Functional gamma-secretase inhibitors reduce beta-amyloid peptide levels in brain

γ-Secretase Activity of Presenilin 1 Regulates Acetylcholine Muscarinic Receptor-mediated Signal Transduction

Familial Alzheimer's disease (FAD) presenilin 1 (PS1) mutations give enhanced calcium responses upon different stimuli, attenuated capacitative calcium entry, an increased sensitivity of cells to undergo apoptosis, and increased gamma-secretase activity. We previously showed that the FAD mutation causing an exon 9 deletion in PS1 results in enhanced basal phospholipase C (PLC) activity (Cedazo-Minguez, A., Popescu, B. O., Ankarcrona, M., Nishimura, T., and Cowburn, R. F. (2002) J. Biol. Chem. 277, 36646-36655). To further elucidate the mechanisms by which PS1 interferes with PLC-calcium signaling, we studied the effect of two other FAD PS1 mutants (M146V and L250S) and two dominant negative PS1 mutants (D257A and D385N) on basal and carbachol-stimulated phosphoinositide (PI) hydrolysis and intracellular calcium concentrations ([Ca2+]i) in SH-SY5Y neuroblastoma cells. We found a significant increase in basal PI hydrolysis in PS1 M146V cells but not in PS1 L250S cells. Both PS1 M146V and PS1 L250S cells showed a significant increase in carbachol-induced [Ca2+]i as compared with nontransfected or wild type PS1 transfected cells. The elevated carbachol-induced [Ca2+]i signals were reversed by the PLC inhibitor neomycin, the ryanodine receptor antagonist dantrolene, the general aspartyl protease inhibitor pepstatin A, and the specific gamma-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester. The cells expressing either PS1 D257A or PS1 D385N had attenuated carbachol-stimulated PI hydrolysis and [Ca2+]i responses. In nontransfected or PS1 wild type transfected cells, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester and pepstatin A also attenuated both carbachol-stimulated PI hydrolysis and [Ca2+]i responses to levels found in PS1 D257A or PS1 D385N dominant negative cells. Our findings suggest that PS1 can regulate PLC activity and that this function is gamma-secretase activity-dependent.

Intraneuronal amyloid-β1-42 production triggered by sustained increase of cytosolic calcium concentration induces neuronal death

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the presence in the brain of senile plaques which contain an amyloid core made of beta-amyloid peptide (Abeta). Abeta is produced by the cleavage of the amyloid precursor protein (APP). Since impairment of neuronal calcium signalling has been causally implicated in ageing and AD, we have investigated the influence of an influx of extracellular calcium on the metabolism of human APP in rat cortical neurones. We report that a high cytosolic calcium concentration, induced by neuronal depolarization, inhibits the alpha-secretase cleavage of APP and triggers the accumulation of intraneuronal C-terminal fragments produced by the beta-cleavage of the protein (CTFbeta). Increase in cytosolic calcium concentration specifically induces the production of large amounts of intraneuronal Abeta1-42, which is inhibited by nimodipine, a specific antagonist of l-type calcium channels. Moreover, calcium release from endoplasmic reticulum is not sufficient to induce the production of intraneuronal Abeta, which requires influx of extracellular calcium mediated by the capacitative calcium entry mechanism. Therefore, a sustained high concentration of cytosolic calcium is needed to induce the production of intraneuronal Abeta1-42 from human APP. Our results show that this accumulation of intraneuronal Abeta1-42 induces neuronal death, which is prevented by a functional gamma-secretase inhibitor.

Chronic treatment with the gamma-secretase inhibitor LY-411,575 inhibits beta-amyloid peptide production and alters lymphopoiesis and intestinal cell differentiation

Inhibition of gamma-secretase, one of the enzymes responsible for the cleavage of the amyloid precursor protein (APP) to produce the pathogenic beta-amyloid (Abeta) peptides, is an attractive approach to the treatment of Alzheimer disease. In addition to APP, however, several other gamma-secretase substrates have been identified (e.g. Notch), and altered processing of these substrates by gamma-secretase inhibitors could lead to unintended biological consequences. To study the in vivo consequences of gamma-secretase inhibition, the gamma-secretase inhibitor LY-411,575 was administered to C57BL/6 and TgCRND8 APP transgenic mice for 15 days. Although most tissues were unaffected, doses of LY-411,575 that inhibited Abeta production had marked effects on lymphocyte development and on the intestine. LY-411,575 decreased overall thymic cellularity and impaired intrathymic differentiation at the CD4(-)CD8(-)CD44(+)CD25(+) precursor stage. No effects on peripheral T cell populations were noted following LY-411,575 treatment, but evidence for the altered maturation of peripheral B cells was observed. In the intestine, LY-411,575 treatment increased goblet cell number and drastically altered tissue morphology. These effects of LY-411,575 were not seen in mice that were administered LY-D, a diastereoisomer of LY-411,575, which is a very weak gamma-secretase inhibitor. These studies show that inhibition of gamma-secretase has the expected benefit of reducing Abeta in a murine model of Alzheimer disease but has potentially undesirable biological effects as well, most likely because of the inhibition of Notch processing.

Phe*-Ala-based pentapeptide mimetics are BACE inhibitors: P2 and P3 SAR

We describe herein the syntheses and evaluation of a series of C-termini pyridyl containing Phe*-Ala-based BACE inhibitors (5-19). In conjunction with four fixed residues at the P1 (Phe), P1' (Ala), P2' (Val), and P2' cap (Pyr.), rather detailed SAR modifications at P2 and P3 positions were pursued. The promising inhibitors emerging from this SAR investigation, 12 and 17 demonstrated very good enzyme potency (IC(50)=45 nM) and cellular activity (IC(50)=0.4 microM).

Notch promotes epithelial-mesenchymal transition during cardiac development and oncogenic transformation

Epithelial-to-mesenchymal transition (EMT) is fundamental to both embryogenesis and tumor metastasis. The Notch intercellular signaling pathway regulates cell fate determination throughout metazoan evolution, and overexpression of activating alleles is oncogenic in mammals. Here we demonstrate that Notch activity promotes EMT during both cardiac development and oncogenic transformation via transcriptional induction of the Snail repressor, a potent and evolutionarily conserved mediator of EMT in many tissues and tumor types. In the embryonic heart, Notch functions via lateral induction to promote a selective transforming growth factor-beta (TGFbeta)-mediated EMT that leads to cellularization of developing cardiac valvular primordia. Embryos that lack Notch signaling elements exhibit severely attenuated cardiac snail expression, abnormal maintenance of intercellular endocardial adhesion complexes, and abortive endocardial EMT in vivo and in vitro. Accordingly, transient ectopic expression of activated Notch1 (N1IC) in zebrafish embryos leads to hypercellular cardiac valves, whereas Notch inhibition prevents valve development. Overexpression of N1IC in immortalized endothelial cells in vitro induces EMT accompanied by oncogenic transformation, with corresponding induction of snail and repression of VE-cadherin expression. Notch is expressed in embryonic regions where EMT occurs, suggesting an intimate and fundamental role for Notch, which may be reactivated during tumor metastasis.

