Nuclear architecture, genome and chromatin organisation in Trypanosoma brucei

Comparative molecular cell biology of phototrophic euglenids and parasitic trypanosomatids sheds light on the ancestor of Euglenozoa

Parasitic trypanosomatids and phototrophic euglenids are among the most extensively studied euglenozoans. The phototrophic euglenid lineage arose relatively recently through secondary endosymbiosis between a phagotrophic euglenid and a prasinophyte green alga that evolved into the euglenid secondary chloroplast. The parasitic trypanosomatids (i.e. Trypanosoma spp. and Leishmania spp.) and the freshwater phototrophic euglenids (i.e. Euglena gracilis) are the most evolutionary distant lineages in the Euglenozoa phylogenetic tree. The molecular and cell biological traits they share can thus be considered as ancestral traits originating in the common euglenozoan ancestor. These euglenozoan ancestral traits include common mitochondrial presequence motifs, respiratory chain complexes containing various unique subunits, a unique ATP synthase structure, the absence of mitochondria-encoded transfer RNAs (tRNAs), a nucleus with a centrally positioned nucleolus, closed mitosis without dissolution of the nuclear membrane and nucleoli, a nuclear genome containing the unusual 'J' base (β-D-glucosyl-hydroxymethyluracil), processing of nucleus-encoded precursor messenger RNAs (pre-mRNAs) via spliced-leader RNA (SL-RNA) trans-splicing, post-transcriptional gene silencing by the RNA interference (RNAi) pathway and the absence of transcriptional regulation of nuclear gene expression. Mitochondrial uridine insertion/deletion RNA editing directed by guide RNAs (gRNAs) evolved in the ancestor of the kinetoplastid lineage. The evolutionary origin of other molecular features known to be present only in either kinetoplastids (i.e. polycistronic transcripts, compaction of nuclear genomes) or euglenids (i.e. monocistronic transcripts, huge genomes, many nuclear cis-spliced introns, polyproteins) is unclear.

African trypanosomes

African trypanosomes cause human African trypanosomiasis and animal African trypanosomiasis. They are transmitted by tsetse flies in sub-Saharan Africa. Although most famous for their mechanisms of immune evasion by antigenic variation, there have been recent important studies that illuminate important aspects of the biology of these parasites both in their mammalian host and during passage through their tsetse fly vector. This Primer overviews current research themes focused on these parasites and discusses how these biological insights and the development of new technologies to interrogate gene function are being used in the search for new approaches to control the parasite. The new insights into the biology of trypanosomes in their host and vector highlight that we are in a ‘golden age’ of discovery for these fascinating parasites.

Conservation and Variation in Strategies for DNA Replication of Kinetoplastid Nuclear Genomes

Introduction:

Understanding how the nuclear genome of kinetoplastid parasites is replicated received experimental stimulus from sequencing of the Leishmania major, Trypanosoma brucei and Trypanosoma cruzi genomes around 10 years ago. Gene annotations suggested key players in DNA replication initiation could not be found in these organisms, despite considerable conservation amongst characterised eukaryotes. Initial studies that indicated trypanosomatids might possess an archaeal-like Origin Recognition Complex (ORC), composed of only a single factor termed ORC1/CDC6, have been supplanted by the more recent identification of an ORC in T. brucei. However, the constituent subunits of T. brucei ORC are highly diverged relative to other eukaryotic ORCs and the activity of the complex appears subject to novel, positive regulation. The availability of whole genome sequences has also allowed the deployment of genome-wide strategies to map DNA replication dynamics, to date in T. brucei and Leishmania. ORC1/CDC6 binding and function in T. brucei displays pronounced overlap with the unconventional organisation of gene expression in the genome. Moreover, mapping of sites of replication initiation suggests pronounced differences in replication dynamics in Leishmania relative to T. brucei.

Conclusion:

Here we discuss what implications these emerging data may have for parasite and eukaryotic biology of DNA replication.

