Dispatching Sonic Hedgehog: Molecular Mechanisms Controlling Deployment

Abstract

The Hedgehog (Hh) family of morphogens direct cell fate decisions during embryogenesis and signal to maintain tissue homeostasis after birth. Hh ligands harbor dual lipid modifications that anchor the proteins into producing cell membranes, effectively preventing ligand release. The transporter-like protein Dispatched (Disp) functions to release these membrane tethers and mobilize Hh ligands to travel toward distant cellular targets. The molecular mechanisms by which Disp achieves Hh deployment are not yet fully understood, but a number of recent publications provide insight into the complex process of Hh release. Herein we review this literature, integrate key discoveries, and discuss some of the open questions that will drive future studies aimed at understanding Disp-mediated Hh ligand deployment.

Keywords: Dispatched; Hedgehog; Sonic Hedgehog; cytonemes; membrane trafficking; morphogen.

Figure 1:. Disp homology and structural analysis.

A) A homology diagram showing the degree of similarity between various RND transporter families and the EST transporter family, which includes PTCH1 and NPC1. Most similar is at the top and least similar at the bottom. The RND transporter subfamily number is indicated in parenthesis to the right of the family name. All RND transporters except for the EST and Disp families are found in prokaryotic organisms. B) Structural diagram of Disp. Transmembrane domains are shown as cylinders. HPE-associated mutations of Disp are indicated with a red star if documented in microform HPE, or a yellow asterisk if in symptomatic HPE. Amino acid changes are indicated [59,64,65]. C) The structure of PTCH1 with cholesterol (red molecule), or in the presence of palmitoylated Shh (red protein). Structures were obtained from the protein structure data bank using structures provided from [22,24] and rendered in The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC..

Figure 2:. Models of Disp intracellular trafficking.

A) Newly synthesized Disp is trafficked to the basolateral surface of cells to be cleaved at the cell surface by Furin [–34,66]. B) Cleavage is thought to allow for internalization of mature Disp with the early endosomal marker Rab5 [34], for eventual apical trafficking. C) Apical Disp may then associate with membrane tethered Hhs for reinternalization in Rab5-positive endosomes [33,38]. Disp/Hhs containing vesicles may then be, D) recycled back to the apical surface in Rab4 positive vesicles [38] for Hh release, or E) sent to the MVB for basolateral surface presentation [33,39].

Figure 3,. Key Figure: Models of Disp mediated Hhs release.

A) Disp/Hh positive exovesicles originating from MVBs are secreted from the basolateral surface of cells and spread across tissue [33,39]. B) Disp/Hhs positive exovesicles targeted to the apical surface of cells may provide a conducive environment for oligomerization of Hhs to form multimers, or allow for the hand off to lipoprotein particles for effective secretion into the extracellular space [38,42,46,47]. C) Disp transfers Shh to Scube2 for release from the producing cell [29,31,67]. D) Disp distributes Hhs by interactions with HSPGs allowing for the passage of Hhs across neighboring cell surfaces [–50]. E) Disp/Hh exovesicles, which may also contain the Hh co-receptor Ihog, are loaded into cytonemes and transported to cytoneme tips for release to target cells [39]. F) Cytonemes containing Disp, Hhs, and co-receptors may pass Hhs to Ptch at receiving cell cytoneme tips [49,51,54].