G-protein-stimulated phospholipase D activity is inhibited by lethal toxin from Clostridium sordellii in HL-60 cells

Clostridium sordellii Lethal-Toxin Autoprocessing and Membrane Localization Activities Drive GTPase Glucosylation Profiles in Endothelial Cells

Clostridium sordellii is a bacterium that can infect humans and cause serious disease and death. The principle virulence factor associated with clinical symptoms is a large protein toxin known as lethal toxin. The mechanism of lethal-toxin intoxication is assumed to be similar to that of the homologous toxins from C. difficile, but very few studies have been done in the context of endothelial cells, a relevant target in C. sordellii infections. This study was designed to test the role of the lethal-toxin enzymatic activities and membrane localization in endothelial cell toxicity and host substrate modification.

Clostridium sordellii infections cause gangrene and edema in humans and gastrointestinal infections in livestock. One of the principle virulence factors is TcsL, a large protein toxin which glucosylates host GTPases to cause cytopathic and cytotoxic effects. TcsL has two enzymatic domains, an N-terminal glucosyltransferase domain (GTD) and an autoprocessing domain responsible for release of the GTD within the cell. The GTD can then use its N-terminal membrane localization domain (MLD) for orientation on membranes and modification of GTPases. This study describes the use of conditionally immortalized murine pulmonary microvascular endothelial cells as a model for the study of TcsL functional activities. Point mutations that disrupt the glucosyltransferase, autoprocessing, or membrane localization activities were introduced into a recombinant version of TcsL, and the activities of these mutants were compared to those of wild-type toxin. We observed that all mutants are defective or impaired in cytotoxicity but differ in their modification of Rac1 and Ras. The data suggest a model where differences in GTPase localization dictate cellular responses to intoxication and highlight the importance of autoprocessing in the function of TcsL.

IMPORTANCEClostridium sordellii is a bacterium that can infect humans and cause serious disease and death. The principle virulence factor associated with clinical symptoms is a large protein toxin known as lethal toxin. The mechanism of lethal-toxin intoxication is assumed to be similar to that of the homologous toxins from C. difficile, but very few studies have been done in the context of endothelial cells, a relevant target in C. sordellii infections. This study was designed to test the role of the lethal-toxin enzymatic activities and membrane localization in endothelial cell toxicity and host substrate modification.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

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.

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

Rac1 inactivation by lethal toxin from Clostridium sordellii modifies focal adhesions upstream of actin depolymerization

Inactivation of different small GTPases upon their glucosylation by lethal toxin from Clostridium sordellii strain IP82 (LT-82) is already known to lead to cell rounding, adherens junction (AJ) disorganization and actin depolymerization. In the present work, we observed that LT-82 induces a rapid dephosphorylation of paxillin, a protein regulating focal adhesion (FA), independently of inactivation of paxillin kinases such as Src, Fak and Pyk2. Among the small GTPases inactivated by this toxin, including Rac, Ras, Rap and Ral, we identified Rac1, as responsible for paxillin dephosphorylation using cells overexpressing Rac1(V12). Rac1 inactivation by LT-82 modifies interactions between proteins from AJ and FA complexes as shown by pull-down assays. We showed that in Triton X-100-insoluble membrane proteins from these complexes, namely E-cadherin, beta-catenin, p120-catenin and talin, are decreased upon LT-82 intoxication, a treatment that also induces a rapid decrease in cell phosphoinositide content. Therefore, we proposed that Rac inactivation by LT-82 alters phosphoinositide metabolism leading to FA and AJ complex disorganization and actin depolymerization.

Fast changes in the functional status of release sites during short-term plasticity: involvement of a frequency-dependent bypass of Rac at Aplysia synapses

