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: 7.41). 12/2010; 14(10):1919-28. DOI: 10.1089/ars.2010.3768
Source: PubMed


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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    • "This radiation induced mitochondrial dysfunction and biogenesis has been shown to be associated with mitophagy induction[75]. Photo-irradiation of individual mitochondria from primary hepatocytes causes altered mitochondrial potential, inner membrane permeablization, excessive ROS generation, and mitophagy induction in a dose-dependent manner and phosphatidylinositol 3-kinase-independent manner[75]. Mitophagy shows a strong correlation with metabolic reprogramming in irradiated cancer cells[76]. "
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    • "The study of mitochondrial quality control in neurons has been hampered by the lack of methods to induce levels of mi­ tochondrial damage that are likely to approximate the extent of damage that might normally arise in a neuron. Other stud­ ies had previously used activation of mt­KR (Yang and Yang, 2011; Wang et al., 2012; Ertürk et al., 2014) or irradiation without a photosensitizer (Kim and Lemasters, 2011) to trigger mitophagy in nonneuronal cells. However, such methods were not applied to the sparsely distributed mitochondria in neuronal processes. "
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    • "Another idea, called 'survival of the slowest', notes that the fate of a mutant depends on its rate of degradation as well as replication, and proposes that defective mitochondria which are metabolically less active generate less oxidative damage in their membranes and for this reason are degraded less frequently than wild-type [66]. Although this hypothesis appears hard to reconcile with mitochondrial dynamics, because evidence shows that dysfunctional mitochondria are preferentially degraded [26,67], instead of being spared, compromised MQC might permit it. "
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