Forward Signaling Mediated by Ephrin-B3 Prevents Contralateral Corticospinal Axons from Recrossing the Spinal Cord Midline

Abstract

To investigate Eph-ephrin bidirectional signaling, a series of mutations were generated in the ephrin-B3 locus. The absence of both forward and reverse signaling resulted in mice with mirror movements as typified by a hopping locomotion. The corticospinal tract was defective as axons failed to respect the midline boundary of the spinal cord and bilaterally innervated both contralateral and ipsilateral motor neuron populations. A second mutation that expresses a truncated ephrin-B3 protein lacking its cytoplasmic domain did not lead to hopping, indicating that reverse signaling is not required for corticospinal innervation. Ephrin-B3 is concentrated at the spinal cord midline, while one of its receptors, EphA4, is expressed in postnatal corticospinal neurons as their fibers pathfind down the contralateral spinal cord. Our data indicate ephrin-B3 functions as a midline-anchored repellent to stimulate forward signaling in EphA4-expressing axons.

Ephrin-B3 is a myelin-based inhibitor of neurite outgrowth

“…Ephrin-B3͞LacZ mice, encoding a chimeric protein with a ␤-gal moiety replacing the intracellular segment of ephrin-B3, and ephrin-B3 knockout mice are described in Yokoyama et al (12). EphA4͞LacZ mice, carrying a LacZ cDNA in place of a functional EphA4 allele, were provided by P. Charnay (15).…”

“…Genetic ablation of ephrin-B3 in mice thus results in bilateral innervation by the CST and a kangaroo-like ''hopping'' phenotype. Knocking out EphA4 produces an identical phenotype, demonstrating a receptor͞ligand Eph͞ephrin relationship in this process (12).…”

“…We, and others, reported that these neurons express the EphA4 receptor tyrosine kinase (RTK) and are prevented from inappropriate recrossing by their resultant sensitivity to the midline expression of the transmembrane ligand ephrin-B3 (12)(13)(14). Genetic ablation of ephrin-B3 in mice thus results in bilateral innervation by the CST and a kangaroo-like ''hopping'' phenotype.…”

EphA4 defines a class of excitatory locomotor-related interneurons

“…Such studies are particularly relevant in mammalian systems where it has been an immense task to characterize the principle constituents of neural networks from both developmental genetics (1) and physiological (10)(11)(12) perspectives. A recent example of a genetic loss-of-function that is related to a distinct abnormal behavioral phenotype is the rabbit-like hopping gait exhibited in mice that have a targeted deletion of the axon guidance molecules EphA4 and ephrinB3 (13)(14)(15). This pronounced phenotype could be reproduced in isolated spinal cords from mutant mice, suggesting that the neuronal network controlling locomotion, also called the central pattern generator (CPG), is genetically reconfigured in the mutants (16).…”

White matter inhibitors in CNS axon regeneration failure

“…During corticospinal tract development, Ephrin-B3 is concentrated at the spinal cord midline and prevents EphA4-expressing axon collaterals of the corticospinal tract from re-crossing the midline since they have already crossed once at the spinomedullary junction (Davy and Soriano, 2005). CST axons in either Ephrin-B3 or EphA4 mutant mice exhibit bilateral innervation of spinal grey matter due to the loss of the midline barrier (Ephrin-B3) or the receptor to sense the barrier (EphA4) (Dottori et al, 1998;Kullander et al, 2001;Yokoyama et al, 2001). Benson et al reported the expression of Ephrin-B3 in postnatal myelinating oligodendrocytes in the mouse spinal cord and provided evidence that this myelin component confers sensitivity to postnatal cortical neurons that express EphA4 (Benson et al, 2005).…”