An amino acid substitution in the Babesia bovis dihydrofolate reductase-thymidylate synthase gene is correlated to cross-resistance against pyrimethamine and WR99210

Stable expression of red fluorescent protein-blasticidin deaminase fusion gene (rfp-bsd) as a selectable marker for DNA transfection in Babesia ovata

The red fluorescent protein (rfp)-blasticidin deaminase (bsd) fusion gene was transfected into Babesia ovata by electroporation with the plasmid DNA and selected with 15 μg/mL of blasticidin S under the in vitro culture condition. The transfected parasite with episomal DNA was selected and cultured for further analysis based on the presence of the rfp-bsd fusion gene by PCR and expression of the fusion protein by immunofluorescence antibody test under fluorescence microscopy for 2 months after the transfection. The results are the first, to our knowledge, to demonstrate the expression and stability of the episomal rfp-bsd fusion gene under the control of actin promoter as a selectable marker for the transfection system in B. ovata.

Establishment of a stable transfection and gene targeting system in Babesia divergens

Babesia divergens is an emerging tick-borne pathogen considered as the principal causative agent of bovine babesiosis in Europe with a notable zoonotic risk to human health. Despite its increasing impact, considerable gaps persist in our understanding of the molecular interactions between this parasite and its hosts. In this study, we address the current limitation of functional genomic tools in B. divergens and introduce a stable transfection system specific to this parasite. We define the parameters for a drug selection system hdhfr-WR99210 and evaluate different transfection protocols for highly efficient generation of transgenic parasites expressing GFP. We proved that plasmid delivery into bovine erythrocytes prior to their infection is the most optimal transfection approach for B. divergens, providing novel evidence of Babesia parasites' ability to spontaneously uptake external DNA from erythrocytes cytoplasm. Furthermore, we validated the bidirectional and symmetrical activity of ef-tgtp promoter, enabling simultaneous expression of external genes. Lastly, we generated a B. divergens knockout line by targeting a 6-cys-e gene locus. The observed dispensability of this gene in intraerythrocytic parasite development makes it a suitable recipient locus for further transgenic application. The platform for genetic manipulations presented herein serves as the initial step towards developing advanced functional genomic tools enabling the discovery of B. divergens molecules involved in host-vector-pathogen interactions.

Inhibitory effects on bovine babesial infection by iron chelator, 1-(N-acetyl-6-aminohexyl)- 3-hydroxy-2-methylpyridin-4-one (CM1), and antimalarial drugs

BACKGROUND

Babesiosis is an infectious disease caused by protozoa of the apicomplexan phylum, genus Babesia. It is a malaria-like parasitic disease that can be transmitted via tick bites. The apicomplexan phylum of eukaryotic microbial parasites has had detrimental impacts on human and veterinary medicine. There are only a few drugs currently available to treat this disease; however, parasitic strains that are resistant to these commercial drugs are increasing in numbers. Plasmodium and Babesia are closely related as they share similar biological features including mechanisms for host cell invasion and metabolism. Therefore, antimalarial drugs may be useful in the treatment of Babesia infections. In addition to antimalarials, iron chelators also inhibit parasite growth. In this study, we aimed to evaluate the in vitro inhibitory efficacy of iron chelator and different antimalarials in the treatment of Babesia bovis.

METHODS

Cytotoxicity of antimalarial drugs; pyrimethamine, artefenomel, chloroquine, primaquine, dihydroarthemisinine, and the iron chelator, 1-(N-acetyl-6-aminohexyl)- 3-hydroxy-2 methylpyridin-4-one (CM1), were evaluated against Madin Darby Bovine Kidney (MDBK) cells and compared to diminazene aceturate, which is the currently available drug for animal babesiosis using an MTT solution. Afterwards, an evaluation of the in vitro growth-inhibitory effects of antimalarial drug concentrations was performed and monitored using a flow cytometer. Half maximal inhibitory concentrations (IC50) of each antimalarial and iron chelator were determined and compared to the antibabesial drug, diminazine aceturate, by interpolation using a curve-fitting technique. Subsequently, the effect of the drug combination was assessed by constructing an isobologram. Values of the sum of fractional inhibitions at 50% inhibition were then estimated.

