Nicastrin binds to membrane-tethered Notch

Processing of Notch and amyloid precursor protein by gamma-secretase is spatially distinct

gamma-Secretase activity is associated with a presenilin (PS)-containing macromolecular complex. Whether PS contains the active site of gamma-secretase has been controversial. One challenge is to find PS that is engaged in the active gamma-secretase complex at the cell surface, where some substrates appear to be processed. In this study, we developed an intact cell photolabeling technique that allows the direct visualization of active gamma-secretase at the cell surface. We demonstrated that active gamma-secretase is present in the plasma membrane. Moreover, the PS1 heterodimer is specifically photolabeled at the cell surface by a potent inhibitor that binds to only the active gamma-secretase. We also explored the cellular processing sites of gamma-secretase for amyloid precursor protein (APP) and Notch by using small molecular probes. MRL631, a gamma-secretase inhibitor that is unable to penetrate the cell membrane, significantly blocks gamma-secretase-mediated Notch cleavage but has little effect on APP processing. These results indicate that Notch is processed at the cell surface and that the majority of APP is processed by intracellular gamma-secretase. Furthermore, the fact that inhibitors first target gamma-secretase in the plasma membrane for Notch processing, and not for APP, will have important implications for drug development to treat Alzheimer's disease and cancer.

Functional reconstitution of gamma-secretase through coordinated expression of presenilin, nicastrin, Aph-1, and Pen-2

The gamma-secretase complex has emerged as an unusual membrane-bound aspartyl protease with the ability to cleave certain substrate proteins at peptide bonds believed to be buried within the hydrophobic environment of the lipid bilayer. This cleavage is responsible for a key biochemical step in signaling from several different cell-surface receptors, and it is also crucial in generating the neurotoxic amyloid peptides that are central to the pathogenesis of Alzheimer's disease. Active gamma-secretase is a multimeric protein complex consisting of at least four different proteins, presenilin, nicastrin, Aph-1, and Pen-2, with presenilin serving as the catalytically active core of the aspartyl protease. Presenilin itself undergoes endoproteolytic maturation, a process that is tightly regulated during the assembly and maturation of gamma-secretase, and that depends on the three cofactors nicastrin, Aph-1, and Pen-2. Recent studies have demonstrated that presenilin and its three cofactors are likely to be the major proteins needed for functional reconstitution of active gamma-secretase and have begun to elucidate the specific functions of the cofactors in the ordered assembly of gamma-secretase.

Functional Implications of the Presenilin Dimerization

Presenilins are the catalytic components of gamma-secretase, an intramembrane-cleaving protease whose substrates include beta-amyloid precursor protein (betaAPP) and the Notch receptors. These type I transmembrane proteins undergo two distinct presenilin-dependent cleavages within the transmembrane region, which result in the production of Abeta and APP intracellular domain (from betaAPP) and the Notch intracellular domain signaling peptide. Most cases of familial Alzheimer's disease are caused by presenilin mutations, which are scattered throughout the coding sequence. Although the underlying molecular mechanism is not yet known, the familial Alzheimer's disease mutations produce a shift in the ratio of the long and short forms of the Abeta peptide generated by the gamma-secretase. We and others have previously shown that presenilin homodimerizes and suggested that a presenilin dimer is at the catalytic core of gamma-secretase. Here, we demonstrate that presenilin transmembrane domains contribute to the formation of the dimer. In-frame substitution of the hydrophilic loop 1, located between transmembranes I and II, which modulates the interactions within the N-terminal fragment/N-terminal fragment dimer, abolishes both presenilinase and gamma-secretase activities. In addition, by reconstituting gamma-secretase activity from two catalytically inactive presenilin aspartic mutants, we provide evidence of an active diaspartyl group assembled at the interface between two presenilin monomers. Under our conditions, this catalytic group mediates the generation of APP intracellular domain and Abeta but not Notch intracellular domain, therefore suggesting that specific diaspartyl groups within the presenilin catalytic core of gamma-secretase mediate the cleavage of different substrates.

Presenilins and gamma-secretase inhibitors affect intracellular trafficking and cell surface localization of the gamma-secretase complex components

The intramembranous cleavage of Alzheimer beta-amyloid precursor protein and the signaling receptor Notch is mediated by the presenilin (PS, PS1/PS2)-gamma-secretase complex, the components of which also include nicastrin, APH-1, and PEN-2. In addition to its essential role in gamma-secretase activity, we and others have reported that PS1 plays a role in intracellular trafficking of select membrane proteins including nicastrin. Here we examined the fate of PEN-2 in the absence of PS expression or gamma-secretase activity. We found that PEN-2 is retained in the endoplasmic reticulum and has a much shorter half-life in PS-deficient cells than in wild type cells, suggesting that PSs are required for maintaining the stability and proper subcellular trafficking of PEN-2. However, the function of PS in PEN-2 trafficking is distinct from its contribution to gamma-secretase activity because inhibition of gamma-secretase activity by gamma-secretase inhibitors did not affect the PEN-2 level or its egress from the endoplasmic reticulum. Instead, membrane-permeable gamma-secretase inhibitors, but not a membrane-impermeable derivative, markedly increased the cell surface levels of PS1 and PEN-2 without affecting that of nicastrin. In support of its role in PEN-2 trafficking, PS1 was also required for the gamma-secretase inhibitor-induced plasma membrane accumulation of PEN-2. We further showed that gamma-secretase inhibitors specifically accelerated the Golgi to the cell surface transport of PS1 and PEN-2. Taken together, we demonstrate an essential role for PSs in intracellular trafficking of the gamma-secretase components, and that selective gamma-secretase inhibitors differentially affect the trafficking of the gamma-secretase components, which may contribute to an inactivation of gamma-secretase.

Notch Oncoproteins Depend on γ-Secretase/Presenilin Activity for Processing and Function

During normal development Notch receptor signaling is important in regulating numerous cell fate decisions. Mutations that truncate the extracellular domain of Notch receptors can cause aberrant signaling and promote unregulated cell growth. We have examined two types of truncated Notch oncoproteins that arise from proviral insertion into the Notch4 gene (Notch4/int-3) or a chromosomal translocation involving the Notch1 gene (TAN-1). Both Notch4/int-3 and TAN-1 oncoproteins lack most or all of their ectodomain. Normal Notch signaling requires gamma-secretase/presenilin-mediated proteolytic processing, but whether Notch oncoproteins are also dependent on gamma-secretase/presenilin activity is not known. We demonstrate that Notch4/int-3-induced activation of the downstream transcription factor, CSL, is abrogated in cells deficient in presenilins or treated with a pharmacological inhibitor of gamma-secretase/presenilins. Furthermore, we find that both Notch4/int-3 and TAN-1 accumulate at the cell surface, where presenilin-dependent cleavage occurs, when gamma-secretase/presenilin activity is inhibited. gamma-Secretase/presenilin inhibition effectively blocks cellular responses to Notch4/int-3, but not TAN-1, apparently because some TAN-1 polypeptides lack transmembrane domains and do not require gamma-secretase/presenilin activity for nuclear access. These studies highlight potential uses and limitations of gamma-secretase/presenilin inhibitors in targeted therapy of Notch-related neoplasms.

