A detergent-insoluble membrane compartment contains Aβ in vivo

Ordered assembly of the amyloid-beta protein (A beta) into amyloid fibrils is a critical step in Alzheimer's disease (AD). To release the amyloidogenic peptide A beta from the Alzheimer amyloid precursor protein (APP), two secretases act sequentially: first, beta-secretase cleaves close to the membrane within the ectodomain and then gamma-secretase cuts within the transmembrane domain(1). The sites of gamma-secretase cleavage are after residues 40 or 42 of A beta. Except in those rare cases of AD caused by a mutation, levels of secreted A beta are not elevated; thus, the secretory pathway may be unaffected, and factors other than the extracellular concentration of A beta may contribute to the aggregation properties of the peptide. A beta is also present in intracellular compartments(2-5). The two gamma-secretase cleavage products, A beta 42 and A beta 40, were found in different compartments: A beta 42 in the endoplasmic reticulum (ER)/intermediate compartment(3-5), and A beta 40 in the trans-Golgi network(2,4) (TGN). The cellular compartments that harbor A beta are target sites for therapeutic intervention. Here we report that in the brain, the principal compartment in which A beta resides is a detergent-insoluble glycolipid-enriched membrane domain (DIG). Also present in the DIC fractions are the endoproteolytic fragments of presenilin-1 (PS1) and APP. The presence of these proteins, which all contribute to the generation of A beta, indicates that the DIG fraction is probably where the intramembranous cleavage of APP occurs.