Identification and functional study of a novel 2-cys peroxiredoxin (BmTPx-1) of Babesia microti

Molecular identification of a thioredoxin peroxidase in Babesia gibsoni with potential against oxidative stress

Babesia gibsoni is the infectious agent of canine babesiosis, a vector-borne infection that poses a global threat to the canine health. As B. gibsoni is an erythrocytic intracellular parasite, the completion of its genome and transcriptome sequencing and analysis facilitates the elucidation of the mechanism of B. gibsoni residue in the erythrocyte. The main function of red blood cells (RBCs) is oxygen delivery; thus, B. gibsoni may be exposed to high levels of oxidative stress. To date, no report is available on the mechanism by which B. gibsoni survives oxidative stress inside the RBCs. In this study, the thioredoxin peroxidase, an important type of peroxidoxin, was identified from B. gibsoni, with 255 amino acids and a molecular weight of 27.7 kDa. There are two conserved "VCP" domains at the N- and C-termini, respectively, indicating that this gene was a 2-Cys peroxiredoxin belonging to the PTZ00137 superfamily. It was named BgTPx-2 and was detected to be located in the B. gibsoni-infected erythrocytes through an indirect immunofluorescence assay using the polyclonal antibody against the recombinant TPx-2. Additionally, its antioxidant activity was analyzed by mixed-function oxidation assay, and BgTPx-2 could protect the pBluescript SK ( +) plasmid from oxidative damage, suggesting an antioxidant function of BgTPx-2. Moreover, the immunogenicity of BgTPx-2 was tested by Western blotting and ELISA using the serum of beagle dogs infected with B. gibsoni, and the positive serum exhibited a detectable and significant antibody response against BgTPx-2 on day 4 and day 9 post-infection, respectively.

A Novel Thioredoxin-Like Protein of Babesia microti Involved in Parasite Pathogenicity

Babesiosis poses a serious threat to immunocompromised individuals and the major etiological species of Babesia for human babesiosis is Babesia microti. Merozoites are a critical stage in the life cycle of Babesia microti. Several merozoite proteins have been demonstrated to play important roles in this process; however, most of the merozoite proteins of B. microti remain unknown. In the present study, we identified a novel merozoite protein of B. microti with similar structure to the thioredoxin (Trx)-like domain of the Trx family, which was named as B. microti Trx-like protein (BmTLP). Western blot assays demonstrated that this protein was expressed by B. microti during the erythrocytic infection process, and its expression peaked on day 7 post-infection in vivo. Immunofluorescence assay further showed that this protein is mainly expressed in B. microti merozoites. BmTLP hold both heparin- and erythrocyte-binding properties, which are critical functions of invasion-related proteins. Immunization with recombinant BmTLP imparted significant protection against B. microti infection in mice. Taken together, these results suggest that the novel merozoite protein, BmTLP, is an important pathogenic molecule of B. microti and may be a possible target for the design of babesiosis control strategy.

The Tick Microbiota Dysbiosis Promote Tick-Borne Pathogen Transstadial Transmission in a Babesia microti-Infected Mouse Model

Ticks are obligate hematophagous ectoparasites. They are important vectors for many pathogens, of both medical and veterinary importance. Antibiotic residues in animal food are known, but very little is known about the effects of antibiotic residues in animals on the microbiome diversity of ticks and tick-borne pathogen transmission. We used a Haemaphysalis longicornis–infested mouse model to evaluate the effect of antibiotic usage on tick microbiome. Nymphal ticks were fed on an antibiotic cocktail-treated or water control mice. Adult ticks molted from nymphs fed on the antibiotic cocktail-treated mouse had a dysbiosed microbiota. Nymphal ticks were also fed on a B. microti–infected mice that had been treated with antibiotic cocktail or water. We found that the B. microti infection in adult ticks with a dysbiosed microbiota (44.7%) was increased compared with the control adult ticks (24.2%) by using qPCR targeting 18S rRNA gene. This may increase the risk of tick-borne pathogens (TBPs) transmission from adult ticks to a vertebrate host. These results show that an antibiotic-treated mouse can induce tick microbiota dysbiosis. Antibiotic treatment of B. microti-infected mouse poses the possibility of increasing transstadial transmission of B. microti from the nymph to the adult H. longicornis. These findings suggest that B. microti transmission may be exacerbated in high antibiotic usage areas.

