Martín Cancela

RNA interference targeting leucine aminopeptidase blocks hatching of Schistosoma mansoni eggs

Schistosoma mansoni leucine aminopeptidase (LAP) is thought to play a central role in hatching of the miracidium from the schistosome egg. We identified two discrete LAPs genes in the S. mansoni genome, and their orthologs in S. japonicum. The similarities in sequence and exon/intron structure of the two genes, LAP1 and LAP2, suggest that they arose by gene duplication and that this occurred before separation of the mansoni and japonicum lineages. The SmLAP1 and SmLAP2 genes have different expression patterns in diverse stages of the cycle; whereas both are equally expressed in the blood dwelling stages (schistosomules and adult), SmLAP2 expression was higher in free living larval (miracidia) and in parasitic intra-snail (sporocysts) stages. We investigated the role of each enzyme in hatching of schistosome eggs and the early stages of schistosome development by RNA interference (RNAi). Using RNAi, we observed marked and specific reduction of mRNAs, along with a loss of exopeptidase activity in soluble parasite extracts against the diagnostic substrate l-leucine-7-amido-4-methylcoumarin hydroxide. Strikingly, knockdown of either SmLAP1 or SmLAP2, or both together, was accompanied by >or=80% inhibition of hatching of schistosome eggs showing that both enzymes are important to the escape of miracidia from the egg. The methods employed here refine the utility of RNAi for functional genomics studies in helminth parasites and confirm these can be used to identify potential drug targets, in this case schistosome aminopeptidases.

Fasciola hepatica leucine aminopeptidase, a promising candidate for vaccination against ruminant fasciolosis.

Leucyl aminopeptidases (LAP) from different parasitic organisms are attracting attention as relevant players in parasite biology, and consequently being considered as candidates for drug and vaccine design. In fact, the highest protection level achieved in ruminant immunization by a native antigen was previously reported by us, using a purified LAP as immunogen in a sheep trial against fasciolosis. Here, we report the cloning of a full-length cDNA from adult F. hepatica encoding a member of the M17 family of LAP (FhLAP) and functional expression and characterization of the corresponding enzyme. FhLAP was closely related to Schistosoma LAPs, but interestingly distant from their mammalian hosts homologues, and was expressed in all stages of the parasite life cycle. The recombinant enzyme, functionally expressed in Escherichia coli, showed a marked amidolytic preference against the synthetic aminopeptidase substrate l-leucine-7-amino-4-methylcoumarin (Leu-AMC) and was also active against Cys-AMC and Met-AMC. Both native and recombinant enzyme were stimulated by the addition of divalent cations predominantly Mn(2+), and strongly inhibited by bestatin and cysteine. Physico-chemical properties, localization by immunoelectron microscopy, MALDI-TOF analysis, and cross-reactivity of anti-rFhLAP immune serum demonstrated that the recombinant enzyme was identical to the previously purified gut-associated LAP from adult F. hepatica. Vaccination trials using rFhLAP for rabbit immunization showed a strong IgG response and a highly significant level of protection after experimental infection with F. hepatica metacercariae, confirming that FhLAP is a relevant candidate for vaccine development.

A distinctive repertoire of cathepsins is expressed by juvenile invasive Fasciola hepatica.

Secreted cysteine proteases are relevant actors in parasite biology, taking part in critical host colonization roles such as traversing tissue barriers, immune evasion and nutrient digestion. In the trematode Fasciola hepatica, the initial step to successful infection of the mammalian host is the excystment of metacercariae and the invasion through the intestinal wall by the newly excysted juveniles (NEJ). While the cathepsin L-like cysteine proteinases secreted by the adult fluke have been extensively characterized, the cataloguing and description of the cathepsins B and L reported in the invasive stages is only sketchy. To identify the cathepsins expressed during excystment and early invasion we constructed cDNA libraries encoding NEJ cathepsins B and L. We found two cathepsin L-like cysteine proteinases (CL3, CL4) and three cathepsins B (CB1, CB2, CB3) which are predominantly expressed in NEJ. Phylogenetic analysis showed that NEJ-expressed cathepsins L constitute a well-defined clade separate from the adult enzymes. Excystment induction resulted in a significant increment in activity towards cathepsin-specific fluorogenic substrates in metacercariae homogenates, consistent with the detection of precursor and mature forms of cathepsins B and L before and after induction. In NEJ culture supernatants, protein and relative activity profiles show subtle changes during the first 48 h, with prevalence of cathepsin L-like activity, although cathepsins CB3 and CL3 were detected by mass spectrometry. Noticeably, the hydrolysis of a substrate with proline in the P2 position was predominant, a property only shared with adult CL2 and vertebrate cathepsin K among the C1A subfamily of cysteine proteases. Collectively these mRNA, protein and enzymatic data demonstrate the existence of a NEJ-specific repertoire of cathepsins expressed early in invasion, distinct to those used by other trematodes, potentially relevant for specific vaccine and chemotherapy design. The diversity of proteases employed by trematodes in the invasion process is discussed.

