Editors' ChoiceNeuroscience

Turning On the Repulsive System

Sci. Signal.  20 Aug 2013:
Vol. 6, Issue 289, pp. ec195
DOI: 10.1126/scisignal.2004644

Spinal cord axons exhibit a complex migration pattern that involves growing toward the floorplate (attraction), then crossing the floorplate, followed by turning (repulsion) and growing up the contralateral side rostrally to the brain. The guidance cue Sonic hedgehog (Shh) functions as an attractant for precrossing commissural axons and as a repulsive cue for postcrossing commissural axons. Wilson and Stoeckli used chick embryos and found that the mRNA and protein for glypican1, which is an ortholog of the Shh positive regulator Dally-like, were present in the floorplate and the dorsal commissural (dl1) axons. Selective knockdown of GPC1 in dl1 neurons, but not in the floorplate, resulted in aberrant migration of dl1 axons, producing a phenotype similar to that seen in the absence of Shh. Consistent with a role for GPC1 in the Shh pathway, combined partial knockdown of GPC1 and Shh resulted in a more severe phenotype than partial knockdown of either protein alone. Knockdown of GPC1 specifically affected the migration of postcrossing dl1 axons, without affecting the precrossing axons, suggesting that GPC1 functioned in the repulsive Shh pathway. Hhip is a co-receptor for Shh that functions in repulsive guidance signaling and is encoded by a Shh target gene. Hhip expression was reduced in the dorsal spinal cord when GPC1 was knocked down, and forced expression of Hhip by electroporation rescued the axon guidance defects caused by GPC1 knockdown. Activation of the canonical Shh pathway by overexpressing Gli transcription factors expanded the domain in which Hhip was expressed, and overexpression of a repressor of Shh signaling reduced Hhip expression, indicating that GPC1 stimulated the transcriptional Shh pathway. A GPC1 mutant that could not be glycosylated or bind Shh failed to rescue Hhip expression or axon guidance defects in response to GPC1 knockdown. Although the Shh transcriptional pathway can target multiple genes, knockdown of GPC1 failed to alter the expression of several other Shh target genes (Ptc1, Boc, and Sfrp1), suggesting that the GPC1-mediated pathway is specific for Hhip. However, overexpression of GPC1 resulted in ectopic expression of Ptc1, Sfrp1, and Hhip. These results suggest that, as dl1 axons cross the floorplate where Shh is produced and concentrations are highest, GPC1 enables the induction of Hhip, which switches the Shh response from attraction to repulsion after the cells cross the midline.

N. H. Wilson, E. T. Stoeckli, Sonic hedgehog regulates its own receptor on postcrossing commissural axons in a Glypican1-dependent manner. Neuron 79, 478–491 (2013). [PubMed]