Biochemistry of the variant surface glycoproteins of salivarian trypanosomes

Columbin inhibits cholesterol uptake in bloodstream forms ofTrypanosoma brucei-A possible trypanocidal mechanism

The diterpenoid furanolactone (columbin) from Aristolochia albida inhibited growth of culture forms of Trypanosoma brucei. In vitro analysis of the compound at 5-250 microg/ml showed complete lysis of the parasites within 10-20 minutes post incubation. At 50 microg/ml, columbin killed about 50% of the parasites which initially appeared swollen under phase contrast microscopy. Also the total amount of cholesterol diminished dose-dependently in the presence of 10-100 microg/ml of columbin after a 3-day incubation period. In vivo analysis of the compound in T. brucei-infected mice revealed that 25 mg/kg administered for 3 consecutive days, completely cleared the parasites from the peripheral circulation. However, columbin could not clear parasites in the cerebrospinal fluid.

Transcription of protein-coding genes in trypanosomes by RNA polymerase I

In eukaryotes, RNA polymerase (pol) II transcribes the protein-coding genes, whereas RNA pol I transcribes the genes that encode the three RNA species of the ribosome [the ribosomal RNAs (rRNAs)] at the nucleolus. Protozoan parasites of the order Kinetoplastida may represent an exception, because pol I can mediate the expression of exogenously introduced protein-coding genes in these single-cell organisms. A unique molecular mechanism, which leads to pre-mRNA maturation by trans-splicing, facilitates pol I-mediated protein-coding gene expression in trypanosomes. Trans-splicing adds a capped 39-nucleotide mini-exon, or spliced leader transcript, to the 5' end of the main coding exon posttranscriptionally. In other eukaryotes, the addition of a 5' cap, which is essential for mRNA function, occurs exclusively as a result of RNA pol II-mediated transcription. Given the assumption that cap addition represents the limiting factor, trans-splicing may have uncoupled the requirement for RNA pol II-mediated mRNA production. A comparison of the alpha-amanitin sensitivity of transcription in naturally occurring trypanosome protein-coding genes reveals that a unique subset of protein-coding genes-the variant surface glycoprotein (VSG) expression sites and the procyclin or the procyclic acidic repetitive protein (PARP) genes-are transcribed by an RNA polymerase that is resistant to the mushroom toxin alpha-amanitin, a characteristic of transcription by RNA pol I. Promoter analysis and a pharmacological characterization of the RNA polymerase that transcribes these genes have strengthened the proposal that the VSG expression sites and the PARP genes represent naturally occurring protein-coding genes that are transcribed by RNA pol I.

The pathology of experimental Trypanosoma evansi infection in the Indonesian buffalo (Bubalus bubalis)

Six Indonesian buffaloes (Bubalus bubalis) were inoculated intravenously with 10(5) Trypanosoma evansi, examined clinically, haematologically and serologically, and then killed 1, 2, 3, 4, 8 or 12 weeks after infection for detailed pathological study. Relapsing fever was related to the waves of parasitaemia and fluctuations of pulse and respiration rates. Anaemic mucous membranes, depression, weakness, refusal to walk, loss of appetite and emaciation were seen. Body weight, packed cell volume, total platelet and red cell counts, and haemoglobin values were below those of two uninfected control buffaloes, as well as below the normal range; on the other hand antibody titres against T. evansi in infected animals were all above those in controls. Emaciation, serous atrophy of fat, hydropericardium, petechial to larger haemorrhages in the pericardium, pneumonia, congested liver and spleen, oedematous enlargement of the superficial lymph nodes and hyperplastic bone marrow were the major gross pathological changes. Histologically, the severity of the disease increased from 1 to 7 weeks after infection and became less obvious at 12 weeks. The most consistent lesions were interstitial pneumonia, interstitial myocarditis, splenic multifocal necrosis, interstitial myositis and hyperplastic bone marrow. The last three lesions appear not to have been reported previously in T. evansi infection in buffaloes or other animals. The clinicopathological findings in this study show that T. evansi is both an intravascular and extravascular parasite.

Parasite-specific antibody responses of ruminants infected with Trypanosoma vivax

Sera from goats and cattle that were infected with two Trypanosoma vivax clones (ILDat 1.2 and ILDat 2.1) derived from different stocks were analysed for antibody activity against the variable surface glycoproteins (VSGs) of the infecting clones by enzyme-linked immune assays (ELISA) and immune lysis. To obtain purified VSG, lysed trypanosomes were separated on dodecyl sulphate-polyacrylamide gels. The gels were copper stained and the VSG protein band was excised from the gel. After destaining, the proteins were electroeluted from the gel slices and used as antigens in ELISA. High titres of IgM and IgG1 antibodies and lytic antibodies against the VSG of the infecting clone were detected. The IgG1 response appeared about 4 days later than the IgM response. IgG2 antibodies were only detected in goats and cattle that were infected with ILDat 1.2. Two goats and two calves that were infected with ILDat 1.2 showed recurrent peaks in lytic activity and of IgM and IgG1 antibody activity to the VSG of the infecting variable antigenic type (VAT). Two goats that were infected with ILDat 2.1 showed a similar pattern, but in two other goats there was a recurrent peak only in the IgM class. Recurrent peaks of antibody activity to the VSG of ILDat 1.2 and ILDat 2.1 were not detected in the sera of goats that had been inoculated with irradiated trypanosomes or that had been infected with an unrelated T. vivax clone. The recurrence of antibody peaks against the VSG of infecting VATs suggests that trypanosomes with completely or partially identical surface determinants reappear during T. vivax infection of ruminants.

