Mice deficient in BACE1, the Alzheimer's beta-secretase, have normal phenotype and abolished beta-amyloid generation

Therapeutic targets in Alzheimer's disease and related tauopathies

Alzheimer's disease is a progressive neurodegenerative disease that is characterized histopathologically by the presence of plaques, mainly composed of Abeta amyloid and the tangles, mainly composed of hyperphosphorylated tau. To date, there is no treatment that can reverse the disease, and all the current therapeutics is directed to cope with the symptoms of the disease. Here we describe the efforts dedicated to attack the plaques and, in more detail, the process of neurofibrillary degeneration, linked to the presence of the hyperphosphorylated microtubule associated protein tau. We have identified the different putative targets for therapeutics and the current knowledge on them.

Current therapeutic options for Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in the developed world. The increasing life expectancy in the last years has led to an increase in the prevalence of this age-related condition and has posed an important medical and social challenge for developed societies. The mainstays of current therapy for AD rely on the cholinergic hypothesis developed more than 20 years ago. These compounds, known as acetylcholinesterase inhibitors (AChEIs), inhibit the cholinesterases and aim at improving the brain synaptic availability of acetylcholine. These drugs have been approved for the treatment of AD based on pivotal clinical trials showing modest symptomatic benefit on cognitive, behavioral, and global measures. Memantine, an NMDA antagonist, has been recently included as a therapeutic option for AD. Memantine can be combined safely with AChEIs for an additional symptomatic benefit. During the last years our understanding of the mechanisms underlying the pathogenesis of AD has markedly expanded. Several putative neuroprotective drugs are thoroughly investigated and many of them have reached the clinical arena. It can be anticipated that some of these drugs will be able to slow/prevent the progression of this condition in the near future.

The Basic Biology of BACE1: A Key Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.

Computational insights into the development of novel therapeutic strategies for Alzheimer's disease

BACKGROUND

β-amyloidosis and oxidative stress have been implicated as root causes of Alzheimer's disease (AD). Current potential therapeutic strategies for the treatment of AD include inhibition of amyloid β (Aβ) production, stimulation of Aβ degradation and prevention of Aβ oligomerization. However, efforts in this direction are hindered by the lack of understanding of the biochemical processes occurring at the atomic level in AD.

DISCUSSION

A radically different approach to achieve this goal would be the application of comprehensive theoretical and computational techniques such as molecular dynamics, quantum mechanics, hybrid quantum mechanics/molecular mechanics, bioinformatics and rotational spectroscopy to investigate complex chemical and physical processes in β-amyloidosis and the oxidative stress mechanism.

CONCLUSION

Results obtained from these studies will provide an atomic level understanding of biochemical processes occurring in AD and advance efforts to develop effective therapeutic strategies for this disease.

Progress in the development of beta-secretase inhibitors for Alzheimer's disease

Since the original identification of BACE in 1999 and until quite recently, BACE was often regarded as a "difficult" drug target, much as renin has proven to be. The reasons for this include the following. First, the long and shallow nature of the substrate binding pocket suggested that it would not be possible to identify small molecule drugs that could have adequate binding affinity. Second, functional groups that typically interact with the active site aspartates are usually highly polarized and, therefore, contribute to reduced CNS localization. Early BACE inhibitors were all designed using knowledge of the peptide substrates and usually contained some variation of a few well-known transition-state isosteres. While these had great impact on fundamental understanding of the enzyme structure and key interaction regions, they were very large, very polar, and had essentially no CNS availability. Continued progress by reducing the peptidic nature of these compounds resulted in incremental advances and has provided compounds that meet, or nearly meet, typical CNS drug-like criteria. The challenges associated with peptidic starting points inspired innovative new approaches to search for different starting points. Several groups employed high concentration screening (ligand concentration 100 microM and higher) to find weak hits after conventional screening (typically at 10 microM) failed to find more potent ones. Fragment-based methods have also been developed to identify even weaker hits (IC50 1 mM and greater). This was accomplished through the evolution and refinement of several detection methodologies including calorimetry, surface plasmon resonance, NMR, and crystallography. Coupled with detailed structural understanding of ligand-enzyme interactions and focus on maintaining ligand efficiency, these developments have resulted in several examples where potency was improved by 10,000-fold to afford compounds with IC50 values < 10 nM and promising drug-like characteristics. Together, all these efforts have afforded a diverse array of chemotypes as BACE inhibitors. Early work focused on improving BACE potency in isolated enzyme assays. However, most of these compounds showed potency reductions in cellular assays. Continued improvements in drug properties and in understanding of the physiologically relevant conditions have resulted in many compounds that show strong potency in both isolated and cellular assays. Several compounds have shown reduction of Abeta using rodent in-vivo models both peripherally and in the brain. Recently, one compound has demonstrated reduction of brain Abeta levels in a non-human primate. Phase I clinical trials were initiated on BACE inhibitor CTS-21166 from CoMentis in July of 2007. This compound derives from the earliest described peptidic inhibitors such as OM99-2 [58] but no details have been reported. In addition to strategies involving small molecule inhibitors of BACE and gamma-secretase to reduce Abeta levels, the application of biological agents has been under investigation since the identification of Abeta. The earliest efforts in this area failed. Despite encouraging results in preclinical models, immunization against Abeta by administration of AN-1792 from Elan led to development of aseptic meningoencephalitis in 6% of the patients receiving the drug. Nevertheless, continued efforts with other biological approaches appear encouraging. Most advanced in clinical trials is bapineuzumab from Elan, which is in Phase III clinical trials. This is a humanized monoclonal antibody against Abeta plaques. A recent monograph is devoted to progress in these areas. Taken together, considerable progress has been made in developing CNS-penetrant agents that reduce AP levels and in providing validation that such agents will be therapeutically beneficial for the treatment of Alzheimer's disease.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

