SummaryThe Leishmania major aquaglyceroporin, LmAQP1, is responsible for the transport of trivalent metalloids, arsenite and antimonite. We have earlier shown that downregulation of LmAQP1 provides resistance to trivalent antimony compounds whereas increased expression of LmAQP1 in drug-resistant parasites can reverse the resistance. In this paper we describe the biochemical characterization of LmAQP1. Expression of LmAQP1 in Xenopus oocytes rendered them permeable to water, glycerol, methylglyoxal, dihydroxyacetone and sugar alcohols. The transport property of LmAQP1 was severely affected when a critical Arg230, located inside the pore of the channel, was altered to either alanine or lysine. Immunofluorescence and immuno-electron microscopy revealed LmAQP1 to be localized to the flagellum of Leishmania promastigotes and in the flagellar pocket membrane and contractile vacuole/spongiome complex of amastigotes. This is the first report of an aquaglyceroporin being localized to the flagellum of any microbe. Leishmania promastigotes and amastigotes expressing LmAQP1 could regulate their volume in response to hypoosmotic stress. Additionally, Leishmania promastigotes overexpressing LmAQP1 were found to migrate faster towards an osmotic gradient. These results taken together suggest that Leishmania LmAQP1 has multiple physiological roles, being involved in solute transport, volume regulation and osmotaxis.

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African trypanosomes produce some prostanoids, especially PGD 2 , PGE 2 and PGF 2a (Kubata et al. 2000, J. Exp. Med. 192: 1327-1338, probably to interfere with the host's physiological response. However, addition of prostaglandin D 2 (but not PGE 2 or PGF 2a ) to cultured bloodstream form trypanosomes led also to a significant inhibition of cell growth. Based on morphological alterations and specific staining methods using vital dyes, necrosis and autophagy were excluded. Here, we report that in bloodstream form trypanosomes PGD 2 induces an apoptosis-like programmed cell death, which includes maintenance of plasma membrane integrity, phosphatidylserine exposure, loss of mitochondrial membrane potential, nuclear chromatin condensation and DNA degradation. The use of caspase inhibitors cannot prevent the cell death, indicating that the process is caspase-independent. Based on these results, we suggest that PGD 2 -induced programmed cell death is part of the population density regulation as observed in infected animals.

Trypanosoma brucei, causative for African sleeping sickness, relies exclusively on glycolysis for ATP production. Under anaerobic conditions, glucose is converted to equimolar amounts of glycerol and pyruvate, which are both secreted from the parasite. As we have shown previously, glycerol transport in T. brucei occurs via specific membrane proteins (Wille, U., Schade, B., and Duszenko, M. (1998) Eur. J. Biochem. 256, 245-250). Here, we describe cloning and biochemical characterization of the three trypanosomal aquaglyceroporins (AQP; TbAQP1-3), which show a 40 -45% identity to mammalian AQP3 and -9. AQPs belong to the major intrinsic protein family and represent channels for small non-ionic molecules. Both TbAQP1 and TbAQP3 contain two highly conserved NPA motifs within the pore-forming region, whereas TbAQP2 contains NSA and NPS motifs instead, which are only occasionally found in AQPs. For functional characterization, all three proteins were heterologously expressed in yeast and Xenopus oocytes. In the yeast fps1⌬ mutant, TbAQPs suppressed hypoosmosensitivity and rendered cells to a hyper-osmosensitive phenotype, as expected for unregulated glycerol channels. Under iso-and hyperosmotic conditions, these cells constitutively released glycerol, consistent with a glycerol efflux function of TbAQP proteins. TbAQP expression in Xenopus oocytes increased permeability for water, glycerol and, interestingly, dihydroxyacetone. Except for urea, TbAQPs were virtually impermeable for other polyols; only TbAQP3 transported erythritol and ribitol. Thus, TbAQPs represent mainly water/glycerol/dihydroxyacetone channels involved in osmoregulation and glycerol metabolism in T. brucei. This function and especially the so far not investigated transport of dihydroxyacetone may be pivotal for the survival of the parasite survival under non-aerobic or osmotic stress conditions.

Recently, we reported the induction of a programmed cell death (PCD) in bloodstream forms of Trypanosoma brucei by prostaglandin D 2 (PGD 2 ). As this prostanoid is readily metabolized in the presence of albumin, we were prompted to investigate if PGD 2 metabolites rather than PGD 2 itself are responsible for the observed PCD. In fact, J series metabolites, especially PGJ 2 and D 12 PGJ 2 , were able to induce PCD more efficiently than PGD 2 . However, the stable PGD 2 analog 17phenyl-trinor-PGD 2 led to the same phenotype as the natural PGD 2 , indicating that the latter induces PCD as well. Interestingly, the intracellular reactive oxygen species (ROS) level increased significantly under J series metabolites treatment and, incubation with N-acetyl-L-cysteine or glutathione reduced ROS production and cell death significantly. We conclude that PGJ 2 and D 12 PGJ 2 formation within the serum represents a mechanism to amplify PGD 2 -induced PCD in trypanosomes via ROS production.