Lack of association between Alzheimer's disease and the promoter region polymorphisms of the nicastrin gene

The biological analysis of nicastrin (NCSTN) shows its crucial role in gamma-cleavage of the amyloid precursor protein. Inhibition of NCSTN demonstrated altered gamma-cleavage activity, suggesting its potential implication in Alzheimer's disease (AD). We sequenced the NCSTN gene promoter region and found two promoter single nucleotide polymorphisms (SNPs) at putative transcription binding sites, -796T/G and -1216C/A. The association study using the promoter SNPs showed no significant genetic effect upon the development of AD. Haplotype analysis with the promoter SNPs and coding SNPs demonstrated no significant difference between familial AD cases and controls. Moreover, the genotype of each promoter SNP did not have an association with age-at-onset in AD. Our investigation suggests that the two promoter SNPs are unrelated to the development of AD, however, further investigation at the promoter region of NCSTN may be necessary to address its potential implication of gene expression in AD.

The presenilin proteins are components of multiple membrane-bound complexes that have different biological activities

Several lines of evidence have indicated that the presenilin proteins function within macromolecular complexes and are necessary for the regulated intramembranous proteolysis of certain type 1 transmembrane proteins, including the amyloid precursor protein, Notch, and p75. Data from multiple complementary experiments now suggest that there may be several distinct presenilin complexes. We show here that presenilin mutations and certain detergents affect the abundance and componentry of the presenilin complexes, and these structural effects correlate with their effects on gamma-secretase activity. Our data suggest that there are at least three complexes, including a approximately 150-kDa nicastrin-aph-1 complex (which is likely to be a precursor complex). There is a stable and abundant intermediate complex of approximately 440 kDa, which contains aph-1, pen-2, nicastrin, and PS1. However, it is the very low abundance, high mass (>/=670 kDa) heteromeric complexes that are associated with the highest gamma-secretase-specific activity.

Elucidation of the interactions between C99, presenilin, and nicastrin by the split-ubiquitin assay

The interactions between C99, presenilin, and nicastrin were investigated by a split-ubiquitin assay. We found that C99 homodimerizes and binds weakly to presenilin and strongly to nicastrin. Domain mapping assays revealed the transmembrane and cytoplasmic carboxy-terminal region of C99 is sufficient for the dimerization of C99 and the interaction between C99 and nicastrin. The extracellular domain of C99 is responsible for binding to presenilin. Nicastrin bound to C99 via its transmembrane domain and carboxy-terminal region. These observations suggest that dimerized (or oligomerized) C99 directly interacts with presenilin, and that this interaction is facilitated by nicastrin.

Presenilin-1 is indirectly implicated in Notch1 cleavage

SUMMARY

It has been previously demonstrated that the Notch1 signalling pathway is impaired in presenilin-1 null cells. This observation suggests a role for presenilin-1 in the Notch1 developmental pathway, possibly through physical interaction. Here, we show that presenilin-1 and Notch1 do not interact directly with each other but are associated in the cell. These findings raise the possibility that the gamma-secretase cleavage occurs via a presenilin complex in association with a putative co-factor specific for the molecule that is being cleaved (e.g. Notch1, (beta-amyloid precursor protein, E-cadherin and ErbB-4, all of which are gamma-secretase substrates).

Nicastrin gene in familial and sporadic Alzheimer's disease

Nicastrin is a protein recently discovered associated to presenilins and involved in the production of amyloid beta peptide that accumulates in Alzheimer's disease (AD) brain. In this study the nicastrin gene was examined for unknown mutations and polymorphisms in 104 patients with familial AD (52 early-onset and 52 late-onset), 174 sporadic AD and 191 healthy neurological controls of Italian origin. The scanning of the nicastrin gene identified a missense mutation (N417Y) in two patients with sporadic AD, in an early-onset familial AD and in a young control subject. Furthermore, we found two silent mutations and four intronic polymorphisms, three of them co-segregating in a single haplotype. We found some differences in the distribution of genotype alterations in the AD population compared to the controls. However, our data together with other evidence did not support the pathological role of missense mutation N417Y.

Notch1 competes with the amyloid precursor protein for gamma-secretase and down-regulates presenilin-1 gene expression

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and Notch1. Based on the fact that APP and Notch are processed by the same gamma-secretase, we postulated that APP and Notch compete for the enzyme activity. In this report, we examined the interactions between APP, Notch, and PS1 using the direct gamma-secretase substrates, Notch 1 Delta extracellular domain (N1DeltaEC) and APP carboxyl-terminal fragment of 99 amino acids, and measured the effects on amyloid-beta protein production and Notch signaling, respectively. Additionally, we tested the hypothesis that downstream effects on PS1 expression may coexist with the competition phenomenon. We observed significant competition between Notch and APP for gamma-secretase activity; transfection with either of two direct substrates of gamma-secretase led to a reduction in the gamma-cleaved products, Notch intracellular domain or amyloid-beta protein. In addition, however, we found that activation of the Notch signaling pathway, by either N1 Delta EC or Notch intracellular domain, induced down-regulation of PS1 gene expression. This finding suggests that Notch activation directly engages gamma-secretase and subsequently leads to diminished PS1 expression, suggesting a complex set of feedback interactions following Notch activation.

Human neurodegenerative disease modeling using Drosophila

A number of approaches have been taken to recreate and to study the role of genes associated with human neurodegenerative diseases in the model organism Drosophila. These studies encompass the polyglutamine diseases, Parkinson's disease, Alzheimer's disease, and tau-associated pathologies. The findings highlight Drosophila as an important model system in which to study the fundamental pathways influenced by these genes and have led to new insights into aspects of pathogenesis and modifier mechanisms.

