BDNF mobilizes synaptic vesicles and enhances synapse formation by disrupting cadherin-beta-catenin interactions

The dynamic role of beta-catenin in synaptic plasticity

In addition to its role in development and cell proliferation, β-catenin has been implicated in neuronal synapse regulation and remodeling. Here we review basic molecular and structural mechanisms of synaptic plasticity, followed by a description of the structure and function of β-catenin. We then describe a role for β-catenin in the cellular processes underlying synaptic plasticity. We also review recent data demonstrating that β-catenin mRNA and protein phosphorylation are dynamically regulated during fear memory consolidation in adult animals. Such alterations are correlated with a change in the association of β-catenin with cadherin, and deletion of the β-catenin gene prevents fear learning. Overall, the extant data suggest that β-catenin may function in mediating the structural changes associated with memory formation. This suggests a general role for β-catenin in synaptic remodeling and stabilization underlying long-term memory in adults, and possible roles for dysfunction in the β-catenin pathway in disorders of memory impairment (e.g. Alzheimer's Disease) and in disturbances in which emotional memories are too strong or resistant to inhibition (e.g. fear learning in Posttraumatic Stress Disorder). Further understanding of the β-catenin pathway may lead to better appreciation for the structural mechanisms underlying learning and memory as well as provide novel therapeutic approaches in memory related disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Protein tyrosine phosphatase σ regulates the synapse number of zebrafish olfactory sensory neurons

The formation and refinement of synaptic connections are key steps of neural development to establish elaborate brain networks. To investigate the functional role of protein tyrosine phosphatase (PTP) σ, we employed an olfactory sensory neuron (OSN)-specific gene manipulation system in combination with in vivo imaging of transparent zebrafish embryos. Knockdown of PTPσ enhanced the accumulation of synaptic vesicles in the axon terminals of OSNs. The exaggerated accumulation of synaptic vesicles was restored to the normal level by the OSN-specific expression of PTPσ, indicating that presynaptic PTPσ is responsible for the regulation of synaptic vesicle accumulation. Consistently, transient expression of a dominant-negative form of PTPσ in OSNs enhanced the accumulation of synaptic vesicles. The exaggerated accumulation of synaptic vesicles was reproduced in transgenic zebrafish lines carrying an OSN-specific expression vector of the dominant-negative PTPσ. By electron microscopic analysis of the transgenic line, we found the significant increase of the number of OSN-mitral cell synapses in the central zone of the olfactory bulb. The density of docked vesicles at the active zone was also increased significantly. Our results suggest that presynaptic PTPσ controls the number of OSN-mitral cell synapses by suppressing their excessive increase.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

The Role of GSK3 in Presynaptic Function

The past ten years of research have identified a number of key roles for glycogen synthase kinase 3 (GSK3) at the synapse. In terms of presynaptic physiology, critical roles for GSK3 have been revealed in the growth and maturation of the nerve terminal and more recently a key role in the control of activity-dependent bulk endocytosis of synaptic vesicles. This paper will summarise the major roles assigned to GSK3 in both immature and mature nerve terminals, the substrates GSK3 phosphorylates to exert its action, and how GSK3 activity is regulated by different presynaptic signalling cascades. The number of essential roles for GSK3, coupled with the numerous signalling cascades all converging to regulate its activity, suggests that GSK3 is a key integrator of multiple inputs to modulate the strength of neurotransmission. Modulation of these pathways may point to potential mechanisms to overcome synaptic failure in neurodegenerative disorders such as Alzheimer's disease.

Cadherin-catenin adhesion complexes at the synapse

Classic cadherins function as key organizers during the formation and remodeling of synapses in the vertebrate central nervous system. Cadherins are Ca2+-dependent homophilic adhesion molecules whose adhesive strength can be regulated by conformational changes, through cadherin's association with intracellular binding proteins, and by the regulation of cadherin turnover and internalization. In this mini-review, we will highlight recent studies on the role of cadherins and their associated partners in regulating synaptic architecture. Moreover, we will discuss molecular mechanisms underlying cadherin turnover and the subsequent impact on synaptic connections.

