Macropinocytic cups function as signal platforms for the mTORC2-AKT pathway to modulate LPS-induced cytokine expression in macrophages

Macropinocytosis is a large-scale endocytosis process that is primarily observed in phagocytes as part of their cellular function to ingest antigens. Once phagocytes encounter gram-negative bacteria, the receptor proteins identify lipopolysaccharides (LPSs), which trigger radical membrane ruffles that gradually change to cup-like structures. The open area of the cups closes to generate vesicles called macropinosomes. The target bacteria are isolated by the cups and engulfed by the cells as the cups close. In addition to its ingestion function, macropinocytosis also regulates the AKT pathway in macrophages. In the current study, we report that macropinocytic cups are critical for LPS-induced AKT phosphorylation (pAKT) and cytokine expression in macrophages. High-resolution scanning electron microscope (SEM) observations detailed the macropinocytic cup structures induced by LPS stimulation. Confocal microscopy revealed that AKT and the kinase molecule mTORC2 were localized in the cups. The biochemical analysis showed that macropinocytosis inhibition blocked LPS-induced pAKT. RNA-Seq, qPCR, and ELISA analyses revealed that the inhibition of macropinocytosis or the AKT pathway causes a decrease in the expression of pro-inflammatory cytokines IL-6 and IL-1α. Moreover, activation of the transcription factor NF-κB, which regulates the cytokine expression downstream of the AKT/IκB pathway, was hindered when macropinocytosis or AKT were inhibited. These results indicate that LPS-induced macropinocytic cups function as signal platforms for the AKT pathway to regulate the cytokine expression by modulating NF-κB activity in LPS-stimulated macrophages. Based on these findings, we propose that macropinocytosis may be a good therapeutic target for controlling cytokine expression.

The SH3 binding site in front of the WH1 domain contributes to the membrane binding of the BAR domain protein endophilin A2

The Bin-Amphiphysin-Rvs (BAR) domain of endophilin binds to the cell membrane and shapes it into a tubular shape for endocytosis. Endophilin has a Src-homology 3 (SH3) domain at their C-terminal. The SH3 domain interacts with the proline-rich motif (PRM) that is found in proteins such as neural Wiskott-Aldrich syndrome protein (N-WASP). Here, we re-examined the binding sites of the SH3 domain of endophilin in N-WASP by machine learning-based prediction and identified the previously unrecognized binding site. In addition to the well-recognized PRM at the central proline-rich region, we found a PRM in front of the N-terminal WASP homology 1 (WH1) domain of N-WASP (NtPRM) as a binding site of the endophilin SH3 domain. Furthermore, the diameter of the membrane tubules in the presence of NtPRM mutant was narrower and wider than that in the presence of N-WASP and in its absence, respectively. Importantly, the NtPRM of N-WASP was involved in the membrane localization of endophilin A2 in cells. Therefore, the NtPRM contributes to the binding of endophilin to N-WASP in membrane remodeling.

Tuftelin1 drives experimental pulmonary fibrosis progression by facilitating stress fiber assembly

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease (ILD) with unknown etiology, characterized by sustained damage repair of epithelial cells and abnormal activation of fibroblasts, the underlying mechanism of the disease remains elusive.

METHODS

To evaluate the role of Tuftelin1 (TUFT1) in IPF and elucidate its molecular mechanism. We investigated the level of TUFT1 in the IPF and bleomycin-induced mouse models and explored the influence of TUFT1 deficiency on pulmonary fibrosis. Additionally, we explored the effect of TUFT1 on the cytoskeleton and illustrated the relationship between stress fiber and pulmonary fibrosis.

RESULTS

Our results demonstrated a significant upregulation of TUFT1 in IPF and the bleomycin (BLM)-induced fibrosis model. Disruption of TUFT1 exerted inhibitory effects on pulmonary fibrosis in both in vivo and in vitro. TUFT1 facilitated the assembly of microfilaments in A549 and MRC-5 cells, with a pronounced association between TUFT1 and Neuronal Wiskott-Aldrich syndrome protein (N-WASP) observed during microfilament formation. TUFT1 can promote the phosphorylation of tyrosine residue 256 (Y256) of the N-WASP (pY256N-WASP). Furthermore, TUFT1 promoted transforming growth factor-β1 (TGF-β1) induced fibroblast activation by increasing nuclear translocation of pY256N-WASP in fibroblasts, while wiskostatin (Wis), an N-WASP inhibitor, suppressed these processes.

CONCLUSIONS

Our findings suggested that TUFT1 plays a critical role in pulmonary fibrosis via its influence on stress fiber, and blockade of TUFT1 effectively reduces pro-fibrotic phenotypes. Pharmacological targeting of the TUFT1-N-WASP axis may represent a promising therapeutic approach for pulmonary fibrosis.

N-WASP-dependent branched actin polymerization attenuates B-cell receptor signaling by increasing the molecular density of receptor clusters

Antigen-induced B-cell receptor (BCR) signaling is critical for initiating and regulating B-cell activation. The actin cytoskeleton plays essential roles in BCR signaling. Upon encountering cell-surface antigens, actin-driven B-cell spreading amplifies signaling, while B-cell contraction following spreading leads to signal attenuation. However, the mechanism by which actin dynamics switch BCR signaling from amplification to attenuation is unknown. Here, we show that Arp2/3-mediated branched actin polymerization is required for mouse splenic B-cell contraction. Contracting B-cells generate centripetally moving actin foci from lamellipodial F-actin networks in the plasma membrane region contacting antigen-presenting surfaces. Actin polymerization driven by N-WASP, but not WASP, initiates these actin foci and facilitates non-muscle myosin II recruitment to the contact zone, creating actomyosin ring-like structures. B-cell contraction increases BCR molecular density in individual clusters, leading to decreased BCR phosphorylation. Increased BCR molecular density reduced levels of the stimulatory kinase Syk, the inhibitory phosphatase SHIP-1, and their phosphorylated forms in individual BCR clusters. These results suggest that N-WASP-activated Arp2/3, coordinating with myosin, generates centripetally moving foci and contractile actomyosin ring-like structures from lamellipodial networks, enabling contraction. B-cell contraction attenuates BCR signaling by pushing out both stimulatory kinases and inhibitory phosphatases from BCR clusters, providing novel insights into actin-facilitated signal attenuation.

The amount of Nck rather than N-WASP correlates with the rate of actin-based motility of Vaccinia virus

IMPORTANCE

Vaccinia virus is a large double-stranded DNA virus and a close relative of Mpox and Variola virus, the causative agent of smallpox. During infection, Vaccinia hijacks its host's transport systems and promotes its spread into neighboring cells by recruiting a signaling network that stimulates actin polymerization. Over the years, Vaccinia has provided a powerful model to understand how signaling networks regulate actin polymerization. Nevertheless, we still lack important quantitative information about the system, including the precise number of viral and host molecules required to induce actin polymerization. Using quantitative fluorescence microscopy techniques, we have determined the number of viral and host signaling proteins accumulating on virions during their egress. Our analysis has uncovered two unexpected new aspects of this process: the number of viral proteins in the virion is not fixed and the velocity of virus movement depends on the level of a single adaptor within the signaling network.

Conditional Knockout of N-WASP Enhanced the Formation of Keratinizing Squamous Cell Carcinoma Induced by KRasG12D

Squamous cell carcinoma (SCC) is one of the most common forms of skin cancer in humans, and Neural Wiskott-Aldrich Syndrome Protein (N-WASP) plays a crucial role in epidermal homeostasis. To elucidate the role of N-WASP in skin cancer, we generated mice which expressed constitutively active KRas (KRasG12D) in keratinocytes with either homozygous (N-WASPKOG12D) or heterozygous (N-WASPHetG12D) N-WASP knockout upon Tamoxifen (TAM) injection. Both the N-WASPKOG12D and N-WASPHetG12D mice had similar body weights and no congenital malformations prior to the injection of TAM. Within 2 weeks of the injections, the N-WASPKOG12D mice exhibited significant reductions in weight coupled with visible tumors at numerous sites, unlike the N-WASPHetG12D mice, which had no visible tumors. We found that both sets of mice had oily, sticky skin and wet eyes 3 weeks after their exposure to TAM, indicating the overproduction of sebum/meibum. At 37 days post TAM injection, several notable observations were made. Tumors collected from the N-WASPKOG12D mice had small- to large-sized keratin pearls that were not observed in the N-WASPHetG12D mice. A Western blot and immunostaining analysis both highlighted significantly higher levels of expression of SCC markers, such as the cytokeratins 8, 17, 18, and 19 and TP63, in the tumors of the N-WASPKOG12D mice compared to those of the latter group. Furthermore, we noted increases in the expression levels of EGFR, P-ERK, GLUT1, P-mTOR, and P-4EBP in the N-WASPKOG12D mice, suggesting that the deletion of N-WASP in the keratinocytes enhanced KRas signaling and glucose uptake, resulting in aggressive tumor formation. Interestingly, a thickening of the epidermal layer within the esophagus and tongue was only observed in the N-WASPKOG12D mice. Immunostaining for PCNA emphasized a significantly higher number of PCNA-positive cells in the skin of the N-WASPKOG12D mice compared to their counterparts, implying that epidermal thickening and enhanced tumorigenesis are due to an increased proliferation of keratinocytes. Through our results, we have established that N-WASP plays a tumor-suppressive role in skin cancer.

