Actin polymerization induced by GTP gamma S in permeabilized neutrophils is induced and maintained by free barbed ends

Repolymerization of actin from actin:thymosin beta4 complex induced by diaphanous related formins and gelsolin

The beta-thymosins are peptides of about 5 kDa molecular mass. Thymosin beta4 (Tbeta4) is the most ubiquitous member of this family and composed of 43 residues. Initially the beta-thymosins were supposed to be specifically produced and released by the thymic gland and to possess hormonal activities modulating the immune response. However, it was later noticed that beta-thymosins are present in the cytoplasm of almost all eukaryotic cells. Especially high concentrations of Tbeta4 were detected in hematopoetic cells, like polymorpho-nuclear leucocytes and in platelets. In these cells the main intracellular function of the beta-thymosins is to bind to monomeric actin and to inhibit its polymerization to filamentous actin. Thus Tbeta4 allows resting eukaryotic cells to maintain a high concentration of monomeric actin, although the intracellular ionic conditions would favor its almost complete polymerization to F-actin. Thereby monomeric actin is sequestered from the dynamic assembly and disassembly processes of the actin cytoskeleton that constantly occur intracellularly.

The beta-thymosins: intracellular and extracellular activities of a versatile actin binding protein family

The beta-thymosins are N-terminally acetylated peptides of about 5 kDa molecular mass and composed of about 40-44 amino acid residues. The first member of the family, thymosin beta4, was initially isolated from thymosin fraction 5, prepared in five steps from calf thymus. Thymosin beta4 was supposed to be specifically produced and released by the thymic gland and to possess hormonal activities modulating the immune response. Various paracrine effects have indeed been reported for these peptides such as cardiac protection, angiogenesis, stimulation of wound healing, and hair growth. Besides these paracrine effects, it was noted that beta-thymosins occur in high concentration in the cytoplasm of many eukaryotic cells and bind to the cytoskeletal component actin. Subsequently it became apparent from in vitro experiments that they preferentially bind to monomeric (G-)actin and stabilize it in its monomeric form. Due to this ability the beta-thymosins are the main intracellular actin sequestering factor, i.e., they posses the ability to remove monomeric actin from the dynamic assembly and disassembly processes of the actin cytoskeleton that constantly occur in activated cells. In this review we will concentrate on the intracellular activity and localization of the beta-thymosins, i.e., their modulating effect on the actin cytoskeleton.

MYOMETRIAL ACTIVATION – COORDINATION, CONNECTIVITY AND CONTRACTILITY

One of the most important stages of pregnancy is the activation of uterine contractions that result in the expulsion of the fetus. The timely onset of labour is clearly important for a healthy start to life but incomplete understanding of the precise mechanisms regulating labour onset have prohibited the development of effective and safe treatments for preterm labour. This review explores the activation of the myometrium at labour onset, focussing on mechanisms of uterine contractility, including those proteins that play an important role in smooth muscle contractility. The review primarily focuses on human work but in the absence of human data describes animal studies. A broad overview of myometrial contraction mechanisms is provided before discussing more detailed aspects and identifying areas where uncertainty remains. Also discussed is the recent application of ‘omics’ based approaches to parturition research, which has facilitated an increase in the understanding of myometrial activation.

Effects of profilin and thymosin beta4 on the critical concentration of actin demonstrated in vitro and in cell extracts with a novel direct assay

The free actin concentration at steady state, Ac, is a variable that determines how actin regulatory proteins influence the extent of actin polymerization. We describe a novel method employing fluorescence anisotropy to directly measure Ac in any sample after the addition of a trace amount of labeled thymosin beta4 or thymosin beta4 peptide. Using this assay, we confirm earlier theoretical work on the helical polymerization of actin and confirm the effects of actin filament-stabilizing drugs and capping proteins on Ac, thereby validating the assay. We also confirm a controversial prior observation that profilin lowers the critical concentration of Mg2+-actin. A general mechanism is proposed to explain this effect, and the first quantitative dose-response curve for the effect of profilin on Ac facilitates its evaluation. This mechanism also predicts the effect of profilin on critical concentration in the presence of the limited amount of capping protein, which is the condition often found in cells, and the effect of profilin on critical concentration in cell extracts is demonstrated for the first time. Additionally, nonlinear effects of thymosin beta4 on the steady state amount of F-actin are explained by the observed changes in Ac. This assay has potential in vivo applications that complement those demonstrated in vitro.

