N-cadherin redistribution during synaptogenesis in hippocampal neurons

Development and molecular organization of dendritic spines and their synapses

Nearly all excitatory input in the hippocampus impinges on dendritic spines which serve as multifunctional compartments that can, at the very least, selectively isolate and amplify incoming signals. Their importance to normal brain function is highlighted by the severe mental impairment observed in most individuals having poorly developed spines (Purpura, Science 1974;186:1126-1128). Distinct groups of membrane proteins, cytoskeletal elements, scaffolding proteins, and second messenger-related proteins are concentrated particularly in dendritic spines, but their ability to generate, maintain, and coordinately regulate spine structure or function is poorly understood. Here we review the unique molecular composition of dendritic spines along with the factors known to influence dendritic spine development in order to construct a model of dendritic spine development in relation to synaptogenesis.

Evidence for direct involvement of beta-catenin in phorbol ester-induced neurite outgrowth in GT1-1 hypothalamic neurones

Gonadotropin-releasing hormone (GnRH) is a pivotal neuroendocrine regulator controlling reproductive functions. However, the scattered distribution of GnRH neurones in the mammalian brain has hindered studies on the development and differentiation of GnRH neurones. In the present study, we used the immortalized GnRH-producing GT1-1 cells to examine whether activation of protein kinase C (PKC) pathway with 12-O-tetradecanoyl-13-acetate (TPA) induces morphological and functional differentiation of GnRH neurones. TPA induced neurite outgrowth and inhibited proliferation of GT1-1 cells that were specifically antagonized by cotreatment of PKC inhibitor, calphostin C. The functional significance of TPA-induced differentiation of GT1-1 cells was manifested in part by the changes in the effects of gamma-aminobutyric acid (GABA) on intracellular Ca2+ levels. In untreated GT1-1 cells, activation of GABA-A receptor with 10 microM muscimol increased intracellular Ca2+ levels, whereas such stimulatory effects disappeared in GT1-1 cells bearing neurites. Accordingly, muscimol could not stimulate GnRH release in TPA-treated GT1-1 cells. To elucidate the molecular mechanism underlying TPA-induced neurite outgrowth, we performed differential display reverse transcription-polymerase chain reaction. Among several genes that are affected by TPA treatment, we found a significant induction of beta-catenin mRNA expression. Along with the rapid induction of beta-catenin protein levels, we observed that beta-catenin was reallocated from cell-cell adhesion sites to the growth cones within 3 h of TPA treatment. Transient transfection studies with green fluorescent protein as a reporter gene demonstrated that beta-catenin overexpression alone can promote neurite outgrowth in GT1-1 cells. Moreover, TPA was found to increase the transcription-activational roles of beta-catenin. Together, these data provide evidence that beta-catenin is involved in the TPA-induced functional differentiation of immortalized GnRH neurones.

Abnormalities of synapses and neurons in the hippocampus of neuropsin-deficient mice

In the present study, we produced null-mutant mice of neuropsin, an extracellular matrix serine protease, to examine the neural functions of this protein particularly in the hippocampus. Golgi-Cox impregnation and Nissl-staining revealed morphological change of cell soma in the mutant mice compared to wild-type mice. However, Golgi-Cox impregnation revealed no apparent change in the dendritic arborization and spine density. Quantitative electronmicroscopic analysis revealed that number of asymmetrical synapses were significantly decreased in the stratum radiatum, the major terminal field of Schaffer-collaterals, whereas free boutons still holding synaptic vesicles but with no synaptic specialization were increased in number in the same microscopic fields. An increased number of parvalbumin-immunoreactive cells (known as fast spiking cells) in mutant was also observed. These results strongly suggest that neuropsin is involved in connectivity of a group of CA1 synapses and consequently in the hippocampal networking.

