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

Design of potent inhibitors of human β-secretase. Part 1

We describe a novel series of potent inhibitors of human beta-secretase. These compounds possess the hydroxyethyl amine transition state isostere. A 2.5A crystal structure of inhibitor 32 bound to BACE is provided.

Presenilin-dependent ErbB4 nuclear signaling regulates the timing of astrogenesis in the developing brain

Embryonic multipotent neural precursors are exposed to extracellular signals instructing them to adopt different fates, neuronal or glial. However, the mechanisms by which precursors integrate these signals to make timely fate choices remained undefined. Here we show that direct nuclear signaling by a receptor tyrosine kinase inhibits the responses of precursors to astrocyte differentiation factors while maintaining their neurogenic potential. Upon neuregulin-induced activation and presenilin-dependent cleavage of ErbB4, the receptor's intracellular domain forms a complex with TAB2 and the corepressor N-CoR. This complex undergoes nuclear translocation and binds promoters of astrocytic genes, repressing their expression. Consistent with this observation, astrogenesis occurs precociously in ErbB4 knockout mice. Our studies define how presenilin-dependent nuclear signaling by a receptor tyrosine kinase directly regulates gene transcription and cell fate. This pathway could be of importance for neural stem cell biology and for understanding the pathogenesis of Alzheimer's disease.

A garden of Notch-ly delights

Over the past decade, the Notch signaling pathway has been shown to be crucially important for normal metazoan development and to be associated with several human inherited and late onset diseases. The realization that altered Notch signaling contributes at various levels to human disease lead in May to the first meeting dedicated solely to Notch signaling in vertebrate development and disease in Madrid, Spain. Hosted by the Cantoblanco Workshops on Biology and organized by Tom Gridley, José Luis de la Pompa and Juan Carlos Izpisúa Belmonte, the meeting covered diverse aspects of this important signaling pathway.

Alzheimer disease: amyloidogenesis, the presenilins and animal models

Alzheimer's disease is the most prevalent form of dementia. Neuropathogenesis is proposed to be a result of the accumulation of amyloid beta peptides in the brain together with oxidative stress mechanisms and neuroinflammation. The presenilin proteins are central to the gamma-secretase cleavage of the amyloid prescursor protein (APP), releasing the amyloid beta peptide. Point mutations in the presenilin genes lead to cases of familial Alzheimer's disease by increasing APP cleavage resulting in excess amyloid beta formation. This review discusses the molecular mechanism of Alzheimer's disease with a focus on the presenilin genes. Alternative splicing of transcripts from these genes and how these may function in several disease states is discussed. There is an emphasis on the importance of animal models in elucidating the molecular mechanisms behind the development of Alzheimer's disease and how the zebrafish, Danio rerio, can be used as a model organism for analysis of presenilin function and Alzheimer's disease pathogenesis.

Studies to investigate the in vivo therapeutic window of the gamma-secretase inhibitor N2-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-L-alaninamide (LY411,575) in the CRND8 mouse

Accumulation of amyloid beta-peptide (Abeta) is considered a key step in the etiology of Alzheimer's disease. Abeta is produced by sequential cleavage of the amyloid precursor protein by beta- and gamma-secretase enzymes. Consequently, inhibition of gamma-secretase provides a promising therapeutic approach to treat Alzheimer's disease. Preclinically, several gamma-secretase inhibitors have been shown to reduce plasma and brain Abeta, although they also produce mechanism-based side effects, including thymus atrophy and intestinal goblet cell hyperplasia. The present studies sought to establish an efficient screen for determining the therapeutic window of gamma-secretase inhibitors and to test various means of maximizing this window. Six-day oral administration of the gamma-secretase inhibitor N(2)-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N(1)-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-l-alaninamide (LY411,575) reduced cortical Abeta(40) in young (preplaque) transgenic CRND8 mice (ED(50) approximately 0.6 mg/kg) and produced significant thymus atrophy and intestinal goblet cell hyperplasia at higher doses (>3 mg/kg). The therapeutic window was similar after oral and subcutaneous administration and in young and aged CRND8 mice. Both the thymus and intestinal side effects were reversible after a 2-week washout period. Three-week treatment with 1 mg/kg LY411,575 reduced cortical Abeta(40) by 69% without inducing intestinal effects, although a previously unreported change in coat color was observed. These studies demonstrate that the 3- to 5-fold therapeutic window for LY411,575 can be exploited to obtain reduction in Abeta levels without induction of intestinal side effects, that intermittent treatment could be used to mitigate side effects, and that a 6-day dosing paradigm can be used to rapidly determine the therapeutic window of novel gamma-secretase inhibitors.

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 Notch signaling pathway in the cnidarian Hydra

Many of the major pathways that govern early development in higher animals have been identified in cnidarians, including the Wnt, TGFbeta and tyrosine kinase signaling pathways. We show here that Notch signaling is also conserved in these early metazoans. We describe the Hydra Notch receptor (HvNotch) and provide evidence for the conservation of the Notch signaling mode via regulated intramembrane proteolysis. We observed that nuclear translocation of the Notch intracellular domain (NID) was inhibited by the synthetic gamma-secretase inhibitor DAPT. Moreover, DAPT treatment of hydra polyps caused distinct differentiation defects in their interstitial stem cell lineage. Nerve cell differentiation proceeded normally but post-mitotic nematocyte differentiation was dramatically reduced. Early female germ cell differentiation was inhibited before exit from mitosis. From these results we conclude that gamma-secretase activity and presumably Notch signaling are required to control differentiation events in the interstitial cell lineage of Hydra.

Concentration-dependent modulation of amyloid-beta in vivo and in vitro using the gamma-secretase inhibitor, LY-450139

LY-450139 is a gamma-secretase inhibitor shown to have efficacy in multiple cellular and animal models. Paradoxically, robust elevations of plasma amyloid-beta (Abeta) have been reported in dogs and humans after administration of subefficacious doses. The present study sought to further evaluate Abeta responses to LY-450139 in the guinea pig, a nontransgenic model that has an Abeta sequence identical to that of human. Male guinea pigs were treated with LY-450139 (0.2-60 mg/kg), and brain, cerebrospinal fluid, and plasma Abeta levels were characterized at 1, 3, 6, 9, and 14 h postdose. Low doses significantly elevated plasma Abeta levels at early time points, with return to baseline within hours. Higher doses inhibited Abeta levels in all compartments at early time points, but elevated plasma Abeta levels at later time points. To determine whether this phenomenon occurs under steady-state drug exposure, guinea pigs were implanted with subcutaneous minipumps delivering LY-450139 (0.3-30 mg/kg/day) for 5 days. Plasma Abeta was significantly inhibited at 10-30 mg/kg/day, but significantly elevated at 1 mg/kg/day. To further understand the mechanism of Abeta elevation by LY-450139, H4 cells overexpressing the Swedish mutant of amyloid-precursor protein and a mouse embryonic stem cell-derived neuronal cell line were studied. In both cellular models, elevated levels of secreted Abeta were observed at subefficacious concentrations, whereas dose-responsive inhibition was observed at higher concentrations. These results suggest that LY-450139 modulates the gamma-secretase complex, eliciting Abeta lowering at high concentrations but Abeta elevation at low concentrations.

