IRE1 plays an essential role in ER stress-mediated aggregation of mutant huntingtin via the inhibition of autophagy flux

Human Molecular Genetics, Jan 2012

Huntington's disease (HD), an inherited neurodegenerative disorder, is caused by an expansion of cytosine-adenine-guanine repeats in the huntingtin gene. The aggregation of mutant huntingtin (mtHTT) and striatal cell loss are representative features to cause uncontrolled movement and cognitive defect in HD. However, underlying mechanism of mtHTT aggregation and cell toxicity remains still elusive. Here, to find new genes modulating mtHTT aggregation, we performed cell-based functional screening using the cDNA expression library and isolated IRE1 gene, one of endoplasmic reticulum (ER) stress sensors. Ectopic expression of IRE1 led to its self-activation and accumulated detergent-resistant mtHTT aggregates. Treatment of neuronal cells with ER stress insults, tunicamycin and thapsigargin, increased mtHTT aggregation via IRE1 activation. The kinase activity of IRE1, but not the endoribonuclease activity, was necessary to stimulate mtHTT aggregation and increased death of neuronal cells, including SH-SY5Y and STHdhQ111/111 huntingtin knock-in striatal cells. Interestingly, ER stress impaired autophagy flux via IRE1-TRAF2 pathway, thus enhancing cellular accumulation of mtHTT. Atg5 deficiency in M5-7 cells increased mtHTT aggregation but blocked ER stress-induced mtHTT aggregation. Further, ER stress markers including p-IRE1 and autophagy markers such as p62 were up-regulated exclusively in the striatal tissues of HD mouse models and in HD patients. Moreover, down-regulation of IRE1 expression rescues the rough-eye phenotype by mtHTT in a HD fly model. These results suggest that IRE1 plays an essential role in ER stress-mediated aggregation of mtHTT via the inhibition of autophagy flux and thus neuronal toxicity of mtHTT aggregates in HD.

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IRE1 plays an essential role in ER stress-mediated aggregation of mutant huntingtin via the inhibition of autophagy flux

Huikyong Lee 2 Jee-Yeon Noh 2 Yumin Oh 2 Youngdoo Kim 2 Jae-Woong Chang 2 Chul-Woong Chung 1 Soon-Tae Lee 0 Manho Kim 0 Hoon Ryu 3 Yong-Keun Jung 2 0 Department of Neurology, Seoul National University Hospital , Seoul 110-799, Korea 1 Biopharmaceutical R&D, LG Life Science Inc. , Daejeon 305-380, Korea 2 Global Research Laboratory, School of Biological Science/Bio-MAX Institute, Seoul National University , 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea 3 Department of Neurology and Pathology, Boston University School of Medicine , Boston, MA 02130, USA Huntington's disease (HD), an inherited neurodegenerative disorder, is caused by an expansion of cytosineadenine-guanine repeats in the huntingtin gene. The aggregation of mutant huntingtin (mtHTT) and striatal cell loss are representative features to cause uncontrolled movement and cognitive defect in HD. However, underlying mechanism of mtHTT aggregation and cell toxicity remains still elusive. Here, to find new genes modulating mtHTT aggregation, we performed cell-based functional screening using the cDNA expression library and isolated IRE1 gene, one of endoplasmic reticulum (ER) stress sensors. Ectopic expression of IRE1 led to its self-activation and accumulated detergent-resistant mtHTT aggregates. Treatment of neuronal cells with ER stress insults, tunicamycin and thapsigargin, increased mtHTT aggregation via IRE1 activation. The kinase activity of IRE1, but not the endoribonuclease activity, was necessary to stimulate mtHTT aggregation and increased death of neuronal cells, including SH-SY5Y and STHdhQ111/111 huntingtin knock-in striatal cells. Interestingly, ER stress impaired autophagy flux via IRE1-TRAF2 pathway, thus enhancing cellular accumulation of mtHTT. Atg5 deficiency in M5-7 cells increased mtHTT aggregation but blocked ER stress-induced mtHTT aggregation. Further, ER stress markers including p-IRE1 and autophagy markers such as p62 were up-regulated exclusively in the striatal tissues of HD mouse models and in HD patients. Moreover, down-regulation of IRE1 expression rescues the rough-eye phenotype by mtHTT in a HD fly model. These results suggest that IRE1 plays an essential role in ER stress-mediated aggregation of mtHTT via the inhibition of autophagy flux and thus neuronal toxicity of mtHTT aggregates in HD. - INTRODUCTION Huntingtons disease (HD) is a late-onset autosomal dominant neurodegenerative disorder characterized by uncontrolled movement, cognitive defect and psychiatric disturbance. The disease manifests at a mean age of 35 years and is fatal after 15 20 years of progressive neurodegeneration. The cause of this disease is an expansion of cytosine-adenineguanine (CAG) repeats in the huntingtin gene, encoding the 350 kDa huntingtin protein and there shows an inverse correlation between expanded CAG length and disease onset (1). The main characteristic of HD pathogenesis is the intraneuronal aggregation of poly-glutamine extended huntingtin (mtHTT) and selective neuronal loss, predominantly in the striatum and other basal ganglia structures (2). The pathogenesis of HD is caused by a combination of both gain and loss of huntingtin function that results in several cellular changes, including transcriptional dysfunction, abnormal vesicular transport, mitochondrial impairment, proteasome inhibition and apoptosis (1,2). The endoplasmic reticulum (ER) is an essential intracellular organelle for protein quality control of synthesizing proteins. Perturbation of ER function due to glucose deprivation, aberrant Ca2+ regulation, viral infection or accumulation of misfolded proteins leads to the unfolded protein response (UPR) to cope with this imbalance of ER homeostasis. When the UPR occurs, ER-resident proteins, such as IRE1, PERK and ATF6, sense it, then activate and transduce their own signals to the nucleus to facilitate protein folding, translational attenuation and ER-associated degradation for cell survival (2). IRE1 is an ER-resident serine/threonine protein kinase. During ER stress, IRE1 dissociates from Bip, is activated via trans-autophosphorylation and then its endoribonuclease activity initiates the unconventional splicing of the mRNA encoding the transcription factor XBP1. XBP1 translocates to the nucleus and up-regulates the expression of a subset of UPR-related genes which regulate protein folding, protein quality control, ER-associated degradation system and ER/ Golgi biogenesis (3). IRE1 also activates NF-kB pathway to induce gene expression encoding mediators of host defense as UPR is sustained (4). However, when the capacity of the quality control system is exceeded by severe or prolonged stress signals, ER undergoes chronic ER stress, which is implicated as one of early pathologic events in HD (5). Under this condition, the prolonged activation of IRE1 induces its interaction with TRAF2 and ASK1 or activates caspase-12, an ER-resident caspase, to lead cell death in neuron cells (5,6). Recent evidence indicates (...truncated)


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Huikyong Lee, Jee-Yeon Noh, Yumin Oh, Youngdoo Kim, Jae-Woong Chang, Chul-Woong Chung, Soon-Tae Lee, Manho Kim, Hoon Ryu, Yong-Keun Jung. IRE1 plays an essential role in ER stress-mediated aggregation of mutant huntingtin via the inhibition of autophagy flux, Human Molecular Genetics, 2012, pp. 101-114, 21/1, DOI: 10.1093/hmg/ddr445