An upstream element of the TamS1 gene is a site of DNA-protein interactions during differentiation to the merozoite in Theileria annulata

ApiAP2 Factors as Candidate Regulators of Stochastic Commitment to Merozoite Production in Theileria annulata

Background

Differentiation of one life-cycle stage to the next is critical for survival and transmission of apicomplexan parasites. A number of studies have shown that stage differentiation is a stochastic process and is associated with a point that commits the cell to a change over in the pattern of gene expression. Studies on differentiation to merozoite production (merogony) in T. annulata postulated that commitment involves a concentration threshold of DNA binding proteins and an auto-regulatory loop.

Principal Findings

In this study ApiAP2 DNA binding proteins that show changes in expression level during merogony of T. annulata have been identified. DNA motifs bound by orthologous domains in Plasmodium were found to be enriched in upstream regions of stage-regulated T. annulata genes and validated as targets for the T. annulata AP2 domains by electrophoretic mobility shift assay (EMSA). Two findings were of particular note: the gene in T. annulata encoding the orthologue of the ApiAP2 domain in the AP2-G factor that commits Plasmodium to gametocyte production, has an expression profile indicating involvement in transmission of T. annulata to the tick vector; genes encoding related domains that bind, or are predicted to bind, sequence motifs of the type 5'-(A)CACAC(A) are implicated in differential regulation of gene expression, with one gene (TA11145) likely to be preferentially up-regulated via auto-regulation as the cell progresses to merogony.

Conclusions

We postulate that the Theileria factor possessing the AP2 domain orthologous to that of Plasmodium AP2-G may regulate gametocytogenesis in a similar manner to AP2-G. In addition, paralogous ApiAP2 factors that recognise 5'-(A)CACAC(A) type motifs could operate in a competitive manner to promote reversible progression towards the point that commits the cell to undergo merogony. Factors possessing AP2 domains that bind (or are predicted to bind) this motif are present in the vector-borne genera Theileria, Babesia and Plasmodium, and other Apicomplexa; leading to the proposal that the mechanisms that control stage differentiation will show a degree of conservation.

The ability of vector-borne Apicomplexan parasites (Babesia, Plasmodium and Theileria) to change from one life-cycle stage to the next is critical for establishment of infection and transmission to new hosts. Stage differentiation steps of both Plasmodium and Theileria are known to involve stochastic transition through an intermediate form to a point that commits the cell to generate the next stage in the life-cycle. In this study we have identified genes encoding ApiAP2 DNA binding proteins in Theileria annulata that are differentially expressed during differentiation from the macroschizont stage, through merozoite production (merogony) to the piroplasm stage. The results provide evidence that the ApiAp2 factor in Theileria that possesses the orthologue of the Plasmodium AP2-G domain may also operate to regulate gametocytogenesis, and that progression to merogony is promoted by the ability of a merozoite DNA binding protein to preferentially up-regulate its own production. In addition, identification of multiple ApiAP2 DNA binding domains that bind related motifs within and across vector-borne Apicomplexan genera lead to the proposal that the mechanisms that promote the transition from asexual to sexual replication will show a degree of conservation.

Evaluation of indirect TaSP enzyme-linked immunosorbent assay for diagnosis of tropical theileriosis in cattle (Bos indicus) and water buffaloes (Bubalus bubalis) in Egypt

The aim of the present study was to evaluate the validity of Theileria annulata surface protein (TaSP)-ELISA, in comparison with traditional microscopic test, for the diagnosis of T. annulata infection among Egyptian baladi cattle (Bos taurus) and water buffaloes (Bubalus bubalis). Molecular confirmation of infection using T. annulata merozoite surface (Tams-1) target amplification by PCR was used as a gold standard. A total of 76 clinically suspected animals including 64 baladi cattle and 12 water buffaloes were investigated in the current study by the three methods. Based on the PCR-confirmed results, the evaluation study revealed higher sensitivity of TaSP-ELISA (72.9% and 75%) as compared to microscopic examination (58.3% and 50%) among cattle and buffaloes, respectively. On the other hand, the specificity of TaSP-ELISA in diagnosis of T. annulata infection was higher (87.5%) in baladi cattle as compared to water buffaloes (37.5%). In conclusion, TaSP-ELISA was shown to be suitable for the diagnosis of T. annulata infection in cattle under field conditions.

