gamma-secretase cleavage is distinct from endoplasmic reticulum degradation of the transmembrane domain of the amyloid precursor protein

The Tao Hong Si Wu Decoction ameliorates diabetes-associated cognitive dysfunction by inhibiting the formation of amyloid plaques

OBJECTIVES

The herbs in Tao Hong Si Wu Decoction (THSWD) are beneficial in the treatment of cognitive impairment. However, the underlying mechanisms of THSWD in treating diabetes-associated cognitive dysfunction (DACD) are not entirely explored. This study is aimed to investigate the therapeutic effects of THSWD in DACD model rats and the underlying mechanism.

METHODS

Ultra-high-phase liquid chromatography was employed to identify the main compounds contained in the THSWD extract. DACD rat model was induced by feeding with a high-sugar and high-fat diet and injecting streptozotocin (35 mg/kg). DACD rats were gavaged with THSWD for 1 week. The learning and memory abilities of the rats were measured by using the Morris water maze. Western blotting was used to detect the changes in DACD rat targets. Statistical methods were used to evaluate the correlation between proteins.

RESULTS

The results show that THSWD effectively reduced the escape latency, hippocampal neuron damage, glycosylated hemoglobin, type A1C, and blood lipid levels in DACD rats. Furthermore, DACD rats showed significantly increased amyloid precursor protein, β-secretase, Aβ1-40 , Aβ1-42 , Tau phosphorylation, and advanced glycation end products (AGEs) expression. However, THSWD treatment can reverse this phenomenon.

CONCLUSIONS

THSWD can improve the learning and memory abilities of DACD rats by inhibiting the expression of AEGs-AGE receptors pathway, which provides an experimental basis for the clinical application of THSWD. In addition, the experiment combines pharmacological and statistical methods, which offers a new perspective for the study of Chinese herbal medicine.

Localization and Processing of the Amyloid-β Protein Precursor in Mitochondria-Associated Membranes

Alteration of mitochondria-associated membranes (MAMs) has been proposed to contribute to the pathogenesis of Alzheimer’s disease (AD). We studied herein the subcellular distribution, the processing, and the protein interactome of the amyloid-β protein precursor (AβPP) and its proteolytic products in MAMs. We reveal that AβPP and its catabolites are present in MAMs in cellular models overexpressing wild type AβPP or AβPP harboring the double Swedish or London familial AD mutations, and in brains of transgenic mice model of AD. Furthermore, we evidenced that both β- and γ-secretases are present and harbor AβPP processing activities in MAMs. Interestingly, cells overexpressing APP_swe show increased ER-mitochondria contact sites. We also document increased neutral lipid accumulation linked to Aβ production and reversed by inhibiting β- or γ-secretases. Using a proteomic approach, we show that AβPP and its catabolites interact with key proteins of MAMs controlling mitochondria and ER functions. These data highlight the role of AβPP processing and proteomic interactome in MAMs deregulation taking place in AD.

Physiological Roles of Ubiquitin Ligases Related to the Endoplasmic Reticulum

Studies on endoplasmic reticulum (ER)-associated degradation (ERAD), in which unfolded proteins accumulated in the ER are selectively transported to the cytosol for degradation by the ubiquitin-proteasome system, have been focused on molecular mechanisms in yeast. In human, disruption of the ER quality control system causes various diseases, such as neurodegenerative disease, lifestyle disease, and cancer. However, there are many ERAD genes with unknown physiological and pathological functions. We identified the novel ubiquitin ligase HRD1 involved in ERAD. HRD1 is expressed in brain neurons and protects against ER stress-induced apoptosis. In familial Parkinson's disease, accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R), a substrate of ubiquitin ligase Parkin involved in ERAD, causes ER stress and apoptosis. We demonstrated that HRD1 promotes ubiquitination and degradation of Pael-R and suppresses ER stress and apoptosis induced by Pael-R. Amyloid precursor protein (APP) is processed into amyloid β (Aβ) in Alzheimer's disease. We found that HRD1 promotes APP ubiquitination and degradation, resulting in decreased generation of Aβ. Furthermore, suppression of HRD1 expression causes APP accumulation and Aβ generation associated with ER stress and apoptosis. Interestingly, HRD1 protein levels significantly decreased in the cerebral cortex of Alzheimer's disease patients, possibly because of its insolubilization. We demonstrated that HRD1 protein was insolubilized by oxidative stress, resulting in the accumulation of HRD1 into the aggresome. In conclusion, oxidative stress-induced HRD1 insolubilization might be involved in a vicious cycle of increased Aβ production and Aβ-induced oxidative stress in Alzheimer's disease pathogenesis.

