Optical Super-Resolution Imaging of β-Amyloid Aggregation In Vitro and In Vivo: Method and Techniques

A Fluorescent Sensor for Quantitative Super-resolution Imaging of Amyloid Fibril Assembly

Many soluble proteins can self-assemble into macromolecular structures called amyloids, a subset of which are implicated in a range of neurodegenerative disorders. The nanoscale size and structural heterogeneity of prefibrillar and early aggregates, as well as mature amyloid fibrils, pose significant challenges for the quantification of amyloid species, identification of their cellular interaction partners and for elucidation of the molecular basis for cytotoxicity. We report a fluorescent amyloid sensor AmyBlink-1 and its application in super-resolution imaging of amyloid structures. AmyBlink-1 exhibits a 5-fold increase in ratio of the green (thioflavin T) to red (Alexa Fluor 647) emission intensities upon interaction with amyloid fibrils. Using AmyBlink-1 , we performed nanoscale imaging of four different types of amyloid fibrils, achieving a resolution of ~30 nm. AmyBlink-1 enables nanoscale visualization and subsequent quantification of morphological features, such as the length and skew of individual amyloid aggregates formed at different times along the amyloid assembly pathway.

A Perspective on Biophysical Studies of Crystallin Aggregation and Implications for Cataract Formation

Lens crystallins are subject to various types of damage during their lifetime which triggers protein misfolding and aggregation, ultimately causing cataracts. There are several models for crystallin aggregation, but a comprehensive picture of the mechanism of cataract is still underway. The complex biomolecular interactions underlying crystallin aggregation have motivated major efforts to resolve the structural details and mechanism of aggregation using multiple biophysical techniques at different resolutions. Together, experimental and computational approaches identify and characterize both amyloidogenic and amorphous aggregates leading to an improved understanding of crystallin aggregation. A rigorous characterization of the aggregation-prone intermediates is crucial in cataract-mediated drug discovery. This Perspective summarizes recent biophysical studies on lens crystallin aggregation. We evaluate the outstanding challenges, future outlook, and rewards in this fertile field of research. With lessons learned from protein folding and multiple pathways of aggregation, we highlight the differences in the overall mechanisms of age-related and congenital cataracts. We expect that a correlation between the existing and developing biophysical techniques would provide a platform to study amyloid architecture in the eye lens and reduce the existing gaps in our understanding of crystallin biophysics.

Tissue Clearing and Expansion Methods for Imaging Brain Pathology in Neurodegeneration: From Circuits to Synapses and Beyond

Studying the structural alterations occurring during diseases of the nervous system requires imaging heterogeneous cell populations at the circuit, cellular and subcellular levels. Recent advancements in brain tissue clearing and expansion methods allow unprecedented detailed imaging of the nervous system through its entire scale, from circuits to synapses, including neurovascular and brain lymphatics elements. Here, we review the state-of-the-art of brain tissue clearing and expansion methods, mentioning their main advantages and limitations, and suggest their parallel implementation for circuits-to-synapses brain imaging using conventional (diffraction-limited) light microscopy -such as confocal, two-photon and light-sheet microscopy- to interrogate the cellular and molecular basis of neurodegenerative diseases. We discuss recent studies in which clearing and expansion methods have been successfully applied to study neuropathological processes in mouse models and postmortem human brain tissue. Volumetric imaging of cleared intact mouse brains and large human brain samples has allowed unbiased assessment of neuropathological hallmarks. In contrast, nanoscale imaging of expanded cells and brain tissue has been used to study the effect of protein aggregates on specific subcellular structures. Therefore, these approaches can be readily applied to study a wide range of brain processes and pathological mechanisms with cellular and subcellular resolution in a time- and cost-efficient manner. We consider that a broader implementation of these technologies is necessary to reveal the full landscape of cellular and molecular mechanisms underlying neurodegenerative diseases.