N Terminus Is Essential for Tropomyosin Functions

Down-regulation of several key actin-binding proteins, such as alpha-actinin, vinculin, gelsolin, and tropomyosins (TMs), is considered to contribute to the disorganized cytoskeleton present in many neoplastic cells. TMs stabilize actin filaments against the gel severing actions of proteins such as cofilin. Among multiple TMs expressed in non-muscle cells, tropomyosin-1 (TM1) isoform induces stress fibers and functions as a suppressor of malignant transformation. However, the molecular mechanisms of TM1-mediated cytoskeletal effects and tumor suppression remain poorly understood. We have hypothesized that the ability of TM1 to stabilize microfilaments is crucial for tumor suppression. In this study, by employing a variant TM1, which contains an N-terminal hemagglutinin epitope tag, we demonstrate that the N terminus is a key determinant of tropomyosin-1 function. Unlike the wild type TM1, the modified protein fails to restore stress fibers and inhibit anchorage-independent growth in transformed cells. Furthermore, the N-terminal modification of TM1 disorganizes the cytoskeleton and delays cytokinesis in normal cells, abolishes binding to F-actin, and disrupts the dimeric associations in vivo. The functionally defective TM1 allows the association of cofilin to stress fibers and disorganizes the microfilaments, whereas wild type TM1 appears to restrict the binding of cofilin to stress fibers. TM1-induced cytoskeletal reorganization appears to be mediated through preventing cofilin interaction with microfilaments. Our studies provide in vivo functional evidence that the N terminus is a critical determinant of TM1 functions, which in turn determines the organization of stress fibers.

Non-hydrolyzable analog of GTP induces activity of Na+ channels via disassembly of cortical actin cytoskeleton

The role of G proteins in regulation of non-voltage-gated Na+ channels in human myeloid leukemia K562 cells was studied by inside-out patch-clamp method. Na+ channels were activated by non-hydrolyzable analog of guanosine triphosphate (GTP), GTPgammaS, known to activate both heterotrimeric and small G proteins. Channel activity was not affected by aluminum fluoride that indiscriminately activates heterotrimeric G proteins. The effect of GTPgammaS was prevented by phalloidin and by G-actin, both interfering with actin disassembly, which indicates that GTPgammaS-induced channel activation was likely due to microfilament disruption. GTPgammaS-activated channels were inactivated by polymerizing actin. These data show, for the first time, that small G proteins can regulate Na+ channels, and an intracellular mechanism mediating their effect involves actin cytoskeleton rearrangements.

Essential requirement of cytosolic phospholipase A(2) for stimulation of NADPH oxidase-associated diaphorase activity in granulocyte-like cells

We have previously established a model of cytosolic phospholipase A(2) (cPLA(2))-deficient differentiated PLB-985 cells (PLB-D cells) and demonstrated that cPLA(2)-generated arachidonic acid (AA) is essential for NADPH oxidase activation. In this study we used this model to investigate the physiological role of cPLA(2) in regulation of NADPH oxidase-associated diaphorase activity. A novel diaphorase activity assay, using 4-iodonitrotetrazolium violet as an electron acceptor, was used in permeabilized neutrophils and PLB-985 cells differentiated toward the granulocytic or monocytic phenotypes. Phorbol 12-myristate 13-acetate, guanosine 5'-3-O- (thio)triphosphate (GTP gamma S), or FMLP stimulated a similar diphenylene iodonium-sensitive diaphorase activity pattern in neutrophils and in differentiated parent PLB-985 cells. This diaphorase activity was not detected in undifferentiated cells, but developed during differentiation. Furthermore, diaphorase activity could not be stimulated in permeabilized neutrophils from X-linked CGD patients and in differentiated gp91(phox)-targeted PLB-985 cells that lacked normal expression of gp91(phox), but was restored to these cells following transduction with retrovirus encoding gp91(phox). The differentiated PLB-D cells showed no diaphorase activity when stimulated by either GTP gamma S or FMLP, and only partial activation when stimulated with phorbol 12-myristate 13-acetate. Diaphorase activity in response to either agonists was fully restored by the addition of 10 microm free AA. The permeabilized cell 4-iodonitrotetrazolium violet reduction assay offers a unique tool for the evaluation of NADPH oxidase-associated diaphorase activity in stimulated whole cells. These results establish an essential and specific physiological requirement of cPLA(2)-generated AA in activation of electron transfer through the FAD reduction center of NADPH oxidase.

