BACE1- and BACE2-expressing human cells: characterization of beta-amyloid precursor protein-derived catabolites, design of a novel fluorimetric assay, and identification of new in vitro inhibitors

Heterologous expression of BACE1 and its interaction with Codonopsis pilosula polysaccharides and Lobetyolin

BACE1, a crucial enzyme in the amyloid-β deposition theory of Alzheimer's disease (AD), is targeted by Codonopsis pilosula, a traditional tonic believed to impede AD onset. However, the specific active compounds responsible for its effects remain elusive. Our prior network pharmacology research identified C. pilosula polysaccharides (CPPS) and Lobetyolin may serve as potential inhibitors of AD by suppressing amyloidogenesis. Here, we recombinantly expressed BACE1 under varied conditions and assessed its activity using Fluorescence Resonance Energy Transfer technology. Through spectroscopy, molecular docking, and dynamics, we elucidated the interactions of CPPS, Lobetyolin, and BACE1. Optimal BACE1 expression occurred at 22 °C with 0.4 mM IPTG for 6 h, yielding a 72 kDa protein. Enzyme kinetics displayed a maximum rate of 4096 μmol/min and a Michaelis constant of 16 mg/mL for BACE1. Spectroscopic analysis revealed differing binding affinities of the compounds at various temperatures, peaking at 293 K. Lobetyolin exhibited superior binding to BACE1 compared to CPPS, driven by hydrophobic and electrostatic forces. Molecular docking and dynamics highlighted hydrophobic amino acids' role in BACE1 interactions with Lobetyolin and CPPS, with binding energy < -1.2 kcal/mol signifying strong affinities. Notably, Lobetyolin and CPPS showed higher BACE1 affinity than APP, with the Lobetyolin-BACE1 complex being the most stable.

Potentially Pathogenic SORL1 Mutations Observed in Autosomal-Dominant Cases of Alzheimer's Disease Do Not Modulate APP Physiopathological Processing

The SORL1 gene encodes LR11/SorLA, a protein that binds β-amyloid precursor protein (APP) and drives its intracellular trafficking. SORL1 mutations, occurring frequently in a subset of familial cases of Alzheimer's disease (AD), have been documented, but their pathogenic potential is not yet clear and questions remain concerning their putative influence on the physiopathological processing of APP. We have assessed the influence of two SORL1 mutations that were described as likely disease-causing and that were associated with either benign (SorLA924) or severe (SorLA511) AD phenotypes. We examined the influence of wild-type and mutants SorLA in transiently transfected HEK293 cells expressing either wild-type or Swedish mutated APP on APP expression, secreted Aβ and sAPPα levels, intracellular Aβ 40 and Aβ42 peptides, APP-CTFs (C99 and C83) expressions, α-, β- and γ-secretases expressions and activities as well as Aβ and CTFs-degrading enzymes. These paradigms were studied in control conditions or after pharmacological proteasomal modulation. We also established stably transfected CHO cells expressing wild-type SorLA and established the colocalization of APP and either wild-type or mutant SorLA. SorLA mutations partially disrupt co-localization of wild-type sorLA with APP. Overall, although we mostly confirmed previous data concerning the influence of wild-type SorLA on APP processing, we were unable to evidence significant alterations triggered by our set of SorLA mutants, whatever the cells or pharmacological conditions examined. Our study , however, does not rule out the possibility that other AD-linked SORL1 mutations could indeed affect APP processing, and that pathogenic mutations examined in the present study could interfere with other cellular pathways/triggers in AD.

Synthesis and Characterization of Andrographolide Derivatives as Regulators of βAPP Processing in Human Cells

Alzheimer's disease (AD) is a devastating neurodegenerative disorder, one of the main characteristics of which is the abnormal accumulation of amyloid peptide (Aβ) in the brain. Whereas β-secretase supports Aβ formation along the amyloidogenic processing of the β-amyloid precursor protein (βAPP), α-secretase counterbalances this pathway by both preventing Aβ production and triggering the release of the neuroprotective sAPPα metabolite. Therefore, stimulating α-secretase and/or inhibiting β-secretase can be considered a promising anti-AD therapeutic track. In this context, we tested andrographolide, a labdane diterpene derived from the plant Andrographis paniculata, as well as 24 synthesized derivatives, for their ability to induce sAPPα production in cultured SH-SY5Y human neuroblastoma cells. Following several rounds of screening, we identified three hits that were subjected to full characterization. Interestingly, andrographolide (8,17-olefinic) and its close derivative 14α-(5',7'-dichloro-8'-quinolyloxy)-3,19-acetonylidene (compound 9) behave as moderate α-secretase activators, while 14α-(2'-methyl-5',7'-dichloro-8'-quinolyloxy)-8,9-olefinic compounds 31 (3,19-acetonylidene) and 37 (3,19-diol), whose two structures are quite similar although distant from that of andrographolide and 9, stand as β-secretase inhibitors. Importantly, these results were confirmed in human HEK293 cells and these compounds do not trigger toxicity in either cell line. Altogether, these findings may represent an encouraging starting point for the future development of andrographolide-based compounds aimed at both activating α-secretase and inhibiting β-secretase that could prove useful in our quest for the therapeutic treatment of AD.

Co-Immobilized Capillary Enzyme Reactor Based on Beta-Secretase1 and Acetylcholinesterase: A Model for Dual-Ligand Screening

We have developed a dual enzymatic system assay involving liquid chromatography-mass spectrometry (LC–MS) to screen AChE and BACE1 ligands. A fused silica capillary (30 cm × 0.1 mm i.d. × 0.362 mm e.d.) was used as solid support. The co-immobilization procedure encompassed two steps and random immobilization. The resulting huAChE+BACE1-ICER/MS was characterized by using acetylcholine (ACh) and JMV2236 as substrates. The best conditions for the dual enzymatic system assay were evaluated and compared to the conditions of the individual enzymatic system assays. Analysis was performed in series for each enzyme. The kinetic parameters (K_Mapp) and inhibition assays were evaluated. To validate the system, galantamine and a β-secretase inhibitor were employed as standard inhibitors, which confirmed that the developed screening assay was able to identify reference ligands and to provide quantitative parameters. The combination of these two enzymes in a single on-line system allowed possible multi-target inhibitors to be screened and identified. The innovative huAChE+BACE1-ICER/MS dual enzymatic system reported herein proved to be a reliable tool to identify and to characterize hit ligands for AChE and BACE1 in an enzymatic competitive environment. This innovative system assay involved lower costs; measured the product from enzymatic hydrolysis directly by MS; enabled immediate recovery of the enzymatic activity; showed specificity, selectivity, and sensitivity; and mimicked the cellular process.

