Exosome Research: A Detailed Overview for Reference
Exosome Research: A Detailed Overview for Reference
I. Introduction to Exosomes
In recent years, exosomes, a class of small extracellular vesicles, have attracted considerable attention in biomedical research. Exosomes are typically described as membrane-bound vesicles approximately 30–150 nm in diameter, with a density of approximately 1.13–1.21 g/mL. They are naturally present in a variety of biological fluids, including blood, saliva, urine, and breast milk.
Exosomes are membrane-enclosed vesicles secreted by living cells and are derived from the late endosomal compartment, also known as multivesicular bodies (MVBs).
Exosomes were first identified approximately 30 years ago. Early studies suggested that exosomes primarily functioned in protein transport, including targeted delivery to recipient cells, exchange of proteins and lipids, and initiation of downstream signaling events. In 2007, researchers reported that exosomes also carry nucleic acids, highlighting their important role in intercellular communication.
Exosomes contain distinct profiles of proteins, RNAs, and lipids, as well as a variety of biologically active signaling molecules. These properties have generated significant interest in their potential applications in the early diagnosis of multiple diseases, contributing to the rapid expansion of the exosome research and biotechnology market.
Exosomes derived from different cell and tissue types can exhibit distinct biological functions because they carry different molecular cargos. For example, tumor cell-derived exosomes may promote angiogenesis, tumor cell proliferation, and immune evasion, whereas dendritic cell-derived exosomes can induce effective antitumor immune responses.
Studies have shown that exosomes contain cell-of-origin-associated proteins, ribosomal RNA (rRNA), and microRNAs (miRNAs). Exosomes can also cross biological barriers and deliver functional nucleic acids between cells, thereby influencing a wide range of biological processes. These properties make exosomes promising candidates for drug delivery systems and gene therapy applications.
Exosomal Protein Composition
Exosomal proteins can broadly be divided into two categories: proteins commonly associated with exosomes regardless of their cellular origin and cell-type-specific proteins.
The first category may be associated with exosome biogenesis and biological functions and includes:
Cytosolic proteins
Proteins involved in intracellular signal transduction
Metabolic enzymes
Heat shock proteins
Tetraspanins
The second category consists of proteins that are enriched in exosomes derived from specific cell types. These cell-of-origin-specific exosomal proteins are often closely associated with the biological functions of the corresponding exosomes. For example, exosomes derived from antigen-presenting cells may contain MHC class II molecules.
II. Exosome Isolation
Exosomes are secreted by living cells and represent a subcellular vesicle population composed primarily of a phospholipid bilayer and associated membrane proteins and molecular cargos.
Because exosomes differ from other cellular components in their morphology, biochemical characteristics, and physical properties, these properties can be exploited for their isolation and purification.
For exosomes derived from cell culture supernatants, one commonly used approach is differential ultracentrifugation. This process first removes cells and large cellular debris through low- and intermediate-speed centrifugation, followed by ultracentrifugation to enrich the exosomal fraction. Density-gradient centrifugation, such as a sucrose gradient, can subsequently be used for further purification based on the buoyant density of vesicles.
More broadly, exosome research generally involves two major steps:
Isolation or capture of small extracellular vesicles
Characterization of their molecular cargos
Common approaches for preliminary exosome isolation include:
Differential and ultracentrifugation
Magnetic bead-based immunocapture
Polymer-based precipitation
Filtration and size-based separation
Following isolation, different analytical techniques can be used for characterization:
Transmission electron microscopy (TEM) for particle morphology and size
Flow cytometry for surface markers
Western blotting and ELISA for protein analysis
qPCR for specific RNA or miRNA analysis
Next-generation sequencing (NGS) for comprehensive RNA profiling
Ultracentrifugation remains one of the most widely used methods for exosome isolation and is often regarded as a conventional reference method. However, it has several practical limitations, including:
Long processing times, potentially requiring several hours
Limited sample throughput
Relatively large starting sample volumes
Potentially low recovery
Labor-intensive operation
Commercial exosome isolation kits are also available and can simplify the isolation process.
