Promotion of neurite and filopodium formation by CD47: roles of integrins, Rac, and Cdc42

Axon extension during development is guided by many factors, but the signaling mechanisms responsible for its regulation remain largely unknown. We have now investigated the role of the transmembrane protein CD47 in this process in N1E-115 neuroblastoma cells. Forced expression of CD47 induced the formation of neurites and filopodia. Furthermore, an Fc fusion protein containing the extracellular region of the CD47 ligand SHPS-1 induced filopodium formation, and this effect was enhanced by CD47 overexpression. SHPS-1-Fc also promoted neurite and filopodium formation triggered by serum deprivation. Inhibition of Rac or Cdc42 preferentially blocked CD47-induced formation of neurites and filopodia, respectively. Overexpression of CD47 resulted in the activation of both Rac and Cdc42. The extracellular region of CD47 was sufficient for the induction of neurite formation by forced expression, but the entire structure of CD47 was required for enhancement of filopodium formation by SHPS-1-Fc. Neurite formation induced by CD47 was also inhibited by a mAb to the integrin beta3 subunit. These results indicate that the interaction of SHPS-1 with CD47 promotes neurite and filopodium formation through the activation of Rac and Cdc42, and that integrins containing the beta3 subunit participate in the effect of CD47 on neurite formation.     

Regulation of Rac1 and Cdc42 activation in thrombin- and collagen-stimulated CHRF-288-11 cells

Rac1 and Cdc42 are members of the Rho family of small GTPases and have been shown to promote the formation of lamellipodia and filopodia at the leading edge of motile cells and affect cell migration. In this study the authors have investigated the activation of Rac1 and Cdc42 by thrombin or collagen using the megakaryocytic cell line, CHRF-288-11. Maximal activation of Rac1 by thrombin or collagen was observed at 3 and 1 min, respectively. Similar results were obtained for the activation of Cdc42 by thrombin or collagen. The activation of Rac1 by thrombin and collagen was inhibited by the calmodulin-specific inhibitor, W7. However, W7 had no effect on the activation of Cdc42 by thrombin or collagen. The less potent calmodulin inhibitor, W5, did not have any effect on Rac1 or Cdc42 activation by thrombin or collagen. Transient over-expression of calmodulin in CHRF cells increased the basal and thrombin-mediated activation of Rac1 when compared to control but had no effect on the basal and thrombin-mediated activation of Cdc42 when compared to control. The results suggest that calmodulin regulates the activation of Rac1 in CHRF-288-11 cells in a manner similar to that in the human platelets.     

PAR-6-PAR-3 mediates Cdc42-induced Rac activation through the Rac GEFs STEF/Tiam1

A polarity complex of PAR-3, PAR-6 and atypical protein kinase C (aPKC) functions in various cell-polarization events, including neuron specification. The small GTPase Cdc42 binds to PAR-6 and regulates cell polarity. However, little is known about the downstream signals of the Cdc42-PAR protein complex. Here, we found that PAR-3 directly interacted with STEF/Tiam1, which are Rac-specific guanine nucleotide-exchange factors, and that STEF formed a complex with PAR-3-aPKC-PAR-6-Cdc42-GTP. Cdc42 induces lamellipodia in a Rac-dependent manner in N1E-115 neuroblastoma cells. Disruption of Cdc42-PAR-6 or PAR-3-STEF binding inhibited Cdc42-induced lamellipodia but not filopodia. The isolated STEF-binding PAR-3 fragment was sufficient to induce lamellipodia independently of Cdc42 and PAR-6. PAR-3 is required for Cdc42-induced Rac activation, but is not essential for lamellipodia formation itself. In cultured hippocampal neurons, STEF accumulated at the tip of the growing axon and colocalized with PAR-3. The spatio-temporal activation and signalling of Cdc42-PAR-6-PAR-3-STEF/Tiam1-Rac seem to be involved in neurite growth and axon specification. We propose that the PAR-6-PAR-3 complex mediates Cdc42-induced Rac activation by means of STEF/Tiam1, and that this process seems to be required for the establishment of neuronal polarity.     

