Effects of heparin and enoxaparin on APP processing and Aβ production in primary cortical neurons from Tg2576 mice

Background

Alzheimer''s disease (AD) is caused by accumulation of Aβ, which is produced through sequential cleavage of β-amyloid precursor protein (APP) by the β-site APP cleaving enzyme (BACE1) and γ-secretase. Enoxaparin, a low molecular weight form of the glycosaminoglycan (GAG) heparin, has been reported to lower Aβ plaque deposition and improve cognitive function in AD transgenic mice.

Methodology/Principal Findings

We examined whether heparin and enoxaparin influence APP processing and inhibit Aβ production in primary cortical cell cultures. Heparin and enoxaparin were incubated with primary cortical cells derived from Tg2576 mice, and the level of APP and proteolytic products of APP (sAPPα, C99, C83 and Aβ) was measured by western blotting. Treatment of the cells with heparin or enoxaparin had no significant effect on the level of total APP. However, both GAGs decreased the level of C99 and C83, and inhibited sAPPα and Aβ secretion. Heparin also decreased the level of β-secretase (BACE1) and α-secretase (ADAM10). In contrast, heparin had no effect on the level of ADAM17.

Conclusions/Significance

The data indicate that heparin and enoxaparin decrease APP processing via both α- and β-secretase pathways. The possibility that GAGs may be beneficial for the treatment of AD needs further study.     

The Large Hydrophilic Loop of Presenilin 1 Is Important for Regulating ��-Secretase Complex Assembly and Dictating the Amyloid �� Peptide (A��) Profile without Affecting Notch Processing

γ-Secretase is an enzyme complex that mediates both Notch signaling and β-amyloid precursor protein (APP) processing, resulting in the generation of Notch intracellular domain, APP intracellular domain, and the amyloid β peptide (Aβ), the latter playing a central role in Alzheimer disease (AD). By a hitherto undefined mechanism, the activity of γ-secretase gives rise to Aβ peptides of different lengths, where Aβ42 is considered to play a particular role in AD. In this study we have examined the role of the large hydrophilic loop (amino acids 320–374, encoded by exon 10) of presenilin 1 (PS1), the catalytic subunit of γ-secretase, for γ-secretase complex formation and activity on Notch and APP processing. Deletion of exon 10 resulted in impaired PS1 endoproteolysis, γ-secretase complex formation, and had a differential effect on Aβ-peptide production. Although the production of Aβ38, Aβ39, and Aβ40 was severely impaired, the effect on Aβ42 was affected to a lesser extent, implying that the production of the AD-related Aβ42 peptide is separate from the production of the Aβ38, Aβ39, and Aβ40 peptides. Interestingly, formation of the intracellular domains of both APP and Notch was intact, implying a differential cleavage activity between the ϵ/S3 and γ sites. The most C-terminal amino acids of the hydrophilic loop were important for regulating APP processing. In summary, the large hydrophilic loop of PS1 appears to differentially regulate the relative production of different Aβ peptides without affecting Notch processing, two parameters of significance when considering γ-secretase as a target for pharmaceutical intervention in AD.     

��-Secretase Inhibitor Potency Is Decreased by Aberrant ��-Cleavage Location of the ��Swedish Mutant�� Amyloid Precursor Protein

The amyloid-β (Aβ) peptide, widely known as the causative molecule of Alzheimer disease (AD), is generated by the sequential cleavage of amyloid precursor protein (APP) by the aspartyl proteases BACE1/β-secretase and presenilin/γ-secretase. Inhibition of BACE1, therefore, is a promising strategy for preventing the progression of AD. However, β-secretase inhibitors (BSIs) exhibit unexpectedly low potency in cells expressing “Swedish mutant” APP (APPswe) and in the transgenic mouse Tg2576, an AD model overexpressing APPswe. The Swedish mutation dramatically accelerates β-cleavage of APP and hence the generation of Aβ; this acceleration has been assumed to underlie the poor inhibitory activity of BSI against APPswe processing. Here, we studied the mechanism by which the Swedish mutation causes this BSI potency decrease. Surprisingly, decreased BSI potency was not observed in an in vitro assay using purified BACE1 and substrates, indicating that the accelerated β-cleavage resulting from the Swedish mutation is not its underlying cause. By focusing on differences between the cell-based and in vitro assays, we have demonstrated here that the potency decrease is caused by the aberrant subcellular localization of APPswe processing and not by accelerated β-cleavage or the accumulation of the C-terminal fragment of β-cleaved APP. Because most patients with sporadic AD express wild type APP, our findings suggest that the wild type mouse is superior to the Tg2576 mouse as a model for determining the effective dose of BSI for AD patients. This work provides novel insights into the potency decrease of BSI and valuable suggestions for its development as a disease-modifying agent.     

The Golgi-Localized γ-Ear-Containing ARF-Binding (GGA) Proteins Alter Amyloid-β Precursor Protein (APP) Processing through Interaction of Their GAE Domain with the Beta-Site APP Cleaving Enzyme 1 (BACE1)

Proteolytic processing of amyloid-β precursor protein (APP) by beta-site APP cleaving enzyme 1 (BACE1) is the initial step in the production of amyloid beta (Aβ), which accumulates in senile plaques in Alzheimer’s disease (AD). Essential for this cleavage is the transport and sorting of both proteins through endosomal/Golgi compartments. Golgi-localized γ-ear-containing ARF-binding (GGA) proteins have striking cargo-sorting functions in these pathways. Recently, GGA1 and GGA3 were shown to interact with BACE1, to be expressed in neurons, and to be decreased in AD brain, whereas little is known about GGA2. Since GGA1 impacts Aβ generation by confining APP to the Golgi and perinuclear compartments, we tested whether all GGAs modulate BACE1 and APP transport and processing. We observed decreased levels of secreted APP alpha (sAPPα), sAPPβ, and Aβ upon GGA overexpression, which could be reverted by knockdown. GGA-BACE1 co-immunoprecipitation was impaired upon GGA-GAE but not VHS domain deletion. Autoinhibition of the GGA1-VHS domain was irrelevant for BACE1 interaction. Our data suggest that all three GGAs affect APP processing via the GGA-GAE domain.     

