Molecular identification and characterization of leucine aminopeptidase 2, an excretory-secretory product of Clonorchis sinensis

Current status and progress in the omics of Clonorchis sinensis

Clonorchis sinensis (C. sinensis) is a fish-borne trematode that inhabits the bile duct of mammals including humans, cats, dogs, rats, and so on. In the complex life cycle of C. sinensis, the worm develops successively in two intermediate hosts in fresh water and one definitive host. What's more, it undergoes eight developmental stages with a distinct morphology. Clonorchiasis, caused by C. sinensis infection, is an important food-borne parasitic disease and one of the most common zoonoses. C. sinensis infection could result in hyperplasia of the bile duct epithelium, obstructive jaundice, gall-stones, cholecystitis and cholangitis, even liver cirrhosis and cholangiocarcinoma. Thus, clonorchiasis is a serious public health problem in endemic areas. Integrated strategies should be adopted in the prevention and control of clonorchiasis due to the epidemiological characteristics. The recent advances in high-throughput technologies have made available the profiling of multiple layers of a biological system, genomics, transcriptomics, proteomics, and metabolomics. These data can help us to get more information about the development, physiology, metabolism, and reproduction of the parasite as well as pathogenesis and parasite-host interactions in clonorchiasis. In the present study, we summarized recent progresses in omics studies on C. sinensis providing insights into the studies and future directions on treating and preventing C. sinensis associated diseases.

Parasite Metalo-aminopeptidases as Targets in Human Infectious Diseases

BACKGROUND

Parasitic human infectious diseases are a worldwide health problem due to the increased resistance to conventional drugs. For this reason, the identification of novel molecular targets and the discovery of new chemotherapeutic agents are urgently required. Metalo- aminopeptidases are promising targets in parasitic infections. They participate in crucial processes for parasite growth and pathogenesis.

OBJECTIVE

In this review, we describe the structural, functional and kinetic properties, and inhibitors, of several parasite metalo-aminopeptidases, for their use as targets in parasitic diseases.

CONCLUSION

Plasmodium falciparum M1 and M17 aminopeptidases are essential enzymes for parasite development, and M18 aminopeptidase could be involved in hemoglobin digestion and erythrocyte invasion and egression. Trypanosoma cruzi, T. brucei and Leishmania major acidic M17 aminopeptidases can play a nutritional role. T. brucei basic M17 aminopeptidase down-regulation delays the cytokinesis. The inhibition of Leishmania basic M17 aminopeptidase could affect parasite viability. L. donovani methionyl aminopeptidase inhibition prevents apoptosis but not the parasite death. Decrease in Acanthamoeba castellanii M17 aminopeptidase activity produces cell wall structural modifications and encystation inhibition. Inhibition of Babesia bovis growth is probably related to the inhibition of the parasite M17 aminopeptidase, probably involved in host hemoglobin degradation. Schistosoma mansoni M17 aminopeptidases inhibition may affect parasite development, since they could participate in hemoglobin degradation, surface membrane remodeling and eggs hatching. Toxoplasma gondii M17 aminopeptidase inhibition could attenuate parasite virulence, since it is apparently involved in the hydrolysis of cathepsin Cs- or proteasome-produced dipeptides and/or cell attachment/invasion processes. These data are relevant to validate these enzymes as targets.

Molecular Mechanisms of Clonorchis sinensis-Host Interactions and Implications for Vaccine Development

Infections caused by Clonorchis sinensis remain a significant public health challenge for both humans and animals, causing pyogenic cholangitis, cholelithiasis, cholecystitis, biliary fibrosis, and even cholangiocarcinoma. However, the strategies used by the parasite and the immunological mechanisms used by the host have not yet been fully understood. With the advances in technologies and the accumulated knowledge of host-parasite interactions, many vaccine candidates against liver flukes have been investigated using different strategies. In this review, we explore and analyze in-depth the immunological mechanisms involved in the pathogenicity of C. sinensis. We highlight the different mechanisms by which the parasite interacts with its host to induce immune responses. All together, these data will allow us to have a better understanding of molecular mechansism of host-parasite interactions, which may shed lights on the development of an effective vaccine against C. sinensis.