Phosphorylation of PACSIN2 at S313 Regulates Podocyte Architecture in Coordination with N-WASP

Changes in the dynamic architecture of podocytes, the glomerular epithelial cells, lead to kidney dysfunction. Previous studies on protein kinase C and casein kinase 2 substrates in neurons 2 (PACSIN2), a known regulator of endocytosis and cytoskeletal organization, reveal a connection between PACSIN2 and kidney pathogenesis. Here, we show that the phosphorylation of PACSIN2 at serine 313 (S313) is increased in the glomeruli of rats with diabetic kidney disease. We found that phosphorylation at S313 is associated with kidney dysfunction and increased free fatty acids rather than with high glucose and diabetes alone. Phosphorylation of PACSIN2 emerged as a dynamic process that fine-tunes cell morphology and cytoskeletal arrangement, in cooperation with the regulator of the actin cytoskeleton, Neural Wiskott-Aldrich syndrome protein (N-WASP). PACSIN2 phosphorylation decreased N-WASP degradation while N-WASP inhibition triggered PACSIN2 phosphorylation at S313. Functionally, pS313-PACSIN2 regulated actin cytoskeleton rearrangement depending on the type of cell injury and the signaling pathways involved. Collectively, this study indicates that N-WASP induces phosphorylation of PACSIN2 at S313, which serves as a mechanism whereby cells regulate active actin-related processes. The dynamic phosphorylation of S313 is needed to regulate cytoskeletal reorganization.

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.

N-WASP-Arp2/3 signaling controls multiple steps of dendrite maturation in Purkinje cells in vivo

During neural development, the actin filament network must be precisely regulated to form elaborate neurite structures. N-WASP tightly controls actin polymerization dynamics by activating an actin nucleator Arp2/3. However, the importance of N-WASP-Arp2/3 signaling in the assembly of neurite architecture in vivo has not been clarified. Here, we demonstrate that N-WASP-Arp2/3 signaling plays a crucial role in the maturation of cerebellar Purkinje cell (PC) dendrites in vivo in mice. N-WASP was expressed and activated in developing PCs. Inhibition of Arp2/3 and N-WASP from the beginning of dendrite formation severely disrupted the establishment of a single stem dendrite, which is a characteristic basic structure of PC dendrites. Inhibition of Arp2/3 after stem dendrite formation resulted in hypoplasia of the PC dendritic tree. Cdc42, an upstream activator of N-WASP, is required for N-WASP-Arp2/3 signaling-mediated PC dendrite maturation. In addition, overactivation of N-WASP is also detrimental to dendrite formation in PCs. These findings reveal that proper activation of N-WASP-Arp2/3 signaling is crucial for multiple steps of PC dendrite maturation in vivo.

Lamellipodia-like actin networks in cells lacking WAVE regulatory complex

Cell migration frequently involves the formation of lamellipodia induced by Rac GTPases activating WAVE regulatory complex (WRC) to drive Arp2/3 complex-dependent actin assembly. Previous genome editing studies in B16-F1 melanoma cells solidified the view of an essential, linear pathway employing the aforementioned components. Here, disruption of the WRC subunit Nap1 (encoded by Nckap1) and its paralog Hem1 (encoded by Nckap1l) followed by serum and growth factor stimulation, or active GTPase expression, revealed a pathway to formation of Arp2/3 complex-dependent lamellipodia-like structures (LLS) that requires both Rac and Cdc42 GTPases, but not WRC. These phenotypes were independent of the WRC subunit eliminated and coincided with the lack of recruitment of Ena/VASP family actin polymerases. Moreover, aside from Ena/VASP proteins, LLS contained all lamellipodial regulators tested, including cortactin (also known as CTTN), the Ena/VASP ligand lamellipodin (also known as RAPH1) and FMNL subfamily formins. Rac-dependent but WRC-independent actin remodeling could also be triggered in NIH 3T3 fibroblasts by growth factor (HGF) treatment or by gram-positive Listeria monocytogenes usurping HGF receptor signaling for host cell invasion. Taken together, our studies thus establish the existence of a signaling axis to Arp2/3 complex-dependent actin remodeling at the cell periphery that operates without WRC and Ena/VASP.

Summary: Rac-dependent actin remodeling can occur in the absence of WAVE regulatory complex, triggered by active Cdc42. WAVE regulatory complex-independent actin structures harbor Arp2/3 complex but not VASP.

Syndecan-4 assists Mycobacterium tuberculosis entry into lung epithelial cells by regulating the Cdc42, N-WASP, and Arp2/3 signaling pathways

Syndecan-4 (SDC4) is a transmembrane heparin sulfate proteoglycan that regulates inflammatory responses, cell motility, cell adhesion and intracellular signaling. In this study, we found that overexpression of SDC4 promoted the infection efficiency of Mycobacterium tuberculosis (Mtb), whereas knockdown of SDC4 reduced the infection efficiency, suggesting that SDC4 assisted Mtb infection of epithelial cells. We also observed that Mtb infection affected the F-actin/G-actin ratio, which was also correlated with SDC4 expression levels. Analysis of the Cdc42, N-WASP, and Arp2/3 signaling pathways during Mtb infection revealed that knockdown of Cdc42 and N-WASP or the addition of ZCL278, Wiskostatin or CK636 (blockers of Cdc42, N-WASP, and Arp2/3, respectively) significantly exacerbated Mtb infection in lung epithelial cells. Taken together, our data indicate that SDC4 assists Mtb infection of epithelial cells by regulating the Cdc42, N-WASP, and Arp2/3 signaling pathways, which regulate the polymerization of the actin cytoskeleton.

FCHSD2 cooperates with CDC42 and N-WASP to regulate cell protrusion formation

Actin-based, finger-like cell protrusions such as microvilli and filopodia play important roles in epithelial cells. Several proteins have been identified to regulate cell protrusion formation, which helps us to learn about the underlying mechanism of this process. FCH domain and double SH3 domains containing protein 2 (FCHSD2) belongs to the FCH and Bin-Amphiphysin-Rvs (F-BAR) protein family, containing an N-terminal F-BAR domain, two SH3 domains, and a C-terminal PDZ domain-binding interface (PBI). Previously, we found that FCHSD2 interacts with WASP/N-WASP and stimulates ARP2/3-mediated actin polymerization in vitro. In the present work, we show that FCHSD2 promotes the formation of apical and lateral cell protrusions in cultured cells. Our data suggest that FCHSD2 cooperates with CDC42 and N-WASP in regulating apical cell protrusion formation. In line with this, biochemical studies reveal that FCHSD2 and CDC42 simultaneously bind to N-WASP, forming a protein complex. Interestingly, the F-BAR domain of FCHSD2 induces lateral cell protrusion formation independently of N-WASP. Furthermore, we show that the ability of FCHSD2 to induce cell protrusion formation requires its plasma membrane-binding ability. In summary, our present work suggests that FCHSD2 cooperates with CDC42 and N-WASP to regulate cell protrusion formation in a membrane-dependent manner.

Actin Assembly around the Shigella-Containing Vacuole Promotes Successful Infection

The enteroinvasive bacterium Shigella flexneri forces its uptake into non-phagocytic host cells through the translocation of T3SS effectors that subvert the actin cytoskeleton. Here, we report de novo actin polymerization after cellular entry around the bacterium-containing vacuole (BCV) leading to the formation of a dynamic actin cocoon. This cocoon is thicker than any described cellular actin structure and functions as a gatekeeper for the cytosolic access of the pathogen. Host CDC42, TOCA-1, N-WASP, WIP, the Arp2/3 complex, cortactin, coronin, and cofilin are recruited to the actin cocoon. They are subverted by T3SS effectors, such as IpgD, IpgB1, and IcsB. IcsB immobilizes components of the actin polymerization machinery at the BCV dependent on its fatty acyltransferase activity. This represents a unique microbial subversion strategy through localized entrapment of host actin regulators causing massive actin assembly. We propose that the cocoon promotes subsequent invasion steps for successful Shigella infection.