Parasitic protozoa and helminths: biological and immunological challenges

Parasitic protozoans and helminths pose considerable medical as well as scientific challenges. Investigations of the complex and very different life cycles of these organisms, their adaptation to the obligate parasitic mode of life, and their ability to face the hostile host environment have resulted in many exciting discoveries. Invasion of host erythrocytes by plasmodial sporozoites and intact skin by schistosomal cercariae are outlined as examples of the elaborate mechanisms of parasitism. Isolation and characterization of single protective antigens or subunit vaccines from these two organisms are examined as models for vaccine development. Finally, developments in exploring gene regulation in protozoans and free and parasitic nematodes are briefly outlined.

Biochemical and immunochemical characterisation of a 20-kilodalton complex of surface-associated antigens from adult Onchocerca gutturosa filarial nematodes

Surface radioiodination of adult Onchocerca parasites reveals a restricted range of proteins associated with the cuticle. We present data to show that prominent among these is a complex of low-molecular-weight proteins which can be released in soluble form by homogenisation of surface-labelled Onchocerca gutturosa in phosphate-buffered saline (PBS). One of this groups of proteins, designated gp20, has a molecular mass of 20,000, is glycosylated with two N-linked carbohydrate side chains, and has a basic pI. Other PBS-soluble, 125I-labelled proteins of similar size appear not to be glycosylated. A distinct group of molecules are released only in the presence of reducing agents, and are likely to be cuticular collagens. The low-molecular-weight components are antigenic and cross-reactive with Onchocerca volvulus infection sera. Cross-reactions are also observed in immunoprecipitation experiments using sera from Brugia-immunised animals and infected humans. Comparative two-dimensional analyses of these immunoprecipitates reveal at least two Onchocerca specific components. As an alternative to radiolabelling and PBS homogenisation, incubation of worms in medium containing the reducing agent 2-mercaptoethanol resulted in a similar set of molecules being released into the medium. Since surface antigens of O. gutturosa from bovines and O. volvulus from humans appear similar in size and are antigenically cross-reactive, the more readily available parasite is being used to study further the properties of these molecules and to provide reagents for raising antisera reactive to the equivalent O. volvulus antigens.

Recent studies of the biology of Trypanosoma vivax

Recent biological investigations of the African trypanosomes have been moving away from their previous preoccupation with the phenomenon of antigenic variation. The feeling has arisen that antigenic variation, as demonstrated by the Trypanozoon and Nannomonas subgenera of trypanosomes, is too extensive, the number of serodemes too large and the coexistence of different species in many areas too complicated, to allow any immunoprophylaxis based on antibodies to variable antigens. This is, of course, not to rule out possible biochemical intervention in the biosynthesis or export of VSG molecules by trypanosomes. However, in the case of T. vivax, more information is required concerning antigenic variation and coat structure in this organism before these avenues of investigation are discarded. Ways of improving the yield of mature metacyclic trypanosomes in vitro must be found, so that the contribution of metacyclic variable antigens to the induction of immunity in T. vivax infection can be elucidated. The number of bloodstream VATs must be determined (perhaps by genetic rather than serological means), as there is evidence both for VAT exhaustion contributing to the self-cure of infected hosts, and for a possible limit to the number of VATs which can be expressed in infections in Africa. In South America nothing is known of the number of serodemes of T. vivax which exist, although such knowledge is obviously required, especially if immunity to bloodstream variants is the more important mechanism of inducing immunity to this trypanosome and true cyclical transmission is rare in, or absent from, that subcontinent. Further, in a fragile organism, with a coat of suspect integrity, the method of VSG packing and the relative exposure of underlying surface molecules seems to hold out even more hope for an immunological intervention based on cell surface but invariant molecules than is the case with T. brucei or T. congolense, although this is being attempted with the latter species. In T. brucei infections the appearance of the non-dividing stumpy population acts as a stimulus to the induction of humoral immune responses. In ruminants, antibody responses to T. vivax, at least as judged from lysis tests, lag behind the appearance of the different VATs by some days. It would be important to determine, therefore, whether, if late bloodstream forms could be induced more frequently in the ruminant, the speed of anti-VAT responses could be enhanced. Whilst self-cure appears to be relatively common in T. vivax infections, it is unlikely that it results in sterile immunity.(ABSTRACT TRUNCATED AT 400 WORDS)

Trypanosoma brucei sspp.: cleavage of variant specific and common glycoproteins during exposure of live cells to trypsin

Intact bloodstream forms of Trypanosoma brucei brucei, T.b. gambiense, and T.b. rhodesiense and procyclic forms of T.b. brucei and T.b. gambiense were incubated in trypsin, solubilized for gel electrophoresis, and analyzed for removal of surface molecules. Silver-stained gels and transfer blots probed with horseradish peroxidase-conjugated or radiolabeled lectins revealed that only three glycoproteins, Gp120p, Gp91p, and Gp23p, were removed from the surface of procyclic forms by trypsin. The variant specific glycoproteins, Gp23b, Gp120b, and in some clones Gp91b were surface molecules cleaved from bloodstream forms. Greater than 90% of the variant specific glycoprotein (VSG) was removed from the surface of all clones studied within 1 hr following the addition of trypsin. The removal of VSG was coincident with appearance of 37 to 50 kDa glycopeptide fragments of VSG with different clones yielding different sized fragments. Detailed kinetic analysis of proteins from whole cell extracts and supernatants of the DuTat 1.1 clone of T.b. rhodesiense using concanavalin A (Con A) and polyclonal antibodies revealed that three major VSG fragments were released during trypsinization. The electrophoretic mobility of the three VSG fragments of DuTat 1.1 was not altered when samples were boiled in sodium dodecyl sulfate to inhibit the endogenous phospholipase C. Antiserum to the cross-reactive determinant bound to intact VSG, but did not bind VSG fragments. Thus, the major Con A binding fragments of DuTat 1.1 VSG and perhaps those of the other clones we studied were probably derived from the N-terminal domain of the molecule. The data suggest that VSG is cleaved by trypsin in situ at the hinge region, but remains attached to the cell surface via weak interaction with neighboring molecules.

