Genetic deletion of a neural cell adhesion molecule variant (N-CAM-180) produces distinct defects in the central nervous system

Genomic structure and promoter activity of the mouse polysialic acid synthase gene (mST8Sia II). Brain-specific expression from a TATA-less GC-rich sequence

The mouse ST8Sia II (mST8Sia II/STX) gene encodes a neural cell adhesion molecule-specific polysialic acid synthase whose expression is regulated during the developmental stages of mouse brain. To elucidate the molecular mechanism by which the expression is tissue-specifically and developmentally regulated, we isolated the complete genomic DNA and characterized the promoter of the gene for mST8Sia II. The gene encoding mST8Sia II was found to span about 80 kilobases and to be composed of six exons. Primer extension and S1 nuclease protection analyses revealed that the transcription started from 167 nucleotides upstream of the translational initiation site. Promoter analyses of the 5'-flanking region of the mST8Sia II gene using a luciferase gene reporter system revealed strong promoter activity in retinoic acid-induced differentiated P19 cells, which highly express the mST8Sia II gene. Deletion analyses demonstrated that the minimal promoter activity detected for the proximal region 325 base pairs upstream from the translational initiation codon (-158 to +167) could be modulated by various sequences within the 9. 5-kilobase 5'-flanking region. The minimal promoter was embedded in a GC-rich domain (74%, GC content), in which two Sp1 binding motifs as well as a long purine-rich region were found, but it lacked TATA and CAAT boxes. The positive regulatory region located between -159 and -659 contained two additional Sp1 binding motifs and a long pyrimidine-rich region. We also found that the minimal promoter region of the mST8Sia II gene was sufficient for expression of a reporter gene in mST8Sia II gene-expressing neural differentiated P19 cells but not in nonexpressing ones. Thus the TATA-less GC-rich minimal promoter region of mST8Sia II probably controls the cell type-specific expression of the mST8Sia II gene.

Mesodermal development in mouse embryos mutant for fibronectin

Three independent mutations were made by homologous recombination in two different regions of the fibronectin (FN) gene; all three appeared to be functional null mutations. The embryonic lethal phenotypes of these mutations were indistinguishable; all three FN mutant strains show mesodermal defects and fail to develop notochord or somites. Nevertheless analysis with lineage markers (Brachyury, sonic hedgehog, Notch-1, and mox-1) showed that both the notochord and the somite lineages were induced at the correct times and places. Furthermore, notochord precursor cells showed extensive cell migration in the absence of FN. However, neither notochord nor somites condensed properly in the absence of FN. These results show that specification of notochordal and somitic mesodermal lineages and significant cell migration are independent of fibronectin but that correct morphogenesis of these structures is FN-dependent.

Cell adhesion molecules and neuronal regeneration

Cell adhesion molecules (CAMs) are multifunctional proteins and are involved in a number of important regulatory processes in the brain, including cell growth, migration and regeneration. Recent studies using model in vitro systems have identified additional binding interactions in which CAMs, particularly those of the Ig superfamily, can participate. Signal transduction pathways are activated following CAM action in the process of neurite outgrowth. Key components in these pathways, such as kinases and phosphatases, are being identified. Receptor phosphatases themselves contain protein motifs characteristic of CAMs and may themselves be involved in adhesion-mediated cell recognition events.

Polysialic acid and the regulation of cell interactions

Polysialic acid, a unique glycosylation of the neural cell adhesion molecule, is highly regulated in its expression. Its function is manifested in the modulation of cell interactions, probably through its unusual physical properties. Recent advances have clarified the enzymatic mechanism of polysialic acid biosynthesis, expanded its role in cell migration and axon guidance, and suggested that it promotes plasticity in the adult nervous system.

Differential adhesion and the initial assembly of the mammalian olfactory nerve

During the initial assembly of the olfactory pathway, the behavior of olfactory axons changes as they grow from the olfactory epithelium toward the telencephalic vesicle. The axons exit the epithelium singly or in small fascicles, and their growth cones are simple and bullet-shaped. Outside the epithelium, they make a sharp dorsal turn and fasciculate into a single nerve; the growth cones remain simple. Upon entering the ventromedial telencephalon, the axons defasciculate, branch extensively, and end in complex, lamellate growth cones which extend toward the ventrolateral aspect of the telencephalic vesicle. The distribution of laminin, collagen-IV, and fibronectin varies in register with these changes in olfactory axon and growth cone behavior. Each of these extracellular matrix molecules influences olfactory neurite outgrowth and growth cone morphology in vitro consistent with its distribution in vivo. The distribution of E-cadherin, L1, neural cell adhesion molecule (NCAM) and the polysialated form of NCAM also varies in register with changes in olfactory axon behavior. In vitro, L1 modulates embryonic olfactory neurite outgrowth and growth cone morphology consistent with its distribution in vivo. Thus, olfactory axon trajectory, fasciculation, and growth cone morphology change within distinct adhesive environments in the nascent olfactory pathway, and some of the molecules that characterize these environments have differential effects upon olfactory neurite growth and growth cone morphology. Consequently, the patterned expression and activity of extracellular matrix and cell surface adhesion molecules may contribute to the initial assembly of the olfactory pathway.

Is there a neural stem cell in the mammalian forebrain?

Neural precursor cells have been of interest historically as the building blocks of the embryonic CNS and, most recently, as substrates for restorative neurological approaches. The majority of previous in vitro studies of the regulation of neural-cell proliferation by polypeptide growth factors, and in vivo studies of neural lineage, argue for the presence of precursors with limited proliferative or lineage potential in the mammalian CNS. This is in contrast to renewable tissues, such as the blood or immune system, skin epithelium and epithelium of the small intestinal crypts, which contain specialized, self-renewing cells known as stem cells. However, recent in vitro and in vivo studies from our and other laboratories lead us to conclude that neural stem cells, with self-renewal and multilineage potential, are present in the embryonic through to adult mammalian forebrain.

