Asymmetric N-cadherin expression results in synapse dysfunction, synapse elimination, and axon retraction in cultured mouse neurons

Cell adhesion molecule immobilized gold surfaces for enhanced neuron-electrode interfaces

To provide a long-term solution for increasing the biocompatibility of neuroprosthetics, approaches to reduce the side effects of invasive neuro-implantable devices are still in need of improvement. Physical, chemical, and bioactive design aspects of the biomaterials are proven to be important for providing proper cell-to-cell, cell-to-material interactions. Particularly, modification of implant surfaces with bioactive cues, especially cell adhesion molecules (CAMs) that capitalize on native neural adhesion mechanisms, are promising candidates in favor of providing efficient interfaces. Within this concept, this study utilized specific CAMs, namely N-Cadherin (Neural cadherin, N-Cad) and neural cell adhesion molecule (NCAM), to enhance neuron-electrode contact by mimicking the cell-to-ECM interactions for improving the survival of cells and promoting neurite outgrowth. For this purpose, representative gold electrode surfaces were modified with N-Cadherin, NCAM, and the mixture (1:1) of these molecules. Modifications were characterized, and the effect of surface modification on both differentiated and undifferentiated neuroblastoma SH-SY5Y cell lines were compared. The findings demonstrated the successful modification of these molecules which subsequently exhibited biocompatible properties as evidenced by the cell viability results. In cell culture experiments, the CAMs displayed promising results in promoting neurite outgrowth compared to conventional poly-l-lysine coated surfaces, especially NCAM and N-Cad/NCAM modified surfaces clearly showed significant improvement. Overall, this optimized approach is expected to provide an insight into the action mechanisms of cells against the local environment and advance processes for the fabrication of alternative neural interfaces.

Plk2 promotes synaptic destabilization through disruption of N-cadherin adhesion complexes during homeostatic adaptation to hyperexcitation

Synaptogenesis in the brain is highly organized and orchestrated by synaptic cellular adhesion molecules (CAMs) such as N-cadherin and amyloid precursor protein (APP) that contribute to the stabilization and structure of synapses. Although N-cadherin plays an integral role in synapse formation and synaptic plasticity, its function in synapse dismantling is not as well understood. Synapse weakening and loss are prominent features of neurodegenerative diseases, and can also be observed during homeostatic compensation to neuronal hyperexcitation. Previously, we have shown that during homeostatic synaptic plasticity, APP is a target for cleavage triggered by phosphorylation by Polo-like kinase 2 (Plk2). Here, we found that Plk2 directly phosphorylates N-cadherin, and during neuronal hyperexcitation Plk2 promotes N-cadherin proteolytic processing, degradation, and disruption of complexes with APP. We further examined the molecular mechanisms underlying N-cadherin degradation. Loss of N-cadherin adhesive function destabilizes excitatory synapses and promotes their structural dismantling as a prerequisite to eventual synapse elimination. This pathway, which may normally help to homeostatically restrain excitability, could also shed light on the dysregulated synapse loss that occurs in cognitive disorders.

Proteomics of the dentate gyrus reveals semantic dementia specific molecular pathology

Semantic dementia (SD) is a clinical subtype of frontotemporal dementia consistent with the neuropathological diagnosis frontotemporal lobar degeneration (FTLD) TDP type C, with characteristic round TDP-43 protein inclusions in the dentate gyrus. Despite this striking clinicopathological concordance, the pathogenic mechanisms are largely unexplained forestalling the development of targeted therapeutics. To address this, we carried out laser capture microdissection of the dentate gyrus of 15 SD patients and 17 non-demented controls, and assessed relative protein abundance changes by label-free quantitative mass spectrometry. To identify SD specific proteins, we compared our results to eight other FTLD and Alzheimer's disease (AD) proteomic datasets of cortical brain tissue, parallel with functional enrichment analyses and protein-protein interactions (PPI). Of the total 5,354 quantified proteins, 151 showed differential abundance in SD patients (adjusted P-value < 0.01). Seventy-nine proteins were considered potentially SD specific as these were not detected, or demonstrated insignificant or opposite change in FTLD/AD. Functional enrichment indicated an overrepresentation of pathways related to the immune response, metabolic processes, and cell-junction assembly. PPI analysis highlighted a cluster of interacting proteins associated with adherens junction and cadherin binding, the cadherin-catenin complex. Multiple proteins in this complex showed significant upregulation in SD, including β-catenin (CTNNB1), γ-catenin (JUP), and N-cadherin (CDH2), which were not observed in other neurodegenerative proteomic studies, and hence may resemble SD specific involvement. A trend of upregulation of all three proteins was observed by immunoblotting of whole hippocampus tissue, albeit only significant for N-cadherin. In summary, we discovered a specific increase of cell adhesion proteins in SD constituting the cadherin-catenin complex at the synaptic membrane, essential for synaptic signaling. Although further investigation and validation are warranted, we anticipate that these findings will help unravel the disease processes underlying SD.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca²⁺-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