In vitro lysis of the bloodstream forms of Trypanosoma brucei gambiense by stearylamine-bearing liposomes

Cytolytic activity of liposomes consisting of stearylamine and phosphatidylcholine (SA/PC-liposomes) was examined in vitro against the bloodstream forms of Trypanosoma brucei gambiense. More than 99% of the cells (2 X 10(6)/ml) were killed within 30 min by treatment with 15 mol% SA/PC-liposomes (100 microM total lipids). As few as 1.2 X 10(12) liposomes per ml (equivalent to 2 nM liposome) showed trypanocidal activity. Fluorescence microscopy of cells treated with the dansylated SA/PC-liposomes suggested that the liposomes bound to and accumulated on the cell surface, eventually damaging the plasma membrane. SA/PC-liposomes showed no significant hemolysis when incubated with human and mouse erythrocytes under conditions that killed greater than 99.9% of the T. b. gambiense trypomastigotes. Human leukocytes were also shown to be less susceptible to SA/PC-liposomes than T. b. gambiense. These results may point to a new direction in strategy for therapy of African trypanosomiasis.

Intracellular transport of a variant surface glycoprotein in Trypanosoma brucei

Trypanosome variant surface glycoproteins (VSGs) have a novel glycan-phosphatidylinositol membrane anchor, which is cleavable by a phosphatidylinositol-specific phospholipase C. A similar structure serves to anchor some membrane proteins in mammalian cells. Using kinetic and ultrastructural approaches, we have addressed the question of whether this structure directs the protein to the cell surface by a different pathway from the classical one described in other cell types for plasma membrane and secreted glycoproteins. By immunogold labeling on thin cryosections we were able to show that, intracellularly, VSG is associated with the rough endoplasmic reticulum, all Golgi cisternae, and tubulovesicular elements and flattened cisternae, which form a network in the area adjacent to the trans side of the Golgi apparatus. Our data suggest that, although the glycan-phosphatidylinositol anchor is added in the endoplasmic reticulum, VSG is nevertheless subsequently transported along the classical intracellular route for glycoproteins, and is delivered to the flagellar pocket, where it is integrated into the surface coat. Treatment of trypanosomes with 1 microM monensin had no effect on VSG transport, although dilation of the trans-Golgi stacks and lysosomes occurred immediately. Incubation of trypanosomes at 20 degrees C, a treatment that arrests intracellular transport from the trans-Golgi region to the cell surface in mammalian cells, caused the accumulation of VSG molecules in structures of the trans-Golgi network, and retarded the incorporation of newly synthesized VSG into the surface coat.

Structural features of antigenic determinants on variant surface glycoproteins from Trypanosoma brucei

The immunochemical structure of two variant surface glycoproteins (VSGs) from Trypanosoma brucei has been studied using monoclonal and polyclonal antibodies. These two VSGs, WaTat 1.1 and WaTat 1.12 have been shown to possess cross-reactive surface-exposed antigenic determinants [Barbet et al., Nature 300, 53-57 (1982)] and similar N-terminal amino acid sequences [Olafson et al., Molec. Biochem. Parasit. 12, 287-298 (1984)]. Monoclonal and polyclonal antibodies were raised against the soluble forms of the two VSGs and against their reduced, alkylated and cyanogen bromide (CNBr) cleaved forms. None of the monoclonal antibodies which bound to the surface of living trypanosomes bound to CNBr fragments of the VSGs nor to denatured VSGs. Polyclonal antibodies raised against denatured and cleaved VSG did not bind to the surface of the living trypanosomes. These results suggest that the variable surface exposed antigenic determinants of VSG are topographically assembled structures. It was also shown that the conserved amino terminal peptides of WaTat 1.1 and WaTat 1.12 do not contain antigenic determinants.

In vivo interaction between Trypanosoma gambiense and leucocytes in mice

Naturally occurring phagocytosis of Trypanosoma gambiense by mouse eosinophils and neutrophils was reported. In vivo and in vitro experiments using monoclonal antibodies confirmed that the phagocytosis is triggered by G1 class antibodies against variable surface antigen. Ultrastructural observation revealed the mode of entry and the intracellular fate of T. gambiense: initial attachment, pseudopodia formation and complete invagination. This phagocytosis resulted in the killing of T. gambiense by mouse eosinophils and neutrophils, suggesting that eosinophils and neutrophils give at least partial protection against infection with T. gambiense in combination with the specific antibodies.

