Editors' ChoiceNeuroscience

Protecting a guidance receptor from cleavage

Sci. Signal.  15 Dec 2015:
Vol. 8, Issue 407, pp. ec368
DOI: 10.1126/scisignal.aae0556

Instructive guidance cues attract or repel growing neurons to shape their growth trajectories. Binding of the repulsive axon guidance cue RGMa to the receptor Neogenin induces intracellular activation of the guanosine triphosphatase RhoA and growth cone collapse. The membrane-associated metalloprotease ADAM17 (also known as TACE) cleaves Neogenin near the membrane, thus releasing the ectodomain and densensitizing neurons to RGMa. van Erp et al. report that the transmembrane protein Lrig2 (leucine-rich repeats and immunoglobulin-like domains 2) coimmunoprecipitated with Neogenin from mouse brain extracts, and both proteins were present at growth cones in a subset of dissociated cortical neurons. Experiments with various truncated forms of Lrig2 and Neogenin in cell lines and in vitro indicated that these proteins interacted through their extracellular domains. The presence of RGMa reduced the interaction between Neogenin and Lrig2. Although exposure of wild-type cortical neurons to RGMa caused activation of RhoA and collapse of growth cones, RGMa did not induce RhoA activation or growth cone collapse when Lrig2 was knocked down by RNA interference. Lrig2 knockdown also reduced the abundance of Neogenin at the cell surface unless the cells were treated with a pharmacological inhibitor of metalloproteases. Addition of the metalloprotease inhibitor reduced the amount of Neogenin shed into the culture medium following RGMa treatment. Culturing primary cortical neurons on RGMa-expressing CHO cells stunted neurite outgrowth, but not when Lrig2 was knocked down in the neurons. In the coculture system, exposure to the metalloprotease inhibitor or knocking down ADAM17 in combination with Lrig2 knockdown resulted in stunted neurites. Lrig2 was required for proper migration of cortical neurons in mouse embryos, and knockdown of Lrig2 improved regeneration following optic nerve injury. In vivo experiments were consistent with Lrig2 cooperating with Neogenin and ADAM17 to regulate axon outgrowth. Thus, Lrig2 protects Neogenin from cleavage by ADAM17, enabling neurons to respond to the repulsive cue RGMa.

S. van Erp, D. M.A. van den Heuvel, Y. Fujita, R. A. Robinson, A. J. C. G. M. Hellemons, Y. Adolfs, E. Y. Van Battum, A. M. Blokhuis, M. Kuijpers, J. A. A. Demmers, H. Hedman, C. C. Hoogenraad, C. Siebold, T. Yamashita, R. J. Pasterkamp, Lrig2 negatively regulates ectodomain shedding of axon guidance receptors by ADAM proteases. Dev. Cell 35, 537–552 (2015). [Online Journal]