BACE knockout mice are healthy despite lacking the primary beta-secretase activity in brain: implications for Alzheimer's disease therapeutics

Development and Structural Modification of BACE1 Inhibitors

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which usually occurs in the elderly. The accumulation of β-amyloid and the formation of neurofibrillary tangles are considered as the main pathogenies of AD. Research suggests that β-secretase 1 (BACE1) plays an important role in the formation of β-amyloid. Discovery of new BACE1 inhibitors has become a significant method to slow down the progression of AD or even cure this kind of disease. This review summarizes the different types and the structural modification of these new BACE1 inhibitors.

Concurrent Treatment with Taxifolin and Cilostazol on the Lowering of β-Amyloid Accumulation and Neurotoxicity via the Suppression of P-JAK2/P-STAT3/NF-κB/BACE1 Signaling Pathways

Taxifolin is a potent flavonoid that exerts anti-oxidative effect, and cilostazol increases intracellular cAMP levels by inhibiting phosphodiesterase 3 that shows antiinflammatory actions. BACE1 (β-site APP cleaving enzyme 1) is the rate-limiting enzyme responsible for the β-cleavage of amyloid precursor proteins to Aβ peptides. In this study, endogenous Aβ and C99 accumulation was explored in N2a Swe cells exposed to 1% FBS medium. Increased Aβ and C99 levels were significantly attenuated by taxifolin alone and in combination with cilostazol. Increased phosphorylated JAK2 at Tyr1007/1008 (P-JAK), phosphorylated STAT3 at Tyr 705 (P-STAT3) expressions and increased expressions of BACE1 mRNA and protein in the activated N2a Swe cells were significantly attenuated by taxifolin (10~50 μM), cilostazol (10~50 μM) alone and in combination at minimum concentrations. In these cells, decreased cytosol IκBα expression was elevated, and increased nuclear NF-κB p65 level and nuclear NF-κB p65 DNA binding activity were significantly reduced by taxifolin and cilostazol in a similar manner. Following STAT3 gene (~70% reduction) knockdown in N2a cells, Aβ-induced nuclear NF-κB and BACE1 expressions were not observed. Taxifolin, cilostazol, or resveratrol significantly stimulated SIRT1 protein expression. In SIRT1 gene-silenced (~50%) N2a cells, taxifolin, cilostazol, and resveratrol all failed to suppress Aβ_1-42-stimulated P-STAT3 and BACE1 expression. Consequently, taxifolin and cilostazol were found to significantly decrease lipopolysaccharide (1–10 μg/ml)-induced iNOS and COX-2 expressions, and nitrite production in cultured BV-2 microglia cells and to increase N2a cell viability. In conclusion, taxifolin and cilostazol strongly inhibited amyloidogenesis in a synergistic manner by suppressing P-JAK2/P-STAT3-coupled NF-κB-linked BACE1 expression via the up-regulation of SIRT1.

Discovery of the 3-Imino-1,2,4-thiadiazinane 1,1-Dioxide Derivative Verubecestat (MK-8931)-A β-Site Amyloid Precursor Protein Cleaving Enzyme 1 Inhibitor for the Treatment of Alzheimer's Disease

Verubecestat 3 (MK-8931), a diaryl amide-substituted 3-imino-1,2,4-thiadiazinane 1,1-dioxide derivative, is a high-affinity β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor currently undergoing Phase 3 clinical evaluation for the treatment of mild to moderate and prodromal Alzheimer's disease. Although not selective over the closely related aspartyl protease BACE2, verubecestat has high selectivity for BACE1 over other key aspartyl proteases, notably cathepsin D, and profoundly lowers CSF and brain Aβ levels in rats and nonhuman primates and CSF Aβ levels in humans. In this annotation, we describe the discovery of 3, including design, validation, and selected SAR around the novel iminothiadiazinane dioxide core as well as aspects of its preclinical and Phase 1 clinical characterization.

New directions for protease inhibitors directed drug discovery

Proteases play crucial roles in various biological processes, and their activities are essential for all living organisms-from viruses to humans. Since their functions are closely associated with many pathogenic mechanisms, their inhibitors or activators are important molecular targets for developing treatments for various diseases. Here, we describe drugs/drug candidates that target proteases, such as malarial plasmepsins, β-secretase, virus proteases, and dipeptidyl peptidase-4. Previously, we reported inhibitors of aspartic proteases, such as renin, human immunodeficiency virus type 1 protease, human T-lymphotropic virus type I protease, plasmepsins, and β-secretase, as drug candidates for hypertension, adult T-cell leukaemia, human T-lymphotropic virus type I-associated myelopathy, malaria, and Alzheimer's disease. Our inhibitors are also described in this review article as examples of drugs that target proteases. © 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 563-579, 2016.

Chemoproteomic profiling reveals that cathepsin D off-target activity drives ocular toxicity of β-secretase inhibitors

Inhibition of β-secretase BACE1 is considered one of the most promising approaches for treating Alzheimer's disease. Several structurally distinct BACE1 inhibitors have been withdrawn from development after inducing ocular toxicity in animal models, but the target mediating this toxicity has not been identified. Here we use a clickable photoaffinity probe to identify cathepsin D (CatD) as a principal off-target of BACE1 inhibitors in human cells. We find that several BACE1 inhibitors blocked CatD activity in cells with much greater potency than that displayed in cell-free assays with purified protein. Through a series of exploratory toxicology studies, we show that quantifying CatD target engagement in cells with the probe is predictive of ocular toxicity in vivo. Taken together, our findings designate off-target inhibition of CatD as a principal driver of ocular toxicity for BACE1 inhibitors and more generally underscore the power of chemical proteomics for discerning mechanisms of drug action.

