Llgl1 mediates timely epicardial emergence and establishment of an apical laminin sheath around the trabeculating cardiac ventricle

During heart development, the embryonic ventricle becomes enveloped by the epicardium, a layer of mesothelium which adheres to the outer apical surface of the heart. This is concomitant with onset of ventricular trabeculation, where a subset of cardiomyocytes lose apicobasal polarity and delaminate basally from the ventricular wall, projecting into the cardiac lumen to begin building the muscle mass necessary for adult cardiac function. Lethal(2) giant larvae homolog 1 (Llgl1) regulates the formation of apical cell junctions and apicobasal polarity, and we investigated its role in ventricular wall maturation, including trabeculation and epicardial establishment. We found thatllgl1mutant zebrafish embryos exhibit aberrantly positioned cardiomyocytes during early trabeculation, some of which extrude apically into the pericardial space. While investigating apical cardiomyocyte extrusion we identified a basal to apical shift in laminin deposition in the ventricular wall. Initially laminin deposition occurs on the luminal (basal) surface of the heart but concomitant with the onset of trabeculation basal laminin is removed and is instead deposited on the exterior (apical) surface of the ventricle. We find that epicardial cells express several laminin subunits as they adhere to the ventricular wall, and show that the epicardium is required for laminin deposition on the ventricular surface. Inllgl1mutants the timing of the basal-apical laminin shift is delayed, in line with a delay in establishment of the epicardial layer. Analysis of earlier epicardial development reveals that while both Llgl1 and laminin are not required for specification of the proepicardial organ, they are instead required for dissemination of epicardial cells to the ventricular surface. Together our analyses reveal an unexpected role for Llgl1 in correct timing of epicardial development, supporting integrity of the myocardial wall during early trabeculation.