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.     

Oral administration of a potent and selective non-peptidic BACE-1 inhibitor decreases beta-cleavage of amyloid precursor protein and amyloid-beta production in vivo

Generation and deposition of the amyloid beta (Abeta) peptide following proteolytic processing of the amyloid precursor protein (APP) by BACE-1 and gamma-secretase is central to the aetiology of Alzheimer's disease. Consequently, inhibition of BACE-1, a rate-limiting enzyme in the production of Abeta, is an attractive therapeutic approach for the treatment of Alzheimer's disease. We have designed a selective non-peptidic BACE-1 inhibitor, GSK188909, that potently inhibits beta-cleavage of APP and reduces levels of secreted and intracellular Abeta in SHSY5Y cells expressing APP. In addition, we demonstrate that this compound can effectively lower brain Abeta in vivo. In APP transgenic mice, acute oral administration of GSK188909 in the presence of a p-glycoprotein inhibitor to markedly enhance the exposure of GSK188909 in the brain decreases beta-cleavage of APP and results in a significant reduction in the level of Abeta40 and Abeta42 in the brain. Encouragingly, subchronic dosing of GSK188909 in the absence of a p-glycoprotein inhibitor also lowers brain Abeta. This pivotal first report of central Abeta lowering, following oral administration of a BACE-1 inhibitor, supports the development of BACE-1 inhibitors for the treatment of Alzheimer's disease.     

BACE1 inhibition reduces endogenous Abeta and alters APP processing in wild-type mice

Accumulation of amyloid beta peptide (Abeta) in brain is a hallmark of Alzheimer's disease (AD). Inhibition of beta-site amyloid precursor protein (APP)-cleaving enzyme-1 (BACE1), the enzyme that initiates Abeta production, and other Abeta-lowering strategies are commonly tested in transgenic mice overexpressing mutant APP. However, sporadic AD cases, which represent the majority of AD patients, are free from the mutation and do not necessarily have overproduction of APP. In addition, the commonly used Swedish mutant APP alters APP cleavage. Therefore, testing Abeta-lowering strategies in transgenic mice may not be optimal. In this study, we investigated the impact of BACE1 inhibition in non-transgenic mice with physiologically relevant APP expression. Existing Abeta ELISAs are either relatively insensitive to mouse Abeta or not specific to full-length Abeta. A newly developed ELISA detected a significant reduction of full-length soluble Abeta 1-40 in mice with the BACE1 homozygous gene deletion or BACE1 inhibitor treatment, while the level of x-40 Abeta was moderately reduced due to detection of non-full-length Abeta and compensatory activation of alpha-secretase. These results confirmed the feasibility of Abeta reduction through BACE1 inhibition under physiological conditions. Studies using our new ELISA in non-transgenic mice provide more accurate evaluation of Abeta-reducing strategies than was previously feasible.     

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

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

Pathogenic A beta induces the expression and activation of matrix metalloproteinase-2 in human cerebrovascular smooth muscle cells

Cerebral amyloid angiopathy (CAA) is a major pathological feature of Alzheimer's disease and related disorders. Human cerebrovascular smooth muscle (HCSM) cells, which are intimately associated with CAA, have been used as an in vitro model system to investigate pathologic interactions with amyloid beta protein (A beta). Previously we have shown that pathogenic forms of A beta induce several pathologic responses in HCSM cells including fibril assembly at the cell surface, increase in the levels of A beta precursor, and apoptotic cell death. Here we show that pathogenic A beta stimulates the expression and activation of matrix metalloproteinase-2 (MMP-2). Furthermore, we demonstrate that the increase in MMP-2 activation is largely caused by increased expression of membrane type-1 (MT1)-MMP expression, the primary MMP-2 activator. Finally, treatment with MMP-2 inhibitors resulted in increased HCSM cell viability in the presence of pathogenic A beta. Our findings suggest that increased expression and activation of MMP-2 may contribute to HCSM cell death in response to pathogenic A beta. In addition, these activities may also contribute to loss of vessel wall integrity in CAA resulting in hemorrhagic stroke. Therefore, further understanding into the role of MMPs in HCSM cell degeneration may facilitate designing therapeutic strategies to treat CAA found in AD and related disorders.     