Subventricular zone-olfactory bulb migratory pathway in the adult mouse: cellular composition and specificity as determined by heterochronic and heterotopic transplantation

To gain insight into cellular and molecular mechanisms subserving neuronal cell migration in the adult mouse forebrain, we have first investigated the cellular composition of the subventricular zone-olfactory bulb pathway (SVZ-OB). The pathway was essentially composed of cells with neuronal and astrocytic identities, neuronal cells being four times more numerous than astrocytes. Neuronal cells (precursors and some young postmitotic neurons) formed continuous cellular strands of migratory cells from the anterior horn of the lateral ventricle to the olfactory bulb. These chains of migrating cells moved within channels formed by the processes of a special subpopulation of astrocytes. The neuronal cells expressed the embryonic form of polysialic acid neural cell adhesion molecule, and the astrocytes were tenascin-C positive, thus preserving an embryonic cellular environment. Through transplantation experiments, the second part of this study attempted to analyze the functional properties of the adult SVZ-OB pathway. Early postnatal (P2-13) cerebellar progenitor cells, taken from a transgenic mouse line in which cerebellar granule cells and molecular layer interneurons (basket/stellate cells) expressed the reporter gene lacZ, were implanted in the SVZ-OB pathway of adult wild-type mice. Unlike grafted SVZ cells that migrate all along the pathway, none of the cerebellar precursors reached the olfactory bulb, although some of them were able to migrate along the caudal one-third of the pathway. The majority (over 67%) of the migrating cells were progenitors that acquired the phenotype of basket/stellate cells. Granule cell progenitors and most granule cells did not survive transplantation. These results show that the adult SVZ-OB pathway is not a "passive generic guidance" for all classes of premigratory neurons. From the two types of grafted cerebellar progenitors, only those with migratory capability and that do not follow radial glial axes are able to translocate along the SVZ-OB pathway. Furthermore, the basket/stellate cell progenitors are specified at the time of grafting: Neither their identity nor the pace of expression of their major distinctive features are influenced by local signals emanating from the adult forebrain.

A knock-out model of paroxysmal nocturnal hemoglobinuria: Pig-a(-) hematopoiesis is reconstituted following intercellular transfer of GPI-anchored proteins

We created a "knockout" embryonic stem cell via targeted disruption of the phosphatidylinositol glycan class A (Pig-a) gene, resulting in loss of expression of cell surface glycosyl phosphatidylinositol-anchored proteins and reproducing the mutant phenotype of the human disease paroxysmal nocturnal hemoglobinuria. Morphogenesis of Pig-a- embryoid bodies (EB) in vitro was grossly aberrant and, unlike EB derived from normal embryonic stem cells, Pig-A EB produced no secondary hematopoietic colonies. Chimeric EB composed of control plus Pig-A- cells, however, appeared normal, and hematopoiesis from knock-out cells was reconstituted. Transfer in situ of glycosyl phosphatidylinositol-anchored proteins from normal to knock-out cells was demonstrated by two-color fluorescent analysis, suggesting a possible mechanism for these functional effects. Hematopoietic cells with mutated PIG-A genes in humans with paroxysmal nocturnal hemoglobinuria may be subject to comparable pathophysiologic processes and amenable to similar therapeutic protein transfer.

mCD24 expression in the developing mouse brain and in zones of secondary neurogenesis in the adult

Interactions mediated by cell surface glycoproteins are considered to be crucial during the formation of the nervous system. Using a monoclonal antibody directed to mCD24, a glycosylphos-phatidylinositol-anchored membrane glycoprotein, we have mapped its distribution throughout the mouse cerebral cortex during development and in young adult. Before birth, mCD24 immunoreactivity was observed in the intermediate zone, the cortical plate and the marginal zone, whereas the ventricular zones were immunonegative. After birth, mCD24 expression declined rapidly in the cortex, except in the corpus callosum (and other commissures in the brain) where immunoreactivity was still found until P20. Furthermore, mCD24 expression was maintained in young adults (until P60, at least) in zones of secondary neurogenesis, such as the granule cells of the dentate gyrus, the subventricular zone lining the anterior part of the lateral ventricles and a zone of cells extending between the striatum and the corpus callosum to the centre of the olfactory bulb. In this area mCD24 and polysialic acid neural cell adhesion molecule stainings were superimposed, and this corresponded to the pathway of migration of the olfactory immature neurons (subependymal layer). A layer of ciliated ependymal cells, lining all the ventricular walls, was also immunoreactive for mCD24. Thus, except for these epithelial-like cells, mCD24 was essentially found associated with differentiating postmitotic neurons. Its spatiotemporal expression, both during development and in the adult, is compatible with a role for this glycoprotein in cell surface recognition and in signalling events occurring during neuronal migration and axonal growth.

Targeted mutation of Ncam to produce a secreted molecule results in a dominant embryonic lethality

The neural cell adhesion molecule (NCAM) is a membrane-associated member of the immunoglobulin superfamily capable of both homophilic and heterophilic binding. To investigate the significance of this binding, a gene targeting strategy in embryonic stem (ES) cells was used to replace the membrane-associated forms of NCAM with a soluble, secreted form of its extracellular domain. Although the heterozygous mutant ES cells were able to generate low coat color chimeric mice, only the wild-type allele was transmitted, suggesting the possibility of dominant lethality. Analysis of chimeric embryos with high level of ES cell contribution revealed severe growth retardation and morphological defects by E8.5-E9.5. The second allele was also targeted, and embryos derived almost entirely from the homozygous mutant ES cells exhibited the same lethal phenotype as observed with heterozygous chimeras. Together, these results indicate that dominant lethality associated with the secreted NCAM does not require the presence of membrane-associated NCAM. Furthermore, the data indicate that potent bioactive cues or signals can be generated by NCAM.

The divergent homeobox gene PBX1 is expressed in the postnatal subventricular zone and interneurons of the olfactory bulb

In the mammalian brain, an important phase of neurogenesis occurs postnatally in the subventricular zone (SVZ). This region consists of a heterogeneous population of cells, some mitotically active, others postmitotic. A subset of mitotically active SVZ precursor cells gives rise to a population of neurons that migrates over a long distance to their final destination, the olfactory bulb. Other SVZ precursor cells continue to proliferate or undergo cell death. The combination of genes that regulates proliferation and cell fate determination of SVZ precursor cells remains to be identified. We have used the rat homolog of the human homeobox gene PBX1 in Northern analysis and in situ hybridization studies to determine the temporal and regional localization of PBX1 expression during embryonic and postnatal rat brain development. PBX1 is expressed embryonically in the telencephalon. In addition, it is expressed at high levels postnatally in the SVZ, in the migratory pathway to the olfactory bulb, and in the layers of the olfactory bulb that are the targets of these migratory neurons. Combining in situ hybridization for PBX1 with immunostaining for markers of cell proliferation (PCNA), postmitotic neurons (class III beta-tubulin), and glia (GFAP), we show that SVZ proliferating cells and their neuronal progeny express rat PBX1 mRNA, whereas glial cells do not express detectable levels of PBX1. The expression of PBX1 in SVZ precursor cells and postmitotic neurons suggests a role for PBX1 in the generation of olfactory bulb interneurons and in mammalian neurogenesis.