CDH2 mutation affecting N-cadherin function causes attention-deficit hyperactivity disorder in humans and mice

Attention-deficit hyperactivity disorder (ADHD) is a common childhood-onset psychiatric disorder characterized by inattention, impulsivity and hyperactivity. ADHD exhibits substantial heritability, with rare monogenic variants contributing to its pathogenesis. Here we demonstrate familial ADHD caused by a missense mutation in CDH2, which encodes the adhesion protein N-cadherin, known to play a significant role in synaptogenesis; the mutation affects maturation of the protein. In line with the human phenotype, CRISPR/Cas9-mutated knock-in mice harboring the human mutation in the mouse ortholog recapitulated core behavioral features of hyperactivity. Symptoms were modified by methylphenidate, the most commonly prescribed therapeutic for ADHD. The mutated mice exhibited impaired presynaptic vesicle clustering, attenuated evoked transmitter release and decreased spontaneous release. Specific downstream molecular pathways were affected in both the ventral midbrain and prefrontal cortex, with reduced tyrosine hydroxylase expression and dopamine levels. We thus delineate roles for CDH2-related pathways in the pathophysiology of ADHD.

Molecular mechanisms of attention-deficit hyperactivity disorder (ADHD) are not fully understood. Here the authors demonstrate a mutation in CDH2, encoding N-cadherin, that is associated with ADHD, and in a mouse model, delineate molecular electrophysiological characteristics associated with this mutation.

Levetiracetam and N-Cadherin Antibody Alleviate Brain Pathology Without Reducing Early Epilepsy Development After Focal Non-convulsive Status Epilepticus in Rats

Focal non-convulsive status epilepticus (fNCSE) is a neurological condition characterized by a prolonged seizure that may lead to the development of epilepsy. Emerging experimental evidence implicates neuronal death, microglial activation and alterations in the excitatory and inhibitory synaptic balance as key features in the pathophysiology following fNCSE. We have previously reported alterations in the excitatory adhesion molecule N-cadherin in rats with fNCSE originating from the hippocampus that subsequently also develop spontaneous seizures. In this study, fNCSE rats were treated intraperitoneally with the conventional anti-epileptic drug levetiracetam in combination with intraparenchymal infusion of N-cadherin antibodies (Ab) for 4 weeks post-fNCSE. The N-cadherin Ab was infused into the fornix and immunohistochemically N-cadherin Ab-stained neurons were detected within the dorsal hippocampal structures as well as in superjacent somatosensory cortex. Continuous levetiracetam treatment for 4 weeks post-fNCSE reduced microglia activation, including cell numbers and morphological changes, partly decreased neuronal cell loss, and excitatory post-synaptic scaffold protein PSD-95 expression in selective hippocampal structures. The additional treatment with N-cadherin Ab did not reverse neuronal loss, but moderately reduced microglial activation, and further reduced PSD-95 levels in the dentate hilus of the hippocampus. Despite the effects on brain pathology within the epileptic focus, neither monotherapy with systemic levetiracetam nor levetiracetam in combination with local N-cadherin Ab administration, reduced the amount of focal or focal evolving into bilateral convulsive seizures, seizure duration, or interictal epileptiform activity during 1 month of continuous electroenephalogram recordings within the hippocampus after fNCSE. Behavioral tests for spatial memory, anxiety, social interaction and anhedonia did not detect gross behavioral differences between fNCSE rats with or without treatment. The results reveal the refractory features of the present rodent model of temporal lobe epilepsy following fNCSE, which supports its clinical value for further therapeutic studies. We identify the persistent development of epilepsy following fNCSE, in spite of partly reduced brain pathology within the epileptic focus.

Stick around: Cell-Cell Adhesion Molecules during Neocortical Development

The neocortex is an exquisitely organized structure achieved through complex cellular processes from the generation of neural cells to their integration into cortical circuits after complex migration processes. During this long journey, neural cells need to establish and release adhesive interactions through cell surface receptors known as cell adhesion molecules (CAMs). Several types of CAMs have been described regulating different aspects of neurodevelopment. Whereas some of them mediate interactions with the extracellular matrix, others allow contact with additional cells. In this review, we will focus on the role of two important families of cell-cell adhesion molecules (C-CAMs), classical cadherins and nectins, as well as in their effectors, in the control of fundamental processes related with corticogenesis, with special attention in the cooperative actions among the two families of C-CAMs.