The virulence of Trypanosoma congolense can be determined by the antibody response of inbred strains of mice

Three inbred strains of mice, BALB/c, C57Bl/6 and CBA/J were infected with three clones of Trypanosoma congolense, DIND/3.1, SAM/28.1 and KAR/57.1, which were obtained from three different stocks. DIND/3.1 was of high virulence for BALB/c and CBA/J but of negligible virulence for C57Bl/6. SAM/28.1 was of high virulence and KAR/57.1 of negligible virulence for the three strains of mice. In each case, high virulence was correlated with a late, transient and low titre protective antibody response measured by complement mediated lysis of live organisms. Negligible virulence was correlated with an early, high titre protective antibody response. Suppression of the antibody response by sub-lethal irradiation or cyclophosphamide treatment of the host turned a trypanosome infection of negligible virulence into one of high virulence. In mice with mixed infections it was shown that highly virulent trypanosomes did not influence the course of infection and antibody response to trypanosomes of negligible virulence and vice-versa. The relationship of total antigen mass to the kinetics of the antibody response suggests that 1000- to 10,000-fold less antigen is required in good responder than in bad responder mice to trigger the immune response. Thus the virulence of T. congolense can be determined by the antibody response of inbred strains of mice. The specificity and dose dependency of this antibody response seem to implicate the involvement of Ir genes.

Trypanosoma brucei brucei, T. brucei gambiense, and T. brucei rhodesiense: common glycoproteins and glycoprotein oligosaccharide heterogeneity identified by lectin affinity blotting and endoglycosidase H treatment

Whole cell extracts of 10 clones of bloodstream forms of African trypanosomes representing two strains of Trypanosoma brucei gambiense, one strain of T. b. rhodesiense and one strain of T. b. brucei were fractionated on sodium dodecyl sulfate-polyacrylamide gels, electrophoretically transferred to nitrocellulose paper, and probed with horseradish peroxidase conjugated lectins to detect glycoproteins. Variant specific glycoproteins of all 10 clones bound peroxidase labeled concanavalin A, but peroxidase labeled wheat germ agglutinin bound to the variant specific glycoproteins of only 3 of the 10 clones examined. In addition, 22 other glycoproteins expressed in common by all clones bound peroxidase labeled concanavalin A; 19 common glycoproteins bound peroxidase labeled wheat germ agglutinin. Lectin binding to transferred glycoproteins was specifically inhibited by appropriate monosaccharides, alpha-methyl mannoside for concanavalin A and N-acetyl glucosamine for wheat germ agglutinin. Prior incubation of blots in endo-beta-N-acetylglucosaminidase H eliminated binding of peroxidase-labeled concanavalin A to most of the 22 common glycoproteins. Two glycoproteins, designated Gp 81 and Gp 110, were the major Endoglycosidase H resistant components. Endoglycosidase H treatment also reduced binding of peroxidase labeled concanavalin A to the variant specific glycoproteins of 7 clones. The variant specific glycoproteins from the 3 clones that bound peroxidase labeled concanavalin A following enzyme treatment were those that bound peroxidase labeled wheat germ agglutinin. These results show that African trypanosomes express a greater number of glycoproteins than has been reported previously and that only a limited number of these glycoproteins bear Endoglycosidase H resistant oligosaccharides.

An assay of membrane-bound Trypanosoma brucei phospholipase using an integral membrane protein substrate and detergent phase separation

The technique of phase separation in a solution of the non-ionic detergent Triton X-114 was used to measure the enzymatic conversion of a membrane protein to a soluble product via removal of a hydrophobic moiety. The substrate was the major surface protein (p63), of Leishmania promastigotes and the enzyme was a phospholipase C purified from Trypanosoma brucei. This membrane-bound enzyme is responsible for the cleavage of the hydrophobic lipid membrane anchor of the variant surface glycoprotein (VSG), of T. brucei. The assay is fast, simple and uses small amounts of reagents. It has been used to determine the pH optimum, thermal resistance, and the sensitivity to inhibitors of the trypanosomal phospholipase.

Trypanosoma (Duttonefla) vivax

Although Trypanosoma vivax was first discovered in 1905 (Ref. 1), the fact that most stocks of this parasite are restricted to ruminant hosts has retarded investigation of this species compared with the experimentally more amenable T. brucei and T. congolense. The veterinary importance of T. vivax (Box 1) and a recent report suggesting that T. vivax may have an even more extended range than previously thought (Box 2) prompts an evaluation of the current knowledge of the biology of this trypanosome.

Physical and immunological analysis of the two domains isolated from a variant surface glycoprotein of Trypanosoma brucei

A specific surface glycoprotein of a variant of Trypanosoma brucei was cleaved with trypsin and the two major domains of the molecule have been purified. We have studied the chemical composition of each domain and compared the data to published results of the specific cDNA sequence. Circular dichroism measurements show that the amino-terminal domain includes preferentially alpha-helical or beta-sheet structure. The physicochemical analyses are supplemented by a prediction of secondary structure and a statistical pattern of hydrophilicity-hydrophobicity. The results are discussed in light of the internal limits that were described in the process of partial gene conversion occurring between the variant gene sequence and related members of the same gene family. Immunoblots with homologous antiserum indicate that the amino-terminal domain is implicated in antigenicity. In addition, immunoblotting with heterologous antiserum on native antigen, tryptic hydrolysates, or purified domains suggests a site of interaction supported by the two domains.

Posttranslational modification and intracellular transport of a trypanosome variant surface glycoprotein

After synthesis on membrane-bound ribosomes, the variant surface glycoprotein (VSG) of Trypanosoma brucei is modified by: (a) removal of an N-terminal signal sequence, (b) addition of N-linked oligosaccharides, and (c) replacement of a C-terminal hydrophobic peptide with a complex glycolipid that serves as a membrane anchor. Based on pulse-chase experiments with the variant ILTat-1.3, we now report the kinetics of three subsequent processing reactions. These are: (a) conversion of newly synthesized 56/58-kD polypeptides to mature 59-kD VSG, (b) transport to the cell surface, and (c) transport to a site where VSG is susceptible to endogenous membrane-bound phospholipase C. We found that the t 1/2 of all three of these processes is approximately 15 min. The comparable kinetics of these processes is compatible with the hypotheses that transport of VSG from the site of maturation to the cell surface is rapid and that VSG may not reach a phospholipase C-containing membrane until it arrives on the cell surface. Neither tunicamycin nor monensin blocks transport of VSG, but monensin completely inhibits conversion of 58-kD VSG to the mature 59-kD form. In the presence of tunicamycin, VSG is synthesized as a 54-kD polypeptide that is subsequently processed to a form with a slightly higher Mr. This tunicamycin-resistant processing suggests that modifications unrelated to N-linked oligosaccharides occur. Surprisingly, the rate of VSG transport is reduced, but not abolished, by dropping the chase temperature to as low as 10 degrees C.