Novel Role of RanBP9 in BACE1 Processing of Amyloid Precursor Protein and Amyloid �� Peptide Generation

Accumulation of the amyloid β (Aβ) peptide derived from the proteolytic processing of amyloid precursor protein (APP) is the defining pathological hallmark of Alzheimer disease. We previously demonstrated that the C-terminal 37 amino acids of lipoprotein receptor-related protein (LRP) robustly promoted Aβ generation independent of FE65 and specifically interacted with Ran-binding protein 9 (RanBP9). In this study we found that RanBP9 strongly increased BACE1 cleavage of APP and Aβ generation. This pro-amyloidogenic activity of RanBP9 did not depend on the KPI domain or the Swedish APP mutation. In cells expressing wild type APP, RanBP9 reduced cell surface APP and accelerated APP internalization, consistent with enhanced β-secretase processing in the endocytic pathway. The N-terminal half of RanBP9 containing SPRY-LisH domains not only interacted with LRP but also with APP and BACE1. Overexpression of RanBP9 resulted in the enhancement of APP interactions with LRP and BACE1 and increased lipid raft association of APP. Importantly, knockdown of endogenous RanBP9 significantly reduced Aβ generation in Chinese hamster ovary cells and in primary neurons, demonstrating its physiological role in BACE1 cleavage of APP. These findings not only implicate RanBP9 as a novel and potent regulator of APP processing but also as a potential therapeutic target for Alzheimer disease.The major defining pathological hallmark of Alzheimer disease (AD)² is the accumulation of amyloid β protein (Aβ), a neurotoxic peptide derived from β- and γ-secretase cleavages of the amyloid precursor protein (APP). The vast majority of APP is constitutively cleaved in the middle of the Aβ sequence by α-secretase (ADAM10/TACE/ADAM17) in the non-amyloidogenic pathway, thereby abrogating the generation of an intact Aβ peptide. Alternatively, a small proportion of APP is cleaved in the amyloidogenic pathway, leading to the secretion of Aβ peptides (37–42 amino acids) via two proteolytic enzymes, β- and γ-secretase, known as BACE1 and presenilin, respectively (1).The proteolytic processing of APP to generate Aβ requires the trafficking of APP such that APP and BACE1 are brought together in close proximity for β-secretase cleavage to occur. We and others have shown that the low density lipoprotein receptor-related protein (LRP), a multifunctional endocytosis receptor (2), binds to APP and alters its trafficking to promote Aβ generation. The loss of LRP substantially reduces Aβ release, a phenotype that is reversed when full-length (LRP-FL) or truncated LRP is transfected in LRP-deficient cells (3, 4). Specifically, LRP-CT lacking the extracellular ligand binding regions but containing the transmembrane domain and the cytoplasmic tail is capable of rescuing amyloidogenic processing of APP and Aβ release in LRP deficient cells (3). Moreover, the LRP soluble tail (LRP-ST) lacking the transmembrane domain and only containing the cytoplasmic tail of LRP is sufficient to enhance Aβ secretion (5). This activity of LRP-ST is achieved by promoting APP/BACE1 interaction (6), although the precise mechanism is unknown. Although we had hypothesized that one or more NPXY domains in LRP-ST might underlie the pro-amyloidogenic processing of APP, we recently found that the 37 C-terminal residues of LRP (LRP-C37) lacking the NPXY motif was sufficient to robustly promote Aβ production independent of FE65 (7). Because LRP-C37 likely acts by recruiting other proteins, we used the LRP-C37 region as bait in a yeast two-hybrid screen, resulting in the identification of 4 new LRP-binding proteins (7). Among these, we focused on Ran-binding protein 9 (RanBP9) in this study, which we found to play a critical role in the trafficking and processing of APP. RanBP9, also known as RanBPM, acts as a multi-modular scaffolding protein, bridging interactions between the cytoplasmic domains of a variety of membrane receptors and intracellular signaling targets. These include Axl and Sky (8), MET receptor protein-tyrosine kinase (9), and β2-integrin LFA-1 (10). Similarly, RanBP9 interacts with Plexin-A receptors to strongly inhibit axonal outgrowth (11) and functions to regulate cell morphology and adhesion (12, 13). Here we show that RanBP9 robustly promotes BACE1 processing of APP and Aβ generation.     

The inhalation anesthetic desflurane induces caspase activation and increases amyloid beta-protein levels under hypoxic conditions

