Discussion

Metabolic adaptation of trypanosomes to changing host environments and carbon source availability is essential for progression in the parasitic life cycle. Using a culture model for fly stage development, we have investigated the impact of carbon source availability on procyclic to metacyclic development. Glucose withdrawal significantly accelerated and glycerol inhibited stage development, suggesting that these carbon sources act as metabolic signals for development in the fly. Global proteome analysis has identified a set of enzymes of citrate metabolism (citrate synthase, CS; aconitase, ACO; glycosomal isocitrate dehydrogenase, IDHg) that are induced by glucose withdrawal conditions and repressed by glycerol. Metabolite analysis of knock out lines deficient in these enzymes provides evidence for a novel pathway feeding glycosomal NAD(P)H production by IDHg. When a KO mutant of IDHg was tested, development arrested in the epimastigote stage and no metacyclic forms were detected. In agreement, fly passage of the same mutant revealed a dramatic and rescueable maturation phenotype with normal midgut infections, but sterile salivary glands. Hence, IDHg plays an essential role for development in the tsetse. In contrast, the complete tricarboxylic acid cycle (TCA) is not essential in the procyclic stage, with and without glucose. The biochemical role in development of a uniquely bi-specific NAD(P)H-dependent IDH (see X. Wang et al. this meeting) will be discussed.