Intergenic regions in the rhoptry associated protein-1 (rap-1) locus promote exogenous gene expression in Babesia bovis

Transient transfection of Babesia ovis using heterologous promoters

Babesia species, etiological agents of babesiosis, a recognized emerging tick-borne disease, are a significant animal and human health concern with a worldwide socio-economic impact. The development of genetic manipulation techniques, such as transfection technology, is pivotal to improve knowledge regarding the biology of these poorly studied parasites towards better disease control strategies. For Babesia ovis, responsible for ovine babesiosis, a tick-borne disease of small ruminants, these tools are not yet available. The present study was based on the existence of interchangeable cross-species functional promoters between Babesia species. Herein, we describe for the first time B. ovis transient transfection using two heterologous promoters, the ef-1α-B intergenic regions from B. bovis and B. ovata. Their ability to drive expression of a reporter luciferase in B. ovis supports their cross-species functionality. Also, the ef-1α-B promoter region from B. ovata resulted in statistically significantly higher luminescence values in comparison to the control, thus a possibly suitable promoter for stable gene expression. Evaluation of transfection efficiency using qPCR demonstrated that higher luminescence levels were due to promoter strength rather than a higher transfection efficiency. These findings represent a step forward in the development of methods for B. ovis genetic manipulation, an undoubtedly necessary tool to study this parasite basic biology, including its life cycle, the parasite interactions with host cells and virulence factors.

Knockout of Babesia bovis rad51 ortholog and its complementation by expression from the BbACc3 artificial chromosome platform

Babesia bovis establishes persistent infections of long duration in cattle, despite the development of effective anti-disease immunity. One mechanism used by the parasite to achieve persistence is rapid antigenic variation of the VESA1 cytoadhesion ligand through segmental gene conversion (SGC), a phenomenon thought to be a form of homologous recombination (HR). To begin investigation of the enzymatic basis for SGC we initially identified and knocked out the Bbrad51 gene encoding the B. bovis Rad51 ortholog. BbRad51 was found to be non-essential for in vitro growth of asexual-stage parasites. However, its loss resulted in hypersensitivity to methylmethane sulfonate (MMS) and an apparent defect in HR. This defect rendered attempts to complement the knockout phenotype by reinsertion of the Bbrad51 gene into the genome unsuccessful. To circumvent this difficulty, we constructed an artificial chromosome, BbACc3, into which the complete Bbrad51 locus was inserted, for expression of BbRad51 under regulation by autologous elements. Maintenance of BbACc3 makes use of centromeric sequences from chromosome 3 and telomeric ends from chromosome 1 of the B. bovis C9.1 line. A selection cassette employing human dihydrofolate reductase enables recovery of transformants by selection with pyrimethamine. We demonstrate that the BbACc3 platform is stably maintained once established, assembles nucleosomes to form native chromatin, and expands in telomere length over time. Significantly, the MMS-sensitivity phenotype observed in the absence of Bbrad51 was successfully complemented at essentially normal levels. We provide cautionary evidence, however, that in HR-competent parasites BbACc3 can recombine with native chromosomes, potentially resulting in crossover. We propose that, under certain circumstances this platform can provide a useful alternative for the genetic manipulation of this group of parasites, particularly when regulated gene expression under the control of autologous elements may be important.

Identification and characterization of interchangeable cross-species functional promoters between Babesia gibsoni and Babesia bovis

The development of transgenic techniques has been reported in many protozoan parasites over the past few years. We recently established a successful transient transfection system for Babesia gibsoni based on Bg 5'-ef-1α promoter. This study investigated 6 homologous and 6 heterologous promoters for B. gibsoni and B. bovis and identified novel interchangeable cross-species functional promoters between B. gibsoni and B. bovis. Ten out of twelve promoters had heterologous promoter function. In particular, Bg 5'-ef-1α and Bg 5'-actin heterologous promoters resulted in a significantly higher luciferase activity than Bb 5'-ef-1α homologous promoter in B. bovis. The present study showed that Bg 5'-actin promoted the highest luciferase activity in both B. gibsoni and B. bovis. The study further indicates that heterologous promoter function widely exists between B. gibsoni and B. bovis. This finding is an important step for future stable transfection construct design and for the production of vaccines based on transfected B. gibsoni and B. bovis parasites.

