Identification of N-WASP homologs in human and rat brain

PRMT5 Interacting Partners and Substrates in Oligodendrocyte Lineage Cells

The protein arginine methyl transferase PRMT5 is an enzyme expressed in oligodendrocyte lineage cells and responsible for the symmetric methylation of arginine residues on histone tails. Previous work from our laboratory identified PRMT5 as critical for myelination, due to its transcriptional regulation of genes involved in survival and early stages of differentiation. However, besides its nuclear localization, PRMT5 is found at high levels in the cytoplasm of several cell types, including oligodendrocyte progenitor cells (OPCs) and yet, its interacting partners in this lineage, remain elusive. By using mass spectrometry on protein eluates from extracts generated from primary oligodendrocyte lineage cells and immunoprecipitated with PRMT5 antibodies, we identified 1196 proteins as PRMT5 interacting partners. These proteins were related to molecular functions such as RNA binding, ribosomal structure, cadherin and actin binding, nucleotide and protein binding, and GTP and GTPase activity. We then investigated PRMT5 substrates using iTRAQ-based proteomics on cytosolic and nuclear protein extracts from CRISPR-PRMT5 knockdown immortalized oligodendrocyte progenitors compared to CRISPR-EGFP controls. This analysis identified a similar number of peptides in the two subcellular fractions and a total number of 57 proteins with statistically decreased symmetric methylation of arginine residues in the CRISPR-PRMT5 knockdown compared to control. Several PRMT5 substrates were in common with cancer cell lines and related to RNA processing, splicing and transcription. In addition, we detected ten oligodendrocyte lineage specific substrates, corresponding to proteins with high expression levels in neural tissue. They included: PRC2C, a proline-rich protein involved in methyl-RNA binding, HNRPD an RNA binding protein involved in regulation of RNA stability, nuclear proteins involved in transcription and other proteins related to migration and actin cytoskeleton. Together, these results highlight a cell-specific role of PRMT5 in OPC in regulating several other cellular processes, besides RNA splicing and metabolism.

Role of ARP2/3 Complex-Driven Actin Polymerization in RSV Infection

Respiratory syncytial virus (RSV) is the leading viral agent causing bronchiolitis and pneumonia in children under five years old worldwide. The RSV infection cycle starts with macropinocytosis-based entry into the host airway epithelial cell membrane, followed by virus transcription, replication, assembly, budding, and spread. It is not surprising that the host actin cytoskeleton contributes to different stages of the RSV replication cycle. RSV modulates actin-related protein 2/3 (ARP2/3) complex-driven actin polymerization for a robust filopodia induction on the infected lung epithelial A549 cells, which contributes to the virus’s budding, and cell-to-cell spread. Thus, a comprehensive understanding of RSV-induced cytoskeletal modulation and its role in lung pathobiology may identify novel intervention strategies. This review will focus on the role of the ARP2/3 complex in RSV’s pathogenesis and possible therapeutic targets to the ARP2/3 complex for RSV.

Molecular and cellular impact of Psoriasin (S100A7) on the healing of human wounds

Psoriasin, which is also known as S100A7, is a member of the S100 protein family, a group of calcium-responsive signalling proteins. Psoriasin expression remains high in patients with psoriasis, whereas it is downregulated in patients with invasive breast carcinoma. This observation suggests that this protein may be a notable marker of keratinocyte function and differentiation during wound healing. The aim of the present study was to determine the cellular impact of Psoriasin in keratinocytes, which are the primary cell type associated with wound healing. Psoriasin expression in wound tissues was examined using reverse transcription-quantitative polymerase chain reaction and immunochemical staining. Knockdown of Psoriasin in HaCaT cells was performed using anti-Psoriasin ribozyme transgenes and the effect on growth, adhesion and migration of keratinocytes was subsequently determined using in vitro cellular functional assays. Psoriasin expression is upregulated in wounds, particularly at the wound edges. The present study demonstrated that Psoriasin is expressed in keratinocytes and is a fundamental regulator of keratinocyte migration. Significant increases in the rate of keratinocyte adhesion, migration and growth were observed in Psoriasin-deficient cells (P<0.01 vs. control). Application of small inhibitors identified the potential association of neural Wiskott-Aldrich syndrome protein, focal adhesion primase and rho-associated protein kinase signalling pathways with Psoriasin-regulated cell adhesion and motility. In conclusion, Psoriasin serves an important role in the wound healing process, suggesting that it may be utilized as a potential wound healing biomarker.

Aldolase sequesters WASP and affects WASP/Arp2/3-stimulated actin dynamics

In addition to its roles in sugar metabolism, fructose-1,6-bisphosphate aldolase (aldolase) has been implicated in cellular functions independent from these roles, termed "moonlighting functions." These moonlighting functions likely involve the known aldolase-actin interaction, as many proteins with which aldolase interacts are involved in actin-dependent processes. Specifically, aldolase interacts both in vitro and in cells with Wiskott-Aldrich Syndrome Protein (WASP), a protein involved in controlling actin dynamics, yet the function of this interaction remains unknown. Here, the effect of aldolase on WASP-dependent processes in vitro and in cells is investigated. Aldolase inhibits WASP/Arp2/3-dependent actin polymerization in vitro. In cells, knockdown of aldolase results in a decreased rate of cell motility and cell spreading, two WASP-dependent processes. Expression of exogenous aldolase rescues these defects. Whether these effects of aldolase on WASP-dependent processes were due to aldolase catalysis or moonlighting functions is tested using aldolase variants defective in either catalytic or actin-binding activity. While the actin-binding deficient aldolase variant is unable to inhibit actin polymerization in vitro and is unable to rescue cell motility defects in cells, the catalytically inactive aldolase is able to perform these functions, providing evidence that aldolase moonlighting plays a role in WASP-mediated processes.

