Ubiquitination of Listeria Virulence Factor InlC Contributes to the Host Response to Infection

Listeria in Pregnancy-The Forgotten Culprit

Listeria monocytogenes is a Gram-positive bacterium that causes listeriosis, a severe foodborne illness that is particularly dangerous during pregnancy. It thrives in diverse environments, including refrigerated conditions and food production facilities, due to its adaptability to varying temperatures, pH levels, and salt concentrations. Its virulence stems from the ability to invade host cells, particularly macrophages and epithelial cells, and avoid, or at least postpone, immune detection by utilizing virulence factors such as internalins, listeriolysin O, and actin assembly-inducing protein. This intracellular motility and biofilm formation make LM a persistent pathogen in food safety and public health. Pregnant women are at a much higher risk of listeriosis, which can result in serious fetal complications such as miscarriage, stillbirth, and preterm labor due to LM's affinity for placental tissues. The vertical transmission of LM from mother to fetus can lead to neonatal listeriosis, which can result in sepsis and meningitis, with high mortality rates if not promptly treated. Early diagnosis and treatment with antibiotics, such as ampicillin or gentamicin, are crucial for maternal and neonatal outcomes.

Cross-regulation of Listeria monocytogenes and the host ubiquitin system in listeriosis

The facultative intracellular bacterium Listeria (L.) monocytogenes may cause severe diseases in humans and animals. The control of listeriosis/L. monocytogenes requires the concerted action of cells of the innate and adaptive immune systems. In this regard, cell-intrinsic immunity of infected cells, activated by the immune responses, is crucial for the control and elimination intracellular L. monocytogenes. Both the immune response against L. monocytogenes and cell intrinsic pathogen control are critically regulated by post-translational modifications exerted by the host ubiquitin system and ubiquitin-like modifiers (Ubls). In this review, we discuss our current understanding of the role of the ubiquitin system and Ubls in listeriosis, as well as future directions of research.

Raising a Bacterium to the Rank of a Model System: The Listeria Paradigm

My scientific career has resulted from key decisions and reorientations, sometimes taken rapidly but not always, guided by discussions or collaborations with amazing individuals from whom I learnt a lot scientifically and humanly. I had never anticipated that I would accomplish so much in what appeared as terra incognita when I started to interrogate the mechanisms underlying the virulence of the bacterium Listeria monocytogenes. All this has been possible thanks to a number of talented team members who ultimately became friends.

A bacterial virulence factor interacts with the splicing factor RBM5 and stimulates formation of nuclear RBM5 granules

L. monocytogenes causes listeriosis, a foodborne disease that is particularly dangerous for immunocompromised individuals and fetuses. Several virulence factors of this bacterial pathogen belong to a family of leucine-rich repeat (LRR)-containing proteins called internalins. Among these, InlP is known for its role in placental infection. We report here a function of InlP in mammalian cell nucleus organization. We demonstrate that bacteria do not produce InlP under in vitro culture conditions. When ectopically expressed in human cells, InlP translocates into the nucleus and changes the morphology of nuclear speckles, which are membrane-less organelles storing splicing factors. Using yeast two-hybrid screen, immunoprecipitation and pull-down experiments, we identify the tumor suppressor and splicing factor RBM5 as a major nuclear target of InlP. InlP inhibits RBM5-induced cell death and stimulate the formation of RBM5-induced nuclear granules, where the SC35 speckle protein redistributes. Taken together, these results suggest that InlP acts as a nucleomodulin controlling compartmentalization and function of RBM5 in the nucleus and that L. monocytogenes has developed a mechanism to target the host cell splicing machinery.

