The Alzheimer beta-amyloid protein precursor/protease nexin-II is cleaved by secretase in a trans-Golgi secretory compartment in human neuroglioma cells

Advances in the cell biology of the trafficking and processing of amyloid precursor protein: impact of familial Alzheimer's disease mutations

The production of neurotoxic amyloid-β peptides (Aβ) is central to the initiation and progression of Alzheimer's disease (AD) and involves sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. APP and the secretases are transmembrane proteins and their co-localisation in the same membrane-bound sub-compartment is necessary for APP cleavage. The intracellular trafficking of APP and the β-secretase, BACE1, is critical in regulating APP processing and Aβ production and has been studied in several cellular systems. Here, we summarise the intracellular distribution and transport of APP and its secretases, and the intracellular location for APP cleavage in non-polarised cells and neuronal models. In addition, we review recent advances on the potential impact of familial AD mutations on APP trafficking and processing. This is critical information in understanding the molecular mechanisms of AD progression and in supporting the development of novel strategies for clinical treatment.

Proteins Found in the Triple-Negative Breast Cancer Secretome and Their Therapeutic Potential

The cancer secretome comprises factors secreted by tumors, including cytokines, growth factors, proteins from the extracellular matrix (ECM), proteases and protease inhibitors, membrane and extracellular vesicle proteins, peptide hormones, and metabolic proteins. Secreted proteins provide an avenue for communication with other tumor cells and stromal cells, and these in turn promote tumor growth and progression. Breast cancer is the most commonly diagnosed cancer in women in the US and worldwide. Triple-negative breast cancer (TNBC) is characterized by its aggressiveness and its lack of expression of the estrogen receptor (ER), progesterone receptor (PR), and HER2, making it unable to be treated with therapies targeting these protein markers, and leaving patients to rely on standard chemotherapy. In order to develop more effective therapies against TNBC, researchers are searching for targetable molecules specific to TNBC. Proteins in the TNBC secretome are involved in wide-ranging cancer-promoting processes, including tumor growth, angiogenesis, inflammation, the EMT, drug resistance, invasion, and development of the premetastatic niche. In this review, we catalog the currently known proteins in the secretome of TNBC tumors and correlate these secreted molecules with potential therapeutic opportunities to facilitate translational research.

Reciprocal modulation between amyloid precursor protein and synaptic membrane cholesterol revealed by live cell imaging

The amyloid precursor protein (APP) has been extensively studied because of its association with Alzheimer's disease (AD). However, APP distribution across different subcellular membrane compartments and its function in neurons remains unclear. We generated an APP fusion protein with a pH-sensitive green fluorescent protein at its ectodomain and a pH-insensitive blue fluorescent protein at its cytosolic domain and used it to measure APP's distribution, subcellular trafficking, and cleavage in live neurons. This reporter, closely resembling endogenous APP, revealed only a limited correlation between synaptic activities and APP trafficking. However, the synaptic surface fraction of APP was increased by a reduction in membrane cholesterol levels, a phenomenon that involves APP's cholesterol-binding motif. Mutations at or near binding sites not only reduced both the surface fraction of APP and membrane cholesterol levels in a dominant negative manner, but also increased synaptic vulnerability to moderate membrane cholesterol reduction. Our results reveal reciprocal modulation of APP and membrane cholesterol levels at synaptic boutons.

Multidisciplinary approaches for targeting the secretase protein family as a therapeutic route for Alzheimer's disease

The continual increase of the aging population worldwide renders Alzheimer's disease (AD) a global prime concern. Several attempts have been focused on understanding the intricate complexity of the disease's development along with the on- andgoing search for novel therapeutic strategies. Incapability of existing AD drugs to effectively modulate the pathogenesis or to delay the progression of the disease leads to a shift in the paradigm of AD drug discovery. Efforts aimed at identifying AD drugs have mostly focused on the development of disease-modifying agents in which effects are believed to be long lasting. Of particular note, the secretase enzymes, a group of proteases responsible for the metabolism of the β-amyloid precursor protein (βAPP) and β-amyloid (Aβ) peptides production, have been underlined for their promising therapeutic potential. This review article attempts to comprehensively cover aspects related to the identification and use of drugs targeting the secretase enzymes. Particularly, the roles of secretases in the pathogenesis of AD and their therapeutic modulation are provided herein. Moreover, an overview of the drug development process and the contribution of computational (in silico) approaches for facilitating successful drug discovery are also highlighted along with examples of relevant computational works. Promising chemical scaffolds, inhibitors, and modulators against each class of secretases are also summarized herein. Additionally, multitarget secretase modulators are also taken into consideration in light of the current growing interest in the polypharmacology of complex diseases. Finally, challenging issues and future outlook relevant to the discovery of drugs targeting secretases are also discussed.

The physical approximation of APP and BACE-1: A key event in alzheimer's disease pathogenesis

Alzheimer's disease (AD) is characterized by the accumulation of insoluble deposits of Amyloid β (Aβ) in brains. Aβ is derived by sequential cleavage of the amyloid precursor protein (APP) by β-site secretase enzyme (BACE-1) and γ-secretase. Proteolytic processing of APP by BACE-1 is the rate-limiting step in Aβ production, and this pathway is a prime target for AD drug development. Both APP and BACE-1 are membrane-spanning proteins, transported via secretory and endocytic pathways; and the physical interaction of APP and BACE-1 during trafficking is a key cell biological event initiating the amyloidogenic pathway. Here, we highlight recent research on intracellular trafficking/sorting of APP and BACE-1, and discuss how dysregulation of these pathways might lead to enhanced convergence of APP and BACE-1, and subsequent β-cleavage of APP. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 340-347, 2018.

