ArgBP2 and the SoHo family of adapter proteins in oncogenic diseases

Impact of Sorbs2 dysfunction on cardiovascular diseases

Despite significant advancements in prevention and treatment over the past decades, cardiovascular diseases (CVDs) remain the leading cause of death worldwide. CVDs involve multifactorial inheritance, but our understanding of the genetic impact on these diseases is still incomplete. Sorbin and SH3 domain-containing protein 2 (Sorbs2) is ubiquitously expressed in various tissues, including the cardiovascular system. Increasing evidence suggests that Sorbs2 malfunction contributes to CVDs. This manuscript will review our current understanding of the potential mechanisms underlying Sorbs2 dysregulation in the development of CVDs.

Identification of RSK substrates using an analog-sensitive kinase approach

The p90 ribosomal S6 kinases (RSK) family of serine/threonine kinases comprises four isoforms (RSK1-4) that lie downstream of the ERK1/2 mitogen-activated protein kinase pathway. RSKs are implicated in fine tuning of cellular processes such as translation, transcription, proliferation, and motility. Previous work showed that pathogens such as Cardioviruses could hijack any of the four RSK isoforms to inhibit PKR activation or to disrupt cellular nucleocytoplasmic trafficking. In contrast, some reports suggest nonredundant functions for distinct RSK isoforms, whereas Coffin-Lowry syndrome has only been associated with mutations in the gene encoding RSK2. In this work, we used the analog-sensitive kinase strategy to ask whether the cellular substrates of distinct RSK isoforms differ. We compared the substrates of two of the most distant RSK isoforms: RSK1 and RSK4. We identified a series of potential substrates for both RSKs in cells and validated RanBP3, PDCD4, IRS2, and ZC3H11A as substrates of both RSK1 and RSK4, and SORBS2 as an RSK1 substrate. In addition, using mutagenesis and inhibitors, we confirmed analog-sensitive kinase data showing that endogenous RSKs phosphorylate TRIM33 at S1119. Our data thus identify a series of potential RSK substrates and suggest that the substrates of RSK1 and RSK4 largely overlap and that the specificity of the various RSK isoforms likely depends on their cell- or tissue-specific expression pattern.

Oncogenic MORC2 in cancer development and beyond

Microrchidia CW-type zinc finger 2 (MORC2) is a member of the MORC superfamily of nuclear proteins. Growing evidence has shown that MORC2 not only participates in gene transcription and chromatin remodeling but also plays a key in human disease and tumor development by regulating the expression of downstream oncogenes or tumor suppressors. The present review provides an updated overview of MORC2 in the aspect of cancer hallmark and therapeutic resistance and summarizes its upstream regulators and downstream target genes. This systematic review may provide a favorable theoretical basis for emerging players of MORC2 in tumor development and new insight into the potential clinical application of basic science discoveries in the future.

The cytoskeleton adaptor protein Sorbs1 controls the development of lymphatic and venous vessels in zebrafish

BACKGROUND

Lymphangiogenesis, the formation of lymphatic vessels, is tightly linked to the development of the venous vasculature, both at the cellular and molecular levels. Here, we identify a novel role for Sorbs1, the founding member of the SoHo family of cytoskeleton adaptor proteins, in vascular and lymphatic development in the zebrafish.

RESULTS

We show that Sorbs1 is required for secondary sprouting and emergence of several vascular structures specifically derived from the axial vein. Most notably, formation of the precursor parachordal lymphatic structures is affected in sorbs1 mutant embryos, severely impacting the establishment of the trunk lymphatic vessel network. Interestingly, we show that Sorbs1 interacts with the BMP pathway and could function outside of Vegfc signaling. Mechanistically, Sorbs1 controls FAK/Src signaling and subsequently impacts on the cytoskeleton processes regulated by Rac1 and RhoA GTPases. Inactivation of Sorbs1 altered cell-extracellular matrix (ECM) contacts rearrangement and cytoskeleton dynamics, leading to specific defects in endothelial cell migratory and adhesive properties.

