β-Sitosterol could serve as a dual inhibitor of Trypanosoma congolense sialidase and phospholipase A2: in vitro kinetic analyses and molecular dynamic simulations

The involvement of Trypanosoma congolense sialidase alongside phospholipase A2 has been widely accepted as the major contributing factor to anemia during African animal trypanosomiasis. The enzymes aid the parasite in scavenging sialic acid and fatty acids necessary for survival in the infected host, but there are no specific drug candidates against the two enzymes. This study investigated the inhibitory effects of β-sitosterol on the partially purified T. congolense sialidase and phospholipase A2. Purification of the enzymes using DEAE cellulose column led to fractions with highest specific activities of 8016.41 and 39.26 µmol/min/mg for sialidase and phospholipase A2, respectively. Inhibition kinetics studies showed that β-sitosterol is non-competitive and an uncompetitive inhibitor of sialidase and phospholipase A2 with inhibition binding constants of 0.368 and 0.549 µM, respectively. Molecular docking of the compound revealed binding energies of - 8.0 and - 8.6 kcal/mol against the sialidase and phospholipase A2, respectively. Furthermore, 100 ns molecular dynamics simulation using GROMACS revealed stable interaction of β-sitosterol with both enzymes. Hydrogen bond interactions between the ligand and Glu284 and Leu102 residues of the sialidase and phospholipase A2, respectively, were found to be the major stabilizing forces. In conclusion, β-sitosterol could serve as a dual inhibitor of T. congolense sialidase and phospholipase A2; hence, the compound could be exploited further in the search for newer trypanocides.

Structural and kinetic characterization of Trypanosoma congolense pyruvate kinase

Trypanosoma are blood-borne parasites and are the causative agents of neglected tropical diseases (NTDs) affecting both humans and animals. These parasites mainly rely on glycolysis for their energy production within the mammalian host, which is why trypanosomal glycolytic enzymes have been pursued as interesting targets for the development of trypanocidal drugs. The structure-function relationships of pyruvate kinases (PYKs) from trypanosomatids (Trypanosoma and Leishmania) have been well-studied within this context. In this paper, we describe the structural and enzymatic characterization of PYK from T. congolense (TcoPYK), the main causative agent of Animal African Trypanosomosis (AAT), by employing a combination of enzymatic assays, thermal unfolding studies and X-ray crystallography.

The trypanocidal benzoxaborole AN7973 inhibits trypanosome mRNA processing

Kinetoplastid parasites-trypanosomes and leishmanias-infect millions of humans and cause economically devastating diseases of livestock, and the few existing drugs have serious deficiencies. Benzoxaborole-based compounds are very promising potential novel anti-trypanosomal therapies, with candidates already in human and animal clinical trials. We investigated the mechanism of action of several benzoxaboroles, including AN7973, an early candidate for veterinary trypanosomosis. In all kinetoplastids, transcription is polycistronic. Individual mRNA 5'-ends are created by trans splicing of a short leader sequence, with coupled polyadenylation of the preceding mRNA. Treatment of Trypanosoma brucei with AN7973 inhibited trans splicing within 1h, as judged by loss of the Y-structure splicing intermediate, reduced levels of mRNA, and accumulation of peri-nuclear granules. Methylation of the spliced leader precursor RNA was not affected, but more prolonged AN7973 treatment caused an increase in S-adenosyl methionine and methylated lysine. Together, the results indicate that mRNA processing is a primary target of AN7973. Polyadenylation is required for kinetoplastid trans splicing, and the EC50 for AN7973 in T. brucei was increased three-fold by over-expression of the T. brucei cleavage and polyadenylation factor CPSF3, identifying CPSF3 as a potential molecular target. Molecular modeling results suggested that inhibition of CPSF3 by AN7973 is feasible. Our results thus chemically validate mRNA processing as a viable drug target in trypanosomes. Several other benzoxaboroles showed metabolomic and splicing effects that were similar to those of AN7973, identifying splicing inhibition as a common mode of action and suggesting that it might be linked to subsequent changes in methylated metabolites. Granule formation, splicing inhibition and resistance after CPSF3 expression did not, however, always correlate and prolonged selection of trypanosomes in AN7973 resulted in only 1.5-fold resistance. It is therefore possible that the modes of action of oxaboroles that target trypanosome mRNA processing might extend beyond CPSF3 inhibition.

Trypanosuppresive effects of ellagic acid and amelioration of the trypanosome-associated pathological features coupled with inhibitory effects on trypanosomal sialidase in vitro and in silico

BACKGROUND

The search for novel antitrypanosomal agents had previously led to the isolation of ellagic acid as a bioactive antitrypanosomal compound using in vitro studies. However, it is not known whether this compound will elicit antitrypanosomal activity in in vivo condition which is usually the next step in the drug discovery process.

PURPOSE

Herein, we investigated the in vivo activity of ellagic acid against bloodstream form of Trypanosoma congolense and its ameliorative effects on trypanosome-induced anemia and organ damage as well as inhibitory effects on trypanosomal sialidase.

