Amyloid-β Precursor Protein Processing Research Articles

Hypoxia/ischemia activate processing of Amyloid Precursor Protein: impact of vascular dysfunction in the pathogenesis of Alzheimer's disease.

Alzheimer's disease (AD) is associated with deficiencies in cerebrovascular functions, e.g. reduced cerebral blood flow and capillary amyloid angiopathy, both of which are evident during the early phase of AD, thus local hypoxia/ischemia could augment the pathogenesis of AD. There is abundant literature revealing that exposures to hypoxia/ischemia increase the amyloidogenic processing of amyloid-β precursor protein (APP) leading to the accumulation of amyloid-β peptides in brain. This hypoxia-induced response has been attributed to a significant increase in the activities of β- and γ-secretases, whereas α-secretase activity decreases in hypoxia. Recent studies have indicated that hypoxia-inducible factor-1α (HIF-1α) stimulates the transcription of the β-secretase 1 (BACE1) gene through the hypoxia-response element in the BACE1 promoter. Moreover, HIF-1α protein can directly interact with the γ-secretase complex and increase its activity in a non-transcriptional manner. Hypoxia/ischemia also trigger endoplasmic reticulum stress and impair autophagy in brain, which consequently can stimulate the expression of presenilin 1 (PS1) and activate γ-secretase. Subsequently, PS1 protein can stabilize HIF-1α protein and in addition, APP intracellular domain peptide is able to induce the expression of HIF-1α. The activation of β- and γ-secretases is an evolutionarily conserved hypoxia response, e.g. it is also present in zebrafish. Given that β- and γ-secretases have many substrates in addition to APP, one could postulate that AD pathology is a byproduct of the rescue process mediated by these two aspartyl proteinases under hypoxic/ischemic conditions. We will review the recent evidence indicating that vascular dysfunctions can provoke AD pathology by activating β- and γ-secretases.

Anti-Viral Properties of Amyloid-β Peptides.

Amyloid-β (Aβ) peptides generated by the amyloidogenic pathway of amyloid-β protein precursor processing contribute significantly to neurodegeneration characteristic of Alzheimer's disease (AD). The involvement of Aβ peptides in the etiology of AD remains a subject of debate. Data published in the last 6 years by three different groups have added a new twist by revealing that Aβ peptides could act as antimicrobial peptides (AMP) in in vitro assays against some common and clinically relevant microorganisms, inhibit replication of seasonal and pandemic strains of influenza A and HSV-1 virus. These observations are of significance with respect to the notion that pathogens may be important contributors to the development of AD, particularly in the case of herpes simplex virus (HSV) infection, which often resides in the same cerebral sites where AD arises. Here, we review the data that support the interpretation that Aβ peptides behave as AMP, with an emphasis on studies concerning HSV-1 and a putative molecular mechanism that suggests that interactions between Aβ peptides and the HSV-1 fusogenic protein gB lead to impairment of HSV-1 infectivity by preventing the virus from fusing with the plasma membrane. A number of avenues for future research are suggested.

Preservation of cell-survival mechanisms by the presenilin-1 K239N mutation may cause its milder clinical phenotype

Presenilin 1 (PSEN1) mutations are the main cause of monogenic Alzheimer's disease. We studied the functional effects of the mutation K239N, which shows incomplete penetrance at the age of 65 years and compared it with the more aggressive mutation E120G. We engineered stable cell lines expressing human PSEN1 wild type or with K239N or E120G mutations. Both mutations induced dysfunction of γ-secretase in the processing of amyloid-β protein precursor, leading to an increase in the amyloid β42/amyloid β40 ratio. Analysis of homeostatic mechanisms showed that K239N induced lower basal and hydrogen peroxide induced intracellular levels of reactive oxygen species than E120G. Similarly, K239N induced lower vulnerability to apoptosis by hydrogen peroxide injury than E120G. Accordingly, the proapoptotic signaling pathways c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase maintained PSEN1-mediated negative regulation in K239N but not in E120G-bearing cells. Furthermore, the activation of the prosurvival signaling pathways mitogen-activated protein kinase/extracellular signal-regulated kinase and phosphoinositide 3-kinase/Akt was lower in E120G-bearing cells. Therefore, preservation of mechanisms regulating cell responses independent of amyloid-β protein precursor processing may account for the milder phenotype induced by the PSEN1 K239N mutation.