Perioperative factors including hypoxia, hypocapnia, and certain anesthetics have been suggested to contribute to Alzheimer disease (AD) neuropathogenesis. Desflurane is one of the most commonly used inhalation anesthetics. However, the effects of desflurane on AD neuropathogenesis have not been previously determined. Here, we set out to assess the effects of desflurane and hypoxia on caspase activation, amyloid precursor protein (APP) processing, and amyloid β-protein (Aβ) generation in H4 human neuroglioma cells (H4 naïve cells) as well as those overexpressing APP (H4-APP cells). Neither 12% desflurane nor hypoxia (18% O₂) alone affected caspase-3 activation, APP processing, and Aβ generation. However, treatment with a combination of 12% desflurane and hypoxia (18% O₂) (desflurane/hypoxia) for 6 h induced caspase-3 activation, altered APP processing, and increased Aβ generation in H4-APP cells. Desflurane/hypoxia also increased levels of β-site APP-cleaving enzyme in H4-APP cells. In addition, desflurane/hypoxia-induced Aβ generation could be reduced by the broad caspase inhibitor benzyloxycarbonyl-VAD. Finally, the Aβ aggregation inhibitor clioquinol and γ-secretase inhibitor L-685,458 attenuated caspase-3 activation induced by desflurane/hypoxia. In summary, desflurane can induce Aβ production and caspase activation, but only in the presence of hypoxia. Pending in vivo confirmation, these data may have profound implications for anesthesia care in elderly patients, and especially those with AD.An estimated 200 million patients worldwide undergo surgery each year. Several reports have suggested that anesthesia and surgery may facilitate development of Alzheimer disease (AD)⁴ (1–3). A recent study also reported that patients having coronary artery bypass graft surgery under general anesthesia are at increased risk for AD as compared with those having percutaneous transluminal coronary angioplasty under local anesthesia (4).Genetic evidence, confirmed by neuropathological and biochemical findings, indicates that excessive production and/or accumulation of amyloid β-protein (Aβ) play a fundamental role in the pathology of AD (reviewed in Refs. 5 and 6). Aβ is produced via serial proteolysis of amyloid precursor protein (APP) by aspartyl protease β-site APP-cleaving enzyme (BACE), or β-secretase, andγ-secretase. BACE cleaves APP to generate a 99-residue membrane-associated C terminus fragment (APP-C99). APP-C99 is further cleaved by γ-secretase to release 4-kDa Aβ and β-amyloid precursor protein intracellular domain (7–9). Presenilin and γ-secretase co-fractionate as a detergent-sensitive, high molecular weight complex (10) that includes at least three other proteins, nicastrin/APH-2, APH-1, and PEN-2, all of which are necessary and sufficient for γ-secretase activity (11–13). Increasing evidence indicates that apoptosis is associated with a variety of neurodegenerative disorders, including AD (Refs. 14–17; reviewed in Ref. 18). Aβ has been shown to cause caspase activation and apoptosis, which can in turn potentiate Aβ generation (16, 19–28). Finally, fibrillar aggregates of Aβ and oligomeric species of Aβ are more neurotoxic (29–37).Perioperative factors, including hypoxia (38–42), hypocapnia (43), and anesthetics (44–47), have been reported to potentially contribute to AD neuropathogenesis. These perioperative factors may also cause post-operative cognitive dysfunction, a dementia associated with surgery and anesthesia, by triggering AD neuropathogenesis.Isoflurane, sevoflurane, and desflurane are the most commonly used inhalation anesthetics. It has been reported that isoflurane enhances the oligomerization and cytotoxicity of Aβ (44) and induces apoptosis (48–51). Our recent studies have shown that a clinically relevant concentration of isoflurane can lead to caspase-3 activation, decrease cell viability, alter APP processing, and increase Aβ generation in human H4 neuroglioma cells overexpressing human APP (45–47). Loop et al. (49) reported that isoflurane and sevoflurane, but not desflurane, can induce caspase activation and apoptosis in human T lymphocytes. However, effects of desflurane and desflurane plus other perioperative risk factors, e.g. hypoxia, on APP processing and Aβ generation have not been assessed.In the present study, we set out to determine effects of desflurane, hypoxia, and the combination of the two (desflurane/hypoxia) on caspase-3 activation, APP processing, and Aβ generation in H4 human neuroglioma cells (H4 naïve cells) and H4 naïve cells stably transfected to express full-length (FL) APP (H4-APP cells). We also investigated whether the caspase inhibitor, Z-VAD, the γ-secretase inhibitor L-685,458, and the Aβ aggregation inhibitor clioquinol could attenuate desflurane/hypoxia-induced caspase-3 activation and Aβ generation.     

Autophagic vacuoles are enriched in amyloid precursor protein-secretase activities: implications for beta-amyloid peptide over-production and localization in Alzheimer's disease

In Alzheimer’s disease (AD), the neuropathologic hallmarks of β-amyloid deposition and neurofibrillary degeneration are associated with early and progressive pathology of the endosomal–lysosomal system. Abnormalities of autophagy, a major pathway to lysosomes for protein and organelle turnover, include marked accumulations of autophagy-related vesicular compartments (autophagic vacuoles or AVs) in affected neurons. Here, we investigated the possibility that AVs contain the proteases and substrates necessary to cleave the amyloid precursor protein (APP) to Aβ peptide that forms β-amyloid, a key pathogenic factor in AD. AVs were highly purified using a well-established metrizamide gradient procedure from livers of transgenic YAC mice overexpressing wild-type human APP. By Western blot analysis, AVs contained APP, βCTF - the β-cleaved carboxyl-terminal domain of APP, and BACE, the protease-mediating β-cleavage of APP. β-Secretase activity measured against a fluorogenic peptide was significantly enriched in the AV fraction relative to whole-liver lysate. Compared to other recovered subcellular fractions, AVs exhibited the highest specific activity of γ-secretase based on a fluorogenic assay and inhibition by a specific inhibitor of γ-secretase, DAPT. AVs were also the most enriched subcellular fraction in levels of the γ-secretase components presenilin and nicastrin. Immunoelectron microscopy demonstrated selective immunogold labeling of AVs with antibodies specific for the carboxyl termini of human Aβ40 and Aβ42. These data indicate that AVs are a previously unrecognized and potentially highly active compartment for Aβ generation and suggest that the abnormal accumulation of AVs in affected neurons of the AD brain contributes to β-amyloid deposition.     

Loss of Function of ATXN1 Increases Amyloid ��-Protein Levels by Potentiating ��-Secretase Processing of ��-Amyloid Precursor Protein

Alzheimer disease (AD) is a devastating neurodegenerative disease with complex and strong genetic inheritance. Four genes have been established to either cause familial early onset AD (APP, PSEN1, and PSEN2) or to increase susceptibility for late onset AD (APOE). To date ∼80% of the late onset AD genetic variance remains elusive. Recently our genome-wide association screen identified four novel late onset AD candidate genes. Ataxin 1 (ATXN1) is one of these four AD candidate genes and has been indicated to be the disease gene for spinocerebellar ataxia type 1, which is also a neurodegenerative disease. Mounting evidence suggests that the excessive accumulation of Aβ, the proteolytic product of β-amyloid precursor protein (APP), is the primary AD pathological event. In this study, we ask whether ATXN1 may lead to AD pathogenesis by affecting Aβ and APP processing utilizing RNA interference in a human neuronal cell model and mouse primary cortical neurons. We show that knock-down of ATXN1 significantly increases the levels of both Aβ40 and Aβ42. This effect could be rescued with concurrent overexpression of ATXN1. Moreover, overexpression of ATXN1 decreased Aβ levels. Regarding the underlying molecular mechanism, we show that the effect of ATXN1 expression on Aβ levels is modulated via β-secretase cleavage of APP. Taken together, ATXN1 functions as a genetic risk modifier that contributes to AD pathogenesis through a loss-of-function mechanism by regulating β-secretase cleavage of APP and Aβ levels.     

Zinc overload enhances APP cleavage and Aβ deposition in the Alzheimer mouse brain

Background

Abnormal zinc homeostasis is involved in β-amyloid (Aβ) plaque formation and, therefore, the zinc load is a contributing factor in Alzheimer''s disease (AD). However, the involvement of zinc in amyloid precursor protein (APP) processing and Aβ deposition has not been well established in AD animal models in vivo.

