Research Highlights

Neuronal injury: Putting the brake on axonal outgrowth

Inhibition of axon outgrowth from CGN by a soluble proteolytic fragment of MAG (dMAG). Copyright © 2007 by the American Society for Biochemistry and Molecular Biology.

The adult central nervous system inhibits de novo axonal outgrowth, restricting the extent to which the brain can recover from injury. Upon injury to the nervous system, axonal outgrowth is thought to be inhibited by interactions between myelin-associated glycoprotein (MAG) and the axonal receptors, gangliosides and the glycosylphosphatidylinositol (GPI)-anchored Nogo-receptors (NgR). However, the relative contribution of each of these interaction partners has been unclear. Reporting to the Journal of Biological Chemistry, Mehta et al. now show that both gangliosides and Nogo receptors contribute separately to inhibition by MAG, but that signaling through both receptors converges at RhoA activation.

Exposure of dorsal root ganglion neurons (DRGNs) to an anti-MAG antibody reversed outgrowth inhibition by 70% in vitro. Addition of an enzyme that releases GPI-anchored proteins or addition of sialidase, which releases MAG-binding sialic acid residues, decreased inhibition by 50% or 20%, respectively, whereas the combination decreased inhibition 70%. This finding suggests that GPI-anchored proteins and sialylated glycans have an additive effect on neurite outgrowth inhibition. Furthermore, the authors noted that the addition of a competitive antagonist of NgR binding (NEP1-40) also reversed outgrowth inhibition by 50%, and exposure to an inhibitor of glycolipid synthesis reversed inhibition by 35%. Thus, NgRs are the primary receptors and sialoglycans secondary receptors responsible for MAG binding and neurite outgrowth inhibition on DRGNs. However, when Mehta et al. repeated these experiments with cerebellar granule neurons (CGNs) and hippocampal neurons (HNs), the inhibition of neurite outgrowth was found to be dependent on gangliosides rather than NgRs, as treatment to release GPI-anchored proteins or treatment with NEP1-40 did not exhibit any effect on inhibition.

Mehta et al. examined the intracellular signaling pathway for growth inhibition and found that blocking the RhoA-activating NgR co-receptor p75NTR led to a 50% reduction in inhibition in DRGNs but not in CGNs, while blocking the Rho kinase downstream of RhoA resulted in an 80% reversal of inhibition in all cell types. These results indicate that the majority of neurite outgrowth inhibition signaling for DRGNs, CGNs and HNs converges at Rho kinase. The findings were confirmed using cells exposed to myelin extracts from MAG-null and wild-type mice.

The central result of the study is the finding that different receptors — and putatively different receptor-ligand glycan recognition mechanisms — are responsible for axonal growth inhibition in different areas of the injured nervous system. At the same time, blocking Rho kinase action may turn out to be a general therapeutic option for the stimulation of neurite growth.

Author: Mirko von Elstermann