Differential expression of alpha-6 and other subunits of laminin binding integrins during development of the murine heart.

The development of the heart from a single heart tube to a four chambered organ with two separated unidirectional flows is a highly complex process. Events like looping, septation, tissue remodelling, and development of valves take place in a time period in which the heart already exerts its pump function. Adhesion of cells to each other and to their extracellular matrix as well as the capability to migrate in such a dynamic environment are extremely important. Integrins and extracellular matrix components have already been implicated in this process. In this report, we describe in detail the differential expression of the alpha-6 integrin subunit during late murine heart development, e.g., in the process from looping to the end of septation. We compare mRNA and protein expression patterns with those of beta-1 and other subunits of laminin-binding integrins, alpha-3 and alpha-7. We show a constant and high expression of alpha-6 in the atrial myocardium and a decrease in expression in the ventricular trabecular myocardium. The compact myocardial wall and the ventricular septum do not express alpha-6, except for the myocardium of the distal outflow tract at early stages. Moreover, we describe expression of this integrin subunit in the endocardial cushions that contribute to the development of the atrioventricular and semilunar valves. We propose a role for the alpha-6-beta-1 laminin receptor in the adhesion of cells to their extracellular matrix at sites of high stress due to cardiac contraction or blood flow induced shear stress. Moreover, site specific endothelial expression within the heart and surrounding extracardiac tissue is discussed. This study suggests a distinct role for alpha-6-beta-1 in the heart and provides insight concerning probably important roles of integrins and their extracellular matrix ligands during embryonic development.