Neural cell contact and axonal growth

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Hyaluronic acid hydrogel immobilized with RGD peptides for brain tissue engineering

In this paper, hyaluronic acid hydrogels with open porous structure have been developed for scaffold of brain tissue engineering. A short peptide sequence of arginine-glycine-aspartic acid (RGD) was immobilized on the backbone of the hydrogels. Both unmodified hydrogels and those modified with RGD were implanted into the defects of cortex in rats and evaluated for their ability to improve tissue reconstruction. After 6 and 12 weeks, sections of brains were processed for DAB and Glees staining. They were also labeled with GFAP and ED1 antibodies, and observed under the SEM for ultrastructral examination. After implanting into the lesion of cortex, the porous hydrogels functioned as a scaffold to support cells infiltration and angiogenesis, simultaneously inhibiting the formation of glial scar. In addition, HA hydrogels modified with RGD were able to promote neurites extension. Our experiments showed that the hyaluronic acid-RGD hydrogel provided a structural, three-dimensional continuity across the defect and favoured reorganization of local wound-repair cells, angiogenesis and axonal growth into the hydrogel scaffold, while there was little evidence of axons regeneration in unmodified hydrogel.

Alignment of glial cells stimulates directional neurite growth of CNS neurons in vitro

Olfactory ensheathing cells (OECs) together with olfactory nerve fibroblasts (ONFs) and neonatal astrocytes are potent stimulators of neurite growth in adulthood and during development, respectively. Since it is known that alignment of glial cells is important for the correct outgrowth of axon tracts, it was hypothesized that the alignment of glial cells stimulates directional and enhanced neurite outgrowth. Adult OEC/ONF and neonatal astrocytes were cultured either on biodegradable poly(d,l)-lactide matrices or in Petri dishes for 4 days. Thereafter neonatal cerebral cortical neurons were added. After a 2-days coculture period the cultures were fixed and processed for a combined MAP-2 and phosphorylated neurofilament (RT97) staining. The neurite growth (neurite elongation and neurite formation) and the neurite direction were assessed. We show that (1). OEC/ONF cultures are more potent in stimulating the length of the longest neurite of cocultured neurons, (2). alignment of glial is achieved in vitro on our biomatrices, (3). aligned glial/biomatrix complexes do not enhance neurite growth, and (4). aligned glial/biomatrix complexes direct neurite outgrowth. These data have significant implications for in vivo experiments focusing on glial transplantation. Transplanting glial/biomatrix complexes may stimulate the directional regrowth of severed axons across a lesion site.

Automated difference image analysis of lamellar ruffling: effect of temperature change on human SH-SY5Y neuroblastoma cells

Quantitation of cell movement and lamellar extension is critical to the study of growth factors, chemotactic agents and signaling cascades. Many studies are conducted by examining the size or number of lamellae in static images of cells. However, these methods do not quantify lamellar behavior over time and may overlook important changes in lamellar function. Most presently available methods for analyzing dynamic aspects of lamellar function examine changes in a cell's 2-dimensional perimeter and are best suited to the analysis of flattened lamellae. However, some cells generate 3-dimensional lamellar ruffles whose behavior is not readily detected using these methods. In the present study we analyze temperature-dependent ruffling of human SH-SY5Y neuroblastoma cells using automated digital subtraction of time-lapse images and quantitation of the resultant 'difference' image, and compare results obtained using this and other approaches. We report that ruffling behavior of SH-SY5Y cells is measurably altered by temperature changes of as little as 1 degrees C, and that these changes are best detected using difference image analysis. Our studies indicate that temperature is a critical variable in studies of SH-SY5Y behavior and that difference image analyses may be an important complement to other methods in the study of lamellar ruffling.

