The Wiskott-Aldrich syndrome protein: forging the link between actin and cell activation

HIV-1 Mediated Cortical Actin Disruption Mirrors ARP2/3 Defects Found in Primary T Cell Immunodeficiencies

During cell movement, cortical actin balances mechanical and osmotic forces to maintain cell function while providing the scaffold for cell shape. Migrating CD4+ T cells have a polarized structure with a leading edge containing dynamic branched and linear F-actin structures that bridge intracellular components to surface adhesion molecules. These actin structures are complemented with a microtubular network beaded with membrane bound organelles in the trailing uropod. Disruption of actin structures leads to dysregulated migration and changes in morphology of affected cells. In HIV-1 infection, CD4+ T cells have dysregulated movement. However, the precise mechanisms by which HIV-1 affects CD4+ T cell movement are unknown. Here, we show that HIV-1 infection of primary CD4+ T cells causes at least four progressive morphological differences as a result of virally induced cortical cytoskeleton disruption, shown by ultrastructural and time lapse imaging. Infection with a ΔNef virus partially abrogated the dysfunctional phenotype in infected cells and partially restored a wild-type shape. The pathological morphologies after HIV-1 infection phenocopy leukocytes which contain genetic determinants of specific T cell Inborn Errors of Immunity (IEI) or Primary Immunodeficiencies (PID) that affect the actin cytoskeleton. To identify potential actin regulatory pathways that may be linked to the morphological deformities, uninfected CD4+ T cell morphology was characterized following addition of small molecule chemical inhibitors. The ARP2/3 inhibitor CK-666 recapitulated three of the four abnormal morphologies we observed in HIV-1 infected cells. Restoring ARP2/3 function and cortical actin integrity in people living with HIV-1 infection is a new avenue of investigation to eradicate HIV-1 infected cells from the body.

T cell functions and organ infiltration by leukemic T cells require cortactin

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that is still fatal in many cases. T cell blasts are characterized by hyperactivation and strong proliferative and migratory capacities. The chemokine receptor CXCR4 is involved in mediating malignant T cell properties, and cortactin has been shown to control CXCR4 surface localization in T-ALL cells. We have previously shown that cortactin overexpression is correlated with organ infiltration and relapse in B-ALL. However, the role of cortactin in T cell biology and T-ALL remains elusive. Here, we analyzed the functional relevance of cortactin for T cell activation and migration and the implications for T-ALL development. We found that cortactin is upregulated in response to T cell receptor engagement and recruited to the immune synapse in normal T cells. Loss of cortactin caused reduced IL-2 production and proliferation. Cortactin-depleted T cells showed defects in immune synapse formation and migrated less due to impaired actin polymerization in response to T cell receptor and CXCR4 stimulation. Leukemic T cells expressed much higher levels of cortactin compared to normal T cells that correlated with greater migratory capacity. Xenotransplantation assays in NSG mice revealed that cortactin-depleted human leukemic T cells colonized the bone marrow significantly less and failed to infiltrate the central nervous system, suggesting that cortactin overexpression drives organ infiltration, which is a major complication of T-ALL relapse. Thus, cortactin could serve as a potential therapeutic target for T-ALL and other pathologies involving aberrant T cell responses.

Review: Bioengineering strategies to probe T cell mechanobiology

T cells play a major role in adaptive immune response, and T cell dysfunction can lead to the progression of several diseases that are often associated with changes in the mechanical properties of tissues. However, the concept that mechanical forces play a vital role in T cell activation and signaling is relatively new. The endogenous T cell microenvironment is highly complex and dynamic, involving multiple, simultaneous cell-cell and cell-matrix interactions. This native complexity has made it a challenge to isolate the effects of mechanical stimuli on T cell activation. In response, researchers have begun developing engineered platforms that recapitulate key aspects of the native microenvironment to dissect these complex interactions in order to gain a better understanding of T cell mechanotransduction. In this review, we first describe some of the unique characteristics of T cells and the mounting research that has shown they are mechanosensitive. We then detail the specific bioengineering strategies that have been used to date to measure and perturb the mechanical forces at play during T cell activation. In addition, we look at engineering strategies that have been used successfully in mechanotransduction studies for other cell types and describe adaptations that may make them suitable for use with T cells. These engineering strategies can be classified as 2D, so-called 2.5D, or 3D culture systems. In the future, findings from this emerging field will lead to an optimization of culture environments for T cell expansion and the development of new T cell immunotherapies for cancer and other immune diseases.

The dock-and-coalesce mechanism for the association of a WASP disordered region with the Cdc42 GTPase

Intrinsically disordered proteins (IDPs) play key roles in signaling and regulation. Many IDPs undergo folding upon binding to their targets. We have proposed that coupled folding and binding of IDPs generally follow a dock-and-coalesce mechanism, whereby a segment of the IDP, through diffusion, docks to its cognate subsite and, subsequently, the remaining segments coalesce around their subsites. Here, by a combination of experiment and computation, we determined the precise form of dock-and-coalesce operating in the association between the intrinsically disordered GTPase-binding domain (GBD) of the Wiskott-Aldrich Syndrome protein and the Cdc42 GTPase. The association rate constants (k_a ) were measured by stopped-flow fluorescence under various solvent conditions. k_a reached 10⁷ m^-1 ·s^-1 at physiological ionic strength and had a strong salt dependence, suggesting that an electrostatically enhanced, diffusion-controlled docking step may be rate limiting. Our computation, based on the transient-complex theory, identified the N-terminal basic region of the GBD as the docking segment. However, several other changes in solvent conditions provided strong evidence that the coalescing step also contributed to determining the magnitude of k_a . Addition of glucose and trifluoroethanol and an increase in temperature all produced experimental k_a values much higher than expected from the effects on the docking rate alone. Conversely, addition of urea led to k_a values much lower than expected if only the docking rate was affected. These results all pointed to k_a being approximately two-thirds of the docking rate constant under physiological solvent conditions.

Switch-like Arp2/3 activation upon WASP and WIP recruitment to an apparent threshold level by multivalent linker proteins in vivo

Actin-related protein 2/3 (Arp2/3) complex activation by nucleation promoting factors (NPFs) such as WASP, plays an important role in many actin-mediated cellular processes. In yeast, Arp2/3-mediated actin filament assembly drives endocytic membrane invagination and vesicle scission. Here we used genetics and quantitative live-cell imaging to probe the mechanisms that concentrate NPFs at endocytic sites, and to investigate how NPFs regulate actin assembly onset. Our results demonstrate that SH3 (Src homology 3) domain-PRM (proline-rich motif) interactions involving multivalent linker proteins play central roles in concentrating NPFs at endocytic sites. Quantitative imaging suggested that productive actin assembly initiation is tightly coupled to accumulation of threshold levels of WASP and WIP, but not to recruitment kinetics or release of autoinhibition. These studies provide evidence that WASP and WIP play central roles in establishment of a robust multivalent SH3 domain-PRM network in vivo, giving actin assembly onset at endocytic sites a switch-like behavior.

