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1.  Actin-Binding Protein 1 Regulates B Cell Receptor-Mediated Antigen Processing and Presentation in Response to B Cell Receptor Activation1 
The BCR serves as both signal transducer and Ag transporter. Binding of Ags to the BCR induces signaling cascades and Ag processing and presentation, two essential cellular events for B cell activation. BCR-initiated signaling increases BCR-mediated Ag-processing efficiency by increasing the rate and specificity of Ag transport. Previous studies showed a critical role for the actin cytoskeleton in these two processes. In this study, we found that actin-binding protein 1 (Abp1/HIP-55/SH3P7) functioned as an actin-binding adaptor protein, coupling BCR signaling and Ag-processing pathways with the actin cytoskeleton. Gene knockout of Abp1 and overexpression of the Src homology 3 domain of Abp1 inhibited BCR-mediated Ag internalization, consequently reducing the rate of Ag transport to processing compartments and the efficiency of BCR-mediated Ag processing and presentation. BCR activation induced tyrosine phosphorylation of Abp1 and translocation of both Abp1 and dynamin 2 from the cytoplasm to plasma membrane, where they colocalized with the BCR and cortical F-actin. Mutations of the two tyrosine phosphorylation sites of Abp1 and depolymerization of the actin cytoskeleton interfered with BCR-induced Abp1 recruitment to the plasma membrane. The inhibitory effect of a dynamin proline-rich domain deletion mutant on the recruitment of Abp1 to the plasma membrane, coimmunoprecipitation of dynamin with Abp1, and coprecipitation of Abp1 with GST fusion of the dyanmin proline-rich domain demonstrate the interaction of Abp1 with dynamin 2. These results demonstrate that the BCR regulates the function of Abp1 by inducing Abp1 phosphorylation and actin cytoskeleton rearrangement, and that Abp1 facilitates BCR-mediated Ag processing by simultaneously interacting with dynamin and the actin cytoskeleton. The Journal of Immunology, 2008, 180: 6685–6695.
PMCID: PMC2855894  PMID: 18453588
2.  Roles of Small GTPase Rac1 in the Regulation of Actin Cytoskeleton during Dengue Virus Infection 
Increased vascular permeability is a hallmark feature in severe dengue virus (DV) infection, and dysfunction of endothelial cells has been speculated to contribute in the pathogenesis of dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Rho-family GTPase Rac1 is a significant element of endothelial barrier function regulation and has been implicated in the regulation of actin remodeling and intercellular junction formation. Yet there is little evidence linking Rac1 GTPase to alteration in endothelial cell function induced by DV infection.
Methods and Findings
Here, we showed that actin is essential for DV serotype 2 (DV2) entry into and release from ECV304 cells, and Rac1 signaling is involved these processes. At early infection, actin cytoskeleton rearranged significantly during 1 hour post infection, and disrupting actin filament dynamics with jasplakinolide or cytochalasin D reduced DV2 entry. DV2 entry induced reduction of Rac1 activity within 1 hour post infection. The expression of dominant-negative forms of Rac1 established that DV2 entry is negatively regulated by Rac1. At late infection, actin drugs also inhibited the DV2 release and induced accumulation of viral proteins in the cytoplasm. Meanwhile, the activity of Rac1 increased significantly with the progression of DV2 infection and was up-regulated in transfected cells expressing E protein. Confocal microscopy showed that DV2 E protein was closely associated with either actin or Rac1 in DV2-infected cells. The interaction between E protein and actin was further confirmed by co-immunoprecipitation assay.
These results defined roles for actin integrity in DV2 entry and release, and indicated evidence for the participation of Rac1 signaling pathways in DV2-induced actin reorganizations and E-actin interaction. Our results may provide further insight into the pathogenesis of DHF/DSS.
Author Summary
An important clinical characteristic of dengue hemorrhagic fever/dengue shock syndrome is increased vascular permeability. Actin cytoskeleton is a significant element of endothelial barrier function regulation. In vitro study showed that dengue virus infection could induce redistributions of actin cytoskeleton. It is not precisely clear the roles of actin and the mechanisms of its reorganization during the infection. Using immunochemical assays, drug inhibition assays and protein interaction profiling methods, we aimed to identify the ways in which dengue virus serotype 2 interacts with actin cytoskeleton. The study showed that dynamic treadmilling of actin is necessary for dengue virus entry, production and release, while small GTPase Rac1 also plays multiple roles during these processes. In addition, we demonstrated the association of viral E protein with actin, indicating a direct effect of viral protein on the structural modifications of actin cytoskeleton. Our results provide evidence for the participation of Rac1 signaling pathways in viral protein-induced actin reorganizations, which may be a mechanism involved in the etiology of dengue hemorrhagic fever.
PMCID: PMC2930870  PMID: 20824170
3.  Wdpcp, a PCP Protein Required for Ciliogenesis, Regulates Directional Cell Migration and Cell Polarity by Direct Modulation of the Actin Cytoskeleton 
PLoS Biology  2013;11(11):e1001720.
Wdpcp, a protein required for both planar cell polarity and ciliogenesis, regulates cell polarity and alignment via direct modulation of the actin cytoskeleton.
Planar cell polarity (PCP) regulates cell alignment required for collective cell movement during embryonic development. This requires PCP/PCP effector proteins, some of which also play essential roles in ciliogenesis, highlighting the long-standing question of the role of the cilium in PCP. Wdpcp, a PCP effector, was recently shown to regulate both ciliogenesis and collective cell movement, but the underlying mechanism is unknown. Here we show Wdpcp can regulate PCP by direct modulation of the actin cytoskeleton. These studies were made possible by recovery of a Wdpcp mutant mouse model. Wdpcp-deficient mice exhibit phenotypes reminiscent of Bardet–Biedl/Meckel–Gruber ciliopathy syndromes, including cardiac outflow tract and cochlea defects associated with PCP perturbation. We observed Wdpcp is localized to the transition zone, and in Wdpcp-deficient cells, Sept2, Nphp1, and Mks1 were lost from the transition zone, indicating Wdpcp is required for recruitment of proteins essential for ciliogenesis. Wdpcp is also found in the cytoplasm, where it is localized in the actin cytoskeleton and in focal adhesions. Wdpcp interacts with Sept2 and is colocalized with Sept2 in actin filaments, but in Wdpcp-deficient cells, Sept2 was lost from the actin cytoskeleton, suggesting Wdpcp is required for Sept2 recruitment to actin filaments. Significantly, organization of the actin filaments and focal contacts were markedly changed in Wdpcp-deficient cells. This was associated with decreased membrane ruffling, failure to establish cell polarity, and loss of directional cell migration. These results suggest the PCP defects in Wdpcp mutants are not caused by loss of cilia, but by direct disruption of the actin cytoskeleton. Consistent with this, Wdpcp mutant cochlea has normal kinocilia and yet exhibits PCP defects. Together, these findings provide the first evidence, to our knowledge, that a PCP component required for ciliogenesis can directly modulate the actin cytoskeleton to regulate cell polarity and directional cell migration.
