Afadin regulates puncta adherentia junction formation and presynaptic differentiation in hippocampal neurons

s-Afadin binds to MAGUIN/Cnksr2 and regulates the localization of the AMPA receptor and glutamatergic synaptic response in hippocampal neurons

A hippocampal mossy fiber synapse implicated in learning and memory is a complex structure in which a presynaptic bouton attaches to the dendritic trunk by puncta adherentia junctions (PAJs) and wraps multiply branched spines. The postsynaptic densities (PSDs) are localized at the heads of each of these spines and faces to the presynaptic active zones. We previously showed that the scaffolding protein afadin regulates the formation of the PAJs, PSDs, and active zones in the mossy fiber synapse. Afadin has two splice variants: l-afadin and s-afadin. l-Afadin, but not s-afadin, regulates the formation of the PAJs but the roles of s-afadin in synaptogenesis remain unknown. We found here that s-afadin more preferentially bound to MAGUIN (a product of the Cnksr2 gene) than l-afadin in vivo and in vitro. MAGUIN/CNKSR2 is one of the causative genes for nonsyndromic X-linked intellectual disability accompanied by epilepsy and aphasia. Genetic ablation of MAGUIN impaired PSD-95 localization and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptor surface accumulation in cultured hippocampal neurons. Our electrophysiological analysis revealed that the postsynaptic response to glutamate, but not its release from the presynapse, was impaired in the MAGUIN-deficient cultured hippocampal neurons. Furthermore, disruption of MAGUIN did not increase the seizure susceptibility to flurothyl, a GABA_A receptor antagonist. These results indicate that s-afadin binds to MAGUIN and regulates the PSD-95-dependent cell surface localization of the AMPA receptor and glutamatergic synaptic responses in the hippocampal neurons and that MAGUIN is not involved in the induction of epileptic seizure by flurothyl in our mouse model.

Necl2/3-mediated mechanism for tripartite synapse formation

Ramified, polarized protoplasmic astrocytes interact with synapses via perisynaptic astrocyte processes (PAPs) to form tripartite synapses. These astrocyte-synapse interactions mutually regulate their structures and functions. However, molecular mechanisms for tripartite synapse formation remain elusive. We developed an in vitro co-culture system for mouse astrocytes and neurons that induced astrocyte ramifications and PAP formation. Co-cultured neurons were required for astrocyte ramifications in a neuronal activity-dependent manner, and synaptically-released glutamate and activation of astrocytic mGluR5 metabotropic glutamate receptor were likely involved in astrocyte ramifications. Astrocytic Necl2 trans-interacted with axonal Necl3, inducing astrocyte-synapse interactions and astrocyte functional polarization by recruiting EAAT1/2 glutamate transporters and Kir4.1 K+ channel to the PAPs, without affecting astrocyte ramifications. This Necl2/3 trans-interaction increased functional synapse number. Thus, astrocytic Necl2, synaptically-released glutamate and axonal Necl3 cooperatively formed tripartite glutamatergic synapses in vitro. Studies on hippocampal mossy fiber synapses in Necl3 knockout and Necl2/3 double knockout mice confirmed these previously unreported mechanisms for astrocyte-synapse interactions and astrocyte functional polarization in vivo.

Nectins and Nectin-like molecules in synapse formation and involvement in neurological diseases

Synapses are interneuronal junctions which form neuronal networks and play roles in a variety of functions, including learning and memory. Two types of junctions, synaptic junctions (SJs) and puncta adherentia junctions (PAJs), have been identified. SJs are found at all excitatory and inhibitory synapses whereas PAJs are found at excitatory synapses, but not inhibitory synapses, and particularly well developed at hippocampal mossy fiber giant excitatory synapses. Both SJs and PAJs are mediated by cell adhesion molecules (CAMs). Major CAMs at SJs are neuroligins-neurexins and Nectin-like molecules (Necls)/CADMs/SynCAMs whereas those at PAJs are nectins and cadherins. In addition to synaptic PAJs, extrasynaptic PAJs have been identified at contact sites between neighboring dendrites near synapses and regulate synapse formation. In addition to SJs and PAJs, a new type of cell adhesion apparatus different from these junctional apparatuses has been identified and named nectin/Necl spots. One nectin spot at contact sites between neighboring dendrites at extrasynaptic regions near synapses regulates synapse formation. Several members of nectins and Necls had been identified as viral receptors before finding their physiological functions as CAMs and evidence is accumulating that many nectins and Necls are related to onset and progression of neurological diseases. We review here nectin and Necls in synapse formation and involvement in neurological diseases.

