a concentration-dependent manner. In contrast, binding of monomeric L1 could only be saturated with itself but not with laminin-1 and with alpha5beta1 integrin. Promotion of neurite outgrowth from PC12 cells cultured on adsorbed trimeric L1 was increased by 100%, whereas on monomeric L1 the increase was only 50% over the control value. Promotion of neurite outgrowth from PC12 cells was specifically inhibited in a concentration-dependent manner by a polyclonal antibody against L1. These findings show that clustering of only three extracellular domains increases considerably L1's binding affinity to different ligands and enhances neurite outgrowth, suggesting that adhesive functions of L1 on the cell surface depend on cluster formation.

The third fibronectin type III repeat is required for L1 to serve as an optimal substratum for neurite extension

As a means of defining functionally important regions of the L1 neuronal cell adhesion molecule, neurite outgrowth from cerebellar neurons was compared on monolayers of L1-negative B28 glioma cells, B28 cells transfected with wild-type human L1, and B28 cells transfected with variant forms of L1. Neurite outgrowth on L1-positive B28 cells is greatly enhanced over that seen on parental B28 cells. Neurite outgrowth on B28 cells expressing L1 variants that lack either the first or the fifth fibronectin type III repeat is comparable to that seen on monolayers expressing wild-type L1. In contrast, B28 cells expressing L1 without the third fibronectin type III repeat do not support neurite outgrowth above the background level seen on parental B28 cells. This suggests that the third fibronectin type III repeat plays a key role in the ability of L1 to promote neurite extension. This is consistent with reports that the third fibronectin type III repeat mediates L1 homomultimerization and integrin binding and that plasmin cleavage within this domain interferes with L1 function by abolishing these molecular interactions.

Nine novel L1 CAM mutations in families with X-linked hydrocephalus

Mutations in the gene for neural cell adhesion molecule L1 are responsible for the highly variable phenotype found in families with X-linked hydrocephalus, MASA syndrome, and spastic paraplegia type I. To date, 32 different mutations have been observed, the majority being unique to individual families. Here, we report nine novel mutations in L1 in 10 X-linked hydrocephalus families. Four mutations truncate the L1 protein and eliminate cell surface expression, and two would produce abnormal L1 through alteration of RNA processing. A further two of these mutations are small in-frame deletions that have occurred through a mechanism involving tandem repeated sequences. Together with a single missense mutation, these latter examples contribute to the growing number of existing mutations that affect short regions of the L1 protein that may have particular functional significance.

A potential role for the plasmin(ogen) system in the posttranslational cleavage of the neural cell adhesion molecule L1

L1 is a neural recognition molecule that promotes neural developmental and regenerative processes. Posttranslational cleavage of L1 is believed to be important for regulating its function in vivo, but little is known of the proteolytic systems responsible. In this study we present evidence that plasmin can regulate both L1 expression and function. The addition of plasmin to cell lines results in a dose-dependent loss of surface L1 expression, with the simultaneous appearance of soluble L1 species. The addition of plasminogen to primary neurons and melanoma cells also resulted in the generation of plasmin and the concomitant release of L1. One product of plasmin-mediated cleavage is an amino-terminal fragment of approximately 140 kDa that has been previously described as a natural posttranslational cleavage product in vivo. This fragment was confirmed to result from cleavage at two sites in the middle of the third fibronectin-like domain of L1. Cleavage at a further site, proximal to the transmembrane domain of L1, was also observed at higher plasmin concentrations. Plasmin was further confirmed to abrogate homophilic L1 interactions required for cellular aggregation. Based on these findings we propose that plasmin is likely to be an important regulator of L1-mediated processes including those documented in the nervous system.