Several β-secretase (BACE) inhibitors exhibit unexplained ocular toxicity in preclinical studies. Here the authors generate a clickable photoaffinity probe to interrogate off-targets in cells and animals, and identify inhibition of cathepsin D as a driver of ocular toxicity.

Stepping closer to treating Alzheimer's disease patients with BACE1 inhibitor drugs

Alzheimer's disease (AD) is the most common age-dependent neurodegenerative disease which impairs cognitive function and gradually causes patients to be unable to lead normal daily lives. While the etiology of AD remains an enigma, excessive accumulation of β-amyloid peptide (Aβ) is widely believed to induce pathological changes and cause dementia in brains of AD patients. BACE1 was discovered to initiate the cleavage of amyloid precursor protein (APP) at the β-secretase site. Only after this cleavage does γ-secretase further cleave the BACE1-cleaved C-terminal APP fragment to release Aβ. Hence, blocking BACE1 proteolytic activity will suppress Aβ generation. Due to the linkage of Aβ to the potential cause of AD, extensive discovery and development efforts have been directed towards potent BACE1 inhibitors for AD therapy. With the recent breakthrough in developing brain-penetrable BACE1 inhibitors, targeting amyloid deposition-mediated pathology for AD therapy has now become more practical. This review will summarize various strategies that have successfully led to the discovery of BACE1 drugs, such as MK8931, AZD-3293, JNJ-54861911, E2609 and CNP520. These drugs are currently in clinical trials and their updated states will be discussed. With the promise of reducing Aβ generation and deposition with no alarming safety concerns, the amyloid cascade hypothesis in AD therapy may finally become validated.

BACE-1 is expressed in the blood-brain barrier endothelium and is upregulated in a murine model of Alzheimer's disease

Endothelial cells of the blood-brain barrier form a structural and functional barrier maintaining brain homeostasis via paracellular tight junctions and specific transporters such as P-glycoprotein. The blood-brain barrier is responsible for negligible bioavailability of many neuroprotective drugs. In Alzheimer's disease, current treatment approaches include inhibitors of BACE-1 (β-site of amyloid precursor protein cleaving enzyme), a proteinase generating neurotoxic β-amyloid. It is known that BACE-1 is highly expressed in endosomes and membranes of neurons and glia. We now provide evidence that BACE-1 is expressed in blood-brain barrier endothelial cells of human, mouse, and bovine origin. We further show its predominant membrane localization by 3D-dSTORM super-resolution microscopy, and by biochemical fractionation that further shows an abluminal distribution of BACE-1 in brain microvessels. We confirm its functionality in processing APP in primary mouse brain endothelial cells. In an Alzheimer's disease mouse model we show that BACE-1 is upregulated at the blood-brain barrier compared to healthy controls. We therefore suggest a critical role for BACE-1 at the blood-brain barrier in β-amyloid generation and in vascular aspects of Alzheimer's disease, particularly in the development of cerebral amyloid angiopathy.

Lamotrigine Reduces β-Site AβPP-Cleaving Enzyme 1 Protein Levels Through Induction of Autophagy

Lamotrigine (LTG), a broad-spectrum anti-epileptic drug widely used in treatment for seizures, shows potential efficacy in Alzheimer's disease (AD) therapy. Chronic LTG treatment rescues the suppressed long-term potentiation, loss of spines and cognitive deficits in AβPP/PS1 mice, known to overexpress a chimeric mouse/human mutant amyloid-β protein precursor (AβPP) and a mutant human presenilin 1 (PS1). These changes are accompanied by reduction of amyloid-β (Aβ) plaques density and of levels of β-C-terminal fragment of AβPP (β-CTF), a fragment of AβPP cleaved by β-secretase. These results suggest LTG treatment reduces Aβ production, possibly through modulation of cleavage of AβPP by β-secretase. However, the underlying mechanisms still remain unclear. In this study, decreased protein levels, but not mRNA levels of β-site AβPP-cleaving enzyme 1 (BACE1), were observed in cultured HEK293 cells and the brains of AβPP/PS1 transgenic mice upon LTG treatment. Moreover, LTG treatment suppressed mammalian target of rapamycin (mTOR) signaling, while enhancing activation of cAMP response element binding protein (CREB), two signaling pathways essential for autophagy induction. LTG treatment increased the numbers of LC3-GFP + puncta and LC3-II levels in HEK293 cells, indicating an induction of autophagy. The downregulation of BACE1 by LTG treatment was prevented by the autophagy inhibitor 3-Methyladenine. Therefore, this study shows that LTG treatment reduces the protein levels of BACE1 through activation of autophagy, possibly via inhibition of mTOR signaling and activation of CREB.

Current pharmacotherapy and putative disease-modifying therapy for Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease of the central nervous system correlated with the progressive loss of cognition and memory. β-Amyloid plaques, neurofibrillary tangles and the deficiency in cholinergic neurotransmission constitute the major hallmarks of the AD. Two major hypotheses have been implicated in the pathogenesis of AD namely the cholinergic hypothesis which ascribed the clinical features of dementia to the deficit cholinergic neurotransmission and the amyloid cascade hypothesis which emphasized on the deposition of insoluble peptides formed due to the faulty cleavage of the amyloid precursor protein. Current pharmacotherapy includes mainly the acetylcholinesterase inhibitors and N-methyl-D-aspartate receptor agonist which offer symptomatic therapy and does not address the underlying cause of the disease. The disease-modifying therapy has garnered a lot of research interest for the development of effective pharmacotherapy for AD. β and γ-Secretase constitute attractive targets that are focussed in the disease-modifying approach. Potentiation of α-secretase also seems to be a promising approach towards the development of an effective anti-Alzheimer therapy. Additionally, the ameliorative agents that prevent aggregation of amyloid peptide and also the ones that modulate inflammation and oxidative damage associated with the disease are focussed upon. Development in the area of the vaccines is in progress to combat the characteristic hallmarks of the disease. Use of cholesterol-lowering agents also is a fruitful strategy for the alleviation of the disease as a close association between the cholesterol and AD has been cited. The present review underlines the major therapeutic strategies for AD with focus on the new developments that are on their way to amend the current therapeutic scenario of the disease.

