Fringe modifies O-fucose on mouse Notch1 at epidermal growth factor-like repeats within the ligand-binding site and the Abruptex region

Fringe plays a key role in the specification of boundaries during development by modulating the ability of Notch ligands to activate Notch receptors. Fringe is a fucose-specific beta1,3-N-acetylglucosaminyltransferase that modifies O-fucose moieties on the epidermal growth factor-like (EGF) repeats of Notch. To investigate how the change in sugar structure caused by Fringe modulates Notch activity, we have analyzed the sites of O-fucose and Fringe modification on mouse Notch1. The extracellular domain of Notch1 has 36 tandem EGF repeats, many of which are predicted to be modified with O-fucose. We recently proposed a broadened consensus sequence for O-fucose, C(2)X(3-5)(S/T)C(3) (where C(2) and C(3) represent the second and third conserved cysteines), significantly expanding the potential number of modification sites on Notch. Here we demonstrate that sites predicted using this broader consensus sequence are modified with O-fucose on mouse Notch1, and we present evidence suggesting that the consensus can be further refined to C(2)X(4-5)(S/T)C(3). In particular, we demonstrate that EGF 12, a portion of the ligand-binding site, is modified with O-fucose and that this site is evolutionarily conserved. We also show that endogenous Fringe proteins in Chinese hamster ovary cells (Lunatic fringe and Radical fringe) as well as exogenous Manic fringe modify O-fucose on many but not all EGF repeats of mouse Notch1. These findings suggest that the Fringes show a preference for O-fucose on some EGF repeats relative to others. This specificity appears to be encoded within the amino acid sequence of the individual EGF repeats. Interestingly, our results reveal that Manic fringe modifies O-fucose both at the ligand-binding site (EGF 12) and in the Abruptex region. These findings provide insight into potential mechanisms by which Fringe action on Notch receptors may influence both the affinity of Notch-ligand binding and cell-autonomous inhibition of Notch signaling by ligand.     

Modulation of receptor signaling by glycosylation: fringe is an O-fucose-beta1,3-N-acetylglucosaminyltransferase

The Notch family of signaling receptors plays key roles in determining cell fate and growth control. Recently, a number of laboratories have shown that O-fucose glycans on the epidermal growth factor (EGF)-like repeats of the Notch extracellular domain modulate Notch signaling. Fringe, a known modifier of Notch function, is an O-fucose specific beta1,3-N-acetylglucosaminyltransferase. The transfer of GlcNAc to O-fucose on Notch by fringe results in the potentiation of signaling by the Delta class of Notch ligands, but causes inhibition of signaling by the Serrate/Jagged class of Notch ligands. Interestingly, addition of a beta1,4 galactose by beta4GalT-1 to the GlcNAc added by fringe is required for Jagged1-induced Notch signaling to be inhibited in a co-culture assay. Thus, both fringe and beta4GalT-1 are modulators of Notch function. Several models have been proposed to explain how alterations in O-fucose glycans result in changes in Notch signaling, and these models are discussed.     

Notch signaling in normal and disease States: possible therapies related to glycosylation

The Notch signaling pathway is involved in a wide variety of highly conserved developmental processes in mammals. Importantly, mutations of the Notch protein and components of its signaling pathway have been implicated in an array of human diseases (T-cell leukemia and other cancers, Multiple Sclerosis, CADASIL, Alagille Syndrome, Spondylocostal Dysostosis). In mammals, Notch becomes activated upon binding of its extracellular domain to ligands (Delta and Jagged/Serrate) that are present on the surface of apposed cells. The extracellular domain of Notch contains up to 36 tandem Epidermal Growth Factor-like (EGF) repeats. Many of these EGF repeats are modified at evolutionarily-conserved consensus sites by an unusual form of O-glycosylation called O-fucose. Work from several groups indicates that O-fucosylation plays an important role in ligand mediated Notch signaling. Recent evidence also suggests that the enzyme responsible for addition of O-fucose to Notch, protein O-fucosyltransferase-1 (POFUT1), may serve a quality control function in the endoplasmic reticulum. Additionally, some of the O-fucose moieties are further elongated by the action of members of the Fringe family of beta-1,3-N-acetylglucosaminyltransferases. The alteration in O-fucose saccharide structure caused by Fringe modulates the response of Notch to its ligands. Thus, glycosylation serves an important role in regulating Notch activity. This review focuses on the role of glycosylation in the normal functioning of the Notch pathway. As well, potential roles for glycosylation in Notch-related human diseases, and possible roles for therapeutic targeting of POFUT1 and Fringe in Notch-related human diseases, are discussed.     