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Neurite fasciculation mediated by complexes of axonin-1 and Ng cell adhesion molecule

Neural cell adhesion molecules composed of immunoglobulin and fibronectin type III-like domains have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic interactions across the extracellular space (axonin- 1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1-NgCAM) that occurs exclusively in the plane of the same membrane (cis-interaction). Using domain deletion mutants we localized the NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized in the Ig domains 2-4 and the third FnIII domain. The NgCAM-NgCAM interaction could be established simultaneously with the axonin-1-NgCAM interaction. In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

Multiple roles of neural cell adhesion molecule, neural cell adhesion molecule-polysialic acid, and L1 adhesion molecules during sensory innervation of the otic epithelium in vitro

To explore the role of cell adhesion molecules in the innervation of the inner ear, antibody perturbation was used on histotypic co-cultures of the ganglionic and epithelial anlagen derived from the otocyst. When unperturbed, these tissues survived and differentiated in this culture system with outgrowth of fasciculated neuronal fibers which expressed neural cell adhesion molecule and L1. The fibers exhibited target choice and penetration, then branching and spreading within the otic epithelium as individual axons. Treatment of the co-cultures, or of the ganglionic anlagen alone, with anti-neural cell adhesion molecule or anti-L1 Fab fragments produced a defasciculation of fibers but did not affect neurite outgrowth. In the co-cultures this defasciculation was accompanied by a small increase in the number of fibers found in inappropriate tissues. However, the antibodies did not prevent fiber entry to the otic epithelium. In contrast, removal of polysialic acid from neural cell adhesion molecule with endoneuraminadase-N, while producing a similar fiber defasciculation, also increased the incidence of fibers entering the epithelium. Nevertheless, once within the target tissue, the individual fibers responded to either Fab or to desialylation by spreading out more rapidly, branching, and growing farther into the epithelium. The findings suggest that fasciculation is not essential for specific sensory fibers to seek out and penetrate the appropriate target, although it may improve their tracking efficiency. Polysialic acid on neural cell adhesion molecule appears to limit initial penetration of the target epithelium. Polysialic acid as well as neural cell adhesion molecule and L1 function are involved in fiber-target interactions that influence the arborization of sensory axons within the otic epithelium.

Integrin alpha1 localization in murine central and peripheral nervous system

The distribution of neurons expressing integrin alpha1 subunit protein (INTalpha1) was examined in adult mouse tissues of not only the central nervous system, but also the sympathetic ganglia, and the adrenal gland by immunohistochemistry and immunoelectron microscopy. INTalpha1-positive neurons were observed in most tissues examined, and most of them were found to coexpress tyrosine hydroxylase (TH) except for Purkinje cells and hippocampal neurons. Expression of INTalpha1 was also observed in the malpositioned cortical neurons in reeler mutants, and appeared not to be affected by the aberrant cell migration of the reeler cortical neurons. In situ hybridization showed that the expression of INTalpha1 mRNA was correlated with synthesis of the INTalpha1 protein in each case, and this finding indicated that expression of the protein was controlled by transcriptional regulation of the INTalpha1 gene.

The immunoglobulin superfamily: an insight on its tissular, species, and functional diversity

The immunoglobulin superfamily (IgSF) is a heterogenic group of proteins built on a common fold, called the Ig fold, which is a sandwich of two beta sheets. Although members of the IgSF share a similar Ig fold, they differ in their tissue distribution, amino acid composition, and biological role. In this paper we report an up-to-date compilation of the IgSF where all known members of the IgSF are classified on the basis of their common functional role (immune system, antibiotic proteins, enzymes, cytokine receptors, etc.) and their distribution in tissue (neural system, extracellular matrix, tumor marker, muscular proteins, etc.), or in species (vertebrates, invertebrates, bacteria, viruses, fungi, and plants). The members of the family can contain one or many Ig domains, comprising two basic types: the constant domain (C), with seven strands, and the variable domain (V), with eight, nine, or ten strands. The different overviews of the IgSF led to the definition of new domain subtypes, mainly concerning the C type, based on the distribution of strands within the two sheets. The wide occurrence of the Ig fold and the much less conserved sequences could have developed from a common ancestral gene and/or from a convergent evolutionary process. Cell adhesion and pattern recognition seem to be the common feature running through the entire family.