Sharing vesicles between central presynaptic terminals: implications for synaptic function

Presynaptic terminals in hippocampal neurons house functionally distinct vesicle pools, the size, structure and biochemical features of which are major determinants of presynaptic strength and performance. In classical models of synaptic function these vesicle pools are synapse-specific, but accumulating evidence is now demonstrating that some vesicles are laterally mobile along axons and readily shared in a functional manner across adjacent terminals. In effect then, these mobile vesicles represent a further class of synapse-spanning vesicle pool, or "superpool". Here we outline the characteristics of this additional pool type, discussing its structural organization within axons and presynaptic terminals as well as its relationship with conventional vesicle pools. We draw comparisons between extrasynaptic vesicle dynamics and the growing literature on extrasynaptic mobility of non-vesicular synaptic elements which, taken together, raise important questions about the operational independence of adjacent release sites. We also examine the functional implications of lateral vesicle sharing, from the notion that extrasynaptic vesicles can contribute to the release capabilities of individual terminals, to its potential role as a substrate for facilitating changes in synaptic weight as a basis for plasticity.

Unraveling mechanisms of homeostatic synaptic plasticity

Homeostatic synaptic plasticity is a negative feedback mechanism that neurons use to offset excessive excitation or inhibition by adjusting their synaptic strengths. Recent findings reveal a complex web of signaling processes involved in this compensatory form of synaptic strength regulation, and in contrast to the popular view of homeostatic plasticity as a slow, global phenomenon, neurons may also rapidly tune the efficacy of individual synapses on demand. Here we review our current understanding of cellular and molecular mechanisms of homeostatic synaptic plasticity.

A vesicle superpool spans multiple presynaptic terminals in hippocampal neurons

Synapse-specific vesicle pools have been widely characterized at central terminals. Here, we demonstrate a vesicle pool that is not confined to a synapse but spans multiple terminals. Using fluorescence imaging, correlative electron microscopy, and modeling of vesicle dynamics, we show that some recycling pool vesicles at synapses form part of a larger vesicle "superpool." The vesicles within this superpool are highly mobile and are rapidly exchanged between terminals (turnover: approximately 4% of total pool/min), significantly changing vesicular composition at synapses over time. In acute hippocampal slices we show that the mobile vesicle pool is also a feature of native brain tissue. We also demonstrate that superpool vesicles are available to synapses during stimulation, providing an extension of the classical recycling pool. Experiments using focal BDNF application suggest the involvement of a local TrkB-receptor-dependent mechanism for synapse-specific regulation of presynaptic vesicle pools through control of vesicle release and capture to or from the extrasynaptic pool.

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.

Wnt signaling mediates experience-related regulation of synapse numbers and mossy fiber connectivities in the adult hippocampus

We investigated how experience regulates the structure of a defined neuronal circuit in adult mice. Enriched environment (EE) produced a robust and reversible increase in hippocampal stratum lucidum synapse numbers, mossy fiber terminal (LMT) numbers, and spine plus synapse densities at LMTs, whereas a distinct mechanism depending on Rab3a promoted LMT volume growth. In parallel, EE increased postsynaptic CA3 pyramidal neuron Wnt7a/b levels. Inhibiting Wnt signaling through locally applied sFRP-1 suppressed the effects of EE on synapse numbers and further reduced synapse numbers in control mice. Wnt7 applied to CA3 mimicked the effects of EE on synapse and LMT numbers. CA3 Wnt7a/b levels were enhanced by excitatory activity and reduced by sFRP-1. Synapse numbers and Wnt7a/b levels peaked in mice aged 6-12 months; a decline in aged mice was reversed by EE. Therefore, behavioral experience specifically regulates adult global stratum lucidum synapse numbers and hippocampal network structure through Wnt signaling.

Scribble interacts with beta-catenin to localize synaptic vesicles to synapses

An understanding of how synaptic vesicles are recruited to and maintained at presynaptic compartments is required to discern the molecular mechanisms underlying presynaptic assembly and plasticity. We have previously demonstrated that cadherin-beta-catenin complexes cluster synaptic vesicles at presynaptic sites. Here we show that scribble interacts with the cadherin-beta-catenin complex to coordinate vesicle localization. Scribble and beta-catenin are colocalized at synapses and can be coimmunoprecipitated from neuronal lysates, indicating an interaction between scribble and beta-catenin at the synapse. Using an RNA interference approach, we demonstrate that scribble is important for the clustering of synaptic vesicles at synapses. Indeed, in scribble knockdown cells, there is a diffuse distribution of synaptic vesicles along the axon, and a deficit in vesicle recycling. Despite this, synapse number and the distribution of the presynaptic active zone protein, bassoon, remain unchanged. These effects largely phenocopy those observed after ablation of beta-catenin. In addition, we show that loss of beta-catenin disrupts scribble localization in primary neurons but that the localization of beta-catenin is not dependent on scribble. Our data supports a model by which scribble functions downstream of beta-catenin to cluster synaptic vesicles at developing synapses.