The effect of primary particle size on biodistribution of inhaled gold nano-agglomerates

Airborne engineered nanoparticles undergo agglomeration, and careful distinction must be made between primary and agglomerate size of particles, when assessing their health effects. This study compares the effects on rats undergoing 15-day inhalation exposure to airborne agglomerates of gold nanoparticles (AuNPs) of similar size distribution and number concentration (1 × 10(6) particles/cm(3)), but two different primary diameters of 7 nm or 20 nm. Inhalation of agglomerates containing 7-nm AuNPs resulted in highest deposition by mass concentration in the lungs, followed by brain regions including the olfactory bulb, hippocampus, striatum, frontal cortex, entorhinal cortex, septum, cerebellum; aorta, esophagus, and kidney. Eight organs/tissues especially the brain retained greater mass concentration of Au after inhalation exposure to agglomerates of 7-nm than 20-nm AuNPs. Macrophage mediated escalation followed by fecal excretion is the major pathway of clearing inhaled AuNPs in the lungs. Microarray analyses of the hippocampus showed mostly downregulated genes, related to the cytoskeleton and neurite outgrowth. Together, results in this study indicate disintegration of nanosized agglomerates after inhalation and show impact of primary size of particles on subsequent biodistribution.

Enteropathogenic Escherichia coli and vaccinia virus do not require the family of WASP-interacting proteins for pathogen-induced actin assembly

The human pathogens enteropathogenic Escherichia coli (EPEC) and vaccinia virus trigger actin assembly in host cells by activating the host adaptor Nck and the actin nucleation promoter neural Wiskott-Aldrich syndrome protein (N-WASP). EPEC translocates effector molecules into host cells via type III secretion, and the interaction between the translocated intimin receptor (Tir) and the bacterial membrane protein intimin stimulates Nck and N-WASP recruitment, leading to the formation of actin pedestals beneath adherent bacteria. Vaccinia virus also recruits Nck and N-WASP to generate actin tails that promote cell-to-cell spread of the virus. In addition to Nck and N-WASP, WASP-interacting protein (WIP) localizes to vaccinia virus tails, and inhibition of actin tail formation upon ectopic expression of WIP mutants led to the suggestion that WIP is required for this process. Similar studies of WIP mutants, however, did not affect the ability of EPEC to form actin pedestals, arguing against an essential role for WIP in EPEC-induced actin assembly. In this study, we demonstrate that Nck and N-WASP are normally recruited by vaccinia virus and EPEC in the absence of WIP, and neither WIP nor the WIP family members CR16 and WIRE/WICH are essential for pathogen induced actin assembly. In addition, although Nck binds EPEC Tir directly, N-WASP is required for its localization during pedestal formation. Overall, these data highlight similar pathogenic strategies shared by EPEC and vaccinia virus by demonstrating a requirement for both Nck and N-WASP, but not WIP or WIP family members in pathogen-induced actin assembly.

The adaptor protein Nck2 mediates Slit1-induced changes in cortical neuron morphology

Slits are multifunctional guidance cues, capable of triggering neurite repulsion, extension, or branching, depending on cell type and developmental context. While the Robo family of Slit receptors is a well-established mediator of axon repulsion, a role for Robos in Slit-mediated neurite growth and branching is not well defined, and the signaling molecules that link Robo to the cytoskeletal changes that drive neurite outgrowth are not well characterized in vertebrates. We show that Slit stimulates cortical dendrite branching, and we report that Slit also triggers a robust increase in the length of cortical axons in vitro. Moreover, neurons derived from Robo1; Robo2 deficient mice do not display an increase in neurite length, indicating that endogenous Robos mediate Slit's growth-promoting effects on both axons and dendrites. We also demonstrate that the SH2/SH3 adaptor proteins Nck1 and Nck2 bind to Robo via an atypical SH3-mediated mechanism. Furthermore, we show that only Nck2 is required for the Slit-induced changes in cortical neuron morphology in vitro. These findings indicate a specific role for Nck2 in linking Robo activation to the cytoskeleton rearrangements that shape cortical neuron morphology.

Systems analysis of endothelial cell plasma membrane proteome of rat lung microvasculature

BACKGROUND

Endothelial cells line all blood vessels to form the blood-tissue interface which is critical for maintaining organ homeostasis and facilitates molecular exchange. We recently used tissue subcellular fractionation combined with several multi-dimensional mass spectrometry-based techniques to enhance identification of lipid-embedded proteins for large-scale proteomic mapping of luminal endothelial cell plasma membranes isolated directly from rat lungs in vivo. The biological processes and functions of the proteins expressed at this important blood-tissue interface remain unexplored at a large scale.

RESULTS

We performed an unbiased systems analysis of the endothelial cell surface proteome containing over 1800 proteins to unravel the major functions and pathways apparent at this interface. As expected, many key functions of plasma membranes in general (i.e., cell surface signaling pathways, cytoskeletal organization, adhesion, membrane trafficking, metabolism, mechanotransduction, membrane fusion, and vesicle-mediated transport) and endothelial cells in particular (i.e., blood vessel development and maturation, angiogenesis, regulation of endothelial cell proliferation, protease activity, and endocytosis) were significantly overrepresented in this proteome. We found that endothelial cells express multiple proteins that mediate processes previously reported to be restricted to neuronal cells, such as neuronal survival and plasticity, axon growth and regeneration, synaptic vesicle trafficking and neurotransmitter metabolic process. Surprisingly, molecular machinery for protein synthesis was also detected as overrepresented, suggesting that endothelial cells, like neurons, can synthesize proteins locally at the cell surface.

CONCLUSION

Our unbiased systems analysis has led to the potential discovery of unexpected functions in normal endothelium. The discovery of the existence of protein synthesis at the plasma membrane in endothelial cells provides new insight into the blood-tissue interface and endothelial cell surface biology.

Mutagenesis of the Shigella flexneri autotransporter IcsA reveals novel functional regions involved in IcsA biogenesis and recruitment of host neural Wiscott-Aldrich syndrome protein

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

Chronic treatment with aripiprazole induces differential gene expression in the rat frontal cortex

Chronic treatment of antipsychotic drugs can modulate gene expression in the brain, which may underscore their clinical efficacy. Aripiprazole is the first approved antipsychotic drug of the class of dopamine D2 receptor partial agonist, which has been shown to have similar efficacy and favourable side-effects profile compared to other antipsychotic drugs. This study aimed to identify differential gene expression induced by chronic treatment of aripiprazole. We used microarray-based gene expression profiling technology, real-time quantitative PCR and Western blot analysis to identify differentially expressed genes in the frontal cortex of rats under 4 wk treatment of aripiprazole (10 mg/kg). We were able to detect ten up-regulated genes, including early growth response gene 1, 2, 4 (Egr1, Egr2, Egr4), chromobox homolog 7 (Cbx7), cannabinoid receptor (Cnr1), catechol-O-methyltransferase (Comt), protein phosphatase 2c, magnesium dependent (Ppm2c), tachykinin receptor 3 (Tacr3), Wiscott-Aldrich syndrome-like gene (Wasl) and DNA methyltransferase 3a (Dnmt3a). Our data indicate that chronic administration of aripiprazole can induce differential expression of genes involved in transcriptional regulation and chromatin remodelling and genes implicated in the pathogenesis of psychosis.

