Efficacy and Concurrent Validity of Computerized Brain Training Based on Everyday Living (BTEL) Based on Instrumental Activities of Living for Cognitively Healthy Old Individuals: A Preliminary Study

Background

Interventions to prevent or attenuate cognitive decline and dementia in older adults are becoming increasingly important. Recently, cognitive training exercise can be via computer or mobile technology for independent or home use. Recent meta-analysis has reported that Computerized Cognitive Training (CCT) is effective at enhancing cognitive function in healthy older and Alzheimer's disease adults, although little is known about individual characteristics of each computerized program.

Objective

We developed a new CCT named Brain Training Based on Everyday Living (BTEL) to enhance cognitive capacity for Instrumental Activities of Daily Living (IADL). We aim to evaluate the efficacy of the BTEL among cognitively healthy old individuals and to explore its concurrent validity and construct concept.

Methods

We conducted a double-blind study where 106 individuals aged 65 years and older (intervened = 53, control = 53) worked on the active and placebo tasks three times a week over three months (clinical trial: UMIN000048730). The main results were examined using ANCOVA and calculating correlation coefficients.

Results

We found no effect on total score of the three tests; however, there was significant effect for the BTEL on: recognition in MMSE, and immediate recall in HDSR. The tasks are associated with prefrontal cortex. In addition, correlations indicated that each BTEL domain had some validity as a cognitive assessment tool. Different from previous CCT, we determined the neuropsychological characteristics of specific cognitive tasks of the BTEL to a certain degree.

Conclusions

We found modest efficacy of the BTEL in cognitively healthy old individuals and confirmed its concurrent validity and the conceptual construct.

A case with burning mouth syndrome followed by dementia with Lewy bodies: a case report

Burning mouth syndrome (BMS) is characterized by persistent oral burning sensations without corresponding organic findings. Dementia with Lewy bodies (DLB) is a common type of dementia and generally presents visual hallucination and parkinsonism as motor dysfunction besides cognitive decline. In this case report, we present a case in which DLB emerged during the treatment for BMS, with a relatively positive outcome for BMS. A 74 years-old female complained of burning pain in her mouth and a subsequent decrease in food intake. Following a diagnosis of BMS, pharmacotherapy was initiated. BMS was much improved with mirtazapine 15 mg and aripiprazole 1.0 mg, leading to the restoration of her food intake by day 180. However, BMS flared up again triggered by deteriorating physical condition of herself and that of her husband. With aripiprazole 1.5 mg and amitriptyline 25 mg, her BMS gradually improved by day 482. However, by day 510, an increase in anxiety was noted, accompanied by the occasionally misidentification of her husband on day 566. Her cognitive impairment and disorientation were also reported by her husband on the day 572, she was then immediately referred to a neurologist specialized dementia and diagnosed with DLB on the day 583. Her treatment was adjusted to include the prescription of rivastigmine which was titrated up to 9.0 mg. Considering the potential impact of amitriptyline on cognitive function, it was reduced and switched to mirtazapine; however, her oral sensations slightly got worse. Following the consultation with her neurologist, amitriptyline 10 mg was reintroduced and aripiprazole was discontinued on day 755. Remarkably, BMS gradually improved without deteriorating DLB. This case indicated the reaffirmed necessity of careful interviews for changes in daily life not only with the patients but also with their families through the medical assessments. It highlights the vigilance regarding potential cognitive decline underlying or induced as an adverse event especially when treating elderly patients with BMS. While the interaction between BMS and DLB remains unclear, this case underscores the importance of prudent diagnosis and constructing collaboration with specialists in managing BMS with the early phase of DLB.

