Protein-tyrosine Kinase and GTPase Signals Cooperate to Phosphorylate and Activate Wiskott-Aldrich Syndrome Protein (WASP)/Neuronal WASP

Role of Wiskott Aldrich syndrome protein in haematological malignancies: genetics, molecular mechanisms and therapeutic strategies

As patients continue to suffer from lymphoproliferative and myeloproliferative diseases known as haematopoietic malignancies can affect the bone marrow, blood, lymph nodes, and lymphatic and non-lymphatic organs. Despite advances in the current treatment, there is still a significant challenge for physicians to improve the therapy of HMs. WASp is an important regulator of actin polymerization and the involvement of WASp in transcription is thought to be linked to the DNA damage response and repair. In some studies, severe immunodeficiency and lymphoid malignancy are caused by WASp mutations or the absence of WASp and these mutations in WAS can alter the function and/or expression of the intracellular protein. Loss-of-function and Gain-of-function mutations in WASp have an impact on cancer malignancies' incidence and onset. Recent studies suggest that depending on the clinical or experimental situation, WASPs and WAVEs can operate as a suppressor or enhancers for cancer malignancy. These dual functions of WASPs and WAVEs in cancer likely arose from their multifaceted role in cells that could be targeted for anticancer drug development. The significant role and their association of WASp in Chronic myeloid leukaemia, Juvenile myelomonocytic leukaemia and T-cell lymphoma is discussed. In this review, we described the structure and function of WASp and its family mechanism, analysing major regulatory effectors and summarising the clinical relevance and drugs that specifically target WASp in disease treatment in various hematopoietic malignancies by different approaches.

A single chlamydial protein reshapes the plasma membrane and serves as recruiting platform for central endocytic effector proteins

Uptake of obligate intracellular bacterial pathogens into mammalian epithelial cells is critically dependent on modulation of the host's endocytic machinery. It is an open question how the invading pathogens generate a membrane-bound vesicle appropriate to their size. This requires extensive deformation of the host plasma membrane itself by pathogen-derived membrane-binding proteins, accompanied by substantial F-actin-based forces to further expand and finally pinch off the vesicle. Here we show that upon adhesion to the host cell, the human pathogenic bacterium Chlamydia pneumoniae secretes the scaffolding effector protein CPn0677, which binds to the inner leaflet of the invaginating host's PM, induces inwardly directed, negative membrane curvature, and forms a recruiting platform for the membrane-deforming BAR-domain containing proteins Pacsin and SNX9. In addition, while bound to the membrane, CPn0677 recruits monomeric G-actin, and its C-terminal region binds and activates N-WASP, which initiates branching actin polymerization via the Arp2/3 complex. Together, these membrane-bound processes enable the developing endocytic vesicle to engulf the infectious elementary body, while the associated actin network generates the forces required to reshape and detach the nascent vesicle from the PM. Thus, Cpn0677 (now renamed SemD) acts as recruiting platform for central components of the endocytic machinery during uptake of chlamydia.

Branching out in different directions: Emerging cellular functions for the Arp2/3 complex and WASP-family actin nucleation factors

The actin cytoskeleton impacts practically every function of a eukaryotic cell. Historically, the best-characterized cytoskeletal activities are in cell morphogenesis, motility, and division. The structural and dynamic properties of the actin cytoskeleton are also crucial for establishing, maintaining, and changing the organization of membrane-bound organelles and other intracellular structures. Such activities are important in nearly all animal cells and tissues, although distinct anatomical regions and physiological systems rely on different regulatory factors. Recent work indicates that the Arp2/3 complex, a broadly expressed actin nucleator, drives actin assembly during several intracellular stress response pathways. These newly described Arp2/3-mediated cytoskeletal rearrangements are coordinated by members of the Wiskott-Aldrich Syndrome Protein (WASP) family of actin nucleation-promoting factors. Thus, the Arp2/3 complex and WASP-family proteins are emerging as crucial players in cytoplasmic and nuclear activities including autophagy, apoptosis, chromatin dynamics, and DNA repair. Characterizations of the functions of the actin assembly machinery in such stress response mechanisms are advancing our understanding of both normal and pathogenic processes, and hold great promise for providing insights into organismal development and interventions for disease.

Targeting the actin nucleation promoting factor WASp provides a therapeutic approach for hematopoietic malignancies

Cancer cells depend on actin cytoskeleton rearrangement to carry out hallmark malignant functions including activation, proliferation, migration and invasiveness. Wiskott–Aldrich Syndrome protein (WASp) is an actin nucleation-promoting factor and is a key regulator of actin polymerization in hematopoietic cells. The involvement of WASp in malignancies is incompletely understood. Since WASp is exclusively expressed in hematopoietic cells, we performed in silico screening to identify small molecule compounds (SMCs) that bind WASp and promote its degradation. We describe here one such identified molecule; this WASp-targeting SMC inhibits key WASp-dependent actin processes in several types of hematopoietic malignancies in vitro and in vivo without affecting naïve healthy cells. This small molecule demonstrates limited toxicity and immunogenic effects, and thus, might serve as an effective strategy to treat specific hematopoietic malignancies in a safe and precisely targeted manner.

Cancer cells proliferate and invade via cytoskeletal proteins such as WASp, exclusively expressed in hematopoietic cells. Here the authors show a specific small molecule compound inhibiting cancer cell activity by WASp degradation and demonstrating its therapeutic potential in vitro and in vivo.

Wiskott-Aldrich Syndrome Protein: Roles in Signal Transduction in T Cells

Signal transduction regulates the proper function of T cells in an immune response. Upon binding to its specific ligand associated with major histocompatibility complex (MHC) molecules on an antigen presenting cell, the T cell receptor (TCR) initiates intracellular signaling that leads to extensive actin polymerization. Wiskott-Aldrich syndrome protein (WASp) is one of the actin nucleation factors that is recruited to TCR microclusters, where it is activated and regulates actin network formation. Here we highlight the research that has focused on WASp-deficient T cells from both human and mice in TCR-mediated signal transduction. We discuss the role of WASp in proximal TCR signaling as well as in the Ras/Rac-MAPK (mitogen-activated protein kinase), PKC (protein kinase C) and Ca2+-mediated signaling pathways.

EFA6B regulates a stop signal for collective invasion in breast cancer

Cancer is initiated by somatic mutations in oncogenes or tumor suppressor genes. However, additional alterations provide selective advantages to the tumor cells to resist treatment and develop metastases. Their identification is of paramount importance. Reduced expression of EFA6B (Exchange Factor for ARF6, B) is associated with breast cancer of poor prognosis. Here, we report that loss of EFA6B triggers a transcriptional reprogramming of the cell-to-ECM interaction machinery and unleashes CDC42-dependent collective invasion in collagen. In xenograft experiments, MCF10 DCIS.com cells, a DCIS-to-IDC transition model, invades faster when knocked-out for EFA6B. In addition, invasive and metastatic tumors isolated from patients have lower expression of EFA6B and display gene ontology signatures identical to those of EFA6B knock-out cells. Thus, we reveal an EFA6B-regulated molecular mechanism that controls the invasive potential of mammary cells; this finding opens up avenues for the treatment of invasive breast cancer.

