Stem cells have the capacity to heal damaged tissues and replace death cells. However, harvesting and employing the right stem cell for the right job, at the right dose, and under the right conditions has proven to be a difficult puzzle to solve.
In particular, the damaged heart has proven rather difficult to heal with stem cells. Many different clinical trials have administered stem cells by direct injection, intracoronary delivery in coronary vessels, or injection into the surface of the heart. These studies have examined the efficacy of stem cells from fat, bone marrow, the heart itself, and other sources. The upshot of these studies is that some strategies work and others do not, but even those that work only work modestly well.
The biggest obstacle is overcoming the hostile environment into which stem cells are introduced when they are administered into the heart after a heart attack. The post-heart attack heart suffers from lots of inflammation, low oxygen concentrations, and the pervasive presence of dangerous molecules. Several laboratories have discovered that preconditioning stem cells by growing them in low-oxygen culture conditions can increase their survival in the post-heart attack heart as can genetically engineering cells to resist increased levels of cell stress. Now, a research team from Ohio State University has designed a different strategy to beat the hostility of the post-heart attack heart.
After a heart attack, oxygen-deprived tissues die and various chemical messengers instruct damaged cells to die. However, data from a host of clinical trials strongly suggests that this dangerous time is the best time to introduce stem cells into the heart. Thus, this window of opportunity is the “best of times and the worst of times” for cell therapy.
As it turns out, about 30% of all mammalian protein-encoding genes are regulated by small molecules called microRNAs (miRNAs). MiRNAs are single-stranded RNA molecules approximately 22 nucleotides in length that bind to messenger RNAs and regulate their translation into protein or half-life. Research has shown that miRNAs have substantial potential as a therapeutic target for the treatment of many diseases, including cardiovascular disease. A good deal of research in laboratory animals and in cultured heart cells that altered expression of miRNAs such as miR-1, miR-133, miR-21 and miR-208 contribute to the development of heart disease. The laboratory of Mahmood Khan, a scientist at the Davis Heart and Lung Research Institute at The Ohio State University Wexner Medical Center, has focused on miR-133a which seems to play a role in slowing fibrosis and cardiac remodeling. Importantly, levels of miR-133a are reduced in the heart tissues of patients who have suffered a heart attack.
Khan and his group predicted that increasing the levels of miR-133a in stem cells as they are cultured might preprogram the cells to survive in the hostile environment of the post-heart attack heart.
Khan’s team began by bioengineering a molecule to induce mesenchymal stem cells (MSCs) to produce miRNA-133a. When transplanted into an animal model of cardiac ischemia, the pre-treated MSCs showed improved survival over non-treated MSCs. These pre-treated MSCs also did a better job at decreasing the global damage to the heart, and increasing the thickness of the left ventricle, the main pumping chamber of the heart.
“We found that the pre-treated MSCs did a better job at decreasing the global damage to the heart, along with improvement in the left-ventricular wall thickness compared to the untreated MSCs,” said Dr. Angelos. “MSCs are a commonly used cell type in current heart failure studies, so our findings are definitely relevant to that work.”
The results of these experiments were published in the March issue of the Journal of Cardiovascular Pharmacology.
While trying to increase cell survival, Khan and his colleagues also addressed the problems surviving stem cells face in the heart – how to help them function along existing heart tissue without getting in the way or fouling things up.
To date, most stem cells are grown in flat culture plates and either injected directly into the heart muscle (typically on the periphery of scar tissue), or infused into the heart via an artery. While most of these stem cells either die or diffuse throughout the body, successfully transplanted stem cells sometimes inadvertently disrupt heart function.
“The heart is a constantly moving, connected matrix of muscle fibers working together to make the heart pump in sync,” said Dr. Angelo’s, an emergency medicine researcher and collaborator of Dr Khan. “Transplanted stem cells may not align with native tissue, potentially disrupting or attenuating signals that keep a steady heartbeat. There’s evidence that this could contribute to arrhythmias.”
To create a more secure environment that allows implanted stem cells successfully engraft into the heart, Drs. Khan and Angelos have used a biodegradeable nanofiber “patch” seeded with human inducible pluripotent stem cells derived cardiomyocytes (hiPSC-CMs). Khan and Angelos chose hiPSC-CMs because these cells are patient derived and can be used to model the heart disease of patients and for autologous stem cell transplantation in patients with failing hearts.
Both aligned nanofiber patch and standard culture plate were seeded with hiPSC-CMs. Both sets of cultures heart muscle cells were compared for calcium signaling (a measure of proper heart muscle function) and synchronous beating. Within two weeks, both stem cell cultures were spontaneously beating like a miniature heart, but the linear grain of the nanofiber formed an aligned pattern of cells that looked and functioned like a healthy heart tissue.
“The cardiomyocytes cultured on a flat plate are scattered and disorganized. Cardiomyocytes grown on the nanofiber scaffolding look more like healthy heart cells, beat more strongly and in greater synchronicity than cells from the flat plate,” said Dr. Khan. “Next, we hope to use what we’ve learned from this study to develop a thicker, multi-layer patch that could help restore thin and weakened heart walls.”
Drs. Khan and Angelos see great potential future clinical applications for their nanofiber bandage. This treatment could potentially bandage the damaged heart muscle of heart patients with the nanofiber cardiac patch. Also, it is possible that they could someday combine the microRNA pre-treatment technique and the patch to give stem cells a survival boost along with a protective structure to improve outcomes.
Khan, his co-workers and his collaborators published this work on May 19 in PLoS ONE.