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Loss of Porcupine impairs convergent extension during gastrulation in zebrafish

Porcupine (Porcn), an O-acyltransferase located in the endoplasmic reticulum (ER), is required for lipidation of Wnt proteins to enable their trafficking from the ER in mammalian cell culture. However, it is unclear whether Porcn is required for trafficking of all members of the Wnt family. In this study, we investigated the function of Porcn in zebrafish embryos. We identified two zebrafish homologs of porcupine, porcn and porcupine-like (porcn-l). Zebrafish porcn, but not porcn-l, restores secretion of Wnt proteins in porcn-deficient mouse L cells. Morpholino-mediated knockdown of porcn in zebrafish embryos impairs convergence and extension (CE) during gastrulation without changing embryonic patterning. Moreover, porcn interacts genetically with wnt5b and wnt11 in regulating CE. By contrast, porcn-deficient embryos do not exhibit phenotypes caused by failure in canonical Wnt signaling, which is activated by several Wnt ligands, including Wnt3a. Furthermore, expression of genes regulated by the canonical Wnt signaling pathway is not perturbed in knockdown embryos relative to that in controls. Although the trafficking and lipidation of ectopically expressed zebrafish Wnt5b and mouse Wnt5a are impaired in porcn-deficient embryos, those of ectopically expressed Wnt3a are less or not affected. In addition, the secretion of Wnt5a is inhibited by less Porcn inhibitor than that of Wnt3a in HEK293T cells. Thus, a decrease of Porcn activity does not equivalently affect trafficking and lipidation of different Wnt proteins in zebrafish embryos and in cultured mammalian cells.