Genetically Encoded Ca 2+ Sensors

Cytosolic Ca²⁺ plays a crucial role as a second messenger in cell signaling, regulating a wide range of cellular processes. Hence, studying its spatiotemporal dynamics has emerged as a key area of research, and fluorescence imaging has become an indispensable method. The first attempt to visualize Ca²⁺ in living specimens was made in the 1960s by microinjecting the bioluminescent Ca²⁺-binding protein aequorin. However, the true breakthrough in Ca²⁺ imaging is often considered to be the development of Ca²⁺ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and its derivatives such as quin2, Fura-2, and Fluo-3, which allowed us to visualize Ca²⁺ responses in living cells by fluorescence microscopy. While these organic sensors are widely used in biological applications, they have notable drawbacks, including uncontrolled cellular localization and challenges in loading into living animals. The advent of genetically encoded Ca²⁺ indicators (GECIs) has overcome these limitations. In this review, we focus on GECIs, tracing their development and comparing them with organic sensors.