Expression of human FE65 in amyloid precursor protein transgenic mice is associated with a reduction in beta-amyloid load

FE65 is an adaptor protein that interacts with the cytoplasmic tail of the amyloid precursor protein (APP). In cultured non-neuronal cells, the formation of the FE65-APP complex is a key element for the modulation of APP processing, signalling and beta-amyloid (Abeta) production. The functions of FE65 in vivo, including its role in the metabolism of neuronal APP, remain to be investigated. In this study, transgenic mice expressing human FE65 were generated and crossbred with APP transgenic mice, known to develop Abeta deposits at 6 months of age. Compared with APP mice, APP/FE65 double transgenic mice exhibited a lower Abeta accumulation in the cerebral cortex as demonstrated by immunohistochemistry and immunoassay, and a lower level of APP-CTFs. The reduced accumulation of Abeta in APP/FE65 double transgenics, compared with APP mice, could be linked to the low Abeta42 level observed at 4 months of age and to the lower APP-CTFs levels. The present work provides evidence that FE65 plays a role in the regulation of APP processing in an in vivo model.     

The disulphide bonds in the catalytic domain of BACE are critical but not essential for amyloid precursor protein processing activity

beta-Site APP-cleaving enzyme (BACE) initiates the processing of the amyloid precursor protein (APP) leading to the generation of beta-amyloid, the main component of Alzheimer's disease senile plaques. BACE (Asp2, memapsin 2) is a type I transmembrane aspartic protease responsible for the beta-secretase cleavage of APP producing a soluble form of the ectodomain (sAPPbeta) and the membrane-bound, carboxy-terminal intermediates C99 and C89. BACE maturation involves cysteine bridge formation, N -glycosylation and propeptide removal. We investigated variants of BACE in which the disulphide bonds of the catalytic domain spanning between Cys216/Cys420, Cys278/Cys443 and Cys330/Cys380 were removed by mutagenesis. When transfected in cultured cells, these mutants showed impaired maturation. Nevertheless, a fraction of mutated protein retained both the competence to mature as well as the activity to process APP. For the generation of a functional enzyme the conserved Cys330/Cys380 bond was the most critical, whereas the two bonds between Cys216/Cys420 and Cys278/Cys443, which are typical for the membrane-bound BACE, appeared to be less important.     

APPepsilon, the epsilon-secretase-derived N-terminal product of the beta-amyloid precursor protein, behaves as a type I protein and undergoes alpha-, beta-, and gamma-secretase cleavages

beta-Amyloid peptide accumulates in the brain of patients affected by sporadic or familial forms of Alzheimer's disease. It derives from the proteolytic attacks of the beta-amyloid precursor protein (betaAPP) by beta- and gamma-secretase activities. An additional epsilon cleavage taking place a few residues C-terminal to the gamma-site has been reported, leading to the formation of an intracellular fragment referred to as APP intracellular domain C50. This epsilon cleavage received particular attention because it resembles the S3 Notch cleavage generating Notch intracellular domain. Indeed, APP intracellular domain, like its Notch counterpart, appears to mediate important physiological functions. gamma and epsilon cleavages on betaAPP appear spatio-temporally linked but pharmacologically distinct and discriminable by mutagenesis approaches. As these cleavages could be seen as either deleterious (gamma-site) or beneficial (epsilon-site), it appears of most interest to set up models aimed at studying these activities separately, particularly to design specific and bioavailable inhibitors. On the other hand, it is important to respect the topology of the substrates in order to examine physiologically relevant cleavages. Here we describe the obtention of cells overexpressing APPepsilon, the epsilon-secretase-derived N-terminal fragment of betaAPP. Interestingly, this N-terminal fragment of betaAPP was shown by biochemical and immunohistochemical approaches to behave as a genuine membrane-bound protein. APPepsilon undergoes constitutive and protein kinase C-regulated alpha-secretase cleavages. Furthermore, APPepsilon is targeted by the beta-secretase beta-site APP-cleaving enzyme and is subsequently cleaved by gamma-secretase. The resulting beta-amyloid peptide production is fully prevented by various gamma-secretase inhibitors. Altogether, our study shows that APPepsilon is a relevant betaAPP derivative to study gamma-secretase activities and to design specific inhibitors without facing any rate-limiting effect of epsilon-secretase-derived cleavage.     

