RNA interference in Haemonchus contortus: Suppression of beta-tubulin gene expression in L3, L4 and adult worms in vitro

Target screening using RNA interference in the sheep abomasal nematode parasite Haemonchus contortus

RNA interference (RNAi) on parasitic nematodes has been described as a valuable tool for screening putative targets that could be used as novel drug and/or vaccine candidates. This study aimed to set up a pipeline to identify potential targets using RNAi for vaccine/anti-parasite therapy development against Haemonchus contortus, a blood-feeding abomasal nematode parasite. The available H. contortus sequence data was mined for targets, which were tested for essentiality using RNAi electroporation assays. A total of 56 genes were identified and tested for knockdown using electroporation of first-stage larvae (L1) H. contortus with the target double-stranded RNA. Electroporation of L1 proved to be effective overall; 17 targets had a strong phenotype and significant reduction in alive H. contortus, and another 24 had a moderate phenotype with a significant reduction in larvae development. A total of 28 targets showed a significant reduction in the development of H. contortus larvae to the infective stage (L3) following the RNAi assay. Down-regulation of target transcript levels was evaluated in some targets by semi-quantitative PCR. Four out of five genes tested showed complete knockdown of mRNA levels via semi-quantitative PCR, whereas the knockdown was partial for one. In conclusion, the results indicate that the RNAi pathway is confirmed in H. contortus and that several target genes have the potential to be investigated further as possible vaccine candidates.

A proteasomal β5 subunit of Haemonchus contortus with a role in the growth, development and life span

BACKGROUND

The proteasome in eukaryotic cells can degrade a variety of proteins and plays an important role in regulating the cell cycle, cell survival and apoptosis. The proteasome receives much attention as a potential chemotherapeutic target for treatment of a variety of infectious parasitic diseases, but few studies of proteasomes have been done on parasitic nematodes.

METHODS

A proteasomal β5 subunit encoding gene (named Hc-pbs-5) and its inferred product (Hc-PBS-5) in Haemonchus contortus were identified and characterized in this study. Then, the transcriptional profiles and anatomical expression were studied using an integrated molecular approach. Finally, a specific proteasome inhibitor bortezomib (BTZ), together with RNA interference (RNAi), was employed to assess the function of Hc-PBS-5.

RESULTS

Bioinformatic analysis revealed that the coding sequence of Hc-pbs-5 was 855 bp long and encoded 284 amino acids (aa). The predicted protein (Hc-PBS-5) had core conservative sequences (65-250 aa) belonging to N-terminal nucleophile (Ntn) family of hydrolases. Real-time PCR results revealed that Hc-pbs-5 was continuously transcribed in eight developmental stages with higher levels at the infective third-stage larvae (L3s) and adult males of H. contortus. Immunohistochemical results revealed that Hc-PBS-5 was expressed in intestine, outer cuticle, muscle cells under the outer cuticle, cervical glands and seminal vesicles of male adults and also in intestine, outer cuticle, cervical glands, uterine wall, eggs and ovaries of female adults of H. contortus. BTZ could reduce proportions of egg hatching, and the fourth-stage larvae (L4s) developed from the exsheathed L3s (xL3s) of H. contortus. In addition, silencing Hc-pbs-5 by soaking the specific double-stranded RNA (dsRNA) could decrease the transcription of Hc-pbs-5 and result in fewer xL3s developing to L4s in vitro.

CONCLUSIONS

These results indicate that proteasomal β5 subunit plays an important role in the growth, development and life span of H. contortus.

Identification and characterisation of the haemozoin of Haemonchus contortus

BACKGROUND

Most haematophagous organisms constantly suck the host's haemoglobin, which produces toxic free haem. This toxic haem aggregation into the nontoxic crystallisation complex known as haemozoin represents one of the most important detoxification pathways in living organisms, but very little is known about the features of haemozoin in parasitic nematodes. Here, we identified and characterised the haemozoin of an economically significant blood-sucking nematode, Haemonchus contortus.

METHODS

Using electron microscopy, spectrophotometry analyses and biochemical approaches, haemozoin crystallisation was identified and characterised in parasitic fourth-stage larvae (L4s) and/or adult worms as well as L4s of in vitro culture.

RESULTS

The haemozoin was formed in intestinal lipid droplets of the parasitic L4s and adult worms. The characterisation of the haemozoin showed regularly spherical structures and had a 400-nm absorption peak. Furthermore, the haemozoin in in vitro cultured L4s was associated with the culture time and concentration of red blood cells added into the medium, and its formation could be inhibited by chloroquine-derived drugs.

CONCLUSIONS

This work provides detailed insight into the haemozoin formation of H. contortus and should have important implications for developing novel therapeutic targets against this parasite or related haematophagous organisms.

Genetic manipulations in helminth parasites

Screening of vaccine or drug target in parasitic helminth is hindered by lack of robust tool for functional studies of parasite protein which account for the availability of only a few anti-helminthic vaccines, diagnostic assay and slower pace of development of an anthelmintic drug. With the piling up of parasite transcriptomic and genomic data, in silico screening for possible vaccine/drug target could be validated by functional characterization of proteins by RNA interference or CRISPR/Cas9. These reverse genetic engineering tools have opened up a better avenue and opportunity for screening parasitic proteins in vitro as well as in vivo. RNA interference provides a technique for silencing targeted mRNA transcript for understanding a gene function in helminth as evidence by work in Caenorhabditis elegans. Recent genetic engineering tool, CRISPR/Cas9 allows knock-out/deletion of the desired gene in parasitic helminths and the other provision it provides in terms of gene knock-in/insertion in parasite genome is still to be explored in future. This manuscript discussed the work that has been carried out on RNAi and CRISPR/Cas9 for functional studies of helminth parasitic proteins.

In vitro culture of parasitic stages of Haemonchus contortus

Haemonchus contortus is a constraint to sheep production. Seeking to reduce the use of hosts and produce parasitic stages in large-scale, a 42-day in vitro culture protocol of H. contortus third-stage larvae was optimized using Dulbecco's modified Eagle's medium (DMEM). In cell-free culture, larvae were maintained at 39.6°C, in acidic media (pH 6.1) for 3 or 6 days with Δ4-dafachronic acid followed by DMEM pH 7.4 supplemented or not with Fildes' reagent. In DMEM pH 7.4 at 37°C, supplementation with Caco-2 cells was compared to Fildes. On Day 14, fourth-stage larvae (L4) development rates in acidic media supplemented (86.8-88.4%) or not (74.4-77.8%) with Fildes and in Caco-2 cell co-culture (92.6%) were similar, and superior to DMEM pH 7.4 with Fildes (0.0%). On Day 21, Caco-2 cell co-culture resulted in higher larvae differentiation (25.0%) and lower degeneration (13.9%) compared to acidic media (1.5-8.1% and 48.6-69.9%, respectively). This is the first report of prolonged in vitro culture of H. contortus larvae using commercial media in co-culture with Caco-2 cells. Although no progression to the adult stage, Caco-2 cell co-culture resulted in morphological differentiation of H. contortus L4 and larval viability for up to 28 days.

