Tip-enhanced Raman spectroscopy of amyloid β at neuronal spines

Unveiling brain disorders using liquid biopsy and Raman spectroscopy

Brain disorders, including neurodegenerative diseases (NDs) and traumatic brain injury (TBI), present significant challenges in early diagnosis and intervention. Conventional imaging modalities, while valuable, lack the molecular specificity necessary for precise disease characterization. Compared to the study of conventional brain tissues, liquid biopsy, which focuses on blood, tear, saliva, and cerebrospinal fluid (CSF), also unveils a myriad of underlying molecular processes, providing abundant predictive clinical information. In addition, liquid biopsy is minimally- to non-invasive, and highly repeatable, offering the potential for continuous monitoring. Raman spectroscopy (RS), with its ability to provide rich molecular information and cost-effectiveness, holds great potential for transformative advancements in early detection and understanding the biochemical changes associated with NDs and TBI. Recent developments in Raman enhancement technologies and advanced data analysis methods have enhanced the applicability of RS in probing the intricate molecular signatures within biological fluids, offering new insights into disease pathology. This review explores the growing role of RS as a promising and emerging tool for disease diagnosis in brain disorders, particularly through the analysis of liquid biopsy. It discusses the current landscape and future prospects of RS in the diagnosis of brain disorders, highlighting its potential as a non-invasive and molecularly specific diagnostic tool.

Gold Nanoparticle-Decorated Catalytic Micromotor-Based Aptassay for Rapid Electrochemical Label-Free Amyloid-β42 Oligomer Determination in Clinical Samples from Alzheimer's Patients

Micromotor (MM) technology offers a valuable and smart on-the-move biosensing microscale approach in clinical settings where sample availability is scarce in the case of Alzheimer's disease (AD). Soluble amyloid-β protein oligomers (AβO) (mainly AβO42) that circulate in biological fluids have been recognized as a molecular biomarker and therapeutic target of AD due to their high toxicity, and they are correlated much more strongly with AD compared to the insoluble Aβ monomers. A graphene oxide (GO)-gold nanoparticles (AuNPs)/nickel (Ni)/platinum nanoparticles (PtNPs) micromotors (MMGO-AuNPs)-based electrochemical label-free aptassay is proposed for sensitive, accurate, and rapid determination of AβO42 in complex clinical samples such as brain tissue, cerebrospinal fluid (CSF), and plasma from AD patients. An approach that implies the in situ formation of AuNPs on the GO external layer of tubular MM in only one step during MM electrosynthesis was performed (MMGO-AuNPs). The AβO42 specific thiolated-aptamer (AptAβO42) was immobilized in the MMGO-AuNPs via Au-S interaction, allowing for the selective recognition of the AβO42 (MMGO-AuNPs-AptAβO42-AβO42). AuNPs were smartly used not only to covalently bind a specific thiolated-aptamer for the design of a label-free electrochemical aptassay but also to improve the final MM propulsion performance due to their catalytic activity (approximately 2.0× speed). This on-the-move bioplatform provided a fast (5 min), selective, precise (RSD < 8%), and accurate quantification of AβO42 (recoveries 94-102%) with excellent sensitivity (LOD = 0.10 pg mL-1) and wide linear range (0.5-500 pg mL-1) in ultralow volumes of the clinical sample of AD patients (5 μL), without any dilution. Remarkably, our MM-based bioplatform demonstrated the competitiveness for the determination of AβO42 in the target samples against the dot blot analysis, which requires more than 14 h to provide qualitative results only. It is also important to highlight its applicability to the potential analysis of liquid biopsies as plasma and CSF samples, improving the reliability of the diagnosis given the heterogeneity and temporal complexity of neurodegenerative diseases. The excellent results obtained demonstrate the analytical potency of our approach as a future tool for clinical/POCT (Point-of-care testing) routine scenarios.

