New research shows that the amino acid asparagine is critical for the survival and replication of the bacterium that causes TB, raising hopes of new treatment strategies.
The study reveals that asparagine is used by the bacterium, Mycobacterium tuberculosis, for two main functions. It provides nitrogen, an important nutrient, and supplies ammonia, which can counteract the acidification that occurs as a defensive mechanism inside a host cell.
Researcher Luiz Pedro de Carvalho of the Medical Research Council's National Institute for Medical Research (NIMR; now part of the Francis Crick Institute) explained: "Nitrogen is a key nutrient for M. tuberculosis - it's present in almost all biological molecules, such as amino acids, which make up proteins, and nucleosides, which make up DNA and RNA and vitamins.
"In addition, acidification of bacteria engulfed by host immune cells is one of the chief responses triggered by the host's immune system to fight diseases such as TB."
Dr de Carvalho and his colleagues demonstrated that asparagine is taken up by M. tuberculosis using at least two different transport systems. They found that the enzyme that breaks down asparagine, called asparginase, is found both inside and outside M. tuberculosis cells, meaning that it carries out nitrogen metabolism and ammonia production in two independent compartments.
"This shows that asparaginase has evolved to cope with two independent but essential functions, working inside M. tuberculosis as a metabolic enzyme and outside the bacterium to help counter the host's immune response," explained Dr de Carvalho.
The researchers used a combination of classic genetics and microbiology techniques, imaging techniques and new methods such as metabolomics (a screening method that allows hundreds of metabolites to be separated, identified and quantified simultaneously), as well as cell and infection biology techniques.
Dr de Carvalho added: "Our study points to a few different avenues that can be exploit to rationally design/screen for new antibiotics.
"A drug that could inhibit M. tuberculosis asparaginase would block nitrogen uptake and metabolism in the bacterium, but would also sensitise it to acidification inside the host cell, therefore hitting two essential functions at the same time. The problem with this approach is that the inhibitor would have to be super selective, to avoid toxicity in humans by affecting the human version of asparaginase.
"Another appealing strategy would involve preventing the asparaginase being exported outside the M. tuberculosis cell. Because this is a unique bacterial process, we wouldn't expect this approach to be toxic to humans. Compounds that block asparaginase export should render M. tuberculosis sensitive to acid, thereby working with the human host's immune system to kill the bacteria."
Dr de Carvalho worked with Gérald Larrouy-Maumus at NIMR and colleagues at the University of Pisa in Italy, the Weill Cornell Medical College in New York, USA, and the Institute of Pharmacology and Structural Biology, the Centre d'Immunologie de Marseille-Luminy, the Curie Institute, the Pasteur Institute and INSERM, all in France.
The paper, Mycobacterium tuberculosis Exploits Asparagine to Assimilate Nitrogen and Resist Acid Stress during Infection, is published in PLOS Pathogens.