Aβ Oligomer Toxicity-Reducing Therapy for the Prevention of Alzheimer's Disease: Importance of the Nrf2 and PPARγ Pathways

Recent studies have revealed that soluble amyloid-β oligomers (AβOs) play a pathogenetic role in Alzheimer's disease (AD). Indeed, AβOs induce neurotoxic and synaptotoxic effects and are also critically involved in neuroinflammation. Oxidative stress appears to be a crucial event underlying these pathological effects of AβOs. From a therapeutic standpoint, new drugs for AD designed to remove AβOs or inhibit the formation of AβOs are currently being developed. However, it is also worth considering strategies for preventing AβO toxicity itself. In particular, small molecules with AβO toxicity-reducing activity have potential as drug candidates. Among such small molecules, those that can enhance Nrf2 and/or PPARγ activity can effectively inhibit AβO toxicity. In this review, I summarize studies on the small molecules that counteract AβO toxicity and are capable of activating Nrf2 and/or PPARγ. I also discuss how these interrelated pathways are involved in the mechanisms by which these small molecules prevent AβO-induced neurotoxicity and neuroinflammation. I propose that AβO toxicity-reducing therapy, designated ATR-T, could be a beneficial, complementary strategy for the prevention and treatment of AD.

Peripheral administration of nanomicelle-encapsulated anti-Aβ oligomer fragment antibody reduces various toxic Aβ species in the brain

BACKGROUND

Although a large amount of evidence has revealed that amyloid β (Aβ), especially Aβ oligomers, protofibrils, and pyroglutamated Aβs, participate primarily in the pathophysiological processes of Alzheimer's disease, most clinical trials of anti-Aβ antibody therapy have never acquired successful efficacy in human clinical trials, partly because peripheral administration of antibody medications was unable to deliver sufficient amounts of the molecules to the brain. Recently, we developed polymeric nanomicelles capable of passing through the blood-brain barrier that function as chaperones to deliver larger amounts of heavy molecules to the brain. Herein, we aimed to evaluate the efficacy of newly developed antibody 6H4 fragments specific to Aβ oligomers encapsulated in polymeric nanomicelles on the development of Alzheimer's disease pathology in Alzheimer's disease model mice at the age of emergence of early Alzheimer's disease pathology.

RESULTS

During the 10-week administration of 6H4 antibody fragments in polymeric nanomicelles, a significant reduction in the amounts of various toxic Aβ species, such as Aβ oligomers, toxic Aβ conformers, and pyroglutamated Aβs in the brain was observed. In addition, immunohistochemistry indicated inhibition of diameters of Aβ plaques, Aβ-antibody immunoreactive areas, and also plaque core formation. Behavioral analysis of the mice model revealed that the 6H4 fragments-polymeric nanomicelle group was significantly better at maintaining long-term spatial reference memory in the probe and platform tests of the water maze, thereby indicating inhibition of the pathophysiological process of Alzheimer's disease.

CONCLUSIONS

The results indicated that the strategy of reducing toxic Aβ species in early dementia owing to Alzheimer's disease by providing sufficient antibodies in the brain may modify Alzheimer's disease progression.

Soluble APP-α and APP-β in cerebrospinal fluid as potential biomarkers for differential diagnosis of mild cognitive impairment

OBJECTIVES

Concentrations of soluble amyloid precursor proteins-α (sAPPα) and -β (sAPPβ) in cerebrospinal fluid (CSF) may reflect the neuropathology of Alzheimer's disease (AD). We previously reported that the concentrations of both sAPPα and sAPPβ were significantly higher in patients with mild cognitive impairment (MCI) due to AD (MCI-AD) than in control subjects without cognitive impairment. The present study analyzed whether these sAPPs are useful in the differential diagnosis of MCI.

METHODS

A modified and sensitive method was used to analyze concentrations of sAPPα and sAPPβ in CSF of patients with MCI-AD (n = 30) and MCI due to other causes (MCI-others) (n = 24). Phosphorylated tau (p-tau) and amyloid β-protein 42 (Aβ42) were also analyzed using standard methods.

RESULTS

CSF concentrations of sAPPα and sAPPβ were significantly higher in the MCI-AD than in the MCI-others group (p < 0.001). Furthermore, concentrations of both sAPPα and sAPPβ were highly correlated with the concentration of p-tau, consistent with our previous report.

CONCLUSIONS

Measurement of both sAPPs in CSF using sensitive methods can be helpful in the precise differential diagnosis of patients with MCI.

