Mechanisms of Smoothened Regulation in Hedgehog Signaling

Abstract

The seven-transmembrane protein, Smoothened (SMO), has shown to be critical for the hedgehog (HH) signal transduction on the cell membrane (and the cilium in vertebrates). SMO is subjected to multiple types of post-translational regulations, including phosphorylation, ubiquitination, and sumoylation, which alter SMO intracellular trafficking and cell surface accumulation. Recently, SMO is also shown to be regulated by small molecules, such as oxysterol, cholesterol, and phospholipid. The activity of SMO must be very well balanced by these different mechanisms in vivo because the malfunction of SMO will not only cause developmental defects in early stages, but also induce cancers in late stages. Here, we discuss the activation and inactivation of SMO by different mechanisms to better understand how SMO is regulated by the graded HH signaling activity that eventually governs distinct development outcomes.

Keywords: cholesterol; hedgehog signaling; phospholipid; phosphorylation; smoothened; ubiquitination.

Figure 1

The highly conserved HH signaling pathway. Shown here is a scheme of the HH pathway. (Left panel), in the absence of HH, PTC inhibits SMO. Ci/GLI is processed into a truncated repressor form that enters the nucleus to block target gene expression. (Right panel), the presence of HH relieves the inhibition of PTC on SMO, leading to dimmerization and cell surface accumulation of SMO. Upon Hh stimulation, full-length Ci is activated to turn on target gene expression.

Figure 2

SMO regulation in HH signaling. Shown here is a scheme of SMO regulation through different mechanisms. In the absence of HH, unphosphorylated SMO is ubiquitinated and degraded through proteosomes and lysosomes. In the presence of HH, SMO is phosphorylated by multiple kinases. Lysine residues in the C-tail of SMO are sumoylated to counteract ubiquitination. Phosphorylation and dimmerization leads to the cell surface accumulation and activation of SMO. Both PI(4)P and chelesterol can activate SMO upon HH stimulation.

Figure 3

SMO ECD structure. Shown here is the structure of SMO ECD domain, adopted from the AlphaFold protein structure database. Critical cysteins are indicated by red stars with the positions numbered.