The role of Alzheimer's disease-related presenilin 1 in intercellular adhesion

Most cases of familial early-onset Alzheimer's disease are caused by mutations in the presenilin 1 (PS1) gene. However, the cellular functions of PS1 are unknown. We showed predominant localization of PS1 to cell-cell contacts of the plasma membrane in human prostate epithelial tissue and in a human epithelial cell line HEp2 stably transfected with an inducible PS1 construct. PS1 co-immunoprecipitated with beta-catenin from cell lysates of stable transfectants. Conversely, PS1 lacking the PS1-beta-catenin interaction site did not co-immunoprecipitate with beta-catenin and was not recruited to the cell-cell contacts. L cells, which do not form tight intercellular contacts, formed clusters of adhered cells after stable transfection with GFP-PS1 cDNA and demonstrated a clear preference for independent aggregation in the mixed cultures. However, L cells transfected with mutant GFP-PS1 constructs, which had a truncated N-terminus of PS1 or deleted PS1-beta-catenin interaction site, failed to form intercellular contacts. In addition, in primary cultures of mouse cortical neurons PS1 was highly concentrated on the surface of extended growth cones. Taken together, our results suggest an important role of PS1 in intercellular adhesion in epithelial cells and neurons.

N-cadherin is regulated by gonadal steroids in the adult hippocampus

In the adult hippocampus, gonadal steroids induce neural remodeling through cellular and molecular mechanisms that are largely unknown. The calcium-dependent cell adhesion molecule N-cadherin, which participates in the developmental organization of the nervous system, has recently been localized to hippocampal synapses and is suspected to participate in adult synaptic physiology. Little is currently known about the regulation of cadherins in the adult central nervous system, although posttranslational modifications are thought to account for variability in N-cadherin expression levels. To evaluate the possibility that gonadal steroids regulate N-cadherin in the adult hippocampus, we examined hippocampal N-cadherin mRNA levels and protein expression in castrated adult male rats treated with testosterone, or its metabolites 17beta-estradiol or dihydrotestosterone. Northern blot analysis indicated increased hippocampal N-cadherin mRNA levels in the adult rat hippocampus after treatment with 17beta-estradiol but not testosterone or dihydrotestosterone. Increased N-cadherin immunoreactivity was observed in CA1 and CA3 pyramidal cells after 17beta-estradiol treatment. Additionally, both 17beta-estradiol and testosterone treatment increased N-cadherin immunoreactivity in the neuropil of the stratum lacunosum-moleculare, which includes apical dendrites from pyramidal cells. In contrast, dihydrotestosterone treatment had no effect on levels of N-cadherin protein expression in CA1 or CA3 pyramidal cells or in the stratum lacunosum-moleculare. These results demonstrate that, in the hippocampus, expression levels of N-cadherin are dynamic in adulthood. To our knowledge, there have been no other demonstrations of steroid regulation of cadherin expression in neural populations. These results suggest a possible adhesive mechanism for steroid-induced plasticity of the adult nervous system.

Regulation of rat hippocampal neural cadherin in the kainic acid induced seizures

Regulation of neural (N-) cadherin expression in the hippocampus was examined by in situ hybridization and immunohistochemistry methods in the rat model of kainic acid (KA) induced seizures. After 12 and 24 h of KA administration, mRNA expression level of N-cadherin decreased in the hippocampal CA1 and CA3 area in parallel with decrease of the number of neural cells. In contrast, after 48 h and 7 days, mRNA expression level recovered partially, although the number of neural cells remained small. In addition, immunohistochemical staining indicated that N-cadherin protein expression of survived neurons increased significantly after 48 h of KA administration. These results indicated that N-cadherin might be involved in neuronal reconstruction at the hippocampus.

N-cadherin is regulated by gonadal steroids in adult sexually dimorphic spinal motoneurons

Gonadal steroids influence the morphology and function of neurons in the adult spinal cord through cellular and molecular mechanisms that are largely unknown. The cadherins are cell adhesion molecules that participate in the formation and organization of the CNS during embryonic development, and recent evidence suggests that the cadherins continue to regulate neural structure and function in adulthood. Using degenerate oligonucleotides coding conserved regions of the catenin-binding domain of classical cadherins in a RT-PCR cloning strategy, we identified several cadherin subtypes, the most frequently cloned being N-, E-, and R-cadherin, suggesting that these are the major classical cadherin subtypes present in the adult male rat lumbosacral spinal cord. We then examined cadherin expression levels of these cadherin subtypes under steroid conditions known to induce plastic changes in spinal motoneurons. Semiquantitative PCR revealed that mRNA levels of N-cadherin, but not E-cadherin or R-cadherin, are elevated in castrated rats treated with testosterone, 17 beta-estradiol, or dihydrotestosterone relative to castrate rats not treated with steroids. Immunolocalization of N-cadherin revealed that steroid treatment increased N-cadherin expression levels in functionally related neural populations whose morphology and function are regulated by steroids. These results suggest a role for N-cadherin in steroid-induced neuroplastic change in the adult lumbar spinal cord.