Advances in the application of genetic manipulation methods to apicomplexan parasites

Apicomplexan parasites such as Babesia, Theileria, Eimeria, Cryptosporidium and Toxoplasma greatly impact animal health globally, and improved, cost-effective measures to control them are urgently required. These parasites have complex multi-stage life cycles including obligate intracellular stages. Major gaps in our understanding of the biology of these relatively poorly characterised parasites and the diseases they cause severely limit options for designing novel control methods. Here we review potentially important shared aspects of the biology of these parasites, such as cell invasion, host cell modification, and asexual and sexual reproduction, and explore the potential of the application of relatively well-established or newly emerging genetic manipulation methods, such as classical transfection or gene editing, respectively, for closing important gaps in our knowledge of the function of specific genes and proteins, and the biology of these parasites. In addition, genetic manipulation methods impact the development of novel methods of control of the diseases caused by these economically important parasites. Transient and stable transfection methods, in conjunction with whole and deep genome sequencing, were initially instrumental in improving our understanding of the molecular biology of apicomplexan parasites and paved the way for the application of the more recently developed gene editing methods. The increasingly efficient and more recently developed gene editing methods, in particular those based on the CRISPR/Cas9 system and previous conceptually similar techniques, are already contributing to additional gene function discovery using reverse genetics and related approaches. However, gene editing methods are only possible due to the increasing availability of in vitro culture, transfection, and genome sequencing and analysis techniques. We envisage that rapid progress in the development of novel gene editing techniques applied to apicomplexan parasites of veterinary interest will ultimately lead to the development of novel and more efficient methods for disease control.

Transient transfection of intraerythrocytic Babesia gibsoni using elongation factor-1 alpha promoter

The development of gene manipulation techniques has been reported in many protozoan parasites over the past few years. However, these techniques have not yet been established for Babesia gibsoni. Here, we report for the first time, the successful transient transfection of B. gibsoni. The plasmid containing the firefly luciferase reporter gene (pBS-ELA) was transfected into B. gibsoni by an AMAXA 4D Nucleofector™ device. Transfection using program FA113 and Lonza buffer SF showed the highest luciferase expression. Twenty micrograms of plasmid produced the highest relative transfection efficiency. The fluorescent protein-expressing parasites were determined by GFP-containing plasmid (pBS-EGA) at 48 and 72h post transfection. This finding is the first step towards a stable transfection method for B. gibsoni, which may contribute to a better understanding of the biology of the parasite.

Identification of interchangeable cross-species function of elongation factor-1 alpha promoters in Babesia bigemina and Babesia bovis

Background

Tick-borne Babesia bigemina is responsible for acute and potentially lethal hemolytic disease in cattle. The development of genetic manipulation tools necessary to the better understanding of parasite biology is currently limited by the lack of a complete parasite genome and experimental tools such as transfection. Effective promoters, required to regulate expression of transgenes, such as the elongation factor-1 alpha (ef-1α), have been identified in other apicomplexans such as Babesia bovis and Plasmodium falciparum.

Methods

The B. bigemina ef-1a locus was defined by searching a partial genome library of B. bigemina (Sanger Institute). Presence of an intron in the 5’ untranslated region was determined by 5’ Rapid Amplification of cDNA Ends (RACE) analysis. Promoter activity was determined by measurement of luciferase expression at several time points after electroporation, efficiency of transfections and normalization of data was determined by quantitative PCR and by the percentage of parasitized erythrocytes.

Results

The ef-1α locus contains two identical head to head ef-1α genes separated by a 1.425 kb intergenic (IG) region. Significant sequence divergence in the regions upstream of the inverted repeats on each side of the B. bigemina IG region suggest independent regulation mechanisms for controlling expression of each of the two ef-1α genes. Plasmid constructs containing the 5’ and 3’ halves of the IG regions controlling the expression of the luciferase gene containing a 3’ region of a B. bigemina rap-1a gene, were generated for the testing of luciferase activity in transiently transfected parasites. Both halves of the ef-1α IG region tested showed the ability to promote high level production of luciferase. Moreover, both B. bigemina ef-1α promoters are also active in transiently transfected B. bovis and conversely, a B. bovis ef-1α promoter is active in transiently transfected B. bigemina.

