Mammalian target of rapamycin complex 1 (mTORC1) plays a role in Pasteurella multocida toxin (PMT)-induced protein synthesis and proliferation in Swiss 3T3 cells

Revealing the lethal effects of Pasteurella multocida toxin on multiple organ systems

Pasteurella multocida toxin (PMT) is one of the most important virulence factors of Pasteurella multocida type D. Pasteurella multocida infection has caused enormous economic losses in the pig farming industry. Although it is well known that this bacterial infection causes progressive atrophic rhinitis, its effects on other organ tissues in pigs are unclear. In this study, PMT was expressed and purified, and the cytotoxic effects of PMT on four types of swine cells, LLC-PK1, PAM, IPEC, and ST, were investigated. LLC-PK1 exhibited the highest sensitivity to the cytotoxic effects of PMT. Our studies revealed that a PMT concentration of 0.1 μg/kg can lead to weight loss, whereas a PMT concentration of 0.5 μg/kg can lead to death in mice. PMT causes damage to the intestines, kidneys, lungs, livers, and spleens of mice. Furthermore, PMT caused acute death in pigs at treatment concentrations greater than 5 μg/kg; at PMT concentration of 2.5 μg/kg, weight loss occurred until death. PMT mainly caused damage to the hearts, lungs, livers, spleens and kidneys of pigs. The organ coefficient showed that damage to the heart and kidneys was the most severe and caused the renal pelvis and renal pyramid to dissolve and become cavitated. Pathology revealed hemorrhage in the lungs, liver, and spleen, and the kidneys were swollen and vacuolated, which was consistent with the damaged target organs in the mice. In conclusion, these findings demonstrate that PMT is extremely toxic in vitro and in vivo, causing damage to various organs of the body, especially the kidneys and lungs. This study provides a theoretical basis for the in-depth exploration of the cytotoxic effects of PMT on target organs.

CXCL8 Knockout: A Key to Resisting Pasteurella multocida Toxin-Induced Cytotoxicity

Pasteurella multocida, a zoonotic pathogen that produces a 146-kDa modular toxin (PMT), causes progressive atrophic rhinitis with severe turbinate bone degradation in pigs. However, its mechanism of cytotoxicity remains unclear. In this study, we expressed PMT, purified it in a prokaryotic expression system, and found that it killed PK15 cells. The host factor CXCL8 was significantly upregulated among the differentially expressed genes in a transcriptome sequencing analysis and qPCR verification. We constructed a CXCL8-knockout cell line with a CRISPR/Cas9 system and found that CXCL8 knockout significantly increased resistance to PMT-induced cell apoptosis. CXCL8 knockout impaired the cleavage efficiency of apoptosis-related proteins, including Caspase3, Caspase8, and PARP1, as demonstrated with Western blot. In conclusion, these findings establish that CXCL8 facilitates PMT-induced PK15 cell death, which involves apoptotic pathways; this observation documents that CXCL8 plays a key role in PMT-induced PK15 cell death.

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.

Acute Conditioning of Antigen-Expanded CD8+ T Cells via the GSK3β-mTORC Axis Differentially Dictates Their Immediate and Distal Responses after Antigen Rechallenge

Simple Summary

Expanded, antigen-experienced CD8⁺ T cells are utilized in immunotherapy to treat infections and cancers. Antigen rechallenge of these cells leads to their re-expansion. The effector functions of re-expanded CD8⁺ T cells are critical for their therapeutic efficacy. We found that acute conditioning of the cells, before antigen rechallenge, impacts their effector function after re-expansion. Our data showed that acute pharmacological modulation of the GSK3β-mTORC axis with TWS119 or rapamycin, but not Torin1, before antigen rechallenge promotes the effector functions of re-expanded CD8⁺ T cells. These findings suggest that acute conditioning of the GSK3β-mTORC axis in expanded CD8⁺ T cells, before antigen rechallenge, can promote the therapeutic performance of re-expanded CD8⁺ T cells.

