Differential contributions of Ng-CAM and N-CAM to cell adhesion in different neural regions

Neuronal and astrocytic protein connections and associated adhesion molecules

Astrocytes are morphologically complex, with a myriad of processes which allow contact with other astrocytes, blood vessels, and neurons. Adhesion molecules expressed by these cells regulate this connectivity. Adhesion molecules are required to form and maintain functional neural circuits, but their importance and mechanisms of action, particularly in astrocyte-neuron contact, remain unresolved. Several studies of neuron-astrocyte connections have demonstrated the vital functions of adhesion molecules, including neuron-glia cell adhesion molecules, astrotactins, and protocadherins. In this review, we provide an overview and perspective of astrocyte-neuron contacts mediated by adhesion molecules in developing neural circuits and synapse formation, especially in the cerebellum. We also outline a novel mechanism of interaction between neurons and astrocytes in the tripartite synapses that has been recently found by our group.

NeuroD1 regulates survival and migration of neuroendocrine lung carcinomas via signaling molecules TrkB and NCAM

Small-cell lung cancer and other aggressive neuroendocrine cancers are often associated with early dissemination and frequent metastases. We demonstrate that neurogenic differentiation 1 (NeuroD1) is a regulatory hub securing cross talk among survival and migratory-inducing signaling pathways in neuroendocrine lung carcinomas. We find that NeuroD1 promotes tumor cell survival and metastasis in aggressive neuroendocrine lung tumors through regulation of the receptor tyrosine kinase tropomyosin-related kinase B (TrkB). Like TrkB, the prometastatic signaling molecule neural cell adhesion molecule (NCAM) is a downstream target of NeuroD1, whose impaired expression mirrors loss of NeuroD1. TrkB and NCAM may be therapeutic targets for aggressive neuroendocrine cancers that express NeuroD1.

Self-organizing circuit assembly through spatiotemporally coordinated neuronal migration within geometric constraints

BACKGROUND

Neurons are dynamically coupled with each other through neurite-mediated adhesion during development. Understanding the collective behavior of neurons in circuits is important for understanding neural development. While a number of genetic and activity-dependent factors regulating neuronal migration have been discovered on single cell level, systematic study of collective neuronal migration has been lacking. Various biological systems are shown to be self-organized, and it is not known if neural circuit assembly is self-organized. Besides, many of the molecular factors take effect through spatial patterns, and coupled biological systems exhibit emergent property in response to geometric constraints. How geometric constraints of the patterns regulate neuronal migration and circuit assembly of neurons within the patterns remains unexplored.

METHODOLOGY/PRINCIPAL FINDINGS

We established a two-dimensional model for studying collective neuronal migration of a circuit, with hippocampal neurons from embryonic rats on Matrigel-coated self-assembled monolayers (SAMs). When the neural circuit is subject to geometric constraints of a critical scale, we found that the collective behavior of neuronal migration is spatiotemporally coordinated. Neuronal somata that are evenly distributed upon adhesion tend to aggregate at the geometric center of the circuit, forming mono-clusters. Clustering formation is geometry-dependent, within a critical scale from 200 µm to approximately 500 µm. Finally, somata clustering is neuron-type specific, and glutamatergic and GABAergic neurons tend to aggregate homo-philically.

CONCLUSIONS/SIGNIFICANCE

We demonstrate self-organization of neural circuits in response to geometric constraints through spatiotemporally coordinated neuronal migration, possibly via mechanical coupling. We found that such collective neuronal migration leads to somata clustering, and mono-cluster appears when the geometric constraints fall within a critical scale. The discovery of geometry-dependent collective neuronal migration and the formation of somata clustering in vitro shed light on neural development in vivo.

The role of stretching in slow axonal transport

Axonal stretching is linked to rapid rates of axonal elongation. Yet the impact of stretching on elongation and slow axonal transport is unclear. Here, we develop a mathematical model of slow axonal transport that incorporates the rate of axonal elongation, protein half-life, protein density, adhesion strength, and axonal viscosity to quantify the effects of axonal stretching. We find that under conditions where the axon (or nerve) is free of a substrate and lengthens at rapid rates (>4 mm day⁻¹), stretching can account for almost 50% of total anterograde axonal transport. These results suggest that it is possible to accelerate elongation and transport simultaneously by increasing either the axon's susceptibility to stretching or the forces that induce stretching. To our knowledge, this work is the first to incorporate the effects of stretching in a model of slow axonal transport. It has relevance to our understanding of neurite outgrowth during development and peripheral nerve regeneration after trauma, and hence to the development of treatments for spinal cord injury.

The promotive effects of thymosin beta4 on neuronal survival and neurite outgrowth by upregulating L1 expression

Thymosin beta(4) (Tbeta4) is a major actin-sequestering peptide widely distributed in mammalian tissues including the nervous system. The presence of this peptide in the nervous system likely plays a role in synaptogensis, axon growth, cell migration, and plastic changes in dendritic spine. However, the effects of Tbeta4 on the survival of neurons and axonal outgrowth have still not been fully understood. So far it is not clear if the effects of Tbeta4 are associated with L1 functions. In the present study, we hypothesized that Tbeta4-induced up-regulation of L1 synthesis could be involved in the survival and axon outgrowth of cultured spinal cord neurons. To test this hypothesis, primarily cultured neurons were prepared from the mouse spinal cord and treated with various concentrations of Tbeta4 ranging from 0.1 to 10 microg/ml. The analysis of L1 mRNA expression and protein synthesis in neurons was then carried out using RT-PCR and western blot assays, respectively. After the addition of Tbeta4 to cultures, cells were then treated with antibodies against distinct domains of L1-Fc. Subsequently, beta-tubulin III and L1 double-labeled indirect immunofluorescence was carried out. Meanwhile, L1 immunofluorescent reactivity was analyzed and compared in cells treated with Tbeta4. Furthermore, the number of beta-tubulin III-positive cells and neurite lengths were measured. We found that Tbeta4 enhanced L1 expression in a dose-dependent manner, and the highest L1 mRNA and protein synthesis in cells increased by more than 2.1- and 2.3-fold in the presence of Tbeta4 at identical concentrations, respectively. Moreover, it also dose dependently enhanced neurite outgrowth and neuronal survival. Compared to conditions without Tbeta4, the length of neurite and neuronal survival increased markedly in presence of 0.5, 1, and 5 microg/ml Tbeta4, respectively, whereas the effects of Tbeta4 were significantly attenuated or inhibited in the process of L1-Fc antibodies treatment. These above results indicate that the promotive effect of Tbeta4 on the survival and neurite outgrowth of cultured spinal cord neurons might be mediated, at least in part via a stimulation of the production of L1 in the neurons.

Cerebellar granule cell migration and the effects of alcohol

In the developing brain the majority of neurons migrate from their birthplace to their final destination. This active movement is essential for the formation of cortical layers and nuclei. The impairment of migration does not affect the viability of neurons but often results in abnormal differentiation. The proper migration of neurons requires the orchestrated activities of multiple cellular and molecular events, such as pathway selection, the activation of specific receptors and channels, and the assembly and disassembly of cytoskeletal components. The migration of neurons is very vulnerable to exposure to environmental toxins, such as alcohol. In this article, we will focus on recent developments in the migration of cerebellar granule cells. First, we will describe when, where and how granule cells migrate through different cortical layers to reach their final destination. Second, we will present how internal programs control the sequential changes in granule cell migration. Third, we will review the roles of external guidance cues and transmembrane signals in granule cell migration. Finally, we will reveal mechanisms by which alcohol exposure impairs granule cell migration.