Author Summary
Cilia are microscopic cell surface hair-like protrusions that can act as antennae to mediate cell signaling. Mutations disrupting ciliogenesis can cause many developmental anomalies associated with syndromes known as “ciliopathies.” Some developmental defects, such as limb polydactyly, arise from disruption of cilia-transduced sonic hedgehog signaling, while other defects, such as aberrant patterning of hair cells in the inner ear, arise from disrupted Wnt signaling resulting in modulation of planar cell polarity (PCP)—a process whereby cells are polarized and aligned. While ciliopathy phenotypes would suggest that cilia are involved in modulating PCP, the mechanistic link between cilia and PCP has been elusive. Our study using a mouse model carrying a mutation in Wdpcp, a gene required for both ciliogenesis and PCP, suggest that Wdpcp modulation of PCP involves interactions with the actin cytoskeleton separate from its function in ciliogenesis. We observe Wdpcp localization in cilia, where it is required for recruitment of proteins essential for ciliogenesis. Wdpcp interacts with Sept2, and is also found in actin filaments, where it regulates actin dynamics essential for PCP. Together, these findings show that PCP regulation by Wdpcp is distinct from its function in ciliogenesis and involves direct modulation of the actin cytoskeleton.
PMCID: PMC3841097  PMID: 24302887
4.  Actin Dynamics Regulate Multiple Endosomal Steps during Kaposi's Sarcoma-Associated Herpesvirus Entry and Trafficking in Endothelial Cells 
PLoS Pathogens  2009;5(7):e1000512.
The role of actin dynamics in clathrin-mediated endocytosis in mammalian cells is unclear. In this study, we define the role of actin cytoskeleton in Kaposi's sarcoma-associated herpesvirus (KSHV) entry and trafficking in endothelial cells using an immunofluorescence-based assay to visualize viral capsids and the associated cellular components. In contrast to infectivity or reporter assays, this method does not rely on the expression of any viral and reporter genes, but instead directly tracks the accumulation of individual viral particles at the nuclear membrane as an indicator of successful viral entry and trafficking in cells. Inhibitors of endosomal acidification reduced both the percentage of nuclei with viral particles and the total number of viral particles docking at the perinuclear region, indicating endocytosis, rather than plasma membrane fusion, as the primary route for KSHV entry into endothelial cells. Accordingly, a viral envelope protein was only detected on internalized KSHV particles at the early but not late stage of infection. Inhibitors of clathrin- but not caveolae/lipid raft-mediated endocytosis blocked KSHV entry, indicating that clathrin-mediated endocytosis is the major route of KSHV entry into endothelial cells. KSHV particles were colocalized not only with markers of early and recycling endosomes, and lysosomes, but also with actin filaments at the early time points of infection. Consistent with these observations, transferrin, which enters cells by clathrin-mediated endocytosis, was found to be associated with actin filaments together with early and recycling endosomes, and to a lesser degree, with late endosomes and lysosomes. KSHV infection induced dynamic actin cytoskeleton rearrangements. Disruption of the actin cytoskeleton and inhibition of regulators of actin nucleation such as Rho GTPases and Arp2/3 complex profoundly blocked KSHV entry and trafficking. Together, these results indicate an important role for actin dynamics in the internalization and endosomal sorting/trafficking of KSHV and clathrin-mediated endocytosis in endothelial cells.
Author Summary
Endocytosis, an essential biological process mediating cellular internalization events, is often exploited by pathogens for their entry into target cells. The role of actin cytoskeleton in clathrin-mediated endocytosis in mammalian cells remains unclear. Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus linked to the development of Kaposi's sarcoma, an endothelial malignancy commonly found in AIDS patients, and several other malignancies. In this study, we found that KSHV uses the clathrin-mediated endocytosis pathway to enter endothelial cells, and this process is regulated by actin dynamics. We found KSHV particles in early and recycling endosomes, and lysosomes, which are docked on actin filaments at the early time points of viral infection. Similarly, transferrin, which enters cells by clathrin-mediated endocytosis, is associated with actin filaments together with early and recycling endosomes, and, to a lesser degree, with late endosomes and lysosomes. Disruption of the actin cytoskeleton and inhibition of regulators of actin nucleation such as Rho GTPases and Arp2/3 complex profoundly blocked KSHV entry and trafficking in endothelial cells. Together, these results define an important role for actin dynamics in multiple endosomal steps during KSHV infection and clathrin-mediated endocytosis in endothelial cells.
PMCID: PMC2702172  PMID: 19593382
5.  Genetic deletion of ABP-120 alters the three-dimensional organization of actin filaments in Dictyostelium pseudopods 
The Journal of Cell Biology  1995;128(5):819-835.
This study extends the observations on the defects in pseudopod formation of ABP-120+ and ABP-120- cells by a detailed morphological and biochemical analysis of the actin based cytoskeleton. Both ABP-120+ and ABP-120- cells polymerize the same amount of F-actin in response to stimulation with cAMP. However, unlike ABP-120+ cells, ABP-120- cells do not incorporate actin into the Triton X-100-insoluble cytoskeleton at 30-50 s, the time when ABP-120 is incorporated into the cytoskeleton and when pseudopods are extended after cAMP stimulation in wild-type cells. By confocal and electron microscopy, pseudopods extended by ABP- 120- cells are not as large or thick as those produced by ABP-120+ cells and in the electron microscope, an altered filament network is found in pseudopods of ABP-120- cells when compared to pseudopods of ABP-120+ cells. The actin filaments found in areas of pseudopods in ABP- 120+ cells either before or after stimulation were long, straight, and arranged into space filling orthogonal networks. Protrusions of ABP-120- cells are less three-dimensional, denser, and filled with multiple foci of aggregated filaments consistent with collapse of the filament network due to the absence of ABP-120-mediated cross-linking activity. The different organization of actin filaments may account for the diminished size of protrusions observed in living and fixed ABP-120- cells compared to ABP-120+ cells and is consistent with the role of ABP- 120 in regulating pseudopod extension through its cross-linking of actin filaments.
PMCID: PMC2120398  PMID: 7876307
6.  Remodelling of Cortical Actin Where Lytic Granules Dock at Natural Killer Cell Immune Synapses Revealed by Super-Resolution Microscopy 
PLoS Biology  2011;9(9):e1001152.
Super-resolution 3D imaging reveals remodeling of the cortical actin meshwork at the natural killer cell immune synapse, which is likely to be important for secretion of lytic granules.
