Arrested Oocyst Maturation in Plasmodium Parasites Lacking Type II NADH:Ubiquinone Dehydrogenase

Abstract

The Plasmodium mitochondrial electron transport chain has received considerable attention as a potential target for new antimalarial drugs. Atovaquone, a potent inhibitor of Plasmodium cytochrome bc 1 , in combination with proguanil is recommended for chemoprophylaxis and treatment of malaria. The type II NADH:ubiquinone oxidoreductase (NDH2) is considered an attractive drug target, as its inhibition is thought to lead to the arrest of the mitochondrial electron transport chain and, as a consequence, pyrimidine biosynthesis, an essential pathway for the parasite. Using the rodent malaria parasite Plasmodium berghei as an in vivo infection model, we studied the role of NDH2 during Plasmodium life cycle progression. NDH2 can be deleted by targeted gene disruption and, thus, is dispensable for the pathogenic asexual blood stages, disproving the candidacy for an anti-malarial drug target. After transmission to the insect vector, NDH2-deficient ookinetes display an intact mitochondrial membrane potential. However, ndh2(؊) parasites fail to develop into mature oocysts in the mosquito midgut. We propose that Plasmodium blood stage parasites rely on glycolysis as the main ATP generating process, whereas in the invertebrate vector, a glucose-deprived environment, the malaria parasite is dependent on an intact mitochondrial respiratory chain.

Apicomplexan Energy Metabolism: Carbon Source Promiscuity and the Quiescence Hyperbole

“…the ETC (NADH dehydrogenase and succinate dehydrogenase) are dispensable in asexual Plasmodium [33,36,43,44].…”

“…hence all apicomplexans are reliant on an alternative, membrane-bound (but not spanning) type II NDH that can transfer electrons from NADH to ubiquinone, but not contribute to transmembrane proton transfer (Figure 2) [9,63,67]. The precise mitochondrial membrane location of the Plasmodium enzyme is unknown, but it is dispensable in P. berghei blood stages [44]. In T. gondii, two mitochondrial NDH isoforms are present on the matrix side of the inner leaflet and are required (but individually dispensable) for ATP regulation and normal tachyzoite proliferation [68][69][70][71].…”

“…This includes differentiation of gametes into ookinetes, invasion of the mid-gut epithelium, oocyst formation, and, ultimately, differentiation into sporozoites [77,78]. The increased reliance on the TCA is supported by upregulation of TCA enzymes in gametocyte stages [79,80] and the profound physiological consequences of genetic disruption and/or chemical inhibition of these enzymes (Figure 1) [26,31,36,43,44,46,51,62,81].…”

“…Deletion of KDH in the insect-transmissible NF54 strain of P. falciparum [36], and SDH [43], NDH [44], ATP synthase [31], or BCKDH [46] in P. berghei, all result in failure of normal post-fertilization progression to ookinetes and/or oocysts, although viable gametes are apparently formed. A similar phenotype is observed in P. berghei PEP carboxylase mutants [51].…”

Mitochondrial ATP synthase is dispensable in blood-stage Plasmodium berghei rodent malaria but essential in the mosquito phase

“…Nevertheless, the current consensus is that tricarboxylic acid cycling, electron transport, and ATP synthesis happen in the parasite mitochondrion, just not very much in asexual blood-stage parasites (16,19,22,23). Indeed, genetic knockout studies have shown that components of the mitochondrial electron-transport chain are dispensable in blood-stage malaria parasites, so long as the ability to regenerate ubiquinone for pyrimidine synthesis is maintained (10,24,25). Electron transport-defective parasites exhibit a phenotype only in the insect stage, where they are unable to complete their development and cannot transmit back to a vertebrate (24,25).…”