Enzymatic characteristics and preventive effect of leucine aminopeptidase against Echinococcus multilocularis

Alveolar echinococcosis, a parasitic zoonotic disease caused by the larval stage of Echinococcus multilocularis infection, is a global epidemic in Eurasia and North America. Leucine aminopeptidase (LAP) of the M17 peptidase family could act on an ideal target antigen in diagnosis and prevention of parasitic diseases (schistosomiasis, malaria, fascioliasis) because of its good immunogenicity. In this study, the bioinformatic and enzymatic characterizations of recombinant Echinococcus multilocularis LAP (rEm-LAP) were evaluated. A prokaryotic expression system for rEm-LAP protein was established and its immunogenicity and preventive efficacy were demonstrated in a BALB/c mice model. This is the first report about the LAP of Echinococcus multilocularis and with a 57.4 KD purified rEm-LAP protein successfully expressed by pCzn1-LAP in Escherichia coli BL-21 cells. Enzymatic analysis results showed optimal rEm-LAP activity at pH 9. Serum indirect ELISA demonstrated that rEm-LAP could induce a Th1 and Th2 mixed-type immunological response and produce high levels of IgG, IgG1, IgG2a, IgM, and IgA. Furthermore, serum IFN-γ and IL-4 secretion were increased compared with the control groups. Finally, vaccination with rEm-LAP significantly decreased both the number and size of the cysts in Echinococcus multilocularis metacestode infected mice model. The current study provides evidence that rEm-LAP could be a potential vaccine antigen of Echinococcus multilocularis.

Molecular characterization and immune protection of the 3-hydroxyacyl-CoA dehydrogenase gene in Echinococcus granulosus

BACKGROUND

Cystic echinococcosis (CE) is a serious parasitic zoonosis caused by the larvae of the tapeworm Echinococcus granulosus. The development of an effective vaccine is one of the most promising strategies for controlling CE.

METHODS

The E. granulosus 3-hydroxyacyl-CoA dehydrogenase (EgHCDH) gene was cloned and expressed in Escherichia coli. The distribution of EgHCDH in protoscoleces (PSCs) and adult worms was analyzed using immunofluorescence. The transcript levels of EgHCDH in PSCs and adult worms were analyzed using quantitative real-time reverse transcription PCR (RT-qPCR). The immune protective effects of the rEgHCDH were evaluated.

RESULTS

The 924-bp open reading frame sequence of EgHCDH, which encodes a protein of approximately 34 kDa, was obtained. RT-qPCR analysis revealed that EgHCDH was expressed in both the PSCs and adult worms of E. granulosus. Immunofluorescence analysis showed that EgHCDH was mainly localized in the tegument of PSCs and adult worms. Western blot analysis showed that the recombinant protein was recognized by E. granulosus-infected dog sera. Animal challenge experiments demonstrated that dogs immunized with recombinant (r)EgHCDH had significantly higher serum IgG, interferon gamma and interleukin-4 concentrations than the phosphate-buffered saline (PBS) control group. The rEgHCDH vaccine was able to significantly reduce the number of E. granulosus and inhibit the segmental development of E. granulosus compared to the PBS control group.

CONCLUSIONS

The results suggest that rEgHCDH can induce partial immune protection against infection with E. granulosus and could be an effective candidate for the development of new vaccines.

Amino acids serve as an important energy source for adult flukes of Clonorchis sinensis