Natural Killer (NK) cells are innate immune cells that secrete lytic granules to directly kill virus-infected or transformed cells across an immune synapse. However, a major gap in understanding this process is in establishing how lytic granules pass through the mesh of cortical actin known to underlie the NK cell membrane. Research has been hampered by the resolution of conventional light microscopy, which is too low to resolve cortical actin during lytic granule secretion. Here we use two high-resolution imaging techniques to probe the synaptic organisation of NK cell receptors and filamentous (F)-actin. A combination of optical tweezers and live cell confocal microscopy reveals that microclusters of NKG2D assemble into a ring-shaped structure at the centre of intercellular synapses, where Vav1 and Grb2 also accumulate. Within this ring-shaped organisation of NK cell proteins, lytic granules accumulate for secretion. Using 3D-structured illumination microscopy (3D-SIM) to gain super-resolution of ∼100 nm, cortical actin was detected in a central region of the NK cell synapse irrespective of whether activating or inhibitory signals dominate. Strikingly, the periodicity of the cortical actin mesh increased in specific domains at the synapse when the NK cell was activated. Two-colour super-resolution imaging revealed that lytic granules docked precisely in these domains which were also proximal to where the microtubule-organising centre (MTOC) polarised. Together, these data demonstrate that remodelling of the cortical actin mesh occurs at the central region of the cytolytic NK cell immune synapse. This is likely to occur for other types of cell secretion and also emphasises the importance of emerging super-resolution imaging technology for revealing new biology.
Author Summary
Natural Killer (NK) cells are immune cells that can recognise and kill virus-infected and cancerous cells. This killing requires an intercellular contact —termed an immune synapse—between the NK cell and its target cell through which molecules can be delivered to trigger lysis. Reorganisation of the NK cell cytoskeleton is essential for the delivery and release at the synapse of granules containing the cytolytic molecules. Understanding precisely how the cytoskeleton is involved in these cytolytic events has been hampered by our inability to resolve cytoskeletal structure at immune synapses by conventional light microscopy. Very recent advances in imaging technology have now provided the resolving power to see previously undetectable cellular structures. Here, we have used 3D super-resolution imaging to observe the structure of the actin cytoskeleton at the NK immune synapse. We found that a dense mesh of actin underlies the immune synapse and that it is remodelled upon NK cell activation. Domains within the actin meshwork open up specifying where the lytic granules dock and also where the microtubule-organising centre moves towards. Thus, actin remodelling occurs at the immune synapse during secretion and this may be important for the regulation of lytic granule secretion.
PMCID: PMC3172219  PMID: 21931537
7.  Regulation of actin cytoskeleton architecture by Eps8 and Abi1 
BMC Cell Biology  2005;6:36.
The actin cytoskeleton participates in many fundamental processes including the regulation of cell shape, motility, and adhesion. The remodeling of the actin cytoskeleton is dependent on actin binding proteins, which organize actin filaments into specific structures that allow them to perform various specialized functions. The Eps8 family of proteins is implicated in the regulation of actin cytoskeleton remodeling during cell migration, yet the precise mechanism by which Eps8 regulates actin organization and remodeling remains elusive.
Here, we show that Eps8 promotes the assembly of actin rich filopodia-like structures and actin cables in cultured mammalian cells and Xenopus embryos, respectively. The morphology of actin structures induced by Eps8 was modulated by interactions with Abi1, which stimulated formation of actin cables in cultured cells and star-like structures in Xenopus. The actin stars observed in Xenopus animal cap cells assembled at the apical surface of epithelial cells in a Rac-independent manner and their formation was accompanied by recruitment of N-WASP, suggesting that the Eps8/Abi1 complex is capable of regulating the localization and/or activity of actin nucleators. We also found that Eps8 recruits Dishevelled to the plasma membrane and actin filaments suggesting that Eps8 might participate in non-canonical Wnt/Polarity signaling. Consistent with this idea, mis-expression of Eps8 in dorsal regions of Xenopus embryos resulted in gastrulation defects.
Together, these results suggest that Eps8 plays multiple roles in modulating actin filament organization, possibly through its interaction with distinct sets of actin regulatory complexes. Furthermore, the finding that Eps8 interacts with Dsh and induced gastrulation defects provides evidence that Eps8 might participate in non-canonical Wnt signaling to control cell movements during vertebrate development.
PMCID: PMC1274305  PMID: 16225669
8.  p57KIP2 control of actin cytoskeleton dynamics is responsible for its mitochondrial pro-apoptotic effect 
Cell Death & Disease  2012;3(5):e311-.
p57 (Kip2, cyclin-dependent kinase inhibitor 1C), often found downregulated in cancer, is reported to hold tumor suppressor properties. Originally described as a cyclin-dependent kinase (cdk) inhibitor, p57KIP2 has since been shown to influence other cellular processes, beyond cell cycle regulation, including cell death and cell migration. Inhibition of cell migration by p57KIP2 is attributed to the stabilization of the actin cytoskeleton through the activation of LIM domain kinase-1 (LIMK-1). Furthermore, p57KIP2 is able to enhance mitochondrial-mediated apoptosis. Here, we report that the cell death promoting effect of p57KIP2 is linked to its effect on the actin cytoskeleton. Indeed, whereas Jasplakinolide, an actin cytoskeleton-stabilizing agent, mimicked p57KIP2's pro-apoptotic effect, destabilizing the actin cytoskeleton with cytochalsin D reversed p57KIP2's pro-apoptotic function. Conversely, LIMK-1, the enzyme mediating p57KIP2's effect on the actin cytoskeleton, was required for p57KIP2's death promoting effect. Finally, p57KIP2-mediated stabilization of the actin cytoskeleton was associated with the displacement of hexokinase-1, an inhibitor of the mitochondrial voltage-dependent anion channel, from the mitochondria, providing a possible mechanism for the promotion of the mitochondrial apoptotic cell death pathway. Altogether, our findings link together two tumor suppressor properties of p57KIP2, by showing that the promotion of cell death by p57KIP2 requires its actin cytoskeleton stabilization function.
PMCID: PMC3366085  PMID: 22592318
p57KIP2; cell migration; cancer; cytoskeleton
9.  Counteracting the activation of pAkt by inhibition of MEK/Erk inhibition reduces actin disruption-mediated apoptosis in PTEN-null PC3M prostate cancer cell lines 
Oncology Letters  2013;6(5):1383-1389.
