ϒ-secretase and LARG mediate distinct RGMa activities to control appropriate layer targeting within the optic tectum.

P Banerjee,P Mehlen,P P Monnier,S Sugita,J Charish,V A Wallace,H Harada,N G Tassew,D Goldschneider

doi:10.1038/cdd.2015.111

Abstract

While a great deal of progress has been made in understanding the molecular mechanisms that regulate retino-tectal mapping, the determinants that target retinal projections to specific layers of the optic tectum remain elusive. Here we show that two independent RGMa-peptides, C- and N-RGMa, activate two distinct intracellular pathways to regulate axonal growth. C-RGMa utilizes a Leukemia-associated RhoGEF (LARG)/Rho/Rock pathway to inhibit axonal growth. N-RGMa on the other hand relies on ϒ-secretase cleavage of the intracellular portion of Neogenin to generate an intracellular domain (NeICD) that uses LIM-only protein 4 (LMO4) to block growth. In the developing tectum (E18), overexpression of C-RGMa and dominant-negative LARG (LARG-PDZ) induced overshoots in the superficial tectal layer but not in deeper tectal layers. In younger embryos (E12), C-RGMa and LARG-PDZ prevented ectopic projections toward deeper tectal layers, indicating that C-RGMa may act as a barrier to descending axons. In contrast both N-RGMa and NeICD overexpression resulted in aberrant axonal-paths, all of which suggests that it is a repulsive guidance molecule. Thus, two RGMa fragments activate distinct pathways resulting in different axonal responses. These data reveal how retinal projections are targeted to the appropriate layer in their target tissue.

Highlights

Because C- and N-Repulsive Guidance Molecule a (RGMa) interact with the same FNIII (3–4) domain of Neogenin to inhibit axonal outgrowth, it was assumed that these peptides activate the same intracellular pathway.[10]
C-RGMa had a decision index of only 0.2 ± 0.1, suggesting that it had no guidance effect. This result was unexpected as (i) C-RGMa is regarded as the guidance peptide of RGMa13 and (ii) this represents the first example of an inhibitory outgrowth protein that is not a repulsive guidance molecule
Both N-RGMa and C-RGMa displayed similar properties with outgrowth reduced by ~ 3-fold compared with laminin alone (Supplementary Figure 1). These results reveal that both C-and N-RGMa have different guidance properties, but they suggest that these two peptides may operate via two distinct intracellular pathways

Summary

Introduction

Received 28.11.2014; revised 11.7.2015; accepted 02.7.2015; Edited by N Bazan; published online 21.8.15 cleavage of the intracellular portion of Neogenin. Υ-secretase generates a Neogenin intracellular domain (NeICD) peptide that interacts with LIM-only protein 4 (LMO4) to block axonal growth. We show that distinct axon guidance effects can be mediated by two different RGMa peptides

Methods

Results

Conclusion