<p>Spinal N-Cadherin/CREB Signaling Contributes to Chronic Alcohol Consumption-Enhanced Postsurgical Pain</p>

Background

It has been reported that N-cadherin and cAMP response element binding protein (CREB) in the spinal cord are critical for synaptogenesis and regulation of excitatory synapse function, which could underlie chronic pain development. The aim of the present study was to investigate the role of spinal N-cadherin/CREB signaling in postsurgical pain chronicity following chronic alcohol consumption.

Methods

C57BL/6 male mice were randomly assigned into different groups. Plantar incision was used to induce postsurgical pain. Chronic alcohol consumption was conducted by giving mice unlimited access to different concentrations of ethanol for five weeks. We measured paw withdrawal thresholds to test postsurgical pain. Using Western blotting, we examined the expression of N-Cadherin and CREB in the spinal dorsal horn. We further performed intrathecal injection of specific N-cadherin and CREB inhibitors to assess the role of spinal N-cadherin/CREB signaling in chronic alcohol consumption-enhanced postsurgical pain.

Results

We observed that the chronic alcohol consumption significantly prolonged postsurgical pain and enhanced plantar incision-increased N-cadherin expression and CREB phosphorylation at the Ser133 in the spinal cord. Intrathecal injection of specific N-cadherin and CREB inhibitors attenuated chronic alcohol consumption-prolonged postsurgical pain.

Conclusion

Our results suggest that spinal N-cadherin/CREB signaling is involved in chronic alcohol consumption-caused postsurgical pain chronicity.

Urokinase-Type Plasminogen Activator Protects Cerebral Cortical Neurons from Soluble Aβ-Induced Synaptic Damage

Soluble amyloid β (Aβ)-induced synaptic dysfunction is an early event in the pathogenesis of Alzheimer's disease (AD) that precedes the deposition of insoluble Aβ and correlates with the development of cognitive deficits better than the number of plaques. The mammalian plasminogen activation (PA) system catalyzes the generation of plasmin via two activators: tissue-type (tPA) and urokinase-type (uPA). A dysfunctional tPA-plasmin system causes defective proteolytic degradation of Aβ plaques in advanced stages of AD. In contrast, it is unknown whether uPA and its receptor (uPAR) contribute to the pathogenesis of this disease. Neuronal cadherin (NCAD) plays a pivotal role in the formation of synapses and dendritic branches, and Aβ decreases its expression in cerebral cortical neurons. Here we show that neuronal uPA protects the synapse from the harmful effects of soluble Aβ. However, Aβ-induced inactivation of the eukaryotic initiation factor 2α halts the transcription of uPA mRNA, leaving unopposed the deleterious effects of Aβ on the synapse. In line with these observations, the synaptic abundance of uPA, but not uPAR, is decreased in the frontal cortex of AD patients and 5xFAD mice, and in cerebral cortical neurons incubated with soluble Aβ. We found that uPA treatment increases the synaptic expression of NCAD by a uPAR-mediated plasmin-independent mechanism, and that uPA-induced formation of NCAD dimers protects the synapse from the harmful effects of soluble Aβ oligomers. These data indicate that Aβ-induced decrease in the synaptic abundance of uPA contributes to the development of synaptic damage in the early stages of AD.SIGNIFICANCE STATEMENT Soluble amyloid β (Aβ)-induced synaptic dysfunction is an early event in the pathogenesis of cognitive deficits in Alzheimer's disease (AD). We found that neuronal urokinase-type (uPA) protects the synapse from the deleterious effects of soluble Aβ. However, Aβ-induced inactivation of the eukaryotic initiation factor 2α decreases the synaptic abundance of uPA, leaving unopposed the harmful effects of Aβ on the synapse. In line with these observations, the synaptic expression of uPA is decreased in the frontal cortex of AD brains and 5xFAD mice, and uPA treatment abrogates the deleterious effects of Aβ on the synapse. These results unveil a novel mechanism of Aβ-induced synaptic dysfunction in AD patients, and indicate that recombinant uPA is a potential therapeutic strategy to protect the synapse before the development of irreversible brain damage.