Profiling Established Cell Lines as a Means to Screening Diversity

Cell lines provide a readily available source of target material for functional and molecular binding screens in drug discovery. The Cell PROFILE® program at NovaScreen® represents an effort to identify receptors and enzymes expressed in established cell lines that are relevant to important drug screening endeavors. In this report, we present data on a selected number of receptors and enzymes for four cell lines studied in this survey. The objective of this survey was not to compare one cell line with another, but to illustrate the diversity of pharmacologic targets and the untapped potential of databases for readily obtainable cell lines. The following cell lines, which are all derived from human tumors, were included in this study (with some relevant pharmacologic/pathologic targets): HT-29, derived from an adenocarcinoma of the colon (colorectal cancer); SK-N-MC, derived from a neuroepithelioma (NPY receptors, apoptosis, HIV type I infection); H-4, derived from a neuroglioma (Alzheimer's disease); and LNCaP, derived from a prostate carcinoma (androgen receptor, prostate cancer). Specific to this survey were receptor-binding assays for androgens, corticotropin-releasing factor, endothelin, GABA, NMDA, somatostatin, and alpha and beta adrenergic ligands, as well as binding sites for ion channels. A comparison of specific binding of these various sites between target tissues routinely used in our assays and established cell lines reveals a diversity of receptors heretofore not reported for the latter and represents a potential database for screening and pharmacologic research.

The Amyloid Precursor Protein is rapidly transported from the Golgi apparatus to the lysosome and where it is processed into beta-amyloid

BACKGROUND

Alzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid peptide (Aβ). Aβ is produced by sequential cleavage of the Amyloid Precursor Protein (APP) by β- and γ-secretases. Many studies have demonstrated that the internalization of APP from the cell surface can regulate Aβ production, although the exact organelle in which Aβ is produced remains contentious. A number of recent studies suggest that intracellular trafficking also plays a role in regulating Aβ production, but these pathways are relatively under-studied. The goal of this study was to elucidate the intracellular trafficking of APP, and to examine the site of intracellular APP processing.

RESULTS

We have tagged APP on its C-terminal cytoplasmic tail with photoactivatable Green Fluorescent Protein (paGFP). By photoactivating APP-paGFP in the Golgi, using the Golgi marker Galactosyltranferase fused to Cyan Fluorescent Protein (GalT-CFP) as a target, we are able to follow a population of nascent APP molecules from the Golgi to downstream compartments identified with compartment markers tagged with red fluorescent protein (mRFP or mCherry); including rab5 (early endosomes) rab9 (late endosomes) and LAMP1 (lysosomes). Because γ-cleavage of APP releases the cytoplasmic tail of APP including the photoactivated GFP, resulting in loss of fluorescence, we are able to visualize the cleavage of APP in these compartments. Using APP-paGFP, we show that APP is rapidly trafficked from the Golgi apparatus to the lysosome; where it is rapidly cleared. Chloroquine and the highly selective γ-secretase inhibitor, L685, 458, cause the accumulation of APP in lysosomes implying that APP is being cleaved by secretases in the lysosome. The Swedish mutation dramatically increases the rate of lysosomal APP processing, which is also inhibited by chloroquine and L685, 458. By knocking down adaptor protein 3 (AP-3; a heterotetrameric protein complex required for trafficking many proteins to the lysosome) using siRNA, we are able to reduce this lysosomal transport. Blocking lysosomal transport of APP reduces Aβ production by more than a third.

CONCLUSION

These data suggests that AP-3 mediates rapid delivery of APP to lysosomes, and that the lysosome is a likely site of Aβ production.

Endoplasmic reticulum enrollment in Alzheimer's disease

Alzheimer's disease (AD) poses a huge challenge for society and health care worldwide as molecular pathogenesis of the disease is poorly understood and curative treatment does not exist. The mechanisms leading to accelerated neuronal cell death in AD are still largely unknown, but accumulation of misfolded disease-specific proteins has been identified as potentially involved. In the present review, we describe the essential role of endoplasmic reticulum (ER) in AD. Despite the function that mitochondria may play as the central major player in the apoptotic process, accumulating evidence highlights ER as a critical organelle in AD. Stress that impairs ER physiology leads to accumulation of unfolded or misfolded proteins, such as amyloid β (Aβ) peptide, the major component of amyloid plaques. In an attempt to ameliorate the accumulation of unfolded proteins, ER stress triggers a protective cellular mechanism, which includes the unfolded protein response (UPR). However, when activation of the UPR is severe or prolonged enough, the final cellular outcome is pathologic apoptotic cell death. Distinct pathways can be activated in this process, involving stress sensors such as the JNK pathway or ER chaperones such as Bip/GRP94, stress modulators such as Bcl-2 family proteins, or even stress effectors such as caspase-12. Here, we detail the involvement of the ER and associated stress pathways in AD and discuss potential therapeutic strategies targeting ER stress.

Structural aspects and physiological consequences of APP/APLP trans-dimerization

The amyloid precursor protein (APP) is one of the key proteins in Alzheimer's disease (AD), as it is the precursor of amyloid β (Aβ) peptides accumulating in amyloid plaques. The processing of APP and the pathogenic features of especially Aβ oligomers have been analyzed in detail. Remarkably, there is accumulating evidence from cell biological and structural studies suggesting that APP and its mammalian homologs, the amyloid precursor-like proteins (APLP1 and APLP2), participate under physiological conditions via trans-cellular dimerization in synaptogenesis. This offers the possibility that loss of synapses in AD might be partially explained by dysfunction of APP/APLPs cell adhesion properties. In this review, structural characteristics of APP trans-cellular interaction will be placed critically in context with its putative physiological functions focusing on cell adhesion and synaptogenesis.

Axonal transport of APP and the spatial regulation of APP cleavage and function in neuronal cells

Over two decades have passed since the original discovery of amyloid precursor protein (APP). While physiological function(s) of APP still remain a matter of debate, consensus exists that the proteolytic processing of this protein represents a critical event in the life of neurons and that abnormalities in this process are instrumental in Alzheimer's disease (AD) pathogenesis. Specific molecular components involved in APP proteolysis have been identified, and their enzymatic activities characterized in great detail. As specific proteolytic fragments of APP are identified and novel physiological effects for these fragments are revealed, more obvious becomes our need to understand the spatial organization of APP proteolysis. Valuable insights on this process have been obtained through the study of non-neuronal cells. However, much less is known about the topology of APP processing in neuronal cells, which are characterized by their remarkably complex cellular architecture and extreme degree of polarization. In this review, we discuss published literature addressing various molecular mechanisms and components involved in the trafficking and subcellular distribution of APP and APP secretases in neurons. These include the relevant machinery involved in their sorting, the identity of membranous organelles in which APP is transported, and the molecular motor-based mechanisms involved in their translocation. We also review experimental evidence specifically addressing the processing of APP at the axonal compartment. Understanding neuron-specific mechanisms of APP processing would help illuminating the physiological roles of APP-derived proteolytic fragments and provide novel insights on AD pathogenesis.

