Expression of multiple cadherins and catenins in the chick optic tectum

Retinoic acid signaling regulates proliferation and lamina formation in the developing chick optic tectum

The retinotectal system has been extensively studied for investigating the mechanism(s) for topographic map formation. The optic tectum, which is composed of multiple laminae, is the major retino recipient structure in the developing avian brain. Laminar development of the tectum results from cell proliferation, differentiation and migration, coordinated in strict temporal and spatial patterns. However, the molecular mechanisms that orchestrate these complex developmental events, have not been fully elucidated. In this study, we have identified the presence of differential retinoic acid (RA) signaling along the rostro-caudal and dorsoventral axis of the tectum. We show for the first time that loss of RA signaling in the anterior optic tectum, leads to an increase in cell proliferation and gross changes in the morphology manifested as defects in lamination. Detailed analysis points to delayed migration of cells as the plausible cause for the defects in lamina formation. Thus, we conclude that in the optic tectum, RA signaling is involved in maintaining cell proliferation and in regulating the formation of the tectal laminae.

New Optical Tools to Study Neural Circuit Assembly in the Retina

During development, neurons navigate a tangled thicket of thousands of axons and dendrites to synapse with just a few specific targets. This phenomenon termed wiring specificity, is critical to the assembly of neural circuits and the way neurons manage this feat is only now becoming clear. Recent studies in the mouse retina are shedding new insight into this process. They show that specific wiring arises through a series of stages that include: directed axonal and dendritic growth, the formation of neuropil layers, positioning of such layers, and matching of co-laminar synaptic partners. Each stage appears to be directed by a distinct family of recognition molecules, suggesting that the combinatorial expression of such family members might act as a blueprint for retinal connectivity. By reviewing the evidence in support of each stage, and by considering their underlying molecular mechanisms, we attempt to synthesize these results into a wiring model which generates testable predictions for future studies. Finally, we conclude by highlighting new optical methods that could be used to address such predictions and gain further insight into this fundamental process.

Cadherins Interact With Synaptic Organizers to Promote Synaptic Differentiation

Classical cadherins, a set of ~20 related recognition and signaling molecules, have been implicated in many aspects of neural development, including the formation and remodeling of synapses. Mechanisms underlying some of these steps have been studied by expressing N-cadherin (cdh2), a Type 1 cadherin, in heterologous cells, but analysis is complicated because widely used lines express cdh2 endogenously. We used CRISPR-mediated gene editing to generate a Human embryonic kidney (HEK)293 variant lacking Cdh2, then compared the behavior of rodent cortical and hippocampal neurons co-cultured with parental, cdh2 mutant and cdh2-rescued 293 lines. The comparison demonstrated that Cdh2 promotes neurite branching and that it is required for three synaptic organizers, neurologin1 (NLGL1), leucine-rich repeat transmembrane protein 2 (LRRtm2), and Cell Adhesion Molecule 1 (Cadm1/SynCAM) to stimulate presynaptic differentiation, assayed by clustering of synaptic vesicles at sites of neurite-293 cell contact. Similarly, Cdh2 is required for a presynaptic organizing molecule, Neurexin1β, to promote postsynaptic differentiation in dendrites. We also show that another Type I cadherin, Cdh4, and a Type II cadherin, Cdh6, can substitute for Cdh2 in these assays. Finally, we provide evidence that the effects of cadherins require homophilic interactions between neurites and the heterologous cells. Together, these results indicate that classical cadherins act together with synaptic organizers to promote synaptic differentiation, perhaps in part by strengthening the intracellular adhesion required for the organizers to act efficiently. We propose that cadherins promote high affinity contacts between appropriate partners, which then enable synaptic differentiation.

The classic cadherins in synaptic specificity

During brain development, billions of neurons organize into highly specific circuits. To form specific circuits, neurons must build the appropriate types of synapses with appropriate types of synaptic partners while avoiding incorrect partners in a dense cellular environment. Defining the cellular and molecular rules that govern specific circuit formation has significant scientific and clinical relevance because fine scale connectivity defects are thought to underlie many cognitive and psychiatric disorders. Organizing specific neural circuits is an enormously complicated developmental process that requires the concerted action of many molecules, neural activity, and temporal events. This review focuses on one class of molecules postulated to play an important role in target selection and specific synapse formation: the classic cadherins. Cadherins have a well-established role in epithelial cell adhesion, and although it has long been appreciated that most cadherins are expressed in the brain, their role in synaptic specificity is just beginning to be unraveled. Here, we review past and present studies implicating cadherins as active participants in the formation, function, and dysfunction of specific neural circuits and pose some of the major remaining questions.

Cadherin-8 expression, synaptic localization, and molecular control of neuronal form in prefrontal corticostriatal circuits

Neocortical interactions with the dorsal striatum support many motor and executive functions, and such underlying functional networks are particularly vulnerable to a variety of developmental, neurological, and psychiatric brain disorders, including autism spectrum disorders, Parkinson's disease, and Huntington's disease. Relatively little is known about the development of functional corticostriatal interactions, and in particular, virtually nothing is known of the molecular mechanisms that control generation of prefrontal cortex-striatal circuits. Here, we used regional and cellular in situ hybridization techniques coupled with neuronal tract tracing to show that Cadherin-8 (Cdh8), a homophilic adhesion protein encoded by a gene associated with autism spectrum disorders and learning disability susceptibility, is enriched within striatal projection neurons in the medial prefrontal cortex and in striatal medium spiny neurons forming the direct or indirect pathways. Developmental analysis of quantitative real-time polymerase chain reaction and western blot data show that Cdh8 expression peaks in the prefrontal cortex and striatum at P10, when cortical projections start to form synapses in the striatum. High-resolution immunoelectron microscopy shows that Cdh8 is concentrated at excitatory synapses in the dorsal striatum, and Cdh8 knockdown in cortical neurons impairs dendritic arborization and dendrite self-avoidance. Taken together, our findings indicate that Cdh8 delineates developing corticostriatal circuits where it is a strong candidate for regulating the generation of normal cortical projections, neuronal morphology, and corticostriatal synapses.

Neural plasticity and proliferation in the generation of antidepressant effects: hippocampal implication

It is widely accepted that changes underlying depression and antidepressant-like effects involve not only alterations in the levels of neurotransmitters as monoamines and their receptors in the brain, but also structural and functional changes far beyond. During the last two decades, emerging theories are providing new explanations about the neurobiology of depression and the mechanism of action of antidepressant strategies based on cellular changes at the CNS level. The neurotrophic/plasticity hypothesis of depression, proposed more than a decade ago, is now supported by multiple basic and clinical studies focused on the role of intracellular-signalling cascades that govern neural proliferation and plasticity. Herein, we review the state-of-the-art of the changes in these signalling pathways which appear to underlie both depressive disorders and antidepressant actions. We will especially focus on the hippocampal cellularity and plasticity modulation by serotonin, trophic factors as brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) through intracellular signalling pathways—cAMP, Wnt/β-catenin, and mTOR. Connecting the classic monoaminergic hypothesis with proliferation/neuroplasticity-related evidence is an appealing and comprehensive attempt for improving our knowledge about the neurobiological events leading to depression and associated to antidepressant therapies.

