Long-Range Guidance of Spinal Commissural Axons by Netrin1 and Sonic Hedgehog from Midline Floor Plate Cells

Abstract

Graphical Abstract Highlights d Floor-plate-derived netrin1 (FP-netrin1) guides commissural axons at long range d FP-netrin1 and Shh collaborate to guide commissural axons in the ventral spinal cord d Both FP-netrin1 and ventricular zone-netrin1 contribute to commissural axon guidance d FP-netrin1 may guide via haptotaxis and/or chemotaxis In Brief Recent studies have queried the role of netrin1 from floor plate (FP-netrin1) in guiding commissural axons. Wu et al. show that, in spinal cord, FP-netrin1 is required and acts at long range to guide commissural axons, collaborating with Shh. SUMMARYAn important model for axon pathfinding is provided by guidance of embryonic commissural axons from dorsal spinal cord to ventral midline floor plate (FP). FP cells produce a chemoattractive activity, comprised largely of netrin1 (FP-netrin1) and Sonic hedgehog (Shh), that can attract the axons at a distance in vitro. netrin1 is also produced by ventricular zone (VZ) progenitors along the axons' route (VZ-netrin1). Recent studies using region-specific netrin1 deletion suggested that FP-netrin1 is dispensable and VZ-netrin1 sufficient for netrin guidance activity in vivo. We show that removing FP-netrin1 actually causes guidance defects in spinal cord consistent with long-range action (i.e., over hundreds of micrometers), and double mutant analysis supports that FP-netrin1 and Shh collaborate to attract at long range. We further provide evidence that netrin1 may guide via chemotaxis or haptotaxis. These results support the model that netrin1 signals at both short and long range to guide commissural axons in spinal cord.

Loss of floor plate Netrin-1 impairs midline crossing of corticospinal axons and leads to mirror movements

“…Shh::cre;Ntn1 lox/lox mice is a viable mouse model, in which Ntn1 depletion is restricted to the floor plate in the regions of the CNS located caudal to the midbrain/hindbrain boundary. Interestingly, midline crossing of spinal cord and hindbrain commissural axons is mostly preserved in this model [11][12][13][14] . Yet, development of the corticospinal axons, which occurs over a protracted period, has not been investigated in these mice.…”

“…This unveils a distinct and unique guidance mechanism for corticospinal axons, since FP-Netrin-1 was proven to be dispensable or mostly dispensable for midline crossing of hindbrain commissural axons and spinal commissural axons respectively. Guidance of these ventral commissural axons, which occurs during early embryonic stages, relies mainly on another source of Netrin-1, produced by the progenitor cells of the ventricular zone [11][12][13][14] . Interestingly, at the time when corticospinal axons cross the CNS midline (around birth), these progenitor cells are virtually absent 19 .…”

“…In Shh::cre;Ntn1 lox/lox mice, disruption of the CST decussation results in abnormal but functional ipsilateral projections. Previous characterization of Shh::cre;Ntn1 lox/lox mice revealed that guidance of commissural axons in the spinal cord was mostly preserved [11][12][13][14] . In Shh::cre;Ntn1 lox/lox mice, we found no alteration of Ntn1 expression in the forebrain ( Supplementary Fig.…”

Early neuronal processes interact with glia to establish a scaffold for orderly innervation of the cochlea

“…Testing such a model is not straightforward as one would not expect axons to become completely lost but instead to be slightly slower or make more mistakes en route. Identifying these types of subtle guidance defects will require analysis of individual axon trajectories and more careful quantification, similar to how a role for floor plate-derived Netrin-1 was demonstrated 8,9 . It is also possible that the glia themselves provide additional synergizing cues that directly impact neurite outgrowth behavior.…”

Neuron subtype-specific effector gene expression in the Motor Ganglion of Ciona

“…More specifically, netrin1 from the floorplate of the developing hindbrain is dispensable for midline crossing (Dominici et al, 2017;Yamauchi et al, 2017), which is mostly regulated by netrin1 distributed along the axon path and derived from ventricular zone progenitor progenitors instead. In the spinal cord, both netrin from both sources act synergistically as both short-and long-range cues to guide midline crossing (Dominici et al, 2017;Moreno-Bravo et al, 2019;Varadarajan et al, 2017;Wu et al, 2019). Furthermore, a role for UNC-6/netrin in C. elegans is instructive for neuronal polarization and defines the site of axonogensis (Adler et al, 2006).…”