Apical secretion of Wnt1 in polarized epithelial cells is regulated by exocyst-mediated trafficking

Wnts are glycosylated proteins secreted from various cell types including mesenchymal, hematopoietic and epithelial cells. Directional secretion of Wnts in polarized epithelial cells is unique; Wnt11 is secreted apically, whereas Wnt5a and Wnt3a are secreted basolaterally. Here, we found that Wnt1 is equivalently secreted both apically and basolaterally in MDCK cells. Wnt1 was modified with a complex- or hybrid-type glycan at Asn29 and Asn359 and the high-mannose- or hybrid-type glycan at Asn316. Although glycosylation of Wnt11 at the N-terminal site was shown to be essential for its apical secretion, glycosylation of Asn29 of Wnt1 was not required. Instead, the apical secretion of Wnt1 was inhibited by knockdown of Sec6 and Sec8, suggesting that Wnt1 is secreted apically via exocyst-mediated transport. Basolateral secretion of Wnt1 was mediated by clathrin and AP-1, in mechanism similar to that used by Wnt5a and Wnt3a. Although Wingless was reported to be transcytosed to the basolateral region in the Drosophila wing disc, transcytosis was not involved in the basolateral secretion of Wnt1. Thus, the polarized secretion of Wnt1 is regulated by different mechanisms than other Wnts.