Traditional Chinese Nootropic Medicine Radix Polygalae and Its Active Constituent Onjisaponin B Reduce β-Amyloid Production and Improve Cognitive Impairments

Decline of cognitive function is the hallmark of Alzheimer’s disease (AD), regardless of the pathological mechanism. Traditional Chinese medicine has been used to combat cognitive impairments and has been shown to improve learning and memory. Radix Polygalae (RAPO) is a typical and widely used herbal medicine. In this study, we aimed to follow the β-amyloid (Aβ) reduction activity to identify active constituent(s) of RAPO. We found that Onjisaponin B of RAPO functioned as RAPO to suppress Aβ production without direct inhibition of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) and γ-secretase activities. Our mechanistic study showed that Onjisaponin B promoted the degradation of amyloid precursor protein (APP). Further, oral administration of Onjisaponin B ameliorated Aβ pathology and behavioral defects in APP/PS1 mice. Taken together, our results indicate that Onjisaponin B is effective against AD, providing a new therapeutic agent for further drug discovery.

[Pharmacological studies on neurodegenerative diseases focusing on refolding and degradation of unfolded proteins in the endoplasmic reticulum]

The endoplasmic reticulum (ER) has physiological roles in the quality control of proteins. Various stresses (e.g., oxidation, aging) to the ER cause accumulation of unfolded/misfolded proteins in the ER lumen, followed by unfolded protein responses (UPR) such as refolding of unfolded protein by chaperons, ER-associated degradation (ERAD), and termination of protein synthesis. In this study, we identified protein-disulfide isomerase (PDI) upregulation by hypoxic stress in the ER of rat brains/astroglial cells. PDI overexpression attenuates hypoxia-induced neuronal apoptosis. In the brain autopsy of patients with sporadic Alzheimer's and Parkinson diseases, PDI was found to be S-nitrosylated, which reduced chaperone activity of PDI, suggesting the involvement of PDI in these diseases. In addition, we identified the novel E3 ubiquitin ligase HRD1 and observed that HRD1 activates degradation of Parkin-associated endothelin receptor-like receptor (Pael-R). HRD1 suppresses ER stress and Pael-R-induced apoptosis. Furthermore, HRD1 ubiquitinates amyloid precursor protein (APP), resulting in the decrease in amyloid β (Aβ) generation. Suppression of HRD1 expression causes APP accumulation and Aβ generation. HRD1 protein significantly decreased in the cerebral cortex of patients with Alzheimer's disease. HRD1 decrease in the brain of patients with Alzheimer's disease could be due to the insolubilization of HRD1 by oxidative stress. Subsequently, we observed that 4-phenylbutyric acid (4-PBA) possesses chaperone activity, which prevents protein aggregation and that 4-(4-methoxyphenyl)butanoic acid, a 4-PBA derivative, increases protective ability against ER stress-induced neuronal death. We believe that 4-PBA and its derivatives are potential candidates for pharmacological intervention for ER stress-induced neurodegenerative diseases.

[Molecular pharmacological studies on the protection mechanism against endoplasmic reticulum stress-induced neurodegenerative disease]

