An investigation into polyglutamine aggregation mechanisms

Protein aggregation is a key mechanism involved in neurodegeneration associated with Alzheimer’s, Parkinson’s and Huntington’s diseases. Nine diseases (including Huntington’s) arise from polyglutamine (polyQ) expansion above a repeat threshold of approximately 37 glutamines, and neuronal toxicity correlates with the process of protein aggregation. The similar toxic gain-of-function mechanism of the nine diseases supports the hypothesis that disease onset and progression is dependent upon polyQ expansion. However, there is an increasing body of literature demonstrating that the protein context of the polyQ tract plays an important modulating role in the disease process. The composition of regions flanking repeats can alter the biochemical and biophysical properties of the polyQ region. Interactions between flanking regions and other molecules can also influence aggregation and cellular localization, which are critical factors for toxicity. More recently, there is evidence that domains flanking the polyQ tract can also aggregate independently of the polyQ tract, and that this significantly alters the rate at which the polyQ regions form fibrillar aggregates and the properties of these aggregates. This thesis investigates the molecular mechanisms leading to polyQ aggregation and the role of protein context in modulating the aggregation pathway. A family of model polyQ proteins were engineered and produced. The proteins have a repeat-length dependent aggregation mechanism, recapitulating the relationship between repeat length and age of disease onset. The stability and structure of the flanking domain were unperturbed when fused to a pathological length polyQ tract, suggesting that protein misfolding within the polyQ tract is the driving force behind the key characteristics of the polyQ diseases. The repeat location and domain architecture affect the rate of polyQ-dependent aggregation, indicating that host protein factors can modulate aggregation. Furthermore, the small heat shock protein, αB-crystallin does not inhibit the aggregation of the model polyQ protein that aggregates by the polyQ-driven mechanism. αB-crystallin does, however, inhibit aggregation of the disease protein ataxin-3 by a mechanism involving interactions with the flanking domain. Together, the results within this thesis have provided insight into the molecular basis of polyQ disease and have shown that the propensity for polyQ aggregation is determined by a complex interplay between the polyQ region, host protein factors and the cellular environment.