Expression of multiple cadherins and catenins in the chick optic tectum

Cadherins form a large family of homophilic cell adhesion molecules that are involved in numerous aspects of neural development. The best-studied neural cadherin, N-cadherin, is concentrated at synapses made by retinal axons in the chick optic tectum and is required for the arborization of retinal axons in their target (retinorecipient) laminae. By analogy, other cadherins might mediate arborization or synaptogenesis in other tectal laminae. Here we consider which cadherins are expressed in tectum, which cells express them, and how their expression is regulated. First, using N-cadherin as a model, we show that synaptic input regulates both cadherin gene expression and the subcellular distribution of cadherin protein. Second, we demonstrate that N-, R-, and T-cadherin are each expressed in distinct laminar patterns during retinotectal synaptogenesis and that N- and R- are enriched in nonoverlapping synaptic subsets. Third, we show that over 20 cadherin superfamily genes are expressed in the tectum during the time that synapses are forming and that many of them are expressed in restricted groups of cells. Finally, we report that both beta-catenin and gamma-catenin (plakoglobin), cytoplasmic proteins required for cadherin signaling, are enriched at synapses and associated with N-cadherin. However, beta- and gamma-catenins are differentially distributed and regulated, and form mutually exclusive complexes. This result suggests that cadherin-based specificity involves multiple cadherin-dependent signaling pathways as well as multiple cadherins.     

High-performance capillary electrophoresis of hyaluronic acid: determination of its amount and molecular mass

Individual GABAergic interneurons in hippocampus can powerfully inhibit more than a thousand excitatory pyramidal neurons. Therefore, control of interneuron excitability provides control over hippocampal networks. We have identified a novel mechanism in hippocampus that weakens excitatory synapses onto GABAergic interneurons. Following stimulation that elicits long-term potentiation at neighboring synapses onto excitatory cells, excitatory synapses onto inhibitory interneurons undergo a long-term synaptic depression (interneuron LTD; iLTD). Unlike most other forms of hippocampal synaptic plasticity, iLTD is not synapse specific: stimulation of an afferent pathway triggers depression not only of activated synapses but also of inactive excitatory synapses onto the same interneuron. These results suggest that high frequency afferent activity increases hippocampal excitability through a dual mechanism, simultaneously potentiating synapses onto excitatory neurons and depressing synapses onto inhibitory neurons.     

Neurotrophins induce formation of functional excitatory and inhibitory synapses between cultured hippocampal neurons

Cell cultures were used to analyze the role of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the development of synaptic transmission. Neurons obtained from embryonic day 18 (E18) rat hippocampus and cultured for 2 weeks exhibited extensive spontaneous synaptic activity. By comparison, neurons obtained from E16 hippocampus expressed very low levels of spontaneous or evoked synaptic activity. Neurotrophin treatment produced a sevenfold increase in the number of functional synaptic connections in the E16 cultures. BDNF induced formation of both excitatory and inhibitory synapses, whereas NT-3 induced formation of only excitatory synapses. These effects were independent of serum or the age of the glia bed used for the culture. They were not accompanied by significant changes in synaptic-vesicle-associated proteins or glutamate receptors. Treatment of the cultures with the neurotrophins for 3 d was sufficient to establish the maximal level of functional synapses. During this period, neurotrophins did not affect the viability or the morphology of the excitatory neurons, although they did produce an increase in the number and length of dendrites of the GABAergic neurons. Remarkably, only BDNF caused an increase in the number of axonal branches and in the total length of the axons of the GABAergic neurons. These results support a unique and differential role for neurotrophins in the formation of excitatory and inhibitory synapses in the developing hippocampus.     