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a mechanism against ER stress, wherein unfolded proteins accumulated in the ER are transported to the cytosol for degradation by the ubiquitin-proteasome system. We identified the novel ubiquitin ligase HRD1 involved in ERAD. HRD1 is expressed in brain neurons and protects against ER stress-induced apoptosis. In familial Parkinson's disease, accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R), a substrate of ubiquitin ligase Parkin involved in ERAD, leads to ER stress and apoptosis. We have demonstrated that HRD1 promotes ubiquitination and degradation of Pael-R and suppresses ER stress and apoptosis induced by Pael-R. Amyloid precursor protein (APP) is processed into amyloid β (Aβ) in Alzheimer's disease. We showed that HRD1 promotes APP ubiquitination and degradation, resulting in decreased generation of Aβ. Furthermore, suppression of HRD1 expression causes APP accumulation and Aβ generation associated with ER stress and apoptosis. Interestingly, HRD1 levels significantly decreased in the cerebral cortex of Alzheimer's disease patients, possibly because of its insolubilization. 4-phenylbutyrate (4-PBA) has been demonstrated to restore normal trafficking and activity of mutant proteins by acting as a chemical chaperone. We demonstrated that 4-PBA possesses chaperone activity in vitro, and this prevents protein aggregation. Furthermore, we revealed that 4-PBA attenuates the activation of ER stress responses and neuronal cell death, suggesting that HRD1 decreases unfolded protein accumulation in the ER. In addition, 4-PBA restores the normal expression of Pael-R protein and suppresses Pael-R-induced ER stress. Therefore, 4-PBA is a potential candidate for use in the pharmacotherapy of several neurodegenerative diseases linked to ER stress.

Loss of HRD1-mediated protein degradation causes amyloid precursor protein accumulation and amyloid-beta generation

Endoplasmic reticulum-associated degradation (ERAD) is a system by which proteins accumulated in the endoplasmic reticulum (ER) are retrotranslocated to the cytosol and degraded by the ubiquitin-proteasome pathway. HRD1 is expressed in brain neurons and acts as an ERAD ubiquitin ligase. Amyloid precursor protein (APP) is processed into amyloid-beta peptides (Abetas) that form plaque deposits in the brains of Alzheimer's disease (AD) patients. We found significantly decreased HRD1 protein levels in the cerebral cortex of AD patients. HRD1 colocalized with APP in brain neurons and interacted with APP through the proline-rich region of HRD1. HRD1 promoted APP ubiquitination and degradation, resulting in decreased generation of Abeta. Furthermore, suppression of HRD1 expression induced APP accumulation that led to increased production of Abeta associated with ER stress. Immunohistochemical analysis revealed that suppression of HRD1 expression inhibited APP aggresome formation, resulting in apoptosis. In addition, we found that the ATF6- and XBP1-induced upregulation of ERAD led to APP degradation and reduced Abeta production. These results suggest that the breakdown of HRD1-mediated ERAD causes Abeta generation and ER stress, possibly linked to AD.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Abeta(25-35) attenuated SREBP level in nuclear extracts of serum-deprived human neuroblastoma cells

Disturbance in cholesterol homeostasis appears to be an important factor in the pathogenesis of neurodegenerative disorders. The aim of the present study was to investigate sterol regulatory element binding protein (SREBP) levels in the nuclear extracts of human neuroblastoma cells and the possible interaction of beta-amyloid peptide (Abeta) and cholesterol with this transcription factor. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in serum-deprived media in the presence or absence of Abeta((25-35)) (1 microM) or cholesterol (300 microM) for 24 h. Nuclear extracts were subjected to SDS-PAGE, and SREBP cleavage product (68 kDa) was detected by immunoblotting. SREBP levels were elevated in the cells incubated 24 h in serum-deprived experimental media and were attenuated by Abeta or cholesterol-supplementation. It is likely that the ability of Abeta to release cholesterol into the medium and downregulate SREBP is due to a feedback mechanism.

Independent inhibition of Alzheimer disease beta- and gamma-secretase cleavage by lowered cholesterol levels

The major molecular risk factor for Alzheimer disease so far identified is the amyloidogenic peptide Abeta(42). In addition, growing evidence suggests a role of cholesterol in Alzheimer disease pathology and Abeta generation. However, the cellular mechanism of lipid-dependent Abeta production remains unclear. Here we describe that the two enzymatic activities responsible for Abeta production, beta-secretase and gamma-secretase, are inhibited in parallel by cholesterol reduction. Importantly, our data indicate that cholesterol depletion within the cellular context inhibits both secretases additively and independently from each other. This is unexpected because the beta-secretase beta-site amyloid precursor protein cleaving enzyme and the presenilin-containing gamma-secretase complex are structurally different from each other, and these enzymes are apparently located in different subcellular compartments. The parallel and additive inhibition has obvious consequences for therapeutic research and may indicate an intrinsic cross-talk between Alzheimer disease-related amyloid precursor protein processing, amyloid precursor protein function, and lipid biology.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

