A ratiometric ER calcium sensor for quantitative comparisons across cell types and subcellular regions.

The endoplasmic reticulum (ER) is an important regulator of in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER levels that allow easier comparisons across conditions. Here, we developed a ratiometric version of our original ER-GCaMP probe that allows for more quantitative comparisons of the concentration of in the ER across cell types and sub-cellular compartments. Using this approach we show that the resting concentration of ER Ca2+ in primary dissociated neurons is substantially lower than that in measured in embryonic fibroblasts.