Membrane affinity of individual toxic protein oligomers determined at the single-molecule level

Tracking heterogenous protein aggregation at nanoscale through fluorescence correlation spectroscopy

Various biophysical techniques have been extensively employed to study protein aggregation due to its significance. Traditionally, these methods detect aggregation at micrometer length scales and micromolar concentrations. However, unlike in vitro, protein aggregation typically occurs at nanomolar concentrations in vivo. Here, using fluorescence correlation spectroscopy (FCS), we captured bromelain aggregation at concentrations as low as ~20 nM, surpassing the detection limit of traditional methods like thioflavin T fluorescence, scattering, and fluorescence microscopy by more than one order of magnitude. Moreover, using thioflavin T fluorescence-based FCS, we have detected larger aggregates at higher bromelain concentrations, which is undetectable in FCS otherwise. Importantly, our study reveals inherent heterogeneity in bromelain aggregation, inaccessible to ensemble-averaged techniques. The presented report may provide a platform for the characterization of premature aggregates at very low protein concentrations, which are thought to be functionally significant species in protein aggregation-induced diseases.

Single-Molecule Maps of Membrane Insertion by Amyloid-β Oligomers Predict Their Toxicity

The interaction of small Amyloid-β (Aβ) oligomers with the lipid membrane is an important component of the pathomechanism of Alzheimer's disease (AD). However, oligomers are heterogeneous in size. How each type of oligomer incorporates into the membrane, and how that relates to their toxicity, is unknown. Here, we employ a single molecule technique called Q-SLIP (Quencher-induced Step Length Increase in Photobleaching) to measure the membrane insertion of each monomeric unit of individual oligomers of Aβ42, Aβ40, and Aβ40-F19-Cyclohexyl alanine (Aβ40-F19Cha), and correlate it with their toxicity. We observe that the N-terminus of Aβ42 inserts close to the center of the bilayer, the less toxic Aβ40 inserts to a shallower depth, and the least toxic Aβ40-F19Cha has no specific distribution. This oligomer-specific map provides a mechanistic representation of membrane-mediated Aβ toxicity and should be a valuable tool for AD research.

A Toxicogenic Interaction between Intracellular Amyloid-β and Apolipoprotein-E

Alzheimer's disease (AD) is associated with the aggregation of amyloid β (Aβ) and tau proteins. Why ApoE variants are significant genetic risk factors remains a major unsolved puzzle in understanding AD, although intracellular interactions with ApoE are suspected to play a role. Here, we show that specific changes in the fluorescence lifetime of fluorescently tagged small Aβ oligomers in rat brain cells correlate with the cellular ApoE content. An inhibitor of the Aβ-ApoE interaction suppresses these changes and concomitantly reduces Aβ toxicity in a dose-dependent manner. Single-molecule techniques show changes both in the conformation and in the stoichiometry of the oligomers. Neural stem cells derived from hiPSCs of Alzheimer's patients also exhibit these fluorescence lifetime changes. We infer that intracellular interaction with ApoE modifies the N-terminus of the Aβ oligomers, inducing changes in their stoichiometry, membrane affinity, and toxicity. These changes can be directly imaged in live cells and can potentially be used as a rapid and quantitative cellular assay for AD drug discovery.

Determining the Stoichiometry of Amyloid Oligomers by Single-Molecule Photobleaching

Small oligomers are the initial intermediates in the pathway to amyloid fibril formation. They have a distinct identity from the monomers as well as from the protofibrils and the fibrils, both in their structure and in their properties. In many cases, they play a crucial biological role. However, due to their transient nature, they are difficult to characterize. "Oligomer" is a diffuse definition, encompassing aggregates of many different sizes, and this lack of precise definition causes much confusion and disagreement between different research groups. Here, we define the small oligomers as "n"-mers with n < 10, which is the size range in which the amyloid proteins typically exist at the initial phase of the aggregation process. Since the oligomers dynamically interconvert into each other, a solution of aggregating amyloid proteins will contain a distribution of sizes. A precise characterization of an oligomeric solution will, therefore, require quantification of the relative population of each size. Size-based separation methods, such as size-exclusion chromatography, are typically used to characterize this distribution. However, if the interconversion between oligomers of different sizes is fast, this would not yield reliable results. Single-molecule photobleaching (smPB) is a direct method to evaluate this size distribution in a heterogeneous solution without separation. In addition, understanding the mechanism of action of amyloid oligomers requires knowing the affinity of each oligomer type to different cellular components, such as the cell membrane. These measurements are also amenable to smPB. Here we show how to perform smPB, both for oligomers in solution and for oligomers attached to the membrane.