Modulators and Inhibitors of γ- and β-Secretases

Most gene mutations associated with Alzheimer's disease point to the metabolism of amyloid precursor protein as a potential cause. The beta- and gamma-secretases are two executioners of amyloid precursor protein processing resulting in amyloid-beta. Significant progress has been made in the selective inhibition of both proteases, regardless of structural information for gamma-secretase. Several peptidic and nonpeptidic leads were identified for both targets.

Gene expression profiles of AML derived stem cells; similarity to hematopoietic stem cells

Tumors contain a fraction of cancer stem cells that maintain the propagation of the disease. The CD34(+)CD38(-) cells, isolated from acute myeloid leukemia (AML), were shown to be enriched leukemic stem cells (LSC). We isolated the CD34(+)CD38(-) cell fraction from AML and compared their gene expression profiles to the CD34(+)CD38(+) cell fraction, using microarrays. We found 409 genes that were at least twofold over- or underexpressed between the two cell populations. These include underexpression of DNA repair, signal transduction and cell cycle genes, consistent with the relative quiescence of stem cells, and chromosomal aberrations and mutations of leukemic cells. Comparison of the LSC expression data to that of normal hematopoietic stem cells (HSC) revealed that 34% of the modulated genes are shared by both LSC and HSC, supporting the suggestion that the LSC originated within the HSC progenitors. We focused on the Notch pathway since Jagged-2, a Notch ligand was found to be overexpressed in the LSC samples. We show that DAPT, an inhibitor of gamma-secretase, a protease that is involved in Jagged and Notch signaling, inhibits LSC growth in colony formation assays. Identification of additional genes that regulate LSC self-renewal may provide new targets for therapy.

Demonstration of a common artifact in immunosorbent assays of brain extracts: Development of a solid-phase extraction protocol to enable measurement of amyloid-β from wild-type rodent brain

In the process of developing species-specific, immunosorbent assays for brain amyloid-beta (Abeta) in non-transgenic animals, we have demonstrated an artifact that impedes accurate quantitation of Abeta in this assay format. Using synthetic peptides, cerebrospinal fluid (CSF), or plasma samples, no nonspecific binding or cross-species immunoreactivity was detected in human or rodent Abeta assays. However, extracts of guinea pig brain (human Abeta sequence) or rat brain (rodent Abeta sequence) demonstrated immunoreactivity regardless of which capture antibody, detection antibody, or reporter method (colorimetric or fluorescent) was used. This immunoreactivity remained even in the absence of a capture antibody. Various blocking conditions failed to resolve the nonspecific binding of detection antibodies in the presence of brain extracts. Fractionation of DEA-extracted guinea pig brain over Sephadex G-50 demonstrated the feasibility of separating specific from nonspecific binding components in the brain extracts. Thus, a solid phase extraction method, compatible with multiple extraction buffers, has been developed to isolate and concentrate Abeta from brain extracts. This isolation method eliminates non-specific binding components from brain extracts and allows for accurate quantitation and robust detection of multiple Abeta peptides in extracts from wild-type animals.

COUP-TFI controls Notch regulation of hair cell and support cell differentiation

The orphan nuclear receptor COUP-TFI (Nr2f1) regulates many aspects of mammalian development, but little is known about its role in cochlear hair cell and Deiter's support cell development. The COUP-TFI knockout (COUP-TFI(-/-)) has a significant increase in hair cell (HC) number in the mid-to-apical turns. The total number of hair cells is not increased over wild type, perhaps because of displaced hair cells and a shortened cochlear duct. This implicates a defect of convergent-extension in the COUP-TFI(-/-) duct. In addition, excess proliferation in the COUP-TFI(-/-) sensory epithelium indicates that the origin of the extra HCs in the apex is complex. Because loss-of-function studies of Notch signaling components have similar phenotypes, we investigated Notch regulation of hair cell differentiation in COUP-TFI(-/-) mice and confirmed misregulation of Notch signaling components, including Jag1, Hes5 and in a manner consistent with reduced Notch signaling, and correlated with increases in hair cell and support cell differentiation. The disruption of Notch signaling by a gamma-secretase inhibitor in an in vitro organ culture system of wild-type cochleae resulted in a reduction in expression of the Notch target gene Hes5 and an increase in hair cell differentiation. Importantly, inhibition of Notch activity resulted in a greater increase in hair cell differentiation in COUP-TFI(-/-) cochlear cultures than in wild-type cultures, suggesting a hypersensitivity to Notch inactivation in COUP-TFI(-/-) cochlea, particularly at the apical turn. Thus, we present evidence that reduced Notch signaling contributes to increases in hair cell and support cell differentiation in COUP-TFI(-/-) mice, and suggest that COUP-TFI is required for Notch regulation of hair cell and support cell differentiation.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Alzheimer's disease

Alzheimer's disease is the most common cause of dementia. Research advances have enabled detailed understanding of the molecular pathogenesis of the hallmarks of the disease--ie, plaques, composed of amyloid beta (Abeta), and tangles, composed of hyperphosphorylated tau. However, as our knowledge increases so does our appreciation for the pathogenic complexity of the disorder. Familial Alzheimer's disease is a very rare autosomal dominant disease with early onset, caused by mutations in the amyloid precursor protein and presenilin genes, both linked to Abeta metabolism. By contrast with familial disease, sporadic Alzheimer's disease is very common with more than 15 million people affected worldwide. The cause of the sporadic form of the disease is unknown, probably because the disease is heterogeneous, caused by ageing in concert with a complex interaction of both genetic and environmental risk factors. This seminar reviews the key aspects of the disease, including epidemiology, genetics, pathogenesis, diagnosis, and treatment, as well as recent developments and controversies.