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A thick actin cocoon forms de novo around the Shigella-containing vacuole upon entry

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The effector IcsB entraps host actin regulators at the vacuole by lipidation

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Cdc42, N-WASP, and the Arp2/3 complex are major actin cocoon regulators

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Cocoon formation promotes subsequent Shigella niche formation and dissemination

1H, 13C, and 15N NMR chemical shift assignment of the complex formed by the first EPEC EspF repeat and N-WASP GTPase binding domain

LEE-encoded effector EspF (EspF) is an effector protein part of enteropathogenic Escherichia coli's (EPEC's) arsenal for intestinal infection. This intrinsically disordered protein contains three highly conserved repeats which together compose over half of the protein's complete amino acid sequence. EPEC uses EspF to hijack host proteins in order to promote infection. In the attack EspF is translocated, together with other effector proteins, to host cell via type III secretion system. Inside host EspF stimulates actin polymerization by interacting with Neural Wiskott-Aldrich syndrome protein (N-WASP), a regulator in actin polymerization machinery. It is presumed that EspF acts by disrupting the autoinhibitory state of N-WASP GTPase binding domain. In this NMR spectroscopy study, we report the ¹H, ¹³C, and ¹⁵N resonance assignments for the complex formed by the first 47-residue repeat of EspF and N-WASP GTPase binding domain. These near-complete resonance assignments provide the basis for further studies which aim to characterize structure, interactions, and dynamics between these two proteins in solution.

FLI1 Induces Megakaryopoiesis Gene Expression Through WAS/WIP-Dependent and Independent Mechanisms; Implications for Wiskott-Aldrich Syndrome

Wiskott–Aldrich Syndrome, WAS/WAVE, is a rare, X-linked immune-deficiency disease caused by mutations in the WAS gene, which together with its homolog, N-WASP, regulates actin cytoskeleton remodeling and cell motility. WAS patients suffer from microthrombocytopenia, characterized by a diminished number and size of platelets, though the underlying mechanism is not fully understood. Here, we identified FLI1 as a direct transcriptional regulator of WAS and its binding partner WIP. Depletion of either WAS or WIP in human erythroleukemic cells accelerated cell proliferation, suggesting tumor suppressor function of both genes in leukemia. Depletion of WAS/WIP also led to a significant reduction in the percentage of CD41 and CD61 positive cells, which mark committed megakaryocytes. RNAseq analysis revealed common changes in megakaryocytic gene expression following FLI1 or WASP knockdown. However, in contrast to FLI1, WASP depletion did not alter expression of late-stage platelet-inducing genes. N-WASP was not regulated by FLI1, yet its silencing also reduced the percentage of CD41+ and CD61+ megakaryocytes. Moreover, combined knockdown of WASP and N-WASP further suppressed megakaryocyte differentiation, indicating a major cooperation of these related genes in controlling megakaryocytic cell fate. However, unlike WASP/WIP, N-WASP loss suppressed leukemic cell proliferation. WASP, WIP and N-WASP depletion led to induction of FLI1 expression, mediated by GATA1, and this may mitigate the severity of platelet deficiency in WAS patients. Together, these results uncover a crucial role for FLI1 in megakaryocyte differentiation, implicating this transcription factor in regulating microthrombocytopenia associated with Wiskott–Aldrich syndrome.

Molecular Basis of LH Action on Breast Cancer Cell Migration and Invasion via Kinase and Scaffold Proteins

Breast cancer (BC) is a major public health problem affecting women worldwide. Approximately 80% of diagnosed cases are hormone-dependent breast cancers. These hormones are known to stimulate tumor development and progression. In this setting, tentative evidence suggests that luteinizing hormone (LH) may also play a role in tumors. In BC cells that express functional LH receptors (LHR), this hormone regulates cell migration and invasion by controlling several kinases that activate actin cytoskeletal proteins. In this article, we show that LH induces phosphorylation of paxillin and its translocation toward the plasmatic membrane, where focal adhesion complexes are assembled. This process is triggered via a rapid extra-gonadal LHR signaling to Src/FAK/paxillin, which results in the phosphorylation/activation of the nucleation promoter factors cortactin and N-WASP. As a consequence, Arp2/3 complexes induce actin polymerization, essential to promote cell adhesion, migration, and invasion, thus enhancing metastatic spread of tumoral cells. Our findings provide relevant information about how gonadotrophins exert their action in BC. This information helps us understand the extragonadal effects of LH on BC metastasis. It may provide new perspectives for therapeutic treatment, especially for women with high serum levels of gonadotrophins.

Latent developmental potential to form limb-like skeletal structures in zebrafish

Changes in appendage structure underlie key transitions in vertebrate evolution. Addition of skeletal elements along the proximal-distal axis facilitated critical transformations, including the fin-to-limb transition that permitted generation of diverse modes of locomotion. Here, we identify zebrafish mutants that form supernumerary long bones in their pectoral fins. These new bones integrate into musculature, form joints, and articulate with neighboring elements. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage patterning, vav2 and waslb, that function in a common pathway. This pathway is required for appendage development across vertebrates, and loss of Wasl in mice causes defects similar to those seen in murine Hox mutants. Concordantly, formation of supernumerary bones requires Hox11 function, and mutations in the vav2/wasl pathway drive enhanced expression of hoxa11b, indicating developmental homology with the forearm. Our findings reveal a latent, limb-like pattern ability in fins that is activated by simple genetic perturbation.

Chlamydia trachomatis TmeA Directly Activates N-WASP To Promote Actin Polymerization and Functions Synergistically with TarP during Invasion

The increasing genetic tractability of Chlamydia trachomatis is accelerating the ability to characterize the unique infection biology of this obligate intracellular parasite. These efforts are leading to a greater understanding of the molecular events associated with key virulence requirements.

Chlamydia trachomatis is a medically significant human pathogen and is an epithelial-tropic obligate intracellular parasite. Invasion of nonprofessional phagocytes represents a crucial step in the infection process and has likely promoted the evolution of a redundant mechanism and routes of entry. Like many other viral and invasive bacterial pathogens, manipulation of the host cell cytoskeleton represents a focal point in Chlamydia entry. The advent of genetic techniques in C. trachomatis, such as creation of complete gene deletions via fluorescence-reported allelic exchange mutagenesis (FRAEM), is providing important tools to unravel the contributions of bacterial factors in these complex pathways. The type III secretion chaperone Slc1 directs delivery of at least four effectors during the invasion process. Two of these, TarP and TmeA, have been associated with manipulation of actin networks and are essential for normal levels of invasion. The functions of TarP are well established, whereas TmeA is less well characterized. We leverage chlamydial genetics and proximity labeling here to provide evidence that TmeA directly targets host N-WASP to promote Arp2/3-dependent actin polymerization. Our work also shows that TmeA and TarP influence separate, yet synergistic pathways to accomplish chlamydial entry. These data further support an appreciation that a pathogen, confined by a reductionist genome, retains the ability to commit considerable resources to accomplish bottle-neck steps during the infection process.

Cortactin in Epithelial-Mesenchymal Transition

Cortactin, a member of the actin-binding protein family, plays an important role in cell movement involving the cytoskeleton, as cell movement mediated by cortactin may induce the epithelial–mesenchymal transition. Cortactin participates in tumor proliferation, migration, and invasion and other related disease processes by binding to different proteins and participating in different pathways and mechanisms that induce the occurrence of these disease processes. Therefore, this article reviews the correlations between cortactin, the actin cytoskeleton, and the epithelial–mesenchymal transition and discusses its clinical importance in tumor therapy.

