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

Beta-secretase (BACE) as a drug target for Alzheimer's disease

Evidence suggests that the beta-amyloid peptide (Abeta) is central to the pathophysiology of Alzheimer's Disease (AD). Amyloid plaques, primarily composed of Abeta, progressively develop in the brains of AD patients, and mutations in three genes (APP, PS1, and PS2) cause early on-set familial AD (FAD) by increasing synthesis of the toxic Abeta42 peptide. Given the strong association between Abeta and AD, therapeutic strategies to lower the concentration of Abeta in the brain should prove beneficial for the treatment of AD. Abeta is a proteolytic product of the large TypeI membrane protein, amyloid precursor protein (APP). Two proteases, called beta- and gamma-secretase, cleave APP to generate the Abeta peptide. For over a decade, the molecular identities of these proteases were unknown. Recently, the gamma-secretase has been tentatively identified as the presenilin proteins, PS1 and PS2, and the beta-secretase has been shown to be the novel transmembrane aspartic protease, beta-site APP Cleaving Enzyme 1 (BACE1; also called Asp2 and memapsin2). BACE2, a novel protease homologous to BACE1, was also identified, and the two BACE enzymes define a new family of transmembrane aspartic proteases. BACE1 exhibits all the properties of the beta-secretase, and as the key enzyme that initiates the formation of Abeta, BACE1 is an attractive drug target for AD. This review discusses the identification and initial characterization of BACE1 and BACE2, and summarizes our current understanding of BACE1 post-translational processing and intracellular trafficking. Finally, recent studies of BACE1 knockout mice, the BACE1 X-ray structure, and implications for BACE1 drug development will be discussed.

Role of CD40 ligand in amyloidosis in transgenic Alzheimer's mice

We have shown that interaction of CD40 with CD40L enables microglial activation in response to amyloid-beta peptide (Abeta), which is associated with Alzheimer's disease (AD)-like neuronal tau hyperphosphorylation in vivo. Here we report that transgenic mice overproducing Abeta, but deficient in CD40L, showed decreased astrocytosis and microgliosis associated with diminished Abeta levels and beta-amyloid plaque load. Furthermore, in the PSAPP transgenic mouse model of AD, a depleting antibody against CD40L caused marked attenuation of Abeta/beta-amyloid pathology, which was associated with decreased amyloidogenic processing of amyloid precursor protein (APP) and increased circulating levels of Abeta. Conversely, in neuroblastoma cells overexpressing wild-type human APP, the CD40-CD40L interaction resulted in amyloidogenic APP processing. These findings suggest several possible mechanisms underlying mitigation of AD pathology in response to CD40L depletion, and validate the CD40-CD40L interaction as a target for therapeutic intervention in AD.

Beta-amyloid production, aggregation, and clearance as targets for therapy in Alzheimer's disease

1. Despite major efforts aimed at elucidating the molecular basis and physiopathology of Alzheimer's disease (AD), there is still no effective treatment available for this devastating disorder. The biological mechanisms underlying the development of AD are complex, as multiple factors appear to modulate (either positively or negatively) the progression of neurodegeneration in the brains of AD patients. Not surprisingly, a number of different therapeutic approaches aimed at distinct aspects of the disease are currently being pursued. Given its central role in the neuropathology of AD, the beta-amyloid peptide (Abeta) is the focus of many such approaches. 2. In this review, we discuss recent developments along three major lines of investigation: (i) identification and characterization of inhibitors of the enzymes involved in proteolytic processing of the amyloid precursor protein and production of Abeta; (ii) identification of the pathways involved in cerebral degradation and clearance of Abeta; and (iii) characterization of small-molecule inhibitors of amyloid aggregation that prevent cerebral amyloid deposition and neurotoxicity. 3. Significant progress has been achieved in these directions, opening up new perspectives toward the development of effective approaches for the treatment or prevention of AD.

gamma-Secretase: characterization and implication for Alzheimer disease therapy

gamma-Secretase is a membrane-bound protease that cleaves within the transmembrane region of amyloid precursor protein to generate the C-termini of the Abeta peptides which are believed to play a central role in the neuropathology of Alzheimer's disease. An in vitro gamma-secretase assay using a recombinant substrate C100Flag has been developed to facilitate the characterization and identification of this enigmatic protease. Biochemical studies establish that gamma-secretase activity is catalyzed by a PS1-containing macromolecular complex. Moreover, the fact that the photoreactive active gamma-secretase inhibitor directed to the active site labels PS1 suggests that PS1 contains the active site of the protease. Presenilin/gamma-secretase as a potential target for AD therapy and its role in regulated intramembrane proteolysis are discussed.

Alzheimer's disease: treatments in discovery and development

Alzheimer's disease is the single biggest unmet medical need in neurology. Current drugs are safe, but of limited benefit to most patients. This review discusses the scientific basis and current status of promising disease-modifying therapies in the discovery and development stages. I describe the major targets of anti-amyloid therapy and the main focus of disease modification approaches. In addition, two new potential treatment approaches supported by retrospective epidemiology are outlined.