Sink Hypothesis and Therapeutic Strategies for Attenuating Aβ Levels

Amyloid β (Aβ) plaque, comprised mainly by Aβ peptides, is an important pathology of Alzheimer’s brains. Major efforts have been devoted to targeting this neurotoxic Aβ peptide for discovering disease-modifying treatments for Alzheimer’s disease. Inasmuch as Aβ is found in both the brain and the periphery, it is hypothesized that there is some form of equilibrium for the Aβ in the brain and the periphery such that Aβ can be transported across the blood-brain barrier. By modulating the periphery Aβ levels, it is predicted that the brain Aβ levels will undergo concomitant changes, forming the basis of the “sink hypothesis” for Aβ lowering strategies. In this review, the significance and implication of this sink hypothesis as well as how the sink hypothesis may contribute to the recent Aβ-based drug discovery in AD are discussed. Ultimately, the validity of the sink hypothesis will be resolved when the appropriate Aβ agents are being tested in the clinic.

The amyloid-beta rise and gamma-secretase inhibitor potency depend on the level of substrate expression

The amyloid-beta (Abeta) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-beta precursor protein (APP) through consecutive proteolytic cleavages by beta-site APP-cleaving enzyme and gamma-secretase. Unexpectedly gamma-secretase inhibitors can increase the secretion of Abeta peptides under some circumstances. This "Abeta rise" phenomenon, the same inhibitor causing an increase in Abeta at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the Abeta rise depends on the beta-secretase-derived C-terminal fragment of APP (betaCTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of Abeta, known as "p3," formed by alpha-secretase cleavage, did not exhibit a rise. In addition to the Abeta rise, low betaCTF or C99 expression decreased gamma-secretase inhibitor potency. This "potency shift" may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, gamma-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of Abeta under conditions of low substrate expression. The Abeta rise was also observed in rat brain after dosing with the gamma-secretase inhibitor BMS-299897. The Abeta rise and potency shift are therefore relevant factors in the development of gamma-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the Abeta rise, including the "incomplete processing" and endocytic models, are discussed.

Mint3/X11gamma is an ADP-ribosylation factor-dependent adaptor that regulates the traffic of the Alzheimer's Precursor protein from the trans-Golgi network

Beta-amyloid peptides (Abeta) are the major component of plaques in brains of Alzheimer's patients, and are they derived from the proteolytic processing of the beta-amyloid precursor protein (APP). The movement of APP between organelles is highly regulated, and it is tightly connected to its processing by secretases. We proposed previously that transport of APP within the cell is mediated in part through its sorting into Mint/X11-containing carriers. To test our hypothesis, we purified APP-containing vesicles from human neuroblastoma SH-SY5Y cells, and we showed that Mint2/3 are specifically enriched and that Mint3 and APP are present in the same vesicles. Increasing cellular APP levels increased the amounts of both APP and Mint3 in purified vesicles. Additional evidence supporting an obligate role for Mint3 in traffic of APP from the trans-Golgi network to the plasma membrane include the observations that depletion of Mint3 by small interference RNA (siRNA) or mutation of the Mint binding domain of APP changes the export route of APP from the basolateral to the endosomal/lysosomal sorting route. Finally, we show that increased expression of Mint3 decreased and siRNA-mediated knockdowns increased the secretion of the neurotoxic beta-amyloid peptide, Abeta(1-40). Together, our data implicate Mint3 activity as a critical determinant of post-Golgi APP traffic.

Downregulation of myosin II-B by siRNA alters the subcellular localization of the amyloid precursor protein and increases amyloid-β deposition in N2a cells

The Alzheimer's disease (AD) brain pathology is characterized by extracellular deposits of amyloid-beta (Abeta) peptides and intraneuronal fibrillar structures. These pathological features may be functionally linked, but the mechanism by which Abeta accumulation relates to neuronal degeneration is still poorly understood. Abeta peptides are fragments cleaved from the amyloid precursor protein (APP), a transmembrane protein ubiquitously expressed in the nervous system. Although the proteolytic processing of APP has been implicated in AD, the physiological function of APP and the subcellular site of APP cleavages remain unknown. The overall structure of the protein and its fast anterograde transport along the axon support the idea that APP functions as a vesicular receptor for cytoskeletal motor proteins. In the current study, we test the hypothesis that myosin II, important contributor to the cytoskeleton of neuronal cells, may influence the trafficking and/or the processing of APP. Our results demonstrate that downregulation of myosin II-B, the major myosin isoform in neurons, is able to increase Abeta deposition, concomitantly altering the subcellular localization of APP. These new insights might be important for the understanding of the function of APP and provide a novel conceptual framework in which to analyze its pathological role.

Alpha-secretase activation--an approach to Alzheimer's disease therapy

The nonamyloidogenic pathway of processing the amyloid precursor protein (APP) involves the cleavage within the amyloid-beta peptide sequence, and thus precludes amyloid-beta formation. The identification of a member of the disintegrin and metalloproteinase family, ADAM10, as an alpha-secretase that prevents plaque formation and hippocampal deficits in vivo gave us the possibility to examine the alpha-secretase as a potential target for the therapy of Alzheimer's disease. Within the priority program Cellular Mechanisms of Alzheimer's Disease, we investigated several approaches to stimulate the alpha-secretase pathway. Two protein convertases were found to be responsible for the removal of the prodomain, and for the formation of the mature enzyme with alpha-secretase activity. The cloning and characterization of the human ADAM10 promoter provided the basis to examine ADAM10 gene expression. We found a common upregulation of ADAM10, APP, and APP-like protein 2 during differentiation of neuronal cells by retinoic acid, and increased alpha-secretase cleavage of the two substrates. Other approaches for enhancing alpha-secretase activity are the reduction of cellular cholesterol and the stimulation of G protein-coupled neuropeptide receptors. Our results suggest medications and dietary regiments which enhance the nonamyloidogenic pathway of APP processing to be a valuable approach to Alzheimer's disease therapy.

