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

A novel mouse model for N-terminal truncated Aβ2-x generation through meprin β overexpression in astrocytes

Neurotoxic amyloid-β (Aβ) peptides cause neurodegeneration in Alzheimer's disease (AD) patients' brains. They are released upon proteolytic processing of the amyloid precursor protein (APP) extracellularly at the β-secretase site and intramembranously at the γ-secretase site. Several AD mouse models were developed to conduct respective research in vivo. Most of these classical models overexpress human APP with mutations driving AD-associated pathogenic APP processing. However, the resulting pattern of Aβ species in the mouse brains differs from those observed in AD patients' brains. Particularly mutations proximal to the β-secretase cleavage site (e.g., the so-called Swedish APP (APPswe) fostering Aβ1-x formation) lead to artificial Aβ production, as N-terminally truncated Aβ peptides are hardly present in these mouse brains. Meprin β is an alternative β-secretase upregulated in brains of AD patients and capable of generating N-terminally truncated Aβ2-x peptides. Therefore, we aimed to generate a mouse model for the production of so far underestimated Aβ2-x peptides by conditionally overexpressing meprin β in astrocytes. We chose astrocytes as meprin β was detected in this cell type in close proximity to Aβ plaques in AD patients' brains. The meprin β-overexpressing mice showed elevated amyloidogenic APP processing detected with a newly generated neo-epitope-specific antibody. Furthermore, we observed elevated Aβ production from endogenous APP as well as AD-related behavior changes (hyperlocomotion and deficits in spatial memory). The novel mouse model as well as the established tools and methods will be helpful to further characterize APP cleavage and the impact of different Aβ species in future studies.

Meprin β knockout reduces brain Aβ levels and rescues learning and memory impairments in the APP/lon mouse model for Alzheimer's disease

β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the major described β-secretase to generate Aβ peptides in Alzheimer's disease (AD). However, all therapeutic attempts to block BACE1 activity and to improve AD symptoms have so far failed. A potential candidate for alternative Aβ peptides generation is the metalloproteinase meprin β, which cleaves APP predominantly at alanine in p2 and in this study we can detect an increased meprin β expression in AD brain. Here, we report the generation of the transgenic APP/lon mouse model of AD lacking the functional Mep1b gene (APP/lon × Mep1b-/-). We examined levels of canonical and truncated Aβ species using urea-SDS-PAGE, ELISA and immunohistochemistry in brains of APP/lon mouse × Mep1b-/-. Additionally, we investigated the cognitive abilities of these mice during the Morris water maze task. Aβ1-40 and 1-42 levels are reduced in APP/lon mice when meprin β is absent. Immunohistochemical staining of mouse brain sections revealed that N-terminally truncated Aβ2-x peptide deposition is decreased in APP/lon × Mep1b-/- mice. Importantly, loss of meprin β improved cognitive abilities and rescued learning behavior impairments in APP/lon mice. These observations indicate an important role of meprin β within the amyloidogenic pathway and Aβ production in vivo.

Caspase Activation and Caspase-Mediated Cleavage of APP Is Associated with Amyloid β-Protein-Induced Synapse Loss in Alzheimer's Disease

Amyloid β-protein (Aβ) toxicity is hypothesized to play a seminal role in Alzheimer's disease (AD) pathogenesis. However, it remains unclear how Aβ causes synaptic dysfunction and synapse loss. We hypothesize that one mechanism of Aβ-induced synaptic injury is related to the cleavage of amyloid β precursor protein (APP) at position D664 by caspases that release the putatively cytotoxic C31 peptide. In organotypic slice cultures derived from mice with a knock-in mutation in the APP gene (APP D664A) to inhibit caspase cleavage, Aβ-induced synaptic injury is markedly reduced in two models of Aβ toxicity. Loss of dendritic spines is also attenuated in mice treated with caspase inhibitors. Importantly, the time-dependent dendritic spine loss is correlated with localized activation of caspase-3 but is absent in APP D664A cultures. We propose that the APP cytosolic domain plays an essential role in Aβ-induced synaptic damage in the injury pathway mediated by localized caspase activation.

Unexpected beta-amyloid production by middle doses of resveratrol through stabilization of APP protein and AMPK-mediated inhibition of trypsin-like proteasome activity in a cell model of Alzheimer's disease

Resveratrol is a drug candidate used for Alzheimer's disease (AD) and shows beneficial effects in various toxicity and production models, although recent clinical trial data did not show satisfactory results. Here we demonstrated the potential side effects of resveratrol in AD. We demonstrated resveratrol concentration- and time-dependent Aβ production using Aβ secreted cellular model and analyzed resveratrol-related molecular signaling. In Swedish mutant of APP (APPsw) stably expressing cells, treatment with a middle dose of resveratrol for 24 h unexpectedly increased Aβ production, but higher concentrations or shorter treatment durations did not. Resveratrol-mediated Aβ production was caused by an increase in APP protein levels associated with proteasome-dependent regulation of APP stability. Inhibition of AMPK, cAMP production, and epac1 attenuated Aβ production and APP increase by resveratrol, which blocked the inhibition of trypsin-like proteasomal activity. In addition, high-dose resveratrol decreased Aβ secretion and β-secretase activity at any treatment duration. Our data suggest that an appropriate dose of resveratrol can paradoxically increase Aβ production via stabilization of APP protein in an AMPK-proteasome signaling-dependent manner, which provides mechanistic insights into prior unsatisfactory clinical outcomes and the future clinical use of resveratrol.

Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Lipids constitute the bulk of the dry mass of the brain and have been associated with healthy function as well as the most common pathological conditions of the brain. Demographic factors, genetics, and lifestyles are the major factors that influence lipid metabolism and are also the key components of lipid disruption in Alzheimer's disease (AD). Additionally, the most common genetic risk factor of AD, APOE ϵ4 genotype, is involved in lipid transport and metabolism. We propose that lipids are at the center of Alzheimer's disease pathology based on their involvement in the blood-brain barrier function, amyloid precursor protein (APP) processing, myelination, membrane remodeling, receptor signaling, inflammation, oxidation, and energy balance. Under healthy conditions, lipid homeostasis bestows a balanced cellular environment that enables the proper functioning of brain cells. However, under pathological conditions, dyshomeostasis of brain lipid composition can result in disturbed BBB, abnormal processing of APP, dysfunction in endocytosis/exocytosis/autophagocytosis, altered myelination, disturbed signaling, unbalanced energy metabolism, and enhanced inflammation. These lipid disturbances may contribute to abnormalities in brain function that are the hallmark of AD. The wide variance of lipid disturbances associated with brain function suggest that AD pathology may present as a complex interaction between several metabolic pathways that are augmented by risk factors such as age, genetics, and lifestyles. Herewith, we examine factors that influence brain lipid composition, review the association of lipids with all known facets of AD pathology, and offer pointers for potential therapies that target lipid pathways.

Alzheimer's Disease and Diabetes: Insulin Signaling as the Bridge Linking Two Pathologies

Alzheimer's (or Alzheimer) disease (AD) is the most prevalent subset of dementia, affecting elderly populations worldwide. The cumulative costs of the AD care are rapidly accelerating as the average lifespan increases. Onset and risk factors for AD and AD-like dementias have been largely unknown until recently. Studies show that chronic type II diabetes mellitus (DM) is closely associated with neurodegeneration, especially AD. Type II DM is characterized by the cells' inability to take up insulin, as well as chronic hyperglycemia. In the central nervous system, insulin has crucial regulatory roles, while chronic hyperglycemia leads to formation and accumulation of advanced glycation end products (AGEs). AGEs are the major contributor to insulin resistance in diabetic cells, due to their regulatory role on sirtuin expression. Insulin activity in the central nervous system is known to interact with key proteins affected in neurodegenerative conditions, such as amyloid-β precursor protein (AβPP or APP), huntingtin-associated protein-1 (HAP1), Abelson helper integration site-1 (AHI1 or Jouberin), kinesin, and tau. Sirtuins have been theorized to be the mechanism for insulin resistance, and have been found to be affected in neurodegenerative conditions as well. There are hints that all these neuronal proteins may be closely related, although the mechanisms remain unclear. This review will gather existing research on these proteins and highlight the link between neurodegenerative conditions and diabetes mellitus.