1. Ultracentrifugation
For a 10 mL cell culture supernatant, the following differential centrifugation workflow can be used as a reference:
Centrifuge at 300 × g for 10 min at 4°C to remove cells.
Transfer the supernatant and centrifuge at 2,000 × g for 20 min at 4°C to further remove cellular debris.
Carefully transfer the resulting supernatant and centrifuge at 10,000 × g for 30 min to remove larger subcellular components.
Perform a further ultracentrifugation step at 10,000 × g for 60 min as described in the original procedure.
Discard the supernatant and retain the resulting pellet.
Resuspend the pellet in 30 mL PBS and mix thoroughly.
Perform another centrifugation step at 10,000 × g for 60 min.
Resuspend the final pellet in approximately 1 mL PBS.
Aliquot the purified exosome suspension into microcentrifuge tubes and store at −80°C until further use.
Note: The centrifugation parameters in the source material appear to contain transcription/OCR inconsistencies. The above translation preserves the stated parameters rather than introducing unverified conditions. Experimental conditions should be optimized according to the sample type, centrifuge rotor, and downstream application.
2. Magnetic Bead-Based Isolation and Immunocapture
Because ultracentrifugation can be time-consuming and labor-intensive, commercially available total extracellular vesicle/exosome isolation reagents can provide a more convenient approach for enriching intact vesicles from different starting materials.
These reagents and associated protocols can be adapted to a wide range of experimental designs, including experiments involving small input volumes and high sample throughput.
Total exosomes or extracellular vesicles enriched from cultured cells using an isolation reagent or ultracentrifugation can be further purified into specific subpopulations through immunomagnetic capture.
For example, CD63-specific Dynabead-based reagents can be used for immunocapture. Alternatively, streptavidin-coated beads can be combined with specific biotinylated antibodies to isolate exosomal subpopulations based on surface antigen recognition.
This strategy allows researchers to selectively enrich exosomal subpopulations according to the surface markers of interest.
III. Exosome Characterization and Analysis
Following isolation and purification, exosomes can be analyzed at both the protein and RNA levels, including studies of microRNAs and long non-coding RNAs (lncRNAs).
Common analytical approaches include:
Western blotting for exosomal protein markers
qPCR for miRNA expression
Microarray analysis for miRNA profiling
NGS for miRNA and small RNA profiling
Transcriptomic analysis for mRNA and other RNA species
Although exosome research has advanced rapidly, a substantial proportion of current applications remain at the basic research stage. Compared with basic characterization studies, relatively fewer studies have progressed to biomarker validation, diagnostic development, or therapeutic applications.
The characterization and quantification of exosomes represent a rapidly developing research and biotechnology field. Circulating biomarkers also present significant opportunities, although substantial challenges remain in standardization, validation, sensitivity, and reproducibility.
The success of noninvasive prenatal testing has further demonstrated the potential of circulating biomarkers and supports continued investigation of their applications in cancer and other disease areas.
1. Protein Analysis
(1) SDS-PAGE
SDS-PAGE can be used to analyze the protein composition and relative abundance of proteins present in isolated exosome preparations.
(2) Western Blotting
Western blotting can be used to detect and evaluate the expression of specific proteins in exosome preparations.
(3) 2D Gel Electrophoresis and Proteomics
Two-dimensional gel electrophoresis (2-DE) and other proteomic approaches can be used to investigate the identity, abundance, and differential expression of proteins associated with exosomes.
2. Transmission Electron Microscopy
Transmission electron microscopy (TEM) can be used to examine the size and morphology of isolated exosomes.
When combined with immunogold labeling, TEM can also help determine the localization of specific markers associated with exosomal membranes.
TEM therefore provides valuable morphological evidence for exosome characterization and can be combined with molecular marker analysis to provide more comprehensive validation.
3. Genetic and RNA-Level Analysis
(1) Real-Time PCR
Real-time quantitative PCR (qPCR) can be used to quantify the expression of specific genes or RNA molecules of interest in exosome preparations.
(2) Transcriptomic Analysis
Transcriptomic analysis based on RNA sequencing or microarray technology can be used to investigate differences in gene transcription profiles associated with exosomes.