Abstract

CD8⁺ T cells protect against tumors and intracellular pathogens. The inflammatory cytokines IL-2, IL-15, and IL-7 are necessary for their expansion. However, elevated serum levels of these cytokines are often associated with cancer, poorer prognosis of cancer patients, and exhaustion of antigen-expanded CD8⁺ T cells. The impact of acute conditioning of antigen-expanded CD8⁺ T cells with these cytokines is unknown. Here, we generated antigen-expanded CD8⁺ T cells using dendritic cells and PC-3 cells. The cells were acutely (18–24 h) conditioned with IL-2 and either the GSK3β inhibitor TWS119, the mTORC1 inhibitor rapamycin, or the mTORC1/2 inhibitor Torin1, then their immediate and post-re-expansion (distal) cytokine responses after antigen rechallenge were evaluated. We found that acute IL-2 conditioning upregulated the immediate antigen-induced cytokine response of the tested cells. Following their re-expansion, however, the cells showed a decreased cytokine response. These IL-2 conditioning-mediated impacts were counteracted with TWS119 or rapamycin but not with Torin1. Our data revealed that the acute conditioning of antigen-expanded CD8⁺ T cells with IL-2 modulates the GSK3β-mTORC signaling axis. This modulation differentially affected the immediate and distal cytokine responses of the cells. The acute targeting of this signaling axis could, therefore, represent a novel strategy for the modulation of antigen-expanded CD8⁺ T cells.

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.

Inhibition of Methamphetamine Self-Administration and Reinstatement by Central Blockade of Angiotensin II Receptor in Rats

The molecular mechanism and treatment of methamphetamine (METH) use disorder remain unclear. The current study aimed to investigate the role of central angiotensin II receptor (ATR) in drug taking and seeking behavior associated with METH use disorder. The effect of an ATR type 1 (AT1R) antagonist, candesartan cilexetil, on the reinforcing and motivational effects of METH was first assessed using the animal model of METH self-administration (SA) and reinstatement. The levels of dopamine D2 receptor (D2R) and AT1R were subsequently examined. Furthermore, the present study determined the expression of microRNAs (miRNAs) by comparing METH SA, METH-yoked, and Saline-yoked groups. The target miRNAs were further overexpressed in the nucleus accumbens (NAc) via a lentivirus vector to investigate the effects of target miRNAs on METH SA maintained under a fixed ratio 1, progressive ratio, and cue/drug reinstatement of METH SA. The potential role of the AT1R-PLCβ-CREB signaling pathway was finally investigated. The results suggest that AT1R blockade effectively reduced METH SA and reinstatement, in conjunction with the counter-regulation of D2R and AT1R. A total of 17 miRNAs targeting Ang II in NAc were found to be associated with the voluntary intake of METH. Furthermore, overexpression of specific miR-219a-5p targeting AT1R-regulated METH SA and reinstatement. The AT1R-PLCβ-CREB signaling pathway was found to be associated with the effect of AT1R on the drug-taking and drug-seeking behavior involving METH use disorder.

Astragaloside II promotes intestinal epithelial repair by enhancing L-arginine uptake and activating the mTOR pathway

Astragaloside II (AS II) extracted from Astragalus membranaceus has been reported to promote tissue wound repair. However, the effect of AS II on inflammatory bowel disease is unknown. We investigated the effects and mechanism of AS II on intestinal wound healing in both in vitro and in vivo models. Human intestinal Caco-2 cells were treated with multiple concentrations of AS II to assess cell proliferation, scratch wound closure, L-arginine uptake, cationic amino acid transporter activity, and activation of the mTOR signaling pathway. These effects were also measured in a mouse model of colitis. AS II promoted wound closure and increased cell proliferation, L-arginine uptake, CAT1 and CAT2 protein levels, total protein synthesis, and phosphorylation of mTOR, S6K, and 4E-BP1 in Caco-2 cells. These effects were suppressed by lysine or rapamycin treatment, suggesting that the enhanced arginine uptake mediates AS II-induced wound healing. Similar results were also observed in vivo. Our findings indicate that AS II can contribute to epithelial barrier repair following intestinal injury, and may offer a therapeutic avenue in treating irritable bowel disease.

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- induced osteoclastogenesis requires mTOR activation

Background

Pasteurella multocida toxin (PMT) is a potent inducer of osteoclast formation. Pigs suffering from an infection with toxigenic Pasteurella multocida strains develop atrophic rhinitis characterised by a loss of turbinate bones and conchae. However, on the molecular level the process of bone loss remains largely uncharacterised.

