Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

Abstract

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.

Keywords: cancer; gene inactivation; mouse model; ubiquitin–proteasome system.

Figure 1

The ubiquitin–proteasome system. (a) The mature free ubiquitin monomer protein is either recycled from the ubiquitinated substrate or cleaved from the polyubiquitin precursor. Both of these reactions are catalyzed by deubiquitinases (DUBs). Ubiquitin is then activated (E1), conjugated (E2), and finally ligated to the cognate substrate via ubiquitin ligases (E3). The polyubiquitinated substrate is later transferred to the proteasome, unfolded, and proteolytically degraded to small peptides or free amino acids. For more details see the text. (b) RING E3s catalyze the direct transfer of ubiquitin from E2∼ubiquitin to the substrate. HECT (homologous to E6AP C-terminus), and RBR (RING-between-RING) E3s accept ubiquitin from E2 to form an E3∼ubiquitin thioester intermediate via transthiolation reaction. For more details see the text.

Figure 2

The modular structure of ubiquitin ligases involved in the Wnt, TGF-β, and Notch pathways.

Figure 3

The Wnt signaling pathway and its regulation by ubiquitination The canonical Wnt signaling pathway is triggered by Wnt ligand binding to the complex of its receptor, Frizzled, and co-receptors LRP5/6. The activated receptor associates with the Dishevelled (DVL) protein and inhibits the β-catenin destruction complex. The stabilized β-catenin translocates and accumulates in the nucleus, where it activates the Wnt-dependent transcriptional program. For details see the text.

Figure 4

The TGF-β signaling pathway and its regulation by ubiqutination TGF-β released from the LTBP1 complex binds to TGF-β receptor II (TGF-βRII) and initiates transphosphorylation and activation of the associated TGF-β receptor I (TGF-βRI). The fully activated receptor transduces the signal to downstream factors belonging to the family of r-Smad (regulatory-Small mothers against decapentaplegic) transcription factors—Smad2 and Smad3. Smad2/3 form a trimer complex with Smad4 (co-Smad), translocate to the nucleus, and initiate transcription of TGF-β target genes. For details see the text.

Figure 5

Notch signaling pathway by ubiqutination upon binding of Jagged/Delta (JAG/DLL) ligands to the Notch receptor (N1); the endocytic system is activated in the ligand-bearing cell. This leads to two sequential proteolytic events. The first is metalloproteinase-dependent (ADAM10/17) and it releases the extracellular domain of the Notch receptor which is afterward engulfed via transendocytosis. The second proteolytic cleavage is membrane bound and dependent on γ-secretase activity. After this step, NICD (the Notch intracellular part) is released to the cytosol and transported to the nucleus where it interacts with the CSL/RBP-Jκ transcription factor and its co-factor Mastermind (MAML1). The complex is tethered to promoters of Notch target genes. For details see the text.

Figure 6

Schematic representation of mouse models of the selected ubiquitin ligases summarizing their physiological role in the Wnt, TGF-β, and Notch signaling pathways.