ZIGIR, a Granule-Specific Zn Indicator, Reveals Human Islet α Cell Heterogeneity.

Numerous mammalian cells contain abundant Zn in their secretory granules, yet available Zn sensors lack the desired specificity and sensitivity for imaging granular Zn. We developed a fluorescent zinc granule indicator, ZIGIR, that possesses numerous desired properties for live cell imaging, including >100-fold fluorescence enhancement, membrane permeability, and selective enrichment to acidic granules. The combined advantages endow ZIGIR with superior sensitivity and specificity for imaging granular Zn. ZIGIR enables separation of heterogenous β cells based on their insulin content and sorting of mouse islets into pure α cells and β cells. In human islets, ZIGIR facilitates sorting of endocrine cells into highly enriched α cells and β cells, reveals unexpectedly high Zn activity in the somatostatin granule of some δ cells, and uncovers variation in the glucagon content among human α cells. We expect broad applications of ZIGIR for studying Zn biology and Zn-rich secretory granules and for engineering β cells with high insulin content for treating diabetes.