Wnt signaling plays a critical role in numerous aspects of metazoan biology and disease. Dependent on the Wnt ligand and receptor repertoire different pathways are triggered, however one critical conserved cascade is the so-called canonical Wnt signaling pathway. In this cascade the binding of a Wnt ligand inhibits β-catenin destruction; as a result, β-catenin accumulates and enters the nucleus; nuclear β-catenin binds to proteins of the TCF/LEF family of transcription factors to activate the transcription of Wnt target genes. A goal of the lab is to provide a comprehensive understanding of the transduction and outputs of canonical Wnt signaling.
Dissecting the Signaling Mechanics
Using Drosophila as the principle workhorse we have an ongoing effort to map the Wnt signaling pathway and identify the components involved. Our efforts have uncovered many key players, including discovering Pangolin (Pan)/dTCF the transcription factor that, in Drosophila, acts together with Armadillo (Arm) to transduce the Wg signal. Others factors include Legless (Lgs), Pygopus (Pygo), Hyrax, the ISWI-NURF complex, Coop, and the kinases Nek2, Smi35A, and Lic. We have also found and characterized components critical for Wnt secretion such as Wntless, the retromer complex, Reggie-1/Flotillin-2, the p24 proteins Emp24 and Éclair, and the myotubularin lipid phosphatases MTM-6 and MTM-9. Our quest to discover new components is continuing.
As well as looking for new components we are working to delineate the signal transduction mechanism, with a focus on the translation of signal into genetic program. Using state of the art techniques – RNA-Seq, ChIP-Seq, CRISPR/Cas9 – we are probing the Wnt response-ome.
Dissecting the roles of Wnt signaling in development
We are leveraging on the insights we have gained in Drosophila to refine our analysis of the roles of Wnt signaling in mammalian development. Pivotal to this was the discovery of Lgs/Bcl9 and Pygo. We coined the term 'chain of adaptors' to describe how they act in the Wnt pathway: Lgs mediates the recruitment of Pygo to the nuclear Arm-Pan complex. This insight enabled us to generate a mutant form of β-catenin that retained the cell adhesion function but was transcriptionally inert – this is now a key reagent in the field refining our understanding of Wnt signaling in mice.
Studies of the roles of Bcl9 and Pygo in mice have also yielded exciting and unexpected results: Bcl9 and Bcl9l (Bcl9/9l) mediate the interaction between β-catenin and Pygopus (Pygo) via two evolutionarily conserved domains, HD1 and HD2, respectively. We generated mouse strains lacking these domains to probe the β-catenin-dependent and β-catenin-independent roles of Bcl9/9l and Pygo during mouse development. While lens development is critically dependent on the presence of the HD1 domain, it is not affected by the lack of the HD2 domain. Unexpectedly therefore Bcl9/9l act in this context in a β-catenin-independent manner as part of a new regulatory circuit together with Pax6, the master regulator of eye development.
Bcl9 and Bcl9L are also critically required for a subset of Wnt target genes in cells of the colon epithelium; interestingly, this subset comprises genes involved in controlling EMT and stem cell-related features. This is relevant as aberrant Wnt signaling in the colon epithelium underlies almost all cases of colorectal cancer. The connection of Wnt signaling to cancer is a major focus of the lab, however investigating its role in basic developmental biology is equally critical.
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