Clonorchiasis, caused by chronic infection with Clonorchis sinensis (C. sinensis), is an important food-borne parasitic disease that seriously afflicts more than 35 million people globally, resulting in a socioeconomic burden in endemic regions. C. sinensis adults long-term inhabit the microaerobic and limited-glucose environment of the bile ducts. Energy metabolism plays a key role in facilitating the adaptation of adult flukes to crowded habitat and hostile environment. To understand energy source for adult flukes, we compared the component and content of free amino acids between C. sinensis-infected and uninfected bile. The results showed that the concentrations of free amino acids, including aspartic acid, serine, glycine, alanine, histidine, asparagine, threonine, lysine, hydroxylysine, and urea, were significantly higher in C. sinensis-infected bile than those in uninfected bile. Furthermore, exogenous amino acids could be utilized by adult flukes via the gluconeogenesis pathway regardless of the absence or presence of exogenous glucose, and the rate-limiting enzymes, such as C. sinensis glucose-6-phosphatase, fructose-1,6-bisphosphatase, phosphoenolpyruvate carboxykinase, and pyruvate carboxylase, exhibited high expression levels by quantitative real-time PCR analysis. Interestingly, no matter whether exogenous glucose was present, inhibition of gluconeogenesis reduced the glucose and glycogen levels as well as the viability and survival time of adult flukes. These results suggest that gluconeogenesis might play a vital role in energy metabolism of C. sinensis and exogenous amino acids probably serve as an important energy source that benefits the continued survival of adult flukes in the host. Our study will be a cornerstone for illuminating the biological characteristics of C. sinensis and the host-parasite interactions.

Clonorchiasis, closely related to cholangiocarcinoma and hepatocellular carcinoma, has led to a negative socioeconomic impact in global areas especially some Asian endemic regions. Owing to the emergence of drug resistance and hypersensitivity reactions after the massive and repeated use of praziquantel as well as the lack of effective vaccines, searching for new strategies that prevent and treat clonorchiasis has become an urgent matter. Clonorchis sinensis, the causative agent of clonorchiasis, long-term inhabits the microaerobic and limited-glucose environment of the bile ducts. Adequate nutrients are essential for adult flukes to resist the adverse condition and survive in the crowed habitat. Studies on energy metabolism of adult flukes are beneficial for further exploring host-parasite interactions and developing novel anti-parasitic drugs. Our results suggest that gluconeogenesis probably plays a vital role in energy metabolism of Clonorchis sinensis and exogenous amino acids might be an essential energy source for adult flukes to successfully survive in the host. Our foundational study opens a new avenue for explaining energy metabolism of Clonorchis sinensis and provides a valuable strategy that the gluconeogenesis pathway will be a potential and novel target for the prevention and treatment of clonorchiasis.

Expression, Tissue Localization and Serodiagnostic Potential of Echinococcus granulosus Leucine Aminopeptidase

Echinococcus granulosus is the causative agent of cystic echinococcosis (CE), a widespread parasitic zoonosis. Leucine aminopeptidases (LAPs) of the M17 peptidase family have important functions in regulating the balance of catabolism and anabolism, cell maintenance, growth and defense. In this study, we presented a bioinformatic characterization and experimentally determined the tissue distribution characteristics of E. granulosus LAP (Eg-LAP), and explored its potential value for diagnosis of CE in sheep based on indirect ELISA. Through fluorescence immunohistochemistry, we found that Eg-LAP was present in the tegument and hooks of PSCs, the whole germinal layer and adult worm parenchymatous tissue. Western blotting results revealed that the recombinant protein could be identified using E. granulosus-infected sheep serum. The diagnostic value of this recombinant protein was assessed by indirect ELISA, and compared with indirect ELISA based on hydatid fluid antigen. The sensitivity and specificity rEgLAP-ELISA were 95.8% (23/24) and 79.09% (87/110), respectively, while using hydatid fluid as antigen showed the values 41.7% (10/24) and 65.45% (72/110). This is the first report concerning leucine aminopeptidase from E. granulosus, and the results showed that Eg-LAP belong to M17 peptidase families, and that it is involved in important biological function of E. granulosus. Furthermore, rEg-LAP is appropriate for diagnosing and monitoring CE in sheep in field. Development of a rapid test using rEg-LAP to diagnose sheep CE deserves further study.