Silver nanoparticle-based SERS sensors for sensitive detection of amyloid-β aggregates in biological fluids

One of the hallmarks of Alzheimer's disease (AD) pathogenesis is the production, aggregation, and deposition of amyloid-β (Aβ) peptide. Surface-enhanced Raman spectroscopy (SERS) is a promising analytical technique capable of providing valuable information on chemical composition and molecule conformations in biological samples. However, one of the main challenges for introducing the SERS technique into the practice is preparation of scalable and at the same time stable nanostructured sensors with uniform spatial distribution of nanoparticles. Herein, we propose SERS platforms for reproducible, sensitive, label-free quantification of amyloid-β aggregates for short-wavelength - 532 and 633 nm - lasers. A SERS sensor - based on silver nanoparticles immobilized into a chitosan film (AgNP/CS) - provided a uniform distribution of AgNPs from a colloidal suspension across the SERS sensor, resulting in nanomolar limits of detection (LODs) for Aβ42 aggregates with a portable 532 nm laser. The laser-induced deposition was used to obtain denser periodic plasmonic sensors (AgNP/LID) with a uniform nanoparticle distribution. The AgNP/LID SERS sensor allowed for 15 pM LOD for Aβ42 aggregates with 633 nm laser. Notably, both nanostructured substrates allowed to distinguish amyloid aggregates from monomers. Therefore, our approach demonstrated applicability of SERS for detection of macromolecular volumetric objects as amyloid-β aggregates for fundamental biological studies as well as for "point-of-care" diagnostics and screening for early stages of neurodegenerative diseases.

A review: Exploring the metabolic and structural characterisation of beta pleated amyloid fibril in human tissue using Raman spectrometry and SAXS

Amyloidosis is a deleterious condition caused by abnormal amyloid fibril build-up in living tissues. To date, 42 proteins that are linked to amyloid fibrils have been discovered. Amyloid fibril structure variation can affect the severity, progression rate, or clinical symptoms of amyloidosis. Since amyloid fibril build-up is the primary pathological basis for various neurodegenerative illnesses, characterization of these deadly proteins, particularly utilising optical techniques have been a focus. Spectroscopy techniques provide significant non-invasive platforms for the investigation of the structure and conformation of amyloid fibrils, offering a wide spectrum of analyses ranging from nanometric to micrometric size scales. Even though this area of study has been intensively explored, there still remain aspects of amyloid fibrillization that are not fully known, a matter hindering progress in treating and curing amyloidosis. This review aims to provide recent updates and comprehensive information on optical techniques for metabolic and proteomic characterization of β-pleated amyloid fibrils found in human tissue with thorough literature analysis of publications. Raman spectroscopy and SAXS are well established experimental methods for study of structural properties of biomaterials. With suitable models, they offer extended information for valid proteomic analysis under physiologically relevant conditions. This review points to evidence that despite limitations, these techniques are able to provide for the necessary output and proteomics indication in order to extrapolate the aetiology of amyloid fibrils for reliable diagnostic purposes. Our metabolic database may also contribute to elucidating the nature and function of the amyloid proteome in development and clearance of amyloid diseases.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Mn-Containing Bioactive Glass-Ceramics: BMP-2-Mimetic Peptide Covalent Grafting Boosts Human-Osteoblast Proliferation and Mineral Deposition

The addition of Mn in bioceramic formulation is gaining interest in the field of bone implants. Mn activates human osteoblast (h-osteoblast) integrins, enhancing cell proliferation with a dose-dependent effect, whereas Mn-enriched glasses induce inhibition of Gram-negative or Gram-positive bacteria and fungi. In an effort to further optimize Mn-containing scaffolds' beneficial interaction with h-osteoblasts, a selective and specific covalent functionalization with a bioactive peptide was carried out. The anchoring of a peptide, mapped on the BMP-2 wrist epitope, to the scaffold was performed by a reaction between an aldehyde group of the peptide and the aminic groups of silanized Mn-containing bioceramic. SEM-EDX, FT-IR, and Raman studies confirmed the presence of the peptide grafted onto the scaffold. In in vitro assays, a significant improvement in h-osteoblast proliferation, gene expression, and calcium salt deposition after 7 days was detected in the functionalized Mn-containing bioceramic compared to the controls.

A unifying framework for amyloid-mediated membrane damage: The lipid-chaperone hypothesis

