Unambiguous identification of histone H1 in Trypanosoma cruzi

Bovine (Bos taurus) humoral immune response against Echinococcus granulosus and hydatid cyst infertility

Echinococcus granulosus, the agent of hydatid disease, presents an indirect life cycle, with canines (mainly dogs) as definitive hosts, and herbivores and human as intermediary ones. In intermediary hosts fertile and infertile cysts develop, but only the first ones develop protoscoleces, the parasite form infective to definitive hosts. We report the presence of bovine IgGs in the germinal layer from infertile cysts (GLIC), in an order of magnitude greater than in the germinal layer from fertile cysts (GLFC). When extracted with salt solutions, bovine IgGs from GLIC are associated with low or with high affinity (most likely corresponding to non specific and antigen specific antibodies, respectively). Specific IgGs penetrate both the cells of the germinal layer and HeLa cultured cells and recognize parasitic proteins. These results, taken together with previous ones from our laboratory, showing induction of apoptosis in the germinal layer of infertile hydatid cysts, provide the first coherent explanation of the infertility process. They also offer the possibility of identifying the parasite antigens recognized, as possible targets for immune modulation.

Histone genes in trypanosomatids

Histone genes in Trypanosomatids are of considerable interest because these flagellates do not condense their chromatin during mitosis. In contrast to higher eukaryotes, histone genes in Trypanosomatids are found on separate chromosomes, and their transcripts are polyadenylated. Sequence similarity of Trypanosomatid core histones with those of higher eukaryotes is found predominantly in the globular region; the N-terminal is highly divergent. Finally, in general, Trypanosomatid histones H1 are of low molecular weight, bearing closest homology to the C-terminal region of the higher eukaryote histones H1. These features constitute interesting targets for a rational approach to the study of these protozoa, as discussed here by Norbel Galanti and colleagues.

Chromatin and nuclear organization in Trypanosoma cruzi

A total of 100 years have passed since the discovery of the protozoan Trypanosoma cruzi, the etiologic agent of Chagas’ disease. Since its discovery, the molecular and cellular biology of this early divergent eukaryote, as well as its interactions with the mammalian and insect hosts, has progressed substantially. It is now clear that this parasite presents unique mechanisms controlling gene expression, DNA replication, cell cycle and differentiation, generating several morphological forms that are adapted to survive in different hosts. In recent years, the relationship between the chromatin structure and nuclear organization with the unusual transcription, splicing, DNA replication and DNA repair mechanisms have been investigated in T. cruzi. This article reviews the relevant aspects of these mechanisms in relation to chromatin and nuclear organization.

Post-translational modifications of Trypanosoma cruzi histone H4

Histone tails provide sites for a variety of post-translational modifications implicated in the control of gene expression and chromatin assembly. As both histones and control of gene expression in trypanosomes are highly divergent compared to most eukaryotes, post-translational modifications of Trypanosoma cruzi histones were investigated. After in vivo incubation of live parasites with radiolabeled precursors, histone H4 mainly incorporates [(3)H]-acetyl, and to a lesser extent [(3)H]-methyl residues. In contrast, histone H3 preferentially incorporates [(3)H]-methyl residues. The modifications of histone H4 were further characterized by mass spectrometry. MALDI-TOF-TOF-MS analysis revealed that peptides from histone H4 amino-terminus, obtained by either endoproteinase Glu-C or endoproteinase Arg-C digestion, contain isoforms with 14 and 42Da additions, suggesting the presence of simultaneous acetylations and/or methylations. Tandem mass spectrometry analysis demonstrated that the N-terminal alanine is methylated, and lysine residues at positions 4, 10, 14 and 57 are acetylated; lysine at position 18 is mono-methylated, while arginine at position 53 is dimethylated. Immunoblotting analyses using specific antibodies raised against synthetic and acetylated peptides of T. cruzi histone H4 indicate that lysine 4 is acetylated in the majority of histone H4, while other acetylations at the N-terminus portion of histone H4 are less abundant.