High-affinity interactions and signal transduction between Aβ oligomers and TREM2

Rare coding variants in the triggering receptor expressed on myeloid cells 2 (TREM2) are associated with increased risk for Alzheimer's disease (AD), but how they confer this risk remains uncertain. We assessed binding of TREM2, AD‐associated TREM2 variants to various forms of Aβ and APOE in multiple assays. TREM2 interacts directly with various forms of Aβ, with highest affinity interactions observed between TREM2 and soluble Aβ42 oligomers. High‐affinity binding of TREM2 to Aβ oligomers is characterized by very slow dissociation. Pre‐incubation with Aβ is shown to block the interaction of APOE. In cellular assays, AD‐associated variants of TREM2 reduced the amount of Aβ42 internalized, and in NFAT assay, the R47H and R62H variants decreased NFAT signaling activity in response to Aβ42. These studies demonstrate i) a high‐affinity interaction between TREM2 and Aβ oligomers that can block interaction with another TREM2 ligand and ii) that AD‐associated TREM2 variants bind Aβ with equivalent affinity but show loss of function in terms of signaling and Aβ internalization.

Super-resolution Imaging of Amyloid Structures over Extended Times by Using Transient Binding of Single Thioflavin T Molecules

Oligomeric amyloid structures are crucial therapeutic targets in Alzheimer's and other amyloid diseases. However, these oligomers are too small to be resolved by standard light microscopy. We have developed a simple and versatile tool to image amyloid structures by using thioflavin T without the need for covalent labeling or immunostaining. The dynamic binding of single dye molecules generates photon bursts that are used for fluorophore localization on a nanometer scale. Thus, photobleaching cannot degrade image quality, allowing for extended observation times. Super-resolution transient amyloid binding microscopy promises to directly image native amyloid by using standard probes and record amyloid dynamics over minutes to days. We imaged amyloid fibrils from multiple polypeptides, oligomeric, and fibrillar structures formed during different stages of amyloid-β aggregation, as well as the structural remodeling of amyloid-β fibrils by the compound epi-gallocatechin gallate.

Insights into Insulin Fibril Assembly at Physiological and Acidic pH and Related Amyloid Intrinsic Fluorescence

Human insulin is a widely used model protein for the study of amyloid formation as both associated to insulin injection amyloidosis in type II diabetes and highly prone to form amyloid fibrils in vitro. In this study, we aim to gain new structural insights into insulin fibril formation under two different aggregating conditions at neutral and acidic pH, using a combination of fluorescence, circular dichroism, Fourier-transform infrared spectroscopy, and transmission electron miscroscopy. We reveal that fibrils formed at neutral pH are morphologically different from those obtained at lower pH. Moreover, differences in FTIR spectra were also detected. In addition, only insulin fibrils formed at neutral pH showed the characteristic blue-green fluorescence generally associated to amyloid fibrils. So far, the molecular origin of this fluorescence phenomenon has not been clarified and different hypotheses have been proposed. In this respect, our data provide experimental evidence that allow identifying the molecular origin of such intrinsic property.

Probing supramolecular protein assembly using covalently attached fluorescent molecular rotors

Changes in microscopic viscosity and macromolecular crowding accompany the transition of proteins from their monomeric forms into highly organised fibrillar states. Previously, we have demonstrated that viscosity sensitive fluorophores termed 'molecular rotors', when freely mixed with monomers of interest, are able to report on changes in microrheology accompanying amyloid formation, and measured an increase in rigidity of approximately three orders of magnitude during aggregation of lysozyme and insulin. Here we extend this strategy by covalently attaching molecular rotors to several proteins capable of assembly into fibrils, namely lysozyme, fibrinogen and amyloid-β peptide (Aβ(1-42)). We demonstrate that upon covalent attachment the molecular rotors can successfully probe supramolecular assembly in vitro. Importantly, our new strategy has wider applications in cellulo and in vivo, since covalently attached molecular rotors can be successfully delivered in situ and will colocalise with the aggregating protein, for example inside live cells. This important advantage allowed us to follow the microscopic viscosity changes accompanying blood clotting and during Aβ(1-42) aggregation in live SH-SY5Y cells. Our results demonstrate that covalently attached molecular rotors are a widely applicable tool to study supramolecular protein assembly and can reveal microrheological features of aggregating protein systems both in vitro and in cellulo not observable through classical fluorescent probes operating in light switch mode.

