Wiscott-Aldrich Syndrome Protein Research Articles

Loss of cytoplasmic actin filaments raises nuclear actin levels to drive INO80C-dependent chromosome fragmentation

Loss of cytosolic actin filaments upon TORC2 inhibition triggers chromosome fragmentation in yeast, which results from altered base excision repair of Zeocin-induced lesions. To find the link between TORC2 kinase and this yeast chromosome shattering (YCS) we performed phosphoproteomics. YCS-relevant phospho-targets included plasma membrane-associated regulators of actin polymerization, such as Las17, the yeast Wiscott-Aldrich Syndrome protein. Induced degradation of Las17 was sufficient to trigger YCS in presence of Zeocin, bypassing TORC2 inhibition. In yeast, Las17 does not act directly at damage, but instead its loss, like TORC2 inhibition, raises nuclear actin levels. Nuclear actin, in complex with Arp4, forms an essential subunit of several nucleosome remodeler complexes, including INO80C, which facilitates DNA polymerase elongation. Here we show that the genetic ablation of INO80C activity leads to partial YCS resistance, suggesting that elevated levels of nuclear G-actin may stimulate INO80C to increase DNA polymerase processivity and convert single-strand lesions into double-strand breaks.

Flow Cytometric Determination of Actin Polymerization in Peripheral Blood Leukocytes Effectively Discriminate Patients With Homozygous Mutation in ARPC1B From Asymptomatic Carriers and Normal Controls.

Actin nucleators initiate formation of actin filaments. Among them, the Arp2/3 complex has the ability to form branched actin networks. This complex is regulated by members of the Wiscott-Aldrich syndrome protein (WASp) family. Polymerization of actin filaments can be evaluated through flow cytometry by fluorescent phalloidin staining before and after stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP). We identified a missense mutation in the gene ARPC1B (Arp2/3 activator subunit) resulting in defective actin polymerization in four patients (three of them were related). All patients (1 male, 3 female) developed microthrombocytopenia, cellular immune deficiency, eczema, various autoimmune manifestations, recurrent skin abscesses and elevated IgE antibodies. Besides four patients with homozygous mutation in ARPC1B, we also identified six heterozygous carriers without clinical disease (3 males, 3 females) within the same family. We developed a functional test to evaluate Arp2/3 complex function, which consists of flow cytometric detection of intracellular polymerized actin after in vitro fMLP stimulation of leukocytes. Median fluorescence intensities of FITC-phalloidin stained actin were measured in monocytes, neutrophils and lymphocytes of patients, carriers, and healthy control subjects. We detected non-efficient actin polymerization in monocytes and neutrophils of homozygous patients compared to carriers or the healthy subjects. In monocytes, the increase in median fluorescence intensities was significantly lower in patients compared to carriers (104 vs. 213%; p < 0.01) and healthy controls (104 vs. 289%; p < 0.01). Similarly, the increase in median fluorescence intensities in neutrophils was significantly increased in the group with carriers (208%; p < 0.01) and healthy controls (238%; p < 0.01) and significantly decreased in the patient's group (94%). Our functional fMLP/phalloidin test can therefore be used as a practical tool to separate symptomatic patients from asymptomatic mutation associated to actin polymerization.

Abstract 4602: Abi1 promotes breast cancer progression by regulating cell-matrix adhesion