Purification and characterisation of membrane-form variant surface glycoproteins of Trypanosoma brucei

Membrane-form variant surface glycoprotein of Trypanosoma brucei can be prepared in the presence of para-chloromercuriphenylsulphonic acid. The membrane-bound enzyme that usually cleaves a lipid from this glycoprotein, thus producing the soluble variant surface glycoprotein, is inhibited by a range of sulphydryl reagents. The effect of such inhibitors, both on cell lysates and on semi-purified enzyme, reveals that the enzyme may have a sulphydryl at or near its active site. Fatty acid analysis and isoelectric point measurements of membrane form and soluble form are presented.

Solution properties of the variant surface glycoprotein of Trypanosoma brucei

The solution properties of the membrane form and soluble form of variant surface glycoproteins from Trypanosoma brucei have been compared. Solution cross-linking studies established that both forms are dimers, although dissociation of membrane-form variant surface glycoprotein can be promoted by certain ionic and zwitterionic detergents. Sedimentation coefficients were measured under a range of conditions, and the results were comparable with the results of solution cross-linking. Stokes radii were measured by gel filtration, allowing a value for the frictional coefficient to be calculated. The two forms show no differences other than those consistent with binding of detergent micelles to the hydrophobic moiety present on membrane form surface glycoprotein. This validates the use of soluble variant surface glycoprotein in X-ray crystallography experiments.

Inhibition of Trypanosoma brucei brucei peptidyl transferase activity by sparsomycin analogs and effects on trypanosome protein synthesis and proliferation

Peptidyl transferase activity of Trypanosoma brucei brucei polyribosomes was competitively inhibited by analogs of sparsomycin (Ki = 1-100 microM). The analogs were also potent inhibitors of [3H]-leucine and [3H]mannose incorporation into the proteins of intact trypanosomes with little or no effect on overall respiratory rate, suggesting a specific site of action for these analogs on protein synthesis. The peptidyl transferase inhibitors were effective at low concentrations at limiting the proliferation of trypanosomes both in vitro and in vivo. The potency of the compounds as inhibitors of cell proliferation was positively correlated with their efficacy as inhibitors of peptidyl transferase activity. One compound, MDL 20828 (1 mg/kg), increased the survival time of T. b. brucei-infected mice 4-fold in the absence of any overt drug toxicity to the hosts.

Trypanosoma brucei: analysis of relapsing populations in sensitive and resistant breeds of cattle

The clone DiTat 1.1 of Trypanosoma brucei brucei was injected into four bovids, and clones obtained from successive waves of parasitemia were used to study the expressed variant-specific surface glycoprotein repertoire. Twenty-four clones were obtained which could be classified into 12 different variable antigen types, in addition to the clone injected, using agglutination or immunofluorescence with monospecific antisera. The variable surface glycoproteins of the 25 clones were extracted using the detergent octyl-beta-D-glucopyranoside in the presence of the protease inhibitor, N-cbz-L-phenylalaninechloromethylketone. The molecular weights varied from 52,000 to 69,000 and the pI from 5.0 to 8.8. The virulence of 14 clones representing 13 variable antigen types was ascertained in mice. The mean survival time ranged from 20.5 to 43.0 days. Clones isolated from early peaks of parasitemia in the bovid were the most virulent while clones derived from later peaks were less virulent. It seems that organisms of diminishing virulence appear in bovids, leading to self-cure of the disease. All clones were sensitive to human serum in a blood infectivity inhibition test. Antibody against all virulent clones appeared in 20 cattle (10 Zebus, 10 Baoulés) which had been injected with T. brucei DiTat 1.1. There was no evidence for parasites of high or low virulence being preferentially expressed in resistant or sensitive hosts.

Expression of two variant surface glycoproteins on individual African trypanosomes during antigen switching

Individual Trypanosoma brucei rhodesiense organisms were observed in the process of switching variant surface glycoproteins (VSG's). During this switch, trypanosomes simultaneously expressed both pre- and postswitch VSG's uniformly over their surface as detected with monoclonal antibodies. Analysis of this switching event showed that trypanosomes expressing any one of three distinct preswitch VSG's could switch to expression of from one to three different postswitch VSG's. Up to 2.7 percent of the trypanosome population was in the process of switching at one time.

