Inhibition of Fc epsilon-RI-mediated activation of rat basophilic leukemia cells by Clostridium difficile toxin B (monoglucosyltransferase)

Glucosyltransferase Activity of Clostridium difficile Toxin B Triggers Autophagy-mediated Cell Growth Arrest

Autophagy is a bulk cell-degradation process that occurs through the lysosomal machinery, and many reports have shown that it participates in microbial pathogenicity. However, the role of autophagy in Clostridium difficile infection (CDI), the leading cause of antibiotics-associated diarrhea, pseudomembranous colitis and even death in severe cases, is not clear. Here we report that the major virulent factor toxin B (TcdB) of Clostridium difficile elicits a strong autophagy response in host cells through its glucosyltransferase activity. Using a variety of autophagy-deficient cell lines, i.e. HeLa/ATG7^−/−, MEF/atg7^−/−, MEF/tsc2^−/−, we demonstrate that toxin-triggered autophagy inhibits host cell proliferation, which contributes to TcdB-caused cytopathic biological effects. We further show that both the PI3K complex and mTOR pathway play important roles in this autophagy induction process and consequent cytopathic event. Although the glucosyltransferase activity of TcdB is responsible for inducing both cell rounding and autophagy, there is no evidence suggesting the causal relationship between these two events. Taken together, our data demonstrate for the first time that the glucosyltransferase enzymatic activity of a pathogenic bacteria is responsible for host autophagy induction and the following cell growth arrest, providing a new paradigm for the role of autophagy in host defense mechanisms upon pathogenic infection.

The Role of Rho GTPases in Toxicity of Clostridium difficile Toxins

Clostridium difficile (C. difficile) is the main cause of antibiotic-associated diarrhea prevailing in hospital settings. In the past decade, the morbidity and mortality of C. difficile infection (CDI) has increased significantly due to the emergence of hypervirulent strains. Toxin A (TcdA) and toxin B (TcdB), the two exotoxins of C. difficile, are the major virulence factors of CDI. The common mode of action of TcdA and TcdB is elicited by specific glucosylation of Rho-GTPase proteins in the host cytosol using UDP-glucose as a co-substrate, resulting in the inactivation of Rho proteins. Rho proteins are the key members in many biological processes and signaling pathways, inactivation of which leads to cytopathic and cytotoxic effects and immune responses of the host cells. It is supposed that Rho GTPases play an important role in the toxicity of C. difficile toxins. This review focuses on recent progresses in the understanding of functional consequences of Rho GTPases glucosylation induced by C. difficile toxins and the role of Rho GTPases in the toxicity of TcdA and TcdB.

Relationship of light scatter change and Cdc42-regulated actin status

BACKGROUND

Cdc42 GTPase has important roles in regulating intracellular actin reorganization. The current methods to monitor actin changes are typically complex and point by point.

METHODS

The effects of Cdc42 inhibitors on the side scatter changes were tested in a newly developed continuous assay using the flow cytometer. Staining with fluorescently labeled phalloidin was used for comparison.

RESULTS

Cdc42-specific inhibitors caused dose-dependent changes of both the right-angle side scatter and the phalloidin-stained actin.

CONCLUSIONS

The right-angle light scatter change can be used as a method to circumvent phalloidin staining and be an early convenient step in screening Cdc42 inhibitors. © 2015 International Clinical Cytometry Society.

Clostridium difficile toxin B inhibits the secretory response of human mast cell line-1 (HMC-1) cells stimulated with high free-Ca²⁺ and GTPγS

Clostridium difficile toxins A and B (TcdA and TcdB) belong to the class of large clostridial cytotoxins and inactivate by glucosylation some low molecular mass GTPases of the Rho-family (predominantly Rho, Rac and Cdc42), known as regulators of the actin cytoskeleton. TcdA and B also represent the main virulence factors of the anaerobic gram-positive bacterium that is the causal agent of pseudomembranous colitis. In our study, TcdB was chosen instead of TcdA for the well-known higher cytotoxic potency. Inactivation of Rho-family GTPases by this toxin in our experimental conditions induced morphological changes and reduction of electron-dense mast cell-specific granules in human mast cell line-1 (HMC-1) cells, but not cell death or permeabilisation of plasma-membranes. Previously reported patch-clamp dialysis experiments revealed that high intracellular free-Ca(2+) and GTPγS concentrations are capable of inducing exocytosis as indicated by significant membrane capacitance (Cm) increases in HMC-1 cells. In this study, we investigated the direct effects of TcdB upon HMC-1 cell "stimulated" Cm increase, as well as on "constitutive" secretion of hexosaminidase and interleukin-16 (IL-16). Compared to untreated control cells, HMC-1 cells incubated with TcdB for 3-24h exhibited a significant reduction of the mean absolute and relative Cm increase in response to free-Ca(2+) and GTPγS suggesting an inhibition of secretory processes by TcdB. In conclusion, the HMC-1 cell line represents a suitable model for the study of direct effects of C. difficile toxins on human mast cell secretory activity.

Communication between 5-HT and small GTPases

Advances over the past decade have improved our understanding of the serotonin (5-HT) biology outside the central nervous system specifically the molecular mechanisms of serotonergic signaling in association with small GTPases. It is now recognized that the communication between 5-HT and GTPases plays important roles in peripheral tissues, vascular cells and are involved in coagulation, hypertension, inflammation, healing and protection. Furthermore, 5-HT receptors as heterotrimeric GTP-binding protein-coupled receptors act as effector protein on the small GTPases. Therefore, the antagonists or agonists of the effector proteins of small GTPases could be useful therapeutic agents for the treatment of several diseases and disorders.

The enterotoxicity of Clostridium difficile toxins

The major virulence factors of Clostridium difficile infection (CDI) are two large exotoxins A (TcdA) and B (TcdB). However, our understanding of the specific roles of these toxins in CDI is still evolving. It is now accepted that both toxins are enterotoxic and proinflammatory in the human intestine. Both purified TcdA and TcdB are capable of inducing the pathophysiology of CDI, although most studies have focused on TcdA. C. difficile toxins exert a wide array of biological activities by acting directly on intestinal epithelial cells. Alternatively, the toxins may target immune cells and neurons once the intestinal epithelial barrier is disrupted. The toxins may also act indirectly by stimulating cells to produce chemokines, proinflammatory cytokines, neuropeptides and other neuroimmune signals. This review considers the mechanisms of TcdA- and TcdB-induced enterotoxicity, and recent developments in this field.

