Why are exosomes getting a lot of attention?
Exosomes in liquid biopsy
For cancer patients, tissue biopsies provide critical information on diagnosis and response to treatment. In many cases, however, it is difficult to get enough tissue for biomarker testing, due to factors such as the patient's health status and location of the tumor.
Liquid biopsy provides an alternative, non-invasive testing solution in such cases. It relies on the detection and isolation of three types of tumor biomarkers in biofluids such as blood and urine - circulating tumor cells (CTCs), cell-free DNA (cfDNA), and exosomes. In many cases, liquid biopsy compliments the information obtained via a traditional tissue biopsy, allowing physicians to take timely and precise decisions for swifter disease management.
Cancer cells shed exosomes into circulation; hence, they are implicated in spreading of the disease and are exploited as tools for diagnosis and therapy. The following properties of exosomes make them good liquid biopsy candidates:
- Exosomes contain proteins and RNAs that might reflect the status of the originating cells.
- Exosomes, due to their membrane-bound structure, can protect its contents from degradation.
- Exosomes can be obtained from body fluids using non-invasive methods.
Exosomal biomarkers for bladder cancer diagnosis: RNA and proteomic profiling of urinary exosomes and other extracellular vesicles from bladder cancer patients identified promising disease biomarkers. Extracellular vesicles were elevated in the urine of bladder cancer patients compared to healthy controls, suggesting their potential as biomarkers for bladder cancer. Another study demonstrated that microRNA profiling in cell-free urine can be used to stage bladder cancer with more than 80% sensitivity and specificity. Moreover, multiple studies comparing proteomic profiles of urinary exosomes and other extracellular vesicles (bladder cancer patients vs. healthy participants) have identified several potential prognostic and diagnostic biomarker proteins for bladder cancer.
Exosomal biomarkers for breast cancer diagnosis: Exosomal cargo molecules such as proteins, microRNAs and long non-coding RNAs (IncRNAs) are being actively investigated as diagnostic or prognostic markers for breast cancer. These molecules are differentially expressed in certain stages or types of breast cancer. For example, the absence of two proteins that control mitochondrial function -SH3GL2 and MFN2 - in serum exosomes is associated with breast cancer and the occurrence of lymph node metastases. Another study reported that two exosomal miRNAs - miR-101 and miR-372 - were elevated in the serum of breast cancer patients compared to healthy controls. A study on the exosomal IncRNA named GAS5 showed that GAS5 accumulated in the exosomes of certain breast cancer cell lines.
Exosomal biomarkers for prostate cancer diagnosis: An invasive tissue biopsy is the only definite diagnosis method for prostate cancer; hence, liquid biopsy would be a reliable non-invasive diagnostic alternative. Several microRNAs, IncRNAs and proteins in serum- and urine-derived exosomes are being characterized as potential prostate cancer biomarkers for diagnosis and to predict treatment response. For example, miRNAs derived from plasma exosomes - miR-1290 and miR-375 - can help predict the prognosis of castration-resistant prostate cancer. Multiple exosomal proteins from prostate cancer cells have also been identified as promising biomarkers; exportin-1, Notch3, PCA3, and LAMTOR1 are few among the many proteins identified.
Exosomes in regenerative medicine
Regenerative medicine is a collective term used to define approaches that aim to regenerate damaged tissue to treat a broad spectrum of degenerative conditions such as myocardial infarction, acute kidney injuries and osteoarthritis. Stem cell transplantation is one such approach, but it comes with certain limitations. These include toxicities caused by the cells getting trapped in lungs, rejection by host, and concerns regarding tumor formation.
Exosome-based regenerative medicine aims to overcome some of these challenges. Stem cells such as mesenchymal/stromal stem cells (MSCs) release exosomes which recapitulate the biological activity of MSCs. Hence, MSC-derived exosomes are now being explored as a better alternative to stem cell therapy. Exosomes are preferred over MSCs due to their: (1) higher safety profile, (2) lower immunogenicity, (3) smaller size enabling migration to the target organ without getting trapped in lungs, (5) inability to directly form tumors.
Several preclinical studies have established the potential of MSC-derived exosomes in regenerative medicine. Macular degeneration is an age-related degenerative disease which occurs due to damage to the macula of the retina. In a comparative animal study, scientists showed that both transplantations of MSCs or MSC-derived exosomes reduced retinal damage and inhibited apoptosis induced by laser injury. A clinical trial is underway to study if these observations can be translated to the clinic.
Exosomes in drug delivery
Exosome-based delivery systems harness the intrinsic ability of exosomes to safely, specifically, and stably deliver its cargo molecules over long distances. The following properties of exosomes make them an ideal drug delivery candidate:
- High specificity: Exosomes derived from a specific progenitor cell will deliver its contents to the specific target cell.
- Small size Being small and native to the body, exosomes can avoid phagocytosis, fuse with the cell membrane, and bypass the engulfment by lysosomes.
- Low immunogenicity: Exosomes do not elicit a strong immune response as they are native to the body.
- High stability: Exosomes are stable in body fluids such as blood allowing them to travel long distances within the body under both physiological and pathological conditions.
- Hydrophilic core: Exosomes can host water-soluble drugs owing to their hydrophilic core.
- Surface binding: Exosomes can bind various proteins and peptides on their surface, a property which could be exploited to deliver therapeutic substances or eliminate adverse peptides.
Exosomes can cross the blood-brain barrier (BBB), a feat they accomplish with more efficiency than other nanotechnology-based drug delivery systems. In a Parkinson's disease model, exosomes loaded with a powerful antioxidant enzyme - catalase - penetrated the BBB and were widely distributed in the brain of animals, resulting in a positive therapeutic response.
These examples demonstrate the rising interest in exosome biology and its potential clinical applications. In the future, we can expect to see these tiny vesicles being exploited to understand and treat a wide range of diseases.
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