Biomimetic Macrocyclic Inhibitors of Human Cathepsin D: Structure-Activity Relationship and Binding Mode Analysis

Human cathepsin D (CatD), a pepsin-family aspartic protease, plays an important role in tumor progression and metastasis. Here, we report the development of biomimetic inhibitors of CatD as novel tools for regulation of this therapeutic target. We designed a macrocyclic scaffold to mimic the spatial conformation of the minimal pseudo-dipeptide binding motif of pepstatin A, a microbial oligopeptide inhibitor, in the CatD active site. A library of more than 30 macrocyclic peptidomimetic inhibitors was employed for scaffold optimization, mapping of subsite interactions, and profiling of inhibitor selectivity. Furthermore, we solved high-resolution crystal structures of three macrocyclic inhibitors with low nanomolar or subnanomolar potency in complex with CatD and determined their binding mode using quantum chemical calculations. The study provides a new structural template and functional profile that can be exploited for design of potential chemotherapeutics that specifically inhibit CatD and related aspartic proteases.

Upregulation of the Sarco-Endoplasmic Reticulum Calcium ATPase 1 Truncated Isoform Plays a Pathogenic Role in Alzheimer's Disease

Dysregulation of the Endoplasmic Reticulum (ER) Ca²⁺ homeostasis and subsequent ER stress activation occur in Alzheimer Disease (AD). We studied the contribution of the human truncated isoform of the sarco-endoplasmic reticulum Ca²⁺ ATPase 1 (S1T) to AD. We examined S1T expression in human AD-affected brains and its functional consequences in cellular and transgenic mice AD models. S1T expression is increased in sporadic AD brains and correlates with amyloid β (Aβ) and ER stress chaperone protein levels. Increased S1T expression was also observed in human neuroblastoma cells expressing Swedish-mutated β-amyloid precursor protein (βAPP) or treated with Aβ oligomers. Lentiviral overexpression of S1T enhances in return the production of APP C-terminal fragments and Aβ through specific increases of β-secretase expression and activity, and triggers neuroinflammation. We describe a molecular interplay between S1T-dependent ER Ca²⁺ leak, ER stress and βAPP-derived fragments that could contribute to AD setting and/or progression.

Methamphetamine regulates βAPP processing in human neuroblastoma cells

Methamphetamine is a potent and highly addictive psychostimulant whose abuse has turned out to be a global health hazard. The multitudinous effects it exerts at the cellular level induces neurotoxic responses in the human brain, ultimately leading to neurocognitive disorders. Strikingly, brain changes, tissue damage and neuropsychological symptoms due to Meth exposure compels and necessitates to link the probability of risk of developing premature Alzheimer's disease, a progressive neurodegenerative disorder characterized by amyloid plaques composed of amyloid-β peptides and clinical dementia. These peptides are derived from sequential cleavages of the β-amyloid precursor protein by β- and γ-secretases. Previous studies reveals evidence for both positive and negative effects of Meth pertaining to cognitive functioning based on the dosage paradigm and duration of exposure revealing a beneficial psychotropic profile under some conditions and deleterious cognitive deficits under some others. In this context, we proposed to examine the effect of Meth on βAPP metabolism and βAPP-cleaving secretases in the human neuroblastoma SH-SY5Y cell line. Our results showed that Meth dose-dependently increases BACE1 expression and catalytic activity, while its effect on the α-cleavage of βAPP and on the expression and catalytic activity of the main α-secretase ADAM10 display a bell-curve shape. To our knowledge, the present study is the first to demonstrate that Meth can control βAPP-cleaving secretases. Moreover, we propose from these findings that the deleterious effect of Meth on cognitive decline might be an outcome of high dosage paradigm whereas acute and short-term drug use which stimulated sAPPα might produce improvements in cognition in disorders such as AD.

How Ligand Protonation State Controls Water in Protein-Ligand Binding

The role of water in protein-ligand binding has been an intensely studied topic in recent years; however, how ligand protonation state change perturbs water has not been considered. Here we show that water dynamics and interactions can be controlled by the protonation state of ligand using continuous constant pH molecular dynamics simulations of two closely related model systems, β-secretase 1 and 2 (BACE1 and BACE2), in complex with a small-molecule inhibitor. Simulations revealed that, upon binding, the inhibitor pyrimidine ring remains deprotonated in BACE1 but becomes protonated in BACE2. Pyrimidine protonation results in water displacement, rigidification of the binding pocket, and shift in the ligand binding mode from water-mediated to direct hydrogen bonding. These findings not only support but also rationalize the most recent structure-selectivity data in BACE1 drug design. Binding-induced protonation state changes are likely common; our work offers a glimpse at how modeling protein-ligand binding while allowing ligand titration can further advance the understanding of water and structure-based drug design.

Increased Plasma Beta-Secretase 1 May Predict Conversion to Alzheimer's Disease Dementia in Individuals With Mild Cognitive Impairment

BACKGROUND

Increased beta-secretase 1 (BACE1) activity has consistently been detected in brain tissue and cerebrospinal fluid of subjects with mild cognitive impairment (MCI) and probable Alzheimer's disease (AD) compared with control subjects. The collection of cerebrospinal fluid by lumbar puncture is invasive. We sought to identify the presence of plasma BACE1 activity and determine potential alterations in subjects with MCI with clinical follow-up examinations for 3 years using patients with diagnosed probable AD dementia compared with healthy control subjects.

