Exosomes: Nanoparticles full of Surprises
If the past few years of exosome research has taught us anything about these tiny biomolecule-filled parcels, it is that ‘what do exosomes do?’ is the wrong question to ask. In the context of cell to cell communication, ‘what don’t exosomes do’ seems to be a better starting point.
As we now know, virtually every cell in the body actively sends and receives exosomes. The role they play in coordinating biological function has turned out to be astonishingly broad and important to health.
For the researchers that first discovered them in the early 1980s, exosomes’ escalating profile would be quite a surprise. Their initial discovery – that exosomes play a role in waste disposal in developing red blood cells – was virtually forgotten until the mid-1990s, when it was shown exosomes are involved in communication between immune cells. Even this first glimpse of exosomes’ important role gave no real hint of the research riches to come.
Perhaps the biggest twist in the exosome discovery story came in 2007, when a Swedish team showed exosomes carry not just proteins and lipids, but genetic material in the form of strips of ribonucleic acid (RNA). In other words, exosomes contained genetic directives that could directly reprogram recipient cells’ behaviour in subtle but significant ways.
As we now know, exosomes are critical to tissue regeneration and to coordinating the immune system’s fight against infection. They are key to the communication between mother and her unborn baby in the first moments of life; they could also turn out to be central to staving off age-related disease into old age.
“Exosome research has matured into one of the most exciting and most surprising research fields in modern biology,” Zoltan Szallasi from Boston Children’s Hospital and Harvard Medical School recently wrote.1
This article surveys some of the latest discoveries that add unexpected new twists to our understanding of the functions of exosomes and their rich medical potential.
Affairs of the heart
After a heart attack, stem cells released from the bone marrow soon flood the site of damage and attempt to initiate repairs. Exosomes are the key to this response, was the surprise discovery researchers reported in Nature Communications in 2019.2
Following a heart attack, heart cells send out a large pulse of exosomes which act as an urgent SOS message. These exosomes carry myocardial microRNAs that are selectively transferred to the bone marrow, where they trigger stem cell release. The stem cells then home in to the cardiac site a bit like an ambulance arriving at the scene of an accident to treat the damage.
But exosomes don’t just send SOS messages out. It is now clear that exosomes from the stem cells are taken up by damaged cardiac cells to trigger a regenerative cascade of repair – a bit like a ‘boost-me’ delivery to cells in peril.
Discovering ways to harness this two-way mechanism could lead to more effective treatments for heart conditions, which is urgently needed. Heart disease remains the biggest single killer worldwide.3 The SOS exosome discovery is just one of the ways exosomes are being pursued in heart health medical research.
Around a decade ago, treatments based on infusing extra stem cells into the body to enhance heart repair seemed to offer great promise. However, the puzzle was that despite the therapy’s beneficial effects, very few stem cells actually became integrated into the heart to replace damaged heart cells. An even more intriguing result was published in 2008, when researchers showed that the liquid medium the stem cells were grown in was just as therapeutically effective as the stem cells themselves – and that a component in the medium 50-200 nanometres in size was responsible for the effect.4
It turned out the stem cells themselves weren’t the main source of the treatment benefit – it was the exosomes they release.5 Therapies based on exosomes released by stem cells are replacing live stem cell treatments.
Remarkable regeneration involving exosomes
The ability to regenerate damaged body parts was supposed to be the rare preserve of salamanders and a few other oddball animals. While regenerating entire lost limbs may be beyond human abilities, US researchers recently made the surprising discovery that humans have a greater regenerative capacity than we thought.7
Contrary to the textbooks, damaged cartilage can regenerate in adult humans, the team showed. The repair process seemed to be orchestrated by microRNAs, suggesting that delivery of microRNAs could counteract cartilage breakdown and reverse osteoarthritis. Exosomes are a rich source of microRNAs and can deliver microRNAs into cells to reprogram the target cell for regeneration.