Over the past thirty years, researchers have highlighted the role played by a class of proteins or polypeptides that forms pathogenic amyloid aggregates in vivo, including i) the amyloid Aβ peptide, which is known to form senile plaques in Alzheimer's disease; ii) α-synuclein, responsible for Lewy body formation in Parkinson's disease and iii) IAPP, which is the protein component of type 2 diabetes-associated islet amyloids. These proteins, known as intrinsically disordered proteins (IDPs), are present as highly dynamic conformational ensembles. IDPs can partially (mis) fold into (dys) functional conformations and accumulate as amyloid aggregates upon interaction with other cytosolic partners such as proteins or lipid membranes. In addition, an increasing number of reports link the toxicity of amyloid proteins to their harmful effects on membrane integrity. Still, the molecular mechanism underlying the amyloidogenic proteins transfer from the aqueous environment to the hydrocarbon core of the membrane is poorly understood. This review starts with a historical overview of the toxicity models of amyloidogenic proteins to contextualize the more recent lipid-chaperone hypothesis. Then, we report the early molecular-level events in the aggregation and ion-channel pore formation of Aβ, IAPP, and α-synuclein interacting with model membranes, emphasizing the complexity of these processes due to their different spatial-temporal resolutions. Next, we underline the need for a combined experimental and computational approach, focusing on the strengths and weaknesses of the most commonly used techniques. Finally, the last two chapters highlight the crucial role of lipid-protein complexes as molecular switches among ion-channel-like formation, detergent-like, and fibril formation mechanisms and their implication in fighting amyloidogenic diseases.

Time-Evolved SERS Signatures of DEP-Trapped Aβ and Zn2+Aβ Peptides Revealed by a Sub-10 nm Electrode Nanogap

Alzheimer's disease (AD) has become highly relevant in aging societies, yet the fundamental molecular basis for AD is still poorly understood. New tools to study the undergoing structural conformation changes of amyloid beta (Aβ) peptides, the pathogenic hallmark of AD, could play a crucial role in the understanding of the underlying mechanisms of misfolding and cytotoxicity of this peptide. It has been recently reported that Zn²⁺ interacts with Aβ and changes its aggregation pathway away from less harmful fibrillar forms to more toxic species. Here, we present a versatile platform based on a set of sub-10 nm nanogap electrodes for the manipulation and sensing of biomolecules in the physiological condition at a low copy number, where molecules are trapped via dielectrophoresis (DEP) across the nanogap, which also serves as a surface-enhanced Raman spectroscopy hotspot. In this study, we demonstrate that our electrode nanogap platform can be used to study the structural difference between Aβ40 and ZnAβ40 peptides at different aggregation stages in the physiologically relevant concentration and in solution phase. The Raman spectroscopic signatures of the DEP-captured neuropeptides prove the device to be attractive as a label-free bioanalytical tool.

Spatial organization of protein aggregates on red blood cells as physical biomarkers of Alzheimer's disease pathology

Quantifying physical differences of protein aggregates implicated in Alzheimer’s disease (AD), in blood, could provide crucial information on disease stages. Here, red blood cells (RBCs) from 50 patients with neurocognitive complaints and 16 healthy individuals were profiled using an atomic force microscope (AFM). AFM measurements revealed patient age– and stage of neurocognitive disorder–dependent differences in size, shape, morphology, assembly, and prevalence of protein aggregates on RBCs, referred to as physical biomarkers. Crystals composed of fibrils were exclusively detected on RBCs for AD patients aged above 80 years. Fibril prevalence was negatively correlated with the cerebrospinal fluid (CSF) β-amyloid (Aβ) 42/40 ratio and was observed to be higher in the Aβ-positive patient category. Using a cutoff of ≥40% fibril prevalence, the CSF Aβ status was classified with 88% accuracy (sensitivity 100%, specificity 73%). The merits and challenges in integrating physical biomarkers in AD diagnosis are discussed.

The Progress of Label-Free Optical Imaging in Alzheimer's Disease Screening and Diagnosis

As the major neurodegenerative disease of dementia, Alzheimer's disease (AD) has caused an enormous social and economic burden on society. Currently, AD has neither clear pathogenesis nor effective treatments. Positron emission tomography (PET) and magnetic resonance imaging (MRI) have been verified as potential tools for diagnosing and monitoring Alzheimer's disease. However, the high costs, low spatial resolution, and long acquisition time limit their broad clinical utilization. The gold standard of AD diagnosis routinely used in research is imaging AD biomarkers with dyes or other reagents, which are unsuitable for in vivo studies owing to their potential toxicity and prolonged and costly process of the U.S. Food and Drug Administration (FDA) approval for human use. Furthermore, these exogenous reagents might bring unwarranted interference to mechanistic studies, causing unreliable results. Several label-free optical imaging techniques, such as infrared spectroscopic imaging (IRSI), Raman spectroscopic imaging (RSI), optical coherence tomography (OCT), autofluorescence imaging (AFI), optical harmonic generation imaging (OHGI), etc., have been developed to circumvent this issue and made it possible to offer an accurate and detailed analysis of AD biomarkers. In this review, we present the emerging label-free optical imaging techniques and their applications in AD, along with their potential and challenges in AD diagnosis.

