Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.     

Cellular localization and signaling activity of beta-catenin in migrating neural crest cells.

In the vertebrate embryo, development of the neural crest is accompanied by sequential changes in cellular adhesiveness, allowing cells to delaminate from the neural epithelium, to undergo migration through extracellular matrix material, and to coalesce into ganglia of the peripheral nervous system. Because of its dual role in cell adhesion, as a link between cadherins and the actin cytoskeleton, and in cell signaling, as a key mediator of the Wnt-signaling pathway, beta-catenin is a good candidate to play a central role in the control of neural crest cell development. In the present study, we analyzed, by using an in vitro culture system, whether the cellular localization and the signaling activity of beta-catenin are regulated in conjunction with cell migration during ontogeny of trunk neural crest cells in the avian embryo. beta-Catenin molecules were found primarily in association with N-cadherin in the regions of intercellular contacts in most migrating neural crest cells, and only early-migrating cells situated in proximity with the dorsal side of the neural tube showed detectable beta-catenin in their nuclei. This finding indicates that beta-catenin may be recruited for signaling in neural crest cells only transiently at the onset of migration and that sustained beta-catenin signals are not necessary for the progression of migration. The nuclear distribution of beta-catenin within crest cells was not affected upon modification of the N-cadherin-mediated cell-cell contacts, revealing that recruitment of beta-catenin for signaling is not driven by changes in intercellular cohesion during migration. Overstimulation of beta-catenin signals in neural crest cells at the time of their migration, using LiCl treatment or coculture with Wnt-1-producing cells, induced nuclear translocation of beta-catenin and Lef-1 up-regulation in neural crest cells and provoked a marked inhibition of cell delamination and migration. The effect of LiCl and exogenous Wnt-1 on neural crest cells could be essentially attributed to a dramatic decrease in integrin-mediated cell-matrix adhesion as well as a massive reduction of cell proliferation. In addition, although it apparently did not affect expression of neural crest markers, Wnt-1 exposure dramatically affected signaling events involving Notch-Delta, presumably also accounting for the strong reduction in cell delamination. In conclusion, our data indicate that beta-catenin functions primarily in cell adhesion events during migration and may be recruited transiently for signaling during delamination possibly to regulate the balance between cell proliferation and cell differentiation.     

Turn-off, drop-out: functional state switching of cadherins.

The classic cadherins are a group of calcium dependent, homophilic cell-cell adhesion molecules that drive morphogenetic rearrangements and maintain the integrity of cell groups through the formation of adherens junctions. The formation and maintenance of cadherin-mediated adhesions is a multistep process and mechanisms have evolved to regulate each step. This suggests that functional state switching plays an important role in development. Among the many challenges ahead is to determine the developmental role that functional state switching plays in tissue morphogenesis and to define the roles of each of the several regulatory interactions that participate in switching. One correlate of the loss of cadherin-mediated adhesion, the "turn-off" of cadherin function, is the exit, or "drop-out" of cells from neural and epithelial layers and their conversion to a motile phenotype. We suggest that epithelial mesenchymal conversions may be initiated by signaling pathways that result in the loss of cadherin function. Tyrosine phosphorylation of beta-catenin is one such mechanism. Enhanced phosphorylation of tyrosine residues on beta-catenin is almost invariably associated with loss of the cadherin-actin connection concomitant with loss of adhesive function. There are several tyrosine kinases and phosphatases that have been shown to have the potential to alter the phosphorylation state of beta-catenin and thus the function of cadherins. Our laboratory has focused on the role of the nonreceptor tyrosine phosphatase PTP1B in regulating the phosphorylation of beta-catenin on tyrosine residues. Our data suggest that PTP1B is crucial for maintenance of N-cadherin-mediated adhesions in embryonic neural retina cells. By using an L-cell model system constitutively expressing N-cadherin, we have worked out many of the molecular interactions essential for this regulatory interaction. Extracellular cues that bias this critical regulatory interaction toward increased phosphorylation of beta-catenin may be a critical component of many developmental events.     

