Hydrogen ion gradients across the mitochondrial, endosomal and plasma membranes in bloodstream forms of trypanosoma brucei solving the three-compartment problem

Venturicidin A affects the mitochondrial membrane potential and induces kDNA loss in Trypanosoma brucei

Neglected tropical diseases caused by trypanosomatid parasites have devastating health and economic consequences, especially in tropical areas. New drugs or new combination therapies to fight these parasites are urgently needed. Venturicidin A, a macrolide extracted from Streptomyces, inhibits the ATP synthase complex of fungi and bacteria. However, its effect on trypanosomatids is not fully understood. In this study, we tested venturicidin A on a panel of trypanosomatid parasites using Alamar Blue assays and found it to be highly active against Trypanosoma brucei and Leishmania donovani, but much less so against Trypanosoma evansi. Using fluorescence microscopy, we observed a rapid loss of the mitochondrial membrane potential in T. brucei bloodstream forms upon venturicidin A treatment. Additionally, we report the loss of mitochondrial DNA in approximately 40%-50% of the treated parasites. We conclude that venturicidin A targets the ATP synthase of T. brucei, and we suggest that this macrolide could be a candidate for anti-trypanosomatid drug repurposing, drug combinations, or medicinal chemistry programs.

The release of host-derived antibodies bound to the variant surface glycoprotein (VSG) of Trypanosoma brucei cannot be explained by pH-dependent conformational changes of the VSG dimer

Background

Trypanosoma brucei is a protozoan parasite that evades the mammalian host's adaptive immune response by antigenic variation of the highly immunogenic variant surface glycoprotein (VSG). VSGs form a dense surface coat that is constantly recycled through the endosomal system. Bound antibodies are separated in the endosome from the VSG and destroyed in the lysosome. For VSGs it has been hypothesized that pH-dependent structural changes of the VSG could occur in the more acidic environment of the endosome and hence, facilitate the separation of the antibody from the VSG.

Methods

We used size exclusion chromatography, where molecules are separated according to their hydrodynamic radius to see if the VSG is present as a homodimer at both pH values. To gain information about the structural integrity of the protein we used circular dichroism spectroscopy by exposing the VSG in solution to a mixture of right- and left-circularly polarized light and analysing the absorbed UV spectra. Evaluation of protein stability and molecular dynamics simulations at different pH values was performed using different computational methods.

Results

We show, for an A2-type VSG, that the dimer size is only slightly larger at pH 5.2 than at pH 7.4. Moreover, the dimer was marginally more stable at lower pH due to the higher affinity (ΔG = 353.37 kcal/mol) between the monomers. Due to the larger size, the predicted epitopes were more exposed to the solvent at low pH. Moderate conformational changes (ΔRMSD = 0.35 nm) in VSG were detected between the dimers at pH 5.2 and pH 7.4 in molecular dynamics simulations, and no significant differences in the protein secondary structure were observed by circular dichroism spectroscopy.

Conclusions

Thus, the dissociation of anti-VSG-antibodies in endosomes cannot be explained by changes in pH.

New insights into the role of acidocalcisomes in trypanosomatids

Acidocalcisomes are electron-dense organelles rich in polyphosphate and inorganic and organic cations that are acidified by proton pumps, and possess several channels, pumps, and transporters. They are present in bacteria and eukaryotes and have been studied in greater detail in trypanosomatids. Biogenesis studies of trypanosomatid acidocalcisomes found that they share properties with lysosome-related organelles of animal cells. In addition to their described roles in autophagy, cation and phosphorus storage, osmoregulation, pH homeostasis, and pathogenesis, recent studies have defined the role of these organelles in phosphate utilization, calcium ion (Ca2+ ) signaling, and bioenergetics, and will be the main subject of this review.

