Ethan Lee

Bio: Ethan Lee is an academic researcher from Vanderbilt University. The author has contributed to research in topics: Wnt signaling pathway & LRP6. The author has an hindex of 40, co-authored 105 publications receiving 7689 citations. Previous affiliations of Ethan Lee include Massachusetts Institute of Technology & Rice University.

Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis.

Abstract: Mechanisms of guanosine triphosphate (GTP) hydrolysis by members of the G protein alpha subunit-p21ras superfamily of guanosine triphosphatases have been studied extensively but have not been well understood. High-resolution x-ray structures of the GTP gamma S and GDP.AlF4- complexes formed by the G protein Gi alpha 1 demonstrate specific roles in transition-state stabilization for two highly conserved residues. Glutamine204 (Gln61 in p21ras) stabilizes and orients the hydrolytic water in the trigonal-bipyramidal transition state. Arginine 178 stabilizes the negative charge at the equatorial oxygen atoms of the pentacoordinate phosphate intermediate. Conserved only in the G alpha family, this residue may account for the higher hydrolytic rate of G alpha proteins relative to those of the p21ras family members. The fold of Gi alpha 1 differs from that of the homologous Gt alpha subunit in the conformation of a helix-loop sequence located in the alpha-helical domain that is characteristic of these proteins; this site may participate in effector binding. The amino-terminal 33 residues are disordered in GTP gamma S-Gi alpha 1, suggesting a mechanism that may promote release of the beta gamma subunit complex when the alpha subunit is activated by GTP.

The roles of APC and Axin derived from experimental and theoretical analysis of the Wnt pathway.

Abstract: Wnt signaling plays an important role in both oncogenesis and development. Activation of the Wnt pathway results in stabilization of the transcriptional coactivator β-catenin. Recent studies have demonstrated that axin, which coordinates β-catenin degradation, is itself degraded. Although the key molecules required for transducing a Wnt signal have been identified, a quantitative understanding of this pathway has been lacking. We have developed a mathematical model for the canonical Wnt pathway that describes the interactions among the core components: Wnt, Frizzled, Dishevelled, GSK3β, APC, axin, β-catenin, and TCF. Using a system of differential equations, the model incorporates the kinetics of protein–protein interactions, protein synthesis/degradation, and phosphorylation/dephosphorylation. We initially defined a reference state of kinetic, thermodynamic, and flux data from experiments using Xenopus extracts. Predictions based on the analysis of the reference state were used iteratively to develop a more refined model from which we analyzed the effects of prolonged and transient Wnt stimulation on β-catenin and axin turnover. We predict several unusual features of the Wnt pathway, some of which we tested experimentally. An insight from our model, which we confirmed experimentally, is that the two scaffold proteins axin and APC promote the formation of degradation complexes in very different ways. We can also explain the importance of axin degradation in amplifying and sharpening the Wnt signal, and we show that the dependence of axin degradation on APC is an essential part of an unappreciated regulatory loop that prevents the accumulation of β-catenin at decreased APC concentrations. By applying control analysis to our mathematical model, we demonstrate the modular design, sensitivity, and robustness of the Wnt pathway and derive an explicit expression for tumor suppression and oncogenicity.

Small-molecule inhibition of Wnt signaling through activation of casein kinase 1α

Abstract: Wnt/β-catenin signaling is critically involved in metazoan development, stem cell maintenance and human disease. Using Xenopus laevis egg extract to screen for compounds that both stabilize Axin and promote β-catenin turnover, we identified an FDA-approved drug, pyrvinium, as a potent inhibitor of Wnt signaling (EC(50) of ∼10 nM). We show pyrvinium binds all casein kinase 1 (CK1) family members in vitro at low nanomolar concentrations and pyrvinium selectively potentiates casein kinase 1α (CK1α) kinase activity. CK1α knockdown abrogates the effects of pyrvinium on the Wnt pathway. In addition to its effects on Axin and β-catenin levels, pyrvinium promotes degradation of Pygopus, a Wnt transcriptional component. Pyrvinium treatment of colon cancer cells with mutation of the gene for adenomatous polyposis coli (APC) or β-catenin inhibits both Wnt signaling and proliferation. Our findings reveal allosteric activation of CK1α as an effective mechanism to inhibit Wnt signaling and highlight a new strategy for targeted therapeutics directed against the Wnt pathway.

The Wnt signaling pathway in development and disease.

Abstract: Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.