nArgBP2, a novel neural member of ponsin/ArgBP2/vinexin family that interacts with synapse-associated protein 90/postsynaptic density-95-associated protein (SAPAP)

nArgBP2 together with GKAP and SHANK3 forms a dynamic layered structure

nArgBP2, a protein whose disruption is implicated in intellectual disability, concentrates in excitatory spine-synapses. By forming a triad with GKAP and SHANK, it regulates spine structural rearrangement. We here find that GKAP and SHANK3 concentrate close to the synaptic contact, whereas nArgBP2 concentrates more centrally in the spine. The three proteins collaboratively form biomolecular condensates in living fibroblasts, exhibiting distinctive layered localizations. nArgBP2 concentrates in the inner phase, SHANK3 in the outer phase, and GKAP partially in both. Upon co-expression of GKAP and nArgBP2, they evenly distribute within condensates, with a notable peripheral localization of SHANK3 persisting when co-expressed with either GKAP or nArgBP2. Co-expression of SHANK3 and GKAP with CaMKIIα results in phase-in-phase condensates, with CaMKIIα at the central locus and SHANK3 and GKAP exhibiting peripheral localization. Additional co-expression of nArgBP2 maintains the layered organizational structure within condensates. Subsequent CaMKIIα activation disperses a majority of the condensates, with an even distribution of all proteins within the extant deformed condensates. Our findings suggest that protein segregation via phase separation may contribute to establishing layered organization in dendritic spines.

Biomolecular condensate assembly of nArgBP2 tunes its functionality to manifest the structural plasticity of dendritic spines

nArgBP2, a candidate gene for intellectual disability, is a postsynaptic protein critical for dendritic spine development and morphogenesis, and its knockdown (KD) in developing neurons severely impairs spine-bearing excitatory synapse formation. Surprisingly, nArgBP2 KD in mature neurons did not cause morphological defects in the existing spines at rest, raising questions of how it functions in mature neurons. We found that unlike its inaction at rest, nArgBP2 KD completely inhibited the enlargement of dendritic spines during chemically induced long-term potentiation (cLTP) in mature neurons. We further found that nArgBP2 forms condensates in dendritic spines and that these condensates are dispersed by cLTP, which spatiotemporally coincides with spine head enlargement. Condensates with CaMKII phosphorylation-deficient mutant or CaMKII inhibition are neither dispersed nor accompanied by spine enlargement during cLTP. We found that nArgBP2 condensates in spines exhibited liquid-like properties, and in heterologous and in vitro expression systems, nArgBP2 undergoes liquid-liquid phase separation via multivalent intermolecular interactions between SH3 domains and proline-rich domains. It also forms coacervates with CaMKIIα, which is rapidly dissembled by calcium/CaMKIIα-dependent phosphorylation. We further showed that the interaction between nArgBP2 and WAVE1 competes with nArgBP2 phase separation and that blocking the nArgBP2-WAVE1 interaction prevents spine enlargement during cLTP. Together, our results suggest that nArgBP2 at rest is confined to the condensates but is released by CaMKIIα-mediated phosphorylation during synaptic plasticity, which regulates its timely interaction with WAVE1 to induce spine head enlargement in mature neurons.

SAPAP Scaffold Proteins: From Synaptic Function to Neuropsychiatric Disorders

Excitatory (glutamatergic) synaptic transmission underlies many aspects of brain activity and the genesis of normal human behavior. The postsynaptic scaffolding proteins SAP90/PSD-95-associated proteins (SAPAPs), which are abundant components of the postsynaptic density (PSD) at excitatory synapses, play critical roles in synaptic structure, formation, development, plasticity, and signaling. The convergence of human genetic data with recent in vitro and in vivo animal model data indicates that mutations in the genes encoding SAPAP1-4 are associated with neurological and psychiatric disorders, and that dysfunction of SAPAP scaffolding proteins may contribute to the pathogenesis of various neuropsychiatric disorders, such as schizophrenia, autism spectrum disorders, obsessive compulsive disorders, Alzheimer's disease, and bipolar disorder. Here, we review recent major genetic, epigenetic, molecular, behavioral, electrophysiological, and circuitry studies that have advanced our knowledge by clarifying the roles of SAPAP proteins at the synapses, providing new insights into the mechanistic links to neurodevelopmental and neuropsychiatric disorders.

Complete Model of Vinculin Suggests the Mechanism of Activation by Helical Super-Bundle Unfurling

To shed light onto the activation mechanism of vinculin, we carried out a detailed refinement of chicken vinculin and compared it to the human protein which is greater than 95% identical. Refinement resulted in a complete and significantly improved model. This model includes important elements such as a pro-rich strap region (PRR) and C-terminus. The conformation of the PRR stabilized by its inter- and intra-molecular contacts shows a dynamic, but relatively stable motif that constitutes a docking platform for multiple molecules. The contact of the C-terminus with the PRR suggests that phosphorylation of Tyr1065 might control activation and membrane binding. Improved electron densities showed the presence of large solvent molecules such as phosphates/sulfates and a head-group of PIP2. The improved model allowed for a computational stability analysis to be performed by the program Corex/Best which located numerous hot-spots of increased and decreased stability. Proximity of the identified binding sites for regulatory partners involved in inducing or suppressing the activation of vinculin to the unstable elements sheds new light onto the activation pathway and differential activation. This stability analysis suggests that the activation pathway proceeds by unfurling of the super-bundle built from four bundles of helices without separation of the Vt region (840-1066) from the head. According to our mechanism, when activating proteins bind at the strap region a separation of N and C terminal bundles occurs, followed by unfurling of the super-bundle and flattening of the general shape of the molecule, which exposes the interaction sites for binding of auxiliary proteins.

Development of an Eight-gene Prognostic Model for Overall Survival Prediction in Patients with Hepatocellular Carcinoma

BACKGROUND AND AIMS

The overall survival (OS) of hepatocellular carcinoma (HCC) remains dismal. Bioinformatic analysis of transcriptome data could identify patients with poor OS and may facilitate clinical decision. This study aimed to develop a prognostic gene model for HCC.

METHODS

GSE14520 was retrieved as a training set to identify differential expressed genes (DEGs) between tumor and adjacent liver tissues in HCC patients with different OS. A DEG-based prognostic model was then constructed and the TCGA-LIHC and ICGC-LIRI datasets were used to validate the model. The area under the receiver operating characteristic curve (AUC) and hazard ratio (HR) of the model for OS were calculated. A model-based nomogram was established and verified.

RESULTS

In the training set, differential expression analysis identified 80 genes dysregulated in oxidation-reduction and metabolism regulation. After univariate Cox and LASSO regression, eight genes (LPCAT1, DHRS1, SORBS2, ALDH5A1, SULT1C2, SPP1, HEY1 and GOLM1) were selected to build the prognostic model. The AUC for 1-, 3- and 5-year OS were 0.779, 0.736, 0.754 in training set and 0.693, 0.689, 0.693 in the TCGA-LIHC validation set, respectively. The AUC for 1- and 3-year OS were 0.767 and 0.705 in the ICGC-LIRI validation set. Multivariate analysis confirmed the model was an independent prognostic factor (training set: HR=4.422, p<0.001; TCGA-LIHC validation set: HR=2.561, p<0.001; ICGC-LIRI validation set: HR=3.931, p<0.001). Furthermore, a nomogram combining the model and AJCC stage was established and validated, showing increased OS predictive efficacy compared with the prognostic model (p=0.035) or AJCC stage (p<0.001).