Abstract Breast cancer is one of the most common diagnosed cancers in the United States. There will be approximately 266,120 new breast cancer cases in the U.S. and over 40,000 patients are predicted to die from the malignant progression of their breast cancer tumors in 2018. Traditional therapeutic methods of the invasive breast cancer include surgery accompanied by chemotherapy. However, following the initial therapy patients often relapse and eventually die from tumor metastasis. Abl interactor 1, or Abi1, is an adaptor protein which regulates actin polymerization through participation in WAVE (Wiscott-Aldrich syndrome protein family verprolin homologous) complex. The WAVE regulatory complex (also called WRC), regulates actin based cellular behavior, such as cellular adhesion and cell migration, by activating Arp2/3 complex and promoting actin polymerization. Previous studies indicated the direct interaction between integrin α4 and Abi1, which suggests a potential role of Abi1 in regulating cell-matrix adhesion signaling by involved in the inside-out integrin signaling pathways. In order to test the above hypothesis, we developed two genetic engineered Abi1 knocked-out breast cell lines from either HER2+ breast cancer cell line SKBR3 or normal breast epithelial cell line MCF10A by using CRISPR-Cas9 technique. After confirming inactivation of Abi1 gene by Western blotting, DNA and RNA sequencing, cells were harvested and analyzed for integrins levels. Western blotting results indicated dysregulation of multiple integrins in the Abi1 knock-out cell lines. The results from subsequent immunofluorescence experiments of the Abi1 knock-out cell lines also suggested changes in cell-matrix adhesion signaling using staining for scaffold protein paxillin and phosphorylated Focal-Adhesion-Kinase (FAK) as readouts. Our findings suggest that Abi1 might functions as a key regulator in breast cancer tumor progression by playing an important role in regulating signal pathways in cell-matrix adhesion. Supported by the Carol Baldwin Foundation of CNY, and in part by NIH-NCI grant No. R01 CA161018. Citation Format: Xiang Li, Angelina Regua, Gennady Bratslavsky, Vladimir A. Kuznetsov, Leszek Kotula. Abi1 promotes breast cancer progression by regulating cell-matrix adhesion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4602.

Abstract 108: Abi1 positively regulates lung metastasis of aggressive breast cancer in PyMT mouse model

Abstract Breast cancer is the most commonly diagnosed non-cutaneous cancer in American women and is estimated to cause 40,000 deaths this year. Despite standard of care, breast cancer patients often relapse after a few years of treatment thus highlighting the need for new molecular targets for improved management of metastatic disease. Abelson interactor 1 (Abi1) is an adaptor protein associated with the WAVE (Wiscott-Aldrich syndrome protein family verprolin homologous) regulatory complex and Arp2/3 (Actin-related proteins 2 and 3)-mediated actin cytoskeleton remodeling. Our analysis of human tumor data indicates that Abi1 is frequently upregulated in invasive breast cancers, is associated with poor survival, and may promote an aggressive breast tumor phenotype. Abi1 has been shown to positively regulate breast cancer cell proliferation, motility, division, and invasion in vitro however its exact role in vivo remains unknown. We therefore hypothesize that Abi1 promotes breast tumor progression and metastasis in a mouse breast cancer model. Using the well-characterized Polyoma Middle T (PyMT) breast cancer model, we conditionally deleted Abi1 from the mammary epithelium and determined the effects on tumor growth kinetics and WAVE complex protein levels. Interestingly, our studies show delayed tumor growth only in mice with heterozygous deletion of Abi1 while homozygote KO mice show relatively unchanged tumor growth. Western blot analyses of Abi1 KO breast tumors show concomitant loss of WAVE complex proteins supporting our previous findings that WAVE complex integrity is dependent on Abi1. Our data also show significant upregulation of Abi2 only in homozygous Abi1 KO mice, suggesting a potential compensatory role for Abi2 that may support primary tumor growth in absence of Abi1. Most interestingly, Abi1 null PyMT mice show significantly reduced incidence of lung metastasis, supporting our hypothesis that Abi1 promotes metastasis of breast cancer cells. In summary, Abi1 loss abrogates lung metastasis in PyMT mice however primary tumor growth remains largely unaffected, possibly due to a compensatory mechanism by a related protein, Abi2. This work will establish the biochemical dynamics between members of the Abi protein family and WAVE complex to improve targeted treatments in aggressive human breast cancer. [Supported by NCI grant R01-CA161018 and Carol M. Baldwin Breast Cancer Research Fund of CNY] Citation Format: Angelina Regua, Isabelle Bichindaritz, Tiffany Caza, Julie R. White, Alexander Nappi, Claudia Mondragon, Mira Krendel, Gennady Bratslavsky, Abirami Sivapiragasam, Leszek Kotula. Abi1 positively regulates lung metastasis of aggressive breast cancer in PyMT mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 108.

HRS-WASH axis governs actin-mediated endosomal recycling and cell invasion.