An antibody to the tetraspan membrane protein CD9 promotes neurite formation in a partially alpha3beta1 integrin-dependent manner

The tetraspan cell surface glycoprotein, CD9, has been implicated in cellular signaling during growth and differentiation in the hematopoietic and nervous systems. Because CD9 expression is induced early in development in sensory and sympathetic neuroblasts, we investigated the role of CD9 in neurite outgrowth. We plated dissociated cells from neonatal sympathetic ganglia on immobilized anti-CD9 antibodies or antibodies against other cell surface molecules. We show here that B2C11, an anti-CD9 antibody that has been shown previously to activate Schwann cells in vitro, promotes robust neurite outgrowth from sympathetic neurons that is greater than that on other antibody surfaces and is comparable to neurite outgrowth on a collagen substratum. In addition, B2C11 causes dramatic morphological changes in neurons and glia from dissociated ganglia, including a flattening of these cells. Because CD9 interacts with integrins in many cell types including Schwann cells, and specifically with the alpha3beta1 integrin in some cells, we tested whether the effect of B2C11 on neurite outgrowth is mediated by this integrin. An anti-alpha3beta1 antibody, Ralph 3-1, attenuates the extent of neurite outgrowth on B2C11 and collagen, but not on laminin. Because the alpha3beta1 integrin has been shown to mediate neurite outgrowth on different substrates, these results provide a functional significance for the CD9-alpha3beta1 interaction; downstream signaling may be activated by this cis interaction on the cell surface in response to external cues that promote neurite outgrowth.

Interference with axonin-1 and NrCAM interactions unmasks a floor-plate activity inhibitory for commissural axons

Axonin-1 and NrCAM were previously shown to be involved in the in vivo guidance of commissural growth cones across the floor plate of the embryonic chicken spinal cord. To further characterize their role in axon pathfinding, we developed a two-dimensional coculture system of commissural and floor-plate explants in which it was possible to study the behavior of growth cones upon floor-plate contact. Although commissural axons readily entered the floor plate under control conditions, perturbations of either axonin-1 or NrCAM interactions prevented the growth cones from entering the floor-plate explants. The presence of antiaxonin-1 resulted in the collapse of commissural growth cones upon contact with the floor plate. The perturbation of NrCAM interactions also resulted in an avoidance of the floor plate, but without inducing growth-cone collapse. Therefore, axonin-1 and NrCAM are crucial for the contact-mediated interaction between commissural growth cones and the floor plate, which in turn is required for the proper guidance of the axons across the ventral midline and their subsequent rostral turn into the longitudinal axis.

Hydrogels containing peptide or aminosugar sequences implanted into the rat brain: influence on cellular migration and axonal growth

Biocompatible polymer matrices for implantation into lesion sites in the brain were synthesized by incorporating peptide or aminosugar sequences into N-(2-hydroxypropyl)methacrylamide (HPMA) hydrogels. RGD peptide sequences were chemically linked to the hydrogel backbone via a glycylglycine spacer; aminosugars were glucosamine (NHGlc) or N-acetylglucosamine residues. Unmodified or sequence containing HPMA hydrogels were implanted into the lesioned optic tract or cerebral cortex of juvenile (17- to 19-day-old) or adult rat brains, respectively. After 10-12 months host animals were perfused and the brains were processed for immunohistochemistry using antibodies to neurofilaments (RT97), laminin, glial fibrillary acidic protein (GFAP), carbonic anhydrase II (CAII), S100 protein, macrophages (ED1), and myelin basic protein (MBP). Unmodified (control) HPMA hydrogels contained no cellular infiltration or axonal growth. Peptide (RGD)- and aminosugar-modified hydrogels showed increased adhesion properties with host neural tissue, were vascularized, and were infiltrated by host nonneuronal cells. Astrocytes (GFAP+) and macrophages (ED1(+)) were the major cell types seen within modified HPMA hydrogels, the largest numbers being found in RGD-containing polymers. CAII+ oligodendroglia were not seen within any of the hydrogel matrices. RT97(+)/MBP- axons grew into both the RGD and NHGlc hydrogel matrices for small distances. The number of axons was greatest in hydrogels implanted into cerebral cortex but in both cortex and optic tract implants the highest density of axons was seen in polymers containing RGD. The findings of this study are discussed in the context of CNS tissue replacement and the construction of bioactive scaffolds to promote regenerative axonal growth across areas of injury in the brain and spinal cord.

