In search of an enzyme: the beta-secretase of Alzheimer's disease is an aspartic proteinase

Potential Enzymatic Targets in Alzheimer's: A Comprehensive Review

Alzheimer's, a degenerative cause of the brain cells, is called as a progressive neurodegenerative disease and appears to have a heterogeneous etiology with main emphasis on amyloid-cascade and hyperphosphorylated tau-cascade hypotheses, that are directly linked with macromolecules called enzymes such as β- & γ-secretases, colinesterases, transglutaminases, and glycogen synthase kinase (GSK-3), cyclin-dependent kinase (cdk-5), microtubule affinity-regulating kinase (MARK). The catalytic activity of the above enzymes is the result of cognitive deficits, memory impairment and synaptic dysfunction and loss, and ultimately neuronal death. However, some other enzymes also lead to these dysfunctional events when reduced to their normal activities and levels in the brain, such as α- secretase, protein kinase C, phosphatases etc; metabolized to neurotransmitters, enzymes like monoamine oxidase (MAO), catechol-O-methyltransferase (COMT) etc. or these abnormalities can occur when enzymes act by other mechanisms such as phosphodiesterase reduces brain nucleotides (cGMP and cAMP) levels, phospholipase A2: PLA2 is associated with reactive oxygen species (ROS) production etc. On therapeutic fronts, several significant clinical trials are underway by targeting different enzymes for development of new therapeutics to treat Alzheimer's, such as inhibitors for β-secretase, GSK-3, MAO, phosphodiesterase, PLA2, cholinesterases etc, modulators of α- & γ-secretase activities and activators for protein kinase C, sirtuins etc. The last decades have perceived an increasing focus on findings and search for new putative and novel enzymatic targets for Alzheimer's. Here, we review the functions, pathological roles, and worth of almost all the Alzheimer's associated enzymes that address to therapeutic strategies and preventive approaches for treatment of Alzheimer's.

Low anticoagulant heparin oligosaccharides as inhibitors of BACE-1, the Alzheimer's β-secretase

Heparin (HP) is a promising agent for anti-Alzheimer's disease (AD), but its anticoagulant activity limits its applications. So a low anticoagulant heparin (LAH) with anti-AD effect is needed. A novel LAH and heparan sulfate (HS) were purified from crude porcine intestinal heparin. Their structures were characterized by nuclear magnetic resonance and liquid chromatography-mass spectrometry. LAH had a relatively high degree of sulfation, but lower than that of HP. 3-O-Sulfated-containing glucosamine residues further confirmed the low anticoagulant activity of LAH. Sixteen oligosaccharides of LAH and HS were prepared and assigned. Evaluation of anti-BACE-1 activities suggested that their potencies were positively correlated with degree of sulfation and polymerization of oligosaccharides. Besides, LAH-derived hexa- to dodecasaccharides was promised to be administrated in vitro as BACE-1 inhibitors. This study presented ideal BACE-1 inhibitors, LAH-derived oligosaccharides, with virtually no anticoagulant activities, which were promised to be excellent leads for treatment of AD.

Neuronal response in Alzheimer's and Parkinson's disease: the effect of toxic proteins on intracellular pathways

Accumulation of protein aggregates is the leading cause of cellular dysfunction in neurodegenerative disorders. Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease, Prion disease and motor disorders such as amyotrophic lateral sclerosis, present with a similar pattern of progressive neuronal death, nervous system deterioration and cognitive impairment. The common characteristic is an unusual misfolding of proteins which is believed to cause protein deposition and trigger degenerative signals in the neurons. A similar clinical presentation seen in many neurodegenerative disorders suggests the possibility of shared neuronal responses in different disorders. Despite the difference in core elements of deposits in each neurodegenerative disorder, the cascade of neuronal reactions such as activation of glycogen synthase kinase-3 beta, mitogen-activated protein kinases, cell cycle re-entry and oxidative stress leading to a progressive neurodegeneration are surprisingly similar. This review focuses on protein toxicity in two neurodegenerative diseases, AD and PD. We reviewed the activated mechanisms of neurotoxicity in response to misfolded beta-amyloid and α-synuclein, two major toxic proteins in AD and PD, leading to neuronal apoptosis. The interaction between the proteins in producing an overlapping pathological pattern will be also discussed.

