Pasteurella multocida Toxin-induced Activation of RhoA Is Mediated via Two Families of Gα Proteins, Gαq and Gα12/13

Activation of Galpha (i) and subsequent uncoupling of receptor-Galpha(i) signaling by Pasteurella multocida toxin

Bacterial protein toxins are powerful tools for elucidating signaling mechanisms in eukaryotic cells. A number of bacterial protein toxins, e.g. cholera toxin, pertussis toxin (PTx), or Pasteurella multocida toxin (PMT), target heterotrimeric G proteins and have been used to stimulate or block specific signaling pathways or to demonstrate the contribution of their target proteins in cellular effects. PMT is a major virulence factor of P. multocida causing pasteurellosis in man and animals and is responsible for atrophic rhinitis in pigs. PMT modulates various signaling pathways, including phospholipase Cbeta and RhoA, by acting on the heterotrimeric G proteins Galpha(q) and Galpha(12/13), respectively. Here we report that PMT is a powerful activator of G(i) protein. We show that PMT decreases basal isoproterenol and forskolin-stimulated cAMP accumulation in intact Swiss 3T3 cells, inhibits adenylyl cyclase activity in cell membrane preparations, and enhances the inhibition of cAMP accumulation caused by lysophosphatidic acid via endothelial differentiation gene receptors. PMT-mediated inhibition of cAMP production is independent of toxin activation of Galpha(q) and/or Galpha(12/13). Although the effects of PMT are not inhibited by PTx, PMT blocks PTx-catalyzed ADP-ribosylation of G(i). PMT also inhibits steady-state GTPase activity and GTP binding of G(i) in Swiss 3T3 cell membranes stimulated by lysophosphatidic acid. The data indicate that PMT is a novel activator of G(i), modulating its GTPase activity and converting it into a PTx-insensitive state.

The C3 domain of Pasteurella multocida toxin is the minimal domain responsible for activation of Gq-dependent calcium and mitogenic signaling

The large 1285-amino-acid protein toxin from Pasteurella multocida (PMT) is a multifunctional single-chain polypeptide that binds to and enters eukaryotic cells and acts intracellularly to promote G(q) and G(12/13) protein-dependent calcium and mitogenic signal transduction. Previous studies indicated that the intracellular activity domain responsible for PMT action was located within the C-terminal 600-700 amino acids. In this study, we have exogenously expressed a series of N- and C-terminal PMT fragments directly in mammalian cells and have used the dual luciferase reporter system to assay for toxin-mediated activation of calcium-calcineurin-NFAT signaling (NFAT-luciferase) and mitogenic serum response signaling (SRE-luciferase). Using this approach, we have defined the last 180 amino acids, which encompass the C3 domain in the crystal structure, as the minimum domain sufficient to activate both NFAT and SRE signaling pathways.

Anion secretion evoked by Pasteurella multocida toxin across rat colon

Stimulation of muscarinic receptors is known to have a biphasic effect on colonic Cl(-) secretion: a short-lasting activation, which is followed by a long-lasting inhibition. In order to find out, which role Gq proteins play in both processes, Pasteurella multocida toxin was used, a known activator of G alpha q. This toxin (1.5 microg/ml) had a dual action on short-circuit current (Isc) across rat distal colon: it stimulated transiently Isc and subsequently down-regulated the Isc evoked by Ca2+-dependent secretagogues such as acetylcholine or ATP. The inactive mutant (P. multocida toxin C1165S), which does not stimulate G alpha q), was ineffective. Cl(-) dependence and sensitivity against bumetanide, a blocker of the Na+-K+-2Cl(-) cotransporter, confirmed that the increase in Isc evoked by the toxin represented Cl(-) secretion. The effect of P. multocida toxin was suppressed by YM-254890 (10(-7) M), a blocker of G alpha q. Experiments with apically permeabilized tissues revealed that the secretory response to P. multocida toxin was concomitant with an increase in basolateral K+ conductance as it is observed for other agonists inducing Ca2+-dependent anion secretion. Consequently, these results suggest that Gq proteins are not only involved in the activation of secretion, e.g. after stimulation of muscarinic or purinergic receptors, but also play a central role in the long-term down-regulation of intestinal secretion after activation of these types of receptors.