Synaptic transmission can be described as a stochastic quantal process defined by three main parameters: N, the number of functional release sites; P, the release probability; and Q, the quantum of response. Many changes in synaptic strength that are observed during expression of short term plasticity rely on modifications in P. Regulation of N has been also suggested. We have investigated at identified cholinergic inhibitory Aplysia synapses the cellular mechanism of post-tetanic potentiation (PTP) expressed under control conditions or after N has been depressed by applying lethal toxin (LT) from Clostridium sordellii or tetanus toxin (TeNT). The analysis of the Ca(2+) dependency, paired-pulse ratio and variance to mean amplitude relationship of the postsynaptic responses elicited at distinct extracellular [Ca(2+)]/[Mg(2+)] elicited during control post-tetanic potentiation (PTP(cont)) indicated that PTP(cont) is mainly driven by an increase in release probability, P. The PTP expressed at TeNT-treated synapses (PTP(TeNT)) was found to be similar to PTP(cont), but scaled to the extent of reduction in N produced by TeNT. Despite LT inducing a decrease in N as TeNT does, the PTP expressed at LT-treated synapses (PTP(LT)) was characterized by exceptionally large amplitude and bi-exponential time course, as compared to PTP(cont) or the PTP(TeNT). Analysis of the Ca(2+) dependency of PTP(LT), paired-pulse ratio and fluctuations in amplitude of the postsynaptic responses elicited during PTP(LT) or the variance to mean amplitude relationship of time-locked postsynaptic responses in a series of subsequent PTP(LT) indicated that an N-driven change is involved in the early phase (1 s time scale) of PTP(LT), while at a later stage PTP(LT) is composed of both N and P increases. Our results suggest that fast switching on of the functional status of the release sites occurs also during the early events of PTP(cont). The early N-driven phase of PTP(LT) is likely to be a functional recovery of the release sites silenced by Rac inactivation. This effect did not appear to result from reversion of LT inhibitory action but from bypassing the step regulated by Rac. Altogether the data suggest that Rac and the intracellular pathway which allows the bypassing of Rac are key players in new forms of short-term plasticity that rely on fast, activity-dependent changes in the functional status of the release sites.

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.

Degeneration and regeneration of murine skeletal neuromuscular junctions after intramuscular injection with a sublethal dose of Clostridium sordellii lethal toxin

Clostridium sordellii lethal toxin (LT), a 250-kDa protein which is the bacteria's major virulence factor, belongs to a family of large clostridial cytotoxins which glucosylate small GTP-binding proteins. Here, we report the results of our ex vivo analysis of the structure and function of skeletal neuromuscular tissue obtained from mice at various times after intramuscular injection of a sublethal dose of LT (0.25 ng/g of body wt). The toxin caused, within 24 h, pronounced localized edema, inflammation, myofibril disassembly, and degeneration of skeletal muscle fibers in the injected area, and it glucosylated the muscle tissue's small GTPases. Regeneration of the damaged fibers was evident 6 to 9 days postinjury and was completed by 60 days. The expression of dystrophin, laminin, and fast and neonatal myosin in regenerating fibers, detected by immunofluorescence microscopy, confirmed that LT does not impair the high regenerative capacity of murine skeletal muscle fibers. Functional studies revealed that LT affects muscle contractility and neuromuscular transmission. However, partial recovery of nerve-evoked muscle twitches and tetanic contractions was observed by day 15 postinjection, and extensive remodeling of the neuromuscular junction's nerve terminals and clusters of muscle acetylcholine receptors was still evident 30 days postinjection. In conclusion, to the best of our knowledge, this is the first report to characterize the degeneration and regeneration of skeletal neuromuscular tissue after in vivo exposure to a large clostridial cytotoxin. In addition, our data may provide an explanation for the severe neuromuscular alterations accompanying wound infections caused by C. sordellii.

Rac GTPase instructs nuclear factor-kappaB activation by conveying the SCF complex and IkBalpha to the ruffling membranes

Nuclear factor-kappaB (NF-kappaB) is a ubiquitously expressed transcription factor that plays a central role in directing a vast range of cellular functions. Its activation is controlled by the Rac GTPase and relies on the coordinated cooperation of the E3-ligase complex SCF(betaTrCP), composed by Skp-1/Cullin-1, Rbx/Roc1, and the beta-TrCP proteins. Recently, Cullin-1 has been reported to form a complex with the activated Rac GTPase. Here, we show that the specific activation of the Rac GTPase, besides directing its own positioning, induces the relocalization of the SCF component Cullin-1 to the ruffling membranes. This occurred only if the ruffles were stimulated by the Rac GTPase and was accompanied by the repositioning to the same intracellular compartment of the SCF protein Skp-1 and the ubiquitin-like molecule Nedd-8. The SCF substrate IkBalpha was also directed to the ruffling membranes in a Rac-dependent way. The novelty of these findings is in respect to the demonstration that the correct positioning at the ruffling membranes is crucial for the subsequent series of events that leads to IkBalpha proteasomal degradation and the resultant activation of NF-kappaB. Consequently, this points to the role of Rac as a docking molecule in NF-kappaB activation.

