scholarly journals Keeping Balance Between Genetic Stability and Plasticity at the Telomere and Subtelomere of Trypanosoma brucei

2021 ◽  
Vol 9 ◽  
Author(s):  
Bibo Li
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Microbial Pathogens ◽  
Yeast Cells ◽  
Genome Integrity ◽  
Environmental Cues ◽  
Telomere Protein ◽  
Nucleoprotein Complexes ◽  
Underlying Mechanisms ◽  
Kinetoplastid Parasite

Telomeres, the nucleoprotein complexes at chromosome ends, are well-known for their essential roles in genome integrity and chromosome stability. Yet, telomeres and subtelomeres are frequently less stable than chromosome internal regions. Many subtelomeric genes are important for responding to environmental cues, and subtelomeric instability can facilitate organismal adaptation to extracellular changes, which is a common theme in a number of microbial pathogens. In this review, I will focus on the delicate and important balance between stability and plasticity at telomeres and subtelomeres of a kinetoplastid parasite, Trypanosoma brucei, which causes human African trypanosomiasis and undergoes antigenic variation to evade the host immune response. I will summarize the current understanding about T. brucei telomere protein complex, the telomeric transcript, and telomeric R-loops, focusing on their roles in maintaining telomere and subtelomere stability and integrity. The similarities and differences in functions and underlying mechanisms of T. brucei telomere factors will be compared with those in human and yeast cells.

scholarly journals Trypanosoma brucei PolIE suppresses telomere recombination

2021 ◽  
Author(s):  
Maiko Tonini ◽  
M. A. G. Rabbani ◽  
Marjia Afrin ◽  
Bibo Li
Keyword(s):  
Trypanosoma Brucei ◽  
Immune Evasion ◽  
Human African Trypanosomiasis ◽  
Antigenic Variation ◽  
Telomere Maintenance ◽  
Genome Integrity ◽  
Major Player ◽  
Elevated Rate ◽  
Major Surface ◽  
Telomere Structure

Telomeres are essential for genome integrity and stability. In T. brucei that causes human African trypanosomiasis, the telomere structure and telomere proteins also influence the virulence of the parasite, as its major surface antigen involved in the host immune evasion is expressed exclusively from loci immediately upstream of the telomere repeats. However, telomere maintenance mechanisms are still unclear except that telomerase-mediated telomere synthesis is a major player. We now identify PolIE as an intrinsic telomere complex component. We find that depletion of PolIE leads to an increased amount of telomere/subtelomere DNA damage, an elevated rate of antigenic variation, and an increased amount of telomere T-circles and C-circles, indicating that PolIE suppresses telomere recombination and helps maintain telomere integrity. In addition, we observe much longer telomere G-rich 3 prime overhangs in PolIE-depleted cells, which is not dependent on telomerase. Furthermore, the level of telomere DNA synthesis is slightly increased in PolIE-depleted cells, which is dependent on telomerase. Therefore, we identify PolIE as a major player for telomere maintenance in T. brucei.

scholarly journals A DOT1B/Ribonuclease H2 protein complex is involved in R-loop processing, genomic integrity and antigenic variation in Trypanosoma brucei

2020 ◽  
Author(s):  
Nicole Eisenhuth ◽  
Tim Vellmer ◽  
Falk Butter ◽  
Christian J. Janzen
Keyword(s):  
Trypanosoma Brucei ◽  
Transcriptional Activation ◽  
Antigenic Variation ◽  
Genome Integrity ◽  
Large Reservoir ◽  
Regulatory Processes ◽  
Associated Proteins ◽  
Expression Site ◽  
Protective Variant ◽  
Regulatory Functions

ABSTRACTThe parasite Trypanosoma brucei periodically changes the expression of protective variant surface glycoproteins (VSGs) to evade its host’s immune system in a process known as antigenic variation. One route to change VSG expression is the transcriptional activation of a previously silent VSG expression site (ES), a subtelomeric region containing the VSG genes. Homologous recombination of a different VSG from a large reservoir into the active ES represents another route. The conserved histone methyltransferase DOT1B is involved in transcriptional silencing of inactive ES and influences ES switching kinetics. The molecular machinery that enables DOT1B to execute these regulatory functions remains elusive, however. To better understand DOT1B-mediated regulatory processes, we purified DOT1B-associated proteins using complementary biochemical approaches. We identified several novel DOT1B-interactors. One of these was the Ribonuclease H2 complex, previously shown to resolve RNA-DNA hybrids, maintain genome integrity, and play a role in antigenic variation. Our study revealed that DOT1B depletion results in an increase in RNA-DNA hybrids, accumulation of DNA damage and recombination-based ES switching events. Surprisingly, a similar pattern of VSG deregulation was observed in Ribonuclease H2 mutants. We propose that both proteins act together in resolving R-loops to ensure genome integrity and contribute to the tightly-regulated process of antigenic variation.