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We evaluated the effects of dihydroxyacetone (DHA) on Trypanosoma brucei bloodstream forms. DHA is considered an energy source for many different cell types. T. brucei takes up DHA readily due to the presence of aquaglyceroporins. However, the parasite is unable to use it as a carbon source because of the absence of DHA kinase (DHAK). We could not find a homolog of the relevant gene in the genomic database of T. brucei and have been unable to detect DHAK activity in cell lysates of the parasite, and the parasite died quickly if DHA was the sole energy source in the medium. In addition, during trypanosome cultivation, DHA induced growth inhibition with a 50% inhibitory concentration of about 1 mM, a concentration that is completely innocuous to mammals. DHA caused cell cycle arrest in the G 2 /M phase of up to 70% at a concentration of 2 mM. Also, DHA-treated parasites showed profound ultrastructural alterations, including an increase of vesicular structures within the cytosol and the presence of multivesicular bodies, myelin-like structures, and autophagy-like vacuoles, as well as a marked disorder of the characteristic mitochondrion structure. Based on the toxicity of DHA for trypanosomes compared with mammals, we consider DHA a starting point for a rational design of new trypanocidal drugs.Trypanosoma brucei is the causative agent of sleeping sickness, a fatal illness that is endemic in many countries in subSaharan Africa. Nearly half a million people are currently infected with the parasite, and 50,000 to 60,000 new cases appear every year (40). Available drugs are not effective against all stages of the disease. Melarsoprol, a trivalent organic arsenical that was discovered in 1949, is still the first line of defense against sleeping sickness. This compound (together with eflornithine, a drug effective only against Trypanosoma gambiense) is the most effective medication for the late stage of the disease (5); however, it is not an ideal drug. Although melarsoprol has a high cure rate (80 to 90%), it remains ineffective in a number of cases. In addition, there has been an increase of treatment failure of up to 30% due to the emergence of melarsoprol resistance (17). Most importantly, however, the drug causes grave side affects. About 10% of patients develop a severe posttreatment reactive encephalopathy, which is fatal in approximately 50% of cases (17,18). This scenario clearly highlights the importance of a rational search for new trypanocidal compounds and/or biochemical targets to develop effective drugs that are safe, easy to use under field conditions, and inexpensive to produce.Recently, the three aquaglyceroporins (TbAQP1 to -3) of T. brucei have been cloned and characterized (38). They show high permeability for water and glycerol compared with other aquaglyceroporins. Surprisingly, however, this study also revealed for the first time that aquaglyceroporins from protozoa are highly permeable for dihydroxyacetone (DHA), a threecarbon sugar. This substance permeates these channels in the same range as, ...

SummaryThe Leishmania major aquaglyceroporin, LmAQP1, is responsible for the transport of antimonite [Sb(III)], an activated form of Pentostam or Glucantime. Downregulation of LmAQP1 provides resistance to trivalent antimony compounds and increased expression of LmAQP1 in drug-resistant parasites can reverse the resistance. Besides metalloid transport, LmAQP1 is also permeable to water, glycerol, methylglyoxal, dihydroxyacetone and sugar alcohols. LmAQP1 also plays a physiological role in volume regulation and osmotaxis. In this study, we examined the role of extracellular C-loop glutamates (Glu143, Glu145 and Glu152) in LmAQP1 activity. Alteration of both Glu143 and Glu145 to alanines did not affect either the biochemical or physiological properties of the protein, suggesting that neither residue is critical for LmAQP1 activity. Alteration of Glu152 to alanine, aspartate and glutamine affected metalloid transport in the order, wild-type > E152Q > E152D > E152A. In fact, axenic amastigotes expressing E152A LmAQP1 accumulated negligible levels of either arsenite [As(III)] or Sb(III). Alteration of Glu152 significantly affected volume regulation and osmotaxis, suggesting that Glu152 is critical for the physiological activity of the parasite. More importantly, alteration of Glu152 to alanine did not affect glycerol permeability. Although the metalloids, As(III) and Sb(III), are believed to be transported through aquaglyceroporin channels as they behave as inorganic molecular mimic of glycerol, this is the first report where metalloid and glycerol transport can be dissected by a single mutation at the extracellular pore entry of LmAQP1 channel.