Differential Properties of the Synaptogenic Activities of the Neurexin Ligands Neuroligin1 and LRRTM2

Synaptic cell adhesion molecules are well established to exhibit synaptogenic activity when overexpressed in target cells, indicating that they are involved in formation and functional maturation of synapses. The postsynaptic adhesion proteins Neuroligin1 and LRRTM2 both induce synaptic vesicle clusters in presynaptic axons in vitro by transsynaptically interacting with neurexins. In neurons, this is accompanied by the induction of glutamatergic, but not GABAergic synapses. Although the synaptogenic activity of Neuroligin1 has been well characterized, the properties of the synaptogenic activities of other synaptic adhesion molecules are largely unknown. In this paper, we now compared characteristics of the synaptogenic activities of Neuroligin1 and LRRTM2 upon overexpression in cultured mouse cortical neurons. Individual cortical neurons were transfected with Neuroligin1 and LRRTM2 expression plasmids, respectively, and synaptic vesicle clustering in contacting axons was examined by immunostaining for the vesicle membrane protein VAMP2. In immature neurons at 6–7 days in vitro (DIV) both Neuroligin1 and LRRTM2 exhibited strong synaptogenic activity. However, upon further neuronal differentiation only LRRTM2 retained significant synaptogenic activity at 12–13 DIV. A similar differential developmental maturation of the synaptogenic activities of Neuroligin1 and LRRTM2 was observed for the induction of glutamatergic synapses, which were detected by co-immunostaining for VGLUT1 and Homer1. Most interestingly, the synaptogenic activity of Neuroligin1 was strongly dependent on the expression and function of the synaptic adhesion molecule N-cadherin in immature neurons. In contrast, the synaptogenic activity of LRRTM2 was independent of N-cadherin expression and function in both immature (6–7 DIV) and more mature neurons (14–15 DIV). Taken together, our results with overexpression in cultured cortical neurons revealed striking differences in the properties of the synaptogenic activities of Neuroligin1 and LRRTM2, although both transsynaptically interact with presynaptic neurexins.

Improved memory and reduced anxiety in δ-catenin transgenic mice

δ-Catenin is abundant in the brain and affects its synaptic plasticity. Furthermore, loss of δ-catenin is related to the deficits of learning and memory, mental retardation (cri-du-chat syndrome), and autism. A few studies about δ-catenin deficiency mice were performed. However, the effect of δ-catenin overexpression in the brain has not been investigated as yet. Therefore we generated a δ-catenin overexpressing mouse model. To generate a transgenic mouse model overexpressing δ-catenin in the brain, δ-catenin plasmid having a Thy-1 promotor was microinjected in C57BL/6 mice. Our results showed δ-catenin transgenic mice expressed higher levels of N-cadherin, β-catenin, and p120-catenin than did wild type mice. Furthermore, δ-catenin transgenic mice exhibited better object recognition, better sociability, and lower anxiety than wild type mice. However, both mice groups showed a similar pattern in locomotion tests. Although δ-catenin transgenic mice show similar locomotion, they show improved sociability and reduced anxiety. These characteristics are opposite to the symptoms of autism or mental retardation, which are caused when δ-catenin is deficient. These results suggest that δ-catenin may alleviate symptoms of autism, Alzheimer's disease and mental retardation.

A Critical Role of Presynaptic Cadherin/Catenin/p140Cap Complexes in Stabilizing Spines and Functional Synapses in the Neocortex

The formation of functional synapses requires coordinated assembly of presynaptic transmitter release machinery and postsynaptic trafficking of functional receptors and scaffolds. Here, we demonstrate a critical role of presynaptic cadherin/catenin cell adhesion complexes in stabilizing functional synapses and spines in the developing neocortex. Importantly, presynaptic expression of stabilized β-catenin in either layer (L) 4 excitatory neurons or L2/3 pyramidal neurons significantly upregulated excitatory synaptic transmission and dendritic spine density in L2/3 pyramidal neurons, while its sparse postsynaptic expression in L2/3 neurons had no such effects. In addition, presynaptic β-catenin expression enhanced release probability of glutamatergic synapses. Newly identified β-catenin-interacting protein p140Cap is required in the presynaptic locus for mediating these effects. Together, our results demonstrate that cadherin/catenin complexes stabilize functional synapses and spines through anterograde signaling in the neocortex and provide important molecular evidence for a driving role of presynaptic components in spinogenesis in the neocortex.

