Polyglutamine aggregation behavior in vitro supports a recruitment mechanism of cytotoxicity

In expanded CAG repeat diseases such as Huntington's disease, proteins cont aining polyglutamine (poly(Gln)) sequences with repeat lengths of about 37 residues or more are associated with development of both disease symptoms a nd neuronal intranuclear inclusions (NIIs). Disease physiology in animal an d cellular models does not always correlate with NE formation, however, and the mechanism by which aggregate for,gr information might lead to cytotoxi city is unknown. To help evaluate various possible mechanisms, we determine d the biophysical properties of a series of simple poly(Gln) peptides. The circular dichroism spectra of poly(Gln) peptides with repeat lengths of fiv e, 15, 28 and 44 residues are all nearly identical and are consistent with a high degree of random coil structure, suggesting that the length-dependen ce of disease is not related to a conformational change in the monomeric st ates of expanded poly(Gln) sequences. In contrast, there is a dramatic incr ease in both the kinetics and the thermodynamic favorability of the spontan eous formation of ordered, amyloid-like aggregates for poly(Gln) peptides w ith repeat lengths of greater than 37 residues. At the same time, poly(Gln) peptides with repeat lengths in the 15-20 residue range, despite their poo r abilities to support spontaneous, self-nucleated aggregation, are capable of efficiently adding to an already-formed aggregate. We also find that mo rphologically small, finely divided aggregates are much more efficient at r ecruiting poly(Gln) peptides than are large aggregates, suggesting a possib le explanation for why disease pathology does not always correlate with the observable NII burden. Together, these data are consistent with a model fo r disease pathology in which critical cellular proteins possessing poly(Gln ) sequences of modest length become inactivated when they are recruited int o aggregates of an expanded poly(Gln) protein. (C) 2001 Academic Press.