A novel substrate for analyzing Alzheimer's disease gamma-secretase

Specific Mutations near the Amyloid Precursor Protein Cleavage Site Increase γ-Secretase Sensitivity and Modulate Amyloid-β Production

Amyloid-β peptides (Aβs) are produced via cleavage of the transmembrane region of the amyloid precursor protein (APP) by γ-secretase and are responsible for Alzheimer's disease. Familial Alzheimer's disease (FAD) is associated with APP mutations that disrupt the cleavage reaction and increase the production of neurotoxic Aβs, i.e., Aβ42 and Aβ43. Study of the mutations that activate and restore the cleavage of FAD mutants is necessary to understand the mechanism of Aβ production. In this study, using a yeast reconstruction system, we revealed that one of the APP FAD mutations, T714I, severely reduced the cleavage, and identified secondary APP mutations that restored the cleavage of APP T714I. Some mutants were able to modulate Aβ production by changing the proportions of Aβ species when introduced into mammalian cells. Secondary mutations include proline and aspartate residues; proline mutations are thought to act through helical structural destabilization, while aspartate mutations are thought to promote interactions in the substrate binding pocket. Our results elucidate the APP cleavage mechanism and could facilitate drug discovery.

Mechanisms of amyloid-β34 generation indicate a pivotal role for BACE1 in amyloid homeostasis

The beta‑site amyloid precursor protein (APP) cleaving enzyme (BACE1) was discovered due to its "amyloidogenic" activity which contributes to the production of amyloid-beta (Aβ) peptides. However, BACE1 also possesses an "amyloidolytic" activity, whereby it degrades longer Aβ peptides into a non‑toxic Aβ34 intermediate. Here, we examine conditions that shift the equilibrium between BACE1 amyloidogenic and amyloidolytic activities by altering BACE1/APP ratios. In Alzheimer disease brain tissue, we found an association between elevated levels of BACE1 and Aβ34. In mice, the deletion of one BACE1 gene copy reduced BACE1 amyloidolytic activity by ~ 50%. In cells, a stepwise increase of BACE1 but not APP expression promoted amyloidolytic cleavage resulting in dose-dependently increased Aβ34 levels. At the cellular level, a mislocalization of surplus BACE1 caused a reduction in Aβ34 levels. To align the role of γ-secretase in this pathway, we silenced Presenilin (PS) expression and identified PS2-γ-secretase as the main γ-secretase that generates Aβ40 and Aβ42 peptides serving as substrates for BACE1's amyloidolytic cleavage to generate Aβ34.

Proteolytically generated soluble Tweak Receptor Fn14 is a blood biomarker for γ-secretase activity

Fn14 is a cell surface receptor with key functions in tissue homeostasis and injury but is also linked to chronic diseases. Despite its physiological and medical importance, the regulation of Fn14 signaling and turnover is only partly understood. Here, we demonstrate that Fn14 is cleaved within its transmembrane domain by the protease γ‐secretase, resulting in secretion of the soluble Fn14 ectodomain (sFn14). Inhibition of γ‐secretase in tumor cells reduced sFn14 secretion, increased full‐length Fn14 at the cell surface, and enhanced TWEAK ligand‐stimulated Fn14 signaling through the NFκB pathway, which led to enhanced release of the cytokine tumor necrosis factor. γ‐Secretase‐dependent sFn14 release was also detected ex vivo in primary tumor cells from glioblastoma patients, in mouse and human plasma and was strongly reduced in blood from human cancer patients dosed with a γ‐secretase inhibitor prior to chimeric antigen receptor (CAR)‐T‐cell treatment. Taken together, our study demonstrates a novel function for γ‐secretase in attenuating TWEAK/Fn14 signaling and suggests the use of sFn14 as an easily measurable pharmacodynamic biomarker to monitor γ‐secretase activity in vivo.

Melanoma-secreted Amyloid Beta Suppresses Neuroinflammation and Promotes Brain Metastasis

Brain metastasis is a significant cause of morbidity and mortality in multiple cancer types and represents an unmet clinical need. The mechanisms that mediate metastatic cancer growth in the brain parenchyma are largely unknown. Melanoma, which has the highest rate of brain metastasis among common cancer types, is an ideal model to study how cancer cells adapt to the brain parenchyma. Our unbiased proteomics analysis of melanoma short-term cultures revealed that proteins implicated in neurodegenerative pathologies are differentially expressed in melanoma cells explanted from brain metastases compared to those derived from extracranial metastases. We showed that melanoma cells require amyloid beta (AB) for growth and survival in the brain parenchyma. Melanoma-secreted AB activates surrounding astrocytes to a pro-metastatic, anti-inflammatory phenotype and prevents phagocytosis of melanoma by microglia. Finally, we demonstrate that pharmacological inhibition of AB decreases brain metastatic burden.

Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence

β-Site APP-cleaving enzyme 1 (BACE1) cleaves amyloid β-protein precursor (APP) at the bond between Met⁶⁷¹ and Asp⁶⁷² (β-site) to generate the carboxyl-terminal fragment (CTFβ/C99). BACE1 also cleaves APP at another bond between Thr⁶⁸¹ and Gln⁶⁸² (β'-site), yielding CTFβ'/C89. Cleavage of CTFβ/C99 by γ-secretase generates Aβ(1-XX), whereas cleavage of CTFβ'/C89 generates Aβ(11-XX). Thus, β'-site cleavage by BACE1 is amyloidolytic rather than amyloidogenic. β' cleavage of mouse APP is more common than the corresponding cleavage of human APP. We found that the H684R substitution within human Aβ, which replaces the histidine in the human protein with the arginine found at the corresponding position in mouse, facilitated β' cleavage irrespective of the species origin of BACE1, thereby significantly increasing the level of Aβ(11-XX) and decreasing the level of Aβ(1-XX). Thus, amino acid substitutions within the Aβ sequence influenced the selectivity of alternative β- or β'-site cleavage of APP by BACE1. In familial Alzheimer's disease (FAD), the APP gene harbors pathogenic variations such as the Swedish (K670N/M671L), Leuven (E682K), and A673V mutations, all of which decrease Aβ(11-40) generation, whereas the protective Icelandic mutation (A673T) increases generation of Aβ(11-40). Thus, A673T promotes β' cleavage of APP and protects subjects against AD. In addition, CTFβ/C99 was cleaved by excess BACE1 activity to generate CTFβ'/C89, followed by Aβ(11-40), even if APP harbored pathogenic mutations. The resultant Aβ(11-40) was more metabolically labile in vivo than Aβ(1-40). Our analysis suggests that some FAD mutations in APP are amyloidogenic and/or amyloidolytic via selection of alternative BACE1 cleavage sites.

