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

Cellular ROS tolerance determines the effect of plumbagin on osteoclast differentiation

Plumbagin is used in traditional medicine because of its anti-inflammatory and anti-microbial properties. As a naphthoquinone, plumbagin triggers the production of reactive oxygen species (ROS). In vitro cancer studies showed that plumbagin triggers apoptosis in cancer cells through ROS production. As cancer-mediated chronic inflammation can affect bone density, it was hypothesized that plumbagin might directly inhibit the formation of bone-resorbing osteoclasts. We previously showed that the effect of plumbagin on osteoclastogenesis differed between bone marrow-derived macrophages and the macrophage cell line RAW 264.7. Although RAW 264.7 macrophages are able to initiate the gene program required for osteoclastogenesis, only primary macrophages successfully differentiate into osteoclasts. Here, we show that RAW 264.7 cells are more sensitive toward plumbagin-induced apoptosis. In the presence of plumbagin and the cytokine RANKL, which triggers ROS production to drive osteoclastogenesis, RAW 264.7 macrophages produce increased amounts of ROS and die. Addition of the ROS scavenger N-acetyl cysteine prevented cell death, linking the failure to differentiate to increased ROS levels. RAW 264.7 cells show reduced expression of genes protective against oxidative stress, while primary macrophages have a higher tolerance toward ROS. Our data suggest that it is indispensable to consider cell (line)-intrinsic properties when studying phytochemicals.

IFN-γ promotes PANoptosis in Pasteurella multocida toxin-induced pneumonia in mice

Interferon-γ (IFN-γ) is a pleiotropic cytokine that regulates diverse biological functions, including modulation of inflammatory response and innate and adaptive immunity. In our study, we found that IFN-γ plays an important role in the regulation of Pasteurella multocida toxin-associated pneumonia. In work described here, we demonstrated that rPMT induced a lethal pneumonia in WT mice and the severity of the pneumonia was substantially alleviated in IFN-γ-deficient mice, IFN-γ deficiency significantly elevated the survival rate and reduced the pathological lesions of the lungs after rPMT challenged. Notably, IFN-γ deficiency significantly decreased myeloperoxidase (MPO) expression abundance in the lung tissue, and the MPO was mainly expressed in the lung tissue injury region of WT mice. More importantly, IFN-γ deficiency impaired the activation of PANoptosis specific markers, including the caspase 3, GSDMD, and MLKL, and reduced the expression of IL-1β. Cumulatively, this study demonstrates that IFN-γ promotes PANoptosis in PMT induced pneumonia in mice, providing a basis for studying the pathogenic mechanism of PMT.

Pasteurella multocida toxin - lessons learned from a mitogenic toxin

The gram-negative, zoonotic bacterium Pasteurella multocida was discovered in 1880 and found to be the causative pathogen of fowl cholera. Pasteurella-related diseases can be found in domestic and wild life animals such as buffalo, sheep, goat, deer and antelope, cats, dogs and tigers and cause hemorrhagic septicemia in cattle, rhinitis or pneumonia in rabbits or fowl cholera in poultry and birds. Pasteurella multocida does not play a major role in the immune-competent human host, but can be found after animal bites or in people with close contact to animals. Toxigenic strains are most commonly found in pigs and express a phage-encoded 146 kDa protein, the Pasteurella multocida toxin (PMT). Toxin-expressing strains cause atrophic rhinitis where nasal turbinate bones are destroyed through the inhibition of bone building osteoblasts and the activation of bone resorbing osteoclasts. After its uptake through receptor-mediated endocytosis, PMT specifically targets the alpha subunit of several heterotrimeric G proteins and constitutively activates them through deamidation of a glutamine residue to glutamate in the alpha subunit. This results in cytoskeletal rearrangement, proliferation, differentiation and survival of cells. Because of the toxin's mitogenic effects, it was suggested that it might have carcinogenic properties, however, no link between Pasteurella infections and cell transformation could be established, neither in tissue culture models nor through epidemiological data. In the recent years it was shown that the toxin not only affects bone, but also the heart as well as basically all cells of innate and adaptive immunity. During the last decade the focus of research shifted from signal transduction processes to understanding how the bacteria might benefit from a bone-destroying toxin. The primary function of PMT seems to be the modulation of immune cell activation which at the same time creates an environment permissive for osteoclast formation. While the disease is restricted to pigs, the implications of the findings from PMT research can be used to explore human diseases and have a high translational potential. In this review our current knowledge will be summarized and it will be discussed what can be learned from using PMT as a tool to understand human pathologies.

