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

Targeting Phosphoinositide 3-Kinase - Five Decades of Chemical Space Exploration

Phosphoinositide 3-kinase (PI3K) plays a key role in a plethora of physiologic processes and controls cell growth, metabolism, immunity, cardiovascular and neurological function, and more. The discovery of wort-mannin as the first potent PI3K inhibitor (PI3Ki) in the 1990s provided rapid identification of PI3K-dependent processes, which drove the discovery of the PI3K/protein kinase B (PKB/Akt)/target of rapamycin (mTOR) pathway. Genetic mouse models and first PI3K isoform-specific inhibitors pinpointed putative therapeutic applications. The recognition of PI3K as target for cancer therapy drove subsequently drug development. Here we provide a brief journey through the emerging roles of PI3K to the development of preclinical and clinical PI3Ki candidates.

A New STAT3-binding Partner, ARL3, Enhances the Phosphorylation and Nuclear Accumulation of STAT3

Signal transducer and activator of transcription 3 (STAT3) is involved in cell proliferation, differentiation, and cell survival during immune responses, hematopoiesis, neurogenesis, and other biological processes. STAT3 activity is regulated by a variety of mechanisms, including phosphorylation and nuclear translocation. To clarify the molecular mechanisms underlying the regulation of STAT3 activity, we performed yeast two-hybrid screening. We identified ARL3 (ADP-ribosylation factor-like 3) as a novel STAT3-binding partner. ARL3 recognizes the DNA-binding domain as well as the C-terminal region of STAT3 in vivo, and their binding was the strongest when both proteins were activated. Importantly, small interfering RNA-mediated reduction of endogenous ARL3 expression decreased IL-6-induced tyrosine phosphorylation, nuclear accumulation, and transcriptional activity of STAT3. These results indicate that ARL3 interacts with STAT3 and regulates the transcriptional activation of STAT3 by influencing its nuclear accumulation of STAT3.

Effects of cholesterol alterations are mediated via G-protein-related pathways in outer hair cells

Cholesterol is an essential component of cell membranes, and determines their rigidity and fluidity. Alterations in membrane cholesterol by MβCD or water-soluble cholesterol affect the stiffness, capacitance, motility, and cell length of outer hair cells (OHCs). This suggests that reconstruction of the cytoskeleton may be induced by cholesterol alterations. In this study, we investigated intracellular signaling pathways involving G proteins to determine whether they modulate the changes in voltage-dependent capacitance caused by cholesterol alterations. Membrane capacitance of isolated guinea pig OHCs were assessed using a two-sine voltage stimulus protocol superimposed onto a voltage ramp (200 ms duration) from -150 to +140 mV. One group of OHCs was treated with 100 μM guanosine 5'-O-(3-thiotriphosphate) tetralithium salt (GTPγS), the GTP analog, administrated into individual cells via patch pipettes. Another group of OHCs was internally perfused with 600 μM guanosine 5'-(β-thio) diphosphate trilithium salt (GDPβS), the GDP analog. A third group was perfused with internal solution only as a control. Application of 1 mM MβCD shifted non-linear capacitance curves to the depolarized direction of the control group with reduction of the peak capacitance (C mpeak). After the 10-min application of MβCD, shifts of voltage at C mpeak (V cmpeak) and reduction of C mpeak were 73.32 ± 11.09 mV and 9.09 ± 2.10 pF, respectively (n = 4). On the other hand, in the GTPγS-treated group, the shift of V cmpeak and reduction of C mpeak were attenuated remarkably. The shift of V cmpeak and reduction of C mpeak in the 10-min application of MβCD were 9.73 ± 10.92 mV and 3.08 ± 1.91 pF, respectively (n = 7). MβCD decreased the cell length by 16.53 ± 4.27 % in the control group and by 6.45 ± 6.22 % in the GTPγS group. In addition, we investigated the effects of GDPβS on cholesterol-treated OHCs. One millimolar cholesterol was externally applied after the 4-min application of 1 mM MβCD because the shift of V-C m function caused by cholesterol alone was small. Application of cholesterol shifted V-C m curves of the control group to the hyperpolarized direction with increase of the C mpeak. After the 10-min application of cholesterol, changes of V cmpeak and C mpeak were -9.19 ± 6.68 mV and 2.14 ± 0.44 pF, respectively (n = 4). On the other hand, in the GDPβS-treated OHCs, the shift of V cmpeak and increase of C mpeak were attenuated markedly. The shift of V cmpeak and increase of C mpeak after 10 min were 5.13 ± 10.46 mV and -0.55 ± 1.39 pF, respectively (n = 6). This study demonstrated that internally perfused GTPγS inhibited the MβCD effects and GDPβS inhibited the cholesterol effects, raising the possibility that G proteins may be involved in outer hair cell homeostasis as well as the possibility that cholesterol response may be G protein mediated. More study is required to clarify the detailed role of G proteins in the relation between cholesterol and the OHC cytoskeleton.