Molecular cloning and characterization of leucine aminopeptidase gene from Taenia pisiformis

Leucine aminopeptidase (LAP, EC: 3.4.11.1) is an important metalloexopeptidase that catalyze the hydrolysis of amino-terminal leucine residues from polypeptides and proteins. In this study, a full length of cDNA encoding leucine aminopeptidase of Taenia pisiformis (TpLAP) was cloned by rapid amplification of cDNA-ends using the polymerase chain reaction (RACE-PCR) method. The full-length cDNA of the TpLAP gene is 1823 bp and contains a 1569 bp ORF encoding 533 amino acids with a putative mass of 56.4 kDa. TpLAP contains two characteristic motifs of the M17LAP family in the C-terminal sequence: the metal binding site 265-[VGKG]-271 and the catalytic domain motif 351-[NTDAEGRL]-357. The soluble GST-TpLAP protein was expressed in Escherichia coli Transetta (DE3) and four specific anti-TpLAP monoclonal antibodies (mAbs) were prepared. In enzymatic assays, the optimal activity was observed at pH 9.5 at 45 °C. GST-TpLAP displayed a hydrolyzing activity for the Leu-pNA substrate with a maximum activity of 46 U/ml. The enzymatic activity was significantly enhanced by Mn²⁺ and completely inhibited by 20 nM bestatin and 0.15 mM EDTA. The native TpLAP was detected specifically in ES components of adult T. pisiformis by western blotting using anti-TpLAP mAb as a probe. Quantitative real-time PCR revealed that the TpLAP gene was expressed at a high level in adult worm tissues, especially in the gravid proglottids (50.71-fold). Immunolocalization analysis showed that TpLAP was located primarily in the subtegumental parenchyma zone and the uterine wall of adult worms. Our results indicate that TpLAP is a new member of the M17LAP family and can be considered as a stage-differentially expressed protein. These findings might provide new insights into the study of the mechanisms of growth, development and survival of T. pisiformis in the final host and have potential value as an attractive target for drug therapy or vaccine intervention.

Sequence analysis and characterization of pyruvate kinase from Clonorchis sinensis, a 53.1-kDa homopentamer, implicated immune protective efficacy against clonorchiasis

BACKGROUND

Clonorchis sinensis, the causative agent of clonorchiasis, is classified as one of the most neglected tropical diseases and affects more than 15 million people globally. This hepatobiliary disease is highly associated with cholangiocarcinoma. As key molecules in the infectivity and subsistence of trematodes, glycolytic enzymes have been targets for drug and vaccine development. Clonorchis sinensis pyruvate kinase (CsPK), a crucial glycolytic enzyme, was characterized in this research.

RESULTS

Differences were observed in the sequences and spatial structures of CsPK and PKs from humans, rats, mice and rabbits. CsPK possessed a characteristic active site signature (IKLIAKIENHEGV) and some unique sites but lacked the N-terminal domain. The predicted subunit molecular mass (Mr) of CsPK was 53.1 kDa. Recombinant CsPK (rCsPK) was a homopentamer with a Mr. of approximately 290 kDa by both native PAGE and gel filtration chromatography. Significant differences in the protein and mRNA levels of CsPK were observed among four life stages of C. sinensis (egg, adult worm, excysted metacercaria and metacercaria), suggesting that these developmental stages may be associated with diverse energy demands. CsPK was widely distributed in adult worms. Moreover, an intense Th1-biased immune response was persistently elicited in rats immunized with rCsPK. Also, rat anti-rCsPK sera suppressed C. sinensis adult subsistence both in vivo and in vitro.

CONCLUSIONS

The sequences and spatial structures, molecular mass, and expression profile of CsPK have been characterized. rCsPK was indicated to be a homopentamer. Rat anti-rCsPK sera suppressed C. sinensis adult subsistence both in vivo and in vitro. CsPK is worthy of further study as a promising target for drug and vaccine development.

A leucine aminopeptidase is involved in kinetoplast DNA segregation in Trypanosoma brucei

The kinetoplast (k), the uniquely packaged mitochondrial DNA of trypanosomatid protists is formed by a catenated network of minicircles and maxicircles that divide and segregate once each cell cycle. Although many proteins involved in kDNA replication and segregation are now known, several key steps in the replication mechanism remain uncharacterized at the molecular level, one of which is the nabelschnur or umbilicus, a prominent structure which in the mammalian parasite Trypanosoma brucei connects the daughter kDNA networks prior to their segregation. Here we characterize an M17 family leucyl aminopeptidase metalloprotease, termed TbLAP1, which specifically localizes to the kDNA disk and the nabelschur and represents the first described protein found in this structure. We show that TbLAP1 is required for correct segregation of kDNA, with knockdown resulting in delayed cytokinesis and ectopic expression leading to kDNA loss and decreased cell proliferation. We propose that TbLAP1 is required for efficient kDNA division and specifically participates in the separation of daughter kDNA networks.