Molecular mechanisms of inhibiting glucosyltransferases for biofilm formation in Streptococcus mutans

Glucosyltransferases (Gtfs) play critical roles in the etiology and pathogenesis of Streptococcus mutans (S. mutans)- mediated dental caries including early childhood caries. Gtfs enhance the biofilm formation and promotes colonization of cariogenic bacteria by generating biofilm extracellular polysaccharides (EPSs), the key virulence property in the cariogenic process. Therefore, Gtfs have become an appealing target for effective therapeutic interventions that inhibit cariogenic biofilms. Importantly, targeting Gtfs selectively impairs the S. mutans virulence without affecting S. mutans existence or the existence of other species in the oral cavity. Over the past decade, numerous Gtfs inhibitory molecules have been identified, mainly including natural and synthetic compounds and their derivatives, antibodies, and metal ions. These therapeutic agents exert their inhibitory role in inhibiting the expression gtf genes and the activities and secretion of Gtfs enzymes with a wide range of sensitivity and effectiveness. Understanding molecular mechanisms of inhibiting Gtfs will contribute to instructing drug combination strategies, which is more effective for inhibiting Gtfs than one drug or class of drugs. This review highlights our current understanding of Gtfs activities and their potential utility, and discusses challenges and opportunities for future exploration of Gtfs as a therapeutic target.

Ratiometric Detection of Mercury (II) Ions in Living Cells Using Fluorescent Probe Based on Bis(styryl) Dye and Azadithia-15-Crown-5 Ether Receptor

Bis(styryl) dye 1 bearing N-phenylazadithia-15-crown-5 ether receptor has been evaluated as a ratiometric fluorescent chemosensor for mercury (II) ions in living cells. In aqueous solution, probe 1 selectively responds to the presence of Hg²⁺ via the changes in the emission intensity as well as in the emission band shape, which is a result of formation of the complex with 1:1 metal to ligand ratio (dissociation constant 0.56 ± 0.15 µM). The sensing mechanism is based on the interplay between the RET (resonance energy transfer) and ICT (intramolecular charge transfer) interactions occurring upon the UV/Vis (380 or 405 nm) photoexcitation of both styryl chromophores in probe 1. Bio-imaging studies revealed that the yellow (500-600 nm) to red (600-730 nm) fluorescence intensity ratio decreased from 4.4 ± 0.2 to 1.43 ± 0.10 when cells were exposed to increasing concentration of mercury (II) ions enabling ratiometric quantification of intracellular Hg²⁺ concentration in the 37 nM-1 μM range.

Vacuolar ATPase depletion contributes to dysregulation of endocytosis in bloodstream forms of Trypanosoma brucei

Background

Vacuolar H⁺-ATPase (V-ATPase) is a highly conserved protein complex which hydrolyzes ATP and pumps protons to acidify vacuolar vesicles. Beyond its role in pH maintenance, the involvement of V-ATPase in endocytosis is well documented in mammals and plants but is less clear in Trypanosoma brucei.

Methods

In this study, the subcellular localization of V-ATPase subunit B (TbVAB) of T. brucei was assessed via in situ N-terminal YFP-tagging and immunofluorescence assays. Transgenic bloodstream forms (BSF) of T. brucei were generated which comprised either a V-ATPase subunit B (TbVAB) conditional knockout or a V-ATPase subunit A (TbVAA) knockdown. Acridine orange and BCECF-AM were employed to assess the roles of V-ATPase in the pH regulation of BSF T. brucei. The endocytic activities of three markers were also characterized by flow cytometry analyses. Furthermore, trypanosomes were counted from trypanolysis treatment groups (either containing 1% or 5% NHS) and endocytosed trypanosome lytic factor (TLF) was also analyzed by an immunoblotting assay.

Results

TbVAB was found to localize to acidocalcisomes, lysosomes and probably also to endosomes of BSF of T. brucei and was demonstrated to be essential for cell growth. TbVAB depletion neutralized acidic organelles at 24 hours post-tetracycline depletion (hpd), meanwhile the steady state intracellular pH increased from 7.016 ± 0.013 to 7.422 ± 0.058. Trypanosomes with TbVAB depletion at 24 hpd were found to take up more transferrin (2.068 ± 0.277 fold) but less tomato lectin (49.31 ± 22.57%) by endocytosis, while no significant change was detected in dextran uptake. Similar endocytic dysregulated phenotypes were also observed in TbVAA knockdown cells. In addition, TbVAB depleted trypanosomes showed a low uptake of TLF and exhibited less sensitive to lysis in both 1% and 5% NHS treatments.

Conclusions

TbVAB is a key component of V-ATPase and was found to play a key function in endocytosis as well as exhibiting different effects in a receptor/cargo dependent manner in BSF of T. brucei. Besides vacuolar alkalinization, the dysregulation of endocytosis in TbVAB depleted T. brucei is considered to contribute to the reduced sensitivity to lysis by normal human serum.