HtrA2 Regulates β-Amyloid Precursor Protein (APP) Metabolism through Endoplasmic Reticulum-associated Degradation

Alzheimer disease-associated beta-amyloid peptide is generated from its precursor protein APP. By using the yeast two-hybrid assay, here we identified HtrA2/Omi, a stress-responsive chaperone-protease as a protein binding to the N-terminal cysteinerich region of APP. HtrA2 coimmunoprecipitates exclusively with immature APP from cell lysates as well as mouse brain extracts and degrades APP in vitro. A subpopulation of HtrA2 localizes to the cytosolic side of the endoplasmic reticulum (ER) membrane where it contributes to ER-associated degradation of APP together with the proteasome. Inhibition of the proteasome results in accumulation of retrotranslocated forms of APP and increased association of APP with HtrA2 and Derlin-1 in microsomal membranes. In cells lacking HtrA2, APP holoprotein is stabilized and accumulates in the early secretory pathway correlating with elevated levels of APP C-terminal fragments and increased Abeta secretion. Inhibition of ER-associated degradation (either HtrA2 or proteasome) promotes binding of APP to the COPII protein Sec23 suggesting enhanced trafficking of APP out of the ER. Based on these results we suggest a novel function for HtrA2 as a regulator of APP metabolism through ER-associated degradation.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Intracellular pathways of folded and misfolded amyloid precursor protein degradation

A number of studies suggest that early events in the maturation of amyloid precursor protein (APP) are important in determining its entry into one of several alternative processing pathways, one of which leads to the toxic protein beta-amyloid (Abeta). In pulse-labeled APP expressing CHO cells two proteolytic systems can degrade newly translated APP: the proteosome and a cysteine protease. When N-glycosylation was inhibited by tunicamycin, the former system is the dominant mechanism of APP degradation. Without tunicamycin present, the cysteine protease is operational: cysteine protease inhibitors completely inhibit APP turnover in cells in which the secretory pathway is interrupted with brefeldin A or when alpha-secretase and endosomal degradation are also pharmacologically blocked. APP immunoprecipitated from cells extracted under mild conditions and labeled in the presence of tunicamycin exhibited greater sensitivity to endoproteinase glu-C (V8) or lys-C than from cells without drug. The V8 fragment missing in tunicamyin treated cells encompassed the KPI inhibitor insertion site but was distinct from the site of N-glycosylation. It is concluded that a conformational change caused by interrupted N-glycosylation shunts newly translated APP into the proteasomal degradation pathway. Pulse-labeled and chased cells showed an additional V8 fragment that was not present in pulsed-labeled cells and was not due to glycosylation since it was also present in cells labeled in the presence of brefeldin. This latter result indicates that an additional, delayed conformational alteration occurs in the endoplasmic reticulum.

Proteasome-mediated effects on amyloid precursor protein processing at the gamma-secretase site

Abeta (beta-amyloid) peptides are found aggregated in the cortical amyloid plaques associated with Alzheimer's disease neuropathology. Inhibition of the proteasome alters the amount of Abeta produced from APP (amyloid precursor protein) by various cell lines in vitro. Proteasome activity is altered during aging, a major risk factor for Alzheimer's disease. In the present study, a human neuroblastoma cell line expressing the C-terminal 100 residues of APP (SH-SY5Y-SPA4CT) was used to determine the effect of proteasome inhibition, by lactacystin and Bz-LLL-COCHO (benzoyl-Leu-Leu-Leu-glyoxal), on APP processing at the gamma-secretase site. Proteasome inhibition caused a significant increase in Abeta peptide levels in medium conditioned by SH-SY5Y-SPA4CT cells, and was also associated with increased cell death. APP is a substrate of the apoptosis-associated caspase 3 protease, and we therefore investigated whether the increased Abeta levels could reflect caspase activation. We report that caspase activation was not required for proteasome-inhibitor-mediated effects on APP (SPA4CT) processing. Cleavage of Ac-DEVD-AMC (N-acetyl-Asp-Glu-Val-Asp-7-amino-4-methylcoumarin), a caspase substrate, was reduced following exposure of SH-SY5Y-SPA4CT cells to lactacystin, and co-treatment of cells with lactacystin and a caspase inhibitor [Z-DEVD-FMK (benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone)] resulted in higher Abeta levels in medium, augmenting those seen with lactacystin alone. This study indicated that proteasome inhibition could increase APP processing specifically at the gamma-secretase site, and increase release of Abeta, in the absence of caspase activation. This indicates that the decline in proteasome function associated with aging would contribute to increased Abeta levels.