Key factors in Alzheimer's disease: beta-amyloid precursor protein processing, metabolism and intraneuronal transport

During the last years it has become evident that the beta-amyloid (Abeta) component of senile plaques may be the key molecule in the pathology of Alzheimer's disease (AD). The source and place of the neurotoxic action of Abeta, however, is still a matter of controversy. The precursor of the beta-amyloid peptide is the predominantly neuronal beta-amyloid precursor protein. We, and others, hypothesize that intraneuronal misregulation of APP leads to an accumulation of Abeta peptides in intracellular compartments. This accumulation impairs APP trafficking, which starts a cascade of pathological changes and causes the pyramidal neurons to degenerate. Enhanced Abeta secretion as a function of stressed neurons and remnants of degenerated neurons provide seeds for extracellular Abeta aggregates, which induce secondary degenerative events involving neighboring cells such as neurons, astroglia and macrophages/microglia. Beta-amyloid precursor protein has a pivotal role in Alzheimer's disease.

Interaction of the cytosolic domains of sorLA/LR11 with the amyloid precursor protein (APP) and beta-secretase beta-site APP-cleaving enzyme

sorLA is a recently identified neuronal receptor for amyloid precursor protein (APP) that is known to interact with APP and affect its intracellular transport and processing. Decreased levels of sorLA in the brain of Alzheimer's disease (AD) patients and elevated levels of amyloid-beta peptide (Abeta) in sorLA-deficient mice point to the importance of the receptor in this neurodegenerative disorder. We analyzed APP cleavage in an APP-shedding assay and found that both sorLA and, surprisingly, a sorLA tail construct inhibited APP cleavage in a beta-site APP-cleaving enzyme (BACE)-dependent manner. In line with this finding, sorLA and the sorLA tail significantly reduced secreted Abeta levels when BACE was overexpressed, suggesting that sorLA influences beta-cleavage. To understand the effect of sorLA on APP cleavage by BACE, we analyzed whether sorLA interacts with APP and/or BACE. Because both full-length sorLA and sorLA C-terminal tail constructs were functionally relevant for APP processing, we analyzed sorLA-APP for a potential cytoplasmatic interaction domain. sorLA and C99 coimmunoprecipitated, pointing toward the existence of a new cytoplasmatic interaction site between sorLA and APP. Moreover, sorLA and BACE also coimmunoprecipitate. Thus, sorLA interacts both with BACE and APP and might therefore directly affect BACE-APP complex formation. To test whether sorLA impacts BACE-APP interactions, we used a fluorescence resonance energy transfer assay to evaluate BACE-APP interactions in cells. We discovered that sorLA significantly reduced BACE-APP interactions in Golgi. We postulate that sorLA acts as a trafficking receptor that prevents BACE-APP interactions and hence BACE cleavage of APP.

Biological roles of APP in the epidermis

The amyloid precursor protein (APP) was initially detected in cells of the central nervous system where it is considered to be involved in the pathogenesis of Alzheimer's disease. However, APP is also found in peripheral organs with exceptionally strong expression in the mammalian epidermis where it fulfils a variety of distinct biological roles. Full length APP appears to facilitate keratinocyte adhesion due to its ability to interact with the extracellular matrix. The C-terminus of APP also serves as adapter protein for binding the motor protein kinesin thereby mediating the centripetal transport of melanosomes in epidermal melanocytes. By the action of alpha-secretase sAPPalpha, the soluble N-terminal portion of APP, is released. sAPPalpha has been shown to be a potent epidermal growth factor thus stimulating proliferation and migration of keratinocytes as well as the exocytic release of melanin by melanocytes. The release of sAPPalpha can be almost completely blocked by inhibiting alpha-secretase with hydroxamic acid-based zinc metalloproteinase inhibitors. In hyperproliferative keratinocytes from psoriatic skin this inhibition results in normalized growth.

The non-amyloidogenic pathway: structure and function of alpha-secretases

The amyloid cascade hypothesis is the most accepted explanation for the pathogenesis of Alzheimer's disease (AD). APP is the precursor of the amyloid beta peptide (Abeta), the principal proteinaceous component of amyloid plaques in brains of Alzheimer's disease patients. Proteolytic cleavage of APP by the alpha-secretase within the Abeta sequence precludes formation of amyloidogenic peptides and leads to a release of soluble APPsalpha which has neuroprotective properties. In several studies, a decreased amount of APPsalpha in the cerebrospinal fluid of AD patients has been observed. Three members of the ADAM family (a disintegrin and metalloproteinase) ADAM-10, ADAM-17 (TACE) and ADAM-9 have been proposed as alpha-secretases. We review the evidence for each of these enzymes acting as a physiologically relevant alpha-secretase. In particular, we focus on ADAM-10, which recently was shown in a transgenic mouse model for AD, to act as an alpha-secretase in vivo. We also discuss the pharmacological up-regulation of alpha-secretases as a possible therapeutic treatment for AD.

Novel alpha-secretase cleavage of Alzheimer's amyloid beta precursor protein in the endoplasmic reticulum of COS7 cells

In the processing of APP, alpha- and beta-secretase pathways compete with each other for cleaving APP. Therefore, physiologically these two secretases are likely to colocalize in the same subcellular compartments. Previously beta-secretase cleavage of APP was found in the endoplasmic reticulum (ER). We herein tested whether alpha-secretase cleavage is also detected in the ER. We used experimental system of COS7 cells transfected with cDNA encoding human APP695, and the cell lysates and media were examined for its proteolytic products. When APP expression is concentrated in the ER by BFA-mediated transport inhibition or by using mutant APP harboring an ER-retrieval motif, alpha-secretase product sAPPalpha was accumulated in the cells. Immunofluorescence microscopy revealed that the ER-targeted APP produced intracellular accumulation of sAPPalpha, colocalizing with an ER marker. These results indicate that alpha-secretase cleavage of APP occurs in the ER. Further we examined the effects of phorbol ester PDBu, a direct activator of PKC, on the alpha-secretase and beta-secretase cleavages of APP occurring in the ER. Treatment with PDBu of COS7 cells transfected with the ER-targeted APP increased production of sAPPalpha and conversely decreased production of beta-secretase product sAPPbeta. Thus, in the ER, alpha-secretase competes with beta-secretase for cleaving APP and such competitive correlation might modulate the production of Abeta42 found in this compartment.