DOI:http://dx.doi.org/10.7554/eLife.29140.001

Actin is one of the most abundant proteins in yeast, mammalian and other eukaryotic cells. It assembles into long chains known as filaments that the cell uses to generate forces for various purposes. For example, actin filaments are needed to pull part of the membrane surrounding the cell inwards to bring molecules from the external environment into the cell by a process called endocytosis.

In yeast, a member of the WASP family of proteins promotes the assembly of actin filaments around the site where endocytosis will occur. To achieve this, WASP interacts with several other proteins including WIP and myosin, a motor protein that moves along actin filaments to generate mechanical forces. However, it was not clear how these proteins work together to trigger actin filaments to assemble at the right place and time.

Sun et al. addressed this question by studying yeast cells with genetic mutations affecting one or more of these proteins. The experiments show that WASP, myosin and WIP are recruited to sites where endocytosis is about to occur through specific interactions with other proteins. For example, a region of WASP known as the proline-rich domain can bind to proteins that contain an “SH3” domain. WASP and WIP arrive first, stimulating actin to assemble in an “all and nothing” manner and attracting myosin to the actin. Further experiments indicate that WASP and WIP need to reach a threshold level before actin starts to assemble.

The findings of Sun et al. suggest that WASP and WIP play key roles in establishing the network of proteins needed for actin filaments to assemble during endocytosis. These proteins are needed for many other processes in yeast and other cells, including mammalian cells. Therefore, the next steps will be to investigate whether WASP and WIP use the same mechanism to operate in other situations.

DOI:http://dx.doi.org/10.7554/eLife.29140.002

Computational spatiotemporal analysis identifies WAVE2 and cofilin as joint regulators of costimulation-mediated T cell actin dynamics

Fluorescence microscopy is one of the most important tools in cell biology research because it provides spatial and temporal information to investigate regulatory systems inside cells. This technique can generate data in the form of signal intensities at thousands of positions resolved inside individual live cells. However, given extensive cell-to-cell variation, these data cannot be readily assembled into three- or four-dimensional maps of protein concentration that can be compared across different cells and conditions. We have developed a method to enable comparison of imaging data from many cells and applied it to investigate actin dynamics in T cell activation. Antigen recognition in T cells by the T cell receptor (TCR) is amplified by engagement of the costimulatory receptor CD28. We imaged actin and eight core actin regulators to generate over a thousand movies of T cells under conditions in which CD28 was either engaged or blocked in the context of a strong TCR signal. Our computational analysis showed that the primary effect of costimulation blockade was to decrease recruitment of the activator of actin nucleation WAVE2 (Wiskott-Aldrich syndrome protein family verprolin-homologous protein 2) and the actin-severing protein cofilin to F-actin. Reconstitution of WAVE2 and cofilin activity restored the defect in actin signaling dynamics caused by costimulation blockade. Thus, we have developed and validated an approach to quantify protein distributions in time and space for the analysis of complex regulatory systems.

Mechanism and rate constants of the Cdc42 GTPase binding with intrinsically disordered effectors

Intrinsically disordered proteins (IDPs) are often involved in signaling and regulatory functions, through binding to cellular targets. Many IDPs undergo disorder-to-order transitions upon binding. Both the binding mechanisms and the magnitudes of the binding rate constants can have functional importance. Previously we have found that the coupled binding and folding of any IDP generally follows a sequential mechanism that we term dock-and-coalesce, whereby one segment of the IDP first docks to its subsite on the target surface and the remaining segments subsequently coalesce around their respective subsites. Here we applied our TransComp method within the framework of the dock-and-coalesce mechanism to dissect the binding kinetics of two Rho-family GTPases, Cdc42 and TC10, with two intrinsically disordered effectors, WASP and Pak1. TransComp calculations identified the basic regions preceding the GTPase binding domains (GBDs) of the effectors as the docking segment. For Cdc42 binding with both WASP and Pak1, the calculated docking rate constants are close to the observed overall binding rate constants, suggesting that basic-region docking is the rate-limiting step and subsequent conformational coalescence of the GBDs on the Cdc42 surface is fast. The possibility that conformational coalescence of the WASP GBD on the TC10 surface is slow warrants further experimental investigation. The account for the differences in binding rate constants among the three GTPase-effector systems and mutational effects therein yields deep physical and mechanistic insight into the binding processes. Our approach may guide the selection of mutations that lead to redesigned binding pathways.

Signaling by the engulfment receptor draper: a screen in Drosophila melanogaster implicates cytoskeletal regulators, Jun N-terminal Kinase, and Yorkie

Draper, the Drosophila melanogaster homolog of the Ced-1 protein of Caenorhabditis elegans, is a cell-surface receptor required for the recognition and engulfment of apoptotic cells, glial clearance of axon fragments and dendritic pruning, and salivary gland autophagy. To further elucidate mechanisms of Draper signaling, we screened chromosomal deficiencies to identify loci that dominantly modify the phenotype of overexpression of Draper isoform II (suppressed differentiation of the posterior crossvein in the wing). We found evidence for 43 genetic modifiers of Draper II. Twenty-four of the 37 suppressor loci and 3 of the 6 enhancer loci were identified. An additional 5 suppressors and 2 enhancers were identified among mutations in functionally related genes. These studies reveal positive contributions to Drpr signaling for the Jun N-terminal Kinase pathway, supported by genetic interactions with hemipterous, basket, jun, and puckered, and for cytoskeleton regulation as indicated by genetic interactions with rac1, rac2, RhoA, myoblast city, Wiskcott-Aldrich syndrome protein, and the formin CG32138, and for yorkie and expanded. These findings indicate that Jun N-terminal Kinase activation and cytoskeletal remodeling collaborate in Draper signaling. Relationships between Draper signaling and Decapentaplegic signaling, insulin signaling, Salvador/Warts/Hippo signaling, apical-basal cell polarity, and cellular responses to mechanical forces are also discussed.

Ubiquitylation-dependent downregulation of Nck regulates its functional activity

The Nck adapter protein is involved in key cellular functions, such as actin polymerization and reorganization, serving as a molecular bridge between the surface complex essential for foreign antigen recognition, the T-cell antigen receptor (TCR), and the actin machinery. However, the mechanisms regulating Nck expression and functions are unknown. In this study, we revealed Nck negative regulation and demonstrated that Nck is ubiquitylated following cellular activation. We identified the molecular determinants and mediators involved in this process. Our data suggest that Nck ubiquitylation might serve as a mechanism controlling Nck-mediated effector functions during cellular activation.