Association study of cathepsin D gene polymorphism in Iranian patients with sporadic late-onset Alzheimer's disease

One of the most prevalent forms of dementia is Alzheimer's disease (AD). Complex inheritance and multifactorial patterns of late-onset AD (LOAD) along with its heterogeneity are due to the presence of different AD-predisposing genes with different influence on disease development among various populations. A key event in the pathogenesis of AD is the deposition of β-amyloid peptide, which is derived from the amyloid precursor protein by β- and γ-secretases. Cathepsin D (CTSD) is an acid protease with β- and γ-secretase-like features in vitro. An exonic C→T polymorphism at position 224 of the CTSD gene (rs: 17571) has been shown to be associated with the enzyme function of CTSD and with AD. Two studies in the German population reported a strong association of this polymorphism with an increased risk of developing AD, while other studies did not confirm this observation. We tested for this association in a case-control study in 100 Iranian sporadic LOAD patients based on diagnostic criteria of DSM-IV-TR and NINCDS-ADRDA and in 100 normal controls without any personal and family history of AD or other related dementias. Polymerase chain reaction-restriction fragment length polymorphism was set up to detect this polymorphism. Our study demonstrated that T-carrying genotype frequency in AD patients is significantly higher than in controls and there was a 2.5-fold increased risk for developing AD in the T-carrying genotype compared to C/C genotype (odds ratio = 2.5, p = 0.010). The odds ratio for subjects with the apolipoprotein E ε4 (APOE ε4) allele was 2.91 (p = 0.003) and carriers of the CTSD T and APOE ε4 alleles had a 6.25-fold increased risk of the disease (p = 0.0). Our results indicate that CTSD genotype is associated with the disease and a combination of the above risk factors significantly alters the risk for developing AD.

Intranasal therapeutic strategies for management of Alzheimer's disease

Alzheimer's disease (AD) is a chronic and progressive age-related irreversible neurodegenerative disorder that represents 70% of all dementia with 35 million cases worldwide. Successful treatment strategies for AD have so far been limited, and present therapy is based on cholinergic replacement therapy and inhibiting glutamate excitotoxicity. In this context, role of neuroprotective drugs has generated considerable interest in management of AD. Recently, direct intranasal (IN) delivery of drug moieties to the central nervous system (CNS) has emerged as a therapeutically viable alternative to oral and parenteral routes. IN delivery bypasses the blood-brain barrier by delivering and targeting drugs to the CNS along the olfactory and trigeminal neural pathways which are in direct contact with both the environment and the CNS. In an attempt to understand how neurotherapeutics/nanoparticulate delivery systems can be transported from the nose to the CNS, the present review sets out to discuss the mechanism of transport from nose to brain. The aim of this review is to discuss and summarize the latest findings of some of the major studies on IN drug delivery in AD models, with a focus on the potential efficacy of neuroprotective treatments.

Extracellular amyloid beta 42 causes necrosis, inhibition of nuclear division, and mitotic disruption under both folate deficient and folate replete conditions as measured by the cytokinesis-block micronucleus cytome assay

Alzheimer's disease is associated with accumulation of extracellular beta amyloid peptide 42 (Aβ42) which may induce DNA damage and reduce cellular regenerative potential. These effects may be exacerbated under conditions of folate deficiency. The aim of this study was to investigate whether extracellular Aβ42 induces DNA damage and cell death in human peripheral lymphocytes and whether there is an interactive effect between extracellular Aβ42 and folic acid status. Peripheral blood lymphocytes were cultured in medium under conditions of both low and high folate (20 and 200 nM, respectively) and challenged with either Aβ42 or the physiologically normal form Aβ40 (both at 5, 10, 15 µM). Genome stability and cytotoxicity events were investigated using the cytokinesis-block micronucleus cytome (CBMN-cyt) assay. Outcome measures scored included the nuclear division index (NDI), necrosis, apoptosis, binucleated cells with micronuclei (MN), nucleoplasmic bridges (NPB), and nuclear buds (NBUD) and abnormally shaped nuclei (circular, (CIR) and horse-shoe, (HS) that may be indicative of mitotic disruption. Folic acid deficiency significantly reduced NDI (P < 0.001) and increased all the DNA damage biomarkers (MN, NPB, NBUD, HS, CIR), (P < 0.001). In contrast, exposure to Aβ40 had no impact on CBMN cytome biomarkers but Aβ42 significantly reduced NDI (P < 0.01), increased necrosis (P < 0.05) and frequency of cells with circular nuclei (P < 0.01). There was no evidence of an interaction between Aβ42 and folic acid with respect to CBMN cytome biomarkers. Extracellular Aβ42 appears to have cytotoxic and cytostatic effects but its effect on chromosomal instability appears to be small relative to folate deficiency.

