title: Variants of mitochondrial autophagy: Types 1 and 2 mitophagy and micromitophagy (Type 3).
abstract: Mitophagy (mitochondrial autophagy), which removes damaged, effete and superfluous mitochondria, has several distinct variants. In Type 1 mitophagy occurring during nutrient deprivation, preautophagic structures (PAS) grow into cup-shaped phagophores that surround and sequester individual mitochondria into mitophagosomes, a process requiring phosphatidylinositol-3-kinase (PI3K) and often occurring in coordination with mitochondrial fission. After sequestration, the outer compartment of the mitophagosome acidifies, followed by mitochondrial depolarization and ultimately hydrolytic digestion in lysosomes. Mitochondrial damage stimulates Type 2 mitophagy. After photodamage to single mitochondria, depolarization occurs followed by decoration and then coalescence of autophagic LC3-containing structures on mitochondrial surfaces. Vesicular acidification then occurs. By contrast to Type 1 mitophagy, PI3K inhibition does not block Type 2 mitophagy. Further, Type 2 mitophagy is not associated with phagophore formation or mitochondrial fission. A third form of self-eating of mitochondria is formation of mitochondria-derived vesicles (MDVs) enriched in oxidized mitochondrial proteins that bud off and transit into multivesicular bodies. Topologically, the internalization of MDV by invagination of the surface of multivesicular bodies followed by vesicle scission into the lumen is a form of microautophagy, or micromitophagy (Type 3 mitophagy). Cell biological distinctions are the basis for these three types of mitophagy. Future studies are needed to better characterize the molecular and biochemical differences between Types 1, 2 and 3 mitophagy.
authors.affiliation: Center for Cell Death, Injury & Regeneration, Department of Drug Discovery & Biomedical Sciences, Medical University of South Carolina, DD504 Drug Discovery Building, 70 President Street, MSC 140, Charleston, SC 29425, United States of America ; Department of Biochemistry & Molecular Biology, Medical University of South Carolina, SC, United States of America ; Institute of Theoretical & Experimental Biophysics, Russian Academy of Sciences, Pushchino, Russian Federation.
authors.forename: John J
authors.initials: JJ
authors.lastname: Lemasters
mesh_terms: D006801:Humans; D053078:Membrane Potential, Mitochondrial; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D063306:Mitophagy / Q000502:physiology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism
chemical_list: D008869:Microtubule-Associated Proteins; D019869:Phosphatidylinositol 3-Kinases; D017382:Reactive Oxygen Species
keywords: 3 MA, 3-methyladenine; Drp1, dynamin-related protein-1; GFP, green fluorescent protein; LC3, microtubule-associated protein-1 light chain-3; LTR, LysoTracker Red; MDV, mitochondria-derived vesicle; MFFR, MitoFluor Far Red; MV633, MitoView 633; Micromitophagy; Mitochondria-derived vesicles; Mitophagy; Nutrient deprivation; PAS, preautophagic structure; PI3K, phosphatidylinositol 3-kinase; Photodamage; Preautophagic structure; TMRM, tetramethyrhodamine methyester; TOM20, transporter of the outer membrane-20; mtDNA, mitochondrial DNA; ΔΨ, membrane potential