Inhibition of N-WASP affects actin-mediated cytokinesis during porcine oocyte maturation

N-WASP is the mammalian ortholog of WASP which is an actin nucleation promoting factor and has been reported to regulate actin nucleation and polymerization for multiple cell activities. However, the expression and functions of N-WASP in porcine oocytes are still unclear. In this study, we showed that N-WASP expressed at all stages during porcine oocyte maturation, and immunofluorescence staining indicated that N-WASP mainly accumulated at the cortex in different stages of meiosis. Inhibition of N-WASP activity by Wiskostatin significantly decreased the rate of first polar body extrusion and disturbed the cell cycle progression of porcine oocytes. Further analysis indicated that cortical actin distribution was interfered by N-WASP inhibition, and this might be through its regulatory roles on the expression and localization of ARP2, a key component of actin nucleator Arp2/3 complex. Moreover, the expression of N-WASP decreased after ROCK activity inhibition, indicating a ROCK-N-WASP-ARP2/3 pathway for actin assembly in porcine oocytes. Taken together, these results suggest that N-WASP is critical for the regulation of actin filaments for cytokinesis during porcine oocyte maturation.

αII-spectrin controls calcium-regulated exocytosis in neuroendocrine chromaffin cells through neuronal Wiskott-Aldrich Syndrome protein interaction

Besides a fundamental structural role at the plasma membrane, spectrin- and actin-based skeletons have been proposed to participate in various processes including vesicular trafficking. Neuroendocrine cells release hormones and neuropeptides through calcium-regulated exocytosis, a process that is coordinated by a fine remodeling of the actin cytoskeleton. We describe here that calcium-regulated exocytosis is impaired in chromaffin and PC12 cells with reduced αII-spectrin expression levels. Using yeast two-hybrid screening, we show that neuronal Wiskott-Aldrich Syndrome protein (N-WASP) is a partner of the αII-spectrin SH3 domain and demonstrate that secretagogue-evoked N-WASP recruitment at cell periphery is blocked in the absence of αII-spectrin. Additionally, experiments performed with ectopically expressed αII-spectrin mutant unable to bind N-WASP indicated that the interaction between SH3 domain and N-WASP is pivotal for neuroendocrine secretion. Our results extend the list of spectrin interactors and strengthen the idea that αII-spectrin is an important scaffold protein that gathers crucial actin-related players of the exocytic machinery.

Concerted regulation of actin polymerization during constitutive secretion by cortactin and PKD2

Constitutive secretion from the trans-Golgi-network (TGN) is facilitated by a concerted regulation of vesicle biogenesis and fission processes. The protein kinase D family (PKD) has been previously described to enhance vesicle fission by modifying the lipid environment. PKD also phosphorylates the actin regulatory protein cortactin at S298 to impair synergistic actin polymerization. We here report additional functions for PKD2 (also known as PRKD2) and cortactin in the regulation of actin polymerization during the fission of transport carriers from the TGN. Phosphorylation of cortactin at S298 impairs the interaction between WIP (also known as WIPF1) and cortactin. WIP stabilizes the autoinhibited conformation of N-WASP (also known as WASL). This leads to an inhibition of synergistic Arp2/3-complex-dependent actin polymerization at the TGN. PKD2 activity at the TGN is controlled by active CDC42-GTP which directly activates N-WASP, inhibits PKD2 and shifts the balance to non-S298-phosphorylated cortactin, which can in turn sequester WIP from N-WASP. Consequently, synergistic actin polymerization at the TGN and constitutive secretion are enhanced.

N-WASP knockdown upregulates inflammatory cytokines expression in human gingival fibroblasts

OBJECTIVE

The neuronal wiskott-aldrich syndrome protein (N-WASP) is a member of the wiskott-aldrich syndrome protein (WASP) family. N-WASP plays a vital role in promoting cell migration, receptor signaling and immune inflammatory responses. This study aimed to observe the changes in the expression of inflammatory factors and involving pathways after N-WASP knockdown in human gingival fibroblasts (HGFs).

DESIGN

Gingival inflammatory condition of N-WASP knockout mice was evaluated by H&E staining. N-WASP in HGFs was knockdown by siRNA and the best knockdown efficiency was determined by qRT-PCR and immunofluorescence. The mRNA levels of interleukin (IL)-6, IL-8, C-C motif ligand 2 (CCL2), superoxide dismutase 2 (SOD2) and prostaglandin endoperoxide synthase 2 (PTGS2) were evaluated by qRT-PCR after N-WASP knockdown with or without mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) inhibitors. The protein levels of IL-6, IL-8 and CCL2 were assessed by ELISA. Western blotting was used to detect the activation of NF-κB and MAPK signaling pathways.

RESULTS

Gingival tissue from N-WASP knockout mice exhibited an inflammatory reaction. The expression of IL-6, IL-8, CCL2, SOD2 and PTGS2 was significantly upregulated after N-WASP knockdown in HGFs for 6, 24 and 48 h, except for the SOD2 at 6 h. N-WASP knockdown significantly activated the signaling pathways of NF-κB and MAPK. The inhibitors of p65, p38, ERK and JNK clearly decreased IL-6, IL-8, CCL2, SOD2 and PTGS2 expression after N-WASP knockdown.

CONCLUSION

These data indicated that N-WASP deficiency in HGFs increases the production of inflammatory cytokine and is regulated via NF-κB and MAPK signaling pathways.

Molecular Actions of Thyroid Hormone on Breast Cancer Cell Migration and Invasion via Cortactin/N-WASP

The thyroid hormone triiodothyronine (T3) plays a fundamental role in growth regulation, differentiation, metabolism and cellular movement. These processes are particularly important considering that deregulation of T3 levels could promote abnormal responsiveness of mammary epithelial cells, which may lead to the development and progression of breast cancer (BC). Once cells migrate and invade different tissues, BC metastasis is the main cause of cancer-related death because it is particularly difficult to revert this multistep process. Cell migration integrates several steps that induce changes in cell structure and morphology to promote BC cell invasion. These sequential steps include actin cytoskeleton remodeling, focal adhesion complex formation and, finally, the turnover of branched actin filament networks. In this article, we demonstrate that T3 has the ability to modify the Epithelial-Mesenchymal Transition process. In addition, we show that T3 induces actin cytoskeleton reorganization, triggers focal adhesion formation and, as a consequence, promotes actin nucleation via non-genomic pathway. These events are specifically modulated by T3 via integrin αvβ3 to FAK/paxillin/cortactin/N-WASP/Arp2/3 complex signaling pathway, increasing cell adhesion, migration and invasion of T-47D BC cells. We suggest that T3 influences the progression of tumor metastasis by controlling signaling pathways that converge in cell motility. This knowledge is crucial for the development of novel therapeutic strategies for BC treatment.

ARP3 Controls the Podocyte Architecture at the Kidney Filtration Barrier

Podocytes, highly specialized epithelial cells, build the outer part of the kidney filtration barrier and withstand high mechanical forces through a complex network of cellular protrusions. Here, we show that Arp2/3-dependent actin polymerization controls actomyosin contractility and focal adhesion maturation of podocyte protrusions and thereby regulates formation, maintenance, and capacity to adapt to mechanical requirements of the filtration barrier. We find that N-WASP-Arp2/3 define the development of complex arborized podocyte protrusions in vitro and in vivo. Loss of dendritic actin networks results in a pronounced activation of the actomyosin cytoskeleton and the generation of over-maturated but less efficient adhesion, leading to detachment of podocytes. Our data provide a model to explain podocyte protrusion morphology and their mechanical stability based on a tripartite relationship between actin polymerization, contractility, and adhesion.

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ARP3-dependent actin assembly is required for podocyte process formation

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Arp2/3 thereby links process formation, podocyte adhesion and mechano-adaptation

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Arp2/3 function is regulated by a reciprocal interplay with actomyosin

Loss of N-WASP drives early progression in an Apc model of intestinal tumourigenesis

N-WASP (WASL) is a widely expressed cytoskeletal signalling and scaffold protein also implicated in regulation of Wnt signalling and homeostatic maintenance of skin epithelial architecture. N-WASP mediates invasion of cancer cells in vitro and its depletion reduces invasion and metastatic dissemination of breast cancer. Given this role in cancer invasion and universal expression in the gastrointestinal tract, we explored a role for N-WASP in the initiation and progression of colorectal cancer. While deletion of N-wasp is not detectably harmful in the murine intestinal tract, numbers of Paneth cells increased, indicating potential changes in the stem cell niche, and migration up the crypt-villus axis was enhanced. Loss of N-wasp promoted adenoma formation in an adenomatous polyposis coli (Apc) deletion model of intestinal tumourigenesis. Thus, we establish a tumour suppressive role of N-WASP in early intestinal carcinogenesis despite its later pro-invasive role in other cancers. Our study highlights that while the actin cytoskeletal machinery promotes invasion of cancer cells, it also maintains normal epithelial tissue function and thus may have tumour suppressive roles in pre-neoplastic tissues. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

WIP/ITSN1 complex is involved in cellular vesicle trafficking and formation of filopodia-like protrusions

WIP (WASP interacting protein) together with N-WASP (neural Wiskott-Aldrich syndrome protein) regulates actin polymerization that is crucial for invadopodia and filopodia formation. Recently, we reported the WIP interaction with ITSN1 which is highly implicated in endo-/exocytosis, apoptosis, mitogenic signaling and cytoskeleton rearrangements. Here we demonstrate that the WIP/ITSN1 complex is involved in the transferrin receptor recycling and partially co-localizes with a marker of the fast recycling endosomes, RAB4. Moreover, ITSN1 recruits WIP to RAB4-positive vesicles upon overexpression. Our data indicate that WIP enhances the interaction of N-WASP with ITSN1 and promotes ITSN1/β-actin association. Moreover, the WIP/ITSN1-L complex facilitates formation of filopodia-like protrusions in MCF-7 cells. Thus, WIP/ITSN1 complex is involved in the cellular vesicle trafficking and actin-dependent membrane processes.