Gamma-secretase inhibitors for Alzheimer's disease: balancing efficacy and toxicity

The amyloid hypothesis, which states that beta-amyloid (Abeta) aggregates cause the onset and progression of Alzheimer's disease (AD), is a leading proposal to explain AD aetiology. Based on this hypothesis, compounds that inhibit gamma-secretase, one of the enzymes responsible for forming Abeta, are potential therapeutics for AD. Preclinical studies clearly establish that gamma-secretase inhibitors can reduce brain Abeta in rodent models. The initial investigation of the effects of a gamma-secretase inhibitor on Abeta-induced cognitive deficits in transgenic mice showed that modest Abeta reductions (15-30%) are sufficient to reverse Abeta-induced cognitive deficits in Tg2576 mice. Extending these studies to other gamma-secretase inhibitors and other models with Abeta-induced cognitive deficits will be important. Unfortunately, gamma-secretase inhibitors also cause abnormalities in the gastrointestinal tract, thymus and spleen in rodents. These changes likely result from inhibition of Notch cleavage, a transmembrane receptor involved in regulating cell-fate decisions. Two recent studies in rodents suggest that Abeta reduction using gamma-secretase inhibitors can be partially separated from Notch inhibition. Given the uncertain Abeta reduction target and the potential for mechanism-based toxicity, biomarkers for efficacy and toxicity would be helpful in clinical trials. The first report of gamma-secretase inhibitors in clinical trials was recently published. In this study, LY-450139 reduced plasma Abeta, but not cerebrospinal fluid Abeta. Taken together, the results of studies to date suggest that gamma-secretase inhibitors have the potential to address a large unmet medical need if the technical challenges can be overcome.

Transgenic analysis of the medaka mesp-b enhancer in somitogenesis

Somitogenesis is a critical step during the formation of metameric structures in vertebrates. Recent studies in mouse, chick, zebrafish and Xenopus have revealed that several factors, such as T-box genes, Notch/Delta, Wnt, retinoic acid and FGF signaling, are involved in the specification of nascent somites. By interacting with these pathways, the Mesp2-like bHLH transcription factors are transiently expressed in the anterior presomitic mesoderm and play a crucial role in somite formation. The regulatory mechanisms of Mesp2 and its related genes during somitogenesis have been studied in mouse and Xenopus. However, the precise mechanism that regulates the transcriptional activity of Mesp2 has yet to be determined. In our current report, we identify the essential enhancer element of medaka mesp-b, an orthologue of mouse Mesp2, using transgenic techniques and embryo manipulation. Our results demonstrate that a region of approximately 2.8 kb, upstream of the mesp-b gene, is responsible for both the initiation and anterior localization of mesp-b transcription within a somite primordium. Furthermore, putative motifs for both T-box transcription factors and Notch/Delta signaling are present in this enhancer region and are essential for activity.

Notch signaling regulates midline cell specification and proliferation in zebrafish

Notochord and floor plate cells are sources of molecules that pattern tissues near the midline, including the spinal cord. Hypochord cells are also found at the midline of anamniote embryos and are important for aorta development. Delta-Notch signaling regulates midline patterning in the dorsal organizer by inhibiting notochord formation and promoting hypochord and possibly floor plate development, but the precise mechanisms by which this regulation occurs are unknown. We demonstrate here that floor plate and hypochord cells arise from distinct regions of the zebrafish shield. Blocking Notch signaling during gastrulation entirely prevented hypochord specification but only reduced the number of floor plate cells that developed compared to control embryos. In contrast, elevation of Notch signaling at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not affected. A cell proliferation assay revealed that Notch signaling maintains dividing floor plate progenitors. Together, our results indicate that Notch signaling regulates allocation of appropriate numbers of different midline cells by different mechanisms.

Novel gamma-secretase inhibitors discovered by library screening of in-house synthetic natural product intermediates

Screening of our in-house compound library comprised of intermediates of natural product synthesis projects resulted in discovering two novel gamma-secretase inhibitors, which coincidently had similar moieties, that is, cyclohexenone and two aryl groups arranged on the core six-membered ring. Structure-activity relationship studies of these compounds were also developed.

Gamma secretase-mediated Notch signaling worsens brain damage and functional outcome in ischemic stroke

Mice transgenic for antisense Notch and normal mice treated with inhibitors of the Notch-activating enzyme gamma-secretase showed reduced damage to brain cells and improved functional outcome in a model of focal ischemic stroke. Notch endangers neurons by modulating pathways that increase their vulnerability to apoptosis, and by activating microglial cells and stimulating the infiltration of proinflammatory leukocytes. These findings suggest that Notch signaling may be a therapeutic target for treatment of stroke and related neurodegenerative conditions.

C-terminal Fragment of Presenilin Is the Molecular Target of a Dipeptidic γ-Secretase-specific Inhibitor DAPT (N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-Butyl Ester)

Gamma-secretase is a multimeric membrane protein complex composed of presenilin (PS), nicastrin, Aph-1 and, Pen-2 that is responsible for the intramembrane proteolysis of various type I transmembrane proteins, including amyloid beta-precursor protein and Notch. The direct labeling of PS polypeptides by transition-state analogue gamma-secretase inhibitors suggested that PS represents the catalytic center of gamma-secretase. Here we show that one of the major gamma-secretase inhibitors of dipeptidic type, N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT), targets the C-terminal fragment of PS, especially the transmembrane domain 7 or more C-terminal region, by designing and synthesizing DAP-BpB (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-(S)-phenylglycine-4-(4-(8-biotinamido)octylamino)benzoyl)benzyl)methylamide), a photoactivable DAPT derivative. We also found that DAP-BpB selectively binds to the high molecular weight gamma-secretase complex in an activity-dependent manner. Photolabeling of PS by DAP-BpB is completely blocked by DAPT or its structural relatives (e.g. Compound E) as well as by arylsulfonamides. In contrast, transition-state analogue inhibitor L-685,458 or alpha-helical peptidic inhibitor attenuated the photolabeling of PS1 only at higher concentrations. These data illustrate the DAPT binding site as a novel functional domain within the PS C-terminal fragment that is distinct from the catalytic site or the substrate binding site.

Synthesis and evaluation of succinoyl-caprolactam γ-secretase inhibitors

The synthesis, evaluation, and structure-activity relationships of a series of succinoyl lactam inhibitors of the Alzheimer's disease gamma-secretase are described. Beginning with a screening hit with broad proteinase activity, optimization provided compounds with both high selectivity for inhibition of gamma-secretase and high potency in cellular assays of A beta reduction. The SAR and early in vivo properties of this series of inhibitors will be presented.

Vertebrate Segmentation: Snail Counts the Time until Morphogenesis

During segmentation of vertebrate embryos, unsegmented mesenchymal mesoderm is divided into epithelial segments called somites. This process is governed by oscillating gene expression of the somite clock. A recent paper identifies the transcription factor Snail as a link between the somite clock and the control of somite morphogenesis.

