Date Published: February 20, 2014
Publisher: Public Library of Science
Author(s): Alexandre Gouzy, Gérald Larrouy-Maumus, Daria Bottai, Florence Levillain, Alexia Dumas, Joshua B. Wallach, Irène Caire-Brandli, Chantal de Chastellier, Ting-Di Wu, Renaud Poincloux, Roland Brosch, Jean-Luc Guerquin-Kern, Dirk Schnappinger, Luiz Pedro Sório de Carvalho, Yannick Poquet, Olivier Neyrolles, Marcel A. Behr.
Mycobacterium tuberculosis is an intracellular pathogen. Within macrophages, M. tuberculosis thrives in a specialized membrane-bound vacuole, the phagosome, whose pH is slightly acidic, and where access to nutrients is limited. Understanding how the bacillus extracts and incorporates nutrients from its host may help develop novel strategies to combat tuberculosis. Here we show that M. tuberculosis employs the asparagine transporter AnsP2 and the secreted asparaginase AnsA to assimilate nitrogen and resist acid stress through asparagine hydrolysis and ammonia release. While the role of AnsP2 is partially spared by yet to be identified transporter(s), that of AnsA is crucial in both phagosome acidification arrest and intracellular replication, as an M. tuberculosis mutant lacking this asparaginase is ultimately attenuated in macrophages and in mice. Our study provides yet another example of the intimate link between physiology and virulence in the tubercle bacillus, and identifies a novel pathway to be targeted for therapeutic purposes.
With nearly 1.3 million lives claimed in 2012, as reported by the World Health Organization, tuberculosis (TB) remains the major cause of death due to a single bacterial pathogen. A better understanding of the interactions between Mycobacterium tuberculosis, the etiologic agent of TB, and its human host may help improve current therapies. In particular, unraveling the microbial mechanisms involved in uptake and catabolism of host-derived nutrients required by the pathogen during its life cycle may identify targets for novel antimicrobials –.
Identifying the nutrients used by M. tuberculosis to assimilate essential elements, such as carbon and nitrogen, is key to understanding host-pathogen interactions in TB. In this context, we recently reported that aspartate is a key nitrogen source used by M. tuberculosis during infection , . Here we further show that asparagine can serve as an additional source of nitrogen for the pathogen through transport by the amino acid permease AnsP2, and subsequent hydrolysis by the asparaginase AnsA (Fig. 6). Furthermore, our results establish a unique link between mycobacterial physiology and virulence since we show AnsA has a dual function in both nitrogen assimilation and in protection against acid stress in vitro and inside host cells (Fig. 4). Our results are most likely relevant from a physiological viewpoint since asparagine is present at 50–60 µM in the human plasma , and is 2- to 4-fold more concentrated in white blood cells , , . In addition, we further show here that asparagine accumulates in the mycobacterial vacuole inside infected macrophages. Altogether, these observations indicate that asparagine is most likely readily accessible to M. tuberculosis during infection in vivo.