Discussion

Neglected Tropical Diseases (NTDs) caused by parasitic nematodes (PNs) impose a global disease burden surpassing both malaria and tuberculosis.  Overreliance on a limited range of anthelmintics heightens the threat of drug resistance and drives the need for novel nematode-NTD control options. Therapeutic exploitation of endocannabinoid (EC) signalling is receiving significant attention in human medicine due to its role in neuromodulatory processes including fertility, motor control and feeding.  Knowledge on nematode EC biology is primarily limited to Caenorhabditis elegans where data demonstrate that EC signalling influences key aspects of neurobiology including homeostasis, ageing and locomotion.  Crucially, mammalian and nematode EC G-protein coupled receptors (EC-GPCRs) appear to differ, providing the potential for selective anthelmintic drug design.  In this study we have exploited nematode genome and transcriptome data to demonstrate that non-mammalian-like EC-GPCRS and EC signalling pathway enzymes display pan-phylum conservation across 96 nematode species (113 genomes), including key PNs of medical and agricultural significance.  ECS pathway protein complement appears to be broadly clade specific; clade 2 nematodes display a reduced complement of EC-GPCRs, clade 12 species predominantly lack the EC-GPCR NPR-19 and degradation enzymes FAAH-1, and -2, and clade 8 nematodes display fewer EC pathway components overall.  In addition, life-stage specific transcriptome analyses for 28 nematode species including Caenorhabditis elegans, Strongyloides stercoralis and Meloidgyne incognita,indicate that EC-GPCRs are expressed in therapeutically relevant life stages of PNs. Interestingly, EC-biosynthesis enzymes and EC-GPCRs are significantly upregulated in the infective larval stages of multiple PN species including Strongyloides stercoralis (Ss).  S. stercoralis targeted mutagenesis studies will reveal the functional importance of EC signalling in nematodes.  Together these data suggest a putative role for EC signalling in PN host seeking and invasion and underscore the appeal of EC signalling as a new, unexploited, avenue for novel nematode-NTD drug target discovery pipelines.