Integrin and neurocan binding to L1 involves distinct Ig domains

The cell adhesion molecule L1, a 200-220-kDa type I membrane glycoprotein of the Ig superfamily, mediates many neuronal processes. Originally studied in the nervous system, L1 is expressed by hematopoietic and many epithelial cells, suggesting a more expanded role. L1 supports homophilic L1-L1 and integrin-mediated cell binding and can also bind with high affinity to the neural proteoglycan neurocan; however, the binding site is unknown. We have dissected the L1 molecule and investigated the cell binding ability of Ig domains 1 and 6. We report that RGD sites in domain 6 support alpha5beta1- or alphavbeta3-mediated integrin binding and that both RGD sites are essential. Cooperation of RGD sites with neighboring domains are necessary for alpha(5)beta(1). A T cell hybridoma and activated T cells could bind to L1 in the absence of RGDs. This binding was supported by Ig domain 1 and mediated by cell surface-exposed neurocan. Lymphoid and brain-derived neurocan were structurally similar. We also present evidence that a fusion protein of the Ig 1-like domain of L1 can bind to recombinant neurocan. Our results support the notion that L1 provides distinct cell binding sites that may serve in cell-cell or cell-matrix interactions.

The cell adhesion molecule L1 is developmentally regulated in the renal epithelium and is involved in kidney branching morphogenesis

We immunopurified a surface antigen specific for the collecting duct (CD) epithelium. Microsequencing of three polypeptides identified the antigen as the neuronal cell adhesion molecule L1, a member of the immunoglobulin superfamily. The kidney isoform showed a deletion of exon 3. L1 was expressed in the mesonephric duct and the metanephros throughout CD development. In the adult CD examined by electron microscopy, L1 was not expressed on intercalated cells but was restricted to CD principal cells and to the papilla tall cells. By contrast, L1 appeared late in the distal portion of the elongating nephron in the mesenchymally derived epithelium and decreased during postnatal development. Immunoblot analysis showed that expression, proteolytic cleavage, and the glycosylation pattern of L1 protein were regulated during renal development. L1 was not detected in epithelia of other organs developing by branching morphogenesis. Addition of anti-L1 antibody to kidney or lung organotypic cultures induced dysmorphogenesis of the ureteric bud epithelium but not of the lung. These results suggest a functional role for L1 in CD development in vitro. We further postulate that L1 may be involved in the guidance of developing distal tubule and in generation and maintenance of specialized cell phenotypes in CD.

Neurexophilin binding to alpha-neurexins. A single LNS domain functions as an independently folding ligand-binding unit

alpha-Neurexins (Ialpha, IIalpha, and IIIalpha) are receptor-like proteins expressed in hundreds of isoforms on the neuronal cell surface. The extracellular domains of alpha-neurexins are composed of six LNS repeats, named after homologous sequences in the Laminin A G domain, Neurexins, and Sex hormone-binding globulin, with three interspersed epidermal growth factor-like domains. Purification of neurexin Ialpha revealed that it is tightly complexed to a secreted glycoprotein called neurexophilin 1. Neurexophilin 1 is a member of a family of at least four genes and resembles a neuropeptide, suggesting a function as an endogenous ligand for alpha-neurexins. We have now used recombinant proteins and knockout mice to investigate which isoforms and domains of different neurexins and neurexophilins interact with each other. We show that neurexophilins 1 and 3 but not 4 (neurexophilin 2 is not expressed in rodents) bind to a single individual LNS domain, the second overall LNS domain in all three alpha-neurexins. Although this domain is alternatively spliced, all splice variants bind, suggesting that alternative splicing does not regulate binding. Using homologous recombination to disrupt the neurexophilin 1 gene, we generated mutant mice that do not express detectable neurexophilin 1 mRNA. Mice lacking neurexophilin 1 are viable with no obvious morbidity or mortality. However, homozygous mutant mice exhibit male sterility, probably because homologous recombination resulted in the co-insertion into the neurexophilin gene of herpes simplex virus thymidine kinase, which is known to cause male sterility. In the neurexophilin 1 knockout mice, neurexin Ialpha is complexed with neurexophilin 3 but not neurexophilin 4, suggesting that neurexophilin 1 is redundant with neurexophilin 3 and that neurexophilins 1 and 3 but not 4 bind to neurexins. This hypothesis was confirmed using expression experiments. Our data reveal that the six LNS and three epidermal growth factor domains of neurexins are independently folding ligand-binding domains that may interact with distinct targets. The results support the notion that neurexophilins represent a family of extracellular signaling molecules that interact with multiple receptors including all three alpha-neurexins.