METHODS

Rats were infected with T. congolense and were treated with 100 and 200mg/kg body weight (BW) of ellagic acid for fourteen days. The levels of parasitemia, packed cell volume and biochemical parameters were measured. Subsequently, T. congolense sialidase was partially purified on DEAE cellulose column and the mode of inhibition of ellagic acid on the T. congolense sialidase determined. Molecular docking study was also conducted to determine the mode of interaction of the ellagic acid to the catalytic domain of T. rangeli sialidase.

RESULTS

At a dose of 100 and 200mg/kg (BW), ellagic acid demonstrated significant (P < 0.05) trypanosuppressive effect for most of the 24 days experimental period. Further, the ellagic acid significantly (P < 0.05) ameliorated the trypanosome-induced anemia, hepatic and renal damages as well as hepatomegaly, splenomegaly and renal hypertrophy. The trypanosome-associated free serum sialic acid upsurge alongside the accompanied membrane bound sialic acid reduction were also significantly (P < 0.05) prevented by the ellagic acid treatment. The T. congolense sialidase was purified to a fold of 6.6 with a yield of 83.8%. The enzyme had a K_M and Vmax of 70.12mg/ml and 0.04µmol/min respectively, and was inhibited in a non-competitive pattern by ellagic acid with an inhibition binding constant of 1986.75μM. However, in molecular docking study, ellagic acid formed hydrogen bonding interaction with major residues R³⁹, R³¹⁸, and W¹²⁴ at the active site of T. rangeli sialidase with a predicted binding free energy of -25.584kcal/mol.

CONCLUSION

We concluded that ellagic acid possesses trypanosuppressive effects and could ameliorate the trypanosome-induced pathological alterations.

Mechanisms controlling anaemia in Trypanosoma congolense infected mice

Background

Trypanosoma congolense are extracellular protozoan parasites of the blood stream of artiodactyls and are one of the main constraints on cattle production in Africa. In cattle, anaemia is the key feature of disease and persists after parasitaemia has declined to low or undetectable levels, but treatment to clear the parasites usually resolves the anaemia.

Methodology/Principal Findings

The progress of anaemia after Trypanosoma congolense infection was followed in three mouse strains. Anaemia developed rapidly in all three strains until the peak of the first wave of parasitaemia. This was followed by a second phase, characterized by slower progress to severe anaemia in C57BL/6, by slow recovery in surviving A/J and a rapid recovery in BALB/c. There was no association between parasitaemia and severity of anaemia. Furthermore, functional T lymphocytes are not required for the induction of anaemia, since suppression of T cell activity with Cyclosporin A had neither an effect on the course of infection nor on anaemia. Expression of genes involved in erythropoiesis and iron metabolism was followed in spleen, liver and kidney tissues in the three strains of mice using microarrays. There was no evidence for a response to erythropoietin, consistent with anaemia of chronic disease, which is erythropoietin insensitive. However, the expression of transcription factors and genes involved in erythropoiesis and haemolysis did correlate with the expression of the inflammatory cytokines Il6 and Ifng.

Conclusions/Significance

The innate immune response appears to be the major contributor to the inflammation associated with anaemia since suppression of T cells with CsA had no observable effect. Several transcription factors regulating haematopoiesis, Tal1, Gata1, Zfpm1 and Klf1 were expressed at consistently lower levels in C57BL/6 mice suggesting that these mice have a lower haematopoietic capacity and therefore less ability to recover from haemolysis induced anaemia after infection.

Trypanosoma congolense procyclins: unmasking cryptic major surface glycoproteins in procyclic forms

In the tsetse fly, the protozoan parasite Trypanosoma congolense is covered by a dense layer of glycosylphosphatidylinositol (GPI)-anchored molecules. These include a protease-resistant surface molecule (PRS), which is expressed by procyclic forms early in infection, and a glutamic acid- and alanine-rich protein (GARP), which appears at later stages. Since neither of these surface antigens is expressed at intermediate stages, we investigated whether a GPI-anchored protein of 50 to 58 kDa, previously detected in procyclic culture forms, might constitute the coat of these parasites. We therefore partially purified the protein from T. congolense Kilifi procyclic forms, obtained an N-terminal amino acid sequence, and identified its gene. Detailed analyses showed that the mature protein consists almost exclusively of 13 heptapeptide repeats (EPGENGT). The protein is densely N glycosylated, with up to 13 high-mannose oligosaccharides ranging from Man(5)GlcNAc(2) to Man(9)GlcNAc(2) linked to the peptide repeats. The lipid moiety of the glycosylphosphatidylinositol is composed of sn-1-stearoyl-2-lyso-glycerol-3-HPO(4)-1-(2-O-acyl)-d-myo-inositol. Heavily glycosylated proteins with similar repeats were subsequently identified in T. congolense Savannah procyclic forms. Collectively, this group of proteins was named T. congolense procyclins to reflect their relationship to the EP and GPEET procyclins of T. brucei. Using an antiserum raised against the EPGENGT repeat, we show that T. congolense procyclins are expressed continuously in the fly midgut and thus form the surface coat of cells that are negative for both PRS and GARP.