Conclusions

Collectively these data demonstrate the existence of two distinct promoters with homologous and heterologous promoter function in B. bigemina and B. bovis which is described for the first time in Babesia species. This study is of significance for development of interspecies stable transfection systems for B. bigemina and for B. bovis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1859-9) contains supplementary material, which is available to authorized users.

Genomic resources for a unique, low-virulence Babesia taxon from China

BACKGROUND

Babesiosis is a socioeconomically important tick-borne disease of animals (including humans) caused by haemoprotozoan parasites. The severity of babesiosis relates to host and parasite factors, particularly virulence/pathogenicity. Although Babesia bovis is a particularly pathogenic species of cattle, there are species of Babesia of ruminants that have limited pathogenicity. For instance, the operational taxonomic unit Babesia sp. Xinjiang (abbreviated here as Bx) of sheep from China is substantially less virulent/pathogenic than B. bovis is in cattle. Although the reason for this distinctiveness is presently unknown, it is possible that Bx has a reduced ability to adhere to cells or evade/suppress immune responses, which might relate to particular proteins, such as the variant erythrocyte surface antigens (VESAs).

RESULTS

We sequenced and annotated the 8.4 Mb nuclear draft genome of Bx and compared it with those of B. bovis and B. bigemina by synteny analysis; we also investigated the genetic relationship of Bx with selected Babesia species and related apicomplexans for which genomic datasets are available, and explored the VESA complement in Bx.

CONCLUSIONS

The availability of the Bx genome now provides unique opportunities to elucidate aspects of the molecular biology, biochemistry and physiology of Bx, and to explore the reason(s) for its limited virulence and/or apparent ability to evade immune attack by the host animal. Moreover, the present genomic resource and an in vitro culture system for Bx raises the prospect of establishing a functional genomic platform to explore essential genes as new intervention targets against babesiosis.

Establishment of transient and stable transfection systems for Babesia ovata

Background

Bovine babesiosis is a tick-borne disease caused by several species of Babesia which produce acute and fatal disease in cattle and affect livestock industry worldwide. Babesia ovata is a benign species widespread in east Asian countries and causes anemia, particularly in cattle which are co-infected with Theileria orientalis. The development of genetic manipulation methods is necessary to improve our understanding of the basic biology of protozoan pathogens toward a better control of disease. Such tools have not been developed for B. ovata, and are the aim of this study.

Methods

In this study we transfected constructs that were designed to evaluate the ability of several B. ovata promoter candidates to drive expression of a reporter luciferase. We found that the elongation factor-1 alpha intergenic region (ef-1α IG) and the actin 5’ non-coding region (NR) had highest promoter activities. To establish a stable transfection system, we generated a plasmid construct in which the ef-1α IG promoter drives gfp expression, and the actin 5’ NR mediates expression of the selectable marker hdhfr. The plasmid was designed for episomal transfection, as well as to integrate by double cross-over homologous recombination into the ef-1α locus. Circular or linearized plasmid was transfected by electroporation into in vitro cultured B. ovata and retention of the plasmid was facilitated by drug selection with 5 nM WR99210 initiated 48 h after transfection.

Results

After one-week cultivation with WR99210, GFP-expressing parasites were observed by fluorescence microscopy. Integration of the plasmid construct into the ef-1α locus was confirmed by PCR, Southern blot analysis, and sequencing of recombination sites. These results confirm successful development of a stable transfection system for B. ovata.

Conclusion

The current study provides a fundamental molecular tool to aid in molecular and cellular studies of B. ovata.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1439-z) contains supplementary material, which is available to authorized users.