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Bacterial toxins as immunomodulators

Bacterial toxins are the causative agent at pathology in a variety of diseases. Although not always the primary target of these toxins, many have been shown to have potent immunomodulatory effects, for example, inducing immune responses to co-administered antigens and suppressing activation of immune cells. These abilities of bacterial toxins can be harnessed and used in a therapeutic manner, such as in vaccination or the treatment of autoimmune diseases. Furthermore, the ability of toxins to gain entry to cells can be used in novel bacterial toxin based immuno-therapies in order to deliver antigens into MHC Class I processing pathways. Whether the immunomodulatory properties of these toxins arose in order to enhance bacterial survival within hosts, to aid spread within the population or is pure serendipity, it is interesting to think that these same toxins potentially hold the key to preventing or treating human disease.

The immunosuppressor mycophenolic acid kills activated lymphocytes by inducing a nonclassical actin-dependent necrotic signal

Mycophenolate mofetil (MMF) is an immunosuppressive agent used in transplantation. Over the last decade, MMF has also emerged as an alternative therapeutic regimen for autoimmune diseases, mainly for patients refractory to other therapies. The active compound of MMF, mycophenolic acid (MPA), depletes the intracellular pool of guanosine tri-phosphate through inosine monophosphate dehydrogenase blockade. The molecular mechanism involved in the elimination of T and B lymphocytes upon inhibition of inosine monophosphate dehydrogenase remains elusive. In this study, we showed that in contrast to the immunosuppressors azathioprine, cyclosporin A, and tacrolimus, MPA killed lymphocytes through the activation of a caspase-independent necrotic signal. Furthermore, the MPA-mediated necrotic signal relied on the transmission of a novel intracellular signal involving Rho-GTPase Cdc42 activity and actin polymerization. In addition to its medical interest, this study sheds light on a novel and atypical molecular mechanism leading to necrotic cell death.

Clostridium difficile toxins A and B directly stimulate human mast cells

Clostridium difficile toxins A and B (TcdA and TcdB) are the causative agents of antibiotic-associated pseudomembranous colitis. Mucosal mast cells play a crucial role in the inflammatory processes underlying this disease. We studied the direct effects of TcdA and TcdB on the human mast cell line HMC-1 with respect to degranulation, cytokine release, and the activation of proinflammatory signal pathways. TcdA and TcdB inactivate Rho GTPases, the master regulators of the actin cytoskeleton. The inactivation of Rho GTPases induced a reorganization of the actin cytoskeleton accompanied by morphological changes of cells. The TcdB-induced reorganization of the actin cytoskeleton in HMC-1 cells reduced the number of electron-dense mast cell-specific granules. Accordingly, TcdB induced the release of hexosaminidase, a marker for degranulation, in HMC-1 cells. The actin rearrangement was found to be responsible for degranulation since latrunculin B induced a comparable hexosaminidase release. In addition, TcdB as well as latrunculin B induced the activation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase 1/2 and also resulted in a p38 MAPK-dependent increased formation of prostaglandins D(2) and E(2). The autocrine stimulation of HMC-1 cells by prostaglandins partially contributed to the degranulation. Interestingly, TcdB-treated HMC-1 cells, but not latrunculin B-treated HMC-1 cells, showed a strong p38 MAPK-dependent increase in interleukin-8 release. Differences in the mast cell responses to TcdB and latrunculin B are probably due to the presence of functionally inactive Rho GTPases in toxin-treated cells. Thus, the HMC-1 cell line is a promising model for studying the direct effects of C. difficile toxins on mast cells independently of the tissue context.

The SPR signal in living cells reflects changes other than the area of adhesion and the formation of cell constructions

Surface plasmon resonance (SPR) sensors detected large angle of resonance (AR) changes, when RBL-2H3 rat mast cells were cultured and activated on a sensor chip. Here, we demonstrated that PAM212 mouse keratinocytes also showed a large change in AR, when EGF-stimulated. We explored these changes due to intracellular reactions, through the relationship between the AR and the area of cell adhesion, using confocal microscopy for RBL-2H3 cells and PAM212 cells. The effect of Mycalolide B and Toxin B, inhibitors for cell motility, on AR was observed using RBL-2H3 cells. Measuring AR in the presence of various numbers of non-stimulated cells demonstrated that AR and cell density were proportional. However, the AR increase in response to antigen was 35% higher than that expected by solely an increase of the cell adhesion area. Moreover, the AR with PAM212 cells decreased following a transient increase in response to EGF, whilst the area of cell adhesion remained at an increased level. Furthermore, the treatment of RBL-2H3 cells with either Mycalolide B or Toxin B slightly inhibited, but never abolished the AR increase induced by antigen. These treatments abolished all morphological changes, including ruffling and the increase of cell adhesion area observed by light microscopy. These results suggest that AR changes reflect intracellular events rather than changes in the size of the area to which cells adhere.

Activation of mitogen-activated protein kinases is essential for hydrogen peroxide -induced apoptosis in retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells are constantly exposed to oxidative injury while clearing byproducts of photoreceptor turnover, a circumstance thought to be responsible for degenerative retinal diseases. The mechanisms of hydrogen peroxide (H(2)O(2))-induced apoptosis in RPE cells are not fully understood. We studied signal transduction mechanisms of H(2)O(2)-induced apoptosis in the human RPE cell line ARPE-19. Activation of two stress kinases (JNK and p38) occurs during H(2)O(2) stimulation, and H(2)O(2)-mediated cell death was significantly reduced by their specific inhibition. Exposure to a lethal dose of H(2)O(2) elicited Bax translocation to the mitochondria and release of apoptosis-inducing factor (AIF) from the mitochondria, both of which were abolished by either JNK- or p38-specific inhibitors. Both H(2)O(2)-induced cell death and JNK/p38 phosphorylation were partially inhibited by C. difficile toxin B, inhibitor of Rho, Rac, and cdc42. Use of pull-down assays revealed that the small GTPase activated by H(2)O(2) is Rac1. This study is the first to demonstrate that H(2)O(2) induces a Rac1/JNK1/p38 signaling cascade, and that JNK and p38 activation is important for H(2)O(2)-induced apoptosis as well as AIF/Bax translocation of RPE cells.

Modification of epithelial cell barrier permeability and intercellular junctions by Clostridium sordellii lethal toxins

Clostridium sordellii lethal toxin (LT) is a glucosyltransferase which inactivates small GTPases from the Rho and Ras families. In the present work, we studied the effects of two variants, LT82 and LT9048, on the integrity of epithelial cell barrier using polarized MCCD (Mouse Cortical Collecting Duct) and MDCK (Madin-Darby Canine Kidney) cells. Our results demonstrate for the first time that LTs have very limited effects on tight junctions. In contrast, we show that both toxins modified the paracellular permeability within 2-4 h. Concomitantly LT82 and LT9048 induced a disorganization of basolateral actin filaments, without modifying apical actin. Both toxins mainly altered adherens junctions by removing E-cadherin-catenin complexes from the membrane to the cytosol. Similar effects on adherens junctions have been observed with other toxins, which directly or indirectly depolymerize actin. Thereby, Rac, a common substrate of both LTs, might play a central role in LT-dependent adherens junction alteration. Here, we show that adherens junction perturbation induced by LTs results neither from a direct effect of toxins on adherens junction proteins nor from an actin-independent Rac pathway, but rather from a Rac-dependent disorganization of basolateral actin cytoskeleton. This further supports that a dynamic equilibrium of cortical actin filaments is essential for functional E-cadherin organization in epithelia.

