Transcriptional control of endocardial-specific gene expression - SUMMARY Congenital heart disease (CHD) is a significant cause of morbidity and mortality worldwide, affecting nearly 1% of all live births, and there is a critical need to understand the cellular and molecular processes that control normal heart development and to understand how dysregulation of those processes leads to CHD. Beyond the burden of CHD, adult cardiovascular disease remains the most common cause of mortality worldwide, and there is an urgent need for strategies to intervene in adult heart disease. The endocardium, the single cell layer of endothelial cells that form the heart’s inner lining, plays several important roles in heart development and homeostasis. The endocardium functions as a signaling source for myocardial growth, is essential for the development of the trabeculae, and is the primary source of cells that undergo endocardial-to-mesenchymal transformation (EMT) to form the valves and membranous ventricular septum. The endocardium also functions as a critical sensor of cardiac injury and is essential for promoting cardiomyocyte cell cycle reentry and heart regeneration in zebrafish, neonatal mice, and to a very limited extent in adult mammals. Perturbations in endocardial gene expression are associated with several forms of congenital heart disease, including hypoplastic left heart syndrome and valve defects. However, the gene regulatory networks that specify the endocardium remain largely undefined and, compared to the development of other endothelial cell types, much less is known about the key transcription factor combinations that control endocardial-specific gene expression in development, homeostasis, and in response to injury or other stresses. Unpublished work presented in this application has identified a large cohort of novel endothelial-specific enhancers, including subsets with activity restricted solely to the endocardium and to the valves. In addition, an endocardial-specific enhancer associated with the slc16a2 gene is activated in response to heart injury throughout the endocardium, including distal to the site of injury. slc16a2 encodes a thyroid hormone transporter . Given the important role of thyroid hormone in suppressing heart regenerative potential in both zebrafish and neonatal mice, the slc16a2 gene, its product, and its enhancer may be important targets for modulating the regenerative potential of the heart. Overall, the enhancers being dissected in this work provide a unique and important set of tools to address the hypothesis that discrete transcription factor combinations bind enhancers and regulate chromatin state to control endocardial- and valve- specific gene expression. More specifically, this project will identify transcriptional regulators of endocardial- restricted gene expression, determine how valve-restricted expression is achieved, and define the regulation and function of slc16a2 during heart injury and regeneration.
