The neural cell adhesion molecule L1 is distinct from the N-CAM related group of surface antigens BSP-2 and D2

Brain tissue responses to neural implants impact signal sensitivity and intervention strategies

Implantable biosensors are valuable scientific tools for basic neuroscience research and clinical applications. Neurotechnologies provide direct readouts of neurological signal and neurochemical processes. These tools are generally most valuable when performance capacities extend over months and years to facilitate the study of memory, plasticity, and behavior or to monitor patients’ conditions. These needs have generated a variety of device designs from microelectrodes for fast scan cyclic voltammetry (FSCV) and electrophysiology to microdialysis probes for sampling and detecting various neurochemicals. Regardless of the technology used, the breaching of the blood–brain barrier (BBB) to insert devices triggers a cascade of biochemical pathways resulting in complex molecular and cellular responses to implanted devices. Molecular and cellular changes in the microenvironment surrounding an implant include the introduction of mechanical strain, activation of glial cells, loss of perfusion, secondary metabolic injury, and neuronal degeneration. Changes to the tissue microenvironment surrounding the device can dramatically impact electrochemical and electrophysiological signal sensitivity and stability over time. This review summarizes the magnitude, variability, and time course of the dynamic molecular and cellular level neural tissue responses induced by state-of-the-art implantable devices. Studies show that insertion injuries and foreign body response can impact signal quality across all implanted central nervous system (CNS) sensors to varying degrees over both acute (seconds to minutes) and chronic periods (weeks to months). Understanding the underlying biological processes behind the brain tissue response to the devices at the cellular and molecular level leads to a variety of intervention strategies for improving signal sensitivity and longevity.

Neuregulin 1 enhances cell adhesion molecule l1 expression in human glioma cells and promotes their migration as a function of malignancy

Similar functions of L1, a cell adhesion molecule, and the cytokine neuregulin 1 (Nrg1) have been suggested in tumorigenesis and the promotion of metastasis. We studied the relationships of Nrg1 and L1 expression in human gliomas. Using immunofluorescence staining on a human glioma tissue microarray, we found a positive correlation between levels of L1 and Nrg1α or Nrg1β expression; expression tended to increase with increasing WHO (World Health Organization) tumor grade. L1 was also found to colocalize with either Nrg1 isoform. In cultures of U87-MG human glioblastoma and human U251 and SHG-44 glioma cells, the base levels of full-length L1 expression were increased by the 2 Nrg1 molecules in the nanomolar range, and Nrg1 siRNA downregulated full-length L1 expression in these tumor cell lines. U87-MG cells treated with either Nrg1 isoform also showed enhanced migration when compared with that treated with vehicle control. In addition, administration of either lapatinib (a dual inhibitor of both the epidermal growth factor receptor and ErbB-2) or erlotinib (an inhibitor of the epidermal growth factor receptor) in combination with either Nrg1α or Nrg1β inhibited the L1 expression elicited by these cytokines in U87-MG cells. Together, our data suggest that Nrg1 regulates L1 expression in gliomas, and that Nrg1 may contribute to malignancy by upregulating the L1 expression in glioblastoma cells, thereby enhancing their migration.

Families of neural adhesion molecules

The neural cell adhesion molecules L1 and N-CAM share a common carbohydrate epitope that is recognized by the monoclonal antibodies L2 and HNK-1. The L2/HNK-1 epitope is also present on the myelin-associated glycoprotein (MAG) and secreted J1 glycoprotein, both of which have been identified as cell adhesion molecules. Each of the four adhesion molecules is differentially expressed during development on distinct cell types. Expression of the L2/HNK-1 epitope is regulated independently of the protein backbone, is phylogenetically conserved, and plays a role in cell-cell and, particularly, cell-substrate interactions. Another set of glycoproteins shares a common carbohydrate epitope designated L3. This epitope is present on the novel adhesion molecule on glia (AMOG), L1 and MAG, but not on J1 and N-CAM. As in the L2/HNK-1 family, the number of glycoproteins expressing this epitope is not yet known. It is therefore possible that heterogeneities in carbohydrate structures are associated with different sets of adhesion molecules and may have functional implications.

Differential expression of phosphacan/RPTPβ isoforms in the developing mouse visual system

The chondroitin sulfate proteoglycan DSD-1-PG/phosphacan represents one of four splice variants of receptor-protein-tyrosine-phosphatase-beta/zeta (RPTPbeta/zeta). This receptor is expressed by glial cells and occurs in two isoforms, RPTPbeta(long) and RPTPbeta(short). The secreted forms phosphacan and phosphacan short isoform (PSI) bind to extracellular matrix and adhesion molecules and might mediate astroglial effects on neuronal differentiation. Phosphacan and RPTPbeta(long) both carry the DSD-1 epitope, a glycosaminoglycan modification that is involved in stimulating neurite outgrowth of embryonic rat mesencephalic and hippocampal neurons in a polycationic environment. Additionally, phosphacan inhibits neurite outgrowth of embryonic DRG neurons in the presence of laminin. In the light of these functional properties we examined the expression patterns of the DSD-1 epitope and phosphacan isoforms in the developing mouse visual system. During retinal development the DSD-1 epitope appears around embryonic day (E)13, peaks around postnatal day (P)6, and is downregulated from P9 to adolescence. By comparison, the phosphacan core protein is first detectable at E12, reaches maximal levels around P14, and persists, although at lower levels, to adulthood. The DSD-1 epitope is restricted to the nerve fiber and the inner plexiform layers. In contrast, the phosphacan core protein immunoreactivity extends from the nerve fiber layer to the outer plexiform layer. The level of expression of the phosphacan/RPTPbeta gene was investigated by reverse-transcriptase polymerase chain reaction. These experiments suggest that there is a shift in the expression patterns of the different phosphacan/RPTPbeta isoforms during late embryonic and postnatal development. In situ hybridization experiments support the conclusion that at least one of the phosphacan/RPTPbeta isoforms in the retina is expressed by neurons.

Dynamics of olfactory learning-induced up-regulation of L1 in the piriform cortex and hippocampus

L1 is a cell adhesion molecule implicated in the formation of neural circuits and synaptic plasticity. We have examined the sequence and time-frame in which modifications in the synaptic expression of L1 occur in the piriform cortex and hippocampus in the course of rule learning of an olfactory discrimination task. Rats were trained to choose the correct odour in a pair to be rewarded with drinking water. Such training requires 6-8 days on average before rats reach maximal performance. We observed a learning-induced L1 up-regulation that occurred at an early training stage in the piriform cortex but only after rule-learning establishment in the hippocampus. We suggest that the dynamics of L1 up-regulation may reflect the functional role of these brain regions in olfactory rule learning.

Accumulation of N-CAM 180 at Contact Sites Between Neuroblastoma Cells and Latex Beads Coated with Extracellular Matrix Molecules

Neuronal cells expressing neural cell adhesion molecule (N-CAM) accumulate the largest N-CAM component (N-CAM 180) at cell - cell contact sites. To test whether this accumulation is induced by interactions at the surface membrane, latex beads coated with several purified adhesion molecules or extracellular matrix (ECM) components were co-cultured with neuroblastoma cells. Beads coated with L1, N-CAM, the L2/HNK-1 carbohydrate epitope-carrying molecules from adult mouse brain or laminin from Engelbreth-Holm-Swarm (EHS) sarcoma did not induce an accumulation of N-CAM 180 or L1 at sites of contact suggesting that these molecules are not directly involved in N-CAM 180 accumulation or that their mobility is required for this process. Beads coated with ECM components of the PF-HR9 cell line induced accumulation of N-CAM 180 at sites of contact with neuroblastoma cells. Accumulation was seen at cell bodies of undifferentiated and differentiated neuroblastoma cells, as well as on neurites and growth cones of differentiated neuroblastoma cells. Accumulation of the neural adhesion molecule L1 was also seen, but less prominently and reproducibly. These observations suggest that molecules of the ECM can directly or indirectly, e.g. via molecules linked to N-CAM 180 on the cell surface, induce accumulation of N-CAM 180.