Isozyme-specific stimulation of phospholipase C-gamma2 by Rac GTPases

The regulation of the two isoforms of phospholipase C-gamma, PLCgamma(1) and PLCgamma(2), by cell surface receptors involves protein tyrosine phosphorylation as well as interaction with adapter proteins and phosphatidylinositol 3,4,5-trisphosphate (PtdInsP(3)) generated by inositol phospholipid 3-kinases (PI3Ks). All three processes may lead to recruitment of the PLCgamma isozymes to the plasma membrane and/or stimulation of their catalytic activity. Recent evidence suggests that PLCgamma may also be regulated by Rho GTPases. In this study, PLCgamma(1) and PLCgamma(2) were reconstituted in intact cells and in a cell-free system with Rho GTPases to examine their influence on PLCgamma activity. PLCgamma(2), but not PLCgamma(1), was markedly activated in intact cells by constitutively active Rac1(G12V), Rac2(G12V), and Rac3(G12V) but not by Cdc42(G12V) and RhoA(G14V). The mechanism of PLCgamma(2) activation was apparently independent of phosphorylation of tyrosine residues known to be modified by PLCgamma(2)-activating protein-tyrosine kinases. Activation of PLCgamma(2) by Rac2(G12V) in intact cells coincided with a translocation of PLCgamma(2) from the soluble to the particulate fraction. PLCgamma isozyme-specific activation of PLCgamma(2) by Rac GTPases (Rac1 approximately Rac2 > Rac3), but not by Cdc42 or RhoA, was also observed in a cell-free system. Herein, activation of wild-type Rac GTPases with guanosine 5'-(3-O-thio)triphosphate caused a marked stimulation of PLCgamma(2) but had no effect on the activity of PLCgamma(1). PLCgamma(1) and PLCgamma(2) have previously been shown to be indiscriminately activated by PtdInsP(3) in vitro. Thus, the results suggest a novel mechanism of PLCgamma(2) activation by Rac GTPases involving neither protein tyrosine phosphorylation nor PI3K-mediated generation of PtdInsP(3).

Clostridial Rho-inhibiting protein toxins

Rho proteins are master regulators of a large array of cellular functions, including control of cell morphology, cell migration and polarity, transcriptional activation, and cell cycle progression. They are the eukaryotic targets of various bacterial protein toxins and effectors, which activate or inactivate the GTPases. Here Rho-inactivating toxins and effectors are reviewed, including the families of large clostridial cytotoxins and C3-like transferases, which inactivate Rho GTPases by glucosylation and ADP-ribosylation, respectively.

A role for Rho and Rac in secretagogue-induced amylase release by pancreatic acini

The actin cytoskeleton has long been implicated in protein secretion. We investigated whether Rho and Rac, known regulators of the cytoskeleton, are involved in amylase secretion by mouse pancreatic acini. Secretagogues, including cholecystokinin (CCK) and the acetylcholine analog carbachol, increased the amount of GTP-bound RhoA and Rac1 and induced translocation from cytosol to a membrane fraction. Immunocytochemistry revealed the translocation of Rho and Rac within the apical region of the cell. Expression by means of adenoviral vectors of dominant-negative Rho (RhoN19), dominant-negative Rac (RacN17), and Clostridium Botulinum C3 exotoxin, which ADP ribosylates and inactivates Rho, significantly inhibited amylase secretion by CCK and carbachol; inhibiting both Rho and Rac resulted in a greater reduction. This inhibitory effect of RhoN19 on CCK-induced amylase secretion was apparent in both the early and late phases of secretion, whereas RacN17 was more potent on the late phase of secretion. None of these three affected the basal Ca2+or the peak intracellular Ca2+concentration stimulated by CCK. Latrunculin, a marine toxin that sequesters actin monomers, time-dependently decreased the total amount of filamentous actin (F-actin) and dose-dependently decreased secretion by secretagogues without affecting Ca2+signaling. These data suggest that Rho and Rac are both involved in CCK-induced amylase release in pancreatic acinar cell possibly through an effect on the actin cytoskeleton.

Clostridium difficile toxins: mechanism of action and role in disease

As the leading cause of hospital-acquired diarrhea, Clostridium difficile colonizes the large bowel of patients undergoing antibiotic therapy and produces two toxins, which cause notable disease pathologies. These two toxins, TcdA and TcdB, are encoded on a pathogenicity locus along with negative and positive regulators of their expression. Following expression and release from the bacterium, TcdA and TcdB translocate to the cytosol of target cells and inactivate small GTP-binding proteins, which include Rho, Rac, and Cdc42. Inactivation of these substrates occurs through monoglucosylation of a single reactive threonine, which lies within the effector-binding loop and coordinates a divalent cation critical to binding GTP. By glucosylating small GTPases, TcdA and TcdB cause actin condensation and cell rounding, which is followed by death of the cell. TcdA elicits effects primarily within the intestinal epithelium, while TcdB has a broader cell tropism. Important advances in the study of these toxins have been made in the past 15 years, and these are detailed in this review. The domains, subdomains, and residues of these toxins important for receptor binding and enzymatic activity have been elegantly studied and are highlighted herein. Furthermore, there have been major advances in defining the role of these toxins in modulating the inflammatory events involving the disruption of cell junctions, neuronal activation, cytokine production, and infiltration by polymorphonuclear cells. Collectively, the present review provides a comprehensive update on TcdA and TcdB's mechanism of action as well as the role of these toxins in disease.

Bacterial cytotoxins: targeting eukaryotic switches

Many bacterial cytotoxins act on eukaryotic cells by targeting the regulators that are involved in controlling the cytoskeleton or by directly modifying actin, with members of the Rho GTPase family being particularly important targets. The actin cytoskeleton, and especially the GTPase 'molecular switches' that are involved in its control, have crucial functions in innate and adaptive immunity, and have pivotal roles in the biology of infection. In this review, we briefly discuss the role of the actin cytoskeleton and the Rho GTPases in host-pathogen interactions, and review the mode of actions of bacterial protein toxins that target these components.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Large clostridial cytotoxins

The large clostridial cytotoxins are a family of structurally and functionally related exotoxins from Clostridium difficile (toxins A and B), C. sordellii (lethal and hemorrhagic toxin) and C. novyi (alpha-toxin). The exotoxins are major pathogenicity factors which in addition to their in vivo effects are cytotoxic to cultured cell lines causing reorganization of the cytoskeleton accompanied by morphological changes. The exotoxins are single-chain protein toxins, which are constructed of three domains: receptor-binding, translocation and catalytic domain. These domains reflect the self-mediated cell entry via receptor-mediated endocytosis, translocation into the cytoplasm, and execution of their cytotoxic activity by an inherent enzyme activity. Enzymatically, the toxins catalyze the transfer of a glucosyl moiety from UDP-glucose to the intracellular target proteins which are the Rho and Ras GTPases. The covalent attachment of the glucose moiety to a conserved threonine within the effector region of the GTPases renders the Rho-GTPases functionally inactive. Whereas the molecular mode of cytotoxic effects is fully understood, the mechanisms leading to inflammatory processes in the context of disease (e.g., antibiotic-associated pseudomembranous colitis caused by Clostridium difficile) are less clear.

