|M.Sc Student||Cohen Yasmin|
|Subject||The Role of Protein Tyrosine Kinase 7 (PTK7) in Regulating|
the Wnt Signaling Pathway
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
The Wnt-signaling network is crucial for regulating cell fate specification, morphogenesis and cell-growth during embryonic development and tissue homeostasis. During embryonic neural development, the "canonical" Wnt/β-catenin pathway regulates neural plate specification, while the "non-canonical" Planar-Cell-Polarity (PCP)/Wnt pathway controls neural plate folding and elongation in vertebrates. The Wnt-signaling pathway is responsible for the closing of the neural tube along the body axis of the developing embryo. Perturbation of Wnt-signaling pathway components disrupts early nervous system development causing neural tube defects (NTDs). NTDs are birth defects of the embryonic nervous system that occur when the neural tube fails to close completely. In the canonical Wnt-pathway, Wnt-ligands bind the membrane bound Frizzled (FZ) and LRP6 receptors, triggering a chain of intracellular events that prevent β-catenin protein degradation, thus enabling translocation of β-catenin to the nucleus, where it activates Wnt-responsive target genes. The non-canonical Wnt-PCP pathway is involved in convergent-extension movement of cells during gastrulation/neurulation and neural tube closure. In the PCP-pathway, different Wnt-ligands bind a subset of FZ receptors to regulate the reorganization of the actin cytoskeleton to promote directed cell movements. Regulatory interactions between these two Wnt-pathways are mediated by the LRP6 protein, and each pathway can mutually inhibit one another. Thus, “too little” or “too much” PCP-activity can induce NTD phenotypes. Only a narrow range of PCPactivity promotes correct neural tube folding. An optimal balance of both canonical and non-canonical Wnt-activities is necessary for correct neural tube closure. Protein Tyrosine Kinase 7 (PTK7) is a transmembrane protein highly conserved amongst metazoans. Recent studies link PTK7 to Wnt-signaling and NTDs. Studies from our lab show that PTK7 is required to maintain LRP6 protein levels. PTK7 regulates canonical and non-canonical Wnt-signaling, controlling posterior neural cell fatespecification, neural convergent-extension and neural tube closure in vertebrate embryos. PTK7 interacts with LRP6 at the membrane to simultaneously promote canonical Wnt-signaling, while inhibiting Wnt-PCP activity. PTK7 proteolytic isoforms were discovered in cancer cells that appear to regulate cancer cell behavior. In this 2 project, we focused on the biological activity of these small PTK7 isoforms in regulating Wnt-activity in a non-cancer system. We examined the function of two PTK7 proteolytic isoforms in-vivo during normal embryonic development of the Xenopus laevis nervous system. We focused on two PTK7 isoforms: (1) The cPTK7 622-1070 protein that has a transmembrane domain, an intracellular domain, but no extracellular domain. (2) The cPTK7 726-1070 protein that has an intracellular domain, but no transmembrane or extracellular domain. PTK7 knockdown Xenopus embryos have an identical NTD phenotype like LRP6 depleted embryos. We found that ectopic expression of either these two isoforms rescued the PTK7 knockdown phenotype similar to ectopic expression of full-length PTK7 protein. These PTK7 isoforms also supported canonical Wnt-signaling, rescuing expression of Wnt-target genes typically lost in PTK7-knockdown embryos. Moreover, we showed that like full-length PTK7 protein, these two isoforms can maintain LRP6 protein levels in the embryo. This study is the first to show that PTK7 isoforms function similar to full-length PTK7 protein in a normal biological system, the developing vertebrate nervous system.