Notch signaling is an evolutionarily conserved mechanism that determines cell fate in a variety of contexts during development. This is achieved through different modes of action that are context dependent. One mode involves boundary formation between two groups of cells. With this mode of action, Notch signaling is central to vertebrate evolution as it drives the segmentation of paraxial mesoderm in the formation of somites, which are the precursors of the vertebra. In this case, boundary formation facilitates a mesenchymal to epithelial transition, leading to the creation of a somite. In addition, the boundary establishes a signaling centre that patterns the somite, a feature that directly impacts on vertebral column formation. Studies in Xenopus, zebrafish, chicken and mouse have established the importance of Notch signaling in somitogenesis, and indeed in mouse how perturbations in somitogenesis affect vertebral column formation. Spondylocostal dysostosis is a congenital disorder characterized by formation of abnormal vertebrae. Here, mutation in Notch pathway genes demonstrates that Notch signaling is also required for normal somite formation and vertebral column development in humans; of particular interest here is mutation of the LUNATIC FRINGE gene, which causes SCD type 3. LUNATIC FRINGE encodes for a fucose-specific β1,3-N-acetylglucosaminyltransferase, which modifies Notch receptors and alters Notch signaling activity. This review will focus on Notch glycolsylation, and the role of LUNATIC FRINGE in somite formation and vertebral column development in mice and humans.

Introduction

Abnormal vertebral segmentation (AVS) is a common congenital abnormality that affects vertebra singly or in multiples. AVS results in a variety of morphologies that affect vertebral size, shape and integrity; these ultimately impact upon the shape and flexure of the spine. AVS may represent an isolated finding or, in the case of multiple AVS is commonly associated with a range of dysmorphic features. In only a small proportion of AVS cases has the genetic component been identified (reviewed in [START_REF] Turnpenny | Abnormal vertebral segmentation and the notch signaling pathway in man[END_REF]). Spondylocostal dysostosis (SCD) represent one of these cases, with mutation in genes of the Notch signalling pathway, segregating with this disorder with a recessive mode of inheritance. Loss of function mutations in genes of the Notch signalling pathway in mouse have demonstrated the essential role that Notch signalling plays in the formation of the vertebral column and in its precursor, the somites. Somite formation occurs in a reiterative manner during embryonic development, and studies in a number of vertebrate species have lead to the elucidation of numerous components of the Notch signalling pathway that function in somite formation (reviewed in [START_REF] Dequéant | Segmental patterning of the vertebrate embryonic axis[END_REF]). Notch signalling is instrumental in two aspects of somitogenesis; segmentation of discrete somites from unsegmented mesenchyme and rostrocaudal patterning of the somite. Disruption of either of these processes has a profound impact on the vertebral column (reviewed in [START_REF] Saga | The making of the somite: molecular events in vertebrate segmentation[END_REF]).

Notch signaling and pathway components

Mechanisms of action

The Notch signaling pathway instructs developmental processes using different mechanisms of action described as lateral inhibition/specification, inductive signaling and boundary formation. Lateral inhibition/specification occurs between largely "equivalent" groups of cells that express both Notch receptor and ligand. Notch signaling amplifies small differences between these cells until distinct biochemical events occur that dictate cell fate, and results in the generation of cells of different types. Lateral inhibition/specification is instrumental in neurogenesis in vertebrates and invertebrates

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(reviewed in [START_REF] Artavanis-Tsakonas | Notch signaling: cell fate control and signal integration in development[END_REF][START_REF] Greenwald | LIN-12/Notch signaling: lessons from worms and flies[END_REF][START_REF] Louvi | Notch signalling in vertebrate neural development[END_REF]). Inductive signaling occurs between non-equivalent cells and despite expression of both ligand and receptors in both populations, signaling occurs directionally to alter cell fate. Lymphopoiesis is perhaps the best-characterized example in vertebrates.

Here, despite the fact that Notch receptors and ligands are expressed in both bone marrow progenitors and the thymic stroma, Notch1 signaling occurs in bone marrow progenitors and is required for their development into T cells [START_REF] Han | Inducible gene knockout of transcription factor recombination signal binding protein-J reveals its essential role in T versus B lineage decision[END_REF][START_REF] Wilson | Notch 1-deficient common lymphoid precursors adopt a B cell fate in the thymus[END_REF] (reviewed in [START_REF] Radtke | Notch regulation of lymphocyte development and function[END_REF]). The role of Notch signaling in boundary formation is the most relevant to this review; here, Notch facilitates the formation of borders between distinct cellular fields. Examples of this exist in disparate developmental contexts; establishment of the dorsoventral wing boundary and the equator in the eye (Drosophila), anteroposterior border formation of the boundary between the dorsal and ventral thalamus (chicken), in rhombomeric boundary function of the hindbrain (zebrafish), and in segmentation of developing somites (Xenopus, snake, zebrafish, chicken, mouse [START_REF] Gomez | Control of segment number in vertebrate embryos[END_REF][START_REF] Cheng | Notch activation regulates the segregation and differentiation of rhombomere boundary cells in the zebrafish hindbrain[END_REF][START_REF] Zeltser | A new developmental compartment in the forebrain regulated by Lunatic fringe[END_REF] (reviewed in [START_REF] Irvine | Fringe, Notch, and making developmental boundaries[END_REF][START_REF] Weinmaster | Modulation of Notch Signaling during Somitogenesis[END_REF]). Somite formation, like embryonic development in general, is reliant on the activity of a number of key signaling mechanisms to drive fundamental processes such as cell proliferation, apoptosis, fate decisions, differentiation and migration. The complexity of the task is contingent on the integration of these processes and the signaling pathways that drive them. The Notch signaling pathway is known to integrate with fibroblast growth factor (FGF), hedgehog (Hh), janus kinase/signal transducers and activators of transcription (Jak/STAT), receptor tyrosine kinase (RTK), transforming growth factor-β (TGF-β), Wnt, and hypoxia pathways [START_REF] Gustafsson | Hypoxia requires notch signaling to maintain the undifferentiated cell state[END_REF] (reviewed in [START_REF] Dequéant | Segmental patterning of the vertebrate embryonic axis[END_REF][START_REF] Hurlbut | Crossing paths with Notch in the hyper-network[END_REF]). The degree of Integration between these signaling pathways and Notch has not been fully realized and the subject of future research will be to unravel these interdependencies.

Notch receptor and signal transduction

The Notch signaling pathway, first identified in Drosophila, is largely conserved through evolution from invertebrates to vertebrates [START_REF] Artavanis-Tsakonas | Notch signaling[END_REF][START_REF] Bray | Notch signalling: a simple pathway becomes complex[END_REF]. Notch is a type I transmembrane receptor that exists on the cell surface as a heterodimer [START_REF] Blaumueller | Intracellular cleavage of Notch leads to a heterodimeric receptor on the plasma membrane[END_REF][START_REF] Logeat | The Notch1 receptor is cleaved constitutively by a furin-like convertase[END_REF]. The receptor is activated upon binding delta or serrate, also type I transmembrane proteins, resident on adjacent cells. This activation of the signaling pathway in trans results in the signal-receiving cell often adopting a different fate to that of the signal-sending cell. In mammals, there are four Notch receptors (Notch1-4) (Fig. 1). The extracellular domains of Notch contain between 29 and 36 EGF-like repeats, associated with ligand binding, and three Lin-12/Notch (LIN) repeats that prevent ligand-independent signaling. The intracellular domain contains two protein interaction domains: the RBPj associated molecule (RAM) domain binds RBPj/CSL, and six ankyrin (ANK) repeats moderate protein interactions. Nuclear localization signals (NLS), a transactivation domain (TAD), and a PEST sequence, are also present to various extents.