Histones and histone modifications in protozoan parasites

Protozoan parasites are early branching eukaryotes causing significant morbidity and mortality in humans and livestock. Single-celled parasites have evolved complex life cycles, which may involve multiple host organisms, and strategies to evade host immune responses. Consequently, two key aspects of virulence that underlie pathogenesis are parasite differentiation and antigenic variation, both of which require changes in the expressed genome. Complicating these requisite alterations in the parasite transcriptome is chromatin, which serves as a formidable barrier to DNA processes including transcription, repair, replication and recombination. Considerable progress has been made in the study of chromatin dynamics in other eukaryotes, and there is much to be gained in extending these analyses to protozoan parasites. Much of the work completed to date has focused on histone acetylation and methylation in the apicomplexans and trypanosomatids. As we describe in this review, such studies provide a unique vantage point of the evolutionary picture of eukaryotic cell development, and reveal unique phenomena that could be exploited pharmacologically to treat protozoal diseases.

Trypanosoma cruzi histone H1 is phosphorylated in a typical cyclin dependent kinase site accordingly to the cell cycle

Histone H1 of most eukaryotes is phosphorylated during the cell cycle progression and seems to play a role in the regulation of chromatin structure, affecting replication and chromosome condensation. In trypanosomatids, histone H1 lacks the globular domain and is shorter when compared with the histone of other eukaryotes. We have previously shown that in Trypanosoma cruzi, the agent of Chagas' disease, histone H1 is phosphorylated and this increases its dissociation from chromatin. Here, we demonstrate using mass spectrometry analysis that T. cruzi histone H1 is only phosphorylated at the serine 12 in the sequence SPKK, a typical cyclin-dependent kinase site. We also found a correlation between the phosphorylation state of histone H1 and the cell cycle. Hydroxyurea and lactacystin, which, respectively, arrest parasites at the G1/S and G2/M stages of the cell cycle, increased the level of histone H1 phosphorylation. Cyclin-dependent kinase-related enzymes TzCRK3, and less intensely the TzCRK1 were able to phosphorylate histone H1 in vitro. Histone H1 dephosphorylation was prevented by treating the parasites with okadaic acid but not with calyculin A. These findings suggest that T. cruzi histone H1 phosphorylation is promoted by cyclin dependent kinases, present during S through G2 phase of the cell cycle, and its dephosphorylation is promoted by specific phosphatases.

Trypanosomatid histones

The histones are responsible for packaging and regulating access to eukaryotic genomes. Trypanosomatids are flagellated protists that diverged early from the eukaryotic lineage and include parasites that cause disease in humans and other mammals. Here, we review the properties of histones in parasitic trypanosomatids, from gene organization and sequence to expression, post-translational modification and function within chromatin. Phylogenetic and experimental analysis indicates that certain specifically conserved histone sequence motifs, particularly within the N-terminal 'tail' domains, possibly represent functionally important modification substrates conserved throughout the eukaryotic lineage. For example, histone H3 contains a highly conserved methylation substrate. Trypanosomatids also possess at least three variant histones. Among these is an orthologue of H2A.Z, a histone involved in protecting 'active' chromatin from silencing in yeast. Histones provide docking platforms for a variety of regulatory factors. The presence of histone modification and variant histones in trypanosomatids therefore represents evidence for a network that provides the discrimination required to regulate transcription, recombination, repair and chromosome replication and segregation.

Chromatin from two classes of platyhelminthes display both protist H1 and higher eukaryote core histones

Histones from the parasitic platyhelminthes, Echinococcus granulosus and Fasciola hepatica, were systematically characterized. Core histones H2A, H2B, H3 and H4, which were identified on the basis of amino acid sequencing and mass spectrometry data, showed conserved electrophoretic patterns. Histones H1, identified on the basis of physicochemical properties, amino acid composition and amino acid sequencing, showed divergence, both in their number and electrophoretic mobilities, between the two species and among other organisms. According to these data, core histones but not H1 histones, would be stabilized during evolution at the level of platyhelminthes.

