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.
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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)