Research Article: RNAseq Analysis of the Parasitic Nematode Strongyloides stercoralis Reveals Divergent Regulation of Canonical Dauer Pathways

Date Published: October 25, 2012

Publisher: Public Library of Science

Author(s): Jonathan D. Stoltzfus, Samuel Minot, Matthew Berriman, Thomas J. Nolan, James B. Lok, Aaron R. Jex.

Abstract: The infectious form of many parasitic nematodes, which afflict over one billion people globally, is a developmentally arrested third-stage larva (L3i). The parasitic nematode Strongyloides stercoralis differs from other nematode species that infect humans, in that its life cycle includes both parasitic and free-living forms, which can be leveraged to investigate the mechanisms of L3i arrest and activation. The free-living nematode Caenorhabditis elegans has a similar developmentally arrested larval form, the dauer, whose formation is controlled by four pathways: cyclic GMP (cGMP) signaling, insulin/IGF-1-like signaling (IIS), transforming growth factor β (TGFβ) signaling, and biosynthesis of dafachronic acid (DA) ligands that regulate a nuclear hormone receptor. We hypothesized that homologous pathways are present in S. stercoralis, have similar developmental regulation, and are involved in L3i arrest and activation. To test this, we undertook a deep-sequencing study of the polyadenylated transcriptome, generating over 2.3 billion paired-end reads from seven developmental stages. We constructed developmental expression profiles for S. stercoralis homologs of C. elegans dauer genes identified by BLAST searches of the S. stercoralis genome as well as de novo assembled transcripts. Intriguingly, genes encoding cGMP pathway components were coordinately up-regulated in L3i. In comparison to C. elegans, S. stercoralis has a paucity of genes encoding IIS ligands, several of which have abundance profiles suggesting involvement in L3i development. We also identified seven S. stercoralis genes encoding homologs of the single C. elegans dauer regulatory TGFβ ligand, three of which are only expressed in L3i. Putative DA biosynthetic genes did not appear to be coordinately regulated in L3i development. Our data suggest that while dauer pathway genes are present in S. stercoralis and may play a role in L3i development, there are significant differences between the two species. Understanding the mechanisms governing L3i development may lead to novel treatment and control strategies.

Partial Text: Parasitic nematodes infect over one billion people worldwide, resulting in vast morbidity [1], as well as causing significant agricultural losses from infections of both animals and plants [2]. The infectious form of many parasitic nematodes, including those causing hookworm disease, filariasis, and strongyloidiasis, is a developmentally arrested third-stage larva (L3i), which is both stress-resistant and long-lived [3]–[5]. Upon entering a suitable host, L3i quickly resume development (activation), eventually forming parasitic adults [4], [5]. The genes and proteins constituting the pathways that control the developmental arrest and activation of L3i represent potential targets for chemotherapy as well as environmental control strategies.

In this study, we determined which homologs of C. elegans genes involved in dauer arrest and/or activation (Figure 2) are present in S. stercoralis and whether these S. stercoralis genes are developmentally regulated in a manner consistent with the regulation of their C. elegans counterparts. Our results have provided important insights into which developmental pathways are conserved between the morphologically similar dauer and L3i stages, thereby illuminating potential mechanisms governing L3i development. In our searches of the S. stercoralis and S. ratti draft genomes as well as our de novo assembled S. stercoralis transcript database, we were able to identify S. stercoralis homologs for nearly every C. elegans gene directly involved in the four canonical dauer pathways. While these pathways are well conserved in metazoans, they regulate a wide variety of functions; thus, we were specifically interested in whether they regulate S. stercoralis L3i arrest and/or activation.