Glycerol suppresses glucose consumption in trypanosomes through metabolic contest

Abstract

Microorganisms must make the right choice for nutrient consumption to adapt to their changing environment. As a consequence, bacteria and yeasts have developed regulatory mechanisms involving nutrient sensing and signaling, known as “catabolite repression,” allowing redirection of cell metabolism to maximize the consumption of an energy-efficient carbon source. Here, we report a new mechanism named “metabolic contest” for regulating the use of carbon sources without nutrient sensing and signaling. Trypanosoma brucei is a unicellular eukaryote transmitted by tsetse flies and causing human African trypanosomiasis, or sleeping sickness. We showed that, in contrast to most microorganisms, the insect stages of this parasite developed a preference for glycerol over glucose, with glucose consumption beginning after the depletion of glycerol present in the medium. This “metabolic contest” depends on the combination of 3 conditions: (i) the sequestration of both metabolic pathways in the same subcellular compartment, here in the peroxisomal-related organelles named glycosomes; (ii) the competition for the same substrate, here ATP, with the first enzymatic step of the glycerol and glucose metabolic pathways both being ATP-dependent (glycerol kinase and hexokinase, respectively); and (iii) an unbalanced activity between the competing enzymes, here the glycerol kinase activity being approximately 80-fold higher than the hexokinase activity. As predicted by our model, an approximately 50-fold down-regulation of the GK expression abolished the preference for glycerol over glucose, with glucose and glycerol being metabolized concomitantly. In theory, a metabolic contest could be found in any organism provided that the 3 conditions listed above are met.

Glycerol, a possible new player in the biology of trypanosomes

“…The molecular mechanism of this glucose preference over proline is currently unknown. Surprisingly, the parasite also developed an absolute preference for glycerol over glucose, already described in the 1960s by Ryley [ 9 ] and recently revisited by our group [ 10 ]. Indeed, glucose is not consumed as long as glycerol is present in the medium.…”

“…We characterised the molecular mechanism of this glycerol preference, which was called “metabolic contest” since it is based on competition between 2 kinases (hexokinase [HK] and glycerol kinase [GK]) for the same substrate (ATP) [ 10 ]. When glucose and glycerol enter the cells via glucose transporters and aquaglyceroporins, they are first phosphorylated to glucose 6-phosphate (G6P) and glycerol 3-phosphate (Gly3P) by HK (step 1 in Fig 1 ) and GK (step 19), respectively.…”

“…Hence, the limitation of the glycosomal ATP pool available to glycosomal kinases offers a situation where a significant excess of one kinase (here GK) can theoretically abolish the metabolic flux through another one (here HK). In this context, the very large excess of GK activity compared to HK activity (74-fold excess) explains the absolute glycerol preference confirmed by genetic manipulations and metabolic approaches [ 10 ]. The competition of given enzymes for a common substrate is a well-known process used to finely tune metabolic fluxes, in particular at metabolic branch points [ 12 ].…”

“…The physiological role of glycerol metabolism in vivo in the fly is still an open question. The preference of glycerol over glucose is based on the huge overexpression of GK, which could be reduced by at least 90% without affecting the glycerol metabolic flux [ 10 ]. Incidentally, glycerol also induces a 2.3-fold reduction of GK expression, without affecting metabolic contest, suggesting that the very high GK level in glucose-rich conditions is not compatible with glycerol-rich conditions, may be due to accumulation of Gly3P or other downstream metabolites, and/or a sharp decrease in the glycosomal ATP/ADP ratio [ 10 ].…”

“…, the cytosolic and glycosomal localisation of phosphoglycerate kinase (PGK, step 7), respectively [ 17 ]. Another major difference resides in the 10-fold higher glycolytic flux in BSF (for example, HK activity is up-regulated 28-fold [ 10 ]), with pyruvate representing up to 95% of the end products excreted from metabolism of glucose [ 18 ] ( Fig 1C ), while PCF mainly convert glucose to excreted acetate and succinate [ 19 ] ( Fig 1A ). Consequently, PGK is responsible for the regeneration of glycosomal ATP in BSF ( Fig 1C ), rather than PEPCK/PPDK as mentioned above for PCF ( Fig 1A ).…”