Aβ production as consequence of cellular death of a human neuroblastoma overexpressing APP

In human brain the Abeta peptide is produced mainly by neurons and the overexpression of amyloid precursor protein (APP) that involves an increase in Abeta secretion, has been observed in some areas of the Alzheimer's disease patients brain. We have generated two stably transfected human neuroblastoma lines which overexpress APP; both of them secreted Abeta and showed morphological changes and cell death with apoptotic program characteristics. Interestingly, coculture experiments with the untransfected human neuroblastoma cell line showed that the Abeta peptide was not responsible for the death in those cell lines; additionally, we indicate that upon cell death, Abeta peptide is secreted into cell medium.

Will preventing protein aggregates live up to its promise as prophylaxis against neurodegenerative diseases?

Protein aggregation and misfolding characterize most age-related neurodegenerative diseases including Alzheimer, Parkinson and Huntington diseases. Protein aggregation has generally been assumed to be responsible for neurodegeneration in these disorders due to association and genetics. However, protein aggregation may, in fact, be an attempt to protect neurons from the stress resulting from the disease etiology. In this review, we weigh the evidence of whether removal of amyloids, aggregates and neuronal inclusions represent a reasonable strategy for protecting neurons.

Nitric oxide in the cerebral cortex of amyloid-precursor protein (SW) Tg2576 transgenic mice

Changes in the amyloid-peptide (Abeta), neuronal and inducible nitric oxide (NO)synthase (nNOS, iNOS), nitrotyrosine, glial fibrillary acidic protein, and lectin from Lycopersicon esculentum (tomato) were investigated in the cerebral cortex of transgenic mice (Tg2576) to amyloid precursor protein (APP), by immunohistochemistry (bright light, confocal, and electron microscopy). The expression of nitrergic proteins and synthesis of nitric oxide were analyzed by immunoblotting and NOS activity assays, respectively. The cerebral cortex of these transgenic mice showed an age-dependent progressive increase in intraneuronal aggregates of Abeta-peptide and extracellular formation of senile plaques surrounded by numerous microglial and reactive astrocytes. Basically, no changes to nNOS reactivity or expression were found in the cortical mantle of either wild or transgenic mice. This reactivity in wild mice corresponded to numerous large type I and small type II neurons. The transgenic mice showed swollen, twisted, and hypertrophic preterminal and terminal processes of type I neurons, and an increase of the type II neurons. The calcium-dependent NOS enzymatic activity was higher in wild than in the transgenic mice. The iNOS reactivity, expression and calcium-independent enzymatic activity increased in transgenic mice with respect to wild mice, and were related to cortical neurons and microglial cells. The progressive elevation of NO production resulted in a specific pattern of protein nitration in reactive astrocytes. The ultrastructural study carried out in the cortical mantle showed that the neurons contained intracellular aggregates of Abeta-peptide associated with the endoplasmic reticulum, mitochondria, and Golgi apparatus. The endothelial vascular cells also contained Abeta-peptide deposits. This transgenic model might contribute to understand the role of the nitrergic system in the biological changes related to neuropathological progression of Alzheimer's disease.

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

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

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

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

Overexpression of nicastrin increases Abeta production

Gamma-secretase cleavage is the final proteolytic step that releases the amyloid beta-peptide (Abeta) from the amyloid beta-protein precursor (APP). Significant evidence indicates that the presenilins (PS) are catalytic components of a high molecular weight gamma-secretase complex. The glycoprotein nicastrin was recently identified as a functional unit of this complex based on 1) binding to PS and 2) the ability to modulate Abeta production following mutation of a conserved DYIGS region. In contrast to the initial report, we find that overexpression of wild-type (WT) nicastrin increases Abeta production, whereas DYIGS mutations (MT) have little or no effect. The increase in Abeta production is associated with an increase in gamma-secretase activity but not with a detectable increase in PS1 levels. Subcellular fractionation studies show that WT but not MT nicastrin matures into buoyant membrane fractions enriched in gamma-secretase activity. These data support the hypothesis that nicastrin is an essential component of the gamma-secretase complex. The finding that WT nicastrin overexpression can increase gamma-secretase activity without altering levels of the presumed catalytic component (PS) of the enzyme may point to a role for nicastrin in facilitating cleavage by regulating substrate interactions with the gamma-secretase complex.

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

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

A role for presenilin 1 in regulating the delivery of amyloid precursor protein to the cell surface

Presenilin 1 (PS1) and presenilin 2 play a critical role in the gamma-secretase processing of amyloid precursor protein (APP) and Notch1. Here, we investigate maturation and intracellular trafficking of APP and other membrane proteins in cells expressing an experimental PS1 deletion mutant (deltaM1,2). Stable expression of deltaM1,2 impairs gamma-secretase processing of Notch1 and delays Abeta secretion. Kinetic studies show enhanced O-glycosylation and sialylation of holo-APP and marked accumulation of APP COOH-terminal fragments (CTFs). Surface biotinylation, live staining, and trafficking studies show increased surface accumulation of holo-APP and CTFs in deltaM1,2 cells resulting from enhanced surface delivery of newly synthesized APP. Expression of a loss-of-function PS1 mutant (D385A) or incubation of cells with gamma-secretase inhibitors also increases surface levels of holo-APP and CTFs. In contrast to APP, glycosylation and surface accumulation of another type I membrane protein, nicastrin, are markedly reduced in deltaM1,2 cells. Finally, expression of deltaM1,2 results in the increased assembly and surface expression of nicotinic acetylcholine receptors, illustrating that PS1's influence on protein trafficking extends beyond APP and other type I membrane protein substrates of gamma-secretase. Collectively, our findings provide evidence that PS1 regulates the glycosylation and intracellular trafficking of APP and select membrane proteins.