Epigenetic rules for expression of cell adhesion molecules during morphogenesis

From very early developmental times, cell adhesion molecules (CAMs) play key roles in linking cells together and regulating cell movement. By virtue of their capacity to link epithelia and condense mesenchyme, CAMs can act as mechanochemical regulators of morphogenesis. In the vertebrate species examined so far, CAMs appear in ordered sequences on cell surfaces during development. In this paper, evidence is reviewed indicating that the sequential expression of CAMs on cell surfaces at a variety of sites of embryonic induction follows a set of modulation rules that are first discernible at early gastrulation. These rules are related to the adhesion of cells in collectives and to the establishment of borders between such collectives. After gastrulation, all mesenchymal conversions employ N-CAM and show changes in its prevalence in a transition N----0----N where 0 means low or undetectable amounts of the CAM (rule I). In contrast, epithelia modulate from a state in which N-CAM and L-CAM appear simultaneously to the expression of only one or the other of these primary CAMs (rule II). At a variety of induction sites, cell collectives obeying rule I are found in proximity to cell collectives obeying rule II. During the morphogenesis of complex structures such as the feather or the optic placode, one can see a recursive application of these rules, reflecting the formation of significant histological boundaries within which the expression of gene products other than CAMs can lead to great morphological diversity. It is suggested that the genes for CAMs are regulated independently from and prior to those specifying intracellular proteins in a given tissue. According to this proposal, the existence of the epigenetic rules governing CAM expression reflects the evolutionary conservation of a key means of establishing tissue and animal form through the mechanochemical regulation of processes such as cell division, movement and death.

Neural cell adhesion molecule (NCAM) and prealbumin in cerebrospinal fluid from depressed patients

The size of the soluble form of the human cerebrospinal fluid (CSF) neural cell adhesion molecule, NCAM-sol, was by gel permeation chromatography estimated to 160-250 kDa. Within the CSF the concentration of NCAM-sol was found about 15-25% increased in lumbar fluid and 25% increased in ventricular fluid, both compared to cisternal fluid. Whereas prealbumin was found evenly distributed in CSF, albumin was relatively enriched in lumbar fluid. The concentrations of NCAM-sol and prealbumin were measured in lumbar CSF from psychiatric patients. Prealbumin was increased 7.2% and NCAM-sol was decreased 15.1% in depressed patients. The changes were partially normalized during recovery from the depression. The findings can be explained by hypothesizing that endogenous depression is associated with an increased choroid plexus activity and CSF production.

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.

The receptor protein tyrosine phosphatase mu, PTPmu, regulates histogenesis of the chick retina

The formation of laminae within the retina requires the coordinate regulation of cell differentiation and migration. The cell adhesion molecule and member of the immunoglobulin superfamily, receptor protein tyrosine phosphatase Mu, PTPmu, is expressed in precursor and early, differentiated cells of the prelaminated retina, and later becomes restricted to the inner plexiform, ganglion cell, and optic fiber layers. Since the timing of PTPmu expression correlates with the peak period of retinal lamination, we examined whether this RPTP could be regulating cell adhesion and migration within the retina, and thus influencing retinal development. Chick retinal organ cultures were infected with herpes simplex viruses encoding either an antisense sequence to PTPmu, wild-type PTPmu, or a catalytically inactive mutant form of PTPmu, and homophilic adhesion was blocked by using a function-blocking antibody. All conditions that perturbed PTPmu dramatically disrupted retinal histogenesis. Our findings demonstrate that catalytic activity and adhesion mediated by PTPmu regulate lamination of the retina, emphasizing the importance of adhesion and signaling via receptor protein tyrosine phosphatases in the developing nervous system. To our knowledge, this is the first demonstration that an Ig superfamily RPTP regulates the lamination of any neural tissue.

Identification of the Serum Complex Which Induces Cerebellar Granule Cell In Vitro Differentiation and Resistance to Excitatory Amino Acids

The protein complex promoting in vitro terminal differentiation of cerebellar granule cells has been isolated from rabbit serum. We designate the complex the neurite outgrowth and adhesion complex (NOAC). The apparent molecular weight, evaluated by gel filtration, is 80 - 100 kDa. Rat cerebellar granule cells cultured in NOAC exhibit much lower glial cell contamination and survive, in their differentiated state, much longer than in 10% foetal calf serum. While they bind tetanus toxin, express specific antigens such as synapsin I, synaptophisin and A2B5, and release [3H]d-aspartate in a fashion similar to that shown by cells cultured in foetal calf serum, they show a 60% reduction in the total number of kainate binding sites. Excitatory amino acid (EAA)-triggered and depolarization-stimulated calcium influx, measured in the presence of different agonists, is 50 - 80% lower in NOAC-cultured cells. NOAC cells are resistant to excitotoxic stimuli carried by EAAs or by depolarizing treatments with 50 mM KCl or 6 microM veratridine. The marked resistance of NOAC-cultured neurons to EAAs can be attributed to decreased calcium entry through EAA-coupled and voltage-gated calcium channels and possibly to other, as yet unidentified, phenotypic properties of these cells. These findings demonstrate that rabbit serum contains one or more polypeptide(s) endowed with the properties of promoting in vitro survival and differentiation of rat cerebellar granule cells and of conferring an EAA-resistant phenotype.

Effects of L1 blockade on sensory axon outgrowth and pathfinding in the chick hindlimb

In the developing chick hindlimb, sensory axons, which grow together in bundles as they extend distally, and the motoneuron axons they encounter express the cell adhesion molecule L1. Following injection of function-blocking anti-L1 antibodies into the limb at stage 25, some sensory axons choose inappropriate peripheral nerves even though motoneuron pathfinding is unaffected. Here, to further elucidate L1's role, we assessed the effects of this perturbation using pathway tracing, immune labeling, confocal microscopy, and electron microscopy. After L1 blockade, sensory axons were still bundled and closely apposed. However, clear signs of decreased adhesion were detectable ultrastructurally. Further, sensory axons grew into the limb more slowly than normal, wandering more widely, branching more frequently, and sometimes extending along inappropriate peripheral nerves. Sensory axons that ultimately projected along different cutaneous nerves showed increased intermixing in the spinal nerves, due to errors in pathfinding and also to a decreased ability to segregate into nerve-specific fascicles. These results suggest that, in the highly complex in vivo environment, as in tissue culture, L1 stimulates axon growth and enhances fasciculation, and that these processes contribute to the orderly, timely, and specific growth of sensory axons into the limb.

Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development

The structurally related cell adhesion molecules L1 and Nr-CAM have overlapping expression patterns in cerebellar granule cells. Here we analyzed their involvement in granule cell development using mutant mice. Nr-CAM-deficient cerebellar granule cells failed to extend neurites in vitro on contactin, a known ligand for Nr-CAM expressed in the cerebellum, confirming that these mice are functionally null for Nr-CAM. In vivo, Nr-CAM-null cerebella did not exhibit obvious histological defects, although a mild size reduction of several lobes was observed, most notably lobes IV and V in the vermis. Mice deficient for both L1 and Nr-CAM exhibited severe cerebellar folial defects and a reduction in the thickness of the inner granule cell layer. Additionally, anti-L1 antibodies specifically disrupted survival and maintenance of Nr-CAM-deficient granule cells in cerebellar cultures treated with antibodies. The combined results indicate that Nr-CAM and L1 play a role in cerebellar granule cell development, and suggest that closely related molecules in the L1 family have overlapping functions.