Common Signal Transduction Molecules Activated by Bacterial Entry into a Host Cell and by Reactive Oxygen Species

Significance: An increasing number of pathogens are acquiring resistance to antibiotics. Efficient antimicrobial drug regimens are important even for the most advanced therapies, which range from cutting-edge invasive clinical protocols, such as robotic surgeries, to the treatment of harmless bacterial diseases and to minor scratches to the skin. Therefore, there is an urgent need to survey alternative antimicrobial drugs that can reinforce or replace existing antibiotics. Recent Advances: Bacterial proteins that are critical for energy metabolism, promising novel anticancer thiourea derivatives, and the use of synthetic molecules that increase the sensitivity of currently used antibiotics are among the recently discovered antimicrobial drugs. Critical Issues: In the development of new drugs, serious consideration should be given to the previous bacterial evolutionary selection caused by antibiotics, by the high proliferation rate of bacteria, and by the simple prokaryotic structure of bacteria. Future Directions: The survey of drug targets has mainly focused on bacterial proteins, although host signaling molecules involved in the treatment of various pathologies may have unknown antimicrobial characteristics. Recent data have suggested that small molecule inhibitors might enhance the effect of antibiotics, for example, by limiting bacterial entry into host cells. Phagocytosis, the mechanism by which host cells internalize pathogens through β-actin cytoskeletal rearrangement, induces calcium signaling, small GTPase activation, and phosphorylation of the phosphatidylinositol 3-kinase-serine/threonine-specific protein kinase B pathway. Antioxid. Redox Signal. 34, 486-503.

Actin regulators in cancer progression and metastases: From structure and function to cytoskeletal dynamics

The cytoskeleton is a central factor contributing to various hallmarks of cancer. In recent years, there has been increasing evidence demonstrating the involvement of actin regulatory proteins in malignancy, and their dysregulation was shown to predict poor clinical prognosis. Although enhanced cytoskeletal activity is often associated with cancer progression, the expression of several inducers of actin polymerization is remarkably reduced in certain malignancies, and it is not completely clear how these changes promote tumorigenesis and metastases. The complexities involved in cytoskeletal induction of cancer progression therefore pose considerable difficulties for therapeutic intervention; it is not always clear which cytoskeletal regulator should be targeted in order to impede cancer progression, and whether this targeting may inadvertently enhance alternative invasive pathways which can aggravate tumor growth. The entire constellation of cytoskeletal machineries in eukaryotic cells are numerous and complex; the system is comprised of and regulated by hundreds of proteins, which could not be covered in a single review. Therefore, we will focus here on the actin cytoskeleton, which encompasses the biological machinery behind most of the key cellular functions altered in cancer, with specific emphasis on actin nucleating factors and nucleation-promoting factors. Finally, we discuss current therapeutic strategies for cancer which aim to target the cytoskeleton.

WHIMP links the actin nucleation machinery to Src-family kinase signaling during protrusion and motility

Cell motility is governed by cooperation between the Arp2/3 complex and nucleation-promoting factors from the Wiskott-Aldrich Syndrome Protein (WASP) family, which together assemble actin filament networks to drive membrane protrusion. Here we identify WHIMP (WAVE Homology In Membrane Protrusions) as a new member of the WASP family. The Whimp gene is encoded on the X chromosome of a subset of mammals, including mice. Murine WHIMP promotes Arp2/3-dependent actin assembly, but is less potent than other nucleation factors. Nevertheless, WHIMP-mediated Arp2/3 activation enhances both plasma membrane ruffling and wound healing migration, whereas WHIMP depletion impairs protrusion and slows motility. WHIMP expression also increases Src-family kinase activity, and WHIMP-induced ruffles contain the additional nucleation-promoting factors WAVE1, WAVE2, and N-WASP, but not JMY or WASH. Perturbing the function of Src-family kinases, WAVE proteins, or Arp2/3 complex inhibits WHIMP-driven ruffling. These results suggest that WHIMP-associated actin assembly plays a direct role in membrane protrusion, but also results in feedback control of tyrosine kinase signaling to modulate the activation of multiple WASP-family members.

The actin cytoskeleton is a collection of protein polymers that assemble and disassemble within cells at specific times and locations. Sophisticated cytoskeletal regulators called nucleation-promoting factors ensure that actin polymerizes when and where it is needed, and many of these factors are members of the Wiskott-Aldrich Syndrome Protein (WASP) family. Several of the 8 known WASP-family proteins function in cell motility, but how the different factors collaborate with one another is not well understood. In this study, we identified WHIMP, a new WASP-family member that is encoded on the X chromosome of a variety of mammals. In mouse cells, WHIMP enhances cell motility by assembling actin filaments that push the plasma membrane forward. Unexpectedly, WHIMP also activates tyrosine kinases, enzymes that stimulate multiple WASP-family members during motility. Our results open new avenues of research into how nucleation factors cooperate during movement and how the molecular activities that underlie motility differ in distinct cell types and organisms.

Rac1 GTPase activates the WAVE regulatory complex through two distinct binding sites

The Rho GTPase Rac1 activates the WAVE regulatory complex (WRC) to drive Arp2/3 complex-mediated actin polymerization, which underpins diverse cellular processes. Here we report the structure of a WRC-Rac1 complex determined by cryo-electron microscopy. Surprisingly, Rac1 is not located at the binding site on the Sra1 subunit of the WRC previously identified by mutagenesis and biochemical data. Rather, it binds to a distinct, conserved site on the opposite end of Sra1. Biophysical and biochemical data on WRC mutants confirm that Rac1 binds to both sites, with the newly identified site having higher affinity and both sites required for WRC activation. Our data reveal that the WRC is activated by simultaneous engagement of two Rac1 molecules, suggesting a mechanism by which cells may sense the density of active Rac1 at membranes to precisely control actin assembly.

Our cells contain a network of filaments made up of a protein called actin. Just like the skeleton that supports our body, the actin ‘cytoskeleton’ gives a cell its shape and strength. Actin filaments are also critical for many other processes including enabling cells to move and divide. The assembly of actin filaments must be properly controlled so that they are formed at the right time and place within the cell.

A complex of proteins known as the WAVE Regulatory Complex (WRC) promotes the assembly of actin filaments. The complex contains a region called the VCA, which is able to bind to and activate another protein to make the new actin filaments. The WRC regulates filament assembly by controlling the availability of the VCA in a way that is similar to opening and closing a safe box. When new actin filaments are not needed, the safe box is closed and the VCA is not available. However, when cells need to make new actin filaments, the WRC is opened to release the VCA region so that it is able to bind to the filament-producing protein.

Previous studies have shown that a protein called Rac1 acts as a key to open the WRC and trigger actin filament assembly. But it remains unclear how this works. A major obstacle to studying this process is that Rac1 and the WRC only weakly interact with each other, which makes it difficult to capture the interaction under experimental conditions.

