Lateral positioning at the dorsal midline: Slit and Roundabout receptors guide Drosophila heart cell migration

Abstract

Heart morphogenesis requires the coordinated regulation of cell movements and cell-cell interactions between distinct populations of cardiac precursor cells. Little is known about the mechanisms that organize cardiac cells into this complex structure. In this study, we analyzed the role of Slit, an extracellular matrix protein and its transmembrane receptors Roundabout (Robo) and Roundabout2 (Robo2) during morphogenesis of the Drosophila heart tube, a process analogous to early heart formation in vertebrates. During heart assembly, two types of progenitor cells align into rows and coordinately migrate to the dorsal midline of the embryo, where they merge to assemble a linear heart tube. Here we show that cardiac-specific expression of Slit is required to maintain adhesion between cells within each row during dorsal migration. Moreover, differential Robo expression determines the relative distance each row is positioned from the dorsal midline. The innermost CBs express only Robo, whereas the flanking pericardial cells express both receptors. Removal of robo2 causes pericardial cells to shift toward the midline, whereas ectopic robo2 in CBs drives them laterally, resulting in an unfused heart tube. We propose a model in which Slit has a dual role during assembly of the linear heart tube, functioning to regulate both cell positioning and adhesive interactions between migrating cardiac precursor cells.cell adhesion ͉ dorsal vessel ͉ heart morphogenesis

Cellular components and signals required for the cardiac outflow tract assembly in Drosophila

“…Thus, one possibility is that COMs are attracted to the heart tip via Slit-Robo signaling and that inability of COMs to interact with the heart tip in a slit mutant background contributes to the slowdown in HANC cell movements. The potential role of Slit in COM attraction is in agreement with expression of Slit in the cardioblasts (26)(27)(28) and Robo2 in COM muscles and is supported by the disrupted COM-heart cell contact in a musclespecific knockdown of robo2 via RNAi (Fig. 4J).…”

“…We first attempted to determine whether the candidate molecules are expressed in migrating HANC cells and the anterior heart tube. It has been shown that Slit is expressed in cardioblasts and in pericardial cells during embryonic stages 14 and 15 along the entire cardiac primordium (26)(27)(28). Slit expression was also observed in the head region in cells that form the invaginating dorsal pouch (38).…”

“…The precise spatial positioning of the OFT components originating from the distinct tissue primordia raises the question of how their assembly is controlled. We reasoned that the Slit-Robo signaling pathway that had been found to play a critical role in cardiac primordia assembly (26)(27)(28) and in the attraction of somatic muscle fibers to their tendonous attachment sites (29) is a good candidate for controlling the OFT morphogenesis. We also inferred from the previous work (30) that the DE-cadherin Shg that is expressed in the heart and involved in cardiac lumen formation may also control interactions between the HANC cells and the anterior cardioblasts.…”

“…To gain insights into signals governing OFT assembly, we first focused on the Slit-Robo signaling pathway, which has been shown to be involved in neural crest cell migration in vertebrates (24)(25), in Drosophila heart morphogenesis (26)(27)(28), and in the attraction of muscles to their attachment sites (29). We also tested the expression and function of DE-cadherin Shotgun (Shg), which is known to play important functions in the morphogenesis of the cardiac tube (30).…”

Molecular Mechanisms Controlling Midline Crossing by Precerebellar Neurons

“…In Drosophila, Slit/ Robo signaling also prevents tracheal, sensory precursor cells, and cardiac cells from approaching the midline (Lundstrom et al, 2004;Orgogozo et al, 2004;Santiago-Martinez et al, 2006). In vertebrates, a similar function for Slit/Robo in the control of neuronal migration at the midline has not been demonstrated.…”