Type Three Secretion System mediated pathogenesis of V. cholerae
Vibrio cholerae is a diverse species found in aquatic environments worldwide, and it is the causative agent of the severe diarrheal disease known as cholera. Epidemic disease in Asia and South America is currently caused only by strains of the O1 or O139 serogroup of V. cholerae. However, a significant amount of world wide, sporadic disease is caused by strains of other serogroups, collectively called non-O1/non-O139 strains. Ribotyping and comparative genomic analyses have shown that these strains are very diverse both phylogenetically and in their genetic content compared to strains of the O1 and O139 serogroups. Unlike epidemic strains, the majority of non-O1/non-O139 strains do not carry the well characterized virulence factors for colonization (toxin co-regulated pilus, TCP) and cholera toxin (CT) production. It is presumed that pathogenic non-O1/non-O139 isolates have acquired novel virulence factors that confer the ability to colonize and cause disease in a TCP/CT independent manner. However, these strains remain largely uncharacterized.
AM-19226 is a clinically isolated, O39 serogroup strain of V. cholerae that does not carry the genes encoding TCP or CT. However, whole genome sequencing of AM-19226 has identified open reading frames (ORFs) having significant similarity to genes encoding the structural components of a Type Three Secretion System (T3SS). These ORFs, named vcs, lie within a ~60kb pathogenicity island that has been found in other non-O1/non-O139 strains. A wide variety of gram-negative, pathogenic bacteria use TTSSs as a conserved mechanism to translocate multiple virulence factors, referred to as T3SS effector proteins, directly into the cytosol of eukaryotic cells. We therefore postulate that the vcs genes represent a previously unidentified mechanism for host cell interaction acquired by V. cholerae. Also within this island are two open reading frames predicted to encode proteins having sequence similarity to ToxR, an important player in the network of regulatory proteins that govern the expression of virulence factors in epidemic O1 and O139 serogroup strains. Although the amino acid sequences of proteins encoding the structural components are highly conserved among T3SSs of different organisms, the sequences of effector proteins typically share limited or no homology. Effector proteins are therefore often unique to a specific T3SS, and their interactions with eukaryotic host cell proteins serve to elicit distinct phenotypes beneficial for the particular bacterial pathogen.
In order to understanding the scope of molecular mechanisms responsible for TTSS mediated disease, we are working to:
1. understand the role of the ToxR paralogs and ToxR itself in T3SS related gene expression.
2. identify in vitro conditions that promote expression of the T3SS genes.
3. identify effector proteins that are required during infection to provide functions critical for colonization and disease.
To accomplish these goals we use several experimental approaches, including genetic and molecular techniques complimented by in vitro mammalian cell culture model systems. In addition, we collaborate with Dr. J. Scott Butler (also in the Dept. of Microbiology & Immunology) in using S. cerevisiae as a model system for the identification of effector proteins and the analyses of their molecular interactions with components of the eukaryotic host machinery.