To overcome this obstacle, Chen et al. tethered a Rac1 molecule to the WRC in order to make the interaction more stable. A technique called cryo-electron microscopy was used to study the three-dimensional shape of this Rac1-WRC complex. Unexpectedly, Rac1 was attached to a different part of the WRC than the site predicted by previous studies. Further experiments showed that Rac1 needs to bind to both of these sites at the same time in order to open the WRC and release VCA, similar to using two keys to open one safe box for increased security.

Some cancers, neurological disorders and other diseases can be caused by defects in WRC and Rac1 activity. Therefore, these findings could lead to new ways to treat these conditions in human patients.

YAP/TAZ-CDC42 signaling regulates vascular tip cell migration

Angiogenesis and vascular remodeling are essential for the establishment of vascular networks during organogenesis. Here we show that the Hippo signaling pathway effectors YAP and TAZ are required, in a gene dosage-dependent manner, for the proliferation and migration of vascular endothelial cells (ECs) during retinal angiogenesis. Intriguingly, nuclear translocation of YAP and TAZ induced by Lats1/2-deletion blocked endothelial migration and phenocopied Yap/Taz-deficient mutants. Furthermore, overexpression of a cytoplasmic form of YAP (YAPS127D) partially rescued the migration defects caused by loss of YAP and TAZ function. Finally, we found that cytoplasmic YAP positively regulated the activity of the small GTPase CDC42, deletion of which caused severe defects in endothelial migration. These findings uncover a previously unrecognized role of cytoplasmic YAP/TAZ in promoting cell migration by activating CDC42 and provide insight into how Hippo signaling in ECs regulates angiogenesis.

Phosphorylation of Drebrin and Its Role in Neuritogenesis

Neuritogenesis is an early event in neuronal development in which newborn neurons first form growth cones, as a prerequisite for the formation of axons and dendrites. Growth cones emerge from segmented regions of the lamellipodium of embryonic neurons and grow away from the cell body leaving behind a neurite that will eventually polarise into an axon or dendrite. Growth cones also function to navigate precise routes through the embryo to locate an appropriate synaptic partner. Dynamic interactions between two components of the neuronal cytoskeleton, actin filaments and microtubules, are known to be essential for growth cone formation and hence neuritogenesis. The molecular mechanisms that coordinate interactions between actin filaments and dynamic microtubules during neuritogenesis are beginning to be understood. One candidate pathway coupling actin filaments to microtubules consists of the actin filament-binding protein drebrin and the microtubule-binding +TIP protein EB3. This pathway is regulated proximally by cyclin-dependent kinase 5 phosphorylation of drebrin but the upstream elements in the pathway have yet to be identified.

Allosteric N-WASP activation by an inter-SH3 domain linker in Nck

Actin filament networks assemble on cellular membranes in response to signals that locally activate neural Wiskott-Aldrich-syndrome protein (N-WASP) and the Arp2/3 complex. An inactive conformation of N-WASP is stabilized by intramolecular contacts between the GTPase binding domain (GBD) and the C helix of the verprolin-homology, connector-helix, acidic motif (VCA) segment. Multiple SH3 domain-containing adapter proteins can bind and possibly activate N-WASP, but it remains unclear how such binding events relieve autoinhibition to unmask the VCA segment and activate the Arp2/3 complex. Here, we have used purified components to reconstitute a signaling cascade driven by membrane-localized Src homology 3 (SH3) adapters and N-WASP, resulting in the assembly of dynamic actin networks. Among six SH3 adapters tested, Nck was the most potent activator of N-WASP-driven actin assembly. We identify within Nck a previously unrecognized activation motif in a linker between the first two SH3 domains. This linker sequence, reminiscent of bacterial virulence factors, directly engages the N-WASP GBD and competes with VCA binding. Our results suggest that animals, like pathogenic bacteria, have evolved peptide motifs that allosterically activate N-WASP, leading to localized actin nucleation on cellular membranes.

Signal Integration during T Lymphocyte Activation and Function: Lessons from the Wiskott-Aldrich Syndrome

Over the last decades, research dedicated to the molecular and cellular mechanisms underlying primary immunodeficiencies (PID) has helped to understand the etiology of many of these diseases and to develop novel therapeutic approaches. Beyond these aspects, PID are also studied because they offer invaluable natural genetic tools to dissect the human immune system. In this review, we highlight the research that has focused over the last 20 years on T lymphocytes from Wiskott–Aldrich syndrome (WAS) patients. WAS T lymphocytes are defective for the WAS protein (WASP), a regulator of actin cytoskeleton remodeling. Therefore, study of WAS T lymphocytes has helped to grasp that many steps of T lymphocyte activation and function depend on the crosstalk between membrane receptors and the actin cytoskeleton. These steps include motility, immunological synapse assembly, and signaling, as well as the implementation of helper, regulatory, or cytotoxic effector functions. The recent concept that WASP also works as a regulator of transcription within the nucleus is an illustration of the complexity of signal integration in T lymphocytes. Finally, this review will discuss how further study of WAS may contribute to solve novel challenges of T lymphocyte biology.

X-linked thrombocytopenia causing mutations in WASP (L46P and A47D) impair T cell chemotaxis

Background

Mutation in the Wiskott-Aldrich syndrome Protein (WASP) causes Wiskott-Aldrich syndrome (WAS), X-linked thrombocytopenia (XLT) and X-linked congenital neutropenia (XLN). The majority of missense mutations causing WAS and XLT are found in the WH1 (WASP Homology) domain of WASP, known to mediate interaction with WIP (WASP Interacting Protein) and CIB1 (Calcium and Integrin Binding).

Results

We analyzed two WASP missense mutants (L46P and A47D) causing XLT for their effects on T cell chemotaxis. Both mutants, WASP_R^L46P and WASP_R^A47D (S1-WASP shRNA resistant) expressed well in Jurkat^WASP-KD T cells (WASP knockdown), however expression of these two mutants did not rescue the chemotaxis defect of Jurkat^WASP-KD T cells towards SDF-1α. In addition Jurkat^WASP-KD T cells expressing these two WASP mutants were found to be defective in T cell polarization when stimulated with SDF-1α. WASP exists in a closed conformation in the presence of WIP, however both the mutants (WASP_R^L46P and WASP_R^A47D) were found to be in an open conformation as determined in the bi-molecular complementation assay. WASP protein undergoes proteolysis upon phosphorylation and this turnover of WASP is critical for T cell migration. Both the WASP mutants were found to be stable and have reduced tyrosine phosphorylation after stimulation with SDF-1α.

Conclusion

Thus our data suggest that missense mutations WASP_R^L46P or WASP_R^A47D affect the activity of WASP in T cell chemotaxis probably by affecting the turnover of the protein.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-014-0091-1) contains supplementary material, which is available to authorized users.

