Editing Notch signaling pathway (section)

=== Pathway ===

Maturation of the notch receptor involves cleavage at the prospective extracellular side during intracellular trafficking in the Golgi complex.<ref name="pmid10899003">{{cite journal | vauthors = Munro S, Freeman M | title = The notch signalling regulator fringe acts in the Golgi apparatus and requires the glycosyltransferase signature motif DXD | journal = Current Biology | volume = 10 | issue = 14 | pages = 813–820 | date = July 2000 | pmid = 10899003 | doi = 10.1016/S0960-9822(00)00578-9 | s2cid = 13909969 | doi-access = free | bibcode = 2000CBio...10..813M }}</ref> This results in a bipartite protein, composed of a large extracellular domain linked to the smaller transmembrane and intracellular domain. Binding of ligand promotes two proteolytic processing events; as a result of proteolysis, the intracellular domain is liberated and can enter the nucleus to engage other DNA-binding proteins and regulate gene expression.

Notch and most of its ligands are transmembrane proteins, so the cells expressing the ligands typically must be adjacent to the notch expressing cell for signaling to occur.{{citation needed|date=May 2007}} The notch ligands are also single-pass transmembrane proteins and are members of the DSL (Delta/Serrate/LAG-2) family of proteins. In ''[[Drosophila melanogaster]]'' (the fruit fly), there are two ligands named [[Delta (ligand)|Delta]] and [[Serrate (ligand)|Serrate]]. In mammals, the corresponding names are [[Delta-like]] and [[Jagged (ligand)|Jagged]]. In mammals there are multiple Delta-like and Jagged ligands, as well as possibly a variety of other ligands, such as F3/contactin.<ref name="Lai" />

In the nematode ''[[Caenorhabditis elegans|C. elegans]]'', two genes encode homologous proteins, ''glp-1'' and ''lin-12''. There has been at least one report that suggests that some cells can send out processes that allow signaling to occur between cells that are as much as four or five cell diameters apart.{{citation needed|date=June 2007}}

The notch extracellular domain is composed primarily of small cystine-rich motifs called [[Epidermal growth factor|EGF]]-like repeats.<ref name="pmid16973733">{{cite journal | vauthors = Ma B, Simala-Grant JL, Taylor DE | title = Fucosylation in prokaryotes and eukaryotes | journal = Glycobiology | volume = 16 | issue = 12 | pages = 158R–184R | date = December 2006 | pmid = 16973733 | doi = 10.1093/glycob/cwl040 | doi-access =  }}</ref>

Notch 1, for example, has 36 of these repeats. Each EGF-like repeat is composed of approximately 40 amino acids, and its structure is defined largely by six conserved cysteine residues that form three conserved disulfide bonds. Each EGF-like repeat can be modified by [[glycans|''O''-linked glycans]] at specific sites.<ref name="pmid12460944">{{cite journal | vauthors = Shao L, Luo Y, Moloney DJ, Haltiwanger R | title = O-glycosylation of EGF repeats: identification and initial characterization of a UDP-glucose: protein O-glucosyltransferase | journal = Glycobiology | volume = 12 | issue = 11 | pages = 763–770 | date = November 2002 | pmid = 12460944 | doi = 10.1093/glycob/cwf085 | doi-access =  }}</ref> An [[glycosylation|''O''-glucose]] sugar may be added between the first and second conserved cysteines, and an [[glycosylation|''O''-fucose]] may be added between the second and third conserved cysteines. These sugars are added by an as-yet-unidentified [[glycosylation|''O''-glucosyltransferase]] (except for [https://www.ncbi.nlm.nih.gov/pubmed/18243100 Rumi]), and [[GDP-fucose Protein O-fucosyltransferase 1|GDP-fucose Protein ''O''-fucosyltransferase 1]] ([[GDP-fucose Protein O-fucosyltransferase 1|POFUT1]]), respectively. The addition of [[glycosylation|''O''-fucose]] by [[GDP-fucose Protein O-fucosyltransferase 1|POFUT1]] is absolutely necessary for notch function, and, without the enzyme to add ''O''-fucose, all notch proteins fail to function properly. As yet, the manner by which the glycosylation of notch affects function is not completely understood.