The role of tyrosine 71 in modulating the flap conformations of BACE1

β-Site amyloid precursor protein cleaving enzyme 1 (BACE1) is a potential target for treating Alzheimer's disease. BACE1's binding site is partially covered by a flexible loop on its N-terminal domain, known as the "flap," which has been found in several conformations in crystal structures of BACE1 and other aspartyl proteases. The side chain of the invariant residue Tyr71 on the flap adopts several rotameric orientations, leading to our hypothesis that the orientation of this residue dictates the movement and conformations available to the flap. We investigated this hypothesis by performing 220 ns of molecular dynamics simulations of bound and unbound wild-type BACE1 as well as the unbound Y71A mutant. Our findings indicate that the flap exhibits various degrees of mobility and adopts different conformations depending on the Tyr71 orientation. Surprisingly, the "self-inhibited" form is stable in our simulations, making it a reasonable target for drug design. The alanine mutant, lacking a large side chain at position 71, displays significant differences in flap dynamics from wild type, freely sampling very open and closed conformations. Our simulations show that Tyr71, in addition to its previously determined functions in catalysis and substrate binding, has the important role of modulating flap conformations in BACE1.

miR-29c regulates BACE1 protein expression

Recently, BACE1 expression was shown to be regulated by microRNAs, small endogenous RNA molecules that regulate protein expression through sequence-specific interaction with messenger RNA. Here, we showed that microRNA-29c (miR-29c), a miRNA that is enriched in the brain and highly expressed in the APPswe/PSΔE9 mouse lowers BACE1 protein in vitro and in transgenic miR-29c mice. In silico analysis identified two putative target sites in the BACE1 mRNA for the miR-29c family. We chose SH-SY5Y, HEK-293T cell lines and miR-29c transgenic mice for these studies to validate our hypothesis. Significantly, over-expression of miR-29c in SH-SY5Y, HEK-293T cell lines and miR-29c transgenic mice downregulated BACE1 protein levels. Our findings suggest that miR-29c may be an endogenous regulator of BACE1 protein expression.

The β-secretase enzyme BACE1 as a therapeutic target for Alzheimer's disease

Amyloid plaques are defining histopathologic lesions in the brains of Alzheimer's disease (AD) patients and are composed of the amyloid-beta peptide, which is widely considered to play a critical role in the pathogenesis of AD. The β-secretase, or β-site amyloid precursor protein cleaving enzyme 1 (BACE1; also called Asp2, memapsin 2), is the enzyme that initiates the generation of amyloid beta. Consequently, BACE1 is an attractive drug target for lowering cerebral levels of amyloid beta for the treatment or prevention of AD. Much has been learned about BACE1 since its discovery over 10 years ago. In the present article, we review BACE1 properties and characteristics, cell biology, in vivo validation, substrates, therapeutic potential, and inhibitor drug development. Studies relating to the physiological functions of BACE1 and the promise of BACE1 inhibition for AD will also be discussed. We conclude that therapeutic inhibition of BACE1 should be efficacious for AD, although careful titration of the drug dose may be necessary to limit mechanism-based side effects.

Computational modeling of substrate specificity and catalysis of the β-secretase (BACE1) enzyme

In this combined MD simulation and DFT study, interactions of the wild-type (WT) amyloid precursor protein (APP) and its Swedish variant (SW), Lys670 → Asn and Met671 → Leu, with the beta-secretase (BACE1) enzyme and their cleavage mechanisms have been investigated. BACE1 catalyzes the rate-limiting step in the generation of 40-42 amino acid long Alzheimer amyloid beta (Aβ) peptides. All key structural parameters such as position of the flap, volume of the active site, electrostatic binding energy, structures, and positions of the inserts A, D, and F and 10s loop obtained from the MD simulations show that, in comparison to the WT-substrate, BACE1 exhibits greater affinity for the SW-substrate and orients it in a more reactive conformation. The enzyme-substrate models derived from the MD simulations were further utilized to investigate the general acid/base mechanism used by BACE1 to hydrolytically cleave these substrates. This mechanism proceeds through the following two steps: (1) formation of the gem-diol intermediate and (2) cleavage of the peptide bond. For the WT-substrate, the overall barrier of 22.4 kcal/mol for formation of the gem-diol intermediate is 3.3 kcal/mol higher than for the SW-substrate (19.1 kcal/mol). This process is found to be the rate-limiting in the entire mechanism. The computed barrier is in agreement with the measured barrier of ca. 18.00 kcal/mol for the WT-substrate and supports the experimental observation that the cleavage of the SW-substrate is 60 times more efficient than the WT-substrate.

Small molecule inhibitors of amyloid β peptide aggregation as a potential therapeutic strategy for Alzheimer's disease

Amyloid β (Aβ) peptides have long been viewed as a potential target for Alzheimer's disease (AD). Aggregation of Aβ peptides in the brain tissue is believed to be an exclusively pathological process. Therefore, blocking the initial stages of Aβ peptide aggregation with small molecules could hold considerable promise as the starting point for the development of new therapies for AD. Recent rapid progresses in our understanding of toxic amyloid assembly provide a fresh impetus for this interesting approach. Here, we discuss the problems, challenges and new concepts in targeting Aβ peptides.

