To catch a killer. What can mycobacterial models teach us about Mycobacterium tuberculosis pathogenesis?

Abstract

Mycobacterium tuberculosis is the causative agent of the global tuberculosis epidemic. To combat this successful human pathogen we need a better understanding of the basic biology of mycobacterial pathogenesis. The use of mycobacterial model systems has the potential to greatly facilitate our understanding of how M. tuberculosis causes disease. Recently, studies using mycobacterial models, including M. bovis BCG, M. marinum, and M. smegmatis have significantly contributed to understanding M. tuberculosis. Specifically, there have been advances in genetic manipulation of M. tuberculosis using inducible promoters and recombineering that alleviate technical limitations in working with mycobacteria. Model systems have helped elucidate how secretion systems function at both the molecular level and during virulence. Mycobacterial models have also led to interesting hypotheses about how M. tuberculosis mediates latent infection and host response. While there is utility in using model systems to understand tuberculosis, each of these models represent distinct mycobacterial species with unique environmental adaptations. Directly comparing findings in model mycobacteria to those in M. tuberculosis will illuminate the similarities and differences between these species and increase our understanding of why M. tuberculosis is such a potent human pathogen.

Figure 1. Recent Advances in Mycobacterial Tool Development

A. The Tet-ON and Tet-OFF inducible mycobacterial promoter systems, for example driving the essential secA1 gene. For the Tet-ON system, the TetR repressor occupies the tetO operator in the absence of anhydrotetracycline (atc) repressing transcription. This is relieved upon atc addition. Tet-OFF allows for transcription in the absence of atc, and transcriptional repression or silencing in the presence of atc. B. Mycobacterial strains lacking the essential secA1 gene. Strains with Tet-ON secA1 are only viable in the presence of atc, while strains with Tet-OFF secA1 are only viable in the absence of atc. C. The pTetCoex system. The inducible Tet-ON promoter drives expression of gfp, while the P_hsp constitutive promoter drives expression of rfip, a newly discovered red fluorescent protein. Black boxes represent terminators, and the black arrow represents a stabilizing gene. D. Tunable expression of GFP and co-expression of RFIP using TetCoex. Left, increase of GFP expression in M. smegmatis over time in the presence of 200ng/ml atc relative to co-expression of RFIP (RFU, relative fluorescent units). Right, tunable expression of GFP in an atc dose-dependent manner, compared to constitutive expression of RFIP. From [9] with permission.

Figure 2. Mycobacterial killing by Ubiquitinated Peptides

Left, Model for killing of M. tuberculosis by ubiquitin peptides by macrophages. In IFNγ activated macrophages, the M. tuberculosis phagosome is thought to fuse with ubiquitin (Ub) -containing vesicles including MVBs (multivesicular bodies generated by the ESCRT complex) and LC-3 positive (red line) autophagosomes, and the ubiquitin coated bacteria are targeted to the lysosome. Right, M. marinum differs from M. tuberculosis in that a population of M. marinum escape the phagosome. Cytosolic M. marinum are coated with ubiquitin in an autophagy-independent manner. The coated M. marinum have two possible fates. First, they can be targeted to the lysosome. Second, the bacteria can shed the ubiquitin coated cell wall (which itself is sent to the lysosome). It is possible that this population then forms actin tails, the actin tail population is not ubiquitinated.