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Huntington's disease (HD) is a progressive, adult-onset neurodegenerative disease caused by a polyglutamine (polyQ) expansion in the N-terminal region of the protein huntingtin (HTT). There are no cures or disease-modifying therapies for HD. HTT has a highly conserved Akt phosphorylation site at serine 421, and prior work in HD models found that phosphorylation at S421 (S421-P) diminishes the toxicity of mutant HTT (mHTT) fragments in neuronal cultures. However, whether S421-P affects the toxicity of mHTT in vivo remains unknown. In this work, we used murine models to investigate the role of S421-P in HTT-induced neurodegeneration. Specifically, we mutated the human mHTT gene within a BAC to express either an aspartic acid or an alanine at position 421, mimicking tonic phosphorylation (mHTT-S421D mice) or preventing phosphorylation (mHTT-S421A mice), respectively. Mimicking HTT phosphorylation strongly ameliorated mHTT-induced behavioral dysfunction and striatal neurodegeneration, whereas neuronal dysfunction persisted when S421 phosphorylation was blocked. We found that S421 phosphorylation mitigates neurodegeneration by increasing proteasome-dependent turnover of mHTT and reducing the presence of a toxic mHTT conformer. These data indicate that S421 is a potent modifier of mHTT toxicity and offer in vivo validation for S421 as a therapeutic target in HD.

Polyglutamine (polyQ) stretches exceeding a threshold length confer a toxic function to proteins that contain them and cause at least nine neurological disorders. The basis for this toxicity threshold is unclear. Although polyQ expansions render proteins prone to aggregate into inclusion bodies, this may be a neuronal coping response to more toxic forms of polyQ. The exact structure of these more toxic forms is unknown. Here we show that the monoclonal antibody 3B5H10 recognizes a species of polyQ protein in situ that strongly predicts neuronal death. The epitope selectively appears among some of the many low-molecular-weight conformational states assumed by expanded polyQ and disappears in higher-molecular-weight aggregated forms, such as inclusion bodies. These results suggest that protein monomers and possibly small oligomers containing expanded polyQ stretches can adopt a conformation that is recognized by 3B5H10 and is toxic or closely related to a toxic species.

Background: Huntington's disease is a neurodegenerative disorder, typically with clinical manifestations in adult years, caused by an expanded polyglutamine-coding repeat in HTT. There are no treatments that delay or prevent the onset or progression of this devastating disease. Objective and Methods: In order to study its pre-symptomatic molecular progression and provide a large mammalian model for determining natural history of the disease and for therapeutic testing, we generated and previously reported on lines of transgenic sheep carrying a full length human HTT cDNA transgene, with expression driven by a minimal HTT promoter. We report here further characterization of our preferred line, OVT73. Results: This line reliably expresses the expanded human huntingtin protein at modest, but readily detectable levels throughout the brain, including the striatum and cortex. Transmission of the 73 unit glutamine coding repeat was relatively stable over three generations. At the first time-point of a longitudinal study, animals sacrificed at 6 months (7 transgenic, 7 control) showed reduced striatum GABAA α1 receptor, and globus pallidus leu-enkephalin immunoreactivity. Two of three 18 month old animals sacrificed revealed cortical neuropil aggregates. Furthermore, neuronal intranuclear inclusions were identified in the piriform cortex of a single 36 month old animal in addition to cortical neuropil aggregates. Conclusions: Taken together, these data indicate that the OVT73 transgenic sheep line will progressively reveal early HD pathology and allow therapeutic testing over a period of time relevant to human patients. [ABSTRACT FROM AUTHOR]

A correction to the article "Identifying polyglutamine protein species in situ that best predict neurodegeneration" that was published in the 2011 issue is presented.