Post-transcriptional Inhibition of Luciferase Reporter Assays by the Nod-like Receptor Proteins NLRX1 and NLRC3

Luciferase reporter assays (LRAs) are widely used to assess the activity of specific signal transduction pathways. Although powerful, rapid and convenient, this technique can also generate artifactual results, as revealed for instance in the case of high throughput screens of inhibitory molecules. Here we demonstrate that the previously reported inhibitory effect of the Nod-like receptor (NLR) protein NLRX1 on NF-κB- and type I interferon-dependent pathways in LRAs was a nonspecific consequence of the overexpression of the NLRX1 leucine-rich repeat (LRR) domain. By comparing luciferase activity and luciferase gene expression using quantitative PCR from the same samples, we showed that NLRX1 inhibited LRAs in a post-transcriptional manner. In agreement, NLRX1 also repressed LRAs if luciferase was expressed under the control of a constitutive promoter, although the degree of inhibition by NLRX1 seemed to correlate with the dynamic inducibility of luciferase reporter constructs. Similarly, we observed that overexpression of another NLR protein, NLRC3, also resulted in artifactual inhibition of LRAs; thus suggesting that the capacity to inhibit LRAs at a post-transcriptional level is not unique to NLRX1. Finally, we demonstrate that host type I interferon response to Sendai virus infection was normal in NLRX1-silenced human HEK293T cells. Our results thus highlight the fact that LRAs are not a reliable technique to assess the inhibitory function of NLRs, and possibly other overexpressed proteins, on signal transduction pathways

A murine systemic model for shigellosis

Summary Shigella spp. are pathogenic bacteria responsible for bacillary dysentery in humans. The major lesions in colonic mucosa are intense inflammation with apoptosis of macrophages and release of pro-inflammatory cytokines. The study of shigellosis is hindered by the natural resistance of rodents to oral infection with Shigella. Therefore, animal models exploit other routes of infection. Here, we describe a novel murine model in which animals receive shigellae via the caudal vein. Mice infected with 5 × 106 (LD50) virulent shigellae died at 48 h post infection, whereas animals receiving non-invasive mutants survived. The liver is the main target of infection, where shigellae induce microgranuloma formation. In mice infected with invasive bacteria, high frequency of apoptotic cells is observed within hepatic microgranulomas along with significant levels of mRNA for pro-inflammatory cytokines such as IL-1β, IL-18, IL-12 and IFN-γ. Moreover, in the blood of these animals high levels of IL-6 and transaminases are detected. Our results demonstrate the intravenous model is suitable for pathogenicity studies and useful to explore the immune response after Shigella infection

Amino Acid Starvation Induced by Invasive Bacterial Pathogens Triggers an Innate Host Defense Program

SummaryAutophagy, which targets cellular constituents for degradation, is normally inhibited in metabolically replete cells by the metabolic checkpoint kinase mTOR. Although autophagic degradation of invasive bacteria has emerged as a critical host defense mechanism, the signals that induce autophagy upon bacterial infection remain unclear. We find that infection of epithelial cells with Shigella and Salmonella triggers acute intracellular amino acid (AA) starvation due to host membrane damage. Pathogen-induced AA starvation caused downregulation of mTOR activity, resulting in the induction of autophagy. In Salmonella-infected cells, membrane integrity and cytosolic AA levels rapidly normalized, favoring mTOR reactivation at the surface of the Salmonella-containing vacuole and bacterial escape from autophagy. In addition, bacteria-induced AA starvation activated the GCN2 kinase, eukaryotic initiation factor 2α, and the transcription factor ATF3-dependent integrated stress response and transcriptional reprogramming. Thus, AA starvation induced by bacterial pathogens is sensed by the host to trigger protective innate immune and stress responses

The bacterial and cellular determinants controlling the recruitment of mTOR to the Salmonella-containing vacuole

Summary Bacterial invasion results in the rapid induction of an acute state of cytosolic amino acid (AA) starvation, provoked by host membrane damage. Bacteria-induced AA starvation, in turn, down-regulates mTOR signaling while triggering autophagy and the integrated stress response pathway dependent on GCN2, eIF2α and ATF3. In Salmonella-infected cells, we now demonstrate that the host AA starvation response program depended on the Salmonella pathogenicity island (SPI)-1, the activity of which was required to damage the Salmonella-containing vacuole (SCV) in the early stage of infection. At a later stage (3–4 hour post-infection), the progressive recruitment of mTOR to the surface of the SCV appeared to be independent of the activity of SPI-2 and of SCV positioning in the cell. Instead, mTOR localization to the SCV required the activity of host AA transporters SLC1A5, SLC3A2 and SLC7A5, resulting in bacterial escape from autophagy. These results expand our understanding of the mechanisms underlying the AA starvation response in Salmonella-infected cells

Adhesion molecules expression and cytokines profiles in ileal tissue of healthy and paratuberculosis affected sheep.

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Microbiota, Innate Immune System, and Gastrointestinal Muscle Ongoing Studies

Aim: To test the activities of culture-extracted or commercially available toll-like receptors (TLRs) ligands to establish their direct impact on target gastrointestinal motor cells. Methods: Short-term and long-term effects of Shigella flexneri M90T and Escherichia coil K-2 strains-extracted lipopolysaccharides (LPS), commercially highly purified LPS (E. coli 0111:B4 and EH100), and Pam2CSK4 and Pam3CSK4, which bind TLR2/6 and TLR1/2 heterodimers, respectively, have been assessed on pure primary cultures of colonic human smooth muscle cells (HSMC). Results: Pathogenic Shigella-LPS and nonpathogenic E. coli K-2-LPS induced a time-dependent decrease of resting cell length and acetylcholine-induced contraction, with both alterations occurring rapidly and being more pronounced in response to the former. However, their effects differed, prolonging HSMC exposure with Shigella-LPS effects maintained throughout the 4 hours of observation compared with E. coli K-2-LPS, which disappeared after 60 minutes of incubation. Similar differences in magnitude and time dependency of myogenic effects were observed between pure TLR4 and TLR2/1 or TLR2/6 ligands. The specific activation of TLR4 with LPS from pathogen or nonpathogen E. coli, 0111:B4 and EH100, respectively, induced smooth muscle alterations that progressively increased, prolonging incubation, whereas TLR2 ligands induced short-term alterations, of a lesser magnitude, which decreased over time. The real-time polymerase chain reaction analysis showed that HSMC express mRNA for TLR1, 2, 4, and 6, substantiating a direct effect of TLR ligands on human colonic smooth muscle. Conclusions: This study highlights that bacterial products can directly affect gastrointestinal motility and that TLRs subtypes may differ in their cellular activity