Effects of soft-diet feeding on BDNF expression in hippocampus of mice

Our previous study showed that mice fed a soft diet after weaning had reduced synaptic connections in the hippocampal formation and impaired spatial learning ability after 3 months of age. We hypothesized that soft-diet feeding during development reduced levels of brain-derived neurotrophic factor (BDNF) protein in the hippocampus, resulting in lower synaptic densities in this region. Male pups of C57BL/6 mice were fed either a solid (hard-diet group) or powdered diet (soft-diet group), starting at weaning. Expression of BDNF protein in the hippocampus and cerebral cortex was evaluated quantitatively with enzyme-linked immunosorbent assay (ELISA) at 1, 3 and 6 months of age. Reduction in BDNF protein levels due to soft diet was detected markedly in the hippocampus of 3- and 6-month-old mice. On the other hand, a soft diet showed no significant effect on BDNF content in the cerebral cortex throughout the ages investigated. Immunohistochemistry of hippocampal formation in 3-month-old mice revealed that intensities of BDNF immunoreactivity in the dentate gyrus granule cell layer and CA1 and CA3 pyramidal cell layers appeared diminished in mice fed the soft diet compared with mice fed the hard diet. These results indicate that insufficient mastication activity during development reduces BDNF protein levels in the hippocampus and influences synaptic plasticity in this region.

Regulation of dendrite and spine morphogenesis and plasticity by catenins

The appropriate regulation of dendrite, spine, and synapse morphogenesis in neurons both during and after development is critical for the formation and maintenance of neural circuits. It is becomingly increasingly clear that the cadherin-catenin cell adhesion complex, a complex that has been widely studied in epithelia, regulates neuronal morphogenesis. More interestingly, the catenins, cytosolic proteins that bind to cadherins, regulate multiple aspects of neuronal morphogenesis including dendrite, spine, and synapse morphogenesis and plasticity, both independent of and dependent on their ability to bind cadherins. In this review, we examine some of the more recent and exciting studies that implicate individual catenins in various aspects of neuronal morphogenesis and plasticity.

Signaling by synaptogenic molecules

Multiple signaling pathways initiate and specify the formation of synapses in the central nervous system. General principles that organize nascent synapses have emerged from the studies in multiple model organisms. These include the synapse-organizing roles of dedicated synaptic adhesion molecules, synaptic signaling following receptor-ligand interactions, and the regulation of synapse formation by secreted molecules. Intracellularly, a range of effectors subsequently regulates signaling steps and cytoskeletal changes. Together, a blueprint of synapse formation is emerging into which these distinct signaling steps will need to be integrated temporally and spatially.

Effects of the beta-amino acid antagonist TAG on thalamocortical inhibition

Chemical transmission at inhibitory synapses in thalamus may involve receptor activation by beta-amino acids and glycine, as well as GABA. Given their hypothesized roles, we investigated effects of the putative beta-amino acid antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide (TAG) on synaptic inhibition in dorsal thalamus. We performed whole-cell recordings in 200-250 microm sections and immunocytochemical (ICC) studies in ventrobasal thalamus of rat brain (P12-P14). Stimulation of medial lemniscus evoked inhibitory postsynaptic currents (IPSCs) which were purely glycinergic or GABA(A)ergic, or most commonly mixed glycinergic and GABA(A)ergic responses, based on abolition by strychnine, bicuculline, or combined antagonism. TAG antagonized mixed IPSCs (IC(50) approximately 70 microM) in a manner distinguishable from classical glycine and GABA(A) receptor antagonists. TAG (250 microM) reduced the amplitude of glycinergic components which had a decay time constant of approximately 9 ms or approximately 230 ms by 45-50%, and a GABA(A)ergic component which had a decay time constant of approximately 40 ms by approximately 60%. As in the glycinergic component, TAG reduced the amplitude of infrequently occurring, pure glycinergic IPSCs. Surprisingly, TAG had no effect on pure GABA(A)ergic IPSCs, with a decay time constant of approximately 20 ms that correlated to kinetics of GABA-activated channels. ICC studies showed co-localization of alpha(1/2) glycine and alpha(4) GABA(A) receptors at inhibitory synapses. Activation of alpha(4) receptors by beta-amino acids may contribute to the GABA(A)ergic component of mixed IPSCs. The short and long-duration glycinergic IPSCs had decay time constants that correlated to the burst durations of single channels opened by beta-amino acids and glycine. Overall, the effects of TAG implicate beta-amino acid involvement in GABA(A)ergic and glycinergic transmission.

