RESUMEN
Spinal commissural axon navigation across the midline in the floor plate requires repulsive forces from local Slit repellents. The long-held view is that Slits push growth cones forward and prevent them from turning back once they became sensitized to these cues after midline crossing. We analyzed with fluorescent reporters Slits distribution and FP glia morphology. We observed clusters of Slit-N and Slit-C fragments decorating a complex architecture of glial basal process ramifications. We found that PC2 proprotein convertase activity contributes to this pattern of ligands. Next, we studied Slit-C acting via PlexinA1 receptor shared with another FP repellent, the Semaphorin3B, through generation of a mouse model baring PlexinA1Y1815F mutation abrogating SlitC but not Sema3B responsiveness, manipulations in the chicken embryo, and ex vivo live imaging. This revealed a guidance mechanism by which SlitC constantly limits growth cone exploration, imposing ordered and forward-directed progression through aligned corridors formed by FP basal ramifications.
Asunto(s)
Interneuronas Comisurales/fisiología , Médula Espinal/crecimiento & desarrollo , Animales , Axones/fisiología , Western Blotting , Embrión de Pollo , Conos de Crecimiento/fisiología , Ratones , Microscopía Fluorescente , Tubo Neural/embriología , Tubo Neural/crecimiento & desarrollo , Médula Espinal/embriologíaRESUMEN
Accurate perception of guidance cues is crucial for cell and axon migration. During initial navigation in the spinal cord, commissural axons are kept insensitive to midline repellents. Upon midline crossing in the floor plate, they switch on responsiveness to Slit and Semaphorin repulsive signals and are thus propelled away and prevented from crossing back. Whether and how the different midline repellents control specific aspects of this navigation remain to be elucidated. We set up a paradigm for live-imaging and super-resolution analysis of PlexinA1, Neuropilin-2, and Robo1/2 receptor dynamics during commissural growth cone navigation in chick and mouse embryos. We uncovered a remarkable program of sensitization to midline cues achieved by unique spatiotemporal sequences of receptor allocation at the growth-cone surface that orchestrates receptor-specific growth-cone behavior changes. This reveals post-translational mechanisms whereby coincident guidance signals are temporally resolved to allow the generation of specific guidance responses.
Asunto(s)
Axones/fisiología , Proteínas del Tejido Nervioso/metabolismo , Semaforinas/metabolismo , Animales , Membrana Celular/metabolismo , Embrión de Pollo , Pollos , Embrión de Mamíferos/metabolismo , Conos de Crecimiento/metabolismo , Ratones , Proteínas del Tejido Nervioso/química , Dominios Proteicos , Receptores de Superficie Celular/química , Receptores de Superficie Celular/metabolismo , Receptores Inmunológicos/metabolismo , Proteínas Recombinantes/metabolismo , Factores de Tiempo , Proteínas RoundaboutRESUMEN
The navigation of commissural axons in the developing spinal cord has attracted multiple studies over the years. Many important concepts emerged from these studies which have enlighten the general mechanisms of axon guidance. The navigation of commissural axons is regulated by a series of cellular territories which provides the diverse guidance information necessary to ensure the successive steps of their pathfinding towards, across, and away from the ventral midline. In this review, we discuss how repulsive forces, by propelling, channelling, and confining commissural axon navigation, bring key contributions to the formation of this neuronal projection.