CONCLUSIONS

Our eight-gene prognostic model and the related nomogram represent as reliable prognostic tools for OS prediction in HCC patients.

Systematic detection of functional proteoform groups from bottom-up proteomic datasets

To a large extent functional diversity in cells is achieved by the expansion of molecular complexity beyond that of the coding genome. Various processes create multiple distinct but related proteins per coding gene - so-called proteoforms - that expand the functional capacity of a cell. Evaluating proteoforms from classical bottom-up proteomics datasets, where peptides instead of intact proteoforms are measured, has remained difficult. Here we present COPF, a tool for COrrelation-based functional ProteoForm assessment in bottom-up proteomics data. It leverages the concept of peptide correlation analysis to systematically assign peptides to co-varying proteoform groups. We show applications of COPF to protein complex co-fractionation data as well as to more typical protein abundance vs. sample data matrices, demonstrating the systematic detection of assembly- and tissue-specific proteoform groups, respectively, in either dataset. We envision that the presented approach lays the foundation for a systematic assessment of proteoforms and their functional implications directly from bottom-up proteomic datasets.

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.

Molecular architecture of postsynaptic Interactomes

The postsynaptic density (PSD) plays an essential role in the organization of the synaptic signaling machinery. It contains a set of core scaffolding proteins that provide the backbone to PSD protein-protein interaction networks (PINs). These core scaffolding proteins can be seen as three principal layers classified by protein family, with DLG proteins being at the top, SHANKs along the bottom, and DLGAPs connecting the two layers. Early studies utilizing yeast two hybrid enabled the identification of direct protein-protein interactions (PPIs) within the multiple layers of scaffolding proteins. More recently, mass-spectrometry has allowed the characterization of whole interactomes within the PSD. This expansion of knowledge has further solidified the centrality of core scaffolding family members within synaptic PINs and provided context for their role in neuronal development and synaptic function. Here, we discuss the scaffolding machinery of the PSD, their essential functions in the organization of synaptic PINs, along with their relationship to neuronal processes found to be impaired in complex brain disorders.

Modified Glutamatergic Postsynapse in Neurodegenerative Disorders

The postsynaptic density (PSD) is a complex subcellular domain important for postsynaptic signaling, function, and plasticity. The PSD is present at excitatory synapses and specialized to allow for precise neuron-to-neuron transmission of information. The PSD is localized immediately underneath the postsynaptic membrane forming a major protein network that regulates postsynaptic signaling and synaptic plasticity. Glutamatergic synaptic dysfunction affecting PSD morphology and signaling events have been described in many neurodegenerative disorders, either sporadic or familial forms. Thus, in this review we describe the main protein players forming the PSD and their activity, as well as relevant modifications in key components of the postsynaptic architecture occurring in Huntington's, Parkinson's and Alzheimer's diseases.

The RNA-binding protein SORBS2 suppresses hepatocellular carcinoma tumourigenesis and metastasis by stabilizing RORA mRNA

BACKGROUND

Numerous studies have revealed that dysregulation of RNA-binding protein (RBP) expression is causally linked with human cancer tumourigenesis. However, the detailed biological effect and underlying mechanisms of most RBPs remain unclear.

METHODS

Expression of sorbin and SH3 domain-containing 2 (SORBS2) in hepatocellular carcinoma (HCC) was detected by qRT-PCR, immunohistochemistry assay and Western blot assay. Proliferation, migration, invasion and cell cycle progression of HCC cells were measured by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, colony-forming assay, Transwell assay and flow cytometry assay respectively. A xenograft model and metastatic model were established to evaluate the proliferation and metastasis of HCC cells in vivo. Western blot assays were performed to assess the expression of epithelial-mesenchymal transition markers. Luciferase reporter assay, RNA immunoprecipitation and pull-down assay elucidated the effect of SORBS2 on one of its downstream genes.

RESULTS

The expression of SORBS2 was significantly decreased in HCC and was associated with metastasis, advanced TNM clinical stage and poor clinical outcome of HCC patients. Furthermore, our results suggested that SORBS2 inhibited HCC cell proliferation, invasion, migration and EMT both in vivo and in vitro. Mechanistically, we revealed that retinoic acid receptor-related orphan receptor (RORA) was a major target of SORBS2 and was critical to sustaining the antitumour effect of SORBS2 on HCC cells. SORBS2 reduced RORA mRNA degradation by directly binding to the 3'UTR of RORA mRNA.

CONCLUSIONS

In this study, we found for the first time that SORBS2 contributed to the suppression of HCC tumourigenesis and metastasis via post-transcriptional regulation of RORA expression as an RBP.

Interneuronal NMDA receptors regulate long-term depression and motor learning in the cerebellum

KEY POINTS

NMDA receptors (NMDARs) are required for long-term depression (LTD) at parallel fibre-Purkinje cell synapses, but their cellular localization and physiological functions in vivo are unclear. NMDARs in molecular-layer interneurons (MLIs), but not granule cells or Purkinje cells, are required for LTD, but not long-term potentiation induced by low-frequency stimulation of parallel fibres. Nitric oxide produced by NMDAR activation in MLIs probably mediates LTD induction. NMDARs in granule cells or Purkinje cells are dispensable for motor learning during adaptation of horizontal optokinetic responses.

ABSTRACT

Long-term potentiation (LTP) and depression (LTD), which serve as cellular synaptic plasticity models for learning and memory, are crucially regulated by N-methyl-d-aspartate receptors (NMDARs) in various brain regions. In the cerebellum, LTP and LTD at parallel fibre (PF)-Purkinje cell (PC) synapses are thought to mediate certain forms of motor learning. However, while NMDARs are essential for LTD in vitro, their cellular localization remains controversial. In addition, whether and how NMDARs mediate motor learning in vivo remains unclear. Here, we examined the contribution of NMDARs expressed in granule cells (GCs), PCs and molecular-layer interneurons (MLIs) to LTD/LTP and motor learning by generating GC-, PC- and MLI/PC-specific knockouts of Grin1, a gene encoding an obligatory GluN1 subunit of NMDARs. While robust LTD and LTP were induced at PF-PC synapses in GC- and PC-specific Grin1 (GC-Grin1 and PC-Grin1, respectively) conditional knockout (cKO) mice, only LTD was impaired in MLI/PC-specific Grin1 (MLI/PC-Grin1) cKO mice. Application of diethylamine nitric oxide (NO) sodium, a potent NO donor, to the cerebellar slices restored LTD in MLI/PC-Grin1 cKO mice, suggesting that NO is probably downstream to NMDARs. Furthermore, the adaptation of horizontal optokinetic responses (hOKR), a cerebellar motor learning task, was normally observed in GC-Grin1 cKO and PC-Grin1 cKO mice, but not in MLI/PC-Grin1 cKO mice. These results indicate that it is the NMDARs expressed in MLIs, but not in PCs or GCs, that play important roles in LTD in vitro and motor learning in vivo.