Antimicrobial peptides: Defending the mucosal epithelial barrier

The recent epidemic caused by aerosolized SARS-CoV-2 virus illustrates the importance and vulnerability of the mucosal epithelial barrier against infection. Antimicrobial proteins and peptides (AMPs) are key to the epithelial barrier, providing immunity against microbes. In primitive life forms, AMPs protect the integument and the gut against pathogenic microbes. AMPs have also evolved in humans and other mammals to enhance newer, complex innate and adaptive immunity to favor the persistence of commensals over pathogenic microbes. The canonical AMPs are helictical peptides that form lethal pores in microbial membranes. In higher life forms, this type of AMP is exemplified by the defensin family of AMPs. In epithelial tissues, defensins, and calprotectin (complex of S100A8 and S100A9) have evolved to work cooperatively. The mechanisms of action differ. Unlike defensins, calprotectin sequesters essential trace metals from microbes, which inhibits growth. This review focuses on defensins and calprotectin as AMPs that appear to work cooperatively to fortify the epithelial barrier against infection. The antimicrobial spectrum is broad with overlap between the two AMPs. In mice, experimental models highlight the contribution of both AMPs to candidiasis as a fungal infection and periodontitis resulting from bacterial dysbiosis. These AMPs appear to contribute to innate immunity in humans, protecting the commensal microflora and restricting the emergence of pathobionts and pathogens. A striking example in human innate immunity is that elevated serum calprotectin protects against neonatal sepsis. Calprotectin is also remarkable because of functional differences when localized in epithelial and neutrophil cytoplasm or released into the extracellular environment. In the cytoplasm, calprotectin appears to protect against invasive pathogens. Extracellularly, calprotectin can engage pathogen-recognition receptors to activate innate immune and proinflammatory mechanisms. In inflamed epithelial and other tissue spaces, calprotectin, DNA, and histones are released from degranulated neutrophils to form insoluble antimicrobial barriers termed neutrophil extracellular traps. Hence, calprotectin and other AMPs use several strategies to provide microbial control and stimulate innate immunity.

Alarmins S100A8/A9 promote intervertebral disc degeneration and inflammation-related pain in a rat model through toll-like receptor-4 and activation of the NF-κB signaling pathway

OBJECTIVE

The molecules released from cells undergoing necrosis are recognized as alarmins, and S100A8/9, a typical alarmin, is associated with several inflammation-related diseases. This study was to investigate the molecular role of S100A8/A9 on the process of intervertebral disc degeneration (IVDD) and inflammation-related pain.

METHODS

The expression pattern of S100A8/A9 in different degenerated human nucleus pulposus (NP) tissues were measured by Real-time quantitative reverse transcription PCR (RT-qPCR) and immunohistochemical (IHC). The effects of S100A8/A9 on matrix production were assessed by RT-qPCR, western blotting, and cell immunofluorescence. Involvement of TLR4 and NF-κB signaling pathways were studied by pharmachemical inhibitors and small interfering RNAs (siRNAs). The development of degenerative and pain features in the IVDD model were examed by IHC and pain-behavior testing.

RESULTS

The expression of S100A8/A9 was significantly elevated in severely degenerated human NP tissue with similar expression pattern of TNF-α. In NP cells, S100A8/A9 increased MMP-3/13, TNF-α, IL-6 expression and inhibited aggrecan and collagen II expression. RT-qPCR and western blotting showed that the regulatory effects of S100A8/A9 on IVD were TLR4 dependent. Pharmacological inhibition or siRNA knockdown of the NF-κB signaling attenuated S100A8/A9-induced upregulation of MMP-3/13, TNF-α and IL-6. In vivo, S100A9 inhibitor treatment inhibited disc-puncture induced IVDD and inflammation-related pain.

CONCLUSIONS

This study showed that S100A8/A9 bound to TLR4 and increased the expression of MMPs, TNF-α, and IL-6 through NF-κB signaling pathways in NP cells. Furthermore, S100A8/A9 inhibitor could prevent development of IVDD and inflammation-related pain in the rat model.

The Characteristics and Function of Internalin G in Listeria monocytogenes

In order to clarified characteristics and function of internalin G (inlG) in Listeria monocytogenes ATCC®19111 (1/2a) (LM), the immune protection of the inlG was evaluated in mice, the homologous recombination was used to construct inlG deletion strains, and their biological characteristics were studied by the transcriptomics analysis. As a result, the immunization of mice with the purified protein achieved a protective effect against bacterial infection. The deletion strain LM-AinlG was successfully constructed with genetic stability. The mouse infection test showed that the virulence of LM was decreased after the deletion of the inlG gene. The deletion strain showed enhanced adhesion to and invasion of Caco-2 cells. Compared to the wild strain, 18 genes were up-regulated, and 24 genes were down-regulated in the LM-AinlG. This study has laid a foundation for further research on the function of inlG and the pathogenesis of LM. In this study, immunization of mice with the purified inlG protein achieved a protective effect against Listeria monocytogenes infection. The virulence of LM-ΔinlG was decreased by mouse infection. However, the adhesion and invasion ability to Caco-2 cell were enhanced. Compared to the wild strain, 18 genes were up-regulated, and 24 genes were down-regulated in the LM-ΔinlG. This study has laid a foundation for further study of the function of the inlG and the listeriosis.