Rho-modifying C3-like ADP-ribosyltransferases

C3-like exoenzymes comprise a family of seven bacterial ADP-ribosyltransferases, which selectively modify RhoA, B, and C at asparagine-41. Crystal structures of C3 exoenzymes are available, allowing novel insights into the structure-function relationships of these exoenzymes. Because ADP-ribosylation specifically inhibits the biological functions of the low-molecular mass GTPases, C3 exoenzymes are established pharmacological tools to study the cellular functions of Rho GTPases. Recent studies, however, indicate that the functional consequences of C3-induced ADP-ribosylation are more complex than previously suggested. In the present review the basic properties of C3 exoenzymes are briefly summarized and new findings are reviewed.

Rac2 is critical for neutrophil primary granule exocytosis

Neutrophil degranulation is important in many inflammatory disorders, although the intracellular mechanisms underlying this process remain poorly understood. The Rho GTPase, Rac2, has been implicated in control of degranulation in earlier studies. We hypothesized that Rac2 selectively regulates neutrophil primary granule release. Using bone marrow and peritoneal exudate neutrophils from rac2(-/-) mice in comparison with similar cells from wild-type C57Bl/6 mice, we found that primary granule myeloperoxidase and elastase release was absent in Rac2(-/-) neutrophils in response to chemoattractant stimulation, cytochalasin B/f-Met-Leu-Phe (CB/fMLP), and CB/leukotriene B4. Rac2(-/-) neutrophils also failed to exhibit mobilization of the primary granule marker CD63+ during CB/fMLP stimulation as determined by confocal microscopy. Priming of Rac2(-/-) neutrophils with tumor necrosis factor (TNF) or by peritoneal elicitation did not rescue the defect in primary granule release. However, phosphorylation of p38 mitogen-activated protein (MAP) kinase in Rac2(-/-) neutrophils was evident in response to CB/fMLP and/or TNF. Primary granule density and morphology were normal in Rac2(-/-) neutrophils. Secondary specific and tertiary granule release, measured by lactoferrin immunoassay and zymography, was normal in response to CB/fMLP and adhesion to fibronectin. These findings suggest an obligatory role for Rac2 in regulation of primary granule release by neutrophils.

The Rac/Cdc42 guanine nucleotide exchange factor beta1Pix enhances mastoparan-activated Gi-dependent pathway in mast cells

Carbachol stimulates granule exocytosis, phospholipase C (PLC), and phospholipase D (PLD) in RBL-2H3hm1 mast cells by a mechanism that involves Galphaq. However, mastoparan stimulates the same responses through Gi protein. Both Gi and Galphaq pathways are suppressed by Clostridium difficile toxin B, suggesting that Rac and Cdc42 small GTPases are also involved. Over-expression of beta1Pix, a guanine nucleotide exchange factor for Rac and Cdc42, enhances mastoparan-but not carbachol-induced hexosaminidase secretion and PLC and PLD activation. Furthermore, cells expressing beta1Pix exhibit elevated levels of mastoparan-stimulated IP3 production. Taken together, these findings implicate beta1Pix in regulating hexoasaminidase secretion and IP3 production in early stage upon mastoparan stimulation.

The small GTP-binding protein RhoA regulates serotonin-induced Na+-current response in the neurons of Aplysia

Application of serotonin (5-HT) induces a slow inward current response in identified neurons of Aplysia ganglia under voltage clamp. The 5-HT-induced current response was depressed in Na+-free media, but augmented in Ca2+-free media, and unaffected by a change in external K+. The 5-HT-induced response was markedly blocked by intracellular injection of guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS). After the injection of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), the responses to 5-HT gradually and significantly increased at the initial period, reached its plateau, and finally decreased. Intracellular injection of Clostridium difficile toxin B, a blocker of small G-protein Rho family members such as Rho (RhoA, RhoB and RhoC), Rac and Cdc42, markedly depressed the 5-HT-induced response. Intracellular injection of Clostridium botulinum C3 exoenzyme, a specific blocker of RhoA, RhoB, RhoC, exhibited a similar depressing effect observed with toxin B. In contrast, intracellular injection of recombinant L63RhoA, a constitutively active form of RhoA, significantly augmented the 5-HT-induced response without affecting the resting membrane. These results suggested that the 5-HT-induced Na+-current response might be facilitated by the activation of Aplysia Rho which is closely homologous to RhoA, RhoB or RhoC in mammalian neuron.

Involvement of Rho family G protein in the cell signaling for sperm incorporation during fertilization of mouse eggs: inhibition by Clostridium difficile toxin B

Sperm-egg interaction was investigated in mouse eggs freed from the zona pellucida and injected with Clostridium difficile toxin B, the inhibitor of Rho family small G proteins. Toxin B reduced in a dose-dependent manner the percentage of eggs associated with sperm fusion on the surface or sperm nucleus decondensation in the ooplasm, examined by injection of a DNA-staining dye into the egg and transfer of the dye to the fused sperm head after recording intracellular Ca(2+) responses for 100 min postinsemination. The mean number of decondensed sperm nuclei per egg was remarkably decreased by approximately 1 microg/ml toxin B in the ooplasm. This was because spermatozoa were arrested at the fusion state without developing to sperm incorporation and tended to lose cytoplasmic continuity to the egg. The fusion-arrested spermatozoa caused transient small Ca(2+) oscillations in most of eggs, while an injected spermatozoon produced repetitive large Ca(2+) spikes unaffected by toxin B. A decrease in the rate of fused spermatozoa and decondensed sperm nuclei was also caused by 20-40 microM cytochalasin D, the inhibitor of actin polymerization. Immunostaining of Rho proteins showed that Rac1 and RhoB are present in the cortical ooplasm, but Cdc42 is absent. Actin filaments in the cortex appeared to be reduced in toxin B-injected eggs. This study suggests that Rho protein(s) regulating actin-based cytoskeletal reorganization is involved in the process leading to sperm incorporation.

