Pasteurella multocida toxin stimulates mitogenesis and cytoskeleton reorganization in Swiss 3T3 fibroblasts

Selective Membrane Redistribution and Depletion of Gαq-Protein by Pasteurella multocida Toxin

Pasteurella multocida toxin (PMT), the major virulence factor responsible for zoonotic atrophic rhinitis, is a protein deamidase that activates the alpha subunit of heterotrimeric G proteins. Initial activation of G alpha-q-coupled phospholipase C-beta-1 signaling by PMT is followed by uncoupling of G alpha-q-dependent signaling, causing downregulation of downstream calcium and mitogenic signaling pathways. Here, we show that PMT decreases endogenous and exogenously expressed G alpha-q protein content in host cell plasma membranes and in detergent resistant membrane (DRM) fractions. This membrane depletion of G alpha-q protein was dependent upon the catalytic activity of PMT. Results indicate that PMT-modified G alpha-q redistributes within the host cell membrane from the DRM fraction into the soluble membrane and cytosolic fractions. In contrast, PMT had no affect on G alpha-s or G beta protein levels, which are not substrate targets of PMT. PMT also had no affect on G alpha-11 levels, even though G alpha-11 can serve as a substrate for deamidation by PMT, suggesting that membrane depletion of PMT-modified G-alpha-q has specificity.

Pasteurella multocida toxin: Targeting mast cell secretory granules during kiss-and-run secretion

Pasteurella multocida toxin (PMT), a virulence factor of the pathogenic Gram-negative bacterium P. multocida, is a 146 kDa protein belonging to the A-B class of toxins. Once inside a target cell, the A domain deamidates the α-subunit of heterotrimeric G-proteins, thereby activating downstream signaling cascades. However, little is known about how PMT selects and enters its cellular targets. We therefore studied PMT binding and uptake in porcine cultured intestinal mucosal explants to identify susceptible cells in the epithelium and underlying lamina propria. In comparison with Vibrio cholera B-subunit, a well-known enterotoxin taken up by receptor-mediated endocytosis, PMT binding to the epithelial brush border was scarce, and no uptake into enterocytes was detected by 2h, implying that none of the glycolipids in the brush border are a functional receptor for PMT. However, in the lamina propria, PMT distinctly accumulated in the secretory granules of mast cells. This also occurred at 4 °C, ruling out endocytosis, but suggestive of uptake via pores that connect the granules to the cell surface. Mast cell granules are known to secrete their contents by a "kiss-and-run" mechanism, and we propose that PMT may exploit this secretory mechanism to gain entry into this particular cell type.

Regulation of Toll-like receptor 4-mediated immune responses through Pasteurella multocida toxin-induced G protein signalling

Background

Lipopolysaccharide (LPS)-triggered Toll-like receptor (TLR) 4-signalling belongs to the key innate defence mechanisms upon infection with Gram-negative bacteria and triggers the subsequent activation of adaptive immunity. There is an active crosstalk between TLR4-mediated and other signalling cascades to secure an effective immune response, but also to prevent excessive inflammation. Many pathogens induce signalling cascades via secreted factors that interfere with TLR signalling to modify and presumably escape the host response. In this context heterotrimeric G proteins and their coupled receptors have been recognized as major cellular targets. Toxigenic strains of Gram-negative Pasteurella multocida produce a toxin (PMT) that constitutively activates the heterotrimeric G proteins Gα_q, Gα₁₃ and Gα_i independently of G protein-coupled receptors through deamidation. PMT is known to induce signalling events involved in cell proliferation, cell survival and cytoskeleton rearrangement.

Results

Here we show that the activation of heterotrimeric G proteins through PMT suppresses LPS-stimulated IL-12p40 production and eventually impairs the T cell-activating ability of LPS-treated monocytes. This inhibition of TLR4-induced IL-12p40 expression is mediated by Gα_i-triggered signalling as well as by Gβγ-dependent activation of PI3kinase and JNK.

Taken together we propose the following model: LPS stimulates TLR4-mediated activation of the NFĸB-pathway and thereby the production of TNF-α, IL-6 and IL-12p40. PMT inhibits the production of IL-12p40 by Gα_i-mediated inhibition of adenylate cyclase and cAMP accumulation and by Gβγ-mediated activation of PI3kinase and JNK activation.