The involvement of axonin-1/SC2 in mediating notochord-derived chemorepulsive activities for dorsal root ganglion neurites

Previous studies have suggested that the developing notochord secretes diffusible axon guidance molecules that repel dorsal root ganglion (DRG) neurites (R. Keynes et al., 1997, Neuron 18, 889-897; K. Nakamoto and T. Shiga, 1998, Dev. Biol. 202, 304-314). Neither notochord-derived chemorepellents nor their receptors on DRG neurites are, however, known. Here we investigated whether cell adhesion molecules (CAMs) of the immunoglobulin/fibronectin type III subfamily present on DRG neurites, including axonin-1/SC2, N-CAM, Ng-CAM, and Nr-CAM, are required for mediating the notochord-derived chemorepulsion. Using collagen gel cocultures of DRGs and notochord explants, we found that an antibody against axonin-1/SC2 diminished the effects of the chemorepulsive activity from the notochord, whereas antibodies against N-CAM, Ng-CAM, and Nr-CAM had no effect. We further showed that the removal of glycosylphosphatidylinositol-anchored cell surface molecules, including axonin-1/SC2, from DRG neurites diminished the effects of the notochord-derived chemorepulsive activity to an extent similar to that of treatment with the anti-axonin-1/SC2 antibody. These results suggest that axonin-1/SC2 expressed on DRG neurites may be involved in mediating the notochord-derived chemorepulsive activity.

Coordinate regulation of cadherin and integrin function by the chondroitin sulfate proteoglycan neurocan

N-cadherin and beta1-integrins play decisive roles in morphogenesis and neurite extension and are often present on the same cell. Therefore, the function of these two types of adhesion systems must be coordinated in time and space to achieve the appropriate cell and tissue organization. We now show that interaction of the chondroitin sulfate proteoglycan neurocan with its GalNAcPTase receptor coordinately inhibits both N-cadherin- and beta1-integrin-mediated adhesion and neurite outgrowth. Furthermore, the inhibitory activity is localized to an NH(2)-terminal fragment of neurocan containing an Ig loop and an HA-binding domain. The effect of neurocan on beta1-integrin function is dependent on a signal originating from the cadherin cytoplasmic domain, possibly mediated by the nonreceptor protein tyrosine kinase Fer, indicating that cadherin and integrin engage in direct cross-talk. In the developing chick, neural retina neurocan is present in the inner plexiform layer from day 7 on, and the GalNAcPTase receptor becomes restricted to the inner nuclear layer and the ganglion cell layer (as well as the fiber layer), the two forming a sandwich. These data suggest that the coordinate inhibition of cadherin and integrin function on interaction of neurocan with its receptor may prevent cell and neurite migration across boundaries.

Characterization and isolation of ductular cells coexpressing neural cell adhesion molecule and Bcl-2 from primary cholangiopathies and ductal plate malformations

It has recently been shown that reactive bile ductules display neuroendocrine features, including immunoreactivity for the neural cell adhesion molecule (NCAM). In this study we have compared the immunohistochemical expression of NCAM with that of HEA-125 (biliary specific) and LKM-1 (hepatocyte specific) and other markers relevant to morphogenesis (Bcl-2, EMA) and cell proliferation (Ki-67) in cryostat sections from different chronic liver diseases and from fetal livers at different gestational ages. In parallel, viable NCAM-positive ductular cells were purified from collagenase digests of cirrhotic livers by immunomagnetic separation and characterized by immunocytochemistry and transmission electron microscopy. We demonstrated that reactive ductules with atypical morphology coexpressed NCAM and Bcl-2 and were found mainly in congenital diseases associated with ductal plate malformation and in primary cholangiopathies. On the contrary, reactive ductules with typical morphology were negative for NCAM/Bcl-2 and positive for EMA. Reactive ductules coexpressing NCAM/Bcl-2 were negative for the proliferation marker Ki-67 and appeared to be directly connected with periportal hepatocytes. In fetal livers NCAM/Bcl-2 was transiently expressed during the early developmental stages of ductal plate (10-16 weeks) and started to disappear as the ductal plate began duplicating. NCAM-positive ductal plate cells were Ki-67 negative, becoming positive in duplicated segments. Thus the histogenesis of ductular reactive cells seems to recapitulate the early stages of biliary ontogenesis. In primary cholangiopathies and ductal plate malformations, these cells do not appear to maturate further, and thus abundant ductular structures coexist with vanishing mature ducts. These NCAM-positive ductular cells were immunopurified from patients with chronic cholestatic liver diseases and showed ultrastructural features consistent with a less differentiated phenotype than mature cholangiocytes. These isolated cells represent a useful model for in vitro studies.

In vitro studies of growth cone behavior support a role for fasciculation mediated by cell adhesion molecules in sensory axon guidance during development

Axonal interactions, which are mediated by cell adhesion molecules (CAMs) as well as other types of membrane proteins, are important for sensory axon pathfinding in the developing chick hindlimb. We have previously shown that injection of antibodies that block the function of either G4/L1 or N-cadherin into the limb, starting when the first sensory axons reach the plexus, alters the segmental pattern of projections along cutaneous nerves. Specific removal of polysialic acid from NCAM using the enzyme endoneuraminidase N (Endo N) also resulted in significant changes in cutaneous projection patterns, while injection of antibodies against NCAM itself had no obvious effect (M. G. Honig and U. S. Rutishauser, 1996, Dev. Biol. 175, 325-337). To help understand the cellular basis for these findings, we developed a tissue culture system in which the axons from dorsal root ganglion explants grow within defined laminin lanes and examined whether the same treatments increased or decreased a growth cone's tendency to be closely associated with neighboring axons. After 2 days in culture, images of the cultures were recorded, antibodies or Endo N was added, and images of the same fields were recaptured an hour later. To quantify the results, growth cones located in defined regions of the laminin lanes were classified, before and after the perturbation, as "free" (i.e., growing primarily on the laminin substratum), "fasciculated" (i.e., growing tightly along other neurites), or "intermediate" (i.e., growing both on the laminin substratum and in contact with other neurites). We found that anti-G4/L1 and anti-N-cadherin, but not anti-NCAM, caused an increase in defasciculated growth cones, whereas Endo N resulted in an increase in fasciculated growth cones. These changes in fasciculation are consistent with the changes in cutaneous projections seen in our previous in ovo perturbations. The results from these tissue culture experiments thus provide strong support for the idea that one mechanism by which CAMs affect sensory axon pathfinding in vivo is by regulating the affinity of sensory growth cones for neighboring axons, which in turn can modulate the growth cone's ability to navigate through the surrounding environment.