Degradome of soluble ADAM10 and ADAM17 metalloproteases

Disintegrin and metalloproteinases (ADAMs) 10 and 17 can release the extracellular part of a variety of membrane-bound proteins via ectodomain shedding important for many biological functions. So far, substrate identification focused exclusively on membrane-anchored ADAM10 and ADAM17. However, besides known shedding of ADAM10, we identified ADAM8 as a protease capable of releasing the ADAM17 ectodomain. Therefore, we investigated whether the soluble ectodomains of ADAM10/17 (sADAM10/17) exhibit an altered substrate spectrum compared to their membrane-bound counterparts. A mass spectrometry-based N-terminomics approach identified 134 protein cleavage events in total and 45 common substrates for sADAM10/17 within the secretome of murine cardiomyocytes. Analysis of these cleavage sites confirmed previously identified amino acid preferences. Further in vitro studies verified fibronectin, cystatin C, sN-cadherin, PCPE-1 as well as sAPP as direct substrates of sADAM10 and/or sADAM17. Overall, we present the first degradome study for sADAM10/17, thereby introducing a new mode of proteolytic activity within the protease web.

Emerging Alternative Proteinases in APP Metabolism and Alzheimer's Disease Pathogenesis: A Focus on MT1-MMP and MT5-MMP

Processing of amyloid beta precursor protein (APP) into amyloid-beta peptide (Aβ) by β-secretase and γ-secretase complex is at the heart of the pathogenesis of Alzheimer’s disease (AD). Targeting this proteolytic pathway effectively reduces/prevents pathology and cognitive decline in preclinical experimental models of the disease, but therapeutic strategies based on secretase activity modifying drugs have so far failed in clinical trials. Although this may raise some doubts on the relevance of β- and γ-secretases as targets, new APP-cleaving enzymes, including meprin-β, legumain (δ-secretase), rhomboid-like protein-4 (RHBDL4), caspases and membrane-type matrix metalloproteinases (MT-MMPs/η-secretases) have confirmed that APP processing remains a solid mechanism in AD pathophysiology. This review will discuss recent findings on the roles of all these proteinases in the nervous system, and in particular on the roles of MT-MMPs, which are at the crossroads of pathological events involving not only amyloidogenesis, but also inflammation and synaptic dysfunctions. Assessing the potential of these emerging proteinases in the Alzheimer’s field opens up new research prospects to improve our knowledge of fundamental mechanisms of the disease and help us establish new therapeutic strategies.

Can brain impermeable BACE1 inhibitors serve as anti-CAA medicine?

Background

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of ß-amyloid peptides (Aß) in and surrounding the wall of microvasculature in the central nervous system, together with parenchymal amyloid plaques collectively referred to as cerebral amyloidosis, which occurs in the brain commonly among the elderly and more frequently in patients with Alzheimer’s disease (AD). CAA is associated with vascular injury and may cause devastating neurological outcomes. No therapeutic approach is available for this lesion to date.

Main body

ß-Secretase 1 (BACE1) is the enzyme initiating Aß production. Brain permeable BACE1 inhibitors targeting primarily at the parenchymal plaque pathology are currently evaluated in clinical trials. This article presents findings in support of a role of BACE1 elevation in the development of CAA, in addition to plaque pathogenesis. The rationale, feasibility, benefit and strategic issues for developing BACE1 inhibitors against CAA are discussed. Brain impermeable compounds are considered preferable as they might exhibit sufficient anti-CAA efficacy without causing significant neuronal/synaptic side effects.

Conclusion

Early pharmacological intervention to the pathogenesis of CAA is expected to provide significant protection for cerebral vascular health and hence brain health. Brain impermeable BACE1 inhibitors should be optimized and tested as potential anti-CAA therapeutics.

Computational approach for the assessment of inhibitory potency against beta-amyloid aggregation

Beta-amyloid (Aβ) plaques are one of the hallmarks of Alzheimer's disease. Their presence in the brain leads to neurodegeneration and memory decline. Therefore, search for new drugs able to decrease formation of such deposits is of great interest. Our previously developed multifunctional compounds inhibited transformation of monomers into fibrils. Herein, we describe the computational approach for the assessment of inhibitory activity against Aβ aggregation. The influence of novel inhibitors on amyloid Aβ_17-42 was studied by employing of molecular docking and all-atom molecular dynamics simulations. We found that the number of intermolecular backbone hydrogen bonds at the end of 100ns MD simulation was correlated with the level of anti-aggregation potency of studied compounds. Such data may be successfully applied to in silico design of novel inhibitors of Aβ aggregation.

Does any drug to treat cancer target mTOR and iron hemostasis in neurodegenerative disorders?

The prevalence of neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Huntington's disease are increased by age. Alleviation of their symptoms and protection of normal neurons against degeneration are the main aspects of the research to establish novel therapeutic strategies. Iron as the one of most important cation not only play important role in the structure of electron transport chain proteins but also has pivotal duties in cellular activities. But disruption in iron hemostasis can make it toxin to neurons which causes lipid peroxidation, DNA damage and etc. In patients with Alzheimer and Parkinson misbalancing in iron homeostasis accelerate neurodegeneration and cause neuroinflmmation. mTOR as the common signaling pathway between cancer and neurodegenerative disorders controls iron uptake and it is in active form in both diseases. Anti-cancer drugs which target mTOR causes iron deficiency and dual effects of mTOR inhibitors can candidate them as a therapeutic strategy to alleviate neurodegeneration/inflammation because of iron overloading.

Aβ-Induced Insulin Resistance and the Effects of Insulin on the Cholesterol Synthesis Pathway and Aβ Secretion in Neural Cells

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) toxicity, tau pathology, insulin resistance, neuroinflammation, and dysregulation of cholesterol homeostasis, all of which play roles in neurodegeneration. Insulin has polytrophic effects on neurons and may be at the center of these pathophysiological changes. In this study, we investigated possible relationships among insulin signaling and cholesterol biosynthesis, along with the effects of Aβ42 on these pathways in vitro. We found that neuroblastoma 2a (N2a) cells transfected with the human gene encoding amyloid-β protein precursor (AβPP) (N2a-AβPP) produced Aβ and exhibited insulin resistance by reduced p-Akt and a suppressed cholesterol-synthesis pathway following insulin treatment, and by increased phosphorylation of insulin receptor subunit-1 at serine 612 (p-IRS-S612) as compared to parental N2a cells. Treatment of human neuroblastoma SH-SY5Y cells with Aβ42 also increased p-IRS-S612, suggesting that Aβ42 is responsible for insulin resistance. The insulin resistance was alleviated when N2a-AβPP cells were treated with higher insulin concentrations. Insulin increased Aβ release from N2a-AβPP cells, by which it may promote Aβ clearance. Insulin increased cholesterol-synthesis gene expression in SH-SY5Y and N2a cells, including 24-dehydrocholesterol reductase (DHCR24) and 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCR) through sterol-regulatory element-binding protein-2 (SREBP2). While Aβ42-treated SH-SY5Y cells exhibited increased HMGCR expression and c-Jun phosphorylation as pro-inflammatory responses, they also showed down-regulation of neuro-protective/anti-inflammatory DHCR24. These results suggest that Aβ42 may cause insulin resistance, activate JNK for c-Jun phosphorylation, and lead to dysregulation of cholesterol homeostasis, and that enhancing insulin signaling may relieve the insulin-resistant phenotype and the dysregulated cholesterol-synthesis pathway to promote Aβ release for clearance from neural cells.