Elucidation of the molecular actions of NCAM and structurally related cell adhesion molecules

The Neural Cell Adhesion Molecule (NCAM) is a founder member of a large family of cell surface glycoproteins that share structural motifs related to immunoglobulin and fibronectin type III (FN III) domains [Walsh and Doherty (1991) (Fig. 1). These glycoproteins have been grouped based on the respective number of each type of domain. In vertebrates members of this family of glycoproteins include L1/NILE, NgCAM, axonin-1/TAG-1, and Thy-1 as well as NCAM. In addition structural homologs of NCAM and L1 have been identified in Drosophila and Grasshoppers [Walsh and Doherty (1991)]. These insect homologs are called fasciclins and a series of mutants corresponding to these aspects of synaptic plasticity [Mayford et al. (1992) Science 256:638-644]. In vertebrates all of these glycoproteins are expressed in the developing nervous system where they have been identified as candidate molecules for mediating axon outgrowth, fasciculation, regeneration, and target recognition. In addition, NCAM is expressed in a number of different tissues and cell types. For example, NCAM is expressed in a dynamic pattern in developing and regenerating adult muscle. In this review we aim to describe important aspects of the role of these CAMS in development of the nervous system, including the neuromuscular junction. Furthermore, we will explore the prospective use of molecular biology, cell biology, and molecular genetic techniques, such as transgenic mice, to understand the role and molecular action of this family of cell adhesion molecules in vivo.

Cortical lesions induce an increase in cell number and PSA-NCAM expression in the subventricular zone of adult rats

The subventricular zone (SVZ) bordering the lateral ventricle is one of the few regions of adult brain that contains dividing cells. These cells can differentiate into neurons in vivo after migration into the olfactory bulb and in vitro in the presence of appropriate growth factors. Little is known, however, about the fate of these cells in vivo after brain injury in adults. We examined cell number and expression of differentiation markers in the SVZ of adult rats after cortical lesions. Aspiration lesions of the sensorimotor cortex in adult rats induced a transient doubling of the number of cells in the SVZ at the level of the striatum without consistent increases in bromodeoxyuridine-labeled cells. Immunoreactivity to the polysialylated neural cell adhesion molecule, expressed by the majority of cells of the SVZ during development, increased dramatically after lesion. In contrast, immunolabeling for molecules found in mature neurons and glia did not increase in the SVZ after lesion, and immunoreactivity for growth factors that induce differentiation of SVZ cells in vitro decreased or remained undetectable, suggesting that lack of appropriate growth factor expression may contribute to the lack of differentiation of the newly accumulated cells in vivo. The data reveal that cells of the SVZ are capable of plasticity in the adult rat after brain injury in vivo and that the newly accumulated cells retain characteristics seen during development.

Young neurons from the adult subependymal zone proliferate and migrate along an astrocyte, extracellular matrix-rich pathway

The subependymal zone (SEZ) of the lateral ventricle of adult rodents has long been known to be mitotically active. There has been increased interest in the SEZ, since it has been demonstrated that neuroepithelial stem cells residing there generate neurons in addition to glia in vitro. In the present study, we have examined parasagittal sections of the adult mouse brain using immunocytochemistry for extracellular matrix (ECM) molecules (tenascin and chondroitin sulfate-containing proteoglycans), glial fibrillary acidic protein (GFAP, a cytoskeletal protein prominently expressed by immature and reactive astrocytes), RC-2 (a radial glial and immature astrocyte cytoskeletal marker), TuJ1 (a class III beta-tubulin isoform expressed solely by postmitotic and adult neurons), nestin (a cytoskeletal protein associated with stem cells), neuron-specific enolase, and bromodeoxyuridine (BrdU, which is taken up by dividing cells). Our results demonstrate that a population of young neurons reside within an ECM-rich, GFAP-positive astrocyte pathway from the rostral SEZ all the way into the olfactory bulb. Furthermore, BrdU labeling studies indicate that there is a high level of cell division along the entire length of this path, and double-labeling studies indicate that neurons committed to a neuronal lineage (i.e., TuJ1+) take up BrdU (suggesting they are in the DNA synthesis phase of the cell cycle), again along the entire length of the SEZ "migratory pathway." Thus, the SEZ appears to retain the ability to produce neurons and glia throughout the life of the animal, functioning as a type of "brain marrow." The implications of these findings are discussed in relation to the role that such a glial/ ECM-rich boundary (as seen in the embryonic cortical subplate and other developing areas) may play in: confining the migratory populations and maintaining them in a persistent state of immaturity; facilitating their migration to the olfactory bulb, where they are incorporated into established adult circuitries; and potentially altering SEZ cell cycle dynamics that eventually lead to cell death.

Occurrence of poly(alpha2,8-deaminoneuraminic acid) in mammalian tissues: widespread and developmentally regulated but highly selective expression on glycoproteins

In tissues of higher organisms homopolymers of alpha2,8-linked N-acetylneuraminic acid can be found as a posttranslational modification on selected proteins. We report here the discovery of homopolymers of alpha2,8-linked deaminoneuraminic acid [poly(alpha2,8-KDN)] in various tissues derived from all three germ layers in vertebrates including mammals. The monoclonal antibody kdn8kdn in conjunction with a bacterial KDNase permitted the detection of poly(alpha2,8-KDN) by immunohistochemistry and immunoblotting. Further evidence for the existence of poly(alpha2,8-KDN) was obtained by gas/liquid chromatography. The poly(alpha2,8-KDN) glycan was detectable in all tissues studied with the exception of mucus-producing cells present in various organs, the extracellular matrix, and basement membranes. However, in certain organs such as muscle, kidney, lung, and brain its expression was developmentally regulated. Despite its widespread tissue distribution, the poly(alpha2,8-KDN) glycan was detected on a single 150-kDa glycoprotein except for a single >350-kDa glycoprotein in kidney, which makes it most distinctive among polysialic acids. The ubiquitous yet selective expression may be indicative of a general function of the poly(alpha2,8-KDN)-bearing glycoproteins.

