Genetically Encoded, Photostable Indicators to Image Dynamic Zn²⁺ Secretion of Pancreatic Islets

As an essential element for living organisms, zinc (Zn²⁺) exerts its biological functions both intracellularly and extracellularly. Previous studies have reported a number of genetically encoded Zn²⁺ indicators (GEZIs), which have been widely used to monitor Zn²⁺ in the cytosol and intracellular organelles. However, it is challenging to localize existing GEZIs to the extracellular space to detect secreted Zn²⁺. Herein, we report two photostable, green fluorescent protein (GFP) based indicators, ZIBG1 and ZIBG2, which respond to Zn²⁺ selectively and have affinities suited for detecting Zn²⁺ secretion from intracellular vesicles. In particular, ZIBG2 can be effectively targeted to the extracellular side of plasma membrane. We applied cell surface-localized ZIBG2 to monitor glucose-induced dynamic Zn²⁺ secretion from mouse insulinoma MIN6 cells and primary mouse and human pancreatic islets. Because Zn²⁺ is co-released with insulin from β-cells, the fluorescence of cell surface-localized ZIBG2 was shown to be a strong indicator for the functional potency of islets. Our work here has thus expanded the use of GEZIs to image dynamic Zn²⁺ secretion in live tissue. Because it is convenient to use genetically encoded indicators for expression over extended periods and for in vivo delivery, we envision future applications of ZIBG2 in development of induced β-cells or islets to advance cell replacement therapies for diabetes and in direct imaging of Zn²⁺ secretion dynamics in vivo.