Influence of technique and transplantation site on rosette formation in rabbit retinal transplants

In order to determine mechanical and host-graft related interactions in the histogenesis of retinal transplants, a new technique for transplanting flat and comparatively large pieces of embryonic rabbit retina into adult rabbit eyes was elaborated. With the procedure, free-floating grafts in the epiretinal space survive, develop and differentiate largely without rosette formation, suggesting that the dissection and transplantation procedure is adequate for obtaining a normal development. On the other hand, subretinal transplants mature at an apparently faster pace than epiretinal transplants, but do not become regularly laminated. Outer segments do not develop well in the epiretinal transplants, whereas they do so in the subretinal ones, suggesting host-graft interactions by means of yet unknown diffusible factors.

Expression pattern of integrin beta 1 subunit in Purkinje cells of rat and cerebellar mutant mice

We found that integrin beta 1 subunit (INT beta 1)-immunoreactive Purkinje cells first appeared caudally at postnatal day (PD) 6 of rat and most Purkinje cells gradually became positive by PD 12. The expression of INT beta 1 was then suppressed in some of these cells, so that the positive Purkinje cells in the adult were organized into parasagittal bands interposed by negative cells throughout the vermis and hemispheres. When Purkinje cells were deprived of their climbing fiber innervation by inferior cerebellar pedunculotomy or by transplantation of cerebellar anlagen into the anterior eye chamber, the subsequent patterning of INT beta 1-positive Purkinje cells was not changed. In both reeler and weaver mice, the INT beta 1-positive parasagittal bands were observed, however, the Purkinje cells in the staggerer mice did not express INT beta 1 at any stage. These data suggest that the expression of INT beta 1 in Purkinje cells is genetically programmed in the developing cerebellum, and that the afferent synaptic inputs by climbing and parallel fibers are not prerequisites for INT beta 1 expression in Purkinje cells. Therefore, the unique distribution patterns of INT beta 1-positive Purkinje cells provides a new marker for postnatal development of rodent cerebella.

Intracellular signaling is changed after clustering of the neural cell adhesion molecules axonin-1 and NgCAM during neurite fasciculation

Neural cell adhesion molecules of the immunoglobulin/fibronectin type III family on axons have been implicated in promotion of neurite outgrowth, fasciculation, and the mediation of specific cell adhesion. The present study demonstrates that two of these molecules on dorsal root ganglion neurons are associated with distinct protein kinases, axonin-1 with the src-related nonreceptor tyrosine kinase fyn and NgCAM with a casein kinase II-related activity and a serine/ threonine kinase related to S6 kinase. When neurites grew without contacts involving axonin-1 and NgCAM, strong fyn kinase activity was associated with axonin-1, whereas the NgCAM-associated kinase activities were low. Clustering of axonin-1 with NgCAM induced by the formation of cell-cell contacts correlated with a reduction of the axonin-1-associated fyn activity and an increased phosphorylation of NgCAM by the associated casein kinase II-related activity. Thus, axonin-1 and NgCAM trigger distinctive intracellular signals during in vitro differentiation depending on their state of association.

Heterophilic interactions of DM-GRASP: GRASP-NgCAM interactions involved in neurite extension

DM-GRASP is an immunoglobulin superfamily cell adhesion molecule that is expressed in both the developing nervous and immune system. Specific populations of neurons respond to DM-GRASP substrates appears to require homophilic interactions between DM-GRASP molecules. We were interested in determining whether DM-GRASP interacts heterophilically with other ligands as well. We have found that eleven proteins from embryonic chick brain membranes consistently bind to and elute from a DM-GRASP-Sepharose affinity column. One of these proteins is DM-GRASP itself, consistent with its known homophilic binding. Another protein, at 130 kD, is immunoreactive with monoclonal antibodies to NgCAM. Other neural cell adhesion molecules were not detected in the eluate. The DM-GRASP-Sepharose eluate also contains a potent neurite stimulating activity, which cannot be accounted for by either DM-GRASP or NgCAM. To investigate the interaction of DM-GRASP and NgCAM, antibodies against DM-GRASP were added to neuronal cultures extending neurites on an NgCAM substrate. The presence of antibodies to DM-GRASP decreased neurite extension on laminin, suggesting that the antibody is not toxic or generally inhibiting motility. We present two possible models for the DM-GRASP-NgCAM association and a hypothesis for neural cell adhesion function that features the dimerization of cell adhesion molecules.