Suppressor Mutations for Presenilin 1 Familial Alzheimer Disease Mutants Modulate γ-Secretase Activities

γ-Secretase is a multisubunit membrane protein complex containing presenilin (PS1) as a catalytic subunit. Familial Alzheimer disease (FAD) mutations within PS1 were analyzed in yeast cells artificially expressing membrane-bound substrate, amyloid precursor protein, or Notch fused to Gal4 transcriptional activator. The FAD mutations, L166P and G384A (Leu-166 to Pro and Gly-384 to Ala substitution, respectively), were loss-of-function in yeast. We identified five amino acid substitutions that suppress the FAD mutations. The cleavage of amyloid precursor protein or Notch was recovered by the secondary mutations. We also found that secondary mutations alone activated the γ-secretase activity. FAD mutants with suppressor mutations, L432M or S438P within TMD9 together with a missense mutation in the second or sixth loops, regained γ-secretase activity when introduced into presenilin null mouse fibroblasts. Notably, the cells with suppressor mutants produced a decreased amount of Aβ42, which is responsible for Alzheimer disease. These results indicate that the yeast system is useful to screen for mutations and chemicals that modulate γ-secretase activity.

The A673T mutation in the amyloid precursor protein reduces the production of β-amyloid protein from its β-carboxyl terminal fragment in cells

Introduction

The A673T mutation in the amyloid precursor protein (APP) protects against Alzheimer’s disease by reducing β-amyloid protein (Aβ) production. This mutation reduced the release of the soluble APP fragment (sAPPβ), which is processed by β-secretase, suggesting a concomitant decrease in the β-carboxyl fragment of APP (C99), which is a direct substrate of γ-secretase for Aβ production. However, it remains controversial whether the level of C99 is significantly reduced in cells expressing APP that carry A673T as the cause of reduced Aβ production. Here, we investigated the effect of the A673T mutation in C99 on γ-cleavage in cells.

Results

We found that the level of C99 in cells expressing APP A673T was indistinctive of that observed in cells expressing wild-type APP, although the release of sAPPβ was significantly reduced in the APP A673T cells. In addition, our reconstituted β-secretase assay demonstrated no significant difference in β-cleavage on an APP fragment carrying the A673T mutation compared with the wild-type fragment. Importantly, cells expressing C99 containing the A673T mutation (C99 A2T; in accordance with the Aβ numbering) produced roughly half the level of Aβ compared with the wild-type C99, suggesting that the C99 A2T is an insufficient substrate of γ-secretase in cells. A cell-free γ-secretase assay revealed that Aβ production from the microsomal fraction of cells expressing C99 A2T was diminished. A sucrose gradient centrifugation analysis indicated that the levels of the C99 A2T that was codistributed with γ-secretase components in the raft fractions were reduced significantly.

Conclusions

Our data indicate that the A673T mutation in APP alters the release of sAPPβ, but not the C99 level, and that the C99 A2T is an inefficient substrate for γ-secretase in cell-based assay.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0247-6) contains supplementary material, which is available to authorized users.

Impact of amyloid precursor protein hydrophilic transmembrane residues on amyloid-beta generation

Amyloid-β (Aβ) peptides are likely the molecular cause of neurodegeneration observed in Alzheimer's disease. In the brain, Aβ42 and Aβ40 are toxic and the most important proteolytic fragments generated through sequential processing of the amyloid precursor protein (APP) by β- and γ-secretases. Impeding the generation of Aβ42 and Aβ40 is thus considered as a promising strategy to prevent Alzheimer's disease. We therefore wanted to determine key parameters of the APP transmembrane sequence enabling production of these Aβ species. Here we show that the hydrophilicity of amino acid residues G33, T43, and T48 critically determines the generation of Aβ42 and Aβ40 peptides (amino acid numbering according to Aβ nomenclature starting with aspartic acid 1). First, we performed a comprehensive mutational analysis of glycine residue G33 positioned within the N-terminal half of the APP transmembrane sequence by exchanging it against the 19 other amino acids. We found that hydrophilicity of the residue at position 33 positively correlated with Aβ42 and Aβ40 generation. Second, we analyzed two threonine residues at positions T43 and T48 in the C-terminal half of the APP-transmembrane sequence. Replacement of single threonine residues by hydrophobic valines inversely affected Aβ42 and Aβ40 generation. We observed that threonine mutants affected the initial γ-secretase cut, which is associated with levels of Aβ42 or Aβ40. Overall, hydrophilic residues of the APP transmembrane sequence decide on the exact initial γ-cut and the amounts of Aβ42 and Aβ40.

Beta amyloid-induced upregulation of death receptor 6 accelerates the toxic effect of N-terminal fragment of amyloid precursor protein

Amyloid precursor protein (APP) plays essential roles in the development of the Alzheimer's disease. Although full-length APP has been thoroughly studied, the role of the cleavage fragments especially the N-terminal fragments (N-APPs) in Alzheimer's disease pathogenesis was still elusive. In this study, we demonstrated that application of recombinant APP₁₈₋₂₈₆ could enhance beta amyloid (Aβ)-induced neuronal injuries which were related to the activation of apoptosis proteins. Aβ treatment could induce a slight increase of N-APPs release. In addition, expression of death receptor 6 (DR6) was increased in Aβ-treated neurons and APP transgenic mice. Moreover, the effect of APP₁₈₋₂₈₆ on Aβ-induced injuries could be suppressed by the application of recombinant DR6₄₁₋₃₄₁ and DR6 antibody. Furthermore, pull-down assay revealed that APP₁₈₋₂₈₆ could bind both exogenous and endogenous DR6. Aβ promoted APP₁₈₋₂₈₆ targeting to neuron which was accompanied with the increase of DR6 expression, whereas downregulation of DR6 by interference RNA could alleviate the binding of N-APPs to neuron and also suppressed Aβ-dependent toxic effect with N-APPs. These results suggested that APP N-terminal fragments might play neurotoxic roles in Aβ-induced neuronal injuries through cell surface DR6.

Loss of PAFAH1B2 reduces amyloid-β generation by promoting the degradation of amyloid precursor protein C-terminal fragments

Amyloid-β peptide (Aβ) is believed to play a central role in the pathogenesis of Alzheimer's disease. In view of the side effects associated with inhibiting the secretases that produce Aβ, new molecular targets are required to provide alternative therapeutic options. We used RNA interference (RNAi) to systematically screen the Drosophila genome to identify genes that modulate Aβ production upon knockdown. RNAi of 41 genes in Drosophila cells significantly lowered Aβ without affecting general secretion or viability. After the γ-secretase complex components, the most potent effect was observed for platelet activating factor acetylhydrolase α (Paf-AHα), and, in mammalian cells, the effect was replicated for its ortholog PAFAH1B2. Knockdown of PAFAH1B2 strongly reduced Aβ secretion from human cells, and this effect was confirmed in primary cells derived from PAFAH1B2 knock-out mice. Reduced Aβ production was not attributable to altered β-amyloid precursor protein (APP) ectodomain shedding but was a result of an enhanced degradation of APP C-terminal fragments (CTFs) in the absence of PAFAH1B2 but not its close homolog PAFAH1B3. Enhanced degradation of APP CTFs was selective because no such effects were obtained for Notch or E-/N-cadherin. Thus, we have identified an important protein that can selectively modify Aβ generation via a novel mechanism, namely enhanced degradation of its immediate precursor. In view of the absence of a neurological phenotype in PAFAH1B2 knock-out mice, targeted downregulation of PAFAH1B2 may be a promising new strategy for lowering Aβ.

