Additional remarks phenotype | Mutant/mutation
The mutant lacks expression of ACO
Protein (function)
The (putative) TCA cycle enzyme aconitase (ACO), which is responsible for the isomeric conversion of citrate to isocitrate
Glucose consumption by Plasmodium-infected RBC increases 10-fold and these stages rely primarily on glycolysis for energy generation. Notwithstanding their dependence on glycolysis, asexual blood stages maintain a single, poorly cristate mitochondrion and are dependent on electron transport chain (ETC) activity for the re-oxidation of inner membrane dehydrogenases and pyrimidine biosynthesis. The maintenance of the mitochondrial ETC is sustained in part, by the oxidation of pyruvate (diverted from glycolysis) and the uptake and catabolism of glutamine. Pyruvate can enter the TCA cycle via two pathways; through anaplerotic reactions involving the CO2-fixing enzyme, phosphoenolpyruvate carboxylase (PEPC), or through the activity of a repurposed branched chain α-keto acid dehydrogenase (BCKDH) complex, which substitutes for the activity of the missing mitochondrial pyruvate dehydrogenase in Plasmodium and other apicomplexan parasites. Despite the essentiality of the mitochondrion, operation of the TCA cycle is not required for intra-erythrocytic growth of P.falciparum.
Plasmodium spp. lack key enzymes involved in gluconeogenesis and all developmental stages are predicted to be dependent on the uptake of sugars. However, in contrast to the asexual blood stages, there is increasing evidence that the mosquito-infective stages of Plasmodium exhibit an increased dependence on the TCA cycle and mitochondrial metabolism.
Specifically, Plasmodium gametocytes develop more complex tubular mitochondrial cristae suggestive of increased mitochondrial function. Metabolomic analyses have confirmed increased TCA metabolism in P.falciparum gametocytes and demonstrated that this is essential for gametocyte maturation. Recent genetic studies have also shown that the TCA cycle is essential for the development of P.falciparum mosquito stages, consistent with earlier work in P.berghei demonstrating that the TCA cycle and the electron transport chain are required for ookinete development and oocyst formation.
In this study a combination of metabolomic and reverse genetic approaches was used to investigate the metabolic changes that occur in key mosquito stages of P.berghei and the potential impact of these changes on parasite infection in the mosquito. We find that these stages are highly sensitive to disruptions in multiple pathways in central carbon metabolism including the TCA cycle, the utilisation of glutamine as a carbon source, intermediary carbon metabolism and coenzyme A (CoA) synthesis.
Phenotype
Normal growth of asexual blood stages and gametocyte production. Normal exflagellation; strongly reduced ookinete production. No oocyst and sporozoite production.
Additional information
From the paper: Despite possessing similar exflagellation rates to wild type parasite, ookinete conversion was found to be severely affected in aco- parasites. To determine if this defect was sex-specific, genetic crosses of aco- parasites were performed with P.berghei lines RMgm-348 (Pb270, p47-, which produces viable male gametes but non-viable female gametes) and RMgm-15 (Pb137, p48/45-, which produces viable female gametes but non-viable male gametes). Surprisingly, given the maternal inheritance of the mitochondrion, aco- parasites were found to produce severely reduced numbers of ookinetes in both crosses, suggesting that all gametes are affected in the absence of a complete TCA cycle.
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