Inhibiting BACE1 to reverse synaptic dysfunctions in Alzheimer's disease

Over the past two decades, many studies have identified significant contributions of toxic β-amyloid peptides (Aβ) to the etiology of Alzheimer's disease (AD), which is the most common age-dependent neurodegenerative disease. AD is also recognized as a disease of synaptic failure. Aβ, generated by sequential proteolytic cleavages of amyloid precursor protein (APP) by BACE1 and γ-secretase, is one of major culprits that cause this failure. In this review, we summarize current findings on how BACE1-cleaved APP products impact learning and memory through proteins localized on glutamatergic, GABAergic, and dopaminergic synapses. Considering the broad effects of Aβ on all three types of synapses, BACE1 inhibition emerges as a practical approach for ameliorating Aβ-mediated synaptic dysfunctions. Since BACE1 inhibitory drugs are currently in clinical trials, this review also discusses potential complications arising from BACE1 inhibition. We emphasize that the benefits of BACE1 inhibitory drugs will outweigh the concerns.

Alzheimer's therapy targeting the β-secretase enzyme BACE1: Benefits and potential limitations from the perspective of animal model studies

Accumulating evidence points to the amyloid-β (Aβ) peptide as the culprit in the pathogenesis of Alzheimer's disease (AD). β-Site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is a protease that is responsible for initiating Aβ production. Although precise mechanisms that trigger Aβ accumulation remain unclear, BACE1 inhibition undoubtedly represents an important intervention that may prevent and/or cure AD. Remarkably, animal model studies with knockouts, virus-delivered small interfering RNAs, immunization and bioavailable small-molecule agents that specifically inhibit BACE1 activity strongly support the idea for the therapeutic BACE1 inhibition. Meanwhile, a growing number of BACE1 substrates besides APP uncover new physiological roles of this protease, raising some concern regarding the safety of BACE1 inhibition. Here, I review recent progress in preclinical studies that have evaluated the efficacies and potential limitations of genetic/pharmacological inhibition of BACE1, with special focus on AD-associated phenotypes including synaptic dysfunction, neuron loss and memory deficits in animal models.

Post-translational regulation of the β-secretase BACE1

β-Secretase, widely known as β-site APP cleaving enzyme 1 (BACE1), is a membrane-associated protease that cleaves amyloid precursor protein (APP) to generate amyloid β-protein (Aβ). As this cleavage is a pathologically relevant event in Alzheimer's disease, BACE1 is considered a viable therapeutic target. BACE1 can be regulated at the transcriptional, post-transcriptional, translational, and post-translational levels. Elucidation of the regulatory pathways of BACE1 is critical, not only for understanding the pathological mechanisms of AD but also developing effective therapeutic strategies to inhibit activity of the protease. This mini-review focuses on the post-translational regulation of BACE1, as modulation at this level is closely associated with both physiological and pathological conditions. Current knowledge on the mechanisms underlying such BACE1 regulation and their implications for therapy are discussed.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

Amyloid Proteins and Their Role in Multiple Sclerosis. Considerations in the Use of Amyloid-PET Imaging

Thioflavin T derivatives are used in positron-emission tomography (PET) studies to detect amyloid protein deposits in patients with Alzheimer disease. These tracers bind extensively to white matter, which suggests that they may be useful in studies of multiple sclerosis (MS), and that proteins resulting from proteolytic processing of the amyloid precursor protein (APP) may contribute to MS. This article reviews data from both clinical and preclinical studies addressing the role of these proteins, whether they are detected in CSF studies or using PET imaging. APP is widely expressed in demyelinated axons and may have a protective effect in MS and in experimental allergic encephalomyelitis in animals. Several mechanisms associated with this increased expression may affect the degree of remyelination in MS. Amyloid-PET imaging may help determine the degree of demyelination and provide information on the molecular changes linked to APP proteolytic processing experienced by patients with MS.

Acceleration of amyloidogenesis and memory impairment by estrogen deficiency through NF-κB dependent beta-secretase activation in presenilin 2 mutant mice

Nearly 7-10 million people are living with Alzheimer's disease (AD) worldwide. Senile plaques composed of β-amyloid (Aβ) are a pathological hallmark of Alzheimer's disease. Presenilin 2 (PS2) mutations increase Aβ generation in the brains of AD patients. The Aβ is generated through the sequential cleavage of amyloid precursor protein by β- and γ-secretases. Additionally, increasing evidences suggest that estrogen can reduce the development of AD via regulation of β-secretases activity and beta-site APP-cleaving enzyme (BACE1) expression. But the underlying correlation mechanism of Aβ generation by PS2 mutations and estrogen remains to be clarified. To investigate the anti-amyloidogenesis effect of estrogen in a PS2 mutative condition, we examined memory impairment in ovariectomized PS2 mutation (N141I) mice in which cognitive function was assessed by the Morris water maze test and passive avoidance test. In addition, Western blot analysis, immunostaining, immunofluorescence staining, ELISA and enzyme activity assays were used to examine the degree of Aβ deposition in the brains. In the present study, Aβ accumulated more in the ovariectomized PS2 mutant mice brain, and greatly worsened memory impairment and glial activation as well as neurogenic inflammation. In parallel with increased memory impairment, activity of β-secretase and expression of the BACE1 increased inovariectomized PS2 mutant mice. Much higher activity of NF-κB was observed by EMSA in ovariectomized PS2 mutant mice. In addition, the Aβ level was decreased by treatment of β-estradiol through inhibiting BACE1 expression in PS2 transfacted PC12 cells. These results suggest that mutation of PS2 can lead to NF-κB mediate amyloidogensis, and this effect can be amplified by the absence of estrogen.

