Understanding Infection by the Bacterium Listeria monocytogenes As a Way to Address Key Issues in Biology
Summary: Pascale Cossart investigates infection by intracellular bacteria; her model is Listeria monocytogenes, one of the best systems for studying invasion, intracellular parasitism, tissue tropism, crossing of host barriers, and bacterial regulation. Her research is at the forefront of infection biology, cell biology, and microbiology.
My research unit analyzes infectious processes by intracellular bacterial pathogens; the bacterial pathogen Listeria monocytogenes is our main model system. My goal is to elucidate the bacterial and host cell factors that are essential for the establishment and persistence of infection and to define the mechanisms in which they are involved. My ongoing research is critical to and a prerequisite for the generation of novel therapeutics against infectious diseases.
I use multidisciplinary approaches that range from postgenomic methods to the most sophisticated techniques of cell biology, in particular imaging to understand at the molecular level the ensemble of interactions taking place during infection. Our ultimate goal is to understand human listeriosis, which requires the use of relevant animal models.
The results of my group and others since my initial studies on bacterial pathogens in 1986 provide an increasingly clearer view of the bacterial and host cell components critical for infection and of the key events taking place at the cell level, at the tissue level, and on the level of the whole animal. These data and the research capacity of my group allow us to now extend our investigations to other bacterial pathogens, leading to the establishment of general concepts in infection biology. My research also significantly contributes to the understanding of fundamental processes in cell biology and microbiology, in particular in RNA-mediated bacterial gene regulation.
L. monocytogenes is responsible for severe food-borne infections, with a mortality rate of 30 percent. After ingestion of contaminated food products, bacteria reach the intestine, cross the intestinal barrier, and disseminate to the brain and to the placenta of pregnant women via blood. L. monocytogenes causes disease by crossing three host barriers: the intestinal barrier, the blood-brain barrier, and the placental barrier. It can also resist innate immune responses and macrophage killing and invade epithelial cells, adapt to the intracellular milieu, and grow inside cells. In addition, bacteria are propelled from cell to cell by an efficient system driven by actin polymerization.
My laboratory has made important contributions in several areas:
- Analysis of bacterial entry into cells through the identification of the bacterial internalization proteins, the internalins that interact with cellular receptors E-cadherin and Met, and the complex signaling pathways involved in cytoskeleton rearrangements and membrane-remodeling events;
- Understanding actin-based motility, with the discovery of ActA;
- Dissection of the many bacterial factors involved in the various steps of cell infection via comparative genomics (i.e., identification and characterization of genes specifically present in pathogenic species and absent in nonpathogenic species); and
- Understanding adaptation of bacteria to the host, with the discovery of an RNA thermosensor and the role in trans of the small transcripts generated by riboswitches.
My group has also generated a relevant transgenic animal model that encompasses the species specificity of the internalin protein for its receptor E-cadherin and that allowed us to understand the crossing of the intestinal barrier by the bacterium in vivo.
On the bacterial side, my present research focuses on (1) the identification of novel virulence factors through the analysis of a series of secreted proteins that are only expressed by the pathogenic L. monocytogenes and not by the nonpathogenic species and (2) bacterial RNA-mediated regulation. We conducted two large-scale studies that used tiling arrays and RNA-seq for analysis of the bacterial transcriptome after growth of the bacteria in various conditions. These studies identified a large number of small RNAs and antisense RNAs. Together with Rotem Sorek (Weizmann Institute), we discovered that some long antisense RNAs act not only as inhibitors of genes but also as mRNAs for divergently oriented genes. We coined the name excludon for this type of structure, which allows the quick and mutually exclusive expression of divergent genes. We are analyzing the role of several small RNAs in virulence and the role of tiny open reading frames that previously were ignored.
On the cellular side, I am pursuing investigations on bacterial entry, with a recent focus on the critical role of clathrin in the early actin rearrangements. Together with Frances Brodsky (University of California, San Francisco), we have shown that clathrin heavy-chain phosphorylation is a critical event for the whole process of entry. We are performing a genome-wide RNAi screen to further understand all the actors in this symphony. We are making a significant effort to understand the downregulation of cytoskeleton rearrangements and phosphoinositide signaling. Particular attention will be given to septins, which are filamentous proteins that modulate entry and can entrap intracytosolic bacteria in cages in a process linked to autophagy.
My group has discovered that L. monocytogenes induces a transient mitochondrial fission and that mitochondrial dynamics are critical for efficient infection. We are investigating the nonclassical pathways underlying the fission event during infection and how this affects infection.
I am also investing much effort in deciphering how bacterial pathogens exploit post-translational modifications (PTMs) for their own benefit during infection. We are examining in particular how L. monocytogenes intercepts the SUMO (small ubiquitin-like modifiers) pathway. We are identifying the proteins that are deSUMOylated and their role during infection. We are also investigating the SUMOylation of several candidate proteins during infection by various pathogens. We are starting to analyze other PTMs, including ISG15.
An important new facet of my research concerns the reprogramming of the host cell transcriptome upon infection. We are investigating how bacteria induce histone modifications and chromatin remodeling. We are anlyzing the factors and pathways involved in the histone modifications. Our goal is to understand how L. monocytogenes, via the bacterial factor LntA, manipulates the BAHD1 heterochromatinization factor that we recently identified. We are also investigating whether epigenetic modifications are maintained once infection is cleared.
As of September 26, 2012