Syndecan-2 induces filopodia by active cdc42Hs

The syndecans, a family of transmembrane heparan sulfate proteoglycans, are ubiquitous molecules whose intracellular function is still unknown. To examine the function of syndecan-2, one of the most abundant heparan sulfate proteoglycan in fibroblasts, we performed transfection studies in COS-1 and Swiss 3T3 cells. Endogenous syndecan-2 colocalized with F-actin in cortical structures. Overexpression of full-length syndecan-2 induced the formation of long filopodia-like structures. These changes correlated with a rearrangement of the actin cytoskeleton, which strongly colocalized with syndecan-2. Overexpression of syndecan-2 lacking the extracellular domain increased the number of microspikes on the cell surface but failed to induce filopodia. Addition of heparin blocked the effect of full-length syndecan-2, suggesting that heparan sulfate chains in the extracellular domain are necessary to induce filopodia. Coexpression of cdc42Hs negative-dominant N17 blocked syndecan-2-induced filopodia and cdc42Hs positive-dominant V12 had a synergic effect. This indicates that active cdc42Hs is necessary for syndecan-2 induction of filopodia. These results provide a link between syndecan-2, actin cytoskeleton, and cdc42Hs.     

Glycosaminoglycan-dependent and -independent inhibition of neurite outgrowth by agrin

Agrin is a proteoglycan that can inhibit neurite outgrowth from multiple neuronal types when present as a substrate. Agrin's neurite inhibitory activity is confined to the N-terminal segment of the protein (agrin N150), which contains heparan sulfate (HS) and chondroitin sulfate (CS) side chains. We have examined the activities of various purified recombinant agrin fragments and their glycosaminoglycan (GAG) side chains in neurite outgrowth inhibition. Inhibitory activity was tested using dissociated chick ciliary ganglion neurons or dorsal root ganglion explants growing on laminin or N-cadherin. Initial experiments demonstrated that agrin N150 lacking GAG chains inhibited neurite outgrowth. Both halves of N150, each containing HS and/or CS side chains, could also inhibit neurite growth. Experiments using agrin fragments in which the GAG acceptor residues were mutated, or using agrin fragments purified from cells deficient in GAG synthesis, demonstrated that inhibition by the N-terminal portion of N150 requires GAGs, but that inhibition from the C-terminal part of N150 does not. Thus, the core protein or other types of glycosylation are important for inhibition from the more C-terminal region. Our results suggest that there are two distinct mechanisms for neurite outgrowth inhibition by agrin, one that is GAG-dependent and one that is GAG-independent.     

The Rho GTPases in Macrophage Motility and Chemotaxis

The GTP-binding proteins, Rho, Rac and Cdc42 are known to regulate actin organisation. Rho induces the assembly of contractile actin-based microfilaments such as stress fibres, Rac regulates the formation of membrane ruffles and lamellipodia, and Cdc42 activation is necessary for the formation of filopodia. In addition, all three proteins can also regulate the assembly of integrin-containing focal adhesion complexes. The orchestration of these distinct cytoskeletal changes is thought to form the basis of the co-ordination of cell motility and we have investigated the roles of Rho family proteins in migration using a model system. We have found that in the macrophage cell line Bacl, the cytokine CSF-1 rapidly induces actin reorganisation: it stimulates the formation of filopodia, lamellipodia and membrane ruffles, as well as the appearance of fine actin cables within the cell. We have shown that Cdc42, Rac and Rho regulate the CSF-1 induced formation of these distinct actin filament-based structures. Using a cell tracking procedure we found that both Rho and Rac were required for CSF-1 stimulated cell translocation. In contrast, inhibition of Cdc42 does not prevent macrophages migrating in response to CSF-1, but does prevent recognition of a CSF-1 concentration gradient, so that cells now migrate randomly rather than up the gradient of this chemotactic cytokine. This implies that Cdc42, and thus probably filopodia, are required for gradient sensing and cell polarisation in macrophages.     