Research progress and limitation analysis of RNA interference in Haemonchus contortus in China

Haemonchus contortus is a highly pathogenic and economically important parasitic nematode that affects small ruminants worldwide. While omics studies hold great promise, there are fewer research tools available for analyzing subsequent gene function studies. RNA interference (RNAi) technology offers a solution to this problem, as it especially allows for the knockout or shutting off of the expression of specific genes. As a result, RNAi technology has been widely used to explore gene function and disease treatment research. In this study, we reviewed the latest advancements in RNAi research on Haemonchus contortus in China, with the aim of providing a reference for the identification of key genes involved in growth and development, anthelmintic resistance, diagnostic markers, and diagnostic drug targets for the treatment of Haemonchus contortus.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

Transgenesis in parasitic helminths: a brief history and prospects for the future

Helminth infections impact the health of hundreds of millions of persons globally and also cause important economic losses in livestock farming. Methodological limitations as well as the low attention given to the study of helminths have impacted biological research and, thus, the procurement of accurate diagnosis and effective treatments. Understanding the biology of helminths using genomic and proteomic approaches could contribute to advances in understanding host-helminth interactions and lead to new vaccines, drugs and diagnostics. Despite the significant advances in genomics in the last decade, the lack of methodological adaptation of current transgenesis techniques has hampered the progression of post-genomic research in helminthology. However, the application of new techniques, such as CRISPR, to the study of trematodes and nematodes has opened new avenues for genome editing-powered functional genomics for these pathogens. This review summarises the historical advances in functional genomics in parasitic helminths and highlights pending limitations that will need to be overcome to deploy transgenesis tools.

A novel BR-SMAD is required for larval development in barber's pole worm Haemonchus contortus

SMAD proteins mediate TGF-β signaling and thereby regulate the metazoan development; however, they are poorly defined in Haemonchus contortus–a common blood-sucking parasitic nematode of small ruminants. Here, we characterized an R-SMAD family protein in H. contortus termed HcSMA2, which is closely related to Caenorhabditis elegans SMA2 (CeSMA2) involved in the bone morphogenetic protein (BMP) signaling. Hcsma2 is transcribed in all developmental stages of H. contortus but highly induced in the adult male worms. The RNA interference with Hcsma2 retarded the transition of infective L3 into L4 larvae. Besides, the bimolecular fluorescence complementation revealed the interaction of HcSMA2 with a TGF-β-activated-R-SMAD (HcDAF8). Together these results show a BMP-like receptor-regulated SMAD in H. contortus that is required for larval differentiation and underscore an adaptive functional repurposing of BMP-signaling in parasitic worms.

RNAi-mediated silencing of Trichinella spiralis serpin-type serine protease inhibitors results in a reduction in larval infectivity

Trichinella spiralis serpin-type serine protease inhibitors (TsSPIs) are expressed in adult worms (AW), newborn larvae (NBL) and muscle larvae (ML) of T. spiralis, with the ML stage demonstrating the highest expression level. This study aims to determine TsSPI functions in larval viability and invasion of intestinal epithelial cells in vitro, as well as their development, survival, and fecundity in vivo via RNAi. TsSPI-specific siRNAs and dsRNA were transfected into ML by incubation. The silencing effect of TsSPI transcription and expression was determined using qPCR and western blot, respectively. After incubation in 60 ng/μL dsRNA–TsSPI for 3 days, larval TsSPI mRNA and protein expression levels were reduced by 68.7% and 68.4% (P < 0.05), respectively. dsRNA-mediated silencing of TsSPI significantly impacted larval invasion into intestinal epithelial cells in vitro but did not affect the survival rate of larvae. After challenge with dsRNA–TsSPI-treated ML, mice exhibited a 56.0% reduction in intestinal AW burden and 56.9% reduction in ML burden (P < 0.05), but NBL production of female AW remained the same (P > 0.05). Our results revealed that RNAi-mediated silencing of TsSPI expression in T. spiralis significantly reduced larval infectivity and survival in the host but had no effect on the survival rate and fecundity. Furthermore, TsSPIs have no effect on the growth and reproduction of parasites but may be directly involved in regulating the interaction of T. spiralis and the host. Therefore, TsSPIs are crucial in the process of T. spiralis larval invasion and parasite survival in the host.

Suppression of hyaluronidase reduces invasion and establishment of Haemonchus contortus larvae in sheep

Haemonchus contortus is a hematophagous endoparasite of small ruminants, which is responsible for huge economic losses in livestock sector. Hyaluronidase produced by infective larvae of H. contortus can degrade hyaluronic acid present in the host’s abomasal tissue. Thus, it facilitates larval tissue invasion and early establishment. We herein explored this ability of hyaluronidase in H. contortus, and tested whether hyaluronidase is utilized as a virulence factor by H. contortus while establishing the infection. We first successfully blocked the hyaluronidase gene in L3 larvae by RNA interference (RNAi), which was subsequently confirmed by qPCR, enzymatic activity, and immunohistochemistry assays. Using these larvae we then conducted in vivo and in vitro assays on sheep to assess the effects of hyaluronidase suppression on larval invasion and establishment of infection. The in vivo assay showed a significant drop in worm burden in siRNA treated group in comparison to control group. During in vitro assay we applied an ovine ex vivo model where siRNA treated group of larvae showed significantly reduced invasion of the abomasal tissue explants as compared to control group. These findings indicate that hyaluronidase plays a key role in host’s tissue invasion and larval establishment, and it is used as a virulence factor by H. contortus while establishing the infection. As an invasive virulence molecule, its functional research is thus conducive to the prevention of haemonchosis.

Toward integrative 'omics of the barber's pole worm and related parasitic nematodes

Advances in nucleic acid sequencing, mass spectrometry and computational biology have facilitated the identification, annotation and analysis of genes, transcripts, proteins and metabolites in model nematodes (Caenorhabditis elegans and Pristionchus pacificus) and socioeconomically important parasitic nematodes (Clades I, III, IV and V). Significant progress has been made in genomics and transcriptomics as well as in the proteomics and lipidomics of Haemonchus contortus (the barber's pole worm) - one of the most pathogenic representatives of the order Strongylida. Here, we review salient aspects of genomics, transcriptomics, proteomics, lipidomics, glycomics and functional genomics, and discuss the rise of integrative 'omics of this economically important parasite. Although our knowledge of the molecular biology, genetics and biochemistry of H. contortus and related species has progressed significantly, much remains to be explored, particularly in areas such as drug resistance, unique/unknown genes, host-parasite interactions, parasitism and the pathogenesis of disease, by integrating the use of multiple 'omics methods. This approach should lead to a better understanding of H. contortus and its relatives at a 'systems biology' level, and should assist in developing new interventions against these parasites.

A daf-7-related TGF-β ligand (Hc-tgh-2) shows important regulations on the development of Haemonchus contortus

BACKGROUND

In most multicellular organisms, the transforming growth factor-β (TGF-β) signalling pathway is involved in regulating the growth and stem cell differentiation. Previous studies have demonstrated the importance of three key molecules in this pathway in the parasitic nematode Haemonchus contortus, including one TGF-β type I receptor (Hc-tgfbr1), one TGF-β type II receptor (Hc-tgfbr2), and one co-Smad (Hc-daf-3), which regulated the developmental transition from the free-living to the parasitic stages of this parasite. However, almost nothing is known about the function of the TGF-β ligand (Hc-tgh-2) of H. contortus.