Reelin in Alzheimer's Disease, Increased Levels but Impaired Signaling: When More is Less.

In the continuing search for proteins that play a role in Alzheimer's disease (AD) and that are related to the pathological hallmarks, those that influence cognitive function and that constitute potential therapeutic targets deserve special interest. Reelin is a signaling protein that is involved in a cascade of cytoplasmic events that control tau phosphorylation and that regulate synaptic neurotransmission, plasticity, and memory. Both Reelin expression and glycosylation are modulated by amyloid-β (Aβ), suggesting that the activity of Reelin could be affected in AD and hence, its possible influence on this pathology should be taken into consideration. The levels of Reelin in the brain of AD patients appear to be altered and interestingly, disrupted Reelin signaling is associated with increased tau phosphorylation as well as with amyloid-β protein precursor processing. We discuss here the somewhat contradictory data regarding Reelin levels in AD and we evaluate the processing of the Reelin receptor, ApoER2, and other downstream events, such as the phosphorylation of the intracellular adapter Dab1. Together with brain Reelin levels, these changes may represent a relevant read-out of Reelin signaling in the human brain.

Chronic Sleep Deprivation Exacerbates Learning-Memory Disability and Alzheimer's Disease-Like Pathologies in AβPP(swe)/PS1(ΔE9) Mice.

Recently, there is an increasing concern over the association between sleep disorders and Alzheimer's disease (AD). Clinical observations have reported that chronic sleep deprivation (SD) may serve as a risk factor for AD. However, the pathological evidence for this assumption is still lacking. In the present study, we examined the potential impacts of chronic SD on learning-memory and AD-related pathologies in AβPP(swe)/PS1(ΔE9) transgenic (TG) mice and their wild-type (WT) littermates. Results indicated that mice (both TG and WT) exposed to 2-month SD showed an altered amyloid-β protein precursor processing, an elevated level of phosphorylated tau protein, and impaired cognitive performance as compared to non-sleep deprivation (NSD) controls. Moreover, the SD-treated TG mice exhibited more amyloid-β(1-42) production and developed more senile plaques in the cortex and hippocampus than NSD-treated TG mice. In addition, SD caused a striking neuronal mitochondrial damage, caspase cascade activation, and neuronal apoptosis in the hippocampus of both TG and WT mice. More importantly, all these behavioral, neuropathological, and biochemical changes induced by chronic SD were long lasting and were irreversible during a 3-month normal housing condition. Collectively, these results indicate that chronic SD impairs learning and memory, exacerbates AD pathologies, and aggravates the mitochondria-mediated neuronal apoptosis in a long-lasting manner. Our findings provide important experimental evidence to prove that chronic SD is a risk factor for AD.

AZD3293: A Novel, Orally Active BACE1 Inhibitor with High Potency and Permeability and Markedly Slow Off-Rate Kinetics.