Cytokine secretion by CD4+ T cells at the immunological synapse requires Cdc42-dependent local actin remodeling but not microtubule organizing center polarity

Cytokine secretion by T lymphocytes plays a central role in mounting adaptive immune responses. However, little is known about how newly synthesized cytokines, once produced, are routed within T cells and about the mechanisms involved in regulating their secretions. In this study, we investigated the role of cytoskeleton remodeling at the immunological synapse (IS) in cytokine secretion. We show that a key regulator of cytoskeleton remodeling, the Rho GTPase Cdc42, controls IFN-γ secretion by primary human CD4+ T lymphocytes. Surprisingly, microtubule organizing center polarity at the IS, which does not depend on Cdc42, is not required for cytokine secretion by T lymphocytes, whereas microtubule polymerization is required. In contrast, actin remodeling at the IS, which depends on Cdc42, controls the formation of the polymerized actin ring at the IS, the dynamic concentration of IFN-γ-containing vesicles inside this ring, and the secretion of these vesicles. These results reveal a previously unidentified role of Cdc42-dependent actin remodeling in cytokine exocytosis at the IS.

Ubiquitylation-dependent negative regulation of WASp is essential for actin cytoskeleton dynamics

The Wiskott-Aldrich syndrome protein (WASp) is a key regulator of actin dynamics during cell motility and adhesion, and mutations in its gene are responsible for Wiskott-Aldrich syndrome (WAS). Here, we demonstrate that WASp is ubiquitylated following T-cell antigen receptor (TCR) activation. WASp phosphorylation at tyrosine 291 results in recruitment of the E3 ligase Cbl-b, which, together with c-Cbl, carries out WASp ubiquitylation. Lysine residues 76 and 81, located at the WASp WH1 domain, which contains the vast majority of WASp gene mutations, serve as the ubiquitylation sites. Disruption of WASp ubiquitylation causes WASp accumulation and alters actin dynamics and the formation of actin-dependent structures. Our data suggest that regulated degradation of activated WASp might be an efficient strategy by which the duration and localization of actin rearrangement and the intensity of T-cell activation are controlled.

Defense of zebrafish embryos against Streptococcus pneumoniae infection is dependent on the phagocytic activity of leukocytes

Severe community acquired pneumonia caused by Streptococcus pneumoniae is the most common cause of death from infection in developing countries. Serotype specific conjugate vaccines have decreased the incidence of invasive infections, but at the same time, disease due to non-vaccine serotypes have increased. New insights into host immune mechanisms against pneumococcus may provide better treatment and prevention strategies. Zebrafish is an attractive vertebrate model for studying host immune responses and infection biology. Here we show that an intravenous challenge with pneumococcus infects zebrafish embryos leading to death in a dose dependent manner. Survival rates correlate with the bacterial burden in the embryos. The production of proinflammatory cytokines is induced in zebrafish after pneumococcal exposure. Importantly, morpholino treated embryos lacking either myeloid cells or the ability to phagocytose bacteria have lowered survival rates compared to wild type embryos after pneumococcal challenge. These data suggest that the survival of zebrafish embryos upon intravenous infection with S. pneumoniae is dependent on the clearance of the bacteria by phagocytosing cells. Additionally, we demonstrate that mutant pneumococci lacking known virulence factors are attenuated in the zebrafish model. Our data demonstrate that zebrafish embryos can be used for study innate immune responses as well as virulence determinants in pneumococcal infections.

Impaired NK-cell migration in WAS/XLT patients: role of Cdc42/WASp pathway in the control of chemokine-induced beta2 integrin high-affinity state

We analyzed the involvement of Wiskott-Aldrich syndrome protein (WASp), a critical regulator of actin cytoskeleton remodeling, in the control of natural killer (NK)-cell migration. NK cells derived from patients with Wiskott-Aldrich syndrome/X-linked thrombocytopenia (WAS/XLT), carrying different mutations in the WASP coding gene, displayed reduced migration through intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), or endothelial cells in response to CXCL12/stromal cell-derived factor-1 and CX3CL1/fractalkine. Inhibition of WAS/XLT NK-cell migration was associated with reduced ability of these cells to up-regulate the expression of CD18 activation neoepitope and to adhere to ICAM-1 or VCAM-1 following chemokine stimulation. Moreover, chemokine receptor or beta1 or beta2 integrin engagement on NK cells rapidly resulted in Cdc42 activation and WASp tyrosine phosphorylation as well as in WASp association with Fyn and Pyk-2 tyrosine kinases. NK-cell pretreatment with wiskostatin, to prevent Cdc42/WASp association, impaired chemokine-induced NK-cell migration through ICAM-1 and beta2 integrin activation-dependent neoepitope expression. These results show that the Cdc42/WASp pathway plays a crucial role in the regulation of NK-cell migration by acting as a critical component of the chemokine-induced inside-out signaling that regulates lymphocyte function-associated antigen-1 function and suggest that after integrin or chemokine receptor engagement WASp function is regulated by the coordinate action of both Cdc42 and tyrosine kinases.

Formins in development: orchestrating body plan origami

Formins, proteins defined by the presence of an FH2 domain and their ability to nucleate linear F-actin de novo, play a key role in the regulation of the cytoskeleton. Initially thought to primarily regulate actin, recent studies have highlighted a role for formins in the regulation of microtubule dynamics, and most recently have uncovered the ability of some formins to coordinate the organization of both the microtubule and actin cytoskeletons. While biochemical analyses of this family of proteins have yielded many insights into how formins regulate diverse cytoskeletal reorganizations, we are only beginning to appreciate how and when these functional properties are relevant to biological processes in a developmental or organismal context. Developmental genetic studies in fungi, Dictyostelium, vertebrates, plants and other model organisms have revealed conserved roles for formins in cell polarity, actin cable assembly and cytokinesis. However, roles have also been discovered for formins that are specific to particular organisms. Thus, formins perform both global and specific functions, with some of these roles concurring with previous biochemical data and others exposing new properties of formins. While not all family members have been examined across all organisms, the analyses to date highlight the significance of the flexibility within the formin family to regulate a broad spectrum of diverse cytoskeletal processes during development.

Acute inhibition of selected membrane-proximal mouse T cell receptor signaling by mitochondrial antagonists

T cells absorb nanometric membrane vesicles, prepared from plasma membrane of antigen presenting cells, via dual receptor/ligand interactions of T cell receptor (TCR) with cognate peptide/major histocompatibility complex (MHC) plus lymphocyte function-associated antigen 1 (LFA-1) with intercellular adhesion molecule 1. TCR-mediated signaling for LFA-1 activation is also required for the vesicle absorption. Exploiting those findings, we had established a high throughput screening (HTS) platform and screened a library for isolation of small molecules inhibiting the vesicle absorption. Follow-up studies confirmed that treatments (1 hour) with various mitochondrial antagonists, including a class of anti-diabetic drugs (i.e., Metformin and Phenformin), resulted in ubiquitous inhibition of the vesicle absorption without compromising viability of T cells. Further studies revealed that the mitochondrial drug treatments caused impairment of specific membrane-proximal TCR signaling event(s). Thus, activation of Akt and PLC-gamma1 and entry of extracellular Ca(2+) following TCR stimulation were attenuated while polymerization of monomeric actins upon TCR triggering progressed normally after the treatments. Dynamic F-actin rearrangement concurring with the vesicle absorption was also found to be impaired by the drug treatments, implying that the inhibition by the drug treatments of downstream signaling events (and the vesicle absorption) could result from lack of directional relocation of signaling and cell surface molecules. We also assessed the potential application of mitochondrial antagonists as immune modulators by probing effects of the long-term drug treatments (24 hours) on viability of resting primary T cells and cell cycle progression of antigen-stimulated T cells. This study unveils a novel regulatory mechanism for T cell immunity in response to environmental factors having effects on mitochondrial function.