The protective effect of crocin on the amyloid fibril formation of Aβ42 peptide in vitro

Aβ is the main constituent of the amyloid plaque found in the brains of patients with Alzheimer's disease. There are two common isoforms of Aβ: the more common form, Aβ40, and the less common but more amyloidogenic form, Aβ42. Crocin is a carotenoid from the stigma of the saffron flower and it has many medicinal properties, including antioxidant effects. In this study, we examined the potential of crocin as a drug candidate against Aβ42 amyloid formation. The thioflavin T-binding assay and electron microscopy were used to examine the effects of crocin on the extension and disruption of Aβ42 amyloids. To further investigate the relationship between crocin and Aβ42 structure, we analyzed peptide conformation using the ANS-binding assay and circular dichroism (CD) spectroscopy. An increase in the thioflavin T fluorescence intensity upon incubation revealed amyloid formation in Aβ42. It was found that crocin has the ability to prevent amyloid formation by decreasing the fluorescence intensity. Electron microscopy data also indicated that crocin decreased the amyloid fibril content of Aβ. The ANS-binding assay showed that crocin decreased the hydrophobic area in incubated Aβ42. CD spectroscopy results also showed that the peptide undergoes a structural change to α-helical and β-turn. Our study shows that the anti-amyloidogenic effect of crocin may be exerted not only by the inhibition of Aβ amyloid formation but also by the disruption of amyloid aggregates. Therefore, crocin could be essential in the search for therapies inhibiting aggregation or disrupting aggregation.

Prion protein expression alters APP cleavage without interaction with BACE-1

The prion protein (PrP) and the beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE-1) are both copper binding proteins, but are associated with two separate neurodegenerative diseases. The role of BACE-1 in the formation of beta-amyloid has made it a major target in attempts to reduce the formation of beta-amyloid in Alzheimer's diseases. However, the suggestion that PrP, normally associated with prion diseases, binds to BACE-1 and reduces its activity has led to the suggestion that the study of this interaction could be of considerable importance to Alzheimer's disease. We therefore undertook to investigate the possible interaction of these two proteins physically and at the level of transcription, translation and APP cleavage. Our findings suggest that mature PrP and BACE-1 do not physically interact, but that altered PrP expression results in altered BACE-1 protein expression and promoter activity. Additionally, overexpression of PrP results in increased cleavage of APP in contrast to previous datas suggesting a reduction. Our findings suggest that any relation between PrP and BACE-1 is indirect. Altered expression of PrP causes changes in the expression of many other proteins which may be as a result of altered copper metabolism.

Aqueous extract of lavender (Lavandula angustifolia) improves the spatial performance of a rat model of Alzheimer's disease

OBJECTIVE

Alzheimer's disease (AD) is one of the most important neurodegenerative disorders. It is characterized by dementia including deficits in learning and memory. The present study aimed to evaluate the effects of aqueous extract of lavender (Lavandula angustifolia) on spatial performance of AD rats.

METHODS

Male Wistar rats were first divided into control and AD groups. Rat model of AD was established by intracerebroventricular injection of 10 μg Aβ1-42 20 d prior to administration of the lavender extract. Rats in both groups were then introduced to 2 stages of task learning (with an interval of 20 d) in Morris water maze, each followed by one probe test. After the first stage of spatial learning, control and AD animals received different doses (50, 100 and 200 mg/kg) of the lavender extract.