Beta-secretase as a target for the treatment of Alzheimer's disease

Finding inhibitors of the first step of the amyloid cascade, A beta (42) generation, is a major goal of Alzheimer's disease drug development. Two target protease activities, beta-and gamma-secretase, were detected more than 10 years ago but progress in this area has been slow because the enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as beta-secretase. The enzyme has been characterized in detail. The X-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Our recent knockout studies show that BACE1 is critical for A beta generation, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism based toxicity. Target-mediated toxicity of beta-secretase inhibition cannot be ruled out, however, as long as the major substrates of this enzyme are unknown. Although various peptidic beta-secretase inhibitors have been published, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes. The focus of this review is progress in the beta-secretase field from the identification of the enzyme in 1999 to the most recent publications.

Abeta as a bioflocculant: implications for the amyloid hypothesis of Alzheimer's disease

Research into Alzheimer's disease (AD) has been guided by the view that deposits of fibrillar amyloid-beta peptide (Abeta) are neurotoxic and are largely responsible for the neurodegeneration that accompanies the disease. This 'amyloid hypothesis' has claimed support from a wide range of molecular, genetic and animal studies. We critically review these observations and highlight inconsistencies between the predictions of the amyloid hypothesis and the published data. We show that the data provide equal support for a 'bioflocculant hypothesis', which posits that Abeta is normally produced to bind neurotoxic solutes (such as metal ions), while the precipitation of Abeta into plaques may be an efficient means of presenting these toxins to phagocytes. We conclude that if the deposition of Abeta represents a physiological response to injury then therapeutic treatments aimed at reducing the availability of Abeta may hasten the disease process and associated cognitive decline in AD.

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.

Enzymic properties of recombinant BACE2

BACE2 (Memapsin 1) is a membrane-bound aspartic protease that is highly homologous with BACE1 (Memapsin 2). While BACE1 processes the amyloid precursor protein (APP) at a key step in generating the beta-amyloid peptide and presumably causes Alzheimer's disease (AD), BACE2 has not been demonstrated to be directly involved in APP processing, and its physiological functions remain to be determined. In vivo, BACE2 is expressed as a precursor protein containing pre-, pro-, protease, transmembrane, and cytosolic domains/peptides. To determine the enzymatic properties of BACE2, two variants of its pro-protease domain, pro-BACE2-T1 (PB2-T1) and pro-BACE2-T2 (PB2-T2), were constructed. They have been expressed in Escherichia coli as inclusion bodies, refolded and purified. These two recombinant proteins have the same N terminus but differ at their C-terminal ends: PB2-T1 ends at Pro466, on the boundary of the postulated transmembrane domain, and PB2-T2 ends at Ser431, close to the homologous ends of other aspartic proteases such as pepsin. While PB2-T1 shares similar substrate specificities with BACE1 and other 'general' aspartic proteases, the specificity of PB2-T2 is more constrained, apparently preferring to cleave at the NH2-terminal side of paired basic residues. Unlike other 'typical' aspartic proteases, which are active only under acidic conditions, the recombinant BACE2, PB2-T1, was active at a broad pH range. In addition, pro-BACE2 can be processed at its in vivo maturation site by BACE1.

Emerging Alzheimer's disease therapies: inhibition of beta-secretase

Among the approaches towards disease modifying treatment of Alzheimer's disease blocking the initial step of the amyloid cascade, Abeta42 generation, has received most attention. Abeta42 generation requires two proteases, beta- and gamma-secretase, and inhibition of these enzymes is a key focus of AD drug development. Progress in this area has been slow, because these enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as beta-secretase. The enzyme has been characterized in detail. The x-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Our recent knockout studies show that BACE1 is critical for Abeta generation, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism based toxicity. However, target-mediated toxicity of beta-secretase inhibition cannot be ruled out, as long as the major substrates of this enzyme are unknown. While various peptidic beta-secretase inhibitors have been published, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes.

Therapeutic strategies for Alzheimer's disease

Alzheimer's disease is a progressive and ultimately fatal neurological disorder for which there is no effective treatment at present. The disease is characterized pathologically by cerebral plaques that contain the amyloid-beta peptide and thread-like neuronal structures composed of the microtubule-associated protein TAU. Both amyloid-beta and TAU are thought to be crucial to pathogenesis, but compelling evidence supports amyloid-beta as the 'prime mover'. The main efforts for developing therapeutics are therefore focused on preventing amyloid-beta production, aggregation or downstream neurotoxic events. The progress of these and other approaches raises the hope that effective agents for the prevention and treatment of Alzheimer's disease will be available in the near future.

