Highly dynamic and sensitive NEMOer calcium indicators for imaging ER calcium signals in excitable cells.

The Endoplasmic/sarcoplasmic reticulum (ER/SR) is central to calcium (Ca(2+)) signaling, yet current genetically encoded Ca(2+) indicators (GECIs) cannot detect elementary Ca(2+) release events from ER/SR, particularly in muscle cells. Here, we report NEMOer, a set of organellar GECIs, to efficiently capture ER Ca(2+) dynamics with increased sensitivity and responsiveness. NEMOer indicators exhibit dynamic ranges an order of magnitude larger than G-CEPIA1er, enabling 2.7-fold more sensitive detection of Ca(2+) transients in both non-excitable and excitable cells. The ratiometric version further allows super-resolution monitoring of local ER Ca(2+) homeostasis and dynamics. Notably, NEMOer-f enabled the inaugural detection of Ca(2+) blinks, elementary Ca(2+) releasing signals from the SR of cardiomyocytes, as well as in vivo spontaneous SR Ca(2+) releases in zebrafish. In summary, the highly dynamic NEMOer sensors expand the repertoire of organellar Ca(2+) sensors that allow real-time monitoring of intricate Ca(2+) dynamics and homeostasis in live cells with high spatiotemporal resolution.