Use of chimeric F3-NCAM molecules to explore the properties of VASE exon in modulating polysialylation and neurite outgrowth

Differential splicing of VASE exon in the fourth immunoglobulin (Ig) domain and attachment to the fifth Ig domain of alpha 2-8 linked sialic acid (PSA) both dramatically change, in opposite manner, Neural Cell Adhesion Molecule (NCAM) functional properties. Reciprocal patterns of VASE and PSA expression suggest that they might be mutually exclusive. Here, we tested whether informations conferring polysialylation reside in NCAM-Ig domains 4 and 5 and the influence of the VASE exon encoded sequence on this process. We also examined if the VASE sequence was still able to inhibit neurite outgrowth when presented out of its normal NCAM context. Constructs have been prepared encoding NCAM-Ig domains 4 (with or without the VASE exon) and 5 fused to the F3 molecule. Stable clones expressing the chimeric molecules or wild type F3 were then obtained in the AtT-20 cell line. Although the chimeric molecules were expressed on the cell surface none of them was bearing PSA. Thus, polysialylation cannot be conferred to proteins by addition of the NCAM-Ig domains 4 and 5 modular motif and in this molecular context, the VASE sequence is not influencing the process. These chimeric molecules, either expressed at the surface of RIN or COS cells or presented as soluble forms, were examined for their effect on neurite outgrowth. In all cases, the length of neurites of sensory neurons was significantly reduced when grown in presence of the VASE containing chimera by comparison with the chimera without VASE or wild type F3. When neurons from NCAM knock-out mice were used for the assay, the VASE inhibition could not be detected. Thus VASE is able to act as a modular motif and NCAM expressed on neurons participates in transducing its effect.

Axonin-1, Nr-CAM, and Ng-CAM play different roles in the in vivo guidance of chick commissural neurons

Immunoglobulin/fibronectin type III-like cell adhesion molecules have been implicated in axon pathfinding based on their expression pattern in the developing nervous system and on their complex interactions described in vitro. The present in vivo study demonstrates that interactions by two of these molecules, axonin-1 on commissural growth cones and Nr-CAM on floor plate cells, are required for accurate pathfinding at the midline. When axonin-1 or Nr-CAM interactions were perturbed, many commissural axons failed to cross the midline and turned instead along the ipsilateral floor plate border. In contrast, though perturbation of Ng-CAM produced a defasciculation of the commissural neurites, it did not affect their guidance across the floor plate.

Developmental changes in epitope accessibility as an indicator of multiple states of an immunoglobulin-like neural cell adhesion molecule

Cell surface molecules with restricted spatial and temporal distributions are good candidates for mediators of the cell-cell interactions that are necessary for the development of the nervous system. A monoclonal antibody (MAb 23A7) was produced that selectively and transiently labeled a limited subset of axons in the chick embryo spinal cord. Determination of the N-terminal amino acid sequence and immunoprecipitation experiments demonstrated that the 23A7 antigen is identical to Bravo/Nr-CAM, a previously described cell adhesion molecule with immunoglobulin-like domains (E.J. de la Rosa, J.F. Kayyem, J.M. Roman, Y.-D. Stierhof, W.J. Dreyer, and U. Schwartz [1989] J. Cell Biol. 111:3087-3096; M. Grumet, V. Mauro, M.P. Goon, G.M. Edelman, and B.A. Cunningham [1991] J. Cell Biol. 113:1399-1412). The temporal distribution of the 23A7 antigen is unusual in that, immunohistochemically, MAb 23A7 binding greatly decreases after 7 days of development, whereas Western blot analysis indicates increasing levels of the antigen until 17 days of development. In contrast, an antiserum against purified Nr-CAM, which also binds only to the 23A7 antigen, labels nearly all the axons in the tissue throughout all the later stages of development. These anomalous observations are apparently not the result of differential sensitivity of the 23A7 epitope to fixation, the use of suboptimal concentrations of the MAb, or selective MAb binding to a subset of Bravo/Nr-CAM molecules produced by alternative splicing of the transcript or by posttranslational modification. These findings could indicate the existence of multiple states of Bravo/Nr-CAM, which during development, vary in the accessibility of their extracellular domains to the MAb. This suggests the existence of multiple conformation or aggregation states of this cell adhesion molecule, each of which might be performing a different function.

