Inhibiting and catalysing amyloid fibrillation at dynamic lipid interfaces

Abstract

Proteins are naturally exposed to diverse interfaces in living organisms, from static solid to dynamic fluid. Solid interfaces can enrich proteins as corona, and then catalyze, retard or hinder amyloid fibrillation. But fluid interfaces abundant in biology have rarely been studied for their correlation with protein fibrillation. Unsaturated fatty acids own growing essential roles in diet, whose fluid interfaces are found in vitro to catalyze amyloid fibrillation under certain physiologic conditions. It is determined by the location of double bonds within alkyl chains as well as the presence of physical shear. Docosahexaenoic acid (DHA) shows low catalysis because its unique alkyl chain does not favor to stabilize cross-β nucleus. Mixtures of different fatty acids also decelerate their catalytic activity. High catalysis poses an unprecedented approach to synthesize biologic nanofibrils as one-dimensional (1D) building blocks of functional hybrids. Fibrillation inhibition implied that appropriate diet would be a preventive strategy for amyloid-related diseases. Thus these results may find their significances in diverse fields of science as chemistry, biotechnology, nanotechnology, nutrition, amyloid pathobiology and nanomedicine.

Introduction

Many native proteins and peptides in living systems can denature and fold into insoluble β-sheet-based aggregates under certain specific environments [1]. Besides serving for diverse biological functions in biofilms, eggshells and silk, these one-dimensional (1D) nanostructures were also identified as amyloid fibrils and associated with many neurodegenerative disorders (e.g. Alzheimer’s, Huntington’s and Parkinson’s diseases) and progressive diseases (e.g. type II diabetes and cystic fibrosis) [2]. Though being far from fully understanding their biological causes, a variety of precautions and therapies have been pursued to retard or hinder disease-related amyloid fibrillation [3]. For example, as there lacks a promising drug that prevents amyloid-β deposits in Alzheimer’s brain, the research community has started to explore alternative approaches of early intervention, such as alteration of human lifestyle and diet [4]. A study published in 2006 suggested that a Mediterranean diet rich in olive oil might help to stave off Alzheimer’s disease [5]. Many food constituents (e.g. curcumin, caffeine) were proposed to be able to serve as disease-modifying agents for Alzheimer’s diseases [6]. Due to the fact that cholesterol catalyzed Aβ42 (the 42-residue form of the amyloid-β peptide) aggregation through a heterogeneous nucleation pathway in the presence of lipid membranes, the link between Alzheimer’s disease and the impairment of cholesterol homeostasis was rationalized in 2018 [7]. In view of complicated food ingredients, it will be highly attractive for both the scientific community and human society to reveal inherent connection between specific food constituents and amyloid fibrillation.

In principle, amyloid fibrillation of proteins follows a nucleation-and-growth mechanism and is influenced by many physiological and environmental factors in-vivo and in-vitro [8]. In particular, alien static solid interfaces are able to interfere with the formation of stable cross-β nucleus, and thereby the nucleation-dependent fibrillation process. Certain nanoparticles (such as CdTe [9], fluorinated nanoparticles and cationic polystyrene nanoparticles [10]) were capable of binding amyloid monomers or oligomers with high affinity and frustrating amyloid fibrillation, owing to depleting free monomers and oligomers for nucleation and blocking self-recognition for monomer addition [11], [12]. Some nanomaterials (e.g. copolymers, cerium oxide, carbon nanotubes and titanium oxide) enriched proteins and their oligomers locally on their surfaces, and hereby enhanced the probability of forming cross-β nucleates for protein fibrillation [13], [14], [15]. Despite these controversial interfacial effects, proper hydrophobic interfaces seemed to favor the formation of cross-β nucleus by enriching proteins as protein corona and exposing hydrophobic groups of proteins for β-sheets organization [8].

Fatty acids are one of the main constituents in plant oils and animal fats, and a type of important food catabolites in living organisms, which can enter the blood circulatory systems and involve in many biological activities, such as maintaining structure of cell membranes, synthesis of prostaglandins and other substances, and multiple functions of brain [16]. Their chemical structures and compositions are determined by their oleochemical sources and physiological activities, e.g. unsaturated fatty acids are abundant in marine fish oils and plant oils. Liang [17] and Johansson [18] suggested that some polyunsaturated fatty acids might stabilize soluble protein aggregates more than insoluble nucleating aggregates, and thus hinder self-recognition for monomer addition, which were further confirmed by in vivo results [19]. However, Perrin [20] indicated that long-chain polyunsaturated fatty acids favored the formation of nucleating aggregates of α-synuclein on their micelle/vesicle surfaces.

Being abundant in biological systems and expecting growing healthy roles in diet, cis unsaturated fatty acids with alkyl chain lengths of 12–24 carbon atoms are in rapidly increasing demand in modern human diet. Herein the double-bond number and location were tuned in unsaturated fatty acids to investigate their influences on amyloid fibrillation of proteins. Having relatively low solubility in water and melting points, they form enormous dynamic fluid interfaces under physiologic conditions.