The L2/HNK-1 Carbohydrate Epitope is Involved in the Preferential Outgrowth of Motor Neurons on Ventral Roots and Motor Nerves

Based on the observation that in adult mice the carbohydrate epitope L2/HNK-1 is detectable on Schwann cells in ventral spinal roots, but only scarcely in dorsal roots (Martini et al., Dev. Biol., 129, 330 - 338, 1988), the possibility was investigated that the carbohydrate is involved in the outgrowth of regenerating motor neuron axons on peripheral nerve substrates expressing the epitope. To monitor whether the L2 carbohydrate remains present during the time periods in which regenerating axons penetrate the denervated distal nerve stumps, the expression of L2 in motor and sensory branches of the femoral nerve was investigated in normal animals and after a crush lesion. During the first two postoperative weeks, L2 immunoreactivity remained high in the myelinating Schwann cells of the motor branch, whereas L2 immunoreactivity was virtually absent in the sensory branch. In a first experimental approach, cryosections of ventral and dorsal spinal roots and of motor and sensory nerves of adult rats and mice were used as substrates for neurite outgrowth. Neurites of motor neurons from chicken embryos were approximately 35% longer after 30 h of maintenance on ventral roots than on dorsal roots. Neurites from sensory neurons had the same length on dorsal as on ventral motors and were as long as neurites from motor neurons grown on dorsal roots. L2 antibodies reduced neurite outgrowth of motor neurons on ventral roots but not on dorsal roots. Neurite outgrowth of sensory neurons on both roots was not altered by the antibodies. Neurite outgrowth of motor neurons on a mixture of the extracellular matrix glycoprotein laminin and the L2 carbohydrate-carrying glycolipid was significantly higher than on the laminin substrate mixture with GD1b ganglioside or sulphatide. L2 antibodies reduced neurite outgrowth of motor neurons by 50% on the L2 glycolipid, but not on GD1b or sulphatide. These observations indicate that the L2 carbohydrate promotes neurite outgrowth of motor neurons in vitro and may thus contribute to the preferential reinnervation of motor nerves by regenerating motor axons in vivo.

Role for myelin-associated glycoprotein as a functional tenascin-R receptor

The expression of the immunoglobulin superfamily member myelin-associated glycoprotein (MAG) and the extracellular matrix glycoprotein tenascin-R (TN-R) by oligodendrocytes overlaps in time and space. The two molecules can be neurite outgrowth-inhibitory or -promoting depending on the neuronal cell type and the environment in which they are presented. Here we show that the two molecules directly bind to each other in vitro and that binding sites on TN-R localize to two domains, the fibrinogen domain and the epidermal growth factor-like repeat domain with the N-terminal cysteine-rich stretch. We further show by a functional assay, namely the repulsion of MAG-transfected Chinese hamster ovary cells (CHO) cells from a TN-R substrate, that MAG is part of the signalling pathway of TN-R for cell repulsion. When coated as a uniform substrate, MAG was inhibitory for neurite outgrowth of hippocampal and cerebellar neurons in vitro, when compared to poly-L-lysine, while TN-R enhanced neurite outgrowth. When added to MAG, TN-R neutralized the neurite outgrowth-inhibitory effects of MAG, presumably by blocking the neurite outgrowth-inhibitory domain of MAG.

Expression of HNK-1 carbohydrate and its binding protein, SBP-1, in apposing cell surfaces in cerebral cortex and cerebellum

Sulfoglucuronyl carbohydrate is the terminal moiety of neolacto-oligosaccharides, expressed on several glycoproteins of the immunoglobulin superfamily involved in cell-cell recognition and on two glycolipids. Sulfoglucuronyl carbohydrate is temporally and spatially regulated in the developing nervous system. It appears to be involved in neural cell recognition and in cell adhesion processes through its interaction with specific proteins on cell surfaces. Previously we have characterized a specific sulfoglucuronyl carbohydrate-binding protein in rat brain. Sulfoglucuronyl carbohydrate binding protein-1 is structurally similar to a 30,000 mol. wt adhesive and neurite outgrowth promoting protein amphoterin [Rauvala and Pihlaskari (1987) J. biol. Chem. 262, p. 16,625]. The pattern of expression of sulfoglucuronyl carbohydrate binding protein-1 in developing rat nervous system was studied to understand the significance of its interaction with sulfoglucuronyl carbohydrate-bearing molecules. Biochemical analyses showed that the expression of sulfoglucuronyl carbohydrate binding protein-1 was developmentally regulated similarly to sulfoglucuronyl carbohydrate. Immunocytochemical localization of sulfoglucuronyl carbohydrate binding protein-1 and sulfoglucuronyl carbohydrate was performed by bright-field and fluorescent confocal laser scanning microscopy. In postnatal day 7 rat cerebellum, sulfoglucuronyl carbohydrate binding protein-1 was primarily associated with neurons of the external and internal granule cell layers. The sulfoglucuronyl carbohydrate binding protein-1 immunoreactivity was absent in Purkinje cell bodies and their dendrites in the molecular layer, as well as in Bergmann glial fibres and in white matter. In contrast, sulfoglucuronyl carbohydrate (reactive with HNK-1 antibody) was localized in processes surrounding granule neurons in the internal granule cell layer. Sulfoglucuronyl carbohydrate was also expressed in Purkinje neurons and their dendrites in the molecular layer and their axonal processes in the white matter. To a lesser extent Bergmann glial fibres were also positive for sulfoglucuronyl carbohydrate. In the cerebral cortex, at embryonic day 21, sulfoglucuronyl carbohydrate binding protein-1 was mainly observed in immature neurons of the cortical plate and subplate and dividing cells near the ventricular zone. Whereas, sulfoglucuronyl carbohydrate was strongly expressed in the fibres of the subplate and marginal zone. Sulfoglucuronyl carbohydrate was also found in the processes surrounding the sulfoglucuronyl carbohydrate binding protein-1-expressing neuronal cell bodies in the cortical plate and in ventricular zone. The specific localization of sulfoglucuronyl carbohydrate binding protein- in cerebellar granule neurons and neurons of the cerebral cortex was also confirmed by immunocytochemistry of the dissociated tissue cell cultures. The complementary localization of sulfoglucuronyl carbohydrate and sulfoglucuronyl carbohydrate binding protein-1, both in cerebral cortex and cerebellum, in apposing cellular structures indicate possible interaction between the two and signalling during the process of cell migration and arrest of migration.

Coordinate expression of L1 and 6B4 proteoglycan/phosphacan is correlated with the migration of mesencephalic dopaminergic neurons in mice

Mesencephalic dopaminergic (DA) neurons of mice are generated from embryonic day 10 to 12 (E10-12) in the ventricular zone of the mesencephalon. They first migrate toward the ventral mesencephalon, and then turn laterally, or tangentially, in the basal part of the mesencephalon. With immunohistochemical analysis of E10-E15 ICR mice, we found that cell adhesion molecule L1 was transiently expressed on the median part of tangential fibers coincident with the lateral migration of DA neurons from E11 to E13, when neurons move along the tangential fibers toward their final destinations: the reticular formation, the substantia nigra pars compact, and the ventral tegmental area. While L1 expression was not observed in DA neurons, they expressed a chondroitin sulfate proteoglycan, 6B4 proteoglycan/phosphacan, which has been shown to bind to L1/Ng-CAM in vitro. These results suggest that the heterophilic interaction between 6B4 proteoglycan on the neurons and L1 on the fibers is involved in the lateral migration of mesencephalic DA neurons in mice.

VASE-encoded peptide modifies NCAM- and L1-mediated neurite outgrowth

Interactions between the neural cell adhesion molecule (NCAM) with NCAM-expressing neurons (trans-interaction) stimulate outgrowth of neurites. The extent of NCAM-triggered neurite outgrowth depends on the presence of 10 amino acids derived from the variable alternatively spliced exon (VASE or pi-exon) in the fourth immunoglobulin-like domain of NCAM (Ig4): NCAM with VASE reduces and without VASE enhances neurite outgrowth in cis- or trans-interaction. We have investigated the role of VASE in neurite outgrowth by characterizing the receptors at the cell surface of cultured cerebellar neurons. Results from experiments with L1 and NCAM antibodies and with cerebellar neurons derived from wild-type or NCAM-deficient mice show that substrate-coated Ig4 with VASE (Ig4+) or without VASE (Ig4-) stimulates neurite outgrowth by a trans-interaction with L1 and that Ig4- promotes neurite outgrowth more strongly than Ig4+ by a transinteraction with NCAM.

