Cadherin cell adhesion molecules in vertebrate neural development

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Heterogeneity of cadherin-8 expression in the neonatal rat striatum: comparison with striatal compartments

Mechanisms of organization of the striatal compartments are poorly understood, although involvement of cell adhesion molecules in the compartmentalization has been suggested. Cadherin-8 distribution in the neonatal rat striatum was immunohistochemically studied using a rabbit anti-cadherin-8 antiserum. Intensity of cadherin-8 immunolabeling in the striatum was heterogeneous from postnatal day 0 to postnatal day 7. At postnatal day 9, cadherin-8 immunoreactivity was so weak that heterogeneity was no longer clearly seen. Cadherin-8 immunoreactivity was not detectable at postnatal day 14. Cadherin-8-rich and cadherin-8-poor areas were identical to calbindin-rich areas and tyrosine hydroxylase-rich patches, respectively, in allocation, indicating that cadherin-8 was predominantly expressed in the striatal matrix. These results suggest that cadherin-8 is involved in formation of the striatal compartmentalized structures during brain development.

Cadherin-8 protein expression in gray matter structures and nerve fibers of the neonatal and adult mouse brain

The topological distribution of mouse cadherin-8 protein in the neonatal and adult mouse brain was studied immunohistochemically using a rabbit antiserum. Cadherin-8 expression was restricted to several areas in neonatal brains constituting particular neural circuits, i.e. the limbic system, the basal ganglia-thalamocortical circuit, and the cerebellum and related nuclei. In addition, the nerve fibers linking some of the cadherin-8-positive areas, i.e. the habenulo-interpeduncular tract, decussation of the dorsal tegmentum, the medial longitudinal fasciculus, transverse pontine fibers, the brachium conjunctivum and the inferior cerebellar peduncle were cadherin-8 positive, as were the spinal tract of the trigeminal nerve, oculomotor nerve, facial nerve and trigeminal nerve. Cadherin-8 expression also showed a patch-like distribution in the intermediate gray layer of the superior colliculus, resembling acetylcholinesterase-rich patches in allocation. Segmentally organized cadherin-8-positive areas were found in the neonatal cerebellar Purkinje cell layer. Some nuclei and fibers in the brainstem and cerebellum, expressing cadherin-8 at neonatal stages, were also stained in the adult mouse brain. These findings suggest that cadherin-8 is involved in the formation of particular neural circuits by connecting areas expressing this molecule with positive nerve fibers, and indicate its possible implication in subdivisional organization in the superior colliculus and cerebellum.

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

Restricted expression of R-cadherin by brain nuclei and neural circuits of the developing chicken brain

Cadherins are a family of Ca(2+)-dependent cell-cell adhesion molecules regulating morphogenesis by a preferentially homophilic binding mechanism. We have previously shown that the expression of R-cadherin in the early chicken forebrain (embryonic days E3-E6) is restricted to particular neuromeres or parts of neuromeres. R-cadherin-expressing neuroblasts born in these areas accumulate in the mantle zone and aggregate in particular (pro-) nuclei (Gänzler and Redies [1995] J. Neurosci. 15:4157-4172). In the present study, these findings are extended to later developmental stages (embryonic days E8, E11, and E15). By immunohistochemical and in situ hybridization techniques, we show that, at these stages of development, R-cadherin expression remains restricted to particular developing gray matter regions and fiber tracts. The R-cadherin-positive fiber tracts connect some of the R-cadherin-positive gray matter areas to form parts of particular neural circuits in the visual, auditory, somatosensory, and motor systems. Moreover, R-cadherin expression reflects the morphologic differentiation of gray matter regions. As brain nuclei become morphologically more distinct, the expression of R-cadherin shows a clearer demarcation of the nuclear boundaries. In addition, R-cadherin expression in some nuclei becomes restricted to particular subregions or to clusters of neurons. In the cerebellum, R-cadherin is expressed in parasagittal stripes. These results suggest that R-cadherin expression reflects the functional and morphologic maturation of gray matter structures and of information processing circuits in the embryonic chicken brain.

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.

Changes in the concentrations of extracellular Mg++ and Ca++ down-regulate E-cadherin and up-regulate alpha 2 beta 1 integrin function, activating keratinocyte migration on type I collagen