Chromatin of Trypanosoma cruzi: in situ analysis revealed its unusual structure and nuclear organization

Chromatin of Trypanosoma cruzi is known to be organized in classical nucleosomal filaments, but surprisingly, these filaments do not fold in visible chromosomes and the nuclear envelope is preserved during cell division. Our hypothesis about the role of chromatin structure in regulating gene expression and, more generally, cell functioning, pressed us to verify if chromatin organization is modulated during the parasite life-cycle. To this end, we analyzed in situ the fine structural organization of T. cruzi chromatin by means of an integrated biophysical approach, using differential scanning calorimetry and fluorescence microscopy. We observed that logarithmic forms exhibit a less condensed chromatin with respect to the stationary ones. Thermal analysis revealed that parasite chromatin is organized in three main levels of condensation, barring from the polynucleosomal filament till to superstructured fibers. Besides, the fluorescence images of nuclei showed a characteristic chromatin distribution, with defined domains localized near to the nuclear envelope. While in stationary parasites, these regions are highly condensed, in logarithmic forms they unfold by extending themselves toward the center of nucleus. These observations suggest that, in comparison with higher eukaryotes, in T. cruzi the nuclear envelope plays an unusual and pivotal role in interphase and in mitosis.

Histone H1 is phosphorylated in non-replicating and infective forms of Trypanosoma cruzi

The nuclear structure changes during the differentiation from growing to infective stages of Trypanosoma cruzi. As histone modifications have been correlated with structural and functional changes of chromatin, we investigated whether histones in T. cruzi are modified during the life cycle of this protozoan parasite. We found that histone H1 isolated from proliferating forms (epimastigotes) and from differentiated/infective forms (trypomastigotes) have a distinct migrating pattern in Triton-acetic acid-urea gel electrophoresis. While epimastigotes contain predominantly a fast migrating form, a slow migrating band is prominent in trypomastigotes. By metabolically labeling the cells with radioactive phosphate, we demonstrated that the slow migrating histone H1 band is phosphorylated, and that after alkaline phosphatase treatment, it migrates as the fast form. Parasites arrested at the onset of the S phase of the cell cycle with hydroxyurea (HU) also predominantly have the phosphorylated form of histone H1, suggesting that phosphorylation occurs in non-replicating stages of T. cruzi. We also found that the phosphorylated histone H1 is more weakly associated with the chromatin, being preferentially released at 150 mM NaCl. Therefore, histone H1 phosphorylation varies during the life cycle of T. cruzi, and might be related to changes in the chromatin structure.

Chromatin and histones from Giardia lamblia: a new puzzle in primitive eukaryotes

The three deepest eukaryote lineages in small subunit ribosomal RNA phylogenies are the amitochondriate Microsporidia, Metamonada, and Parabasalia. They are followed by either the Euglenozoa (e.g., Euglena and Trypanosoma) or the Percolozoa as the first mitochondria-containing eukaryotes. Considering the great divergence of histone proteins in protozoa we have extended our studies of histones from Trypanosomes (Trypanosoma cruzi, Crithidia fasciculata and Leishmania mexicana) to the Metamonada Giardia lamblia, since Giardia is thought to be one of the most primitive eukaryotes. In the present work, the structure of G. lamblia chromatin and the histone content of the soluble chromatin were investigated and compared with that of higher eukaryotes, represented by calf thymus. The chromatin is present as nucleosome filaments which resemble the calf thymus array in that they show a more regular arrangement than those described for Trypanosoma. SDS-polyacrylamide gel electrophoresis and protein characterization revealed that the four core histones described in Giardia are in the same range of divergence with the histones from other lower eukaryotes. In addition, G. lamblia presented an H1 histone with electrophoretic mobility resembling the H1 of higher eukaryotes, in spite of the fact that H1 has a different molecular mass in calf thymus. Giardia also presents a basic protein which was identified as an HU-like DNA-binding protein usually present in eubacteria, indicating a chimaeric composition for the DNA-binding protein set in this species. Finally, the phylogenetic analysis of selected core histone protein sequences place Giardia divergence before Trypanosoma, despite the fact that Trypanosoma branch shows an acceleration in the evolutionary rate pointing to an unusual evolutionary behavior in this lineage.

