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

Abstract

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.

Ubiquitin mediated regulation of the E3 ligase GP78 by Mahogunin in trans affects mitochondrial homeostasis

“…By contrast, this balance is pushed more towards fission when mitochondria are depolarized or there is excessive reactive oxygen species (ROS) generation in these organelles. This ultimately culminates in elimination of dysfunctional mitochondria through mitophagy (Twig et al, 2008;Narendra et al, 2008;Wang et al, 2012;Frank et al, 2012).…”

Quality control of mitochondria during aging: Is there a good and a bad side of mitochondrial dynamics?

“…Interestingly, it was shown recently in yeast that a-synuclein induced cytotoxicity and reduction in chronological lifespan depends on mitophagy, indicating that mitophagy could also have detrimental effects [59]. To our knowledge only when applying mild and transient oxidative stress conditions mitophagy is specifically induced in mammalian cells without concomitant induction of bulk autophagy [50].…”

“…These results further strengthen the general view that mitochondrial dynamics is strictly regulated at several levels (for a review see [63]). Hyperfusion of mitochondria was further observed under numerous stress conditions and after starvation [37,38,50,57,64]. One mechanism mediating this dynamic change in mitochondrial morphology is linked to protein kinase A dependent phosphorylation of DRP1, resulting in the inhibition of mitochondrial fission which was shown to reduce the extent of mitophagy [37,38,50].…”

“…Hyperfusion of mitochondria was further observed under numerous stress conditions and after starvation [37,38,50,57,64]. One mechanism mediating this dynamic change in mitochondrial morphology is linked to protein kinase A dependent phosphorylation of DRP1, resulting in the inhibition of mitochondrial fission which was shown to reduce the extent of mitophagy [37,38,50]. It should be noted that mitochondrial hyperfusion after starvation was not only shown in several tumor cell lines but also in primary mouse hepatocytes [37] and thus represents one of the very few examples where primary tissues have been investigated.…”

“…In this way dysfunctional mitochondria become spatially separated from the intact network and, importantly, distinguishable from intact mitochondria. This is needed to prime them for their selective degradation by autophagy (mitophagy) and several reports have confirmed that mitochondrial fission is indeed required for the selective removal of mitochondria in mammals [37,38,[47][48][49][50]. Damaged mitochondria are subsequently marked by stabilization of PINK1 at the outer membrane and by recruitment of PARKIN [17,18] which represent early steps in mitophagy.…”