Contributions of Wiskott-Aldrich syndrome family cytoskeletal regulatory adapters to immune regulation

Cytoskeletal structure and dynamic rearrangement are integrally involved in coupling external stimuli to the orchestrated network of molecular interactions and cellular responses required for T-cell effector function. Members of the Wiskott-Aldrich syndrome protein (WASp) family are now widely recognized as cytoskeletal scaffolding adapters that coordinate the transmission of stimulatory signals to downstream induction of actin remodeling and cytoskeletal-dependent T-cell responses. In this review, we discuss the structural and functional properties of the WASp family members, with an emphasis on the roles of these proteins in the molecular pathways underpinning T-cell activation. The contributions of WASp family proteins and the cytoskeletal reorganization they evoke to expression of specific T-cell effector functions and the implications of such activity to normal immune responses and to the immunologic deficits manifested by Wiskott-Aldrich syndrome patients are also described.

Recent advances in the biology of WASP and WIP

WASP, the product of the gene mutated in Wiskott-Aldrich syndrome, is expressed only in hematopoietic cells and is the archetype of a family of proteins that include N-WASP and Scar/WAVE. WASP plays a critical role in T cell activation and actin reorganization. WASP has multiple protein-interacting domains. Through its N-terminal EVH1 domain WASP binds to its partner WASP interacting protein (WIP) and through its C-terminal end it interacts with and activates the Arp2/3 complex. In lymphocytes, most of WASP is sequestered with WIP and binding to WIP is essential for the stability of WASP. The central proline-rich region of WASP serves as docking site to several adaptor proteins. Through these multiple interactions WASP integrates many cellular signals to actin cytoskeleton remodeling. In this review, we have summarized recent developments in the biology of WASP and the role of WIP in regulating WASP function. We also discuss WASP-independent functions of WIP.

Dysregulation of innate immunity: hereditary periodic fever syndromes

The hereditary periodic fever syndromes encompass a rare group of diseases that have lifelong recurrent episodes of inflammatory symptoms and an acute phase response in common. Clinical presentation can mimic that of lymphoproliferative disorders and patients often go undiagnosed for many years. These syndromes follow an autosomal inheritance pattern, and the major syndromes are linked to specific genes, most of which are involved in regulation of the innate immune response through pathways of apoptosis, nuclear factor kappaBeta activation and cytokine production. In others, the link between the protein involved and inflammation is less clear. The recurrent inflammation can lead to complications, such as renal impairment due to amyloidosis and vasculitis, visual impairment, hearing loss, and joint destruction, depending on the specific syndrome. In recent years, treatment options for these diseases have improved significantly. Early establishment of an accurate diagnosis and start of appropriate therapy improves prognosis in these patients.

Identification of interaction partners for individual SH3 domains of Fas ligand associated members of the PCH protein family in T lymphocytes

Pombe Cdc15 homology (PCH) family proteins are regarded as key elements for linking membrane-associated processes to cytoskeletal elements and thus play a major role in exo- and endocytosis and organelle trafficking. We previously reported that, via their SH3 domains, several members of the PCH proteins interact with the proline-rich region of Fas ligand (FasL, CD95L), a key death factor in immune cells. Since protein-protein interactions that govern the storage and transport of FasL-associated vesicles are largely unknown, the present study was performed to identify other potential binding partners for SH3 domains of FasL-interacting PCH proteins. To this end, individual SH3 domains were expressed as GST fusion proteins and used to precipitate associated proteins from leukemic T cell lines and activated human T cell blasts. 87 protein bands representing 34 individual proteins were identified by mass spectrometry. The presented list of candidate interactors not only highlights the role of PCH proteins as adapters between vesicular membranes and the cytoskeleton but also points to an involvement of these proteins in the regulation of signalling events in T lymphocytes.

Improvement of migratory defects in a murine model of Wiskott-Aldrich syndrome gene therapy

Wiskott-Aldrich syndrome (WAS) is an X-linked hematological disease characterized by immunodeficiency, eczema, and thrombocytopaenia, and shows promise for treatment with hematopoietic stem cell gene therapy. The immunopathology of WAS is attributable at least in part to defects of cell migration and localization as a result of chemotactic, adhesive, and chemokinetic defects. Whereas previous studies using either gammaretroviral or lentiviral vectors have demonstrated variable correction of T-cell proliferation and dendritic cell (DC) cytoarchitecture, we have used a lentiviral vector expressing an eGFP-WASp fusion protein to test the potential for restoration of cell migratory defects. Multilineage expression of the fusion transgene was present for up to 10 months after primary engraftment, and also in secondary recipients analyzed after a further 9 months. Transduced bone marrow-derived dendritic cells (BMDCs) demonstrated recovery of podosome numbers and turnover, while B cells, BMDCs, and Langerhans cells (LCs) exhibited enhanced chemotactic responses to specific stimuli. As an indication of functionality in vivo, splenic marginal zone B cells and a cutaneous contact hypersensitivity (CHS) response to dinitrofluorobenzene (DNFB) were both partially restored. These proof of principle experiments demonstrate that WAS protein (WASp) transgene expression can be successfully maintained long term in primary and secondary recipients, and that it is associated with a significant repair of migratory defects.

Development of lentiviral gene therapy for Wiskott Aldrich syndrome

BACKGROUND

Wiskott Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency. This complex disease is characterised by microthrombocytopenia, recurrent infections, eczema and is associated with a high incidence of autoimmunity and of lymphoid malignancies. WAS is attracting growing attention not only because it highlights the rich cellular and systems biology revolving around cytoskeletal regulation but also because it is candidate for a haematopoietic stem cell gene therapy indication.

OBJECTIVES

As several groups are developing this novel approach, this review discusses the state of the art and challenges in clinical development of gene therapy for WAS, with particular regard to biosafety.

METHODS

In spite of the successes of haematopoietic gene therapy for genetic immune deficiencies, there is a need for more efficient transduction protocols and for vectors with a superior safety profile. Preclinical studies have provided reasonable expectations that haematopoietic gene therapy with a self-inactivated HIV-1-derived vector using the native gene promoter for expression of the WAS transgene will be safe and will lead to the restoration of WAS protein in the haematopoietic and immune system at levels sufficient to provide an improvement in the condition of WAS patients.

CONCLUSIONS

Phase I/II clinical studies will soon be initiated in several European centres to assess the safety and efficacy of this lentiviral vector in WAS patients.