Capping of the barbed ends of actin filaments by a high-affinity profilin-actin complex

Profilin, a ubiquitous 12 to 15-kDa protein, serves many functions, including sequestering monomeric actin, accelerating nucleotide exchange on actin monomers, decreasing the critical concentration of the barbed end of actin filaments, and promoting actin polymerization when barbed ends are free. Most previous studies have focused on profilin itself rather than its complex with actin. A high-affinity profilin-actin complex (here called profilactin) can be isolated from a poly-(L)-proline (PLP) column by sequential elution with 3 M and 7 M urea. Profilactin inhibited the elongation rate of pyrenyl-G-actin from filament seeds in a concentration- and time-dependent manner. Much greater inhibition of elongation was observed with spectrin-F-actin than gelsolin-F-actin seeds, suggesting that the major effect of profilactin was due to capping the barbed ends of actin filaments. Its dissociation constant for binding to filament ends was 0.3 microM; the on- and off-rate constants were estimated to be 1.7 x 10(3) M-1 s-1 and 4.5 x 10(-4) s-1, respectively. Purified profilin (obtained by repetitive applications to a PLP column and assessed by silver-stained polyacylamide gels) did not slow the elongation rate of pyrenyl-G-actin from filament seeds. Capping protein could not be detected by Western blotting in the profilactin preparation, but low concentrations of gelsolin did contaminate our preparation. However, prolonged incubation with either calcium or EGTA did not affect capping activity, implying that contaminating gelsolin-actin complexes were not primarily responsible for the observed capping activity. Reapplication of the profilactin preparation to PLP-coupled Sepharose removed both profilin and actin and concurrently eliminated its capping activity. Profilactin that was reapplied to uncoupled Sepharose retained its capping activity. Phosphatidylinositol-4,5-bisphosphate (PIP2) was the most potent phosphoinositol in reducing the capping activity of profilactin. Dissociation of the tight profilactin complex may serve as a unique mechanism by which profilin helps regulate actin filament growth.

Erythrocyte membrane fractions contain free barbed filament ends despite sufficient concentrations of retained capper(s) to prevent barbed end growth

Many cellular functions depend on rapid cytoskeletal rearrangements localized to specific cytoplasmic domains. Tight regulation of the submembranous microfilament network is accomplished in large part in erythrocytes and granulocytes by actin binding proteins that cap the fast-growing barbed filament ends. Study of this dynamic system is necessarily hampered by the confounding perturbations of cell lysis and dilution. In this paper, we characterize the functional properties of the membrane-associated spectrin-actin complex from human erythrocytes as it exists after hypotonic lysis. Purified spectrin-actin "seeds" extracted from erythrocyte membranes effectively nucleated actin elongation from their barbed ends. However, polymerization from spectrin-actin complexes associated with the membrane fraction prematurely slowed despite the presence of G-actin in great excess of the critical monomer concentration. The addition of cytochalasin B decreased (rather than augmented) the slowing of elongation attributable to the membrane fraction, indicating that capping of barbed filament ends (not monomer sequestration) was the major mechanism underlying this effect. The paradoxical implication of our findings is that, despite the presence of excess capper(s) in the membrane fraction, the membrane-associated spectrin-actin seeds were not capped until after dilution into physiological ionic strength buffer containing monomeric actin. Furthermore, by comparing the degrees of contamination of the extracted and membrane-associated spectrin-actin preparations, it appeared that recognized capping proteins (including gelsolin and capping protein beta2) were not the predominant cappers found in the membrane pellet after hypotonic lysis. We hypothesize that the barbed ends of membrane-associated spectrin-actin complexes, while not excluding actin monomers, may be selectively inaccessible to certain cappers (perhaps simply as the result of steric hindrance). Growth from such complexes in vivo could be limited by the availability of polymerization-competent G-actin.