Modulation of Aneuploidy in Leishmania donovani during Adaptation to Different In Vitro and In Vivo Environments and Its Impact on Gene Expression

Aneuploidy is usually deleterious in multicellular organisms but appears to be tolerated and potentially beneficial in unicellular organisms, including pathogens. Leishmania, a major protozoan parasite, is emerging as a new model for aneuploidy, since in vitro-cultivated strains are highly aneuploid, with interstrain diversity and intrastrain mosaicism. The alternation of two life stages in different environments (extracellular promastigotes and intracellular amastigotes) offers a unique opportunity to study the impact of environment on aneuploidy and gene expression. We sequenced the whole genomes and transcriptomes of Leishmania donovani strains throughout their adaptation to in vivo conditions mimicking natural vertebrate and invertebrate host environments. The nucleotide sequences were almost unchanged within a strain, in contrast to highly variable aneuploidy. Although high in promastigotes in vitro, aneuploidy dropped significantly in hamster amastigotes, in a progressive and strain-specific manner, accompanied by the emergence of new polysomies. After a passage through a sand fly, smaller yet consistent karyotype changes were detected. Changes in chromosome copy numbers were correlated with the corresponding transcript levels, but additional aneuploidy-independent regulation of gene expression was observed. This affected stage-specific gene expression, downregulation of the entire chromosome 31, and upregulation of gene arrays on chromosomes 5 and 8. Aneuploidy changes in Leishmania are probably adaptive and exploited to modulate the dosage and expression of specific genes; they are well tolerated, but additional mechanisms may exist to regulate the transcript levels of other genes located on aneuploid chromosomes. Our model should allow studies of the impact of aneuploidy on molecular adaptations and cellular fitness.IMPORTANCE Aneuploidy is usually detrimental in multicellular organisms, but in several microorganisms, it can be tolerated and even beneficial. Leishmania-a protozoan parasite that kills more than 30,000 people each year-is emerging as a new model for aneuploidy studies, as unexpectedly high levels of aneuploidy are found in clinical isolates. Leishmania lacks classical regulation of transcription at initiation through promoters, so aneuploidy could represent a major adaptive strategy of this parasite to modulate gene dosage in response to stressful environments. For the first time, we document the dynamics of aneuploidy throughout the life cycle of the parasite, in vitro and in vivo We show its adaptive impact on transcription and its interaction with regulation. Besides offering a new model for aneuploidy studies, we show that further genomic studies should be done directly in clinical samples without parasite isolation and that adequate methods should be developed for this.

Single-molecule analysis of DNA replication reveals novel features in the divergent eukaryotes Leishmania and Trypanosoma brucei versus mammalian cells

Leishmania and Trypanosoma are unicellular parasites that possess markedly original biological features as compared to other eukaryotes. The Leishmania genome displays a constitutive 'mosaic aneuploidy', whereas in Trypanosoma brucei, the megabase-sized chromosomes are diploid. We accurately analysed DNA replication parameters in three Leishmania species and Trypanosoma brucei as well as mouse embryonic fibroblasts (MEF). Active replication origins were visualized at the single molecule level using DNA molecular combing. More than one active origin was found on most DNA fibres, showing that the chromosomes are replicated from multiple origins. Inter-origin distances (IODs) were measured and found very large in trypanosomatids: the mean IOD was 160 kb in T. brucei and 226 kb in L. mexicana. Moreover, the progression of replication forks was faster than in any other eukaryote analyzed so far (mean velocity 1.9 kb/min in T. brucei and 2.4-2.6 kb/min in Leishmania). The estimated total number of active DNA replication origins in trypanosomatids is ~170. Finally, 14.4% of unidirectional replication forks were observed in T. brucei, in contrast to 1.5-1.7% in Leishmania and 4% in MEF cells. The biological significance of these original features is discussed.

Mono-allelic VSG expression by RNA polymerase I in Trypanosoma brucei: expression site control from both ends?

Trypanosoma brucei is a vector borne, lethal protistan parasite of humans and livestock in sub-Saharan Africa. Antigenic variation of its cell surface coat enables the parasite to evade adaptive immune responses and to live freely in the blood of its mammalian hosts. The coat consists of ten million copies of variant surface glycoprotein (VSG) that is expressed from a single VSG gene, drawn from a large repertoire and located near the telomere at one of fifteen so-called bloodstream expression sites (BESs). Thus, antigenic variation is achieved by switching to the expression of a different VSG gene. A BES is a tandem array of expression site-associated genes and a terminal VSG gene. It is polycistronically transcribed by a multifunctional RNA polymerase I (RNAPI) from a short promoter that is located 45-60 kb upstream of the VSG gene. The mechanism(s) restricting VSG expression to a single BES are not well understood. There is convincing evidence that epigenetic silencing and transcription attenuation play important roles. Furthermore, recent data indicated that there is regulation at the level of transcription initiation and that, surprisingly, the VSG mRNA appears to have a role in restricting VSG expression to a single gene. Here, we review BES expression regulation and propose a model in which telomere-directed, epigenetic BES silencing is opposed by BES promoter-directed, activated RNAPI transcription.