The Notch signaling pathway is deceptively simple, it lacks second messengers as the intracellular domain (ICD) of the Notch receptor participates directly in target gene transcription (Fig. 2). However it is becoming increasingly clear that signaling specificity brings complexity and this is achieved through the action of Notch signaling modifiers (reviewed in [START_REF] Bray | Notch signalling: a simple pathway becomes complex[END_REF]). Notch is synthesized as a single polypeptide. Specific epidermal growth factor (EGF)-like repeats are modified by O-fucosylation in the endoplasmic reticulum (ER) by protein O-fucosyltransferase 1 (Pofut1). As Notch traffics through the Golgi, it may be further modified by elongation of the O-linked fucose with an N-acetylglucosamine (GlcNAc); this is catalysed by a fucose-specific β1,3-N-acetylglucosaminyltransferase (β1,3-GlcNAc-transferase; fringe) [START_REF] Fleming | Serrate-mediated activation of Notch is specifically blocked by the product of the gene fringe in the dorsal compartment of the Drosophila wing imaginal disc[END_REF][START_REF] Panin | Fringe modulates Notchligand interactions[END_REF]. In the trans-Golgi network, Notch is cleaved (S1 cleavage) by a furin-like convertase [START_REF] Blaumueller | Intracellular cleavage of Notch leads to a heterodimeric receptor on the plasma membrane[END_REF][START_REF] Logeat | The Notch1 receptor is cleaved constitutively by a furin-like convertase[END_REF]. The N-terminal extracellular truncation (ECN) and C-terminal transmembrane and intracellular domain (TMIC) fragments form a heterodimer through non-covalent interactions and traffic to the cell surface [START_REF] Rand | Calcium depletion dissociates and activates heterodimeric notch receptors[END_REF][START_REF] Sanchez-Irizarry | Notch subunit heterodimerization and prevention of ligand-independent proteolytic activation depend, respectively, on a novel domain and the LNR repeats[END_REF]. Here, the ECN is available to interact with ligand in trans. Ligand binding results in proteolytic

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ACCEPTED MANUSCRIPT

cleavage (S2 cleavage) by disintegrin and metalloprotease domain (ADAM) proteases ADAM10 and ADAM17, and in the separation of the Notch heterodimer through transendocytosis of the ECN into the signal-sending cell [START_REF] Brou | A novel proteolytic cleavage involved in Notch signaling: the role of the disintegrin-metalloprotease TACE[END_REF][START_REF] Mumm | A ligand-induced extracellular cleavage regulates gamma-secretase-like proteolytic activation of Notch1[END_REF][START_REF] Parks | Ligand endocytosis drives receptor dissociation and activation in the Notch pathway[END_REF][START_REF] Nichols | DSL ligand endocytosis physically dissociates Notch1 heterodimers before activating proteolysis can occur[END_REF][START_REF] Pan | Kuzbanian controls proteolytic processing of Notch and mediates lateral inhibition during Drosophila and vertebrate neurogenesis[END_REF][START_REF] Hartmann | The disintegrin/metalloprotease ADAM 10 is essential for Notch signalling but not for alphasecretase activity in fibroblasts[END_REF]. This produces a transmembrane bound fragment with the extracellular truncation (EXT), which is subsequently cleaved (S3 cleavage) by the γ-secretase complex [START_REF] Okochi | Presenilins mediate a dual intramembranous gammasecretase cleavage of Notch-1[END_REF][START_REF] Schroeter | Notch-1 signalling requires ligandinduced proteolytic release of intracellular domain[END_REF][START_REF] Strooper | A presenilin-1dependent gamma-secretase-like protease mediates release of Notch intracellular domain[END_REF] resulting in the release of the Notch intracellular domain (ICD). Notch ICD translocates to the nucleus where it forms a complex with the DNA-binding protein Rbpj/CSL (CBF1-Suppressor of Hairless-Lag1) [START_REF] Hsieh | Truncated mammalian Notch1 activates CBF1/RBPJk-repressed genes by a mechanism resembling that of Epstein-Barr virus EBNA2[END_REF][START_REF] Jarriault | Signalling downstream of activated mammalian Notch[END_REF]. The interaction between ICD and CSL leads to displacement of transcriptional co-repressors previously complexed with CSL [START_REF] Hsieh | Truncated mammalian Notch1 activates CBF1/RBPJk-repressed genes by a mechanism resembling that of Epstein-Barr virus EBNA2[END_REF][START_REF] Kao | A histone deacetylase corepressor complex regulates the Notch signal transduction pathway[END_REF][START_REF] Oswald | p300 acts as a transcriptional coactivator for mammalian Notch-1[END_REF][START_REF] Taniguchi | LIM protein KyoT2 negatively regulates transcription by association with the RBP-J DNA-binding protein[END_REF][START_REF] Zhou | SKIP, a CBF1-associated protein, interacts with the ankyrin repeat domain of NotchIC To facilitate[END_REF]. Mastermind-like (MAML) is recruited to the Notch ICD-CSL complex ( [START_REF] Nam | Structural basis for cooperativity in recruitment of MAML coactivators to Notch transcription complexes[END_REF][START_REF] Wilson | Crystal structure of the CSL-Notch-Mastermind ternary complex bound to DNA[END_REF], engages proteins required for transcriptional activation such as the histone acetyltransferase p300, and activates Notch target genes [START_REF] Oswald | p300 acts as a transcriptional coactivator for mammalian Notch-1[END_REF][START_REF] Fryer | Mastermind mediates chromatin-specific transcription and turnover of the Notch enhancer complex[END_REF][START_REF] Wallberg | p300 and PCAF act cooperatively to mediate transcriptional activation from chromatin templates by notch intracellular domains in vitro[END_REF]. MAML also recruits CycC:CDK8 (cyclinC: cyclin dependent kinase8) to this binary complex, which leads to Notch ICD phosphorylation and degradation ( [START_REF] Fryer | Mastermind mediates chromatin-specific transcription and turnover of the Notch enhancer complex[END_REF][START_REF] Fryer | Mastermind recruits CycC:CDK8 to phosphorylate the Notch ICD and coordinate activation with turnover[END_REF]. Known direct targets of Notch include members of the hairy/enhancer-of-split (HES), HES-related (HERP) and MESP families of bHLH transcription factors, Lfng, Cyclin D1, p21 WAF1 , Nodal, PTEN and GFAP [START_REF] Cole | Clock regulatory elements control cyclic expression of Lunatic fringe during somitogenesis[END_REF][START_REF] Ge | Notch signaling promotes astrogliogenesis via direct CSL-mediated glial gene activation[END_REF][START_REF] Iso | HES and HERP families: multiple effectors of the Notch signaling pathway[END_REF][START_REF] Krebs | Characterization of Notch3-deficient mice: normal embryonic development and absence of genetic interactions with a Notch1 mutation[END_REF][START_REF] Morales | Periodic Lunatic fringe expression is controlled during segmentation by a cyclic transcriptional enhancer responsive to notch signaling[END_REF][START_REF] Rangarajan | Notch signaling is a direct determinant of keratinocyte growth arrest and entry into differentiation[END_REF][START_REF] Ronchini | Induction of cyclin D1 transcription and CDK2 activity by Notch(ic): implication for cell cycle disruption in transformation by Notch(ic)[END_REF][START_REF] Terasaki | Transgenic analysis of the medaka mesp-b enhancer in somitogenesis[END_REF][START_REF] Whelan | CBF-1 (RBP-Jkappa) binds to the PTEN promoter and regulates PTEN gene expression[END_REF][START_REF] Yasuhiko | Tbx6mediated Notch signaling controls somite-specific Mesp2 expression[END_REF].

DSL ligands trans-activate and cis-inhibit Notch signalling

Notch receptors bind DSL (Delta/Serrate/Lag2) ligands. In mammals there are five DSL ligands that can be grouped into two classes (Delta-like and Serrate-like) based on homology to Drosophila Delta or Serrate. Dll1, Dll3 and Dll4 are Delta-like while Jagged1 and Jagged2 are Serrate-like ligands (Fig 1). The extracellular domain is characterized by a signal peptide, an amino-terminal domain (with limited homology between different DSL ligands), a DSL domain and a series of EGF-like repeats [START_REF] Nye | Vertebrate ligands for Notch[END_REF]. C-terminal to the transmembrane domain is an intracellular region lacking any recognizable motifs except for a PDZ ligand-binding domain present in Dll1, Dll4 and Jagged1. The DSL domain is related to an EGF-like repeat and is required for receptor binding in trans. Relative affinities of DSL ligands binding in trans with Notch receptors have not been exhaustively determined; however, it is known that Dll1 interacts with Notch1 and Notch2, Jagged1 with Notch1-3, and Jagged2 with Notch2 [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF][START_REF] Sakamoto | Intracellular cellautonomous association of Notch and its ligands: a novel mechanism of Notch signal modification[END_REF][START_REF] Shimizu | Binding of Delta1, Jagged1, and Jagged2 to Notch2 rapidly induces cleavage, nuclear translocation, and hyperphosphorylation of Notch2[END_REF][START_REF] Shimizu | Mouse jagged1 physically interacts with notch2 and other notch receptors. Assessment by quantitative methods[END_REF][START_REF] Shimizu | Physical interaction of Delta1, Jagged1, and Jagged2 with Notch1 and Notch3 receptors[END_REF]. Despite the absence of recognizable motifs in the intracellular domain of DSL ligands, this region is required to activate Notch in trans. It facilitates endocytosis of the ligand in association with the ECN part of Notch, into the ligand-expressing cell [START_REF] Parks | Ligand endocytosis drives receptor dissociation and activation in the Notch pathway[END_REF][START_REF] Nichols | DSL ligand endocytosis physically dissociates Notch1 heterodimers before activating proteolysis can occur[END_REF][START_REF] Klueg | Ligand-receptor interactions and trans-endocytosis of Delta, Serrate and Notch: members of the Notch signalling pathway in Drosophila[END_REF][START_REF] Muskavitch | Delta-notch signaling and Drosophila cell fate choice[END_REF]. This endocytosis requires the intracellular region of the DSL ligand to be ubiquitylated; this is achieved through the action of two E3-ubiquitin ligases, Mindbomb and Neuralized [START_REF] Deblandre | Xenopus neuralized is a ubiquitin ligase that interacts with XDelta1 and regulates Notch signaling[END_REF][START_REF] Itoh | Mind bomb is a ubiquitin ligase that is essential for efficient activation of Notch signaling by Delta[END_REF][START_REF] Lai | Drosophila neuralized is a ubiquitin ligase that promotes the internalization and degradation of delta[END_REF].

