Review: Modulating Factors in Amyloid-b Fibril Formation J. McLaurin,* , ² D.-S. Yang,* C. M. Yip,‡ , § ,¶ and P. E. Fraser* , \ ,1 *Centre for Research in Neurodegenerative Diseases, ‡Institute for Biomaterials and Biomedical Engineering, ²Department of Laboratory Medicine and Pathobiology, §Department of Biochemistry, ¶ Department of Chemical Engineering, and \Department of Medical Biophysics, University of Toronto, Toronto, Ontario, M5S 3H2, Canada Received February 28, 2000, and in revised form May 24, 2000 Amyloid formation is a key pathological feature of Alzheimer’s disease and is considered to be a major contributing factor to neurodegeneration and clin- ical dementia. Amyloid is found as both diffuse and senile plaques in the parenchyma of the brain and is composed primarily of the 40- to 42-residue amy- loid-b (Ab) peptides. The characteristic amyloid fi- ber exhibits a high b-sheet content and may be gen- erated in vitro by the nucleation-dependent self- association of the Ab peptide and an associated conformational transition from random to b-confor- mation. Growth of the fibrils occurs by assembly of the Ab seeds into intermediate protofibrils, which in turn self-associate to form mature fibers. This multistep process may be influenced at various stages by factors that either promote or inhibit Ab fiber formation and aggregation. Identification of these factors and understanding the driving forces behind these interactions as well as the structural motifs necessary for these interactions will help to elucidate potential sites that may be targeted to prevent amyloid formation and its associated toxic- ity. This review will discuss some of the modulating factors that have been identified to date and their role in fibrillogenesis. © 2000 Academic Press Key Words: amyloid-b peptides; amyloid; atomic force microscopy; b-structure; circular dichroism spectroscopy; electron microscopy. INTRODUCTION The Ab peptide is derived from the larger amyloid precursor protein (APP) as a normal cleavage prod- uct (Esch et al., 1990; Haass et al., 1992). It is gen- erated by cleavage of APP at two locations by pro- teases denoted as the b- and g-secretases. The initial proteolysis by the b-secretase results in a residual C-terminal fragment containing the transmem- brane and cytoplasmic domains of APP, which un- dergoes an additional, intramembranous cleavage by a g-secretase to release Ab. The g-secretase site, possibly mediated by one or more proteases, is het- erogenous and produces Ab fragments that can vary in length from 39 to 43 residues. However, the two predominant species are peptides spanning residues 1–40 (Ab40) or 1–42 (Ab42). These minor differ- ences in Ab cleavage have profound consequences on their propensity to aggregate and form fibers with the longer Ab42 representing the more amyloido- genic species. An alternate, non-Ab-generating pathway regulated by the so-called a-secretase also has been observed. This particular protease cleaves Ab in the vicinity of the leucine (residue 17) to generate the “p3” peptide and effectively eliminate Ab processing. The identity of g-secretase is un- known although protease inhibitor studies have demonstrated that Ab40 and Ab42 may be gener- ated from separate pathways or possibly in distinct cellular compartments. In neurons, Ab42 appears to be generated, at least in part, in the endoplasmic reticulum while the trans-Golgi network is consid- ered to be one of the primary sites of Ab40 produc- tion (Hartmann et al., 1997; Cook et al., 1997; Tien- ari et al., 1997; Wild-Bode et al., 1997). In vitro structural studies of Ab using synthetic peptides have demonstrated the spontaneous as- sembly into amyloid-like fibers. The fibrils are of varying lengths, unbranched, and 70 –120 Å in di- ameter with a characteristic b-sheet conformation and protofilament organization. A number of models have been proposed for the fibrils, but it is generally accepted that the major b-sheet regions of the pep- tide are formed by the hydrophobic internal residues 17–21 and C-terminal residues 29 – 42 (Kirschner et al., 1987; Halverson et al., 1990; Inouye et al., 1993). Fibrillogenesis is a two-step reaction involving an initial slow, lag period that reflects the thermody- 1 To whom correspondence should be addressed at Centre for Research in Neurodegenerative Diseases, University of Toronto, 6 Queen’s Park Crescent, Toronto, Ontario, M5S 3H2, Canada. Fax: (416) 978-1878. E-mail: paul.fraser@utoronto.ca. Journal of Structural Biology 130, 259 –270 (2000) doi:10.1006/jsbi.2000.4289, available online at http://www.idealibrary.com on 259 1047-8477/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.