N- and E-cadherin expression in human ovarian and urogenital duct development

OBJECTIVE

To investigate expression of N- and E-cadherin in the developing human ovary.

DESIGN

The expression of N- and E-cadherin was analyzed in 18 human fetal ovaries between 8 and 20 weeks' gestation using immunohistochemistry. Fetal human male and rat urogenital tracts were used for comparison of expression.

SETTING

Academic research institute.

PATIENT(S)

Women undergoing termination of pregnancy.

INTERVENTION(S)

Immunofluorescent analysis of cadherin expression.

RESULT(S)

In fetal ovary, N- and E-cadherins were expressed at all gestations with overlapping but not identical patterns. Expression was associated with germ cells and adjacent somatic cells, including within newly formed primordial follicles, but neither cadherin was expressed in the somatic cell cords. The epithelia of the müllerian and wolffian ducts expressed only N- and E-cadherin, respectively, in a mutually exclusive fashion. This pattern of cadherin expression was found to be conserved between human and rat fetuses of both genders.

CONCLUSION(S)

The demonstration of N- and E-cadherin expression in the human fetal ovary indicates likely roles in gonadal development from germ cell proliferation to primordial follicle formation, as well as in the development of the urogenital ducts of both genders. This is consistent with animal studies identifying cadherins as key regulators of early germ cell development.

Trans-synaptic adhesion between NGL-3 and LAR regulates the formation of excitatory synapses

Synaptic adhesion molecules regulate multiple steps of synapse formation and maturation. The great diversity of neuronal synapses predicts the presence of a large number of adhesion molecules that control synapse formation through trans-synaptic and heterophilic adhesion. We identified a previously unknown trans-synaptic interaction between netrin-G ligand-3 (NGL-3), a postsynaptic density (PSD) 95-interacting postsynaptic adhesion molecule, and leukocyte common antigen-related (LAR), a receptor protein tyrosine phosphatase. NGL-3 and LAR expressed in heterologous cells induced pre- and postsynaptic differentiation in contacting axons and dendrites of cocultured rat hippocampal neurons, respectively. Neuronal overexpression of NGL-3 increased presynaptic contacts on dendrites of transfected neurons. Direct aggregation of NGL-3 on dendrites induced coclustering of excitatory postsynaptic proteins. Knockdown of NGL-3 reduced the number and function of excitatory synapses. Competitive inhibition by soluble LAR reduced NGL-3-induced presynaptic differentiation. These results suggest that the trans-synaptic adhesion between NGL-3 and LAR regulates excitatory synapse formation in a bidirectional manner.

Influence of brain-derived neurotrophic factor on pathfinding of dentate granule cell axons, the hippocampal mossy fibers

Mossy fibers, the dentate granule cell axons, are generated throughout an animal's lifetime. Mossy fiber paths and synapses are primarily restricted to the stratum lucidum within the CA3 region. Brain-derived neurotrophic factor (BDNF), a neurotrophin family protein that activates Trk neurotrophin receptors, is highly expressed in the stratum lucidum in an activity-dependent manner. The addition of a Trk neurotrophin receptor inhibitor, K252a, to cultured hippocampal slices induced aberrant extension of mossy fibers into ectopic regions. BDNF overexpression in granule cells ameliorated the mossy fiber pathway abnormalities caused by a submaximal dose of K252a. A similar rescue was observed when BDNF was expressed in CA3 pyramidal cells, most notably in mossy fibers distal to the expression site. These findings are the first to clarify the role of BDNF in mossy fiber pathfinding, not as an attractant cue but as a regulator, possibly acting in a paracrine manner. This effect of BDNF may be as a signal for new fibers to fasciculate and extend further to form synapses with neurons that are far from active BDNF-expressing synapses. This mechanism would ensure the emergence of new independent dentate gyrus-CA3 circuits by the axons of new-born granule cells.