Gamma-secretase: substrates and inhibitors

The amyloid beta-protein (Abeta) deposited in Alzheimer's disease (AD), the most common form of dementia in the elderly, is a secreted proteolytic product of the amyloid beta-protein precursor (APP). Generation of Abeta from the APP requires two sequential proteolytic events, beta-secretase cleavage to generate the amino terminus, followed by gamma-secretase cleavage to generate the carboxyl terminus. Because this process is a central event in the pathogenesis of AD, gamma-secretase is believed to be an excellent therapeutic target. Gamma-secretase activity has been demonstrated to be membrane-associated, with the cleavage site primarily determined by the location of the substrate with respect to the membrane. It has also been shown that this unusual proteolytic activity not only occurs for APP, but also for proteins involved in morphogenic processes or cell proliferation and differentiation such as Notch and ErbB4. Thus far, all gamma-secretase substrates are involved in some form of nuclear signaling. These recent findings have important implications for the development of pharmacological interventions that target gamma-secretase.

Continuing strategies for inhibiting Alzheimer's gamma-secretase

Gamma-secretase processing of the amyloid-beta precursor protein (APP) releases the amyloid-beta peptide, which is widely held to be involved in the pathogenesis of Alzheimer's disease. This protease is apparently a complex of integral membrane proteins that includes the multi-pass presenilin. Transition-state analogue inhibitors of gamma-secretase are important molecular probes of the enzyme active site. We have identified new transition-state analogues, (hydroxyethy) urea peptidomimetics, that inhibit gamma-secretase activity at submicromolar concentrations in cell culture. The inhibitory activity of a family of such compounds provided further support that gamma-secretase has loose sequence specificity at the active site, and one of these compounds allowed partial purification of the protease complex. In addition, because the site of gamma-secretase cleavage of APP lies within its single transmembrane domain, we designed short peptides based on this domain which assume a helical conformation. These peptides inhibited gamma-secretase in the low micromolar range in cell culture, suggesting that they indeed mimick the APP substrate conformation.

Lipid rafts play an important role in A beta biogenesis by regulating the beta-secretase pathway

The Alzheimer's amyloid beta protein (A beta) precursor (APP) is proteolytically cleaved by beta-secretase to N- and C-terminal fragments sAPPbeta and CTFbeta, respectively. Subsequently, CTFbeta is cleaved by gamma-secretase to generate A beta. We previously showed that the levels of secreted A beta and sAPPbeta were significantly reduced upon removal of glycosylphosphatidylinositol (GPI)-anchored proteins from either primary brain cells or Chinese hamster ovary cultures. The results indicated that GPI-anchored proteins facilitated beta-secretase activity. In this report, we strengthen the previous findings by demonstrating that CTFbeta, like sAPPbeta, is also reduced upon removal of GPI-anchored proteins and that sAPPbeta does not accumulate in an intracellular compartment. This facilitation pathway does not appear to be important for the processing of a disease-linked mutant form of APP (670NL), known to be a superior beta-secretase substrate. A novel aspartyl protease, BACE, responsible for beta-secretase activity in the brain is not GPI-anchored. However, BACE in brain membranes accumulate in lipid rafts, a compartment marked by the accumulation of GPI-anchored proteins. This finding is consistent with the hypothesis that BACE interacts with GPI-anchored proteins that facilitate its activity possibly by chaperoning it into lipid rafts.

Novel therapeutic strategies provide the real test for the amyloid hypothesis of Alzheimer's disease

The amyloid and tangle cascade hypothesis is the dominant explanation for the pathogenesis of Alzheimer's disease (AD). A complete knowledge of the metabolic pathways leading to beta-amyloid (A beta) production and clearance in vivo and of the pathological events that lead to fibril formation and deposition into plaques is crucial for the development of an 'anti-amyloid' therapeutic strategy. Important advances in this respect have been achieved recently, revealing new candidate drug targets. Among the most promising potential treatments are beta- and gamma-secretase inhibitors, A beta vaccination, Cu-Zn chelators, cholesterol-lowering drugs and non-steroidal anti-inflammatory drugs. Now, the major question is whether these drugs will work in the clinic.

Biogenesis and metabolism of Alzheimer's disease Abeta amyloid peptides

Biochemical and genetic evidence indicates the balance of biogenesis/clearance of Abeta amyloid peptides is altered in Alzheimer's disease. Abeta is derived, by two sequential cleavages, from the receptor-like amyloid precursor protein (APP). The proteases involved are beta-secretase, identified as the novel aspartyl protease BACE, and gamma-secretase, a multimeric complex containing the presenilins (PS). Gamma-secretase can release either Abeta40 or the more aggregating and cytotoxic Abeta42. Secreted Abeta peptides become either degraded by the metalloproteases insulin-degrading enzyme (IDE) and neprilysin or metabolized through receptor uptake mediated by apolipoprotein E. Therapeutic approaches based on secretase inhibition or amyloid clearance are currently under development.