Regions of Drosophila Notch that contribute to ligand binding and the modulatory influence of Fringe

Two glycosyltransferases that transfer sugars to epidermal growth factor (EGF) domains, OFUT1 and Fringe, regulate Notch signaling. To characterize the impact of glycosylation at the 23 consensus O-fucose sites in Drosophila Notch, we conducted deletion mapping and site-specific mutagenesis and then assayed the binding of soluble forms of Notch to cell-surface ligands. Our results support the conclusion that EGF11 and EGF12 are essential for ligand binding, but indicate that other EGF domains also make substantial contributions to ligand binding. Characterization of Notch deletion constructs and O-fucose site mutants further revealed that no single site or region can account for the influence of Fringe on Notch-ligand binding. Additionally, we observed an influence of Fringe on a Notch fragment including only 4 of its 36 EGF domains (EGF10-13). Together, our observations imply that glycosylation influences Notch-ligand interactions through a distributive mechanism that involves local interactions with multiple EGF domains and led us to suggest a structural model for how Notch interacts with its ligands.     

Biological functions of glycosyltransferase genes involved in O-fucose glycan synthesis

Rare types of glycosylation often occur in a domain-specific manner and are involved in specific biological processes. Well-known examples of such modification are O-linked fucose (O-fucose) and O-linked glucose (O-glucose) glycans on epidermal growth factor (EGF) domains. In particular, O-fucose glycans are reported to regulate the functions of EGF domain-containing proteins such as urinary-type plasminogen activator and Notch receptors. Two glycosyltransferases catalyze the initiation and elongation of O-fucose glycans. The initiation process is catalyzed by O-fucosyltransferase 1, which is essential for Notch signalling in both Drosophila and mice. O-fucosyltransferase 1 can affect the folding, ligand interaction and endocytosis of Notch receptors, and both the glycosyltransferase and non-catalytic activities of O-fucosyltransferase 1 have been reported. The elongation of O-fucose monosaccharide is catalyzed by Fringe-related genes, which differentially modulate the interaction between Notch and two classes of ligands, namely, Delta and Serrate/Jagged. In this article, we have reviewed the recent reports addressing the distinctive features of the glycosyltransferases and O-glycans present on the EGF domains.     

In vitro reconstitution of the modulation of Drosophila Notch-ligand binding by Fringe

Notch signaling plays critical roles in animal development and physiology. The activation of Notch receptors by their ligands is modulated by Fringe-dependent glycosylation. Fringe catalyzes the addition of N-acetylglucosamine in a beta1,3 linkage onto O-fucose on epidermal growth factor-like domains. This modification of Notch by Fringe influences the binding of Notch ligands to Notch receptors. However, prior studies have relied on in vivo glycosylation, leaving unresolved the question of whether addition of N-acetylglucosamine is sufficient to modulate Notch-ligand interactions on its own, or whether instead it serves as a precursor to subsequent post-translational modifications. Here, we describe the results of in vitro assays using purified components of the Drosophila Notch signaling pathway. In vitro glycosylation and ligand binding studies establish that the addition of N-acetylglucosamine onto O-fucose in vitro is sufficient both to enhance Notch binding to the Delta ligand and to inhibit Notch binding to the Serrate ligand. Further elongation by galactose does not detectably influence Notch-ligand binding in vitro. Consistent with these observations, carbohydrate compositional analysis and mass spectrometry on Notch isolated from cells identified only N-acetylglucosamine added onto Notch in the presence of Fringe. These observations argue against models in which Fringe-dependent glycosylation modulates Notch signaling by acting as a precursor to subsequent modifications and instead establish the simple addition of N-acetylglucosamine as a basis for the effects of Fringe on Drosophila Notch-ligand binding.     