nArgBP2-SAPAP-SHANK, the core postsynaptic triad associated with psychiatric disorders

Despite the complex genetic architecture, a broad spectrum of psychiatric disorders can still be caused by mutation(s) in the same gene. These disorders are interrelated with overlapping causative mechanisms including variations in the interaction among the risk-associated proteins that may give rise to the specific spectrum of each disorder. Additionally, multiple lines of evidence implicate an imbalance between excitatory and inhibitory neuronal activity (E/I imbalance) as the shared key etiology. Thus, understanding the molecular mechanisms underlying E/I imbalance provides essential insight into the etiology of these disorders. One important class of candidate risk genes is the postsynaptic scaffolding proteins, such as nArgBP2, SAPAP, and SHANK that regulate the actin cytoskeleton in dendritic spines of excitatory synapses. This review will cover and discuss recent studies that examined how these proteins, especially nArgBP2, are associated with psychiatric disorders. Next, we propose a possibility that variations in the interaction among these proteins in a specific brain region might contribute to the onset of diverse phenotypes of psychiatric disorders.

The assembly of scaffolding proteins, key regulators of many signaling pathways, found in the brain’s synapses underpin a diverse range of neuropsychiatric disorders. Sunghoe Chang and colleagues from Seoul National University, South Korea, review how these postsynaptic proteins regulate the cellular cytoskeleton in nerve cell protrusions to maintain the balance between excitatory and inhibitory inputs in the brain. They discuss how perturbations in three particular proteins can cause an imbalance in synaptic signals that leads to conditions such as bipolar disorder, schizophrenia and autism. The authors propose that these proteins form a “core scaffolding triad” and interact in different ways to cause different mental illnesses. Dysregulation of these proteins could explain how mutations in the same genes, depending on whether they boost or decrease gene expression, contribute to the onset of diverse psychiatric disorders.

Sorbin and SH3 domain-containing protein 2 (SORBS2) is a component of the acto-myosin ring at the apical junctional complex in epithelial cells

SORBS2 is a scaffolding protein associated with Abl/Arg non-receptor tyrosine kinase pathways and is known to interact with actin and several other cytoskeletal proteins in various cell types. Previous BioID proximity labeling of tight and adherens junction proteins suggested that SORBS2 is a component of the apical junction complex of epithelial cells. We asked whether SORBS2 plays a previously unappreciated role in controlling perijunctional actin and tight junction barrier function. Using super resolution imaging we confirmed that SORBS2 is localized at the apical junction complex but farther from the membrane than ZO-1 and located partially overlapping both the tight- and adherens junctions with a periodic concentration that alternates with myosin IIB in polarized epithelial cells. Overexpression of GFP-SORBS2 recruited alpha-actinin, vinculin and N-WASP, and possibly CIP4 to cellular junctions. However, CRISPR-Cas9 knock-out of SORBS2 did not alter the localization- or immunofluorescent staining intensity of these or several other junctional- and cytoskeletal proteins. SORBS2 knock-out also did not affect the barrier function as measured by TER and dextran flux; nor did it change actin-dependent junction re-assembly as measured by Ca²⁺-switch and Latrunculin-B wash-out assays. The kinetics of HGF-induced cell scattering and wound healing, and dextran flux increase induced by PDGF also were unaffected by SORBS2 knock-out. SORBS2 concentrates with apical junctional actin that accumulates in response to knock-down of ZO-1 and ZO-2. In spite of our finding that SORBS2 is clearly a component of the apical junction complex, it does not appear to be required for either normal tight- or adherens junction assembly, structure or function or for growth factor-mediated changes in tight junction dynamics.

Biochemical Purification of Binding Partners of Synaptic Scaffold Proteins

The chemical synapse displays specialized intercellular adhesion between pre- and potsynaptic plasma membranes mediated by synaptic cell adhesion proteins. In this asymmetric cell adhesion, pre- and postsynapses have their own unique functions; the presynaptic terminal releases neurotransmitter, which diffuses through the synaptic cleft and is received by receptors accumulated at the postsynapse. Such distinct modes of actions of pre- and postsynapses in synaptic neurotransmission are the rate-limiting factors in signal processing in the brain, and thus protein-protein interactions within the pre- and postsynaptic scaffold are of particular importance for brain function by regulating the pre- and postsynaptic function. In the present paper, we outline a method to screen for binding partners of synaptic scaffold proteins biochemically.

nArgBP2 regulates excitatory synapse formation by controlling dendritic spine morphology

Neural Abelson-related gene-binding protein 2 (nArgBP2) was originally identified as a protein that directly interacts with synapse-associated protein 90/postsynaptic density protein 95-associated protein 3 (SAPAP3), a postsynaptic scaffolding protein critical for the assembly of glutamatergic synapses. Although genetic deletion of nArgBP2 in mice leads to manic/bipolar-like behaviors resembling many aspects of symptoms in patients with bipolar disorder, the actual function of nArgBP2 at the synapse is completely unknown. Here, we found that the knockdown (KD) of nArgBP2 by specific small hairpin RNAs (shRNAs) resulted in a dramatic change in dendritic spine morphology. Reintroducing shRNA-resistant nArgBP2 reversed these defects. In particular, nArgBP2 KD impaired spine-synapse formation such that excitatory synapses terminated mostly at dendritic shafts instead of spine heads in spiny neurons, although inhibitory synapse formation was not affected. nArgBP2 KD further caused a marked increase of actin cytoskeleton dynamics in spines, which was associated with increased Wiskott-Aldrich syndrome protein-family verprolin homologous protein 1 (WAVE1)/p21-activated kinase (PAK) phosphorylation and reduced activity of cofilin. These effects of nArgBP2 KD in spines were rescued by inhibiting PAK or activating cofilin combined with sequestration of WAVE. Together, our results suggest that nArgBP2 functions to regulate spine morphogenesis and subsequent spine-synapse formation at glutamatergic synapses. They also raise the possibility that the aberrant regulation of synaptic actin filaments caused by reduced nArgBP2 expression may contribute to the manifestation of the synaptic dysfunction observed in manic/bipolar disorder.

Impaired Dendritic Development and Memory in Sorbs2 Knock-Out Mice

Intellectual disability is a common neurodevelopmental disorder characterized by impaired intellectual and adaptive functioning. Both environmental insults and genetic defects contribute to the etiology of intellectual disability. Copy number variations of SORBS2 have been linked to intellectual disability. However, the neurobiological function of SORBS2 in the brain is unknown. The SORBS2 gene encodes ArgBP2 (Arg/c-Abl kinase binding protein 2) protein in non-neuronal tissues and is alternatively spliced in the brain to encode nArgBP2 protein. We found nArgBP2 colocalized with F-actin at dendritic spines and growth cones in cultured hippocampal neurons. In the mouse brain, nArgBP2 was highly expressed in the cortex, amygdala, and hippocampus, and enriched in the outer one-third of the molecular layer in dentate gyrus. Genetic deletion of Sorbs2 in mice led to reduced dendritic complexity and decreased frequency of AMPAR-miniature spontaneous EPSCs in dentate gyrus granule cells. Behavioral characterization revealed that Sorbs2 deletion led to a reduced acoustic startle response, and defective long-term object recognition memory and contextual fear memory. Together, our findings demonstrate, for the first time, an important role for nArgBP2 in neuronal dendritic development and excitatory synaptic transmission, which may thus inform exploration of neurobiological basis of SORBS2 deficiency in intellectual disability.