Facilitation of Ca(2+)-dependent exocytosis by Rac1-GTPase in bovine chromaffin cells

Rho family GTPases are primary mediators of cytoskeletal reorganization, although they have also been reported to regulate cell secretion. Yet, the extent to which Rho family GTPases are activated by secretory stimuli in neural and neuroendocrine cells remains unknown. In bovine adrenal chromaffin cells, we found Rac1, but not Cdc42, to be rapidly and selectively activated by secretory stimuli using an assay selective for the activated GTPases. To examine effects of activated Rac1 on secretion, constitutively active mutants of Rac1 (Rac1-V12, Rac1-L61) were transiently expressed in adrenal chromaffin cells. These mutants facilitated secretory responses elicited from populations of intact and digitonin-permeabilized cells as well as from cells under whole cell patch clamp. A dominant negative Rac1 mutant (Rac1-N17) produced no effect on secretion. Expression of RhoGDI, a negative regulator of Rac1, inhibited secretory responses while overexpression of effectors of Rac1, notably, p21-activated kinase (Pak1) and actin depolymerization factor (ADF) promoted evoked secretion. In addition, expression of effector domain mutants of Rac1-V12 that exhibit reduced activation of the cytoskeletal regulators Pak1 and Partner of Rac1 (POR1) resulted in a loss of Rac1-V12-mediated enhancement of evoked secretion. These findings suggest that Rac1, in part, functions to modulate secretion through actions on the cytoskeleton. Consistent with this hypothesis, the actin modifying drugs phalloidin and jasplakinolide enhanced secretion, while latrunculin-A inhibited secretion and eliminated the secretory effects of Rac1-V12. In summary, Rac1 was activated by secretory stimuli and modulated the secretory pathway downstream of Ca2+ influx, partly through regulation of cytoskeletal organization.

Bacterial toxins that modify the actin cytoskeleton

Bacterial pathogens utilize several strategies to modulate the organization of the actin cytoskeleton. Some bacterial toxins catalyze the covalent modification of actin or the Rho GTPases, which are involved in the control of the actin cytoskeleton. Other bacteria produce toxins that act as guanine nucleotide exchange factors or GTPase-activating proteins to modulate the nucleotide state of the Rho GTPases. This latter group of toxins provides a temporal modulation of the actin cytoskeleton. A third group of bacterial toxins act as adenylate cyclases, which directly elevate intracellular cAMP to supra-physiological levels. Each class of toxins gives the bacterial pathogen a selective advantage in modulating host cell resistance to infection.

Effects of large clostridial cytotoxins on activation of RBL 2H3-hm1 mast cells indicate common and different roles of Rac in FcepsilonRI and M1-receptor signaling

Using Rho GTPases-inhibiting clostridial cytotoxins, we showed recently in RBL cells that the GTPase Rac is involved in FcepsilonRI (high-affinity receptor for IgE) signaling and receptor-mediated calcium mobilization, including influx via calcium release-activated calcium channels. Here, we studied the role of Rho GTPases in muscarinic M1 receptor signaling in RBL 2H3-hm1 cells. Clostridium difficile toxin B, which inactivates Rho, Rac, and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac but not Rho, blocked M1-mediated exocytosis, indicating that Rac but not Rho is involved in the regulation of receptor-mediated exocytosis. Although antigen-induced FcepsilonRI stimulation caused tyrosine phosphorylation of the Rac guanine nucleotide exchange factor Vav, M1 stimulation by carbachol activated Rac independently of Vav. The Rac-inactivating toxins blocked M1 receptor-induced membrane translocation of the pleckstrin homology domain of protein kinase B, which is a phosphoinositide 3-kinase effector. The M1-induced calcium release from internal stores was not affected by toxin B; however, the subsequent calcium influx from the extracellular space was inhibited. The data suggest that besides capacitative calcium entry, the M1 signaling pathway activates further calcium entry channels with mechanisms that are not affected by the inhibition of Rac.

[Mechanism of production and release of tumor necrosis factor implicated in inflammatory diseases]

Tumor necrosis factor (TNF) is a potent inflammatory cytokine involved in many pathophysiological conditions including rheumatoid arthritis and Crohn's disease. Despite recent evidence regarding signal transduction via TNF receptor and its biological actions, the mechanism of TNF release remains poorly understood. To clarify how production and release of TNF are regulated, we focused on mast cells and microglia which are involved in allergic inflammation and brain damage or recovery, respectively. In RBL-2H3 mast cells, anti-allergic drugs including azelastine inhibited the release of TNF more potently than degranulation in response to antigen or ionomycin. It was also demonstrated that TNF releasing steps are regulated via the PKC alpha-dependent pathway. Furthermore, Rho GTPases, possibly Rac, were shown to be involved in antigen-induced TNF transcription through activating PKC beta I. In cultured rat brain microglia, we found that extracellular ATP triggers the release of TNF via the P2X7 receptor. ERK and JNK are also involved in ATP-induced TNF transcription, while p38 regulates the transport of TNF mRNA from the nucleus to the cytosol. Additionally, JNK and p38, but not ERK, are activated via the P2X7 receptor. A better understanding of the specific pathways that regulate TNF release for each effector cell may offer further possible therapeutic targets for inflammatory diseases.

Differential actions of PAR2 and PAR1 in stimulating human endothelial cell exocytosis and permeability: the role of Rho-GTPases

Endothelial cell proteinase activated receptors (PARs) belong to a family of heterotrimeric G protein-coupled receptors that are implicated in leukocyte accumulation and potentiation of reperfusion injury. We characterized the effect and the signal transduction pathways recruited after stimulation of endothelial PAR2. We used von Willebrand Factor (vWF) release and monolayer permeability to peroxidase to report Weibel-Palade body (WPB) exocytosis and pore formation, respectively. Human umbilical vein endothelial cells (HUVECs) were stimulated with the selective PAR2 agonist peptide SLIGRL-NH2 or PAR1 agonist peptide TFLLR-NH2. PAR2 stimulation resulted in WPB exocytosis like PAR1 stimulation but, unlike PAR1, failed to increase monolayer permeability. BAPTA-AM inhibited PAR2-induced exocytosis, indicating a PAR2 calcium-dependent signal in ECs. Moreover, PAR2-like PAR1-stimulated exocytosis requires actin cytoskeleton remodeling, because vWF release is inhibited if the cells were pretreated with Jasplakinolide. Rho-GTPase activity is required for PAR-stimulated exocytosis, because inactivation of this family of actin-regulatory proteins with Clostridium difficile toxin B blocked exocytosis. Expression of dominant-negative mutant Cdc42(17N) inhibited exocytosis whereas neither dominant-negative Rac(17N) expression nor C3 exotoxin treatment affected vWF release. PAR2 stimulated RhoA-GTP weakly compared with the PAR1 agonist. We conclude that both PAR2 and PAR1 elicit WP body exocytosis in a calcium and Cdc42 GTPase-dependent manner. In contrast, the differential effect of PAR1 versus PAR2 activation to increase monolayer permeability correlates with weak RhoA activation by the PAR2 agonist.