FRAT1 peptide decreases Abeta production in swAPP(751) cells

Recently, LiCl has been shown to inhibit amyloid beta peptide secretion in association with diminished glycogen synthase kinase beta (GSK3beta) activity. However, it remains unclear if direct inhibition of GSK3beta activity will result in decreased Abeta production. Frequently rearranged in advanced T-cell lymphomas 1 (FRAT1) protein is a negative regulator of GSK3alpha/beta kinase activity. To examine whether direct inhibition of GSK3alpha/beta kinase activity can lower Abeta production, a FRAT1 peptide was expressed in swAPP(751) cells that produce high levels of Abeta. Our data demonstrate that cellular expression of FRAT1 peptide in swAPP(751) cells increases both GSK3alpha and beta phosphorylation on Ser21 and Ser9, respectively, while inhibiting kinase activity of both isoforms. Moreover, as a result of FRAT1 expression, the production of both total Abeta and Abeta(1-42) was significantly decreased. Thus, we provide evidence that direct regulation of GSK3alpha/beta by FRAT1 peptide significantly decreases Abeta production in swAPP(751) cells.

Mouse models of Alzheimer's disease: the long and filamentous road

Alzheimer's disease (AD) is characterized by memory impairment leading to dementia, deposition of amyloid plaques and neurofibrillary tangles (NFTs), and neuronal loss. The major component of plaques is the amyloid beta peptide, A beta, whereas NFTs contain hyperphosphorylated forms of the microtubule-associated protein tau (tau). Familial AD (FAD) mutations either elevate A beta synthesis by favoring 'secretase' of the Alzheimer beta-amyloid precursor protein (APP) or enhance the fibrillogenic properties of this peptide. Mutations in the tau gene cause a different disease denoted FTPD-17, but suggest that the aberrant forms of tau seen in AD are unlikely to be benign. These findings imply a complex pathogenic cascade in AD and important goals of transgenic modeling are to capture and stratify this pathogenic process. Several laboratories have created APP transgenic (Tg) mice that exhibit AD-like amyloid pathology and A beta burdens. These Tg lines also exhibit deficits in spatial reference and/or working memory, with immunization against A beta attenuating both AD-associated phenotypes. Tangle-like pathologies are observed in mice expressing FTPD-17 mutant forms of tau, but florid tau pathologies based upon the wild type (wt) tau isoforms present in AD have proven more elusive. Creation of animal models with robust amyloid and tau pathologies, yet free of irrelevant confounding pathologies, remains a major objective in this field.

A conserved GXXXG motif in APH-1 is critical for assembly and activity of the gamma-secretase complex

The multipass membrane protein APH-1, found in the gamma-secretase complex together with presenilin, nicastrin, and PEN-2, is essential for Notch signaling in Caenorhabditis elegans embryos and is required for intramembrane proteolysis of Notch and beta-amyloid precursor protein in mammalian and Drosophila cells. In C. elegans, a mutation of the conserved transmembrane Gly123 in APH-1 (mutant or28) leads to a notch/glp-1 loss-of-function phenotype. In this study, we show that the corresponding mutation in mammalian APH-1aL (G122D) disrupts the physical interaction of APH-1aL with hypoglycosylated immature nicastrin and the presenilin holoprotein as well as with mature nicastrin, presenilin, and PEN-2. The G122D mutation also reduced gamma-secretase activity in intramembrane proteolysis of membrane-tethered Notch. Moreover, we found that the conserved transmembrane Gly122, Gly126, and Gly130 in the fourth transmembrane region of mammalian APH-1aL are part of the membrane helix-helix interaction GXXXG motif and are essential for the stable association of APH-1aL with presenilin, nicastrin, and PEN-2. These findings suggest that APH-1 plays a GXXXG-dependent scaffolding role in both the initial assembly and subsequent maturation and maintenance of the active gamma-secretase complex.

Adipsin, a biomarker of gastrointestinal toxicity mediated by a functional gamma-secretase inhibitor

Functional gamma-secretase inhibitors (FGSIs) can block the cleavage of several transmembrane proteins including amyloid precursor protein (APP), and the cell fate regulator Notch-1. FGSIs, by inhibiting APP processing, block the generation of amyloid beta (Abeta) peptides and may slow the development of Alzheimer's disease. FGSIs used to inhibit APP processing may disrupt Notch processing, thus interfering with cell fate determination. Described herein is a FGSI-mediated gastrointestinal toxicity characterized by cell population changes in the ileum of rats, which are indicative of Notch signaling disruption. Microarray analysis of ileum from FGSI-treated rats revealed differential expression responses in a number of genes indicative of Notch signaling perturbation, including the serine protease adipsin. We were able to show that FGSI-treated rats had elevated levels of adipsin protein in gastrointestinal contents and feces, and by immunohistochemistry demonstrated that adipsin containing ileum crypt cells were increased in FGSI-treated rats. The mouse Adipsin proximal promoter contains a putative binding site for the Notch-induced transcriptional regulator Hes-1, which we demonstrate is able to bind Hes-1. Additional studies in 3T3-L1 preadipocytes demonstrate that this FGSI inhibits Hes-1 expression while up-regulating adipsin expression. Overexpression of Hes-1 was able to down-regulate adipsin expression and block pre-adipocyte differentiation. We propose that adipsin is a Hes-1-regulated gene that is de-repressed during FGSI-mediated disruption of Notch/Hes-1 signaling. Additionally, the aberrant expression of adipsin, and its presence in feces may serve as a noninvasive biomarker of gastrointestinal toxicity associated with perturbed Notch signaling.