We have demonstrated recently that shifts in the concentrations of extracellular Mg++ and Ca++ occur during cutaneous injury in vivo. These shifts correlate well with the timing of migration of various cell types involved in wound healing, including keratinocytes. In the present study, we examined the potential of such cation shifts to activate the keratinocyte migratory phenotype. In modified Boyden chamber migration assays, alpha 2 beta 1 integrin-mediated migration of human keratinocytes (HaCaT) on type I collagen was supported by Mg++ but not by Ca++ alone. Migration could be increased up to twofold, however, by using both cations in combination, as long as the Mg++ concentration was in the optimal range for migration in Mg++ only (1-3 mM) and Ca++ was present at concentrations of approximately 0.1-1 mM. Further examination of this divalent-cation-induced migratory keratinocyte phenotype demonstrated that, as Mg++ is elevated and Ca++ is reduced, mature E-cadherin and cell-cell contacts are reduced and the alpha 2 beta 1 integrin is redistributed from cell-cell contacts to the periphery. These in vitro observations corroborate what occurs in vivo at the keratinocyte migrating front during wound healing. Together these data suggest that changes in the concentrations of extracellular Mg++ and Ca++ can regulate the competitive interplay between Ca(++)-dependent E-cadherin-mediated and Mg(++)-dependent alpha 2 beta 1-integrin-mediated adhesion, promoting the development of an activated keratinocyte phenotype.

Cadherins, steroids and cancer

The cadherins are a family of calcium-dependent cell adhesion molecules which are thought to be key regulators of morphogenesis. This review contains a discussion of the structure, function and regulation of these cell adhesion molecules. In particular, we discuss recent studies that demonstrate the ability of steroids to modulate cadherin levelsin vivo. We speculate that steroids and estrogenic organochlorines exert their diverse morphoregulatory actions on tissues by altering cadherin levels.

Cadherins and catenins: interactions and functions in embryonic development

During the past year, the family of cadherin cell-adhesion molecules has increased in number and diversity. Recent studies have also emphasized how cadherin activity can be regulated by the dynamic association with intracellular components, the catenins, and with extracellular molecules that are linked to different cell-signaling pathways. Finally, the initial steps have been taken towards identifying the function of cadherins in vivo, including their potential roles in early embryonic development.

Purification and characterization of NCAD90, a soluble endogenous form of N-cadherin, which is generated by proteolysis during retinal development and retains adhesive and neurite-promoting function

The cadherins are calcium-dependent cell adhesion molecules which regulate cell-cell interactions during morphogenesis. During development, cadherin expression is subject to dynamic patterns of regulation. We have previously demonstrated that expression of N-cadherin, the predominant cadherin of neural tissues, is sharply down-regulated during development of the retina and brain during later stages of histogenesis (Lagunowich and Grunwald, Dev Biol 135:158-171, 1989; Lagunowich et al., J Neurosci Res 32:202-208, 1992), and that this down-regulation is due to multiple factors, including decreased mRNA levels and turnover apparently mediated by endogenous metalloproteolytic activity (Roark et al., Development 114:973-984, 1992). In the present study, we describe metabolic studies which provide direct biochemical evidence for turnover of 130-kDa N-cadherin in embryonic retina tissues, yielding a soluble 90-kDa N-terminal fragment. We demonstrate that this form of N-cadherin, which we refer to as NCAD90, accumulates in vivo during development. We further demonstrate that purified NCAD90, obtained from embryonic vitreous humor, retains biological function and promotes cell adhesion and neurite growth in a dose-dependent fashion among chick embryo neural retina cells when present in a substrate-bound form. The morphology of retinal cells and neurites grown on a substrate of NCAD90 differs strikingly from that seen on a laminin substrate, in a manner similar to that described for intact 130-kDa N-cadherin. We conclude that proteolysis of N-cadherin at the cell surface during embryonic retinal histogenesis is an endogenous mechanism for regulating N-cadherin expression which generates a novel and functional form of the protein. The results further indicate that an intact cytoplasmic domain is not essential for all cadherin functions.

Differential expression of N- and R-cadherin in functional neuronal systems and other structures of the developing chicken brain

Cadherins are a family of cell surface molecules mediating calcium-dependent cell-cell adhesion in a variety of tissues. More than a dozen cadherins are expressed in the vertebrate brain. To obtain insight into the biological significance of this diversity in cadherin expression, we mapped the expression of N- and R-cadherin in the brain of the developing chicken embryo (days 2-19 of incubation) by immunohistochemical and in situ hybridization techniques. Whereas the expression of N- and R-cadherin is relatively uniform or weak in early (about 2-5 days of incubation) and late development (15 days of incubation to hatching stage), these two molecules are differentially expressed in specific nuclei and fiber tracts between days 6-11 of incubation. For example, in the mes- and diencephalon, one of the tectofugal pathways and its target nuclei, here called the tecto-pretecto-rotundal system, express N-cadherin. R-cadherin is expressed by a different tectofugal system, the tectoisthmic pathway. The other tectofugal systems express neither N- nor R-cadherin. In addition, a small number of other mes- and diencephalic nuclei express N- or R-cadherin. On the basis of these results and experimental evidence from other studies, we speculate that the two cadherins are involved in the formation and segregation of particular functional systems within the vertebrate central nervous system (CNS) by regulating the formation of nuclei, and the pathfinding and/or the selective fasciculation of neurites. Apart from neuronal elements, a variety of vascular and ependymal structures also express N-cadherin or R-cadherin, e.g., the parenchymal blood vessels, the choroid plexus, the floor and roof plates, and the ventricular lining. These findings suggest that the two cadherins play a variety of roles during the development of neuronal and nonneuronal epithelial structures throughout CNS development.

