The role of endocytosis in regulating L1-mediated adhesion

N-BAR and F-BAR proteins-endophilin-A3 and PSTPIP1-control clathrin-independent endocytosis of L1CAM

Recent advances in the field demonstrate the high diversity and complexity of endocytic pathways. In the current study, we focus on the endocytosis of L1CAM. This glycoprotein plays a major role in the development of the nervous system, and is involved in cancer development and is associated with metastases and poor prognosis. Two L1CAM isoforms are subject to endocytosis: isoform 1, described as a clathrin-mediated cargo; isoform 2, whose endocytosis has never been studied. Deciphering the molecular machinery of isoform 2 internalisation should contribute to a better understanding of its pathophysiological role. First, we demonstrated in our cellular context that both isoforms of L1CAM are mainly a clathrin-independent cargo, which was not expected for isoform 1. Second, the mechanism of L1CAM endocytosis is specifically mediated by the N-BAR domain protein endophilin-A3. Third, we discovered PSTPIP1, an F-BAR domain protein, as a novel actor in this endocytic process. Finally, we identified galectins as endocytic partners and negative regulators of L1CAM endocytosis. In summary, the interplay of the BAR proteins endophilin-A3 and PSTPIP1, and galectins fine tune the clathrin-independent endocytosis of L1CAM.

Intrinsic disorder in integral membrane proteins

The well-defined roles and specific protein-protein interactions of many integral membrane proteins (IMPs), such as those functioning as receptors for extracellular matrix proteins and soluble growth factors, easily align with considering IMP structure as a classical "lock-and-key" concept. Nevertheless, continued advances in understanding protein conformation, such as those which established the widespread existence of intrinsically disordered proteins (IDPs) and especially intrinsically disordered regions (IDRs) in otherwise three-dimensionally organized proteins, call for ongoing reevaluation of transmembrane proteins. Here, we present basic traits of IDPs and IDRs, and, for some select single-span IMPs, consider the potential functional advantages intrinsic disorder might provide and the possible conformational impact of disease-associated mutations. For transmembrane proteins in general, we highlight several investigational approaches, such as biophysical and computational methods, stressing the importance of integrating them to produce a more-complete mechanistic model of disorder-containing IMPs. These procedures, when synergized with in-cell assessments, will likely be key in translating in silico and in vitro results to improved understanding of IMP conformational flexibility in normal cell physiology as well as disease, and will help to extend their potential as therapeutic targets.

An antibody against L1 cell adhesion molecule inhibits cardiotoxicity by regulating persistent DNA damage

Targeting the molecular pathways underlying the cardiotoxicity associated with thoracic irradiation and doxorubicin (Dox) could reduce the morbidity and mortality associated with these anticancer treatments. Here, we find that vascular endothelial cells (ECs) with persistent DNA damage induced by irradiation and Dox treatment exhibit a fibrotic phenotype (endothelial-mesenchymal transition, EndMT) correlating with the colocalization of L1CAM and persistent DNA damage foci. We demonstrate that treatment with the anti-L1CAM antibody Ab417 decreases L1CAM overexpression and nuclear translocation and persistent DNA damage foci. We show that in whole-heart-irradiated mice, EC-specific p53 deletion increases vascular fibrosis and the colocalization of L1CAM and DNA damage foci, while Ab417 attenuates these effects. We also demonstrate that Ab417 prevents cardiac dysfunction-related decrease in fractional shortening and prolongs survival after whole-heart irradiation or Dox treatment. We show that cardiomyopathy patient-derived cardiovascular ECs with persistent DNA damage show upregulated L1CAM and EndMT, indicating clinical applicability of Ab417. We conclude that controlling vascular DNA damage by inhibiting nuclear L1CAM translocation might effectively prevent anticancer therapy-associated cardiotoxicity.

Tissue mechanics, an important regulator of development and disease

A growing body of work describes how physical forces in and around cells affect their growth, proliferation, migration, function and differentiation into specialized types. How cells receive and respond biochemically to mechanical signals is a process termed mechanotransduction. Disease may arise if a disruption occurs within this mechanism of sensing and interpreting mechanics. Cancer, cardiovascular diseases and developmental defects, such as during the process of neural tube formation, are linked to changes in cell and tissue mechanics. A breakdown in normal tissue and cellular forces activates mechanosignalling pathways that affect their function and can promote disease progression. The recent advent of high-resolution techniques enables quantitative measurements of mechanical properties of the cell and its extracellular matrix, providing insight into how mechanotransduction is regulated. In this review, we will address the standard methods and new technologies available to properly measure mechanical properties, highlighting the challenges and limitations of probing different length-scales. We will focus on the unique environment present throughout the development and maintenance of the central nervous system and discuss cases where disease, such as brain cancer, arises in response to changes in the mechanical properties of the microenvironment that disrupt homeostasis. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

The human nephrin Y1139RSL motif is essential for podocyte foot process organization and slit diaphragm formation during glomerular development

Nephrin is an immunoglobulin-type cell-adhesion molecule with a key role in the glomerular interpodocyte slit diaphragm. Mutations in the nephrin gene are associated with defects in the slit diaphragm, leading to early-onset nephrotic syndrome, typically resistant to treatment. Although the endocytic trafficking of nephrin is essential for the assembly of the slit diaphragm, nephrin's specific endocytic motifs remain unknown. To search for endocytic motifs, here we performed a multisequence alignment of nephrin and identified a canonical YXXØ-type motif, Y¹¹³⁹RSL, in the nephrin cytoplasmic tail, expressed only in primates. Using site-directed mutagenesis, various biochemical methods, single-plane illumination microscopy, a human podocyte line, and a human nephrin-expressing zebrafish model, we found that Y¹¹³⁹RSL is a novel endocytic motif and a structural element for clathrin-mediated nephrin endocytosis that functions as a phosphorylation-sensitive signal. We observed that Y¹¹³⁹RSL motif-mediated endocytosis helps to localize nephrin to specialized plasma membrane domains in podocytes and is essential for normal foot process organization into a functional slit diaphragm between neighboring foot processes in zebrafish. The importance of nephrin Y¹¹³⁹RSL for healthy podocyte development was supported by population-level analyses of genetic variations at this motif, revealing that such variations are very rare, suggesting that mutations in this motif have autosomal-recessive negative effects on kidney health. These findings expand our understanding of the mechanism underlying nephrin endocytosis and may lead to improved diagnostic tools or therapeutic strategies for managing early-onset, treatment-resistant nephrotic syndrome.