The role of polysialic acid in migration of olfactory bulb interneuron precursors in the subventricular zone

Transplantation studies have been used to show that tangential migration of olfactory bulb interneuron precursors is retarded in NCAM-mutant mice, and that this defect reflects loss of NCAM polysialic acid (PSA). In contrast, radial migration of cells within the bulb did not require PSA. Reciprocal transplantations between wild-type and mutant mice have revealed that the mutation affects the in vivo migration environment in the subventricular zone, and not movement of individual cells. However, in vitro migration of the cells into a PSA-negative collagen matrix environment was also PSA dependent. The surprisingly similar results obtained in the in vivo and in vitro environments is consistent with the observation that migration of subventricular cells occurs as streams of closely apposed cells in which the PSA-positive cells appear to serve as their own migration substrate.

Molecular cloning of a developmentally regulated N-acetylgalactosamine alpha2,6-sialyltransferase specific for sialylated glycoconjugates

A cDNA encoding a novel sialyltransferase has been isolated employing the polymerase chain reaction using degenerate primers to conserved regions of the sialylmotif that is present in all eukaryotic members of the sialyltransferase gene family examined to date. The cDNA sequence revealed an open reading frame coding for 305 amino acids, making it the shortest sialyltransferase cloned to date. This open reading frame predicts all the characteristic structural features of other sialyltransferases including a type II membrane protein topology and both sialylmotifs, one centrally located and the second in the carboxyl-terminal portion of the cDNA. When compared with all other sialyltransferase cDNAs, the predicted amino acid sequence displays the lowest homology in the sialyltransferase gene family. Northern analysis shows this sialyltransferase to be developmentally regulated in brain with expression persisting through adulthood in spleen, kidney, and lung. Stable transfection of the full-length cDNA in the human kidney carcinoma cell line 293 produced an active sialyltransferase with marked specificity for the sialoside, Neu5Ac-alpha2,3Gal-beta1,3GalNAc and glycoconjugates carrying the same sequence such as G(M1b) and fetuin. The disialylated tetrasaccharide formed by reacting the sialyltransferase with the aforementioned sialoside was analyzed by one- and two-dimensional 1H and 13C NMR spectroscopy and was shown to be the Neu5Ac-alpha2,3Gal-beta1,3(Neu5Ac-alpha2,6)GalNAc sialoside. This indicates that the enzyme is a GalNAc alpha-2,6-sialyltransferase. Since two other ST6GalNAc sialyltransferase cDNAs have been isolated, this sialyltransferase has been designated ST6GalNAc III. Of these three, ST6GalNAc III displays the most restricted acceptor specificity and is the only sialyltransferase cloned to date capable of forming the developmentally regulated ganglioside G(D1alpha) from G(M1b).

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.

Chain migration of neuronal precursors

In the brain of adult mice, cells that divide in the subventricular zone of the lateral ventricle migrate up to 5 millimeters to the olfactory bulb where they differentiate into neurons. These migrating cells were found to move as chains through a well-defined pathway, the rostral migratory stream. Electron microscopic analysis of serial sections showed that these chains contained only closely apposed, elongated neuroblasts connected by membrane specializations. A second cell type, which contained glial fibrillary acidic protein, ensheathed the chains of migrating neuroblasts. Thus, during chain migration, neural precursors moved associated with each other and were not guided by radial glial or axonal fibers.

The role of polysialic acid and other carbohydrate polymers in neural structural plasticity

Polysialic acid (PSA) fulfills several criteria for a molecule involved in structural plasticity, including expression in regions capable of plasticity, re-expression in structures undergoing synaptic rearrangement in the adult, downregulation following innervation, and regulation by activity. In addition, removal of PSA reduces the capacity for structural plasticity. PSA may be paradigmatic for other large polymeric carbohydrates, such as glycosaminoglycans and proteoglycans, which also are highly charged and can be extensively hydrated. These carbohydrates may affect structural plasticity by altering cell-cell and/or cell-matrix interactions by increasing intermolecular spacing through hydration.

Granule cell specification in the developing mouse brain as defined by expression of the zinc finger transcription factor RU49

The creation of specific neuronal cell types within the developing brain is a critical and unsolved biological problem. Precedent from invertebrate development, and from vertebrate myogenesis and lymphogenesis, has established that cell specification often involves transcription factors that are expressed throughout the differentiation of a given cell type. In this study, we have identified in Zn2+ finger transcription factor RU49 as a definitive marker for the cerebellar granule neuron lineage. Thus, RU49 is expressed in the earliest granule cell progenitors at the rhombic lip as they separate from the ventricular zone of the neural tube to generate a secondary proliferative matrix, and it continues to be expressed in differentiating and mature granule neurons. Proliferating granule cell progenitors isolated from the rhombic lip at E14 or from the external germinal layer at P6 continue to express RU49 in vitro. Both the olfactory bulb and dentate gyrus granule cell lineages also express this factor as they are generated with the developing brain. RU49 binds a novel bipartite DNA-binding element in a manner consistent with chemical rules governing the DNA-binding specificity of this class of transcription factor. The novel biochemical properties of RU49 and its restricted expression within the three lineages of CNS granule neurons suggest that RU49 may play a critical role in their specification. Furthermore, these results raise the interesting possibility that the generation of these three neuronal populations to form displaced germinative zones within the developing brain may reflect their use of a common developmental mechanism involving RU49.

The cellular function of MASH1 in autonomic neurogenesis

Using primary cultures and immortalized multipotential stem cell lines derived from wild-type and Mash1 mutant neural crest cells, we have analyzed the cellular function of MASH1 in autonomic neurogenesis. We present evidence for the existence of a precursor expressing MASH1 and neuronal markers such as neurofilament, neuron-specific tubulin, and tetanus toxin receptor. This cell has a nonneuronal morphology. Differentiation of this precursor to neurons that express markers such as SCG10, peripherin, and neuron-specific enolase is dependent upon MASH1 function. These data imply that the differentiation of autonomic neurons from uncommitted neural crest cells occurs in several sequential steps. Moreover, they suggest that MASH1 does not commit multipotent cells to a neural fate, like its Drosophila achaete-scute counterparts, but rather promotes the differentiation of a committed neuronal precursor.

