Coordinate Synthesis and Export of Lipid Virulence Factors in Mycobacterium Tuberculosis
Description:... Mycobacterium tuberculosis is a highly successful human pathogen and is estimated to infect 2 billion people worldwide, resulting in about 2 million deaths per year. M. tuberculosis synthesizes and secretes a wide array of lipid virulence factors that are central to its pathogenesis. Several of these lipids are associated with the cell wall and are surface-exposed. These surface-exposed polyketide lipids are important in modulating the host immune system as well as for protection from host radicals. The overall goal of the work presented here is to better understand the synthesis, export and regulation of these virulence lipids. The first part of this thesis describes the coordinate synthesis and transport of a critical surface-exposed lipid virulence factor, phthiocerol dimycocerosate (PDIM). PDIM is synthesized by a large number of enzymes including PpsE and transported to the cell wall via MmpL7. We show that PpsE interacts with MmpL7 suggesting coupled synthesis and export of PDIM. We propose that the synthesis and transport proteins form a complex that promotes efficient lipid export. The second part of this thesis describes a novel mechanism in which virulence polyketide synthesis is regulated by metabolite availability. Using a powerful mass spectrometric approach that simultaneously monitors hundreds of lipids, we found that the production of two lipid virulence factors, PDIM and sulfolipid-1 (SL-1), are coupled and co-regulated by the concentration of a common metabolite, methyl malonyl CoA. Consistent with this view, increased levels of methyl malonyl CoA led to increased abundance and mass of both PDIM and SL-1. Furthermore, perturbation of methyl malonyl CoA metabolism led to attenuation of the bacteria in mice. M. tuberculosis likely grows on host lipids during infection suggesting that methyl malonyl CoA production by the bacteria is high in vivo due to either increased flux through the citric acid cycle or the catabolism of odd-chain fatty acids. Interestingly, PDIM is produced in high mass forms during infection suggesting that, indeed, high levels of methyl malonyl CoA exist in vivo. We propose that growth of M. tuberculosis on fatty acids during infection leads to increased flux of methyl malonyl CoA through lipid biosynthetic pathways, resulting in increased virulence lipid synthesis during infection.
Show description