Functional γ-secretase complex assembly in Golgi/trans-Golgi network: interactions among presenilin, nicastrin, Aph1, Pen-2, and γ-secretase substrates

Gamma-secretase is a proteolytic complex whose substrates include Notch, beta-amyloid precursor protein (APP), and several other type I transmembrane proteins. Presenilin (PS) and nicastrin are known components of this high-molecular-weight complex, and recent genetic screens in invertebrates have identified two additional gene products, Aph1 and Pen-2, as key factors in gamma-secretase activity. Here, we examined the interaction of the components of the gamma-secretase complex in Chinese hamster ovary cells stably expressing human forms of APP, PS1, Aph1, and Pen-2. Subcellular fractionation of membrane vesicles and subsequent coimmunoprecipitation of individual gamma-secretase components revealed that interactions among all proteins occurred in the Golgi/trans-Golgi network (TGN) compartments. Furthermore, incubation of the Golgi/TGN-enriched vesicles resulted in de novo generation of amyloid beta-protein and APP intracellular domain. Immunofluorescent staining of the individual gamma-secretase components supported our biochemical evidence that the gamma-secretase components assemble into the proteolytically active gamma-secretase complex in the Golgi/TGN compartment.

Amyloid precursor protein (APP) and the biology of proteolytic processing: relevance to Alzheimer's disease

The processing of amyloid precursor protein (APP) generates amyloid-beta (Abeta) peptides 1-40 and 1-42. The latter is neurotoxic and its accumulation results in amyloid fibril formation and the generation of senile plaques, the hallmark of Alzheimer's disease (AD). Whilst there has been considerable progress made in understanding the generation of Abeta by alpha-, beta- and gamma-secretase activity on APP, recently enzymes involved in the degradation of Abeta have been identified including neprilysin and insulin-degrading enzyme (IDE). We review the pathways involved in proteolytic processing of APP and discuss the potential implications of aberrant proteolysis on neurodegeneration. It is conceivable that single nucleotide polymorphisms (SNPs) in the regulatory regions of genes in these proteolytic cascades, which alter their expression, could contribute to some of the age-related changes seen in AD.

The transmembrane domain region of nicastrin mediates direct interactions with APH-1 and the gamma-secretase complex

Nicastrin (NCT) is a type I integral membrane protein that is one of the four essential components of the gamma-secretase complex, a protein assembly that catalyzes the intramembranous cleavage of the amyloid precursor protein and Notch. Other gamma-secretase components include presenilin-1 (PS1), APH-1, and PEN-2, all of which span the membrane multiple times. The mechanism by which NCT associates with the gamma-secretase complex and regulates its activity is unclear. To avoid the misfolding phenotype often associated with introducing deletions or mutations into heavily glycosylated and disulfide-bonded proteins such as NCT, we produced chimeras between human (hNCT) and Caenorhabditis elegans NCT (ceNCT). Although ceNCT did not associate with human gamma-secretase components, all of the ceNCT/hNCT chimeras interacted with gamma-secretase components from human, C. elegans, or both, indicating that they folded correctly. A region at the C-terminal end of hNCT, encompassing the last 50 residues of its ectodomain, the transmembrane domain, and the cytoplasmic domain was important for mediating interactions with human PS1, APH-1, and PEN-2. This finding is consistent with the fact that the bulk of the gamma-secretase complex proteins resides within the membrane, with relatively small extramembranous domains. Finally, hNCT associated with hAPH-1 in the absence of PS, consistent with NCT and APH-1 forming a subcomplex prior to association with PS1 and PEN-2 and indicating that the interactions between NCT with PS1 may be indirect or stabilized by the presence of APH-1.

A presenilin dimer at the core of the gamma-secretase enzyme: insights from parallel analysis of Notch 1 and APP proteolysis

Notch receptors and the amyloid precursor protein are type I membrane proteins that are proteolytically cleaved within their transmembrane domains by a presenilin (PS)-dependent gamma-secretase activity. In both proteins, two peptide bonds are hydrolyzed: one near the inner leaflet and the other in the middle of the transmembrane domain. Under saturating conditions the substrates compete with each other for proteolysis, but not for binding to PS. At least some Alzheimer's disease-causing PS mutations reside in proteins possessing low catalytic activity. We demonstrate (i) that differentially tagged PS molecules coimmunoprecipitate, and (ii) that PS N-terminal fragment dimers exist by using a photoaffinity probe based on a transition state analog gamma-secretase inhibitor. We propose that gamma-secretase contains a PS dimer in its catalytic core, that binding of substrate is at a site separate from the active site, and that substrate is cleaved at the interface of two PS molecules.

Positive and negative regulation of the gamma-secretase activity by nicastrin in a murine model

Nicastrin is a component of the gamma-secretase complex that has been shown to adhere to presenilin-1 (PS1), Notch, and APP. Here we demonstrate that Nicastrin-deficient mice showed a phenotype that is indistinguishable from PS1/PS2 double knock-out mice, whereas heterozygotes were healthy and viable. Fibroblasts derived from Nicastrin-deficient embryos were unable to generate amyloid beta-peptide and failed to release the intracellular domain of APP- or Notch1-Gal4-VP16 fusion proteins. Additionally, C- and N-terminal fragments of PS1 and the C-terminal fragments of PS2 were not detectable in Nicastrin-null fibroblasts, whereas full-length PS1 accumulated in null fibroblasts, indicating that Nicastrin is required for the endoproteolytic processing of presenilins. Interestingly, cells derived from Nicastrin heterozygotes produced relatively higher levels of amyloid beta-peptide whether the source was endogenous mouse or transfected human APP. These data demonstrate that Nicastrin is essential for the gamma-secretase cleavage of APP and Notch in mammalian cells and that Nicastrin has both positive and negative functions in the regulation of gamma-secretase activity.

Presenilin-1 interacts directly with the beta-site amyloid protein precursor cleaving enzyme (BACE1)

A neuropathological hallmark of Alzheimer's disease is the presence of amyloid plaques. The major constituent of these plaques, occurring largely in brain areas important for memory and cognition, is the 40-42 amyloid residues (Abeta). Abeta is derived from the amyloid protein precursor after cleavage by the recently identified beta-secretase (BACE1) and the putative gamma-secretase complex containing presenilin 1 (PS1). In an attempt to develop a functional secretase enzymatic assay in yeast we demonstrate a direct binding between BACE1 and PS1. This interaction was confirmed in vivo using coimmunoprecipitation and colocalization studies in human cultured cells. Our results show that PS1 preferably binds immature BACE1, thus possibly acting as a functional regulator of BACE1 maturation and/or activity.