Immunohistochemical expression of cell adhesion molecule L1 in hemimegalencephaly

We demonstrated immunohistochemically an abnormal expression of the neural cell adhesion molecule L1 in 10 developing brains of children with hemimegalencephaly (HM) aged from 36 weeks gestation to 10 years of age, comparing them with 23 controls aged from 13 weeks of gestation to 14 years. There was dense L1 expression in focal regions of the molecular layer beneath leptomeningeal glioneuronal heterotopia, in areas of cerebral cortex with large neurons, and in the disorganized or neuronal heterotopic sites in the white matter in HM. L1 was also heterogeneously enhanced in the abnormal cortex after 1 year of age, suggesting that axonal growth was delayed. These changes persisted into the older age group in the abnormal areas of cortex in HM. The cell bodies of many enlarged neurons in HM were immunopositive for L1, whereas L1 was usually localized to the processes of normal neurons. The delayed L1 immunoreactivity and enlarged L1-immunopositive neurons may be closely related to the pathogenesis of unilateral megalencephaly with cortical dysplasia and heterotopia.

In vitro oligodendrogliotrophic properties of cell adhesion molecules in the immunoglobulin superfamily: myelin-associated glycoprotein and N-CAM

To determine if cell recognition molecules interact trophically with oligodendrocytes (OCs), their effect as growth substrates for differentiating oligodendroblasts was studied in primary culture. Oligodendroblasts purified from postnatal rat cerebrum by immunopanning were plated on substratum-bound cell adhesion molecules or extracellular matrix glycoproteins in chemically defined medium in which OCs terminally differentiate but survive poorly. Growth on myelin-associated glycoprotein (MAG) and neural cell adhesion molecule (N-CAM) selectively increased the number of viable cells per culture 2 weeks after plating as much as tenfold and sixfold, respectively, over background survival on an albumin substrate, whereas L1, tenascin-R, tenascin-C, fibronectin, and laminin were ineffective. Neither MAG nor N-CAM stimulated bromodeoxyuridine incorporation into cultures, indicating that enhanced proliferation did not contribute to better survival. Compared to growth on polyornithine alone, oligodendroblast differentiation in the added presence of MAG or N-CAM was qualitatively unchanged; > 90% of surviving cells developed into OCs that matured further by immunocytochemical and morphological criteria. A striking difference, however, was the quantitative effect of MAG and N-CAM substrates on oligodendrite outgrowth, increasing myelin-like membrane formation two- to threefold (> 8 x 10(3) microns2/cell). These findings support the concept that autotypic or heterotypic cell contact-mediated signaling by recognition molecules at the OC surface contributes trophic support of myelinogenesis.

The effect of continuous intraventricular infusion of L1 and NCAM antibodies on spatial learning in rats

Recent studies suggest a role of the neural cell adhesion molecules L1 and NCAM in mechanisms of memory storage. In the present study we analyzed the effect of continuous intraventricular infusion of polyclonal antibodies directed against L1 (antiL1) or NCAM (antiNCAM) on the performance of male Wistar rats during the acquisition and retention of a spatial learning task (Morris water-maze). In this task animals have to learn the spatial position of a hidden escape platform in a water tank to escape onto it. During acquisition of the task animals with continuous infusion of antiNCAM - but not those infused with antiL1 - showed day-dependent attenuated learning in comparison to controls (P = 0.001). Control animals were either injected with vehicle (PBS) or with polyclonal antibodies raised against liver cell membrane. When the escape platform was removed during the retention test (transfer test), the performance of animals continuously infused with antiL1 as well as those continuously infused with antiNCAM showed an impaired search pattern when compared with the performance of control animals (P = 0.001 and 0.04, respectively). Whereas control animals spent up to 46% of their time searching for the platform in the correct quadrant, the time antiL1- and antiNCAM-infused animals spent in this quadrant was closer to chance level (30.5% and 36.5%), respectively). The present data provide additional support for an involvement of the two adhesion molecules L1 and NCAM in synaptic plasticity underlying memory storage.

C-erbB2/neu transfection induces gap junctional communication incompetence in glial cells

Astrocytes form functional networks that participate in active signaling in which external stimuli are generated and amplified in many of the same ways as in neurons. Gap junctions between astrocytes offer the structural avenue by which the electrical and metabolic signals are propagated from one cell to another. Little is known about the trafficking, assembly, and degradation mechanisms of the major astrocytic gap junction protein connexin43. We have studied a glial cell line transfected with the C-erbB2/neu oncogene (neu+), finding severe interruption of gap junctional communication after stable transfection. Evidence from Western blotting and phosphorylation studies showed that the processing of connexin43 to its higher phosphorylated isoforms is disturbed. Confocal laser imaging indicates that the major deficit in the neu+ cells is attributable to a lack in plaque assembly of connexin43. Because the neu+ cells also lack N-CAM proteins and because work from others has indicated a close relationship between communication competence and constitutive CAM expression, our data suggest that expression of C-erbB2/neu oncogene alters cell-cell association via CAM proteins, which thereby affects gap junction plaque assembly and appropriate phosphorylation of connexin43.

Immunohistochemical expression of cell adhesion molecule L1 during development of the human brain

We demonstrated the expression of neural cell adhesion molecule, L1, during human brain development by immunostaining with anti-L1 fibronectin domain antibody. In the human cerebellum, the inner half of the external granular and molecular layers, mostly parallel fibres, and the Purkinje cell layer were immunoreactive for L1 from the early foetal period of 13-15 gestational weeks (GW). Immunoreactivity was strongest in the molecular layer in the perinatal period. The subcortical white matter and dentate layer were immunoreactive in foetal life. In the human cerebrum, the outer half of the molecular layer, together with Cajal-Retzius cell bodies, were positively stained until 30-34 GW, when afferent fibres develop over the entire cortex. The fibres in the white matter were strongly immunopositive in the fascicles until early infancy. These results suggest that L1 is temporally and spatially expressed in the developing brain, and may play important roles in neural cell migration, neurite elongation, and axonal fasciculation.

Distinct effects of recombinant tenascin-R domains in neuronal cell functions and identification of the domain interacting with the neuronal recognition molecule F3/11

We have identified distinct domains of the rat extracellular matrix glycoprotein tenascin-R using recombinant fragments of the molecule that confer neuronal cell functions. In short-term adhesion assays (0.5 h), cerebellar neurons adhered best to the fragment representing the fibrinogen knob (FG), but also the fibronectin type III (FN) repeats 1-2 and 6-8. FG, FN1-2 and FN3-5 were the most repellent fragments for neuronal cell bodies. Neurites and growth cones were strongly repelled from areas coated with fragments containing the cysteine-rich stretch and the EGF-like domains (EGF-L), FN1-2, FN3-5 and FG. Polarization of morphology of hippocampal neurons was exclusively associated with FG, while EGF-L prevented neurite outgrowth altogether. The binding site of the neuronal receptor for tenascin-R, the immunoglobulin superfamily adhesion molecule F3/11, was localized to EGF-L. The combined observations show distinct, but also overlapping functions for the different tenascin-R domains. They further suggest the existence of multiple neuronal tenascin-R receptors which influence the response of neurons to their extracellular matrix environment.

CD9 of mouse brain is implicated in neurite outgrowth and cell migration in vitro and is associated with the alpha 6/beta 1 integrin and the neural adhesion molecule L1

We describe here a novel monoclonal antibody (mab H6) which recognizes CD9, an integral cell surface constituent previously described in cells of the hematopoietic lineage and involved in the aggregation of platelets. Mab H6 was raised against membranes of immature mouse astrocytes and reacted with a protein of 25-27 kD in detergent extracts of adult mouse brain membranes. Sequence analysis of the N-terminal amino acids revealed an identity of 96% with CD9 from mouse kidney. CD9 was localized in the central and peripheral mouse nervous systems: in the spinal cord of 11-day-old mouse embryos, CD9 was strongly expressed in the floor and roof plates. In the adult mouse sciatic nerve, myelin sheaths were highly CD9-immunoreactive. Mab H6 reacted with the cell surfaces of both glial cells and neurons in culture and inhibited migration of neuronal cell bodies, neurite fasciculation and outgrowth of astrocytic processes from cerebellar microexplants. Neurite outgrowth from isolated small cerebellar neurons was increased in the presence of mab H6 on substrate-coated laminin, but not on substrate-coated poly-L-lysine. Addition of mab H6 elicited an increase in intracellular Ca2+ concentration in these cells on substrate-coated laminin. Immunoprecipitates of CD9 from cultured mouse neuroblastoma N2A cells contained the alpha 6/beta 1 integrin. Moreover, preparations of CD9 immunoaffinity-purified from adult mouse brain using a mab H6 column contained the neural adhesion molecule L1, but not other neural adhesion molecules. CD9 bound to L1, but not to NCAM or MAG. Both the alpha 6/beta 1 integrin and L1 could be induced to coredistribute with CD9 on the surface of cultured neuroblastoma N2A cells. The combined observations suggest that CD9 can associate with L1 and alpha 6/beta 1 integrin to influence neural cell interactions in vitro.