Polysialylated neural cell adhesion molecule expression by neurons and astroglial processes in the rat dentate gyrus declines dramatically with increasing age

The expression of polysialylated neurons in the dentate gyrus of the hippocampal formation of young (postnatal day 40), mature (postnatal day 80) and aged (postnatal day 540) male Wistar rats has been investigated by immunohistochemical techniques employing a monoclonal antibody specific for neural cell adhesion molecule-linked alpha 2,8 polysialic acid. A strong immunoreactivity was found on the cell bodies, dendrites and axons of granule-like neuronal cells at the border between the hilar region and the granule cell layer of the young rat. In the mature animal the number of immunoreactive neurons declined dramatically and were virtually absent in the aged group. Using an alternative fixation procedure, glial fibrillary acidic protein-positive and polysialylated astroglia processes were found in close proximity to the dendrites of the polysialylated granule-like cells. The number of astroglial processes traversing the granule cell layer showed a similar age-dependent decline to that observed with the polysialylated neurons. Glial fibrillary acidic protein-positive and polysialylated stellate astroglia were present throughout the hippocampal formation, but did not show the marked age-dependent decline observed with the astroglial processes in the granule cell layer. The neuronal dendrites and astroglial processes exhibited a strict numerical ratio in the young and mature animal and, in double immunofluorescence studies with anti-polysialic acid and anti-glial fibrillary acidic protein, the astroglial processes exhibited apparent points of cell and/or dendritic contact. These findings suggest that loss of polysialylated astroglial processes precedes the decline in polysialylated dentate neurons.

Cell and molecular analysis of the developing and adult mouse subventricular zone of the cerebral hemispheres

The subventricular zone (SVZ) of the lateral ventricle remains mitotically active in the adult mammalian central nervous system (CNS). Recent studies have suggested that this region may contain neuronal precursors (neural stem cells) in adult rodents. A variety of neuronal and glial markers as well as three extracellular matrix (ECM) markers were examined with the hope of understanding factors that may affect the growth and migration of neurons from this region throughout development and in the adult. This study has characterized the subventricular zone of late embryonic, postnatal, and adult mice using several neuronal markers [TuJ1, nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), neuron-specific enolase (NSE)], glial markers [RC-2, vimentin, glial fibrillary acidic protein (GFAP), galactocerebroside (Gal-C)], ECM markers [tenascin-C (TN-C), chondroitin sulfate, a chondroitin sulfate proteoglycan termed dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG)], stem-cell marker (nestin), and proliferation-specific marker [bromodeoxyuridine (BrdU)]. TuJ1+ and nestin+ cells (neurons and stem cells, respectively) persist in the region into adulthood, although the numbers of these cells become more sparse as the animal develops, and they appear to be immature compared to the cells in surrounding forebrain structures (e.g., not expressing NSE and having few, if any, processes). Likewise, NADPH-d+ cells are found in and around the SVZ during early postnatal development but become more sparse in the proliferative zone through maturity, and, by adulthood, only a few labeled cells can be found at the border between the SVZ and surrounding forebrain structures (e.g., the striatum), and even smaller numbers of positive cells can be found within the adult SVZ proper. BrdU labeling also seems to decrease significantly after the first postnatal week, but it still persists in the SVZ of adult animals. The disappearance of RC-2+ (radial) glia during postnatal development and the persistence of glial-derived ECM molecules such as tenascin and chondroitin sulfate proteoglycans (as well as other "boundary" molecules) in the adult SVZ may be associated with a persistence of immaturity, cell death, and a lack of cell emigration from the SVZ in the adult.

Effect of polysialic acid on the behavior of retinal ganglion cell axons during growth into the optic tract and tectum

We have demonstrated previously that the polysialic acid (PSA) moiety of the neural cell adhesion molecule (NCAM) can regulate peripheral nerve branching during development. In particular, it was found that specific enzymatic removal of PSA from motor axons causes them to form tight fascicles that are less responsive to normal guidance cues. In the present study, the role of PSA in the behavior of axons in the central nervous system has been examined through an analysis of chick optic axons during development. Unlike peripheral axons, which generally grow in a PSA-free environment, PSA was found to be present both on retinal ganglion cell axons and their environment in the tract and tectum. Furthermore, the enzymatic removal of PSA from the optic axons caused them to defasciculate in the tract/tectal region. This response was morphologically similar to targeting corrections made by these axons at a later stage when PSA levels have decreased, suggesting that the PSA may serve to shield them from responding prematurely to some guidance cues in their target region.

Regeneration in the adult mammalian CNS: guided by development

In the past year, the roles and mechanisms of molecules involved in cell survival (glial-derived neurotrophic growth factor), growth cone guidance (netrins and semaphorins), axonal outgrowth and sorting (neural cadherin, polysialylated neural cell adhesion molecules, and L1), and neuronal connectivity (cell adhesion molecules, dystroglycan, and agrin) have been described during development and, to a limited extent, in the mature CNS. Evidence is now emerging that some developmental events, such as the expression of polysialylated neural cell adhesion molecule and L1, are recapitulated during adult CNS regeneration. These results suggest new avenues to address more accurately the challenges of axonal regrowth in the adult mammalian CNS.

Wiring, dysmorphogenesis and epilepsy: a hypothesis

Cerebral cortical dysgenesis has been found by magnetic resonance imaging to be the second most common pathology underlying medically refractory chronic partial epilepsy. Patients with the latter condition form the largest group in specialist epilepsy clinics. The pathogenesis of the epilepsy in cortical dysgenesis remains largely obscure. The most popular current hypothesis holds neuronal misconnection secondary to neuronal malpositioning culpable for seizure activity. However, a review of the published literature of cortical dysgenesis and an analysis of newer magnetic resonance and histopathological data, suggests that this view is no longer tenable. A modified hypothesis is proposed in which neuronal connectivity itself is postulated to be the primary motive force in both cerebral morphogenesis and epileptogenesis in cases of cortical dysgenesis. This hypothesis leads to the generation of a model for cortical development and directly testable predictions of intercellular connectivity, as well as a potential tool for the prediction of the possibility of freedom from seizure activity after surgical resection of dysgenetic lesions in individual cases.