Immunoproteomic analysis of Schistosoma japonicum schistosomulum proteins recognized by immunoglobulin G in the sera of susceptible and non-susceptible hosts

The aim of this study was to search for immunogenic schistosomula proteins in the hope of identifying novel intervention targets. Schistosomula proteins were analyzed by immunoproteomic which the probes were sera derived from BALB/c mice (susceptible hosts) and Microtus fortis (resistant hosts). A total of 116 immunoreactive proteins recognized by 10 days post-infected BALB/c mice, M. fortis sera, and uninfected M. fortis sera were selected for further analysis. Finally, 95 protein spots were identified by mass spectrometry (MS) analysis. Bioinformatics analysis showed that the differentially identified immunogenic proteins participated mainly in cytoskeleton organization, cell motility, energy metabolism, responses to stimuli, and protein folding. Many of these proteins were the tegument or excretory-secretory products of schistosomes reported in previous studies. Among of them, Schistosoma japonicum DnaJ (Hsp40) homologue (SjDnaJ) was successfully expressed and the purified recombinant product was evaluated by immunoprotective experiment. After immunization of BALB/c mice with recombinant SjDnaJ, it could induce 34.5% and 48.9% reductions in the numbers of worms and eggs in the liver. These results contribute to a better understanding of the molecular mechanisms underlying the host-parasite relationship and provide a major dataset to facilitate the further development of new vaccine candidates and/or diagnostic markers for schistosomiasis.

BIOLOGICAL SIGNIFICANCE

Schistosomiasis is caused by parasitic blood-dwelling flukes in tropical and subtropical areas, and it is one of the world's most prevalent tropical diseases. The lack of effective vaccine and reliable diagnostic methods make this disease difficult to control. In China, S. japonicum can infect more than 40 different susceptible mammals for this parasite. However, M. fortis is the only known mammal where the schistosome cannot develop and it exhibits no significant pathological effects. Many studies' results showed that native antibodies against S. japonicum are present in M. fortis that may have important anti-schistosomiasis roles during the infection process. The aim of this study was to search for immunogenic schistosomula proteins in the hope of identifying novel intervention targets. We present a comparative immunoproteomics analysis of the proteins recognized by susceptible and resistant host antibodies before and 10-days after infections. The results of this analysis will be helpful for identifying the key molecules required for the survival and development of schistosomes. At the same time, the study contributes to a better understanding of the molecular mechanisms underlying the host-parasite relationship associated with schistosomes and they also provide a major dataset to facilitate the further development of new diagnostic assays and/or vaccine candidates for schistosomiasis.

Systemic and local mucosal immune responses induced by orally delivered Bacillus subtilis spore expressing leucine aminopeptidase 2 of Clonorchis sinensis

Human clonorchiasis caused by Clonorchis sinensis (C. sinensis) has been increasingly prevalent in recent years so that an effective measure is essential and urgent to control the infectious disease. Oral delivery of antigens from C. sinensis may be an important approach to effectively induce both systemic and local immune responses to anti-infection of the parasite. In the current study, we used Bacillus subtilis (B. subtilis) spores as a delivery vehicle to introduce leucine aminopeptidase 2 of C. sinensis (CsLAP2), an excretory/secretory antigen with high immunogenicity, expressing on their surface. SDS-PAGE, western blotting, and flow cytometry indicated that CsLAP2 was successfully expressed on the surface of B. subtilis spores (CotC-CsLAP2 spores). BALB/c mice were treated with spores intragastrically. On day 31 after the treatment, we found that mice intragastrically treated with CotC-CsLAP2 spores exhibited higher IgG, IgG1, IgG2a, and IgA level in sera as well as higher sIgA level in bile and intestinal lavage fluid compared to mice orally administrated with spores not expressing CsLAP2 (CotC spores) and naïve mice. The peak titer of IgG/IgA presented on day 31/49 after oral administration. IgG1 level was lower than IgG2a in group administrated with CotC-CsLAP2 spores. sIgA-secreting cells were obviously observed in intestinal epithelium of mice orally treated with CotC-CsLAP2 spores. After incubated with CotC-CsLAP2, the levels of IFN-γ, IL-6, IL-10, IL-17A, and TNF significantly increased in the supernatant of splenocytes isolated from mice orally treated with CotC-CsLAP2 spores, while there was no statistically significant difference of IL-4 level representing Th2 response among the groups. Our study demonstrated that oral administration of CsLAP2 delivered by B. subtilis spore elicited obvious systemic and local mucosal immunity. Secretory IgA and Th1-Th17 cellular immunity might involved in mechanisms of the immune response.

