Exosomes: A big role for a small player

Tiny vesicles of macromolecules (30-120nm) secreted by cells called exosomes have come to the forefront of cancer, neurodegeneration, immunology, and more research areas. They were first described by Rose Johnstone’s group in 1983 in the context of transferrin receptor secretion by sheep reticulocytes. As technologies to study these vesicles improved, exosomes went from a niche mechanism to playing a broad signaling role after the discovery by Valadi et al that the RNAs inside can be used to exchange information between cells. Further investigation revealed that they also contain proteins and are found in a variety of body fluids including saliva, blood, and urine.

Like traditional vesicles, exosomes are formed in the endosomal pathway. During late endosomal maturation, intraluminal vesicles containing a variety of macromolecules form inside the endosome. Rather than fusing with the lysosome afterward, the late endosome fuses with the plasma membrane, releasing the intraluminal vesicles as exosomes.

Applications for cancer, immunology, and diagnostics

Recent work has uncovered a variety of roles for exosomes. For instance, Cheng et al showed that they play a role in cardiac tissue repair after an infarction, sending miRNAs that mobilize bone marrow progenitor cells. Also, Ramkumar Menon’s group at the University of Texas Medical Branch found that maternal exosomes can be delivered to the developing fetus and may impact tissue function. In cancer, Chen and colleagues discovered that exosomal PD-L1 secreted by metastatic lesions enhances tumor immunosuppression and is associated with poor clinical response. Lastly, Fan et al recently showed that cancer cells can use an alternative exosomal development pathway to create pro-tumorigenic exosomes.   

Due to their payloads, tropism, and ubiquity, exosomes also have exciting applications for diagnostics and drug delivery. Since exosomes contain proteins and nucleic acids from their origin, they have been used to diagnose and monitor disease. An example is exosomal CD63, which can be used to identify melanoma. Exosomal tau can also be used to detect the onset of early Alzheimer’s disease.

Besides diagnostics, there is potential for improving drug delivery. Drawbacks of current generation nanoparticle therapies include pre-mature drug release and high immunogenicity. However, exosomes have low immunogenicity, can be loaded with drug payload, and can be cell-targeted through their surface molecules. Exosomes are also small enough to pass through the blood-brain barrier, so there is potential for neurodegenerative and brain tumor treatment.

Methods to study exosomes

To isolate and study exosomes, there are a few biochemical methods including sucrose gradient and differential ultracentrifugation. Besides their small size, a challenge is their lability. Though cost-effective, differential ultracentrifugation methods may distort membranes and destroy others, lowering yields. By contrast, sucrose gradient centrifugation creates a “cushion” while helping remove contaminants and protocols have been developed to maximize yield and improve the workflow.

Once isolated, flow cytometry is a popular means of characterizing exosomes. Examining the cell surface markers can provide information regarding the origin of an exosome and potential tropism. For instance, the presence of ICAM-1, B7.2, and MHC Class II indicates a dendritic cell origin.

There is much more to learn about exosomes and Proteintech offers a variety of helpful markers that can aid you in your journey. Below are a few popular markers:

References:

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