In vivo assessment of brain interstitial fluid with microdialysis reveals plaque-associated changes in amyloid-beta metabolism and half-life

Soluble amyloid-beta (Abeta) peptide converts to structures with high beta-sheet content in Alzheimer's disease (AD). Soluble Abeta is released by neurons into the brain interstitial fluid (ISF), in which it can convert into toxic aggregates. Because assessment of ISF Abeta levels may provide unique insights into Abeta metabolism and AD, an in vivo microdialysis technique was developed to measure it. Our Abeta microdialysis technique was validated ex vivo with human CSF and then in vivo in awake, freely moving mice. Using human amyloid precursor protein (APP) transgenic mice, we found that, before the onset of AD-like pathology, ISF Abeta in hippocampus and cortex correlated with levels of APP in those tissues. After the onset of Abeta deposition, significant changes in the ISF Abeta40/Abeta42 ratio developed without changes in Abeta1-x. These changes differed from changes seen in tissue lysates from the same animals. By rapidly inhibiting Abeta production, we found that ISF Abeta half-life was short ( approximately 2 hr) in young mice but was twofold longer in mice with Abeta deposits. This increase in half-life, without an increase in steady-state levels, suggests that inhibition of Abeta synthesis reveals a portion of the insoluble Abeta pool that is in dynamic equilibrium with ISF Abeta. This now measurable in vivo pool is a likely target for new diagnostic and therapeutic strategies.

Distinct presenilin-dependent and presenilin-independent ?-secretases are responsible for total cellular A? production

gamma-Secretase is the second of two proteolytic enzymes involved in the liberation of the beta-amyloid peptide (Abeta) from the amyloid precursor protein (APP). gamma-Secretase cleavage occurs at several intracellular sites, including the Golgi network and the endoplasmic reticulum/intermediate compartment (ER/IC) to produce multiple forms of the Abeta peptide that can be either secreted from the cell or remain intracellular. To date, most evidence has suggested that members of the presenilin protein family are required for gamma-secretase activity. Although it seems that presenilins are indeed necessary for the production of most secreted and intracellular Abeta particularly that generated in downstream organelles, it was shown recently that a presenilin-independent gamma-secretase is active in the ER/IC and is responsible for the production of a portion of intracellular Abeta42. We discuss the implications of this finding for the understanding of presenilin biology and speculate on the putative identity of the presenilin-independent cleavage activity.

Generation of the beta-amyloid peptide and the amyloid precursor protein C-terminal fragment gamma are potentiated by FE65L1

Members of the FE65 family of adaptor proteins, FE65, FE65L1, and FE65L2, bind the C-terminal region of the amyloid precursor protein (APP). Overexpression of FE65 and FE65L1 was previously reported to increase the levels of alpha-secretase-derived APP (APPs alpha). Increased beta-amyloid (A beta) generation was also observed in cells showing the FE65-dependent increase in APPs alpha. To understand the mechanism for the observed increase in both A beta and APPs alpha given that alpha-secretase cleavage of a single APP molecule precludes A beta generation, we examined the effects of FE65L1 overexpression on APP C-terminal fragments (APP CTFs). Our data show that FE65L1 potentiates gamma-secretase processing of APP CTFs, including the amyloidogenic CTF C99, accounting for the ability of FE65L1 to increase generation of APP C-terminal domain and A beta 40. The FE65L1 modulation of these processing events requires binding of FE65L1 to APP and APP CTFs and is not because of a direct effect on gamma-secretase activity, because Notch intracellular domain generation is not altered by FE65L1. Furthermore, enhanced APP CTF processing can be detected in early endosome vesicles but not in endoplasmic reticulum or Golgi membranes, suggesting that the effects of FE65L1 occur at or near the plasma membrane. Finally, although FE65L1 increases APP C-terminal domain production, it does not mediate the APP-dependent transcriptional activation observed with FE65.

Gamma-secretase activity is dispensable for mesenchyme-to-epithelium transition but required for podocyte and proximal tubule formation in developing mouse kidney

Notch signaling is involved in pronephros development in Xenopus and in glomerulogenesis in mice. However, owing to early lethality in mice deficient for some Notch pathway genes and functional redundancy for others, a role for Notch signaling during early stages of metanephric development has not been defined. Using an antibody specific to the N-terminal end of gamma-secretase-cleaved Notch1, we found evidence for Notch1 activation in the comma and S-shaped bodies of the mouse metanephros. We therefore cultured mouse metanephroi in the presence of a gamma-secretase inhibitor, N-S-phenyl-glycine-t-butyl ester (DAPT), to block Notch signaling. We observed slightly reduced ureteric bud branching but normal mesenchymal condensation and expression of markers indicating that mesenchyme induction had occurred. However, fewer renal epithelial structures were observed, with a severe deficiency in proximal tubules and glomerular podocytes, which are derived from cells in which activated Notch1 is normally present. Distal tubules were present but in reduced numbers, and this was accompanied by an increase in intervening, non-epithelial cells. After a transient 3-day exposure to DAPT, proximal tubules expanded, but podocyte differentiation failed to recover after removal of DAPT. These observations suggest that gamma-secretase activity, probably through activation of Notch, is required for maintaining a competent progenitor pool as well as for determining the proximal tubule and podocyte fates.

Differential glycosylation regulates processing of lipoprotein receptors by gamma-secretase

The low density lipoprotein (LDL) receptor-related protein 1 (LRP1) belongs to a growing number of cell surface proteins that undergo regulated proteolytic processing that culminates in the release of their intracellular domain (ICD) by the intramembranous protease gamma-secretase. Here we show that LRP1 is differentially glycosylated in a tissue-specific manner and that carbohydrate addition reduces proteolytic cleavage of the extracellular domain and, concomitantly, ICD release. The apolipoprotein E (apoE) receptor-2 (apoER2), another member of the LDL receptor family with functions in cellular signal transmission, also undergoes sequential proteolytic processing, resulting in intracellular domain release into the cytoplasm. The penultimate processing step also involves cleavage of the apoER2 extracellular domain. The rate at which this cleavage step occurs is determined by the glycosylation state of the receptor, which in turn is regulated by the alternative splicing of an exon encoding several O-linked sugar attachment sites. These findings suggest a role for differential and tissue-specific glycosylation as a physiological switch that modulates the diverse biological functions of these receptors in a cell-type specific manner.

