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

The endoplasmic reticulum (ER) is an important regulator of Ca2+ in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER Ca2+ homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER Ca2+ 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 Ca2+ 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.