SIGNIFICANCE STATEMENT

Copy number variations of the SORBS2 gene are linked to intellectual disability, but the neurobiological mechanisms are unknown. We found that nArgBP2, the only neuronal isoform encoded by SORBS2, colocalizes with F-actin at neuronal dendritic growth cones and spines. nArgBP2 is highly expressed in the cortex, amygdala, and dentate gyrus in the mouse brain. Genetic deletion of Sorbs2 in mice leads to impaired dendritic complexity and reduced excitatory synaptic transmission in dentate gyrus granule cells, accompanied by behavioral deficits in acoustic startle response and long-term memory. This is the first study of Sorbs2 function in the brain, and our findings may facilitate the study of neurobiological mechanisms underlying SORBS2 deficiency in the development of intellectual disability.

Microchidia protein 2, MORC2, downregulates the cytoskeleton adapter protein, ArgBP2, via histone methylation in gastric cancer cells

ArgBP2 is an adapter protein that plays an important role in actin-dependent processes such as cell adhesion and migration. However, its function and regulation mechanisms in gastric cancer have not yet been investigated. Here, we showed the low expression of ArgBP2 mRNA level in gastric tumor samples and its repressive function in the proliferation, migration, and invasion of gastric cancer cells. Then, we cloned and identified ArgBP2 promoter and verified that MORC2 bound to the promoter. Moreover, we demonstrated that MORC2 enhanced the recruitment of EZH2, which promoted the tri-methylation of H3K27, leading to the transcriptional repression of ArgBP2. Our results might thus contribute to understanding the molecular mechanisms of ArgBP2 regulation and suggesting ArgBP2 as a potential therapeutic target for gastric cancer.

Arg kinase-binding protein 2 (ArgBP2) interaction with α-actinin and actin stress fibers inhibits cell migration

Cell migration requires dynamic remodeling of the actomyosin network. We report here that an adapter protein, ArgBP2, is a component of α-actinin containing stress fibers and inhibits migration. ArgBP2 is undetectable in many commonly studied cancer-derived cell lines. COS-7 and HeLa cells express ArgBP2 (by Western analysis), but expression was detectable only in approximately half the cells by immunofluorescence. Short term clonal analysis demonstrated 0.2-0.3% of cells switch ArgBP2 expression (on or off) per cell division. ArgBP2 can have a fundamental impact on the actomyosin network: ArgBP2 positive COS-7 cells, for example, are clearly distinguishable by their denser actomyosin (stress fiber) network. ArgBP2γ binding to α-actinin appears to underlie its ability to localize to stress fibers and decrease cell migration. We map a small α-actinin binding region in ArgBP2 (residues 192-228) that is essential for these effects. Protein kinase A phosphorylation of ArgBP2γ at neighboring Ser-259 and consequent 14-3-3 binding blocks its interaction with α-actinin. ArgBP2 is known to be down-regulated in some aggressively metastatic cancers. Our work provides a biochemical explanation for the anti-migratory effect of ArgBP2.

Phosphorylation at Y1065 in vinculin mediates actin bundling, cell spreading, and mechanical responses to force

Vinculin is an essential structural adaptor protein that localizes to sites of adhesion and is involved in a number of cell processes including adhesion, spreading, motility, force transduction, and cell survival. The C-terminal vinculin tail domain (Vt) contains the necessary structural components to bind and cross-link actin filaments. Actin binding to Vt induces a conformational change that promotes dimerization through the C-terminal hairpin of Vt and enables actin filament cross-linking. Here we show that Src phosphorylation of Y1065 within the C-terminal hairpin regulates Vt-mediated actin bundling and provide a detailed characterization of Y1065 mutations. Furthermore, we show that phosphorylation at Y1065 plays a role in cell spreading and the response to the application of mechanical force.

Oligomerization and phosphorylation dependent regulation of ArgBP2 adaptive capabilities and associated functions

ArgBP2 (Arg-Binding Protein 2/SORBS2) is an adaptor protein involved in cytoskeleton associated signal transduction, thereby regulating cell migration and adhesion. These features are associated with its antitumoral role in pancreatic cancer cells. Tyrosine phosphorylation of ArgBP2, mediated by c-Abl kinase and counterbalanced by PTP-PEST phosphatase, regulates many of its interactions. However, the exact mechanisms of action and of regulation of ArgBP2 remain largely unknown. We found that ArgBP2 has the capacity to form oligomers which are destabilized by tyrosine phosphorylation. We could show that ArgBP2 oligomerization involves the binding of one of its SH3 domains to a specific proline rich cluster. ArgBP2 self-association increases its binding to some of its molecular partners and decreased its affinity for others. Hence, the phosphorylation/oligomerization state of ArgBP2 directly regulates its functions by modulating its adaptive capabilities. Importantly, using a human pancreatic cancer cell model (MiaPaCa-2 cells), we could validate that this property of ArgBP2 is critical for its cytoskeleton associated functions. In conclusions, we describe a new mechanism of regulation of ArgBP2 where tyrosine phosphorylation of the protein interfere with a SH3 mediated self-interaction, thereby controlling its panel of interacting partners and related functions.

Vinculin regulation of F-actin bundle formation: what does it mean for the cell?

Vinculin is an essential cell adhesion protein, found at both focal adhesions and adherens junctions, where it couples transmembrane proteins to the actin cytoskeleton. Vinculin is involved in controlling cell shape, motility and cell survival, and has more recently been shown to play a role in force transduction. The tail domain of vinculin (Vt) has the ability to both bind and bundle actin filaments. Binding to actin induces a conformational change in Vt believed to promote formation of a Vt dimer that is able to crosslink actin filaments. We have recently provided additional evidence for the actin-induced Vt dimer and have shown that the vinculin carboxyl (C)-terminal hairpin is critical for both the formation of the Vt dimer and for bundling F-actin. We have also demonstrated the importance of the C-terminal hairpin in cells as deletion of this region impacts both adhesion properties and force transduction. Intriguingly, we have identified bundling deficient variants of vinculin that show different cellular phenotypes. These results suggest additional role(s) for the C-terminal hairpin, distinct from its bundling function. In this commentary, we will expand on our previous findings and further investigate these actin bundling deficient vinculin variants.

