Cloning and characterization of three Eimeria tenella lipid phosphate phosphatases

Although lipid phosphate phosphatases (LPPs) play an important role in cellular signaling in addition to lipid biosynthesis, little is thus far known about parasite LPPs. In this study, we characterized three Eimeria tenella cDNA clones encoding LPP named EtLPP1, EtLPP2 and EtLPP3. Key structural features previously described in LPPs, including the three conserved domains proposed as catalytic sites, a single conserved N-glycosylation site, and putative transmembrane domains were discovered in the three resulting EtLPP amino acid sequences. Expression of His6-tagged EtLPP1, -2, and -3 in HEK293 cells produced immunoreactive proteins with variable molecular sizes, suggesting the presence of multiple forms of each of the three EtLPPs. The two faster-migrating protein bands below each of the three EtLPP proteins were found to be very similar to the porcine 35-kDa LPP enzyme in their molecular size and the extent of their N-glycosylation, suggesting that the three EtLPPs are partially N-glycosylated. Kinetic analyses of the activity of the three enzymes against PA, LPA, C1P and S1P showed that Km values for each of the substrates were (in μM) 284, 46, 28, and 22 for EtLPP1; 369, 179, 237, and 52 for EtLPP2; and 355, 83, and 260 for EtLPP3. However, EtLPP3 showed negligible activity on S1P. These results confirmed that the three EtLPPs have broad substrate specificity. The results also indicated that despite structural similarities, the three EtLPPs may play distinct functions through their different models of substrate preference. Furthermore, particularly high expression levels of the three EtLPP genes were detected in the sporozoite stage of the E. tenella life cycle (p<0.001), suggesting that their encoded proteins might play an important biological function in the sporozoite stage.

Malaria parasite-synthesized heme is essential in the mosquito and liver stages and complements host heme in the blood stages of infection

Heme metabolism is central to malaria parasite biology. The parasite acquires heme from host hemoglobin in the intraerythrocytic stages and stores it as hemozoin to prevent free heme toxicity. The parasite can also synthesize heme de novo, and all the enzymes in the pathway are characterized. To study the role of the dual heme sources in malaria parasite growth and development, we knocked out the first enzyme, δ-aminolevulinate synthase (ALAS), and the last enzyme, ferrochelatase (FC), in the heme-biosynthetic pathway of Plasmodium berghei (Pb). The wild-type and knockout (KO) parasites had similar intraerythrocytic growth patterns in mice. We carried out in vitro radiolabeling of heme in Pb-infected mouse reticulocytes and Plasmodium falciparum-infected human RBCs using [4-¹⁴C] aminolevulinic acid (ALA). We found that the parasites incorporated both host hemoglobin-heme and parasite-synthesized heme into hemozoin and mitochondrial cytochromes. The similar fates of the two heme sources suggest that they may serve as backup mechanisms to provide heme in the intraerythrocytic stages. Nevertheless, the de novo pathway is absolutely essential for parasite development in the mosquito and liver stages. PbKO parasites formed drastically reduced oocysts and did not form sporozoites in the salivary glands. Oocyst production in PbALASKO parasites recovered when mosquitoes received an ALA supplement. PbALASKO sporozoites could infect mice only when the mice received an ALA supplement. Our results indicate the potential for new therapeutic interventions targeting the heme-biosynthetic pathway in the parasite during the mosquito and liver stages.

We demonstrated about two decades ago that the malaria parasite could make heme on its own, although it imports heme from red blood cell hemoglobin during the blood stages of infection. We investigated the role of parasite-synthesized heme in all stages of parasite growth by knocking out two genes in the heme-biosynthetic pathway of Plasmodium berghei that infects mice. We found that the parasite-synthesized heme complements the function of hemoglobin-heme during the blood stages. The parasite-synthesized heme appears to be a backup mechanism. The parasite incorporates both sources of heme into hemozoin, a detoxification product, and into mitochondrial cytochromes. The parasite-synthesized heme is, however, absolutely essential for parasite growth during the mosquito and liver stages. We restored the sporozoite formation and liver-stage development of the knockout parasites by providing the missing metabolite. Thus, the heme-biosynthetic pathway could be a target for antimalarial therapies in the mosquito and liver stages of infection. The knockout parasite could also be tested for its potential as a genetically attenuated sporozoite vaccine.