Development of Functional Genomic Tools in Trematodes: RNA Interference and Luciferase Reporter Gene Activity in Fasciola hepatica

The growing availability of sequence information from diverse parasites through genomic and transcriptomic projects offer new opportunities for the identification of key mediators in the parasite–host interaction. Functional genomics approaches and methods for the manipulation of genes are essential tools for deciphering the roles of genes and to identify new intervention targets in parasites. Exciting advances in functional genomics for parasitic helminths are starting to occur, with transgene expression and RNA interference (RNAi) reported in several species of nematodes, but the area is still in its infancy in flatworms, with reports in just three species. While advancing in model organisms, there is a need to rapidly extend these technologies to other parasites responsible for several chronic diseases of humans and cattle. In order to extend these approaches to less well studied parasitic worms, we developed a test method for the presence of a viable RNAi pathway by silencing the exogenous reporter gene, firefly luciferase (fLUC). We established the method in the human blood fluke Schistosoma mansoni and then confirmed its utility in the liver fluke Fasciola hepatica. We transformed newly excysted juveniles of F. hepatica by electroporation with mRNA of fLUC and three hours later were able to detect luciferase enzyme activity, concentrated mainly in the digestive ceca. Subsequently, we tested the presence of an active RNAi pathway in F. hepatica by knocking down the exogenous luciferase activity by introduction into the transformed parasites of double-stranded RNA (dsRNA) specific for fLUC. In addition, we tested the RNAi pathway targeting an endogenous F. hepatica gene encoding leucine aminopeptidase (FhLAP), and observed a significant reduction in specific mRNA levels. In summary, these studies demonstrated the utility of RNAi targeting reporter fLUC as a reporter gene assay to establish the presence of an intact RNAi pathway in helminth parasites. These could facilitate the study of gene function and the identification of relevant targets for intervention in organisms that are by other means intractable. More specifically, these results open new perspectives for functional genomics of F. hepatica, which hopefully can lead to the development of new interventions for fascioliasis.

Survey of transcripts expressed by the invasive juvenile stage of the liver fluke Fasciola hepatica

BackgroundThe common liver fluke Fasciola hepatica is the agent of a zoonosis with significant economic consequences in livestock production worldwide, and increasing relevance to human health in developing countries. Although flukicidal drugs are available, re-infection and emerging resistance are demanding new efficient and inexpensive control strategies. Understanding the molecular mechanisms underlying the host-parasite interaction provide relevant clues in this search, while enlightening the physiological adaptations to parasitism. Genomics and transcriptomics are still in their infancy in F. hepatica, with very scarce information available from the invasive newly excysted juveniles (NEJ). Here we provide an initial glimpse to the transcriptomics of the NEJ, the first stage to interact with the mammalian host.ResultsWe catalogued more than 500 clusters generated from the analysis of F. hepatica juvenile expressed sequence tags (EST), several of them not detected in the adult stage. A set of putative F. hepatica specific transcripts, and a group of sequences conserved exclusively in flatworms were identified. These novel sequences along with a set of parasite transcripts absent in the host genomes are putative new targets for future anti-parasitic drugs or vaccine development.Comparisons of the F. hepatica sequences with other metazoans genomes or EST databases were consistent with the basal positioning of flatworms in the bilaterian phylogeny. Notably, GC content, codon usage and amino acid frequencies are remarkably different in Schistosomes to F. hepatica and other trematodes.Functional annotation of predicted proteins showed a general representation of diverse biological functions. Besides proteases and antioxidant enzymes expected to participate in the early interaction with the host, various proteins involved in gene expression, protein synthesis, cell signaling and mitochondrial enzymes were identified. Differential expression of secreted protease gene family members between juvenile and adult stages may respond to different needs during host colonization.ConclusionThe knowledge of the genes expressed by the invasive stage of Fasciola hepatica is a starting point to unravel key aspects of this parasites biology. The integration of the emerging transcriptomics, and proteomics data and the advent of functional genomics tools in this organism are positioning F. hepatica as an interesting model for trematode biology.