Tafenoquine Is a Promising Drug Candidate for the Treatment of Babesiosis

Due to drug resistance, commonly used anti-Babesia drugs have limited efficacy against babesiosis and inflict severe side effects. Tafenoquine (TAF) was approved by the U.S. Food and Drug Administration in 2018 for the radical cure of Plasmodium vivax infection and for malaria prophylaxis. Here, we evaluated the efficacy of TAF for the treatment of Babesia infection and elucidated the suspected mechanisms of TAF activity against Babesia parasites. Parasitemia and survival rates of Babesia rodhaini-infected BALB/c and SCID mice were used to explore the role of the immune response in Babesia infection after TAF treatment. Parasitemia, survival rates, body weight, vital signs, complete blood count, and blood biochemistry of B. gibsoni-infected splenectomized dogs were determined to evaluate the anti-Babesia activity and side effects of TAF. Then, to understand the mechanism of TAF activity, hydrogen peroxide was used as an oxidizer for short-term B. rodhaini incubation in vitro, and the expression levels of antioxidant enzymes were confirmed using B. microti-infected mice by reverse transcription-quantitative PCR (qRT-PCR). Acute B. rodhaini and B. gibsoni infections were rapidly eliminated with TAF administration. Repeated administration of TAF or a combination therapy with other antibabesial agents is still needed to avoid a potentially fatal recurrence for immunocompromised hosts. Caution about hyperkalemia should be taken during TAF treatment for Babesia infection. TAF possesses a babesicidal effect that may be related to drug-induced oxidative stress. Considering the lower frequency of glucose-6-phosphate dehydrogenase deficiency in animals compared to that in humans, TAF use on Babesia-infected farm animals and pets is eagerly anticipated.

A cysteine protease of Babesia microti and its interaction with tick cystatins

Babesiosis is a tick-borne protozoonosis caused by Babesia, which can cause fever, hemolytic anemia, hemoglobinuria, and even death. Babesia microti is a parasite found in rodents and can be pathogenic to humans. In this study, the full-length cDNA of a B. microti cysteine protease (BmCYP) was expressed and the recombinant rBmCYP protein analyzed and characterized. BmCYP is encoded by an ORF of 1.3 kb, with a predicted molecular weight of 50 kDa and a theoretical pI of 8.5. The amino acid sequence of BmCYP exhibits an identity of 32.9 to 35.2% with cysteine proteases of Babesia ovis, Babesia bovis, and Theileria, respectively. The results of the proteinase assays show that rBmCYP has cysteine protease enzymatic activity. In addition, we demonstrate that tick cystatins rRhcyst-1 and rRhcyst-2 were able to effectively inhibit the activity of rBmCYP; the inhibition rates were 57.2% and 30.9%, respectively. Tick cystatins Rhcyst-1 and Rhcyst-2 were differentially expressed in ticks that fed on Babesia-infected mice relative to non-infected control ticks. Our results suggest that BmCYP is a functional enzyme with cysteine protease enzymatic activity and may be involved in tick-B. microti interactions.

A Novel Thioredoxin-Dependent Peroxiredoxin (TPx-Q) Plays an Important Role in Defense Against Oxidative Stress and Is a Possible Drug Target in Babesia microti

Thioredoxin peroxidases (TPxs) are ubiquitous cysteine-based peroxidases that reduce peroxides as part of antioxidant defenses and redox signaling and are essential for Babesia microti protection against adverse environment agents like reactive oxygen species (ROS) and reactive nitrogen species (RNS). To better systematically understand TPxs, we identified a novel 2-Cys peroxiredoxin-Q (BmTPx-Q) of B. microti. The full-length BmTPx-Q gene is 653 bp that consists of an intact open reading frame of 594 bp that encodes a 197-amino acid protein. The predicted protein has a molecular weight of 22.3 kDa and an isoelectric point of 9.18. Moreover, BmTPx-Q showed low identity at the amino acid level to other peroxiredoxins (Prxs) among the currently known subfamilies. The recombinant BmTPx-Q protein (rBmTPx-Q) was expressed in Escherichia coli and purified with beads. The native protein BmTPx-Q was detected using mouse anti-BmTPx-Q polyclonal serum with western blotting and indirect immunofluorescence assay (IFA). In addition, enzyme activity was observed using nicotinamide adenine dinucleotide phosphate (NADPH) as substrate and triggered the NADPH-dependent reduction of the Trx/TrxR system. It was also discovered that BmTPx-Q mainly exists as a monomer whether under its native or functional states. In addition, when incubated with Chloroquine diphosphate salt for 24 h in vitro, the expression of BmTPx-Q showed a marked downward trend with the increase of drug concentration. These results suggest that B. microti uses BmTPx-Q to reduce and detoxify hydrogen peroxides to survive and proliferate inside the host. Furthermore, BmTPx-Q showed the lowest identity with host enzymes and could be a potential drug target for the development of novel strategies to control B. microti infection.