Identification of the border between fibronectin type III homologous repeats 2 and 3 of the neural cell adhesion molecule L1 as a neurite outgrowth promoting and signal transducing domain

To determine the domains of the neural cell adhesion molecule L1 involved in neurite outgrowth, we have generated monoclonal antibodies against L1 and investigated their effects on neurite outgrowth of small cerebellar neurons in culture. When the 10 antibodies were coated as substrate, only antibody 557.B6, which recognizes an epitope represented by a synthetic peptide comprising amino acids 818 to 832 at the border between the fibronectin type III homologous repeats 2 and 3, was as efficacious as L1 in promoting neurite outgrowth, increasing intracellular levels of Ca2+, and stimulating the turnover of inositol phosphates. These findings suggest that neurite outgrowth and changes in these second messengers are correlated. Such a correlation was confirmed by the ability of Ca2+ channel antagonists and pertussis toxin to inhibit neurite outgrowth on L1 and antibody 557.B6. These observations indicate for the first time a distinct site on cell surface-bound L1 as a prominent signal-transducing domain through which the recognition events appear to be funneled to trigger neurite outgrowth, increase turnover of inositol phosphates, and elevate intracellular levels of Ca2+.

Several extracellular domains of the neural cell adhesion molecule L1 are involved in homophilic interactions

The neural cell adhesion molecule L1 is a multidomain protein that plays important roles in cell adhesion, migration, and neurite outgrowth. It can interact with itself by a self-binding, i.e., homophilic adhesion mechanism (Kadmon et al.: J Cell Biol 110: 193-208, 1990a). To determine the domains of L1 involved in homophilic binding, we have generated protein fragments of L1 in a prokaryotic and a eukaryotic expression system and used these covalently coupled to fluorescent microspheres to quantify aggregation between them by cytofluorometric analysis. Protein fragments containing the first and second Ig-like domains and the third fibronectin type III homologous repeat showed avid self-binding. Ig-like domains III and IV also showed some self-binding, whereas Ig-like domains V and VI and fibronectin type III homologous repeats 1 and 2 as well as 4 and 5 were less or not active. Binding between different domains was also observed: fibronectin type III homologous repeats 4 and 5 interacted with Ig-like domains I and II, and fibronectin type III homologous repeats 3-5 interacted with all Ig-like domains. These results were confirmed by experiments testing the binding of fragment-conjugated microspheres to substrate-coated L1 or to cell surface-expressed L1 on cultured neurons. Binding of L1 to itself was interfered with by all protein fragments tested, suggesting that also less avidly binding domains of L1 contribute to homophilic binding. These observations indicate prominent functional roles of both Ig-like domains and fibronectin type III homologous repeats in homophilic binding of L1.

Evidence for cis interaction and cooperative signalling by the heat-stable antigen nectadrin (murine CD24) and the cell adhesion molecule L1 in neurons

L1 is a transmembranal homophilic cell adhesion molecule of the immunoglobulin superfamily expressed by neural and lymphoid cells. The heat-stable antigen (HSA, murine CD24) nectadrin is a highly and heterogeneously glycosylated glycophosphatidylinositol-linked differentiation antigen of haematopoietic and neural cells. L1 and nectadrin have been shown to mediate cell adhesion and intracellular Ca2+ signals in neurons and B lymphoblasts, respectively. Here we show that nectadrin is co-expressed with L1 in murine cerebellar granule cell neurons and neuroblastoma N2A cells. Purified nectadrin bound to L1 with an apparent binding ratio of five nectadrin molecules to one L1 molecule at saturation. Binding between nectadrin and purified N-CAM was not observed. In co-capping experiments nectadrin co-redistributed with L1 and N-CAM. Since in these cells N-CAM and L1 cohere by cis-binding nectadrin appears to join the L1-N-CAM complex through binding to L1. Antibodies to each L1 and nectadrin evoked small increases in the intracellular Ca2+ concentration. However, when both antibodies were added together or in tandem to the cells, a strong intracellular Ca2+ signal was measured that was at least 6- and 10-fold stronger than the signal separately induced by L1 and nectadrin antibodies respectively. Such a cooperative effect was not observed in B lymphoblasts, using the same antibodies, or in neurons, using a combination of L1 and Thy-1 antibodies. Both the weak Ca2+ signal mediated by L1 alone and the enhanced signal jointly triggered by antibodies to L1 and nectadrin were inhibited by phorbol 12-myristate 13-acetate and were not significantly affected by Ni2+ and Cd2+ cations, suggesting that they are related to one another and involve recruitment of intracellular Ca2+. Nectadrin therefore appears to join a functional complex of neuronal adhesion molecules and to potentiate the signal transduction pathway of L1, possibly in response to neuron-neuron contact formation.

The L2/HNK-1 carbohydrate mediates adhesion of neural cells to laminin

The L2/HNK-1 carbohydrate epitope shared by several neural adhesion molecules has been implicated in cell-to-cell and cell-to-laminin adhesion (Keilhauer et al., Nature, 316, 728-730, 1985; Künemund et al., J. Cell Biol., 106, 213-223, 1988). As demonstrated previously for chicken retinal ganglion cells (Cole et al., Neurosci. Lett., 93, 170-175, 1988), cerebral cortex astrocytes or cerebellar neurons could not be shown to adhere to the substrate-bound L2/HNK-1 carbohydrate. The cell-bound L2/HNK-1 carbohydrate, however, was a potent mediator of astrocytic and neuronal cell adhesion to laminin, which was strongly reduced in the presence of the L2/HNK-1 carbohydrate-carrying glycolipids or Fab fragments of a monoclonal antibody against it. Inhibition of adhesion could not be observed in the presence of the negatively charged gangliosides or sulphatide, but in the presence of heparin. To investigate whether the L2/HNK-1 carbohydrate and heparin use the same or different binding sites on laminin, adhesion of cells to laminin was determined in the presence of heparin and Fab fragments of a monoclonal L2 antibody, which gave an additive value of inhibition as compared to the inhibition caused by the single compounds. This result, as well as studies of the binding of the L2/HNK-1 glycolipids to laminin in the presence of heparin, indicates that the L2/HNK-1 carbohydrate and heparin are implicated in different aspects of neural cell adhesion to laminin.

Use of large-scale hydrazinolysis in the preparation of N-linked oligosaccharide libraries: application to brain tissue

In this report, we describe the preparation of a library of N-linked glycans from whole murine brain obtained by the large-scale hydrazinolysis of an acetone powder of the tissue followed by chromatographic procedures. 84% of the characterized oligosaccharides were found to be anionic, the remainder neutral. The anionic species were successively neutralized by neuraminidase (29%), aq. hydrofluoric acid (30%), and methanolysis (26%), indicating that approximately equal portions were sensitive to desialylation, dephosphorylation and desulfation, respectively. The presence of the sulfated fraction was confirmed by direct 35SO4 metabolic labelling. A residual partially characterized fraction was found to be anionic through possession of carboxylic acid groups, unrelated to sialic acid. The purified oligosaccharides, in the absence of their original protein conjugates, were shown to retain those immunological characteristics essential for recognition by a specific monoclonal antibody, LS (412), that is known to recognize a carbohydrate epitope present on a number of neural adhesion molecules and functional in neural cell adhesion. These properties confirm the viability of scaling up the size of the hydrazinolysis procedure and adapting it to whole tissue for the production of glycan libraries and for the probing of structures of interest.

Monoclonal antibody that recognizes the carbohydrate portion of cell adhesion molecule L1 influences calcium current in cultured neurons

A monoclonal antibody (mAb), 2E12, against the neural cell adhesion molecule L1 recognized the 200 kDa component of L1. The epitope of L1 reacting with mAb 2E12 was localized in its carbohydrate chain, judging from the results of experiments on glycopeptidase F treatment. The physiological effect of adding mAbL1 (2E12) to cultured mouse dorsal root ganglion neurons was studied using patch-clamp techniques. The binding of mAbL1 (2E12) to the neurons expressing L1 molecule induced an inward current inhibited by calcium channel blockers such as nifedipine and Lanthanum. It was also found that the mAbL1 (2E12) leads to a rise in the intracellular Ca2+ concentration ([Ca2+]i) in cultured neurons. This rise seems to be due to an influx of extracellular Ca2+, since treatment with EGTA abolished those phenomena. L-type calcium channel blockers such as nifedipine and cadmium, as well as inward current, blocked the effect of mAbL1 (2E12). These results suggest that the carbohydrate chain of L1 glycoprotein is directly involved in the induction of calcium current, and that the L1 molecule may play a prominent role in regulation of the Ca2+ channel.