Substrate determinants in the C99 juxtamembrane domains differentially affect γ-secretase cleavage specificity and modulator pharmacology

The molecular mechanisms governing γ-secretase cleavage specificity are not fully understood. Herein, we demonstrate that extending the transmembrane domain of the amyloid precursor protein-derived C99 substrate in proximity to the cytosolic face strongly influences γ-secretase cleavage specificity. Sequential insertion of leucines or replacement of membrane-anchoring lysines by leucines elevated the production of Aβ42, whilst lowering production of Aβ40. A single insertion or replacement was sufficient to produce this phenotype, suggesting that the helical length distal to the ε-site is a critical determinant of γ-secretase cleavage specificity. Replacing the lysine at the luminal membrane border (K28) with glutamic acid (K28E) increased Aβ37 and reduced Aβ42 production. Maintaining a positive charge with an arginine replacement, however, did not alter cleavage specificity. Using two potent and structurally distinct γ-secretase modulators (GSMs), we elucidated the contribution of K28 to the modulatory mechanism. Surprisingly, whilst lowering the potency of the non-steroidal anti-inflammatory drug-type GSM, the K28E mutation converted a heteroaryl-type GSM to an inverse GSM. This result implies the proximal lysine is critical for the GSM mechanism and pharmacology. This region is likely a major determinant for substrate binding and we speculate that modulation of substrate binding is the fundamental mechanism by which GSMs exert their action.

Utility of an improved model of amyloid-beta (Aβ₁₋₄₂) toxicity in Caenorhabditis elegans for drug screening for Alzheimer's disease

Background

The definitive indicator of Alzheimer’s disease (AD) pathology is the profuse accumulation of amyloid-ß (Aß) within the brain. Various in vitro and cell-based models have been proposed for high throughput drug screening for potential therapeutic benefit in diseases of protein misfolding. Caenorhabditis elegans offers a convenient in vivo system for examination of Aß accumulation and toxicity in a complex multicellular organism. Ease of culturing and a short life cycle make this animal model well suited to rapid screening of candidate compounds.

Results

We have generated a new transgenic strain of C. elegans that expresses full length Aß_1-42. This strain differs from existing Aß models that predominantly express amino-truncated Aß_3-42. The Aß_1-42 is expressed in body wall muscle cells, where it oligomerizes, aggregates and results in severe, and fully penetrant, age progressive-paralysis. The in vivo accumulation of Aß_1-42 also stains positive for amyloid dyes, consistent with in vivo fibril formation. The utility of this model for identification of potential protective compounds was examined using the investigational Alzheimer’s therapeutic PBT2, shown to be neuroprotective in mouse models of AD and significantly improve cognition in AD patients. We observed that treatment with PBT2 provided rapid and significant protection against the Aß-induced toxicity in C. elegans.

Conclusion

This C. elegans model of full length Aß_1-42 expression can now be adopted for use in screens to rapidly identify and assist in development of potential therapeutics and to study underlying toxic mechanism(s) of Aß.

Alzheimer's disease-linked mutations in presenilin-1 result in a drastic loss of activity in purified γ-secretase complexes

BACKGROUND

Mutations linked to early onset, familial forms of Alzheimer's disease (FAD) are found most frequently in PSEN1, the gene encoding presenilin-1 (PS1). Together with nicastrin (NCT), anterior pharynx-defective protein 1 (APH1), and presenilin enhancer 2 (PEN2), the catalytic subunit PS1 constitutes the core of the γ-secretase complex and contributes to the proteolysis of the amyloid precursor protein (APP) into amyloid-beta (Aβ) peptides. Although there is a growing consensus that FAD-linked PS1 mutations affect Aβ production by enhancing the Aβ1-42/Aβ1-40 ratio, it remains unclear whether and how they affect the generation of APP intracellular domain (AICD). Moreover, controversy exists as to how PS1 mutations exert their effects in different experimental systems, by either increasing Aβ1-42 production, decreasing Aβ1-40 production, or both. Because it could be explained by the heterogeneity in the composition of γ-secretase, we purified to homogeneity complexes made of human NCT, APH1aL, PEN2, and the pathogenic PS1 mutants L166P, ΔE9, or P436Q.

METHODOLOGY/PRINCIPAL FINDINGS

We took advantage of a mouse embryonic fibroblast cell line lacking PS1 and PS2 to generate different stable cell lines overexpressing human γ-secretase complexes with different FAD-linked PS1 mutations. A multi-step affinity purification procedure was used to isolate semi-purified or highly purified γ-secretase complexes. The functional characterization of these complexes revealed that all PS1 FAD-linked mutations caused a loss of γ-secretase activity phenotype, in terms of Aβ1-40, Aβ1-42 and APP intracellular domain productions in vitro.

CONCLUSION/SIGNIFICANCE

Our data support the view that PS1 mutations lead to a strong γ-secretase loss-of-function phenotype and an increased Aβ1-42/Aβ1-40 ratio, two mechanisms that are potentially involved in the pathogenesis of Alzheimer's disease.

Amyloid precursor protein (APP) mediated regulation of ganglioside homeostasis linking Alzheimer's disease pathology with ganglioside metabolism

Gangliosides are important players for controlling neuronal function and are directly involved in AD pathology. They are among the most potent stimulators of Aβ production, are enriched in amyloid plaques and bind amyloid beta (Aβ). However, the molecular mechanisms linking gangliosides with AD are unknown. Here we identified the previously unknown function of the amyloid precursor protein (APP), specifically its cleavage products Aβ and the APP intracellular domain (AICD), of regulating GD3-synthase (GD3S). Since GD3S is the key enzyme converting a- to b-series gangliosides, it therefore plays a major role in controlling the levels of major brain gangliosides. This regulation occurs by two separate and additive mechanisms. The first mechanism directly targets the enzymatic activity of GD3S: Upon binding of Aβ to the ganglioside GM3, the immediate substrate of the GD3S, enzymatic turnover of GM3 by GD3S was strongly reduced. The second mechanism targets GD3S expression. APP cleavage results, in addition to Aβ release, in the release of AICD, a known candidate for gene transcriptional regulation. AICD strongly down regulated GD3S transcription and knock-in of an AICD deletion mutant of APP in vivo, or knock-down of Fe65 in neuroblastoma cells, was sufficient to abrogate normal GD3S functionality. Equally, knock-out of the presenilin genes, presenilin 1 and presenilin 2, essential for Aβ and AICD production, or of APP itself, increased GD3S activity and expression and consequently resulted in a major shift of a- to b-series gangliosides. In addition to GD3S regulation by APP processing, gangliosides in turn altered APP cleavage. GM3 decreased, whereas the ganglioside GD3, the GD3S product, increased Aβ production, resulting in a regulatory feedback cycle, directly linking ganglioside metabolism with APP processing and Aβ generation. A central aspect of this homeostatic control is the reduction of GD3S activity via an Aβ-GM3 complex and AICD-mediated repression of GD3S transcription.