RESULTS

In the first stage of experiment, the latency to locate the hidden platform in AD group was significantly higher than that in control group. However, in the second stage of experiment, control and AD rats that received distilled water (vehicle) showed similar performance, indicating that the maze navigation itself could improve the spatial learning of AD animals. Besides, in the second stage of experiment, control and AD rats that received lavender extract administration at different doses (50, 100, and 200 mg/ kg) spent less time locating the platform (except for the AD rats with 50 mg/kg extract treatment), as compared with their counterparts with vehicle treatment, respectively. In addition, lavender extract significantly improved the performance of control and AD rats in the probe test, only at the dose of 200 mg/kg, as compared with their counterparts with vehicle treatment.

CONCLUSION

The lavender extract can effectively reverse spatial learning deficits in AD rats.

Selective amyloid-beta lowering agents

The amyloid-beta peptide (Abeta), implicated in the pathogenesis of Alzheimer's disease (AD), is produced through sequential proteolysis of the Abeta precursor protein (APP) by beta- and gamma-secretases. Thus, blocking either of these two proteases, directly or indirectly, is potentially worthwhile toward developing AD therapeutics. beta-Secretase is a membrane-tethered pepsin-like aspartyl protease suitable for structure-based design, whereas gamma-secretase is an unusual, heterotetrameric membrane-embedded aspartyl protease. While gamma-secretase inhibitors entered clinical trials first due to their superior pharmacological properties (for example, brain penetration) over beta-secretase inhibitors, it has since become clear that gamma-secretase inhibitors can cause mechanism-based toxicities owing to interference with the proteolysis of another gamma-secretase substrate, the Notch receptor. Strategies for targeting Abeta production at the gamma-secretase level without blocking Notch signalling will be discussed. Other strategies utilizing cell-based screening have led to the identification of novel Abeta lowering agents that likewise leave Notch proteolysis intact. The mechanism by which these agents lower Abeta is unknown, but these compounds may ultimately reveal new targets for AD therapeutics.

Heparin Derivatives as Inhibitors of BACE-1, the Alzheimer's β-Secretase, with Reduced Activity against Factor Xa and Other Proteases

Heparan sulfate (HS) regulates processing of the amyloid precursor protein by the Alzheimer's beta-secretase (BACE-1). An HS analogue, porcine intestinal mucosal heparin, was systematically modified at the principal positions of O-sulfation and N-sulfation/acetylation and tested for BACE-1 inhibitory and anti factor Xa activities. The derivative with the highest anti-BACE-1 to anti-Xa activity ratio contained N-acetyl and 2-O- and 6-O-sulfates and also exhibited attenuated activities against cathepsin-D and renin, two other structurally related aspartyl proteases.

Syntaxin 5 interacts specifically with presenilin holoproteins and affects processing of betaAPP in neuronal cells

The specific roles of syntaxin 5 (Syx 5) in the interaction with presenilin (PS) and the accumulation of beta-amyloid precursor protein (betaAPP), as well as the secretion of beta-amyloid peptide (Abeta peptide) were examined in NG108-15 cells. Syx 5, which localizes from the endoplasmic reticulum (ER) to the Golgi, bound to PS holoproteins, while the other Syxs studied did not. Among familial Alzheimer's disease (FAD)-linked PS mutants, PS1deltaE9, which lacks the endoproteolytic cleavage site, showed markedly decreased binding to Syx 5. The interaction domains in Syx 5 were mapped to the transmembrane region and to the cytoplasmic region containing the alpha-helical domains, which are distinct from the H3 (SNARE motif). Among all of the Syxs examined, only overexpression of Syx 5 resulted in the accumulation of betaAPP in the ER to cis-Golgi compartment, an attenuation of the amount of the C-terminal fragment (APP-CTF) of betaAPP, and a reduction in the secretion of Abeta peptides. Furthermore, co-expression of Syx 5 with C99 resulted in an increase in APP-CTF and suppressed Abeta secretion. Taken together, these results indicate that Syx 5 may play a specific role in the modulation of processing and/or trafficking of FAD-related proteins in neuronal cells by interaction with PS holoproteins in the early secretory compartment of neuronal cells.