Transport of ethanolamine and its incorporation into the variant surface glycoprotein of bloodstream forms of Trypanosoma brucei

The membrane-attached form of the variant surface glycoprotein (mf-VSG) of bloodstream forms of Trypanosoma brucei is anchored to the plasma membrane by a hydrophobic C-terminal lipo-oligosaccharide containing ethanolamine. Analysis by polyacrylamide gel electrophoresis showed that several different cloned T. brucei strains (strain EATRO 110 and variants 117 and 118 of strain 427) incorporated [3H]ethanolamine into both mf-VSG and the soluble VSG derived from it, but not into other proteins. Other trypanosomatids, e.g. Leishmania mexicana promastigotes, T. cruzi epimastigotes, and T. brucei procyclic forms, did not incorporate ethanolamine into cellular proteins. Thus, [3H]ethanolamine can be used as a specific biosynthetic label for T. brucei VSG polypeptides. The time course of incorporation of [3H]ethanolamine into VSG showed a lag period of about 15 min. Double-labelling experiments using [3H]ethanolamine and H3[32P]O4 demonstrated that ethanolamine labelled only the C-terminal moiety and was not incorporated into other portions of the VSG molecule. Cellular uptake of ethanolamine occurred via a specific carrier-mediated transport system having a Vmax of 132 pmol min-1 mg-1 protein and a Km of 3.7 microM. The properties of this transport system are consistent with the possibility that ethanolamine is derived entirely from the host.

Molecular biology of trypanosome antigenic variation

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Isolation and cell-free synthesis of variant surface glycoproteins from Trypanosoma congolense

Two Trypanosoma congolense stocks, 1/148 FLY and TREU 921, were cloned in A/J strain mice immunosuppressed with cyclophosphamide. The cloned populations, AmNat 1.1 and AmNat 3.1, each characterized by a different variant antigen type, were checked for homogeneity by the indirect fluorescent antibody test using 6-day antisera developed in rabbits. The variant surface glycoproteins (VSGs) from both AmNat clones were purified to homogeneity. Electrophoresis on sodium dodecyl sulfate (SDS)-polyacrylamide gradient gels revealed that the apparent Mr values of the two VSGs were 51 700 (AmNat 1.1) and 49 900 (AmNat 3.1). Monospecific antisera prepared in rabbits to each VSG were used to confirm the homogeneity of the clones by the indirect fluorescent antibody test. The VSGs were susceptible to endoglycosidase H digestion, indicating the presence of high-mannose type oligosaccharides in these glycoproteins. The apparent Mr values of the endoglycosidase H-digested VSGs were 48 800 and 46 900 for AmNat 1.1 and 3.1, respectively. Poly(A+)-enriched RNA isolated from each clone was assayed for template activity using a mRNA-dependent rabbit reticulocyte lysate for in vitro protein synthesis. Radioactively labeled polypeptides were initially characterized by SDS-polyacrylamide gradient gel electrophoresis and visualized by fluorography. VSG-specific translation products were immunoprecipitated with IgGs isolated from the homologous monospecific antisera and analyzed on SDS-polyacrylamide gradient gels. The apparent Mr values for the AmNat 1.1 and 3.1 precursor VSGs synthesized in vitro were 39 000 and 43 000, respectively.

Rapid processing of the carboxyl terminus of a trypanosome variant surface glycoprotein

The variant surface glycoprotein of the parasite Trypanosoma brucei contains a glycolipid of unknown structure covalently attached to its COOH terminus. We have shown, by using metabolic labeling with [35S]methionine or [3H]myristic acid, precipitation with specific antibodies, and NaDodSO4/polyacrylamide gel electrophoresis, that this glycolipid is attached to the variant surface glycoprotein polypeptide within 1 min after its translation.

Purification and regulatory properties of phosphofructokinase from Trypanosoma (Trypanozoon) brucei brucei

Phosphofructokinase (EC 2.7.1.11) from Trypanosoma (Trypanozoon) brucei brucei was purified to homogeneity by using a three-step procedure that may be performed within 1 day. Proteolysis, which removes a fragment of Mr approx. 2000, may occur during the purification, but this can be prevented by including antipain, an inhibitor of cysteine proteinases, in the buffers during the purification. The subunits of the enzyme appear to be identical in size, with an Mr of 49 000. The Mr of the native enzyme was estimated to be approx. 220 000, suggesting a tetrameric structure. Kinetic studies showed the activity to depend hyperbolically on the concentration of ATP but sigmoidally on the concentration of fructose 6-phosphate. Although cyclic AMP, AMP and ADP stimulated the enzyme activity at low concentrations of fructose 6-phosphate, the last two nucleotides were inhibitory at high concentrations of this substrate. Phosphoenolpyruvate behaved as an allosteric inhibitor of the phosphofructokinase. Citrate, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate and Pi did not influence significantly the activity of the enzyme.

Conservation of structure detected in two trypanosome surface glycoproteins by amino acid sequence alignment

The predominant molecule exposed to antibody on the surface of Trypanosoma brucei is a glycoprotein of about 60 000 molecular weight which varies in amino acid sequence. The complete sequences of two such variable surface glycoproteins (VSGs) from randomly isolated, different antigenic types of trypanosomes were compared by amino acid sequence alignment. Homologous sequences were found distributed over various regions of the VSGs. Particularly good homology was observed between residues 16-34, 91-115, 177-194 and 254-345 from the N-terminus, in addition to the known conserved region close to the C-terminus. Homology was also demonstrated in the corresponding regions of the cDNA sequences by matrix analysis.

Selective telomere activation and the control of antigen gene expression in trypanosomes

African trypanosomes escape the immune defence of their mammalian host by changing their antigenic surface coat. Antigenic variation occurs through differential gene activation: only one antigen gene is transcribed at a time, among a large collection of specific sequences. This transcription always takes place in a telomere, but it seems that different telomeres can be used alternatively as the gene expression site. Since the trypanosome genome is made up of numerous chromosomes, it would appear that a highly selective process allows the activation of only one telomere at a time. This process seems linked to the differential inactivation of a peculiar telomeric DNA modification system. Two mechanisms allow antigen genes to be expressed. First, a gene copy can be inserted in the expression site by replacing the formerly expressed gene. This is due to gene conversion, whose extent can vary considerably, according to the degree of homology between the recombining partners. The second mechanism involves the activation of another telomere along with deactivation of the telomere containing the previously expressed gene. This form of activation can occur without apparent DNA rearrangement. The alternate use of these mechanisms leads to rapid changes in the antigen gene repertoire, due to gain and loss of different sequences, and to alteration of their activation rate.

