1
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Li B. Unwrap RAP1's Mystery at Kinetoplastid Telomeres. Biomolecules 2024; 14:67. [PMID: 38254667 PMCID: PMC10813129 DOI: 10.3390/biom14010067] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2023] [Revised: 12/27/2023] [Accepted: 12/27/2023] [Indexed: 01/24/2024] Open
Abstract
Although located at the chromosome end, telomeres are an essential chromosome component that helps maintain genome integrity and chromosome stability from protozoa to mammals. The role of telomere proteins in chromosome end protection is conserved, where they suppress various DNA damage response machineries and block nucleolytic degradation of the natural chromosome ends, although the detailed underlying mechanisms are not identical. In addition, the specialized telomere structure exerts a repressive epigenetic effect on expression of genes located at subtelomeres in a number of eukaryotic organisms. This so-called telomeric silencing also affects virulence of a number of microbial pathogens that undergo antigenic variation/phenotypic switching. Telomere proteins, particularly the RAP1 homologs, have been shown to be a key player for telomeric silencing. RAP1 homologs also suppress the expression of Telomere Repeat-containing RNA (TERRA), which is linked to their roles in telomere stability maintenance. The functions of RAP1s in suppressing telomere recombination are largely conserved from kinetoplastids to mammals. However, the underlying mechanisms of RAP1-mediated telomeric silencing have many species-specific features. In this review, I will focus on Trypanosoma brucei RAP1's functions in suppressing telomeric/subtelomeric DNA recombination and in the regulation of monoallelic expression of subtelomere-located major surface antigen genes. Common and unique mechanisms will be compared among RAP1 homologs, and their implications will be discussed.
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Affiliation(s)
- Bibo Li
- Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, College of Arts and Sciences, Cleveland State University, 2121 Euclid Avenue, Cleveland, OH 44115, USA;
- Case Comprehensive Cancer Center, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
- Department of Inflammation and Immunity, Lerner Research Institute, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195, USA
- Center for RNA Science and Therapeutics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
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2
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Transcription Dependent Loss of an Ectopically Expressed Variant Surface Glycoprotein during Antigenic Variation in Trypanosoma brucei. mBio 2022; 13:e0384721. [PMID: 35229632 PMCID: PMC8941856 DOI: 10.1128/mbio.03847-21] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
In the mammalian host, Trypanosoma brucei is coated in a single-variant surface glycoprotein (VSG) species. Stochastic switching of the expressed VSG allows the parasite to escape detection by the host immune system. DNA double-strand breaks (DSB) trigger VSG switching, and repair via gene conversion results in an antigenically distinct VSG being expressed from the single active bloodstream-form expression site (BES). The single active BES is marked by VSG exclusion 2 (VEX2) protein. Here, we have disrupted monoallelic VSG expression by stably expressing a second telomeric VSG from a ribosomal locus. We found that cells expressing two VSGs contained one VEX2 focus that was significantly larger in size than the wild-type cells; this therefore suggests the ectopic VSG is expressed from the same nuclear position as the active BES. Unexpectedly, we report that in the double VSG-expressing cells, the DNA sequence of the ectopic copy is lost following a DSB in the active BES, despite it being spatially separated in the genome. The loss of the ectopic VSG is dependent on active transcription and does not disrupt the number or variety of templates used to repair a BES DSB and elicit a VSG switch. We propose that there are stringent mechanisms within the cell to reinforce monoallelic expression during antigenic variation. IMPORTANCE The single-cell parasite Trypanosoma brucei causes the fatal disease human African trypanosomiasis and is able to colonize the blood, fat, skin, and central nervous system. Trypanosomes survive in the mammalian host owing to a dense protective protein coat that consists of a single-variant surface glycoprotein species. Stochastic switching of one VSG for an immunologically distinct one enables the parasite to escape recognition by the host immune system. We have disrupted monoallelic antigen expression by expressing a second VSG and report that following DSB-triggered VSG switching, the DNA sequence of the ectopic VSG is lost in a transcription-dependent manner. We propose that there are strict requirements to ensure that only one variant antigen is expressed following a VSG switch, which has important implications for understanding how the parasite survives in the mammalian host.
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3
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Maudlin IE, Kelly S, Schwede A, Carrington M. VSG mRNA levels are regulated by the production of functional VSG protein. Mol Biochem Parasitol 2021; 241:111348. [PMID: 33352254 PMCID: PMC7871013 DOI: 10.1016/j.molbiopara.2020.111348] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2020] [Revised: 12/15/2020] [Accepted: 12/16/2020] [Indexed: 02/06/2023]
Abstract
The bloodstream form of Trypanosoma brucei persists in mammalian hosts through a population survival strategy depending on antigenic variation of a cell surface coat composed of the variant surface glycoprotein (VSG). The integrity of the VSG coat is essential and blocking its synthesis results in a cell division cycle arrest just prior to cytokinesis. This observation indicates that VSG levels are monitored and that the cell has mechanisms to respond to a disruption of synthesis. Here, the regulation of VSG mRNA levels has been investigated by first measuring VSG mRNA copy number, and second using ectopic expression of VSG transgenes containing premature termination codons. The findings are that (i) VSG mRNA copy number varies with the identity of the VSG and (ii) a pathway detects synthesis of non-functional VSG protein and results in an increase in VSG mRNA levels.
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Affiliation(s)
- Isabella E Maudlin
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, United Kingdom
| | - Steve Kelly
- Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, United Kingdom
| | - Angela Schwede
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, United Kingdom.
| | - Mark Carrington
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, United Kingdom.
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4
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African trypanosomes expressing multiple VSGs are rapidly eliminated by the host immune system. Proc Natl Acad Sci U S A 2019; 116:20725-20735. [PMID: 31554700 PMCID: PMC6789922 DOI: 10.1073/pnas.1905120116] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Many parasites escape the host immune system by undergoing antigenic variation, a process in which surface antigens are regularly shed and replaced by new ones. Trypanosoma brucei employs multiple sophisticated molecular mechanisms to ensure the expression of a homogeneous VSG coat. We generated a mutant parasite that expresses multiple distinct VSGs and studied the consequences of having a multi-VSG coat during an infection. We showed that expression of multiple VSGs makes the parasites more vulnerable to the immune response, which can now control the trypanosomes from the onset of the infection, allowing most mice to survive. In the future, trypanosome infections may be treated using drugs that generate parasites with multi-VSG coats. Trypanosoma brucei parasites successfully evade the host immune system by periodically switching the dense coat of variant surface glycoprotein (VSG) at the cell surface. Each parasite expresses VSGs in a monoallelic fashion that is tightly regulated. The consequences of exposing multiple VSGs during an infection, in terms of antibody response and disease severity, remain unknown. In this study, we overexpressed a high-mobility group box protein, TDP1, which was sufficient to open the chromatin of silent VSG expression sites, to disrupt VSG monoallelic expression, and to generate viable and healthy parasites with a mixed VSG coat. Mice infected with these parasites mounted a multi-VSG antibody response, which rapidly reduced parasitemia. Consequently, we observed prolonged survival in which nearly 90% of the mice survived a 30-d period of infection with undetectable parasitemia. Immunodeficient RAG2 knock-out mice were unable to control infection with TDP1-overexpressing parasites, showing that the adaptive immune response is critical to reducing disease severity. This study shows that simultaneous exposure of multiple VSGs is highly detrimental to the parasite, even at the very early stages of infection, suggesting that drugs that disrupt VSG monoallelic expression could be used to treat trypanosomiasis.