The Role of Oxidative Stress-Induced Epigenetic Alterations in Amyloid-β Production in Alzheimer's Disease

An increasing number of studies have proposed a strong correlation between reactive oxygen species (ROS)-induced oxidative stress (OS) and the pathogenesis of Alzheimer's disease (AD). With over five million people diagnosed in the United States alone, AD is the most common type of dementia worldwide. AD includes progressive neurodegeneration, followed by memory loss and reduced cognitive ability. Characterized by the formation of amyloid-beta (Aβ) plaques as a hallmark, the connection between ROS and AD is compelling. Analyzing the ROS response of essential proteins in the amyloidogenic pathway, such as amyloid-beta precursor protein (APP) and beta-secretase (BACE1), along with influential signaling programs of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and c-Jun N-terminal kinase (JNK), has helped visualize the path between OS and Aβ overproduction. In this review, attention will be paid to significant advances in the area of OS, epigenetics, and their influence on Aβ plaque assembly. Additionally, we aim to discuss available treatment options for AD that include antioxidant supplements, Asian traditional medicines, metal-protein-attenuating compounds, and histone modifying inhibitors.

Activation of mTOR: a culprit of Alzheimer's disease?

Alzheimer's disease (AD) is characterized by cognitive impairment in clinical presentation, and by β-amyloid (Aβ) production and the hyper-phosphorylation of tau in basic research. More highlights demonstrate that the activation of the mammalian target of rapamycin (mTOR) enhances Aβ generation and deposition by modulating amyloid precursor protein (APP) metabolism and upregulating β- and γ-secretases. mTOR, an inhibitor of autophagy, decreases Aβ clearance by scissoring autophagy function. mTOR regulates Aβ generation or Aβ clearance by regulating several key signaling pathways, including phosphoinositide 3-kinase (PI3-K)/protein kinase B (Akt), glycogen synthase kinase 3 [GSK-3], AMP-activated protein kinase (AMPK), and insulin/insulin-like growth factor 1 (IGF-1). The activation of mTOR is also a contributor to aberrant hyperphosphorylated tau. Rapamycin, the inhibitor of mTOR, may mitigate cognitive impairment and inhibit the pathologies associated with amyloid plaques and neurofibrillary tangles by promoting autophagy. Furthermore, the upstream and downstream components of mTOR signaling are involved in the pathogenesis and progression of AD. Hence, inhibiting the activation of mTOR may be an important therapeutic target for AD.

Endoplasmic reticulum stress promotes amyloid-beta peptides production in RGC-5 cells

Endoplasmic reticulum (ER) stress has been implicated in various neurodegenerative diseases, including Alzheimer's disease. We have previously observed amyloid production in the retina of the Tg2576 transgenic mouse model of Alzheimer's disease. In this study, we used tunicamycin-induced ER stress in RGC-5 cells, a cell line identical to the photoreceptor cell line 661W, to investigate the effect of ER stress on production of amyloid-beta (Abeta) peptides. We found that the mRNA level of amyloid-beta precursor protein (APP) remained stable, while the protein level of amyloid-beta precursor protein (APP) was decreased, the amyloid-beta precursor protein cleaving enzymes beta-site APP-cleaving enzyme 1 and presenilin 1 were upregulated, Abeta1-40 and Abeta1-42 production were increased, and reactive oxygen species production and apoptosis markers were elevated following induction of ER stress. The protein level of Abeta degradation enzymes, neprilysin, endothelin-converting enzyme 1, and endothelin-converting enzyme 2 remained unchanged during the prolonged ER stress, showing that the generation of Abeta did not result from reduction of proteolysis by these enzymes. Inclusion of group II caspase inhibitor, Z-DEVD-FMK, increased the ER stress mediated Abeta production, suggesting that they are generated by a caspase-independent mechanism. Our findings provided evidence of a role of ER stress in Abeta peptide overproduction and apoptotic pathway activation in RGC-5 cells.

Altered processing of amyloid precursor protein in cells undergoing apoptosis

Altered proteolysis of amyloid precursor protein is an important determinant of pathology development in Alzheimer's disease. Here, we describe the detection of two novel fragments of amyloid precursor protein in H4 neuroglioma cells undergoing apoptosis. Immunoreactivity of these 25-35 kDa fragments to two different amyloid precursor protein antibodies suggests that they contain the amyloid-β region and an epitope near the C-terminus of amyloid precursor protein. Generation of these fragments is associated with cleavage of caspase-3 and caspase-7, suggesting activation of these caspases. Studies in neurons undergoing DNA damage-induced apoptosis also showed similar results. Inclusion of caspase inhibitors prevented the generation of these novel fragments, suggesting that they are generated by a caspase-dependent mechanism. Molecular weight prediction and immunoreactivity of the fragments generated suggested that such fragments could not be generated by cleavage at any previously identified caspase, secretase, or calpain site on amyloid precursor protein. Bioinformatic analysis of the amino acid sequence of amyloid precursor protein revealed that fragments fitting the observed size and immunoreactivity could be generated by either cleavage at a novel, hitherto unidentified, caspase site or at a previously identified matrix metalloproteinase site in the extracellular domain. Proteolytic cleavage at any of these sites leads to a decrease in the generation of α-secretase cleaved secreted APP, which has both anti-apoptotic and neuroprotective properties, and thus may contribute to neurodegeneration in Alzheimer's disease.

Amyloid precursor protein (APP) regulates synaptic structure and function

The amyloid precursor protein (APP) plays a critical role in Alzheimer's disease (AD) pathogenesis. APP is proteolytically cleaved by β- and γ-secretases to generate the amyloid β-protein (Aβ), the core protein component of senile plaques in AD. It is also cleaved by α-secretase to release the large soluble APP (sAPP) luminal domain that has been shown to exhibit trophic properties. Increasing evidence points to the development of synaptic deficits and dendritic spine loss prior to deposition of amyloid in transgenic mouse models that overexpress APP and Aβ peptides. The consequence of loss of APP, however, is unsettled. In this study, we investigated whether APP itself plays a role in regulating synaptic structure and function using an APP knock-out (APP-/-) mouse model. We examined dendritic spines in primary cultures of hippocampal neurons and CA1 neurons of hippocampus from APP-/- mice. In the cultured neurons, there was a significant decrease (~35%) in spine density in neurons derived from APP-/- mice compared to littermate control neurons that were partially restored with sAPPα-conditioned medium. In APP-/- mice in vivo, spine numbers were also significantly reduced but by a smaller magnitude (~15%). Furthermore, apical dendritic length and dendritic arborization were markedly diminished in hippocampal neurons. These abnormalities in neuronal morphology were accompanied by reduction in long-term potentiation. Strikingly, all these changes in vivo were only seen in mice that were 12-15 months in age but not in younger animals. We propose that APP, specifically sAPP, is necessary for the maintenance of dendritic integrity in the hippocampus in an age-associated manner. Finally, these age-related changes may contribute to AD pathology independent of Aβ-mediated synaptic toxicity.

APP processing in Alzheimer's disease

An important pathological feature of Alzheimer's disease (AD) is the presence of extracellular senile plaques in the brain. Senile plaques are composed of aggregations of small peptides called β-amyloid (Aβ). Multiple lines of evidence demonstrate that overproduction/aggregation of Aβ in the brain is a primary cause of AD and inhibition of Aβ generation has become a hot topic in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavages first by β-secretase and then by γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα and preclude Aβ generation. Cleavage of APP by caspases may also contribute to AD pathologies. Therefore, understanding the metabolism/processing of APP is crucial for AD therapeutics. Here we review current knowledge of APP processing regulation as well as the patho/physiological functions of APP and its metabolites.