METHODS

Seventy-five patients with probable AD, 96 individuals with MCI, and 53 age-matched and sex-matched healthy control subjects were recruited from three independent international academic memory clinics and AD research expert centers. Plasma BACE1 activity was measured by a synthetic fluorescence substrate enzyme-linked immunosorbent assay. BACE1 protein expression was assessed by Western blotting using three different antibodies that recognize the epitopes of the N-terminus, C-terminus, and full-length BACE1.

RESULTS

Compared with healthy control subjects, plasma BACE1 activity (V_max) significantly increased by 53.2% in subjects with MCI and by 68.9% in patients with probable AD. Subjects with MCI who converted to probable AD dementia at follow-up examinations exhibited significantly higher BACE1 activity compared with cognitively stable MCI nonconverters and showed higher levels of BACE1 activity than patients with AD.

CONCLUSIONS

Plasma BACE1 activity is significantly increased in MCI converters and patients with probable AD. The sensitivities and specificities of BACE1 activity for the patients were 84% and 88%, respectively. Our results indicate that plasma BACE1 activity may be a biomarker for AD risk and could predict progression from prodromal to probable AD dementia.

The Polyherbal Wattana Formula Displays Anti-Amyloidogenic Properties by Increasing α-Secretase Activities

Alzheimer's disease is characterized by the deposition of insoluble amyloid-β peptides produced from the β-amyloid precursor protein (βAPP). Because α-secretase cleavage by ADAM10 and ADAM17 takes place in the middle of Aβ, its activation is considered as a promising anti-AD therapeutic track. Here we establish that the polyherbal Wattana formula (WNF) stimulates sAPPα production in cells of neuronal and non-neuronal origins through an increase of both ADAM10 and ADAM17 catalytic activities with no modification of BACE1 activity and expression. This effect is blocked by specific inhibition or genetic depletion of these disintegrins and we show that WNF up-regulates ADAM10 transcription and ADAM17 maturation. In addition, WNF reduces Aβ40 and Aβ42 generation in human cell lines. Altogether, WNF presents all the characteristics of a potent preventive anti-Alzheimer formula. Importantly, this natural recipe, currently prescribed to patients for the treatment of other symptoms without any secondary effect, can be tested immediately for further clinical studies.

MT5-MMP Promotes Alzheimer's Pathogenesis in the Frontal Cortex of 5xFAD Mice and APP Trafficking in vitro

We previously reported that deficiency of membrane-type five matrix metalloproteinase (MT5-MMP) prevents amyloid pathology in the cortex and hippocampus of 5xFAD mice, and ameliorates the functional outcome. We have now investigated whether the integrity of another important area affected in Alzheimer's disease (AD), the frontal cortex, was also preserved upon MT5-MMP deficiency in 4-month old mice at prodromal stages of the pathology. We used the olfactory H-maze (OHM) to show that learning impairment associated with dysfunctions of the frontal cortex in 5xFAD was prevented in bigenic 5xFAD/MT5-MMP^-/- mice. The latter exhibited concomitant drastic reductions of amyloid beta peptide (Aβ) assemblies (soluble, oligomeric and fibrillary) and its immediate precursor, C99. Simultaneously, astrocyte reactivity and tumor necrosis factor alpha (TNF-α) levels were also lowered. Moreover, MT5-MMP deficiency induced a decrease in N-terminal soluble fragments of amyloid precursor protein (APP), including soluble APPα (sAPPα), sAPPβ and the MT5-MMP-linked fragment of 95 kDa, sAPP95. However, the lack of MT5-MMP did not affect the activity of β- and γ-secretases. In cultured HEKswe cells, transiently expressed MT5-MMP localized to early endosomes and increased the content of APP and Aβ40 in these organelles, as well as Aβ levels in cell supernatants. This is the first evidence that the pro-amyloidogenic features of MT5-MMP lie, at least in part, on the ability of the proteinase to promote trafficking into one of the amyloidogenic subcellular loci. Together, our data further support the pathogenic role of MT5-MMP in AD and that its inhibition improves the functional and pathological outcomes, in this case in the frontal cortex. These data also support the idea that MT5-MMP could become a novel therapeutic target in AD.

Aβ42 oligomers modulate β-secretase through an XBP-1s-dependent pathway involving HRD1

The aspartyl protease β-site APP cleaving enzyme, BACE1, is the rate-limiting enzyme involved in the production of amyloid-β peptide, which accumulates in both sporadic and familial cases of Alzheimer’s disease and is at the center of gravity of the amyloid cascade hypothesis. In this context, unravelling the molecular mechanisms controlling BACE1 expression and activity in both physiological and pathological conditions remains of major importance. We previously demonstrated that Aβ controlled BACE1 transcription in an NFκB-dependent manner. Here, we delineate an additional cellular pathway by which natural and synthetic Aβ42 oligomers enhance active X-box binding protein XBP-1s. XBP-1s lowers BACE1 expression and activity indirectly, via the up-regulation of the ubiquitin-ligase HRD1 that acts as an endogenous down-regulator of BACE1. Thus, we delineate a novel pathway by which cells could compensate for Aβ42 oligomers production and thus, associated toxicity, by triggering a compensatory mechanism aimed at lowering BACE-1-mediated Aβ production by a molecular cascade involving XBP-1s and HRD1. It thus identifies HRD1 as a potential target for a novel Aβ-centered therapeutic strategy.

ADAM30 Downregulates APP-Linked Defects Through Cathepsin D Activation in Alzheimer's Disease

Although several ADAMs (A disintegrin-like and metalloproteases) have been shown to contribute to the amyloid precursor protein (APP) metabolism, the full spectrum of metalloproteases involved in this metabolism remains to be established. Transcriptomic analyses centred on metalloprotease genes unraveled a 50% decrease in ADAM30 expression that inversely correlates with amyloid load in Alzheimer's disease brains. Accordingly, in vitro down- or up-regulation of ADAM30 expression triggered an increase/decrease in Aβ peptides levels whereas expression of a biologically inactive ADAM30 (ADAM30(mut)) did not affect Aβ secretion. Proteomics/cell-based experiments showed that ADAM30-dependent regulation of APP metabolism required both cathepsin D (CTSD) activation and APP sorting to lysosomes. Accordingly, in Alzheimer-like transgenic mice, neuronal ADAM30 over-expression lowered Aβ42 secretion in neuron primary cultures, soluble Aβ42 and amyloid plaque load levels in the brain and concomitantly enhanced CTSD activity and finally rescued long term potentiation alterations. Our data thus indicate that lowering ADAM30 expression may favor Aβ production, thereby contributing to Alzheimer's disease development.