Regulated binding of PTP1B-like phosphatase to N-cadherin: control of cadherin-mediated adhesion by dephosphorylation of beta-catenin

Cadherins are a family of cell-cell adhesion molecules which play a central role in controlling morphogenetic movements during development. Cadherin function is regulated by its association with the actin containing cytoskeleton, an association mediated by a complex of cytoplasmic proteins, the catenins: alpha, beta, and gamma. Phosphorylated tyrosine residues on beta-catenin are correlated with loss of cadherin function. Consistent with this, we find that only nontyrosine phosphorylated beta-catenin is associated with N-cadherin in E10 chick retina tissue. Moreover, we demonstrate that a PTP1B-like tyrosine phosphatase associates with N-cadherin and may function as a regulatory switch controlling cadherin function by dephosphorylating beta-catenin, thereby maintaining cells in an adhesion-competent state. The PTP1B-like phosphatase is itself tyrosine phosphorylated. Moreover, both direct binding experiments performed with phosphorylated and dephosphorylated molecules, and treatment of cells with tyrosine kinase inhibitors indicate that the interaction of the PTP1B-like phosphatase with N-cadherin depends on its tyrosine phosphorylation. Concomitant with the tyrosine kinase inhibitor-induced loss of the PTP1B-like phosphatase from its association with N-cadherin, phosphorylated tyrosine residues are retained on beta-catenin, the association of N-cadherin with the actin containing cytoskeleton is lost and N-cadherin-mediated cell adhesion is prevented. Tyrosine phosphatase inhibitors also result in the accumulation of phosphorylated tyrosine residues on beta-catenin, loss of the association of N-cadherin with the actin-containing cytoskeleton, and prevent N-cadherin mediated adhesion, presumably by directly blocking the function of the PTP1B-like phosphatase. We previously showed that the binding of two ligands to the cell surface N-acetylgalactosaminylphosphotransferase (GalNAcPTase), the monoclonal antibody 1B11 and a proteoglycan with a 250-kD core protein, results in the accumulation of phosphorylated tyrosine residues on beta-catenin, uncoupling of N-cadherin from its association with the actin containing cytoskeleton, and loss of N-cadherin function. We now report that binding of these ligands to the GalNAcPTase results in the absence of the PTP1B-like phosphatase from its association with N-cadherin as well as the loss of the tyrosine kinase and tyrosine phosphatase activities that otherwise co-precipitate with N-cadherin. Control antibodies and proteoglycans have no such effect. This effect is similar to that observed with tyrosine kinase inhibitors, suggesting that the GalNAcPTase/proteoglycan interaction inhibits a tyrosine kinase, thereby preventing the phosphorylation of the PTP1B-like phosphatase, and its association with N-cadherin. Taken together these data indicate that a PTP1B-like tyrosine phosphatase can regulate N-cadherin function through its ability to dephosphorylate beta-catenin and that the association of the phosphatase with N-cadherin is regulated via the interaction of the GalNAcPTase with its proteoglycan ligand. In this manner the GalNAcPTase-proteoglycan interaction may play a major role in morphogenetic cell and tissue interactions during development.     

Regulation of beta-catenin levels and localization by overexpression of plakoglobin and inhibition of the ubiquitin-proteasome system

beta-Catenin and plakoglobin (gamma-catenin) are closely related molecules of the armadillo family of proteins. They are localized at the submembrane plaques of cell-cell adherens junctions where they form independent complexes with classical cadherins and alpha-catenin to establish the link with the actin cytoskeleton. Plakoglobin is also found in a complex with desmosomal cadherins and is involved in anchoring intermediate filaments to desmosomal plaques. In addition to their role in junctional assembly, beta-catenin has been shown to play an essential role in signal transduction by the Wnt pathway that results in its translocation into the nucleus. To study the relationship between plakoglobin expression and the level of beta-catenin, and the localization of these proteins in the same cell, we employed two different tumor cell lines that express N-cadherin, and alpha- and beta-catenin, but no plakoglobin or desmosomal components. Individual clones expressing various levels of plakoglobin were established by stable transfection. Plakoglobin overexpression resulted in a dose-dependent decrease in the level of beta-catenin in each clone. Induction of plakoglobin expression increased the turnover of beta-catenin without affecting RNA levels, suggesting posttranslational regulation of beta-catenin. In plakoglobin overexpressing cells, both beta-catenin and plakoglobin were localized at cell-cell junctions. Stable transfection of mutant plakoglobin molecules showed that deletion of the N-cadherin binding domain, but not the alpha-catenin binding domain, abolished beta-catenin downregulation. Inhibition of the ubiquitin-proteasome pathway in plakoglobin overexpressing cells blocked the decrease in beta-catenin levels and resulted in accumulation of both beta-catenin and plakoglobin in the nucleus. These results suggest that (a) plakoglobin substitutes effectively with beta-catenin for association with N-cadherin in adherens junctions, (b) extrajunctional beta-catenin is rapidly degraded by the proteasome-ubiquitin system but, (c) excess beta-catenin and plakoglobin translocate into the nucleus.     