Effects of cross-linked profilin:beta/gamma-actin on the dynamics of the microfilament system in cultured cells

There is evidence that the profilin:actin complex is the immediate precursor in the formation of actin filaments in cells. This paper describes the cell morphology and microfilament distribution after microinjection of covalently cross-linked profilin:beta/gamma-actin (PxA) in two different cell lines. Injected cells were either kept unstimulated or stimulated with platelet-derived growth factor (PDGF) before fixation and visualization of filamentous actin. After injection of low doses of PxA, the cells displayed an actin organization characterized by a clearance of diffuse fluorescence from a region immediately interior of ruffling edges and the appearance of small dots of fluorescence in the same region. At higher concentrations, PxA effectively inhibited outgrowth of lamellae and microspikes, and there was a drastic reduction of actin staining in the zone behind the advancing edge. This effect is reminiscent of the effect of cytochalasin B on fibroblasts and the growth cone of neuronal cells. As in these cases, there remained a rim of actin-dependent fluorescence on the very edge of the membrane lamella, particularly in the PxA-treated fibroblasts. The interference of PxA with the formation of surface structures was pronounced after PDGF stimulation. Here, PxA effectively eliminated the enhancement of the ruffling activity in the cell edges and on the dorsal surface of the cells. In contrast to PxA, injection of non-cross-linked profilin:beta/gamma-actin had no apparent effect on cell morphology and microfilament distribution except for an increased concentration of filamentous actin in one of the cell lines.

Function of the cytoskeleton in human neutrophils and methods for evaluation

The cytoskeleton plays a critical role in the determination of cell shape and serves as a scaffold for critical cellular enzymes and adhesion molecules. It provides structural integrity for the cell and regulates the function of many biochemical events that are critical to cellular function. The microfilamentous cytoskeleton participates in force generation necessary for shape change and motion. In neutrophils and other motile cells, polymerization of actin likely drives extension of the lamellae and participates in force generation through interaction with myosin, by polymerization alone and by osmotic mechanisms. Here, we will focus on the microfilamentous cytoskeleton in the neutrophil and briefly review its function as well as some direct and indirect methods that have been used to asses its role in neutrophil function. The discussion will address general approaches and leaves the details of the methods to the references.

Regulation of the RhoA pathway in human endothelial cell spreading on type IV collagen: role of calcium influx

We have shown that nonvoltage-operated Ca(2+) entry regulates human umbilical vein endothelial cell adhesion, migration, and proliferation on type IV collagen. We now demonstrate a requirement for Ca(2+) influx for activation of the RhoA pathway during endothelial cell spreading on type IV collagen. Reorganization of actin into stress fibers was complete when the cells where fully spread at 90 minutes. No actin organization into stress fibers was seen in endothelial cells plated on type I collagen, indicating a permissive effect of type IV collagen. CAI, a blocker of nonvoltage-operated Ca(2+) channels, prevented development of stress fiber formation in endothelial cells on type IV collagen. This permissive effect was augmented by Ca(2+) influx, as stimulated by 0. 5 microM thapsigargin or 0.1 microM ionomycin, yielding faster development of actin stress fibers. Ca(2+) influx and actin rearrangement in response to thapsigargin and ionomycin were abrogated by CAI. Activated, membrane-bound RhoA is a substrate for C3 exoenzyme which ADP-ribosylates and inactivates RhoA, preventing actin stress fiber formation. Pretreatment of endothelial cells with C3 exoenzyme prevented basal and thapsigargin-augmented stress fiber formation. While regulation of Ca(2+) influx did not alter RhoA translocation, it reduced in vitro ADP-ribosylation of RhoA (P(2)<0. 05), suggesting Ca(2+) influx is needed for RhoA activation during spreading on type IV collagen; no Ca(2+) regulated change in RhoA was seen in HUVECs spreading on type I collagen matrix. Blockade of Ca(2+) influx of HUVEC spread on type IV collagen also reduced tyrosine phosphorylation of p190Rho-GAP and blocked thapsigargin-enhanced binding of p190Rho-GAP to focal adhesion kinase. Thus, Ca(2+) influx is necessary for RhoA activation and for linkage of the RhoA/stress fiber cascade to the focal adhesion/focal adhesion kinase pathway during human umbilical vein endothelial cell spreading on type IV collagen.

Inducible recruitment of Cdc42 or WASP to a cell-surface receptor triggers actin polymerization and filopodium formation

BACKGROUND

Cdc42, a GTP-binding protein of the Rho family, controls actin cytoskeletal organization and helps to generate actin-based protruding structures, such as filopodia. In vitro, Cdc42 regulates actin polymerization by facilitating the creation of free barbed ends - the more rapidly growing ends of actin filaments - and subsequent elongation at these ends. The Wiskott- Aldrich syndrome protein, WASP, which has a pleckstrin-homology domain and a Cdc42/Rac-binding motif, has been implicated in cell signaling and cytoskeleton reorganization. We have investigated the consequences of local recruitment of activated Cdc42 or WASP to the plasma membrane.

