The tuberculosis-causing bacterium, Mycobacterium tuberculosis (Mtb), has the ability to overcome alveolar macrophages in the lungs. New findings in Cell Reports now reveal a key signaling mechanism that allows the bacteria to defeat these immune cells. The work comes from a team led by the University of Alabama at Birmingham.
Previous studies have shown that an Mtb protein known as tuberculosis necrotizing toxin (TNT) is secreted into the cytosol of Mtb-infected macrophages, where it functions to hydrolyze the key substrate NAD+. This enzymatic activity of TNT subsequently causes necrosis of the macrophages. However, how the loss of NAD+ and necrosis are connected remains unknown.
In their work, the team finds that when NAD+ is depleted by TNT, a necroptosis (programmed necrosis) pathway is activated—particularly through two key mediator proteins, RIPK3 and MLKL. This activation bypasses two upstream components of the pathway. The necroptosis was also associated with depolarized mitochondria and impaired ATP synthesis, known hallmarks of Mtb-induced cell death.
Surprisingly, this phenomenon was also observed even in uninfected macrophages. In the absence of TNT or Mtb, NAD+ depletion to a critical threshold was enough to trigger necroptosis.
Perhaps NAD+ may offer a new avenue for the treatment of tuberculosis, the team thought. Sure enough, when they replenished the NAD+ levels in the infected macrophages, they observed a 3-fold increase in the mitochondrial membrane potential and cell viability.
Ensuring mitochondrial health in the infected macrophages also improved viability. This involved adding compounds that either increased the number of mitochondria, increased the rate of mitochondrial respiration or prevented mitochondrial permeability.
"The finding that NAD+ depletion triggers programmed cell death to kill macrophages infected with M. tuberculosis reveals strategies for host-targeted approaches to treat tuberculosis," said study senior co-author Michael Niederweis.