Methodology/Principal Findings

In the present study, APP and presenilin 1 (PS1) double transgenic mice were treated with a high dose of zinc (20 mg/ml ZnSO4 in drinking water). This zinc treatment increased APP expression, enhanced amyloidogenic APP cleavage and Aβ deposition, and impaired spatial learning and memory in the transgenic mice. We further examined the effects of zinc overload on APP processing in SHSY-5Y cells overexpressing human APPsw. The zinc enhancement of APP expression and cleavage was further confirmed in vitro.

Conclusions/Significance

The present data indicate that excess zinc exposure could be a risk factor for AD pathological processes, and alteration of zinc homeostasis is a potential strategy for the prevention and treatment of AD.     

Alzheimer Disease A�� Production in the Absence of S-Palmitoylation-dependent Targeting of BACE1 to Lipid Rafts

Alzheimer disease β-amyloid (Aβ) peptides are generated via sequential proteolysis of amyloid precursor protein (APP) by BACE1 and γ-secretase. A subset of BACE1 localizes to cholesterol-rich membrane microdomains, termed lipid rafts. BACE1 processing in raft microdomains of cultured cells and neurons was characterized in previous studies by disrupting the integrity of lipid rafts by cholesterol depletion. These studies found either inhibition or elevation of Aβ production depending on the extent of cholesterol depletion, generating controversy. The intricate interplay between cholesterol levels, APP trafficking, and BACE1 processing is not clearly understood because cholesterol depletion has pleiotropic effects on Golgi morphology, vesicular trafficking, and membrane bulk fluidity. In this study, we used an alternate strategy to explore the function of BACE1 in membrane microdomains without altering the cellular cholesterol level. We demonstrate that BACE1 undergoes S-palmitoylation at four Cys residues at the junction of transmembrane and cytosolic domains, and Ala substitution at these four residues is sufficient to displace BACE1 from lipid rafts. Analysis of wild type and mutant BACE1 expressed in BACE1 null fibroblasts and neuroblastoma cells revealed that S-palmitoylation neither contributes to protein stability nor subcellular localization of BACE1. Surprisingly, non-raft localization of palmitoylation-deficient BACE1 did not have discernible influence on BACE1 processing of APP or secretion of Aβ. These results indicate that post-translational S-palmitoylation of BACE1 is not required for APP processing, and that BACE1 can efficiently cleave APP in both raft and non-raft microdomains.Alzheimer disease-associated β-amyloid (Aβ)³ peptides are derived from the sequential proteolysis of β-amyloid precursor protein (APP) by β- and γ-secretases. The major β-secretase is an aspartyl protease, termed BACE1 (β-site APP-cleaving enzyme 1) (1–4). BACE1 cleaves APP within the extracellular domain of APP, generating the N terminus of Aβ. In addition, BACE1 also cleaves to a lesser extent within the Aβ domain between Tyr¹⁰ and Glu¹¹ (β′-cleavage site). Processing of APP at these sites results in the shedding/secretion of the large ectodomain (sAPPβ) and generating membrane-tethered C-terminal fragments +1 and +11 (β-CTF) (5). The multimeric γ-secretase cleaves at multiple sites within the transmembrane domain of β-CTF, generating C-terminal heterogeneous Aβ peptides (ranging in length between 38 and 43 residues) that are secreted, as well as cytosolic APP intracellular domains (6). In addition to BACE1, APP can be cleaved by α-secretase within the Aβ domain between Lys¹⁶ and Leu¹⁷, releasing sAPPα and generating α-CTF. γ-Secretase cleavage of α-CTF generates N-terminal truncated Aβ, termed p3.Genetic ablation of BACE1 completely abolishes Aβ production, establishing BACE1 as the major neuronal enzyme responsible for initiating amyloidogenic processing of APP (4, 7). Interestingly, both the expression and activity of BACE1 is specifically elevated in neurons adjacent to senile plaques in brains of individuals with Alzheimer disease (8). In the past few years additional substrates of BACE1 have been identified that include APP homologues APLP1 and APLP2 (9), P-selectin glycoprotein ligand-1 (10), β-galactoside α2,6-sialyltransferase (11), low-density lipoprotein receptor-related protein (12), β-subunits of voltage-gated sodium channels (13), and neuregulin-1 (14, 15), thus extending the physiological function of BACE1 beyond Alzheimer disease pathogenesis.BACE1 is a type I transmembrane protein with a long extracellular domain harboring a catalytic domain and a short cytoplasmic tail. BACE1 is synthesized as a proenzyme, which undergoes post-translational modifications that include removal of a pro-domain by a furin-like protease, N-glycosylation, phosphorylation, S-palmitoylation, and acetylation, during the transit in the secretory pathway (16–20). In non-neuronal cells the majority of BACE1 localizes to late Golgi/TGN and endosomes at steady-state and a fraction of BACE1 also cycles between the cell surface and endosomes (21). The steady-state localization of BACE1 is consistent with the acidic pH optimum of BACE1 in vitro, and BACE1 cleavage of APP is observed in the Golgi apparatus, TGN, and endosomes (22–25). BACE1 endocytosis and recycling are mediated by the GGA family of adaptors binding to a dileucine motif (⁴⁹⁶DISLL) in its cytoplasmic tail (21, 26–31). Phosphorylation at Ser⁴⁹⁸ within this motif modulates GGA-dependent retrograde transport of BACE1 from endosomes to TGN (21, 26–31).Over the years, a functional relationship between cellular cholesterol level and Aβ production has been uncovered, raising the intriguing possibility that cholesterol levels may determine the balance between amyloidogenic and non-amyloidogenic processing of APP (32–34). Furthermore, several lines of evidence from in vitro and in vivo studies indicate that cholesterol- and sphingolipid-rich membrane microdomains, termed lipid rafts, might be the critical link between cholesterol levels and amyloidogenic processing of APP. Lipid rafts function in the trafficking of proteins in the secretory and endocytic pathways in epithelial cells and neurons, and participate in a number of important biological functions (35). BACE1 undergoes S-palmitoylation (19), a reversible post-translational modification responsible for targeting a variety of peripheral and integral membrane proteins to lipid rafts (36). Indeed, a significant fraction of BACE1 is localized in lipid raft microdomains in a cholesterol-dependent manner, and addition of glycosylphosphatidylinositol (GPI) anchor to target BACE1 exclusively to lipid rafts increases APP processing at the β-cleavage site (37, 38). Antibody-mediated co-patching of cell surface APP and BACE1 has provided further evidence for BACE1 processing of APP in raft microdomains (33, 39). Components of the γ-secretase complex also associate with detergent-resistant membrane (DRM) fractions enriched in raft markers such as caveolin, flotillin, PrP, and ganglioside GM1 (40). The above findings suggest a model whereby APP is sequentially processed by BACE1 and γ-secretase in lipid rafts.Despite the accumulating evidence, cleavage of APP by BACE1 in non-raft membrane regions cannot be unambiguously ruled out because of the paucity of full-length APP (APP FL) and BACE1 in DRM isolated from adult brain and cultured cells (41). Moreover, it was recently reported that moderate reduction of cholesterol (<25%) displaces BACE1 from raft domains, and increases BACE1 processing by promoting the membrane proximity of BACE1 and APP in non-raft domains (34). Nevertheless, this study also found that BACE1 processing of APP is inhibited with further loss of cholesterol (>35%), consistent with earlier studies (32, 33). Nevertheless, given the pleiotropic effects of cholesterol depletion on membrane properties and vesicular trafficking of secretory and endocytic proteins (42–47), unequivocal conclusions regarding BACE1 processing of APP in lipid rafts cannot be reached based on cholesterol depletion studies.In this study, we explored the function of BACE1 in lipid raft microdomains without manipulating cellular cholesterol levels. In addition to the previously reported S-palmitoylation sites (Cys⁴⁷⁸/Cys⁴⁸²/Cys⁴⁸⁵) within the cytosolic tail of BACE1 (19), we have identified a fourth site (Cys⁴⁷⁴) within the transmembrane domain of BACE1 that undergoes S-palmitoylation. A BACE1 mutant with Ala substitution of all four Cys residues (BACE1-4C/A) fails to associate with DRM in cultured cells, but is not otherwise different from wtBACE1 in terms of protein stability, maturation, or subcellular localization. Surprisingly, APP processing and Aβ generation were unaffected in cells stably expressing the BACE1-4C/A mutant. Finally, we observed an increase in the levels of APP CTFs in detergent-soluble fractions of BACE1-4C/A as compared with wtBACE1 cells. Thus, our data collectively indicate a non-obligatory role of S-palmitoylation and lipid raft localization of BACE1 in amyloidogenic processing of APP.     

