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Method Article
This paper proposes a new generation of multiparametric analytical platforms with increased throughput for the characterization of extracellular vesicle subsets. The method is based on a combination of multiplexed biosensing methods with metrological and morphomechanical analyses by atomic force microscopy, coupled with Raman spectroscopy, to qualify vesicular targets trapped on a microarray biochip.
Extracellular vesicles (EVs) are membrane-derived, tiny vesicles produced by all cells that range from 50 to several hundreds of nanometers in diameter and are used as a means of intercellular communication. They are emerging as promising diagnostic and therapeutic tools for a variety of diseases. There are two main biogenesis processes used by cells to produce EVs with differences in size, composition, and content. Due to their high complexity in size, composition, and cell origin, their characterization requires a combination of analytical techniques. This project involves the development of a new generation of multiparametric analytical platforms with increased throughput for the characterization of subpopulations of EVs. To achieve this goal, the work starts from the nanobioanalytical platform (NBA) established by the group, which allows an original investigation of EVs based on a combination of multiplexed biosensing methods with metrological and morphomechanical analyses by atomic force microscopy (AFM) of vesicular targets trapped on a microarray biochip. The objective was to complete this EV investigation with a phenotypic and molecular analysis by Raman spectroscopy. These developments enable the proposal of a multimodal and easy-to-use analytical solution for the discrimination of EV subsets in biological fluids with clinical potential
The growing interest in EV research in diagnosis and in therapeutics1,2,3,4,5, combined with the challenges this field faces, has resulted in the development and implementation of a large variety of approaches and techniques for quantifying or characterizing these vesicles. The most widely used methods for EV identification are protein-specific immunoblotting and proteomics to confirm the origin of EVs, transmission electron microscopy (TEM) to confirm their structure, and nanoparticle tracking analysis (NTA) to quantify their number and size distribution in a sample volume.
None of these techniques on their own, however, give all the information required to characterize EV subsets. The inherent heterogeneity of EVs due to the diversity in their biochemical and physical properties impedes global analyses that are reliable and reproducible, especially for EVs contained in a mixture (crude sample). Detection and characterization methods are, therefore, needed for EVs, both individually and generally to complement other methods that are faster but not selective6.
High-resolution imaging by TEM (or cryoTEM) or AFM allows the determination of the morphology and metrology of EVs with a nanometric resolution7,8,9,10,11,12. However, the main limitation of the use of electron microscopy for biological objects, such as EVs, is the need for a vacuum to carry out the study which requires the fixation and dehydration of the sample. Such preparation makes it difficult to translate from the structures observed to the in-solution EV morphology. To avoid this dehydration of the sample, the technique of cryoTEM is the most suitable for EV characterization13. It is widely used for determining the ultrastructure of EVs. The immunolabeling of vesicles by biofunctionalized gold nanoparticles also makes it possible to identify specific subpopulations of EVs and distinguish them from other particles present in a complex biological sample. However, due to the low number of EVs analyzed by electronic microscopy, it is often difficult to perform a characterization that is representative of a complex and heterogeneous sample.
To reveal this size heterogeneity, the International Society for Extracellular Vesicles (ISEV) suggests analyzing a sufficient number of widefield images, accompanied by smaller images, to reveal individual EVs with high resolution14. AFM is an alternative to optical approaches and electronic diffraction techniques for the study of EVs. This technique uses a sharp tip held by a flexible cantilever that scans the sample deposited on one support, line by line, and adjusts the distance between the tip and the elements present through a feedback loop. This makes it possible to characterize the topography of the sample and collect morphomechanical information15,16,17,18. The EVs can be scanned by AFM either after being deposited on an atomically flat substrate or after having been captured on a specific substrate functionalized by antibodies, peptides, or aptamers to characterize the various subpopulations18,19. Due to its ability to quantify and simultaneously probe the structure, biomechanics, and membranous biomolecular content of EVs within complex biological samples without the need for pretreatment, labeling, or dehydration, AFM is now increasingly used to characterize EVs in a fine and multiparametric manner under physiological conditions of temperature and medium.
This paper proposes a methodology using a core gold biochip capable of being (bio)chemically functionalized in a multiplexed format. This substrate is the cornerstone of a powerful analytical platform combining the biodetection of EV subsets by surface plasmon resonance, and once the EVs are adsorbed/grafted or immunocaptured on the chip, AFM enables the metrological and morphomechanical characterization of the EVs. Coupled with the Raman signature of the EV subsets captured on the chip, this analytical platform enables the qualification of the EVs present in biological samples in a label-free manner and without any need for preanalytical steps. This paper shows that the combination of powerful techniques, assisted by a highly rigorous methodology in substrate preparation and data acquisition, makes the EV analysis deep, definitive, and robust.