Mechanism of cytotoxicity mediated by the C31 fragment of the amyloid precursor protein

The cytoplasmic tail of the amyloid precursor protein (APP) contains two putatively cytotoxic peptides, Jcasp and C31, derived by caspase cleavage of APP. Jcasp is a fragment starting from the epsilon-secretase site to position 664, while C31 is a fragment from position 665 to the C-terminus. Our studies now showed that compared to C31, Jcasp appeared to play a minor role in cytotoxicity. In particular, inhibition of Jcasp generation by treatment of gamma-secretase inhibitor did not lead to any attenuation of C31-induced toxicity. Secondly, because C31 toxicity is largely absent in cells lacking endogenous APP, we determined, using a split beta-galactosidase complementary assay to monitor protein-protein interactions, the presence of APP associated complexes. Our results demonstrated that both APP homomeric and C31/APP heteromeric complexes were correlated with cell death, indicating that C31 complexes with APP to recruit the interacting partners that initiate the signals related to cellular toxicity.

p120 catenin recruits cadherins to gamma-secretase and inhibits production of Abeta peptide

The gamma-secretase complex cleaves many transmembrane proteins, including amyloid precursor protein, EphB and ErbB tyrosine kinase receptors, Notch1 receptors, and adhesion factors. Presenilin 1, the catalytic subunit of gamma-secretase, associates with the cadherin/catenin cell-cell adhesion/communication system and promotes cadherin processing (Georgakopoulos, A., et al. (1999) Mol. Cell 4, 893-902; Marambaud, P., et al. (2002) EMBO J. 21, 1948-1956), but the mechanism by which gamma-secretase and cadherins associate is unclear. Here we report that p120 catenin (p120ctn), a component of the cadherin-catenin complex, recruits gamma-secretase to cadherins, thus stimulating their processing while inhibiting production of Abeta peptide and the amyloid precursor protein intracellular domain. This function of p120ctn depends on both p120ctn-cadherin and p120ctn-presenilin 1 binding, indicating that p120ctn is the central factor that bridges gamma-secretase and cadherin-catenin complexes. Our data show that p120ctn is a unique positive regulator of the gamma-secretase processing of cadherins and a negative regulator of the amyloid precursor protein processing. Furthermore, our data suggest that specific members of the gamma-secretase complex may be used to recruit different substrates and that distinct PS1 sequences are required for processing of APP and cadherins.

Caspase-3 is enriched in postsynaptic densities and increased in Alzheimer's disease

Progressive synaptic degeneration and neuron loss are major structural correlates of cognitive impairment in Alzheimer's disease (AD). The mechanisms by which synaptic degeneration in AD occurs have not been established. The activation of proteins within the caspase family has been implicated in AD-associated neurodegeneration, and synaptically localized caspase activity could play a role in the synaptic degeneration and loss found in AD. We used synaptosomal fractionation with Western blotting and immunohistochemistry to examine the anatomical, subcellular, and subsynaptic expression patterns of caspase 3 in both the anterior cingulate cortex and hippocampus of control and AD patients. In both control and AD cases, there was a selective enrichment of caspase- 3 at synapses, particularly in the postsynaptic density (PSD) fractions. Compared with controls, AD patients exhibited significant increases in synaptic procaspase- 3 and active caspase-3 expression levels that were most evident in the PSD fractions. These data demonstrate for the first time the preferential localization and increase of caspase-3 in the PSD fractions in AD and suggest an important role for caspase 3 in synapse degeneration during disease progression.

Long-term prevention of Alzheimer's disease-like behavioral deficits in PDAPP mice carrying a mutation in Asp664

The deficits of Alzheimer's disease (AD) are believed to result, at least in part, from neurotoxicity of beta-amyloid (Abeta), a set of 38-43 amino acid fragments derived from the beta-amyloid precursor protein (APP). In addition, APP generates the APP-C31 and Jcasp toxic fragments intracellularly by cleavage at Asp664. We reported that mutation of Asp664 to Ala in a FAD-human APP transgene prevented AD-like deficits but did not affect Abeta production or deposition in PDAPP mice, arguing that D664A plays a crucial role in the generation of AD-like deficits. Whether D664A simply delays or completely prevents AD-like deficits, however, remained undefined. To address this question, we performed behavioral studies longitudinally on a pretrained mouse cohort at 9 and 13 months (mo) of age. While behavioral deficits were present in PDAPP mice, performance of Tg PDAPP(D664A) mice was not significantly different from non-Tg littermates' across all ages tested. Moreover, aberrant patterns in non-cognitive components of behavior in PDAPP mice were ameliorated in PDAPP(D664A) animals as well. A trend towards poorer retention at 9 mo and poorer learning at 13 mo that did not reach statistical significance was observed in PDAPP(D664A) mice. These results support and extend recent studies showing that cleavage of APP at Asp664 (or protein-protein interactions dependent on Asp664) is a crucial event in the generation of AD-like deficits in PDAPP mice. Our results thus further demonstrate that the D664A mutation either completely precludes, or markedly delays (beyond 13 mo) the appearance of AD-like deficits in this mouse model of AD.

Two isoforms of PSAP/MTCH1 share two proapoptotic domains and multiple internal signals for import into the mitochondrial outer membrane

Presenilin 1-associated protein (PSAP) was first identified as a protein that interacts with presenilin 1. It was later reported that PSAP is a mitochondrial protein that induces apoptosis when overexpressed in cultured cells. PSAP is also known as mitochondrial carrier homolog 1 (Mtch1). In this study, we show that there are two proapoptotic PSAP isoforms generated by alternative splicing that differ in the length of a hydrophilic loop located between two predicted transmembrane domains. Using RT-PCR and Western blot assays, we determined that both isoforms are expressed in human and rat tissues as well as in culture cells. Our results indicate that PSAP is an integral mitochondrial outer membrane protein, although it contains a mitochondrial carrier domain conserved in several inner membrane carriers, which partially overlaps one of the predicted transmembrane segments. Deletion of this transmembrane segment impairs mitochondrial import of PSAP. Replacement of this segment with each of two transmembrane domains, with opposite membrane orientations, from an unrelated protein indicated that one of them allowed mitochondrial localization of the PSAP mutant, whereas the other one did not. Our interpretation of these results is that PSAP contains multiple mitochondrial targeting motifs dispersed along the protein but that a transmembrane domain in the correct position and orientation is necessary for membrane insertion. The amino acid sequence within this transmembrane domain may also be important. Furthermore, two independent regions in the amino terminal side of the protein are responsible for its proapoptotic activity. Possible implications of these findings in PSAP function are discussed.

Deficits in synaptic transmission and learning in amyloid precursor protein (APP) transgenic mice require C-terminal cleavage of APP

Synaptic dysfunction has been shown to be one of the earliest correlates of disease progression in animal models of Alzheimer's disease. Amyloid-beta protein (Abeta) is thought to play an important role in disease-related synaptic dysfunction, but the mechanism by which Abeta leads to synaptic dysfunction is not understood. Here we describe evidence that cleavage of APP in the C terminus may be necessary for the deficits present in APP transgenic mice. In APP transgenic mice with a mutated cleavage site at amino acid 664, normal synaptic transmission, synaptic plasticity, and learning were maintained despite the presence of elevated levels of APP, Abeta42, and even plaque accumulation. These results indicate that cleavage of APP may play a critical role in the development of synaptic and behavioral dysfunction in APP transgenic mice.

