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PubMed1983

Isolation of an actin polymerization stimulator from bovine thyroid plasma membranes.

An actin polymerization stimulator was purified from bovine thyroid plasma membranes by DNase I affinity column chromatography. Although the molecular weight of the protein was about 42,000 (42K) by sodium dodecyl sulfate polyacrylamide gel electrophoresis, it did not comigrate with actin. In the presence of 30 mM KC1, the 42K protein facilitated formation of actin filaments when analyzed by a centrifugation method, accelerated the initial phase of actin polymerization as measured in an Ostwald viscometer and increased the length of filaments as shown by electron microscopy. The 42K protein also accelerated the initial phase of actin polymerization in the presence of 100 mM KC1 and 2 mM MgCl2 but did not affect the final viscosity. The effect of the 42K protein was diminished by 5 uM cytochalasin B or 1 uM cytochalasin D. This 42K protein may anchor actin filaments onto the thyroid plasma membrane.

Actins↗

PubMed1997

Conformational and functional studies of three gelsolin subdomain-1 synthetic peptides and their implication in actin polymerization.

Gelsolin, a calcium and inositol phospholipid-sensitive protein, regulates actin filament length. Its activity is complex (capping, severing, etc.) and is supported by several functional domains. The N-terminal domain alone (S1), in particular, is able to impede actin polymerization. Our investigations were attempted to precise this inhibitory process by using synthetic peptides as models mimicking gelsolin S1 activity. Three peptides issued from S1 and located in gelsolin-actin interfaces were synthesized. The peptides (15-28, 42-55, and 96-114 sequences) were tested for their conformational and actin binding properties. Although the three peptides interact well with actin, only peptide 42-55 affects actin polymerization. A detailed kinetic study shows that the latter peptide essentially inhibits the nucleation step during actin polymerization. In conclusion, the present work shows that the binding of a synthetic peptide to a small sequence located outside the actin-actin interface is essential in the actin polymerization process.

Actins↗

PubMed2001

How is actin polymerization nucleated in vivo?

Actin polymerization in vivo is dependent on free barbed ends that act as nuclei. Free barbed ends can arise in vivo by nucleation from the Arp2/3 complex, uncapping of barbed ends on pre-existing filaments or severing of filaments by cofilin. There is evidence that each mechanism operates in cells. However, different cell types use different combinations of these processes to generate barbed ends during stimulated cell motility. Here, I describe recent attempts to define the relative contributions of these three mechanisms to actin nucleation in vivo. The rapid increase in the number of barbed ends during stimulation is not due to any single mechanism. Cooperation between capping proteins, cofilin and the Arp2/3 complex is necessary for the development of protrusive force at the leading edge of the cell: uncapping and cofilin severing contributing barbed ends, whereas activity of the Arp2/3 complex is necessary, but not sufficient, for lamellipod extension. These results highlight the need for new methods that enable the direct observation of actin nucleation and so define precisely the relative contributions of the three processes to stimulated cell motility.

Actin-Related Protein 2↗

PubMed1993

Effects of adenosine and its analogues on actin polymerization in human polymorphonuclear leucocytes.

1. The effects of adenosine and its analogues on actin polymerization in human polymorphonuclear leucocytes (PMN) induced by three different chemotactic stimulants, platelet-activating factor (PAF), N-formyl-methionyl-leucyl-phenylalanine (FMLP) and an activated fragment of C5 (C5a) were investigated. 2. Adenosine and its analogues inhibited the actin polymerization induced by these three agents in a concentration-dependent manner and theophylline, a competitive antagonist at adenosine receptors, abolished these inhibitory effects. 3. The adenosine analogue 5'-N-ethylcarboxamideadenosine (NECA) was a more potent inhibitor of actin polymerization than either L-N6-phenylisopropyladenosine (PIA) or adenosine itself; the rank order of potency of these agonists was characteristic of adenosine A2 receptors. 4. Adenosine deaminase (ADA) abolished the inhibitory effect of adenosine and augmented PAF-induced actin polymerization. 5. It was concluded that, at physiological concentrations, adenosine inhibits actin polymerization in PMN via activation of PMN surface membrane adenosine A2 receptors and thus modulates chemotactic stimulus-induced PMN motility.