Trans fatty acids enhance amyloidogenic processing of the Alzheimer amyloid precursor protein (APP)

Hydrogenation of oils and diary products of ruminant animals leads to an increasing amount of trans fatty acids in the human diet. Trans fatty acids are incorporated in several lipids and accumulate in the membrane of cells. Here we systematically investigate whether the regulated intramembrane proteolysis of the amyloid precursor protein (APP) is affected by trans fatty acids compared to the cis conformation. Our experiments clearly show that trans fatty acids compared to cis fatty acids increase amyloidogenic and decrease nonamyloidogenic processing of APP, resulting in an increased production of amyloid beta (Aβ) peptides, main components of senile plaques, which are a characteristic neuropathological hallmark for Alzheimer's disease (AD). Moreover, our results show that oligomerization and aggregation of Aβ are increased by trans fatty acids. The mechanisms identified by this in vitro study suggest that the intake of trans fatty acids potentially increases the AD risk or causes an earlier onset of the disease.

APP dimer formation is initiated in the endoplasmic reticulum and differs between APP isoforms

The amyloid precursor protein (APP) is part of a larger gene family, which has been found to form homo- or heterotypic complexes with its homologues, whereby the exact molecular mechanism and origin of dimer formation remains elusive. In order to assess the cellular location of dimerization, we have generated a cell culture model system in CHO-K1 cells, stably expressing human APP, harboring dilysine-based organelle sorting motifs [KKAA-endoplasmic reticulum (ER); KKFF-Golgi], accomplishing retention within early secretory compartments. We show that APP exists as disulfide-bonded dimers upon ER retention after it was isolated from cells, and analyzed by SDS-polyacrylamide gel electrophoresis under non-reducing conditions. In contrast, strong denaturing and reducing conditions, or deletion of the E1 domain, resulted in the disappearance of those dimers. Thus we provide first evidence that a fraction of APP can associate via intermolecular disulfide bonds, likely generated between cysteines located in the extracellular E1 domain. We particularly visualize APP dimerization itself and identified the ER as subcellular compartment of its origin using biochemical or split GFP approaches. Interestingly, we also found that minor amounts of SDS-resistant APP dimers were located to the cell surface, revealing that once generated in the oxidative environment of the ER, dimers remained stably associated during transport. In addition, we show that APP isoforms encompassing the Kunitz-type protease inhibitor (KPI) domain exhibit a strongly reduced ability to form cis-directed dimers in the ER, whereas trans-mediated cell aggregation of Drosophila Schneider S2-cells was isoform independent. Thus, suggesting that steric properties of KPI-APP might be the cause for weaker cis-interaction in the ER, compared to APP695. Finally, we provide evidence that APP/APLP1 heterointeractions are likewise initiated in the ER.

Lysine 624 of the amyloid precursor protein (APP) is a critical determinant of amyloid β peptide length: support for a sequential model of γ-secretase intramembrane proteolysis and regulation by the amyloid β precursor protein (APP) juxtamembrane region

γ-Secretase is a multiprotein intramembrane cleaving aspartyl protease (I-CLiP) that catalyzes the final cleavage of the amyloid β precursor protein (APP) to release the amyloid β peptide (Aβ). Aβ is the primary component of senile plaques in Alzheimer's disease (AD), and its mechanism of production has been studied intensely. γ-Secretase executes multiple cleavages within the transmembrane domain of APP, with cleavages producing Aβ and the APP intracellular domain (AICD), referred to as γ and ε, respectively. The heterogeneous nature of the γ cleavage that produces various Aβ peptides is highly relevant to AD, as increased production of Aβ 1-42 is genetically and biochemically linked to the development of AD. We have identified an amino acid in the juxtamembrane region of APP, lysine 624, on the basis of APP695 numbering (position 28 relative to Aβ) that plays a critical role in determining the final length of Aβ peptides released by γ-secretase. Mutation of this lysine to alanine (K28A) shifts the primary site of γ-secretase cleavage from 1-40 to 1-33 without significant changes to ε cleavage. These results further support a model where ε cleavage occurs first, followed by sequential proteolysis of the remaining transmembrane fragment, but extend these observations by demonstrating that charged residues at the luminal boundary of the APP transmembrane domain limit processivity of γ-secretase.

Amyloid precursor protein mutation E682K at the alternative β-secretase cleavage β'-site increases Aβ generation

BACE1 cleaves the amyloid precursor protein (APP) at the β-cleavage site (Met₆₇₁–Asp₆₇₂) to initiate the generation of amyloid peptide Aβ. BACE1 is also known to cleave APP at a much less well-characterized β′-cleavage site (Tyr₆₈₁–Glu₆₈₂). We describe here the identification of a novel APP mutation E682K located at this β′-site in an early onset Alzheimer's disease (AD) case. Functional analysis revealed that this E682K mutation blocked the β′-site and shifted cleavage of APP to the β-site, causing increased Aβ production. This work demonstrates the functional importance of APP processing at the β′-site and shows how disruption of the balance between β- and β′-site cleavage may enhance the amyloidogenic processing and consequentially risk for AD. Increasing exon- and exome-based sequencing efforts will identify many more putative pathogenic mutations without conclusive segregation-based evidence in a single family. Our study shows how functional analysis of such mutations allows to determine the potential pathogenic nature of these mutations. We propose to classify the E682K mutation as probable pathogenic awaiting further independent confirmation of its association with AD in other patients.

Substrate docking to γ-secretase allows access of γ-secretase modulators to an allosteric site

γ-Secretase generates the peptides of Alzheimer's disease, Aβ₄₀ and Aβ₄₂, by cleaving the amyloid precursor protein within its transmembrane domain. γ-Secretase also cleaves numerous other substrates, raising concerns about γ-secretase inhibitor off-target effects. Another important class of drugs, γ-secretase modulators, alter the cleavage site of γ-secretase on amyloid precursor protein, changing the Aβ₄₂/Aβ₄₀ ratio, and are thus a promising therapeutic approach for Alzheimer's disease. However, the target for γ-secretase modulators is uncertain, with some data suggesting that they function on γ-secretase, whereas others support their binding to the amyloid precursor. In this paper we address this controversy by using a fluorescence resonance energy transfer-based assay to examine whether γ-secretase modulators alter Presenilin-1/γ-secretase conformation in intact cells in the absence of its natural substrates such as amyloid precursor protein and Notch. We report that the γ-secretase allosteric site is located within the γ-secretase complex, but substrate docking is needed for γ-secretase modulators to access this site.

γ-Secretase modulators have promise in the treatment of Alzheimer's disease, but their molecular target is uncertain. Here, fluorescence resonance energy transfer is used to determine that the γ-secretase allosteric site is within the γ-secretase complex and that substrate docking is required for modulators to access the site.