Rho family GTPases are required for activation of Jak/STAT signaling by G protein-coupled receptors

As do cytokine receptors and receptor tyrosine kinases, G protein-coupled receptors (GPCRs) signal to Janus kinases (Jaks) and signal transducers and activators of transcription (STATs). However, the early biochemical events linking GPCRs to this signaling pathway have been unclear. Here we show that GPCR-stimulated Rac activity and the subsequent generation of reactive oxygen species are necessary for activating tyrosine phosphorylation of Jaks and STAT-dependent transcription. The requirement for Rac activity can be overcome by addition of hydrogen peroxide. Expression of activated mutants of Rac1 is sufficient to activate Jak2 and STAT-dependent transcription, and the activation of Jak2 correlates with the ability of Rac1 to bind to NADPH oxidase subunit p67(phox). We further show that GPCR agonists stimulate tyrosine phosphorylation of STAT1 and STAT3 proteins in a Rac-dependent manner. The tyrosine phosphorylation of STAT3 is biphasic; the first peak of phosphorylation is weak and correlates with rapid activation of Jaks by GPCRs, whereas the second peak is stronger and requires the synthesis of an autocrine factor. Rho also plays an essential role in the induction of STAT transcriptional activity. Our results highlight a novel role for Rho GTPases in mediating the regulatory effects of GPCRs on STAT-dependent gene expression.

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.

Actin machinery of phagocytic cells: universal target for bacterial attack

Uptake of microorganisms by eukaryotic cells depends on proper functioning of the actin machinery. It creates a driving force for the cell membrane deformations necessary for ingestion and killing of microbes by phagocytes. Therefore, specific alterations in the activity of the actin apparatus could be favorable for pathogenic bacteria, representing an efficient mechanism in their virulence. Such alterations are supposed to be achieved in two principle ways. One is accomplished via binding of bacterial ligands to certain surface receptors, which initiate subsequent actin cytoskeleton rearrangements. Another is to introduce cytoskeleton-targeted products directly into eukaryotic cells and in this way modulate the activity of the actin apparatus. Indeed, Legionella and some other intracellular parasites possess ligands able to stimulate certain receptors on the surface of phagocytes and possess devices suitable for translocation of effector molecules into eukaryotic cytoplasm. The results of such events could be increased uptake of these microbes and their subsequent transportation to permit multiplication in their intracellular niche. On the contrary, representatives of Clostridium and a number of other extracellular pathogens create products which penetrate eukaryotic cells and disorganize the actin cytoskeleton network, thus making uptake of these pathogens by phagocytes impossible.

Positive modulation by Ras of interleukin-1beta-mediated nitric oxide generation in insulin-secreting clonal beta (HIT-T15) cells

In the present study, we have shown that exposure of insulin-secreting clonal beta (HIT-T15) cells to interleukin-1beta (IL-1beta) results in a time- and concentration-dependent increase in nitric oxide (NO) release. These effects by IL-1beta on NO release were mediated by induction of inducible nitric oxide synthase (iNOS) from the cells. Preincubation of HIT cells with Clostridium sordellii lethal toxin-82, which irreversibly glucosylates and inactivates small G-proteins, such as Ras, Rap, Ral, and Rac, but not Cdc42, completely abolished IL-1beta-induced NO release. Pre-exposure of HIT cells to C. sordellii lethal toxin-9048, which monoglucosylates and inhibits Ras, Cdc42, Rac, and Rap, but not Ral, also attenuated IL-1beta-mediated NO release. These data indicate that activation of Ras and/or Rac may be necessary for IL-1beta-mediated NO release. Preincubation of HIT cells with C. difficile toxin-B, which monoglucosylates Rac, Cdc42, and Rho, had no demonstrable effects on IL-mediated NO release, ruling out the possibility that Rac may be involved in this signaling step. Further, two structurally dissimilar inhibitors of Ras function, namely manumycin A and damnacanthal, inhibited, in a concentration-dependent manner, the IL-1beta-mediated NO release from these cells. Together, our data provide evidence, for the first time, that Ras activation is an obligatory step in IL-1beta-mediated NO release and, presumably, the subsequent dysfunction of the pancreatic beta cell. Our data also provide a basis for future investigations to understand the mechanism of cytokine-induced beta cell death leading to the onset of insulin-dependent diabetes mellitus.