Immunocytochemical localization of cell adhesion molecules in the developing and mature olfactory system

The localization of Ca+(+)-independent cell adhesion molecules (CAMs) in the developing and mature olfactory epithelium and bulb is reviewed. The CAMs included in this article are the neural cell adhesion molecule (N-CAM), the 180 kD component of N-CAM (N-CAM 180), the embryonic form of N-CAM (E-N-CAM), L1 glycoproteins, J1 glycoproteins, and the adhesion molecule on glia (AMOG). In addition, the expression of the L2-HNK-1 carbohydrate epitope, shared by N-CAM, L1, J1 and myelin-associated glycoprotein (MAG) in the adult olfactory epithelium and bulb has also been documented. For the localization of these molecules at the light and electron microscopic levels, immunocytochemical techniques were used and are described in detail. During development and organogenesis, the olfactory system exhibits a pattern of CAM expression similar to the general pattern described for the developing nervous system. In the adult olfactory system, however, a significant retention of CAMs characteristic for developmental and morphogenetic processes, such as E-N-CAM, AMOG, as well as the high molecular weight components of J1 glycoproteins, can be observed. The retention of these embryonic features are most likely associated with the cell turnover and high plasticity of this system. Moreover, the predominance of N-CAM 180 with respect to other components of N-CAM, as well as the absence of the L2/HNK-1 carbohydrate epitope, are also particular traits of the primary olfactory system which could be associated with its exceptional properties.

EMA: a developmentally regulated cell-surface glycoprotein of CNS neurons that is concentrated at the leading edge of growth cones

To identify cell-surface molecules that mediate interactions between neurons and their environment during neural development, we used monoclonal antibody techniques to define a developmentally regulated antigen in the central nervous system of the mouse. The antibody we produced (2A1) immunolabels cells throughout the central nervous system; we analyzed its distribution in the developing cerebral cortex, where it is expressed on cells very soon after they complete mitosis and leave the periventricular proliferative zone. Expression continues into adult life. The antibody also labels the epithelium of the choroid plexus and the renal proximal tubules, but does not label neurons of the peripheral nervous system in the dorsal root ganglia. In dissociated cell culture of embryonic cerebral cortex, 2A1 labels the surface of neurons but not glia. Immunolabeling of neurons in tissue culture is particularly prominent on the edge of growth cones, including filopodia and the leading edge of lamellipodia, when observed with either immunofluorescence or freeze-etch immunoelectron microscopy. Immunopurification with 2A1 of a CHAPS-extracted membrane preparation from brains of neonatal mice produces a broad (32-36 kD) electrophoretic band and a less prominent 70 kD band that are sensitive to N-glycosidase but not endoglycosidase H. Thus the 2A1 antibody recognizes a developmentally regulated, neuronal cell surface glycoprotein (or glycoproteins) with complex N-linked oligosaccharide side chains. We have termed the glycoprotein antigen EMA because of its prominence on the edge membrane of growth cones. EMA is similar to the M6 antigen (Lagenaur et al: J. Neurobiol. 23:71-88, 1992) in apparent molecular weight, distribution in tissue sections, and immunoreactivity on Western blots, suggesting that the two antigens are similar or identical. Expression of EMA is a very early manifestation of neuronal differentiation; its distribution on growth cones suggests a role in mediating the interactions between growth cones and the external cues that guide them.

Involvement of neuronal cell surface molecule B2 in the formation of retinal plexiform layers

The B2 molecule is a 220 kd neuronal cell surface protein of Xenopus, recognized by monoclonal antibody B2 (MAb B2). Immunohistochemistry using MAb B2 revealed that the B2 molecule was expressed in both the inner and outer plexiform layers within the neural retina. During development of the neural retina, the B2 molecule first appeared at stages 35/36 in the newly formed plexiform layers. When embryonic eyes were cultured in the presence of anti-B2 antiserum (Fab fragments), the formation of the retinal plexiform layers was impeded. These data suggest that the cell surface molecule B2 plays a role in the development of retinal plexiform layers.

Cell-cell contact modulates expression of cell adhesion molecule L1 in PC12 cells

The effect of cell contact on the expression of cell adhesion molecule L1 was investigated. The rat pheochromocytoma cell line PC12 cells were cultured at various densities in the presence or absence of the nerve growth factor. The addition of the nerve growth factor promoted the expression of L1. The expression of L1 was higher when the cells were cultured at high density than when done at low density both in the presence or absence of the nerve growth factor. Immunohistochemical staining of L1 showed that the expression of L1 was higher in the cells contacting each other. These results show that cell interaction affects the expression of cell adhesion molecule L1 in the PC12 cells.

Expression and function of the neural cell adhesion molecule L1 in mouse leukocytes

The neural cell adhesion molecule L1 is a cell surface glycoprotein of the immunoglobulin superfamily which mediates adhesion between neural cells. The possibility that similar cell-cell recognition mechanisms may be shared by the nervous and immune systems prompted us to study the expression and function of L1 in cells of the hematopoietic system. Immunofluorescence analysis using monoclonal L1 antibody revealed that the molecule is expressed in the bone marrow, spleen, and thymus of the mouse. This observation was confirmed by amplifying cDNA derived from these organs by the polymerase chain reaction with L1-specific oligonucleotide primers. Two-color fluorescence analysis indicated that bone marrow lymphoid and granulocyte precursor cells express low and high levels of L1, respectively. In the thymus L1 is primarily expressed by mature cells that have a strong expression of CD3 and in the spleen both B cells and T cells express L1. The possible function of L1 in lymphoid cells was studied using subcloned ESb-MP lymphoma cells having high or low densities of L1 on the cell surface as well as activated splenic B lymphoblasts. Parental and subcloned ESb-MP cells that strongly expressed L1 could form homotypic aggregates in the presence of low Ca2+ levels, whereas subcloned ESb-MP cells with a weak expression of L1 did not aggregate, suggesting that L1 mediates the Ca(2+)-independent aggregation of the parental ESb-MP cells. Furthermore, the aggregation of activated B lymphoblasts under physiological concentrations of Ca2+ and Mg2+ was inhibited by 30% in the presence of Fab fragments of polyclonal L1 antibodies, implying that L1 also mediates adhesion among normal lymphoid cells. A possible role of L1 on lymphocytes in stimulating the innervation of lymphoid organs is discussed.

Down-regulation of myelin-associated glycoprotein on Schwann cells by interferon-gamma and tumor necrosis factor-alpha affects neurite outgrowth

To investigate the influence of inflammatory cytokines on the potential of peripheral nerves to regenerate, we analyzed the effect of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on the ability of immortalized Schwann cells to mediate outgrowth of neurites from primary DRG neurons. We found that IFN-gamma and TNF-alpha synergistically inhibited the neurite outgrowth-promoting properties of the Schwann cells by specifically down-regulating myelin-associated glycoprotein (MAG) at the levels of mRNA and cell surface protein by approximately 60%. Antibodies to MAg inhibited the outgrowth of neurites on Schwann cells to the same extent as treatment with the two cytokines. Since MAG appears to be involved in both neurite outgrowth and myelination, our findings may provide evidence for a mechanism, by which inflammatory cytokines interfere with Schwann cell-neuron interactions.

The 200/220 kDa antigen recognized by monoclonal antibody (MAb) UJ127.11 on neural tissues and tumors is the human L1 adhesion molecule

MAb UJ127.11, raised against 16 week human fetal brain, recognizes an antigen present primarily on normal and tumor tissues derived from the neuroectoderm. The antigen has previously been identified as a 220/240 kDa cell surface glycoprotein as determined by immunoprecipitation studies. We show here, that the 220/240 kDa antigen is the human L1 cell adhesion molecule and by Western blot analysis actually has a calculated molecular weight of between 200-220 kDa. Immunocytochemical studies with UJ127.11 and an antibody (5G3) recently utilized to isolate human L1 from brain indicate that both reagents have very similar binding profiles. The binding of radiolabelled UJ127.11 to its target antigen can be blocked by the addition of a rabbit anti-human L1 antiserum. Furthermore, sequential immunoprecipitation and Western blot analysis shows that UJ127.11 and the rabbit anti-human L1 antiserum recognize identical proteins.