Release activity-dependent control of vesicle endocytosis by the synaptic adhesion molecule N-cadherin

At synapses in the mammalian brain, continuous information transfer requires the long-term maintenance of homeostatic coupling between exo- and endocytosis of synaptic vesicles. Because classical endocytosis is orders of magnitude slower than the millisecond-range exocytosis of vesicles, high frequency vesicle fusion could potentially compromise structural stability of synapses. However, the molecular mechanisms mediating the tight coupling of exo- and endocytosis are largely unknown. Here, we investigated the role of the transsynaptic adhesion molecules N-cadherin and Neuroligin1 in regulating vesicle exo- and endocytosis by using activity-induced FM4–64 staining and by using synaptophysin-pHluorin fluorescence imaging. The synaptic adhesion molecules N-cadherin and Neuroligin1 had distinct impacts on exo- and endocytosis at mature cortical synapses. Expression of Neuroligin1 enhanced vesicle release in a N-cadherin-dependent way. Most intriguingly, expression of N-cadherin enhanced both vesicle exo- and endocytosis. Further detailed analysis of N-cadherin knockout neurons revealed that the boosting of endocytosis by N-cadherin was largely dependent on preceding high levels of vesicle release activity. In summary, regulation of vesicle endocytosis was mediated at the molecular level by N-cadherin in a release activity-dependent manner. Because of its endocytosis enhancing function, N-cadherin might play an important role in the coupling of vesicle exo- and endocytosis.

Long-term depression-associated signaling is required for an in vitro model of NMDA receptor-dependent synapse pruning

Activity-dependent pruning of synaptic contacts plays a critical role in shaping neuronal circuitry in response to the environment during postnatal brain development. Although there is compelling evidence that shrinkage of dendritic spines coincides with synaptic long-term depression (LTD), and that LTD is accompanied by synapse loss, whether NMDA receptor (NMDAR)-dependent LTD is a required step in the progression toward synapse pruning is still unknown. Using repeated applications of NMDA to induce LTD in dissociated rat neuronal cultures, we found that synapse density, as measured by colocalization of fluorescent markers for pre- and postsynaptic structures, was decreased irrespective of the presynaptic marker used, post-treatment recovery time, and the dendritic location of synapses. Consistent with previous studies, we found that synapse loss could occur without apparent net spine loss or cell death. Furthermore, synapse loss was unlikely to require direct contact with microglia, as the number of these cells was minimal in our culture preparations. Supporting a model by which NMDAR-LTD is required for synapse loss, the effect of NMDA on fluorescence colocalization was prevented by phosphatase and caspase inhibitors. In addition, gene transcription and protein translation also appeared to be required for loss of putative synapses. These data support the idea that NMDAR-dependent LTD is a required step in synapse pruning and contribute to our understanding of the basic mechanisms of this developmental process.

Postsynaptic Y654 dephosphorylation of β-catenin modulates presynaptic vesicle turnover through increased n-cadherin-mediated transsynaptic signaling

Synaptic adhesion molecules, which coordinately control structural and functional changes at both sides of synapses, are important for synaptogenesis and synaptic plasticity. Because they physically form homophilic or heterophilic adhesions across synaptic junctions, these molecules can initiate transsynaptic communication in both anterograde and retrograde directions. Using optical imaging approaches, we investigated whether an increase in postsynaptic N-cadherin could correspondingly alter the function of connected presynaptic terminals. Postsynaptic expression of β-catenin Y654F, a phosphorylation-defective form with enhanced binding to N-cadherin, is sufficient to increase postsynaptic surface levels of N-cadherin and consequently promote presynaptic reorganizations. Such reorganizations include increases in the densities of the synaptic vesicle protein, Synaptotagmin 1 and the active zone scaffold protein, Bassoon, the number of active boutons and the size of the total recycling vesicle pool. In contrast, synaptic vesicle turnover is significantly impaired, preventing the exchange of synaptic vesicles with adjacent boutons. Together, N-cadherin-mediated retrograde signaling, governed by phosphoregulation of postsynaptic β-catenin Y654, coordinately modulates presynaptic vesicle dynamics to enhance synaptic communication in mature neurons. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 61-74, 2017.

Properties of Retinal Precursor Cells Grown on Vertically Aligned Multiwalled Carbon Nanotubes Generated for the Modification of Retinal Implant-Embedded Microelectrode Arrays

