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For over a decade RNA interference (RNAi) has been an important molecular tool for functional genomics studies in parasitic flatworms. Despite this, our understanding of RNAi dynamics in many flatworm parasites, such as the temperate liver fluke (Fasciola hepatica), remains rudimentary. The ability to maintain developing juvenile fluke in vitro provides the opportunity to perform functional studies during development of the key pathogenic life stage. Here, we investigate the RNAi competence of developing juvenile liver fluke. Firstly, all life stages examined possess, and express, core candidate RNAi effectors encouraging the hypothesis that all life stages of F. hepatica are RNAi competent. RNAi effector analyses supported growing evidence that parasitic flatworms have evolved a separate clade of RNAi effectors with unknown function. Secondly, we assessed the impact of growth/development during in vitro culture on RNAi in F. hepatica juveniles and found that during the first week post-excystment liver fluke juveniles exhibit quantitatively lower RNAi mediated transcript knockdown when maintained in growth inducing media. This did not appear to occur in older in vitro juveniles, suggesting that rapidly shifting transcript dynamics over the first week following excystment alters RNAi efficacy after a single 24 h exposure to double stranded (ds)RNA. Finally, RNAi efficiency was found to be improved through use of a repeated dsRNA exposure methodology that has facilitated silencing of genes in a range of tissues, thereby increasing the utility of RNAi as a functional genomics tool in F. hepatica., Graphical abstract Image 1, Highlights • All F. hepatica life stages express the RNAi machinery for a functional pathway. • RNAi-mediated knockdown in juvenile F. hepatica is affected by in vitro growth. • Repeated dsRNA exposures ensure longevity of transcript and protein knockdown. • This provides a basis for a robust RNAi platform in F. hepatica juveniles.

The liver fluke

The majority of anthelmintics dysregulate neuromuscular function, a fact most prominent for drugs against nematode parasites. In contrast to the strong knowledge base for nematode neurobiology, resource and tool deficits have prevented similar advances in flatworm parasites since those driven by bioimaging, immunocytochemistry, and neuropeptide biochemistry 20-30 years ago. However, recent developments are encouraging a renaissance in liver fluke neurobiology that can now support flukicide discovery. Emerging data promote neuromuscular signalling components, and especially G protein-coupled receptors (GPCRs), as next-generation targets. Here, we summarise these data and expose some of the new opportunities to accelerate progress towards GPCR-targeted flukicides for Fasciola hepatica.

Expanding 'omics' datasets for parasitic nematodes have accelerated the identification of putative drug targets derived from the nematode nervous system. However, novel drug target validation is hampered by the absence of adequate localisation, functional characterisation, and receptor deorphanisation tools in key nematode pathogens. Reverse genetics techniques have advanced to encompass transgenic, targeted mutagenesis, gene silencing (RNA interference), and genome editing (CRISPR/Cas9) approaches in Caenorhabditis elegans. Unfortunately the translation to nematode pathogens has been slow, such that parasite-focused toolbox development and optimisation is critical. Here we review the discovery, localisation, and functional characterisation toolkit available for parasitic nematode neuropeptide research, and assess the scope and limitations of the tools and techniques for novel nematicide discovery.