CONCLUSIONS

Overall, using in vitro and in vivo assays, we identify Sorbs1 as an important regulator of venous and lymphatic angiogenesis independently of the Vegfc signaling axis. These results provide a better understanding of the complexity found within context-specific vascular and lymphatic development.

Bivalent activity of super-enhancer RNA LINC02454 controls 3D chromatin structure and regulates glioma sensitivity to temozolomide

Glioma cell sensitivity to temozolomide (TMZ) is critical for effective treatment and correlates with patient survival, although mechanisms underlying this activity are unclear. Here, we reveal a new mechanism used by glioma cells to modulate TMZ sensitivity via regulation of SORBS2 and DDR1 genes by super-enhancer RNA LINC02454. We report that LINC02454 activity increases glioma cell TMZ sensitivity by maintaining long-range chromatin interactions between SORBS2 and the LINC02454 enhancer. By contrast, LINC02454 activity also decreased glioma cell TMZ sensitivity by promoting DDR1 expression. Our study suggests a bivalent function for super-enhancer RNA LINC02454 in regulating glioma cell sensitivity to TMZ.

Novel Insights into the Role of Chromatin Remodeler MORC2 in Cancer

A newly discovered chromatin remodeler, MORC2, is a Microrchidia (MORC) family member. MORC2 acts as a chromatin remodeler by binding to the DNA and changing chromatin conformation using its ATPase domain. MORC2 is highly expressed in a variety of human cancers. It controls diverse signaling pathways essential for cancer development through its target genes and interacting partners. MORC2 promotes cancer cells' growth, invasion, and migration by regulating the expression of genes involved in these processes. MORC2 is localized primarily in the nucleus and is also found in the cytoplasm. In the cytoplasm, MORC2 interacts with adenosine triphosphate (ATP)-citrate lyase (ACLY) to promote lipogenesis and cholesterogenesis in cancer. In the nucleus, MORC2 interacts with the transcription factor c-Myc to control the transcription of genes involved in glucose metabolism to drive cancer cell migration and invasion. Furthermore, MORC2 recruits on to the promoters of tumor suppressor genes to repress their transcription and expression to promote oncogenesis. In addition to its crucial function in oncogenesis, it plays a vital role in DNA repair. Overall, this review concisely summarizes the current knowledge about MORC2-regulated molecular pathways involved in cancer.

Characterization of p38α signaling networks in cancer cells using quantitative proteomics and phosphoproteomics

p38α (encoded by MAPK14) is a protein kinase that regulates cellular responses to almost all types of environmental and intracellular stresses. Upon activation, p38α phosphorylates many substrates both in the cytoplasm and nucleus, allowing this pathway to regulate a wide variety of cellular processes. While the role of p38α in the stress response has been widely investigated, its implication in cell homeostasis is less understood. To investigate the signaling networks regulated by p38α in proliferating cancer cells, we performed quantitative proteomic and phosphoproteomic analyses in breast cancer cells in which this pathway had been either genetically targeted or chemically inhibited. Our study identified with high confidence 35 proteins and 82 phosphoproteins (114 phosphosites) that are modulated by p38α, and highlighted the implication of various protein kinases, including MK2 and mTOR, in the p38α-regulated signaling networks. Moreover, functional analyses revealed an important contribution of p38α to the regulation of cell adhesion, DNA replication and RNA metabolism. Indeed, we provide experimental evidence supporting that p38α facilitates cancer cell adhesion, and showed that this p38α function is likely mediated by the modulation of the adaptor protein ArgBP2. Collectively, our results illustrate the complexity of the p38α regulated signaling networks, provide valuable information on p38α-dependent phosphorylation events in cancer cells, and document a mechanism by which p38α can regulate cell adhesion.

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.

