In the canonical Notch signaling pathway, intramembrane cleavage by γ-secretase serves to release an intracellular domain of Notch that has activity in the nucleus through binding to transcription factors. In addition, we showed that Notch also supplies signals to Delta, a major Notch ligand, to release the intracellular domain of Delta by γ-secretase from the cell membrane, which then translocates to the nucleus, where it mediates the transcription of specific genes. Therefore, the Notch-Delta signaling pathway is bi-directional and similar mechanisms regulated by γ-secretase are involved in both directions. Recently, it was demonstrated that many type 1 transmembrane proteins including Notch, Delta and amyloid precursor protein (APP) are substrates for γ-secretase and release intracellular domains of these proteins from cell membranes. These observations that the common enzyme, γ-secretase, modulates proteolysis and the turnover of possible signaling molecules have led to the attractive hypothesis that mechanisms similar to the Notch-Delta signaling pathway may widely contribute to γ-secretase-regulated signaling pathways, including APP signaling which leads to Alzheimers disease. Here, we review the molecular mechanisms of the Notch-Delta signaling pathway in a bi-directional manner, and discuss the recent progress in understanding the biology of γ-secretase-regulated signaling with respect to neurodegeneration.
Keywords: Notch, Delta, γ-secretase, amyloid precursor protein (APP), type 1 transmembrane proteins, the regulated intramembrane proteolysis (RIP) mechanism