Coexpression of Actin-Related Protein 2 and Wiskott-Aldrich Syndrome Family Verproline-Homologous Protein 2 in Adenocarcinoma of the Lung

PURPOSE

Highly invasive and metastatic cancer cells, such as adenocarcinoma of the lung cells, form irregular protrusions by assembling a branched network of actin filaments. In mammalian cells, the actin-related protein 2 and 3 (Arp2/3) complex initiates actin assembly to form lamellipodial protrusions by binding to Wiskott-Aldrich syndrome (WASP)/WASP family verproline-homologous protein 2 (WAVE2). In this study, colocalization of Arp2 and WAVE2 in adenocarcinoma of the lung was investigated to elucidate its prognostic value.

EXPERIMENTAL DESIGN

Immunohistochemical staining of Arp2 and WAVE2 was done on mirror sections of 115 adenocarcinomas of the lung from pathologic stage IA to IIIA classes. Kaplan-Meier disease-free survival and overall survival curves were analyzed to determine the prognostic significance of the coexpression of Arp2 and WAVE2.

RESULTS

Immunoreactivity for both Arp2 and WAVE2 was detected in the same cancer cells in 78 (67.8%) of the 115 lung cancer specimens. The proportion of cancer cells expressing both Arp2 and WAVE2 was significantly higher in cases with lymph-node metastasis (P = 0.0046), and significantly lower in bronchioloalveolar carcinomas (P < 0.0001). The patients whose cancer cells coexpressed them had a shorter disease-free survival time (P < 0.0001) and overall survival time (P < 0.0001). Multivariate Cox regression analysis revealed that coexpression of Arp2 and WAVE2 is an independent risk factor for tumor recurrence.

CONCLUSIONS

Coexpression of Arp2 and WAVE2 is correlated with poorer patient outcome, and may be involved in the mechanism of cancer metastasis.

WICH, a member of WASP-interacting protein family, cross-links actin filaments

In yeast, Verprolin plays an important role in rearrangement of the actin cytoskeleton. There are three mammalian homologues of Verprolin, WIP, CR16, and WICH, and all of them bind actin and Wiskott-Aldrich syndrome protein (WASP) and/or neural-WASP. Here, we describe a novel function of WICH. In vitro co-sedimentation analysis revealed that WICH not only binds to actin filaments but also cross-links them. Fluorescence and electron microscopy detected that this cross-linking results in straight bundled actin filaments. Overexpression of WICH alone in cultured fibroblast caused the formation of thick actin fibers. This ability of WICH depended on its own actin cross-linking activity. Importantly, the actin cross-linking activity of WICH was modified through a direct association with N-WASP. Taken together, these data suggest that WICH induces a bundled form of actin filament with actin cross-linking activity and the association with N-WASP suppresses that activity. WICH thus appears to be a novel actin bundling protein.

Zyxin, axin, and Wiskott-Aldrich syndrome protein are adaptors that link the cadherin/catenin protein complex to the cytoskeleton at adherens junctions in the seminiferous epithelium of the rat testis

During spermatogenesis, the movement of germ cells across the seminiferous epithelium is associated with extensive junction restructuring. Yet the underlying mechanism (or mechanisms) that regulates these events is largely unknown. If the molecular architecture of the cell-cell actin-based adherens junction (AJ), such as ectoplasmic specialization (ES) and tubulobulbar complex- two testis-specific AJ types, is known, many functional mechanistic studies can be designed. We thus undertook an investigation to study 3 adaptors in the seminiferous epithelium: zyxin, axin, and Wiskott-Aldrich syndrome protein (WASP). All 3 adaptors were shown to be products of Sertoli and germ cells. Zyxin was shown to be a stage-specific protein that was most prominent during stages V-VII and restricted mostly to pachytene spermatocytes, but it could also be detected at the site of basal and apical ectoplasmic specialization (ES). Zyxin, axin, and WASP were shown to be structurally linked to the N-cadherin/beta-catenin/alpha-actinin/actin complex but not to the nectin-3/afadin or the beta 1-integrin-mediated protein complexes. Interestingly, zyxin, axin, and WASP are also structurally linked to vimentin (an intermediate filament protein) and alpha-tubulin (the subunit of a microtubule), which suggests that they have a role (or roles) in the regulation of the dynamics of the desmosome-like junction and microtubule. These results illustrate that zyxin, axin, and WASP are adaptors in both AJs and intermediate filament-based desmosome-like junctions. This raises the possibility that classic cadherins are also associated with vimentin-based intermediate filaments via these adaptors in the testis. While virtually no N-cadherin was found to associate with vimentin in the seminiferous tubules, it did associate with vimentin when testis lysates were used. Interestingly, about 5% of the E-cadherin associated with vimentin in isolated seminiferous tubules, and about 50% of the E-cadherin in the testis used vimentin as its attachment site. These data suggest that cadherins in the testis, unlike those in other epithelia, use different attachment sites to anchor the cadherin/catenin complex to the cytoskeleton. The levels of zyxin, axin, and WASP were also assessed during AF-2364-mediated AJ disruption of the testis, which illustrated a time-dependent protein reduction that was similar to the trends observed in nectin-3 and afadin but was the opposite of those observed for N-cadherin and beta-catenin, which were induced. Collectively, these results illustrate that while these adaptors are structurally associated with the cadherin/catenin complex in the testis, they are regulated differently.