Inhibition of amyloid beta-peptide production by blockage of beta-secretase cleavage site of amyloid precursor protein

Amyloid beta-peptide (Abeta) is implicated as the major causative agent in Alzheimer's disease (AD). Abeta is produced by the processing of the amyloid precursor protein (APP) by BACE1 (beta-secretase) and gamma-secretase. Many inhibitors have been developed for the secretases. However, the inhibitors will interfere with the processing of not only APP but also of other secretase substrates. In this study, we describe the development of inhibitors that prevent production of Abeta by specific binding to the beta-cleavage site of APP. We used the hydropathic complementarity (HC) approach for the design of short peptide inhibitors. Some of the HC peptides were bound to the substrate peptide (Sub W) corresponding to the beta-cleavage site of APP and blocked its cleavage by recombinant human BACE1 (rhBACE1) in vitro. In addition, HC peptides specifically inhibited the cleavage of Sub W, and not affecting other BACE1 substrates. Chemical modification allowed an HC peptide (CIQIHF) to inhibit the processing of APP as well as the production of Abeta in the treated cells. Such novel APP-specific inhibitors will provide opportunity for the development of drugs that can be used for the prevention and treatment of AD with minimal side effects.     

New pyrazolyl and thienyl aminohydantoins as potent BACE1 inhibitors: exploring the S2' region

The proteolytic enzyme β-secretase (BACE1) plays a central role in the synthesis of the pathogenic β-amyloid in Alzheimer's disease. SAR studies of the S2' region of the BACE1 ligand binding pocket with pyrazolyl and thienyl P2' side chains are reported. These analogs exhibit low nanomolar potency for BACE1, and demonstrate >50- to 100-fold selectivity for the structurally related aspartyl proteases BACE2 and cathepsin D. Small groups attached at the nitrogen of the P2' pyrazolyl moiety, together with the P3 pyrimidine nucleus projecting into the S3 region of the binding pocket, are critical components to ligand's potency and selectivity. P2' thiophene side chain analogs are highly potent BACE1 inhibitors with excellent selectivity against cathepsin D, but only modest selectivity against BACE2. The cell-based activity of these new analogs tracked well with their increased molecular binding with EC(50) values of 0.07-0.2 μM in the ELISA assay for the most potent analogs.     

Suppression of APP-containing vesicle trafficking and production of β-amyloid by AID/DHHC-12 protein

The metabolism of amyloid β-protein precursor (APP) is regulated by various cytoplasmic and/or membrane-associated proteins, some of which are involved in the regulation of intracellular membrane trafficking. We found that a protein containing Asp–His–His–Cys (DHHC) domain, alcadein and APP interacting DHHC protein (AID)/DHHC-12, strongly inhibited APP metabolism, including amyloid β-protein (Aβ) generation. In cells expressing AID/DHHC-12, APP was tethered in the Golgi, and APP-containing vesicles disappeared from the cytoplasm. Although DHHC domain-containing proteins are involved in protein palmitoylation, a AID/DHHC-12 mutant of which the enzyme activity was impaired by replacing the DHHC sequence with Ala–Ala–His–Ser (AAHS) made no detectable difference in the generation and trafficking of APP-containing vesicles in the cytoplasm or the metabolism of APP. Furthermore, the mutant AID/DHHC-12 significantly increased non-amyloidogenic α-cleavage of APP along with activation of a disintegrin and metalloproteinase 17, a major α-secretase, suggesting that protein palmitoylation involved in the regulation of α-secretase activity. AID/DHHC-12 can modify APP metabolism, including Aβ generation in multiple ways by regulating the generation and/or trafficking of APP-containing vesicles from the Golgi and their entry into the late secretary pathway in an enzymatic activity-independent manner, and the α-cleavage of APP in the enzymatic activity-dependent manner.     

