Unraveling the complexity of γ-secretase

Abstract

γ-Secretase was initially defined as a proteolytic activity that cleaves within the transmembrane of the amyloid precursor protein (APP) to produce the amyloid β-peptide of Alzheimer's disease. The discovery of mutations in APP and the presenilins associated with familial Alzheimer's disease and their effects on APP processing dovetailed with pharmacological studies on γ-secretase, leading to the revelation that presenilins are unprecedented membrane-embedded aspartyl proteases. Other members of what became known as the γ-secretase complex were subsequently identified. In parallel with these advances, connections between presenilins and Notch receptors essential to metazoan development became evident, resulting in the concurrent realization that γ-secretase also carries out intramembrane proteolysis of Notch as part of its signaling mechanism. Substantial progress has been made toward elucidating how γ-secretase carries out complex processing of transmembrane domains, how it goes awry in familial Alzheimer's disease, the scope of its substrates, and the atomic details of its structure. Critical questions remain for future study, toward further unraveling the complexity of this unique membrane-embedded proteolytic machine and its roles in biology and disease.

Keywords: Alzheimer’s disease; Amyloid; Membrane proteins; Notch; Protease.

Figure 1.

Substrates for γ-secretase. (A) Proteolytic processing of APP and Notch. The ectodomain of APP is first shed by β-secretase. Alternatively, APP can be cut within the Aβ region by α-secretases (not shown). The remaining membrane-associated stub is cleaved at least twice in the transmembrane region, at the γ site to produce Aβ and at the ε site to produce the intracellular domain (AICD). The Notch receptor is first processed in the secretory pathway at the S1 site by furin (not shown) to produce a heterodimer. Upon activation by ligand, this heterodimer is first cleaved at the S2 site by metalloproteases to shed the ectodomain, and the remaining membrane-associated stub is cleaved within the transmembrane domain at the S3 and S4 sites. The transmembrane cleavage events are carried out by the presenilin-containing γ-secretase complex. (B) A partial list of the ~90 other type I integral membrane proteins cleaved within their transmembrane regions by γ-secretase.

Figure 2.

Components and assembly of the γ-secretase complex. γ-Secretase is composed of four different integral membrane proteins: presenilin, nicastrin, Aph-1, and Pen-2. Presenilin undergoes endoproteolysis into an N-terminal fragment (NTF) and a C-terminal fragment (CTF) that remain associated. Two conserved aspartates within adjacent transmembrane domains are essential for both presenilin endoproteolysis and γ-secretase activity.

Figure 3.

Processive proteolysis of the APP transmembrane domain by γ-secretase. An endoproteolytic activity of the enzyme cleaves at the ε site within the transmembrane domain close to the membrane-cytosol interface to give long Aβ peptides Aβ48 and Aβ49 and the APP intracellular domain (AICD). The carboxypeptidase activity of γ-secretase then trims Aβ48 and Aβ49 in 3–4 amino acid increments along two pathways: (A) Aβ49→Aβ46→Aβ43→Aβ40 and (B) Aβ48→Aβ45→Aβ42→Aβ38.

Figure 4.

First detailed structure of the γ-secretase complex determined by cryo-EM, single-particle analysis, and image reconstruction. (A) Nicastrin: green; Aph-1: yellow; Pen-2: magenta; PSEN1: cyan (NTF) and aquamarine (CTF), with catalytic aspartates in red. PSEN1 TMD 2 was not resolved. (B) The active site on PSEN1, with D257 in TMD 6 and D385 in TMD 7 in close proximity. PDB: 5A63.

Figure 5.

γ-Secretase bound to APP-derived substrate. APP substrate is located inside PSEN1. The substrate TMD assumes a β-strand conformation near the cytoplasmic side as it interacts with the active site. PSEN1 TMD 2 is now visible (foreground), as is the cytoplasmic side of TMD 6 (cf. Figure 4). PDB: 6IYC.