The last decade has been marked by tremendous progress in our understanding of the cell biology of mitochondria, with the identification of molecules and mechanisms that regulate their fusion, fission, motility, and the architectural transitions within the inner membrane. More importantly, the manipulation of these machineries in tissues has provided links between mitochondrial dynamics and physiology. Indeed, just as the proteins required for fusion and fission were identified, they were quickly linked to both rare and common human diseases. This highlighted the critical importance of this emerging field to medicine, with new hopes of finding drugable targets for numerous pathologies, from neurodegenerative diseases to inflammation and cancer. In the midst of these exciting new discoveries, an unexpected new aspect of mitochondrial cell biology has been uncovered; the generation of small vesicular carriers that transport mitochondrial proteins and lipids to other intracellular organelles. These mitochondrial-derived vesicles (MDVs) were first found to transport a mitochondrial outer membrane protein MAPL to a subpopulation of peroxisomes. However, other MDVs did not target peroxisomes and instead fused with the late endosome, or multivesicular body. The Parkinson's disease-associated proteins Vps35, Parkin, and PINK1 are involved in the biogenesis of a subset of these MDVs, linking this novel trafficking pathway to human disease. In this review, we outline what has been learned about the mechanisms and functional importance of MDV transport and speculate on the greater impact of these pathways in cellular physiology.

Antigen presentation is essential for establishing immune tolerance and for immune responses against infectious disease and cancer. Although antigen presentation can be mediated by autophagy, here we demonstrate a pathway for mitochondrial antigen presentation (MitAP) that relies on the generation and trafficking of mitochondrial-derived vesicles (MDVs) rather than on autophagy/mitophagy. We find that PINK1 and Parkin, two mitochondrial proteins linked to Parkinson's disease (PD), actively inhibit MDV formation and MitAP. In absence of PINK1 or Parkin, inflammatory conditions trigger MitAP in immune cells, both in vitro and in vivo. MitAP and the formation of MDVs require Rab9 and Sorting nexin 9, whose recruitment to mitochondria is inhibited by Parkin. The identification of PINK1 and Parkin as suppressors of an immune-response-eliciting pathway provoked by inflammation suggests new insights into PD pathology.

Peroxisomes function together with mitochondria in a number of essential biochemical pathways, from bile acid synthesis to fatty acid oxidation. Peroxisomes grow and divide from pre-existing organelles, but can also emerge de novo in the cell. The physiological regulation of de novo peroxisome biogenesis remains unclear, and it is thought that peroxisomes emerge from the endoplasmic reticulum in both mammalian and yeast cells. However, in contrast to the yeast system, a number of integral peroxisomal membrane proteins are imported into mitochondria in mammalian cells in the absence of peroxisomes, including Pex3, Pex12, Pex13, Pex14, Pex26, PMP34 and ALDP. Overall, the mitochondrial localization of peroxisomal membrane proteins in mammalian cells has largely been considered a mis-targeting artefact in which de novo biogenesis occurs exclusively from endoplasmic reticulum-targeted peroxins. Here, in following the generation of new peroxisomes within human patient fibroblasts lacking peroxisomes, we show that the essential import receptors Pex3 and Pex14 target mitochondria, where they are selectively released into vesicular pre-peroxisomal structures. Maturation of pre-peroxisomes containing Pex3 and Pex14 requires fusion with endoplasmic reticulum-derived vesicles carrying Pex16, thereby providing full import competence. These findings demonstrate the hybrid nature of newly born peroxisomes, expanding their functional links to mitochondria.