L1 cell adhesion molecule as a therapeutic target in cancer

L1 cell adhesion molecule (L1CAM) is the prototype member of the L1-family of closely related neural adhesion molecules. L1CAM is differentially expressed in the normal nervous system as well as pathological tissues and displays a wide range of biological activities. In human malignancies, L1CAM plays a vital role in tumor growth, invasion and metastasis. Recently, increasing evidence has suggested that L1CAM exerts a variety of functions at different steps of tumor progression through a series of signaling pathways. In addition, L1CAM has been identified as a promising target for cancer therapy by using synthetic and natural inhibitors. In this review, we provide an up-to-date overview of the role of L1CAM involved in cancers and the rationale for L1CAM as a novel molecular target for cancer therapy.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

Involvement of SARA in Axon and Dendrite Growth

SARA (Smad Anchor for Receptor Activation) plays a crucial role in Rab5-mediated endocytosis in cell lines localizing to early endosomes where it regulates morphology and function. Here, we analyzed the role of SARA during neuronal development and tested whether it functions as a regulator of endocytic trafficking of selected axonal and membrane proteins. Suppression of SARA perturbs the appearance of juxtanuclear endocytic recycling compartments and the neurons show long axons with large growth cones. Furthermore, surface distribution of the cell adhesion molecule L1 in axons and the fusion of vesicles containing transferring receptor (TfR) in dendrites were increased in neurons where SARA was silenced. Conversely, SARA overexpression generated large early endosomes and reduced neurite outgrowth. Taken together, our findings suggest a significant contribution of SARA to key aspects of neuronal development, including neurite formation.

Rabex-5 protein regulates the endocytic trafficking pathway of ubiquitinated neural cell adhesion molecule L1

Ubiquitination of integral membrane proteins is a common posttranslational modification used to mediate endocytosis and endocytic sorting of cell surface proteins in eukaryotic cells. Ubiquitin (Ub)-binding proteins (UBPs) regulate the stability, function, and localization of ubiquitinated cell surface proteins in the endocytic pathway. Here, I report that the immunoglobulin superfamily cell adhesion molecule L1 undergoes ubiquitination and dephosphorylation on the plasma membrane upon L1 antibody-induced clustering, which mimics L1-L1 homophilic binding, and that these modifications are critical for obtaining the maximal rate of internalization and trafficking to the lysosome, but not to the proteasome. Notably, L1 antibody-induced clustering leads to the association of ubiquitinated L1 with Rabex-5, a UBP and guanine nucleotide exchange factor for Rab5, via interaction with the motif interacting with Ub (MIU) domain, but not the A20-type zinc finger domain. This interaction specifically depends on the presence of an Ub moiety on lysine residues in L1. Rabex-5 expression accelerates the internalization rates of L1(WT) and L1(Y1176A), a tyrosine-based motif mutant, but not L1(K11R), an ubiquitination-deficient mutant, leading to the accumulation of ubiquitinated L1 on endosomes. In contrast, RNA interference-mediated knockdown of Rabex-5 impairs the internalizations of L1(WT) and L1(Y1176A), but not L1(K11R) from the plasma membrane. Overall, these results provide a novel mechanistic insight into how Rabex-5 regulates internalization and postendocytic trafficking of ubiquitinated L1 destined for lysosomal degradation.

Activation of ezrin/radixin/moesin mediates attractive growth cone guidance through regulation of growth cone actin and adhesion receptors

The development of a functioning neural network relies on responses of axonal growth cones to molecular guidance cues that are encountered en route to their target tissue. Nerve growth factor (NGF) and neurotrophin-3 serve as attractive cues for chick embryo sensory growth cones in vitro and in vivo, but little is known about the actin-binding proteins necessary to mediate this response. The evolutionarily conserved ezrin/radixin/moesin (ERM) family of proteins can tether actin filaments to the cell membrane when phosphorylated at a conserved threonine residue. Here we show that acute neurotrophin stimulation rapidly increases active phospho-ERM levels in chick sensory neuron growth cone filopodia, coincident with an increase in filopodial L1 and β-integrin. Disrupting ERM function with a dominant-negative construct (DN-ERM) results in smaller and less motile growth cones with disorganized actin filaments. Previously, we found that NGF treatment increases actin-depolymerizing factor (ADF)/cofilin activity and growth cone F-actin (Marsick et al., 2010). Here, we show this F-actin increase, as well as attractive turning to NGF, is blocked when ERM function is disrupted despite normal activation of ADF/cofilin. We further show that DN-ERM expression disrupts leading edge localization of active ADF/cofilin and free F-actin barbed ends. Moreover, filopodial phospho-ERM levels are increased by incorporation of active ADF/cofilin and reduced by knockdown of L1CAM.Together, these data suggest that ERM proteins organize actin filaments in sensory neuron growth cones and are crucial for neurotrophin-induced remodeling of F-actin and redistribution of adhesion receptors.

Endocytosis and endosomes at the crossroads of regulating trafficking of axon outgrowth-modifying receptors

In neurons, many receptors must be localized correctly to axons or dendrites for proper function. During development, receptors for nerve growth and guidance are targeted to axons and localized to growth cones where receptor activation by ligands results in promotion or inhibition of axon growth. Signaling outcomes downstream of ligand binding are determined by the location, levels and residence times of receptors on the neuronal plasma membrane. Therefore, the mechanisms controlling the trafficking of these receptors are crucial to the proper wiring of circuits. Membrane proteins accumulate on the axonal surface by multiple routes, including polarized sorting in the trans Golgi network, sorting in endosomes and removal by endocytosis. Endosomes also play important roles in the signaling pathways for both growth-promoting and -inhibiting molecules: signaling endosomes derived from endocytosis are important for signaling from growth cones to cell bodies. Growth-promoting neurotrophins and growth-inhibiting Nogo-A can use EHD4/Pincher-dependent endocytosis at the growth cone for their respective retrograde signaling. In addition to retrograde transport of endosomes, anterograde transport to axons in endosomes also occurs for several receptors, including the axon outgrowth-promoting cell adhesion molecule L1/NgCAM and TrkA. L1/NgCAM also depends on EHD4/Pincher-dependent endocytosis for its axonal polarization. In this review, we will focus on receptors whose trafficking has been reported to be modulated by the EHD4/Pincher family of endosomal regulators, namely L1/NgCAM, Trk and Nogo-A. We will first summarize the pathways underlying the axonal transport of these proteins and then discuss the potential roles of EHD4/Pincher in mediating their endocytosis.

A novel strategy to isolate ubiquitin conjugates reveals wide role for ubiquitination during neural development

Ubiquitination has essential roles in neuronal development and function. Ubiquitin proteomics studies on yeast and HeLa cells have proven very informative, but there still is a gap regarding neuronal tissue-specific ubiquitination. In an organism context, direct evidence for the ubiquitination of neuronal proteins is even scarcer. Here, we report a novel proteomics strategy based on the in vivo biotinylation of ubiquitin to isolate ubiquitin conjugates from the neurons of Drosophila melanogaster embryos. We confidently identified 48 neuronal ubiquitin substrates, none of which was yet known to be ubiquitinated. Earlier proteomics and biochemical studies in non-neuronal cell types had identified orthologs to some of those but not to others. The identification here of novel ubiquitin substrates, those with no known ubiquitinated ortholog, suggests that proteomics studies must be performed on neuronal cells to identify ubiquitination pathways not shared by other cell types. Importantly, several of those newly found neuronal ubiquitin substrates are key players in synaptogenesis. Mass spectrometry results were validated by Western blotting to confirm that those proteins are indeed ubiquitinated in the Drosophila embryonic nervous system and to elucidate whether they are mono- or polyubiquitinated. In addition to the ubiquitin substrates, we also identified the ubiquitin carriers that are active during synaptogenesis. Identifying endogenously ubiquitinated proteins in specific cell types, at specific developmental stages, and within the context of a living organism will allow understanding how the tissue-specific function of those proteins is regulated by the ubiquitin system.