Molecular Association of the Neural Adhesion Molecules L1 and N-CAM in the Surface Membrane of Neuroblastoma Cells is Shown by Chemical Cross-linking

The two neural cell adhesion molecules L1 and N-CAM could be shown to be associated in the surface membrane of cultured neuroblastoma cells by chemical cross-linking with 3,3'-dithiobis(sulphosuccinimidyl-propionate) and subsequent immunopurification and precipitation using antibodies to L1 and N-CAM. Glycoproteins recognized in neuroblastoma cells by antibodies to mouse liver membranes were not chemically cross-linked to L1 or N-CAM. These observations suggest that a molecular association between the two molecules may be the basis for their functional cooperativity (Kadmon et al., 1990a,b, J. Cell Biol., 110, 193-208; 209-218).

Identification of a cDNA clone specific for the oligodendrocyte-derived repulsive extracellular matrix molecule J1-160/180

A cDNA clone specific for the oligodendrocyte-derived extracellular matrix glycoproteins J1-160/180 was obtained from a lambda ZAPII expression library using polyclonal antibodies generated against mouse J1-160. The library was constructed from poly(A)(+)-RNA isolated from O1 antigen-positive rat oligodendrocytes. The cDNA clone expressed a fusion protein that was recognized by the J1-160/180-specific monoclonal antibodies 596, 619, and 620, and, weakly, 597. The fusion protein was not recognized by polyclonal antibodies to mouse J1/tenascin. The cDNA clone with an insert of approximately 5.6 kb in size contained the nucleotide sequence coding for the amino acid sequence of the N-terminus of a tryptic peptide derived from mouse J1-160. The developmental and tissue distribution of the mRNA recognized by the cDNA clone is in agreement with the described expression of the J1-160/180 proteins.

Induction of NILE/L1 glycoprotein during neuronal differentiation of the embryonal carcinoma cell line EC1003

A new clone of the mouse embryonal carcinoma cell line 1003 (EC 1003.16) can be maintained in an undifferentiated state in serum-containing medium. Shifting these cells to serum-free, hormonally defined medium causes them to differentiate morphologically and acquire a number of molecular properties characteristic of neurons. Whereas undifferentiated cells lack the NILE/L1 glycoprotein, expression of this neuronal cell adhesion molecule is induced in the differentiating cells. Message for NILE/L1 becomes detectable after 5 days in serum-free medium, and cell-surface NILE/L1 can first be seen at this same time. Changes in two other cell adhesion molecules occur in parallel with the induction of NILE/L1. Fibronectin receptor is present on undifferentiated cells, but is down-regulated by the differentiating neurons. The neural cell adhesion molecule (N-CAM) undergoes a shift from the very adhesive adult form to the less adhesive, highly sialylated embryonic form. These changes would appear to emphasize the role of NILE/L1 in adhesive interactions involving differentiating neurons. Some changes in ganglioside expression also occur during EC 1003.16 differentiation. Undifferentiated cells express the D 1.1 ganglioside but lack gangliosides that are reactive with the monoclonal antibody A2B5. Differentiating cells lose D 1.1 and become A2B5-positive. Since D 1.1 is characteristic of undifferentiated neuroepithelial cells and A2B5 reactivity is a marker for several types of differentiated neurons, these changes in vitro appear to mimic events that occur in vivo.

Isolation and sequence of partial cDNA clones of human L1: homology of human and rodent L1 in the cytoplasmic region

We have isolated cDNA clones coding for the human homologue of the neuronal cell adhesion molecule L1. The nucleotide sequence of the cDNA clones and the deduced primary amino acid sequence of the carboxy terminal portion of the human L1 are homologous to the corresponding sequences of mouse L1 and rat NILE glycoprotein, with an especially high sequences identity in the cytoplasmic regions of the proteins. There is also protein sequence homology with the cytoplasmic region of the Drosophila cell adhesion molecule, neuroglian. The conservation of the cytoplasmic domain argues for an important functional role for this portion of the molecule.

Retention of J1/tenascin and the polysialylated form of the neural cell adhesion molecule (N-CAM) in the adult olfactory bulb

To gain insight into the cellular and molecular mechanisms underlying neurogenesis in adult mouse olfactory bulb, several adhesion molecules expressed by glial cells and neurons were investigated. In the germinal zone of the olfactory bulb, the subependymal layer of the rostral region of the lateral ventricles, two adhesion molecules are detectable that are characteristic of early morphogenetic events: J1/tenascin and the polysialylated form, the so-called embryonic form, of N-CAM. The polysialylated form of N-CAM is expressed by most cells in the subependymal layer, and by some astrocytes and neurons in the granular layer adjacent to the subependymal layer. This suggests that bipotential precursor cells retain expression of the embryonic form during their migration from the subependymal layer and during the first stages of differentiation into neurons and glia. Expression of the polysialylated form of N-CAM is also retained in monolayer cultures of six-day-old olfactory bulbs, 55 days after seeding in vitro. J1/tenascin was detectable in the subependymal layer using two monoclonal antibodies. The immunostaining pattern was different between the two antibodies and more restricted to the subependymal layer than when staining with polyclonal J1 antibodies was performed, indicating that J1/tenascin exists in distinct isoforms. Finally, our observations suggest that, in the adult olfactory bulb, L1 is not only a neuron-neuron adhesion molecule, but it may also be involved in neuron-glia interactions, since it is found at contact sites between these two cell types. L1, therefore, may be a neuron-glia adhesion molecule in some parts of the CNS, while it is not in others.

Isolation and biochemical characterization of a neural proteoglycan expressing the L5 carbohydrate epitope

The monoclonal L5 antibody reacts with an N-glycosidically linked carbohydrate structure which is present on the neural cell adhesion molecule L1, neural chondroitin sulfate proteoglycans, and other not yet identified glycosylated proteins. Using this antibody, we isolated and characterized proteoglycans from adult mouse brain and cultured astrocytes biosynthetically labeled with Na2 35SO4 and a 3H-amino acid mixture. Our data suggest that the L5 proteoglycans of both sources are identical in their biochemical properties. The apparent molecular mass of the L5 proteoglycan is approximately 500 kDa. Digestion of the iodinated L5 proteoglycan from mouse brain and of the [35S]methionine-labeled L5 proteoglycan from cultured astrocytes with proteinase-free chondroitinases ABC and AC revealed three major core proteins with apparent molecular masses of approximately 380, 360, and 260 kDa. These represent molecularly distinct protein cores.

Highly sialylated N-CAM is expressed in adult mouse optic nerve and retina

The localization of the neural cell adhesion molecule (N-CAM) and its highly sialylated form, which is prevalent in young tissues and has therefore been called embryonic neural cell adhesion molecule, was studied in the developing and adult mouse optic nerve and retina immunohistologically and immunochemically. At embryonic and early postnatal ages, neuroblasts and young postmitotic neurons, Müller cells and astrocytes in the retina, and retinal ganglion cell axons and all glial cells in the optic nerve express highly sialylated neural cell adhesion molecule. Beginning with the third postnatal week, highly sialylated neural cell adhesion molecule disappears from retinal ganglion cell axons in the optic nerve and from neuronal cell bodies and processes in the retina. In addition, it is not detectable on oligodendrocytes in 3-week-old animals. However, highly sialylated neural cell adhesion molecule continues to be expressed in the adult optic nerve and retina by astrocytes and Müller cells. On these cells it is only absent from cell membranes contacting basal lamina. Weakly sialylated neural cell adhesion molecule, in contrast, is expressed by all cell types of retinal and optic nerve during development and in the adult. The loss of highly sialylated neural cell adhesion molecule from neurons and oligodendrocytes must therefore be considered as a cell type-specific conversion of the so-called embryonic to the adult form of neural cell adhesion molecule and does not simply reflect the disappearance of neural cell adhesion molecule from these cells. Weakly sialylated neural cell adhesion molecule, however, is absent from outer segments of photoreceptor cells and, as is the case for the highly sialylated form, from glial cell surfaces contacting basal lamina. Thus, the expression of highly sialylated neural cell adhesion molecule by pre- and postmitotic neurons and by oligodendrocytes is restricted mainly to the period of histogenetic events in retina and optic nerve, i.e. cell division, cell migration, dendritic and axonal growth and synaptogenesis. In addition to the observation that this form of neural cell adhesion molecule is less adhesive than the weakly sialylated, adult form, it is likely that highly sialylated neural cell adhesion molecule plays an important role during dynamic morphogenetic events. Furthermore, the expression of highly sialylated neural cell adhesion molecule by astrocytes and Müller cells in adult optic nerves and retinae suggests some histogenetically plastic functions for these cells in the adult mouse visual system.

