Review
Mechanistic Insights into the Generation and Transduction of Hedgehog Signaling

https://doi.org/10.1016/j.tibs.2020.01.006Get rights and content

Highlights

  • The Hedgehog (HH) signaling pathway is essential for the development of animals and is involved in many human cancers.

  • The secreted ligand HH binds its receptor Patched-1 (PTCH1) and releases the inhibition of Smoothened (SMO) by PTCH1; PTCH1 regulates SMO catalytically via sterols.

  • How HH recognizes and inhibits PTCH1, how SMO is regulated, and how the HH signal is transduced into cells remain unknown and controversial.

  • Recent structural studies on HH–PTCH1 complexes and SMO in different conformations reveal the mechanism of PTCH1 recognition and inhibition by HH and provide insights into the mechanism of SMO activation.

Cell differentiation and proliferation require Hedgehog (HH) signaling and aberrant HH signaling causes birth defects or cancers. In this signaling pathway, the N-terminally palmitoylated and C-terminally cholesterylated HH ligand is secreted into the extracellular space with help of the Dispatched-1 (DISP1) and Scube2 proteins. The Patched-1 (PTCH1) protein releases its inhibition of the oncoprotein Smoothened (SMO) after binding the HH ligand, triggering downstream signaling events. In this review, we discuss the recent structural and biochemical studies on four major components of the HH pathway: the HH ligand, DISP1, PTCH1, and SMO. This research provides mechanistic insights into how HH signaling is generated and transduced from the cell surface into the intercellular space and will aid in facilitating the treatment of HH-related diseases.

Section snippets

Physiology of the HH Signaling Pathway

The HH signaling pathway plays a vital role in the developmental patterning of animal embryogenesis and regeneration of adult tissues [1., 2., 3., 4.] and the HH ligand functions in a paracrine signaling (see Glossary) manner to regulate the HH signaling pathway [5]. Importantly, abnormal activation of HH signaling is implicated in many cancers, including basal cell carcinoma (BCC) and medulloblastoma (MB) [6]. Because of the complexity of the HH signaling pathway, the molecular mechanism of

Overview of the HH Signaling Pathway

Although the HH signaling pathway is conserved in metazoan cells, the HH family contains different numbers of homologs owing to evolutionary divergence [7]. For example, while in the fruit fly there is only one HH ligand, there are three HH members in mammals: Sonic Hedgehog (SHH), Desert Hedgehog (DHH), and Indian Hedgehog (IHH). All three mammalian HH ligands share high sequence homology and function as initial ligands that trigger HH signaling.

In the first step of the signaling pathway, the

The Release and Trafficking of HH Ligand from Producing Cells to Receiving Cells

There are three disp genes and three scube genes in humans, and the DISP1 and Scube2 proteins are well studied among their homologs. DISP1 contains 1524 amino acids, including 12 TMs, two ECDs, and N-terminal and C-terminal cytosolic flexible regions (Figure 2, top). Previous work showed that a proprotein convertase named Furin can cleave the ECD-I of DISP1 triggering HH-N release, that this cleavage is required for the activation of DISP1 in vivo, and that mutations of the cleavage site

PTCH1 and N-Terminal Palmitoylation of HH-N

N-terminal palmitoylation has been shown to be indispensable for HH signaling in both vertebrates and Drosophila by differentiation assays [47., 48., 49.] and GLI-dependent HH signaling assays [15,32] and embryonic development in both Drosophila and mice can be interfered with by abolishing the palmitoylation of HH-N [10,11,49., 50., 51., 52.]. The indicated assays showed that fatty-acylated SHH-N is far more active than unacylated SHH-N. Moreover, inhibitors of HHAT that block the

The Putative Mechanism of Palmitoylated HH-Mediated Signaling

PTCH1 catalytically controls the activity of SMO without direct interaction [66]; PTCH1 may transport some small molecules, changing their concentrations in the cell membrane. These small molecules may stimulate or inhibit SMO, further regulating HH signaling. Since PTCH1 shares a very similar topology with the putative LDL-derived cholesterol transporter NPC1, it has been predicted that PTCH1 serves as a sterol transporter to regulate the HH signal [16]. Structural analysis revealed a putative

Signal Transduction from the Cell Surface into the Intercellular Space

The mechanism of how the HH protein triggers GLI activation via the PTCH1–SMO system remains unclear. As a member of the class-F GPCR family, SMO includes 7-TMs and a CRD at the amino terminus [75] (Figure 1). Previous studies showed cholesterol, or its derivatives such as 20(S)-hydroxycholesterol, can bind the CRD of SMO on the cell surface to directly stimulate the HH signal [76., 77., 78., 79., 80.]. Importantly, the CRD of SMO has been shown to be covalently modified by cholesterol at

Concluding Remarks

The recent structural and functional studies have provided molecular insights into the generation and regulation of HH signaling (Figure 5). Particularly, the technological breakthrough of cryo-EM accelerated these discoveries. Structures of DISP and its complex with unmodified HH-N provided a primary understanding of HH-N secretion, 1:1 PTCH1–HH-N and 2:1 PTCH1–HH-N complexes explained the physiological importance of the modifications of HH-N and revealed the inhibition mechanism of PTCH1 by

Acknowledgments

We apologize to our colleagues whose work has not been mentioned owing to space limitations. We thank E. Coutavas, L. Friedberg, J. Jiang, and P. Schmiege for their time and efforts on the manuscript preparation. This work was supported by NIH grants R01 GM135343 and P01 HL020948. X.Q. is the recipient of DDBrown Fellow of the Life Sciences Research Foundation. X.L. is a Damon Runyon-Rachleff Innovator supported by the Damon Runyon Cancer Research Foundation (DRR-53-19) and a Rita C. and

Glossary

CAM-related/downregulated by oncogenes (CDO) and Brother of CDO (BOC)
co-receptors of HH on the cell surface that can complex with HH via the calcium-mediated interface and upregulate HH signaling.
Dispatched-1 (DISP1)
a membrane protein that facilitates the secretion of native HH from producing cells. DISP is a structural homolog of PTCH, including 12 TMs and two ECDs.
Glioma-associated oncogene (GLI)
a zinc-finger transcription factor that mediates transcriptional responses to HH signaling. In

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