PI4K2 beta/AP-1-Based TGN-Endosomal Sorting Regulates Wnt Signaling

Abstract

Endosomal membrane traffic serves crucial roles in cell physiology, signaling, and development [1-4]. 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 [5]. 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, 7]. The Wnt pathway is a major developmental signaling cascade and depends on endosomal sorting in Wnt-sending cells [8-10]. Whether TGN/endosomal sorting modulates signaling downstream of Frizzled (Fz) receptors in Wnt-receiving cells is unknown. Here, we identify PI4-kinase type 2 beta (PI4K2 beta) as a regulator of TGN/endosomal sorting and Wnt signaling. PI4K2 beta and AP-1 interact directly and are required for efficient sorting between endosomes and the TGN. Zebrafish embryos depleted of PI4K2 beta or AP-1 lack pectoral fins due to defective Wnt signaling. Rescue experiments demonstrate requirements for PI4K2 beta-AP-1 complex formation and PI4K2 beta-mediated PI(4)P synthesis. Furthermore, PI4K2 beta binds to the Fz-associated component Dishevelled (DvI) and regulates endosomal recycling of Fz receptors and Wnt target gene expression. These data reveal an evolutionarily conserved role for PI4K2 beta and AP-1 in coupling phosphoinositide metabolism to AP-1-mediated sorting and Wnt signaling.