Impact of the ADHD-susceptibility gene CDH13 on development and function of brain networks

Attention-deficit/hyperactivity disorder (ADHD) is a common, early onset and enduring neuropsychiatric disorder characterized by developmentally inappropriate inattention, hyperactivity, increased impulsivity and motivational/emotional dysregulation with similar prevalence rates throughout different cultural settings. Persistence of ADHD into adulthood is associated with considerable risk for co-morbidities such as depression and substance use disorder. Although the substantial heritability of ADHD is well documented the etiology is characterized by a complex coherence of genetic and environmental factors rendering identification of risk genes difficult. Genome-wide linkage as well as single nucleotide polymorphism (SNP) and copy-number variant (CNV) association scans recently allow to reliably define aetiopathogenesis-related genes. A considerable number of novel ADHD risk genes implicate biological processes involved in neurite outgrowth and axon guidance. Here, we focus on the gene encoding Cadherin-13 (CDH13), a cell adhesion molecule which was replicably associated with liability to ADHD and related neuropsychiatric conditions. Based on its unique expression pattern in the brain, we discuss the molecular structure and neuronal mechanisms of Cadherin-13 in relation to other cadherins and the cardiovascular system. An appraisal of various Cadherin-13-modulated signaling pathways impacting proliferation, migration and connectivity of specific neurons is also provided. Finally, we develop an integrative hypothesis of the mechanisms in which Cadherin-13 plays a central role in the regulation of brain network development, plasticity and function. The review concludes with emerging concepts about alterations in Cadherin-13 signaling contributing to the pathophysiology of neurodevelopmental disorders.

Regulation of cadherin expression in nervous system development

This review addresses our current understanding of the regulatory mechanisms for classical cadherin expression during development of the vertebrate nervous system. The complexity of the spatial and temporal expression patterns is linked to morphogenic and functional roles in the developing nervous system. While the regulatory networks controlling cadherin expression are not well understood, it is likely that the multiple signaling pathways active in the development of particular domains also regulate the specific cadherins expressed at that time and location. With the growing understanding of the broader roles of cadherins in cell-cell adhesion and non-adhesion processes, it is important to understand both the upstream regulation of cadherin expression and the downstream effects of specific cadherins within their cellular context.

Linkage of N-cadherin to multiple cytoskeletal elements revealed by a proteomic approach in hippocampal neurons

The CNS synapse is an adhesive junction differentiated for chemical neurotransmission and is equipped with presynaptic vesicles and postsynaptic neurotransmitter receptors. Cell adhesion molecule cadherins not only maintain connections between pre- and postsynaptic membranes but also modulate the efficacy of synaptic transmission. Although the components of the cadherin-mediated adhesive apparatus have been studied extensively in various cell systems, the complete picture of these components, particularly at the synaptic junction, remains elusive. Here, we describe the proteomic assortment of the N-cadherin-mediated synaptic adhesion apparatus in cultured hippocampal neurons. N-cadherin immunoprecipitated from Triton X-100-solubilized neuronal extract contained equal amounts of β- and α-catenins, as well as F-actin-related membrane anchor proteins such as integrins bridged with α-actinin-4, and Na(+)/K(+)-ATPase bridged with spectrins. A close relative of β-catenin, plakoglobin, and its binding partner, desmoplakin, were also found, suggesting that a subset of the N-cadherin-mediated adhesive apparatus also anchors intermediate filaments. Moreover, dynein heavy chain and LEK1/CENPF/mitosin were found. This suggests that internalized pools of N-cadherin in trafficking vesicles are conveyed by dynein motors on microtubules. In addition, ARVCF and NPRAP/neurojungin/δ2-catenin, but not p120ctn/δ1-catenin or plakophilins-1, -2, -3, -4 (p0071), were found, suggesting other possible bridges to microtubules. Finally, synaptic stimulation by membrane depolarization resulted in an increased 93-kDa band, which corresponded to proteolytically truncated β-catenin. The integration of three different classes of cytoskeletal systems found in the synaptic N-cadherin complex may imply a dynamic switching of adhesive scaffolds in response to synaptic activity.

Cell adhesion, the backbone of the synapse: "vertebrate" and "invertebrate" perspectives

Synapses are asymmetric intercellular junctions that mediate neuronal communication. The number, type, and connectivity patterns of synapses determine the formation, maintenance, and function of neural circuitries. The complexity and specificity of synaptogenesis relies upon modulation of adhesive properties, which regulate contact initiation, synapse formation, maturation, and functional plasticity. Disruption of adhesion may result in structural and functional imbalance that may lead to neurodevelopmental diseases, such as autism, or neurodegeneration, such as Alzheimer's disease. Therefore, understanding the roles of different adhesion protein families in synapse formation is crucial for unraveling the biology of neuronal circuit formation, as well as the pathogenesis of some brain disorders. The present review summarizes some of the knowledge that has been acquired in vertebrate and invertebrate genetic model organisms.

Synaptic loss and retention of different classic cadherins with LTP-associated synaptic structural remodeling in vivo

Cadherins are synaptic cell adhesion molecules that contribute to persistently enhanced synaptic strength characteristic of long-term potentiation (LTP). What is relatively unexplored is how synaptic activity of the kind that induces LTP-associated remodeling of synapse structure affects localization of cadherins, particularly in mature animals in vivo, details which could offer insight into how different cadherins contribute to synaptic plasticity. Here, we use a well-described in vivo LTP induction protocol that produces robust synaptic morphological remodeling in dentate gyrus of adult rats in combination with confocal and immunogold electron microscopy to localize cadherin-8 and N-cadherin at remodeled synapses. We find that the density and size of cadherin-8 puncta are significantly diminished in the potentiated middle molecular layer (MML) while concurrently, N-cadherin remains tightly clustered at remodeled synapses. These changes are specific to the potentiated MML, and occur without any change in density or size of synaptophysin puncta. Thus, the loss of cadherin-8 probably represents selective removal from synapses rather than overall loss of synaptic junctions. Together, these findings suggest that activity-regulated loss and retention of different synaptic cadherins could contribute to dual demands of both flexibility and stability in synapse structure that may be important for synaptic morphological remodeling that accompanies long-lasting plasticity.