Plaque-associated disruption of CSF and plasma amyloid-beta (Abeta) equilibrium in a mouse model of Alzheimer's disease

To better understand amyloid-beta (Abeta) metabolism in vivo, we assessed the concentration of Abeta in the CSF and plasma of APP(V717F) (PDAPP) transgenic mice, a model that develops age-dependent Alzheimer's disease (AD)-like pathology. In 3-month-old mice, prior to the development of Abeta deposition in the brain, there was a highly significant correlation between Abeta levels in CSF and plasma. In 9-month-old-mice, an age at which some but not all mice have developed Abeta deposition, there was also a significant correlation between CSF and plasma Abeta; however, the correlation was not as strong as that present in young mice. In further exploring CSF and plasma Abeta levels in 9-month-old mice, levels of CSF Abeta were found to correlate highly with Abeta burden. Analysis of the CSF: plasma Abeta ratio revealed a selective two-fold increase in plaque versus non-plaque bearing mice, strongly suggesting a plaque-mediated sequestration of soluble Abeta in brain. Interestingly, in 9-month-old mice, a significant correlation between CNS and plasma Abeta was limited to mice lacking Abeta deposition. These findings suggest that there is a dynamic equilibrium between CNS and plasma Abeta, and that plaques create a new equilibrium because soluble CNS Abeta not only enters the plasma but also deposits onto amyloid plaques in the CNS.     

Lipid peroxidation up-regulates BACE1 expression in vivo: a possible early event of amyloidogenesis in Alzheimer's disease

Increased lipid peroxidation is shown to be an early event of Alzheimer's disease (AD). However, it is not clear whether and how increased lipid peroxidation might lead to amyloidogenesis, a hallmark of AD. Glutathione peroxidase 4 (Gpx4) is an essential antioxidant defense enzyme that protects an organism against lipid peroxidation. Gpx4+/- mice show increased lipid peroxidation in brain, as evidenced by their elevated levels of 4-hydroxy-2-nonenal. To understand the role of lipid peroxidation in amyloidogenesis, we studied secretase activities in Gpx4+/- mice as a function of age. Both young (6 months) and middle-aged (17-20 months) Gpx4+/- mice had higher levels of beta-secretase activity than their age-matched wildtype controls, and the increased beta-secretase activity in Gpx4+/- mice was a result of up-regulation of beta-site amyloid precursor protein cleavage enzyme 1 (BACE1) expression at the protein level. The high level of BACE1 protein led to increased endogenous beta-amyloid (Abeta)(1-40) in middle-aged Gpx4+/- mice. We further studied amyloidogenesis in APPGpx4+/- mice. Our data indicate that APPGpx4+/- mice had significantly increased amyloid plaque burdens and increased Abeta(1-40) and Abeta(1-42) levels compared with APPGpx4+/+ mice. Therefore, our results indicate that increased lipid peroxidation leads to increased amyloidogenesis through up-regulation of BACE1 expression in vivo, a mechanism that may be important in pathogenesis of AD at early stages.     

Effect of tumor necrosis factor-alpha converting enzyme (TACE) and metalloprotease inhibitor on amyloid precursor protein metabolism in human neurons

Tumor necrosis factor-alpha (TNF-alpha) is implicated in inflammatory processes and much effort is being directed at inhibiting the release of TNF-alpha for treatment of inflammatory conditions. In this context, the drug CP-661,631 has been developed to inhibit the TNF-alpha converting enzyme (TACE). However, TACE is also implicated in amyloid precursor protein secretion. Amyloid precursor protein (APP) undergoes constitutive and regulated secretion by alpha-secretase endoproteolytic cleavage within the amyloid beta peptide (Abeta) domain. Alternative cleavage at the N- and C-terminus of the Abeta domain by beta- and gamma-secretases results in the production of Abeta. In many cellular and in vivo animal models, increased secretion of APP results in a concomitant decrease in the production of Abeta suggesting that the two pathways are intricately linked. However, in human primary neuron cultures, increased APP secretion is not associated with a decrease in total Abeta production. To determine if the use of CP-661,631 may enhance amyloidogenic processing in human brain, we have assessed the effect of CP-661,631 on APP metabolism in primary cultures of human neurons. Our results show that CP-661,631 effectively prevents regulated APP secretion but does not increase total Abeta levels in human primary neuron cultures.     