Actins↗

PubMed2004

A neural Wiskott-Aldrich Syndrome protein-mediated pathway for localized activation of actin polymerization that is regulated by cortactin.

Activation of the epidermal growth factor (EGF) receptor can stimulate actin polymerization via the Arp2/3 complex using a number of signaling pathways, and specific stimulation conditions may control which pathways are activated. We have previously shown that localized stimulation of EGF receptor with EGF bound to beads results in localized actin polymerization and protrusion. Here we show that the actin polymerization is dependent upon activation of the Arp2/3 complex by neural Wiskott-Aldrich Syndrome protein (N-WASP) via Grb2 and Nck2. Suppression of Grb2 or Nck2 results in loss of localization of N-WASP at the activation site and reduced actin polymerization. Although cortactin has been found to synergize with N-WASP for Arp2/3-dependent actin polymerization in vitro, we find that cortactin can restrict N-WASP localization around EGF-bead-induced protrusions. In addition, cortactin-deficient cells have increased lamellipod dynamics but show reduced net translocation, suggesting that cortactin can contribute to cell polarity by controlling the extent of Arp2/3 activation by WASP family members and the stability of the F-actin network.

Actins↗

PubMed1995

ADP-induced platelet aggregation and actin polymerization. Involvement of GpIIb/IIIa and the effect of Mg2+.

We have investigated the effects of Mg2+ (added to platelet rich plasma [PRP] as 10mM MgCl2 or MgSO4) on the platelet aggregation and actin polymerization that occurs in response to adenosine diphosphate (ADP). The PRP was prepared from blood containing hirudin as anticoagulant. Mg2+ added before 1 microM ADP completely inhibited aggregation and markedly inhibited actin polymerization. Mg2+ (10mM) added before 10 microM ADP converted irreversible aggregation into a reversible response; similarly, apparently irreversible actin polymerization was converted to a reversible response in which polymerization was followed by some actin depolymerization. Mg2+ added after inducing platelet aggregation with 10 microM ADP produced parallel disaggregation of platelets and actin depolymerization. Actin polymerization occurs immediately on adding ADP to PRP (in association with shape change) and further polymerization occurs in association with platelet aggregation. When aggregation (and the actin polymerization associated with this) was prevented by adding M148, a monoclonal antibody directed at the GpIIb/IIIa complex, or simply by avoiding stirring the sample, Mg2+ had no effect on actin polymerization/depolymerization. Thus Mg2+ only affected the changes in actin that were associated with the aggregation response. This was in contrast to iloprost (which acts at the PGI2 receptor to stimulate adenylate cyclase) which induced rapid actin depolymerization when added after ADP stimulation of platelets under circumstances where platelet aggregation was avoided. These results show that Mg2+ affects actin polymerization as well as platelet aggregation, and that it affects the actin polymerization associated with aggregation but not that associated with shape change (in contrast to iloprost which inhibits both).(ABSTRACT TRUNCATED AT 250 WORDS)

Actins↗

PubMed1997

Colchicine activates actin polymerization by microtubule depolymerization.

Swiss 3T3 fibroblasts were treated with the microtubule-disrupting agent colchicine to study any interaction between microtubule dynamics and actin polymerization. Colchicine increased the amount of filamentous actin (F-actin), in a dose- and time-dependent manner with a significant increase at 1 h by about 130% over control level. Confocal microscopic observation showed that colchicine increased F-actin contents by stress fiber formation without inducing membrane ruffling. Colchicine did not activate phospholipase C and phospholipase D, whereas lysophosphatidic acid did, indicating that colchicine may have a different mechanism of actin polymerization regulation from LPA. A variety of microtubule-disrupting agents stimulated actin polymerization in Swiss 3T3 and Rat-2 fibroblasts as did colchicine, but the microtubule-stabilizing agent taxol inhibited actin polymerization induced by the above microtubule-disrupting agents. In addition, colchicine-induced actin polymerization was blocked by two protein phosphatase inhibitors, okadaic acid and calyculin A. These results suggest that microtubule depolymerization activates stress fiber formation by serine/threonine dephosphorylation in fibroblasts.

