Conformation-specific perturbation of membrane dynamics by structurally distinct oligomers of Alzheimer's amyloid-β peptide

A turn for the worse: Aβ β-hairpins in Alzheimer's disease

Amyloid-β (Aβ) oligomers are a cause of neurodegeneration in Alzheimer's disease (AD). These soluble aggregates of the Aβ peptide have proven difficult to study due to their inherent metastability and heterogeneity. Strategies to isolate and stabilize homogenous Aβ oligomer populations have emerged such as mutations, covalent cross-linking, and protein fusions. These strategies along with molecular dynamics simulations have provided a variety of proposed structures of Aβ oligomers, many of which consist of molecules of Aβ in β-hairpin conformations. β-Hairpins are intramolecular antiparallel β-sheets composed of two β-strands connected by a loop or turn. Three decades of research suggests that Aβ peptides form several different β-hairpin conformations, some of which are building blocks of toxic Aβ oligomers. The insights from these studies are currently being used to design anti-Aβ antibodies and vaccines to treat AD. Research suggests that antibody therapies designed to target oligomeric Aβ may be more successful at treating AD than antibodies designed to target linear epitopes of Aβ or fibrillar Aβ. Aβ β-hairpins are good epitopes to use in antibody development to selectively target oligomeric Aβ. This review summarizes the research on β-hairpins in Aβ peptides and discusses the relevance of this conformation in AD pathogenesis and drug development.

Amyloid oligomers and their membrane toxicity - A perspective study

Amyloidosis is a condition involving a disparate group of pathologies characterized by the extracellular deposition of insoluble fibrils composed of broken-down proteins. These proteins can accumulate locally, causing peculiar symptoms, or in a widespread way, involving many organs and. causing severe systemic failure. The damage that is created is related not only to the accumulation of. amyloid fibrils but above all to the precursor oligomers of the fibrils that manage to enter the cell in a very particular way. This article analyzes the current state of research related to the entry of these oligomers into the cell membrane and the theories related to their toxicity. The paper proposed here not only aims to review the contents in the literature but also proposes a new vision of amyloid toxicity. that could occur in a multiphase process catalyzed by the cell membrane itself. In this process, the denaturation of the lipid bilayer is followed by the stabilization of a pore through energetically favorable self-assembly processes which are achieved through particular oligomeric structures.

Delineating the Role of GxxxG Motif in Amyloidogenesis: A New Perspective in Targeting Amyloid-Beta Mediated AD Pathogenesis

The pursuit of a novel structural motif that can shed light on the key functional attributes is a primary focus in the study of protein folding disorders. Decades of research on Alzheimer's disease (AD) have centered on the Amyloid β (Aβ) pathway, highlighting its significance in understanding the disorder. The diversity in the Aβ pathway and the possible silent tracks which are yet to discover, makes it exceedingly intimidating to the interdisciplinary scientific community. Over the course of AD research, Aβ has consistently been at the forefront of scientific inquiry and discussion. In this review, we epitomize the role of a potential structural motif (GxxxG motif) that may provide a new horizon to the Aβ conflict. We emphasize on how comprehensive understanding of this motif from a structure-function perspective may pave the way for designing novel therapeutics intervention in AD and related diseases.

Probing the Hydrophobic Region of a Lipid Bilayer at Specific Depths Using Vibrational Spectroscopy

A novel spectroscopic approach for studying the flexibility and mobility in the hydrophobic interior of lipid bilayers at specific depths is proposed. A set of test compounds featuring an azido moiety and a cyano or carboxylic acid moiety, connected by an alkyl chain of different lengths, was synthesized. FTIR data and molecular dynamics calculations indicated that the test compounds in a bilayer are oriented so that the cyano or carboxylic acid moiety is located in the lipid head-group region, while the azido group stays inside the bilayer at the depth determined by its alkyl chain length. We found that the asymmetric stretching mode of the azido group (νN3) can serve as a reporter of the membrane interior dynamics. FTIR and two-dimensional infrared (2DIR) studies were performed at different temperatures, ranging from 22 to 45 °C, covering the Lβ-Lα phase transition temperature of dipalmitoylphosphatidylcholine (∼41 °C). The width of the νN3 peak was found to be very sensitive to the phase transition and to the temperature in general. We introduced an order parameter, SN3, which characterizes restrictions to motion inside the bilayer. 2DIR spectra of νN3 showed different extents of inhomogeneity at different depths in the bilayer, with the smallest inhomogeneity in the middle of the leaflet. The spectral diffusion dynamics of the N3 peak was found to be dependent on the depth of the N3 group location in the bilayer. The obtained results enhance our understanding of the bilayer dynamics and can be extended to investigate membranes with more complex compositions.

Alpha Synuclein: Neurodegeneration and Inflammation

Alpha-Synuclein (α-Syn) is one of the most important molecules involved in the pathogenesis of Parkinson's disease and related disorders, synucleinopathies, but also in several other neurodegenerative disorders with a more elusive role. This review analyzes the activities of α-Syn, in different conformational states, monomeric, oligomeric and fibrils, in relation to neuronal dysfunction. The neuronal damage induced by α-Syn in various conformers will be analyzed in relation to its capacity to spread the intracellular aggregation seeds with a prion-like mechanism. In view of the prominent role of inflammation in virtually all neurodegenerative disorders, the activity of α-Syn will also be illustrated considering its influence on glial reactivity. We and others have described the interaction between general inflammation and cerebral dysfunctional activity of α-Syn. Differences in microglia and astrocyte activation have also been observed when in vivo the presence of α-Syn oligomers has been combined with a lasting peripheral inflammatory effect. The reactivity of microglia was amplified, while astrocytes were damaged by the double stimulus, opening new perspectives for the control of inflammation in synucleinopathies. Starting from our studies in experimental models, we extended the perspective to find useful pointers to orient future research and potential therapeutic strategies in neurodegenerative disorders.

