Measuring human beta-secretase (BACE1) activity using homogeneous time-resolved fluorescence

Advancement in Analytical Techniques for Determining the Activity of β-Site Amyloid Precursor Protein Cleaving Enzyme 1

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. The pathogenesis is still not fully clear. One of the main histopathological manifestations is senile plaques formed by β-amyloid (Aβ) accumulation. Aβ is generated from the sequential proteolysis of amyloid precursor protein (APP) by β-secretase [i.e. β-site APP cleaving enzyme 1 (BACE1)] and γ-secretase, with a rate-limiting step controlled by BACE1 activity. Therefore, inhibiting BACE1 activity has become a potential therapeutic strategy for AD. The development of reliable detection methods for BACE1 activity plays an important role in early diagnosis of AD and evaluation of the therapeutic effect of new drugs for AD. This article has reviewed the recent advances in BACE1 activity detection techniques. The challenges of applying these analysis techniques to early clinical diagnosis of AD and development trends of the detection techniques have been prospected.

Role of Natural Compounds and Target Enzymes in the Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) is a progressive neurological condition. The rising prevalence of AD necessitates the rapid development of efficient therapy options. Despite substantial study, only a few medications are capable of delaying the disease. Several substances with pharmacological activity, derived from plants, have been shown to have positive benefits for the treatment of AD by targeting various enzymes, such as acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), β-secretase, γ-secretase, and monoamine oxidases (MAOs), which are discussed as potential targets. Medicinal plants have already contributed a number of lead molecules to medicine development, with many of them currently undergoing clinical trials. A variety of medicinal plants have been shown to diminish the degenerative symptoms associated with AD, either in their raw form or as isolated compounds. The aim of this review was to provide a brief summary of AD and its current therapies, followed by a discussion of the natural compounds examined as therapeutic agents and the processes underlying the positive effects, particularly the management of AD.

Screening Platform Based on Inductively Coupled Plasma Mass Spectrometry for β-Site Amyloid Protein Cleaving Enzyme 1 (BACE1) Inhibitors

β-Site amyloid protein cleaving enzyme 1 (BACE1) is a promising therapeutic target for developing inhibitors to alleviate Alzheimer's disease (AD). Herein, we established an inductively coupled plasma mass spectrometry (ICPMS)-based inhibitor screening platform. A biotin-labeled lanthanide-coded peptide probe (LCPP; biotin-PEG₂-EVNLDAEC-DOTA-Ln) was designed to determine the activity of BACE1 and evaluate the degree of inhibition of inhibitors. The platform was first validated with two commercially available inhibitors (BSI I and BSI IV) in terms of IC₅₀ values and then applied to two newly designed inhibitors (inhibitors II and III) based on the crystal structure of BACE1 interacting with inhibitor I, and each of them contained an acylguanidine core structure. We found that their inhibition effects were improved as evaluated by the sensitive and accurate LCPP-ICPMS platform, demonstrating its ability for new drug screening.

Is It the Twilight of BACE1 Inhibitors?

β-secretase (BACE1) has been regarded as a prime target for the development of amyloid beta (Aβ) lowering drugs in the therapy of Alzheimer´s disease (AD). Although the enzyme was discovered in 1991 and helped to formulate the Aβ hypothesis as one of the very important features of AD etiopathogenesis, progress in AD treatment utilizing BACE1 inhibitors has remained limited. Moreover, in the last years, major pharmaceutical companies have discontinued clinical trials of five BACE1 inhibitors that had been strongly perceived as prospective. In our review, the Aβ hypothesis, the enzyme, its functions, and selected substrates are described. BACE1 inhibitors are classified into four generations. Those that underwent clinical trials displayed adverse effects, including weight loss, skin rashes, worsening of neuropsychiatric symptoms, etc. Some inhibitors could not establish a statistically significant risk-benefit ratio, or even scored worse than placebo. We still believe that drugs targeting BACE1 may still hide some potential, but a different approach to BACE1 inhibition or a shift of focus to modulation of its trafficking and/or post-translational modification should now be followed.