Background. To analyze the biocompatibility of vertically aligned multiwalled carbon nanotubes (MWCNT), used as nanomodification to optimize the properties of prostheses-embedded microelectrodes that induce electrical stimulation of surviving retinal cells. Methods. MWCNT were synthesized on silicon wafers. Their growth was achieved by iron particles (Fe) or mixtures of iron-platinum (Fe-Pt) and iron-titanium (Fe-Ti) acting as catalysts. Viability, growth, adhesion, and gene expression of L-929 and retinal precursor (R28) cells were analyzed after nondirect and direct contact. Results. Nondirect contact had almost no influence on cell growth, as measured in comparison to reference materials with defined levels of cytotoxicity. Both cell types exhibited good proliferation properties on each MWCNT-coated wafer. Viability ranged from 95.9 to 99.8%, in which better survival was observed for nonfunctionalized MWCNT generated with the Fe-Pt and Fe-Ti catalyst mixtures. R28 cells grown on the MWCNT-coated wafers showed a decreased gene expression associated with neural and glial properties. Expression of the cell cycle-related genes CCNC, MYC, and TP53 was slightly downregulated. Cultivation on plasma-treated MWCNT did not lead to additional changes. Conclusions. All tested MWCNT-covered slices showed good biocompatibility profiles, confirming that this nanotechnology is a promising tool to improve prostheses bearing electrodes which connect with retinal tissue.

Cadherins and catenins in dendrite and synapse morphogenesis

Neurons are highly polarized specialized cells. Neuronal integrity and functional roles are critically dependent on dendritic architecture and synaptic structure, function and plasticity. The cadherins are glycosylated transmembrane proteins that form cell adhesion complexes in various tissues. They are associated with a group of cytosolic proteins, the catenins. While the functional roles of the complex have been extensively investigates in non-neuronal cells, it is becoming increasingly clear that components of the complex have critical roles in regulating dendritic and synaptic architecture, function and plasticity in neurons. Consistent with these functional roles, aberrations in components of the complex have been implicated in a variety of neurodevelopmental disorders. In this review, we discuss the roles of the classical cadherins and catenins in various aspects of dendrite and synapse architecture and function and their relevance to human neurological disorders. Cadherins are glycosylated transmembrane proteins that were initially identified as Ca(2+)-dependent cell adhesion molecules. They are present on plasma membrane of a variety of cell types from primitive metazoans to humans. In the past several years, it has become clear that in addition to providing mechanical adhesion between cells, cadherins play integral roles in tissue morphogenesis and homeostasis. The cadherin family is composed of more than 100 members and classified into several subfamilies, including classical cadherins and protocadherins. Several of these cadherin family members have been implicated in various aspects of neuronal development and function. (1-3) The classical cadherins are associated with a group of cytosolic proteins, collectively called the catenins. While the functional roles of the cadherin-catenin cell adhesion complex have been extensively investigated in epithelial cells, it is now clear that components of the complex are well expressed in central neurons at different stages during development. (4,5) Recent exciting studies have shed some light on the functional roles of cadherins and catenins in central neurons. In this review, we will provide a brief overview of the cadherin superfamily, describe cadherin family members expressed in central neurons, cadherin-catenin complexes in central neurons and then focus on role of the cadherin-catenin complex in dendrite morphogenesis and synapse morphogenesis, function and plasticity. The final section is dedicated to discussion of the emerging list of neural disorders linked to cadherins and catenins. While the roles of cadherins and catenins have been examined in several different types of neurons, the focus of this review is their role in mammalian central neurons, particularly those of the cortex and hippocampus. Accompanying this review is a series of excellent reviews targeting the roles of cadherins and protocadherins in other aspects of neural development.

Learning From Animal Models of Obsessive-Compulsive Disorder

Obsessive-compulsive disorder (OCD) affects 2%-3% of the population worldwide and can cause significant distress and disability. Substantial challenges remain in the field of OCD research and therapeutics. Approved interventions alleviate symptoms only partially, with 30%-40% of patients being resistant to treatment. Although the etiology of OCD is still unknown, research evidence points toward the involvement of cortico-striato-thalamocortical circuitry. This review focuses on the most recent behavioral, genetics, and neurophysiologic findings from animal models of OCD. Based on evidence from these models and parallels with human studies, we discuss the circuit hyperactivity hypothesis for OCD, a potential circuitry dysfunction of action termination, and the involvement of candidate genes. Adding a more biologically valid framework to OCD will help researchers define and test new hypotheses and facilitate the development of targeted therapies based on disease-specific mechanisms.

