Laying the Genetic and Molecular Foundation for the Study of Fusobacterium Nucleatum in Relation to Human Health and Disease
Casasanta, Michael Anthony
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Fusobacterium nucleatum is a Gram-negative, anaerobic bacterium that is a member of the human oral microbiota. Although it is a normal resident of the mouth, it is associated with a number of human diseases including: sepsis, inflammatory bowel disease (IBD), and colorectal cancer (CRC). Despite the important association of F. nucleatum with human health and disease, remarkably little is known about the molecular mechanisms underlying these infections. This knowledge gap can, in part, be attributed to a lack of molecular tools and experimental workflows. Creating the genetic tools to fill this knowledge gap is an imperative undertaking for the future development of treatments for diseases involving F. nucleatum. Previous work in the field has assigned functions to just a handful of Fusobacterium proteins (Fap2, FadA), and only two of those proteins have a well-defined role in the host-pathogen relationship. This dissertation contains work that lays the molecular and genetic foundation for future studies involving F. nucleatum by creating a unique gene deletion system while simultaneously establishing broadly applicable experimental workflows and molecular tools to study initial bacterial attachment and invasion processes crucial to Fusobacterium virulence. Marker-less gene deletions confirm the importance of Fap2 in host-cell attachment and invasion and suggest a lesser role in invasion for FadA, representing a significant revision to the Fusobacterium-host relationship. Also, our system allows for the overexpression and purification of virulence factors directly from Fusobacterium for the first time. This permits us to study aspects of Fusobacterium protein biology that were previously impossible and will provide further insights into the nature of Fusobacterium virulence. A custom suite of molecular tools was also developed to facilitate recombinant expression of these proteins in general laboratory settings using simple E. coli protein expression systems. We have used these new technologies to express and purify a number of potential Fusobacterium virulence factors as detailed in this dissertation. Also contained in this dissertation is the application of these breakthroughs to probe the function of a novel F. nucleatum outer membrane phospholipase, FplA. Phospholipases are important virulence factors in a number of well-studied human pathogens including Pseudomonas aeruginosa and Legionella pneumophila, where they interfere with host cellular signaling processes to increase intracellular bacterial survival. Our data show that FplA is a Class A1 phospholipase (PLA1) with robust catalytic activity capable of binding to and cleaving a number of lipid types. Additionally, we show that it has the ability to bind to important host signaling lipids including phosphatidylinositol 3, 5-bisphosphate and phosphatidylinositol 3, 4, 5-triphosphate. These data suggest FplA may play a role in manipulating the intracellular processes of host cells. Taken together, work in this dissertation provides tools and experimental frameworks for the future study of F. nucleatum pathogenesis while identifying and initially characterizing a new, potentially significant, virulence factor in FplA.
- Doctoral Dissertations