A role for the cadherin family of cell adhesion molecules in hippocampal long-term potentiation

The cadherins are a family of cell-cell adhesion molecules that mediate Ca2+-dependent homophilic interactions between cells and transduce signals by interacting with cytoplasmic proteins. In the hippocampus, immunostaining combined with confocal microscopy revealed that both neural- (N-) and epithelial- (E-) cadherin are present at synaptic sites, implying a role in synaptic function. Pretreatment of hippocampal slices with antibodies (Abs) raised against the extracellular domain of either N-cad or E-cad had no effect on basal synaptic properties but significantly reduced long-term potentiation (LTP). Infusion of antagonistic peptides containing the His-Ala-Val (HAV) consensus sequence for cadherin dimerization also attenuated LTP induction without affecting previously established LTP. Because the intense synaptic stimulation associated with LTP induction might transiently deplete extracellular Ca2+ and hence potentially destabilize cadherin-cadherin interactions, we examined whether slices could be protected from inhibition by N-cad Abs or HAV peptides by raising the extracellular Ca2+ concentration. Indeed, we found that high extracellular Ca2+ prevented the block of LTP by these agents. Taken together, these results indicate that cadherins are involved in synaptic plasticity, and the stability of cadherin-cadherin bonds may be regulated by synaptic stimulation.     

Association between a transmembrane protein tyrosine phosphatase and the cadherin-catenin complex

Cadherins are calcium-dependent cell adhesion molecules that play fundamental roles in embryonic development, tissue morphogenesis, and cancer. A prerequisite for their function is association with the actin cytoskeleton via the catenins. Tyrosine phosphorylation of beta-catenin, which correlates with a reduction in cadherin-dependent cell adhesion, may provide cells with a mechanism to regulate cadherin activity. Here we report that beta-catenin immune precipitates from PC12 cells contain tyrosine phosphatase activity which dephosphorylates beta-catenin in vitro. In addition, we show that a member of the leukocyte antigen-related protein (LAR)-related transmembrane tyrosine phosphatase family (LAR-PTP) associates with the cadherin-catenin complex. This association required the amino-terminal domain of beta-catenin but does not require the armadillo repeats, which mediate association with cadherins. The interaction also is detected in PC9 cells, which lack alpha-catenin. Thus, the association is not mediated by alpha-catenin or by cadherins. Interestingly, LAR-PTPs are phosphorylated on tyrosine in a TrkA-dependent manner, and their association with the cadherin-catenin complex is reduced in cells treated with NGF. We propose that changes in tyrosine phosphorylation of beta-catenin mediated by TrkA and LAR-PTPs control cadherin adhesive function during processes such as neurite outgrowth.     

Apoptosis of endothelial cells is associated with paracrine induction of adhesion molecules: evidence for an interleukin-1beta-dependent paracrine loop