Significance of the cell adhesion molecules and sialic acid in neurodegeneration

The central nervous system (CNS) is a complex and precise mechanism that controls the most highest functions of the body. All of them depend on the cellular and molecular interactions called by neurobiologists "cellular plasticity". The CNS is a flexible structure but its regeneration after damage is strongly limited. Better understanding of cellular and molecular basis of brain repair can open new way in the development of therapeutic tools for neurodegeneration. Among many molecules that participate in the formation of neuronal networks, neural cell adhesion molecule (NCAM) and its sialylated derivative seem to play crucial role in the life of brain. In particular, polysialylated cell adhesion molecule (PSA-NCAM) is proposed to participate in the neuroprotective response in neurodegeneration by reducing of AMPA/NMDA receptors sensitivity to glutamate and facilitating disconnection of cell-cell interactions. These mechanisms protect from excitotoxic damage and promote dendritic/spine re-growth. This review briefly focuses on the expression and role of PSA-NCAM in neurodegenerative diseases and its potential application in therapy.     

Immunoglobulin superfamily molecules in the nervous system.

Among the various types of membrane molecules involved in cell-cell interactions in the nervous system, we have focused in this review upon membrane proteins belonging to the immunoglobulin superfamily (IgSF). IgSF molecules are distinctive in that: (1) a large percentage of known neural adhesion molecules belongs to the IgSF; (2) they are homologous in structure (Ig domain), yet exhibit large variation of function in cell-cell interactions. The structure of IgSF molecules is briefly summarized in Section II, and each member of the IgSF which has been found in the nervous system is reviewed in Section III. In Section IV, we have discussed possible properties of yet-unknown nervous system IgSF molecules, on the assumption that nervous system IgSF molecules thus far discovered comprise only a small portion of those existing. Discussion is based upon an analogy with the immune system and upon knowledge of cell-cell interactions in the development of the nervous system. Our principal aims in this review are to summarize knowledge of neural IgSF molecules and to discuss the possibility that some IgSF molecules may encode in their structures instructions for recognizing, or for being recognized by, target neural cells. Further growth of knowledge of IgSF molecules may yield insights into the patterns of cell-cell interactions underlying the formation of neuronal circuits during development.     

Integrins,cadherins, and catenins: molecular cross-talk in cancer cells

In the past decade, there have been major advances in the understanding of some of the mechanisms underlying tumour differentiation, invasion, and metastasis, in which cell–cell and cell–matrix adhesion molecules play a critical role. Cadherin/catenin complex and the integrins are the prime mediators of cell adhesion in normal and transformed cells, cadherin/catenin being largely responsible for intercellular adhesion and integrins for cell–extracellular matrix interactions. Intercellular and cell–matrix adhesion mediated by cadherin/catenin and integrins is likely to play a role in the control of both structural morphology and functional differentiation; hence, any loss of this control mechanism may well facilitate the neoplastic process. Indeed, in cancer cells, there is a co-ordinated down-regulation of both integrins and cadherins which correlates with tumour dedifferentiation. However, the expression and cellular localization of catenins do not always correlate with cadherin expression, since the catenins are rather promiscuous molecules which interact not only with E-cadherin, but also with growth regulatory and signalling molecules such as epidermal growth factor receptor and the adenomatous polyposis coli gene product. © 1998 John Wiley & Sons, Ltd.     

Cell adhesion receptors - signaling capacity and exploitation by bacterial pathogens

Cell adhesion receptors play an essential role in multicellular organisms by mediating the direct association of cells with each other and with proteins of the extracellular matrix. Members of different protein families such as integrins, cadherins, immunoglobulin superfamily cell adhesion molecules (IgCAMs), selectins, and syndecans not only support the structural integrity of cells and tissues, but also contribute to the transduction of signals. Interestingly, several of these molecules are exploited by bacterial pathogens to establish tight contact with eukaryotic cells. Using the example of integrins, cadherins, and IgCAMs, this review illustrates the signaling capacity of cell adhesion receptors and highlights a number of bacterial adhesins that are known to engage these receptors. Where applicable, the role of the receptor-adhesin interaction in the course of the infection is discussed.     