Cell adhesion molecules regulate guidance of dorsal root ganglion axons in the marginal zone and their invasion into the mantle layer of embryonic spinal cord

In order to elucidate the mechanisms regulating the projections of dorsal root ganglion (DRG) axons in the dorsal funiculus and invasion into target regions in the mantle layer (prospective gray matter) of the spinal cord, we examined the interactions between DRG axons and spinal cord. DRG neurons were dissociated from chick embryos and cultured for 1-2 days on cryostat sections of the spinal cord at embryonic day 5 (E5) or at E9. E5 and E9 DRG neurons extended neurites onto both marginal zone (prospective white matter) and mantle layer (prospective gray matter) of the spinal cord, suggesting that both of these regions are permissive for neurite growth. When E5 DRG neurites approached cryosections of E5 spinal cord from outside, most of them ran in the marginal zone without invading the mantle layer. In contrast, about half of E9 DRG neurites entered the mantle layer after crossing the marginal zone of E9 spinal cord. These growth patterns of DRG neurites on spinal marginal zone and mantle layer are similar to the pathway formation of DRG axons at comparable stages in vivo; DRG axons run exclusively in the prospective dorsal funiculus before E6, and enter the mantle layer (prospective dorsal horn) to reach the target regions by E9. Perturbation of functions of Ng-CAM, Nr-CAM, and axonin-1/SC2 by adding the specific antibodies in the culture medium increased the ratio of DRG neurites entering the mantle layer of E5 spinal cord, suggesting that these cell adhesion molecules are involved in keeping DRG neurites in the marginal zone. Taken together with the expression of Ng-CAM, Nr-CAM, and axonin-1/SC2, these CAMs on DRG axons may regulate the guidance of these axons in the marginal zone before E6, and the subsequent decrease in the relative levels of these CAMs might allow DRG axons to invade the target mantle layer.

Ethanol-exposed central neurons fail to migrate and undergo apoptosis

Prenatal exposure of human brain to ethanol impairs neuronal migration and differentiation and causes mental retardation. The present results indicate that the adverse effects of ethanol on brain development may be partly due to the ethanol-induced disturbance of neuronal interaction with laminin, a protein involved in neuronal migration and axon guidance. This report shows that physiological concentrations (IC50 = 28 mM) of ethanol inhibit neurite outgrowth and neuronal migration of the rat cerebellar granule neurons on a laminin substratum. The ethanol-treated granule neurons undergo apoptosis, degrade their laminin substratum, and appear to release and bind increased amounts of the B2-chain-derived peptides along their surfaces. A protease inhibitor aprotinin, and the NMDA receptor channel, and voltage-gated calcium channel antagonist MK801 partially protect cerebellar granule neurons from ethanol-induced neurotoxicity. These results imply that ethanol-treated granule neurons resemble the granule neurons of the homozygous weaver mouse cerebellum with respect to their apoptosis, laminin expression, and partial rescue by approtinin and MK-801. Thus, ethanol may influence neuronal survival and neurite outgrowth via molecular pathways similar to those involved in neuronal death in other neurodegenerative processes of the central nervous system.

Sprouting adult CNS cholinergic axons express NILE and associate with astrocytic surfaces expressing neural cell adhesion molecule

To assess the cellular and molecular substrates for cholinergic axon growth in the adult central nervous system (CNS), we implanted grafts of control and nerve growth factor (NGF)-producing genetically modified fibroblasts within the striatum of rats. Sprouting cholinergic axonal processes that grew into grafts of NGF-producing fibroblasts were fasciculated and followed the surface of astrocytic processes for long distances within the grafts. The close and long distance anatomical relationship between the sprouted axons and the astrocytes supported previous ultrastructural evidence that astrocytes may serve as a cellular substrate for sprouting cholinergic axons in vivo. The sprouted axon processes were associated with the expression of nerve growth factor-inducible large external (NILE) glycoprotein on their surfaces. NILE expression was not seen in control grafts where there was an absence of cholinergic ingrowth. NILE has been demonstrated to play a role in axon fasciculation in a number of other neural systems. The astrocytic processes in both control and NGF-producing fibroblast grafts expressed neural cell adhesion molecule (NCAM), suggesting that NCAM-mediated adhesion may be responsible for the close relationship between the axons and astrocytes within the grafts. NGF-induced heterotypic interactions between neuronal NILE and astroglial NCAM may also be required for adult cholinergic axonal sprouting.

Synergistic neurite-outgrowth promoting activity of two related axonal proteins, Bravo/Nr-CAM and G4/Ng-CAM in chicken retinal explants

In the developing chicken retina, optic fibres migrating to the tectum express on their surfaces several cell adhesion molecules, including Bravo/Nr-CAM and G4/Nr-CAM and G4/Ng-CAM. We have previously described differential distribution along the retinotectal projection and differential modulation by environmental cues for Bravo and G4 and here we further compare the characteristics of these immunoglobulin superfamily molecules. From day 6 of embryonic development (E6) to 20 (E20), Bravo and G4 were found to coexist in the retinal optic fibre layer. However, while G4 staining was confined to that layer, as development proceeded Bravo staining spread to plexiform layers and some radial structures of the retina. G4 displayed a dose-dependent neurite-outgrowth promoting activity for E6 retinal explants, while Bravo did not support neurite growth. Surprisingly, when the retinal explants were grown on mixtures of the two molecules, a much more vigorous growth of neurites was seen, revealing a synergistic effect. We propose that Bravo and G4, as well as other axonal surface molecules, affect axonal growth in different ways when they are present in combination than when they are alone.

Embryonic expression patterns of the neural cell adhesion molecule gene are regulated by homeodomain binding sites

During development of the vertebrate nervous system, the neural cell adhesion molecule (N-CAM) is expressed in a defined spatiotemporal pattern. We have proposed that the expression of N-CAM is controlled, in part, by proteins encoded by homeobox genes. This hypothesis has been supported by previous in vitro experiments showing that products of homeobox genes can both bind to and transactivate the N-CAM promoter via two homeodomain binding sites, HBS-I and HBS-II. We have now tested the hypothesis that the N-CAM gene is a target of homeodomain proteins in vivo by using transgenic mice containing native and mutated N-CAM promoter constructs linked to a beta-galactosidase reporter gene. Segments of the 5' flanking region of the mouse N-CAM gene were sufficient to direct expression of the reporter gene in the central nervous system in a pattern consistent with that of the endogenous N-CAM gene. For example, at embryonic day (E) 11, beta-galactosidase staining was found in postmitotic neurons in dorsolateral and ventrolateral regions of the spinal cord; at E14.5, staining was seen in these neurons throughout the spinal cord. In contrast, mice carrying an N-CAM promoter-reporter construct with mutations in both homeodomain binding sites (HBS-I and HBS-II) showed altered expression patterns in the spinal cord. At E11, beta-galactosidase expression was seen in the ventrolateral spinal cord, but was absent in the dorsolateral areas, and at E 14.5, beta-galactosidase expression was no longer detected in any cells of the cord. Homeodomain binding sites found in the N-CAM promoter thus appear to be important in determining specific expression patterns of N-CAM along the dorsoventral axis in the developing spinal cord. These experiments suggest that the N-CAM gene is an in vivo target of homeobox gene products in vertebrates.

Functional analysis of posttranslational cleavage products of the neuron-glia cell adhesion molecule, Ng-CAM

Neuron-glia cell adhesion molecule (Ng-CAM) mediates cell adhesion between neurons homophilically and between neurons and glia heterophilically; it also promotes neurite outgrowth. In the chick brain, Ng-CAM is detected as glycoproteins of 190 and 210 kD (Ng-CAM200) with posttranslational cleavage products of 135 kD (F135, which contains most of the extracellular region) and 80 kD (F80, which includes the transmembrane and the cytoplasmic domains). To examine the functions of each of these components, we have expressed Ng-CAM200, F135, and F80 in murine L cells, and F135 and F80 as GST fusion proteins in the pGEX vector in bacteria. Appropriately transfected L cells expressed each of these proteins on their surfaces; F135 was also found in the media of cells transfected with Ng-CAM200 and F135. In addition to binding homophilically, cells transfected with Ng-CAM200 and F135 bound heterophilically to untransfected L cells, suggesting that there is a ligand for Ng-CAM on fibroblasts that may be related to the glial ligand. Detailed studies using the transfected cells and the fusion proteins indicated that both the homophilic and the heterophilic binding activities of Ng-CAM are localized in the F135 fragment of the molecule. The results also indicated that proteolytic cleavage of Ng-CAM200 is not required either for its expression on the cell surface or for cell adhesion and that there is an "anchor" for F135 on L cells (and presumably on neurons). In contrast to the cell binding results, the F80 but not the F135 fusion protein enhanced the outgrowth of neurites from dorsal root ganglion cells; this activity was associated with the FnIII repeats of F80. The observations that a protein corresponding to F135 contains the cell aggregation sites whereas one corresponding to the F80 has the ability to promote neurite outgrowth suggest that proteolytic cleavage may be an important event in regulating these Ng-CAM activities during embryonic development and neural regeneration.