Design principles of insect and vertebrate visual systems

A century ago, Cajal noted striking similarities between the neural circuits that underlie vision in vertebrates and flies. Over the past few decades, structural and functional studies have provided strong support for Cajal's view. In parallel, genetic studies have revealed some common molecular mechanisms controlling development of vertebrate and fly visual systems and suggested that they share a common evolutionary origin. Here, we review these shared features, focusing on the first several layers-retina, optic tectum (superior colliculus), and lateral geniculate nucleus in vertebrates; and retina, lamina, and medulla in fly. We argue that vertebrate and fly visual circuits utilize common design principles and that taking advantage of this phylogenetic conservation will speed progress in elucidating both functional strategies and developmental mechanisms, as has already occurred in other areas of neurobiology ranging from electrical signaling and synaptic plasticity to neurogenesis and axon guidance.

Molecular and cellular mechanisms of lamina-specific axon targeting

The specificity of synaptic connections is directly related to the functional integrity of neural circuits. Long-range axon guidance and topographic mapping mechanisms bring axons into spatial proximity of target cells and thus limit the number of potential synaptic partners. Synaptic specificity is then achieved by extracellular short-range guidance cues and cell-surface recognition cues. Neural activity may enhance the precision and strength of specific circuit connections. Here, we focus on one of the final steps of synaptic matchmaking: the targeting of synaptic layers and the mutual recognition of axons and dendrites within these layers.

T-cadherin structures reveal a novel adhesive binding mechanism

Vertebrate genomes encode 19 classical cadherins and about 100 nonclassical cadherins. Adhesion by classical cadherins depends on binding interactions in their N-terminal EC1 domains, which swap N-terminal beta-strands between partner molecules from apposing cells. However, strand-swapping sequence signatures are absent from nonclassical cadherins, raising the question of how these proteins function in adhesion. Here, we show that T-cadherin, a glycosylphosphatidylinositol (GPI)-anchored cadherin, forms dimers through an alternative nonswapped interface near the EC1-EC2 calcium-binding sites. Mutations within this interface ablate the adhesive capacity of T-cadherin. These nonadhesive T-cadherin mutants also lose the ability to regulate neurite outgrowth from T-cadherin-expressing neurons. Our findings reveal the likely molecular architecture of the T-cadherin homophilic interface and its requirement for axon outgrowth regulation. The adhesive binding mode used by T-cadherin may also be used by other nonclassical cadherins.

Differential gene expression in the developing human macula: microarray analysis using rare tissue samples

The macula is a unique and important region in the primate retina that achieves high resolution and color vision in the central visual field. We recently reported data obtained from microarray analysis of gene expression in the macula of the human fetal retina (Kozulin et al., Mol Vis 15:45–59, 1). In this paper, we describe the preliminary analyses undertaken to visualize differences and verify comparability of the replicates used in that study, report the differential expression of other gene families obtained from the analysis, and show the reproducibility of our findings in several gene families by quantitative real-time PCR.

N-cadherin prodomain cleavage regulates synapse formation in vivo

Cadherins are initially synthesized bearing a prodomain that is thought to limit adhesion during early stages of biosynthesis. Functional cadherins lack this prodomain, raising the intriguing possibility that cells may utilize prodomain cleavage as a means to temporally or spatially regulate adhesion after delivery of cadherin to the cell surface. In support of this idea, immunostaining for the prodomain of zebrafish N-cadherin revealed enriched labeling at neuronal surfaces at the soma and along axonal processes. To determine whether post-translational cleavage of the prodomain affects synapse formation, we imaged Rohon-Beard cells in zebrafish embryos expressing GFP-tagged wild-type N-cadherin (NCAD-GFP) or a GFP-tagged N-cadherin mutant expressing an uncleavable prodomain (PRON-GFP) rendering it nonadhesive. NCAD-GFP accumulated at synaptic microdomains in a developmentally regulated manner, and its overexpression transiently accelerated synapse formation. PRON-GFP was much more diffusely distributed along the axon and its overexpression delayed synapse formation. Our results support the notion that N-cadherin serves to stabilize pre- to postsynaptic contacts early in synapse development and suggests that regulated cleavage of the N-cadherin prodomain may be a mechanism by which the kinetics of synaptogenesis are regulated.

In vitro guidance of retinal axons by a tectal lamina-specific glycoprotein Nel

Nel is a glycoprotein containing five chordin-like and six epidermal growth factor-like domains and is strongly expressed in the nervous system. In this study, we have examined expression patterns and in vitro functions of Nel in the chicken retinotectal system. We have found that in the developing tectum, expression of Nel is localized in specific laminae that retinal axons normally do not enter, including the border between the retinorecipient and non-retinorecipient laminae. Nel-binding activity is detected on retinal axons both in vivo and in vitro, suggesting that retinal axons express a receptor for Nel. In vitro, Nel inhibits retinal axon outgrowth and induces growth cone collapse and axon retraction. These results indicate that Nel acts as an inhibitory guidance cue for retinal axons, and suggest its roles in the establishment of the lamina-specificity in the retinotectal projection.

Cadherin-8 and N-cadherin differentially regulate pre- and postsynaptic development of the hippocampal mossy fiber pathway

Cells sort into regions and groups in part by their selective surface expression of particular classic cadherins during development. In the nervous system, cadherin-based sorting can define axon tracts, restrict axonal and dendritic arbors to particular regions or layers, and may encode certain aspects of synapse specificity. The underlying model has been that afferents and their targets hold in common the expression of a particular cadherin, thereby providing a recognition code of homophilic cadherin binding. However, most neurons express multiple cadherins, and it is not clear whether multiple cadherins all act similarly in shaping neural circuitry. Here we asked how two such cadherins, cadherin-8 and N-cadherin, influence the guidance and differentiation of hippocampal mossy fibers. Using organotypic hippocampal cultures, we find that cadherin-8 regulates mossy fiber fasciculation and targeting, but has little effect on CA3 dendrites. In contrast, N-cadherin regulates mossy fiber fasciculation, but has little impact on axonal growth and targeting. However, N-cadherin is essential for CA3 dendrite arborization. Both cadherins are required for formation of proper numbers of presynaptic terminals. Mechanistically, such differential actions of these two cadherins could, in theory, reflect coupling to distinct intracellular binding partners. However, we find that both cadherins bind beta-catenin in dentate gyrus (DG). This suggests that cadherins may engage different intracellular signaling cascades downstream of beta-catenin, coopt different extracellular binding partners, or target distinct subcellular domains. Together our findings demonstrate that cadherin-8 and N-cadherin are critical for generating the mossy fiber pathway, but that each contributes differentially to afferent and target differentiation, thereby complementing one another in the assembly of a synaptic circuit.