Conclusions

On the basis of the experiments with PMT this study gives an example of a pathogen-induced interaction between G protein-mediated and TLR4-triggered signalling and illustrates how a bacterial toxin is able to interfere with the host’s immune response.

Rho/ROCK-dependent inhibition of 3T3-L1 adipogenesis by G-protein-deamidating dermonecrotic toxins: differential regulation of Notch1, Pref1/Dlk1, and β-catenin signaling

The dermonecrotic toxins from Pasteurella multocida (PMT), Bordetella (DNT), Escherichia coli (CNF1-3), and Yersinia (CNFY) modulate their G-protein targets through deamidation and/or transglutamination of an active site Gln residue, which results in activation of the G protein and its cognate downstream signaling pathways. Whereas DNT and the CNFs act on small Rho GTPases, PMT acts on the α subunit of heterotrimeric G_q, G_i, and G_12/13 proteins. We previously demonstrated that PMT potently blocks adipogenesis and adipocyte differentiation in a calcineurin-independent manner through downregulation of Notch1 and stabilization of β-catenin and Pref1/Dlk1, key proteins in signaling pathways strongly linked to cell fate decisions, including fat and bone development. Here, we report that similar to PMT, DNT, and CNF1 completely block adipogenesis and adipocyte differentiation by preventing upregulation of adipocyte markers, PPARγ and C/EBPα, while stabilizing the expression of Pref1/Dlk1 and β-catenin. We show that the Rho/ROCK inhibitor Y-27632 prevented or reversed these toxin-mediated effects, strongly supporting a role for Rho/ROCK signaling in dermonecrotic toxin-mediated inhibition of adipogenesis and adipocyte differentiation. Toxin treatment was also accompanied by downregulation of Notch1 expression, although this inhibition was independent of Rho/ROCK signaling. We further show that PMT-mediated downregulation of Notch1 expression occurs primarily through G_12/13 signaling. Our results reveal new details of the pathways involved in dermonecrotic toxin action on adipocyte differentiation, and the role of Rho/ROCK signaling in mediating toxin effects on Wnt/β-catenin and Notch1 signaling, and in particular the role of G_q and G_12/13 in mediating PMT effects on Rho/ROCK and Notch1 signaling.

Pasteurella multocida and immune cells

Pasteurella multocida was first discovered by Perroncito in 1878 and named after Louis Pasteur who first isolated and described this Gram-negative bacterium as the cause of fowl disease in 1880. Subsequently, P. multocida was also found to cause atrophic rhinitis in pigs, haemorrhagic septicaemia in cattle and respiratory diseases in many other animals. Among other factors such as lipopolysaccharide, outer membrane proteins and its capsule, the protein toxin (PMT) of P. multocida is an important virulence factor that determines the immunological response of the host's immune system. However, the exact molecular mechanisms taking place in cells of the innate and adaptive immune system are largely unknown for any of these virulence factors. Due to the obvious function of PMT on cells of the porcine skeletal system where it causes bone destruction, PMT was regarded as an osteolytic protein toxin. However, it remained unclear what the actual benefit for the bacteria would be. Recently, more attention was drawn to the osteoimmunological effects of PMT and the interplay between bone and immune cells. This review summarises the knowledge of effects of P. multocida virulence factors on the host's immune system.

Pasteurella multocida toxin interaction with host cells: entry and cellular effects

The mitogenic dermonecrotic toxin from Pasteurella multocida (PMT) is a 1285-residue multipartite protein that belongs to the A-B family of bacterial protein toxins. Through its G-protein-deamidating activity on the α subunits of heterotrimeric G(q)-, G(i)- and G(12/13)-proteins, PMT potently stimulates downstream mitogenic, calcium, and cytoskeletal signaling pathways. These activities lead to pleiotropic effects in different cell types, which ultimately result in cellular proliferation, while inhibiting cellular differentiation, and account for the myriad of physiological outcomes observed during infection with toxinogenic strains of P. multocida.