Reversal of Alzheimer's-like pathology and behavior in human APP transgenic mice by mutation of Asp664

The deficits characteristic of Alzheimer's disease (AD) are believed to result, at least in part, from the neurotoxic effects of beta-amyloid peptides, a set of 39-43 amino acid fragments derived proteolytically from beta-amyloid precursor protein (APP). APP also is cleaved intracytoplasmically at Asp-664 to generate a second cytotoxic peptide, APP-C31, but whether this C-terminal processing of APP plays a role in the pathogenesis of AD is unknown. Therefore, we compared elements of the Alzheimer's phenotype in transgenic mice modeling AD with vs. without a functional Asp-664 caspase cleavage site. Surprisingly, whereas beta-amyloid production and plaque formation were unaltered, synaptic loss, astrogliosis, dentate gyral atrophy, increased neuronal precursor proliferation, and behavioral abnormalities were completely prevented by a mutation at Asp-664. These results suggest that Asp-664 plays a critical role in the generation of Alzheimer-related pathophysiological and behavioral changes in human APP transgenic mice, possibly as a cleavage site or via protein-protein interactions.

Abeta induces cell death by direct interaction with its cognate extracellular domain on APP (APP 597-624)

Amyloid beta-peptide (Abeta) is postulated to play a central role in the pathogenesis of Alzheimer's disease. We recently proposed a pathway of Abeta-induced toxicity that is APP dependent and involves the facilitation of APP complex formation by Abeta. The APP-dependent component requires cleavage of APP at position 664 in the cytoplasmic domain, presumably by caspases or caspase-like proteases, with release of a potentially cytotoxic C31 peptide. In this study we show that Abeta interacted directly and specifically with membrane-bound APP to facilitate APP homo-oligomerization. Using chimeric APP molecules, this interaction was shown to take place between Abeta and its homologous sequence on APP. Consistent with this finding, we demonstrated that Abeta also facilitated the oligomerization of beta-secretase cleaved APP C-terminal fragment (C99). We found that the YENPTY domain in the APP cytoplasmic tail and contained within C31 is critical for this cell death pathway. Deletion or alanine- scanning mutagenesis through this domain significantly attenuated cell death apparently without affecting either APP dimerization or cleavage at position 664. This indicated that sequences within C31 are required after its release from APP. As the YENPTY domain has been shown to interact with a number of cytosolic adaptor molecules, it is possible that the interaction of APP, especially dimeric forms of APP, with these molecules contribute to cell death.

Caspase cleaved presenilin-1 is part of active gamma-secretase complexes

gamma-Secretase is a key enzyme involved in the processing of the beta-amyloid precursor protein into amyloid beta-peptides (Abeta). Abeta accumulates and forms plaques in Alzheimer's disease (AD) brains. A progressive neurodegeneration and cognitive decline occurs during the course of the disease, and Abeta is believed to be central for the molecular pathogenesis of AD. Apoptosis has been implicated as one of the mechanisms behind the neuronal cell loss seen in AD. We have studied preservation and activity of the gamma-secretase complex during apoptosis in neuroblastoma cells (SH-SY5Y) exposed to staurosporine (STS). We report that the known components (presenilin, Nicastrin, Aph-1 and Pen-2) interact and form active gamma-secretase complexes in apoptotic cells. In addition, the fragments corresponding to the PS1 N-terminal fragment and the caspase-cleaved PS1 C-terminal fragment (PS1-caspCTF) were found to form active gamma-secretase complexes when co-expressed in presenilin (PS) knockout cells. Interestingly, PS1-caspCTF replaced the normal PS1 C-terminal fragment and was co-immunoprecipitated with the gamma-secretase complex in SH-SY5Y cells exposed to STS. In addition, Abeta was detected in medium from apoptotic HEK APP(swe) cells. Together, the data show that gamma-secretase complexes containing PS1-caspCTF are active, and suggest that this proteolytic activity is also important in dying cells and may affect the progression of AD.

Binding kinetics and duration of in vivo action of novel prolyl oligopeptidase inhibitors

Prolyl oligopeptidase (POP) is a serine protease that specifically hydrolyses small peptides at the carboxyl end of the proline residue. POP has gained pharmaceutical interest, since its inhibitors have been shown to have antiamnesic properties in rat. We examined the effect of the 2(S)-substituents CN and COCH(2)OH at the P1 site of the parent inhibitors isophthalic acid 2(S)-(cyclopentanecarbonyl)pyrrolidine-l-prolyl-pyrrolidine amide and 4-phenylbutanoyl-l-prolyl-pyrrolidine and bulky 5-t-butyl group at the P2 site l-prolyl residue of the parent inhibitor 4-phenylbutanoyl-l-prolyl-pyrrolidine on the binding kinetics to the enzyme. In addition, we studied the duration of POP inhibition in the rat tissues in vivo after i.p. administration. CN and COCH(2)OH substituents at the P1 site pyrrolidine group were found to greatly increase the affinity of the inhibitor and the enzyme-inhibitor complex half-life. In addition, 5-t-butyl group at the P2 site l-prolyl residue increased the dissociation half-life of the enzyme-inhibitor complex, without much affecting the inhibitory potency. The duration of the inhibition in the rat tissues followed the inhibition kinetic properties in that the compounds with fast dissociation produced shorter inhibition in the rat tissues than the compounds with slow dissociation. The duration of POP inhibition of compounds was evidently not governed by their serum clearance. The fact that the in vivo pharmacodynamic behaviour of POP inhibitors can be predicted by their in vitro-properties may be of importance when designing therapeutically useful POP inhibitors.

Caspase inhibition therapy abolishes brain trauma-induced increases in Abeta peptide: implications for clinical outcome

The detrimental effects of traumatic brain injury (TBI) on brain tissue integrity involve progressive axonal damage, necrotic cell loss, and both acute and delayed apoptotic neuronal death due to activation of caspases. Post-injury accumulation of amyloid precursor protein (APP) and its toxic metabolite amyloid-beta peptide (Abeta) has been implicated in apoptosis as well as in increasing the risk for developing Alzheimer's disease (AD) after TBI. Activated caspases proteolyze APP and are associated with increased Abeta production after neuronal injury. Conversely, Abeta and related APP/Abeta fragments stimulate caspase activation, creating a potential vicious cycle of secondary injury after TBI. Blockade of caspase activation after brain injury suppresses apoptosis and improves neurological outcome, but it is not known whether such intervention also prevents increases in Abeta levels in vivo. The present study examined the effect of caspase inhibition on post-injury levels of soluble Abeta, APP, activated caspase-3, and caspase-cleaved APP in the hippocampus of nontransgenic mice expressing human Abeta, subjected to controlled cortical injury (CCI). CCI produced brain tissue damage with cell loss and elevated levels of activated caspase-3, Abeta(1-42) and Abeta(1-40), APP, and caspase-cleaved APP fragments in hippocampal neurons and axons. Post-CCI intervention with intracerebroventricular injection of 100 nM Boc-Asp(OMe)-CH(2)F (BAF, a pan-caspase inhibitor) significantly reduced caspase-3 activation and improved histological outcome, suppressed increases in Abeta and caspase-cleaved APP, but showed no significant effect on overall APP levels in the hippocampus after CCI. These data demonstrate that after TBI, caspase inhibition can suppress elevations in Abeta. The extent to which Abeta suppression contributes to improved outcome following inhibition of caspases after TBI is unclear, but such intervention may be a valuable therapeutic strategy for preventing the long-term evolution of Abeta-mediated pathology in TBI patients who are at risk for developing AD later in life.