Roles of F-BAR/PCH proteins in the regulation of membrane dynamics and actin reorganization

The Pombe Cdc15 Homology (PCH) proteins have emerged in many species as important coordinators of signaling pathways that regulate actomyosin assembly and membrane dynamics. The hallmark of the PCH proteins is the presence of a Fes/CIP4 homology-Bin/Amphiphysin/Rvsp (F-BAR) domain; therefore they are commonly referred to as F-BAR proteins. The prototype F-BAR protein, Cdc15p of Schizosaccharomyces pombe, has a role in the formation of the contractile actomyosin ring during cytokinesis. Vertebrate F-BAR proteins have an established role in binding phospholipids and they participate in membrane deformations, for instance, during the internalization of transmembrane receptors. This way the F-BAR proteins will function as linkers between the actin polymerization apparatus and the machinery regulating membrane dynamics. Interestingly, some members of the F-BAR proteins are implicated in inflammatory or neurodegenerative disorders and the observations can be expected to have clinical implications for the treatment of the diseases.

Wiskott Aldrich syndrome protein (WASP) and N-WASP are critical for T cell development

Although T cell dysfunction and lymphopenia are key features of immunodeficient patients with the Wiskott-Aldrich syndrome and Wiskott-Aldrich syndrome protein (WASP)-deficient mice, T cell development appears relatively normal. We hypothesized that N-WASP, a ubiquitously expressed homologue of WASP, may serve a redundant function with WASP. To examine the unique and redundant activities of WASP and N-WASP, we generated ES cells devoid of WASP and N-WASP [double knockout (DKO)] and used the RAG-2-deficient blastocyst complementation system to generate DKO lymphocytes. Moreover, we mated WASP KO mice with mice containing a conditionally targeted N-WASP allele and used the Cre-loxP system to generate mice lacking WASP and N-WASP in T cells [conditional DKO (cDKO)]. In both systems, N-WASP-deficient cells were indistinguishable from WT cells. In contrast, T cell development in DKO and cDKO mice was markedly altered, as shown by thymic hypocellularity and reduced numbers of peripheral T cells. We found that the combined activity of WASP and N-WASP was important for CD4(-)CD8(-) double-negative (DN)-to-CD4(+)CD8(+) double-positive (DP) cell transition, and this may be partly explained by reduced cycling DN3 cells. In addition, decreased migratory responses of CD4(+)CD8(-) and CD4(-)CD8(+) single-positive (SP) cells and increased percentage of CD69(low)CD24(low) and CD62L(low) SP cells in cDKO cells imply retention of SP cells in the thymus. In summary, this study suggests that, although WASP serves a unique role for peripheral T cell function, T cell development depends on the combined activity of WASP and N-WASP.

T Cell Responses in Mammalian Diaphanous-related Formin mDia1 Knock-out Mice

Activated T cells rapidly assemble filamentous (F-) actin networks in response to ligation of the T cell receptor or upon interaction with adhesive stimuli in order to facilitate cell migration and the formation of the immune synapse. Branched filament assembly is crucial for this process and is dependent upon activation of the Arp2/3 complex by the actin nucleation-promoting factor Wiskott-Aldrich Syndrome protein (WASp). Genetic disruption of the WAS gene has been linked to hematopoietic malignancies and various cytopenias. Although the contributions of WASp and Arp2/3 to T cell responses are fairly well characterized, the role of the mammalian Diaphanous (mDia)-related formins, which both nucleate and processively elongate non-branched F-actin, has not been demonstrated. Here, we report the effects on T cell development and function following the knock out of the murine Drf1 gene encoding the canonical formin p140mDia1. Drf1(-/-) mice develop lymphopenia characterized by diminished T cell populations in lymphoid tissues. Consistent with a role for p140mDia1 in the regulation of the actin cytoskeleton, isolated Drf1(-/-) splenic T cells adhered poorly to extracellular matrix proteins and migration in response to chemotactic stimuli was completely abrogated. Both integrin and chemokine receptor expression was unaffected by Drf1(-/-) targeting. In response to proliferative stimuli, both thymic and splenic Drf1(-/-) T cells failed to proliferate; ERK1/2 activation was also diminished in activated Drf1(-/-) T cells. These data suggest a central role for p140mDia1 in vivo in dynamic cytoskeletal remodeling events driving normal T cell responses.

Sequential gene profiling of basal cell carcinomas treated with imiquimod in a placebo-controlled study defines the requirements for tissue rejection

An analysis of basal cell carcinoma subjected to local application of imiquimod revealed that most transcripts stimulated by imiquimod involve the activation of cellular innate and adaptive immune-effector mechanisms.

Background

Imiquimod is a Toll-like receptor-7 agonist capable of inducing complete clearance of basal cell carcinoma (BCC) and other cutaneous malignancies. We hypothesized that the characterization of the early transcriptional events induced by imiquimod may provide insights about immunological events preceding acute tissue and/or tumor rejection.

Results

We report a paired analysis of adjacent punch biopsies obtained pre- and post-treatment from 36 patients with BCC subjected to local application of imiquimod (n = 22) or vehicle cream (n = 14) in a blinded, randomized protocol. Four treatments were assessed (q12 applications for 2 or 4 days, or q24 hours for 4 or 8 days). RNA was amplified and hybridized to 17.5 K cDNA arrays. All treatment schedules similarly affected the transcriptional profile of BCC; however, the q12 × 4 days regimen, associated with highest effectiveness, induced the most changes, with 637 genes unequivocally stimulated by imiquimod. A minority of transcripts (98 genes) confirmed previous reports of interferon-α involvement. The remaining 539 genes portrayed additional immunological functions predominantly involving the activation of cellular innate and adaptive immune-effector mechanisms. Importantly, these effector signatures recapitulate previous observations of tissue rejection in the context of cancer immunotherapy, acute allograft rejection and autoimmunity.

Conclusion

This study, based on a powerful and reproducible model of cancer eradication by innate immune mechanisms, provides the first insights in humans into the early transcriptional events associated with immune rejection. This model is likely representative of constant immunological pathways through which innate and adaptive immune responses combine to induce tissue destruction.

Regulation of T-cell activation by the cytoskeleton

To become activated, T cells must efficiently recognize antigen-presenting cells or target cells through several complex cytoskeleton-dependent processes, including integrin-mediated adhesion, immunological-synapse formation, cellular polarization, receptor sequestration and signalling. The actin and microtubule systems provide the dynamic cellular framework that is required to orchestrate these processes and ultimately contol T-cell activation. Here, we discuss recent advances that have furthered our understanding of the crucial importance of the T-cell cytoskeleton in controlling these aspects of T-cell immune recognition.

Pombe Cdc15 homology proteins: regulators of membrane dynamics and the actin cytoskeleton

Pombe Cdc15 homology (PCH) proteins have emerged in many species as important coordinators of signalling pathways that regulate actomyosin assembly and membrane dynamics. For example, the prototype PCH protein, Cdc15p of Schizosaccharomyces pombe, has a role in assembly of the contractile ring, which is needed to separate dividing cells. Recently, mammalian PCH proteins have been found to bind phospholipids and to participate in membrane deformation. These findings suggest that PCH proteins are crucial linkers of membrane dynamics and actin polymerization, for example, during the internalization of transmembrane receptors. Intriguingly, some members of the PCH protein family are mutated in neurodegenerative and inflammatory diseases, which has implications for the identification of cures for such disorders.

