Mitophagy selectively degrades individual damaged mitochondria after photoirradiation

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.