SSCP analysis of the P2 purine transporter TcoAT1 gene of Trypanosoma congolense leads to a simple PCR-RFLP test allowing the rapid identification of diminazene resistant stocks

Analyses were made on a Trypanosoma congolense contig coding a putative P2-like nucleoside transporter (the contig was named in this study TcoAT1). The sequence includes a start and stop codon and presents a high similarity with the gene TbAT1 of T. brucei (Smallest Sum Probability 2.8e-136). To investigate a possible link between point mutations and diminazene aceturate (DA) resistance in mice, the TcoAT1 putative genes of 26 T. congolense strains, characterised for DA sensitivity in the single dose mouse test, were screened by means of the Single Strand Conformation Polymorphism technique (SSCP). Results showed that the SSCP profiles of 23 out of 26 (88.5%) T. congolense strains were confirmed by the sensitivity test in mice with the commonly accepted criterion for sensitivity to diminazene being a CD80 of 20mg/kg in the mouse test. The remaining T. congolense strains showed a resistant SSCP profile and relapsed in mice after treatment at doses lower than 20mg/kg indicating that the SSCP is more sensitive than the single dose mouse test for the detection of resistance to diminazene. However, none of the strains used in this study showed a sensitive SSCP profile while they were resistant in the single dose mouse test. The sequencing of the TcoAT1 gene of two sensitive, two intermediate and two resistant strains allowed the set up of a PCR-RFLP test for the discrimination between sensitive and resistant strains confirming the SSCP results for the 26 strains of this study.

Azaanthraquinone inhibits respiration and in vitro growth of long slender bloodstream forms of Trypanosoma congolense

An ethanolic extract of Mitracarpus scaber was found to possess in vitro and in vivo trypanocidal activity against Trypanosoma congolense. At a dosage of 50 mg kg(-1) day(-1) in normal saline for 5 days, the extract cured Balbc mice infected with T. congolense without any relapse. The isolated active component benz(g)isoquinoline 5,10 dione (Azaanthraquinone) (AQ) purified from the extract was found to inhibit glucose-dependent cellular respiration and glycerol-3-phosphate-dependent mitochondrial O(2) assimilation of the long bloodstream forms of Trypanosoma congolense. On account of the pattern of inhibition, the target could be the mitochondrial electron transport system composed of glyceraldehyde 3-phosphate dehydrogenase (G3PDH). The azaanthraquinone specifically inhibited the reduced coenzyme Q(1)-dependent O(2) uptake of the mitochondria with respect to ubiquinone. The susceptible site could be due to ubiquinone redox system which links the two enzyme activities.

Glycosylphosphatidylinositol-anchored surface molecules of Trypanosoma congolense insect forms are developmentally regulated in the tsetse fly

Procyclic culture forms of Trypanosoma congolense have been shown to express a glutamic acid/alanine-rich protein (GARP) on their surface. By labelling T. congolense procyclic culture forms with glycosylphosphatidylinositol (GPI) precursors, we show that GARP is bound to the membrane by a GPI anchor and demonstrate the presence of two additional GPI-anchored surface molecules of 24-34 and 58 kDa that are abundantly expressed. The 24-34 kDa molecule, which is recognised by monoclonal antibodies that bind to the surface of living trypanosomes, is resistant to proteolysis, suggesting that it consists (predominantly) of non-proteinaceous material. We have therefore named it protease-resistant surface molecule (PRS). In common with the EP and GPEET procyclins of Trypanosoma brucei, the relative expression of the T. congolense GPI-anchored molecules changes during parasite development in the tsetse fly. PRS is abundantly expressed by procyclic trypanosomes in the midgut shortly after infection, but is downregulated in established midgut forms and completely absent from the epimastigote form in the proboscis. In contrast, GARP is downregulated in parasites in the tsetse fly midgut, but upregulated in the epimastigote form. Unexpectedly, 14 days post-infection, procyclic forms frequently are negative for both PRS and GARP, suggesting that they might be expressing another stage-specific surface antigen at this point in the life cycle.

Trypanosoma congolense: expression of a heat shock protein 70 and initial evaluation as a diagnostic antigen for bovine trypanosomosis

A 69-kDa immunodominant protein of Trypanosoma congolense was identified as a member of the hsp70 family that is homologous to mammalian BiP. We report here the expression of the gene encoding the T. congolense BiP in a bacterial system. Dot blotting of the truncated recombinant proteins confirmed that BiP antigenicity is mainly located in the C-terminal third of the molecule. A recombinant fragment corresponding to this region was used as an antigen in an indirect ELISA and an initial evaluation of its diagnostic potential for bovine trypanosomosis was performed. The test showed limited sensitivity for detection of primary-infected cattle but was capable of accurately detecting secondary infections. As BiP and its derivatives may be produced at low cost under stable forms allowing standardization of the tests, they warrant further evaluation as antigens fro serodiagnosis of bovine trypanosomosis.