Colonization of the tsetse fly midgut with commensal Kosakonia cowanii Zambiae inhibits trypanosome infection establishment

Tsetse flies (Glossina spp.) vector pathogenic trypanosomes (Trypanosoma spp.) in sub-Saharan Africa. These parasites cause human and animal African trypanosomiases, which are debilitating diseases that inflict an enormous socio-economic burden on inhabitants of endemic regions. Current disease control strategies rely primarily on treating infected animals and reducing tsetse population densities. However, relevant programs are costly, labor intensive and difficult to sustain. As such, novel strategies aimed at reducing tsetse vector competence require development. Herein we investigated whether Kosakonia cowanii Zambiae (Kco_Z), which confers Anopheles gambiae with resistance to Plasmodium, is able to colonize tsetse and induce a trypanosome refractory phenotype in the fly. Kco_Z established stable infections in tsetse’s gut and exhibited no adverse effect on the fly’s survival. Flies with established Kco_Z infections in their gut were significantly more refractory to infection with two distinct trypanosome species (T. congolense, 6% infection; T. brucei, 32% infection) than were age-matched flies that did not house the exogenous bacterium (T. congolense, 36% infected; T. brucei, 70% infected). Additionally, 52% of Kco_Z colonized tsetse survived infection with entomopathogenic Serratia marcescens, compared with only 9% of their wild-type counterparts. These parasite and pathogen refractory phenotypes result from the fact that Kco_Z acidifies tsetse’s midgut environment, which inhibits trypanosome and Serratia growth and thus infection establishment. Finally, we determined that Kco_Z infection does not impact the fecundity of male or female tsetse, nor the ability of male flies to compete with their wild-type counterparts for mates. We propose that Kco_Z could be used as one component of an integrated strategy aimed at reducing the ability of tsetse to transmit pathogenic trypanosomes.

Tsetse flies transmit pathogenic African trypanosomes, which are the causative agents of socio-economically devastating human and animal African trypanosomiases. These diseases are currently controlled in large part by reducing the population size of tsetse vectors through the use of insecticides, traps and sterile insect technique. However, logistic and monetary hurdles often preclude the prolonged application of procedures necessary to maintain these control programs. Thus, novel strategies, including those aimed at sustainably reducing the ability of tsetse to transmit trypanosomes, are presently under development. Herein we stably colonize tsetse flies with a bacterium (Kosakonia cowanii Zambiae, Kco_Z) that acidifies their midgut, thus rendering the environment inhospitable to infection with two distinct, epidemiologically important trypanosome strains as well as an entomopathogenic bacteria. In addition to inducing a trypanosome refractory phenotype, colonization of tsetse with Kco_Z exerts only a modest fitness cost on the fly. Taken together, these findings suggest that Kco_Z could be applied to enhance the effectiveness of currently employed tsetse control programs.

The silicon trypanosome: a test case of iterative model extension in systems biology

The African trypanosome, Trypanosoma brucei, is a unicellular parasite causing African Trypanosomiasis (sleeping sickness in humans and nagana in animals). Due to some of its unique properties, it has emerged as a popular model organism in systems biology. A predictive quantitative model of glycolysis in the bloodstream form of the parasite has been constructed and updated several times. The Silicon Trypanosome is a project that brings together modellers and experimentalists to improve and extend this core model with new pathways and additional levels of regulation. These new extensions and analyses use computational methods that explicitly take different levels of uncertainty into account. During this project, numerous tools and techniques have been developed for this purpose, which can now be used for a wide range of different studies in systems biology.

Molecular basis for the reverse reaction of African human trypanosomes glycerol kinase

The glycerol kinase (GK) of African human trypanosomes is compartmentalized in their glycosomes. Unlike the host GK, which under physiological conditions catalyzes only the forward reaction (ATP-dependent glycerol phosphorylation), trypanosome GK can additionally catalyze the reverse reaction. In fact, owing to this unique reverse catalysis, GK is potentially essential for the parasites survival in the human host, hence a promising drug target. The mechanism of its reverse catalysis was unknown; therefore, it was not clear if this ability was purely due to its localization in the organelles or whether structure-based catalytic differences also contribute. To investigate this lack of information, the X-ray crystal structure of this protein was determined up to 1.90 Å resolution, in its unligated form and in complex with three natural ligands. These data, in conjunction with results from structure-guided mutagenesis suggests that the trypanosome GK is possibly a transiently autophosphorylating threonine kinase, with the catalytic site formed by non-conserved residues. Our results provide a series of structural peculiarities of this enzyme, and gives unexpected insight into the reverse catalysis mechanism. Together, they provide an encouraging molecular framework for the development of trypanosome GK-specific inhibitors, which may lead to the design of new and safer trypanocidal drug(s).