Presenilin 1 and Presenilin 2 Target γ-Secretase Complexes to Distinct Cellular Compartments

γ-Secretase complexes achieve the production of amyloid peptides playing a key role in Alzheimer disease. These proteases have many substrates involved in important physiological functions. They are composed of two constant subunits, nicastrin and PEN2, and two variable ones, presenilin (PS1 or PS2) and APH1 (APH1aL, APH1aS, or APH1b). Whether the composition of a given γ-secretase complex determines a specific cellular targeting remains unsolved. Here we combined a bidirectional inducible promoter and 2A peptide technology to generate constructs for the temporary, stoichiometric co-expression of six different combinations of the four γ-secretase subunits including EGFP-tagged nicastrin. These plasmids allow for the formation of functional γ-secretase complexes displaying specific activities and maturations. We show that PS1-containing γ-secretase complexes were targeted to the plasma membrane, whereas PS2-containing ones were addressed to the trans-Golgi network, to recycling endosomes, and, depending on the APH1-variant, to late endocytic compartments. Overall, these novel constructs unravel a presenilin-dependent subcellular targeting of γ-secretase complexes. These tools should prove useful to determine whether the cellular distribution of γ-secretase complexes contributes to substrate selectivity and to delineate regulations of their trafficking.

Melatonin regulates the transcription of βAPP-cleaving secretases mediated through melatonin receptors in human neuroblastoma SH-SY5Y cells

Melatonin is involved in the control of various physiological functions, such as sleep, cell growth and free radical scavenging. The ability of melatonin to behave as an antioxidant, together with the fact that the Alzheimer-related amyloid β-peptide (Aβ) triggers oxidative stress through hydroxyl radical-induced cell death, suggests that melatonin could reduce Alzheimer's pathology. Although the exact etiology of Alzheimer's disease (AD) remains to be established, excess Aβ is believed to be the primary contributor to the dysfunction and degeneration of neurons that occurs in AD. Aβ peptides are produced via the sequential cleavage of β-secretase β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase (PS1/PS2), while α-secretase (ADAM10) prevents the production of Aβ peptides. We hypothesized that melatonin could inhibit BACE1 and PS1/PS2 and enhance ADAM10 expression. Using the human neuronal SH-SY5Y cell line, we found that melatonin inhibited BACE1 and PS1 and activated ADAM10 mRNA level and protein expression in a concentration-dependent manner and mediated via melatonin G protein-coupled receptors. Melatonin inhibits BACE1 and PS1 protein expressions through the attenuation of nuclear factor-κB phosphorylation (pNF-κB). Moreover, melatonin reduced BACE1 promoter transactivation and consequently downregulated β-secretase catalytic activity. The present data show that melatonin is not only a potential regulator of β/γ-secretase but also an activator of α-secretase expression through the activation of protein kinase C, thereby favoring the nonamyloidogenic pathway over the amyloidogenic pathway. Altogether, our findings suggest that melatonin may be a potential therapeutic agent for reducing the risk of AD in humans.

Study on Aβ34 biology and detection in transgenic mice brains

The β-amyloid precursor protein undergoes cleavages by β- and γ-secretasses yielding amyloid-β peptides (Aβ) that accumulate in Alzheimer's disease. Subsequently, Aβ peptides are targets of additional truncations or endoproteolytic cleavages explaining the diversity of Aβ-related fragments recovered in cell media or pathologic human fluids. Here, we focused on Aβ1-34 (Aβ34) that has been detected both in vitro and in vivo and that derives from the hydrolysis of Aβ by β-secretase. We have obtained and fully characterized by immunologic and biochemical approaches, a polyclonal antibody that specifically recognizes the C-terminus of Aβx-34. We present immunohistochemical evidence for the presence of Aβx-34 in the brain of 3xTg mice and Alzheimer's disease-affected human brains. Finally, we demonstrate a neprilysin-mediated degradation process of Aβ34 and the ability of synthetic Aβ34 to protect HEK cells overexpressing either wild type or Swedish-mutated β-amyloid precursor protein from apoptosis.

A cellular model of amyloid precursor protein processing and amyloid-β peptide production

BACKGROUND

A hallmark pathologic feature of Alzheimer's disease (AD) is accumulation of neuritic senile plaques in the brain parenchyma. Neurotoxic plaque cores are composed predominantly of amyloid-β (Aβ) peptides of 40 and 42 amino acids in length, formed by sequential cleavage of amyloid precursor protein (APP) by β-, and γ-secretases. There is a great interest in approaches to modulate Aβ peptide production and develop therapeutic interventions to reduce Aβ levels to halt or slow the progression of neurodegeneration.

NEW METHOD

We characterized and present the BE(2)-M17 human neuroblastoma cell line as a novel in vitro model of the APP-cleavage cascade to support future (1) functional studies of molecular regulators in Aβ production, and (2) high-throughput screening assays of new pharmacotherapeutics.

RESULTS

In BE(2)-M17 cells, both RNA (i.e., RT-PCR, RNA sequencing) and protein analyses (i.e., Western blots, ELISA), show endogenous expression of critical components of the amyloidogenic pathway, APP-cleavage intermediates CTF83 and CTF99, and final cleavage products Aβ40 and Aβ42. We further report effects of retinoic acid-mediated differentiation on morphology and gene expression in this cell line.

COMPARISON WITH EXISTING METHOD(S)

In contrast to primary isolates or other cell lines reported in current literature, BE(2)-M17 not only sustains baseline expression of the full contingent of APP-processing components, but also remains stably adherent during culture, facilitating experimental manipulations.