In vivo characterization of Abeta(40) changes in brain and cerebrospinal fluid using the novel gamma-secretase inhibitor N-[cis-4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) in the rat

Plaques in the parenchyma of the brain containing Abeta peptides are one of the hallmarks of Alzheimer's disease. These Abeta peptides are produced by the final proteolytic cleavage of the amyloid precursor protein by the intramembraneous aspartyl protease gamma-secretase. Thus, one approach to lowering levels of Abeta has been via the inhibition of the gamma-secretase enzyme. Here, we report a novel, bioavailable gamma-secretase inhibitor, N-[cis-4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) that displayed oral pharmacokinetics suitable for once-a-day dosing. It was able to markedly reduce Abeta in the brain and cerebrospinal fluid (CSF) in the rat, with ED(50) values of 6 and 10 mg/kg, respectively. Time-course experiments using MRK-560 demonstrated these reductions in Abeta could be maintained for 24 h, and comparable temporal reductions in rat brain and CSF Abeta(40) further suggested that these two pools of Abeta are related. This relationship between the brain and CSF Abeta was maintained when MRK-560 was dosed once a day for 2 weeks, and accordingly, when all the data for the dose-response curve and time courses were correlated, a strong association was observed between the brain and CSF Abeta levels. These results demonstrate that MRK-560 is an orally bioavailable gamma-secretase inhibitor with the ability to markedly reduce Abeta peptide in the brain and CSF of the rat and confirm the utility of the rat for assessing the effects of gamma-secretase inhibitors on central nervous system Abeta(40) levels in vivo.

A faster migrating variant masquerades as NICD when performing in vitro gamma-secretase assays with bacterially expressed Notch substrates

Intramembrane proteolysis is a new and rapidly growing field. In vitro assays utilizing recombinant substrates for gamma-secretase, an intramembrane-cleaving enzyme, are critically important in order to characterize the biochemical properties of this unusual enzyme. Several recombinant Notch proteins of varying length are commonly used as in vitro substrates for CHAPSO-solubilized gamma-secretase. Here we report that several recombinant Notch constructs undergo limited or no proteolysis in vitro. Instead, upon incubation with or without gamma-secretase, variants of the intact protein migrate during SDS-PAGE at the location expected for the gamma-secretase specific cleavage products. In addition, we show that addition of aspartyl- and gamma-secretase specific protease inhibitors are able to retard the formation of these variants independent of gamma-secretase, which could lead to the erroneous conclusion that Notch cleavage by solubilized gamma-secretase was achieved in vitro even when no proteolysis occurred. In contrast, substrates produced in mammalian or insect cells are cleaved efficiently in vitro. These observations suggest that in vitro studies reliant on recombinant, bacterially produced Notch TMD should be performed with the inclusion of additional controls able to differentiate between actual cleavage and this potential artifact.

Ligand-Dependent Cleavage of the P75 Neurotrophin Receptor Is Necessary for NRIF Nuclear Translocation and Apoptosis in Sympathetic Neurons

The p75 neurotrophin receptor regulates neuronal survival, promoting it in some contexts yet activating apoptosis in others. The mechanism by which the receptor elicits these differential effects is poorly understood. Here, we demonstrate that p75 is cleaved by gamma-secretase in sympathetic neurons, specifically in response to proapoptotic ligands. This cleavage resulted in ubiquitination and subsequent nuclear translocation of NRIF, a DNA binding protein essential for p75-mediated apoptosis. Inhibition of gamma-secretase or expression of a mutant p75 resistant to this protease prevented receptor proteolysis, blocked NRIF nuclear entry, and prevented apoptosis. In contrast, overexpression of the p75 ICD resulted in NRIF nuclear accumulation and apoptosis. The receptor proteolysis and NRIF nuclear localization were also observed in vivo during naturally occurring cell death in the superior cervical ganglia. These results indicate that p75-mediated apoptosis requires gamma-secretase dependent release of its ICD, which facilitates nuclear translocation of NRIF.

New prospects and strategies for drug target discovery in neurodegenerative disorders

The future of neurodegenerative therapeutics development depends upon effective disease modification strategies centered on carefully investigated targets. Pharmaceutical research endeavors that probe for a much deeper understanding of disease pathogenesis, and explain how adaptive or compensatory mechanisms might be engaged to delay disease onset or progression, will produce the needed breakthroughs. Below, we discuss the prospects for new targets emerging out of the study of brain disease genes and their associated pathogenic pathways. We describe a general experimental paradigm that we are employing across several mouse models of neurodegenerative disease to elucidate molecular determinants of selective neuronal vulnerability. We outline key elements of our target discovery program and provide examples of how we integrate genomic technologies, neuroanatomical methods, and mouse genetics in the search for neurodegenerative disease targets.

SorLA signaling by regulated intramembrane proteolysis

The single-transmembrane receptor SorLA/LR11 contains binding domains typical for lipoprotein receptors and a VPS10 domain, which binds the neuropeptide head-activator. This undecapeptide enhances proliferation of neuronal precursor cells in a SorLA-dependent manner. Using specific inhibitors we found previously that head activator activates shedding of SorLA by the metalloprotease TACE close to the transmembrane domain releasing the large extra-cellular part of the receptor. Here we show that the remaining COOH-terminal membrane fragment of SorLA is processed by gamma-secretase. Inhibition of gamma-secretase by specific inhibitors or overexpression of dominant negative presenilin mutants and knock out of the presenilin genes led to accumulation of the SorLA membrane fragment and also of full-length SorLA in the membrane. In an in vitro assay we observed the gamma-secretase-dependent release of the two soluble cleavage products, the SorLA cytoplasmic domain and the SorLA beta-peptide. These processing steps are reminiscent of a novel signaling pathway that has been described for the notch receptor. Here, the notch cytoplasmic domain is released into the cytoplasm by the gamma-secretase and migrates to the nucleus where it acts as a transcriptional regulator. In parallel we found that a fusion protein of the released cytoplasmic tail of SorLA with EGFP located to the nucleus only if the nuclear localization signal of SorLA was intact. In a reporter gene assay the cytoplasmic domain of SorLA acted as a transcriptional activator indicating that SorLA might directly regulate transcription after activation by gamma-secretase.

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.

Oscillations of the snail genes in the presomitic mesoderm coordinate segmental patterning and morphogenesis in vertebrate somitogenesis

The segmented body plan of vertebrate embryos arises through segmentation of the paraxial mesoderm to form somites. The tight temporal and spatial control underlying this process of somitogenesis is regulated by the segmentation clock and the FGF signaling wavefront. Here, we report the cyclic mRNA expression of Snail 1 and Snail 2 in the mouse and chick presomitic mesoderm (PSM), respectively. Whereas Snail genes' oscillations are independent of NOTCH signaling, we show that they require WNT and FGF signaling. Overexpressing Snail 2 in the chick embryo prevents cyclic Lfng and Meso 1 expression in the PSM and disrupts somite formation. Moreover, cells mis-expressing Snail 2 fail to express Paraxis, remain mesenchymal, and are thereby inhibited from undergoing the epithelialization event that culminates in the formation of the epithelial somite. Thus, Snail genes define a class of cyclic genes that coordinate segmentation and PSM morphogenesis.

