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

Abstract

Intracellular parasites of the phylum Apicomplexa are dependent on the scavenging of essential amino acids from their hosts. We previously identified a large family of apicomplexan-specific plasma membrane-localized amino acid transporters, the ApiATs, and showed that the Toxoplasma gondii transporter TgApiAT1 functions in the selective uptake of arginine. TgApiAT1 is essential for parasite virulence, but dispensable for parasite growth in medium containing high concentrations of arginine, indicating the presence of at least one other arginine transporter. Here we identify TgApiAT6-1 as the second arginine transporter. Using a combination of parasite assays and heterologous characterisation of TgApiAT6-1 in Xenopus laevis oocytes, we demonstrate that TgApiAT6-1 is a general cationic amino acid transporter that mediates both the high-affinity uptake of lysine and the low-affinity uptake of arginine. TgApiAT6-1 is the primary lysine transporter in the disease-causing tachyzoite stage of T. gondii and is essential for parasite proliferation. We demonstrate that the uptake of cationic amino acids by TgApiAT6-1 is ‘trans-stimulated’ by cationic and neutral amino acids and is likely promoted by an inwardly negative membrane potential. These findings demonstrate that T. gondii has evolved overlapping transport mechanisms for the uptake of essential cationic amino acids, and we draw together our findings into a comprehensive model that highlights the finely-tuned, regulated processes that mediate cationic amino acid scavenging by these intracellular parasites.

An Uninvited Seat at the Dinner Table: How Apicomplexan Parasites Scavenge Nutrients from the Host

“…This hypothesis was supported in a recent publication in which TgApiAT6-1 was identified as the transporter that mediates TgApiAT1-independent L-arginine uptake in T. gondii [75]. Specifically, two parallel studies on TgApiAT6-1 have demonstrated that this protein is a general cationic amino acid transporter that mediates both the high-affinity uptake of L-lysine and the low-affinity uptake of L-arginine [75,76]. Furthermore, TgApiAT6-1 is the sole L-lysine transporter in T. gondii, explaining the indispensability reported in the corresponding gene [76].…”

“…However, TgApiAT1 is dispensable for parasite growth in a medium containing high concentrations of L-arginine, suggesting the pres-ence of at least one other L-arginine transporter. This hypothesis was supported in a recent publication in which TgApiAT6-1 was identified as the transporter that mediates TgApiAT1-independent L-arginine uptake in T. gondii [75]. Specifically, two parallel studies on TgApiAT6-1 have demonstrated that this protein is a general cationic amino acid transporter that mediates both the high-affinity uptake of L-lysine and the low-affinity uptake of L-arginine [75,76].…”

“…Taken together, TgApiAT1 abundance is upregulated in response to limited availability of L-arginine but downregulated upon limitation of a range of nutrients such as glucose, Lglutamine, and L-tyrosine, indicating that TgApiAT1 abundance regulation is perhaps part of a more general starvation response. Integrating their findings with research published by Augusto et al [111], Fairweather et al [75] proposed an appealing model to explain how an apicomplexan parasite responds to changes in the availability of a key nutrient. Augusto et al [111] demonstrated that intracellular T. gondii induces nutrient starvation in host cells, activating an integrated stress response (ISR) pathway in the infected cells.…”

“…If the ratio of L-arginine:L-lysine in the host cell is instead high, L-arginine can compete with L-lysine for uptake by TgApiAT6-1, resulting in a decreased role for TgApiAT1 that translates into downregulation of its abundance. Overall, TgApiAT1 may contribute substantially to L-arginine uptake when L-arginine levels are limited, while TgApiAT6-1 may ensure uptake of L-arginine and L-lysine under nutrient-rich conditions [75].…”

Toxoplasma metabolic flexibility in different growth conditions

“…the parasite sensing conditional changes. This expands on recent evidence that metabolic pathways are in fact essential only when parasites are grown in low-nutrient in vitro conditions [6][7][8][9]. Here we review some of the major metabolic pathways on which T. gondii depends for survival, and we interrogate the metabolic flexibility in the context of nutrient availability to explore the physiological relevance of in vitro systems during infection.…”