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Glycosylation and development: A frog hops into the gap

X. laevis mesoderm formation is inhibited after injection of xGalntl-1 mRNA (B). Copyright 2008 by The Company of Biologists Ltd.

Mucin-type O-glycosylation of proteins is initiated by serine or threonine N-acetylgalactosaminylation and is known to modulate the development of species such as the fruit fly. Due to enzymatic redundancy, it has been difficult to ascribe unique roles or substrates to the 14 described members of the human N-acetylgalactosamine (GalNAc) transferase family. Research by Herr et al. in Development now describes the effects of transforming growth factor- receptor (TGFBR) O-glycosylation on Xenopus laevis embryogenesis.

By microarray screening, the authors identified a xGalntl-1 gene sharing 72% homology with the mammalian glycosyltransferase N-acetylgalactosaminyltransferase-like 1. Further analysis revealed that embryonic X. laevis mucin-type O-glycosylation coincided with xGalntl-1 expression in neural structures such as the forebrain and the mediolateral spinal chord. Thus, mucin-type O-glycosylation may inhibit mesodermal development, but promote the development of ectoderm from which neural structures emerge. This idea was validated by the observation that when xGalntl-1 mRNA was injected into the X. laevis 4-cell stage it induced neural marker gene expression, but inhibited dorsal and ventral mesodermal marker gene expression. This suggests that mucin-type O-glycosylation interferes with Activin/Nodal signaling — a TGF- subtype known to induce mesoderm germ layer development. Indeed, co-injection of xGalntl-1 morpholinos stimulated Activin mRNA-induced mesoderm gene expression, and xGalntl-1 mRNA injection counteracted the co-injection of Activin mRNA.

In addition to germ layer development, mucin-type O-glycosylation may also influence cell signaling involving bone morphogenetic proteins (BMPs), which are a TGF- superfamily subgroup that establish the dorsoventral axis of the embryo. These results partially support a hypothesis that had been previously confirmed in zebrafish: that injection of xGalntl-1 mRNA into the 4-cell stage leads to a more dorsalized embryo, but falls short of the strong dorsalization effects of a BMP antagonist such as Chordin. This indicates that O-glycosylation attenuates, but does not abrogate, BMP signaling in X. laevis.

Screening a panel of type TGFBR subunits, Herr et al. found that O-glycosylation by xGalntl-1 inhibited signal transduction through the ActR-IIb and ActR-IIa receptors, which reverted back to normal upon treatment with the mucin-type O-glycosylation inhibitor benzyl-GalNAc. Binding studies showed that O-glycosylation prevented ActR-IIb and ActR-IIa binding to type I TGFBR subunit Alk-4, which mediates Activin/Nodal signaling, and the subunits Alk-3 and Alk-6, which transmit BMP signals. These results indicate that the inhibitory effects of O-glycosylation on Activin/Nodal and BMP signaling are due to TGFBR assembly regulation.

Injection of mammalian Galntl-1 mRNA into X. laevis embryos and expression of xGalntl 1 in human embryonic kidney cells both mimicked the effects found in the X. laevis model system, showing that galntl-1-induced regulation of TGF- signaling components might be a general feature of vertebrate development. The study of Herr et al. is complemented by recent progress in the ability to track developmental glycosylation, and future research may add to the evidence that glycosylation types — such as fucosylation — regulate signaling pathways that guide development.

Author: Mirko von Elstermann