Attaching-and-Effacing Pathogens Exploit Junction Regulatory Activities of N-WASP and SNX9 to Disrupt the Intestinal Barrier

BACKGROUND & AIMS

Neural Wiskott-Aldrich Syndrome protein (N-WASP) is a key regulator of the actin cytoskeleton in epithelial tissues and is poised to mediate cytoskeletal-dependent aspects of apical junction complex (AJC) homeostasis. Attaching-and-effacing (AE) pathogens disrupt this homeostasis through translocation of the effector molecule early secreted antigenic target-6 (ESX)-1 secretion-associated protein F (EspF). Although the mechanisms underlying AJC disruption by EspF are unknown, EspF contains putative binding sites for N-WASP and the endocytic regulator sorting nexin 9 (SNX9). We hypothesized that N-WASP regulates AJC integrity and AE pathogens use EspF to induce junction disassembly through an N-WASP- and SNX9-dependent pathway.

METHODS

We analyzed mice with intestine-specific N-WASP deletion and generated cell lines with N-WASP and SNX9 depletion for dynamic functional assays. We generated EPEC and Citrobacter rodentium strains complemented with EspF bearing point mutations abolishing N-WASP and SNX9 binding to investigate the requirement for these interactions.

RESULTS

Mice lacking N-WASP in the intestinal epithelium showed spontaneously increased permeability, abnormal AJC morphology, and mislocalization of occludin. N-WASP depletion in epithelial cell lines led to impaired assembly and disassembly of tight junctions in response to changes in extracellular calcium. Cells lacking N-WASP or SNX9 supported actin pedestals and type III secretion, but were resistant to EPEC-induced AJC disassembly and loss of transepithelial resistance. We found that during in vivo infection with AE pathogens, EspF must bind both N-WASP and SNX9 to disrupt AJCs and induce intestinal barrier dysfunction.

CONCLUSIONS

Overall, these studies show that N-WASP critically regulates AJC homeostasis, and the AE pathogen effector EspF specifically exploits both N-WASP and SNX9 to disrupt intestinal barrier integrity during infection.

VEGF Triggers the Activation of Cofilin and the Arp2/3 Complex within the Growth Cone

A crucial neuronal structure for the development and regeneration of neuronal networks is the axonal growth cone. Affected by different guidance cues, it grows in a predetermined direction to reach its final destination. One of those cues is the vascular endothelial growth factor (VEGF), which was identified as a positive effector for growth cone movement. These positive effects are mainly mediated by a reorganization of the actin network. This study shows that VEGF triggers a tight colocalization of cofilin and the Arp2/3 complex to the actin cytoskeleton within chicken dorsal root ganglia (DRG). Live cell imaging after microinjection of GFP (green fluorescent protein)-cofilin and RFP (red fluorescent protein)-LifeAct revealed that both labeled proteins rapidly redistributed within growth cones, and showed a congruent distribution pattern after VEGF supplementation. Disruption of signaling upstream of cofilin via blocking LIM-kinase (LIMK) activity resulted in growth cones displaying regressive growth behavior. Microinjection of GFP-p16b (a subunit of the Arp2/3 complex) and RFP-LifeAct revealed that both proteins redistributed into lamellipodia of the growth cone within minutes after VEGF stimulation. Disruption of the signaling to the Arp2/3 complex in the presence of VEGF by inhibition of N-WASP (neuronal Wiskott–Aldrich–Scott protein) caused retraction of growth cones. Hence, cofilin and the Arp2/3 complex appear to be downstream effector proteins of VEGF signaling to the actin cytoskeleton of DRG growth cones. Our data suggest that VEGF simultaneously affects different pathways for signaling to the actin cytoskeleton, since activation of cofilin occurs via inhibition of LIMK, whereas activation of Arp2/3 is achieved by stimulation of N-WASP.

Attaching-and-Effacing Pathogens Exploit Junction Regulatory Activities of N-WASP and SNX9 to Disrupt the Intestinal Barrier

BACKGROUND & AIMS

Neural Wiskott-Aldrich Syndrome protein (N-WASP) is a key regulator of the actin cytoskeleton in epithelial tissues and is poised to mediate cytoskeletal-dependent aspects of apical junction complex (AJC) homeostasis. Attaching-and-effacing (AE) pathogens disrupt this homeostasis through translocation of the effector molecule early secreted antigenic target-6 (ESX)-1 secretion-associated protein F (EspF). Although the mechanisms underlying AJC disruption by EspF are unknown, EspF contains putative binding sites for N-WASP and the endocytic regulator sorting nexin 9 (SNX9). We hypothesized that N-WASP regulates AJC integrity and AE pathogens use EspF to induce junction disassembly through an N-WASP- and SNX9-dependent pathway.

METHODS

We analyzed mice with intestine-specific N-WASP deletion and generated cell lines with N-WASP and SNX9 depletion for dynamic functional assays. We generated EPEC and Citrobacter rodentium strains complemented with EspF bearing point mutations abolishing N-WASP and SNX9 binding to investigate the requirement for these interactions.

RESULTS

Mice lacking N-WASP in the intestinal epithelium showed spontaneously increased permeability, abnormal AJC morphology, and mislocalization of occludin. N-WASP depletion in epithelial cell lines led to impaired assembly and disassembly of tight junctions in response to changes in extracellular calcium. Cells lacking N-WASP or SNX9 supported actin pedestals and type III secretion, but were resistant to EPEC-induced AJC disassembly and loss of transepithelial resistance. We found that during in vivo infection with AE pathogens, EspF must bind both N-WASP and SNX9 to disrupt AJCs and induce intestinal barrier dysfunction.

CONCLUSIONS

Overall, these studies show that N-WASP critically regulates AJC homeostasis, and the AE pathogen effector EspF specifically exploits both N-WASP and SNX9 to disrupt intestinal barrier integrity during infection.

SP1-induced up-regulation of lncRNA SNHG14 as a ceRNA promotes migration and invasion of clear cell renal cell carcinoma by regulating N-WASP

Clear cell renal cell carcinoma (ccRCC) is one of the leading causes of genitourinary cancer-related death, largely due to the metastasis of ccRCC. Previous profiling study showed that lncRNAs are critical regulators in lots of cancers. However, the roles of specific lncRNAs in ccRCC migration and invasion are still unknown. In this study, we utilized the high-throughput genome sequencing to identify the potential differentially expressed lncRNAs in ccRCC and further determined the underlying regulatory mechanism. We found that lncRNA SNHG14 was significantly up-regulated in ccRCC cell lines in contrast to normal renal epithelial cells. By performing bioinformatics analysis and luciferase reporter assays, we revealed that the transcription factor SP1 can bind to the promoter region of SNHG14, resulting in the overexpression of SNHG14 in ccRCC. Functionally, enhanced expression of lncRNA SNHG14 promoted cell migration and invasion through promoting N-WASP protein level. More importantly, RT-qPCR and in situ RNA FISH analysis showed that SNHG14 was predominantly abundant in the cytoplasm of ccRCC cells. The subsequent RNA immunoprecipitation assay, and gain or loss-function assays showed that SNHG14 functioned as ceRNA to regulate N-WASP expression and cell motility ability via a miR-203-dependent manner. Our results imply that SNHG14 is a critical lncRNA that promotes ccRCC migration and invasion via sponging miR-203 and elevating N-WASP. Therefore, SNHG14 could serve as a promising therapeutic target for ccRCC.