The effect of cholesterol and monosialoganglioside (GM1) on the release and aggregation of amyloid beta-peptide from liposomes prepared from brain membrane-like lipids

In order to investigate the influence of cholesterol (Ch) and monosialoganglioside (GM1) on the release and subsequent deposition/aggregation of amyloid beta peptide (Abeta)-(1-40) and Abeta-(1-42), we have examined Abeta peptide model membrane interactions by circular dichroism, turbidity measurements, and transmission electron microscopy (TEM). Model liposomes containing Abeta peptide and a lipid mixture composition similar to that found in the cerebral cortex membranes (CCM-lipid) have been prepared. In all, four Abeta-containing liposomes were investigated: CCM-lipid; liposomes with no GM1 (GM1-free lipid); those with no cholesterol (Ch-free lipid); liposomes with neither cholesterol nor GM1 (Ch-GM1-free lipid). In CCM liposomes, Abeta was rapidly released from membranes to form a well defined fibril structure. However, for the GM1-free lipid, Abeta was first released to yield a fibril structure about the membrane surface, then the membrane became disrupted resulting in the formation of small vesicles. In Ch-free lipid, a fibril structure with a phospholipid membrane-like shadow formed, but this differed from the well defined fibril structure seen for CCM-lipid. In Ch-GM1-free lipid, no fibril structure formed, possibly because of membrane solubilization by Abeta. The absence of fibril structure was noted at physiological extracellular pH (7.4) and also at liposomal/endosomal pH (5.5). Our results suggest a possible role for both Ch and GM1 in the membrane release of Abeta from brain lipid bilayers.

Generation of the beta-amyloid peptide and the amyloid precursor protein C-terminal fragment gamma are potentiated by FE65L1

Members of the FE65 family of adaptor proteins, FE65, FE65L1, and FE65L2, bind the C-terminal region of the amyloid precursor protein (APP). Overexpression of FE65 and FE65L1 was previously reported to increase the levels of alpha-secretase-derived APP (APPs alpha). Increased beta-amyloid (A beta) generation was also observed in cells showing the FE65-dependent increase in APPs alpha. To understand the mechanism for the observed increase in both A beta and APPs alpha given that alpha-secretase cleavage of a single APP molecule precludes A beta generation, we examined the effects of FE65L1 overexpression on APP C-terminal fragments (APP CTFs). Our data show that FE65L1 potentiates gamma-secretase processing of APP CTFs, including the amyloidogenic CTF C99, accounting for the ability of FE65L1 to increase generation of APP C-terminal domain and A beta 40. The FE65L1 modulation of these processing events requires binding of FE65L1 to APP and APP CTFs and is not because of a direct effect on gamma-secretase activity, because Notch intracellular domain generation is not altered by FE65L1. Furthermore, enhanced APP CTF processing can be detected in early endosome vesicles but not in endoplasmic reticulum or Golgi membranes, suggesting that the effects of FE65L1 occur at or near the plasma membrane. Finally, although FE65L1 increases APP C-terminal domain production, it does not mediate the APP-dependent transcriptional activation observed with FE65.

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.

A novel mechanism for the regulation of amyloid precursor protein metabolism

Modifier of cell adhesion protein (MOCA; previously called presenilin [PS] binding protein) is a DOCK180-related molecule, which interacts with PS1 and PS2, is localized to brain areas involved in Alzheimer's disease (AD) pathology, and is lost from the soluble fraction of sporadic Alzheimer's disease (AD) brains. Because PS1 has been associated with gamma-secretase activity, MOCA may be involved in the regulation of beta-amyloid precursor protein (APP) processing. Here we show that the expression of MOCA decreases both APP and amyloid beta-peptide secretion and lowers the rate of cell-substratum adhesion. In contrast, MOCA does not lower the secretion of amyloid precursor-like protein (APLP) or several additional type 1 membrane proteins. The phenotypic changes caused by MOCA are due to an acceleration in the rate of intracellular APP degradation. The effect of MOCA expression on the secretion of APP and cellular adhesion is reversed by proteasome inhibitors, suggesting that MOCA directs nascent APP to proteasomes for destruction. It is concluded that MOCA plays a major role in APP metabolism and that the effect of MOCA on APP secretion and cell adhesion is a downstream consequence of MOCA-directed APP catabolism. This is a new mechanism by which the expression of APP is regulated.

