Expression site attenuation mechanistically links antigenic variation and development in Trypanosoma brucei

Batram, Christopher; Jones, Nicholas Blurton; Janzen, Christian J.; Markert, Sebastian M.; Engstler, Markus

doi:10.7554/elife.02324

Cited by 88 publications

(127 citation statements)

References 41 publications

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“…Another recent report demonstrated that a VSG, transcribed at a chromosomeinternal site by T7-phage polymerase, transiently silenced the native VSG (25). This same report demonstrated repression spreading along the VSG-ES in a DOT1B, histone methyltransferase-dependent manner (25). Spreading of a repressed domain may also explain why we see derepression of both bicistronic GFP and NPT reporters during VEX1 knockdown (Fig.…”

Section: Discussionsupporting

confidence: 70%

“…We now show that this repression is VEX1-dependent and also that homologous sequences can promote VEX1-dependent repression. Another recent report demonstrated that a VSG, transcribed at a chromosomeinternal site by T7-phage polymerase, transiently silenced the native VSG (25). This same report demonstrated repression spreading along the VSG-ES in a DOT1B, histone methyltransferase-dependent manner (25).…”

Section: Discussionmentioning

confidence: 77%

“…For example, a histone H3 variant, a bloodstream stagespecific modified DNA base known as J or hydroxymethyluracil (22,23), the chromatin remodeling ISWI complex (24), the histone H3K76 trimethyltransferase DOT1B (25), and the telomere-associated protein RAP1 (26) all facilitate VSG-ES silencing. In addition, cohesin function facilitates maintenance of the active VSG-ES (27), and inositol phosphate signaling impacts VSG-ES regulation (28).…”

mentioning

confidence: 99%

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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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Section: Discussionsupporting

confidence: 70%

Section: Discussionmentioning

confidence: 77%

mentioning

confidence: 99%

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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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“…It has recently been proposed that antigenic variation is linked with differentiation in T. brucei [44,45]. Overexpression of a second ectopic VSG from an inducible promoter can result in transient silencing of the active ES proceeding upwards from the telomere.…”

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

confidence: 99%

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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“…This monoallelic VSG expression ensures that after a VSG switching event, the originally active VSG no longer is expressed on the cell surface. Several mechanisms of VSG expression regulation have been identified, including specialized localization of the active ES at an extranucleolar ES body (ESB) that is enriched with RNA polymerase I (24), regulated transcription elongation along ESs (25,26), modulation of ES chromatin structure (27)(28)(29)(30)(31)(32), modulation of ES promoter activities (33)(34)(35)(36)(37), and telomere protein-mediated telomeric silencing (38,39). VSG expression regulation has been reviewed elsewhere recently (40, 41) and will not be discussed here in detail.…”

Section: Antigenic Variation In T Bruceimentioning

confidence: 99%

DNA Double-Strand Breaks and Telomeres Play Important Roles in Trypanosoma brucei Antigenic Variation

2015

Eukaryot Cell

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Human-infecting microbial pathogens all face a serious problem of elimination by the host immune response. Antigenic variation is an effective immune evasion mechanism where the pathogen regularly switches its major surface antigen. In many cases, the major surface antigen is encoded by genes from the same gene family, and its expression is strictly monoallelic. Among pathogens that undergo antigenic variation, Trypanosoma brucei (a kinetoplastid), which causes human African trypanosomiasis, Plasmodium falciparum (an apicomplexan), which causes malaria, Pneumocystis jirovecii (a fungus), which causes pneumonia, and Borrelia burgdorferi (a bacterium), which causes Lyme disease, also express their major surface antigens from loci next to the telomere. Except for Plasmodium, DNA recombination-mediated gene conversion is a major pathway for surface antigen switching in these pathogens. In the last decade, more sophisticated molecular and genetic tools have been developed in T. brucei, and our knowledge of functions of DNA recombination in antigenic variation has been greatly advanced. VSG is the major surface antigen in T. brucei. In subtelomeric VSG expression sites (ESs), VSG genes invariably are flanked by a long stretch of upstream 70-bp repeats. Recent studies have shown that DNA double-strand breaks (DSBs), particularly those in 70-bp repeats in the active ES, are a natural potent trigger for antigenic variation in T. brucei. In addition, telomere proteins can influence VSG switching by reducing the DSB amount at subtelomeric regions. These findings will be summarized and their implications will be discussed in this review.

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Expression site attenuation mechanistically links antigenic variation and development in Trypanosoma brucei

Cited by 88 publications

References 41 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

DNA Double-Strand Breaks and Telomeres Play Important Roles in Trypanosoma brucei Antigenic Variation

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