Mycobacterium tuberculosis wears what it eats

Abstract

Mycobacterium tuberculosis remains one of the most pernicious of human pathogens. Current vaccines are ineffective, and drugs, although efficacious, require prolonged treatment with constant medical oversight. Overcoming these problems requires a greater appreciation of M. tuberculosis in the context of its host. Upon infection of either macrophages in culture or animal models, the bacterium realigns its metabolism in response to the new environments it encounters. Understanding these environments, and the stresses that they place on M. tuberculosis, should provide insights invaluable for the development of new chemo- and immunotherapeutic strategies.

Figure 1. The main features of the human TB granuloma

A fully-formed human TB granuloma is an extremely stratified structure. The center of the granuloma is caseous in nature and rich in lipids, thought to be derived from the lipids present in foamy macrophages. The caseum is surrounded by a macrophage-rich layer that contains foamy macrophages, multi-nucleate giant cells and epithelioid macrophages. Mtb bacilli are observed in many of these cells. This structure is frequently encased in a fibrous capsule of collagen and other extracellular matrix proteins. Lymphocytes tend to be restricted to the periphery of the granuloma outside the fibrous outer layer.

Figure 2. Diagram of the Mtb-containing phagosome

Mtb resides in a phagosome that is arrested at the early endosomal stage of maturation and fails to fuse with mature lysosomes. The pH of the vacuole is pH 6.4 and the compartment remains fusigenic and continues to mix with the recycling endosomal system (Clemens and Horwitz, 1996; Sturgill-Koszycki et al., 1996; Sturgill-Koszycki et al., 1994). Endosomal cargo, such as transferrin, can be visualized passing through the Mtb-containing vacuole. The trafficking of LDL bound to the LDL receptor mirrors that of transferrin. LDL is likely processed by the lipoprotein lipase in the early endosome (Heeren et al., 2001), which could render the cholesterol core accessible to the bacterium. In the host cell, cholesterol is transported into the cytosol and complexed with cholesterol-binding proteins prior to its esterification and incorporation into lipid droplets within the leaflets of the ER membrane. At later stages in foamy macrophages, Mtb has been observed within the lipid droplets in the cells indicating that there can be direct communication between the Mtb-containing vacuole and the droplets (Peyron et al., 2008). This association is consistent with genetic evidence indicating that Mtb exploits cholesterol as a carbon source during infection (Pandey and Sassetti, 2008).

Figure 3. Venn diagram of the universally-expressed transcriptome

The Venn diagrams illustrate those genes with conserved expression and regulation inside macrophage across the panel of MTC clinical isolates. The gene expression profiles of 17 MTC strains 24h post-infection of resting macrophages and macrophages activated overnight with interferon-γ were determined by microarray. Normalized expression ratios for each strain were determined by comparison of RNA from intracellular bacteria versus control bacteria of the same strain in medium (Homolka et al., 2010). This enabled the delineation of the macrophage-specific response for each individual isolate. The Venn diagrams illustrate the activation-dependent (magenta) and independent (green) genes, together with those genes up or down-regulated independent of macrophage status across clinical isolates (merged). Genes in the core transcriptome were selected based on trending up or down in all strains (up or down >1.2-fold in 15 of 17 strains).

Figure 4. Outline of the pathways relevant to C₃ metabolism

The metabolism of cholesterol, methyl-branched fatty acids and odd-chain length lipids will raise the intracellular levels of the C₃ compounds propionate or propionyl-CoA, which Mtb finds highly toxic. The bacterium has developed three different strategies to detoxify propionyl-CoA. Isocitrate lyase activity has been suborned to fulfill the function of methylcitrate lyase in the last step of the methyl citrate cycle to generate the TCA cycle intermediate succinate. The methylmalonyl pathway has also been mobilized to metabolize propionyl-CoA to produce succinyl-CoA via the VitB12-dependent activity of methylmalonyl-CoA mutase. Finally, intermediates from the methylmalonyl pathway can be incorporated directly into the abundant, methyl-branched lipids of the bacterial cell wall, such as PDIM and SL-1.