Protein Kinase A in cellular migration-Niche signaling of a ubiquitous kinase

Cell migration requires establishment and maintenance of directional polarity, which in turn requires spatial heterogeneity in the regulation of protrusion, retraction, and adhesion. Thus, the signaling proteins that regulate these various structural processes must also be distinctly regulated in subcellular space. Protein Kinase A (PKA) is a ubiquitous serine/threonine kinase involved in innumerable cellular processes. In the context of cell migration, it has a paradoxical role in that global inhibition or activation of PKA inhibits migration. It follows, then, that the subcellular regulation of PKA is key to bringing its proper permissive and restrictive functions to the correct parts of the cell. Proper subcellular regulation of PKA controls not only when and where it is active but also specifies the targets for that activity, allowing the cell to use a single, promiscuous kinase to exert distinct functions within different subcellular niches to facilitate cell movement. In this way, understanding PKA signaling in migration is a study in context and in the elegant coordination of distinct functions of a single protein in a complex cellular process.

RNA-binding protein SORBS2 suppresses clear cell renal cell carcinoma metastasis by enhancing MTUS1 mRNA stability

RNA-binding proteins (RBPs) predominantly contribute to abnormal posttranscriptional gene modulation and disease progression in cancer. Sorbin and SH3 domain-containing 2 (SORBS2), an RBP, has been reported to be a potent tumor suppressor in several cancer types. Through integrative analysis of clinical specimens, we disclosed that the expression level of SORBS2 was saliently decreased in metastatic tissues and positively correlated with overall survival. We observed that overexpression of SORBS2 brought about decreased metastatic capacity in ccRCC cell lines. Transcriptome-wide analysis revealed that SORBS2 notably increased microtubule-associated tumor-suppressor 1 gene (MTUS1) expression. In-depth mechanistic exploring discovered that the Cys2-His2 zinc finger (C2H2-ZnF) domain of SORBS2 directly bound to the 3′ untranslated region (3′UTR) of MTUS1 mRNA, which increased MTUS1 mRNA stability. In addition, we identified that MTUS1 regulated microtubule dynamics via promoting KIF2C^S192 phosphorylation by Aurora B. Together, our research identified SORBS2 as a suppressor of ccRCC metastasis by enhancing MTUS1 mRNA stability, providing a novel understanding of RBPs during ccRCC progression.

Cbl-Associated Protein CAP contributes to correct formation and robust function of the Drosophila heart tube

The formation of a tube-like structure is a basic step in the making of functional hearts in vertebrates and invertebrates and therefore, its understanding provides important information on heart development and function. In Drosophila, the cardiac tube originates from two bilateral rows of dorsally migrating cells. On meeting at the dorsal midline, coordinated changes in cell shape and adhesive properties transform the two sheets of cells into a linear tube. ECM and transmembrane proteins linked to the cytoskeleton play an important role during these dynamic processes. Here we characterize the requirement of Cbl-Associated Protein (CAP) in Drosophila heart formation. In embryos, CAP is expressed in late migrating cardioblasts and is located preferentially at their luminal and abluminal periphery. CAP mutations result in irregular cardioblast alignment and imprecisely controlled cardioblast numbers. Furthermore, CAP mutant embryos show a strongly reduced heart lumen and an aberrant shape of lumen forming cardioblasts. Analysis of double heterozygous animals reveals a genetic interaction of CAP with Integrin- and Talin-encoding genes. In post-embryonic stages, CAP closely colocalizes with Integrin near Z-bands and at cell-cell contact sites. CAP mutants exhibit a reduced contractility in larval hearts and show a locally disrupted morphology, which correlates with a reduced pumping efficiency. Our observations imply a function of CAP in linking Integrin signaling with the actin cytoskeleton. As a modulator of the cytoskeleton, CAP is involved in the establishment of proper cell shapes during cardioblast alignment and cardiac lumen formation in the Drosophila embryo. Furthermore, CAP is required for correct heart function throughout development.

