ABSTRACT
In eukaryotic cells, different organelles interact at membrane contact sites stabilized by tethers. Mitochondrial mitofusin 2 (MFN2) acts as a membrane tether that interacts with an unknown partner on the endoplasmic reticulum (ER). In this work, we identified the MFN2 splice variant ERMIT2 as the ER tethering partner of MFN2. Splicing of MFN2 produced ERMIT2 and ERMIN2, two ER-specific variants. ERMIN2 regulated ER morphology, whereas ERMIT2 localized at the ER-mitochondria interface and interacted with mitochondrial mitofusins to tether ER and mitochondria. This tethering allowed efficient mitochondrial calcium ion uptake and phospholipid transfer. Expression of ERMIT2 ameliorated the ER stress, inflammation, and fibrosis typical of liver-specific Mfn2 knockout mice. Thus, ER-specific MFN2 variants display entirely extramitochondrial MFN2 functions involved in interorganellar tethering and liver metabolic activities.
Subject(s)
Calcium , Endoplasmic Reticulum , GTP Phosphohydrolases , Mitochondria , Mitochondrial Proteins , Animals , Mice , Calcium/metabolism , Endoplasmic Reticulum/metabolism , GTP Phosphohydrolases/genetics , GTP Phosphohydrolases/metabolism , Liver/metabolism , Mitochondria/metabolism , Mitochondrial Membranes/metabolism , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Protein Isoforms , Mice, Knockout , Humans , Mice, Inbred C57BL , HeLa Cells , Alternative Splicing , Endoplasmic Reticulum StressABSTRACT
Mitochondria amplify activation of caspases during apoptosis by releasing cytochrome c and other cofactors. This is accompanied by fragmentation of the organelle and remodeling of the cristae. Here we provide evidence that Optic Atrophy 1 (OPA1), a profusion dynamin-related protein of the inner mitochondrial membrane mutated in dominant optic atrophy, protects from apoptosis by preventing cytochrome c release independently from mitochondrial fusion. OPA1 does not interfere with activation of the mitochondrial "gatekeepers" BAX and BAK, but it controls the shape of mitochondrial cristae, keeping their junctions tight during apoptosis. Tightness of cristae junctions correlates with oligomerization of two forms of OPA1, a soluble, intermembrane space and an integral inner membrane one. The proapoptotic BCL-2 family member BID, which widens cristae junctions, also disrupts OPA1 oligomers. Thus, OPA1 has genetically and molecularly distinct functions in mitochondrial fusion and in cristae remodeling during apoptosis.
Subject(s)
Apoptosis/genetics , GTP Phosphohydrolases/metabolism , Membrane Fusion/physiology , Mitochondria/metabolism , Mitochondrial Membranes/metabolism , Animals , Cell Line , GTP Phosphohydrolases/genetics , Mice , Mice, Knockout , Mitochondria/genetics , Mitochondria/ultrastructure , Mitochondrial Membranes/ultrastructure , Proto-Oncogene Proteins c-bcl-2/metabolism , Signal Transduction/physiology , Tight Junctions/metabolism , Tight Junctions/ultrastructure , bcl-2 Homologous Antagonist-Killer Protein/metabolism , bcl-2-Associated X Protein/metabolismABSTRACT
The regulated equilibrium between mitochondrial fusion and fission is essential to maintain integrity of the organelle. Mechanisms of mitochondrial fusion are largely uncharacterized in mammalian cells. It is unclear whether OPA1, a dynamin-related protein of the inner membrane mutated in autosomal dominant optic atrophy, participates in fusion or fission. OPA1 promoted the formation of a branched network of elongated mitochondria, requiring the integrity of both its GTPase and C-terminal coiled-coil domain. Stable reduction of OPA1 levels by RNA interference resulted in small, fragmented, and scattered mitochondria. Levels of OPA1 did not affect mitochondrial docking, but they correlated with the extent of fusion as measured by polyethylene glycol mitochondrial fusion assays. A genetic analysis proved that OPA1 was unable to tubulate and fuse mitochondria lacking the outer membrane mitofusin 1 but not mitofusin 2. Our data show that OPA1 functionally requires mitofusin 1 to regulate mitochondrial fusion and reveal a specific functional difference between mitofusin 1 and 2.