Substrate-mediated regulation of the arginine transporter of Toxoplasma gondii

Abstract

Intracellular parasites, such as the apicomplexan Toxoplasma gondii, are adept at scavenging nutrients from their host. However, there is little understanding of how parasites sense and respond to the changing nutrient environments they encounter during an infection. TgApiAT1, a member of the apicomplexan ApiAT family of amino acid transporters, is the major uptake route for the essential amino acid L-arginine (Arg) in T. gondii. Here, we show that the abundance of TgApiAT1, and hence the rate of uptake of Arg, is regulated by the availability of Arg in the parasite’s external environment, increasing in response to decreased [Arg]. Using a luciferase-based ‘biosensor’ strain of T. gondii, we demonstrate that the expression of TgApiAT1 varies between different organs within the host, indicating that parasites are able to modulate TgApiAT1-dependent uptake of Arg as they encounter different nutrient environments in vivo. Finally, we show that Arg-dependent regulation of TgApiAT1 expression is post-transcriptional, mediated by an upstream open reading frame (uORF) in the TgApiAT1 transcript, and we provide evidence that the peptide encoded by this uORF is critical for mediating regulation. Together, our data reveal the mechanism by which an apicomplexan parasite responds to changes in the availability of a key nutrient.

Coordinated action of multiple transporters in the acquisition of essential cationic amino acids by the intracellular parasite Toxoplasma gondii

“…The data in the present study indicate that Tg ApiAT6-1 functions as this alternative Arg transporter. We have recently shown that the expression of Tg ApiAT1 is up-regulated under Arg-limiting conditions and expressed at low levels under Arg-replete conditions [ 31 ]. The differential expression of Tg ApiAT1 may therefore allow these parasites to survive when Arg levels are limited, while Tg ApiAT6-1 may ensure regulated uptake of Arg and Lys under nutrient-rich conditions.…”

“…Based on our findings, and on several other recent studies into Arg uptake in T . gondii [ 27 , 31 , 41 ], we can now propose a comprehensive model for the uptake of cationic amino acids into these parasites ( Fig 7 ). The scavenging of AA + by parasites results in a depletion of these amino acids in the host cell cytosol, causing upregulation of the host CAT1 AA + transporter [ 41 ].…”

“…in the cells of organs with high Arg catabolism such as the liver [ 45 ]; Fig 7 , right), Lys uptake by Tg ApiAT6-1 will out-compete Arg uptake. The parasite responds by upregulating Tg ApiAT1, which enables sufficient Arg uptake for parasite proliferation [ 31 ]. If the ratio of Arg:Lys in the host cell is high ( e .…”

“…g . in the cells of organs like the kidneys that function in the net synthesis of Arg [ 46 ]; Fig 7 , left), Arg can compete with Lys for uptake by Tg ApiAT6-1, resulting in an increased role for Tg ApiAT6-1 in Arg uptake in such environments (and a decreased role for Tg ApiAT1, corresponding to its decreased abundance; [ 31 ]).…”

“…Where applicable, anhydrotetracycline (ATc) was added to a final concentration of 0.5 μg/ml with 0.025% (v/v) ethanol added to relevant vehicle controls. In experiments wherein we varied the concentration of Lys in the growth medium, we generated homemade RPMI medium containing a range of [Lys], as described previously [ 31 ].…”