Regulation of the development of tectal neurons and their projections by transcription factors Brn3a and Pax7

The rostral part of the dorsal midbrain, known as the superior colliculus in mammals or the optic tectum in birds, receives a substantial retinal input and plays a diverse and important role in sensorimotor integration. However, little is known about the development of specific subtypes of neurons in the tectum, particularly those which contribute tectofugal projections to the thalamus, isthmic region, and hindbrain. Here we show that two homeodomain transcription factors, Brn3a and Pax7, are expressed in mutually exclusive patterns in the developing and mature avian midbrain. Neurons expressing these factors are generated at characteristic developmental times, and have specific laminar fates within the tectum. In mice expressing betagalactosidase targeted to the Pou4f1 (Brn3a) locus, Brn3a-expressing neurons contribute to the ipsilateral but not the contralateral tectofugal projections to the hindbrain. Using misexpression of Brn3a and Pax7 by electroporation in the chick tectum, combined with GFP reporters, we show that Brn3a determines the laminar fate of subsets of tectal neurons. Furthermore, Brn3a regulates the development of neurons contributing to specific ascending and descending tectofugal pathways, while Pax7 globally represses the development of tectofugal projections to nearly all brain structures.

Lamina-specific axonal projections in the zebrafish tectum require the type IV collagen Dragnet

The mechanisms underlying the precise targeting of tectal layers by ingrowing retinal axons are largely unknown. In zebrafish, individual axons choose one of four retinorecipient layers upon entering the tectum and remain restricted to this layer, despite continual remodeling and shifting of their terminal arbors. In dragnet mutants, by contrast, a large fraction of retinal axons aberrantly trespass between layers or form terminal arbors that span two layers. The dragnet gene, drg, encodes collagen IV(alpha5) (Col4a5), a basement membrane component lining the surface of the tectum. Heparan sulfate proteoglycans (HSPGs) are normally associated with the tectal basement membrane but are dispersed in the dragnet mutant tectum. Zebrafish boxer (extl3) mutants, which are deficient in HSPG synthesis, show laminar targeting defects similar to those in dragnet. Our results show that the collagen IV sheet anchors secreted factors at the surface of the tectum, which serve as guidance cues for retinal axons.

Sequence analysis and expression mapping of the rat clustered protocadherin gene repertoires

Three closely-linked clusters of protocadherin (Pcdh) genes (alpha, beta, and gamma) encoding more than 50 distinct mRNAs have been identified in humans and mice, and proposed to play important roles in neuronal connectivity in the CNS. The human and mouse Pcdh alpha and gamma clusters each span a region of about 300 kb genomic DNA, and are each organized into a tandem array of more than a dozen highly-similar "variable" exons, and three downstream "constant" exons. Little is known about the expression patterns of the alpha and gamma repertoires in the CNS. Here, we comprehensively analyzed the one megabase rat Pcdh genomic DNA sequences at the nucleotide level using various computational methods. We found that the clustered rat Pcdh genes display strict orthologous relationships with those of mice but not humans. Moreover, each rat Pcdh variable exon is preceded by a distinct promoter. We designed two complete sets of isoform-specific probes and extensively mapped the expression patterns for each member of the alpha and gamma repertoires in the adult rat CNS by non-isotopic in situ hybridization experiments. We found that most alpha and gamma mRNA isoforms are broadly expressed in similar patterns in subsets of cells (with some displaying interesting cortical layer-specific expression) throughout various CNS regions, including the olfactory bulb, cerebral cortex, hippocampus, cerebellum, and spinal cord. The broad expression of most alpha or gamma mRNAs throughout various regions of the CNS is consistent with the hypothesis that these genes may be used for neurons to establish their individuality and also provide the adhesive diversity required for complex synaptic connectivity in the mammalian CNS.

Labeled lines in the retinotectal system: markers for retinorecipient sublaminae and the retinal ganglion cell subsets that innervate them

Axons of retinal ganglion cells (RGCs) carry visual information to the brain. In most vertebrates, the major synaptic target of RGCs is the optic tectum. In the chick, RGC axons form synapses in just 4 of 16 histologically recognizable laminae (the retinorecipient laminae [RRLs]), and arbors of individual RGCs are confined to a single RRL. To analyze the development and function of these parallel pathways, markers are required that selectively label them. Here, we have identified molecular markers for individual RRLs and for RGCs that project to them. Some of the markers may mediate or modulate signaling through the separate pathways: neuropeptides (substance P, neuromedin B, somatostatin-I and -II) and their receptors (substance P receptor), neurotransmitter synthetic enzymes (choline acetyltransferase) and the corresponding receptors (acetylcholine receptor beta2) and calcium-binding proteins (parvalbumin and calbindin). Other markers are adhesive proteins that could mediate selective connectivity of RGC subsets within specific RRLs (cadherin-7, cadherin-11, reelin and neuropilin-1). We further show that RGC subsets whose axons project to specific RRLs are heterogeneous with respect to the retinal sublaminae within which their dendrites arborize. Our results define laminar-specified circuits from retina to brain and support a model in which RGCs transmit information from multiple sources to single central laminae, where it can be integrated.

The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Synaptic cell adhesion molecules (SCAMs) are mostly membrane-anchored molecules with extracellular domains that extend into the synaptic cleft. Prototypical SCAMs interact with homologous or heterologous molecules on the surface of adjacent cells, ensuring the precise apposition of pre- and postsynaptic elements. More recent definitions of SCAMs often include molecules involved in axon pathfinding, cell recognition and synaptic differentiation events, making SCAMs functionally and molecularly a highly diverse group. In this review, we summarize the proposed in vivo functions of a large variety of SCAMs. We mainly focus on results obtained from analyses of genetic model organisms, mostly mouse knockout mutants, lacking expression of the respective candidate genes. In contrast to the substantial effect yielded by some knockouts of molecules involved in synaptic vesicle release, no SCAM mutant has been reported thus far that shows a prominently altered structure of the majority of synapses or even lacks synapses altogether. This surprising resilience of synaptic structure might be explained by a high redundancy between different SCAMs, by the assumption that the crucial molecular players in synapse structure have yet to be discovered or by a grand variability in the mechanisms of synapse formation that underlies the diversity of synapses. Whatever the final answer turns out to be, the genetic dissection of the SCAM superfamilies has led to a much better understanding of the different steps required to form, differentiate and modify a synapse.

Versican in the developing brain: lamina-specific expression in interneuronal subsets and role in presynaptic maturation

Chondroitin sulfate proteoglycans (CSPGs) of the extracellular matrix help stabilize synaptic connections in the postnatal brain and impede regeneration after injury. Here, we show that a CSPG of the lectican family, versican, also promotes presynaptic maturation in the developing brain. In the embryonic chick optic tectum, versican is expressed selectively by subsets of interneurons confined to the retinorecipient laminae, in which retinal axons arborize and form synapses. It is a major receptor for the Vicia villosa B4 lectin (VVA), shown previously to inhibit invasion of the retinorecipient lamina by retinal axons (Inoue and Sanes, 1997). In vitro, versican promotes enlargement of presynaptic varicosities in retinal axons. Depletion of versican in ovo, by RNA interference, results in retinal arbors with smaller than normal varicosities. We propose that versican provides a lamina-specific cue for presynaptic maturation and discuss the related but distinct effects of versican depletion and VVA blockade.