3T3 Cells↗

PubMed1987

Actin polymerization and its relationship to locomotion and chemokinetic response in maturing human promyelocytic leukemia cells.

We studied actin polymerization in the HL-60 human promyelocytic leukemia cell line during induced myeloid maturation and its relationship to the rate of locomotion (ROL). The percent G-actin (of total actin) was measured by DNAase I inhibition, F-actin was determined by fluorescence-activated cell sorter (FACS) analysis of nitrobenzoxadiazol (NBD)-phallacidin-stained cells, and ROL was measured by computer-assisted analysis of the tracks of individual cells. Uninduced HL-60 cells moved slowly (2.3 +/- 1.0 microns/min) and showed no change in ROL or in the state of actin polymerization when stimulated by formyl-methionyl-leucyl-phenylalanine (fMLP). Nonstimulated cells induced to differentiate with dimethylformamide had no change in the degree of actin polymerization but exhibited a mean (m) ROL similar to normal human polymorphonuclear leukocytes (PMN) (8.6 +/- 1.4 micron/min [HL-60 cells] v 7.8 +/- 1.8 microns/min [PMN]. When induced HL-60 cells were stimulated with fMLP, actin polymerization occurred. The F-actin content increased, as determined by FACS analysis of NBD-phallacidin-stained cells, and the percentage of G-actin decreased, as determined by a 24.5% decrease in DNAase I inhibitory activity. However, induced HL-60 cells stimulated with fMLP did not increase their mROL. These studies show that, unlike normal human PMN, chemotactic peptides can cause an intracellular biochemical change that is not associated with a chemokinetic response in induced HL-60 cells. The HL-60 cell line may be a useful model to study the development of chemotactic peptide-mediated actin polymerization during myeloid cell maturation.

Actins↗

PubMedDate not supplied

Glycosylated and nonglycosylated recombinant human granulocyte colony-stimulating factor differently modifies actin polymerization in neutrophils.

AIM: Several neutrophil functions can be modified by rhG-CSF administration. Neutrophil morphology changes in the course of treatment with Filgrastim (nonglycosylated rhG-CSF), along with impairment of chemotaxis. Both morphology and chemotaxis are not affected by treatment with Lenograstim (glycosylated rhG-CSF). Thus, we evaluated actin polymerization in neutrophils induced by treatment with the two forms of rhG-CSF. In fact, actin polymerization is crucial for neutrophil motility. MATERIALS AND METHODS: We evaluated twelve healthy subjects undergoing peripheral blood stem cells (PBSC) mobilization for allogeneic transplantation to HLA-identical siblings. Neutrophils were isolated by peripheral venous blood before and after administration of either Filgrastim (six PBSC donors) or Lenograstim (six PBSC donors). Actin polymerization was investigated by a flow cytometric assay, using FITC-phalloidin as a specific probe for F-actin, and two parameters were measured: spontaneous actin polymerization in resting neutrophils; fMLP-stimulated actin polymerization. Results were expressed as relative F-actin content. Fifteen blood donors were studied as a control group. RESULTS: Filgrastim administration induced an increased relative F-actin content in resting neutrophils; however, no further actin polymerization was observed after fMLP stimulation. Neutrophils from subjects treated with Lenograstim showed a normal behaviour in terms of both spontaneous and stimulated actin polymerization. CONCLUSIONS: Glycosylated and nonglycosylated rhG-CSF differently affect actin polymerization in newly generated neutrophils. Such effects may explain some previous findings concerning both morphology and chemotactic properties and may be due to different effects of the two forms of rhG-CSF on proteins involved in neutrophil motility regulation.

Actins↗

PubMed1998

Mechanism of Cdc42-induced actin polymerization in neutrophil extracts.

