Review: a role for the FGF receptor in the axonal growth response stimulated by cell adhesion molecules?

7,8-dihydroxyflavone ameliorates cognitive and motor deficits in a Huntington's disease mouse model through specific activation of the PLCγ1 pathway

Huntington's disease (HD) is a fatal neurodegenerative disease with motor, cognitive and psychiatric impairment. Dysfunctions in HD models have been related to reduced levels of striatal brain-derived neurotrophic factor (BDNF) and imbalance between its receptors TrkB and p75(NTR). Thus, molecules with activity on the BDNF/TrkB/p75 system can have therapeutic potential. 7,8-Dihydroxyflavone (7,8-DHF) was described as a TrkB agonist in several models of neuro-degenerative diseases, however, its TrkB activation profile needs further investigation due to its pleiotropic properties and divergence from BDNF effect. To investigate this, we used in vitro and in vivo models of HD to dissect TrkB activation upon 7,8-DHF treatment. 7,8-DHF treatment in primary cultures showed phosphorylation of TrkBY816 but not TrkBY515 with activation of the PLCγ1 pathway leading to morphological and functional improvements. Chronic administration of 7,8-DHF delayed motor deficits in R6/1 mice and reversed deficits on the Novel Object Recognition Test (NORT) at 17 weeks. Morphological and biochemical analyses revealed improved striatal levels of enkephalin, and prevention of striatal volume loss. We found a TrkBY816 but not TrkBY515 phosphorylation recovery in striatum concordant with in vitro results. Additionally, 7,8-DHF normalized striatal levels of induced and neuronal nitric oxide synthase (iNOS and nNOS, respectively) and ameliorated the imbalance of p75/TrkB. Our results provide new insights into the mechanism of action of 7,8-DHF suggesting that its effect through the TrkB receptor in striatum is via selective phosphorylation of its Y816 residue and activation of PLCγ1 pathway, but pleiotropic effects of the drug also contribute to its therapeutic potential.

Generation, characterization and preclinical studies of a human anti-L1CAM monoclonal antibody that cross-reacts with rodent L1CAM

L1 cell adhesion molecule (L1CAM) is aberrantly expressed in malignant tumors and plays important roles in tumor progression. Thus, L1CAM could serve as a therapeutic target and anti-L1CAM antibodies may have potential as anticancer agents. However, L1CAM is expressed in neural cells and the druggability of anti-L1AM antibody must be validated at the earliest stages of preclinical study. Here, we generated a human monoclonal antibody that is cross-reactive with mouse L1CAM and evaluated its pharmacokinetic properties and anti-tumor efficacy in rodent models. First, we selected an antibody (Ab4) that binds human and mouse L1CAM from the human naïve Fab library using phage display, then increased its affinity 45-fold through mutation of 3 residues in the complementarity-determining regions (CDRs) to generate Ab4M. Next, the affinity of Ab4M was increased 1.8-fold by yeast display of single-chain variable fragment containing randomly mutated light chain CDR3 to generate Ab417. The affinities (K_D) of Ab417 for human and mouse L1CAM were 0.24 nM and 79.16 pM, respectively. Ab417 specifically bound the Ig5 domain of L1CAM and did not exhibit off-target activity, but bound to the peripheral nerves embedded in normal human tissues as expected in immunohistochemical analysis. In a pharmacokinetics study, the mean half-life of Ab417 was 114.49 h when a single dose (10 mg/kg) was intravenously injected into SD rats. Ab417 significantly inhibited tumor growth in a human cholangiocarcinoma xenograft nude mouse model and did not induce any adverse effect in in vivo studies. Thus, Ab417 may have potential as an anticancer agent.