Mismatched appositions of presynaptic and postsynaptic components in isolated hippocampal neurons

To determine whether presynaptic input is necessary for postsynaptic differentiation, we isolated hippocampal neurons in microisland culture and thus deprived pyramidal cells of GABA input and GABAergic neurons of glutamate input. We find that glutamate input is necessary for clustering the AMPA-type glutamate receptor but not for clustering the NMDA receptor or the associated PSD-95 family scaffold in GABAergic cells; GABA input is not necessary for clustering the GABA(A) receptor or gephyrin in pyramidal cells. Isolated neurons showed a surprising mismatch of presynaptic and postsynaptic components. For example, in isolated pyramidal neurons, although GABA(A) receptor clusters covered <4% of the dendritic surface and presynaptic boutons covered <12%, a full two-thirds of the GABA(A) receptor clusters were localized inappropriately opposite the non-GABAergic, presumed glutamatergic, terminals. Furthermore, inhibitory and excitatory postsynaptic components were segregated into separate clusters in isolated cells and apposed to separate boutons of a single axon. Thus, GABA(A) receptors were clustered opposite some terminals, whereas NMDA receptors were clustered opposite other terminals of a single axon. These results suggest the involvement of a synaptogenic signal common to glutamate and GABA synapses that permits experimentally induced mismatching of presynaptic and postsynaptic components in isolated neurons, as well as a second specificity-conferring signal that mediates appropriate matching in mixed cultures.

Cadherins: molecular codes for axon guidance and synapse formation

The formation of the myriad of neuronal connections within the vertebrate nervous system relies on expression of molecular tags that match extending axon populations with synaptic target sites. Recent work suggests that cadherins, a group of calcium-dependent cell adhesion molecules, are candidates to serve such a role. The diversity of the cadherin family in the nervous system allows for a multitude of interactions to specify neuronal connections. Specific cadherin types demarcate subpopulations of developing axons that interconnect within neuronal circuits. Expression of different cadherin species at select synapse populations raises exciting prospects for this molecule class in controlling adhesive interactions during synaptogenesis and plasticity. Regulation of cadherin-mediated adhesive strength is an attractive mechanism to explain the different cadherin functions in axon growth and at synapses.

Increasing numbers of synaptic puncta during late-phase LTP: N-cadherin is synthesized, recruited to synaptic sites, and required for potentiation

It is an open question whether new synapses form during hippocampal LTP. Here, we show that late-phase LTP (L-LTP) is associated with a significant increase in numbers of synaptic puncta identified by synaptophysin and N-cadherin, an adhesion protein involved in synapse formation during development. During potentiation, protein levels of N-cadherin are significantly elevated and N-cadherin dimerization is enhanced. The increases in synaptic number and N-cadherin levels are dependent on cAMP-dependent protein kinase (PKA) and protein synthesis, both of which are also required for L-LTP. Blocking N-cadherin adhesion prevents the induction of L-LTP, but not the early-phase of LTP (E-LTP). Our data suggest that N-cadherin is synthesized during the induction of L-LTP and recruited to newly forming synapses. N-cadherin may play a critical role in L-LTP by holding nascent pre-and postsynaptic membranes in apposition, enabling incipient synapses to acquire function and contribute to potentiation.

Cadherin-mediated adhesion at the interneuronal synapse

One of the recent advances in the molecular definition of a synapse has been the identification of cadherins as major structural components. The presence of classic (N- and E-) cadherins in the synaptic complex is not surprising considering the ultrastructural similarities between interneuronal synapses and the adhesive junctions formed between epithelial cells. However, the role of these adhesion molecules and their junctions in this context is likely to encompass both developmental and physiological phenomena that are unique to the synapse. Moreover, the recent finding that a much broader family of cadherin-related receptors is also located at the synaptic complex has fuelled speculation that cadherins have a role in generation of specificity in synaptic connectivity as well as structure.