SET protein (TAF1beta, I2PP2A) is involved in neuronal apoptosis induced by an amyloid precursor protein cytoplasmic subdomain

When overexpressed, a short cytoplasmic domain of the amyloid precursor protein (APP), normally unmasked in the brain of Alzheimer's disease patients, activates caspase-3 and induces neuronal death. Death induction by this "Jcasp" domain is lost when tyrosine 653 is changed into an aspartate, suggesting specific interactions with unknown partners. To identify these putative partners and start to elucidate the mechanisms involved in Jcasp-induced cell death, we internalized a biotinylated version of the peptide into primary neurons and analyzed intracellular interacting proteins by pull-down and mass spectrometry. We find that SET protein, also called template-activating factor (TAF1beta) or phosphatase 2A inhibitor 2 (I2(PP2A)), specifically binds Jcasp early after internalization and that SET and Jcasp interact directly in vitro. Down-regulation of SET reduces Jcasp-induced cell death, confirming a role of this protein in Jcasp-induced apoptosis. Conversely, SET gain of function increases cell death, which suggests that SET level is crucial for neuronal survival/death. Taken together, these results suggest that SET is part of a neuronal apoptotic pathway related to Alzheimer's disease and provides a new entry in the analysis of this pathology.

Proteasome-mediated effects on amyloid precursor protein processing at the gamma-secretase site

Abeta (beta-amyloid) peptides are found aggregated in the cortical amyloid plaques associated with Alzheimer's disease neuropathology. Inhibition of the proteasome alters the amount of Abeta produced from APP (amyloid precursor protein) by various cell lines in vitro. Proteasome activity is altered during aging, a major risk factor for Alzheimer's disease. In the present study, a human neuroblastoma cell line expressing the C-terminal 100 residues of APP (SH-SY5Y-SPA4CT) was used to determine the effect of proteasome inhibition, by lactacystin and Bz-LLL-COCHO (benzoyl-Leu-Leu-Leu-glyoxal), on APP processing at the gamma-secretase site. Proteasome inhibition caused a significant increase in Abeta peptide levels in medium conditioned by SH-SY5Y-SPA4CT cells, and was also associated with increased cell death. APP is a substrate of the apoptosis-associated caspase 3 protease, and we therefore investigated whether the increased Abeta levels could reflect caspase activation. We report that caspase activation was not required for proteasome-inhibitor-mediated effects on APP (SPA4CT) processing. Cleavage of Ac-DEVD-AMC (N-acetyl-Asp-Glu-Val-Asp-7-amino-4-methylcoumarin), a caspase substrate, was reduced following exposure of SH-SY5Y-SPA4CT cells to lactacystin, and co-treatment of cells with lactacystin and a caspase inhibitor [Z-DEVD-FMK (benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone)] resulted in higher Abeta levels in medium, augmenting those seen with lactacystin alone. This study indicated that proteasome inhibition could increase APP processing specifically at the gamma-secretase site, and increase release of Abeta, in the absence of caspase activation. This indicates that the decline in proteasome function associated with aging would contribute to increased Abeta levels.

Brain Prolyl Endopeptidase Expression in Aging, APP Transgenic Mice and Alzheimer’s Disease

Prolyl endopeptidase (PEP) is believed to inactivate neuropeptides that are present in the extracellular space. However, the intracellular localization of PEP suggests additional, yet unidentified physiological functions for this enzyme. Here we studied the expression, enzymatic activity and subcellular localization of PEP in adult and aged mouse brain as well as in brains of age-matched APP transgenic Tg2576 mice and in brains of Alzheimer's disease patients. In mouse brain PEP was exclusively expressed by neurons and displayed region- and age-specific differences in expression levels, with the highest PEP activity being present in cerebellum and a significant increase in hippocampal but not cortical or cerebellar PEP activity in aged mouse brain. In brains of young APP transgenic Tg2576 mice, hippocampal PEP activity was increased compared to wild-type littermates in the pre-plaque phase but not in aged mice with beta-amyloid plaque pathology. This "accelerated aging" with regard to hippocampal PEP expression in young APP transgenic mice might be one factor contributing to the observed cognitive deficits in these mice in the pre-plaque phase and could also explain in part the cognition-enhancing effects of PEP inhibitors in several experimental paradigms.

The endocytotic pathway is required for increased A beta 42 secretion during apoptosis

Secretion and progressive cerebral accumulation of beta-amyloid peptides (A beta), which derive by endoproteolytic ('amyloidogenic') processing of beta-amyloid precursor protein (APP), are felt to represent collectively an early and necessary event in the pathogenesis of Alzheimer's disease. APP amyloidogenic processing can occur via secretory or endocytotic pathways, but the relative contribution of these pathways to A beta secretion remains to be established. The effect of apoptosis on amyloidogenic processing and A beta secretion similarly is incompletely understood. We tested the hypothesis that APP processing by the endocytotic pathway represents a stress-related neural cell response, by comparing A beta secretion after induction of apoptosis in PC12 cells transfected either for endocytosis-competent or -deficient APP. Newly prepared adenoviral vectors encompassing targeted mutagenesis of the cytoplasmic tail YENP tetrapeptide sequence, which serves as the principal APP internalization signal, were used to express endocytosis-deficient holoprotein. We report that the endocytotic pathway is required for the generation and secretion of A beta 42, and that secretion of this neurotoxic peptide increases significantly during apoptosis. We demonstrate additionally that more A beta 40 apparently is generated in secretory compartments during apoptosis when APP processing by the endocytotic pathway is impaired.

Amino-truncated amyloid beta-peptide (Abeta5-40/42) produced from caspase-cleaved amyloid precursor protein is deposited in Alzheimer's disease brain

Caspase activation and apoptosis are implicated in Alzheimer's disease (AD). In view of the finding that the amyloid precursor protein (APP) undergoes caspase-mediated cleavage in the cytoplasmic region, we analyzed amyloid beta-peptide (Abeta) production in human neuronal and nonneuronal cells expressing wild-type APP and the caspase-cleaved form of APP (APPDeltaC). Biochemical analyses, including immunoprecipitation/mass spectrometry, revealed that APPDeltaC-expressing cells secrete increased levels of amino-terminally truncated Abeta5-40/42 and reduced levels of Abeta1-40/42, compared with wild-type APP-expressing cells. We propose that Abeta5-40/42 is derived from alternative beta-cleavage of APP by alpha-secretase-like protease(s), based on data from treatment of cells with inhibitors of BACE and alpha-secretase. Apoptosis induction resulted in this alternative cleavage of APP in wild-type APP-expressing cells. Moreover, immunohistochemical staining of the AD brain with an end-specific antibody to Abeta5-40/42 revealed peptide deposits in vascular lesions with amyloid angiopathy. The data collectively suggest that caspase cleavage of APP leads to increased production and deposition of Abeta5-40/42 in the AD brain, and highlight the significance of amino-truncated Abeta in the pathogenesis of AD.

Long-term accumulation of amyloid-beta, beta-secretase, presenilin-1, and caspase-3 in damaged axons following brain trauma

Plaques composed of amyloid beta (Abeta) have been found within days following brain trauma in humans, similar to the hallmark plaque pathology of Alzheimer's disease (AD). Here, we evaluated the potential source of this Abeta and long-term mechanisms that could lead to its production. Inertial brain injury was induced in pigs via head rotational acceleration of 110 degrees over 20 ms in the coronal plane. Animals were euthanized at 3 hours, 3 days, 7 days, and 6 months post-injury. Immunohistochemistry and Western blot analyses of the brains were performed using antibodies specific for amyloid precursor protein (APP), Abeta peptides, beta-site APP-cleaving enzyme (BACE), presenilin-1 (PS-1), caspase-3, and caspase-mediated cleavage of APP (CCA). Substantial co-accumulation for all of these factors was found in swollen axons at all time points up to 6 months following injury. Western blot analysis of injured brains confirmed a substantial increase in the protein levels of these factors, particularly in the white matter. These data suggest that impaired axonal transport due to trauma induces long-term pathological co-accumulation of APP with BACE, PS-1, and activated caspase. The abnormal concentration of these factors may lead to APP proteolysis and Abeta formation within the axonal membrane compartment.