Asymmetric tyrosine kinase arrangements in activation or autophosphorylation of receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) play important roles in the control of many cellular processes including cell proliferation, cell adhesion, angiogenesis, and apoptosis. Ligand-induced dimerization of RTKs leads to autophosphorylation and activation of RTKs. Structural studies have shown that while isolated ectodomains of several RTKs form symmetric dimers the isolated cytoplasmic kinase domains of epidermal growth factor receptor (EGFR) and fibroblast growth factor receptor (FGFR) form asymmetric dimers during their activation. Binding of one kinase molecule of EGFR to a second kinase molecule asymmetrically leads to stimulation of kinase activity and enhanced autophosphorylation. Furthermore, the structures of the kinase domain of FGFR1 and FGFR2 reveal the formation of asymmetric interfaces in the processes of autophosphorylation at their specific phosphotyrosine (pY) sites. Disruption of asymmetric dimer interface of EGFR leads to reduction in enzymatic activity and drastic reduction of autophosphorylation of FGFRs in ligand-stimulated live cells. These studies demonstrate that asymmetric dimer formation is as a common phenomenon critical for activation and autophosphorylation of RTKs.

Fibronectin type III (FN3) modules of the neuronal cell adhesion molecule L1 interact directly with the fibroblast growth factor (FGF) receptor

The neuronal cell adhesion molecule (CAM) L1 promotes axonal outgrowth, presumably through an interaction with the fibroblast growth factor receptor (FGFR). The present study demonstrates a direct interaction between L1 fibronectin type III (FN3) modules I-V and FGFR1 immunoglobulin (Ig) modules II and III by surface plasmon resonance analysis. Binding of L1 to FGFR1 was enhanced by adenosine 5'-triphosphate (ATP), adenylylmethylenediphosphonate (AMP-PCP), and guanosine-5'-triphosphate (GTP), but not adenosine monophosphate (AMP). The L1-FN3 modules were capable of activating FGFR1, reflected by receptor phosphorylation, and this resulted in the induction of differentiation of primary neurons, reflected by neurite outgrowth. Furthermore, ATP modulated L1-induced neuronal differentiation and FGFR1 phosphorylation through regulation of the L1-FGFR1 interaction.

Pre- and postsynaptic actions of L1-CAM in nicotinic pathways

Cell adhesion molecules (CAMs) have long been known to guide axon outgrowth and pathfinding. More recent evidence indicates they contribute to synapse formation as well. The L1 family of IgCAMs has been implicated in long-term potentiation, learning, and some features of synaptic structure. We show here that L1 is localized in nicotinic pathways at both pre- and postsynaptic sites. In the chick ciliary ganglion, postsynaptic L1 is associated with nicotinic receptors and potentiates their downstream signaling. Postsynaptic L1 is also important for aligning presynaptic structures over the postsynaptic cell. Dominant negative experiments suggest this latter effect depends on homophilic interactions with presynaptic L1. At the neuromuscular junction L1 is also found presynaptically where dominant negative experiments again indicate a role in aligning presynaptic structures over postsynaptic receptors, both in culture and in vivo. These findings identify new roles for L1 at nicotinic synapses and underscore the multipotency of L1-CAMs.

PTPσ promotes retinal neurite outgrowth non-cell-autonomously

The receptor-like protein tyrosine phosphatase (RPTP) PTPsigma controls the growth and targeting of retinal axons, both in culture and in ovo. Although the principal actions of PTPsigma have been thought to be cell-autonomous, the possibility that RPTPs related to PTPsigma also have non-cell-autonomous signaling functions during axon development has also been supported genetically. Here we report that a cell culture substrate made from purified PTPsigma ectodomains supports retinal neurite outgrowth in cell culture. We show that a receptor for PTPsigma must exist on retinal axons and that binding of PTPsigma to this receptor does not require the known, heparin binding properties of PTPsigma. The neurite-promoting potential of PTPsigma ectodomains requires a basic amino acid domain, previously demonstrated in vitro as being necessary for ligand binding by PTPsigma. Furthermore, we demonstrate that heparin and oligosaccharide derivatives as short as 8mers, can specifically block neurite outgrowth on the PTPsigma substrate, by competing for binding to this same domain. This is the first direct evidence of a non-cell-autonomous, neurite-promoting function of PTPsigma and of a potential role for heparin-related oligosaccharides in modulating neurite promotion by an RPTP.