Transmembrane proteins in the sorting endosome are either recycled to their point of origin or destined for lysosomal degradation. Lysosomal sorting is mediated by interaction of ubiquitylated transmembrane proteins with the endosomal sorting complex required for transport (ESCRT) machinery. In this study, we uncover an alternative role for the ESCRT-0 component hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) in promoting the constitutive recycling of transmembrane proteins. We find that endosomal localization of the actin nucleating factor Wiscott-Aldrich syndrome protein and SCAR homologue (WASH) requires HRS, which occupies adjacent endosomal subdomains. Depletion of HRS results in defective constitutive recycling of epidermal growth factor receptor and the matrix metalloproteinase MT1-MMP, leading to their accumulation in internal compartments. We show that direct interactions with endosomal actin are required for efficient recycling and use a model system of chimeric transferrin receptor trafficking to show that an actin-binding motif can counteract an ubiquitin signal for lysosomal sorting. Directed receptor recycling is used by cancer cells to achieve invasive migration. Accordingly, abrogating HRS- and actin-dependent MT1-MMP recycling results in defective matrix degradation and invasion of triple-negative breast cancer cells.

Abstract 3404: Loss of Abi1 abrogates lung metastasis in the PyMT mouse model of breast cancer

Abstract This study aims to elucidate the role of Abelson interactor 1 (Abi1), a key protein in the WAVE regulatory complex, in mammary carcinogenesis and metastasis. Breast cancer is the second-leading cause of mortality in women in the United States with an estimated ~200,000 new cases and over 40,000 deaths this year. Despite current treatment modalities, breast cancer patients often relapse after only a few years of treatment thus emphasizing the need for better therapeutic targets. Abi1 is an adaptor protein mainly associated with the WAVE (Wiscott-Aldrich syndrome protein family verprolin homologous) regulatory complex and Arp2/3 (Actin-related proteins 2 and 3)-mediated actin cytoskeleton remodeling. Our bioinformatic and gene expression analyses of human tumor data indicates that Abi1 is frequently upregulated in invasive breast cancers, is associated with poor survival, and may promote an aggressive breast tumor phenotype. Downregulation of Abi1 also abrogates motility and invasion of breast cancer cells, most likely through inactivation of both Src and PI3 kinase as well as certain matrix metalloproteases. We therefore hypothesize that Abi1 positively regulates breast tumor progression and invasion through dysregulation of these cell signaling pathways. To determine the role of Abi1 in breast tumor progression, we used a Cre-lox system to conditionally delete Abi1 in the mammary tissue of Polyoma Middle T (PyMT) breast cancer mice. Abi1 knockout (KO) and control mice were palpated bi-weekly to determine tumor latency and tumors were measured with a caliper to determine total tumor burden. We are currently analyzing changes in tumor kinetics as a result of conditional Abi1 knockout in the mammary epithelium of PyMT mice. Our preliminary studies of indicate slowed tumor growth in Abi1 KO PyMT mice. Western blot analyses of Abi1 KO mammary tumors indicate concomitant loss of WAVE complex proteins supporting our previous findings that WAVE complex integrity is dependent on Abi1. Most interestingly, Abi1 null PyMT mice exhibit significantly reduced incidence of lung metastasis, supporting our hypothesis that Abi1 promotes invasion by breast cancer cells. In summary, Abi1 loss leads to reduction of lung metastasis in PyMT mice, possibly through inactivation of key cell signaling and proliferation pathways such as Src and PI3 kinase. This work will establish Abi1 as a potential prognostic marker and therapeutic target in metastatic breast cancer. Citation Format: Angelina Regua, Isabelle Bichindaritz, Tiffany Caza, Julie White, Robert Adamiecki, Mira Krendel, Gennady Bratslavsky, Leszek Kotula. Loss of Abi1 abrogates lung metastasis in the PyMT mouse model of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3404. doi:10.1158/1538-7445.AM2017-3404

Actin foci facilitate activation of the phospholipase C-γ in primary T lymphocytes via the WASP pathway.

Wiscott Aldrich Syndrome protein (WASP) deficiency results in defects in calcium ion signaling, cytoskeletal regulation, gene transcription and overall T cell activation. The activation of WASP constitutes a key pathway for actin filament nucleation. Yet, when WASP function is eliminated there is negligible effect on actin polymerization at the immunological synapse, leading to gaps in our understanding of the events connecting WASP and calcium ion signaling. Here, we identify a fraction of total synaptic F-actin selectively generated by WASP in the form of distinct F-actin 'foci'. These foci are polymerized de novo as a result of the T cell receptor (TCR) proximal tyrosine kinase cascade, and facilitate distal signaling events including PLCγ1 activation and subsequent cytoplasmic calcium ion elevation. We conclude that WASP generates a dynamic F-actin architecture in the context of the immunological synapse, which then amplifies the downstream signals required for an optimal immune response.