Peptide-based, irreversible inhibitors of gamma-secretase activity

The characterization of the enzymes responsible for amyloid beta-peptide (Abeta) production is considered to be a primary goal towards the development of future therapeutics for the treatment of Alzheimer's disease. Inhibitors of gamma-secretase activity were critical in demonstrating that the presenilins (PSs) likely comprised at least part of the active site of the gamma-secretase enzyme complex, with two highly conserved membrane aspartates presumably acting as catalytic residues. However, whether or not these aspartates are actually the catalytic residues of the enzyme complex or are merely essential for normal PS function and/or maturation is still unknown. In this paper, we report the development of reactive inhibitors of gamma-secretase activity that are functionally irreversible. Since such inhibitors have been shown to bind catalytic residues in other aspartyl proteases (e.g., HIV protease), they might be used to determine if the transmembrane aspartates of PSs are involved directly in substrate cleavage.

Overexpression of nicastrin increases Abeta production

Gamma-secretase cleavage is the final proteolytic step that releases the amyloid beta-peptide (Abeta) from the amyloid beta-protein precursor (APP). Significant evidence indicates that the presenilins (PS) are catalytic components of a high molecular weight gamma-secretase complex. The glycoprotein nicastrin was recently identified as a functional unit of this complex based on 1) binding to PS and 2) the ability to modulate Abeta production following mutation of a conserved DYIGS region. In contrast to the initial report, we find that overexpression of wild-type (WT) nicastrin increases Abeta production, whereas DYIGS mutations (MT) have little or no effect. The increase in Abeta production is associated with an increase in gamma-secretase activity but not with a detectable increase in PS1 levels. Subcellular fractionation studies show that WT but not MT nicastrin matures into buoyant membrane fractions enriched in gamma-secretase activity. These data support the hypothesis that nicastrin is an essential component of the gamma-secretase complex. The finding that WT nicastrin overexpression can increase gamma-secretase activity without altering levels of the presumed catalytic component (PS) of the enzyme may point to a role for nicastrin in facilitating cleavage by regulating substrate interactions with the gamma-secretase complex.

Nicastrin is required for assembly of presenilin/gamma-secretase complexes to mediate Notch signaling and for processing and trafficking of beta-amyloid precursor protein in mammals

Recent studies indicate that nicastrin (NCT) and presenilins form functional components of a multimeric gamma-secretase complex required for the regulated intramembraneous proteolysis of Notch and beta-amyloid (Abeta) precursor protein (APP). To determine whether nicastrin is required for proteolytic processing of Notch and APP in mammals and the role of nicastrin in presenilin/gamma-secretase complex assembly, we generated nicastrin-deficient (NCT-/-) mice and derived fibroblasts from NCT-/- embryos. Nicastrin-null embryos died by embryonic day 10.5 and exhibited several patterning defects, including abnormal somite segmentation, phenotypes that are reminiscent of embryos lacking Notch1 or both presenilins. Importantly, secretion of Abeta peptides is abolished in NCT-/- fibroblasts, whereas it is reduced by approximately 50% in NCT+/- cells; the failure to generate Abeta peptides in NCT-/- cells is accompanied by destabilization of the presenilin/gamma-secretase complex and accumulation of APP-C-terminal fragments. Moreover, APP trafficking analysis in NCT-/- fibroblasts revealed a significant delay in the rate of APP reinternalization compared with that of control cells. Together, these results establish that nicastrin is an essential component of the multimeric gamma-secretase complex in mammals required for both gamma-secretase activity and APP trafficking and suggest that nicastrin may be a valuable therapeutic target for Alzheimer's disease.

Presenilin 1 and presenilin 2 have differential effects on the stability and maturation of nicastrin in Mammalian brain

The presenilins and nicastrin form high molecular mass, multimeric protein complexes involved in the intramembranous proteolysis of several proteins. Post-translational glycosylation and trafficking of nicastrin is necessary for the activity of these complexes. We report here that although there are differences in the post-translational processing of nicastrin in neurons and glia, both of the presenilins are required for the physiological post-translational modification and for the correct subcellular distribution of nicastrin. Absence of presenilin 1 (PS1) is associated with dramatic reductions in the level of mature glycosylated nicastrin and with redistribution of nicastrin away from the cell surface. In contrast, absence of presenilin 2 (PS2) is associated with only modest reductions in the levels of immature nicastrin. It is notable that these differential effects parallel the differential effects of null mutations in PS1 and PS2 on APP and Notch processing. Our data therefore suggest that the differential interactions of PS1 and PS2 with nicastrin reflect different functions for the PS1 and PS2 complexes.

Different cofactor activities in gamma-secretase assembly: evidence for a nicastrin-Aph-1 subcomplex

The gamma-secretase complex is required for intramembrane cleavage of several integral membrane proteins, including the Notch receptor, where it generates an active signaling fragment. Four putative gamma-secretase components have been identified-presenilin (Psn), nicastrin (Nct), Aph-1, and Pen-2. Here, we use a stepwise coexpression approach to investigate the role of each new component in gamma-secretase assembly and activation. Coexpression of all four proteins leads to high level accumulation of mature Psn and increased proteolysis of Notch. Aph-1 and Nct may form a subcomplex that stabilizes the Psn holoprotein at an early step in gamma-secretase assembly. Subcomplex levels of Aph-1 are down-regulated by stepwise addition of Psn, suggesting that Aph-1 might not enter the mature complex. In contrast, Pen-2 accumulates proportionally with Psn, and is associated with Psn endoproteolysis during gamma-secretase assembly. These results demonstrate that Aph-1 and Pen-2 are essential cofactors for Psn, but that they play different roles in gamma-secretase assembly and activation.

Ets transcription factors ER81 and Elk1 regulate the transcription of the human presenilin 1 gene promoter

We have previously defined a crucial DNA element controlling 90% of the expression of the presenilin 1 gene at (-35 to +6). This region contains an Ets transcription factor binding motif, and a 2-base pair alteration within the core sequence (GGAA to TTAA) of the Ets consensus also reduced transcription by over 90%. We have shown that Ets1/2 transcription factors bind specifically to the -10 Ets element and activate PS1 transcription. The identification of other transcription factors recognizing specifically this promoter area should provide insights into the regulation of PS1. We have used the -10 Ets element as a bait in yeast one hybrid screening of a human brain cDNA library. This assay selected three factors from the Ets family: Ets2, ER81 and Elk1. We show that in vitro translated ER81 indeed binds specifically to the -10 region of the PS1 promoter and that ER81 activates by two- to threefold the basal transcription of a presenilin-1 promoter-chloramphenicol acetyltransferase reporter synthetic gene (-119, +178)PS1CAT in transient infection assays in neuroblastoma cells (SK-N-SH). GABPalpha, a member of the Ets family closely related to Ets2 and also containing a pointed domain, only increased PS1 transcription by about twofold. Cotransfection of GABPbeta together with GABPalpha did not increase PS1 transcription. However, GABPbeta alone activated PS1 transcription by two- to threefold. In contrast, the more distantly related Ets factor Elk1 repressed PS1 transcription very effectively.

