Dscam Switches Slit Repulsion To Mild Attraction Via Robo Receptors

Background and Aim : Axon guidance is a critical part of neural development, the process that generates and shapes the nervous system, from the earliest stages of embryogenesis to the last years of life. Axons navigate to reach their correct targets via different axon guidance cues, such as Netrins and Slits. The axonal growth cone contains receptors that distinguish these guidance molecules and translates them into attractive or repulsive responses. Slit repels axons from the CNS midline by binding to the Robo (Roundabout) receptor. Netrin acts as an attractant through fra/DCC/Unc-40 and Dscam (Down syndrome cell adhesion molecule) receptors. However, genetic evidence shows that Dscam also responds to other ligand(s).Methods : In vivo, slit-FL and slit-N transgenes have differential effects on motor neuron innervation of muslces that are mediated by Dscam. Overexpression of slit-FL and slit-N at the CNS midline in a robo mutant background leads to an increase in axon attraction to the midline. We interpret this result as evidence that Slit has an attractive as well as a repulsive function. To test the hypothesis that Dscam can act as an attractive receptor for Slit, we employed a range of slit transgenes to analyze attractive functions of slit in axon guidance. Our data argues that Slit needs to be processed to act as an axonal attractant in vivo. We examined the consequence of removing the Robo binding domain (LRR2) from Slit in vivo, hoping to observe the attractive function masked by Slit’s repulsive activity. Instead, the transgene appears only to inhibit Dscam activity, supporting a model that Dscam requires Robo as a co-receptor.Results : We have identified Slit as an additional ligand for Dscam using both cell overlay and immunoprecipitation assays. Our results show that the Dscam only binds to the Slit N-terminal fragment (Slit-N), in a domain distinct from the Robo binding site. I have demonstrated that Robo preferentially binds full length Slit (Slit-FL), but in the presence of Dscam binds Slit-N. We believe that this Slit-N dependent Dscam-Robo complex modifies Robo signaling.Conclusion : Our data shows that Dscam binds to proteolytically processed Slit and converts repulsion to mild attraction in the presence of Robo receptors. Our work also indicates that, as seen for other ligands, Slit can act as both an attractant and repellent via distinct receptors, and indicates how the complexity of the nervous system can arise through a relatively small number of ligands.

کلیدواژه ها:

Central Nervous System ، Chemoattractant axon guidance ، pioneer axons ، Axon regeneration