Functional consequences of mutations in postsynaptic scaffolding proteins and relevance to psychiatric disorders

Functional studies on postsynaptic scaffolding proteins at excitatory synapses have revealed a plethora of important roles for synaptic structure and function. In addition, a convergence of recent in vivo functional evidence together with human genetics data strongly suggest that mutations in a variety of these postsynaptic scaffolding proteins may contribute to the etiology of diverse human psychiatric disorders such as schizophrenia, autism spectrum disorders, and obsessive-compulsive spectrum disorders. Here we review the most recent evidence for several key postsynaptic scaffolding protein families and explore how mouse genetics and human genetics have intersected to advance our knowledge concerning the contributions of these important players to complex brain function and dysfunction.

Cell biological characterization of a multidomain adaptor protein, ArgBP2, in epithelial NMuMG cells, and identification of a novel short isoform

ArgBP2 is a member of the SoHo (sorbin-homology) family of adaptor proteins believed to play roles in cell adhesion, cytoskeletal organization, and signaling. We show here a novel splicing isoform of ArgBP2, i.e., ArgBP2™, composed of only three SH3 (src-homology 3) domains and structurally similar to vinexinß. We then characterized the biochemical and cell biological properties of ArgBP2 to compare these with vinexin. Similar to vinexin, ArgBP2 was enriched at focal adhesions in REF52 fibroblast cells and induced anchorage-dependent extracellular signal-regulated kinase activation in NIH3T3 fibroblast cells. In epithelial NMuMG cells, immunofluorescence analyses revealed localization of ArgBP2 at tight junctions (TJs), whereas vinexin was distributed in cytoplasm as well as cell-cell boundaries. During TJ formation, recruitment of ZO-1 to TJs was followed by ArgBP2. Based on mutation analyses, a second SH3 domain was found to be important for ArgBP2 localization to the cell-cell contact sites. These data suggest some role of ArgBP2 in NMuMG cells at TJs that may be distinct from the function of vinexin.

CDK5 is essential for soluble amyloid β-induced degradation of GKAP and remodeling of the synaptic actin cytoskeleton

The early stages of Alzheimer's disease are marked by synaptic dysfunction and loss. This process results from the disassembly and degradation of synaptic components, in particular of scaffolding proteins that compose the post-synaptic density (PSD), namely PSD95, Homer and Shank. Here we investigated in rat frontal cortex dissociated culture the mechanisms involved in the downregulation of GKAP (SAPAP1), which links the PSD95 complex to the Shank complex and cytoskeletal structures within the PSD. We show that Aβ causes the rapid loss of GKAP from synapses through a pathway that critically requires cdk5 activity, and is set in motion by NMDAR activity and Ca(2+) influx. We show that GKAP is a direct substrate of cdk5 and that its phosphorylation results in polyubiquitination and proteasomal degradation of GKAP and remodeling (collapse) of the synaptic actin cytoskeleton; the latter effect is abolished in neurons expressing GKAP mutants that are resistant to phosphorylation by cdk5. Given that cdk5 also regulates degradation of PSD95, these results underscore the central position of cdk5 in mediating Aβ-induced PSD disassembly and synapse loss.

Arg/Abl-binding protein, a Z-body and Z-band protein, binds sarcomeric, costameric, and signaling molecules

ArgBP2 (Arg/Abl-Binding Protein) is expressed at high levels in the heart and is localized in the Z-bands of mature myofibrils. ArgBP2 is a member of a small family of proteins that also includes vinexin and CAP (c-Cbl-associated protein), all characterized by having one sorbin homology (SOHO) domain and three C-terminal SH3 domains. Antibodies directed against ArgBP2 also react with the Z-bodies of myofibril precursors: premyofibrils and nascent myofibrils. Expression in cardiomyocytes of plasmids encoding Yellow Fluorescent Protein (YFP) fused to either full length ArgBP2, the SOHO, mid-ArgBP or the SH3 domains of ArgBP2 led to Z-band targeting of the fusion proteins, whereas an N-terminal fragment lacking these domains did not target to Z-bands. Although ArgBP2 is not found in skeletal muscle cells, YFP-ArgBP2 did target to Z-bodies and Z-bands in cultured myotubes. GST-ArgBP2-SH3 bound actin, α-actinin and vinculin proteins in blot overlays, cosedimentation assays, and EM negative staining techniques. Over-expression of ArgBP2 and ArgBP2-SH3 domains, but not YFP alone, led to loss of myofibrils in cardiomyocytes. Fluorescence recovery after photobleaching was used to measure the rapid dynamics of both the full length and some truncated versions of ArgBP2. Our results indicate that ArgBP2 may play an important role in the assembly and maintenance of myofibrils in cardiomyocytes.

Vinculin, an adapter protein in control of cell adhesion signalling

Vinculin, discovered in 1979 (Geiger, 1979), is an adapter protein with binding sites for more than 15 proteins. Biochemical and structural analyses have contributed to detailed knowledge about potential binding partners and the understanding of how their binding may be regulated. Despite all this information the molecular basis of how vinculin acts in cells and controls a wide variety of signals remains elusive. This review aims to highlight recent discoveries with an emphasis on how vinculin is involved in the coordination of a network of signals.

DNA methylation in vulnerability to post-traumatic stress in rats: evidence for the role of the post-synaptic density protein Dlgap2

Post-traumatic stress disorder (PTSD) is unique among psychiatric disorders since there is an explicit requirement for the presence of a well-defined precipitating environmental event. This suggests the participation of adaptable molecular processes such as epigenetic modifications, including acetylation and methylation of histones and DNA methylation. In the present study we investigated whether changes in DNA methylation are associated with the effects of traumatic stressor, using a validated PTSD rat model. Screening of genomic DNA methylation patterns revealed that maladaptation to traumatic stress is associated with numerous changes in the methylation pattern of rat hippocampus. Of the differentially methylated genes revealed by this global screening, Disks Large-Associated Protein (Dlgap2) was of special interest, demonstrating an increase in a specific methylation site which was associated with a reduction in its gene expression in PTSD-like compared to non-PTSD-like rats. The association between the methylation rate and Dlgap2 expression was further substantiated by re-dividing the rats according to their methylation state. A significantly higher expression was observed in the non-methylated compared to methylated rats. In addition, taking all rats as one group revealed a significant correlation between their behavioural stress responses and Dlgap2 transcript levels. These results suggest that alterations in global methylation pattern are involved in behavioural adaptation to environmental stress and pinpoint Dlgap2 as a possible target in PTSD.

BmCAP, a silkmoth gene encoding multiple protein isoforms characterized by SoHo and SH3 domains: expression analysis during ovarian follicular development

CAP/ArgBP2/vinexin family proteins, adaptor proteins characterized by three SH3 domains at their C-termini and a SoHo domain towards their N-termini, are known to regulate cell adhesion, cytoskeletal organization, and growth factor signaling. Here we present the isolation and ovarian expression of the BmCAP gene which encodes CAP/ArgBP2/vinexin family proteins in the silkmoth, Bombyx mori. Screening for full-length cDNA clones identified three mRNA isoforms, BmCAP-A1, BmCAP-A2 and BmCAP-B, which show expression throughout ovarian follicular development. Using an antibody raised against a unique region between the SoHo and SH3 domains, BmCAP-A protein isoforms were identified that show specific expression in different compartments of the ovarian follicles. Immunofluorescence staining of the cells of the follicular epithelium establishes a dynamic pattern of BmCAP-A protein localization during choriogenesis. During early choriogenesis, BmCAP-A has a diffuse localization in the cytoplasm but could also be found concentrated at the apical and basal sides at the cell-cell junctions. During late choriogenesis, the diffuse cytoplasmic staining of BmCAP-A disappears while the staining pattern at the apical side resembles a blueprint for the eggshell surface structure. We suggest that BmCAP-A isoforms have important functions during ovarian development, which involve not only the traditional roles in actin organization or cell-cell adhesion but also the regulation of secretion of chorion proteins and the sculpting of the chorion surface.