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.

Afadin (AF6) in cancer progression: A multidomain scaffold protein with complex and contradictory roles

Adherens (AJ) and tight junctions (TJ) maintain cell-cell adhesions and cellular polarity in normal tissues. Afadin, a multi-domain scaffold protein, is commonly found in both adherens and tight junctions, where it plays both structural and signal-modulating roles. Afadin is a complex modulator of cellular processes implicated in cancer progression, including signal transduction, migration, invasion, and apoptosis. In keeping with the complexities associated with the roles of adherens and tight junctions in cancer, afadin exhibits both tumor suppressive and pro-metastatic functions. In this review, we will explore the dichotomous roles that afadin plays during cancer progression.

Autism and Williams syndrome: Dissimilar socio-cognitive profiles with similar patterns of abnormal gene expression in the blood

LAY ABSTRACT

Autism spectrum disorders and Williams syndrome are complex cognitive conditions exhibiting quite opposite features in the social domain: whereas people with autism spectrum disorders are mostly hyposocial, subjects with Williams syndrome are usually reported as hypersocial. At the same time, autism spectrum disorders and Williams syndrome share some common underlying behavioral and cognitive deficits. It is not clear, however, which genes account for the attested differences (and similarities) in the socio-cognitive domain. In this article, we adopted a comparative molecular approach and looked for genes that might be differentially (or similarly) regulated in the blood of people with these conditions. We found a significant overlap between genes dysregulated in the blood of patients compared to neurotypical controls, with most of them being upregulated or, in some cases, downregulated. Still, genes with similar expression trends can exhibit quantitative differences between conditions, with most of them being more dysregulated in Williams syndrome than in autism spectrum disorders. Differentially expressed genes are involved in aspects of brain development and function (particularly dendritogenesis) and are expressed in brain areas (particularly the cerebellum, the thalamus, and the striatum) of relevance for the autism spectrum disorder and the Williams syndrome etiopathogenesis. Overall, these genes emerge as promising candidates for the similarities and differences between the autism spectrum disorder and the Williams syndrome socio-cognitive profiles.

Polarity Protein AF6 Controls Hepatic Glucose Homeostasis and Insulin Sensitivity by Modulating IRS1/AKT Insulin Pathway in an SHP2-Dependent Manner

Insulin resistance is a major contributing factor in the development of metabolic disease. Although numerous functions of the polarity protein AF6 (afadin and MLLT4) have been identified, a direct effect on insulin sensitivity has not been previously described. We show that AF6 is elevated in the liver tissues of dietary and genetic mouse models of diabetes. We generated liver-specific AF6 knockout mice and show that these animals exhibit enhanced insulin sensitivity and liver glycogen storage, whereas overexpression of AF6 in wild-type mice by adenovirus-expressing AF6 led to the opposite phenotype. Similar observations were obtained from in vitro studies. In addition, we discovered that AF6 directly regulates IRS1/AKT kinase-mediated insulin signaling through its interaction with Src homology 2 domain-containing phosphatase 2 (SHP2) and its regulation of SHP2's tyrosine phosphatase activity. Finally, we show that knockdown of hepatic AF6 ameliorates hyperglycemia and insulin resistance in high-fat diet-fed or db/db diabetic mice. These results demonstrate a novel function for hepatic AF6 in the regulation of insulin sensitivity, providing important insights about the metabolic role of AF6.