A growing body of pathological, biomarker, genetic, and mechanistic data suggests that amyloid accumulation, as a result of changes in production, processing, and/or clearance of brain amyloid-β peptide (Aβ) concentrations, plays a key role in the pathogenesis of Alzheimer’s disease (AD). Beta-secretase 1 (BACE1) mediates the first step in the processing of amyloid-β protein precursor (AβPP) to Aβ peptides, with the soluble N terminal fragment of AβPP (sAβPPβ) as a direct product, and BACE1 inhibition is an attractive target for therapeutic intervention to reduce the production of Aβ. Here, we report the in vitro and in vivo pharmacological profile of AZD3293, a potent, highly permeable, orally active, blood-brain barrier (BBB) penetrating, BACE1 inhibitor with unique slow off-rate kinetics. The in vitro potency of AZD3293 was demonstrated in several cellular models, including primary cortical neurons. In vivo in mice, guinea pigs, and dogs, AZD3293 displayed significant dose- and time-dependent reductions in plasma, cerebrospinal fluid, and brain concentrations of Aβ40, Aβ42, and sAβPPβ. The in vitro potency of AZD3293 in mouse and guinea pig primary cortical neuronal cells was correlated to the in vivo potency expressed as free AZD3293 concentrations in mouse and guinea pig brains. In mice and dogs, the slow off-rate from BACE1 may have translated into a prolongation of the observed effect beyond the turnover rate of Aβ. The preclinical data strongly support the clinical development of AZD3293, and patients with AD are currently being recruited into a combined Phase 2/3 study to test the disease-modifying properties of AZD3293.

Cholesterol Potentiates <i>β</i>-Amyloid Genesis in Cultured Human Umbilical Vein Endothelial Cells

Cerebral Amyloid Angiopathy (CAA) occurs commonly among the elderly and almost invariably in patients with Alzheimer’s Disease (AD). The β-amyloid peptides (Aβ) are produced via the amy-loidogenic processing of β-Amyloid Precursor Protein (APP) by β-secretase-1 (BACE1) and γ- secretase. Vascular endothelial cells are lately shown to possess the molecular machinery of Aβ production, which might participate in the development of CAA. Hypercholesterolemia is considered a risk factor for AD, whereas less is known if cholesterol may modulate endothelial Aβ production. In the present study we verified the amyloidogenic capability of Human Umbilical Vein Endothelial Cells (HUVECs) in vitro and explored the effect of cholesterol exposure on their amy-loidogenic potential. Cholesterol treatments at 12.5 and 25 mg/dL significantly elevated APP, BACE1 and APP β-CTF protein levels and β-site APP cleavage activity in cell lysates, and Aβ40 levels in culture medium. However, coincubation with cholesterol at 50 and 100 mg/dL attenuated the viability of the cultured cells and diminished their amyloidogenic capability. These findings suggest that high cholesterol exposure is stressful to vascular endothelial cells, and at a certain dosage range can promote an amyloidogenic response in these cells.

Physical and functional interaction between the α- and γ-secretases: A new model of regulated intramembrane proteolysis.

Many single-transmembrane proteins are sequentially cleaved by ectodomain-shedding α-secretases and the γ-secretase complex, a process called regulated intramembrane proteolysis (RIP). These cleavages are thought to be spatially and temporally separate. In contrast, we provide evidence for a hitherto unrecognized multiprotease complex containing both α- and γ-secretase. ADAM10 (A10), the principal neuronal α-secretase, interacted and cofractionated with γ-secretase endogenously in cells and mouse brain. A10 immunoprecipitation yielded γ-secretase proteolytic activity and vice versa. In agreement, superresolution microscopy showed that portions of A10 and γ-secretase colocalize. Moreover, multiple γ-secretase inhibitors significantly increased α-secretase processing (r = -0.86) and decreased β-secretase processing of β-amyloid precursor protein. Select members of the tetraspanin web were important both in the association between A10 and γ-secretase and the γ → α feedback mechanism. Portions of endogenous BACE1 coimmunoprecipitated with γ-secretase but not A10, suggesting that β- and α-secretases can form distinct complexes with γ-secretase. Thus, cells possess large multiprotease complexes capable of sequentially and efficiently processing transmembrane substrates through a spatially coordinated RIP mechanism.