Results

Recently it was found that PMT activates the serine/threonine kinase mammalian target of rapamycin (mTOR) in fibroblasts. Using RAW264.7 macrophages, we investigated the role of the mTOR complex 1 (mTORC1) in PMT-mediated osteoclast formation. PMT induces the differentiation of RAW264.7 macrophages into multinucleated, tartrate resistant acid phosphatase (TRAP) positive osteoclasts that are capable to resorb bone. In the presence of the mTORC1 inhibitor rapamycin, PMT was significantly less able to induce the formation of TRAP-positive osteoclasts. Accordingly, the resulting resorption of bone was strongly reduced. A major target of mTOR is the 70 kDa ribosomal protein S6 kinase 1 (p70 S6K1). Activated p70 S6K1 decreases the expression of programmed cell death protein 4 (PDCD4), a negative transcriptional regulator of osteoclastogenesis, at the protein and gene level. Ultimately this results in the activation of c-Jun, a component of the activator protein 1 (AP-1) complex, which is a major transcription factor for the induction of osteoclast-specific genes. We now demonstrate that c-Jun and its downstream target, the osteoclast-specific bone degrading protease cathepsin K, are upregulated upon PMT treatment in an mTOR-dependent manner.

Conclusions

Activation of mTOR signalling plays a central role in the formation of osteoclasts through the bacterial toxin PMT. On the molecular level, PMT-induced activation of mTOR leads to down regulation of PDCD4, a known repressor of AP-1 complex, culminating in the activation of c-Jun, an essential transcription factor for triggering osteoclastogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-015-0117-7) contains supplementary material, which is available to authorized users.

mTOR pathway is involved in ADP-evoked astrocyte activation and ATP release in the spinal dorsal horn in a rat neuropathic pain model

BACKGROUND

ATP/ADP-evoked spinal astrocyte activation plays a vital role in the development of neuropathic pain. We aim to investigate the role of mammalian target of rapamycin (mTOR) pathway on the spinal astrocyte activation in the neuropathic pain development in rats.

METHODS

Sprague Dawley (SD) rats were subjected to chronic constriction of the sciatic nerve (CCI). Rapamycin or ADP was intrathecally injected daily to explore their effects on spinal astrocyte activation and pain development. Expression of glial fibrillary acidic protein (GFAP) and mTOR in the spinal dorsal horn was assessed by immunohistochemistry. Von Frey hairs and Hargreaves paw withdrawal test were conducted to evaluate mechanical allodynia and thermal sensitivity, respectively. Firefly luciferase ATP assay was used to assess the change of ATP level in cerebrospinal fluid (CSF) and medium of cultured astrocytes.

RESULTS

GFAP expression was enhanced in the ipsilateral spinal dorsal horn from day 3 after surgery. GFAP and mTOR expression in the rat spinal dorsal horn on post-surgical day 14 was enhanced by daily intrathecal injection of ADP, which was inhibited by rapamycin. Rapamycin decreased lower mechanical pain threshold and the thermal withdrawal latency. Intrathecal injection of ADP enhanced the ATP release, which was partially inhibited by rapamycin. Study of cultured astrocytes indicated that ATP could be released from astrocytes.

CONCLUSION

Our data demonstrated that ADP enhanced neuropathic pain in CCI rats, which was inhibited by rapamycin. This study indicates that targeting mTOR pathway could serve as a novel therapeutic strategy in neuropathic pain management.

G protein-coupled receptors and the regulation of autophagy

Autophagy is an important catabolic cellular process that eliminates damaged and unnecessary cytoplasmic proteins and organelles. Basal autophagy occurs during normal physiological conditions, but the activity of this process can be significantly altered in human diseases. Thus, defining the regulatory inputs and signals that control autophagy is essential. Nutrients are key modulators of autophagy. Although autophagy is generally accepted to be regulated in a cell-autonomous fashion, recent studies suggest that nutrients can modulate autophagy in a systemic manner by inducing the secretion of hormones and neurotransmitters that regulate G protein-coupled receptors (GPCRs). Emerging studies show that GPCRs also regulate autophagy by directly detecting extracellular nutrients. We review the role of GPCRs in autophagy regulation, highlighting their potential as therapeutic drug targets.