JMY expression by Sertoli cells contributes to mediating spermatogenesis in mice

Sertoli cells are crucial for spermatogenesis in the seminiferous epithelium because their actin cytoskeleton supports vesicular transport, cell junction formation, protein anchoring, and spermiation. Here, we show that a junction-mediating and actin-regulatory protein (JMY) affects the blood-tissue barrier (BTB) function through remodeling of the Sertoli cell junctional integrity and it also contributes to controlling endocytic vesicle trafficking. These functions are critical for the maintenance of sperm fertility since loss of Sertoli cell-specific Jmy function induced male subfertility in mice. Specifically, these mice have (a) impaired BTB integrity and spermatid adhesion in the seminiferous tubules; (b) high incidence of sperm structural deformity; and (c) reduced sperm count and poor sperm motility. Moreover, the cytoskeletal integrity was compromised along with endocytic vesicular trafficking. These effects impaired junctional protein recycling and reduced Sertoli cell BTB junctional integrity. In addition, JMY interaction with actin-binding protein candidates α-actinin1 and sorbin and SH3 domain containing protein 2 was related to JMY activity, and in turn, actin cytoskeletal organization. In summary, JMY affects the control of spermatogenesis through the regulation of actin filament organization and endocytic vesicle trafficking in Sertoli cells.

A temporal transcriptome and methylome in human embryonic stem cell-derived cardiomyocytes identifies novel regulators of early cardiac development

Stem cell-based cardiogenesis has become a powerful tool to enhance our understanding of cardiac development and test novel therapeutics for cardiovascular diseases. However, transcriptional and epigenetic regulation of multiple transitional stages from pluripotent cells to committed cardiomyocytes has not yet been fully characterized. To characterize how transcription factors, lincRNAs and DNA methylation change at temporal developmental stages, and identify potential novel regulators during cardiogenesis. We utilized a previously reported protocol that yields human cardiomyocytes (hCM) with more than 90% purity from human Embryonic Stem Cells (hESC). Leveraging the purity of cells resulting from this protocol, we systematically examined how gene expression and DNA methylation programs change at temporal developmental stages during cardiogenesis. Our results provide a comprehensive view of expression changes during cardiogenesis that extend previous studies, allowing us to identify key transcription factors as well as lincRNAs that are strongly associated with cardiac differentiation. Moreover, we incorporated a simple but powerful method to screen for novel regulators of cardiogenesis solely based on expression changes and found four novel cardiac-related transcription factors, i.e., SORBS2, MITF, DPF3, and ZNF436, which have no or few prior literature reports and we were able to validate using siRNA. Our strategy of identifying novel regulators of cardiogenesis can also be easily implemented in other stem cell-based systems. Our results provide a valuable resource for understanding cardiogenesis that extends previous findings by leveraging the purity of our cell lines, which allowed us to identify four novel cardiac-related regulators.

HSF1, in association with MORC2, downregulates ArgBP2 via the PRC2 family in gastric cancer cells

Arg Kinase-binding protein 2 (ArgBP2) is considered to be a scaffold protein that coordinates multiple signaling pathways converging on cell adhesion and actin cytoskeletal organization. It also plays an important role in blocking cancer metastasis as a potential tumor suppressor. However, its regulation mechanisms in tumor migration, especially in gastric cancer, are not fully understood. Here, we identified an ArgBP2 enhancer and showed that heat shock factor 1 (HSF1) directly interacted with microrchidia CW-type zinc finger 2 (MORC2) and bound to the enhancer of ArgBP2. HSF1 was found to promote proliferation, migration and invasion of gastric cancer cells. HSF1 or/and MORC2 increased recruitment of the polycomb repressive complex 2 (PRC2), particularly enhancer of zeste homolog 2 (EZH2), to the ArgBP2 enhancer and catalyzed tri-methylation of lysine 27 on histone H3 (H3K27me3), leading to transcriptional repression of ArgBP2. In addition, HSF1 and MORC2-induced migration and invasion in gastric cancer cells was dependent on ArgBP2 or EZH2. Clinical data exhibited a negative correlation of ArgBP2 with MORC2, HSF1, and EZH2. Our results thus contribute to the knowledge of the regulatory mechanism of HSF1 in down-regulating ArgBP2, providing new insight into the HSF1&MORC2-PRC2-ArgBP2 signaling pathway and a better understanding of their functions in gastric cancer cells.

