The expression of gluconeogenic enzymes is typically repressed when glucose is available. The protozoan parasite Toxoplasma gondii utilizes host glucose to sustain high rates of intracellular replication. However, despite their preferential utilization of glucose, intracellular parasites constitutively express two isoforms of the gluconeogenic enzyme fructose 1,6-bisphosphatase (TgFBP1 and TgFBP2). The rationale for constitutive expression of FBPases in T. gondii remains unclear. We find that conditional knockdown of TgFBP2 results in complete loss of intracellular growth in vitro under glucose-replete conditions and loss of acute virulence in mice. TgFBP2 deficiency was rescued by expression of catalytically active FBPase and was associated with altered glycolytic and mitochondrial TCA cycle fluxes, as well as dysregulation of glycolipid, amylopectin, and fatty acid biosynthesis. Futile cycling between gluconeogenic and glycolytic enzymes may constitute a regulatory mechanism that allows T. gondii to rapidly adapt to changes in nutrient availability in different host cells.

Toxoplasma gondii is a widespread protozoan parasite infecting nearly all warm-blooded organisms. Asexual reproduction of the parasite within its host cells is achieved by consecutive lytic cycles, which necessitates biogenesis of significant energy and biomass. Here we show that glucose and glutamine are the two major physiologically important nutrients used for the synthesis of macromolecules (ATP, nucleic acid, proteins, and lipids) in T. gondii, and either of them is sufficient to ensure the parasite survival. The parasite can counteract genetic ablation of its glucose transporter by increasing the flux of glutamine-derived carbon through the tricarboxylic acid cycle and by concurrently activating gluconeogenesis, which guarantee a continued biogenesis of ATP and biomass for host-cell invasion and parasite replication, respectively. In accord, a pharmacological inhibition of glutaminolysis or oxidative phosphorylation arrests the lytic cycle of the glycolysis-deficient mutant, which is primarily a consequence of impaired invasion due to depletion of ATP. Unexpectedly, however, intracellular parasites continue to proliferate, albeit slower, notwithstanding a simultaneous deprivation of glucose and glutamine. A growth defect in the glycolysis-impaired mutant is caused by a compromised synthesis of lipids, which cannot be counterbalanced by glutamine but can be restored by acetate. Consistently, supplementation of parasite cultures with exogenous acetate can amend the lytic cycle of the glucose transport mutant. Such plasticity in the parasite's carbon flux enables a growth-and-survival trade-off in assorted nutrient milieus, which may underlie the promiscuous survival of T. gondii tachyzoites in diverse host cells. Our results also indicate a convergence of parasite metabolism with cancer cells.Toxoplasma gondii is an opportunistic intracellular pathogen of the phylum Apicomplexa that inflicts life-threatening acute and chronic infections in humans and animals (1). The parasite causes ocular and cerebral toxoplasmosis and sporadic abortion/stillbirth as a consequence of severe tissue necrosis in individuals with a poor or weakened immunity, e.g. neonates, AIDS, and organ-transplantation patients. The lytic cycle of T. gondii starts with the active energy-dependent invasion of host cells by the tachyzoite stage that is followed by rapid intracellular replication, egress by host-cell lysis, and reinvasion of adjacent cells. Upon infection of host cells in vitro, one tachyzoite can usually produce 32-64 daughter cells within a nonfusogenic vacuole, which provides a safe niche for the parasite replication and an interface for acquisition of host resources (1). Such an efficient asexual reproduction and concomitant expansion of the parasite vacuole requires a significant nutritional import and biomass synthesis within the parasite. Moreover, tachyzoites likely need to adjust metabolism according to variable bioenergetic demands of extracellular and intracellular states as well as to the nutritional cues in distinct...

is considered to be one of the most successful intracellular pathogens, because it can reproduce in varied nutritional milieus, encountered in diverse host cell types of essentially any warm-blooded organism. Our earlier work demonstrated that the acute (tachyzoite) stage of depends on cooperativity of glucose and glutamine catabolism to meet biosynthetic demands. Either of these two nutrients can sustain the parasite survival; however, what determines the metabolic plasticity has not yet been resolved. Here, we reveal two discrete phosphoenolpyruvate carboxykinase (PEPCK) enzymes in the parasite, one of which resides in theiochondrion (PEPCK), whereas the other protein is otxpressed in achyzoites (PEPCK). Parasites with an intact glycolysis can tolerate genetic deletions ofPEPCK as well as of PEPCK, indicating their nonessential roles for tachyzoite survival.PEPCK can also be ablated in a glycolysis-deficient mutant, while PEPCK is refractory to deletion. Consistent with this, the lytic cycle of a conditional mutant of PEPCK in the glycolysis-impaired strain was aborted upon induced repression of the mitochondrial isoform, demonstrating its essential role for the glucose-independent survival of parasites. Isotope-resolved metabolomics of the conditional mutant revealed defective flux of glutamine-derived carbon into RNA-bound ribose sugar as well as metabolites associated with gluconeogenesis, entailing a critical nodal role of PEPCK in linking catabolism of glucose and glutamine with anabolic pathways. Our data also suggest a homeostatic function ofPEPCK in cohesive operation of glycolysis and the tricarboxylic acid cycle in a normal glucose-replete milieu. Conversely, we found that the otherwise integrative enzyme pyruvate carboxylase (PyC) is dispensable not only in glycolysis-competent but also in glycolysis-deficient tachyzoites despite a mitochondrial localization. Last but not least, the observed physiology of tachyzoites appears to phenocopy cancer cells, which holds promise for developing common therapeutics against both threats.

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