Rapid and direct transport of cell surface APP to the lysosome defines a novel selective pathway

Background

A central feature of Alzheimer's disease is the cleavage of the amyloid precursor protein (APP) to form beta-amyloid peptide (Aβ) by the β-secretase and γ-secretase enzymes. Although this has been shown to occur after endocytosis of APP from the cell surface, the exact compartments of APP processing are not well defined. We have previously demonstrated that APP and γ-secretase proteins and activity are highly enriched in purified rat liver lysosomes. In order to examine the lysosomal distribution and trafficking of APP in cultured cells, we generated constructs containing APP fused to a C-terminal fluorescent protein tag and N-terminal HA-epitope tag. These were co-transfected with a panel of fluorescent-protein tagged compartment markers.

Results

Here we demonstrate using laser-scanning confocal microscopy that although APP is present throughout the endosomal/lysosomal system in transfected Cos7 and neuronal SN56 cell lines as well as in immunostained cultured mouse neurons, it is enriched in the lysosome. We also show that the Swedish and London mutations reduce the amount of APP in the lysosome. Surprisingly, in addition to its expected trafficking from the cell surface to the early and then late endosomes, we find that cell-surface labelled APP is transported rapidly and directly from the cell surface to lysosomes in both Cos7 and SN56 cells. This rapid transit to the lysosome is blocked by the presence of either the London or Swedish mutations.

Conclusions

These results demonstrate the presence of a novel, rapid and specific transport pathway from the cell surface to the lysosomes. This suggests that regulation of lysosomal traffic could regulate APP processing and that the lysosome could play a central role in the pathophysiology of Alzheimer's disease.

An APP inhibitory domain containing the Flemish mutation residue modulates gamma-secretase activity for Abeta production

Gamma-secretase is an aspartyl protease that cleaves multiple substrates within their transmembrane domains. Gamma-secretase processes the amyloid precursor protein (APP) to generate gamma-amyloid (Agamma) peptides associated with Alzheimer's disease. Here, we show that APP possesses a substrate inhibitory domain (ASID) that negatively modulates gamma-secretase activity for Agamma production by binding to an allosteric site within the gamma-secretase complex. Alteration of this ASID by deletion or mutation, as is seen with the Flemish mutation (A21G), reduces its inhibitory potency and promotes Agamma production. Notably, peptides derived from ASID show selective inhibition of gamma-secretase activity for Agamma production over Notch1 processing. Therefore, this mode of regulation represents an unprecedented mechanism for modulating gamma-secretase, providing insight into the molecular basis of Alzheimer's disease pathogenesis and a potential strategy for the development of therapeutics.

alpha-secretase mediated conversion of the amyloid precursor protein derived membrane stub C99 to C83 limits Abeta generation

The Swedish mutation within the amyloid precursor protein (APP) causes early-onset Alzheimer's disease due to increased cleavage of APP by BACE1. While beta-secretase shedding of Swedish APP (APPswe) largely results from an activity localized in the late secretory pathway, cleavage of wild-type APP occurs mainly in endocytic compartments. However, we show that liberation of Abeta from APPswe is still dependent on functional internalization from the cell surface. Inspite the unchanged overall beta-secretase cleaved soluble APP released from APP(swe) secretion, mutations of the APPswe internalization motif strongly reduced C99 levels and substantially decreased Abeta secretion. We point out that alpha-secretase activity-mediated conversion of C99 to C83 is the main cause of this Abeta reduction. Furthermore, we demonstrate that alpha-secretase cleavage of C99 even contributes to the reduction of Abeta secretion of internalization deficient wild-type APP. Therefore, inhibition of alpha-secretase cleavage increased Abeta secretion through diminished conversion of C99 to C83 in APP695, APP695swe or C99 expressing cells.

Induced dimerization of the amyloid precursor protein leads to decreased amyloid-beta protein production

The amyloid precursor protein (APP) plays a central role in Alzheimer disease (AD) pathogenesis because sequential cleavages by beta- and gamma-secretase lead to the generation of the amyloid-beta (Abeta) peptide, a key constituent in the amyloid plaques present in brains of AD individuals. In several studies APP has recently been shown to form homodimers, and this event appears to influence Abeta generation. However, these studies have relied on APP mutations within the Abeta sequence itself that may affect APP processing by interfering with secretase cleavages independent of dimerization. Therefore, the impact of APP dimerization on Abeta production remains unclear. To address this question, we compared the approach of constitutive cysteine-induced APP dimerization with a regulatable dimerization system that does not require the introduction of mutations within the Abeta sequence. To this end we generated an APP chimeric molecule by fusing a domain of the FK506-binding protein (FKBP) to the C terminus of APP. The addition of the synthetic membrane-permeant drug AP20187 induces rapid dimerization of the APP-FKBP chimera. Using this system we were able to induce up to 70% APP dimers. Our results showed that controlled homodimerization of APP-FKBP leads to a 50% reduction in total Abeta levels in transfected N2a cells. Similar results were obtained with the direct precursor of beta-secretase cleavage, C99/SPA4CT-FKBP. Furthermore, there was no modulation of different Abeta peptide species after APP dimerization in this system. Taken together, our results suggest that APP dimerization can directly affect gamma-secretase processing and that dimerization is not required for Abeta production.

Upregulation of CRABP1 in human neuroblastoma cells overproducing the Alzheimer-typical Abeta42 reduces their differentiation potential

BACKGROUND

Alzheimer's disease (AD) is characterized by neurodegeneration and changes in cellular processes, including neurogenesis. Proteolytic processing of the amyloid precursor protein (APP) plays a central role in AD. Owing to varying APP processing, several beta-amyloid peptides (Abeta) are generated. In contrast to the form with 40 amino acids (Abeta40), the variant with 42 amino acids (Abeta42) is thought to be the pathogenic form triggering the pathological cascade in AD. While total-Abeta effects have been studied extensively, little is known about specific genome-wide effects triggered by Abeta42 or Abeta40 derived from their direct precursor C99.

METHODS

A combined transcriptomics/proteomics analysis was performed to measure the effects of intracellularly generated Abeta peptides in human neuroblastoma cells. Data was validated by real-time polymerase chain reaction (real-time PCR) and a functional validation was carried out using RNA interference.

RESULTS

Here we studied the transcriptomic and proteomic responses to increased or decreased Abeta42 and Abeta40 levels generated in human neuroblastoma cells. Genome-wide expression profiles (Affymetrix) and proteomic approaches were combined to analyze the cellular response to the changed Abeta42- and Abeta40-levels. The cells responded to this challenge with significant changes in their expression pattern. We identified several dysregulated genes and proteins, but only the cellular retinoic acid binding protein 1 (CRABP1) was up-regulated exclusively in cells expressing an increased Abeta42/Abeta40 ratio. This consequently reduced all-trans retinoic acid (RA)-induced differentiation, validated by CRABP1 knock down, which led to recovery of the cellular response to RA treatment and cellular sprouting under physiological RA concentrations. Importantly, this effect was specific to the AD typical increase in the Abeta42/Abeta40 ratio, whereas a decreased ratio did not result in up-regulation of CRABP1.