Phosphorylation site dynamics of early T-cell receptor signaling

In adaptive immune responses, T-cell receptor (TCR) signaling impacts multiple cellular processes and results in T-cell differentiation, proliferation, and cytokine production. Although individual protein-protein interactions and phosphorylation events have been studied extensively, we lack a systems-level understanding of how these components cooperate to control signaling dynamics, especially during the crucial first seconds of stimulation. Here, we used quantitative proteomics to characterize reshaping of the T-cell phosphoproteome in response to TCR/CD28 co-stimulation, and found that diverse dynamic patterns emerge within seconds. We detected phosphorylation dynamics as early as 5 s and observed widespread regulation of key TCR signaling proteins by 30 s. Development of a computational model pointed to the presence of novel regulatory mechanisms controlling phosphorylation of sites with central roles in TCR signaling. The model was used to generate predictions suggesting unexpected roles for the phosphatase PTPN6 (SHP-1) and shortcut recruitment of the actin regulator WAS. Predictions were validated experimentally. This integration of proteomics and modeling illustrates a novel, generalizable framework for solidifying quantitative understanding of a signaling network and for elucidating missing links.

Src kinases regulate de novo actin polymerization during exocytosis in neuroendocrine chromaffin cells

The cortical actin network is dynamically rearranged during secretory processes. Nevertheless, it is unclear how de novo actin polymerization and the disruption of the preexisting actin network control transmitter release. Here we show that in bovine adrenal chromaffin cells, both formation of new actin filaments and disruption of the preexisting cortical actin network are induced by Ca²⁺ concentrations that trigger exocytosis. These two processes appear to regulate different stages of exocytosis; whereas the inhibition of actin polymerization with the N-WASP inhibitor wiskostatin restricts fusion pore expansion, thus limiting the release of transmitters, the disruption of the cortical actin network with cytochalasin D increases the amount of transmitter released per event. Further, the Src kinase inhibitor PP2, and cSrc SH2 and SH3 domains also suppress Ca²⁺-dependent actin polymerization, and slow down fusion pore expansion without disturbing the cortical F-actin organization. Finally, the isolated SH3 domain of c-Src prevents both the disruption of the actin network and the increase in the quantal release induced by cytochalasin D. These findings support a model where a rise in the cytosolic Ca²⁺ triggers actin polymerization through a mechanism that involves Src kinases. The newly formed actin filaments would speed up the expansion of the initial fusion pore, whereas the preexisting actin network might control a different step of the exocytosis process.

Binding and function of phosphotyrosines of the Ephrin A2 (EphA2) receptor using synthetic sterile α motif (SAM) domains

The sterile α motif (SAM) domain of the ephrin receptor tyrosine kinase, EphA2, undergoes tyrosine phosphorylation, but the effect of phosphorylation on the structure and interactions of the receptor is unknown. Studies to address these questions have been hindered by the difficulty of obtaining site-specifically phosphorylated proteins in adequate amounts. Here, we describe the use of chemically synthesized and specifically modified domain-length peptides to study the behavior of phosphorylated EphA2 SAM domains. We show that tyrosine phosphorylation of any of the three tyrosines, Tyr(921), Tyr(930), and Tyr(960), has a surprisingly small effect on the EphA2 SAM structure and stability. However, phosphorylation at Tyr(921) and Tyr(930) enables differential binding to the Src homology 2 domain of the adaptor protein Grb7, which we propose will lead to distinct functional outcomes. Setting up different signaling platforms defined by selective interactions with adaptor proteins thus adds another level of regulation to EphA2 signaling.

Tyrosine kinases, drugs, and Shigella flexneri dissemination

Shigella flexneri is an enteropathogenic bacterium responsible for approximately 100 million cases of severe dysentery each year. S. flexneri colonization of the human colonic epithelium is supported by direct spread from cell to cell, which relies on actin-based motility. We have recently uncovered that, in intestinal epithelial cells, S. flexneri actin-based motility is regulated by the Bruton's tyrosine kinase (Btk). Consequently, treatment with Ibrutinib, a specific Btk inhibitor currently used in the treatment of B-cell malignancies, effectively impaired S. flexneri spread from cell to cell. Thus, therapeutic intervention capitalizing on drugs interfering with host factors supporting the infection process may represent an effective alternative to treatments with antimicrobial compounds.

Multiscale modeling of cell shape from the actin cytoskeleton

The actin cytoskeleton is a dynamic structure that constantly undergoes complex reorganization events during many cellular processes. Mathematical models and simulations are powerful tools that can provide insight into the physical mechanisms underlying these processes and make predictions that can be experimentally tested. Representation of the interactions of the actin filaments with the plasma membrane and the movement of the plasma membrane for computation remains a challenge. Here, we provide an overview of the different modeling approaches used to study cytoskeletal dynamics and highlight the differential geometry approach that we have used to implement the interactions between the plasma membrane and the cytoskeleton. Using cell spreading as an example, we demonstrate how this approach is able to successfully capture in simulations, experimentally observed behavior. We provide a perspective on how the differential geometry approach can be used for other biological processes.

The adaptor molecule SAP plays essential roles during invariant NKT cell cytotoxicity and lytic synapse formation

The adaptor molecule signaling lymphocytic activation molecule-associated protein (SAP) plays critical roles during invariant natural killer T (iNKT) cell ontogeny. As a result, SAP-deficient humans and mice lack iNKT cells. The strict developmental requirement for SAP has made it difficult to discern its possible involvement in mature iNKT cell functions. By using temporal Cre recombinase-mediated gene deletion to ablate SAP expression after completion of iNKT cell development, we demonstrate that SAP is essential for T-cell receptor (TCR)-induced iNKT cell cytotoxicity against T-cell and B-cell leukemia targets in vitro and iNKT-cell-mediated control of T-cell leukemia growth in vivo. These findings are not restricted to the murine system: silencing RNA-mediated suppression of SAP expression in human iNKT cells also significantly impairs TCR-induced cytolysis. Mechanistic studies reveal that iNKT cell killing requires the tyrosine kinase Fyn, a known SAP-binding protein. Furthermore, SAP expression is required within iNKT cells to facilitate their interaction with T-cell targets and induce reorientation of the microtubule-organizing center to the immunologic synapse (IS). Collectively, these studies highlight a novel and essential role for SAP during iNKT cell cytotoxicity and formation of a functional IS.