Generation of aggregation prone N-terminally truncated amyloid β peptides by meprin β depends on the sequence specificity at the cleavage site

BACKGROUND

The metalloprotease meprin β cleaves the Alzheimer's Disease (AD) relevant amyloid precursor protein (APP) as a β-secretase reminiscent of BACE-1, however, predominantly generating N-terminally truncated Aβ2-x variants.

RESULTS

Herein, we observed increased endogenous sAPPα levels in the brains of meprin β knock-out (ko) mice compared to wild-type controls. We further analyzed the cellular interaction of APP and meprin β and found that cleavage of APP by meprin β occurs prior to endocytosis. The N-terminally truncated Aβ2-40 variant shows increased aggregation propensity compared to Aβ1-40 and acts even as a seed for Aβ1-40 aggregation. Additionally, we observed that different APP mutants affect the catalytic properties of meprin β and that, interestingly, meprin β is unable to generate N-terminally truncated Aβ peptides from Swedish mutant APP (APPswe).

CONCLUSION

Concluding, we propose that meprin β may be involved in the generation of N-terminally truncated Aβ2-x peptides of APP, but acts independently from BACE-1.

Meta-analysis of BACE1 gene rs638405 polymorphism and the risk of Alzheimer's disease in Caucasion and Asian population

Recent studies showed the β-site amyloid precursor protein cleaving enzyme (BACE) is associated with Alzheimer's disease (AD). However, studies investigating the association of single-nucleotide polymorphism (SNP) in exon 5 of BACE1 (rs638405, C786G, Val262) with AD are controversial. Therefore we conducted this meta-analysis to clarify the association. Relevant studies were identified on PubMed, Cochrane library and CNKI from established through July 2015 according to the inclusion criteria. Odds ratios (ORs) with 95% confidence intervals (CIs) and five genetic models were applied to assess the association. A total of 13 studies composed of 2538 AD patients and 3020 controls were included in this study. Significant association of SNP rs638405 with AD was found in overall population among allelic genetic model (G vs. C: OR=1.11, 95%CI=1.02-1.20, P=0.01), codominant genetic model (GG vs. CC: OR=1.22, 95%CI=1.04-1.44, P=0.02) and recessive genetic model (GG vs. GC+ CC: OR=1.25, 95%CI=1.10-1.42, P=0.0008). Besides, subgroup analysis indicated significant association among Asian population (allelic genetic model, G vs. C, OR=1.18, 95%CI=1.04-1.34, P=0.01; codominant genetic model, GG vs. CC, OR=1.43, 95%CI=1.08-1.89, P=0.01 and recessive genetic model, GG vs. GC+ CC, OR=1.40, 95%CI=1.09-1.78, P=0.008) and Caucasion population (recessive genetic model, GG vs. GC+ CC, OR=1.20, 95%CI=1.02-1.39, P=0.02). Our analysis demonstrated that GG genotype and G allele of BACE1 gene rs638405 probably increase the risk of AD.

Neurological dysfunctions associated with altered BACE1-dependent Neuregulin-1 signaling

Inhibition of BACE1 is being pursued as a therapeutic target to treat patients suffering from Alzheimer's disease because BACE1 is the sole β-secretase that generates β-amyloid peptide. Knowledge regarding other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. Neuregulin-1 (Nrg1) is a BACE1 substrate and BACE1 cleavage of Nrg1 is critical for signaling functions in myelination, remyelination, synaptic plasticity, normal psychiatric behaviors, and maintenance of muscle spindles. This review summarizes the most recent discoveries associated with BACE1-dependent Nrg1 signaling in these areas. This body of knowledge will help to provide guidance for preventing unwanted Nrg1-based side effects following BACE1 inhibition in humans. To initiate its signaling cascade, membrane anchored Neuregulin (Nrg), mainly type I and III β1 Nrg1 isoforms and Nrg3, requires ectodomain shedding. BACE1 is one of such indispensable sheddases to release the functional Nrg signaling fragment. The dependence of Nrg on the cleavage by BACE1 is best manifested by disrupting the critical role of Nrg in the control of axonal myelination, schizophrenic behaviors as well as the formation and maintenance of muscle spindles.

The role and therapeutic targeting of α-, β- and γ-secretase in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly and its prevalence is set to increase rapidly in coming decades. However, there are as yet no available drugs that can halt or even stabilize disease progression. One of the main pathological features of AD is the presence in the brain of senile plaques mainly composed of aggregated β amyloid (Aβ), a derivative of the longer amyloid precursor protein (APP). The amyloid hypothesis proposes that the accumulation of Aβ within neural tissue is the initial event that triggers the disease. Here we review research efforts that have attempted to inhibit the generation of the Aβ peptide through modulation of the activity of the proteolytic secretases that act on APP and discuss whether this is a viable therapeutic strategy for treating AD.