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.

Beta-catenin expression in human neural cell lines following exposure to cytokines and growth factors

Beta-catenin acts as a key mediator of the Wnt/Wingless signaling pathway involved in cell proliferation, differentiation and survival. Recent studies have shown that an unstable interaction between beta-catenin and the mutant presenilin-1 induces neuronal apoptosis, and that beta-catenin levels are decreased in the brains of patients with Alzheimer's disease (AD). Since activated microglia and astrocytes play a role in the process of neuronal degeneration in AD, the cytokine/growth factor-regulated expression of beta-catenin in human neural cell lines, including NTera2 teratocarcinoma-derived differentiated neurons (NTera2-N), IMR-32 neuroblastoma, SKN-SH neuroblastoma and U-373MG astrocytoma, was studied quantitatively following exposure to epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, interferon (IFN)-gamma, transforming growth factor (TGF)-beta1, dibutyryl cyclic adenosine 3',5'-cyclic monophosphate (cAMP) (dbcAMP) or phorbol 12-myristate 13-acetate (PMA). Beta-catenin mRNA expressed constitutively in all of these cell lines was unaffected by treatment with any factors examined. In contrast, beta-catenin protein levels were reduced markedly in NTera2-N cells by exposure to dbcAMP, EGF or bFGF, and in U-373MG cells by treatment with dbcAMP or PMA, but were unaffected in any cell lines by BDNF, TNF-alpha, IL-1beta, IL-6, IFN-gamma or TGF-beta1. These results indicate that beta-catenin is expressed constitutively in human neural cells and downregulated at a protein level by a set of growth factors in a cell type-specific manner.

Loss of cadherin-11 adhesion receptor enhances plastic changes in hippocampal synapses and modifies behavioral responses

Cadherins organize symmetrical junctions between the pre- and postsynaptic membranes in central synapses. One of them, cadherin-11 (cad11), is expressed in the limbic system of the brain, most strongly in the hippocampus. Immunohistochemical studies of the hippocampus showed that cad11 proteins were densely distributed in its synaptic neuropil zones; in cultured hippocampal neurons, their distribution often overlapped with that of synaptophysin, and also occasionally with that of GluR1 at spines. To assess the role of cad11 in synaptic formation and/or function, we analyzed brains of cad11-deficient mice. In these mice, long-term potentiation (LTP) in the CA1 region of the hippocampus was, unexpectedly, enhanced; and the level of LTP saturation was increased. In behavioral tests, the mutant mice showed reduced fear- or anxiety-related responses. These results suggest that the cad11-mediated junctions may modulate synaptic efficacy, confining its dynamic changes to a limited range, or these junctions are required for normal development of synaptic organization in the hippocampus.

Developmentally regulated expression of Thy-1 in structures of the mouse sensory-motor system

Thy-1 is a cell-surface molecule of the immunoglobulin superfamily which is expressed at high levels in the mature nervous system. Thy-1 has been implicated in regulating axonal outgrowth and synaptic function, but little is known regarding its cellular localization and expression in the central nervous system (CNS) during development or in adulthood. In this study, Thy-1 gene expression and protein localization were examined in sensory-motor and related areas of the adult and postnatally developing mouse CNS. Thy-1 mRNA expression was restricted to neurons; immunoreactivity was densely distributed throughout the neuropil of all regions examined, often delineated the neuronal plasmalemma, and labeled axons in white matter tracts of the brain and spinal cord. In adulthood, immunolabeling was regionally widespread and was present relatively homogeneously throughout all cell-dense layers of sensory-motor cortex, throughout most thalamic nuclei, globus pallidus, and spinal cord. Developmentally, however, Thy-1 expression and localization exhibited a spatially and temporally staggered sequence leading to the adult pattern. In sensory-motor cortex, Thy-1 expression in layer V preceded expression in other layers; in the barrel field, labeling of barrel septa preceeded a gradually increasing intensity of immunolabeling of barrel centers; in the thalamus, Thy-1 exhibited a differential onset and temporal pattern of expression across different nuclei associated with motor, sensory, or limbic systems; in the caudate nucleus, Thy-1 expression was greatest during the first postnatal week of life before declining during subsequent development. Taken together, the adult distribution and developmental patterns leading to it form a unique profile in comparison with other structurally related glycosyl-phosphatidylinositol (GPI)-anchored neural cell adhesion molecules. The pattern and timing of Thy-1 expression across layers and nuclei during early postnatal development are more complex than previously recognized, thus perhaps reflecting varied roles for Thy-1 in aspects of structural or functional maturation which proceed independently of the timing of neurogenesis, migration, and dendritic and axonal growth.