Single Molecule Measurements of the Accessibility of Molecular Surfaces

An important measure of the conformation of protein molecules is the degree of surface exposure of its specific segments. However, this is hard to measure at the level of individual molecules. Here, we combine single molecule photobleaching (smPB, which resolves individual photobleaching steps of single molecules) and fluorescence quenching techniques to measure the accessibility of individual fluorescently labeled protein molecules to quencher molecules in solution. A quencher can reduce the time a fluorophore spends in the excited state, increasing its photostability under continuous irradiation. Consequently, the photo-bleaching step length would increase, providing a measure for the accessibility of the fluorophore to the solvent. We demonstrate the method by measuring the bleaching step-length increase in a lipid, and also in a lipid-anchored peptide (both labelled with rhodamine-B and attached to supported lipid bilayers). The fluorophores in both molecules are expected to be solvent-exposed. They show a near two-fold increase in the step length upon incubation with 5 mM tryptophan (a quencher of rhodamine-B), validating our approach. A population distribution plot of step lengths before and after addition of tryptophan show that the increase is not always homogenous. Indeed there are different species present with differential levels of exposure. We then apply this technique to determine the solvent exposure of membrane-attached N-terminus labelled amylin (h-IAPP, an amyloid associated with Type II diabetes) whose interaction with lipid bilayers is poorly understood. hIAPP shows a much smaller increase of the step length, signifying a lower level of solvent exposure of its N-terminus. Analysis of results from individual molecules and step length distribution reveal that there are at least two different conformers of amylin in the lipid bilayer. Our results show that our method (“Q-SLIP”, Quenching-induced Step Length increase in Photobleaching) provides a simple route to probe the conformational states of membrane proteins at a single molecule level.

Illuminating amyloid fibrils: Fluorescence-based single-molecule approaches

The aggregation of proteins into insoluble filamentous amyloid fibrils is a pathological hallmark of neurodegenerative diseases that include Parkinson's disease and Alzheimer's disease. Since the identification of amyloid fibrils and their association with disease, there has been much work to describe the process by which fibrils form and interact with other proteins. However, due to the dynamic nature of fibril formation and the transient and heterogeneous nature of the intermediates produced, it can be challenging to examine these processes using techniques that rely on traditional ensemble-based measurements. Single-molecule approaches overcome these limitations as rare and short-lived species within a population can be individually studied. Fluorescence-based single-molecule methods have proven to be particularly useful for the study of amyloid fibril formation. In this review, we discuss the use of different experimental single-molecule fluorescence microscopy approaches to study amyloid fibrils and their interaction with other proteins, in particular molecular chaperones. We highlight the mechanistic insights these single-molecule techniques have already provided in our understanding of how fibrils form, and comment on their potential future use in studying amyloid fibrils and their intermediates.

Single Molecule Characterization of Amyloid Oligomers

The misfolding and aggregation of polypeptide chains into β-sheet-rich amyloid fibrils is associated with a wide range of neurodegenerative diseases. Growing evidence indicates that the oligomeric intermediates populated in the early stages of amyloid formation rather than the mature fibrils are responsible for the cytotoxicity and pathology and are potentially therapeutic targets. However, due to the low-populated, transient, and heterogeneous nature of amyloid oligomers, they are hard to characterize by conventional bulk methods. The development of single molecule approaches provides a powerful toolkit for investigating these oligomeric intermediates as well as the complex process of amyloid aggregation at molecular resolution. In this review, we present an overview of recent progress in characterizing the oligomerization of amyloid proteins by single molecule fluorescence techniques, including single-molecule Förster resonance energy transfer (smFRET), fluorescence correlation spectroscopy (FCS), single-molecule photobleaching and super-resolution optical imaging. We discuss how these techniques have been applied to investigate the different aspects of amyloid oligomers and facilitate understanding of the mechanism of amyloid aggregation.