Two-way Dispatched function in Sonic hedgehog shedding and transfer to high-density lipoproteins

Abstract

SummaryThe Sonic hedgehog (Shh) signaling pathway controls embryonic development and tissue homeostasis after birth. This requires regulated solubilization of dual-lipidated, firmly plasma membrane-associated Shh precursors from producing cells. Although it is firmly established that the resistance-nodulation-division transporter Dispatched (Disp) drives this process, it is less clear how lipidated Shh solubilization from the plasma membrane is achieved. We previously showed that Disp enhances proteolytic Shh solubilization from its lipidated terminal peptide anchors. This process, called shedding, converts tightly membrane-associated hydrophobic Shh precursors into delipidated soluble proteins. We show here that Disp-mediated Shh shedding is modulated by a serum factor that we identify as high-density lipoprotein (HDL). In addition to serving as soluble sinks for free membrane cholesterol, HDLs also accept the cholesterolmodified Shh peptide from Disp. The cholesteroylated Shh peptide is required and sufficient for Disp-mediated transfer because mCherry linked to cholesteroylated peptides associates with HDL in a Disp-dependent manner, but an N-palmitoylated Shh variant that lacks C-cholesterol does not. Disp-mediated Shh transfer to HDL is finalized by proteolytic processing of the palmitoylated N-terminal membrane anchor. Unlike dual processed soluble Shh with moderate bioactivity, HDL-associated N-processed Shh is highly bioactive. We suggest that the purpose of different post-translationally modified soluble Shh forms generated from one dual-lipidated cellular precursor is to fine-tune cellular responses in a tissue type-specific and time-specific manner.