Requirement of RNA synthesis for pathfinding by growing axons

The effects of actinomycin D were studied in cultured grasshopper embryos at different stages of development by following the outgrowth patterns of identified neurones known as aCC, pCC, and Q1. When administered at stages occurring before 31% of embryonic development, actinomycin D (0.05-0.10 microM for 24-48 hours) prevented axon extension, whereas it did not affect the development of the nervous system in embryos older than 34% of development. At 31-34% of development, actinomycin D perturbed pathfinding of aCC without blocking axon extension. Thus, only 22% of the aCCs (n = 271) in embryos treated with actinomycin D extended an axon along the intersegmental nerve as in control embryos. In the remaining embryos, aCC failed to turn into the intersegmental nerve root; its growth cone remained in the longitudinal connective, above or below the turning point. Neurones of the group caudal to the intersegmental nerve root could extend along either the anterior or posterior commissure of the next posterior segment. In contrast to the observations made with aCC, only 1.2% of pCC (n = 166) and 0.0% of Q1 (n = 45) in embryos treated with actinomycin D showed axon growth along aberrant pathways. The position of the growth cones of most pCCs and all Q1s observed were in various points along their normal pathway. Both pCC and Q1, as a population, showed an extension rate significantly lower than that of their control counterparts. The effect of actinomycin D on aCC pathway choice was probably mediated by inhibition of RNA synthesis, because incorporation of uridine into RNA was reduced by 40%. The labelling of several monoclonal antibodies (1C10, 3B11, 7F7) that recognise surface glycoproteins (lachesin, fasciclin I, and REGA-1) involved in nervous system development of grasshopper embryos was suppressed. Our results suggest that the navigation of some axons along different pathways requires the synthesis of new mRNA.

Expression of four immunoglobulin superfamily adhesion molecules (L1, Nr-CAM/Bravo, neurofascin/ABGP, and N-CAM) in the developing mouse spinal cord

To identify cell adhesion molecules (CAMs) expressed by mammalian motoneurons, we applied the polymerase chain reaction to a murine motor neuron-like cell line, NSC-34. Using primers derived from a group of L1-related CAMs, we cloned two alternatively spliced forms of mouse L1, which differ by a 12-base-pair insert, plus putative murine orthologs of the chicken cell adhesion molecules Nr-CAM/Bravo and neurofascin. All four mRNAs are expressed in NSC-34 cells, but only neurofascin and the insert-minus form of L1 are expressed in its neuroblastoma parent, N18TG2. Analysis of RNA in neonatal tissues reveals expression largely restricted to the brain and spinal cord. In situ hybridization histochemistry of spinal cord shows that motoneurons express L1, Nr-CAM, and neurofascin as well as N-CAM. L1 and N-CAM RNAs are detected throughout the period studied (from embryonic day [E]11 to postnatal day [P]28), whereas Nr-CAM is expressed only at early ages (< E15) and neurofascin is predominantly expressed postnatally. Moreover, each CAM is expressed by distinct subsets of neighboring cells and at distinct times. For example, Nr-CAM mRNA is present in floor plate cells of embryonic spinal cord, whereas neurofascin is expressed by a subset of glia postnatally. Finally, we show that each CAM has a distinct spatiotemporal pattern of expression in dorsal root ganglia.

Rapid expression and transport of embryonic N-CAM in dentate gyrus following entorhinal cortex lesion: ultrastructural analysis

Neural cell adhesion molecules are known to be important in axon guidance and synapse formation in the developing brain. The embryonic form of neural cell adhesion molecule (eN-CAM) is reexpressed in the outer molecular layer (OML) of the dentate gyrus following entorhinal cortex (ERC) lesion. Ultrastructural analysis revealed localization of eN-CAM to the membrane of granule-cell dendritic membranes and occasionally axons within the denervated zone. Because eN-CAM is expressed rapidly (within 2 days) after ERC lesion, we were interested in the temporal sequence of expression. Denervated hippocampi (12, 15, 24, and 48 hours post-ERC lesion) were stained with anti-eN-CAM and processed for immunoelectron microscopy. At 12 hours, there was no evidence of staining for eN-CAM. By 15 hours after lesion, membranes of both dendrites and axons throughout the molecular layer exhibited moderate eN-CAM staining, and dendritic cytoplasm was heavily labeled. Twenty-four hours following lesion, plasma membrane staining of eN-CAM on both axons and dendrites had increased in intensity within the OML, whereas membrane eN-CAM staining was diminished in the inner molecular layer (IML), and the intradendritic cytoplasmic staining disappeared. By 48 hours after lesion, eN-CAM staining had disappeared from the IML but remained intense and widely distributed in the OML. These findings suggest a rapid transport of de novo synthesized protein. A generalized reaction appears to occur immediately following denervation, and eN-CAM is up-regulated in the complete expanse of the dendritic membrane, despite the fact that only the OML is denervated.(ABSTRACT TRUNCATED AT 250 WORDS)

N-cadherin and Ng-CAM/8D9 are involved serially in the migration of newly generated neurons into the adult songbird brain

In the adult avian forebrain, neurons continue to be produced in the subependymal zone (SZ), from which they migrate upon radial fibers. To identify ligands regulating this process, we studied N-cadherin and Ng-CAM/8D9 expression in HVC, a neurogenic region of the canary neostriatum. N-cadherin was relatively restricted to the SZ and was expressed by dividing, [3H]thymidine-labeled precursor cells. However, cellular N-cadherin was down-regulated prior to neuronal migration from the SZ. Addition of anti-N-cadherin Fab hastened neuronal migration from adult SZ explants, without influencing neuronal number. Unlike N-cadherin, Ng-CAM/8D9 was expressed by migrating neurons. Anti-8D9 Fab inhibited neuronal migration upon cultured ependymoglia, which did not express Ng-CAM/8D9. Thus, the departure of new neurons from the adult SZ may require their suppression of N-cadherin, whereas their subsequent migration and survival may depend upon neuronal expression of Ng-CAM/8D9 and its interaction with a heterophilic radial cell receptor.

cDCC (chicken homologue to a gene deleted in colorectal carcinoma) is an epithelial adhesion molecule expressed in the basal cells and involved in epithelial-mesenchymal interaction

Cloning of human DCC (deleted in colorectal carcinoma, Fearon et al., 1990) showed that it is an immunoglobulin superfamily member homologous to neural cell adhesion molecules (N-CAM). To explore the normal function of this molecule, we have cloned a chicken homologue to DCC (cDCC) and raised an antibody to DCC. cDCC is a protein of 160 kDa with an expression pattern distinct from those of other immunoglobulin family members including N-CAM and Ng-CAM. Transgene expression of cDCC in fibroblasts led to increased cell-cell adhesion. Localization studies in chicken and mouse embryos showed that DCC is expressed in the epithelia of skin, gut, lung, and bladder. In adult, the expression of DCC is limited to the basal layer of stratified epithelium in skin, crypt regions of intestinal villi, and stem cells in mammary duct. Cell aggregation assay using embryonic chicken skin epithelial cells and antibody to DCC showed it is a Ca2+ independent cell adhesion molecule. In epithelial-mesenchymal interactions during feather morphogenesis, antibody to DCC suppressed the formation of dermal condensations and the polarized localization of N-CAM and fibronectin. These results implied that DCC is an epithelial cell adhesion molecule required for mediating critical functions in epithelial-epithelial and epithelial-mesenchymal interactions.