Lactic acid induces aberrant amyloid precursor protein processing by promoting its interaction with endoplasmic reticulum chaperone proteins

Background

Lactic acid, a natural by-product of glycolysis, is produced at excess levels in response to impaired mitochondrial function, high-energy demand, and low oxygen availability. The enzyme involved in the production of β-amyloid peptide (Aβ) of Alzheimer''s disease, BACE1, functions optimally at lower pH, which led us to investigate a potential role of lactic acid in the processing of amyloid precursor protein (APP).

Methodology/Principal Findings

Lactic acid increased levels of Aβ40 and 42, as measured by ELISA, in culture medium of human neuroblastoma cells (SH-SY5Y), whereas it decreased APP metabolites, such as sAPPα. In cell lysates, APP levels were increased and APP was found to interact with ER-chaperones in a perinuclear region, as determined by co-immunoprecipitation and fluorescence microscopy studies. Lactic acid had only a very modest effect on cellular pH, did increase the levels of ER chaperones Grp78 and Grp94 and led to APP aggregate formation reminiscent of aggresomes.

Conclusions/Significance

These findings suggest that sustained elevations in lactic acid levels could be a risk factor in amyloidogenesis related to Alzheimer''s disease through enhanced APP interaction with ER chaperone proteins and aberrant APP processing leading to increased generation of amyloid peptides and APP aggregates.     

A Peptide Binding to the β-Site of APP Improves Spatial Memory and Attenuates Aβ Burden in Alzheimer’s Disease Transgenic Mice

Amyloid precursor protein cleaving enzyme 1 (BACE1), an aspartyl protease, initiates processing of the amyloid precursor protein (APP) into β-amyloid (Aβ); the peptide likely contributes to development of Alzheimer’s disease (AD). BACE1 is an attractive therapeutic target for AD treatment, but it exhibits other physiological activities and has many other substrates besides APP. Thus, inhibition of BACE1 function may cause adverse side effects. Here, we present a peptide, S1, isolated from a peptide library that selectively inhibits BACE1 hydrolytic activity by binding to the β-proteolytic site on APP and Aβ N-terminal. The S1 peptide significantly reduced Aβ levels in vitro and in vivo and inhibited Aβ cytotoxicity in SH-SY5Y cells. When applied to APPswe/PS1dE9 double transgenic mice by intracerebroventricular injection, S1 significantly improved the spatial memory as determined by the Morris Water Maze, and also attenuated their Aβ burden. These results indicate that the dual-functional peptide S1 may have therapeutic potential for AD by both reducing Aβ generation and inhibiting Aβ cytotoxicity.     

Detection of Neuritic Plaques in Alzheimer's Disease Mouse Model

Alzheimer''s disease (AD) is the most common neurodegenerative disorder leading to dementia. Neuritic plaque formation is one of the pathological hallmarks of Alzheimer''s disease. The central component of neuritic plaques is a small filamentous protein called amyloid β protein (Aβ)¹, which is derived from sequential proteolytic cleavage of the beta-amyloid precursor protein (APP) by β-secretase and γ-secretase. The amyloid hypothesis entails that Aγ-containing plaques as the underlying toxic mechanism in AD pathology². The postmortem analysis of the presence of neuritic plaque confirms the diagnosis of AD. To further our understanding of Aγ neurobiology in AD pathogenesis, various mouse strains expressing AD-related mutations in the human APP genes were generated. Depending on the severity of the disease, these mice will develop neuritic plaques at different ages. These mice serve as invaluable tools for studying the pathogenesis and drug development that could affect the APP processing pathway and neuritic plaque formation. In this protocol, we employ an immunohistochemical method for specific detection of neuritic plaques in AD model mice. We will specifically discuss the preparation from extracting the half brain, paraformaldehyde fixation, cryosectioning, and two methods to detect neurotic plaques in AD transgenic mice: immunohistochemical detection using the ABC and DAB method and fluorescent detection using thiofalvin S staining method.     

