ARGININE SENSING IN THE HUMAN PATHOGEN LEISHMANIA

Protozoan of the genus Leishmania are the causative agents of leishmaniasis in humans. These parasites cycle between promastigotes in the sand fly mid-gut and amastigotes in phagolysosome of mammalian macrophages. During infection, parasites up-regulate macrophage nitric oxide synthase and arginase activity, both of which use arginine as a substrate. These elevated activities depleted macrophage arginine pools, a situation that invading Leishmania cannot tolerate as this is an essential amino acid. Leishmania donovani imports exogenous arginine via a mono-specific amino acid transporter (AAP3) and utilizes it primarily through the polyamine pathway to provide precursors for trypanothione biosynthesis. Here we report the discovery of an arginine availability pathway that senses the lack of environmental arginine. Depletion of arginine from promastigote and amastigote growth media induces a rapid up-regulation in AAP3 expression and activity, as well as a few other genes. Significantly, this arginine availability pathway is also activated in parasites during macrophage infection. Phosphoproteomic analyses of L. donovani promastigotes have implicated a mitogen activated protein kinase 2 (MPK2)-mediated signaling cascade in this response and L. mexicana mutants lacking MPK2 are unable to respond to arginine deprivation. In addition, these mutants cannot differentiate axenically into amastigotes and do not survive as amastigotes in peritoneal macrophages, and failed to establish an infection in mice. We propose that sensing arginine levels plays a critical role in Leishmania virulence by activating a rapid metabolic reaction for salvaging this amino acid in response to the lower arginine concentration in the macrophage phagolysosome. This response further promotes amastigote survival by further depleting the macrophage arginine pool, thereby suppressing production of cytotoxic nitric oxide.