Collagenolytic Activities of the Major Secreted Cathepsin L Peptidases Involved in the Virulence of the Helminth Pathogen, Fasciola hepatica

Background The temporal expression and secretion of distinct members of a family of virulence-associated cathepsin L cysteine peptidases (FhCL) correlates with the entry and migration of the helminth pathogen Fasciola hepatica in the host. Thus, infective larvae traversing the gut wall secrete cathepsin L3 (FhCL3), liver migrating juvenile parasites secrete both FhCL1 and FhCL2 while the mature bile duct parasites, which are obligate blood feeders, secrete predominantly FhCL1 but also FhCL2. Methodology/Principal Findings Here we show that FhCL1, FhCL2 and FhCL3 exhibit differences in their kinetic parameters towards a range of peptide substrates. Uniquely, FhCL2 and FhCL3 readily cleave substrates with Pro in the P2 position and peptide substrates mimicking the repeating Gly-Pro-Xaa motifs that occur within the primary sequence of collagen. FhCL1, FhCL2 and FhCL3 hydrolysed native type I and II collagen at neutral pH but while FhCL1 cleaved only non-collagenous (NC, non-Gly-X-Y) domains FhCL2 and FhCL3 exhibited collagenase activity by cleaving at multiple sites within the α1 and α2 triple helix regions (Col domains). Molecular simulations created for FhCL1, FhCL2 and FhCL3 complexed to various seven-residue peptides supports the idea that Trp67 and Tyr67 in the S2 subsite of the active sites of FhCL3 and FhCL2, respectively, are critical to conferring the unique collagenase-like activity to these enzymes by accommodating either Gly or Pro residues at P2 in the substrate. The data also suggests that FhCL3 accommodates hydroxyproline (Hyp)-Gly at P3-P2 better than FhCL2 explaining the observed greater ability of FhCL3 to digest type I and II collagens compared to FhCL2 and why these enzymes cleave at different positions within the Col domains. Conclusions/Significance These studies further our understanding of how this helminth parasite regulates peptidase expression to ensure infection, migration and establishment in host tissues.

The major cathepsin L secreted by the invasive juvenile Fasciola hepatica prefers proline in the S2 subsite and can cleave collagen.

Secreted cysteine proteases are major players in host-parasite interactions; in Fasciola hepatica, a distinct group of cathepsins L was found to be predominantly expressed in the juvenile stages, but their enzymatic properties were unknown. Cathepsin L3 (FhCL3) is a main component of the juvenile secretory products and may participate in invasion. To characterize the biochemical properties, the proenzyme was expressed in the methylotrophic yeast Hansenula polymorpha and the mature enzyme was obtained from the culture medium. FhCL3 exhibited optimal activity and stability at neutral pH and a noticeable restricted substrate specificity with 70-fold preference for Tos-Gly-Pro-Arg-AMC over typical cathepsin substrates with hydrophobic or aliphatic residues in the S2 position. Accordingly, FhCL3 efficiently cleaved type I collagen over different pH and temperature conditions, but it did not cleave immunoglobulin. While most cathepsin cysteine proteinases are unable to digest collagen, mammalian cathepsin K, adult F. hepatica FhCL2 and the plant zingipain can also cleave collagen and substrates with Pro in P2 position, but only FhCL3 and zingipain hydrolyze these substrates with the highest efficiency. Molecular modeling and structural comparisons of the collagen cleaving cathepsins indicated that the strong substrate selectivity observed might be due to steric restrictions imposed by bulky aromatic residues at the S2-S3 subsites. The remarkable similarities of the active site clefts highlight the evolutive constrains acting on enzyme function. The presence of a collagen cleaving enzyme in F. hepatica juvenile stages is suggestive of a role in tissue invasion, an essential feature for the establishment of the parasites in their host.

Fasciola hepatica mucin-encoding gene: expression, variability and its potential relevance in host-parasite relationship.