METHODS

Here, the temporal transcription profiles of Hc-tgh-2 at eight different developmental stages and spatial expression patterns of Hc-TGH-2 in adult female and male worms of H. contortus have been examined by real-time PCR and immunohistochemistry, respectively. In addition, RNA interference (RNAi) by soaking was employed to assess the importance of Hc-tgh-2 in the development from exsheathed third-stage larvae (xL3s) to fourth-stage larvae (L4s) in H. contortus.

RESULTS

Hc-tgh-2 was continuously transcribed in all eight developmental stages of H. contortus studied with the highest level in the infective third-stage larvae (iL3) and Hc-TGH-2 was located in the muscle of the body wall, intestine, ovary of adult females and testes of adult males. Silencing Hc-tgh-2 by the specific double-stranded RNA (dsRNA), decreased the transcript level of Hc-tgh-2 and resulted in fewer xL3s developing to L4s in vitro.

CONCLUSIONS

These results suggested that the TGF-β ligand, Hc-TGH-2, could play important roles in the developmental transition from the free-living (L3s) to the parasitic stage (L4s). Furthermore, it may also take part in the processes such as digestion, absorption, host immune response and reproductive development in H. contortus adults.

The potential for vaccines against scour worms of small ruminants

This review addresses the research landscape regarding vaccines against scour worms, particularly Trichostrongylus spp. and Teladorsagia circumcincta. The inability of past research to deliver scour-worm vaccines with reliable and reproducible efficacy has been due in part to gaps in knowledge concerning: (i) host-parasite interactions leading to development of type-2 immunity, (ii) definition of an optimal suite of parasite antigens, and (iii) rational formulation and administration to induce protective immunity against gastrointestinal nematodes (GIN) at the site of infestation. Recent 'omics' developments enable more systematic analyses. GIN genomes are reaching completion, facilitating "reverse vaccinology" approaches that have been used successfully for the Rhipicephalus australis vaccine for cattle tick, while methods for gene silencing and editing in GIN enable identification and validation of potential vaccine antigens. We envisage that any efficacious scour worm vaccine(s) would be adopted similarly to "Barbervax™" within integrated parasite management schemes. Vaccines would therefore effectively parallel the use of resistant animals, and reduce the frequency of drenching and pasture contamination. These aspects of integration, efficacy and operation require updated models and validation in the field. The conclusion of this review outlines an approach to facilitate an integrated research program.

Recent advances in functional genomics for parasitic nematodes of mammals

Human-parasitic nematodes infect over a quarter of the world's population and are a major cause of morbidity in low-resource settings. Currently available treatments have not been sufficient to eliminate infections in endemic areas, and drug resistance is an increasing concern, making new treatment options a priority. The development of new treatments requires an improved understanding of the basic biology of these nematodes. Specifically, a better understanding of parasitic nematode development, reproduction and behavior may yield novel drug targets or new opportunities for intervention such as repellents or traps. Until recently, our ability to study parasitic nematode biology was limited because few tools were available for their genetic manipulation. This is now changing as a result of recent advances in the large-scale sequencing of nematode genomes and the development of new techniques for their genetic manipulation. Notably, skin-penetrating gastrointestinal nematodes in the genus Strongyloides are now amenable to transgenesis, RNAi and CRISPR/Cas9-mediated targeted mutagenesis, positioning the Strongyloides species as model parasitic nematode systems. A number of other mammalian-parasitic nematodes, including the giant roundworm Ascaris suum and the tissue-dwelling filarial nematode Brugia malayi, are also now amenable to transgenesis and/or RNAi in some contexts. Using these tools, recent studies of Strongyloides species have already provided insight into the molecular pathways that control the developmental decision to form infective larvae and that drive the host-seeking behaviors of infective larvae. Ultimately, a mechanistic understanding of these processes could lead to the development of new avenues for nematode control.

Elucidating the molecular and developmental biology of parasitic nematodes: Moving to a multiomics paradigm

In the past two decades, significant progress has been made in the sequencing, assembly, annotation and analyses of genomes and transcriptomes of parasitic worms of socioeconomic importance. This progress has somewhat improved our knowledge and understanding of these pathogens at the molecular level. However, compared with the free-living nematode Caenorhabditis elegans, the areas of functional genomics, transcriptomics, proteomics and metabolomics of parasitic nematodes are still in their infancy, and there are major gaps in our knowledge and understanding of the molecular biology of parasitic nematodes. The information on signalling molecules, molecular pathways and microRNAs (miRNAs) that are known to be involved in developmental processes in C. elegans and the availability of some molecular resources (draft genomes, transcriptomes and some proteomes) for selected parasitic nematodes provide a basis to start exploring the developmental biology of parasitic nematodes. Indeed, some studies have identified molecules and pathways that might associate with developmental processes in related, parasitic nematodes, such as Haemonchus contortus (barber's pole worm). However, detailed information is often scant and 'omics resources are limited, preventing a proper integration of 'omic data sets and comprehensive analyses. Moreover, little is known about the functional roles of pheromones, hormones, signalling pathways and post-transcriptional/post-translational regulations in the development of key parasitic nematodes throughout their entire life cycles. Although C. elegans is an excellent model to assist molecular studies of parasitic nematodes, its use is limited when it comes to explorations of processes that are specific to parasitism within host animals. A deep understanding of parasitic nematodes, such as H. contortus, requires substantially enhanced resources and the use of integrative 'omics approaches for analyses. The improved genome and well-established in vitro larval culture system for H. contortus provide unprecedented opportunities for comprehensive studies of the transcriptomes (mRNA and miRNA), proteomes (somatic, excretory/secretory and phosphorylated proteins) and lipidomes (e.g., polar and neutral lipids) of this nematode. Such resources should enable in-depth explorations of its developmental biology at a level, not previously possible. The main aims of this review are (i) to provide a background on the development of nematodes, with a particular emphasis on the molecular aspects involved in the dauer formation and exit in C. elegans; (ii) to critically appraise the current state of knowledge of the developmental biology of parasitic nematodes and identify key knowledge gaps; (iii) to cover salient aspects of H. contortus, with a focus on the recent advances in genomics, transcriptomics, proteomics and lipidomics as well as in vitro culturing systems; (iv) to review recent advances in our knowledge and understanding of the molecular and developmental biology of H. contortus using an integrative multiomics approach, and discuss the implications of this approach for detailed explorations of signalling molecules, molecular processes and pathways likely associated with nematode development, adaptation and parasitism, and for the identification of novel intervention targets against these pathogens. Clearly, the multiomics approach established recently is readily applicable to exploring a wide range of interesting and socioeconomically significant parasitic worms (including also trematodes and cestodes) at the molecular level, and to elucidate host-parasite interactions and disease processes.