The actin cytoskeleton is important in the maintenance of cellular homeostasis and in signal transduction pathways leading to cell growth and apoptotic cell death in eukaryotic cells. Disruption of actin dynamics is associated with morphological changes in cancer cells. Deletion of phosphatase and tensin homolog (PTEN), a tumor suppressor gene involved in the regulation of the cell cycle and apoptosis, leads to cytoskeleton disruption and double-strand breaks (DSBs). To study the mechanism(s) of actin disruption-mediated apoptosis and its potential application for anticancer therapy, PTEN-null PC3M prostate cancer cells were treated with latrunculin B (LB). LB induced destabilization of the actin microfilament and apoptosis in a dose-dependent manner, as demonstrated by morphological changes and nuclear condensation in the PC3M cells. In addition, it resulted in an increase in the levels of γH2AX recruitment, implicating the induction of DNA damage, including DSBs. Induction of Bax, with little effect on Bcl-2 expression, indicated that actin disruption causes apoptosis through activation of Bax signaling in PC3M cells. Treatment with U20126, a mitogen-activated protein kinase kinase (MEK) inhibitor, resulted in attenuated induction of DSBs and apoptosis through activation of protein kinase B (Akt), suggesting that LB-mediated actin dysfunction induces DSBs via the MEK/extracellular signal-regulated kinase (Erk) pathway in cells. Therefore, counteracting activation of phosphorylated Akt stemming from the inhibition of MEK/Erk resulted in attenuation of actin disruption-induced apoptotic events in the PC3M cells. The results of this study provide information not only for use in delineation of the molecular association between actin disruption and tumorigenesis, but also for the development of a strategy for actin-based anticancer chemotherapy against highly metastatic prostate cancer.
PMCID: PMC3813806  PMID: 24179529
latrunculin B; PTEN-null PC3M prostate cancer cell lines; apoptosis; pAkt; MEK/Erk; actin
10.  Human MCF10A Mammary Epithelial Cells Undergo Apoptosis following Actin Depolymerization That Is Independent of Attachment and Rescued by Bcl-2 
Molecular and Cellular Biology  2001;21(19):6529-6536.
Many tumor cells are impaired in adhesion-regulated apoptosis, which contributes to their metastatic potential. However, suppression of this apoptotic pathway in untransformed cells is not mediated only by adhesion to the extracellular matrix but also through the resulting ability to spread and adopt a distinct morphology. Since cell spreading is dependent on the integrity of the actin microfilament cytoskeleton, we sought to determine if actin depolymerization was sufficient to induce apoptosis, even in the presence of continuous attachment. For this study, we used a human mammary epithelial cell line (MCF10A), which is immortalized but remains adhesion dependent for survival. Treatment of MCF10A cells with latrunculin-A (LA), an inhibitor of actin polymerization, rapidly led to disruption of the actin cytoskeleton and caused cell rounding but preserved attachment. Initiation of apoptosis in LA-treated MCF10A cells was detected by mitochondrial localization of the Bax apoptotic protein, which was prevented by overexpression of Bcl-2. DNA fragmentation and poly(ADP-ribose) polymerase (PARP) cleavage in LA-treated MCF10A cells indicated progression to the execution phase of apoptosis. The MDA-MB-453 cell line, which was derived from a metastatic human mammary tumor, was resistant to PARP cleavage and loss of viability in response to actin depolymerization. Stable overexpression of Bcl-2 in the untransformed MCF10A cells was able to recapitulate the resistance to apoptosis found in the tumor cell line. We demonstrate that inhibition of actin polymerization is sufficient to stimulate apoptosis in attached MCF10A cells, and we present a novel role for Bcl-2 in cell death induced by direct disruption of the actin cytoskeleton.
PMCID: PMC99799  PMID: 11533241
11.  Structural and Functional Dissection of the Abp1 ADFH Actin-binding Domain Reveals Versatile In Vivo Adapter FunctionsD⃞ 
Molecular Biology of the Cell  2005;16(7):3128-3139.
Abp1 is a multidomain protein that regulates the Arp2/3 complex and links proteins involved in endocytosis to the actin cytoskeleton. All of the proposed cellular functions of Abp1 involve actin filament binding, yet the actin binding site(s) on Abp1 have not been identified, nor has the importance of actin binding for Abp1 localization and function in vivo been tested. Here, we report the crystal structure of the Saccharomyces cerevisiae Abp1 actin-binding actin depolymerizing factor homology (ADFH) domain and dissect its activities by mutagenesis. Abp1-ADFH domain and ADF/cofilin structures are similar, and they use conserved surfaces to bind actin; however, there are also key differences that help explain their differential effects on actin dynamics. Using point mutations, we demonstrate that actin binding is required for localization of Abp1 in vivo, the lethality caused by Abp1 overexpression, and the ability of Abp1 to activate Arp2/3 complex. Furthermore, we genetically uncouple ABP1 functions that overlap with SAC6, SLA1, and SLA2, showing they require distinct combinations of activities and interactions. Together, our data provide the first structural and functional view of the Abp1–actin interaction and show that Abp1 has distinct cellular roles as an adapter, linking different sets of ligands for each function.
PMCID: PMC1165398  PMID: 15872087
12.  Arabidopsis Actin-Depolymerizing Factor-4 Links Pathogen Perception, Defense Activation and Transcription to Cytoskeletal Dynamics 
PLoS Pathogens  2012;8(11):e1003006.
The primary role of Actin-Depolymerizing Factors (ADFs) is to sever filamentous actin, generating pointed ends, which in turn are incorporated into newly formed filaments, thus supporting stochastic actin dynamics. Arabidopsis ADF4 was recently shown to be required for the activation of resistance in Arabidopsis following infection with the phytopathogenic bacterium Pseudomonas syringae pv. tomato DC3000 (Pst) expressing the effector protein AvrPphB. Herein, we demonstrate that the expression of RPS5, the cognate resistance protein of AvrPphB, was dramatically reduced in the adf4 mutant, suggesting a link between actin cytoskeletal dynamics and the transcriptional regulation of R-protein activation. By examining the PTI (PAMP Triggered Immunity) response in the adf4 mutant when challenged with Pst expressing AvrPphB, we observed a significant reduction in the expression of the PTI-specific target gene FRK1 (Flg22-Induced Receptor Kinase 1). These data are in agreement with recent observations demonstrating a requirement for RPS5 in PTI-signaling in the presence of AvrPphB. Furthermore, MAPK (Mitogen-Activated Protein Kinase)-signaling was significantly reduced in the adf4 mutant, while no such reduction was observed in the rps5-1 point mutation under similar conditions. Isoelectric focusing confirmed phosphorylation of ADF4 at serine-6, and additional in planta analyses of ADF4's role in immune signaling demonstrates that nuclear localization is phosphorylation independent, while localization to the actin cytoskeleton is linked to ADF4 phosphorylation. Taken together, these data suggest a novel role for ADF4 in controlling gene-for-gene resistance activation, as well as MAPK-signaling, via the coordinated regulation of actin cytoskeletal dynamics and R-gene transcription.
