MICA:Characterization of graft-host cellular niche and crosstalk to augment cardiomyocyte-based cellular therapy to treat heart failure.

In a heart attack, the heart muscle dies which impairs the pumping ability of the heart. Heart failure ensues which carries 1 in 2 chances of death within 5 years of diagnosis. With an ageing population, heart failure is increasingly common with up to ~1 million UK citizens affected currently. At this moment, heart transplantation is the only definitive cure for heart failure but only ~200 heart transplants are performed each year in the UK. Thus, heart failure is a rapidly growing unmet clinical need.

Unfortunately, the heart is the least replicative organ in the body and is unable to restore its pumping action. Stem cells are cells that can turn into any cell type in the body, and so adding new heart muscle using a stem cell-based approach holds great promise to reinstate the pumping ability of the heart. Stem cell-based heart cells have been shown in animals to restore some heart muscle.

The epicardium, the outer layer of the heart, is essential for cardiac development as it contributes to supportive cells of the heart muscle and some blood vessels. Furthermore, the epicardial cells are constantly talking to the cardiac cells. This active to-and-fro communication between cells has been shown to be crucial for healthy heart development in mammals. We recently showed that combining stem cell-derived epicardium in combination with stem cell-derived heart cells improve heart function when delivered shortly after a heart attack.

However, stem-cell-based heart cells alone were unable to improve cardiac function when the heart attack was long ago with resultant scarring and heart failure. We reasoned that the host heart's scarred environment greatly challenges the stem cell-derived heart cells' ability to merge well (i.e engraft) with the host heart. When stem cell-derived heart cells merge well with the host tissue, the cells will form new heart muscle in continuity with the host tissue. This is required to successfully restore the pumping action of the failing heart.

Based on the data from our early studies, we hypothesized that the host heart is constantly talking to the engrafted heart cells and vice versa, as they attempt to restore the failing heart. The host-graft communication will be able to affect both host's and transplanted heart cells' behaviour. We also hypothesized that this communication is altered by different host environments (i.e. scarred) and when different types of heart cells are introduced into the host heart. So as to improve the pumping action of failing hearts, we need to understand the host-graft crosstalk at the genomic level. This will enable us to alter the host-graft communication, in order to improve the heart cells' engraftment and reinstate the pumping ability of the heart.

Thus, the key goals of this study are first to identify the unique genomic signatures of infarcted rat hearts with and without cell therapy. This will enable us to understand how the individual cells are talking to each other and changing their behaviour, in response to the environment and themselves. Secondly, we will identify a panel of small molecules that will improve the ability of the stem cells-derived heart cells to engraft and restore the heart's pumping ability. Thirdly, we aim to test whether the small molecules will improve the heart cells' engraftment within human cardiac tissues in the laboratory setting. We will also use ageing & infarcted human tissues to best simulate the clinical trials setting.

Testing the engraftment of human heart cells with complex human heart tissues in the laboratory is unique. This gives us the chance to address any challenges prior to clinical trials. Overall, this project is a key step to developing stem cells-derived heart cells as a commonplace heart failure treatment - a more accessible treatment alternative to heart transplantation.

1 in 2 patients will die within 5 years from a heart failure diagnosis as there is no definitive cure to date. Human embryonic stem cells (hESC)-derived cardiomyocytes have great potential for cardiac repair and regeneration following myocardial infarction (MI) although challenges e.g. cell survival, engraftment, maturation and electrical integration remain. Successful 'primary remuscularization' of the infarcted heart was observed when (hESC)-derived cardiomyocytes were introduced soon after MI. Recently, we improved outcomes by co-delivering (hESC)-derived epicardium, a key stromal cell enabling the epicardial-myocardial crosstalk necessary for cardiac embryogenesis. However, the clinical challenge remains chronic heart failure. Previous attempts at remuscularizing the chronically infarcted hearts with (hESC)-derived cardiomyocytes alone have shown no benefit. We reasoned that the hostile environment of chronically infarcted myocardium would pose a greater engraftment challenge to the (hESC)-derived cardiomyocytes.

Our preliminary testing with alternative cellular approaches e.g. species-matched (neonatal rat cardiomyocytes) or combination cellular therapy (hESC-cardiomyocytes and hESC-epicardium) showed that cellular therapy could regenerate the chronically infarcted rat heart, albeit to a lesser extent than in the subacute MI setting.

We hypothesise that the graft-host tissue niche and its crosstalk play an instrumental role in determining successful cellular integration as well as overall cardiac functional recovery. Unfortunately, the graft-host cellular niche and its crosstalk remain poorly understood. In this application, we propose to define the graft-host niche and delineate their crosstalk in chronically failing rat hearts at the transcriptomic and spatial levels, in order to identify the principal barriers to donor-recipient integration. We will then test ways to improve cardiomyocyte integration using complex in vitro human heart systems.