Cbl-associated protein is tyrosine phosphorylated by c-Abl and c-Src kinases

Background

The c-Cbl-associated protein (CAP), also known as ponsin, localizes to focal adhesions and stress fibers and is involved in signaling events. Phosphorylation has been described for the other two members of the sorbin homology family, vinexin and ArgBP2, but no data exist about the putative phosphorylation of CAP. According to previous findings, CAP binds to tyrosine kinase c-Abl. However, it is not known if CAP is a substrate of c-Abl or other tyrosine kinases or if phosphorylation regulates its localization.

Results

We here show that CAP is Tyr phosphorylated by and interacts with both c-Abl and c-Src. One major phosphorylation site, Tyr360, and two minor contributors Tyr326 and Tyr632 were identified as Abl phosphorylation sites, whereas Src preferentially phosphorylates Tyr326 and Tyr360. Phosphorylation of CAP was not necessary for its localization to focal adhesions and stress fibers, but Tyr326Phe substitution alters the function of CAP during cell spreading.

Conclusion

This is the first demonstration of phosphorylation of CAP by any kinase. Our findings suggest that coordinated action of Src and Abl might regulate the function of CAP and reveal a functional role especially for the Src-mediated Tyr phosphorylation of CAP in cell spreading.

CIP4 is a new ArgBP2 interacting protein that modulates the ArgBP2 mediated control of WAVE1 phosphorylation and cancer cell migration

ArgBP2 is a multi-adapter protein involved in signal transduction associated to the cytoskeleton and was shown to regulate the migration and adhesion of pancreatic cancer cells thereby modulating their tumorigenicity. Here we describe the interaction of ArgBP2 with CIP4, a new associated protein identified by yeast two-hybrid. We found that both proteins modulated their reciprocal tyrosine phosphorylation catalyzed by the non-receptor tyrosine kinase c-Abl. We observed that, like ArgBP2, CIP4 directly interacted with WAVE1 and could enhance its phosphorylation by c-Abl. ArgBP2 and CIP4 acted synergistically to increase WAVE1 tyrosine phosphorylation. Finally, we could show that CIP4 was dispensable for the ArgBP2 induced blockade of cell migration whereas its overexpression was deleterious for this important function of ArgBP2.

ArgBP2 and the SoHo family of adapter proteins in oncogenic diseases

ArgBP2, a member of the SoHo family of adapter proteins, is a regulator of actin-dependent processes such as cell adhesion and migration. Recent data from our lab revealed that by regulating adhesion and migration of pancreatic cancer cells, ArgBP2 is endowed with an anti-tumoral function. We could show that part of the molecular mechanism involved the interaction of ArgBP2 with the Arp2/3 activator WAVE1, the tyrosine phosphatase PTP-PEST, and the tyrosine kinase c-Abl. As ArgBP2 shares common structural organization and overlapping functions with the two other members of this protein family, CAP and Vinexin, it raises the question whether these two other proteins could also be involved in cancer diseases. The control of cell migration being an important issue in tumor treatment, these recent findings suggest that ArgBP2 family-dependent signaling pathways represents potential targets for the development of therapeutic strategies, and highlight the importance of elucidating their molecular mechanisms of cytoskeletal regulation.

ArgBP2-dependent signaling regulates pancreatic cell migration, adhesion, and tumorigenicity

The poor prognosis of pancreatic cancer is due to rapid locoregional invasion, the early development of metastases, and the limited efficacy of current therapies. To date, none of the identified oncogenes and suppressors involved in this disease have led to efficient treatments. Here, we describe that the scaffold protein ArgBP2 is repressed during oncogenic transformation of the pancreas. We could show, using a pancreatic cancer cell line model, that this repression of ArgBP2 participates in the progression of this disease. Interestingly, in vitro analyses revealed that the antitumoral potential of ArgBP2 is linked to the control of cell adhesion and migration rather than to the regulation of cell proliferation or sensitivity to apoptosis. Moreover, we could detail part of the molecular mechanism responsible by identifying new ArgBP2-interacting proteins, and show that this function is partly achieved by the control of a WAVE/PTP-PEST/c-Abl signaling complex. These findings point to a new mechanism of pancreatic cancer progression leading to invasion and metastasis and suggest that the ArgBP2 signaling pathway could represent a new target for cancer therapy.

Identification of synaptic activity-dependent genes by exposure of cultured cortical neurons to tetrodotoxin followed by its withdrawal

Activity-dependent gene expression is one of the key mechanisms of synaptic plasticity that form the basis of higher order functions such as learning and memory. In the present study, we surveyed for activity-dependent genes by analyzing gene expression changes accompanying reversible inhibition of synaptic activity by tetrodotoxin (TTX) using two types of DNA microarrays; our focused oligo DNA microarray "Synaptoarray" and the commercially available high-density array. Cerebral cortical cells from E18 rat embryos were cultured for 14 days to ensure synaptogenesis, then treated with 1 muM TTX for 48 hr without detectable effect on cell viability. Synaptic density estimated by the amount of Synapsin I and Synaptotagmin I was decreased 21-24% by TTX treatment, but recovered to the control level 48 hr after TTX withdrawal. Comparison of gene expression profiles by competitive hybridization of fluorescently labeled cRNA from TTX-treated and control cells showed an overall downregulation of the genes on the Synaptoarray by TTX-treatment with different recovery rates after TTX withdrawal. With 16 representative genes, microarray data were validated by real-time PCR analysis. Genes most severely downregulated by TTX and upregulated above the control level at 5 hr after TTX withdrawal were munc13-1 (involved in docking and priming of synaptic vesicles) and Shank2 (involved in the postsynaptic scaffold). In addition, comprehensive screening at 5 hr after TTX withdrawal using high density arrays resulted in additional identification of Rgs2, a regulator of trimeric G-protein signaling, as an activity-dependent gene. These three genes are thus likely to be key factors in the regulation of synaptic plasticity. (c) 2007 Wiley-Liss, Inc.

