CK1 in Developmental Signaling: Hedgehog and Wnt

Abstract

The casein kinase 1 (CK1) family of serine (Ser)/threonine (Thr) protein kinases participates in a myriad of cellular processes including developmental signaling. Hedgehog (Hh) and Wnt pathways are two major and evolutionarily conserved signaling pathways that control embryonic development and adult tissue homeostasis. Deregulation of these pathways leads to many human disorders including birth defects and cancer. Here, I review the role of CK1 in the regulation of Hh and Wnt signal transduction cascades from the membrane reception systems to the transcriptional effectors. In both Hh and Wnt pathways, multiple CK1 family members regulate signal transduction at several levels of the pathways and play either positive or negative roles depending on the signaling status, individual CK1 isoforms involved, and the specific substrates they phosphorylate. A common mechanism underlying the control of CK1-mediated phosphorylation of Hh and Wnt pathway components is the regulation of CK1/substrate interaction within large protein complexes. I will highlight this feature in the context of Hh signaling and draw interesting parallels between the Hh and Wnt pathways.

Keywords: CK1; Cancer; Development; GSK3; Hh; Kinase; PKA; Phosphorylation; Signaling; Smo; Wnt; β-Catenin.

Fig. 1

CK1 family of kinases. (A) Family tree of CK1 isoforms from Drosophila and mammals. (B) Schematic drawings of mammalian CK1 family members with kinase domains depicted in grey. Numbers in parentheses indicate the lengths of CK1 isoforms generated from alternative splicing.

Fig. 2. Hh and Wnt pathways

(Top) Drosophila Hh pathway. In the “signaling off “ state, Ci and its kinases PKA, GSK3 and CK1 form a large protein complex scaffolded by Cos2 and Fu, leading to its phosphorylation and proteolysis to generate a truncated repressor form that inhibits the expression of Hh target genes. In the “signaling on “ state, Hh binds Ptc and releases its inhibition of Smo, leading to Smo phosphorylation and activation. Smo interacts with Cos2/Fu and promotes Fu phosphorylation and activation but inhibits PKA/CK1-mediated Ci phosphorylation and processing. Activated Fu converts full-length Ci into an active form that stimulates the expression of Hh target genes. (Bottom) Wnt/β-catenin pathway. In the “signaling off “ state, β-catenin and its kinases GSK3 and CK1α form a destruction complex scaffolded by Axin and APC, leading to its phosphorylation and degradation. In the “signaling on “ state, Wnt binds Fz and LRP5/6, leading to phosphorylation of LRP5/6 by GSK3 and CK1. LRP5/6 phosphorylation on the PPPSP motifs recruits the destruction complex through Axin, leading to a blockage of β-catenin phosphorylation. As a consequence, β-catenin accumulates, translocate to the nucleus, and binds TCF to turn on Wnt responsive genes.

Fig. 3. CK1 phosphorylates both Ci/Gli and Smo in the Hh pathway

(Top) Diagrams of Ci, mouse Gli2, and human Gli3 showing the PKA/GSK3/CK1 phosphorylation clusters in Ci/Gli. Putative Slimb/β-TRCP binding sites in Ci/Gli are underlined. Grey and blue boxes denote a Zn-finger DNA binding domain and a trans-activation domain, respectively. Phosphorylation sites for the indicated kinases are color-coded. (Bottom) Diagrams showing the Smo phosphorylation sites in Drosophila (top) and mammalian Smo (bottom). Grey boxes indicate transmembrane helixes. The red box in Drosophila Smo C-tail denotes the SAID domain. The phosphorylation clusters are underlined and phosphorylation sites for the indicated kinases are color-coded.

Fig. 4. Phosphorylation of β-catenin and LRP6 by CK1 and GSK3

(A) Diagram of βcatenin with the CK1 and GSK3 phosphorylation cluster shown underneath. β-TRCP binding site is boxed. Grey boxes denote the Armadillo (Arm) repeats and the black box indicates the destruction box. (B) Diagram of LRP6 with the membrane proximal S/T rich sequence and the first PPPSP motif shown underneath. Filled, open, and grey boxes indicate the transmembrane domain, S/T rich domain, and multiple PPPSP motifs.