Stochastic induction of Theileria annulata merogony in vitro by chloramphenicol

Theileria annulata inhabits the cytoplasm of bovine leukocytes where it can be found as a multinucleated schizont. The schizont is the pathogenic stage of the life cycle and by interfering with host signalling pathways, it induces unlimited host cell proliferation and protection against apoptosis. In the infected animal, the schizont differentiates to the merozoite life cycle stage in a process called merogony. This takes place within the host leukocyte, resulting in the production of merozoites that are subsequently released by leukocyte lysis. In established cultures of T. annulata-transformed cells, merogony does not spontaneously occur, but the process can be activated by a shift in temperature. In this study we show that chloramphenicol induces schizont differentiation in proliferating T. annulata-transformed cells. We demonstrate that chloramphenicol-induced merogony is inherently asynchronous and has a quantitative basis. The process is accompanied by the down-regulation of schizont-specific surface proteins, de novo expression of merozoite-specific markers such as Tamr1 and Tams1 and the morphological hallmarks of merogony. Chloramphenicol-induced parasite differentiation was found to be associated with diminished proliferation potential and extensive morphological changes of the host cell, including increased numbers of pseudopodia. Significantly, chloramphenicol treatment can accelerate merogony induced by elevated temperature, supporting postulation that the differentiation event is a stochastic process that can be manipulated to alter the outcome of parasitic infection.

A Theileria annulata DNA binding protein localized to the host cell nucleus alters the phenotype of a bovine macrophage cell line

The apicomplexan parasite Theileria annulata is the only intracellular eukaryote that is known to induce the proliferation of mammalian cells. However, as the parasite undergoes stage differentiation, host cell proliferation is inhibited, and the leukocyte is eventually destroyed. We have isolated a parasite gene (SuAT1) encoding an AT hook DNA binding polypeptide that has a predicted signal peptide, PEST motifs, nuclear localization signals, and domains which indicate interaction with regulatory components of the higher eukaryotic cell cycle. The polypeptide is localized to the nuclei of macroschizont-infected cells and was detected at significant levels in cells that were undergoing parasite stage differentiation. Transfection of an uninfected transformed bovine macrophage cell line, BoMac, demonstrated that SuAT1 can modulate cellular morphology and alter the expression pattern of a cytoskeletal polypeptide in a manner similar to that found during the infection of leukocytes by the parasite. Our findings indicate that Theileria parasite molecules that are transported to the leukocyte nucleus have the potential to modulate the phenotype of infected cells.

Identification of a 40S Ribosomal protein (S17) that is differentially expressed between the macroschizont and piroplasm stages of Theileria annulata

The nucleotide and protein sequence of the 40S ribosomal protein S17 (RibS17) of the protozoan parasite Theileria annulata has been determined. Southern blot analysis showed the gene was single copy and comparative sequence analysis revealed that the predicted polypeptide had high sequence homology with the RibS17 from other organisms. Northern blot analysis showed that there was a 3-fold increase in the level of RibS17 RNA between the macroschizont and the piroplasm stage of the lifecycle, whereas, there was no difference in expression between the sporozoite and the macroschizont stages. Antisera to the purified fusion protein, corresponding to the terminal 50 amino acids of the protein sequence, were raised in rabbits. Western analysis detected a polypeptide of the predicted size that was more abundant in the piroplasm stage compared with the macroschizont stage. Immunofluorescence analysis with the same antisera revealed a strong signal in the macroschizont and piroplasm stages, but the antiserum did not cross-react with the bovine host cells. The antisera did, however, cross-react with Toxoplasma gondii tachyzoites and Plasmodium falciparum merozoites. The possible functional significance of the stage related increase in abundance of a ribosomal protein is discussed.

Characterisation of a cluster of genes encoding Theileria annulata AT hook DNA-binding proteins and evidence for localisation to the host cell nucleus

Infection of bovine leukocytes by the apicomplexan parasite Theileria annulata results in alteration of host cell gene expression and stimulation of host cell proliferation. At present, the parasite-derived factors involved in these processes are unknown. Recently, we described the characterisation of a parasite gene (TashAT2), whose polypeptide product bears AT hook DNA-binding motifs and may be transported from the parasite to the host nucleus. We now describe the isolation of a further two genes (TashAT1 and TashAT3) that are very closely related to TashAT2. All three TashAT genes are located together in a tight cluster, interspersed by two further small open reading frames, all facing head to tail. TashAT2 was shown to be expressed in all T. annulata cell lines examined, whereas TashAT1 and TashAT3 were expressed in the sporozoite stage of the parasite, and also in infected cell lines, where their expression was found to vary between different cell lines. Evidence for transport was provided by antisera raised against TashAT1 and TashAT3 that reacted with the host nucleus of T. annulata-infected cells. Reactivity was particularly strong against the host nuclei of the T. annulata-infected cloned cell line D7B12, which is attenuated for differentiation. A polypeptide in the size range predicted for TashAT3 was preferentially detected in host enriched D7B12 nuclear extracts. DNA-binding analysis demonstrated that fusion proteins containing the AT hook region of either TashAT1 or TashAT2 bound preferentially to AT rich DNA.