Regulation of beta-amyloid precursor protein expression by brain-derived neurotrophic factor involves activation of both the Ras and phosphatidylinositide 3-kinase signalling pathways

Brain-derived neurotrophic factor (BDNF) stimulates beta-amyloid precursor protein (APP) promoter activity by a Ras-dependent mechanism in TrkB-expressing SH-SY5Y cells. To determine the signalling pathways involved in the BDNF-induced response, we have analysed the ability of TrkB mutated forms to mediate promoter stimulation. Brain-derived neurotrophic factor causes a significant induction of promoter activity and mutation K540R in the active site of TrkB completely abolishes the neurotrophin-induced response. A substitution of the Y484 residue by phenylalanine, which blocks binding of Shc, reduces the activation of APP promoter by BDNF by approximately 50% whereas mutation Y785P, which blocks binding of phospholipase C gamma, does not affect the response. In addition, the phosphatidylinositide 3-kinase (PI3K)-specific inhibitors wortmannin and LY294002 reduced BDNF-induced activation. In agreement with a participation of both Ras/MAPK- and PI3K/Akt-mediated mechanisms, transient expression of constitutive active forms of Ras, PI3K and other components of both signalling pathways led to a significant increase of APP promoter activity. Furthermore, the stimulation of the APP promoter by BDNF was completely precluded by expression of dominant-negative forms of Ras and PI3K effectors. Taken together, our results suggest that simultaneous activation of at least two signalling pathways, Ras/MAPK and PI3K/Akt, is necessary to mediate a full activation of the APP promoter by BDNF.     

Incorporation of β-amyloid protein through the bovine ileal epithelium before and after weaning: Model for orally transmitted amyloidoses

To determine the mechanism of bovine intestinal incorporation of the pathogen, and the pathogenesis of prion protein in the early stage, cows suckling and weaning were orally given a fusion protein of Aβ-EGFP. Aβ-EGFP was incorporated through the villous columnar epithelial cells and accumulated in crypt patches in the ileum of suckling cows. The sites of the uptake and accumulation of Aβ-EGFP are very close to the peripheral nervous system; however, such uptake of Aβ-EGFP was not observed in 6-month-old post-weaning cows. The present study, therefore, suggests that the weaning period is very important for the risk of transmission.     

Neurotoxic traffic: uncovering the mechanics of amyloid production in Alzheimer's disease

Alzheimer's disease (AD) is thought by many to result from the accumulation of the neurotoxic amyloid-β (Aβ) peptide in brain parenchyma. The process by which Aβ is proteolytically derived from the larger amyloid precursor protein (APP) has been the focus of much attention in the AD research field over the past decade. Recently, several of the proteins directly involved in the generation of Aβ have been identified and characterized providing a number of viable therapeutic targets for the treatment of AD. However, the cellular mechanisms by which these proteins interact in the proteolytic processing of APP have not been well defined, nor are they readily apparent when one considers what is known about the intracellular localization and trafficking of the various participants. This article will review the underlying cell biology of Aβ production and discuss the mechanistic options for APP processing given the current knowledge of the proteases involved.     

Novel mutation in the human HPRT1 gene and the Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report a novel point mutation that led to HGprt-related neurological dysfunction (HND) in a family in which there was a missense mutation in exon 6 of the coding region of the HPRT1 gene: g.34938G>T, c.403G>T, p.D135Y. Molecular diagnosis is consistent with the genetic heterogeneity of the HPRT1 gene responsible for HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.