The cell adhesion protein P-selectin glycoprotein ligand-1 is a substrate for the aspartyl protease BACE1

The aspartyl protease BACE1 cleaves the amyloid precursor protein and the sialyltransferase ST6Gal I and is important in the pathogenesis of Alzheimer's disease. The normal function of BACE1 and additional physiological substrates have not been identified. Here we show that BACE1 acts on the P-selectin glycoprotein ligand 1 (PSGL-1), which mediates leukocyte adhesion in inflammatory reactions. In human monocytic U937 and human embryonic kidney 293 cells expressing endogenous or transfected BACE1, PSGL-1 was cleaved by BACE1 to generate a soluble ectodomain and a C-terminal transmembrane fragment. No evidence of the cleavage fragment was seen in primary cells derived from mice deficient in BACE1. By using deletion constructs and enzymatic deglycosylation of the C-terminal PSGL-1 fragments, the cleavage site in PSGL-1 was mapped to the juxtamembrane region within the ectodomain. In an in vitro assay BACE1 catalyzed the formation of the PSGL-1 products seen in vivo. The cleavage occurred at a Leu-Ser peptide bond as identified by mass spectrometry using a synthetic peptide. We conclude that PSGL-1 is an additional substrate for BACE1.

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.

Treatment of Alzheimer's disease: current status and new perspectives

Alzheimer's disease (AD) is the most common neurodegenerative disorder and the most prevalent cause of dementia with ageing. Pharmacological treatment of AD is based on the use of acetylcholinesterase inhibitors, which have beneficial effects on cognitive, functional, and behavioural symptoms of the disease, but their role in AD pathogenesis is unknown. Other pharmacological therapies are becoming available--including the recently approved drug memantine, an NMDA channel blocker indicated for advanced AD. Here, we review clinical features of the available cholinesterase inhibitors (donepezil, rivastigmine, and galantamine) including their pharmacological properties, the evidence for switching from one agent to another, "head to head" studies, and the emerging evidence for the use of memantine in AD. New therapeutic approaches--including those more closely targeted to the pathogenesis of the disease--will also be reviewed. These potentially disease modifying treatments include amyloid-beta-peptide vaccination, secretase inhibitors, cholesterol-lowering drugs, metal chelators, and anti-inflammatory agents.

Demonstration by FRET of BACE interaction with the amyloid precursor protein at the cell surface and in early endosomes

Amyloid-beta peptide, which accumulates in senile plaques in Alzheimer's disease, is derived from the amyloid precursor protein (APP) by proteolytic processing. beta-secretase (Asp2), which cleaves APP at the N-terminus of amyloid-beta, has recently been identified to be the protease BACE. In the present study, we examined the subcellular localization of interactions between APP and BACE by using both double immunofluorescence and a fluorescence resonance energy transfer (FRET) approach. Cell surface APP and BACE, studied by using antibodies directed against their ectodomains in living H4 neuroglioma cells co-transfected with APP and BACE, showed exquisite co-localization and demonstrated a very close interaction by FRET analysis. The majority of cell surface APP and BACE were internalized after 15 minutes, but they remained strongly co-localized together in the early endosomal compartment, where FRET analysis demonstrated a continued close interaction. By contrast, at later timepoints, almost no co-localization or FRET was observed in lysosomal compartments. To determine whether the APP-BACE interaction on cell surface and endosomes contributed to amyloid-beta synthesis, we labeled cell surface APP and demonstrated detectable levels of labeled amyloid-beta within 30 minutes. APP-Swedish mutant protein enhanced amyloid-beta synthesis from cell surface APP, consistent with the observation that it is a better BACE substrate than wild-type APP. Taken together, these data confirm a close APP-BACE interaction in early endosomes, and highlight the cell surface as an additional potential site of APP-BACE interaction.

Current status of metals as therapeutic targets in Alzheimer's disease

There is accumulating evidence that interactions between beta-amyloid and copper, iron, and zinc are associated with the pathophysiology of Alzheimer's disease (AD). A significant dyshomeostasis of copper, iron, and zinc has been detected, and the mismanagement of these metals induces beta-amyloid precipitation and neurotoxicity. Chelating agents offer a potential therapeutic solution to the neurotoxicity induced by copper and iron dyshomeostasis. Currently, the copper and zinc chelating agent clioquinol represents a potential therapeutic route that may not only inhibit beta-amyloid neurotoxicity, but may also reverse the accumulation of neocortical beta-amyloid. A Phase II double-blind clinical trial of clioquinol with B12 supplementation will be published soon, and the results are promising. This article summarizes the role of transition metals in amyloidgenesis and reviews the potential promise of chelation therapy as a treatment for AD.

Vaccines for Alzheimer's disease: how close are we?

Alzheimer's disease is a neurodegenerative disorder characterised by a progressive loss of cognitive function. Despite the considerable progress being made, a complete description of the molecular pathology of this disease has yet to be elucidated. The evidence indicates that abnormal processing and extracellular deposition of the longer form of the beta-amyloid (Abeta) peptide (Abeta(1-42), a proteolytic derivative of the amyloid precursor protein [APP]) is implicated in the pathogenesis of Alzheimer's disease. In this respect, recent use of experimental mouse models, in which the mice develop some aspects of Alzheimer's disease in a reproducible fashion, has provided a new opportunity for a multidisciplinary and invasive analysis of mechanisms behind the amyloid pathology and its role in Alzheimer's disease. It has been demonstrated, using a single transgenic mouse model system that overexpresses the human mutated APP gene, that an immunisation against Abeta(1-42) causes a marked reduction in the amyloid burden in the brain. The follow-up research provided more evidence that both active and passive Abeta immunisation also reduces cognitive dysfunction in transgenic mouse models of Alzheimer's disease. Other studies using different approaches - such as secretase, cholesterol and Abeta metalloprotein inhibitors or NSAIDs - but all targeting the abnormal metabolism of Abeta have confirmed in each case that a significant reduction of amyloid plaque burden can be achieved in transgenic mouse models of Alzheimer's disease. This research strongly supports the notion that abnormal Abeta processing is essential to the pathogenesis of Alzheimer's disease and provides a crucial platform for the development and detailed testing of potential treatments in experimental models before each of these approaches can be proposed as a therapy for Alzheimer's disease. Although the first clinical trial of active immunisation with a pre-aggregated synthetic Abeta(42) preparation (AN-1792 vaccine) met with some setbacks and was discontinued after several patients experienced meningoencephalitis, the follow-up analysis of the effect of immunisation against Abeta in humans revealed a powerful effect of vaccination in the clearance of amyloid plaques from the cerebral cortex.

