Inaugural Article: Identification of the nuclear receptor DAF-12 as a therapeutic target in parasitic nematodes

Parasitic nematodes constitute a large family of pathogens that infect hosts ranging from plants and animals to people, causing great economic loss and worldwide health threats (1, 2). One of the most problematic parasites, Strongyloides stercoralis, is estimated to infect 100–200 million people. Primary infections are often asymptomatic and clinically silent in immunocompetent individuals. However, once the immune system is compromised (e.g., by corticosteroid therapy), the parasite establishes autoinfection cycles that result in a frequently fatal disseminated strongyloidiasis (3, 4). Hookworms (Ancylostoma and Necator spp.) are other parasitic nematodes that affect >1 billion people and are the dominant cause for iron-deficient anemia worldwide (2). Oral administration of anthelmintics such as benzimidazoles (microtuble toxins) and ivermectin (a neurotoxin) is currently the preferred treatment for nematode infections (5). However, no reliable options exist for treating the more severe form of disseminated strongyloidiasis (6). Moreover, resistance to the anthelmintics has become widespread in animals and is beginning to occur in humans (2, 7). Therefore, studying the mechanisms that govern nematode life cycles is an attractive approach to identifying new therapeutic targets. Infection of hosts by parasitic nematodes is mediated by infective larvae, which in S. stercoralis and hookworm are the third stage or L3 larvae (iL3) (4, 5). Interestingly, iL3 larvae resemble the dauer larvae of the free-living nematode Caenorhabditis elegans in that they are all nonfeeding, developmentally arrested, dormant filariform larvae with a sealed buccal capsule and thickened body wall cuticle, enabling them to survive environmental challenges (8). Like C. elegans dauer larvae, iL3 recover from their arrested development once they enter a proper environment (i.e., their respective hosts) (9). To date, the molecular mechanisms controlling the recovery of iL3 parasites are poorly understood. In C. elegans, dauer diapause is controlled by endocrine signals in response to environmental cues. In favorable environments, reproductive development requires an insulin/IGF-1 (II-S) and TGFβ signaling network that converges on a downstream steroid hormone–nuclear receptor signaling pathway mediated by the nuclear receptor DAF-12 (10). Previously, we have shown that steroid hormones named dafachronic acids (DAs), which are de novo synthesized from dietary sterols by the cytochrome P450 DAF-9, activate DAF-12 and promote dauer recovery in C. elegans (11). For parasitic nematodes, it has also been reported that the induction by the host of the II-S pathway is essential for iL3 recovery (12, 13). Based on the similarities between free-living and parasitic worms, we hypothesized that the steroid hormone/nuclear receptor signal (i.e., DA/DAF-12) would also be conserved and control the dauer-like iL3 diapause in parasitic nematodes (Fig. 1A). In the present study, we identified the DAF-12 orthologs in several parasitic species and showed that these receptors respond to DAs. Remarkably, administration of DAs caused a significant proportion of worms in the postfree-living generation of S. stercoralis to form supernumerary free-living but unviable stage 4 larvae. There was a corresponding and profound reduction in the population of the pathogenic S. stercoralis iL3, suggesting that these compounds or their congeners might be used as a new class of drugs for treatment of disseminated strongyloidiasis. Finally, we report the X-ray crystal structure of the DAF-12 ligand-binding domain for S. stercoralis. The structural basis for ligand binding to DAF-12, which represents one of the hundreds of nuclear receptors found in nematodes, should provide a powerful tool for drug development. Fig. 1. Conserved nuclear receptor signaling pathway controls dauer/iL3 diapause. (A) Schematic for signaling pathways that control dauer/iL3 diapause. The chemical structures for dafachronic acid ligands for DAF-12 are also shown. (B) Sequence similarities of .