Planarian fibroblast growth factor receptor homologs expressed in stem cells and cephalic ganglions

The strong regenerative capacity of planarians is considered to reside in the totipotent somatic stem cell called the 'neoblast'. However, the signal systems regulating the differentiation/growth/migration of stem cells remain unclear. The fibroblast growth factor (FGF)/FGF receptor (FGFR) system is thought to mediate various developmental events in both vertebrates and invertebrates. We examined the molecular structures and expression of DjFGFR1 and DjFGFR2, two planarian genes closely related to other animal FGFR genes. DjFGFR1 and DjFGFR2 proteins contain three and two immunoglobulin-like domains, respectively, in the extracellular region and a split tyrosine kinase domain in the intracellular region. Expression of DjFGFR1 and DjFGFR2 was observed in the cephalic ganglion and mesenchymal space in intact planarians. In regenerating planarians, accumulation of DjFGFR1-expressing cells was observed in the blastema and in fragments regenerating either a pharynx or a brain. In X-ray-irradiated planarians, which had lost regenerative capacity, the number of DjFGFR1-expressing cells in the mesenchymal space decreased markedly. These results suggest that the DjFGFR1 protein may be involved in the signal systems controlling such aspects of planarian regeneration as differentiation/growth/migration of stem cells.

Patterning the optic neuroepithelium by FGF signaling and Ras activation

During vertebrate embryogenesis, the neuroectoderm differentiates into neural tissues and also into non-neural tissues such as the choroid plexus in the brain and the retinal pigment epithelium in the eye. The molecular mechanisms that pattern neural and non-neural tissues within the neuroectoderm remain unknown. We report that FGF9 is normally expressed in the distal region of the optic vesicle that is destined to become the neural retina, suggesting a role in neural patterning in the optic neuroepithelium. Ectopic expression of FGF9 in the proximal region of the optic vesicle extends neural differentiation into the presumptive retinal pigment epithelium, resulting in a duplicate neural retina in transgenic mice. Ectopic expression of constitutively active Ras is also sufficient to convert the retinal pigment epithelium to neural retina, suggesting that Ras-mediated signaling may be involved in neural differentiation in the immature optic vesicle. The original and the duplicate neural retinae differentiate and laminate with mirror-image polarity in the absence of an RPE, suggesting that the program of neuronal differentiation in the retina is autonomously regulated. In mouse embryos lacking FGF9, the retinal pigment epithelium extends into the presumptive neural retina, indicating a role of FGF9 in defining the boundary of the neural retina.

Dysregulation of the neural cell adhesion molecule and neuropsychiatric disorders

Cell adhesion molecule proteins play a diverse role in neural development, signal transduction, structural linkages to extracellular and intracellular proteins, synaptic stabilization, neurogenesis, and learning. Three basic mRNA isoforms and potent posttranslational modifications differentially regulate these neurobiological properties of the neural cell adhesion molecule (N-CAM). Abnormal concentrations of N-CAM 105-115 kDa (cN-CAM), N-CAM variable alternative spliced exon (VASE), and N-CAM secreted exon (SEC) are related to schizophrenia and bipolar neuropsychiatric disorders. These N-CAM isoforms provide potential mechanisms for expression of multiple neurobiological alterations between controls and individuals with schizophrenia or bipolar illness. Multiple processes can trigger the dysregulation of N-CAM isoforms. Differences in neuropil volume, neuronal diameter, gray matter thickness, and ventricular size can be related to N-CAM neurobiological properties in neuropsychiatric disorders. Potential test of the N-CAM dysregulation hypothesis of neuropsychiatric disorder is whether ongoing dysregulation of N-CAM would cause cognitive impairments, increased lateral ventricle volume, and decreased hippocampal volume observed in schizophrenia and to a lesser extent in bipolar disorder. An indirect test of this theory conducted in animal experiments lend support to this N-CAM hypothesis. N-CAM dysregulation is consistent with a synaptic abnormality that could underlie the disconnection between brain regions consistent with neuroimaging reports. Synapse stability and plasticity may be part of the molecular neuropathology of these disorders.