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.

Regulation of I(Cl,swell) in neuroblastoma cells by G protein signaling pathways

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) activated the I(Cl,swell) anion channel in N1E115 neuroblastoma cells in a swelling-independent manner. GTPgammaS-induced current was unaffected by ATP removal and broadly selective tyrosine kinase inhibitors, demonstrating that phosphorylation events do not regulate G protein-dependent channel activation. Pertussis toxin had no effect on GTPgammaS-induced current. However, cholera toxin inhibited the current approximately 70%. Exposure of cells to 8-bromoadenosine 3',5'-cyclic monophosphate did not mimic the effect of cholera toxin, and its inhibitory action was not prevented by treatment of cells with an inhibitor of adenylyl cyclase. These results demonstrate that GTPgammaS does not act through Galpha(i/o) GTPases and that Galpha(s)/Gbetagamma G proteins inhibit the channel and/or channel regulatory mechanisms through cAMP-independent mechanisms. Swelling-induced activation of I(Cl,swell) was stimulated two- to threefold by GTPgammaS and inhibited by 10 mM guanosine 5'-O-(2-thiodiphosphate). The Rho GTPase inhibitor Clostridium difficile toxin B inhibited both GTPgammaS- and swelling-induced activation of I(Cl,swell). Taken together, these findings indicate that Rho GTPase signaling pathways regulate the I(Cl,swell) channel via phosphorylation-independent mechanisms.

Cytoskeletal alterations in Dictyostelium induced by expression of human cdc42

The rho family of small G proteins has been shown to be involved in controlling actin filament dynamics in cells. To evaluate the functional overlap between human and Dictyostelium G proteins, we conditionally expressed constitutively active human cdc42 (V12-cdc42) in Dictyostelium cells. Upon induction, cells adopted a unique morphology: a flattened shape with wrinkles running from the cell edge toward the center. The appearance of these wrinkles is highly dynamic so that the cells cycle between the wrinkled and relatively normal morphologies. Phalloidin staining indicates that the stellate wrinkles contain dense actin structures and also that numerous filopods project vertically from the center of these cells. Consistent with the hypothesis that cdc42 induces actin polymerization in vivo, cells expressing V12-cdc42 show an increase in the amount of F-actin associated with the cytoskeleton. This is accompanied by an increase in the association of the actin-binding proteins 34-kDa bundler, ABP-120 and alpha-actinin with the cytoskeleton. In conclusion, human cdc42 has various effects on the Dictyostelium actin cytoskeleton consistent with a conserved role of small GTPases in control of the cytoskeleton.

ATP-dependent membrane assembly of F-actin facilitates membrane fusion

We recently established an in vitro assay that monitors the fusion between latex-bead phagosomes and endocytic organelles in the presence of J774 macrophage cytosol (). Here, we show that different reagents affecting the actin cytoskeleton can either inhibit or stimulate this fusion process. Because the membranes of purified phagosomes can assemble F-actin de novo from pure actin with ATP (), we focused here on the ability of membranes to nucleate actin in the presence of J774 cytosolic extracts. For this, we used F-actin sedimentation, pyrene actin assays, and torsional rheometry, a biophysical approach that could provide kinetic information on actin polymerization and gel formation. We make two major conclusions. First, under our standard in vitro conditions (4 mg/ml cytosol and 1 mM ATP), the presence of membranes actively catalyzed the assembly of cytosolic F-actin, which assembled into highly viscoelastic gels. A model is discussed that links these results to how the actin may facilitate fusion. Second, cytosolic actin paradoxically polymerized more under ATP depletion than under high-ATP conditions, even in the absence of membranes; we discuss these data in the context of the well described, large increases in F-actin seen in many cells during ischemia.

