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

Synaptic zinc ion (Zn) has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn in the brain of awake animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn. In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn (FRISZ) displayed a substantial Zn-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 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 in the nervous system.