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Wnt signaling is implicated in a variety of developmental and pathological processes. The molecular mechanisms governing the secretion of Wnt ligands remain to be elucidated. Wntless, an evolutionarily conserved multipass transmembrane protein, is a dedicated secretion factor of Wnt proteins that participates in Drosophila melanogaster embryogenesis. In this study, we show that Xenopus laevis Wntless (XWntless) regulates the secretion of a specific Wnt ligand, XWnt4, and that this regulation is specifically required for eye development in Xenopus. Moreover, the Retromer complex is required for XWntless recycling to regulate the XWnt4-mediated eye development. Inhibition of Retromer function by Vps35 morpholino (MO) resulted in various Wnt deficiency phenotypes, affecting mesoderm induction, gastrulation cell movements, neural induction, neural tube closure, and eye development. Overexpression of XWntless led to the rescue of Vps35 MO-mediated eye defects but not other deficiencies. These results collectively suggest that XWntless and the Retromer complex are required for the efficient secretion of XWnt4, facilitating its role in Xenopus eye development.
Dynamic expression pattern of XWntless during Xenopus embryogenesis. (A) Sequence similarity among Drosophila Wntless (DWntless), Xenopus Wntless (XWntless), and human Wntless (hWntless). (B) Temporal expression pattern of XWntless was analyzed by RT-PCR. Numbers indicate developmental stages. E, egg stage; −, without reverse transcription. (C) Spatial expression pattern of XWntless was analyzed by RT-PCR of dissected tissues of gastrulaembryo. −, without reverse transcription; WE, whole embryo; AC, animal cap explant; D, dorsal marginal explant; V, ventral marginal explant. Xbra was used for marginal expression control, Chordin for dorsal marginal expression control, Msx1 for animal cap and ventral marginal expression control, and ODC for loading control. (D to I) Expression pattern of XWntless was analyzed by in situ hybridization. (D and E) XWntless is expressed at the animal hemisphere at blastula and gastrula stage. XWntless is not detected at the dorsal and ventral marginal zone of gastrulaembryo (arrow and arrowhead, respectively). (F) At neurula stage, XWntless is expressed broadly throughout the dorsal side of the embryo and strongly at the borders of lateral and anterior neural plate. (G) At stage 22, XWntless is expressed at the eyeprimordium and neural tube. (H and I) At stage 32, XWntless is expressed at the liver (black arrowhead), heart (white arrowhead), pronephros (arrow in panel I), and dorsal neural tube, including brain and spinal cord (arrow in panel H). Panel I is an enlarged image of the portion of panel H enclosed by the white dashed line. (J and K) XWntless sense probe was hybridized as a control. (L to N) Eye expression of XWntless is dynamically changing. At stages 23 and 27, XWntless is expressed broadly at the eye field, and at stage 32, XWntless is restricted to distinct regions, the center and the border of the eye. WLS, XWntless; st., stage.
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