Snapshots of mitochondrial fission imaged by cryo-scanning transmission electron tomography.

Mitochondria undergo constant remodeling via fission, fusion, extension and degradation. Fission, in particular, depends on the accumulation of mitochondrial fission factor (MFF) and subsequent recruitment of the dynamin-related protein DRP1 (also known as DNM1L). We used cryo-scanning transmission electron tomography (cryo-STET) to investigate mitochondrial morphologies in MFF mutant (MFF-/-) mouse embryonic fibroblast (MEF) cells in ATP-depleting conditions that normally induce fission. The capability of cryo-STET to image through the cytoplasmic volume to a depth of 1 µm facilitated visualization of intact mitochondria and their surroundings. We imaged changes in mitochondrial morphology and cristae structure, as well as contacts with the endoplasmic reticulum (ER), degradative organelles and the cytoskeleton at stalled fission sites. We found disruption of the outer mitochondrial membrane at contact sites with the ER and degradative organelles at sites of mitophagy. We identified fission sites where the inner mitochondrial membrane is already separated while the outer membrane is still continuous. Although MFF is a general fission factor, these observations demonstrate that mitochondrial fission can proceed to the final stage in its absence. The use of cryo-STET allays concerns about the loss of structures due to sample thinning required for tomography using cryo-transmission electron microscopy.