CONCLUSIONS

Our evidence supports the use of BE(2)-M17 as a novel, human, cell-based model of the APP processing pathway that offers a potential streamlined approach to dissect molecular functions of endogenous regulatory pathways, and perform mechanistic studies to identify modulators of Aβ production.

The β-secretase-derived C-terminal fragment of βAPP, C99, but not Aβ, is a key contributor to early intraneuronal lesions in triple-transgenic mouse hippocampus

Triple-transgenic mice (3xTgAD) overexpressing Swedish-mutated β-amyloid precursor protein (βAPP(swe)), P310L-Tau (Tau(P301L)), and physiological levels of M146V-presenilin-1 (PS1(M146V)) display extracellular amyloid-β peptides (Aβ) deposits and Tau tangles. More disputed is the observation that these mice accumulate intraneuronal Aβ that has been linked to synaptic dysfunction and cognitive deficits. Here, we provide immunohistological, genetic, and pharmacological evidences for early, age-dependent, and hippocampus-specific accumulation of the β-secretase-derived βAPP fragment C99 that is observed from 3 months of age and enhanced by pharmacological blockade of γ-secretase. Notably, intracellular Aβ is only detectable several months later and appears, as is the case of C99, in enlarged cathepsin B-positive structures, while extracellular Aβ deposits are detected ~12 months of age and beyond. Early C99 production occurs mainly in the CA1/subicular interchange area of the hippocampus corresponding to the first region exhibiting plaques and tangles in old mice. Furthermore, the comparison of 3xTgAD mice with double-transgenic mice bearing the βAPP(swe) and Tau(P301L) mutations but expressing endogenous PS1 (2xTgAD) demonstrate that C99 accumulation is not accounted for by a loss of function triggered by PS1 mutation that would have prevented C99 secondary cleavage by γ-secretase. Together, our work identifies C99 as the earliest βAPP catabolite and main contributor to the intracellular βAPP-related immunoreactivity in 3xTgAD mice, suggesting its implication as an initiator of the neurodegenerative process and cognitive alterations taking place in this mouse model.

Green tea catechin leads to global improvement among Alzheimer's disease-related phenotypes in NSE/hAPP-C105 Tg mice

Amyloid β (Αβ) has been reported to be responsible for the functional and structural abnormalities of Alzheimer's disease (AD) through the induction of oxidative stress. The aim of this study was to determine whether or not treatment of transgenic (Tg) mice with green tea catechin (GTC), a radical scavenger, improves AD phenotypes. To test this, 7-month-old Tg mice were treated with a low (1 mg) or high (10 mg) dose of GTC for 6 months. Surprisingly, GTC-treated Tg mice exhibited significant decreases in behavioral impairment, Aβ-42 production, APP-C99/89 expression, γ-secretase component and Wnt protein levels, γ-secretase activity and MAPK activation. In contrast, the levels of APP-C83 protein and enzyme activities (α-secretase, neprilysin and Pin1) were elevated in the GTC-treated groups. Moreover, GTC-treated groups showed lower levels of total cholesterol and low-density lipoprotein cholesterol, whereas the level of high-density lipoprotein cholesterol increased. These results provide the first experimental evidence that GTC improves AD phenotypes, thereby suggesting that GTC can be used in the prevention of AD or treatment of AD patients.

Nuclear factor-κB regulates βAPP and β- and γ-secretases differently at physiological and supraphysiological Aβ concentrations

Anatomical lesions in Alzheimer disease-affected brains mainly consist of senile plaques, inflammation stigmata, and oxidative stress. The nuclear factor-κB (NF-κB) is a stress-activated transcription factor that is activated around senile plaques. We have assessed whether NF-κB could be differentially regulated at physiological or supraphysiological levels of amyloid β (Aβ) peptides. Under these experimental conditions, we delineated the putative NF-κB-dependent modulation of all cellular participants in Aβ production, namely its precursor βAPP (β-amyloid precursor protein) and the β- and γ-secretases, the two enzymatic machines involved in Aβ genesis. Under physiological conditions, NF-κB lowers the transcriptional activity of the promoters of βAPP, β-secretase (β-site APP-cleaving enzyme 1, BACE1), and of the four protein components (Aph-1, Pen-2, nicastrin, presenilin-1, or presenilin-2) of the γ-secretase in HEK293 cells. This was accompanied by a reduction of both protein levels and enzymatic activities, thereby ultimately yielding lower amounts of Aβ and AICD (APP intracellular domain). In stably transfected Swedish βAPP-expressing HEK293 cells triggering supraphysiological concentrations of Aβ peptides, NF-κB activates the transcription of βAPP, BACE1, and some of the γ-secretase members and increases protein expression and enzymatic activities, resulting in enhanced Aβ production. Our pharmacological approach using distinct NF-κB kinase modulators indicates that both NF-κB canonical and alternative pathways are involved in the control of Aβ production. Overall, our data demonstrate that under physiological conditions, NF-κB triggers a repressive effect on Aβ production that contributes to maintaining its homeostasis, while NF-κB participates in a degenerative cycle where Aβ would feed its own production under pathological conditions.

α-Secretase-derived fragment of cellular prion, N1, protects against monomeric and oligomeric amyloid β (Aβ)-associated cell death

In physiological conditions, both β-amyloid precursor protein (βAPP) and cellular prion (PrP(c)) undergo similar disintegrin-mediated α-secretase cleavage yielding N-terminal secreted products referred to as soluble amyloid precursor protein-α (sAPPα) and N1, respectively. We recently demonstrated that N1 displays neuroprotective properties by reducing p53-dependent cell death both in vitro and in vivo. In this study, we examined the potential of N1 as a neuroprotector against amyloid β (Aβ)-mediated toxicity. We first show that both recombinant sAPPα and N1, but not its inactive parent fragment N2, reduce staurosporine-stimulated caspase-3 activation and TUNEL-positive cell death by lowering p53 promoter transactivation and activity in human cells. We demonstrate that N1 also lowers toxicity, cell death, and p53 pathway exacerbation triggered by Swedish mutated βAPP overexpression in human cells. We designed a CHO cell line overexpressing the London mutated βAPP (APP(LDN)) that yields Aβ oligomers. N1 protected primary cultured neurons against toxicity and cell death triggered by oligomer-enriched APP(LDN)-derived conditioned medium. Finally, we establish that N1 also protects neurons against oligomers extracted from Alzheimer disease-affected brain tissues. Overall, our data indicate that a cellular prion catabolite could interfere with Aβ-associated toxicity and that its production could be seen as a cellular protective mechanism aimed at compensating for an sAPPα deficit taking place at the early asymptomatic phase of Alzheimer disease.