Author Summary
The activation and regulation of the plant immune system requires the coordinated function of numerous pre-formed and inducible cellular responses. Following pathogen perception, plants not only activate specific defense-associated signaling, such as resistance (R) genes, but also redirect basic cellular machinery to support innate immune signaling. Within each of these processes, the actin cytoskeleton has been demonstrated to play a significant role in structural-based defense signaling in plants in response to pathogen infection. Most notably, the actin cytoskeleton of plants has been shown to play a role in structural-based defense signaling following fungal pathogen infection. Recent work from our laboratory has demonstrated that the actin cytoskeleton of Arabidopsis mediates defense signaling following perception of the phytopathogenic bacterium Pseudomonas syringae. Using a combination of genetic and cell biology-based approaches, we found that ADF4, a regulator of actin cytoskeletal dynamics, is required for the specific activation of R-gene-mediated signaling. By analyzing the activation of signaling following pathogen perception, we have identified substantial crosstalk between recognition of pathogen virulence factors (e.g., effector proteins) and the regulation of R-gene transcription. In total, our work highlights the intimate relationship between basic cellular processes and the perception and activation of defense signaling following pathogen infection.
PMCID: PMC3493479  PMID: 23144618
13.  Structural Insights into the Inhibition of Actin-Capping Protein by Interactions with Phosphatidic Acid and Phosphatidylinositol (4,5)-Bisphosphate 
PLoS Computational Biology  2012;8(11):e1002765.
The actin cytoskeleton is a dynamic structure that coordinates numerous fundamental processes in eukaryotic cells. Dozens of actin-binding proteins are known to be involved in the regulation of actin filament organization or turnover and many of these are stimulus-response regulators of phospholipid signaling. One of these proteins is the heterodimeric actin-capping protein (CP) which binds the barbed end of actin filaments with high affinity and inhibits both addition and loss of actin monomers at this end. The ability of CP to bind filaments is regulated by signaling phospholipids, which inhibit the activity of CP; however, the exact mechanism of this regulation and the residues on CP responsible for lipid interactions is not fully resolved. Here, we focus on the interaction of CP with two signaling phospholipids, phosphatidic acid (PA) and phosphatidylinositol (4,5)-bisphosphate (PIP2). Using different methods of computational biology such as homology modeling, molecular docking and coarse-grained molecular dynamics, we uncovered specific modes of high affinity interaction between membranes containing PA/phosphatidylcholine (PC) and plant CP, as well as between PIP2/PC and animal CP. In particular, we identified differences in the binding of membrane lipids by animal and plant CP, explaining previously published experimental results. Furthermore, we pinpoint the critical importance of the C-terminal part of plant CPα subunit for CP–membrane interactions. We prepared a GST-fusion protein for the C-terminal domain of plant α subunit and verified this hypothesis with lipid-binding assays in vitro.
Author Summary
The actin cytoskeleton is a prominent feature of eukaryotes and plays a central role in many essential aspects of their lives. This highly malleable structure responds to a wide range of stimuli with rapid changes in organization or dynamics. These responses are thought to be mediated by dozens of actin-binding proteins, the biochemical activities of which have been demonstrated to be tightly controlled by other proteins and/or signal transduction mediators. In this study, we investigated the structural aspects of inhibition of actin-capping protein (CP) by phosphatidic acid (PA) and phosphatidylinositol (4,5)-bisphosphate (PIP2). We employed diverse computational methods in combination with experimental approaches to reveal mechanistic details of the direct interaction of CP with the phospholipid membrane containing either PA or PIP2. Importantly, we found several differences between PA/PIP2–CP interactions from two distinct species, Arabidopsis and chicken, that enable us to explain and expand upon previously published results. Our new data shed light on the nature of interactions between peripheral membrane proteins and PA-containing lipid bilayers. In addition to a description of the phospholipid-mediated regulation of CP activity, our work also significantly contributes to the ongoing debate on structural details of protein interactions with phospholipids.
PMCID: PMC3486809  PMID: 23133367
14.  Actin-Induced Hyperactivation of the Ras Signaling Pathway Leads to Apoptosis in Saccharomyces cerevisiae 
Molecular and Cellular Biology  2006;26(17):6487-6501.
Recent research has revealed a conserved role for the actin cytoskeleton in the regulation of aging and apoptosis among eukaryotes. Here we show that the stabilization of the actin cytoskeleton caused by deletion of Sla1p or End3p leads to hyperactivation of the Ras signaling pathway. The consequent rise in cyclic AMP (cAMP) levels leads to the loss of mitochondrial membrane potential, accumulation of reactive oxygen species (ROS), and cell death. We have established a mechanistic link between Ras signaling and actin by demonstrating that ROS production in actin-stabilized cells is dependent on the G-actin binding region of the cyclase-associated protein Srv2p/CAP. Furthermore, the artificial elevation of cAMP directly mimics the apoptotic phenotypes displayed by actin-stabilized cells. The effect of cAMP elevation in inducing actin-mediated apoptosis functions primarily through the Tpk3p subunit of protein kinase A. This pathway represents the first defined link between environmental sensing, actin remodeling, and apoptosis in Saccharomyces cerevisiae.
PMCID: PMC1592845  PMID: 16914733
15.  Filamin Is Required for Ring Canal Assembly and Actin Organization during Drosophila Oogenesis 
The Journal of Cell Biology  1999;146(5):1061-1074.
The remodeling of the actin cytoskeleton is essential for cell migration, cell division, and cell morphogenesis. Actin-binding proteins play a pivotal role in reorganizing the actin cytoskeleton in response to signals exchanged between cells. In consequence, actin-binding proteins are increasingly a focus of investigations into effectors of cell signaling and the coordination of cellular behaviors within developmental processes. One of the first actin-binding proteins identified was filamin, or actin-binding protein 280 (ABP280). Filamin is required for cell migration (Cunningham et al. 1992), and mutations in human α-filamin (FLN1; Fox et al. 1998) are responsible for impaired migration of cerebral neurons and give rise to periventricular heterotopia, a disorder that leads to epilepsy and vascular disorders, as well as embryonic lethality. We report the identification and characterization of a mutation in Drosophila filamin, the homologue of human α-filamin. During oogenesis, filamin is concentrated in the ring canal structures that fortify arrested cleavage furrows and establish cytoplasmic bridges between cells of the germline. The major structural features common to other filamins are conserved in Drosophila filamin. Mutations in Drosophila filamin disrupt actin filament organization and compromise membrane integrity during oocyte development, resulting in female sterility. The genetic and molecular characterization of Drosophila filamin provides the first genetic model system for the analysis of filamin function and regulation during development.
