Geno- and phenotypic characteristics of a transfected Babesia bovis 6-Cys-E knockout clonal line

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.

Disruption of a DNA fragment that encodes the microneme adhesive repeat domain-containing region of the BBOV_III011730 does not affect the blood stage growth of Babesia bovis in vitro

Babesia bovis, an intraerythrocytic hemoprotozoan parasite, causes the most pathogenic form of bovine babesiosis, negatively impacting the cattle industry. Comprehensive knowledge of B. bovis biology is necessary for developing control methods. In cattle, B. bovis invades the red blood cells (RBCs) and reproduces asexually. Micronemal proteins, which bind to sialic acid of host cells via their microneme adhesive repeat (MAR) domains, are believed to play a key role in host cell invasion by apicomplexan parasites. In this study, we successfully deleted the region encoding MAR domain of the BBOV_III011730 by integrating a fusion gene of enhanced green fluorescent protein-blasticidin-S-deaminase into the genome of B. bovis. The transgenic B. bovis, lacking the MAR domain of the BBOV_III011730, invaded bovine RBCs in vitro and grew at rates similar to the parental line. In conclusion, our study revealed that the MAR domain is non-essential for the intraerythrocytic development of B. bovis in vitro.

Comparison of high throughput RNA sequences between Babesia bigemina and Babesia bovis revealed consistent differential gene expression that is required for the Babesia life cycle in the vertebrate and invertebrate hosts

Bovine babesiosis caused by Babesia bigemina and Babesia bovis is an economically important disease that affects cattle worldwide. Both B. bigemina and B. bovis are transovarially transmitted by Rhipicephalus ticks. However, little is known regarding parasite gene expression during infection of the tick vector or mammalian host, which has limited the development of effective control strategies to alleviate the losses to the cattle industry. To understand Babesia gene regulation during tick and mammalian host infection, we performed high throughput RNA-sequencing using samples collected from calves and Rhipicephalus microplus ticks infected with B. bigemina. We evaluated gene expression between B. bigemina blood-stages and kinetes and compared them with previous B. bovis RNA-seq data. The results revealed similar patterns of gene regulation between these two tick-borne transovarially transmitted Babesia parasites. Like B. bovis, the transcription of several B. bigemina genes in kinetes exceeded a 1,000-fold change while a few of these genes had a >20,000-fold increase. To identify genes that may have important roles in B. bigemina and B. bovis transovarial transmission, we searched for genes upregulated in B. bigemina kinetes in the genomic datasets of B. bovis and non-transovarially transmitted parasites, Theileria spp. and Babesia microti. Using this approach, we identify genes that may be potential markers for transovarial transmission by B. bigemina and B. bovis. The findings presented herein demonstrate common Babesia genes linked to infection of the vector or mammalian host and may contribute to elucidating strategies used by the parasite to complete their life cycle.

A Culture-Adapted Strain of Babesia bovis Has Reduced Subpopulation Complexity and Is Unable to Complete Its Natural Life Cycle in Ticks

Babesia bovis natural field strains are composed of several geno-phenotypically distinct subpopulations. This feature, together with possible epigenetic modifications, may facilitate adaptation to variable environmental conditions. In this study we compare geno-phenotypical features among long-term (more than 12 years) (LTCP) and short-term cultured B. bovis parasites (STCP) derived from the B. bovis S74-T3Bo strain. LTCPs intraerythrocytic forms are smaller in size than STCPs and have faster in vitro growth rate. In contrast to its parental strain, the LTCP lack expression of the sexual stage specific 6cysA and 6cysB proteins and are unable to develop sexual forms upon in vitro sexual stage induction. Consistently, in contrast to its parental strain, LTCPs have reduced virulence and are not transmissible to cattle by vector competent Rhipicephalus microplus (R. microplus). Similar to previous comparisons among attenuated and virulent B. bovis strains, the LTCP line has decreased genomic diversity compared to the STCP line. Thus, LTCP may contribute to our understanding of adaptive mechanisms used by the parasites in response to environmental changes, protective immunity, virulence, and transmission by ticks. In addition, LTCPs may be considered as candidates for a non-tick transmissible vaccine against bovine babesiosis.

