SUMMARY Proper heart development is essential for efficient heart function throughout life. Congenital heart defects (CHDs) are the most common congenital malformations, occurring in up to 1% of live births and 10% of still births. CHDs can also lead to increased risk of cardiovascular diseases in adults. During vertebrate heart development, cardiomyocyte progenitors are surrounded by a rich extracellular matrix (ECM). Mutations in several ECM proteins in humans can lead to CHDs. However, the mechanisms underlying CHDs associated with mutations that affect the ECM remain poorly understood. Signals from the ECM are detected by Integrin receptors on target cells. Our preliminary analysis of zebrafish carrying loss-of-function mutations in both the Integrin alpha5 and Integrin alpha4 genes indicate that these Integrins are reiteratively required to promote proper cardiac morphogenesis and ventricle size. The overall goal of this proposal is to elucidate mechanisms by which cardiomyocytes interpret and integrate signals from the ECM to execute proper cardiomyocyte migration and heart growth. The Specific Aims of this proposal are: Aim 1 - Determine mechanisms by which Integrin signaling mediates proper cardiomyocyte migration. This aim will use histological techniques and confocal time lapse microscopy of live embryos to test the hypothesis that Integrin signaling promotes directed cardiomyocyte migration by establishing an organized, polarized epithelium; Aim 2 - Determine the role of Integrins in ventricle development. This aim combines cardiomyocyte number quantification and temporal differentiation assays to test the hypothesis that Integrins drive heart growth by promoting addition of later- differentiating second heart field (SHF) cardiomyocytes to the ventricle and outflow tract; Aim 3 - Determine the tissue-specific and temporal requirements for Integrins in promoting cardiomyocyte migration and SHF addition. This aim uses novel transgenic lines to spatially and temporally regulate Integrin expression to test the hypothesis that Integrin signaling acts specifically in the cardiomyocytes, first to promote cardiomyocyte migration, and later to promote second heart field addition. By illuminating how different cardiomyocyte populations integrate and interpret signals from the surrounding ECM, our results may provide insight into the etiology of CHDs in humans, and can be applied to novel tissue engineering therapies aimed at restoring efficient heart function.