Alzheimer's disease-related amyloid-β induces synaptotoxicity in human iPS cell-derived neurons

Human induced pluripotent stem cell (iPSC)-derived neurons have been proposed to be a highly valuable cellular model for studying the pathomechanisms of Alzheimer's disease (AD). Studies employing patient-specific human iPSCs as models of familial and sporadic forms of AD described elevated levels of AD-related amyloid-β (Aβ). However, none of the present AD iPSC studies could recapitulate the synaptotoxic actions of Aβ, which are crucial early events in a cascade that eventually leads to vast brain degeneration. Here we established highly reproducible, human iPSC-derived cortical cultures as a cellular model to study the synaptotoxic effects of Aβ. We developed a highly efficient immunopurification procedure yielding immature neurons that express markers of deep layer cortical pyramidal neurons and GABAergic interneurons. Upon long-term cultivation, purified cells differentiated into mature neurons exhibiting the generation of action potentials and excitatory glutamatergic and inhibitory GABAergic synapses. Most interestingly, these iPSC-derived human neurons were strongly susceptible to the synaptotoxic actions of Aβ. Application of Aβ for 8 days led to a reduction in the overall FM4–64 and vGlut1 staining of vesicles in neurites, indicating a loss of vesicle clusters. A selective analysis of presynaptic vesicle clusters on dendrites did not reveal a significant change, thus suggesting that Aβ impaired axonal vesicle clusters. In addition, electrophysiological patch-clamp recordings of AMPA receptor-mediated miniature EPSCs revealed an Aβ-induced reduction in amplitudes, indicating an impairment of postsynaptic AMPA receptors. A loss of postsynaptic AMPA receptor clusters was confirmed by immunocytochemical stainings for GluA1. Incubation with Aβ for 8 days did not result in a significant loss of neurites or cell death. In summary, we describe a highly reproducible cellular AD model based on human iPSC-derived cortical neurons that enables the mechanistic analysis of Aβ-induced synaptic pathomechanisms and the development of novel therapeutic approaches.

δ-Catenin Regulates Spine Architecture via Cadherin and PDZ-dependent Interactions

The ability of neurons to maintain spine architecture and modulate it in response to synaptic activity is a crucial component of the cellular machinery that underlies information storage in pyramidal neurons of the hippocampus. Here we show a critical role for δ-catenin, a component of the cadherin-catenin cell adhesion complex, in regulating spine head width and length in pyramidal neurons of the hippocampus. The loss of Ctnnd2, the gene encoding δ-catenin, has been associated with the intellectual disability observed in the cri du chat syndrome, suggesting that the functional roles of δ-catenin are vital for neuronal integrity and higher order functions. We demonstrate that loss of δ-catenin in a mouse model or knockdown of δ-catenin in pyramidal neurons compromises spine head width and length, without altering spine dynamics. This is accompanied by a reduction in the levels of synaptic N-cadherin. The ability of δ-catenin to modulate spine architecture is critically dependent on its ability to interact with cadherin and PDZ domain-containing proteins. We propose that loss of δ-catenin during development perturbs synaptic architecture leading to developmental aberrations in neural circuit formation that contribute to the learning disabilities in a mouse model and humans with cri du chat syndrome.

Cadherin-based transsynaptic networks in establishing and modifying neural connectivity

It is tacitly understood that cell adhesion molecules (CAMs) are critically important for the development of cells, circuits, and synapses in the brain. What is less clear is what CAMs continue to contribute to brain structure and function after the early period of development. Here, we focus on the cadherin family of CAMs to first briefly recap their multidimensional roles in neural development and then to highlight emerging data showing that with maturity, cadherins become largely dispensible for maintaining neuronal and synaptic structure, instead displaying new and narrower roles at mature synapses where they critically regulate dynamic aspects of synaptic signaling, structural plasticity, and cognitive function. At mature synapses, cadherins are an integral component of multiprotein networks, modifying synaptic signaling, morphology, and plasticity through collaborative interactions with other CAM family members as well as a variety of neurotransmitter receptors, scaffolding proteins, and other effector molecules. Such recognition of the ever-evolving functions of synaptic cadherins may yield insight into the pathophysiology of brain disorders in which cadherins have been implicated and that manifest at different times of life.

N-cadherin participated in invasion and metastasis of human esophageal squamous cell carcinoma via taking part in the formation of vasculogenic mimicry

Vasculogenic mimicry (VM) refers to the unique ability of highly aggressive tumor cells to mimic the pattern of embryonic vasculogenic networks, and the presence of VM correlates to an increased risk of metastasis and poor clinical outcome of cancers. Several key molecules, including N-cadherin, have been implicated in VM. However, the role of N-cadherin in the formation of VM in esophageal squamous cell carcinoma (ESCC) had not been elucidated. In this study, firstly we aimed to identify VM patterns in ESCC tissues and to explore their clinical significance. VM was present in 12 out of 56 samples, and ESCC with lymph node metastasis had a higher incidence of VM than that without lymph node metastasis. More importantly, VM channels were associated with the expression of N-cadherin in ESCC tissues. In order to further explore the role of N-cadherin in VM formation and invasion and metastasis in ESCC, secondly, we silenced the expression of N-cadherin with small hairpin RNA in ESCC cell line KYSE-70; herein, we showed that KYSE-70 cells with N-cadherin silencing lost not only the capacity to form tube-like structures on collagen (VM) but also the invasion, metastasis and proliferation ability in KYSE-70 cells in vitro. Taken together, antivascular therapies targeting tumor cell VM may be an effective approach to the treatment of patients with highly metastatic ESCC.