A Role for Cdc42 in Macrophage Chemotaxis

Three members of the Rho family, Cdc42, Rac, and Rho are known to regulate the organization of actin-based cytoskeletal structures. In Bac1.2F5 macrophages, we have shown that Rho regulates cell contraction, whereas Rac and Cdc42 regulate the formation of lamellipodia and filopodia, respectively. We have now tested the roles of Cdc42, Rac, and Rho in colony stimulating factor-1 (CSF-1)–induced macrophage migration and chemotaxis using the Dunn chemotaxis chamber. Microinjection of constitutively activated RhoA, Rac1, or Cdc42 inhibited cell migration, presumably because the cells were unable to polarize significantly in response to CSF-1. Both Rho and Rac were required for CSF-1–induced migration, since migration speed was reduced to background levels in cells injected with C3 transferase, an inhibitor of Rho, or with the dominant-negative Rac mutant, N17Rac1. In contrast, cells injected with the dominant-negative Cdc42 mutant, N17Cdc42, were able to migrate but did not polarize in the direction of the gradient, and chemotaxis towards CSF-1 was abolished.

We conclude that Rho and Rac are required for the process of cell migration, whereas Cdc42 is required for cells to respond to a gradient of CSF-1 but is not essential for cell locomotion.

     

Trans-interactions of nectins induce formation of filopodia and Lamellipodia through the respective activation of Cdc42 and Rac small G proteins

Nectins and afadin constitute a novel cell-cell adhesion system that plays a cooperative role with cadherins in the organization of adherens junctions (AJs). Nectins are Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecules, and afadin is a nectin- and actin filament-binding protein that connects nectins to the actin cytoskeleton. Rac and Cdc42 small G proteins have been implicated in the organization of AJs, but their modes of action remain unknown. The trans-interaction of E-cadherin has recently been shown to induce the activation of Rac, but not that of Cdc42. We show here that the trans-interactions of nectins induce the formation of filopodia and lamellipodia through the respective activation of Cdc42 and Rac. The Cdc42 activation is necessary, but not sufficient, for the Rac-induced formation of lamellipodia, whereas the Rac activation is not necessary for the Cdc42-induced formation of filopodia. These effects of nectins require their cytoplasmic tail but not their association with afadin. We propose here the functional relationship between nectins and the small G proteins in the organization of AJs.     

The Ras-related protein Cdc42Hs and bradykinin promote formation of peripheral actin microspikes and filopodia in Swiss 3T3 fibroblasts.

The Ras-related protein Cdc42 plays a role in yeast cell budding and polarity. Two related proteins, Rac1 and RhoA, promote formation in mammalian cells of membrane ruffles and stress fibers, respectively, which contain actin microfilaments. We now show that microinjection of the related human Cdc42Hs into Swiss 3T3 fibroblasts induced the formation of peripheral actin microspikes, determined by staining with phalloidin. A proportion of these microspikes was found to be components of filopodia, as analyzed by time-lapse phase-contrast microscopy. The formation of filopodia was also found to be promoted by Cdc42Hs microinjection. This was followed by activation of Rac1-mediated membrane ruffling. Treatment with bradykinin also promoted formation of microspikes and filopodia as well as subsequent effects similar to that seen upon Cdc42Hs microinjection. These effects of bradykinin were specifically inhibited by prior microinjection of dominant negative Cdc42HsT17N, suggesting that bradykinin acts by activating cellular Cdc42Hs. Since filopodia have been ascribed an important sensory function in fibroblasts and are required for guidance of neuronal growth cones, these results indicate that Cdc42Hs plays an important role in determining mammalian cell morphology.     

Cdc42 signaling regulated by dopamine D2 receptor correlatively links specific brain regions of hippocampus to cocaine addiction

BackgroundHippocampus plays critical roles in drug addiction. Cocaine-induced modifications in dopamine receptor function and the downstream signaling are important regulation mechanisms in cocaine addiction. Rac regulates actin filament accumulation while Cdc42 stimulates the formation of filopodia and neurite outgrowth. Based on the region specific roles of small GTPases in brain, we focused on the hippocampal subregions to detect the regulation of Cdc42 signaling in long-term morphological and behavioral adaptations to cocaine.MethodsGenetically modified mouse models of Cdc42, dopamine receptor D1 (D1R) and D2 (D2R) and expressed Cdc42 point mutants that are defective in binding to and activation of its downstream effector molecules PAK and N-WASP were generated, respectively, in CA1 or dentate gyrus (DG) subregion.ResultsCocaine induced upregulation of Cdc42 signaling activity. Cdc42 knockout or mutants blocked cocaine-induced increase in spine plasticity in hippocampal CA1 pyramidal neurons, leading to a decreased conditional place preference (CPP)-associated memories and spatial learning and memory in water maze. Cdc42 knockout or mutants promoted cocaine-induced loss of neurogenesis in DG, leading to a decreased CPP-associated memories and spatial learning and memory in water maze. Furthermore, by using D1R knockout, D2R knockout, and D2R/Cdc42 double knockout mice, we found that D2R, but not D1R, regulated Cdc42 signaling in cocaine-induced neural plasticity and behavioral changes.ConclusionsCdc42 acts downstream of D2R in the hippocampus and plays an important role in cocaine-induced neural plasticity through N-WASP and PAK-LIMK-Cofilin, and Cdc42 signaling pathway correlatively links specific brain regions (CA1, dentate gyrus) to cocaine-induced CPP behavior.     