SORBS1 suppresses tumor metastasis and improves the sensitivity of cancer to chemotherapy drug

Tumor metastasis and invasion are both hallmarks of cancer malignancy and the leading cause of cancer death. Here we show that the adaptor protein SORBS1 (Sorbin and SH3 domain-containing protein 1, also known as CAP/ponsin) is expressed at low levels in clinical cancer samples. In addition, low-level expression of SORBS1 was significantly associated with poor clinical outcomes and the increased tumor cell invasive capacity in breast cancer patients. We demonstrate that depletion of SORBS1 increases protrusions and filopodium-like protrusions (FLPs) formation, as well as the migratory and invasive abilities of cancer cells, via activation of JNK/cJun. Furthermore, silencing of SORBS1 promotes the epithelial-to-mesenchymal transition (EMT) process and attenuates chemical drug sensitivity especially that to cisplatin, by inhibition of p53 in breast cancer cells. Thus, we illustrate that SORBS1 is a potential inhibitor of metastasis in cancer and may be a promising target in chemotherapy.

The distribution of vinculin to lipid rafts plays an important role in sensing stiffness of extracellular matrix

Extracellular matrix (ECM) stiffness regulates cell differentiation, survival, and migration. Our previous study has shown that the interaction of the focal adhesion protein vinculin with vinexin α plays a critical role in sensing ECM stiffness and regulating stiffness-dependent cell migration. However, the mechanism how vinculin-vinexin α interaction affects stiffness-dependent cell migration is unclear. Lipid rafts are membrane microdomains that are known to affect ECM-induced signals and cell behaviors. Here, we show that vinculin and vinexin α can localize to lipid rafts. Cell-ECM adhesion, intracellular tension, and a rigid ECM promote vinculin distribution to lipid rafts. The disruption of lipid rafts with Methyl-β-cyclodextrin impaired the ECM stiffness-mediated regulation of vinculin behavior and rapid cell migration on rigid ECM. These results indicate that lipid rafts play an important role in ECM-stiffness regulation of cell migration via vinculin.

Large-Scale Screening of Preferred Interactions of Human Src Homology-3 (SH3) Domains Using Native Target Proteins as Affinity Ligands

The Src Homology-3 (SH3) domains are ubiquitous protein modules that mediate important intracellular protein interactions via binding to short proline-rich consensus motifs in their target proteins. The affinity and specificity of such core SH3 - ligand contacts are typically modest, but additional binding interfaces can give rise to stronger and more specific SH3-mediated interactions. To understand how commonly such robust SH3 interactions occur in the human protein interactome, and to identify these in an unbiased manner we have expressed 324 predicted human SH3 ligands as full-length proteins in mammalian cells, and screened for their preferred SH3 partners using a phage display-based approach. This discovery platform contains an essentially complete repertoire of the ∼300 human SH3 domains, and involves an inherent binding threshold that ensures selective identification of only SH3 interactions with relatively high affinity. Such strong and selective SH3 partners could be identified for only 19 of these 324 predicted ligand proteins, suggesting that the majority of human SH3 interactions are relatively weak, and thereby have capacity for only modest inherent selectivity. The panel of exceptionally robust SH3 interactions identified here provides a rich source of leads and hypotheses for further studies. However, a truly comprehensive characterization of the human SH3 interactome will require novel high-throughput methods based on function instead of absolute binding affinity.

Interaction of atypical cadherin Fat1 with SoHo adaptor proteins CAP/ponsin and ArgBP2

Mammalian Fat1 is a giant atypical cadherin/tumor suppressor involved in the regulation of cellular orientation, migration, and growth. Fat1 is implicated in the development of the brain, eye, and kidney. Altered expression or mutations of FAT1 are also associated with cancer and facioscapulohumeral muscular dystrophy (FSHD). Yet, the mechanistic functions of this pathway remain incompletely understood. Here, we report the identification of Sorbin-homology (SoHo) proteins as novel interaction partners of Fat1 by virtue of a yeast-two-hybrid screen. SoHo proteins play diverse roles as adaptor proteins in cell signaling, cell adhesion and sarcomere architecture, including altered expression in cancer and FSHD. Specifically, we found SoHo proteins CAP/ponsin-1 and -2 (Sorbs1) and ArgBP2 (Sorbs2) to interact with the cytoplasmic domain of Fat1. We mapped the interaction to a prolin-rich classic type II PXXP motif within Fat1 and to the three Src-homology (SH3) domains within SoHo proteins using mutant expression in yeast, pulldown assays, and cell culture. Functionally, endogenous ponsin-2 expression of NRK-52E cells at cellular leading edges was lost upon knockdown of Fat1. In summary, our data point to an interaction of Fat1 with SoHo proteins that is able to recruit SoHo proteins to sites of Fat1 expression.