Recent Advances in Molecular Genetic Tools for Babesia

Development of in vitro culture and completion of genome sequencing of several Babesia parasites promoted the efforts to establish transfection systems for these parasites to dissect the gene functions. It has been more than a decade since the establishment of first transfection for Babesia bovis, the causative agent of bovine babesiosis. However, the number of genes that were targeted by genetic tools in Babesia parasites is limited. This is partially due to the low efficiencies of these methods. The recent adaptation of CRISPR/Cas9 for genome editing of Babesia bovis can accelerate the efforts for dissecting this parasite’s genome and extend the knowledge on biological aspects of erythrocytic and tick stages of Babesia. Additionally, glmS ribozyme as a conditional knockdown system is available that could be used for the characterization of essential genes. The development of high throughput genetic tools is needed to dissect the function of multigene families, targeting several genes in a specific pathway, and finally genome-wide identification of essential genes to find novel drug targets. In this review, we summarized the current tools that are available for Babesia and the genes that are being targeted by these tools. This may draw a perspective for the future development of genetic tools and pave the way for the identification of novel drugs or vaccine targets.

Development of a stable transgenic Theileria equi parasite expressing an enhanced green fluorescent protein/blasticidin S deaminase

Theileria equi, an intraerythrocytic protozoan parasite, causes equine piroplasmosis, a disease which negatively impacts the global horse industry. Genetic manipulation is one of the research tools under development as a control method for protozoan parasites, but this technique needs to be established for T. equi. Herein, we report on the first development of a stable transgenic T. equi line expressing enhanced green fluorescent protein/blasticidin S deaminase (eGFP/BSD). To express the exogenous fusion gene in T. equi, regulatory regions of the elongation factor-1 alpha (ef-1α) gene were identified in T. equi. An eGFP/BSD-expression cassette containing the ef-1α gene promoter and terminator regions was constructed and integrated into the T. equi genome. On day 9 post-transfection, blasticidin-resistant T. equi emerged. In the clonal line of T. equi obtained by limiting dilution, integration of the eGFP/BSD-expression cassette was confirmed in the designated B-locus of the ef-1α gene via PCR and Southern blot analyses. Parasitaemia dynamics between the transgenic and parental T. equi lines were comparable in vitro. The eGFP/BSD-expressing transgenic T. equi and the methodology used to generate it offer new opportunities for better understanding of T. equi biology, with the add-on possibility of discovering effective control methods against equine piroplasmosis.

The key to egress? Babesia bovis perforin-like protein 1 (PLP1) with hemolytic capacity is required for blood stage replication and is involved in the exit of the parasite from the host cell

Bovine babesiosis is a tick-borne disease caused by apicomplexan parasites of the Babesia genus that represents a major constraint to livestock production worldwide. Currently available vaccines are based on live parasites which have archetypal limitations. Our goal is to identify candidate antigens so that new and effective vaccines against Babesia may be developed. The perforin-like protein (PLP) family has been identified as a key player in cell traversal and egress in related apicomplexans and it was also identified in Babesia, but its function in this parasite remains unknown. The aim of this work was to define the PLP family in Babesia and functionally characterize PLP1, a representative member of the family in Babesia bovis. Bioinformatic analyses demonstrate a variable number of plp genes (four to eight) in the genomes of six different Babesia spp. and conservation of the family members at the secondary and tertiary structure levels. We demonstrate here that Babesia PLPs contain the critical domains present in other apicomplexan PLPs to display the lytic capacity. We then focused on the functional characterization of PLP1 of B. bovis, both in vitro and in vivo. PLP1 is expressed and exposed to the host immune system during infection and has high hemolytic capacity under a wide range of conditions in vitro. A B. bovis plp1 knockout line displayed a decreased growth rate in vitro compared with the wild type strain and a peculiar phenotype consisting of multiple parasites within a single red blood cell, although at low frequency. This phenotype suggests that the lack of PLP1 has a negative impact on the mechanism of egression of the parasite and, therefore, on its capacity to proliferate. It is possible that PLP1 is associated with other proteins in the processes of invasion and egress, which were found to have redundant mechanisms in related apicomplexans. Future work will be focused on unravelling the network of proteins involved in these essential parasite functions.