(3) miRNA Analysis
RNA sequencing or microarray analysis can be used to characterize exosome-associated miRNA expression profiles and investigate the biological functions of specific miRNAs.
(4) lncRNA Analysis
RNA sequencing or microarray analysis can also be used to profile exosome-associated long non-coding RNAs (lncRNAs) and investigate the potential relationships between specific lncRNAs, exosomes, and disease-related regulatory pathways.
IV. Isolation and Analysis of Exosomal RNA
1. Exosome Isolation
For cell culture-derived exosome studies, cells should be cultured under conditions that minimize contamination from exogenous extracellular vesicles present in serum.
When serum-containing culture medium is used, the serum should be appropriately exosome-depleted or extracellular-vesicle-depleted before use.
A representative isolation workflow is as follows:
Collect the cell suspension or conditioned culture medium in a suitable centrifuge tube.
Centrifuge at 300 × g for 10 min at 4°C to pellet cells.
Carefully transfer the supernatant to an ultracentrifuge tube. If necessary, dilute or supplement the sample with PBS according to the experimental design.
Centrifuge the sample at 16,500 × g for 20 min at 4°C to further remove cells and cellular debris.
Filter the resulting supernatant through a 0.2 μm filter to remove particles larger than approximately 200 nm.
Transfer the filtered supernatant to a new ultracentrifuge tube and ultracentrifuge at 120,000 × g for 70 min at 4°C to pellet the exosomal fraction.
Carefully discard the supernatant.
To maximize recovery, gently resuspend the exosome-enriched pellet several times in a small volume of an appropriate buffer, typically around 3 × 50 μL, depending on the downstream application.
The choice of resuspension buffer should be determined by the intended downstream analysis:
Protein or RNA extraction: an appropriate lysis buffer may be preferred.
Electron microscopy: PBS or another suitable imaging buffer may be used.
Flow cytometry: an appropriate particle-compatible buffer may be used.
Functional assays: the buffer should be selected based on the requirements of the biological assay.
Exosome Isolation from Biological Fluids
Exosomes can also be isolated from biological fluids such as plasma using similar approaches.
For viscous samples, dilution with PBS may be necessary before centrifugation and filtration.
The source material also indicates that, under certain conditions, the centrifugation force may be increased to approximately 29,500 × g, and the final ultracentrifugation step may be extended to 90 min.
If further purification is required, exosome-enriched particles can be separated using a sucrose density gradient. Exosomes are typically enriched in fractions corresponding to a density of approximately 1.13–1.19 g/mL.
Note: Several parameters in this section appear to have been affected by OCR or source-text corruption. The values above should therefore be treated as reference information rather than a validated universal protocol.
2. Exosome Identification by Electron Microscopy
To further remove contaminating proteins and enrich the exosomal fraction, the exosome-containing pellet can be re-pelleted by ultracentrifugation at 120,000 × g for 70 min at 4°C, followed by resuspension in PBS.
A small aliquot of the sample can be used for protein extraction and total protein quantification. It is recommended to reserve the majority of the intact exosome preparation in PBS on ice or at −80°C for subsequent experiments.
For TEM analysis, approximately 10 μg of intact exosomal protein resuspended in PBS can be applied to a Formvar/carbon-coated nickel grid.
A representative immunogold-labeling workflow is outlined below:
Place a small drop of the exosome suspension on a suitable sealing film.
Gently place the Formvar/carbon-coated nickel grid onto the sample drop for approximately 30–60 min. Ensure that the coated side of the grid faces the sample.
Wash the grid sequentially using several drops of PBS. Use filter paper to gently wick away excess liquid without directly contacting the coated surface.
Fix the sample by placing the grid onto a drop of 2% paraformaldehyde for approximately 10 min.
Repeat the washing step before immunolabeling.
Incubate the grid with an appropriate primary antibody. Common antibodies include anti-CD63 and anti-MHC class II antibodies.
Following primary antibody incubation, wash the grid using PBS containing 0.1% bovine serum albumin (BSA).
Incubate the grid with an appropriate 10-nm gold-conjugated secondary antibody.
Wash the grid with PBS.
Fix the labeled preparation with 2.5% glutaraldehyde for approximately 10 min.
Wash the grid with deionized water.
