The γ-secretase complex
BACE1-digested APP CTFs are subsequently cleaved by the γ-secretase complex to release Aβ. In addition, γ-secretase cleaves a series of functionally important substrates such as NOTCH [
138] and tyrosinase [
139]. γ-secretase activity is produced from a high molecular weight complex consisting of at least four transmembrane components: presenilin (PS, with two mammalian homologs as PS1 and PS2), nicastrin, anterior pharynx-defective-1 (APH-1), and presenilin enhancer-2 (PEN2) [
140,
141]. PS1/2 have an undetermined number of transmembrane domains [
142] and undergo endoproteolytic cleavage to generate PS amino-terminal fragments (NTFs) and CTFs. PS NTF and CTF form a functional heterodimer, each contributing a highly conserved aspartate residue indispensable for γ-secretase activity [
143]. Nicastrin is a type I transmembrane glycoprotein considered to be the scaffolding protein within the γ-secretase complex. Nicastrin may also act as the γ-secretase receptor [
144]. The seven-transmembrane APH-1 interacts with nicastrin to form a stable intermediate in an early assembly stage of the γ-secretase complex [
140,
141], whereas the two-pass transmembrane component PEN2 regulates PS endoproteolysis [
145,
146]. Each of these four components is necessary for γ-secretase activity and deficiency in any of these factors dramatically impairs the enzymatic activity.
Assembly of the γ-secretase complex and its trafficking route within the cell
Although aspects of γ-secretase complex formation and distribution have yet to be fully elucidated, a predicted model for the assembly, maturation, and trafficking of the γ-secretase complex has been proposed. In this model, the four components of the γ-secretase complex are synthesized in the ER, where they await incorporation into stable complex. Complex formation initially begins with the association of APH-1 and nicastrin to form an early intermediate subcomplex. Full-length PS is then incorporated into and stabilized within the complex. Subsequent assembly of PEN2 drives the conversion of the full-length PS into active NTF/CTF heterodimers in ER and Golgi compartments [
140,
147]. During its transit through Golgi/TGN, nicastrin undergoes complete glycosylation, thus transforming the γ-secretase complex into its mature form. The γ-secretase complex in large part shuttles between the ER and Golgi, whereas stabilized mature γ-secretase complex is transported to the plasma membrane. γ-secretase at the membrane can then undergo endocytosis to endosomes, and subsequent trafficking to late endosomes/multivesicular bodies or the lysosome (Figure
1) [
148‐
151].
Factors regulating γ-secretase trafficking and activity
In addition to its function as the catalytic core of the γ-secretase complex, PS1 has been shown to modulate trafficking of several transmembrane proteins including APP [
75]. Since PS1 interacts with a series of trafficking modulator proteins such as RAB family members and PLD1, PS1 might exert its trafficking regulation through these interactions [
152]. Conversely, these trafficking modulators may also reciprocally regulate PS1 trafficking. For example, we find that PLD1 interacts with PS1 and overexpression of PLD1 promotes cell surface accumulation of PS1 in an APP-independent manner [
153]. A trafficking regulation role has been proposed for APP [
154] and we also find that APP can reciprocally regulate the intracellular trafficking of PS1 and other γ-secretase components. APP deficiency results in elevated transportation of PS1/γ-secretase from TGN to the cell surface, thereby enhancing NOTCH cleavage [
153]. Additionally, APP can function as a kinesin-I membrane receptor and mediates axonal transport of PS1 and BACE1 [
155].
The recently identified postsynaptic protein ARC also interacts with PS1 and participates in sorting PS1 to early and recycling endosomes to enhance amyloidogenic APP processing. Aβ production can be blocked by interrupting ARC association with PS1/γ-secretase and genetic deletion of ARC decreases Aβ load in a transgenic AD mouse model [
156].
The G protein-coupled receptor (GPCR) β2-adrenergic receptor is associated with PS1 where agonist-induced β2-adrenergic receptor endocytosis mediates trafficking of PS1/γ-secretase to late endosomes and lysosomes to enhance Aβ production [
157]. Another GPCR, GPR3 is also found to be a modulator of Aβ production: overexpression of GPR3 increases the formation and cell surface localization of the mature γ-secretase components, accompanied with stimulated Aβ generation but not NOTCH processing. In contrast, GPR3 deficiency prevents Aβ accumulation both
in vitro and
in vivo. Moreover, GPR3 is highly expressed in normal human brain regions implicated in AD and its level is elevated in sporadic AD brain [
158]. Interestingly, recently it was suggested that both β2-adrenergic receptor and GPR3 exert their effects on Aβ generation through interacting with β-arrestin 2, a member of the arrestin family [
159]. Arrestins are GPCR-interacting scaffold proteins and can inhibit G protein coupling to GPCRs, leading to GPCR desensitization and even mediating a G protein-independent signaling pathway [
160]. β-arrestin 2 is found to interact with APH-1, redistribute the γ-secretase complex toward detergent-resistent membranes and increase its catalytic activity [
159]. The degree of Aβ production stimulation is also correlated with GPCR-β-arrestin 2 binding and receptor trafficking to endocytic vesicles [
161]. Moreover, a subset of GPCRs including GPR3 and PTGER2, are found to be associated with APP when internalized via β-arrestin 2, thereby increasing APP amyloidogenic processing as well [
161]. Coincidently, another arrestin family member, β-arrestin 1, can interact with APH-1 and facilitate the formation of nicastrin/APH-1 subcomplex and mature γ-secretase complex. β-arrestin 1 expression is upregulated in the brains of sporadic AD patients and transgenic AD mice, whereas deletion of β-arrestin 1 in AD mice results in reduced Aβ generation, diminished Aβ pathology and improved cognition [
162].
The retention in ER sorting receptor 1 (RER1), a membrane factor mediating protein retrieval from Golgi to the ER, has been found to regulate γ-secretase trafficking. In one study, RER1 was found to bind to the transmembrane region of nicastrin and retrieve unincorporated nicastrin from the cis-Golgi back to the ER. Since RER1 competes with APH-1 for nicastrin binding, depletion of RER1 resulted in elevated γ-secretase complex assembly/activity and increased Aβ secretion [
163]. In another study, RER1 was found to bind to the first transmembrane domain of PEN2 and retrieve unassembled PEN2 to the ER [
164]. Both studies suggest that RER1 may be critical in monitoring quality control during γ-secretase complex assembly. Recently, RER1 overexpression was also shown to decrease both γ-secretase localization on the cell surface and Aβ secretion, whereas RER1 downregulation had opposite effects [
165]. Furthermore, increased RER1 levels could decrease mature APP and increase immature APP, resulting in less surface accumulation of APP [
165].
ATP-binding cassette transporter-2 (ABCA2) is a component responsible for retrograde trafficking of lipoprotein-derived cholesterol from late endosome/lysosome to the ER, and has been genetically linked to AD [
166,
167]. It has been recently shown that ABCA2 depletion can reduce γ-secretase complex formation due to alterations in protein levels, post-translational modification, and subcellular localization of nicastrin, thus affecting the γ-processing of APP and Aβ generation [
168].