Measuring serotonin binding to its receptors via charge transfer to ANAP

Serotonin (5-HT) is a vital intercellular messenger with diverse signaling functions throughout the human body. We have characterized and implemented a novel, fluorescence-based method of measuring 5-HT binding to gain a fuller understanding of the interactions between 5-HT and its receptors. This method involves expression of 5-HT receptor proteins with the fluorescent, non-canonical amino acid L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (ANAP) incorporated into the ligand binding site. ANAP fluorescence was quenched in solution by both 5-HT and dopamine. Time-resolved photoluminescence and transient absorption spectroscopy confirmed that ANAP quenching by 5-HT involved a rapidly reversible charge-transfer process. Based on density functional theory calculations, this process likely involved a ANAP reduction by 5-HT as assessed using whole-cell patch clamp. To test this method on intact receptors in a cellular context, we expressed 5-HT3A receptors (5-HT-gated ion channels) in HEK293T cells with ANAP inserted co-translationally into the transmitter binding site. Fluorescently labeled 5-HT3A receptors were functional and activated by 5-HT. Addition of 5-HT caused a concentration-dependent quenching of fluorescence from ANAP-tagged channels expressed in intact cells and unroofed plasma membranes, demonstrating the utility of this method for measuring 5-HT binding to its receptors. Collectively, these results delineate a technique for measuring transmitter binding that can be widely adopted to measure 5-HT binding not only to 5-HT3 receptors, but to any 5-HT receptor, transporter, or binding protein.