Identification of Trypanosoma brucei components involved in trypanolysis by normal human serum

Normal human serum (NHS) confers human resistance to infection by the parasite Trypanosoma brucei owing to the trypanolytic activity of apolipoprotein L1 (APOL1), present in two serum complexes termed Trypanolytic Factors (TLF-1 and -2). In order to identify parasite components involved in the intracellular trafficking and activity of TLFs, an inducible RNA interference (RNAi) genomic DNA library constructed in bloodstream form T. brucei was subjected to RNAi induction and selection for resistant parasites under NHS conditions favouring either TLF-1 or TLF-2 uptake. While TLF-1 conditions readily selected the haptoglobin-haemoglobin (HP-HB) surface receptor TbHpHbR as expected, given its known ability to bind TLF-1, under TLF-2 conditions no specific receptor for TLF-2 was identified. Instead, the screen allowed the identification of five distinct factors expected to be involved in the assembly of the vacuolar proton pump V-ATPase and consecutive endosomal acidification. These data confirm that lowering the pH during endocytosis is required for APOL1 toxic activity.

Mechanism of Trypanosoma brucei gambiense resistance to human serum

The African parasite Trypanosoma brucei gambiense accounts for 97% of human sleeping sickness cases. T. b. gambiense resists the specific human innate immunity acting against several other tsetse-fly-transmitted trypanosome species such as T. b. brucei, the causative agent of nagana disease in cattle. Human immunity to some African trypanosomes is due to two serum complexes designated trypanolytic factors (TLF-1 and -2), which both contain haptoglobin-related protein (HPR) and apolipoprotein LI (APOL1). Whereas HPR association with haemoglobin (Hb) allows TLF-1 binding and uptake via the trypanosome receptor TbHpHbR (ref. 5), TLF-2 enters trypanosomes independently of TbHpHbR (refs 4, 5). APOL1 kills trypanosomes after insertion into endosomal/lysosomal membranes. Here we report that T. b. gambiense resists TLFs via a hydrophobic β-sheet of the T. b. gambiense-specific glycoprotein (TgsGP), which prevents APOL1 toxicity and induces stiffening of membranes upon interaction with lipids. Two additional features contribute to resistance to TLFs: reduction of sensitivity to APOL1 requiring cysteine protease activity, and TbHpHbR inactivation due to a L210S substitution. According to such a multifactorial defence mechanism, transgenic expression of T. b. brucei TbHpHbR in T. b. gambiense did not cause parasite lysis in normal human serum. However, these transgenic parasites were killed in hypohaptoglobinaemic serum, after high TLF-1 uptake in the absence of haptoglobin (Hp) that competes for Hb and receptor binding. TbHpHbR inactivation preventing high APOL1 loading in hypohaptoglobinaemic serum may have evolved because of the overlapping endemic area of T. b. gambiense infection and malaria, the main cause of haemolysis-induced hypohaptoglobinaemia in western and central Africa.

Dissecting the catalytic mechanism of Trypanosoma brucei trypanothione synthetase by kinetic analysis and computational modeling

In pathogenic trypanosomes, trypanothione synthetase (TryS) catalyzes the synthesis of both glutathionylspermidine (Gsp) and trypanothione (bis(glutathionyl)spermidine (T(SH)₂)). Here we present a thorough kinetic analysis of Trypanosoma brucei TryS in a newly developed phosphate buffer system at pH 7.0 and 37 °C, mimicking the physiological environment of the enzyme in the cytosol of bloodstream parasites. Under these conditions, TryS displays K_m values for GSH, ATP, spermidine, and Gsp of 34, 18, 687, and 32 μm, respectively, as well as K_i values for GSH and T(SH)₂ of 1 mm and 360 μm, respectively. As Gsp hydrolysis has a K_m value of 5.6 mm, the in vivo amidase activity is probably negligible. To obtain deeper insight in the molecular mechanism of TryS, we have formulated alternative kinetic models, with elementary reaction steps represented by linear kinetic equations. The model parameters were fitted to the extensive matrix of steady-state data obtained for different substrate/product combinations under the in vivo-like conditions. The best model describes the full kinetic profile and is able to predict time course data that were not used for fitting. This system's biology approach to enzyme kinetics led us to conclude that (i) TryS follows a ter-reactant mechanism, (ii) the intermediate Gsp dissociates from the enzyme between the two catalytic steps, and (iii) T(SH)₂ inhibits the enzyme by remaining bound at its product site and, as does the inhibitory GSH, by binding to the activated enzyme complex. The newly detected concerted substrate and product inhibition suggests that TryS activity is tightly regulated.