Lentiviral vectors targeting WASp expression to hematopoietic cells, efficiently transduce and correct cells from WAS patients

Gene therapy has been proposed as a potential treatment for Wiskott-Aldrich syndrome (WAS), a severe primary immune deficiency characterized by multiple hematopoietic-specific cellular defects. In order to develop an optimal lentiviral gene transfer cassette for this application, we compared the performance of several internal promoters in a variety of cell lineages from human WAS patients. Vectors using endogenous promoters derived from short (0.5 kb) or long (1.6 kb) 5' flanking sequences of the WAS gene, expressed the transgene in T, B, dendritic cells as well as CD34(+) progenitor cells, but functioned poorly in non-hematopoietic cells. Defects of T-cell proliferation and interleukin-2 production, and the cytoskeletal anomalies in WAS dendritic cells were also corrected. The levels of reconstitution were comparable to those obtained following transduction with similar lentiviral vectors incorporating constitutive PGK-1, EF1-alpha promoters or the spleen focus forming virus gammaretroviral LTR. Thus, native regulatory sequences target the expression of the therapeutic WAS transgene to the hematopoietic system, as is naturally the case for WAS, and are effective for correction of multiple cellular defects. These vectors may have significant advantages for clinical application in terms of natural gene regulation, and reduction in the potential for adverse mutagenic events.

The systemic autoinflammatory diseases: inborn errors of the innate immune system

The autoinflammatory syndromes are a newly recognized group of immune disorders that lack the high titers of self-reactive antibodies and T cells characteristic of classic autoimmune disease. Nevertheless, patients with these illnesses experience unprovoked inflammatory disease in the absence of underlying infection. Here we discuss recent advances in eight Mendelian autoinflammatory diseases. The causative genes and the proteins they encode play a critical role in the regulation of innate immunity. Both pyrin and cryopyrin, the proteins mutated in familial Mediterranean fever and the cryopyrinopathies, respectively, are involved in regulation of the proinflammatory cytokine, IL-1beta, and may influence the activity of the transcription factor, NFkappaB. NOD2, the Blau syndrome protein, shares certain domains with cryopyrin and appears to be a sensor of intracellular bacteria. PSTPIP1, mutated in the syndrome of pyogenic arthritis with pyoderma gangrenosum and acne, interacts both with pyrin and a protein tyrosine phosphatase to regulate innate and adaptive immune responses. Somewhat unexpectedly, mutations in the p55 TNF receptor lead not to immunodeficiency but to dramatic inflammatory disease, the mechanisms of which are still under investigation. Finally, the discovery of the genetic basis of the hyperimmunoglobulinemia D with periodic fever syndrome has provided a fascinating but incompletely understood link between cholesterol biosynthesis and autoinflammation. In this manuscript, we summarize the current state of the art with regard to the diagnosis, pathogenesis, and treatment of these inborn errors of the innate immune system.

Specific patterns of Cdc42 activity are related to distinct elements of T cell polarization

T cell polarization toward and within the cellular interface with an APC is critical for effective T cell activation. The Rho family GTPase Cdc42 is a central regulator of cellular polarization. Using live-cell imaging, we characterized the spatiotemporal patterns of Cdc42 activity and their physiological regulation. Using three independent means of experimental manipulation of Cdc42 activity, we established that Cdc42 is a critical regulator of T cell actin dynamics, TCR clustering, and cell cycle entry. Using quantification of three-dimensional data, we could relate distinct spatiotemporal patterns of Cdc42 activity to specific elements of T cell activation. This result suggests that Cdc42 activity in specific locations at specific times is most critical for its function in T cell activation.

Pathogenesis of familial periodic fever syndromes or hereditary autoinflammatory syndromes

Familial periodic fever syndromes, otherwise known as hereditary autoinflammatory syndromes, are inherited disorders characterized by recurrent episodes of fever and inflammation. The general hypothesis is that the innate immune response in these patients is wrongly tuned, being either too sensitive to very minor stimuli or turned off too late. The genetic background of the major familial periodic fever syndromes has been unraveled, and through research into the pathophysiology, a clearer picture of the innate immune system is emerging. After an introduction on fever, interleukin-1beta and inflammasomes, which are involved in the majority of these diseases, this manuscript offers a detailed review of the pathophysiology of the cryopyrin-associated periodic syndromes, familial Mediterranean fever, the syndrome of pyogenic arthritis, pyoderma gangrenosum and acne, Blau syndrome, TNF-receptor-associated periodic syndrome and hyper-IgD and periodic fever syndrome. Despite recent major advances, there are still many questions to be answered regarding the pathogenesis of these disorders.

CD2BP1 modulates CD2-dependent T cell activation via linkage to protein tyrosine phosphatase (PTP)-PEST

Human CD2 regulates T cell activation and adhesion via mechanisms yet to be fully understood. This study focuses on CD2BP1, a CD2 cytoplasmic tail-binding protein preferentially expressed in hematopoetic cells. Structural and functional analyses suggest that CD2BP1 acts as a scaffold protein, participating in regulation of the actin cytoskeleton. In this study, using a murine Ag-specific primary T cell transduction system to assess CD69, IL-2, and IFN-gamma expression, we provide evidence that CD2BP1 directly and negatively impacts T cell activation via isolated CD2 triggering or TCR stimulation dependent on coordinate CD2 engagement. Disruption of protein tyrosine phosphatase-PEST and/or CD2BP1 association with the CD2 signalsome rescues T cells from the inhibitory effect of CD2 crosslinking. The overexpression of CD2BP1 selectively attenuates phospholipase Cgamma1, ERK1/2, and p38 phosphorylation without abrogating CD2-independent TCR stimulation. This study provides new insight on the regulation of T cell activation and may have implications for autoimmune processes known to be associated with CD2BP1 mutations.