Theileria annulata: identification, by differential mRNA display, of modulated host and parasite gene expression in cell lines that are competent or attenuated for differentiation to the merozoite

To identify both host and parasite genes that show altered expression during differentiation of Theileria annulata from the macroschizont to the merozoite stage of the life cycle, the RNA profiles of two T. annulata-infected clonal cell lines (D7 and D7B12) with the same genetic background have been compared by RNA display. In the cloned cell line D7, T. annulata differentiates from the macroschizont to the merozoite at 41 degrees C, whereas in the cell line D7B12, which was derived by recloning D7, the parasite does not differentiate. Therefore, genes that show altered expression levels in either clone could be modulated by the differentiation event and are possible candidates for regulators of this process. Differential display was carried out initially on RNA extracted from D7 and D7B12 macroschizont-infected cells cultured at 37 degrees C and secondly on RNA extracted from the two cell lines incubated at 41 degrees C to induce differentiation to the merozoite. The first procedure identified 29 cDNA fragments that displayed altered levels between D7 and D7B12, 9 of which were confirmed to be differentially expressed by Northern blot analysis. Of these 9 gene fragments, 8 were found to be of host origin, while 1 was parasite derived. The second RNA display analysis identified 14 transcripts that showed altered levels during a differentiation time course, of which 6 were confirmed to be differentially expressed between D7B12 cells and differentiating D7 cells by Northern blot analysis. Of these 6 gene fragments, 1 was of host and 5 were of parasite origin. The parasite genes either showed levels of RNA consistent with constitutive gene expression or, in one case, a genuine upregulation of mRNA associated with the differentiation process.

Temporal co-ordination of macroschizont and merozoite gene expression during stage differentiation of Theileria annulata

The bovine parasite, Theileria annulata has a complex life-cycle involving the expression and repression of genes during development of its morphologically distinct life-cycle stages. In order to detail the molecular events that occur during differentiation of the intracellular multinucleate macroschizont to the extra-cellular uninucleate merozoite, we have isolated two genes, Tash1 and Tash2 which are differentially expressed during differentiation. Nuclear run on data show that Tash1 gene expression is controlled, at least in part, at the level of transcription. Immunofluorescence data identify the macroschizont as the location for both Tash1 and Tash2 gene products. Northern blot analysis of these genes indicated that their mRNA levels decrease during differentiation in vitro, at a time point coincident with major elevation in the mRNA levels of the merozoite antigen, Tams1, shown previously to be associated with commitment to merozoite production. Furthermore, experiments where cultures were incubated at 41 degrees C for 4 days and replaced at 37 degrees C for 2 days demonstrated that re-expression of Tash1 occurred and is probably linked to reversion to the macroschizont and decreased expression of Tams1. These results imply that the control of macroschizont and merozoite gene expression during differentiation is closely co-ordinated temporally. In addition, a comparison of Tash2 and Tams1 expression has indicated that translational or post-translational control of gene expression may operate in the undifferentiated macroschizont.

Transient transfection of Theileria annulata

We have developed a method to transiently transfect infective, uninucleate, Theileria annulata sporozoites. Transfection vectors have been constructed using a number of T. annulata 5' gene flanking sequences linked to the enhanced green fluorescence protein (eGFP) reporter gene. Sporozoites were transfected with these constructs using the lipid transfection agent SuperFect, then allowed to infect purified bovine mononuclear cells (PBMs). Green fluorescence was observed in developing trophozoites, 36-40 h post infection, using constructs containing the upstream regions of the T. annulata Hsp70, T. annulata merozite surface antigen 1 (TamS1) and T. annulata macroschizont-specific AT hook-containing protein2 (TashAT2) genes. A construct with the 5' TamS1 upstream sequence in reverse orientation gave no detectable fluorescence indicating fluorescence was derived by expression from the T. annulata promoter. A cytomegalovirus (CMV) promoter construct showed no activity in this stage of the parasite. However, when this construct was introduced directly into schizont-infected cells by electroporation, fluorescence was observed in the bovine cells but not the schizont. We describe the significance of these results in relation to novel control strategies and the fundamental biology of Theileria parasites.

Genomic organisation and developmentally regulated expression of an apicomplexan aspartyl proteinase

The cDNA for an aspartyl proteinase, termed eimepsin, was isolated from an Eimeria tenella sporulated oocyst library and the deduced amino acid sequence found to be almost identical to a previously described aspartyl proteinase from E. acervulina (97.4% amino acid identity). An E. tenella cosmid clone covering the entire eimepsin gene was cloned and characterised. Sequencing revealed that the eimepsin gene spans 2.9 kb and consists of 18 exons and 17 introns. The 5' flanking region sequence of the gene contains a putative transcriptional promoter sequence (TATAAA box) and three potential transcription initiator sites (Inr sites). Expression of eimepsin at the mRNA and protein level is developmentally regulated during oocyst sporulation. The eimepsin transcript was detected in unsporulated oocysts and increased in abundance during the early part of sporulation when the oocyst undergoes nuclear division and blast formation. Thereafter, the level of the eimepsin transcript decreases and in the excysted sporozoite, no eimepsin-specific RNA was detected. Expression of eimepsin lags behind transcript expression by some hours, and the protein accumulates in the oocyst during sporocyst and sporozoite formation.