Cell migration from the olfactory placode and the ontogeny of the neuroendocrine compartments

The olfactory placode and its derivative, the olfactory pit, give rise to several different populations of migrating cells, which contribute to drive the organization of the prosencephalon, but also to form a part of the central neuroendocrine compartments. Some cell types are seemingly transient and can play a role in the establishment of the final connections. The understanding of the mechanisms involved in the migration and differentiation of these cell populations can give an insight on the interplay between peripheral structures and central nervous system and on the mechanisms of commitment, phenotype selection and control for neuroendocrine cells able to selectively "colonize" the brain.

Restricted expression of the irreC-rst protein is required for normal axonal projections of columnar visual neurons

The 104 kDa irreC-rst protein, a member of the immunoglobulin superfamily, mediates homophilic adhesion in cell cultures. In larval optic chiasms, the protein is found on recently formed axon bundles, not on older ones. In developing visual neuropils, it is present in all columnar domains of specific layers. The number of irreC-rst-positive neuropil stratifications increases until the midpupal stage. Immunoreactivity fades thereafter. The functional importance of the restricted expression pattern is demonstrated by the severe projection errors of axons in the first and second optic chiasms in loss of function mutants and in transformants that express the irreC-rst protein globally. Epigenesis of the phenotypes can be explained partially on the bases of homophilic irreC-rst interactions.

Expression cloning of a human polysialyltransferase that forms the polysialylated neural cell adhesion molecule present in embryonic brain

Polysialic acid is a developmentally regulated posttranslational modification of the neural cell adhesion molecule (N-CAM). It has been suggested that this large anionic carbohydrate modulates the adhesive property of N-CAM, but the precise function of polysialic acid is not known. Here we describe the isolation and functional expression of a cDNA encoding a human polysialyltransferase. For this expression cloning, COS-1 cells were cotransfected with a human fetal brain cDNA library and a cDNA encoding human N-CAM. Transfected COS-1 cells were stained with a monoclonal antibody specific for polysialic acid and enriched by fluorescence-activated cell sorting. Sibling selection of recovered plasmids resulted in a cDNA clone that directs the expression of polysialic acid on the cell surface. The deduced amino acid sequence indicates that the polysialyltransferase shares a common sequence motif with other sialyltransferases cloned so far. The polysialyltransferase is, however, distinct by having two clusters of basic amino acids. The amount of the polysialyltransferase transcripts correlates well with the formation of polysialic acid in various human tissues, and is abundant in the fetal brain but not in the adult brain. Moreover, HeLa cells stably expressing polysialic acid and N-CAM promoted neurite outgrowth and sprouting. These results indicate that the cloned polysialyltransferase forms polysialylated, embryonic N-CAM, which is critical for plasticity of neural cells.

Induction of NCAM expression in mouse olfactory keratin-positive basal cells in vitro

The expression of neural cell adhesion molecule (NCAM) by olfactory keratin+ basal cells was investigated in vitro. In primary cultures from embryonic day 14.5 or newborn mouse olfactory epithelium (OE), flat keratin+ basal cells were negative for NCAM-immunostaining. Transforming-growth factor-beta 1, beta 2 (TGF-beta 1, beta 2) or a high concentration of Ca2+ induced these cells to express NCAM. We obtained a cell line, designated DBC1.2, from embryonic day 14.5 mouse OE. All DBC1.2 cells were positive for keratin-immunostaining, whereas nearly all cells were negative for NCAM-immunostaining. Therefore, DBC1.2 cells seemed to derive from dark basal cells. DBC1.2 cells were also induced to express NCAM by the treatment with TGF-beta s or a high Ca2+ concentration. Western blotting revealed that the components of the NCAM expressed by DBC1.2 were 120 and 140 kDa, but not 180 kDa, isoforms. Since NCAM is the cell adhesion molecule mainly expressed in nervous tissues, these results suggest a cell lineage relationship or interactions between olfactory keratin+ basal cells and olfactory sensory neurons.

Platelet-derived growth factor is a survival factor for PSA-NCAM+ oligodendrocyte pre-progenitor cells

Mature oligodendroglia, which synthesize and express lipids and proteins characteristic of myelin, are generated from precursor cells which are formed in germinal matrix, then migrate widely through the neuraxis. We now demonstrate that these precursor cells can be recognized at a very early stage by their surface expression of polysialylated neural cell adhesion molecules (PSA-NCAM), and only later bind anti-ganglioside antibodies that had previously been used to recognize "O-2A" oligodendroglial precursor cells. PSA-NCAM expression by these cells is likely to be of functional significance, since a recent study demonstrated that O-2A cells become immobile when stripped of PSA-NCAM. Platelet-derived growth factor (PDGF) proved to be a survival factor for these PSA-NCAM+cells, and in a defined medium, PDGF was sufficient to ensure maturation of immunopurified PSA-NCAM+cells to oligodendroglia.

A molecular approach to the pathophysiology of the X chromosome-linked Kallmann's syndrome

The human KAL gene is responsible for the X chromosome-linked Kallmann's syndrome, which consists of an association between hypogonadotropic hypogonadism and anosmia (or hyposmia). Additional symptoms are occasionally observed. The olfactory defect is associated with hypoplasia of the olfactory bulbs and tracts. The hypogonadism may be due to a defect in the embryonic migratory process of GnRH-synthesizing neurones from the olfactory pits up to the brain. The human and chicken KAL genes have been isolated. From the amino acid sequences deduced, it has been postulated that the KAL protein is an extracellular matrix component, with putative antiprotease activity and adhesion function. Various point mutations and, in a few cases, deletions of KAL have been detected in patients. By in situ hybridization, KAL expression has been studied during embryonic development in the chick. From embryonic day 2 (ED2) to ED8, the KAL gene is expressed in various endodermal, mesodermal and ectodermal derivatives, whereas the expression from ED8 is almost entirely restricted to definite neuronal populations in the central nervous system, most of which still express the gene after hatching. According to such a spatiotemporal pattern of expression, we suggest that the KAL gene is involved both in morphogenetic events and in late neuronal differentiation and/or neuronal trophicity. With respect to the olfactory system, the KAL gene is expressed in the mitral cells of the olfactory bulbs from ED8 onwards. In contrast, no expression of the KAL gene is detected at any stage in either the embryonic olfactory epithelium or the surrounding nasal mesenchyme. Therefore, assuming that similar conditions are found in the human embryo, we suggest that the olfactory anomaly in X-linked Kallmann's syndrome results from a central target cell defect. Current hypotheses regarding the pathophysiology of the GnRH deficiency are also discussed. In situ hybridization experiments in the human embryo, as well as characterization of the KAL protein, are in progress.