Small molecule targeting Cdc42-intersectin interaction disrupts Golgi organization and suppresses cell motility

Signaling through the Rho family of small GTPases has been intensely investigated for its crucial roles in a wide variety of human diseases. Although RhoA and Rac1 signaling pathways are frequently exploited with the aid of effective small molecule modulators, studies of the Cdc42 subclass have lagged because of a lack of such means. We have applied high-throughput in silico screening and identified compounds that are able to fit into the surface groove of Cdc42, which is critical for guanine nucleotide exchange factor binding. Based on the interaction between Cdc42 and intersectin (ITSN), a specific Cdc42 guanine nucleotide exchange factor, we discovered compounds that rendered ITSN-like interactions in the binding pocket. By using in vitro binding and imaging as well as biochemical and cell-based assays, we demonstrated that ZCL278 has emerged as a selective Cdc42 small molecule modulator that directly binds to Cdc42 and inhibits its functions. In Swiss 3T3 fibroblast cultures, ZCL278 abolished microspike formation and disrupted GM130-docked Golgi structures, two of the most prominent Cdc42-mediated subcellular events. ZCL278 reduces the perinuclear accumulation of active Cdc42 in contrast to NSC23766, a selective Rac inhibitor. ZCL278 suppresses Cdc42-mediated neuronal branching and growth cone dynamics as well as actin-based motility and migration in a metastatic prostate cancer cell line (i.e., PC-3) without disrupting cell viability. Thus, ZCL278 is a small molecule that specifically targets Cdc42-ITSN interaction and inhibits Cdc42-mediated cellular processes, thus providing a powerful tool for research of Cdc42 subclass of Rho GTPases in human pathogenesis, such as those of cancer and neurological disorders.

Bruton's tyrosine kinase regulates Shigella flexneri dissemination in HT-29 intestinal cells

Shigella flexneri is a Gram-negative intracellular pathogen that infects the intestinal epithelium and utilizes actin-based motility to spread from cell to cell. S. flexneri actin-based motility has been characterized in various cell lines, but studies in intestinal cells are limited. Here we characterized S. flexneri actin-based motility in HT-29 intestinal cells. In agreement with studies conducted in various cell lines, we showed that S. flexneri relies on neural Wiskott-Aldrich Syndrome protein (N-WASP) in HT-29 cells. We tested the potential role of various tyrosine kinases involved in N-WASP activation and uncovered a previously unappreciated role for Bruton's tyrosine kinase (Btk) in actin tail formation in intestinal cells. We showed that Btk depletion led to a decrease in N-WASP phosphorylation which affected N-WASP recruitment to the bacterial surface, decreased the number of bacteria displaying actin-based motility, and ultimately affected the efficiency of spread from cell to cell. Finally, we showed that the levels of N-WASP phosphorylation and Btk expression were increased in response to infection, which suggests that S. flexneri infection not only triggers the production of proinflammatory factors as previously described but also manipulates cellular processes required for dissemination in intestinal cells.

Disease-associated missense mutations in the EVH1 domain disrupt intrinsic WASp function causing dysregulated actin dynamics and impaired dendritic cell migration

Wiskott Aldrich syndrome (WAS), an X-linked immunodeficiency, results from loss-of-function mutations in the human hematopoietic cytoskeletal regulator gene WAS. Many missense mutations in the Ena Vasp homology1 (EVH1) domain preserve low-level WAS protein (WASp) expression and confer a milder clinical phenotype. Although disrupted binding to WASp-interacting protein (WIP) leads to enhanced WASp degradation in vivo, the intrinsic function of EVH1-mutated WASp is poorly understood. In the present study, we show that, despite mediating enhanced actin polymerization compared with wild-type WASp in vitro, EVH1 missense mutated proteins did not support full biologic function in cells, even when levels were restored by forced overexpression. Podosome assembly was aberrant and associated with dysregulated lamellipodia formation and impaired persistence of migration. At sites of residual podosome-associated actin polymerization, localization of EVH1-mutated proteins was preserved even after deletion of the entire domain, implying that WIP-WASp complex formation is not absolutely required for WASp localization. However, retention of mutant proteins in podosomes was significantly impaired and associated with reduced levels of WASp tyrosine phosphorylation. Our results indicate that the EVH1 domain is important not only for WASp stability, but also for intrinsic biologic activity in vivo.

Activation of Stat3 in endothelial cells following hypoxia-reoxygenation is mediated by Rac1 and protein Kinase C

Stat3 is an important transcription factor that regulates both proinflammatory and anit-apoptotic pathways in the heart. This study examined the mechanisms of activation of Stat3 in human endothelial cells following hypoxia/reoxygenation (H/R). By expression of constitutively active Rac1 mutant protein, and by RNA silencing of Rac1, we found that Stat3 forms a multiprotein complex with Rac1 and PKC in an H/R-dependent manner, which at least in part, appears to regulate Stat3 S727 phosphorylation. Selective inhibition of PKC with calphostin C produces a marked suppression of Stat3 S727 phosphorylation. The association of Stat3 with Rax1 occurs predominantly at the cell membrane, but also inside the nucleus, and occurs through the binding of the coiled-coil domain of Stat3 to the 54 NH(2)-terminal residues of Rac1. Transfection with a peptide comprising the NH(2)-terminal 17 amino acid residues of Rac1-dependent signaling pathways resulting in physical association between Rac1 and Stat3 and the formation of a novel multiprotein complex with PKC.

A mathematical model of CR3/TLR2 crosstalk in the context of Francisella tularensis infection

Complement Receptor 3 (CR3) and Toll-like Receptor 2 (TLR2) are pattern recognition receptors expressed on the surface of human macrophages. Although these receptors are essential components for recognition by the innate immune system, pathogen coordinated crosstalk between them can suppress the production of protective cytokines and promote infection. Recognition of the virulent Schu S4 strain of the intracellular pathogen Francisella tularensis by host macrophages involves CR3/TLR2 crosstalk. Although experimental data provide evidence that Lyn kinase and PI3K are essential components of the CR3 pathway that influences TLR2 activity, additional responsible upstream signaling components remain unknown. In this paper we construct a mathematical model of CR3 and TLR2 signaling in response to F. tularensis. After demonstrating that the model is consistent with experimental results we perform numerical simulations to evaluate the contributions that Akt and Ras-GAP make to ERK inhibition. The model confirms that phagocytosis-associated changes in the composition of the cell membrane can inhibit ERK activity and predicts that Akt and Ras-GAP synergize to inhibit ERK.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.

The small GTPase RhoA regulates the contraction of smooth muscle tissues by catalyzing the assembly of cytoskeletal signaling complexes at membrane adhesion sites

The activation of the small GTPase RhoA is necessary for ACh-induced actin polymerization and airway smooth muscle (ASM) contraction, but the mechanism by which it regulates these events is unknown. Actin polymerization in ASM is catalyzed by the actin filament nucleation activator, N-WASp and the polymerization catalyst, Arp2/3 complex. Activation of the small GTPase cdc42, a specific N-WASp activator, is also required for actin polymerization and tension generation. We assessed the mechanism by which RhoA regulates actin dynamics and smooth muscle contraction by expressing the dominant negative mutants RhoA T19N and cdc42 T17N, and non-phosphorylatable paxillin Y118/31F and paxillin ΔLD4 deletion mutants in SM tissues. Their effects were evaluated in muscle tissue extracts and freshly dissociated SM cells. Protein interactions and cellular localization were analyzed using proximity ligation assays (PLA), immunofluorescence, and GTPase and kinase assays. RhoA inhibition prevented ACh-induced cdc42 activation, N-WASp activation and the interaction of N-WASp with the Arp2/3 complex at the cell membrane. ACh induced paxillin phosphorylation and its association with the cdc42 GEFS, DOCK180 and α/βPIX. Paxillin tyrosine phosphorylation and its association with βPIX were RhoA-dependent, and were required for cdc42 activation. The ACh-induced recruitment of paxillin and FAK to the cell membrane was dependent on RhoA. We conclude that RhoA regulates the contraction of ASM by catalyzing the assembly and activation of cytoskeletal signaling modules at membrane adhesomes that initiate signaling cascades that regulate actin polymerization and tension development in response to contractile agonist stimulation. Our results suggest that the RhoA-mediated assembly of adhesome complexes is a fundamental step in the signal transduction process in response to agonist -induced smooth muscle contraction.