Low-density lipoprotein receptor-related protein levels and endocytic function are reduced by overexpression of the FE65 adaptor protein, FE65L1

The FE65 adaptor protein family was identified in two-hybrid screens as proteins that bind the cytoplasmic domain of the amyloid precursor protein (APP). Studies have shown that FE65 binding to APP modulates APP processing. Increased levels of alpha-secretase derived secreted APP (APPsalpha) and beta-amyloid (Abeta) were recovered from conditioned media upon FE65L1 or FE65 overexpression. These effects were associated with an increase in the ratio of mature/immature APP and increased cell-surface APP. FE65 has also been reported to bind low-density lipoprotein receptor-related protein (LRP). Here we show that FE65L1 overexpression results in decreased LRP steady state levels, LRPs, and LRP endocytic receptor function. These changes in LRP protein levels are not due to decreased transcription of LRP. Furthermore, pulse/chase experiments demonstrate that changes in LRP protein only occurred 12-18 h after translation. We conclude that the decreases in LRP levels likely reflect routing of LRP away from the cell surface into a degradative pathway. Previous studies suggested that LRP plays an important role for Abeta production of Kunitz protease inhibitor forms of APP in the endocytic pathway. These data show that FE65L1 can differentially affect the metabolic fate of APP and LRP. In addition, these data suggest that the LRP decrease observed in FE65L1 overexpressing cells may in part contribute to altered APP processing.

The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease

Investigators studying the primary culprit responsible for Alzheimer disease have, for the past two decades, primarily focused on amyloid-beta (Abeta). Here, we put Abeta on trial and review evidence amassed by the prosecution that implicate Abeta and also consider arguments and evidence gathered by the defense team who are convinced of the innocence of their client. As in all trials, the arguments provided by the prosecution and defense revolve around the same evidence, with opposing interpretations. Below, we present a brief synopsis of the trial for you, the jury, to decide the verdict. Amyloid-beta: guilty or not-guilty?

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

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

Secretases as targets for drug design in Alzheimer's disease

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

Demonstration by fluorescence resonance energy transfer of two sites of interaction between the low-density lipoprotein receptor-related protein and the amyloid precursor protein: role of the intracellular adapter protein Fe65

Amyloid-beta, the major constituent of senile plaques in Alzheimer's disease, is derived from the amyloid precursor protein (APP) by proteolysis. Kunitz protease inhibitor (KPI) containing forms of APP (APP751/770) interact with a multifunctional endocytic receptor, the low-density lipoprotein receptor-related protein (LRP), which modulates its proteolytic processing affecting production of amyloid-beta. We used fluorescence resonance energy transfer (FRET) using labeled LRP and APP in H4 cell line to examine the subcellular localization and the molecular domains involved in the APP-LRP interaction. KPI-containing forms of APP (APP770) demonstrated FRET with LRP that was sensitive to the LRP inhibitor receptor-associated protein (RAP), suggesting an interaction between the extracellular domains of APP770 and LRP. APP695 also interacts with LRP to lesser degree (as measured by extracellular domain probes), and this ectodomain interaction is not altered by RAP. By using C-terminally tagged LRP and APP, we demonstrate a second site of interaction between the C termini of both APP695 and APP770 and the C terminus of LRP, and that the interactions at these regions are not sensitive to RAP. We next examined the possibility that the C-termini APP-LRP interaction was mediated by Fe65, an adaptor protein that interacts with the cytoplasmic tails of LRP and APP. FRET studies confirmed a close proximity between the amino Fe65 phosphotyrosine binding (PTB) domain and LRP cytoplasmic domain and between the carboxyl Fe65 PTB domain and the APP cytoplasmic domain. These findings demonstrate that LRP interaction with APP occurs via both extracellular and intracellular protein interaction domains.

Endosomal sorting of amyloid precursor protein-P-selectin chimeras influences secretase processing

Amyloid beta protein, the major component of the senile plaques in Alzheimer's disease, is generated by secretory and endocytic processing of amyloid precursor protein. Internalized amyloid precursor protein either recycles to the plasma membrane, where alpha-secretase resides, or moves to acidic compartment(s) for beta-secretase exposure. While the trans-Golgi network contains beta-secretase activity, recent examination of the subcellular distribution of this proteinase, called BACE, has led to the suggestion that beta-secretase activity might also reside at the plasma membrane and in endosomes. To examine the role of endocytic compartments in beta-secretase processing of amyloid precursor protein, the wild-type and endosomal sorting mutant P-selectin cytoplasmic domains were used to control movement of amyloid precursor protein through endosomes. Amyloid precursor protein/P-selectin, which is sorted from early to late endosomes, undergoes significantly less alpha-secretase cleavage, and more beta-secretase cleavage, than amyloid precursor protein/P-selectin768A, a mutant that recycles more efficiently to the cell surface. Our results demonstrate that endosomal sorting influences relative exposure of the amyloid precursor protein/P-selectin chimeras to alpha- and beta-secretase activities, and suggest that, because delivery to late endocytic compartments favors beta-secretase processing of amyloid precursor protein, there is likely limited beta-secretase activity in early endosomes or at the cell surface. We propose that the trans-Golgi network may be involved in both secretory and endocytic generation of amyloid beta protein.