BACE1 as a potential biomarker for Alzheimer's disease

The diagnosis of Alzheimer's disease (AD) relies principally on clinical criteria for probable and possible AD as defined by the NINCDS-ADRDRA. The field is desperately lacking of biological markers to assist with AD diagnosis and verification of treatment efficacy. According to the Consensus Report of the Working Group on Molecular and Biochemical Markers of Alzheimer's Disease, in order to qualify as a biomarker the sample in question must adhere to certain basic requirements, including the ability to: reflect AD pathology and differentiate it from other dementia with an 80% sensitivity; be reliable and reproducible; be easy to perform and analyze; remain relatively inexpensive. Beta secretases are crucial enzymes in the pathogenesis of AD. Given its primary role in brain amyloidogenesis and its ubiquitous expression, one may consider measuring peripheral BACE1 levels and activity as biomarkers of AD, like performed in the brain and cerebrospinal fluid. However, very little is known about the periphery and whether peripheral BACE1 is involved in AD pathogenesis or mirrors AD progression. Moreover, no investigation has focused on the possibility of monitoring peripheral BACE1 to assess the efficiency of BACE1 inhibitors during the course of clinical trials. Part of the problem may be attributed to the lack of sensitive molecular tools which are absolutely necessary to use BACE1 as a biomarker. In this review we evaluate the progress and feasibility of developing BACE1 as a biomarker for AD in different tissues.

The novel membrane protein TMEM59 modulates complex glycosylation, cell surface expression, and secretion of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases alpha- and beta-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid beta peptide (Abeta). At present, little is known about the cellular mechanisms that control APP shedding and Abeta generation. Here, we identified a novel protein, transmembrane protein 59 (TMEM59), as a new modulator of APP shedding. TMEM59 was found to be a ubiquitously expressed, Golgi-localized protein. TMEM59 transfection inhibited complex N- and O-glycosylation of APP in cultured cells. Additionally, TMEM59 induced APP retention in the Golgi and inhibited Abeta generation as well as APP cleavage by alpha- and beta-secretase cleavage, which occur at the plasma membrane and in the endosomes, respectively. Moreover, TMEM59 inhibited the complex N-glycosylation of the prion protein, suggesting a more general modulation of Golgi glycosylation reactions. Importantly, TMEM59 did not affect the secretion of soluble proteins or the alpha-secretase like shedding of tumor necrosis factor alpha, demonstrating that TMEM59 did not disturb the general Golgi function. The phenotype of TMEM59 transfection on APP glycosylation and shedding was similar to the one observed in cells lacking conserved oligomeric Golgi (COG) proteins COG1 and COG2. Both proteins are required for normal localization and activity of Golgi glycosylation enzymes. In summary, this study shows that TMEM59 expression modulates complex N- and O-glycosylation and suggests that TMEM59 affects APP shedding by reducing access of APP to the cellular compartments, where it is normally cleaved by alpha- and beta-secretase.

BACE1 and BACE2 enzymatic activities in Alzheimer's disease

beta-Secretase is the rate limiting enzymatic activity in the production of the amyloid-beta peptide (Abeta) and is thought to be involved in Alzheimer's disease (AD) pathogenesis. Although BACE1 (beta-site APP Cleaving Enzyme 1, EC 3.4.23.46) has received significant attention, the related BACE2 (EC 3.4.23.45) has not. Though BACE2 is also expressed in the brain, its potential role in AD has not been resolved. In this study, we compared the activities of both BACE1 and BACE2, which were isolated from the same samples of frontal cortex from both AD-affected individuals and age-matched controls. BACE1 activity showed a significant positive correlation with the amount of extractable Abeta, and BACE1 protein and activity were significantly increased in AD cases. Unexpectedly, there were substantial total amounts of BACE2 protein and enzymatic activity in the human brain. BACE2 activity did not change significantly in the AD brain, and was not related to Abeta concentration. These data indicate that BACE1 likely accounts for most of the Abeta produced in the human brain, and that BACE2 activity is not a likely contributor. However, as both forms of BACE compete for the same substrate pool, even small changes in BACE2 activity could have consequences for human disease.

Pharmacological evidences for DFK167-sensitive presenilin-independent gamma-secretase-like activity

Amyloid-beta (Abeta) peptides production is thought to be a key event in the neurodegenerative process ultimately leading to Alzheimer's disease (AD) pathology. A bulk of studies concur to propose that the C-terminal moiety of Abeta is released from its precursor beta-amyloid precursor protein by a high molecular weight enzymatic complex referred to as gamma-secretase, that is composed of at least, nicastrin (NCT), Aph-1, Pen-2, and presenilins (PS) 1 or 2. They are thought to harbor the gamma-secretase catalytic activity. However, several lines of evidence suggest that additional gamma-secretase-like activities could potentially contribute to Abeta production. By means of a quenched fluorimetric substrate (JMV2660) mimicking the beta-amyloid precursor protein sequence targeted by gamma-secretase, we first show that as expected, this probe allows monitoring of an activity detectable in several cell systems including the neuronal cell line telencephalon specific murine neurons (TSM1). This activity is reduced by DFK167, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), and LY68458, three inhibitors known to functionally interact with PS. Interestingly, JMV2660 but not the unrelated peptide JMV2692, inhibits Abeta production in an in vitrogamma-secretase assay as expected from a putative substrate competitor. This activity is enhanced by PS1 and PS2 mutations known to be responsible for familial forms of AD and reduced by aspartyl mutations inactivating PS or in cells devoid of PS or NCT. However, we clearly establish that residual JMV2660-hydrolysing activity could be recovered in PS- and NCT-deficient fibroblasts and that this activity remained inhibited by DFK167. Overall, our study describes the presence of a proteolytic activity displaying gamma-secretase-like properties but independent of PS and still blocked by DFK167, suggesting that the PS-dependent complex could not be the unique gamma-secretase activity responsible for Abeta production and delineates PS-independent gamma-secretase activity as a potential additional therapeutic target to fight AD pathology.