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5
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Ridewood S, Ooi CP, Hall B, Trenaman A, Wand NV, Sioutas G, Scherwitzl I, Rudenko G. The role of genomic location and flanking 3'UTR in the generation of functional levels of variant surface glycoprotein in Trypanosoma brucei. Mol Microbiol 2017; 106:614-634. [PMID: 28906055 PMCID: PMC5698767 DOI: 10.1111/mmi.13838] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/12/2017] [Indexed: 12/21/2022]
Abstract
Trypanosoma brucei faces relentless immune attack in the mammalian bloodstream, where it is protected by an essential coat of Variant Surface Glycoprotein (VSG) comprising ∼10% total protein. The active VSG gene is in a Pol I‐transcribed telomeric expression site (ES). We investigated factors mediating these extremely high levels of VSG expression by inserting ectopic VSG117 into VSG221 expressing T. brucei. Mutational analysis of the ectopic VSG 3′UTR demonstrated the essentiality of a conserved 16‐mer for mRNA stability. Expressing ectopic VSG117 from different genomic locations showed that functional VSG levels could be produced from a gene 60 kb upstream of its normal telomeric location. High, but very heterogeneous levels of VSG117 were obtained from the Pol I‐transcribed rDNA. Blocking VSG synthesis normally triggers a precise precytokinesis cell‐cycle checkpoint. VSG117 expression from the rDNA was not adequate for functional complementation, and the stalled cells arrested prior to cytokinesis. However, VSG levels were not consistently low enough to trigger a characteristic ‘VSG synthesis block’ cell‐cycle checkpoint, as some cells reinitiated S phase. This demonstrates the essentiality of a Pol I‐transcribed ES, as well as conserved VSG 3′UTR 16‐mer sequences for the generation of functional levels of VSG expression in bloodstream form T. brucei.
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Affiliation(s)
- Sophie Ridewood
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Cher-Pheng Ooi
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Belinda Hall
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Anna Trenaman
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Nadina Vasileva Wand
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Georgios Sioutas
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Iris Scherwitzl
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
| | - Gloria Rudenko
- Department of Life Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington, London SW7 2AZ, UK
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6
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Pinger J, Chowdhury S, Papavasiliou FN. Variant surface glycoprotein density defines an immune evasion threshold for African trypanosomes undergoing antigenic variation. Nat Commun 2017; 8:828. [PMID: 29018220 PMCID: PMC5635023 DOI: 10.1038/s41467-017-00959-w] [Citation(s) in RCA: 45] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2017] [Accepted: 08/02/2017] [Indexed: 11/15/2022] Open
Abstract
Trypanosoma brucei is a protozoan parasite that evades its host's adaptive immune response by repeatedly replacing its dense variant surface glycoprotein (VSG) coat from its large genomic VSG repertoire. While the mechanisms regulating VSG gene expression and diversification have been examined extensively, the dynamics of VSG coat replacement at the protein level, and the impact of this process on successful immune evasion, remain unclear. Here we evaluate the rate of VSG replacement at the trypanosome surface following a genetic VSG switch, and show that full coat replacement requires several days to complete. Using in vivo infection assays, we demonstrate that parasites undergoing coat replacement are only vulnerable to clearance via early IgM antibodies for a limited time. Finally, we show that IgM loses its ability to mediate trypanosome clearance at unexpectedly early stages of coat replacement based on a critical density threshold of its cognate VSGs on the parasite surface. Trypanosoma brucei evades the host immune system through replacement of a variant surface glycoprotein (VSG) coat. Here, the authors show that VSG replacement takes several days to complete, and the parasite is vulnerable to the host immune system for a short period of time during the process.
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Affiliation(s)
- Jason Pinger
- The Rockefeller University, Laboratory of Lymphocyte Biology, 1230 York Avenue, New York, NY, 10065, USA.
- The David Rockefeller Graduate School, 1230 York Avenue, New York, NY, 10065, USA.
| | - Shanin Chowdhury
- The Rockefeller University, Laboratory of Lymphocyte Biology, 1230 York Avenue, New York, NY, 10065, USA
| | - F Nina Papavasiliou
- The Rockefeller University, Laboratory of Lymphocyte Biology, 1230 York Avenue, New York, NY, 10065, USA.
- Division of Immune Diversity, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg, 69120, Germany.
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7
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Structural basis for the shielding function of the dynamic trypanosome variant surface glycoprotein coat. Nat Microbiol 2017; 2:1523-1532. [PMID: 28894098 DOI: 10.1038/s41564-017-0013-6] [Citation(s) in RCA: 37] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2016] [Accepted: 07/26/2017] [Indexed: 01/30/2023]
Abstract
The most prominent defence of the unicellular parasite Trypanosoma brucei against the host immune system is a dense coat that comprises a variant surface glycoprotein (VSG). Despite the importance of the VSG family, no complete structure of a VSG has been reported. Making use of high-resolution structures of individual VSG domains, we employed small-angle X-ray scattering to elucidate the first two complete VSG structures. The resulting models imply that the linker regions confer great flexibility between domains, which suggests that VSGs can adopt two main conformations to respond to obstacles and changes of protein density, while maintaining a protective barrier at all times. Single-molecule diffusion measurements of VSG in supported lipid bilayers substantiate this possibility, as two freely diffusing populations could be detected. This translates into a highly flexible overall topology of the surface VSG coat, which displays both lateral movement in the plane of the membrane and variation in the overall thickness of the coat.
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8
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Hartel AJW, Glogger M, Jones NG, Abuillan W, Batram C, Hermann A, Fenz SF, Tanaka M, Engstler M. N-glycosylation enables high lateral mobility of GPI-anchored proteins at a molecular crowding threshold. Nat Commun 2016; 7:12870. [PMID: 27641538 PMCID: PMC5031801 DOI: 10.1038/ncomms12870] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2016] [Accepted: 08/09/2016] [Indexed: 01/17/2023] Open
Abstract
The protein density in biological membranes can be extraordinarily high, but the impact of molecular crowding on the diffusion of membrane proteins has not been studied systematically in a natural system. The diversity of the membrane proteome of most cells may preclude systematic studies. African trypanosomes, however, feature a uniform surface coat that is dominated by a single type of variant surface glycoprotein (VSG). Here we study the density-dependence of the diffusion of different glycosylphosphatidylinositol-anchored VSG-types on living cells and in artificial membranes. Our results suggest that a specific molecular crowding threshold (MCT) limits diffusion and hence affects protein function. Obstacles in the form of heterologous proteins compromise the diffusion coefficient and the MCT. The trypanosome VSG-coat operates very close to its MCT. Importantly, our experiments show that N-linked glycans act as molecular insulators that reduce retarding intermolecular interactions allowing membrane proteins to function correctly even when densely packed.
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Affiliation(s)
- Andreas J. W. Hartel
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Marius Glogger
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Nicola G. Jones
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Wasim Abuillan
- Physical Chemistry of Biosystems, Institute of Physical Chemistry, University of Heidelberg, Heidelberg 69120, Germany
| | - Christopher Batram
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Anne Hermann
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Susanne F. Fenz
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
| | - Motomu Tanaka
- Physical Chemistry of Biosystems, Institute of Physical Chemistry, University of Heidelberg, Heidelberg 69120, Germany
- Institute for Integrated Cell-Material Science (WPI iCeMS), Kyoto University, Kyoto 606-8501, Japan
| | - Markus Engstler
- Department of Cell and Developmental Biology, Theodor-Boveri-Institute, Biocenter, University of Würzburg, Würzburg 97074, Germany
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9
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DNA Recombination Strategies During Antigenic Variation in the African Trypanosome. Microbiol Spectr 2016; 3:MDNA3-0016-2014. [PMID: 26104717 DOI: 10.1128/microbiolspec.mdna3-0016-2014] [Citation(s) in RCA: 51] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Survival of the African trypanosome in its mammalian hosts has led to the evolution of antigenic variation, a process for evasion of adaptive immunity that has independently evolved in many other viral, bacterial and eukaryotic pathogens. The essential features of trypanosome antigenic variation have been understood for many years and comprise a dense, protective Variant Surface Glycoprotein (VSG) coat, which can be changed by recombination-based and transcription-based processes that focus on telomeric VSG gene transcription sites. However, it is only recently that the scale of this process has been truly appreciated. Genome sequencing of Trypanosoma brucei has revealed a massive archive of >1000 VSG genes, the huge majority of which are functionally impaired but are used to generate far greater numbers of VSG coats through segmental gene conversion. This chapter will discuss the implications of such VSG diversity for immune evasion by antigenic variation, and will consider how this expressed diversity can arise, drawing on a growing body of work that has begun to examine the proteins and sequences through which VSG switching is catalyzed. Most studies of trypanosome antigenic variation have focused on T. brucei, the causative agent of human sleeping sickness. Other work has begun to look at antigenic variation in animal-infective trypanosomes, and we will compare the findings that are emerging, as well as consider how antigenic variation relates to the dynamics of host-trypanosome interaction.