Design and synthesis of statine-Containing BACE inhibitors

Utilizing structure-based techniques and solid-phase synthesis, statine-based tetrapeptide BACE inhibitors were designed and synthesized using a heptapeptide BACE transition-state mimetic, 1, as the starting point. Structure-activity relationship studies at the P(3), P(2), and P(2)' positions as well as the N-terminal capping group on scaffold 5 led to the discovery of potent inhibitors 27, 32, and 34 (IC(50) <100 nM). In addition, computational analysis and the X-ray structure of BACE-inhibitor 38 are discussed.

Amyloid precursor protein (APP) and the biology of proteolytic processing: relevance to Alzheimer's disease

The processing of amyloid precursor protein (APP) generates amyloid-beta (Abeta) peptides 1-40 and 1-42. The latter is neurotoxic and its accumulation results in amyloid fibril formation and the generation of senile plaques, the hallmark of Alzheimer's disease (AD). Whilst there has been considerable progress made in understanding the generation of Abeta by alpha-, beta- and gamma-secretase activity on APP, recently enzymes involved in the degradation of Abeta have been identified including neprilysin and insulin-degrading enzyme (IDE). We review the pathways involved in proteolytic processing of APP and discuss the potential implications of aberrant proteolysis on neurodegeneration. It is conceivable that single nucleotide polymorphisms (SNPs) in the regulatory regions of genes in these proteolytic cascades, which alter their expression, could contribute to some of the age-related changes seen in AD.

Heparan sulfate regulates amyloid precursor protein processing by BACE1, the Alzheimer's beta-secretase

Cleavage of amyloid precursor protein (APP) by the Alzheimer's β-secretase (BACE1) is a key step in generating amyloid β-peptide, the main component of amyloid plaques. Here we report evidence that heparan sulfate (HS) interacts with β-site APP-cleaving enzyme (BACE) 1 and regulates its cleavage of APP. We show that HS and heparin interact directly with BACE1 and inhibit in vitro processing of peptide and APP substrates. Inhibitory activity is dependent on saccharide size and specific structural characteristics, and the mechanism of action involves blocking access of substrate to the active site. In cellular assays, HS specifically inhibits BACE1 cleavage of APP but not alternative cleavage by α-secretase. Endogenous HS immunoprecipitates with BACE1 and colocalizes with BACE1 in the Golgi complex and at the cell surface, two of its putative sites of action. Furthermore, inhibition of cellular HS synthesis results in enhanced BACE1 activity. Our findings identify HS as a natural regulator of BACE1 and suggest a novel mechanism for control of APP processing.

Characterization of the ectodomain shedding of the beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1)

Generation of the amyloid peptide through proteolytic processing of the amyloid precursor protein by beta- and gamma-secretases is central to the etiology of Alzheimer's disease. beta-secretase, known more widely as the beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), has been identified as a transmembrane aspartic proteinase, and its ectodomain has been reported to be cleaved and secreted from cells in a soluble form. The extracellular domains of many diverse proteins are known to be cleaved and secreted from cells by a process known as ectodomain shedding. Here we confirm that the ectodomain of BACE1 is secreted from cells and that this processing is up-regulated by agents that activate protein kinase C. A metalloproteinase is involved in the cleavage of BACE1 as hydroxamic acid-based metalloproteinase inhibitors abolish the release of shed BACE1. Using potent and selective inhibitors, we demonstrate that ADAM10 is a strong candidate for the BACE1 sheddase. In addition, we show that the BACE1 sheddase is distinct from alpha-secretase and, importantly, that inhibition of BACE1 shedding does not influence amyloid precursor protein processing at the beta-site.

Alzheimer's disease: molecular understanding predicts amyloid-based therapeutics

Degenerative diseases of the brain were long considered among the most obscure and intractable of human maladies. However, recent advances in understanding their mechanisms have brought us to the verge of potential disease-modifying agents. This progress is perhaps best exemplified by the case of Alzheimer's disease. The application of molecular pathology and genetics has led to the recognition that the four genes implicated to date in familial Alzheimer's disease all chronically elevate cerebral levels of the amyloid beta-protein (Abeta). Accordingly, small molecule inhibitors of the beta- and gamma-secretases, the proteases that generate Abeta from its precursor, are under active development, and some have shown in vivo efficacy in mouse models. An alternative approach, active or passive immunization against Abeta, has received extensive pre-clinical validation in mice, but an effective preparation free of significant side effects in humans is still awaited. Several other potential therapies are also reviewed here. If one or more of these varied approaches is ultimately proven to slow or prevent dementia, Alzheimer's disease will become a salient example of the successful application of reductionist biology to the most complex of organs, the human cerebral cortex.