The limbic system-associated membrane protein (LAMP) selectively mediates interactions with specific central neuron populations

The limbic system-associated membrane protein (LAMP) is a 64–68 × 10(3) M(r) glycoprotein that is expressed by subsets of neurons that are functionally interconnected. LAMP exhibits characteristics that are indicative of a developmentally significant protein, such as an early and restricted pattern of expression and the ability to mediate specific fiber-target interactions. A potential, selective adhesive mechanism by which LAMP may regulate the formation of specific circuits is investigated in the present experiments. LAMP is readily released from intact membranes by phosphatidyl inositol-specific phospholipase C. Purified, native LAMP, isolated by PI-PLC digestion and immunoaffinity chromatography, is capable of mediating fluorescent Covasphere aggregation via homophilic binding. To test the ability of LAMP to selectively facilitate substrate adhesion and growth of neurons from LAMP-positive, in contrast to LAMP-negative regions of the developing brain, purified LAMP was dotted onto nitrocellulose-coated dishes and test cells plated. Limbic neurons from perirhinal cortex bind specifically to substrate-bound LAMP within 4 hours, forming small cell aggregates with short neuritic processes that continue to grow through a 48 hour period of monitoring. Preincubation of cells with anti-LAMP has a modest effect on cell binding but significantly reduces initiation of process growth. Non-limbic neurons from somatosensory cortex and olfactory bulb fail to bind or extend processes on the LAMP substrate to any significant extent. All cell populations bind equally well and form neurites on poly-D-lysine and laminin. The present results provide direct evidence that LAMP can specifically facilitate interactions with select neurons in the CNS during development. The data support the concept that patterned expression of unique cell adhesion molecules in functionally related regions of the mammalian brain can regulate circuit formation.

Isolation of a neural chondroitin sulfate proteoglycan with neurite outgrowth promoting properties

Proteoglycans are expressed in various tissues on cell surfaces and in the extracellular matrix and display substantial heterogeneity of both protein and carbohydrate constituents. The functions of individual proteoglycans of the nervous system are not well characterized, partly because specific reagents which would permit their isolation are missing. We report here that the monoclonal antibody 473HD, which binds to the surface of early differentiation stages of murine astrocytes and oligodendrocytes, reacts with the chondroitin sulfate/dermatan sulfate hybrid epitope DSD-1 expressed on a central nervous system chondroitin sulfate proteoglycan designated DSD-1-PG. When purified from detergent-free postnatal days 7 to 14 mouse brain extracts, DSD-1-PG displays an apparent molecular mass between 800-1,000 kD with a prominent core glycoprotein of 350-400 kD. Polyclonal anti-DSD-1-PG antibodies and monoclonal antibody 473HD react with the same molecular species as shown by immunocytochemistry and sequential immunoprecipitation performed on postnatal mouse cerebellar cultures, suggesting that the DSD-1 epitope is restricted to one proteoglycan. DSD-1-PG promotes neurite outgrowth of embryonic day 14 mesencephalic and embryonic day 18 hippocampal neurons from rat, a process which can be blocked by monoclonal antibody 473HD and by enzymatic removal of the DSD-1-epitope. These results show that the hybrid glycosaminoglycan structure DSD-1 supports the morphological differentiation of central nervous system neurons.

Use of ELISA to G4 antigen to quantitate neurite outgrowth in the chick both in vivo and in vitro

The G4 glycoprotein is found on the earliest developing neurite tracts of the chick embryo. An ELISA is introduced here to quantify the amount of G4-expressing neurites in the picogram range. In this double-sandwich assay, an anti-G4 monoclonal antibody fixes the G4 antigen to the plastic surface, which then is detected by a polyclonal antiserum; nonspecific background is decreased by competitive displacement. The sensitivity of the assay allows us to follow quantitatively the very first neurite growth in embryonic heads, trunks, retinae, and brains. G4-based neurite growth is shown to occur earlier in heads than in trunks; in brain it is nearly 10-fold higher than in the retina by embryonic day 8. By determination of acetylcholinesterase (AChE) activities from the same homogenates, our earlier histochemical findings are verified now on a quantitative basis, again showing that AChE consistently precedes G4 antigen. Moreover, as an in vitro example, the G4 ELISA is applied to the nerve growth factor (NGF) standard bioassay on dorsal root ganglia; the half-maximal response is reached at approximately 10 ng/ml of NGF for G4-based neurite growth and at approximately 1 ng/ml of NGF for AChE expression, respectively.