Acute gamma-secretase inhibition improves contextual fear conditioning in the Tg2576 mouse model of Alzheimer's disease

Transgenic mice (Tg2576) overexpressing the Swedish mutation of the human amyloid precursor protein display biochemical, pathological, and behavioral markers consistent with many aspects of Alzheimer's disease, including impaired hippocampal function. Impaired, hippocampal-dependent, contextual fear conditioning (CFC) is observed in mice as young as 20 weeks of age. This impairment can be attenuated after treatment before training with the phosphodiesterase-4 inhibitor rolipram (0.1 mg/kg, i.p.). A rolipram-associated improvement is also observed in the littermate controls, suggesting that the effect of rolipram is independent of beta-amyloid. Acute treatment before training (but not after training or before testing) with the gamma-secretase inhibitor (GSI) N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine-t-butylester (DAPT), at a dose that reduces brain concentrations of beta-amyloid (100 mg/kg), attenuates the impairment in 20- to 65-week-old Tg2576 mice. Importantly, DAPT had no effect on performance of control littermates. These data are supportive of a role of beta-amyloid in the impairment of CFC in Tg2576 mice. Furthermore, they suggest that acute treatment with GSI may provide improved cognitive functioning as well as disease-modifying effects in Alzheimer's disease.

Physiological regulation of the beta-amyloid precursor protein signaling domain by c-Jun N-terminal kinase JNK3 during neuronal differentiation

Beta-amyloid precursor protein (APP) is a conserved and ubiquitous transmembrane glycoprotein strongly implicated in the pathogenesis of Alzheimer's disease but whose normal biological function is unknown. Analogy to the Notch protein suggests that APP is a cell-surface receptor that signals via sequential proteolytic cleavages that release its intracellular domain (AICD) to the nucleus. Because these cleavages are major targets for therapeutic inhibition, it is critical to elucidate their physiological function. AICD is stabilized by Fe65, interacts with the transcriptional factor Tip60, and translocates to the nucleus. Here, we show that endogenous AICD in primary neurons is detectable only during a short period of time during differentiation in culture. During this transient rise, a portion of AICD localizes to the nucleus. Subsequently, phosphorylation of the APP cytoplasmic domain at threonine 668 appears to disrupt the stabilizing interaction with Fe65 and thus downregulate AICD-mediated signaling. Furthermore, we find that the neuron-specific c-Jun N-terminal kinase JNK3, but not JNK1 or JNK2, mediates a substantial portion of this phosphorylation. We conclude that endogenous AICD undergoes tight temporal regulation during the differentiation of neurons and is negatively regulated by JNK3 via phosphorylation of APP at Thr668.

Effect of gamma-secretase inhibitors on muscarinic receptor-mediated calcium signaling in human salivary epithelial cells

Altered intracellular Ca(2+) signaling has been observed in cells derived from Alzheimer's disease patients, and a possible link between gamma-secretase activity and the content of intracellular Ca(2+) stores has been suggested. To test this hypothesis we studied the effects of several gamma-secretase inhibitors on muscarinic receptor-mediated intracellular calcium release in the human salivary gland cell line HSG. Although several inhibitors in the peptide aldehyde class partially blocked carbachol-induced Ca(2+) transients, these effects did not appear to be due to gamma-secretase inhibition, and overall we found no evidence that inhibition of gamma-secretase activity had any significant effect on agonist-induced intracellular calcium release in HSG cells. In complementary experiments with presenilin-null cells we found that the reconstitution of gamma-secretase activity by transfection with wild-type presenilin 1 likewise had no significant effect on thapsigargin-induced Ca(2+) release. In a test of the specific hypothesis that the level of APP intracellular domain (AICD), the intracellular fragment of the beta-amyloid precursor protein (APP) resulting from gamma-secretase cleavage, can modulate the Ca(2+) content of the endoplasmic reticulum, we were unable to demonstrate any effect of APP small interfering RNA on the magnitude of carbachol-induced intracellular calcium release in HSG cells. Together our data cast considerable doubt on the hypothesis that there is a direct link between gamma-secretase activity and the content of intracellular Ca(2+) stores.

Amyloid Precursor Protein and Notch Intracellular Domains are Generated after Transport of their Precursors to the Cell Surface

Alzheimer's disease is characterized by brain deposition of extracellular amyloid beta-peptide (Abeta)-containing plaques. The cellular site of gamma-secretase activity, which releases Abeta and the corresponding amyloid precursor protein intracellular domain (AICD), remains controversial. Proposed cleavage sites range from the endoplasmic reticulum (ER), the Golgi apparatus, and the cell surface to endosomal compartments. We now used C99-green fluorescent protein (GFP), a fluorescent reporter substrate for gamma-secretase activity and monitored AICD production in living cells. C99-GFP is efficiently cleaved by gamma-secretase, and AICD-GFP is released into the cytosol. Inhibiting gamma-secretase results in accumulation of C99-GFP in early endosomes. By blocking selective transport steps along the secretory pathway, we demonstrate that gamma-secretase does not cleave its substrates in the ER, the Golgi/trans-Golgi network, or in secretory vesicles. In contrast, inhibition of endocytosis did not inhibit cleavage of C99-GFP. Similar results were obtained for another gamma-secretase substrate, NotchDeltaE. Our results suggest that intracellular domains are generated by gamma-secretase at the plasma membrane and/or early endosomes.

Current Pharmacotherapy for Alzheimer's Disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease that affects approximately 4.5 million people in the United States. The mainstays of current pharmacotherapy for AD are compounds aimed at increasing the levels of acetylcholine in the brain, thereby facilitating cholinergic neurotransmission through inhibition of the cholinesterases. These drugs, known as acetylcholinesterase inhibitors (AChEIs), were first approved by the U.S. Food and Drug Administration (FDA) in 1995 based on clinical trials showing modest symptomatic benefit on cognitive, behavioral, and global measures. In 2004 the FDA approved memantine, an NMDA antagonist, for treating dementia symptoms in moderate to severe AD cases. In clinical practice, memantine may be co-administered with an AChEI, although neither drug individually or in combination affects the underlying pathophysiology of dementia. Dementia in AD results from progressive synaptic loss and neuronal death. As knowledge of the mechanisms responsible for neurodegeneration in AD increases, it is anticipated that neuroprotective drugs to slow or prevent neuronal dysfunction and death will be developed to complement current symptomatic treatments.