Measurement of cellular beta-site of APP cleaving enzyme 1 activity and its modulation in neuronal assay systems

Amyloid-beta peptide (Abeta), a putatively causative agent of Alzheimer's disease (AD), is proteolytically derived from beta-amyloid precursor protein (APP). Here we describe cellular assays to detect the activity of the key protease beta-site of APP cleaving enzyme 1 (BACE1) based on an artificial reporter construct containing the BACE1 cleavage site of APP. These methods allow identification of inhibitors and indirect modulators of BACE1. In primary neuronal cultures transfected with human APP constructs (huAPP), Abeta production was modified by BACE1 inhibitors similarly to the production of endogenous murine Abeta in wild-type cells and to that of different transgenic neurons. To further improve the assay, we substituted the extracellular domain of APP by secreted alkaline phosphatase (SEAP). SEAP was easily quantified in the cell culture supernatants after cleavage of SEAP-APP by BACE1 or alpha-secretases. To render the assay specific for BACE1, the alpha-secretase cleavage site of SEAP-APP was eliminated either by site-directed mutagenesis or by substituting the transmembrane part of APP by the membrane domain of the erythropoietin receptor (EpoR). The pharmacology of these constructs was characterized in detail in HEK293 cells (human embryonic kidney cell line), and the SEAP-APP-EpoR construct was also introduced into primary murine neurons and there allowed specific measurement of BACE1 activity.

Etazolate, a neuroprotective drug linking GABA(A) receptor pharmacology to amyloid precursor protein processing

Pharmacological modulation of the GABA(A) receptor has gained increasing attention as a potential treatment for central processes affected in Alzheimer disease (AD), including neuronal survival and cognition. The proteolytic cleavage of the amyloid precursor protein (APP) through the alpha-secretase pathway decreases in AD, concurrent with cognitive impairment. This APP cleavage occurs within the beta-amyloid peptide (Abeta) sequence, precluding formation of amyloidogenic peptides and leading to the release of the soluble N-terminal APP fragment (sAPPalpha) which is neurotrophic and procognitive. In this study, we show that at nanomolar-low micromolar concentrations, etazolate, a selective GABA(A) receptor modulator, stimulates sAPPalpha production in rat cortical neurons and in guinea pig brains. Etazolate (20 nM-2 microM) dose-dependently protected rat cortical neurons against Abeta-induced toxicity. The neuroprotective effects of etazolate were fully blocked by GABA(A) receptor antagonists indicating that this neuroprotection was due to GABA(A) receptor signalling. Baclofen, a GABA(B) receptor agonist failed to inhibit the Abeta-induced neuronal death. Furthermore, both pharmacological alpha-secretase pathway inhibition and sAPPalpha immunoneutralization approaches prevented etazolate neuroprotection against Abeta, indicating that etazolate exerts its neuroprotective effect via sAPPalpha induction. Our findings therefore indicate a relationship between GABA(A) receptor signalling, the alpha-secretase pathway and neuroprotection, documenting a new therapeutic approach for AD treatment.

Beta-secretase: structure, function, and evolution

The most popular current hypothesis is that Alzheimer's disease (AD) is caused by aggregates of the amyloid peptide (Abeta), which is generated by cleavage of the Abeta protein precursor (APP) by beta-secretase (BACE-1) followed by gamma-secretase. BACE-1 cleavage is limiting for the production of Abeta, making it a particularly good drug target for the generation of inhibitors that lower Abeta. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Abeta generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective drug-like molecule targeting novel mechanisms of BACE-1 regulation.

Nicotine leads to improvements in behavioral impairment and an increase in the nicotine acetylcholine receptor in transgenic mice

Nicotine is the principal psychoactive ingredient in cigarette smoke, and has been associated with health problems in humans. However, the pure form of nicotine may prove to be a valuable pharmaceutical agent for the treatment of AD. However, the beneficial effects of nicotine remain a matter of much controversy. In order to clarify this issue, 12-month-old transgenic mice, expressing neuron-specific enolase (NSE)-controlled APPsw, were treated with low, middle, and high doses of nicotine for 6 months. Herein, we have concluded that the nicotine-treated groups evidenced improvements in behavior and increases in the nicotine acetylcholine receptor, nAchRalpha7. These findings provide experimental evidence that nicotine effects an improvement in impaired memory, and that this improvement is associated with an increase in nAchRalpha7. Thus, nicotine may prove a good preventative or therapeutic modality for AD patients.

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.

