Mitochondrial dynamics and quality control have a central role in the maintenance of cellular integrity. Mitochondrial ubiquitin ligase membrane-associated RING-CH (MARCH5) regulates mitochondrial dynamics. Here, we show that mitochondrial adaptation to stress is driven by MARCH5-dependent quality control on acetylated Mfn1. Under mitochondrial stress conditions, levels of Mfn1 were elevated twofold and depletion of Mfn1 sensitized these cells to apoptotic death. Interestingly, overexpression of Mfn1 also promoted cell death in these cells, indicating that a fine tuning of Mfn1 levels is necessary for cell survival. MARCH5 binds Mfn1 and the MARCH5-dependent Mfn1 ubiquitylation was significantly elevated under mitochondrial stress conditions along with an increase in acetylated Mfn1. The acetylation-deficient K491R mutant of Mfn1 showed weak interaction with MARCH5 as well as reduced ubiquitylation. Neither was observed in the acetylation mimetic K491Q mutant. In addition, MARCH5-knockout mouse embryonic fibroblast and MARCH5H43W-expressing HeLa cells lacking ubiquitin ligase activity experienced rapid cell death upon mitochondrial stress. Taken together, a fine balance of Mfn1 levels is maintained by MARCH5-mediated quality control on acetylated Mfn1, which is crucial for cell survival under mitochondria stress conditions.

No abstract

Infection of hepatitis B virus (HBV) increase the incidence of chronic liver disease and hepatocellular carcinoma (HCC). The hepatitis B viral x (HBx) protein encoded by the HBV genome contributes to the pathogenesis of HCC and thus, negative regulation of HBx is beneficial for the alleviation of the disease pathogenesis. MARCH5 is a mitochondrial E3 ubiquitin ligase and here, we show that high MARCH5 expression levels are correlated with improved survival in HCC patients. MARCH5 interacts with HBx protein mainly accumulated in mitochondria and targets it for degradation. The N-terminal RING domain of MARCH5 was required for the interaction with HBx, and MARCH5H43W lacking E3 ligase activity failed to reduce HBx protein levels. High expression of HBx results in the formation of protein aggregates in semi-denaturing detergent agarose gels and MARCH5 mediates the elimination of protein aggregates through the proteasome pathway. HBx-induced ROS production, mitophagy, and cyclooxygenase-2 gene expression were suppressed in the presence of high MARCH5 expression. These results suggest MARCH5 as a target for alleviating HBV-mediated liver disease.

No abstract

We have previously shown that prolonged mitochondrial elongation triggers cellular senescence. Here, we report that enforced mitochondrial elongation by hFis1 depletion caused a severe defect in cell cycle progression through G2/M phase (~3-fold reduction in mitotic index; p < 0.01). Reintroduction of Myc-hFis1 to these cells induced mitochondrial fragmentation and restored the cell cycle, indicating that morphodynamic changes of mitochondria closely link to the cell cycle. In hFis1-knockdown cells, cell cycle regulators governing the G2/M phase, including cyclin A, cyclin B1, cyclin-dependent kinase1 (Cdk1), polo-like kinase1 (Plk1), aurora kinase A and Mad2, were significantly suppressed (2- to 10-fold). Notably, however, when mitochondrial fragmentation was induced by double knockdown of hFis1 and Opa1, the cells regained their ability to enter mitosis, and cell cycle regulators were rebounded. Reconstitution of the cyclin B1/Cdk1 complex, a major regulator of the G2/M transition, failed to restore mitotic entry in hFis1-depleted cells. In contrast, expression of Plk1, an upstream regulator of the cyclin B1/Cdk1 complex, or FoxM1 (forkhead box M1), a master transcriptional factor for the cell cycle regulators of G2/M phase, restored the cell cycle in these cells. Our findings suggest that mitochondrial fission molecule hFis1 ensures the proper cell division by interplay with the cell cycle machinery.

Alzheimer’s disease (AD) has no cure. Earlier, we showed that partial inhibition of mitochondrial complex I (MCI) with the small molecule CP2 induces an adaptive stress response, activating multiple neuroprotective mechanisms. Chronic treatment reduced inflammation, Aβ and pTau accumulation, improved synaptic and mitochondrial functions, and blocked neurodegeneration in symptomatic APP/PS1 mice, a translational model of AD. Here, using serial block-face scanning electron microscopy (SBFSEM) and three-dimensional (3D) EM reconstructions combined with Western blot analysis and next-generation RNA sequencing, we demonstrate that CP2 treatment also restores mitochondrial morphology and mitochondria-endoplasmic reticulum (ER) communication, reducing ER and unfolded protein response (UPR) stress in the APP/PS1 mouse brain. Using 3D EM volume reconstructions, we show that in the hippocampus of APP/PS1 mice, dendritic mitochondria primarily exist as mitochondria-on-a-string (MOAS). Compared to other morphological phenotypes, MOAS have extensive interaction with the ER membranes, forming multiple mitochondria-ER contact sites (MERCS) known to facilitate abnormal lipid and calcium homeostasis, accumulation of Aβ and pTau, abnormal mitochondrial dynamics, and apoptosis. CP2 treatment reduced MOAS formation, consistent with improved energy homeostasis in the brain, with concomitant reductions in MERCS, ER/UPR stress, and improved lipid homeostasis. These data provide novel information on the MOAS-ER interaction in AD and additional support for the further development of partial MCI inhibitors as a disease-modifying strategy for AD.