INAUGURAL ARTICLE by a Recently Elected Academy Member:Secreted Wingless-interacting molecule (Swim) promotes long-range signaling by maintaining Wingless solubility

Wnt proteins comprise a conserved family of secreted signaling molecules with key functions during embryonic development and adult homeostasis (1). Wnt signaling is initiated by the binding of Wnt to its receptors, Frizzled (Fz) and Arrow/LRP5/6 (2, 3), which leads to the stabilization and subsequent translocation of β-catenin to the nucleus where it associates with members of the Tcf/Lef family of transcription factors to regulate target genes (4, 5). Aberrant Wnt signaling contributes to a variety of developmental abnormalities and adult diseases (6); therefore, elucidating the mechanisms of Wnt signal transduction is an area of intense research. Wnt proteins can act as morphogens (7–9), secreted molecules that affect tissue organization by providing spatial information in the form of a concentration gradient. In the developing Drosophila wing imaginal disc, Wingless (Wg) is secreted by a narrow stripe of cells at the dorsoventral (DV) boundary. It then diffuses through a field of cells to activate transcription of high-threshold target genes such as senseless (sens) close to the Wg-producing cells and low-threshold genes such as distalless (dll) farther away (10–12). This finding implies that Wg travels away from its site of production through the extracellular environment while maintaining an active conformation. Wnt proteins are dually lipid modified by the covalent attachment of palmitic acid (9, 13, 14) and palmitoleic acid (15), causing them to be hydrophobic (14), and Wnts may be membrane-associated under many circumstances (16, 17). This finding presents a paradox: how is it possible that Wnts affect cells at a distance if their lipid moieties confer high affinity for cell membranes? Experiments in Drosophila show that a membrane-tethered form of Wg is sufficient to function as a short-range inducer as it can rescue wg-null embryos and induce transcription of short- but not long-range target genes in larval wing discs (12). This absence of transcriptional activation of long-range Wg target genes indicates that diffusion away from the site of production is necessary for proper Wg signaling. Extracellular Wg has been detected in membranous lipoprotein particles (18), which may contribute to Wg transport; however, Wg signaling activity of these particles has not been reported. Another possibility is that Wnts diffuse freely from membranes, but this possibility raises the question of how a hydrophobic protein is able to travel through the aqueous extracellular environment without aggregating. In this paper, we describe an extracellular Wg binding protein called Secreted Wg-interacting molecule (Swim), a putative member of the Lipocalin family of transport proteins. We show that Swim binds to Wg with nanomolar affinity in a palmitate-dependent manner and maintains Wg solubility and signaling activity. Reduction of swim expression in vivo shortens the distribution of extracellular Wg and leads to impaired long-range Wg signaling activity. These data suggest a unique carrier function of Swim to mediate the morphogenetic activity of the hydrophobic Wg protein.