The evolution of organellar calcium mapping technologies

Intracellular Ca²⁺ fluxes are dynamically controlled by the co-involvement of multiple organellar pools of stored Ca²⁺. Endolysosomes are emerging as physiologically critical, yet underexplored, sources and sinks of intracellular Ca²⁺. Delineating the role of organelles in Ca²⁺ signaling has relied on chemical fluorescent probes and electrophysiological strategies. However, the acidic endolysosomal environment presents unique issues, which preclude the use of traditional chemical reporter strategies to map lumenal Ca²⁺. Here, we broadly address the current state of knowledge about organellar Ca²⁺ pools. We then outline the application of traditional probes, and their sensing paradigms. We then discuss how a new generation of probes overcomes the limitations of traditional Ca²⁺probes, emphasizing their ability to offer critical insights into endolysosomal Ca²⁺, and its feedback with other organellar pools.