Extracellular Vesicles: How Understanding the Body’s ‘Secret Messengers’ Sheds Light on Disease

2023 marks the fortieth anniversary of the discovery of exosomes – tiny extracellular vesicles that are increasingly being recognized as playing an important role in health and disease (Harding et al., 2013).

We take a closer look at these ‘secret messengers’ to find out what extracellular vesicles are, what they do, and how they can be studied and harnessed to treat a wide range of conditions.

What are Exosomes?

Extracellular vesicles are a small, spherical nanoparticles surrounded by a lipid bilayer and packed with a variety of biologically active molecules including proteins, messenger RNAs (mRNAs), microRNAs, lipids and more.

They are produced by almost all cell types and have been found in a wide range of biofluids including blood, lymph, urine, and even tears. The most well-known of these vesicles are exosomes, which are typically 30-150 nm in diameter (Doyle and Wang, 2019).

Rather than simply being inert bags of molecules that randomly move through the body, extracellular vesicles are highly targeted biological messengers that can relay information between cells at a distance. They recognize and bind to specific target cells through surface protein interactions, following which they are internalized and release their contents.

Exosomes and other extracellular vesicle are thought to play an active role in regulating physiological processes, for example by modulating signals responsible for cell death, survival, cell differentiation, and in some cases, regulating the immune response. Alongside this normal role in maintaining health, exosomes have been implicated in a number of diseases, particularly cancer (Jablonska et al., 2019).

Clinical Applications of Exosomes

The wide-ranging contents and properties of exosomes have sparked great interest from biomedical researchers who are keen to harness their abilities to diagnose, monitor and treat diseases such as cancer and beyond. The ability to detect exosomes in easily accessed biofluids such as blood and urine is another plus.

Here are some examples of the clinical applications of exosomes that are currently being explored.

Exosomes as biomarkers of disease

The contents of exosomes resemble that of the parental cells from which they are released. They can therefore provide a snapshot of what is going on inside the cell and provide valuable information about the progression of disease (Wang et al., 2022)

Tumor-derived exosomes, sometimes known as oncosomes, have been found to contain a number of bioactive molecules that enhance the cancer growth and metastasis.

The contents of these exosomes vary depending on the cancer stage and type, offering the potential to use them as biomarkers, providing insightful information relating to the diagnosis, prognosis, and spread of cancer.

Exosomes are also emerging as exciting potential biomarkers in neurodegenerative conditions and other brain disorders (Mathew et al., 2021).

Exosomes as therapeutic targets

Exosomes have been shown to play a role in long-distance cell communication processes underlying cancer metastasis by altering tumor microenvironments and setting up pre-metastatic niches in which migrating tumor cells can grow.

Understanding more about these pro-metastatic exosomes could enable researchers to develop therapeutics that interfere with these processes in a range of ways, such as blocking exosome production or secretion, breaking down the cargo they carry, or stopping exosome-mediated intercellular communication by preventing fusion to cells (Tai et al., 2018).

Using exosomes for drug delivery

The ability of exosomes to mediate intercellular communication at a distance, along with their small size and high target selectivity, makes them an attractive drug delivery mechanism (Moon and CHange, 2022).

Whether naturally produced or synthetic, exosomes can be loaded with a range of therapeutic agents, including mRNA or proteins and engineered to deliver to them to specific cells. The fact that they’re derived from the body also makes them less immunogenic than many current drug delivery platforms, and they also protect their precious cargo from degradation by extracellular enzymes.

Technical Challenges in Studying Extracellular Vesicles

Although there is plenty of excitement around clinical applications of exosomes and other extracellular vesicles, we are still some way off realizing the potential of these intercellular messengers due to the technical limitations of current methods for isolating and studying them.

Despite their ubiquity, the small size and relative scarcity of exosomes make purification a challenge, requiring arduous separation techniques such as ultracentrifugation.

Blood plasma and serum are currently the most promising sources for exosome analysis and can easily be obtained, compared with more invasive samples such as cerebrospinal fluid (CSF). But many proteins in blood bind to exosomes, making it difficult to get a pure sample.

Analyzing the contents of extracellular vesicles is also challenging. This is particularly true for proteomic analysis, due to the low abundance of proteins.

While advances in PCR technology have made it possible to detect extremely low concentrations of RNA or DNA, down to single molecules, most proteomics techniques are unable to cope with the dynamic range of proteins present in exosomes and other vesicles.

In order to unlock the biological secrets hidden within exosomes, there’s an urgent need for robust, scalable deep proteomic profiling technologies that can cope with the technical demands of exosome isolation and downstream analysis.

To solve this challenge, we have developed a novel automated workflow based on our highly sensitive TrueDiscovery™ mass spectrometry platform, combining size-exclusion chromatography, exosome concentration, and optimized mass spectrometry. This enables deep identification and quantification of thousands of exosomal proteins from just 200 µl of plasma or serum.

We’ll be writing more in detail about this in a future post, but you can explore our poster from the 2023 AACR conference describing how we applied this workflow to studying exosomes in samples from patients with melanoma to further understand the biology of the disease and discover novel biomarkers.

A Promising Future for Extracellular Vesicle Proteomics

Extracellular vesicles are a new, exciting area of biology with much promise, not only in cancer but other diseases too. Our unbiased, deep mass spectrometry proteomics approach opens up the possibility of large-scale automated analysis of exosomes and other vesicles in blood, as well as other sample types.

Through large-scale deep proteomics, we can now begin to unlock insights that will deepen our understanding of the role of these important molecular messengers in health and disease, and support the next generation of cancer biomarkers, diagnostics and therapies.

References:

Harding CV, Heuser JE, Stahl PD. Exosomes: looking back three decades and into the future. J Cell Biol. 2013 Feb 18;200(4):367-71. doi: 10.1083/jcb.201212113.

Doyle LM, Wang MZ. Overview of Extracellular Vesicles, Their Origin, Composition, Purpose, and Methods for Exosome Isolation and Analysis. Cells. 2019 Jul 15;8(7):727. doi: 10.3390/cells8070727.

Jablonska J, Pietrowska M, Ludwig S, Lang S, Thakur BK. Challenges in the Isolation and Proteomic Analysis of Cancer Exosomes-Implications for Translational Research. Proteomes. 2019 May 15;7(2):22. doi: 10.3390/proteomes7020022.

Wang, X., Huang, J., Chen, W. et al. The updated role of exosomal proteins in the diagnosis, prognosis, and treatment of cancer. Exp Mol Med 54, 1390–1400 (2022). https://doi.org/10.1038/s12276-022-00855-4

Mathew B, Mansuri MS, Williams KR, Nairn AC. Exosomes as Emerging Biomarker Tools in Neurodegenerative and Neuropsychiatric Disorders-A Proteomics Perspective. Brain Sci. 2021 Feb 19;11(2):258. doi: 10.3390/brainsci11020258

Tai YL, Chen KC, Hsieh JT, Shen TL. Exosomes in cancer development and clinical applications. Cancer Sci. 2018 Aug;109(8):2364-2374. doi: 10.1111/cas.13697

Moon B, Chang S. Exosome as a Delivery Vehicle for Cancer Therapy. Cells. 2022 Jan 18;11(3):316. doi: 10.3390/cells11030316