RESULTS

We used an activated Cdc42 protein that could be recruited to an engineered membrane receptor by adding rapamycin as a bridge, and added antibody-coupled beads to aggregate these receptors. Inducible recruitment of Cdc42 to clusters of receptors stimulated actin polymerization, resulting in the formation of membrane protrusions. Cdc42-induced protrusions were enriched in the vasodilator-stimulated phosphoprotein VASP and the focal-adhesion-associated proteins zyxin and ezrin. The Cdc42 effector WASP could also induce the formation of protrusions, albeit of different morphology.

CONCLUSIONS

This is the first demonstration that the local recruitment of activated Cdc42 or its downstream effector, WASP, to a membrane receptor in whole cells is sufficient to trigger actin polymerization that results in the formation of membrane protrusions. Our data suggest that Cdc42-induced actin-based protrusions result from the local and serial recruitment of cytoskeletal proteins including zyxin, VASP, and ezrin.

Chemoattractant-induced lamellipod extension

The mammary adenocarcinoma cell line MTLn3 is chemotactic towards epidermal growth factor (EGF), and this induced motility is thought to promote breast cancer invasion and metastasis. Stimulation of MTLn3 cells with EGF results in the extension of a flat, thin structure filled with filamentous actin and termed a lamellipod. Lamellipod extension is dependent on actin polymerization and is localized to the border of adherent cells. The structure of EGF-stimulated lamellipods in MTLn3 cells is well suited to analysis of chemoattractant-stimulated protrusion. Actin polymerization occurs within 200 nm of the extending edge of the lamellipod. Although extension of the lamellipod is not dependent upon interaction with the substratum, stabilization of the extended lamellipod is dependent on an adhesive substratum. Dorsal ruffling is suppressed during lamellipod extension. Tyrosine phosphorylation is reduced in preexisting focal contacts compared to new contacts induced by EGF stimulation. The coordination of turnover of focal contacts with lamellipod extension is proposed to result in polarized cell motility in response to gradients of chemoattractants.

Nucleation and elongation of actin filaments in the presence of high speed supernate from neutrophil lysates: modulating effects of Ca2+ and phosphatidylinositol-4,5-bisphosphate

Cell motility depends on the rapid growth of cortical actin filaments whose barbed ends are capped in the resting cell. High speed supernates (HSS) of dilute neutrophil lysates contain actin monomers and/or oligomers that can be induced to polymerize by certain stimuli. We questioned whether some of the actin remaining in the supernate after high speed centrifugation exists as occult nucleation sites which can elongate when uncapped. Phosphatidylinositol-4,5-bisphosphate (PIP2) may play a critical role as an intracellular messenger in cytoskeletal rearrangement after stimulation by removing cappers from barbed filament ends. The experiments reported here examine the separate and interactive effects of PIP2 micelles and micromolar [Ca2+] on the rates of nucleation and elongation of pyrenyl-G-actin in the presence of HSS. HSS slowed the nucleation and elongation rates of gel-filtered pyrenyl-G-actin polymerized at submicromolar [Ca2+]. Under these conditions, PIP2 only slightly increased the number of nucleation sites, but delayed the slowing of the elongation rate in the presence of HSS. Nucleating activity in HSS could be induced by the addition of micromolar [Ca2+] and totally abolished by immunoprecipitation of gelsolin from HSS; incubation of HSS with PIP2 at micromolar [Ca2+] slightly decreased the number of calcium-induced nucleation sites in the supernate. Incubation of HSS with PIP2 before the addition of calcium led to a greater reduction in Ca2+-inducible nucleation sites. HSS possessed more nucleation sites after simultaneous exposure to PIP2 and Ca2+, followed by chelation of Ca2+ with EGTA, than HSS preincubated at micromolar [Ca2+] without PIP2. At submicromolar [Ca2+], PIP2 only generated a few barbed end nucleation sites in the HSS, but lessened the gradual slowing of elongation seen with HSS in the absence of PIP2, presumably by preventing capping by capping protein-beta2 in the supernate. Pointed end nucleating sites in HSS, attributable to gelsolin, could be created by adding micromolar [Ca2+]. The preincubation of HSS with PIP2 in the absence of micromolar [Ca2+] decreased the number of Ca2+-inducible nucleation sites in the HSS. Under conditions mimicking the sequential rise and fall of cytosolic [Ca2+] after stimulation, PIP2 accelerated actin polymerization despite the inhibitory action of HSS by maintaining Ca2+-activated nucleation sites. These observations suggest that a possible role for PIP2 in modulating cytoskeletal growth in vivo may be to regulate nucleation sites activated by sequential changes in cytosolic [Ca2+].