Expression of the adhesion molecules L1, N-CAM and J1/tenascin during development of the murine small intestine

We have previously studied the immunohistological localization of the three adhesion molecules L1, N-CAM and J1/tenascin in adult mouse small intestine and shown that L1 expression in epithelial crypt cells underlies the adhesion of these cells to one another [63]. To obtain further insight into the functional roles of L1, N-CAM and J1/tenascin in this organ we studied their expression starting at embryonic day 14 during embryonic and early postnatal morphogenesis and during epithelial cell migration in the adult. Expression of L1 was restricted to neural cells until approximately postnatal day 5, when L1 started to be detectable on crypt but not on villus cells, predominantly on the basolateral membrane infoldings. As in brain, L1-specific mRNA was approximately 6 kb in size. L1 from intestine appears to differ from the brain-derived equivalent in possessing a higher level of glycosylation. N-CAM was detectable from embryonic day 14 onward in neural and also in mesenchymal cells. Expression by smooth muscle cells decreased during development. In the villus core, N-CAM was strongly detectable at contact sites between smooth muscle cells forming the cellular scaffold of the villus. From embryonic day 14 onward, N-CAM appeared in both 180- and 140-kDa forms. J1/tenascin was present in both neural and mesenchymal cells from embryonic day 14 onward. Starting at embryonic day 17, J1/tenascin appeared concentrated at the boundary between mesenchyme and epithelium in an increasing gradient from the crypt base to the villus top. From embryonic day 14 onward J1/tenascin consisted of the 190- and 220-kDa components. J1/tenascin from intestine differed from brain-derived J1 in its carbohydrate composition. These observations show that the three adhesion molecules are expressed by distinct cell populations and may serve as cell-type-specific markers in pathologically altered intestinal tissue.

Immunocytological localization of the highly polysialylated form of the neural cell adhesion molecule during development of the murine cerebellar cortex

The expression of the highly polysialylated form of the neural cell adhesion molecule (N-CAM)--the so-called embryonic N-CAM (E-N-CAM)--was investigated in the developing and adult mouse cerebellar cortex by immunohistology and immunocytology at the light and electron microscopic levels. E-N-CAM was never (from embryonic day 14 to postnatal day 15) detectable in the germinal zone of neuroblasts destined to form or forming the external granular layer and was only observed once small cerebellar interneurons had become postmitotic before the beginning of granule cell migration. Granule cells expressed E-N-CAM on cell bodies, axons, and leading and trailing processes also during migration but ceased to reveal detectable levels of E-N-CAM at the end of migration after having reached their final position in the internal granular layer. Other cerebellar cell types, such as Purkinje cells, Bergmann glia, astrocytes, oligodendrocytes, and most prominently, stellate and basket cells, also expressed E-N-CAM, but became E-N-CAM-negative during the third and fourth postnatal weeks, coinciding with overt cessation of cerebellar histogenesis. Thus, except for neuroblasts, E-N-CAM appeared characteristic of growing and moving cellular structures, in agreement with the notion that the highly polysialylated form of N-CAM is less adhesive than the adult form.

Immunoelectron microscopic localization of cell adhesion molecule L1 in developing rat pyramidal tract

The glycoprotein L1 is a cell adhesion molecule that has been proposed to function in the peripheral nervous system in axon fasciculation and onset of myelination. In this report we localize L1 during the development of a major central pathway: the pyramidal tract. The (sub)cellular localization of L1 was determined both by pre-embedding staining on Vibratome sections and by immunogold labelling on ultracryosections in developing rat pyramidal tract at the fifth cervical segment. On arrival at the fifth cervical segment, i.e. at postnatal day 1, growth cones of pioneer fibres did not exhibit L1-immunoreactivity. In the contact zone between pyramidal tract growth cones and glial processes no L1-immunoreactivity was observed. A clear L1-immunoreactivity was noted on small unmyelinated other axons situated in the entrance area of the pyramidal tract growth cones. Also on later arriving, i.e. between postnatal days 2 and 10, small unmyelinated fasciculating pyramidal tract axons L1 were present. It is our impression that L1 is localized in an irregular patchy way on the outer side of the axonal membrane. During the onset of myelination, i.e. between postnatal days 10 and 14, L1 could not be detected on axons ensheathed by oligodendrocytic processes. When myelination is largely completed, i.e. at postnatal day 21, the L1 antigen could be localized within the axoplasma of both unmyelinated and myelinated pyramidal tract axons. Furthermore, L1 could be observed occasionally on small unmyelinated pyramidal tract axons. Whereas compact myelin was always L1-negative, L1 was noted periaxonally between the axolemma and compact myelin and at (para)nodal regions at the contact zone between axolemma and oligodendrocytic processes. From these results it may be deduced that: (1) L1 is involved in fasciculation of outgrowing later arriving pyramidal tract fibres: (2) L1 is not involved in the onset of myelination in this central tract; (3) L1 might play an additional adhesive role in myelinated rat pyramidal tract.

Immunocytochemical localization of the neural cell adhesion molecules L1, N-CAM, and J1 in Pacinian corpuscles of the mouse during development, in the adult and during regeneration

The immunocytochemical localization of the neural cell adhesion molecules L1, N-CAM and J1/tenascin was investigated by light and electron microscopical techniques in murine Pacinian corpuscles during development, in the adult and in the regenerating state. In adult corpuscles, L1 was present only at contact sites between the sensory axon and inner core lamellae. From birth, the earliest stage tested, until day 7, L1 was additionally expressed on lamellar processes of the inner core cells. N-CAM was expressed in developing and adult corpuscles on lamellae and somata of the inner and outer core cells at their contact sites but was hardly detectable at contact sites between axolemma and inner core lamellae. J1/tenascin was found only in association with the extracellular material of the inner core, especially with the two radial clefts and the boundary space between inner and outer core. In developing corpuscles, J1/tenascin became detectable on extracellular material with the onset of inner core differentiation at approximately day 2. After transection or crush of the sciatic nerve, L1 disappeared from the corpuscles but reappeared with regrowing axons at contact sites between axonal membranes and inner core cells. At any regenerative stage inner core cells remained L1-negative. In denervated and reinnervated corpuscles the expression pattern of N-CAM and J1/tenascin did not differ from the normal adult. These observations suggest that a sensory organ, the Pacinian corpuscle, differs from the sciatic nerve and the neuromuscular junction in that its expression of adhesion molecules remains the same in the denervated state as in the innervated adult. Furthermore, in the denervated Pacinian corpuscle, adhesion molecule expression does not resemble that of any developmental stage tested. Thus, other cures than regulation of adhesion molecule expression patterns might be involved in the successful reinnervation of sensory corpuscles.

A protein kinase activity is associated with and specifically phosphorylates the neural cell adhesion molecule L1

The neural cell adhesion molecule L1 is a phosphorylated integral membrane glycoprotein that is recovered from adult mouse brain by immunoaffinity chromatography as a set of polypeptides with apparent molecular masses of 200, 180, 140, 80, and 50 kilodaltons (L1-200, L1-180, L1-140, L1-80, and L1-50, respectively). In the present study, we show that two kinase activities are associated with immunopurified L1: One specifically phosphorylates L1-200 and L1-80 but not L1-180, L1-140, or L1-50. This pattern of phosphorylation corresponds to the one described for L1 after metabolic phosphate incorporation into cultures of cerebellar cells. In both cases, serine is the main amino acid that is labeled by radioactive phosphate. The kinase activity is not activated by Ca2+, calmodulin, phosphatidylserine, diolein, cyclic AMP, or cyclic GMP, a result suggesting that the enzyme is distinct from Ca2+/calmodulin-dependent kinases, from protein kinase C, or from cyclic AMP/cyclic GMP-dependent kinases and may belong to the independent kinase group. The other kinase phosphorylates only casein but not L1, utilizes GTP as well as ATP, and is strongly inhibited by heparin. Because the primary structure of the L1 protein does not contain consensus sequences characteristic for known kinases, we believe that the catalytic activities detectable in immunopurified L1 are due to kinases that are strongly enough associated with L1 to withstand the stringent purification procedures.