Molecular biology of Pasteurella multocida toxin

Pasteurella multocida toxin (PMT) is the causative agent of progressive atrophic rhinitis in swine. The 146 kDa single-chain toxin harbours discrete domains important for receptor binding, internalisation and biological activity. The molecular basis of the toxin's activity is the deamidation of a specific glutamine residue in the α-subunit of heterotrimeric G proteins. This results in an inhibition of the inherent GTPase activity leading to a constitutively active phenotype of the G protein. Due to the ability of the toxin to act on various families of heterotrimeric G proteins, a large subset of signal transduction pathways is stimulated.

Membrane interaction of Pasteurella multocida toxin involves sphingomyelin

Pasteurella multocida toxin (PMT) is an AB toxin that causes pleiotropic effects in targeted host cells. The N-terminus of PMT (PMT-N) is considered to harbor the membrane receptor binding and translocation domains responsible for mediating cellular entry and delivery of the C-terminal catalytic domain into the host cytosol. Previous studies have implicated gangliosides as the host receptors for PMT binding. To gain further insight into the binding interactions involved in PMT binding to cell membranes, we explored the role of various membrane components in PMT binding, utilizing four different approaches: (a) TLC-overlay binding experiments with (125) I-labeled PMT, PMT-N or the C-terminus of PMT; (b) pull-down experiments using reconstituted membrane liposomes with full-length PMT; (c) surface plasmon resonance analysis of PMT-N binding to reconstituted membrane liposomes; (d) and surface plasmon resonance analysis of PMT-N binding to HEK-293T cell membranes without or with sphingomyelinase, phospholipase D or trypsin treatment. The results obtained revealed that, in our experimental system, full-length PMT and PMT-N did not bind to gangliosides, including monoasialogangliosides GM(1) , GM(2) or GM(3) , but instead bound to membrane phospholipids, primarily the abundant sphingophospholipid sphingomyelin or phosphatidylcholine with other lipid components. Collectively, these studies demonstrate the importance of sphingomyelin for PMT binding to membranes and suggest the involvement of a protein co-receptor.

Cellular and molecular action of the mitogenic protein-deamidating toxin from Pasteurella multocida

The mitogenic toxin from Pasteurella multocida (PMT) is a member of the dermonecrotic toxin family, which includes toxins from Bordetella, Escherichia coli and Yersinia. Members of the dermonecrotic toxin family modulate G-protein targets in host cells through selective deamidation and/or transglutamination of a critical active site Gln residue in the G-protein target, which results in the activation of intrinsic GTPase activity. Structural and biochemical data point to the uniqueness of PMT among these toxins in its structure and action. Whereas the other dermonecrotic toxins act on small Rho GTPases, PMT acts on the α subunits of heterotrimeric G(q) -, G(i) - and G(12/13) -protein families. To date, experimental evidence supports a model in which PMT potently stimulates various mitogenic and survival pathways through the activation of G(q) and G(12/13) signaling, ultimately leading to cellular proliferation, whilst strongly inhibiting pathways involved in cellular differentiation through the activation of G(i) signaling. The resulting cellular outcomes account for the global physiological effects observed during infection with toxinogenic P. multocida, and hint at potential long-term sequelae that may result from PMT exposure.

Pasteurella multocida toxin-stimulated osteoclast differentiation is B cell dependent

Pasteurella multocida is a Gram-negative bacillus that infects a number of wild and domestic animals, causing respiratory diseases. Toxigenic Pasteurella multocida strains produce a protein toxin (PMT) that leads to atrophic rhinitis in swine due to enhanced osteoclastogenesis and the inhibition of osteoblast function. We show that PMT-induced osteoclastogenesis is promoted by an as-yet-uncharacterized B-cell population. The toxin, however, is not acting at the level of hematopoietic stem cells, since purified CD117(+) cells from murine hematopoietic progenitor cells cultivated with PMT did not mature into osteoclasts. The early macrophages contained within this cell population (CD117(+)/CD11b(+)) did not further differentiate into osteoclasts but survived and were able to phagocytose. Within the CD117(-) population, however, we detected PMT-induced generation of a B220(+)/CD19(+) and B220(+)/IgM(+) B-cell population that was able to take up fluorescently labeled PMT. Using purified B-cell and macrophage populations, we show that these B cells are needed to efficiently generate osteoclasts from macrophages. Cells of the immune system are thought to affect osteoclast formation and function by secreting cytokines and growth factors. We show here that PMT-stimulated B cells produce elevated levels of the osteoclastogenic factors interleukin-1β (IL-1β), IL-6, tumor necrosis factor alpha, and receptor activator of nuclear factor receptor ligand (RANKL) compared to B cells generated through incubation with IL-7. These results suggest that the osteoclastic properties characteristic for PMT may result from a cross talk between bone cells and lymphoid cells and that B cells might be an important target of Pasteurella multocida.

