Developmental regulation of Strongyloides stercoralis infectious third-stage larvae by canonical dauer pathways

Parasitic nematodes inflict a vast global disease burden in humans as well as animals and plants of agricultural importance; understanding how these worms infect their hosts has significant health and economic implications. In humans, soil-transmitted parasitic nematodes cause hookworm disease and strongyloidiasis, and vector-transmitted parasitic nematodes cause filariasis. The infectious form of the species causing these diseases is a developmentally arrested third-stage larva (L3i). Molecular mechanisms governing L3i developmental arrest and activation within a host have been poorly understood. An analogous developmentally arrested third-stage larva—the dauer larva—forms during stressful environmental conditions in the free-living nematode Caenorhabditis elegans and is controlled by four cellular signaling pathways. The dauer hypothesis posits that similar mechanisms regulate dauer and L3i development. The parasitic nematode Strongyloides stercoralis was used to test the dauer hypothesis because its life cycle includes both parasitic and free-living forms. To investigate the role of canonical dauer pathway homologs in regulating L3i arrest and activation, this study utilized transcriptome sequencing (RNAseq), transgenesis, and pharmacological studies. Transcripts encoding cyclic guanosine monophosphate (cGMP) pathway components were coordinately up-regulated in L3i. Application of membrane-permeable 8-bromo-cGMP resulted in activation of L3i and modulation of ligand transcripts in other pathways. In comparison to C. elegans, S. stercoralis has few genes encoding insulin/IGF-1 -like signaling (IIS) ligands, several of which have transcripts modulated during L3i development. Application of the phosphatidylinositol-3 kinase inhibitor, LY294002, prevented L3i activation in host-like conditions. The S. stercoralis transcriptome includes seven homologs of the single C. elegans dauer transforming growth factor β (TGFβ) ligand, three of which are only expressed in L3i. Although the C. elegans nuclear hormone receptor ligand delta7-dafachronic acid (DA) stimulates L3i activation, putative DA biosynthetic genes were not coordinately regulated in L3i development. These data demonstrate that S. stercoralis has homologs for nearly every component in the four canonical dauer pathways, that cGMP signaling may transduce host cues during L3i activation, and that IIS regulates L3i arrest and activation. However, dauer TGFβ signaling appears to function in L3i arrest, an opposite role than in C. elegans, and endogenous DA regulation of L3i development remains largely unexplored