Mitochondrial metabolism of sexual and asexual blood stages of the malaria parasite Plasmodium falciparum

Abstract

Background: The carbon metabolism of the blood stages of Plasmodium falciparum, comprising rapidly dividing asexual stages and non-dividing gametocytes, is thought to be highly streamlined, with glycolysis providing most of the cellular ATP. However, these parasitic stages express all the enzymes needed for a canonical mitochondrial tricarboxylic acid (TCA) cycle, and it was recently proposed that they may catabolize glutamine via an atypical branched TCA cycle. Whether these stages catabolize glucose in the TCA cycle and what is the functional significance of mitochondrial metabolism remains unresolved. Results: We reassessed the central carbon metabolism of P. falciparum asexual and sexual blood stages, by metabolically labeling each stage with 13 C-glucose and 13 C-glutamine, and analyzing isotopic enrichment in key pathways using mass spectrometry. In contrast to previous findings, we found that carbon skeletons derived from both glucose and glutamine are catabolized in a canonical oxidative TCA cycle in both the asexual and sexual blood stages. Flux of glucose carbon skeletons into the TCA cycle is low in the asexual blood stages, with glutamine providing most of the carbon skeletons, but increases dramatically in the gametocyte stages. Increased glucose catabolism in the gametocyte TCA cycle was associated with increased glucose uptake, suggesting that the energy requirements of this stage are high. Significantly, whereas chemical inhibition of the TCA cycle had little effect on the growth or viability of asexual stages, inhibition of the gametocyte TCA cycle led to arrested development and death.

Apicomplexan Energy Metabolism: Carbon Source Promiscuity and the Quiescence Hyperbole

“…Despite heavy utilization of glycolysis, TCA metabolism does occur in asexual Plasmodium[ 1 5 _ T D $ D I F F ] (Figure 2), although generally at low turnover [26]. Flux through these pathways greatly increases in sexual and insect stages (reviewed below) [26,46] and a switch from anaerobic to aerobic metabolism might also allow survival of so-called 'dormant' parasites (Box 1).…”

“…Flux through these pathways greatly increases in sexual and insect stages (reviewed below) [26,46] and a switch from anaerobic to aerobic metabolism might also allow survival of so-called 'dormant' parasites (Box 1). Together, these observations highlight that mitochondrial activity is important in Plasmodium, with different stages drawing on different metabolic functions.…”

“…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.…”

“…Indeed, glucose uptake in P. falciparum-infected red blood cells (RBCs) has been observed to increase 75-100-fold compared to uninfected RBC [22][23][24][25]. Up to 93% of glucose is converted directly to lactate in asexual stages [26][ 4 _ T D $ D I F F ] and is ultimately excreted into the surrounding host cell. Perturbation of glucose uptake is detrimental to parasite growth [27,28], suggesting that glycolysis is an important part of the parasite's strategy for rapid proliferation.…”