Netrin-1 Acts as a Repulsive Guidance Cue for Sensory Axonal Projections toward the Spinal Cord

Abstract

During early development, the ventral spinal cord expresses chemorepulsive signals that act on dorsal root ganglion (DRG) axons to help orient them toward the dorsolateral part of the spinal cord. However, the molecular nature of this chemorepulsion is mostly unknown. We report here that netrin-1 acts as an early ventral spinal cord-derived chemorepellent for DRG axons. In the developing mouse spinal cord, netrin-1 is expressed in the floor plate of the spinal cord, and the netrin receptor Unc5c is expressed in DRG neurons. We show that human embryonic kidney cell aggregates secreting netrin-1 repel DRG axons and that netrin-1-deficient ventral spinal cord explants lose their repulsive influence on DRG axons. In embryonic day 10 netrin-1 mutant mice, we find that DRG axons exhibit transient misorientation. Furthermore, by means of gain-of-function analyses, we show that ectopic netrin-1 in the dorsal and intermediate spinal cord prevents DRG axons from being directed toward the dorsal spinal cord. Together, these findings suggest that netrin-1 contributes to the formation of the initial trajectories of developing DRG axons as a repulsive guidance cue.

Expression of seven members of the ADAM family in developing chicken spinal cord

“…Wnt/b-catenin signaling promotes sensory neurogenesis in early neural crest cells (Lee et al, 2004), while bone morphogenic protein (BMP) signals induce neural crest cells to differentiate into sympathetic neurons (Reissmann et al, 1996). Semaphorin3A, axonin-1/SC2, and netrin-1 are required as repulsive guidance cues for the formation of sensory axonal projections to the spinal cord (Masuda et al, 2003(Masuda et al, , 2008.…”

Netrin-1 signaling for sensory axons

“…7,8 First, in the mouse embryo at embryonic day (E) 10-11.5 when many DRG axons orient themselves to reach the DREZ, netrin-1 is strongly expressed in the floor plate of the ventral spinal cord but not in the dorsal spinal cord (Fig. 1A).…”

“…We clearly showed that netrin-1 exerts a chemorepulsive activity toward developing DRG axons and that the ventral spinal cord-derived repulsive activity depends on netrin-1 in vitro. 8 Additional evidence for a chemorepulsive role of netrin-1 came from the observation of DRG axonal trajectories in netrin-1-deficient mice. 7,8 In netrin-1-deficient embryos at E10, we showed that some DRG axons became misoriented toward the ventral spinal cord, probably because of the absence of netrin-1 proteins in the ventral spinal cord (Fig.…”

“…8 In addition, both postnatal and adult DRG neurons respond to netrin-1-induced axon inhibition. 9 Consistent with these results, DRG neurons at not only later developmental stages (E13.5) but also postnatal stages express the repulsion-mediating netrin-1 receptor Unc5c.…”

“…9 Consistent with these results, DRG neurons at not only later developmental stages (E13.5) but also postnatal stages express the repulsion-mediating netrin-1 receptor Unc5c. 8,9 Generally, lesioning of the dorsal column projection of sensory axons results in a complete lack of regeneration. The possible explanation for the complete lack of regeneration is that the environment, the lesion site itself and/or oligodendrocytes adjacent to the lesion, may be non-permissive for regenerating axons.…”