Myosin waves and a mechanical asymmetry guide the oscillatory migration of Drosophila cardiac progenitors

Heart development begins with the formation of a tube, as cardiac progenitors migrate from opposite sides of the embryo and meet medially. Defective cardiac progenitor movements cause congenital heart defects. However, the mechanisms of cell migration during early heart development remain poorly understood. We investigated the mechanisms of movement of the Drosophila cardiac progenitors, the cardioblasts. Using quantitative time-lapse microscopy, we found that cardioblasts did not advance monotonically. Instead, cardioblasts took periodic forward and backward steps as they migrated. The forward steps were greater in both amplitude and duration, resulting in net forward movement of the cells. The molecular motor non-muscle myosin II displayed an alternating pattern of localization between the leading and trailing ends of migrating cardioblasts, forming oscillatory waves that traversed the cells. The alternating pattern of myosin polarity was associated with the alternative contraction and relaxation of the leading and trailing edges of the cell. Mathematical modelling predicted that forward migration requires the presence of a stiff boundary at the trailing edge of the cardioblasts. Consistent with this, we found a supracellular actin cable at the trailing edge of the cardioblasts. When we reduced the tension sustained by the actin cable, the amplitude of the backward steps of cardioblasts increased, thus reducing the net forward speed of migration. Together, our results indicate that periodic cell shape changes coupled with a polarized actin cable produce asymmetrical forces that guide cardioblast migration.