Expression of cytosolic peroxiredoxins in Plasmodium berghei ookinetes is regulated by environmental factors in the mosquito bloodmeal

The Plasmodium ookinete develops over several hours in the bloodmeal of its mosquito vector where it is exposed to exogenous stresses, including cytotoxic reactive oxygen species (ROS). How the parasite adapts to these challenging conditions is not well understood. We have systematically investigated the expression of three cytosolic antioxidant proteins, thioredoxin-1 (Trx-1), peroxiredoxin-1 (TPx-1), and 1-Cys peroxiredoxin (1-Cys Prx), in developing ookinetes of the rodent parasite Plasmodium berghei under various growth conditions. Transcriptional profiling showed that tpx-1 and 1-cys prx but not trx-1 are more strongly upregulated in ookinetes developing in the mosquito bloodmeal when compared to ookinetes growing under culture conditions. Confocal immunofluorescence imaging revealed comparable expression patterns on the corresponding proteins. 1-Cys Prx in particular exhibited strong expression in mosquito-derived ookinetes but was not detectable in cultured ookinetes. Furthermore, ookinetes growing in culture upregulated tpx-1 and 1-cys prx when challenged with exogenous ROS in a dose-dependent fashion. This suggests that environmental factors in the mosquito bloodmeal induce upregulation of cytosolic antioxidant proteins in Plasmodium ookinetes. We found that in a parasite line lacking TPx-1 (TPx-1KO), expression of 1-Cys Prx occurred significantly earlier in mosquito-derived TPx-1KO ookinetes when compared to wild type (WT) ookinetes. The protein was also readily detectable in cultured TPx-1KO ookinetes, indicating that 1-Cys Prx at least in part compensates for the loss of TPx-1 in vivo. We hypothesize that this dynamic expression of the cytosolic peroxiredoxins reflects the capacity of the developing Plasmodium ookinete to rapidly adapt to the changing conditions in the mosquito bloodmeal. This would significantly increase its chances of survival, maturation and subsequent escape. Our results also emphasize that environmental conditions must be taken into account when investigating Plasmodium-mosquito interactions.

The malaria parasite Plasmodium is transmitted by Anopheles mosquitoes. Within the midgut of the insect, it is exposed to multiple environmental stresses, including cytotoxic reactive oxygen species (ROS). To avoid destruction, the parasite develops into a motile ookinete capable of leaving the midgut. Yet, ookinete development lasts over several hours and requires the parasite to adapt to an increasingly challenging environment. Here we show that ookinetes of the rodent parasite Plasmodium berghei during development in the mosquito midgut increase the expression of the protective antioxidant proteins peroxiredoxin-1 (TPx-1) and 1-Cys peroxiredoxin (1-Cys Prx). This upregulation was also inducible in cultured ookinetes by challenging them with ROS. This suggests that ookinetes actively modulate the expression of their antioxidant proteins in response to the changing conditions in the mosquito. We also found that ookinetes lacking TPx-1 (TPx-1KO) upregulated 1-Cys Prx expression significantly earlier than wild type ookinetes. This indicates that the TPx-1KO parasites compensate for the loss of TPx-1 by altering the expression pattern of the functionally related 1-Cys Prx. The observed dynamic regulation of the cytosolic antioxidant proteins may help the Plasmodium ookinete to adapt to rapidly changing environmental conditions and thus to increase the probability of survival, maturation and escape from the mosquito midgut.

Role of the endogenous antioxidant system in the protection of Schistosoma mansoni primary sporocysts against exogenous oxidative stress

