A Compact Model of huntingtin Toxicity

Division of Neurobiology, Department of Psychiatry, Children's Medical Surgical Center, Johns Hopkins University School of Medicine, Baltimore, Mayland 21287, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 03/2011; 286(10):8188-96. DOI: 10.1074/jbc.M110.192013
Source: PubMed


Huntington disease results from an expanded polyglutamine region in the N terminus of the huntingtin protein. HD pathology
is characterized by neuronal degeneration and protein inclusions containing N-terminal fragments of mutant huntingtin. Structural
information is minimal, though it is believed that mutant huntingtin polyglutamine adopts β structure upon conversion to a
toxic form. To this end, we designed mammalian cell expression constructs encoding compact β variants of Htt exon 1 N-terminal
fragment and tested their ability to aggregate and induce toxicity in cultured neuronal cells. In parallel, we performed molecular
dynamics simulations, which indicate that constructs with expanded polyglutamine β-strands are stabilized by main-chain hydrogen
bonding. Finally, we found a correlation between the reactivity to 3B5H10, an expanded polyglutamine antibody that recognizes
a compact β rich hairpin structure, and the ability to induce cell toxicity. These data are consistent with an important role
for a compact β structure in mutant huntingtin-induced cell toxicity.

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    • "It seems more likely that the MW8 antibody was labelling both aggregated and aggregating Htt, whereas ubiquitin labelled only Htt that was already aggregated. It is also notable that we could observe MW8-labelled cells before any aggregates were visible supporting the idea that there is an oligomerisation of Htt before visible aggregates form [43], [44], [45]. Again, this was not seen with ubiquitin staining, supporting our suggestion that the protein was not ubiquitinated before it aggregated. "
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    ABSTRACT: Abnormal insoluble ubiqitinated protein aggregates are found in the brains of Huntington's disease (HD) patients and in mice transgenic for the HTT mutation. Here, we describe the earliest stages of visible NII formation in brains of R6/2 mice killed between 2 and 6 weeks of age. We found that huntingtin-positive aggregates formed rapidly (within 24-48 hours) in a spatiotemporal manner similar to that we described previously for ubiquitinated inclusions. However, in most neurons, aggregates are not ubiquitinated when they first form. It has always been assumed that mutant huntingtin is recognised as 'foreign' and consequently ubiquitinated and targeted for degradation by the ubiquitin-proteasome system pathway. Our data, however, suggest that aggregation and ubiquitination are separate processes, and that mutant huntingtin fragment is not recognized as 'abnormal' by the ubiquitin-proteasome system before aggregation. Rather, mutant Htt appears to aggregate before it is ubiquitinated, and then either aggregated huntingtin is ubiquitinated or ubiquitinated proteins are recruited into aggregates. Our findings have significant implications for the role of the ubiquitin-proteasome system in the formation of aggregates, as they suggest that this system is not involved until after the first aggregates form.
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