Application of intact cell-based NFAT-beta-lactamase reporter assay for Pasteurella multocida toxin-mediated activation of calcium signaling pathway

Pasteurella multocida toxin (PMT) stimulates and subsequently uncouples phospholipase C beta1 (PLCbeta1) signal transduction through its selective action on the alpha subunit of the Gq-protein. Here, we describe the application of an NFAT-beta-lactamase reporter assay as a functional readout for PMT-induced activation of the Gq-protein-coupled PLCbeta1-IP(3)-Ca(2+) signaling pathway. Use of the NFAT-beta-lactamase reporter assay with a cell-permeable fluorogenic substrate provides high sensitivity due to the absence of endogenous beta-lactamase activity in mammalian cells. This assay system was optimized for cell density, dose and time exposure of PMT stimulation. It is suited for quantitative characterization of PMT activity in mammalian cells and for use as a high-throughput screening method for PMT deletion and point mutants suitable for vaccine development. This method has application's for diagnostic screening of clinical isolates of toxinogenic P. multocida.

Action of Pasteurella multocida toxin on Galpha(q) is persistent and independent of interaction with G-protein-coupled receptors

Pasteurella multocida toxin (PMT) activates Galpha(q) and facilitates stimulation of inositol phosphate accumulation induced by agonists via G(q)-coupled membrane receptors. Here, we studied the effects of PMT on agonist-induced GTPgammaS binding to G(q) in cell membranes and a role of G-protein-coupled receptors in the action of PMT. Pre-treatment of Swiss 3T3 cells with PMT increased bombesin or vasopressin-induced GTPgammaS-binding in cell membranes by about 50 to 150%. Increase in agonist-stimulated GTPgammaS-binding caused by PMT pretreatment was specific for Galpha(q) and not observed with Galpha(11). PMT-induced effects on GTPgammaS-binding were persistent after removing the toxin or in the presence of anti-PMT antibody. Stimulation of agonist-induced GTPgammaS-binding by PMT was independent of phosphorylation of the C-terminal tyrosine356 of Galpha(q). Activation of phospholipase C by PMT occurred via Galpha(q) which was fused to the alpha(1b)-adrenoceptor and also with a C-terminally deleted Galpha(q), which is not able to interact with G protein-coupled membrane receptors. The data indicate that activation of Galpha(q) by PMT is persistent and independent of a functional interaction of G(q) with G-protein-coupled receptors.

Calcineurin-independent inhibition of 3T3-L1 adipogenesis by Pasteurella multocida toxin: suppression of Notch1, stabilization of beta-catenin and pre-adipocyte factor 1

Pasteurella multocida toxin (PMT) is a potent mitogen and a specific activator of Gq-dependent signalling pathways. PMT impairs osteoblast differentiation and causes bone loss and fat reduction in vivo. We examined the effect of PMT on cell signalling pathways involved in 3T3-L1 adipocyte differentiation. We demonstrate that PMT treatment before or together with differentiation induction factors inhibits adipogenesis and prevents upregulation of important adipocyte markers - peroxisome-proliferator-activated receptor gamma (PPARgamma) and CAATT enhancer-binding protein alpha (C/EBPalpha). Moreover, PMT completely downregulates PPARgamma and C/EBPalpha expression in mature adipocytes. Differentiation of pre-adipocytes into adipocytes requires the suppression of pre-adipocyte factor 1 (Pref1) and Wnt signalling, along with the degradation of beta-catenin. PMT prevents downregulation of Pref1 and beta-catenin under differentiation-inducing conditions. In addition, PMT treatment downregulates expression of Notch1, a protein responsible for cell fate decision and implicated in regulation of adipogenesis in 3T3-L1 cells. PMT action on adipogenesis was not reversed by cyclosporin A, an inhibitor of Galphaq-PLC-calcium-dependent calcineurin activation. Our results reveal new pathways involved in PMT action on cellular physiology and differentiation. Our study further demonstrates that the effect of PMT on Pref1/PPARgamma/C/EBPalpha expression and adipogenesis does not occur just through activation of the Galphaq-calcium-calcineurin pathway, but involves Wnt/beta-catenin and Notch1 signalling pathways, two signalling pathways strongly linked to cancer predisposition, neurological and immunological dysfunctions, and fat and bone development.

Histidine Residues at the Active Site of the Pasteurella multocida Toxin

We have investigated histidine residues near the active site of the mitogenic Pasteurella multocida toxin. Mutation of H1202 or H1228 had little effect, while the effect of mutation on H1223 depended on the amino acid substituted. Mutation of H1205 caused complete loss of activity, indicating its importance in PMT activity.