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10
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Ramey-Butler K, Ullu E, Kolev NG, Tschudi C. Synchronous expression of individual metacyclic variant surface glycoprotein genes in Trypanosoma brucei. Mol Biochem Parasitol 2015; 200:1-4. [PMID: 25896436 DOI: 10.1016/j.molbiopara.2015.04.001] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2015] [Revised: 04/03/2015] [Accepted: 04/08/2015] [Indexed: 11/27/2022]
Abstract
One distinctive feature of the Trypanosoma brucei life cycle is the presence of two discrete populations that are based on differential expression of variant surface glycoproteins (VSGs). Both are adapted to the environmental pressures they face and more importantly, both contribute directly to transmission. Metacyclics in the tsetse fly enable transmission to a new mammalian host, whereas bloodstream trypanosomes must avoid immune destruction to the extent that sufficient numbers are available for transmission, when the insect vector takes a blood meal. At present, there are few investigations on the molecular aspects of parasite biology in the tsetse vector and specifically about the activation of metacyclic VSG gene expression. Here we used an established in vitro differentiation system based on the overexpression of the RNA-binding protein 6 (RBP6), to monitor two metacyclic VSGs (VSG 397 and VSG 653) during development from procyclics to infectious metacyclic forms. We observed that activation of these two mVSGs was simultaneous both at the transcript and protein level, and manifested by the appearance of only one of the mVSGs in individual cells.
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Affiliation(s)
- Kiantra Ramey-Butler
- Department of Internal Medicine, Yale School of Medicine, New Haven, CT 06536, USA
| | - Elisabetta Ullu
- Department of Internal Medicine, Yale School of Medicine, New Haven, CT 06536, USA; Department of Cell Biology, Yale School of Medicine, New Haven, CT 06536, USA
| | - Nikolay G Kolev
- Department of Epidemiology of Microbial Diseases, Yale School of Public Health, New Haven, CT 06536, USA.
| | - Christian Tschudi
- Department of Epidemiology of Microbial Diseases, Yale School of Public Health, New Haven, CT 06536, USA
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11
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Abstract
Studies on Variant Surface Glycoproteins (VSGs) and antigenic variation in the African trypanosome, Trypanosoma brucei, have yielded a remarkable range of novel and important insights. The features first identified in T. brucei extend from unique to conserved-among-trypanosomatids to conserved-among-eukaryotes. Consequently, much of what we now know about trypanosomatid biology and much of the technology available has its origin in studies related to VSGs. T. brucei is now probably the most advanced early branched eukaryote in terms of experimental tractability and can be approached as a pathogen, as a model for studies on fundamental processes, as a model for studies on eukaryotic evolution or often all of the above. In terms of antigenic variation itself, substantial progress has been made in understanding the expression and switching of the VSG coat, while outstanding questions continue to stimulate innovative new approaches. There are large numbers of VSG genes in the genome but only one is expressed at a time, always immediately adjacent to a telomere. DNA repair processes allow a new VSG to be copied into the single transcribed locus. A coordinated transcriptional switch can also allow a new VSG gene to be activated without any detectable change in the DNA sequence, thereby maintaining singular expression, also known as allelic exclusion. I review the story behind VSGs; the genes, their expression and switching, their central role in T. brucei virulence, the discoveries that emerged along the way and the persistent questions relating to allelic exclusion in particular.
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Affiliation(s)
- David Horn
- Division of Biological Chemistry & Drug Discovery, College of Life Sciences, University of Dundee, Dow Street, Dundee DD1 5EH, UK.
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12
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Batram C, Jones NG, Janzen CJ, Markert SM, Engstler M. Expression site attenuation mechanistically links antigenic variation and development in Trypanosoma brucei. eLife 2014; 3:e02324. [PMID: 24844706 PMCID: PMC4027811 DOI: 10.7554/elife.02324] [Citation(s) in RCA: 61] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
We have discovered a new mechanism of monoallelic gene expression that links antigenic variation, cell cycle, and development in the model parasite Trypanosoma brucei. African trypanosomes possess hundreds of variant surface glycoprotein (VSG) genes, but only one is expressed from a telomeric expression site (ES) at any given time. We found that the expression of a second VSG alone is sufficient to silence the active VSG gene and directionally attenuate the ES by disruptor of telomeric silencing-1B (DOT1B)-mediated histone methylation. Three conserved expression-site-associated genes (ESAGs) appear to serve as signal for ES attenuation. Their depletion causes G1-phase dormancy and reversible initiation of the slender-to-stumpy differentiation pathway. ES-attenuated slender bloodstream trypanosomes gain full developmental competence for transformation to the tsetse fly stage. This surprising connection between antigenic variation and developmental progression provides an unexpected point of attack against the deadly sleeping sickness. DOI:http://dx.doi.org/10.7554/eLife.02324.001 African sleeping sickness is a potentially lethal disease that is caused by a parasite called T. brucei and spread by tsetse flies. Like many of the parasites that cause tropical diseases, T. brucei employs genetic trickery to evade the immune systems of humans and other mammals. This involves changing the variant surface glycoprotein (VSG) coat that surrounds the parasite on a regular basis in order to remain one step ahead of the immune system of its host: while the immune system looks for invaders wearing a particular coat, the parasites are spreading through the host in a completely different coat. To infect other hosts, the parasite must undergo changes that allow it to re-infect the tsetse fly. Therefore, besides the ‘antigenic variation’ that allows it to change its surface coat when it is in the blood of its host, T. brucei must undergo a more fundamental metamorphosis before it is capable of colonizing the tsetse fly. However, many details of the changes that allow the parasites to re-infect flies are not understood. T. brucei has several hundred VSG genes clustered in about 15 regions known as expression sites, but only a single expression site is active at any given time. Each expression site also contains a number of other genes known as expression site-associated genes (ESAGs). Antigenic variation can occur as a result of different VSG genes within the same expression site being expressed as proteins, or when the active expression site is silenced and another expression site is activated. This is another process that is not fully understood. Batram et al. now reveal that the expression of VSG genes, antigenic variation and the changes that allow the parasites to re-infect flies are all related to each other. This suggests that the expression site could provide a new point of attack in the fight against African sleeping sickness. DOI:http://dx.doi.org/10.7554/eLife.02324.002
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Affiliation(s)
- Christopher Batram
- Department of Cell and Developmental Biology, University of Würzburg, Würzburg, Germany
| | - Nicola G Jones
- Department of Cell and Developmental Biology, University of Würzburg, Würzburg, Germany
| | - Christian J Janzen
- Department of Cell and Developmental Biology, University of Würzburg, Würzburg, Germany
| | - Sebastian M Markert
- Department of Cell and Developmental Biology, University of Würzburg, Würzburg, Germany
| | - Markus Engstler
- Department of Cell and Developmental Biology, University of Würzburg, Würzburg, Germany
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13
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Symmetry Breaking in a Model of Antigenic Variation with Immune Delay. Bull Math Biol 2012; 74:2488-509. [DOI: 10.1007/s11538-012-9763-8] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2011] [Accepted: 07/27/2012] [Indexed: 11/25/2022]
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14
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Blyuss KB. The effects of symmetry on the dynamics of antigenic variation. J Math Biol 2012; 66:115-37. [DOI: 10.1007/s00285-012-0508-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2011] [Revised: 01/15/2012] [Indexed: 11/24/2022]
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15
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Smith TK, Vasileva N, Gluenz E, Terry S, Portman N, Kramer S, Carrington M, Michaeli S, Gull K, Rudenko G. Blocking variant surface glycoprotein synthesis in Trypanosoma brucei triggers a general arrest in translation initiation. PLoS One 2009; 4:e7532. [PMID: 19855834 PMCID: PMC2762041 DOI: 10.1371/journal.pone.0007532] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2009] [Accepted: 09/22/2009] [Indexed: 01/22/2023] Open
Abstract
Background The African trypanosome Trypanosoma brucei is covered with a dense layer of Variant Surface Glycoprotein (VSG), which protects it from lysis by host complement via the alternative pathway in the mammalian bloodstream. Blocking VSG synthesis by the induction of VSG RNAi triggers an unusually precise precytokinesis cell-cycle arrest. Methodology/Principal Findings Here, we characterise the cells arrested after the induction of VSG RNAi. We were able to rescue the VSG221 RNAi induced cell-cycle arrest through expression of a second different VSG (VSG117 which is not recognised by the VSG221 RNAi) from the VSG221 expression site. Metabolic labeling of the arrested cells showed that blocking VSG synthesis triggered a global translation arrest, with total protein synthesis reduced to less than 1–4% normal levels within 24 hours of induction of VSG RNAi. Analysis by electron microscopy showed that the translation arrest was coupled with rapid disassociation of ribosomes from the endoplasmic reticulum. Polysome analysis showed a drastic decrease in polysomes in the arrested cells. No major changes were found in levels of transcription, total RNA transcript levels or global amino acid concentrations in the arrested cells. Conclusions The cell-cycle arrest phenotype triggered by the induction of VSG221 RNAi is not caused by siRNA toxicity, as this arrest can be alleviated if a second different VSG is inserted downstream of the active VSG221 expression site promoter. Analysis of polysomes in the stalled cells showed that the translation arrest is mediated at the level of translation initiation rather than elongation. The cell-cycle arrest induced in the presence of a VSG synthesis block is reversible, suggesting that VSG synthesis and/or trafficking to the cell surface could be monitored during the cell-cycle as part of a specific cell-cycle checkpoint.