Emerging role of human microbiome in cancer development and response to therapy: special focus on intestinal microflora

In recent years, there has been a greater emphasis on the impact of microbial populations inhabiting the gastrointestinal tract on human health and disease. According to the involvement of microbiota in modulating physiological processes (such as immune system development, vitamins synthesis, pathogen displacement, and nutrient uptake), any alteration in its composition and diversity (i.e., dysbiosis) has been linked to a variety of pathologies, including cancer. In this bidirectional relationship, colonization with various bacterial species is correlated with a reduced or elevated risk of certain cancers. Notably, the gut microflora could potentially play a direct or indirect role in tumor initiation and progression by inducing chronic inflammation and producing toxins and metabolites. Therefore, identifying the bacterial species involved and their mechanism of action could be beneficial in preventing the onset of tumors or controlling their advancement. Likewise, the microbial community affects anti-cancer approaches’ therapeutic potential and adverse effects (such as immunotherapy and chemotherapy). Hence, their efficiency should be evaluated in the context of the microbiome, underlining the importance of personalized medicine. In this review, we summarized the evidence revealing the microbiota's involvement in cancer and its mechanism. We also delineated how microbiota could predict colon carcinoma development or response to current treatments to improve clinical outcomes.

Oral and intestinal bacterial exotoxins: Potential linked to carcinogenesis

Growing evidence suggests that imbalances in resident microbes (dysbiosis) can promote chronic inflammation, immune-subversion, and production of carcinogenic metabolites, thus leading to neoplasia. Yet, evidence to support a direct link of individual bacteria species to human sporadic cancer is still limited. This chapter focuses on several emerging bacterial toxins that have recently been characterized for their potential oncogenic properties toward human orodigestive cancer and the presence of which in human tissue samples has been documented. These include cytolethal distending toxins produced by various members of gamma and epsilon Proteobacteria, Dentilisin from mammalian oral Treponema, Pasteurella multocida toxin, two Fusobacterial toxins, FadA and Fap2, Bacteroides fragilis toxin, colibactin, cytotoxic necrotizing factors and α-hemolysin from Escherichia coli, and Salmonella enterica AvrA. It was clear that these bacterial toxins have biological activities to induce several hallmarks of cancer. Some toxins directly interact with DNA or chromosomes leading to their breakdowns, causing mutations and genome instability, and others modulate cell proliferation, replication and death and facilitate immune evasion and tumor invasion, prying specific oncogene and tumor suppressor pathways, such as p53 and β-catenin/Wnt. In addition, most bacterial toxins control tumor-promoting inflammation in complex and diverse mechanisms. Despite growing laboratory evidence to support oncogenic potential of selected bacterial toxins, we need more direct evidence from human studies and mechanistic data from physiologically relevant experimental animal models, which can reflect chronic infection in vivo, as well as take bacterial-bacterial interactions among microbiome into consideration.