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

FBP11 regulates nuclear localization of N-WASP and inhibits N-WASP-dependent microspike formation

WASP family proteins are involved in cortical actin cytoskeleton reorganization. Neural Wiskott-Aldrich syndrome protein (N-WASP), a ubiquitously expressed WASP homologous protein, directly binds with Cdc42, activating Arp2/3 complex. In this study, we show that N-WASP-dependent microspike formation is inhibited by formin binding protein 11 (FBP11). Endogenous FBP11 localizes with nuclear-speckles, and co-localization of N-WASP and FBP11 was observed when they were co-expressed. Epidermal growth factor (EGF) induced actin-microspike formation in COS7 cells. However, transient expression of FBP11 suppressed N-WASP-dependent actin-microspike formation by trapping N-WASP in the nucleus. These results indicate that FBP11 regulates localization of N-WASP, thus negatively regulating the function of N-WASP in the cytoplasm.

A Cortactin-CD2-associated protein (CD2AP) complex provides a novel link between epidermal growth factor receptor endocytosis and the actin cytoskeleton

Growth factor regulation of the cortical actin cytoskeleton is fundamental to a wide variety of cellular processes. The cortical actin-associated protein, cortactin, regulates the formation of dynamic actin networks via the actin-related protein (Arp)2/3 complex and hence is a key mediator of such responses. In order to reveal novel roles for this versatile protein, we used a proteomics-based approach to isolate cortactin-interacting proteins. This identified several proteins, including CD2-associated protein (CD2AP), as targets for the cortactin Src homology 3 domain. Co-immunoprecipitation of CD2AP with cortactin occurred at endogenous expression levels, was transiently induced by epidermal growth factor (EGF) treatment, and required the cortactin Src homology 3 domain. The CD2AP-binding site for cortactin mapped to the second of three proline-rich regions. Because CD2AP is closely related to Cbl-interacting protein of 85 kDa (CIN85), which regulates growth factor receptor down-regulation via complex formation with Cbl and endophilin, we investigated whether the CD2AP-cortactin complex performs a similar function. EGF treatment of cells led to transient association of Cbl and the epidermal growth factor receptor (EGFR) with a constitutive CD2AP-endophilin complex. Cortactin was recruited into this complex with slightly delayed kinetics compared with Cbl and the EGFR. Immunofluorescence analysis revealed that the EGFR, CD2AP, and cortactin co-localized in regions of EGF-induced membrane ruffles. Therefore, by binding both CD2AP and the Arp2/3 complex, cortactin links receptor endocytosis to actin polymerization, which may facilitate the trafficking of internalized growth factor receptors.

Involvement of Wiskott-Aldrich syndrome protein family verprolin-homologous protein (WAVE) and Rac1 in the phagocytosis of amyloid-beta(1-42) in rat microglia

Alzheimer's disease (AD) is characterized by the accumulation of extracellular amyloid-beta (A beta) fibrils with microglia. Recently, there has been great interest in the microglial phagocytosis of A beta, because the microglial pathway is considered to be one of the A beta clearance pathways in the brain parenchyma. However, the mechanism of microglial phagocytosis of A beta is not fully understood and, thus, was investigated in this study. At one minute after exposure to A beta(1-42) (A beta 42), A beta immunoreactivity was detected at the cell surface of microglia. After 1 h, marked immunoreactivity was observed in the cytosolic vesicles. At 12 h, delayed phagocytosis of fibrillar A beta 42 was also observed with the formation of a large phagocytic cup. The microglial cell shape rapidly changed to an ameboid form during the process of phagocytosis. Although neither neural Wiskott-Aldrich syndrome protein (N-WASP) nor WASP interacting SH3 protein (WISH) immunoreactivity was co-localized with filamentous actin (F-actin) distribution, both WASP family verprolin-homologous protein (WAVE) and Rac1 immunoreactivity was co-localized with F-actin in the lamellipodia of phogocytic microglia. These results suggest that WAVE and Rac1 participate in the phagocytosis of A beta 42 by microglia.

Signaling mechanisms that regulate actin-based motility processes in the nervous system

Actin-based motility is critical for nervous system development. Both the migration of neurons and the extension of neurites require organized actin polymerization to push the cell membrane forward. Numerous extracellular stimulants of motility and axon guidance cues regulate actin-based motility through the rho GTPases (rho, rac, and cdc42). The rho GTPases reorganize the actin cytoskeleton, leading to stress fiber, filopodium, or lamellipodium formation. The activity of the rho GTPases is regulated by a variety of proteins that either stimulate GTP uptake (activation) or hydrolysis (inactivation). These proteins potentially link extracellular signals to the activation state of rho GTPases. Effectors downstream of the rho GTPases that directly influence actin polymerization have been identified and are involved in neurite development. The Arp2/3 complex nucleates the formation of new actin branches that extend the membrane forward. Ena/VASP proteins can cause the formation of longer actin filaments, characteristic of growth cone actin morphology, by preventing the capping of barbed ends. Actin-depolymerizing factor (ADF)/cofilin depolymerizes and severs actin branches in older parts of the actin meshwork, freeing monomers to be re-incorporated into actively growing filaments. The signaling mechanisms by which extracellular cues that guide axons to their targets lead to direct effects on actin filament dynamics are becoming better understood.

Filopodia are conduits for melanosome transfer to keratinocytes

Melanosomes are specialized melanin-synthesizing organelles critical for photoprotection in the skin. Melanosome transfer to keratinocytes, which involves whole organelle donation to another cell, is a unique biological process and is poorly understood. Time-lapse digital movies and electron microscopy show that filopodia from melanocyte dendrites serve as conduits for melanosome transfer to keratinocytes. Cdc42, a small GTP-binding protein, is known to mediate filopodia formation. Melanosome-enriched fractions isolated from human melanocytes expressed the Cdc42 effector proteins PAK1 and N-WASP by western blotting. Expression of constitutively active Cdc42(Cdc42V12) in melanocytes co-cultured with keratinocytes induced a highly dendritic phenotype with extensive contacts between melanocytes and keratinocytes through filopodia, many of which contained melanosomes. These results suggest a unique role for filopodia in organelle transport and, in combination with our previous work showing the presence of SNARE proteins and rab3a on melanosomes, suggest a novel model system for melanosome transfer to keratinocytes.