Selective stabilization and synaptic specificity: a new cell-biological model

How are appropriate connections between neurons sorted from the overwhelming surplus of potential, yet inappropriate, connections? Despite the apparently improbable nature of the process, brains wire themselves with a high degree of reproducibility that has been conserved across evolutionary history. Here, we outline a viable cell-biological model for generating synaptic specificity that features selection of nascent synapses based on adhesion and recognition. This process uses the highly dynamic and stochastic nature of intracellular trafficking to generate reproducible patterns of synaptic connectivity.

An isoform-specific allele of Drosophila N-cadherin disrupts a late step of R7 targeting

Drosophila N-cadherin is required for the formation of precise patterns of connections in the fly brain. Alternative splicing is predicted to give rise to 12 N-cadherin isoforms. We identified an N-cadherin allele, N-cad(18Astop), that eliminates the six isoforms containing alternative exon 18A and demonstrate that it strongly disrupts the connections of R7 photoreceptor neurons. During the first half of pupal development, N-cadherin is required for R7 growth cones to terminate within a temporary target layer in the medulla. N-cadherin isoforms containing exon 18B are sufficient for this initial targeting. By contrast, 18A isoforms are preferentially expressed in R7 during the second half of pupal development and are necessary for R7 to terminate in the appropriate synaptic layer in the medulla neuropil. Transgene rescue experiments suggest that differences in isoform expression, rather than biochemical differences between isoforms, underlie the 18A isoform requirement in R7 neurons.

Expression of Catenin family members CTNNA1, CTNNA2, CTNNB1 and JUP in the primate prefrontal cortex and hippocampus

Members of the catenin family of proteins are thought to play a major role in the folding and lamination of the cerebral cortex. We have used in situ hybridization to determine the cellular expression patterns of four members of this family, Alpha-E-, Alpha-N-, Beta-, and Gamma-catenins (CTNNA1, CTNNA2, CTNNB1, and JUP respectively) in the adult primate dorsolateral prefrontal cortex (DLPFC) and hippocampus. CTNNA2, CTNNB1, and JUP mRNAs were detected in all layers of the DLPFC and in all neuronal subregions of the hippocampal formation, however CTNNA1 mRNA, coding for an 'epithelial' specific catenin, was not detected in any region of the cortex or hippocampus. CTNNA2, a 'neuronal-specific' catenin, and CTNNB1 mRNAs were abundant in both DLPFC and hippocampus, with a distinct neuronal localization. CTNNA2 mRNA was concentrated in both granular/stellate cells and large pyramidal cell bodies, while CTNNB1 expression was more strongly associated with granular cell bodies throughout the DLPFC, with expression in pyramidal cells confined mainly to cortical Layers III and VI. CTNNA2 and CTNNB1 mRNAs were also abundant in the granule cells of the dentate gyrus and pyramidal cells of Ammon's horn, apparently co-expressed in the same neurons. JUP mRNA was rather diffusely localized in the DLPFC without the distinct laminar patterns seen for CTNNA2 and CTNNB1 but was distinctly localized in the granule cells of the dentate gyrus and pyramidal cells of Ammon's horn. These studies demonstrate a distinct neuronal pattern of gene expression for catenin family members in primate brain structures characterized by high degrees of folding and strong lamination. The high level expression of these transcripts supports the notion of a major role for catenins even in the adult brain. Such an understanding is also important in view of the multiple interactions that catenins have with many other proteins in the adult and ageing brain. This may also have implications for understanding the pathogenesis of neurodegenerative diseases such as Alzheimer's disease, as well as emerging neuronal stem cell therapies.

A GFP-based genetic screen reveals mutations that disrupt the architecture of the zebrafish retinotectal projection

The retinotectal projection is a premier model system for the investigation of molecular mechanisms that underlie axon pathfinding and map formation. Other important features, such as the laminar targeting of retinal axons, the control of axon fasciculation and the intrinsic organization of the tectal neuropil, have been less accessible to investigation. In order to visualize these processes in vivo, we generated a transgenic zebrafish line expressing membrane-targeted GFP under control of the brn3c promoter/enhancer. The GFP reporter labels a distinct subset of retinal ganglion cells (RGCs), which project mainly into one of the four retinorecipient layers of the tectum and into a small subset of the extratectal arborization fields. In this transgenic line, we carried out an ENU-mutagenesis screen by scoring live zebrafish larvae for anatomical phenotypes. Thirteen recessive mutations in 12 genes were discovered. In one mutant, ddl, the majority of RGCs fail to differentiate. Three of the mutations, vrt, late and tard, delay the orderly ingrowth of retinal axons into the tectum. Two alleles of drg disrupt the layer-specific targeting of retinal axons. Three genes, fuzz, beyo and brek, are required for confinement of the tectal neuropil. Fasciculation within the optic tract and adhesion within the tectal neuropil are regulated by vrt, coma, bluk, clew and blin. The mutated genes are predicted to encode molecules essential for building the intricate neural architecture of the visual system.

Selective capability of SynCAM and neuroligin for functional synapse assembly

Synaptic cell adhesion is central for synapse formation and function. Recently, the synaptic cell adhesion molecules neuroligin 1 (NL1) and SynCAM were shown to induce presynaptic differentiation in cocultured neurons when expressed in a non-neuronal cell. However, it is uncertain how similar the resulting artificial synapses are to regular synapses. Are these molecules isofunctional, or do all neuronal cell adhesion molecules nonspecifically activate synapse formation? To address these questions, we analyzed the properties of artificial synapses induced by NL1 and SynCAM, compared the actions of these molecules with those of other neuronal cell adhesion molecules, and examined the functional effects of NL1 and SynCAM overexpression in neurons. We found that only NL1 and SynCAM specifically induced presynaptic differentiation in cocultured neurons. The induced nerve terminals were capable of both spontaneous and evoked neurotransmitter release, suggesting that a full secretory apparatus was assembled. By all measures, SynCAM- and NL1-induced artificial synapses were identical. Overexpression in neurons demonstrated that only SynCAM, but not NL1, increased synaptic function in immature developing excitatory neurons after 8 d in vitro. Tests of chimeric molecules revealed that the dominant-positive effect of SynCAM on synaptic function in developing neurons was mediated by its intracellular cytoplasmic tail. Interestingly, morphological analysis of neurons overexpressing SynCAM or NL1 showed the opposite of the predictions from electrophysiological results. In this case, only NL1 increased the synapse number, suggesting a role for NL1 in morphological synapse induction. These results suggest that both NL1 and SynCAM act similarly and specifically in artificial synapse induction but that this process does not reflect a shared physiological function of these molecules.

