Nuclear import of beta-Catenin in Wnt-signaling

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Project Summary Wnt/Wingless (Wg)-signaling plays important roles in intercellular signaling in all metazoan organisms. It functions in many critical processes, such as organ patterning, growth control, cell polarity, and stem cell maintenance. When deregulated it is linked to diseases ranging from congenital heart disease and aberrant vasculogenesis to cancer. Recent studies in my lab have shown that Intraflagellar Transport complex A (IFT-A) proteins modulate canonical Wnt/Wg-signaling independently of the ciliary role of IFTs. We demonstrated that they do so together with Kinesin2 (as they do in ciliary functions) and promote nuclear translocation of β- catenin upon Wnt-pathway activation and act downstream of β-catenin stabilization. Kinesin-2 and IFT-A proteins act as a complex during Wg-signaling in Drosophila and mammals. Mutants of both, Kinesin 2 and IFT-A, affect Wg/Wnt-signaling in the same manner, and they interact genetically and physically. Kap3, a kinesin associated protein that serves as the bridging factor between Kinesin 2 and IFT-A in ciliary function is also required in the same manner and is involved in the formation of a physical complex of Kinesin 2-Kap3- IFTs. The IFT-A protein IFT140 then directly binds to β-catenin, called Armadillo/Arm in Drosophila. Upon pathway stimulation by Wg/Wnt and resulting pathway activation, all these factors co-localize with each other and β-catenin, and bind together to micro-tubules (MTs). Single or double mutant cells for Kinesin-2, IFT-As, or Kap3 fail to properly activate Wg/Wnt-signaling targets in both Drosophila and MEFs. In addition, axin double mutant backgrounds with IFT-A or Kinesin-2 reveal high levels of cytoplasmic Arm/β-catenin but fail to activate Wg/Wnt targets, due to reduced nuclear β-catenin localization. These data indicate that the Kinesin-2/IFT-A complex promotes nuclear localization of Arm/β-catenin by protecting it from a cytoplasmic tether/inhibitor. We have thus identified a mechanistic function of the Kinesin-2/IFT-A complex in Wnt-signaling, independent of their role in the cilium. As the associated mechanism(s) are conserved in mammalian cells, these observations are potentially amenable to drug treatment. However, several questions remain: (i) How is the Kinesin-2/IFT-A complex promoting nuclear localization; our data suggest it is by movement along microtubules, but this needs to be refined; and (ii) what is the factor that competes with IFT140 binding to Arm/β-catenin and serves an `inhibitory' function for Arm/β-catenin nuclear localization. We will address in Aim 1 the cell biology of this mechanism using an elegant ex vivo Wnt-signaling system, and Aim 2 is tailored to identify antagonistic factor(s) that compete with IFT140 binding. Our preliminary data suggest that this knowledge can serve as an entry-point for new drug development to inhibit overactive Wnt/β-catenin signaling. Thus, information acquired in this application will advance our mechanistic understanding of Wnt/β-catenin signaling, and potentially also lead to follow up studies to develop new drugs and thus benefit the treatment in disease associated contexts.