Abstract
Arginine is an essential amino acid for the human pathogen Leishmania but not to its host. Thus, the mechanism by which this protozoan parasite regulates cellular homeostasis of arginine is critical for its survival and virulence. In a previous study, we cloned and functionally characterized a high affinity arginine-specific transporter, LdAAP3, from Leishmania donovani. In this investigation, we have characterized the relationship between arginine transport via LdAAP3 and amino acid availability. Starving promastigotes for amino acids decreased the cellular level of most amino acids including arginine but also increased the abundance of both LdAAP3 mRNA and protein and up-regulated arginine transport activity. Genetic obliteration of the polyamine biosynthesis pathway for which arginine is the sole precursor caused a significant decrease in the rate of arginine transport. Cumulatively, we established that LdAAP3 expression and activity changed whenever the cellular level of arginine changed. Our findings have led to the hypothesis that L. donovani promastigotes have a signaling pathway that senses cellular concentrations of arginine and subsequently activates a mechanism that regulates LdAAP3 expression and activity. Interestingly, this response of LdAAP3 to amino acid availability in L. donovani is identical to that of the mammalian cation amino acid transporter 1. Thus, we conjecture that Leishmania mimics the host response to amino acid availability to improve virulence.
Highlights
L-Arginine is a metabolically versatile cationic amino acid that provides a precursor function for the synthesis of a variety of bioactive molecules in all organisms
Amino Acid Starvation Up-regulates LdAAP3 Expression and Activity—When mid log phase L. donovani promastigotes were transferred from Earl’s-based medium 199 to Earl’s salt solution supplemented with only 5 mM glucose, the initial rate of arginine transport increased as a function of time of starvation (Fig. 1A)
Tions almost completely inhibited this up-regulation of arginine transport rates, indicating that the augmentation in arginine transport rates could be ascribed to decreased amino acid availability (Fig. 1B)
Summary
Starving promastigotes for amino acids decreased the cellular level of most amino acids including arginine and increased the abundance of both LdAAP3 mRNA and protein and up-regulated arginine transport activity. Our findings have led to the hypothesis that L. donovani promastigotes have a signaling pathway that senses cellular concentrations of arginine and subsequently activates a mechanism that regulates LdAAP3 expression and activity This response of LdAAP3 to amino acid availability in L. donovani is identical to that of the mammalian cation amino acid transporter 1. Arginine is an essential amino acid for Leishmania [8], and its metabolism and homeostasis depends on supply from external pools This amino acid has only two functions in the parasite serving as a monomeric precursor for polypeptide synthesis and to funnel carbon atoms into the polyamine biosynthetic pathway [9]. The results of this work indicate that arginine transport in L. donovani is regulated by a mechanism that senses changes in the level of the intracellular pool of arginine
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