A study reveals how M. tuberculosis escapes death in macrophages

July 10, 2018

Bacteria that cause the devastating tuberculosis disease have the ability to escape destruction and develop after being engulfed by lung macrophages, immune cells that are supposed to destroy the pathogens. Researchers from the University of Alabama at Birmingham have now described the main biochemical steps between the bacterium Mycobacterium tuberculosis and the macrophage responsible for this ability.

Michael Niederweis, Ph.D., and his colleagues say that this knowledge reveals targeted patient strategies. to treat tuberculosis, which kills 1.4 million people and infects another 10 million each year. Niederweis is a professor of microbiology at UAB, and the study appears in the journal Cell Reports .

Three years ago, the Niederweis group described the first toxin ever found in M. tuberculosis despite 132 years of study. This contrasts with almost all other pathogenic bacteria, whose toxins contribute to disease or death.

They named the necrotizing toxin of tuberculous toxin, or TNT, and showed that TNT is the founding member of a new clbad of unrecognized toxins. than 600 bacterial and fungal species. Yersinia pestis, the bacterium responsible for the bubonic plague of the Black Death in medieval Europe.

The Niederweis team discovered that TNT enzymatically hydrolyzes NAD +, a vital coenzyme in all living cells, and this loss of NAD + in macrophages necrotic cell death of the macrophage, which releases the bacterium M tuberculosis to infect more cells. This necrotic death has diverted the macrophage from its normal path to destroy the engulfed bacteria. lysosomal degradation of bacteria within the macrophage and programmed cell death, or apoptosis, of the macrophage

The present study shows how TNT helps to win the battle between M. tuberculosis and the human immune system to control the fate of infected macrophages -; a critical fight that determines the outcome of the infection.

Macrophages have genes for a canonical pathway to programmed necrosis, known as necroptosis, which can be activated by the immune system. The Niederweis group discovered that the depletion of NAD + by hydrolysis of TNT activated two key mediators of this pathway, RIPK3 and MLKL, while avoiding two components upstream of the pathway. At the same time, the powers of the cell called mitochondria have depolarized, and the synthesis of ATP, the molecule that provides most of the chemical energy to the cells, has been impaired.

Surprisingly, researchers exhausted NAD + levels in uninfected macrophages. inhibiting an enzyme in the NAD + recovery pathway, necrosis via the RIPK3 and MLKL pathway was also activated. This showed that depletion of NAD + alone, even when TNT and M. tuberculosis were not present, was sufficient to induce necroptosis.

This discovery led to a question: would reconstitution of NAD + in a macrophage infected with M. tuberculosis alleviate cytotoxicity? caused by TNT? The researchers found that adding nicotinamide, a precursor of NAD +, to cell culture of infected macrophages tripled macrophage viability. Similarly, the addition of compounds to protect mitochondria in infected macrophages -; either by increasing the number of mitochondria, increasing the rate of mitochondrial respiration or by preventing the formation of mitochondrial permeability -; has also increased mitochondrial membrane potential and cell viability three to four times in infected macrophages. In addition, the four treatments limit the intracellular growth of M. tuberculosis bacteria. "The discovery that NAD + depletion triggers programmed cell death to kill macrophages infected with M. tuberculosis," said Niederweis, "reveals strategies for targeted approaches to the host.

These could include the use of FDA-approved drugs that reduce necroptosis, NAD + reconstitution for patients or the use of reagents that promote mitochondrial function, all in combination with antibacterial drugs used to treat tuberculosis These patient-targeted strategies could also be applied to other bacterial and fungal pathogens that deplete NAD +. "Perhaps even more important," said Niederweis, "the role of RIPK3 as a sensor Cell energy could play a role in other diseases. NAD + deficiency is a common pathological factor, such as type 2 diabetes and various neurological and cardiac diseases. "

Niederweis stated that the research" was the result of a very productive team work. "Pajuelo, Ph.D., and Norberto Gonzalez-Juarbe, Ph.D., a postdoctoral fellow in the laboratory of Carlos J. Orihuela, Ph.D., badociate professor of microbiology at UAB