Advances in the cellular and molecular biology of the beta-amyloid protein in Alzheimer's disease

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as "secretases" participate in APP processing. An enzymatic activity, beta-secretase, cleaves APP on the amino side of Abeta producing a large secreted derivative, sAPPbeta, and an Abeta-bearing membrane-associated C-terminal derivative, CTFbeta, which is subsequently cleaved by the second activity, gamma-secretase, to release Abeta. Alternatively, a third activity, alpha-secretase, cleaves APP within Abeta to the secreted derivative sAPPalpha and membrane-associated CTFalpha. The predominant secreted APP derivative is sAPPalpha in most cell-types. Most of the secreted Abeta is 40 residues long (Abeta40) although a small percentage is 42 residues in length (Abeta42). However, the longer Abeta42 aggregates more readily and was therefore considered to be the pathologically important form. Advances in our understanding of APP processing, trafficking, and turnover will pave the way for better drug discovery for the eventual treatment of AD. In addition, APP gene regulation and its interaction with other proteins may provide useful drug targets for AD. The emerging knowledge related to the normal function of APP will help in determining whether or not the AD associated changes in APP metabolism affect its function. The present review summarizes our current understanding of APP metabolism and function and their relationship to other proteins involved in AD.

Potential novel targets for Alzheimer pharmacotherapy: I. Secretases

The prevailing major theory of Alzheimer's disease (AD) is that insoluble amyloid beta-peptide (Abeta) found in the cerebral plaques characteristic of the disease is causative or is at least a contributing factor. According to this theory, inhibition of aberrant Abeta production should prevent or at least limit the extent of AD pathophysiology. As three 'secretase' enzymes (alpha, beta and gamma) catalyse the proteolytic cleavage of amyloid precursor protein (APP) (the precursor protein of Abeta), one or more secretases have become targets for potential novel AD pharmacotherapy. Secretase inhibitors have been designed and are in various stages of development. The clinical trials of these compounds will, if positive, result in drugs with dramatically better clinical efficacy or, if negative, will force a reassessment of the theory about the role of Abeta in AD.

A non-amyloidogenic function of BACE-2 in the secretory pathway

beta-Site amyloid precursor protein cleavage enzyme (BACE)-1 and BACE-2 are members of a novel family of membrane-bound aspartyl proteases. While BACE-1 is known to cleave beta-amyloid precursor protein (betaAPP) at the beta-secretase site and to be required for the generation of amyloid beta-peptide (Abeta), the role of its homologue BACE-2 in amyloidogenesis is less clear. We now demonstrate that BACE-1 and BACE-2 have distinct specificities in cleavage of betaAPP in cultured cells. Radiosequencing of the membrane-bound C-terminal cleavage product revealed that BACE-2 cleaves betaAPP in the middle of the Abeta domain between phenylalanines 19 and 20, resulting in increased secretion of APPs-alpha- and p3-like products and reduced production of Abeta species. This cleavage can occur in the Golgi and later secretory compartments. We also demonstrate that BACE-1-mediated cleavage of betaAPP at Asp1 of the Abeta domain can occur as early as in the endoplasmic reticulum, while cleavage at Glu11 occurs in later compartments. These data indicate that the distinct specificities of BACE-1 and BACE-2 in their cleavage of betaAPP differentially affect the generation of Abeta.

Physiopathological modulators of amyloid aggregation and novel pharmacological approaches in Alzheimer's disease

The biological mechanisms underlying the neuropathology of Alzheimer's disease (AD) are complex, as several factors likely contribute to the development of the disease. Therefore, it is not surprising that a number of different possible therapeutic approaches addressing distinct aspects of this disease are currently being investigated. Among these are ways to prevent amyloid aggregation and/or deposition, to prevent neuronal degeneration, and to increase brain neurotransmitter levels. Here, we discuss possible roles of endogenous modulators of Abeta aggregation in the physiopathology of AD and some of the strategies currently under consideration to interfere with brain levels of beta-amyloid, its aggregation and neurotoxicity.

Increased expression of the amyloid precursor beta-secretase in Alzheimer's disease

Beta-secretase cleavage represents the first step in the generation of Abeta polypeptides and initiates the amyloid cascade that leads to neurodegeneration in Alzheimer's disease. By comparative Western blot analysis, we show a 2.7-fold increase in protein expression of the beta-secretase enzyme BACE in the brain cortex of Alzheimer's disease patients as compared to age-matched controls. Similarly, we found the levels of the amyloid precursor protein C-terminal fragment produced by beta-secretase to be increased by nearly twofold in Alzheimer's disease cortex.

BACE1 interacts with nicastrin

Beta-amyloid peptide (Abeta) is generated through the proteolytic cleavage of beta-amyloid precursor protein (APP) by beta- and gamma-secretases. The beta-secretase, BACE1, initiates Abeta formation followed by gamma-cleavage within the APP transmembrane domain. Although BACE1 localizes in the transGolgi network (TGN), its physiological substrates and modulators are not known. In addition, the relationship to other secretase(s) also remains unidentified. Here, we demonstrate that BACE1 binds to nicastrin, a component of gamma-secretase complexes, in vitro, and that nicastrin activates beta-secretase activity in COS-7 cells.

