Discussion

Knowledge of small RNA structure and function has greatly increased in the last decade, from higher organisms to parasitic helminths. Genome and transcriptome data has identified different classes of nematode small RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNA) and PIWI-interacting RNAs (piRNAs). These are involved in negatively regulating expressed genes and/or transposable elements, however their precise functions are currently unknown. Here I will focus on miRNAs, which modulate gene expression by binding with partial sequence complementarity to sites predominantly in the 3’UTR of target mRNAs. Genetic studies in C. elegans first showed that miRNAs were required for correct development. We profiled the expression of miRNAs in all parasitic stages of the highly pathogenic gastrointestinal nematode Haemonchus contortus using microarrays. miRNAs enriched in L3 and L4 larval stages are conserved in nematodes or other organisms, and using miRNA inhibitors, we found that L3-enriched miRNAs act to suppress development and maintain an arrested state. In contrast, many miRNAs abundantly-expressed in adult male and female worms are species specific, suggesting potential roles in adaptation and survival within the host. miRNAs have also been identified in excretory-secretory (ES) products and in extracellular vesicles released from a range of parasitic nematodes, stimulating interest in small RNAs as possible modulators of host immunity. I will discuss recent work that aims to determine the functions of small RNAs within parasitic nematodes and in host-parasite interactions.