My laboratory is focused on understanding the role of the Wnt signaling pathway during embryogenesis and tumorigenesis.
Understanding the development of an organism from a fertilized egg into a multi-cellular organism with proper polarity includingdorso-ventral, anterior-posterior and left-right symmetry remains a challenge for biologists. This embryological process is tightly regulated temporally and spatially and results from interplay between several signaling pathways, and one key signaling pathway required is the Wnt pathway. Wnt signaling has been demonstrated to regulate critical cell fate determination, proliferation, behavior, adhesion, migration and polarity during development. Wnt and its signaling components, in addition to playing a crucial role in embryogenesis have been implicated in tumorigenesis and play causative roles in human colon cancers. We are dissecting theWnt signaling pathway using a multidisciplinary approach drawing on techniques from molecular biology, biochemistry, cell biology and embryology. Our primary system is the Xenopus laevis (frog), along with use of mammalian culture cells.
The Wnt signaling cascade has evolved as a complex series of independent signaling modules and to date comprises of three signaling modules; a canonical, a non-canonical and a Wnt/Ca2+ pathway. The canonical pathway regulates cell fate determination and primary axis formation through gene transcription, the non-canonical pathway regulates cell movements through modification of the actin cytoskeleton and the Wnt/Ca2+ pathway impacts both cell movements and cell fate determination.
We are focusing on studies to define the molecular basis of Wnt signaling by analyzing the role of Dishevelled (Dvl), a key component. For canonical signaling, we described the individual amino acid residues required for the binding of Axin, an important negative regulator of canonical Wnt signaling, to Dvl. We further deciphered the individual amino acids that were required for the targeting of Dvl to vesicles and were required for canonical pathway activation; this work represented a collaborative study with theOverduin group at the University of Colorado. For non-canonical signaling, we demonstrated that the small GTPase Rho can be biochemically activated by a Wnt/Fz/Dvl cascade in mammalian cells and Xenopus embryos, and we identified a novel effector for Rho activation downstream of Dvl. This protein, Daam1, is a member of the formin family of cytoskeletal proteins, and we showed that Daam1 has a critical role in mediating gastrulation cell movements. We further demonstrated that the Rac GTPase could also be biochemically activated by a Wnt/Fz/Dvl cascade, and that Jun kinase functions downstream of Rac in this pathway. This Rac/Junkinase pathway was activated independently of Daam1/Rho activation, leading us to propose that the activation of Rho and Rac were independent and parallel signaling pathways required for gastrulation cell movements. Together, these studies have deciphered the contributions of individual domains of Dvl in transducing Wnt signaling. We demonstrated a pathway similar to the Planar Cell Polarity pathway of Drosophila for the regulation of gastrulation and defined biochemical pathways for the regulation of cell polarity and gastrulation movements by Wnt signaling. Our demonstration of the roles of Rho and Rac as mediating effects of Wnt signaling on the cytoskeleton may provide an important avenue for the understanding of aspects of Wnt-mediated tumorigenesis.
We are now continuing our studies by analyzing a number of new factors that we have identified that impact the canonical or the non-canonical pathways. These studies will provide a clearer understanding of the mechanisms of Wnt signaling and remains pivotal to our understanding of the molecular nature of embryology and cancer formation.