Beta-secretase processing in the trans-Golgi network preferentially generates truncated amyloid species that accumulate in Alzheimer's disease brain

The amyloid beta (A beta) peptide that accumulates in Alzheimer's disease brain is derived from the proteolytic processing of the amyloid precursor protein by beta- and gamma-secretase activities. The beta-secretase enzyme beta-site amyloid precursor protein-cleaving enzyme (BACE) generates the N terminus of A beta by cleavage at either Asp(1) (beta-site) or Glu(11) (beta'-site), ultimately leading to the production of full-length A beta 1-40/42 or truncated A beta 11-40/42. The functional significance of this variable cleavage site specificity as well as the relative pathological impact of full-length versus N-terminally truncated A beta remains largely unknown. In our analysis of BACE reactivity in cell culture, we found that the preference of the protease for either beta- or beta'-cleavage was strongly dependent on intracellular localization. Within the endoplasmic reticulum, beta-site proteolysis predominated, whereas in the trans-Golgi network, beta'-cleavage was favored. Furthermore, the contrasting cleavage site specificities of BACE were not simply due to differences in organelle pH or the oligosaccharide composition of the glycoproteins involved. Examination of post-mortem brain specimens revealed significant levels of A beta 11-40/42 within insoluble amyloid pools. Taken together, these data support an important role for beta'-cleavage in the process of cerebral amyloid deposition and localize the processing event to the trans-Golgi network.

Localization of beta-secretase memapsin 2 in the brain of Alzheimer's patients and normal aged controls

Chronic accumulation of beta-amyloid in the brain has been shown to result in complex molecular and cellular changes that accompany neurodegeneration in Alzheimer's disease (AD). In this study, we examined the expression of a newly identified beta-secretase, memapsin 2 (M2) or beta-site APP cleaving enzyme in deparaffinized sections from 10 AD patients and 10 aged matched controls and in frozen samples of parietal cortex from 11 AD and 8 controls. M2 is mainly expressed in neurons, with high levels in CA4 to CA2 regions and transentorhinal cortex and low or intermediate levels in CA1, subiculum, and granule cells of the dentate gyrus. The majority of AD brains showed an increase of M2 expression in the CA1, but a decrease in the transentorhinal cortex. A subset of controls and AD patients had high M2 expression in parietal neocortex. Double-staining revealed that senile plaques are not directly associated with the soma of M2-expressing neurons. Neurofibrillary tangles were associated with lower M2 expression in AD. These data indicate that beta-secretase M2 may not be straightforwardly involved in amyloid plaque formation in AD brain.

Splice variants of the beta-site APP-cleaving enzyme BACE1 in human brain and pancreas

BACE is the beta-secretase responsible for the first step in amyloidogenic processing of the amyloid precursor protein APP. We have identified two BACE isoforms, BACE1B and BACE1C, lacking 25 and 44 amino acids, respectively. Whereas the BACE1B transcript is present in human pancreas and brain, the BACE1C transcript is found in pancreas only. In transfected cells both BACE1A, which encodes the originally described full-length BACE1 protein and the close homolog BACE2 localized mainly to post-Golgi membranes. In contrast, the two shorter isoforms were found in the endoplasmic reticulum only, and they did not display beta-secretase activity. Using RNase protection we in addition show that the major pancreatic transcript is BACE1A. This suggests that the known absence of beta-secretase activity in the pancreas is not due to a missing BACE1A transcript.

Mouse models of Alzheimer's disease: a quest for plaques and tangles

Many genetically altered mice have been designed to help understand the role of specific gene mutations in the pathogenesis of Alzheimer's disease (AD) based on the realization that specific mutations in the genes for amyloid precursor protein--the presenilins and tau--are associated with early-onset familial AD or, in the case of tau mutations, other neurodegenerative diseases with neurofibrillary tangles. However, attempts to reproduce the neuropathology of AD in the mouse have been frustrating. Transgenic designs emphasizing amyloid precursor protein produced mice that develop amyloid plaques, but neurodegeneration and neurofibrillary tangles failed to form. Strategies emphasizing tau resulted in increased phosphorylation of tau and tangle formation, although amyloid plaques were absent. Nevertheless, crossing transgenic animals expressing mutated tau and amyloid precursor protein has produced a mouse that closely recapitulates the neuropathology of AD. A review of the various murine models, their role in understanding the pathogenesis of AD and their use in testing therapeutic regimens, is provided.

Beta-amyloid peptide in regulated secretory vesicles of chromaffin cells: evidence for multiple cysteine proteolytic activities in distinct pathways for beta-secretase activity in chromaffin vesicles