Contrast the sample using 2% uranyl acetate for approximately 15 min.
Embed or stabilize the preparation using 0.13% methylcellulose containing 0.4% uranyl acetate for approximately 10 min.
Remove excess liquid and allow the grid to air-dry for approximately 5 min.
Examine the prepared grid using a transmission electron microscope or store the grid for subsequent analysis.
TEM can also be performed without immunolabeling because exosomes can be identified based on their characteristic size and morphology. However, combining particle size, morphology, and membrane-marker analysis provides stronger evidence for exosome characterization.
3. Exosome Characterization by Flow Cytometry
Because exosomes are generally smaller than the detection range of conventional flow cytometers, direct detection can be challenging. One commonly used strategy is to first capture exosomes onto antibody-coated magnetic or latex beads, followed by flow cytometric analysis.
Commercial bead products can be used, or antibody-coated beads can be prepared in the laboratory. Depending on the cellular origin of the exosomes and the research objective, different antibodies can be selected for bead coating.
Commonly used exosomal markers include CD63 and MHC class II.
Preparation of Antibody-Coated Beads
For laboratory-prepared antibody-coated beads, the source protocol describes the use of 4 μm latex beads coated with anti-CD63 antibody.
A representative procedure is:
Add 25 μL of 4 μm latex beads (approximately 30 × 10⁶ beads) to 100 μL of 2× MES buffer.
Centrifuge at 3,000 × g for 15–20 min and wash the beads.
Resuspend the bead pellet in 100 μL MES buffer.
Prepare an antibody mixture containing an amount of antibody corresponding to approximately 12.5 μg, together with an equal volume of MES buffer.
Add the antibody mixture to the beads and incubate overnight at room temperature with gentle mixing.
Wash the antibody-coated beads three times with PBS by centrifugation at approximately 3,000 × g for 20 min.
Resuspend the beads in 100 μL storage buffer to obtain a final concentration of approximately 300,000 beads/μL.
Exosome Capture and Flow Cytometric Detection
For each sample and antibody condition:
Use an amount of antibody-coated beads corresponding to at least approximately 100,000 beads for an exosome preparation containing the desired amount of exosomal protein.
Incubate the exosome-bead complexes in a total volume of approximately 300 μL PBS overnight at 4°C with gentle mixing.
Add 300 μL of 200 mM glycine and incubate for approximately 30 min.
Wash the exosome-bead complexes twice using washing buffer consisting of PBS containing 1–3% serum, centrifuging at approximately 600 × g for 10 min.
Incubate the exosome-bead complexes with the appropriate IgG control antibody at 4°C.
Wash the complexes twice using the washing buffer described above.
Add 90 μL washing buffer and 10 μL of the selected detection antibody to the exosome-bead complexes. Suitable antibodies may include anti-CD9, anti-CD63, or anti-CD81.
Incubate for approximately 40 min with gentle mixing.
Wash the exosome-bead complexes twice using the washing buffer.
Add approximately 300 μL washing buffer and acquire the samples using a flow cytometer.
Different types of beads, including latex beads and magnetic beads, can be used. When magnetic beads are used instead of latex beads, the workflow may differ slightly. For example, the blocking step may not be required, and bead washing can be performed using a magnetic separation rack rather than centrifugation.
For laboratory-prepared antibody-coated magnetic beads, the source protocol describes a storage buffer containing 0.1% glycine and 0.1% sodium azide in PBS, pH 7.2.
Important Consideration for Flow Cytometry
It is particularly important to use serum-depleted or exosome-depleted serum when preparing the flow cytometry washing buffer. This helps minimize contamination from serum-derived extracellular vesicles and prevents them from interfering with exosome analysis.
When antibody-coated beads are used for exosome characterization, it is also important to recognize that the assay detects only the exosomal subpopulation captured by the specific antibody used. Therefore, the results should not automatically be interpreted as representing the entire exosome population.
4. Exosome Detection by Western Blotting
Western blotting is a well-established method for detecting exosome-associated proteins. In exosome research, particular attention should be paid to appropriate protein extraction and selection of multiple positive and negative markers.
Protein Extraction
Add an appropriate protein lysis buffer to the isolated exosome pellet.