Regulation of phospholipase D (PLD) in growth plate chondrocytes by 24R,25-(OH)2D3 is dependent on cell maturation state (resting zone cells) and is specific to the PLD2 isoform

Many of the effects of 1alpha,25-(OH)2D3 and 24R,25-(OH)2D3 on costochondral chondrocytes are mediated by the protein kinase C (PKC) signal transduction pathway. 1alpha,25-(OH)2D3 activates PKC in costochondral growth zone chondrocytes through a specific membrane receptor (1alpha,25-mVDR), involving rapid increases in diacylglycerol via a phospholipase C (PLC)-dependent mechanism. 24R,25-(OH)2D3 activates PKC in resting zone chondrocytes. Although diacylglycerol is increased by 24R,25-(OH)2D3, PLC is not involved, suggesting a phospholipase D (PLD)-dependent mechanism. Here, we show that resting zone and growth zone cells express mRNAs for PLD1a, PLD1b, and PLD2. Both cell types have PLD activity, but levels are higher in resting zone cells. 24R,25-(OH)2D3, but not 24S,25-(OH)2D3 or 1alpha,25-(OH)2D3, stimulates PLD activity in resting zone cells within 3 min via nongenomic mechanisms. Neither 1alpha,25-(OH)2D3 nor 24R,25-(OH)2D3 affected PLD in growth zone cells. Basal and 24R,25-(OH)2D3-stimulated PLD were inhibited by the PLD inhibitors wortmannin and EDS. Inhibition of phosphatidylinositol 3-kinase (PI 3-kinase), PKC, phosphatidylinositol-specific PLC (PI-PLC), and phosphatidylcholine-specific PLC (PC-PLC) had no effect on PLD activity. Thus, 24R,25-(OH)2D3 stimulates PLD, and PI 3-kinase, PI-PLC and PKC are not involved, whereas PLD is required for stimulation of PKC by 24R,25-(OH)2D3. Pertussis toxin, GDPbetaS, and GTPgammaS had no effect on 24R,25-(OH)2D3-dependent PLD when added to cell cultures, indicating that G-proteins are not involved. These data show that PKC activation in resting zone cells is mediated by PLD and suggest that a functional 24R,25-(OH)2D3-mVDR is required. The results also support the conclusion that the 24R,25-(OH)2D3-responsive PLD is PLD2, since this PLD isoform is G-protein-independent.

Evidence for Rho protein regulation of renal tubular epithelial cell function

BACKGROUND

Rho proteins are small guanine 5'-triphosphate (GTP)-binding proteins felt to be important regulators of several aspects of cell function, including the organization of the actin cytoskeleton. The effects of Rho proteins on the regulation of renal tubular epithelial cell function are not known.

METHODS

Selected bacterial toxins that inhibit Rho protein function were used to examine the effect of Rho in cultured renal tubular epithelial cells.

RESULTS

Clostridium difficile toxin A significantly and dose dependently inhibited LLC-PK(1) cell (3)H-thymidine uptake and healing of small wounds made in confluent monolayers, and it induced apoptosis. A second Clostridium difficile toxin (toxin B) that acted via a different receptor also impaired LLC-PK(1) thymidine uptake and wound healing, and it induced apoptosis. A third bacterial toxin, C3 toxin from Clostridium botulinum, also impaired LLC-PK(1) thymidine uptake and stimulated apoptosis in LLC-PK(1) cells. Since Rho inhibition disrupted organization of the actin cytoskeleton, we examined the effects of another agent that disrupted the actin cytoskeleton (cytochalasin D) and found significant dose-dependent effects that impaired LLC-PK1 thymidine uptake and wound healing and that induced apoptosis. The effects of toxin A and cytochalasin D to induce apoptosis were not associated with significant changes in expression of Bcl-2, BAD, or BAK proteins and were significantly attenuated by a pancaspase inhibitor.

CONCLUSIONS

Our results suggest that Rho proteins are important endogenous regulators of several aspects of renal tubular epithelial cell function, including proliferation, migration, and apoptosis. Further studies are needed to clarify the cellular mechanisms of Rho regulation of renal epithelial cell function.