Implications of amyloid precursor protein and subsequent beta-amyloid production to the pharmacotherapy of Alzheimer's disease

Alzheimer's disease is the most common type of dementia in older people. It is highly prevalent, affecting 35-45% of those aged 85 years or older. This disease has devastating consequences to patients, their families, caregivers, and the health care system. Much has been learned about its pathobiology, which has led to the beta-amyloid (Abeta) hypothesis. This hypothesis continues to be the predominant postulate of the pathobiology of Alzheimer's disease. Under this hypothesis, abnormal accumulation of Abeta is followed by a cascade of neurotoxic effects, which eventually result in neurodegeneration and development of Alzheimer's disease. This is thought to be the result of altered processing of the amyloid precursor protein (APP), preferentially by beta- and gamma-secretase enzymes rather than nonamyloidogenic processing by alpha-secretase. The growing body of knowledge regarding the processing of APP to various forms of Abeta has resulted in new approaches to the investigation of putative anti-Alzheimer's disease compounds, including immune-based therapies and various agents that can positively affect APP processing.

Secretases as targets for the treatment of Alzheimer's disease: the prospects

The amyloid hypothesis is still used to explain the pathogenesis of Alzheimer's disease. Despite all progress made, however, the molecular causes of the amyloid pathology, and of the tau pathology, tend to be ignored in most patients with this disorder (sporadic, late-onset). Mutant genes for amyloid precursor protein (APP) or presenilin cause early-onset familial Alzheimer's disease (<1% of all cases) and have helped to elucidate APP processing and amyloid-peptide formation by alpha, beta, and gamma secretases. Inhibition of production of amyloid peptides by inhibitors of beta and gamma secretases has been suggested as the rational and most specific therapeutic approach. Alternatively, or additionally, the activation of alpha secretase would increase non-amyloidogenic processing of APP. Here we review fundamental, genetic, and clinical arguments on which the therapeutic strategies for design of secretase agonists and antagonists are based, with special attention to physiological model systems to assess the potential of current efforts.

Structure-based design and synthesis of HIV-1 protease inhibitors employing beta-D-mannopyranoside scaffolds

A preliminary account on the structure-based design, synthesis and evaluation of peptidomimetic inhibitors of HIV-1 protease containing beta-D-mannopyranoside scaffolds is given. The compounds prepared had IC(50) values in the micromolar range. The results provide a platform for the development of more potent carbohydrate-based inhibitors of HIV-1 and other aspartic proteases.

Glycosaminoglycans and beta-amyloid, prion and tau peptides in neurodegenerative diseases

Protein aggregation into dense filamentous inclusions is a characteristic feature of many etiologically diverse neurodegenerative disorders including Alzheimer's disease (AD), spongiform encephalopathies, and tauopathies. Thus, beta-amyloid peptide (Abeta) accumulates within senile amyloid plaques in AD, protease-resistant prion protein constitutes the amyloid deposits in spongiform encephalopathies and tau protein gives rise to neurofibrillary tangles (NFT) both in AD and in tauopathies. Curiously, these abnormal protein inclusions contain, in addition to their major peptide components, some associated sulfated glycosaminoglycans (sGAG). Here we discuss the proposal that the binding of sGAG to aggregate-forming peptides may modify the pathogenic process depending on their subcellular localization.

The brain in Down syndrome

Down syndrome (trisomy 21) is a genetic disease with developmental brain abnormalities resulting in early mental retardation and precocious, age dependent Alzheimer-type neurodegeneration. We tried to discuss the role of neurodevelopmental abnormalities in connection with aberrant expression of genes on chromosome 21 including amyloid precursor protein (APP), CuZn superoxide dismutase (SOD1) and glial-derived S100 beta protein for neurodegeneration in DS. In this model, alterations in developmental pathways due to aberrant gene expression can impair cellular homeostasis and predispose to neurodegeneration of certain brain regions and types of nerve cells, involving cholinergic, serotonergic and catecholaminergic transmission, by shifting balance toward a pro-apoptotic state.