Molecular Targeting and Translational Research for New Therapeutic Strategies on Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. AD treatments currently approved are commonly regarded as purely symptomatic. Therefore, development of effective disease modifying treatments is eagerly required. Pathological hallmarks of AD brain include amyloid-β (Aβ) accumulation, neurofibrillary tangle (NFT), synaptic loss, and neurodegeneration. Understanding of AD pathogenesis provides deep insight into the therapeutic approach for AD. In this review, we introduce our current translational researches for new therapeutic strategies against AD. We recently revealed that a key molecule for actin assembly, Wiscott-Aldrich syndrome protein family verprolin-homologous protein (WAVE), is accumulated into NFT in an Aβ pathology-dependent manner. Because synaptic function is maintained by precise regulation of actin assembly, abnormal accumulation of WAVE induced by Aβ pathology may expand the NFT pathology to synaptic dysfunction. These results imply that lowering of brain Aβ at the early stage of AD pathogenesis is a promising therapeutic approach. As the approach for the elimination of brain Aβ, we have studied microglial Aβ phagocytosis and proposed its therapeutic application for AD treatment. We demonstrated that anti-Aβ antibody and galantamine promote microglial Aβ phagocytosis by binding to Fc receptors and nicotinic acetylcholine receptors on microglia, respectively. On the other hand, we demonstrated that the transplantation with microglia effectively eliminate brain Aβ and microglia-like Aβ phagocytic cells were successfully prepared from bone marrow derived cells by the stimulation with macrophage colony-stimulating factor. Thus, we have studied cell therapeutic strategy aim at its clinical application for AD treatment by the activation of endogenous microglia and transplantation of exogenous microglia-like Aβ phagocytic cells.

Estrogen Effects on Actin Cytoskeletal and Endocytic Proteins Associated With Tubulobulbar Complex Disruption in Rat Testes

Tubulobulbar complexes (TBCs), evaginations of mature spermatids, penetrate into the surrounding Sertoli cell cytoplasm of testis seminiferous epithelium during rat spermatogenesis. These structures prepare mature spermatids for their release into the seminiferous tubular lumen via a process called spermiation. Based on their functions of transient attachment and endocytosis, many actin-regulatory and endocytic proteins are associated with TBCs. Previously, exogenous 17β-estradiol administration to adult male rats showed spermiation failure that was attributed to TBC disruption. To determine the molecular basis of estrogen-induced TBC disruption, we examined the expressions and localizations of actin-regulatory proteins, endocytic proteins, Rho-GTPases, and phosphorylation in TBCs during sperm release. Results demonstrated absence of neural Wiscott Aldrich syndrome protein, cortactin, adaptor-related protein complex 2 sigma-1 subunit, dynamin 2, cell division control protein 42, and phosphocortactin in the concavity of spermatid head where TBCs are present without change in their protein expression levels. Absence of these proteins could have led to collapse of the TBC structure which is involved in its formation and function.

Phospholipase D (PLD) drives cell invasion, tumor growth and metastasis in a human breast cancer xenograph model

Breast cancer is one of the most common malignancies in human females in the world. One protein that has elevated enzymatic lipase activity in breast cancers in vitro is phospholipase D (PLD), which is also involved in cell migration. We demonstrate that the PLD2 isoform, which was analyzed directly in the tumors, is crucial for cell invasion that contributes critically to the growth and development of breast tumors and lung metastases in vivo. We used three complementary strategies in a SCID mouse model and also addressed the underlying molecular mechanism. First, the PLD2 gene was silenced in highly metastatic, aggressive breast cancer cells (MDA-MB-231) with lentivirus-based shRNA, which were xenotransplanted in SCID mice. The resulting mouse primary mammary tumors were reduced in size (65%, p<0.05) and their onset delayed when compared to control tumors. Second, we stably overexpressed PLD2 in low-invasive breast cancer cells (MCF-7) with a biscistronic MIEG retroviral vector and observed that these cells were converted into a highly aggressive phenotype, as primary tumors that formed following xenotransplantation were larger, grew faster and developed lung metastases more readily. Third, we implanted osmotic pumps into SCID xenotransplanted mice that delivered two different small-molecule inhibitors of PLD activity (FIPI and NOPT). These inhibitors led to significant (>70%, p<0.05) inhibition of primary tumor growth, metastatic axillary tumors and lung metastases. In order to define the underlying mechanism, we determined that the machinery of PLD-induced cell invasion is mediated by phosphatidic acid (PA), WASp, Grb2 and Rac2 signaling events that ultimately affect actin polymerization and cell invasion. In summary, this study shows that PLD has a central role in the development, metastasis and level of aggressiveness of breast cancer, raising the possibility that PLD2 could be used as a new therapeutic target.

