Pyroglutamylated Amyloid-β Peptide Reverses Cross β-Sheets by a Prion-Like Mechanism

Abstract

The amyloid hypothesis causatively relates the fibrillar deposits of amyloid β peptide (Aβ) to Alzheimer’s disease (AD). More recent data, however, identify the soluble oligomers as the major cytotoxic entities. Pyroglutamylated Aβ (pE-Aβ) is present in AD brains and exerts augmented neurotoxicity, which is believed to result from its higher β-sheet propensity and faster fibrillization. While this concept is based on a set of experimental results, others have reported similar β-sheet contents in unmodified and pyroglutamylated Aβ, and slower aggregation of pE-Aβ as compared to unmodified Aβ, leaving the issue unresolved. Here, we assess the structural differences between Aβ and pE-Aβ peptides that may underlie their distinct cytotoxicities. Transmission electron microscopy identifies a larger number of prefibrillar aggregates of pE-Aβ at early stages of aggregation and suggests that pE-Aβ affects the fibrillogenesis even at low molar fractions. Circular dichroism and FTIR data indicate that while the unmodified Aβ readily forms β-sheet fibrils in aqueous media, pE-Aβ displays increased α-helical and decreased β-sheet propensity. Moreover, isotope-edited FTIR spectroscopy shows that pE-Aβ reverses β-sheet formation and hence fibrillogenesis of the unmodified Aβ peptide via a prion-like mechanism. These data provide a novel structural mechanism for pE-Aβ hypertoxicity; pE-Aβ undergoes faster formation of prefibrillar aggregates due to its increased hydrophobicity, thus shifting the initial stages of fibrillogenesis toward smaller, hypertoxic oligomers of partial α-helical structure.

Mitochondrial dysfunction induced by a post-translationally modified amyloid linked to a familial mutation in an alternative model of neurodegeneration

“…Moreover, it is conceivable that the intermediate conformational behavior of ABri E could be tilted to more amyloidogenic conformations by existing fibrillar structures of ABri pE acting as seeding elements capable of accelerating and/or enhancing the formation of more pathogenic elements, a mechanism resembling the infectivity of prion diseases, which is currently considered as a significant contributor to the mechanisms of AD pathogenesis [80]. In this light, in vitro and in vivo evidence supporting the seeding capabilities of pE-modified peptides as well as consequential propagation via prion-like mechanisms has been presented using truncated forms of Aβ [28, 81] and substantiated by complementary biophysical studies [82]. …”

Enhanced Fibril Fragmentation of N‐Terminally Truncated and Pyroglutamyl‐Modified Aβ Peptides

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N-terminal truncation and pyroglutamyl (pE) formation are naturally occurring chemical modifications of the Ab peptide in Alzheimers disease.W es how herein that these two modifications significantly reduce the fibril length and the transition midpoint of thermal unfolding of the fibrils,but they do not substantially perturb the fibrillary peptide conformation. [4][5][6] Pharmacological inhibition of glutamyl cyclase,t he enzyme that catalyzes the formation of the pE lactam ring, reduces amyloid plaque deposition in vivo and retards memory decline in mice, [1] but mixed results have been published on the biophysical effects of these modifications.S everal studies report pyroglutamylated Ab to exhibit accelerated fibril formation; [7][8][9][10] while others claim inhibitory activities, [11] an enhanced propensity to form small, cytotoxic oligomers, [8,10,12,13] or differential effects on the Ab40 and Ab42 peptide variants. [1][2][3] Depending on the type of analysis,1 0-50 %o ft he deposited Ab is pE-modified in human AD.

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“…No comparable EM images were obtained with fibrils from unmodified Ab,and there was also Our data have several implications.F irst, they establish that pE modification consistently accelerates Ab fibrillation in series of peptides extending to positions 40 and 42. [11] Inhibitory activity was previously assumed based on TEM data showing the formation of long filaments to be prevented by pEAb. [11] Inhibitory activity was previously assumed based on TEM data showing the formation of long filaments to be prevented by pEAb.…”

Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid β peptides

“…In the 1:1 combination, this α-helical propensity is augmented, corroborating the notion that AβpE 3-42 is able to retard amyloid fibrillogenesis by opposing α-helix to β-sheet structural transition. 65 …”

“…The window was mounted on a sample holder and spectra were measured on a Vector-22 FTIR spectrometer (Bruker Optics, Billerica, MA, USA) equipped with a liquid nitrogen-cooled Hg-Cd-Te detector at 2 cm −1 nominal resolution at 25 °C, as described. 65 Following measurements of the spectra of dry peptides, the CaF 2 window was dismounted, placed horizontally, and 120 μL of D 2 O-based buffer were added to the dry peptide. A 50 μm Teflon spacer and a second CaF 2 window were placed on top of the sample and tightly sealed.…”