Arginine sensing and transport is uncoupled in Leishmania donovani parasites

Visceral leishmaniasis is a deadly disease caused by Leishmania donovani. L. donovani imports exogenous arginine via a mono-specific arginine transporter (AAP3) and utilizes it primarily to provide precursors for trypanothione biosynthesis. The depletion of arginine from promastigote and amastigote growth media induced a rapid up-regulation in AAP3 expression and activity, as well as a few other genes by activation of a Protein Kinase A (PKA) signaling cascade of arginine deprivation response (ADR) pathway. Arginine is a positively charged amino acid with a guanidine cap at the distal end. We postulated that some of best known arginine transport inhibitors have conserved guanidino groups in their side chains and some of these transport inhibitors were also structural analogues of arginine. To understand whether arginine sensing and transport are coupled; we used a panel of compounds with conserved and modified amidino-group including these arginine transport inhibitors (canavanine, N-Methyl L-arginine acetate (NMLAA) and pentamidine) to check their effect on ADR. L. donovani promastigotes and THP-1 infected amastigotes. We identified that pentamidine and canavanine downregulated ADR response; While NMLAA and other amidino-group modified compounds had no effect on ADR both in promastigotes and intracellular amastigotes. Additionally, pentamidine and canavanine also mimicked the presence of arginine thus downregulating ADR response in arginine starved promastigotes and intracellular amastigotes. Our data shows that arginine sensing and transport are uncoupled. The arginine sensor responds to both arginine deprivation and sufficiency by either activating or deactivating ADR by modulating the expression of AAP3.