PMCID: PMC2169474  PMID: 10477759
filamin; ABP280; Drosophila; actin; ring canals
16.  Modulation of Actin Mechanics by Caldesmon and Tropomyosin 
The ability of cells to sense and respond to physiological forces relies on the actin cytoskeleton, a dynamic structure that can directly convert forces into biochemical signals. Because of the association of muscle actin-binding proteins (ABPs) may affect F-actin and hence cytoskeleton mechanics, we investigated the effects of several ABPs on the mechanical properties of the actin filaments. The structural interactions between ABPs and helical actin filaments can vary between interstrand interactions that bridge azimuthally adjacent actin monomers between filament strands (i.e. by molecular stapling as proposed for caldesmon) or, intrastrand interactions that reinforce axially adjacent actin monomers along strands (i.e. as in the interaction of tropomyosin with actin). Here, we analyzed thermally driven fluctuations in actin’s shape to measure the flexural rigidity of actin filaments with different ABPs bound. We show that the binding of phalloidin increases the persistence length of actin by 1.9-fold. Similarly, the intrastrand reinforcement by smooth and skeletal muscle tropomyosins increases the persistence length 1.5-and 2- fold respectively. We also show that the interstrand crosslinking by the C-terminal actin-binding fragment of caldesmon, H32K, increases persistence length by 1.6-fold. While still remaining bound to actin, phosphorylation of H32K by ERK abolishes the molecular staple (Foster et al. 2004. J Biol Chem 279;53387–53394) and reduces filament rigidity to that of actin with no ABPs bound. Lastly, we show that the effect of binding both smooth muscle tropomyosin and H32K is not additive. The combination of structural and mechanical studies on ABP-actin interactions will help provide information about the biophysical mechanism of force transduction in cells.
PMCID: PMC2975105  PMID: 18000881
actin-binding proteins; persistence length; flexural rigidity; cytoskeleton; filament mechanics
17.  Inside view of cell locomotion through single-molecule: fast F-/G-actin cycle and G-actin regulation of polymer restoration 
The actin cytoskeleton drives cell locomotion and tissue remodeling. The invention of live-cell fluorescence single-molecule imaging opened a window for direct viewing of the actin remodeling processes in the cell. Since then, a number of unanticipated molecular functions have been revealed. One is the mechanism of F-actin network breakdown. In lamellipodia, one third of newly polymerized F-actin disassembles within 10 seconds. This fast F-actin turnover is facilitated by the filament severing/disrupting activity involving cofilin and AIP1. Astoundingly fast dissociation kinetics of the barbed end interactors including capping protein suggests that F-actin turnover might proceed through repetitive disruption/reassembly of the filament near the barbed end. The picture of actin polymerization is also being revealed. At the leading edge of the cell, Arp2/3 complex is highly activated in a narrow edge region. In contrast, mDia1 and its related Formin homology proteins display a long-distance directional molecular movement using their processive actin capping ability. Recently, these two independently-developed projects converged into a discovery of the spatiotemporal coupling between mDia1-mediated filament nucleation and actin disassembly. Presumably, the local concentration fluctuation of G-actin regulates the actin nucleation efficiency of specific actin nucleators including mDia1. Pharmacological perturbation and quantitative molecular behavior analysis synergize to reveal hidden molecular linkages in the actin turnover cycle and cell signaling.
PMCID: PMC3417570  PMID: 20075609
single-molecule imaging; actin turnover; mDia1; pharmacokinetic simulation; G-actin
18.  Mechanism of synergistic activation of Arp2/3 complex by cortactin and N-WASP 
eLife  2013;2:e00884.
Nucleation promoting factors (NPFs) initiate branched actin network assembly by activating Arp2/3 complex, a branched actin filament nucleator. Cellular actin networks contain multiple NPFs, but how they coordinately regulate Arp2/3 complex is unclear. Cortactin is an NPF that activates Arp2/3 complex weakly on its own, but with WASP/N-WASP, another class of NPFs, potently activates. We dissect the mechanism of synergy and propose a model in which cortactin displaces N-WASP from nascent branches as a prerequisite for nucleation. Single-molecule imaging revealed that unlike WASP/N-WASP, cortactin remains bound to junctions during nucleation, and specifically targets junctions with a ∼160-fold increased on rate over filament sides. N-WASP must be dimerized for potent synergy, and targeted mutations indicate release of dimeric N-WASP from nascent branches limits nucleation. Mathematical modeling shows cortactin-mediated displacement but not N-WASP recycling or filament recruitment models can explain synergy. Our results provide a molecular basis for coordinate Arp2/3 complex regulation.
eLife digest
Cells constantly sense, and react to, their environments. They can monitor or alter their surroundings by taking up or secreting various substances, and may also migrate toward food supplies, or toward signaling molecules—for example, to clot blood or heal wounds. These actions depend on the cytoskeleton, a protein meshwork that gives cells their shape; allows them to transport materials into, out of, or across their cytoplasms; and enables them to move.
The filaments of the cytoskeleton are constructed from several different types of proteins, one of which is called actin. In response to signals, actin can assemble into linear filaments, or can form branches with one end anchored on an existing filament. Branch formation requires the Arp2/3 complex, which initiates and anchors branches on existing filaments, and also various ‘nucleation-promoting factors’ (NPFs), which turn on the branching activity of the Arp2/3 complex.
Two types of NPFs have been identified: type I interact with individual actin molecules, while type II bind to actin filaments. Previous work has shown that type I NPFs—including the N-WASP protein—have a specialized domain called VCA that binds to both the Arp2/3 complex and to actin molecules. VCA brings actin molecules to the branch site, which initiates branch formation, but how N-WASP collaborates with type II NPFs to build branches is not well understood.
Helgeson and Nolen now examine how a type II NPF called cortactin works with the Arp2/3 complex and N-WASP to construct new branches on actin filaments in vitro. Cortactin appears to displace the VCA domain of N-WASP to stimulate branch formation, and then to remain associated with—and stabilize—the growing branch. Helgeson and Nolen suggest that these NPFs work together to create branches using an “obligatory displacement” model. According to this scheme, N-WASP (or another type I NPF), the Arp2/3 complex and two actin molecules are bound at the site of a future branch on an actin filament, poised for branch formation. However, before more actin molecules can be added, N-WASP must be released, either slowly on its own—as Smith et al. also report in findings published concurrently in eLife—or rapidly with the help of cortactin or other type II NPFs.
Although the rationale for N-WASP removal is not yet understood, type I NPFs are generally attached to the plasma membrane. When N-WASP releases the mother filament, the membrane should no longer be able to block the addition of actin molecules to a growing branch.
PMCID: PMC3762189  PMID: 24015358
Arp2/3; actin; WASP; cortactin; single molecule; N-WASP; Mouse
19.  Regulation of Actin Cytoskeleton Dynamics in Cells 
Molecules and cells  2010;29(4):311-325.