Encompassing receptor flexibility in virtual screening using ensemble docking-based hybrid QSAR: discovery of novel phytochemicals for BACE1 inhibition

Mimicking receptor flexibility during receptor-ligand binding is a challenging task in computational drug design since it is associated with a large increase in the conformational search space. In the present study, we have devised an in silico design strategy incorporating receptor flexibility in virtual screening to identify potential lead compounds as inhibitors for flexible proteins. We have considered BACE1 (β-secretase), a key target protease from a therapeutic perspective for Alzheimer's disease, as the highly flexible receptor. The protein undergoes significant conformational transitions from open to closed form upon ligand binding, which makes it a difficult target for inhibitor design. We have designed a hybrid structure-activity model containing both ligand based descriptors and energetic descriptors obtained from molecular docking based on a dataset of structurally diverse BACE1 inhibitors. An ensemble of receptor conformations have been used in the docking study, further improving the prediction ability of the model. The designed model that shows significant prediction ability judged by several statistical parameters has been used to screen an in house developed 3-D structural library of 731 phytochemicals. 24 highly potent, novel BACE1 inhibitors with predicted activity (Ki) ≤ 50 nM have been identified. Detailed analysis reveals pharmacophoric features of these novel inhibitors required to inhibit BACE1.

BACE1 (β-secretase) inhibitors for the treatment of Alzheimer's disease

BACE1 (β-secretase, memapsin 2, Asp2) has emerged as a promising target for the treatment of Alzheimer's disease. BACE1 is an aspartic protease which functions in the first step of the pathway leading to the production and deposition of amyloid-β peptide (Aβ). Its gene deletion showed only mild phenotypes. BACE1 inhibition has direct implications in the Alzheimer's disease pathology without largely affecting viability. However, inhibiting BACE1 selectively in vivo has presented many challenges to medicinal chemists. Since its identification in 2000, inhibitors covering many different structural classes have been designed and developed. These inhibitors can be largely classified as either peptidomimetic or non-peptidic inhibitors. Progress in these fields resulted in inhibitors that contain many targeted drug-like characteristics. In this review, we describe structure-based design strategies and evolution of a wide range of BACE1 inhibitors including compounds that have been shown to reduce brain Aβ, rescue the cognitive decline in transgenic AD mice and inhibitor drug candidates that are currently in clinical trials.

Design and Synthesis of Novel Arylketo-containing P1-P3 Linked Macro-cyclic BACE-1 Inhibitors

A series of arylketo-containing P1-P3 linked macrocyclic BACE-1 inhibitors were designed, synthesized, and compared with compounds with a previously known and extensively studied corresponding P2 isophthalamide moiety with the aim to improve on permeability whilst retaining the enzyme- and cell-based activities. Several inhibitors displayed substantial increases in Caco-2 cell-based permeability compared to earlier synthesized inhibitors and notably also with retained activities, showing that this approach might yield BACE-1 inhibitors with improved properties.

Axonal and Schwann cell BACE1 is equally required for remyelination of peripheral nerves

Inhibition of β-site APP cleaving enzyme 1 (BACE1) is being pursued as a therapeutic target for treating patients with Alzheimer's disease because BACE1 is the sole β-secretase for generating β-amyloid peptide. Knowledge regarding the other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. BACE1 deficiency in mice causes hypomyelination during development and impairs remyelination in injured sciatic nerves. Since BACE1 is expected to be ubiquitously expressed, we asked whether axonal or Schwann cell BACE1 is required for optimal remyelination. By swapping sciatic nerve segments from BACE1-null mice with the corresponding wild-type nerve segments or vice versa, we tested how a deficiency of BACE1 in Schwann cells or axons affects remyelination. Our results show that BACE1 in axons and Schwann cells is similarly important for remyelination of regenerated axons. Nerve injury induces BACE1 transcription and protein levels are elevated in Schwann cells. Expression of type I neuregulin 1 (Nrg1), rather than type III Nrg1, was induced by Schwann cells, and the abolished Nrg1 cleavage in BACE1-null Schwann cells contributed to decreased remyelination of regenerated axons. Hence, this study is the first to demonstrate the equal importance of axonal and Schwann cell BACE1 for remyelination of injured nerves.

The potent BACE1 inhibitor LY2886721 elicits robust central Aβ pharmacodynamic responses in mice, dogs, and humans

BACE1 is a key protease controlling the formation of amyloid β, a peptide hypothesized to play a significant role in the pathogenesis of Alzheimer's disease (AD). Therefore, the development of potent and selective inhibitors of BACE1 has been a focus of many drug discovery efforts in academia and industry. Herein, we report the nonclinical and early clinical development of LY2886721, a BACE1 active site inhibitor that reached phase 2 clinical trials in AD. LY2886721 has high selectivity against key off-target proteases, which efficiently translates in vitro activity into robust in vivo amyloid β lowering in nonclinical animal models. Similar potent and persistent amyloid β lowering was observed in plasma and lumbar CSF when single and multiple doses of LY2886721 were administered to healthy human subjects. Collectively, these data add support for BACE1 inhibition as an effective means of amyloid lowering and as an attractive target for potential disease modification therapy in AD.

β-site amyloid precursor protein cleaving enzyme 1(BACE1) regulates Notch signaling by controlling the cleavage of Jagged 1 (Jag1) and Jagged 2 (Jag2) proteins

BACE1 is a type I transmembrane aspartyl protease that cleaves amyloid precursor protein at the β-secretase site to initiate the release of β-amyloid peptide. As a secretase, BACE1 also cleaves additional membrane-bound molecules by exerting various cellular functions. In this study, we showed that BACE1 can effectively shed the membrane-anchored signaling molecule Jagged 1 (Jag1).Wealso mapped the cleavage sites of Jag1 by ADAM10 and ADAM17. Although Jag1 shares a high degree of homology with Jag2 in the ectodomain region, BACE1 fails to cleave Jag2 effectively, indicating a selective cleavage of Jag1. Abolished cleavage of Jag1 in BACE1-null mice leads to enhanced astrogenesis and, concomitantly, reduced neurogenesis. This characterization provides biochemical evidence that the Jag1-Notch pathway is under the control of BACE1 activity

