Elongation Kinetics of Polyglutamine Peptide Fibrils: A Quartz Crystal Microbalance with Dissipation Study

Abnormally expanded polyglutamine domains in proteins are associated with several neurodegenerative diseases, including Huntington's disease. Expansion of the polyglutamine (polyQ) domain facilitates aggregation of the affected protein, and several studies directly link aggregation to neurotoxicity. Studies of synthetic polyQ peptides have contributed substantially to our understanding of the mechanism of aggregation. In this report, polyQ fibrils were immobilized onto a sensor, and their elongation by polyQ peptides of various length and conformation was examined using quartz crystal microbalance with dissipation monitoring (QCM-D). The rate of elongation increased as the peptide length increased from 8 to 24 glutamines (Q8, Q20, and Q24). Monomer conformation affected elongation rates: insertion of a β-turn template d-Pro-Gly in the center of the peptide increased elongation rates several-fold, while insertion of Pro-Pro dramatically slowed elongation. Dissipation measurements of the QCM-D provided qualitative information about mechanical properties of the elongating fibrils. These data showed clear differences in the characteristics of the elongating aggregates, depending on the specific identity of the associating polyQ peptide. Elongation rates were sensitive to the pH and ionic strength of the buffer. Comparison of QCM-D data with those obtained by optical waveguide lightmode spectroscopy revealed that very little water was associated with the elongation of fibrils by the peptide containing d-Pro-Gly, but a significant amount of water was associated when the fibrils were elongated by Q20. Together, the data indicate that elongation of polyQ fibrils can occur without full consolidation to the fibril structure, resulting in variations to the aggregate structure during elongation.