Structure and variation within variant surface glycoproteins of Trypanosoma brucei

Variant surface glycoproteins of the African trypanosomes are members of a multigene family which show extraordinary amino sequence diversity. The extent of this diversity and the significance of homologies both in the amino acid sequence and in the post-translational modifications are discussed in the light of what is predicted for the structure of these molecules and what is now known from X-ray crystallographic analysis.

Antigenic variation in its biological context

The biology of antigenic variation is discussed, and the problems that must be solved to provide a full understanding of antigenic variation are considered. These are (i) the induction of v.s.g. synthesis in the salivary glands of the tsetse fly; (ii) the nature of the restriction on v.s.g. genes that allows only some of them to be expressed in the salivary glands; (iii) the nature of 'predominance' in v.s.g. expression in the mammalian host, and the mechanism by which it operates; (iv) the repression of v.s.g. synthesis in the insect midgut; (v) the anamnestic response that produces expression of the ingested variant in the first patent parasitaemia in the mammalian host; (vi) the mechanism by which only one v.s.g. gene at a time is expressed; (vii) the relationship if any of v.s.g. structure to v.s.g.-associated differences in growth rate and host range; (viii) the role of v.s.g. release within the life cycle and to pathogenesis.

Structure of the variant glycoproteins and surface coat of Trypanosoma brucei

The pathogenic African trypanosomes have a unique mechanism for antigenic variation. Each cell is covered by a surface coat consisting of about seven million essentially identical glycoprotein molecules drawn from a large repertoire of variants, each encoded by an individual gene. Amino acid sequence variation extends throughout the molecule but reduces from the amino terminus to the carboxy terminus, where certain features, especially the grouping of cysteine residues, are quite conserved. The range of diversity within the thousand or so variant glycoprotein genes that exist in each cell is large. New variants may arise instantaneously by segmental gene conversion. Variant surface glycoproteins are synthesized with amino terminal signal sequences and hydrophobic carboxy terminal tails. The tails are extraordinarily conserved. After synthesis, they are replaced by a complex glycolipid structure in which myristic (dodecanoic) acid serves to anchor the polypeptide to the surface membrane. Enzymic cleavage of myristic acid releases variant glycoproteins from the surface coat.

Purification of major variable-surface glycoproteins from African trypanosomes by reverse-phase high-performance liquid chromatography

Reverse-phase high-performance liquid chromatography (RP-HPLC) was used in a one-step procedure to purify and analyze several different major variable-surface glycoproteins (VSGs) from lysates of African trypanosomes. RP-HPLC was used to fractionate lysates of trypanosomes and the VSG localized to the major peak of the elution profile using a rabbit antiserum to the cross-reacting determinant of the VSG. Polyacrylamide gel electrophoresis of HPLC fractions showed that the purity of isolated VSGs was equivalent to or better than that attained using conventional purification procedures. The elution positions of purified VSGs from a variety of cloned trypanosomes were identical, indicating the presence of a common hydrophobic feature on the surface of these highly polymorphic antigens. Preliminary experiments have shown that purification of VSG from trypanosome lysates may be scaled up to preparative levels. The results show that RP-HPLC is a useful procedure for rapid preparation of highly purified trypanosome VSGs and that analysis of their various molecular forms will be facilitated by the application of HPLC methods.

Identification and characterization of a major surface antigen of Giardia lamblia

The surface antigens of Giardia lamblia trophozoites were characterized by crossed immunoelectrophoresis, radioiodination, and immunoprecipitation. Crossed immunoelectrophoretic analysis of trophozoites with hyperimmune rabbit anti-trophozoite antiserum revealed a prominent precipitin peak that disappeared upon adsorption of the antiserum with live or formaldehyde-fixed trophozoites. This peak was intensely labeled when the antigen was derived from surface-radioiodinated trophozoites. An antiserum monospecific for the antigen contained in this precipitin peak was prepared. The precipitin peak was shown to contain an antigen with an apparent molecular weight of 82,000 by Western blotting. The antiserum also detected this 82,000-molecular-weight antigen on nitrocellulose blots of trophozoites analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On radioiodination of live trophozoites, an iodinated molecule of 82,000 apparent molecular weight was resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was immunoprecipitated by the monospecific antiserum. Preliminary characterization of this antigen with the monospecific antiserum in crossed immunoelectrophoresis revealed that the surface antigen is hydrophobic and thus may be anchored in the plasma membrane, and that it is heat sensitive, but only partially sensitive to pronase or periodate. This antigen was shared by the four G. lamblia strains examined.

6 A-resolution X-ray structure of a variable surface glycoprotein from Trypanosoma brucei

The variable surface glycoprotein (VSG) is the predominant component of the surface coat of the African trypanosome. The expression of antigenically distinct VSGs on minor populations during infection allows the parasite to escape the host immune response. Purification of the protein is facilitated by the enzymatic release of a soluble form of VSG (sVSG) which occurs on cell lysis. The soluble form is a dimer with an approximate molecular weight of 120,000-130,000. Partial proteolysis of sVSG reveals a protease-sensitive link between an amino-terminal domain which comprises about two-thirds of the molecule, and a C-terminal domain which contains the membrane attachment site. We have obtained crystals suitable for high-resolution structural analysis from preparations of three sVSG: MITat 1.2, ILTat 1.25 and ILTat 1.22. The crystal structure of the dimer of the MITat 1.2 amino-terminal domain has been solved to 6 A resolution. We report here that the dimer is an unusual 90 A rod-like molecule composed of a helical bundle of at least four 80 A-long alpha-helices.

