The First Step of Amyloidogenic Aggregation Fabio Castello, † Salvador Casares, ‡ Maria J. Ruedas-Rama, † and Angel Orte* ,† † Department of Physical Chemistry, Faculty of Pharmacy, University of Granada, Campus Cartuja, 18071 Granada, Spain ‡ Department of Physical Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva, 18071 Granada, Spain * S Supporting Information ABSTRACT: The structural and dynamic characterization of the on-pathway intermediates involved in the mechanism of amyloid fibril formation is one of the major remaining biomedical challenges of our time. In addition to mature fibrils, various oligomeric structures are implicated in both the rate-limiting step of the nucleation process and the neuronal toxicity of amyloid deposition. Single-molecule fluorescence spectroscopy (SMFS) is an excellent tool for extracting most of the relevant information on these molecular systems, especially advanced multiparameter approaches, such as pulsed interleaved excitation (PIE). In our investigations of an amyloidogenic SH3 domain of α-spectrin, we have found dynamic oligomerization, even prior to incubation. Our single-molecule PIE experiments revealed that these species are small, mostly dimeric, and exhibit a loose and dynamic molecular organization. Furthermore, these experiments have allowed us to obtain quantitative information regarding the oligomer stability. These pre-amyloidogenic oligomers may potentially serve as the first target for fibrillization-prevention strategies. ■ INTRODUCTION The high impact of neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and many others, 1,2 on both the quality of life of those affected and the world economy is currently one of the major issues in the field of biomedicine. The common hallmark of these diseases is the deposition of fibrillar proteinaceous plaques called amyloid fibrils. The structural and dynamic characterization of the on-pathway intermediates involved in the mechanism of amyloid fibril formation remains to be determined. These early oligomeric structures represent one of the most interesting therapeutic targets because they play a central role in the neuronal toxicity of amyloid plaques. 3 Understanding the nature and formation mechanism of these aggregates is therefore crucial for the development of chemical or biological treatments that successfully reduce the strong impact of amyloid diseases. One of the major difficulties hindering the study of the mechanism of amyloid fibril formation via conventional techniques is the transient nature of the heterogeneous population of the various formed species. 4-6 Conventional biophysical tools for the study of amyloid fibril formation provide bulk information on the ensemble of the population, which is typically sufficient to assess the overall aggregation kinetics and evaluate hypotheses for broad mechanisms of aggregation. This family of techniques typically comprises thioflavin T fluorescence emission and turbidimetry, among others. Nevertheless, these techniques are not sufficient to unravel the intrinsic heterogeneity of preamyloid aggregates. Additional knowledge on the population of these aggregates has been acquired using alternative approaches, which allow for a more complete characterization than classical techniques. For example, dynamic light scattering (DLS) 7 and fluorescence correlation spectroscopy 8 provide insights into the distributions of species based on the diffusional properties. Imaging techniques, such as atomic force microscopy, 9 total internal reflection fluorescence microscopy, 10 and superresolution fluorescence microscopy, 11 have been recently employed to directly probe the shape of aggregates, monitor fibril growth, and directly visualize the localization of aggregates within live cells. Among novel biophysical techniques, single-molecule fluo- rescence spectroscopy (SMFS) allows for the investigation of biomolecular interactions at the molecular level. The ability to individually probe molecules within a large population provides the direct and exciting advantage of immediately revealing the heterogeneity of the population. 12,13 These advantages offer new possibilities to elucidate the mechanism for the amyloid fibril formation process. Direct detection of aggregated species has been achieved utilizing dual-color excitation SMFS schemes 14 and multiparametric single-molecule spectroscopy. 15 Burstwise analysis of coincident single-molecule fluorescence events upon dual-color excitation 14 has provided in-depth information on the size distributions and kinetics of aggregation. 16 This approach has been successfully applied to studies on disease-related proteins. The aggregate populations, thermodynamic properties, and the effect of aggregation- inhibiting chaperonins have been studied for amyloid-β peptides, 17 which represent the primary component of amyloid plaques in Alzheimer’s disease. Similarly, monitoring the kinetics of neuroserpin polymerization at the single-molecule level has provided insights into the underlying aggregation Received: February 27, 2015 Revised: June 1, 2015 Article pubs.acs.org/JPCB © XXXX American Chemical Society A DOI: 10.1021/acs.jpcb.5b01957 J. Phys. Chem. B XXXX, XXX, XXX-XXX