Effect of NO-generating compound NaNO2 on ultrastructure of synaptic vesicles of glutamatergic synapses

Ultrastructure of synaptic vesicles in axon terminals of granule cells from isolated cerebellum of Rana temporaria frogs under the influence of NO-generating compound NaNO2 in various concentrations and electrical stimulation was evaluated by the method of electron microscopy. NO-generating compound in low concentration induced translocation of synaptic vesicles and formation of small clusters. The size and structure of synaptic vesicles remained unchanged under these conditions. Increasing the concentration of NaNO2 led to swelling of synaptic vesicles, formation of arranged heaps from individual vesicles or fusion of their content. Electrical stimulation of the cerebellum in the presence of NaNO2 increased damage to synaptic vesicles. These experimental data model some stages observed in stroke. The formation of clusters from synaptic vesicles is a compensatory and adaptive response maintaining the structure of synaptic vesicles and protecting neurons from high concentrations of glutamate. Glutamate produces a toxic effect on nerve cells and glial cells of the cerebellum under pathological conditions, which is accompanied by impairment of signal transduction from presynaptic to postsynaptic neurons.

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell-cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate beta-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell-cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate beta-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

Synapses are regulated by the cytoplasmic tyrosine kinase Fer in a pathway mediated by p120catenin, Fer, SHP-2, and beta-catenin

Localization of presynaptic components to synaptic sites is critical for hippocampal synapse formation. Cell adhesion–regulated signaling is important for synaptic development and function, but little is known about differentiation of the presynaptic compartment. In this study, we describe a pathway that promotes presynaptic development involving p120catenin (p120ctn), the cytoplasmic tyrosine kinase Fer, the protein phosphatase SHP-2, and β-catenin. Presynaptic Fer depletion prevents localization of active zone constituents and synaptic vesicles and inhibits excitatory synapse formation and synaptic transmission. Depletion of p120ctn or SHP-2 similarly disrupts synaptic vesicle localization with active SHP-2, restoring synapse formation in the absence of Fer. Fer or SHP-2 depletion results in elevated tyrosine phosphorylation of β-catenin. β-Catenin overexpression restores normal synaptic vesicle localization in the absence of Fer or SHP-2. Our results indicate that a presynaptic signaling pathway through p120ctn, Fer, SHP-2, and β-catenin promotes excitatory synapse development and function.

Formin-dependent synaptic growth: evidence that Dlar signals via Diaphanous to modulate synaptic actin and dynamic pioneer microtubules

The diaphanous gene is the founding member of a family of Diaphanous-related formin proteins (DRFs). We identified diaphanous in a screen for genes that are necessary for the normal growth and stabilization of the Drosophila neuromuscular junction (NMJ). Here, we demonstrate that diaphanous mutations perturb synaptic growth at the NMJ. Diaphanous protein is present both presynaptically and postsynaptically. However, genetic rescue experiments in combination with additional genetic interaction experiments support the conclusion that dia is necessary presynaptically for normal NMJ growth. We then document defects in both the actin and microtubule cytoskeletons in dia mutant nerve terminals. In so doing, we define and characterize a population of dynamic pioneer microtubules within the NMJ that are distinct from the bundled core of microtubules identified by the MAP1b-like protein Futsch. Defects in both synaptic actin and dynamic pioneer microtubules are correlated with impaired synaptic growth in dia mutants. Finally, we present genetic evidence that Dia functions downstream of the presynaptic receptor tyrosine phosphatase Dlar and the Rho-type GEF (guanine nucleotide exchange factor) trio to control NMJ growth. Based on the established function of DRFs as Rho-GTPase-dependent regulators of the cell cytoskeleton, we propose a model in which Diaphanous links receptor tyrosine phosphatase signaling at the plasma membrane to growth-dependent modulation of the synaptic actin and microtubule cytoskeletons.

Transducing Neuronal Activity into Dendritic Spine Morphology: New Roles for p38 MAP Kinase and N-cadherin

Synaptic plasticity depends on the generation, modification and disconnection of synapses. An excitatory synapse is connected to a specialized dendritic compartment called a spine, which undergoes activity-induced remodeling. Here, we discuss a signaling pathway that transduces neuronal activity into the remodeling of spine through p38 mitogen-activated protein kinase (MAPK) and N-cadherin. Dendritic spines change their morphology and density in response to neuronal activity. In the early phase, posttranslational modifications of synaptic molecules regulate spine morphology, whereas activity-induced gene products reduce spine density in the late phase. One of the targets of these mechanisms is N-cadherin. An activity-induced protocadherin, arcadlin, stimulates thousand and one 2β (TAO2β) kinase, which in turn activates p38 MAPK through MAPK kinase 3 (MEK3), resulting in the endocytosis of N-cadherin and the decrease in spine number. This pathway also underlies the mechanism of the spine decrease in neuronal disorders, such as Alzheimer's disease and epilepsy. Development of new p38 MAPK inhibitors brings a ray of hope with respect to the development of more effective therapies for these patients.