RESULTS

Results indicate that all drugs tested could safely inhibit babesia parasite growth, as high as 2500 μM were non-toxic to mammalian cells. Although no drugs inhibited B. bovis more effectively than diminazine aceturate in this experiment, in vitro growth inhibition results with IC50 values of pyrimethamine 6.25 ± 2.59 μM, artefenomel 2.56 ± 0.67 μM, chloroquine 2.14 ± 0.76 μM, primaquine 22.61 ± 6.72 μM, dihydroarthemisinine 4.65 ± 0.22 μM, 1-(N-acetyl-6-aminohexyl)- 3-hydroxy-2 methylpyridin-4-one (CM1) 9.73 ± 1.90 μM, and diminazine aceturate 0.42 ± 0.01 μM, confirm that all drugs could inhibit B. bovis and could be used as alternative treatments for bovine babesial infection. Furthermore, the efficacy of a combination of the iron chelator, CM1, in combination with artefenomel dihydroarthemisinin or chloroquine, and artefenomel in combination with the iron chelator, CM1, dihydroarthemisinin or chloroquine, exhibited synergism against B. bovis in vitro.

CONCLUSION

Our evaluation of the inhibitory efficacy of the iron chelator CM1, antimalarial drugs, and a combination of these drugs against B. bovis could be potentially useful in the development and discovery of a novel drug for the treatment of B. bovis in the future.

Theileria equi isolates vary in susceptibility to imidocarb dipropionate but demonstrate uniform in vitro susceptibility to a bumped kinase inhibitor

Background

The apicomplexan hemoparasite Theileria equi is a causative agent of equine piroplasmosis, eradicated from the United States in 1988. However, recent outbreaks have sparked renewed interest in treatment options for infected horses. Imidocarb dipropionate is the current drug of choice, however variation in clinical response to therapy has been observed.

Methods

We quantified the in vitro susceptibility of two T. equi isolates and a lab generated variant to both imidocarb dipropionate and a bumped kinase inhibitor compound 1294. We also evaluated the capacity of in vitro imidocarb dipropionate exposure to decrease susceptibility to that drug. The efficacy of imidocarb dipropionate for clearing infection in four T. equi infected ponies was also assessed.

Results

We observed an almost four-fold difference in imidocarb dipropionate susceptibility between two distinct isolates of T. equi. Four ponies infected with the less susceptible USDA Florida strain failed to clear the parasite despite two rounds of treatment. Importantly, a further 15-fold decrease in susceptibility was produced in this strain by continuous in vitro imidocarb dipropionate exposure. Despite a demonstrated difference in imidocarb dipropionate susceptibility, there was no difference in the susceptibility of two T. equi isolates to bumped kinase inhibitor 1294.

Conclusions

The observed variation in imidocarb dipropionate susceptibility, further reduction in susceptibility caused by drug exposure in vitro, and failure to clear T. equi infection in vivo, raises concern for the emergence of drug resistance in clinical cases undergoing treatment. Bumped kinase inhibitors may be effective as alternative drugs for the treatment of resistant T. equi parasites.

Comparative genomic analysis and phylogenetic position of Theileria equi

Background

Transmission of arthropod-borne apicomplexan parasites that cause disease and result in death or persistent infection represents a major challenge to global human and animal health. First described in 1901 as Piroplasma equi, this re-emergent apicomplexan parasite was renamed Babesia equi and subsequently Theileria equi, reflecting an uncertain taxonomy. Understanding mechanisms by which apicomplexan parasites evade immune or chemotherapeutic elimination is required for development of effective vaccines or chemotherapeutics. The continued risk of transmission of T. equi from clinically silent, persistently infected equids impedes the goal of returning the U. S. to non-endemic status. Therefore comparative genomic analysis of T. equi was undertaken to: 1) identify genes contributing to immune evasion and persistence in equid hosts, 2) identify genes involved in PBMC infection biology and 3) define the phylogenetic position of T. equi relative to sequenced apicomplexan parasites.

Results

The known immunodominant proteins, EMA1, 2 and 3 were discovered to belong to a ten member gene family with a mean amino acid identity, in pairwise comparisons, of 39%. Importantly, the amino acid diversity of EMAs is distributed throughout the length of the proteins. Eight of the EMA genes were simultaneously transcribed. As the agents that cause bovine theileriosis infect and transform host cell PBMCs, we confirmed that T. equi infects equine PBMCs, however, there is no evidence of host cell transformation. Indeed, a number of genes identified as potential manipulators of the host cell phenotype are absent from the T. equi genome. Comparative genomic analysis of T. equi revealed the phylogenetic positioning relative to seven apicomplexan parasites using deduced amino acid sequences from 150 genes placed it as a sister taxon to Theileria spp.

