Oxidation of cofilin mediates T cell hyporesponsiveness under oxidative stress conditions

Oxidative stress leads to impaired T cell activation. A central integrator of T cell activation is the actin-remodelling protein cofilin. Cofilin is activated through dephosphorylation at Ser3. Activated cofilin enables actin dynamics through severing and depolymerization of F-actin. Binding of cofilin to actin is required for formation of the immune synapse and T cell activation. Here, we showed that oxidatively stressed human T cells were impaired in chemotaxis- and costimulation-induced F-actin modulation. Although cofilin was dephosphorylated, steady-state F-actin levels increased under oxidative stress conditions. Mass spectrometry revealed that cofilin itself was a target for oxidation. Cofilin oxidation induced formation of an intramolecular disulfide bridge and loss of its Ser3 phosphorylation. Importantly, dephosphorylated oxidized cofilin, although still able to bind to F-actin, did not mediate F-actin depolymerization. Impairing actin dynamics through oxidation of cofilin provides a molecular explanation for the T cell hyporesponsiveness caused by oxidative stress.     

Ankycorbin: a novel actin cytoskeleton-associated protein

BACKGROUND: Actin cytoskeleton structures are essential for a wide variety of cell functions, including cell shape change, cell motility, cell adhesion, cell polarity and cytokinesis. Many actin filament (F-actin)-binding proteins have been isolated and implicated in the maintenance and reorganization of actin cytoskeleton structures. RESULTS: We purified here a novel protein with a molecular mass of about 125 kDa (p125) from rat liver. We cloned its cDNA from a mouse kidney cDNA library and determined its nucleotide and deduced amino acid sequences. p125 was a protein of 979 amino acids with a calculated Mr of 108 847. p125 contained six ankyrin repeats in the N-terminal region and a domain predicted to form a coiled-coil structure in the C-terminal region. We named p125 ankycorbin (ankyrin repeat- and coiled-coil structure-containing protein). Northern blot analysis indicated that ankycorbin was ubiquitously expressed in all the tissues examined. Immunofluorescence and immunoelectron microscope analyses revealed that ankycorbin was associated with the cortical actin cytoskeleton structures in terminal web and cell-cell adhesion sites and stress fibres. However, ankycorbin did not directly bind to F-actin as estimated by the F-actin co-sedimentation assay. CONCLUSIONS: These results indicate that ankycorbin is indirectly associated with the actin cytoskeleton structures, presumably through an unidentified factor and suggest that it is involved in their maintenance and/or reorganization.     

Ras/PI3kinase/cofilin-independent activation of human CD45RA+ and CD45RO+ T cells by superagonistic CD28 stimulation

T cell activation requires costimulation of TCR/CD3 plus accessory receptors (e.g. CD28). A hallmark of costimulation is the dynamic reorganization of the actin cytoskeleton, important for receptor polarization in the immunological synapse. The classical model of T cell costimulation was challenged by the detection of superagonistic anti-CD28 antibodies. These induce T cell proliferation and--as demonstrated here--production of IFN-gamma, CD25 and CD69 even in the absence of TCR/CD3 coligation. Here, we analyzed whether superagonistic CD28 stimulation induces costimulatory signaling events. Costimulation leads to phosphorylation of the actin-bundling protein L-plastin and dephosphorylation of the actin-reorganizing protein cofilin. Cofilin binds to F-actin only in its dephosphorylated form. Binding of cofilin to F-actin leads to depolymerization or severing of F-actin. The latter ends up in smaller F-actin fragments, which can be elongated at the free barbed ends. This results in enhanced actin polymerization. Dephosphorylation of cofilin requires activation of Ras and PI3Kinase. Interestingly, superagonistic CD28 stimulation activates human peripheral blood T cells independently of Ras and PI3Kinase. Accordingly, it does not lead to cofilin dephosphorylation and receptor polarization. Likewise, L-plastin is not phosphorylated. Thus, superagonistic CD28 stimulation does not mimic costimulation. Instead, it leads to a Ras/PI3Kinase/cofilin-independent state of "unpolarized T cell activation".     