Developmental lead exposure disturbs expression of synaptic neural cell adhesion molecules in herring gull brains

Neurobehavioral testing of herring gull chicks (Larus argentatus) in both laboratory and field studies indicates that lead exposure during critical periods of development causes neurological deficits that may compromise survival in the wild. Accumulating evidence suggests that lead impairs neurodevelopment, in part, by altering the expression of cell adhesion molecules (CAMs) responsible for the proper formation and maintenance of neural structure and synaptic function. We examined the adhesion molecules NCAM, L1, and N-cadherin in gull brains to determine whether these CAMs are altered by lead exposure and might serve as markers of developmental neurotoxicity. One-day-old chicks were collected from nesting colonies and were laboratory housed. On post-hatching day (PHD) 2, chicks were given 100 mg/kg lead acetate or saline (intraperitoneally). Birds were killed on PHD 34, 44, or 55 (blood-lead levels averaged 27.4, 20.8, and 19.5 microg/dl, respectively). Brains were removed and stored at -70 degrees C until analysis. Expression of CAMs was determined in synaptosomal preparations by Western blotting and the activity of NCAM-associated sialyltransferase (ST) was determined in purified whole brain golgi apparatus. Elevation in synaptosomal polysialylated NCAM expression and a significant increase in golgi ST activity was observed in lead-treated animals at PHD 34. Reductions in synaptosomal N-cadherin were observed at PHD 34 and 44, while L1 expression appeared unaffected by lead at any time-point. By 55 days post-hatching, no differences in N-cadherin expression, polysialylated NCAM expression or NCAM-associated ST activity were seen in lead-treated animals as compared with age-matched control animals. Lead-induced disruption of CAM expression during early neurodevelopment may contribute to behavioral deficits observed in herring gulls in both the laboratory and the field, and may serve as a marker for heavy metal exposure during postnatal development.

Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.

N-cadherin influences migration of oligodendrocytes on astrocyte monolayers

Oligodendrocyte cell migration is required for the development of the nervous system and the repopulation of demyelinated lesions in the adult central nervous system. We have investigated the role of the calcium-dependent adhesion molecules, the cadherins, in oligodendrocyte-astrocyte interaction and oligodendrocyte progenitor migration. Immunostaining demonstrated the expression of N-cadherin on the surfaces of both oligodendrocytes and astrocytes, and oligodendrocyte-like cells adhered to and spread on N-cadherin substrates. The blocking of cadherin function by antisera or specific peptides reduced adhesion of oligodendroglia to astrocyte monolayers, diminished contact time between oligodendrocyte processes and individual astrocytes, and significantly increased the migration of oligodendrocyte-like cells on astrocyte monolayers. Furthermore, a soluble cadherin molecule without adhesive properties increased oligodendroglial proliferation on various extracellular matrix substrates. These data suggest that cadherins are at least partially responsible for the poor migration-promoting properties of astrocytes and that decreasing cell-cell adhesion might effect repopulation of demyelinated multiple sclerosis lesions by oligodendrocyte progenitors.