The dynamic remolding of the actin cytoskeleton is a critical part of most cellular activities, and malfunction of cytoskeletal proteins results in various human diseases. The transition between two forms of actin, monomeric or G-actin and filamentous or F-actin, is tightly regulated in time and space by a large number of signaling, scaffolding and actin-binding proteins (ABPs). New ABPs are constantly being discovered in the post-genomic era. Most of these proteins are modular, integrating actin binding, protein-protein interaction, membrane-binding, and signaling domains. In response to extracellular signals, often mediated by Rho family GTPases, ABPs control different steps of actin cytoskeleton assembly, including filament nucleation, elongation, severing, capping, and depolymerization. This review summarizes structure-function relationships among ABPs in the regulation of actin cytoskeleton assembly.
PMCID: PMC3910092  PMID: 20446344
actin-binding protein; actin cytoskeleton; BAR protein; F-actin; filament cross-linking; filament nucleation and elongation; G-actin; Rho-GTPase; WH2
20.  The Plant Actin Cytoskeleton Responds to Signals from Microbe-Associated Molecular Patterns 
PLoS Pathogens  2013;9(4):e1003290.
Plants are constantly exposed to a large and diverse array of microbes; however, most plants are immune to the majority of potential invaders and susceptible to only a small subset of pathogens. The cytoskeleton comprises a dynamic intracellular framework that responds rapidly to biotic stresses and supports numerous fundamental cellular processes including vesicle trafficking, endocytosis and the spatial distribution of organelles and protein complexes. For years, the actin cytoskeleton has been assumed to play a role in plant innate immunity against fungi and oomycetes, based largely on static images and pharmacological studies. To date, however, there is little evidence that the host-cell actin cytoskeleton participates in responses to phytopathogenic bacteria. Here, we quantified the spatiotemporal changes in host-cell cytoskeletal architecture during the immune response to pathogenic and non-pathogenic strains of Pseudomonas syringae pv. tomato DC3000. Two distinct changes to host cytoskeletal arrays were observed that correspond to distinct phases of plant-bacterial interactions i.e. the perception of microbe-associated molecular patterns (MAMPs) during pattern-triggered immunity (PTI) and perturbations by effector proteins during effector-triggered susceptibility (ETS). We demonstrate that an immediate increase in actin filament abundance is a conserved and novel component of PTI. Notably, treatment of leaves with a MAMP peptide mimic was sufficient to elicit a rapid change in actin organization in epidermal cells, and this actin response required the host-cell MAMP receptor kinase complex, including FLS2, BAK1 and BIK1. Finally, we found that actin polymerization is necessary for the increase in actin filament density and that blocking this increase with the actin-disrupting drug latrunculin B leads to enhanced susceptibility of host plants to pathogenic and non-pathogenic bacteria.
Author Summary
The cytoskeleton is a dynamic platform for sensing and responding to a diverse array of biotic and abiotic stresses. The nature and timing of the changes in actin organization range from excessive bundling, to massive depolymerization, to new filament assembly, depending on the particular signal and the responding cell type. Here, we use the Arabidopsis–Pseudomonas pathosystem to dissect pathogen-derived cues that elicit changes in the plant host-cell cytoskeleton. Overall, we provide the first evidence that the actin cytoskeleton rearranges in response to a phytopathogenic bacterium and we quantified the temporal response of epidermal cells to Pseudomonas syringae pv. tomato DC3000 strains and susceptible Arabidopsis mutants, using a robust set of tools for measuring changes in actin organization. An immediate but transient increase in actin filament abundance was associated with pattern-triggered immunity. This response could be mimicked with microbe-associated molecular pattern peptide treatments. Second, we observed a late increase in actin filament bundling that appears to be part of effector-triggered susceptibility. We dissected the initial steps involved in the host-cell signaling pathway and demonstrated that FLS2, BAK1, and BIK1 were required for the actin response. Collectively, these findings demonstrate that rapid changes in host-cell cytoskeleton organization occur in response to receptor-mediated signaling during plant innate immunity.
PMCID: PMC3616984  PMID: 23593000
21.  Prostaglandins temporally regulate cytoplasmic actin bundle formation during Drosophila oogenesis 
Molecular Biology of the Cell  2014;25(3):397-411.
Tight regulation of actin remodeling is essential for development, and misregulation results in disease. Cytoskeletal dynamics are regulated by prostaglandins (PGs)—lipid signals. PGs temporally regulate actin remodeling during Drosophila oogenesis, at least in part, by modulating the activity of the actin elongation factor Enabled.
Prostaglandins (PGs)—lipid signals produced downstream of cyclooxygenase (COX) enzymes—regulate actin dynamics in cell culture and platelets, but their roles during development are largely unknown. Here we define a new role for Pxt, the Drosophila COX-like enzyme, in regulating the actin cytoskeleton—temporal restriction of actin remodeling during oogenesis. PGs are required for actin filament bundle formation during stage 10B (S10B). In addition, loss of Pxt results in extensive early actin remodeling, including actin filaments and aggregates, within the posterior nurse cells of S9 follicles; wild-type follicles exhibit similar structures at a low frequency. Hu li tai shao (Hts-RC) and Villin (Quail), an actin bundler, localize to all early actin structures, whereas Enabled (Ena), an actin elongation factor, preferentially localizes to those in pxt mutants. Reduced Ena levels strongly suppress early actin remodeling in pxt mutants. Furthermore, loss of Pxt results in reduced Ena localization to the sites of bundle formation during S10B. Together these data lead to a model in which PGs temporally regulate actin remodeling during Drosophila oogenesis by controlling Ena localization/activity, such that in S9, PG signaling inhibits, whereas at S10B, it promotes Ena-dependent actin remodeling.
PMCID: PMC3907279  PMID: 24284900
22.  Polyphosphoinositides-dependent regulation of the osteoclast actin cytoskeleton and bone resorption 
BMC Cell Biology  2004;5:19.
Gelsolin, an actin capping protein of osteoclast podosomes, has a unique function in regulating assembly and disassembly of the podosome actin filament. Previously, we have reported that osteopontin (OPN) binding to integrin αvβ3 increased the levels of gelsolin-associated polyphosphoinositides, podosome assembly/disassembly, and actin filament formation. The present study was undertaken to identify the possible role of polyphosphoinositides and phosphoinositides binding domains (PBDs) of gelsolin in the osteoclast cytoskeletal structural organization and osteoclast function.