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.

SORBS2 transcription is activated by telomere position effect-over long distance upon telomere shortening in muscle cells from patients with facioscapulohumeral dystrophy

DNA is organized into complex three-dimensional chromatin structures, but how this spatial organization regulates gene expression remains a central question. These DNA/chromatin looping structures can range in size from 10-20 kb (enhancers/repressors) to many megabases during intra- and inter-chromosomal interactions. Recently, the influence of telomere length on chromatin organization prior to senescence has revealed the existence of long-distance chromatin loops that dictate the expression of genes located up to 10 Mb from the telomeres (Telomere Position Effect-Over Long Distances [TPE-OLD]). Here, we demonstrate the existence of a telomere loop at the 4q35 locus involving the sorbin and SH3 domain-containing protein 2 gene, SORBS2, a skeletal muscle protein using a modification of the chromosome conformation capture method. The loop reveals a cis-acting mechanism modifying SORBS2 transcription. The expression of this gene is altered by TPE-OLD in myoblasts from patients affected with the age-associated genetic disease, facioscapulohumeral muscular dystrophy (FSHD1A, MIM 158900). SORBS2 is expressed in FSHD myoblasts with short telomeres, while not detectable in FSHD myoblasts with long telomeres or in healthy myoblasts regardless of telomere length. This indicates that TPE-OLD may modify the regulation of the 4q35 locus in a pathogenic context. Upon differentiation, both FSHD and healthy myotubes express SORBS2, suggesting that SORBS2 is normally up-regulated by maturation/differentiation of skeletal muscle and is misregulated by TPE-OLD-dependent variegation in FSHD myoblasts. These findings provide additional insights for the complexity and age-related symptoms of FSHD.

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.

Vinexin β Interacts with Hepatitis C Virus NS5A, Modulating Its Hyperphosphorylation To Regulate Viral Propagation

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is essential for HCV genome replication and virion production and is involved in the regulation of multiple host signaling pathways. As a proline-rich protein, NS5A is capable of interacting with various host proteins containing Src homology 3 (SH3) domains. Previous studies have suggested that vinexin, a member of the sorbin homology (SoHo) adaptor family, might be a potential binding partner of NS5A by yeast two-hybrid screening. However, firm evidence for this interaction is lacking, and the significance of vinexin in the HCV life cycle remains unclear. In this study, we demonstrated that endogenously and exogenously expressed vinexin β coimmunoprecipitated with NS5A derived from different HCV genotypes. Two residues, tryptophan (W307) and tyrosine (Y325), in the third SH3 domain of vinexin β and conserved Pro-X-X-Pro-X-Arg motifs at the C terminus of NS5A were indispensable for the vinexin-NS5A interaction. Furthermore, downregulation of endogenous vinexin β significantly suppressed NS5A hyperphosphorylation and decreased HCV replication, which could be rescued by expressing a vinexin β short hairpin RNA-resistant mutant. We also found that vinexin β modulated the hyperphosphorylation of NS5A in a casein kinase 1α-dependent on manner. Taken together, our findings suggest that vinexin β modulates NS5A phosphorylation via its interaction with NS5A, thereby regulating HCV replication, implicating vinexin β in the viral life cycle.

IMPORTANCE

Hepatitis C virus (HCV) nonstructural protein NS5A is a phosphoprotein, and its phosphorylation states are usually modulated by host kinases and other viral nonstructural elements. Additionally, cellular factors containing Src homology 3 (SH3) domains have been reported to interact with proline-rich regions of NS5A. However, it is unclear whether there are any relationships between NS5A phosphorylation and the NS5A-SH3 interaction, and little is known about the significance of this interaction in the HCV life cycle. In this work, we demonstrate that vinexin β modulates NS5A hyperphosphorylation through the NS5A-vinexin β interaction. Hyperphosphorylated NS5A induced by vinexin β is casein kinase 1α dependent and is also crucial for HCV propagation. Overall, our findings not only elucidate the relationships between NS5A phosphorylation and the NS5A-SH3 interaction but also shed new mechanistic insight on Flaviviridae NS5A (NS5) phosphorylation. We believe that our results may afford the potential to offer an antiviral therapeutic strategy.