Biological Evaluation of Newly Synthesized Biaryl Guanidine Derivatives to Arrest β-Secretase Enzymatic Activity Involved in Alzheimer’s Disease

Proteases BACE1 (β-secretases) enzymes have been recognized as a promising target associated with Alzheimer's disease (AD). This study was carried out on the principles of molecular docking, chemical synthesis, and enzymatic inhibition of BACE1 enzymes via biaryl guanidine-based ligands. Based on virtual screening, thirteen different compounds were synthesized and subsequently evaluated via in vitro and in vivo studies. Among them, 1,3-bis(5,6-difluoropyridin-3-yl)guanidine (compound (9)) was found the most potent (IC₅₀ = 97 ± 0.91 nM) and active to arrest (99%) β-secretase enzymes (FRET assay). Furthermore, it was found to improve the novel object recognition test and Morris water maze test significantly (p < 0.05). Improved pharmacokinetic parameters, viz., Log P_o/w (1.76), Log S (-2.73), and better penetration to the brain (BBB permeation) with zero Lipinski violation, made it possible to hit the BACE1 as a potential therapeutic source for AD.

Pharmacophore Modeling and Docking Studies to Investigate Potential Leads for the Development of β -Secretase APP Cleavage Enzyme-1 (BACE-1) Inhibitors

Background:

Alzheimer’s disease is a medical condition with detrimental brain health. It is majorly diagnosed in aging individuals plaque in β) characterized by accumulated Amyloidal beta (A 1 BACE) 1 secretase APP cleavage enzyme βneurological areas. The ) is the target of choice that can be exploited to find drugs against Alzheimer’s disease.

Methods:

A series of BACE-1 inhibitors with reported binding constant were considered for the development of a feature based pharmacophore model.

Results:

The good correlation coefficient (r=0.91) and RMSD of 0.93 was observed with 30 compounds in training set. The model was validated internally (r2test=0.76) as well as externally by Fischer validation. The pharmacophore based virtual screening retrieved compounds that were docked and biologically evaluated.

Conclusion:

The three structurally diverse molecules were tested by in-vitro method. The pyridine derivative with highest fit value (6.9) exhibited IC50 value of 2.70 µM and thus was found to be the most promising lead molecule as BACE-1 inhibitor.

Design and synthesis of donepezil analogues as dual AChE and BACE-1 inhibitors

Multi-target-directed ligands (MTDLs) centered on β-secretase 1 (BACE-1) inhibition are emerging as innovative therapeutics in addressing the complexity of neurodegenerative diseases. A new series of donepezil analogues was designed, synthesized and evaluated as MTDLs against neurodegenerative diseases. Profiling of donepezil, a potent acetylcholinesterase (hAChE) inhibitor, into BACE-1 inhibition was achieved through introduction of backbone amide linkers to the designed compounds which are capable of hydrogen-bonding with BACE-1 catalytic site. In vitro assays and molecular modeling studies revealed the dual mode of action of compounds 4-6 against hAChE and BACE-1. Notably, compound 4 displayed potent hAChE inhibition (IC₅₀ value of 4.11 nM) and BACE-1 inhibition (IC₅₀ value of 18.3 nM) in comparison to donepezil (IC₅₀ values of 6.21 and 194 nM against hAChE and BACE-1, respectively). Moreover, 4 revealed potential metal chelating property, low toxicity on SH-SY5Y neuroblastoma cells and ability to cross the blood-brain barrier (BBB) in PAMPA-BBB assay which renders 4 a potential lead for further optimization of novel small ligands for the treatment of Alzheimer's disease.

Design, Synthesis, and Biological Evaluation of 1-Benzylamino-2-hydroxyalkyl Derivatives as New Potential Disease-Modifying Multifunctional Anti-Alzheimer's Agents

The multitarget approach is a promising paradigm in drug discovery, potentially leading to new treatment options for complex disorders, such as Alzheimer's disease. Herein, we present the discovery of a unique series of 1-benzylamino-2-hydroxyalkyl derivatives combining inhibitory activity against butyrylcholinesterase, β-secretase, β-amyloid, and tau protein aggregation, all related to mechanisms which underpin Alzheimer's disease. Notably, diphenylpropylamine derivative 10 showed balanced activity against both disease-modifying targets, inhibition of β-secretase (IC_{50  hBACE-1} = 41.60 μM), inhibition of amyloid β aggregation (IC_50 Aβ = 3.09 μM), inhibition of tau aggregation (55% at 10 μM); as well as against symptomatic targets, butyrylcholinesterase inhibition (IC_{50  hBuChE} = 7.22 μM). It might represent an encouraging starting point for development of multifunctional disease-modifying anti-Alzheimer's agents.