The glycosylphosphatidylinositol-PLC in Trypanosoma brucei forms a linear array on the exterior of the flagellar membrane before and after activation

Bloodstream forms of Trypanosoma brucei contain a glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) that cleaves the GPI-anchor of the variable surface glycoprotein (VSG). Its location in trypanosomes has been controversial. Here, using confocal microscopy and surface labelling techniques, we show that the GPI-PLC is located exclusively in a linear array on the outside of the flagellar membrane, close to the flagellar attachment zone, but does not co-localize with the flagellar attachment zone protein, FAZ1. Consequently, the GPI-PLC and the VSG occupy the same plasma membrane leaflet, which resolves the topological problem associated with the cleavage reaction if the VSG and the GPI-PLC were on opposite sides of the membrane. The exterior location requires the enzyme to be tightly regulated to prevent VSG release under basal conditions. During stimulated VSG release in intact cells, the GPI-PLC did not change location, suggesting that the release mechanism involves lateral diffusion of the VSG in the plane of the membrane to the fixed position of the GPI-PLC.

African trypanosomes cause sleeping sickness, for which current therapy is inadequate. The parasite protects its surface from the host immune system by regularly switching its surface coat. The glycosylphosphatidylinositol-PLC only occurs in the bloodstream form, where it removes the surface coat after it enters the tsetse fly vector. Activation of the enzyme in the bloodstream would be fatal for the parasite and it is, therefore, a potential drug target. However, therapeutic strategies have been hampered by confusion over the location of the GPI-PLC despite great effort by many labs. We have used a wide variety of techniques, including one completely novel method, that exploits the dependence of detection for partially buried surface proteins on the temperature of fixation, to identify the location of the GPI-PLC in relation to other markers unequivocally. All approaches consistently show that the GPI-PLC is located exclusively in the outer leaflet of the plasma membrane covering the flagellum, where it is confined to a narrow linear array adjacent to the flagellar attachment zone. Our data have resolved the question of how enzyme and substrate meet and also suggest that chemotherapeutic agents would be able to target the GPI-PLC in its exterior location.

The mitochondrion is a site of trypanocidal action of the aromatic diamidine DB75 in bloodstream forms of Trypanosoma brucei

Human African trypanosomiasis (HAT) is a fatal tropical disease caused by infection with protozoans of the species Trypanosoma brucei gambiense and T. b. rhodesiense. An oral prodrug, DB289, is a promising new therapy undergoing phase III clinical trials for early-stage HAT. DB289 is metabolically converted to the active trypanocidal diamidine DB75 [2,5-bis(4-amidinophenyl)furan]. We previously determined that DB75 inhibits yeast mitochondrial function (C. A. Lanteri, B. L. Trumpower, R. R. Tidwell, and S. R. Meshnick, Antimicrob. Agent Chemother. 48:3968-3974, 2004). The purpose of this study was to investigate if DB75 targets the mitochondrion of T. b. brucei bloodstream forms. DB75 rapidly accumulates within the mitochondria of living trypanosomes, as indicated by the fluorescent colocalization of DB75 with a mitochondrion-specific dye. Fluorescence-activated cell sorting analysis of rhodamine 123-stained living trypanosomes shows that DB75 and other trypanocidal diamidines (pentamidine and diminazene) collapse the mitochondrial membrane potential. DB75 inhibits ATP hydrolysis within T. brucei mitochondria and appears to inhibit the oligomycin-sensitive F 1 F 0-ATPase and perhaps other ATPases. DB75 is most likely not an inhibitor of electron transport within trypanosome mitochondria, since DB75 fails to inhibit mitochondrial respiration when glycerol-3-phosphate is used as the respiratory substrate. However, DB75 inhibits whole-cell respiration (50% inhibitory concentration, 20 microM) at drug concentrations and incubation durations that also result in the dissipation of the mitochondrial membrane potential. Taken together, these findings suggest that the mitochondrion is a target of the trypanocidal action of DB75.