Control mechanisms of the H2A genes expression in Trypanosoma cruzi

In a previous report we have described that the T. cruzi histone H2A gene is encoded in two independent gene clusters located in a single chromosome. In the present paper we show that both gene cluster are actively transcribed as two sized classes of polyadenylated mRNAs demonstrating, moreover, the existence of alternative splicing sites and microheterogeneities at the polyadenylation site. We also describe that while the expression of the H2A genes in the non replicative trypomastigote forms is only residual, in the replicative forms there is constitutive transcription of these genes and that the transcription is not associated to DNA replication. The data show, moreover, that in the replicative forms the steady state levels of the H2A mRNAs are controlled at a post-transcriptional level which is associated to DNA replication.

Histone synthesis in Trypanosoma cruzi

Trypanosoma cruzi is an ancient, parasitic eukaryote which does not undergo chromatin condensation during cell division. This behavior may be explained if one considers the strong amino acid sequence divergence of Trypanosoma histones compared to higher eukaryotes. In the latter organisms histone synthesis is coupled to DNA replication. Considering the nonconserved amino acid sequence of T. cruzi histones, as well as the absence of chromatin condensation in this organism, we have studied histone synthesis in relation to DNA replication in this parasite. We have found that core histones and a fraction of histone H1 are synthesized concomitantly to DNA replication. However, another fraction of histone H1 is constitutively synthesized.

Presence of an unusually high concentration of an ubiquitinated histone-like protein in Trypanosoma cruzi

The conjugation of ubiquitin to histones H2A and H2B has been established in higher eukaryotes and has been related to changes in chromatin organization. In Trypanosoma cruzi, no condensation of chromatin occurs during mitosis. In order to determine the presence of histone ubiquitination in T. cruzi epimastigotes, histones were extracted from chromatin and analyzed by three electrophoretic systems: acid-urea, triton-acid-urea and sodium-dodecyl-sulphate polyacrylamide gel. The immunochemical detection of ubiquitin-histone conjugates by Western blotting showed a strong reaction with a slow migrating band of M(r) 19 kDa. The high percentage of ubiquitin-histone conjugates present in T. cruzi chromatin may be related to the inability of this parasite to condense chromatin into a 30 nm fiber.

Histone H1 expression varies during the Leishmania major life cycle

The deduced amino acid sequence of Leishmania major sw3 cDNA reveals the presence of characteristic histone H1 amino acid motifs. However, the open reading frame is of an unusually small size for histone H1 (105 amino acids) because it lacks the coding potential for the central hydrophobic globular domain of linker histones present in other eukaryotes. Here, we provide biochemical evidence that the SW3 protein is indeed a L. major nuclear histone H1, and that it is differentially expressed during the life cycle of the parasite. Due to its high lysine content, the SW3 protein can be purified to a high degree from L. major nuclear lysates with 5% perchloric acid, a histone H1 preparative method. Using an anti-SW3 antibody, this protein is detected as a 17 kDa or as a 17/19 kDa doublet in the nuclear subfraction in different L. major strains. The nuclear localization of the SW3 protein is further supported by immunofluorescence studies. During in vitro promastigote growth, both the sw3 cytoplasmic mRNA and its protein progressively accumulate within parasites from early log phase to stationary phase. Within amastigotes, the high level of H1 expression is maintained but decreases when amastigotes differentiate into promastigotes. Together, these observations suggest that the different levels of this histone H1 protein could influence the varying degrees of chromatin condensation during the life-cycle of the parasite, and provide us with tools to study this mechanism.

Properties of the histones and functional aspects of the soluble chromatin of epimastigote Trypanosoma cruzi

The amino acid composition of all histones of Trypanosoma cruzi was analyzed, and the terminology of the histones of higher eukaryotes adopted. One chromatin associated protein, previously considered to be a variant of histone H1, could not be clearly identified, and shows features of core histones as well as of histone H1. An improved method for the isolation of intact nuclei and the production of soluble chromatin in T. cruzi was established. The chromatin of T. cruzi is relatively instable and histone H1 is easily lost during experimental manipulations. Histone H1 dissociates completely at a relatively low NaCl concentration of 380 mM, leading to an open nucleosome filament which does not condense. The influence of histone H1 of T. cruzi and of rat liver on the compaction pattern of the chromatin was investigated by homologous and heterologous reconstitution experiments, and analysed by electron microscopy. It could be shown that histone H1 of T. cruzi induces nucleosome filaments of T. cruzi as well as those of rat liver to condense. The same is true for histone H1 of rats. It can be concluded that T. cruzi has a functional histone H1.