Design, Synthesis, and Biological Evaluation of 2-(Benzylamino-2-Hydroxyalkyl)Isoindoline-1,3-Diones Derivatives as Potential Disease-Modifying Multifunctional Anti-Alzheimer Agents

The complex nature of Alzheimer’s disease calls for multidirectional treatment. Consequently, the search for multi-target-directed ligands may lead to potential drug candidates. The aim of the present study is to seek multifunctional compounds with expected activity against disease-modifying and symptomatic targets. A series of 15 drug-like various substituted derivatives of 2-(benzylamino-2-hydroxyalkyl)isoindoline-1,3-diones was designed by modification of cholinesterase inhibitors toward β-secretase inhibition. All target compounds have been synthesized and tested against eel acetylcholinesterase (eeAChE), equine serum butyrylcholinesterase (eqBuChE), human β-secretase (hBACE-1), and β-amyloid (Aβ-aggregation). The most promising compound, 12 (2-(5-(benzylamino)-4-hydroxypentyl)isoindoline-1,3-dione), displayed inhibitory potency against eeAChE (IC₅₀ = 3.33 μM), hBACE-1 (43.7% at 50 μM), and Aβ-aggregation (24.9% at 10 μM). Molecular modeling studies have revealed possible interaction of compound 12 with the active sites of both enzymes—acetylcholinesterase and β-secretase. In conclusion: modifications of acetylcholinesterase inhibitors led to the discovery of a multipotent anti-Alzheimer’s agent, with moderate and balanced potency, capable of inhibiting acetylcholinesterase, a symptomatic target, and disease-modifying targets: β-secretase and Aβ-aggregation.

A peptide–WS2 nanosheet based biosensing platform for determination of β-secretase and screening of its inhibitors

β-Secretase (BACE1) is an important drug target in the treatment of Alzheimer's disease (AD).

Design, synthesis and biological evaluation of new phthalimide and saccharin derivatives with alicyclic amines targeting cholinesterases, beta-secretase and amyloid beta aggregation

The complexity of Alzheimer's disease (AD) calls for search of multifunctional compounds as potential candidates for effective therapy. A series of phthalimide and saccharin derivatives linked by different alicyclic fragments (piperazine, hexahydropyrimidine, 3-aminopyrrolidine or 3-aminopiperidine) with phenylalkyl moieties attached have been designed, synthesized, and evaluated as multifunctional anti-AD agents with cholinesterase, β-secretase and β-amyloid inhibitory activities. In vitro studies showed that the majority of saccharin derivatives with piperazine moiety and one phthalimide derivative with 3-aminopiperidine fragment exhibited inhibitory potency toward acetylcholinesterase (AChE) with EeAChE IC₅₀ values ranging from 0.83 μM to 19.18 μM. The target compounds displayed inhibition of human β-secretase-1 (hBACE1) ranging from 26.71% to 61.42% at 50 μM concentration. Among these compounds, two multifunctional agents (26, [2-(2-(4-benzylpiperazin-1-yl)ethyl)benzo[d]isothiazol-3(2H)-one 1,1-dioxide] and 52, 2-(2-(3-(3,5-difluorobenzylamino)piperidin-1-yl)ethyl)isoindoline-1,3-dione) have been identified. Compound 26 exhibited the highest inhibitory potency against EeAChE (IC₅₀ = 0.83 μM) and inhibitory activity against hBACE1 (33.61% at 50 μM). Compound 52 is a selective AChE inhibitor (IC_{50 AChE} = 6.47 μM) with BACE1 inhibitory activity (26.3% at 50 μM) and it displays the most significant Aβ anti-aggregating properties among all the obtained compounds (39% at 10 μM). Kinetic and molecular modeling studies indicate that 26 may act as non-competitive AChE inhibitor able to interact with both catalytic and peripheral active site of the enzyme.

