Toxin secretion and trafficking by Mycobacterium tuberculosis

Area of Science:

• Microbiology
• Cell Biology
• Immunology

Background:

• The tuberculosis necrotizing toxin (TNT) is a key virulence factor of Mycobacterium tuberculosis (Mtb) responsible for macrophage cytotoxicity.
• TNT, derived from the outer membrane protein CpnT, must reach the host cell cytosol to exert its toxic effect.
• The precise mechanisms governing TNT secretion and intracellular trafficking remain largely unelucidated.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the secretion and trafficking of the tuberculosis necrotizing toxin (TNT) from Mycobacterium tuberculosis (Mtb).
• To identify the specific type VII secretion systems involved in TNT export and its subsequent delivery into the host cell cytosol.
• To elucidate the roles of Mtb secretion systems in phagosomal membrane permeabilization, facilitating TNT entry.

Main Methods:

• Comprehensive analysis of the five type VII secretion systems (ESX systems) in Mtb.
• Genetic manipulation and mutant analysis to assess the function of ESX systems in CpnT export and TNT activity.
• Investigation of phagosomal membrane permeabilization in Mtb-infected macrophages.

Main Results:

• The ESX-4 secretion system is essential for the export of CpnT and the surface presentation of TNT.
• The ESX-1, ESX-2, and ESX-4 secretion systems are collectively required for the permeabilization of the phagosomal membrane.
• Coordinated action of ESX-1, ESX-2, and ESX-4 enables the trafficking of TNT into the cytosol of Mtb-infected macrophages.

Conclusions:

• The ESX-4 system plays a critical role in Mtb virulence by mediating TNT export.
• ESX-2 and ESX-4, previously uncharacterized in this context, are vital for phagosomal membrane disruption.
• Mtb employs a sophisticated interplay between toxin secretion and phagosomal escape, mediated by multiple ESX systems, to facilitate host cell killing.