A serine/threonine-specific protein kinase of Haemonchus contortus with a role in the development

In the free-living nematode Caenorhabditis elegans, the serine/threonine-specific protein kinase, AKT, is known to play a key role in dauer formation, life-span, and stress-resistance through the insulin-like signaling pathway. Although the structure and function of AKT-coding genes of C. elegans are understood, this is not the case for homologous genes in parasitic nematodes. In the present study, we explored a C. elegans akt-1 gene homolog in the parasitic nematode Haemonchus contortus, investigated its transcript isoforms (Hc-akt-1a and Hc-akt-1b), and studied expression and function using both homologous and heterologous functional genomic tools. In C. elegans, we showed that the predicted promoter of Hc-akt-1 drives substantial expression in ASJ neurons of the N2 (wild-type) strain. In H. contortus (Haecon-5 stain), RNAi (soaking) led to a significantly decreased transcript abundance for both Hc-akt-1a and Hc-akt-1b, and reduced larval development in larval stages in vitro. Chemical inhibition was also shown to block larval development. Taken together, the evidence from this study points to a key functional role for Hc-akt-1 in H. contortus.

A TGF-β type II receptor that associates with developmental transition in Haemonchus contortus in vitro

Background

The TGF-β signalling pathway plays a key role in regulating dauer formation in the free-living nematode Caenorhabditis elegans, and previous work has shown that TGF-β receptors are involved in parasitic nematodes. Here, we explored the structure and function of a TGF-β type II receptor homologue in the TGF-β signalling pathway in Haemonchus contortus, a highly pathogenic, haematophagous parasitic nematode.

Methodology/Principal findings

Amino acid sequence and phylogenetic analyses revealed that the protein, called Hc-TGFBR2 (encoded by the gene Hc-tgfbr2), is a member of TGF-β type II receptor family and contains conserved functional domains, both in the extracellular region containing cysteine residues that form a characteristic feature (CXCX₄C) of TGF-β type II receptor and in the intracellular regions containing a serine/threonine kinase domain. The Hc-tgfbr2 gene was transcribed in all key developmental stages of H. contortus, with particularly high levels in the infective third-stage larvae (L3s) and male adults. Immunohistochemical results revealed that Hc-TGFBR2 was expressed in the intestine, ovary and eggs within the uterus of female adults, and also in the testes of male adults of H. contortus. Double-stranded RNA interference (RNAi) in this nematode by soaking induced a marked decrease in transcription of Hc-tgfbr2 and in development from the exsheathed L3 to the fourth-stage larva (L4) in vitro.

Conclusions/Significance

These results indicate that Hc-TGFBR2 plays an important role in governing developmental processes in H. contortus via the TGF-β signalling pathway, particularly in the transition from the free-living to the parasitic stages.

Haemonchus contortus is a gastrointestinal parasitic nematode that causes major economic losses in small ruminants. Here, we investigated the structure and function of a TGF-β type II receptor homologue (Hc-TGFBR2) and its role in regulating H. contortus development. The results showed that the Hc-tgfbr2 gene was transcribed in all developmental stages of H. contortus, with the highest level in L3s and male adults; the encoded protein Hc-TGFBR2 was expressed in the intestine and gonads of adult stages of this nematode. The transcriptional abundance of Hc-tgfbr2 decreased significantly following knockdown by RNA interference in xL3s of H. contortus, which also caused a marked reduction in the number of xL3s developing to L4s in vitro. These findings reveal that the TGF-β type II receptor (Hc-TGFBR2) associates with development of H. contortus, particularly in its transition from the free-living to the parasitic stage.

The role of carbon dioxide in nematode behaviour and physiology

Carbon dioxide (CO2) is an important sensory cue for many animals, including both parasitic and free-living nematodes. Many nematodes show context-dependent, experience-dependent and/or life-stage-dependent behavioural responses to CO2, suggesting that CO2 plays crucial roles throughout the nematode life cycle in multiple ethological contexts. Nematodes also show a wide range of physiological responses to CO2. Here, we review the diverse responses of parasitic and free-living nematodes to CO2. We also discuss the molecular, cellular and neural circuit mechanisms that mediate CO2 detection in nematodes, and that drive context-dependent and experience-dependent responses of nematodes to CO2.

Terror in the dirt: Sensory determinants of host seeking in soil-transmitted mammalian-parasitic nematodes

Infection with gastrointestinal parasitic nematodes is a major cause of chronic morbidity and economic burden around the world, particularly in low-resource settings. Some parasitic nematode species, including the human-parasitic threadworm Strongyloides stercoralis and human-parasitic hookworms in the genera Ancylostoma and Necator, feature a soil-dwelling infective larval stage that seeks out hosts for infection using a variety of host-emitted sensory cues. Here, we review our current understanding of the behavioral responses of soil-dwelling infective larvae to host-emitted sensory cues, and the molecular and cellular mechanisms that mediate these responses. We also discuss the development of methods for transgenesis and CRISPR/Cas9-mediated targeted mutagenesis in Strongyloides stercoralis and the closely related rat parasite Strongyloides ratti. These methods have established S. stercoralis and S. ratti as genetic model systems for gastrointestinal parasitic nematodes and are enabling more detailed investigations into the neural mechanisms that underlie the sensory-driven behaviors of this medically and economically important class of parasites.

Gene silencing by RNA interference in the ectoparasitic mite, Psoroptes ovis

The presence of components of the RNA interference (RNAi) pathway in Psoroptes ovis, an ectoparasitic mite responsible for psoroptic mange, was investigated through interrogation of the P. ovis genome. Homologues of transcripts representing critical elements for achieving effective RNAi in the mite, Tetranychus urticae and the model organisms Caenorhabditis elegans and Drosophila melanogaster were identified and, following the development of a non-invasive immersion method of double stranded RNA delivery, gene silencing by RNAi was successfully demonstrated in P. ovis. Significant reductions in transcript levels were achieved for three target genes which encode the Group 2 allergen (Pso o 2), mu-class glutathione S-transferase (PoGST-mu1) and beta-tubulin (Poβtub). This is the first demonstration of RNAi in P. ovis and provides a mechanism for mining transcriptomic and genomic datasets for novel control targets against this economically important ectoparasite.

A TGF-β type I receptor-like molecule with a key functional role in Haemonchus contortus development

Here we investigated the gene of a transforming growth factor (TGF)-β type I receptor-like molecule in Haemonchus contortus, a highly pathogenic and economically important parasitic nematode of small ruminants. Designated Hc-tgfbr1, this gene is transcribed in all developmental stages of H. contortus, and the encoded protein has glycine-serine rich and kinase domains characteristic of a TGF-β family type I receptor. Expression of a GFP reporter driven by the putative Hc-tgfbr1 promoter localised to two intestinal rings, the anterior-most intestinal ring (int ring I) and the posterior-most intestinal ring (int ring IX) in Caenorhabditis elegans in vivo. Heterologous genetic complementation using a plasmid construct containing Hc-tgfbr1 genomic DNA failed to rescue the function of Ce-daf-1 (a known TGF-β type I receptor gene) in a daf-1-deficient mutant strain of C. elegans. In addition, a TGF-β type I receptor inhibitor, galunisertib, and double-stranded RNA interference (RNAi) were employed to assess the function of Hc-tgfbr1 in the transition from exsheathed L3 (xL3) to the L4 of H. contortus in vitro, revealing that both galunisertib and Hc-tgfbr1-specific double-stranded RNA could retard L4 development. Taken together, these results provide evidence that Hc-tgfbr1 is involved in developmental processes in H. contortus in the transition from the free-living to the parasitic stage.