Expression of E-, P-, n-cadherins and catenins in human bladder carcinoma cell lines

PURPOSE

Cadherins are cell surface glycoproteins that mediate Ca2+-dependent, homophilic cell-cell adhesion. The classical cadherins, E-, P- and N-cadherins, are known to self-associate from their extracellular domain, while their cytoplasmic domain interacts with either beta-catenin or plakoglobin (gamma-catenin), which in turn is bound to alpha-catenin that links the complex to the actin cytoskeleton. The aim of the present study was to analyze the expression of E-, P- and N-cadherins and catenins in human bladder carcinoma cells.

MATERIALS AND METHODS

Five human bladder carcinoma cell lines, representing a variety of differentiation states, were grown in cell culture. We performed a cell aggregation assay, specific for biological cadherin activity. The expression of cadherins and catenins was analyzed by immunocytochemistry, Western blotting and RT-PCR. The interactions between cadherins and catenins were assessed by immunoprecipitation.

RESULTS

We observed a reduced E-cadherin expression in the poorly differentiated and invasive-tumor derived cells. Interestingly, immunofluorescence study reveals the persistent localization of catenins at intercellular contacts in two E-cadherin deficient cell lines (T24 and TCCSUP) which yet exhibit an epithelial-like morphology and a calcium-dependent adhesive capacity. This suggests that other cadherin(s) are expressed in these both cell lines. P-cadherin, another epithelial cadherin, is expressed only in E-cadherin positive cells. On the other hand, N-cadherin is present at cell-cell borders in the very anaplastic cell lines, T24 and TCCSUP, and is able to link beta-catenin or plakoglobin.

CONCLUSION

These results indicate that N-cadherin may participate in intercellular adhesion, while facilitating bladder tumorigenesis.

Analysis of the L1-deficient mouse phenotype reveals cross-talk between Sema3A and L1 signaling pathways in axonal guidance

In humans, defects of the corticospinal tract have been attributed to mutations in the gene encoding L1 CAM, a phenotype that is reproduced in L1-deficient mice. Using coculture assays, we report that Sema3A secreted from the ventral spinal cord repels cortical axons from wild-type but not from L1-deficient mice. L1 and neuropilin-1 (NP-1) form a stable complex, and their extracellular domains can directly associate. Thus, L1 is a component of the Sema3A receptor complex, and L1 mutations may disrupt Sema3A signaling in the growth cone, leading to guidance errors. Addition of soluble L1Fc chimeric molecules does not restore Sema3A responsiveness of L1-deficient axons; instead, it converts the repulsion of wild-type axons into an attraction, further supporting a function for L1 in the Sema3A transducing pathways within the growth cone.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

Cadherins and their connections: adhesion junctions have broader functions

Cadherins - a family of cell-cell adhesion molecules - are linked to the actin cytoskeleton via intervening proteins. Recent results address molecular explanations for observed cadherin behavior, point to signals that regulate adhesion by modulating elements of the cadherin-associated complex, challenge the belief that different cadherins generally cannot cross-adhere, and highlight instructive roles for cadherins in cell signaling and differentiation.

I-type lectins in the nervous system

The number of animal lectins, basically defined upon their interaction with specific carbohydrate structures, is growing considerably during the last few years. Among these proteins the recently identified subfamily of I-type lectins consists of mainly transmembranous glycoproteins belonging to the immunoglobulin superfamily. Most of the I-type lectins participate in cell adhesion events, as are the different sialoadhesins recognizing sialylated glycan structures, which represent the best characterized subgroup. I-type lectins are abundant in the nervous system and have been implicated in a number of morphogenetic processes as fundamental as axon growth, myelin formation and growth factor signaling. In the present review, we summarize the structural and functional properties of I-type lectins expressed in neural tissues with a main focus on the sialoadhesin myelin-associated glycoprotein, the neural cell adhesion molecule and the fibroblast growth factor receptors.