Ethno-veterinary medicine: screening of Nigerian medicinal plants for trypanocidal properties

Trypanosoma congolense and T. brucei bloodstream form parasites were propagated axenically in suitable standard media at 34 degrees C. The effects of 33 plant extracts, fractions and pure compounds were evaluated on two clones of T. brucei and drug-sensitive and multi-drug-resistant clones of T. congolense. The cytotoxic activity of the trypanocidal extracts was also evaluated on calf aorta endothelial cells in vitro. Of the extracts tested, 22% killed T. congolense IL 1180 at a concentration of 100 microg/ml while 18% killed 90-100% of T. brucei ILTat 1.4 at the same concentration. However, 6% of the active extracts killed 93% of a dyskinetoplastid form of T. brucei IL Tat 1.1, indicating that the intact kinetoplast is a target of some of the compounds tested. Of the 12 extracts that displayed activity against drug sensitive trypanosomes, 66.7% had trypanocidal activity on a multi-drug-resistant clone, T. congolense IL 3338. The extracts of Eugenia uniflora, Acacia artaxacantha, Terminalia ivorensis, T. superba and Alchornea cordifolia had median lethal concentrations of between 13 and 69 microg/ml on both the drug-sensitive, IL 1180 and multi-drug-resistant clone, IL 3338. The median lethal doses of the active plant extracts on the calf aorta endothelial cells varied between 112 and 13750 microg/ml while the calculated selective indices ranged between 0.71 and 246.8 indicating bright prospects for the development of some of these extracts as potential trypanocidal agents.

Characterisation of the loci encoding the glutamic acid and alanine rich protein of Trypanosoma congolense

We have characterised the organisation of genes encoding the glutamate and alanine rich protein (GARP) surface coat of the procyclic and epimastigote stages of Trypanosoma congolense in the tsetse fly. The GARP genes are arranged at two, possibly physically linked, loci, one of which exhibits allelic variation. One locus contains a single GARP gene, whilst both alleles of the other have a large tandem array of polycistronically transcribed GARP genes. Sequence analysis has revealed that there are very few coding differences between different GARP genes. A sequence related to the Trypanosoma brucei expression site associated gene 4 (encoding a transmembrane protein with a cytoplasmic adenylate cyclase domain) has been identified within a region at the downstream flank of one locus. There is no evidence that, within the single trypanosome, GARP genes are as diverse as the procyclin genes that encode a corresponding coat in T. brucei.

Pathways of glucose catabolism in procyclic Trypanosoma congolense

Studies of respiration on glucose in procyclic Trypanosoma congolense in the presence of rotenone, antimycin, cyanide, salicylhydroxamic acid and malonate have indicated the presence of NADH dehydrogenase, cytochrome b-c1, cytochrome aa3, trypanosome alternate oxidase and NADH fumarate reductase/succinate dehydrogenase pathway that contributes electrons to coenzyme Q of the respiratory chain. The rotenone sensitive NADH dehydrogenase, the trypanosome alternate oxidase, and cytochrome aa3 accounted for 24.5 +/- 6.5, 36.2 +/- 4.2 and 54.1 +/- 5.5% respectively of the total respiration. Activities of lactate dehydrogenase, NAD(+)-linked malic enzyme and pyruvate kinase were less than 6 nanomoles/min/mg protein suggesting that they play a minor role in energy metabolism of the parasite. Phosphoenolpyruvate carboxykinase, pyruvate dehydrogenase, succinate dehydrogenase, NADP(+)-linked malic enzyme, NADH fumarate reductase, malate dehydrogenase, and alpha-ketoglutarate dehydrogenase and glycerol kinase on the other hand had specific activities greater than 60 nanomoles/min/mg protein. These enzyme activities could account for the production of pyruvate, acetate, succinate and glycerol. The results further show that the amount of glycerol produced was 35-48% of the combined total of pyruvate, acetate and succinate produced. It is apparent that some of the glycerol 3-phosphate produced in glycolysis in the presence of salicylhydroxamic acid is dephosphorylated to form glycerol while the rest is oxidised via cytochrome aa3 to form acetate, succinate and pyruvate.

Catabolism of proline by procyclic culture forms of Trypanosoma congolense

The effect of various metabolic inhibitors on the rate of oxygen consumption by procyclic culture forms of Trypanosoma congolense utilizing proline as substrate was investigated. Cyanide inhibited the rate of oxygen consumption by 81.0 +/- 6.7%, malonate inhibited the rate by 51.6 +/- 1.6% and Antimycin A by 73.1 +/- 5.9%. A combination of cyanide and malonate inhibited the rate of oxygen consumption by 84.9 +/- 6.7% while a combination of antimycin A and malonate inhibited the rate by 81.6 +/- 7.6%. Rotenone had no effect on the rate of respiration except when the intact cells were first permeabilized by digitonin after which rotenone decreased the rate of respiration by 20-30%. Salicylhydroxamate (SHAM) did not have any effect on the rate of oxygen consumption. Enzymes involved in the catabolism of proline with high activities were: proline dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, fumarase, NADP-linked malic enzyme, alanine aminotransferase and malate dehydrogenase. Activities of 1-pyrroline-5 carboxylate dehydrogenase, glutamate dehydrogenase, aspartate aminotransferase and NAD-linked malic enzyme were detectable but lower. The end products of proline catabolism were alanine and glutamate. Unlike the case in Trypanosoma brucei brucei aspartate was not detected. Possible pathways of proline catabolism in procyclic culture forms of T. congolense and of electron transfer are proposed.