Expression of L1 and N-CAM cell adhesion molecules during development of the mouse olfactory system

The expression of the neural adhesion molecules L1 and N-CAM has been studied in the embryonic and early postnatal olfactory system of the mouse in order to gain insight into the function of these molecules during development of a neural structure which retains neuronal turnover capacities throughout adulthood. N-CAM was slightly expressed and L1 was not significantly expressed in the olfactory placode on Embryonic Day 9, the earliest stage tested. Rather, N-CAM was strongly expressed in the mesenchyme underlying the olfactory placode. In the developing nasal pit, L1 and N-CAM were detectable in the developing olfactory epithelium, but not in regions developing into the respiratory epithelium. At early developmental stages, expression of the so-called embryonic form of N-CAM (E-N-CAM) coincides with the expression of N-CAM, whereas at later developmental stages and in the adult it is restricted to a smaller number of sensory cell bodies and axons, suggesting that the less adhesive embryonic form is characteristic of morphogenetically dynamic neuronal structures. Moreover, E-N-CAM is highly expressed at contact sites between olfactory axons and their target cells in the glomeruli of the olfactory bulb. L1 and N-CAM 180, the component of N-CAM that accumulates at cell contacts by interaction with the cytoskeleton are detectable as early as the first axons extend toward the primordial olfactory bulb. L1 remains prominent throughout development on axonal processes, both at contacts with other axons and with ensheathing cells. Contrary to N-CAM 180 which remains detectable on differentiating sensory neuronal cell bodies, L1 is only transiently expressed on these and is no longer detectable on primary olfactory neuronal cell bodies in the adult. Furthermore, whereas throughout development L1 has a molecular form similar to that seen in other parts of the developing and adult central nervous systems, N-CAM and, in particular, N-CAM 180 retain their highly sialylated form at least partially throughout all ages studied. These observations suggest that E-N-CAM and N-CAM 180 are characteristic of developmentally active structures and L1 may not only be involved in neurite outgrowth, but also in stabilization of contacts among fasciculating axons and between axons and ensheathing cells, as it has previously been found in the developing peripheral nervous system.

Neural cell adhesion molecules influence second messenger systems

We have investigated the influence of the neural cell adhesion molecules L1 and N-CAM on second messenger systems using a PC12 rat pheochromocytoma cell line as a model and triggering cell surface receptors by specific antibody binding. Antibodies directed against L1 and N-CAM, but not against other cell surface components, reduce intracellular levels of the inositol phosphates IP2 and IP3, while intracellular levels of cAMP are unaffected. Antibodies against L1 and N-CAM also reduce intracellular pH and increase intracellular Ca2+ by opening Ca2+ channels in a pertussis toxin-inhibitable manner, suggesting the involvement of a G protein in the signal transduction process. Cross-linking of the adhesion molecules on the surface membrane is not required for the effects to occur. Furthermore, adhesion of single PC12 cells to each other elicits effects on intracellular pH and Ca2+ similar to those seen after application, underscoring the physiological significance of the observed changes.

Immunohistological localization of the adhesion molecules L1, N-CAM, and MAG in the developing and adult optic nerve of mice

The localization of the cell adhesion molecules L1, neural cell adhesion molecule (N-CAM), and myelin-associated glycoprotein (MAG) was studied immunohistologically at the light and electron microscopic levels and immunochemically in the developing and adult mouse optic nerve and retina. The neural adhesion molecule L1 is strongly expressed on the shafts of fasciculating unmyelinated axons at all ages studied from embryonic day 15 through adulthood. Growth cones of retinal ganglion cell axons were weakly L1-positive or L1-negative when contacting glial cells. Unmyelinated axons were not only L1-positive when contacting each other but also when contacting glia, whereas contacts between glial cells were L1-negative at all developmental unmyelinated retinal nerve fiber layer or in the unmyelinated optic nerve head became L1-negative when enwrapped by myelin in the optic nerve proper. At all stages of development N-CAM showed profuse labeling on fasciculating axons, growth cones, and their contact sites with glial cells as well as contacts between glial cells. In contrast to L1, axons remained N-CAM-positive when becoming myelinated. Sometimes, N-CAM was found in compact myelin. However, N-CAM was absent from glial surfaces contacting basement membranes at the interface to meninges, blood vessels, and the vitreous body of the eye. MAG was first detectable intracellularly in oligodendrocytes associated with the endoplasmic reticulum and Golgi apparatus before it became apparent at the cell surface. There it was present on oligodendrocytes prior and during the first stages of ensheathment of axons, both on cell body and processes. After formation of compact myelin MAG remained strongly expressed periaxonally and was only weakly detectable in noncompacted myelin including inner mesaxon and paranodal loops. None of the adhesion molecules was detectable on extracellular matrix, in the meninges, or on endothelial cells. Immunochemical analysis of antigen expression at different developmental stages was in agreement with the immunohistological data. We infer from these observations that L1 is involved in stabilization not only of axon-axon, but also axon-glia contacts, while the more dynamic structure of the growth cone generally expresses less L1. A differential expression of L1 along the course of an axon--being present on its unmyelinated, but absent on its myelinated part--further supports the notion that L1 may be involved in the stabilization of axonal fascicles but not of axon-myelin contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of cell adhesion molecules in the olfactory system of the adult mouse: presence of the embryonic form of N-CAM

The expression of the neural cell adhesion molecules N-CAM and L1 was investigated in the olfactory system of the mouse using immunocytochemical and immunochemical techniques. In the olfactory epithelium, globose basal cells and olfactory neurons were stained by the polyclonal N-CAM antibody reacting with all three components of N-CAM (N-CAM total) in their adult and embryonic states. Dark basal cells and supporting cells were not found positive for N-CAM total. The embryonic form of N-CAM (E-N-CAM) was only observed on the majority of globose basal cells, the precursor cells of olfactory neurons, and some neuronal elements, probably immature neurons, since they were localized adjacent to the basal cell layer. Differentiated neurons in the olfactory epithelium did not express E-N-CAM. In contrast to N-CAM total, the 180-kDa component of N-CAM (N-CAM180) and E-N-CAM, L1 was not detectable on cell bodies in the olfactory epithelium. L1 and N-CAM180 were strongly expressed on axons leaving the olfactory epithelium. Olfactory axons were also labeled by antibodies to N-CAM180 and L1 in the lamina propria and the nerve fiber and glomerular layers of the olfactory bulb, but only some axons showed a positive immunoreaction for E-N-CAM. Ensheathing cells in the olfactory nerve were observed to bear some labeling for N-CAM total, L1, and N-CAM180, but not E-N-CAM. In the olfactory bulb, L1 was not present on glial cells. In contrast, N-CAM180 was detectable on some glia and N-CAM total on virtually all glia. Glia in the nerve fiber layer were labeled by E-N-CAM antibody only at the external glial limiting membrane. In the glomerular layer, E-N-CAM expression was particularly pronounced at contacts between olfactory axons and target cells. The presence of E-N-CAM in the adult olfactory epithelium and bulb was confirmed by Western blot analysis. The continued presence of E-N-CAM in adulthood on neuronal precursor cells, a subpopulation of olfactory axons, glial cells at the glia limitans, and contacts between olfactory axons and their target cells indicates the retention of embryonic features in the mammalian olfactory system, which may underlie its remarkable regenerative capacity.

Adhesion of neural cells to extracellular matrix constituents. Involvement of glycosaminoglycans and cell adhesion molecules

Single cell suspensions of early postnatal mouse cerebellum adhere to substrate-bound culture supernatants of the teratocarcinoma cell line PF-HR9 and can be inhibited to adhere by antibodies to the neural cell adhesion molecules L1 and N-CAM. Adhesion can also be inhibited by the glycosaminoglycans heparin and heparan sulfate, and less by chondroitin sulfate or hyaluronic acid. Heparinase treatment of cells, but not of HR9 substrate, reduces adhesion. Adhesion does not appear to be mediated by laminin, a constituent of HR9 extracellular matrix, since L1 and N-CAM antibodies do not interfere with cell adhesion on EHS sarcoma laminin as substrate and since antibodies to EHS sarcoma laminin partially inhibit adhesion to HR9 extracellular matrix which contains laminin. Of the other extracellular matrix constituents analysed in HR9 culture supernatants (collagen type IV, a heparan sulfate proteoglycan and fibronectin) none could be shown to promote adhesion, when coated as substrate, suggesting that yet unidentified compounds are responsible for L1- or N-CAM-mediated cell adhesion. These experiments show for the first time that extracellular matrix constituents can act as binding partners for the neural cell adhesion molecules L1 and N-CAM.