Developmental studies in insects and vertebrates have begun to explain how commissural axons manage to traverse the midline only once. Proteins such as the netrins attract their growth cones to the midline. Then, as they cross, the growth cones upregulate Roundabout (Robo), whose interaction with its ligand Slit generates repulsion. However, there is also growing evidence that growth cones lose their attraction for netrin as they cross the midline, although they often continue to express the netrin receptor DCC. In Science, Stein and Tessier-Lavigne report that the interaction between Slit and Robo might provide the key to the silencing of this attraction.

The authors exposed spinal neurons from stage 22 Xenopus embryos to Slit2 and netrin-1 in vitro. They showed that the growth cones were attracted towards netrin-1, but the attraction was silenced in the presence of Slit2. Surprisingly, the growth cones were not repelled by Slit2, either on its own or in conjunction with netrin-1. This is interesting because it confirms that the silencing of attraction to netrin-1 is not simply a net response to competing attractive and repulsive forces. Stage 28 growth cones were repelled by Slit2 but were not attracted by netrin-1, perhaps reflecting changes in responsiveness during neuronal maturation.

Removal of the cytoplasmic domains of DCC or Robo prevented Slit2-mediated silencing of attraction to netrin-1. By constructing chimeric proteins, the authors showed that silencing could be achieved even when the Robo cytoplasmic domain was activated through a different ectodomain such as Met, which binds hepatocyte growth factor (also known as scatter factor) rather than Slit. Therefore, silencing seems to be mediated by activation of Robo rather than through a direct interaction between Slit2 and DCC. Indeed, immunoprecipitation experiments confirmed that Slit2 promotes the association of Robo with DCC.

Stein and Tessier-Lavigne propose that the binding of Slit to Robo induces a conformational change that enables the cytoplasmic domain of Robo to interact with that of DCC, thereby silencing the attraction between netrin-1 and its receptor. So, activation of Robo by Slit2 could both inhibit the attraction of growth cones to the midline and induce their repulsion away from it, providing a simple switch to prevent them from being presented with conflicting signals.