What Are Exosomes and Why Are Exosomes Important?
What Are Exosomes? Definition of Exosomes
Exosomes are defined as nanometre-sized vesicles, being packages of biomolecules ranging from 40-150 nanometres in size that are released by virtually every cell type in the body. Once thought to be a kind of refuse disposal system for cells, exosomes are now known to be far more important than that. Exosomes have been shown to be key mediators of cell to cell communication, delivering a distinct cargo of lipids, proteins and nucleic acids that reflects their cell of origin.
The exosomes released by regenerative cells such as stem cells, for example, are potent drivers of healing and repair. Whereas exosomes secreted from diseased cells could be used to detect and diagnose conditions such as cancers at their earliest and most readily treatable phase.
Exosomes offer a potential new paradigm in the diagnosis and treatment of disease. A broad range of exosome-based biomedical applications are now beginning to be assessed in clinical trials.
In this article, we describe in detail what exosomes are and how they are produced, highlight their importance in health and disease, and touch upon their potential contribution to medicine.
What do we find inside Exosomes
An exosome is a type of extracellular vesicle (EV), a family of nanometre-scale, biomolecule-filled, lipid-wrapped packages released by most cells.1
Every exosomes’ contents typically include lipids; genetic information, in the form of various types of ribonucleic acid (RNA); and proteins including enzymes, growth factors, receptors and cytokines. This cargo is contained within a phospholipid bilayer membrane, the same substance cell membranes are made from.
Exosomes are found in all body fluids, and play a key role in carrying messages and molecules from one cell to another, in a target-specific manner. The recipient cell may be proximal to the sender, or at a distant site in the body.
Cells release a range of EVs, of various sizes and with various functions. Exosomes are among the smallest vesicles, generally ranging from 40 to 150 nanometres in size.2
How are exosomes formed?
Whereas some EVs bud directly from the cell’s surface, exosome formation has classically been considered to be initiated deep inside the cell.3
Exosome production starts with a process called endocytosis, in which sections of the cell’s membrane buds off from the internal side of the membrane. These sections coalesce inside the cell to form compartments called endosomes.
Inside the endosome, the internal membrane budding process repeats, until the endosome is filled with the tiny vesicles that will become exosomes. At this stage of their development, the vesicles are known as intraluminal vesicles (ILVs). The entire vesicle-filled structure is known as a multivesicular body (MVB).
As the ILVs form, they are loaded with their particular cargo of lipids, proteins and RNAs. The full details of this process are still being elucidated, but more than one mechanism for ILV cargo loading has been shown to operate. One pathway involves a multi-protein machine called the endosomal sorting complex required for transport (ESCRT).4 Although the exact details of the process are still to be pinned down, the ESCRT contributes to the selection and transfer of specific biomolecules into the ILVs.
However, cells also use ECSRT-independent pathways to load exosomes.4 When researchers completely block ESCRT function in cells, exosome production is not shut down completely, with only the production of specific ILV types disrupted. One ESCRT-independent mechanism may involve ‘lipid rafts’, transiently-formed sections of membrane enriched in certain lipids, protein receptors and other biomolecules that can bind certain compounds to be loaded into the ILVs.
Once the development and maturation of the MVB is complete, it can travel to and fuse with the cell’s membrane, ejecting its content of vesicles out of the cell as it does so. EVs formed and released via this process are called exosomes.
Fusion with the cell’s membrane and release of its vesicle cargo is not the only potential fate of MVBs. They can also fuse with a structure inside the cell called the lysosome, which breaks down the MVB and recycles its content. The regulatory processes by which the MVB’s fate is decided are not yet known.
However, the biochemical details of the exosome production process do continue to be unravelled. In August 2019, researchers in the Netherlands showed that blocking a protein called NDRG1, known to be involved in trafficking endosomes to the lysosome, increases the number of functional exosomes the cell releases.6 Boosting levels of a protein called Rab27a, which plays a role in docking the MVB with the cell membrane, also increases exosome release rates.7
Upon release into the extracellular milieu, exosomes join the other EVs circulating the body. Once they have been released, there is no way to definitively tell whether an EV of exosome size has been released via the internal MVB pathway, or whether they have simply budded directly outward off of the cell’s surface. In fact, regardless of their mode of formation, EVs in this size range appear to be functionally alike.8 In this article, all vesicles in the small size range will be called exosomes.
What do Exosomes do?
Exosomes can travel all around the body. A small fraction of exosomes break down rapidly after release, releasing growth factors and other substances that can activate receptors on nearby cells.3
But most exosomes persist for longer periods. They are found in every bodily fluid and are able to cross even tightly regulated boundaries, including the blood-brain barrier. The end of their journey comes when an exosome is taken up by a recipient cell. This process can be highly selective, with certain exosomes are only taken up by a specific target cell type.
Exosomes and their target cells can interact in a number of different ways. In some cases, the exosome interacts solely with a receptor on the cell’s outer surface to trigger a response. Alternately, the exosome can directly fuse with the cell’s membrane, releasing its cargo into the cell as it does so.
A third possibility is that exosomes enter the cell in a process similar to the way they were originally formed but in reverse. The exosomes can be partially engulfed by the cell’s membrane, and then appear to roll across the cell’s surface before being internalised via endocytosis. Once inside the cell, the exosome can fuse with the endosome membrane to release its cargo into the cytoplasm of the cell.