Transgenic models of Alzheimer's disease: learning from animals

As the scope of the problem of Alzheimer's disease (AD) grows due to an aging population, research into the devastating condition has taken on added urgency. Rare inherited forms of AD provide insight into the molecular pathways leading to degeneration and have made possible the development of transgenic animal models. Several of these models are based on the overexpression of amyloid precursor protein (APP), presenilins, or tau to cause production and accumulation of amyloid-beta into plaques or hyperphosphorylated tau into neurofibrillary tangles. Producing these characteristic neuropathological lesions in animals causes progressive neurodegeneration and in some cases similar behavioral disruptions to those seen in AD patients. Knockout models of proteins involved in AD have also been generated to explore the native functions of these genes and examine whether pathogenesis is due to loss of function or toxic gain of function in these systems. Although none of the transgenic lines models the human condition exactly, the ability to study similar pathological processes in living animals have provided numerous insights into disease mechanisms and opportunities to test therapeutic agents. This chapter reviews animal models of AD and their contributions to developing therapeutic approaches for AD.

New pathways in drug discovery for alzheimer’s disease

Specific treatments for Alzheimer's disease (AD) were first introduced in the 1990s using the acetyl-cholinesterase inhibitors. More recently, the N-methyl-D-aspartate (NMDA) antagonist memantine has become available. Although these treatments do provide a modest improvement in the cognitive abnormalities present in AD, their pharmacology is based on manipulation of neurotransmitter systems, and there is no compelling evidence that they interfere with the underlying pathogenic process. Pathologic and genetic data have led to the hypothesis that a peptide called amyloid ss(Abeta) plays a primary role in the pathophysiology of AD. Several investigational therapies targeting Abeta are now undergoing clinical trials. This paper reviews the available data regarding Abeta-directed therapies that are in the clinic and summarizes the approach to biomarkers and clinical trial designs that can provide evidence of modification of the underlying disease process.

Modulation of neurosteroid production in human neuroblastoma cells by Alzheimer's disease key proteins

Studies performed with animals suggest neurosteroid involvement in neuroprotection. However in humans, the role of neurosteroidogenesis in the regulation of degenerative processes is unknown. To determine whether cellular factors intervening in degenerative mechanisms may interfere with the process of neurosteroidogenesis in humans, we combined pulse-chase experiments with HPLC and continuous flow scintillation detection to compare neurosteroid production in normal and transfected SH-SY5Y cells with key proteins involved in Alzheimer's disease (AD). Microscope analyses revealed that cell morphology was unchanged in stably transfected SH-SY5Y cells overexpressing human native tau (hTau40), mutant tau (P301L), and wild-type amyloid precursor protein (APPwt) compared to controls. Biochemical investigations showed that hTau40 enhanced progesterone (PROG), 17OHPROG, testosterone, and 3alpha-androstanediol neosynthesis from pregnenolone. In contrast, tau with the pathogenic P301L mutation was devoid of action on neurosteroidogenesis. Overexpression of APPwt inhibited PROG formation, did not affect 17OHPROG and testosterone, but increased 3alpha-androstanediol and estradiol synthesis. Extracellular treatment of control cells with aggregated amyloid peptide mimicked the action of APPwt expression on PROG but not on 3alpha-androstanediol and estradiol production. Moreover, PROG biosynthesis in APPwt cells was up-regulated in the presence of a gamma-secretase inhibitor. Our results provide the first evidence for the regulation of neurosteroid biosynthesis by key proteins involved in the etiology of AD. The data suggest that pathogenic factors may induce neurodegeneration in humans through the reduction of the synthesis of endogenous neuroprotective neurosteroids in nerve cells.

Lactacystin decreases amyloid-β peptide production by inhibiting β-secretase activity

The human amyloid precursor protein (APP) is processed by the nonamyloidogenic and the amyloidogenic catabolic pathways. The sequential cleavage of APP by the beta- and gamma-secretase activities, known as the amyloidogenic processing of APP, leads to the formation of the amyloid-beta peptide (Abeta). Abeta is the main constituent of the amyloid core of senile plaques, a typical hallmark of Alzheimer's disease. In addition to secretases, other cellular proteolytic activities, like the proteasome, might participate in the metabolism of APP. We investigated the consequence of proteasome inhibition on the amyloidogenic processing of human APP. CHO cells and primary cultures of rat cortical neurons expressing human APP or a protein corresponding to its beta-cleaved C-terminal fragment (C99) were treated with lactacystin, an irreversible inhibitor of the chymotrypsin-like activity of the proteasome. Lactacystin significantly decreased the level of Abeta produced from APP in both cellular models, whereas the production of Abeta from C99 was not affected. Lactacystin did not inhibit gamma-secretase activity but was found to inhibit the beta-cleavage of APP, leading to a proportional decrease in Abeta production. Although lactacystin did not inhibit the catalytic activity of recombinant BACE1, a decrease in neuronal beta-secretase activity was measured after treatment with lactacystin.

Amyloid precursor-like protein 1 influences endocytosis and proteolytic processing of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) is a key regulatory step in the generation of the Alzheimer disease amyloid beta peptide (Abeta). The molecular mechanisms underlying the control of APP shedding remain little understood but are in part dependent on the low density lipoprotein receptor-related protein (LRP), which is involved in APP endocytosis. Here, we show that the APP homolog APLP1 (amyloid precursor-like protein 1) influences APP shedding. In human embryonic kidney 293 cells expression of APLP1 strongly activated APP shedding by alpha-secretase and slightly reduced beta-secretase cleavage. As revealed by domain deletion analysis, the increase in APP shedding required the NPTY amino acid motif within the cytoplasmic domain of APLP1. This motif is conserved in APP and is essential for the endocytosis of APP and APLP1. Unrelated membrane proteins containing similar endocytic motifs did not affect APP shedding, showing that the increase in APP shedding was specific to APLP1. In LRP-deficient cells APLP1 no longer induced APP shedding, suggesting that in wild-type cells APLP1 interferes with the LRP-dependent endocytosis of APP and there by increases APP alpha-cleavage. In fact, an antibody uptake assay revealed that expression of APLP1 reduced the rate of APP endocytosis. In summary, our study provides a novel mechanism for APP shedding, in which APLP1 affects the endocytosis of APP and makes more APP available for alpha-secretase cleavage.

In vitro characterization of a gamma-secretase radiotracer in mammalian brain

Inhibition of gamma-secretase is a potential therapeutic target for Alzheimer's disease (AD). The present studies have characterized the in vitro properties of a radiolabeled small molecule gamma-secretase inhibitor, [3H]compound D (Yan et al., 2004, J. Neurosci.24, 2942-2952) in mammalian brain. [3H]Compound D was shown to bind with nanomolar affinity (Kd = 0.32-1.5 nM) to a single population of saturable sites in rat, rhesus and human brain cortex homogenates, the density of binding sites ranging from 4 to 7 nM across the species. Competition studies with a structurally diverse group of gamma-secretase inhibitors with a wide range of binding affinities showed that the binding affinities of these compounds correlated well with their ability to inhibit gamma-secretase in vitro. Autoradiographic studies showed that the specific binding of [3H]compound D was widely distributed throughout adult rat, rhesus and normal human brain. There did not appear to be any difference in distribution of [3H]compound D specific binding sites in AD cortex compared with control human cortex as measured using tissue section autoradiography, nor any correlation between gamma-secretase binding and plaque burden as measured immunohistochemically. [3H]compound D is a useful tool to probe the expression and pharmacology of gamma-secretase in mammalian brain.

