The O-fucose glycan in the ligand-binding domain of Notch1 regulates embryogenesis and T cell development

Mechanistic insights into Notch receptor signaling from structural and biochemical studies

Notch proteins are the receptors in a highly conserved signal transduction system used to communicate signals between cells that contact each other. Studies investigating structure-function relationships in Notch signaling have gained substantial momentum in recent years. Here, we summarize the current understanding of the molecular logic of Notch signal transduction, emphasizing structural and biochemical studies of Notch receptors, their ligands, and complexes of intracellular Notch proteins with their target transcription factors. Recent advances in the structure-based modulation of Notch-signaling activity are also discussed.

Notch1 signaling plays a role in regulating precursor differentiation during CNS remyelination

In the developing CNS, Notch1 and its ligand, Jagged1, regulate oligodendrocyte differentiation and myelin formation, but their role in repair of demyelinating lesions in diseases such as multiple sclerosis remains unresolved. To address this question, we generated a mouse model in which we targeted Notch1 inactivation to oligodendrocyte progenitor cells (OPCs) using Olig1Cre and a floxed Notch1 allele, Notch1(12f). During CNS development, OPC differentiation was potentiated in Olig1Cre:Notch1(12f/12f) mice. Importantly, in adults, remyelination of demyelinating lesions was also accelerated, at the expense of proliferation within the progenitor population. Experiments in vitro confirmed that Notch1 signaling was permissive for OPC expansion but inhibited differentiation and myelin formation. These studies also revealed that astrocytes exposed to TGF-beta1 restricted OPC maturation via Jagged1-Notch1 signaling. These data suggest that Notch1 signaling is one of the mechanisms regulating OPC differentiation during CNS remyelination. Thus, Notch1 may represent a potential therapeutical avenue for lesion repair in demyelinating disease.

Regulation of Notch signaling during T- and B-cell development by O-fucose glycans

Notch signaling is required for the development of all T cells and marginal zone (MZ) B cells. Specific roles in T- and B-cell differentiation have been identified for different Notch receptors, the canonical Delta-like (Dll) and Jagged (Jag) Notch ligands, and downstream effectors of Notch signaling. Notch receptors and ligands are post-translationally modified by the addition of glycans to extracellular domain epidermal growth factor-like (EGF) repeats. The O-fucose glycans of Notch cell-autonomously modulate Notch-ligand interactions and the strength of Notch signaling. These glycans are initiated by protein O-fucosyltransferase 1 (Pofut1), and elongated by the transfer of N-acetylglucosamine (GlcNAc) to the fucose by beta1,3GlcNAc-transferases termed lunatic, manic, or radical fringe. This review discusses T- and B-cell development from progenitors deficient in O-fucose glycans. The combined data show that Lfng and Mfng regulate T-cell development by enhancing the interactions of Notch1 in T-cell progenitors with Dll4 on thymic epithelial cells. In the spleen, Lfng and Mfng cooperate to modify Notch2 in MZ B progenitors, enhancing their interaction with Dll1 on endothelial cells and regulating MZ B-cell production. Removal of O-fucose affects Notch signaling in myelopoiesis and lymphopoiesis, and the O-fucose glycan in the Notch1 ligand-binding domain is required for optimal T-cell development.

Ion mobility-mass spectrometry analysis of isomeric carbohydrate precursor ions

The rapid separation of isomeric precursor ions of oligosaccharides prior to their analysis by mass spectrometry to the nth power (MS(n)) was demonstrated using an ambient pressure ion mobility spectrometer (IMS) interfaced with a quadrupole ion trap. Separations were not limited to specific types of isomers; representative isomers differing solely in the stereochemistry of sugars, in their anomeric configurations, and in their overall branching patterns and linkage positions could be resolved in the millisecond time frame. Physical separation of precursor ions permitted independent mass spectra of individual oligosaccharide isomers to be acquired to at least MS(3), the number of stages of dissociation limited only practically by the abundance of specific product ions. IMS-MS(n) analysis was particularly valuable in the evaluation of isomeric oligosaccharides that yielded identical sets of product ions in tandem mass spectrometry experiments, revealing pairs of isomers that would otherwise not be known to be present in a mixture if evaluated solely by MS dissociation methods alone. A practical example of IMS-MS(n) analysis of a set of isomers included within a single high-performance liquid chromatography fraction of oligosaccharides released from bovine submaxillary mucin is described.

Notch signaling: the core pathway and its posttranslational regulation

Notch signaling controls numerous cell-fate specification events in multicellular organisms, and dysregulated Notch signaling causes several diseases with underlying developmental defects. A key step in Notch receptor activation is its intramembrane proteolysis, which releases an intracellular fragment that participates directly in transcriptional regulation of nuclear target genes. Despite the apparent simplicity of this mechanism, a host of posttranslational processes regulate Notch activity during its synthesis and secretion, ligand-dependent activation at the surface, endocytic trafficking, and degradation. This review describes the core developmental logic of Notch signaling and how regulatory mechanisms tailor Notch pathway outputs to specific developmental scenarios.