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Biomarker-based dissection of neurodegenerative diseases

The diagnosis of neurodegenerative diseases within neurology and psychiatry are hampered by the difficulty in getting biopsies and thereby validating the diagnosis by pathological findings. Biomarkers for other types of disease have been readily adopted into the clinical practice where for instance troponins are standard tests when myocardial infarction is suspected. However, the use of biomarkers for neurodegeneration has not been fully incorporated into the clinical routine. With the development of cerebrospinal fluid (CSF) biomarkers that reflect pathological events within the central nervous system (CNS), important clinical diagnostic tools are becoming available. This review summarizes the most promising biomarker candidates that may be used to monitor different types of neurodegeneration and protein inclusions, as well as different types of metabolic changes, in living patients in relation to the clinical phenotype and disease progression over time. Our aim is to provide the reader with an updated lexicon on currently available biomarker candidates, how far they have come in development and how well they reflect pathogenic processes in different neurodegenerative diseases. Biomarkers for specific pathogenetic processes would also be valuable tools both to study disease pathogenesis directly in patients and to identify and monitor the effect of novel treatment strategies.

Differential regulation of BACE1 expression by oxidative and nitrosative signals

Background

It is well established that both cerebral hypoperfusion/stroke and type 2 diabetes are risk factors for Alzheimer's disease (AD). Recently, the molecular link between ischemia/hypoxia and amyloid precursor protein (APP) processing has begun to be established. However, the role of the key common denominator, namely nitric oxide (NO), in AD is largely unknown. In this study, we investigated redox regulation of BACE1, the rate-limiting enzyme responsible for the β-cleavage of APP to Aβ peptides.

Results

Herein, we studied events such as S-nitrosylation, a covalent modification of cysteine residues by NO, and H₂O₂-mediated oxidation. We found that NO and H₂O₂ differentially modulate BACE1 expression and enzymatic activity: NO at low concentrations (<100 nM) suppresses BACE1 transcription as well as its enzymatic activity while at higher levels (0.1-100 μM) NO induces S-nitrosylation of BACE1 which inactivates the enzyme without altering its expression. Moreover, the suppressive effect on BACE1 transcription is mediated by the NO/cGMP-PKG signaling, likely through activated PGC-1α. H₂O₂ (1-10 μM) induces BACE1 expression via transcriptional activation, resulting in increased enzymatic activity. The differential effects of NO and H₂O₂ on BACE1 expression and activity are also reflected in their opposing effects on Aβ generation in cultured neurons in a dose-dependent manner. Furthermore, we found that BACE1 is highly S-nitrosylated in normal aging brains while S-nitrosylation is markedly reduced in AD brains.

Conclusion

This study demonstrates for the first time that BACE1 is highly modified by NO via multiple mechanisms: low and high levels of NO suppress BACE1 via transcriptional and post translational regulation, in contrast with the upregulation of BACE1 by H₂O₂-mediated oxidation. These novel NO-mediated regulatory mechanisms likely protect BACE1 from being further oxidized by excessive oxidative stress, as from H₂O₂ and peroxynitrite which are known to upregulate BACE1 and activate the enzyme, resulting in excessive cleavage of APP and Aβ generation; they likely represent the crucial house-keeping mechanism for BACE1 expression/activation under physiological conditions.

Potent BACE-1 inhibitor design using pharmacophore modeling, in silico screening and molecular docking studies

BACKGROUND

Beta-site amyloid precursor protein cleaving enzyme (BACE-1) is a single-membrane protein belongs to the aspartyl protease class of catabolic enzymes. This enzyme involved in the processing of the amyloid precursor protein (APP). The cleavage of APP by BACE-1 is the rate-limiting step in the amyloid cascade leading to the production of two peptide fragments Aβ40 and Aβ42. Among two peptide fragments Aβ42 is the primary species thought to be responsible for the neurotoxicity and amyloid plaque formation that lead to memory and cognitive defects in Alzheimer's disease (AD). AD is a ravaging neurodegenerative disorder for which no disease-modifying treatment is currently available. Inhibition of BACE-1 is expected to stop amyloid plaque formation and emerged as an interesting and attractive therapeutic target for AD.

METHODS

Ligand-based computational approach was used to identify the molecular chemical features required for the inhibition of BACE-1 enzyme. A training set of 20 compounds with known experimental activity was used to generate pharmacophore hypotheses using 3D QSAR Pharmacophore Generation module available in Discovery studio. The hypothesis was validated by four different methods and the best hypothesis was utilized in database screening of four chemical databases like Maybridge, Chembridge, NCI and Asinex. The retrieved hit compounds were subjected to molecular docking study using GOLD 4.1 program.

RESULTS

Among ten generated pharmacophore hypotheses, Hypo 1 was chosen as best pharmacophore hypothesis. Hypo 1 consists of one hydrogen bond donor, one positive ionizable, one ring aromatic and two hydrophobic features with high correlation coefficient of 0.977, highest cost difference of 121.98 bits and lowest RMSD value of 0.804. Hypo 1 was validated using Fischer randomization method, test set with a correlation coefficient of 0.917, leave-one-out method and decoy set with a goodness of hit score of 0.76. The validated Hypo 1 was used as a 3D query in database screening and retrieved 773 compounds with the estimated activity value <100 nM. These hits were docked into the active site of BACE-1 and further refined based on molecular interactions with the essential amino acids and good GOLD fitness score.

CONCLUSION

The best pharmacophore hypothesis, Hypo 1, with high predictive ability contains chemical features required for the effective inhibition of BACE-1. Using Hypo 1, we have identified two compounds with diverse chemical scaffolds as potential virtual leads which, as such or upon further optimization, can be used in the designing of new BACE-1 inhibitors.