Cholesterol-lowering drugs reduce APP processing to Aβ by inducing APP dimerization

Amyloid beta (Aβ) is a major component of amyloid plaques, which are a key pathological hallmark found in the brains of Alzheimer’s disease (AD) patients. We show that statins are effective at reducing Aβ in human neurons from nondemented control subjects, as well as subjects with familial AD and sporadic AD. Aβ is derived from amyloid precursor protein (APP) through sequential proteolytic cleavage by BACE1 and γ-secretase. While previous studies have shown that cholesterol metabolism regulates APP processing to Aβ, the mechanism is not well understood. We used iPSC-derived neurons and bimolecular fluorescence complementation assays in transfected cells to elucidate how altering cholesterol metabolism influences APP processing. Altering cholesterol metabolism using statins decreased the generation of sAPPβ and increased levels of full-length APP (flAPP), indicative of reduced processing of APP by BACE1. We further show that statins decrease flAPP interaction with BACE1 and enhance APP dimerization. Additionally, statin-induced changes in APP dimerization and APP-BACE1 are dependent on cholesterol binding to APP. Our data indicate that statins reduce Aβ production by decreasing BACE1 interaction with flAPP and suggest that this process may be regulated through competition between APP dimerization and APP cholesterol binding.     

GGA1 overexpression attenuates amyloidogenic processing of the amyloid precursor protein in Niemann-Pick type C cells

Alzheimer’s disease (AD) and a rare inherited disorder of cholesterol transport, Niemann-Pick type C (NPC) share several similarities including aberrant APP processing and increased Aβ production. Previously, we have shown that the AD-like phenotype in NPC model cells involves cholesterol-dependent enhanced APP cleavage by β-secretase and accumulation of both APP and BACE1 within endocytic compartments. Since retrograde transport of BACE1 from endocytic compartments to the trans-Golgi network (TGN) is regulated by the Golgi-localized γ-ear containing ADP ribosylation factor-binding protein 1 (GGA1), we analyzed in this work a potential role of GGA1 in the AD-like phenotype of NPC1-null cells. Overexpression of GGA1 caused a shift in APP processing towards the non-amyloidogenic pathway by increasing the localization of APP at the cell surface. However, the observed effect appear to be independent on the subcellular localization and phosphorylation state of BACE1. These findings show that the AD-like phenotype of NPC model cells can be partly reverted by promoting a non-amyloidogenic processing of APP through the upregulation of GGA1 supporting its preventive role against AD.     

FLZ Alleviates the Memory Deficits in Transgenic Mouse Model of Alzheimer’s Disease via Decreasing Beta-Amyloid Production and Tau Hyperphosphorylation

Alzheimer’s disease (AD) is the most common cause of dementia worldwide and mainly characterized by the aggregated β-amyloid (Aβ) and hyperphosphorylated tau. FLZ is a novel synthetic derivative of natural squamosamide and has been proved to improve memory deficits in dementia animal models. In this study, we aimed to investigate the mechanisms of FLZ’s neuroprotective effect in APP/PS1 double transgenic mice and SH-SY5Y (APPwt/swe) cells. The results showed that treatment with FLZ significantly improved the memory deficits of APP/PS1 transgenic mice and decreased apoptosis of SH-SY5Y (APPwt/swe) cells. FLZ markedly attenuated Aβ accumulation and tau phosphorylation both in vivo and in vitro. Mechanistic study showed that FLZ interfered APP processing, i.e., FLZ decreased β-amyloid precursor protein (APP) phosphorylation, APP-carboxy-terminal fragment (APP-CTF) production and β-amyloid precursor protein cleaving enzyme 1 (BACE1) expression. These results indicated that FLZ reduced Aβ production through inhibiting amyloidogenic pathway. The mechanistic study about FLZ’s inhibitory effect on tau phosphorylation revealed t the involvement of Akt/glycogen synthase kinase 3β (GSK3β) pathway. FLZ treatment increased Akt activity and inhibited GSK3β activity both in vivo and in vitro. The inhibitory effect of FLZ on GSK3β activity and tau phosphorylation was suppressed by inhibiting Akt activity, indicating that Akt/GSK3β pathway might be the possible mechanism involved in the inhibitory effect of FLZ on tau hyperphosphorylation. These results suggested FLZ might be a potential anti-AD drug as it not only reduced Aβ production via inhibition amyloidogenic APP processing pathway, but also attenuated tau hyperphosphoylation mediated by Akt/GSK3β.     

Anesthetic propofol attenuates the isoflurane-induced caspase-3 activation and Aβ oligomerization

Accumulation and deposition of β-amyloid protein (Aβ) are the hallmark features of Alzheimer''s disease. The inhalation anesthetic isoflurane has been shown to induce caspase activation and increase Aβ accumulation. In addition, recent studies suggest that isoflurane may directly promote the formation of cytotoxic soluble Aβ oligomers, which are thought to be the key pathological species in AD. In contrast, propofol, the most commonly used intravenous anesthetic, has been reported to have neuroprotective effects. We therefore set out to compare the effects of isoflurane and propofol alone and in combination on caspase-3 activation and Aβ oligomerization in vitro and in vivo. Naïve and stably-transfected H4 human neuroglioma cells that express human amyloid precursor protein, the precursor for Aβ; neonatal mice; and conditioned cell culture media containing secreted human Aβ40 or Aβ42 were treated with isoflurane and/or propofol. Here we show for the first time that propofol can attenuate isoflurane-induced caspase-3 activation in cultured cells and in the brain tissues of neonatal mice. Furthermore, propofol-mediated caspase inhibition occurred when there were elevated levels of Aβ. Finally, isoflurane alone induces Aβ42, but not Aβ40, oligomerization, and propofol can inhibit the isoflurane-mediated oligomerization of Aβ42. These data suggest that propofol may mitigate the caspase-3 activation by attenuating the isoflurane-induced Aβ42 oligomerization. Our findings provide novel insights into the possible mechanisms of isoflurane-induced neurotoxicity that may aid in the development of strategies to minimize potential adverse effects associated with the administration of anesthetics to patients.     