LincRNA-Cox2 regulates IL6/JAK3/STAT3 and NF-κB P65 pathway activation in Listeria monocytogenes-infected RAW264.7 cells

Listeria monocytogenes (Lm) can lead to high mortality rates relative to other foodborne pathogens. Lm-induced inflammation is partly characterized by macrophage activation. Long non-coding RNAs (lncRNAs) have important roles in various biological processes. However, it is unknown how lncRNAs regulate the host response to Lm infection. To identify the role of lncRNA in Lm infection, we used in vitro and in vivo models. We found that lincRNA-Cox2 was highly expressed in Lm-infected RAW264.7 cells. LincRNA-Cox2 knockdown resulted in reduced proinflammatory cytokines, apoptosis, migration ability and enhanced phagocytosis of Lm. LincRNA-Cox2 knockdown also reduced the phosphorylation of Janus kinase 3 (JAK3) and signal transducer and activator of transcription (STAT3) and the nuclear translocation of nuclear factor (NF)-κB P65, which are known to be involved in inflammatory responses. Experimentally inhibiting the protein and phosphorylation levels of STAT3 resulted in reduced proinflammatory cytokines and enhanced phagocytosis of Lm by the RAW264.7 cells. Our research suggests that lincRNA-Cox2 plays important roles in inflammation, the phagocytic function and cell migration ability of RAW264.7 cells by activating interleukin (IL)-6/JAK3/STAT3 signaling, and lincRNA-Cox2 also regulates NF-κB P65 nuclear translocation. Our research provides new insights into the regulatory role of lincRNA-Cox2 in Lm infection.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Distribution of PDLIM1 at actin-rich structures generated by invasive and adherent bacterial pathogens

The enteric bacterial pathogens Listeria monocytogenes (Listeria) and enteropathogenic Escherichia coli (EPEC) remodel the eukaryotic actin cytoskeleton during their disease processes. Listeria generate slender actin-rich comet/rocket tails to move intracellularly, and later, finger-like membrane protrusions to spread amongst host cells. EPEC remain extracellular, but generate similar actin-rich membranous protrusions (termed pedestals) to move atop the host epithelia. These structures are crucial for disease as diarrheal (and systemic) infections are significantly abrogated during infections with mutant strains that are unable to generate the structures. The current repertoire of host components enriched within these structures is vast and diverse. In this protein catalog, we and others have found that host actin crosslinkers, such as palladin and α-actinin-1, are routinely exploited. To expand on this list, we set out to investigate the distribution of PDLIM1, a scaffolding protein and binding partner of palladin and α-actinin-1, during bacterial infections. We show that PDLIM1 localizes to the site of initial Listeria entry into cells. Following this, PDLIM1 localizes to actin filament clouds surrounding immotile bacteria, and then colocalizes with actin once the comet/rocket tails are generated. Unlike palladin or α-actinin-1, PDLIM1 is maintained within the actin-rich core of membrane protrusions. Conversely, α-actinin-1, but not PDLIM1 (or palladin), is enriched at the membrane invagination that internalizes the Listeria-containing membrane protrusion. We also show that PDLIM1 is a component of the EPEC pedestal core and that its recruitment is dependent on the bacterial effector Tir. Our findings highlight PDLIM1 as another protein present within pathogen-induced actin-rich structures.

Pathogenic Biohacking: Induction, Modulation and Subversion of Host Transcriptional Responses by Listeria monocytogenes

During infection, the foodborne bacterial pathogen Listeria monocytogenes dynamically influences the gene expression profile of host cells. Infection-induced transcriptional changes are a typical feature of the host-response to bacteria and contribute to the activation of protective genes such as inflammatory cytokines. However, by using specialized virulence factors, bacterial pathogens can target signaling pathways, transcription factors, and epigenetic mechanisms to alter host gene expression, thereby reprogramming the response to infection. Therefore, the transcriptional profile that is established in the host is delicately balanced between antibacterial responses and pathogenesis, where any change in host gene expression might significantly influence the outcome of infection. In this review, we discuss the known transcriptional and epigenetic processes that are engaged during Listeria monocytogenes infection, the virulence factors that can remodel them, and the impact these processes have on the outcome of infection.