Central angiotensin II stimulation promotes β amyloid production in Sprague Dawley rats

Background

Stress and various stress hormones, including catecholamines and glucocorticoids, have recently been implicated in the pathogenesis of Alzheimer's disease (AD), which represents the greatest unresolved medical challenge in neurology. Angiotensin receptor blockers have shown benefits in AD and prone-to-AD animals. However, the mechanisms responsible for their efficacy remain unknown, and no studies have directly addressed the role of central angiotensin II (Ang II), a fundamental stress hormone, in the pathogenesis of AD. The present study focused on the role of central Ang II in amyloidogenesis, the critical process in AD neuropathology, and aimed to provide direct evidence for the role of this stress hormone in the pathogenesis of AD.

Methodology/Principal Findings

Increased central Ang II levels during stress response were modeled by intracerebroventricular (ICV) administration of graded doses of Ang II (6 ng/hr low dose, 60 ng/hr medium dose, and 600 ng/hr high dose, all delivered at a rate of 0.25 µl/hr) to male Sprague Dawley rats (280–310 g) via osmotic pumps. After 1 week of continuous Ang II infusion, the stimulation of Ang II type 1 receptors was accompanied by the modulation of amyloid precursor protein, α-, β-and γ-secretase, and increased β amyloid production. These effects could be completely abolished by concomitant ICV infusion of losartan, indicating that central Ang II played a causative role in these alterations.

Conclusions/Significance

Central Ang II is essential to the stress response, and the results of this study suggest that increased central Ang II levels play an important role in amyloidogenesis during stress, and that central Ang II-directed stress prevention and treatment might represent a novel anti-AD strategy.

Current perspectives on pharmacotherapy of Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a daunting public health threat that has prompted the scientific community's ongoing efforts to decipher the underlying disease mechanism, and thereafter, target this therapeutically. Although basic research in AD has made remarkable progress over the past two decades, currently available drugs can only improve cognitive symptoms temporarily; no treatment can reverse, stop, or even slow this inexorable neurodegenerative process. Numerous disease-modifying strategies targeting the production and clearance of Aβ, as well as modulation of abnormal aggregation of tau filaments, are currently in clinical trials .

AREAS COVERED

this review provides an overview of a wide array of therapeutic approaches under investigation, and the perspectives developed in the last 10 years.

EXPERT OPINION

While it is not possible to predict the success of any individual program, one or more are likely to prove effective. Indeed, it seems reasonable to predict that in the not-too-distant future, a synergistic combination of agents will have the capacity to alter the neurodegenerative cascade and reduce the global impact of this devastating disease. The scientific community must acknowledge that Alzheimer's disease is a complex multifactorial disorder, and thus a single target or pathogenic pathway is unlikely to be identified. The major aim should be to design ligands with pluripotent pharmacological activities.

APP processing in Alzheimer's disease

An important pathological feature of Alzheimer's disease (AD) is the presence of extracellular senile plaques in the brain. Senile plaques are composed of aggregations of small peptides called β-amyloid (Aβ). Multiple lines of evidence demonstrate that overproduction/aggregation of Aβ in the brain is a primary cause of AD and inhibition of Aβ generation has become a hot topic in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavages first by β-secretase and then by γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα and preclude Aβ generation. Cleavage of APP by caspases may also contribute to AD pathologies. Therefore, understanding the metabolism/processing of APP is crucial for AD therapeutics. Here we review current knowledge of APP processing regulation as well as the patho/physiological functions of APP and its metabolites.

Inhibition of beta-secretase in vivo via antibody binding to unique loops (D and F) of BACE1

β-Secretase (BACE1) is an attractive drug target for Alzheimer disease. However, the design of clinical useful inhibitors targeting its active site has been extremely challenging. To identify alternative drug targeting sites we have generated a panel of BACE1 monoclonal antibodies (mAbs) that interfere with BACE1 activity in various assays and determined their binding epitopes. mAb 1A11 inhibited BACE1 in vitro using a large APP sequence based substrate (IC(50) ∼0.76 nm), in primary neurons (EC(50) ∼1.8 nm), and in mouse brain after stereotactic injection. Paradoxically, mAb 1A11 increased BACE1 activity in vitro when a short synthetic peptide was used as substrate, indicating that mAb 1A11 does not occupy the active-site. Epitope mapping revealed that mAb 1A11 binds to adjacent loops D and F, which together with nearby helix A, distinguishes BACE1 from other aspartyl proteases. Interestingly, mutagenesis of loop F and helix A decreased or increased BACE1 activity, identifying them as enzymatic regulatory elements and as potential alternative sites for inhibitor design. In contrast, mAb 5G7 was a potent BACE1 inhibitor in cell-free enzymatic assays (IC(50) ∼0.47 nm) but displayed no inhibitory effect in primary neurons. Its epitope, a surface helix 299-312, is inaccessible in membrane-anchored BACE1. Remarkably, mutagenesis of helix 299-312 strongly reduced BACE1 ectodomain shedding, suggesting that this helix plays a role in BACE1 cellular biology. In conclusion, this study generated highly selective and potent BACE1 inhibitory mAbs, which recognize unique structural and functional elements in BACE1, and uncovered interesting alternative sites on BACE1 that could become targets for drug development.

β-Secretase activity in rat astrocytes: translational block of BACE1 and modulation of BACE2 expression

BACE1 and BACE2 are two closely related membrane-bound aspartic proteases. BACE1 is widely recognized as the neuronal β-secretase that cleaves the amyloid-β precursor protein, thus allowing the production of amyloid-β, i.e. the peptide that has been proposed to trigger the neurodegenerative process in Alzheimer's disease. BACE2 has ubiquitous expression and its physiological and pathological role is still unclear. In light of a possible role of glial cells in the accumulation of amyloid-β in brain, we have investigated the expression of these two enzymes in primary cultures of astrocytes. We show that astrocytes possess β-secretase activity and produce amyloid-β because of the activity of BACE2, but not BACE1, the expression of which is blocked at the translational level. Finally, our data demonstrate that changes in the astrocytic phenotype during neuroinflammation can produce both a negative as well as a positive modulation of β-secretase activity, also depending on the differential responsivity of the brain regions.

