MITOL prevents ER stress-induced apoptosis by IRE1α ubiquitylation at ER-mitochondria contact sites.

Keisuke Takeda,Takashi Uehara,Takao Iwawaki,Shigeru Yanagi,Aoi Uda,Toshifumi Fukuda,Naoki Ito,Satoshi Ishido,Ryoko Inatome,Isshin Shiiba,Shun Nagashima,Takeshi Tokuyama,Nobuko Matsushita

doi:10.15252/embj.2018100999

Abstract

Unresolved endoplasmic reticulum (ER) stress shifts the unfolded protein response signaling from cell survival to cell death, although the switching mechanism remains unclear. Here, we report that mitochondrial ubiquitin ligase (MITOL/MARCH5) inhibits ER stress‐induced apoptosis through ubiquitylation of IRE1α at the mitochondria‐associated ER membrane (MAM). MITOL promotes K63‐linked chain ubiquitination of IRE1α at lysine 481 (K481), thereby preventing hyper‐oligomerization of IRE1α and regulated IRE1α‐dependent decay (RIDD). Therefore, under ER stress, MITOL depletion or the IRE1α mutant (K481R) allows for IRE1α hyper‐oligomerization and enhances RIDD activity, resulting in apoptosis. Similarly, in the spinal cord of MITOL‐deficient mice, ER stress enhances RIDD activity and subsequent apoptosis. Notably, unresolved ER stress attenuates IRE1α ubiquitylation, suggesting that this directs the apoptotic switch of IRE1α signaling. Our findings suggest that mitochondria regulate cell fate under ER stress through IRE1α ubiquitylation by MITOL at the MAM.

Highlights

The endoplasmic reticulum (ER) is an intracellular compartment essential for the maturation of proteins
Since an imbalance in mitochondrial homeostasis has been shown to cause ER stress, we suspected that the morphological abnormalities in the ER of 4-OHT-treated MITOLF/F MEFs could have resulted from mitochondrial fragmentation
Drp1 knockdown by shDrp1#1 and shDrp1#2 rescued the morphological abnormalities in the mitochondria of MITOLF/F MEFs treated with 4-OHT, whereas Drp1 silencing did not affect the morphological abnormalities in the ER of these cells (Fig EV1E–G), suggesting that the role of MITOL in the maintenance of ER homeostasis is independent of its function in mitochondrial dynamics via Drp1 regulation

Summary

Introduction

The endoplasmic reticulum (ER) is an intracellular compartment essential for the maturation of proteins. Various physiological and pathological changes require enhanced ER functions, such as protein folding, removal of unfolded proteins, and reduction of oxidative stress. An imbalance in ER homeostasis causes ER stress and triggers the unfolded protein response (UPR), which is initiated by three ER proteins, PERK, ATF6, and IRE1a. The UPR is involved in the recovery of ER homeostasis through ERAD and ER chaperones (Travers et al, 2000). Under persistent or severe ER stress, an alternative UPR triggers apoptosis (Shore et al, 2011; Tabas & Ron, 2011). UPR-mediated apoptosis is responsible for the pathogenesis of human diseases including diabetes mellitus, heart failure, and neurodegenerative diseases (Yoshida, 2007). Understanding the mechanisms of UPR regulation may contribute to the development of therapies for these diseases

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