Proteolysis of chimeric beta-amyloid precursor proteins containing the Notch transmembrane domain yields amyloid beta-like peptides

gamma-Secretase is an unusual intramembranous protease that has been reported to cleave the beta-amyloid precursor protein (APP) near the middle of its transmembrane domain (TMD) but cleave Notch near the cytoplasmic end of its TMD. To ascertain whether the TMD sequence of the substrate determines where gamma-secretase cleaves and whether the region just before the TMD participates in recognition by the enzyme, we expressed chimeric human APP molecules containing either the TMD or pre-TMD regions of Notch or other transmembrane proteins. APP chimeras bearing either the Notch or the amyloid precursor-like protein-2 TMD released similar amounts of approximately 4-kDa amyloid beta-peptide (Abeta)-like peptides as did intact APP. Mass spectrometry revealed that the principal Abeta-like peptide ended at residue 40, indicating cleavage at the middle of the Notch TMD in the chimera. Generation of Abeta-like peptides was significantly decreased when the APP TMD was replaced by those of SREBP-1 or human epithelial growth factor receptor 3. Replacement of the APP pre-TMD region (Abeta 10-28) with that of SREBP-1 increased generation of Abeta-like peptides, while those of human epithelial growth factor receptor 3 or amyloid precursor-like protein-2 decreased it. We conclude that gamma-secretase can cleave near the middle of the Notch TMD, that Abeta-like peptides may arise during Notch processing, and that the pre-TMD sequence of the substrate influences recognition or binding by the enzyme.

The intramembrane cleavage site of the amyloid precursor protein depends on the length of its transmembrane domain

Proteolytic processing of the amyloid precursor protein by beta-secretase generates C99, which subsequently is cleaved by gamma-secretase, yielding the amyloid beta peptide (A beta). This gamma-cleavage occurs within the transmembrane domain (TMD) of C99 and is similar to the intramembrane cleavage of Notch. However, Notch and C99 differ in their site of intramembrane cleavage. The main gamma-cleavage of C99 occurs in the middle of the TMD, whereas the cleavage of Notch occurs close to the C-terminal end of the TMD, making it unclear whether both are cleaved by the same protease. To investigate whether gamma-cleavage always occurs in the middle of the TMD of C99 or may also occur at the end of the TMD, we generated C99-mutants with an altered length of the TMD and analyzed their gamma-cleavage in COS7 cells. The C terminus of A beta and thus the site of gamma-cleavage were determined by using monoclonal antibodies and mass spectrometry. Compared with C99-wild type (wt), most mutants with an altered length of the TMD changed the cleavage site of gamma-secretase, whereas control mutants with mutations outside the TMD did not. Thus, the length of the whole TMD is a major determinant for the cleavage site of gamma-secretase. Moreover, the C99-mutants were not only cleaved at one site but at two sites within their TMD. One cleavage site was located around the middle of the TMD, regardless of its actual length. An additional cleavage occurred within the N-terminal half of their TMD and thus at the opposite side of the Notch cleavage site.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Retention of the Alzheimer's amyloid precursor fragment C99 in the endoplasmic reticulum prevents formation of amyloid beta-peptide