Cdc42, activated with GTPgammaS, induces actin polymerization in supernatants of lysed neutrophils. This polymerization, like that induced by agonists, requires elongation at filament barbed ends. To determine if creation of free barbed ends was sufficient to induce actin polymerization, free barbed ends in the form of spectrin-actin seeds or sheared F-actin filaments were added to cell supernatants. Neither induced polymerization. Furthermore, the presence of spectrin-actin seeds did not increase the rate of Cdc42-induced polymerization, suggesting that the presence of Cdc42 did not facilitate polymerization from spectrin-actin seeds such as might have been the case if Cdc42 inhibited capping or released G-actin from a sequestered pool. Electron microscopy revealed that Cdc42-induced filaments elongated rapidly, achieving a mean length greater than 1 micron in 15 s. The mean length of filaments formed from spectrin-actin seeds was <0.4 micron. Had spectrin-actin seeds elongated at comparable rates before they were capped, they would have induced longer filaments. There was little change in mean length of Cdc42-induced filaments between 15 s and 5 min, suggesting that the increase in F-actin over this time was due to an increase in filament number. These data suggest that Cdc42 induction of actin polymerization requires both creation of free barbed ends and facilitated elongation at these ends.

Actin Cytoskeleton↗

PubMed1995

Actin polymerization localizes to the activated epidermal growth factor receptor in the plasma membrane, independent of the cytosolic free calcium transient.

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of A431 cells, leading to a approximately 30% rise in cellular filamentous actin levels. EGF-induced actin polymerization depends upon EGF receptor (EGFR) tyrosine kinase activity, since the selective tyrosine kinase inhibitor AG213 abolishes EGF-induced actin polymerization. In accordance, confocal laser scanning microscopy shows that newly assembled actin filaments localize selectively to the tyrosine-phosphorylated EGFR in the plasma membrane, since actin polymerization is not observed at the internalized tyrosine-phosphorylated EGFR. Actin binding proteins (ABP's) are generally believed to regulate actin filament assembly. Ca2+ is known as one of the important regulatory factors for the activity of ABP's in vitro [15]. Therefore, we investigated the importance of the EGF-induced transient rise in [Ca2+]i for the regulation of actin polymerization in vivo. Continuous high [Ca2+]i in the millimolar range induces a prominent rise in cellular filamentous actin levels to approximately 50% over control cells. However, actin polymerization is unimpaired under conditions which effectively block the EGF-induced [Ca2+]i transient. These data demonstrate that EGF-induced actin polymerization localizes to the activated EGFR in the membrane skeleton, independent of the cytosolic free calcium transient.

Actins↗

PubMed1996

Studies on the thermokinetic characterisation of actin polymerization and the effect of cisplatin.

The microcalorimetic measurement for studying the thermokinetic of actin polymerization in vitro was employed for the first time. The thermodynamic parameters of actin polymerization at 310.15 K are reported as delta Hm = 49.26 kJ/mol, delta Gm = 25.62 kJ/mol and delta Sm = 241.54 J/K.mol. The thermogram shows that polymerization of actin is a complicate process in which multistep reactions occur corresponding to exothermic and endothermic effects. Cisplatin, at lower concentration, affects the polymerization of G-actin resulting in a decrease of delta Hm; and, at higher concentration, induce the crosslinking and depolymerization of F-actin in the equilibrium system of G/F. The experimental results show that as the concentration of cisplatin increases, the sign of delta Hm changes from positive to negative, i.e. the magnitude and sign of delta Hm of actin polymerization depends on the concentration of cisplatin.

Actins↗

PubMed1990

Correlation between chemotactic peptide-induced changes in chlorotetracycline fluorescence and F-actin content in human neutrophils: a role for membrane-associated calcium in the regulation of actin polymerization?

Several observations indicate that the triggering event for receptor-mediated actin polymerization takes place in or close to the plasma membrane. Stimulation of human neutrophils with the chemotactic peptide formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe) causes rapid and transient changes in both chlorotetracycline (CTC) fluorescence and the cellular content of filamentous actin (F-actin), thus suggesting a regulatory role for membrane-bound calcium in actin polymerization. In the present study, tetracaine, a proposed antagonist to membrane-bound calcium, totally inhibited the rebinding of the membrane calcium released by fMet-Leu-Phe. This was accompanied by a magnified and sustained increase in the cellular content of F-actin. In agreement, N-ethylmaleimide, an inhibitor of motile functions, completely abolished the fMet-Leu-Phe-triggered changes in both CTC fluorescence and F-actin content and rapidly reversed the responses when added after the peptide. The tumor promoter phorbol-12-myristate-13-acetate, caused only small changes in CTC fluorescence and F-actin content, and reduced a subsequent fMet-Leu-Phe-induced CTC response and actin polymerization. Inhibition of the breakdown of phosphatidylinositol 4,5-bisphosphate, by calcium depletion, had no significant effects on the fMet-Leu-Phe-induced CTC response and alterations in F-actin content, whereas pretreatment with pertussis toxin totally inhibited both these responses. Consequently, the strong correlation between changes in CTC fluorescence and F-actin content, found in this study, suggests a triggering or modulating role of membrane-associated calcium on actin polymerization in human neutrophils.

