RNA interference in parasitic nematodes – from genome to control

Abstract

Teladorsagia circumcincta is a parasitic nematode which is a major cause of ovine parasitic gastroenteritis in temperate climatic regions. The parasite has developed resistance to the major anthelmintic drug classes and this challenges its future control. Vaccination is a potential alternative control method since sheep are able to develop protective immunity against this parasite. Although potential vaccine candidates have been revealed, the increasing gene datasets suggest that vaccinetarget selection may be aided by screening methods such as RNAi. This is a reverse genetic mechanism that causes highly specific gene silencing which was initially described and applied to defining gene function in Caenorhabditis elegans. Nevertheless, its application was more difficult than anticipated in parasitic nematodes because of the inconsistency of the silencing effect. In the unsuccessful cases, did the dsRNA penetrate the parasite and activate the RNAi pathway? Thus far, there are no internal controls that indicate the activation of the pathway. Are the RNAi pathway genes constantly transcribed or are they ‘switched on’ in response to the dsRNA exposure? The initial aim of the study was to determine potential marker genes in the RNAi pathway that could indicate the activation of the pathway in C. elegans. After the exposure to dsRNA from two target genes, the transcript levels of three candidate marker genes (Ce-dcr-1, Ce-ego-1 and Ce-rsd-3) were examined and showed that exposure to dsRNA has no effect on the transcript levels of these genes making them inappropriate markers for the activation of the RNAi pathway. The two target-genes were Ce-cpr-4 and Ce-sod-4 which had been proven to be consistently susceptible and refractory to RNAi, respectively. Another aim of the project was to develop an RNAi platform in T. circumcincta for use as a screening method for potential vaccine candidates. The targets selected for the in vitro RNAi included: five members of the Activation-associated Secreted Proteins (ASPs); a Macrophage migration Inhibitory Factor-like (Tci-mif-1) and a Surface Associated Antigen gene (Tci-saa-1), all of which have been associated with vaccine-induced protective immunity. The selection of the ASPs was based on a bioinformatic and transcriptomic analysis of the ASPs in T. circumcincta. The results showed successful knock-down only for three out of five ASP targets after 1 hour of soaking in gene-specific double stranded RNA (dsRNA) which illustrates the inconsistency and the target specificity of RNAi in T. circumcincta which has been observed in the past with other parasitic nematodes. Inconsistencies were also observed within the successful ASP targets with the results not being reproducible after several successful experiments. Potential reasons for the inconsistencies were examined with the duration of larval storage being a critical factor. Larvae stored for a short or long period of time were susceptible and refractory to RNAi, respectively. Experiments were also conducted to investigate how the ASPs relate to extracellular microvesicles (EMVs). These vesicles are considered to play an important role in the intercellular communication between parasites and their hosts, and thus represent potentially useful vaccine and/or drug targets. Transmission electron microscopy (TEM) confirmed that EMVs are excreted / secreted by the parasite and the proteomic analysis revealed several types of proteins within the vesicles such as: ASPs, Actins, Metallopeptidases, and RAB proteins. A comparative analysis of EMVs, EMV-free ES (Excretory / Secretory) and total ES products showed that approximately 35% of the proteins found in the vesicles could also be identified in EMV-free ES and in total ES products, whilst the remaining 65% were present only in EMVs.