Agrin triggers the clustering of raft-associated acetylcholine receptors through actin cytoskeleton reorganization

Cholesterol/sphingolipid-rich membrane microdomains or membrane rafts have been implicated in various aspects of receptor function such as activation, trafficking and synapse localization. More specifically in muscle, membrane rafts are involved in AChR (acetylcholine receptor) clustering triggered by the neural factor agrin, a mechanism considered integral to NMJ (neuromuscular junction) formation. In addition, actin polymerization is required for the formation and stabilization of AChR clusters in muscle fibres. Since membrane rafts are platforms sustaining actin nucleation, we hypothesize that these microdomains provide the suitable microenvironment favouring agrin/MuSK (muscle-specific kinase) signalling, eliciting in turn actin cytoskeleton reorganization and AChR clustering. However, the identity of the signalling pathways operating through these microdomains still remains unclear. In this work, we attempted to identify the interactions between membrane raft components and cortical skeleton that regulate, upon signalling by agrin, the assembly and stabilization of synaptic proteins of the postsynaptic membrane domain at the NMJ. We provide evidence that in C2C12 myotubes, agrin triggers the association of a subset of membrane rafts enriched in AChR, the -MuSK and Cdc42 (cell division cycle 42) to the actin cytoskeleton. Disruption of the liquid-ordered phase by methyl-β-cyclodextrin abolished this association. We further show that actin and the actin-nucleation factors, N-WASP (neuronal Wiscott-Aldrich syndrome protein) and Arp2/3 (actin-related protein 2/3) are transiently associated with rafts on agrin engagement. Consistent with these observations, pharmacological inhibition of N-WASP activity perturbed agrin-elicited AChR clustering. Finally, immunoelectron microscopic analyses of myotube membrane uncovered that AChRs were constitutively associated with raft nanodomains at steady state that progressively coalesced on agrin activation. These rearrangements of membrane domains correlated with the reorganization of cortical actin cytoskeleton through concomitant and transient recruitment of the Arp2/3 complex to AChR-enriched rafts. The present observations support the notion that membrane rafts are involved in AChR clustering by promoting local actin cytoskeleton reorganization through the recruitment of effectors of the agrin/MuSK signalling cascade. These mechanisms are believed to play an important role in vivo in the formation of the NMJ.

CR16 forms a complex with N-WASP in human testes

The expression of corticosteroids and regional expression16 (CR16) and neural Wiscott-Aldrich syndrome protein (N-WASP) was studied in the testes of men with idiopathic azoospermia by (1) immunohistochemistry and Western blot to detect the expression of CR16 and N-WASP protein, (2) reverse transcription with the polymerase chain reaction (RT-PCR) to determine the amounts of CR16 and N-WASP mRNA, and (3) double-staining immunofluorescence and laser scanning confocal microscopy (LSCM) to demonstrate the co-localization of CR16 and N-WASP proteins. Immunohistochemistry revealed CR16 and N-WASP proteins in the Sertoli cells, specifically in the Sertoli cell-spermatogenic cell junctions (SspJs) in the epithelium of the seminiferous tubules. RT-PCR indicated that the relative amounts of CR16 and N-WASP mRNA were significantly decreased in men with idiopathic azoospermia, although the ratio of CR16 and N-WASP mRNA was approximately 1:1 in both groups of men. Western blot showed the presence of the CR16 monomer and N-WASP in all tissues examined, but the relevant bands were paler in men with idiopathic azoospermia than those in healthy men. The distribution of CR16 and N-WASP was similar in the epithelium of seminiferous tubules based on double-staining immunofluorescence and LSCM, thereby suggesting that CR16 and N-WASP were co-localized in the SspJs as a CR16/N-WASP complex. The decreased levels of CR16 and N-WASP in the testes of men with idiopathic azoospermia compared with healthy men and the co-localization at the SspJs as a CR16/N-WASP complex in the epithelium of seminiferous tubules suggest that CR16 and N-WASP play a role in the pathogenesis of azoospermia.

