Accumulation of the amyloid-beta protein (Abeta) in the cerebral cortex is an early and invariant event in the pathogenesis of Alzheimer's disease. The final step in the generation of Abeta from the beta-amyloid precursor protein is an apparently intramembranous proteolysis by the elusive gamma-secretase(s). The most common cause of familial Alzheimer's disease is mutation of the genes encoding presenilins 1 and 2, which alters gamma-secretase activity to increase the production of the highly amyloidogenic Abeta42 isoform. Moreover, deletion of presenilin-1 in mice greatly reduces gamma-secretase activity, indicating that presenilin-1 mediates most of this proteolytic event. Here we report that mutation of either of two conserved transmembrane (TM) aspartate residues in presenilin-1, Asp 257 (in TM6) and Asp 385 (in TM7), substantially reduces Abeta production and increases the amounts of the carboxy-terminal fragments of beta-amyloid precursor protein that are the substrates of gamma-secretase. We observed these effects in three different cell lines as well as in cell-free microsomes. Either of the Asp --> Ala mutations also prevented the normal endoproteolysis of presenilin-1 in the TM6 --> TM7 cytoplasmic loop. In a functional presenilin-1 variant (carrying a deletion in exon 9) that is associated with familial Alzheimer's disease and which does not require this cleavage, the Asp 385 --> Ala mutation still inhibited gamma-secretase activity. Our results indicate that the two transmembrane aspartate residues are critical for both presenilin-1 endoproteolysis and gamma-secretase activity, and suggest that presenilin 1 is either a unique diaspartyl cofactor for gamma-secretase or is itself gamma-secretase, an autoactivated intramembranous aspartyl protease.

␥-Secretase catalyzes the intramembrane proteolysis of Notch, ␤-amyloid precursor protein, and other substrates as part of a new signaling paradigm and as a key step in the pathogenesis of Alzheimer's disease. This unusual protease has eluded identification, though evidence suggests that the presenilin heterodimer comprises the catalytic site and that a highly glycosylated form of nicastrin associates with it. The formation of presenilin heterodimers from the holoprotein is tightly gated by unknown limiting cellular factors. Here we show that Aph-1 and Pen-2, two recently identified membrane proteins genetically linked to ␥-secretase, associate directly with presenilin and nicastrin in the active protease complex. Coexpression of all four proteins leads to marked increases in presenilin heterodimers, full glycosylation of nicastrin, and enhanced ␥-secretase activity. These findings suggest that the four membrane proteins comprise the limiting components of ␥-secretase and coassemble to form the active enzyme in mammalian cells.

The beta-amyloid precursor protein (beta-APP), which is involved in the pathogenesis of Alzheimer's disease, and the Notch receptor, which is responsible for critical signalling events during development, both undergo unusual proteolysis within their transmembrane domains by unknown gamma-secretases. Here we show that an affinity reagent designed to interact with the active site of gamma-secretase binds directly and specifically to heterodimeric forms of presenilins, polytopic proteins that are mutated in hereditary Alzheimer's and are known mediators of gamma-secretase cleavage of both beta-APP and Notch. These results provide evidence that heterodimeric presenilins contain the active site of gamma-secretase, and validate presenilins as principal targets for the design of drugs to treat and prevent Alzheimer's disease.