Lemur tail kinase 1 (LMTK1) regulates the endosomal localization of β-secretase BACE1

Lemur tail kinase 1 (LMTK1), previously called apoptosis-associated tyrosine kinase (AATYK), is an endosomal Ser/Thr kinase. We recently reported that LMTK1 regulates axon outgrowth, dendrite arborization and spine formation via Rab11-mediated vesicle transport. Rab11, a small GTPase regulating recycling endosome trafficking, is shown to be associated with late-onset Alzheimer's disease (LOAD). In fact, genome-wide association studies identified many proteins regulating vesicle transport as risk factors for LOAD. Furthermore, LMTK1 has been reported to be a risk factor for frontotemporal dementia. Then, we hypothesized that LMTK1 contributes to AD development through vesicle transport and examined the effect of LMTK1 on the cellular localization of AD-related proteins, amyloid precursor protein (APP) and β-site APP cleaving enzyme 1 (BACE1). The β-cleavage of APP by BACE1 is the initial and rate-limiting step in Aβ generation. We found that LMTK1 accumulated BACE1, but not APP, to the perinuclear endosomal compartment, whereas the kinase-negative (kn) mutant of LMTK1A did not. The β-C-terminal fragment was prone to increase under overexpression of LMTK1A kn. Moreover, the expression level of LMTK1A was reduced in AD brains. These results suggest the possibility that LMTK1 is involved in AD development through the regulation of the proper endosomal localization of BACE1.

[Surgical Treatment of Aortic Stenosis Combined with Bilateral Pheochromocytomas in a Dialysis Patient]

We report a case of a dialysis patient with severe aortic stenosis(AS) along with bilateral pheochromocytomas. A 52-year-old man presented with syncope and was diagnosed with severe AS. Although aortic valve replacement(AVR) was scheduled, bilateral pheochromocytomas were found during preoperative examination. There was a high possibility of developing hemodynamical crisis during AVR, and we planned to perform adrenalectomy prior to AVR. To avoid circulatory collapse just after adrenalectomy, balloon aortic valvuloplasty (BAV) was performed beforehand. Two weeks after the adrenalectomy, AVR was performed in a stable condition.

[Euglycemic Diabetic Ketoacidosis Caused by a Sodium-glucose Co-transporter (SGLT) 2 Inhibitor after Coronary Artery Bypass Grafting]

A 65-year-old woman with type Ⅱ diabetes and unstable angina presented with chest pain due to in-stent restenosis. Her regular medication comprised an sodium-glucose co-transporter( SGLT) 2 inhibitor. Because of unstable hemodynamic status, semi-emergency coronary artery bypass grafting (CABG) was performed. Postoperatively, the cardiac and hemodynamic status stabilized, but there was progression of metabolic acidosis. Based on the presence of massive urinary ketone bodies without hyper glycosuria, the patient was diagnosed with euglycemic diabetic ketoacidosis( DKA) caused by an SGLT2 inhibitor. Ketoacidosis without elevated blood glucose( i.e., euglycemic DKA) has been reported to be associated with intake of an SGLT2 inhibitor, which promoted glucose excretion in the urine. Our patient developed euglycemic DKA due to the progression of myocardial ischemia and surgical stress. Guidelines in other countries have stipulated that SGLT2 inhibitor should be stopped 24 hours preoperatively. In our case, euglycemic DKA occurred even when the SGLT2 inhibitor was stopped for more than 24 hours preoperatively. Further studies on the withdrawal of an SGLT2 inhibitor in the appropriate perioperative period are required.

TFEB-mediated Enhancement of the Autophagy-lysosomal Pathway Dually Modulates the Process of Amyloid β-Protein Generation in Neurons