Functional cis-heterodimers of N- and R-cadherins

Classical cadherins form parallel cis-dimers that emanate from a single cell surface. It is thought that the cis-dimeric form is active in cell-cell adhesion, whereas cadherin monomers are likely to be inactive. Currently, cis-dimers have been shown to exist only between cadherins of the same type. Here, we show the specific formation of cis-heterodimers between N- and R-cadherins. E-cadherin cannot participate in these complexes. Cells coexpressing N- and R-cadherins show homophilic adhesion in which these proteins coassociate at cell-cell interfaces. We performed site- directed mutagenesis studies, the results of which support the strand dimer model for cis-dimerization. Furthermore, we show that when N- and R-cadherins are coexpressed in neurons in vitro, the two cadherins colocalize at certain neural synapses, implying biological relevance for these complexes. The present study provides a novel paradigm for cadherin interaction whereby selective cis-heterodimer formation may generate new functional units to mediate cell-cell adhesion.

Molecular mechanisms that control endothelial cell contacts

Endothelial cell contacts control the permeability of the blood vessel wall. This allows the endothelium to form a barrier for solutes, macromolecules, and leukocytes between the vessel lumen and the interstitial space. Loss of this barrier function in pathophysiological situations can lead to extracellular oedema. The ability of leukocytes to enter tissue at sites of inflammation is dependent on molecular mechanisms that allow leukocytes to adhere to the endothelium and to migrate through the endothelial cell layer and the underlying basal lamina. It is a commonly accepted working hypothesis that inter-endothelial cell contacts are actively opened and closed during this process. Angiogenesis is another important process that requires well-controlled regulation of inter-endothelial cell contacts. The formation of new blood vessels by sprouting from pre-existing vessels depends on the loosening of established endothelial cell contacts and the migration of endothelial cells that form the outgrowing sprouts. This review focuses on the molecular composition of endothelial cell surface proteins and proteins of the cytoskeletal undercoat of the plasma membrane at sites of inter-endothelial cell contacts and discusses the current knowledge about the potential role of such molecules in the regulation of endothelial cell contacts.

Molecular modification of N-cadherin in response to synaptic activity

The relationship between adhesive interactions across the synaptic cleft and synaptic function has remained elusive. At certain CNS synapses, pre- to postsynaptic adhesion is mediated at least in part by neural (N-) cadherin. Here, we demonstrate that upon depolarization of hippocampal neurons in culture by K+ treatment, or application of NMDA or alpha-latrotoxin, synaptic N-cadherin dimerizes and becomes markedly protease resistant. These properties are indices of strong, stable, enhanced cadherin-mediated intercellular adhesion. N-cadherin retained protease resistance for at least 2 hr after recovery, while other surface molecules, including other cadherins, were completely degraded. The acquisition of protease resistance and dimerization of N-cadherin is not dependent on new protein synthesis, nor is it accompanied by internalization of N-cadherin. By immunocytochemistry, we found that high K+ selectively induces surface dispersion of N-cadherin, which, after recovery, returns to synaptic puncta. N-cadherin dispersion under K+ treatment parallels the rapid expansion of the presynaptic membrane consequent to the massive vesicle fusion that occurs with this type of depolarization. In contrast, with NMDA application, N-cadherin does not disperse but does acquire enhanced protease resistance and dimerizes. Our data strongly suggest that synaptic adhesion is dynamically and locally controlled, and modulated by synaptic activity.

Target-specific factors regulate the formation of glutamatergic transmitter release sites in cultured neocortical neurons

Synapse formation in the mammalian CNS is thought to involve specific target recognition processes between presynaptic and postsynaptic neurons leading to the establishment of defined neuronal circuits. To study the role of target neuron-specific factors in synaptogenesis, we used cocultures of presynaptic explants and dissociated target neurons from rat neocortex, which enabled us to selectively vary the postsynaptic target neurons. Cocultures containing target neurons that were obtained early during development [embryonic day 16 (E16)] were compared to cocultures containing target neurons that were obtained at a later embryonic stage (E19). Postsynaptic currents (PSCs) were evoked in target neurons by maximal extracellular stimulation in the presynaptic explant. The mean amplitudes of AMPA and NMDA receptor-mediated PSCs were sixfold reduced in E16 target neurons, whereas the mean amplitudes of GABA(A) receptor-mediated PSCs did not differ between E16 and E19 target neurons. This reduction was in part caused by an apparently twofold reduction in mean quantal amplitude, as shown by recording AMPA receptor-mediated miniature PSCs. In addition, a reduced number of glutamatergic release sites in E16 target neurons was revealed by synapsin I immunostaining of dendritic presynaptic terminals. No differences in mean release probability were observed between E16 and E19 target neurons. Thus, the formation of glutamatergic transmitter release sites was strongly influenced by target neuron-specific factors. The formation of functional GABAergic synapses, however, was independent of the type of target neurons, suggesting specific retrograde signaling during the establishment of glutamatergic synapses.