Amyloid precursor protein associates with a nicastrin-dependent docking site on the presenilin 1-gamma-secretase complex in cells demonstrated by fluorescence lifetime imaging

Gamma-secretase cleavage is the final enzymatic step generating beta-amyloid via intramembranous cleavage of the amyloid precursor protein (APP). Presenilin (PS), initially identified as a gene in which mutations account for the vast majority of early-onset autosomal dominant Alzheimer's disease, is a major component of gamma-secretase. Enzymatic activity also depends on nicastrin, Aph-1, and Pen-2. We propose a model in which gamma-secretase components assemble, interact with substrates initially at a docking site, and then cleave and release substrates. To test this model, we developed a novel morphological technique on the basis of advanced fluorescence microscopy methods, fluorescence lifetime imaging microscopy (FLIM). FLIM allows us to examine protein-protein "proximity" in intact cells. We show that, although the strongest colocalization of APP and PS1 is in the perinuclear area, the strongest interactions detected by FLIM are at or near the cell surface. We also found that APP-PS1 interactions occur even when gamma-secretase inhibitors or "dominant-negative" PS1 mutations are used to block gamma-secretase activity. Finally, using nicastrin RNA interference, we demonstrate that nicastrin is critical for APP association with PS1. We interpret these results to suggest that there is a noncatalytic docking site closely associated with PS1-gamma-secretase.

APP intracellular domain formation and unaltered signaling in the presence of familial Alzheimer's disease mutations

One of the cardinal neuropathological findings in brains from Alzheimer's disease (AD) patients is the occurrence of amyloid beta-peptide (Abeta) deposits. The gamma-secretase-mediated intramembrane proteolysis event generating Abeta also results in the release of the APP intracellular domain (AICD), which may mediate nuclear signaling. It was recently shown that AICD starts at a position distal to the site predicted from gamma-secretase cleavage within the membrane. This novel site, the epsilon site, is located close to the inner leaflet of the membrane bilayer. The relationship between proteolysis at the gamma and epsilon sites has not been fully characterized. Here we studied AICD signaling in intact cells using a chimeric C99 molecule and a luciferase reporter system. We show that the release of AICD from the membrane takes place in a compartment downstream of the endoplasmic reticulum, is dependent on presenilin proteins, and can be inhibited by treatment with established gamma-secretase inhibitors. Moreover, we find that AICD signaling remains unaltered from C99 derivatives containing mutations associated with increased Abeta42 production and familial AD. These findings indicate that there are very similar routes for Abeta and AICD formation but that FAD-linked mutations in APP primarily affect gamma-secretase-mediated Abeta42 formation, and not AICD signaling.

Peptide-based, irreversible inhibitors of gamma-secretase activity

The characterization of the enzymes responsible for amyloid beta-peptide (Abeta) production is considered to be a primary goal towards the development of future therapeutics for the treatment of Alzheimer's disease. Inhibitors of gamma-secretase activity were critical in demonstrating that the presenilins (PSs) likely comprised at least part of the active site of the gamma-secretase enzyme complex, with two highly conserved membrane aspartates presumably acting as catalytic residues. However, whether or not these aspartates are actually the catalytic residues of the enzyme complex or are merely essential for normal PS function and/or maturation is still unknown. In this paper, we report the development of reactive inhibitors of gamma-secretase activity that are functionally irreversible. Since such inhibitors have been shown to bind catalytic residues in other aspartyl proteases (e.g., HIV protease), they might be used to determine if the transmembrane aspartates of PSs are involved directly in substrate cleavage.

Notch mediates TGF alpha-induced changes in epithelial differentiation during pancreatic tumorigenesis

Notch signaling regulates cell fate decisions in a wide variety of adult and embryonic tissues. Here we show that Notch pathway components and Notch target genes are upregulated in invasive pancreatic cancer, as well as in pancreatic cancer precursors from both mouse and human. In mouse pancreas, ectopic Notch activation results in accumulation of nestin-positive precursor cells and expansion of metaplastic ductal epithelium, previously identified as a precursor lesion for pancreatic cancer. Notch is also activated as a direct consequence of EGF receptor activation in exocrine pancreas and is required for TGF alpha-induced changes in epithelial differentiation. These findings suggest that Notch mediates the tumor-initiating effects of TG alpha by expanding a population of undifferentiated precursor cells.

The gamma-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester reduces A beta levels in vivo in plasma and cerebrospinal fluid in young (plaque-free) and aged (plaque-bearing) Tg2576 mice

Acute, s.c. administration of a gamma-secretase inhibitor, N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT), to young PDAPP mice dose dependently decreases cortical amyloid-beta (A beta). The present studies replicated these findings in Tg2576 mice and examined further whether DAPT would reduce cerebrospinal fluid (CSF) A beta comparably in young (plaque-free) and aged (plaque-bearing) mice. In the first study, vehicle or DAPT (10, 30, or 100 mg/kg s.c.) administered to young Tg2576 mice (6 months old) dose dependently reduced A beta peptide levels in the cortex as seen previously in the PDAPP mice. Additionally, a dose-dependent decrease in plasma A beta levels was evident. The same dosing regime was applied next to aged mice (17 months old) to assess A beta changes in the CSF in addition to plasma and brains. DAPT dose dependently reduced A beta levels in the CSF and plasma, but not in the brain wherein A beta levels were 400 to 500 times higher than those in young mice, consistent with a large pool of A beta extracted from amyloid deposits. In subsequent studies, effects of oral DAPT (100 or 200 mg/kg) were examined concurrently in young and aged mice. DAPT reduced A beta levels in CSF and plasma to a similar extent at both ages. In contrast, DAPT reduced brain A beta levels primarily in young mice, with minimal effects in aged mice. These results demonstrate that A beta levels in CSF and plasma decrease dose dependently after gamma-secretase inhibition, and this response is not affected by amyloid plaque burden. We conclude that CSF and plasma A beta may offer a clinically applicable, mechanism-based biomarker for inhibitors of A beta production.