Subcellular localization of presenilin 2 endoproteolytic C-terminal fragments

Mutations in the genes that encode the presenilin 1 and 2 (PS1 and PS2) proteins cause the majority of familial Alzheimer's disease (FAD). Differential cleavage of the presenilins results in a generation of at least two C-terminal fragments (CTFs). An increase in the smaller of these two CTFs is one of the few changes in presenilin processing associated with FAD mutations in both PS1 and PS2. Interestingly, the phosphorylation of PS2 modulates the production of the smaller, caspase-derived PS2 CTF, which indicates that the generation of this fragment is a regulated, physiologic event. To date, there is no data concerning the subcellular distribution of the caspase-derived PS2 CTF. Because this fragment is normally present at levels that are difficult to detect, we have used cell lines in which the production of wild-type or N141I mutant PS2 is controlled by a tetracycline-regulated promoter in order to assess the subcellular localization of the caspase CTF in relation to the larger, constitutive PS2 CTF and to PS2 holoprotein. We have found that when levels of PS2 are low, the constitutive CTF colocalizes with markers consistent with localization in the early Golgi-ER-Golgi intermediate compartment (ERGIC) while the caspase CTF colocalizes with markers for the endoplasmic reticulum (ER). Following induction of wild-type or mutant PS2, when the levels of PS2 are high, the primary localization of the constitutive CTF appears to shift from the early Golgi-ERGIC in addition to the ER. Interestingly, while the induction of wild-type PS2 resulted in the localization of the caspase CTF primarily in the ER, the induction of mutant PS2 resulted in the localization of the caspase CTF to both the ER and the early Golgi-ERGIC. In summary, these data suggest that the two presenilin 2 CTFs have different patterns of subcellular localization and that the N141I PS2 mutation alters the localization pattern of the PS2 caspase fragment.

Demonstration by fluorescence resonance energy transfer of two sites of interaction between the low-density lipoprotein receptor-related protein and the amyloid precursor protein: role of the intracellular adapter protein Fe65

Amyloid-beta, the major constituent of senile plaques in Alzheimer's disease, is derived from the amyloid precursor protein (APP) by proteolysis. Kunitz protease inhibitor (KPI) containing forms of APP (APP751/770) interact with a multifunctional endocytic receptor, the low-density lipoprotein receptor-related protein (LRP), which modulates its proteolytic processing affecting production of amyloid-beta. We used fluorescence resonance energy transfer (FRET) using labeled LRP and APP in H4 cell line to examine the subcellular localization and the molecular domains involved in the APP-LRP interaction. KPI-containing forms of APP (APP770) demonstrated FRET with LRP that was sensitive to the LRP inhibitor receptor-associated protein (RAP), suggesting an interaction between the extracellular domains of APP770 and LRP. APP695 also interacts with LRP to lesser degree (as measured by extracellular domain probes), and this ectodomain interaction is not altered by RAP. By using C-terminally tagged LRP and APP, we demonstrate a second site of interaction between the C termini of both APP695 and APP770 and the C terminus of LRP, and that the interactions at these regions are not sensitive to RAP. We next examined the possibility that the C-termini APP-LRP interaction was mediated by Fe65, an adaptor protein that interacts with the cytoplasmic tails of LRP and APP. FRET studies confirmed a close proximity between the amino Fe65 phosphotyrosine binding (PTB) domain and LRP cytoplasmic domain and between the carboxyl Fe65 PTB domain and the APP cytoplasmic domain. These findings demonstrate that LRP interaction with APP occurs via both extracellular and intracellular protein interaction domains.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Detection of the membrane-retained carboxy-terminal tail containing polypeptides of the amyloid precursor protein in tissue from Alzheimer's disease brain

A major hallmark of Alzheimer's disease (AD) is the presence of extracellular amyloid plaques consisting primarily of amyloid beta peptide (A beta) which is derived from a larger beta-amyloid precursor protein (APP). APP is processed via secretory and endosomal/lysosomal pathways by a group of proteases called secretases. During the processing of APP, the carboxy-terminal tail fragment has been suggested to remain within the cell. To investigate the fate of this fragment, we generated an antibody specific for a nine amino acid residue, the sequence of which was derived from the carboxy-terminal putative cytoplasmic tail of APP. Computer analysis of the entire APP gene, searching for regions of greatest antigenicity, surface probability, hydrophilicity, and presence of beta turns, indicated that the cytoplasmic tail region is an immunodominant region of APP. The peptide coupled to keyhole limpet hemocyanin protein, produced a very high titer antibody (1:1 x 10(6)). To evaluate the specificity of the antibody, immunoprecipitation of in vitro transcribed and translated DNA encoding the carboxy-terminal amino acids of APP in wheat germ extract was carried out. A single immunoprecipitated band of the correct size was seen by SDS-PAGE. The antibody was also able to specifically detect the accumulation of the stable C-terminal tail containing fragments of APP in neurites of the amygdala and hippocampus regions of the human brain tissue from AD subjects, but did not react with age-matched control normal brain tissue. The localization of the C-terminal tail of APP within the brain tissue of AD patients underscores the likely importance of the C-terminus in the pathogenesis of AD.

Alpha-secretase activity of the disintegrin metalloprotease ADAM 10. Influences of domain structure

Disintegrin metalloproteases from different organisms form the ADAM (a disintegrin and metalloprotease) family. All members display a common domain organization and possess four potential functions: proteolysis, cell adhesion, cell fusion, and cell signaling. Members of the ADAM family are responsible for the proteolytic cleavage of transmembrane proteins and release of their extracellular domain. The proteolytic process is referred to as ectodomain shedding, which is activated by phorbol esters and inhibited by hydroxamic acid-based inhibitors. We have shown that the disintegrin metalloprotease ADAM 10 has both constitutive and regulated alpha-secretase activity. Expression of a dominant negative mutant of ADAM 10 in HEK cells decreases the secretion of APPs alpha. In order to investigate the influence of distinct protein domains of ADAM 10 on alpha-secretase activity, several deletion mutants of ADAM 10 were constructed. Our findings demonstrate that the deletion of the disintegrin domain results in a mutant ADAM 10 with remaining alpha-secretase activity, whereas the deletion of the prodomain destroys the proteolytic activity of ADAM 10.