Genome of the avirulent human-infective trypanosome--Trypanosoma rangeli

Background

Trypanosoma rangeli is a hemoflagellate protozoan parasite infecting humans and other wild and domestic mammals across Central and South America. It does not cause human disease, but it can be mistaken for the etiologic agent of Chagas disease, Trypanosoma cruzi. We have sequenced the T. rangeli genome to provide new tools for elucidating the distinct and intriguing biology of this species and the key pathways related to interaction with its arthropod and mammalian hosts.

Methodology/Principal Findings

The T. rangeli haploid genome is ∼24 Mb in length, and is the smallest and least repetitive trypanosomatid genome sequenced thus far. This parasite genome has shorter subtelomeric sequences compared to those of T. cruzi and T. brucei; displays intraspecific karyotype variability and lacks minichromosomes. Of the predicted 7,613 protein coding sequences, functional annotations could be determined for 2,415, while 5,043 are hypothetical proteins, some with evidence of protein expression. 7,101 genes (93%) are shared with other trypanosomatids that infect humans. An ortholog of the dcl2 gene involved in the T. brucei RNAi pathway was found in T. rangeli, but the RNAi machinery is non-functional since the other genes in this pathway are pseudogenized. T. rangeli is highly susceptible to oxidative stress, a phenotype that may be explained by a smaller number of anti-oxidant defense enzymes and heat-shock proteins.

Conclusions/Significance

Phylogenetic comparison of nuclear and mitochondrial genes indicates that T. rangeli and T. cruzi are equidistant from T. brucei. In addition to revealing new aspects of trypanosome co-evolution within the vertebrate and invertebrate hosts, comparative genomic analysis with pathogenic trypanosomatids provides valuable new information that can be further explored with the aim of developing better diagnostic tools and/or therapeutic targets.

Comparative genomics is a powerful tool that affords detailed study of the genetic and evolutionary basis for aspects of lifecycles and pathologies caused by phylogenetically related pathogens. The reference genome sequences of three trypanosomatids, T. brucei, T. cruzi and L. major, and subsequent addition of multiple Leishmania and Trypanosoma genomes has provided data upon which large-scale investigations delineating the complex systems biology of these human parasites has been built. Here, we compare the annotated genome sequence of T. rangeli strain SC-58 to available genomic sequence and annotation data from related species. We provide analysis of gene content, genome architecture and key characteristics associated with the biology of this non-pathogenic trypanosome. Moreover, we report striking new genomic features of T. rangeli compared with its closest relative, T. cruzi, such as (1) considerably less amplification on the gene copy number within multigene virulence factor families such as MASPs, trans-sialidases and mucins; (2) a reduced repertoire of genes encoding anti-oxidant defense enzymes; and (3) the presence of vestigial orthologs of the RNAi machinery, which are insufficient to constitute a functional pathway. Overall, the genome of T. rangeli provides for a much better understanding of the identity, evolution, regulation and function of trypanosome virulence determinants for both mammalian host and insect vector.

The epigenome of Trypanosoma brucei: a regulatory interface to an unconventional transcriptional machine

The epigenome represents a major regulatory interface to the eukaryotic genome. Nucleosome positions, histone variants, histone modifications and chromatin associated proteins all play a role in the epigenetic regulation of DNA function. Trypanosomes, an ancient branch of the eukaryotic evolutionary lineage, exhibit some highly unusual transcriptional features, including the arrangement of functionally unrelated genes in large, polymerase II transcribed polycistronic transcription units, often exceeding hundreds of kilobases in size. It is generally believed that transcription initiation plays a minor role in regulating the transcript level of genes in trypanosomes, which are mainly regulated post-transcriptionally. Recent advances have revealed that epigenetic mechanisms play an essential role in the transcriptional regulation of Trypanosoma brucei. This suggested that the modulation of gene activity, particularly that of pol I transcribed genes, is, indeed, an important control mechanism, and that the epigenome is critical in regulating gene expression programs that allow the successful migration of this parasite between hosts, as well as the continuous evasion of the immune system in mammalian hosts. A wide range of epigenetic signals, readers, writers and erasers have been identified in trypanosomes, some of which have been mapped to essential genetic functions. Some epigenetic mechanisms have also been observed to be unique to trypanosomes. We review recent advances in our understanding of epigenetic control mechanisms in T. brucei, the causative agent of African sleeping sickness, and highlight the utility of epigenetic targets in the possible development of new therapies for human African trypanosomiasis.