The principle of the proposed approach is to prepare a gold substrate, to adsorb/graft or capture the EV subtypes, and to scan them by AFM to estimate the size and morphology of each EV subset. Additionally, those adsorbed EVs are analyzed by Raman spectroscopy. This substrate can, indeed, present three types of interfaces of growing complexity: naked, chemically functionalized, or ligand microarrays. Before describing the different steps of the protocol, readers are referred to the schematic presentation of the nanobioanalytical platform (NBA) approach in Figure 1, combining surface plasmon resonance imaging (SPRi), AFM, and spectroscopy.
Figure 1: The NanoBioAnalytical platform. The approach combines (A) surface plasmon resonance imaging, (B) atomic force microscopy, and infrared/Raman (nano)spectroscopy, all engaged on the same substrate-a multiplexed gold chip. Abbreviations: NBA = NanoBioAnalytical platform; SPRi =surface plasmon resonance imaging; AFM = atomic force microscopy; EV = extracellular vesicle. Please click here to view a larger version of this figure.
The core gold biochip constitutes the heart of the platform since all the label-free characterization techniques are conducted on this biochip. According to the needs of the EV characterization (either global/total EVs or EV subsets) and the limitations/demands of the methods used, three types of gold biochip surfaces have been developed: either "naked," chemically functionalized "C11/C16," or ligand-biofunctionalized, called "ligand" gold surface.
The naked biochip, called "naked," enables the simple adsorption of EVs on gold. It is possible to select the buffer used and realize this adsorption either in a passive way (incubation and then rinsing steps) or to monitor it under flow (in SPRi). Moreover, this passive adsorption can be realized either on the whole chip (as a macroarray) or localized in microarrays using a micropipette spotter. The "under flow procedure" allows investigators to follow the kinetics and the level of EV adsorption. This approach on the naked gold substrate is adopted when the chemical layer interface may interfere with the analytical method (e.g., for Raman spectroscopy).
The chemically functionalized biochip, called "C11/C16," is used to create a dense and robust "carpet" of EVs covalently bound on the gold surface by forming primary amide bonds with the thiolates when the objective is to have a global view of the EV sample. Indeed, in this case, the gold is functionalized by a thiolate mixture of mercapto-1-undecanol (11-MUOH: "C11") and mercapto-1-hexadecanoic acid (16-MHA: "C16"), and a fraction of the thiolates are chemically activated to establish covalent binding with the targets. Again, this strategy can be realized either passively (incubation and then rinsing steps, either in "macroarray" or in multiple microarrays using a micropipette spotter) or under flow rates (in SPRi) to follow the kinetics and level of EV grafting on the gold surface.
The ligand-biofunctionalized biochip, called "ligands," is chemically activated to covalently graft different ligands (e.g., antibodies, receptors) to selectively capture (with affinity) different EV subsets that coexist in the biological sample.
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1. Gold substrate preparation
NOTE: Three types of surfaces are produced on gold chips: 1) naked surface, 2) chemically functionalized, and 3) biofunctionalized (ligands grafted on C11C16 layer). They will be called "naked," "C11C16," and "ligands," respectively, from this point onward.
Figure 2: Biochip and manual spotter. Gold biochip (left), micropipette spotter (middle), and the biochip after spotting with ligand droplets of 300 nL each (right). Please click here to view a larger version of this figure.
Figure 3: Preconcentration tests to determine the optimal pH for ligand grafting. The sensorgram presents the level of interaction as a function of time for one ligand injected randomly (at different pH values) at the same concentration over 2 min on the surface. OG is the detergent, which allows the baseline to be recovered between each injection. Here, the sensorgram indicates that pH 6 allowed the most ligand grafting, with a SPRi signal of 1091 RU. Abbreviations: OG = octyl glucoside; RU = response unit. Please click here to view a larger version of this figure.
2. Surface plasmon resonance imaging
Figure 4: SPRi CCD image of the biochip. (A,B) Multiplexed biochip after albumin passivation. (A) A chip with no default; (B) some defects that appeared on the chip: fusion of spots (i), weak grafting (ii), or dust or "contaminants" (iii). The ROIs, in color in the spots (one color per ligand family), were chosen to avoid those "contaminants". When spots merged, they were noted and either ignored or named as "mixture of ligands 1 and 2". (C) Naked gold chip without microarrays for the experiment examining the adsorption of EVs on gold. The blue arrow indicates the flow direction. This chip did not present spots, and the ROIs were chosen to register the reflectivity signal from line 1 (L1, red circles) to line 4 (L4, purple circles) during the sample injection. Scale bar = 1 mm for all three images. Abbreviations: SPRi = surface plasmon resonance imaging; CCD = charge-coupled device; ROIs = regions of interest; EVs = extracellular vesicles. Please click here to view a larger version of this figure.
