Cardiac morphogenesis: matrix metalloproteinase coordination of cellular mechanisms underlying heart tube formation and directionality of looping.

During heart organogenesis, the spatiotemporal organization of the extracellular matrix (ECM) undergoes significant remodeling. Because matrix metalloproteinases (MMPs) are known to be key regulators of cell-matrix interactions, we analyzed the role(s) of MMPs, and specifically MMP-2, in early heart development. Both MMP-2 neutralizing antibody and the broad-spectrum MMP inhibitor Ilomastat in a temporal manner, when applied between chick embryonic stages 5 (primitive streak stage) to stage 12 ( approximately 16-somites), produced severe heart tube defects. Exposure to the MMP inhibitor at stage 5 produced various degrees of cardia bifida. At the seven-somite stage, MMP-2/Ilomastat inhibition caused a shift in normal left-right patterning of cell proliferation within the dorsal mesocardium and mesoderm of the anterior heart field that correlated with a change in looping direction. MMP inhibition at the 10- to 12-somite stage resulted in an arrest of heart tube bending by inhibiting the breakdown of the dorsal mesocardial ECM. The experimental observations suggest that MMP activity regulates the coordination of early heart organogenesis by affecting ventral closure of the heart and gut tubes, asymmetric cell proliferation in the dorsal mesocardium to drive looping direction, and ECM degradation within the dorsal mesocardium allowing looping to proceed toward completion.