Evidence for the binding of Ng-CAM to laminin

Ng-CAM is a cell adhesion molecule mediating neuron-glia and neuron-neuron adhesion via different binding mechanisms. While its binding can be homophilic as demonstrated by the self-aggregation of Ng-CAM coated beads (Covaspheres), Ng-CAM has also been shown to bind to glia by a heterophilic mechanism. In the present study, we found that the extent of Ng-CAM Covasphere aggregation was strongly diminished in the presence of the extracellular matrix glycoprotein laminin. When proteolytic fragments of laminin were tested, the P1' fragment (obtained from the short arms by pepsin treatment) was found to inhibit aggregation of Ng-CAM-Covaspheres while the elastase fragments E3 and E8 (from the long arm) were ineffective. To provide other means of analyzing interactions between laminin and Ng-CAM, the two proteins were covalently linked to differently fluorescing Covaspheres and tested for coaggregation. Laminin-Covaspheres coaggregated with Ng-CAM-Covaspheres, and this binding was inhibited both by anti-Ng-CAM and by anti-laminin antibodies. Covaspheres coated with other proteins including BSA and fibronectin did not coaggregate with Ng-CAM-Covaspheres. Moreover, using a solid phase binding assay, we found that 125I-labeled Ng-CAM bound to laminin and to Ng-CAM but not to fibronectin. The results suggest that regions in the short arms of laminin can bind to Ng-CAM. To test whether Ng-CAM present on neurons could be involved in binding to laminin, adhesion of neurons to substrates coated with various proteins was tested in the presence of specific antibodies. Anti-Ng-CAM Fab' fragments inhibited neuronal binding to laminin but not binding to fibronectin. The combined results open the possibility that Ng-CAM on the surface of neurons may mediate binding to laminin in vivo, and that interactions with laminin can modulate homophilic Ng-CAM binding.

NILE/L1 and NCAM-polysialic acid expression on growing axons of isolated neurons

The neuron adhesion molecules NILE/L1 and NCAM may be involved in axonal guidance and cell recognition. To investigate all exposed membrane domains of single neurons, something which has not previously been done for any adhesion molecule, we used digitally processed scanning electron microscopy with a high-energy backscatter electron detector. This allowed a quantitative analysis of immunogold staining densities on all surfaces of isolated rat hippocampal neurons in culture to study NILE/L1 and NCAM expression independent of potentially inductive innervation. During early stages of neuritic extension, all growth cones showed similar NILE/L1 expression, but as soon as a single process extended farther than the others (by 20 hours), this putative axon and its growth cone generally showed a stronger level of NILE/L1 immunogold labeling than the other neurites. This is the earliest evidence of plasma membrane differentiation between axons and dendrites. With further neuritic growth, the relative NILE/L1 expression on axons and their growth cones continued to increase. In contrast to some earlier reports, NILE/L1 was expressed on axonal growth cones growing on both polylysine-coated glass and astrocyte substrates. Strong immunostaining for NCAM-related polysialic acid (PSA) was found on axonal growth cones and filopodia, suggesting that the homophilic adhesive action of NCAM may be reduced during axonal growth. PSA showed greater labeling on distal axons than on other areas of the neuron, indicating a variable NCAM-mediated adhesion on different regions of the same cell. Neither NILE/L1, NCAM, nor PSA appeared to show regional differences in axons fasciculating or defasciculating on themselves. A strong intercellular heterogeneity of NILE/L1, NCAM, and PSA expression levels on neurons in the same culture dish was found, suggesting that subsets of cells from the hippocampus may express biologically relevant differences in adhesion molecules compared to neighboring neurons. In light of the growing body of evidence pointing to the multifaceted array of homophilic and heterophilic binding interactions that NILE/L1 and NCAM may exhibit, and the functional importance of molecular densities, the quantitative data here support the hypothesis that sufficient cellular and subcellular heterogeneity exists for these molecules to be involved in some aspects of axonal guidance.

Apical polarization of N-CAM in retinal pigment epithelium is dependent on contact with the neural retina

The retinal pigment epithelium (RPE) is unique among epithelia in that its apical surface does not face a lumen, but, instead, is specialized for interaction with the neural retina. The molecules involved in the interaction of the RPE with the neural retina are not known. We show here that the neural cell adhesion molecule (N-CAM) is found both on the apical surface of RPE in situ and on the outer segments of photoreceptors, fulfilling an important requisite for an adhesion role between both structures. Strikingly, culture of RPE results in rapid redistribution of N-CAM to the basolateral surface. This is not due to an isoform shift, since the N-CAM expressed by cultured cells (140 kD) is the same as that expressed by RPE in vivo. Rather, the reversed polarity of N-CAM appears to result from the disruption of the contact between the RPE and the photoreceptors of the neural retina. We suggest that N-CAM in RPE and photoreceptors participate in these interactions.

Expression patterns of the cell adhesion molecule Nr-CAM during histogenesis of the chick nervous system

Neuron-glia-related cell adhesion molecule (Nr-CAM) is a recently characterized cell adhesion molecule in the family of immunoglobulin-related molecules of which the neural cell adhesion molecule, N-CAM, is the prototype. Nr-CAM shares structural properties with another member of this family (neuron-glia CAM, Ng-CAM) and both molecules exhibit homophilic and heterophilic binding properties. To understand better the role of such molecules in development, we have examined the sites of synthesis and expression of Nr-CAM by means of in situ hybridization and immunohistochemistry. Both methods indicated that Nr-CAM is expressed only in the nervous system. The molecule was observed on neurons in both the peripheral and central nervous systems and on epithelial floor plate cells in the spinal cord, but it was absent in the germinal zones. The protein was present on perikarya, but was found preferentially on axonal tracts. As observed for messenger RNAs specifying other cell adhesion molecules, messenger RNA for Nr-CAM was localized in the perikarya. The temporal expression of Nr-CAM was correlated with various neural morphoregulatory events, including cell proliferation and migration, axonal outgrowth and myelination. The molecule was expressed during the onset of neurogenesis at embryonic day 3 in the floor plate epithelium, and then on postmitotic ventral horn motor neurons of the spinal cord. At later stages, it was expressed throughout the spinal cord but disappeared from the floor plate. In the cerebellum, Nr-CAM was found on granule and Purkinje neurons and afferent fibers. Both local and projection neurons in the optic tectum, as well as axonal pathways throughout the telencephalon, expressed Nr-CAM. In the peripheral nervous system, Nr-CAM was expressed strongly in sensory and autonomic ganglia and in the enteric nervous system. At the onset of myelination, there was a general decrease in staining for Nr-CAM protein in the central nervous system but not in the periphery. Comparison of the expression of Nr-CAM to that of the structurally related Ng-CAM showed considerable overlap in their distributions, although there were differences in the levels at which each CAM was observed in particular structures. For example, sympathetic ganglia stained more intensely for Nr-CAM protein than for Ng-CAM. This differential but co-distributed pattern is consistent with the idea that although similar cell adhesion molecules have independent binding specificities, they may have related functions that act synergistically in the development of the nervous system.

