MicroRNAs: novel targets for cardiac regenerative medicine

Abstract

In recent years, much research has been done on stem cell therapy for the injured heart. While the effects of stem cell therapy are promising, there is still much room for improvement as only a low percentage of transplanted cells actually engraft in the injured heart. This low transplantation efficiency is commonly attributed to the extremely poor survival and functional integration of the transplanted cells, and signifies the need for novel strategies that increase the effectiveness of cell-based therapies, thereby potentiating cardiac repair. In the past few years it has become apparent that microRNAs (miRNAs) are key regulators of stem cell function and tightly regulate various pathophysiological processes in the heart. In this thesis, the potential role of miRNAs in various processes which are necessary to improve cell therapy and enhance cardiac regeneration is investigated. The focus lies on the use of adult human cardiomyocyte progenitor cells (hCMPCs) as they are one of the most promising options for treating the injured heart. They can differentiate into all cell types of the heart and, after transplantation in the infarcted heart, prevent further deterioration of heart function. Chapter 2 compares the miRNA expression profile of proliferating hCMPCs with hCMPCs differentiated into cardiomyocytes, and shows that miR-1 and miR-499 greatly enhance the cardiomyogenic differentiation of hCMPCs by targeting Sox6, a transcription factor involved in muscle development. In Chapter 3, it is demonstrated that miR-1 is also involved in the angiogenic differentiation of hCMPCs, and targets Spred1, a negative regulator of growth factor-induced ERK/MAP kinase activation. These first two chapters indicate that miRNAs can be used to direct and improve the differentiation of hCMPCs into the three cardiovascular lineages. In Chapter 4, miR-155 is shown to inhibit necrotic cell death in hCMPCs, which is mediated by targeting RIP1, a protein required for programmed necrotic cell death. The same miRNA is investigated in Chapter 5, where it is demonstrated that miR-155 inhibits hCMPC migration by targeting MMP16, an enzyme involved in the breakdown of extracellular matrix. These two chapters show that miRNAs can be used to increase hCMPC survival and regulate their migratory capacity, providing the possibility of improving cell survival and cell retention upon cell injection. Chapter 6 provides a detailed overview and small history of the various applications of small RNA technology in vivo, including the current methods for in vivo modulation of miRNA function. In Chapter 7, it is shown, by using a specific miRNA inhibitor, that inhibition of miR-214 enhances angiogenesis. MiR-214 negatively regulates angiogenesis by targeting QKI, a protein critical for vascular development, and thereby negatively regulates the expression and secretion of several pro-angiogenic growth factors. This study indicates that miRNA modulation may be used to enhance vascularization, an essential regenerative process necessary to achieve effective cell engraftment and survival. Taken together, this thesis shows that miRNAs are able to regulate various cellular processes that are of great importance to the success of cell-based therapy, and therefore, miRNAs provide novel therapeutic targets to promote cardiac regeneration.