Fasting hyperglycaemia with oral glucose tolerance in acute Trypanosoma congolense infection of rats

Intraperitoneal inoculation of rats with Trypanosoma congolense (Federe strain) produced a sustained parasitaemia from days 7 to 23 post-infection (pi). The fasting tail-blood glucose (FBG) concentrations in the infected animals increased (p<0.05) from 3.8+/-0.2 mmol/l on day 0 pi to 4.6+/-0.2, 4.9+/-0.2 and 5.8+/-0.3 mmol/l on days 7, 10 and 17 pi and decreased (p<0.05) to 3.1+/-0.8 and 2.9+/-0.7 mmol/l on days 20 and 23 pi, but the values in the uninfected controls varied between 3.8+/-0.3 mmol/l on day 0 pi and 3.9+/-0.2 mmol/l on day 23 pi. After oral glucose intake (1.0 g/kg) and determination of tail-blood glucose (BG) concentrations after 2 h, the percentage increase in BG from FBG was either comparable in infected and uninfected groups (days 7, 20, 23 pi) or lower (p<0.05) in the infected group (days 10, 17), suggesting the same rate of tissue glucose delivery in both groups or a faster rate in the infected group. Therefore, oral glucose tolerance in the infected rat was not impaired, but there was initial fasting hyperglycaemia followed by fasting hypoglycaemia in the later stage.

The accumulation and compartmentalization of isometamidium chloride in Trypanosoma congolense, monitored by its intrinsic fluorescence

Interaction of the trypanocide isometamidium chloride with components of Trypanosoma congolense results in characteristic shifts in the intrinsic fluorescence of the drug. The specificity of this interaction was investigated by analysing the effects of various physicochemical manipulations on its fluorescence properties. The characteristic shifts involved a preferential increase in the intensity of one emission peak over the other, resulting in a systematic increase in the ratio of fluorescence intensities. These effects were apparently due to constraints on fluorophore free rotation in the solution (that is, viscosity). Purified DNA produced similar effects in a saturable manner displaying high affinity for the drug, indicating that the constraint involves binding of the drug to high-affinity binding sites within the DNA. Such binding sites were demonstrated in lysates derived from trypanosomal cells. The binding sites were associated with macromolecular species (M(r) > 12000), and were partly disrupted by thermal denaturation and proteolysis. Treatment with DNase 1 produced high levels of disruption of the binding sites (> 85%), indicating an involvement of DNA in the binding. BSA demonstrated weak non-specific binding of the drug. Entry of drug into live trypanosomal cells (monitored by 14C-labelled drug uptake) was paralleled by fluorescence shifts observed under comparable conditions of drug concentration and buffer conditions. Both systems (fluorescence shifts and accumulation of labelled drug) indicated the presence of a saturable membrane transporter with high affinity for the drug. We conclude that monitoring the fluorescence shifts of isometamidium constitutes a sensitive and highly specific probe for entry of the drug into trypanosomal cells, thereby enabling resolution of the transport events involved.

Flagellum-mediated adhesion of Trypanosoma congolense to bovine aorta endothelial cells

We studied the interaction between Trypanosoma congolense and bovine aorta endothelial (BAE) cell monolayers. Our findings suggest that trypanosomes adhere predominantly to the flattened, peripheral cell surface domains as well as to filamentous endothelial outgrowths that are present during in vitro cultivation in non-confluent monolayers. Adhesion is mediated exclusively by the flagellum in a distinct geometrical order with respect to the flagellar cytoskeleton. Thus, it is possible to define exactly the trypanosomal cell surface domain involved in the attachment process. After 24-48 h of cultivation on monolayers, trypanosomes start to develop short, filopodia-like flagellar protrusions, which serve as additional elements in assisting parasite attachment. Small filaments (3-5 nm) also serve as cross-links between flagellar and endothelial cell surface membranes. Lectin-gold labeling shows that these cross-links contain sialic acid residues. In vitro assays confirm that sialic acid is involved in the adhesion process, whereas the extracellular matrix (ECM) proteins fibronectin, collagen, laminin and vitronectin are not. The presence of T. congolense exhibits a mitogenic effect on BAE cells.

A colorimetric assay for trypanosome viability and metabolic function

We have adapted a tetrazolium salt (MTT) colorimetric cytotoxicity assay to the assessment of viability and metabolic function in cultured African trypanosomes. Trypomastigotes of Trypanosoma congolense and T. brucei rhodesianse were harvested from the blood of parasitemic rats and cultured under axenic conditions that support trypanosome viability and growth. Analysis of serial dilutions of these bloodstream forms indicated that the assay could detect 10(4) parasites. To assess the effect of lymphoid cytokines on trypomastigote viability, 10(5) freshly harvested parasites were cultured with a wide range of dilutions of human recombinant IL-1, IL-3, IL-6, interferon-gamma (IFN gamma) or tumor necrosis factor-alpha (TNF alpha), or bovine recombinant IFN gamma or TNF alpha for 24, 48 or 96 h. These cytokines had no apparent growth enhancing or inhibitory effect on the trypomastigotes compared with growth in supplemented medium alone. This assay has several advantages over traditional counting methods, including increased sensitivity and rapid, repeatable quantitation. This adaptation of the MTT colorimetric assay should be useful in screening drugs and host-derived factors for growth-modulating effects on trypanosomes and other extracellular protozoan parasites.