Fasciola hepatica is the causative agent of fasciolosis, a zoonosis with significant impact both in human and animal health. Understanding the basic processes of parasite biology, especially those related to interactions with its host, will contribute to control F. hepatica infections and hence liver pathology. Mucins have been described as important mediators for parasite establishment within its host, due to their key roles in immune evasion. In F. hepatica, mucin expression is upregulated in the mammalian invasive newly excysted juvenile (NEJ) stage in comparison with the adult stage. Here, we performed sequencing of mucin cDNAs prepared from NEJ RNA, resulting in six different cDNAs clusters. The differences are due to the presence of a tandem repeated sequence of 66 bp encoded by different exons. Two groups of apomucins one with three and the other with four repeats, with 459 and 393 bp respectively, were identified. These cDNAs have open reading frames encoding Ser-Thr enriched proteins with an N-terminal signal peptide, characteristic of apomucin backbone. We cloned a 4470 bp gene comprising eight exons and seven introns that encodes all the cDNA variants identified in NEJs. By real time polymerase chain reaction and high-resolution melting approaches of individual flukes we infer that fhemuc-1 is a single-copy gene, with at least two different alleles. Our data suggest that both gene polymorphism and alternative splicing might account for apomucin variability in the fhemuc-1 gene that is upregulated in NEJ invasive stage. The relevance of this variation in host-parasite interplay is discussed.

Expression of the histone chaperone SET/TAF-Iβ during the strobilation process of Mesocestoides corti (Platyhelminthes, Cestoda).

The histone chaperone SET/TAF-Iβ is implicated in processes of chromatin remodelling and gene expression regulation. It has been associated with the control of developmental processes, but little is known about its function in helminth parasites. In Mesocestoides corti, a partial cDNA sequence related to SET/TAF-Iβ was isolated in a screening for genes differentially expressed in larvae (tetrathyridia) and adult worms. Here, the full-length coding sequence of the M. corti SET/TAF-Iβ gene was analysed and the encoded protein (McSET/TAF) was compared with orthologous sequences, showing that McSET/TAF can be regarded as a SET/TAF-Iβ family member, with a typical nucleosome-assembly protein (NAP) domain and an acidic tail. The expression patterns of the McSET/TAF gene and protein were investigated during the strobilation process by RT-qPCR, using a set of five reference genes, and by immunoblot and immunofluorescence, using monospecific polyclonal antibodies. A gradual increase in McSET/TAF transcripts and McSET/TAF protein was observed upon development induction by trypsin, demonstrating McSET/TAF differential expression during strobilation. These results provided the first evidence for the involvement of a protein from the NAP family of epigenetic effectors in the regulation of cestode development.

Antigen B from Echinococcus granulosus enters mammalian cells by endocytic pathways

Background Cystic hydatid disease is a zoonosis caused by the larval stage (hydatid) of Echinococcus granulosus (Cestoda, Taeniidae). The hydatid develops in the viscera of intermediate host as a unilocular structure filled by the hydatid fluid, which contains parasitic excretory/secretory products. The lipoprotein Antigen B (AgB) is the major component of E. granulosus metacestode hydatid fluid. Functionally, AgB has been implicated in immunomodulation and lipid transport. However, the mechanisms underlying AgB functions are not completely known. Methodology/Principal findings In this study, we investigated AgB interactions with different mammalian cell types and the pathways involved in its internalization. AgB uptake was observed in four different cell lines, NIH-3T3, A549, J774 and RH. Inhibition of caveolae/raft-mediated endocytosis causes about 50 and 69% decrease in AgB internalization by RH and A549 cells, respectively. Interestingly, AgB colocalized with the raft endocytic marker, but also showed a partial colocalization with the clathrin endocytic marker. Finally, AgB colocalized with an endolysosomal tracker, providing evidence for a possible AgB destination after endocytosis. Conclusions/Significance The results indicate that caveolae/raft-mediated endocytosis is the main route to AgB internalization, and that a clathrin-mediated entry may also occur at a lower frequency. A possible fate for AgB after endocytosis seems to be the endolysosomal system. Cellular internalization and further access to subcellular compartments could be a requirement for AgB functions as a lipid carrier and/or immunomodulatory molecule, contributing to create a more permissive microenvironment to metacestode development and survival.