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.     

An O-fucose site in the ligand binding domain inhibits Notch activation

Two glycosyltransferases that transfer sugars to EGF domains, OFUT1 and Fringe, regulate Notch signaling. However, sites of O-fucosylation on Notch that influence Notch activation have not been previously identified. Moreover, the influences of OFUT1 and Fringe on Notch activation can be positive or negative, depending on their levels of expression and on whether Delta or Serrate is signaling to Notch. Here, we describe the consequences of eliminating individual, highly conserved sites of O-fucose attachment to Notch. Our results indicate that glycosylation of an EGF domain proposed to be essential for ligand binding, EGF12, is crucial to the inhibition of Serrate-to-Notch signaling by Fringe. Expression of an EGF12 mutant of Notch (N-EGF12f) allows Notch activation by Serrate even in the presence of Fringe. By contrast, elimination of three other highly conserved sites of O-fucosylation does not have detectable effects. Binding assays with a soluble Notch extracellular domain fusion protein and ligand-expressing cells indicate that the NEGF12f mutation can influence Notch activation by preventing Fringe from blocking Notch-Serrate binding. The N-EGF12f mutant can substitute for endogenous Notch during embryonic neurogenesis, but not at the dorsoventral boundary of the wing. Thus, inhibition of Notch-Serrate binding by O-fucosylation of EGF12 might be needed in certain contexts to allow efficient Notch signaling.     

Highly conserved O-fucose sites have distinct effects on Notch1 function

The extracellular domain of mouse Notch1 contains 36 tandem epidermal growth factor-like (EGF) repeats, many of which are modified with O-fucose. Previous work from several laboratories has indicated that O-fucosylation plays an important role in ligand mediated Notch activation. Nonetheless, it is not clear whether all, or a subset, of the EGF repeats need to be O-fucosylated. Three O-fucose sites are invariantly conserved in all Notch homologues with 36 EGF repeats (within EGF repeats 12, 26, and 27). To investigate which O-fucose sites on Notch1 are important for ligand-mediated signaling, we mutated the three invariant O-fucose sites in mouse Notch1, along with several less highly conserved sites, and evaluated their ability to transduce Jagged1- and Delta1-mediated signaling in a cell-based assay. Our analysis revealed that mutation of any of the three invariant O-fucose sites resulted in significant changes in both Delta1 and Jagged1 mediated signaling, but mutations in less highly conserved sites had no detectable effect. Interestingly, mutation of each invariant site gave a distinct effect on Notch function. Mutation of the O-fucose site in EGF repeat 12 resulted in loss of Delta1 and Jagged1 signaling, while mutation of the O-fucose site in EGF repeat 26 resulted in hyperactivation of both Delta1 and Jagged1 signaling. Mutation of the O-fucose site in EGF repeat 27 resulted in faulty trafficking of the Notch receptor to the cell surface and a decreased S1 processing of the receptor. These results indicate that the most highly conserved O-fucose sites in Notch1 are important for both processing and ligand-mediated signaling in the context of a cell-based signaling assay.     

Modulation of receptor signaling by glycosylation: fringe is an O-fucose-β1,3-N-acetylglucosaminyltransferase

The Notch family of signaling receptors plays key roles in determining cell fate and growth control. Recently, a number of laboratories have shown that O-fucose glycans on the epidermal growth factor (EGF)-like repeats of the Notch extracellular domain modulate Notch signaling. Fringe, a known modifier of Notch function, is an O-fucose specific β1,3-N-acetylglucosaminyltransferase. The transfer of GlcNAc to O-fucose on Notch by fringe results in the potentiation of signaling by the Delta class of Notch ligands, but causes inhibition of signaling by the Serrate/Jagged class of Notch ligands. Interestingly, addition of a β1,4 galactose by β4GalT-1 to the GlcNAc added by fringe is required for Jagged1-induced Notch signaling to be inhibited in a co-culture assay. Thus, both fringe and β4GalT-1 are modulators of Notch function. Several models have been proposed to explain how alterations in O-fucose glycans result in changes in Notch signaling, and these models are discussed.     

