The serotonin-gated ion channel (5-HT3R) mediates excitatory neuronal communication in the gut and the brain. It is the target for setrons, a class of competitive antagonists widely used as antiemetics, and is involved in several neurological diseases. Cryo-electron microscopy of the 5-HT3R in complex with serotonin or setrons revealed that the protein has access to a wide conformational landscape. However, assigning known high-resolution structures to actual states contributing to the physiological response remains a challenge. In the present study, we used voltage-clamp fluorometry (VCF) to measure simultaneously, for 5-HT3R expressed at a cell membrane, conformational changes by fluorescence and channel opening by electrophysiology. Four positions identified by mutational screening report motions around and outside the serotonin-binding site through incorporation of cysteine-tethered rhodamine dyes with or without a nearby quenching tryptophan. VCF recordings show that the 5-HT3R has access to four families of conformations endowed with distinct fluorescence signatures: "restinglike" without ligand, "inhibited-like" with setrons, "pre-active-like" with partial agonists and "activelike" (open channel) with partial and strong agonists. Data are remarkably consistent with cryo-EM structures, the fluorescence partners matching respectively Apo, setron-bound, 5-HT bound-closed and 5-HT-bound-open conformations. Data show that strong agonists promote a concerted motion of all fluorescently labelled sensors during activation, while partial agonists, especially when lossof-function mutations are engineered, stabilize both active and pre-active conformations. In conclusion, VCF, though the monitoring of electrophysiologically silent conformational changes, illuminates allosteric mechanisms contributing to signal transduction and their differential regulation by important classes of physiological and clinical effectors.
Significance Statement
High-resolution structures of serotonin-gated receptors (5-HT3AR) have evidenced a wide range of conformations that are challenging to annotate to physiologically relevant states. Voltageclamp fluorometry allows to investigate the activation of 5-HT3AR by simultaneously following molecular motions and electrophysiological states at the plasma membrane. Here, we developed four fluorescent sensors reporting conformational changes at the serotonin binding site and at the extracellular domain and transmembrane domain interface. Investigation of a series of agonists, partial agonists and antagonists show that strong agonists promote a concerted motion of the whole protein during activation, while antagonists and partial agonists stabilize distinct closed-channel conformations. Data offer insights into allosteric mechanisms, unravelling the conformational dynamics of the receptors and helping to annotate high-resolution static structures.