G protein beta subunit-null mutants are impaired in phagocytosis and chemotaxis due to inappropriate regulation of the actin cytoskeleton

Chemotaxis and phagocytosis are basically similar in cells of the immune system and in Dictyostelium amebae. Deletion of the unique G protein beta subunit in D. discoideum impaired phagocytosis but had little effect on fluid-phase endocytosis, cytokinesis, or random motility. Constitutive expression of wild-type beta subunit restored phagocytosis and normal development. Chemoattractants released by cells or bacteria trigger typical transient actin polymerization responses in wild-type cells. In beta subunit-null cells, and in a series of beta subunit point mutants, these responses were impaired to a degree that correlated with the defect in phagocytosis. Image analysis of green fluorescent protein-actin transfected cells showed that beta subunit- null cells were defective in reshaping the actin network into a phagocytic cup, and eventually a phagosome, in response to particle attachment. Our results indicate that signaling through heterotrimeric G proteins is required for regulating the actin cytoskeleton during phagocytic uptake, as previously shown for chemotaxis. Inhibitors of phospholipase C and intracellular Ca2+ mobilization inhibited phagocytosis, suggesting the possible involvement of these effectors in the process.

GTPgammaS-induced actin polymerisation in vitro: ATP- and phosphoinositide-independent signalling via Rho-family proteins and a plasma membrane-associated guanine nucleotide exchange factor

In a cell-free system from neutrophil cytosol GTP(γ)S can induce an increase in the number of free filament barbed ends and massive actin polymerisation and cross-linking. GTP(γ)S stimulation was susceptible to an excess of GDP, but not Bordetella pertussis toxin and could not be mimicked by aluminium fluoride, myristoylated GTPgammaS. Gialpha2 or Gbeta1gamma2 subunits of trimeric G proteins. In contrast, RhoGDI and Clostridium difficile toxin B (inactivating Rho family proteins) completely abrogated the effect of GTPgammaS. When recombinant, constitutively activated and GTPgammaS-loaded Rac1, RhoA, or Cdc42 proteins alone or in combination were probed at concentrations >100 times the endogenous, however, they were ineffective. Purified Cdc42/Rac-interactive binding (CRIB) domain of WASP or C3 transferase did not prevent actin polymerisation by GTPgammaS. The action of GTPgammaS was blocked by mM [Mg2+], unless a heat- and trypsin-sensitive component present in neutrophil plasma membrane was added. Liberation of barbed ends seems therefore to be mediated by a toxin B-sensitive cytosolic Rho-family protein, requiring a membrane-associated guanine nucleotide exchange factor (GEF) for its activation by GTPgammaS under physiologic conditions. The inefficiency of various protein kinase and phosphatase inhibitors (staurosporine, genistein, wortmannin, okadaic acid and vanadate) and removal of ATP by apyrase, suggests that phosphate transfer reactions are not required for the downstream propagation of the GTPgammaS signal. Moreover, exogenously added phosphoinositides failed to induce actin polymerisation and a PtdIns(4,5)P2-binding peptide did not interfere with the response to GTPgammaS. The speed and simplicity of the presented assay applicable to protein purification techniques will facilitate the further elucidation of the molecular partners involved in actin polymerisation.

Corequirement of specific phosphoinositides and small GTP-binding protein Cdc42 in inducing actin assembly in Xenopus egg extracts

Both phosphoinositides and small GTP-binding proteins of the Rho family have been postulated to regulate actin assembly in cells. We have reconstituted actin assembly in response to these signals in Xenopus extracts and examined the relationship of these pathways. We have found that GTPgammaS stimulates actin assembly in the presence of endogenous membrane vesicles in low speed extracts. These membrane vesicles are required, but can be replaced by lipid vesicles prepared from purified phospholipids containing phosphoinositides. Vesicles containing phosphatidylinositol (4,5) bisphosphate or phosphatidylinositol (3,4,5) trisphosphate can induce actin assembly even in the absence of GTPgammaS. RhoGDI, a guanine-nucleotide dissociation inhibitor for the Rho family, inhibits phosphoinositide-induced actin assembly, suggesting the involvement of the Rho family small G proteins. Using various dominant mutants of these G proteins, we demonstrate the requirement of Cdc42 for phosphoinositide-induced actin assembly. Our results suggest that phosphoinositides may act to facilitate GTP exchange on Cdc42, as well as to anchor Cdc42 and actin nucleation activities. Hence, both phosphoinositides and Cdc42 are required to induce actin assembly in this cell-free system.

