A transcription-independent epigenetic mechanism is associated with antigenic switching in<i>Trypanosoma brucei</i>

Aresta‐Branco, Francisco; Pimenta, Silvia; Figueiredo, Luísa M.

doi:10.1093/nar/gkv1459

Cited by 36 publications

(41 citation statements)

References 57 publications

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“…Although the active VSG-ES is specifically depleted of nucleosomes (11,12), silent VSG-ESs are similarly located in the extranucleolar space in bloodstream-form cells, and neither active nor silent VSG-ESs show an appreciable association with the nuclear envelope (13). An HMG chromatin protein that is enriched and inversely correlated with nucleosome abundance at the ESB and in the nucleolus (14) appears to maintain open chromatin at these sites (15). In addition, a highly SUMOylated focus is specific to the site of the ESB (16).…”

mentioning

confidence: 99%

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

Glover

Hutchinson

Alsford

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

184

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Allelic exclusion underpins antigenic variation and immune evasion in African trypanosomes. These bloodstream parasites use RNA polymerase-I (pol-I) to transcribe just one telomeric variant surface glycoprotein (VSG) gene at a time, producing superabundant and switchable VSG coats. We identified trypanosome VSG exclusion-1 (VEX1) using a genetic screen for defects in telomere-exclusive expression. VEX1 was sequestered by the active VSG and silencing of other VSGs failed when VEX1 was either ectopically expressed or depleted, indicating positive and negative regulation, respectively. Positive regulation affected VSGs and nontelomeric pol-I-transcribed genes, whereas negative regulation primarily affected VSGs. Negative regulation by VEX1 also affected telomeric pol-I-transcribed reporter constructs, but only when they contained blocks of sequence sharing homology with a pol-I-transcribed locus. We conclude that restricted positive regulation due to VEX1 sequestration, combined with VEX1-dependent, possibly homology-dependent silencing, drives a "winner-takes-all" mechanism of allelic exclusion.epigenetic | monoallelic | silencing | telomere | Trypanosoma brucei C ells often restrict expression to a single allele of a gene or gene family. This allelic exclusion underpins antigenic variation in pathogens, including trypanosomes that cause sleeping sickness (1) and Plasmodium parasites that cause malaria (2). Allelic exclusion is also essential for singular olfactory receptor expression and a sense of smell in metazoa (3). Although many factors have been identified that are required for the expression of one allele or for the silencing of other alleles in these systems, our understanding of the mechanisms by which expression and silencing are established and coordinated remains incomplete (1-3).The African trypanosome Trypanosoma brucei is a flagellated parasitic protozoan transmitted among mammalian hosts by tsetse flies. In addition to causing trypanosomiasis in humans, a fatal and neglected tropical disease, these parasites also cause nagana in cattle. Antigenic variation is essential for persistent bloodstream infection in the face of host adaptive immune defenses and has long been a paradigm for studies on allelic exclusion (1); parasite immune evasion depends upon singular variant surface glycoprotein (VSG) gene expression and VSG switching. Although multiple subtelomeric VSGs are available for expression (4), only one is transcribed (5). Both active and silent VSGs are located at the ends of polycistronic transcription units known as expression sites (ESs) (6). Notably, VSG-ES promoters (6) recruit RNA polymerase-I (pol-I) that typically transcribes ribosomal RNA genes (7). Indeed, the active VSG-ES is associated with an extranucleolar focus of pol-I known as the expression-site body (ESB) (8-10). Although the active VSG-ES is specifically depleted of nucleosomes (11, 12), silent VSG-ESs are similarly located in the extranucleolar space in bloodstream-form cells, and neither active nor silent VSG-ESs show an ...

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mentioning

confidence: 99%

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

Glover

Hutchinson

Alsford

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

184

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“…HMO1 coats active Pol I-transcription units with a mutually exclusive distribution to the linker histone H1 and appears to stabilize chromatin depleted of nucleosomes [29,32]. Consistent with this, in T. brucei, TDP1 appears to help maintain an active chromatin state at the ES, even if ES transcription itself is inducibly blocked [33].…”

Section: How Is Vsg Expression Controlled In a Monoallelic Fashion?mentioning

confidence: 66%

How to create coats for all seasons: elucidating antigenic variation in African trypanosomes