Cadherin activity is required for activity-induced spine remodeling

Neural activity induces the remodeling of pre- and postsynaptic membranes, which maintain their apposition through cell adhesion molecules. Among them, N-cadherin is redistributed, undergoes activity-dependent conformational changes, and is required for synaptic plasticity. Here, we show that depolarization induces the enlargement of the width of spine head, and that cadherin activity is essential for this synaptic rearrangement. Dendritic spines visualized with green fluorescent protein in hippocampal neurons showed an expansion by the activation of AMPA receptor, so that the synaptic apposition zone may be expanded. N-cadherin-venus fusion protein laterally dispersed along the expanding spine head. Overexpression of dominant-negative forms of N-cadherin resulted in the abrogation of the spine expansion. Inhibition of actin polymerization with cytochalasin D abolished the spine expansion. Together, our data suggest that cadherin-based adhesion machinery coupled with the actin-cytoskeleton is critical for the remodeling of synaptic apposition zone.

Synaptic adhesion molecules

Formation, differentiation and plasticity of synapses, the specialized cell-cell contacts through which neurons communicate, all require interactions between pre- and post-synaptic partners. Several synaptically localized adhesion molecules potentially capable of mediating these interactions have been identified recently. Functional studies suggest roles for some of them in target recognition (e.g. SYG-1 and sidekicks), formation and alignment of synaptic specializations (e.g. SynCAM, neuroligin and neurexin), and regulation of synaptic structure and function (e.g. cadherins and syndecan).

OL-protocadherin expression in the visual system of the chicken embryo

The expression of OL-protocadherin, a homotypically binding cell adhesion molecule, was mapped in the visual system of the chicken embryo at intermediate to late stages of development (11-19 days of incubation). The expression was compared with that of four classic cadherins, described previously. OL-protocadherin is expressed by the isthmooptic nucleus, its retinopetal projection, and possibly its retinal target neurons, the amacrine cells. Ganglion cells begin to express OL-protocadherin at relatively late stages of development. The layers of the optic tectum, the projection neurons in the stratum griseum centrale, and the tectofugal pathways show differential OL-protocadherin immunoreactivity. Several of the diencephalic target nuclei of the tectothalamic projection, such as the principal pretectal nucleus, subpretectal nucleus, and nucleus rotundus, contain distinct subregions or populations of neurons expressing OL-protocadherin. In these centers, the expression pattern of OL-protocadherin differs from that of the four classic cadherins, though it shows partial overlap with them. Other retinorecipient and/or tectorecipient nuclei (ventral geniculate nucleus, lateral dorsolateral nucleus, superficial synencephalic nucleus, pretectal area, and griseum tectale) also show a differential immunoreactivity for OL-protocadherin and other cadherins. Some of these nuclei and the optic tectum display a similar sequence of cadherin expression from superficial to deep layers, in a pattern that may reflect mutual interconnections. This result indicates a partial conservation of cadherin expression across interconnected embryonic divisions, from the mesencephalon to the ventral thalamus. In conclusion, OL-protocadherin is a marker for specific functional gray matter structures and neural circuits in the chicken visual system. J. Comp. Neurol. 470:240-255, 2004.

Selective synaptic cadherin expression by traced neurons of the chicken visual system

The stable and specific locking-in of pre- and postsynaptic membranes in synaptogenesis may be mediated by integral membrane proteins, such as members of the cadherin family. Cadherins are ideal candidate molecules for mediating synaptic specificity because they are differentially expressed in functionally connected brain structures. We studied the expression of four classic cadherins (R-cadherin, N-cadherin, cadherin-6B and cadherin-7) at the synaptic level on the somata and the proximal neurites of identified neuron populations that were traced selectively in the developing chicken visual system. Three major findings were observed. (1) Synapses on somata of shepherd's crook cells of the optic tectum are associated preferentially with one cadherin subtype. (2) In an isthmic nucleus that contains a mixed population of cells expressing different cadherins, somatic synapses tend to express the same cadherin subtype as the rest of the cell. (3) In the oculomotor complex, two cadherin subtypes are expressed only by synapses on the axon hillock. However, another neuron type that projects from the tectum to the isthmic nucleus does not show such selective synaptic cadherin staining. Our findings support the idea that a cadherin-based adhesive mechanism can mediate synaptic specificity.

Chronic electroconvulsive seizure up-regulates beta-catenin expression in rat hippocampus: role in adult neurogenesis

BACKGROUND

Beta-catenin was discovered as a cytoskeletal protein, constituting a link between the cadherins to the actin cytoskeleton. Aside from this function, beta-catenin is a key effector molecule in the Wnt signaling pathway, serving as a downstream transcription factor.

METHODS

In this study, we examined the influence of electroconvulsive seizures (ECS) on the expression of beta-catenin, as well as expression of Wnt-2, in rat hippocampus. Repeated administration of generalized seizures increased levels of beta-catenin immunoreactivity in the subgranular zone of the hippocampus. To assess the relationship of beta-catenin to cell division in the dentate gyrus of the adult rat hippocampus, colocalization of beta-catenin with a marker of cell division was examined.

RESULTS

Beta-catenin immunoreactivity was consistently localized in newborn cells in this region, indicating a possible role in cell division and differentiation in adult hippocampus. We also found that ECS treatment significantly increased levels of Wnt-2, one of the ligands that activates beta-catenin signaling.

CONCLUSIONS

These results demonstrate that ECS increases Wnt-beta-catenin signaling and suggest that this pathway could mediate in part the neuronal adaptations underlying the therapeutic action of this treatment paradigm.

Internal structure of the nucleus rotundus revealed by mapping cadherin expression in the embryonic chicken visual system

The nucleus rotundus is the largest nucleus of the avian thalamus. It is an important center of visual information processing and conveys information from the optic tectum to the ectostriatum in the telencephalon. The nucleus rotundus is generally believed to contain internal divisions processing information on color, form, motion, and looming of visual objects. The detailed arrangement of these internal divisions is unclear. Here, we map the expression of four classic cadherins (N-cadherin, R-cadherin, cadherin-6B, and cadherin-7), which are markers for specific functional gray matter divisions and their fiber connections in the vertebrate brain. Results show that each cadherin is expressed by one coherent part of the nucleus rotundus that is connected to other brain structures by fiber tracts expressing the same subtype of cadherin. Overall, the expression of the four cadherins encompasses almost the entire nucleus rotundus. The four cadherin-expressing parts show different degrees of overlap. For example, the cadherin-6B part and the cadherin-7 part overlap extensively, whereas the R-cadherin part and the cadherin-6B part show little overlap and are partially complementary. Regions with shallow gradients of cadherin expression alternate with regions that show relatively abrupt changes in cadherin expression. At some points, changes of cadherin expression are also arranged in a pinwheel-like fashion, alternating between clockwise and counterclockwise orientations. In general, these results are reminiscent of the organization of functional modules in the mammalian visual cortex. It is speculated that each domain of cadherin expression corresponds to a functional domain, which processes a specific stimulus feature.

