WNT signaling is crucial for embryonic development and adult tissue homeostasis, with aberrant signaling resulting in developmental disorders and disease, including cancer. Although much is known, a deeper mechanistic understanding of this signaling cascade will improve our understanding of cancer formation, progression and metastasis, allowing for the development of more effective therapeutics. WNT/b-catenin signaling is driven by the stabilization of the transcriptional co-activator, b-catenin. In the absence of WNT ligand, a cytosolic destruction complex phosphorylates, ubiquitylates and degrades b-catenin. In the presence of WNT ligand, the WNT receptors, Frizzled and LRP6, and intracellular proteins form an alternative complex called the WNT signalosome. This results in b-catenin accumulation and activation of b-catenin target genes. Recent data demonstrate that upon WNT ligand engagement, the signalosome is endocytosed. Although conflicting data exist within the literature, a consensus is beginning to emerge that clathrin-dependent endocytosis of the signalosome results in signalosome degradation. This training proposal and my thesis project is devoted to elucidating the molecular events and dynamics of signalosome formation, stabilization and endocytosis in normal cells and in cancer, with an emphasis on kinases. In the first half of my graduate training, I utilized a gain-of- function screen of the kinome to identify AAK1 as a negative regulator of WNT signaling. I demonstrated that AAK1 activates a transcription independent negative feedback loop to promote LRP6 internalization, resulting in WNT signaling downregulation. In the course of these studies, we demonstrated that AAK1 promotes the phosphorylation of a clathrin adapter protein, AP2M1, 8-10 hrs post-WNT3A and that AAK1 and AP2M1 interact with the tumor suppressor, WTX. My lab previously discovered the WTX tumor suppressor as a component of the signalosome and b-catenin destruction complex. Therefore, I will define comprehensive WNT3A and WTX- dependent changes to the phosphoproteome by quantitative mass spectrometry and test whether WTX regulates signalosome endocytosis via AAK1. Additionally, CSNK1g is known to regulate phosphorylation of LRP6, an essential step for signalosome formation. CSNK1g has 3 isoforms, CSNK1g1/2/3, all identified as understudied kinases. My preliminary data suggest each isoform functions differently to activate WNT signaling and promote LRP6 internalization. A main focus for the remainder of my graduate work will be to functionally characterize the role of each CSNK1g isoform in regulating WNT signaling, define comprehensive protein-protein interaction networks and evaluate isoform specific changes to the WNT-driven phosphoproteome. Because this work is descriptive in nature, I expect it to be submitted for publication in 14 months. To summarize, the precise role of endocytosis in WNT signaling remains unclear, with numerous questions surrounding the mechanism(s) and components of endocytosis and its effects on signaling. This work, and my future postdoctoral work, will provide me training in and experience in the mechanisms of WNT signaling and feedback attenuation.
Various mechanisms drive hyperactivity of the WNT signaling pathway in cancer. Defining and understanding these mechanisms will facilitate cancer treatment and improve patient outcome. This training proposal aims to clarify the roles of the WTX tumor suppressor and the druggable CSNK1g kinase isoforms in WNT signaling regulation.