CONCLUSION

We conclude that increasing the Abeta42/Abeta40 ratio up-regulates CRABP1, which in turn reduces the differentiation potential of the human neuroblastoma cell line SH-SY5Y, but increases cell proliferation. This work might contribute to the better understanding of AD neurogenesis, currently a controversial topic.

Amyloid beta-protein precursor juxtamembrane domain regulates specificity of gamma-secretase-dependent cleavages

Amyloid beta-protein (Abeta), the major component of cerebral plaques associated with Alzheimer disease, is derived from amyloid beta-protein precursor (APP) through sequential proteolytic cleavage involving beta- and gamma-secretase. The intramembrane cleavage of APP by gamma-secretase occurs at two major sites, gamma and epsilon, although the temporal and/or mechanistic relationships between these cleavages remain unknown. In our attempt to address this issue, we uncovered an important regulatory role for the APP luminal juxtamembrane domain. We demonstrated in cell-based assays that domain replacements in this region can greatly reduce secreted Abeta resulting from gamma-cleavage without affecting the epsilon-cleavage product. This Abeta reduction is likely due to impaired proteolysis at the gamma-cleavage site. Further analyses with site-directed mutagenesis identified two juxtamembrane residues, Lys-28 and Ser-26 (Abeta numbering), as the critical determinants for efficient intramembrane proteolysis at the gamma-site. Consistent with the growing evidence that epsilon-cleavage of APP precedes gamma-processing, longer Abeta species derived from the gamma-cleavage-deficient substrates were detected intracellularly. These results indicate that the luminal juxtamembrane region of APP is an important regulatory domain that modulates gamma-secretase-dependent intramembrane proteolysis, particularly in differentiating gamma- and epsilon-cleavages.

In vitro gamma-secretase cleavage of the Alzheimer's amyloid precursor protein correlates to a subset of presenilin complexes and is inhibited by zinc

The gamma-secretase complex mediates the final proteolytic event in Alzheimer's disease amyloid-beta biogenesis. This membrane complex of presenilin, anterior pharynx defective, nicastrin, and presenilin enhancer-2 cleaves the C-terminal 99-amino acid fragment of the amyloid precursor protein intramembranously at gamma-sites to form C-terminally heterogeneous amyloid-beta and cleaves at an epsilon-site to release the intracellular domain or epsilon-C-terminal fragment. In this work, two novel in vitro gamma-secretase assays are developed to further explore the biochemical characteristics of gamma-secretase activity. During development of a bacterial expression system for a substrate based on the amyloid precursor protein C-terminal 99-amino acid sequence, fragments similar to amyloid-beta and an epsilon-C-terminal fragment were observed. Upon purification this substrate was used in parallel with a transfected source of substrate to measure gamma-secretase activity from detergent extracted membranes. With these systems, it was determined that recovery of size-fractionated cellular and tissue-derived gamma-secretase activity is dependent upon detergent concentration and that activity correlates to a subset of high molecular mass presenilin complexes. We also show that by changing the solvent environment with dimethyl sulfoxide, detection of epsilon-C-terminal fragments can be elevated. Lastly, we show that zinc causes an increase in the apparent molecular mass of an amyloid precursor protein gamma-secretase substrate and inhibits its cleavage. These studies further refine our knowledge of the complexes and biochemical factors needed for gamma-secretase activity and suggest a mechanism by which zinc dysregulation may contribute to Alzheimer's disease pathogenesis.

Longer forms of amyloid beta protein: implications for the mechanism of intramembrane cleavage by gamma-secretase

Gamma-cleavage of beta-amyloid precursor protein (APP) in the middle of the cell membrane generates amyloid beta protein (Abeta), and epsilon-cleavage, approximately 10 residues downstream of the gamma-cleavage site, releases the APP intracellular domain (AICD). A significant link between generation of Abeta and AICD and failure to detect AICD41-99 led us to hypothesize that epsilon-cleavage generates longer Abetas, which are then processed to Abeta40/42. Using newly developed gel systems and an N-end-specific monoclonal antibody, we have identified the longer Abetas (Abeta1-43, Abeta1-45, Abeta1-46, and Abeta1-48) within the cells and in brain tissues. The production of these longer Abetas as well as Abeta40/42 is presenilin dependent and is suppressed by {1S-benzyl-4R-[1S-carbamoyl-2-phenylethylcarbamoyl-1S-3-methylbutylcarbamoyl]-2R-hydroxy-5-phenylpentyl}carbamic acid tert-butyl ester, a transition state analog inhibitor for aspartyl protease. In contrast, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, a potent dipeptide gamma-secretase inhibitor, builds up Abeta1-43 and Abeta1-46 intracellularly, which was also confirmed by mass spectrometry. Notably, suppression of Abeta40 appeared to lead to an increase in Abeta43, which in turn brings an increase in Abeta46, in a dose-dependent manner. We therefore propose an alpha-helical model in which longer Abeta species generated by epsilon-cleavage is cleaved at every three residues in its carboxyl portion.

Truncated carboxyl-terminal fragments of beta-amyloid precursor protein are processed to amyloid beta-proteins 40 and 42

We previously showed that beta-amyloid precursor protein (APP) is cleaved not only in the middle of the membrane (gamma-cleavage) but also at novel cleavage sites close to the membrane/cytoplasmic boundary (epsilon-cleavage), releasing APP intracellular domains (AICDs) 49-99 and 50-99. To learn more about the relationship between gamma- and epsilon-cleavage, C-terminally truncated carboxyl-terminal fragments (CTFs) of APP, especially CTFs1-48 and 1-49 (the postulated products that are generated by epsilon-cleavage), were transiently expressed in CHO cells. Most importantly, the cells expressing CTF1-49 secreted predominantly amyloid beta-protein (Abeta) 40, while those expressing CTF1-48 secreted preferentially Abeta42. This supports our assumption that epsilon-cleavage precedes Alphabeta production and that preceding epsilon-cleavage determines the preference for the final Abeta species. The gamma-secretase inhibitors, L-685,458 and DAPT, suppressed Abeta production from CTF1-49. Regarding Abeta production from CTF1-48, L-685,458 suppressed it, but DAPT failed to do so. A dominant negative mutant of presenilin 1 suppressed the production of Abeta40 and 42 from both CTFs1-48 and 1-49. These data should shed significant light into the mechanism of Abeta production.

Cell-type-specific processing of the amyloid precursor protein by Presenilin during Drosophila development

The cleavage of proteins within their transmembrane domain by Presenilin (PS) has an important role in different signalling pathways and in Alzheimer's disease. Nevertheless, not much is known about the regulation of PS activity. It has been suggested that substrate recognition by the PS complex depends only on the size of the extracellular domain independent of the amino-acid sequence and that PS activity is constitutive in all cells that express the minimal components of the complex. We report here the development of an in vivo reporter system that allowed us to analyse the processing of human amyloid precursor protein (APP) and the Notch receptor tissue specifically during Drosophila development in the living organism. Using this system, we demonstrate differences between APP and Notch processing and show that PS-mediated cleavage of APP can be regulated in different cell types independent of the size of the extracellular domain.