Ooi

Rudenko

2017

Emerging Topics in Life Sciences

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Extracellular parasites of the mammalian bloodstream face considerable challenges including incessant assault by the immune system. African trypanosomes are consummate survivors in this inclement environment and are renowned for their supremely sophisticated strategy of antigenic variation of their protective surface coat during the course of chronic infections. Recent developments are making us realize how complex this antigenic machinery is and are allowing us to tackle previously intractable problems. However, many of the simplest (and arguably the most important) questions still remain unanswered! How is the extraordinary heterogeneity of variant surface glycoprotein variants during a chronic infection generated?The bloodstream form of Trypanosoma brucei is coated with a dense covering of ∼10 7 variant surface glycoprotein (VSG) molecules, which extend as antigenically variable rods connected to the cell surface via glycosylphosphatidylinositol (GPI) anchors [1]. This GPI attachment allows VSGs to move freely over the cell surface [2]. Tight packing of these VSGs produces a protective barrier shielding invariant surface proteins from recognition by host antibodies and preventing trypanosome lysis by the alternative pathway of the complement system [3]. Extraordinarily high rates of recycling of surface VSG allow selective removal of host cell molecules including antibodies and complement from the trypanosome surface, thereby functioning as a 'coat-cleaning ' machine [4]. This prolongs trypanosome survival in rising antibody titres and facilitates escape from macrophages [5].Each trypanosome has a vast repertoire of VSG genes [6], differing in number and content between different strains, presumably as a consequence of diversifying forces. For example, the 'VSGnome' of T. brucei 427 contains more than 2500 different VSG genes, of which more than 80% are not immediately functional [7]. Switching the active VSG can entail transcriptional switches between the ∼15 telomeric VSG expression site (ES) transcription units (Figure 1). More importantly, DNA rearrangements and predominantly gene conversions can copy new VSGs (or segments of VSGs) into the active ES, thereby resulting in an antigenic switch [8] (Figure 1). This extraordinary ability to construct novel patchwork, or mosaic, VSGs is key for trypanosome survival [9]. The continuous recombination of various permutations and combinations of sections of VSG genes (or pseudogenes) allows the trypanosome to construct an almost infinite number of antigenically distinct VSG coat types.A given wave of trypanosome parasitaemia is normally predominantly composed of a major VSG variant, although it has long been known that minor VSG variants are also present within each wave. However, there is possibly phenomenal heterogeneity within these infection waves, as next-generation RNA sequencing of RNA transcripts has documented an extraordinary variety of minor VSG variants [10]. These experiments argue that a considerable percentage of the available VSG repertoire is disp...

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“…As a control, immunofluorescence was also performed in a distinct WT strain (i.e. not lacking TbRH1) in which a transcription elongation blockade within BES1 has silenced this ES and predominantly activated BES3 (28, 45, 46), containing VSG121 (Fig.3 C,D). Most ( ∼ 68%) Tbrh1 -/- cells stained only with anti-VSG121 antiserum, though a minority ( ∼ 32%) were VSG221-VSG121 double expressers.…”

Section: Resultsmentioning

confidence: 99%

RibonucleaseH1-targeted R-loops in surface antigen gene expression sites can direct trypanosome immune evasion

Briggs

Crouch

Lemgruber

et al. 2018

Preprint

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30Switching of the Variant Surface Glycoprotein (VSG) in Trypanosoma brucei provides a crucial host immune 31 evasion strategy that is catalysed both by transcription and recombination reactions, each operating within 32 specialised telomeric VSG expression sites (ES). VSG switching is likely triggered by events focused on the 33 single actively transcribed ES, from a repertoire of around 15, but the nature of such events is unclear. Here 34 we show that RNA-DNA hybrids, called R-loops, form preferentially within sequences termed the 70 bp 35 repeats in the actively transcribed ES, but spread throughout the active and inactive ES in the absence of 36 RNase H1, which degrades R-loops. Loss of RNase H1 also leads to increased levels of VSG coat switching 37 and replication-associated genome damage, some of which accumulates within the active ES. This work 38indicates VSG ES architecture elicits R-loop formation, and that these RNA-DNA hybrids connect T. brucei 39 immune evasion by transcription and recombination. 40 41 Author summary 42All pathogens must survive eradication by the host immune response in order to continue infections and be 43 passed on to a new host. Changes in the proteins expressed on the surface of the pathogen, or on the 44 surface of the cells the pathogen infects, is a widely used strategy to escape immune elimination. 45Understanding how this survival strategy, termed antigenic variation, operates in any pathogen is critical, 46 both to understand interaction between the pathogen and host and disease progression. A key event in 47 antigenic variation is the initiation of the change in expression of the surface protein gene, though how this 48 occurs has been detailed in very few pathogens. Here we examine how changes in expression of the surface 49 coat of the African trypanosome, which causes sleeping sickness disease, are initiated. We reveal that 50 specialised nucleic acid structures, termed R-loops, form around the expressed trypanosome surface 51 protein gene and increase in abundance after mutation of an enzyme that removes them, leading to 52 increased changes in the surface coat in trypanosome cells that are dividing. We therefore shed light on the 53 earliest acting events in trypanosome antigenic variation. 54 55 56 Introduction 57

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A transcription-independent epigenetic mechanism is associated with antigenic switching inTrypanosoma brucei

Cited by 36 publications

References 57 publications

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

How to create coats for all seasons: elucidating antigenic variation in African trypanosomes

RibonucleaseH1-targeted R-loops in surface antigen gene expression sites can direct trypanosome immune evasion

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