Trypanosoma brucei: differential requirement of membrane potential for import of proteins into mitochondria in two developmental stages

Trypanosome alternative oxidase (TAO) and the cytochrome oxidase (COX) are two developmentally regulated terminal oxidases of the mitochondrial electron transport chain in Trypanosoma brucei. Here, we have compared the import of TAO and cytochrome oxidase subunit IV (COIV), two stage-specific nuclear encoded mitochondrial proteins, into the bloodstream and procyclic form mitochondria of T. brucei to understand the import processes in two different developmental stages. Under in vitro conditions TAO and COIV were imported and processed into isolated mitochondria from both the bloodstream and procyclic forms. With mitochondria isolated from the procyclic form, the import of TAO and COIV was dependent on the mitochondrial inner membrane potential (delta psi) and required protein(s) on the outer membrane. Import of these proteins also depended on the presence of both internal and external ATP. However, import of TAO and COIV into isolated mitochondria from the bloodstream form was not inhibited after the mitochondrial delta psi was dissipated by valinomycin, CCCP, or valinomycin and oligomycin in combination. In contrast, import of these proteins into bloodstream mitochondria was abolished after the hydrolysis of ATP by apyrase or removal of the ATP and ATP-generating system, suggesting that import is dependent on the presence of external ATP. Together, these data suggest that nuclear encoded proteins such as TAO and COIV are imported in the mitochondria of the bloodstream and the procyclic forms via different mechanism. Differential import conditions of nuclear encoded mitochondrial proteins of T. brucei possibly help it to adapt to different life forms.

Energy metabolism and its compartmentation in Trypanosoma brucei

African trypanosomes are parasitic protozoa of the order of Kinetoplastida, which cause sleeping sickness and nagana. Trypanosomes are not only of scientific interest because of their clinical importance, but also because these protozoa contain several very unusual biological features, such as their special energy metabolism. The energy metabolism of Trypanosoma brucei differs significantly from that of its host, not only because it comprises distinct enzymes and metabolic pathways, but also because some of the glycolytic enzymes are localized in organelles called glycosomes. Furthermore, the energy metabolism changes drastically during the complex life cycle of this parasite. This review will focus on the recent advances made in understanding the process of ATP production in T. brucei during its life cycle and the consequences of the special subcellular compartmentation.

Trypanosome lytic factor, a subclass of high-density lipoprotein, forms cation-selective pores in membranes

Trypanosome lytic factor 1 (TLF1) is a subclass of human high-density lipoprotein that kills some African trypanosomes thereby protecting humans from infection. We have shown that TLF1 is a 500 kDa HDL complex composed of lipids and at least seven different proteins. Here we present evidence outlining a new paradigm for the mechanism of lysis; TLF1 forms cation-selective pores in membranes. We show that the replacement of external Na+ (23 Da) with the larger tetramethylammonium+, choline+ and tetraethylammonium+ ions (74 Da, 104 Da and 130 Da) ameliorates the osmotically driven swelling and lysis of trypanosomes by TLF1. Confirmation of cation pore-formation was obtained using small unilamellar vesicles incubated with TLF1; these showed the predicted change in membrane potential expected from an influx of sodium ions. Using planar lipid bilayer model membranes made from trypanosome lipids, which allow the detection of single channels, we found that TLF1 forms discrete ion-conducting channels (17 pS) that are selective for potassium ions over chloride ions. We propose that the initial influx of extracellular Na+ down its concentration gradient promotes the passive entry of Cl- through preexisting Cl- channels. The net influx of both Na+ and Cl- create an osmotic imbalance that leads to passive water diffusion. This loss of osmoregulation results in cytoplasmic vacuolization, cell swelling and ultimately trypanosome lysis.

