Current Views on the Roles of O-Glycosylation in Controlling Notch-Ligand Interactions

Abstract

The 100th anniversary of Notch discovery in Drosophila has recently passed. The Notch is evolutionarily conserved from Drosophila to humans. The discovery of human-specific Notch genes has led to a better understanding of Notch signaling in development and diseases and will continue to stimulate further research in the future. Notch receptors are responsible for cell-to-cell signaling. They are activated by cell-surface ligands located on adjacent cells. Notch activation plays an important role in determining the fate of cells, and dysregulation of Notch signaling results in numerous human diseases. Notch receptors are primarily activated by ligand binding. Many studies in various fields including genetics, developmental biology, biochemistry, and structural biology conducted over the past two decades have revealed that the activation of the Notch receptor is regulated by unique glycan modifications. Such modifications include O-fucose, O-glucose, and O-N-acetylglucosamine (GlcNAc) on epidermal growth factor-like (EGF) repeats located consecutively in the extracellular domain of Notch receptors. Being fine-tuned by glycans is an important property of Notch receptors. In this review article, we summarize the latest findings on the regulation of Notch activation by glycosylation and discuss future challenges.

Significant Roles of Notch O-Glycosylation in Cancer

“…The EGF repeats are defined by the presence of six conserved cysteine residues forming three disulfide bonds that are essential for its three-dimensional structure. EGF repeats can be modified with O-glycans at distinct sites [9][10][11][12]20,21] O-Glucose monosaccharides are added by POGLUT2 or POGLUT3 to a serine residue within the consensus sequence, C 3 -X-N-T-X-G-S-F-X-C 4 . O-GlcNAc monosaccharides are added to a serine or threonine residue within the consensus sequence, C 5 -X-X-G-X-(S/T)-G-X-X-C 6 , and can be modified with galactose and sialic acid in mammals.…”

“…The NECD contains 29–36 EGF repeats (36 in NOTCH1 and NOTCH2, 34 in NOTCH3, and 29 in NOTCH4) that are responsible for ligand-binding interactions [ 8 , 9 ]. These EGF repeats harbor sites for the addition of both N -linked and O -linked glycans [ 10 , 11 , 12 ].…”

“…The EGF repeats are defined by the presence of six conserved cysteine residues forming three disulfide bonds that are essential for its three-dimensional structure. EGF repeats can be modified with O -glycans at distinct sites [ 9 , 10 , 11 , 12 , 20 , 21 ]. The three major types of O -linked glycosylation found on the EGF repeats of Notch pathway components are O -fucosylation, O -glucosylation, and O - N -acetylglucosamine (GlcNAc) modification, which exist as monosaccharides or extended forms.…”

Shooting at Moving and Hidden Targets—Tumour Cell Plasticity and the Notch Signalling Pathway in Head and Neck Squamous Cell Carcinomas

“…It was further observed that LFng and MFng proteins suppressed Jagged1-induced signalling, and DLL1 or JAG1 ligands were able to induce RFng [ 334 ]. The mechanisms of O-glycosylation by Fringe proteins on NOTCH receptor interactions are still under intense investigation, as recently reviewed by [ 335 ]. Additionally, transformed squamous carcinoma cells can express significant amounts of JAG1 and 2 proteins, the role of which remains unclear: are these able to trigger Notch-signalling via auto-stimulation, by cis-acting cell–cell-interactions?…”

POGLUT2 and POGLUT3 O-glucosylate multiple EGF repeats in fibrillin-1, -2, and LTBP1 and promote secretion of fibrillin-1

“…Decades of research have been performed analyzing O-glycosylation of EGF repeats and the significance of the Oglycans. This research has been dominated by investigations of the Notch receptors (16)(17)(18)(19) which structurally resemble fibrillins and LTBPs in that they have extracellular domains with as many as 36 tandem EGF repeats. Notch EGF repeats are heavily Oglycosylated by three enzymes: Protein O-Glucosyltransferase 1 (POGLUT1), Protein O-Fucosyltransferase 1 (POFUT1), and EGF Domain Specific O-Linked N-Acetylglucosamine Transferase (EOGT) (Fig.…”