Rac GTPase plays an essential role in exocytosis by controlling the fusion competence of release sites

The role of small GTPases of the Rho family in synaptic functions has been addressed by analyzing the effects of lethal toxin (LT) from Clostridium sordellii strain IP82 (LT82) on neurotransmitter release at evoked identified synapses in the buccal ganglion of Aplysia. LT82 is a large monoglucosyltranferase that uses UDP-glucose as cofactor and glucosylates Rac (a small GTPase related to Rho), and Ras, Ral, and Rap (three GTPases of the Ras family). Intraneuronal application of LT (50 nm) rapidly inhibits evoked acetylcholine (ACh) release as monitored electrophysiologically. Injection of the catalytic domain of the toxin similarly blocked ACh release, but not when key amino acids needed for glucosylation were mutated. Intraneuronal application of competitive nucleotide sugars that differentially prevent glucosylation of Rac- and Ras-related GTPases, and the use of a toxin variant that affects a different spectrum of small GTPases, established that glucosylation of Rac is responsible for the reduction in ACh release. To determine the quantal release parameters affected by Rac glucosylation, we developed a nonstationary analysis of the fluctuations in postsynaptic response amplitudes that was performed before and after the toxin had acted or during toxin action. The results indicate that neither the quantal size nor the average probability for release were affected by lethal toxin action. ACh release blockage by LT82 was only caused by a reduction in the number of functional release sites. This reveals that after docking of synaptic vesicles, vesicular Rac stimulates a membrane effector (or effectors) essential for the fusion competence of the exocytotic sites.

Activation of Rac, Cdc42 and other downstream signalling molecules by Bartonella bacilliformis during entry into human endothelial cells

Bartonella bacilliformis is an intracellular bacterial pathogen of human endothelial cells. In vitro incubation of B. bacilliformis with human endothelial cells leads to the formation of filamentous actin extensions (filopodia) within 30 min, followed by formation of membrane rufflings or lamellipodia within 1 h of incubation. By immunofluorescence, F-actin phalloidin staining and anti-Rac antibodies were shown to co-localize in the membrane rufflings, indicating the recruitment of activated Rac at lamellipodia. Preincubation of endothelial cells with the Clostridial toxin, TcdB-10463, which inactivates the Rho-family GTPases, Rho, Rac and Cdc42, inhibited the entry of B. bacilliformis by 50-90%. Preincubation of endothelial cells with the Clostridial toxin, TcsL-1522, which specifically inactivates Rac and, to a lesser extent, Cdc42, but not Rho, inhibited entry by 30-40%. A 3.4-5.0-fold increase in activated (GTP-bound) -intracellular Rac and Cdc42 was observed in affinity precipitation assays. Increased kinase activity of p21-activated kinase (PAK), a specific downstream effector of activated Rac/Cdc42 was also observed during the time course of infection. Activation of SAPK/JNK-1 and 2, and p38 MAPKs in signalling pathways, was also detected during infection with Bartonella, as was increased binding activity of AP-1 transcription factor.

Interactions of the Trichoderma reesei rho3 with the secretory pathway in yeast and T. reesei

We recently isolated from the filamentous fungus Trichoderma reesei (Hypocrea jecorina) a gene encoding RHOIII as a multicopy suppressor of the yeast temperature-sensitive secretory mutation, sec15-1. To characterize this gene further, we tested its ability to suppress other late-acting secretory mutations. The growth defect of yeast strains with sec1-1, sec1-11, sec3-2, sec6-4 and sec8-9 mutations was suppressed. Expression of rho3 also improved the impaired actin organization of sec15-1 cells at +38 degrees C. Overproduction of yeast Rho3p using the same expression vector as T. reesei RHOIII appeared to be toxic in sec3-101, sec5-24, sec8-9, sec10-2 and sec15-1 cells. When expressed from the GAL1 promoter, RHO3 suppressed the growth defect of sec1 at the restrictive temperature and inhibited the growth of sec3-101 at the permissive temperature. Disruption of the rho3 gene in the T. reesei genome did not affect the hyphal or colony morphology nor the cellular cytoskeleton organization. Furthermore, the growth of T. reesei was not affected on glucose by the rho3 disruption. Instead, both growth and protein secretion of T. reesei in cellulose cultures was remarkably decreased in rho3 disruptant strains when compared with the parental strain. These results suggest that rho3 is involved in secretion processes in T. reesei.

Phospholipases stimulate secretion in RBL mast cells

Roles for glycerophospholipids in exocytosis have been proposed, but remain controversial. Phospholipases are stimulated following the activation of the high-affinity receptor for immunoglobulin E (IgE) in mast cells. To study the biochemical sequelae that lead to degranulation, broken cell systems were employed. We demonstrate that the addition of three distinct types of exogenous phospholipases (i.e., bcPLC, scPLD, and tfPLA(2)), all of which hydrolyze phosphatidylcholine (PC), trigger degranulation in permeabilized RBL-2H3 cells, a mucosal mast cell line. Production of bioactive lipids by these phospholipases promotes release of granule contents through the plasma membrane and acts downstream of PKC, PIP(2), and Rho subfamily GTPases in regulated secretion. These exogenous phospholipase-induced degranulation pathways circumvent specific factors activated following stimulation of the IgE receptor as well as in ATP- and GTP-dependent intracellular pathways. Taken together, these results suggest that regulated secretion may be achieved in vitro in the absence of cytosolic factors via phospholipase activation and that products of PC hydrolysis can promote exocytosis in mast cells.

Delivery of proteins into living cells by reversible membrane permeabilization with streptolysin-O

The pore-forming toxin streptolysin O (SLO) can be used to reversibly permeabilize adherent and nonadherent cells, allowing delivery of molecules with up to 100 kDa mass to the cytosol. Using FITC-labeled albumin, 10(5)-10(6) molecules were estimated to be entrapped per cell. Repair of toxin lesions depended on Ca(2+)-calmodulin and on intact microtubules, but was not sensitive to actin disruption or to inhibition of protein synthesis. Resealed cells were viable for days and retained the capacity to endocytose and to proliferate. The active domains of large clostridial toxins were introduced into three different cell lines. The domains were derived from Clostridium difficile B-toxin and Clostridium sordelli lethal toxin, which glycosylate small G-proteins, and from Clostridium botulinum C2 toxin, which ADP-ribosylates actin. After delivery with SLO, all three toxins disrupted the actin cytoskeleton to cause rounding up of the cells. Glucosylation assays demonstrated that G-proteins Rho and Ras were retained in the permeabilized cells and were modified by the respective toxins. Inactivation of these G-proteins resulted in reduced stimulus-dependent granule secretion, whereas ADP-ribosylation of actin by the C. botulinum C2-toxin resulted in enhanced secretion in cells. The presented method for introducing proteins into living cells should find multifaceted application in cell biology.