Antioxidants produced by the parasite Schistosoma mansoni are believed to be involved in the maintenance of cellular redox balance, thus contributing to larval survival in their intermediate snail host, Biomphalaria glabrata. Here, we focused on specific antioxidant enzymes, including glutathione-S-transferases 26 and 28 (GST26 and 28), glutathione peroxidase (GPx), peroxiredoxin 1 and 2 (Prx1 and 2) and Cu/Zn superoxide dismutase (SOD), known to be involved in cellular redox reactions, in an attempt to evaluate their endogenous antioxidant function in the early-developing primary sporocyst stage of S. mansoni. Previously we demonstrated a specific and consistent RNA interference (RNAi)-mediated knockdown of GST26 and 28, Prx1 and 2, and GPx transcripts, and an unexpected elevation of SOD transcripts in sporocysts treated with gene-specific double-stranded (ds)RNA. In the present followup study, in vitro transforming sporocysts were exposed to dsRNAs for GST26 and 28, combined Prx1/2, GPx, SOD or green-fluorescent protein (GFP, control) for 7 days in culture, followed by assessment of the effects of specific dsRNA treatments on protein levels using semi-quantitative Western blot analysis (GST26, Prx1/2 only), and larval susceptibility to exogenous oxidative stress in in vitro killing assays. Significant decreases (80% and 50%) in immunoreactive GST26 and Prx1/2, respectively, were observed in sporocysts treated with specific dsRNA, compared to control larvae treated with GFP dsRNA. Sporocysts cultured with dsRNAs for GST26, GST28, Prx1/2 and GPx, but not SOD dsRNA, were significantly increased in their susceptibility to H₂O₂ oxidative stress (60–80% mortalities at 48 hr) compared to GFP dsRNA controls (∼18% mortality). H₂O₂-mediated killing was abrogated by bovine catalase, further supporting a protective role for endogenous sporocyst antioxidants. Finally, in vitro killing of S. mansoni sporocysts by hemocytes of susceptible NMRI B. glabrata snails was increased in larvae treated with Prx1/2, GST26 and GST28 dsRNA, compared to those treated with GFP or SOD dsRNAs. Results of these experiments strongly support the hypothesis that endogenous expression and regulation of larval antioxidant enzymes serve a direct role in protection against external oxidative stress, including immune-mediated cytotoxic reactions. Moreover, these findings illustrate the efficacy of a RNAi-type approach in investigating gene function in larval schistosomes.

Species of the human blood fluke Schistosoma are estimated to infect approximately 200 million people worldwide, resulting in loss of health, vitality and productivity mainly among the world's poorest inhabitants. Since snail intermediate hosts represent an essential part of the flukes' life cycle, an understanding of the strategies used by the intramolluscan schistosome larvae to survive within this host may provide novel approaches for disrupting larval development and thus transmission to humans. Anti-oxidant enzymes produced by the parasite Schistosoma mansoni are believed to play a critical role in the maintenance of cellular redox balance, contributing to larval survival in their snail host, Biomphalaria glabrata. In this study, we have incorporated a RNA interference approach attempting to knock down specific anti-oxidant enzymes, including gluthatione-S-transferases 26 and 28 (GST26 and 28), gluthatione peroxidase (GPx), peroxiredoxins 1 and 2 (Prx1/2) and superoxide dismutase (SOD), and to evaluate their endogenous anti-oxidant function in the sporocyst stage of S. mansoni. Results clearly demonstrated a significantly higher susceptibility of antioxidant double-stranded (ds)RNA-treated larvae to in vitro H₂O₂ treatment or hemocytic encapsulation compared to GFP dsRNA controls. Taken together, our findings support the hypothesis that endogenous expression and regulation of larval antioxidant enzymes serve a direct role in protection against external oxidative stress, including immune-mediated cytotoxic reactions.

Serine protease activity in developmental stages of Eimeria tenella

A number of complex processes are involved in Eimeria spp. survival, including control of sporulation, intracellular invasion, evasion of host immune responses, successful reproduction, and nutrition. Proteases have been implicated in many of these processes, but the occurrence and functions of serine proteases have not been characterized. Bioinformatic analysis suggests that the Eimeria tenella genome contains several serine proteases that lack homology to trypsin. Using RT-PCR, a gene encoding a subtilisin-like and a rhomboid protease-like serine protease was shown to be developmentally regulated, both being poorly expressed in sporozoites (SZ) and merozoites (MZ). Casein substrate gel electrophoresis of oocyst extracts during sporulation demonstrated bands of proteolytic activity with relative molecular weights (Mr) of 18, 25, and 45 kDa that were eliminated by coincubation with serine protease inhibitors. A protease with Mr of 25 kDa was purified from extracts of unsporulated oocysts by a combination of affinity and anion exchange chromatography. Extracts of SZ contained only a single band of inhibitor-sensitive proteolytic activity at 25 kDa, while the pattern of proteases from extracts of MZ was similar to that of oocysts except for the occurrence of a 90 kDa protease, resistant to protease inhibitors. Excretory-secretory products (ESP) from MZ contained AEBSF (4-[2-Aminoethyl] benzenesulphonyl fluoride)-sensitive protease activity with a specific activity about 10 times greater than that observed in MZ extracts. No protease activity was observed in the ESP from SZ. Pretreatment of SZ with AEBSF significantly reduced SZ invasion and the release of the microneme protein, MIC2. The current results suggest that serine proteases are present in all the developmental stages examined.

