Both α-cleavage and β-cleavage generate short APP C-terminal fragments that are further processed by γ-secretase. Distinct from α-/β-secretases, γ-activity involves a large proteinase complex consisting of at least four major protein components (Presenilin1 or Presenilin2, PEN2, APH1 and Nicastrin) (Vetrivel et al. 2006
Presenilins (PSs) were identified and cloned in the mid-1990s (Levy-Lahad et al. 1995
; Sherrington et al. 1995
). Genetic mutations of PSs are found in a large portion of familiar AD (FAD) cases, indicating its crucial role in AD. Although other proteins are required for the γ-secretase complex, PSs are believed to contain the actual protease activity (Wolfe et al. 1999
; Wen et al.
2008; Ahn et al.
2010a). PSs are multi-transmembrane proteins and can be cleaved at the cytoplasmic loop between the 6th
transmembrane regions to generate an N terminal and a C terminal fragment during post-translational maturation (Thinakaran et al. 1996
). The two fragments interact with each other after the cleavage and they are both necessary for γ-secretase activity. Transgenic mice overexpressing PSs with FAD mutations show significantly increased Aβ42 levels, suggesting that PS mutations probably induce AD by producing more of the hydrophobic Aβ42 form (Duff et al. 1996
; Xia et al.
Nicastrin, identified as a protein that interacts with PS in 2000 (Yu et al. 2000
), is a type I membrane glycoprotein with a large ectodomain (Perrin et al.
2009). Nicastrin undergoes a glycosylation/maturation process that causes a conformation change in its ectodomain, which is crucial for the assembly and maturation of the γ-secretase complex and γ-activity (Shirotani et al. 2003
; Chavez-Gutierrez et al. 2008
). Mature nicastrin can bind to the ectodomain of APP CTFs derived through α-/β-secretase cleavage and may act as a substrate receptor of γ-secretase (Shirotani et al. 2003
; Kaether et al. 2004
; Shah et al. 2005
PEN2 and APH1 are another two γ-secretase complex components that were originally identified as the enhancers of PSs (Francis et al. 2002
). APH1 is a multiple transmembrane protein with seven transmembrane domains and a cytosolic C terminus (Fortna et al. 2004
). APH1 interacts with immature nicastrin and PS to form a relatively stable pre-complex which is then translocated to the trans-Golgi from the ER/cis-Golgi for further maturation (Lee et al. 2002
; Kimberly et al.
2003; Takasugi et al. 2003
; Jankowsky et al.
2004). PEN2 is a hairpin-like protein with two transmembrane domains and with both ends in the lumen (Crystal et al. 2003
; Takasugi et al. 2003
). PEN2 is found to mediate the endoproteolysis of PS (Luo et al. 2003
). Enhanced γ-secretase activity is also observed when PEN2 is exogenously expressed (Shiraishi et al. 2004
The γ-secretase complex is assembled in sequential steps. Nicastrin and APH1 initially form a subcomplex (Shirotani et al. 2004
) and then PS binds to the Nicastrin-APH1 subcomplex (Takasugi et al. 2003
). The joining of PEN2 results in a conformation-dependent activation of γ-secretase (Kimberly et al.
2003; Takasugi et al. 2003
). Nicastrin, PEN2, APH1 and PS interact with each other and also mutually modulate each other (Lee et al. 2002
; Steiner et al. 2002
; Pasternak et al. 2003
; Kaether et al. 2004
). Nicastrin ablation leads to decreased levels of APH1, PEN2 and PS1 fragments, accompanied by increased levels of immature full-length PS1 (Zhang et al. 2005
). Downregulation of APH1, PEN2 or nicastrin also reduces the processing of PS and results in impaired γ-cleavage of APP and Notch (Francis et al. 2002
). PS deficiency also results in decreased levels of PEN2 and APH1, as well as impaired glycosylation/maturation of Nicastrin (Zhang et al. 2005
γ-secretase cleaves APP at multiple sites and in sequential steps to generate Aβ peptides of different lengths (). The majority of Aβ peptides produced are 40 amino acids long, however, peptides ranging from 38 to 43 amino acids are found in vivo
. Besides the dominant γ-cleavage site at 40 and 42 residues, ζ-cleavage at 46 and
-cleavage at 49 residues are also thought to be mediated by γ-secretase (Weidemann et al. 2002
; Zhao et al. 2004
; Raben et al.
2005; Sato et al. 2005
). Accordingly, various AICDs (C50, C53, C57 and C59) can be generated during these multi-site cleavages executed by γ-secretase. However, all of the endogenous AICD forms are rarely detected, probably due to their very rapid degradation (Ag 2000
; Lu et al. 2000
; Sastre et al. 2001
; Yu et al. 2001
; Sato et al. 2003
). Interestingly, as the substrate of γ-secretase, APP itself can regulate the intracellular trafficking and cell surface delivery of PS1 (Kuzuya et al.
2007; Liu et al. 2009
). In addition, APP has been found to possess a domain that negatively modulates γ-secretase activity in Aβ production by binding to an allosteric site within the γ-secretase complex (Kuzuya et al.
2007; Ahn et al.
2010b; Zhang and Xu 2010
). These results reveal a novel mutual regulation between γ-secretase and its substrate.
In addition to cleaving APP, γ-secretase cleaves many other single transmembrane proteins within the transmembrane domain (Lee et al. 2002
). One of the most important γ-secretase substrates, Notch, can release its intracellular domain (NICD) upon γ-cleavage. NICD is well-known as a signal molecule that transactivates a number of genes critical to development (Kopan et al. 1996
; Schroeter et al. 1998
). Mice with ablation of PS1, Nicastrin, or APH1A show Notch-deficient-like lethal phenotypes and neuronal tube formation defects (Shen et al. 1997
; Donoviel et al. 1999
; Li et al. 2003
; Ma et al. 2005
). Postnatal forebrain-specific inactivation of PS1 in APP transgenic mice, although preventing Aβ accumulation, failed to rescue contextual memory long-term. Conditional inactivation of γ-secretase components in the forebrain resulted in progressive memory impairment and age-related neurodegeneration (Dominguez et al.
2005; Saura et al. 2005
; Serneels et al. 2009
; Tabuchi et al. 2009