Photorhabdus luminescens genes induced upon insect infection

BACKGROUND

Photorhabdus luminescens is a Gram-negative luminescent enterobacterium and a symbiote to soil nematodes belonging to the species Heterorhabditis bacteriophora. P.luminescens is simultaneously highly pathogenic to insects. This bacterium exhibits a complex life cycle, including one symbiotic stage characterized by colonization of the upper nematode gut, and a pathogenic stage, characterized by release from the nematode into the hemocoel of insect larvae, resulting in rapid insect death caused by bacterial toxins. P. luminescens appears to sense and adapt to the novel host environment upon changing hosts, which facilitates the production of factors involved in survival within the host, host-killing, and -exploitation.

RESULTS

A differential fluorescence induction (DFI) approach was applied to identify genes that are up-regulated in the bacterium after infection of the insect host Galleria mellonella. For this purpose, a P. luminescens promoter-trap library utilizing the mCherry fluorophore as a reporter was constructed, and approximately 13,000 clones were screened for fluorescence induction in the presence of a G. mellonella larvae homogenate. Since P. luminescens has a variety of regulators that potentially sense chemical molecules, like hormones, the screen for up-regulated genes or operons was performed in vitro, excluding physicochemical signals like oxygen, temperature or osmolarity as variables. Clones (18) were obtained exhibiting at least 2.5-fold induced fluorescence and regarded as specific responders to insect homogenate. In combination with a bioinformatics approach, sequence motifs were identified in these DNA-fragments that are similar to 29 different promoters within the P. luminescens genome. By cloning each of the predicted promoters upstream of the reporter gene, induction was verified for 27 promoters in vitro, and for 24 promoters in viable G. mellonella larvae. Among the validated promoters are some known to regulate the expression of toxin genes, including tccC1 (encoding an insecticidal toxin complex), and others encoding putative toxins. A comparably high number of metabolic genes or operons were observed to be induced upon infection; among these were eutABC, hutUH, and agaZSVCD, which encode proteins involved in ethanolamine, histidine and tagatose degradation, respectively. The results reflect rearrangements in metabolism and the use of other metabolites available from the insect. Furthermore, enhanced activity of promoters controlling the expression of genes encoding enzymes linked to antibiotic production and/or resistance was observed. Antibiotic production and resistance may influence competition with other bacteria, and thus might be important for a successful infection. Lastly, several genes of unknown function were identified that may represent novel pathogenicity factors.

CONCLUSION

We show that a DFI screen is useful for identifying genes or operons induced by chemical stimuli, such as diluted insect homogenate. A bioinformatics comparison of motifs similar to known promoters is a powerful tool for identifying regulated genes or operons. We conclude that signals for the regulation of those genes or operons induced in P. luminescens upon insect infection may represent a wide variety of compounds that make up the insect host. Our results provide insight into the complex response to the host that occurs in a bacterial pathogen, particularly reflecting the potential for metabolic shifts and other specific changes associated with virulence.

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.

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.

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.