The ''O''-glucose on notch can be further elongated to a trisaccharide with the addition of two [[xylose]] sugars by [[xylose|xylosyltransferases]], and the [[glycosylation|''O''-fucose]] can be elongated to a tetrasaccharide by the ordered addition of an [[N-acetylglucosamine]] (GlcNAc) sugar by an [[N-acetylglucosamine|N-Acetylglucosaminyltransferase]] called [[Fringe Genes|Fringe]], the addition of a [[galactose]] by a [[galactose|galactosyltransferase]], and the addition of a [[sialic acid]] by a [[sialic acid|sialyltransferase]].<ref name="pmid17132502">{{cite book | vauthors = Lu L, Stanley P | chapter = Roles of O-Fucose Glycans in Notch Signaling Revealed by Mutant Mice | title = Functional Glycomics | series = Methods in Enzymology | volume = 417 | pages = 127–136 | year = 2006 | pmid = 17132502 | doi = 10.1016/S0076-6879(06)17010-X | isbn = 9780121828226 }}</ref>

To add another level of complexity, in mammals there are three Fringe GlcNAc-transferases, named lunatic fringe, manic fringe, and radical fringe. These enzymes are responsible for something called a "fringe effect" on notch signaling.<ref name="pmid17215308">{{cite journal | vauthors = Thomas GB, van Meyel DJ | title = The glycosyltransferase Fringe promotes Delta-Notch signaling between neurons and glia, and is required for subtype-specific glial gene expression | journal = Development | volume = 134 | issue = 3 | pages = 591–600 | date = February 2007 | pmid = 17215308 | doi = 10.1242/dev.02754 | doi-access = free }}</ref> If Fringe adds a GlcNAc to the [[glycosylation|''O''-fucose]] sugar then the subsequent addition of a galactose and sialic acid will occur. In the presence of this tetrasaccharide, notch signals strongly when it interacts with the Delta ligand, but has markedly inhibited signaling when interacting with the Jagged ligand.<ref name="pmid12826675">{{cite journal | vauthors = LaVoie MJ, Selkoe DJ | title = The Notch ligands, Jagged and Delta, are sequentially processed by alpha-secretase and presenilin/gamma-secretase and release signaling fragments | journal = The Journal of Biological Chemistry | volume = 278 | issue = 36 | pages = 34427–34437 | date = September 2003 | pmid = 12826675 | doi = 10.1074/jbc.M302659200 | doi-access = free }}</ref> The means by which this addition of sugar inhibits signaling through one ligand, and potentiates signaling through another is not clearly understood.

Once the notch extracellular domain interacts with a ligand, an ADAM-family [[metalloprotease]] called ADAM10, cleaves the notch protein just outside the membrane.<ref name="pmid19726682">{{cite journal | vauthors = van Tetering G, van Diest P, Verlaan I, van der Wall E, Kopan R, Vooijs M | title = Metalloprotease ADAM10 is required for Notch1 site 2 cleavage | journal = The Journal of Biological Chemistry | volume = 284 | issue = 45 | pages = 31018–31027 | date = November 2009 | pmid = 19726682 | pmc = 2781502 | doi = 10.1074/jbc.M109.006775 | doi-access = free }}</ref> This releases the extracellular portion of notch (NECD), which continues to interact with the ligand. The ligand plus the notch extracellular domain is then [[endocytosis|endocytosed]] by the ligand-expressing cell. There may be signaling effects in the ligand-expressing cell after endocytosis; this part of notch signaling is a topic of active research.{{citation needed|reason=This claim needs a reliable source; Bogart was a famous actor, and his major biographies don't mention snooker.|date=August 2016}} After this first cleavage, an enzyme called [[Gamma-secretase|γ-secretase]] (which is implicated in [[Alzheimer's disease]]) cleaves the remaining part of the notch protein just inside the inner leaflet of the [[cell membrane]] of the notch-expressing cell. This releases the intracellular domain of the notch protein (NICD), which then moves to the [[Cell nucleus|nucleus]], where it can regulate gene expression by activating the [[transcription factor]] [[RBPJ|CSL]]. It was originally thought that these CSL proteins suppressed Notch target transcription. However, further research showed that, when the intracellular domain binds to the complex, it switches from a repressor to an activator of transcription.<ref>{{cite book |doi=10.1038/npg.els.0004194 |chapter=Drosophila ''Patterning'': Delta-Notch Interactions |title=Encyclopedia of Life Sciences |year=2006 | vauthors = Desbordes S, López-Schier H |isbn=0470016175 }}</ref> Other proteins also participate in the intracellular portion of the notch signaling cascade.<ref name="pmid22223095">{{cite journal | vauthors = Borggrefe T, Liefke R | title = Fine-tuning of the intracellular canonical Notch signaling pathway | journal = Cell Cycle | volume = 11 | issue = 2 | pages = 264–276 | date = January 2012 | pmid = 22223095 | doi = 10.4161/cc.11.2.18995 | doi-access = free }}</ref>