The Yersinia pseudotuberculosis cytotoxic necrotizing factor (CNFY) selectively activates RhoA

The cytotoxic necrotizing factors (CNF)1 and CNF2 from pathogenic Escherichia coli strains activate RhoA, Rac1, and Cdc42 by deamidation of Gln63 (RhoA) or Gln61 (Rac and Cdc42). Recently, a novel cytotoxic necrotizing factor termed CNFY was identified in Yersinia pseudotuberculosis strains (Lockman, H. A., Gillespie, R. A., Baker, B. D., and Shakhnovich, E. (2002) Infect. Immun. 70, 2708-2714). We amplified the cnfy gene from genomic DNA of Y. pseudotuberculosis, cloned and expressed the recombinant protein, and studied its activity. Recombinant GST-CNFY induced morphological changes in HeLa cells and caused an upward shift of RhoA in SDS-PAGE, as is known for GST-CNF1 and GST-CNF2. Mass spectrometric analysis of GST-CNFY-treated RhoA confirmed deamidation at Glu63. Treatment of RhoA, Rac1, and Cdc42 with GST-CNFY decreased their GTPase activities, indicating that all of these Rho proteins could serve as substrates for GST-CNFY in vitro. In contrast, RhoA, but not Rac or Cdc42, was the substrate of GST-CNFY in culture cells. GST-CNFY caused marked stress fiber formation in HeLa cells after 2 h. In contrast to GST-CNF1, formation of filopodia or lamellipodia was not induced with GST-CNFY. Accordingly, effector pull-down experiments with lysates of toxin-treated cells revealed strong activation of RhoA but no activation of Rac1 or Cdc42 after 6 h of GST-CNFY-treatment. Moreover, in rat hippocampal neurons, GST-CNFY results in the retraction of neurites, indicating RhoA activation. In contrast, no activation of Rac or Cdc42 was found. Altogether, our data suggest that CNFY from Y. pseudotuberculosis is a strong, selective activator of RhoA, which can be used as a powerful tool for constitutive RhoA activation without concomitant activation of Rac1 or Cdc42.     

The GTPase-activating protein n-chimaerin cooperates with Rac1 and Cdc42Hs to induce the formation of lamellipodia and filopodia.

n-Chimaerin is a GTPase-activating protein (GAP) mainly for Rac1 and less so for Cdc42Hs in vitro. The GAP activity of n-chimaerin is regulated by phospholipids and phorbol esters. Microinjection of Rac1 and Cdc42Hs into mammalian cells induces formation of the actin-based structures lamellipodia and filopodia, respectively, with the former being prevented by coinjection of the chimaerin GAP domain. Strikingly, microinjection of the full-length n-chimaerin into fibroblasts and neuroblastoma cells induces the simultaneous formation of lamellipodia and filopodia. These structures undergo cycles of dissolution and formation, resembling natural morphological events occurring at the leading edge of fibroblasts and neuronal growth cones. The effects of n-chimaerin on formation of lamellipodia and filopodia were inhibited by dominant negative Rac1(T17N) and Cdc42Hs(T17N), respectively. n-Chimaerin's effects were also inhibited by coinjection with Rho GDP dissociation inhibitor or by treatment with phorbol ester. A mutant n-chimaerin with no GAP activity and impaired p21 binding was ineffective in inducing morphological changes, while a mutant lacking GAP activity alone was effective. Microinjected n-chimaerin colocalized in situ with F-actin. Taken together, these results suggest that n-chimaerin acts synergistically with Rac1 and Cdc42Hs to induce actin-based morphological changes and that this action involves Rac1 and Cdc42Hs binding but not GAP activity. Thus, GAPs may have morphological functions in addition to downregulation of GTPases.     