Conclusions

The EMA family does not fit the paradigm for classical antigenic variation, and we propose a novel model describing the role of the EMA family in persistence. T. equi has lost the putative genes for host cell transformation, or the genes were acquired by T. parva and T. annulata after divergence from T. equi. Our analysis identified 50 genes that will be useful for definitive phylogenetic classification of T. equi and closely related organisms.

Cloning, expression, and characterization of Babesia gibsoni dihydrofolate reductase-thymidylate synthase: inhibitory effect of antifolates on its catalytic activity and parasite proliferation

Dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a well-validated antifolate drug target in certain pathogenic apicomplexans, but not in the genus Babesia, including Babesia gibsoni. Therefore, we isolated, cloned, and expressed the wild-type B. gibsoni dhfr-ts gene in Escherichia coli and evaluated the inhibitory effect of antifolates on its enzyme activity, as well as on in vitro parasite growth. The full-length gene consists of a 1,548-bp open reading frame encoding a 58.8-kDa translated peptide containing DHFR and TS domains linked together in a single polypeptide chain. Each domain contained active-site amino acid residues responsible for the enzymatic activity. The expressed soluble recombinant DHFR-TS protein was approximately 57 kDa after glutathione S-transferase (GST) cleavage, similar to an approximately 58-kDa native enzyme identified from the parasite merozoite. The non-GST fusion recombinant DHFR enzyme revealed K(m) values of 4.70 +/- 0.059 (mean +/- standard error of the mean) and 9.75 +/- 1.64 microM for dihydrofolic acid (DHF) and NADPH, respectively. Methotrexate was a more-potent inhibitor of the enzymatic activity (50% inhibition concentration [IC(50)] = 68.6 +/- 5.20 nM) than pyrimethamine (IC(50) = 55.0 +/- 2.08 microM) and trimethoprim (IC(50) = 50 +/- 12.5 microM). Moreover, the antifolates' inhibitory effects on DHFR enzyme activity paralleled their inhibition of the parasite growth in vitro, indicating that the B. gibsoni DHFR could be a model for studying antifolate compounds as potential drug candidates. Therefore, the B. gibsoni DHFR-TS is a molecular antifolate drug target.

Genome sequence of Babesia bovis and comparative analysis of apicomplexan hemoprotozoa

Babesia bovis is an apicomplexan tick-transmitted pathogen of cattle imposing a global risk and severe constraints to livestock health and economic development. The complete genome sequence was undertaken to facilitate vaccine antigen discovery, and to allow for comparative analysis with the related apicomplexan hemoprotozoa Theileria parva and Plasmodium falciparum. At 8.2 Mbp, the B. bovis genome is similar in size to that of Theileria spp. Structural features of the B. bovis and T. parva genomes are remarkably similar, and extensive synteny is present despite several chromosomal rearrangements. In contrast, B. bovis and P. falciparum, which have similar clinical and pathological features, have major differences in genome size, chromosome number, and gene complement. Chromosomal synteny with P. falciparum is limited to microregions. The B. bovis genome sequence has allowed wide scale analyses of the polymorphic variant erythrocyte surface antigen protein (ves1 gene) family that, similar to the P. falciparum var genes, is postulated to play a role in cytoadhesion, sequestration, and immune evasion. The ∼150 ves1 genes are found in clusters that are distributed throughout each chromosome, with an increased concentration adjacent to a physical gap on chromosome 1 that contains multiple ves1-like sequences. ves1 clusters are frequently linked to a novel family of variant genes termed smorfs that may themselves contribute to immune evasion, may play a role in variant erythrocyte surface antigen protein biology, or both. Initial expression analysis of ves1 and smorf genes indicates coincident transcription of multiple variants. B. bovis displays a limited metabolic potential, with numerous missing pathways, including two pathways previously described for the P. falciparum apicoplast. This reduced metabolic potential is reflected in the B. bovis apicoplast, which appears to have fewer nuclear genes targeted to it than other apicoplast containing organisms. Finally, comparative analyses have identified several novel vaccine candidates including a positional homolog of p67 and SPAG-1, Theileria sporozoite antigens targeted for vaccine development. The genome sequence provides a greater understanding of B. bovis metabolism and potential avenues for drug therapies and vaccine development.