Lunatic fringe, manic fringe, and radical fringe recognize similar specificity determinants in O-fucosylated epidermal growth factor-like repeats

Notch signaling is a component of a wide variety of developmental processes in many organisms. Notch activity can be modulated by O-fucosylation (mediated by protein O-fucosyltransferase-1) and Fringe, a beta1,3-N-acetylglucosaminyltransferase that modifies O-fucose in the context of epidermal growth factor-like (EGF) repeats. Fringe was initially described in Drosophila, and three mammalian homologues have been identified, Manic fringe, Lunatic fringe, and Radical fringe. Here for the first time we have demonstrated that, similar to Manic and Lunatic, Radical fringe is also a fucose-specific beta1,3-N-acetylglucosaminyltransferase. The fact that three Fringe homologues exist in mammals raises the question of whether and how these enzymes differ. Although Notch contains numerous EGF repeats that are predicted to be modified by O-fucose, previous studies in our laboratory have demonstrated that not all O-fucosylated EGF repeats of Notch are further modified by Fringe, suggesting that the Fringe enzymes can differentiate between them. In this work, we have sought to identify specificity determinants for the recognition of an individual O-fucosylated EGF repeat by the Fringe enzymes. We have also sought to determine differences in the biochemical behavior of the Fringes with regard to their in vitro enzymatic activities. Using both in vivo and in vitro experiments, we have found two amino acids that appear to be important for the recognition of an O-fucosylated EGF repeat by all three mammalian Fringes. These amino acids provide an initial step toward defining sequences that will allow us to predict which O-fucosylated EGF repeats are modified by the Fringes.     

Modulation of notch-ligand binding by protein O-fucosyltransferase 1 and fringe

Notch receptors are glycoproteins that mediate a wide range of developmental processes. Notch is modified in its epidermal growth factor-like domains by the addition of fucose to serine or threonine residues. O-Fucosylation is mediated by protein O-fucosyltransferase 1, and down-regulation of this enzyme by RNA interference or mutation of the Ofut1 gene in Drosophila or by mutation of the Pofut1 gene in mouse prevents Notch signaling. To investigate the molecular basis for the requirement for O-linked fucose on Notch, we assayed the ability of tagged, soluble forms of the Notch extracellular domain to bind to its ligands, Delta and Serrate. Down-regulation of OFUT1 by RNA interference in Notch-secreting cells inhibits both Delta-Notch and Serrate-Notch binding, demonstrating a requirement for O-linked fucose for efficient binding of Notch to its ligands. Conversely, overexpression of OFUT1 in cultured cells increases Serrate-Notch binding but inhibits Delta-Notch binding. These effects of OFUT1 are consistent with the consequences of OFUT1 overexpression on Notch signaling in vivo. Intriguingly, they are also opposite to, and are suppressed by, expression of the glycosyltransferase Fringe, which specifically modifies O-linked fucose. Thus, Notch-ligand interactions are dependent upon both the presence and the type of O-fucose glycans.     

Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1

Fringe O-fucose-beta1,3-N-acetylglucosaminyltransferases modulate Notch signaling by potentiating signaling induced by Delta-like ligands, while inhibiting signaling induced by Serrate/Jagged1 ligands. Based on binding studies, the differential effects of Drosophila fringe (DFng) on Notch signaling are thought to result from alterations in Notch glycosylation that enhance binding of Delta to Notch but reduce Serrate binding. Here, we report that expression of mammalian fringe proteins (Lunatic [LFng], Manic [MFng], or Radical [RFng] Fringe) increased Delta1 binding and activation of Notch1 signaling in 293T and NIH 3T3 cells. Although Jagged1-induced signaling was suppressed by LFng and MFng, RFng enhanced signaling induced by either Delta1 or Jagged1, underscoring the diversity of mammalian fringe glycosyltransferases in regulating signaling downstream of different ligand-receptor combinations. Interestingly, suppression of Jagged1-induced Notch1 signaling did not correlate with changes in Jagged1 binding as found for Delta1. Our data support the idea that fringe glycosylation increases Delta1 binding to potentiate signaling, but we propose that although fringe glycosylation does not reduce Jagged1 binding to Notch1, the resultant ligand-receptor interactions do not effectively promote Notch1 proteolysis required for activation of downstream signaling events.     