In addition to DSL ligands trans-activating Notch, they can also inhibit signalling when expressed in the same cell (in cis) as Notch [START_REF] De Celis | The Abruptex domain of Notch regulates negative interactions between Notch, its ligands and Fringe[END_REF][START_REF] Henrique | Maintenance of neuroepithelial progenitor cells by Delta-Notch signalling in the embryonic chick retina[END_REF]. When coexpressed, Notch1 interacts with Dll1 or Jagged1 not on the cell surface but inside the cell [START_REF] Sakamoto | Intracellular cellautonomous association of Notch and its ligands: a novel mechanism of Notch signal modification[END_REF]. The mechanism through which this occurs is not known, however co-expression of Lfng (β1,3 N-acetlyglucosyltransferase) inhibits this cell autonomous interaction [START_REF] Sakamoto | Intracellular cellautonomous association of Notch and its ligands: a novel mechanism of Notch signal modification[END_REF]. Recently it has been shown that the DSL ligand Dll3 is distinct; it does not trans-activate Notch but rather interacts with Notch only in cis to inhibit signalling, and it is predominantly expressed in the cis-Golgi [START_REF] Geffers | Divergent functions and distinct localization of the Notch ligands DLL1 and DLL3 in vivo[END_REF][START_REF] Ladi | The divergent DSL ligand Dll3 does not activate Notch signaling but cell autonomously attenuates signaling induced by other DSL ligands[END_REF].

Notch and DSL ligands are modified by glycosylation

O-glycosylation of Notch

The N-terminal region of Notch receptors contain tandem EGF-like repeats that are small protein domains of 40 amino acids with six conserved cysteines that form three conserved disulfide bonds [START_REF] Campbell | Epidermal growth factor-like modules[END_REF]. EGF-like repeats are of two types: human EGF-like (hEGF) and complement Clr-like (cEGF) [START_REF] Wouters | Evolution of distinct EGF domains with specific functions[END_REF]. The EGF-like repeats of Notch are the hEGF type and

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this subtype is present in proteins that are shed from the cell surface and mediate intercellular signaling [START_REF] Wouters | Evolution of distinct EGF domains with specific functions[END_REF]. Specific EGF-like repeats of Notch.undergo O-linked glycosylation with the addition of O-glucose and O-fucose (Fig. 3). These modifications occur on the hydroxyls of serine or threonine residues in the EGF-like repeats; O-glucose sites are located between the first and second conserved cysteine residues (C1XSXPC2) and O-fucose sites between the second and third conserved cysteine residues (C2X4-5S/TC3) [START_REF] Harris | O-linked fucose and other post-translational modifications unique to EGF modules[END_REF][START_REF] Shao | Fringe modifies O-fucose on mouse Notch1 at epidermal growth factor-like repeats within the ligand-binding site and the Abruptex region[END_REF]. Both O-glucose and O-fucose can be elongated by the sequential action of other glycosyltransferases to yield a trisaccharide or a tetrasaccharide, respectively (Fig. 3). The structure, based on other EGF-associated O-glucose trisaccharide, is predicted to be Xylα1,3Xylα1,3Glc (where Xyl is xylose and Glc is glucose, in which glucose is attached to serine [START_REF] Moloney | Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules[END_REF]. The tetrasaccharide attached to Notch is Sia-α2,3/6-Gal-β1,4-GlcNAc-β1,3-Fuc-α1-O-Ser (where Sia is sialic acid, Gal is galactose, GlcNAc is N-acetylglucosamine and Fuc is fucose, in which fucose is attached to serine or threonine [START_REF] Moloney | Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules[END_REF]. O-glucosyltransferase activity using an EGF-like repeat as the acceptor and UDP-[( 3)H]glucose as the donor was identified in mammalian cells and tissues; however, the gene encoding this activity was not idenitifed [START_REF] Shao | O-glycosylation of EGF repeats: identification and initial characterization of a UDP-glucose: protein O-glucosyltransferase[END_REF]. Only recently, a genetic screen designed to isolate temperature-sensitive mutants of Drosophila that affected Notch signaling identified rumi, a mutant that completely lacks O-glycosyltransferase activity [START_REF] Acar | Rumi is a CAP10 domain glycosyltransferase that modifies Notch and is required for Notch signaling[END_REF]. The rumi gene encodes a CAP10 domain protein O-glycosyltransferase that acts in the endoplasminc reticulum (ER) lumen to regulate Notch folding. In the absence of rumi, the ability of Notch to be expressed on the cell surface and to bind Delta is not impaired, however S2 cleavage of Notch does not occur. This suggests that O-glucose modification by rumi is required for the ligand-induced conformational change that is required for S2 cleavage of Notch in Drosophila.

O-fucosyltransferase catalyses the addition of O-fucose to serine and threonine residues of Notch EGF-like repeats (Ofut1 in Drosophila and Pofut1 in mammals). In Drosophila, loss of Ofut1 phenocopies the loss of fringe and indicates that the absence of O-fucose is equivalent to the absence of elongated O-fucose. In mouse loss of Pofut1 confirms that O-fucosyltransferase activity essential for Notch function; however, the phenotype is more similar to that generated when Notch signaling is blocked through, for example, the loss of RBPj/CSL [START_REF] Moloney | Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules[END_REF][START_REF] Shi | Protein O-fucosyltransferase 1 is an essential component of Notch signaling pathways[END_REF][START_REF] Sasamura | neurotic, a novel maternal neurogenic gene, encodes an O-fucosyltransferase that is essential for Notch-Delta interactions[END_REF][START_REF] Okajima | Modulation of notch-ligand binding by protein Ofucosyltransferase 1 and fringe[END_REF][START_REF] Okajima | Regulation of notch signaling by o-linked fucose[END_REF]. While most fucosyltransferases are type II membrane proteins in the trans-Golgi, Ofut1/Pofut1 is a soluble protein in ER [START_REF] Luo | O-fucosylation of notch occurs in the endoplasmic reticulum[END_REF][START_REF] Okajima | Chaperone activity of protein Ofucosyltransferase 1 promotes notch receptor folding[END_REF]. Functions for Ofut1, independent of, and dependent on transferase-activity, have been proposed that include folding of Notch in the ER, endocytic trafficking of Notch, and Notchligand binding [START_REF] Sasamura | neurotic, a novel maternal neurogenic gene, encodes an O-fucosyltransferase that is essential for Notch-Delta interactions[END_REF][START_REF] Okajima | Modulation of notch-ligand binding by protein Ofucosyltransferase 1 and fringe[END_REF][START_REF] Okajima | Chaperone activity of protein Ofucosyltransferase 1 promotes notch receptor folding[END_REF][START_REF] Sasaki | Polarized exocytosis and transcytosis of Notch during its apical localization in Drosophila epithelial cells[END_REF][START_REF] Sasamura | The Ofucosyltransferase O-fut1 is an extracellular component that is essential for the constitutive endocytic trafficking of Notch in Drosophila[END_REF]. Recent evidence provides compelling support for the noncatalytic activity of Ofut1 as a regulator of Notch trafficking to the cell surface, probably through action as a chaperone in the ER to promote the proper folding of the extracellular domain of Notch [START_REF] Okajima | Contributions of chaperone and glycosyltransferase activities of O-fucosyltransferase 1 to Notch signaling[END_REF]. In addition, expression of Ofut1 (lacking catalytic activity) rescued Notch receptor activity and led to phenotypes mimicking loss of fringe activity (see below) [START_REF] Okajima | Contributions of chaperone and glycosyltransferase activities of O-fucosyltransferase 1 to Notch signaling[END_REF]. In contrast to Drosophila Ofut1, mammalian Pofut1 is required for the generation of optimally functional Notch receptors, but is not an essential chaperone for stable, cell surface expression of Notch receptors [START_REF] Stahl | Roles of Pofut1 and Ofucose in mammalian notch signaling[END_REF].