Analysis of transmembrane domain mutants is consistent with sequential cleavage of Notch by gamma-secretase

gamma-Secretase is a lipid-embedded, intramembrane-cleaving aspartyl protease that cleaves its substrates twice within their transmembrane domains (TMD): once near the cytosolic leaflet (at S3/epsilon) and again in the middle of the TMD (at S4/gamma). To address whether this unusual process occurs in two independent or interdependent steps, we investigated how mutations at the S3/epsilon site in Notch1-based substrates impact proteolysis. We demonstrate that such mutations greatly inhibit not only gamma-secretase-mediated cleavage at S3 but also at S4, independent of their impact on NICD stability. These results, together with our previous observations, suggest that hydrolysis at the center of the Notch transmembrane domain (S4/gamma) is dependent on the S3/epsilon cleavage. Notch (and perhaps all gamma-secretase substrates) may be cleaved by sequential proteolysis starting at S3.

Protease inhibitors as potential disease-modifying therapeutics for Alzheimer’s disease

The current lack of an effective treatment for Alzheimer's disease (AD) has fuelled an intense search for novel therapies for this neurodegenerative condition. Aberrant production or decreased clearance of amyloid-beta peptides is widely accepted to be causative for AD. Amyloid-beta peptides are produced by sequential processing of the beta-amyloid precursor protein by the two aspartyl-type proteases beta-secretase and gamma-secretase. Because proteases are generally classified as druggable, these secretases are a centre of attraction for various drug discovery efforts. Although a large number of specific drug-like gamma-secretase inhibitors have been discovered, progress towards the clinic has been slowed by the broad substrate specificity of this unusual intramembrane-cleaving enzyme. In particular, the Notch receptor depends on gamma-secretase for its signalling function and, thus, gamma-secretase inhibition produces distinct phenotypes related to a disturbance of this pathway in preclinical animal models. The main task now is to define the therapeutic window in man between desired central efficacy and Notch-related side effects. In contrast, most studies with knockout animals have indicated that beta-secretase inhibition may have minimal adverse effects; however, the properties of the active site of this enzyme make it difficult to find small-molecule inhibitors that bind with high affinity. In most instances, inhibitors are large and peptidic in nature and, therefore, unsuitable as drug candidates. Thus, there are many issues associated with the development of protease inhibitors for AD that must be addressed before they can be used to test the 'amyloid cascade hypothesis' in the clinic. The outcomes of such trials will provide new directions to the scientific community and hopefully new treatment options for AD patients.

gamma-Secretase substrate selectivity can be modulated directly via interaction with a nucleotide-binding site

gamma-Secretase is an unusual protease with an intramembrane catalytic site that cleaves many type I membrane proteins, including the amyloid beta-protein (Abeta) precursor (APP) and the Notch receptor. Genetic and biochemical studies have identified four membrane proteins as components of gamma-secretase: heterodimeric presenilin composed of its N- and C-terminal fragments, nicastrin, Aph-1, and Pen-2. Here we demonstrated that certain compounds, including protein kinase inhibitors and their derivatives, act directly on purified gamma-secretase to selectively block cleavage of APP- but not Notch-based substrates. Moreover, ATP activated the generation of the APP intracellular domain and Abeta, but not the generation of the Notch intracellular domain by the purified protease complex, and was a direct competitor of the APP-selective inhibitors, as were other nucleotides. In accord, purified gamma-secretase bound specifically to an ATP-linked resin. Finally, a photoactivable ATP analog specifically labeled presenilin 1-C-terminal fragments in purified gamma-secretase preparations; the labeling was blocked by ATP itself and APP-selective gamma-secretase inhibitors. We concluded that a nucleotide-binding site exists within gamma-secretase, and certain compounds that bind to this site can specifically modulate the generation of Abeta while sparing Notch. Drugs targeting the gamma-secretase nucleotide-binding site represent an attractive strategy for safely treating Alzheimer disease.

Determination of guinea-pig cortical gamma-secretase activity ex vivo following the systemic administration of a gamma-secretase inhibitor

(2S)-2-{[(3,5-Diflurophenyl)acetyl]amino}-N-[(3S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepin-3-yl]propanamide (compound E) is a gamma-secretase inhibitor capable of reducing amyloid beta-peptide (1-40) and amyloid beta-peptide (1-42) levels. In this study we investigated the effect of in vivo administration of compound E on guinea-pig plasma, CSF and cortical amyloid beta-peptide (1-40) concentration. Using repeated sampling of CSF, compound E (30 mg/kg p.o.) was shown to cause a time-dependent decrease in CSF amyloid beta-peptide (1-40) levels, which was maximal at 3 h (70% inhibition), compared to baseline controls. After 3 h administration, compound E (3, 10 and 30 mg/kg p.o.), reduced plasma, CSF and DEA-extracted cortical amyloid beta-peptide (1-40) levels by 95, 97 and 99%; 26, 48 and 78%; 32, 33, and 47%, respectively, compared to vehicle control values. In the same animals, compound E (3, 10 and 30 mg/kg p.o.) inhibited cortical gamma-secretase activity, determined ex vivo using the recombinant substrate C100Flag, by 40, 71 and 79% of controls, respectively. These data demonstrate the value of determining not only the extent by which systemic administration of a gamma-secretase inhibitor reduces amyloid beta-peptide, but also the inhibition of brain gamma-secretase activity, as a more direct estimate of enzyme occupancy.

The intracellular domain of CD44 promotes the fusion of macrophages

Macrophages seed all tissues in which they have the ability, in specific and rare instances, to fuse with themselves and to differentiate into osteoclasts in bone or into giant cells in chronic inflammatory reactions. Although these cells play a central role in osteoporosis and in foreign body rejection, respectively, the molecular mechanism used by macrophages to fuse remains poorly understood. Macrophages might also fuse with somatic and tumor cells to promote tissue repair and metastasis, respectively. We reported that CD44 expression is highly induced in macrophages at the onset of fusion in which it plays a role. We report now that the intracellular domain of CD44 (CD44ICD) is cleaved in macrophages undergoing fusion and that presenilin inhibitors prevent the release of CD44ICD and fusion. We also show that CD44ICD promotes the fusion of tissue macrophages and bone marrow-derived macrophages. Finally, we report that CD44ICD is localized in the nucleus of macrophages in which it promotes the activation of NF-kappaB. These observations open avenues to study the role of CD44ICD in blood cells and tumors.