Dynamic spatiotemporal patterns of alternative splicing of an F-actin scaffold protein, afadin, during murine development

An F-actin scaffold protein, afadin, comprises two splice variants called l-afadin (a long isoform) and s-afadin (a short isoform). It is known that in adult tissues, l-afadin is ubiquitously expressed while s-afadin is restrictedly expressed in brain. In cultured cortical neurons, l-afadin potentiates axonal branching whereas s-afadin blocks axonal branching by functioning as a naturally occurring dominant-negative isoform that forms a heterodimer with l-afadin. However, the temporal and spatial expression pattern of s-afadin during development or across multiple tissues and organs has not been fully understood. In this study, using Western blotting and RT-qPCR techniques and the fluorescent splicing reporters, we examined the detailed expression patterns of l- and s-afadin isoforms across various tissues and cell types, including rat organs at developmental stages, primary cultured neuronal and non-neuronal cells prepared from the developing rat brain, and in neurons in vitro generated from P19 embryonal carcinoma (EC) cells. Both mRNA and protein of s-afadin were abundantly expressed in various regions of rat neuronal tissues, and their expression dynamically changed during development in vivo. The expression of s-afadin was also detected in primary rat cortical neurons, but not in astrocytes or fibroblasts, and the neuronal expression increased during neuronal maturation in vitro. The dynamic alternative splicing event of afadin during development was successfully visualized with the newly developed fluorescent splicing reporter plasmids at a single cell level. Moreover, s-afadin was undetectable in undifferentiated EC cells, and the expression was induced upon differentiation of the cells into neurons. These data suggest that spatiotemporal and dynamic alternative splicing produces differential expression patterns of afadin isoforms and that alternative splicing of afadin is controlled by the neuron-specific regulator(s) whose activity is triggered and dynamically altered during neuronal differentiation and maturation.

Differential regional and subcellular localization patterns of afadin splice variants in the mouse central nervous system

AF6/afadin is an F-actin scaffold protein that plays essential roles in the organization of cell-cell junctions of polarized epithelia. Afadin comprises two major isoforms produced by alternative splicing - a longer isoform l-afadin, having the F-actin-binding domain, and a shorter isoform s-afadin, harboring the amino acid sequences unique to the isoform but lacking the F-actin-binding domain. We recently identified functional differences between l- and s-afadin isoforms in the regulation of axon branching in primary cultured cortical neurons; the former potentiates and the latter blocks axon branching. Previous biochemical reports indicate differences in tissue and cell-type distributions of isoforms, and it was shown that l-afadin is ubiquitously expressed in various tissues and cell-types, while s-afadin is predominantly expressed in neuronal tissues and cultured neurons. However, the spatial expression pattern of s-afadin across neuronal tissues or within neurons has not been revealed because no antibody specific for s-afadin is yet available. In this study, we report the generation and characterization of an antibody that specifically distinguishes s-afadin from l-afadin, and its application to investigate the expression profile of s-afadin in primary cultured neurons and tissue cryosections of adult mouse brain and retina. We describe differential regional and subcellular localization patterns of l- and s-afadin isoforms in the mouse central nervous system.

PAR3-PAR6-atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3-PAR6-aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3-PAR6-aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.

Involvement of l-afadin, but not s-afadin, in the formation of puncta adherentia junctions of hippocampal synapses

A hippocampal mossy fiber synapse has a complex structure in which presynaptic boutons attach to the dendritic trunk by puncta adherentia junctions (PAJs) and wrap multiply-branched spines, forming synaptic junctions. It was previously shown that afadin regulates the formation of the PAJs cooperatively with nectin-1, nectin-3, and N-cadherin. Afadin is a nectin-binding protein with two splice variants, l-afadin and s-afadin: l-afadin has an actin filament-binding domain, whereas s-afadin lacks it. It remains unknown which variant is involved in the formation of the PAJs or how afadin regulates it. We showed here that re-expression of l-afadin, but not s-afadin, in the afadin-deficient cultured hippocampal neurons in which the PAJ-like structure was disrupted, restored this structure as estimated by the accumulation of N-cadherin and αΝ-catenin. The l-afadin mutant, in which the actin filament-binding domain was deleted, or the l-afadin mutant, in which the αΝ-catenin-binding domain was deleted, did not restore the PAJ-like structure. These results indicate that l-afadin, but not s-afadin, regulates the formation of the hippocampal synapse PAJ-like structure through the binding to actin filaments and αN-catenin. We further found here that l-afadin bound αN-catenin, but not γ-catenin, whereas s-afadin bound γ-catenin, but hardly αN-catenin. These results suggest that the inability of s-afadin to form the hippocampal synapse PAJ-like structure is due to its inability to efficiently bind αN-catenin.