Fringe elongates O-fucose of Notch EGF-like repeats

Fringe was identified in Drosophila as a gene that affected formation of the dorsoventral boundary during wing formation [START_REF] Irvine | fringe, a Boundary-specific signaling molecule, mediates interactions between dorsal and ventral cells during Drosophila wing development[END_REF]. Genetic analysis revealed that fringe potentates delta-dependent Notch signaling and inhibits serrate-dependent Notch signaling [START_REF] Fleming | Serrate-mediated activation of Notch is specifically blocked by the product of the gene fringe in the dorsal compartment of the Drosophila wing imaginal disc[END_REF][START_REF] Panin | Fringe modulates Notchligand interactions[END_REF][START_REF] Bruckner | Glycosyltransferase activity of Fringe modulates Notch-Delta interactions[END_REF][START_REF] Moloney | Fringe is a glycosyltransferase that modifies Notch[END_REF]. Fringe encodes a fucose-specific β1,3-N-acetylglucosaminyltransferase (β1,3-GlcNAc-transferase). Fringe activity requires UDP-GlcNAc as a donor and O-linked fucose in the alpha conformation as an acceptor [START_REF] Moloney | Fringe is a glycosyltransferase that modifies Notch[END_REF]. In mammals, three genes encode for fringe activity; lunatic fringe (Lfng), manic fringe (Mfng) and radical fringe (Rfng) each of which is

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capable of complementing mutations in Drosophila fringe (Dfng) [START_REF] Cohen | Fringe boundaries coincide with Notch-dependent patterning centres in mammals and alter Notch-dependent development in Drosophila[END_REF][START_REF] Johnston | A family of mammalian Fringe genes implicated in boundary determination and the Notch pathway[END_REF]. Fringe shows considerable preference for a properly folded EGF-like repeat as a substrate, and can glycosylate EGF-like repeats on Notch receptors and DSL ligands [START_REF] Moloney | Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules[END_REF][START_REF] Shao | O-glycosylation of EGF repeats: identification and initial characterization of a UDP-glucose: protein O-glucosyltransferase[END_REF][START_REF] Bruckner | Glycosyltransferase activity of Fringe modulates Notch-Delta interactions[END_REF][START_REF] Panin | Notch ligands are substrates for protein O-fucosyltransferase-1 and Fringe[END_REF].

Mammalian Fringe proteins

Catalytic activity

The catalytic activity of fringe proteins has been compared, and despite the use of different acceptor substrates, Lfng has a significantly higher catalytic efficiency than Mfng, Rfng or DLfng [START_REF] Moloney | Fringe is a glycosyltransferase that modifies Notch[END_REF][START_REF] Rampal | Lunatic fringe, manic fringe, and radical fringe recognize similar specificity determinants in O-fucosylated epidermal growth factor-like repeats[END_REF]. Given that the substrate greatly affects the measured catalytic activity of the fringe enzymes, the current picture of relative activity may not be representative of specific fringe-notch interactions. Indeed, not all O-fucosylated EGF-like repeats are modified by Fringe, suggesting that Fringe may recognize amino acid sequences in individual EGF-like repeats [START_REF] Shao | Fringe modifies O-fucose on mouse Notch1 at epidermal growth factor-like repeats within the ligand-binding site and the Abruptex region[END_REF]. Of those O-fucose EGF-like repeats that are modified by fringe activity, the three mammalian fringes display similar specificity with regard to the EGF-like repeats that are modified [START_REF] Rampal | Lunatic fringe, manic fringe, and radical fringe recognize similar specificity determinants in O-fucosylated epidermal growth factor-like repeats[END_REF]; distinctions have been reported, with Mfng showing the least ability to modify O-fucose EGF-like repeats, but it is currently unclear if these are due to differences in catalytic activity of the fringes, or substrate recognition [START_REF] Bruckner | Glycosyltransferase activity of Fringe modulates Notch-Delta interactions[END_REF][START_REF] Cohen | Fringe boundaries coincide with Notch-dependent patterning centres in mammals and alter Notch-dependent development in Drosophila[END_REF].

Impact on Notch signaling

The three mammalian fringes have the capacity to modify four receptors; therefore, the effects of fringe activity on Notch signaling in mammals are more complex and not yet fully understood. Generally speaking, as in Drosophila, fringe activity enhances delta-mediated Notch signaling and inhibits jagged-mediated Notch activity [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF][START_REF] Yang | Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1[END_REF]. Current exceptions are: Rfng potentiates Jagged1-mediated Notch1 signaling [START_REF] Yang | Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1[END_REF]. Mfng does not enhance Notch2 signaling [START_REF] Shimizu | Manic fringe and lunatic fringe modify different sites of the Notch2 extracellular region, resulting in different signaling modulation[END_REF], and Lfng potentiates Jagged1-mediated Notch2 signaling [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF]. These data are incomplete and further uncertainty ensues when gain-and loss-of-function studies with Lfng in chick and mouse embryos conclude that Lfng inhibits Notch1 signaling [START_REF] Dale | Periodic notch inhibition by lunatic fringe underlies the chick segmentation clock[END_REF][START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF][START_REF] Shifley | Oscillatory lunatic fringe activity is crucial for segmentation of the anterior but not posterior skeleton[END_REF].

Fringe activity alters the efficacy of Notch-ligand binding in trans to affect Notch signaling [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF][START_REF] Bruckner | Glycosyltransferase activity of Fringe modulates Notch-Delta interactions[END_REF][START_REF] Yang | Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1[END_REF]. Fringe activity increases the capacity of Notch1 to bind to Dll1 but does not alter Notch1-Jagged1 binding [START_REF] Yang | Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1[END_REF]. These binding assays relied on in vivo glycosylation so it was unclear if further modification of Notch, subsequent to the addition of GlcNAc by fringe, affected Notch-ligand binding. In vitro reconstitution assays demonstrated that addition of GlcNAc by fringe is sufficient to alter the interaction of Notch with its ligands, and that further elongation of the GlcNAc-1,3-Fuc disaccharide by Gal did not alter Notch-ligand binding in vitro [START_REF] Xu | In vitro reconstitution of the modulation of Drosophila notch-ligand binding by fringe[END_REF]. In contrast, Chen et al [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF] using showed that Fringe modulation of Jagged1-induced Notch signaling requires the addition of Gal to form the trisaccharide O-Fuc-GlcNAc-Gal on Notch. These disparate findings may reflect the fact that binding of Drosophila ligands with Notch was assayed in one study [START_REF] Xu | In vitro reconstitution of the modulation of Drosophila notch-ligand binding by fringe[END_REF], and Notch signaling in rodent cells was examined in the other [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF].

using Chinese hamster ovary (CHO) glycosylation mutants in a co-culture assay, showed that Fringe modulation of Jagged1-induced Notch signaling requires the addition of Gal to form the trisaccharide O-Fuc-GlcNAc-Gal on Notch [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF]. The galactosyltransferase b4GalT-1 was shown to be necessary to obtain the Fringe-mediated effects on Notch signaling in this assay [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF].J. Chen et al. / Gene Expression Patterns 6 (2006) 376-382 377 [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF][START_REF] Chen | Expression of Notch signaling pathway genes in mouse embryos lacking beta4galactosyltransferase-1[END_REF] As previously discussed, Notch receptors and DSL ligands also interact in cis leading to inhibition of signalling [START_REF] Sakamoto | Intracellular cellautonomous association of Notch and its ligands: a novel mechanism of Notch signal modification[END_REF][START_REF] Geffers | Divergent functions and distinct localization of the Notch ligands DLL1 and DLL3 in vivo[END_REF][START_REF] Ladi | The divergent DSL ligand Dll3 does not activate Notch signaling but cell autonomously attenuates signaling induced by other DSL ligands[END_REF]. Expression of Lfng inhibits the intracellular interaction of Notch1 with Dll1 or Jagged1 [START_REF] Sakamoto | Intracellular cellautonomous association of Notch and its ligands: a novel mechanism of Notch signal modification[END_REF] but not with Dll3 [START_REF] Ladi | The divergent DSL ligand Dll3 does not activate Notch signaling but cell autonomously attenuates signaling induced by other DSL ligands[END_REF]. The mechanism of

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ligand-dependent cis-inhibition of Notch signaling, and the role of fringe activity in this inhibition, is unknown.