Actin-capping protein is involved in controlling organization of actin cytoskeleton together with ADF/cofilin, profilin and F-actin crosslinking proteins in fission yeast

Actin-capping protein (CP) is a heterodimeric protein which is expressed in various eukaryotic cells. CP binds to the barbed end of the actin filaments in vitro and inhibits both the association and dissociation of actin monomers at this end. However, the cellular role of CP has not been uncovered. Here we investigated the function of CP in fission yeast cells. The fission yeast CP is composed of Acp1 and Acp2. It was found that Acp2 accumulated as cortical dots at the cell ends during interphase and the mid-region of mitotic cells, which disappeared in the absence of Acp1 or F-actin. Acp1 and Acp2, when co-over-expressed, decreased F-actin structures in cells, and cytokinesis was often interrupted in these cells. On the other hand, disruption of one of the CP genes affected the distribution of F-actin patches at cell ends and decreased the rate of actin depolymerization in vivo. Moreover, genetic analysis showed that CP controls actin dynamics together with ADF/cofilin and profilin. In addition, CP is likely involved in assembling the F-actin contractile ring and F-actin patch with F-actin-crosslinking proteins.     

Directional actin polymerization associated with spotted fever group Rickettsia infection of Vero cells.

Members of the spotted fever group (SFG) of rickettsiae spread rapidly from cell to cell by an unknown mechanism(s). Staining of Rickettsia rickettsii-infected Vero cells with rhodamine phalloidin demonstrated unique actin filaments associated with one pole of intracellular rickettsiae. F-actin tails greater than 70 microns in length were seen extending from rickettsiae. Treatment of infected cells with chloramphenicol eliminated rickettsia-associated F-actin tails, suggesting that de novo protein synthesis of one or more rickettsial proteins is required for tail formation. Rickettsiae were coated with F-actin as early as 15 min postinfection, and tail formation was detected by 30 min. A survey of virulent and avirulent species within the SFG rickettsiae demonstrated that all formed actin tails. Typhus group rickettsiae, which do not spread directly from cell to cell, lacked F-actin tails entirely or exhibited only very short tails. Transmission electron microscopy demonstrated fibrillar material in close association with R. rickettsii but not Rickettsia prowazekii. Biochemical evidence that actin polymerization plays a role in movement was provided by showing that transit of R. rickettsii from infected cells into the cell culture medium was inhibited by treatment of host cells with cytochalasin D. These data suggest that the cell-to-cell transmission of SFG rickettsiae may be aided by induction of actin polymerization in a fashion similar to that described for Shigella flexneri and Listeria monocytogenes.     

Interaction of immunoglobulin with actin

Actin can form specific, direct associations with immunoglobulin resulting in soluble complexes or cross-linked matrices. This interaction can be detected by four in vitro assays using purified components: (1) actin enhances the cytophilic activity of guinea pig IgG2; (2) in solutions of low ionic strength, actin and IgG2 co-precipitate: (3) soluble complexes exist in 0.1 M KCl as revealed by the displacement of actin from its expected sedimentation pattern in a gradient of sucrose when in the presence of IgG 1, IgG2, or IgM; (4) immunoglobulin (IgG1, IgG2, BGG)‡: increases the viscosity of F-actin solutions, presumably by crosslinking F-actin filaments. These data suggest that direct interaction of a cytoskeletal protein with a cell surface receptor is possible.     

Psidin, a conserved protein that regulates protrusion dynamics and cell migration

Dynamic assembly and disassembly of actin filaments is a major driving force for cell movements. Border cells in the Drosophila ovary provide a simple and genetically tractable model to study the mechanisms regulating cell migration. To identify new genes that regulate cell movement in vivo, we screened lethal mutations on chromosome 3R for defects in border cell migration and identified two alleles of the gene psidin (psid). In vitro, purified Psid protein bound F-actin and inhibited the interaction of tropomyosin with F-actin. In vivo, psid mutations exhibited genetic interactions with the genes encoding tropomyosin and cofilin. Border cells overexpressing Psid together with GFP-actin exhibited altered protrusion/retraction dynamics. Psid knockdown in cultured S2 cells reduced, and Psid overexpression enhanced, lamellipodial dynamics. Knockdown of the human homolog of Psid reduced the speed and directionality of migration in wounded MCF10A breast epithelial monolayers, whereas overexpression of the protein increased migration speed and altered protrusion dynamics in EGF-stimulated cells. These results indicate that Psid is an actin regulatory protein that plays a conserved role in protrusion dynamics and cell migration.     