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Affiliation(s)
- Terry K. Smith
- Centre for Biomolecular Sciences, University of St. Andrews, Fife, Scotland, United Kingdom
| | - Nadina Vasileva
- The Peter Medawar Building for Pathogen Research, University of Oxford, Oxford, United Kingdom
| | - Eva Gluenz
- Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom
| | - Stephen Terry
- The Peter Medawar Building for Pathogen Research, University of Oxford, Oxford, United Kingdom
| | - Neil Portman
- Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom
| | - Susanne Kramer
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Mark Carrington
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | | | - Keith Gull
- Sir William Dunn School of Pathology, University of Oxford, Oxford, United Kingdom
| | - Gloria Rudenko
- The Peter Medawar Building for Pathogen Research, University of Oxford, Oxford, United Kingdom
- * E-mail:
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16
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Figueiredo LM, Janzen CJ, Cross GA. A histone methyltransferase modulates antigenic variation in African trypanosomes. PLoS Biol 2008; 6:e161. [PMID: 18597556 PMCID: PMC2443197 DOI: 10.1371/journal.pbio.0060161] [Citation(s) in RCA: 122] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2008] [Accepted: 05/23/2008] [Indexed: 11/18/2022] Open
Abstract
To evade the host immune system, several pathogens periodically change their cell-surface epitopes. In the African trypanosomes, antigenic variation is achieved by tightly regulating the expression of a multigene family encoding a large repertoire of variant surface glycoproteins (VSGs). Immune evasion relies on two important features: exposing a single type of VSG at the cell surface and periodically and very rapidly switching the expressed VSG. Transcriptional switching between resident telomeric VSG genes does not involve DNA rearrangements, and regulation is probably epigenetic. The histone methyltransferase DOT1B is a nonessential protein that trimethylates lysine 76 of histone H3 in Trypanosoma brucei. Here we report that transcriptionally silent telomeric VSGs become partially derepressed when DOT1B is deleted, whereas nontelomeric loci are unaffected. DOT1B also is involved in the kinetics of VSG switching: in ΔDOT1B cells, the transcriptional switch is so slow that cells expressing two VSGs persist for several weeks, indicating that monoallelic transcription is compromised. We conclude that DOT1B is required to maintain strict VSG silencing and to ensure rapid transcriptional VSG switching, demonstrating that epigenetics plays an important role in regulating antigenic variation in T. brucei. The surface of Trypanosoma brucei, a unicellular parasite that lives in the bloodstream of its mammalian host, is coated with glycoprotein (VSG) molecules. To evade elimination by the immune system, this parasite replaces its coat with one tailored from another glycoprotein variant. Even though there are hundreds of VSG genes in the genome, this process, called antigenic variation, works because all are silenced except for the one that encodes the current coat. In this work, we show that the chromatin-modifying enzyme DOT1B helps to epigenetically regulate the number of VSGs each parasite can have at a time at the surface and how fast each parasite can switch from one coat to another. In parasites lacking DOT1B, silent VSG genes become partially active and the switch from one VSG to another slows down, allowing two different VSGs to appear on the surface of an individual parasite at the same time. Our studies reveal the importance of epigenetics in regulating VSG genes and provide new insights toward the understanding of this unique survival device. Antigenic variation in Trypanosoma brucei relies on monoallelic expression of a multigene family. New evidence shows that a chromatin-modifying enzyme prevents simultaneous expression of different proteins at the parasite's surface.
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Affiliation(s)
- Luisa M Figueiredo
- Laboratory of Molecular Parasitology, the Rockefeller University, New York, New York, United States of America
| | | | - George A.M Cross
- Laboratory of Molecular Parasitology, the Rockefeller University, New York, New York, United States of America
- * To whom correspondence should be addressed. E-mail:
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17
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Mansfield JM, Paulnock DM. Genetic manipulation of African trypanosomes as a tool to dissect the immunobiology of infection. Parasite Immunol 2008; 30:245-53. [PMID: 18208450 DOI: 10.1111/j.1365-3024.2007.01003.x] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The variant surface glycoprotein (VSG) coat of African trypanosomes exhibits immunobiological functions distinct from its prominent role as a variant surface antigen. In order to address questions regarding immune stealth effects of VSG switch-variant coats, and the innate immune system activating effects of shed VSG substituents, several groups have genetically modified the ability of trypanosomes to express or release VSG during infection of the mammalian host. The role of mosaic surface coats expressed by VSG switch-variants (VSG double-expressors) in escaping early immune detection, and the role of VSG glycosylphosphatidylinositol (GPI) anchor substituents in regulating host immunity have been revealed, respectively, by stable co-expression of an exogenous VSG gene in trypanosomes expressing an endogenous VSG gene, and by knocking out the genetic locus for GPI-phospholipase C (PLC) that releases VSG from the membrane. Both approaches to genetic modification of African trypanosomes have suggested interesting and unexpected immunobiological effects associated with surface coat molecules.
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Affiliation(s)
- J M Mansfield
- Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA.
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18
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Lythgoe KA, Morrison LJ, Read AF, Barry JD. Parasite-intrinsic factors can explain ordered progression of trypanosome antigenic variation. Proc Natl Acad Sci U S A 2007; 104:8095-100. [PMID: 17463092 PMCID: PMC1876577 DOI: 10.1073/pnas.0606206104] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2006] [Indexed: 11/18/2022] Open
Abstract
Pathogens often persist during infection because of antigenic variation in which they evade immunity by switching between distinct surface antigen variants. A central question is how ordered appearance of variants, an important determinant of chronicity, is achieved. Theories suggest that it results directly from a complex pattern of transition connectivity between variants or indirectly from effects such as immune cross-reactivity or differential variant growth rates. Using a mathematical model based only on known infection variables, we show that order in trypanosome infections can be explained more parsimoniously by a simpler combination of two key parasite-intrinsic factors: differential activation rates of parasite variant surface glycoprotein (VSG) genes and density-dependent parasite differentiation. The model outcomes concur with empirical evidence that several variants are expressed simultaneously and that parasitaemia peaks correlate with VSG genes within distinct activation probability groups. Our findings provide a possible explanation for the enormity of the recently sequenced VSG silent archive and have important implications for field transmission.