Soluble L1CAM promotes breast cancer cell adhesion and migration in vitro, but not invasion

Background

Neural recognition molecule L1CAM, which is a key protein involved in early nervous system development, is known to be abnormally expressed and shed in several types of cancers where it participates in metastasis and progression. The distinction of L1CAM presence in cancerous vs. normal tissues has suggested it to be a new target for cancer treatment. Our current study focused on the potential role of soluble L1CAM in breast cancer cell adhesion to extracellular matrix proteins, migration, and invasion.

Results

We found L1 expression levels were correlated with breast cancer stage of progression in established data sets of clinical samples, and also were high in more metastatic breast cancer cell lines MDA-MB-231 and MDA-MB-435, but low in less migratory MDA-MB-468 cells. Proteolysis of L1 into its soluble form (sL1) was detected in cell culture medium from all three above cell lines, and can be induced by PMA activation. Over-expression of the L1 ectodomain in MDA-MB-468 cells by using a lentiviral vector greatly increased the amount of sL1 released by those cells. Concomitantly, cell adhesion to extracellular matrix and cell transmigration ability were significantly promoted, while cell invasion ability through Matrigel™ remained unaffected. On the other hand, attenuating L1 expression in MDA-MB-231 cells by using a shRNA lentiviral vector resulted in reduced cell-matrix adhesion and transmigration. Similar effects were also shown by monoclonal antibody blocking of the L1 extracellular region. Moreover, sL1 in conditioned cell culture medium induced a directional migration of MDA-MB-468 cells, which could be neutralized by antibody treatment.

Conclusions

Our data provides new evidence for the function of L1CAM and its soluble form in promoting cancer cell adhesion to ECM and cell migration. Thus, L1CAM is validated further to be a potential early diagnostic marker in breast cancer progression and a target for breast cancer therapy.

Alterations of EHD1/EHD4 protein levels interfere with L1/NgCAM endocytosis in neurons and disrupt axonal targeting

Axon growth is regulated by many proteins, including adhesion molecules, which need to be trafficked correctly to axons. The adhesion molecule L1/neuron-glia cell adhesion molecule (NgCAM) travels to axons via an endocytosis-dependent pathway (transcytosis), traversing somatodendritic endosomes. The Eps15 homology domain (EHD) family proteins (EHD1-EHD4) play important roles in endosomal recycling and possibly in endocytosis. We investigated whether EHD1 regulates L1/NgCAM trafficking in neurons. Both short hairpin-mediated downregulation and overexpression of EHD1 led to dendritic mistargeting of NgCAM. Downregulation of EHD1 showed increased endosomal accumulation of NgCAM, whereas, surprisingly, overexpression of EHD1 led to impairment of L1/NgCAM internalization in neurons but not in fibroblasts. Transferrin internalization, however, was unaffected. At longer overexpression times of EHD1, NgCAM endocytosis returned to normal, suggesting rapid upregulation of compensatory endocytic pathways. EHD1 is capable of hetero-oligomerization, and an endogenous complex of EHD1 and EHD4 was identified previously. We therefore tested whether short-term overexpression of other EHD family members showed a similar endocytosis defect. Expression of EHD4, but not of EHD3, also caused a defect in L1/NgCAM endocytosis. Oligomerization of EHD1 was required to cause NgCAM endocytosis defects, and simultaneous expression of EHD1 and EHD4 rescued NgCAM endocytosis. Therefore, balanced levels of EHD1-EHD4 are important for NgCAM endocytosis in neurons. Our data suggest that EHD1 plays roles in both endosomal recycling and a specialized endocytosis pathway in neurons used by NgCAM. We propose that EHD1 and EHD4 act as hetero-oligomeric complexes in this pathway.

Ubiquitination and endocytosis of cell adhesion molecule DM-GRASP regulate its cell surface presence and affect its role for axon navigation

DM-GRASP, cell adhesion molecule of the immunoglobulin superfamily, has been shown to promote growth and navigation of axons. We here demonstrate that clustering of DM-GRASP in the plasma membrane induces its rapid internalization via dynamin- and clathrin-dependent endocytosis, which is controlled by phosphatidylinositol 3-kinase and mitogen-activated protein kinase ERK. The clustering of DM-GRASP activates ERK; the intensity and duration of ERK activation by DM-GRASP do not depend on rapid clathrin-mediated internalization of DM-GRASP. Moreover, the preference of retinal ganglion cell axons for DM-GRASP-coated micro-lanes requires clathrin-mediated endocytosis for the appropriate axonal turning reactions at substrate borders. Because the intracellular domain of DM-GRASP does not contain motifs for direct interactions with the endocytosis machinery, we performed a yeast two-hybrid screen to identify intracellular proteins mediating the uptake of DM-GRASP and isolated ubiquitin. Immunoprecipitation of DM-GRASP coexpressed with ubiquitin revealed that one or two ubiquitin(s) are attached to the intracellular domain of cell surface-resident DM-GRASP. Furthermore, elevated ubiquitination levels result in a decrease of cell surface-resident DM-GRASP as well as in the amount of total DM-GRASP. The endocytosis rate is not affected, but the delivery to multivesicular bodies is increased, indicating that DM-GRASP ubiquitination enhances its sorting into the degradation pathway. Together, our data show that ubiquitination and endocytosis of DM-GRASP in concert regulate its cell surface concentration, which is crucial for its function in axon navigation.

NCAM is ubiquitylated, endocytosed and recycled in neurons

The neural cell adhesion molecule NCAM plays an important role during neural development and in the adult brain. To study the intracellular trafficking of NCAM in neurons, two major isoforms, NCAM140 or NCAM180, were expressed in primary cortical neurons and in the rat B35 neuroblastoma cell line. NCAM was endocytosed and subsequently recycled to the plasma membrane, whereas only a minor fraction was degraded in lysosomes. In cortical neurons, endocytosis of NCAM was detected in the soma, neurites and growth cones in a developmentally regulated fashion. Furthermore, we found that NCAM is mono-ubiquitylated at the plasma membrane and endocytosis was significantly increased in cells overexpressing ubiquitin. Therefore, we propose that ubiquitylation represents an endocytosis signal for NCAM.

The L1 cell adhesion molecule as a target for radioimmunotherapy

Monoclonal antibodies directed against the L1 cell adhesion molecule were shown recently to inhibit growth of target tumor cells in vitro and the growth of tumor cells in vivo in nude mice. The biologic functions of L1 in tumor cells, which include growth-promoting activity linked to endocytosis and cellular processing of the L1 cell surface protein, make this protein an attractive target for antibodies. This update deals with recent results on L1 expression in normal tissues and in the tumors that were investigated until now. L1 expression outside of the nervous system is highly restricted to peripheral nerve bundles and kidney-collecting tubule cells. In tumors, L1 overexpression is not ubiquitous. It is prevalent in neuroblastomas and in malignant ovarian tumors, and is also found in certain subtypes of other nonneuroendocrine and nongynecologic tumors, such as renal-cell carcinomas. The structure of the L1 protein and what is known about its functional role in tumors will be described in this paper. L1 is not only a novel tumor marker, but it appears to have growth-promoting and antiapoptotic functions and may contribute to a more malignant phenotype. The preclinical studies and the clinical study to evaluate tumor-targeting properties and potential for therapy of radiolabeled anti-L1 antibodies will be described to date. Some of these studies underline the importance of L1 endocytosis for the targeting of radiolabeled antibodies.