Regenerative stem cell derived exosomes – rich in certain microRNAs – are also being investigated for their ability to promote repair of hard-to-treat injuries such as cartilage and tendons.8 A team in Singapore recently showed that, in an animal model of osteoarthritis, exosomes from mesenchymal stem cells trigger a well-orchestrated multifaceted pattern of cartilage repair.9 In separate research, a 3D-printed scaffold of extracellular matrix impregnated with MSC-derived exosomes effectively regenerated damaged cartilage in animal studies.10
Skin is the largest organ of the human body and acts as a protective barrier against all sorts of potential threats – from microorganisms, to chemicals, to physical damage – while also helping to regulate body temperature.
Skin is one part of the human body that usually regenerates to an extent after injury – as we see when we cut ourselves or we have an operation. But the complex healing process can be derailed in many ways, from the formation of excess scar tissue (fibrosis and keloid) to conditions such as diabetic ulcers where the wound fails to heal up. Exosomes are showing great promise in enhanced wound healing, from accelerating wound closure to reducing scarring, to complete healing of chronic wounds.11,12
Damage to peripheral nerves – the network of nerves outside of the brain – afflicts over 20 million people in the US alone. Peripheral nerve damage has many possible causes, from physical injury, to autoimmune diseases such as diabetes and multiple sclerosis. Damaged nerves generally do not regenerate well, and doctors’ ability to repair them is extremely limited. But recent research suggests that exosomes from stem cells could assist.13 The microRNAs the exosomes carry can promote peripheral nerve regeneration. Engineered exosomes with enhanced levels of key miRNAs could further promote this effect, either alone or with nerve guidance conduit technologies that steer the two severed ends of a nerve to re-join.
Recent research even hints that exosomes could promote repair of damage to the spinal cord itself, by shutting down the chronic inflammation that quickly sets in following spinal trauma and which causes further damage while hindering recovery.14
It seems exosomes will offer many more regenerative surprises still to come
Cancer and exosomes – yin and yang
Cancer is one of the largest causes of death and morbidity in people as they age. Just as some cells in our body produce ‘good’ exosomes, diseased cells such as cancer cells can release ‘bad’ exosomes.
‘Bad’ exosomes from cancer cells turn out to be powerful mediators for promoting cancer cell survival and spread (metastasis) – both harmful for the patient. However, these ‘bad’ exosomes could also turn out to be cancer’s Achilles heel, according to the latest exosome research.
One line of attack is to use circulating cancer exosomes as a way to detect cancers earlier, when the cancer can be more effectively treated by conventional therapies. Checking patient blood samples for diagnostic cancer exosomes is potentially a simple and accurate way to check the whole body for hidden early stage cancers. The cancer exosomes can reveal not just the presence of cancer, but also indicate cancer type and prognosis. Exosomes in urine or semen samples, for example, have great potential for non-invasively differentiating aggressive prostate cancers from low risk tumours, saving many men from unnecessary risky surgery.15,16
The ‘bad’ exosomes cancer cells release often contain specific contents that modify behaviour of surrounding cells to favour the growing tumour.17
Tumour cells are well known to express proteins on their surface, including one called PD-L1, that suppress the immune system, preventing the person’s immune system from attacking the growing tumour. This year researchers showed cancers also release exosomes laden with PD-L1 proteins into the bloodstream to systemically suppress immune system activation.18 These discoveries show the power of exosomes in our bodies – for good or bad.
Developing new drugs to knock out cancer exosomes are not the only possibility. US medical device company Aethlon Medical recently received US Food and Drug Administration approval to start testing a device that can clear from the bloodstream the exosomes that head and neck cancer cells release to resist chemotherapy drugs.19
Fundamental exosome research continues to bring new insights that can be leveraged to attack cancer. For example, the exosomes that cancer cells release are preferentially taken up by other cancer cells – which makes them excellent drug delivery vehicles for delivering anticancer drugs, researchers reported in mid-2019. The exosomes produced by cancer cells grown in the lab could be loaded with the anticancer drug doxorubicin.20 When injected into rodents, the exosomes preferentially accumulated in and killed cancer cells – including cancer stem cells, which are typically highly resistant to chemotherapeutics and make cancer difficult to eradicate. Selectively delivering these drugs into cancer cells maximises the tumour killing effect while minimising the side effects to healthy cells.
