A plant/fungal-type phosphoenolpyruvate carboxykinase located in the parasite mitochondrion ensures glucose-independent survival of Toxoplasma gondii

Abstract

Toxoplasma gondii 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 T. gondii 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 the mitochondrion (TgPEPCK_mt), whereas the other protein is not expressed in tachyzoites (TgPEPCK_net). Parasites with an intact glycolysis can tolerate genetic deletions of TgPEPCK_mt as well as of TgPEPCK_net, indicating their nonessential roles for tachyzoite survival. TgPEPCK_net can also be ablated in a glycolysis-deficient mutant, while TgPEPCK_mt is refractory to deletion. Consistent with this, the lytic cycle of a conditional mutant of TgPEPCK_mt 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_mt in linking catabolism of glucose and glutamine with anabolic pathways. Our data also suggest a homeostatic function ofTgPEPCK_mt 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 (TgPyC) 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 T. gondii tachyzoites appears to phenocopy cancer cells, which holds promise for developing common therapeutics against both threats.

Keywords: Toxoplasma gondii; cancer metabolism; gluconeogenesis; glutamine catabolism; glycolysis; intracellular parasitism; parasite metabolism; phosphoenolpyruvate carboxykinase; pyruvate carboxylase (PC); tricarboxylic acid cycle (TCA cycle) (Krebs cycle).