Actins↗

PubMed1996

Dephosphorylation of a 34kd triton-insoluble F-actin pool protein is associated with phorbol ester-induced actin polymerization in human polymorphonuclear leukocytes.

Activation of human polymorphonuclear leukocytes (PMNs) by chemotactic peptide (FMLP) or phorbol ester (PMA) results in actin reorganization and PMN motility. Evidence suggests that PMA and FMLP activate PMN actin reorganization by different mechanisms. For example, the protein phosphatase inhibitor, okadaic acid (OA), inhibits PMA- but not FMLP-induced actin rearrangement, suggesting protein dephosphorylation is key to PMA but not FMLP actin changes and that PMN actin reorganization occurs by multiple mechanisms. Further support for multiple actin polymerization mechanisms is the recent description of distinct F-actin pools coexisting with G-actin in PMNs, Triton insoluble F-actin (TIF) and Triton soluble F-actin (TSF). These studies examine quantitative actin pool-specific actin polymerization in PMA- and FMLP-activated PMNs using quantitative SDS-PAGE and the phosphorylation of proteins in each actin pool using 32P orthophosphate (32P) labeling. The results show: (1) OA alone has no effect on actin pool content; (2) PMA induces actin growth only in the TIF pool similar to results with FMLP, and (3) OA pretreatment has no effect on FMLP actin polymerization, but inhibits PMA-induced changes. 32P results show that in basal PMNs, multiple phosphoproteins are found in the TIF including a protein of MW 34kd (pp34), the TSF pool contains a pp34 and a pp69 and the G-actin pool a pp34. PMA induces dephosphorylation of pp34 in the TIF (0.59 +/- 0.14 x basal, n = 3). OA prior to PMA prevents TIF pp34 dephosphorylation and actin shifts between the TIF, TSF, and G pools. OA alone results in phosphorylation of pp34 in all actin pools but no shift in actin content. The results show that (1) phosphoproteins exist in all three actin pools of PMNs-TIF-actin, TSF-actin, and G-actin; (2) both PMA and FMLP cause quantitatively identical actin polymerization in the TIF; and (3) in contrast, PMA but not FMLP TIF growth requires dephosphorylation of a pp34. This as yet unidentified phosphoprotein appears crucial to PMA- but not FMLP-induced actin polymerization.

Actins↗

PubMed1997

Actin polymerization is induced by Arp2/3 protein complex at the surface of Listeria monocytogenes.

The pathogenic bacterium Listeria monocytogenes is capable of directed movement within the cytoplasm of infected host cells. Propulsion is thought to be driven by actin polymerization at the bacterial cell surface, and moving bacteria leave in their wake a tail of actin filaments. Determining the mechanism by which L. monocytogenes polymerizes actin may aid the understanding of how actin polymerization is controlled in the cell. Actin assembly by L. monocytogenes requires the bacterial surface protein ActA and protein components present in host cell cytoplasm. We have purified an eight-polypeptide complex that possesses the properties of the host-cell actin polymerization factor. The pure complex is sufficient to initiate ActA-dependent actin polymerization at the surface of L. monocytogenes, and is required to mediate actin tail formation and motility. Two subunits of this protein complex are actin-related proteins (ARPs) belonging to the Arp2 and Arp3 subfamilies. The Arp3 subunit localizes to the surface of stationary bacteria and the tails of motile bacteria in tissue culture cells infected with L. monocytogenes; this is consistent with a role for the complex in promoting actin assembly in vivo. The activity and subunit composition of the Arp2/3 complex suggests that it forms a template that nucleates actin polymerization.