Beta-catenin is required for memory consolidation

Identified for its role in development, β-catenin has been implicated in neuronal synapse regulation and remodeling. We examined β-catenin expression in the adult mouse brain and its role in amygdala-dependent learning and memory. We found alterations in β-catenin mRNA and protein phosphorylation during fear memory consolidation. Such alterations correlated with a change in the association of β-catenin with cadherin. Pharmacologically, this consolidation was enhanced with lithium-mediated facilitation of β-catenin. Genetically, the role of β-catenin was confirmed with site-specific deletions of floxed β-catenin in the amygdala. Baseline locomotor, anxiety-related behaviors, and the acquisition or expression of conditioned fear were normal. However, amygdala-specific deletion prevented the normal transfer of newly formed fear learning into long-term memory. Thus, β-catenin within the amygdala may be required for the normal consolidation, but not acquisition, of fear memory. This suggests a general role for β-catenin in synaptic remodeling and stabilization underlying long-term memory in adults.

Cadherins and catenins at synapses: roles in synaptogenesis and synaptic plasticity

Synapse formation involves reciprocal interactions between cells resulting in formation of a structure optimized for efficient information transfer. Recent work has implicated constituents of the cadherin-catenin cell-adhesion complex in both synapse formation and plasticity. In this review, we describe recent interesting discoveries on mechanisms of cadherin complex function, in addition to regulating adhesion, that are relevant for understanding the role of this complex in synaptogenesis and plasticity. We describe how this complex acts via (i) recruitment/stabilization of intracellular partners; (ii) regulation of intracellular signaling pathways; (iii) regulation of cadherin surface levels, stability and turnover; (iv) stabilization of receptors; and (v) regulation of gene expression. These exciting discoveries provide insights into novel functional roles of the complex beyond regulating cell adhesion.

Signalling by neurotrophins and hepatocyte growth factor regulates axon morphogenesis by differential beta-catenin phosphorylation

Tyrosine phosphorylation of beta-catenin, a component of adhesion complexes and of the Wnt pathway, affects cell adhesion, migration and gene transcription. By reducing beta-catenin availability using shRNA-mediated gene silencing or expression of intracellular N-cadherin, we show that beta-catenin is required for axon growth downstream of brain-derived neurotrophic factor (BDNF) signalling and hepatocyte growth factor (HGF) signalling. We demonstrate that the receptor tyrosine kinases (RTKs) Trk and Met interact with and phosphorylate beta-catenin. Stimulation of Trk receptors by neurotrophins (NTs) results in phosphorylation of beta-catenin at residue Y654, and increased axon growth and branching. Conversely, pharmacological inhibition of Trk or expression of a Y654F mutant blocks these effects. beta-catenin phosphorylated at Y654 colocalizes with the cytoskeleton at growth cones. However, HGF, which also increases axon growth and branching, induces beta-catenin phosphorylation at Y142 and a nuclear localization. Interestingly, dominant-negative DeltaN-TCF4 abolishes the effects of HGF in axon growth and branching, but not that of NTs. We conclude that NT- and HGF-signalling differentially phosphorylate beta-catenin, targeting this protein to distinct compartments to regulate axon morphogenesis by TCF4-transcription-dependent and -independent mechanisms. These results place beta-catenin downstream of growth-factor-RTK signalling in axon differentiation.

Actin in action: the interplay between the actin cytoskeleton and synaptic efficacy

Synapse regulation exploits the capacity of actin to function as a stable structural component or as a dynamic filament. Beyond its well-appreciated role in eliciting visible morphological changes at the synapse, the emerging picture points to an active contribution of actin to the modulation of the efficacy of pre- and postsynaptic terminals. Moreover, by engaging distinct pools of actin and divergent signalling pathways, actin-dependent morphological plasticity could be uncoupled from modulation of synaptic strength. The aim of this Review is to highlight some of the recent progress in elucidating the role of the actin cytoskeleton in synaptic function.

