Mitochondrial dynamics quantitatively revealed by STED nanoscopy with an enhanced squaraine variant probe

Mitochondria play a critical role in generating energy to support the entire lifecycle of biological cells, yet it is still unclear how their morphological structures evolve to regulate their functionality. Conventional fluorescence microscopy can only provide ∼300 nm resolution, which is insufficient to visualize mitochondrial cristae. Here, we developed an enhanced squaraine variant dye (MitoESq-635) to study the dynamic structures of mitochondrial cristae in live cells at superresolution. The low saturation intensity and high photostability make it ideal for long-term, high-resolution STED nanoscopy. We demonstrate the time-lapsed imaging of the mitochondrial inner membrane over 50 minutes in living HeLa cells at 35.2 nm resolution for the first time. The forms of the cristae during mitochondrial fusion and fission can be clearly resolved. Our study demonstrates the emerging capability of optical STED nanoscopy to investigate intracellular physiological processes at nanoscale resolution for long periods of time with minimal phototoxicity.