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A Genetically Encoded Far-Red Fluorescent Indicator for Imaging Synaptically-Released Zn2+

Synaptic Zn2+ has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn2+ in the brain in live animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn2+. In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn2+ dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn2+ (FRISZ) displayed a substantial Zn2+-specific turn-on response and low micromolar affinity. We genetically anchored FRISZ to the mammalian extracellular membrane via a transmembrane α-helix. We further successfully used membrane-tethered FRISZ (FRISZ-TM) to image synaptic Zn2+ dynamics in response to sound in the primary auditory cortex (A1) in awake mice. This study thus establishes a new technology for studying the roles of synaptic Zn2+ in the nervous system.