Robo1 and 2 Repellent Receptors Cooperate to Guide Facial Neuron Cell Migration and Axon Projections in the Embryonic Mouse Hindbrain.

Graphical abstract The wild-type developmental time course of facial branchiomotor nucleus is shown on the left side of the hindbrain, and the altered development in Robo1/2 double mutants on the right. Slit-expressing regions are shown by red (−) signs in the ventral midline (floor plate) and dorsal midline. The time course of cell migration is shown by a color gradient of green (E10.5) to blue to purple (E13.5). In wild type, the motor neurons first differentiate in r4, then they project primary axons to their dorsal exit point in r4 (green arrow), followed by a secondary migration of the neuron cell bodies caudally and then dorsally into r6 (curved arrow). In Robo1/2 double mutants, some primary axons project toward the dorsal exit point, but other axons project ventrally into then longitudinally within the floor plate, with many of these midline projections representing long secondary axons. Mutant neuron cell bodies migrate along three pathways, including some undertaking the normal caudal migration to r6, while other neuron cell bodies follow their axons streaming either dorsally toward the exit point or ventrally into the floor plate. Highlights • Robo1 and 2 functions were tested in facial neuron cell body migration and axon projections. • In Robo1/2 double-mutant mouse embryos, facial cell bodies and axons abnormally shifted into the hindbrain floor plate. • Some motor neurons projected two or more long axons. • Cell body tangential migration into the caudal hindbrain was disrupted. • Robo receptor-mediated repulsion is required for facial axon projection and cell body migration. Abstract The facial nerve is necessary for our ability to eat, speak, and make facial expressions. Both the axons and cell bodies of the facial nerve undergo a complex embryonic developmental pattern involving migration of the cell bodies caudally and tangentially through rhombomeres, and simultaneously the axons projecting to exit the hindbrain to form the facial nerve. Our goal in this study was to test the functions of the chemorepulsive receptors Robo1 and Robo2 in facial neuron migration and axon projection by analyzing genetically marked motor neurons in double-mutant mouse embryos through the migration time course, E10.0–E13.5. In Robo1/2 double mutants, axon projection and cell body migration errors were more severe than in single mutants. Most axons did not make it to their motor exit point, and instead projected into and longitudinally within the floor plate. Surprisingly, some facial neurons had multiple axons exiting and projecting into the floor plate. At the same time, a subset of mutant facial cell bodies failed to migrate caudally, and instead either streamed dorsally toward the exit point or shifted into the floor plate. We conclude that Robo1 and Robo2 have redundant functions to guide multiple aspects of the complex cell migration of the facial nucleus, as well as regulating axon trajectories and suppressing formation of ectopic axons. [ABSTRACT FROM AUTHOR]