Mature NOTCH1 heterodimer on the cell surface is activated by one of its ligands: DLL1 (Cordle et al. 2008, Jarriault et al. 1998), DLL4 (Benedito et al. 2009), JAG1 (Li et al. 1998, Benedito et al. 2009) or JAG2 (Luo et al. 1997, Shimizu et al. 2000), expressed in trans on a neighboring cell. Thus, a ligand-expressing cell is a signal-sending cell, while the NOTCH1 expressing cell is a signal-receiving cell. If NOTCH1 has undergone Fringe modification in the Golgi, it is preferentially activated by Delta ligands (Yang et al. 2005), DLL1 and DLL4.
Upon binding to NOTCH1 on a neighboring cell, NOTCH ligands are ubiquitinated by Mindbomb (MIB1 and MIB2) and/or Neuralized (NEURL and NEURL1B) E3 ubiquitin ligases and endocytosed (Koo et al. 2007, Koo et al. 2005, Itoh et al. 2003, Lai et al. 2001, Koutelou et al. 2008, Song et al. 2006). Endocytosis of ubiquitinated ligands is thought to mechanically stretch the bound NOTCH1 receptor, exposing a cleavage site S2 that is recognized by ADAM10 and/or ADAM17 metalloprotease (van Tetering et al. 2009, Brou et al. 2000, Hartmann et al. 2002, Pan et al. 1997). S2 cleavage of NOTCH1 produces the NEXT1 fragment which is further cleaved at an S3 cleavage site by the gamma-secretase complex, resulting in release of the NOTCH1 intracellular domain (NICD1) into the cytosol (de Strooper et al. 1999, Schroeter et al. 1998, Huppert et al. 2000). NICD1 produced by activation of NOTCH1 in response to in trans presented Delta and Jagged ligands (DLL/JAG) traffics to the nucleus where it acts as a transcription regulator.
NOTCH1 signaling can also be activated by ligands other than DLL1, DLL4, JAG1 and JAG2. CNTN1 (Contactin-1), transiently expressed during central and peripheral nervous system development, activates NOTCH1 and NOTCH2 in trans, promoting oligodendrocyte maturation and myelination (Hu et al. 2003). DNER (Delta and Notch-like epidermal growth factor-related receptor) is a transmembrane protein specifically expressed in dendrites and cell bodies of postmitotic neurons. Activation of NOTCH1 by DNER in trans may play an important role in development of the central nervous system by influencing differentiation of astrocytes (Eiraku et al. 2005). Activation of NOTCH1 by both CNTN1 and DNER is Deltex (DTX)-dependent and results in gamma-secretase mediated release of NICD1. Three members of the Deltex protein family: DTX1, DTX2 and DTX4 possess a domain involved in binding cdc10/ankyrin repeats of NOTCH. DTX proteins are considered as positive regulators of NOTCH signaling, although the exact mechanism has not been elucidated (Matsuno et al. 1998, Kishi et al. 2001).In addition, DTX can mediate downregulation of NOTCH signaling by recruiting non-visual beta-arrestins to NOTCH (Mukherjee et al. 2005), thereby trigerring NOTCH ubiquitination. DTX proteins are negatively regulated by ITCH (AIP4) ubiquitin ligase (Chastagner et al. 2006).
NOTCH1 signaling in the signal-receiving cell can be turned off in cis by expression of NOTCH ligands DLL/JAG (Cordle et al. 2008, Sprinzak et al. 2010), as well as DLK1 (Baladron et al. 2005, Bray et al. 2008). Formation of NOTCH1:ligand complexes in cis prevents interaction of NOTCH1 with ligands expressed in trans, resulting in the inhibition of NOTCH signaling. In the signal-sending cell, NOTCH signaling can be negatively regulated by the protein NUMB, which is asymmetrically distributed during cell division (Rhyu et al. 1994). NUMB recruits ITCH ubiquitin ligase to NOTCH1 and promotes sorting of NOTCH1 through late endosomes for degradation (McGill et al. 2009, Chastagner et al. 2008).