The canonical Notch signaling pathway: unfolding the activation mechanism

Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, the misregulation or loss of Notch signaling underlies a wide range of human disorders, from developmental syndromes to adult-onset diseases and cancer. Notch signaling is remarkably robust in most tissues even though each Notch molecule is irreversibly activated by proteolysis and signals only once without amplification by secondary messenger cascades. In this Review, we highlight recent studies in Notch signaling that reveal new molecular details about the regulation of ligand-mediated receptor activation, receptor proteolysis, and target selection.

A Notch updated

Cell–cell signaling mediated by the Notch receptor is iteratively involved in numerous developmental contexts, and its dysregulation has been associated with inherited genetic disorders and cancers. The core components of the signaling pathway have been identified for some time, but the study of the modulation of the pathway in different cellular contexts has revealed many layers of regulation. These include complex sugar modifications in the extracellular domain as well as transit of Notch through defined cellular compartments, including specific endosomes.

Notch signalling in the paraxial mesoderm is most sensitive to reduced Pofut1 levels during early mouse development

Background

The evolutionarily conserved Notch signalling pathway regulates multiple developmental processes in a wide variety of organisms. One critical posttranslational modification of Notch for its function in vivo is the addition of O-linked fucose residues by protein O-fucosyltransferase 1 (POFUT1). In addition, POFUT1 acts as a chaperone and is required for Notch trafficking. Mouse embryos lacking POFUT1 function die with a phenotype indicative of global inactivation of Notch signalling. O-linked fucose residues on Notch can serve as substrates for further sugar modification by Fringe (FNG) proteins. Notch modification by Fringe differently affects the ability of ligands to activate Notch receptors in a context-dependent manner indicating a complex modulation of Notch activity by differential glycosylation. Whether the context-dependent effects of Notch receptor glycosylation by FNG reflect different requirements of distinct developmental processes for O-fucosylation by POFUT1 is unclear.

Results

We have identified and characterized a spontaneous mutation in the mouse Pofut1 gene, referred to as "compact axial skeleton" (cax). Cax carries an insertion of an intracisternal A particle retrotransposon into the fourth intron of the Pofut1 gene and represents a hypomorphic Pofut1 allele that reduces transcription and leads to reduced Notch signalling. Cax mutant embryos have somites of variable size, showed partly abnormal Lfng expression and, consistently defective anterior-posterior somite patterning and axial skeleton development but had virtually no defects in several other Notch-regulated early developmental processes outside the paraxial mesoderm that we analyzed.

Conclusion

Notch-dependent processes apparently differ with respect to their requirement for levels of POFUT1. Normal Lfng expression and anterior-posterior somite patterning is highly sensitive to reduced POFUT1 levels in early mammalian embryos, whereas other early Notch-dependent processes such as establishment of left-right asymmetry or neurogenesis are not. Thus, it appears that in the presomitic mesoderm (PSM) Notch signalling is particularly sensitive to POFUT1 levels. Reduced POFUT1 levels might affect Notch trafficking or overall O-fucosylation. Alternatively, reduced O-fucosylation might preferentially affect sites that are substrates for LFNG and thus important for somite formation and patterning.

Role of unusual O-glycans in intercellular signaling

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucin-type O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the epidermal growth factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.

Characterizing the glycome of the mammalian immune system

The outermost layer of all immune cells, the glycocalyx, is composed of a complex mixture of glycoproteins, glycolipids and lectins, which specifically recognize particular glycan epitopes. As the glycocalyx is the cell's primary interface with the external environment many biologically significant events can be attributed to glycan recognition. For this reason the rapidly expanding glycomics field is being increasingly recognized as an important component in our quest to better understand the functioning of the immune system. In this review, we highlight the current status of immune cell glycomics, with particular attention being paid to T- and B-lymphocytes and dendritic cells. We also describe the strategies and methodologies used to define immune cell glycomes.