Development and characterization of beta-secretase monolithic micro-immobilized enzyme reactor for on-line high-performance liquid chromatography studies

beta-Site APP cleavage enzyme 1 (BACE-1) is a transmembrane aspartyl protease that cleaves the amyloid-beta precursor protein (APP), which is abundant in neurons. BACE-1 is required for the generation of amyloid-beta (Abeta) peptides implicated in the pathogenesis of Alzheimer's disease (AD). It is widely believed that halting the production of Abeta peptide, by inhibition of BACE-1, is an attractive therapeutic modality for the treatment of Alzheimer's disease. BACE-1 has never been immobilized before. In the present study, for the first time, human recombinant beta-secretase micro-immobilised enzyme reactor (hrBACE-1-micro-IMER) was prepared by using an in situ immobilisation procedure on an ethylendiamine monolithic convective interaction media (EDA-CIM) disk. The activity and kinetic parameters of the hrBACE-1-micro-IMER were investigated by insertion in a HPLC system with fluorescent and mass detection. The micro-IMER was characterized in terms of units of immobilised hrBACE-1 and best mobile phase conditions for activity, by using as substrate casein-FITC and JMV2236, a peptide mimicking the Swedish-mutated APP (amyloid precursor protein) sequence. The characterization of the hrBACE-1-micro-IMER in terms of number of enzymatic active units after covalent linking to the solid matrix was performed by using the JMV2236 peptide as substrate in a HPLC-MS system. JMV2236 was injected into the hrBACE-1-micro-IMER and enzymatically cleaved; the product of the enzymatic cleavage and the remaining non-cleaved substrate were collected on a C18 column trap and switched to the LC-electrospray ionization MS system for kinetic constants determination. Inhibition studies were carried out. The effect of donepezil and pepstatin A, as BACE-1 inhibitors, was evaluated by simultaneous injection of the compounds with the peptidic substrate. The relative IC(50) values were found in agreement with that derived by the conventional fluorescence method, confirming the applicability of this new IMER for on-line inhibition studies. The main advantages of the hrBACE-1-micro-IMER approach over the conventional methods were found to be the increased enzyme efficiency, stability and the decreased time of analysis.

Alkalizing drugs induce accumulation of amyloid precursor protein by-products in luminal vesicles of multivesicular bodies

Amyloid precursor protein (APP) metabolism is central to the pathogenesis of Alzheimer disease. We showed recently that the amyloid intracellular domain (AICD), which is released by gamma-secretase cleavage of APP C-terminal fragments (CTFs), is strongly increased in cells treated with alkalizing drugs (Vingtdeux, V., Hamdane, M., Bégard, S., Loyens, A., Delacourte, A., Beauvillain, J.-C., Buée, L., Marambaud, P., and Sergeant, N. (2007) Neurobiol. Dis. 25, 686-696). Herein, we aimed to determine the cell compartment in which AICD accumulates. We show that APP-CTFs and AICD are present in multivesicular structures. Multivesicular bodies contain intraluminal vesicles (known as exosomes) when released in the extracellular space. We demonstrate that APP, APP-CTFs, and AICD are integrated and secreted within exosomes in differentiated neuroblastoma and primary neuronal culture cells. Together with recent data showing that amyloid-beta is also found in exosomes, our data show that multivesicular bodies are essential organelles for APP metabolism and that all APP metabolites can be secreted in the extracellular space.

Receptor tyrosine kinases positively regulate BACE activity and Amyloid-β production through enhancing BACE internalization

Amyloid-beta (Abeta) peptide, the primary constituent of senile plaques in Alzheimer's disease (AD), is generated by beta-secretase- and gamma-secretase-mediated sequential proteolysis of the amyloid precursor protein (APP). The aspartic protease, beta -site APP cleavage enzyme (BACE), has been identified as the main beta-secretase in brain but the regulation of its activity is largely unclear. Here, we demonstrate that both BACE activity and subsequent Abeta production are enhanced after stimulation of receptor tyrosine kinases (RTKs), such as the receptors for epidermal growth factor (EGF) and nerve growth factor (NGF), in cultured cells as well as in mouse hippocampus. Furthermore, stimulation of RTKs also induces BACE internalization into endosomes and Golgi apparatus. This enhancement of BACE activity and Abeta production upon RTK activation could be specifically inhibited by Src family kinase inhibitors and by depletion of endogenous c-Src with RNAi, and could be mimicked by over-expressed c-Src. Moreover, blockage of BACE internalization by a dominant negative form of Rab5 also abolished the enhancement of BACE activity and Abeta production, indicating the requirement of BACE internalization for the enhanced activity. Taken together, our study presents evidence that BACE activity and Abeta production are under the regulation of RTKs and this is achieved via RTK-stimulated BACE internalization, and suggests that an aberration of such regulation might contribute to pathogenic Abeta production.

Intracellular pH regulates amyloid precursor protein intracellular domain accumulation

The amyloid precursor protein (APP) metabolism is central to pathogenesis of Alzheimer's disease (AD). Parenchymal amyloid deposits, a neuropathological hallmark of AD, are composed of amyloid-beta peptides (Abeta). Abeta derives from the amyloid precursor protein (APP) by sequential cleavages by beta- and gamma-secretases. Gamma-secretase cleavage releases the APP intracellular domain (AICD), suggested to mediate a nuclear signaling. Physiologically, AICD is seldom detected and thus supposed to be rapidly degraded. The mechanisms responsible of its degradation remain unknown. We used a pharmacological approach and showed that several alkalizing drugs induce the accumulation of AICD in neuroblastoma SY5Y cell lines stably expressing APP constructs. Moreover, alkalizing drugs induce AICD accumulation in naive SY5Y, HEK and COS cells. This accumulation is not mediated by the proteasome or metallopeptidases and is not the result of an increased gamma-secretase activity since the gamma-secretase cleavage of Notch1 and N-Cadherin is not affected by alkalizing drug treatments. Altogether, our data demonstrate for the first time that alkalizing drugs induce the accumulation of AICD, a mechanism likely mediated by the endosome/lysosome pathway.

A novel approach to identifying beta-secretase inhibitors: bis-statine peptide mimetics discovered using structure and spot synthesis

Beta-secretase 1 (BACE1) is an aspartic protease believed to play a critical role in Alzheimer's disease. Inhibitors of this enzyme have been designed by incorporating the non-cleavable hydroxyethylene and statine isosteres into peptides corresponding to BACE1 substrate sequences. We sought to develop new methods to quickly characterize and optimize inhibitors based on the statine core. Minimal sequence requirements for binding were first established using both crystallography and peptide spot synthesis. These shortened peptide inhibitors were then optimized by using spot synthesis to perform iterative cycles of substitution and deletion. The present study resulted in the identification of novel "bis-statine" inhibitors shown by crystallography to have a unique binding mode. Our results demonstrate the application of peptide spot synthesis as an effective method for enhancing peptidomimetic drug discovery.