Transfection of Babesia bovis by Double Selection with WR99210 and Blasticidin-S and Its Application for Functional Analysis of Thioredoxin Peroxidase-1

Genetic manipulation is an essential technique to analyze gene function; however, limited methods are available for Babesia bovis, a causative pathogen of the globally important cattle disease, bovine babesiosis. To date, two stable transfection systems have been developed for B. bovis, using selectable markers blasticidin-S deaminase (bsd) or human dihydrofolate reductase (hdhfr). In this work, we combine these two selectable markers in a sequential transfection system. Specifically, a parent transgenic B. bovis line which episomally expresses green fluorescent protein (GFP) and human dihydrofolate reductase (hDHFR), was transfected with a plasmid encoding a fusion protein consisting of red fluorescent protein (RFP) and blasticidin-S deaminase (BSD). Selection with WR99210 and blasticidin-S resulted in the emergence of parasites double positive for GFP and RFP. We then applied this method to complement gene function in a parasite line in which thioredoxin peroxidase-1 (Bbtpx-1) gene was knocked out using hDHFR as a selectable marker. A plasmid was constructed harboring both RFP-BSD and Bbtpx-1 expression cassettes, and transfected into a Bbtpx-1 knockout (KO) parasite. Transfectants were independently obtained by two transfection methods, episomal transfection and genome integration. Complementation of Bbtpx-1 resulted in full recovery of resistance to nitrosative stress, via the nitric oxide donor sodium nitroprusside, which was impaired in the Bbtpx-1 KO parasites. In conclusion, we developed a sequential transfection method in B. bovis and subsequently applied this technique in a gene complementation study. This method will enable broader genetic manipulation of Babesia toward enhancing our understanding of the biology of this parasite.

Rhoptry-associated protein (rap-1) genes in the sheep pathogen Babesia sp. Xinjiang: Multiple transcribed copies differing by 3' end repeated sequences

Sheep babesiosis occurs mainly in tropical and subtropical areas. The sheep parasite Babesia sp. Xinjiang is widespread in China, and our goal is to characterize rap-1 (rhoptry-associated protein 1) gene diversity and expression as a first step of a long term goal aiming at developing a recombinant subunit vaccine. Seven different rap-1a genes were amplified in Babesia sp. Xinjiang, using degenerate primers designed from conserved motifs. Rap-1b and rap-1c gene types could not be identified. In all seven rap-1a genes, the 5' regions exhibited identical sequences over 936 nt, and the 3' regions differed at 28 positions over 147 nt, defining two types of genes designated α and β. The remaining 3' part varied from 72 to 360 nt in length, depending on the gene. This region consists of a succession of two to ten 36 nt repeats, which explains the size differences. Even if the nucleotide sequences varied, 6 repeats encoded the same stretch of amino acids. Transcription of at least four α and two β genes was demonstrated by standard RT-PCR.

Differentiation between Israeli B. bovis vaccine strain and field isolates

The present study demonstrated for the first time the ability to distinguish between the Israeli Babesia bovis vaccine strain and field isolates. The existence of an additional EcoRI restriction site in the rhoptry-associated protein-1 (rap-1) gene, which is unique to the Israeli vaccine strain, and the abolition of one of the HaeIII restriction sites in the rap-1 gene of the vaccine strain enabled distinction between the Israeli B. bovis vaccine strain and field isolates, and this was the basis for polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) development. ClustalW sequence alignment of RAP-1-deduced amino acids of the Israeli B. bovis strains and of field isolates showed that the total sequence identity among the RAP-1 amino acid sequences ranged from 97.5% to 100%. However, comparison between amino acids of RAP-1 of the Israeli vaccine strain and of field isolates, on the one hand, and B. bovis strains from Argentina, Mexico, Brazil, and USA, on the other hand, revealed 90% identity. The PCR-RFLP assay offered the great advantage of being able to distinguish between vaccine and field isolates in mixtures and provide new insight into the molecular epidemiology of B. bovis infections in Israel.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

SUMMARY Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

Bovine babesiosis in the 21st century: advances in biology and functional genomics

Bovine babesiosis caused by the protozoan parasite, Babesia bovis, remains a significant cause of avoidable economic losses to the livestock industry in many countries throughout the world. The molecular mechanisms underlying the pathophysiology of severe disease in susceptible cattle are not well understood and the tools available to study the biology of the parasite, including technologies for genetic manipulation, have only recently been developed. Recent availability of multiple parasite genomes and bioinformatic tools, in combination with the development of new biological reagents, will facilitate our better understanding of the parasite. This will ultimately assist in the identification of novel targets for the development of new therapeutics and vaccines. Here we describe some recent advances in Babesia research and highlight some important challenges for the future.