RNA interference mediated knockdown of Brugia malayi UDP-Galactopyranose mutase severely affects parasite viability, embryogenesis and in vivo development of infective larvae

Background

Galactofuranose is an essential cell surface component present in bacteria, fungi and several nematodes such as Caenorhabditis spp., Brugia spp., Onchocerca spp. and Strongyloides spp. This sugar maintains the integrity of parasite surface and is essential for virulence. UDP-Galactopyranose mutase (bmugm) plays a key role in Galf biosynthesis by catalyzing conversion of UDP-Galactopyranose into UDP-galactofuranose and knockout studies of the gene in Leishmania major, Mycobacterium and Aspergillus fumigatus displayed attenuated virulence while RNA interference study in C. elegans exhibited detrimental effects. Presence of UGM in several prokaryotic and eukaryotic microbial pathogens and its absence in higher eukaryotes renders it an attractive drug target. In the present study, RNA interference studies have been carried out to validate bmugm as an antifilarial drug target.

Methods

RNA interference studies using two different sequences of siRNAs targeting bmugm were carried out. The in vitro gene silencing of adult B. malayi parasites was undertaken to observe the effects on parasites. Infective larvae were also exposed to siRNAs and their in vivo development in jirds was observed.

Results

The in vitro gene silencing induced by siRNA1 and 2 individually as well as together knocked down the bmugm gene expression causing impaired viability of the exposed worms along with extremely reduced motility, abridged microfilarial release and adversely effected embryogenesis. The combinatorial in vitro gene silencing revealed marginally better results than both the siRNAs individually. Thus, infective larvae were treated with siRNA combination which showed downregulation of bmugm mRNA expression resulting into sluggish larval movements and/or death. The siRNA-treated actively motile larvae when inoculated intraperitoneally into jirds demonstrated highly reduced transformation of these larvae into adult worms with detrimental effects on embryogenesis. The effects of gene silencing were long-lasting as the adult worms developed from siRNA-treated larvae showed noticeable knockdown in the target gene expression.

Conclusions

The validation studies undertaken here conclude that bmugm is essential for the proper development and survival of the parasite and support its candidature as an antifilarial drug target.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-1967-1) contains supplementary material, which is available to authorized users.

Genomes of parasitic nematodes (Meloidogyne hapla, Meloidogyne incognita, Ascaris suum and Brugia malayi) have a reduced complement of small RNA interference pathway genes: knockdown can reduce host infectivity of M. incognita

The discovery of RNA interference (RNAi) as an endogenous mechanism of gene regulation in a range of eukaryotes has resulted in its extensive use as a tool for functional genomic studies. It is important to study the mechanisms which underlie this phenomenon in different organisms, and in particular to understand details of the effectors that modulate its effectiveness. The aim of this study was to identify and compare genomic sequences encoding genes involved in the RNAi pathway of four parasitic nematodes: the plant parasites Meloidogyne hapla and Meloidogyne incognita and the animal parasites Ascaris suum and Brugia malayi because full genomic sequences were available-in relation to those of the model nematode Caenorhabditis elegans. The data generated was then used to identify some potential targets for control of the root knot nematode, M. incognita. Of the 84 RNAi pathway genes of C. elegans used as model in this study, there was a 42-53 % reduction in the number of effectors in the parasitic nematodes indicating substantial differences in the pathway between species. A gene each from six functional groups of the RNAi pathway of M. incognita was downregulated using in vitro RNAi, and depending on the gene (drh-3, tsn-1, rrf-1, xrn-2, mut-2 and alg-1), subsequent plant infection was reduced by up to 44 % and knockdown of some genes (i.e. drh-3, mut-2) also resulted in abnormal nematode development. The information generated here will contribute to defining targets for more robust nematode control using the RNAi technology.

Functional Genomics Tools for Haemonchus contortus and Lessons From Other Helminths

The availability of genome and transcriptome data for parasitic nematodes, including Haemonchus contortus, has highlighted the need to develop functional genomics tools. Comparative genomic analysis, particularly using data from the free-living nematode Caenorhabditis elegans, can help predict gene function. Reliable approaches to study function directly in parasitic nematodes are currently lacking. However, gene knockdown by RNA interference (RNAi) is being successfully used in schistosome and planarian species to define gene functions. Lessons from these systems may be applied to improve RNAi in H. contortus. Previous studies in H. contortus and related nematodes demonstrated reliable RNAi-mediated silencing of some genes, but not others. Current data suggest that susceptibility to RNAi in these nematodes is limited to genes expressed in sites accessible to the environment, such as the gut, amphids and excretory cell. Therefore, RNAi is functional in H. contortus, but improvements are needed to develop this system as a functional genomics platform. Here, we summarize RNAi studies on H. contortus and discuss the optimization of RNA delivery and improvements to culture methods to enhance larval development, protein turnover and the induction of phenotypic effects in vitro. The transgenic delivery of RNA or dominant-negative gene constructs and the recently developed CRISPR/Cas genome-editing technique are considered as potential alternative approaches for gene knockout. This is a key time to devote greater effort in progressing from genome to function, to improve our understanding of the biology of Haemonchus and identify novel targets for parasite control.

RNAi-mediated gene knockdown by microinjection in the model entomopathogenic nematode Heterorhabditis bacteriophora

Background

Parasitic nematodes threaten the health of humans and livestock and cause a major financial and socioeconomic burden to modern society. Given the widespread distribution of diseases caused by parasitic nematodes there is an urgent need to develop tools that will elucidate the genetic complexity of host-parasite interactions. Heterorhabditis bacteriophora is a parasitic nematode that allows simultaneous monitoring of nematode infection processes and host immune function, and offers potential as a tractable model for parasitic nematode infections. However, molecular tools to investigate these processes are required prior to its widespread acceptance as a robust model organism. In this paper we describe microinjection in adult H. bacteriophora as a suitable means of dsRNA delivery to knockdown gene transcripts.

Methods

RNA interference was used to knockdown four genes by injecting dsRNA directly into the gonad of adult hermaphrodite nematodes. RNAi phenotypes were scored in the F1 progeny on the fifth day post-injection, and knockdown of gene-specific transcripts was quantified with real-time quantitative RT-PCR (qRT-PCR).

Results

RNAi injection in adult hermaphrodites significantly decreased the level of target transcripts to varying degrees when compared with controls. The genes targeted by RNAi via injection included cct-2, nol-5, dpy-7, and dpy-13. In each case, RNAi knockdown was confirmed phenotypically by examining the progeny of injected animals, and also confirmed at the transcriptional level by real-time qRT-PCR.