STED imaging of tau filaments in Alzheimer's disease cortical grey matter

Alzheimer's disease (AD) involves the propagation of filaments of tau protein throughout the cerebral cortex. Imaging tau filaments and oligomers in human brain at high resolution would help contribute insight into the mechanism and progression of tauopathic diseases. STED microscopy is a nano-scale imaging technique and we aimed to test the abilities of this method for resolving tau structures within human brain. Using autopsied 50μm AD brain sections, we demonstrate that STED microscopy can resolve immunolabelled tau filaments at 77nm resolution. Ribbon-like tau filaments imaged by STED appeared smooth along their axis with limited axial undulations. STED also resolved 70-80nm wide tau puncta. Of the fluorophores tested, STAR635p was optimal for STED imaging in this tissue. This was in part due to brain tissue autofluorescence within the lower wavelength ranges (488-590nm). Further, the stability and minimal photobleaching of STAR635p allowed STED z-stacks of neurons packed with tau filaments (neurofibrillary tangles) to be collated. There was no loss of x-y image resolution of individual tau filaments through the 20μm z-stack. This demonstrates that STED can contribute to nano-scale analysis and characterisation of pathologies within banked human autopsied brain tissue. Resolving tau structures at this level of resolution provides promising avenues for understanding mechanisms of pathology propagation in the different tauopathies as well as illuminating what contributes to disease heterogeneity.

From single-molecule spectroscopy to super-resolution imaging of the neuron: a review

For more than 20 years, single-molecule spectroscopy has been providing invaluable insights into nature at the molecular level. The field has received a powerful boost with the development of the technique into super-resolution imaging methods, ca. 10 years ago, which overcome the limitations imposed by optical diffraction. Today, single molecule super-resolution imaging is routinely used in the study of macromolecular function and structure in the cell. Concomitantly, computational methods have been developed that provide information on numbers and positions of molecules at the nanometer-scale. In this overview, we outline the technical developments that have led to the emergence of localization microscopy techniques from single-molecule spectroscopy. We then provide a comprehensive review on the application of the technique in the field of neuroscience research.

A Guide to Structured Illumination TIRF Microscopy at High Speed with Multiple Colors

Optical super-resolution imaging with structured illumination microscopy (SIM) is a key technology for the visualization of processes at the molecular level in the chemical and biomedical sciences. Although commercial SIM systems are available, systems that are custom designed in the laboratory can outperform commercial systems, the latter typically designed for ease of use and general purpose applications, both in terms of imaging fidelity and speed. This article presents an in-depth guide to building a SIM system that uses total internal reflection (TIR) illumination and is capable of imaging at up to 10 Hz in three colors at a resolution reaching 100 nm. Due to the combination of SIM and TIRF, the system provides better image contrast than rival technologies. To achieve these specifications, several optical elements are used to enable automated control over the polarization state and spatial structure of the illumination light for all available excitation wavelengths. Full details on hardware implementation and control are given to achieve synchronization between excitation light pattern generation, wavelength, polarization state, and camera control with an emphasis on achieving maximum acquisition frame rate. A step-by-step protocol for system alignment and calibration is presented and the achievable resolution improvement is validated on ideal test samples. The capability for video-rate super-resolution imaging is demonstrated with living cells.

Construction of specific magnetic resonance imaging/optical dual-modality molecular probe used for imaging angiogenesis of gastric cancer

The purpose of the study was to construct specific magnetic resonance imaging (MRI)/optical dual-modality molecular probe. Tumor-bearing animal models were established. MRI/optical dual-modality molecular probe was construed by coupling polyethylene glycol (PEG)-modified nano-Fe₃O₄ with specific targeted cyclopeptide GX1 and near-infrared fluorescent dyes Cy5.5. MRI/optical imaging effects of the probe were observed and the feasibility of in vivo double-modality imaging was discussed. It was found that, the double-modality probe was of high stability; tumor signal of the experimental group tended to be weak after injection of the probe, but rose to a level which was close to the previous level after 18 h (p > 0.05). We successively completed the construction of an ideal MRI/optical dual-modality molecular probe. MRI/optical dual-modality molecular probe which can selectively gather in gastric cancer is expected to be a novel probe used for diagnosing gastric cancer in the early stage.

Estimation of the lag time in a subsequent monomer addition model for fibril elongation

The lag time for dock–lock fibril elongation can be estimated from kinetic parameters.