Six years in the past, Michael Niederweis, Ph.D., described the primary identified toxin of the lethal pathogen Mycobacterium tuberculosis (Mtb), an exotoxin that had gone undetected for 132 years.
Now Niederweis and colleagues on the University of Alabama at Birmingham describe the mechanism of secretion and trafficking of that toxin, TNT, which is the main cytotoxicity issue for the pathogen that infects 9 million folks a yr and kills greater than 1 million.
Their report, revealed in Nature Communications, identifies key secretion systems utilized by Mtb in pathogenesis, which makes them candidate targets for therapeutics.
When Mtb micro organism are inhaled right into a human lung, they’re engulfed by host lung macrophages and trapped inside intracellular vesicles known as phagosomes. Mtb prevents the phagosome from merging with a lysosome, which in any other case would kill the micro organism. Instead, Mtb safely grows contained in the phagosomes, then breaks out of the phagosomes into the cytosol of the macrophage, and it makes use of the TNT toxin to kill the macrophage by necroptosis, releasing the micro organism to contaminate different cells.
Exotoxin export from Mtb or comparable micro organism face two obstacles—first, the toxin protein must be transported throughout the micro organism’s cytoplasmic membrane, after which, it must be transported throughout the micro organism’s outer membrane. To do that, micro organism make use of all kinds of specialised secretion methods—basically molecular machines that acknowledge and transport the bacterial toxins or different proteins concerned in quite a lot of bacterial features, reminiscent of adherence or scavenging.
Toxin secretion mechanisms have been described in virtually all main bacterial pathogens, however there was one notable exception—Mtb.
Now, Niederweis, David Pajuelo, Ph.D., Uday Tak, Ph.D., Lei Zhang, Ph.D., UAB Department of Microbiology, have used a complete genetic evaluation to study which of the 5 kind VII secretion methods in Mtb is used for TNT secretion. They additionally found which three secretion methods are important, appearing in live performance, to permeabilize the phagosome membrane. That sieve-like harm permits TNT to enter the macrophage cytosol, the place it depletes NAD+ to induce necroptotic demise.
The 5 kind VII secretion methods in Mtb are generally known as ESX-1, ESX-2, ESX-3, ESX-4 and ESX-5. Each of those secretion methods is a fancy of between seven and greater than 15 proteins that type the molecular equipment. Up to now, no roles have been identified for ESX-2 and ESX-4.
TNT begins its journey as part of a bigger protein, CpnT. Only when CpnT reaches the outer membrane does the poisonous TNT break free from the bigger protein. Niederweis and colleagues discovered that each export to the cell floor and secretion of CpnT/TNT into the cytosol of macrophages contaminated with Mtb require the ESX-4 system. This is the primary identified molecular operate for the ESX-4 system in Mtb.
While it was identified that ESX-1 participates in permeabilizing the phagosome membrane, the UAB researchers surprisingly discovered that ESX-2 and ESX-4 are additionally required to behave in live performance with ESX-1 to rupture the phagosomal membrane and allow trafficking of TNT into the cytosol of Mtb-infected macrophages.
“Thus, our study identifies not only the system required for the secretion of the only known exotoxin of Mtb,” mentioned Niederweis, “but also establishes new molecular roles for the two previously uncharacterized type VII secretion systems, ESX-2 and ESX-4, in phagosomal rupture, a critical step in Mtb pathogenesis.”
In the Nature Communications paper, the researchers additionally suggest a mannequin for TNT manufacturing, export, secretion and trafficking by Mtb, and so they level out the steps of this course of which might be nonetheless unclear.
“The paradigm-changing discovery that both the ESX-2 and ESX-4 systems need to act in concert with the ESX-1 system to permeabilize the phagosomal membrane raises important questions regarding the molecular mechanism of this process and the regulation of these activities,” Niederweis mentioned. “This study presents a major advancement in our understanding of protein secretion and trafficking by Mtb and will certainly stimulate further research in these important areas of Mtb biology.”
David Pajuelo et al, Toxin secretion and trafficking by Mycobacterium tuberculosis, Nature Communications (2021). DOI: 10.1038/s41467-021-26925-1
University of Alabama at Birmingham
How the pathogen Mycobacterium tuberculosis secretes and traffics its solely identified exotoxin (2021, November 24)
retrieved 24 November 2021
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