Axonal glycoproteins with immunoglobulin- and fibronectin type III-related domains in vertebrates: structural features, binding activities, and signal transduction

The L1- and F11-like axonal glycoproteins, implicated in neurite outgrowth and fasciculation, are members of the Ig superfamily comprising multiple fibronectin type III-like domains. Their Ig-like and fibronectin type III-related domains are likely to be composed of seven beta-strands arranged in two opposing beta-sheets of highly similar topology. Whereas the F11-like molecules lack a transmembrane sequence and are anchored in the plasma membrane by a glycosylphosphatidylinositol, the L1-like molecules comprise cytoplasmic domains with highly conserved sequence motifs. Most of the latter proteins occur in different isoforms generated by alternative pre-mRNA splicing, which has not been documented for molecules of the F11 subgroup. L1-like proteins undergo heterophilic as well as homophilic interactions, whereas only the former mode of binding was observed for F11-like proteins. Evidence is accumulating that these Ig superfamily molecules with fibronectin type III-like domains are interacting in a complex manner with each other and molecules of the extracellular matrix. Investigations assigning structure to function reveal that their individual extracellular domains serve distinct binding activities. Recent studies also suggest that L1 and NCAM are implicated in the transduction of transmembrane signals.

Cholinesterases regulate neurite growth of chick nerve cells in vitro by means of a non-enzymatic mechanism

Cholinesterases present homologies with some cell adhesion molecules; however, it is unclear whether and how they perform adhesive functions. Here, we provide the first direct evidence showing that neurite growth in vitro from various neuronal tissues of the chick embryo can be modified by some, but not all, anticholinesterase agents. By quantifying the neuritic G4 antigen in tectal cell cultures, the effect of anticholinesterases on neurite growth is directly compared with their cholinesterase inhibitory action. BW 284C51 and ethopropazine, inhibiting acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), respectively, strongly decrease neurite growth in a dose-dependent manner. However, echothiophate which inhibits both cholinesterases, does not change neuritic growth. These quantitative data are supplemented by morphological observations in retinal explant cultures grown on striped laminin carpets, viz., defasciculation of neurite bundles by BW 284C51 and Bambuterol occurs, indicating that these drugs disturb adhesive mechanisms. These data strongly suggest that a) cholinesterases can participate in regulating axonal growth, b) both AChE and BChE can perform such a nonsynaptic function, and c) this function is not the result of the enzyme activity per se, since at least one drug was found that inhibits all cholinesterase activities but not neurite growth. Thus, a secondary site on cholinesterase molecules must be responsible for adhesive functions.

Pre- and postnatal development of the primary visual cortex of the common marmoset. II. Formation, remodelling, and elimination of synapses as overlapping processes

During ontogenesis changes in the numerical density of synapses are usually assumed to depend essentially on variations in the formation of synapses. Only the final adjustment to adult synapse densities is thought to include the elimination of synapses in some brain regions of certain species. Here, we focus attention on quantitative aspects of synapse elimination throughout development of area 17 of marmoset monkeys (Callithrix jacchus). Mature synapses, various precursor forms, and indicators of lysosomal degradation of synapses were quantitatively analysed by electron microscopy and morphometric methods. A total number of about 135 x 10(9) synapses was calculated for area 17 in each adult hemisphere corresponding to a volume density of 600 x 10(6) synapses/mm3. At 3 months of age, the respective values were 508 x 10(9)/area and 1,159 x 10(6)/mm3, while at birth these values were 69 x 10(9)/area and 328 x 10(6)/mm3. Consequently, at least three out of four synapses are eliminated between 3 months and adulthood. However, the real number of synapses being eliminated during development is probably much larger if the time course of lysosomal degradation is additionally taken into account. The frequency of lysosomes in presynaptic endings is highest before net-elimination of synapses occurs, i.e., between 1 and 3 months. This suggests that lysosomal degradation is not directly responsible for the majority of synapses removed during ontogenesis but apparently represents a second mechanism for synapse remodelling and elimination. Thus, it appears from this study that remodelling and elimination of synapses are quantitatively as important as their formation, and accompany synaptogenesis from its very onset onwards.

