Mitochondria are not fixed structures; they continuously divide and fuse. Fusion is mediated by mitofusins (MFN1 and MFN2) on the outer membrane and OPA1 on the inner membrane. Fission is mediated by DRP1, which is recruited to the outer membrane by adaptor proteins including MFF, FIS1, and MID49/51. The two processes do opposite things to the network. Fusion mixes matrix contents, so a mildly damaged mitochondrion can be rescued by complementation with healthy proteins and mtDNA from its neighbors. Fission divides the network into smaller units, which allows a damaged segment to be isolated, its membrane potential to be tested, and if it fails, its removal by mitophagy. The balance between fission and fusion therefore determines whether stress is diluted across the network or concentrated into a removable unit. Chronic stress typically shifts the balance toward fission, producing the fragmented mitochondrial networks seen in many stressed and diseased cells.
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Mitochondrial Stress, Quality Control, and Cell Fate
Fission and fusion as opposing repair strategies
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Watch the network as the balance shifts. When fusion dominates, the mitochondria form long interconnected tubules, and a damaged segment can be rescued because its contents mix with healthy neighbors. When fission dominates, DRP1 constricts the tubules and the network fragments into small units. This is not damage in itself; it is a strategy. Fragmentation isolates a damaged segment so its membrane potential can be tested, and if it fails, the segment is removed by mitophagy while the rest of the network survives. The important point is that fusion dilutes damage and fission concentrates it, and the cell shifts the balance depending on how severe and how sustained the stress is.
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