Influence of Pasteurella multocida Toxin on the differentiation of dendritic cells into osteoclasts

Dendritic cells (DC) are antigen-presenting cells that connect the innate and adaptive immune system to ensure an efficient immune response during the course of an infection. Recently, DC came into the spotlight as a potential source of osteoclast progenitors, especially under (auto)inflammatory conditions. The virulence factor Pasteurella multocida Toxin (PMT) causes atrophic rhinitis in pigs, a disease characterised by a severe reduction of nasal bone. Our group and others have shown the potential of PMT in mediating differentiation of monocytes/macrophages into bone-resorbing osteoclasts. However, whether DC are target cells for PMT-induced osteoclast differentiation, is currently unknown. Using different murine DC model systems, we investigated the ability of PMT to induce osteoclast formation in DC. Similar to our previous observations in macrophages, PMT was endocytosed by DC and triggered intracellular deamidation of residue Q209 of the Gq alpha subunit. Still, PMT failed to induce prolonged secretion of osteoclastogenic cytokines and osteoclast formation; instead PMT-treated DC secreted interleukin-12 (IL-12), an inhibitor of osteoclastogenesis. In this study, we show that in comparison to bone marrow-derived macrophages, PMT induces maturation of DC through increased expression of the activation markers CD80 and CD86. As maturation of DC prevents their transdifferentiation into osteoclasts, we hypothesize that PMT, a potent osteoclastogenic toxin, fails to trigger osteoclastogenesis in DC due to its effect on DC maturation and IL-12 production.

Pasteurella multocida Toxin Triggers RANKL-Independent Osteoclastogenesis

Bone remodeling is a continuous process to retain the structural integrity and function of the skeleton. A tight coupling is maintained between osteoclast-mediated resorption of old or damaged bones and osteoblast-mediated formation of new bones for bone homeostasis. While osteoblasts differentiate from mesenchymal stem cells, osteoclasts are hematopoietic in origin and derived from myeloid precursor cells. Osteoclast differentiation is driven by two cytokines, cytokine receptor activator of NF-κB ligand (RANKL), and macrophage colony-stimulating factor. Imbalances in the activity of osteoblasts and osteoclasts result in the development of bone disorders. Bacterially caused porcine atrophic rhinitis is characterized by a loss of nasal ventral conche bones and a distortion of the snout. While Bordetella bronchiseptica strains cause mild and reversible symptoms, infection of pigs with toxigenic Pasteurella multocida strains causes a severe and irreversible decay. The responsible virulence factor Pasteurella multocida toxin (PMT) contains a deamidase activity in its catalytical domain that constitutively activates specific heterotrimeric G proteins to induce downstream signaling cascades. While osteoblasts are inhibited by the toxin, osteoclasts are activated, thus skewing bone remodeling toward excessive bone degradation. Still, the mechanism by which PMT interferes with bone homeostasis, and the reason for this unusual target tissue is not yet well understood. Here, we show that PMT has the potential to differentiate bone marrow-derived macrophages into functional osteoclasts. This toxin-mediated differentiation process is independent of RANKL, a cytokine believed to be indispensable for triggering osteoclastogenesis, as addition of osteoprotegerin to PMT-treated macrophages does not show any effect on PMT-induced osteoclast formation. Although RANKL is not a prerequisite, toxin-primed macrophages show enhanced responsiveness to low concentrations of RANKL, suggesting that the PMT-generated microenvironment offers conditions where low concentrations of RANKL lead to an increase in the number of osteoclasts resulting in increased resorption. PMT-mediated release of the osteoclastogenic cytokines such as IL-6 and TNF-α, but not IL-1, supports the differentiation process. Although the production of cytokines and the subsequent activation of signaling cascades are necessary for PMT-mediated differentiation into osteoclasts, they are not sufficient and PMT-induced activation of G protein signaling is essential for efficient osteoclastogenesis.

Pasteurella multocida Toxin Manipulates T Cell Differentiation

Pasteurella multocida causes various diseases in a broad range of wild and domestic animals. Toxigenic strains of the serotypes A and D produce an AB protein toxin named Pasteurella multocida toxin (PMT). PMT constitutively activates the heterotrimeric G protein subunits Gαq, Gα13, and Gαi through deamidation of a glutamine residue, which results in cytoskeletal rearrangements as well as increased proliferation and survival of the host cell. In human monocytes, PMT alters the lipopolysaccharide (LPS)-induced activation toward a phenotype that suppresses T cell activation. Here we describe that the toxin also modulates CD4-positive T helper (Th) cells directly. PMT amplifies the expansion of Th cells through enhanced cell cycle progression and suppression of apoptosis and manipulates the differentiation of Th subclasses through activation of Signal Transducers and Activators of Transcription (STAT) family members and induction of subtype-specific master transcription factors. A large population of toxin-treated T cells is double-positive for Foxp3 and RORγt, the transcription factors expressed by Treg and Th17 cells, respectively. This suggests that these cells could have the potential to turn into Th17 cells or suppressive Treg cells. However, in terms of function, the PMT-differentiated cells behave as inflammatory Th17 cells that produce IL-17 and trigger T cell proliferation.