Alterations in drug transport in resistant Trypanosoma congolense

The transport of isometamidium chloride (Samorin) in Trypanosoma congolense which were either sensitive or resistant to this widely used trypanocide was studied in vitro. Significantly lower amounts of drug were accumulated over time by resistant than by sensitive trypanosomes. While no direct evidence could be obtained, indirect observations implied the involvement of an increased efflux of drug from the resistant trypanosomes. In both the resistant and sensitive parasites, drug transport was found to be mediated by an energy-dependent, specific process, presumably receptor-mediated. However, the specificity of the putative receptors was altered in the drug-resistant parasites. It is proposed that an alteration or replacement of a specific receptor in isometamidium chloride-resistant T. congolense results in an increased efflux of the drug and that this increased efflux at least partially mediates the reduction in sensitivity to the compound.

Fluorescence analysis of the interaction of isometamidium with Trypanosoma congolense

Isometamidium chloride (Samorin) is the only compound recommended for prophylaxis against bovine trypanosomiasis in sub-Saharan Africa. The fluorescence property of this compound was used to investigate the interaction of the molecule with in vitro-derived bloodstream forms of Trypanosoma congolense IL 1180. Incubation of isometamidium with trypanosomes at 37 degrees C for 180 min resulted in a gradual alteration of the lambda max. with time (from 600 to 584 nm) and an increase in the intensity of trypanosome-associated fluorescence of approx. 2-fold. The alteration in fluorescence was temperature-dependent and inhibited by the addition of N-ethylmaleimide. In contrast, with intact cells addition of digitonin caused a rapid increase in fluorescence intensity to approximately four times that observed with intact cells. Uptake of isometamidium was also determined using radiolabelled drug; the results indicated that the time course of the uptake process resembled the fluorescence profile and was temperature-dependent. The results therefore indicate that the alteration of fluorescence is due to interaction of isometamidium with an intracellular component(s) and that isometamidium is transported across the plasma membrane via a protein carrier. The data also indicate that the described fluorescence technique can be used to investigate the role of membrane transport in resistance to isometamidium.

Kinetic modelling of isometamidium chloride (Samorin) uptake by Trypanosoma congolense

Clones of Trypanosoma congolense which express resistance to the widely used trypanocide isometamidium chloride accumulate less of the drug than clones which are sensitive to drug treatment. A mathematical model has been developed which was able to predict theoretical lines representing the uptake kinetics in trypanosomes which were sensitive to isometamidium, as well as for resistant trypanosomes in which reduced accumulation was a result of either reduced uptake or enhanced efflux of the drug. Data from drug uptake experiments were then fitted to these theoretical lines. While the value for drug efflux could not be separated from the dissociation constant of the trypanosomes for isometamidium, it was demonstrated that reduced accumulation is not a result of reduced uptake of isometamidium by drug-resistant trypanosomes.

Transport of isometamidium (Samorin) by drug-resistant and drug-sensitive Trypanosoma congolense

The uptake kinetics of a 14C-labelled trypanocidal compound isometamidium chloride (Samorin, RMB Animal Health Ltd, UK) was measured in drug-resistant and drug-sensitive Trypanosoma congolense. It was established that drug uptake was significantly more rapid and quantitatively greater in drug-sensitive parasites. There was clear evidence that drug uptake in both the resistant and sensitive trypanosomes was by a specific, receptor-mediated process. This specific drug transport was energy-dependent, being sensitive to metabolic inhibition with SHAM/glycerol. Significant differences in drug transport were observed which could be correlated with resistance to isometamidium. The optimal pH for drug accumulation was lowered in the resistant trypanosomes; this finding, along with an observed change in specificity for the related compound homidium bromide, suggested that the specific receptor for isometamidium is altered in the resistant trypanosomes, possibly resulting in a reduction in drug uptake. In addition to these alterations in drug uptake, efflux of isometamidium also appears to occur in the resistant trypanosomes. Both a reduction in incubation temperature and metabolic inhibition increased the level of trypanosome-associated isometamidium in the resistant parasites. This was in contrast to observations using drug-sensitive parasites. Furthermore, the addition of calcium flux-modulating agents to the incubation medium also resulted in an increase in accumulation by the resistant parasites.

Trypanosoma (Nannomonas) congolense: changes in respiratory metabolism during the life cycle

All four life cycle stages (bloodstream, procyclic, epimastigote, and metacyclic) of Trypanosoma congolense IL 3000 were assayed with an oxygen electrode (polarograph) for the presence of terminal oxidases and carbon-source preference. In addition, these stages were used for histochemical analysis of mitochondrial activity using rhodamine 123, nitroblue tetrazolium, and diaminobenzidine. Morphometry was used to compare mitochondrial volumes and surface area among the different life cycle stages. It was found that in contrast to epimastigote forms, which were metabolically almost identical to procyclic forms, metacyclic forms showed characteristics of, and seemed preadapted to, differentiation into the bloodstream stage. While mitochondrial NAD+ diaphorase activity and an electrochemical potential were detected in all life cycle stages, metacyclic metabolism was glucose-based and terminal oxidase activity was primarily dependent upon the trypanosome alternative oxidase with the contribution of cyanide-sensitive respiration accounting for only 20-30% of the total respiratory capacity.