Presenilin endoproteolysis mediated by an aspartyl protease activity pharmacologically distinct from gamma-secretase

Presenilin (PS)-dependent gamma-secretase cleavage is the final proteolytic step in generating amyloid beta protein (A beta), a key peptide involved in the pathogenesis of Alzheimer's disease. PS undergoes endoproteolysis by an unidentified 'presenilinase' to generate the functional N-terminal and C-terminal fragment heterodimers (NTF/CTF) that may harbor the gamma-secretase active site. To better understand the relationship between presenilinase and gamma-secretase, we characterized the biochemical properties of presenilinase and compared them with those of gamma-secretase. Similar to gamma-secretase, presenilinase was most active at acidic pH 6.3. Aspartyl protease inhibitor pepstatin A blocked presenilinase activity with an IC50 of approximately 1 microM. Difluoroketone aspartyl protease transition state analogue MW167 was relatively selective for presenilinase (IC50 < 1 microM) over gamma-secretase (IC50-16 microM). Importantly, removing the transition state mimicking moiety simultaneously abolished both presenilinase and gamma-secretase inhibition, suggesting that presenilinase, like gamma-secretase, is an aspartyl protease. Interestingly, several of the most potent gamma-secretase inhibitors (IC50 = 0.3 or 20 nM) failed to block presenilinase activity. Although de novo generation of PS1 fragments coincided with production of A beta in vitro, blocking presenilinase activity without reducing pre-existing fragment levels permitted normal de novo generation of A beta and amyloid intracellular domain. Therefore, presenilinase has characteristics of an aspartyl protease, but this activity is distinct from gamma-secretase.

Contribution of presenilin/gamma-secretase to calsenilin-mediated apoptosis

Mutant presenilins cause early-onset of familial Alzheimer's disease and render cells vulnerable to apoptosis. Calsenilin/DREAM/KChIP3 is a multifunctional calcium-binding protein that interacts with presenilin and mediates calcium-mediated apoptosis. In the present study, we report that the calsenilin-mediated apoptosis is regulated by presenilin. The expression of calsenilin was highly up-regulated in neuronal cells undergoing Abeta42-triggered cell death. The incidence of calsenilin-mediated apoptosis was diminished in presenilin-1(-/-) mouse embryonic fibroblast cells or neuronal cells stably expressing a loss-of-function presenilin-1 mutant. On the contrary, an array of familial Alzheimer's disease-associated presenilin mutants (gain-of-function) increased calsenilin-induced cell death. Moreover, gamma-secretase inhibitors, including compound E and DAPT, decreased the calsenilin-induced cell death. These results suggest that the pro-apoptotic activity of calsenilin coordinates with presenilin/gamma-secretase activity to play a crucial role in the neuronal death of Alzheimer's disease.

Sulindac sulfide is a noncompetitive gamma-secretase inhibitor that preferentially reduces Abeta 42 generation

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been known to reduce risk for Alzheimer's disease. In addition to the anti-inflammatory effects of NSAIDs to block cylooxygenase, it has been shown recently that a subset of NSAIDs selectively inhibits the secretion of highly amyloidogenic Abeta42 from cultured cells, although the molecular target(s) of NSAIDs in reducing the activity of gamma-secretase for Abeta42 generation (gamma(42)-secretase) still remain unknown. Here we show that sulindac sulfide (SSide) directly acts on gamma-secretase and preferentially inhibits the gamma(42)-secretase activity derived from the 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate-solubilized membrane fractions of HeLa cells, in an in vitro gamma-secretase assay using recombinant amyloid beta precursor protein C100 as a substrate. SSide also inhibits activities for the generation of Abeta40 as well as for Notch intracellular domain at higher concentrations. Notably, SSide displayed linear noncompetitive inhibition profiles for gamma(42)-secretase in vitro. Our data suggest that SSide is a direct inhibitor of gamma-secretase that preferentially affects the gamma(42)-secretase activity.

Differential effects of inhibitors on the gamma-secretase complex. Mechanistic implications

Gamma-secretase is a protease complex of four integral membrane proteins, with presenilin (PS) as the apparent catalytic component, and this enzyme processes the transmembrane domains of a variety of substrates, including the amyloid beta-protein precursor and the Notch receptor. Here we explore the mechanisms of structurally diverse gamma-secretase inhibitors by examining their ability to displace an active site-directed photoprobe from PS heterodimers. Most gamma-secretase inhibitors, including a potent inhibitor of the PS-like signal peptide peptidase, blocked the photoprobe from binding to PS1, indicating that these compounds either bind directly to the active site or alter it through an allosteric interaction. Conversely, some reported inhibitors failed to displace this interaction, demonstrating that these compounds do not interfere with the protease by affecting its active site. Differential effects of the inhibitors with respect to photoprobe displacement and in cell-based and cell-free assays suggest that these compounds are important mechanistic tools for deciphering the workings of this intramembrane-cleaving protease complex and its similarity to other polytopic aspartyl proteases.

Aspartic proteases involved in Alzheimer's disease

Alzheimer's disease afflicts every tenth human aged over 65. Despite the dramatic progress that has been made in understanding the disease, the exact cause of Alzheimer's disease is still unknown. Most gene mutations associated with Alzheimer's disease point at the same culprits: amyloid precursor protein and ultimately amyloid beta. The enigmatic proteases alpha-,beta-, and gamma-secretase are the three executioners of amyloid precursor protein processing, and disruption of their delicate balance is suspected to result in Alzheimer's disease. Significant progress has been made in the selective control of these proteases, regardless of the availability of structural information. Not even the absence of a robust cell-free assay for gamma-secretase could hamper the identification of nonpeptidic inhibitors of this enzyme for long. Within five years, four distinctly different structural moieties were developed and the first drug candidates are in clinical trials. Unfortunately, selective inhibition of amyloid beta formation remains a crucial issue because fundamental fragments of the gamma-secretase complex are important for other signaling events. This problem makes beta-secretase inhibition and alpha-secretase induction even more appealing.

Insulin-degrading enzyme regulates the levels of insulin, amyloid beta-protein, and the beta-amyloid precursor protein intracellular domain in vivo

Two substrates of insulin-degrading enzyme (IDE), amyloid beta-protein (Abeta) and insulin, are critically important in the pathogenesis of Alzheimer's disease (AD) and type 2 diabetes mellitus (DM2), respectively. We previously identified IDE as a principal regulator of Abeta levels in neuronal and microglial cells. A small chromosomal region containing a mutant IDE allele has been associated with hyperinsulinemia and glucose intolerance in a rat model of DM2. Human genetic studies have implicated the IDE region of chromosome 10 in both AD and DM2. To establish whether IDE hypofunction decreases Abeta and insulin degradation in vivo and chronically increases their levels, we characterized mice with homozygous deletions of the IDE gene (IDE --). IDE deficiency resulted in a >50% decrease in Abeta degradation in both brain membrane fractions and primary neuronal cultures and a similar deficit in insulin degradation in liver. The IDE -- mice showed increased cerebral accumulation of endogenous Abeta, a hallmark of AD, and had hyperinsulinemia and glucose intolerance, hallmarks of DM2. Moreover, the mice had elevated levels of the intracellular signaling domain of the beta-amyloid precursor protein, which was recently found to be degraded by IDE in vitro. Together with emerging genetic evidence, our in vivo findings suggest that IDE hypofunction may underlie or contribute to some forms of AD and DM2 and provide a mechanism for the recently recognized association among hyperinsulinemia, diabetes, and AD.

