Mitophagy is triggered by mild oxidative stress in a mitochondrial fission dependent manner.

Mitochondrial dysfunction is linked to apoptosis, aging, cancer, and a number of neurodegenerative and muscular disorders. The interplay between mitophagy and mitochondrial dynamics has been linked to the removal of dysfunctional mitochondria ensuring mitochondrial quality control. An open question is what role mitochondrial fission plays in the removal of mitochondria after mild and transient oxidative stress; conditions reported to result in moderately elevated reactive oxygen species (ROS) levels comparable to physical activity. Here we show that applying such conditions led to fragmentation of mitochondria and induction of mitophagy in mouse and human cells. These conditions increased ROS levels only slightly and neither triggered cell death nor led to a detectable induction of non-selective autophagy. Starvation led to hyperfusion of mitochondria, to high ROS levels, and to the induction of both non-selective autophagy and to a lesser extent to mitophagy. We conclude that moderate levels of ROS specifically trigger mitophagy but are insufficient to trigger non-selective autophagy. Expression of a dominant-negative variant of the fission factor DRP1 blocked mitophagy induction by mild oxidative stress as well as by starvation. Taken together, we demonstrate that in mammalian cells under mild oxidative stress a DRP1-dependent type of mitophagy is triggered while a concomitant induction of non-selective autophagy was not observed. We propose that these mild oxidative conditions resembling well physiological situations are thus very helpful for studying the molecular pathways governing the selective removal of dysfunctional mitochondria.

Mitochondrion-derived reactive oxygen species lead to enhanced amyloid beta formation.

AIMS: Intracellular amyloid beta (Aß) oligomers and extracellular Aß plaques are key players in the progression of sporadic Alzheimer's disease (AD). Still, the molecular signals triggering Aß production are largely unclear. We asked whether mitochondrion-derived reactive oxygen species (ROS) are sufficient to increase Aß generation and thereby initiate a vicious cycle further impairing mitochondrial function. RESULTS: Complex I and III dysfunction was induced in a cell model using the respiratory inhibitors rotenone and antimycin, resulting in mitochondrial dysfunction and enhanced ROS levels. Both treatments lead to elevated levels of Aß. Presence of an antioxidant rescued mitochondrial function and reduced formation of Aß, demonstrating that the observed effects depended on ROS. Conversely, cells overproducing Aß showed impairment of mitochondrial function such as comprised mitochondrial respiration, strongly altered morphology, and reduced intracellular mobility of mitochondria. Again, the capability of these cells to generate Aß was partly reduced by an antioxidant, indicating that Aß formation was also ROS dependent. Moreover, mice with a genetic defect in complex I, or AD mice treated with a complex I inhibitor, showed enhanced Aß levels in vivo. INNOVATION: We show for the first time that mitochondrion-derived ROS are sufficient to trigger Aß production in vitro and in vivo. CONCLUSION: Several lines of evidence show that mitochondrion-derived ROS result in enhanced amyloidogenic amyloid precursor protein processing, and that Aß itself leads to mitochondrial dysfunction and increased ROS levels. We propose that starting from mitochondrial dysfunction a vicious cycle is triggered that contributes to the pathogenesis of sporadic AD.

Mitophagy in yeast is independent of mitochondrial fission and requires the stress response gene WHI2.

Dysfunctional mitochondria show a reduced capacity for fusion and, as mitochondrial fission is maintained, become spatially separated from the intact network. By that mechanism, dysfunctional mitochondria have been proposed to be targeted for selective degradation by mitophagy, thereby providing a quality control system for mitochondria. In yeast, conflicting results concerning the role of mitochondrial dynamics in mitophagy have been reported. Here, we investigate the effects on mitophagy of altering mitochondrial fission and fusion, using biochemical, as well as fluorescence-based, assays. Rapamycin-induced mitophagy was shown to depend upon the autophagy-related proteins Atg11, Atg20 and Atg24, confirming that a selective type of autophagy occurred. Both fragmentation of mitochondria and inhibition of oxidative phosphorylation were not sufficient to trigger mitophagy, and neither deletion of the fission factors Dnm1, Fis1, Mdv1 or Caf4 nor expression of dominant-negative variants of Dnm1 impaired mitophagy. The diminished mitophagy initially observed in a Δfis1 mutant was not due to the absence of Fis1 but rather due to a secondary mutation in WHI2, which encodes a factor reported to function in the general stress response and the Ras-protein kinase A (PKA) signaling pathway. We propose that, in yeast, mitochondrial fission is not a prerequisite for the selective degradation of mitochondria, and that mitophagy is linked to the general stress response and the Ras-PKA signaling pathway.

Nix is a selective autophagy receptor for mitochondrial clearance.

Autophagy is the cellular homeostatic pathway that delivers large cytosolic materials for degradation in the lysosome. Recent evidence indicates that autophagy mediates selective removal of protein aggregates, organelles and microbes in cells. Yet, the specificity in targeting a particular substrate to the autophagy pathway remains poorly understood. Here, we show that the mitochondrial protein Nix is a selective autophagy receptor by binding to LC3/GABARAP proteins, ubiquitin-like modifiers that are required for the growth of autophagosomal membranes. In cultured cells, Nix recruits GABARAP-L1 to damaged mitochondria through its amino-terminal LC3-interacting region. Furthermore, ablation of the Nix:LC3/GABARAP interaction retards mitochondrial clearance in maturing murine reticulocytes. Thus, Nix functions as an autophagy receptor, which mediates mitochondrial clearance after mitochondrial damage and during erythrocyte differentiation.