Abnormalities of the autophagy-lysosomal pathway (ALP) have been implicated in the pathology of Alzheimer's disease (AD). Activation of TFEB (transcription factor EB), a master regulator of the ALP, leads to ALP facilitation. The present study sought to clarify whether TFEB-mediated ALP facilitation influences the process of amyloid β-protein (Aβ) generation in neurons. TFEB was overexpressed in mature rat primary cortical neurons via recombinant adenoviruses, without (basal conditions) or with co-overexpression of wild-type amyloid precursor protein (APP) or its β-C-terminal fragment (β-CTF). We confirmed that TFEB overexpression upregulated the lysosomal proteins, cathepsin D and LAMP-1. In TFEB-expressing neurons, protein levels of ADAM10 were profoundly increased, whereas those of APP, BACE1, or γ-secretase complex proteins were unaffected. However, TFEB did not affect ADAM10 mRNA levels. TFEB overexpression had different effects on Aβ production depending on the expression level of APP or β-CTF: TFEB slightly decreased Aβ secretion under basal conditions; clearly increased α-CTF levels and marginally increased β-CTF levels with modest increases in secreted Aβ in APP-expressing neurons; and caused a remarkable increase in β-CTF levels with a significant increase in secreted Aβ in β-CTF-expressing neurons. Inhibition of proteasomes, but not lysosomes, markedly increased β-CTF levels in β-CTF-expressing neurons. These results collectively indicate that TFEB modulates Aβ production not only by increasing α-secretase processing of APP through ADAM10 upregulation but also by augmenting β-CTF levels possibly via altered proteasome-mediated catabolism. Thus, TFEB-mediated ALP enhancement appears to have dual, but opposite, effects on Aβ production in neurons.

Re-evaluation of soluble APP-α and APP-β in cerebrospinal fluid as potential biomarkers for early diagnosis of dementia disorders

Background

Because soluble (or secreted) amyloid precursor protein-β (sAPPβ) and -α (sAPPα) possibly reflect pathological features of Alzheimer’s disease (AD), they are potential biomarker candidates for dementia disorders, including AD and mild cognitive impairment (MCI) due to AD (MCI-AD). However, controversial results have been reported regarding their alterations in the cerebrospinal fluid (CSF) of AD and MCI-AD patients. In this study, we re-assessed the utility of sAPPα and sAPPβ in CSF as diagnostic biomarkers of dementia disorders.

Methods

We used a modified and sensitive detection method to analyze sAPPs levels in CSF in four groups of patients: AD (N = 33), MCI-AD (N = 17), non-AD dementia (N = 27), and disease controls (N = 19). Phosphorylated tau (p-tau), total tau, and Aβ42 were also analyzed using standard methods.

Results

A strong correlation was observed between sAPPα and sAPPβ, consistent with previous reports. Both sAPPα and sAPPβ were highly correlated with p-tau and total tau, suggesting that sAPPs possibly reflect neuropathological changes in the brain. Levels of sAPPα were significantly higher in MCI-AD cases compared with non-AD and disease control cases, and those of sAPPβ were also significantly higher in MCI-AD and AD cases relative to other cases. A logistic regression analysis indicated that sAPPα and sAPPβ have good discriminative power for the diagnosis of MCI-AD.

Conclusions

Our findings collectively suggest that both sAPPs are pathologically relevant and potentially useful biomarkers for early and accurate diagnosis of dementia disorders. We also suggest that careful measurement is important in assessing the diagnostic utility of CSF sAPPs.

Electronic supplementary material

The online version of this article (10.1186/s40364-017-0108-5) contains supplementary material, which is available to authorized users.

Mitochondria are devoid of amyloid β-protein (Aβ)-producing secretases: Evidence for unlikely occurrence within mitochondria of Aβ generation from amyloid precursor protein

Mitochondrial dysfunction is implicated in the pathological mechanism of Alzheimer's disease (AD). Amyloid β-protein (Aβ), which plays a central role in AD pathogenesis, is reported to accumulate within mitochondria. However, a question remains as to whether Aβ is generated locally from amyloid precursor protein (APP) within mitochondria. We investigated this issue by analyzing the expression patterns of APP, APP-processing secretases, and APP metabolites in mitochondria separated from human neuroblastoma SH-SY5Y cells and those expressing Swedish mutant APP. APP, BACE1, and PEN-2 protein levels were significantly lower in crude mitochondria than microsome fractions while those of ADAM10 and the other γ-secretase complex components (presenilin 1, nicastrin, and APH-1) were comparable between fractions. The crude mitochondrial fraction containing substantial levels of cathepsin D, a lysosomal marker, was further separated via iodixanol gradient centrifugation to obtain mitochondria- and lysosome-enriched fractions. Mature APP, BACE1, and all γ-secretase complex components (in particular, presenilin 1 and PEN-2) were scarcely present in the mitochondria-enriched fraction, compared to the lysosome-enriched fraction. Moreover, expression of the β-C-terminal fragment (β-CTF) of APP was markedly low in the mitochondria-enriched fraction. Additionally, immunocytochemical analysis showed very little co-localization between presenilin 1 and Tom20, a marker protein of mitochondria. In view of the particularly low expression levels of BACE1, γ-secretase complex proteins, and β-CTF in mitochondria, we propose that it is unlikely that Aβ generation from APP occurs locally within this organelle.

