Properties of polyglutamine expansion in vitro and in a cellular model forHuntington's disease

Eight neurodegenerative diseases have been shown to be caused by the expans ion of a polyglutamine stretch in specific target proteins that lead to a g ain in toxic property. Most of these diseases have some features in common. A pathological threshold of 35-40 glutamine residues is observed in five o f the diseases. The mutated proteins (or a polyglutamine-containing subfrag ment) form ubiquitinated aggregates in neurons of patients or mouse models, in most cases within the nucleus. We summarize the properties of a monoclo nal antibody that recognizes specifically, in a Western blot, polyglutamine stretches longer than 35 glutamine residues with an affinity that increase s with polyglutamine length. This indicates that the pathological threshold observed in five diseases corresponds to a conformational change creating a pathological epitope, most probably involved in the aggregation property of the carrier protein. We also show that a fragment of a normal protein ca rrying 38 glutamine residues is able to aggregate into regular fibrils in v itro. Finally, we present a cellular model in which the induced expression of a mutated full-length huntingtin protein leads to the formation of nucle ar inclusions that share many characteristics with those observed in patien ts: those inclusions are ubiquitinated and contain only an N-terminal fragm ent of huntingtin. This model should thus be useful in studying a processin g step that is likely to be important in the pathogenicity of mutated hunti ngtin.