Suppression of the caspase cleavage of β-amyloid precursor protein by its cytoplasmic phosphorylation

beta-Amyloid precursor protein (APP) is a type I transmembrane protein. Its cleavages by beta- and gamma-secretases yield beta-amyloid, which is the main constituent of senile plaques in Alzheimer's disease (AD). In apoptotic cells and AD brains, APP is alternatively cleaved by caspases in the cytoplasmic region after the Asp664 residue (with respect to the numbering conversion for the APP695 isoform). Caspase-cleaved fragments of APP are cytotoxic and have been implicated in AD pathogenesis; however, the mechanisms regulating the cleavage have not been studied. APP is constitutively phosphorylated at Thr668 in brain. In the present study, we demonstrate that APP phosphorylated at Thr668 is less vulnerable to cytoplasmic cleavage by caspase-3 and caspase-8. This suggests that APP phosphorylation suppresses the generation of caspase-cleaved fragments of APP in the brain and that perturbation of this phosphorylation may be involved in APP-mediated neurotoxicity.

Fe65 is not involved in the platelet-derived growth factor-induced processing of Alzheimer's amyloid precursor protein, which activates its caspase-directed cleavage

The proteolytic processing of the precursor of the beta-amyloid peptides (APP) is believed to be a key event in the pathogenesis of Alzheimer's disease. This processing is activated through a pathway involving the PDGF receptor, Src, and Rac1. In this paper, we demonstrate that this pathway specifically acts on APP and requires the YENPTY motif present in the APP cytosolic domain. Considering that several results indicate that the adaptor proteins interacting with this domain affect the processing of APP, we examined their possible involvement in the PDGF-induced pathway. By using an APP-Gal4 reporter system, we observed that the overexpression of Fe65 activates APP-Gal4 cleavage, whereas X11 stabilizes APP. Although mDab1 and Jip1 have no effect, Shc induces a strong activation of APP cleavage, and the contemporary exposure of cells to PDGF causes a dramatic cooperative effect. The analysis of point mutations of the APP YENPTY motif indicates that Fe65 and PDGF function through different mechanisms. In fact, Fe65 requires the integrity of APP695 Tyr682 residue, whereas PDGF effect is dependent upon the integrity of Asn684. Furthermore, the mutation of Asp664 of APP, which is the target site for the caspase-directed APP cleavage, strongly decreases the effect of Fe65. This suggests that Fe65 activates the cleavage of APP by caspases, and in fact, caspase inhibitor Z-VEVD decreases the APP cleavage induced by Fe65. On the contrary, the effects of Shc overexpression, like those of PDGF, are completely absent in the presence of compound X and require the integrity of the Asn684 residue of APP695. The involvement of Shc in the pathway regulating APP processing is confirmed by the effects of constitutively active and dominant negative mutants of Src and Rac1.

Protein accumulation in traumatic brain injury

Traumatic brain injury (TBI) is one of the most devastating diseases in our society, accounting for a high percentage of mortality and disability. A major consequence of TBI is the rapid and long-term accumulation of proteins. This process largely reflects the interruption of axonal transport as a result of extensive axonal injury. Although many proteins are found accumulating after TBI, three have received particular attention; beta-amyloid precursor protein and its proteolytic products, amyloid-beta (Abeta) peptides, neurofilament proteins, and synuclein proteins. Massive coaccumulations of all of these proteins are found in damaged axons throughout the white matter after TBI. Additionally, these proteins form aggregates in other neuronal compartments and in brain parenchyma after brain trauma. Interestingly, TBI is also an epigenetic risk factor for developing neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. Here, the similarities and differences of these accumulations with pathologies of neurodegenerative diseases will be explored. In addition, the potential deleterious roles of protein accumulations on functional outcome and progressive neurodegeneration following TBI will be examined.

Amyloid beta protein toxicity mediated by the formation of amyloid-beta protein precursor complexes

The amyloid-beta protein precursor, a type 1 transmembrane protein, gives rise to the amyloid beta-protein, a neurotoxic peptide postulated to be involved in the pathogenesis of Alzheimer's disease. Here, we show that soluble amyloid beta protein accelerates amyloid precursor protein complex formation, a process that contributes to neuronal cell death. The mechanism of cell death involves the recruitment of caspase-8 to the complex, followed by intracytoplasmic caspase cleavage of amyloid precursor protein. In vivo, the levels of soluble amyloid beta protein correlated with caspase-cleaved fragments of the amyloid precursor protein in brains of Alzheimer's disease subjects. These findings suggest that soluble amyloid beta protein-induced multimerization of the amyloid precursor protein may be another mechanism by which amyloid beta protein contributes to synapse loss and neuronal cell death seen in Alzheimer's disease.

Caspase cleavage of the amyloid precursor protein modulates amyloid beta-protein toxicity

The amyloid beta-protein precursor (APP) is proteolytically cleaved to generate the amyloid beta-protein (Abeta), the principal constituent of senile plaques found in Alzheimer's disease (AD). In addition, Abeta in its oligomeric and fibrillar forms have been hypothesized to induce neuronal toxicity. We and others have previously shown that APP can be cleaved by caspases at the C-terminus to generate a potentially cytotoxic peptide termed C31. Furthermore, this cleavage event and caspase activation were increased in the brains of AD, but not control, cases. In this study, we show that in cultured cells, Abeta induces caspase cleavage of APP in the C-terminus and that the subsequent generation of C31 contributes to the apoptotic cell death associated with Abeta. Interestingly, both Abeta toxicity and C31 pathway are dependent on the presence of APP. Both APP-dependent Abeta toxicity and C31-induced apoptotic cell death involve apical or initiator caspases-8 and -9. Our results suggest that Abeta-mediated toxicity initiates a cascade of events that includes caspase activation and APP cleavage. These findings link C31 generation and its potential cell death activity to Abeta cytotoxicity, the leading mechanism proposed for neuronal death in AD.

Caspase activation increases beta-amyloid generation independently of caspase cleavage of the beta-amyloid precursor protein (APP)

The amyloid precursor protein (APP) undergoes "alternative" proteolysis mediated by caspases. Three major caspase recognition sites have been identified in the APP, i.e. one at the C terminus (Asp720) and two at the N terminus (Asp197 and Asp219). Caspase cleavage at Asp720 has been suggested as leading to increased production of Abeta. Thus, we set out to determine which putative caspase sites in APP, if any, are cleaved in Chinese hamster ovary cell lines concurrently with the increased Abeta production that occurs during apoptosis. We found that cleavage at Asp720 occurred concurrently with caspase 3 activation and the increased production of total secreted Abeta and Abeta1-42 in association with staurosporine- and etoposide-induced apoptosis. To investigate the contribution of caspase cleavage of APP to Abeta generation, we expressed an APP mutant truncated at Asp720 that mimics APP caspase cleavage at the C-terminal site. This did not increase Abeta generation but, in contrast, dramatically decreased Abeta production in Chinese hamster ovary cells. Furthermore, the ablation of caspase-dependent cleavage at Asp720, Asp197, and Asp219 (by site-directed mutagenesis) did not prevent enhanced Abeta production following etoposide-induced apoptosis. These findings indicate that the enhanced Abeta generation associated with apoptosis does not require cleavage of APP at its C-terminal (Asp720) and/or N-terminal caspase sites.