Notch activity in neural cells triggered by a mutant allele with altered glycosylation

The receptor protein Notch is inactive in neural precursor cells despite neighboring cells expressing ligands. We investigated specification of the R8 neural photoreceptor cells that initiate differentiation of each Drosophila ommatidium. The ligand Delta was required in R8 cells themselves, consistent with a lateral inhibitor function for Delta. By contrast, Delta expressed in cells adjacent to R8 could not activate Notch in R8 cells. The split mutation of Notch was found to activate signaling in R8 precursor cells, blocking differentiation and leading to altered development and neural cell death. split did not affect other, inductive functions of Notch. The Ile578-->Thr578 substitution responsible for the split mutation introduced a new site for O-fucosylation on EGF repeat 14 of the Notch extracellular domain. The O-fucose monosaccharide did not require extension by Fringe to confer the phenotype. Our results suggest functional differences between Notch in neural and non-neural cells. R8 precursor cells are protected from lateral inhibition by Delta. The protection is affected by modifications of a particular EGF repeat in the Notch extracellular domain. These results suggest that the pattern of neurogenesis is determined by blocking Notch signaling, as well as by activating Notch signaling.     

Galactose differentially modulates lunatic and manic fringe effects on Delta1-induced NOTCH signaling

NOTCH signaling induced by Delta1 (DLL1) and Jagged1 (JAG1) NOTCH ligands is modulated by the β3N-acetylglucosaminyl transferase Fringe. LFNG (Lunatic Fringe) and MFNG (Manic Fringe) transfer N-acetylglucosamine (GlcNAc) to O-fucose attached to EGF-like repeats of NOTCH receptors. In co-culture NOTCH signaling assays, LFNG generally enhances DLL1-induced, but inhibits JAG1-induced, NOTCH signaling. In mutant Chinese hamster ovary (CHO) cells that do not add galactose (Gal) to the GlcNAc transferred by Fringe, JAG1-induced NOTCH signaling is not inhibited by LFNG or MFNG. In mouse embryos lacking B4galt1, NOTCH signaling is subtly reduced during somitogenesis. Here we show that DLL1-induced NOTCH signaling in CHO cells was enhanced by LFNG, but this did not occur in either Lec8 or Lec20 CHO mutants lacking Gal on O-fucose glycans. Lec20 mutants corrected with a B4galt1 cDNA became responsive to LFNG. By contrast, MFNG promoted DLL1-induced NOTCH signaling better in the absence of Gal than in its presence. This effect was reversed in Lec8 cells corrected by expression of a UDP-Gal transporter cDNA. The MFNG effect was abolished by a DDD to DDA mutation that inactivates MFNG GlcNAc transferase activity. The binding of soluble NOTCH ligands and NOTCH1/EGF1-36 generally reflected changes in NOTCH signaling caused by LFNG and MFNG. Therefore, the presence of Gal on O-fucose glycans differentially affects DLL1-induced NOTCH signaling modulated by LFNG versus MFNG. Gal enhances the effect of LFNG but inhibits the effect of MFNG on DLL1-induced NOTCH signaling, with functional consequences for regulating the strength of NOTCH signaling.     