Fast turnover of L1 adhesions in neuronal growth cones involving both surface diffusion and exo/endocytosis of L1 molecules

We investigated the interplay between surface trafficking and binding dynamics of the immunoglobulin cell adhesion molecule L1 at neuronal growth cones. Primary neurons were transfected with L1 constructs bearing thrombin-cleavable green fluorescent protein (GFP), allowing visualization of newly exocytosed L1 or labeling of membrane L1 molecules by Quantum dots. Intracellular L1-GFP vesicles showed preferential centrifugal motion, whereas surface L1-GFP diffused randomly, revealing two pathways to address L1 to adhesive sites. We triggered L1 adhesions using microspheres coated with L1-Fc protein or anti-L1 antibodies, manipulated by optical tweezers. Microspheres coupled to the actin retrograde flow at the growth cone periphery while recruiting L1-GFP molecules, of which 50% relied on exocytosis. Fluorescence recovery after photobleaching experiments revealed a rapid recycling of L1-GFP molecules at L1-Fc (but not anti-L1) bead contacts, attributed to a high lability of L1-L1 bonds at equilibrium. L1-GFP molecules truncated in the intracellular tail as well as neuronal cell adhesion molecules (NrCAMs) missing the clathrin adaptor binding sequence showed both little internalization and reduced turnover rates, indicating a role of endocytosis in the recycling of mature L1 contacts at the base of the growth cone. Thus, unlike for other molecules such as NrCAM or N-cadherin, diffusion/trapping and exo/endocytosis events cooperate to allow the fast renewal of L1 adhesions.

Modulation of semaphorin signaling by Ig superfamily cell adhesion molecules

During axon navigation, growth cones continuously interact with molecular cues in their environment, some of which control adherence and bundle assembly, others axon elongation and direction. Growth cone responses to these different environmental cues are tightly coordinated during the development of neuronal projections. Several recent studies show that axon sensitivity to guidance cues is modulated by extracellular and intracellular signals. This regulation may enable different classes of cues to combine their effects and may also represent important means for diversifying pathway choices and for compensating for the limited number of guidance cues. This chapter focuses on the modulation exerted by Ig Super-family cell adhesion molecules (IgSFCAMs) on guidance cues of the class III secreted semaphorins.

Cell adhesion molecule L1 modulates nerve-growth-factor-induced CGRP-IR fiber sprouting

Overexpression of nerve growth factor (NGF) using adenoviruses (Adts) after spinal cord injury induces extensive regeneration and sprouting of calcitonin-gene-related peptide immunoreactive (CGRP-IR) fibers, whereas overexpression of cell adhesion molecules (CAMs) has no effect on the normal distribution of these fibers. Interestingly, co-expression of cell adhesion molecule L1 and NGF significantly decreases (p<0.0001) CGRP-IR fiber sprouting within the spinal cord, when compared to NGF alone. Co-expression of cell adhesion molecules NCAM or N-cadherin had no effect on NGF-induced CGRP-IR fiber sprouting. These data demonstrate that reduced sprouting is specific to L1 co-expression and not other cell adhesion molecules. In vitro studies carried out to address potential mechanisms show that neurite outgrowth over astrocytes overexpressing L1 in the presence of NGF is comparable to controls, indicating that other factors present in vivo might be involved in the L1-mediated reduction in sprouting. One potential factor is semaphorin 3A (sema3A), which mediates growth cone collapse of CGRP-positive axons. Recent studies have shown that L1 is important in sema3A receptor signaling for cortical neurons. In our study, co-expression of sema3A indeed reduces neurite outgrowth from DRG neurons by about 40% on L1-expressing astrocytes. Based on these results, we hypothesize that overexpression of L1 potentiates sema3A signaling resulting in reduced sprouting.

The cell adhesion molecule neuroplastin-65 inhibits hippocampal long-term potentiation via a mitogen-activated protein kinase p38-dependent reduction in surface expression of GluR1-containing glutamate receptors

Neuroplastin-65 is a brain-specific, synapse-enriched member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. Previous studies highlighted the importance of neuroplastin-65 for long-term potentiation (LTP), but the mechanism was unclear. Here, we show how neuroplastin-65 activation of mitogen-activated protein kinase p38 (p38MAPK) modified synapse strength by altering surface glutamate receptor expression. Organotypic hippocampal slice cultures treated with the complete extracellular fragment of neuroplastin-65 (FcIg1-3) sustained an increase in the phosphorylation of p38MAPK and an inability to induce LTP at hippocampal synapses. The LTP block was reversed by application of the p38MAPK inhibitor SB202190, suggesting that p38MAPK activation occurred downstream of neuroplastin-65 binding and upstream of the loss of LTP. Further investigation revealed that the mechanism underlying neuroplastin-65-dependent prevention of LTP was a p38MAPK-dependent acceleration of the loss of surface-exposed glutamate receptor subunits that was reversed by pretreatment with the p38MAPK inhibitor SB202190. Our results indicate that neuroplastin-65 binding and associated stimulation of p38MAPK activity are upstream of a mechanism to control surface glutamate receptor expression and thereby influence plasticity at excitatory hippocampal synapses.

Ethanol inhibits L1 cell adhesion molecule activation of mitogen-activated protein kinases

Inhibition of the functions of L1 cell adhesion molecule (L1) by ethanol has been implicated in the pathogenesis of the neurodevelopmental aspects of the fetal alcohol syndrome (FAS). Ethanol at pharmacological concentrations has been shown to inhibit L1-mediated neurite outgrowth of rat post-natal day 6 cerebellar granule cells (CGN). Extracellular signal-related kinases (ERK) 1/2 activation occurs following L1 clustering. Reduction in phosphoERK1/2 by inhibition of mitogen-activated protein kinase kinase (MEK) reduces neurite outgrowth of cerebellar neurons. Here, we examine the effects of ethanol on L1 activation of ERK1/2, and whether this activation occurs via activation of fibroblast growth factor receptor 1 (FGFR1). Ethanol at 25 mm markedly inhibited ERK1/2 activation by both clustering L1 with cross-linked monoclonal antibodies, or by L1-Fc chimeric proteins. Clustering L1 with subsequent ERK1/2 activation did not result in tyrosine phosphorylation of the FGFR1. In addition, inhibition of FGFR1 tyrosine kinase blocked basic fibroblast growth factor (bFGF) activation of ERK1/2, but did not affect activation of ERK1/2 by clustered L1. We conclude that ethanol disrupts the signaling pathway between L1 clustering and ERK1/2 activation, and that this occurs independently of the FGFR1 pathway in cerebellar granule cells.