Granzyme A produces bioactive IL-1β through a nonapoptotic inflammasome-independent pathway

Bacterial components are recognized by the immune system through activation of the inflammasome, eventually causing processing of the proinflammatory cytokine interleukin-1? (IL-1?), a pleiotropic cytokine and one of the most important mediators of inflammation, through the protease caspase-1. Synthesis of the precursor protein and processing into its bioactive form are tightly regulated, given that disturbed control of IL-1? release can cause severe autoinflammatory diseases or contribute to cancer development. We show that the bacterial Pasteurella multocida toxin (PMT) triggers Il1b gene transcription in macrophages independently of Toll-like receptor signaling through RhoA/Rho-kinase-mediated NF-?? activation. Furthermore, PMT mediates signal transducer and activator of transcription (STAT) protein-controlled granzyme A (a serine protease) expression in macrophages. The exocytosed granzyme A enters target cells and mediates IL-1? maturation independently of caspase-1 and without inducing cytotoxicity. These findings show that macrophages can induce an IL-1?-initiated immune response independently of inflammasome activity.

What a difference a Dalton makes: bacterial virulence factors modulate eukaryotic host cell signaling systems via deamidation

Pathogenic bacteria commonly deploy enzymes to promote virulence. These enzymes can modulate the functions of host cell targets. While the actions of some enzymes can be very obvious (e.g., digesting plant cell walls), others have more subtle activities. Depending on the lifestyle of the bacteria, these subtle modifications can be crucially important for pathogenesis. In particular, if bacteria rely on a living host, subtle mechanisms to alter host cellular function are likely to dominate. Several bacterial virulence factors have evolved to use enzymatic deamidation as a subtle posttranslational mechanism to modify the functions of host protein targets. Deamidation is the irreversible conversion of the amino acids glutamine and asparagine to glutamic acid and aspartic acid, respectively. Interestingly, all currently characterized bacterial deamidases affect the function of the target protein by modifying a single glutamine residue in the sequence. Deamidation of target host proteins can disrupt host signaling and downstream processes by either activating or inactivating the target. Despite the subtlety of this modification, it has been shown to cause dramatic, context-dependent effects on host cells. Several crystal structures of bacterial deamidases have been solved. All are members of the papain-like superfamily and display a cysteine-based catalytic triad. However, these proteins form distinct structural subfamilies and feature combinations of modular domains of various functions. Based on the diverse pathogens that use deamidation as a mechanism to promote virulence and the recent identification of multiple deamidases, it is clear that this enzymatic activity is emerging as an important and widespread feature in bacterial pathogenesis.

Implications of non-canonical G-protein signaling for the immune system

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which consist of three subunits α, β, and γ, function as molecular switches to control downstream effector molecules activated by G protein-coupled receptors (GPCRs). The GTP/GDP binding status of Gα transmits information about the ligand binding state of the GPCR to intended signal transduction pathways. In immune cells heterotrimeric G proteins impact signal transduction pathways that directly, or indirectly, regulate cell migration, activation, survival, proliferation, and differentiation. The cells of the innate and adaptive immune system abundantly express chemoattractant receptors and lesser amounts of many other types of GPCRs. But heterotrimeric G-proteins not only function in classical GPCR signaling, but also in non-canonical signaling. In these pathways the guanine exchange factor (GEF) exerted by a GPCR in the canonical pathway is replaced or supplemented by another protein such as Ric-8A. In addition, other proteins such as AGS3-6 can compete with Gβγ for binding to GDP bound Gα. This competition can promote Gβγ signaling by freeing Gβγ from rapidly rebinding GDP bound Gα. The proteins that participate in these non-canonical signaling pathways will be briefly described and their role, or potential one, in cells of the immune system will be highlighted.