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Affiliation(s)
- Katrina A. Lythgoe
- *Institutes of Evolution, Immunology, and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, United Kingdom; and
| | - Liam J. Morrison
- Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, University of Glasgow, 120 University Place, Glasgow G12 8TA, United Kingdom
| | - Andrew F. Read
- *Institutes of Evolution, Immunology, and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, United Kingdom; and
| | - J. David Barry
- Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, University of Glasgow, 120 University Place, Glasgow G12 8TA, United Kingdom
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19
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Glover L, Horn D. Repression of polymerase I-mediated gene expression at Trypanosoma brucei telomeres. EMBO Rep 2006; 7:93-9. [PMID: 16311518 PMCID: PMC1369228 DOI: 10.1038/sj.embor.7400575] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2005] [Revised: 09/15/2005] [Accepted: 10/10/2005] [Indexed: 02/08/2023] Open
Abstract
The African trypanosome, Trypanosoma brucei, is a flagellated pathogenic protozoan that branched early from the eukaryotic lineage. Unusually, it uses RNA polymerase I (Pol I) for mono-telomeric expression of variant surface glycoprotein (VSG) genes in bloodstream-form cells. Many other subtelomeric VSG genes are reversibly repressed, but no repressive DNA sequence has been identified in any trypanosomatid. Here, we show that artificially seeded de novo telomeres repress Pol I-dependent gene expression in mammalian bloodstream and insect life-cycle stages of T. brucei. In a telomeric VSG expression site, repression spreads further along the chromosome and this effect is specific to the bloodstream stage. We also show that de novo telomere extension is telomerase dependent and that the rate of extension correlates with the expression level of the adjacent gene. Our results show constitutive telomeric repression in T. brucei and indicate that an enhanced, developmental stage-specific repression mechanism controls antigenic variation.
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Affiliation(s)
- Lucy Glover
- London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK
| | - David Horn
- London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK
- Tel: +44 20 7927 2352; Fax: +44 20 7636 8739; E-mail:
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20
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Frank SA, Barbour AG. Within-host dynamics of antigenic variation. INFECTION GENETICS AND EVOLUTION 2006; 6:141-6. [PMID: 16461018 DOI: 10.1016/j.meegid.2004.10.005] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/01/2004] [Revised: 10/29/2004] [Accepted: 10/30/2004] [Indexed: 11/21/2022]
Abstract
Genomes of some parasites contain dozens of alternative and highly diverged surface antigens, of which only a single one is expressed in any cell. Individual cells occasionally change expression of their surface antigen, allowing them to escape immune surveillance. These switches appear to occur in a partly random way, creating a diverse set of antigenic variants. In spite of this diversity, the parasitemia develops as a series of outbreaks, in which each outbreak is dominated by relatively few antigenic types. Host-specific immunity eventually clears the dominant antigenic types, and a new outbreak follows from antigenic types that have apparently been present all along at low frequency. This pattern of sequential dominance by different antigenic types remains unexplained. We review the five most prominent theories, which have developed mainly from studies of the protozoans Trypanosoma and Plasmodium, and the bacterial spirochete Borrelia. The most promising theories depend on some combination of mechanisms to create favored connectivity pathways through the matrix of transitions between variants. Favored pathways may arise from biased switches at the molecular level of gene expression or from biases imposed by immune selection. We illustrate the concept of connectivity pathways by reanalysis of data on transitions between variants from Borrelia hermsii.
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Affiliation(s)
- Steven A Frank
- Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697-2525, USA.
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21
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Morrison LJ, Majiwa P, Read AF, Barry JD. Probabilistic order in antigenic variation of Trypanosoma brucei. Int J Parasitol 2006; 35:961-72. [PMID: 16000200 DOI: 10.1016/j.ijpara.2005.05.004] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2005] [Revised: 05/02/2005] [Accepted: 05/05/2005] [Indexed: 10/25/2022]
Abstract
Antigenic variation in African trypanosomes displays a degree of order that is usually described as 'semi-predictable' but which has not been analysed in statistical detail. It has been proposed that, during switching, the variable antigen type (VAT) being inactivated can influence which VAT is subsequently activated. Antigenic variation proceeds by the differential activation of members of the large archive of distinct variable surface glycoprotein (VSG) genes. The most popular model for ordered expression of VATs invokes differential activation probabilities for individual VSG genes, dictated in part by which of the four types of genetic locus they occupy. We have shown, in pilot experiments in cattle, correlation between the timing of appearance of VSG-specific mRNA and of lytic antibodies corresponding to seven VSGs encoded by single-copy genes. We have then determined the times of appearance of VAT-specific antibodies, as a measure of appearance of the VATs, in a statistically significant number of mouse infections (n=22). There is a surprisingly high degree of order in temporal appearance of the VATs, indicating that antigenic variation proceeds through order in the probability of activation of each VAT. In addition, for the few examples of each available, the locus type inhabited by the silent 'donor' VSG plays a significant role in determination of order. We have analysed in detail previously published data on VATs appearing in first relapse peaks, and find that the variant being switched off does not influence which one is being switched on. This differs from what has been reported for Plasmodium falciparum var antigenic variation. All these features of trypanosome antigenic variation can be explained by a one-step model in which, following an initial deactivation event, the switch process and the imposition of order early in infection arise from the inherent activation probabilities of the specific VSG being switched on.
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MESH Headings
- Animals
- Antigenic Variation/genetics
- Antigens, Protozoan/immunology
- Cattle
- Chronic Disease
- DNA, Complementary/genetics
- DNA, Protozoan/genetics
- Female
- Genes, Protozoan
- Mice
- Mice, Inbred BALB C
- Parasitemia/immunology
- Parasitemia/parasitology
- Polymerase Chain Reaction/methods
- RNA, Protozoan/genetics
- Trypanosoma brucei brucei/genetics
- Trypanosoma brucei brucei/immunology
- Trypanosoma brucei brucei/metabolism
- Trypanosomiasis, African/immunology
- Trypanosomiasis, African/parasitology
- Trypanosomiasis, Bovine/immunology
- Trypanosomiasis, Bovine/parasitology
- Variant Surface Glycoproteins, Trypanosoma/genetics
- Variant Surface Glycoproteins, Trypanosoma/metabolism
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Affiliation(s)
- Liam J Morrison
- Wellcome Centre for Molecular Parasitology, University of Glasgow, 56 Dumbarton Rd, Glasgow, G11 6NU, Scotland, UK
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22
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Abstract
African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.
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Affiliation(s)
- J M Mansfield
- Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA.
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23
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Gupta S. Parasite immune escape: new views into host-parasite interactions. Curr Opin Microbiol 2005; 8:428-33. [PMID: 15993644 DOI: 10.1016/j.mib.2005.06.011] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2005] [Accepted: 06/21/2005] [Indexed: 11/25/2022]
Abstract
For parasites of humans and animals that rely on vectors or on sexual contact for transmission, it is particularly important that infection does not to terminate before the occurrence of the crucial event that completes its lifecycle (e.g. another mosquito bite). For chronic infection to occur, it is essential that the parasite avoids clearance by the host immune system. Much progress has been made in elucidating the immunological interactions and the molecular mechanisms involved in the process of immune evasion. Mathematical models have also been invaluable in understanding these processes, particularly in the generation of new ideas about a complex form of immune evasion known as antigenic variation whereby a major target of the host immune response is varied during the course of a single infection to avoid recognition.
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Affiliation(s)
- Sunetra Gupta
- Department of Zoology, University of Oxford, Oxford, OX1 3PS, United Kingdom.
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24
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Dubois ME, Demick KP, Mansfield JM. Trypanosomes expressing a mosaic variant surface glycoprotein coat escape early detection by the immune system. Infect Immun 2005; 73:2690-7. [PMID: 15845470 PMCID: PMC1087325 DOI: 10.1128/iai.73.5.2690-2697.2005] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Host resistance to African trypanosomiasis is partially dependent on an early and strong T-independent B-cell response against the variant surface glycoprotein (VSG) coat expressed by trypanosomes. The repetitive array of surface epitopes displayed by a monotypic surface coat, in which identical VSG molecules are closely packed together in a uniform architectural display, cross-links cognate B-cell receptors and initiates T-independent B-cell activation events. However, this repetitive array of identical VSG epitopes is altered during the process of antigenic variation, when former and nascent VSG proteins are transiently expressed together in a mosaic surface coat. Thus, T-independent B-cell recognition of the trypanosome surface coat may be disrupted by the introduction of heterologous VSG molecules into the coat structure. To address this hypothesis, we transformed Trypanosoma brucei rhodesiense LouTat 1 with the 117 VSG gene from Trypanosoma brucei brucei MiTat 1.4 in order to produce VSG double expressers; coexpression of the exogenous 117 gene along with the endogenous LouTat 1 VSG gene resulted in the display of a mosaic VSG coat. Results presented here demonstrate that the host's ability to produce VSG-specific antibodies and activate B cells during early infection with VSG double expressers is compromised relative to that during infection with the parental strain, which displays a monotypic coat. These findings suggest a previously unrecognized mechanism of immune response evasion in which coat-switching trypanosomes fail to directly activate B cells until coat VSG homogeneity is achieved. This process affords an immunological advantage to trypanosomes during the process of antigenic variation.