Rac and phosphatidylinositol 3-kinase regulate the protein kinase B in Fc epsilon RI signaling in RBL 2H3 mast cells

FcepsilonRI signaling in rat basophilic leukemia cells depends on phosphatidylinositol 3-kinase (PI3-kinase) and the small GTPase Rac. Here, we studied the functional relationship among PI3-kinase, its effector protein kinase B (PKB), and Rac using inhibitors of PI3-kinase and toxins inhibiting Rac. Wortmannin, an inhibitor of PI3-kinase, blocked FcepsilonRI-mediated tyrosine phosphorylation of phospholipase Cgamma, inositol phosphate formation, calcium mobilization, and secretion of hexosaminidase. Similarly, Clostridium difficile toxin B, which inactivates all Rho GTPases including Rho, Rac and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac (possibly Cdc42) but not Rho, blocked these responses. Stimulation of the FcepsilonRI receptor induced a rapid increase in the GTP-bound form of Rac. Whereas toxin B inhibited the Rac activation, PI3-kinase inhibitors (wortmannin and LY294002) had no effect on activation of Rac. In line with this, wortmannin had no effect on tyrosine phosphorylation of the guanine nucleotide exchange factor Vav. Wortmannin, toxin B, and lethal toxin inhibited phosphorylation of PKB on Ser(473). Similarly, translocation of the pleckstrin homology domain of PKB tagged with the green fluorescent protein to the membrane, which was induced by activation of the FcepsilonRI receptor, was blocked by inhibitors of PI3-kinase and Rac inactivation. Our results indicate that in rat basophilic leukemia cells Rac and PI3-kinase regulate PKB and suggest that Rac is functionally located upstream and/or parallel of PI3-kinase/PKB in FcepsilonRI signaling.

Effects of latrunculin reveal requirements for the actin cytoskeleton during secretion from mast cells

To investigate the role of the actin cytoskeleton in exocytosis, we have tested the effects of latrunculin B, a microfilament-disrupting drug, on secretion from intact and permeabilised rat peritoneal mast cells. The toxin strongly inhibited secretion from intact cells (attached or in suspension) responding to a polybasic agonist, compound 48/80. However, this effect was revealed only after a profound depletion of actin filaments. This was achieved by a long (1 h) exposure of cells to the drug before activation, together with its presence during activation. Maximal inhibition of secretion by such treatment was 85% at 40 microgram/ml latrunculin B. These results indicate that minimal actin structures are essential for the exocytotic response. In contrast, stimulus-induced cell spreading was prevented by latrunculin (5 microgram/ml) applied either before or after activation. The effects of the toxin on intact cells were fully reversible. The responses of permeabilised cells were affected differentially: secretion induced by calcium was more sensitive to latrunculin than that induced by GTP-gamma-S. The calcium response, therefore, is more dependent upon the integrity of the actin cytoskeleton than the response induced by GTP-gamma-S. Again, maximal inhibitory effects (approximately 65 and 25% at 40 microgram/ml) were observed only when cells were exposed to the toxin both before and after permeabilisation. Since the permeabilised cells system focuses on the final steps of exocytosis, the incomplete inhibition suggests that actin plays a modulatory rather than a central role at this stage.

Microbial toxins and the glycosylation of rho family GTPases

Large clostridial cytotoxins act on cells by glycosylating low molecular mass GTPases using nucleotide-sugars as the sugar donor. These toxins are important virulence factors in human and animal diseases, but are also valuable cell biology tools. Recent findings shed some light on their mode of action and provide new insights into the structure/activity relationship of these bacterial toxins.

Inhibition of calcium release-activated calcium current by Rac/Cdc42-inactivating clostridial cytotoxins in RBL cells

Using large clostridial cytotoxins as tools, the role of Rho GTPases in activation of RBL 2H3 hm1 cells was studied. Clostridium difficile toxin B, which glucosylates Rho, Rac, and Cdc42 and Clostridium sordellii lethal toxin, which glucosylates Rac and Cdc42 but not Rho, inhibited the release of hexosaminidase from RBL cells mediated by the high affinity antigen receptor (FcepsilonRI). Additionally, toxin B and lethal toxin inhibited the intracellular Ca(2+) mobilization induced by FcepsilonRI-stimulation and thapsigargin, mainly by reducing the influx of extracellular Ca(2+). In patch clamp recordings, toxin B and lethal toxin inhibited the calcium release-activated calcium current by about 45%. Calcium release-activated calcium current, the receptor-stimulated Ca(2+) influx, and secretion were inhibited neither by the Rho-ADP-ribosylating C3-fusion toxin C2IN-C3 nor by the actin-ADP-ribosylating Clostridium botulinum C2 toxin. The data indicate that Rac and Cdc42 but not Rho are not only involved in late exocytosis events but are also involved in Ca(2+) mobilization most likely by regulating the Ca(2+) influx through calcium release-activated calcium channels activated via FcepsilonRI receptor in RBL cells.

Rho GTPases as targets of bacterial protein toxins

Several bacterial toxins target Rho GTPases, which constitute molecular switches in several signaling processes and master regulators of the actin cytoskeleton. The biological activities of Rho GTPases are blocked by C3-like transferases, which ADP-ribosylate Rho at Asn41, but not Rac or Cdc42. Large clostridial cytotoxins (e. g., Clostridium difficile toxin A and B) glucosylate Rho GTPases at Thr37 (Rho) or Thr35 (Rac/Cdc42), thereby inhibiting Rho functions by preventing effector coupling. The 'injected' toxins ExoS, YopE and SptP from Pseudomonas aeruginosa, Yersinia and Salmonella ssp., respectively, which are transferred into the eukaryotic target cells by the type-III secretion system, inhibit Rho functions by acting as Rho GAP proteins. Rho GTPases are activated by the cytotoxic necrotizing factors CNF1 and CNF2 from Escherichia coli and by the dermonecrotizing toxin DNT from B. bronchiseptica. These toxins deamidate/transglutaminate Gln63 of Rho to block the intrinsic and GAP-stimulated GTP hydrolysis, thereby constitutively activating the GTPases. Rho GTPases are also activated by SopE, a type-III system injected protein from Salmonella ssp., that acts as a GEF protein.

p38 MAP kinase activation by Clostridium difficile toxin A mediates monocyte necrosis, IL-8 production, and enteritis