Cellular functions of WASP family proteins at a glance

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family function as nucleation-promoting factors for the ubiquitously expressed Arp2/3 complex, which drives the generation of branched actin filaments. Arp2/3-generated actin regulates diverse cellular processes, including the formation of lamellipodia and filopodia, endocytosis and/or phagocytosis at the plasma membrane, and the generation of cargo-laden vesicles from organelles including the Golgi, endoplasmic reticulum (ER) and the endo-lysosomal network. Recent studies have also identified roles for WASP family members in promoting actin dynamics at the centrosome, influencing nuclear shape and membrane remodeling events leading to the generation of autophagosomes. Interestingly, several WASP family members have also been observed in the nucleus where they directly influence gene expression by serving as molecular platforms for the assembly of epigenetic and transcriptional machinery. In this Cell Science at a Glance article and accompanying poster, we provide an update on the subcellular roles of WHAMM, JMY and WASH (also known as WASHC1), as well as their mechanisms of regulation and emerging functions within the cell.

The F-Actin Binding Protein Cortactin Regulates the Dynamics of the Exocytotic Fusion Pore through its SH3 Domain

Upon cell stimulation, the network of cortical actin filaments is rearranged to facilitate the neurosecretory process. This actin rearrangement includes both disruption of the preexisting actin network and de novo actin polymerization. However, the mechanism by which a Ca²⁺ signal elicits the formation of new actin filaments remains uncertain. Cortactin, an actin-binding protein that promotes actin polymerization in synergy with the nucleation promoting factor N-WASP, could play a key role in this mechanism. We addressed this hypothesis by analyzing de novo actin polymerization and exocytosis in bovine adrenal chromaffin cells expressing different cortactin or N-WASP domains, or cortactin mutants that fail to interact with proline-rich domain (PRD)-containing proteins, including N-WASP, or to be phosphorylated by Ca²⁺-dependent kinases, such as ERK1/2 and Src. Our results show that the activation of nicotinic receptors in chromaffin cells promotes cortactin translocation to the cell cortex, where it colocalizes with actin filaments. We further found that, in association with PRD-containing proteins, cortactin contributes to the Ca²⁺-dependent formation of F-actin, and regulates fusion pore dynamics and the number of exocytotic events induced by activation of nicotinic receptors. However, whereas the actions of cortactin on the fusion pore dynamics seems to depend on the availability of monomeric actin and its phosphorylation by ERK1/2 and Src kinases, cortactin regulates the extent of exocytosis by a mechanism independent of actin polymerization. Together our findings point out a role for cortactin as a critical modulator of actin filament formation and exocytosis in neuroendocrine cells.

N-WASP promotes invasion and migration of cervical cancer cells through regulating p38 MAPKs signaling pathway

Neural Wiskott-Aldrich syndrome protein (N-WASP) is an important member of the WASP family involved in the actin cytoskeleton reorganization. Recent evidence suggests that N-WASP may play important roles in tumor progression and metastasis. However, the contribution of N-WASP to cervical cancer is still unknown. The present study focused on elucidating the role of N-WASP in the malignant behavior of cervical cancer cells. We found that N-WASP overexpressed in cervical cancer tissues compared with paired paracancerous tissues and normal tissues, and similar results were observed in several cervical cancer cell lines. Furthermore, we demonstrated that overexpression of N-WASP facilitated migration and invasion of cervical cancer cells, while downregulation of N-WASP resulted in decreased cell migration and invasion. In addition, the data showed that N-WASP might promote invasion and migration of cervical cancer cells via regulating the activity of p38 MAPKs pathway. Altogether, the study suggested that N-WASP might serve as an oncogene in cervical cancer, and provided novel insights into the mechanism that how N-WASP promoted invasion and migration of cervical cancer cells.

CDC42-interacting protein 4 promotes metastasis of nasopharyngeal carcinoma by mediating invadopodia formation and activating EGFR signaling

Background

Nasopharyngeal carcinoma (NPC) is a common malignancy in Southern China and Southeast Asia. In this study, we investigated the functional and molecular mechanisms by which CDC42-interacting protein 4 (CIP4) influences NPC.

Methods

The expression levels of CIP4 were examined by Western blot, qRT-PCR or IHC. MTT assay was used to detect the proliferative rate of NPC cells. The invasive abilities were examined by matrigel and transwell assay. The metastatic abilities of NPC cells were revealed in BALB/c nude mice.

Results

We report that CIP4 is required for NPC cell motility and invasion. CIP4 promotes the activation of N-WASP that controls invadopodia formation and activates EGFR signaling, which induces downstream MMP2 (matrix metalloproteinase 2) upregulation. In addition, CIP4 could promote NPC metastasis by activating the EGFR pathway. In nude mouse models, distant metastasis was significantly inhibited in CIP4-silenced groups. High CIP4 expression is an independent adverse prognostic factor of overall survival (OS) and distant metastasis-free survival (DMFS).

Conclusion

We identify the critical role of CIP4 in metastasis of NPC which suggest that CIP4 may be a potential therapeutic target of NPC patients.

Myogenic differentiation depends on the interplay of Grb2 and N-WASP

Myogenesis requires a well-coordinated withdrawal from cell cycle, morphological changes and cell fusion mediated by actin cytoskeleton. Grb2 is an adaptor protein whose central SH2 domain binds to phosphorylated tyrosine residues of activated receptors and activates intracellular signaling pathway, while its N-terminal and C-terminal SH3 domains bind to proline rich proteins such as N-WASP (Neural-Wiskott Aldrich Syndrome Protein). We found that the expression of Grb2 was increased at the beginning of differentiation and remained constant during differentiation in C2C12 myoblasts. Knocking down endogenous Grb2 expression caused a significant increase in the fusion index and expression of MyHC, a terminal differentiation marker when compared with the control. Over expression of Grb2 in C2C12 (C2C12^Grb2-Myc) reduced myotube formation and expression of MyHC. Similarly over expression of Grb2^P49L-Myc (N-terminal SH3 domain mutant) or Grb2^R86K-Myc (SH2 domain mutant) inhibited myogenic differentiation of C2C12 cells. However, the expression of Grb2^P206L-Myc (C-terminal SH3 domain mutant) did not inhibit myotube formation and expression of MyHC. This suggests that the C-terminal SH3 domain of Grb2 is critical for the inhibition of myogenic differentiation. The C2C12^Grb2-Myc cells have reduced phalloidin staining at late stages of differentiation. Expression of N-WASP in C2C12^Grb2-Myc cells rescued the myogenic defect and increased phalloidin staining (increased F-actin) in these cells. Thus our results suggest that Grb2 is a negative regulator of myogenesis and reduces myogenic differentiation by inhibiting actin polymerization/remodeling through its C-terminal SH3 domain.

Expression of N-WASP is regulated by HiF1α through the hypoxia response element in the N-WASP promoter

Cancer cell migration and invasion involves temporal and spatial regulation of actin cytoskeleton reorganization, which is regulated by the WASP family of proteins such as N-WASP (Neural- Wiskott Aldrich Syndrome Protein). We have previously shown that expression of N-WASP was increased under hypoxic conditions. In order to characterize the regulation of N-WASP expression, we constructed an N-WASP promoter driven GFP reporter construct, N-WASP^pro-GFP. Transfection of N-WASP^pro-GFP construct and plasmid expressing HiF1α (Hypoxia Inducible factor 1α) enhanced the expression of GFP suggesting that increased expression of N-WASP under hypoxic conditions is mediated by HiF1α. Sequence analysis of the N-WASP promoter revealed the presence of two hypoxia response elements (HREs) characterized by the consensus sequence 5′-GCGTG-3′ at -132 bp(HRE1) and at -662 bp(HRE2) relative to transcription start site (TSS). Site-directed mutagenesis of HRE1(-132) but not HRE2(-662) abolished the HiF1α induced activation of N-WASP promoter. Similarly ChIP assay demonstrated that HiF1α bound to HRE1(-132) but not HRE2(-662) under hypoxic condition. MDA-MB-231 cells but not MDA-MB-231^KD cells treated with hypoxia mimicking agent, DMOG showed enhanced gelatin degradation. Similarly MDA-MB-231^KD(N-WASP^pro-N-WASP^R) cells expressing N-WASP^R under the transcriptional regulation of WT N-WASP^pro but not MDA-MB-231^KD(N-WASP^proHRE1-N-WASP^R) cells expressing N-WASP^R under the transcriptional regulation of N-WASP^proHRE1 showed enhanced gelatin degradation when treated with DMOG. Thus indicating the importance of N-WASP in hypoxia induced invadopodia formation. Thus, our data demonstrates that hypoxia-induced activation of N-WASP expression is mediated by interaction of HiF1α with the HRE1(-132) and explains the role of N-WASP in hypoxia induced invadopodia formation.