RhoG GTPase Controls a Pathway That Independently Activates Rac1 and Cdc42Hs

RhoG is a member of the Rho family of GTPases that shares 72% and 62% sequence identity with Rac1 and Cdc42Hs, respectively. We have expressed mutant RhoG proteins fused to the green fluorescent protein and analyzed subsequent changes in cell surface morphology and modifications of cytoskeletal structures. In rat and mouse fibroblasts, green fluorescent protein chimera and endogenous RhoG proteins colocalize according to a tubular cytoplasmic pattern, with perinuclear accumulation and local concentration at the plasma membrane. Constitutively active RhoG proteins produce morphological and cytoskeletal changes similar to those elicited by a simultaneous activation of Rac1 and Cdc42Hs, i.e., the formation of ruffles, lamellipodia, filopodia, and partial loss of stress fibers. In addition, RhoG and Cdc42Hs promote the formation of microvilli at the cell apical membrane. RhoG-dependent events are not mediated through a direct interaction with Rac1 and Cdc42Hs targets such as PAK-1, POR1, or WASP proteins but require endogenous Rac1 and Cdc42Hs activities: coexpression of a dominant negative Rac1 impairs membrane ruffling and lamellipodia but not filopodia or microvilli formation. Conversely, coexpression of a dominant negative Cdc42Hs only blocks microvilli and filopodia, but not membrane ruffling and lamellipodia. Microtubule depolymerization upon nocodazole treatment leads to a loss of RhoG protein from the cell periphery associated with a reversal of the RhoG phenotype, whereas PDGF or bradykinin stimulation of nocodazole-treated cells could still promote Rac1- and Cdc42Hs-dependent cytoskeletal reorganization. Therefore, our data demonstrate that RhoG controls a pathway that requires the microtubule network and activates Rac1 and Cdc42Hs independently of their growth factor signaling pathways.     

Rho family GTPases and neuronal growth cone remodelling: relationship between increased complexity induced by Cdc42Hs, Rac1, and acetylcholine and collapse induced by RhoA and lysophosphatidic acid.

Rho family GTPases have been assigned important roles in the formation of actin-based morphologies in nonneuronal cells. Here we show that microinjection of Cdc42Hs and Rac1 promoted formation of filopodia and lamellipodia in N1E-115 neuroblastoma growth cones and along neurites. These actin-containing structures were also induced by injection of Clostridium botulinum C3 exoenzyme, which abolishes RhoA-mediated functions such as neurite retraction. The C3 response was inhibited by coinjection with the dominant negative mutant Cdc42Hs(T17N), while the Cdc42Hs response could be competed by coinjection with RhoA. We also demonstrate that the neurotransmitter acetylcholine (ACh) can induce filopodia and lamellipodia on neuroblastoma growth cones via muscarinic ACh receptor activation, but only when applied in a concentration gradient. ACh-induced formation of filopodia and lamellipodia was inhibited by preinjection with the dominant negative mutants Cdc42Hs(T17N) and Rac1(T17N), respectively. Lysophosphatidic acid (LPA)-induced neurite retraction, which is mediated by RhoA, was inhibited by ACh, while C3 exoenzyme-mediated neurite outgrowth was inhibited by injection with Cdc42Hs(T17N) or Rac1(T17N). Together these results suggest that there is competition between the ACh- and LPA-induced morphological pathways mediated by Cdc42Hs and/or Rac1 and by RhoA, leading to either neurite development or collapse.     