Although their lifetime circulating the body may only be a matter of hours, the influence that they have upon the recipient cell can be very long-lived.
What role do exosomes play in the body?
When exosomes were first described in the mid-1980s, they were thought to represent a garbage disposal system, a method by which cells could jettison molecules they no longer need. Exosomes may play a role in cellular waste disposal in some cases, but they also have much more significant functions, subsequent research has shown.1
The first hint of exosomes’ deeper role came in the 1990s when exosomes were discovered to play a specific role in communication between immune cells, activating T cells to recognise and attack pathogens.9
But the real breakthrough came another decade later. In 2007, researchers in Sweden made the seminal discovery that exosomes ferry proteins and RNA between cells, a process which produced a measurable change in the receiving cell’s behaviour.10 This discovery showed that exosomes did not just passively deliver messages; their RNA cargo could directly reprogram the recipient cell’s behaviour.
This discovery triggered a rapidly escalating spiral of interest in exosomes among researchers. Particularly in the last 5 years, exosomes have been shown to be a key line of cell-to-cell communication involving virtually every cell type in the body.
For example, exosomes are one mechanism by which immune cells coordinate their response to an infection. More generally, exosomes help coordinate a whole-body response to tissue damage and disease. They also appear to be behind the whole-body benefits of exercise.
The list of specific roles that exosomes have been shown to play in the body is now growing rapidly. Although an exhaustive list is beyond the scope of this article, the following examples are intended to give a snapshot of exosomes’ diversity of function.
Several recent studies have shown the roles exosomes play in supporting heart health, for example. Following a heart attack, damaged heart cells release exosomes that trigger an increase in the number of regenerative progenitor cells released from the bone marrow into circulation. These progenitor cells can then increase the level of cardiac repair.11
However, that’s not the only exosome-based interaction taking place following a heart attack. Another study published in 2019 shows the powerful pro-healing properties of exosomes released by regenerative stem cells in our body. After a heart attack, exosomes secreted by heart muscle stem cells provided cardioprotective effects, activated regenerative signals, and augment cardiac repair.12
Exosomes also play a key role in development during the first phases of life. Recent research has shown the key role exosomes play in communication between the cells of a pregnant mother and the cells of her foetus.13 Foetal cells use exosomes to signal to the mother that its organs have matured, triggering labour. Exosomes also play a key role in directing the organisation of neurons and the establishment of neural circuits in the developing brain.14
The more research that is done into exosome function, the more we discover the pivotal role they play in development, in maintaining health, in the processes of aging, and in disease.
Can Exosomes help with aging and disease?
Reflecting the fact that exosomes are conduits for the cells producing them, exosomes released by diseased cells have been implicated in the development and cell-to-cell spread of conditions including neurodegeneration and cancer.15, 16
But even in the absence of specific diseases, there is also an overall shift in exosome populations as we age. The balance of exosomes found circulating in the body gradually changes; the number of disease-promoting inflammatory exosomes rises, while the number of regenerative exosomes declines.1
In 2017, researchers from the National Institute on Aging (NIA) in Maryland, US, and her colleagues took the first longitudinal look at the changes of exosome levels to individual humans over time.17 Studying 75 healthy individuals aged between 30 and 64 years old, the researchers compared each individual’s exosome levels in two blood samples taken 5 years apart.
The researchers saw a clear decline in levels of circulating exosomes with age. But when it comes to aging, it’s not simply the number of EVs in circulation around the body that changes, the NIA researchers reported. The type of exosome in circulation, and their influence on the other cells, also changes as we age, they discovered.
For example, they noted significant differences in the way older individuals’ exosomes were taken up by immune cells. The team showed that exosomes isolated from older subjects were more readily taken up by immune cells called B cells. These cells are the body’s antibody-producing factories, essential for fighting off infections. The researchers also showed that these aged exosomes more effectively activated a second type of immune cell called monocytes, a type of white blood cell.
The findings tie in with the fact that aging in general, and age-related disease in particular, is associated with chronic inflammation, a condition linked to an overactive immune system that results in systemic stress and impaired self-repair around the body. “This data suggests that exosomes may transmit inflammatory signals to immune cells, which may, in turn, continue the inflammatory cascade,” the team reported in the paper.
One implication of the work is that exosomes could be ideal biomarkers of overall health in aging humans. Patterns of exosome levels in the blood could become a convenient way to get a quick and minimally-invasive read-out of an individual patient’s well-being.
The discovery also adds to the growing list of evidence that infusions of regenerative exosomes, from pro-reparative cells such as stem cells, could help to counter many age-related diseases, by restoring circulating exosomes levels to a more youthful state.18
For the past decade or more, there has been a lot of activity around the therapeutic potential of stem cells; regenerative cells thought to be capable of replacing diseased cells. It has now become clear that the exosomes that stem cells release, rather than the cells themselves, are the real source of health benefit.19
The cargo of RNA, lipids and proteins the exosomes carry appear to activate self-repair mechanisms in recipient tissues. Infusions of exosomes have been proven to slow down aging in mice, extending the animals’ lifespan and healthspan. The first human clinical trials of exosome therapies are now underway.20
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