Netrin 1 and Dcc regulate oligodendrocyte process branching and membrane extension via Fyn and RhoA

The molecular mechanisms underlying the elaboration of branched processes during the later stages of oligodendrocyte maturation are not well understood. Here we describe a novel role for the chemotropic guidance cue netrin 1 and its receptor deleted in colorectal carcinoma (Dcc) in the remodeling of oligodendrocyte processes. Postmigratory, premyelinating oligodendrocytes express Dcc but not netrin 1, whereas mature myelinating oligodendrocytes express both. We demonstrate that netrin 1 promotes process extension by premyelinating oligodendrocytes in vitro and in vivo. Addition of netrin 1 to mature oligodendrocytes in vitro evoked a Dcc-dependent increase in process branching. Furthermore, expression of netrin 1 and Dcc by mature oligodendrocytes was required for the elaboration of myelin-like membrane sheets. Maturation of oligodendrocyte processes requires intracellular signaling mechanisms involving Fyn, focal adhesion kinase (FAK), neuronal Wiscott-Aldrich syndrome protein (N-WASP) and RhoA; however, the extracellular cues upstream of these proteins in oligodendrocytes are poorly defined. We identify a requirement for Src family kinase activity downstream of netrin-1-dependent process extension and branching. Using oligodendrocytes derived from Fyn knockout mice, we demonstrate that Fyn is essential for netrin-1-induced increases in process branching. Netrin 1 binding to Dcc on mature oligodendrocytes recruits Fyn to a complex with the Dcc intracellular domain that includes FAK and N-WASP, resulting in the inhibition of RhoA and inducing process remodeling. These findings support a novel role for netrin 1 in promoting oligodendrocyte process branching and myelin-like membrane sheet formation. These essential steps in oligodendroglial maturation facilitate the detection of target axons, a key step towards myelination.

Vesicular Egress of Non-Enveloped Lytic Parvoviruses Depends on Gelsolin Functioning

The autonomous parvovirus Minute Virus of Mice (MVM) induces specific changes in the cytoskeleton filaments of infected permissive cells, causing in particular the degradation of actin fibers and the generation of “actin patches.” This is attributed to a virus-induced imbalance between the polymerization factor N-WASP (Wiscott-Aldrich syndrome protein) and gelsolin, a multifunctional protein cleaving actin filaments. Here, the focus is on the involvement of gelsolin in parvovirus propagation and virus-induced actin processing. Gelsolin activity was knocked-down, and consequences thereof were determined for virus replication and egress and for actin network integrity. Though not required for virus replication or progeny particle assembly, gelsolin was found to control MVM (and related H1-PV) transport from the nucleus to the cell periphery and release into the culture medium. Gelsolin-dependent actin degradation and progeny virus release were both controlled by (NS1)/CKIIα, a recently identified complex between a cellular protein kinase and a MVM non-structural protein. Furthermore, the export of newly synthesized virions through the cytoplasm appeared to be mediated by (virus-modified) lysomal/late endosomal vesicles. By showing that MVM release, like entry, is guided by the cytoskeleton and mediated by vesicles, these results challenge the current view that egress of non-enveloped lytic viruses is a passive process.

Mutagenesis of theShigella flexneriAutotransporter IcsA Reveals Novel Functional Regions Involved in IcsA Biogenesis and Recruitment of Host Neural Wiscott-Aldrich Syndrome Protein

The IcsA (VirG) protein of Shigella flexneri is a polarly localized, outer membrane protein that is essential for virulence. Within host cells, IcsA activates the host actin regulatory protein, neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn recruits the Arp2/3 complex, which nucleates host actin to form F-actin comet tails and initiate bacterial motility. Linker insertion mutagenesis was undertaken to randomly introduce 5-amino-acid in-frame insertions within IcsA. Forty-seven linker insertion mutants were isolated and expressed in S. flexneri Delta icsA strains. Mutants were characterized for IcsA protein production, cell surface expression and localization, intercellular spreading, F-actin comet tail formation, and N-WASP recruitment. Using this approach, we have identified a putative autochaperone region required for IcsA biogenesis, and our data suggest an additional region, not previously identified, is required for N-WASP recruitment.