Pasteurella multocida toxin (PMT) activates RhoGTPases, induces actin polymerization and inhibits migration of human dendritic cells, but does not influence macropinocytosis

Dendritic cells (DCs) are considered as one of the principal initiators of immune responses. In their immature state, they migrate into peripheral tissue in order to uptake antigen and to patrol for danger signals. Upon maturation, they acquire the ability to migrate to the lymph nodes and present the captured antigens to T cells in order to direct the development of specific immune responses. There is evidence that microbial compounds interfere with proper functions of DCs in order to block innate and specific immunity. Here we characterized the influence of Pasteurella multocida toxin (PMT) on monocyte-derived DCs. Using pull-down assays with recombinant rhotekin or p21-activated kinase, we demonstrated the activation of RhoGTPases by PMT in DCs. Moreover, PMT induced changes in DC morphology and actin polymerization, impaired chemotaxin-induced actin re-organization and inhibited their migration response. However, macropinocytosis was not influenced by PMT. In summary, these data indicate that PMT inhibits proper function of the motility machinery in DCs, which might limit the development of adaptive immune surveillance during infection with Pasteurella multocida.

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

Pasteurella multocida toxin (PMT) is a potent mitogen, which is known to activate phospholipase Cbeta by stimulating the alpha-subunit of the heterotrimeric G protein G(q). PMT also activates RhoA and RhoA-dependent pathways. Using YM-254890, a specific inhibitor of G(q/11), we studied whether activation of RhoA involves G proteins other than G(q/11). YM-254890 inhibited PMT or muscarinic M3-receptor-mediated stimulation of phospholipase Cbeta at similar concentrations in HEK293m3 cells. In these cells, PMT-induced RhoA activation and enhancement of RhoA-dependent luciferase activity were partially inhibited by YM-254890. In Galpha(q/11)-deficient fibroblasts, PMT induced activation of RhoA, increase in RhoA-dependent luciferase activity, and increase in ERK phosphorylation. None of these effects were influenced by YM-254890. However, RhoA activation by PMT was inhibited by RGS2, RGS16, lscRGS, and dominant negative G(13)(GA), indicating involvement of Galpha(12/13) in the PMT effect on RhoA. In Galpha(12/13) gene-deficient cells, PMT-induced stimulation of RhoA, luciferase activity, and ERK phosphorylation were blocked by YM-254890, indicating the involvement of G(q). Infection with a virus harboring the gene of Galpha(13) reconstituted the increase in RhoA-dependent luciferase activity by PMT even in the presence of YM-254890. The data show that YM-254890 is able to block PMT activation of Galpha(q) and indicate that, in addition to Galpha(q), the Galpha(12/13) G proteins are targets of PMT.

Bacterial toxins that modulate host cell-cycle progression

The mammalian cell cycle is involved in many processes--such as immune responses, maintenance of epithelial barrier functions, and cellular differentiation--that affect the growth and colonization of pathogenic bacteria. Therefore it is not surprising that many bacterial pathogens manipulate the host cell cycle with respect to these functions. Cyclomodulins are a growing family of bacterial toxins and effectors that interfere with the eukaryotic cell cycle. Here, we review some of these cyclomodulins such as cytolethal distending toxins, vacuolating cytotoxin, the polyketide-derived macrolide mycolactone, cycle-inhibiting factor, cytotoxic necrotizing factors, dermonecrotic toxin, Pasteurella multocida toxin and cytotoxin-associated antigen A. We describe and compare their effects on the mammalian cell cycle and their putative role in disease, commensalism and symbiosis. We also discuss a possible link between these cyclomodulins and cancer.

Action of Pasteurella multocida toxin depends on the helical domain of Galphaq

Pasteurella multocida produces a 146-kDa protein toxin (PMT), which activates multiple cellular signal transduction pathways, resulting in the activation of phospholipase Cbeta, RhoA, Jun kinase, and extracellular signal-regulated kinase. Using Galpha(q)/Galpha(11) -deficient cells, it was shown that the PMT-induced pleiotropic effects are mediated by Galpha(q) but not by the highly related Galpha(11) protein (Zywietz, A., Gohla, A., Schmelz, M., Schultz, G., and Offermanns, S. (2001) J. Biol. Chem. 276, 3840-3845). Here we studied the molecular basis of the unique specificity of PMT to distinguish between Galpha(q) and/or Galpha(11). Infection of Galpha(q) -deficient cells with retrovirus-encoding Galpha(q) caused reconstitution of PMT-induced activation of phospholipase Cbeta, whereas Galpha(11) -encoding virus did not reconstitute PMT activity. Chimeras between Galpha(q) and/or Galpha(11) revealed that a peptide region of Galpha(q), covering amino acid residues 105-113, is essential for the action of PMT to activate phospholipase Cbeta. Exchange of glutamine 105 or asparagine 109 of Galpha(11), which are located in the all-helical domain of the Galpha subunit, with the equally positioned histidines of Galpha(q), renders Galpha(11) capable of transmission PMT-induced phospholipase Cbeta activation. The data indicate that the all-helical domain of Galpha(q) is essential for the action of PMT and suggest an essential functional role of this domain in signal transduction via G(q) proteins.