The site of a missense mutation in the extracellular Ig or FN domains of L1CAM influences infant mortality and the severity of X linked hydrocephalus

The L1 cell adhesion molecule (L1CAM) plays an important role in axon growth, fasciculation, and neural migration. Mutations in the L1CAM gene produce a phenotype characterised by X linked hydrocephalus, mental retardation, spastic paraplegia, adducted thumbs, and agenesis of the corpus callosum. We have conducted a detailed analysis of the phenotypic effects of missense mutations in the extracellular portion of L1CAM, following a study that differentiated between "key" amino acid residues critical for maintaining the conformation of the extracellular immunoglobulin type C-like (Ig) or fibronectin type III-like (FN) domains and surface residues of less certain significance. We have analysed the data from 71 published cases and seven patients whose mutations were detected in our laboratory to determine if the site of a missense mutation in the Ig or FN domains correlated with the severity of hydrocephalus, presence of adducted thumbs, or survival past infancy. Mutations affecting the key residues in either type of domain were more likely to produce a phenotype with severe hydrocephalus, adducted thumbs, and lifespan less than one year than were mutations affecting surface residues. In addition, mutations affecting the FN domains were more likely than those affecting Ig domains to produce a phenotype with severe hydrocephalus, with less certain effects on adducted thumbs and lifespan. Mutations in key residues of the FN domains were particularly deleterious to infant survival. These data provide information that may be useful in predicting some aspects of the phenotypic effects of certain L1CAM mutations.

Cell-cell interactions during transendothelial migration of tumor cells

A key event in cancer metastasis is the transendothelial migration of tumor cells. This process involves multiple adhesive interactions between tumor cells and the endothelium. After adhering to the surface of endothelial cells, tumor cells must penetrate the endothelial junction, which contains high concentrations of the cell adhesion molecules VE-cadherin and PECAM-1. Studies using an in vitro model system, consisting of melanoma cells which are seeded onto a monolayer of endothelial cells cultured on Matrigel, have revealed reorganization of the cytoskeleton and dynamic changes in the cell shape of both tumor and endothelial cells. The initial stages of transmigration are characterized by numerous membrane blebs protruding from the basolateral surfaces of the melanoma cells. Contact regions also show an abundance of microfilaments arising from the underlying endothelial cells. These adhesive interactions lead to the redistribution of both VE-cadherin and PECAM-1 and, consequently, a localized dissolution of the endothelial junction. The penetration of the endothelial junction is initiated by melanoma pseudopods. Despite the disappearance of VE-cadherin from the retracting endothelial junction, heterotypic contacts between the tumor cell and its surrounding endothelial cells show a high concentration of pan-cadherin staining, suggesting that transmigration of melanoma cells might yet be facilitated by interactions with another member of the cadherin family. Upon adhesion to the Matrigel, melanoma cells begin to spread and invade the matrix material, while the endothelial cells extend processes over the melanoma cells to reform the monolayer. Interestingly, the leading margins of these endothelial processes contain a high concentration ofN-cadherin. VE-cadherin and PECAM-1 reappear only when the advancing endothelial processes meet to reform the endothelial junction. Together, these observations suggest that endothelial cells actively participate in the transmigration of tumor cells and specific cadherins are involved in different steps of this complex process.

