UC Irvine

The heart is a vital organ that pumps blood throughout the body by constantly, cyclically contracting. Unfortunately, the heart can be vulnerable a wide range of diseases and injuries. In order to truly predict, prevent, or treat diseases in the heart, the myocardial environment and how it affects myocardial repair and pathways that lead to cardiac diseases should be thoroughly investigated. However, the complex structure, genetic and environmental factors, and inaccessibility in vivo make it difficult to fully elucidate the mechanisms behind cardiac remodeling and repair. Thus, to truly study the heart, it is necessary to develop in vitro models that can closely recapitulate the myocardial environment to methodically investigate the isolated mechanisms.

The heart has a unique dynamic mechanical environment that is closely linked to its highly organized structure and specialized cells. This mechanical environment is contributed to the unique composition of cell types and the resultant structure of the tissue. Interestingly, in some heart diseases, this organization and cellular composition is disrupted, which results in loss of efficient heart function. Additionally, as the heart contracts, all the resident cells are also exposed to unique mechanical strains which can affect their function and organization. To create an accurate in vitro model of the myocardium, the mechanical strains and cellular composition of the heart should be considered. However, it is not fully known what cells contribute to the highly organized structure of the heart and how the cells are affected by mechanical strain.

To address these problems, we recapitulated in vitro both the dynamic mechanical environment of the heart and its cellular composition to demonstrate how these factors can affect the organization, morphology, and viability of cardiac cells. In order to understand how cardiac cells organize in healthy or pathological heart tissue, we applied cyclic strain to co-cultures of the two most dominant cell types within the heart, cardiomyocytes and fibroblasts. Our results illustrates that cardiomyocytes and fibroblasts do influence the organization of one another. Additionally, to recreate in vitro the organization or lack thereof observed in healthy and fibrotic heart tissue respectively, we examined tissue exposed to cyclic strain with various densities of cardiomyocytes and fibroblasts. Furthermore, when we investigated if intercellular junctions could affect organization, there were no changes in the overall orientation of the two cell types when junction formation was inhibited with drugs. Thus, either these intercellular junctions are more important in the myocardium or they play a secondary role in organization relative to the amount of cells present.

Additionally, in this dissertation we examined how mechanics mimicking the myocardial environment affects cells with and without a LMNA mutation. To investigate this, we used patient specific cells and subjected them to cyclic strains before quantifying the cell viability/proliferation, cytoskeleton and extracellular matrix organization, proportion of dysmorphic nuclei, and nuclear shape. Though our results indicated that cyclic strain alone was insufficient to cause any significant differences that could explain the mechanisms that lead to heart diseases in these patients with LMNA mutations, we were able to observe these differences in cells with a severe type of the LMNA mutation.

Overall, our results highlight the influence that different cell types have on heart tissue organization. Additionally, we emphasize the importance of the unique mechanical environment of the heart when examining cardiac tissue organization and cells with genetic mutations. The work done in this dissertation will serve as a foundation for future models of the heart that study cardiac remodeling, repair, and diseases.