Ubiquitin-specific protease-14 reduces cellular aggregates and protects against mutant huntingtin-induced cell degeneration: involvement of the proteasome and ER stress-activated kinase IRE1α.

Huntington's disease (HD) is an autosomal inherited neurological disease caused by a CAG-repeat expansion in the first exon of huntingtin gene encoding for the huntingtin protein (Htt). In HD, there is an accumulation of intracellular aggregates of mutant Htt that negatively influence cellular functions. The aggregates contain ubiquitin, and part of the HD pathophysiology could result from an imbalance in cellular ubiquitin levels. Deubiquitinating enzymes are important for replenishing the ubiquitin pool, but less is known about their roles in brain diseases. We show here that overexpression of the ubiquitin-specific protease-14 (Usp14) reduces cellular aggregates in mutant Htt-expressing cells mainly via the ubiquitin proteasome system. We also observed that the serine-threonine kinase IRE1 involved in endoplasmic reticulum (ER) stress responses is activated in mutant Htt-expressing cells in culture as well as in the striatum of mutant Htt transgenic (BACHD) mice. Usp14 interacted with IRE1 in control cells but less in mutant Htt-expressing cells. Overexpression of Usp14 in turn was able to inhibit phosphorylation of IRE1α in mutant Htt-overexpressing cells and to protect against cell degeneration and caspase-3 activation. These results show that ER stress-mediated IRE1 activation is part of mutant Htt toxicity and that this is counteracted by Usp14 expression. Usp14 effectively reduced cellular aggregates and counteracted cell degeneration indicating an important role of this protein in mutant Htt-induced cell toxicity.

GADD34 mediates cytoprotective autophagy in mutant huntingtin expressing cells via the mTOR pathway.

Increased protein aggregation and altered cell signaling accompany many neurodegenerative diseases including Huntington's disease (HD). Cell stress is counterbalanced by signals mediating cell repair but the identity of these are not fully understood. We show here that the mammalian target of rapamycin (mTOR) pathway is inhibited and cytoprotective autophagy is activated in neuronal PC6.3 cells at 24 h after expression of mutant huntingtin proteins. Tuberous sclerosis complex (TSC) 1/2 interacted with growth arrest and DNA damage protein 34 (GADD34), which caused TSC2 dephosphorylation and induction of autophagy in mutant huntingtin expressing cells. However, GADD34 and autophagy decreased at later time points, after 48 h of transfection with the concomitant increase in mTOR activity. Overexpression of GADD34 counteracted these effects and increased cytoprotective autophagy and cell survival. These results show that GADD34 plays an important role in cell protection in mutant huntingtin expressing cells. Modulation of GADD34 and the TSC pathway may prove useful in counteracting cell degeneration accompanying HD and other neurodegenerative diseases.

Downregulation of NF-kappaB signaling by mutant huntingtin proteins induces oxidative stress and cell death.

Accumulation of abnormal proteins and endoplasmic reticulum stress accompany neurodegenerative diseases including Huntington's disease. We show that the expression of mutant huntingtin proteins with extended polyglutamine repeats differentially affected endoplasmic reticulum signaling cascades linked to the inositol-requiring enzyme-1 (IRE1) pathway. Thus, the p38 and c-Jun N-terminal kinase pathways were activated, while the levels of the nuclear factor-kappaB-p65 (NF-kappaB-p65) protein decreased. Downregulation of NF-kappaB signaling was linked to decreased antioxidant levels, increased oxidative stress, and enhanced cell death. Concomitantly, calpain was activated, and treatment with calpain inhibitors restored NF-kappaB-p65 levels and increased cell viability. The calpain regulator, calpastatin, was low in cells expressing mutant huntingtin, and overexpression of calpastatin counteracted the deleterious effects caused by N-terminal mutant huntingtin proteins. These results show that calpastatin and an altered NF-kappaB-p65 signaling are crucial factors involved in oxidative stress and cell death mediated by mutant huntingtin proteins.

NF-kappaB-dependent regulation of brain-derived neurotrophic factor in hippocampal neurons by X-linked inhibitor of apoptosis protein.

X chromosome-linked inhibitor of apoptosis protein (XIAP) is an anti-apoptotic protein enhancing cell survival. Brain-derived neurotrophic factor (BDNF) also promotes neuronal viability but the links between XIAP and BDNF have remained unclear. We show here that the overexpression of XIAP increases BDNF in transgenic mice and cultured rat hippocampal neurons, whereas downregulation of XIAP by silencing RNA decreased BDNF. XIAP also stimulated BDNF signaling, as shown by increased phosphorylation of the TrkB receptor and the downstream molecule, cAMP response element-binding protein. The mechanism involved nuclear factor-kappaB (NF-kappaB) activation and blocking of NF-kappaB signaling inhibited the increased activities of BDNF promoters I and IV by XIAP. In neuronal cultures XIAP also upregulated interleukin (IL)-6, which is an NF-kappaB-responsive gene. The addition of IL-6 elevated whereas incubation with IL-6-blocking antibodies reduced BDNF in the neurons. BDNF itself activated NF-kappaB in the neurons at higher concentrations. The data show that XIAP has trophic effects on hippocampal neurons by increasing BDNF and TrkB activity. The results reveal a cytokine network in the brain involving BDNF, IL-6 and XIAP interconnected via the NF-kappaB system.