Molecular anatomy of the postsynaptic density

The postsynaptic density (PSD) is a structure composed of both membranous and cytoplasmic proteins localized at the postsynaptic plasma membrane of excitatory synapses. Biochemical and molecular biological studies have identified a number of proteins present in the PSD. Glutamate receptors are important constituents of the PSD and membrane proteins involved in synaptic signal transduction and cell adhesion are also essential components. Scaffolding proteins containing multiple protein interaction motifs are thought to provide the framework of the PSD through their interactions with both membrane proteins and the cytoplasmic proteins. Among the cytoplasmic signaling molecules, calcium-calmodulin-dependent protein kinase II stands out as a major component of the PSD and its dynamic translocation to the PSD in response to neuronal activity is crucial in synaptic signal transduction. Recent advancements in molecular biological, structural and electrophysiological techniques have enabled us to directly measure the number, distribution and interactions of PSD molecules with high sensitivity and precision. In this review, I describe the structure and molecular composition of the PSD as well as the molecular interactions between the major constituents. This information will be combined with recent quantitative analyses of the PSD protein contents per synapse, in order to provide a current view of the PSD molecular architecture and its dynamics.

CAP interacts with cytoskeletal proteins and regulates adhesion-mediated ERK activation and motility

CAP/Ponsin belongs to the SoHo family of adaptor molecules that includes ArgBP2 and Vinexin. These proteins possess an N-terminal sorbin homology (SoHo) domain and three C-terminal SH3 domains that bind to diverse signaling molecules involved in a variety of cellular processes. Here, we show that CAP binds to the cytoskeletal proteins paxillin and vinculin. CAP localizes to cell-extracellular matrix (ECM) adhesion sites, and this process requires binding to vinculin. Overexpression of CAP induces the aggregation of paxillin, vinculin and actin at cell-ECM adhesion sites. Moreover, CAP inhibits adhesion-dependent processes such as cell spreading and focal adhesion turnover, whereas a CAP mutant that is unable to localize to cell-ECM adhesion sites is incapable of exerting these effects. Finally, depletion of CAP by siRNA-mediated knockdown leads to enhanced cell spreading, migration and the activation of the PAK/MEK/ERK pathway in REF52 cells. Taken together, these results indicate that CAP is a cytoskeletal adaptor protein involved in modulating adhesion-mediated signaling events that lead to cell migration.

Differential control of postsynaptic density scaffolds via actin-dependent and -independent mechanisms

Organization and dynamic remodeling of postsynaptic density (PSD) are thought to be critical in postsynaptic signal transduction, but the underlying molecular mechanisms are not well understood. We show here that four major scaffolding molecules, PSD-95, GKAP, Shank, and PSD-Zip45, show distinct instability in total molecular content per synapse. Fluorescence recovery after photobleaching also confirmed their distinct turnover rates. Among the PSD molecules examined, PSD-95 was most stable, but its elimination did not influence the dynamics of its direct binding partner GKAP. Multiple interactions of scaffolding molecules with the actin cytoskeleton have suggested their importance in both maintenance and remodeling of the PSD. Indeed, acute pharmacological disruption of F-actin rapidly eliminated the dynamic fraction of GKAP, Shank, and PSD-Zip45, without changing synaptic localization of PSD-95. GKAP content in synapses increased after pharmacological enhancement of neuronal activity, whereas Shank and PSD-Zip45 content showed reduction. Inhibition of F-actin dynamics prevented activity-dependent redistribution of all three scaffolds. We also assessed involvement of glutamate receptors in the regulation of PSD dynamics. Genetic manipulations eliminating either NMDA receptors or metabotropic glutamate receptors did not primarily influence mobility of their binding scaffolds. These results collectively indicate a critical role of filamentous actin in determining the extent of dynamic reorganization in PSD molecular composition.

The postsynaptic density

Glutamatergic synapses in the central nervous system are characterized by an electron-dense web underneath the postsynaptic membrane; this web is called the postsynaptic density (PSD). PSDs are composed of a dense network of several hundred proteins, creating a macromolecular complex that serves a wide range of functions. Prominent PSD proteins such as members of the MaGuk or ProSAP/Shank family build up a dense scaffold that creates an interface between clustered membrane-bound receptors, cell adhesion molecules and the actin-based cytoskeleton. Moreover, kinases, phosphatases and several proteins of different signalling pathways are specifically localized within the spine/PSD compartment. Small GTPases and regulating proteins are also enriched in PSDs being the molecular basis for regulated structural changes of cytoskeletal components within the synapse in response to external or internal stimuli, e.g. synaptic activation. This synaptic rearrangement (structural plasticity) is a rapid process and is believed to underlie learning and memory formation. The characterization of synapse/PSD proteins is especially important in the light of recent data suggesting that several mental disorders have their molecular defect at the synapse/PSD level.

Protein kinase A-dependent increase in WAVE2 expression induced by the focal adhesion protein vinexin

The focal adhesion protein vinexin is a member of a family of adaptor proteins that are thought to participate in the regulation of cell adhesion, cytoskeletal reorganization, and growth factor signaling. Here, we show that vinexin beta increases the amount of and reduces the mobility on SDS-PAGE of Wiskott-Aldrich syndrome protein family verprolin-homologous protein (WAVE) 2 protein, which is a key factor modulating actin polymerization in migrating cells. This mobility retardation disappeared after in vitro phosphatase treatment. Co-immunoprecipitation assays revealed the interaction of vinexin beta with WAVE2 as well as WAVE1 and N-WASP. Vinexin beta interacts with the proline-rich region of WAVE2 through the first and second SH3 domains of vinexin beta. Mutations disrupting the interaction impaired the ability of vinexin beta to increase the amount of WAVE2 protein. Treatments with proteasome inhibitors increased the amount of WAVE2, but did not have an additive effect with vinexin beta. Inhibition of protein kinase A (PKA) activity suppressed the vinexin-induced increase in WAVE2 protein, while activation of PKA increased WAVE2 expression without vinexin beta. These results suggest that vinexin beta regulates the proteasome-dependent degradation of WAVE2 in a PKA-dependent manner.

Involvement of palladin and alpha-actinin in targeting of the Abl/Arg kinase adaptor ArgBP2 to the actin cytoskeleton

Palladin and alpha-actinin are major components of stress fiber dense bodies, cardiomyocyte Z-discs and neuronal synapses. They function as structural molecules and cytoskeletal regulators but also as docking sites to other proteins. Both antisense and transient overexpression experiments have shown that palladin plays an important role in the regulation of actin cytoskeleton. ArgBP2 is a multi-domain scaffolding protein which shares both the tissue distribution and subcellular localization with palladin. ArgBP2 is directly linked to intracellular signaling cascades by its interaction with Abl family kinases, Pyk2 and the ubiquitin ligase Cbl. It has several actin associated binding partners and has been shown to regulate cytoskeletal dynamics. Here, we show by in vivo and in vitro methods that palladin's amino-terminal poly-proline sequences directly interact with the first carboxy-terminal SH3 domain of ArgBP2. We further demonstrate a direct interaction between alpha-actinin and the amino-terminal segment of ArgBP2. Immunoprecipitation and targeting assays suggest that a three-way complex of the proteins occurs in vivo. The interactions provide an explanation to the previously observed Z-disc-specific localization of ArgBP2 and indicate interplay between signaling adaptors and structural proteins of the Z-disc.

