Deciphering molecular mechanisms underlying the unconventional extracellular mitochondria release

2018 Fiscal Year Annual Research Report

• Project/Area Number: 17F17413
• Research Institution: Osaka University
• Principal Investigator: 望月 秀樹 大阪大学, 医学系研究科, 教授 (90230044)
• Co-Investigator(Kenkyū-buntansha): CHOONG CHI JING 大阪大学, 医学(系)研究科(研究院), 外国人特別研究員
• Project Period (FY): 2017-11-10 – 2020-03-31
• Keywords: Mitochondria / Parkin / Rotenone / Mitophagy

Outline of Annual Research Achievements

It is generally accepted that healthy cells degrade their own damaged mitochondria through a process called mitophagy. However, recent studies reported mitochondria can be transferred out of specific cells into the extracellular space. Here we study how mitochondrial toxin-induced stress and presence of mitophagy-related protein parkin affect the extracellular mitochondrial release. Mitochondrial toxin rotenone-induced mitochondrial quality impairment promotes extracellular release of mitochondria. How the extracellular vesicles containing sequestered mitochondria bud directly at the plasma membrane and the size range point to an apparently microvesicle-mediated process. We then addressed the involvement of parkin, a cytosolic E3 ligase commonly mutated in recessive familial parkinsonisms which functions in mitochondrial quality control pathway by translocating to depolarized mitochondria and inducing their elimination by mitophagy, in the event of extracellular mitochondria release. Importantly, the level of parkin-directed mitophagy regulates extracellular mitochondria release. Taken together, overwhelming or perturbation of mitophagy pathway above certain threshold contributes to mitochondria expulsion, an alternative option for removal of damaged organelles from cells, for ultimate mitochondrial quality control.

Current Status of Research Progress

1: Research has progressed more than it was originally planned.

Reason

By performing time-lapse confocal imaging on a stable cell line PC12-pDsRed2mito with fluorescent-labeled mitochondria, we observed release of mitochondria, entrapped in membrane-surrounded vesicles from cells into extracellular space upon mitochondrial toxin rotenone-induced stress. Electron micrograph confirmed the presence of both morphologically intact mitochondria with swollen appearance and typical cristae, surrounded by cytoplasm in membrane vesicles around 500nm in diameter in the rotenone-treated samples. Increased mitochondrial DNA as well as mitochondrial protein translocase of outer membrane 20 (Tom20) in the rotenone-treated samples suggest that rotenone-induced mitochondrial quality impairment promotes extracellular release of mitochondria. Overexpression of parkin, which has a pivotal role in mitophagy regulation, suppresses while its knockdown exacerbates the extracellular mitochondrial release under both basal and stress condition.

Strategy for Future Research Activity

In order to probe clinical relevance, human fibroblasts and CSF samples obtained from control subjects and patients carrying parkin mutation will be used for assessment. Shotgun proteomics analysis will be performed to unravel detailed underlying mechanisms.