And even the native exosomes released by stem cells, such as those being investigated for heart disease, could have a role as a cancer therapy. US-based researchers showed this year that embryonic stem cell exosomes can enhance doxorubicin’s efficacy against solid tumours including breast cancer.21 The exosomes alone had no cancer cell killing effect, but seemed to sensitise the cells toward the drug. The combination exosome-doxorubicin therapy killed 49% of cancer cells, compared to 38% of cells treated with doxorubicin alone.
Unwinding aging
One of the youngest fields of science research has grown recently around the realisation that aging, and the onset of age-related diseases, is not a fixed process but one that can be manipulated and delayed.
In 2019, a team including academics from Stanford University and the University of California, Los Angeles (UCLA), reported that their FDA-approved human clinical trial of a three-drug combination therapy had wound back participants’ biological age by about two years.22 Improvement in participants’ health scores suggested the treatment had extended their ‘health span,’ the number of years they could expect to remain fit and healthy into old age.
As we look into the mechanisms of biological maintenance and repair, and how they become dysregulated and damaged with age, exosomes appear to play a major positive role.
One of the most striking discoveries in anti-aging research is that when old mice are given cell free blood plasma from young mice, there is a dramatic rejuvenating effect.23 Although the mice don’t live any longer, they become stronger, healthier and even smarter. As researchers home in on the substances in young blood responsible for the effect, exosomes come to the fore.
For example, a 2019 mouse study showed when an exosome-borne enzyme abundant in the blood of young animals was given to older animals, it boosted their health and lifespan.24 In mice and humans alike, as the body ages it becomes less efficient at producing NAD+, a key component of cellular metabolism. Using exosomes to top up older mice’s levels of an enzyme called eNAMPT back to youthful levels enhances NAD+ production and counteracts aging, the study showed.
The regenerative effects of exosomes isolated from stem cells are also being closely studied for their potential to extend health span. In late 2019, researchers showed these exosomes could rejuvenate a corrosive type of cell called senescent cells.25 These are cells that have acquired some damage and stopped dividing, but persist in a zombie-like state contributing to chronic low-level inflammation. The exosome treatment rejuvenated these cells and alleviated the problem.
In fact, the immune system plays a surprisingly critical role in the process of aging, we are discovering.
Targeting the thymus, a major centre of T cell production in the body, was how the recent three-drug therapy reversed aging in healthy middle-aged men. Exosomes are intimately involved in thymus maintenance, deliver a key signalling molecule called Wnt4. Falling Wnt4 levels – which happens as we age – contributes to thymus degeneration. A research team in Europe recently engineered cells to produce exosomes enriched in Wnt4 and a related microRNA, miR27b.26 In mice, these exosomes can counteract thymus degeneration, a 2019 study showed. These engineered exosomes are a promising strategy for rejuvenating the human immune system to counteract age related disease, the researchers concluded.
And even more recently, a January 2020 study showed that stem cells themselves can be rejuvenated with exosomes.27 The team in Spain showed that when mesenchymal stem cells (MSCs) from old animals were treated with exosomes produced by young MSCs, the old MSCs recovered the regenerative capacity of young cells. In the reverse experiment, young MSCs treated with the exosomes from old MSCs started to behave like old cells. The study adds yet another way in which the exosomes from young and healthy populations of MSCs might restore the natural mechanisms of tissue recovery and repair in older individuals, counteracting age-related disease.
Therapies that can repair damage and restore function in aged tissues, winding back the body clock, could be available surprisingly soon.
Conclusion
Although this article has focused on just a handful of topics, there is much more to exosome science than just heart health, cancer therapy and the mechanisms of aging and rejuvenation. Eye-opening or even jaw-dropping exosome discoveries are being made at an ever-more rapid rate, across multiple areas of health and biomedicine.28 Despite the recent progress unlocking exosomes’ secrets, exosomes remain an up and coming young field of research, and there are undoubtedly big surprises still to come.
Ultimately, however, the one milestone that will be anything but surprising is when exosome-based treatments become commonplace in modern medicine.
References:
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