Actin-Related Protein 2↗

PubMed1996

Actin polymerization in human eosinophils, unlike human neutrophils, depends on intracellular calcium mobilization.

Eosinophils represent major effector cells in the allergic inflammation. In contrast to neutrophils, the mechanism of eosinophil activation during the inflammatory response is poorly understood. In this study, the relation between calcium fluxes, chemotaxis, and actin polymerization in eosinophils from healthy non-atopic donors was investigated. Pre-incubation of eosinophils with the intracellular calcium chelator BAPTA dose-dependently prevented an increase in the intracellular calcium concentration ([Ca2+]i), whereas the depletion of extracellular calcium in the test medium had no effect. The chemotactic response of eosinophils, which was measured by the modified boyden chamber technique upon stimulation with RANTES, C5a and PAF, was dose-dependently inhibited by the chelation of intracellular calcium as well as inactivation of the cells in Ca2+ -depleted medium. To evaluate whether other cell functions which are involved in the migratory response of eosinophils might be dependent on intracellular and extracellular calcium, actin polymerization was investigated. Flow-cytometric measurement of F-actin with NBD-phallacidin revealed that actin polymerization in human eosinophils in response to RANTES, C5a, and PAF was dose-dependently inhibited by the intracellular calcium chelator BAPTA. Since it is well known that actin polymerization in neutrophils is not affected by chelation of intracellular calcium, actin polymerization in these cells was investigated under the same conditions as for eosinophils. In contrast to eosinophils, BAPTA did not inhibit actin polymerization in neutrophils. In summary, these data demonstrate that intracellular calcium fluxes represent a prerequisite for eosinophil chemotaxis and actin polymerization in human eosinophils. Furthermore, regulation of actin polymerization in eosinophils differed from that of neutrophils on the level of intracellular calcium fluxes.

Actins↗

PubMed2003

Itk functions to control actin polymerization at the immune synapse through localized activation of Cdc42 and WASP.

Actin polymerization at the immune synapse is required for T cell activation and effector function; however, the relevant regulatory pathways remain poorly understood. We showed previously that binding to antigen presenting cells (APCs) induces localized activation of Cdc42 and Wiskott-Aldrich Syndrome protein (WASP) at the immune synapse. Several lines of evidence suggest that Tec kinases could interact with WASP-dependent actin regulatory processes. Since T cells from Rlk-/-, Itk-/-, and Rlk-/- x Itk-/- mice have defects in signaling and development, we asked whether Itk or Rlk function in actin polymerization at the immune synapse. We find that Itk-/- and Rlk-/- x Itk-/- T cells are defective in actin polymerization and conjugate formation in response to antigen-pulsed APCs. Itk functions downstream of the TCR, since similar defects were observed upon TCR engagement alone. Using conformation-specific probes, we show that although the recruitment of WASP and Arp2/3 complex to the immune synapse proceeds normally, the localized activation of Cdc42 and WASP is defective. Finally, we find that the defect in Cdc42 activation likely stems from a requirement for Itk in the recruitment of Vav to the immune synapse. Our results identify Itk as a key element of the pathway leading to localized actin polymerization at the immune synapse.

Actins↗

PubMed1983

The tightly bound divalent cation regulates actin polymerization.

The polymerization characteristics of Ca++-actin and Mg++-actin were studied by measuring initial rates of polymerization upon addition of phalloidin-stabilized nuclei and neutral salt. Under conditions where the effects of divalent cation exchange were minimized, CaCl2 and MgCl2 were found to be equally effective in polymerizing actin. Mg++-actin was found to nucleate and polymerize more readily than Ca++-actin, having a forward rate constant about twice that of Ca++-actin under a variety of polymerizing conditions. The critical concentration for Ca++-actin is approximately 20 times that for Mg++-actin under equivalent conditions. These data imply that the polymer of Mg++-actin must be more stable than that of Ca++-actin, having a depolymerization rate constant about 10 fold lower. Since Mg++ is probably the tightly-bound cation in vivo, whereas Ca++-actin has been more widely studied in vitro, it would appear that actin in its physiological state is probably more polymerizable and more stable in the polymer form than previously considered.

Actins↗