Clostridium difficile toxin A causes acute neutrophil infiltration and intestinal mucosal injury. In cultured cells, toxin A inactivates Rho proteins by monoglucosylation. In monocytes, toxin A induces IL-8 production and necrosis by unknown mechanisms. We investigated the role of mitogen-activated protein (MAP) kinases in these events. In THP-1 monocytic cells, toxin A activated the 3 main MAP kinase cascades within 1 to 2 minutes. Activation of p38 was sustained, whereas stimulation of extracellular signal-regulated kinases and c-Jun NH(2)-terminal kinase was transient. Rho glucosylation became evident after 15 minutes. IL-8 gene expression was reduced by 70% by the MEK inhibitor PD98059 and abrogated by the p38 inhibitor SB203580 or by overexpression of dominant-negative mutants of the p38-activating kinases MKK3 and MKK6. SB203580 also blocked monocyte necrosis and IL-1beta release caused by toxin A but not by other toxins. Finally, in mouse ileum, SB203580 prevented toxin A-induced neutrophil recruitment by 92% and villous destruction by 90%. Thus, in monocytes exposed to toxin A, MAP kinase activation appears to precede Rho glucosylation and is required for IL-8 transcription and cell necrosis. p38 MAP kinase also mediates intestinal inflammation and mucosal damage induced by toxin A.

The actin cytoskeleton and neurotransmitter release: an overview

Here we review evidence that actin and its binding partners are involved in the release of neurotransmitters at synapses. The spatial and temporal characteristics of neurotransmitter release are determined by the distribution of synaptic vesicles at the active zones, presynaptic sites of secretion. Synaptic vesicles accumulate near active zones in a readily releasable pool that is docked at the plasma membrane and ready to fuse in response to calcium entry and a secondary, reserve pool that is in the interior of the presynaptic terminal. A network of actin filaments associated with synaptic vesicles might play an important role in maintaining synaptic vesicles within the reserve pool. Actin and myosin also have been implicated in the translocation of vesicles from the reserve pool to the presynaptic plasma membrane. Refilling of the readily releasable vesicle pool during intense stimulation of neurotransmitter release also implicates synapsins as reversible links between synaptic vesicles and actin filaments. The diversity of actin binding partners in nerve terminals suggests that actin might have presynaptic functions beyond synaptic vesicle tethering or movement. Because most of these actin-binding proteins are regulated by calcium, actin might be a pivotal participant in calcium signaling inside presynaptic nerve terminals. However, there is no evidence that actin participates in fusion of synaptic vesicles.

A Rho-related GTPase is involved in Ca(2+)-dependent neurotransmitter exocytosis

Rho, Rac, and Cdc42 monomeric GTPases are well known regulators of the actin cytoskeleton and phosphoinositide metabolism and have been implicated in hormone secretion in endocrine cells. Here, we examine their possible implication in Ca(2+)-dependent exocytosis of neurotransmitters. Using subcellular fractionation procedures, we found that RhoA, RhoB, Rac1, and Cdc42 are present in rat brain synaptosomes; however, only Rac1 was associated with highly purified synaptic vesicles. To determine the synaptic function of these GTPases, toxins that impair Rho-related proteins were microinjected into Aplysia neurons. We used lethal toxin from Clostridium sordellii, which inactivates Rac; toxin B from Clostridium difficile, which inactivates Rho, Rac, and Cdc42; and C3 exoenzyme from Clostridium botulinum and cytotoxic necrotizing factor 1 from Escherichia coli, which mainly affect Rho. Analysis of the toxin effects on evoked acetylcholine release revealed that a member of the Rho family, most likely Rac1, was implicated in the control of neurotransmitter release. Strikingly, blockage of acetylcholine release by lethal toxin and toxin B could be completely removed in <1 s by high frequency stimulation of nerve terminals. Further characterization of the inhibitory action produced by lethal toxin suggests that Rac1 protein regulates a late step in Ca(2+)-dependent neuroexocytosis.

Interactions between Fc(epsilon)RI and lipid raft components are regulated by the actin cytoskeleton

Previous studies showed that crosslinking of IgE-Fc(epsilon)RI complexes on RBL-2H3 mast cells causes their association with isolated detergent-resistant membranes, also known as lipid rafts, in a cholesterol-dependent process that precedes initiation of signaling by these receptors. To investigate these interactions on intact cells, we examined the co-redistribution of raft components with crosslinked IgE-Fc(epsilon)RI using confocal microscopy. After several hours of crosslinking at 4 degrees C, the glycosylphosphatidylinositol-linked protein Thy-1 and the Src-family tyrosine kinase Lyn co-redistribute with IgE-Fc(epsilon)RI in large patches at the plasma membrane. Under these conditions, F-actin also undergoes dramatic co-segregation with Fc(epsilon)RI and raft components but is dispersed following a brief warm-up to 37 degrees C. When crosslinking of IgE-Fc(epsilon)RI is initiated at higher temperatures, co-redistribution of raft components with patched Fc(epsilon)RI is not readily detected unless stimulated F-actin polymerization is inhibited by cytochalasin D. In parallel, cytochalasin D converts transient antigen-stimulated tyrosine phosphorylation to a more sustained response. Sucrose gradient analysis of lysed cells reveals that crosslinked IgE-Fc(epsilon)RI remains associated with lipid rafts throughout the time course of the transient phosphorylation response but undergoes a time-dependent shift to higher density that is prevented by cytochalasin D. Our results indicate that interactions between Lyn and crosslinked IgE-Fc(epsilon)RI are regulated by stimulated F-actin polymerization, and this is best explained by a segregation of anchored raft components from more mobile ones.

Bacterial toxins inhibiting or activating small GTP-binding proteins

Amino acids located on the switch 1 or switch 2 domains of small GTPases of the Ras and Rho family are targets of several bacterial toxins. Exoenzyme C3 from Clostridium botulinum ADP-ribosylates specifically Rho at R43 and prevents the recruitment of Rho on the cell membrane. This blocks the downstream effects of the Rho GTPase. However, exoenzyme C3 is not a toxin, and chimeric proteins fusing C3 with the B moiety of either diphtheria toxin or Pseudomonas aeruginosa exotoxin A have been produced to intoxicate cells with low concentration of C3. C. difficile toxin B modifies by glucosylation Rho on T37 and Rac and Cdc42 on T35. Glucosylation of Rho, Rac, and Cdc42 blocks the binding of these GTPases on their downstream effectors. C. sordellii lethal toxin modifies Ras, Rap, and Rac on T35 by glucosylation. Cytotoxic necrotizing factor 1 (CNF1), from uropathogenic Escherichia coli strains, deamidates Q63 of Rho into E63, thereby blocking the intrinsic or GAP-mediated GTPase of Rho. This allows permanent activation of Rho. Thus, Rho GTPases are targets for three different toxin activities. Molecular mechanisms of these toxins are discussed.