Paradigm shift redefining molecular, metabolic and structural events in Alzheimer's disease involves a proposed contribution by transition metals. Defined lengthy preclinical stage provides new hope to circumvent advancement of disease- and age-related neurodegeneration

It is estimated that 5.5 Million North Americans suffer from varying degrees of Alzheimer's disease (AD) and by the year 2050 it may be one in 85 people globally (100 Million). It will be shown that heavy metal toxicity plays a significant role in sporadic AD. Although current literature speaks to involvement of metal ions (via Fenton reaction), studies and reviewers have yet to link cellular events including known structural changes such as amyloid plaque development to this metal toxicity the way it is proposed here. Contrary to the current AD model which positions BACE1 (β-secretase) as an aberrant or AD-advancing enzyme, it is proposed herein that the neuron's protective counteraction to this metal toxicity is, in fact, a justified increase in BACE1 activity and amyloid precursor protein (APP) processing to yield more secreted APP (sAPP) and β-amyloid peptide in response to metal toxicity. This new perspective which justifies a functional role for APP, BACE1 enzyme activity and the peptide products from this activity may at first appear to be counterintuitive. Compelling evidence, however, is presented and a mechanism is shown herein that validate BACE1 recruitment and the resulting β-amyloid protein as strategic countermeasures serving the cell effectively against neuro-impeding disease. It is proposed that β-amyloid peptide chelates and sequesters free heavy metals in the extracellular medium to aggregate as amyloid plaque while unchelated β-amyloid migrates across the cell membrane to chelate intracellular free divalent metals. The sequestered intracellular metal is subsequently chaperoned as a metallo-peptide to cross the plasma membrane and aggregate as amyloid plaques extracellularly. The BACE1 countermeasure is not genetic or metabolic aberration; and this novel conclusion demonstrates that it must not be inhibited as currently targeted. APP, BACE1, β-amyloid peptide, and sAPP play positive roles against the preclinical oxidative load that predates AD symptoms for as long as 20 years. A healthy neuron may tolerate free metal toxicity, such as iron in the case of injury-induced amyloid, for as long as twenty years due to this very BACE1 activity. In later stages, the uncontrolled metals and ROS are compounded by other factors which together overcome this BACE1/β-amyloid protein countermeasure. This results in a sudden increase in IL-1 leading to Tau's hyperphosphorylation as cited and eventually to Tau dissociation from the microtubule cytoskeleton interrupting cell trafficking. At this later stage of AD the β-amyloid protein which once served as a vehicle to escort toxic metals to the extracellular medium and a trap to form a relatively benign extraneuronal disposal site is no longer translocated due to interruption of trafficking and now accumulates intracellularly facilitating hyper-oxidative ROS levels and contributes to irreversible neuron apoptosis.

The normal and pathologic roles of the Alzheimer's β-secretase, BACE1

As the most common neurodegenerative disease, therapeutic avenues for the treatment and prevention of Alzheimer's Disease are highly sought after. The aspartic protease BACE1 is the initiator enzyme for the formation of Aβ, a major constituent of amyloid plaques that represent one of the hallmark pathological features of this disorder. Thus, targeting BACE1 for disease-modifying AD therapies represents a rationale approach. The collective knowledge acquired from investigations of BACE1 deletion mutants and characterization of BACE1 substrates has downstream significance not only for the discovery of AD drug therapies but also for predicting side effects of BACE1 inhibition. Here we discuss the identification and validation of BACE1 as the β-secretase implicated in AD, in addition to information regarding BACE1 cell biology, localization, substrates and potential physiological functions derived from BACE1 knockout models.

Intraperitoneal injection of the pancreatic peptide amylin potently reduces behavioral impairment and brain amyloid pathology in murine models of Alzheimer's disease

Amylin, a pancreatic peptide, and amyloid-beta peptides (Aβ), a major component of Alzheimer's disease (AD) brain, share similar β-sheet secondary structures, but it is not known whether pancreatic amylin affects amyloid pathogenesis in the AD brain. Using AD mouse models, we investigated the effects of amylin and its clinical analog, pramlintide, on AD pathogenesis. Surprisingly, chronic intraperitoneal (i.p.) injection of AD animals with either amylin or pramlintide reduces the amyloid burden as well as lowers the concentrations of Aβ in the brain. These treatments significantly improve their learning and memory assessed by two behavioral tests, Y maze and Morris water maze. Both amylin and pramlintide treatments increase the concentrations of Aβ1-42 in cerebral spinal fluid (CSF). A single i.p. injection of either peptide also induces a surge of Aβ in the serum, the magnitude of which is proportionate to the amount of Aβ in brain tissue. One intracerebroventricular injection of amylin induces a more significant surge in serum Aβ than one i.p. injection of the peptide. In 330 human plasma samples, a positive association between amylin and Aβ1-42 as well as Aβ1-40 is found only in patients with AD or amnestic mild cognitive impairment. As amylin readily crosses the blood-brain barrier, our study demonstrates that peripheral amylin's action on the central nervous system results in translocation of Aβ from the brain into the CSF and blood that could be an explanation for a positive relationship between amylin and Aβ in blood. As naturally occurring amylin may play a role in regulating Aβ in brain, amylin class peptides may provide a new avenue for both treatment and diagnosis of AD.