Multifunctional role of protein kinase C in regulating the formation and maturation of specific synapses

Target-dependent increases in axon growth and varicosities accompany the formation of functional synapses between Aplysia sensory neurons and specific postsynaptic neurons (L7 and not L11). The enhanced growth is regulated in part by a target-dependent increase in the secretion of sensorin, the sensory neuron neuropeptide. We report here that protein kinase C (PKC) activity is required for synapse formation by sensory neurons with L7 and for the target-dependent increases in sensorin synthesis and secretion. Blocking PKC activity reversibly blocked synapse formation and axon growth of sensory neurons contacting L7, but did not affect axon growth of sensory neurons contacting L11 or axon growth of the postsynaptic targets. Blocking PKC activity also blocked the target-induced increase in sensorin synthesis and secretion. Sensorin then activates additional signaling pathways required for synapse maturation and synapse-associated growth. Exogenous anti-sensorin antibody blocked target-induced activation and translocation into sensory neuron nuclei of p42/44 mitogen-activated protein kinase (MAPK), attenuated synapse maturation, and curtailed growth of sensory neurons contacting L7, but not the growth of sensory neurons contacting L11. Inhibitors of MAPK or phosphoinositide 3-kinase also attenuated synapse maturation and curtailed growth and varicosity formation of sensory neurons contacting L7, but not growth of sensory neurons contacting L11. These results suggest that PKC activity regulated by specific cell-cell interactions initiates the formation of specific synapses and the subsequent synthesis and release of a neuropeptide to activate additional signaling pathways required for synapse maturation.

Catenins: playing both sides of the synapse

Synapses of the central nervous system (CNS) are specialized cell-cell junctions that mediate intercellular signal transmission from one neuron to another. The directional nature of signal relay requires synaptic contacts to be morphologically asymmetric with distinct protein components, while changes in synaptic communication during neural network formation require synapses to be plastic. Synapse morphology and plasticity require a dynamic actin cytoskeleton. Classical cadherins, which are junctional proteins associated with the actin cytoskeleton, localize to synapses and regulate synaptic adhesion, stability and remodeling. The major intracellular components of cadherin junctions are the catenin proteins, and increasing evidence suggests that cadherin-catenin complexes modulate an array of synaptic processes. Here we review the role of catenins in regulating the development of pre- and postsynaptic compartments and function in synaptic plasticity, with particular focus on their role in regulating the actin cytoskeleton.

Slit1a inhibits retinal ganglion cell arborization and synaptogenesis via Robo2-dependent and -independent pathways

Upon arriving at their targets, developing axons cease pathfinding and begin instead to arborize and form synapses. To test whether CNS arborization and synaptogenesis are controlled by Slit-Robo signaling, we followed single retinal ganglion cell (RGC) arbors over time. ast (robo2) mutant and slit1a morphant arbors had more branch tips and greater arbor area and complexity compared to wild-type and concomitantly more presumptive presynaptic sites labeled with YFP-Rab3. Increased arborization in ast was phenocopied by dominant-negative Robo2 expressed in single RGCs and rescued by full-length Robo2, indicating that Robo2 acts cell-autonomously. Time-lapse imaging revealed that ast and slit1a morphant arbors stabilized earlier than wild-type, suggesting a role for Slit-Robo signaling in preventing arbor maturation. Genetic analysis showed that Slit1a acts both through Robo2 and Robo2-independent mechanisms. Unlike previous PNS studies showing that Slits promote branching, our results show that Slits inhibit arborization and synaptogenesis in the CNS.

Brain-derived neurotrophic factor and its receptor tropomyosin-related kinase B in the mechanism of action of antidepressant therapies

The focus of this review is to critically examine and review the literature on the role of brain-derived neurotrophic factor (BDNF) and its primary receptor, tropomyosin-related kinase B (TrkB), in the actions of pharmacologically diverse antidepressant treatments for depression. This will include a review of the studies on the regulation of BDNF and TrkB by different types of antidepressant drug treatments and animal in models of depression, as well as altered levels of BDNF and TrkB in the blood and postmortem brain of patients with depression. Results from clinical and basic studies have demonstrated that stress and depression decrease BDNF expression and neurogenesis and antidepressant treatment reverses or blocks these effects, leading to the neurotrophic hypothesis of depression. Clinical studies demonstrate an association between BDNF levels and several disorders, including depression, epilepsy, bipolar disorder, Parkinson's and Alzheimer's diseases. Physical activity and diet exert neurotrophic effects and positively modulate BDNF levels. A common single nucleotide polymorphism (SNP) in the BDNF gene, a methionine substitution for valine, is associated with alterations in brain anatomy and memory, but what role it has in clinical disorders is unclear. Findings suggest that early childhood events and adult stress produce neurodegenerative alterations in the brain that can eventually cause breakdown of information processing in the neuronal networks regulating mood. Antidepressant treatments elevate activity-dependent neuronal plasticity by activating BDNF, thereby gradually restoring network function and ultimately mood.

