Has2 expression in heart forming regions is independent of BMP signaling

Heart septation and valve malformations constitute the most common birth defects. These cardiac structures arise from the endocardial cushions through dynamic interactions between cells and the extracellular matrix (cardiac jelly). Targeted deletion of the hyaluronan synthase-2 (Has2) gene in mice results in an absence of cardiac jelly and endocardial cushions, a loss of vascular integrity, and embryonic death at E9.5. Despite the requirements for Has2 and its synthetic product hyaluronan (HA) in the developing cardiovascular system, little is known about the normal expression pattern of Has2 or the factors regulating Has2 gene transcription during development. Bmp signaling is an important regulator of cardiac myogenesis, and is also important for endocardial cushion formation. The current study defines the embryonic expression pattern of Has2 and explores the regulation of Has2 gene expression by Bmp signaling. In situ hybridization studies demonstrate dynamic Has2 expression patterns during myocardial cell development and cardiac tube formation, formation of the cardiac endocardial cushions, and cushion invasion by valve primordial cells. Despite overlapping regional expression of Bmp2 in the late gastrula anterior lateral endoderm and Has2 in the adjacent cardiogenic mesoderm, application of noggin-expressing CHO cells beneath the endoderm failed to perturb normal Has2 expression. Thus, in contrast to many genes expressed in the heart forming region, regulation of Has2 in the cardiogenic mesoderm is independent of Bmp signaling.