Nematicidal activity of two monoterpenoids and SER-2 tyramine receptor of Caenorhabditis elegans

In vitro cultures of two nematodes (Caenorhabditis elegans and Ascaris suum) were established to study the nematicidal activity of three monoterpenoids (thymol, carvacrol and p-cymene). Toxicity of thymol and carvacrol was found for the two nematodes tested. The study was then aimed to address whether nematode tyramine receptor (TyrR) could interact with the two compounds by using HEK293 mammalian cells transfected with a C. elegans TyrR (ser-2) sequence, in hope of developing a high-throughput cell-based platform for future screening of new antihelminthic compounds. SER-2 expression and functionality in the transfected cells was first confirmed by green fluorescent protein tagging, competitive receptor binding, intracellular cyclic AMP, and intracellular calcium [Ca(2+)](i) mobilization assays. Thymol and carvacrol were then tested and demonstrated to interact with TyrR in desensitizing SER-2 for tyramine activation in [Ca(2+)](i) mobilization assay, and in translocating SER-2 from membrane to cytoplasm in receptor internalization assay. Receptor internalization activity of thymol and carvacrol was significantly blocked in cells expressing mutant SER-2 with the S210A/S214A double mutations, thus confirming specificity of the interactions. In summary, the current study showed that the nematicidal activity of thymol and carvacrol might be mediated through TyrR as the two compounds could trigger the signaling cascade downstream from the receptor in cells expressing wild-type but not a mutant SER-2. The TyrR-expressing cell system may prove to be a good screening platform for developing new antihelmintic compounds that may overcome parasite drug resistance, especially when such chemicals are used in combination with commercial drugs.