Activation of transfected M1 or M3 muscarinic acetylcholine receptors induces cell-cell adhesion of Chinese hamster ovary cells expressing endogenous cadherins

Expression of endogenous cadherins by Chinese hamster ovary (CHO) cells has not been previously reported. However, we observed that CHO cells adhere to one another upon activation of transfected muscarinic acetylcholine receptors (mAChR), suggesting that the cells express endogenous cadherins. A 160-base pair RT-PCR product with 100% homology to the cytoplasmic domain of human E-cadherin was amplified from CHO cells. A second RT-PCR product amplified from these cells has 92% homology to the cytoplasmic domain of human cadherin-9 and 86% homology to the cytoplasmic domain of human cadherin-6. Western blotting indicates that CHO cells express a 165-kDa protein recognized by E-cadherin antibodies and a 120-kDa protein recognized by an antibody to the cadherin C-terminus sequence. The ability of transfected mAChR subtypes to regulate cadherin-mediated adhesion of CHO cells was tested by measuring the permeation of horseradish peroxidase across confluent CHO cell monolayers, by microscopic examination of the cells, and by aggregation assays. Cell-cell adhesion is induced within 15 min of activating transfected M1 or M3 mAChR which functionally couple to protein kinase C (PKC). In contrast, CHO cell adhesion is not affected by activating transfected M2 mAChR which functionally couple to other effectors. Activation of PKC with phorbol esters also induces cell-cell adhesion of all CHO sublines tested. Immunofluorescence assays reveal that endogenous cadherins redistribute on the plasma membrane of CHO cells following mAChR or PKC activation. Inactivation of cadherins by removal of extracellular Ca2+ abrogates adhesion induced by mAChR or PKC activation. Our demonstration that activation of only odd-numbered mAChR subtypes induces cadherin-mediated adhesion suggests that the unique responses of cells to M1 or M3 mAChR stimulation may involve cadherin activation.

Estrogen promotes the initial migration and inception of NgCAM-dependent calcium-signaling by new neurons of the adult songbird brain

The adult avian forebrain continues to generate neurons from ventricular zone (VZ) precursor cells, whose neuronal progeny then migrate into the brain parenchyma. Migrating neurons respond to the Ig-family adhesion molecule NgCAM with increments in cytosolic calcium, and migration is disrupted by anti-NgCAM Ig. The calcium response to NgCAM is developmentally restricted to bipolar migrants during a period spanning 6 to 9 DIV. This period corresponds to the postmitotic age at which new neurons leave the adult VZ to traverse a subjacent layer of estrogen-receptive "gatekeeper" neurons. Since neuronal passage through this layer occurs concurrently with the onset of NgCAM-dependent calcium signaling, we asked whether acquisition of the calcium response to NgCAM required estrogen exposure. Among neurons arising from explants of the adult finch neostriatal VZ, only those supplemented with estrogen developed calcium responses to NgCAM; neither explants raised in the absence of estrogen, nor those supplemented with testosterone, did so. Neurons in all three groups expressed NgCAM, had equivalent baseline calcium levels, and responded identically to K+-depolarization. Nonetheless, many more neurons migrated from explants of both finch and canary VZ raised in estrogen-supplemented media than from their estrogen-deprived counterparts, even though no effect of estrogen on neuronal survival per se was noted. These findings suggest that estrogen encourages the initial departure and assumption of signal competence by neurons arising from the adult avian VZ, thereby promoting their parenchymal recruitment and migration success.

