MAG and OMgp Synergize with Nogo-A to Restrict Axonal Growth and Neurological Recovery after Spinal Cord Trauma

Abstract

Functional recovery after adult CNS damage is limited in part by myelin inhibitors of axonal regrowth. Three molecules, Nogo-A, MAG, and OMgp, are produced by oligodendrocytes and share neuronal receptor mechanisms through NgR1 and PirB. While each has an axon-inhibitory role in vitro, their in vivo interactions and relative potencies have not been defined. Here, we compared mice singly, doubly, or triply mutant for these three myelin inhibitor proteins. The myelin extracted from Nogo-A mutant mice is less inhibitory for axons than is that from wild-type mice, but myelin lacking MAG and OMgp is indistinguishable from control. However, myelin lacking all three inhibitors is less inhibitory than Nogo-A-deficient myelin, uncovering a redundant and synergistic role for all three proteins in axonal growth inhibition. Spinal cord injury studies revealed an identical in vivo hierarchy of these three myelin proteins. Loss of Nogo-A allows corticospinal and raphespinal axon growth above and below the injury, as well as greater behavioral recovery than in wild-type or heterozygous mutant mice. In contrast, deletion of MAG and OMgp stimulates neither axonal growth nor enhanced locomotion. The triple-mutant mice exhibit greater axonal growth and improved locomotion, consistent with a principal role for Nogo-A and synergistic actions for MAG and OMgp, presumably through shared receptors. These data support the hypothesis that targeting all three myelin ligands, as with NgR1 decoy receptor, provides the optimal chance for overcoming myelin inhibition and improving neurological function.