Phosphoglycerate kinase inhibits epithelial cell invasion by group B streptococci

Group B streptococci (GBS) are opportunistic human pathogens that cause infection and invasive disease in newborns, pregnant women and non-pregnant adults. The internalization of GBS into eukaryotic cells occurs in an actin-microfilament dependent process. The objective of our study was to understand what host cell and/or bacterial factors may be involved in this process. We focused on alpha-actinin, an actin binding protein closely associated with cytoplasmic F-actin in the eukaryotic cell, to determine if it is involved in actin recruitment upon GBS internalization. Initial work revealed that GBS does not recruit alpha-actinin. However, it was found that alpha-actinin antibodies bound to the surface of the GBS, suggesting GBS possess surface-exposed actin binding protein(s). Slide agglutination experiments revealed that when the bacteria were emulsified with F-actin, visible agglutination occurred, further suggesting the presence of an actin binding protein on the GBS cell. Western blot analysis found that anti-alpha-actinin antibodies bound to a 42 kDa protein; mass spectra analysis identified this protein as GBS phosphoglycerate kinase (PGK). Competitive binding assays suggest that the PGK-actin interaction is not a factor in the initial binding of GBS to epithelial cells, however, treating epithelial cells with PGK prior to performing an invasion assay inhibited GBS internalization. This occurred in a dose dependent manner with 10 microg/mL of PGK inhibiting invasion by over 70%, and 50 microg/mL PGK inhibits GBS invasion completely.     

A GFP-actin reporter line to explore microfilament dynamics across the malaria parasite lifecycle

Malaria parasite motility relies on an internal parasite actomyosin motor that, when linked to the host cell substrate, propels motile zoites forward. Despite their key role in this process, attempts to visualize actin microfilaments (F-actin) during motility and under native microscopy conditions have not to date been successful. Towards facilitating their visualization we present here a Plasmodium berghei transgenic line in which a green fluorescent protein (GFP)-actin fusion is constitutively expressed through the lifecycle. Focused investigation of the largest motile form, the insect stage ookinete, demonstrates a large cytosolic pool of actin with no obvious F-actin structures. However, following treatment with the actin filament-stabilizing drug Jasplakinolide, we show evidence for concentration of F-actin dynamics in the parasite pellicle and at polar apices. These observations support current models for gliding motility and establish a cellular tool for further exploration of the diverse roles actin is thought to play throughout parasite development.     

Small heat shock protein with apparent molecular mass 20 kDa (Hsp20, HspB6) is not a genuine actin-binding protein

The interaction of recombinant human small heat shock protein with apparent molecular mass 20 kDa (Hsp20, HspB6) with actin was investigated. Wild type Hsp20 and its S16D mutant mimicking phosphorylation of Hsp20 by cyclic nucleotide-dependent protein kinases do not affect the rate and extent of actin polymerization. Ultracentrifugation of the mixture of Hsp20 (or its S16D mutant) with isolated F-actin or F-actin containing tropomyosin, calponin or α-actinin resulted in co-sedimentation of less than 0.04 mol of Hsp20 monomer per mol of actin. Myofibrils of skeletal, cardiac or smooth muscle bound less than 0.04 mol of Hsp20 monomer per mol of actin and this stoichiometry was independent of phosphorylation or mutation of Ser16 of Hsp20. Since Hsp20 is not a genuine actin-binding protein, the earlier described correlation between Hsp20 phosphorylation and smooth muscle relaxation cannot be explained by direct interaction of Hsp20 with actin.     