WICH, a novel verprolin homology domain-containing protein that functions cooperatively with N-WASP in actin-microspike formation

We describe a novel protein that contains a verprolin-homology (V) region, through which several actin-regulating proteins, including Wiskott-Aldrich syndrome protein (WASP) family members, bind directly to actin. The amino acid sequence is homologous to the sequences of WASP-interacting protein (WIP) and CR16, both of which associate with WASP and/or N-WASP, and thus these three proteins constitute a new protein family. We named the protein WICH (WIP- and CR16-homologous protein). WICH associates strongly with N-WASP but only weakly with WASP via its C-terminal WASP-interacting (W) region. Ectopic expression of WICH induces actin-microspike formation through cooperation with N-WASP. In addition, expression of the W fragment of WICH suppresses microspike formation induced by N-WASP, indicating an essential role for WICH in N-WASP-induced microspike formation.

The rat homologue of Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP) associates with actin filaments, recruits N-WASP from the nucleus, and mediates mobilization of actin from stress fibers in favor of filopodia formation

We cloned and characterized the rat homologue of the Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP). Rat WIP shows 86% amino acid sequence identity to human WIP. Northern analyses revealed two major mRNA species of 5.0 and 3.8 kb, which were ubiquitously expressed, though predominantly in spleen and lung. Minor species of 2.4, 1.8, 1.4, and 1.1 kb were also detected in some tissues and cell lines. Thus, WIP is subject to tissue-specific alternative splicing. WIP bound to N-WASP in vivo, as revealed by co-immunoprecipitation. Expression of WIP in rat fibroblasts revealed a clear co-localization with actin stress fibers. However, expression in tumor cells lacking actin cables did not restore these structures. Interestingly, co-expression of WIP and N-WASP resulted in redistribution of N-WASP, abrogating its dominant nuclear expression and leading to co-localization with WIP in the perinuclear area and with actin in membrane protrusions. Moreover, stress fibers and, concomitantly, the associated WIP were largely dissolved. Very similar effects were seen upon epidermal growth factor stimulation of serum-starved cells. Our results suggest that WIP might be involved in transmitting mitogenic signals to cytoskeletal functions, perhaps by modulating the subcellular localization of N-WASP. Interaction of N-WASP with WIP may in turn lead to mobilization of actin from stress fibers and nucleation of new actin filaments in filopodia.

WASP and N-WASP in human platelets differ in sensitivity to protease calpain

Mutations of Wiskott-Aldrich syndrome protein (WASP) underlie the severe thrombocytopenia and immunodeficiency of the Wiskott-Aldrich syndrome. WASP, a specific blood cell protein, and its close homologue, the broadly distributed N-WASP, function in dynamic actin polymerization processes. Here it is demonstrated that N-WASP is expressed along with WASP, albeit at low levels, in human blood cells. The presence of approximately 160 nmol/L rapidly acting N-WASP molecules may explain the normal capacity of WASP-negative patient platelets for early agonist-induced aggregation and filopodia formation. Ex vivo experiments revealed a significant difference between WASP and N-WASP in sensitivity to calpain, the Ca++-dependent protease activated in agonist-stimulated platelets. Through the use of a series of calpain-containing broken cell systems, it is shown that WASP is cleaved in a Ca++-dependent reaction inhibitable by calpeptin and E64d and that N-WASP is not cleaved, suggesting that the cleavage of WASP by calpain functions in normal platelets as part of a Ca++-dependent switch mechanism that terminates the surface projection phase of blood cell activation processes.

Cortactin: coupling membrane dynamics to cortical actin assembly

Exposure of cells to a variety of external signals causes rapid changes in plasma membrane morphology. Plasma membrane dynamics, including membrane ruffle and microspike formation, fusion or fission of intracellular vesicles, and the spatial organization of transmembrane proteins, is directly controlled by the dynamic reorganization of the underlying actin cytoskeleton. Two members of the Rho family of small GTPases, Cdc42 and Rac, have been well established as mediators of extracellular signaling events that impact cortical actin organization. Actin-based signaling through Cdc42 and Rac ultimately results in activation of the actin-related protein (Arp) 2/3 complex, which promotes the formation of branched actin networks. In addition, the activity of both receptor and non-receptor protein tyrosine kinases along with numerous actin binding proteins works in concert with Arp2/3-mediated actin polymerization in regulating the formation of dynamic cortical actin-associated structures. In this review we discuss the structure and role of the cortical actin binding protein cortactin in Rho GTPase and tyrosine kinase signaling events, with the emphasis on the roles cortactin plays in tyrosine phosphorylation-based signal transduction, regulating cortical actin assembly, transmembrane receptor organization and membrane dynamics. We also consider how aberrant regulation of cortactin levels contributes to tumor cell invasion and metastasis.

N-WASP deficiency reveals distinct pathways for cell surface projections and microbial actin-based motility

The Wiskott-Aldrich syndrome protein (WASP) family of molecules integrates upstream signalling events with changes in the actin cytoskeleton. N-WASP has been implicated both in the formation of cell-surface projections (filopodia) required for cell movement and in the actin-based motility of intracellular pathogens. To examine N-WASP function we have used homologous recombination to inactivate the gene encoding murine N-WASP. Whereas N-WASP-deficient embryos survive beyond gastrulation and initiate organogenesis, they have marked developmental delay and die before embryonic day 12. N-WASP is not required for the actin-based movement of the intracellular pathogen Listeria but is absolutely required for the motility of Shigella and vaccinia virus. Despite these distinct defects in bacterial and viral motility, N-WASP-deficient fibroblasts spread by using lamellipodia and can protrude filopodia. These results imply a crucial and non-redundant role for N-WASP in murine embryogenesis and in the actin-based motility of certain pathogens but not in the general formation of actin-containing structures.

Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency characterized by thrombocytopenia with small platelets, eczema, recurrent infections, autoimmune disorders, IgA nephropathy, and an increased incidence of hematopoietic malignancies. The identification of the responsible gene, WASP (Wiskott-Aldrich Syndrome Protein), revealed clinical heterogeneity of the syndrome, and showed that X-linked thrombocytopenia without, or with only mild immunodeficiency and eczema, is also caused by mutations of WASP. The study of WASP and its mutations demonstrates how a single gene defect can cause multiple and complex clinical symptoms.