Transduction of TAT/full-length gelsolin and PBDs containing gelsolin peptides into osteoclasts demonstrated: 1) F-actin enriched patches; 2) disruption of actin ring; 3) an increase in the association polyphosphoinositides (PPIs) with the transduced peptides containing PBDs. The above-mentioned effects were more pronounced with gelsolin peptide containing 2 tandem repeats of PBDs (PBD (2)). Binding of PPIs to the transduced peptides has resulted in reduced levels of PPIs association with the endogenous gelsolin, and thereby disrupted the actin remodeling processes in terms of podosome organization in the clear zone area and actin ring formation. These peptides also exhibited a dominant negative effect in the formation of WASP-Arp2/3 complex indicating the role of phosphoinositides in WASP activation. The TAT-PBD gelsolin peptides transduced osteoclasts are functionally defective in terms of motility and bone resorption.
Taken together, these data demonstrate that transduction of PBD gelsolin peptides into osteoclasts produced a dominant negative effect on actin assembly, motility, and bone resorption. These findings indicate that phosphoinositide-mediated signaling mechanisms regulate osteoclast cytoskeleton, podosome assembly/disassembly, actin ring formation and bone resorption activity of osteoclasts.
PMCID: PMC441372  PMID: 15142256
osteoclasts; actin ring; podosomes; phosphoinositides; gelsolin; WASP; Arp2/3
23.  SAPK2/p38-dependent F-Actin Reorganization Regulates Early Membrane Blebbing during Stress-induced Apoptosis  
The Journal of Cell Biology  1998;143(5):1361-1373.
In endothelial cells, H2O2 induces the rapid formation of focal adhesion complexes at the ventral face of the cells and a major reorganization of the actin cytoskeleton into dense transcytoplasmic stress fibers. This change in actin dynamics results from the activation of the mitogen-activated protein (MAP) kinase stress-activated protein kinase-2/p38 (SAPK2/p38), which, via MAP kinase-activated protein (MAPKAP) kinase-2/3, leads to the phosphorylation of the actin polymerization modulator heat shock protein of 27 kD (HSP27). Here we show that the concomitant activation of the extracellular signal-regulated kinase (ERK) MAP kinase pathway by H2O2 accomplishes an essential survival function during this process. When the activation of ERK was blocked with PD098059, the focal adhesion complexes formed under the plasma membrane, and the actin polymerization activity led to a rapid and intense membrane blebbing. The blebs were delimited by a thin F-actin ring and contained enhanced levels of HSP27. Later, the cells displayed hallmarks of apoptosis, such as DEVD protease activities and internucleosomal DNA fragmentation. Bleb formation but not apoptosis was blocked by extremely low concentrations of the actin polymerization inhibitor cytochalasin D or by the SAPK2 inhibitor SB203580, indicating that the two processes are not in the same linear cascade. The role of HSP27 in mediating membrane blebbing was assessed in fibroblastic cells. In control fibroblasts expressing a low level of endogenous HSP27 or in fibroblasts expressing a high level of a nonphosphorylatable HSP27, H2O2 did not induce F-actin accumulation, nor did it generate membrane blebbing activity in the presence or absence of PD098059. In contrast, in fibroblasts that expressed wild-type HSP27 to a level similar to that found in endothelial cells, H2O2 induced accumulation of F-actin and caused bleb formation when the ERK pathway was inhibited. Cis-platinum, which activated SAPK2 but induced little ERK activity, also induced membrane blebbing that was dependent on the expression of HSP27. In these cells, membrane blebbing was not followed by caspase activation or DNA fragmentation. We conclude that the HSP27-dependent actin polymerization–generating activity of SAPK2 associated with a misassembly of the focal adhesions is responsible for induction of membrane blebbing by stressing agents.
PMCID: PMC2133090  PMID: 9832563
SAPK2/p38; HSP27; F-actin; blebbing; apoptosis
24.  Cytochalasin B Modulates Macrophage-Mediated Inflammatory Responses 
Biomolecules & Therapeutics  2014;22(4):295-300.
The actin cytoskeleton plays an important role in macrophage-mediated inflammatory responses by modulating the activation of Src and subsequently inducing nuclear factor (NF)-κB translocation. In spite of its critical functions, few papers have examined how the actin cytoskeleton can be regulated by the activation of toll-like receptor (TLR). Therefore, in this study, we further characterized the biological value of the actin cytoskeleton in the functional activation of macrophages using an actin cytoskeleton disruptor, cytochalasin B (Cyto B), and explored the actin cytoskeleton’s involvement in morphological changes, cellular attachment, and signaling events. Cyto B strongly suppressed the TLR4-mediated mRNA expression of inflammatory genes such as cyclooxygenase (COX)-2, tumor necrosis factor (TNF)-α, and inducible nitric oxide (iNOS), without altering cell viability. This compound also strongly suppressed the morphological changes induced by lipopolysaccharide (LPS), a TLR4 ligand. Cyto B also remarkably suppressed NO production under non-adherent conditions but not in an adherent environment. Cyto B did not block the co-localization between surface glycoprotein myeloid differentiation protein-2 (MD2), a LPS signaling glycoprotein, and the actin cytoskeleton under LPS conditions. Interestingly, Cyto B and PP2, a Src inhibitor, enhanced the phagocytic uptake of fluorescein isothiocyanate (FITC)-dextran. Finally, it was found that Cyto B blocked the phosphorylation of vasodilator-stimulated phosphoprotein (VASP) at 1 min and the phosphorylation of heat shock protein 27 (HSP27) at 5 min. Therefore, our data suggest that the actin cytoskeleton may be one of the key components involved in the control of TLR4-mediated inflammatory responses in macrophages.
PMCID: PMC4131529  PMID: 25143807
Actin cytoskeleton; Inflammation; Cytochalasin B; Macrophages; TLR4
25.  Evidence for a Conformational Change in Actin Induced by Fimbrin (N375) Binding  
The Journal of Cell Biology  1997;139(2):387-396.
Fimbrin belongs to a superfamily of actin cross-linking proteins that share a conserved 27-kD actin-binding domain. This domain contains a tandem duplication of a sequence that is homologous to calponin. Calponin homology (CH) domains not only cross-link actin filaments into bundles and networks, but they also bind intermediate filaments and some signal transduction proteins to the actin cytoskeleton. This fundamental role of CH domains as a widely used actin-binding domain underlines the necessity to understand their structural interaction with actin. Using electron cryomicroscopy, we have determined the three-dimensional structure of F-actin and F-actin decorated with the NH2-terminal CH domains of fimbrin (N375). In a difference map between actin filaments and N375-decorated actin, one end of N375 is bound to a concave surface formed between actin subdomains 1 and 2 on two neighboring actin monomers. In addition, a fit of the atomic model for the actin filament to the maps reveals the actin residues that line, the binding surface. The binding of N375 changes actin, which we interpret as a movement of subdomain 1 away from the bound N375. This change in actin structure may affect its affinity for other actin-binding proteins and may be part of the regulation of the cytoskeleton itself. Difference maps between actin and actin decorated with other proteins provides a way to look for novel structural changes in actin.
PMCID: PMC2139807  PMID: 9334343

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