Cadherin-mediated adhesion depends on the association of its cytoplasmic domain with the actin-containing cytoskeleton. This interaction is mediated by a group of cytoplasmic proteins: alpha-and beta- or gamma- catenin. Phosphorylation of beta-catenin on tyrosine residues plays a role in controlling this association and, therefore, cadherin function. Previous work from our laboratory suggested that a nonreceptor protein tyrosine phosphatase, bound to the cytoplasmic domain of N-cadherin, is responsible for removing tyrosine-bound phosphate residues from beta-catenin, thus maintaining the cadherin-actin connection (). Here we report the molecular cloning of the cadherin-associated tyrosine phosphatase and identify it as PTP1B. To definitively establish a causal relationship between the function of cadherin-bound PTP1B and cadherin-mediated adhesion, we tested the effect of expressing a catalytically inactive form of PTP1B in L cells constitutively expressing N-cadherin. We find that expression of the catalytically inactive PTP1B results in reduced cadherin-mediated adhesion. Furthermore, cadherin is uncoupled from its association with actin, and beta-catenin shows increased phosphorylation on tyrosine residues when compared with parental cells or cells transfected with the wild-type PTP1B. Both the transfected wild-type and the mutant PTP1B are found associated with N-cadherin, and recombinant mutant PTP1B binds to N-cadherin in vitro, indicating that the catalytically inactive form acts as a dominant negative, displacing endogenous PTP1B, and rendering cadherin nonfunctional. Our results demonstrate a role for PTP1B in regulating cadherin-mediated cell adhesion.     

Expression of a candidate cadherin in T lymphocytes

Cadherins are homotypic adhesion molecules that classically mediate interactions between cells of the same type in solid tissues. In addition, E-cadherin is able to support homotypic adhesion of epidermal Langerhans cells to keratinocytes (Tang, A., Amagai, M., Granger, L. G., Stanley, J. R. & Udey, M. C. (1993) Nature (London) 361, 82-85) and heterotypic adhesion of mucosal epithelial cells to E-cadherin-negative intestinal intraepithelial T lymphocytes. Thus, we hypothesized that cadherins may play a wider role in cell-to-cell adhesion events involving T lymphocytes. We searched for a cadherin or cadherins in T lymphocytes with a pan-cadherin antiserum and antisera against alpha- or beta-catenin, molecules known to associate with the cytoplasmic domain of cadherins. The anti-beta-catenin antisera coimmunoprecipitated a radiolabeled species in T-lymphocyte lines that had a molecular mass of 129 kDa and was specifically immunoblotted with the pan-cadherin antiserum. Also, the pan-cadherin antiserum directly immunoprecipitated a 129-kDa radiolabeled species from an 125I surface-labeled Jurkat human T-cell leukemic cell line. After V8 protease digestion, the peptide map of this pan-cadherin-immunoprecipitated, 129-kDa species exactly matched that of the 129-kDa species coimmunoprecipitated with the beta-catenin antiserum. These results demonstrate that T lymphocytes express a catenin-associated protein that appears to be a member of the cadherin superfamily and may contribute to T cell-mediated immune surveillance.     

Epithelial cell adhesion molecule (Ep-CAM) modulates cell-cell interactions mediated by classic cadherins

The contribution of noncadherin-type, Ca2+-independent cell-cell adhesion molecules to the organization of epithelial tissues is, as yet, unclear. A homophilic, epithelial Ca2+-independent adhesion molecule (Ep-CAM) is expressed in most epithelia, benign or malignant proliferative lesions, or during embryogenesis. Here we demonstrate that ectopic Ep-CAM, when expressed in cells interconnected by classic cadherins (E- or N-cadherin), induces segregation of the transfectants from the parental cell type in coaggregation assays and in cultured mixed aggregates, respectively. In the latter assay, Ep-CAM-positive transfectants behave like cells with a decreased strength of cell-cell adhesion as compared to the parental cells. Using transfectants with an inducible Ep-CAM-cDNA construct, we demonstrate that increasing expression of Ep-CAM in cadherin-positive cells leads to the gradual abrogation of adherens junctions. Overexpression of Ep-CAM has no influence on the total amount of cellular cadherin, but affects the interaction of cadherins with the cytoskeleton since a substantial decrease in the detergent-insoluble fraction of cadherin molecules was observed. Similarly, the detergent-insoluble fractions of alpha- and beta-catenins decreased in cells overexpressing Ep-CAM. While the total beta-catenin content remains unchanged, a reduction in total cellular alpha-catenin is observed as Ep-CAM expression increases. As the cadherin-mediated cell-cell adhesions diminish, Ep-CAM-mediated intercellular connections become predominant. An adhesion-defective mutant of Ep-CAM lacking the cytoplasmic domain has no effect on the cadherin-mediated cell-cell adhesions. The ability of Ep-CAM to modulate the cadherin-mediated cell-cell interactions, as demonstrated in the present study, suggests a role for this molecule in development of the proliferative, and probably malignant, phenotype of epithelial cells, since an increase of Ep-CAM expression was observed in vivo in association with hyperplastic and malignant proliferation of epithelial cells.     