The role of Ras-ERK-IL-1β signaling pathway in upregulation of elastin expression by human coronary artery smooth muscle cells cultured in 3D scaffolds

Incorporation of endogenous elastin, a key structural component of the vascular extracellular matrix (ECM), is an important requirement for engineered vascular tissues. In addition to providing elastic recoil of the tissue, elastin influences cell function and promotes cell signaling by interacting with specific cell surface receptors. Although progress has been made in understanding the mechanisms of in vivo elastin expression and incorporation into fibers, it is notably absent from engineered vessels. Recently we showed that the three-dimensional (3D) scaffold topography was able to upregulate elastin synthesis by human coronary artery smooth muscle cells (HCASMC). The present study was undertaken to explore the molecular mechanisms responsible for 3D scaffold-induced elastin gene transcription. Here, we show several lines of evidence that signal transduction pathway leading to elastin gene expression by HCASMC cultured in synthetic 3D scaffolds to be strikingly different from two-dimensional (2D) surfaces. In 3D scaffolds, α5β1 integrin engagement by HCASMC was significantly reduced and the putative focal adhesion kinase (FAK) was poorly phosphorylated concomitant with FAK and protein tyrosine kinase Pyk2 downregulation. FAK-associated adhesion proteins vinculin and paxillin were also significantly downregulated by the 3D scaffold topography. Furthermore, contrary to 2D cultures, HCASMC cultured on 3D scaffolds had no Rho activation suggesting pliability of the elastomeric synthetic scaffold. Elastin expression in 3D cultures followed Ras-ERK1/2 signal transduction pathway and was further dependent on endogenously expressed interleukin-1β (IL-1β). Blocking of ERK1/2 activation using a pharmacologic inhibitor reduced both elastin and IL-1β gene expressions in 3D cultures. Transient transfection of IL-1β using siRNA, however, did not affect ERK1/2 activation but downregulated elastin gene expression suggesting that endogenous IL-1β acts downstream from ERK1/2. Taken together, results of the present study provide evidence that endogenous IL-1β play a role in elastin gene upregulation and, that this upregulation is mediated by the Ras-ERK1/2 pathway in 3D cultures.

Nck and Cdc42 co-operate to recruit N-WASP to promote FcγR-mediated phagocytosis

The adaptor protein Nck has been shown to link receptor ligation to actin-based signalling in a diverse range of cellular events, such as changes in cell morphology and motility. It has also been implicated in phagocytosis. However, its molecular role in controlling actin remodelling associated with phagocytic uptake remains to be clarified. Here, we show that Nck, which is recruited to phagocytic cups, is required for Fcγ receptor (FcγR)- but not complement receptor 3 (CR3)-induced phagocytosis. Nck recruitment in response to FcγR ligation is mediated by the phosphorylation of tyrosine 282 and 298 in the ITAM motif in the cytoplasmic tail of the receptor. In the absence of FcγR phosphorylation, there is also no recruitment of N-WASP or Cdc42 to phagocytic cups. Nck promotes FcγR-mediated phagocytosis by recruiting N-WASP to phagocytic cups. Efficient phagocytosis, however, only occurs, if the CRIB domain of N-WASP can also interact with Cdc42. Our observations demonstrate that Nck and Cdc42 collaborate to stimulate N-WASP-dependent FcγR-mediated phagocytosis.

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.

Cortactin, an actin binding protein, regulates GLUT4 translocation via actin filament remodeling

Insulin regulates glucose uptake into fat and skeletal muscle cells by modulating the translocation of GLUT4 between the cell surface and interior. We investigated a role for cortactin, a cortical actin binding protein, in the actin filament organization and translocation of GLUT4 in Chinese hamster ovary (CHO-GLUT4myc) and L6-GLUT4myc myotube cells. Overexpression of wild-type cortactin enhanced insulin-stimulated GLUT4myc translocation but did not alter actin fiber formation. Conversely, cortactin mutants lacking the Src homology 3 (SH3) domain inhibited insulin-stimulated formation of actin stress fibers and GLUT4 translocation similar to the actin depolymerizing agent cytochalasin D. Wortmannin, genistein, and a PP1 analog completely blocked insulin-induced Akt phosphorylation, formation of actin stress fibers, and GLUT4 translocation indicating the involvement of both PI3-K/Akt and the Src family of kinases. The effect of these inhibitors was even more pronounced in the presence of overexpressed cortactin suggesting that the same pathways are involved. Knockdown of cortactin by siRNA did not inhibit insulin-induced Akt phosphorylation but completely inhibited actin stress fiber formation and glucose uptake. These results suggest that the actin binding protein cortactin is required for actin stress fiber formation in muscle cells and that this process is absolutely required for translocation of GLUT4-containing vesicles to the plasma membrane.

The chemotactic defect in wiskott-Aldrich syndrome macrophages is due to the reduced persistence of directional protrusions

Wiskott-Aldrich syndrome protein (WASp) is an actin nucleation promoting factor that is required for macrophages to directionally migrate towards various chemoattractants. The chemotaxis defect of WASp-deficient cells and its activation by Cdc42 in vivo suggest that WASp plays a role in directional sensing, however, its precise role in macrophage chemotaxis is still unclear. Using shRNA-mediated downregulation of WASp in the murine monocyte/macrophage cell line RAW/LR5 (shWASp), we found that WASp was responsible for the initial wave of actin polymerization in response to global stimulation with CSF-1, which in Dictyostelium discoideum amoebae and carcinoma cells has been correlated with the ability to migrate towards chemoattractants. Real-time monitoring of shWASp cells, as well as WASp^−/− bone marrow-derived macrophages (BMMs), in response to a CSF-1 gradient revealed that the protrusions from WASp-deficient cells were directional, showing intact directional sensing. However, the protrusions from WASp-deficient cells demonstrated reduced persistence compared to their respective control shRNA and wild-type cells. Further examination showed that tyrosine phosphorylation of WASp was required for both the first wave of actin polymerization following global CSF-1 stimulation and proper directional responses towards CSF-1. Importantly, the PI3K, Rac1 and WAVE2 proteins were incorporated normally in CSF-1 – elicited protrusions in the absence of WASp, suggesting that membrane protrusion driven by the WAVE2 complex signaling is intact. Collectively, these results suggest that WASp and its phosphorylation play critical roles in coordinating the actin cytoskeleton rearrangements necessary for the persistence of protrusions required for directional migration of macrophages towards CSF-1.