Conclusions

Here we describe for the first time the successful use of microinjection to knockdown gene transcripts in H. bacteriophora. This technique can be used widely to study the molecular basis of parasitism.

Recent Advances in Elucidating Nematode Moulting - Prospects of Using Oesophagostomum dentatum as a Model

There are major gaps in our knowledge of many molecular biological processes that take place during the development of parasitic nematodes, in spite of the fact that understanding such processes could lead to new ways of treating and controlling parasitic diseases via the disruption of one or more biological pathways in the parasites. Progress in genomics, transcriptomics, proteomics and bioinformatics now provides unique opportunities to investigate the molecular basis of key developmental processes in parasitic nematodes. The porcine nodule worm, Oesophagostomum dentatum, represents a large order (Strongylida) of socioeconomically important nematodes, and provides a useful platform for exploring molecular developmental processes, particularly given that this nematode can be grown and maintained in culture in vitro for periods longer than most other nematodes of this order. In this article, we focus on the moulting process (ecdysis) in nematodes; review recent advances in our understanding of molecular aspects of moulting in O. dentatum achieved by using integrated proteomic-bioinformatic tools and discuss key implications and future prospects for research in this area, also with respect to developing new anti-nematode interventions and biotechnological outcomes.

Gene silencing by RNA interference in the house dust mite, Dermatophagoides pteronyssinus

This is the first report of gene silencing by RNA interference (RNAi) in the European house dust mite, Dermatophagoides pteronyssinus, Trouessart, 1897. Using a non-invasive immersion method first developed for the honey bee mite, Varroa destructor, a significant reduction in the expression of D. pteronyssinus glutathione-S-transferase mu-class 1 enzyme (DpGST-mu1) was achieved following overnight immersion in double stranded RNA encoding DpGST-mu1. Although no detrimental phenotypic changes were observed following silencing, this technique can now be used to address fundamental physiological questions and assess the potential therapeutic benefit in silencing D. pteronyssinus target genes in selected domestic situations of high human-mite interface.

Impact of next-generation technologies on exploring socioeconomically important parasites and developing new interventions

High-throughput molecular and computer technologies have become instrumental for systems biological explorations of pathogens, including parasites. For instance, investigations of the transcriptomes of different developmental stages of parasitic nematodes give insights into gene expression, regulation and function in a parasite, which is a significant step to understanding their biology, as well as interactions with their host(s) and disease. This chapter (1) gives a background on some key parasitic nematodes of socioeconomic importance, (2) describes sequencing and bioinformatic technologies for large-scale studies of the transcriptomes and genomes of these parasites, (3) provides some recent examples of applications and (4) emphasizes the prospects of fundamental biological explorations of parasites using these technologies for the development of new interventions to combat parasitic diseases.

A potential role for the interaction of Wolbachia surface proteins with the Brugia malayi glycolytic enzymes and cytoskeleton in maintenance of endosymbiosis

The human filarial parasite Brugia malayi harbors an endosymbiotic bacterium of the genus Wolbachia. The Wolbachia represent an attractive target for the control of filarial induced disease as elimination of the bacteria affects molting, reproduction and survival of the worms. The molecular basis for the symbiotic relationship between Wolbachia and their filarial hosts has yet to be elucidated. To identify proteins involved in this process, we focused on the Wolbachia surface proteins (WSPs), which are known to be involved in bacteria-host interactions in other bacterial systems. Two WSP-like proteins (wBm0152 and wBm0432) were localized to various host tissues of the B. malayi female adult worms and are present in the excretory/secretory products of the worms. We provide evidence that both of these proteins bind specifically to B. malayi crude protein extracts and to individual filarial proteins to create functional complexes. The wBm0432 interacts with several key enzymes involved in the host glycolytic pathway, including aldolase and enolase. The wBm0152 interacts with the host cytoskeletal proteins actin and tubulin. We also show these interactions in vitro and have verified that wBm0432 and B. malayi aldolase, as well as wBm0152 and B. malayi actin, co-localize to the vacuole surrounding Wolbachia. We propose that both WSP protein complexes interact with each other via the aldolase-actin link and/or via the possible interaction between the host's enolase and the cytoskeleton, and play a role in Wolbachia distribution during worm growth and embryogenesis.

In vitro gene silencing of independent phosphoglycerate mutase (iPGM) in the filarial parasite Brugia malayi

BACKGROUND

The phosphoglycerate mutase (PGM) enzyme catalyzes the interconversion of 2- and 3-phosphoglycerate in the glycolytic /gluconeogenic pathways that are present in the majority of cellular organisms. They can be classified as cofactor-dependent PGM (dPGM) or cofactor-independent PGM (iPGM). Vertebrates, yeasts, and many bacteria have only dPGM, while higher plants, nematodes, archaea, and many other bacteria have only iPGM. A small number of bacteria, including Escherichia coli and certain archaea and protozoa, contain both forms. The silencing of ipgm in Caenorhabditis elegans (C. elegans) has demonstrated the importance of this enzyme in parasite viability and, therefore, its potential as an anthelmintic drug target. In this study, the role of the Brugia malayi (B. malayi) ipgm in parasite viability, microfilaria release, embryogenesis, and in vivo development of infective larvae post-gene silencing was explored by applying ribonucleic acid (RNA) interference studies.

RESULTS

The in vitro ipgm gene silencing by small interfering RNA (siRNA) leads to severe phenotypic deformities in the intrauterine developmental stages of female worms with a drastic reduction (~90%) in the motility of adult parasites and a significantly reduced (80%) release of microfilariae (mf) by female worms in vitro. Almost half of the in vitro-treated infective L3 displayed sluggish movement. The in vivo survival and development of siRNA-treated infective larvae (L3) was investigated in the peritoneal cavity of jirds where a ~45% reduction in adult worm establishment was observed.

CONCLUSION

The findings clearly suggest that iPGM is essential for both larval and adult stages of B. malayi parasite and that it plays a pivotal role in female worm embryogenesis. The results thus validate the Bm-iPGM as a putative anti-filarial drug target.

Genomics and emerging drug discovery technologies

As in other areas of the biological sciences, a decisive move into the genomics era is underway for the study of helminths. As sequencing technologies improve and costs continue to drop, genomes are becoming available and will eventually be taken for granted by parasitologists and drug discovery researchers as being core knowledge for any organism of interest. Within a decade, it is plausible to envision the availability of draft or complete genomes from 100 nematode and platyhelminth species. Helminth genome sequencing offers substantial challenges to assembly and annotation because of size, repeat and gene structure and sequence polymorphisms. Research programmes in parasitology will need to adapt to effectively make use of these genomes and to identify the most promising opportunities for the application of genomic information to the development of control strategies, including anthelmintics. Immediate avenues for the use of genomes include superior description of gene and protein expression and function through microarrays, proteomics and RNA interference.