Proteomics analysis and protein expression during sporozoite excystation of Cryptosporidium parvum (Coccidia, Apicomplexa)

Cryptosporidiosis, caused by coccidian parasites of the genus Cryptosporidium, is a major cause of human gastrointestinal infections and poses a significant health risk especially to immunocompromised patients. Despite intensive efforts for more than 20 years, there is currently no effective drug treatment against these protozoa. This study examined the zoonotic species Cryptosporidium parvum at two important stages of its life cycle: the non-excysted (transmissive) and excysted (infective) forms. To increase our understanding of the molecular basis of sporozoite excystation, LC-MS/MS coupling with a stable isotope N-terminal labeling strategy using iTRAQ reagents was used on soluble fractions of both non-excysted and excysted sporozoites, i.e. sporozoites both inside and outside oocysts were examined. Sporozoites are the infective stage that penetrates small intestinal enterocytes. Also to increase our knowledge of the C. parvum proteome, shotgun sequencing was performed on insoluble fractions from both non-excysted and excysted sporozoites. In total 303 C. parvum proteins were identified, 56 of which, hitherto described as being only hypothetical proteins, are expressed in both excysted and non-excysted sporozoites. Importantly we demonstrated that the expression of 26 proteins increases significantly during excystation. These excystation-induced proteins included ribosomal proteins, metabolic enzymes, and heat shock proteins. Interestingly three Apicomplexa-specific proteins and five Cryptosporidium-specific proteins augmented in excysted invasive sporozoites. These eight proteins represent promising targets for developing vaccines or chemotherapies that could block parasite entry into host cells.

Disulfide bridges in the mesophilic triosephosphate isomerase from Giardia lamblia are related to oligomerization and activity

Triosephosphate isomerase from the mesophile Giardia lamblia (GlTIM) is the only known TIM with natural disulfide bridges. We previously found that oxidized and reduced thiol states of GlTIM are involved in the interconversion between native dimers and higher oligomeric species, and in the regulation of enzymatic activity. Here, we found that trophozoites and cysts have different oligomeric species of GlTIM and complexes of GlTIM with other proteins. Our data indicate that the internal milieu of G. lamblia is favorable for the formation of disulfide bonds. Enzyme mutants of the three most solvent exposed Cys of GlTIM (C202A, C222A, and C228A) were prepared to ascertain their contribution to oligomerization and activity. The data show that the establishment of a disulfide bridge between two C202 of two dimeric GlTIMs accounts for multimerization. In addition, we found that the establishment of an intramonomeric disulfide bond between C222 and C228 abolishes catalysis. Multimerization and inactivation are both reversed by reducing conditions. The 3D structure of the C202A GlTIM was solved at 2.1 A resolution, showing that the environment of the C202 is prone to hydrophobic interactions. Molecular dynamics of an in silico model of GlTIM when the intramonomeric disulfide bond is formed, showed that S216 is displaced 4.6 A from its original position, causing loss of hydrogen bonds with residues of the active-site loop. This suggests that this change perturb the conformational state that aligns the catalytic center with the substrate, inducing enzyme inactivation.

A calcium-dependent protein kinase regulates Plasmodium ookinete access to the midgut epithelial cell

Plasmodium parasites are fertilized in the mosquito midgut and develop into motile zygotes, called ookinetes, which invade the midgut epithelium. Here we show that a calcium-dependent protein kinase, CDPK3, of the rodent malarial parasite (Plasmodium berghei) is produced in the ookinete stage and has a critical role in parasite transmission to the mosquito vector. Targeted disruption of the CDPK3 gene decreased ookinete ability to infect the mosquito midgut by nearly two orders of magnitude. Electron microscopic analyses demonstrated that the disruptant ookinetes could not access midgut epithelial cells by traversing the layer covering the cell surface. An in vitro migration assay showed that these ookinetes lack the ability to migrate through an artificial gel, suggesting that this defect caused their failure to access the epithelium. In vitro migration assays also suggested that this motility is induced in the wild type by mobilization of intracellular stored calcium. These results indicate that a signalling pathway involving calcium and CDPK3 regulates ookinete penetration of the layer covering the midgut epithelium. Because humans do not possess CDPK family proteins, CDPK3 is a good target for blocking malarial transmission to the mosquito vector.