Modulation of calcium by inhibitory systems in the developing auditory midbrain

Inhibitory synaptic transmission is of fundamental importance during the maturation of central auditory circuits, and their subsequent ability to process acoustic information. The present study investigated the manner in which inhibitory transmission regulates intracellular free calcium levels in the gerbil inferior colliculus using a brain slice preparation. Inhibitory and excitatory postsynaptic potentials were evoked by electrical stimulation of the ascending afferents at the level of the dorsal nucleus of the lateral lemniscus. Pharmacologically isolated inhibitory synaptic potentials were able to attenuate a calcium rise in collicular neurons that was generated by depolarizing current injection. In addition, GABA(A) and glycine receptor antagonists typically led to an increase of calcium in collicular neurons during electrical stimulation of the ascending afferent pathway at the level of the dorsal nucleus of the lateral lemniscus. Bath application of GABA or muscimol, a GABA(A) receptor agonist, evoked a brief hyperpolarization followed by a long-lasting depolarization in inferior colliculus neurons. This treatment also induced a transient calcium increase that correlated with the membrane depolarization phase. Baclofen, a GABA(B) receptor agonist, had no effect on either membrane potential or calcium levels. Ratiometric measures indicated that the muscimol-evoked rise in calcium was approximately 150 nM above basal levels. The muscimol-evoked responses were completely antagonized by bicuculline and attenuated by picrotoxin. Together, these results suggest that inhibitory synaptic transmission participates in the regulation of postsynaptic calcium during the developmental period. Inhibitory transmission may attenuate a calcium influx that is evoked by excitatory synapses, but it can also produce a modest influx of calcium when activated alone. These mechanisms may help to explain the influence of inhibitory transmission on the development of postsynaptic properties.     

Neuronal circuits are subdivided by differential expression of type-II classic cadherins in postnatal mouse brains

A number of type-II classic cadherin cell-cell adhesion molecules are expressed in the brain. To investigate their roles in brain morphogenesis, we selected three type-II cadherins, cadherin-6 (cad6), -8 (cad8) and -11 (cad11), and mapped their expressions in the forebrain and other restricted regions of postnatal mouse brains. In the cerebral cortex, each cortical area previously defined was delineated by a specific combinatorial expression of these cadherins. The thalamus and other subcortical regions of the forebrain were also subdivided by differential expression of the three cadherins; e.g., the medial geniculate body expressed only cad6; the ventral posterior thalamic nucleus, cad6/cad11; and the anteroventral thalamic nucleus, cad6/cad8. Likewise, in the olivocerebellar system, each subdivision of the inferior olive expressed a unique set of the three cadherins, and the cerebellar cortex had parasagittal stripes of cad8/cad11 expressions. Close analysis of these cadherin expression patterns revealed that they are correlated with neuronal connection patterns. Examples of these correlations include that cad6 delineates the auditory projection system, cad6/cad8/ cad11 are expressed by part of the Papez circuit, and cad6/cad8 are expressed by subdivisions of the olivo-nuclear circuit. Together with the recent finding that the cadherin adhesion system is localized in synaptic junctions, our findings support the notion that cadherin-mediated cell-cell adhesion plays a role in selective interneuronal connections during neural network formation.     

