Netrin1 establishes multiple boundaries for axon growth in the developing spinal cord

Abstract

The canonical model for netrin1 function proposed that it acted as a long-range chemotropic axon guidance cue. In the developing spinal cord, floor-plate (FP)-derived netrin1 was thought to act as a diffusible attractant to draw commissural axons to the ventral midline. However, our recent studies have shown that netrin1 is dispensable in the FP for axon guidance. We have rather found that netrin1 acts locally: netrin1 is produced by neural progenitor cells (NPCs) in the ventricular zone (VZ), and deposited on the pial surface as a haptotactic adhesive substrate that guides Dcc+ axon growth. Here, we further demonstrate that this netrin1 pial-substrate has an early role orienting pioneering spinal axons, directing them to extend ventrally. However, as development proceeds, commissural axons choose to grow around a boundary of netrin1 expressing cells in VZ, instead of continuing to extend alongside the netrin1 pial-substrate in the ventral spinal cord. This observation suggests netrin1 may supply a more complex activity than pure adhesion, with netrin1-expressing cells also supplying a growth boundary for axons. Supporting this possibility, we have observed that additional domains of netrin1 expression arise adjacent to the dorsal root entry zone (DREZ) in E12.5 mice that are also required to sculpt axonal growth. Together, our studies suggest that netrin1 provides “hederal” boundaries: a local growth substrate that promotes axon extension, while also preventing local innervation of netrin1-expressing domains.

Deriving Dorsal Spinal Sensory Interneurons from Human Pluripotent Stem Cells

“…However, the number of ROBO3 + neurons was lower than we anticipated in the BMP4-directed hEBs compared with the RA control. Mouse Robo3 is present at high levels on commissural neurons and their axons throughout the dorsal and intermediate spinal cord ( Sabatier et al., 2004 , Varadarajan and Butler, 2017 , Varadarajan et al., 2017 ). However, monkey Robo3 is primarily present on intermediate spinal neurons, and that the pioneering commissural axons appear to stem from this intermediate population ( Figure 6 D′).…”

Control of Growth Cone Polarity, Microtubule Accumulation, and Protrusion by UNC-6/Netrin and Its Receptors in Caenorhabditis elegans

“…Recent studies in the vertebrate spinal cord have shown that expression of Netrin-1 in the floorplate is not required for ventral commissural axon ventral guidance, (Dominici et al 2017;Varadarajan and Butler 2017;Yamauchi et al 2017). Rather, expression in ventricular cells is important, possibly in a contact-mediated haptotactic event.…”

DCCmutation update: Congenital mirror movements, isolated agenesis of the corpus callosum, and developmental split brain syndrome

“…However, recent reports have revised this model by demonstrating that Ntn1 produced by neural progenitors in the ventral ventricular zone of hindbrain and spinal cord neuroepithelium are essential for commissural axon extension toward the ventral midline in embryonic mice (Dominici et al., ; Varadarajan, et al., ; Yamauchi et al., ). These reports propose that, in mice, commissural axons are directed via Dcc‐dependent haptotaxis toward the ventral midline by Ntn1 produced and deposited at the pial surface of the hindbrain and spinal cord, not via a diffusible gradient of Ntn1 emanating from the floor plate (Dominici et al., ; Varadarajan & Butler, ; Varadarajan et al., ; Yamauchi et al., ). Therefore, regardless of how Ntn1 is presented to axons, it is clear that it functions to direct Dcc‐expressing commissural axons toward the ventral midline of the hindbrain and spinal cord during development.…”