Metalloproteases and Proteolytic Processing

Proteolytic enzymes constitute around 2% of the human genome and are involved in all stages of cell and organism development from fertilization through to cell death. In the human genome the major classes of peptidases are represented by cysteine-, serine- and metalloenzymes, which possess a wide spectrum of substrate specificity and physiological functions. The identification of many novel peptidases from genome sequencing programmes has suggested potential new therapeutic targets. In addition, several well characterised peptidases were recently shown to possess new and unexpected biological roles in neuroinflammation, cancer and angiogenesis, cardiovascular diseases and neurodegeneration. This chapter will briefly characterize the main classes of metallopeptidases and their roles in health and disease. Particular attention will be paid to the angiotensin-converting enzyme (ACE), neprilysin (NEP) and adamalysin (ADAM) families of proteases and their pathophysiological roles with a particular emphasis on cancer and neurodegeneration. The roles and mechanisms of protein shedding which primarily involve the ADAMs family of metallopeptidases will be explained using amyloid protein precursor (APP) processing cascades as a well characterized example. The therapeutic significance of modulating (activating or inhibiting) metallopeptidase activity will be a particular focus of this chapter.

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.

Progress in the development of new drugs in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease with a global prevalence estimated at 26.55 million in 2006. During the past decades, several agents have been approved that enhance cognition of AD patients. However, the effectiveness of these treatments are limited or controversial and they do not modify disease progression. Recent advances in understanding AD pathogenesis have led to the development of numerous compounds that might modify the disease process. AD is mainly characterized neuropathologically by the presence of two kinds of protein aggregates: extracellular plaques of Abeta-peptide and intracellular neurofibrillary tangles. Abeta and tau could interfere in an original way contributing to a cascade of events leading to neuronal death and transmitter deficits. Investigation for novel therapeutic approaches targeting the presumed underlying pathogenic mechanisms is major focus of research. Antiamyloid agents targeting production, accumulation, clearance, or toxicity associated with Abeta peptide, are some approaches under investigation to limit extracellular plaques of Abeta-peptide accumulation. We can state as an example: Abeta passive and active immunization, secretases modulation, Abeta degradation enhancement, or antiaggregation and antifibrillization agents. Tau-related therapies are also under clinical investigation but few compounds are available. Another alternative approach under development is neuroprotective agents such as antioxidants, anti-inflammatory drugs, compounds acting against glutamate mediated neurotoxicity. Neurorestorative approaches through neurotrophin or cell therapy also represent a minor avenue in AD research. Finally, statins, receptor for advanced glycation end products inhibitors, thiazolidinediones, insulin, and hormonal therapies are some other ways of research for a therapeutic approach of Alzheimer's disease. Taking into account AD complexity, it becomes clear that polypharmacology with drugs targeting different sites could be the future treatment approach and a majority of the recent drugs under evaluation seems to act on multiple targets. This article exposes general classes of disease-modifying therapies under investigation.

[Treatment of Alzheimer's disease and future approaches]

The progressive neuronal loss in Alzheimer's disease leads to neurochemical abnormalities which provide the basis for symptomatic treatments. Four cholinesterase inhibitors were released in this indication. Meta-analyses have confirmed a beneficial effect on cognitive functioning and activities of daily living. The NMDA receptor antagonist, memantine, was also approved for the treatment of moderate to severe and may be associated. Progress in the patho-physiology of the disease offers some hope of new treatments acting on the cerebral lesions. The amyloid hypothesis allowed the emergence of active or passive immunotherapies, and of secretase inhibitors or modulators. Recent studies have targeted the P tau protein. The brain plasticity and the uses of stem cells offer more distant hope.

Secretases as therapeutic targets for Alzheimer's disease

Accumulation of amyloid-β (Aβ) is widely accepted as the key instigator of Alzheimer's disease (AD). The proposed mechanism is that accumulation of Aβ results in inflammatory responses, oxidative damages, neurofibrillary tangles and, subsequently, neuronal/synaptic dysfunction and neuronal loss. Given the critical role of Aβ in the disease process, the proteases that produce this peptide are obvious targets. The goal would be to develop drugs that can inhibit the activity of these targets. Protease inhibitors have proved very effective for treating other disorders such as AIDS and hypertension. Mutations in APP (amyloid-β precursor protein), which flanks the Aβ sequence, cause early-onset familial AD, and evidence has pointed to the APP-to-Aβ conversion as a possible therapeutic target. Therapies aimed at modifying Aβ-related processes aim higher up the cascade and are therefore more likely to be able to alter the progression of the disease. However, it is not yet fully known whether the increases in Aβ levels are merely a result of earlier events that were already causing the disease.

