Sugars of the Glycosylation Salvage Pathway Control Wnt in the Early Embryo
It has been estimated that approximately 50% of all proteins in a cell are glycosylated. Glycoconjugate biosynthesis requires activation of monosaccharides, which can be imported into the cell, derived from other sugars within the cell, or salvaged from degraded glycans. There is growing recognition that the contribution of the salvage pathway to glycosylation is substantial. Free N-Acetylglucosamine (GlcNAc) is converted into Uridine diphosphate GlcNAc (UDP-GlcNAc) by the salvage pathway for use in N-linked glycosylation. The first step in this pathway is the conversion of GlcNAc into GlcNAc-6-P by N-Acetylglucosamine Kinase (NAGK) which accounts for more than 80% of the free GlcNAc-6-P in the cell. GlcNAc-6-P mutase converts GlcNAc-6-P into GlcNAc-1-P, which is converted into UDP-GlcNAc by UDP-GlcNAc pyrophosphorylase. Previous work in our lab demonstrated that overexpression of components of the salvage pathway induced a Wnt-like phenotype in early Xenopus laevis embryos. To confirm the role of the salvage pathway in regulating Wnt signaling, we tested the effects sugars (and an inhibitor) of the salvage pathway in Xenopus laevis and Danio rerio (zebrafish) as well as in a cell-based, Wnt reporter human cell line. We found that addition of salvage pathway sugars resulted in activation of the Wnt pathway. Conversely, inhibition of the salvage pathway resulted in inhibition of the Wnt pathway. We conclude that alterations in the activity of the glycosylation salvage pathway may control Wnt activities.