Spondylocostal dysostosis (SCD) is caused by mutation of the fucose-specific β1,3 N-acetylglucosaminyltransferase LFNG

Spondylocostal dysostoses are a group of vertebral malsegmentation disorders

The vertebral column is characterized by the linear arrangement of regularly shaped vertebrae, bound together and supported by tendons and muscles. The vertebrae, tendons and muscles of the axial skeleton all differentiate from somites, which are derived from mesoderm. Somites condense into epithelial spheres on either side of the embryonic midline in a reiterative manner from caudally located presomitic mesoderm (PSM) during embryonic development. The ventral half of the somite (sclerotome) reverts to mesenchyme, which migrates around the neural tube and notochord and becomes partitioned into rostral and caudal halves; the caudal half of one somite combines with the rostral half of its neighboring somite to form an individual vertebra. These mesenchymal cells differentiate into cartilage, which later ossifies forming definitive vertebrae. These aspects of somite patterning and vertebral column formation have been well reviewed [START_REF] Brent | Developmental regulation of somite derivatives: muscle, cartilage and tendon[END_REF][START_REF] Christ | Segmentation of the vertebrate body[END_REF][START_REF] Monsoro-Burq | Sclerotome development and morphogenesis: when experimental embryology meets genetics[END_REF]. Abnormal vertebral segmentation (AVS) is a common congenital abnormality with an estimated prevalence of 0.5-1/1000 [START_REF] Jaskwhich | Congenital scoliosis[END_REF][START_REF] Mcmaster | Natural history of congenital kyphosis and kyphoscoliosis. A study of one hundred and twelve patients[END_REF][START_REF] Shands | The incidence of scoliosis in the state of Delaware; a study of 50,000 minifilms of the chest made during a survey for tuberculosis[END_REF][START_REF] Wynne-Davies | Congenital vertebral anomalies: aetiology and relationship to spina bifida cystica[END_REF]. Many manifestations of AVS exist resulting in uneven or fused vertebrae that can be present once in the vertebral column or in multiples; AVS follows Mendelian inheritance (autosomal dominant and recessive) and also can occur sporadically [START_REF] Turnpenny | Abnormal vertebral segmentation and the notch signaling pathway in man[END_REF]. The term spondylocostal dysostosis (SCD) is applied to a wide variety of radiological features that include multiple AVS. These include contiguous involvement of greater than ten vertebrae, often affecting all spinal regions. Despite asymmetric misalignment of some ribs and the presence of rib fusions, there is a basic overall symmetry to the shape of the thorax. Individuals with SCD have reduced stature and nonprogressive kyphosis [START_REF] Turnpenny | Abnormal vertebral segmentation and the notch signaling pathway in man[END_REF]. Genetic mapping and candidate gene sequencing approaches have identified causative mutations in four genes. SCDO1 (OMIM 277300) [START_REF] Bulman | Mutations in the human delta homologue, DLL3, cause axial skeletal defects in spondylocostal dysostosis[END_REF] represents around 20% of all cases, and is due to mutation of the DLL3 gene, with 25 distinct causative mutations identified to date [START_REF] Bulman | Mutations in the human delta homologue, DLL3, cause axial skeletal defects in spondylocostal dysostosis[END_REF][START_REF] Bonafe | A cluster of autosomal recessive spondylocostal dysostosis caused by three newly identified DLL3 mutations segregating in a small village[END_REF][START_REF] Turnpenny | Novel mutations in DLL3, a somitogenesis gene encoding a ligand for the Notch signalling pathway, cause a consistent pattern of abnormal vertebral segmentation in spondylocostal dysostosis[END_REF][START_REF] Whittock | Pseudodominant inheritance of spondylocostal dysostosis type 1 caused by two familial delta-like 3 mutations[END_REF] and unpublished). Mutation in MESP2 (SCDO2; OMIM 608681 [START_REF] Whittock | Mutated MESP2 Causes Spondylocostal Dysostosis in Humans[END_REF]), LFNG (SCDO3; OMIM 609813 [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]) and HES7 (SCDO4; [START_REF] Sparrow | Mutation of HAIRY-AND-ENHANCER-OF-SPLIT-7 in humans causes spondylocostal dysostosis[END_REF]) appear to occur less frequently as they are currently represented by single cases. All four genes are part of the Notch signaling pathway; DLL3 encodes an inhibitory ligand of Notch and MESP2, LFNG and HES7 are Notch target genes expressed in the PSM.

Mutation of LFNG cause SCDO3

The proband presented with extensive congenital vertebral anomalies (Fig 4), long, slender fingers, and camptodactyly of the left index finger. Multiple vertebral ossification centers in the thoracic spine, with fitted angular shapes were apparent. The severe foreshortening of the spine was emphasized by the comparison of the patient's arm span (186.5 cm) with adult height (155 cm) [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]. Sequence analysis did not identify mutations in DLL3 or MESP2; therefore a candidate gene approach was used to identify the genetic cause of SCD in this patient. A search for mouse mutants that recapitulated the SCD phenotype suggested that LFNG was indeed a good candidate for causing SCD. The phenotypes of embryos lacking Dll3 and Lfng are virtually identical [START_REF] Zhang | Segmentation defects of Notch pathway mutants and absence of a synergistic phenotype in lunatic fringe/radical fringe double mutant mice[END_REF], indicating that murine somitogenesis is similarly dependent on each gene. Moreover, Lfng gene expression is severely disrupted in Dll3 null embryos, suggesting that Lfng expression is dependent on Dll3 function [START_REF] Dunwoodie | Axial skeletal defects caused by mutation in the spondylocostal dysplasia/pudgy gene Dll3 are associated with disruption of the segmentation clock within the presomitic mesoderm[END_REF][START_REF] Kusumi | Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes[END_REF]. A homozygous missense mutation (c.564C>A) in exon 3 was detected, resulting in the substitution of leucine for phenylalanine (F188L) [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]. The proband's consanguineous parents had normal spinal anatomy, and were both heterozygous for the mutant allele. The substituted phenylalanine residue is absolutely conserved in all known fringe proteins from Drosophila melanogaster to humans [START_REF] Correia | Molecular genetic analysis of the glycosyltransferase Fringe in Drosophila[END_REF] and is close to the Mn 2+ -binding active site of the enzyme [START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF] which is DDD 200-202 in human LFNG. Prediction of the structural consequence of the F188L substitution, based on the highly homologous Manic Fringe [START_REF] Jinek | Structural insights into the Notch-modifying glycosyltransferase Fringe[END_REF] suggests that the conserved phenylalanine residue (F188) is not directly involved in uridine diphosphate (UDP)-GlcNAc (donor) or protein binding. Rather, it is likely to reside in a helix that packs against the strand containing the Mn 2+ -ligating residues D200 and the nearby D202. F188 is predicted to form an aromatic cluster with residues F196 and H198, and thus the F188L mutation may either cause steric perturbation of the Mn 2+ -binding site by altered packing of the smaller mutant (leucine) residue or cause electronic disruption of the enzymatic reaction by removal of required π-π interactions associated with the aromatic ring [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]. Functional analysis of the LFNG F188L variant demonstrated that it did not function like wildtype LFNG and thus it is considered that c.564C>A in exon 3 represent a null mutation in LFNG [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]. Firstly localization of wildtype and F188L LFNG, was compared by expressing wildtype mouse LFNG or the F187L, which is the mouse version of F188L in C2C12 myoblasts cells. LFNG is normally localized to the Golgi where it adds β-1,3-Nacetylglucosamine to O-fucosylated EGF-like repeats of Notch [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF]. This was confirmed with wildtype LFNG, it accumulated in a Golgi-like pattern in the cell and colocalised with GM130, which marks the cis-Golgi compartment. In contrast F187L was neither enriched in a Golgi-like pattern nor was there colocalization with GM130. Next, the ability of LFNG to modify Notch1 signaling was determined using a coculture assay [START_REF] Chapman | High levels of Notch signaling down-regulate Numb and Numblike[END_REF]. Notch1 signaling is enhanced by LFNG when trans-activated by the Dll1 ligand and conversely attenuated when activated by the Jagged1 ligand [START_REF] Hicks | Fringe differentially modulates Jagged1 and Delta1 signalling through Notch1 and Notch2[END_REF][START_REF] Yang | Fringe glycosyltransferases differentially modulate Notch1 proteolysis induced by Delta1 and Jagged1[END_REF]. It was demonstrated that LFNG F187L did not enhance Dll1-Notch1 signaling and did not reduce Jagged1-Notch1 signaling like wildtype LFNG. Finally the GlcNAc-transferase activity of F187L was examined in vitro using UDP-[3H]GlcNAc as the donor and pNp-fucose as the acceptor for the reaction; F187L lacked GlcNAc-transferase activity [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF].

