Mitochondrial respiration relies on electron transport, an essential yet dangerous process in that it leads to the generation of reactive oxygen species (ROS). ROS can be neutralized within the mitochondria through enzymatic activity, yet the mechanism for steady-state removal of oxidized mitochondrial protein complexes and lipids is not well understood. We have previously characterized vesicular profiles budding from the mitochondria that carry selected cargo. At least one population of these mitochondria-derived vesicles (MDVs) targets the peroxisomes; however, the fate of the majority of MDVs was unclear. Here, we demonstrate that MDVs carry selected cargo to the lysosomes. Using a combination of confocal and electron microscopy, we observe MDVs in steady state and demonstrate that they are stimulated as an early response to oxidative stress, the extent of which is determined by the respiratory status of the mitochondria. Delivery to the lysosomes does not require mitochondrial depolarization and is independent of ATG5 and LC3, suggesting that vesicle delivery complements mitophagy. Consistent with this, ultrastructural analysis of MDV formation revealed Tom20-positive structures within the vesicles of multivesicular bodies. These data characterize a novel vesicle transport route between the mitochondria and lysosomes, providing insights into the basic mechanisms of mitochondrial quality control.

Mitochondria and peroxisomes share a number of common biochemical processes, including the beta oxidation of fatty acids and the scavenging of peroxides. Here, we identify a new outer-membrane mitochondria-anchored protein ligase (MAPL) containing a really interesting new gene (RING)-finger domain. Overexpression of MAPL leads to mitochondrial fragmentation, indicating a regulatory function controlling mitochondrial morphology. In addition, confocal- and electron-microscopy studies of MAPL-YFP led to the observation that MAPL is also incorporated within unique, DRP1-independent, 70-100 nm diameter mitochondria-derived vesicles (MDVs). Importantly, vesicles containing MAPL exclude another outer-membrane marker, TOM20, and vesicles containing TOM20 exclude MAPL, indicating that MDVs selectively incorporate their cargo. We further demonstrate that MAPL-containing vesicles fuse with a subset of peroxisomes, marking the first evidence for a direct relationship between these two functionally related organelles. In contrast, a distinct vesicle population labeled with TOM20 does not fuse with peroxisomes, indicating that the incorporation of specific cargo is a primary determinant of MDV fate. These data are the first to identify MAPL, describe and characterize MDVs, and define a new intracellular transport route between mitochondria and peroxisomes.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits distribution, and reproduction in any medium, provided the original author and source are credited. This license does not permit commercial exploitation without specific permission.The modification of proteins by the small ubiquitin-like modifier (SUMO) is known to regulate an increasing array of cellular processes. SUMOylation of the mitochondrial fission GTPase dynamin-related protein 1 (DRP1) stimulates mitochondrial fission, suggesting that SUMOylation has an important function in mitochondrial dynamics. The conjugation of SUMO to its substrates requires a regulatory SUMO E3 ligase; however, so far, none has been functionally associated with the mitochondria. By using biochemical assays, overexpression and RNA interference experiments, we characterized the mitochondrial-anchored protein ligase (MAPL) as the first mitochondrial-anchored SUMO E3 ligase. Furthermore, we show that DRP1 is a substrate for MAPL, providing a direct link between MAPL and the fission machinery. Importantly, the large number of unidentified mitochondrial SUMO targets suggests a global role for SUMOylation in mitochondrial function, placing MAPL as a crucial component in the regulation of multiple conjugation events.

Mitochondria-derived vesicles (MDVs) have been shown to transport cargo from the mitochondria to the peroxisomes. Mitochondria and peroxisomes share common functions in the oxidation of fatty acids and the reduction of damaging peroxides. Their biogenesis is also linked through both the activation of master transcription factors such as PGC-1alpha and the common use of fission machinery, including DRP1, Mff, and hFis1. We have previously shown that MDVs are formed independently of the known mitochondrial fission GTPase Drp1 and are enriched for a mitochondrial small ubiquitin-like modifier (SUMO) E3 ligase called MAPL (mitochondrial-anchored protein ligase). Here, we demonstrate that the retromer complex, a known component of vesicle transport from the endosome to the Golgi apparatus, regulates the transport of MAPL from mitochondria to peroxisomes. An unbiased screen shows that Vps35 and Vps26 are found in complex with MAPL, and confocal imaging reveals Vps35 recruitment to mitochondrial vesicles. Silencing of Vps35 or Vps26A leads to a significant reduction in the delivery of MAPL to peroxisomes, placing the retromer within a novel intracellular trafficking route and providing insight into the formation of MAPL-positive MDVs.

The mechanisms that ensure an equal inheritance of cellular organelles during mitosis are an important area of study in cell biology. For the mitochondria fragment during mitosis, however, the cellular links that signal these changes are largely unknown. We recently identified a SUMO protease, SenP5, that deSUMOylates a number of mitochondrial targets, including the dynamin-related fission GTPase, DRP1. In interphase, SenP5 resides primarily within the nucleoli, in addition to a cytosolic pool. Here we report the relocalization of SenP5 from the nucleoli to the mitochondrial surface at G 2 /M transition prior to nuclear envelope breakdown. The recruitment of SenP5 results in a significant loss in mitochondrial SUMOylation, and a concomitant increase in the labile pool of DRP1 that drives mitochondrial fragmentation. Importantly, silencing of SenP5 leads to an arrest in the cell cycle precisely at the time when the protease is translocated to the mitochondria. These data indicate that transition of SenP5 to the mitochondria plays an important role in mitochondrial fragmentation during mitosis. The altered intracellular localization of SenP5 represents the first example of the mitochondrial recruitment of a SUMO protease and provides new insights into the mechanisms of interorganellar communication during the cell cycle.The regulation of the cell cycle is based upon a number of critical checkpoints that ensure the cell is healthy, the DNA is correctly replicated, there is sufficient metabolic energy, and that the organelles are properly partitioned during mitosis. Each of the cell cycle checkpoints are maintained through precise signaling cascades, whose activities determine whether the cycle proceeds, remains quiescent, or whether the cell may enter into apoptotic death. A complete understanding of all cell cycle checkpoints is critical for the identification of new therapeutic targets for both cancer and for the development of regenerative technologies. Recently, genetic models in Drosophila melanogaster have identified at least two novel retrograde signaling pathways that ensure sufficient metabolic capacity and health at the G 1 /S checkpoint (1, 2). Mutations in a component of electron transport chain complex IV led to a 60% decrease in cellular ATP, thereby activating AMP-activated protein kinase and p53-dependent degradation of cyclin E (1). In a parallel pathway, the increased production of cellular ROS through mutations in a component of complex I led to the activation of the c-Jun NH 2 -terminal kinase (JNK)-FOXO cascade that up-regulates the cyclin E inhibitor Dacapo, causing cell cycle arrest at G 1 /S (2). These two pathways highlight the emerging importance of the mitochondria as an essential component of intracellular signaling cascades and cell cycle regulation.The mitochondria cannot be formed de novo, therefore the proliferation and segregation of these organelles during mitosis may be just as important to cellular survival as their role in signaling the health of the cell during G 1 . Mitochondr...