Constant pH Molecular Dynamics Reveals pH-Modulated Binding of Two Small-Molecule BACE1 Inhibitors

Targeting β-secretase (BACE1) with small-molecule inhibitors offers a promising route for treatment of Alzheimer's disease. However, the intricate pH dependence of BACE1 function and inhibitor efficacy has posed major challenges for structure-based drug design. Here we investigate two structurally similar BACE1 inhibitors that have dramatically different inhibitory activity using continuous constant pH molecular dynamics (CpHMD). At high pH, both inhibitors are stably bound to BACE1; however, within the enzyme active pH range, only the iminopyrimidinone-based inhibitor remains bound, while the aminothiazine-based inhibitor becomes partially dissociated following the loss of hydrogen bonding with the active site and change of the 10s loop conformation. The drastically lower activity of the second inhibitor is due to the protonation of a catalytic aspartate and the lack of a propyne tail. This work demonstrates that CpHMD can be used for screening pH-dependent binding profiles of small-molecule inhibitors, providing a new tool for structure-based drug design and optimization.

Recent development of multifunctional agents as potential drug candidates for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a complex and progressive neurodegenerative disorder. The available therapy is limited to the symptomatic treatment and its efficacy remains unsatisfactory. In view of the prevalence and expected increase in the incidence of AD, the development of an effective therapy is crucial for public health. Due to the multifactorial aetiology of this disease, the multi-target-directed ligand (MTDL) approach is a promising method in search for new drugs for AD. This review updates information on the development of multifunctional potential anti-AD agents published within the last three years. The majority of the recently reported structures are acetylcholinesterase inhibitors, often endowed with some additional properties. These properties enrich the pharmacological profile of the compounds giving hope for not only symptomatic but also causal treatment of the disease. Among these advantageous properties, the most often reported are an amyloid-β antiaggregation activity, inhibition of β-secretase and monoamine oxidase, an antioxidant and metal chelating activity, NOreleasing ability and interaction with cannabinoid, NMDA or histamine H3 receptors. The majority of novel molecules possess heterodimeric structures, able to interact with multiple targets by combining different pharmacophores, original or derived from natural products or existing therapeutics (tacrine, donepezil, galantamine, memantine). Among the described compounds, several seem to be promising drug candidates, while others may serve as a valuable inspiration in the search for new effective therapies for AD.

Use of high-throughput mass spectrometry to reduce false positives in protease uHTS screens

As a label-free technology, mass spectrometry (MS) enables assays to be generated that monitor the conversion of substrates with native sequences to products without the requirement for substrate modifications or indirect detection methods. Although traditional liquid chromatography (LC)-MS methods are relatively slow for a high-throughput screening (HTS) paradigm, with cycle times typically ≥ 60 s per sample, the Agilent RapidFire High-Throughput Mass Spectrometry (HTMS) System, with a cycle time of 5-7 s per sample, enables rapid analysis of compound numbers compatible with HTS. By monitoring changes in mass directly, HTMS assays can be used as a triaging tool by eliminating large numbers of false positives resulting from fluorescent compound interference or from compounds interacting with hydrophobic fluorescent dyes appended to substrates. Herein, HTMS assays were developed for multiple protease programs, including cysteine, serine, and aspartyl proteases, and applied as a confirmatory assay. The confirmation rate for each protease assay averaged <30%, independent of the primary assay technology used (i.e., luminescent, fluorescent, and time-resolved fluorescent technologies). Importantly, >99% of compounds designed to inhibit the enzymes were confirmed by the corresponding HTMS assay. Hence, HTMS is an effective tool for removing detection-based false positives from ultrahigh-throughput screening, resulting in hit lists enriched in true actives for downstream dose response titrations and hit-to-lead efforts.

The zymogen of plasmepsin V from Plasmodium falciparum is enzymatically active

Plasmepsin V, a membrane-bound aspartic protease present in Plasmodium falciparum, is involved in the export of malaria parasite effector proteins into host erythrocytes and therefore is a potential target for antimalarial drug development. The present study reports the bacterial recombinant expression and initial characterization of zymogenic and mature plasmepsin V. A 484-residue truncated form of proplasmepsin (Glu37-Asn521) was fused to a fragment of thioredoxin and expressed as inclusion bodies. Refolding conditions were optimized and zymogen was processed into a mature form via cleavage at the Asn80-Ala81 peptide bond. Mature plasmepsin V exhibited a pH optimum of 5.5-7.0 with Km and kcat of 4.6 μM and 0.24s(-1), respectively, at pH 6.0 using the substrate DABCYL-LNKRLLHETQ-E(EDANS). Furthermore, the prosegment of proplasmepsin V was shown to be nonessential for refolding and inhibition. Unexpectedly, unprocessed proplasmepsin V was enzymatically active with slightly reduced substrate affinity (∼ 2-fold), and similar pH optimum as well as turnover compared to the mature form. Both zymogenic and mature plasmepsin V were partially inhibited by pepstatin A as well as several KNI aspartic protease inhibitors while certain metals strongly inhibited activity. Overall, the present study provides the first report on the nonessentiality of the prosegment for plasmepsin V folding and activity, and therefore, subsequent characterization of its structure-function relationships of both zymogen and mature forms in the development of novel inhibitors with potential antimalarial activities is warranted.

