Caenorhabditis elegans muscle Cys-loop receptors as novel targets of terpenoids with potential anthelmintic activity.
PLoS Negl Trop Dis 2019;
13:e0007895. [PMID:
31765374 PMCID:
PMC6901230 DOI:
10.1371/journal.pntd.0007895]
[Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2019] [Revised: 12/09/2019] [Accepted: 10/31/2019] [Indexed: 11/27/2022] Open
Abstract
The anthelmintic treatment of nematode infections remains the pillar of worm control in both human and veterinary medicine. Since control is threatened by the appearance of drug resistant nematodes, there is a need to develop novel compounds, among which phytochemicals constitute potential anthelmintic agents. Caenorhabditis elegans has been pivotal in anthelmintic drug discovery and in revealing mechanisms of drug action and resistance. By using C. elegans, we here revealed the anthelmintic actions of three plant terpenoids -thymol, carvacrol and eugenol- at the behavioral level. Terpenoids produce a rapid paralysis of worms with a potency rank order carvacrol > thymol > eugenol. In addition to their paralyzing activity, they also inhibit egg hatching, which would, in turn, lead to a broader anthelmintic spectrum of activity. To identify drug targets, we performed an in vivo screening of selected strains carrying mutations in receptors involved in worm locomotion for determining resistance to the paralyzing effect of terpenoids. The assays revealed that two Cys-loop receptors with key roles in worm locomotion -Levamisole sensitive nicotinic receptor (L-AChR) and GABA(A) (UNC-49) receptor- are involved in the paralyzing effects of terpenoids. To decipher the mechanism by which terpenoids affect these receptors, we performed electrophysiological studies using a primary culture of C. elegans L1 muscle cells. Whole cell recordings from L1 cells demonstrated that terpenoids decrease macroscopic responses of L-AChR and UNC-49 receptor to their endogenous agonists, thus acting as inhibitors. Single-channel recordings from L-AChR revealed that terpenoids decrease the frequency of opening events, probably by acting as negative allosteric modulators. The fact that terpenoids act at different receptors may have important advantages regarding efficacy and development of resistance. Thus, our findings give support to the use of terpenoids as either an alternative or a complementary anthelmintic strategy to overcome the ever-increasing resistance of parasites to classical anthelmintic drugs.
Parasitic nematodes (roundworms) are of major significance as human pathogens and have important economic impact worldwide due to considerable losses in livestock and food crops. Drug treatment of nematode infections (anthelmintic drugs) are the pillar of worm control in human and veterinary medicine. Due to the appearance of drug resistant nematodes, there is a need of developing novel drugs, among which phytochemicals, that have environmental sustainability advantages, may constitute potential anthelmintic compounds. As parasitic nematodes are not ideal laboratory animals, the free-living nematode Caenorhabditis elegans, which shares many physiological characteristics with parasites and is sensitive to anthelmintic drugs, has emerged as a model organism for anthelmintic drug discovery. We found that three terpenoid compounds present in essential oil of plants–thymol, carvacrol and eugenol–produce rapid paralysis of C. elegans and inhibit egg hatching, thus mediating both rapid and long-term anthelmintic effects. By testing mutant worms that lack receptor proteins essential for locomotion we identified two different muscle receptors, nicotinic and GABA receptors, as terpenoid targets of the paralyzing effects. Electrophysiological studies from C. elegans cultured muscle cells demonstrated that terpenoids inhibit the function of these receptors. Thus, by modulating two receptors with key roles in worm motility, these terpenoids emerge as novel anthelmintic compounds.
Collapse