Cell-cell adhesion by homophilic interaction of the neuronal recognition molecule axonin-1

The axonal surface glycoprotein axonin-1, which occurs both as a glycosyl-phosphatidylinositol-anchored membrane-bound form and a secreted form, promotes neurite outgrowth and is thought to be involved in axon-guidance mechanisms in the developing nervous system. Recently, we have demonstrated that the neurite-outgrowth-promoting activity of axonin-1, presented as a substratum for cultured neurons, is mediated by a heterophilic interaction with the axonal glycoprotein neuronglia cell-adhesion molecule (Ng-CAM). Here we present evidence for homophilic (like-like) binding among axonin-1 molecules. Axonin-1 was heterologously expressed in myeloma cells. Clonal cell lines, with exposed membrane-bound axonin-1 at their surface, formed large multicellular aggregates. Incubations of transfected and parental myeloma cells, under a series of different conditions, revealed homophilic axonin-1/axonin-1 interactions across the intermembrane space as the molecular mechanism promoting stable cell-cell contacts. Using structural and functional characterisation, recombinant axonin-1 was very similar to native axonin-1, suggesting that homophilic axonin-1 interactions are also established in neurons. The capability of axonin-1 to interact with both Ng-CAM and other axonin-1 molecules might contribute to the formation of macromolecular networks at contact sites of growth cones and axons, comprising molecules of both membranes, and thus represent a mechanism for regulating neurite outgrowth and pathfinding.

UNC-5, a transmembrane protein with immunoglobulin and thrombospondin type 1 domains, guides cell and pioneer axon migrations in C. elegans

The unc-5 gene is required for guiding pioneering axons and migrating cells along the body wall in C. elegans. In mutants, dorsal migrations are disrupted, but ventral and longitudinal movements are largely unaffected. The gene was tagged for molecular cloning by transposon insertions. Based on genomic and cDNA sequencing, the gene encodes UNC-5, a transmembrane protein of 919 aa. The predicted extracellular N-terminus comprises two immunoglobulin and two thrombospondin type 1 domains. Except for an SH3-like motif, the large intracellular C-terminus is novel. Mosaic analysis shows that unc-5 acts in migrating cells and pioneering neurons. We propose that UNC-5 is a transmembrane receptor expressed on the surface of motile cells and growth cones to guide dorsal movements.

Specificity of cell adhesion in development: the cadherin superfamily

Cadherins are major cell-surface receptors involved in specific cell adhesion during development. Recent results reveal the existence of a growing array of related molecules involved in various forms of cell-cell adhesion, including that mediated by desmosomes. Comparisons with other families of adhesion receptors suggest testable models for functions of the emerging cadherin superfamily in development and disease.

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.

The chicken neural extracellular matrix molecule restrictin: similarity with EGF-, fibronectin type III-, and fibrinogen-like motifs

Restrictin is a chick neural extracellular matrix protein implicated in neural cell attachment and found to be associated with the cell surface recognition protein F11. Here we show by cDNA cloning that restrictin is a large multidomain protein composed of 4 structural motifs. At the N-terminus restrictin contains a cysteine-rich segment of about 140 aa that might link restrictin monomers into oligomers. This region is followed by 4.5 epidermal growth factor-like repeats and then by 9 consecutive motifs that are similar to fibronectin type III motifs. At the C-terminus restriction is related to the beta and gamma chains of fibrinogen, including similarity to a calcium-binding segment. Restrictin shows substantial sequence similarity with tenascin (cytotactin) throughout the polypeptide, and like tenascin, it forms oligomeric structures, as revealed by electron microscopy of immunoaffinity-purified restriction. The cell attachment site of restrictin is mapped to the C-terminal region by antibody perturbation experiments.