8.A.56 The Wntless Protein (Wls) Family
wls, (srt, evi) of Drosophila is a segment polarity gene required for wingless (wg)-dependent patterning processes, acting in both wg-sending cells and wg-target cells. In non-neuronal cells, Wls directs wg secretion. The Wls traffic loop encompasses the Golgi, the cell surface, an endocytic compartment and a retrograde route leading back to the Golgi, and involves clathrin-mediated endocytosis and the retromer complex (a conserved protein complex consisting of Vps35 and Vps26) (Bänziger et al. 2006). In neuronal cells (the larval motorneuron NMJ), the wg signal moves across the synapse via the release of Wls-containing exosome-like vesicles (Korkut et al. 2009). Postsynaptic Wls is required for the trafficking of fz2 through the fz2-interacting protein Grip. Sprinter (Srt) is required for secretioin of wingless (Wg) (Goodman et al. 2006) as Wg is retained by evi mutant cells (Bartscherer et al. 2006).
Wnt proteins comprise a large class of secreted signaling molecules with key roles during embryonic development and throughout adult life. Porcupine and Wntless/Evi/Sprinter are required in Wnt-producing cells for the processing and secretion of many Wnt proteins, and secretion occurs independently of lipid modification (Ching et al. 2008). All Wnts, except WntD, require Wls for secretion. All Wnts, except WntD, also contain a conserved serine residue (in Wg S239), which is essential for their functional and physical interaction with Wls. Finally, all Wnts except WntD, require the acyltransferase Porcupine for activity and for functionally interacting with Wls. Thus, Por-mediated lipidation of the S239-equivalent residue is essential for the interaction with, and secretion by, Wls (Herr and Basler 2012; Tang et al. 2012). Together with the cargo receptor Evi/WIs, Wnts are transported through endosomal compartments onto exosomes, a process that requires the R-SNARE Ykt6 (Gross et al. 2012). the Bro1-domain-containing protein Myopic (Mop) is indispensable for endosomal trafficking of Wg and Wls. Reductions in Mop leads to trapping of Wg and Wls in the early endosomes (Pradhan-Sundd and Verheyen 2014).
Xenopus laevis Wntless (XWntless) regulates the secretion of a specific Wnt ligand, XWnt4, and this regulation is required for eye development. The Retromer complex is required for XWntless recycling to regulate the XWnt4-mediated eye development. Inhibition of Retromer function by Vps35 morpholino (MO) results in various Wnt deficiency phenotypes, affecting mesoderm induction, gastrulation cell movements, neural induction, neural tube closure, and eye development (Kim et al. 2009). Disrupting the secretion of human Wnt5a also induced ER stress in mammalian cells, and a C-terminal KKVY-motif of Wg is required for its retrograde Golgi-to-ER transport, thus inducing ER stress. However, ER stress resulting from Wnt secretion impairment could be readily explained by its inability to leave the ER, and not resulting from Golgi-to-ER retrograde transport (Tang 2016).