Research Article: De novo biosynthesis of sterols and fatty acids in the Trypanosoma brucei procyclic form: Carbon source preferences and metabolic flux redistributions

Date Published: May 29, 2018

Publisher: Public Library of Science

Author(s): Yoann Millerioux, Muriel Mazet, Guillaume Bouyssou, Stefan Allmann, Tiila-Riikka Kiema, Eloïse Bertiaux, Laetitia Fouillen, Chandan Thapa, Marc Biran, Nicolas Plazolles, Franziska Dittrich-Domergue, Aline Crouzols, Rik K. Wierenga, Brice Rotureau, Patrick Moreau, Frédéric Bringaud, Dominique Soldati-Favre.


De novo biosynthesis of lipids is essential for Trypanosoma brucei, a protist responsible for the sleeping sickness. Here, we demonstrate that the ketogenic carbon sources, threonine, acetate and glucose, are precursors for both fatty acid and sterol synthesis, while leucine only contributes to sterol production in the tsetse fly midgut stage of the parasite. Degradation of these carbon sources into lipids was investigated using a combination of reverse genetics and analysis of radio-labelled precursors incorporation into lipids. For instance, (i) deletion of the gene encoding isovaleryl-CoA dehydrogenase, involved in the leucine degradation pathway, abolished leucine incorporation into sterols, and (ii) RNAi-mediated down-regulation of the SCP2-thiolase gene expression abolished incorporation of the three ketogenic carbon sources into sterols. The SCP2-thiolase is part of a unidirectional two-step bridge between the fatty acid precursor, acetyl-CoA, and the precursor of the mevalonate pathway leading to sterol biosynthesis, 3-hydroxy-3-methylglutaryl-CoA. Metabolic flux through this bridge is increased either in the isovaleryl-CoA dehydrogenase null mutant or when the degradation of the ketogenic carbon sources is affected. We also observed a preference for fatty acids synthesis from ketogenic carbon sources, since blocking acetyl-CoA production from both glucose and threonine abolished acetate incorporation into sterols, while incorporation of acetate into fatty acids was increased. Interestingly, the growth of the isovaleryl-CoA dehydrogenase null mutant, but not that of the parental cells, is interrupted in the absence of ketogenic carbon sources, including lipids, which demonstrates the essential role of the mevalonate pathway. We concluded that procyclic trypanosomes have a strong preference for fatty acid versus sterol biosynthesis from ketogenic carbon sources, and as a consequence, that leucine is likely to be the main source, if not the only one, used by trypanosomes in the infected insect vector digestive tract to feed the mevalonate pathway.

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Trypanosoma brucei is a hemoparasitic unicellular eukaryote that causes Human African Trypanosomiasis (HAT), also known as sleeping sickness. The disease, fatal if untreated, is endemic in 36 countries in sub-Saharan Africa, with about 70 million people living at risk of infection [1]. The T. brucei life cycle is complex and the parasite must adapt to several dynamic micro-environments encountered both in the insect vector, tsetse fly, and in the mammalian hosts. This leads to substantial morphological and metabolic changes, including adaptation of their lipid and energy metabolism. Here, we will focus on the insect midgut procyclic stage (PCF) of the parasite by providing a comprehensive analysis of its fatty acid and sterol de novo biosynthesis from available carbon sources.

The human and livestock pathogen Trypanosoma brucei has maintained and developed the ability to produce de novo fatty acids through the mitochondrial FASII and microsomal elongase system, as well as sterols through the mevalonate pathway, which are all essential for growth of the mammalian and insect stages of the parasite (for reviews see [16, 17, 30, 42]). Here we have performed a comprehensive metabolic analysis of all possible carbon sources feeding fatty acid and sterol biosynthesis in PCF, with the aim to complete the metabolic map, to study the regulatory interplay between the different branches of the corresponding metabolic network, and to determine the real contribution of each branch in vivo in the insect vector.