N- and C-terminal domains of beta-catenin, respectively, are required to initiate and shape axon arbors of retinal ganglion cells in vivo

We used deletion mutants to study beta-catenin function in axon arborization of retinal ganglion cells (RGCs) in live Xenopus laevis tadpoles. A deletion mutant betacatDeltaARM consists of the N- and C-terminal domains of wild-type beta-catenin that contain, respectively, alpha-catenin and postsynaptic density-95 (PSD-95)/discs large (Dlg)/zona occludens-1 (ZO-1) (PDZ) binding sites but lacks the central armadillo repeat region that binds cadherins and other proteins. Expression of DeltaARM in RGCs of live tadpoles perturbed axon arborization in two distinct ways: some RGC axons did not form arbors, whereas the remaining RGC axons formed arbors with abnormally long and tangled branches. Expression of the N- and C-terminal domains of beta-catenin separately in RGCs resulted in segregation of these two phenotypes. The axons of RGCs overexpressing the N-terminal domain of beta-catenin developed no or very few branches, whereas axons of RGCs overexpressing the C-terminal domain of beta-catenin formed arbors with long, tangled branches. Additional analysis revealed that the axons of RGCs that did not form arbors after overexpression of DeltaARM or the N-terminal domain of beta-catenin were frequently mistargeted within the tectum. These results suggest that interactions of the N-terminal domain of beta-catenin with alpha-catenin and of the C-terminal domain with PDZ domain-containing proteins are required, respectively, to initiate and shape axon arbors of RGCs in vivo.

Spatial organization of the pigeon tectorotundal pathway: an interdigitating topographic arrangement

The retinotectofugal system is the main visual pathway projecting upon the telencephalon in birds and many other nonmammalian vertebrates. The ascending tectal projection arises exclusively from cells located in layer 13 of the optic tectum and is directed bilaterally toward the thalamic nucleus rotundus. Although previous studies provided evidence that different types of tectal layer 13 cells project to different subdivisions in Rt, apparently without maintaining a retinotopic organization, the detailed spatial organization of this projection remains obscure. We reexamined the pigeon tectorotundal projection using conventional tracing techniques plus a new method devised to perform small deep-brain microinjections of crystalline tracers. We found that discrete injections involving restricted zones within one subdivision retrogradely label a small fraction of layer 13 cells that are distributed throughout the layer, covering most of the tectal representation of the contralateral visual field. Double-tracer injections in one subdivision label distinct but intermingled sets of layer 13 neurons. These results, together with the tracing of tectal axonal terminal fields in the rotundus, lead us to propose a novel "interdigitating" topographic arrangement for the tectorotundal projection, in which intermingled sets of layer 13 cells, presumably of the same particular class and distributed in an organized fashion throughout the surface of the tectum, terminate in separate regions within one subdivision. This spatial organization has significant consequences for the understanding of the physiological and functional properties of the tectofugal pathway in birds.

Identification and localization of multiple classic cadherins in developing rat limbic system

Classic cadherins are multifunctional adhesion proteins that play roles in tissue histogenesis, neural differentiation, neurite outgrowth and synapse formation. Several lines of evidence suggest that classic cadherins may establish regional or laminar recognition cues by virtue of their differential expression and tight, and principally homophilic, cell adhesion. As a first step toward investigating the role this family plays in generating limbic system connectivity, we used RT-PCR to amplify type I and type II classic cadherins present in rat hippocampus during the principal period of synaptogenesis. We identified nine different cadherins, one of which, cadherin-9, is novel in hippocampus. Using in situ hybridization, we compared the cellular and regional distribution of five of the cadherins (N, 6, 8, 9 and 10) during the first two postnatal weeks in hippocampus, subiculum, entorhinal cortex, cingulate cortex, anterior thalamus, hypothalamus and amygdala. We find that each cadherin is differentially distributed in distinct, but highly overlapping fields that largely correspond to known anatomical boundaries and are often coordinately expressed in interconnected regions. For example, cadherin-6 expression defines CA1 and its principal target, the subiculum; cadherin-10 is differentially expressed in CA1 and CA3 in a manner correlating with the organization of interconnecting Schaffer collateral axons; and cadherin-9 shows a striking concentration in CA3. Some cadherin mRNAs are highly restricted to particular anatomical fields over the entire time course, while others are more broadly expressed and become concentrated within particular domains coincident with the timing of afferent ingrowth. Our data indicate that classic cadherins are sufficiently diverse and differentially distributed to support a role in cell surface recognition and adhesion during the formation of limbic system connectivity.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Early enucleation does not alter the gross morphology of identified projection neurons in the chicken optic tectum

During development of the nervous system, neurotrophic interactions are essential for the synchronization of substructures such as retina and visual midbrain. However, morphological studies of postsynaptic elements after ablation of afferents are sparse. We investigated the effect of uni- and bilateral eye anlagen removal on identified projection neurons in the chicken optic tectum. Without retinal input, neurons in the stratum griseum centrale express their specific cell adhesions molecules, retain large dendritic fields and form specialized dendritic endings; however, the latter are deformed and extend over a much larger area. Our results show that even monosynaptically innervated tectal neurons develop largely independently from trophic retinal inputs and only become dependent on these after synaptic contact with retinal afferents.

Up-regulation of cadherin-2 and cadherin-4 in regenerating visual structures of adult zebrafish

Cadherins are homophilic cell adhesion molecules that control development of a variety of tissues and maintenance of adult structures. In this study, we examined expression of zebrafish cadherin-2 (Cdh2, N-cadherin) and cadherin-4 (Cdh4, R-cadherin) in the visual system of adult zebrafish after eye or optic nerve lesions using immunocytochemistry and immunoblotting. Both Cdh2 and Cdh4 immunoreactivities were specifically up-regulated in regenerating retina and/or the optic pathway. Furthermore, temporal expression patterns of these two cadherins were distinct during the regeneration of the injured tissues. Cadherins have been shown to regulate axonal outgrowth in the developing nervous system, but this is the first report, to our knowledge, of increased cadherin expression associated with axonal regeneration in the vertebrate central nervous system. Our results suggest that both Cdh2 and Cdh4 may be important for regeneration of injured retinal ganglion cell axons.