In summary these studies demonstrate that F1887L did not accumulate in the Golgi, did not modify Notch1 signaling, and lacked GlcNAc-transferase activity. LFNG D202A was used as a control in these assays as it lacks transferase activity [START_REF] Moloney | Fringe is a glycosyltransferase that modifies Notch[END_REF][START_REF] Chen | Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1[END_REF]. F187L was functionally equivalent to D202A except that D202A localized normally to the Golgi [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF]. These findings strongly support the hypothesis that F188L is non-functional and causes SCD in this patient.

Lfng-null mice model SCDO3 and define the developmental origins of the vertebral anomalies Lfng expression in mouse

In mouse during gastrulation, Lfng transcripts are localized to posterior nascent mesoderm with expression becoming restricted to stripes either side of the node [START_REF] Przemeck | Node and midline defects are associated with left-right development in Delta1 mutant embryos[END_REF]. This expression appears coincident with Mesp2, implicating Lfng in head-trunk boundary formation and demonstrating the existence of molecular prepatterning in nascent mesoderm [START_REF] Kusumi | Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes[END_REF]. Later in the PSM, Lfng transcripts are present in two domains; a broad caudal domain characterized by oscillatory expression, and a rostral domain that narrows in synchrony with somite formation (Fig. 5A). These two domains of Lfng expression are driven by distinct regulatory elements and the identification of functional CSL binding sites demonstrates that Lfng is a direct target of Notch signaling [START_REF] Cole | Clock regulatory elements control cyclic expression of Lunatic fringe during somitogenesis[END_REF][START_REF] Morales | Periodic Lunatic fringe expression is controlled during segmentation by a cyclic transcriptional enhancer responsive to notch signaling[END_REF]. Like Lfng, other targets of Notch signaling are expressed in an oscillatory pattern in the PSM such as Hes1, Hes5, Hes7, Hey1 [START_REF] Dunwoodie | Axial skeletal defects caused by mutation in the spondylocostal dysplasia/pudgy gene Dll3 are associated with disruption of the segmentation clock within the presomitic mesoderm[END_REF][START_REF] Bessho | Dynamic expression and essential functions of Hes7 in somite segmentation[END_REF][START_REF] Jouve | Notch signalling is required for cyclic expression of the hairy-like gene HES1 in the presomitic mesoderm[END_REF][START_REF] Leimeister | Oscillating expression of c-Hey2 in the presomitic mesoderm suggests that the segmentation clock may use combinatorial signaling through multiple interacting bHLH factors[END_REF][START_REF] Palmeirim | Avian hairy gene expression identifies a molecular clock linked to vertebrate segmentation and somitogenesis[END_REF]; they oscillate in caudal PSM in phase with the detection of Notch1 ICD which is the cleaved, activated form of Notch1 [START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF][START_REF] Huppert | Analysis of Notch function in presomitic mesoderm suggests a gamma-secretase-independent role for presenilins in somite differentiation[END_REF].

Somitogenesis relies on the oscillatory progression rostrally of activated Notch1 in

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the PSM. A key element of this oscillatory Notch1 activation is the cyclical expression of the transcriptional repressor Hes7 (reviewed in [START_REF] Kageyama | Oscillator mechanism of notch pathway in the segmentation clock[END_REF]. Hes7, like Lfng, is a target gene of Notch1 signaling; Hes7 protein inhibits its own transcription as well as that of Lfng. These interdependent negative regulatory mechanisms by Hes7 and Lfng are important to maintain the oscillatory activation of Notch1 in the PSM. Recently Shifley and Cole have demonstrated that processing of the mouse Lfng preprotein reduces the half-life of the enzyme, thus facilitating the rapid oscillations of Lfng activity and the segmentation clock that drives somitogenesis [START_REF] Shifley | Lunatic fringe protein processing by proprotein convertases may contribute to the short protein half-life in the segmentation clock[END_REF].

In the rostral PSM Lfng expression is also activated by Mesp2, Lfng then inhibits Notch1 signaling in this region creating an interface between cells undergoing Notch1 signaling and cells which are not; this interface marks the site where the next somite boundary will form (Fig. 5C) [START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF]. In addition, the rostral PSM cells that express Lfng adopt a rostral fate due to co-expression with Mesp2; Mesp2 acts as a rostral determinant as it restricts the expression of the caudal determinant, Dll1 to the caudal half-somite [START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF]130,[START_REF] Takahashi | Mesp2 initiates somite segmentation through the Notch signalling pathway[END_REF] (Fig. 5B). The expression of Lfng is altered in the PSM of Notch pathway mutants; further supporting the finding that Lfng is a target of Notch signaling [START_REF] Dunwoodie | Axial skeletal defects caused by mutation in the spondylocostal dysplasia/pudgy gene Dll3 are associated with disruption of the segmentation clock within the presomitic mesoderm[END_REF][START_REF] Kusumi | Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes[END_REF][START_REF] Barrantes | Interaction between Notch signalling and Lunatic fringe during somite boundary formation in the mouse[END_REF][START_REF] Mustonen | Lunatic fringe, FGF, and BMP regulate the Notch pathway during epithelial morphogenesis of teeth[END_REF].

In addition to its expression in the PSM, Lfng is expressed in a number of other locations in the embryo and postnatal mouse, these include: the segmenting hindbrain, ventricular zone of the developing cortex neural crest cells, olfactory placode, inner ear, lung, kidney, urogenital epithelium and prostate, ovary, thymus and developing T cells, small intestine skin, hair follicle and teeth [START_REF] Cohen | Fringe boundaries coincide with Notch-dependent patterning centres in mammals and alter Notch-dependent development in Drosophila[END_REF][START_REF] Johnston | A family of mammalian Fringe genes implicated in boundary determination and the Notch pathway[END_REF][START_REF] Mustonen | Lunatic fringe, FGF, and BMP regulate the Notch pathway during epithelial morphogenesis of teeth[END_REF][START_REF] Favier | Localisation of members of the notch system and the differentiation of vibrissa hair follicles: receptors, ligands, and fringe modulators[END_REF][START_REF] Grishina | BMP7 inhibits branching morphogenesis in the prostate gland and interferes with Notch signaling[END_REF][START_REF] Hahn | Lunatic fringe null female mice are infertile due to defects in meiotic maturation[END_REF][START_REF] Ishii | Demarcation of early mammalian cortical development by differential expression of fringe genes[END_REF][START_REF] Koch | Subversion of the T/B lineage decision in the thymus by lunatic fringe-mediated inhibition of Notch-1[END_REF][START_REF] Leimeister | Expression of Notch pathway genes in the embryonic mouse metanephros suggests a role in proximal tubule development[END_REF][START_REF] Morsli | Development of the mouse inner ear and origin of its sensory organs[END_REF][START_REF] Piscione | Expression of Hairy/Enhancer of Split genes, Hes1 and Hes5, during murine nephron morphogenesis[END_REF][START_REF] Pouyet | Dynamic Lunatic fringe expression is correlated with boundaries formation in developing mouse teeth[END_REF][START_REF] Schröder | Expression of Notch pathway components in fetal and adult mouse small intestine[END_REF][START_REF] Thélu | Notch signalling is linked to epidermal cell differentiation level in basal cell carcinoma, psoriasis and wound healing[END_REF][START_REF] Thélu | Differential expression pattern of the three Fringe genes is associated with epidermal differentiation[END_REF][START_REF] Tsukumo | Lunatic fringe controls T cell differentiation through modulating notch signaling[END_REF][START_REF] Van Tuyl | Overexpression of lunatic fringe does not affect epithelial cell differentiation in the developing mouse lung[END_REF][START_REF] Visan | Regulation of T lymphopoiesis by Notch1 and Lunatic fringe-mediated competition for intrathymic niches[END_REF][START_REF] Zhang | A mutation in the Lunatic fringe gene suppresses the effects of a Jagged2 mutation on inner hair cell development in the cochlea[END_REF].

