A highly conserved signal transduction pathway that orchestrates the growth and development of tissues is mediated by the secreted morphogen Sonic Hedgehog (Shh). Shh is autocleaved in the endoplasmic reticulum into its secreted biologically active N-terminal fragment (ShhN). The events leading to processing and secretion of Shh are under complex regulation, and particularly, our knowledge of the receptors involved in the Shh secretion pathway is incomplete. In this issue (p. 3832), Valerie Wallace and colleagues report the identification of a new Shh-trafficking receptor. Sortilin (Sort1) is a member of the VPS10P-domain receptor family, known to target ligands to numerous cellular compartments. The authors find that Shh and Sort1 co-immunoprecipitate and interact, as detected by performing a proximity ligation assay, and that they colocalise at the Golgi. The overexpression of Sort1 leads to redistribution of ShhN to the Golgi and reduces Shh secretion. Further, loss of Sort1 in a processing-deficient Shh-mutant mouse model partially rescues the patterning defects in optic-field width observed in the mutant mouse. Conversely, expression of a C-terminal truncated form of Sort1 in retinal ganglion cells results in a decrease in astrocyte proliferation in the optic nerve, a Shh-dependent process. Taken together, this study identifies a new molecular pathway to control the intracellular sorting and cellular release of active Shh.