Signaling cascades of Pasteurella multocida toxin in immune evasion

Pasteurella multocida toxin (PMT) is a protein toxin found in toxigenic strains of Pasteurella multocida. PMT is the causative agent for atrophic rhinitis in pigs, a disease characterized by loss of nasal turbinate bones due to an inhibition of osteoblast function and an increase in osteoclast activity and numbers. Apart from this, PMT acts as a strong mitogen, protects from apoptosis and has an impact on the differentiation and function of immune cells. Many signaling pathways have been elucidated, however, the effect of these signaling cascades as a means to subvert the host’s immune system are just beginning to unravel.

Pasteurella multocida: from zoonosis to cellular microbiology

In a world where most emerging and reemerging infectious diseases are zoonotic in nature and our contacts with both domestic and wild animals abound, there is growing awareness of the potential for human acquisition of animal diseases. Like other Pasteurellaceae, Pasteurella species are highly prevalent among animal populations, where they are often found as part of the normal microbiota of the oral, nasopharyngeal, and upper respiratory tracts. Many Pasteurella species are opportunistic pathogens that can cause endemic disease and are associated increasingly with epizootic outbreaks. Zoonotic transmission to humans usually occurs through animal bites or contact with nasal secretions, with P. multocida being the most prevalent isolate observed in human infections. Here we review recent comparative genomics and molecular pathogenesis studies that have advanced our understanding of the multiple virulence mechanisms employed by Pasteurella species to establish acute and chronic infections. We also summarize efforts being explored to enhance our ability to rapidly and accurately identify and distinguish among clinical isolates and to control pasteurellosis by improved development of new vaccines and treatment regimens.

Substrate specificity of Pasteurella multocida toxin for α subunits of heterotrimeric G proteins

Pasteurella multocida is the causative agent of a number of epizootic and zoonotic diseases. Its major virulence factor associated with atrophic rhinitis in animals and dermonecrosis in bite wounds is P. multocida toxin (PMT). PMT stimulates signal transduction pathways downstream of heterotrimeric G proteins, leading to effects such as mitogenicity, blockade of apoptosis, or inhibition of osteoblast differentiation. On the basis of Gα(i2), it was demonstrated that the toxin deamidates an essential glutamine residue of the Gα(i2) subunit, leading to constitutive activation of the G protein. Here, we studied the specificity of PMT for its G-protein targets by mass spectrometric analyses and by utilizing a monoclonal antibody, which recognizes specifically G proteins deamidated by PMT. The studies revealed deamidation of 3 of 4 families of heterotrimeric G proteins (Gα(q/11), Gα(i1,2,3), and Gα(12/13) of mouse or human origin) by PMT but not by a catalytic inactive toxin mutant. With the use of G-protein fragments and chimeras of responsive or unresponsive G proteins, the structural basis for the discrimination of heterotrimeric G proteins was studied. Our results elucidate substrate specificity of PMT on the molecular level and provide evidence for the underlying structural reasons of substrate discrimination.

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.

Toxin-induced RhoA activity mediates CCL1-triggered signal transducers and activators of transcription protein signaling

RhoA is reportedly involved in signal transducers and activators of transcription (STAT)-dependent transcription. However, the pathway connecting the GTPase and STAT signaling has not been characterized. Here, we made use of bacterial toxins, which directly activate Rho GTPases to analyze this pathway. Cytotoxic necrotizing factors (CNFs) are produced by pathogenic Escherichia coli strains and by Yersinia pseudotuberculosis. They activate small GTPases of the Rho family by deamidation of a glutamine, which is crucial for GTP hydrolysis. We show that RhoA activation leads to phosphorylation and activation of STAT3 and identify signal proteins involved in this pathway. RhoA-dependent STAT3 stimulation requires ROCK and Jun kinase activation as well as AP1-induced protein synthesis. The secretion of one or more factors activates the JAK-STAT pathway in an auto/paracrine manner. We identify CCL1/I-309 as an essential cytokine, which is produced and secreted upon RhoA activation and which is able to activate STAT3-dependent signaling pathways.