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Affiliation(s)
- Melissa E Dubois
- Department of Bacteriology, University of Wisconsin-Madison, 1925 Willow Drive, Madison, WI 53706, USA
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25
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Engstler M, Boshart M. Cold shock and regulation of surface protein trafficking convey sensitization to inducers of stage differentiation in Trypanosoma brucei. Genes Dev 2004; 18:2798-811. [PMID: 15545633 PMCID: PMC528899 DOI: 10.1101/gad.323404] [Citation(s) in RCA: 145] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Transmission of a protozoan parasite from a vertebrate to invertebrate host is accompanied by cellular differentiation. The signals from the environment that trigger the process are poorly understood. The model parasite Trypanosoma brucei proliferates in the mammalian bloodstream and in the tsetse fly. On ingestion by the tsetse, the trypanosome undergoes a rapid differentiation that is marked by replacement of the variant surface glycoprotein (VSG) coat with GPI-anchored EP and GPEET procyclins. Here we show that a cold shock of DeltaT > 15 degrees C is sufficient to reversibly induce high-level expression of the insect stage-specific EP gene in the mammalian bloodstream stages of T. brucei. The 3'-UTR of the EP mRNA is necessary and sufficient for the increased expression. During cold shock, EP protein accumulates in the endosomal compartment in the proliferating, slender, bloodstream stage, whereas the EP is present on the plasma membrane in the quiescent, stumpy, bloodstream stage. Thus, there is a novel developmentally regulated cell surface access control mechanism for a GPI-anchored protein. In addition to inducing EP expression, cold shock results in the acquisition of sensitivity to micromolar concentrations of cis-aconitate and citrate by stumpy but not slender bloodstream forms. The cis-aconitate and citrate commit stumpy bloodstream cells to differentiation to the procyclic stage along with rapid initial proliferation. We propose a hierarchical model of three events that regulate differentiation after transmission to the tsetse: sensing the temperature change, surface access of a putative receptor, and sensing of a chemical cue.
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Affiliation(s)
- Markus Engstler
- Ludwig-Maximilians-Universität, Department Biologie I, Genetik, 80638 München, Germany
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26
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Abstract
African trypanosomes are protozoan parasites that reside in the mammalian bloodstream where they constantly confront the immune responses directed against them. They keep one-step-ahead of the immune system by continually switching from the expression of one variant surface glycoprotein (VSG) on their surface to the expression of another immunologically distinct VSG-a phenomenon called antigenic variation. About 1000 VSG genes (VSGs) and pseudo-VSGs are scattered throughout the trypanosome genome, all of which are transcriptionally silent except for one. Usually, the active VSG has been recently duplicated and translocated to one of about 20 potential bloodstream VSG expression sites (B-ESs). Each of the 20 potential B-ESs is adjacent to a chromosomal telomere, but only one B-ES is actively transcribed in a given organism. Recent evidence suggests the active B-ES is situated in an extra-nucleolar body of the nucleus where it is transcribed by RNA polymerase I. Members of another group of about 20 telomere-linked VSG expression sites (the M-ESs) are expressed only during the metacyclic stage of the parasite in its tsetse fly vector. Progress in sequencing the African trypanosome genome has led to additional insights on the organization of genes within both groups of ESs that may ultimately suggest better ways to control or eliminate this deadly pathogen.
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Affiliation(s)
- John E Donelson
- Department of Biochemistry, University of Iowa, Iowa City, IA 52242, USA.
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27
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Triggs VP, Bangs JD. Glycosylphosphatidylinositol-dependent protein trafficking in bloodstream stage Trypanosoma brucei. EUKARYOTIC CELL 2003; 2:76-83. [PMID: 12582124 PMCID: PMC141176 DOI: 10.1128/ec.2.1.76-83.2003] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
We have previously demonstrated that glycosylphosphatidylinositol (GPI) anchors strongly influence protein trafficking in the procyclic insect stage of Trypanosoma brucei (M. A. McDowell, D. A. Ransom, and J. D. Bangs, Biochem. J. 335:681-689, 1998), where GPI-minus variant surface glycoprotein (VSG) reporters have greatly reduced rates of endoplasmic reticulum (ER) exit but are ultimately secreted. We now demonstrate that GPI-dependent trafficking also occurs in pathogenic bloodstream trypanosomes. However, unlike in procyclic trypanosomes, truncated VSGs lacking C-terminal GPI-addition signals are not secreted but are mistargeted to the lysosome and degraded. Failure to export these reporters is not due to a deficiency in secretion of these cells since the N-terminal ATPase domain of the endogenous ER protein BiP is efficiently secreted from transgenic cell lines. Velocity sedimentation experiments indicate that GPI-minus VSG dimerizes similarly to wild-type VSG, suggesting that degradation is not due to ER quality control mechanisms. However, GPI-minus VSGs are fully protected from degradation by the cysteine protease inhibitor FMK024, a potent inhibitor of the major lysosomal protease trypanopain. Immunofluorescence of cells incubated with FMK024 demonstrates that GPI-minus VSG colocalizes with p67, a lysosomal marker. These data suggest that in the absence of a GPI anchor, VSG is mistargeted to the lysosome and subsequently degraded. Our findings indicate that GPI-dependent transport is a general feature of secretory trafficking in both stages of the life cycle. A working model is proposed in which GPI valence regulates progression in the secretory pathway of bloodstream stage trypanosomes.
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Affiliation(s)
- Veronica P Triggs
- Department of Biomolecular Chemistry, University of Wisconsin-Madison Medical School, Madison, Wisconsin 53706, USA
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28
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Abstract
The haploid nuclear genome of the African trypanosome, Trypanosoma brucei, is about 35 Mb and varies in size among different trypanosome isolates by as much as 25%. The nuclear DNA of this diploid organism is distributed among three size classes of chromosomes: the megabase chromosomes of which there are at least 11 pairs ranging from 1 Mb to more than 6 Mb (numbered I-XI from smallest to largest); several intermediate chromosomes of 200-900 kb and uncertain ploidy; and about 100 linear minichromosomes of 50-150 kb. Size differences of as much as four-fold can occur, both between the two homologues of a megabase chromosome pair in a specific trypanosome isolate and among chromosome pairs in different isolates. The genomic DNA sequences determined to date indicated that about 50% of the genome is coding sequence. The chromosomal telomeres possess TTAGGG repeats and many, if not all, of the telomeres of the megabase and intermediate chromosomes are linked to expression sites for genes encoding variant surface glycoproteins (VSGs). The minichromosomes serve as repositories for VSG genes since some but not all of their telomeres are linked to unexpressed VSG genes. A gene discovery program, based on sequencing the ends of cloned genomic DNA fragments, has generated more than 20 Mb of discontinuous single-pass genomic sequence data during the past year, and the complete sequences of chromosomes I and II (about 1 Mb each) in T. brucei GUTat 10.1 are currently being determined. It is anticipated that the entire genomic sequence of this organism will be known in a few years. Analysis of a test microarray of 400 cDNAs and small random genomic DNA fragments probed with RNAs from two developmental stages of T. brucei demonstrates that the microarray technology can be used to identify batteries of genes differentially expressed during the various life cycle stages of this parasite.
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Affiliation(s)
- N M El-Sayed
- The Institute for Genomic Research (TIGR), 9712 Medical Center Drive, Rockville, MD 20850, USA.
