Spying on organelle Ca2+ in living cells: the mitochondrial point of view

Over the past years, the use of genetically encoded Ca2+ indicators (GECIs), derived from aequorin and green fluorescent protein, has profoundly transformed the study of Ca2+ homeostasis in living cells leading to novel insights into functional aspects of Ca2+ signalling. Particularly relevant for a deeper understanding of these key aspects of cell pathophysiology has been the possibility of imaging changes in Ca2+ concentration not only in the cytoplasm, but also inside organelles. In this review, we will provide an overview of the ongoing developments in the use of GECIs, with particular focus on mitochondrially targeted probes. Indeed, due to recent advances in organelle Ca2+ imaging with GECIs, mitochondria are now at the centre of renewed interest: they play key roles both in the physiology of the cell and in multiple pathological conditions relevant to human health.