Role of L1 in neural development: what the knockouts tell us

Mutations in the cell adhesion molecule L1 cause severe developmental anomalies in the human nervous system. Recent descriptions of L1 gene knock-out mice from three research groups demonstrate that these mice are strikingly similar to humans with mutations in the L1 gene. In both humans and mice there are defects in the development of the corticospinal tract and cerebellar vermis, hydrocephalus, and impaired learning. The production of a viable animal model for X-linked hydrocephalus suggests that unanswerable questions posed by the human disease will finally be approachable using modern experimental methods.

Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules

We analyzed changes in the expression of mRNAs for the axonal growth-promoting cell recognition molecules L1.1, L1.2, and neural cell adhesion molecule (NCAM) after a rostral (proximal) or caudal (distal) spinal cord transection in adult zebrafish. One class of cerebrospinal projection nuclei (represented by the nucleus of the medial longitudinal fascicle, the intermediate reticular formation, and the magnocellular octaval nucleus) showed a robust regenerative response after both types of lesions as determined by retrograde tracing and/or in situ hybridization for GAP-43. A second class (represented by the nucleus ruber, the nucleus of the lateral lemniscus, and the tangential nucleus) showed a regenerative response only after proximal lesion. After distal lesion, upregulation of L1.1 and L1.2 mRNAs, but not NCAM mRNA expression, was observed in the first class of nuclei. The second class of nuclei did not show any changes in their mRNA expression after distal lesion. After proximal lesion, both classes of brain nuclei upregulated L1.1 mRNA expression (L1.2 and NCAM were not tested after proximal lesion). In the glial environment distal to the spinal lesion, labeling for L1.2 mRNA but not L1.1 or NCAM mRNAs was increased. These results, combined with findings in the lesioned retinotectal system of zebrafish (Bernharnhardt et al., 1996), indicate that the neuron-intrinsic regulation of cell recognition molecules after axotomy depends on the cell type as well as on the proximity of the lesion to the neuronal soma. Glial reactions differ for different regions of the CNS.

Adhesion molecules and inherited diseases of the human nervous system

Mutations in the human genes for the adhesion molecules Po, L1, and merosin cause severe abnormalities in nervous system development. Po and merosin are required for normal myelination in the nervous system, and L1 is essential for development of major axon pathways such as the corticospinal tract and corpus callosum. While mutations that lead to a loss of the adhesive function of these molecules produce severe phenotypes, mutations that disrupt intracellular signals or intracellular interactions are also deleterious. Geneticists have found that more than one clinical syndrome can be caused by mutations in each of these adhesion molecules, confirming that these proteins are multifunctional. This review focuses on identifying common mechanisms by which mutations in adhesion molecules alter neural development.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Neural cell recognition molecule L1: from cell biology to human hereditary brain malformations

The neural cell recognition molecule L1 is a member of the immunoglobulin superfamily implicated in embryonic brain development. L1 is engaged in complex extracellular interactions, with multiple binding partners on cell surfaces and in the extracellular matrix. It is the founder of a neural group of related cell surface receptors that share with L1 a highly conserved cytoplasmic domain that associates with the cytoskeleton. Phenotypic analyses of human patients with mutations in the L1 gene and characterizations of L1-deficient mice suggest that L1 is important for embryonic brain histogenesis, in particular the development of axon tracts.

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.

A conserved role for L1 as a transmembrane link between neuronal adhesion and membrane cytoskeleton assembly

The L1-family of cell adhesion molecules is involved in many important aspects of nervous system development. Mutations in the human L1-CAM gene cause a complicated array of neurological phenotypes; however, the molecular basis of these effects cannot be explained by a simple loss of adhesive function. Human L1-CAM and its Drosophila homolog neuroglian are rather divergent in sequence, with the highest degree of amino acid sequence conservation between segments of their cytoplasmic domains. In an attempt to elucidate the fundamental functions shared between these distantly related members of the L1-family, we demonstrate here that the extracellular domains of mammalian L1-CAMs and Drosophila neuroglian are both able to induce the aggregation of transfected Drosophila S2 cells in vitro. To a limited degree they even interact with each other in cell adhesion and neurite outgrowth assays. The cytoplasmic domains of human L1-CAM and neuroglian are both able to interact with the Drosophila homolog of the cytoskeletal linker protein ankyrin. Moreover the recruitment of ankyrin to cell-cell contacts is completely dependent on L1-mediated cell adhesion. These findings support a model of L1 function in which the phenotypes of human L1-CAM mutations results from a disruption of the link between the extracellular environment and the neuronal cytoskeleton.

