Bacteria of the genus Brucella are the etiologic agents of brucellosis, a worldwide zoonosis that is highly transmissible to humans. Due to its high infectivity through inhalation, brucellae are included in the CDC Category B list of Select Agents. Virulence of this pathogen mostly depends upon its ability to survive and replicate within macrophages of the infected host. Following phagocytosis, it is commonly accepted that intracellular brucellae avoid fusion of their vacuole, the Brucella-containing vacuole (BCV), with lysosomes via segregation from the degradative endocytic pathway. BCVs then interact with subcompartments of the endoplasmic reticulum, known as ER exit sites (ERES) and eventually fuse with the core ER to generate an ER-derived organelle permissive for replication. The VirB type IV secretion system, a major determinant of Brucella virulence that is induced inside the intermediate BCV, is required for the conversion of the BCV into a replicative organelle, likely through the translocation of effector molecules into the macrophage that modulate host functions. Yet, none of these effectors have been identified, impairing advances in the understanding of Brucella molecular pathogenesis. ? ? The proposed early segregation of BCV from the endocytic pathway is inconsistent with the fact that BCV acidification is required for bacterial survival, possibly through the induction of the virB operon. Given this discrepancy, we have further investigated BCV interactions with endocytic compartments, using confocal live cell microscopy methods that have not yet been applied to Brucella. Due to the sensitivity of these techniques, we found that intermediate BCVs significantly interact with the late endosomal/lysosomal compartment, including lysosomes, as judged by the recruitment on BCV of the small GTPase Rab7, its effector RILP, and BCV accessibility to fluid phase markers delivered from lysosomes. Moreover, such interactions are required for further trafficking to the ER and bacterial replication, likely by providing an inducing environment for expression of the VirB apparatus. This work has been published in Traffic (Starr et al., Traffic 9 (5): 678694). To further characterize Brucella intracellular trafficking and identify additional host factors this pathogen subverts, we have undertaken a more systematic and unbiaised approach, based on screening siRNA knock-down libraries of mammalian genes involved in intracellular membrane trafficking. Cells treated with siRNAs against 140 genes encoding membrane trafficking proteins were infected and screened by microscopy for defects in bacterial trafficking and replication. Thirty-five siRNAs against genes involved in the endocytic and secretory pathways, in cytoskeleton homeostasis and clathrin-mediated endocytosis significantly impaired Brucella replication but not entry, confirming the important roles of both the endocytic and secretory machineries in Brucella intracellular survival strategy. ? ? To identify VirB effector molecules and further characterize the molecular mechanisms of Brucella intracellular survival, we have been using bioinformatics tools in collaboration with the RTB/RTS Genomics Unit at RML to select genes that encode candidate effectors, based on their unicity to Brucella and the presence of motifs related to type IV secreted proteins in homologous VirB systems, or on the presence of eukaryotic-like motifs consistent with expected functions for these effectors. Seventeen candidates have been identified in the Brucella genome and are being tested for translocation into mammalian cells using an assay developed by the group of Renee Tsolis at the University of California at Davis. Additionally, we have been studying in collaboration with Dr Tsolis the function of the identified Brucella effector VceC, which is translocated by intracellular bacteria upon infection. Ectopic expression of a GFP-VceC fusion in HeLa cells induced the specific vacuolation of the endoplasmic reticulum, an effect consistent with Brucella subversion of this compartment.
| Celli, Jean (2012) LRSAM1, an E3 Ubiquitin ligase with a sense for bacteria. Cell Host Microbe 12:735-6 |
| Celli, Jean (2008) Intracellular localization of Brucella abortus and Francisella tularensis in primary murine macrophages. Methods Mol Biol 431:133-45 |
| Starr, Tregei; Ng, Tony W; Wehrly, Tara D et al. (2008) Brucella intracellular replication requires trafficking through the late endosomal/lysosomal compartment. Traffic 9:678-94 |