Rab5 and Rab11 mediate transferrin and anti-variant surface glycoprotein antibody recycling in Trypanosoma brucei

The mammalian-infective bloodstream form of Trypanosoma brucei possesses a highly active endocytotic system. Evasion of the host immune response by T. brucei is dependent on antigenic variation of VSG (variant surface glycoprotein), but additional mechanisms for removal of surface-bound antibody also operate. Four Rab proteins, Tb (trypanosomal) RAB4, 5A, 5B and 11 are located to the endosomal system; TbRAB5A and TbRAB11 co-localize with internalized anti-VSG antibody and transferrin. A live cell assay was used to record a single cycle of endocytosis of anti-VSG IgG and transferrin, their subsequent degradation within the endosomal system and exocytosis of the products. TbRAB5A and TbRAB11 were involved in the overall process of endocytosis, degradation and exocytosis, whereas TbRAB5B and TbRAB4 were not implicated. The kinetics of anti-VSG IgG and transferrin recycling depend on the nucleotide state of TbRAB5A and TbRAB11. These data, together with previous work, suggest that IgG and transferrin initially enter a TbRAB5A sorting endosome and are most probably recycled subsequently via a TbRAB11-dependent step. Analysis of the recycled IgG and transferrin demonstrated extensive degradation of these recycled proteins. Degradation of transferrin was enhanced in cells expressing increased amounts of TbRAB5A or TbRAB11 with a Ser-->Asn mutation, but was decreased when active TbRAB11 was overexpressed. The extent of degradation of anti-VSG IgG was found to be unaffected by mutant Rab protein expression. The presence of an efficient mechanism for the removal of IgG bound to the external surface of T. brucei and its subsequent proteolysis within the recycling system suggests a role for this pathway in immune evasion.

A vacuolar-type H+-pyrophosphatase governs maintenance of functional acidocalcisomes and growth of the insect and mammalian forms of Trypanosoma brucei

Vacuolar proton pyrophosphatases (V-H(+)-PPases) are electrogenic proton pumps found in many organisms of considerable industrial, environmental, and clinical importance. V-H(+)-PPases of several parasites were shown to be associated with acidic vacuoles named acidocalcisomes, which contain polyphosphate and calcium. In this work we functionally characterized a Trypanosoma brucei V-H(+)-PPase gene by using double-stranded RNA interference methodology to produce inducible V-H(+)-PPase-deficient strains of procyclic and bloodstream forms (PFiVP1 and BFiVP1). Acidocalcisomes of these mutated parasites lost acidity and contained 90% less polyphosphate. PFiVP1 did not release calcium after the addition of nigericin, and its total acidity was reduced by 70%. This mutant also failed to stabilize its intracellular pH on exposure to external basic pH >7.4 and recovered from intracellular acidification at a slower rate and to a more acidic final intracellular pH. In the absence of T. brucei V-H(+)-PPase expression, PFiVP1 and BFiVP1 grew at a slower rate with doubling times of 27 h instead of 15 h, and 10 h instead of 7.5 h, respectively. Moreover, BFiVP1 could not grow over 5 x 10(5) cells/ml corresponding to a cell density reduction of five times for bloodstream form stationary phase growth.

Factors that determine the plasma-membrane potential in bloodstream forms of Trypanosoma brucei

The plasma-membrane potential (Delta(psi)p) in bloodstream forms of Trypanosoma brucei was studied using several different radiolabelled probes: 86Rb+ and [14C]SCN- were used to report Delta(psi)p directly because they distribute in easily measured quantities across the plasma membrane only, and [3H]methyltriphenylphosphonium (MePh3P+) was used to report Delta(psi)p only when Delta(psi)m had been abolished with FCCP because it reports the algebraic sum of the two potentials when used alone. The unperturbed Delta(psi)p had a value of -82 mV and was found to be essentially identical with, and determined almost completely by, the potassium diffusion potential, as evidenced by: (a) the lack of effect of valinomycin on the value obtained under appropriate conditions when any of these probes were used; (b) the close agreement of this measured value with that predicted from the measured distribution of K+ across the plasma membrane (-76 mV); (c) the large effect of changes in the extracellular K+ concentration by substitution with Na+ on Delta(psi)p together with the complete lack of effect of substitution of extracellular Na+ by the choline cation or substitution of extracellular Cl- by the gluconate anion on Delta(psi)p. The contribution to Delta(psi)p by electrogenic pumping of Na+/K+-ATPase was found to be small (of the order of 6 mV). H+ was not found to be pumped across the plasma membrane or to contribute to Delta(psi)p.