Quantitative imaging of mitochondrial redox conditions at the single-organelle level

Mitochondria are morphologically and functionally heterogeneous and dynamically adapt to the current metabolic status of their hosting cell. Moreover, they are prominent sources but also sensitive targets of redox modulation and oxidative stress. Such subcellular ROS/redox signals are considered pivotal aspects in health and disease. Yet, their deciphering requires advanced optical tools. Here we took advantage of transgenic redox-indicator mice expressing a mitochondria-targeted reduction/oxidation-sensitive green fluorescent protein (roGFPm) in excitatory projection neurons. By excitation-ratiometric two-photon microscopy we quantified in acute brain slices the redox conditions of individual mitochondria. After developing adequate redox sensor calibrations and solving laser-mediated bleaching issues, we finally chose caudoputamen, which showed the most promising mitochondrial arrangement for our imaging approach. Confirming the reliability of single-mitochondria redox imaging, we characterized the interplay of redox state and mitochondrial morphology. In general, roGFPm was more oxidized in spherical than in filamentous mitochondria. Acute hypoxia reverted mitochondria to a more roundish shape and evoked a reducing shift. Furthermore, the fraction of spherical mitochondria increased with aging. Around postnatal day (pd)350, a significantly higher fraction of roundish mitochondria was present in females than in males. In addition, from pd150 on, female mice showed lower degrees of roGFPm oxidation than males. Both findings might be linked to estrogen levels, which decrease in female mice with reproductive senescence around pd350. In view of the pivotal role of mitochondria for cellular wellbeing and their involvement in various neuropathologies, the established single-organelle redox-imaging approach will foster further detailed studies.