Intranuclear Degradation of Polyglutamine Aggregates by the Ubiquitin-Proteasome System

Abstract

Huntington disease and its related autosomal-dominant polyglutamine (pQ) neurodegenerative diseases are characterized by intraneuronal accumulation of protein aggregates. Studies on protein aggregates have revealed the importance of the ubiquitin-proteasome system as the front line of protein quality control (PQC) machinery against aberrant proteins. Recently, we have shown that the autophagy-lysosomal system is also involved in cytoplasmic aggregate degradation, but the nucleus lacked this activity. Consequently, the nucleus relies entirely on the ubiquitin-proteasome system for PQC. According to previous studies, nuclear aggregates possess a higher cellular toxicity than do their cytoplasmic counterparts, however degradation kinetics of nuclear aggregates have been poorly understood. Here we show that nuclear ubiquitin ligases San1p and UHRF-2 each enhance nuclear pQ aggregate degradation and rescued pQ-induced cytotoxicity in cultured cells and primary neurons. Moreover, UHRF-2 is associated with nuclear inclusion bodies in vitro and in vivo. Our data suggest that UHRF-2 is an essential molecule for nuclear pQ degradation as a component of nuclear PQC machinery in mammalian cells. Huntington disease (HD)2 and related polyglutamine (pQ) diseases are caused by the expansion of trinucleotide repeats encoding pQ within the mutant gene product (1). The pQ length dependence of disease onset and severity in pQ diseases correlates strongly with the tendency of expanded pQ proteins to aggregate in disease models (2, 3). Because of this pQ chain, the gene product assembles into oligomers, further aggregates to form microscopically visible inclusion bodies (IBs), and shows pathological extensity in diseased brains (4).Aggregated forms of pQ-expanded huntingtin (Htt) can disrupt cellular function in a variety of ways, including inactivation of transcription factors (5, 6) and impairment of the ubiquitin proteasome system (UPS) (7). The precise mechanism of UPS inhibition remains to be solved, but could be of particular interest, because it has been shown to occur in in vivo disease models (8). UPS impairment triggers aggresome formation (9), an active cellular mechanism to enrich cytoplasmic aggregates and autophagy-lysosomal components to the microtubule-organizing center by retrograde transport, which in turn enhances the efficiency and selectivity of autophagic degradation of cytoplasmic aggregates supporting the UPS as an alternative protein quality control (PQC) system (10, 11). However, autophagy is ineffective in clearing nuclear aggregates (12), and there is no known nuclear PQC mechanism other than the UPS. Therefore, the nucleus is a relatively protected environment for aggregates than the cytoplasm.The majority of the pQ proteins are functional in the nucleus, and the strong correlation between aggregate toxicity and nuclear translocation of pQ proteins has led to the hypothesis that the nucleus is the primary center for action of these proteotoxins (13). This was shown by redirecting nuclear ataxin-1 or andro...

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Firefly luciferase mutants as sensors of proteome stress

“…Moreover, the over-expression of San1p or UHRF-2 has been shown to enhance degradation of polyQ aggregates in cultured cells and primary neurons (Iwata et al, 2009) which underscores the role of the nucleus in preventing the accumulation of misfolded proteins.…”

“…Although a mammalian homolog of San1p has not been identified so far, another nuclear E3 ligase called UHRF-2 has been identified as a San1p functional ortholog in mammals (Iwata et al, 2009). Moreover, the over-expression of San1p or UHRF-2 has been shown to enhance degradation of polyQ aggregates in cultured cells and primary neurons (Iwata et al, 2009) which underscores the role of the nucleus in preventing the accumulation of misfolded proteins.…”