Reconstitution of clathrin-independent endocytosis at the apical domain of permeabilized MDCK II cells: requirement for a Rho-family GTPase

This paper studies the endocytosis of ricin at the apical pole of polarized MDCK II cells after permeabilization of the cells basolaterally with streptolysin O. Ricin endocytosis after the addition of cytosol with an ATP-regenerating system was 2-3-fold higher than after the addition of a transport medium. A similar increase in ricin endocytosis was obtained by reconstitution of dialyzed cytosol with the nonhydrolyzable GTP analog, GTP gamma S, in the presence of an ATP-regenerating system. The nonhydrolyzable GDP analog, GDP beta S, did not increase ricin uptake. In contrast to the data obtained with ricin, GTP gamma S was found to inhibit apical transferrin uptake in MDCK II cells transfected with the human transferrin receptor, and the data thus imply that GTP gamma S supports clathrin-independent endocytosis. Electron microscopy (EM) demonstrated that free endocytic vesicles were formed from the apical pole of permeabilized MDCK II cells in the presence of GTP gamma S and that both a ricin-HRP conjugate, HRP, and cationized gold were endocytosed. Ricin endocytosis in the presence of intact cytosol, as well as GTP gamma S-stimulated ricin uptake, was inhibited by Clostridium botulinum C3 transferase, an enzyme found to inactivate Rho proteins. The data demonstrate that apical clathrin-independent endocytosis functions in the presence of GTP gamma S, and suggest that one or more of the small GTP binding proteins of the Rho family is involved in regulation of the apical clathrin-independent endocytosis in MDCK II cells.

Profilin enhances Cdc42-induced nucleation of actin polymerization

We find that profilin contributes in several ways to Cdc42-induced nucleation of actin filaments in high speed supernatant of lysed neutrophils. Depletion of profilin inhibited Cdc42-induced nucleation; re-addition of profilin restored much of the activity. Mutant profilins with a decreased affinity for either actin or poly-l-proline were less effective at restoring activity. Whereas Cdc42 must activate Wiskott-Aldrich Syndrome protein (WASP) to stimulate nucleation by the Arp2/3 complex, VCA (verpolin homology, cofilin, and acidic domain contained in the COOH-terminal fragment of N-WASP) constitutively activates the Arp2/3 complex. Nucleation by VCA was not inhibited by profilin depletion. With purified N-WASP and Arp2/3 complex, Cdc42-induced nucleation did not require profilin but was enhanced by profilin, wild-type profilin being more effective than mutant profilin with reduced affinity for poly-l-proline. Nucleation by the Arp2/3 complex is a function of the free G-actin concentration. Thus, when profilin addition decreased the free G-actin concentration, it inhibited Cdc42- and VCA-induced nucleation. However, when profilin was added with G-actin in a ratio that maintained the initial free G-actin concentration, it increased the rate of both Cdc42- and VCA-induced nucleation. This enhancement, also seen with purified proteins, was greatest when the free G-actin concentration was low. These data suggest that under conditions present in intact cells, profilin enhances nucleation by activated Arp2/3 complex.

Molecular mechanisms of platelet exocytosis: role of SNAP-23 and syntaxin 2 and 4 in lysosome release

On stimulation by strong agonists, platelets release the contents of 3 storage compartments in 2 apparent waves of exocytosis. The first wave is the release of α- and dense core granule contents and the second is the release of lysosomal contents. Using a streptolysin O-permeabilized platelet exocytosis assay, we show that hexosaminidase release is stimulated by either Ca++ or by GTP-γ-S. This release step retains the same temporal separation from serotonin release as seen in intact platelets. This assay system was also used to dissect the molecular mechanisms of lysosome exocytosis. Lysosome release requires adenosine triphosphate and the general membrane fusion protein, N-ethylmaleimide sensitive factor. Uniquely, 2 syntaxin t-SNAREs, syntaxin 2 and 4, which localize to granules and open canalicular membranes, together with the general target membrane SNAP receptor (t-SNARE) protein SNAP-23 appear to make up the heterodimeric t-SNAREs required for lysosome exocytosis. These studies further show that regardless of stimuli (Ca++or GTP-γ-S) serotonin and hexosaminidase release requires the same membrane fusion machinery.