Thoroughly pipette and vortex-mix the sample.
To facilitate further disruption of the exosomal membrane, the sample may be subjected to three 5-min cycles of sonication in a water bath, with vortex mixing between cycles.
Centrifuge the sample at approximately 13,000 × g for 5 min at room temperature.
Transfer the supernatant to a new microcentrifuge tube.
Western Blot Analysis
Determine the total protein concentration using an appropriate protein assay.
Load approximately 20–50 μg total protein per lane, depending on the experimental design.
Perform gel electrophoresis to separate proteins according to molecular weight.
Transfer the separated proteins to an appropriate membrane.
Block the membrane and wash according to the selected Western blot protocol.
Incubate the membrane with antibodies against exosome-enriched proteins.
Detect target proteins using a chemiluminescence imaging system and analyze the resulting signal using appropriate imaging software.
Common Exosomal Markers
There is no single universal marker that can independently define all exosomes. Instead, several proteins enriched in extracellular vesicles are commonly used for characterization.
Common markers include:
Tetraspanins
CD9
CD63
CD81
Cytoskeleton-associated proteins
Ezrin
Multivesicular body/exosome biogenesis-associated proteins
TSG101
ALIX
Other commonly used markers include:
Flotillin
HSC70
Rab family proteins
Cell-type-specific markers may also be useful. For example:
A33 — associated with intestinal epithelial cells
CD3 — associated with T cells
MHC class II — associated with antigen-presenting cells
Therefore, the appropriate marker panel should be selected according to the cellular origin of the exosomes being investigated.
Negative Markers and Contamination Assessment
Because other intracellular compartments can also generate vesicles, it is recommended to assess proteins associated with other organelles to evaluate sample purity.
Examples include:
Endoplasmic reticulum markers
Calnexin
GRP78
Golgi apparatus marker
GM130
The absence or low abundance of these non-exosomal compartment markers can provide evidence that the preparation contains limited contamination from vesicles or proteins derived from other intracellular compartments.
5. Exosomal RNA Isolation and RNA Analysis
Isolated exosome pellets can be lysed using an appropriate RNA lysis buffer, followed by RNA extraction.
Different RNA isolation kits can be selected according to the downstream application. Column-based extraction methods can provide broad RNA recovery and are commonly used for exosomal RNA analysis.
The source material describes the use of the miRCURY RNA Isolation Kit from Exiqon as one example, although other commercially available RNA extraction kits may also be suitable depending on the experimental requirements.
RNase-Free Handling
Because exosomal RNA is highly susceptible to RNase degradation, RNA isolation should be performed under RNase-free conditions.
Recommended practices include:
Use RNase-free tubes and pipette tips.
Use RNase-free reagents whenever possible.
Clean work surfaces and equipment before RNA extraction.
Minimize unnecessary exposure of RNA samples to the environment.
Keep purified RNA appropriately stored after extraction.
Representative RNA Extraction Procedure
Using a miRCURY RNA extraction kit as an example:
Resuspend the exosome pellet in approximately 350 μL lysis buffer.
Vortex-mix the sample for approximately 15 sec.
Add 200 μL of 95% ethanol and vortex-mix for approximately 10 sec.
Place an RNA extraction column into a collection tube.
Transfer the lysate to the column and centrifuge at approximately 14,000 × g for 1 min.
Add approximately 400 μL washing buffer to the column and centrifuge at approximately 14,000 × g for 1 min.
Repeat the washing step as required by the kit protocol.
Perform a final centrifugation at approximately 14,000 × g for 2 min to ensure that the membrane is completely dry.
Transfer the column to a clean, RNase-free microcentrifuge tube.
Add approximately 50 μL elution buffer directly to the column membrane.
Incubate briefly and centrifuge at approximately 200 × g for 2 min, followed by 14,000 × g for 1 min, to elute the RNA.
Proceed immediately with downstream RNA analysis or store the purified RNA at −80°C.
The purified exosomal RNA can subsequently be used for applications such as:
RT-qPCR
miRNA profiling
lncRNA analysis
mRNA analysis
RNA sequencing
Transcriptomic studies