Glyceroneogenesis: an unexpected metabolic pathway for glutamine in Schistosoma mansoni sporocysts

To date, glyceroneogenesis has only been described in mammals but we demonstrate in this paper that it could exist in the invertebrate Schistosoma mansoni, the parasitic helminth transmitted by fresh water molluscs and responsible for the major human endemic disease, schistosomiasis. Glyceroneogenesis is a phosphoenolpyruvate carboxykinase (PEPCK)-dependent process by which glycerol can be produced from precursors like glutamine and therefore represents a truncated gluconeogenic pathway. We have previously demonstrated the possible central role of glutamine in mollusc-schistosome interactions. In this work, we show that glutamine effectively promotes in vitro survival and protein synthesis in sporocysts, the intramolluscan larval stages of S. mansoni, possibly through its role as an energy source. However, the demonstration that PEPCK is massively expressed in these larval forms as compared to adult parasites, together with the observation that 3-mercaptopicolinate, a specific inhibitor of PEPCK, significantly reduces the effect of glutamine on sporocyst growth, suggest that glutamine could also be used for glucose or glycerol production. Results of [14C] glutamine incorporation confirmed that neosynthesis of glucose and mainly of glycerol occurred in sporocysts and was dependent on PEPCK activity. Therefore, our results strongly indicate that glyceroneogenesis could exist in schistosomes. Several hypotheses can be proposed concerning the importance of glycerol for the adaptation of this helminth to its host osmotic and energetic environment.

Effect of hydrogen peroxide and other protease inhibitors on Cryptosporidium parvum excystation and in vitro development

This study was undertaken to observe the effects of hydrogen peroxide on Cryptosporidium parvum oocysts with respect to protease activity in comparison to known protease inhibitors. In assessing the possible mechanisms of action of hydrogen peroxide, treatment effectiveness was analyzed using 3 assays and the potential roles of proteases and cations were considered. Treatment of C. parvum oocysts with hydrogen peroxide inhibited protease activity up to 50% compared with untreated controls. Treatment of oocysts with chemicals that affect sulfhydryls, including N-ethylmaleimide and dithiolthreitol, inhibited protease activity by >90%. Treatment of oocysts with these chemicals, along with the protease inhibitors, phenylmethylsulfonyl fluoride (PMSF), ethylenediamine-tetraacetic acid, and cystatin, inhibited protease activity as well as in vitro excystation and infection in a cell culture assay. Several mechanisms may result in the successful inhibition of infection and excystation by hydrogen peroxide treatment, including: oxidation of oocyst wall proteins or lipids, chelating of cations necessary for infection, or hydroxyl radical-induced DNA damage to sporozoites, or both.

Role of anti-oxidant preparation (oxi-guard) on Cryptosporidium parvum infection in albino mice

Oxi-guard is a commercial anti-oxidant preparation in the form of capsules. One hundred and twenty albino mice were divided into three groups, first (50 mice) received oxi-guard for one week before infection with Cryptosporidium oocysts and continued for 50 days post-infection (P.I.), second (50 mice) did not receive oxi-guard and infected with Cryptosporidium oocysts and third (20 mice) was neither infected, nor receiving oxi-guard, a control group. Histopathological examination of small intestines and counting of endogenous stages of Cryptosporidium in the epithelial surface were done. Malonyldialdehyde (MDA) levels in small intestines were measured in all mice. It had been observed that the administration of oxi-guard led to significant increase in the numbers of endogenous stages of Cryptosporidium in the small intestines of group (1) in comparison with group (2). MDA levels showed a significant increase on the 15th, 20th and 25th days P.I. in group (2) and they did not show any significant changes in group (1) in comparison with the control group. As regards the activity of endogenous anti-oxidant enzymes in Cryptosporidium parvum oocysts, there was a significant increase in superoxide dismutase in group (2) of mice compared with its level in group (1). Finally, it can be concluded that the administration of oxi-guard had led to eradication of free radicals which in turn prevented their lethal effects on the parasite leading to increased parasitic colonization in the small intestine of mice.