Assessment of cervical lymph node metastasis in esophageal carcinoma using ultrasonography

Cadherins are Ca2+-dependent cell-cell adhesion molecules, and are involved in the formation and maintenance of the organocellular architecture. Using a combination of molecular biologic and biochemical methods, we analyzed cadherins expressed on cultured human malignant lymphoma cell lines (adult T cell lymphomas, human T cell leukemia virus type 1-negative T cell lines, and thymus-derived lymphoma cell lines), and obtained evidence that N-cadherin is the major cadherin expressed on these cells. These cells were found to form cell aggregates in a Ca2+-dependent manner, and more importantly to coaggregate and adhere with cells expressing N-cadherin, suggesting that N-cadherin on lymphoma cells is functionally active. Therefore, N-cadherin expressed on lymphoma cells could underlie the frequent invasion of these cells into the mesenchymal tissue in the skin and the central nervous system.     

Desmosomal cadherin binding domains of plakoglobin

Plakoglobin is a major component of both desmosomes and adherens junctions. At these sites it binds to the cytoplasmic domains of cadherin cell-cell adhesion proteins and regulates their adhesive and cytoskeletal binding functions. Plakoglobin also forms distinct cytosolic protein complexes that function in pathways of tumor suppression and cell fate determination. Recent studies in Xenopus suggest that cadherins inhibit the signaling functions of plakoglobin presumably by sequestering this protein at the membrane and depleting its cytosolic pool. To understand the reciprocal regulation between desmosomal cadherins (desmoglein and desmocollin) and plakoglobin, we have sought to identify the binding domains involved in the formation of these protein complexes. Plakoglobin comprises 13 central repeats flanked by amino-terminal and carboxyl-terminal domains. Our results show that repeats 1-4 are involved in binding desmoglein-1. In contrast, the interaction of plakoglobin with desmocollin-1a is sensitive to deletion of either end of the central repeat domain. The binding sites for two adherens junction components, alpha-catenin and classical cadherins, overlap these sites. Competition among these proteins for binding sites on plakoglobin may therefore account for the distinct composition of adherens junctions and desmosomes.     

Calcium-permeable AMPA receptors mediate long-term potentiation in interneurons in the amygdala

Fear conditioning is a paradigm that has been used as a model for emotional learning in animals. The cellular correlate of fear conditioning is thought to be associative N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity within the amygdala. Here we show that glutamatergic synaptic transmission to inhibitory interneurons in the basolateral amygdala is mediated solely by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In contrast to AMPA receptors at inputs to pyramidal neurons, these receptors have an inwardly rectifying current-voltage relationship, indicative of a high permeability to calcium. Tetanic stimulation of inputs to interneurons caused an immediate and sustained increase in the efficacy of these synapses. This potentiation required a rise in postsynaptic calcium, but was independent of NMDA receptor activation. The potentiation of excitatory inputs to interneurons was reflected as an increase in the amplitude of the GABA(A)-mediated inhibitory synaptic current in pyramidal neurons. These results demonstrate that excitatory synapses onto interneurons within a fear conditioning circuit show NMDA-receptor independent long-term potentiation. This plasticity might underlie the increased synchronization of activity between neurons in the basolateral amygdala after fear conditioning.     

Cadherins promote skeletal muscle differentiation in three-dimensional cultures

The cell-cell adhesion molecule N-cadherin, with its associated catenins, is expressed by differentiating skeletal muscle and its precursors. Although N-cadherin's role in later events of skeletal myogenesis such as adhesion during myoblast fusion is well established, less is known about its role in earlier events such as commitment and differentiation. Using an in vitro model system, we have determined that N-cadherin- mediated adhesion enhances skeletal muscle differentiation in three-dimensional cell aggregates. We transfected the cadherin-negative BHK fibroblastlike cell line with N-cadherin. Expression of exogenous N-cadherin upregulated endogenous beta-catenin and induced strong cell-cell adhesion. When BHK cells were cultured as three-dimensional aggregates, N-cadherin enhanced withdrawal from the cell cycle and stimulated differentiation into skeletal muscle as measured by increased expression of sarcomeric myosin and the 12/101 antigen. In contrast, N-cadherin did not stimulate differentiation of BHK cells in monolayer cultures. The effect of N-cadherin was not unique since E-cadherin also increased the level of sarcomeric myosin in BHK aggregates. However, a nonfunctional mutant N-cadherin that increased the level of beta-catenin failed to promote skeletal muscle differentiation suggesting an adhesion-competent cadherin is required. Our results suggest that cadherin-mediated cell-cell interactions during embryogenesis can dramatically influence skeletal myogenesis.     