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.

The actions of Pasteurella multocida toxin on neuronal cells

Pasteurella multocida toxin (PMT) activates the G-proteins Gα_i(1-3), Gα_q, Gα_11, Gα₁₂ and Gα₁₃ by deamidation of specific glutamine residues. A number of these alpha subunits have signalling roles in neurones. Hence we studied the action of this toxin on rat superior cervical ganglion (SCG) neurones and NG108-15 neuronal cells. Both Gα_q and Gα₁₁ could be identified in SCGs with immunocytochemistry. PMT had no direct action on Kv7 or Cav2 channels in SCGs. However PMT treatment enhanced muscarinic receptor mediated inhibition of M-current (Kv7.2 + 7. 3) as measured by a 19-fold leftward shift in the oxotremorine-M concentration–inhibition curve. Agonists of other receptors, such as bradykinin or angiotensin, that inhibit M-current did not produce this effect. However the amount of PIP₂ hydrolysis could be enhanced by PMT for all three agonists. In a transduction system in SCGs that is unlikely to be affected by PMT, Go mediated inhibition of calcium current, PMT was ineffective whereas the response was blocked by pertussis toxin as expected.

M1 muscarinic receptor evoked calcium mobilisation in transformed NG108-15 cells was enhanced by PMT. The calcium rises evoked by uridine triphosphate acting on endogenous P2Y₂ receptors in NG108-15 cells were enhanced by PMT. The time and concentration dependence of the PMT effect was different for the resting calcium compared to the calcium rise produced by activation of P2Y₂ receptors. PMT's action on these neuronal cells would suggest that if it got into the brain, symptoms of a hyperexcitable nature would be seen, such as seizures.

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Pasteurella multocida toxin (PMT) activates a range of G-protein alpha subunits.

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PMT increased muscarinic receptor mediated suppression of Kv7 potassium current in sympathetic neurones.

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PMT enhances both muscarinic and purinergic receptor mediated calcium mobilisation in NG108-15 cells.

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Both these events are mediated by the G-proteins Gq or G11.

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We would predict that the symptoms of central nervous system PMT toxicity would be hyperexcitable events such as seizures.

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.

Pasteurella multocida toxin (PMT) upregulates CTGF which leads to mTORC1 activation in Swiss 3T3 cells

Pasteurella multocida toxin (PMT) is a mitogenic protein that hijacks cellular signal transduction pathways via deamidation of heterotrimeric G proteins. We previously showed that rPMT activates mTOR signaling via a Gαq/11/PLCβ/PKC mediated pathway, leading in part to cell proliferation and migration. Herein, we show that mTOR and MAPK, but not membrane-associated tyrosine kinases, are activated in serum-starved 3T3 cells by an autocrine/paracrine substance(s) secreted into the conditioned medium following rPMT treatment. Surprisingly, this diffusible factor(s) is capable of activating mTOR and MAPK pathways even in MEF Gαq/11 double knockout cells. Microarray analysis identified connective tissue growth factor (CTGF) mRNA as the most upregulated gene in rPMT-treated serum-starved 3T3 cells relative to untreated cells. These results were further confirmed using RT-PCR and Western blot analyses. In accord with rPMT-induced mTOR activation, upregulation of CTGF protein was observed in WT MEF, but not in Gαq/11 double knockout MEF cells. Although CTGF expression is regulated by TGFβ, rPMT did not activate TGFβ pathway. In addition, MEK inhibitors U0126 or PD98059, but not mTOR specific inhibitors, rapamycin and Torin 1, inhibited rPMT-induced upregulation of CTGF. Importantly, CTGF overexpression in serum-starved 3T3 cells using adenovirus led to phosphorylation of ribosomal protein S6, a downstream target of mTOR. However, despite the ability of CTGF to activate the mTOR pathway, upregulation of CTGF alone could not induce morphological changes as those observed in rPMT-treated cells. Our findings reveal that CTGF plays an important role, but there are additional factors involved in the mitogenic action of PMT.