The neural cell adhesion molecule (NCAM) in development and plasticity of the nervous system

The neural cell adhesion molecule (NCAM) is a member of the immunoglobulin superfamily and is strongly expressed in the nervous system. NCAM is found in three major forms, of which two--NCAM-140 and NCAM-180--are transmembrane proteins, while the third--NCAM-120--is attached to the membrane via a glycosylphosphatidyl inositol anchor. In addition, soluble NCAM forms exist in brain, cerebrospinal fluid, and plasma. NCAM mediates cell adhesion through homophilic as well as through heterophilic interactions. Following NCAM binding, transmembrane signalling is believed to be activated, resulting in increased intracellular calcium. By mediating cell adhesion to other cells and to the extracellular matrix and by activating intracellular signaling pathways, NCAM influences cell migration, neurite extension, and fasciculation, and possibly formation of synapses in the brain. From studies on NCAM knock-out mice, NCAM have been shown to be crucial for the formation of the olfactory bulb and the mossy fiber system in the hippocampus. In addition, NCAM is important for neuronal plasticity in the adult brain associated with learning and regeneration.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Functional cooperation of beta1-integrins and members of the Ig superfamily in neurite outgrowth induction

Neurite outgrowth is a central aspect of the ontogenetic formation of neural networks and is regulated by distinct groups of cell surface molecules. One protein involved in neurite elongation and fasciculation is the neural Ig superfamily member F11/contactin. We have shown previously that F11 promotes neurite extension of chick tectal neurons by interaction with the tectal receptor NrCAM, a member of the L1 subgroup of the Ig superfamily. By contrast, it does not induce outgrowth of retinal neurons despite the fact that these cells also express NrCAM, suggesting that in retinal cells the F11-NrCAM interaction alone is not sufficient to induce neurite extension. In this report we present a novel image analysis procedure to quantify neurite outgrowth and use it to demonstrate that F11 enhances the fibronectin-induced outgrowth response of embryonic retinal neurons. We reveal that NrCAM is the neuronal receptor mediating the enhanced outgrowth of retinal neurons, whereas the related F11-binding molecule NgCAM is not involved. Furthermore, we provide evidence that a beta1-integrin may represent the fibronectin-dependent receptor that cooperates indirectly with the F11-NrCAM pathway. Our results support the concept of a combinatorial labeling of cells in nervous system histogenesis by different classes of cell surface proteins, in particular by integrins and molecules of the Ig superfamily.

Polysialylated NCAM expression during motor axon outgrowth and myogenesis in the fetal rat

Polysialylation of the neural cell adhesion molecule (NCAM) converts it into an anti-adhesive molecule, attenuating intercellular adhesion and repelling apposed membranes. Previous studies have demonstrated that interaxonal repulsion, or defasciculation, induced by polysialylated NCAM (PSA-NCAM) expressed along outgrowing chick motor axons promotes intramuscular branching and facilitates differential guidance of segregating axonal populations. In the present study, we have examined the expression of PSA-NCAM in a developing mammalian motor system during axonal outgrowth, separation of distinct axonal populations, and intramuscular branching. Furthermore, we provide the first clear demonstration of the spatiotemporal modulation of PSA-NCAM expression on myotubes during each stage of myogenesis. Immunohistochemical labelling was used to compare the spatiotemporal pattern of PSA-NCAM expression with those of total-NCAM, the cell adhesion molecule L1, and growth associated protein (GAP-43) during development of the phrenic nerve and diaphragm of fetal rats (embryonic days, E11-E19). During segregation of phrenic and brachial axonal populations at the brachial plexus (E12.5-E13), PSA-NCAM expression was restricted to phrenics, being absent from brachial motoneurons. Both populations labelled equivalently for NCAM, L1, and GAP-43. We postulate that PSA-NCAM may be a component of the molecular machinery that specifically guides phrenic motoneuron growth at the brachial plexus. During diaphragmatic morphogenesis, PSA-NCAM expression: (i) remained high within the phrenic nerve throughout intramuscular branching; (ii) was transiently up-regulated on myotubes during myotube separation associated with primary and secondary myogenesis; (iii) was restricted to those regions of primary and secondary myotube membranes, which were juxtaposed and about to separate. These data suggest a role for PSA-NCAM in the guidance of specific subsets of mammalian motoneurons and in intramuscular branching, and demonstrate an intimate correlation between PSA-NCAM expression and myotube separation.