Multiple roles of afadin in the ultrastructural morphogenesis of mouse hippocampal mossy fiber synapses

A hippocampal mossy fiber synapse, which is implicated in learning and memory, has a complex structure in which mossy fiber boutons attach to the dendritic shaft by puncta adherentia junctions (PAJs) and wrap around a multiply-branched spine, forming synaptic junctions. Here, we electron microscopically analyzed the ultrastructure of this synapse in afadin-deficient mice. Transmission electron microscopy analysis revealed that typical PAJs with prominent symmetrical plasma membrane darkening undercoated with the thick filamentous cytoskeleton were observed in the control synapse, whereas in the afadin-deficient synapse, atypical PAJs with the symmetrical plasma membrane darkening, which was much less in thickness and darkness than those of the control typical PAJs, were observed. Immunoelectron microscopy analysis revealed that nectin-1, nectin-3, and N-cadherin were localized at the control typical PAJs, whereas nectin-1 and nectin-3 were localized at the afadin-deficient atypical PAJs to extents lower than those in the control synapse and N-cadherin was localized at their nonjunctional flanking regions. These results indicate that the atypical PAJs are formed by nectin-1 and nectin-3 independently of afadin and N-cadherin and that the typical PAJs are formed by afadin and N-cadherin cooperatively with nectin-1 and nectin-3. Serial block face-scanning electron microscopy analysis revealed that the complexity of postsynaptic spines and mossy fiber boutons, the number of spine heads, the area of postsynaptic densities, and the density of synaptic vesicles docked to active zones were decreased in the afadin-deficient synapse. These results indicate that afadin plays multiple roles in the complex ultrastructural morphogenesis of hippocampal mossy fiber synapses.

Localization of nectin-2δ at perivascular astrocytic endfoot processes and degeneration of astrocytes and neurons in nectin-2 knockout mouse brain

Nectins are Ca²⁺-independent immunoglobulin-like cell-cell adhesion molecules. In the nervous system, among four members (nectin-1, -2, -3, and -4), nectin-1 and -3 are asymmetrically localized at puncta adherentia junctions formed between the mossy fiber terminals and the dendrites of CA3 pyramidal neurons in the mouse hippocampus and heterophilic trans-interactions between nectin-1 and nectin-3 are involved in the selective interaction of axons and dendrites of cultured neurons. By contrast, nectin-2, which has two splicing variants, nectin-2α and -2δ, has not been well characterized in the brain. We showed here that nectin-2α was expressed in both cultured mouse neurons and astrocytes whereas nectin-2δ was selectively expressed in the astrocytes. Nectin-2δ was localized at the adhesion sites between adjacent cultured astrocytes, but in the brain it was localized on the plasma membranes of astrocytic perivascular endfoot processes facing the basement membrane of blood vessels. Genetic ablation of nectin-2 caused degeneration of astrocytic perivascular endfoot processes and neurons in the cerebral cortex. These results uncovered for the first time the localization and critical functions of nectin-2 in the brain.

Graphene Oxide Nanosheets Reshape Synaptic Function in Cultured Brain Networks

Graphene offers promising advantages for biomedical applications. However, adoption of graphene technology in biomedicine also poses important challenges in terms of understanding cell responses, cellular uptake, or the intracellular fate of soluble graphene derivatives. In the biological microenvironment, graphene nanosheets might interact with exposed cellular and subcellular structures, resulting in unexpected regulation of sophisticated biological signaling. More broadly, biomedical devices based on the design of these 2D planar nanostructures for interventions in the central nervous system require an accurate understanding of their interactions with the neuronal milieu. Here, we describe the ability of graphene oxide nanosheets to down-regulate neuronal signaling without affecting cell viability.