Characterization of the histones of Trypanosoma brucei brucei bloodstream forms

Five groups of histones were shown in Trypanosoma brucei brucei, displaying qualitative and quantitative differences between two stages of the parasite's life-cycle. The influence of the histones of T. b. brucei bloodstream forms on the compaction pattern of the chromatin was investigated and their extractability in diluted acids and their amino acid composition were analysed. While nonhistone proteins barely influenced the formation of higher-order chromatin structures, the histone H1-like proteins were essential for the regular spacing of the nucleosomes and the salt-dependent condensation of the nucleosome filament. Differences were seen in the amino-acid composition of histones of bloodstream forms as compared to procyclic culture forms and higher eukaryotes which may explain the disparities seen in the condensation of the chromatin between the two stages of the life cycle as well as the lack of a salt-dependent condensation into a 30 nm fiber. They point to an alternative method of organizing and processing the genetic information in the nucleus of the trypanosome as compared to higher eukaryotes, the possible hosts of the parasite.

The chromatin of trypanosomes

The nuclear chromatin of trypanosomes is organised in the form of nucleosome filaments. When soluble chromatin is prepared under suitable conditions, a regular array of nucleosomes can be shown by electron microscopy. Chromatin of blood stream as well as procyclic culture forms of Trypanosoma brucei brucei and of T. cruzi shows limited compaction at salt concentrations increasing from 1 to 100 mM. No 30 nm fibres, typical for higher eukaryotes, are formed. Digestion of the nuclear chromatin with micrococcal nuclease and analysis of the histone proteins with various techniques reveal that the basic organisation of the trypanosome chromatin is similar but not identical as compared to that of higher eukaryotes. Distinct differences are present with respect to biochemical properties of the histones as well as to their interaction with the DNA. The primary structure of the histones also differs significantly from that found in other lower and higher eukaryotes. The function of the recently described H1-like proteins in trypanosomes is currently being investigated. The differences that have already been found in the structure and compaction of the trypanosome chromatin compared to that of higher eukaryotes lead us to expect differences of gene expression which, in turn, might offer targets for the control of trypanosomiasis.

A gene family encoding heterogeneous histone H1 proteins in Trypanosoma cruzi

A gene family encoding a set of histone H1 proteins in Trypanosoma cruzi is described. The sequence of 3 genomic and 4 cDNA clones revealed the presence of several motifs characteristic of histone H1, although heterogeneity at the polypeptide level was evident. The clones encode histone H1 proteins of an unusually small size (74-97 amino acids), which lack the globular domain found in histone H1 of higher eukaryotes. All histone H1 mRNAs from T. cruzi are polyadenylated, although no typical polyadenylation signal was found. Furthermore, the genes encoding the histone H1 proteins in T. cruzi are found in a tandem array containing 15-20 gene copies per haploid genome. This tandem array is located on a large chromosome of 2.2 Mb.

Biochemical and functional characterization of histone H1-like proteins in procyclic Trypanosoma brucei brucei

Four variants and/or posttranslational modifications of histone H1-like proteins of Trypanosoma brucei brucei procyclic culture forms were extracted with 0.25 N HCl from isolated nuclei and analyzed by two-dimensional gel electrophoresis. The amino acid composition of these proteins, their ability to space nucleosomes regularly and to induce salt-dependent condensation of the chromatin indicated their histone H1 nature. On the other hand, the histone H1-like proteins clearly differed from their higher-eukaryote counterparts by their weak interaction with DNA under low-salt conditions. As a consequence, intact nucleosome filaments were prepared according to a new preparation protocol especially adapted to the unstable chromatin of T. b. brucei. Our results indicate that the biochemical properties of the histone H1-like proteins contribute to the structural and functional differences between the chromatin of procyclic T. b. brucei and that of higher eukaryotes.