VSG switching in Trypanosoma brucei: antigenic variation analysed using RNAi in the absence of immune selection.

Trypanosoma brucei relies on antigenic variation of its variant surface glycoprotein (VSG) coat for survival. We show that VSG switching can be efficiently studied in vitro using VSG RNAi in place of an immune system to select for switch variants. Contrary to models predicting an instant switch after inhibition of VSG synthesis, switching was not induced by VSG RNAi and occurred at a rate of 10(-4) per division. We find a highly reproducible hierarchy of VSG activation, which appears to be capable of resetting, whereby more than half of the switch events over 12 experiments were to one of two VSGs. We characterized switched clones according to switch mechanism using marker genes in the active VSG expression site (ES). Transcriptional switches between ESs were the preferred switching mechanism, whereby at least 10 of the 17 ESs identified in T. brucei 427 can be functionally active in vitro. We could specifically select for switches mediated by DNA rearrangements by inducing VSG RNAi in the presence of drug selection for the active ES. Most of the preferentially activated VSGs could be activated by multiple mechanisms. This VSG RNAi-based procedure provides a rapid and powerful means for analysing VSG switching in African trypanosomes entirely in vitro.

Variant surface glycoprotein RNA interference triggers a precytokinesis cell cycle arrest in African trypanosomes.

Trypanosoma brucei is a protozoan parasite that causes African sleeping sickness. T. brucei multiplies extracellularly in the bloodstream, relying on antigenic variation of a dense variant surface glycoprotein (VSG) coat to escape antibody-mediated lysis. We investigated the role of VSG in proliferation and pathogenicity by using inducible RNA interference to ablate VSG transcript down to 1-2% normal levels. Inhibiting VSG synthesis in vitro triggers a rapid and specific cell cycle checkpoint blocking cell division. Parasites arrest at a discrete precytokinesis stage with two full-length flagella and opposing flagellar pockets, without undergoing additional rounds of S phase and mitosis. A subset (<10%) of the stalled cells have internal flagella, indicating that the progenitors of these cells were already committed to cytokinesis when VSG restriction was sensed. Although there was no obvious VSG depletion in vitro after 24-h induction of VSG RNA interference, there was rapid clearance of these cells in vivo. We propose that a stringent block in VSG synthesis produces stalled trypanosomes with a minimally compromised VSG coat, which can be targeted by the immune system. Our data indicate that VSG protein or transcript is monitored during cell cycle progression in bloodstream-form T. brucei and describes precise precytokinesis cell cycle arrest. This checkpoint before cell division provides a link between the protective VSG coat and cell cycle progression and could function as a novel parasite safety mechanism, preventing extensive dilution of the protective VSG coat in the absence of VSG synthesis.

Bloodstream form-specific up-regulation of silent vsg expression sites and procyclin in Trypanosoma brucei after inhibition of DNA synthesis or DNA damage.

The African trypanosome Trypanosoma brucei transcribes the active variant surface glycoprotein (VSG) gene from one of about 20 VSG expression sites (ESs). In order to study ES control, we made reporter lines with a green fluorescent protein gene inserted behind the promoter of different ESs. We attempted to disrupt the silencing machinery, and we used fluorescence-activated cell sorter analysis for the rapid and sensitive detection of ES up-regulation. We find that a range of treatments that either block nuclear DNA synthesis, like aphidicolin, or modify DNA-like cisplatin and 1-methyl-3-nitro-1-nitrosoguanidine results in up-regulation of silent ESs. Aphidicolin treatment was the most effective, with almost 80% of the cells expressing green fluorescent protein from a silent ES. All of these treatments blocked the cells in S phase. In contrast, a range of toxic chemicals had little or no effect on expression. These included berenil and pentamidine, which selectively cleave the mitochondrial kinetoplast DNA, the metabolic inhibitors suramin and difluoromethylornithine, and the mitotic inhibitor rhizoxin. Up-regulation also affected other RNA polymerase I (pol I) transcription units, as procyclin genes were also up-regulated after cells were treated with either aphidicolin or DNA-modifying agents. Strikingly, this up-regulation of silent pol I transcription units was bloodstream form-specific and was not observed in insect form T. brucei. We postulate that the redistribution of a limiting bloodstream form-specific factor involved in both silencing and DNA repair results in the derepression of normally silenced pol I transcription units after DNA damage.

Delineation of the regulated Variant Surface Glycoprotein gene expression site domain of Trypanosoma brucei.

The African trypanosome Trypanosoma brucei is protected in the bloodstream of the mammalian host by a dense Variant Surface Glycoprotein (VSG) coat. Although an individual cell has hundreds of VSG genes, the active VSG is transcribed in a mutually exclusive fashion from one of about twenty telomeric VSG expression sites. Expression sites are regulated domains flanked by 50 bp repeat arrays and extensive tracts of repetitive elements. We have integrated exogenous rDNA and expression site promoters upstream of the 50 bp repeats of the VO2 VSG expression site. Transcription from both types of exogenous promoter is downregulated and comparable to promoters targeted into the VSG Basic Copy arrays. We show that the upstream exogenous rDNA promoter escapes VSG expression site control, as switching the downstream VO2 VSG expression site on and off does not affect its activity. Therefore, the 50 bp repeat arrays appear to be the boundary of the regulated expression site domain.

The architecture of variant surface glycoprotein gene expression sites in Trypanosoma brucei.

Trypanosoma brucei evades the immune system by switching between Variant Surface Glycoprotein (VSG) genes. The active VSG gene is transcribed in one of approximately 20 telomeric expression sites (ESs). It has been postulated that ES polymorphism plays a role in host adaptation. To gain more insight into ES architecture, we have determined the complete sequence of Bacterial Artificial Chromosomes (BACs) containing DNA from three ESs and their flanking regions. There was variation in the order and number of ES-associated genes (ESAGs). ESAGs 6 and 7, encoding transferrin receptor subunits, are the only ESAGs with functional copies in every ES that has been sequenced until now. A BAC clone containing the VO2 ES sequences comprised approximately half of a 330 kb 'intermediate' chromosome. The extensive similarity between this intermediate chromosome and the left telomere of T. brucei 927 chromosome I, suggests that this previously uncharacterised intermediate size class of chromosomes could have arisen from breakage of megabase chromosomes. Unexpected conservation of sequences, including pseudogenes, indicates that the multiple ESs could have arisen through a relatively recent amplification of a single ES.