Generation and characterization of a lysosomally targeted, genetically encoded Ca(2+)-sensor

Distinct spatiotemporal Ca(2+) signalling events regulate fundamental aspects of eukaryotic cell physiology. Complex Ca(2+) signals can be driven by release of Ca(2+) from intracellular organelles that sequester Ca(2+) such as the ER (endoplasmic reticulum) or through the opening of Ca(2+)-permeable channels in the plasma membrane and influx of extracellular Ca(2+). Late endocytic pathway compartments including late-endosomes and lysosomes have recently been observed to sequester Ca(2+) to levels comparable with those found within the ER lumen. These organelles harbour ligand-gated Ca(2+)-release channels and evidence indicates that they can operate as Ca(2+)-signalling platforms. Lysosomes sequester Ca(2+) to a greater extent than any other endocytic compartment, and signalling from this organelle has been postulated to provide ‘trigger’ release events that can subsequently elicit more extensive Ca(2+) signals from stores including the ER. In order to investigate lysosomal-specific Ca(2+) signalling a simple method for measuring lysosomal Ca(2+) release is essential. In the present study we describe the generation and characterization of a genetically encoded, lysosomally targeted, cameleon sensor which is capable of registering specific Ca(2+) release in response to extracellular agonists and intracellular second messengers. This probe represents a novel tool that will permit detailed investigations examining the impact of lysosomal Ca(2+) handling on cellular physiology.