Paradoxical Condensation of Copper with Elevated ��-Amyloid in Lipid Rafts under Cellular Copper Deficiency Conditions: IMPLICATIONS FOR ALZHEIMER DISEASE*

Redox-active copper is implicated in the pathogenesis of Alzheimer disease (AD), β-amyloid peptide (Aβ) aggregation, and amyloid formation. Aβ·copper complexes have been identified in AD and catalytically oxidize cholesterol and lipid to generate H₂O₂ and lipid peroxides. The site and mechanism of this abnormality is not known. Growing evidence suggests that amyloidogenic processing of the β-amyloid precursor protein (APP) occurs in lipid rafts, membrane microdomains enriched in cholesterol. β- and γ-secretases, and Aβ have been identified in lipid rafts in cultured cells, human and rodent brains, but the role of copper in lipid raft amyloidogenic processing is presently unknown. In this study, we found that copper modulates flotillin-2 association with cholesterol-rich lipid raft domains, and consequently Aβ synthesis is attenuated via copper-mediated inhibition of APP endocytosis. We also found that total cellular copper is associated inversely with lipid raft copper levels, so that under intracellular copper deficiency conditions, Aβ·copper complexes are more likely to form. This explains the paradoxical hypermetallation of Aβ with copper under tissue copper deficiency conditions in AD.Imbalance of metal ions has been recognized as one of the key factors in the pathogenesis of Alzheimer disease (AD).² Aberrant interactions between copper or zinc with the β-amyloid peptide (Aβ) released into the glutamatergic synaptic cleft vicinity could result in the formation of toxic Aβ oligomers and aggregation into plaques characteristic of AD brains (reviewed in Ref. 1). Copper, iron, and zinc are highly concentrated in extracellular plaques (2, 3), and yet brain tissues from AD (4–6) and human β-amyloid precursor protein (APP) transgenic mice (7–10) are paradoxically copper deficient compared with age-matched controls. Elevation of intracellular copper levels by genetic, dietary, and pharmacological manipulations in both AD transgenic animal and cell culture models is able to attenuate Aβ production (7, 9, 11–15). However, the underlying mechanism is at present unclear.Abnormal cholesterol metabolism is also a contributing factor in the pathogenesis of AD. Hypercholesterolemia increases the risk of developing AD-like pathology in a transgenic mouse model (16). Epidemiological and animal model studies show that a hypercholesterolemic diet is associated with Aβ accumulation and accelerated cognitive decline, both of which are further aggravated by high dietary copper (17, 18). In contrast, biochemical depletion of cholesterol using statins, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, and methyl-β-cyclodextrin, a cholesterol sequestering agent, inhibit Aβ production in animal and cell culture models (19–25).Cholesterol is enriched in lipid rafts, membrane microdomains implicated in Aβ generation from APP cleavage by β- and γ-secretases. Recruitment of BACE1 (β-secretase) into lipid rafts increases the production of sAPP_β and Aβ (23, 26). The β-secretase-cleaved APP C-terminal fragment (β-CTF), and γ-secretase, a multiprotein complex composed of presenilin (PS1 or PS2), nicastrin (Nct), PEN-2 and APH-1, colocalize to lipid rafts (27). The accumulation of Aβ in lipid rafts isolated from AD and APP transgenic mice brains (28) provided further evidence that cholesterol plays a role in APP processing and Aβ generation.Currently, copper and cholesterol have been reported to modulate APP processing independently. However, evidence indicates that, despite tissue copper deficiency, Aβ·Cu²⁺ complexes form in AD that catalytically oxidize cholesterol and lipid to generate H₂O₂ and lipid peroxides (e.g. hydroxynonenal and malondialdehyde), which contribute to oxidative damage observed in AD (29–35). The underlying mechanism leading to the formation of pathological Aβ·Cu²⁺ complexes is unknown. In this study, we show that copper alters the structure of lipid rafts, and attenuates Aβ synthesis in lipid rafts by inhibition of APP endocytosis. We also identify a paradoxical inverse relationship between total cellular copper levels and copper distribution to lipid rafts, which appear to possess a privileged pool of copper where Aβ is more likely to interact with Cu²⁺ under copper-deficiency conditions to form Aβ·Cu²⁺ complexes. These data provide a novel mechanism by which cellular copper deficiency in AD could foster an environment for potentially adverse interactions between Aβ, copper, and cholesterol in lipid rafts.     

Intracellular Trafficking of Presenilin 1 Is Regulated by ��-Amyloid Precursor Protein and Phospholipase D1

