Date Published: January 4, 2016
Publisher: Public Library of Science
Author(s): Mennatallah M. Y. Albarqi, Jonathan D. Stoltzfus, Adeiye A. Pilgrim, Thomas J. Nolan, Zhu Wang, Steven A. Kliewer, David J. Mangelsdorf, James B. Lok, Timothy G. Geary.
The complex life cycle of the parasitic nematode Strongyloides stercoralis leads to either developmental arrest of infectious third-stage larvae (iL3) or growth to reproductive adults. In the free-living nematode Caenorhabditis elegans, analogous determination between dauer arrest and reproductive growth is governed by dafachronic acids (DAs), a class of steroid hormones that are ligands for the nuclear hormone receptor DAF-12. Biosynthesis of DAs requires the cytochrome P450 (CYP) DAF-9. We tested the hypothesis that DAs also regulate S. stercoralis development via DAF-12 signaling at three points. First, we found that 1 μM Δ7-DA stimulated 100% of post-parasitic first-stage larvae (L1s) to develop to free-living adults instead of iL3 at 37°C, while 69.4±12.0% (SD) of post-parasitic L1s developed to iL3 in controls. Second, we found that 1 μM Δ7-DA prevented post-free-living iL3 arrest and stimulated 85.2±16.9% of larvae to develop to free-living rhabditiform third- and fourth-stages, compared to 0% in the control. This induction required 24–48 hours of Δ7-DA exposure. Third, we found that the CYP inhibitor ketoconazole prevented iL3 feeding in host-like conditions, with only 5.6±2.9% of iL3 feeding in 40 μM ketoconazole, compared to 98.8±0.4% in the positive control. This inhibition was partially rescued by Δ7-DA, with 71.2±16.4% of iL3 feeding in 400 nM Δ7-DA and 35 μM ketoconazole, providing the first evidence of endogenous DA production in S. stercoralis. We then characterized the 26 CYP-encoding genes in S. stercoralis and identified a homolog with sequence and developmental regulation similar to DAF-9. Overall, these data demonstrate that DAF-12 signaling regulates S. stercoralis development, showing that in the post-parasitic generation, loss of DAF-12 signaling favors iL3 arrest, while increased DAF-12 signaling favors reproductive development; that in the post-free-living generation, absence of DAF-12 signaling is crucial for iL3 arrest; and that endogenous DA production regulates iL3 activation.
Strongyloides stercoralis is a parasitic nematode that infects both humans and dogs and is the causative agent of strongyloidiasis, which predominately afflicts socio-economically disadvantaged people in developing countries [1–3]. While chronic strongyloidiasis is often asymptomatic or accompanied by low-grade gastrointestinal symptoms, S. stercoralis infection in immunocompromised or corticosteroid-treated patients can progress to hyperinfection and disseminated strongyloidiasis, which can be fatal [4,5]. Understanding the mechanisms regulating the development of S. stercoralis may lead to improved diagnostic, control, and treatment strategies.
The UPD strain of S. stercoralis, originally isolated from naturally infected dogs in 1976, was maintained and cultured as previously described [64,65].
In this study, we hypothesized that DA signaling in S. stercoralis, through the nuclear hormone receptor DAF-12, would stimulate reproductive growth and development, while decreased DAF-12 activity, resulting from a reduction in DA, would cause developmental arrest. We interrogated several developmental checkpoints in S. stercoralis to determine whether DA signaling regulates the parasite’s developmental program. Our data suggest that DA regulation of DAF-12 signaling plays an important role in the development of iL3 in this pathogen.