Elucidating the developmental biology of Haemonchus contortus and other nematodes using a multi-omics approach

© 2019 Guangxu MaAn appraisal of current literature (Chapter 1) revealed that many parasitic worms are pathogens of animals, causing major diseases and socioeconomic losses worldwide. Efforts to control roundworms (nematodes) are often compromised by widespread resistance to currently used treatments. Thus, there is a clear need to work toward new interventions, preferably based on a deep understanding of the molecular biology of nematodes and/or the relationship that they have with their host animals. The predominant focus of the present thesis was on exploring aspects of the developmental biology of parasitic nematodes using advanced molecular (‘omic) and bioinformatic technologies, with an emphasis on the barber’s pole worm (Haemonchus contortus) - one of the most economically important parasites of ruminant livestock. The specific aims were: (1) to establish molecular data sets (resources) for H. contortus; (2) to explore RNA transcription and protein expression profiles in the developmental transition from free-living to parasitic larvae of H. contortus under well-defined conditions in vitro; (3) to construct dauer-like signalling pathways in H. contortus and some other nematodes (ascaridoids); and (4) to elucidate dauer-like signalling and the involvement of bile acid-like dafachronic acids during nematode development. All of these aims were achieved. Addressing aim 1, comprehensive molecular (transcriptomic, proteomic and lipidomic) resources were established for H. contortus using advanced nucleic acid sequencing or mass spectrometry techniques (Chapters 2-4). Addressing aim 2, these resources were utilised for in-depth explorations of molecular changes during the developmental switch from the free-living to the parasitic stage of H. contortus (Chapter 5). This work revealed extensive alterations in transcription and protein expression. There was a discordance between the mRNA transcription and protein expression changes, which appeared to relate to microRNA regulation at the post-transcriptional level and genes involved in signal transduction and signalling molecule interactions. Comparative studies with C. elegans indicated that molecules and canonical dauer signalling pathways integrate environmental cues and developmental processes in H. contortus. Addressing aim 3, gene homologues involved in the dauer signalling pathways were inferred for H. contortus using genomic, transcriptomic and proteomic data sets (Chapter 6). Transcriptomic and proteomic studies of such homologues indicated similar gene transcription and protein phosphorylation profiles between the infective stage of H. contortus and the dauer stage of Caenorhabditis elegans. Although reduced sets of genes encoding G protein-coupled receptors, insulin-like peptides and cholesterol transporters were identified in H. contortus, similar functional roles of the signalling pathways were proposed for parasitic and free-living nematodes. Addressing aim 4, the “dauer hypothesis” was tested in H. contortus using an integrated ‘omics approach (Chapter 7). The dauer-like signalling pathways were shown to be activated during the developmental transition from the free-living to the parasitic stages of H. contortus, and were linked to an amplification of a 3-keto bile acid-like steroid hormone (i.e., dafachronic acid). This hormone bound to a nuclear receptor DAF-12 in vitro, and exhibited stimulatory effects on the larval activation and development of this parasite. These stimulatory effects appeared to be associated with a modulation of the dauer-like signalling cascades and lipid (glycerolipid and glycerophospholipid) metabolism. Specific chemical inhibition of dafachronic acid biosynthesis disrupted lipid metabolism and compromised larval activation and development, suggesting key roles for the hormone signalling module in the development of H. contortus. This work “opened the door” to exploring homologous signalling pathways in biologically distinct (ascaridoid) nematodes, including Toxocara canis (causing toxocariasis) and Ascaris suum (causing acariasis) (Chapters 8 and 9). In conclusion, this thesis showed that an integrated use of these resources allows detailed explorations of signalling molecules, molecular processes and pathways likely associated with nematode development, adaptation and parasitism, and provides opportunities to identify novel intervention targets (Chapter 10). Although this work was focused mainly on H. contortus and two ascaridoid species, the multi-omics approach established herein could be readily used to explore 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