Mutation of the E6-AP Ubiquitin Ligase Reduces Nuclear Inclusion Frequency While Accelerating Polyglutamine-Induced Pathology in SCA1 Mice

Abstract

Mutant ataxin-1, the expanded polyglutamine protein causing spinocerebellar ataxia type 1 (SCA1), aggregates in ubiquitin-positive nuclear inclusions (NI) that alter proteasome distribution in affected SCA1 patient neurons. Here, we observed that ataxin-1 is degraded by the ubiquitin-proteasome pathway. While ataxin-1 [2Q] and mutant ataxin-1 [92Q] are polyubiquitinated equally well in vitro, the mutant form is three times more resistant to degradation. Inhibiting proteasomal degradation promotes ataxin-1 aggregation in transfected cells. And in mice, Purkinje cells that express mutant ataxin-1 but not a ubiquitin-protein ligase have significantly fewer NIs. Nonetheless, the Purkinje cell pathology is markedly worse than that of SCA1 mice. Taken together, NIs are not necessary to induce neurodegeneration, but impaired proteasomal degradation of mutant ataxin-1 may contribute to SCA1 pathogenesis.

Imperfect CAG Repeats Form Diverse Structures in SCA1 Transcripts

“…The SCA1 pathology correlates with the appearance of nuclear aggregates containing the mutated protein (7) but whether the aggregate formation is a symptom or cause of the disease is still a matter of debate (8 -10). The aberrant ataxin-1 either in its soluble form or in the form of aggregates was shown to be toxic to neurons supporting the notion that the poly-Q tract itself may play a critical role in SCA1 pathogenesis (7)(8)(9)(10)(11)(12). It may interfere with neuronal transcription by interacting with specific transcription factors in a poly-Q-dependent manner and induce apoptotic cell death (13).…”

“…2C). Only at the temperature of 70°C and higher the structure formed by the helices H4 and 14 12-1-1-1-14 ⌬ ϭ Ϫ2 29 (CAG) 16 CAUCAGCAU (CAG) 10 16-1-1-1-10 ⌬ ϭ ϩ6 (CAG) 12 CAU (CAG) 16 12-1-16 ⌬ ϭ Ϫ4 30 (CAG) 12 CAUCAGCAU (CAG) 15 12-1-1-1-15 ⌬ ϭ Ϫ3 (CAG) 13 CAUCAGCAU (CAG) 14 13-1-1-1-14 ⌬ ϭ Ϫ1 31 (CAG) 11 CAUCAGCAUCAGCAU (CAG) 15 11-1-1-1-1-1-15 ⌬ ϭ Ϫ4 32 (CAG) 14 CAUCAGCAU (CAG) 15 14-1-1-1-15 ⌬ ϭ Ϫ1 (CAG) 12 CAUCAGCAU (CAG) 17 12- H3 was melted and all internucleotide bonds of that region showed similar reactivity. The structures formed by the repeats containing asymmetrically located interruptions are somewhat different (Fig.…”

“…(CAG) 45 -CAT-CAG-CAT-(CAG) 10 for which their total length (58 repeats) suggested an early onset of the disease at 22 years of age, whereas the actual age at SCA1 onset was 50 years (30). In addition, one of the SCA1 variants analyzed in this study, which belonged to an unaffected carrier, had 43 repeats in the 23-1-1-1-17 configuration (Fig.…”

RNAi: a potential therapy for the dominantly inherited nucleotide repeat diseases

“…30 This would imply that nuclear localization is essential for pathology; however, mutations in E6-AP ubiquitin ligase reduce nuclear inclusion frequency, but accelerate disease, in this transgenic model. 31 These observations indicate that the soluble form of mutant ataxin-1 is pathogenic when the protein localizes to the nucleus but that nuclear aggregate formation is not essential for disease progression. Similar findings have been made in a culture model of CAG-repeat expansion disease.…”

Molecular chaperones as regulators of cell death

“…Although the protein aggregation and formation of inclusion bodies are commonly observed among these diseases, recent reports indicated that intracellular aggregation does not confer the cellular toxicity (Saudou et al, 1998;Cummings et al, 1999;Chun et al, 2002;Watase et al, 2002), rather the protective effect as they sequester the insoluble aggregation of mutant proteins (Arrasate et al, 2004). Finally, Kayed et al (2003) showed that the monomeric or oligomeric fibrous conformation of mutant proteins termed amyloid might induce neuronal toxicity, which is commonly seen among these diseases.…”