Two pathways through Cdc42 couple the N-formyl receptor to actin nucleation in permeabilized human neutrophils

We developed a permeabilization method that retains coupling between N-formyl-methionyl-leucyl-phenylalanine tripeptide (FMLP) receptor stimulation, shape changes, and barbed-end actin nucleation in human neutrophils. Using GTP analogues, phosphoinositides, a phosphoinositide-binding peptide, constitutively active or inactive Rho GTPase mutants, and activating or inhibitory peptides derived from neural Wiskott-Aldrich syndrome family proteins (N-WASP), we identified signaling pathways leading from the FMLP receptor to actin nucleation that require Cdc42, but then diverge. One branch traverses the actin nucleation pathway involving N-WASP and the Arp2/3 complex, whereas the other operates through active Rac to promote actin nucleation. Both pathways depend on phosphoinositide expression. Since maximal inhibition of the Arp2/3 pathway leaves an N17Rac inhibitable alternate pathway intact, we conclude that this alternate involves phosphoinositide-mediated uncapping of actin filament barbed ends.

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.

Rho-family GTPases require the Arp2/3 complex to stimulate actin polymerization in Acanthamoeba extracts

BACKGROUND

Actin filaments polymerize in vivo primarily from their fast-growing barbed ends. In cells and extracts, GTPgammaS and Rho-family GTPases, including Cdc42, stimulate barbed-end actin polymerization; however, the mechanism responsible for the initiation of polymerization is unknown. There are three formal possibilities for how free barbed ends may be generated in response to cellular signals: uncapping of existing filaments; severing of existing filaments; or de novo nucleation. The Arp2/3 complex localizes to regions of dynamic actin polymerization, including the leading edges of motile cells and motile actin patches in yeast, and in vitro it nucleates the formation of actin filaments with free barbed ends. Here, we investigated actin polymerization in soluble extracts of Acanthamoeba.

RESULTS

Addition of actin filaments with free barbed ends to Acanthamoeba extracts is sufficient to induce polymerization of endogenous actin. Addition of activated Cdc42 or activation of Rho-family GTPases in these extracts by the non-hydrolyzable GTP analog GTPgammaS stimulated barbed-end polymerization, whereas immunodepletion of Arp2 or sequestration of Arp2 using solution-binding antibodies blocked Rho-family GTPase-induced actin polymerization.

CONCLUSIONS

For this system, we conclude that the accessibility of free barbed ends regulates actin polymerization, that Rho-family GTPases stimulate polymerization catalytically by de novo nucleation of free barbed ends and that the primary nucleation factor in this pathway is the Arp2/3 complex.

In vitro approaches to study actin and microtubule dependent cell processes

Actin and microtubules represent complex polymer systems that play essential roles during many cellular processes including chromosome segregation, cytokinesis and motility. The dynamic nature of actin and microtubules together with their regulation by a myriad of proteins makes their study both fascinating and challenging. Over the past few years there has been an increasing move towards development of in vitro systems to facilitate the elucidation of the molecular basis of actin and microtubule dependent cell processes. This review focuses on some of the recent developments using in vitro assays to dissect the cellular role of the actin and microtubule cytoskeleton.

Microquantification of cellular and in vitro F-actin by rhodamine phalloidin fluorescence enhancement

Based on the enhancement of rhodamine phalloidin fluorescence after its binding to actin filaments we have developed a technique to quantify F-actin, drastically (>> 100 times) reducing consumption of the expensive fluorescent dye and sample material in comparison to previous methods. Depolymerization of F-actin is prevented by utilizing short incubation times and stabilization of the filaments by actin-binding proteins or formaldehyde. Equilibrium and kinetic mathematical models relating rhodamine fluorescence with F-actin concentrations were used to predict the optimal assay conditions. The method has been applied to measure relative and absolute F-actin concentrations in cytosolic fractions and stimulus-induced actin polymerization in neutrophils. The cells were lysed with octy1-beta-D-glucopyranoside, which is compatible with the assay due to its high critical micelle concentration. As the assay takes less than 1 h and eliminates all previously required washing or extraction steps, it is faster and much simpler than any other presented up to now for quantification of filamentous actin. Moreover, the method is unique for reliable and easy F-actin measurements in cell-free systems.