Antigenic changes in Trypanosoma brucei on transmission by tsetse fly

Cell-line specific RNA editing patterns in Trypanosoma brucei suggest a unique mechanism to generate protein variation in a system intolerant to genetic mutations

Trypanosoma brucei possesses a highly complex RNA editing system that uses guide RNAs to direct the insertion and deletion of uridines in mitochondrial mRNAs. These changes extensively alter the target mRNAs and can more than double them in length. Recently, analyses showed that several of the edited genes possess the capacity to encode two different protein products. The overlapped reading frames can be accessed through alternative RNA editing that shifts the translated reading frame. In this study, we analyzed the editing patterns of three putative dual-coding genes, ribosomal protein S12 (RPS12), the 5′ editing domain of NADH dehydrogenase subunit 7 (ND7 5′), and C-rich region 3 (CR3). We found evidence that alternatively 5′-edited ND7 5′ and CR3 transcripts are present in the transcriptome, providing evidence for the use of dual ORFs in these transcripts. Moreover, we found that CR3 has a complex set of editing pathways that vary substantially between cell lines. These findings suggest that alternative editing can work to introduce genetic variation in a system that selects against nucleotide mutations.

Mitochondrial dual-coding genes in Trypanosoma brucei

Trypanosoma brucei is transmitted between mammalian hosts by the tsetse fly. In the mammal, they are exclusively extracellular, continuously replicating within the bloodstream. During this stage, the mitochondrion lacks a functional electron transport chain (ETC). Successful transition to the fly, requires activation of the ETC and ATP synthesis via oxidative phosphorylation. This life cycle leads to a major problem: in the bloodstream, the mitochondrial genes are not under selection and are subject to genetic drift that endangers their integrity. Exacerbating this, T. brucei undergoes repeated population bottlenecks as they evade the host immune system that would create additional forces of genetic drift. These parasites possess several unique genetic features, including RNA editing of mitochondrial transcripts. RNA editing creates open reading frames by the guided insertion and deletion of U-residues within the mRNA. A major question in the field has been why this metabolically expensive system of RNA editing would evolve and persist. Here, we show that many of the edited mRNAs can alter the choice of start codon and the open reading frame by alternative editing of the 5’ end. Analyses of mutational bias indicate that six of the mitochondrial genes may be dual-coding and that RNA editing allows access to both reading frames. We hypothesize that dual-coding genes can protect genetic information by essentially hiding a non-selected gene within one that remains under selection. Thus, the complex RNA editing system found in the mitochondria of trypanosomes provides a unique molecular strategy to combat genetic drift in non-selective conditions.

In African trypanosomes, many of the mitochondrial mRNAs require extensive RNA editing before they can be translated. During this process, each edited transcript can undergo hundreds of cleavage/ligation events as U-residues are inserted or deleted to generate a translatable open reading frame. A major paradox has been why this incredibly metabolically expensive process would evolve and persist. In this work, we show that many of the mitochondrial genes in trypanosomes are dual-coding, utilizing different reading frames to potentially produce two very different proteins. Access to both reading frames is made possible by alternative editing of the 5’ end of the transcript. We hypothesize that dual-coding genes may work to protect the mitochondrial genes from mutations during growth in the mammalian host, when many of the mitochondrial genes are not being used. Thus, the complex RNA editing system may be maintained because it provides a unique molecular strategy to combat genetic drift.

Antigenic variation in cyclically transmitted Trypanosoma brucei. Variable antigen type composition of the first parasitaemia in mice bitten by trypanosome-infected Glossina morsitans

Tsetse flies were infected with 5 different variable antigen types (VATs) or with a mixture of VATs of the AnTAR 1 serodeme of Trypanosoma brucei. Metacyclic forms from the salivary glands of infected flies were used to initiate infections in mice. Immunofluorescence and trypanolysis reactions employing 24 monospecific antisera were used to analyse the VATs present in the mice following cyclical transmission. Regardless of the VAT used to infect tsetse flies, the first VATs detectable in the bloodstream were those previously identified as metacyclic VATs (M-VATs). These were present until at least 5 days after infection, at which time lytic antibodies against at least 2 of the M-VATs were detectable in the blood of infected mice. In mice immunosuppressed by X-irradiation the M-VATs were detectable in the bloodstream for longer periods, but the percentage of the population labelled with anti-metacyclic sera showed a decrease on day 5 as in non-irradiated animals. The VAT ingested by the tsetse was always detectable early during the first parasitaemia following cyclical transmission and was usually the first VAT detected after the M-VATs. Neutralization of selected M-VATs before infecting mice resulted in elimination of the neutralized M-VAT from the first parasitaemia but had no effect on the expression of other VATs in the early infection.

