Alzheimer's disease is characterized by the deposits of the 4-kDa amyloid ␤ peptide (A␤). The A␤ protein precursor (APP) is cleaved by ␤-secretase to generate a C-terminal fragment, CTF␤, which in turn is cleaved by ␥-secretase to generate A␤. Alternative cleavage of the APP by ␣-secretase at A␤16/17 generates the C-terminal fragment, CTF␣. In addition to A␤, endoproteolytic cleavage of CTF␣ and CTF␤ by ␥-secretase should yield a C-terminal fragment of 57-59 residues (CTF␥). However, CTF␥ has not yet been reported in either brain or cell lysates, presumably due to its instability in vivo. We detected the in vitro generation of A␤ as well as an ϳ6-kDa fragment from guinea pig brain membranes. We have provided biochemical and pharmacological evidence that this 6-kDa fragment is the elusive CTF␥, and we describe an in vitro assay for ␥-secretase activity. The fragment migrates with a synthetic peptide corresponding to the 57-residue CTF␥ fragment. Three compounds previously identified as ␥-secretase inhibitors, pepstatin-A, MG132, and a substrate-based difluoroketone (t-butoxycarbonyl-Val-Ile-(S)-4-amino-3-oxo-2,2-difluoropentanoyl-Val-Ile-OMe), reduced the yield of CTF␥, providing additional evidence that the fragment arises from ␥-secretase cleavage. Consistent with reports that presenilins are the elusive ␥-secretases, subcellular fractionation studies showed that presenilin-1, CTF␣, and CTF␤ are enriched in the CTF␥-generating fractions. The in vitro ␥-secretase assay described here will be useful for the detailed characterization of the enzyme and to screen for ␥-secretase inhibitors.

The amyloid b-protein (Ab) deposited in Alzheimer's disease (AD) is a normally secreted proteolytic product of the amyloid b-protein precursor (APP). Generation of Ab from the APP requires two sequential proteolytic events: an initial b-secretase cleavage at the amino terminus of the Ab sequence followed by g-secretase cleavage at the carboxyl terminus of Ab. We describe the development of a robust in vitro assay for g-secretase cleavage by showing de novo Ab production in vitro and establish that this assay monitors authentic gamma-secretase activity by documenting the production of a cognate g-CTF, confirming the size of the Ab produced by mass spectrometry, and inhibiting cleavage in this system with multiple inhibitors that alter g-secretase activity in living cells. Using this assay, we demonstrate that the g-secretase activity 1) is tightly associated with the membrane, 2) can be solubilized, 3) has a pH optimum of 6.8 but is active from pH 6.0 to pH >8.4, and 4) ascertain that activities of the g-40 and g-42 are indeed pharmacologically distinct. These studies should facilitate the purification of the protease or proteases that are responsible for this unusual activity, which is a major therapeutic target for the treatment of AD.

The Alzheimer's amyloid beta protein (A beta) precursor (APP) is proteolytically cleaved by beta-secretase to N- and C-terminal fragments sAPPbeta and CTFbeta, respectively. Subsequently, CTFbeta is cleaved by gamma-secretase to generate A beta. We previously showed that the levels of secreted A beta and sAPPbeta were significantly reduced upon removal of glycosylphosphatidylinositol (GPI)-anchored proteins from either primary brain cells or Chinese hamster ovary cultures. The results indicated that GPI-anchored proteins facilitated beta-secretase activity. In this report, we strengthen the previous findings by demonstrating that CTFbeta, like sAPPbeta, is also reduced upon removal of GPI-anchored proteins and that sAPPbeta does not accumulate in an intracellular compartment. This facilitation pathway does not appear to be important for the processing of a disease-linked mutant form of APP (670NL), known to be a superior beta-secretase substrate. A novel aspartyl protease, BACE, responsible for beta-secretase activity in the brain is not GPI-anchored. However, BACE in brain membranes accumulate in lipid rafts, a compartment marked by the accumulation of GPI-anchored proteins. This finding is consistent with the hypothesis that BACE interacts with GPI-anchored proteins that facilitate its activity possibly by chaperoning it into lipid rafts.

The Alzheimer's amyloid protein (A␤) is released from the larger amyloid ␤-protein precursor (APP) by unidentified enzymes referred to as ␤-and ␥-secretase. ␤-Secretase cleaves APP on the amino side of A␤ producing a large secreted derivative (sAPP␤) and an A␤-bearing C-terminal derivative that is subsequently cleaved by ␥-secretase to release A␤. Alternative cleavage of the APP by ␣-secretase at A␤16/17 releases the secreted derivative sAPP␣. In yeast, ␣-secretase activity has been attributed to glycosylphosphatidylinositol (GPI)-anchored aspartyl proteases. To examine the role of GPIanchored proteins, we specifically removed these proteins from the surface of mammalian cells using phosphatidylinositol-specific phospholipase C (PI-PLC). PI-PLC treatment of fetal guinea pig brain cultures substantially reduced the amount of A␤40 and A␤42 in the medium but had no effect on sAPP␣. A mutant CHO cell line (gpi85), which lacks GPI-anchored proteins, secreted lower levels of A␤40, A␤42, and sAPP␤ than its parental line (GPI؉). When this parental line was treated with PI-PLC, A␤40, A␤42, and sAPP␤ decreased to levels similar to those observed in the mutant line, and the mutant line was resistant to these effects of PI-PLC. These findings provide strong evidence that one or more GPI-anchored proteins play an important role in ␤-secretase activity and A␤ secretion in mammalian cells. The cell-surface GPI-anchored protein(s) involved in A␤ biogenesis may be excellent therapeutic target(s) in Alzheimer's disease.The amyloid that is invariably deposited in Alzheimer's disease (AD) 1 is composed of an approximately 4-kDa peptide (amyloid ␤-peptide, A␤) that is derived from a larger protein referred to as the amyloid ␤-protein precursor (APP) (1, 2). APP is a type I integral membrane glycoprotein with a large Nterminal extracellular domain, a single transmembrane domain, and a short cytoplasmic tail. The A␤ peptide begins 99 amino acids from the C terminus of APP, and it extends from the extracellular region to a point half-way through the APP membrane-spanning domain (1). A␤ is released from APP by cleavage on its N-and C-terminal ends by ␤-and ␥-secretase, respectively. ␤-Secretase cleavage before residue 1 of A␤ (672 of APP770) also releases the secreted derivative sAPP␤, whereas an alternative cleavage before residue 17 by ␣-secretase releases sAPP␣. In most culture systems tested, the predominant cleavage product is sAPP␣, and this may serve to prevent the production of A␤ (1). The proteolytic processing of APP to sAPP and A␤ is regulated by protein kinase C (3), protein tyrosine kinase (4), muscarinic receptors (5), and estrogens (6). The regulatory pathways involved are cell type-dependent, have little or no effect in some cell types, and normally stimulate the secretion of sAPP␣ while simultaneously reducing the secretion of A␤ (2). A metalloproteinase related to the tumor necrosis factor-␣ converting enzyme (7,8) can cleave APP to sAPP␣ upon activation of PKC by phorbol esters 2 (9, 10). Strong evidence that A␤ plays...