In vitro culture systems for the study of apicomplexan parasites in farm animals

In vitro culture systems represent powerful tools for the study of apicomplexan parasites such as Cryptosporidium, Eimeria, Sarcocystis, Neospora, Toxoplasma, Besnoitia, Babesia and Theileria, all with high relevance for farm animals. Proliferative stages of these parasites have been cultured in vitro employing a large variety of cell culture and explant approaches. For some, such as Cryptosporidium and Eimeria, the sexual development has been reproduced in cell cultures, while for others, animal experimentation is required to fulfill the life cycle. In vitro cultures have paved the way to exploit the basic biology of these organisms, and had a major impact on the development of tools for diagnostic purposes. With the aid of in vitro cultivation, studies on host-parasite interactions, on factors involved in innate resistance, stage conversion and differentiation, genetics and transfection technology, vaccine candidates and drug effectiveness could be carried out. The use of transgenic parasites has facilitated high-throughput screening of anti-microbial compounds that are active against the proliferative stages. Here, we review the basic features of cell culture-based in vitro systems for apicomplexan parasites that are relevant for farm animals, and discuss their applications with a focus on drug identification and studies of stage differentiation.

Characterization of the unusual bidirectional ves promoters driving VESA1 expression and associated with antigenic variation in Babesia bovis

Rapid clonal antigenic variation in Babesia bovis involves the variant erythrocyte surface antigen-1 (VESA1) protein expressed on the infected-erythrocyte surface. Because of the significance of this heterodimeric protein for demonstrated mechanisms of parasite survival and virulence, there is a need to understand how expression of the ves multigene family encoding this protein is controlled. As an initial step toward this goal, we present here initial characterization of the ves promoter driving transcription of VESA1a and -1b subunits. A series of transfection constructs containing various sequence elements from the in vivo locus of active ves transcription (LAT) were used to drive expression of the firefly luciferase gene in a dual luciferase-normalized assay. The results of this approach reveal the presence of two bidirectional promoter activities within the 434-bp intergenic region (IGr), influenced by putative regulatory sequences embedded within the flanking ves1α and ves1β genes. Repressor-like effects on the apposing gene were observed for intron 1 of both ves1α and ves1β. This effect is apparently not dependent upon intronic promoter activity and acts only in cis. The expression of genes within the ves family is likely modulated by local elements embedded within ves coding sequences outside the intergenic promoter region in concert with chromatin modifications. These results provide a framework to help us begin to understand gene regulation during antigenic variation in B. bovis.

Recent insights into alteration of red blood cells by Babesia bovis: moovin' forward

Over the past decade or so, our understanding of the biology of apicomplexan parasites has increased dramatically, particularly in the case of malaria. Notable achievements are the availability of complete genome sequences, transcriptome and proteome profiles and the establishment of in vitro transfection techniques for asexual-stage malaria parasites. Interestingly, despite their major economic importance and striking similarities with malaria, Babesia parasites have been relatively ignored, but change is on the horizon. Here, we bring together recent work on Babesia bovis parasites which are beginning to unravel the molecular mechanisms that underlie the pathogenesis of babesiosis and highlight some opportunities and challenges that lie ahead.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

Cellular adhesive phenomena in apicomplexan parasites of red blood cells

The apicomplexan parasites Babesia and Plasmodium are related, yet phylogenetically distinct haemoprotozoa that infect red blood cells and cause severe diseases of major human and veterinary importance. A variety of cellular and molecular interactions are pivotal in many aspects of the pathogenicity of these two parasites. Comparison of the cellular and molecular mechanisms that culminate in accumulation of parasitised red blood cells in the microvasculature of cattle infected with Babesia bovis (babesiosis) and humans infected with Plasmodium falciparum (falciparum malaria) is particularly instructive given the striking similarities in the pathophysiology of these two important medical and veterinary diseases. While such adhesive phenomena have been studied extensively in malaria, they have received relatively little attention in babesiosis. In this review, we summarise the findings of more than 25 years of research into cellular adhesive phenomena in malaria and speculate on how this body of work can now be applied to Babesia parasites. Such information is fundamental if we are to learn more about the biology of Babesia parasites, the cellular and molecular mechanisms by which they cause infection and disease and how to develop novel therapeutic strategies or vaccines for both Babesia and malaria infections.