Comparative tissue pharmacokinetics and efficacy of moxidectin, abamectin and ivermectin in lambs infected with resistant nematodes: Impact of drug treatments on parasite P-glycoprotein expression

The high level of resistance to the macrocyclic lactones has encouraged the search for strategies to optimize their potential as antiparasitic agents. There is a need for pharmaco-parasitological studies addressing the kinetic-dynamic differences between various macrocyclic lactones under standardized in vivo conditions. The current work evaluated the relationship among systemic drug exposure, target tissue availabilities and the pattern of drug accumulation within resistant Haemonchus contortus for moxidectin, abamectin and ivermectin. Drug concentrations in plasma, target tissues and parasites were measured by high performance liquid chromatography. Additionally, the efficacy of the three molecules was evaluated in lambs infected with resistant nematodes by classical parasitological methods. Furthermore, the comparative determination of the level of expression of P-glycoprotein (P-gp2) in H. contortus recovered from lambs treated with each drug was performed by real time PCR. A longer persistence of moxidectin (P < 0.05) concentrations in plasma was observed. The concentrations of the three compounds in the mucosal tissue and digestive contents were significant higher than those measured in plasma. Drug concentrations were in a range between 452 ng/g (0.5 day post-treatment) and 32 ng/g (2 days post-treatment) in the gastrointestinal (GI) contents (abomasal and intestinal). Concentrations of the three compounds in H. contortus were in a similar range to those observed in the abomasal contents (positive correlation P = 0.0002). Lower moxidectin concentrations were recovered within adult H. contortus compared to abamectin and ivermectin at day 2 post-treatment. However, the efficacy against H. contortus was 20.1% (ivermectin), 39.7% (abamectin) and 89.6% (moxidectin). Only the ivermectin treatment induced an enhancement on the expression of P-gp2 in the recovered adult H. contortus, reaching higher values at 12 and 24 h post-administration compared to control (untreated) worms. This comparative pharmacological evaluation of three of the most used macrocyclic lactones compounds provides new insights into the action of these drugs.

RNAi-mediated silencing of paramyosin expression in Trichinella spiralis results in impaired viability of the parasite

BACKGROUND

Trichinella spiralis expresses paramyosin (Ts-PMY) not only as a structural protein but also as an immunomodulatory protein to protect the worm from being attacked by host complement components. In this study, the functions of PMY in the viability and the growth development of T. spiralis were confirmed at the first time by silencing the gene function with RNA interference technique.

METHODS AND FINDINGS

To understand its functions in the viability of the worm, we used RNA interference to silence the expression of Ts-pmy mRNA and protein in the parasite. Significant silencing of Ts-pmy mRNA expression in larval and adult T. spiralis was achieved by siRNA and dsRNA through soaking and electroporation. Electroporation of T. spiralis larvae with 8 µM siRNA1743 or 100 ng/µl dsRNA-PF3 resulted in 66.3% and 60.4% decrease in Ts-pmy transcript and 52.0% and 64.7% decrease in Ts-PMY protein expression, respectively, compared with larvae treated with irrelevant control siRNA or dsRNA. Larvae treated with siRNA1743 displayed significant reduction in molting (40.8%) and serious surface damage as detected with SYTOX fluorescent staining. Infection of mice with larvae electroporated with Ts-pmy siRNA1743 resulted in 37.6% decrease in adult worm burden and 23.2% decrease in muscle larvae burden compared with mice infected with control siRNA-treated larvae. In addition, adult worms recovered from mice infected with siRNA-treated larvae released 24.8% less newborn larvae.

CONCLUSION

It is the first time RNAi was used on T. spiralis to demonstrate that silencing PMY expression in T. spiralis significantly reduces the parasite's viability and infectivity, further confirming that Ts-PMY plays an important role in the survival of T. spiralis and therefore is a promising target for vaccine development.

The soluble proteome phenotypes of ivermectin resistant and ivermectin susceptible Haemonchus contortus females compared

Anthelmintics in the absence of vaccines have underpinned a parasite control strategy for over 50 years. However, the continued development of anthelmintic resistance (AR) threatens this control. Measuring early AR is difficult as there many routes that resistance can arise from within multi-nematode populations operating complex metabolism capabilities coupled to different drug management pressures. There is an urgent need to identify and measure early resistance in the field situation. Proteomic profiling of expressed soluble proteins offers a new approach to reveal a drug resistant phenotype within a complex protein pattern. The hypothesis under test was that established differences in drug response phenotypes between nematode isolates can also be measured in their comparative proteomes. As a case study, proteomic differences were measured between an ivermectin resistant and susceptible adult female Haemonchus contortus. Adult H. contortus females were extracted from the abomasa of six lambs. The nematodes had been maintained in the lambs as monospecific isolates of either ivermectin susceptible or ivermectin resistant worms. Comparative analysis of the soluble proteome was completed along with immuno-proteomic analysis using pooled infection sera from the lambs. Following image analysis, spots of interest were excised and analysed by peptide mass fingerprinting and the proteins putatively identified using BLAST. Overall, a relative increase in the expression of proteins involved in the detoxification metabolic area was observed in the resistant isolate. In addition, Western blotting analysis also revealed differences in immuno-reactivity profiles between resistant and susceptible isolates. It can be concluded from this study that proteomic differences can be detectable between ivermectin susceptible and a resistant isolates of H. contortus, which could be further explored using other isolates to confirm if proteomic based fingerprinting offers molecular phenotyping or a new panel of resistance biomarkers.

Functional genomics approaches in parasitic helminths

As research on parasitic helminths is moving into the post-genomic era, an enormous effort is directed towards deciphering gene function and to achieve gene annotation. The sequences that are available in public databases undoubtedly hold information that can be utilized for new interventions and control but the exploitation of these resources has until recently remained difficult. Only now, with the emergence of methods to genetically manipulate and transform parasitic worms will it be possible to gain a comprehensive understanding of the molecular mechanisms involved in nutrition, metabolism, developmental switches/maturation and interaction with the host immune system. This review focuses on functional genomics approaches in parasitic helminths that are currently used, to highlight potential applications of these technologies in the areas of cell biology, systems biology and immunobiology of parasitic helminths.

The development of RNA interference (RNAi) in gastrointestinal nematodes

Despite the utility of RNAi for defining gene function in Caenorhabditis elegans and early successes reported in parasitic nematodes, RNAi has proven to be stubbornly inconsistent or ineffective in the animal parasitic nematodes examined to date. Here, we summarise some of our experiences with RNAi in parasitic nematodes affecting animals and discuss the available data in the context of our own unpublished work, taking account of mode of delivery, larval activation, site of gene transcription and the presence/absence of essential RNAi pathway genes as defined by comparisons to C. elegans. We discuss future directions briefly including the evaluation of nanoparticles as a means to enhance delivery of interfering RNA to the target worm tissue.