Brain pyroglutamate amyloid-β is produced by cathepsin B and is reduced by the cysteine protease inhibitor E64d, representing a potential Alzheimer's disease therapeutic

Pyroglutamate amyloid-β peptides (pGlu-Aβ) are particularly pernicious forms of amyloid-β peptides (Aβ) present in Alzheimer's disease (AD) brains. pGlu-Aβ peptides are N-terminally truncated forms of full-length Aβ peptides (flAβ(1-40/42)) in which the N-terminal glutamate is cyclized to pyroglutamate to generate pGlu-Aβ(3-40/42). β-secretase cleavage of amyloid-β precursor protein (AβPP) produces flAβ(1-40/42), but it is not yet known whether the β-secretase BACE1 or the alternative β-secretase cathepsin B (CatB) participate in the production of pGlu-Aβ. Therefore, this study examined the effects of gene knockout of these proteases on brain pGlu-Aβ levels in transgenic AβPPLon mice, which express AβPP isoform 695 and have the wild-type (wt) β-secretase activity found in most AD patients. Knockout or overexpression of the CatB gene reduced or increased, respectively, pGlu-Aβ(3-40/42), flAβ(1-40/42), and pGlu-Aβ plaque load, but knockout of the BACE1 gene had no effect on those parameters in the transgenic mice. Treatment of AβPPLon mice with E64d, a cysteine protease inhibitor of CatB, also reduced brain pGlu-Aβ(3-42), flAβ(1-40/42), and pGlu-Aβ plaque load. Treatment of neuronal-like chromaffin cells with CA074Me, an inhibitor of CatB, resulted in reduced levels of pGlu-Aβ(3-40) released from the activity-dependent, regulated secretory pathway. Moreover, CatB knockout and E64d treatment has been previously shown to improve memory deficits in the AβPPLon mice. These data illustrate the role of CatB in producing pGlu-Aβ and flAβ that participate as key factors in the development of AD. The advantages of CatB inhibitors, especially E64d and its derivatives, as alternatives to BACE1 inhibitors in treating AD patients are discussed.

Aβ reduction in BACE1 heterozygous null 5XFAD mice is associated with transgenic APP level

Background

The β-secretase, BACE1, cleaves APP to initiate generation of the β-amyloid peptide, Aβ, that comprises amyloid plaques in Alzheimer’s disease (AD). Reducing BACE1 activity is an attractive therapeutic approach to AD, but complete inhibition of BACE1 could have mechanism-based side-effects as BACE1^−/− mice show deficits in axon guidance, myelination, memory, and other neurological processes. Since BACE1^+/− mice appear normal there is interest in determining whether 50% reduction in BACE1 is potentially effective in preventing or treating AD. APP transgenic mice heterozygous for BACE1 have decreased Aβ but the extent of reduction varies greatly from study to study. Here we assess the effects of 50% BACE1 reduction on the widely used 5XFAD mouse model of AD.

Results

50% BACE1 reduction reduces Aβ42, plaques, and BACE1-cleaved APP fragments in female, but not in male, 5XFAD/BACE1^+/− mice. 5XFAD/BACE1^+/+ females have higher levels of Aβ42 and steady-state transgenic APP than males, likely caused by an estrogen response element in the transgene Thy-1 promoter. We hypothesize that higher transgenic APP level in female 5XFAD mice causes BACE1 to no longer be in excess over APP so that 50% BACE1 reduction has a significant Aβ42 lowering effect. In contrast, the lower APP level in 5XFAD males allows BACE1 to be in excess over APP even at 50% BACE1 reduction, preventing lowering of Aβ42 in 5XFAD/BACE1^+/− males. We also developed and validated a dot blot assay with an Aβ42-selective antibody as an accurate and cost-effective alternative to ELISA for measuring cerebral Aβ42 levels.

Conclusions

50% BACE1 reduction lowers Aβ42 in female 5XFAD mice only, potentially because BACE1 is not in excess over APP in 5XFAD females with higher transgene expression, while BACE1 is in excess over APP in 5XFAD males with lower transgene expression. Our results suggest that greater than 50% BACE1 inhibition might be necessary to significantly lower Aβ, given that BACE1 is likely to be in excess over APP in the human brain. Additionally, in experiments using the 5XFAD mouse model, or other Thy-1 promoter transgenic mice, equal numbers of male and female mice should be used, in order to avoid artifactual gender-related differences.

BACE1 inhibitor drugs in clinical trials for Alzheimer's disease

β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is the β-secretase enzyme required for the production of the neurotoxic β-amyloid (Aβ) peptide that is widely considered to have a crucial early role in the etiology of Alzheimer’s disease (AD). As a result, BACE1 has emerged as a prime drug target for reducing the levels of Aβ in the AD brain, and the development of BACE1 inhibitors as therapeutic agents is being vigorously pursued. It has proven difficult for the pharmaceutical industry to design BACE1 inhibitor drugs that pass the blood–brain barrier, however this challenge has recently been met and BACE1 inhibitors are now in human clinical trials to test for safety and efficacy in AD patients and individuals with pre-symptomatic AD. Initial results suggest that some of these BACE1 inhibitor drugs are well tolerated, although others have dropped out because of toxicity and it is still too early to know whether any will be effective for the prevention or treatment of AD. Additionally, based on newly identified BACE1 substrates and phenotypes of mice that lack BACE1, concerns have emerged about potential mechanism-based side effects of BACE1 inhibitor drugs with chronic administration. It is hoped that a therapeutic window can be achieved that balances safety and efficacy. This review summarizes the current state of progress in the development of BACE1 inhibitor drugs and the evaluation of their therapeutic potential for AD.

New type of BACE1 siRNA delivery to cells

BACKGROUND

Small interfering RNA (siRNA) gene therapy is a new molecular approach in the search for an efficient therapy for Alzheimer disease (AD), based on the principle of RNA interference. Reducing BACE activity can have great therapeutic potential for the treatment of AD. In this study, receptor-mediated delivery was used to deliver opioid peptide-conjugated BACE 1 to INR-32 human neuroblastoma cells.

