SlitC-PlexinA1 mediates iterative inhibition for orderly passage of spinal commissural axons through the floor plate.

Hugo Ducuing,Karine Kindbeiter,Servane Tauszig-Delamasure,Aurora Pignata,Valerie Castellani,Céline Delloye-Bourgeois,Thibault Gardette,Muriel Bozon,Olivier Thoumine

doi:10.7554/elife.63205

Abstract

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.

Highlights

During neural circuit formation, axons are thought to navigate series of intermediate targets or choice points until they reach their final destination (Comer et al, 2019; Raper and Mason, 2010)
We recently reported using Atto-647N-conjugated green fluorescent protein (GFP) nanobodies and STED microscopy that the membrane pool of PlxnA1WT concentrates at the front of growth cones navigating the floor plate (FP) (Pignata et al, 2019)
PlxnA1 mutation altering perception of SlitC alleviates these constrains, which results in abnormally dynamic growth cones that actively explore the FP space

Summary

Introduction

During neural circuit formation, axons are thought to navigate series of intermediate targets or choice points until they reach their final destination (Comer et al, 2019; Raper and Mason, 2010). Intermediate targets attract the axons, and second, set their response to the ‘next-step’ guidance cues that will specify their upcoming trajectory at the exit (Aviles and Stoeckli, 2016; Zuniga and Stoeckli, 2017) They provide a variety of extracellular cues to guide the axons along the navigation (Nawabi and Castellani, 2011; Neuhaus-Follini and Bashaw, 2015). In various in vivo manipulations of guidance signaling, axons are observed to skip or incompletely achieve the intermediate target crossing, prematurely re-orienting their trajectory, which compromises the formation of neural circuits (Zou et al, 2000; Long et al, 2004; Chen et al, 2008; Philipp et al, 2012; Delloye-Bourgeois et al, 2015; Yang et al, 2018; Bagri et al, 2002; Friocourt and Chedotal, 2017).

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