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29
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Vanhamme L, Poelvoorde P, Pays A, Tebabi P, Van Xong H, Pays E. Differential RNA elongation controls the variant surface glycoprotein gene expression sites of Trypanosoma brucei. Mol Microbiol 2000; 36:328-40. [PMID: 10792720 DOI: 10.1046/j.1365-2958.2000.01844.x] [Citation(s) in RCA: 107] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The protozoan parasite Trypanosoma brucei develops antigenic variation to escape the immune response of its host. To this end, the trypanosome genome contains multiple telomeric expression sites competent for transcription of variant surface glycoprotein genes, but as a rule only a single antigen is expressed at any time. We used reverse transcription-PCR (RT-PCR) to analyse transcription of different segments of the expression sites in different variant clones of two independent strains of T. brucei. The results indicated that RNA polymerase is installed and active at the beginning of many, if not all, expression sites simultaneously, but that a progressive arrest of RNA elongation occurs in all but one site. This defect is linked to inefficient RNA processing and RNA release from the nucleus. Therefore, functional transcription in the active site appears to depend on the selective recruitment of a RNA elongation/processing machinery.
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Affiliation(s)
- L Vanhamme
- Laboratory of Molecular Parasitology, IBMM, University of Brussels, 12, rue des Pr. Jeener et Brachet, B6041 Gosselies, Belgium
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Zitzmann N, Mehlert A, Carrouée S, Rudd PM, Ferguson MA, Carroué S. Protein structure controls the processing of the N-linked oligosaccharides and glycosylphosphatidylinositol glycans of variant surface glycoproteins expressed in bloodstream form Trypanosoma brucei. Glycobiology 2000; 10:243-9. [PMID: 10704523 DOI: 10.1093/glycob/10.3.243] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The variant surface glycoproteins (VSGs) of Trypanosoma brucei are a family of homodimeric glycoproteins that adopt similar shapes. An individual trypanosome expresses one VSG at a time in the form of a dense protective mono-layer on the plasma membrane. VSG genes are expressed from one of several polycistronic transcription units (expression sites) that contain several expression site associated genes. We used a transformed trypanosome clone expressing two different VSGs (VSG121 and VSG221) from the same expression site (that of VSG221) to establish whether the genotype of the trypanosome clone or the VSG structure itself controls VSG N-linked oligosaccharide and GPI anchor glycan processing. In-gel release and fluorescent labeling of N-linked oligosaccharides and on-blot fluorescent labeling and release of GPI anchor glycans were employed to compare the carbohydrate structures of VSG121 and VSG221 when expressed individually in wild-type trypanosome clones and when expressed together in the transformed trypanosome clone. The data indicate that the genotype of the trypanosome clone has no effect on the N-linked oligosaccharide structures present on a given VSG variant and only a minor effect on the GPI anchor glycans. The latter is most likely an effect of changes in inter-VSG packing when two VGSs are expressed simultaneously. Thus, N-linked oligosaccharide and GPI anchor processing enzymes appear to be constitutively expressed in bloodstream form African trypanosomes and the tertiary and quaternary structures of the VSG homodimers appear to dictate the processing and glycoform microheterogeneity of surface-expressed VSGs.
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Affiliation(s)
- N Zitzmann
- Division of Molecular Parasitology and Biological Chemistry, Department of Biochemistry, The Wellcome Trust Building, University of Dundee, Dundee DD1 5EH, UK
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Cardoso De Almeida ML, Geuskens M, Pays E. Cell lysis induces redistribution of the GPI-anchored variant surface glycoprotein on both faces of the plasma membrane of Trypanosoma brucei. J Cell Sci 1999; 112 ( Pt 23):4461-73. [PMID: 10564663 DOI: 10.1242/jcs.112.23.4461] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
African trypanosomes are coated by 10 million copies of a single variant specific glycoprotein (VSG) which are anchored in the plasma membrane by glycosylphosphatidylinositol (GPI). A GPI-specific phospholipase C (GPI-PLC) triggers fast VSG release upon cell lysis but in vivo it is safely controlled and topologically concealed from its substrate by being intracellular. One enigmatic aspect of GPI-PLC action therefore consists of how it could gain access to the VSG in the exoplasmic leaflet of the membrane. The data presented herewith disclose an unexpected possible solution for this puzzle: upon cell rupture the VSG invades the cytoplasmic face of the plasma membrane which thus becomes double coated. This unusual VSG rearrangement was stable in ruptured plasma membrane from GPI-PLC null mutant trypanosomes but transiently preceded VSG release in wild-type parasites. The formation of double coat membrane (DCM) was independent of the presence or activation of GPI-PLC, occurred both at 4 degrees C and 30 degrees C and was unaffected by the classical inhibitor of VSG release, p-choromercuryphenylsulfonic acid (PCM). DCMs conserved the same coat thickness and association with subpellicular microtubules as in intact cells and were prone to form vesicles following gradual detachment of the latter. Our data also demonstrate that: (i) GPI-PLC expressed by one trypanosome only targets its own plasma membrane, being unable to release VSG of another parasite; (ii) DCMs concomitantly formed from trypanosomes expressing different VSGs do not intermix, an indication that DCM might be refractory to membrane fusion.
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Affiliation(s)
- M L Cardoso De Almeida
- Laboratory of Molecular Parasitology, Free University of Brussels, rue des Chevaux, B-1640 Rhode-St-Genèse, Belgium. mlcalmei@alize. ulb.ac.be
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Rudenko G. Genes involved in phenotypic and antigenic variation in African trypanosomes and malaria. Curr Opin Microbiol 1999; 2:651-6. [PMID: 10607631 DOI: 10.1016/s1369-5274(99)00039-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Large polymorphic gene families that are involved in clonal phenotypic variation have been identified in both African trypanosomes and malaria parasites. Many of these gene families are necessary for host adaptation, allowing the parasite to infect different species of host or types of host cells. In many cases, switching between these functionally variable proteins also results in antigenic variation.
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Affiliation(s)
- G Rudenko
- Department of Zoology, Wellcome Trust Centre for the Epidemiology of Infectious Disease, University of Oxford, Oxford, OX1 3FY, UK.
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33
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Frank SA. A model for the sequential dominance of antigenic variants in African trypanosome infections. Proc Biol Sci 1999; 266:1397-401. [PMID: 10445294 PMCID: PMC1690072 DOI: 10.1098/rspb.1999.0793] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Trypanosoma brucei infects various domestic and wild mammals in equatorial Africa. The parasite's genome contains several hundred alternative and highly diverged surface antigens, of which only a single one is expressed in any cell. Individual cells occasionally change expression of their surface antigen, allowing them to escape immune surveillance. These switches appear to occur in a partly random way, creating a diverse set of antigenic variants. In spite of this diversity, the parasitaemia develops as a series of outbreaks, each outbreak dominated by relatively few antigenic types. Host-specific immunity eventually clears the dominant antigenic types and a new outbreak follows from antigenic types that have apparently been present all along at low frequency. This pattern of sequential dominance by different antigenic types remains unexplained. I use a mathematical model of parasitaemia and host immunity to show that small variations in the rate at which each type switches to other types can explain the observations. My model shows that randomly chosen switch rates do not provide sufficiently ordered parasitaemias to match the observations. Instead, minor modifications of switch rates by natural selection are required to develop a sequence of ordered parasitaemias.