Differential localization of VE- and N-cadherins in human endothelial cells: VE-cadherin competes with N-cadherin for junctional localization

The two major cadherins of endothelial cells are neural (N)-cadherin and vascular endothelial (VE)- cadherin. Despite similar level of protein expression only VE-cadherin is located at cell-cell contacts, whereas N-cadherin is distributed over the whole cell membrane. Cotransfection of VE-cadherin and N-cadherin in CHO cells resulted in the same distribution as that observed in endothelial cells indicating that the behavior of the two cadherins was not cell specific but related to their structural characteristics. Similar amounts of alpha- and beta-catenins and plakoglobin were associated to VE- and N-cadherins, whereas p120 was higher in the VE-cadherin complex. The presence of VE-cadherin did not affect N-cadherin homotypic adhesive properties or its capacity to localize at junctions when cotransfectants were cocultured with cells transfected with N-cadherin only. To define the molecular domain responsible for the VE-cadherin-dominant activity we prepared a chimeric construct formed by VE-cadherin extracellular region linked to N-cadherin intracellular domain. The chimera lost the capacity to exclude N-cadherin from junctions indicating that the extracellular domain of VE-cadherin alone is not sufficient for the preferential localization of the molecule at the junctions. A truncated mutant of VE-cadherin retaining the full extracellular domain and a short cytoplasmic tail (Arg621-Pro702) lacking the catenin-binding region was able to exclude N-cadherin from junctions. This indicates that the Arg621-Pro702 sequence in the VE-cadherin cytoplasmic tail is required for N-cadherin exclusion from junctions. Competition between cadherins for their clustering at intercellular junctions in the same cell has never been described before. We speculate that, in the endothelium, VE- and N-cadherin play different roles; whereas VE-cadherin mostly promotes the homotypic interaction between endothelial cells, N-cadherin may be responsible for the anchorage of the endothelium to other surrounding cell types expressing N-cadherin such as vascular smooth muscle cells or pericytes.     

Cadherin expression in the retina and retinofugal pathways of the chicken embryo

The expression of two calcium-dependent adhesion molecules of the cadherin superfamily (cadherin-6B and cadherin-7) was mapped in the embryonic neural retina and retinofugal pathways of the chicken embryo and compared with the expression of R-cadherin, N-cadherin, and B-cadherin, studied previously. Whereas B-cadherin is only found in Miller glia, the other four cadherins are each expressed by specific subpopulations of retinal neurons. For example, different (but partly overlapping) populations of bipolar cells express R-cadherin, cadherin-6B, and cadherin-7. Cadherin-6B and cadherin-7 are also expressed by subsets of amacrine cells. In the inner plexiform layer, cadherin-6B and cadherin-7 immunoreactivities are restricted to specific sublaminae associated with synapsin-I-positive nerve terminals. In addition, cadherin-6B and cadherin-7 are expressed by a subset of ganglion cells that project to several retinorecipient nuclei forming part of the accessory optic system (e.g., nucleus of the basal optic root and external pretectal nucleus). Together with their connecting fiber tracts, these nuclei also express cadherin-6B and cadherin-7 in their neurons and neuropile. The expression patterns of the two cadherins overlap but show distinct differences. Some other visual nuclei express cadherin-7 but not cadherin-6B. The expression patterns differ from those previously described for N- and R-cadherin. Together, these results demonstrate that cadherins could provide a system of adhesive cues that specify developing retinal circuits and other functional connections and subsystems in the embryonic chicken visual system.