Tripchlorolide Attenuates β-amyloid Generation via Suppressing PPARγ-Regulated BACE1 Activity in N2a/APP695 Cells

Due to its apparent rate-limiting function, BACE1 (β-secretase) appears to be a prime target for prevention of amyloid-β (Aβ) generation in brains with Alzheimer's disease (AD). The activity of BACE1 is regulated by peroxisome proliferator-activated receptor-γ (PPARγ), a transcription factor binding site of the BACE1 promoter, indicating that PPARγ may be a potential target for AD treatment. Several studies have demonstrated that PPARγ activation is involved in the immunostimulation of amyloid-β precursor protein processing by nonsteroidal anti-inflammatory drugs (NSAIDs). The present study found that tripchlorolide (T4), with a similar chemical structure to that of NSAIDs, decreased the levels of Aβ secreted in N2a-APP695 cells. T4 treatment reduced the mRNA and protein levels of BACE1 and the protein level of sAPPβ, a cleaved N-terminal fragment of APP by BACE1. The treatment also translocated PPARγ from cytoplasm to nuclear. Intriguingly, T4, like pioglitazone (a PPARγ agonist), suppressed the BACE1 activity in N2a-APP695 cells, which was attenuated by GW9662 (a PPARγ antagonist). These results indicate that T4 may be a PPARγ agonist to enhance the binding of nuclear PPARγ to the BACE1 promoter, which may in turn inhibit the transcription and translation of BACE1, suppress the activity of BACE1, and ultimately attenuate the generation of Aβ. Due to its capability to alter Aβ generation and to protect central neural system against the neurotoxicity of Aβ, T4 may serve as a promising agent in modulating Aβ-related pathology in Alzheimer's disease.

Interaction of ApoE3 and ApoE4 isoforms with an ITM2b/BRI2 mutation linked to the Alzheimer disease-like Danish dementia: Effects on learning and memory

Mutations in Amyloid β Precursor Protein (APP) and in genes that regulate APP processing – such as PSEN1/2 and ITM2b/BRI2 – cause familial dementia, such Familial Alzheimer disease (FAD), Familial Danish (FDD) and British (FBD) dementias. The ApoE gene is the major genetic risk factor for sporadic AD. Three major variants of ApoE exist in humans (ApoE2, ApoE3, and ApoE4), with the ApoE4 allele being strongly associated with AD. ITM2b/BRI2 is also a candidate regulatory node genes predicted to mediate the common patterns of gene expression shared by healthy ApoE4 carriers and late-onset AD patients not carrying ApoE4. This evidence provides a direct link between ITM2b/BRI2 and ApoE4. To test whether ApoE4 and pathogenic ITM2b/BRI2 interact to modulate learning and memory, we crossed a mouse carrying the ITM2b/BRI2 mutations that causes FDD knocked-in the endogenous mouse Itm2b/Bri2 gene (FDDKI mice) with human ApoE3 and ApoE4 targeted replacement mice. The resultant ApoE3, FDDKI/ApoE3, ApoE4, FDDKI/ApoE4 male mice were assessed longitudinally for learning and memory at 4, 6, 12, and 16–17months of age. The results showed that ApoE4-carrying mice displayed spatial working/short-term memory deficits relative to ApoE3-carrying mice starting in early middle age, while long-term spatial memory of ApoE4 mice was not adversely affected even at 16–17months, and that the FDD mutation impaired working/short-term spatial memory in ApoE3-carrying mice and produced impaired long-term spatial memory in ApoE4-carrying mice in middle age. The present results suggest that the FDD mutation may differentially affect learning and memory in ApoE4 carriers and non-carriers.

The Association of Amyloid-β Protein Precursor With α- and β-Secretases in Mouse Cerebral Cortex Synapses Is Altered in Early Alzheimer’s Disease

Amyloid-β peptides (Aβ), the proposed triggers of synaptic dysfunction in early Alzheimer's disease (AD), derive from the endoproteolytic cleavage of amyloid-β precursor protein (APP) by β-secretases (BACE1), whereas APP cleavage by α-secretases (ADAM10) abrogates Aβ formation. We now mapped the synaptic localization of APP, ADAM10, and BACE1 in the mouse cerebral cortex. All three proteins were present in cortical synapses and subsynaptic fractionation revealed that APP was located mainly in the pre-synaptic active zone (53%) and in the post-synaptic density (37%), whereas ADAM10 was enriched in the post-synaptic density (61%) and BACE1 was concentrated in extra-synaptic regions (72%). Immunocytochemistry analysis further showed that APP and BACE1 were co-localized in about 30% of both glutamatergic and GABAergic terminals, whereas few terminals were endowed with ADAM10. This distribution is modified in a mouse model of early AD based on Aβ1-42-intracerebroventricular injection, where the synaptic levels of APP and ADAM10 increased by 30%, whereas BACE1 levels were reduced. This suggests that, in early AD, there are compensatory mechanisms to avoid Aβ overload in cortical synapses favoring the non-amyloidogenic processing of APP.