Aβ-oligomers: A potential therapeutic target for Alzheimer's disease

The cascade of amyloid formation relates to multiple complex events at the molecular level. Previous research has established amyloid plaque deposition as the leading cause of Alzheimer's disease (AD) pathogenesis, detected mainly in aged population. The primary components of the plaques are two alloforms of amyloid-beta (Aβ), Aβ_1-42 and Aβ_1-40 peptides. Recent studies have provided considerable evidence contrary to the previous claim indicating that amyloid-beta oligomers (AβOs) as the main culprit responsible for AD-associated neurotoxicity and pathogenesis. In this review, we have discussed the primary features of AβOs, such as assembly formation, the kinetics of oligomer formation, interactions with various membranes/membrane receptors, the origin of toxicity, and oligomer-specific detection methods. Recently, the discovery of rationally designed antibodies has opened a gateway for using synthesized peptides as a grafting component in the complementarity determining region (CDR) of antibodies. Thus, the Aβ sequence motif or the complementary peptide sequence in the opposite strand of the β-sheet (extracted from the Protein Data Bank: PDB) helps design oligomer-specific inhibitors. The microscopic event responsible for oligomer formation can be targeted, and thus prevention of the overall macroscopic behaviour of the aggregation or the associated toxicity can be achieved. We have carefully reviewed the oligomer formation kinetics and associated parameters. Besides, we have depicted a thorough understanding of how the synthesized peptide inhibitors can impede the early aggregates (oligomers), mature fibrils, monomers, or a mixture of the species. The oligomer-specific inhibitors (peptides or peptide fragments) lack in-depth chemical kinetics and optimization control-based screening. In the present review, we have proposed a hypothesis for effectively screening oligomer-specific inhibitors using the chemical kinetics (determining the kinetic parameters) and optimization control strategy (cost-dependent analysis). Further, it may be possible to implement the structure-kinetic-activity-relationship (SKAR) strategy instead of structure-activity-relationship (SAR) to improve the inhibitor's activity. The controlled optimization of the kinetic parameters and dose usage will be beneficial for narrowing the search window for the inhibitors.

Taylor Dispersion Analysis and Atomic Force Microscopy Provide a Quantitative Insight into the Aggregation Kinetics of Aβ (1-40)/Aβ (1-42) Amyloid Peptide Mixtures

Aggregation of amyloid β peptides is known to be one of the main processes responsible for Alzheimer's disease. The resulting dementia is believed to be due in part to the formation of potentially toxic oligomers. However, the study of such intermediates and the understanding of how they form are very challenging because they are heterogeneous and transient in nature. Unfortunately, few techniques can quantify, in real time, the proportion and the size of the different soluble species during the aggregation process. In a previous work (Deleanu et al. Anal. Chem. 2021, 93, 6523-6533), we showed the potential of Taylor dispersion analysis (TDA) in amyloid speciation during the aggregation process of Aβ (1-40) and Aβ (1-42). The current work aims at exploring in detail the aggregation of amyloid Aβ (1-40):Aβ (1-42) peptide mixtures with different proportions of each peptide (1:0, 3:1, 1:1, 1:3, and 0:1) using TDA and atomic force microscopy (AFM). TDA allowed for monitoring the kinetics of the amyloid assembly and quantifying the transient intermediates. Complementarily, AFM allowed the formation of insoluble fibrils to be visualized. Together, the two techniques enabled us to study the influence of the peptide ratios on the kinetics and the formation of potentially toxic oligomeric species.

Distinct types of amyloid-β oligomers displaying diverse neurotoxicity mechanisms in Alzheimer's disease

Soluble oligomers of amyloid-β (Aβ) are recognized as key pernicious species in Alzheimer's disease (AD) that cause synaptic dysfunction and memory impairments. Numerous studies have identified various types of Aβ oligomers having heterogeneous peptide length, size distribution, structure, appearance, and toxicity. Here, we review the characteristics of soluble Aβ oligomers based on their morphology, size, and structural reactivity toward the conformation-specific antibodies and then describe their formation, localization, and cellular effects in AD brains, in vivo and in vitro. We also summarize the mechanistic pathways by which these soluble Aβ oligomers cause proteasomal impairment, calcium dyshomeostasis, inhibition of long-term potentiation, apoptosis, mitochondrial damage, and cognitive decline. These cellular events include three distinct molecular mechanisms: (i) high-affinity binding with the receptors for Aβ oligomers such as N-methyl- d-aspartate receptors, cellular prion protein, nerve growth factor, insulin receptors, and frizzled receptors; (ii) the interaction of Aβ oligomers with the lipid membranes; (iii) intraneuronal accumulation of Aβ by α7-nicotinic acetylcholine receptors, apolipoprotein E, and receptor for advanced glycation end products. These studies indicate that there is a pressing need to carefully examine the role of size, appearance, and the conformation of oligomers in identifying the specific mechanism of neurotoxicity that may uncover potential targets for designing AD therapeutics.