WASP and WAVE family proteins: key molecules for rapid rearrangement of cortical actin filaments and cell movement

Reorganization of cortical actin filaments plays critical roles in cell movement and pattern formation. Recently, the WASP and WAVE family proteins WASP and N-WASP, and WAVE1, WAVE2 and WAVE3 have been shown to regulate cortical actin filament reorganization in response to extracellular stimuli. These proteins each have a verprolin-homology (V) domain, cofilin-homology (C) domain and an acidic (A) region at the C-terminus, through which they activate the Arp2/3 complex, leading to rapid actin polymerization. N-WASP is usually present as an inactive form in which the VCA region is masked. Cooperative binding of Cdc42 and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) exposes the VCA region, activating N-WASP. In addition to this activation mechanism, WISH also activates N-WASP independently of Cdc42 and PtdIns(4,5)P(2), by binding to the proline-rich region of N-WASP. N-WASP activation induces formation of filopodia in vivo. In contrast, the ubiquitously expressed form of WAVE2 is activated downstream of Rac, leading to formation of lamellipodia. In this case, IRSp53 transmits a signal from Rac to WAVE2 through formation of a ternary Rac-IRSp53-WAVE2 complex. Thus, N-WASP, which is activated downstream of Cdc42 or independently by WISH, induces formation of filopodia and WAVE2, which is activated via IRSp53 downstream of Rac, induces formation of lamellipodia.

A novel neural Wiskott-Aldrich syndrome protein (N-WASP) binding protein, WISH, induces Arp2/3 complex activation independent of Cdc42

We identified a novel adaptor protein that contains a Src homology (SH)3 domain, SH3 binding proline-rich sequences, and a leucine zipper-like motif and termed this protein WASP interacting SH3 protein (WISH). WISH is expressed predominantly in neural tissues and testis. It bound Ash/Grb2 through its proline-rich regions and neural Wiskott-Aldrich syndrome protein (N-WASP) through its SH3 domain. WISH strongly enhanced N-WASP-induced Arp2/3 complex activation independent of Cdc42 in vitro, resulting in rapid actin polymerization. Furthermore, coexpression of WISH and N-WASP induced marked formation of microspikes in Cos7 cells, even in the absence of stimuli. An N-WASP mutant (H208D) that cannot bind Cdc42 still induced microspike formation when coexpressed with WISH. We also examined the contribution of WISH to a rapid actin polymerization induced by brain extract in vitro. Arp2/3 complex was essential for brain extract-induced rapid actin polymerization. Addition of WISH to extracts increased actin polymerization as Cdc42 did. However, WISH unexpectedly could activate actin polymerization even in N-WASP-depleted extracts. These findings suggest that WISH activates Arp2/3 complex through N-WASP-dependent and -independent pathways without Cdc42, resulting in the rapid actin polymerization required for microspike formation.

Two tandem verprolin homology domains are necessary for a strong activation of Arp2/3 complex-induced actin polymerization and induction of microspike formation by N-WASP

All WASP family proteins share a common C terminus that consists of the verprolin homology domain (V), cofilin homology domain (C), and acidic region (A), through which they activate Arp2/3 complex-induced actin polymerization. In this study, we characterized the Arp2/3 complex-mediated actin polymerization activity of VCA fragments of all of the WASP family proteins: WASP, N-WASP, WAVE1, WAVE2, and WAVE3. All of the VCA fragments stimulated the nucleating activity of Arp2/3 complex. Among them, N-WASP VCA, which possesses two tandem V motifs, had a more potent activity than other VCA proteins. The chimeric protein experiments revealed that the V motif was more important to the activation potency than the CA region; two V motifs were required for full activity of N-WASP. COS7 cells overexpressing N-WASP form microspikes in response to epidermal growth factor. However, when a chimeric protein in which the VCA region of N-WASP is replaced with WAVE1 VCA was overexpressed, microspike formation was suppressed. Interestingly, when the N-WASP VCA region was replaced with WAVE1 VCA, having two V motifs, this chimeric protein could induce microspike formation. These results indicate that strong activation of Arp2/3 complex by N-WASP is mainly caused by its two tandem V motifs, which are essential for actin microspike formation.

Cortactin localization to sites of actin assembly in lamellipodia requires interactions with F-actin and the Arp2/3 complex

Cortactin is an actin-binding protein that is enriched within the lamellipodia of motile cells and in neuronal growth cones. Here, we report that cortactin is localized with the actin-related protein (Arp) 2/3 complex at sites of actin polymerization within the lamellipodia. Two distinct sequence motifs of cortactin contribute to its interaction with the cortical actin network: the fourth of six tandem repeats and the amino-terminal acidic region (NTA). Cortactin variants lacking either the fourth tandem repeat or the NTA failed to localize at the cell periphery. Tandem repeat four was necessary for cortactin to stably bind F-actin in vitro. The NTA region interacts directly with the Arp2/3 complex based on affinity chromatography, immunoprecipitation assays, and binding assays using purified components. Cortactin variants containing the NTA region were inefficient at promoting Arp2/3 actin nucleation activity. These data provide strong evidence that cortactin is specifically localized to sites of dynamic cortical actin assembly via simultaneous interaction with F-actin and the Arp2/3 complex. Cortactin interacts via its Src homology 3 (SH3) domain with ZO-1 and the SHANK family of postsynaptic density 95/dlg/ZO-1 homology (PDZ) domain-containing proteins, suggesting that cortactin contributes to the spatial organization of sites of actin polymerization coupled to selected cell surface transmembrane receptor complexes.

Profilin is required for sustaining efficient intra- and intercellular spreading of Shigella flexneri

The ability of Shigella to mediate actin-based motility within the host cell is a prominent pathogenic feature of bacillary dysentery. The ability is dependent on the interaction of VirG with neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn mediates recruitment of Arp2/3 complex and several actin-related proteins. In the present study, we show that profilin I is essential to the rapid movement of Shigella in epithelial cells, for which the capacity of profilin to interact with G-actin and N-WASP is critical. In COS-7 cells overexpressing either mutated profilin H119E, which failed to bind G-actin, or H133S, which is unable to interact with poly-l-proline, Shigella motility was significantly inhibited. Similarly, depletion of profilin from Xenopus egg extracts resulted in a decrease in bacterial motility that was completely rescued by adding back profilin I but not H119E or H133S. In COS-7 cells overexpressing a N-WASP mutant lacking the proline-rich domain (Deltap) unable to interact with profilin, the actin tail formation of intracellular Shigella was inhibited. In N-WASP-depleted extracts, addition of Deltap but not full-length N-WASP was unable to restore the bacterial motility. Furthermore, in a plaque formation assay with Madin-Darby canine kidney cell monolayers infected by Shigella, Madin-Darby canine kidney cells stably expressing H119E, H133S, or Deltap reduced the bacterial cell-to-cell spreading. These results indicate that profilin I associated with N-WASP is an essential host factor for sustaining efficient intra- and intercellular spreading of Shigella.

