Similar Mechanisms Regulated by γ-Secretase are Involved in Both Directions of the Bi-Directional Notch-Delta Signaling Pathway as well as Play a Potential Role in Signaling Events Involving Type 1 Transmembrane Proteins

In the canonical Notch signaling pathway, intramembrane cleavage by gamma-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 gamma-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 gamma-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 gamma-secretase and release intracellular domains of these proteins from cell membranes. These observations that the common enzyme, gamma-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 gamma-secretase-regulated signaling pathways, including APP signaling which leads to Alzheimer's 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 gamma-secretase-regulated signaling with respect to neurodegeneration.