Vector-transmitted blood parasites cause some of the most widely distributed, serious, and poorly controlled diseases globally, including the most severe form of human malaria caused by Plasmodium falciparum. In livestock, tick-transmitted blood parasites include the protozoa Theileria parva, the cause of East Coast fever and Babesia bovis, the cause of tick fever, to which well over half of the world's cattle population are at risk. There is a critical need to better understand the mechanisms by which these parasites are transmitted, persist, and cause disease in order to optimize methods for control, including development of vaccines. This manuscript presents the genome sequence of B. bovis, and provides a whole genome comparative analysis with P. falciparum and T. parva. Genome-wide characterization of the B. bovis antigenically variable ves1 family reveals interesting differences in organization and expression from the related P. falciparum var genes. The second largest gene family (smorf) in B. bovis was newly discovered and may itself be involved in persistence, highlighting the utility of this approach in gene discovery. Organization and structure of the B. bovis genome is most similar to that of Theileria, and despite common features in clinical outcome is limited to microregional similarity with P. falciparum. Comparative gene analysis identifies several previously unknown proteins as homologs of vaccine candidates in one or more of these parasites, and candidate genes whose expression might account for unique properties such as the ability of Theileria to reversibly transform leukocytes.

Expressed sequence tag (EST) analysis of the erythrocytic stages of Babesia bovis

Expressed sequence tags (ESTs) provide an efficient way to identify large numbers of genes expressed in a specific stage of the life cycle of an organism. Here we analysed approximately 13,000 ESTs derived from the erythrocytic stage of the apicomplexan parasite Babesia bovis. The ESTs were clustered in order to obtain information on the expression level of a gene and to increase sequence length and reliability. A total of 3522 clusters were obtained and annotated using BLAST algorithms. The clusters were estimated to represent approximately 2600 genes of which in total approximately 2.1 Mbp sequence information was obtained. Expression levels of the genes, as determined by the numbers of ESTs contained within a cluster, were compared to those of their closest homologs in the erythrocytic stage of Plasmodium falciparum and Toxoplasma gondii tachyzoites. Pathways that are represented relatively abundant in B. bovis are, amongst others, the purine salvage pathway (displaying characteristics not identified before in apicomplexans), isoprenoid biosynthesis in the apicoplast and many genes encoding mitochondrial proteins. Especially remarkable in the latter group are the F-type ATPases - which are hardly expressed in P. falciparum and T. gondii - and two highly expressed glycerol-3-phosphate dehydrogenases creating a shuttle possibly controlling the cytoplasmic NADH/NAD+ -ratio. A comparison of known antigenic proteins from Australian and American strains of B. bovis with the Israel strain used here identifies considerable sequence variation in the rhoptry associated protein-1 (RAP-1), merozoite surface proteins of the variable merozoite surface antigen (VMSA) family and spherical body proteins. Analysis of the EST clusters representing the variable erythocyte surface antigen family reveals many variant transcripts of which a few are dominant. Two putative pseudogenes also seem to be transcribed at high levels.

A Babesia bovis merozoite protein with a domain architecture highly similar to the thrombospondin-related anonymous protein (TRAP) present in Plasmodium sporozoites

Recognition and invasion of host cells is a key step in the life-cycle of all apicomplexan parasites. The thrombospondin-related anonymous protein (TRAP) of Plasmodium sporozoites is directly involved in both processes and shares conserved adhesive domains with micronemal transmembrane proteins of other apicomplexans. Here, we report the cloning and characterization of a Babesia bovis TRAP homologue (BbTRAP). It was predicted to be a type 1 transmembrane protein containing a von Willebrand Factor A domain (vWFA), a thrombospondin type 1 domain (TSP1), a conserved transmembrane region and a conserved cytoplasmic C-terminus, thus resembling the domain arrangement of Plasmodium TRAP. In contrast to Plasmodium TRAP, BbTRAP was shown to be present during the asexual erythrocytic cycle, being located mainly at the apical side of merozoites. Polyclonal rabbit antisera directed against synthetic peptides derived from the TSP1 domain or the C-terminal end of the ectodomain were shown to inhibit erythrocyte invasion in vitro. Both antisera recognized a 75 kDa protein in merozoite extracts as well as in a protein fraction that was secreted into the extracellular milieu during in vitro invasion of erythrocytes.