L1-mediated branching is regulated by two ezrin-radixin-moesin (ERM)-binding sites, the RSLE region and a novel juxtamembrane ERM-binding region

We investigated how the neural cell adhesion molecule L1 mediates neurite outgrowth through L1-L1 homophilic interactions. Wild-type L1 and L1 with mutations in the cytoplasmic domain (CD) were introduced into L1 knock-out neurons, and transfected neurons were grown on an L1 substrate. Neurite length and branching were compared between wild-type L1 and L1CD mutations. Surprisingly, the L1CD is not required for L1-mediated neurite outgrowth but plays a critical role in neurite branching, through both the juxtamembrane region and the RSLE region. We demonstrate that both regions serve as ezrin-moesin-radixin-binding sites. A truncation mutant that deletes 110 of 114 amino acids of the L1CD still supports neurite outgrowth on an L1 substrate, suggesting that a coreceptor binds to L1 in cis and mediates neurite outgrowth and that L1-ankyrin interactions are not essential for neurite initiation or outgrowth. These data are consistent with a model in which L1 can influence L1-mediated neurite outgrowth and branching through both the L1CD and a coreceptor.

RanBPM is an L1-interacting protein that regulates L1-mediated mitogen-activated protein kinase activation

A yeast two-hybrid screen using the last 28 amino acids of the cytoplasmic domain of the neural cell adhesion molecule L1 identified RanBPM as an L1-interacting protein. RanBPM associates with L1 in vivo and the N-terminal region of RanBPM (N-RanBPM), containing the SPRY domain, is sufficient for the interaction with L1 in a glutathione S-transferase pull-down assay. L1 antibody patching dramatically changes the subcellular localization of N-RanBPM in transfected COS cells. Overexpression of N-RanBPM in COS cells reduces L1-triggered extracellular signal-regulated kinase 1/2 activation by 50% and overexpression of N-RanBPM in primary neurons inhibits L1-mediated neurite outgrowth and branching. These data suggest that RanBPM is an adaptor protein that links L1 to the extracellular signal-regulated kinase/MAPK pathway.

Tetanus neurotoxin-mediated cleavage of cellubrevin impairs epithelial cell migration and integrin-dependent cell adhesion

A role for endocytosis and exocytosis in cell migration has been proposed but not yet demonstrated. Here, we show that cellubrevin (Cb), an early endosomal v-SNARE, mediates trafficking in the lamellipod of migrating epithelial cells and partially colocalizes with markers of focal contacts. Expression of tetanus neurotoxin, which selectively cleaves Cb, significantly reduced the speed of migrating epithelial cells. Furthermore, expression of tetanus neurotoxin enhanced the adhesion of epithelial cells to collagen, laminin, fibronectin, and E-cadherin; altered spreading on collagen; and impaired the recycling of beta1 integrins. These results suggest that Cb-dependent membrane trafficking participates in cell motility through the regulation of cell adhesion.

Cleavage of L1 in exosomes and apoptotic membrane vesicles released from ovarian carcinoma cells

PURPOSE

The L1 adhesion molecule (CD171) is overexpressed in human ovarian and endometrial carcinomas and is associated with bad prognosis. Although expressed as a transmembrane molecule, L1 is released from carcinoma cells in a soluble form. Soluble L1 is present in serum and ascites of ovarian carcinoma patients. We investigated the mode of L1 cleavage and the function of soluble L1.

EXPERIMENTAL DESIGN

We used ovarian carcinoma cell lines and ascites from ovarian carcinoma patients to analyze soluble L1 and L1 cleavage by Western blot analysis and ELISA.

RESULTS

We find that in ovarian carcinoma cells the constitutive cleavage of L1 proceeds in secretory vesicles. We show that apoptotic stimuli like C2-ceramide, staurosporine, UV irradiation, and hypoxic conditions enhance L1-vesicle release resulting in elevated levels of soluble L1. Constitutive cleavage of L1 is mediated by a disintegrin and metalloproteinase 10, but under apoptotic conditions multiple metalloproteinases are involved. L1 cleavage occurs in two types of vesicles with distinct density features: constitutively released vesicles with similarity to exosomes and apoptotic vesicles. Both types of L1-containing vesicles are present in the ascites fluids of ovarian carcinoma patients. Soluble L1 from ascites is a potent inducer of cell migration and can trigger extracellular signal-regulated kinase phosphorylation.

CONCLUSIONS

We suggest that tumor-derived vesicles may be an important source for soluble L1 that could regulate tumor cell function in an autocrine/paracrine fashion.

Pathological missense mutations of neural cell adhesion molecule L1 affect neurite outgrowth and branching on an L1 substrate

A number of pathological missense mutations of L1CAM have been shown to disrupt L1-L1 homophilic binding and/or affect surface expression. To investigate whether these mutations disrupt L1-mediated neurite outgrowth, cerebellar neurons from L1 knockout mice are transfected with WT human L1 or L1 mutant constructs, and grown on an L1 substrate. Various parameters of neurite growth are quantified. Most L1 mutations do not affect neurite length significantly but several mutations cause a significant decrease in branching. Comparison of these data with data on L1 expression levels and homophilic binding strength show that changes in neurite growth cannot be simply explained by reductions in either of these parameters. Our results suggest that a coreceptor is involved in L1-mediated neurite outgrowth. Some pathological mutations have little effect on L1 mediated neurite growth, so it is unlikely that a failure of L1-mediated neurite outgrowth is the principle cause of brain defects in patients with L1 mutations.

Cranial neural crest recycle surface integrins in a substratum-dependent manner to promote rapid motility

Cell migration is essential for proper development of numerous structures derived from embryonic neural crest cells (NCCs). Although the migratory pathways of NCCs have been determined, the molecular mechanisms regulating NCC motility remain unclear. NCC migration is integrin dependent, and recent work has shown that surface expression levels of particular integrin alpha subunits are important determinants of NCC motility in vitro. Here, we provide evidence that rapid cranial NCC motility on laminin requires integrin recycling. NCCs showed both ligand- and receptor-specific integrin regulation in vitro. On laminin, NCCs accumulated internalized laminin but not fibronectin receptors over 20 min, whereas on fibronectin neither type of receptor accumulated internally beyond 2 min. Internalized laminin receptors colocalized with receptor recycling vesicles and were subsequently recycled back to the cell surface. Blocking receptor recycling with bafilomycin A inhibited NCC motility on laminin, indicating that substratum-dependent integrin recycling is essential for rapid cranial neural crest migration.

Semaphorin3A-induced receptor endocytosis during axon guidance responses is mediated by L1 CAM

During axon navigation, Semaphorin3A-induced growth cone retraction is correlated with endocytosis. Although its function remains elusive, we showed previously that the cell adhesion molecule of the immunoglobulin super family L1 associates with Neuropilin-1 (NP-1) the Sema3A-binding subunit of the receptor complex and is required for Sema3A to elicit axonal repulsive responses. We report here that upon Sema3A binding to NP-1, L1 and NP-1 are co-internalized through a clathrin-dependent mechanism mediated by L1. We show that in COS7 cells, L1/NP-1 endocytosis is correlated with a cell contraction similar to that observed with the Plexin (Plex)/NP-1 or Plex/NP1/L1 complexes. In neuronal cultures, a L1-mimetic peptide able to switch Sema3A repulsive responses to attraction blocks both endocytosis and growth cone collapse. Similarly, in the COS7 cell model, peptide application prevents both the Sema3-induced L1/NP-1 internalization and cell collapse. These studies demonstrate that the L1/NP-1 complex is able to confer a biological response to Sema3A with L1 mediating receptor internalization following ligand activation. They also reveal that endocytosis controlled by L1/NP-1 cis and trans interactions is pivotal in Sema3A-mediated axon guidance.

