β-Secretase (BACE, Asp-2) is a transmembrane aspartic proteinase responsible for cleaving the amyloid precursor protein (APP) to generate the soluble ectodomain sAPPβ and its C-terminal fragment CTFβ. CTFβ is subsequently cleaved by γ-secretase to produce the neurotoxic/synaptotoxic amyloid-β peptide (Aβ) that accumulates in Alzheimer's disease. Indirect evidence has suggested that amyloidogenic APP processing may preferentially occur in lipid rafts. Here, we show that relatively little wild-type BACE is found in rafts prepared from a human neuroblastoma cell line (SH-SY5Y) by using Triton X-100 as detergent. To investigate further the significance of lipid rafts in APP processing, a glycosylphosphatidylinositol (GPI) anchor has been added to BACE, replacing the transmembrane and C-terminal domains. The GPI anchor targets the enzyme exclusively to lipid raft domains. Expression of GPIBACE substantially up-regulates the secretion of both sAPPβ and amyloid-β peptide over levels observed from cells overexpressing wild-type BACE. This effect was reversed when the lipid rafts were disrupted by depleting cellular cholesterol levels. These results suggest that processing of APP to the amyloid-β peptide occurs predominantly in lipid rafts and that BACE is the rate-limiting enzyme in this process. The processing of the APP₆₉₅isoform by GPI-BACE was up-regulated 20-fold compared with wild-type BACE, whereas only a 2-fold increase in the processing of APP_751/770was seen, implying a differential compartmentation of the APP isoforms. Changes in the local membrane environment during aging may facilitate the cosegregation of APP and BACE leading to increased β-amyloid production.

The steady‐state level of amyloid β‐peptide (Aβ) represents a balance between its biosynthesis from the amyloid precursor protein (APP) through the action of the β‐ and γ‐secretases and its catabolism by a variety of proteolytic enzymes. Recent attention has focused on members of the neprilysin (NEP) family of zinc metalloproteinases in amyloid metabolism. NEP itself degrades both Aβ₁₋₄₀and Aβ₁₋₄₂in vitroandin vivo, and this metabolism is prevented by NEP inhibitors. Other NEP family members, for example endothelin‐converting enzyme, may contribute to amyloid catabolism and may also play a role in neuroprotection. Another metalloproteinase, insulysin (insulin‐degrading enzyme) has also been advocated as an amyloid‐degrading enzyme and may contribute more generally to metabolism of amyloid‐forming peptides. Other candidate enzymes proposed include angiotensin‐converting enzyme, some matrix metalloproteinases, plasmin and, indirectly, thimet oligopeptidase (endopeptidase‐24.15). This review critically evaluates the evidence relating to proteinases implicated in amyloid catabolism. Therapeutic strategies aimed at promoting Aβ degradation may provide a novel approach to the therapy of Alzheimer's disease.

Amyloidogenic processing of the amyloid precursor protein (APP) by β- and γ-secretases generates several biologically active products, including amyloid-β (Aβ) and the APP intracellular domain (AICD). AICD regulates transcription of several neuronal genes, especially the Aβ-degrading enzyme, neprilysin (NEP). APP exists in several alternatively spliced isoforms, APP(695), APP(751), and APP(770). We have examined whether each isoform can contribute to AICD generation and hence up-regulation of NEP expression. Using SH-SY5Y neuronal cells stably expressing each of the APP isoforms, we observed that only APP(695) up-regulated nuclear AICD levels (9-fold) and NEP expression (6-fold). Increased NEP expression was abolished by a β- or γ-secretase inhibitor but not an α-secretase inhibitor. This correlated with a marked increase in both Aβ(1-40) and Aβ(1-42) in APP(695) cells as compared with APP(751) or APP(770) cells. Similar phenomena were observed in Neuro2a but not HEK293 cells. SH-SY5Y cells expressing the Swedish mutant of APP(695) also showed an increase in Aβ levels and NEP expression as compared with wild-type APP(695) cells. Chromatin immunoprecipitation revealed that AICD was associated with the NEP promoter in APP(695), Neuro2a, and APP(Swe) cells but not APP(751) nor APP(770) cells where AICD was replaced by histone deacetylase 1 (HDAC1). AICD occupancy of the NEP promoter was replaced by HDAC1 after treatment of the APP(695) cells with a β- but not an α-secretase inhibitor. The increased AICD and NEP levels were significantly reduced in cholesterol-depleted APP(695) cells. In conclusion, Aβ and functional AICD appear to be preferentially synthesized through β-secretase action on APP(695).

Lipid rafts are membrane domains, more ordered than the bulk membrane and enriched in cholesterol and sphingolipids. They represent a platform for protein-lipid and protein–protein interactions and for cellular signaling events. In addition to their normal functions, including membrane trafficking, ligand binding (including viruses), axonal development and maintenance of synaptic integrity, rafts have also been implicated in the pathogenesis of several neurodegenerative diseases including Alzheimer’s disease (AD). Lipid rafts promote interaction of the amyloid precursor protein (APP) with the secretase (BACE-1) responsible for generation of the amyloid β peptide, Aβ. Rafts also regulate cholinergic signaling as well as acetylcholinesterase and Aβ interaction. In addition, such major lipid raft components as cholesterol and GM1 ganglioside have been directly implicated in pathogenesis of the disease. Perturbation of lipid raft integrity can also affect various signaling pathways leading to cellular death and AD. In this review, we discuss modulation of APP cleavage by lipid rafts and their components, while also looking at more recent findings on the role of lipid rafts in signaling events.