Method for the determination of the levels of β-amyloid peptide in the CSF sampled from freely moving rats

INTRODUCTION

In the present study, a model was developed to determine the effect of secretase inhibition on beta-amyloid peptide (Abeta) levels in the cerebrospinal fluid (CSF) of freely moving adult rats.

METHODS

Rats were chronically implanted with a cannula into the cisterna magna and CSF samples were collected at different time points from the same animal without anaesthesia. The levels of CSF Abeta were measured by a sandwich ELISA assay.

RESULTS

Administration of DAPT, a functional gamma-secretase inhibitor, resulted in a substantial reduction of Abeta40 and Abeta42, confirming the in vivo functionality of the CSF as a biomarker source for endogenous APP processing modulation by secretase inhibitors.

DISCUSSION

Thus, the present work provides clear evidence for the usefulness of CSF sampling from the freely moving rat model for testing the effectiveness of small molecule inhibitors of Abeta production.

Inhibition of angiogenesis and tumor growth by beta and gamma-secretase inhibitors

The involvement of beta-secretase and gamma-secretase in producing the beta-amyloid component of senile plaques found in the brain of Alzheimer's patients has fueled a major research effort to design selective inhibitors of these proteases. Interestingly, gamma-secretase cleaves several proteins including Notch, E-cadherin, CD44 and ErbB-4 (erythroblastic leukemia viral oncogene homolog 4), which are important modulators of angiogenesis. The beta-amyloid precursor protein, which is cleaved by beta-secretase and gamma-secretase to produce beta-amyloid, is highly expressed in the endothelium of neoforming vessels suggesting that it might play a role during angiogenesis. These data prompted us to explore the effects of beta and gamma-secretase inhibitors of different structures on angiogenesis and tumor growth. Both the gamma and beta-secretase inhibitors tested reduce endothelial cell proliferation without inducing cellular toxicity, suppress the formation of capillary structures in vitro and oppose the sprouting of microvessel outgrowths in the rat aortic ring model of angiogenesis. Moreover, they potently inhibit the growth and vascularization of human glioblastoma and human lung adenocarcinoma tumors xenotransplanted into nude mice. Altogether these data suggest that the gamma and beta-secretases play an essential role during angiogenesis and that inhibitors of the beta and gamma-secretases may constitute new classes of anti-angiogenic and anti-tumoral compounds.

Longer forms of amyloid beta protein: implications for the mechanism of intramembrane cleavage by gamma-secretase

Gamma-cleavage of beta-amyloid precursor protein (APP) in the middle of the cell membrane generates amyloid beta protein (Abeta), and epsilon-cleavage, approximately 10 residues downstream of the gamma-cleavage site, releases the APP intracellular domain (AICD). A significant link between generation of Abeta and AICD and failure to detect AICD41-99 led us to hypothesize that epsilon-cleavage generates longer Abetas, which are then processed to Abeta40/42. Using newly developed gel systems and an N-end-specific monoclonal antibody, we have identified the longer Abetas (Abeta1-43, Abeta1-45, Abeta1-46, and Abeta1-48) within the cells and in brain tissues. The production of these longer Abetas as well as Abeta40/42 is presenilin dependent and is suppressed by {1S-benzyl-4R-[1S-carbamoyl-2-phenylethylcarbamoyl-1S-3-methylbutylcarbamoyl]-2R-hydroxy-5-phenylpentyl}carbamic acid tert-butyl ester, a transition state analog inhibitor for aspartyl protease. In contrast, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, a potent dipeptide gamma-secretase inhibitor, builds up Abeta1-43 and Abeta1-46 intracellularly, which was also confirmed by mass spectrometry. Notably, suppression of Abeta40 appeared to lead to an increase in Abeta43, which in turn brings an increase in Abeta46, in a dose-dependent manner. We therefore propose an alpha-helical model in which longer Abeta species generated by epsilon-cleavage is cleaved at every three residues in its carboxyl portion.

Different thresholds of Notch signaling bias human precursor cells toward B-, NK-, monocytic/dendritic-, or T-cell lineage in thymus microenvironment

Notch receptors are involved in lineage decisions in multiple developmental scenarios, including hematopoiesis. Here, we treated hybrid human-mouse fetal thymus organ culture with the gamma-secretase inhibitor 7 (N-[N-(3,5-difluorophenyl)-l-alanyl]-S-phenyl-glycine t-butyl ester) (DAPT) to establish the role of Notch signaling in human hematopoietic lineage decisions. The effect of inhibition of Notch signaling was studied starting from cord blood CD34(+) or thymic CD34(+)CD1(-), CD34(+)CD1(+), or CD4ISP progenitors. Treatment of cord blood CD34(+) cells with low DAPT concentrations results in aberrant CD4ISP and CD4/CD8 double-positive (DP) thymocytes, which are negative for intracellular T-cell receptor beta (TCRbeta). On culture with intermediate and high DAPT concentrations, thymic CD34(+)CD1(-) cells still generate aberrant intracellular TCRbeta(-) DP cells that have undergone DJ but not VDJ recombination. Inhibition of Notch signaling shifts differentiation into non-T cells in a thymic microenvironment, depending on the starting progenitor cells: thymic CD34(+)CD1(+) cells do not generate non-T cells, thymic CD34(+)CD1(-) cells generate NK cells and monocytic/dendritic cells, and cord blood CD34(+)Lin(-) cells generate B, NK, and monocytic/dendritic cells in the presence of DAPT. Our data indicate that Notch signaling is crucial to direct human progenitor cells into the T-cell lineage, whereas it has a negative impact on B, NK, and monocytic/dendritic cell generation in a dose-dependent fashion.

Lithium chloride increases the production of amyloid-beta peptide independently from its inhibition of glycogen synthase kinase 3

Glycogen synthase kinase 3 (GSK3) is able to phosphorylate tau at many sites that are found to be phosphorylated in paired helical filaments in Alzheimer disease. Lithium chloride (LiCl) efficiently inhibits GSK3 and was recently reported to also decrease the production of amyloid-beta peptide (Abeta) from its precursor, the amyloid precursor protein. Therefore, lithium has been proposed as a combined therapeutic agent, inhibiting both the hyperphosphorylation of tau and the production of Abeta. Here, we demonstrate that the inhibition of GSK3 by LiCl induced the nuclear translocation of beta-catenin in Chinese hamster ovary cells and rat cultured neurons, in which a decrease in tau phosphorylation was observed. In both cellular models, a nontoxic concentration of LiCl increased the production of Abeta by increasing the beta-cleavage of amyloid precursor protein, generating more substrate for an unmodified gamma-secretase activity. SB415286, another GSK3 inhibitor, induced the nuclear translocation of beta-catenin and slightly decreased Abeta production. It is concluded that the LiCl-mediated increase in Abeta production is not related to GSK3 inhibition.