Enhancement of potency and efficacy of NADA by PKC-mediated phosphorylation of vanilloid receptor

The search for an endogenous ligand for the vanilloid receptor (VR or TRPV1) has led to the identification of N-arachidonyl dopamine (NADA). This study investigates the role of protein kinase C (PKC)-mediated phosphorylation on NADA-induced membrane currents in Xenopus oocytes heterologously expressing TRPV1 and in dorsal root ganglion (DRG) neurons. In basal state, current induced by 10 microM NADA is 5-10% of the current induced by 1 microM capsaicin or protons at pH 5. However, PKC activator, phorbol 12,13-dibutyrate (PDBu) strongly potentiated ( approximately 15-fold) the NADA-induced current. Repeated application of NADA at short intervals potentiated its own response approximately fivefold in a PKC-dependent manner. PKC inhibitor, bisindolylmaleimide (BIM, 500 nM), a mutant TRPV1 (S800A/S502A), and maximal activation of PKC abolished the potentiation induced by repeated application of NADA. As a further confirmation that NADA could stimulate PKC, pretreatment with NADA potentiated the response of protons at pH 5 (approximately 20 fold), which was dramatically reduced in the mutant TRPV1. In DRG neurons, capsaicin (100 nM) induced a approximately 15 mV depolarization and initiated a train of action potentials compared with 1 microM NADA that produced a approximately 5 mV response. Pretreatment with PDBu induced significantly larger depolarization and potentiated NADA-induced current. Furthermore, exposure of NADA to the intracellular surface of the membrane-induced larger currents suggesting inaccessibility to the intracellular binding site might contribute to its weaker action. These results indicate that NADA is a potent agonist of VR when the receptor is in the PKC-mediated phosphorylation state.

The mechanism of axon growth: what we have learned from the cell adhesion molecule L1

Cell adhesion molecules (CAMs) are not just an inert glue that mediates static cell-cell and cell-extracellular matrix (ECM) adhesion; instead, their adhesivity is dynamically controlled to enable a cell to migrate through complex environmental situations. Furthermore, cell migration requires distinct levels of CAM adhesivity in various subcellular regions. Recent studies on L1, a CAM in the immunoglobulin superfamily, demonstrate that cell adhesion can be spatially regulated by the polarized internalization and recycling of CAMs. This article examines the molecular mechanism of axon growth, with a particular focus on the role of L1 trafficking in the polarized adhesion and migration of neuronal growth cones.

Soluble cell adhesion molecule L1-Fc promotes locomotor recovery in rats after spinal cord injury

Previous studies suggest that the cell adhesion molecule L1 promotes neurite growth by neutralizing white matter associated inhibitors of axonal growth. We made a soluble chimeric dimer by linking mouse L1 to human Fc. This L1-Fc construct (40 microg/mL) markedly facilitated neurite outgrowth, as well as neuronal adhesion to white matter on frozen sections of spinal cord. We applied L1-Fc intrathecally (200 microg/mL at 0.5 microL/h) to rat spinal cords for 2 weeks after a 25-mm weight drop contusion of the T13 spinal cord. Initial experiments indicated that L1-Fc is present in the spinal cord after 2 weeks of intrathecal infusion and significantly improved locomotor recovery by 6-12 weeks after injury. We then randomized 45 rats to intrathecal infusion of L1-Fc (L1), phosphate-buffered saline controls (PBS), and a mouse monoclonal IgM antibody (M1). By 12 weeks after injury, L1-treated rats recovered significantly (p < 0.005) better locomotor function (BBB score 10.57 +/- 0.25, n = 14) than PBS-treated rats (BBB score 9.00 +/- 0.33, n = 14) or M1-treated (BBB score 8.71 +/- 0.16, n = 14). Only two rats of 22 treated with saline recovered weight-supported ambulation. Of 20 L1-Fc-treated rats, however, 18 recovered weight-supported walking by 12 weeks. The L1-Fc-treated rats also showed more consistent hindlimb contact placing than saline controls. We injected biotinylated dextran amine (BDA) into the motor cortices of 14 rats treated with L1-Fc to label corticospinal axons, comparing these with 13 rats treated with saline. In saline-treated rats, BDA-labeled corticospinal axons often grew up to the impact edge and occasionally into the impact site. L1-treated rats showed longer corticospinal tract growth at the injury site. Three rats had BDA-labeled axons that extended beyond the impact center. One L1-Fc-treated rat showed axonal extension and synapse formation in cord distal to the injury. These results indicate that soluble L1-Fc promotes axonal growth and functional recovery after spinal cord injury. However, the limited corticospinal tract growth across the injury site cannot account for the observed locomotor recovery. Thus, L1 may be stimulating growth of other motor tracts or protecting axons and neurons. More studies are required to elucidate the mechanisms of L1-Fc-induced locomotor recovery.

L1-dependent neuritogenesis involves ankyrinB that mediates L1-CAM coupling with retrograde actin flow

The cell adhesion molecule L1 (L1-CAM) plays critical roles in neurite growth. Its cytoplasmic domain (L1CD) binds to ankyrins that associate with the spectrin-actin network. This paper demonstrates that L1-CAM interactions with ankyrinB (but not with ankyrinG) are involved in the initial formation of neurites. In the membranous protrusions surrounding the soma before neuritogenesis, filamentous actin (F-actin) and ankyrinB continuously move toward the soma (retrograde flow). Bead-tracking experiments show that ankyrinB mediates L1-CAM coupling with retrograde F-actin flow in these perisomatic structures. Ligation of the L1-CAM ectodomain by an immobile substrate induces L1CD-ankyrinB binding and the formation of stationary ankyrinB clusters. Neurite initiation preferentially occurs at the site of these clusters. In contrast, ankyrinB is involved neither in L1-CAM coupling with F-actin flow in growth cones nor in L1-based neurite elongation. Our results indicate that ankyrinB promotes neurite initiation by acting as a component of the clutch module that transmits traction force generated by F-actin flow to the extracellular substrate via L1-CAM.

New insights into the diversity and function of neuronal immunoglobulin superfamily molecules

Immunoglobulin superfamily (IgSF) proteins are implicated in diverse steps of brain development, including neuronal migration, axon pathfinding, target recognition and synapse formation, as well as in the maintenance and function of neuronal networks in the adult. We provide here a review of recent findings on the diversity and the role of transmembrane and secreted members of IgSF proteins in the nervous system. We illustrate that the complexity of IgSF protein function results from various different levels of regulation including regulation of gene expression, protein localization, and protein interactions.

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.