Cadherins in the central nervous system

The central nervous system (CNS) is divided into diverse embryological and functional compartments. The early embryonic CNS consists of a series of transverse subdivisions (neuromeres) and longitudinal domains. These embryonic subdivisions represent histogenetic fields in which neurons are born and aggregate in distinct cell groups (brain nuclei and layers). Different subsets of these aggregates become selectively connected by nerve fiber tracts and, finally, by synapses, thus forming the neural circuits of the functional systems in the CNS. Recent work has shown that 30 or more members of the cadherin family of morphoregulatory molecules are differentially expressed in the developing and mature brain at almost all stages of development. In a regionally specific fashion, most cadherins studied to date are expressed by the embryonic subdivisions of the early embryonic brain, by developing brain nuclei, cortical layers and regions, and by fiber tracts, neural circuits and synapses. Each cadherin shows a unique expression pattern that is distinct from that of other cadherins. Experimental evidence suggests that cadherins contribute to CNS regionalization, morphogenesis and fiber tract formation, possibly by conferring preferentially homotypic adhesiveness (or other types of interactions) between the diverse structural elements of the CNS. Cadherin-mediated adhesive specificity may thus provide a molecular code for early embryonic CNS regionalization as well as for the development and maintenance of functional structures in the CNS, from embryonic subdivisions to brain nuclei, cortical layers and neural circuits, down to the level of individual synapses.     

Induction and patterning of the neural plate

The development of the vertebrate nervous system begins when ectoderm on the dorsal surface of the embryo is induced by the organizer to form the neural plate. During neural induction, the organizer appears to specify a region of ectoderm that forms neural tissue, to induce morphogenesis within a region of ectoderm called the notoplate, and to pattern the neural plate to form different parts of the nervous system along the dorsal-ventral (D- V) and anterior-posterior axis (A-P). Recent studies have shown that all of these events can proceed at least partially via signals that pass from the organizer into ectoderm by so-called planar induction. Recent studies have also addressed the molecular signals that potential underlie the generation of the neural A-P axis. Although there is good evidence that polypeptide growth factors can induce different mesodermal cell types, it is not clear as yet whether these molecules provide A-P positional information for dorsal tissues, including the nervous system. Retinoic acid has also been shown to have the properties of a patterning signal and may act via homeobox genes, but the role of endogenous retinoic acid has not been established. In addition, we discuss the induction of the most anterior end of the neural plate, which is perhaps the least understood aspect of neural induction. With the recent isolation of vertebrate forebrain specific markers, the induction of this part of the nervous system can be explored in future experiments.     

Effects of chronic stress on contextual fear conditioning and the hippocampal expression of the neural cell adhesion molecule, its polysialylation, and L1

Chronic stress has been shown to induce time-dependent neurodegeneration in the hippocampus, ranging from a reversible damage to a permanent neuronal loss. This damage has been proposed to impair cognitive function in hippocampus-dependent learning tasks. In this study, we have used a 21-day restraint stress procedure in rats, previously reported to induce reversible atrophy of apical dendrites of CA3 pyramidal cells, to assess whether it may influence subsequent performance in the contextual fear conditioning task under experimental conditions involving high stress levels (1 mA shock intensity as the unconditioned stimulus). In addition, we were interested in the study of the possible cellular and molecular mechanisms involved in the reversible phase of neural damage. Cell adhesion molecules of the immunoglobulin superfamily, such as the neural cell adhesion molecule and L1, are cell-surface macromolecules that, through their recognition and adhesion properties, regulate cell-cell interactions and have been reported to play a key role in cognitive functioning. A second aim of this study was to evaluate whether chronic stress would modulate the expression of the neural cell adhesion molecule, its polysialylation, and L1 in the hippocampus. The results showed that chronic stress facilitated subsequent contextual fear conditioning. They also showed that chronically stressed rats displayed reduced hippocampal neural cell adhesion molecule, but increased polysialylated expression as well as a trend towards exhibiting increased L1 expression.In summary, these results support the view that a 21-day chronic stress regimen predisposes individuals to develop enhanced contextual fear conditioning responses. They also indicate that cell adhesion molecules might play a role in the structural remodelling that occurs in the hippocampus as a consequence of chronic stress exposure.     

Expression of cadherin adhesion molecules on human gametes

The presence of cadherins, Ca(2+)-dependent cell-cell adhesion molecules which may be involved in gamete interaction, was investigated in human gametes. Expression of cadherin molecules was demonstrated using an anti-pan-cadherin antibody and specific antibodies against the three classical cadherins: E- (epithelial), P- (placental) and N- (neural) cadherins. Samples of 48 h old unfertilized oocytes and spermatozoa from in-vitro fertilizing semen samples were lysed and separated by electrophoresis. Localization of cadherins was determined on intact, fixed, permeabilized spermatozoa and oocytes by immunocytochemisty assessed by confocal microscopy. Immunoblotting with the pan-cadherin antibody revealed a single band of approximately 120 kDa in spermatozoa (whether 'fresh', capacitated, or frozen-thawed) and oocyte extracts. Oocytes presented all three classical cadherins with the appropriate molecular weights of 120-130 kDa. In sperm lysate we demonstrated the presence of E-cadherin but not N-cadherin. The anti-P antibody detected a 90 kDa peptide as the only immunoreactive antigen. Following immunocytochemistry of human oocytes all cadherin molecules were allocated predominantly to the plasma membrane with only traces in the cytoplasm. In spermatozoa, several staining patterns were observed with each of the pan-cadherin, N-cadherin and E-cadherin antibodies mostly confined to different head regions. We conclude that cadherin molecules are present on plasma membranes of both human spermatozoa and oocytes and may play a role in the intricate recognition process preceding gamete fusion.     