Exploring interactions between lipids and amyloid-forming proteins: A review on applying fluorescence and NMR techniques

A hallmark of Alzheimer's, Parkinson's, and other amyloid diseases is the assembly of amyloid proteins into amyloid aggregates or fibrils. In many cases, the formation and cytotoxicity of amyloid assemblies are associated with their interaction with cell membranes. Despite studied for many years, the characterization of the interaction is challenged for reasons on the multiple aggregation states of amyloid-forming proteins, transient and weak interactions in the complex system. Although several strategies such as computation biology, spectroscopy, and imaging methods have been performed, there is an urgent need to detail the molecular mechanism in different time scales and high resolutions. This review highlighted the recent applications of fluorescence, solution and solid-state NMR in exploring the interactions between amyloid protein and membranes attributing to their advantages of high sensitivity and atomic resolution.

Surface-enhanced Raman scattering sensing platform for detecting amyloid-β peptide interaction with an aggregation inhibitor

Soluble, small amyloid-β oligomers (AβO) are recognized as significant contributors to the pathology of Alzheimer's disease (AD). Although drugs for treating AD symptoms have been approved, no therapy targeting amyloid-β (Aβ) capable of modifying the course of the disease is available. In an effort to develop a label-free method for screening new anti-AD therapeutic agents, we show the use of a surface-enhanced Raman scattering (SERS) active substrate for detecting the interactions between Aβ peptides and spin-labeled fluorine (SLF), a peptide aggregation inhibitor. Changes in the peak positions and intensity ratios of two spectral peaks near 1600cm^-1 and 2900cm^-1 can be used to monitor the molecular interactions between SLF and Aβ. This study demonstrates the potential of SERS spectroscopy for rapidly screening and identifying new anti-Aβ therapeutic agents.

A label-free reusable aptasensor for Alzheimer's disease

To early effectively detect amyloid-beta (Aβ) oligomers, a label-free reusable aptasensor was designed. This aptasensor based on a luminescent nanoscale lanthanum-based metal-organic framework (L-MOF)-armored single-stranded DNA antibody (MOF-armored-anti-DNA antibody) as signal tags and aptamer bound to magnetic beads (Apt-MB) as capture probe. The reusable aptasensor combines signal tag and capture probe with antigen-antibody interaction. When the reusable aptasensor is formed, the strong fluorescence intensity of L-MOF will "turn off" by photo-induced electron transfer from excited states to an unfilled d shell of iron cations on the nanoparticle surface. Upon the presence of Aβ oligomers in serum samples, they can be especially distinguished with the Aβ oligomers aptamer in capture probes and then signal tags are released into the solution for developing the fluorescence aptasensor under excitation/emission 365 nm/430 nm. Meanwhile, the aptamer was recovered from the complex of Aβ oligomers/Apt-MB by heat treatment. When the temperature returns to room temperature, the recovered aptamer in the capture probe can once again bound to the MOF-armored-anti-DNA antibody for reuse. The label-free reusable aptasensor system detection has high sensitivity and selectivity toward Aβ oligomers (LOD = 0.4 pg/mL) and an excellent linear range (0.001-100 ng/mL). This strategy is a fruitful step for the development of reusable aptasensor and may turn on new avenues for the applications of Aβ oligomer detection in clinical diagnosis.Graphical abstract.

Identifying Fibrillization State of Aβ Protein via Near-Field THz Conductance Measurement

Progressive Alzheimer's disease is correlated with the oligomerization and fibrillization of the amyloid beta (Aβ) protein. We identify the fibrillization stage of the Aβ protein through label-free near-field THz conductance measurements in a buffer solution. Frequency-dependent conductance was obtained by measuring the differential transmittance of the time-domain spectroscopy in the THz range with a molar concentration of monomer, oligomer, and fibrillar forms of the Aβ protein. Conductance at the lower frequency limit was observed to be high in monomers, reduced in oligomers, and dropped to an insulating state in fibrils and increased proportionally with the Aβ protein concentration. The monotonic decrease in the conductance at low frequency was dominated by a simple Drude component in the monomer with concentration and nonlinear conductance behaviors in the oligomer and fibril. By extracting the structural localization parameter, a dimensionless constant, with the modified Drude-Smith model, we defined a dementia quotient (DQ) value (0 < D_e < 1) as a discrete metric for a various Aβ proteins at a low concentration of 0.1 μmol/L; DQ = 1.0 ± 0.002 (fibril by full localization, mainly by Smith component), DQ = 0.64 ± 0.013 (oligomer by intermixed localization), and DQ = 0.0 ± 0.000 (monomer by Drude component). DQ values were discretely preserved independent of the molar concentration or buffer variation. This provides plenty of room for the label-free diagnosis of Alzheimer's disease using the near-field THz conductance measurement.