Expression of polysialylated N-CAM in the central nervous system of adult canaries and its possible relation to function

Polysialylated neuronal cell adhesion molecule (PSA-N-CAM) is a cell surface molecule associated with neurons that undergo changes in configuration or spatial translocation. In both cases, this molecule is thought to reduce the adhesivity of these cells or of their processes, which can thereby insinuate themselves into the existing parenchyma. We used a monoclonal antibody specific to PSA to offer what we believe is the first account of the distribution of PSA-N-CAM in the adult songbird brain. This antibody stained a diversity of cell classes and processes, as follows: 1) a subset of ventricular zone cells; 2) migrating cells thought to be neuroblasts; 3) a subset of differentiated neurons; 4) some brain surface astrocytes; 5) some tanycytes; 6) the neuropil of some regions; 7) some axonal fibers; and 8) possibly some synapses. Our results demonstrate also, for the first time, the wide distribution of a very numerous population of migrating cells in the telencephalon and the seasonal regulation of PSA-N-CAM expression in a part of the adult brain known to undergo seasonal changes in cell recruitment and function. However, we did not find PSA-N-CAM associated with young migrating cells in the high vocal center (HVC), nor was there PSA-N-CAM in the robust nucleus of the archistriatum (RA), which is known to receive new axonal endings from HVC. In these instances spatial translocation may occur with the assistance of other surface molecules.

Neuronal stem cells in the brain of adult vertebrates

It is generally assumed that neurogenesis in the central nervous system ceases before or soon after birth. In the last three decades, however, several studies have reported that new neurons continue to be added into the brain of adult fish, frogs, reptiles, birds and mammals. The precursor cells that give rise to the neurons generated in adulthood are generally located in the walls of the brain ventricles. From these proliferative regions, neuronal precursors migrate toward their final targets where they differentiate; they often traverse long distances through complex brain parenchyma. The identity of the neuronal precursors in the brains of adult animals is still unknown. Experiments in adult birds suggest that proliferating radial cells may be the neuronal precursors. In adult mice, cells present in the subventricular zone can generated neurons in vivo and in vitro. These neuronal precursors can be induced to proliferate in vitro when exposed to growth factors and retain their potential to differentiate into neurons and glia. Whether these putative neural stem cells can differentiate into multiple neuronal types remains to be determined. The neuronal precursors of the adult brain could be used as a source of cells for neuronal transplantation. In addition, these cells could be manipulated in vivo or in vitro to introduce genes into the brain. Adult neurogenesis offers new experimental opportunities to study neuronal birth, migration and differentiation and for the treatment of neurological diseases.

Neural cell adhesion molecule (NCAM) as a quantitative marker in synaptic remodeling

The neural cell adhesion molecule (NCAM) participates in adhesion and neuritic outgrowth during nervous system development. In the adult brain, NCAM is considered to be involved in neuronal sprouting and synaptic remodeling. The NCAM concentration of brain tissue has proved to be a useful marker of these processes, especially when viewed in comparison with the concentration of a marker of mature synapses, e.g. D3-protein (SNAP-25) or synaptophysin. The present review focusses on studies of adult brain in which NCAM concentration estimates and NCAM/D3 ratios have been used to evaluate the rate of synaptic remodeling in brain damage and degenerative diseases.

Polysialylation as a regulator of neural plasticity in rodent learning and aging

Although generally accepted to play an important role in development, the precise functional significance of NCAM remains to be elucidated. Correlative and interventive studies suggest a role for polysialylated NCAM in neurite elaboration. In the adult NCAM polysialylation continues to be expressed in regions of the central nervous system which retain neuroplastic potential. During memory formation modulation of polysialylation on the synapse-enriched isoform of NCAM occurs in the hippocampus. The polysialylated neurons of this structure have been located at the border of the granule cell layer and hilar region of the dentate and their number increases dramatically during memory consolidation. The converse is also true for a profound decline in the basal number of polysialylated neurons occurs with ageing when neural plasticity becomes attenuated. In conclusion, it is suggested that NCAM polysialylation regulates ultrastructural plasticity associated with synaptic elaboration.

Posterior extension of the chick nephric (Wolffian) duct: the role of fibronectin and NCAM polysialic acid

The nephric duct of the chick embryo starts to form at about stage 10 of Hamburger and Hamilton ([1951] J. Morphol. 88:49-92) and extends posteriorly, fusing with the cloaca at about the end of the third day of incubation (HH stage 17). Evidence from the literature suggests that the extension involves active migration of the posterior tip. This investigation concerned some molecules that might control this migration: fibronectin, vitronectin, the beta 1 integrin receptor, and NCAM polysialic acid. The concentration of fibronectin in the extracellular matrix was found by immunocytochemistry to be negligible at the posterior end of the duct; treatment of the living embryo with GRGDS failed to halt further extension of the duct; SEM examination of embryos treated with the synthetic peptides of fibronectin GRGDS, GRDGS, SDGR, and GRGES, or with vitronectin, revealed negligible morphological effects on the duct. It is concluded that there is yet no evidence that fibronectin is an important factor in duct migration. NCAM polysialic acid had a similar distribution to fibronectin, but treatment of the living embryo with Endo-N caused cessation of extension of the duct. Endo-N is an enzyme that specifically degrades PSA without affecting the NCAM polypeptide itself. It is suggested therefore that PSA may play an important role in duct extension. The synthetic peptides of fibronectin each produced distinctive patterns of blebbing on the surfaces of cells in trunk mesoderm, but the duct cells were unaffected. GRGES and SDGR caused blebbing on cells in the somites and the anterior segmental plate, though not on cells in the posterior segmental plate. This suggests that integrin receptors change in the anterior segmental plate as the mesoderm forms somites from somitomeres.