A Partial Down-regulation of WASP Is Sufficient to Inhibit Podosome Formation in Dendritic Cells

The Wiskott Aldrich syndrome protein (WASP) is a hematopoietic-specific cytoskeletal regulator that is necessary for induction of normal immunity. In the context of effective gene therapy for WAS, cellular models of human WASP deficiency are important for definition of the threshold of protein expression required for optimal activity. Using lentiviral vector-mediated RNA interference (RNAi), we were able to down-regulate the levels of human WASP in cell lines and primary cells. In dendritic cells (DC), RNAi-induced WASP deficiency did not impair phenotypic maturation but perturbed cytoskeletal organization. As a result, podosomes, which are actin-rich structures present in immature adherent DC, were formed less efficiently and motility was disturbed. Overall, treatment of cells with RNAi recapitulated the phenotype of cells derived from patients or animals with inactivating mutations of the WAS gene. Interestingly, reduction of the levels of WASP to about 60% of normal was sufficient to inhibit the formation of podosomes in DC, implying that this cell type requires near-normal levels of WASP to sustain physiological cytoskeleton-dependent activities.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

An electrostatic steering mechanism of Cdc42 recognition by Wiskott-Aldrich syndrome proteins

The specific and rapid formation of protein complexes is essential for diverse cellular processes such as remodeling of actin filaments in response to the interaction between Rho GTPases and the Wiskott-Aldrich syndrome proteins (WASp and N-WASp). Although Cdc42, TC10, and other members of the Rho family have been implicated in binding to and activating the WAS proteins, the exact nature of such a protein-protein recognition process has remained obscure. Here, we describe a mechanism that ensures rapid and selective long-range Cdc42-WASp recognition. The crystal structure of TC10, together with mutational and bioinformatic analyses, proved that the basic region of WASp and two unique glutamates in Cdc42 generate favorable electrostatic steering forces that control the accelerated WASp-Cdc42 association reaction. This process is a prerequisite for WASp activation and a critical step in temporal regulation and integration of WASp-mediated cellular responses.

Tyrosine kinase activity and remodelling of the actin cytoskeleton are co-temporally required for degranulation by cytotoxic T lymphocytes

In this study, we examined the contribution of the actin cytoskeleton to T-cell receptor (TCR)-initiated signalling in cytotoxic T lymphocytes (CTLs). We demonstrate that cytoskeletal remodelling is required for sustaining TCR-stimulated signals that lead to degranulation by CTLs. Disruption of the actin cytoskeleton in CTLs already undergoing signalling responses results in an almost immediate loss of essentially all protein tyrosine phosphorylation. This signal reversal is not restricted to tyrosine phosphorylation, as disruption of the actin cytoskeleton also reverses the phosphorylation of the more downstream serine/threonine kinase extracellular signal regulated kinase (Erk). An intact cytoskeleton and cell spreading are not sufficient for maintaining signals, as stabilization of actin filaments, at a point when peak tyrosine phosphorylation is occurring, also leads to the rapid loss of protein tyrosine phosphorylation. Disruption of tyrosine kinase activity after TCR signals are maximally induced causes the immediate reversal of tyrosine phosphorylation as well as cytoskeletal disruption, as indicated by loss of cell spreading, adhesion and CTL degranulation. Taken together, our results indicate that actin remodelling occurs co-temporally with ongoing tyrosine kinase activity, leading to CTL degranulation. We hypothesize that continuous actin remodelling is important for sustaining productive signals, even after downstream signalling molecules such as Erk have been activated, and that the actin cytoskeleton is not solely required for initiating and maintaining the T cell in contact with its stimulus.

The verprolin family of proteins: Regulators of cell morphogenesis and endocytosis

The verprolin family of proteins, WIP, CR16 and WIRE/WICH, has emerged as critical regulators of cytoskeletal organisation in vertebrate cells. The founding father of the family, verprolin, was originally identified in budding yeast and later shown to be needed for actin polymerisation during polarised growth and during endocytosis. The vertebrate verprolins regulate actin dynamics either by binding directly to actin, by binding the WASP family of proteins or by binding to other actin regulating proteins. Interestingly, also the vertebrate verprolins have been implicated in endocytosis, demonstrating that most of the functional modules in this fascinating group of proteins have been conserved from yeast to man.

WIP and WASP play complementary roles in T cell homing and chemotaxis to SDF-1α

Homing of lymphocytes to tissues is a biologically important multistep process that involves selectin-dependent rolling, integrin-dependent adhesion and chemokine-directed chemotaxis. The actin cytoskeleton plays a central role in lymphocyte adhesion and motility. Wiskott-Aldrich syndrome protein (WASP), the product of the gene mutated in Wiskott-Aldrich syndrome, and its partner, the Wiskott-Aldrich syndrome protein-interacting protein (WIP), play important roles in actin re-organization in T lymphocytes. We used mice with disruption of the WASP and WIP genes to examine the role of WASP and WIP in T cell homing. T cell homing to spleen and lymph nodes in vivo was deficient in WASP-/- and WIP-/- mice and severely impaired in WASP-/-WIP-/- double knockout (DKO) mice. Deficiency of WASP, WIP or both did not interfere with selectin-dependent rolling or integrin-dependent adhesion of T cells in vitro. Chemotaxis to stromal cell-derived factor-1alpha (SDF-1alpha) in vitro was mildly reduced in T cells from WASP-/- mice. In contrast, it was significantly impaired in T cells from WIP-/- mice and severely reduced in T cells from DKO mice. Cellular F-actin increase following SDF-1alpha stimulation was normal in WASP-/- and WIP-/- T cells, but severely reduced in T cells from DKO mice. Actin re-organization and polarization in response to SDF-1alpha was abnormal in T cells from all knockout mice. Early biochemical events following SDF-1alpha stimulation that are important for chemotaxis and that included phosphorylation of Lck, cofilin, PAK1 and extracellular regulated kinase (Erk) and GTP loading of Rac-1 were examined in T cells from DKO mice and found to be normal. These results suggest that WASP and WIP are not essential for T lymphocyte rolling and adhesion, but play important and partially redundant roles in T cell chemotaxis in vitro and homing in vivo and function downstream of small GTPases.

Juxtanodin: an oligodendroglial protein that promotes cellular arborization and 2',3'-cyclic nucleotide-3'-phosphodiesterase trafficking

In the process of screening cell-type-specific genes, we identified juxtanodin (JN), an oligodendroglial protein featuring a putative C-terminal actin-binding domain. At the cellular level, JN in the rat CNS colocalized with 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase), a cytoskeleton-related oligodendroglial protein. In the myelin sheath, JN was found mainly in the abaxon and the lateral few terminal loops. Its apposition to the myelinated axon, through the latter, defined an axonal subregion, herewith termed juxtanode, at the Ranvier node-paranode junction. During forebrain ontogenesis, JN expression paralleled that of MBPs but lagged behind CNPase. Juxtanodin transfection promoted arborization of cultured OLN-93 cells and augmented endogenous CNPase expression and transport to the process arbors of cultured primary oligodendrocyte precursors. These results reveal JN as a cytoskeleton-related oligodendroglial protein that delineates the juxtanode and might serve oligodendrocyte motility, differentiation, or myelin-axon signaling. Functionally, JN may be involved in CNS myelination and/or specialization of the node of Ranvier.