Pasteurella multocida toxin as a tool for studying Gq signal transduction

Pasteurella multocida toxin (PMT) stimulates and subsequently uncouples phospholipase C (PLC) signal transduction through its selective action on the Galphaq subunit. This review summarizes what is currently known about the molecular action of PMT on Gq and the resulting cellular effects. Examples are presented illustrating the use of PMT as a powerful tool for dissecting the molecular mechanisms involving pertussis toxin (PT)-insensitive heterotrimeric G proteins.

His1205 and His1223 are essential for the activity of the mitogenic Pasteurella multocida toxin

Pasteurella multocida produces a 146-kDa protein toxin (PMT), which activates multiple cellular signal-transduction pathways, resulting in the activation of PLCbeta, Rho, JNK, and ERK. In addition to an essential cysteine residue at position 1165, PMT contains several histidine residues in the catalytically important C-terminal part of the protein. To elucidate the role of the histidine residues, we treated PMT with the histidine-modifying substance diethyl pyrocarbonate (DEPC). DEPC inhibited PMT in a time- and concentration-dependent manner, suggesting that one or several histidine residues are essential for the biological activity of PMT. In experiments in which PMT was directly delivered into the cytosol of EBL cells by electroporation, we show that DEPC treatment inhibits the catalytically important histidine residues. Leucine substitutions of eight individual histidine residues in the C-terminal catalytic domain of PMT were constructed, and the effect on the biological activity of PMT was analyzed by determining PLCbeta, Rho, and ERK activation. Substitution of two histidine residues, H1205 and H1223, led to inactivation of the resulting PMT proteins, indicating that H1205 and H1223 play an important role in biological activity of the toxin. In addition, we show that the mutant toxins appear to be correctly folded, as judged by protease digestion. The precise function of H1205 and H1223 is not yet known. However, treatment of PMT with the cation chelating substance 1,10-phenantroline led to inactivation of the toxin, indicating that the essential histidine residues and cysteine 1165 might be involved in metal ion binding.

Biological activity of a C-terminal fragment of Pasteurella multocida toxin

The protein toxin of Pasteurella multocida PMT is a potent mitogen and activator of phospholipase Cbeta. In this study different toxin fragments were investigated. A C-terminal fragment encompassing amino acids 581 through 1285 (PMT581C) was constructed, which was inactive toward intact embryonic bovine lung (EBL) cells after addition to culture medium but caused reorganization of the actin cytoskeleton and rounding up of cells when introduced into the cells by electroporation. As the holotoxin, the toxin fragment PMT581C induced an increase in total inositol phosphate levels after introduction into the cell by electroporation. A C-terminal fragment shorter than PMT581C as well as N-terminal fragments were inactive. Exchange of cysteine-1165 for serine in the holotoxin resulted in a complete loss of the ability to increase inositol phosphate levels. Correspondingly, the mutated toxin fragment PMT581C.C1165S was inactive after cell introduction by electroporation, suggesting an essential role of Cys-1165 in the biological activity of the toxin.