BACE2 plays a role in the insulin receptor trafficking in pancreatic ß-cells

BACE1 (β-site amyloidogenic cleavage of precursor protein-cleaving enzyme 1) is a β-secretase protein that plays a central role in the production of the β-amyloid peptide in the brain and is thought to be involved in the Alzheimer's pathogenesis. In type 2 diabetes, amyloid deposition within the pancreatic islets is a pathophysiological hallmark, making crucial the study in the pancreas of BACE1 and its homologous BACE2 to understand the pathological mechanisms of this disease. The objectives of the present study were to characterize the localization of BACE proteins in human pancreas and determine their function. High levels of BACE enzymatic activity were detected in human pancreas. In normal human pancreas, BACE1 was observed in endocrine as well as in exocrine pancreas, whereas BACE2 expression was restricted to β-cells. Intracellular analysis using immunofluorescence showed colocalization of BACE1 with insulin and BACE2 with clathrin-coated vesicles of the plasma membrane in MIN6 cells. When BACE1 and -2 were pharmacologically inhibited, BACE1 localization was not altered, whereas BACE2 content in clathrin-coated vesicles was increased. Insulin internalization rate was reduced, insulin receptor β-subunit (IRβ) expression was decreased at the plasma membrane and increased in the Golgi apparatus, and a significant reduction in insulin gene expression was detected. Similar results were obtained after specific BACE2 silencing in MIN6 cells. All these data point to a role for BACE2 in the IRβ trafficking and insulin signaling. In conclusion, BACE2 is hereby presented as an important enzyme in β-cell function.

High-level expression of a human β-site APP cleaving enzyme in transgenic tobacco chloroplasts and its immunogenicity in mice

Plastid transformation has to date been applied to the expression of heterologous genes involved in agronomic traits and to the production of useful recombinant proteins. Here, we report a feasibility study for producing the human β-site APP cleaving enzyme (BACE) via transformation of tobacco chloroplasts. Stable integration of human BACE into the plastome was confirmed by PCR. Genomic Southern blot analysis detected the presence of the tobacco aadA and human BACE genes between trnI and trnA in the plastome. Northern blot analysis revealed that the aadA and BACE genes were both properly transcribed into a dicistronic transcriptional unit. Human BACE protein expression in transplastomic tobacco was determined by western blot analysis. ELISA analysis revealed that, based on a dilution series of E. coli-derived BACE as a standard, transplastomic lines accumulated BACE to levels of 2.0% of total soluble proteins. When mice were gavaged with the transplastomic tobacco extracts, they showed an immune response against the BACE antigen. The successful production of plastid-based BACE protein has the potential for developing a plant-based vaccine against Alzheimer disease.

Inhibiting β-secretase activity in Alzheimer's disease cell models with single-chain antibodies specifically targeting APP

The Amyloid-β (Aβ) peptide is produced from the amyloid precursor protein (APP) by sequential proteolytic cleavage of APP first by β-secretase and then by γ-secretase. β-Site APP cleaving enzyme-1 (BACE-1) is the predominant enzyme involved in β-secretase processing of APP and is a primary therapeutic target for treatment of Alzheimer's disease. While inhibiting BACE-1 activity has obvious therapeutic advantages, BACE-1 also cleaves numerous other substrates with important physiological activity. Thus, blanket inhibition of BACE-1 function may have adverse side effects. We isolated a single chain variable fragment (scFv) from a human-based scFv yeast display library that selectively inhibits BACE-1 activity toward APP by binding the APP substrate at the proteolytic site. We selected the iBSEC1 scFv, since it recognizes the BACE-1 cleavage site on APP but does not bind the adjacent highly antigenic N-terminal of Aβ, and thus it will target APP but not soluble Aβ. When added to 7PA2 cells, a mammalian cell line that overexpresses APP, the iBSEC1 scFv binds APP on the cell surface, reduces toxicity induced by APP overexpression, and reduces both intracellular and extracellular Aβ levels by around 50%. Since the iBSEC1 scFv does not contain the antibody F(c) region, this construct does not pose the risk of exacerbating inflammation in the brain as faced with full-length monoclonal antibodies for potential therapeutic applications.

Beta-secretase inhibitor GRL-8234 rescues age-related cognitive decline in APP transgenic mice

Alzheimer disease is intimately linked to an excess amount of amyloid-β (Aβ) in the brain. Thus, therapeutic inhibition of Aβ production is an attractive clinical approach to treat this disease. Here we provide the first direct experimental evidence that the treatment of Tg2576 transgenic mice with an inhibitor of β-secretase, GRL-8234, rescues the age-related cognitive decline. We demonstrated that the injected GRL-8234 effectively enters the brain and rapidly decreases soluble Aβ in the brain of Tg2576 mice. The rescue of cognition, which was observed only after long-term inhibitor treatment ranging from 5 to 7.5 mo, was associated with a decrease of brain amyloid-β plaque load. We also found no accumulation of amyloid-β precursor protein after several months of inhibitor treatment. These observations substantiate the idea that Aβ accumulation plays a major role in the cognitive decline of Tg2576 mice and support the concept of Aβ reduction therapy as a treatment of AD.

IGF-1 reduces BACE-1 expression in PC12 cells via activation of PI3-K/Akt and MAPK/ERK1/2 signaling pathways

Insulin-like growth factor 1 (IGF-1) stimulates α-secretase processing of amyloid precursor protein (APP) and decreases Aβ production. Little is known about the relationship between IGF-1 and β-site amyloid precursor protein cleaving enzyme 1 (BACE-1), the protease essential for the production of β-amyloid peptides (Aβ). Here, we investigated the effect of IGF-1 on BACE-1 in PC12 cells. Quantitative polymerase chain reaction analysis and western blot showed that treatment of cells with IGF-1 significantly decreased the levels of BACE-1 mRNA and protein. Furthermore, IGF-1 increased the phosphorylation of Akt and ERK1/2. The presence of the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002 and the mitogen-activated protein kinase kinases (MEK) inhibitor PD98059 blocked the effect of IGF-1 on BACE-1. Our data indicated that IGF-1-induced reduction of BACE-1 might involve the PI3-K/Akt and MAPK/ERK1/2 signaling pathways.