Threonine 576 residue of amyloid-β precursor protein regulates its trafficking and processing

Deposition of amyloid-β (Aβ) in the brain is the main culprit of Alzheimer's disease (AD). Aβ is derived from sequential proteolytic cleavage of amyloid-β precursor protein (APP). Newly synthesized APP is transported from endoplasmic reticulum to the plasma membrane via trans-Golgi network (TGN) after post-translational modification including N- and O-glycosylation. APP is internalized through clathrin-dependent endocytosis from the plasma membrane to the early endosomes. In this study, we investigated the regulation of APP trafficking and processing by mutating three threonine residues known as O-glycosylation sites. We separately mutated three threonine residues of APP695 into alanines (T291A, T292A, and T576A) and expressed them in HeLa cells. Among these APP mutants, only T576A mutant showed reduced cell surface levels, indicating this residue regulates its trafficking. We also confirmed that trafficking from TGN to the plasma membrane was decreased in T576A mutant. Consistent with these observations, T576A mutant accumulated in the early endosomes, and the secreted Aβ level was increased. Thus, these results indicate that threonine 576 residue of APP regulates its trafficking and processing.

Double-Edged Roles of Nitric Oxide Signaling on APP Processing and Amyloid-β Production In Vitro: Preliminary Evidence from Sodium Nitroprusside.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is thought to be caused in part by the age-related accumulation of amyloid-β (Aβ) in the brain. Recent findings have revealed that nitric oxide (NO) modulates the processing of amyloid-β precursor protein (APP) and alters Aβ production; however, the previously presented data are contradictory and the underlying molecular mechanisms are still incomplete. Here, using human SH-SY5Y neuroblastoma cells stably transfected with wild-type APPwt695, we found that NO, derived from NO donor sodium nitroprusside (SNP), bi-directionally modulates APP processing in vitro. The data from ELISA and Western blot (WB) tests indicated that SNP at lower concentrations (0.01 and 0.1μM) inhibits BACE1 expression, thus consequently suppresses APP β-cleavage and decreases Aβ production. In contrast, SNP at higher concentrations (10 and 20μM) biases the APP processing toward the amyloidogenic pathway as evidenced by an increased BACE1 but a decreased ADAM10 expression, together with an elevated Aβ secretion. This bi-directional modulating activity of SNP on APP processing was completely blocked by specific NO scavenger c-PTIO, indicating NO-dependent mechanisms. Moreover, the anti-amyloidogenic activity of SNP is sGC/cGMP/PKG-dependent as evidenced by its reversal by sGC/PKG inhibitions, whereas the amyloidogenic activity of SNP is peroxynitrite-related and can be reversed by peroxynitrite scavenger uric acid. In summary, these present findings predict a double-edged role of NO in APP processing in vitro. Low (physiological) levels of NO inhibit the amyloidogenic processing of APP, whereas extra-high (pathological) concentrations of NO favor the amyloidogenic pathway of APP processing. This preliminary study may provide further evidence to clarify the molecular roles of NO and NO-related signaling in AD and supply potential molecular targets for AD treatment.

Understanding the cholesterol metabolism-perturbing effects of docosahexaenoic acid by gas chromatography-mass spectrometry targeted metabonomic profiling.

