The main interest of our group is to understand host-pathogen interactions on the molecular level. We are studying molecular and cellular events that enable microbial pathogens to evade host defense mechanisms. In particular, we are interested in how obligate intracellular pathogens modulate host cell apoptosis pathways, signal transduction and vesicular trafficking. We are using the Coxiella burnetii as a model system. C. burnetii is the causative agent of human Q-fever, a worldwide zoonotic disease. These small Gram-negative bacteria are classified in the g subdivision of proteobacteria. C. burnetii are typically transmitted to humans by inhalation of infectious material transmitted from domestic livestock. Q-fever is often an asymptomatic or mild flu-like illness, but can develop into an atypical pneumonia or hepatitis. Furthermore, the infection can also result in chronic Q-fever which is typically characterized by bacterial endocarditis and is potentially fatal.
Vero cells infected with Coxiella burnetii stained with DAPI to visualize DNA.
After uptake into mammalian cells by microfilament-dependent endocytosis, C. burnetii remains in a membrane-bound vacuole. This C. burnetii-containing vacuole (CCV) initially appears to mature similarly to phagosomes containing avirulent bacteria, undergoing fusion with endosomes and lysosomes, resulting in the formation of a phagolysosomal compartment. However, C. burnetii delays the maturation of the CCV probably through interactions between the early CCV and the autophagic pathway. How C. burnetii mediates establishment of the phagolysosomal-like compartment in which it resides and replicates, is not well understood. However, bacterial protein synthesis is required, suggesting that bacterial proteins may directly influence biogenesis of the C. burnetii-occupied vacuole.
This scheme describes the maturation of C. burnetii-containing phagosomes (CCV). Within minutes after uptake into macrophages, the CCV fuses with early endosomes (EE). Later the CCV fuses with late endosomes (LE) and finally with lysosomes (Lys). However, the phagosome maturation is delayed, possibly by interaction with the autophagic pathway (AP). The resulting phagolysosome contains hydrolases and has a low pH (4.5-5.5). C. burnetii requires this hostile environment, which normally “kills” bacteria, in order to replicate
C. burnetii has recently been shown to inhibit host cell apoptosis. Apoptosis is a programmed cell death pathway that is crucial for immune system maintenance and removal of damaged or infected cells. It was demonstrated that C. burnetii infection inhibits the induction of the intrinsic cell death pathway by preventing cytochrome C release from mitochondria and consequently inhibiting caspase 3 activation. Another study showed that C. burnetii infection inhibited not only the intrinsic but also the extrinsic apoptosis pathway. However, the mechanism(s) of C. burnetii-induced inhibition of host cell apoptosis is not well understood.
Adapted from Faherty C. S. and Maurelli A. T., Trends in Microbiology (2008)
Sequencing of the C. burnetii genome revealed the presence of genes encoding a type IV secretion system (T4SS) that is related to the Dot/Icm system of Legionella pneumophila. The facultative intracellular bacterium L. pneumophila requires the Dot/Icm system to ensure its survival inside host cells. Using L. pneumophila as a surrogate host, we recently demonstrated that several different C. burnetii proteins with ankyrin repeat-containing domains could be delivered into the host cells by the L. pneumophila T4SS. Additionally, we showed that one of these proteins was translocated into host cells during C. burnetii infection, which validated that the C. burnetii proteins shown to be translocated by the L. pneumophila system represent bone fide type IV effectors. Because ankyrin repeat-containing proteins (Ank) are mainly found in eukaryotes and only rarely in bacteria, it has been suggested that the C. burnetii Ank proteins could be mimicking factors that regulate host cellular functions. However, no function for any C. burnetii T4SS effector protein has been assigned.
The T4SS is a multi-protein complex spanning inner and outer bacterial membrane and the host cell membrane. This protein complex allows translocation of bacterial proteins into the host cell cytosol in order to manipulate the host cell allowing the bacteria to survive and multiply intracellular.
Our current research
Our work indicates that one T4SS effector inhibits host cell apoptosis, while two other C. burnetii Ank proteins show nuclear localization, suggesting that C. burnetii T4SS effector proteins affect host cell signal transduction pathways.
Our current projects
1.) Analysis of the T4SS effector-induced inhibition of apoptosis
To determine the diverse mechanisms employed by C. burnetii to prevent host cell apoptosis, we will analyze the function of C. burnetii T4SS substrates that interfere with signaling through the intrinsic and extrinsic apoptotic pathways.
2.) Characterisation of host cell pathways altered by effectors of Brucella, Chlamydia, and Coxiella: identification of novel therapeutic targets
Within the ERA-NET 3rd call consortium CELLPATH we will characterize the molecular and cellular function of effector proteins from Coxiella burnetii, so that new therapeutic approaches, vaccines, and novel diagnostics, can be developed.
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3.) Identification of host cell factors required for establishment of the replicative C. burnetii-containing vacuole (CCV)
Host cell proteins that are crucial for maintaining endo-lysosomal compartments should be analyzed in their role in establishing the CCV. The goal is to understand what Coxiella requires to create a phagolysosomal compartment suitable for replication.
For more information please see: http://www.SPP1580.uni-bonn.de