Inhibition of pentameric ligand‐gated ion channels by lophotoxin, a diterpenoid from coral Lophogorgia chilensis

Abstract

Lophotoxin is a member of a cyclic diterpenoid family of toxins, devoid of nitrogen, that are isolated from the soft coral Lophogorgia. Previous work shows irreversible binding of lophotoxin to nicotinic acetylcholine receptors (nAChRs), utilizing membranes prepared from the electric organs of Torpedo californica; the inhibition is thought to be mediated by an electrophilic addition of the toxin with a hydroxyl of a conserved tyrosine (Tyr 190) of this family of receptors. According to molecular modeling, a carbon located in the epoxide moiety of the structure, may form a covalent bond that will irreversibly inhibit nAChRs. This unique family of toxins appear to bind to the agonist site of the nAChR, since binding can be prevented by reversible agonists, such as epibatidine, or competitive antagonists, such as MLA. To investigate receptor interactions of lophotoxin with neuronal nicotinic acetylcholine receptors (nAChRs) and related pentameric ligand‐gated ion channels, we employed HEK cells transfected with cDNA's encoding one of three receptor subtypes: α7‐nAChR, α4β2‐nAChR and 5HT3AR, along with a fluorescent reporter, and a receptor extracellular domain surrogate, the soluble, acetylcholine binding protein (AChBP).We found that lophotoxin not only inhibited α7‐nAChR, α4β2‐nAChR function and AChBP ligand binding, as expected, but surprisingly also function of the serotonin gated ion channel (5HT3A). Our results suggest that these diterpinoid toxins may function not only as self‐defense agents against predators attacking branching points of the coral exoskeleton, but may also inhibit proliferation and attachment of neighboring species that could be a threat for this coral, since serotonin activation has been related to induction of germinal vesicle breakdown in bivalves and spawning in coral species. The mechanism and molecular determinants of lophotoxin interactions with pentameric ligand‐gated ion channels are still unclear, but may be resolved through X‐ray crystallographic and mass spectrometry studies with the soluble receptor surrogate, AChBP.Support or Funding InformationCONACYT graduate fellowship to GACHThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.