Transgenic Babesia bovis lacking 6-Cys sexual-stage genes as the foundation for non-transmissible live vaccines against bovine babesiosis

Babesia bovis, a tick-borne apicomplexan parasite responsible for bovine babesiosis has a complex life cycle including sexual development in its Rhipicephalus microplus vector. Understanding the molecular mechanisms involved in sexual development is essential for developing future-generation transmission blocking vaccines (TBVs) and/or non-transmissible attenuated live vaccines. The widely conserved members of the 6-Cys gene family likely play roles in the development of sexual stages of B. bovis, and are candidates for developing novel TBV. The recently defined sexual markers 6-CysA and 6-CysB of B. bovis are strain-conserved and exclusively surface-expressed in tick-stage parasites. However, the high level of sequence identity among the 6-Cys A and 6-Cys B proteins (52% identity), together with similar 6-Cys domain distribution and sub-cellular localization, are suggestive of redundant function. We hypothesized that disruption of both 6-CysA and 6-CysB in B. bovis would result in unaltered ability of the parasite to invade and grow in red blood cells (RBCs), with concomitant loss of the transmission phenotype. Taking advantage of their contiguous genome localization, we generated a double gene-knockout system to disrupt a 3287 bp region encompassing both 6-CysA and 6-CysB genes using a single transfection plasmid. The resulting red-fluorescent ΔAΔB 6-Cys B. bovis transgenic parasite line was able to grow continuously in bovine RBCs in vitro at a similar rate to wild-type parasites, demonstrating that the 6-CysA and 6-CysB genes are not required for the development of blood-stage parasites. This novel gene manipulation approach will allow future experiments aimed at determining the tick-transmission phenotype of parasites lacking tick-stage genes. Parasites deficient in genes required for sexual reproduction could be the foundation for genetically-defined, non-transmissible live vaccines against bovine babesiosis. Developing a non-tick transmissible live vaccine based on attenuated parasites unable to express critical 6-Cys genes and including a molecular vaccine marker could help reduce the burden of bovine babesiosis globally.

Stable transformation of Babesia bigemina and Babesia bovis using a single transfection plasmid

Babesia bigemina and Babesia bovis, are the two major causes of bovine babesiosis, a global neglected disease in need of improved methods of control. Here, we describe a shared method for the stable transfection of these two parasites using electroporation and blasticidin/blasticidin deaminase as a selectable marker. Stably transfected B. bigemina and B. bovis were obtained using a common transfection plasmid targeting the enhanced green fluorescent protein-BSD (egfp-bsd) fusion gene into the elongation factor-1α (ef-1α) locus of B. bigemina and B. bovis under the control of the B. bigemina ef-1α promoter. Sequencing, Southern blotting, immunoblotting and immunofluorescence analysis of parasite-infected red blood cells, demonstrated that the egfp-bsd gene was expressed and stably integrated solely into the ef-1α locus of both, B. bigemina and B. bovis. Interestingly, heterologous B. bigemina ef-1α sequences were able to drive integration into the B. bovis genome by homologous recombination, and the stably integrated B. bigemina ef-1α-A promoter is fully functional in B. bovis. Collectively, the data provides a new tool for genetic analysis of these parasites, and suggests that the development of vaccine platform delivery systems based on transfected B. bovis and B. bigemina parasites using homologous and heterologous promoters is feasible.

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.