Presenilin-Dependent γ-Secretase on Plasma Membrane and Endosomes Is Functionally Distinct

Abstract

The presenilin (PS)/gamma-secretase complex, which contains not only PS but also Aph-1, PEN-2, and nicastrin, mediates proteolysis of the transmembrane domain of beta-amyloid protein precursor (betaAPP). Intramembrane proteolysis occurs at the interface between the membrane and cytosol (epsilon-site) and near the middle of the transmembrane domain (gamma-site), generating the betaAPP intracellular domain (AICD) and Alzheimer disease-associated Abeta, respectively. Both cleavage sites exhibit some diversity. Changes in the precision of gamma-cleavage, which potentially results in secretion of pathogenic Abeta42, have been intensively studied, while those of epsilon-cleavage have not. Although a number of PS-associated factors have been identified, it is unclear whether any of them physiologically regulate the precision of cleavage by PS/gamma-secretase. Moreover, there is currently no clear evidence of whether PS/gamma-secretase function differs according to the subcellular site. Here, we show that endocytosis affects the precision of PS-dependent epsilon-cleavage in cell culture. Relative production of longer AICDepsilon49 increases on the plasma membrane, whereas that of shorter AICDepsilon51 increases on endosomes; however, this occurs without a concomitant major change in the precision of cleavage at gamma-sites. Moreover, very similar changes in the precision of epsilon-cleavage are induced by alteration of the pH. Our findings demonstrate that the precision of epsilon-cleavage by PS/gamma-secretase changes depending upon the conditions and the subcellular location. These results suggest that the precision of cleavage by the PS/gamma-secretase complex may be physiologically regulated by the subcellular location and conditions.

Evidence that Enzyme Processivity Mediates Differential Aβ Production by PS1 and PS2

“…However, as with most reports, the study of TMP21 only examined PS1-containing complexes so it remains unclear whether TMP21 or some other factor might be important for regulating the different PS1- and PS2-mediated AICD:Aβ product ratios. In addition, factors such as enzyme and substrate sub-cellular localization as well as pH have been reported to influence microheterogeneity in ε-site cleavage specificity [42], although no influence on the AICD:Aβ (i.e., ε-site vs. γ-site) product ratio has been noted. …”

Presenilin mutations linked to familial Alzheimer's disease cause an imbalance in phosphatidylinositol 4,5-bisphosphate metabolism

“…Alternatively, PIP 2 may modulate the trafficking of the ␥-secretase complex (or its individual components), thereby influencing the final activity of the complex. Recent reports demonstrate that the targeting of the ␥-secretase complex into different subcellular sites has direct consequences on the functional properties of the ␥-secretase enzyme (38,48). Although the exact mechanism of the PIP 2 effect on ␥-secretase activity remains to be elucidated, modulation of PIP 2 may represent an approach to control A␤ production.…”

Trafficking and Proteolytic Processing of APP

“…The evidence supporting such a model is manifold and, because of space constraints, only key arguments will be highlighted: (1) APP/C99/C83 are not cleaved by g-secretase in the ER (Cupers et al 2001;Maltese et al 2001;Grimm et al 2003;Kaether et al 2006b); (2) by immuno-electron microscopy, PS1 is not detected in Golgi or TGN but in ER and PM (Rechards et al 2003); whereas the ER pool most likely reflects unassembled PS1, the PM pool has assembled, active complex; (3) only the mature, glycosylated NCT, not immature unglycosylated NCT, is present in the fully assembled, active g-secretase complex Kaether et al 2002), and therefore g-secretase-associated NCTmust have passed the Golgi; (4) g-secretase has been shown by various methods, including cell surface biotinylation, binding of biotinylated inhibitors specific for the active complex, and microscopic techniques, to be present at the PM (Kaether et al 2002;Tarassishin et al 2004;Chyung et al 2005). It has been shown that the 1-cleavage of APP differs in endosomes and PM (Fukumori et al 2006), suggesting that g-secretase has different properties depending on its subcellular localization, maybe because of differences in pH or lipid composition. One can speculate that changing the ratio of gsecretase present in the different subcellular compartments might have an impact on AD pathology.…”