Sensitive and continuous screening of inhibitors of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) at single SPR chips

The development of new methods that meet the demand of high-throughput, high-fidelity screening of hit compounds is important to researching modalities of important diseases such as neurological disorders, HIV, and cancer. A surface plasmon resonance- (SPR-) based method capable of continuously screening enzyme inhibitors at a single chip with antibody-amplified signal enhancement has been developed. The proof of concept is demonstrated by monitoring the cleavage of chip-confined peptide substrates [a segment of the amyloid precursor protein (APP) with the Swiss mutation] by β-site APP-cleaving enzyme 1 (BACE1). In the presence of a noninhibitor, BACE1 clips the peptide substrate at the cleavage site, detaching a fragment that is homologous to the N-terminus of the amyloid beta (Aβ) peptide. Consequently, a subsequent injection of the Aβ antibody does not lead to any molecular recognition or SPR signal change at the chip. In contrast, suppression of the BACE1 activity by a strong inhibitor leaves the peptide substrate intact, and the subsequent antibody attachment produces an easily detectable SPR signal. Compared to the widely used FRET (fluorescence resonance energy transfer) assay, the method reported here is more cost-effective, as unlabeled peptide is used as the BACE1 substrate. Furthermore, the assay is faster (each screening cycle lasts for ca. 1.5 h) and can be continuously carried out at a single, regenerable SPR chip for more than 30 h. Consequently, excellent reproducibility (RSD < 5%) and throughput can be attained. Two inhibitors were screened, and their half-maximal inhibitory concentrations (IC50) determined by the SPR method were in excellent agreement with values deduced from ELISA and mass spectrometry.

Ameliorative effects of curculigoside from Curculigo orchioides Gaertn on learning and memory in aged rats

This study was designed to evaluate the ameliorating effects of curculigoside from Curculigo orchioides Gaertn on learning and memory in aged rats. In the present study, the ameliorating effects of curculigoside were determined through animal behaviour studies (including step-down test and Y-maze test), and the possible mechanisms were explored by evaluation of the activity of acetylcholinesterase (AchE) and determination of the expression of BACE1. Oral adminstration of the curculigoside (20, 40 mg/kg/day) for 14 days can significantly improve the latency and number of errors in aged rats based on the behaviour study results. In addition, the activity of AchE can be decreased by treatment of the curculigoside (10, 20, 40 mg/kg/day). Moreover, the expression of BACE1 can be down-regulated in the hippocampus of aged rats treated with curculigoside. The results of our present work have indicated that curculigoside can improve cognitive function in aged animals, possibly by decreasing the activity of AchE in the cerebra and inhibiting the expression of BACE1 in the hippocampus. In conclusion, our results suggested that curculigoside can be possible developed as a new drug for the treatment of Alzheimer's disease in the future.

A miniaturized 1536-well format gamma-secretase assay

gamma-Secretase is an aspartyl protease that cleaves multiple substrates including the amyloid precursor protein (APP) and the Notch proteins. Abnormal proteolysis of APP is involved in the pathogenesis of Alzheimer's disease (AD) and overactive Notch signaling plays an oncogenic role in a variety of cancers. gamma-Secretase has emerged as a promising target for drug development in the treatment of AD and cancer. Assays with increased capacity for high-throughput screening would allow for quicker screening of chemical libraries and facilitate inhibitor development. We have developed a homogeneous time-resolved fluorescence (HTRF)-based assay that makes use of a novel biotinylated recombinant APP substrate and solubilized membrane preparation as the source of the gamma-secretase enzyme. The assay was miniaturized to a 1536-well format and validated in a pilot screen against a library of approximately 3,000 compounds. The overall assay performance was robust due to a calculated Z' factor of 0.74 and its demonstrated ability to identify known gamma-secretase inhibitors such as pepstatin A. This validated assay can readily be used for primary screening against large chemical libraries searching for novel inhibitors of gamma-secretase activity that may represent potential therapeutics for AD and a variety of neoplasms.