Synthesis of new 3-alkoxy-7-amino-4-chloro-isocoumarin derivatives as new beta-amyloid peptide production inhibitors and their activities on various classes of protease

A series of new 7-substituted-4-chloro-3-alkoxy isocoumarin derivatives were synthesized and evaluated as inhibitors of representative classes of proteases: serine protease (alpha-chymotrypsin, trypsin), cysteine protease (Caspase-3), and aspartyl protease (HIV-protease), 20S proteasome and also as inhibitors of amyloid peptide gamma-secretase-mediated production. Protease inhibition selectivity is directly related to the structure of the substituent at the 7-position of the isocoumarin nucleus. 7-Nitro-isocoumarin derivatives (4c, 4d, 4f) are potent alpha-chymotrypsin inhibitors but slightly active or inactive on HIV-protease, as well as on cysteine protease. In contrast, only derivatives bearing a free amino (5d, 5f) or a substituted amino group (6f) at the 7-position of the isocoumarin nucleus, were found weakly active or inactive on alpha-chymotrypsin, trypsin, Caspase-3 and HIV-protease, but prevent gamma-secretase-mediated production of Abeta 40/42 amyloid peptides, which is known to be involved in Alzheimer's disease. Moreover, the most active compounds on beta-amyloid peptide production [JLK6 (5d), JLK2 (5f) and JLK7 (6f)] show only weak or moderate inhibitory activity on the 20S proteasome. The obtained results suggest that the described new isocoumarin analogues could be of interest, since compounds like JLK6 (5d), JLK2 (5f) and JLK7 (6f) can be considered as possible hits for the development of new agents directed towards Alzheimer's disease.

In vitro studies of Flemish, Dutch, and wild-type beta-amyloid provide evidence for two-staged neurotoxicity

Mutations in the beta-amyloid (Abeta) sequence of the amyloid precursor protein gene (APP) present with variable disease phenotypes. While patients with the Dutch APP mutation (E693Q) have predominantly hemorrhagic strokes, Flemish APP (A692G) patients develop both strokes and Alzheimer's disease (AD). To determine whether these diverse clinical and pathological presentations are due to mutant Abeta or APP, we studied the effect of Flemish, Dutch, and wild-type Abeta/APP on phosphorylation of specific tau epitopes observed in AD. No effect was observed in differentiated SH-SY5Y cells either stably expressing APP or treated with synthetic Abeta(12-42). However, we did observe a paradoxical temporal difference in the neurotoxic potential of mutant and wild-type Abeta. While long 24-h incubation at physiological levels of Abeta (2 microM) showed a higher amount of apoptosis for Dutch Abeta, a short 2-h incubation showed elevated apoptosis for Flemish and wild-type Abeta. The altered aggregating properties of Abeta, with Dutch Abeta aggregating faster and Flemish Abeta slower than wild type, elucidated a discrete two-phase Abeta neurotoxicity. We propose here that, at least in vitro, Abeta might be neurotoxic in an initial phase due to its soluble oligomeric or other early toxic Abeta intermediate(s), which is perhaps distinct from the late neurotoxicity incurred by aggregated larger assemblies of Abeta.

Caspase cleavage of members of the amyloid precursor family of proteins

The synapse loss and neuronal cell death characteristic of Alzheimer's disease (AD) are believed to result in large part from the neurotoxic effects of beta-amyloid peptide (Abeta), a 40-42 amino acid peptide(s) derived proteolytically from beta-amyloid precursor protein (APP). However, APP is also cleaved intracellularly to generate a second cytotoxic peptide, C31, and this cleavage event occurs in vivo as well as in vitro and preferentially in the brains of AD patients (Lu et al. 2000). Here we show that APPC31 is toxic to neurons in primary culture, and that like APP, the APP family members APLP1 and possibly APLP2 are cleaved by caspases at their C-termini. The carboxy-terminal peptide derived from caspase cleavage of APLP1 shows a degree of neurotoxicity comparable to APPC31. Our results suggest that even though APLP1 and APLP2 cannot generate Abeta, they may potentially contribute to the pathology of AD by generating peptide fragments whose toxicity is comparable to that of APPC31.

Caspase-3-mediated cleavage of amyloid precursor protein and formation of amyloid Beta peptide in traumatic axonal injury

Immunohistochemical studies demonstrate accumulation of the beta-amyloid precursor protein (APP) within injured axons following traumatic brain injury (TBI). Despite such descriptions, little is known about the ultimate fate of accumulating APP at sites of traumatic axonal injury (TAI). Recently, caspase-3-mediated cleavage of APP and subsequent Abeta deposition was linked to apoptotic neuronal death pathways in hippocampal neurons following ischemic and excitotoxic brain injury. Given that (1) APP is known to accumulate within traumatically injured axons, (2) caspase-3 activation has been demonstrated in traumatic axonal injury (TAI), and (3) recent studies have identified a caspase-3 cleavage site within APP, we initiated the current investigation to determine whether caspase-3-mediated cleavage of APP occurs in TAI. We further assessed whether these events were found in relation to Abeta peptide formation. To this end, we employed antibodies targeting APP, the caspase-3-mediated breakdown product of APP proteolysis, and the Abeta peptide. Rats were subjected to impact acceleration TBI (6 h to 10 days survival), and their brains were processed for single-label bright field and multiple double-label immunofluorescent paradigms using the above antibodies. By 12 h postinjury, caspase-3-mediated APP proteolysis (CMAP) was demonstrated within the medial lemniscus (ML) and medial longitudinal fasciculus (MLF) in axons undergoing TAI, identified by their concomitant APP accumulation. Immunoreactivity for CMAP persisted up to 48 h postinjury in the ML and MLF, but was notably reduced by 10 days following injury. Further, CMAP was colocalized with Abeta formation in foci of TAI. The current study demonstrates that caspase-3 cleavage of APP occurs in TAI and is associated with formation of Abeta peptide. These findings are of interest given recent epidemiological studies supporting an association between TBI and later risk for AD development.

beta-Secretase cleavage of the amyloid precursor protein mediates neuronal apoptosis caused by familial Alzheimer's disease mutations

The amyloid precursor protein (APP) is cleaved by two enzymes, beta-secretase and gamma-secretase, to generate the pathological amyloid beta (Abeta) peptide. Expression of familial Alzheimer's disease (FAD) mutants of APP in primary neurons causes both intracellular accumulation of the C-terminal beta-secretase cleavage product of APP and increased secretion of Abeta, and eventually results in apoptotic death of the cells. To determine whether either of these two processing products of APP is involved in this apoptotic pathway, we first modeled experimentally the accumulation of the beta-secretase cleavage product in neurons. The C-terminal 100 amino acids (C100) of APP, with and without a signal peptide, was expressed in cells via recombinant herpes simplex virus (HSV) vectors. Both transgene products were targeted to the membrane, and both caused apoptosis in the neurons, implicating the beta-secretase cleavage product of APP in apoptosis caused by FAD APPs. Expression in neurons of a mutant of FAD APP that inhibited beta-secretase cleavage inhibited its ability to cause apoptosis. However, expression in neurons of a mutant of FAD APP that inhibited gamma-secretase cleavage did not inhibit the ability of this mutant to cause apoptosis. These data suggested that the C-terminal beta-secretase cleavage product of APP, but not Abeta, mediates the apoptosis caused by FAD mutants of APP. Consistent with this hypothesis, C31, which is generated from the beta-secretase cleavage product, itself caused neuronal apoptosis. Inhibitors of caspases 3, 6 and 8, but not of caspase 9, inhibited the apoptosis caused by FAD mutants of APP. It may be inferred from these data that beta-secretase cleavage of FAD mutants of APP allows the appropriate caspase access to its site of action to produce C31, which directly causes neuronal apoptosis.