In vivo Ca2+ imaging in the endoplasmic reticulum with GAP sensors

Abstract

Proper functioning of organelles such as the endoplasmic reticulum (ER) or the Golgi apparatus requires luminal accumulation of Ca2+ at high concentrations. Direct intra-organellar measurement of Ca2+ concentrations in intact preparations is essential to address complex physiological questions. Few genetically encoded Ca2+ indicators have been developed for high-Ca2+ organelles, and none of these have been used in vivo in the context of transgenic expression. We have recently developed a new family of fluorescent Ca2+ sensors based on the fusion of two Aequorea victoria proteins, GFP and apoaequorin, named GAP. Here we report the generation of a novel low affinity Ca2+ indicator, optimized for measurements in high Ca2+ concentration environments. Transgenic animals (mice and flies) expressing the ER-targeted sensor enabled monitoring fast physiological Ca2+ responses under minimally disturbing conditions, both ex vivo and in vivo.The applicability of the sensor was demonstrated under three experimental paradigms: i) ER Ca2+ oscillations in cultured astrocytes, ii) ex vivo functional mapping of cholinergic receptors triggering ER Ca2+ release in acute hippocampal slices from transgenic mice, and iii) in vivo sarcoplasmic reticulum Ca2+ dynamics in the muscle of transgenic flies. Our results provide proof of concept for the suitability of the new biosensors to monitor Ca2+ dynamics inside intracellular organelles under physiological conditions and open an avenue to explore complex Ca2+ signalling in animal models of health and disease.