MATERIAL AND METHODS

An INR-32 human neuroblastoma cell line was stably transfected to express the APP cDNA coding fragment containing the predicted sites for cleavage by α, β, or γ-secretase. This was then treated with BACE 1 siRNA to silence BACE gene expression. BACE gene transcription and translation was determined using BACE-1 siRNA cross-linked with opioid peptide, together with RT-PCR, Western blot analysis, and ELISA.

RESULTS

Receptor-mediated delivery was used to introduce BACE1 siRNA to the APP - INR 32 human neuroblastoma cells. Decreased BACE mRNA and protein expression were observed after the cells were transfected with BACE1 siRNA.

CONCLUSIONS

Delivery of BACE1 siRNA appears to specifically reduce the cleavage of APP by inhibiting BACE1 activity.

Structure-based design, synthesis and biological evaluation of novel β-secretase inhibitors containing a pyrazole or thiazole moiety as the P3 ligand

We describe structure-based design, synthesis, and biological evaluation of a series of novel inhibitors bearing a pyrazole (compounds 3a-h) or a thiazole moiety (compounds 4a-e) as the P3 ligand. We have also explored Boc-β-amino-l-alanine as a novel P2 ligand. A number of inhibitors have displayed β-secretase inhibitory potency. Inhibitor 4c has shown potent BACE1 inhibitory activity, Ki=0.25nM, cellular EC50 of 194nM, and displayed good selectivity over BACE2. A model of 4c was created based upon the X-ray structure of 2-bound β-secretase which revealed critical interactions in the active site.

Two novel DNA motifs are essential for BACE1 gene transcription

BACE1 gene encodes for β-Site amyloid β precursor protein (APP)-cleaving enzyme1, which is required for generating amyloid β protein(Aβ). Deposition of Aβ in brain plays an essential role in Alzheimer's Disease (AD) pathogenesis. BACE1 gene has a tissue-specific expression pattern and its expression is tightly regulated at transcriptional level. Core promoter is a minimal DNA sequence to direct transcription initiation and serves as a converging platform for the vast network of regulatory events. Here we identified the core promoter of human BACE1 gene, which is a 71 nucleotides region absent of typical known core promoter elements and is sufficient to initiate a basal transcription. Two novel DNA motifs, designated TCE1 and TCE2, were found to be involved in activating the transcription of human BACE1 gene in a synergistic way. Two single nucleotide mutations in these motifs completely abolished the promoter activity. In conclusion, our studies have demonstrated that novel DNA motif TCE1 and TCE2 in human BACE1 gene promoter are two essential cis-acting elements for BACE1 gene transcription. Studies on how these two motifs being regulated by different stimuli could provide insights into the molecular mechanisms underlying AD pathogenesis and pharmaceutical potentials of targeting these motifs for AD treatment.

Retinal Toxicity Induced by a Novel β-secretase Inhibitor in the Sprague-Dawley Rat

β-Secretase 1 (BACE1) represents an attractive target for the treatment of Alzheimer’s disease. In the course of development of a novel small molecule BACE1 inhibitor (AMG-8718), retinal thinning was observed in a 1-month toxicity study in the rat. To further understand the lesion, an investigational study was conducted whereby rats were treated daily with AMG-8718 for 1 month followed by a 2-month treatment-free phase. The earliest detectable change in the retina was an increase in autofluorescent granules in the retinal pigment epithelium (RPE) on day 5; however, there were no treatment-related light microscopic changes observed in the neuroretina and no changes observed by fundus autofluorescence or routine ophthalmoscopic examination after 28 days of dosing. Following 2 months of recovery, there was significant retinal thinning attributed to loss of photoreceptor nuclei from the outer nuclear layer. Electroretinographic changes were observed as early as day 14, before any microscopic evidence of photoreceptor loss. BACE1 knockout rats were generated and found to have normal retinal morphology indicating that the retinal toxicity induced by AMG-8718 was likely off-target. These results suggest that AMG-8718 impairs phagolysosomal function in the rat RPE, which leads to photoreceptor dysfunction and ultimately loss of photoreceptors.

Knock-in of human BACE1 cleaves murine APP and reiterates Alzheimer-like phenotypes

Key neuropathological hallmarks of Alzheimer's disease (AD) are elevated levels of amyloid β-peptide (Aβ) species generated via amyloid precursor protein (APP) endoproteolysis and cleavage by the rate-limiting β-site enzyme 1 (BACE1). Because rodents do not develop amyloid pathologies, we here investigated whether AD-like endophenotypes can be created in mice by expression of human bace1. To avoid pitfalls of existing models, we introduced hbace1 via knock-in under the control of the CaMKII α promoter into the safe HPRT locus. We report amyloidogenic processing of murine APP in the hBACE1 mice (termed PLB4), resulting in the formation of toxic APP metabolites that accumulate intra- and extraneuronally in hippocampus and cortex. Pronounced accumulation of Aβ*56 and Aβ hexamers in the absence of plaque deposition was detected in brain tissue from symptomatic PLB4 mice. Heightened levels of inflammation (gliosis) also appeared in several AD-related brain regions (dentate gyrus, hippocampal area CA1, piriform and parietal cortices) at 6 and 12 months of age. Behaviorally, deficits in habituation to a novel environment and semantic-like memory (social transmission of food preference) were detected from 3 to 4 months of age. Impairments in spatial learning strategies in long-term reference (water maze) and working memory (Y-maze) tasks presented at 6 months, and were distinct from reductions in locomotor activity and anxiety. Overall, our data indicate for the first time that targeted, subtle forebrain-specific expression through single gene knock-in of hBACE1 is sufficient to generate AD-relevant cognitive impairments amid corresponding histopathologies, confirming human BACE as the key parameter in amyloid pathogenesis.