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Affiliation(s)
- S A Frank
- Department of Ecology and Evolutionary Biology, University of California, Irvine 92697-2525, USA
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34
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Alarcon CM, Pedram M, Donelson JE. Leaky transcription of variant surface glycoprotein gene expression sites in bloodstream african trypanosomes. J Biol Chem 1999; 274:16884-93. [PMID: 10358034 DOI: 10.1074/jbc.274.24.16884] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Trypanosoma brucei undergoes antigenic variation by periodically switching the expression of its variant surface glycoprotein (VSG) genes (vsg) among an estimated 20-40 telomere-linked expression sites (ES), only one of which is fully active at a given time. We found that in bloodstream trypanosomes one ES is transcribed at a high level and other ESs are expressed at low levels, resulting in organisms containing one abundant VSG mRNA and several rare VSG RNAs. Some of the rare VSG mRNAs come from monocistronic ESs in which the promoters are situated about 2 kilobases upstream of the vsg, in contrast to the polycistronic ESs in which the promoters are located 45-60 kilobases upstream of the vsg. The monocistronic ES containing the MVAT4 vsg does not include the ES-associated genes (esag) that occur between the promoter and the vsg in polycistronic ESs. However, bloodstream MVAT4 trypanosomes contain the mRNAs for many different ESAGs 6 and 7 (transferrin receptors), suggesting that polycistronic ESs are partially active in this clone. To explain these findings, we propose a model in which both mono- and polycistronic ESs are controlled by a similar mechanism throughout the parasite's life cycle. Certain VSGs are preferentially expressed in metacyclic versus bloodstream stages as a result of differences in ESAG expression and the proximity of the promoters to the vsg and telomere.
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Affiliation(s)
- C M Alarcon
- Department of Biochemistry, University of Iowa, Iowa City, Iowa 52242, USA
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35
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Abstract
Antigenic variation in Trypanosoma brucei is a highly sophisticated survival strategy involving switching between the transcription of one of an estimated thousand variant surface glycoprotein (VSG) genes. Switching involves either transcriptional control, resulting in switching between different VSG expression sites; or DNA rearrangement events slotting previously inactive VSG genes into an active VSG expression site. In recent years, considerable progress has been made in techniques allowing us to genetically modify infective bloodstream form trypanosomes. This is allowing us to reengineer VSG expression sites, and look at the effect on the mechanisms subsequently used for antigenic variation. We can now begin a dissection of a highly complicated survival strategy mediated by many different mechanisms operating simultaneously.
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Affiliation(s)
- G Rudenko
- Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, University of Oxford, Oxford, England.
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Hoek M, Xu H, Cross GA. Trypanosoma brucei: generation of specific antisera to recombinant variant surface glycoproteins. Exp Parasitol 1999; 91:199-202. [PMID: 9990349 DOI: 10.1006/expr.1998.4369] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
MESH Headings
- Animals
- Antibodies, Protozoan/biosynthesis
- Antibodies, Protozoan/immunology
- Blotting, Western
- Cross Reactions
- DNA Primers
- DNA, Protozoan/chemistry
- Electrophoresis, Polyacrylamide Gel
- Fluorescent Antibody Technique, Indirect
- Gene Expression Regulation
- Immune Sera/biosynthesis
- Immune Sera/immunology
- Immunodominant Epitopes/chemistry
- Immunodominant Epitopes/immunology
- Immunodominant Epitopes/isolation & purification
- Polymerase Chain Reaction
- Recombinant Proteins/chemistry
- Recombinant Proteins/immunology
- Recombinant Proteins/isolation & purification
- Sensitivity and Specificity
- Trypanosoma brucei brucei/immunology
- Trypanosomiasis, African/diagnosis
- Trypanosomiasis, African/immunology
- Variant Surface Glycoproteins, Trypanosoma/chemistry
- Variant Surface Glycoproteins, Trypanosoma/immunology
- Variant Surface Glycoproteins, Trypanosoma/isolation & purification
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Affiliation(s)
- M Hoek
- Laboratory of Molecular Parasitology, Rockefeller University, New York, New York 10021, USA
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37
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Abstract
Recent advances in transfection technology have been exploited to address fundamental questions relating to secretory trafficking in African trypanosomes. Targeted gene disruptions and ectopic expression of the major stage-specific surface proteins have provided unexpected insights into both the function and assembly of the essential parasite surface coats. A growing list of novel secretory cargo molecules, as well as advances in the characterization of trypanosomal secretory machinery, provide a unique model system for the study of eukaryotic secretory processes.
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Affiliation(s)
- J D Bangs
- Department of Medical Microbiology, University of Wisconsin-Madison, 1300 University Avenue, Madison, WI 53706, USA.
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Rudenko G, Cross M, Borst P. Changing the end: antigenic variation orchestrated at the telomeres of African trypanosomes. Trends Microbiol 1998; 6:113-6. [PMID: 9582937 DOI: 10.1016/s0966-842x(97)01200-6] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
African trypanosomes express the gene encoding their variant surface glycoprotein (VSG) surface coat from one of many telomeric expression sites. This genomic location at chromosome ends not only allows easy exchange of VSG gene cassettes using various mechanisms of DNA recombination but also appears to play a role in VSG gene expression site control.
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Affiliation(s)
- G Rudenko
- Dept of Molecular Biology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
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39
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Affiliation(s)
- E Pays
- Department of Molecular Biology, University of Brussels 67, Rhode St Genèse, Belgium.
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40
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Turner CM. Trypanosomes with multicoloured coats. ACTA ACUST UNITED AC 1997; 13:247-8. [PMID: 15275060 DOI: 10.1016/s0169-4758(97)01071-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Affiliation(s)
- C M Turner
- Division of Infection and Immunity, IBLS, Joseph Black Building, University of Glasgow, Glasgow, UK.
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Davies KP, Carruthers VB, Cross GA. Manipulation of the vsg co-transposed region increases expression-site switching in Trypanosoma brucei. Mol Biochem Parasitol 1997; 86:163-77. [PMID: 9200123 DOI: 10.1016/s0166-6851(97)02853-3] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Disruption of a region of DNA in Trypanosoma brucei immediately upstream of the expressed telomere-proximal variant surface glycoprotein gene (vsg), known as the co-transposed region (CTR), can cause a dramatic increase in the rate at which the active expression site (ES) is switched off and a new ES is switched on. Deletion of most of the CTR in two ESs caused a greater than 100-fold increase in the rate of ES switching, to about 1.3 x 10(-4) per generation. A more dramatic effect was observed when the entire CTR and the 5' coding region of the expressed vsg221 were deleted. In this case a new ES was activated within a few cell divisions. This switch also occurred in cell lines where a second vsg had been inserted into the ES, prior to CTR deletion. These cell lines, which stably co-expressed the inserted and endogenous Vsgs, in equal amounts, did not differ from the wild-type in growth rate or switching frequency, suggesting that simultaneous expression of two Vsgs has no intrinsic effect. CTR deletion did not disturb the inserted vsg117. We tentatively conclude that it was not the disruption of the vsg221 in itself that destabilized the ES. All of the observed switches occurred without additional detectable DNA rearrangements in the switched ES. Deletion of the 70-bp repeats and/or a vsg pseudogene upstream of the CTR did not affect ES stability. Several speculative interpretations of these observation are offered, the most intriguing of which is that the CTR plays some role in modulating chromatin conformation at an ES.
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Affiliation(s)
- K P Davies
- Laboratory of Molecular Parasitology, Rockefeller University, New York, NY 10021, USA
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42
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Abstract
Trypanosoma brucei can undergo antigenic variation by switching between distinct telomeric variant surface glycoprotein gene (vsg) expression sites (ESs) or by replacing the active vsg. DNA rearrangements have often been associated with ES switching, but it is unclear if such rearrangements are necessary or whether ES inactivation always accompanies ES activation. To explore these issues, we derived ten independent clones, from the same parent, that had undergone a similar vsg activation event. This was achieved in the absence of an immune response, in vitro, using cells with selectable markers integrated into an ES. Nine of the ten clones had undergone ES switching. Such heritable changes in transcription state occurred at a frequency of approximately 6 x 10(-7). Comparison of switched and un-switched clones highlighted the dynamic nature of T. brucei telomeres, but changes in telomere length were not specifically associated with ES switching. Mapping within and beyond the ESs revealed no detectable DNA rearrangements, indicating that rearrangements are not necessary for ES activation/inactivation. Examination of individual cells indicated that ES activation consistently accompanied inactivation of the previously active ES. In some cases, however, we found cells that appeared to have efficiently established the switched state but which subsequently, at a frequency of approximately 2 x 10(-3), generated cells expressing both pre- and post-switch vsgs. These results show that ES activation/inactivation is usually a coupled process but that cells can inherit a propensity to uncouple these events.
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Affiliation(s)
- D Horn
- Laboratory of Molecular Parasitology, Rockefeller University, New York, NY 10021-6399, USA
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