Regulation of connexin43 gap junctional communication by phosphatidylinositol 4,5-bisphosphate

Cell-cell communication through connexin43 (Cx43)-based gap junction channels is rapidly inhibited upon activation of various G protein-coupled receptors; however, the mechanism is unknown. We show that Cx43-based cell-cell communication is inhibited by depletion of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P(2)) from the plasma membrane. Knockdown of phospholipase Cbeta3 (PLCbeta3) inhibits PtdIns(4,5)P(2) hydrolysis and keeps Cx43 channels open after receptor activation. Using a translocatable 5-phosphatase, we show that PtdIns(4,5)P(2) depletion is sufficient to close Cx43 channels. When PtdIns(4,5)P(2) is overproduced by PtdIns(4)P 5-kinase, Cx43 channel closure is impaired. We find that the Cx43 binding partner zona occludens 1 (ZO-1) interacts with PLCbeta3 via its third PDZ domain. ZO-1 is essential for PtdIns(4,5)P(2)-hydrolyzing receptors to inhibit cell-cell communication, but not for receptor-PLC coupling. Our results show that PtdIns(4,5)P(2) is a key regulator of Cx43 channel function, with no role for other second messengers, and suggest that ZO-1 assembles PLCbeta3 and Cx43 into a signaling complex to allow regulation of cell-cell communication by localized changes in PtdIns(4,5)P(2).

Crystal structures reveal a thiol protease-like catalytic triad in the C-terminal region of Pasteurella multocida toxin

Pasteurella multocida toxin (PMT), one of the virulence factors produced by the bacteria, exerts its toxicity by up-regulating various signaling cascades downstream of the heterotrimeric GTPases Gq and G12/13 in an unknown fashion. Here, we present the crystal structure of the C-terminal region (residues 575-1,285) of PMT, which carries an intracellularly active moiety. The overall structure of C-terminal region of PMT displays a Trojan horse-like shape, composed of three domains with a "feet"-,"body"-, and "head"-type arrangement, which were designated C1, C2, and C3 from the N to the C terminus, respectively. The C1 domain, showing marked similarity in steric structure to the N-terminal domain of Clostridium difficile toxin B, was found to lead the toxin molecule to the plasma membrane. The C3 domain possesses the Cys-His-Asp catalytic triad that is organized only when the Cys is released from a disulfide bond. The steric alignment of the triad corresponded well to that of papain or other enzymes carrying Cys-His-Asp. PMT toxicities on target cells were completely abrogated when one of the amino acids constituting the triad was mutated. Our results indicate that PMT is an enzyme toxin carrying the cysteine protease-like catalytic triad dependent on the redox state and functions on the cytoplasmic face of the plasma membrane of target cells.

Modulation of host cell gene expression through activation of STAT transcription factors by Pasteurella multocida toxin

The Pasteurella multocida toxin (PMT) is highly mitogenic and has potential carcinogenic properties. PMT causes porcine atrophic rhinitis that is characterized by bone resorption and loss of nasal turbinates, but experimental nasal infection also leads to excess proliferation of bladder epithelial cells. PMT acts intracellularly and activates phospholipase C-linked signals and MAPK pathways via the heterotrimeric Galpha(q) and Galpha(12/13) proteins. We found that PMT induces activation of STAT proteins, and we identified STAT1, STAT3, and STAT5 as new targets of PMT-induced Galpha(q) signaling. Inhibition of Janus kinases completely abolished STAT activation. PMT-dependent STAT phosphorylation remained constitutive for at least 18 h. PMT caused down-regulation of the expression of the suppressor of cytokine signaling-3, indicating a novel mechanism to maintain activation of STATs. Moreover, stimulation of Swiss 3T3 cells with PMT increased transcription of the cancer-associated STAT-dependent gene cyclooxygenase-2. Because constitutive activation of STATs has been found in a number of cancers, our findings offer a new mechanism for a carcinogenic role of PMT.

Crystallization and preliminary crystallographic studies of the Pasteurella multocida toxin catalytic domain

The C-terminal catalytic domain of Pasteurella multocida toxin, which is the virulence factor of the organism in P. multocida, has been expressed, purified and subsequently crystallized using the sitting-drop vapour-diffusion technique. Native diffraction data to 1.9 A resolution were obtained at the BL44XU beamline of SPring-8 from a flash-frozen crystal at 100 K. The crystals belong to space group C2, with unit-cell parameters a = 111.0, b = 150.4, c = 77.1 A, beta = 105.5 degrees, and are likely to contain one C-PMT (726 residues) per asymmetric unit.