Author + information
- Jennifer L. Hall, PhD∗ ()
- Lillehei Heart Institute, Department of Medicine and Integrative Biology and Physiology, University of Minnesota, Minneapolis, Minnesota
- ↵∗Reprint requests and correspondence:
Dr. Jennifer L. Hall, Department of Medicine and Integrative Biology and Physiology, Lillehei Heart Institute, University of Minnesota, Cancer and Cardiovascular Research Building, 2231 6th Street SE, Minneapolis, Minnesota 55455.
Exosomes are transport vesicles released from cells that contain deoxyribonucleic acid, ribonucleic acid (RNA), and protein (1). Exosomes have captured our attention with the explosion of diagnostics to measure the molecular profiles inside these vesicles with the purpose of detecting, diagnosing, and treating disease.
In addition to diagnostics, therapeutic delivery with exosomes is a topic on the minds of many. After all, exosomes represent a proven, experienced transportation system that provides a safe haven for circulating small molecules with a built-in docking system (1). If one pushes diagnostics and delivery aside, the simple, elegant fact remains that exosomes regulate basic daily functions, including blood coagulation (2). Exosomes also orchestrate responses to specific biological conditions, including ischemia. Exosomes do not always contain protective contents, nor do they always promote a protective response. For example, exosomes are secreted by cancerous cells that perpetuate the cancer by suppressing immune responses. Today, many scientists and clinicians are working to define the roles of exosomes in the setting of disease.
The study by Vicencio et al. (3) in this issue of the Journal provides new evidence of a cardioprotective effect of exosomes. Specifically, these authors demonstrate compelling new data that delivering plasma exosome fractions to rats before an ischemic event is cardioprotective. In fact, infarct size is reduced from ∼48% to 25% with delivery of exosomes (3). The data further suggest that the exosomes evoked cardioprotection via a heat shock protein (HSP), known as HSP70, on the surface of the exosome that is bound to toll-like receptor 4 (TLR4) on recipient cells. The recipient cells were not identified.
HSP levels are elevated after a myocardial infarction or after an ischemic event. Vicencio et al. (3) show that HSP70 resides on the surface of the rat and human exosomes where it binds to TLR4 on the plasma membrane of recipient cells, leading to cardioprotection through extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) phosphorylation, p38 mitogen-activated protein kinase (p38MAPK) activation, and phosphorylation of a second HSP, HSP27. They demonstrated that blockade of HSP70 with a neutralizing antibody prevented the cardioprotective effects and the ERK1/2 and p38MAPK phosphorylation.
Vicencio et al. (3) also evaluated the delivery of an exosome fraction to rats through the tail vein before occlusion of the left anterior descending (LAD) artery and showed a significant reduction in infarct size. A limitation of this study design was the lack of 2 controls: LAD occlusion alone and delivery of donor rat plasma lacking exosomes. Exosomes were delivered to a cohort of rats before LAD occlusion; these rats were then compared to a second group of rats that received an infusion of phosphate-buffered saline before the LAD occlusion (3). Including a control with no tail vein injection before LAD would have been helpful in the interpretation of the results (to determine if the injections were altering the response to the LAD occlusion).
Second, including a control where the authors lowered the exosomes in the circulation would have been ideal and would have served as a much stronger control compared with the phosphate-buffered saline used in the study. Adding a donor fraction of exosomes from a healthy rat, as was done, essentially doubles the amount of exosomes circulating in the rat (since no exosomes were removed in the study design). Thus the animals receiving phosphate-buffered saline still have exosomes. Of note, the Langendorff preparation used as a second series of experiments in this study will also contain exosomes that are released from cells in the heart that circulate in the perfusate. Given the field is still relatively young at understanding the therapeutic effects of exosomes and standardizing the “exosome fraction,” scientists in this field will need to push the boundaries in rigid controls and standards for defining exosome fractions.
State-of-the-art technology has allowed scientists to define exosomes with greater precision since the advent of their discovery more than 30 years ago. Exosomes are considered to be 40 to 120 nm in size (1). Markers used to define exosomes include tetraspanins (including TSPAN29 and TSPAN30), endosomal sorting complexes required for transport (ESCRT) components, programmed cell death 6-interacting protein (PDCD6IP), tumor susceptibility gene 101 (TSG101), flotillin, and milk fat globule-epidermal growth factor (EGF) 8 protein (MFG-E8) (1). Cluster of differentiation (CD) 81 and CD63 have also been used consistently in the literature to mark exosomes and were used in this study by Vicencio et al. (3). Microvesicles have a much larger range in size (50 to 1000 nm) and are typically measured with integrins, selectins, and CD40 ligand (1). Both microvesicles and exosomes carry messenger RNA, micro RNA, noncoding RNAs, and proteins (1). Vicencio et al. (3) show the mean size of the particles in their Figure 1D and they report it is around 75 nm for rats and humans. However, in their Figure 1B (3), you can see the particle counts in both rat and human rise above the 120 nm particle size. Again, better standardization and characterization of the exosomes in this study may have been useful.
Finally, the exosomes isolated in this study were from healthy rats and humans. It is not clear if exosomes or microparticles isolated from non-healthy rats and humans would produce the same beneficial effects.
Overall, the study from Vicencio et al. (3) raises many new important questions for the field while providing evidence of cardioprotection from exosomes in the setting of ischemia and reperfusion.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Hall is a consultant for University of South Florida Health, Tampa, Florida.
- American College of Cardiology Foundation
- Del Conde I.,
- Shrimpton C.N.,
- Thiagarajan P.,
- Lopez J.A.
- Vicencio J.M.,
- Yellon D.M.,
- Sivaraman V.,
- et al.