Regulation of platelet Rac1 and Cdc42 activation through interaction with calmodulin

Rac1 and Cdc42 are members of the Rho family of small GTPases and have been shown to induce lamellipodia and filopodia formation, respectively. This leads to changes in cytoskeleton organization and as a consequence affects cell migration. In the present work we demonstrate that endogenous Rac1 and Cdc42 interact with calmodulin (CaM) in a Ca(2+)-dependent fashion. The interaction of Rac1 and Cdc42 with CaM was shown to be direct. This novel interaction was further confirmed in platelets using co-immunoprecipitation studies. Using CaM database analysis and in vitro peptide competition assays we have identified a 14 amino acid region in Rac1 that is essential for CaM binding. The scrambled form of the peptide did not bind CaM demonstrating specificity of the predicted CaM binding region in Rac1. A similar region capable of binding CaM exists in Cdc42. Furthermore, using the optimal activation time-point for each GTPase, the role of CaM in the function of Rac1 and Cdc42 was examined. Results demonstrate that in human platelets, thrombin caused maximal activation of Rac1 and Cdc42 at ~60 s and ~25 s respectively. The potent CaM antagonist W7 abolished thrombin-mediated activation of Rac1. However, addition of W7 resulted in the activation of Cdc42 over basal and W7 did not inhibit thrombin-mediated activation of Cdc42. The less potent CaM inhibitor, W5, did not have any effect on Rac1 and Cdc42 activation. The results demonstrate that in platelets, binding of CaM to Rac1 increases its activation while its binding to Cdc42 reduces the activation of this GTPase. This suggests an important role for CaM in coordinating Rac1 and Cdc42 activation and in the regulation of cytoskeleton remodeling.     

Integrin engagement suppresses RhoA activity via a c-Src-dependent mechanism

The Rho family GTPases Cdc42, Rac1 and RhoA control many of the changes in the actin cytoskeleton that are triggered when growth factor receptors and integrins bind their ligands [1] [2]. Rac1 and Cdc42 stimulate the formation of protrusive structures such as membrane ruffles, lamellipodia and filopodia. RhoA regulates contractility and assembly of actin stress fibers and focal adhesions. Although prolonged integrin engagement can stimulate RhoA [3] [4] [5], regulation of this GTPase by early integrin-mediated signals is poorly understood. Here we show that integrin engagement initially inactivates RhoA, in a c-Src-dependent manner, but has no effect on Cdc42 or Rac1 activity. Additionally, early integrin signaling induces activation and tyrosine phosphorylation of p190RhoGAP via a mechanism that requires c-Src. Dynamic modulation of RhoA activity appears to have a role in motility, as both inhibition and activation of RhoA hinder migration [6] [7] [8]. Transient suppression of RhoA by integrins may alleviate contractile forces that would otherwise impede protrusion at the leading edge of migrating cells.     

Gene targeting implicates Cdc42 GTPase in GPVI and non-GPVI mediated platelet filopodia formation, secretion and aggregation

Background

Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in actin cytoskeleton regulation. We have shown previously that Rac1 is involved in regulation of platelet secretion and aggregation. However, the role of Cdc42 in platelet activation remains controversial. This study was undertaken to better understand the role of Cdc42 in platelet activation.

Methodology/Principal Findings

We utilized the Mx-cre;Cdc42^lox/lox inducible mice with transient Cdc42 deletion to investigate the involvement of Cdc42 in platelet function. The Cdc42-deficient mice exhibited a significantly reduced platelet count than the matching Cdc42^+/+ mice. Platelets isolated from Cdc42^−/−, as compared to Cdc42^+/+, mice exhibited (a) diminished phosphorylation of PAK1/2, an effector molecule of Cdc42, (b) inhibition of filopodia formation on immobilized CRP or fibrinogen, (c) inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin, (d) inhibition of CRP, collagen or thrombin induced platelet aggregation, and (e) minimal phosphorylation of Akt upon stimulation with CRP or thrombin. The bleeding times were significantly prolonged in Cdc42^−/− mice compared with Cdc42^+/+ mice.

Conclusion/Significance

Our data demonstrate that Cdc42 is required for platelet filopodia formation, secretion and aggregation and therefore plays a critical role in platelet mediated hemostasis and thrombosis.     

