Article

Mitophagy selectively degrades individual damaged mitochondria after photoirradiation

Center for Cell Death, Injury, and Regeneration, Department of Pharmaceutical and Biomedical Sciences, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
Antioxidants & Redox Signaling (Impact Factor: 7.67). 12/2010; 14(10):1919-28. DOI: 10.1089/ars.2010.3768
Source: PubMed

ABSTRACT Damaged and dysfunctional mitochondria are proposed to be removed by autophagy. However, selective degradation of damaged mitochondria by autophagy (mitophagy) has yet to be experimentally verified. In this study, we investigated the cellular fate of individual mitochondria damaged by photoirradiation in hepatocytes isolated from transgenic mice expressing green fluorescent protein fused to microtubule-associated protein 1 light chain 3, a marker of forming and newly formed autophagosomes. Photoirradiation with 488-nm light induced mitochondrial depolarization (release of tetramethylrhodamine methylester [TMRM]) in a dose-dependent fashion. At lower doses of light, mitochondria depolarized transiently with re-polarization within 3 min. With greater light, mitochondrial depolarization became irreversible. Irreversible, but not reversible, photodamage induced autophagosome formation after 32±5 min. Photodamage-induced mitophagy was independent of TMRM, as photodamage also induced mitophagy in the absence of TMRM. Photoirradiation with 543-nm light did not induce mitophagy. As revealed by uptake of LysoTracker Red, mitochondria weakly acidified after photodamage before a much stronger acidification after autophagosome formation. Photodamage-induced mitophagy was not blocked by phosphatidylinositol 3-kinase inhibition with 3-methyladenine (10 mM) or wortmannin (100 nM). In conclusion, individual damaged mitochondria become selectively degraded by mitophagy, but photodamage-induced mitophagic sequestration occurs independently of the phosphatidylinositol 3-kinase signaling pathway, the classical upstream signaling pathway of nutrient deprivation-induced autophagy.

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    • "Mitochondrial specific degradation, termed mitophagy, requires the coordination of cytosolic factors and signals on the outer mitochondrial membrane (OMM). The process of mitophagy is remarkably specific; by uncoupling a single mitochondrion via photo-irradiation, this and only this mitochondrion will be degraded (Kim and Lemasters, 2011). Further to this, damaging the entire mitochondrial pool using an uncoupler (carbonyl cyanide 3-chlorophenylhydrazone) promotes mitochondrial degradation while leaving other organelles intact (Narendra et al., 2008). "
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    • "This notes that the fate of a mutant depends on its rate of degradation as well as growth and proposes that defective mitochondria are degraded less frequently than wild-type. However, this hypothesis has problems with mitochondrial dynamics, as fission and fusion break the required link between genotype and phenotype (Kowald & Kirkwood, 2011), and evidence shows that dysfunctional mitochondria are preferentially degraded (Twig et al., 2008; Kim & Lemasters, 2011), instead of being spared. It is also suggested that the reduced genome size of the deletion mutant confers a selection advantage (Wallace, 1992; Lee et al., 1998). "
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    ABSTRACT: Selective mitochondrial degradation through autophagy (mitophagy) has emerged as an important homeostatic mechanism in a variety of organisms and contexts. Complete clearance of mitochondria can be observed during normal maturation of certain mammalian cell types, and during certain forms of neuronal cell death. In recent years, autophagy dysregulation has been implicated in toxin-injured dopaminergic neurons as well as in major genetic models of Parkinson's disease (PD), including α-synuclein, leucine-rich repeat kinase 2 (LRRK2), parkin, PTEN-induced kinase 1 (PINK1), and DJ-1. Indeed, PINK1-parkin interactions may form the basis of a mechanism by which dissipation of the inner mitochondrial membrane potential can trigger selective mitochondrial targeting for autophagy. Multiple signals are likely to exist, however, depending upon the trigger for mitophagy. Similarly, the regulation of basal or injury-induced autophagy does not always follow canonical pathways described for nutrient deprivation. Implications of this regulatory diversity are discussed in the context of neuronal function and survival. Further studies are needed to address whether alterations in autophagy regulation play a directly injurious role in PD pathogenesis, or if the observed changes reflect impaired, appropriate, or excessive autophagic responses to other forms of cellular injury.
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