A key factor in Alzheimer's disease (AD) is the beta-secretase activity that is required for the production of beta-amyloid (Abeta) peptide from its amyloid precursor protein (APP) precursor. In this study, the majority of Abeta secretion from neuronal chromaffin cells was found to occur via the regulated secretory pathway, compared with the constitutive secretory pathway; therefore, beta-secretase activity in the regulated secretory pathway was examined for the production and secretion of Abeta in chromaffin cells obtained from in vivo adrenal medullary tissue. The presence of Abeta(1-40) in APP-containing chromaffin vesicles, which represent regulated secretory vesicles, was demonstrated by radioimmunoassay (RIA) and reverse-phase high-performance liquid chromatography. These vesicles also contain Abeta(1-42), measured by RIA. Significantly, regulated secretion of Abeta(1-40) from chromaffin cells represented the majority of secreted Abeta (> 95% of total secreted Abeta), compared with low levels of constitutively secreted Abeta(1-40). These results indicate the importance of Abeta production and secretion in the regulated secretory pathway as a major source of extracellular Abeta. Beta-secretase activity in isolated chromaffin vesicles was detected with the substrate Z-Val-Lys-Met-/MCA (methylcoumarinamide) that contains the beta-secretase cleavage site. Optimum beta-secretase activity in these vesicles required reducing conditions and acidic pH (pH 5-6), consistent with the in vivo intravesicular environment. Evidence for cysteine protease activity was shown by E64c inhibition of Z-Val-Lys-Met-MCA-cleaving activity, and E64c inhibition of Abeta(1-40) production in isolated chromaffin vesicles. Chromatography resolved the beta-secretase activity into two distinct proteolytic pathways consisting of: (i) direct cleavage of the beta-secretase site at Met-/Asp by two cysteine proteolytic activities represented by peaks Il-A and Il-B, and (ii) an aminopeptidase-dependent pathway represented by peak I cysteine protease activity that cleaves between Lys-/Met, followed by Met-aminopeptidase that would generate the beta-secretase cleavage site. Treatment of chromaffin cells in primary culture with the cysteine protease inhibitor E64d reduced the production of the beta-secretase product, a 12-14 kDa C-terminal APP fragment. In addition, BACE 1 and BACE 2 were detected in chromaffin vesicles; BACE 1 represented a small fraction of total beta-secretase activity in these vesicles. These results illustrate that multiple cysteine proteases, in combination with BACE 1, contribute to beta-secretase activity in the regulated secretory pathway. These results complement earlier findings for BACE 1 as beta3-secretase for Abeta production in the constitutive secretory pathway that provides basal secretion of Abeta into conditioned media. These findings suggest that drug inhibition of several proteases may be required for reducing Abeta levels as a potential therapeutic approach for AD.

Substrate and inhibitor profile of BACE (beta-secretase) and comparison with other mammalian aspartic proteases

The full-length and ectodomain forms of beta-site APP cleavage enzyme (BACE) have been cloned, expressed in Sf9 cells, and purified to homogeneity. This aspartic protease cleaves the amyloid precursor protein at the beta-secretase site, a critical step in the Alzheimer's disease pathogenesis. Comparison of BACE to other aspartic proteases such as cathepsin D and E, napsin A, pepsin, and renin revealed little similarity with respect to the substrate preference and inhibitor profile. On the other hand, these parameters are all very similar for the homologous enzyme BACE2. Based on a collection of decameric substrates, it was found that BACE has a loose substrate specificity and that the substrate recognition site in BACE extends over several amino acids. In common with the aspartic proteases mentioned above, BACE prefers a leucine residue at position P1. Unlike cathepsin D etc., BACE accepts polar or acidic residues at positions P2'0 and P1 but prefers bulky hydrophobic residues at position P3. BACE displays poor kinetic constants toward its known substrates (wild-type substrate, SEVKM/DAEFR, K(m) = 7 microm, K(cat) = 0.002 s(-1); Swedish mutant, SEVNL/DAEFR, K(m) = 9 microm, K(cat) = 0.02 s(-1)). A new substrate (VVEVDA/AVTP, K(m) = 1 microm, K(cat) = 0.004) was identified by serendipity.

It's a knockout! Male infertility and neuropathology

An increasing number of genetically engineered animals are produced worldwide for use in both basic and applied research. Here, I provide an update of some of the latest mouse knockouts in The Jackson Laboratory Transgenic/Targeted Mutation Database (TBASE), concentrating on those associated with male infertility and neuropathology.

Secretases as targets for drug design in Alzheimer's disease

Alzheimer's disease accounts for the majority of dementia in the elderly. Worldwide, approximately 20 million people are suffering from this devastating disease, with no effective treatment currently available. For efficient drug design, it is important to identify the molecular mechanisms underlying the pathology of the disease. An invariant feature in the pathology of Alzheimer's disease is the amyloid-beta peptide. Amyloid-beta is produced by endoproteolytic cleavage of the amyloid precursor protein by beta- and gamma-secretase. In the past 2 years, the protein responsible for beta-secretase activity has been isolated and researchers are close to identifying gamma-secretase. These recent achievements in Alzheimer's disease research have provided helpful tools for the development of therapeutics.

Preclinical and clinical challenges in the development of disease-modifying therapies for Alzheimer's disease

The neurodegenerative disease first described almost 100 years ago by Alois Alzheimer is predicted to be one of the major health problems of the 21st century. Alzheimer's disease (AD) is a progressive dementia characterized by global cognitive decline and is defined pathologically by amyloid plaques and neurofibrillary tangles. Major unmet medical need has encouraged pharmaceutical companies to invest in AD drug development. Promising novel approaches are under way, assisted by recent advances in animal models and an increased understanding of pathophysiology. However, demonstration of disease modification and identification of at-risk individuals are among the significant challenges facing those working in AD drug development.