gamma-Secretase is a membrane-associated endoprotease that catalyzes the final step in the processing of Alzheimer's beta-amyloid precursor protein (APP), resulting in the release of amyloid beta-peptide (Abeta). The molecular identity of gamma-secretase remains in question, although recent studies have implicated the presenilins, which are membrane-spanning proteins localized predominantly in the endoplasmic reticulum (ER). Based on these observations, we have tested the hypothesis that gamma-secretase cleavage of the membrane-anchored C-terminal stump of APP (i.e. C99) occurs in the ER compartment. When recombinant C99 was expressed in 293 cells, it was localized mainly in the Golgi apparatus and gave rise to abundant amounts of Abeta. Co-expression of C99 with mutant forms of presenilin-1 (PS1) found in familial Alzheimer's disease resulted in a characteristic elevation of the Abeta(42)/Abeta(40) ratio, indicating that the N-terminal exodomain of APP is not required for mutant PS1 to influence the site of gamma-secretase cleavage. Biogenesis of both Abeta(40) and Abeta(42) was almost completely eliminated when C99 was prevented from leaving the ER by addition of a di-lysine retention motif (KKQN) or by co-expression with a dominant-negative mutant of the Rab1B GTPase. These findings indicate that the ER is not a major intracellular site for gamma-secretase cleavage of C99. Thus, by inference, PS1 localized in this compartment does not appear to be active as gamma-secretase. The results suggest that presenilins may acquire the characteristics of gamma-secretase after leaving the ER, possibly by assembling with other proteins in peripheral membranes.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Protease inhibitors divert amyloid precursor protein to the secretory pathway

Addition of cysteine protease inhibitors to cells expressing amyloid precursor protein (APP) resulted in a >2-fold increase in appearance of the secreted extracellular domain of APP in the media. This was accounted for by increased flux of APP into the secretory pathway since protease inhibitors also caused a twofold increase in newly translated, incompletely glycosylated APP detected by pulse-labeling. These results show that a portion of newly translated APP molecules are normally rapidly degraded by cysteine protease(s) but can enter the secretory pathway when degradation is inhibited. Newly translated APP molecules are thus still competent for posttranslational processing in distal cellular compartments. Their degradation thus may not result from misfolding but merely susceptibility to an endoplasmic reticulum localized cysteine protease.

A distinct ER/IC gamma-secretase competes with the proteasome for cleavage of APP

The deposition of amyloid-beta peptides (Abeta) in senile plaques (SPs) is a central pathological feature of Alzheimer's disease (AD). Since SPs are composed predominantly of Abeta1-42, which is more amyloidogenic in vitro, the enzymes involved in generating Abeta1-42 may be particularly important to the pathogenesis of AD. In contrast to Abeta1-40, which is generated in the trans-Golgi network and other cytoplasmic organelles, intracellular Abeta1-42 is produced in the endoplasmic reticulum/intermediate compartment (ER/IC), where it accumulates in a stable insoluble pool. Since this pool of insoluble Abeta1-42 may play a critical role in AD amyloidogenesis, we sought to determine how the production of intracellular Abeta is regulated. Surprisingly, the production of insoluble intracellular Abeta1-42 was increased by a putative gamma-secretase inhibitor as well as by an inhibitor of the proteasome. We further demonstrate that this increased generation of Abeta1-42 in the ER/IC is due to a reduction in the turnover of Abeta-containing APP C-terminal fragments. We conclude that the proteasome is a novel site for degradation of ER/IC-generated APP fragments. Proteasome inhibitors may augment the availability of APP C-terminal fragments for gamma-secretase cleavage and thereby increase production of Abeta1-42 in the ER/IC. Based on the organelle-specific differences in the generation of Abeta by gamma-secretase, we conclude that intracellular ER/IC-generated Abeta1-42 and secreted Abeta1-40 are produced by different gamma-secretases. Further, the fact that a putative gamma-secretase inhibitor had opposite effects on the production of secreted and intracellular Abeta may have important implications for AD drug design.

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

Proteolytic processing of Alzheimer's disease amyloid precursor protein (APP) by beta-secretase leads to A4CT (C99), which is further cleaved by the as yet unknown protease called gamma-secretase. To study the enzymatic properties of gamma-secretase independently of beta-secretase, A4CT together with an N-terminal signal peptide (SPA4CT) may be expressed in eukaryotic cells. However, in all existing SPA4CT proteins the signal peptide is not correctly cleaved upon membrane insertion. Here, we report the generation of a mutated SPA4CT protein that is correctly cleaved by signal peptidase and, thus, identical to the APP-derived A4CT. This novel SPA4CT protein is processed by gamma-secretase in the same manner as APP-derived A4CT and might be valuable for the generation of transgenic animals showing amyloid pathology.