Decreased expression of the embryonic form of the neural cell adhesion molecule in schizophrenic brains

The regulated expression of neural cell adhesion molecule (NCAM) isoforms in the brain is critical for many neurodevelopmental processes including neurulation, axonal outgrowth, and the establishment of neuronal connectivity. We have investigated the expression of the major adult isoforms of NCAM (NCAM-180, NCAM-140, and NCAM-120) and its embryonic highly polysialylated isoform (PSA-NCAM) in the hippocampal region of postmortem brains from 10 schizophrenic and 11 control individuals. Immunohistochemical analysis with a monoclonal antibody recognizing the PSA-NCAM revealed immunoreactivity primarily in the dentate gyrus and in a subset of cells in the hilus region. We have observed a 20-95% reduction in the number of hilar PSA-NCAM-immunoreactive cells in the great majority of schizophrenic brains. The change in PSA-NCAM immunoreactivity is not obvious in other hippocampal subfields. Western blots of tissues from the hippocampal region (as well as from the frontal cortex) probed with a polyclonal antibody recognizing all NCAM isoforms did not reveal significant changes in the overall expression of NCAM, suggesting that the decrease in PSA-NCAM-immunoreactive cells may be related to post-translational processing of the molecule. The expression of this embryonic form of NCAM has been proposed to be related to synaptic rearrangement and plasticity. Therefore, the decrease in PSA-NCAM immunoreactivity in schizophrenic hippocampi may suggest an altered plasticity of this structure in a large proportion of schizophrenic brains. These findings may bear significance to the "neurodevelopmental hypothesis" of schizophrenia.

Molecular characterization of eukaryotic polysialyltransferase-1

Polysialic acid (PSA) is a dynamically regulated product of post-translational modification of the neural cell adhesion molecule, NCAM. Presence of the large anionic carbohydrate modulates NCAM binding properties and, by increasing the intercellular space, influences interactions between other cell surface molecules. PSA expression underlies cell type- and developmental-specific alterations and correlates with stages of cellular motility. In the adult, PSA becomes restricted to regions of permanent neural plasticity and regenerating neural and muscle tissues. Recent data implicate its important function in spatial learning and memory, and in tumour biology. Here we describe the molecular characterization of polysialyltransferase-1, the key enzyme of eukaryotic PSA synthesis. In reconstitution experiments, the newly cloned enzyme induces PSA synthesis in all NCAM-expressing cell lines. Our data therefore represent convincing evidence that the polycondensation of alpha-2,8-linked sialic acids in mammals is the result of a single enzymatic activity and provide a new basis for studying the functional role of PSA in neuro- and tumour biology.

Mutations disrupting neuronal connectivity in the Drosophila visual system

The photoreceptor neurons (R cells) of the Drosophila compound eye elaborate a precise array of neuronal connections in the brain. These projections exhibit target specificity and create topographic maps (retinotopy). We have screened histologically for mutations disrupting R cell connectivity in developing tissue. Eighty mutations were isolated from over 6000 ethylmethane sulfonate-mutagenized lines. Characterization of these mutations included genetic mosaic analysis to determine whether the gene is required in the retina or in the optic ganglia. Most mutations were found to affect connectivity indirectly by disrupting development more generally in the eye or brain. Genes were identified as candidates for playing direct roles in R cell connectivity by affecting axonal outgrowth (eddy), target recognition (limbo and nonstop), and retinotopy (limbo).

Glycoproteins and memory formation

One-trial passive avoidance training in day-old chicks results in a biochemical cascade occurring in two forebrain regions, the intermediate medial hyperstriatum ventrale and the lobus parolfactorius. This cascade, initiated by synaptic transients, results in the activation of immediate early genes and culminates in the de novo synthesis of a family of pre- and post-synaptic membrane glycoproteins, that, inserted into the membrane, serve in the remodelling of synaptic connectivity which is a requirement for the brain representations constituting long-term memory. There are two waves of glycoprotein synthesis consequent on training, the first occurring within an hour of the training experience and the second 5.5-8 h post-training. Blocking synthesis during these time windows results in amnesia for the task. Amongst the glycoproteins involved are two cell adhesion molecules, NCAM and L1. Injection of antibodies to L1 result in amnesia if injected during either time window, but not outside these times; antibodies to NCAM result in amnesia only if injected at the 5.5-h timepoint. I interpret these results as indicating that de novo synthesis of NCAM during the second time window is necessary for producing a persistent memory trace.

Embryonic (PSA) N-CAM reveals chains of migrating neuroblasts between the lateral ventricle and the olfactory bulb of adult mice

In the brain of adult mice, cell division persists in the subventricular zone (SVZ) of the lateral ventricles. These SVZ cells migrate rostrally 3-5 mm to the olfactory bulb, where they differentiate into neurons. We have investigated the distribution of PSA-N-CAM in the adult mouse forebrain. Immunoreactivity for PSA-N-CAM precisely reveals the migratory pathway of SVZ cells. This pathway of PSA-N-CAM positive cells starts in the lateral wall of the lateral ventricle, where immunopositive cells form weblike patterns. The PSA-N-CAM positive pathway extends rostrally between the corpus callosum and the striatum into the anterior ventral telencephalon, and then into the core of the olfactory bulb. Experiments in which [3H]-thymidine was injected systemically indicated that the majority of the dividing cells on the SVZ of the lateral ventricle and along the migratory pathway are positive to PSA-N-CAM or closely associated with PSA-N-CAM. Microinjection of [3H]-thymidine into the SVZ of the lateral ventricle to label a small patch of dividing SVZ cells shows that neuroblasts that migrated away from the injection site are positive or are closely associated with other cells that are positive for PSA-N-CAM. Migrating cells are tethered together, forming long chains of immunopositive cells. The migratory pathway is formed by 30-40 of these immunopositive chains. Radially oriented individual PSA-N-CAM positive cells were observed in the olfactory bulb. These cells seem to have broken away from chains of immunopositive cells in the core of the olfactory bulb and to be migrating to more superficial layers. Little is known about the mechanisms of tangential migration during development and in adulthood. The cell-cell arrangement revealed by PSA-N-CAM staining suggests new models for this form of neuronal migration. PSA-N-CAM localization along the migratory pathway to the olfactory bulb suggests that in the adult brain this molecule plays a role in migration of neuronal precursors.