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.

Protein interactions of the vesicular glutamate transporter VGLUT1

Exocytotic release of glutamate depends upon loading of the neurotransmitter into synaptic vesicles by vesicular glutamate transporters, VGLUTs. The major isoforms, VGLUT1 and 2, exhibit a complementary pattern of expression in synapses of the adult rodent brain that correlates with the probability of release and potential for plasticity. Indeed, expression of different VGLUT protein isoforms confers different properties of release probability. Expression of VGLUT1 or 2 protein also determines the kinetics of synaptic vesicle recycling. To identify molecular determinants that may be related to reported differences in VGLUT trafficking and glutamate release properties, we investigated some of the intrinsic differences between the two isoforms. VGLUT1 and 2 exhibit a high degree of sequence homology, but differ in their N- and C-termini. While the C-termini of VGLUT1 and 2 share a dileucine-like trafficking motif and a proline-, glutamate-, serine-, and threonine-rich PEST domain, only VGLUT1 contains two polyproline domains and a phosphorylation consensus sequence in a region of acidic amino acids. The interaction of a VGLUT1 polyproline domain with the endocytic protein endophilin recruits VGLUT1 to a fast recycling pathway. To identify trans-acting cellular proteins that interact with the distinct motifs found in the C-terminus of VGLUT1, we performed a series of in vitro biochemical screening assays using the region encompassing the polyproline motifs, phosphorylation consensus sites, and PEST domain. We identify interactors that belong to several classes of proteins that modulate cellular function, including actin cytoskeletal adaptors, ubiquitin ligases, and tyrosine kinases. The nature of these interactions suggests novel avenues to investigate the modulation of synaptic vesicle protein recycling.

Downregulation of FOXO3a promotes tumor metastasis and is associated with metastasis-free survival of patients with clear cell renal cell carcinoma

PURPOSE

To explore the mechanisms underlying clear-cell renal cell carcinoma (ccRCC) metastasis using transcriptional profiling and bioinformatics analysis of ccRCC samples, and to elucidate the role of FOXO3a in ccRCC metastasis.

EXPERIMENTAL DESIGN

Gene expression profiling was performed using four primary metastatic and five primary nonmetastatic ccRCC samples. The mRNA and protein levels of FOXO3a in ccRCC samples were investigated by real-time reverse transcription PCR and immunohistochemistry, respectively. The association between metastasis-free survival of patients with ccRCC and FOXO3a mRNA levels was analyzed. Biologic functions of FOXO3a in renal cancer cell lines were investigated. The influence of FOXO3a on tumor metastasis was also studied in vivo orthotopic xenograft tumor model. Finally, the mechanism by which FOXO3a attenuation could increase invasion and migration of tumor cells was explored.

RESULTS

Bioinformatics analysis of the profiling data identified FOXO3a as a key factor in ccRCC metastasis. FOXO3a expression was decreased in primary metastatic ccRCC samples. Patients with low FOXO3a mRNA levels had poor metastasis-free survival (P = 0.003). Knocking down FOXO3a induced tumor cell invasion and migration in the nonmetastatic ccRCC cells. Induced FOXO3a overexpression in SN12-PM6 cells could inhibit tumor metastasis in vivo. Downregulation of FOXO3a increased SNAIL1 expression, thereby activating the epithelial-mesenchymal transition (EMT) of RCC cell lines.

CONCLUSIONS

The loss of FOXO3a induced EMT of tumor cells by upregulating SNAIL1, which promoted tumor cells metastasis in vitro and in vivo. Thus, FOXO3a could be considered as an independent prognostic factor in ccRCC metastasis and could be a marker of occult metastases.