The small GTP-binding protein Rac promotes the dissociation of gelsolin from actin filaments in neutrophils

Gelsolin is an actin filament-capping protein that has been shown to play a key role in cell migration. Here we have studied the involvement of phosphoinositide 3-kinase (PI 3-kinase) and GTP-binding proteins (G-proteins) in the regulation of gelsolin-actin interactions in neutrophils. Inhibition of PI 3-kinase activity in vivo by wortmannin did not affect the dissociation of actin-gelsolin (1:1) complexes induced by neutrophil stimulation with N-formyl-Met-Leu-Phe. Guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) indirectly promoted the dissociation of actin-gelsolin complexes in a cell-free system using neutrophil cytosol, and this effect was blocked by the GDP dissociation inhibitor for Rho (Rho-GDI). The GTPgammaS-loaded ialpha2 and the beta1gamma2 subunits of heterotrimeric G-proteins (Gialpha2 and Gbeta1gamma2) also triggered actin-gelsolin dissociation in a Rho-GDI-sensitive manner. GTP-loaded activated Rac, but not activated Rho, induced the dissociation of cytosolic actin-gelsolin complexes. The guanine nucleotide exchange on Rac was increased by addition of GTPgammaS-loaded Gialpha2 or Gbeta1gamma2 to neutrophil cytosol. These findings suggest that activation of Rac by G-protein-coupled receptors in neutrophils triggers uncapping of actin filaments, independently of PI 3-kinase.

High concentrations of phosphatidylinositol-4,5-bisphosphate may promote actin filament growth by three potential mechanisms: inhibiting capping by neutrophil lysates, severing actin filaments and removing capping protein-beta2 from barbed ends

Cell locomotion requires rapid growth of cortical actin filaments whose barbed ends are capped in the resting cell. Phosphatidylinositol-4,5-bisphosphate (PIP2) may play a critical role as an intracellular messenger in cytoskeletal rearrangement after stimulation. We have examined the effects of PIP2 micelles on the Ca2+-independent actin filament capping activity in high speed supernatants of neutrophil lysates which we had previously demonstrated to be almost entirely due to capping protein-beta2, a homologue of cap Z. High concentrations of PIP2 totally prevented the capping of exogenous spectrin-F-actin seeds by dilute supernatants of neutrophil extracts. Capping could also be inhibited, albeit less effectively, by PIP and PI, but not by other phospholipids. When incubated with filaments in the absence of supernatant, PIP2 increased the number of growing ends. PIP2 also uncapped previously capped actin filaments, as demonstrated by incubating supernatant-capped and uncapped seeds with and without PIP2 and then comparing the initial elongation rates after addition of pyrenyl-G-actin. Incubation of capped seeds with high concentrations of PIP2 increased the number of free barbed ends to a level comparable to that of the uncapped seeds exposed to PIP2. PIP2 caused uncapping to occur too quickly to be explained simply by the off-rate of capping protein-beta2, implying that PIP2 interacted directly with capping protein on the filament ends. In fact, PIP2 transiently uncapped capped seeds in the presence of excess free capping protein. From our data, we estimate that millimolar concentrations of PIP2 (almost 100-fold higher than the amount predicted from the effective concentration in purified systems) would be required to inhibit all the capping protein-beta2 in the cytosol. This discrepancy probably results, in large part, from sequestration of PIP2 by other PIP2-binding proteins in the cytoplasm. If PIP2 mediates differential cytoskeletal growth after chemoattractant stimulation in vivo, very high concentrations may be required subjacent to the plasma membrane for regional severing and uncapping of actin filaments to occur quickly near the perturbed membrane.