Infrared and Raman chemical imaging and spectroscopy at the nanoscale

The advent of nanotechnology, and the need to understand the chemical composition at the nanoscale, has stimulated the convergence of IR and Raman spectroscopy with scanning probe methods, resulting in new nanospectroscopy paradigms. Here we review two such methods, namely photothermal induced resonance (PTIR), also known as AFM-IR and tip-enhanced Raman spectroscopy (TERS). AFM-IR and TERS fundamentals will be reviewed in detail together with their recent crucial advances. The most recent applications, now spanning across materials science, nanotechnology, biology, medicine, geology, optics, catalysis, art conservation and other fields are also discussed. Even though AFM-IR and TERS have developed independently and have initially targeted different applications, rapid innovation in the last 5 years has pushed the performance of these, in principle spectroscopically complimentary, techniques well beyond initial expectations, thus opening new opportunities for their convergence. Therefore, subtle differences and complementarity will be highlighted together with emerging trends and opportunities.

Raman spectroscopy and neuroscience: from fundamental understanding to disease diagnostics and imaging

Neuroscience would directly benefit from more effective detection techniques, leading to earlier diagnosis of disease. The specificity of Raman spectroscopy is unparalleled, given that a molecular fingerprint is attained for each species. It also allows for label-free detection with relatively inexpensive instrumentation, minimal sample preparation, and rapid sample analysis. This review summarizes Raman spectroscopy-based techniques that have been used to advance the field of neuroscience in recent years.

Noncontact tip-enhanced Raman spectroscopy for nanomaterials and biomedical applications

Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical techniques for probing vibrational states with nanoscale resolution. While TERS may be a source of unique information about chemical structure and interactions, it has a limited use for materials with rough or sticky surfaces. Development of the TERS approach utilizing a non-contact scanning probe microscopy mode can significantly extend the number of applications. Here we demonstrate a proof of the concept and feasibility of a non-contact TERS approach and test it on various materials. Our experiments show that non-contact TERS can provide 10 nm spatial resolution and a Raman signal enhancement factor of 10⁵, making it very promising for chemical imaging of materials with high aspect ratio surface patterns and biomaterials.

Double-edged effects of aluminium ions on amyloid fibrillation of hen egg-white lysozyme

Elucidating the effects of Al(III) ions on amyloid fibrillation is important to understand the association between metal ions and Alzheimer's disease. Here, Raman spectroscopy was applied to investigate amyloid fibrillation of hen egg-white lysozymes during thermal incubation with Al(III) ions or acids, combined with atomic force microscopy and thioflavin T fluorescence assays. Kinetics of conformational changes in lysozymes were assessed by monitoring six characteristic Raman spectral markers. The peak of Phe residues at 1003 cm^-1 and two bands of Trp residues at 759 cm^-1 and 1340-1360 cm^-1 corresponded to the lysozyme tertiary structure, whereas two NC_αC stretching vibrations at 899 cm^-1 and 935 cm^-1 and an amide I band were associated with the lysozyme skeleton. There may be a four-stage transformation mechanism underlying the kinetics of amyloid fibrillation of lysozymes with the thermal/Al(III) treatment. Comparison of kinetics under thermal/Al(III) and thermal/acid conditions revealed double-edged roles of Al(III) ions in amyloid fibrillation of lysozymes. Specifically, in addition to postponing α-helix degradation, Al(III) ions accelerated conformational transformations from α-helices to organized β-sheets. The present investigation sheds light on the controversial effects of Al(III) ions on amyloid fibrillation of lysozymes.