The neurotoxicity of amyloid β-protein oligomers is reversible in a primary neuron model

Alzheimer’s disease (AD) is characterized by the accumulation of extracellular amyloid β-protein (Aβ) and intracellular hyperphosphorylated tau proteins. Recent evidence suggests that soluble Aβ oligomers elicit neurotoxicity and synaptotoxicity, including tau abnormalities, and play an initiating role in the development of AD pathology. In this study, we focused on the unclarified issue of whether the neurotoxicity of Aβ oligomers is a reversible process. Using a primary neuron culture model, we examined whether the neurotoxic effects induced by 2-day treatment with Aβ42 oligomers (Aβ-O) are reversible during a subsequent 2-day withdrawal period. Aβ-O treatment resulted in activation of caspase-3 and eIF2α, effects that were considerably attenuated following Aβ-O removal. Immunocytochemical analyses revealed that Aβ-O induced aberrant phosphorylation and caspase-mediated cleavage of tau, both of which were mostly reversed by Aβ-O removal. Furthermore, Aβ-O caused intraneuronal dislocation of β-catenin protein and a reduction in its levels, and these alterations were partially reversed upon Aβ-O withdrawal. The dislocation of β-catenin appeared to reflect synaptic disorganization. These findings indicate that removal of extracellular Aβ-O can fully or partially reverse Aβ-O-induced neurotoxic alterations in our neuron model. Accordingly, we propose that the induction of neurotoxicity by Aβ oligomers is a reversible process, which has important implications for the development of AD therapies.

Amyloid beta-protein and lipid rafts: focused on biogenesis and catabolism

Cerebral accumulation of amyloid β-protein (Aβ) is thought to play a key role in the molecular pathology of Alzheimer's disease (AD). Three secretases (β-, γ-, and α-secretase) are proteases that control the production of Aβ from amyloid precursor protein. Increasing evidence suggests that cholesterol-rich membrane microdomains termed 'lipid rafts' are involved in the biogenesis and accumulation of Aβ as well as Aβ-mediated neurotoxicity. γ-Secretase is enriched in lipid rafts, which are considered an important site for Aβ generation. Additionally, Aβ-degrading peptidases located in lipid rafts, such as neprilysin, appear to play a role in Aβ catabolism. This mini-review focuses on the roles of lipid rafts in the biogenesis and catabolism of Aβ, covering recent research on the relationship between lipid rafts and the three secretases or Aβ-degrading peptidases. Furthermore, the significance of lipid rafts in Aβ aggregation and neurotoxicity is briefly summarized.

Reduction of β-amyloid accumulation by reticulon 3 in transgenic mice

Inhibition of the β-secretase, BACE1, which cleaves amyloid precursor protein (APP) to produce β-amyloid protein (Aβ), is thought to be a feasible therapeutic strategy for Alzheimer's disease. Reticulon (RTN) proteins such as RTN3 have been identified as membrane proteins that interact with BACE1 and inhibit its Aβ-generating activity. In this study, we investigated whether RTN3 can regulate Aβ production in vivo, using transgenic (Tg) mice expressing APP with Swedish and London mutations (APP Tg mice) and those expressing RTN3; the latter mice showed ~1.4-fold higher expression levels of RTN3 protein in the cerebral cortex than non-Tg controls. We analyzed the brains of single APP Tg and double APP/RTN3 Tg mice at the age of approximately 15 months. The levels of secreted APP-β, a direct BACE1 cleavage product of APP, in Tris-soluble fraction were considerably reduced in the hippocampus and cerebral cortex of APP/RTN3 Tg mice relative to those in APP Tg mice. Immunohistochemical analyses demonstrated that Aβ burden and plaques were significantly (by approximately 50%) decreased in both the hippocampus and cerebral cortex of double Tg mice compared to APP Tg mice. Furthermore, the levels of guanidine-soluble Aβ40 and Aβ42 in these brain regions of APP/RTN3 Tg mice were relatively lower than those in APP Tg mice. These findings indicate that even a small increase in RTN3 expression exerts suppressive effects on amyloidogenic processing of APP and Aβ accumulation through modulation of BACE1 activity in vivo, and suggest that induction of RTN3 might be an effective therapeutic strategy against Alzheimer's disease.