Differential expression of neuron-glia cell adhesion molecule (Ng-CAM) on developing axons and growth cones of interneurons in the chick embryo spinal cord: an immunoelectron microscopic study

To elucidate the role of neuron-glia cell adhesion molecule (Ng-CAM) in axonal pathway formation of avian spinal interneurons, we have examined the ultrastructural expression of Ng-CAM in the developing spinal cord, by using a preembedding immunocytochemical method. Ng-CAM immunoreactivity was punctate and was restricted to cell surfaces. In accordance with our previous light microscopic observations (Shiga et al., '90), the earliest developing spinal interneurons were Ng-CAM-positive on their cell bodies, axons, and growth cones. Axons and growth cones that were either fasciculated or in contact with each other strongly expressed Ng-CAM, thus indicating the possible involvement of Ng-CAM in fasciculation of axons and in the contact guidance of growth cones along preexisting axons. By using higher resolution immunoelectron microscopy, the present study has also revealed new information on the subcellular localization of Ng-CAM on developing spinal interneurons, neuroepithelial cells, and floor plate cells. Although Ng-CAM immunoreactivity was prominent on both axons and growth cones, these structures were Ng-CAM-negative when they contacted the basal lamina around the spinal cord. By contrast, Ng-CAM was detectable on the surface of both neuroepithelial cells and floor plate cells only when they made contact with the Ng-CAM-positive axons and growth cones of interneurons. These results suggest that the subcellular distribution of Ng-CAM is regulated differentially, depending on the apposing cell surfaces, and that such differential and developmentally regulated expression may contribute to the elongation, fasciculation, and guidance of spinal axons.

Regional distribution of neural cell adhesion molecule (N-CAM) and L1 in human and rodent hippocampus

Cell surface adhesion molecules N-CAM and L1 are implicated in central nervous system (CNS) cell migration and axon outgrowth in in vitro and in vivo developmental studies. These molecules show a differential distribution during CNS development, thus suggesting that they subserve different roles in process outgrowth and tissue organization. A variety of N-CAM isoforms are known, and individual N-CAMs undergo posttranslational modification. Such changes and the potential for generating numerous molecules may mediate development of specific neural cell contacts and circuitry. We evaluated immunohistochemical staining of polyclonal antibodies to L1 and N-CAM, as well as monoclonal antibodies directed against embryonic N-CAM and the 140 and 180 kDa species of N-CAM in human, rat, and mouse hippocampus. Staining patterns in the three species were qualitatively similar, but staining in the mouse hippocampus was quantitatively greater for some epitopes. A distinctive pattern of staining was found, corresponding to the known anatomy of the structure. Total N-CAM staining was intense in the hilus and inner molecular layer (ML) of the dentate gyrus with lighter staining in the dentate outer ML. The mossy fiber tract (MFT), comprising axons traveling from the dentate granule cells to CA3 pyramidal cells, was strongly stained by polyclonal antibody to N-CAM. There was abundant staining of the stratum radiatum (SR) and stratum oriens (SO) of CA1, but stratum lacunosum moleculare (LM) showed very little staining. The monoclonal antibody 12F11, which recognizes the 140 and 180 kDa forms of N-CAM, intensely stained the MFT, hilus, and inner ML.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular localization of the neural cell adhesion molecule L1 in adult rat neuroendocrine and endocrine tissues: comparisons with NCAM

The tissue distribution and cellular localization of the neural cell adhesion molecule L1 was determined by immunocytochemistry at the optical and ultrastructural levels in adult rat neuroendocrine tissues and pancreatic endocrine cells. L1 was found to be abundant in the neurohypophysis but undetectable in the rest of the pituitary gland. It was barely detectable in the normal rat endocrine pancreas, but a rat pancreatic insulinoma cell line was found by immunofluorescence to express low levels of L1. In the adrenal medulla, it was present on a sub-population of chromaffin cells and its density appeared to be lower on surfaces exposed to the extracellular matrix. Double immunolabelling showed this sub-population to consist of noradrenergic chromaffin cells. Adrenergic chromaffin cells were found not to express L1. In addition, the tissue distribution and cellular localization of NCAM mRNAs was determined by in situ hybridization, extending our previous studies on the cellular expression of NCAM proteins in endocrine and neuroendocrine tissues. This confirmed that the NCAM message has a wider cellular distribution than L1 within the hypophysis and the adrenal gland. In addition to secretory cells, L1 immunoreactivity was detected in glial cells, in particular in the pituicytes of the neurohypophysis, which further distinguishes them from astrocytes, their counterparts in the central nervous system. These data are discussed in terms of the different embryological origins of the various endocrine tissues examined and also in terms of the specific design constraints imposed on these tissues during their development.

Expression of cell adhesion molecules and catecholamine synthesizing enzymes in the developing rat adrenal gland

Cell adhesion molecules play a major role in determining tissue architecture during histogenesis. This immunocytochemical study of the adrenal gland examines the embryonic and early postnatal cellular expression of two neural cell adhesion molecules, NCAM and L1, which are widely expressed in brain and have been found also to be expressed in the adult rat adrenal gland. In parallel, antibodies directed against two neuroendocrine cell markers, tyrosine hydroxylase and phenylethanolamine N-methyltransferase, were employed to verify the phenotypic nature of developing chromaffin cells in order to correlate cell adhesion molecule expression with the state of chromaffin cell differentiation. NCAM was found to be expressed by chromoblasts within extra-adrenal blastema (i.e. before their migration into the cortical primordium) at the 16th day of embryonic life. It continued to be expressed by all developing chromaffin cells after their infiltration into the developing adrenal gland at all ages. L1 was also expressed by chromoblasts in extra-adrenal sites, but was found only in a subpopulation of chromaffin cells within the cortical primordium from the 16th embryonic day onwards. Those chromoblasts which expressed L1 constituted relatively large compact cell clusters within the gland at this stage, while intra-adrenal chromaffin cells not expressing L1 were dispersed in small cell groups. L1 was also strongly expressed by nerve fibres (and their surrounding Schwann cells) which appeared to innervate cell groups as early as the 16th embryonic day. Both extra- and intra-adrenal chromoblasts expressed tyrosine hydroxylase, but the large L1-positive cell aggregates were less intensely immunoreactive for tyrosine hydroxylase than were cells in small groups. PNMT expression was restricted to L1-negative intra-adrenal chromoblasts present in small groups. Ultrastructural observations demonstrated that cells expressing L1 contained few secretory granules at the 18th embryonic day. It is concluded from these data that these chromoblasts are the precursors of the noradrenergic cells found in the mature gland. In addition, the arrangement of noradrenergic chromaffin cells in the form of homotypic cell groups throughout the course of histogenesis of the adrenal medulla is likely to be a direct consequence of the exclusive co-expression of both NCAM and L1 by this subpopulation of maturing chromaffin cells.

