Reviving cells after a heart attack

October 15, 2020

HSCI researchers show the healing mechanisms of extracellular vesicles using a heart-on-a-chip

Extracellular vesicles (EVs) — nanometer-sized packages that travel between cells to deliver cues and cargo — are promising tools for the next generation of therapies for a range of conditions from autoimmune and neurodegenerative diseases to cancer and tissue injury. EVs derived from stem cells have already been shown to help heart cells recover after a heart attack, but exactly how they help and whether the beneficial effect is specific to EVs derived from stem cells has remained a mystery.

Now, HSCI researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have unraveled potential mechanisms behind the healing power of EVs and demonstrated their capacity to not only revive cells after a heart attack, but also keep cells functioning during a heart attack. The researchers demonstrated this functionality in human tissue using a heart-on-a-chip model with embedded sensors that continuously tracked the contractions of the tissue.

The research is published in the journal Science Translational Medicine.

“Our organ-on-chip technology has progressed to the point where we can now fight drug targets instead of fighting the chip design,” said HSCI Principal Faculty member Kit Parker, senior author of the study. “With this study, we have mimicked a human disease on a chip with human cells and developed a novel therapeutic approach to treat it.”

A new source of EVs

Heart attacks, or myocardial infarctions, occur when blood flow to the heart is blocked. It would seem that the best way to treat a heart attack is to restore blood flow but that process actually may cause more damage to the cells in the heart. So-called ischemia-reperfusion injury (IRI) or reoxygenation injury, happens when blood supply returns to tissue after a period of a lack of oxygen.

“The cellular response to IRI involves multiple mechanisms, such as calcium and proton overload, oxidative stress, mitochondrial dysfunction and more,” said Moran Yadid, lead author of the paper. “This complex set of processes poses a challenge for the development of effective therapies that can address each of these problems.”

That’s where the endothelial-derived EVs (EEVs) come in. Because these vesicles are derived from vascular tissue, which is uniquely tuned to sense stress under low oxygen conditions, the researchers hypothesized that the cargo they carry could provide direct protection to cardiac muscle.

The researchers mapped the entire set of EEV proteins that can be delivered by the vesicles.

“Surprisingly, even though these vesicles are only a hundred and fifty nanometers in diameter, they contain almost 2,000 different proteins,” said Yadid. “A lot of these proteins relate to metabolic processes like respiration, mitochondrial function, signaling and homeostasis. In other words, a lot of processes that relate to the cardiac response to stress. So, rather than one molecule that is therapeutic, we think that the exosomes contain a cocktail of molecules and proteins that can, all together, help the cell maintain homeostasis, deal with the stress, modify metabolic action and reduce the amount of injury.”

Testing with a heart-on-a-chip

The team tested the effect of EEVs on human heart tissue using the heart-on-a-chip model developed by the Disease Biophysics Group at SEAS led by Parker. Organ-on-chip platforms mimic the structure and function of native tissue and allow researchers to observe, in real time, the effects of injuries and treatments in human tissue. Here, the researchers simulated a myocardial infarction and reoxygenation on chips that were infused with EEVs and those that were not.

The researchers found that in tissues treated with EEVs, the cardiomyocytes could better adapt to stress conditions and sustain a higher workload. After injury, the heart tissue treated with EEVs had half as many dead cells and had a contractile force four times higher than the untreated tissue after injury.

The team also found that injured cardiomyocytes that had been treated with EEVs exhibited a set of proteins that was more similar to the uninjured ones compared with untreated cells. Surprisingly, the team also observed that cells treated with EEVs continued to contract even without oxygen.

"Exosomal cell therapies might be beneficial when the traditional model of one molecule, one target just won't cure the disease,” said Parker. “With the vesicles we administered, we believe we are taking a shotgun approach to hitting a network of drug targets. With our organ-on-a-chip platform, we will be poised to use synthetic exosomes in a therapeutic manner that may be more efficient and amenable to more reliable manufacturing."

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This story was originally published on the Harvard SEAS website on October 14, 2020.

Source article: Yadid, M. et al. (2020). Endothelial extracellular vesicles contain protective proteins and rescue ischemia-reperfusion injury in a human heart-on-chip. Science Translational Medicine. DOI: 10.1126/scitranslmed.aax8005

This study was supported by the Harvard Materials Research Science and Engineering Center and the National Science Foundation, and the National Center for Advancing Translational Sciences.