Aspartic proteases involved in Alzheimer's disease

Alzheimer's disease afflicts every tenth human aged over 65. Despite the dramatic progress that has been made in understanding the disease, the exact cause of Alzheimer's disease is still unknown. Most gene mutations associated with Alzheimer's disease point at the same culprits: amyloid precursor protein and ultimately amyloid beta. The enigmatic proteases alpha-,beta-, and gamma-secretase are the three executioners of amyloid precursor protein processing, and disruption of their delicate balance is suspected to result in Alzheimer's disease. Significant progress has been made in the selective control of these proteases, regardless of the availability of structural information. Not even the absence of a robust cell-free assay for gamma-secretase could hamper the identification of nonpeptidic inhibitors of this enzyme for long. Within five years, four distinctly different structural moieties were developed and the first drug candidates are in clinical trials. Unfortunately, selective inhibition of amyloid beta formation remains a crucial issue because fundamental fragments of the gamma-secretase complex are important for other signaling events. This problem makes beta-secretase inhibition and alpha-secretase induction even more appealing.

gamma-Secretase activity requires the presenilin-dependent trafficking of nicastrin through the Golgi apparatus but not its complex glycosylation

Nicastrin and presenilin are two major components of the gamma-secretase complex, which executes the intramembrane proteolysis of type I integral membrane proteins such as the amyloid precursor protein (APP) and Notch. Nicastrin is synthesized in fibroblasts and neurons as an endoglycosidase-H-sensitive glycosylated precursor protein (immature nicastrin) and is then modified by complex glycosylation in the Golgi apparatus and by sialylation in the trans-Golgi network (mature nicastrin). These modifications are not observed with exogenously overexpressed nicastrin. Under normal cell culture conditions, only mature nicastrin is expressed at the cell surface and binds to the presenilin heterodimers. Mature nicastrin has a half-life of more than 24 hours. In the absence of presenilin 1 and 2, nicastrin remains entirely endoglycosidase H sensitive, is retained in the endoplasmic reticulum and is slowly degraded. Single presenilin 1 or presenilin 2 deficiency affects glycosylation of nicastrin to a lesser extent than the combined presenilin deficiencies, suggesting a correlation between either the transport of nicastrin out of the endoplasmic reticulum or the concomitant complex glycosylation of nicastrin, and gamma-secretase activity. However, when complex glycosylation of nicastrin was inhibited using mannosidase I inhibitors, gamma-secretase cleavage of APP or Notch was not inhibited and the immature nicastrin still associates with presenilin and appears at the cell surface. Complex glycosylation of nicastrin is therefore not needed for gamma-secretase activity. Because the trafficking of nicastrin to the Golgi apparatus is dependent on presenilins, our data point to a central role of presenilin in nicastrin maturation/localization, which could help to partially resolve the 'spatial paradox'.

Familial Alzheimer disease-linked presenilin 1 variants enhance production of both Abeta 1-40 and Abeta 1-42 peptides that are only partially sensitive to a potent aspartyl protease transition state inhibitor of "gamma-secretase"

Presenilin 1 (PS1) plays an essential role in intramembranous "gamma-secretase" processing of several type I membrane proteins, including the beta-amyloid precursor proteins (APP) and Notch1. In this report, we examine the activity of two familial Alzheimer's disease-linked PS1 variants on the production of secreted Abeta peptides and the effects of L-685,458, a potent gamma-secretase inhibitor, on inhibition of Abeta peptides from cells expressing these PS1 variants. We now report that PS1 variants enhance the production and secretion of both Abeta1-42 and Abeta1-40 peptides. More surprisingly, whereas the IC(50) for inhibition of Abeta1-40 peptide production from cells expressing wild-type PS1 is approximately 1.5 microm, cells expressing the PS1deltaE9 mutant PS1 exhibit an IC(50) of approximately 4 microm. Immunoprecipitation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry reveal that the levels of Abeta1-43 peptides are elevated in medium of PS1deltaE9 cells treated with higher concentrations of inhibitor. The differential effects of wild-type and mutant PS1 on gamma-secretase production of Abeta peptides and the disparity in sensitivity of these peptides to a potent gamma-secretase suggest that PS may be necessary, but not sufficient, to catalyze hydrolysis at the scissile bonds that generate the termini of Abeta1-40 and Abeta1-42 peptides.

APH-1 interacts with mature and immature forms of presenilins and nicastrin and may play a role in maturation of presenilin.nicastrin complexes

APH-1 and PEN-2 genes modulate the function of nicastrin and the presenilins in Caenorhabditis elegans. Preliminary studies in transfected mammalian cells overexpressing tagged APH-1 proteins suggest that this genetic interaction is mediated by a direct physical interaction. Using the APH-1 protein encoded on human chromosome 1 (APH-1(1)L; also known as APH-1a) as an archetype, we report here that endogenous forms of APH-1 are predominantly expressed in intracellular membrane compartments, including the endoplasmic reticulum and cis-Golgi. APH-1 proteins directly interact with immature and mature forms of the presenilins and nicastrin within high molecular weight complexes that display gamma- and epsilon-secretase activity. Indeed APH-1 proteins can bind to the nicastrin delta312-369 loss of function mutant, which does not undergo glycosylation maturation and is not trafficking beyond the endoplasmic reticulum. The levels of expression of endogenous APH-1(1)L can be suppressed by overexpression of any other members of the APH-1 family, suggesting that their abundance is coordinately regulated. Finally, although the absence of APH-1 destabilizes the presenilins, in contrast to nicastrin and PEN-2, APH-1 itself is only modestly destabilized in cells lacking functional expression of presenilin 1 or presenilin 2. Taken together, our data suggest that APH-1 proteins, and APH-1(1) in particular, may have a role in the initial assembly and maturation of presenilin.nicastrin complexes.

