Evolution of exosomes: From cellular garbage trucks to special delivery couriers
Jill Russell, Ph.D.
When scientists first identified exosomes – lipid vesicles ejected by cells – they were thought to serve as tiny garbage haulers, filled with unneeded cellular junk.
Decades of research have shown exosomes instead to be engaged in communication among cells to influence a variety of ends. An invited review of the field by MD Anderson researchers published this month in Science captures the remarkable breadth of activity and biomedical potential of these extracellular vesicles.
“This is an exciting field because exosomes are found in all biological fluids and are secreted by all cells. Understanding exosomes’ function and biology could lead to novel diagnostic and therapeutic strategies for many different diseases,” says review co-author Raghu Kalluri, M.D., Ph.D., chair of Cancer Biology and director of the Metastasis Research Center. Kalluri leads a laboratory of 20 scientists who focus on biology and function of exosomes.
Exosomes play role in cancer, other diseases
Exosomes were discovered 30 years ago, but the recent explosion in exosome research has been driven by advances in technology that allow the virus-sized particles (approximately 40 to 160 nanometers in diameter) to be detected and isolated.
Exosomes are extracellular vesicles formed through double invagination of the cell membrane. Think of them as microscopic semi-trucks that carry cargo to cells. This cargo can include a mix of membrane proteins, cystolic and nuclear proteins, extracellular matrix proteins, metabolites, messenger RNA, noncoding RNA species, and DNA. The cargo exosomes carry depends upon their cells of origin, the cellular microenvironment, and the size of the exosome.
Exosomes form an intercellular communication network that can alter the biological response of recipient cells (for example, inducing cell survival, apoptosis, or immunomodulation) depending in part on the exosomal cargo.
Studies covered in the review article suggest that exosomes could play roles in:
reproduction and development (e.g., promoting sperm maturation, inhibiting HIV-1 infection in sperm, preventing infection in the placenta, and promoting postnatal health and growth via breast milk)
adaptive and innate immune responses to cancer and to infectious agents
metabolic diseases and cardiovascular fitness
neurodegeneration (e.g., promoting or limiting the accumulation of unfolded or misfolded proteins in the brain)
cancer (e.g., influencing neoplasia, tumor growth, metastasis, paraneoplastic syndromes, and resistance to therapy)
Exosomes also might serve as a “Trojan horse” for viruses to infiltrate cells and can pass easily through the blood-brain barrier.
Exosomes are continuously generated by and taken up by cells. Their cargo is varied but can include DNA, RNA, proteins and metabolites. From Kalluri & LeBleu Science 07 Feb 2020: Vol. 367, Issue 6478, eaau6977. Reprinted with permission from AAAS.
What role can exosomes play in cancer treatment, diagnosis?
Because exosomes are found in all bodily fluids, researchers are working to develop minimally invasive diagnostic strategies using liquid biopsies.
“Exosomes can give us a diagnostic window to detect and monitor disease. What we learn from the culture dish is being extended now to provide insights about what is happening in the body,” Kalluri says.
For example, one study showed that elevated levels of miR-21 in exosomes from urine samples were associated with bladder and prostate cancer. Future assays may combine detection of exosomal proteins, lipids, RNA, and microRNA (miRNA) for comprehensive diagnostic and prognostic evaluation and to enhance the specificity and sensitivity of these tools.
Researchers also are working to harness exosomes as a vehicle to deliver drugs to cells. Mouse studies have shown that exosomes can efficiently enter cells with minimal immune clearance and are well-tolerated at levels greater than normally present in the body.
Mesenchymal stromal cell-derived exosomes themselves could be therapeutic agents and have been used to treat a patient with graft-versus-host disease. Exosomes can be engineered to deliver therapeutic payloads, such as short interfering RNA (siRNA), antisense oligonucleotides, chemotherapeutic compounds, and immune modulators.
Enriching ligands on the surface of engineered exosomes allows specific cell types to be targeted. Cell-specific exosomes can deliver miRNA or siRNA payloads to target oncogenes for which there are no effective pharmacological agents.
Pancreatic cancer clinical trial planned
Previously, synthetic lipid nanoparticles were used to deliver siRNA to cells, but these nanoparticles can cause a toxic immune response in humans and tend to accumulate in the liver, limiting the diseases that can be targeted.
However, Kalluri’s team intravenously injected MSC-derived exosomes carrying KrasG12D siRNA to treat pancreatic cancer in multiple animal models. This research forms the foundation for the first-in-human exosome Phase I clinical trial, planned to start in March 2020 at MD Anderson (clinicaltrials.gov, NCT03608631).
“This is a vast area of research, and we are striving to understand the biology of exosomes and find new ways to diagnose disease early and noninvasively, track disease progression, understand how metastasis is initiated, and develop novel therapies with minimal side effects,” says Kalluri.