Silencing of essential genes by RNA interference in Haemonchus contortus

In this study we assessed three technologies for silencing gene expression by RNA interference (RNAi) in the sheep parasitic nematode Haemonchus contortus. We chose as targets five genes that are essential in Caenorhabditis elegans (mitr-1, pat-12, vha-19, glf-1 and noah-1), orthologues of which are present and expressed in H. contortus, plus four genes previously tested by RNAi in H. contortus (ubiquitin, tubulin, paramyosin, tropomyosin). To introduce double-stranded RNA (dsRNA) into the nematodes we tested (1) feeding free-living stages of H. contortus with Escherichia coli that express dsRNA targetting the test genes; (2) electroporation of dsRNA into H. contortus eggs or larvae; and (3) soaking adult H. contortus in dsRNA. For each gene tested we observed reduced levels of mRNA in the treated nematodes, except for some electroporation conditions. We did not observe any phenotypic changes in the worms in the electroporation or dsRNA soaking experiments. The feeding method, however, elicited observable changes in the development and viability of larvae for five of the eight genes tested, including the ‘essential’ genes, Hc-pat-12, Hc-vha-19 and Hc-glf-1. We recommend the E. coli feeding method for RNAi in H. contortus and provide recommendations for future research directions for RNAi in this species.

Considering RNAi experimental design in parasitic helminths

Almost a decade has passed since the first report of RNA interference (RNAi) in a parasitic helminth. Whilst much progress has been made with RNAi informing gene function studies in disparate nematode and flatworm parasites, substantial and seemingly prohibitive difficulties have been encountered in some species, hindering progress. An appraisal of current practices, trends and ideals of RNAi experimental design in parasitic helminths is both timely and necessary for a number of reasons: firstly, the increasing availability of parasitic helminth genome/transcriptome resources means there is a growing need for gene function tools such as RNAi; secondly, fundamental differences and unique challenges exist for parasite species which do not apply to model organisms; thirdly, the inherent variation in experimental design, and reported difficulties with reproducibility undermine confidence. Ideally, RNAi studies of gene function should adopt standardised experimental design to aid reproducibility, interpretation and comparative analyses. Although the huge variations in parasite biology and experimental endpoints make RNAi experimental design standardization difficult or impractical, we must strive to validate RNAi experimentation in helminth parasites. To aid this process we identify multiple approaches to RNAi experimental validation and highlight those which we deem to be critical for gene function studies in helminth parasites.

Gene silencing in parasites: current status and future prospects

Parasitic diseases cause important losses in public and veterinary health worldwide. Novel drugs, more reliable diagnostic techniques and vaccine candidates are urgently needed. Due to the complexity of parasites and the intricate relationship with their hosts, development of successful tools to fight parasites has been very limited to date. The growing information on individual parasite genomes is now allowing the use of a broader range of potential strategies to gain deeper insights into the host-parasite relationship and has increased the possibilities to develop molecular-based tools in the field of parasitology. Nevertheless, functional studies of respective genes are still scarce. The RNA interference phenomenon resulting in the regulation of protein expression through the specific degradation of defined mRNAs, and more specifically the possibility of artificially induce it, has shown to be a powerful tool for the investigation of proteins function in many organisms. Recent advances in the design and delivery of targeting molecules allow efficient and highly specific gene silencing in different types of parasites, pointing out this technology as a powerful tool for the identification of novel vaccine candidates or drug targets at the high-throughput level in the near future, and could enable researchers to functionally annotate parasite genomes. The aim of this review is to provide a comprehensive overview on the current advances and pitfalls in gene silencing mechanisms, techniques, applications and prospects in animal parasites.

RNAi mediated silencing of ATPase RNA helicase gene in adult filarial parasite Brugia malayi impairs in vitro microfilaria release and adult parasite viability

The DExD/H box families of RNA helicases are a multifunctional group of proteins involved in unwinding of inter- and intra-molecular base-paired regions. Successful knockdown of DEAD box RNA helicase gene (BmL3-Helicase) of human lymphatic filarial parasite Brugia malayi was done with specifically designed and chemically synthesized siRNA of <20bp to observe the role of enzyme in parasite biology and its worth as an antifilarial drug target. We made efforts to deliver siRNA into parasite by both electroporation and soaking that resulted into diminished helicase gene expression associated with decreased parasite motility, viability (97%) and release of microfilariae (81.0% reduction) from adult females in vitro. The specific gene knockdown also resulted into death of adult male worms in addition to phenotypic deformities in female worm intrauterine stages. RT-PCR of siRNA treated worms revealed a complete knockdown of BmL3-Helicase transcription within 16h. The present findings thus illustrate that targeting helicase gene of B. malayi would not only interfere with embryogenesis and microfilarial production but also result into decreased motility and viability of microfilariae and adult parasites. The B. malayi helicase enzyme thus represents a possible antifilarial drug target.

An eye on RNAi in nematode parasites

RNA interference (RNAi) has revolutionised approaches to gene function determination. From a parasitology perspective, gene function studies have the added dimension of providing validation data, increasingly deemed essential to the initial phases of drug target selection, pre-screen development. Notionally advantageous to those working on nematode parasites is the fact that Caenorhabditis elegans research spawned RNAi discovery and continues to seed our understanding of its fundamentals. Unfortunately, RNAi data for nematode parasites illustrate variable and inconsistent susceptibilities which undermine confidence and exploitation. Now well-ensconced in an era of nematode parasite genomics, we can begin to unscramble this variation.

Vaccinomics for the major blood feeding helminths of humans

Approximately one billion people are infected with hookworms and/or blood flukes (schistosomes) in developing countries. These two parasites are responsible for more disability adjusted life years lost than most other neglected tropical diseases (NTDs), and together, are second only to malaria. Although anthelmintic drugs are effective and widely available, they do not protect against reinfection, resistant parasites are likely to emerge, and mass drug administration programs are unsustainable. Therefore, there is a pressing need for the development of vaccines against these parasites. In recent years, there have been major advances in our understanding of hookworms and schistosomes at the molecular level through the use of "omics" technologies. The secretomes of these parasites have been characterized using transcriptomics, genomics, proteomics, and newly developed gene manipulation and silencing techniques, and the proteins of interest are now the target of novel antigen discovery approaches, notably immunomics. This research has resulted in the discovery, development, and early stage clinical trials of subunit vaccines against hookworms and schistosomes.

P-glycoproteins of Haemonchus contortus: development of real-time PCR assays for gene expression studies

P-glycoproteins (P-gps) are proteins that function as efflux pumps, removing lipophilic xenobiotic compounds from cells. There is evidence that P-gps play a role in the resistance of parasitic nematodes to anthelmintic drugs such as benzimidazoles and macrocyclic lactones. As anthelmintic resistance becomes more common, it is important to identify candidate resistance genes with the aim of understanding the molecular basis of resistance, and of developing assays to detect these resistance-associated changes. We identified several sequences from the genome of the parasite Haemonchus contortus with convincing homology to the known P-gp coding genes of the model nematode Caenorhabditis elegans. Nine of these sequences were successfully amplified by polymerase chain reaction (PCR) and shown to be most similar to the C. elegans sequences for pgp-1, pgp-2, pgp-3, pgp-4, pgp-9, pgp-10, pgp-11, pgp-12 and pgp-14. These partial P-gp sequences from H. contortus were used to design and optimize a quantitative real-time PCR assay to investigate potential changes in the expression levels of P-gp transcripts associated with drug resistance. No significant changes in P-gp mRNA expression levels were found in a rapidly selected ivermectin-resistant parasite isolate compared to its drug-sensitive parent, but the assay has the potential to be used on other isolates in the future to further investigate resistance-associated changes in P-gp gene expression.