Gamma-secretase inhibitors and Alzheimer's disease

The existence of pathogenic mutations in beta-APP and the presenilin genes provides strong support for the hypothesis that Abeta production and deposition contribute to the etiology of Alzheimer's disease (AD). The heterogeneous carboxyl termini of Abeta molecules deposited in the hippocampus, cortex and cerebrovasculature of AD patients are generated by gamma-secretase. The gamma-secretase that generates the termini in vivo is a complex of proteins containing presenilin as an integral component. Drugs that modulate the production of Abeta by inhibiting gamma-secretase could provide an effective therapy for AD, but like most disease targets, the gamma-secretase appears to have more than a single function. The use of potent inhibitors has aided the discovery and characterization of gamma-secretase functions and reinforced the concept that a successful drug must demonstrate selectivity for lowering Abeta without disrupting the function of gamma-secretase targets. The discovery of drugs that can selectively inhibit beta-APP cleavage is an important objective. This review focuses on studies that enhance our understanding of the effects of inhibiting gamma-secretase and provide direction for developing effective and selective gamma-secretase inhibitors as drugs to treat AD.

The levels of mature glycosylated nicastrin are regulated and correlate with gamma-secretase processing of amyloid beta-precursor protein

Nicastrin, a type-I transmembrane glycoprotein, is a necessary component of the high molecular weight presenilin (PS) complexes that mediate intramembranous cleavage of beta-amyloid precursor protein (betaAPP) and Notch. Nicastrin undergoes trafficking-dependent glycosylation maturation, and PS1 interacts preferentially with these maturely glycosylated forms of nicastrin. We investigated the effects of differing levels of the immature and mature endoglycosidase-H-resistant forms of nicastrin on Abeta40- and Abeta42-peptide secretion in several cell lines stably expressing a mutant nicastrin (D336A/Y337A) that increases Abeta secretion. There was no correlation between Abeta secretion and the level of over-expression of the immature forms of nicastrin. The total level of mature nicastrin remained constant, but mutant nicastrin replaced endogenous mature nicastrin in varying degrees. Differences in the levels of mature mutant nicastrin positively correlated with Abeta secretion, but did not influence either betaAPP trafficking or processing by alpha- and beta-secretases. Proper trafficking and terminal maturation of nicastrin is therefore a necessary event for the regulated intramembranous proteolysis of betaAPP.

Association analysis between Alzheimer's disease and the Nicastrin gene polymorphisms

The biological study of the Nicastrin protein shows its crucial role in the pathogenesis of Alzheimer's disease (AD). We tested the hypothesis that the Nicastrin (NCSTN) gene might be genetically associated with AD. The association analysis of two single nucleotide polymorphisms (SNPs) in the coding region (cSNPs) of NCSTN were performed in an Italian population. No evidence of association was obtained between the two SNPs investigated in sporadic and familial AD cases under the stratification of currently known genetic risk factors including the apolipoprotein E (APOE), the presenilins and the beta-amyloid precursor protein. The result suggests no apparent synergic interaction between the NCSTN and APOE epsilon 4 in the risk to develop the late onset sporadic form of AD. But considering its biological effects, the result can not exclude the NCSTN as candidate for genetic factor in AD. Further genetic study of the NCSTN would be necessary to evaluate the potential genetic involvement in AD.

Alleles at the Nicastrin locus modify presenilin 1- deficiency phenotype

Presenilin 1 (PS1), presenilin 2, and nicastrin form high molecular weight complexes that are necessary for the endoproteolysis of several type 1 transmembrane proteins, including amyloid precursor protein (APP) and the Notch receptor, by apparently similar mechanisms. The cleavage of the Notch receptor at the "S3-site" releases a C-terminal cytoplasmic fragment (Notch intracellular domain) that acts as the intracellular transduction molecule for Notch activation. Missense mutations in the presenilins cause familial Alzheimer's disease by augmenting the "gamma-secretase" cleavage of APP and overproducing one of the proteolytic derivatives, the Abeta peptide. Null mutations in PS1 inhibit both gamma-secretase cleavage of APP and S3-site cleavage of the Notch receptor. Mice lacking PS1 function have defective Notch signaling and die perinatally with severe skeletal and brain deformities. We report here that a genetic modifier on mouse distal chromosome 1, coinciding with the locus containing Nicastrin, influences presenilin-mediated Notch S3-site cleavage and the resultant Notch phenotype without affecting presenilin-mediated APP gamma-site cleavage. Two missense substitutions of residues conserved among vertebrates have been identified in nicastrin. These results indicate that Notch S3-site cleavage and APP gamma-site cleavage are distinct presenilin-dependent processes and support a functional interaction between nicastrin and presenilins in vertebrates. The dissociation of Notch S3-site and APP gamma-site cleavage activities will facilitate development of gamma-secretase inhibitors for treatment of Alzheimer's disease.

Presenilin 1 mutations activate gamma 42-secretase but reciprocally inhibit epsilon-secretase cleavage of amyloid precursor protein (APP) and S3-cleavage of notch

The presenilin 1 (PS1) and presenilin 2 (PS2) proteins are necessary for proteolytic cleavage of the amyloid precursor protein (APP) within its transmembrane domain. One of these cleavage events (termed gamma-secretase) generates the C-terminal end of the Abeta-peptide by proteolysis near residue 710 or 712 of APP(770). Another event (termed gamma-like or epsilon-secretase cleavage) cleaves near residue 721 at approximately 2-5 residues inside the cytoplasmic membrane boundary to generate a series of stable, C-terminal APP fragments. This latter cleavage is analogous to S3-cleavage of Notch. We report here that specific mutations in the N terminus, loop, or C terminus of PS1 all increase the production of Abeta(42) but cause inhibition of both epsilon-secretase cleavage of APP and S3-cleavage of Notch. These data support the hypothesis that epsilon-cleavage of APP and S3-cleavage of Notch are similar events. They also argue that, although both the gamma-site and the epsilon-site cleavage of APP are presenilin-dependent, they are likely to be independent catalytic events.

Mammalian APH-1 interacts with presenilin and nicastrin and is required for intramembrane proteolysis of amyloid-beta precursor protein and Notch

Presenilin and nicastrin are essential components of the gamma-secretase complex that is required for the intramembrane proteolysis of an increasing number of membrane proteins including the amyloid-beta precursor protein (APP) and Notch. By using co-immunoprecipitation and nickel affinity pull-down approaches, we now show that mammalian APH-1 (mAPH-1), a conserved multipass membrane protein, physically associates with nicastrin and the heterodimers of the presenilin amino- and carboxyl-terminal fragments in human cell lines and in rat brain. Similar to the loss of presenilin or nicastrin, the inactivation of endogenous mAPH-1 using small interfering RNAs results in the decrease of presenilin levels, accumulation of gamma-secretase substrates (APP carboxyl-terminal fragments), and reduction of gamma-secretase products (amyloid-beta peptides and the intracellular domains of APP and Notch). These data indicate that mAPH-1 is probably a functional component of the gamma-secretase complex required for the intramembrane proteolysis of APP and Notch.