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

2015 ◽  
Vol 14 (3) ◽  
pp. 196-205 ◽  
Author(s):  
Bibo Li
Keyword(s):  
Trypanosoma Brucei ◽  
Surface Antigen ◽  
Antigenic Variation ◽  
Dna Recombination ◽  
Double Strand Breaks ◽  
Microbial Pathogens ◽  
Dna Double Strand Breaks ◽  
Content Type ◽  
Strand Breaks ◽  
Major Surface

ABSTRACTHuman-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, andBorrelia burgdorferi(a bacterium), which causes Lyme disease, also express their major surface antigens from loci next to the telomere. Except forPlasmodium, 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 inT. brucei, and our knowledge of functions of DNA recombination in antigenic variation has been greatly advanced. VSG is the major surface antigen inT. brucei. In subtelomeric VSG expression sites (ESs),VSGgenes 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 inT. 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.

scholarly journals Developmental competence and antigen switch frequency can be uncoupled in Trypanosoma brucei

2019 ◽  
Vol 116 (45) ◽  
pp. 22774-22782 ◽  
Author(s):  
Kirsty R. McWilliam ◽  
Alasdair Ivens ◽  
Liam J. Morrison ◽  
Monica R. Mugnier ◽  
Keith R. Matthews
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Experimental Studies ◽  
Developmental Competence ◽  
Parasite Development ◽  
Mammalian Host ◽  
Mechanical Transmission ◽  
African Trypanosomes ◽  
Infection Dynamic ◽  
Developmental Capacity

African trypanosomes use an extreme form of antigenic variation to evade host immunity, involving the switching of expressed variant surface glycoproteins by a stochastic and parasite-intrinsic process. Parasite development in the mammalian host is another feature of the infection dynamic, with trypanosomes undergoing quorum sensing (QS)-dependent differentiation between proliferative slender forms and arrested, transmissible, stumpy forms. Longstanding experimental studies have suggested that the frequency of antigenic variation and transmissibility may be linked, antigen switching being higher in developmentally competent, fly-transmissible, parasites (“pleomorphs”) than in serially passaged “monomorphic” lines that cannot transmit through flies. Here, we have directly tested this tenet of the infection dynamic by using 2 experimental systems to reduce pleomorphism. Firstly, lines were generated that inducibly lose developmental capacity through RNAi-mediated silencing of the QS signaling machinery (“inducible monomorphs”). Secondly, de novo lines were derived that have lost the capacity for stumpy formation by serial passage (“selected monomorphs”) and analyzed for their antigenic variation in comparison to isogenic preselected populations. Analysis of both inducible and selected monomorphs has established that antigen switch frequency and developmental capacity are independently selected traits. This generates the potential for diverse infection dynamics in different parasite populations where the rate of antigenic switching and transmission competence are uncoupled. Further, this may support the evolution, maintenance, and spread of important trypanosome variants such as Trypanosoma brucei evansi that exploit mechanical transmission.

scholarly journals Antigenic variation in clones of Trypanosoma brucei grown in immune-deficient mice.

1985 ◽  
Vol 47 (3) ◽  
pp. 684-690 ◽  
Author(s):  
P J Myler ◽  
A L Allen ◽  
N Agabian ◽  
K Stuart
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Deficient Mice

scholarly journals Ku Heterodimer-Independent End Joining in Trypanosoma brucei Cell Extracts Relies upon Sequence Microhomology

2007 ◽  
Vol 6 (10) ◽  
pp. 1773-1781 ◽  
Author(s):  
Peter Burton ◽  
David J. McBride ◽  
Jonathan M. Wilkes ◽  
J. David Barry ◽  
Richard McCulloch
Keyword(s):  
Homologous Recombination ◽  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Bacterial Species ◽  
Nonhomologous End Joining ◽  
Dna Double Strand Breaks ◽  
End Joining ◽  
Strand Breaks ◽  
Cell Extracts ◽  
Ligase Iv

ABSTRACT DNA double-strand breaks (DSBs) are repaired primarily by two distinct pathways: homologous recombination and nonhomologous end joining (NHEJ). NHEJ has been found in all eukaryotes examined to date and has been described recently for some bacterial species, illustrating its ancestry. Trypanosoma brucei is a divergent eukaryotic protist that evades host immunity by antigenic variation, a process in which homologous recombination plays a crucial function. While homologous recombination has been examined in some detail in T. brucei, little work has been done to examine what other DSB repair pathways the parasite utilizes. Here we show that T. brucei cell extracts support the end joining of linear DNA molecules. These reactions are independent of the Ku heterodimer, indicating that they are distinct from NHEJ, and are guided by sequence microhomology. We also demonstrate bioinformatically that T. brucei, in common with other kinetoplastids, does not encode recognizable homologues of DNA ligase IV or XRCC4, suggesting that NHEJ is either absent or mechanistically diverged in these pathogens.

