Wnts belong to a family of secreted proteins that activate multiple receptor-mediated signal transduction pathways in both embryos and adults. My interest in Wnts began when, as a very junior faculty member, I made the serendipitous observation that overexpressing Wnt1 in early Xenopus embryos stimulates the formation of a second Spemann organizer, resulting in two-headed tadpoles. Investigating the biochemical and cellular processes that explain this dramatic phenotype enabled our lab to identify many of the core components of the evolutionarily conserved Wnt/ß-catenin pathway.
The arc of research in our lab has evolved from the early days of examining Wnt signaling in development to a current focus on the roles of Wnt signaling in stem and progenitor cells in regenerative processes, the involvement of Wnt signaling in diseases, and the development of new therapies derived from modulation of Wnt signaling.
Wnt Signaling in Stem and Progenitor Cells
Experiments in Xenopus, zebrafish (Figure 1), mice, and other species have shown that the activation of ß-catenin signaling during regeneration is conserved during evolution. Intriguingly, enhancing Wnt/ß-catenin signaling generally accelerates regeneration. More recently, we have been focusing on Wnts in human embryonic stem cells (hESCs)~and induced pluripotent stem cells, which enable us to ask questions regarding the control of self-renewal and differentiation. Among our early findings, we have demonstrated that Wnt signaling promotes differentiation rather than self-renewal in hESCs.
Studying stem and progenitor cells in model organisms has also yielded some surprises. For example, we used zebrafish embryos to assist Charles Murry, University of Washington, in identifying new regulators of cardiogenesis. In ongoing studies (with Irwin Bernstein, Fred Hutchinson Cancer Research Center, and Gordon Keller, University of Toronto), we have identified key roles for Wnt signaling in hematopoietic progenitor cell formation in vivo.
Probing the Genome for Novel Modulators of Wnt Signaling
Evidence from several labs indicates that the core components of the Wnt/ß-catenin pathway are augmented by tissue-specific activators and inhibitors. Our laboratory is increasingly focused on identifying whether these context-dependent modulators of Wnt signaling are involved in disease and how knowledge of these modulators can be used to identify or target Wnt-based therapies. Addressing these questions has been facilitated by the emergence of new high-throughput technologies. For example, we have established robotic facilities that allow us to conduct high-throughput screens with small interfering RNAs (siRNAs) and with small molecules. Moreover, we are constructing new tools to investigate how regulated proteolysis of AXIN1 plays important roles in controlling Wnt/ß-catenin signaling.
In addition to conducting high-throughput genetic and small-molecule screens, we have employed affinity purification followed by tandem mass spectrometry to construct protein interaction networks in the Wnt pathway. Indeed, we have characterized more than 80 different protein complexes, including complexes containing the noncanonical Wnt pathway proteins SCRIB, NOS1AP, and VANGL, and the complex containing the E3 ubiquitinase MINDBOMB1 and the Wnt receptor RYK. In a significant advance over standard screening methods, we pioneered the integration of proteomic data (which define complexes of proteins but do not reveal protein function) with siRNA data (which identify functional genes but not how the encoded proteins interact to form a signaling network). By integrating two different screens, which compensate for each other’s weaknesses, we have developed a general approach for accelerating the validation of hits from these screens (Figure 2).
These advanced screening tools have time and again enabled us to identify new endogenous regulators of Wnt/ß-catenin signaling that had not been detected by genetic screens. For example, our discovery that the FDA-approved drug Riluzole regulates ß-catenin signaling led us to the unexpected determination that an indirect cellular target of Riluzole, metabotropic glutamate receptor 1 (GRM1), is involved in repressing ß-catenin signaling. High-throughput screening also identified Bruton's tyrosine kinase as a negative regulator of Wnt/β-catenin signaling and WIKI4 as a novel small-molecule inhibitor of Wnt/ß-catenin signaling. Thus, by using a wide range of screening tools, we continue to find new modulators of Wnt/ß-catenin signaling.
Therapeutic Modulation of Wnt Signaling in Disease
We have a long-standing interest in Wnt signaling in cancer, most notably melanoma. We found that elevated nuclear β-catenin levels correlate with improved survival in melanoma patients, an unexpected result because in colorectal cancer, elevated β-catenin correlates with a poorer prognosis. We built on these initial investigations with our discovery that Riluzole modulates ß-catenin signaling; it is in clinical trials for treatment of melanoma, conducted by an independent lab at Rutgers. We also demonstrated that a targeted BRAFV600E inhibitor, vemurafenib, which was recently approved by the FDA as a therapy for melanoma, requires endogenous ß-catenin to promote melanoma apoptosis. Elevating ß-catenin signaling enhances the efficacy of vemurafenib. A striking finding by our lab is that melanoma cells that are resistant to vemurafenib can be sensitized to undergo apoptosis by reducing levels of AXIN1, a negative regulator of ß-catenin. These data clearly establish the importance of Wnt/ß-catenin signaling in the design of appropriate therapies.
Wnt signaling may also be a useful therapeutic target to enhance neurogenesis in the adult brain. We previously showed that traumatic brain injury leads to the transient activation of Wnt/ß-catenin signaling in neural progenitor populations in the mouse brain. More recently, we discovered that a widely prescribed drug activates Wnt/ß-catenin signaling in germinal zones of the brain, which may have implications for therapies for neurological disease and injury.
In summary, our laboratory focuses on identifying the functions and mechanisms of action of Wnt signaling networks in vertebrates and leveraging these insights to better understand the roles of Wnt signaling in human diseases. This approach has contributed to the discovery and understanding of therapies that improve patient outcomes and has potential for further important insights.
Grants from the National Institutes of Health and awards from the Alzheimer's Association and the Department of Defense provided partial support for some of these projects.
As of February 27, 2013