Interactions between surface-bound actin and complement,platelets, and neutrophils

Actin exists as globular (G) monomers or polymeric filaments (F) in the cytoplasm of eukaryotic cells, mediating cell morphologic changes and motility. Large amounts of this protein may be released out to the extracellular compartment during tissue injury, but little is known about its role in biomaterial-related inflammation. We immobilized actin to methylated glass, methylated and aminated silicon, and gold model surfaces and studied the subsequent blood serum deposition and complement activation, generation of reactive oxygen species (ROS), and adhesion and aggregation of neutrophils and platelets. Null ellipsometry showed that approximately one monolayer of G-actin can be immobilized onto the model surfaces and that actin in buffer polymerized on top of this by the addition of K(+) and Mg(2+) ions to form a thicker layer of firmly bound F-actin. After serum incubation, F-actin bound low amounts of anti-complement factor 1q (anti-C1q). Cell responses upon contact with actin-coated surfaces were analyzed by luminol-amplified chemiluminescence, lumi-aggregometry, and fluorescence microscopy. It was shown that surface-triggered aggregation, spreading, and generation of ROS are down-regulated and comparable to the response by adsorbed albumin. However, F-actin on gold surfaces recruited platelets in a C1q-dependent manner. We conclude that in vitro adsorbed actin is a weak complement, platelet, and neutrophil activator, but that F-actin associates with both C1q and platelets.     

Cytotoxic effects by microinjection of ADP-ribosylated skeletal muscle G-actin in PtK2 cells in the absence ofClostridium perfringens iota toxin

The ADP-ribosylating toxinsClostridium botulinum C2 toxin andC. perfringens iota toxin, which ADP-ribosylate monomeric G-actin at Arg-177 but not the polymeric F-actin, induce depolymerization of the actin cytoskeleton in cultured cells. Since ADP-ribosylated G-actin has properties of a barbed-end-capping protein, we studied whether the ADP-ribosylated actin affects the actin cytoskeleton of PtK2 cells even in the absence of ADP-ribosylating toxin. Skeletal muscle actin was ADP-ribosylated byC. perfringens iota toxin and the toxin was removed using an anti-iota toxin antibody. Microinjection of ADP-ribosylated actin caused retraction of the cell body, redistribution and depolymerization of the actin cytoskeleton in a concentration-and time-dependent manner. The finding that ADP-ribosylated actin affects per se the actin cytoskeleton explains the cytopathic effects of ADP-ribosylating toxins on microfilaments, although F-actin is not directly modified by the toxins.     

Myosin Mg-ATPase of molluscan muscles is slightly activated by F-actin under catch state in vitro

Molluscan muscle twitchin, a titin/connectin-related giant protein, regulates interactions between actin and myosin filaments at low Ca²⁺ concentrations. When it is dephosphorylated, actin filaments tightly bind to myosin filaments, resulting in the catch state known as the state of high passive tension with very low energy consumption. Yet when twitchin is phosphorylated actin filaments detach from the myosin filaments, resulting in relaxation of the catch. Here, steady-state Mg-ATPase activities of purified myosin were measured under various conditions: without twitchin, with dephosphorylated twitchin, or with phosphorylated twitchin; with or without phalloidin-stabilized F-actin; and at various Ca²⁺ concentrations. At low Ca²⁺ concentration, Mg-ATPase was activated by F-actin only in the presence of dephosphorylated twitchin (catch state). The activation was about two orders lower than that fully activated by Ca²⁺ and F-actin. In the absence of F-actin, twitchin and its phosphorylation state did not affect Mg-ATPase activities in any of the conditions we tested. Based on these results, we propose a molecular mechanism for the catch, where twitchin alone does not interact with the myosin catalytic motor domain but its complex with F-actin does, forming the bridge between actin and myosin filaments and the myosin slowly hydrolyzes Mg-ATP in the catch state.     

