Figure 1

A simplified diagram of canonical Notch signaling: A: membrane and cytoplasmic events. In ligand expressing cells, ligands are ubiquitinated (UQ) by E3 ligases Mindbomb and Neuralized, endocytosed and "activated". "Active" ligands bind Notch receptors, dissociating N^EC from N™. The complex ligand- N^EC is trans-endocytosed into the ligand-expressing cell, perhaps providing mechanical energy to separate N^EC from N™. Some ligands expressed in cis can bind Notch on the same cell, causing cis-inhibition. Ligand-induced N^EC separation unmasks the ADAM cleavage site (red), which is cleaved by ADAM10 or ADAM17, producing N^EXT and a short peptide which is released. N^EXT is cleaved by γ-secretase, at the membrane or during endocytosis, generating N^IC. This process is facilitated by adaptor-associated kinase AAK1 [101] and may require mono-ubiquitination. The release of N^IC from endosomes (or the selection of cleavage site by γ-secretase) may require endosome acidification (H⁺) by aquaporin Bib. The stability of N^IC is regulated by factors such as Pin-1 prolyl isomerase and NLK kinase. Endocytosis can lead to ligand-independent Notch activation catalyzed by γ-secretase. In the absence of non-visual β-arrestin Kurz, Deltex may lead to Notch endocytosis and activation. The amount of Notch available at the membrane is controlled by many endocytosis-recycling mechanisms. Several E3 ligases (Itch, CBL, Nedd4, the Deltex-Kurz complex) can target Notch for degradation. The ESCRT complex and lgd in Drosophila (and presumably their homologues in mammals) control Notch degradation, and their loss causes accumulation of Notch in endosomes and ligand-independent activation. In actively dividing cells, Numb/ACBD3 asymmetrically partitions to one daughter cell, causing selective Notch degradation in it. GSI, monoclonal antibodies (mAbs) to Notch receptors and ligands and Notch decoy molecules have been used effectively in vivo to inhibit Notch signaling. B: nuclear events. N^IC is transported to the nucleus, where it causes the dissociation of the co-repressor complex including SHARP, SKIP and several other proteins (CoR) from CSL. Notch, CSL and MAML form a tertiary complex which in turn recruites p300 and other coactivators (CoA) to the chromatin and forming the NTC that activates transcription. The NTC can form heterodimers on the chromatin with other NTCs or supramolecular complexes with other transcription factors, modulating the choice of genes regulated by Notch. Dominant negative (DN) MAML constructs or peptidomimetic agents have been used in vivo to inhibit Notch-mediated transcriptional activation (see reference 5 for review).