Abstract
gamma-Secretase, which is responsible for the intramembranous cleavage of Alzheimer beta-amyloid precursor protein and the signaling receptor Notch, is a multiprotein complex consisting of at least four components: presenilin (PS); nicastrin (Nct); APH-1 (anterior pharynx-defective-1); and presenilin enhancer-2 (PEN-2). Presenilin 1 (PS1) is known to be essential for the stability, interaction, and trafficking of the other PS1/gamma-secretase components. However, the precise functions of the other components remain elusive. Here, we investigated the functions of Nct within the PS1/gamma-secretase complex. We demonstrated that the loss of Nct expression in the embryonic fibroblast cells (Nct KO cells) results in dramatically decreased levels of APH-1, PEN-2, and PS1 fragments accompanied by a significant accumulation of full-length PS1. In the absence of Nct, PEN-2 and full-length PS1 are subjected to proteasome-mediated degradation, whereas the degradation of APH-1 is mediated by both proteasomal and lysosomal pathways. Unlike the case of wild type cells in which the gamma-secretase complex mainly locates in the trans-Golgi network, the majority of residual PEN-2, APH-1, and the uncleaved full-length PS1 in Nct KO cells reside in the endoplasmic reticulum, which remain associated with each other in the absence of Nct. Interestingly, significant amounts of full-length PS1 and PEN-2, but not APH-1, are detected on the plasma membrane in Nct KO cells, suggesting the Nct-independent cell surface delivery of the PEN-2.PS1. Finally, the diminished PEN-2 protein level in Nct-deficient cells can be partially restored by overexpression of exogenous PS1, APH-1, or PEN-2 individually or collectively, indicating a dispensable role for Nct in controlling PEN-2 level. Taken together, our study demonstrates a critical role of Nct in the stability and proper intracellular trafficking of other components of the PS1/ gamma-secretase complex but not in maintaining the association of PEN-2, APH-1, and full-length PS1.
Highlights
␥-Secretase, which is responsible for the intramembranous cleavage of Alzheimer -amyloid precursor protein and the signaling receptor Notch, is a multiprotein complex consisting of at least four components: presenilin (PS); nicastrin (Nct); APH-1; and presenilin enhancer-2 (PEN-2)
Consistent with previous notions that the Nct KO exhibits similar phenotypes to those of the PS double KO [36], we found that the levels of Presenilin 1 (PS1) NTF, APH-1, and PEN-2 were dramatically decreased in Nct KO cells
PEN-2 levels were reduced in both Nct KO fibroblasts and PS1/PS2 double knockout (PS 2KO) cells, APH-1 level was dramatically reduced in Nct KO cells but remained stable in PS 2KO cells
Summary
␥-Secretase, which is responsible for the intramembranous cleavage of Alzheimer -amyloid precursor protein and the signaling receptor Notch, is a multiprotein complex consisting of at least four components: presenilin (PS); nicastrin (Nct); APH-1 (anterior pharynx-defective-1); and presenilin enhancer-2 (PEN-2). Biochemical evidence demonstrates that ␥-secretase activity resides in a high molecular weight multiprotein complex composed of at least four components: presenilin 1 or 2 (PS1 or PS2), nicastrin (Nct), APH-1a or b and PEN-2 [2, 3]. Nct appears to interact more closely with APH-1 because down-regulation of Nct by small interfering RNA (siRNA) results in a reduced APH-1 level [15, 16] Based on these observations, it has been proposed that Nct may first bind to APH-1 to form a Nct1⁄7APH-1 precursor subcomplex [17, 18] prior to further association with PS1 and PEN-2 or with PS11⁄7PEN-2 subcomplex. Roles of Nicastrin in Controlling PS/␥-Secretase Complex (TGN) to the cell surface upon treatments with certain ␥-secretase inhibitors further sustains this notion [19]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