The Pasteurella multocida toxin: a new paradigm for the link between bacterial infection and cancer

The concept that bacterial infection could cause cancer has only recently become accepted because of the strong epidemiological and molecular evidence for a major carcinogenic role played by Helicobacter pylori. However, information on other potential bacterial carcinogens is very limited and thereby unconvincing. A different approach is to assess bacteria for potentially pro-carcinogenic properties. The Pasteurella multocida toxin (PMT) has many properties that mark it out as a potential carcinogen. PMT is a highly potent mitogen and has been demonstrated to block apoptosis. PMT modifies and activates members of three of the four families of heterotrimeric G-proteins, all of which have potential roles in carcinogenesis. Many signalling components downstream of these G-proteins are known proto-oncogenes and have been shown to be activated by PMT. These include, amongst others, the Rho GTPase, focal adhesion kinase, cyclooxygenase-2, β-catenin signalling and calcium signalling. PMT action potentially influences many of the acquired Hanahan/Weinberg capabilities necessary for oncogenic transformation. Although there is little evidence that PMT might have a role in human cancer, it serves as an important and novel paradigm for a bacterial link to cancer.

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.

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.

Bacterial protein toxins that modify host regulatory GTPases

Many bacterial pathogens produce protein toxins to outmanoeuvre the immune system of the host. Some of these proteins target regulatory GTPases such as those belonging to the RHO family, which control the actin cytoskeleton of the host cell. In this Review, I discuss a diversity of mechanisms that are used by bacterial effectors and toxins to modulate the activity of host GTPases, with a focus on covalent modifications such as ADP-ribosylation, glucosylation, adenylylation, proteolysis, deamidation and transglutamination.

Pasteurella multocida Toxin-induced Pim-1 expression disrupts suppressor of cytokine signalling (SOCS)-1 activity

Pasteurella multocida Toxin (PMT) is a mitogenic protein toxin that manipulates signal transduction cascades of mammalian host cells and upregulates Janus kinase (JAK) and signal transducers of transcription (STAT) activity. Here we show that in the presence of PMT, increased levels of suppressors of cytokine signalling-1 (SOCS-1) proteins significantly enhance STAT activity. This occurs via PMT-induced expression of the serine/threonine kinase Pim-1 and subsequent threonine phosphorylation of SOCS-1. The ability of SOCS-1 to act as an E3 ubiquitin ligase is regulated by its phosphorylation status. Thus, the tyrosine kinase JAK2 cannot be marked for proteasomal degradation by threonine phosphorylated SOCS-1. Consequently, the expression levels of JAK2 are increased, eventually leading to hyperactivity of JAK2 and its target, the transcription factor STAT3. Eventually this causes increased anchorage-independent cell growth that correlates with the expression levels of SOCS-1. Interestingly, endogenous SOCS-1 production after Toll-like receptor activation also causes STAT3 hyperactivation. Thus we hypothesize that P. multocida Toxin alters host cell signalling using mechanisms that have so far only been known to be employed by oncogenic viral kinases to avoid host immune defence mechanisms.

Effector prediction in host-pathogen interaction based on a Markov model of a ubiquitous EPIYA motif

Background

Effector secretion is a common strategy of pathogen in mediating host-pathogen interaction. Eight EPIYA-motif containing effectors have recently been discovered in six pathogens. Once these effectors enter host cells through type III/IV secretion systems (T3SS/T4SS), tyrosine in the EPIYA motif is phosphorylated, which triggers effectors binding other proteins to manipulate host-cell functions. The objectives of this study are to evaluate the distribution pattern of EPIYA motif in broad biological species, to predict potential effectors with EPIYA motif, and to suggest roles and biological functions of potential effectors in host-pathogen interactions.