Inhibitors of Abeta production: solid-phase synthesis and SAR of alpha-hydroxycarbonyl derivatives

Inhibitors of amyloid-beta (Abeta) protein production have been widely pursued as a potential treatment for Alzheimer's disease. Following the identification of a 5 microM screening hit, SAR was initiated using solid-phase synthetic techniques. Two series of alpha-hydroxy esters and ketones which are sub-micromolar inhibitors of Abeta production were identified. The most potent alpha-hydroxyketone identified is approximately 30-fold more potent than the initial lead.

Physiologic and pathologic events mediated by intramembranous and juxtamembranous proteolysis

Intramembranous proteolysis (IP) is a recently recognized mechanism for transmembrane signal transduction that involves proteolysis of transmembrane proteins within their membrane-spanning domains. Juxtamembranous proteolysis (JP) is similar, but proteolytic cleavage of a transmembrane protein occurs at a site close to, but not within, the transmembrane domain of the target protein. In both IP and JP, a soluble domain of a transmembrane protein is released from its membrane tether. This domain can then transmit a signal either locally or at some distance from the site of cleavage. In certain signaling pathways, JP and IP are linked. JP on one side of the membrane results in secondary IP, which then releases a signaling domain from the membrane. Whereas well-characterized proteases such as caspases, the proteasome, and metalloprotease disintegrins, have been implicated in JP, three families of multipass membrane proteases (MpMPs) have now been shown to carry out IP. Recent studies of events mediated by IP and JP indicate that they regulate key cellular signaling events including pathways involved in sterol regulation, cell fate selection, and growth regulation. Moreover, IP and JP have important roles in certain diseases such as Alzheimer's disease. Because some of the proteases mediating IP and JP can be selectivity inhibited, inhibitors targeting these proteases are likely to alter both physiologic and pathologic events triggered by IP and JP.

JNK-interacting protein-1 promotes transcription of A beta protein precursor but not A beta precursor-like proteins, mechanistically different than Fe65

Processing of the amyloid beta protein precursor (A beta PP) by the beta and gamma secretases leads to the production of two small peptides, amyloid beta and the A beta PP intracellular domain (AID, or called elsewhere AICD). Whereas the role of amyloid beta in the pathogenesis of Alzheimer's disease has been studied extensively, only recently has information begun to accumulate as to the role of AID. Functions identified for AID include its ability to trigger apoptosis and a role in regulating gene transcription, particularly in combination with the A beta PP binding protein Fe65. Here, we report that AID in combination with Janus kinase interacting protein-1 (JIP-1) can activate gene expression. We demonstrate that the mechanism is different from activation in combination with Fe65 by first showing that although Fe65 enters the nucleus in the absence of full-length A beta PP, JIP-1 does not. Additionally, JIP-1-induced activation is Tip60 independent, whereas a complex with AID, Fe65, and Tip60 is formed for Fe65-induced activation. Finally, and probably most interestingly, we show that although the A beta PP family members APLP1 and APLP2 (for amyloid beta precursor-like protein) can cause activation in combination with Fe65, APLP1 and APLP2 show little or no activation in combination with JIP-1. This activity for the AID fragment may help explain the unique functions of A beta PP relative to its other family members, and changes in gene expression found in Alzheimer's disease.

Current pharmacologic options for patients with Alzheimer's disease

BACKGROUND: The aim of the current study was to provide general practitioners with an overview of the available treatment options for Alzheimer's disease (AD). Since general practitioners provide the majority of medical care for AD patients, they should be well versed in treatment options that can improve function and slow the progression of symptoms. DESIGN: Biomedical literature related to acetylcholinesterase inhibitors (AChEIs) was surveyed. In the United States, there are four AChEIs approved for the treatment of AD: tacrine, donepezil, rivastigmine, and galantamine. There are other agents under investigation, but at present, AChEIs are the only approved drug category for AD treatment. MEASUREMENTS AND MAIN RESULTS: AD is becoming a major public health concern and underdiagnosis is a significant problem (with only about half of AD patients being diagnosed and only half of those diagnosed actually being treated). Clinical trials have demonstrated that patients with AD who do not receive active treatment decline at more rapid rates than those who do. CONCLUSIONS: Given that untreated AD patients show decline in three major areas (cognition, behavior, and functional ability), if drug treatment is able to improve performance, maintain baseline performance over the long term, or allow for a slower rate of decline in performance, each of these outcomes should be viewed a treatment success.

Current drugs and future hopes in the treatment of Alzheimer's disease

In spite of several years of experience with the use of cholinesterase inhibitors for treatment of symptoms of Alzheimer's disease their influence on disease progression remains still unclear. New cholinesterase inhibitors should provide an additional neuroprotective activity, because only substances which stop neuronal death can influence disease progression. New treatment strategies are focusing on amyloid processing, preventing the occurrence of toxic A beta(1-42) peptide. These procedures include the vaccination trials, but their clinical usefulness has to be proven. Also strategies focussing on neurofibrillary pathologies should be explored in detail. Drug development for Alzheimer's disease should include all pathological events associated with neurodegeneration, like oxidative stress, neuroinflammation or disturbances in growth factor signaling. Abnormal protein aggregation as a common feature of different neurodegenerative diseases might also be a promising drug target. Beside beta sheet breakers directed against beta-amyloid deposition the endogenous protein beta-synuclein or derivatives of it might be able to counteract aggregation of alpha-synuclein as well as of amyloid beta protein. Interaction with alpha-synuclein deserves special attention because it might be an early step of synaptic degeneration. Due to the complexity of the disease combination of different drugs might be the most promising way to go. The parallel development of early biological markers should enable intervention in pre-symptomatic disease stages.

Gene discovery and validation for neurodegenerative diseases

Treatment of neurodegenerative diseases, such as Alzheimer's disease, multiple sclerosis, Parkinson's disease and amyotrophic lateral sclerosis (ALS), represents a major challenge for the pharmaceutical industry. These disorders have common and unique molecular pathological characteristics that result in serious reductions in nervous-system functionality. Key to developing novel and efficacious therapeutics is the discovery of new gene targets. Genomic, proteomics and bioinformatic analyses are identifying vast amounts of genes whose expression is associated with the pathology of a specific disease. Extensive validation studies performed in parallel with drug development are crucial for the selection of appropriate target genes. This review outlines some of the current progress in gene discovery for neurodegenerative disease.

