The adult human heart cannot regenerate itself after injury, and the death of cardiac muscle cells, known as cardiomyocytes, irreversibly weakens the heart and limits its ability to pump blood. Heart disease is a major global health problem, myocardial infarction annually affects more than one million people in the U.S. alone, and there is still no effective treatment. The study was published in Nature Biomedical Engineering.

Researchers have turned their focus to stem cell transplantation for cardiomyocyte replacement and recovery of heart function, but studies have shown that implanted stem cells have difficulty surviving and differentiating into cardiomyocytes to repair the damaged muscle. When stem cells were differentiated into cardiomyocytes before implantation, heart function improved, but with a complication: the implanted cardiomyocytes did not contract synchronously with the heart, thus causing potentially lethal arrhythmias (abnormal heart rhythm).

They show that the cardiomyocytes derived from human pluripotent stem cells (derived in turn from a small sample of blood) could be a powerful, untapped source of therapeutic microvesicles that could lead to safe and effective treatments of damaged hearts.

Cell-secreted microvesicles are easy to isolate and can be frozen and stored for long periods of time. Such an "off-the-shelf" product has several major advantages over cell therapy;

1) It can be used immediately in an acute-care setting, unlike cells that can take months to isolate and grow

2) It does not cause arrhythmia (which often occurs when cells are transplanted)

3) The regulatory path towards clinical application is much simpler than for cell-based therapy.

It is well known from numerous clinical studies that most of the implanted stem cells are washed away within hours of the treatment, but there still are beneficial effects. This has led to the informal "hit-and-run" hypothesis, meaning that the cells deliver their cargo of regulatory molecules before leaving the site of injury. "Consistent with this hypothesis, we postulated that the benefits of cell therapy of the heart could be coming from the secreted bioactive molecules, rather than the cells themselves," says Vunjak-Novakovic.

The interdisciplinary team, which included bioengineers, clinicians, and systems biology scientists, derived cardiomyocytes from adult human stem cells and cultured these cells to allow them to secrete extracellular vesicles. The vesicles secreted by undifferentiated stem cells were used for comparison. The researchers then used next-generation sequencing to read their messages and instructions.

They found that the extracellular vesicles from cardiomyocytes–but not from stem cells–contained cardiogenic and vasculogenic microRNAs that are very powerful regulatory molecules. Building on the expertise of Vunjak-Novakovic's lab in biomaterials and hydrogels, the team encapsulated the vesicles in a collagen-based patch that slowly released them over the course of four weeks when implanted onto the injured heart in rat models of myocardial infarction.

The team plans next to determine how much the properties of extracellular vesicles change with the maturation of cardiomyocytes derived from pluripotent stem cells, and what would be the optimal developmental stage, or level of maturity, of cardiomyocytes to maximize therapeutic outcomes.  They are also exploring the specific mechanisms behind the therapeutic effects observed in this study. They should be able to extend their use to a range of cardiovascular diseases, and significantly advance the field of cell-free heart therapy.

The researchers monitored the heart to measure blood-pumping function and look for any signs of arrhythmia. They were really excited to find that not only did the hearts treated with cardiomyocyte extracellular vesicles experienced much fewer arrhythmias, but they also recovered cardiac function most effectively and most completely, says Vunjak-Novakovic. In fact, by four weeks after treatment, the hearts treated with extracellular vesicles had a similar cardiac function as those that were never injured.