Notch was first described in 1919 and has been implicated in a variety of developmental processes including somite formation, border definition, anterior and posterior identification, neurogenesis, angiogenesis, and lymphoid development.1-3 The discovery of Notch originated with the description of a mutant fly phenotype characterized by notches in fly wing margins.1 Subsequent studies revealed that the neurogenic phenotype of Notch absence is neuronal hyperplasia due to the increased inability of dermoblasts to differentiate from ectodermal cells to neuroblasts.4-6
|Figure 1. Heterodimeric, membrane-bound Notch is activated by Delta or Jagged ligands thus eliciting cleavage of the intracellular Notch fragment by Presenilin. The Notch fragment translocates to the nucleus, binds various regulator factors including CBF-1, and modifies transcription of gene loci such as HES. HES gene products then influence development via their action on the expression of AS-C genes.
Notch is a large (~300 kDa), single pass membrane protein expressed in both embryonic and adult cells.7 The extracellular domain of the protein is composed of 29 to 36 EGF-like repeats and three cysteine-rich Notch/Lin-12 repeats involved in ligand binding. The intracellular domain contains one RAM domain, six ankyrin repeats, one to two nuclear localization sequences, and one PEST domain (involved in degradation and phosphorylation).8,9 Notch is cleaved in the trans-Golgi by a furin-like protease, thus generating extracellular and intracellular fragments that associate non-covalently forming heterodimers on the cell surface.10 Notch signaling occurs following interaction with Delta, Jagged-1, and Jagged-2 ligands.8 Notch and Delta or Jagged binding results in cleavage of Notch within the membrane by Presenilin-1 gamma-secretase activity. The resultant active intracellular Notch fragment translocates to the nucleus to regulate transcription.8,11
A large number of both positive and negative regulators of Notch activity bind the ankyrin repeat region of the intracellular Notch fragment.8 The RAM domain, as well as the ankyrin repeat region, binds the transcriptional repressor, C-promoter binding factor 1 (CBF1). The Notch/CBF1 complex possesses histone acetylase and deacetylase activity thus enabling it to access the genome and affect transcription.8 It binds the Hairy and Enhancer of Split (HES) gene locus upregulating expression to yield a number of basic helix-loop-helix (bHLH) proteins.8,12 bHLH proteins are capable of targeting the Achaete-Scute gene complex (AS-C) which is essential for proper differentiation of pro-neural cells to neural precursor cells.13 The net action of Notch ligands, Notch, and associated downstream effector molecules (through AS-C) is to inhibit pro-neural cells from becoming neuronal cells.14
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