A single immunoglobulin-like domain of the human neural cell adhesion molecule L1 supports adhesion by multiple vascular and platelet integrins

The neural cell adhesion molecule L1 has been shown to function as a homophilic ligand in a variety of dynamic neurological processes. Here we demonstrate that the sixth immunoglobulin-like domain of human L1 (L1-Ig6) can function as a heterophilic ligand for multiple members of the integrin superfamily including alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. The interaction between L1-Ig6 and alphaIIbbeta3 was found to support the rapid attachment of activated human platelets, whereas a corresponding interaction with alphavbeta3 and alphavbeta1 supported the adhesion of umbilical vein endothelial cells. Mutation of the single Arg-Gly-Asp (RGD) motif in human L1-Ig6 effectively abrogated binding by the aforementioned integrins. A L1 peptide containing this RGD motif and corresponding flanking amino acids (PSITWRGDGRDLQEL) effectively blocked L1 integrin interactions and, as an immobilized ligand, supported adhesion via alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. Whereas beta3 integrin binding to L1-Ig6 was evident in the presence of either Ca2+, Mg2+, or Mn2+, a corresponding interaction with the beta1 integrins was only observed in the presence of Mn2+. Furthermore, such Mn2+-dependent binding by alpha5beta1 and alphavbeta1 was significantly inhibited by exogenous Ca2+. Our findings suggest that physiological levels of calcium will impose a hierarchy of integrin binding to L1 such that alphavbeta3 or active alphaIIbbeta3 > alphavbeta1 > alpha5beta1. Given that L1 can interact with multiple vascular or platelet integrins it is significant that we also present evidence for de novo L1 expression on blood vessels associated with certain neoplastic or inflammatory diseases. Together these findings suggest an expanded and novel role for L1 in vascular and thrombogenic processes.

The leukocyte/neuron cell surface antigen OX2 binds to a ligand on macrophages

The OX2 membrane glycoprotein contains two immunoglobulin superfamily (IgSF) domains and seems likely to interact with other cell surface proteins. A soluble chimeric protein with the two IgSF domains of OX2 engineered onto domains 3 + 4 of rat CD4 antigen was expressed. To detect possible weak interactions, the chimeric protein was coupled to fluorescent covaspheres to provide a highly avid display of OX2. The OX2 covaspheres bound macrophages but not other cell types. The specificity of the interaction was demonstrated by blocking with Fab fragments of the OX2 monoclonal antibody (mAb). A new mAb, MRC OX88, was raised against macrophages which also blocked the interaction and presumably recognizes the ligand. The epitope for the MRC OX2 mAb and a site for ligand binding were mapped to domain 1 by site-directed mutagenesis. The OX2 antigen is present on thymocytes, some lymphocytes, neurons and endothelial cells; thus, it has the potential to mediate interactions between these cell types and macrophages.

Neural cell adhesion molecule L1: signaling pathways and growth cone motility

The neural cell adhesion molecule L1 plays a key role in nervous system development including neuronal migration, neurite growth, and axonal fasciculation. L1 is expressed on most developing axons, and homophilic binding of L1 molecules on adjacent axons is likely to play a key role in axon extension. It is now well documented that a number of second-messenger systems are involved in L1-stimulated neurite growth in vitro. However, it is unclear how L1 homophilic or heterophilic binding trigger signals that regulate the mechanical forces that produce axon extension. In this report, we will review recent advances in understanding L1-associated signals, L1 interactions with the cytoskeleton, and the molecular mechanisms underlying growth cone motility.

