Diffusion and function of Wnt ligands

Wnts are secreted, lipidated glycoproteins that are essential for cell–cell communication in development and tissue homeostasis throughout metazoans [1–3]. The “W” in Wnt comes from the wingless (wg) gene, which is required for proper formation of most tissues and appendages in Drosophila [4,5]. Immunostainings showed gradients of Wg protein emanating from Wg-producing cells in embryos and larval imaginal discs [6–9]. In the mid-1990s, elegant genetic studies in the wing imaginal discs supported the hypothesis that Wg protein could act as a diffusible morphogen by specifying different cell fates along a concentration gradient [10,11]. Although this evidence seemed compelling, caveats included perdurance from earlier Wg expression throughout the presumptive wing and the potential spread of the signal through the division of Wg-producing cells [4,12]. Still, Wg has commonly been considered a paradigm for a secreted protein that can act as a diffusible morphogen across a developing tissue [13–15]. The importance of Wg diffusion was tested by Alexandre and colleagues in 2014, when they replaced the endogenous Wg gene with a cDNA encoding a fusion of the neurotactin transmembrane domain and Wg (NRT-Wg; Fig 1). Tethering Wg to the plasma membrane of Wgproducing cells resulted in a dramatic restriction of the Wg protein gradient in late-stage wing imaginal discs [16]. NRT-Wg homozygotes, i.e., flies in which NRT-Wg is their only source of Wg, survive to adulthood with relatively mild phenotypes, such as slightly smaller wings [16]. This is surprising, as wg is essential for viability and patterning at embryonic and larval stages [4]. The authors suggested that perdurance of Wg target gene expression from earlier stages could provide the information necessary for proper patterning [16]. A recent preprint reports that NRT-Wg is more stable than Wg, in part because of increased expression of its receptor, frizzled 2 (Fz2) [17], which can stabilize Wg [8,17]. These nuances do not detract from the seminal conclusion of Alexandre and colleagues, i.e., that tethering of Wg to its producing cells has almost no effect on the patterning of the fly. Although NRT-Wg flies make it to adulthood, they are sterile and developmentally delayed [16]. In addition, as shown by a new report in this issue, NRT-Wg flies have severe defects in cell fate specification, proliferation, and morphology of the adult intestine [18]. Wg is normally expressed at high levels at the midgut–hindgut boundary (MHB) of the intestine, where it activates Wg target genes at a distance from the MHB [18]. The area of Wg target gene expression is severely reduced in NRT-Wg intestines. The structure of the MHB is drastically altered, as are the longitudinal and circular muscles surrounding the intestine [18]. Epithelial patterning and proliferation are also disrupted. These defects could be phenocopied by a reduction of Wg signaling, and NRT-Wg phenotypes were not rescued by overexpression of a NRT-Wg transgene. NRT-Wg flies had decreased longevity compared with controls, which was more severe when fed a sucrose-only diet [18]. Taken together, the data support a model in which Wg spreading is essential for proper development and function of the intestine. NRT-Wg flies also