Advances in surface-enhanced Raman spectroscopy (SERS) substrates for lipid and protein characterization: sensing and beyond

Surface-enhanced Raman spectroscopy (SERS) has become an essential ultrasensitive analytical tool for biomolecular analysis of small molecules, macromolecular proteins, and even cells. SERS enables label-free, direct detection of molecules through their intrinsic Raman fingerprint. In particular, protein and lipid bilayers are dynamic three-dimensional structures that necessitate label-free methods of characterization. Beyond direct detection and quantitation, the structural information contained in SERS spectra also enables deeper biophysical characterization of biomolecules near metallic surfaces. Therefore, SERS offers enormous potential for such systems, although making measurements in a nonperturbative manner that captures the full range of interactions and activity remains a challenge. Many of these challenges have been overcome through advances in SERS substrate development, which have expanded the applications and targets of SERS for direct biomolecular quantitation and biophysical characterization. In this review, we will first discuss different categories of SERS substrates including solution-phase, solid-supported, tip-enhanced Raman spectroscopy (TERS), and single-molecule substrates for biomolecular analysis. We then discuss detection of protein and biological lipid membranes. Lastly, biophysical insights into proteins, lipids and live cells gained through SERS measurements of these systems are reviewed.

PIP2 Phospholipid-Induced Aggregation of Tau Filaments Probed by Tip-Enhanced Raman Spectroscopy

The morphology and secondary structure of peptide fibers formed by aggregation of tubulin-associated unit (Tau) fragments (K18), in the presence of the inner cytoplasmic membrane phosphatidylinositol component (PIP₂ ) or heparin sodium (HS) as cofactors, are determined with nanoscale (<10 nm) spatial resolution. By means of tip-enhanced Raman spectroscopy (TERS), the inclusion of PIP₂ lipids in fibers is determined based on the observation of specific C=O ester vibration modes. Moreover, analysis of amide I and amide III bands suggests that the parallel β-sheet secondary structure content is lower and the random coil content is higher for fibers grown from the PIP₂ cofactor instead of HS. These observations highlight the occurrence of some local structural differences between these fibers. This study constitutes the first nanoscale structural characterization of Tau/phospholipid aggregates, which are implicated in deleterious mechanisms on neural membranes in Alzheimer's disease.

Nanoscale Discrimination between Toxic and Nontoxic Protein Misfolded Oligomers with Tip-Enhanced Raman Spectroscopy

Highly toxic protein misfolded oligomers associated with neurological disorders such as Alzheimer's and Parkinson's diseases are nowadays considered primarily responsible for promoting synaptic failure and neuronal death. Unraveling the relationship between structure and neurotoxicity of protein oligomers appears pivotal in understanding the causes of the pathological process, as well as in designing novel diagnostic and therapeutic strategies tuned toward the earliest and presymptomatic stages of the disease. Here, it is benefited from tip-enhanced Raman spectroscopy (TERS) as a surface-sensitive tool with spatial resolution on the nanoscale, to inspect the spatial organization and surface character of individual protein oligomers from two samples formed by the same polypeptide sequence and different toxicity levels. TERS provides direct assignment of specific amino acid residues that are exposed to a large extent on the surface of toxic species and buried in nontoxic oligomers. These residues, thanks to their outward disposition, might represent structural factors driving the pathogenic behavior exhibited by protein misfolded oligomers, including affecting cell membrane integrity and specific signaling pathways in neurodegenerative conditions.

Ascent of atomic force microscopy as a nanoanalytical tool for exosomes and other extracellular vesicles

Over the last 30 years, atomic force microscopy (AFM) has made several significant contributions to the field of biology and medicine. In this review, we draw our attention to the recent applications and promise of AFM as a high-resolution imaging and force sensing technology for probing subcellular vesicles: exosomes and other extracellular vesicles. Exosomes are naturally occurring nanoparticles found in several body fluids such as blood, saliva, cerebrospinal fluid, amniotic fluid and urine. Exosomes mediate cell-cell communication, transport proteins and genetic content between distant cells, and are now known to play important roles in progression of diseases such as cancers, neurodegenerative disorders and infectious diseases. Because exosomes are smaller than 100 nm (about 30-120 nm), the structural and molecular characterization of these vesicles at the individual level has been challenging. AFM has revealed a new degree of complexity in these nanosized vesicles and generated growing interest as a nanoscale tool for characterizing the abundance, morphology, biomechanics, and biomolecular make-up of exosomes. With the recent interest in exosomes for diagnostic and therapeutic applications, AFM-based characterization promises to contribute towards improved understanding of these particles at the single vesicle and sub-vesicular levels. When coupled with complementary methods like optical super resolution STED and Raman, AFM could further unlock the potential of exosomes as disease biomarkers and as therapeutic agents.