Uncovering bleomycin-induced genomic alterations and underlying mechanisms in yeast

2021 ◽  
Author(s):  
Dao-Qiong Zheng ◽  
Yu-Ting Wang ◽  
Ying-Xuan Zhu ◽  
Huan Sheng ◽  
Ke-Jing Li ◽  
...  
Keyword(s):  
Genome Sequencing ◽  
Mutation Rate ◽  
Chromosomal Rearrangements ◽  
Yeast Cells ◽  
Antitumor Antibiotic ◽  
Induced Mutations ◽  
Genomic Alterations ◽  
Translesion Dna Synthesis ◽  
Elevated Rate ◽  
Underlying Mechanisms

Bleomycin (BLM) is a widely used chemotherapeutic drug. BLM-treated cells showed an elevated rate of mutations, but the underlying mechanisms remained unclear. In this study, the global genomic alterations in BLM-treated cells were explored in the yeast Saccharomyces cerevisiae . Using genetic assay and whole-genome sequencing, we found that the mutation rate could be greatly elevated in S. cerevisiae cells that underwent Zeocin TM (a BLM member) treatment. One-base deletion and T to G substitution at the 5’-GT-3’ motif was the most striking signature of Zeocin TM -induced mutations. This was mainly the result of translesion DNA synthesis involving Rev1 and polymerase ζ. Zeocin TM treatment led to the frequent loss of heterozygosity and chromosomal rearrangements in the diploid strains. The breakpoints of recombination events were significantly associated with certain chromosomal elements. Lastly, we identified multiple genomic alterations that contributed to BLM resistance in the Zeocin TM -treated mutants. Overall, this study provides new insights into the genotoxicity and evolutional effects of BLM. Importance Bleomycin is an antitumor antibiotic that can mutate genomic DNA. Using yeast models in combination with genome sequencing, the mutational signatures of Zeocin TM (a member of the bleomycin family) are disclosed. Translesion-synthesis polymerases are crucial for the viability of Zeocin TM -treated yeast cells at the sacrifice of a higher mutation rate. We also confirmed that multiple genomic alterations were associated with the improved resistance to Zeocin TM , providing novel insights into how bleomycin resistance is developed in cells.

A gene from the variant surface glycoprotein expression site encodes one of several transmembrane adenylate cyclases located on the flagellum of Trypanosoma brucei

1992 ◽  
Vol 12 (3) ◽  
pp. 1218-1225
Author(s):  
P Paindavoine ◽  
S Rolin ◽  
S Van Assel ◽  
M Geuskens ◽  
J C Jauniaux ◽  
...  
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Trypanosoma Brucei ◽  
Membrane Fraction ◽  
Transcription Unit ◽  
Bloodstream Form ◽  
Surface Glycoprotein ◽  
Yeast Cells ◽  
Variant Surface Glycoprotein ◽  
Polycistronic Transcription ◽  
Expression Site

The bloodstream form of Trypanosoma brucei contains transcripts of at least four genes showing partial sequence homology to the genes for eucaryotic adenylate and guanylate cyclases (S. Alexandre, P. Paindavoine, P. Tebabi, A. Pays, S. Halleux, M. Steinert, and E. Pays, Mol. Biochem. Parasitol. 43:279-288, 1990). One of these genes, termed ESAG 4, belongs to the polycistronic transcription unit of the variant surface glycoprotein (VSG) gene. Whereas ESAG 4 is transcribed only in the bloodstream form of the parasite, the three other genes, GRESAG 4.1, 4.2, and 4.3, are also expressed in procyclic (insect) forms. These genes differ primarily in a region presumed to encode a large extracellular domain. We show here that ESAG 4-related glycoproteins of about 150 kDa can be found in the trypanosome membrane, that they are detected, by light and electron gold immunocytochemistry, only at the surface of the flagellum, and that the products of at least two of these genes, ESAG 4 and GRESAG 4.1, can complement a Saccharomyces cerevisiae mutant for adenylate cyclase. The recombinant cyclases are associated with the yeast membrane fraction and differ with respect to their activation by calcium: while the GRESAG 4.1 and yeast cyclases are inhibited by calcium, the ESAG 4 cyclase is stimulated. ESAG 4 thus most probably encodes the calcium-activated cyclase that has been found to be expressed only in the bloodstream form of T. brucei (S. Rolin, S. Halleux, J. Van Sande, J. E. Dumont, E. Pays, and M. Steinert. Exp. Parasitol. 71:350-352, 1990). Our data suggest that the trypanosome cyclases are not properly regulated in yeast cells.

scholarly journals Escaping the immune system by DNA repair and recombination in African trypanosomes

Open Biology ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 190182 ◽  
Author(s):  
Núria Sima ◽  
Emilia Jane McLaughlin ◽  
Sebastian Hutchinson ◽  
Lucy Glover
Keyword(s):  
Immune Response ◽  
Dna Repair ◽  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Surface Glycoprotein ◽  
Variant Surface Glycoprotein ◽  
Surface Coat ◽  
African Trypanosomes ◽  
Active Expression ◽  
Expression Site

African trypanosomes escape the mammalian immune response by antigenic variation—the periodic exchange of one surface coat protein, in Trypanosoma brucei the variant surface glycoprotein (VSG), for an immunologically distinct one. VSG transcription is monoallelic, with only one VSG being expressed at a time from a specialized locus, known as an expression site. VSG switching is a predominantly recombination-driven process that allows VSG sequences to be recombined into the active expression site either replacing the currently active VSG or generating a ‘new’ VSG by segmental gene conversion. In this review, we describe what is known about the factors that influence this process, focusing specifically on DNA repair and recombination.

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