Stage-specific mechanisms for activation and expression of variant surface glycoprotein genes in Trypanosoma brucei

Abstract

The African trypanosomes undergo extensive development as they proceed through their life cycle, producing a range of stages with distinct functions. In response to the somewhat treacherous, extracellular environments occupied by trypanosomes in their rather different hostsmammals and tsetse flieselaborate survival mechanisms are utilized. The best known of these is antigenic variation (reviewed in [ 1-31), Briefly, the bloodstream form trypanosomes and the metacyclic stage are coated by a layer of the variant surface glycoprotein (VSG). By continual switching of the VSG, the trypanosome population evades complete destruction by the specific anti-VSG immune response. There is a repertoire of more than 1000 silent VSG genes (basic copy, BC genes) in each trypanosome and these are expressed differentially by the mutually exclusive use of bloodstream expression sites (B-ES), which are located at telomeres [4, 51. Most BC genes therefore cannot be expressed where they are and are activated by duplication into B-ES, forming temporary expression-linked copies (ELCs) [6, 71. However, some BC genes normally reside at telomeres and these can be activated it1 sitir (if they occupy a silent B-ES) or by moving into another B-ES [8, 91. Each B-ES is long (at least 40 kb), as dcfined by transcriptional start regions, and contains a collection of expression-site associated genes (ESAGs), which are not related to, but are co-ordinately transcribed with, VSG genes [4,5, 101.Being essential for survival in the mammal, the VSG is first synthesized in the metacyclic stage in the tsetse fly, and it is now clear that VSG genes are activated and expressed very differently in the metacyclic and bloodstream stages.

Transcriptional Regulation of Metacyclic Variant Surface Glycoprotein Gene Expression during the Life Cycle of Trypanosoma brucei

“…Nothing is reported about its activation and expression in the metacyclic stage, but results imply that the MVAT4 promoter is not stage regulated and the gene must therefore be under posttranscriptional regulation during the trypanosome life cycle (1). This is different from what we have observed for the 1.22 M-VSG gene promoter and what we predict for metacyclic promoters in general (6,70). It should be borne in mind that the MVAT4-expressing trypanosomes are rare, having required very extensive selection in the laboratory (1).…”

“…The principal preadaptation is the acquisition of the VSG surface coat, which is necessary for the parasite to tackle both the nonspecific and specific immune mechanisms of the mammal (18). When synthesis of the surface coat initiates in the metacyclic stage, a specific subset of VSG genes is activated, giving rise to a mixture of variable antigen types, which we believe strongly enhances the probability of this population being transmitted in the field into reservoir animals already partially immune to the VSG repertoires of local trypanosomes (6). We have investigated activation and regulation of expression of M-VSG genes and have uncovered a system radically different from that used for VSG activation and expression in the bloodstream.…”

“…We have argued elsewhere (6) that metacyclic trypanosomes are likely to encounter anti-VSG antibody on entering their usual host, i.e., domestic and game animals, which are probably capable of overcoming trypanosome infection and self-curing by generating antibodies against the entire VSG repertoire of local trypanosomes (5,48). Only as antibodies against individual VSGs wane can the metacyclic population gain entry (6). Transmission of the parasite would therefore be greatly enhanced by the metacyclic population expressing a mixture of VSGs rather than just one.…”

Ex Vivo and In Vitro Identification of a Consensus Promoter for VSG Genes Expressed by Metacyclic-Stage Trypanosomes in the Tsetse Fly

“…MVSG gene promoters are also transcriptionally silent during the procyclic stage and under normal circumstances can be reactivated only by differentiation to the metacyclic form in the salivary glands of the tsetse fly. The random nature of MVSG activation during metacyclic development ensures heterogeneity with respect to the VSG coats expressed by the parasite population and, as argued previously, is likely to enhance the probability of reestablishing infection in a previously infected host (3,4). Like BESs, MESs are telomeric, with no coding sequences between the MVSG gene and the telomeric tract, and their transcription is resistant to ␣-amanitin (5).…”

Activity of a Trypanosome Metacyclic Variant Surface Glycoprotein Gene Promoter Is Dependent upon Life Cycle Stage and Chromosomal Context

“…Further, clonal analysis shows that M-VSG genes are activated in situ at the metacyclic stage, without undergoing duplicative transposition, the mechanism associated with activation of most bloodstream VSG genes (41). This system for randomly and poly-clonally expressing M-VSG genes is dominant over the separate bloodstream system and may facilitate establishment of infection in partially immune hosts in the field (4).…”