Transcriptional Regulation of Telomeric Expression Sites and Antigenic Variation in Trypanosomes

Abstract

Introduction: Trypanosoma brucei uses antigenic variation to evade the host antibody clearance by periodically changing its Variant Surface Glycoprotein (VSGs) coat. T. brucei encode over 2,500 VSG genes and pseudogenes, however they transcribe only one VSG gene at time from one of the 20 telomeric Expression Sites (ESs). VSGs are transcribed in a monoallelic fashion by RNA polymerase I from an extranucleolar site named ES body. VSG antigenic switching occurs by transcriptional switching between telomeric ESs or by recombination of the VSG gene expressed. VSG expression is developmentally regulated and its transcription is controlled by epigenetic mechanisms and influenced by a telomere position effect.

Conclusion: Here, we discuss 1) the molecular basis underlying transcription of telomeric ESs and VSG antigenic switching; 2) the current knowledge of VSG monoallelic expression; 3) the role of inositol phosphate pathway in the regulation of VSG expression and switching; and 4) the developmental regulation of Pol I transcription of procyclin and VSG genes.

Keywords: Allelic exclusion; Antigenic variation; Epigenetic; RNA polymerase I; Telomere position effect; Transcriptional regulation; Trypanosoma; Variant surface glycoproteins.

Fig. (1)

T. brucei life cycle and mechanisms of antigenic variation. A) T. brucei slender BFs periodically switch its VSG coat to evade antibody clearance. Parasites expressing different individual VSGs (gray lines) may occur at the same time during infection [58, 153]. Colored thicker lines indicate combined parasitemia. Antibodies against variant types are produced periodically. At high density, slenders differentiate to non-dividing stumpies, which after uptake by tsetse flies develop into PFs in the fly midgut and into Epimastigotes Forms (EF) and then to Metacyclic Forms (MF) in the fly’s salivary glands. PFs and EFs express procyclins but MFs resume VSG expression and are infective to mammals. A sexual stage may occur between EF and MF [154]. B) Antigenic variation occurs by transcriptional switching between telomeric ESs or by recombination of VSG genes. Recombination may happen by gene conversion or segmental gene conversion between ES VSG genes and VSG genes or pseudogenes at other loci including intrachromosomal VSG arrays and minichromosomes.

Fig. (2)

Genomic organization of Pol I and Pol II transcribed genes in T. brucei. A) Pol II transcribed genes are organized in long cotranscribed gene clusters that are separated by SSRs in 11 megabase (Mb) size chromosomes (chr). Arrows indicate direction of Pol II transcription. VSG genes or pseudogenes are organized in subtelomeric regions or megabase chromosomes or at the telomeres of minichromosomes. B) Telomeric BESs and MESs and procyclin loci. BESs contain ESAG genes which are located between the promoter (arrow) and 70 bp repeats. Pseudogenes are indicated by ψ. VSG gene are near telomeric repeats. MES lack ESAGs and are expressed in MFs. PF express procyclins from two procyclin loci which encode EPs or GPEET procyclins.

Fig. (3)

Mechanisms of telomeric ES transcriptional regulation. A) Telomeric ESs are regulated by proteins that function in chromatin regulation, signaling or in nuclear lamina (Table 1). Various chromatin regulatory proteins repress transcription near the promoter in repressed ESs. Telomeric ES proteins repress transcription of the whole ES and thus the VSG genes. A repressive gradient occurs which is strong near the telomeres (thick bars) and weak distal the telomeres (thin bars). The precise binding sites of some telomeric ES proteins are unknown. However, TRF and RAP1 bind to telomeric repeats [71], PIP5Pase binds to and colocalizes with RAP1 and telomeric repeats [70], TRAF and VEX1 co-localizes with TRF [79]. ORC1/CDC6 binds to telomeric repeats independently of RAP1 and TRF [135, 136]. Note that VEX1 is only present in the active ES but its knockdown or overexpression derepress silent ESs [79]. PIP5K and PLC are not in the nucleus but regulate ES transcription [70], as it does the NUP-1 and 2 (indicated by dotted brackets) [128, 129]. B) VSG regulation involves molecules that are present at the active ES and positively regulate ES transcription, i.e. TDP-1, VEX1 and SUMOylation (SUMO), and the active ES is depleted of nucleosomes. In contrast, silent ESs are enriched in histones, base J and chromatin regulatory proteins (depicted in A) which regulates Pol I elongation throughout the ES. Transcription initiates in silent ESs but does not elongate through the ES, and Pol I occupancy is higher in the active ES (indicated by size of Pol I, red circle) [73].