Optical Recording of Zn2+ Dynamics in the Mitochondrial Matrix and Intermembrane Space with the GZnP2 Sensor

The zinc ion (Zn 2+ ) is emerging as an important signaling molecule. Here, we engineered an improved Zn 2+ probe GZnP2 based on a previously developed fluorescent sensor GZnP1 to provide a higher fluorescent readout (2-fold higher) that is proportional to cellular labile Zn 2+ concentrations. We further developed a set of GZnP2 derived imaging tools to determine the labile Zn 2+ concentrations in the mitochondrial matrix, mitochondrial intermembrane space (IMS), and cytosol in four different cell lines (HeLa, Cos-7, HEK293, and INS-1). The labile Zn 2+ concentration in the matrix was less than 1 pM, while the labile Zn 2+ concentration in the IMS was comparable to the cytosol (∼100 pM). With these sensors, we showed that upon exposure to high Zn 2+, only the cytosol and the IMS were overloaded with Zn 2+, while the mitochondrial matrix was unable to sequester excess labile Zn 2+ in depolarized INS-1 cells. This work highlighted the importance of distinguishing the labile Zn 2+ concentrations and dynamics between the mitochondrial matrix and IMS.