Interference with Axonin-1 and NrCAM Interactions Unmasks a Floor-Plate Activity Inhibitory for Commissural Axons

Abstract

Axonin-1 and NrCAM were previously shown to be involved in the in vivo guidance of commissural growth cones across the floor plate of the embryonic chicken spinal cord. To further characterize their role in axon pathfinding, we developed a two-dimensional coculture system of commissural and floor-plate explants in which it was possible to study the behavior of growth cones upon floor-plate contact. Although commissural axons readily entered the floor plate under control conditions, perturbations of either axonin-1 or NrCAM interactions prevented the growth cones from entering the floor-plate explants. The presence of antiaxonin-1 resulted in the collapse of commissural growth cones upon contact with the floor plate. The perturbation of NrCAM interactions also resulted in an avoidance of the floor plate, but without inducing growth-cone collapse. Therefore, axonin-1 and NrCAM are crucial for the contact-mediated interaction between commissural growth cones and the floor plate, which in turn is required for the proper guidance of the axons across the ventral midline and their subsequent rostral turn into the longitudinal axis.

Semaphorin 3d guides laterality of retinal ganglion cell projections in zebrafish

“…Similarly, Xenopus RGC axons are attracted to Netrin as they exit the eye, lose responsiveness to Netrin as they cross the midline, and become repelled by Netrin as they approach the optic tectum (Shewan et al, 2002). Both sensitivity and modality of growth cone responses to particular cues can be modulated by a number of factors, including expression levels of specific receptor components (Hong et al, 1999;Julien et al, 2005;Keleman et al, 2002;Wolman et al, 2004), activation of other surface molecules (Hopker et al, 1999;Kantor et al, 2004;Sabatier et al, 2004;Stein and Tessier-Lavigne, 2001;Stoeckli et al, 1997), and levels of intracellular signaling molecules such as cyclic nucleotides (Polleux et al, 2000;Song et al, 1998;Song et al, 1997), indicating that both intrinsic and extrinsic factors are probably involved in determining how RGC growth cones respond to midline Sema3d.…”

Ephrin-B3, a ligand for the receptor EphB3, expressed at the midline of the developing neural tube

“…Third, an activity of the¯oor plate may repel axons, and thereby prevent ipsilateral axons from crossing the midline, or prevent contralateral axons from re-crossing. Indeed, recent studies have detected a contact-dependent repellent activity in oor plate (Stoeckli et al, 1997). In principle, the roof plate expression of ephrin-B3 could similarly act to inhibit axons from crossing to the contralateral side of the neural tube.…”

DSCAM functions as a netrin receptor in commissural axon pathfinding

“…The YFP plasmid was electroporated into the neural tube along with wildtype DSCAM or mutant DSCAM⌬C. Open-book preparations of spinal cords at stage 23 were immunostained with the anti-axonin-1 antibody (35,36). More than 90% of the axons expressing YFP stained positive for axonin-1 (Fig.…”