Host-derived glucose and its transporter in the obligate intracellular pathogen Toxoplasma gondii are dispensable by glutaminolysis

Abstract

Toxoplasma gondii, as an obligate intracellular and promiscuous pathogen of mammalian cells, utilizes host sugars for energy and to generate glycoconjugates that are important to its survival and virulence. Here, we report that T. gondii glucose transporter (TgGT1) is proficient in transporting mannose, galactose, and fructose besides glucose, and serves as a major hexose transporter at its plasma membrane. Toxoplasma harbors 3 additional putative sugar transporters (TgST1-3), of which TgST2 is expressed at its surface, whereas TgST1 and TgST3 are intracellular. Surprisingly, TgGT1 and TgST2 are nonessential to the parasite as their ablations inflict only a 30% or no defect in its intracellular growth, respectively. Indeed, Toxoplasma can also tolerate the deletion of both genes while incurring no further growth phenotype. Unlike ⌬tgst2, the modest impairment in ⌬tggt1 and ⌬tggt1/⌬tgst2 mutants is because of a minor delay in their intracellular replication, which is a direct consequence of the abolished import of glucose. The ⌬tggt1 displays an attenuated motility in defined minimal media that is rescued by glutamine. TgGT1-complemented parasites show an entirely restored growth, motility, and sugar import. The lack of exogenous glucose in ⌬tggt1 culture fails to accentuate its intrinsic growth defect and prompts it to procure glutamine to sustain its metabolism. Unexpectedly, in vivo virulence of ⌬tggt1 in mice remains unaffected. Taken together, our data demonstrate that glucose is nonessential for T. gondii tachyzoites, underscore glutamine is a complement substrate, and provide a basis for understanding the adaptation of T. gondii to diverse host cells.glucose transport ͉ glutamine metabolism ͉ genetic manipulation

Toxoplasmaaldolase is required for metabolism but dispensable for host-cell invasion

“…In light of the present findings, we conclude that if ALD-adhesin interactions occur in vivo, they do not play an essential role in invasion but rather aldolase is primarily important in energy metabolism. Our finding that aldolase is dispensable in the absence of glucose is consistent with the observation that growth is normal following deletion of the major glucose transporters in T. gondii, as long as glutamine is present (30). Whether ALD is required for long-term growth of T. gondii under these conditions is uncertain, although it might be necessary to participate in gluconeogenesis, a pathway that is complete in the genome (www.toxodb.org).…”

Toxoplasma gondii Development of Its Replicative Niche: in Its Host Cell and Beyond

“…Deletion of the TgGT1 gene results in a significant defect in glucose uptake and a defect in parasite motility and replication (10). The requirement for glucose in parasite motility is linked to the observation that during motility, glycolytic enzymes relocalize to the inner membrane complex (a membranous structure that lies directly adjacent to the plasma membrane and serves as an anchor for the actomyosin machinery to propel the parasite into the host cell), suggesting that glucose provides the energy needed for invasion (8,9).…”

“…The requirement for glucose in parasite motility is linked to the observation that during motility, glycolytic enzymes relocalize to the inner membrane complex (a membranous structure that lies directly adjacent to the plasma membrane and serves as an anchor for the actomyosin machinery to propel the parasite into the host cell), suggesting that glucose provides the energy needed for invasion (8,9). Surprisingly, loss of TgGT1 had no impact on virulence (10), suggesting that Toxoplasma uses other carbon sources to generate ATP.…”

“…Identification of this other carbon source came from the observation that motility of the TgGT1 knockout parasites could be restored by the addition of glutamine to the media (10). Together, these data suggested that parasites could generate ATP through either glycolysis or glutaminolysis.…”

Amino Acid Homeostasis Modulates Salicylic Acid–Associated Redox Status and Defense Responses inArabidopsis

“…The Gln depletion in infected host tissues might also be due to the specific acquisition and usage of amino acids by the pathogen, as revealed in human pathosystems (Hofreuter et al, 2008;Blume et al, 2009), although it appears that this direct pathogen drain might contribute only partially to this depletion, especially during the early stages of infection. Interestingly, expression of plastidic GS2, the major isoform of GS for Gln biosynthesis in leaves, was found to be downregulated during pathogen infection and senescence, which, in turn, causes a Gln deficiency in the chloroplast, a major site for Gln biosynthesis in normal plants.…”