Molecular identification, immunolocalization, and characterization of Clonorchis sinensis calmodulin

One cDNA clone (Cs18h09) encoding Clonorchis sinensis calmodulin (CsCaM) was isolated from our adult cDNA plasmid library. The open reading frame of CsCaM contains 450 bp which encodes 149 amino acids. CsCaM protein comprises four calcium-binding EF-hand motifs. The amino acid sequence of CsCaM shares very high homology with other species. Quantitative RT-polymerase chain reaction (PCR) revealed that CsCaM mRNA was constitutively transcribed in development cycle stages of the parasite, including adult worm, metacercaria, excysted metacercaria, and egg. In addition, recombinant CsCaM (rCsCaM) was expressed as a soluble protein and anti-rCsCaM rat serum could detect CsCaM in the C. sinensis somatic extracts but not in the C. sinensis excretory-secretory products (ESPs). Moreover, immunolocalization assay showed that CsCaM was located in tegument, intestine, pharynx, and eggs. Furthermore, rCsCaM was found to bind calcium ion (Ca2+) and magnesium (Mg2+) in electrophoretic mobility shift assay. Ca2+ binding increased the ability of rCsCaM to bind the hydrophobic fluorescent probe 8-anilinonaphthalene-1-sulphonate, causing a blue shift in the fluorescence emission from 540 to 515 nm with an excitation wavelength of 380 nm and substantial increase in fluorescence intensity but not Mg2+. Collectively, here we showed the basic characterization of CsCaM and inferred that CsCaM could be a Ca2+ sensor protein, and CsCaM may possibly participate in growth and development of adult worm and egg of C. sinensis through binding Ca2+.

The biochemical and immunological characterization of two serpins from Clonorchis sinensis

Serpins (serine proteinase inhibitors) are evidenced to regulate numerous biological processes such as immunoregulation in parasitic helminths. The functions of serpins from Clonorchis sinensis remain unclear to date. In this study, two serpin genes, respectively denominated as CsproSERPIN and CsSERPIN2, had been selected from metacercaria cDNA library of C. sinensis. The biochemical activities of both recombinant proteins (rCsproSERPIN and rCsSERPIN2) were analyzed by assays of inhibition on some serine or cysteine proteases, the results showed that rCsproSERPIN significantly inhibited trypsin, chymotrypsin and thrombin, while rCsSERPIN2 inhibited only chymotrypsin. Moreover, cytokine and antibody measurements indicated that rats subcutaneously immunized with rCsproSERPIN and rCsSERPIN2 respectively developed a strong IFN-γ production and IgG2a levers of sera were higher than IgG1. Besides, immunoblot assays revealed that the rCsproSERPIN and rCsSERPIN2 could be recognized by the sera of rats infected with C. sinensis and the sera of rabbits immunized by excretory/secretory products. Furthermore, immunofluorescence assays illuminated the two were similarly localized in the reproductive organs such as vitelline glands, testis and eggs in adult stage. In short, all the results collectively indicated that CsproSERPIN and CsSERPIN2 might play important role in the parasite development by preventing the parasite from digestion by exogenous serine proteases, as well as CsproSERPIN and CsSERPIN2 probably involved in immunoregulation of host by inducing Th1-biased type cytokines in rats.