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

Abstract

The inability of CNS axons to regenerate after traumatic spinal cord injury is due, in part, to the inhibitory effects of myelin. The three major previously identified constituents of this activity (Nogo, myelin-associated glycoprotein, and oligodendrocyte myelin glycoprotein) were isolated based on their potent inhibition of axon outgrowth in vitro. All three myelin components transduce their inhibitory signals through the same Nogo receptor͞p75 neurotrophin receptor͞LINGO-1 (NgR1͞p75͞LINGO-1) complex. In this study, we considered that molecules known to act as repellants in vertebrate embryonic axonal pathfinding may also inhibit regeneration. In mice, ephrin-B3 functions during development as a midline repellant for axons of the corticospinal tract. We therefore investigated whether this repellant was expressed in the adult spinal cord and retained inhibitory activity. We demonstrate that ephrin-B3 is expressed in postnatal myelinating oligodendrocytes and, by using primary CNS neurons, show that ephrin-B3 accounts for an inhibitory activity equivalent to that of the other three myelin-based inhibitors, acting through p75, combined. Our data describe a known vertebrate axon guidance molecule as a myelinbased inhibitor of neurite outgrowth.spinal cord injury ͉ regeneration ͉ axon ͉ Eph receptor T he corticospinal tract (CST) is the major spinal axon bundle responsible for fine locomotor control. Damage to this tract contributes to the permanent loss of motor control that is the most visible hallmark of spinal cord injury (SCI). These, and other axons of the CNS damaged in SCI, do not regenerate, in part, because of the nonpermissive environment of myelin in the mature CNS (1). Constituents of this repressive activity in myelin include Nogo, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp) (reviewed in ref.2). The recently identified Nogo receptor (NgR1) is a glycosylphosphatidylinositol-linked molecule that binds the 66-aa extracellular loop of Nogo (Nogo-66) and transduces its inhibitory activity when expressed in neurons (3). Surprisingly, MAG and OMgp also bind the NgR1 (4-6) in a complex with the p75 neurotrophin receptor and the newly characterized leucine rich repeat transmembrane protein LINGO-1. p75 and LINGO-1 are requisite signaling components for inhibition by Nogo-66, MAG, and OMgp as p75 Ϫ/Ϫ cerebellar granule neurons (CGNs) or dorsal root ganglion (DRG) neurons exhibit reduced sensitivity to these three inhibitors and to inhibition by CNS myelin (7), and NgR1͞p75 complexes confer sensitivity to these inhibitors only in the presence of LINGO-1 (8).Although it is thought that Nogo, MAG, and OMgp may act in the uninjured CNS to control aberrant sprouting and stabilize mature connections (9), their normal and developmental roles remain a mystery. Axon repellants such as semaphorins, slits, and ephrins play critical roles in tract formation during embryonic development (10). We previously proposed that such molecules may function as inhibitors of regeneration in the...

Myelin-derived ephrinB3 restricts axonal regeneration and recovery after adult CNS injury

“…Members of this family are up-regulated following CNS injury (18,19). EphA4 has been implicated in the response to injury both as an astrocyte and as a neuronally expressed protein, but its functional role in recovery from neuronal injury is not clear (20)(21)(22)(23). Macrophage EphB3 has also been implicated in adult axon regeneration (24).…”

“…Of potential interest, ephrinB3 is known to be expressed by myelinating oligodendrocytes and to inhibit axonal extension (21). Additionally, ephrinB3 is known to function during development as a midline repellent for axons of the corticospinal tract in the spinal cord (17,25,26).…”

Induction of corticospinal regeneration by lentiviral trkB-induced Erk activation

“…Among environmental mechanisms limiting CNS axonal growth are a number of myelin-associated proteins (1)(2)(3)(4)(5) and inhibitory proteoglcyans of the extracellular matrix (6)(7)(8). Modification of the CNS milieu enhances the sprouting or regeneration of several classes of axons (5,9,10).…”

White matter inhibitors in CNS axon regeneration failure

“…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). Moreover, in vitro neurite outgrowth studies demonstrated that a significant part of myelin inhibition is mediated by Ephrin-B3, which remarkably equals to the inhibitory activity of Nogo, MAG, and OMgp combined -at least for the postnatal cortical neurons tested (Benson et al, 2005).…”

“…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). Moreover, in vitro neurite outgrowth studies demonstrated that a significant part of myelin inhibition is mediated by Ephrin-B3, which remarkably equals to the inhibitory activity of Nogo, MAG, and OMgp combined -at least for the postnatal cortical neurons tested (Benson et al, 2005). Interestingly, an independent group reported robust axonal regeneration and improved functional recovery in mice lacking EphA4, the receptor for Ephrin-B3 during CST development (Goldshmit et al, 2004).…”