Signal transduction through tyrosine-phosphorylated C-terminal fragments of amyloid precursor protein via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The proteolytic processing of amyloid precursor protein (APP) through the formation of membrane-bound C-terminal fragments (CTFs) and of soluble beta-amyloid peptides likely influences the development of Alzheimer's disease (AD). We show that in human brain a subset of CTFs are tyrosine-phosphorylated and form stable complexes with the adaptor protein ShcA. Grb2 is also part of these complexes, which are present in higher amounts in AD than in control brains. ShcA immunoreactivity is also greatly enhanced in patients with AD and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. A higher amount of phospho-ERK1,2, likely as result of the ShcA activation, is present in AD brains. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well known ShcA and ERK1,2 activator and a regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally generated. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Gamma-secretase-mediated proteolysis in cell-surface-receptor signalling

Many cell-surface receptors transmit signals to the nucleus through complex protein cascades. By contrast, the Notch signalling pathway uses a relatively direct mechanism, in which the intracellular domain of the receptor is liberated by intramembrane cleavage and translocates to the nucleus. This critical cleavage is mediated by the gamma-secretase complex, and new findings reveal that this mechanism is used by various receptors, although many questions remain about the biochemical details.

Mature glycosylation and trafficking of nicastrin modulate its binding to presenilins

Nicastrin is an integral component of the high molecular weight presenilin complexes that control proteolytic processing of the amyloid precursor protein and Notch. We report here that nicastrin is most probably a type 1 transmembrane glycoprotein that is expressed at moderate levels in the brain and in cultured neurons. Immunofluorescence studies demonstrate that nicastrin is localized in the endoplasmic reticulum, Golgi, and a discrete population of vesicles. Glycosidase analyses reveal that endogenous nicastrin undergoes a conventional, trafficking-dependent maturation process. However, when highly expressed in transfected cells, there is a disproportionate accumulation of the endo-beta-N-acetylglucosaminidase H-sensitive, immature form, with no significant increase in the levels of the fully mature species. Immunoprecipitation revealed that presenilin-1 interacts preferentially with mature nicastrin, suggesting that correct trafficking and co-localization of the presenilin complex components are essential for activity. These findings demonstrate that trafficking and post-translational modifications of nicastrin are tightly regulated processes that accompany the assembly of the active presenilin complexes that execute gamma-secretase cleavage. These results also underscore the caveat that simple overexpression of nicastrin in transfected cells may result in the accumulation of large amounts of the immature protein, which is apparently unable to assemble into the active complexes capable of processing amyloid precursor protein and Notch.

Notch signaling as a therapeutic target

Signals transduced by Notch receptors influence differentiation and proliferation in a wide variety of cell types. Activation of a Notch signal by one of several ligands triggers a series of proteolytic cleavages that release the intracellular region of Notch from the membrane, allowing it ultimately to translocate to the nucleus and activate the transcription of downstream target genes. Recent studies have elucidated the roles of several key proteins that participate in and modulate these central events in Notch signal transduction. These advances offer a variety of potential avenues to manipulate Notch signaling for therapeutic purposes in the treatment of cancer and in stem cell maintenance.

aph-1 and pen-2 are required for Notch pathway signaling, gamma-secretase cleavage of betaAPP, and presenilin protein accumulation

Presenilins are components of the gamma-secretase protein complex that mediates intramembranous cleavage of betaAPP and Notch proteins. A C. elegans genetic screen revealed two genes, aph-1 and pen-2, encoding multipass transmembrane proteins, that interact strongly with sel-12/presenilin and aph-2/nicastrin. Human aph-1 and pen-2 partially rescue the C. elegans mutant phenotypes, demonstrating conserved functions. The human genes must be provided together to rescue the mutant phenotypes, and the inclusion of presenilin-1 improves rescue, suggesting that they interact closely with each other and with presenilin. RNAi-mediated inactivation of aph-1, pen-2, or nicastrin in cultured Drosophila cells reduces gamma-secretase cleavage of betaAPP and Notch substrates and reduces the levels of processed presenilin. aph-1 and pen-2, like nicastrin, are required for the activity and accumulation of gamma-secretase.

Presenilin and nicastrin regulate each other and determine amyloid beta-peptide production via complex formation

Amyloid beta-peptide (Abeta) is generated by the consecutive cuts of two membrane-bound proteases. Beta-secretase cuts at the N terminus of the Abeta domain, whereas gamma-secretase mediates the C-terminal cut. Recent evidence suggests that the presenilin (PS) proteins, PS1 and PS2, may be gamma-secretases. Because PSs principally exist as high molecular weight protein complexes, biologically active gamma-secretases likely require other cofactors such as nicastrin (Nct) for their activities. Here we show that preferentially mature Nct forms a stable complex with PSs. Furthermore, we have down-regulated Nct levels by using a highly specific and efficient RNA interference approach. Very similar to a loss of PS function, down-regulation of Nct levels leads to a massive accumulation of the C-terminal fragments of the beta-amyloid precursor protein. In addition, Abeta production was markedly reduced. Strikingly, down-regulation of Nct destabilized PS and strongly lowered levels of the high molecular weight PS1 complex. Interestingly, absence of the PS1 complex in PS1(-/-) cells was associated with a strong down-regulation of the levels of mature Nct, suggesting that binding to PS is required for trafficking of Nct through the secretory pathway. Based on these findings we conclude that Nct and PS regulate each other and determine gamma-secretase function via complex formation.

Complex N-glycosylated form of nicastrin is stabilized and selectively bound to presenilin fragments

The transmembrane glycoprotein nicastrin is a component of presenilin (PS) protein complex that is involved in gamma-cleavage of beta APP and site-3 cleavage of Notch. PS undergoes endoproteolysis, and the proteolytic fragments are incorporated into the high molecular weight protein complexes that are highly stabilized. Here we show that Endo H-resistant, N-glycosylated form of nicastrin (p150-NCT) is highly stabilized and selectively bound to PS fragments. Moreover, loss-of-function mutations of nicastrin inhibited formation of fully glycosylated p150-NCT as well as stabilization of nicastrin, suggesting that glycosylation and stabilization of nicastrin polypeptides are tightly correlated with its function.