Visualization of beta-secretase cleavage in living cells using a genetically encoded surface-displayed [corrected] FRET probe

The human beta-secretase, BACE, plays a key role in the generation of pathogenic amyloid beta-peptide (Abeta) in Alzheimer's disease and has been identified as an ideal target for therapy. Previous studies reported the monitoring of BACE activity in vitro utilizing chemical synthesized sensors. Here we describe the first genetically encoded FRET probe that can detect BACE activity in vivo. The FRET probe was constructed with the BACE substrate site (BSS) and two mutated green fluorescent proteins. In living cell, the FRET probe was directed to the secretory pathway and anchored on the cell surface to measure BACE enzymatic activity. The results show that the FRET probe can be cleaved by BACE effectively in vivo, suggesting that the probe can be used for real-time monitoring of BACE activity. This assay provides a novel platform for BACE inhibitor screening in vivo.

Multiwell fluorometric and colorimetric microassays for the evaluation of beta-secretase (BACE-1) inhibitors

The amyloid beta (Abeta) peptide is responsible for toxic amyloid plaque formation and is central to the aetiology of Alzheimer's disease (AD). It is generated by proteolytic processing of the amyloid precursor protein (APP) by beta-secretase (BACE-1) and gamma-secretase. Consequently, inhibition of BACE-1, a rate-limiting enzyme in the production of Abeta, is an attractive therapeutic approach to the treatment of Alzheimer's disease. This paper reports on improved microtiter plate-based fluorescence and colorimetric assays for the high-throughput screening (HTS) of BACE-1 inhibitors achieved by employing, for the first time, casein fluorescein isothiocyanate (casein-FITC) and N-alpha-benzoyl-D,L-arginine p-nitroanilide (BAPNA) as substrates, since they are known to be readily available and convenient substrates for proteases. The methods are based on the fluorescence enhancement following casein-FITC proteolysis and the visible absorbance of the p-nitroaniline (pNA) produced by BAPNA hydrolysis, with both reactions catalysed by BACE-1. Casein-FITC is a high-affinity substrate (K (m) = 110 nM) for BACE-1, more so than the Swedish (SW) type peptide (a peptide containing the Swedish mutant of APP, a familiar mutation that enhances Abeta production). BACE-1 catalysis of casein-FITC proteolysis exhibited Michaelis-Menten kinetic. Therefore, it was found that BACE-1 was saturable with casein-FITC that was processed in a time- and pH-dependent manner with greater catalytic efficiency than observed for the SW peptide. The enantioselective hydrolysis of L-BAPNA by BACE-1 was observed. L-BAPNA was hydrolysed ten times more efficiently by BACE-1 than the WT (wild-type peptide). The novel methods were validated using a FRET assay as an independent reference method. Therefore, in order to select new leads endowed with multifunctional activities, drugs for Alzheimer's disease (AD) - potent acetylcholinesterase (AChE) inhibitors - were tested for BACE-1 inhibition using the proposed validated assays. Among these, donepezil, besides being an acetylcholinesterase inhibitor, was also found to be a BACE-1 inhibitor that displayed submicromolar potency (170 nM).

Heparin Activates β-Secretase (BACE1) of Alzheimer's Disease and Increases Autocatalysis of the Enzyme