X11α impairs γ- but not β-cleavage of amyloid precursor protein

The phosphotyrosine binding domain of the neuronal protein X11alpha/mint-1 binds to the C-terminus of amyloid precursor protein (APP) and inhibits catabolism to beta-amyloid (Abeta), but the mechanism of this effect is unclear. Coexpression of X11alpha or its PTB domain with APPswe inhibited secretion of Abeta40 but not APPsbetaswe, suggesting inhibition of gamma- but not beta-secretase. To further probe cleavage(s) inhibited by X11alpha, we coexpressed beta-secretase (BACE-1) or a component of the gamma-secretase complex (PS-1Delta9) with APP, APPswe, or C99, with and without X11alpha, in HEK293 cells. X11alpha suppressed the PS-1Delta9-induced increase in Abeta42 secretion generated from APPswe or C99. However, X11alpha did not impair BACE-1-mediated proteolysis of APP or APPswe to C99. In contrast to impaired gamma-cleavage of APPswe, X11alpha or its PTB domain did not inhibit gamma-cleavage of NotchDeltaE to NICD (the Notch intracellular domain). The X11alpha PDZ-PS.1Delta9 interaction did not affect gamma-cleavage activity. In a cell-free system, X11alpha did not inhibit the catabolism of APP C-terminal fragments. These data suggest that X11alpha may inhibit Abeta secretion from APP by impairing its trafficking to sites of active gamma-secretase complexes. By specifically targeting substrate instead of enzyme X11alpha may function as a relatively specific gamma-secretase inhibitor.

Novel cadherin-related membrane proteins, Alcadeins, enhance the X11-like protein-mediated stabilization of amyloid beta-protein precursor metabolism

Previously we found that X11-like protein (X11L) associates with amyloid beta-protein precursor (APP). X11L stabilizes APP metabolism and suppresses the secretion of the amyloid beta-protein (Abeta) that are the pathogenic agents of Alzheimer's disease (AD). Here we found that Alcadein (Alc), a novel membrane protein family that contains cadherin motifs and originally reported as calsyntenins, also interacted with X11L. Alc was abundant in the brain and occurred in the same areas of the brain as X11L. X11L could simultaneously associate with APP and Alc, resulting in the formation of a tripartite complex in brain. The tripartite complex stabilized intracellular APP metabolism and enhanced the X11L-mediated suppression of Abeta secretion that is due to the retardation of intracellular APP maturation. X11L and Alc also formed another complex with C99, a carboxyl-terminal fragment of APP cleaved at the beta-site (CTFbeta). The formation of the Alc.X11L.C99 complex inhibited the interaction of C99 with presenilin, which strongly suppressed the gamma-cleavage of C99. In AD patient brains, Alc and APP were particularly colocalized in dystrophic neurites in senile plaques. Deficiencies in the X11L-mediated interaction between Alc and APP and/or CTFbeta enhanced the production of Abeta, which may be related to the development or progression of AD.

gamma-Secretase cleavage site specificity differs for intracellular and secretory amyloid beta

The final step in A beta generation is the cleavage of the C-terminal 99 amino acid residues of the amyloid precursor protein by gamma-secretase. gamma-Secretase activity is closely linked to the multi-transmembrane-spanning proteins presenilin 1 and presenilin 2. To elucidate whether the cleavage site specificities of gamma-secretase leading to the formation of secreted and intracellular A beta are identical, we made use of point mutations close to the gamma-cleavage site, known to have a dramatic effect on the 42/40 ratio of secreted A beta. We found that the selected point mutations only marginally influenced the 42/40 ratio of intracellular A beta, suggesting differences in the gamma-secretase cleavage site specificity for the generation of secreted and intracellular A beta. The analysis of the subcellular compartments involved in the generation of intracellular A beta revealed that A beta is not generated in the early secretory pathway in the human SH-SY5Y neuroblastoma cell line. In this study we identified late Golgi compartments to be involved in the generation of intracellular A beta. Moreover, we demonstrate that the presence of processed PS1 is not sufficient to obtain gamma-secretase processing of the truncated amyloid precursor protein construct C99, proposing the existence of an additional factor downstream of the endoplasmic reticulum and early Golgi required for the formation of an active gamma-secretase complex.

No alterations of hippocampal neuronal number and synaptic bouton number in a transgenic mouse model expressing the beta-cleaved C-terminal APP fragment

Previous studies in the literature have resulted in conflicting reports on the potential neurotoxicity of the beta-cleaved Alzheimer's disease C-terminal fragment (beta-CTF) of beta-amyloid precursor protein in vivo. To readdress this question by rigorous quantitative methods, we analyzed transgenic mice expressing human beta-CTF with the I45F mutation (SPA4CT) under control of the prion protein promoter by stereological techniques. The transgene was expressed in hippocampus and cortex in large pyramidal neurons and in dentate gyrus granule cells. Proteolytic processing of beta-CTF released Abeta. However, most of it remained uncleaved. Neurodegeneration was evaluated by investigating the numbers of hippocampal pyramidal and granule neurons, as well as the number of synaptophysin-immunopositive presynaptic boutons in the hippocampus of 15-month-old SPA4CT mice with design-based stereological techniques. The analyses showed that a fourfold higher expression of the transgene compared to murine APP levels had no effect on the numbers of both neurons and synaptophysin-immunopositive presynaptic boutons. These data implicate that expression of beta-CTF per se is not neurotoxic, and that other mechanisms are responsible for the neurotoxic events in Alzheimer's disease brain.

The transmembrane domain of the amyloid precursor protein in microsomal membranes is on both sides shorter than predicted

The amyloid precursor protein is cleaved within its ectodomain by beta-amyloid-converting enzyme (BACE) yielding C99, which is further cleaved by gamma-secretase within its putative transmembrane domain (TMD). Because it is difficult to envisage how a protease may cleave within the membrane, alternative mechanisms have been proposed for gamma-cleavage in which the TMD is shorter than predicted or positioned such that the gamma-cleavage site is accessible to cytosolic proteases. Here, we have biochemically determined the length of the TMD of C99 in microsomal membranes. Using a single cysteine mutagenesis scan of C99 combined with cysteine modification with a membrane-impermeable labeling reagent, we identified which residues are accessible to modification and thus located outside of the membrane. We find that in endoplasmic reticulum-derived microsomes the TMD of C99 consists of 12 residues that span from residues 37 to 48, which is N- and C-terminally shorter than predicted. Thus, the gamma-cleavage sites are positioned around the middle of the lipid bilayer and are unlikely to be accessible to cytosolic proteases. Moreover, the center of the TMD is positioned at the gamma-cleavage site at residue 42. Our data are consistent with a model in which gamma-secretase is a membrane protein that cleaves at the center of the membrane.