Essential role of neural Wiskott-Aldrich syndrome protein in neurite extension in PC12 cells and rat hippocampal primary culture cells

Neural Wiskott-Aldrich syndrome protein (N-WASP) is an actin-regulating protein that induces filopodium formation downstream of Cdc42. It has been shown that filopodia actively extend from the growth cone, a guidance apparatus located at the tip of neurites, suggesting their role in neurite extension. Here we examined the possible involvement of N-WASP in the neurite extension process. Since verprolin, cofilin homology and acidic region (VCA) of N-WASP is known to be required for the activation of Arp2/3 complex that induces actin polymerization, we prepared a mutant (Deltacof) lacking four amino acid residues in the cofilin homology region. The corresponding residues in WASP had been reported to be mutated in some Wiskott-Aldrich syndrome patients. Expression of Deltacof N-WASP suppressed neurite extension of PC12 cells. In support of this, the VCA region of Deltacof cannot activate Arp2/3 complex enough compared with wild-type VCA. Furthermore, H208D mutant, which has been shown unable to bind to Cdc42, also works as a dominant negative mutant in neurite extension assay. Interestingly, the expression of H208D-Deltacof double mutant has no significant dominant negative effect. Finally, the expression of the Deltacof mutant also severely inhibited the neurite extension of primary neurons from rat hippocampus. Thus, N-WASP is thought to be a general regulator of the actin cytoskeleton indispensable for neurite extension, which is probably caused through Cdc42 signaling and Arp2/3 complex-induced actin polymerization.

Identification of two human WAVE/SCAR homologues as general actin regulatory molecules which associate with the Arp2/3 complex

WAVE/SCAR protein was identified as a protein which has similarity to WASP and N-WASP, especially in its C terminal. Recently, WAVE/SCAR protein has been shown to cooperate with the Arp2/3 complex, a nucleation core for actin polymerization in vitro. However, in spite of its general function, WAVE/SCAR expression is mainly restricted to the brain, suggesting the existence of related molecule(s). We here identified two human WAVE/SCAR homologues, which cover other organs. We named the original WAVE1 and newly identified ones WAVE2 and WAVE3. WAVE2 had a very wide distribution with strong expression in peripheral blood leukocytes and mapped on chromosome Xp11.21, next to the WASP locus. WAVE3 and WAVE1 had similar distributions. WAVE3 was strongly expressed in brain and mapped on chromosome 13q12. WAVE1 was mapped on chromosome 6q21-22. Ectopically expressed WAVE2 and WAVE3 induced actin filament clusters in a similar manner with WAVE1. These actin cluster formations were suppressed by deletion of their C-terminal VPH (verproline homology)/WH2 (WASP homology 2) domain. Further, WAVE2 and WAVE3 associate with the Arp2/3 complex as does WAVE1. Our identification of WAVE homologues suggests that WAVE family proteins have general function for regulating the actin cytoskeleton in many tissues.

The Wiskott-Aldrich syndrome protein (WASP): roles in signaling and cytoskeletal organization

The Wiskott-Aldrich Syndrome (WAS) is a rare X-linked primary immunodeficiency that is characterized by recurrent infections, hematopoietic malignancies, eczema, and thrombocytopenia. A variety of hematopoietic cells are affected by the genetic defect, including lymphocytes, neutrophils, monocytes, and platelets. Early studies noted both signaling and cytoskeletal abnormalities in lymphocytes from WAS patients. Following the identification of WASP, the gene mutated in patients with this syndrome, and the more generally expressed WASP homologue N-WASP, studies have demonstrated that WASP-family molecules associate with numerous signaling molecules known to alter the actin cytoskeleton. WASP/N-WASP may depolymerize actin directly and/or serve as an adaptor or scaffold for these signaling molecules in a complex cascade that regulates the cytoskeleton.

The interaction between N-WASP and the Arp2/3 complex links Cdc42-dependent signals to actin assembly

Although small GTP-binding proteins of the Rho family have been implicated in signaling to the actin cytoskeleton, the exact nature of the linkage has remained obscure. We describe a novel mechanism that links one Rho family member, Cdc42, to actin polymerization. N-WASP, a ubiquitously expressed Cdc42-interacting protein, is required for Cdc42-stimulated actin polymerization in Xenopus egg extracts. The C terminus of N-WASP binds to the Arp2/3 complex and dramatically stimulates its ability to nucleate actin polymerization. Although full-length N-WASP is less effective, its activity can be greatly enhanced by Cdc42 and phosphatidylinositol (4,5) bisphosphate. Therefore, N-WASP and the Arp2/3 complex comprise a core mechanism that directly connects signal transduction pathways to the stimulation of actin polymerization.

Syndapin I, a synaptic dynamin-binding protein that associates with the neural Wiskott-Aldrich syndrome protein

The GTPase dynamin has been clearly implicated in clathrin-mediated endocytosis of synaptic vesicle membranes at the presynaptic nerve terminal. Here we describe a novel 52-kDa protein in rat brain that binds the proline-rich C terminus of dynamin. Syndapin I (synaptic, dynamin-associated protein I) is highly enriched in brain where it exists in a high molecular weight complex. Syndapin I can be involved in multiple protein-protein interactions via a src homology 3 (SH3) domain at the C terminus and two predicted coiled-coil stretches. Coprecipitation studies and blot overlay analyses revealed that syndapin I binds the brain-specific proteins dynamin I, synaptojanin, and synapsin I via an SH3 domain-specific interaction. Coimmunoprecipitation of dynamin I with antibodies recognizing syndapin I and colocalization of syndapin I with dynamin I at vesicular structures in primary neurons indicate that syndapin I associates with dynamin I in vivo and may play a role in synaptic vesicle endocytosis. Furthermore, syndapin I associates with the neural Wiskott-Aldrich syndrome protein, an actin-depolymerizing protein that regulates cytoskeletal rearrangement. These characteristics of syndapin I suggest a molecular link between cytoskeletal dynamics and synaptic vesicle recycling in the nerve terminal.

