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

The endoplasmic reticulum (ER) is an important regulator of ${\text{Ca}}^{2+}$ in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER ${\text{Ca}}^{2+}$ homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER ${\text{Ca}}^{2+}$ 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 ${\text{Ca}}^{2+}$ 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.