Ca2+ Imaging of Intracellular Organelles: Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a complex and highly dynamic three-dimensional intracellular membranous system, which acts as a dynamic calcium store in the majority of eukaryotic cells. The special arrangement of intra-ER Ca²⁺ buffers, characterized by low affinity for Ca²⁺ , in combination with SERCA pump activity keeps intraluminal Ca²⁺ ([Ca²⁺ ]_L ) at ∼0.1–0.8 mM (Cell Calcium 38:303–310, 2005), thus creating a steep electrochemical gradient aimed at the cytosol. Activation of ER Ca²⁺ channels results in Ca²⁺ release, which contributes to [Ca²⁺ ]_i elevation, whereas SERCA-dependent Ca²⁺ uptake assists termination of cytosolic Ca²⁺ signals. In addition, the continuous luminal space can act as a travelling route for free Ca²⁺ ions (“Ca²⁺ tunnels”), thus bypassing cytosolic Ca²⁺ buffers and preventing mitochondrial Ca²⁺ uptake or loss of Ca²⁺ over the plasma membrane. Furthermore, changes in [Ca²⁺ ]_L regulate ER-resident chaperones, responsible for postranslational protein processing. Thus, [Ca²⁺ ]_L integrates various signalling events and establishes a link between fast signalling, associated with the ER Ca²⁺ release/uptake, and long-lasting adaptive responses relying primarily on the regulation of protein synthesis. This paper overviews modern techniques for the imaging of [Ca²⁺ ]_L using synthetic fluorescent Ca²⁺ dyes. The methods for ER dye loading, with a particular emphasis on employment of ER targeted esterases (the Targeted-Esterase induced Dye loading, TED) to increase specific accumulation of the probes within the ER lumen are described in detail.