BACE1 is an aspartic protease that generates the N-terminus of the beta-amyloid protein (Alphabeta) from the beta-amyloid precursor protein (APP). BACE1 is a key target for Alzheimer drug development. However, little is known about the physiological regulation of the enzyme. Heparin can promote beta-secretase cleavage of APP in neuroblastoma cells. However, heparin has also been reported to directly inhibit BACE1 activity in vitro. To clarify the role of heparin in regulating BACE1, we examined the effect of heparin on the activity of recombinant human BACE1 (rBACE1) in vitro. Low concentrations (1 microg/mL) of heparin were found to stimulate rBACE1, increasing enzyme V(max) and decreasing the K(M). In contrast, higher concentrations of heparin (10 or 100 microg/mL) were inhibitory. Heparin affinity chromatography demonstrated that heparin interacted strongly with the zymogen form of rBACE1 and bound to a peptide homologous to the N-terminal pro sequence of BACE1. Mature (pro sequence cleaved) enzyme lacked the capacity to be stimulated by heparin, indicating that the pro domain was necessary for the stimulation by heparin. Furthermore, in the presence of stimulatory concentrations of heparin, there was an increase in autocatalytic cleavage of the protease domain and a subsequent loss of enzyme activity in vitro. Our results strongly suggest that heparin stimulates the partially active BACE1 zymogen, and we propose that the activation is mediated by high-affinity binding of heparin to the pro domain. Our study provides evidence that heparan sulfate proteoglycans could regulate the rate of Alphabeta production in vivo.

Characterization of Autocatalytic Conversion of Precursor BACE1 by Heteronuclear NMR Spectroscopy

Accumulation of the cytotoxic 40- to 42-residue beta-amyloid peptide represents the primary pathological process in Alzheimer's disease (AD). BACE1 (beta-site APP cleaving enzyme 1) is responsible for the initial required step in the neuronal amyloidogenic processing of beta-amyloid precursor protein and is a major drug target for the therapeutic intervention of AD. In the present study, BACE1 is initially synthesized as an immature precursor protein containing part of the pre domain and the entire pro domain, and undergoes autocatalytic conversion to yield the well-folded mature BACE1 enzyme. To understand the mechanism of the conversion and the role of the pro domain, we monitored the autocatalytic conversion of BACE1 by heteronuclear NMR spectroscopy and used chemical shift perturbations as a probe to study the structural changes accompanying the autocatalytic conversion. NMR data revealed local conformational changes from a partially disordered to a well-folded conformation associated with the conversion. The conformational changes are largely concentrated in the NH(2)-terminal lobe. Conversely, the active site conformations are conserved during the autocatalytic conversion. The precursor and mature BACE1 proteins were further characterized for their ability to interact with a substrate-based transition state BACE1 peptide inhibitor. The precursor BACE1 rapidly adopted the bound conformation in the presence of the inhibitor, which is identical to the bound conformation of the mature protein. The interaction of the inhibitor with both the precursor BACE1 and the fully processed BACE1 is in slow exchange on the NMR time scale, indicating a tight binding interaction. Overall, the NMR data demonstrated that the pro domain does not hinder inhibitor binding and may assist in the proper folding of the protein. The fully processed BACE1 represents a high quality well-folded protein which is highly stable over a long period of time, and is suitable for evaluation of inhibitor binding by NMR for drug intervention.

A Continuous Time-Resolved Fluorescence Assay for Identification of BACE1 Inhibitors

The aspartic protease beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) mediates the production of the neurotoxic amyloid beta peptide and is therefore considered an important drug target for treatment of Alzheimer's disease. We describe a new homogeneous time-resolved fluorescence quenching assay for the identification of BACE1 inhibitors that is characterized by minimal compound interference and allows both kinetic and end-point measurements. A fluorescent Eu-chelate as fluorescent donor, coupled to the N-terminus of a peptide containing the amyloid precursor protein Swedish mutation with a quenching molecule at the C-terminus as acceptor, is used as substrate. Upon peptide cleavage by BACE1, the energy transfer between donor and acceptor molecules is interrupted, leading to increased fluorescence emission of the donor. Compound interference, a common problem in fluorescence assays, is minimized with this technology because of the large Stoke's shift and the time-resolved fluorescence emission of the Eu-chelate. The assay reproduced IC50 values of known inhibitors and detected them also as hits in a screening campaign. A high signal-to-noise ratio of 289 and a Z' factor of 0.76 make this assay suitable for high-throughput screening.

Searching for the most effective screening system to identify cell-active inhibitors of β-secretase

The β-secretase BACE1 is an attractive drug target for reducing the level of the Alzheimers disease-promoting Aβ peptide in the brain. Whereas potent peptidomimetic in vitro inhibitors of BACE1 have been designed, screening approaches to identify cell-permeable small molecule inhibitors have had limited success so far. In the present minireview we summarize existing screening methods, discuss their scope of application in the drug discovery process and compare them to a novel cell-based screening system to identify BACE1 inhibitors by a positive yeast growth selection.