A genetically encoded far-red fluorescent indicator for imaging synaptically released Zn 2 .

Synaptic zinc ion (Zn 2+ ) has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn 2+ in the brain of awake animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn 2+ . In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn 2+ dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn 2+ (FRISZ) displayed a substantial Zn 2+ -specific turn-on response and low-micromolar affinity. We genetically anchored FRISZ to the mammalian extracellular membrane via a transmembrane (TM) ⍺ helix and characterized the resultant FRISZ-TM construct at the mammalian cell surface. We used FRISZ-TM to image synaptic Zn 2+ in the auditory cortex in acute brain slices and awake mice in response to electric and sound stimuli, respectively. Thus, this study establishes a technology for studying the roles of synaptic Zn 2+ in the nervous system.