Matrix metalloproteinase-7 modulates synaptic vesicle recycling and induces atrophy of neuronal synapses

Matrix metalloproteinase-7 (MMP-7) belongs to a family of zinc dependent endopeptidases that are expressed in a variety of tissues including the brain. MMPs are known to be potent mediators of pericellular proteolysis and likely mediators of dynamic remodelling of neuronal connections. While an association between proteases and the neuronal synapse is emerging, a full understanding of this relationship is lacking. Here, we show that MMP-7 alters the structure and function of presynaptic terminals without affecting neuronal survival. Bath application of recombinant MMP-7 to cultured rat neurons induced long-lasting inhibition of vesicular recycling as measured by synaptotagmin 1 antibody uptake assays and FM4-64 optical imaging. MMP-7 application resulted in reduced abundance of vesicular and active zone proteins locally within synaptic terminals although their general levels remained unaltered. Finally, chronic application of the protease resulted in synaptic atrophy, including smaller terminals and fewer synaptic vesicles, as determined by electron microscopy. Together these results suggest that MMP-7 is a potent modulator of synaptic vesicle recycling and synaptic ultrastructure and that elevated levels of the enzyme, as may occur with brain inflammation, may adversely influence neurotransmission.

Share and share alike: trading of presynaptic elements between central synapses

Central presynaptic terminals harbour synaptic vesicles (SVs) and synapse-specific proteins necessary for neurotransmission. Classically, these elements were thought to reside more or less stably at individual mature synapses, giving rise to the idea that each terminal was essentially an independent functional unit. However, emerging evidence from fluorescence imaging studies in hippocampal cultured neurons is now challenging this view, suggesting that neighbouring synapses along axons share vesicles, and also other synaptic elements, at high levels. This raises the possibility that control of import and export might be an important regulatory target for the maintenance of release sites, modulation of synaptic efficacy and formation of new synaptic contacts. Here, temporal synaptic stability and the functional consequences for presynaptic operation will be considered.

Persistence of long-term memory storage requires a late protein synthesis- and BDNF- dependent phase in the hippocampus

Persistence is the most characteristic attribute of long-term memory (LTM). To understand LTM, we must understand how memory traces persist over time despite the short-lived nature and rapid turnover of their molecular substrates. It is widely accepted that LTM formation is dependent upon hippocampal de novo protein synthesis and Brain-Derived Neurotrophic Factor (BDNF) signaling during or early after acquisition. Here we show that 12 hr after acquisition of a one-trial associative learning task, there is a novel protein synthesis and BDNF-dependent phase in the rat hippocampus that is critical for the persistence of LTM storage. Our findings indicate that a delayed stabilization phase is specifically required for maintenance, but not formation, of the memory trace. We propose that memory formation and memory persistence share some of the same molecular mechanisms and that recurrent rounds of consolidation-like events take place in the hippocampus for maintenance of the memory trace.

Role of glycogen synthase kinase-3beta in early depressive behavior induced by mild traumatic brain injury

Traumatic brain injury (TBI) is a triggering event for a set of pathophysiological changes and concomitant depressive behavior. Glycogen synthase kinase-3 (GSK-3) is a potent in vivo regulator of cell apoptosis and, in addition, is implicated in depressive behavior. In this study, we investigated the role of GSK-3 in the physiological model of mild TBI (mTBI) at both the cellular and behavior levels. mTBI resulted in increased phosphorylation of inhibitory site serine(9) of GSK-3beta, which coincided with increased serine(473) phosphorylation of its upstream kinase PKB and accumulation of its downstream target beta-catenin in the hippocampus. mTBI induced a depressive behavior which was evident as early as 24 h post-injury. Pretreatment with GSK-3 inhibitors, lithium, or L803-mts prevented mTBI-induced depression. We suggest that mTBI elicits a pro-survival cascade of PKB/GSK-3beta/beta-catenin as part of a rehabilitation program. Furthermore, the use of selective GSK-3 inhibitors may have therapeutic benefits in treatment conditions associated with brain injury.