Actin cytoskeleton remodelling by sex steroids in neurones

Cell morphology and its interaction with the extracellular environment are integrated processes involving a number of intracellular controllers orchestrating cytoskeletal proteins and their interaction with the cell membrane and anchorage proteins. Sex steroids are effective regulators of cell morphology and tissue organisation, and recent evidence indicates that this is obtained through the regulation of the actin cytoskeleton. Intriguingly, many of these regulatory actions related to cell morphology are achieved through the rapid, nonclassical signalling of sex steroid receptors to kinase cascades, independently from nuclear alteration of gene expression or protein synthesis. The identification of the mechanistic basis for these rapid actions on cell cytoskeleton has special relevance for the characterisation of the effects of sex steroids under physiological conditions, such as for the development of neurone/neurone interconnections and dendritic spine density. This is considered to be critical for gender-specific differences in brain function and dysfunction. Recent advancements in the characterisation of the molecular basis of the extranuclear signalling of sex steroids help to clarify the role of oestrogen and progesterone in the brain, and may turn out to be of relevance for clinical purposes. This review highlights the regulatory effects of oestrogens and progesterone on actin cytoskeleton and neurone morphology, as well as recent progresses in the characterisation of these mechanisms, providing insights and working hypotheses on possible clinical applications for the modulation of these pathways in the central nervous system.

Tyrosine phosphorylation of WASP promotes calpain-mediated podosome disassembly

Podosomes are actin-based adhesions involved in migration of cells that have to cross tissue boundaries such as myeloid cells. The Wiskott Aldrich Syndrome Protein regulates de novo actin polymerization during podosome formation and it is cleaved by the protease calpain during podosome disassembly. The mechanisms that may induce the Wiskott Aldrich Syndrome Protein cleavage by calpain remain undetermined. We now report that in myeloid cells, tyrosine phosphorylation of the Wiskott Aldrich Syndrome Protein-tyrosine291 (Human)/tyrosine293 (mouse) not only enhances Wiskott Aldrich Syndrome Protein-mediated actin polymerization but also promotes its calpain-dependent degradation during podosome disassembly. We also show that activation of the Wiskott Aldrich Syndrome Protein leading to podosome formation occurs independently of tyrosine phosphorylation in spleen-derived dendritic cells. We conclude that tyrosine phosphorylation of the Wiskott Aldrich Syndrome Protein integrates dynamics of actin and cell adhesion proteins during podosome disassembly required for mobilization of myeloid cells during the immune response.

Bacterial subversion of host actin dynamics at the plasma membrane

Invasion of non-phagocytic cells by a number of bacterial pathogens involves the subversion of the actin cytoskeletal remodelling machinery to produce actin-rich cell surface projections designed to engulf the bacteria. The signalling that occurs to induce these actin-rich structures has considerable overlap among a diverse group of bacteria. The molecular organization within these structures act in concert to internalize the invading pathogen. This dynamic process could be subdivided into three acts - actin recruitment, engulfment, and finally, actin disassembly/internalization. This review will present the current state of knowledge of the molecular processes involved in each stage of bacterial invasion, and provide a perspective that highlights the temporal and spatial control of actin remodelling that occurs during bacterial invasion.

Regulation of WASp by phosphorylation: Activation or other functions?

Wiskott-Aldrich Syndrome protein (WASp) is an actin nucleation-promoting factor that regulates actin polymerisation via the Arp2/3 complex. Its mutation in human syndromes has led to extensive studies on the regulation and activities of this molecule. Several mechanisms for the regulation of WASp activity have been proposed, however, the role of tyrosine phosphorylation remains controversial, particularly due to inconsistencies between results obtained through biochemical and cell biological approaches. In this mini-review, we are addressing the major aspects of WASp regulation with an emphasis on the role of tyrosine phosphorylation on WASp activities.

Regulation of tyrosine phosphorylation in macrophage phagocytosis and chemotaxis

Macrophages display a large variety of surface receptors that are critical for their normal cellular functions in host defense, including finding sites of infection (chemotaxis) and removing foreign particles (phagocytosis). However, inappropriate regulation of these processes can lead to human diseases. Many of these receptors utilize tyrosine phosphorylation cascades to initiate and terminate signals leading to cell migration and clearance of infection. Actin remodeling dominates these processes and many regulators have been identified. This review focuses on how tyrosine kinases and phosphatases regulate actin dynamics leading to macrophage chemotaxis and phagocytosis.

Functional cooperation between the proteins Nck and ADAP is fundamental for actin reorganization

T cell antigen receptor (TCR) activation triggers profound changes in the actin cytoskeleton. In addition to controlling cellular shape and polarity, this process regulates vital T cell responses, such as T cell adhesion, motility, and proliferation. These depend on the recruitment of the signaling proteins Nck and Wiskott-Aldrich syndrome protein (WASp) to the site of TCR activation and on the functional properties of the adapter proteins linker for activation of T cells (LAT) and SH2-domain-containing leukocyte protein of 76 kDa (SLP76). We now demonstrate that Nck is necessary but insufficient for the recruitment of WASp. We show that two pathways lead to SLP76-dependent actin rearrangement. One requires the SLP76 acidic domain, crucial to association with the Nck SH2 domain, and another requires the SLP76 SH2 domain, essential for interaction with the adhesion- and degranulation-promoting adapter protein ADAP. Functional cooperation between Nck and ADAP mediates SLP76-WASp interactions and actin rearrangement. We also reveal the molecular mechanism linking ADAP to actin reorganization.

β1 integrins regulate fibroblast chemotaxis through control of N-WASP stability

Chemotactic migration of fibroblasts towards growth factors, such as during development and wound healing, requires precise spatial coordination of receptor signalling. However, the mechanisms regulating this remain poorly understood. Here, we demonstrate that β1 integrins are required both for fibroblast chemotaxis towards platelet-derived growth factor (PDGF) and growth factor-induced dorsal ruffling. Mechanistically, we show that β1 integrin stabilises and spatially regulates the actin nucleating endocytic protein neuronal Wiskott–Aldrich syndrome protein (N-WASP) to facilitate PDGF receptor traffic and directed motility. Furthermore, we show that in intact cells, PDGF binding leads to rapid activation of β1 integrin within newly assembled actin-rich membrane ruffles. Active β1 in turn controls assembly of N-WASP complexes with both Cdc42 and WASP-interacting protein (WIP), the latter of which acts to stabilise the N-WASP. Both of these protein complexes are required for PDGF internalisation and fibroblast chemotaxis downstream of β1 integrins. This represents a novel mechanism by which integrins cooperate with growth factor receptors to promote localised signalling and directed cell motility.