Pleiotropic effects of Pasteurella multocida toxin are mediated by Gq-dependent and -independent mechanisms. involvement of Gq but not G11

Pasteurella multocida toxin (PMT) is a highly potent mitogen for a variety of cell types. PMT has been shown to induce various cellular signaling processes, and it has been suggested to function through the heterotrimeric G-proteins G(q)/G(11). To analyze the role of G(q)/G(11) in the action of PMT, we have studied the effect of the toxin in Galpha(q)/Galpha(11) double-deficient fibroblasts as well as in fibroblasts lacking only Galpha(q) or Galpha(11). Interestingly, formation of inositol phosphates in response to PMT was exclusively dependent on Galpha(q) but not on the closely related Galpha(11). Although Galpha(q)/Galpha(11) double-deficient and Galpha(q)-deficient cells did not respond with any production of inositol phosphates to PMT, PMT was still able to induce various other cellular effects in these cells, including the activation of Rho, the Rho-dependent formation of actin stress fibers and focal adhesions, as well as the stimulation of c-Jun N-terminal kinase and extracellular signal-regulated kinase. These data show that PMT leads to a variety of cellular effects that are mediated only in part by the heterotrimeric G-protein G(q).

Differential modulation and subsequent blockade of mitogenic signaling and cell cycle progression by Pasteurella multocida toxin

The intracellularly acting protein toxin of Pasteurella multocida (PMT) causes numerous effects in cells, including activation of inositol 1,4,5-trisphosphate (IP(3)) signaling, Ca(2+) mobilization, protein phosphorylation, morphological changes, and DNA synthesis. The direct intracellular target of PMT responsible for activation of the IP(3) pathway is the G(q/11)alpha-protein, which stimulates phospholipase C (PLC) beta1. The relationship between PMT-mediated activation of the G(q/11)-PLC-IP(3) pathway and its ability to promote mitogenesis and cellular proliferation is not clear. PMT stimulation of p42/p44 mitogen-activated protein kinase occurs upstream via G(q/11)-dependent transactivation of the epidermal growth factor receptor. We have further characterized the effects of PMT on the downstream mitogenic response and cell cycle progression in Swiss 3T3 and Vero cells. PMT treatment caused dramatic morphological changes in both cell lines. In Vero cells, limited multinucleation, nuclear fragmentation, and disruption of cytokinesis were also observed; however, a strong mitogenic response occurred only with Swiss 3T3 cells. Significantly, this mitogenic response was not sustained. Cell cycle analysis revealed that after the initial mitogenic response to PMT, both cell types subsequently arrested primarily in G(1) and became unresponsive to further PMT treatment. In Swiss 3T3 cells, PMT induced up-regulation of c-Myc; cyclins D1, D2, D3, and E; p21; PCNA; and the Rb proteins, p107 and p130. In Vero cells, PMT failed to up-regulate PCNA and cyclins D3 and E. We also found that the initial PMT-mediated up-regulation of several of these signaling proteins was not sustained, supporting the subsequent cell cycle arrest. The consequences of PMT entry thus depend on the differential regulation of signaling pathways within different cell types.

Localization of the intracellular activity domain of Pasteurella multocida toxin to the N terminus

We have shown that Pasteurella multocida toxin (PMT) directly causes transient activation of Gqalpha protein that is coupled to phosphatidylinositol-specific phospholipase Cbeta1 in Xenopus oocytes (B. A. Wilson, X. Zhu, M. Ho, and L. Lu, J. Biol. Chem. 272:1268-1275, 1997). We found that antibodies directed against an N-terminal peptide of PMT inhibited the toxin-induced response in Xenopus oocytes, but antibodies against a C-terminal peptide did not. To test whether the intracellular activity domain of PMT is localized to the N terminus, we conducted a deletion mutational analysis of the PMT protein, using the Xenopus oocyte system as a means of screening for toxin activity. Using PCR and conventional cloning techniques, we cloned from a toxinogenic strain of P. multocida the entire toxA gene, encoding the 1,285-amino-acid PMT protein, and expressed the recombinant toxin as a His-tagged fusion protein in Escherichia coli. We subsequently generated a series of N-terminal and C-terminal deletion mutants and expressed the His-tagged PMT fragments in E. coli. These proteins were screened for cytotoxic activity on cultured Vero cells and for intracellular activity in the Xenopus oocyte system. Only the full-length protein without the His tag exhibited activity on Vero cells. The full-length PMT and N-terminal fragments containing the first 500 residues elicited responses in oocytes, but the C-terminal 780 amino acid fragment did not. Our results confirm that the intracellular activity domain of PMT is localized to the N-terminal 500 amino acids of the protein and that the C terminus is required for entry into cells.