A Synthetic Fluorescent Mitochondria-Targeted Sensor for Ratiometric Imaging of Calcium in Live Cells.

Ca2+ handling by mitochondria is crucial for cell life and the direct measure of mitochondrial Ca2+ concentration in living cells is of pivotal interest. Genetically-encoded indicators greatly facilitated this task, however they require demanding delivery procedures. On the other hand, existing mitochondria-targeted synthetic Ca2+ indicators are plagued by several drawbacks, for example, non-specific localization, leakage, toxicity. Here we report the synthesis and characterization of a new fluorescent Ca2+ sensor, named mt-fura-2, obtained by coupling two triphenylphosphonium cations to the molecular backbone of the ratiometric Ca2+ indicator fura-2. Mt-fura-2 binds Ca2+ with a dissociation constant of ≈1.5 μm in vitro. When loaded in different cell types as acetoxymethyl ester, the probe shows proper mitochondrial localization and accurately measures matrix [Ca2+ ] variations, proving its superiority over available dyes. We describe the synthesis, characterization and application of mt-fura-2 to cell types where the delivery of genetically-encoded indicators is troublesome.