Targeting axons to specific fiber tracts in vivo by altering cadherin expression

In brain development, neurons have to be connected with specific postsynaptic neurons to establish functional neuronal circuits. Cadherins are cell adhesion molecules, which mark developing neuronal circuits. Each member of this class of molecules is expressed only on a restricted set of fiber fascicles that connect gray matter structures to form functional neural circuits. In view of their expression patterns, cadherins have been postulated to play a functional role in the proper establishment of fiber connections. We chose the chicken optic tectum to analyze the instructive potential of cadherins in axonal pathfinding. Three tectofugal pathways, the tectothalamic, tectobulbar, and tectoisthmic tracts, exit the dorsal mesencephalon via the brachium of the superior colliculus, a large fiber structure, which can be divided in specific subtracts that are characterized by the selective expression of N-cadherin, cadherin-7, cadherin-6B, or R-cadherin. By using in vivo electroporation, we overexpressed each of the cadherins in tectal projection neurons between embryonic days 6 and 11. Cotransfection with green fluorescent protein expression plasmid allowed us to assess the pathway choice, which the transgenic axons had made. Quantification based on confocal laser scanning microscopic images revealed that transgenic axons selectively fasciculated with tectofugal tracts specified by the matching type of cadherin. This is the first direct evidence that cadherins mediate differential axonal pathfinding in vivo, possibly by a preferentially homotypic adhesive mechanism.

Developmental expression of beta-catenin in mouse retina

Recent studies have shown that catenins play a pivotal role in neuronal signalling during vertebrate development. In order to study the significance of beta-catenin in the developing mouse retina, the localization of beta-catenin was examined by immunohistochemistry from embryonic day (E) 12 to adult mice. Immunoreactivity for beta-catenin was found in ganglion cells of the retina at E12, and extended to the inner and outer plexiform layer as well as the ganglion-cell layer with the strongest immunolabelling from E16 through to postnatal day (P) 5. The immunoreactivity of ganglion cells was distributed on the cell surface. Thereafter, the immunoreactivity gradually decreased, being limited to the inner plexiform layer and ganglion-cell layer, including the nerve-fiber layer in P10. By P16, the weak immunoreactivity was detected in the inner plexiform layer and ganglion-cell layer, and almost disappeared in the adult retina. No distinct immunoreactivity was found in the retinal pigment epithelium. The reverse transcription polymerase chain reaction showed that beta-catenin messenger ribonuclic acid was detected at E12, E16, P1 and P16, and thereafter markedly decreased, being weakest in the adult. These findings show that beta-catenin is expressed during development at the sites of synaptic connections of inner and outer plexiform layers, and on the ganglion cells and their fibers in the retina, suggesting that beta-catenin might play an important role in the synapse formation and ganglion-cell development during the morphogenesis of the retina.

The cadherin family of cell adhesion molecules: multiple roles in synaptic plasticity

Cadherins are cell adhesion molecules that are critically important for establishing brain structure and connectivity during early development. They are enriched at synapses and, by virtue of a number of properties including homophilic recognition and molecular diversity, have been implicated in the generation of synaptic specificity. Cadherins also participate in remodeling synaptic architecture and modifying the strength of the synaptic signal, thereby retaining an active role in synaptic structure, function, and plasticity, which extends beyond initial development. Cadherins have been implicated in the induction of long-term potentiation (LTP) of hippocampal synaptic strength, a cellular model for learning and memory. LTP is associated with the synthesis and recruitment of N-cadherin to newly forming synaptic junctions, induces molecular changes to N-cadherin indicative of augmented adhesive force, and can be prevented when cadherin adhesion is blocked. NMDA receptor activation, which is critically required for synaptic plasticity, may provide a signal that regulates the molecular configuration of synaptic N-cadherin, and therefore the strength of adhesion across the synaptic cleft. Additionally, there exists at the synapse a pool of surface cadherins that is untethered to the actin cytoskeleton and capable of a rapid and reversible dispersion along the plasmalemma under conditions of strong activity. These observations suggest that synaptic activity dynamically regulates both the strength and the localization of cadherin-cadherin bonds across the synaptic junctional interface, changes that may be crucial for regulating synaptic plasticity.

Protein tyrosine phosphatase-mu differentially regulates neurite outgrowth of nasal and temporal neurons in the retina

Cell adhesion molecules play an important role in the development of the visual system. The receptor-type protein tyrosine phosphatase, PTPmu is a cell adhesion molecule that mediates cell aggregation and may signal in response to adhesion. PTPmu is expressed in the chick retina during development and promotes neurite outgrowth from retinal ganglion cell (RGC) axons in vitro (Burden-Gulley and Brady-Kalnay, 1999). The axons of RGC neurons form the optic nerve, which is the sole output from the retina to the optic tectum in the chick. In this study, we observed that PTPmu expression in RGC axons occurs as a step gradient, with temporal axons expressing the highest level of PTPmu. PTPmu expression in the optic tectum occurred as a smooth descending gradient from anterior to posterior regions during development. Because temporal RGC axons innervate anterior tectal regions, PTPmu may regulate the formation of topographic projections to the tectum. In agreement with this hypothesis, a differential response of RGC neurites to a PTPmu substrate was also observed: RGCs of temporal retina were unable to extend neurites on PTPmu compared with neurites of nasal retina. When given a choice between PTPmu and a second substrate, the growth cones of temporal neurites clustered at the PTPmu border and stalled, thus avoiding additional growth on the PTPmu substrate. In contrast, PTPmu was permissive for growth of nasal neurites. Finally, application of soluble PTPmu to retinal cultures resulted in the collapse of temporal but not nasal growth cones. Therefore, PTPmu may specifically signal to temporal RGC axons to cease their forward growth after reaching the anterior tectum, thus allowing for subsequent innervation of deeper tectal layers.

Interaction of synaptic scaffolding molecule and Beta -catenin

Synaptic scaffolding molecule (S-SCAM) is a synaptic membrane-associated guanylate kinase with inverted domain organization (MAGI) that interacts with NMDA receptor subunits and neuroligin. In epithelial cells, the non-neuronal isoform of S-SCAM (MAGI-1) is localized at tight or adherens junctions. Recent studies have revealed that the polarized targeting of MAGI-1 to the lateral membrane is mediated by its C-terminal region and that MAGI-1 interacts with beta-catenin in epithelial cells. In this article, we report that S-SCAM interacts with beta-catenin in neurons. beta-Catenin is coimmunoprecipitated with S-SCAM from rat brain. Both S-SCAM and beta-catenin are localized at synapses and are partially colocalized. The C-terminal region of S-SCAM binds to the C-terminal region of beta-catenin. We have tested how the interaction between S-SCAM and beta-catenin plays a role in the synaptic targeting of S-SCAM and beta-catenin. S-SCAM is targeted to synapses via the C-terminal postsynaptic density-95/Dlg-A/ZO-1 (PDZ) domain. beta-Catenin is targeted to synapses with armadillo repeats. The overexpressed C-terminal region of beta-catenin blocks the synaptic targeting of S-SCAM. The overexpressed C-terminal region of S-SCAM is partially targeted to synapses and forms a small number of clusters. In the presence of overexpressed beta-catenin, the C-terminal region of S-SCAM forms more clusters at synapses. These data suggest that the synaptic targeting of S-SCAM is mediated by the interaction with beta-catenin.