Beta-amyloid catabolism: roles for neprilysin (NEP) and other metallopeptidases?

The steady-state level of amyloid beta-peptide (Abeta) represents a balance between its biosynthesis from the amyloid precursor protein (APP) through the action of the beta- and gamma-secretases and its catabolism by a variety of proteolytic enzymes. Recent attention has focused on members of the neprilysin (NEP) family of zinc metalloproteinases in amyloid metabolism. NEP itself degrades both Abeta(1-40) and Abeta(1-42) in vitro and in vivo, and this metabolism is prevented by NEP inhibitors. Other NEP family members, for example endothelin-converting enzyme, may contribute to amyloid catabolism and may also play a role in neuroprotection. Another metalloproteinase, insulysin (insulin-degrading enzyme) has also been advocated as an amyloid-degrading enzyme and may contribute more generally to metabolism of amyloid-forming peptides. Other candidate enzymes proposed include angiotensin-converting enzyme, some matrix metalloproteinases, plasmin and, indirectly, thimet oligopeptidase (endopeptidase-24.15). This review critically evaluates the evidence relating to proteinases implicated in amyloid catabolism. Therapeutic strategies aimed at promoting A,beta degradation may provide a novel approach to the therapy of Alzheimer's disease.

Activity-dependent isolation of the presenilin- gamma -secretase complex reveals nicastrin and a gamma substrate

Presenilin heterodimers apparently contain the active site of gamma-secretase, a polytopic aspartyl protease involved in the transmembrane processing of both the Notch receptor and the amyloid-beta precursor protein. Although critical to embryonic development and the pathogenesis of Alzheimer's disease, this protease is difficult to characterize, primarily because it is a multicomponent complex of integral membrane proteins. Here the functional gamma-secretase complex was isolated by using an immobilized active site-directed inhibitor of the protease. Presenilin heterodimers and nicastrin bound specifically to this inhibitor under conditions tightly correlating with protease activity, whereas several other presenilin-interacting proteins (beta-catenin, calsenilin, and presenilin-associated protein) did not bind. Moreover, anti-nicastrin antibodies immunoprecipitated gamma-secretase activity from detergent-solubilized microsomes. Unexpectedly, C83, the major endogenous amyloid-beta precursor protein substrate of gamma-secretase, was also quantitatively associated with the complex. These results provide direct biochemical evidence that nicastrin is a member of the active gamma-secretase complex, indicate that beta-catenin, calsenilin, and presenilin-associated protein are not required for gamma activity, and suggest an unprecedented mechanism of substrate-protease interaction.

Lack of association between cathepsin D genetic polymorphism and Alzheimer disease in a Spanish sample

Cathepsin D (catD) is an intracellular aspartyl protease that exhibits beta and gamma secretase-like activity to cleave amyloid precursor protein into beta amyloid peptide. The T-allele of a biallelic (alleles C and T) polymorphism in the exon 2 of the catD gene has been found to be associated with increased risk of Alzheimer disease (AD) in two independent German populations. Other groups have been unable to replicate this association in Caucasian American and Northern Ireland populations. Moreover, a small and no significant tendency for the T-allele to be protective for AD has been demonstrated in Caribbean Hispanics. A case control study utilizing a clinically well-defined group of 311 sporadic AD patients and 346 control subjects was performed to test this association in an ethnically homogeneous population from Spain. We did not observe any association between the T-allele of the catD gene and the disease. Furthermore, catD was not predictive of AD in an interactive fashion when considering apolipoprotein E, age, or gender.

Presence of BACE1 and BACE2 in muscle fibres of patients with sporadic inclusion-body myositis

Sporadic inclusion-body myositis (IBM) is the most common, progressive muscle disease of older individuals. We investigated the presence of BACE1 and BACE2-two beta secretases that cleave amyloid-beta-precursor protein-in muscle-biopsy samples from patients with IBM and from controls. On immunofluorescence, BACE1 and BACE2 co-localised with amyloid beta in IBM vacuolated muscle fibres, but were not found in controls. Immunoblotting showed increased BACE2 but not BACE1 in patients with IBM compared with controls. Our study suggests that both of these proteases might participate in processing of amyloid-beta-precursor protein in IBM muscle fibres.