Regulation of actin polymerization in cell-free systems by GTPgammaS and Cdc42

We have established a cell-free system to investigate pathways that regulate actin polymerization. Addition of GTPgammaS to lysates of polymorphonuclear leukocytes (PMNs) or Dictyostelium discoideum amoeba induced formation of filamentous actin. The GTPgammaS appeared to act via a small G-protein, since it was active in lysates ofD. discoideum mutants missing either the alpha2- or beta-subunit of the heterotrimeric G-protein required for chemoattractant-induced actin polymerization in living cells. Furthermore, recombinant Cdc42, but not Rho or Rac, induced polymerization in the cell-free system. The Cdc42-induced increase in filamentous actin required GTPgammaS binding and was inhibited by a fragment of the enzyme PAK1 that binds Cdc42. In a high speed supernatant, GTPgammaS alone was ineffective, but GTPgammaS-loaded Cdc42 induced actin polymerization, suggesting that the response was limited by guanine nucleotide exchange. Stimulating exchange by chelating magnesium, by adding acidic phospholipids, or by adding the exchange factors Cdc24 or Dbl restored the ability of GTPgammaS to induce polymerization. The stimulation of actin polymerization did not correlate with PIP2 synthesis.

Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

We are investigating structure-function relationships in profilin and actin by site-specific mutagenesis using a yeast, Saccharomyces cerevisiae, expression system to produce wild-type and mutant proteins. This paper shows that deleting proline 96 and threonine 97, which are located close to the major actin binding site on profilin, did not significantly alter the interaction between profilin and phosphatidylinositol 4,5-bisphosphate, nor did it affect the profilin:poly(L-proline) interaction. The mutant protein, however, had a lower capacity to bind to actin in vitro than wild-type profilin, though it showed a slightly increased profilin-enhanced nucleotide exchange on the actin. When microinjected into Swiss 3T3 mouse fibroblasts or porcine aortic endothelial cells, the mutant profilin did not change the organization of the microfilament system like the wild-type profilin did. This provides further evidence that profilin controls microfilament organization in the cell by interacting directly with actin.

Receptor-induced translocation of activated guanine-nucleotide-binding protein alpha i subunits to the cytoskeleton in myeloid differentiated human leukemia (HL-60) cells

The regulation of the cytoskeletal localization of guanine-nucleotide-binding protein alpha i subunits by formyl peptide receptors was studied in myeloid differentiated human leukemia (HL-60) cells. Stimulation of formyl peptide receptors with N-formyl-Met-Leu-Phe (fMet-Leu-Phe) transiently increased the amount of alpha i subunits in the Triton X-100-insoluble cytoskeleton. Similar to the biphasic regulation of the actin content, fMet-Leu-Phe ( > or = 10 nM) rapidly increased the cytoskeletal alpha i content (about threefold at 30 s), which was followed by a rapid reversal to control levels. The formyl peptide receptor increased the cytoskeletal content of both alpha i subtypes, alpha i2 and alpha i3- present in HL-60 cells. In cells permeabilized with Staphylococcus aureus alpha-toxin, fMet-Leu-Phe increased binding of the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), to cytoskeletal proteins in a pertussis-toxin-sensitive manner, which was completely abolished by the F-actin-disrupting agent, cytochalasin B. Using the photoreactive GTP analogue, m-acetylanilido-GTP, the formyl peptide receptor-regulated GTP binding sites at the cytoskeleton were identified as 40-kDa proteins, the molecular size of alpha i subunits. Cytoskeleton prepared from stimulated cells did not exhibit increased GTP[S] binding, which suggests that activated alpha i subunits are translocated to the cytoskeleton. Finally, in alpha-toxin-permeabilized HL-60 cells, fMet-Leu-Phe and GTP[S] cooperatively stimulated actin polymerization. In conclusion, evidence is provided that chemoattractant receptors cause translocation of activated alpha i subunits to the cytoskeleton coincidentally with F-actin formation. The data therefore argue for a potential role of translocated alpha i subunits in the process of receptor-induced actin polymerization.

Signal transduction and actin filament organization

Small GTP-binding proteins of the Rho family appear to integrate extracellular signals from diverse receptor types and initiate rearrangements of F-actin. Active members of the Rho family, Rho and Rac, are now joined by Cdc42 which induces filopodia. Downstream of the Rho family proteins, actin polymerization may be induced by an increase in the availability of actin filament barbed ends. Actin organization may be affected by exposure of actin-binding sites on proteins such as vinculin and ezrin.