Reduced accumulation of isometamidium by drug-resistant Trypanosoma congolense

The accumulation of the trypanocide isometamidium chloride (Samorin, RMB Animal Health Ltd., UK) by a range of clones of Trypanosoma congolense with varying sensitivity to the drug, was measured by methods based on the fluorescence of isometamidium. Fluorescence microscopy and flow cytometry showed a reduction in drug accumulation by resistant clones. Fluorescence spectrophotometry demonstrated an inverse correlation between the intensity of cell-associated fluorescence and the level of resistance of the clones expressed in vivo. The addition of the metabolic inhibitor SHAM/glycerol to the incubation medium resulted in a reduction of this apparent difference in drug accumulation between the clones; those clones which were sensitive to isometamidium showed a reduction in fluorescence while a percentage increase in fluorescence was observed as clones became more resistant to the trypanocide. These observations may be of value for the in vitro detection of resistant T. congolense populations and may also be used to estimate the mean level of resistance in a given sample. The results also imply that decreased accumulation of isometamidium by drug-resistant clones of the parasite may be responsible for the reduction in sensitivity.

Identification of midgut trypanolysin and trypanoagglutinin in Glossina palpalis sspp. (Diptera: Glossinidae)

A midgut trypanolysin and an agglutinin from Glossina palpalis subspecies were isolated and partially characterized using anion-exchange chromatography and polyacrylamide gel electrophoresis. FPLC fractions of midgut extracts of Glossina palpalis palpalis caused agglutination and lysis of two trypanosome species (Trypanosoma congolense and Trypanosoma brucei brucei), although Glossina palpalis gambiensis caused only agglutination. The trypanolysin and agglutinin were active only in the posterior midguts, were heat labile above 50 degrees C, had a periodic cycle of 'activity' in response to bloodmeal intake and were not affected by protease inhibitors or trypsin but were inactivated by pronase. The lytic substance contained two proteins with approximate molecular weights (Mr) of 12,000 and 10,000 Da respectively. The agglutinin had an approximate Mr of 67,000 Da. Gamma-irradiation of the two subspecies caused a temporary inhibition of trypanolytic and agglutinin activities in midgut extracts.

Trypanosoma congolense: an in vitro assay to distinguish drug-resistant from drug-sensitive populations

An in vitro assay to distinguish drug-resistant from drug-sensitive populations of Trypanosoma congolense has been developed. The incorporation of radiolabelled hypoxanthine by procyclic trypanosomes in vitro was measured after 48 h exposure to different concentrations of trypanocides. In the presence of either isometamidium chloride (Samorin) or diminazene aceturate (Berenil), the ability of procyclics of a drug-sensitive stock (TREU 1627) to incorporate hypoxanthine at 28 degrees C was impaired to a much greater extent than that of procyclics of a drug-resistant stock (TREU 1467), when compared with control organisms grown in the absence of drugs. Serum from a rabbit given 1 mg/kg Samorin also inhibited incorporation of radiolabel in TREU 1627 procyclics more severely than in TREU 1467 procyclics, although the difference between stocks was not substantial. When used with cultured blood-stream forms maintained at 35 degrees C, the assay could distinguish the stocks in the presence of Samorin, but no difference was detected between the populations in their incorporation of hypoxanthine after exposure to Berenil.

Endocytosis and intracellular occurrence of the variant surface glycoprotein in Trypanosoma congolense

Trypanosoma congolense bloodstream forms were examined for binding sites of polyclonal anti-variant surface glycoprotein (VSG) antibodies using immunoelectron microscopy. Besides the surface, the antibodies labeled intracellular vesicles, the tubular membrane system, secondary lysosomes, and the digestive vacuole. Protein A gold (PAG), peroxidase gold (POG), anti-VSG antibodies preincubated with PAG, ferritin, concanavalin A-ferritin, and microperoxidase were examined for their suitability as endocytosis tracers in combination with immunoelectron microscopy. Endocytosis of PAG and POG was most effective and was mediated by vesicles transporting the tracer to secondary lysosomes. Gold particles eventually accumulated in the digestive vacuole. Apparantly only low amounts of VSG were internalized during endocytosis. VSG export from the cell interior to the flagellar pocket was not observed during excessive endocytosis of PAG, whereas after incubation with substances causing the formation of filopodia by binding to the surface coat, VSG-labeled vesicles were present near the flagellar pocket.

Purification of African trypanosomes can cause biochemical changes in the parasites

Bloodstream forms of African trypanosomes are routinely purified from blood components by a combination of centrifugation and chromatography on DEAE cellulose at pH 8.0. Here we report that the nonphysiological conditions used for DEAE chromatography of the parasites result in changes in the ATP levels of the trypanosomes and an enhanced release from the parasites of proteins such as variable surface glycoprotein, peptidase, and phospholipase. Some of these changes can be reduced by the addition of nucleosides to the elution buffer and, after the elution of the parasites, by immediate readjustment of the external pH to the normal physiological level of blood (pH 7.4).