The role of cell adhesion molecules in synaptic plasticity and memory

Studies in the past few years suggest that cell adhesion molecules may play signaling as well as structural roles at adult synapses during plasticity. The observation that many adhesion molecules are expressed both pre-synaptically and post-synaptically raises the possibility that information about synaptic activity might simultaneously be communicated to both sides of the synapse, circumventing the need for distinct anterograde and retrograde messengers.

Cloning and expression of mouse Cadherin-7, a type-II cadherin isolated from the developing eye

We report the molecular cloning of Cadherin-7 from the embryonic mouse eye. The deduced amino acid sequence shows it to be a type-II cadherin similar to Xenopus F-cadherin and chick Cadherin-7. The mouse Cadherin-7 gene maps to chromosome 1, outside the conserved linkage group of cadherin genes on chromosome 8. Cadherin-7 is expressed throughout the entire period of neural development and mRNA levels are developmentally regulated in both the embryonic and the postnatal central nervous system (CNS). In adult mice, Cadherin-7 expression is restricted to the CNS, with highest levels in the retina. In the developing eye, Cadherin-7 mRNA is found only in the neural retina. It is expressed by all retinal neuroblasts from E11 onward, but becomes progressively restricted to neurons in the inner neuroblast and developing ganglion cell layers (GCL). In the adult retina it is confined to subpopulations of cells in the GCL and to amacrine cells in the inner part of the inner nuclear layer. This expression pattern suggests a role for Cadherin-7 in mouse retinal development, particularly in the formation and maintenance of the GCL.

Effects of axotomy on the expression and ultrastructural localization of N-cadherin and neural cell adhesion molecule in the quail ciliary ganglion: an in vivo model of neuroplasticity

Postganglionic nerve crush of the avian ciliary ganglion induces detachment of preganglionic terminals from the soma of the injured ciliary neurons, followed by reattachment at about the same time that the postganglionic axons regenerate to their targets. In order to determine the role played by cell adhesion molecules in this response, we have studied injury-induced changes in the amount and distribution of N-cadherin and neural cell adhesion molecule, together with modifications in the expression of their messenger RNAs. Both N-cadherin and neural cell adhesion molecule immunoreactivities associated with postsynaptic specializations decreased between one and three days following postganglionic nerve crush, preceding the detachment of the preganglionic boutons. Immunoreactivities subsequently increased between 13 and 20 days, in parallel with restoration of synaptic contacts on the ganglion cells and the progressive reinnervation of the peripheral targets. In contrast to the rapid decrease in immunoreactivity, the messenger RNA levels of N-cadherin and neural cell adhesion molecule both increased after crush, and remained elevated throughout the 20-day period of the experiment. These results are consistent with roles for N-cadherin and neural cell adhesion molecule in the maintenance of synaptic contacts. The rapid regulation of these proteins in injury-induced synaptic plasticity occurs at the post-transcriptional level, whereas longer term regulation associated with the re-establishment of synapses may be promoted by the increased levels of gene expression.

Structure, development, and plasticity of dendritic spines

Dendritic spines are distinguished by their shapes, subcellular composition, and synaptic receptor subtypes. Recent studies show that actin-dependent movements take place in spine heads, that spines emerge from stubby and shaft synapses after dendritic filopodia disappear, and that spines can form without synaptic activation, are maintained by optimal activation, and are lost with excessive activation or during degeneration.

Homophilic adhesion by cadherins

Cadherins mediate cell-cell adhesion through homophilic interactions. High-resolution structures have greatly enhanced our understanding of this phenomenon over the past few years. Nonetheless, some of the original concepts about cadherin interactions need revision, with the new structural and additional mutagenesis data currently available. Furthermore, in vivo studies on cadherins have provided supplementary information.