SPECs, small binding proteins for Cdc42

The Rho GTPase, Cdc42, regulates a wide variety of cellular activities including actin polymerization, focal complex assembly, and kinase signaling. We have identified a new family of very small Cdc42-binding proteins, designated SPECs (for Small Protein Effector of Cdc42), that modulates these regulatory activities. The two human members, SPEC1 and SPEC2, encode proteins of 79 and 84 amino acids, respectively. Both contain a conserved N-terminal region and a centrally located CRIB (Cdc42/Rac Interactive Binding) domain. Using a yeast two-hybrid system, we found that both SPECs interact strongly with Cdc42, weakly with Rac1, and not at all with RhoA. Transfection analysis revealed that SPEC1 inhibited Cdc42-induced c-Jun N-terminal kinase (JNK) activation in COS1 cells in a manner that required an intact CRIB domain. Immunofluorescence experiments in NIH-3T3 fibroblasts demonstrated that both SPEC1 and SPEC2 showed a cortical localization and induced the formation of cell surface membrane blebs, which was not dependent on Cdc42 activity. Cotransfection experiments demonstrated that SPEC1 altered Cdc42-induced cell shape changes both in COS1 cells and in NIH-3T3 fibroblasts and that this alteration required an intact CRIB domain. These results suggest that SPECs act as novel scaffold molecules to coordinate and/or mediate Cdc42 signaling activities.     

Antagonistic cross-talk between Rac and Cdc42 GTPases regulates generation of reactive oxygen species

Cross-talk between Rho GTPase family members (Rho, Rac, and Cdc42) plays important roles in modulating and coordinating downstream cellular responses resulting from Rho GTPase signaling. The NADPH oxidase of phagocytes and nonphagocytic cells is a Rac GTPase-regulated system that generates reactive oxygen species (ROS) for the purposes of innate immunity and intracellular signaling. We recently demonstrated that NADPH oxidase activation involves sequential interactions between Rac and the flavocytochrome b(558) and p67(phox) oxidase components to regulate electron transfer from NADPH to molecular oxygen. Here we identify an antagonistic interaction between Rac and the closely related GTPase Cdc42 at the level of flavocytochrome b(558) that regulates the formation of ROS. Cdc42 is unable to stimulate ROS formation by NADPH oxidase, but Cdc42, like Rac1 and Rac2, was able to specifically bind to flavocytochrome b(558) in vitro. Cdc42 acted as a competitive inhibitor of Rac1- and Rac2-mediated ROS formation in a recombinant cell-free oxidase system. Inhibition was dependent on the Cdc42 insert domain but not the Switch I region. Transient expression of Cdc42Q61L inhibited ROS formation induced by constitutively active Rac1 in an NADPH oxidase-expressing Cos7 cell line. Inhibition of Cdc42 activity by transduction of the Cdc42-binding domain of Wiscott-Aldrich syndrome protein into human neutrophils resulted in an enhanced fMetLeuPhe-induced oxidative response, consistent with inhibitory cross-talk between Rac and Cdc42 in activated neutrophils. We propose here a novel antagonism between Rac and Cdc42 GTPases at the level of the Nox proteins that modulates the generation of ROS used for host defense, cell signaling, and transformation.     

Cholesterol oxides mediated changes in cytoskeletal organisation involves Rho GTPases small star, filled

The small GTPases Rho, Rac, and Cdc42 regulate the actin cytoskeleton in all eukaryotic cells. In this study we have evaluated the effect of cholesterol oxides (7-ketocholesterol and 25-hydroxycholesterol) on cell migration, cell adhesion, and cytoskeletal organisation of lens epithelial cells (LEC). Effects of cholesterol oxides on cytoskeleton were evaluated by immunofluorescence confocal microscopy. The 7-ketocholesterol induced cell arborisation, with bundling of vimentin and tubulin in the cell processes and formation of filopodia and stress fibres. Cells treated with 25-hydroxycholesterol showed a collapse of vimentin filaments towards the nucleus and formation of lamellipodia. In addition, cells treated with 7-ketocholesterol or 25-hydroxycholesterol showed decreased migration. The effects of cholesterol oxides on cytoskeletal proteins involve the activation of the small GTPases Rho, Rac, and Cdc42. Indeed, formation of both filopodia and stress fibres induced by 7-ketocholesterol is inhibited by overexpressing dominant negatives forms of Cdc42 and RhoA, respectively. Similarly, the collapse of vimentin intermediate filament network and the formation of lamellipodia, induced by 25-hydroxycholesterol, is inhibited by overexpressing dominant negatives forms of Rac1. The effects of cholesterol oxides described in this study for LEC are also observed for at least two other cell lines (H36CE and U373), suggesting that this may represent a general mechanism whereby cholesterol oxides induces cytoskeletal disorganisation.