Editors' ChoiceCell Biology

Deacylated Wnt forms inactive oligomers

Sci. Signal.  07 Apr 2015:
Vol. 8, Issue 371, pp. ec85
DOI: 10.1126/scisignal.aab2664

Lipid modifications are necessary for the proper function of ligands of the Wnt family. For example, palmitoleoylation, a form of acylation, on Ser209 is required for the secretion of Wnt3a and its binding to receptors. Zhang et al. found that the secreted α/β hydrolase Notum antagonized Wnt8 and Wnt3a signaling in Xenopus laevis embryos and in HEK293T cells, respectively. The presence of Notum had no effect on Wnt3a secretion from HEK293T cells. Wnt3a purified from the conditioned medium of HEK293T cells expressing recombinant Wnt3a alone or Wnt3a and a catalytically inactive form of Notum [Notum(S239A)] was hydrophobic, palmitoleoylated, and activated a Wnt-responsive reporter. In contrast, Wnt3a purified from the conditioned medium of cells coexpressing Wnt3A and wild-type Notum was hydrophilic, unpalmitoleoylated, and did not activate the reporter. Purified Notum depalmitoleoylated Wnt3a in vitro, but Notum(S239A) did not. Whereas palmitoleoylated Wnt3a from conditioned medium was monomeric, depalmitoleoylated Wnt3a formed large, soluble oligomers, indicating that palmitoleoylation was required for Wnt3a to retain its proper tertiary structure. During specification of the vertebrate anteroposterior axis, Wnt signaling promotes tail formation, and Wnt antagonists promote specification of head tissues and neural induction. Notum expression in Xenopus embryos was consistent with a role in head formation and neural induction, and injecting Notum, but not Notum(S239A), mRNA into embryos caused enlargement of the head. Conversely, morpholino-mediated knockdown of Notum impaired head formation, and this defect was overcome by co-injection of mRNA encoding the Wnt antagonist Dkk1 or a morpholino targeting the Wnt signaling mediator β-catenin. Notum was also required in the prospective ectoderm for proper neural induction in Xenopus embryos. Thus, Notum-mediated depalmitoleoylation of Wnts is likely a physiologically relevant mechanism of Wnt antagonism in vivo.

X. Zhang, S.-M. Cheong, N. G. Amado, A. H. Reis, B. T. MacDonald, M. Zebisch, E. Y. Jones, J. G. Abreu, X. He, Notum is required for neural and head induction via Wnt deacylation, oxidation, and inactivation. Dev. Cell 32, 719–730 (2015).[PubMed]