Results

A hidden Markov model (HMM) of five amino acids was built for the EPIYA-motif based on the eight known effectors. Using this HMM to search the non-redundant protein database containing 9,216,047 sequences, we obtained 107,231 sequences with at least one EPIYA motif occurrence and 3115 sequences with multiple repeats of the EPIYA motif. Although the EPIYA motif exists among broad species, it is significantly over-represented in some particular groups of species. For those proteins containing at least four copies of EPIYA motif, most of them are from intracellular bacteria, extracellular bacteria with T3SS or T4SS or intracellular protozoan parasites. By combining the EPIYA motif and the adjacent SH2 binding motifs (KK, R4, Tarp and Tir), we built HMMs of nine amino acids and predicted many potential effectors in bacteria and protista by the HMMs. Some potential effectors for pathogens (such as Lawsonia intracellularis, Plasmodium falciparum and Leishmania major) are suggested.

Conclusions

Our study indicates that the EPIYA motif may be a ubiquitous functional site for effectors that play an important pathogenicity role in mediating host-pathogen interactions. We suggest that some intracellular protozoan parasites could secrete EPIYA-motif containing effectors through secretion systems similar to the T3SS/T4SS in bacteria. Our predicted effectors provide useful hypotheses for further studies.

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

Pasteurella multocida toxin activates various heterotrimeric G proteins by deamidation

Pasteurella multocida produces a 146-kDa protein toxin (Pasteurella multocida toxin, PMT), which stimulates diverse cellular signal transduction pathways by activating heterotrimeric G proteins. PMT deamidates a conserved glutamine residue of the α-subunit of heterotrimeric G proteins that is essential for GTP-hydrolysis, thereby arresting the G protein in the active state. The toxin substrates are Gα(q) Gα(13) and the Gα(i)-family proteins. Activation of these α-subunits causes stimulation of phospholipase Cβ, Rho-guanine nucleotide exchange factors or inhibition of adenylyl cyclase. This article provides the current knowledge on PMT concerning the structure-function analysis based on the crystal structure and recently elucidated molecular mode of action. Furthermore, the impact of PMT on cellular signaling is discussed.

Pasteurella multocida toxin activation of heterotrimeric G proteins by deamidation

Pasteurella multocida toxin is a major virulence factor of Pasteurella multocida, which causes pasteurellosis in men and animals and atrophic rhinitis in rabbits and pigs. The approximately 145 kDa protein toxin stimulates various signal transduction pathways by activating heterotrimeric G proteins of the Galpha(q), Galpha(i), and Galpha(12/13) families by using an as yet unknown mechanism. Here, we show that Pasteurella multocida toxin deamidates glutamine-205 of Galpha(i2) to glutamic acid. Therefore, the toxin inhibits the intrinsic GTPase activity of Galpha(i) and causes persistent activation of the G protein. A similar modification is also evident for Galpha(q), but not for the closely related Galpha(11), which is not a substrate of Pasteurella multocida toxin. Our data identify the alpha-subunits of heterotrimeric G proteins as the direct molecular target of Pasteurella multocida toxin and indicate that the toxin does not act like a protease, which was suggested from its thiol protease-like catalytic triad, but instead causes constitutive activation of G proteins by deamidase activity.

Signal transduction--receptors, mediators, and genes

The 2008 annual meeting of the Signal Transduction Society covered a broad spectrum of topics, with signaling in immune cells as the special focus of the meeting. Many of the immune signaling talks concerned B and T lymphocytes in particular; the role of inflammatory cytokines in cancer progression was also addressed. Neoplastic development was also discussed with regard to aspects of cell cycle control, aging, and transformation. Topics extended to signaling pathways induced by bacteria, viruses, and environmental toxins, as well as those involved in differentiation, morphogenesis, and cell death. This international and interdisciplinary scientific gathering induced lively discussions and close interactions between participants.

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.

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.