Discussion

Aquaglyceroporins (AQPs) transport water and glycerol and play important roles in drug-uptake in pathogenic trypanosomatids. For example, AQP2 in the human-infectious African trypanosome, Trypanosoma brucei gambiense, is responsible for melarsoprol and pentamidine-uptake, and AQP2-defects are responsible for melarsoprol treatment-failure. To further probe the roles of these transporters, we assembled a T. b. brucei strain lacking all three AQP-genes. Triple-null aqp1-2-3 T. b. brucei displayed only a very moderate growth defect in vitro, established infections in mice and recovered effectively from hypotonic-shock. The triple-null trypanosomes did, however, display glycerol uptake and efflux defects; they failed to utilise glycerol as a carbon-source and displayed increased sensitivity to salicylhydroxamic acid; an inhibitor of trypanosome alternative oxidase (TAO) that increases intracellular glycerol. Notably, disruption of AQP2 alone also generated cells with glycerol transport defects. As predicted by our findings, re-expression of AQP2 and reversal of melarsoprol-resistance in a clinical isolate reduced SHAM-sensitivity, consistent with repair of a glycerol-efflux defect. Thus, African trypanosome AQPs are not required for viability or osmoregulation but do make important contributions to drug-uptake and glycerol-flux. This improved understanding of AQP-function, and AQP2-mediated drug-uptake and glycerol-efflux in particular, could be exploited. For example, therapies alternating TAO-inhibitors with pentamidine or using TAO-inhibitors and melarsoprol sequentially could be more effective than previously anticipated and could mitigate the further emergence and spread of melarsoprol-resistance.