Discovery of unconventional kinetochores in kinetoplastids

The kinetochore is the macromolecular protein complex that directs chromosome segregation in eukaryotes. It has been widely assumed that the core kinetochore consists of proteins that are common to all eukaryotes. However, no conventional kinetochore components have been identified in any kinetoplastid genome, thus challenging this assumption of universality. Here, we report the identification of 19 kinetochore proteins (KKT1–19) in Trypanosoma brucei. The majority is conserved among kinetoplastids, but none of them has detectable homology to conventional kinetochore proteins. These proteins instead have a variety of features not found in conventional kinetochore proteins. We propose that kinetoplastids build kinetochores using a distinct set of proteins. These findings provide important insights into the longstanding problem of the position of the root of the eukaryotic tree of life.

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Conventional kinetochore proteins cannot be identified in any kinetoplastid genome

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19 kinetochore proteins were identified in Trypanosoma brucei

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Kinetoplastids possess unconventional kinetochores

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This discovery supports the hypothesis that kinetoplastids branched very early

Evolutionary cell biology of chromosome segregation: insights from trypanosomes

Faithful transmission of genetic material is essential for the survival of all organisms. Eukaryotic chromosome segregation is driven by the kinetochore that assembles onto centromeric DNA to capture spindle microtubules and govern the movement of chromosomes. Its molecular mechanism has been actively studied in conventional model eukaryotes, such as yeasts, worms, flies and human. However, these organisms are closely related in the evolutionary time scale and it therefore remains unclear whether all eukaryotes use a similar mechanism. The evolutionary origins of the segregation apparatus also remain enigmatic. To gain insights into these questions, it is critical to perform comparative studies. Here, we review our current understanding of the mitotic mechanism in Trypanosoma brucei, an experimentally tractable kinetoplastid parasite that branched early in eukaryotic history. No canonical kinetochore component has been identified, and the design principle of kinetochores might be fundamentally different in kinetoplastids. Furthermore, these organisms do not appear to possess a functional spindle checkpoint that monitors kinetochore-microtubule attachments. With these unique features and the long evolutionary distance from other eukaryotes, understanding the mechanism of chromosome segregation in T. brucei should reveal fundamental requirements for the eukaryotic segregation machinery, and may also provide hints about the origin and evolution of the segregation apparatus.

TDP1 is an HMG chromatin protein facilitating RNA polymerase I transcription in African trypanosomes

Unusually for a eukaryote, Trypanosoma brucei transcribes its variant surface glycoprotein (VSG) gene expression sites (ESs) in a monoallelic fashion using RNA polymerase I (Pol I). It is still unclear how ES transcription is controlled in T. brucei. Here, we show that the TDP1 architectural chromatin protein is an essential high mobility group box (HMGB) protein facilitating Pol I transcription in T. brucei. TDP1 is specifically enriched at the active compared with silent VSG ES and immediately downstream of ribosomal DNA promoters and is abundant in the nucleolus and the expression site body subnuclear compartments. Distribution of TDP1 at Pol I-transcribed loci is inversely correlated with histones. Depletion of TDP1 results in up to 40–90% reduction in VSG and rRNA transcripts and a concomitant increase in histones H3, H2A and H1 at these Pol I transcription units. TDP1 shares features with the Saccharomyces cerevisiae HMGB protein Hmo1, but it is the first architectural chromatin protein facilitating Pol I-mediated transcription of both protein coding genes as well as rRNA. These results show that TDP1 has a mutually exclusive relationship with histones on actively transcribed Pol I transcription units, providing insight into how Pol I transcription is controlled.

