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 the mitochondrial fission factor (MFF) and subsequent recruitment of the dynamin-related protein Drp1. We used cryo-scanning transmission electron tomography (cryo-STET) to investigate mitochondrial morphologies in MFF mutant cells (MFF ^-/- ) 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 provides visualization of mitochondria and their surroundings intact. We imaged changes in mitochondrial morphology and cristae structure and contacts with endoplasmic reticulum, degradative organelles, and cytoskeleton at stalled fission sites. We found disruption of the outer membrane at contact sites with endoplasmic reticulum and degradative organelles at sites of mitophagy. We identified fission sites where the inner mitochondrial membrane is already separated while the outer is still continuous. While 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 cryo-TEM tomography.