Mitochondrial fusion in human cells is efficient, requires the inner membrane potential, and is mediated by mitofusins.
Mitochondrial fusion remains a largely unknown process despite its observation by live microscopy and the identification of few implicated proteins. Using green and red fluorescent proteins targeted to the mitochondrial matrix, we show that mitochondrial fusion in human cells is efficient and achieves complete mixing of matrix contents within 12 h. This process is maintained in the absence of a functional respiratory chain, despite disruption of microtubules or after significant reduction of cellular ATP levels. In contrast, mitochondrial fusion is completely inhibited by protonophores that dissipate the inner membrane potential. This inhibition, which results in rapid fragmentation of mitochondrial filaments, is reversible: small and punctate mitochondria fuse to reform elongated and interconnected ones upon withdrawal of protonophores. Expression of wild-type or dominant-negative dynamin-related protein 1 showed that fragmentation is due to dynamin-related protein 1-mediated mitochondrial division. On the other hand, expression of mitofusin 1 (Mfn1), one of the human Fzo homologues, increased mitochondrial length and interconnectivity. This process, but not Mfn1 targeting, was dependent on the inner membrane potential, indicating that overexpressed Mfn1 stimulates fusion. These results show that human mitochondria represent a single cellular compartment whose exchanges and interconnectivity are dynamically regulated by the balance between continuous fusion and fission reactions.
Legros
Frédéric
F
Institut National de la Santé et de la Recharche Médicale U523, Institut de Myologie, Groupe Hospitalier Pitié-Salpêtrière, 75651 Paris, France.
Lombès
Anne
A
Frachon
Paule
P
Rojo
Manuel
M
D000255:Adenosine Triphosphate / Q000378:metabolism; D003001:Cloning, Molecular; D003598:Cytoskeletal Proteins / Q000378:metabolism; D004579:Electron Transport; D020558:GTP Phosphohydrolases; D005799:Genes, Dominant; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D008164:Luminescent Proteins / Q000378:metabolism; D008561:Membrane Fusion; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000378:metabolism* / Q000502:physiology*; D026901:Membrane Transport Proteins; D008856:Microscopy, Fluorescence; D008928:Mitochondria / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins / Q000378:metabolism* / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D013997:Time Factors; D014162:Transfection; D014407:Tumor Cells, Cultured; D049411:Utrophin; D000097573:Red Fluorescent Protein
D000255:Adenosine Triphosphate; D003598:Cytoskeletal Proteins; D020558:GTP Phosphohydrolases; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins; D049411:Utrophin; D000097573:Red Fluorescent Protein; C470017:Mfn1 protein, human