CRASH syndrome: mutations in L1CAM correlate with severity of the disease

X-linked hydrocephalus, MASA syndrome and certain forms of X-linked spastic paraplegia and agenesis of corpus callosum are now known to be due to mutations in the gene for the neural cell adhesion molecule L1 (19, 30). As a result, these syndromes have recently been reclassified as CRASH syndrome, an acronym for Corpus callosum hypoplasia, Retardation, Adducted thumbs, Spasticity and Hydrocephalus (8). A comparison of existing case reports with molecular genetic analysis reveals a striking correlation between the type of mutation in the L1CAM gene and the severity of the disease. Mutations that produce truncations in the extracellular domain of the L1 protein are more likely to produce severe hydrocephalus, grave mental retardation or early death than point mutations in the extracellular domain or mutations affecting only the cytoplasmic domain of the protein. While less severe than extracellular truncations, point mutations in the extracellular domain do produce more severe neurologic problems than mutations in just the cytoplasmic domain.

Tyrosine phosphorylation at a site highly conserved in the L1 family of cell adhesion molecules abolishes ankyrin binding and increases lateral mobility of neurofascin

This paper presents evidence that a member of the L1 family of ankyrin-binding cell adhesion molecules is a substrate for protein tyrosine kinase(s) and phosphatase(s), identifies the highly conserved FIGQY tyrosine in the cytoplasmic domain as the principal site of phosphorylation, and demonstrates that phosphorylation of the FIGQY tyrosine abolishes ankyrin-binding activity. Neurofascin expressed in neuroblastoma cells is subject to tyrosine phosphorylation after activation of tyrosine kinases by NGF or bFGF or inactivation of tyrosine phosphatases with vanadate or dephostatin. Furthermore, both neurofascin and the related molecule Nr-CAM are tyrosine phosphorylated in a developmentally regulated pattern in rat brain. The FIGQY sequence is present in the cytoplasmic domains of all members of the L1 family of neural cell adhesion molecules. Phosphorylation of the FIGQY tyrosine abolishes ankyrin binding, as determined by coimmunoprecipitation of endogenous ankyrin and in vitro ankyrin-binding assays. Measurements of fluorescence recovery after photobleaching demonstrate that phosphorylation of the FIGQY tyrosine also increases lateral mobility of neurofascin expressed in neuroblastoma cells to the same extent as removal of the cytoplasmic domain. Ankyrin binding, therefore, appears to regulate the dynamic behavior of neurofascin and is the target for regulation by tyrosine phosphorylation in response to external signals. These findings suggest that tyrosine phosphorylation at the FIGQY site represents a highly conserved mechanism, used by the entire class of L1-related cell adhesion molecules, for regulation of ankyrin-dependent connections to the spectrin skeleton.

Role of Csk in neural differentiation of the embryonic carcinoma cell line P19

To examine the neural function of Csk (C-terminal Src kinase), a membrane-targeted form of Csk (Src/Csk) and its kinase-defective variant (DK-Src/Csk) were expressed in the embryonic carcinoma cell line P19. Expression of Src/Csk, but not DK-Src/Csk, caused reduction of the specific activities of Src and Fyn in the differentiated P19 cells. During neural differentiation, the specific activity of Src was elevated in the control P19 cells, whereas the activation was completely eliminated in the Src/Csk transfectant. In normally differentiated P19 cells, cross-linking of a cell adhesion molecule, L1, induced a short-term activation of Src and Fyn. In the Src/Csk transfectant, L1 stimulation induced delayed activation of Src and Fyn peaking at much lower levels than in the control cells. Src/Csk transfectants developed normally in the initial stages of neural differentiation, but exhibited an apparent defect in cell-to-cell interaction, i.e. neurite fasciculation and aggregation of cell bodies, in the latter stages. These findings imply that Csk is involved in the regulation of Src family kinases that play roles in cell-to-cell interaction mediated by cell adhesion molecules.