Identification of a nonchordate-type classic cadherin in vertebrates: chicken Hz-cadherin is expressed in horizontal cells of the neural retina and contains a nonchordate-specific domain complex.

Classic cadherins mediate calcium-dependent cell-cell adhesion in a variety of animals, but there are marked differences in their domain structures between chordate and nonchordate animals. The extracellular domain of chordate-type classic cadherins (type I and type II classic cadherins) consists of five tandem repeats of conserved sequences called EC domains, whereas that of nonchordate-type classic cadherins (designated as type III classic cadherin) contains a variable number of EC domains, followed by a characteristic domain complex made of laminin-A globular domains and EGF-like repeats. In the present study, we identified a novel vertebrate type III cadherin showing high sequence similarity to Drosophila N-cadherin, and named this molecule chicken Hz-cadherin (cHz-cadherin), because of the distinct expression in horizontal cells of the neural retina. cHz-cadherin functioned as an adhesion molecule when introduced into cultured cells. Database search revealed one cHz-cadherin homologue in zebrafish and two in puffer fish, but none in other vertebrate species examined. These observations indicate that type III classic cadherins have been conserved in vertebrate species, being expressed by limited cells types, but lost in particular phylogenic groups of the vertebrates.     

Adhesion and mobility of embryonic and tumoral cells]

Regulation of a number of adhesion molecules during neural crest cell migration was studied. The neural crest, a transient embryonic neural epithelium structure, undergoes mesenchymal transformation (epithelial-mesenchymal transition). The cells then migrate, giving rise to a variety of elements including the peripheral nervous system and melanocytes. During migration, neural crest cells do not express functional cell Adhesion Molecules but interact specifically with cell-binding domains in fibronectin molecules. A rat bladder carcinoma cell line was used as an in vitro model to study conversion of epithelial cells to a migratory fibroblast-like state. Conversion can be induced by culture on collagen or exposure to acidic Fibroblast Growth Factor (aFGF). Furthermore, constitutive fibroblast-like transformation can be induced by transfection with cDNA encoding aFGF. Growth factor-producing clones exhibit increased invasive and metastatic properties as compared with non-FGF-producing control cells. This model may provide increased understanding of the role of the different adhesion molecules in processes involving cell remodeling, such as tumor spread and development of metastases.     

Programmed Cell Death in the Developing Nervous System

Virtually all cell populations in the vertebrate nervous system undergo massive "naturally-occurring" or "programmed" cell death (PCD) early in development. Initially neurons and glia are overproduced followed by the demise of approximately one-half of the original cell population. In this review we highlight current hypotheses regarding how large-scale PCD contributes to the construction of the developing nervous system. More germane to the theme of this symposium, we emphasize that the survival of cells during PCD depends critically on their ability to access "trophic" molecular signals derived primarily from interactions with other cells. Here we review the cell-cell interactions and molecular mechanisms that control neuronal and glial cell survival during PCD, and how the inability of such signals to suppress PCD may contribute to cell death in some diseases such as spinal muscular atrophy. Finally, by using neurotrophic factors (e.g. CNTF, GDNF) and genes that control the cell death cascade (e.g. Bcl-2) as examples, we underscore the importance of studying the mechanisms that control neuronal and glial cell survival during normal development as a means of identifying molecules that prevent pathology-induced cell death. Ultimately this line of investigation could reveal effective strategies for arresting neuronal and glial cell death induced by injury, disease, and/or aging in humans.     

cadherin-6 message expression in the nervous system of developing zebrafish.