Syk regulates multiple signaling pathways leading to CX3CL1 chemotaxis in macrophages

Several studies have clearly established the importance of the interaction between macrophages and CX3CL1 in the progression of disease. A previous study demonstrated that Syk was required for CX3CL1-mediated actin polymerization and chemotaxis. Here, we delineated the signaling cascade of Syk-mediated cell migration in response to CX3CL1. Inhibition of Syk in bone marrow-derived macrophages or reduction of Syk expression using siRNA in RAW/LR5 cells indicated that Syk was required for the activation of PI3K, Cdc42, and Rac1. Also, reduction in WASP or WAVE2 levels, common downstream effectors of Cdc42 or Rac1, resulted in impaired cell migration to CX3CL1. Syk indirectly regulated WASP tyrosine phosphorylation through Cdc42 activation. Altogether, our data identify that Syk mediated chemotaxis toward CX3CL1 by regulating both Rac1/WAVE2 and Cdc42/WASP pathways, whereas Src family kinases were required for proper WASP tyrosine phosphorylation.

Cdc42-interacting protein 4 promotes breast cancer cell invasion and formation of invadopodia through activation of N-WASp

In the earliest stages of metastasis, breast cancer cells must reorganize the cytoskeleton to affect cell shape change and promote cell invasion and motility. These events require the cytoskeletal regulators Cdc42 and Rho, their effectors such as N-WASp/WAVE, and direct inducers of actin polymerization such as Arp2/3. Little consideration has been given to molecules that shape the cell membrane. The F-BAR proteins CIP4, TOCA-1, and FBP17 generate membrane curvature and act as scaffolding proteins for activated Cdc42 and N-WASp. We found that expression of CIP4, but not TOCA-1 or FBP17, was increased in invasive breast cancer cell lines in comparison with weakly or noninvasive breast cancer cell lines. Endogenous CIP4 localized to the leading edge of migrating cells and to invadopodia in cells invading gelatin. Because CIP4 serves as a scaffolding protein for Cdc42, Src, and N-WASp, we tested whether loss of CIP4 could result in decreased N-WASp function. Interaction between CIP4 and N-WASp was epidermal growth factor responsive, and CIP4 silencing by small interfering RNA caused decreased tyrosine phosphorylation of N-WASp at a Src-dependent activation site (Y256). CIP4 silencing also impaired the migration and invasion of MDA-MB-231 cells and was associated with decreased formation of invadopodia and gelatin degradation. This study presents a new role for CIP4 in the promotion of migration and invasion of MDA-MB-231 breast cancer cells and establishes the contribution of F-BAR proteins to cancer cell motility and invasion.

Extra-nuclear signaling of ERalpha to the actin cytoskeleton in the central nervous system

Cell morphology is controlled by a complex and redundant array of intracellular signaling pathways devoted to the regulation of the actin cytoskeleton and of its relationship with the cell membrane and the extracellular matrix. Sex steroids are effective regulators of cell morphology and tissue organization, and recent evidence indicates that this is obtained through the regulation of the cytoskeleton. Intriguingly, many of these regulatory actions related to cell morphology are achieved through rapid, non-classical signaling of sex steroid receptors to kinase cascades, independently from nuclear alteration of gene expression or protein synthesis. The identification of the mechanistic basis for these rapid actions on cell cytoskeleton has special relevance for the characterization of the effects of sex steroids in physiological conditions, such as their role in the control of brain cell remodeling. Brain cell morphology is controlled by estrogens that regulate the development of neuron/neuron interconnections and dendritic spine density. This is thought to be critical for gender-specific differences in brain function and dysfunction. The recent advancements in the characterization of the molecular basis of the extra-nuclear signaling of estrogen helps to understand the role of estrogen in the brain, and may in the future turn out to be of relevance for clinical purposes. This review highlights the regulatory effects on the cytoskeleton and cell morphology of estrogens as well as the recent advances in the characterization of these mechanisms, providing insights and working hypotheses on possible clinical applications for the modulation of these pathways in the central nervous system.

Physical mechanisms of signal integration by WASP family proteins

The proteins of the Wiskott-Aldrich syndrome protein (WASP) family are activators of the ubiquitous actin nucleation factor, the Arp2/3 complex. WASP family proteins contain a C-terminal VCA domain that binds and activates the Arp2/3 complex in response to numerous inputs, including Rho family GTPases, phosphoinositide lipids, SH3 domain-containing proteins, kinases, and phosphatases. In the archetypal members of the family, WASP and N-WASP, these signals are integrated through two levels of regulation, an allosteric autoinhibitory interaction, in which the VCA is sequestered from the Arp2/3 complex, and dimerization/oligomerization, in which multi-VCA complexes are better activators of the Arp2/3 complex than monomers. Here, we review the structural, biochemical, and biophysical details of these mechanisms and illustrate how they work together to control WASP activity in response to multiple inputs. These regulatory principles, derived from studies of WASP and N-WASP, are likely to apply broadly across the family.

A nucleator arms race: cellular control of actin assembly

For over a decade, the actin-related protein 2/3 (ARP2/3) complex, a handful of nucleation-promoting factors and formins were the only molecules known to directly nucleate actin filament formation de novo. However, the past several years have seen a surge in the discovery of mammalian proteins with roles in actin nucleation and dynamics. Newly recognized nucleation-promoting factors, such as WASP and SCAR homologue (WASH), WASP homologue associated with actin, membranes and microtubules (WHAMM), and junction-mediating regulatory protein (JMY), stimulate ARP2/3 activity at distinct cellular locations. Formin nucleators with additional biochemical and cellular activities have also been uncovered. Finally, the Spire, cordon-bleu and leiomodin nucleators have revealed new ways of overcoming the kinetic barriers to actin polymerization.

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.

Regulation of podosome dynamics by WASp phosphorylation: implication in matrix degradation and chemotaxis in macrophages

Podosomes, adhesion structures capable of matrix degradation, have been linked with the ability of cells to perform chemotaxis and invade tissues. Wiskott-Aldrich Syndrome protein (WASp), an effector of the RhoGTPase Cdc42 and a Src family kinase substrate, regulates macrophage podosome formation. In this study, we demonstrate that WASp is active in podosomes by using TIRF-FRET microscopy. Pharmacological and RNA interference approaches suggested that continuous WASp activity is required for podosome formation and function. Rescue experiments using point mutations demonstrate an absolute requirement for Cdc42 binding to WASp in podosome formation. Although tyrosine phosphorylation was not absolutely required for podosome formation, phosphorylation did regulate the rate of podosome nucleation and actin filament stability. Importantly, WASp tyrosine phosphorylation does not alter WASp activation, instead phosphorylation appears to be important for the restriction of WASp activity to podosomes. In addition, the matrix-degrading ability of cells requires WASp phosphorylation. Chemotactic responses to CSF-1 were also attenuated in the absence of endogenous WASp, which could not be rescued with either tyrosine mutation. These results suggest a more complex role for tyrosine phosphorylation than simply in the regulation of WASp activity, and suggest a link between podosome dynamics and macrophage migration.