Wieffer, M., Cibrián Uhalte, E., Posor, Y., Otten, C., Branz, K., Schütz, I., Mössinger, J., Schu, P., Abdelilah-Seyfried, S., Krauß, M., and Haucke, V. (2013) PI4K2beta/AP-1-Based TGN-Endosomal Sorting Regulates Wnt Signaling. Current biology : CB. 23(21):2185-2190.
Endosomal membrane traffic serves crucial roles in cell physiology, signaling, and development [1, 2, 3 and 4]. Sorting between endosomes and the trans-Golgi network (TGN) is regulated among other factors by the adaptor AP-1, an essential component of multicellular organisms . Membrane recruitment of AP-1 requires phosphatidylinositol 4-phosphate [PI(4)P], though the precise mechanisms and PI4 kinase isozyme (or isozymes) involved in generation of this PI(4)P pool remain unclear [6 and 7]. The Wnt pathway is a major developmental signaling cascade and depends on endosomal sorting in Wnt-sending cells [ 8, 9 and 10]. Whether TGN/endosomal sorting modulates signaling downstream of Frizzled (Fz) receptors in Wnt-receiving cells is unknown. Here, we identify PI4-kinase type 2β (PI4K2β) as a regulator of TGN/endosomal sorting and Wnt signaling. PI4K2β and AP-1 interact directly and are required for efficient sorting between endosomes and the TGN. Zebrafish embryos depleted of PI4K2β or AP-1 lack pectoral fins due to defective Wnt signaling. Rescue experiments demonstrate requirements for PI4K2β-AP-1 complex formation and PI4K2β-mediated PI(4)P synthesis. Furthermore, PI4K2β binds to the Fz-associated component Dishevelled (Dvl) and regulates endosomal recycling of Fz receptors and Wnt target gene expression. These data reveal an evolutionarily conserved role for PI4K2β and AP-1 in coupling phosphoinositide metabolism to AP-1-mediated sorting and Wnt signaling.