Alzheimer's beta-secretase, beta-site amyloid precursor protein-cleaving enzyme, is responsible for cleavage secretion of a Golgi-resident sialyltransferase

The deposition of amyloid beta-peptide (A beta) in the brain is closely associated with the development of Alzheimer's disease. A beta is generated from the amyloid precursor protein (APP) by sequential action of beta-secretase (BACE1) and gamma-secretase. Although BACE1 is distributed among various other tissues, its physiological substrates other than APP have yet to be identified. ST6Gal I is a sialyltransferase that produces a sialyl alpha 2,6galactose residue, and the enzyme is secreted out of the cell after proteolytic cleavage. We report here that BACE1 is involved in the proteolytic cleavage of ST6Gal I, on the basis of the following observations. ST6Gal I was colocalized with BACE1 in the Golgi apparatus by immunofluorescence microscopy, suggesting that BACE1 acts on ST6Gal I within the same intracellular compartment. When BACE1 was overexpressed with ST6Gal I in COS cells, the secretion of ST6Gal I markedly increased. When APP(SW) (Swedish familial Alzheimer's disease mutation), a preferable substrate for BACE1, was coexpressed with ST6Gal I in COS cells, the secretion of ST6Gal I significantly decreased, suggesting that that the beta-cleavage of overexpressed APP(SW) competes with ST6Gal I processing. In addition, BACE1-Fc (Fc, the hinge and constant region of IgG) chimera cleaved protein A-ST6Gal I fusion protein in vitro. Thus, we conclude that BACE1 is responsible for the cleavage and secretion of ST6Gal I.

Relationship between beta amyloid peptide generating molecules and neprilysin in Alzheimer disease and normal brain

beta-Amyloid peptide (Abeta) is generated by two cleavages of amyloid precursor protein (APP). The initial cleavage by BACE is followed by gamma-secretase cleavage of the C-terminal APP fragment. Presenilin-1 (PS-1) is intimately related to gamma-secretase. Once formed, Abeta is mainly broken down by neprilysin. To estimate vulnerability to Abeta senile plaque formation, we measured the relative mRNA levels of APP695, APP751, APP770, BACE, presenilin-1 (PS-1) and neprilysin in nine brain areas and in heart, liver, spleen and kidney in a series of Alzheimer disease (AD) and control cases. Each of the mRNAs was expressed in every tissue examined. APP695 was the dominant APP isoform in brain. Compared with controls, APP695 and PS-1 mRNA levels were significantly elevated in high plaque areas of AD brain, while neprilysin mRNA levels were significantly reduced. BACE levels were not significantly different in AD compared with control brain. In peripheral organs, there were no significant differences in any of the mRNAs between AD and control cases. APP isoforms were differently expressed in the periphery than in brain, with APP 751>770>695. Neprilysin mRNA levels were much higher, while APP695 and PS-1 mRNA levels were much lower in the periphery than in brain. The data suggest that, in the periphery, the capacity to degrade Abeta is srong, accounting for the failure of Abeta deposits to form. In plaque prone areas of AD brain, the capacity to degrade Abeta is weak, while the capacity to generate Ab is upregulated. In plaque resistant areas of brain, a closer balance exists, but there is some tendency towards lower degrading and higher synthesizing capacity in AD brain compared with control brain. Overall, the data indicate that effectiveness of degradation by neprilysin may be a key factor in determining whether Abeta deposits develop.

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.

The transmembrane domain of the Alzheimer's beta-secretase (BACE1) determines its late Golgi localization and access to beta -amyloid precursor protein (APP) substrate

Release of Abeta peptides from beta-amyloid precursor protein (APP) requires sequential cleavage by two endopeptidases, beta- and gamma-secretases. beta-Secretase was recently identified as a novel membrane-bound aspartyl protease, named BACE1, Asp2, or memapsin 2. Employing confocal microscopy and subcellular fractionation, we have found that BACE1 is largely situated in the distal Golgi membrane with a minor presence in the endoplasmic reticulum, endosomes, and plasma membrane in human neuroblastoma SHEP cells and in mouse Neuro-2a cell lines expressing either endogenous mouse BACE1 or additional exogenous human BACE1. The major cellular beta-secretase activity is located in the late Golgi apparatus, consistent with its cellular localization. Furthermore, we demonstrate that the single transmembrane domain of BACE1 alone determines the retention of BACE1 to the Golgi compartments, through examination of recombinant proteins of various BACE1 fragments fused to a reporter green fluorescence protein. In addition, we show that the transmembrane domain of BACE1 is required for the access of BACE1 enzymatic activity to the cellular APP substrate and hence for the optimal generation of the C-terminal fragment of APP (CTF99). The results suggest a molecular and cell biological mechanism for the regulation of beta-secretase activity in vivo.