N- and R-cadherin expression in the optic nerve of the chicken embryo

Cadherins are a family of molecules mediating Ca(2+)-dependent cell-cell adhesion in various tissues. N- and R-cadherin are expressed in the chick embryonic CNS and differ in their expression pattern during development. Here we focus on the differential expression of N- and R-cadherin in the early optic nerve. N-cadherin is expressed by the retinal neurites growing through the optic nerve. R-cadherin is expressed by the early optic nerve glia, which derives from the optic stalk neuroepithelium and corresponds to an immature form of the type-1 astrocyte described in rat optic nerve. The close contact between the plasma membranes of the retinal neurites and the optic nerve glia is believed to be important in guiding retinal axons through the optic nerve. Using neuroblastoma cell lines transfected with R-cadherin, we demonstrate that the N-cadherin-positive retinal axons can use R-cadherin as a substrate for axon elongation. These results suggest that the R-cadherin expressed by the early optic nerve glia might provide a molecular substrate for the growth of N-cadherin-positive retinal axons through the optic nerve.

Expression of N-cadherin mRNA during development of the mouse brain

The expression of N-cadherin mRNA was mapped in the brain of mice between embryonic day 12 (E12) and the adult stage by in situ hybridization of digoxigenin-labeled riboprobe. Two phases of N-cadherin expression can be distinguished. During the first phase (about E12 to E16), expression is ubiquitous throughout the brain and most prominent in the proliferative neuroepithelium. During the second phase (about E16 to postnatal day 6), N-cadherin expression is restricted to particular nuclei or laminae that share common functional features and neuroanatomical connections. Several of the N-cadherin-positive structures receive direct afferents from retinal ganglion cells or from the superior colliculus. Others belong to the reticular system and to the limbic system of the brain. In neocortex, N-cadherin is expressed by deeper layer cells. In the adult brain, only low levels of N-cadherin expression remain in very few types of cells, for example in the Purkinje cells of the cerebellum. These results are similar to data from chicken brain and suggest that the generalized expression of N-cadherin during the early phase and the restriction expression of this molecule in particular functional systems during the later phase is, at least in part, phylogenetically conserved between chicken and mouse. Moreover, the results show that N-cadherin expression extends to phylogenetically newer structures, e.g., the mammalian neocortex.

T-cadherin 2: molecular characterization, function in cell adhesion, and coexpression with T-cadherin and N-cadherin

Cadherins are integral membrane glycoproteins that mediate calcium-dependent, homophilic cell-cell adhesion and are implicated in controlling tissue morphogenesis. T-cadherin is anchored to the membrane through a glycosyl phosphatidylinositol (Ranscht B, Dours-Zimmermann MT: Neuron 7:391-402, 1991) and expressed in a restricted pattern in developing embryos (Ranscht B, Bronner-Fraser M: Development 111:15-22, 1991). We report here the molecular and functional characterization of the T-cadherin isoform, T-cadherin 2 (Tcad-2) and the expression of the corresponding mRNA. Tcad-2 cDNA differs in its 3' nucleotide sequence from T-cadherin cDNA and encodes a protein in which the carboxy terminal Leu of T-cadherin is substituted by Lys and extended by the amino acids SerPheProTyrVal. By RNase protection, mRNAs encoding the T-cadherin isoforms are coexpressed in heart, muscle, liver, skin, somites, and in neural tissue. Many tissues contain both T-cadherin and Tcad-2 mRNAs in conjunction with N-cadherin transcripts, and T-cadherins and N-cadherin proteins are coexpressed on the surface of individual neurons in vitro. Expression in Chinese hamster ovary cells (CHO) revealed that Tcad-2 is a glycosyl phosphatidylinositol-anchored membrane protein that functions in calcium-dependent, homophilic cell adhesion. The identification of a functional T-cadherin isoform and the coexpression of T-cadherins and N-cadherin by individual cells suggest that specific adhesive interactions of embryonic cells may involve a complex interplay between multiple cadherins.

Cell adhesion molecules, second messengers and axonal growth

Recent studies on NCAM-related molecules suggest that individual cell adhesion molecules might function to both promote axonal growth during development and maintain synaptic structure in the adult. Evidence that differential alternative splicing contributes to this apparent bifunctionality of cell adhesion molecules is discussed.

Neural cell contact and axonal growth

The past year has seen significant advances in the identification and characterization of novel vertebrate cell-surface components implicated in neural cell adhesion and axonal growth. These new cell-surface-associated proteins can be categorized into three major structural groups: the cadherins, the integrins and the immunoglobulin superfamily. Studies on the biological function of these proteins indicate that they participate in developmental events of the embryonic nervous system.