The Notch pathway is an evolutionally conserved signalling pathway which plays an
important role in diverse developmental and physiological processes. These include cell-fate
determination, tissue patterning and morphogenesis, cell differentiation, proliferation and cell
death (1). The Notch pathway is named after the Drosophila mutants that showed irregular
notches of missing tissue at the insect wing blade tips (2). The Notch gene was cloned in
1985 (3). Proteins of the Notch families are single-pass transmembrane proteins that function
both as cell surface receptors and nuclear transcriptional regulators (1). Four Notch receptors
(Notch 1-4) have been identified in mammals. Mature Notch receptors are non-covalent
heterodimers consisting of an extracellular subunit (NEC) and a transmembrane subunit
(NTM). NEC possess multiple EGF-like repeats and three specialized Lin-Notch repeats (LNR)
that forms a tight hydrophobic interaction with extracellular stump of NTM. This region masks
an ‘A disintegrin and metalloprotease’ (ADAM) cleavage site. The region where these two
subunits interact is called the heterodimerization domain (HD) (4).
Notch ligands are also transmembrane proteins with multiple EGF-like repeats, a short
cytoplasmic tail and a specialized delta-serrate-lag2 (DSL) domain at the N-terminus. There
are five canonical Notch ligands – Jagged (JAG1 and JAG2), Delta-like (DLL1, DLL3,
DLL4) in mammals (5,6,7,8,9,10). Notch signalling activation occurs upon ligand-receptor
binding, which are expressed on two adjacent cells. Ligand binding causes dissociation of
NEC from NTM, unmasking the ADAM cleavage site (1,4). The NEC fragment is trans-
endocytosed into the ligand expressing cells. The full-length receptor minus the NEC fragment
is cleaved at the membrane by ADAM17 generating an intermediate, Notch extracellular
truncation (NEXT) (1,4,11). This is further cleaved by γ-secretase that generates an active
Notch intracellular fragment (NIC) or Notch intracellular domain (NICD) (1,12). The γ-
secretase complex is composed of PSEN1, PSEN2, PSENEN, NCSTN and APH1 (A or B)
(1,13). Following these two cleavage steps, the NICD is released into the cytoplasm and
translocates into the nucleus to regulate transcription of Notch target genes (1,12,14). Upon
translocation into the nucleus, NICD binds to RBPJ which is a constitutive repressor of Notch
signalling. RBPJ represses Notch target gene expression by recruiting a co-repressor
complex, which includes NCOR1, NCOR2, SNW1, CIR, HDAC1, HDAC2, SPEN and FHL1
and SAP30 (1,4,12,15-21). NICD binding to RBPJ replaces the co-repressor complex with a
co-activator complex which includes MAML1-3, EP300 (1,22,23,24) and SNW1 (1,25,26).
Primary Notch target genes include two families of transcriptional factors – Hes, including
HES1 (15,27) and HES5(15,28) as well as Hey including HEY1 and HEY2 (29,30,31). Other
Notch target genes include CCND1 (27,32), CDKN1A(1,33) , GATA3(1,34) and
CNTN1 acts as a functional ligand of Notch. This trans-extracellular interaction causes
gamma-secretase-dependent nuclear translocation of the NICD. This signalling is involved
in oligodendrocyte precursor cell differentiation and upregulation of myelin-related protein
In addition to the canonical Notch pathway, there is increasing evidence showing RBPJ
independent non-canonical pathways (37).
been fully characterized. Physical interaction of NOTCH-1IC with LCK- PI3K (38,39) may
mediate non-nuclear cross-talk with AKT, leading to survival signalling.(40,41). Notch
stimulation through AKT pathway leads to down regulation of MYC expression (42,43).
Activation of SRC/STAT3 pathway by Notch signaling is dependent on the expression
of Notch effector HES1 transcription factor. The induction of HES1 enhanced SRC
phosphorylation. This activated SRC kinase was found to be responsible for the enhanced
phosphorylation of STAT3 (44). The HES1 and HES5 proteins associate with and facilitate
the complex formation between JAK2 and STAT3, thus promoting STAT3 phosphorylation
and activation (45). The activated STAT3 translocates from the cytoplasm to the nucleus
(45,46) and induces transcriptional activation of target gene expression (including HIF1A)
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