APP processing and synaptic plasticity in presenilin-1 conditional knockout mice

We have developed a presenilin-1 (PS1) conditional knockout mouse (cKO), in which PS1 inactivation is restricted to the postnatal forebrain. The PS1 cKO mouse is viable and exhibits no gross abnormalities. The carboxy-terminal fragments of the amyloid precursor protein differentially accumulate in the cerebral cortex of cKO mice, while generation of beta-amyloid peptides is reduced. Expression of Notch downstream effector genes, Hes1, Hes5, and Dll1, is unaffected in the cKO cortex. Although basal synaptic transmission, long-term potentiation, and long-term depression at hippocampal area CA1 synapses are normal, the PS1 cKO mice exhibit subtle but significant deficits in long-term spatial memory. These results demonstrate that inactivation of PS1 function in the adult cerebral cortex leads to reduced Abeta generation and subtle cognitive deficits without affecting expression of Notch downstream genes.

BACE2 functions as an alternative alpha-secretase in cells

BACE1 and BACE2 define a new subfamily of membrane-anchored aspartyl proteases. Both endoproteases share similar structural organization including a prodomain, a catalytic domain formed via DTG and DSG active site motifs, a single transmembrane domain, and a short C-terminal tail. BACE1 has been identified as the Alzheimer's beta-secretase, whereas BACE2 was mapped to the Down's critical region of human chromosome 21. Herein we show that purified BACE2 can be autoactivated in vitro. Purified BACE2 cleaves human amyloid precursor protein (APP) sequences at the beta-secretase site, and near the alpha-secretase site, mainly at A beta-Phe(20)--Ala(21) and also at A beta-Phe(19)--Phe(20). Alternatively, in cells BACE2 has a limited effect on the beta-secretase site but efficiently cleaves the sequences near the alpha-secretase site. The in vitro specificity of APP processing by BACE2 is distinct from that observed in cells. BACE2 localizes in the endoplasmic reticulum, Golgi, trans-Golgi network, endosomes, and plasma membrane, and its cellular localization patterns depend on the presence of its transmembrane domain. BACE2 chimeras that increase localization of BACE2 in the trans-Golgi network do not change its APP processing patterns. Thus, BACE2 can be distinguished from BACE1 on the basis of autoprocessing of the prosegment, APP processing specificity, and subcellular localization patterns.

A portrait of Alzheimer secretases--new features and familiar faces

The amyloid beta-peptide (Abeta) is a principal component of the cerebral plaques found in the brains of patients with Alzeheimer's disease (AD). This insoluble 40- to 42-amino acid peptide is formed by the cleavage of the Abeta precursor protein (APP). The three proteases that cleave APP, alpha-, beta-, and gamma-secretases, have been implicated in the etiology of AD. beta-Secretase is a membrane-anchored protein with clear homology to soluble aspartyl proteases, and alpha-secretase displays characteristics of certain membrane-tethered metalloproteases. gamma-Secretase is apparently an oligomeric complex that includes the presenilins, which may be the catalytic component of this protease. Identification of the alpha-, beta-, and gamma-secretases provides potential targets for designing new drugs to treat AD.

The amyloidogenic pathway of amyloid precursor protein (APP) is independent of its cleavage by caspases

Amyloid beta-protein (A beta) is the main constituent of senile plaques in Alzheimer's disease and is derived by proteolysis from the amyloid precursor protein (APP). Generation and secretion of both A beta 40 and A beta 42 isoforms depend largely on internalization of APP and occurs mainly in the endocytic pathway. Evidence has also been presented (Gervais, F. G., Xu, D., Robertson, G. S., Vaillancourt, J. P., Zhu, Y., Huang, J., LeBlanc, A., Smith, D., Rigby, M., Shearman, M. S., Clarke, E. E., Zheng, H., Van der Ploeg, L. H. T., Ruffolo, S. C., Thornberry, N. A., Xanthoudakis, S., Zamboni, R. J., Roy, S., and Nicholson, D. W. (1999) Cell, 97, 395--406) that caspase cleavage of APP at its cytosolic tail affects its processing such that it is redirected to a more amyloidogenic pathway, resulting in enhanced A beta generation. However, caspase cleavage of APP also results in loss of its endocytosis signal (YENP), an event that would predict a decline in internalization and a concomitant decrease, not an increase, in A beta generation. In the present work, we examined whether caspase cleavage of APP is relevant to amyloidogenesis. We found that 1) caspase cleavage of APP results in reduced internalization and, accordingly, a decline in A beta secretion; 2) masking of the caspase site in APP did not affect A beta levels and, 3) caspase activation in cells by serum withdrawal did not increase A beta secretion. Thus, caspase cleavage of APP is unlikely to play a direct role in amyloidogenesis.

From symptomatic treatments to causative therapy?

The search for genes that predispose individuals to develop common chronic diseases such as asthma, diabetes and Alzheimer's promises to give insights into their molecular pathogenesis. This will lead to the development of therapies that modulate the pathology, rather than the physiology of these diseases. As academia and the pharmaceutical industry increasingly focus on this challenge, the genetic dissection of Alzheimer's is spearheading attempts to shift the therapeutic paradigm away from symptomatic to curative treatments.

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.