Cadherins are cell surface adhesion molecules that play important roles in development of a variety of tissues including the nervous system. In this study, we analyzed expression pattern of cadherin-6, a member of the type II cadherin subfamily, in the embryonic zebrafish nervous system using in situ hybridization methods. cadherin-6 message is first expressed by the neural keel, then by restricted regions in the brain and spinal cord. cadherin-6 expression in the brain transiently delineates specific brain regions. In the peripheral nervous system, cadherin-6 message is expressed by the neurogenic placodes and the dorsal root ganglia. As development proceeds, cadherin-6 expression domain and/or expression levels increased in the embryonic nervous system. Our results show that cadherin-6 expression in the zebrafish developing nervous system is both spatially and temporally regulated, implicating a role for cadherin-6 in the formation of these nervous structures.     

A temporospatial map of adhesive molecules in the organ of Corti of the mouse cochlea

In the nervous system, several classes of cell-surface and extracellular matrix molecules have been implicated in processes such as neural growth, fasciculation, pathfinding, target recognition and synaptogenesis, which require cell-to-cell or cell-to-substrate binding. In the developing mouse cochlea, little is known about the types of cell-surface and extracellular matrix molecules existing along the neural growth paths or their possible roles in development. Whole mount and sectioned cochlear tissue were immunolabeled for six different adhesive molecules - neural cell adhesion molecule (NCAM), polysialic acid (PSA), neural cell adhesion molecule L1, E-cadherin, syndecan-1 and tenascin-C. A temporospatial map of adhesive molecule distribution in the basal turns of the mouse cochlea was generated. Distributions of adhesive molecules were compared to each other and to the known progress of neural development in the region. This comparison demonstrated differences in the complements of adhesive molecules between the inner and outer hair cell regions, and variations in the expressions of adhesion molecules among different types of nerve fibers. In addition, developmental changes in the adhesive environment around and beneath the outer hair cells coincided with the known timing of the appearance of morphologically defined efferent synapses. These observations raise the possibility that molecular differences at the cell surface of inner and outer hair cells are one way that ingrowing neurites distinguish different environments to determine their growth routes and synaptic partners in the cochlea. In addition these observations demonstrate the potential for differential signaling of afferent and efferent innervation by altering the microenvironments in which synapses are formed.     

Central role of the alpha4beta1 integrin in the coordination of avian truncal neural crest cell adhesion, migration, and survival.

Based on functional and histological studies, the fibronectin receptor of the integrin family alpha4beta1 has been ascribed a critical role during neural crest cell migration in the vertebrate embryo. In the present study, because integrins have been shown to participate in multiple basic cellular processes, including cell adhesion, migration, survival, proliferation, and differentiation, we have reexamined in detail the role of alpha4beta1 during avian truncal neural crest cell migration. RT-PCR and immunocytochemical studies revealed that migrating neural crest cells but not premigratory cells explanted in vitro expressed detectable levels of alpha4 messengers and proteins suggesting that alpha4beta1 expression was induced at the time of the initiation of the migration phase. In agreement with this observation, antibody inhibition of alpha4beta1 activity in vitro resulted in a strong, immediate and sustained reduction of neural crest cell motion on fibronectin, as judged on videomicroscopy analyses, but apparently did not prevent their delamination from the neural tube. However, alpha4beta1 appeared to exhibit a broader role in the control of cell migration on a variety of extracellular matrix molecules, presumably by regulating cellular events downstream from integrins. Moreover, blocking alpha4beta1 function caused a severe increase in apoptotic cell death among the neural crest population without influencing notably cell proliferation. Collectively, these results indicate that, notwithstanding its critical implication in cell motion, alpha4beta1 integrin could play a central role in neural crest cell development by coordinating multiple cellular events, such as cell adhesion, locomotion, and survival.     

Developmental Origin of Neural Stem Cells: The Glial Cell That Could

Since their identification, there has been tremendous interest in adult neural stem cells, in part based upon their potential therapeutic uses in understanding and treating neurological disorders. But what's the origin of these cells in the embryo? We outline here the onset of neural specification in the vertebrate embryo and describe the molecular mechanisms regulating patterning of the central nervous system (CNS). We trace the lineage of the multipotential stem cell of the nervous system from embryonic neuroepithelial cell to adult astrocyte-like neural stem cell. As these stem cells emerge throughout development and in the adult, they appear to be predetermined to a specific neuronal or glial fate. Finally, we compare the properties of embryonic stem cell-derived neural stem cells and CNS-derived neural stem cells.     

Cadherins in development: cell adhesion, sorting, and tissue morphogenesis

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell-cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell-cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.