Tropomyosin-1 protects endothelial cell–cell junctions against cigarette smoke extract through F-actin stabilization in EA.hy926 cell line

The aim of the study was to estimate the effect of cigarette smoke extract (CSE) on EA.hy926 endothelial cells in culture in the context of maintenance of cell–cell junctions through the structural stabilization of the actin cytoskeleton. In the present study, F-actin was stabilized by the overexpression of tropomyosin-1, which is known to stabilize actin filaments in muscle and non-muscle cells. Our study showed that the stabilization of F-actin significantly increased the survival of cells treated with 25% CSE. In addition, after stabilization of F-actin the migratory potential of EA.hy926 cells subjected to CSE treatment was increased. Our results also showed increased fluorescence intensity of alpha- and beta-catenin after CSE treatment in cells which had stabilized F-actin. Analysis of fluorescence intensity of Zonula occludens-1 did not reveal any significant differences when EA.hy926 cells overexpressing tropomyosin-1 were compared with those lacking overexpression. It would appear that overexpression of tropomyosin-1 preserved the structure of actin filaments in the cells treated with CSE. In conclusion, the present study demonstrates that stabilization of F-actin protects EA.hy926 cells against CSE-induced loss of both adherens and tight junctions. The data presented in this study suggest that overexpression of tropomyosin-1 stabilizes the organizational structure of actin filaments and helps preserve the endothelial barrier function under conditions of strong oxidative stress.     

Myopodin is an F-actin bundling protein with multiple independent actin-binding regions

The assembly of striated muscle myofibrils is a multistep process in which a variety of proteins is involved. One of the first and most important steps in myofibrillogenesis is the arrangement of thin myofilaments into ordered I-Z-I brushes, requiring the coordinated activity of numerous actin binding proteins. The early expression of myopodin prior to sarcomeric α-actinin, as well as its binding to actin, α-actinin and filamin indicate an important role for this protein in actin cytoskeleton remodelling with the precise function of myopodin in this process yet remaining to be resolved. While myopodin was previously described as a protein capable of cross-linking actin filaments into thick bundles upon transient transfections, it has remained unclear whether myopodin alone is capable of bundling actin, or if additional proteins are involved. We have therefore investigated the in vitro actin binding properties of myopodin. High speed cosedimentation assays with skeletal muscle actin confirmed direct binding of myopodin to F-actin and showed that this interaction is mediated by at least two independent actin binding sites, found in all myopodin isoforms identified to date. Furthermore, low-speed cosedimentation assays revealed that not only full length myopodin, but also the fragment containing only the second binding site, bundles microfilaments in the absence of accessory proteins. Ultrastructural analysis demonstrated that this bundling activity resembled that of α-actinin. Biochemical experiments revealed that bundling was not achieved by myopodin’s ability to dimerize, indicating the presence of two individual F-actin binding sites within the second binding segment. Thus full length myopodin contains at least three F-actin binding sites. These data provide further understanding of the mechanisms by which myopodin contributes to actin reorganization during myofibril assembly.

     

不同转移潜能肿瘤细胞胸腺素β10表达水平及微丝结构的差异性研究

目的 研究胸腺素β10(Tlaymosinβ10,Tβ10)表达水平与人类肿瘤转移潜能的相关性以及细胞内微丝骨架的变化差异。方法 利用Northern-blot和细胞爬片免疫组织化学的方法检测4组9种具有不同转移潜能的人类肿瘤细胞系中Tβ10 mRNA和蛋白的表达水平;通过组织化学的方法显示细胞内微丝骨架的变化。结果 Tβ10的mRNA和蛋白表达水平在高转移潜能的人类肺癌、黑色素瘤和乳腺癌细胞中高于相应的不／低转移肿瘤细胞;高转移性癌与不／低转移癌细胞比较,前者细胞内肌动蛋白多聚体减少,微丝骨架杂乱、模糊,而低转移癌系微丝骨架粗大、结构清晰、排列整齐。结论 在所研究的人类肿瘤中,Tβ10 mRNA和蛋白表达水平与肿瘤的转移能力呈正相关性;肿瘤转移潜能的增高伴有肌动蛋白多聚体的丢失和微丝结构的解聚,微丝骨架的这种变化与Tβ10的表达增高具有相关性。     

Propionate induces polymorphonuclear leukocyte activation and inhibits formylmethionyl-leucyl-phenylalanine-stimulated activation.

