Drug reveals hidden leucine uptake pathway in tuberculosis bacteria

Amino acids are the building blocks of life, and for the bacterium that causes tuberculosis, Mycobacterium tuberculosis (Mtb), they are central to growth and disease. Scientists have long known that Mtb makes many of its own amino acids, but how the bacterium takes up amino acids from its environment — especially the branched chain amino acids (BCAAs) — has been less clear. BCAAs are important not only for protein building but also feed into production of key cell-wall lipid precursors such as acetyl-CoA and propionyl-CoA. To probe this question, Nisheeth Agarwal and colleagues used a chemical genomic approach: they screened an FDA-approved repurposed library of small molecule inhibitors against an auxotrophic laboratory strain, Mtb mc 2 6206, which is genetically altered to lack leuC-leuD and panC-panD and therefore cannot make certain nutrients on its own. From that screen they identified semapimod. Semapimod stood out because it selectively inhibited growth of the auxotrophic Mtb mc 2 6206, while showing no growth effect on the wild-type pathogenic strain Mtb H 37 Rv in standard laboratory tests. This unexpected specificity prompted further investigation into how semapimod works and what it reveals about amino acid handling in Mtb.

Following the initial screen, the team performed a series of experiments to trace semapimod’s effects. A 24-hour exposure of Mtb mc 2 6206 to semapimod triggered massive transcriptional reprogramming: more than 450 genes changed their expression, spanning many metabolic activities. Those broad shifts suggested that semapimod interferes with a fundamental aspect of bacterial metabolism rather than a single narrow target. By combining transport and functional assays, the researchers affirmed that semapimod inhibits L-leucine uptake in Mtb mc 2 6206. The data indicate that semapimod’s action involves targeting a protein tied to the cell-wall lipid biosynthesis pathway, linking external amino acid uptake to membrane lipid processes. Importantly, while semapimod had no detectable effect on growth of Mtb H 37 Rv in vitro, treating mice infected with Mtb H 37 Rv with semapimod produced a significant reduction in bacterial load in lungs and spleen, showing an in vivo benefit that was not predicted by the laboratory growth tests.

These findings point to a previously underappreciated balance in Mtb between making L-leucine inside the cell and importing it from outside. The work suggests that, alongside endogenous biosynthesis, Mtb has a coordinated uptake machinery for L-leucine that is important for survival inside a host. That machinery appears connected to cell-wall lipid biosynthesis, a pathway already known to be crucial for Mtb virulence and persistence. The study also highlights how repurposing known drugs and applying chemical genomic screens can reveal unexpected vulnerabilities: semapimod selectively exposed a dependence in an auxotrophic strain and showed efficacy in infected animals despite lacking in vitro activity against the wild-type strain. Together, the results encourage further exploration of leucine uptake and associated lipid pathways as potential targets for tuberculosis intervention, and they underline the importance of testing candidate compounds in models that mimic the host environment rather than relying solely on laboratory culture outcomes.