Selecting cells with different Alzheimer's disease gamma-secretase activity using FACS. Differential effect on presenilin exon 9 gamma- and epsilon-cleavage

The ultimate step in Alzheimer's disease Abeta generation involves gamma-secretase, which releases Abeta from its membrane-bound precursor. A similar presenilin-dependent proteolytic activity is implicated in the release of the Notch intracellular domain. We have developed a novel assay for gamma-secretase activity based on green fluorescent protein detection. This involves cotransfection of a substrate-activator based on the amyloid precursor protein or the Notch sequence and a fluorescent reporter gene. Stable fluorescent cell populations were selected by fluorescent activated cell sorting and characterized. This assay enabled the identification and sorting of populations, which differ in their levels of gamma-secretase activity, with high fluorescent cells producing more Abeta than low fluorescent cells. Specific gamma-secretase inhibitors, L-685,458 and MW167, reduced cell fluorescence in a dose-dependent manner that paralleled inhibition of Abeta secretion. Overexpression of presenilin 1 increased the cell fluorescence. Cells expressing presenilin with different aspartate mutations (D257A, D385A and D257A/D385A) or exon 9 deletion mutation showed reduced fluorescence. The single aspartate mutations showed a concomitant reduction in Abeta secretion, whereas the D257A/D385A and DeltaE9 mutations had no effect on Abeta secretion.

Failure of the interaction between presenilin 1 and the substrate of gamma-secretase to produce Abeta in insect cells

Aggregates of beta-amyloid peptide (Abeta) are the major component of the amyloid core of the senile plaques observed in Alzheimer's disease (AD). Abeta results from the amyloidogenic processing of its precursor, the amyloid precursor protein (APP), by beta- and gamma-secretase activities. If beta-secretase has recently been identified and termed BACE, the identity of gamma-secretase is still obscure. Studies with knock-out mice showed that presenilin 1 (PS1), of which mutations are known to be the first cause of inherited AD, is mandatory for the gamma-secretase activity. However, the proteolytic activity of PS1 remains a matter of debate. Here we used transfected Sf9 insect cells, a cellular model lacking endogenous beta- and/or gamma-secretase activities, to characterize the role of BACE and PS1 in the amyloidogenic processing of human APP. We show that, in Sf9 cells, BACE performs the expected beta-secretase cleavage of APP, generating C99. We also show that C99, which is a substrate of gamma-secretase, tightly binds to the human PS1. Despite this interaction, Sf9 cells still do not produce Abeta. This strongly argues against a direct proteolytic activity of PS1 in APP processing, and points toward an implication of PS1 in trafficking/presenting its substrate to the gamma-secretase.

The amyloid precursor protein (APP)-cytoplasmic fragment generated by gamma-secretase is rapidly degraded but distributes partially in a nuclear fraction of neurones in culture

The gamma-secretase cleavage is the last step in the generation of the beta-amyloid peptide (Abeta) from the amyloid precursor protein (APP). The Abeta precipitates in the amyloid plaques in the brain of Alzheimer's disease patients. The fate of the intracellular APP carboxy-terminal stub generated together with Abeta has been, in contrast, only poorly documented. The analogies between the processing of APP and other transmembrane proteins like SREBP and Notch suggests that this intracellular fragment could have important signalling functions. We demonstrate here that APP-C59 is rapidly degraded (half-life approximately 5 min) when overexpressed in baby hamster kidney cells or primary cultures of neurones by a mechanism that is not inhibited by endosomal/lysosomal or proteasome inhibitors. Furthermore, APP-C59 binds to the DNA binding protein Fe65, although this does not increase the half-life of APP-C59. Finally, we demonstrate that a fraction of APP-C59 becomes redistributed to the nuclear detergent-insoluble pellet, in which the transcription factor SP1 is also present. Overall our results reinforce the analogy between Notch and APP processing, and suggest that the APP intracellular domain, like the Notch intracellular domain, could have a role in signalling events from the plasma membrane to the nucleus.

The discrepancy between presenilin subcellular localization and gamma-secretase processing of amyloid precursor protein

We investigated the relationship between PS1 and gamma-secretase processing of amyloid precursor protein (APP) in primary cultures of neurons. Increasing the amount of APP at the cell surface or towards endosomes did not significantly affect PS1-dependent gamma-secretase cleavage, although little PS1 is present in those subcellular compartments. In contrast, almost no gamma-secretase processing was observed when holo-APP or APP-C99, a direct substrate for gamma-secretase, were specifically retained in the endoplasmic reticulum (ER) by a double lysine retention motif. Nevertheless, APP-C99-dilysine (KK) colocalized with PS1 in the ER. In contrast, APP-C99 did not colocalize with PS1, but was efficiently processed by PS1-dependent gamma-secretase. APP-C99 resides in a compartment that is negative for ER, intermediate compartment, and Golgi marker proteins. We conclude that gamma-secretase cleavage of APP-C99 occurs in a specialized subcellular compartment where little or no PS1 is detected. This suggests that at least one other factor than PS1, located downstream of the ER, is required for the gamma-cleavage of APP-C99. In agreement, we found that intracellular gamma-secretase processing of APP-C99-KK both at the gamma40 and the gamma42 site could be restored partially after brefeldin A treatment. Our data confirm the "spatial paradox" and raise several questions regarding the PS1 is gamma-secretase hypothesis.

Beta-amyloid peptide expression is sufficient for myotube death: implications for human inclusion body myopathy

Inclusion body myositis (sIBM) is the most common disorder of skeletal muscle in aged humans. It shares biochemical features with Alzheimer's disease, including congophilic deposits, which are immunoreactive for beta-amyloid peptide (Abeta) and C'-terminal betaAPP epitopes. However, the etiology of myofiber loss and the role of intracellular Abeta in IBM is unknown. Here we report correlative evidence for apoptotic cell death in myofibers of IBM patients that exhibit pronounced Abeta deposition. HSV-1-mediated gene transfer of Abeta(42) into cultured C2C12 myotubes resulted in a 12.6-fold increase in dUTP-labeled and condensed nuclei over nonexpressing myotubes (P < 0.05). The C'-terminal betaAPP domain C99 also induced myotube apoptosis, but to a significantly lesser extent than Abeta. Apoptosis specific to Abeta-expressing myotubes was also demonstrated through DNA fragmentation, decreased mitochondrial function and the loss of membrane phospholipid polarity. Myotubes laden with Abeta(42), but not other transgene products, developed cytoplasmic inclusions consisting of fibrillar material. Furthermore, injection of normal mouse gastrocnemius muscle with HSV-encoding Abeta cDNA resulted in TUNEL-positive myofibers with pyknotic nuclei. We conclude that Abeta is sufficient to induce apoptosis in myofibers both in vivo and in vitro and suggest it may contribute to myofiber loss and muscle dysfunction in patients with IBM.