Regulation of Smoothened Trafficking and Abundance in Hedgehog Signaling.

Jianhang Jia,Jin Jiang

doi:10.3389/fcell.2022.847844

Jianhang Jia, Jin Jiang

Open Access

https://doi.org/10.3389/fcell.2022.847844

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Abstract

The GPCR-family protein Smoothened (Smo) is essential for Hedgehog (Hh) signal transduction in both insects and vertebrates. The regulation of subcellular localization and abundance of Smo is a critical step in Hh signaling. Recent studies have demonstrated that Smo is subjected to ubiquitination mediated by multiple E3 ubiquitin ligases, leading to Smo endocytosis and subsequent degradation through the proteasome- and lysosome-mediated pathways in Drosophila. Ubiquitination of Smo also promotes its ciliary exit in mammalian cells. Hh inhibits Smo ubiquitination by blocking E3 ligase recruitment and promoting Smo deubiquitination through the ubiquitin-specific protease 8 (USP8) in Drosophila. Inhibition of Smo ubiquitination by Hh promotes Smo cell surface accumulation in Drosophila and ciliary accumulation in mammalian cells. Interestingly, Hh also induces sumoylation of Smo in both Drosophila and mammalian cells, which promotes Smo cell surface/ciliary accumulation. This review focuses on how ubiquitination and sumoylation regulate Smo intracellular trafficking and abundance and how these processes are regulated by Hh.

Highlights

The Hedgehog (Hh) morphogen controls embryonic development and adult tissue homeostasis in species ranging from insects to mammals (Ingham and McMahon, 2001; Ingham et al, 2011; Jiang and Hui, 2008)
Many scaffolding proteins and enzymes are involved in Smo ubiquitination, sumoylation, and intracellular trafficking, as discussed in this review; how these events are sophisticatedly controlled by Hh signaling has not been fully understood
Because vertebrate Hh signaling occurs in primary cilia and may utilize different mechanisms to regulate Smo, further studies in vertebrate systems are needed to explore the link between the various enzymes involved in ubiquitination and sumoylation and their possible involvement in human diseases

Summary

INTRODUCTION

The Hedgehog (Hh) morphogen controls embryonic development and adult tissue homeostasis in species ranging from insects to mammals (Ingham and McMahon, 2001; Ingham et al, 2011; Jiang and Hui, 2008). In the absence of Hh, these E3s bind SmoCT through their HECT domains to promote Smo ubiquitination, internalization, and degradation whereas Hh-induced phosphorylation of SmoCT by PKA/CK1 inhibits E3 recruitment and Smo ubiquitination, leading to Smo cell surface accumulation (Figure 2A) (Li et al, 2018b). Aside from the demonstrated role of HERC4 in the regulation of mammalian Smo ubiquitination and stability in cultured NIH3T3 cells and NSCLC cells (Jiang et al, 2019; Sun et al, 2019), a genome-wide screen for modifiers of Hh pathway activity in cultured cells identified a pair of genes encoding a transmembrane protein MEGF8 and a RING family E3 ligase MGRN1 whose loss-offunction resulted in an increased response to Shh due to elevated Smo levels at the cell surface and primary cilia (Kong et al, 2020). Mice homozygous for Megf or Mgrn mutation exhibited increased Smo abundance and elevated sensitivity to Hh signaling whereas mice heterozygous for either Megf or Mgrn mutation were normal but double heterozygous embryos exhibited an incompletely penetrant congenital heart defects (CHDs), revealing delicate genetic interactions between Megf and Mgrn that affect Hh signaling strength through modulating Smo abundance (Kong et al, 2020)

REGULATION OF SMO UBIQUITINATION BY DUBS

REGULATION OF SMO TRAFFICKING AND ABUNDANCE BY SUMOYLATION

REGULATION OF SMO BY ENDOCYTIC PATHWAY COMPONENTS

CONCLUSION AND PERSPECTIVES