Gamma-secretase/presenilin inhibitors for Alzheimer's disease phenocopy Notch mutations in Drosophila

Signaling from the Notch (N) receptor is essential for proper cell-fate determinations and tissue patterning in all metazoans. N signaling requires a presenilin (PS)-dependent transmembrane-cleaving activity that is closely related or identical to the gamma-secretase proteolysis of the amyloid-beta precursor protein (APP) involved in Alzheimer's disease pathogenesis. Here, we show that N-[N-(3,5-difluorophenacetyl)-L-alanyl]-(S)-phenylglycine t-butyl ester, a potent gamma-secretase inhibitor reported to reduce amyloid-beta levels in transgenic mice, prevents N processing, translocation, and signaling in cell culture. This compound also induces developmental defects in Drosophila remarkably similar to those caused by genetic reduction of N. The appearance of this phenocopy depends on the timing and dose of compound exposure, and effects on N-dependent signaling molecules established its biochemical mechanism of action in vivo. Other gamma-secretase inhibitors caused similar effects. Thus, the three-dimensional structure of the drug-binding site(s) in Drosophila gamma-secretase is remarkably conserved vis-à-vis the same site(s) in the mammalian enzyme. These results show that genetics and developmental biology can help elucidate the in vivo site of action of pharmacological agents and suggest that organisms such as Drosophila may be used as simple models for in vivo prescreening of drug candidates.

Medicinal chemistry approaches for the treatment and prevention of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, which is characterised by progressive deterioration of memory and higher cortical functions that ultimately result in total degradation of intellectual and mental activities. Modern strategies in the search of new therapeutic approaches are based on the morphological and biochemical characteristics of AD, and focused on following directions: agents that compensate the hypofunction of cholinergic system, agents that interfere with the metabolism of beta-amyloid peptide, agents that protect nerve cells from toxic metabolites formed in neurodegenerative processes, agents that activate other neurotransmitter systems that indirectly compensate for the deficit of cholinergic functions, agents that affect the process of the formation of neurofibrillary tangles, anti-inflammatory agents that prevent the negative response of nerve cells to the pathological process. The goal of the present review is the validation and an analysis from the point of view of medicinal chemistry of the principles of the directed search of drugs for the treatment and prevention of AD and related neurodegenerative disorders. It is based on systematization of the data on biochemical and structural similarities in the interaction between physiologically active compounds and their biological targets related to the development of such pathologies. The main emphasis is on cholinomimetic, anti-amyloid and anti-metabolic agents, using the data that were published during the last 3 to 4 years, as well as the results of clinical trials presented on corresponding websites.

Growth suppression of pre-T acute lymphoblastic leukemia cells by inhibition of notch signaling

Constitutive NOTCH signaling in lymphoid progenitors promotes the development of immature T-cell lymphoblastic neoplasms (T-ALLs). Although it is clear that Notch signaling can initiate leukemogenesis, it has not previously been established whether continued NOTCH signaling is required to maintain T-ALL growth. We demonstrate here that the blockade of Notch signaling at two independent steps suppresses the growth and survival of NOTCH1-transformed T-ALL cells. First, inhibitors of presenilin specifically induce growth suppression and apoptosis of a murine T-ALL cell line that requires presenilin-dependent proteolysis of the Notch receptor in order for its intracellular domain to translocate to the nucleus. Second, a 62-amino-acid peptide derived from a NOTCH coactivator, Mastermind-like-1 (MAML1), forms a transcriptionally inert nuclear complex with NOTCH1 and CSL and specifically inhibits the growth of both murine and human NOTCH1-transformed T-ALLs. These studies show that continued growth and survival of NOTCH1-transformed lymphoid cell lines require nuclear access and transcriptional coactivator recruitment by NOTCH1 and identify at least two steps in the Notch signaling pathway as potential targets for chemotherapeutic intervention.

Gamma-secretase inhibitors and Alzheimer's disease

The existence of pathogenic mutations in beta-APP and the presenilin genes provides strong support for the hypothesis that Abeta production and deposition contribute to the etiology of Alzheimer's disease (AD). The heterogeneous carboxyl termini of Abeta molecules deposited in the hippocampus, cortex and cerebrovasculature of AD patients are generated by gamma-secretase. The gamma-secretase that generates the termini in vivo is a complex of proteins containing presenilin as an integral component. Drugs that modulate the production of Abeta by inhibiting gamma-secretase could provide an effective therapy for AD, but like most disease targets, the gamma-secretase appears to have more than a single function. The use of potent inhibitors has aided the discovery and characterization of gamma-secretase functions and reinforced the concept that a successful drug must demonstrate selectivity for lowering Abeta without disrupting the function of gamma-secretase targets. The discovery of drugs that can selectively inhibit beta-APP cleavage is an important objective. This review focuses on studies that enhance our understanding of the effects of inhibiting gamma-secretase and provide direction for developing effective and selective gamma-secretase inhibitors as drugs to treat AD.

Lens defects and age-related fiber cell degeneration in a mouse model of increased AbetaPP gene dosage in Down syndrome

Early-onset cataract and Alzheimer's disease occur with high frequency in Down syndrome (trisomy 21), the most common chromosome duplication in human live births. Previously, we used in vivo and lens organ culture models to demonstrate Alzheimer pathophysiology in oxidative stress-related lens degeneration. Currently, well-characterized Alzheimer transgenic mouse models are used to extend these findings. Here, we report on mice carrying a complete copy of a wild-type human AbetaPP (hAbetaPP) gene from the Down syndrome critical region on chromosome 21. hAbetaPP mice produce fiber cell membrane defects similar to those described in human cataracts and increased age-related lens degeneration. hAbetaPP expression and mRNA alternative splicing in human and mouse lens and cornea favor longer, potentially more amyloidogenic forms. Endogenous mouse AbetaPP expression is increased in transgenic lenses, consistent with the cycle of oxidative stress proposed in the mechanism of Alzheimer pathophysiology. Alternative splicing previously designated as neuron-specific occurs in human lens and cornea, and is maintained by hAbetaPP expressed in mouse tissues. These present data implicate AbetaPP in fiber cell formation and in early-onset cataracts in Down syndrome. Finally, our findings provide further support for our hypothesis that Alzheimer pathophysiology contributes to the cataract formation that is increasing in the aging population.

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.

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.