Control of Actin Assembly Dynamics in Cell Motility

Control of Actin Assembly Dynamics in Cell Motility

Opposing Effects of PKCθ and WASp on Symmetry Breaking and Relocation of the Immunological Synapse

The immunological synapse (IS) is a junction between the T cell and antigen-presenting cell and is composed of supramolecular activation clusters (SMACs). No studies have been published on naive T cell IS dynamics. Here, we find that IS formation during antigen recognition comprises cycles of stable IS formation and autonomous naive T cell migration. The migration phase is driven by PKCtheta, which is localized to the F-actin-dependent peripheral (p)SMAC. PKCtheta(-/-) T cells formed hyperstable IS in vitro and in vivo and, like WT cells, displayed fast oscillations in the distal SMAC, but they showed reduced slow oscillations in pSMAC integrity. IS reformation is driven by the Wiscott Aldrich Syndrome protein (WASp). WASp(-/-) T cells displayed normal IS formation but were unable to reform IS after migration unless PKCtheta was inhibited. Thus, opposing effects of PKCtheta and WASp control IS stability through pSMAC symmetry breaking and reformation.

Phosphorylation of a Wiscott-Aldrich Syndrome Protein-associated Signal Complex Is Critical in Osteoclast Bone Resorption

The activities of different kinases have been correlated to the phosphorylation of Wiscott-Aldrich syndrome protein (WASP) by studies in multiple cell systems. The purpose of this study was to elucidate the regulatory mechanisms involved in WASP phosphorylation and the resulting sealing ring formation in osteoclasts. The phosphorylation state of WASP and WASP-interacting proteins was determined in osteoclasts treated with osteopontin or expressing either constitutively active or kinase-defective Src by adenovirus-mediated delivery. In vitro kinase analysis of WASP immunoprecipitates exhibited phosphorylation of c-Src, PYK2, WASP, protein-tyrosine phosphatase (PTP)-PEST, and Pro-Ser-Thr phosphatase-interacting protein (PSTPIP). Phosphorylation of these proteins was increased in osteopontin-treated and constitutively active Src-expressing osteoclasts. Pulldown analysis with glutathione S-transferase-fused proline-rich regions of PTP-PEST revealed coprecipitation of WASP, PYK2, c-Src, and PSTPIP proteins with the N-terminal region (amino acids 294-497) of PTP-PEST. Similarly, interaction of the same signaling proteins, as well as PTP-PEST, was observed with glutathione S-transferase-fused proline-rich regions of WASP. Furthermore, osteopontin stimulation or constitutively active Src expression resulted in serine phosphorylation and inhibition of WASP-associated PTP-PEST. The inhibition of PTP-PEST was accompanied by an increase in tyrosine phosphorylation of WASP and other associated signaling proteins. Experiments with an inhibitor to phosphatase and small interference RNA to PTP-PEST confirmed the involvement of PTP-PEST in sealing ring formation and bone resorption. WASP, which is identified in the sealing ring of resorbing osteoclasts, also demonstrates colocalization with c-Src, PYK2, PSTPIP, and PTP-PEST in immunostaining analyses. Our findings suggest that both tyrosine kinase(s) and the tyrosine phosphatase PTP-PEST coordinate the formation of the sealing ring and thus the bone-resorbing function of osteoclasts.

C-Abl Interacts with the WAVE2 Signaling Complex to Induce Membrane Ruffling and Cell Spreading

c-Abl Interacts with the WAVE2 Signaling Complex to Induce Membrane Ruffling and Cell Spreading

The Structural Basis of Actin Interaction with Multiple WH2/β-Thymosin Motif-Containing Proteins

Participation of actin in cellular processes relies on the dynamics of filament assembly. Filament elongation is fed by monomeric actin in complex with either profilin or a Wiscott-Aldrich syndrome protein (WASP) homology domain 2 (WH2)/beta-thymosin (betaT) domain. WH2/betaT motif repetition (typified by ciboulot) or combination with nonrelated domains (as found in N-WASP) results in proteins that yield their actin to filament elongation. Here, we report the crystal structures of actin bound hybrid proteins, constructed between gelsolin and WH2/betaT domains from ciboulot or N-WASP. We observe the C-terminal half of ciboulot domain 2 bound to actin. In solution, we show that cibolout domains 2 and 3 bind to both G- and F-actin, and that whole ciboulot forms a complex with two actin monomers. In contrast, the analogous portion of N-WASP WH2 domain 2 is detached from actin, indicating that the C-terminal halves of the betaT and WH2 motifs are not functionally analogous.