Interactions between Trypanosoma brucei and Babesia spp. and Plasmodium spp. in mice

Swiss mice with chronic Trypanosoma brucei infections become refractory to subsequent infection with Babesia microti and B. rodhaini. Infection with B. microti 7 days after T. brucei resulted in an obvious inhibition of the babesia parasitaemias and this inhibition became more profound as the time interval between the infections increased, until at 17-20 days the parasitaemias were totally abolished. Even after intravenous injection of large numbers of parasites parasitaemias were inhibited. Similar inhibition was obtained in BALB/c mice but not in C57BL/6 mice. Mice with established T. brucei infections also showed reduced susceptibility to B. rodhaini. In mice similarly infected with T. brucei and the malaria parasites Plasmodium chabaudi chabaudi and P. c. adami the pre-patent periods were noticeably prolonged but the subsequent parasitaemias were unaffected. Infections with P. yoelii were unaffected. Trypanosoma brucei infections were not affected by the intracellular parasites. Among the mechanisms investigated to explain these findings were changes in red blood cell populations, cross-reacting antigens, the release of toxic factors and the generation of activated oxygen species. None of these could account for the inhibition observed.

The polypeptide profiles of strains of the Trypanosoma subgenera Schizotrypanum and Trypanozoon: peptideme characterization

Examination, by SDS-PAGE, of the polypeptide profiles of 36 Brazilian strains of Trypanosoma (Schizotrypanum) cruzi has revealed five peptidemes (subpopulations having similar polypeptide profiles) recognizing differences not apparent from zymodeme analysis. T. cruzi zymodeme (Z)1 has been subdivided into peptidemes 3, 4 and 5 (one example of Z2 was included in peptideme 5). Peptideme 1 consisted mostly of Z2 strains except for one example each of Z1 and Z3. One strain of Z2 (Tc 003) was sufficiently dissimilar to merit separation into peptideme 2. Peptideme characterization was not possible with the Trypanozoon trypanosomes since there were no significant differences in the polypeptide profiles of 56 strains from Africa and South America; their profiles could only be compared in the molecular weight ranges 94,000 to 70,000 and below 36,000 because of distortion resulting from the variant specific surface glycoproteins. It was possible to differentiate between eight VATs of T. (Trypanozoon) brucei (BruTat 2 to 9) by isoelectric focusing of their soluble proteins.

Antigenic variation in cyclically transmittedTrypanosoma brucei. Variable antigen type composition of metacyclic trypanosome populations from the salivary glands ofGlossina morsitans

Tsetse flies (Glossina morsitans) were fed on the blood of mice containing any one of 5 variable antigen types (VATs) ofTrypanosoma bruceiAnTAR 1 serodeme. The VATs of the metacyclic trypanosomes subsequently detected in the flies' saliva probes were investigated using monospecific antisera to AnTAR 1 VATs in indirect immunofluorescence and trypanolysis reactions; these sera included 3 raised against AnTats 1.6, 1.30 and 1.45, previously identified as components of the metacyclic population (M-VATs), and against the 5 VATs originally ingested by the flies. The percentage of metacyclics reacting with a particular M-VAT antiserum remained more or less constant (AnTat 1.6, 6·0–8·3%; AnTat 1.30, 13·7–18·2%; AnTat 1.45, 2·0–8·0%), regardless of the age of the fly or the ingested VAT. As these 3 VATs accounted for no more than 30% of the metacyclic population, the existence of at least one more VAT is envisaged. The ingested VAT could not be detected among the AnTAR 1 metacyclic trypanosomes.

Antigenic analysis by immunofluorescence of in vitro-produced metacyclics of Trypanosoma brucei and their infections in mice

The antigenic types in populations of Metacyclic trypanosomes of Trypanosoma brucei isolated from Glossina morsitans head-salivary gland trypanosome cultures and bloodstream forms in the early parasitemias produced from whole culture supernatant fluids containing metacyclic forms, were analyzed by the indirect fluorescent antibody test using clone-specific antisera. Metacyclic trypanosomes in cultures initiated with cloned bloodstream forms with heterogeneous with respect to their variable antigenic type (VAT). Trypanosomes comprising early parasitemias in immunosuppressed mice infected with metacyclics produced in cultures also had a range of VATs. Three of the VATs detected in the early parasitemias in mice have also been identified by other investigators in tsetse fly-transmitted populations of the same stock.