Short-chain carboxylic acids (SCCA) are metabolic by-products of bacterial pathogens which can alter cytoplasmic pH and inhibit a variety of polymorphonuclear leukocyte (PMN) motile functions. Since cytoskeletal F-actin alterations are central to PMN mobility, in this study we examined the effects of SCCA on cytoskeletal F-actin. Initially, we tested nine SCCA (formate, acetate, propionate, butyrate, valerate, caproate, lactate, succinate, and isobutyrate). We document here that while eight altered cytoplasmic pH, only six altered cytoskeletal F-actin. We then selected one SCCA that altered both F-actin and cytoplasmic pH (propionate) and one SCCA that altered only cytoplasmic pH (lactate) for further study. Propionate, but not lactate, caused an irregular cell shape and F-actin distribution. Furthermore, propionate, but not lactate, inhibited formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated PMN polarization, F-actin localization, and cytoplasmic pH oscillation. Propionate-induced changes in cytoskeletal F-actin and cytoplasmic acidification were not affected by the fMLP receptor antagonist N-t-BOC-1-methionyl-1-leucyl-1-phenylalanine; however, alkalinization was affected. Pertussis toxin treatment completely inhibited propionate-induced changes in F-actin but had no effect on propionate-induced cytoplasmic pH oscillation. These results indicate that propionate (i) bypasses the fMLP receptor and G protein(s) to induce cytoplasmic pH oscillation, (ii) operates through G protein(s) to induce actin oscillation, cell shape changes (to irregular), and F-actin localization, and (iii) inhibits fMLP-stimulated cytoplasmic pH and actin oscillation, PMN polarization, and F-actin localization.     

Identification and functional analysis of the gene for type I myosin in fission yeast

BACKGROUND: Type I myosin is highly conserved among eukaryotes, and apparently plays important roles in a number of cellular processes. In the budding yeast, two myosin I species have been identified and their role in F-actin assembly has been inferred. RESULTS: We cloned the fission yeast myo1 gene, which apparently encoded a myosin I protein. Disruption of myo1 was not lethal, but it caused growth retardation at high and low temperatures, sensitivity to a high concentration of KCl, and aberrance in cell morphology associated with an abnormal distribution of F-actin patches. An abnormal deposition of cell wall materials was also seen. Homothallic myo1Delta cells could mate, but heterothallic myo1Delta cells were poor in conjugation. Myo1p was necessary for the encapsulation of spores. The tail domain of Myo1p was pivotal for its function. Calmodulin could bind to Myo1p through the IQ domain at the neck. CONCLUSIONS: Myo1p appears to control the redistribution of F-actin patches during the cell cycle. Loss of Myo1p function is likely to slow down the actin assembly/disassembly process, which results in a failure of the actin cycle to catch up with other events in both the mitotic and meiotic cell cycles, including extension of the conjugation tubes.     

Actin cytoskeleton reorganization correlates with cofilin nuclear expression and ultrastructural changes in cho aa8 cell line after apoptosis and mitotic catastrophe induction by doxorubicin

The effect of doxorubicin on the expression of cofilin and actin in CHO AA8 cells was estimated by fluorescence and electron microscopy. The presence of cofilin and actin was observed particularly in the nuclei of cells by different modes after treatment by doxorubicin. Cells undergoing mitotic catastrophe expressed some entirely characteristic features together with overlapping elements of other types of cell death. Additionally, the authors suggest that, as defined here, reorganization of F-actin might be involved in all cell death processes. Changes in the nuclear expression of cofilin are related to F-actin cytoplasm-nuclear translocation and its intranuclear dynamic reorganization.     

Nuclear F-actin is required for AcMNPV nucleocapsid morphogenesis.

During nucleocapsid assembly, filamentous actin (F-actin) colocalizes with the major capsid protein of Autographa californica M nucleopolyhedrovirus (AcMNPV) within nuclei of infected lepidopteran host cells. Cytochalasin D (CD) disrupts actin filaments and prevents assembly of progeny AcMNPV, suggesting that nuclear F-actin is essential for nucleocapsid morphogenesis. Direct proof for this hypothesis was provided by the demonstration that two AcMNPV recombinants engineered to express either wild-type- or CD-resistant actin at equivalent rates were differentially sensitive to CD. The AcMNPV requirement for nuclear F-actin is unique among intracellular pathogens and may constitute a significant host range factor.