Polyglutamine Expansion Accelerates the Dynamics of Ataxin-1 and Does Not Result in Aggregate Formation

Abstract

BackgroundPolyglutamine expansion disorders are caused by an expansion of the polyglutamine (polyQ) tract in the disease related protein, leading to severe neurodegeneration. All polyQ disorders are hallmarked by the presence of intracellular aggregates containing the expanded protein in affected neurons. The polyQ disorder SpinoCerebellar Ataxia 1 (SCA1) is caused by a polyQ-expansion in the ataxin-1 protein, which is thought to lead to nuclear aggregates.Methodology/Principal FindingsUsing advanced live cell fluorescence microscopy and a filter retardation assay we show that nuclear accumulations formed by polyQ-expanded ataxin-1 do not resemble aggregates of other polyQ-expanded proteins. Instead of being static, insoluble aggregates, nuclear accumulations formed by the polyQ-expanded ataxin-1 showed enhanced intracellular kinetics as compared to wild-type ataxin-1. During mitosis, ataxin-1 accumulations redistributed equally among daughter cells, in contrast to polyQ aggregates. Interestingly, polyQ expansion did not affect the nuclear-cytoplasmic shuttling of ataxin-1 as proposed before.Conclusions/SignificanceThese results indicate that polyQ expansion does not necessarily lead to aggregate formation, and that the enhanced kinetics may affect the nuclear function of ataxin-1. The unexpected findings for a polyQ-expanded protein and their consequences for ongoing SCA1 research are discussed.

Cohesin can remain associated with chromosomes during DNA replication

“…This is not a problem when imaging for short time intervals (<10 min) but is a major problem for longer recovery periods during which significant protein synthesis occurs. Previous studies have incubated cells in the protein synthesis inhibitor cyclohexamide to mitigate this issue (Gerlich et al, 2006;Kimura and Cook, 2001;Krol et al, 2008); however, under these conditions cells cannot progress through the cell cycle and relevant interacting proteins may become depleted.…”

Polyglutamine Expansion Alters the Dynamics and Molecular Architecture of Aggregates in Dentatorubropallidoluysian Atrophy

“…However, these dynamics may represent variations in the abil- ity of the aggregates to exchange with the external mobile protein pool (outside of the inclusion). Thus, we employed a complementary assay, inverted FRAP (iFRAP), which reports on the exchange of fluorescently labeled species from aggregates or with the neighboring environment or compartment (39). In brief, the whole cell, leaving the inclusion of interest intact, is completely photobleached.…”

“…This model may apply to some other polyQ diseases, and it resonates with previous observations on ataxin-1. Nuclear inclusions of ataxin-1 with a pathological polyQ length contain both fast-and slow-exchanging components, and the former ones contain high ubiquitin levels (19,26,39). By contrast, Htt exon-1 with an expanded polyQ tract forms immobile intranuclear inclusions (19), underscoring the importance of the protein context in polyQ diseases.…”

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“…An earlier description of ataxin-1 selfassociation had identified this property almost exclusively with the AXH domain (Burright et al, 1997). This incorrect assumption has lead subsequent investigators to describe the self association region as partially overlapping the AXH domain (see for instance Krol et al, 2008). We now have clearly shown that this is not the case.…”

“…One aspect that distinguishes ataxin-1 from most other proteins is its ability to form small dense nuclear bodies (Skinner et al, 1997;Matilla et al, 1997). These bodies have been variously described as nuclear structures (Skinner et al, 1997), foci (Tsai et al, 2004), inclusions (Dovey et al, 2004), nuclear accumulations (Krol et al, 2008), or aggresomes (Latonen, 2011) with little consensus on the terminology used. Here, we preferred to name these structures foci as they are formed not just by the mutant expanded protein but also by the non-expanded protein or even by ataxin-1 without the incriminating polyQ tract (Tsai et al, 2004).…”