N-cadherin regulates molecular organization of excitatory and inhibitory synaptic circuits in adult hippocampus in vivo

N-Cadherin and β-catenin form a transsynaptic adhesion complex required for spine and synapse development. In adulthood, N-cadherin mediates persistent synaptic plasticity, but whether the role of N-cadherin at mature synapses is similar to that at developing synapses is unclear. To address this, we conditionally ablated N-cadherin from excitatory forebrain synapses in mice starting in late postnatal life and examined hippocampal structure and function in adulthood. In the absence of N-cadherin, β-catenin levels were reduced, but numbers of excitatory synapses were unchanged, and there was no impact on number or shape of dendrites or spines. However, the composition of synaptic molecules was altered. Levels of GluA1 and its scaffolding protein PSD95 were diminished and the density of immunolabeled puncta was decreased, without effects on other glutamate receptors and their scaffolding proteins. Additionally, loss of N-cadherin at excitatory synapses triggered increases in the density of markers for inhibitory synapses and decreased severity of hippocampal seizures. Finally, adult mutant mice were profoundly impaired in hippocampal-dependent memory for spatial episodes. These results demonstrate a novel function for the N-cadherin/β-catenin complex in regulating ionotropic receptor composition of excitatory synapses, an appropriate balance of excitatory and inhibitory synaptic proteins and the maintenance of neural circuitry necessary to generate flexible yet persistent cognitive and synaptic function.

Candidate genes and functional noncoding variants identified in a canine model of obsessive-compulsive disorder

BACKGROUND

Obsessive-compulsive disorder (OCD), a severe mental disease manifested in time-consuming repetition of behaviors, affects 1 to 3% of the human population. While highly heritable, complex genetics has hampered attempts to elucidate OCD etiology. Dogs suffer from naturally occurring compulsive disorders that closely model human OCD, manifested as an excessive repetition of normal canine behaviors that only partially responds to drug therapy. The limited diversity within dog breeds makes identifying underlying genetic factors easier.

RESULTS

We use genome-wide association of 87 Doberman Pinscher cases and 63 controls to identify genomic loci associated with OCD and sequence these regions in 8 affected dogs from high-risk breeds and 8 breed-matched controls. We find 119 variants in evolutionarily conserved sites that are specific to dogs with OCD. These case-only variants are significantly more common in high OCD risk breeds compared to breeds with no known psychiatric problems. Four genes, all with synaptic function, have the most case-only variation: neuronal cadherin (CDH2), catenin alpha2 (CTNNA2), ataxin-1 (ATXN1), and plasma glutamate carboxypeptidase (PGCP). In the 2 Mb gene desert between the cadherin genes CDH2 and DSC3, we find two different variants found only in dogs with OCD that disrupt the same highly conserved regulatory element. These variants cause significant changes in gene expression in a human neuroblastoma cell line, likely due to disrupted transcription factor binding.

CONCLUSIONS

The limited genetic diversity of dog breeds facilitates identification of genes, functional variants and regulatory pathways underlying complex psychiatric disorders that are mechanistically similar in dogs and humans.

The Caenorhabditis elegans microtubule minus-end binding homolog PTRN-1 stabilizes synapses and neurites

Microtubule dynamics facilitate neurite growth and establish morphology, but the role of minus-end binding proteins in these processes is largely unexplored. CAMSAP homologs associate with microtubule minus-ends, and are important for the stability of epithelial cell adhesions. In this study, we report morphological defects in neurons and neuromuscular defects in mutants of the C. elegans CAMSAP, ptrn-1. Mechanosensory neurons initially extend wild-type neurites, and subsequently remodel by overextending neurites and retracting synaptic branches and presynaptic varicosities. This neuronal remodeling can be activated by mutations known to alter microtubules, and depends on a functioning DLK-1 MAP kinase pathway. We found that PTRN-1 localizes to both neurites and synapses, and our results suggest that alterations of microtubule structures caused by loss of PTRN-1 function activates a remodeling program leading to changes in neurite morphology. We propose a model whereby minus-end microtubule stabilization mediated by a functional PTRN-1 is necessary for morphological maintenance of neurons. DOI: http://dx.doi.org/10.7554/eLife.01637.001.