Trypanosoma congolense: differentiation to metacyclic trypanosomes in culture depends on the concentration of glutamine or proline

The effect of glutamine on the number of metacyclic trypanosomes produced in insect form cultures of Trypanosoma congolense TREU 1457 was investigated. When cultured in the absence of glutamine, trypanosomes did not develop to metacyclic forms. While metacyclics were produced in all cultures maintained with glutamine in the concentration range 2-20 mM, optimum numbers were obtained between 4 and 12 mM glutamine. This pattern was constant in flasks cultured for over two months. The substitution of glutamine by proline in culture medium did not influence the number of metacyclic forms produced. Removal of proline or glutamine from cultures producing metacyclics caused a rapid fall in production of metacyclic forms which could be reversed by re-introduction of either nutrient. Cultures of another stock of T. congolense responded similarly when maintained in different concentrations of glutamine, showing that the effect may be a general one.

Surface carbohydrates of procyclic forms of African trypanosomes studied using fluorescence activated cell sorter analysis and agglutination with lectins

Living culture form procyclics of Trypanosoma brucei brucei, T.b. rhodesiense, T.b. gambiense, T. congolense and T. simiae were tested for binding of eight different lectins. The binding of fluorescein isothiocyanate (FITC)-conjugated lectins was measured using a fluorescence activated cell sorter (FACS) and by agglutination with unlabelled lectins. Five of the lectins failed to bind to any of the procyclic organisms in both tests. All parasites bound concanavalin A (Con A) and all T.b. brucei, T.b. rhodesiense and T. congolense procyclics bound Ricinus communis agglutinin 120 (RCA) and wheat germ agglutinin (WGA). Trypanosoma b. gambiense procyclics failed to bind RCA and thus could be easily discriminated from other subspecies of T. brucei. Similarly, T. simiae did not bind WGA, unlike T. congolense, the other species of the genus Nannomonas. All positive reactions were inhibited by 0.2 M concentrations of the relevant sugars. The results indicate that all species and subspecies of the procyclic culture forms tested have surface-exposed structures resembling alpha-D-mannose moieties and that T.b. brucei, T.b. rhodesiense and T. congolense have surface-exposed molecules resembling D-galactose and N-acetyl D-glucosamine (or sialic acid) moieties. Molecules resembling D-galactose and N-acetyl D-glucosamine residues are absent or inaccessible in T.b. gambiense and T. simiae respectively. A group of T. congolense clones of parasite stocks isolated at Kilifi on the Kenyan coast showed quantitatively different binding of RCA when compared to the other T. congolense clones tested indicating that these organisms differ in surface carbohydrate structure.

Variant surface glycoproteins of Trypanosoma congolense bloodstream and metacyclic forms are anchored by a glycolipid tail

The variant surface glycoproteins (VSGs) of both metacyclic and bloodstream forms of Trypanosoma congolense are shown to be anchored to the plasma membrane through a glycolipid similar to that found in Trypanosoma brucei. Release of soluble VSG from both metacyclic and bloodstream forms is associated with the exposure of an antigenic determinant homologous to the cross-reacting determinant of T. brucei VSGs. Release of soluble VSG of T. congolense can be achieved by lysates of both bloodstream and metacyclic forms of T. congolense, by lysates of T. brucei bloodstream forms, but not by lysates of procyclic forms.

In vitro studies on the biosynthesis of the surface glycoprotein of Trypanosoma congolense

Tritiated leucine, glucosamine, mannose, and galactose were incorporated into the variant specific surface glycoprotein (VSG) of Trypanosoma congolense in vitro. The uptake of the precursors is shown by SDS-polyacrylamide electrophoresis and fluorography, by assay of the radioactivity in immunoprecipitates obtained with specific antisera, and by the isolation of the labeled antigens by affinity chromatography on concanavalin A-sepharose and isoelectric focusing. The in vitro labeled VSG exhibits the same degree of microheterogeneity as that observed in the VSG isolated from trypanosomes grown in animals. Analysis of the incorporated sugars after hydrolysis of the glycoprotein showed that glucosamine and mannose were utilized in biosynthesis of the carbohydrate moiety directly whereas galactose was converted possibly to other intermediates before being incorporated into the antigen. Tunicamycin completely prevented the incorporation of the radiolabeled sugars into the surface glycoprotein. The unglycosylated VSG with a molecular weight of 47 kDa had completely lost its size heterogeneity.

Comparative aspects of purine metabolism in some African trypanosomes

Some enzymes of purine salvage were detected in the cell-free preparations from bloodstream forms of African trypanosomes: Trypanosoma vivax; T. brucei and T. congolense. Extracts of trypanosomes cleave adenosine and inosine hydrolytically except in T. congolense where adenosine cleavage was mediated by a phosphorylase. All the trypanosomes apparently lacked adenosine deaminase. Adenine aminohydrolase was found only in T. vivax while adenosine monophosphate deaminase was detected in T. brucei and T. congolense. There was no detectable adenosine kinase activity in T. brucei. A pathway is proposed for the metabolism of purines in these trypanosomes.