The gene encoding nicastrin, a major gamma-secretase component, modifies risk for familial early-onset Alzheimer disease in a Dutch population-based sample

Nicastrin regulates gamma-secretase cleavage of the amyloid precursor protein by forming complexes with presenilins, in which most mutations causing familial early-onset Alzheimer disease (EOAD) have been found. The gene encoding nicastrin (NCSTN) maps to 1q23, a region that has been linked and associated with late-onset Alzheimer disease (LOAD) in various genome screens. In 78 familial EOAD cases, we found 14 NCSTN single-nucleotide polymorphisms (SNPs): 10 intronic SNPs, 3 silent mutations, and 1 missense mutation (N417Y). N417Y is unlikely to be pathogenic, since it did not alter amyloid beta secretion in an in vitro assay and its frequency was similar in case and control subjects. However, SNP haplotype estimation in two population-based series of Dutch patients with EOAD (n=116) and LOAD (n=240) indicated that the frequency of one SNP haplotype (HapB) was higher in the group with familial EOAD (7%), compared with the LOAD group (3%) and control group (3%). In patients with familial EOAD without the APOE epsilon4 allele, the HapB frequency further increased, to 14%, resulting in a fourfold increased risk (odds ratio = 4.1; 95% confidence interval 1.2-13.3; P=.01). These results are compatible with an important role of gamma-secretase dysfunction in the etiology of familial EOAD.

Prioritizing the proteome: identifying pharmaceutically relevant targets

Considerable attention is now being placed on prioritizing the proteome as the point of delivery for genomic information. Some of the challenges faced in prioritizing efforts from a pharmaceutical perspective, when presented with an incomplete proteome picture, are described. Examples of pharmaceutically relevant proteins are used to illustrate an informatics-based analysis of the proteome using knowledge of known drug targets. We show how results can be maximized by linking informatics approaches to experimental techniques and describe methods that can be used for prioritization within unprecedented protein families using, for example, single nucleotide polymorphism data and knowledge of disease pathways.

Notch cleavage: Nicastrin helps Presenilin make the final cut

Presenilin is thought to be the proteolytic component of gamma-secretase, responsible for the intramembranous cleavage of substrates that include the activated Notch receptor. Recent studies have identified the novel protein Nicastrin as another essential component of the Presenilin/gamma-secretase complex.

Gamma-secretase-like cleavages of Notch and beta APP are mutually exclusive in human cells

Presenilins 1 and 2 are two transmembrane proteins that seem necessary for controlling the proteolytic cleavages of two substrates, betaAPP and Notch, giving rise to Abeta (amyloid beta-peptide) and NICD (Notch Intracellular Domain), respectively. It is a matter for discussion whether presenilins act directly as the cleaving enzyme (referred to as gamma-secretase) or indirectly as a regulator of the substrates/enzymes trafficking to the permissive cell compartment where gamma-secretase cleavage could occur. Here we examined whether betaAPP and Notch undergo mutually exclusive proteolytic events in HEK293 cells or whether they behave as substrates able to compete for a single protease. We show that the overexpression of mDeltaE-Notch-1 does not influence the endogenous recovery of secreted and intracellular Abeta nor those derived from betaAPP-overexpressing HEK293 cells. We establish, conversely, that increasing amounts of betaAPP do not modify the steady-state generation of NICD nor affect the kinetic of production. These data indicate that the proteolytic cleavages leading to the productions of Abeta and NICD are mutually exclusive events in HEK293 cells, and suggest that distinct proteolytic activities contribute to betaAPP and Notch processing.

Aph-2/Nicastrin: an essential component of gamma-secretase and regulator of Notch signaling and Presenilin localization

The Notch signaling pathway plays a role in cell fate specification in many metazoans. A critical aspect of Notch activation involves proteolysis of the Notch receptor. This cleavage event requires Presenilin as a component of a large multiprotein complex, gamma-secretase. This complex mediates a similar cleavage event of the beta-amyloid precursor protein (APP). The transmembrane protein Nicastrin has been found to associate with Presenilin, Notch, and APP. Recent biochemical and genetic studies have focused on elucidating the function of this protein.

Nicastrin is required for gamma-secretase cleavage of the Drosophila Notch receptor

Nicastrin is genetically linked to Notch/lin-12 signaling in C. elegans and is part of a large multiprotein complex along with Presenilin. Here we describe the isolation and characterization of Drosophila Nicastrin (Nic) mutants. Nic mutants and tissue clones display characteristic Notch-like phenotypes. Genetic and inhibitor studies indicate a function for Nicastrin in the gamma-secretase step of Notch processing, similar to Presenilin. Further, Nicastrin is genetically required for signaling from membrane-anchored activated Notch. In the absence of Nicastrin, Presenilin is destabilized and mature C-terminal subunits are absent. Nicastrin might recruit gamma-secretase substrates into the proteolytic complex as a prerequisite for Presenilin maturation and active complex assembly.

Drosophila nicastrin is essential for the intramembranous cleavage of notch

The catalytic subunit of gamma-secretase is thought to be Presenilin, which is required for both the cleavage of APP and in the processing of Notch. Presenilin is found in a multisubunit complex that also contains Nicastrin. Nicastrin has been implicated in APP processing, but its role in Notch signaling remains unclear. Here we show that Drosophila Nicastrin is required for Notch signaling, and acts specifically at the S3 cleavage step. Partially processed Notch accumulates apically in nicastrin and presenilin mutant follicle cells. nicastrin and presenilin mutations also disrupt the spectrin cytoskeleton, suggesting that the gamma-secretase complex has another function in Drosophila in addition to its role in processing Notch and APP.

The presenilins

The presenilins are evolutionarily conserved transmembrane proteins that regulate cleavage of certain other proteins in their transmembrane domains. The clinical significance of this regulation is shown by the contribution of presenilin mutations to 20-50% of early-onset cases of inherited Alzheimer's disease. Although the precise molecular mechanism underlying presenilin function or dysfunction remains elusive, presenilins are thought to be part of a complex of proteins that has 'gamma-secretase cleavage' activity, which is clearly central in the pathogenesis of Alzheimer's disease. Mutations in presenilins increase the production of the longer isoforms of amyloid beta peptide, which are neurotoxic and prone to self-aggregation. Biochemical studies indicate that the presenilins do not act alone but operate within large heteromeric protein complexes, whose components and enzymatic core are the subject of much study and controversy; one essential component is nicastrin. The presenilin primary sequence is remarkably well conserved in eukaryotes, suggesting some functional conservation; indeed, defects caused by mutations in the nemotode presenilin homolog can be rescued by human presenilin.