Over the past few decades, docosahexaenoic acid (DHA) has gained special attention for management of cholesterol-associated metabolic disorders and neurodegenerative diseases such as Alzheimer's disease (AD) owing to its neuroprotective, anti-inflammatory and hypolipidemic properties. Several epidemiological studies have reported the effect of DHA in reducing the risk of developing AD by lowering cholesterol. Hypercholesterolemia is a pro-amyloidogenic factor influencing the enzymatic processing of amyloid-β precursor protein (AβPP) to toxic β-amyloid. However, the mechanism by which DHA modulates the cholesterol pathway has not been established. Thus, the objective of this study was to investigate the mechanism of regulation of cholesterol metabolism by DHA in an AβPP695 overexpressing AD cell model. A gas chromatography/mass spectrometry method was developed and validated for the targeted profiling of 11 cholesterol metabolites in DHA-treated Chinese hamster ovary wild-type (CHO-wt) and AβPP695 overexpressing (CHO-AβPP695) cells. The differential metabolite profiles between DHA- and vehicle-treated groups were further analyzed using fold change values of the ratio of concentration of metabolites in CHO-AβPP695 to CHO-wt cells. Effect of DHA on key rate-limiting enzymatic activities within the cholesterol pathway was established using biochemical assays. Our results showed that DHA reduced the levels of key cholesterol anabolites and catabolites in CHO-AβPP695 cells as compared to CHO-wt cells. Further enzymatic studies revealed that the cholesterol-lowering effect of DHA was mediated by regulating HMG-CoA reductase and squalene epoxidase enzyme activities. We demonstrate for the first time the dual effects of DHA in inhibiting HMG-CoA reductase and squalene epoxidase and modulating the sterol biosynthesis axis of the cholesterol pathway in AβPP695 overexpressing AD. Our novel findings underscore the potential of DHA as a multi-target hypocholesterolemic agent for the prophylaxis of AD and other cholesterol-associated diseases.

Genetic Variation in δ-Opioid Receptor Associates with Increased β- and γ-Secretase Activity in the Late Stages of Alzheimer’s Disease

The agonist-induced activation of human δ-opioid receptor (δOR) has been shown to increase β- (BACE1) and γ-secretase activities leading to increased production of amyloid-β (Aβ) peptide. We have recently shown that phenylalanine to cysteine substitution at amino acid 27 in δOR (δOR-Phe27Cys) increases amyloid-β protein precursor processing through altered endocytic trafficking. Also, a genetic meta-analysis of the δOR-Phe27Cys variation (rs1042114) in two independent Alzheimer's disease (AD) patient cohorts indicated that the heterozygosity of δOR-Phe27Cys increases the risk of AD. Here, we investigated α-, β-, and γ-secretase activities in human brain with respect to δOR-Phe27Cys variation in the temporal cortex of 71 subjects with varying degree of AD-related neurofibrillary pathology (Braak stages I-VI). As a result, a significant increase in β- (p = 0.03) and γ- (p = 0.01), but not α-secretase, activities was observed in late stage AD samples (Braak stages V-VI), which were heterozygous for δOR-Phe27Cys as compared to the δOR-Phe27 and δOR-Cys27 homozygotes. The augmented β-secretase activity was not associated with increased mRNA expression or protein levels of BACE1 in the late stage AD patients, who were heterozygous for the δOR-Phe27Cys variation. These findings suggest that δOR-Phe27Cys variation modulates β- and γ-secretase activity in the late stages of AD likely via post-translational mechanisms other than alterations in the mRNA or protein levels of BACE1, or, in the expression of γ-secretase complex components.

Data for the effects of ER and Golgi stresses on the ER-Golgi SNARE Syntaxin5 expression and on the βAPP processing in cultured hippocampal neurons.

This paper reports the data for the effects of organelle stresses on the ER–Golgi-soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (ER–Golgi SNAREs) syntaxin 5 (Syx5) in neuronal cells. Quantitative as well as qualitative data are presented here to verify the upregulation of Syntaxin 5 (Syx5) under ER and Golgi stresses in hippocampal neurons. Changes in the processing of β-amyloid precursor protein (βAPP) under ER stress were analyzed by immunological assays. In addition, our data shows the specific increase of Syx5 expression under ER and Golgi stresses. Interpretation of our data and further extensive insights into the role of Syx5 in βAPP processing under organelle stress can be found in "ER and Golgi stresses increase ER-Golgi SNARE Syntaxin5: Implications for organelle stress and βAPP processing" [1].