Homophilic and heterophilic binding activities of Nr-CAM, a nervous system cell adhesion molecule

Nr-CAM is a membrane glycoprotein that is expressed on neurons. It is structurally related to members of the N-CAM superfamily of neural cell adhesion molecules having six immunoglobulin-like domains and five fibronectin type III repeats in the extracellular region. We have found that the aggregation of chick brain cells was inhibited by anti-Nr-CAM Fab' fragments, indicating that Nr-CAM can act as a cell adhesion molecule. To clarify the mode of action of Nr-CAM, a mouse fibroblast cell line L-M(TK-) (or L cells) was transfected with a DNA expression construct encoding an entire chicken Nr-CAM cDNA sequence. After transfection, L cells expressed Nr-CAM on their surface and aggregated. Aggregation was specifically inhibited by anti-Nr-CAM Fab' fragments. To check the specificity of this aggregation, a fusion protein (FGTNr) consisting of glutathione S-transferase linked to the six immunoglobulin domains and the first fibronectin type III repeat of Nr-CAM was expressed in Escherichia coli. Addition of FGTNr to the transfected cells blocked their aggregation. Further analysis using a combination of cell aggregation assays, binding of cells to FGTNr-coated substrates, aggregation of FGTNr-coated Covaspheres and binding of FGTNr-coated Covaspheres to FGTNr-coated substrates revealed that Nr-CAM mediates two types of cell interactions: a homophilic, divalent cation-independent binding, and a heterophilic, divalent cation-dependent binding. Homophilic binding was demonstrated between transfected L cells, between chick embryo brain cells and FGTNr, and between Covaspheres to which FGTNr was covalently attached. Heterophilic binding was shown to occur between transfected and untransfected L cells, and between FGTNr and primary chick embryo fibroblasts; in all cases, it was dependent on the presence of either calcium or magnesium. Primary chick embryo glia or a human glial cell line did not bind to FGTNr-coated substrates. The results indicate that Nr-CAM is a cell adhesion molecule of the nervous system that can bind by two distinct mechanisms, a homophilic mechanism that can mediate interactions between neurons and a heterophilic mechanism that can mediate binding between neurons and other cells such as fibroblasts.

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.

Structure of the axonal surface recognition molecule neurofascin and its relationship to a neural subgroup of the immunoglobulin superfamily

The chick axon-associated surface glycoprotein neurofascin is implicated in axonal growth and fasciculation as revealed by antibody perturbation experiments. Here we report the complete cDNA sequence of neurofascin. It is composed of four structural elements: At the NH2 terminus neurofascin contains six Ig-like motifs of the C2 subcategory followed by four fibronectin type III (FNIII)-related repeats. Between the FNIII-like repeats and the plasma membrane spanning region neurofascin contains a domain 75-amino acid residues-long rich in proline, alanine and threonine which might be the target of extensive O-linked glycosylation. A transmembrane segment is followed by a 113-amino acid residues-long cytoplasmic domain. Sequence comparisons indicate that neurofascin is most closely related to chick Nr-CAM and forms with L1 (Ng-CAM) and Nr-CAM a subgroup within the vertebrate Ig superfamily. Sequencing of several overlapping cDNA probes reveals interesting heterogeneities throughout the neurofascin polypeptide. Genomic Southern blots analyzed with neurofascin cDNA clones suggest that neurofascin is encoded by a single gene and its pre-mRNA might be therefore alternatively spliced. Northern blot analysis with domain specific probes showed that neurofascin mRNAs of about 8.5 kb are expressed throughout development in embryonic brain but not in liver. Isolation of neurofascin by immunoaffinity chromatography results in several molecular mass components. To analyze their origin the amino-terminal sequences of several neurofascin components were determined. The NH2-terminal sequences of the 185, 160, and 110-135 kD components are all the same as the NH2 termini predicted by the cDNA sequence, whereas the other neurofascin components start with a sequence found in a putative alternatively spliced segment between the Ig- and FNIII-like part indicating that they are derived by proteolytic cleavage. A combination of enzymatic and chemical deglycosylation procedures and the analysis of peanut lectin binding reveals O- and N-linked carbohydrates on neurofascin components which might generate additional heterogeneity.

Structure, expression, and function of Ng-CAM, a member of the immunoglobulin superfamily involved in neuron-neuron and neuron-glia adhesion

The neuron-glia cell adhesion molecule (Ng-CAM) mediates neuron-neuron adhesion by a homophilic mechanism and neuron-astrocyte adhesion by a heterophilic mechanism. The protein is expressed on neurons and Schwann cells but not on astrocytes. It is most prevalent during development on cell bodies of migrating neurons and on axons during formation of nerves. Ng-CAM expression is greatly increased following nerve injury. Anti-Ng-CAM antibodies inhibited migration of granule cells along Bergmann glia in cerebellar explants and fasciculation of neurites in outgrowths from explants of dorsal root ganglia. The combined results indicate that Ng-CAM on neurons binds to Ng-CAM on adjacent neurons and to as yet unidentified ligands on astrocytes. Ng-CAM is synthesized in chicken neurons from a 6 kb mRNA as Mr approximately 200,000 forms which are cleaved to yield two components of Mr 135,000 and 80,000. It is glycosylated and can be phosphorylated. Amino acid sequence analysis indicates that it contains six immunoglobulin domains, five fibronectin type III repeats, a transmembrane domain and a cytoplasmic region. Structural analyses indicate that Ng-CAM is most closely related to the mammalian glycoprotein L1 but significant differences between them strongly suggest that they are not equivalent molecules. The recent identification of another structurally related molecule in the chicken called Nr-CAM underscores the notion that these molecules are members of a subfamily of neural cell adhesion molecules within the immunoglobulin superfamily that have related or complementary functions in the nervous system.

Intraretinal pathfinding of ganglion cell axons is perturbed by a monoclonal antibody specific for a G4/Ng-CAM-like cell adhesion molecule

To identify molecular components involved in directed axonal outgrowth and in neural pattern formation, hybridoma technology was employed using the visual system of the chicken as a model system. Using cell surface protein fractions as immunogens, we obtained the monoclonal antibody mAb C4, which binds to a 135 kDa cell surface glycoprotein of the high-mannose or complex type. Within the retina, the C4 antigen is found exclusively in the optic fiber layer. Immuno-double labeling of retinal whole mounts with a glial marker and mAb C4 suggests that the C4 antigen is restricted to ganglion cell axons but not found on Müller glial endfeet. Biochemical and histological data reveal similarities between the C4-antigen and G4/NgCAM. Addition of mAb C4 to retina explants cultured on a striped carpet of tectal cell membranes leads to defasciculation of outgrowing axons, suggesting that the C4 antigen serves as an axon cell adhesion molecule (Ax-CAM). Axon elongation on neighboring axons can be also inhibited by the application of mAb C4 to embryonic retina whole mounts in vitro. The aberrant axon growth into incorrect retina layers observed under these conditions suggests that the C4 antigen functions as a guiding cue for the generation of the retinal optic fiber layer.

Molecular structure and functional testing of human L1CAM: an interspecies comparison

The rodent, avian, and insect L1-like cell adhesion molecules are members of the immunoglobulin superfamily that have been implicated in axon growth. We have isolated an L1-like molecule from human brain and found that it also supports neurite growth in vitro. We have also cloned and sequenced the entire coding region of human L1CAM and found that it shows a very high degree of homology to mouse L1cam, with 92% identity at the amino acid level. This similarity suggests that L1CAM is an important molecule in normal human nervous system development and nerve regeneration. Overall, there is substantially less homology to chick Ng-CAM; they are 40% identical at the amino acid level but many regions are highly conserved. Comparison of the sequences from human, mouse, chick, and Drosophila indicates that the L1 immunoglobulin domain 2 and fibronectin type III domain 2 are strongly conserved and thus are likely functionally important.