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Expression of N-WASP expression is enhanced under hypoxia conditions.

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N-WASP is essential for hypoxia induced invasion.

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HiF1α binds to hypoxia response element (HRE) in N-WASP promoter.

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HRE1 is essential for hypoxia induced invadopodia activity

Growth-arrest-specific 7C protein inhibits tumor metastasis via the N-WASP/FAK/F-actin and hnRNP U/β-TrCP/β-catenin pathways in lung cancer

Growth-arrest-specific 7 (GAS7) belongs to a group of adaptor proteins that coordinate the actin cytoskeleton. Among human GAS7 isoforms, only GAS7C possesses a Src homology 3 domain. We report here that GAS7C acts as a migration suppressor and can serve as a prognostic biomarker in lung cancer. GAS7C overexpression reduces lung cancer migration, whereas GAS7C knockdown enhances cancer cell migration. Importantly, ectopically overexpressed GAS7C binds tightly with N-WASP thus inactivates the fibronectin/integrin/FAK pathway, which in turn leads to the suppression of F-actin dynamics. In addition, overexpression of GAS7C sequesters hnRNP U and thus decreases the level of β-catenin protein via the β-TrCP ubiquitin-degradation pathway. The anti-metastatic effect of GAS7C overexpression was also confirmed using lung cancer xenografts. Our clinical data indicated that 23.6% (25/106) of lung cancer patients showed low expression of GAS7C mRNA which correlated with a poorer overall survival. In addition, low GAS7C mRNA expression was detected in 60.0% of metastatic lung cancer patients, indicating an association between low GAS7C expression and cancer progression. A significant inverse correlation between mRNA expression and promoter hypermethylation was also found, which suggests that the low level of GAS7C expression was partly due to promoter hypermethylation. Our results provide novel evidence that low GAS7C correlates with poor prognosis and promotes metastasis in lung cancer. Low GAS7C increases cancer cell motility by promoting N-WASP/FAK/F-actin cytoskeleton dynamics. It also enhances β-catenin stability via hnRNP U/β-TrCP complex formation. Therefore, GAS7C acts as a metastasis suppressor in lung cancer.

Cdc42 in actin dynamics: An ordered pathway governed by complex equilibria and directional effector handover

The small GTPase, Cdc42, is a key regulator of actin dynamics, functioning to connect multiple signals to actin polymerization through effector proteins of the Wiskott-Aldrich syndrome protein (WASP) and Transducer of Cdc42-dependent actin assembly (TOCA) families. WASP family members serve to couple Cdc42 with the actin nucleator, the Arp2/3 complex, via direct interactions. The regulation of these proteins in the context of actin dynamics has been extensively studied. Studies on the TOCA family, however, are more limited and relatively little is known about their roles and regulation. In this commentary we highlight new structural and biophysical insight into the involvement of TOCA proteins in the pathway of Cdc42-dependent actin dynamics. We discuss the biological implications of the low affinity interactions between the TOCA family and Cdc42, as well as probing the sequential binding of TOCA1 and the WASP homolog, N-WASP, to Cdc42. We place our current research in the context of the wealth of biophysical, structural and functional data from earlier studies pertaining to the Cdc42/N-WASP/Arp2/3 pathway of actin polymerization. Finally, we describe the molecular basis for a sequential G protein-effector handover from TOCA1 to N-WASP.

Paxillin, a novel controller in the signaling of estrogen to FAK/N-WASP/Arp2/3 complex in breast cancer cells

Breast cancer is the major cause of cancer-related death in women. Its treatment is particularly difficult when metastasis occurs. The ability of cancer cells to move and invade the surrounding environment is the basis of local and distant metastasis. Cancer cells are able to remodel the actin cytoskeleton, which requires the recruitment of numerous structural and regulatory proteins that modulate actin filaments dynamics, including Paxillin or the Neural Wiskott-Aldrich Syndrome Protein (N-WASP). We show that 17-β estradiol (E2) induces phosphorylation of Paxillin and its translocation toward membrane sites where focal adhesion complexes are assembled. This cascade is triggered by a Gαi1/Gβ protein-dependent signaling of estrogen receptor α (ERα) to c-Src, focal adhesion kinase (FAK) and Paxillin. Within this complex, activated Paxillin recruits the small GTPase Cdc42, which triggers N-WASP phosphorylation. This results in the redistribution of Arp2/3 complexes at sites where membrane structures related to cell movement are formed. Recruitment of Paxillin, Cdc42 and N-WASP is necessary for cell adhesion, migration and invasion induced by E2 in breast cancer cells. In parallel, we investigated whether Raloxifene (RAL), a selective estrogen receptor modulator (SERMs), could inhibit or revert the effects of E2 in breast cancer cell movement. We found that, in the presence of E2, RAL acts as an ER antagonist and displays an inhibitory effect on estrogen-promoted cell adhesion and migration via FAK/Paxillin/N-WASP. Our findings identify an original mechanism through which estrogen regulates breast cancer cell motility and invasion via Paxillin. These results may have clinical relevance for the development of new therapeutic strategies for cancer treatment.

Oocyte-specific deletion of N-WASP does not affect oocyte polarity, but causes failure of meiosis II completion

STUDY QUESTION

There is an unexplored physiological role of N-WASP (neural Wiskott-Aldrich syndrome protein) in oocyte maturation that prevents completion of second meiosis.

SUMMARY ANSWER

In mice, N-WASP deletion did not affect oocyte polarity and asymmetric meiotic division in first meiosis, but did impair midbody formation and second meiosis completion.

WHAT IS KNOWN ALREADY

N-WASP regulates actin dynamics and participates in various cell activities through the RHO-GTPase-Arp2/3 (actin-related protein 2/3 complex) pathway, and specifically the Cdc42 (cell division cycle 42)-N-WASP-Arp2/3 pathway. Differences in the functions of Cdc42 have been obtained from in vitro compared to in vivo studies.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

By conditional knockout of N-WASP in mouse oocytes, we analyzed its in vivo functions by employing a variety of different methods including oocyte culture, immunofluorescent staining and live oocyte imaging. Each experiment was repeated at least three times, and data were analyzed by paired-samples t-test.

MAIN RESULTS AND THE ROLE OF CHANCE

Oocyte-specific deletion of N-WASP did not affect the process of oocyte maturation including spindle formation, spindle migration, polarity establishment and maintenance, and homologous chromosome or sister chromatid segregation, but caused failure of cytokinesis completion during second meiosis (P < 0.001 compared to control). Further analysis showed that a defective midbody may be responsible for the failure of cytokinesis completion.

LIMITATIONS, REASONS FOR CAUTION

The present study did not include a detailed analysis of the mechanisms underlying the results, which will require more extensive further investigations.

WIDER IMPLICATIONS OF THE FINDINGS

N-WASP may play an important role in mediating and co-ordinating the activity of the spindle (midbody) and actin (contractile ring constriction) when cell division occurs. The findings are important for understanding the regulation of oocyte meiosis completion and failures in this process that affect oocyte quality.

LARGE SCALE DATA

None.

STUDY FUNDING AND COMPETING INTERESTS

This work was supported by the National Basic Research Program of China (No. 2012CB944404) and the National Natural Science Foundation of China (Nos 30930065, 31371451, 31272260 and 31530049). There are no potential conflicts of interests.

RTVP-1 regulates glioma cell migration and invasion via interaction with N-WASP and hnRNPK

Glioblastoma (GBM) are characterized by increased invasion into the surrounding normal brain tissue. RTVP-1 is highly expressed in GBM and regulates the migration and invasion of glioma cells. To further study RTVP-1 effects we performed a pull-down assay using His-tagged RTVP-1 followed by mass spectrometry and found that RTVP-1 was associated with the actin polymerization regulator, N-WASP. This association was further validated by co-immunoprecipitation and FRET analysis. We found that RTVP-1 increased cell spreading, migration and invasion and these effects were at least partly mediated by N-WASP. Another protein which was found by the pull-down assay to interact with RTVP-1 is hnRNPK. This protein has been recently reported to associate with and to inhibit the effect of N-WASP on cell spreading. hnRNPK decreased cell migration, spreading and invasion in glioma cells. Using co-immunoprecipitation we validated the interactions of hnRNPK with N-WASP and RTVP-1 in glioma cells. In addition, we found that overexpression of RTVP-1 decreased the association of N-WASP and hnRNPK. In summary, we report that RTVP-1 regulates glioma cell spreading, migration and invasion and that these effects are mediated via interaction with N-WASP and by interfering with the inhibitory effect of hnRNPK on the function of this protein.

