Mitochondrial Metabolism of Glucose and Glutamine Is Required for Intracellular Growth of Toxoplasma gondii

Abstract

Summary Toxoplasma gondii proliferates within host cell vacuoles where the parasite relies on host carbon and nutrients for replication. To assess how T. gondii utilizes these resources, we mapped the carbon metabolism pathways in intracellular and egressed parasite stages. We determined that intracellular T. gondii stages actively catabolize host glucose via a canonical, oxidative tricarboxylic acid (TCA) cycle, a mitochondrial pathway in which organic molecules are broken down to generate energy. These stages also catabolize glutamine via the TCA cycle and an unanticipated γ-aminobutyric acid (GABA) shunt, which generates GABA and additional molecules that enter the TCA cycle. Chemically inhibiting the TCA cycle completely prevents intracellular parasite replication. Parasites lacking the GABA shunt exhibit attenuated growth and are unable to sustain motility under nutrient-limited conditions, suggesting that GABA functions as a short-term energy reserve. Thus, T. gondii tachyzoites have metabolic flexibility that likely allows the parasite to infect diverse cell types.

Apicomplexan Energy Metabolism: Carbon Source Promiscuity and the Quiescence Hyperbole

“…However, flux from both sources is much greater in T. gondii, and therefore, unlike Plasmodium, T. gondii tachyzoite proliferation is sensitive to TCA cycle and ETC perturbations. Disruption of BCKDH attenuates parasite intracellular development and virulence [46], while inhibition of Aco [54] or NDH [68] arrests growth, suggesting that an active respiratory chain is essential for tachyzoite growth, as previously hypothesized [4,54,82]. However, as above, the lethal effects of Aco inhibition may be due to citrate accumulation and cytotoxicity, or inhibition of Aco activity in the T. gondii apicoplast (Figure 3 and Box 3).…”

“…Consistent with this, BCKDH disruption (leading to reduced mitochondrial and cytosolic acetyl-CoA) leads to increased gluconeogenic (and glycolytic) flux [46]. Interestingly, Aco inhibition (leading to increased cytosolic acetyl-CoA) does not [54].…”

“…Unlike Toxoplasma gondii (Figure 3), Plasmodium spp. do not appear to possess the final enzyme of the GABA shunt, SSDH, although GABA is still produced [26] and a putative mitochondrial transporter exists [54]. In this regard, GABA may play a role in mitochondrial metabolism by contributing to transamination reactions that could play a role in TCA regulation.…”

“…Isocitrate Dehydrogenase (IDH) The apicomplexan mitochondria have lost NAD + -dependent IDH and instead rely on the retained NADP + -dependent form of IDH for oxidation of isocitrate to /-ketoglutarate (Figure 2) [4,26,36,53,54]. The generated NADPH may contribute to defense against ROS damage, NADPH-dependent enzyme activity, and the electrochemical gradient through conversion to NADH by NAD(P) + transhydrogenase (Figure 2) [9,55], although the localization of the latter is as yet unclear.…”

“…However, as above, the lethal effects of Aco inhibition may be due to citrate accumulation and cytotoxicity, or inhibition of Aco activity in the T. gondii apicoplast (Figure 3 and Box 3). Intriguingly, fatty acid biosynthesis is slightly reduced upon fluoroacetate treatment [54].…”