ADAM10-mediated cleavage of L1 adhesion molecule at the cell surface and in released membrane vesicles

Cells can release membrane components in a soluble form and as membrane vesicles. L1, an important molecule for cell migration of neural and tumor cells, is released by membrane-proximal cleavage, and soluble L1 promotes cell migration. Release of L1 is enhanced by shedding inducers such as phorbol ester and pervanadate, but it is also enhanced by depletion of cellular cholesterol with methyl-beta-cyclodextrin (MCD). How such different compounds can induce shedding is presently unknown. We show here that ADAM10 is involved in L1 cleavage, which occurs at the cell surface and in the Golgi apparatus. MCD and pervanadate treatment induced the release of microvesicles containing full-length L1 and the active form of ADAM10. L1 cleavage occurred in isolated vesicles. L1-containing microvesicles could trigger haptotactic cell migration. Only the neural L1 form carrying the RSLE signal for clathrin-dependent endocytosis was recruited and cleaved in vesicles. Phorbol ester treatment activated L1 cleavage predominantly at the cell surface. Our results provide evidence for two pathways of L1 cleavage, based on ADAM10 localization, that can be activated differentially: 1) direct cleavage at the cell surface, and 2) release and cleavage in secretory vesicles most likely derived from the Golgi apparatus. The findings establish a novel role for ADAM10 as a vesicle-based protease.

Peptide antagonists of ethanol inhibition of l1-mediated cell-cell adhesion

Ethanol inhibits cell-cell adhesion mediated by the L1 cell adhesion molecule. 1-Octanol potently antagonizes this cellular action of ethanol and also prevents ethanol-induced dysmorphology and cell death in mouse whole embryo culture. NAPVSIPQ (NAP) and SALLRSIPA (SAL) are active peptide fragments of two neuroprotective proteins: activity-dependent neuroprotective protein and activity-dependent neurotrophic factor. NAP and SAL are neuroprotective at femtomolar concentrations against a variety of neurotoxins and also prevent ethanol teratogenesis in mice. To explore the cellular basis for this action, we asked whether NAP and SAL antagonize ethanol inhibition of L1 adhesion. Aggregation assays were carried out in ethanol-sensitive, human L1-transfected NIH/3T3 cells in the absence and presence of NAP and SAL. Neither NAP nor SAL altered L1 adhesion or L1 expression; however, both peptides potently and completely antagonized the inhibition of L1 adhesion by 100 mM ethanol (EC(50): NAP, 6 x 10(-14) M; SAL, 4 x 10(-11) M). NAP also antagonized ethanol inhibition of cell-cell adhesion in bone morphogenetic protein-7-treated NG108-15 cells. In L1-expressing NIH/3T3 cells, SAL antagonism was reversible and could be overcome by increasing concentrations of ethanol. In contrast, NAP antagonism was irreversible and could not be overcome by increasing agonist concentration. Two scrambled NAP peptides (ASPNQPIV and PNIQVASP) were not antagonists at concentrations as high as 10(-7) M. Thus, two structurally unrelated classes of compounds, alcohols and small polypeptides, share two common actions: antagonism of ethanol inhibition of L1-mediated cell adhesion and prevention of ethanol teratogenesis. These findings support the hypothesis that ethanol inhibition of L1 adhesion contributes to ethanol teratogenesis.

The neural cell adhesion molecule L1 potentiates integrin-dependent cell migration to extracellular matrix proteins

The L1 adhesion molecule regulates axon growth and is mutated in the X-linked mental retardation syndrome CRASH (acronym for corpus callosum agenesis, retardation, aphasia, spastic paraplegia, hydrocephalus). A novel role for L1 as a potentiator of neuronal cell migration to extracellular matrix proteins through beta1 integrins and intracellular signaling to mitogen-activated protein (MAP) kinase was identified. L1 potentiated haptotactic migration of B35 neuroblastoma cells toward fibronectin, vitronectin, and laminin through the signaling intermediates c-Src, phosphatidylinositol-3 kinase, and MAP kinase. L1 potentiated migration toward fibronectin through alpha5beta1 integrin in human embryonic kidney 293 cells and depended on determinants of L1 endocytosis: dynamin I, c-Src, and the AP2/clathrin binding site (Arg-Ser-Leu-Glu) in the neuronal splice form of L1. L1 clustering on the cell surface enhanced the internalization of activated beta1 integrins and L1 into distinct endocytic vesicles. L1-potentiated migration, enhancement of beta1 integrin endocytosis, and activation of MAP kinase were coordinately inhibited by mutation of an RGD sequence in the sixth immunoglobulin-like domain of L1. Moreover, three CRASH mutations in the L1 cytoplasmic domain (1194L, S1224L, Y1229H), two of which interfere with ankyrin association, inhibited L1-potentiated migration and MAP kinase activation. Function-blocking antibodies to L1 and beta1 integrin retarded the migration of 5-bromo-2'-deoxyuridine-labeled mouse cerebellar granule cells in slice cultures, underscoring the potential physiological relevance of these findings. These studies suggest that L1 functionally interacts with beta1 integrins to potentiate neuronal migration toward extracellular matrix proteins through endocytosis and MAP kinase signaling, and that impairment of this function by L1 cytoplasmic domain mutations may contribute to neurological deficits in CRASH.

L1 endocytosis is controlled by a phosphorylation-dephosphorylation cycle stimulated by outside-in signaling by L1

Dynamic regulation of the cell surface expression of adhesion molecules is an important mechanism for controlling neuronal growth cone motility and guidance. Clathrin-mediated vesicular internalization of L1 via the tyrosine-based endocytosis motif YRSL regulates adhesion and signaling by this Ig superfamily molecule. Here, we present evidence that tyrosine-1176 (Y1176) of the YRSL motif is phosphorylated in vivo. The nonreceptor tyrosine kinase (p60src) is implicated in L1-mediated neurite outgrowth, and we find that p60src phosphorylates Y1176 in vitro. Phosphorylation of Y1176 prevents L1 binding to AP-2, an adaptor required for clathrin-mediated internalization of L1. mAb 74-5H7 recognizes the sequence immediately NH2-terminal to the tyrosine-based motif and binds L1 only when Y1176 is dephosphorylated. 74-5H7 identifies a subset of L1 present at points of cell-cell contact and in vesicle-like structures that colocalize with an endocytosis marker. L1-L1 binding or L1 cross-linking induces a rapid increase in 74-5H7 immunoreactivity. Our data suggest a model in which homophilic binding or L1 cross-linking triggers transient dephosphorylation of the YRSL motif that makes L1 available for endocytosis. Thus, the regulation of L1 endocytosis through dephosphorylation of Y1176 is a critical regulatory point of L1-mediated adhesion and signaling.