Protein O-fucosyltransferase 2 adds O-fucose to thrombospondin type 1 repeats

O-Fucose is an unusual form of glycosylation found on epidermal growth factor-like (EGF) repeats and thrombospondin type 1 repeats (TSRs) in many secreted and transmembrane proteins. Recently O-fucose on EGF repeats was shown to play important roles in Notch signaling. In contrast, physiological roles for O-fucose on TSRs are unknown. In the accompanying paper (Luo, Y., Nita-Lazar, A., and Haltiwanger, R. S. (2006) J. Biol. Chem. 281, 9385-9392), we demonstrated that an enzyme distinct from protein O-fucosyltransferase 1 adds O-fucose to TSRs. A known homologue of O-fucosyltransferase 1 is putative protein O-fucosyltransferase 2. The cDNA sequence encoding O-fucosyltransferase 2 was originally identified during a data base search for fucosyltransferases in Drosophila. Like O-fucosyltransferase 1, O-fucosyltransferase 2 is conserved from Caenorhabditis elegans to humans. Although O-fucosyltransferase 2 was assumed to be another protein O-fucosyltransferase, no biochemical characterization existed supporting this contention. Here we show that RNAi-mediated reduction of the O-fucosyltransferase 2 message significantly decreased TSR-specific O-fucosyltransferase activity in Drosophila S2 cells. We also found that O-fucosyltransferase 2 is predominantly localized in the endoplasmic reticulum compartment of these cells. Furthermore, we expressed recombinant Drosophila O-fucosyltransferase 2 and showed that it O-fucosylates TSRs but not EGF repeats in vitro. These results demonstrate that O-fucosyltransferase 2 is in fact a TSR-specific O-fucosyltransferase.     

O-linked N-acetylglucosamine is present on the extracellular domain of notch receptors

Rare types of glycosylation often occur in a domain-specific manner and are involved in specific biological processes. In particular, O-fucose glycans are reported to regulate the functions of EGF domain-containing proteins such as Notch receptors. In the course of mass spectrometric analysis of O-glycans displayed on Drosophila Notch receptors expressed in S2 cells, we found an unusual O-linked N-acetylhexosamine (HexNAc) modification which occurs at a site distinct from those of O-fucose and O-glucose glycosylations. Modification site mapping by mass spectrometry and amino acid substitution studies revealed that O-HexNAc modification occurs on a serine or threonine located between the fifth and sixth cysteines within the EGF domain. This modification occurs simultaneously along with other closely positioned O-glycosylations. This modification was determined to be O-beta-GlcNAc by galactosyltransferase labeling and beta-N-acetyl-hexosaminidase digestion experiments and by immunoblotting with a specific antibody. O-GlcNAc modification occurs at multiple sites on Notch epidermal growth factor repeats. O-GlcNAc modification was also found on the extracellular domain of Delta, a ligand for Notch receptors. Although the O-GlcNAc modification is known to regulate a wide range of cellular processes, the list of known modified proteins has previously been limited to intracellular proteins in animals. Thus, the finding of O-GlcNAc modification in extracellular environments predicts a distinct glycosylation process that might be associated with a novel regulatory mechanism for Notch receptor activity.     

O-glucose trisaccharide is present at high but variable stoichiometry at multiple sites on mouse Notch1

Notch activity is regulated by both O-fucosylation and O-glucosylation, and Notch receptors contain multiple predicted sites for both. Here we examine the occupancy of the predicted O-glucose sites on mouse Notch1 (mN1) using the consensus sequence C(1)XSXPC(2). We show that all of the predicted sites are modified, although the efficiency of modifying O-glucose sites is site- and cell type-dependent. For instance, although most sites are modified at high stoichiometries, the site at EGF 27 is only partially glucosylated, and the occupancy of the site at EGF 4 varies with cell type. O-Glucose is also found at a novel, non-traditional consensus site at EGF 9. Based on this finding, we propose a revision of the consensus sequence for O-glucosylation to allow alanine N-terminal to cysteine 2: C(1)XSX(A/P)C(2). We also show through biochemical and mass spectral analyses that serine is the only hydroxyamino acid that is modified with O-glucose on EGF repeats. The O-glucose at all sites is efficiently elongated to the trisaccharide Xyl-Xyl-Glc. To establish the functional importance of individual O-glucose sites in mN1, we used a cell-based signaling assay. Elimination of most individual sites shows little or no effect on mN1 activation, suggesting that the major effects of O-glucose are mediated by modification of multiple sites. Interestingly, elimination of the site in EGF 28, found in the Abruptex region of Notch, does significantly reduce activity. These results demonstrate that, like O-fucose, the O-glucose modifications of EGF repeats occur extensively on mN1, and they play important roles in Notch function.