4-NBT, a specific inhibitor of Babesia microti thioredoxin reductase, affects parasite biochemistry and proteomic properties

Babesia microti is an emerging zoonotic pathogen that is transmitted by ticks and parasites and propagates in mammalian erythrocytes. Thioredoxin reductase (TrxR) plays a crucial role in B. microti survival by maintaining cellular redox homeostasis. In the present study, 4-nitro-2,1,3-benzothiadiazole (4-NBT) was selected as a specific B. microti TrxR inhibitor by comparing rat and parasite TrxR inhibition levels. Reactive oxygen species (ROS) levels were evaluated using flow cytometry, and in B. microti treated with 4-NBT, ROS levels increased with increasing inhibitor concentration. Furthermore, the inhibitor treatment increased lipid peroxidation and protein carbonyl levels, thus indicating a state of oxidative stress. While B. microti treated with 4-NBT appeared to lose the ability to multiply in mice, the fastigium of parasitemia between the treated and control groups was comparable. Furthermore, a TUNEL assay showed that 4-NBT induces apoptosis in B. microti. Proteomic analysis of B. microti treated with 4-NBT detected 960 proteins. Label-free quantitative proteomic analysis identified 118 proteins that were significantly up-regulated and 37 that were significantly down-regulated in the treatment group relative to the control. Of the differential proteins, proteasome and ribosomal subunit expression was up-regulated, thus suggesting that redundant proteins may be damaged by oxidation and waiting for degradation, while proteins for subsistence are waiting for de novo synthesis. Moreover, the findings obtained herein suggest that the DNA and lipids were also damaged and awaiting synthesis or repair. In conclusion, TrxR dysfunction in B. microti results in the breakdown of redox homeostasis and promotes apoptosis.

Babesia microti thioredoxin 3 is an effective antioxidant and involved in the response to antiprotozoal drugs

The intra-erythrocytic apicomplexan Babesia microti is the predominant pathogen that causes human babesiosis, an infectious disease that occurs worldwide. B. microti relies on the antioxidant including thioredoxin system to maintain the redox balance during the erythrocytic stage. In the present study, the full-length B. microti thioredoxin 3 (BmTrx3) gene was cloned, expressed in vitro, and its response to antiprotozoal drugs were tested. The full-length BmTrx3 was 663 bp and contained an intact open reading frame of 567 bp. The encoded polypeptide was 188 amino acids and the predicted molecular weight of the protein was 21.7 kDa. A conserved thioredoxin-like family domain was found in BmTrx3. The expression of BmTrx3 was upregulated on both the third and eighth day post-infection in mice, whereas expression was downregulated during the beginning and later stages. Western blot analysis showed that mouse anti-BmTrx3 serum could recognize the native BmTrx3 in parasite lysates and that the mouse anti-B. microti serum could recognize the recombinant BmTrx3 protein. Immunofluorescence microscopy showed that BmTrx3 localized in the cell cytoplasm of B. microti merozoites in B. microti-infected red blood cells. The results of bovine insulin reduction assay indicated the enzyme activity of the purified recombinant BmTrx3 protein. The anti-malaria drug chloroquine significantly inhibited the expression of BmTrx3, however, another anti-malaria drug qunine, and a known anti-babesiosis drug clindamycin, induced significantly higher upregulation of BmTrx3 mRNA. The results of the present study demonstrate that BmTrx3 is a functional enzyme with antioxidant activity and may be involved in the response of B. microti to anti-parasite drugs.

Molecular characterization of Babesia microti thioredoxin (BmTrx2) and its expression patterns induced by antiprotozoal drugs

Background

Human babesiosis is an infectious disease that is epidemic in various regions all over the world. The predominant causative pathogen of this disease is the intra-erythrocytic parasite Babesia microti. The thioredoxin system is one of the major weapons that is used in the resistance to the reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced by host immune system. In other intra-erythrocytic apicomplexans like the malaria parasite Plasmodium falciparum, anti-oxidative proteins are promising targets for the development of anti-parasitic drugs. However, to date, the sequences and biological properties of thioredoxins and thioredoxin-like molecules of B. microti remain unknown. Understanding the molecular characterization and function of B. microti thioredoxins may help to develop anti-Babesia drugs and controlling babesiosis.