Constitutive alpha-secretase cleavage of the beta-amyloid precursor protein in the furin-deficient LoVo cell line: involvement of the pro-hormone convertase 7 and the disintegrin metalloprotease ADAM10

The beta-amyloid precursor protein (betaAPP) undergoes a physiological cleavage triggered by one or several proteolytic activities referred to as alpha-secretases, leading to the secretion of sAPPalpha. Several lines of evidence indicate that the alpha-secretase cleavage is a highly regulated process. Thus, besides constitutive production of sAPPalpha, several studies have reported on protein kinase C-regulated sAPPalpha secretion. Studies aimed at identifying alpha-secretase(s) candidates suggest the involvement of enzymes belonging to the pro-hormone convertases and disintegrin families. The delineation of respective contributions of proteolytic activities in constitutive and regulated sAPPalpha secretion is rendered difficult by the fact that the overall regulated response always includes the basal constitutive counterpart that cannot be selectively abolished. Here we report on the fact that the furin-deficient LoVo cells are devoid of regulated PKC-dependent sAPPalpha secretion and therefore represent an interesting model to study exclusively the constitutive sAPPalpha secretion. We show here, by a pharmacological approach using selective inhibitors, that pro-hormone convertases and proteases of the ADAM (disintegrin metalloproteases) family participate in the production/secretion of sAPPalphas in LoVo cells. Transfection analysis allowed us to further establish that the pro-hormone convertase 7 and ADAM10 but not ADAM17 (TACE, tumour necrosis factor alpha-converting enzyme) likely contribute to constitutive sAPPalpha secretion by LoVo cells.

Regulation of APP cleavage by alpha-, beta- and gamma-secretases

Proteolytic cleavage of the amyloid protein from the amyloid protein precursor (APP) by APP secretases is a key event in Alzheimer's disease (AD) pathogenesis. alpha-Secretases cleave APP within the amyloid sequences, whereas beta- and gamma-secretases cleave on the N- and C-terminal ends respectively. The transmembrane aspartyl protease BACE has been identified as beta-secretase and several proteases (ADAM-10, TACE, PC7) may be alpha-secretases. A number of studies have suggested that presenilins could be gamma-secretases, although this remains to be demonstrated conclusively. Inhibition of beta- and gamma-secretase, or stimulation of alpha-secretase, is a rational strategy for therapeutic intervention in AD.

Overexpression of Rab11 or constitutively active Rab11 does not affect sAPPalpha and Abeta secretions by wild-type and Swedish mutated betaAPP-expressing HEK293 cells

Presenilins 1 and 2 are two homologous proteins which, when mutated, appear responsible for most of the early-onset familial forms of Alzheimer's disease. Among various functional aspects, presenilins appear to behave as chaperoning partners of a series of proteins including the beta-amyloid precursor protein. Recently, presenilins were shown to interact with Rab11, a GTPase involved in intracellular transport. This suggested that Rab11-presenilin interaction could influence the routing of betaAPP and thereby modulate its maturation. In this context, we examined whether overexpression of Rab11 or its constitutively active mutant Rab11Q70L could affect betaAPP maturation in human HEK293 cells. We show here that the overexpression of both Rab11-related proteins does not modify the recovery of secreted sAPPalpha or Abeta in HEK293 cells expressing wild-type betaAPP or betaAPP harboring the Swedish double mutation. These data indicate that Rab11 does not influence betaAPP processing in HEK293 cells. However, it does not preclude the possibility for Rab11 to modulate other presenilin-mediated functions in human cells.

Proteolytic processing and cell biological functions of the amyloid precursor protein

Recent research has identified some key players involved in the proteolytic processing of amyloid precursor protein (APP) to amyloid beta-peptide, the principal component of the amyloid plaques in Alzheimer patients. Interesting parallels exists with the proteolysis of other proteins involved in cell differentiation, cholesterol homeostasis and stress responses. Since the cytoplasmic domain of APP is anchored to a complex protein network that might function in axonal elongation, dendritic arborisation and neuronal cell migration, the proteolysis of APP might be critically involved in intracellular signalling events.

Modulation of beta-amyloid precursor protein processing by the low density lipoprotein receptor-related protein (LRP). Evidence that LRP contributes to the pathogenesis of Alzheimer's disease

Beta-amyloid peptide (Abeta), which plays a central role in the pathogenesis of Alzheimer's disease, is derived from the transmembrane beta-amyloid precursor protein (APP) by proteolytic processing. Although mechanisms associated with Abeta generation are not fully understood, it is known that Abeta can be generated within endosomal compartments upon internalization of APP from the cell surface. The low density lipoprotein receptor-related protein (LRP) was previously shown to mediate the endocytosis of APP isoforms containing the Kunitz proteinase inhibitor domain (Kounnas, M. Z., Moir, R. D., Rebeck, G. W., Bush, A. I., Argraves, W. S., Tanzi, R. E., Hyman, B. T., and Strickland, D. K. (1995) Cell 82, 331-340; Knauer, M. F., Orlando, R. A., and Glabe, C. G. (1996) Brain Res. 740, 6-14). The objective of the current study was to test the hypothesis that LRP-mediated internalization of cell surface APP can modulate APP processing and thereby affect Abeta generation. Here, we show that long term culturing of cells in the presence of the LRP-antagonist RAP leads to increased cell surface levels of APP and a significant reduction in Abeta synthesis. Further, restoring LRP function in LRP-deficient cells results in a substantial increase in Abeta production. These findings demonstrate that LRP contributes to Abeta generation and suggest novel pharmacological approaches to reduce Abeta levels based on selective LRP blockade.

Identification of a novel aspartic protease (Asp 2) as beta-secretase

The Alzheimer's disease beta-amyloid peptide (Abeta) is produced by excision from the type 1 integral membrane glycoprotein amyloid precursor protein (APP) by the sequential actions of beta- and then gamma-secretases. Here we report that Asp 2, a novel transmembrane aspartic protease, has the key activities expected of beta-secretase. Transient expression of Asp 2 in cells expressing APP causes an increase in the secretion of the N-terminal fragment of APP and an increase in the cell-associated C-terminal beta-secretase APP fragment. Mutation of either of the putative catalytic aspartyl residues in Asp 2 abrogates the production of the fragments characteristic of cleavage at the beta-secretase site. The enzyme is present in normal and Alzheimer's disease (AD) brain and is also found in cell lines known to produce Abeta. Asp 2 localizes to the Golgi/endoplasmic reticulum in transfected cells and shows clear colocalization with APP in cells stably expressing the 751-amino-acid isoform of APP.