The ␤-amyloid precursor protein (APP) is a ubiquitous receptor-like molecule without a known function. However, the recent recognition that APP and Notch undergo highly similar proteolytic processing has suggested a potential signaling function for APP. After ligand binding, Notch is cleaved by the ADAM-17 metalloprotease followed by an intramembrane cleavage mediated by ␥-secretase. The ␥-secretase cut releases the Notch intracellular domain (NICD), which enters the nucleus and modulates transcription. Because APP is processed similarly by ADAM-17 and ␥-secretase, we reasoned that the APP intracellular domain (AICD) has a role analogous to the NICD. We therefore generated a plasmid encoding the AICD sequence and studied the subcellular localization of the expressed protein (C60). Our results demonstrate that the cytoplasmic domain of APP is a highly labile fragment that is stabilized by forming complexes with Fe65 and can then enter the nucleus in neurons and non-neural cells. These findings strongly support the hypothesis that APP signals in the nucleus in a manner analogous to the function of Notch.The ␤-amyloid precursor protein (APP) 1 is a type I transmembrane protein that is proteolytically processed by three enzymatic activities (1). The large ectodomain is first cleaved at one of three sites close to the membrane by either ␣-or ␤-secretase to liberate a soluble APP extracellular piece (termed ␣-or ␤-APP s ). In doing so, ␣-secretase generates an 83-residue carboxyl-terminal fragment (CTF) C83, whereas ␤-secretase cuts at the other two sites to generate an 89-or 99-residue CTF (C89 and C99, respectively) (1, 2). These 3 CTFs are retained in the membrane and become substrates for an unusual intramembrane cleavage mediated by ␥-secretase, producing a heterogeneous set of products. The best characterized of these is the amyloid ␤-protein (A␤) derived from C99, which accumulates to high abundance in senile plaques and appears to play a central role in the etiology of Alzheimer's disease (AD) (1). Whereas the several A␤ species have been studied in great detail, the other products generated by ␥-secretase have received scant attention. One fragment of particular interest is the APP intracellular domain (AICD), the ϳ6-kDa extreme C terminus of APP that results from the ␥-secretase cleavage of the C83, C89, or C99 fragments.The potential importance of AICD has recently been emphasized by the recognition of similarities between APP and another type I transmembrane protein, Notch (3). Notch is a cell surface receptor that plays a critical role in many cell fate decisions during embryogenesis and adulthood (4). Following binding to its prototypical ligand Delta, Notch undergoes two cleavages, the first of which is performed by the metalloprotease ADAM-17, also called tumor necrosis factor-␣ converting enzyme (5, 6). It is subsequently cut by the presenilin-dependent ␥-secretase (7), releasing the Notch intracellular domain (NICD), a fragment analogous to AICD. After release from the membrane, the NICD fragment a...

Presenilin heterodimers apparently contain the active site of ␥-secretase, a polytopic aspartyl protease involved in the transmembrane processing of both the Notch receptor and the amyloid-␤ precursor protein. Although critical to embryonic development and the pathogenesis of Alzheimer's disease, this protease is difficult to characterize, primarily because it is a multicomponent complex of integral membrane proteins. Here the functional ␥-secretase complex was isolated by using an immobilized active site-directed inhibitor of the protease. Presenilin heterodimers and nicastrin bound specifically to this inhibitor under conditions tightly correlating with protease activity, whereas several other presenilin-interacting proteins (␤-catenin, calsenilin, and presenilin-associated protein) did not bind. Moreover, anti-nicastrin antibodies immunoprecipitated ␥-secretase activity from detergentsolubilized microsomes. Unexpectedly, C83, the major endogenous amyloid-␤ precursor protein substrate of ␥-secretase, was also quantitatively associated with the complex. These results provide direct biochemical evidence that nicastrin is a member of the active ␥-secretase complex, indicate that ␤-catenin, calsenilin, and presenilin-associated protein are not required for ␥ activity, and suggest an unprecedented mechanism of substrate-protease interaction.

Perihaematomal oedema (PHO) is an important pathophysiological marker of secondary injury in intracerebral haemorrhage (ICH). In this Review, we describe a novel method to conceptualize PHO formation within the framework of Starling's principle of movement of fluid across a capillary wall. We consider progression of PHO through three stages, characterized by ionic oedema (stage 1) and progressive vasogenic oedema (stages 2 and 3). In this context, possible modifiers of PHO volume and their value in identifying patients who would benefit from therapies that target secondary injury are discussed; the practicalities of using neuroimaging to measure PHO volume are also considered. We examine whether PHO can be used as a predictor of neurological outcome following ICH, and we provide an overview of emerging therapies. Our discussion emphasizes that PHO has clinical relevance both as a therapeutic target, owing to its augmentation of the mass effect of a haemorrhage, and as a surrogate marker for novel interventions that target secondary injury.

The ␥-secretase complex is an unusual multimeric protease responsible for the intramembrane cleavage of a variety of type 1 transmembrane proteins, including the ␤-amyloid precursor protein and Notch. Genetic and biochemical data have revealed that this protease consists of the presenilin heterodimer, a highly glycosylated form of nicastrin, and the recently identified gene products, Aph-1 and Pen-2. Whereas current evidence supports the notion that presenilin comprises the active site of the protease and that the other three components are members of the active complex required for proteolytic activity, the individual roles of the three co-factors remain unclear. Here, we demonstrate that endogenous Aph-1 interacts with an immature species of nicastrin, forming a stable intermediate early in the assembly of the ␥-secretase complex, prior to the addition of presenilin and Pen-2. Our data suggest 1) that Aph-1 is involved in the early stages of ␥-secretase assembly through the stabilization and perhaps glycosylation of nicastrin and by scaffolding nicastrin to the immature ␥-secretase complex, and 2) that presenilin, and later Pen-2, bind to this intermediate during the formation of the mature protease.

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