Excessive accumulation of β-amyloid peptides in the brain is a major cause for the pathogenesis of Alzheimer disease. β-Amyloid is derived from β-amyloid precursor protein (APP) through sequential cleavages by β- and γ-secretases, whose enzymatic activities are tightly controlled by subcellular localization. Delineation of how intracellular trafficking of these secretases and APP is regulated is important for understanding Alzheimer disease pathogenesis. Although APP trafficking is regulated by multiple factors including presenilin 1 (PS1), a major component of the γ-secretase complex, and phospholipase D1 (PLD1), a phospholipid-modifying enzyme, regulation of intracellular trafficking of PS1/γ-secretase and β-secretase is less clear. Here we demonstrate that APP can reciprocally regulate PS1 trafficking; APP deficiency results in faster transport of PS1 from the trans-Golgi network to the cell surface and increased steady state levels of PS1 at the cell surface, which can be reversed by restoring APP levels. Restoration of APP in APP-deficient cells also reduces steady state levels of other γ-secretase components (nicastrin, APH-1, and PEN-2) and the cleavage of Notch by PS1/γ-secretase that is more highly correlated with cell surface levels of PS1 than with APP overexpression levels, supporting the notion that Notch is mainly cleaved at the cell surface. In contrast, intracellular trafficking of β-secretase (BACE1) is not regulated by APP. Moreover, we find that PLD1 also regulates PS1 trafficking and that PLD1 overexpression promotes cell surface accumulation of PS1 in an APP-independent manner. Our results clearly elucidate a physiological function of APP in regulating protein trafficking and suggest that intracellular trafficking of PS1/γ-secretase is regulated by multiple factors, including APP and PLD1.An important pathological hallmark of Alzheimer disease (AD)⁴ is the formation of senile plaques in the brains of patients. The major components of those plaques are β-amyloid peptides (Aβ), whose accumulation triggers a cascade of neurodegenerative steps ending in formation of senile plaques and intraneuronal fibrillary tangles with subsequent neuronal loss in susceptible brain regions (1, 2). Aβ is proteolytically derived from the β-amyloid precursor protein (APP) through sequential cleavages by β-secretase (BACE1), a novel membrane-bound aspartyl protease (3, 4), and by γ-secretase, a high molecular weight complex consisting of at least four components: presenilin (PS), nicastrin (NCT), anterior pharynx-defective-1 (APH-1), and presenilin enhancer-2 (PEN-2) (5, 6). APP is a type I transmembrane protein belonging to a protein family that includes APP-like protein 1 (APLP1) and 2 (APLP2) in mammals (7, 8). Full-length APP is synthesized in the endoplasmic reticulum (ER) and transported through the Golgi apparatus. Most secreted Aβ peptides are generated within the trans-Golgi network (TGN), also the major site of steady state APP in neurons (9–11). APP can be transported to the cell surface in TGN-derived secretory vesicles if not proteolyzed to Aβ or an intermediate metabolite. At the cell surface APP is either cleaved by α-secretase to produce soluble sAPPα (12) or reinternalized for endosomal/lysosomal degradation (13, 14). Aβ may also be generated in endosomal/lysosomal compartments (15, 16). In contrast to neurotoxic Aβ peptides, sAPPα possesses neuroprotective potential (17, 18). Thus, the subcellular distribution of APP and proteases that process it directly affect the ratio of sAPPα to Aβ, making delineation of the mechanisms responsible for regulating trafficking of all of these proteins relevant to AD pathogenesis.Presenilin (PS) is a critical component of the γ-secretase. Of the two mammalian PS gene homologues, PS1 and PS2, PS1 encodes the major form (PS1) in active γ-secretase (19, 20). Nascent PSs undergo endoproteolytic cleavage to generate an amino-terminal fragment (NTF) and a carboxyl-terminal fragment (CTF) to form a functional PS heterodimer (21). Based on observations that PSs possess two highly conserved aspartate residues indispensable for γ-secretase activity and that specific transition state analogue γ-secretase inhibitors bind to PS1 NTF/CTF heterodimers (5, 22), PSs are believed to be the catalytic component of the γ-secretase complex. PS assembles with three other components, NCT, APH-1, and PEN-2, to form the functional γ-secretase (5, 6). Strong evidence suggests that PS1/γ-secretase resides principally in the ER, early Golgi, TGN, endocytic and intermediate compartments, most of which (except the TGN) are not major subcellular sites for APP (23, 24). In addition to generating Aβ and cleaving APP to release the APP intracellular domain, PS1/γ-secretase cleaves other substrates such as Notch (25), cadherin (26), ErbB4 (27), and CD44 (28), releasing their respective intracellular domains. Interestingly, PS1/γ-secretase cleavage of different substrates seems to occur at different subcellular compartments; APP is mainly cleaved at the TGN and early endosome domains, whereas Notch is predominantly cleaved at the cell surface (9, 11, 29). Thus, perturbing intracellular trafficking of PS1/γ-secretase may alter interactions between PS1/γ-secretase and APP, contributing to either abnormal Aβ generation and AD pathogenesis or decreased access of PS1/γ-secretase to APP such that Aβ production is reduced. However, mechanisms regulating PS1/γ-secretase trafficking warrant further investigation.In addition to participating in γ-secretase activity, PS1 regulates intracellular trafficking of several membrane proteins, including other γ-secretase components (nicastrin, APH-1, and PEN-2) and the substrate APP (reviewed in Ref. 30). Intracellular APP trafficking is highly regulated and requires other factors such as mint family members and SorLA (2). Moreover, we recently found that phospholipase D1 (PLD1), a phospholipid-modifying enzyme that regulates membrane trafficking events, can interact with PS1, and can regulate budding of APP-containing vesicles from the TGN and delivery of APP to the cell surface (31, 32). Interestingly, Kamal et al. (33) identified an axonal membrane compartment that contains APP, BACE1, and PS1 and showed that fast anterograde axonal transport of this compartment is mediated by APP and kinesin-I, implying a traffic-regulating role for APP. Increased APP expression is also shown to decrease retrograde axonal transport of nerve growth factor (34). However, whether APP indeed regulates intracellular trafficking of proteins including BACE1 and PS1/γ-secretase requires further validation. In the present study we demonstrate that intracellular trafficking of PS1, as well as that of other γ-secretase components, but not BACE1, is regulated by APP. APP deficiency promotes cell surface delivery of PS1/γ-secretase complex and facilitates PS1/γ-secretase-mediated Notch cleavage. In addition, we find that PLD1 also regulates intracellular trafficking of PS1 through a different mechanism and more potently than APP.     

Neuronal protein trafficking associated with Alzheimer disease: From APP and BACE1 to glutamate receptors

Aberrant and/or cumulative amyloid-beta (Aβ) production, resulting from proteolytic processing of the amyloid precursor protein (APP) by β and γ-secretases, have been postulated to be a main etiological basis of Alzheimer disease (AD). A number of proteins influence the subcellular trafficking itinerary of APP and the β-site APP-cleaving enzyme (BACE1) between the cell surface, endosomes and the trans-Golgi network (TGN). Available evidence suggests that co-residence of APP and BACE1 in the endosomal compartments promotes amyloidogenesis. Retrograde transport of APP out of the endosome to the TGN reduces Aβ production, while APP routed to and kept at the cell surface enhances its non-amyloidogenic, α-secretase-mediated processing. Changes in post-Golgi membrane trafficking in aging neurons that may influence APP processing is particularly relevant to late-onset, idiopathic AD. Dystrophic axons are key features of AD pathology, and impaired axonal transport could play crucial roles in the pathogenesis of idiopathic AD. Recent evidence has also indicated that Aβ-induced synaptic defects and memory impairment could be explained by a loss of both AMPA and NMDA receptors through endocytosis. Detail understanding of factors that influence these neuronal trafficking processes will open up novel therapeutic avenues for preventing or delaying the onset of symptomatic AD.Key words: amyloid precursor protein (APP), β-site APP cleaving enzyme 1 (BACE1), endosome, glutamate receptors, trans-Golgi network (TGN)