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.

Modulation of dorsal root ganglion development by ErbB signaling and the scaffold protein Sorbs3

The multipotent cells of the vertebrate neural crest (NC) arise at the dorsal aspect of the neural tube, then migrate throughout the developing embryo and differentiate into diverse cell types, including the sensory neurons and glia of the dorsal root ganglia (DRG). As multiple cell types are derived from this lineage, it is ideal for examining mechanisms of fate restriction during development. We have isolated a mutant, ouchless, that specifically fails to develop DRG neurons, although other NC derivatives develop normally. This mutation affects the expression of Sorbs3, a scaffold protein known to interact with proteins involved in focal adhesions and several signaling pathways. ouchless mutants share some phenotypic similarities with mutants in ErbB receptors, EGFR homologs that are implicated in diverse developmental processes and associated with several cancers; and ouchless interacts genetically with an allele of erbb3 in DRG neurogenesis. However, the defect in ouchless DRG neurogenesis is distinct from ErbB loss of function in that it is not associated with a loss of glia. Both ouchless and neurogenin1 heterozygous fish are sensitized to the effects of ErbB chemical inhibitors, which block the development of DRG in a dose-dependent manner. Inhibitors of MEK show similar effects on DRG neurogenesis. We propose a model in which Sorbs3 helps to integrate ErbB signals to promote DRG neurogenesis through the activation of MAPK and upregulation of neurogenin1.

PP2A regulatory subunit Bα controls endothelial contractility and vessel lumen integrity via regulation of HDAC7

To supply tissues with nutrients and oxygen, the cardiovascular system forms a seamless, hierarchically branched, network of lumenized tubes. Here, we show that maintenance of patent vessel lumens requires the Bα regulatory subunit of protein phosphatase 2A (PP2A). Deficiency of Bα in zebrafish precludes vascular lumen stabilization resulting in perfusion defects. Similarly, inactivation of PP2A-Bα in cultured ECs induces tubulogenesis failure due to alteration of cytoskeleton dynamics, actomyosin contractility and maturation of cell-extracellular matrix (ECM) contacts. Mechanistically, we show that PP2A-Bα controls the activity of HDAC7, an essential transcriptional regulator of vascular stability. In the absence of PP2A-Bα, transcriptional repression by HDAC7 is abrogated leading to enhanced expression of the cytoskeleton adaptor protein ArgBP2. ArgBP2 hyperactivates RhoA causing inadequate rearrangements of the EC actomyosin cytoskeleton. This study unravels the first specific role for a PP2A holoenzyme in development: the PP2A-Bα/HDAC7/ArgBP2 axis maintains vascular lumens by balancing endothelial cytoskeletal dynamics and cell-matrix adhesion.

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.

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.

An integrative functional genomic and gene expression approach revealed SORBS2 as a putative tumour suppressor gene involved in cervical carcinogenesis

Human papillomavirus (HPV) types 16 and 18 are known to play a major role in cervical carcinogenesis. However, additional genetic alterations are required for the development and progression of cervical cancer. Our aim was to identify genes which are consistently down-regulated in cervical cancers (CxCa) and which are likely to contribute to malignant transformation. Microarray analyses of RNA from high-grade cervical precancers (CIN2/3) and CxCa were performed to screen for putative tumour suppressor genes (TSG) in predefined regions on chromosomes 4 and 10. Validation of the candidate genes was done by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) in 16 normal cervical tissues, 14 CIN2/3 and 16 CxCa. The two most promising genes, SORBS2 and CALML5, were expressed ectopically in various cell lines in order to analyse their functional activity. Reconstitution of SORBS2 expression resulted in a significant reduction in cell proliferation, colony formation and anchorage-independent growth in CaSki, HPKII and HaCaT cells, whereby anchorage-independent growth could only be investigated for CaSki cells. SORBS2 had no effect on cell migration. In contrast, reconstitution of CALML5 expression did not influence the phenotype of all cell lines tested. None of the genes could induce senescence or apoptosis. Our results underline a possible role of SORBS2 as a TSG in cervical carcinogenesis.

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.

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.