A functional role of postsynaptic density-95-guanylate kinase-associated protein complex in regulating Shank assembly and stability to synapses

Postsynaptic density (PSD) proteins include scaffold, cytoskeletal, and signaling proteins that structurally and functionally interact with glutamate receptors and other postsynaptic membrane proteins. The molecular mechanisms regulating the assembly of PSD proteins and their associations with synapses are still widely unknown. We investigated the molecular mechanisms of Shank1 targeting and synapse assembly by looking at the function of guanylate kinase-associated protein (GKAP) and PSD-95 interactions. Shank1 when it is not associated to GKAP, which binds to the Shank PSD-95-Discs Large-zona occludens-1 domain, forms filamentous and fusiform structures in which the Src homology 3 domain specifically interacts with the ankyrin repeat domain, thus allowing its multimerization via a novel form of intermolecular interaction. Surprisingly, in both COS-7 cells and hippocampal neurons, GKAP forms insoluble aggregates with Shank that colocalize with heat shock protein 70 and neurofilaments, two markers of the aggresomes in which misfolded proteins accumulate. However, the two proteins are organized in clusters in COS cells and synaptic clusters in neurons when both are overexpressed and associated with wild-type PSD-95, but not with palmitoylation-deficient PSD-95. Synaptic activity in neurons induces the formation of Shank and GKAP intracellular aggregation and degradation. Similarly, the overexpression of a GKAP mutant that is incapable of binding PSD-95 induces Shank aggregation and degradation in neurons. Our data suggest a possible functional and structural role of the PSD-95-GKAP complex in Shank and PSD protein assembly and stability to synapses.

The Stomatin/Prohibitin/Flotillin/HflK/C Domain of Flotillin-1 Contains Distinct Sequences That Direct Plasma Membrane Localization and Protein Interactions in 3T3-L1 Adipocytes

Flotillin-1 is a lipid raft-associated protein that has been implicated in various cellular processes. We examined the subcellular distribution of flotillin-1 in different cell types and found that localization is cell type-specific. Flotillin-1 relocates from a cytoplasmic compartment to the plasma membrane upon the differentiation of 3T3-L1 adipocytes. To delineate the structural determinants necessary for its localization, we generated a series of truncation mutants of flotillin-1. Wild type flotillin-1 has two putative hydrophobic domains and is localized to lipid raft microdomains at the plasma membrane. Flotillin-1 fragments lacking the N-terminal hydrophobic stretch are excluded from the lipid raft compartments but remain at the plasma membrane. On the other hand, mutants with the second hydrophobic region deleted fail to traffic to the plasma membrane but are instead found in intracellular granule-like structures. Flotillin-1 specifically interacts with the adaptor protein CAP, the Src family kinase Fyn, and cortical F-actin in lipid raft microdomains in adipocytes. Furthermore, CAP and Fyn associate with different regions in the N-terminal sequences of flotillin-1. These results furthered our understanding for how flotillin-1 can function as a molecular link between lipid rafts of the plasma membrane and a multimeric signaling complex at the actin cytoskeleton.

Nectin-like molecule-1/TSLL1/SynCAM3: a neural tissue-specific immunoglobulin-like cell-cell adhesion molecule localizing at non-junctional contact sites of presynaptic nerve terminals, axons and glia cell processes

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules and comprise a family of four members. At the mossy fiber terminals of hippocampus, nectin-1 and nectin-3 localize at the presynaptic and postsynaptic sides of synaptic junctions, respectively, and their trans-interactions play a role in formation of synapses in cooperation with N-cadherin. Nectins are associated with the actin cytoskeleton through afadin, a nectin- and actin-filament-binding protein. Five nectin-like molecules (Necls) which have domain structures similar to those of nectins have been identified and here we characterize Necl-1/TSLL1/SynCAM3, from now on referred to as Necl-1. Tissue distribution analysis showed that Necl-1 was specifically expressed in the neural tissue. Immunofluorescence and immunoelectron microscopy revealed that Necl-1 localized at the contact sites among axons, their terminals, and glia cell processes that cooperatively formed synapses, axon bundles and myelinated axons. Necl-1 showed Ca2+-independent homophilic cell-cell adhesion activity. It furthermore showed Ca2+-independent heterophilic cell-cell adhesion activity with Necl-2/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1 from now on referred to as Necl-2, nectin-1 and nectin-3, but not with Necl-5 or nectin-2. The C-terminal cytoplasmic region of Necl-1 did not bind afadin but bound membrane-associated guanylate kinase subfamily members that contain the L27 domain, including Dlg3, Pals2 and CASK. These results indicate that Necl-1 is a neural-tissue-specific Ca2+-independent immunoglobulin-like cell-cell adhesion molecule which potentially has membrane-associated guanylate kinase subfamily member-binding activity and localizes at the non-junctional cell-cell contact sites.

The Abl/Arg substrate ArgBP2/nArgBP2 coordinates the function of multiple regulatory mechanisms converging on the actin cytoskeleton

ArgBP2, and its brain-specific splice variant, nArgBP2, are interactors and substrates of Abl/Arg tyrosine kinases and of the ubiquitin ligase Cbl. They are members of a family of adaptor proteins that colocalize with actin on stress fibers and at cell-adhesion sites, including neuronal synapses. We show here that their NH2-terminal region, which contains a sorbin homology domain domain, interacts with spectrin, and we identify binding proteins for their COOH-terminal SH3 domains. All these binding partners participate in the regulation of the actin cytoskeleton. These include dynamin, synaptojanin, and WAVE isoforms, as well as WAVE regulatory proteins. At least two of the ArgBP2/nArgBP2 binding partners, synaptojanin 2B and WAVE2, undergo ubiquitination and Abl-dependent tyrosine phosphorylation. ArgBP2/nArgBP2 knockdown in astrocytes produces a redistribution of focal adhesion proteins and an increase in peripheral actin ruffles, whereas nArgBP2 overexpression produces a collapse of the actin cytoskeleton. Thus, ArgBP2/nArgBP2 is a scaffold protein that control the balance between adhesion and motility by coordinating the function of multiple signaling pathways converging on the actin cytoskeleton.

Distinct spatiotemporal expression of SAPAP transcripts in the developing rat brain: a novel dendritically localized mRNA

The four members of the family of synapse-associated protein 90/postsynaptic density-95-associated proteins (SAPAP1-4) are adapter proteins of postsynaptic density (PSD). They interact with different synaptic scaffolding proteins, cytoskeletal components, and signalling components, and are therefore considered to assemble functional multiprotein units at synapses. Here, we analyzed the spatiotemporal expression of SAPAP1-SAPAP4 genes in postnatal rat brain by in situ hybridization. All four genes are expressed in many brain areas, leading to overlapping yet distinct mRNA distribution patterns. Moreover, two mRNAs encoding distinct SAPAP3 isoforms exhibit basically identical postnatal expression patterns. In the hippocampus, SAPAP1, SAPAP2, and SAPAP4 transcripts are restricted to cell body zones, whereas SAPAP3 mRNAs are also detected in molecular layers. Thus, SAPAP3 is one of the few PSD components whose local synthesis in dendrites may contribute to an input-specific adaptation of dendritic spine function.