A role for the small GTPase Rac1 in vaccinia actin-based motility

Vaccinia virus dissemination relies on the recruitment of the nucleation promoting factor N-WASP underneath cell-associated extracellular virus (CEVs) and subsequent recruitment and activation of the ARP2/3 complex, a major actin nucleator of the host cell. We have recently discovered that, in addition to the N-WASP/ARP2/3 pathway, vaccinia actin-based motility also relies on the small GTPase Rac1 and its downstream effector the formin-type actin nucleator FHOD1. Here we discuss the potential signaling mechanisms supporting the integration of the N-WASP/ARP2/3 and Rac1/FHOD1 pathways. We suggest the existence of a receptor tyrosine kinase family member that would integrate the Src-dependent activation of the N-WASP/ARP2/3 pathway and the GTP exchange factor-dependent activation of the Rac1/FHOD1 pathway.

Molecular and Cellular Mechanisms of Shigella flexneri Dissemination

The intracellular pathogen Shigella flexneri is the causative agent of bacillary dysentery in humans. The disease is characterized by bacterial invasion of intestinal cells, dissemination within the colonic epithelium through direct spread from cell to cell, and massive inflammation of the intestinal mucosa. Here, we review the mechanisms supporting S. flexneri dissemination. The dissemination process primarily relies on actin assembly at the bacterial pole, which propels the pathogen throughout the cytosol of primary infected cells. Polar actin assembly is supported by polar expression of the bacterial autotransporter family member IcsA, which recruits the N-WASP/ARP2/3 actin assembly machinery. As motile bacteria encounter cell-cell contacts, they form plasma membrane protrusions that project into adjacent cells. In addition to the ARP2/3-dependent actin assembly machinery, protrusion formation relies on formins and myosins. The resolution of protrusions into vacuoles occurs through the collapse of the protrusion neck, leading to the formation of an intermediate membrane-bound compartment termed vacuole-like protrusions (VLPs). VLP formation requires tyrosine kinase and phosphoinositide signaling in protrusions, which relies on the integrity of the bacterial type 3 secretion system (T3SS). The T3SS is also required for escaping double membrane vacuoles through the activity of the T3SS translocases IpaB and IpaC, and the effector proteins VirA and IcsB. Numerous factors supporting envelope biogenesis contribute to IcsA exposure and maintenance at the bacterial pole, including LPS synthesis, membrane proteases, and periplasmic chaperones. Although less characterized, the assembly and function of the T3SS in the context of bacterial dissemination also relies on factors supporting envelope biogenesis. Finally, the dissemination process requires the adaptation of the pathogen to various cellular compartments through transcriptional and post-transcriptional mechanisms.

Invadopodia proteins, cortactin, N-WASP and WIP differentially promote local invasiveness in ameloblastoma

BACKGROUND

Cell migration and invasion through interstitial tissues are dependent upon several specialized characteristics of the migratory cell notably generation of proteolytic membranous protrusions or invadopodia. Ameloblastoma is a benign odontogenic epithelial neoplasm with a locally infiltrative behaviour. Cortactin and MMT1-MMP are two invadopodia proteins implicated in its local invasiveness. Other invadopodia regulators, namely N-WASP, WIP and Src kinase remain unclarified. This study addresses their roles in ameloblastoma.

MATERIALS AND METHOD

Eighty-seven paraffin-embedded ameloblastoma cases (20 unicystic, 47 solid/multicystic, 3 desmoplastic and 17 recurrent) were subjected to immunohistochemistry for expression of cortactin, N-WASP, WIP, Src kinase and F-actin, and findings correlated with clinicopathological parameters.

RESULTS

Invadopodia proteins (except Src kinase) and F-actin were widely detected in ameloblastoma (cortactin: n = 73/87, 83.9%; N-WASP: n = 59/87; 67.8%; WIP: n = 77/87; 88.5%; and F-actin: n = 87/87, 100%). Protein localization was mainly cytoplasmic and/or membranous, and occasionally nuclear for F-actin. Cortactin, which functions as an actin-scaffolding protein, demonstrated significantly higher expression levels within ameloblastoma tumoral epithelium than in stroma (P < 0.05). N-WASP, which coordinates actin polymerization and invadopodia-mediated extracellular matrix degradation, was overexpressed in the solid/multicystic subtype (P < 0.05). WIP, an upstream regulator of N-WASP, and F-actin were significantly upregulated along the tumour invasive front compared to tumour centres (P < 0.05). Except for males with cortactin overexpression, other clinical parameters (age, ethnicity and anatomical site) showed no significant correlations.

CONCLUSIONS

Present results suggest that local invasiveness of ameloblastoma is dependent upon the migratory potential of its tumour cells as defined by their distribution of cortactin, N-WASP and WIP in correlation with F-actin cytoskeletal dynamics.

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.

Viruses that ride on the coat-tails of actin nucleation

Actin nucleation drives a diversity of critical cellular processes and the motility of a select group of viral pathogens. Vaccinia virus and baculovirus, Autographa californica multiple nucleopolyhedrovirus, recruit and activate the cellular actin nucleator, the Arp2/3 complex, at the surface of virus particles thereby instigating highly localized actin nucleation. The extension of these filaments provides a mechanical force that bestows the ability to navigate the intracellular environment and promote their infectious cycles. This review outlines the viral and cellular proteins that initiate and regulate the signalling networks leading to viral modification of the actin cytoskeleton and summarizes recent insights into the role of actin-based virus transport.

Role of N-WASP in Endothelial Monolayer Formation and Integrity

Endothelial cells (ECs) form a monolayer that serves as a barrier between the blood and the underlying tissue. ECs tightly regulate their cell-cell junctions, controlling the passage of soluble materials and immune cells across the monolayer barrier. We studied the role of N-WASP, a key regulator of Arp2/3 complex and actin assembly, in EC monolayers. We report that N-WASP regulates endothelial monolayer integrity by affecting the organization of cell junctions. Depletion of N-WASP resulted in an increase in transendothelial electrical resistance, a measure of monolayer integrity. N-WASP depletion increased the width of cell-cell junctions and altered the organization of F-actin and VE-cadherin at junctions. N-WASP was not present at cell-cell junctions in monolayers under resting conditions, but it was recruited following treatment with sphingosine-1-phosphate. Taken together, our results reveal a novel role for N-WASP in remodeling EC junctions, which is critical for monolayer integrity and function.

Triggering actin polymerization in Xenopus egg extracts from phosphoinositide-containing lipid bilayers

Xenopus egg extracts are a powerful tool to reconstitute complex cell biological processes using a cell-free strategy. When used in conjunction with liposomes and supported lipid bilayers, they can recapitulate the biochemical activities occurring at the cytosol/plasma membrane interface of the cell that underlie remodeling of the actin cytoskeleton. We use these in vitro systems to elucidate how membranes and proteins collaborate to make the appropriate actin structure at a given time and place. We have recently broadened the types of membrane substrate used, and also optimized protocols for preparation of Xenopus egg extracts for actin assembly assays from membranes. Tuning the lipid composition and curvature appropriately demands an appreciation of the native phospholipid and curvature environments that can form transiently in cells. Supported lipid bilayers on glass coverslips that contain phosphatidylserine and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) make actin bundles termed filopodia-like structures that contain fascin and have vasodilator-stimulated phosphoprotein (VASP) at their growing tips, indicating that these resemble filopodia growing from the plasma membrane. The combination of PI(4,5)P2 and phosphatidylinositol 3-phosphate in curved liposomes or supported bilayers on glass nanospheres uses Snx9, Cdc42, N-WASP (neuronal-Wiskott-Aldrich syndrome protein), and Arp2/3 complex for actin polymerization suggesting that this membrane may mimic the progression from plasma membrane to endosomes. Here we describe how to prepare high-speed supernatant frog egg extracts and phosphoinositide-containing liposomes and supported lipid bilayers that can assemble actin structures. We also describe the methods we use to assay actin polymerization using microscopy and spectrofluorometry and our protocol for immunodepleting specific proteins from extracts.