- An injectable gel that slowly releases short gene sequences known as microRNAs into the heart muscle has been developed.
- The gel can restart replication in existing cardiomyocytes.
- With more heart cells dividing and reproducing, mice treated with this gel after a heart attack showed improved recovery.
A new approach using injectable gel that releases short gene sequences known as microRNAs into the heart muscle can restart replication in existing cardiomyocytes.
The cells that contract the heart muscle and enable it to beat do not regenerate after injury. After a heart attack, there is a dramatic loss of these heart muscle cells and those that survive cannot effectively replicate. The heart pumps less blood with each beat, with fewer of these contractile cells, known as cardiomyocytes, leading to the increased mortality associated with heart disease.
New Method To Enable Replication of Heart Muscle Cells
MicroRNAs that target signaling pathways related to cell proliferation inhibit some of the inherent "stop" signals that keep cardiomyocytes from replicating. This resulted in cardiomyocytes reactivating their proliferative potential.
With more heart cells dividing and reproducing, mice treated with this gel after a heart attack showed improved recovery in key clinically relevant categories.
Dosage Influences Effectiveness of MicroRNA Based Therapy
MicroRNA-based therapies have been studied in the past, but delivering the right dose to the right place has been a consistent challenge.
"Biologic drugs turn over very fast," said Edward Morrisey, Scientific Director of the Penn Institute for Regenerative Medicine in Penn Medicine said. "The microRNAs that we used last less than eight hours in the bloodstream, so having a high local concentration has strong advantages."
Their short lifespan means that if patients were treated systemically, they would need to be injected frequently with large doses to ensure that a sufficient amount of microRNAs reaches their target in the heart. And because these microRNAs are designed to promote cell proliferation, there would be a risk of tumor-producing, off-target effects.
Jason Burdick, Professor in Bioengineering in Penn Engineering said, "We want to design the right material for a specific drug and application. The most important traits of this gel are that its shear-thinning and self-healing."
Shear-thinning means it has bonds that can be broken by mechanical stress, making it more fluid and allowing it to flow through a syringe or catheter.
Self-healing means that when that stress is removed, the gel's bonds re-form, allowing it to stay in place within the heart muscle.
"In addition to the bonds that give the gel its consistency, the gel features attachment sites that keep the microRNAs in place. As the gel breaks down, it loses its grip on the microRNAs, which can slip out of the gel and into the cardiomyocytes," said Leo Wang, a graduate student in Burdick's lab.
While encapsulated, the microRNAs are also protected from degradation, maximizing the time period that they can be effective without the risk of them invading off-target cells. "There's likely a time window that the cardiomyocytes are susceptible to this stimulus --maybe a week or two after injury," Morrisey said. "We want to promote proliferation for a short period and then stop."
Testing The Gel in Mice
To test their gel, the researchers used three types of mouse models. The first group was normal, healthy mice. Within a few days after injecting the gel, their heart tissue showed increased biomarkers of cardiomyocyte proliferation.
The second group was "Confetti mice," so called because they are genetically engineered such that they have individual cardiomyocytes that randomly express one of four different fluorescent proteins. These fluorescent labels allowed the researchers to see that individual cardiomyocyte were indeed dividing in response to the microRNA- gel treatment.
After inducing heart attacks in the mice and introducing the microRNA-gel, the researchers could see that single red, yellow or green cardiomyocytes had become clusters, ranging from two to eight cells of the same color.
The third group was mice in which heart attacks were also induced so that clinically relevant outcomes of the treatment could be studied. These mice showed improved recovery as compared to controls, including higher ejection fraction -- more blood pumped with each beat -- and smaller increases in heart size.
Enlarged hearts are a common consequence of heart attacks, with the expanded area composed of non-contractile scar tissue.
Further Testing in Human Heart Cells
With promising results in mice, next steps for the researchers will involve testing human heart cells in vitro and conduct physiological experiments in animals with more human-like hearts, such as pigs.
More than a potentially life-prolonging treatment itself, the researchers see this microRNA-gel approach as representing a new, more direct avenue for precision regenerative medicine.
"We're seeing a change in approaches for regenerative medicine, using alternatives to stem cell delivery," Burdick said. "Here, instead of introducing new cells that can have their own delivery challenges, we're simply turning on repair mechanisms in cells that survive injury in the heart and other tissues."
- Leo L. Wang, Ying Liu, Jennifer J. Chung, Tao Wang, Ann C. Gaffey, Minmin Lu, Christina A. Cavanaugh, Su Zhou, Rahul Kanade, Pavan Atluri, Edward E. Morrisey and Jason A. Burdick. 'Sustained miRNA delivery from an injectable hydrogel promotes cardiomyocyte proliferation and functional regeneration after ischaemic injury.' Nature Biomedical Engineering (2017). doi:10.1038/s41551-017-0157-y.