Somitic defects in mouse

Lfng is the only mammalian fringe protein required for normal somitogenesis [START_REF] Zhang | Segmentation defects of Notch pathway mutants and absence of a synergistic phenotype in lunatic fringe/radical fringe double mutant mice[END_REF][START_REF] Evrard | lunatic fringe is an essential mediator of somite segmentation and patterning[END_REF][START_REF] Zhang | Defects in somite formation in lunatic fringe-deficient mice[END_REF]. Lfng-null mice have a reduced viability at birth and before weaning; their shortened tail consists of few vertebrae and their body is truncated (Fig. 6). The axial skeleton of these mice is severely disrupted along its length consisting of hemi-vertebra, and fusion of vertebrae, neural arches and ribs. These skeletal defects are presaged by anomalies in their precursor tissues, the somites. Somites are divided and patterned in order to impart flexibility and regularity on the components of the vertebral column, in addition they instruct the position of the dorsal route ganglia and the trajectories of the spinal nerves. Cells of the sclerotome condense in the caudal half of each somite and in doing so restrict the passage of neural crest cells and spinal nerve axons through the rostral part of the somite (reviewed [START_REF] Bronner-Fraser | Rostrocaudal differences within the somites confer segmental pattern to trunk neural crest migration[END_REF]). In the absence of Lfng, the normal alternating density of sclerotome and the regular spacing of dorsal route ganglia and spinal nerve axons are disrupted. These defects are highly reminiscent of those generated when other components of the Notch signaling pathway, such as Dll3, Mesp2, Hes7 and Rippley2, are absent during mouse development [START_REF] Dunwoodie | Axial skeletal defects caused by mutation in the spondylocostal dysplasia/pudgy gene Dll3 are associated with disruption of the segmentation clock within the presomitic mesoderm[END_REF][START_REF] Bessho | Dynamic expression and essential functions of Hes7 in somite segmentation[END_REF][START_REF] Morimoto | The negative regulation of Mesp2 by mouse Ripply2 is required to establish the rostro-caudal patterning within a somite[END_REF][START_REF] Saga | Mesp2: a novel mouse gene expressed in the presegmented mesoderm and essential for segmentation initiation[END_REF]. Lfng is a direct target of Notch, and already noted Lfng inhibits Notch signaling in the PSM of mouse and chick embryos [START_REF] Dale | Periodic notch inhibition by lunatic fringe underlies the chick segmentation clock[END_REF][START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF][START_REF] Shifley | Oscillatory lunatic fringe activity is crucial for segmentation of the anterior but not posterior skeleton[END_REF]. It is therefore consistent with this that Notch1 signaling is disrupted in the PSM of Lfng-null embryos and accordingly so is the expression of other known targets of Notch (Mesp2, Hes5) [START_REF] Morimoto | The Mesp2 transcription factor establishes segmental borders by suppressing Notch activity[END_REF][START_REF] Evrard | lunatic fringe is an essential mediator of somite segmentation and patterning[END_REF]. More recently in an effort to dissect further the roles of Lfng in somitogenesis Shifley et al [START_REF] Shifley | Oscillatory lunatic fringe activity is crucial for segmentation of the anterior but not posterior skeleton[END_REF] deleted the oscillatory expression of Lfng, leaving the gene regulatory elements that control expression in the rostral PSM. In the absence of oscillatory Lfng expression in the caudal PSM, Notch1 activation does not cycle and instead is widespread. In addition, the mice developed AVS in the thoracic and lumbar spine, while the sacral and tail vertebrae are only minimally affected. This indicates that oscillatory Lfng expression and activation of Notch1 are necessary for normal segmentation and patterning of somites in the thoracic and lumbar regions, and largely dispensable during formation of the more caudal somites.

Whilst defects in somitogenesis and vertebral column formation are the most striking in mice lacking Lfng, functions for Lfng have also been uncovered in the inner ear and ovary. Lfng is expressed in non-sensory supporting cells in the mouse cochlea [START_REF] Morsli | Development of the mouse inner ear and origin of its sensory organs[END_REF] and whilst deletion of Lfng has no effect on hair cell number or pattering, loss of Lfng does suppress the production of supernumerary hair cells that arise in the absence of the Notch

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ligand Jagged2 [START_REF] Zhang | A mutation in the Lunatic fringe gene suppresses the effects of a Jagged2 mutation on inner hair cell development in the cochlea[END_REF]. In addition, Lfng is expressed in the granulosa cells of the developing ovarian follicles and in its absence folliculogenesis is abnormal with the progression of oocytes through meiosis slow and incomplete [START_REF] Hahn | Lunatic fringe null female mice are infertile due to defects in meiotic maturation[END_REF]. Infertility may be variable in its penetrance as some Lfng-null females are reportedly fertile [START_REF] Xu | Not all lunatic fringe null female mice are infertile[END_REF].

Conclusions and perspectives

There has been incredible progress made as many of the genetic effectors of somitogenesis have been elucidated. Prominent in this increased understanding is the Notch signaling pathway, and the uncovering of the role that it plays in somite formation and patterning. The fucose-specific β1,3-N-acetylglucosaminyltransferase Lfng emerges as a prominent component of Notch1 signaling in the PSM and there is now an understanding of interdependency between target genes of Notch signaling, such as Hes7 and Lfng, and the oscillatory activity of the Notch1 receptor in the PSM. In addition in the rostral PSM there is an understanding of how the transcription factor Mesp2 controls Lfng expression, and in doing so narrows the region of Notch1 signaling in the rostral PSM; this is providing great insights into the processes of somite segmentation and pattering. It is still not clear how Lfng expression leads to inhibition of Notch1 in the PSM, as in vitro studies show that Lfng potentiates Notch1 signalling when the receptor is trans-activated by the Dll1 ligand. Since Dll1 is the activating ligand of Notch1 in the PSM one might expect Notch1 activity to be potentiated by Lfng. It is clear that an understanding of Lfng modification of Notch1 ligands, and its potential role in ligand-dependent cis-inhibition of Notch1 will need to be achieved in order to address this conundrum. Interaction between Notch signalling components and FGF and Wnt signalling are clearly important for somiogenesis [START_REF] Niwa | The initiation and propagation of Hes7 oscillation are cooperatively regulated by Fgf and notch signaling in the somite segmentation clock[END_REF][START_REF] Aulehla | A beta-catenin gradient links the clock and wavefront systems in mouse embryo segmentation[END_REF][START_REF] Wahl | FGF signaling acts upstream of the NOTCH and WNT signaling pathways to control segmentation clock oscillations in mouse somitogenesis[END_REF] (reviewed in [START_REF] Dequéant | Segmental patterning of the vertebrate embryonic axis[END_REF]). Effort is being made to establish a hierarchy between FGF, Notch and Wnt signalling in the PSM; this may be difficult to fully appreciate given the degree of negative feedback mechanisms that bind these pathways together. It is indeed this interdependency that likely underpins the robustness of somitogenesis.

The mouse has been at the forefront of research into somitogenesis as genetic modification has enabled definitive roles for many genes to be established (reviewed in [START_REF] Dequéant | Segmental patterning of the vertebrate embryonic axis[END_REF][START_REF] Gridley | The long and short of it: somite formation in mice[END_REF]). Clinical research into the genetic causes of AVS has clearly benefited from these developmental genetic studies in mice. For example, the discoveries showing that mutation of DLL3, MESP2, LFNG and very recently HES7, cause the congenital AVS disorder SCD were underpinned by the findings that these genes are each required for somite formation in mouse. Genetic analysis in mouse has shown that Notch, FGF and Wnt signalling are central to somitogenesis, therefore components of these pathways represent candidate genes for causing SCD (independent of DLL3, MESP2, LFNG or HES7) as well as other AVS disorders.

Figure legends

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showing similar segmental anomalies. Reprinted from Sparrow et al [START_REF] Sparrow | Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype[END_REF], © 2005 by The American Society of Human Genetics.