The cell biology of Alzheimer's disease: uncovering the secrets of secretases

Progress has been made in characterizing the secretases involved in endoproteolytic processing of the beta-amyloid precursor protein - the precursor of the amyloid beta-peptide (Abeta), which is the main constituent of amyloid plaques that form in the brains of patients with Alzheimer's disease. It is now thought that Abeta is pivotal in the pathogenesis of Alzheimer's disease, and that reducing brain Abeta levels may help to treat or prevent the disease. Two essential factors for the proteolytic generation of Abeta have been identified, beta-secretase and the presenilins, which might aid the design of drugs against this disease.

Compartmentalization of beta-secretase (Asp2) into low-buoyant density, noncaveolar lipid rafts

Recent epidemiological studies show a reduced prevalence of Alzheimer's disease (AD) in patients treated with inhibitors of cholesterol biosynthesis. Moreover, the cholesterol-transport protein, apolipoprotein E4, and elevated cholesterol are important risk factors for AD. Additionally, in vitro and in vivo studies show that intracellular cholesterol levels can modulate the processing of amyloid precursor protein (APP) to beta-amyloid, the major constituent of senile plaques. Cholesterol plays a crucial role in maintaining lipid rafts in a functional state. Lipid rafts are cholesterol-enriched membrane microdomains implicated in signal transduction, protein trafficking, and proteolytic processing. Since APP, beta-amyloid, and the putative gamma-secretase, presenilin-1 (PS-1), have all been found in lipid rafts, we hypothesized that the recently identified beta-secretase, Asp2 (BACE1), might also be present in rafts. Here, we report that recombinant Asp2 expressed in three distinct cell lines is raft associated. Using both detergent and nondetergent methods, Asp2 protein and activity were found in a light membrane raft fraction that also contained other components of the amyloidogenic pathway. Immunoisolation of caveolin-containing vesicles indicated that Asp2 was present in a unique raft population distinct from caveolae. Finally, depletion of raft cholesterol abrogated association of Asp2 with the light membrane fraction. These observations are consistent with the raft localization of APP processing and suggest that the partitioning of Asp2 into lipid rafts may underlie the cholesterol sensitivity of beta-amyloid production.

A genomic perspective on human proteases as drug targets

Of the approximately 400 known human proteases, approximately 14% are under investigation as drug targets. Although the total is certain to rise during the finishing phase of the human genome project, the initial annotation of the approximately 30,000 human proteome set includes approximately 500 proteases. Bioinformatic analysis can now be performed on complete human protease families and will soon include comparisons with mice and fish. New sequences will require evaluation of their function in normal physiology and human disease. By revealing details such as splice variants and population polymorphisms, genomic sequence information will have a central role in the validation of protease drug targets.

Prodomain processing of Asp1 (BACE2) is autocatalytic

Generation of the amyloid peptide through proteolytic processing of the amyloid precursor protein by beta- and gamma-secretases is central to the etiology of Alzheimer's disease. The highly elusive beta-secretase was recently identified as a transmembrane aspartic proteinase, Asp2 (BACE). The Asp2 homolog Asp1 (BACE2/DRAP) has also been reported to exhibit beta-secretase cleavage of amyloid precursor protein. Most aspartic proteinases are generated as inactive proenzymes, requiring removal of the prodomain to generate active proteinase. Here we show that prodomain processing of Asp1 occurs between Leu(62) and Ala(63) and is autocatalytic. Asp1 cleaved a maltose-binding protein-Asp1 prodomain fusion protein and a synthetic peptide at this site. Mutation of one of the conserved catalytic aspartic acid residues in the active site of Asp1 to asparagine (D110N) abolished this cleavage. Mutation of P(1)' and P(2)' residues in the substrate to phenylalanine reduced cleavage at this site. Asp1 expressed in cells was the mature form, and prodomain processing occurred intramolecularly within the endoplasmic reticulum/early Golgi. Interestingly, a proportion of mature Asp1 was expressed on the cell surface. When full-length Asp1(D110N) was expressed in COS-7 cells, it was not processed, suggesting that no other proteinase can activate Asp1 in these cells.