Amyloid-modifying therapies for Alzheimer's disease: therapeutic progress and its implications

Alzheimer's disease (AD) is the most prevalent form of dementia, affecting an estimated 4.8 million people in North America. For the past decade, the amyloid cascade hypothesis has dominated the field of AD research. This theory posits that the deposition of amyloid-beta protein (Abeta) in the brain is the key pathologic event in AD, which induces a series of neuropathological changes that manifest as cognitive decline and eventual dementia. Based on this theory, interventions that reduce Abeta burden in the brain would be expected to alleviate both the neuropathological changes and dementia, which characterize AD. Several diverse pharmacological strategies have been developed to accomplish this. These include inhibiting the formation of Abeta, preventing the aggregation of Abeta into insoluble aggregates, preventing the entry of Abeta into the brain from the periphery and enhancing the clearance of Abeta from the central nervous system. To date, no amyloid-modifying therapy has yet been successful in phase 3 clinical trials; however, several trials are currently underway. This article provides a review of the status of amyloid-modifying therapies and the implications for the amyloid cascade hypothesis.

BACE1 gene promoter single-nucleotide polymorphisms in Alzheimer's disease

Alzheimer's disease (AD) is the most neurodegenerative disorder leading to dementia. Neuritic plaque formation in brains is a hallmark of AD pathogenesis. Amyloid beta protein (Abeta) is the central component of neuritic plaques. Processing beta-amyloid precursor protein (APP) at the beta-secretase site by the beta-site APP cleaving enzyme 1 (BACE1) is essential for generation of Abeta. Elevation of BACE1 activity and expression has been reported in AD brains. However, no mutation in the BACE1 coding sequence has been identified in AD cases. Human BACE1 expression is tightly regulated at the transcription and translation level. To determine whether there is any single-nucleotide polymorphisms in the BACE1 gene promoter region affecting BACE1 expression in AD pathogenesis, in this study, we screened 2.6 kb of the human BACE1 gene promoter region from late-onset AD patients and found that there was no significant association between single-nucleotide polymorphisms and AD cases.

Design of an orally efficacious hydroxyethylamine (HEA) BACE-1 inhibitor in a preclinical animal model

In this Letter, we describe our efforts to design HEA BACE-1 inhibitors that are highly permeable coupled with negligible levels of permeability-glycoprotein activity. These efforts culminate in producing 16 which lowers Αβ by 28% and 32% in the cortex and CSF, respectively, in the preclinical wild type Hartley guinea pig animal model when dosed orally at 30mpk BID for 2.5days.

BACE1-/- mice exhibit seizure activity that does not correlate with sodium channel level or axonal localization

Background

BACE1 is a key enzyme in the generation of the Aβ peptide that plays a central role in the pathogenesis of Alzheimer's disease. While BACE1 is an attractive therapeutic target, its normal physiological function remains largely unknown. Examination of BACE1^-/- mice can provide insight into this function and also help anticipate consequences of BACE1 inhibition. Here we report a seizure-susceptibility phenotype that we have identified and characterized in BACE1^-/- mice.

Results

We find that electroencephalographic recordings reveal epileptiform abnormalities in some BACE1^-/- mice, occasionally including generalized tonic-clonic and absence seizures. In addition, we find that kainic acid injection induces seizures of greater severity in BACE1^-/- mice relative to BACE1^+/+ littermates, and causes excitotoxic cell death in a subset of BACE1^-/- mice. This hyperexcitability phenotype is variable and appears to be manifest in approximately 30% of BACE1^-/- mice. Finally, examination of the expression and localization of the voltage-gated sodium channel α-subunit Na_v1.2 reveals no correlation with BACE1 genotype or any measure of seizure susceptibility.

Conclusions

Our data indicate that BACE1 deficiency predisposes mice to spontaneous and pharmacologically-induced seizure activity. This finding has implications for the development of safe therapeutic strategies for reducing Aβ levels in Alzheimer's disease. Further, we demonstrate that altered sodium channel expression and axonal localization are insufficient to account for the observed effect, warranting investigation of alternative mechanisms.

A modified preparation (LMK03) of the oriental medicine Jangwonhwan reduces Abeta(1-42) level in the brain of Tg-APPswe/PS1dE9 mouse model of Alzheimer disease

ETHNOPHARMACOLOGICAL RELEVANCE

The oriental medicine Jangwonhwan, which is a boiled extract of 12 medicinal herbs/mushroom, has been prescribed for patients with cognitive dysfunction. Recently, a modified recipe of Jangwonhwan (LMK02-Jangwonhwan) consisting of seven medicinal plants/mushroom, was shown to have a therapeutic potential to ameliorate AD-like pathology.

AIM OF THE STUDY

It was investigated whether a further reduction of Jangwonhwan (LMK03-Jangwonhwan) retains the potency to suppress the AD-like pathology.

MATERIALS AND METHODS

The transgenic mice of Alzheimer disease, Tg-APPswe/PS1dE9, were fed LMK03-Jangwonhwan consisting of two of the herbs, white Poria cocos (Schw.) Wolf and Angelica gigas Nakai, which could protect the AD-like pathology at 300 mg/kg/day of dose for 3 months. In vitro cell biological study, immunohistological and ELISA (enzyme-linked immunosorbent assay) analyses were used to assess its neuroprotective effects against Abeta-induced cell death, and the Abeta accumulation and plaque deposition in the brain.

