Impaired ERAD and ER stress are early and specific events in polyglutamine toxicity

Abstract

Protein misfolding, whether caused by aging, environmental factors, or genetic mutations, is a common basis for neurodegenerative diseases. The misfolding of proteins with abnormally long polyglutamine (polyQ) expansions causes several neurodegenerative disorders, such as Huntington's disease (HD). Although many cellular pathways have been documented to be impaired in HD, the primary triggers of polyQ toxicity remain elusive. We report that yeast cells and neuron-like PC12 cells expressing polyQ-expanded huntingtin (htt) fragments display a surprisingly specific, immediate, and drastic defect in endoplasmic reticulum (ER)-associated degradation (ERAD). We further decipher the mechanistic basis for this defect in ERAD: the entrapment of the essential ERAD proteins Npl4, Ufd1, and p97 by polyQ-expanded htt fragments. In both yeast and mammalian neuron-like cells, overexpression of Npl4 and Ufd1 ameliorates polyQ toxicity. Our results establish that impaired ER protein homeostasis is a broad and highly conserved contributor to polyQ toxicity in yeast, in PC12 cells, and, importantly, in striatal cells expressing full-length polyQ-expanded huntingtin. Polyglutamine (polyQ) expansions in proteins are the basis for at least nine different neurodegenerative disorders, including Huntington's disease (HD) (Orr and Zoghbi 2007). The proteins carrying polyQ expansions are broadly expressed, but each disease is characterized by the vulnerability of a particular subset of neurons. Interactions between sequences flanking the polyQ expansion and the proteome unique to distinct neurons must determine the specific character of each disease. However, in virtually every case, toxicity ensues when the expansion reaches ∼40 residues. Further, the age of disease onset decreases and the severity of disease progression increases as the length of the polyQ expansion increases. Thus, even though each disease is distinct, there must be common underlying toxic mechanisms related to polyQmediated misfolding.To investigate polyQ toxicity, we used a combination of yeast, PC12, and striatal cell models. We and others have developed yeast models that express N-terminal fragments of huntingtin (htt exonI) (Krobitsch and Lindquist 2000;Muchowski et al. 2000;Meriin et al. 2002; Duennwald et al. 2006a,b). Our yeast model recapitulates major features of neuronal polyQ pathology, including the hallmark feature of increasing toxicity with increasing polyQ length (Duennwald et al. 2006b). Thus, the yeast model presents the opportunity to identify factors that specifically determine polyQ toxicity in a genetically tractable model organism.Numerous cellular pathways, such as transcriptional regulation (Riley and Orr 2006), vesicular transport (Gunawardena and Goldstein 2005), and protein turnover (Bence et al. 2001;Bennett et al. 2007) are impaired by polyQ expansion proteins. It remains unclear, however, which of these cellular defects are initial and specific triggers of polyQ toxicity. Here, we focused on how the well-established polyQ-induced defect ...

The Endoplasmic Reticulum (ER)-associated Degradation System Regulates Aggregation and Degradation of Mutant Neuroserpin

“…involved in the degradation of mutant neuroserpin strongly suggests that VCP/p97 cooperates with Hrd1 and gp78 for neuroserpin degradation. The N-terminal fragments of polyQ-expanded huntingtin (Nhtt) and ataxin-3 inhibit the function of ERAD and retrotranslocation (33,34). In our observations, we also found that the polyQ-expanded Nhtt or ataxin-3 significantly stimulates mutant neuroserpin aggregation and perturbs its proteasomal degradation (data not shown).…”

“…In our observations, we also found that the polyQ-expanded Nhtt or ataxin-3 significantly stimulates mutant neuroserpin aggregation and perturbs its proteasomal degradation (data not shown). As the polyQexpanded Nhtt and ataxin-3 affect ERAD by influencing the VCP complex (33,34), and VCP deficiency significantly decreases mutant neuroserpin degradation rates (Fig. 2D), these data further suggest that the ERAD machinery is involved in mutant neuroserpin degradation.…”

Combined chemical–genetic approach identifies cytosolic HSP70 dependence in rhabdomyosarcoma

“…Precedent for such a connection has been established previously. For example, HSP70 inhibition might lead to the formation of cytosolic aggregates, and previous work demonstrated that poly-glutamine aggregates directly interfere with components of the ER-associated degradation (ERAD) pathway, which in turn induces the UPR (42). We also showed previously that perturbing cytosolic HSP70 disables ERAD and triggers the UPR, particularly when membrane-integrated, misfolded ER proteins must be cleared (43).…”

Targeting Axonal Protein Synthesis in Neuroregeneration and Degeneration

“…The role of the UPR in the nervous system in physiological or neurodegenerative conditions remains only partially understood and somewhat controversial (for a discussion see [77]), but recent evidence suggests that ER stress is a common neuronal response to injury or neurodegenerative stimuli and that the UPR is a general mechanism in neurodegeneration. Examples for neurodegenerative diseases for which a UPR contribution has been proposed include Parkinson's disease [78], Huntington's disease [79], and AD [80]. More specifically, a role of axonal ER stress or UPR has been is suggested by the finding of elevated ER stress markers within motorneuron axons in a mouse model for ALS [81], and in response to axotomy [82] or ischemic injury [83].…”