Histone H1 plays a role in heterochromatin formation and VSG expression site silencing in Trypanosoma brucei

The African sleeping sickness parasite Trypanosoma brucei evades the host immune system through antigenic variation of its variant surface glycoprotein (VSG) coat. Although the T. brucei genome contains ∼1500 VSGs, only one VSG is expressed at a time from one of about 15 subtelomeric VSG expression sites (ESs). For antigenic variation to work, not only must the vast VSG repertoire be kept silent in a genome that is mainly constitutively transcribed, but the frequency of VSG switching must be strictly controlled. Recently it has become clear that chromatin plays a key role in silencing inactive ESs, thereby ensuring monoallelic expression of VSG. We investigated the role of the linker histone H1 in chromatin organization and ES regulation in T. brucei. T. brucei histone H1 proteins have a different domain structure to H1 proteins in higher eukaryotes. However, we show that they play a key role in the maintenance of higher order chromatin structure in bloodstream form T. brucei as visualised by electron microscopy. In addition, depletion of histone H1 results in chromatin becoming generally more accessible to endonucleases in bloodstream but not in insect form T. brucei. The effect on chromatin following H1 knock-down in bloodstream form T. brucei is particularly evident at transcriptionally silent ES promoters, leading to 6–8 fold derepression of these promoters. T. brucei histone H1 therefore appears to be important for the maintenance of repressed chromatin in bloodstream form T. brucei. In particular H1 plays a role in downregulating silent ESs, arguing that H1-mediated chromatin functions in antigenic variation in T. brucei.

Trypanosoma brucei causes African sleeping sickness, endemic to sub-Saharan Africa. Bloodstream form T. brucei is covered with a dense coat of variant surface glycoprotein (VSG). Only one VSG is expressed at a time out of a vast repertoire of ∼1500 VSGs. The active VSG is transcribed in a telomeric VSG expression site (ES), and VSG switching allows immune evasion. Exactly how monoallelic exclusion of VSG ESs operates, and how switching between ESs is mediated remains mysterious, although epigenetics and chromatin structure clearly play a major role. The linker histone H1 is thought to orchestrate higher order chromatin structure in eukaryotes, but its exact function is unclear. We investigated the role of histone H1 in the regulation of antigenic variation in T. brucei. We show that histone H1 is associated with chromatin and is required for higher order chromatin structure. Depletion of histone H1 results in derepression of silent VSG ES promoters, indicating that H1-mediated chromatin functions in antigenic variation in T. brucei.

Telomeres, tethers and trypanosomes

Temporal and spatial organization of the nucleus is critical for the control of transcription, mRNA processing and the assembly of ribosomes. This includes the occupancy of specific territories by mammalian chromosomes, the presence of subnuclear compartments such as the nucleolus and Cajal bodies and the division of chromatin between active and inactive states. These latter are commonly associated with the location of DNA within euchromatin and heterochromatin respectively; critically these distinctions arise through modifications to chromatin-associated proteins, including histones, as well as the preferential localization of heterochromatin at the nuclear periphery. Most research on nuclear organization has focused on metazoa and fungi; however, recent technical advances have made more divergent eukaryotes accessible to study, with some surprising results. For example, the organization of heterochromatin is mediated in metazoan nuclei in large part by lamins, the prototypical intermediate filament proteins. Despite the presence of heterochromatin, detected both biochemically and by EM in most eukaryotic organisms, until this year lamins were thought to be restricted to metazoan taxa, and the proteins comprising the lamina in other lineages were unknown. Recent work indicates the presence of lamin orthologs in amoeba, while trypanosomatids possess a large coiled-coil protein, NUP-1, that performs functions analogous to lamins. These data indicate that the presence of a nuclear lamina is substantially more widespread than previously thought, with major implications for the evolution of eukaryotic gene expression mechanisms. We discuss these and other recent findings on the organization of nuclei in diverse organisms, and the implications of these findings for the evolutionary origin of eukaryotes.

Epigenetic mechanisms, nuclear architecture and the control of gene expression in trypanosomes

The control of gene expression, and more significantly gene cohorts, requires tight transcriptional coordination and is an essential feature of probably all cells. In higher eukaryotes, the mechanisms used involve controlled modifications to both local and global DNA environments, principally through changes in chromatin structure as well as cis-element-driven mechanisms. Although the mechanisms regulating chromatin in terms of transcriptional permissiveness and the relation to developmental programmes and responses to the environment are becoming better understood for animal and fungal cells, it is only just beginning to become clear how these processes operate in other taxa, including the trypanosomatids. Recent advances are now illuminating how African trypanosomes regulate higher-order chromatin structure, and, further, how these mechanisms impact on the expression of major surface antigens that are of fundamental importance to life-cycle progression. It is now apparent that several mechanisms are rather more similar between animal and fungal cells and trypanosomes than it originally appeared, but some aspects do involve gene products unique to trypanosomes. Therefore, both evolutionarily common and novel mechanisms cohabit in trypanosomes, offering both important biological insights and possible therapeutic opportunity.