Structure and Aggregation Mechanisms in Amyloids

Abstract

The aggregation of a polypeptide chain into amyloid fibrils and their accumulation and deposition into insoluble plaques and intracellular inclusions is the hallmark of several misfolding diseases known as amyloidoses. Alzheimer′s, Parkinson′s and Huntington’s diseases are some of the approximately 50 amyloid diseases described to date. The identification and characterization of the molecular species critical for amyloid formation and disease development have been the focus of intense scrutiny. Methods such as X-ray and electron diffraction, solid-state nuclear magnetic resonance spectroscopy (ssNMR) and cryo-electron microscopy (cryo-EM) have been extensively used and they have contributed to shed a new light onto the structure of amyloid, revealing a multiplicity of polymorphic structures that generally fit the cross-β amyloid motif. The development of rational therapeutic approaches against these debilitating and increasingly frequent misfolding diseases requires a thorough understanding of the molecular mechanisms underlying the amyloid cascade. Here, we review the current knowledge on amyloid fibril formation for several proteins and peptides from a kinetic and thermodynamic point of view, the structure of the molecular species involved in the amyloidogenic process, and the origin of their cytotoxicity.

Oligomerization Profile of Human Transthyretin Variants with Distinct Amyloidogenicity

“…At the early stages of the oligomerization process (15 h of incubation), the SDS-PAGE gels showed that the most abundant species were 1-mers (22.2% and 21.5% for TTRV30M and TTRL55P, respectively, compared to 33.0% for TTRwt) and 2-mers (32.0% and 28.2% for TTRV30M and TTRL55P, respectively, compared to 29.2% for TTRwt), with TTRL55P showing higher amounts of larger-sized oligomers (>3-mers) and HMW species ( Figure 2). Nonetheless, regardless of the protein variant and oligomerization kinetics, the same pattern of linear polymerization was observed for the three TTR variants, suggesting a downhill polymerization type of oligomerization mechanism through multiple sequential steps, with successive addition of monomeric subunits (1-mers) to the growing low-molecular-weight (LMW) oligomer [55], at least until the formation of octamers (8-mers) [9]. Additionally, in order to characterize the nature and morphology of the TTR aggregates formed, we have analyzed the samples by thioflavin-T (ThT) extrinsic fluorescence and transmission electron microscopy (TEM).…”

“…TTR is implicated in several acquired and hereditary amyloid pathologies (ATTR, TTR amyloidosis) [1,8,9]. Whereas wild-type TTR (TTRwt) is associated with acquired amyloidosis with mainly cardiac involvement (ATTRwt) [10], different variants of TTR are the cause of hereditary amyloidosis involving the peripheral nervous system, autonomic nervous system, heart, eye, leptomeninges and vasculature of the brain [11,12].…”

“…More than 140 amyloidogenic mutations have been described [13], with TTRV30M being the most common amyloidogenic mutation leading to polyneuropathy [14,15], while the TTRL55P variant is associated with one of the most aggressive forms of ATTR polyneuropathy, with an early age of onset at 15-20 years old [16]. TTR amyloidosis shares common characteristics with other amyloid diseases, such as Alzheimer's, spongiform encephalopathies, or Parkinson's, where soluble peptides or proteins undergo conformational changes and aggregate into insoluble and highly stable amyloid fibrils [9,17]. Interestingly, the structural and functional dissimilarities between different amyloidogenic proteins lead to amyloid fibrils that share common structural characteristics, namely, the formation of long unbranched filaments with a cross-β-sheet conformation, i.e., β-strands oriented perpendicularly to the fibril main axis [9,17].…”

“…TTR amyloidosis shares common characteristics with other amyloid diseases, such as Alzheimer's, spongiform encephalopathies, or Parkinson's, where soluble peptides or proteins undergo conformational changes and aggregate into insoluble and highly stable amyloid fibrils [9,17]. Interestingly, the structural and functional dissimilarities between different amyloidogenic proteins lead to amyloid fibrils that share common structural characteristics, namely, the formation of long unbranched filaments with a cross-β-sheet conformation, i.e., β-strands oriented perpendicularly to the fibril main axis [9,17]. Despite the structural resemblance, several distinctive features at the molecular level may explain why the same protein can lead to the manifestation of different pathological phenotypes and amyloid deposition in different organs [18,19].…”

“…Previous studies on a wide range of amyloidogenic diseases, including Alzheimer's, type II diabetes, light-chain amyloidosis (AL), and ATTR, among others, have shown that soluble oligomeric structures and precursors of amyloid fibrils present cytotoxic activity [9,[20][21][22][23][24]. Therefore, the accumulation of amyloid deposits is often seen as the last stage of disease pathogenesis, with the intermediate soluble oligomers and aggregates having an important role in triggering cell death.…”