N-WASP deficiency impairs EGF internalization and actin assembly at clathrin-coated pits

WASP and WAVE family proteins promote actin polymerization by stimulating Arp2/3-complex-dependent filament nucleation. Unlike WAVE proteins, which are known to drive the formation of protrusions such as lamellipodia and membrane ruffles, vertebrate cell functions of WASP or N-WASP are less well established. Recent work demonstrated that clathrin-coated pit invagination can coincide with assembly of actin filaments and with accumulation of N-WASP and Arp2/3 complex, but the relevance of their recruitment has remained poorly defined. We employed two-colour total internal reflection microscopy to study the recruitment and dynamics of various components of the actin polymerization machinery and the epidermal growth factor receptor signalling machinery during clathrin-coated pit internalization in control cells and cells genetically deficient for functional N-WASP. We found that clathrin-coated pit endocytosis coincides with the recruitment of N-WASP, Arp2/3 complex and associated proteins, but not of WAVE family members. Actin accumulation at clathrin-coated pits requires the Arp2/3 complex, since Arp2/3 complex sequestration in the cytosol abolished any detectable actin assembly. The absence of N-WASP caused a significant reduction in the frequencies of actin and Arp2/3 complex accumulations at sites of clathrin-coated pit invagination and vesicle departure. Although N-WASP was not essential for Arp2/3-complex-mediated actin assembly at these sites or for EGF receptor-mediated endocytosis, N-WASP deficiency caused a marked reduction of EGF internalization. We conclude that the assembly of WASP subfamily proteins and associated factors at sites of clathrin-coated pit invagination amplifies actin accumulations at these sites promoting efficient internalization of ligands via clathrin-mediated endocytosis.

The Role of SPECs, Small Cdc42-binding Proteins, in F-actin Accumulation at the Immunological Synapse

SPEC1 and SPEC2 are structurally similar Cdc42-binding proteins of 79 and 84 amino acid residues, respectively. We investigated the role of SPEC2 in T cell function due to its high mRNA expression in lymphocytes. Western blot analysis revealed abundant SPEC2 protein in lymphocytes, which in glutathione S-transferase-capture experiments specifically interacted with only GTP-bound Cdc42. Immunofluorescence experiments revealed that the SPEC2 protein was diffusely localized in the cytoplasm and at the cell membrane in unstimulated Jurkat T cells and Raji B cells. Recruitment of SPEC2 within Jurkat T cells to the antigen-presenting cell interface occurred following incubation with staphylococcal enterotoxin E superantigen-loaded B cells and colocalized there with F-actin and Cdc42. T cell receptor (TCR) activation studies using anti-CD3 antibody-coated polystyrene beads showed that SPEC2 was recruited to the site of bead contact, which was not observed with anti-major histocompatibility complex antibody-coated beads. Accumulation of SPEC2 following TCR engagement occurred as early as 5 min, before obvious F-actin accumulation. Biochemical studies with Jurkat T cells demonstrated that N-terminal cysteine residues in SPEC2 were palmitoylated. Overexpression studies of the related SPEC1 showed that it also was recruited to the activated TCR. Mutational analysis revealed that localization of SPEC1 to the TCR required two N-terminal cysteine residues. Furthermore, a SPEC1 Cdc42 Rac-interacting binding mutant, containing an intact N terminus but defective in Cdc42 binding, completely blocked F-actin accumulation at the activated TCR. Taken together these results suggest that SPECs may play important roles in Cdc42-mediated F-actin accumulation at the immunological synapse.

WASP- mice exhibit defective immune responses to influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis BCG

OBJECTIVE

To quantify the immune response of WASP- mice to three different pathogens: influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis.

METHODS

Primary and secondary T-cell responses to influenza A virus were quantified via tetramer assays. Viral clearance from lung was also measured. Lethality of intranasal inoculation with luminescent S. pneumoniae was assessed by dose escalation and direct luminescence imaging. After intravenous inoculation with M. bovis, residual mycobacteria in lung, liver, and spleen were measured by standard culture methods.

RESULTS

The reduced secondary T-cell response to influenza A virus correlates with a relative but not absolute loss of splenic T and B cells similar to that seen in clinical Wiskott-Aldrich Syndrome (WAS), and slower clearance of virus from lung. The reduced magnitude of the secondary T-cell response correlates with a progressive loss of influenza-specific T cells after primary inoculation. WASP- mice show an increased susceptibility to lethal pneumonia after intranasal inoculation with S. pneumoniae, which is among the most frequent causes of clinical complications in WAS patients. WASP- mice clear M. bovis bacille Calmette-Guerin (BCG) more slowly from lung, liver, and spleen. Bone marrow-derived macrophages, however, show normal ex vivo cytokine secretion in response to M. bovis.

CONCLUSIONS

These results demonstrate that WASP- mice are functionally immunodeficient in regard to three different pathogens, and provide relevant end points for the study of treatment modalities in this model. They also suggest a specific physiologic mechanism, failure to accumulate memory T cells, for at least one of the defective immune responses.

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

Molecular defects that affect platelet dense granules

Platelet dense granules form using mechanisms shared by melanosomes in melanocytes and by subsets of lysosomes in more generalized cells. Consequently, disorders of platelet dense granules can reveal how organelles form and move within cells. Models for the study of new vesicle formation include isolated delta-storage pool deficiency, combined alphadelta-storage pool deficiency, Hermansky-Pudlak syndrome (HPS), Chediak-Higashi syndrome, Griscelli syndrome, thrombocytopenia absent radii syndrome, and Wiskott-Aldrich syndrome. The molecular bases of dense granule deficiency are known for the seven subtypes of HPS, as well as for Chediak-Higashi syndrome, Griscelli syndrome, and Wiskott-Aldrich syndrome. The gene products involved in these disorders help elucidate the generalized process of the formation of vesicles from extant membranes such as the Golgi.

Caenorhabditis elegans WASP and Ena/VASP proteins play compensatory roles in morphogenesis and neuronal cell migration

We report here that WASP and Ena/VASP family proteins play overlapping roles in C. elegans morphogenesis and neuronal cell migration. Specifically, these studies demonstrate that UNC-34/Ena plays a role in morphogenesis that is revealed only in the absence of WSP-1 function and that WSP-1 has a role in neuronal cell migration that is revealed only in the absence of UNC-34/Ena activity. To identify additional genes that act in parallel to unc-34/ena during morphogenesis, we performed a screen for synthetic lethals in an unc-34 null mutant background utilizing an RNAi feeding approach. To our knowledge, this is the first reported RNAi-based screen for genetic interactors. As a result of this screen, we identified a second C. elegans WASP family protein, wve-1, that is most homologous to SCAR/WAVE proteins. Animals with impaired wve-1 function display defects in gastrulation, fail to undergo proper morphogenesis, and exhibit defects in neuronal cell migrations and axon outgrowth. Reducing wve-1 levels in either unc-34/ena or wsp-1 mutant backgrounds also leads to a significant enhancement of the gastrulation and morphogenesis defects. Thus, unc-34/ena, wsp-1, and wve-1 play overlapping roles during embryogenesis and unc-34/ena and wsp-1 play overlapping roles in neuronal cell migration. These observations show that WASP and Ena/VASP proteins can compensate for each other in vivo and provide the first demonstration of a role for Ena/VASP proteins in gastrulation and morphogenesis. In addition, our results provide the first example of an in vivo role for WASP family proteins in neuronal cell migrations and cytokinesis in metazoans.