The neural cell adhesion molecule L1 potentiates integrin-dependent cell migration to extracellular matrix proteins

The L1 adhesion molecule regulates axon growth and is mutated in the X-linked mental retardation syndrome CRASH (acronym for corpus callosum agenesis, retardation, aphasia, spastic paraplegia, hydrocephalus). A novel role for L1 as a potentiator of neuronal cell migration to extracellular matrix proteins through beta1 integrins and intracellular signaling to mitogen-activated protein (MAP) kinase was identified. L1 potentiated haptotactic migration of B35 neuroblastoma cells toward fibronectin, vitronectin, and laminin through the signaling intermediates c-Src, phosphatidylinositol-3 kinase, and MAP kinase. L1 potentiated migration toward fibronectin through alpha5beta1 integrin in human embryonic kidney 293 cells and depended on determinants of L1 endocytosis: dynamin I, c-Src, and the AP2/clathrin binding site (Arg-Ser-Leu-Glu) in the neuronal splice form of L1. L1 clustering on the cell surface enhanced the internalization of activated beta1 integrins and L1 into distinct endocytic vesicles. L1-potentiated migration, enhancement of beta1 integrin endocytosis, and activation of MAP kinase were coordinately inhibited by mutation of an RGD sequence in the sixth immunoglobulin-like domain of L1. Moreover, three CRASH mutations in the L1 cytoplasmic domain (1194L, S1224L, Y1229H), two of which interfere with ankyrin association, inhibited L1-potentiated migration and MAP kinase activation. Function-blocking antibodies to L1 and beta1 integrin retarded the migration of 5-bromo-2'-deoxyuridine-labeled mouse cerebellar granule cells in slice cultures, underscoring the potential physiological relevance of these findings. These studies suggest that L1 functionally interacts with beta1 integrins to potentiate neuronal migration toward extracellular matrix proteins through endocytosis and MAP kinase signaling, and that impairment of this function by L1 cytoplasmic domain mutations may contribute to neurological deficits in CRASH.

L1 mediated homophilic binding and neurite outgrowth are modulated by alternative splicing of exon 2

The neural cell adhesion molecule (CAM) L1 is a member of the immunoglobulin superfamily that has been implicated in neuronal adhesion, neurite outgrowth, and axon guidance. The clinical importance of L1 is illustrated by pathological mutations that lead to hydrocephalus, mental retardation, motor defects, and early mortality. The L1 gene is composed of 28 exons, including exons 2 and 27 that are spliced alternatively, and mutations in exon 2 are associated with severe neurological abnormalities in humans. To elucidate the role of L1 exon 2, a recombinant Fc fusion protein called Delta2L1 was constructed lacking the second exon in the extracellular domain of L1. When bound to fluorescent beads, L1 exhibited homophilic binding while Delta2L1 did not. However, L1 beads coaggregated with the Delta2L1 beads. Similarly, in cell binding studies, L1 bound to L1 and Delta2L1 did not bind to Delta2L1 but it bound moderately to L1. Given the reduced binding of Delta2L1, we tested its effect on neurons. By comparison to L1, a lower percentage of dissociated neurons extended neurites on Delta2L1, and there was a modest decrease in the length of the neurites that grew. Neurite outgrowth from reaggregated neurons was much less robust on Delta2L1 than on L1. The combined results indicate that Delta2L1 does not bind homophilically but it can interact with L1 containing exon 2. The reduced binding and neurite promoting activity of Delta2L1 provides an explanation for certain pathological mutations in L1 that lead to clinically apparent disease in the absence of the normal form of L1 in the nervous system.

The role of endocytic l1 trafficking in polarized adhesion and migration of nerve growth cones

Motility of the nerve growth cone is highly dependent on its dynamic interactions with the microenvironment mediated by cell adhesion molecules (CAMs). These adhesive interactions can be spatially regulated by changing the density and avidity of CAMs on the growth cone. Previous studies have shown that L1, a member of the immunoglobulin superfamily of CAMs, is endocytosed at the central domain of the growth cone followed by centrifugal vesicular transport and reinsertion into the plasma membrane of the leading edge. The present paper focuses on the functional significance of endocytic L1 trafficking in dorsal root ganglia neurons in vitro. We demonstrate that the rate of L1-based neurite growth has a positive correlation with the amount of endocytosed L1 in the growth cone, whereas stimulation of neurite growth via an N-cadherin-dependent mechanism does not increase L1 endocytosis. A growth cone that migrates on an L1 substrate exhibits a steep gradient of L1-mediated adhesion (strong adhesion at the growth cone's leading edge and weak adhesion at the central domain). This gradient of L1 adhesion is attenuated after inhibition of L1 endocytosis in the growth cone by intracellular loading of a function-blocking antibody against alpha-adaptin, a subunit of the clathrin-associated AP-2 adaptor. Inhibition of L1 endocytosis by this antibody also decreased the rate of L1-dependent growth cone migration. These results indicate that the growth cone actively translocates CAMs to create spatial asymmetry in adhesive interactions with its environment and that this spatial asymmetry is important for growth cone migration.

The role of endocytic l1 trafficking in polarized adhesion and migration of nerve growth cones

Motility of the nerve growth cone is highly dependent on its dynamic interactions with the microenvironment mediated by cell adhesion molecules (CAMs). These adhesive interactions can be spatially regulated by changing the density and avidity of CAMs on the growth cone. Previous studies have shown that L1, a member of the immunoglobulin superfamily of CAMs, is endocytosed at the central domain of the growth cone followed by centrifugal vesicular transport and reinsertion into the plasma membrane of the leading edge. The present paper focuses on the functional significance of endocytic L1 trafficking in dorsal root ganglia neurons in vitro. We demonstrate that the rate of L1-based neurite growth has a positive correlation with the amount of endocytosed L1 in the growth cone, whereas stimulation of neurite growth via an N-cadherin-dependent mechanism does not increase L1 endocytosis. A growth cone that migrates on an L1 substrate exhibits a steep gradient of L1-mediated adhesion (strong adhesion at the growth cone's leading edge and weak adhesion at the central domain). This gradient of L1 adhesion is attenuated after inhibition of L1 endocytosis in the growth cone by intracellular loading of a function-blocking antibody against alpha-adaptin, a subunit of the clathrin-associated AP-2 adaptor. Inhibition of L1 endocytosis by this antibody also decreased the rate of L1-dependent growth cone migration. These results indicate that the growth cone actively translocates CAMs to create spatial asymmetry in adhesive interactions with its environment and that this spatial asymmetry is important for growth cone migration.

Immunoglobulin superfamily receptors: cis-interactions, intracellular adapters and alternative splicing regulate adhesion

The immunoglobulin domain is a module found in vertebrates and invertebrates. Its ability to form linear rods when deployed in series, combined with its propensity to bind specifically to other proteins has made it ideal for building cell surface receptors and cell adhesion molecules. These features have resulted in the incorporation of immunoglobulin domains into many hundreds of cell surface molecules. Recently three major advances have been made in understanding immunoglobulin receptors. One is the recognition that their intracellular binding partners are likely to link to multiple cell surface molecules, allowing cross-talk or oligomeric complex formation. A second, but related phenomenon, is their participation in cis-interactions on the extracellular surface that regulate signaling or adhesion. The third is the dramatic ability to form dozens to thousands of different isoforms via alternative splicing. Although antibodies may have been the first example of immunoglobulin-domain-containing proteins using cis-interactions to form receptor like molecules, and the grandest instance of diversity production from limited genetic material, these are clearly old ideas in this superfamily.