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.

Enhancement of neurite outgrowth by the soluble form of human L1 (neural cell adhesion molecule)

L1, a neural cell adhesion molecule, is involved in neurite outgrowth, migration and fasciculation. Although L1 is a membrane glycoprotein expressed on neural cells, the soluble form of L1 is generated in vivo by proteolysis. In the present study, a stable transfectant of Chinese hamster ovary (CHO) cells secreting human L1 without cytoplasmic and membrane spanning domains was generated, and the function of the secreted L1 was examined. Explants from embryonic chick brain stem were cultured on a substrate coated with polyethylenimine (PEI) alone, on substrate-bound L1 or in medium containing soluble L1. The neurites induced by L1, both the substrate-bound form and the soluble form, were 2-3 times longer than those cultured on PEI. The ability of the soluble L1 to induce neurite formation was slightly greater than that of the substrate L1. The present results demonstrated that neurite outgrowth was induced not only by substrate-bound L1 but also by soluble L1. Soluble L1 could be a pharmaceutical candidate for the promotion of nerve regeneration.

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.

Tyrosine phosphorylation of L1 family adhesion molecules: implication of the Eph kinase Cek5

The L1 family comprises transmembrane cell adhesion molecules of the immunoglobulin superfamily that play an important role in neuronal migration and axon outgrowth, fasciculation, and myelination. Consistent with a crucial role in developmental processes, mutations in L1 cause severe brain malformations. Although L1 activates intracellular signaling pathways, little is known about the membrane proximal events of L1 signaling. The cytoplasmic domains of L1 family proteins contain several conserved tyrosine residues that are potential targets for receptor tyrosine kinases. Here, we report that the L1 family protein Ng-CAM is phosphorylated on tyrosine in embryonic day 13 chicken retina. This is the first demonstration of in vivo tyrosine phosphorylation of an L1-like molecule. Because chicken embryo kinase 5 (Cek5) is a receptor tyrosine kinase expressed in neuronal processes and activated in the chicken embryonic retina, we have analyzed the possible role of Cek5 in L1 phosphorylation. The rat glioblastoma cell line B28 was stably transfected with human L1. Additional transient transfection with Cek5 cDNA led to expression of Cek5 in its tyrosine-phosphorylated, activated form. Biochemical analysis revealed that L1 is phosphorylated on tyrosine in Cek5-transfected cells but not in control transfectants. Furthermore, direct phosphorylation of the L1 cytoplasmic domain by Cek5 was demonstrated in an in vitro kinase assay. Tyrosine phosphorylation may represent a novel mechanism of signal cascade initiation through L1.

Expression of an L1-related cell adhesion molecule on developing CNS fiber tracts in zebrafish and its functional contribution to axon fasciculation

E587 antigen, an L1-related cell adhesion molecule, is expressed by growing axons and has previously been shown to enhance axon growth and to mediate fasciculation of axons from newborn retinal ganglion cells in goldfish. In zebrafish, the monoclonal antibody E17 against E587 antigen stains all axons in the primary tracts and commissures from 17 h postfertilization (pf) onward and axons which are added subsequently to this scaffold. Moreover, Fab fragments of an E587 antiserum (E587 Fabs) injected into the ventricle of 30-h pf zebrafish embryos caused a marked defasciculation of distinct axon bundles in the posterior commissure, in hindbrain commissures, and in longitudinal tracts of the hindbrain, where they also caused increased crossings between fascicles. The regulated expression of E587 antigen by all developing axons and the effects caused by E587 Fabs show that E587 antigen contributes to the formation of tight and orderly fascicles in the developing CNS.