Extra-virgin olive oil attenuates amyloid-β and tau pathologies in the brains of TgSwDI mice

Extra-virgin olive oil (EVOO) is one of the main elements of Mediterranean diet. Several studies have suggested that EVOO has several health promoting effects that could protect from and decrease the risk of Alzheimer’s disease (AD). In this study, we investigated the effect of consumption of EVOO-enriched diet on amyloid- and tau-related pathological alterations that are associated with the progression of AD and cerebral amyloid angiopathy (CAA) in TgSwDI mice. Feeding mice with EVOO-enriched diet for 6months, beginning at an age before amyloid-β (Aβ) accumulation starts, has significantly reduced total Aβ and tau brain levels with a significant improvement in mouse cognitive behavior. This reduction in brain Aβ was explained by the enhanced Aβ clearance pathways and reduced brain production of Aβ via modulation of amyloid-β precursor protein processing. On the other hand, although feeding mice with EVOO-enriched diet for 3months, beginning at an age after Aβ accumulation starts, showed improved clearance across the blood–brain barrier and significant reduction in Aβ levels, it did not affect tau levels or improve cognitive functions of TgSwDI mouse. Collectively, results of this study suggest that the long-term consumption of EVOO-containing diet starting at early age provides a protective effect against AD and its related disorder CAA.

Mutation of conserved aspartates affects maturation of both aspartate mutant and endogenous presenilin 1 and presenilin 2 complexes.

Presenilin (PS1 and PS2) holoproteins are transiently incorporated into low molecular weight (MW) complexes. During subsequent incorporation into a higher MW complex, they undergo endoproteolysis to generate stable N- and C-terminal fragments. Mutation of either of two conserved aspartate residues in transmembrane domains inhibits both presenilin-endoproteolysis and the proteolytic processing of beta-amyloid precursor protein and Notch. We show that although PS1/PS2 endoproteolysis is not required for inclusion into the higher MW N- and C-terminal fragment-containing complex, aspartate mutant holoprotein presenilins are not incorporated into the high MW complexes. Aspartate mutant presenilin holoproteins also preclude entry of endogenous wild type PS1/PS2 into the high MW complexes but do not affect the incorporation of wild type holoproteins into lower MW holoprotein complexes. These data suggest that the loss of function effects of the aspartate mutants result in altered PS complex maturation and argue that the functional presenilin moieties are contained in the high molecular weight complexes.

Herpes simplex virus type 2 infection induces AD-like neurodegeneration markers in human neuroblastoma cells

Herpes simplex virus (HSV) types 1 and 2 are neurotropic viruses that establish lifelong latent infections in neurons. Mounting evidence suggests that HSV-1 infection is involved in the pathogenesis of Alzheimer's disease (AD). The relationships between other herpesvirus infections and events associated with neurodegeneration have not, however, been extensively studied. The present work reports that HSV-2 infection leads to the strong accumulation of hyperphosphorylated tau and the amyloid-β peptides Aβ40 and Aβ42 (all major pathological hallmarks of AD) in human SK-N-MC neuroblastoma cells. Infection is also associated with a marked reduction in the amount of Aβ40 secreted and in the proteolytic fragments of the amyloid-β precursor protein (APP) (secreted APPα and the α-C-terminal fragment). These results indicate that HSV-2 infection inhibits the nonamyloidogenic pathway of APP processing and impairs Aβ secretion in these cells. In addition, HSV-2 induces the accumulation of intracellular autophagic compartments containing Aβ due to a failure in the late stages of autophagy. To our knowledge, this is the first report to show that HSV-2 infection strongly alters the tau phosphorylation state, APP processing, and autophagic process in human neuroblastoma cells, leading to the appearance of AD-like neurodegeneration markers.

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.