Recent advances in our understanding of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome initially was described in 1937 and then again in 1954 as an X-linked disorder associated with thrombocytopenia, eczema, and recurrent infections. It remained mysterious how different cell lineages could be affected in this syndrome and, more importantly, how the phenotypic features could be so protean. We now know that the features associated with Wiskott-Aldrich syndrome include dysfunction of nearly all effector arms of the immune system, as well as thrombocytopenia with platelet dysfunction. As a consequence of these abnormalities, children and adults with this syndrome have recurrent bleeding, recurrent and significant infections with common and opportunistic organisms, autoimmune disease, and lymphoreticular malignancies. In 1994, the gene that is defective in Wiskott-Aldrich syndrome was identified and found to be a gene with limited homology to any known gene families. In the past 4 years, much has been learned about the role of this protein in cellular function and T-cell responses specifically. This article reviews some recent clinical findings relevant to Wiskott-Aldrich syndrome, the proposed cellular role of this molecule, its biochemical interactions, and genotype-phenotype considerations.

The essential role of profilin in the assembly of actin for microspike formation

Profilin was first identified as an actin monomer binding protein; however, recent reports indicate its involvement in actin polymerization. To date, there is no direct evidence of a functional role in vivo for profilin in actin cytoskeletal reorganization. Here, we prepared a profilin mutant (H119E) defective in actin binding, but retaining the ability to bind to other proteins. This mutant profilin I suppresses actin polymerization in microspike formation induced by N-WASP, the essential factor in microspike formation. Profilin associates both in vivo and in vitro with N-WASP at proline-rich sites different from those to which Ash/Grb2 binds. This association between profilin and N-WASP is required for N-WASP-induced efficient microspike elongation. Moreover, we succeeded in reconstituting microspike formation in permeabilized cells using profilin I combined with N-WASP and its regulator, Cdc42. These findings provide the first evidence that profilin is a key molecule linking a signaling network to rapid actin polymerization in microspike formation.

Characterization of the Wiskott-Aldrich syndrome protein and its role in the disease

Wiskott-Aldrich syndrome is an X-linked disorder characterized by thrombocytopenia, eczema and immunodeficiency. The Wiskott-Aldrich syndrome protein and the gene that encodes it have been identified by positional cloning and the protein has been shown to contain a pleckstrin-homology domain, a GTPase-binding domain, a proline-rich region and a verprolin/cofilin homology domain. Subsequent studies suggest that the protein is involved in signal transduction and the regulation of the cytoskeleton.

A 5′ Regulatory Sequence Containing Two Ets Motifs Controls the Expression of the Wiskott-Aldrich Syndrome Protein (WASP) Gene in Human Hematopoietic Cells

The recently-identified Wiskott-Aldrich syndrome protein gene (WASP) is responsible for the Wiskott-Aldrich X-linked immunodeficiency as well as for isolated X-linked thrombocytopenia (XLT). To characterize the regulatory sequences of the WASP gene, we have isolated, sequenced and functionally analyzed a 1.6-Kb DNA fragment upstream of the WASP coding sequence. Transfection experiments showed that this fragment is capable of directing efficient expression of the reporter chloramphenicol acetyltransferase (CAT) gene in all human hematopoietic cell lines tested. Progressive 5′ deletions showed that the minimal sequence required for hematopoietic-specific expression consists of 137 bp upstream of the transcription start site. This contains potential binding sites for several hematopoietic transcription factors and, in particular, two Ets-1 consensus that proved able to specifically bind to proteins present in nuclear extracts of Jurkat cells. Overexpression of Ets-1 in HeLa resulted in transactivation of the CAT reporter gene under the control of WASP regulatory sequences. Disruption of the Ets-binding sequences by side-directed mutagenesis abolished CAT expression in Jurkat cells, indicating that transcription factors of the Ets family play a key role in the control of WASP transcription.

A 5′ Regulatory Sequence Containing Two Ets Motifs Controls the Expression of the Wiskott-Aldrich Syndrome Protein (WASP) Gene in Human Hematopoietic Cells

The recently-identified Wiskott-Aldrich syndrome protein gene (WASP) is responsible for the Wiskott-Aldrich X-linked immunodeficiency as well as for isolated X-linked thrombocytopenia (XLT). To characterize the regulatory sequences of the WASP gene, we have isolated, sequenced and functionally analyzed a 1.6-Kb DNA fragment upstream of the WASP coding sequence. Transfection experiments showed that this fragment is capable of directing efficient expression of the reporter chloramphenicol acetyltransferase (CAT) gene in all human hematopoietic cell lines tested. Progressive 5′ deletions showed that the minimal sequence required for hematopoietic-specific expression consists of 137 bp upstream of the transcription start site. This contains potential binding sites for several hematopoietic transcription factors and, in particular, two Ets-1 consensus that proved able to specifically bind to proteins present in nuclear extracts of Jurkat cells. Overexpression of Ets-1 in HeLa resulted in transactivation of the CAT reporter gene under the control of WASP regulatory sequences. Disruption of the Ets-binding sequences by side-directed mutagenesis abolished CAT expression in Jurkat cells, indicating that transcription factors of the Ets family play a key role in the control of WASP transcription.

Induction of filopodium formation by a WASP-related actin-depolymerizing protein N-WASP

Cdc42 is a small GTPase of the Rho family which regulates the formation of actin filaments to generate filopodia. Although there are several proteins such as PAK, ACK and WASP (Wiskott-Aldrich syndrome protein) that bind Cdc42 directly, none of these can account for the filopodium formation induced by Cdc42. Here we demonstrate that before it can induce filopodium formation, Cdc42 must bind a WASP-related protein, N-WASP, that is richest in neural tissues but is expressed ubiquitously. N-WASP induces extremely long actin microspikes only when co-expressed with active Cdc42, whereas WASP, which is expressed in haematopoietic cells, does not, despite the structural similarities between WASP and N-WASP. In a cell-free system, addition of active Cdc42 significantly stimulates the actin-depolymerizing activity of N-WASP, creating free barbed ends from which actin polymerization can then take place. This activation seems to be caused by exposure of N-WASP's actin-depolymerizing region induced by Cdc42 binding.