Figure 1

 1 Figure legendsFigure 1 Mammalian Notch receptors and DSL ligands. The 4 Notch receptors (Notch1-4) and 5 DSL ligands (Dll1, Dll3, Dll4, Jagged1 and Jagged2) are type I transmembrane proteins with their carboxy terminus in the cytosol. Notch Receptors: N-terminal to the transmembrane domain, Notch receptors have 29 to

Figure 2

 2 Figure 2 Notch1 signalling in mammalian somitogenesis. Notch1 is synthesised as a single polypeptide. (A) Specific EGF-like repeats are modified by O-fucosylation in the endoplasmic reticulum (ER) by protein O-fucosyltransferase 1 (Pofut1). As the Notch1 polypeptide passes through the Golgi, these O-fucosylated EGFlike repeats may undergo elongation with the addition of GlcNAc by lunatic fringe (Lfng). (B) S1 cleavage by a furin-like convertase occurs in the trans-Golgi network (TGN). Unlike Dll1, the divergent ligand Dll3 is located in the cis-Golgi and not at the cell surface. (C) The Notch1 heterodimer consisting of the N-terminal extracellular truncation (ECN) and Cterminal transmembrane and intracellular domain (TMIC) traffics to the cell surface. (D) The DSL ligand Dll1 binds Notch1 ECN in trans and activates S2 cleavage by the disintegrin and metallopeptidase domain (ADAM) proteases ADAM10 or ADAM17. (E) This releases Notch1 ECN, which bound to Dll1, is trans-endocytosed into the signalsending cell. (F) The membrane anchored Notch1 extracellular truncation (EXT) may undergo ubiquitylation (ub) facilitating endocytosis. Notch1 EXT undergoes S3 cleavage in the transmembrane domain, this is mediated by γ-secretase and releases Notch1 ICD. (G)Notch1 ICD binds the DNA-binding protein CSL, this triggers the release of corepressor (CoR) proteins and histone deacetylases (HDACs), and facilities the binding of coactivators (CoA) which results in transcription of target genes such as Mesp2, Lfng and Hes7.[START_REF] Bray | Notch signalling: a simple pathway becomes complex[END_REF][START_REF] Blaumueller | Intracellular cleavage of Notch leads to a heterodimeric receptor on the plasma membrane[END_REF][START_REF] Ilagan | SnapShot: notch signaling pathway[END_REF][START_REF] Osborne | Notch signalling during peripheral T-cell activation and differentiation[END_REF].

Figure 3 O

 3 Figure 3 O-linked glycosylation sites of mouse Notch1 EGF-like repeats. EGF-like repeats that contain consensus sites for O-linked fucose (blue), O-linked glucose (yellow), or both (green) are indicated. Identified O-fucose glycans include sialic acidα(2,3)Galβ(1,4)GlcNAcβ(1,3)Fuc-O and O-glucose glycans include Xylα(1,3)Xylα(1,3)O-Glc-O. The N-terminus of Notch1 is at the top. Standardised symbols and colors were used from the Consortium for Functional Glycomics to represent glycans. [73, 74, 170].

Figure 4

 4 Figure 4 Vertebral anomalies caused by mutation of LFNG. Radiograph (A) and T2-weighted coronal MRI images (B, C) in the vertebral plane. (A) Severe vertebral segmentation anomalies throughout the vertebral column. (B) Thoracic spine showing vertebral centres with a fitted angular shape. (C) Cervical and lumbar spine

Figure 5

 5 Figure 5 Cycling expression of Lfng. Dorsal views of whole-mount RNA in situ hybridization preparations of mouse embryo presomitic mesoderm showing Lfng expression (purple), rostral at the top. (A) Cycling Lfng expression is represented in three phases at E9.5. Embryos have been arranged to display selected stages in the temporal sequence of expression. (B) Lfng expression in embryos, bisected into axial halves. The left half was fixed, while the right half was cultured for 45 min. Rostral bands of Lfng expression display an anteriograde shift, and caudal regions of expression also change. Arrowheads indicate rostral Lfng bands of expression. Asterisks show caudal expression. (C) A schematic depiction of the genetic activities involved in Mesp2 activation and the subsequent events regulated by Mesp2.Mesp2 is expressed in the anterior presomitic mesoderm by the cooperative function of Tbx6 and Notch signaling (NICD dotted line). The Notch signal oscillates in the presomitic mesoderm, thus the dotted line moves anteriorly (white arrow). Once Mesp2 protein is generated, NICD is suppressed and several genes are activated that establish the segmental border and rostral-caudal polarity within the somites. S1, formed somite; S0, forming somite; S-1, S-2, somite primordia. Asterisks show caudal presomitic mesodermal expression. Figures5A,Bis reproduced from Kusumi et al[START_REF] Kusumi | Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes[END_REF], and 5C from Saga et al[130].

Figure 6

 6 Figure[START_REF] Louvi | Notch signalling in vertebrate neural development[END_REF] Skeletal phenotype of Lfng-null mice. a, Photograph of 8-week-old wild-type and mutant littermates. Mutant animals have a very small and bent tail and their body axis is shortened from the neck to the rump. b-e, Alcian blue/Alizarin red skeletal preparations of newborn wild-type and mutant mice. b, Side view with limbs removed (WT, wild type; -/-, homozygous mutant). c, Dorsal and ventral views of the cervical, thoracic and lumbar regions, with the ventral ribs and sternum removed. c, Dissected ventral ribs and sternum. Reprinted by permission from Macmillan Publishers Ltd: Nature, Evrard et al[START_REF] Evrard | lunatic fringe is an essential mediator of somite segmentation and patterning[END_REF], copyright (1998).

  36 epidermal growth factor (EGF)-like repeats that are required for ligand binding, three LIN12/Notch repeats (LNR) that prevent ligand-independent signaling, and a heterodimerisation domain (overlapping parallel lines) which contains the S2 cleavage site. C-terminal to the transmembrane domain the receptors have an RBPj associated molecule (RAM) domain that binds RBPj/CSL, and six ankyrin (ANK) repeats which generally moderate protein interactions and are hydroxylated by factor inhibiting HIF (FIH) in Notch1-3. There is a nuclear localisation sequence (NLS) in the RAM domain and Notch1-3 have a second NLS. There is a transcription activation domain (TAD), and a C-terminal polypeptide enriched in proline, glutamine, serine and threonine residues (PEST) that is associated with protein degradation. A TAD has not been identified in Notch 3 or Notch4. Notch 4 also lacks a second NLS and PEST sequence. DSL Ligands: N-terminal to the transmembrane domain, the DLS ligands (excluding Dll3) have a Delta/Serrate/Lag (DLS) domain and EGF-like repeats that interacts with the EGF-like repeats of Notch. Jagged1-2 have a von Willebrand factor type C domain (VWC) which is generally involved in multiprotein complexes. C-terminal to the transmembrane domain the proteins are unstructured and, excluding Dll3 and Jagged2, there is a PDZ ligand-binding domain (PDZLDB) at the C-terminus. This figure was assembled using the following: ExPASy http://www.expasy.org/sprot/, SMART http://smart.embl-heidelberg.de/, PESTfind Analysis Webtool https://emb1.bcc.univie.ac.at/toolbox/pestfind/pestfind-analysis-webtool.htm,[START_REF] Ascano | The C-terminal PDZ-ligand of JAGGED1 is essential for cellular transformation[END_REF][START_REF] Coleman | Asparaginyl hydroxylation of the notch ankyrin repeat domain by factor inhibiting hypoxia inducible factor[END_REF][162][START_REF] Ehebauer | Highresolution crystal structure of the human Notch 1 ankyrin domain[END_REF][START_REF] Kurooka | Roles of the ankyrin repeats and C-terminal region of the mouse notch1 intracellular region[END_REF][START_REF] Oberg | The Notch intracellular domain is ubiquitinated and negatively regulated by the mammalian Sel-10 homolog[END_REF][START_REF] Tamura | Physical interaction between a novel domain of the receptor Notch and the transcription factor RBP-J kappa/Su(H)[END_REF][START_REF] Zheng | Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways[END_REF].

Acknowledgements

I apologise to those who have made contributions to our understanding of this research area and who were not cited in this review. I thank Marcus Cremonese and Gavin Chapman for their assistance with the figures and an anonymous reviewer. I acknowledge support through National Health and Medical Research Council (NHMRC) project grant 404804, and Senior Research Fellowships from Pfizer Foundation Australia and the NHMRC.