Methods

The full-length B. microti thioredoxin 2 (BmTrx2) gene was obtained using a rapid amplification of cDNA ends (RACE) method, and the deduced BmTrx2 amino acid sequence was analyzed using regular bioinformatics tools. Recombinant BmTrx2 protein was expressed in vitro and purified using His-tag protein affinity chromatography resins. Reverse transcription PCR, quantitative real-time PCR and Western blot were employed to detect the expression and native proteins of BmTrx2. Indirect immunofluorescence assay was used to localize BmTrx2 in B. microti. Bovine insulin reduction assays were used to determine the enzyme activity of the purified recombinant BmTrx2 protein.

Results

The full-length BmTrx2 was 564 bp with a 408 bp open reading frame encoding a protein of 135 amino acids. The predicted molecular weight of the protein was 15.5 kDa. A conserved thioredoxin-like family domain was found in BmTrx2. The expression of BmTrx2 was upregulated on both the third and eighth day post-infection in mice, whereas expression was downregulated during the beginning and later stages. The results of Western blot analysis showed the native BmTrx2 in parasite lysates could be detected by mouse anti-BmTrx2 serum and that the recombinant BmTrx2 protein could be recognized by serum of B. microti-infected mice. Immunofluorescence microscopy showed that BmTrx2 localized in the cell cytoplasm of B. microti merozoites in B. microti-infected red blood cells. The results of bovine insulin reduction assay indicated the purified recombinant BmTrx2 protein possesses antioxidant enzyme activity. Dihydroartemisinin and quinine, known anti-malaria drugs, and clindamycin, a known anti-babesiosis drug, induced significantly higher upregulation of BmTrx2 mRNA.

Conclusions

Our results indicate that BmTrx2 is a functional enzyme with antioxidant activity and may be involved in the response of B. microti to anti-parasite drugs.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2619-9) contains supplementary material, which is available to authorized users.

Babesia microti Aldo-keto Reductase-Like Protein Involved in Antioxidant and Anti-parasite Response

The intraerythrocytic apicomplexan Babesia microti is the primary causative agent of human babesiosis, which is an infectious disease that occurs in various regions around the world. Although the aldo-keto reductases (AKRs) of this parasite have been sequenced and annotated, their biological properties remain unknown. AKRs are a superfamily of enzymes with diverse functions in the reduction of aldehydes and ketones. In the present study, we cloned the full-length cDNA of a B. microti aldo-keto reductase-like protein (BmAKR) and analyzed the deduced amino acid sequence of the BmAKR protein. This protein has a conserved AKR domain with an N-terminal signal sequence. Bmakr was upregulated on the 8th day after infection, whereas it was downregulated during the later stages. The recombinant protein of BmAKR was expressed in a glutathione S-transferase-fused soluble form in Escherichia coli. Western blot analysis showed that the mouse anti-BmAKR antibody recognized native BmAKR from a parasite lysate. Immunofluorescence microscopy localized BmAKR to the cytoplasm of B. microti merozoites in mouse RBCs in this study. Bmakr expression was significantly upregulated in the presence of oxidant stress. Atovaquone, a known anti-babesiosis drug, and robenidine, a known anti-coccidiosis drug, induced upregulation of Bmakr mRNA, thereby suggesting that Bmakr may be involved in anti-parasite drug response.

Identification of 2-Cys Peroxiredoxin (BmTPx-2) as Antioxidant Active Molecule from Babesia microti

Peroxiredoxins (Prxs) are a family of antioxidant enzymes that reduce peroxides in the presence of thioredoxin, thioredoxin reductase, and nicotinamide adenine dinucleotide phosphate (NADPH) to resist oxidative stress. In this study, we identified and isolated a 2-Cys Prx designated as ‘BmTPx-2’ from Babesia microti, with a full-length cDNA of 826 bp and an open reading frame of 756 bp, which encodes a 251-amino acid protein. BLAST analysis demonstrated that BmTPx-2 shows the typical features of members of the 2-Cys Prx family, which includes harboring two conserved VCP motifs with Cys¹⁰¹ and Cys²²¹ conserved cysteine residues. Recombinant BmTPx-2 was expressed in Escherichia coli and analyzed by western blot. The antioxidant activity of BmTPx-2 was demonstrated using a mixed-function oxidation system and oxidation of NADPH. Furthermore, BmTPx-2 mRNA expression level in parasites at the erythrocytes and tick stages were analyzed by real-time fluorescence quantitative PCR. Peak BmTPx-2 mRNA transcription was detected 8 days after infection at the erythrocyte stage, but not at the tick stage. Taken together, this study characterized BmTPx-2 from B. microti as an antioxidant molecule that was specifically transcribed at the erythrocyte stage.