RESULTS

In vitro study with SH-SY5Y neuroblastoma cells showed that LMK03-Jangwonhwan could protect from cytotoxicity induced by hydrogen peroxide or oligomeric Abeta(1-42). Tg-APPswe/PS1dE9 mice were administered LMK03-Jangwonhwan at 300 mg/kg/day for 3 months from 4.5 months of age. Immunohistological and ELISA analyses showed that LMK03-Jangwonhwan partially reduced Abeta(1-42)and Abeta(1-40) levels and beta-amyloid plaque deposition in the brain of Tg-APPswe/PS1dE9 mice. However, LMK03-Jangwonhwan poorly suppressed accumulation of reactive oxidative stress in the hippocampus of Tg-APPswe/PS1dE9 mice and inefficiently improved the expression of phospho-CREB and calbindin, the cellular factors that were down-regulated in AD-like brains.

CONCLUSIONS

These results suggest that LMK03-Jangwonhwan has a potency to inhibit AD-like pathology at a detectable level, but LMK03 is not likely to retain the major ability of LMK02-Jangwonhwan to modify AD pathology in several AD-related molecular parameters.

Current therapies and new strategies for the management of Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder that currently affects millions of Americans. There is no cure at present and no real long-term hope for patients with AD. While partially effective in improving symptoms, currently available treatments approved by the US Food and Drug Administration (FDA) do not halt progression of AD, or address the underlying mechanism of the disease, in part because the etiology of AD is still an active area of investigation. Identification of risk factors and the pathogenic mechanism of AD hold the promise of bringing forth novel treatments and perhaps even a cure. In this review, we will summarize some of the risk factors for AD, AD diagnosis, and current treatments. Novel therapeutic strategies such as inhibition of beta-amyloid peptide (Abeta), tau-mediated pathogenesis, and receptors for advanced glycation end products (RAGE), as well as neuroprotective and anti-inflammatory approaches and the impact of cholesterol-lowering, botanical, and nutritional agents are also reviewed.

Pharmacogenetic features of cathepsin B inhibitors that improve memory deficit and reduce beta-amyloid related to Alzheimer's disease

Beta-amyloid (Abeta) in the brain is a major factor involved in Alzheimer's disease (AD) that results in severe memory deficit. Our recent studies demonstrate pharmacogenetic differences in the effects of inhibitors of cathepsin B to improve memory and reduce Abeta in different mouse models of AD. The inhibitors improve memory and reduce brain Abeta in mice expressing the wild-type (WT) beta-secretase site of human APP, expressed in most AD patients. However, these inhibitors have no effect in mice expressing the rare Swedish (Swe) mutant amyloid precursor protein (APP). Knockout of the cathepsin B decreased brain Abeta in mice expressing WT APP, validating cathepsin B as the target. The specificity of cathepsin B to cleave the WT beta-secretase site, but not the Swe mutant site, of APP for Abeta production explains the distinct inhibitor responses in the different AD mouse models. In contrast to cathepsin B, the BACE1 beta-secretase prefers to cleave the Swe mutant site. Discussion of BACE1 data in the field indicate that they do not preclude cathepsin B as also being a beta-secretase. Cathepsin B and BACE1 could participate jointly as beta-secretases. Significantly, the majority of AD patients express WT APP and, therefore, inhibitors of cathepsin B represent candidate drugs for AD.

Is BACE1 a suitable therapeutic target for the treatment of Alzheimer's disease? Current strategies and future directions

Alzheimer's disease (AD) is characterized by the extracellular deposition of the β-amyloid protein (Aβ). Aβ is a fragment of a much larger precursor protein, the amyloid precursor protein (APP). Sequential proteolytic cleavage of APP by β-secretase and γ-secretase liberates Aβ from APP. The aspartyl protease BACE1 (β-site APP-cleaving enzyme 1) catalyses the rate-limiting step in the production of Aβ, and as such it is considered to be a major target for drug development in Alzheimer's disease. However, the development of a BACE1 inhibitor therapy is problematic for two reasons. First, BACE1 has been found to have important physiological roles. Therefore, inhibition of the enzyme could have toxic consequences. Second, the active site of BACE1 is relatively large, and many of the bulky compounds that are needed to inhibit BACE1 activity are unlikely to cross the blood-brain barrier. This review focuses on the structure BACE1, current therapeutic strategies based on developing active-site inhibitors, and new approaches to therapy involving targeting the expression or post-translational regulation of BACE1.

BACE1 deficiency causes altered neuronal activity and neurodegeneration

BACE1 is required for the release of beta-amyloid (Abeta) in vivo, and inhibition of BACE1 activity is targeted for reducing Abeta generation in Alzheimer's patients. To further our understanding of the safe use of BACE1 inhibitors in human patients, we aimed to study the physiological functions of BACE1 by characterizing BACE1-null mice. Here, we report the finding of spontaneous behavioral seizures in BACE1-null mice. Electroencephalographic recordings revealed abnormal spike-wave discharges in BACE1-null mice, and kainic acid-induced seizures also occurred more frequently in BACE1-null mice compared with their wild-type littermates. Biochemical and morphological studies showed that axonal and surface levels of Na(v)1.2 were significantly elevated in BACE1-null mice, consistent with the increased fast sodium channel current recorded from BACE1-null hippocampal neurons. Patch-clamp recording also showed altered intrinsic firing properties of isolated BACE1-null hippocampal neurons. Furthermore, population spikes were significantly increased in BACE1-null brain slices, indicating hyperexcitability of BACE1-null neurons. Together, our results suggest that increased sodium channel activity contributes to the epileptic behaviors observed in BACE1-null mice. The knowledge from this study is crucial for the development of BACE1 inhibitors for Alzheimer's therapy and to the applicative study of epilepsy.