Notch and presenilin regulate cellular expansion and cytokine secretion but cannot instruct Th1/Th2 fate acquisition

Recent reports suggested that Delta1, 4 and Jagged1, 2 possessed the ability to instruct CD4⁺ T cell into selection of Th1 or Th2 fates, respectively, although the underlying mechanism endowing the cleaved Notch receptor with memory of ligand involved in its activation remains elusive. To examine this, we prepared artificial antigen-presenting cells expressing either DLL1 or Jag1. Although both ligands were efficient in inducing Notch2 cleavage and activation in CD4⁺ T or reporter cells, the presence of Lunatic Fringe in CD4⁺ T cells inhibited Jag1 activation of Notch1 receptor. Neither ligand could induce Th1 or Th2 fate choice independently of cytokines or redirect cytokine-driven Th1 or Th2 development. Instead, we find that Notch ligands only augment cytokine production during T cell differentiation in the presence of polarizing IL-12 and IL-4. Moreover, the differentiation choices of naïve CD4⁺ T cells lacking γ-secretase, RBP-J, or both in response to polarizing cytokines revealed that neither presenilin proteins nor RBP-J were required for cytokine-induced Th1/Th2 fate selection. However, presenilins facilitate cellular proliferation and cytokine secretion in an RBP-J (and thus, Notch) independent manner. The controversies surrounding the role of Notch and presenilins in Th1/Th2 polarization may reflect their role as genetic modifiers of T-helper cells differentiation.

Roles of Pofut1 and O-fucose in mammalian Notch signaling

Mammalian Notch receptors contain 29-36 epidermal growth factor (EGF)-like repeats that may be modified by protein O-fucosyltransferase 1 (Pofut1), an essential component of the canonical Notch signaling pathway. The Drosophila orthologue Ofut1 is proposed to function as both a chaperone required for stable cell surface expression of Notch and a protein O-fucosyltransferase. Here we investigate these dual roles of Pofut1 in relation to endogenous Notch receptors of Chinese hamster ovary and murine embryonic stem (ES) cells. We show that fucosylation-deficient Lec13 Chinese hamster ovary cells have wild type levels of Pofut1 and cell surface Notch receptors. Nevertheless, they have reduced binding of Notch ligands and low levels of Delta1- and Jagged1-induced Notch signaling. Exogenous fucose but not galactose rescues both ligand binding and Notch signaling. Murine ES cells lacking Pofut1 also have wild type levels of cell surface Notch receptors. However, Pofut1-/- ES cells do not bind Notch ligands or exhibit Notch signaling. Although overexpression of fucosyltransferase-defective Pofut1 R245A in Pofut1-/- cells partially rescues ligand binding and Notch signaling, this effect is not specific. The same rescue is achieved by an unrelated, inactive, endoplasmic reticulum glucosidase. Therefore, mammalian Notch receptors require Pofut1 for the generation of optimally functional Notch receptors, but, in contrast to Drosophila, Pofut1 is not required for stable cell surface expression of Notch. Importantly, we also show that, under certain circumstances, mammalian Notch receptors are capable of signaling in the absence of Pofut1 and O-fucose.

In vivo consequences of deleting EGF repeats 8-12 including the ligand binding domain of mouse Notch1

BACKGROUND

Notch signaling is highly conserved in the metazoa and is critical for many cell fate decisions. Notch activation occurs following ligand binding to Notch extracellular domain. In vitro binding assays have identified epidermal growth factor (EGF) repeats 11 and 12 as the ligand binding domain of Drosophila Notch. Here we show that an internal deletion in mouse Notch1 of EGF repeats 8-12, including the putative ligand binding domain (lbd), is an inactivating mutation in vivo. We also show that maternal and zygotic Notch1(lbd/lbd) mutant embryos develop through gastrulation to mid-gestation.

RESULTS

Notch1(lbd/lbd) embryos died at mid-gestation with a phenotype indistinguishable from Notch1 null mutants. In embryonic stem (ES) cells, Notch1(lbd) was expressed on the cell surface at levels equivalent to wild type Notch1, but Delta1 binding was reduced to the same level as in Notch1 null cells. In an ES cell co-culture assay, Notch signaling induced by Jagged1 or Delta1 was reduced to a similar level in Notch1(lbd) and Notch1 null cells. However, the Notch1(lbd/lbd) allele was expressed similarly to wild type Notch1 in Notch1(lbd/lbd) ES cells and embryos at E8.75, indicating that Notch1 signaling is not essential for the Notch1 gene to be expressed. In addition, maternal and zygotic Notch1 mutant blastocysts developed through gastrulation.

CONCLUSION

Mouse Notch1 lacking the ligand binding domain is expressed at the cell surface but does not signal in response to the canonical Notch ligands Delta1 and Jagged1. Homozygous Notch1(lbd/lbd) mutant embryos die at approximately E10 similar to Notch1 null embryos. While Notch1 is expressed in oocytes and blastocysts, Notch1 signaling via canonical ligands is dispensable during oogenesis, blastogenesis, implantation and gastrulation.

Functions of O-fucosyltransferase in Notch trafficking and signaling: towards the end of a controversy?

The precise role of the O-fucosyltransferase Ofut1 in Notch-receptor trafficking has remained controversial. A recent study sheds new light on the non-catalytic activity of Ofut1 and provides further evidence that Ofut1 acts as a chaperone in the endoplasmic reticulum.