Antigenic variation in parasites

Antigenic variation is a powerful survival strategy adapted by certain species of parasitic protozoa to allow them to survive in the immunized host. It is exemplified by the African trypanosomes, which provide far and away the best characterized and most studied system of this kind. Why have the trypanosomes developed antigenic variation to such a sophisticated level? Because the trypanosome lives its life in the bloodstream of its mammalian host and is therefore in continuous conflict with the host's immune system. Antigenic variation represents its whole survival strategy, with some help provided by its ability to immunosuppress the host. The importance of antigenic variation to the trypanosome is underscored by the estimate that up to 10% of the trypanosome genome may be devoted to variant antigen genes (Van der Ploeget al.1982). Most other parasitic protozoa prefer a less confrontational existence and usually adopt an intracellular home for at least a part of their life-cycle within the mammalian host. That being the case, do other parasitic protozoa need antigenic variation within their armorarium ? The answer seems to be yes, although the reasons why are by no means clear. For example, the stages in the life-cycle which exhibit antigenic variation might be expected to be those which are released free into the bloodstream – in malaria, sporozoites and merozoites, for example. Yet there seems to be no evidence for phenotypic variation at all in these stages. Rather, it is the intracellular stages which, in bothPlasmodiumandBabesia, seem to elaborate molecules which are expressed at the surface of the parasitized cell, and which are capable of both eliciting an immune response and of avoiding the con- sequences of such a response by phenotypic antigenic variation. Why are such antigens expressed, and what is their functional significance?

Amino terminal sequence homology among variant surface glycoproteins of African trypanosomes

Amino terminal amino acid sequences were determined for eight variable surface glycoproteins purified from successive parasitemias of cloned Trypanosoma brucei. These sequences were compared by sequence alignment analysis with each other and with amino terminal sequences of variable surface glycoproteins from other clones of T. brucei, T. congolense and T. equiperdum. Contrary to previously held views, a remarkable degree of sequence homology was found among all sequences. In several cases the homology was greater between glycoproteins from different species than between glycoproteins isolated from successive parasite peaks, suggesting a common primordial gene for trypanosome variable surface glycoproteins. This amino terminal homology, taken together with homologies found in other regions of variant surface glycoproteins strongly suggests that their tertiary structures are conserved.

Complete nucleotide sequence of an unusual mobile element from trypanosoma brucei

The complete nucleotide sequence of a mobile element from Trypanosoma brucei is presented along with the sequence of its target site, which shows that the insertion has generated a 7 base pair direct repeat. The cloned copy of the element is a dimeric structure, one end of each monomer consisting of a stretch of 14 A residues preceded by a putative trypanosome polyadenylation signal. Six base pairs of DNA of unknown origin are found in the dimer between the two copies of the element. Evidence suggests that the element is present in the genome mainly as a monomer whose sequence is conserved across several species of trypanosome. The element contains an open reading frame encoding the same 160 amino acid protein in both sequenced copies and is extensively transcribed from both strands.

Purification and properties of the membrane form of variant surface glycoproteins (VSGs) from Trypanosoma brucei

Variant surface glycoproteins (VSG) of Trypanosoma brucei are released in a water soluble form on impairment of membrane integrity. We have previously shown that this release is the result of an enzyme-mediated event which converts the hydrophobic membrane form VSG into the hydrophilic water-soluble form. We now present further details of the methods by which membrane form VSG ( mfVSG ) may be isolated, uncontaminated by water-soluble VSG ( sVSG ). The sensitivity to different metal ions of the enzyme that mediated the conversion event is discussed, and some biochemical characteristics of different mfVSG preparations are presented.

Release and purification of Trypanosoma brucei variant surface glycoprotein

Conditions affecting the solubilization of variant surface glycoprotein (VSG) from Trypanosoma brucei have been investigated. The results obtained form the basis for a convenient and efficient method for VSG purification. VSG release from the cell surface was temperature-dependent, following osmotic lysis at 0 degree C, and was inhibited by low concentrations of Zn2+ but not by tosyl-lysine chloromethyl-ketone (TLCK), phenylmethylsulfonylfluoride (PMSF), or iodoacetamide. These and other results eliminated the possibility that release was due to proteolytic cleavage of the C-terminal hydrophobic tail present on newly synthesized VSG. Bolton and Hunter reagent reacted with several components on living cells.

Are there two classes of VSG gene in Trypanosoma brucei?

Antigenic variation in the African trypanosomes involves the sequential expression of genes coding for different variant surface glycoproteins (VSGs) (reviewed in refs 1-3). When expression of some VSG genes is switched on, a newly duplicated copy of the expressed gene has been observed within the trypanosome genome, which is not found after the gene's expression is switched off again. The duplicated copy has therefore been called an expression-linked copy (ELC). The expression of the gene appears to be strictly coupled to the presence of the ELC. This has led to the hypothesis that the duplicative transposition generating the ELC may itself be responsible for the control of VSG expression. With other VSG genes, expression-linked duplication has not been observed, and expression is clearly not controlled in this way. Data are presented here which demonstrate that either of these observations may be obtained with a single VSG gene, depending on the chance selection of particular clones from antigenically switched populations. Thus, the different observations do not imply the existence of two distinct classes of VSG gene controlled by different mechanisms, but different aspects of processes common to all VSG genes.