Figure 5: SPRi experiments of EV injection onto a biochip. (A) Capture experiment on a multiplexed biochip showing the reflectivity signals of different ligands. Here, the signal-to-noise ratio for the different ligands was very good (and especially on the antiCD41 spots) since the response of the negative control was negligible. (B) Adsorption experiment of EVs on a naked biochip. Sensorgram presenting the conditioning of the chip with two flushes of buffer and OG cleaning (1), with the EV sample injection (2), and the reflectivity signal after EV interaction (3). On this biochip, there was no negative control, but the reflectivity signal (its kinetics, its stability after injection) was high, meaning that those EVs were able to adsorb and remain stable on the gold chip. Abbreviations: EV = extracellular vesicle; OG = octyl glucoside. Please click here to view a larger version of this figure.
3. Atomic force microscopy
Figure 6: Biochip characterization by AFM. After the SPRi experiment, the chip was either fixed and dried or maintained in liquid for AFM characterization. (A) The machined glass slide (with two perpendicular positioning wedges, indicated with a "w" on the picture) presenting a mask fitting with the localization of the 16 biochip microarrays. By light exposure and transparency, once installed for the AFM characterization, the glass slide enables the AFM tip to be placed on the desired spot to characterize it. (B) The biochip installed on the "mask" slide and under a drop of buffer to scan in liquid conditions. (C) SPRi image of the 16 microarrays. (D) One microarray imaged by optical microscopy after the immunocapture of biofunctionalized calibration nanoparticles of 920 nm in diameter. The white squares indicate the sampling of the different areas scanned by AFM in each spot of interest to make the AFM characterization robust. Scale bars = (C) 1 mm, (D) 500 µm. Abbreviations: AFM = atomic force microscopy; SPRi = surface plasmon resonance imaging. Please click here to view a larger version of this figure.
4. Raman spectroscopy
NOTE: For Raman spectroscopy, replace the glass slide used as substrate with a slide of CaF2, which has a negligible Raman signature.
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Determination of the optimum pH conditions for ligand grafting
The different ligands used to prepare the biochips are tested as a function of the pH and their availability to interact with the thiolate chemical layer (Figure 3). The ligands are diluted in acetate buffer at different pH values and injected on the biochip chemically functionalized with a C11C16 layer. The solutions are injected randomly on the surface, and a detergent (OG at 40 mM) is injected after each...
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The recent methods for EV identification that are the most widely used are protein-specific immunoblotting to confirm the origin of EVs, TEM to confirm their structure, and NTA to quantify their number and size distribution in a sample volume3. Nevertheless, the high interest in EVs in (bio)medical research and the limitations of existing analytical tools have prompted the scientific community to develop new methods for EV characterization, discrimination, and quantification.
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The authors have no conflicts of interest to disclose.
Kelly Aubertin and Fabien Picot from the IVETh core facility (Paris) are acknowledged for the Raman imaging experiments. Thierry Burnouf (Taipei Medical University, Taiwan) and Zuzana Krupova (From Helincourt, France) are acknowledged for providing the EV samples derived from blood platelet and bovine milk samples, respectively. The work was supported by the region Bourgogne Franche-Comté and the EUR EIPHI graduate school (NOVICE project, 2021-2024). Part of this work was done using the CLIPP platform and in RENATECH clean room facilities in FEMTO-ENGINEERING, for which we thank Rabah Zeggari.
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Name | Company | Catalog Number | Comments |
CD41a antibody | Diaclone SAS (France) | 447528 | |
CP920 | Microparticles GmbH, Germany | 448303 | |
DXR3xi | Thermo Fisher Scientific | T1502 | |
EDC | Sigma | A6272 | |
Ethanolamine | Sigma | P5368-10PAK | |
Evs derived from platelet concentrates | Collaboration : Pr T. Burnouf (TMU, Taipei) | S2889 | |
Evs from bovine milk | Collaboration : Dr Z. Krupova (Excilone, Helincourt - France) | 3450 | |
Glutaraldehyde | Sigma | 56845 | |
Gwyddion | 853.223.020 | ||
Magnetron sputtering | PLASSYS | SAB5300165 | |
mercapto-1-hexadecanoic acid | Sigma | G5882 | |
Mercapto-1-undecanol | Sigma | O8001 | |
Mountains SPIP ones | Digital Surf | ||
NanoWizard 3 Bioscience | Bruker-JPK | ||
Octyl Glucoside (OG) | Sigma | ||
Ovalbumine antibody | Sigma | ||
Phosphate Buffer Saline (PBS) | Sigma | ||
Rat Albumin Serum (RSA) | Sigma | ||
Sodium acetate buffer | Sigma | ||
SPR-Biacore 3000 | GE Healthcare/ Cytiva life sciences | ||
SPRi Biochip | MIMENTO technology platform | The biochips were produced in-house in the clean room, Besancon | |
SPRi Plex II | Horiba Scientific | ||
Sulfo-NHS | Sigma |
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