Potential repurposing of oncology drugs for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia, affecting about 30 million people worldwide. Despite recent advances in understanding its molecular pathology, no mechanism-based drugs are currently available that can halt the progression of AD. Because amyloid-β-peptide (Aβ), a primary component of senile plaques, is thought to be a central pathogenic culprit, several disease-modifying therapies are being developed, including inhibitors of Aβ-producing proteases and immunotherapies with anti-Aβ antibodies. Drug repositioning or repurposing is regarded as a complementary and reasonable approach to identify new drug candidates for AD. This commentary will discuss the clinical relevance of an attractive candidate compound reported in a recent paper by Hayes et al. (BMC Medicine 2013) as well as perspectives regarding the possible repositioning of oncology drugs for the treatment of AD. See related research article here http://www.biomedcentral.com/1741-7015/11/81.

Chronic cladribine administration increases amyloid beta peptide generation and plaque burden in mice

Background

The clinical uses of 2-chloro-2′-deoxyadenosine (2-CDA) or cladribine which was initially prescribed to patients with hematological and lymphoid cancers is now extended to treat patients with multiple sclerosis (MS). Previous data has shown that 2-CDA has high affinity to the brain and readily passes through the blood brain barrier reaching CSF concentrations 25% of that found in plasma. However, whether long-term administration of 2-CDA can lead to any adverse effects in patients or animal models is not yet clearly known.

Methodology

Here we show that exposure of 2-CDA to CHO cells stably expressing wild-type APP751 increased generation and secretion of amyloid β peptide (Aβ) in to the conditioned medium. Interestingly, increased Aβ levels were noticed even at non-toxic concentrations of 2-CDA. Remarkably, chronic treatment of APdE9 mice, a model of Alzheimer's disease with 2-CDA for 60 days increased amyloid plaque burden by more than 1-fold. Increased Aβ generation appears to result from increased turnover of APP as revealed by cycloheximide-chase experiments. Additionally, surface labeling of APP with biotin and immunoprecipitation of surface labeled proteins with anti-biotin antibody also indicated increased APP at the cell surface in 2-CDA treated cells compared to controls. Increased turnover of APP by 2-CDA in turn might be a consequence of decreased protein levels of PIN 1, which is known to regulate cis-trans isomerization and phosphorylation of APP. Most importantly, like many other oncology drugs, 2-CDA administration led to significant delay in acquiring a reward-based learning task in a T maze paradigm.

Conclusions

Taken together, these data provide compelling evidence for the first time that chronic 2-CDA administration can increase amyloidogenic processing of APP leading to robustly increased plaque burden which may be responsible for the observed deficits in learning skills. Thus chronic treatment of mice with 2-CDA can have deleterious effects in vivo.

Neuronal β-amyloid generation is independent of lipid raft association of β-secretase BACE1: analysis with a palmitoylation-deficient mutant

β-Secretase, BACE1 is a neuron-specific membrane-associated protease that cleaves amyloid precursor protein (APP) to generate β-amyloid protein (Aβ). BACE1 is partially localized in lipid rafts. We investigated whether lipid raft localization of BACE1 affects Aβ production in neurons using a palmitoylation-deficient mutant and further analyzed the relationship between palmitoylation of BACE1 and its shedding and dimerization. We initially confirmed that BACE1 is mainly palmitoylated at four C-terminal cysteine residues in stably transfected neuroblastoma cells. We found that raft localization of mutant BACE1 lacking the palmitoylation modification was markedly reduced in comparison to wild-type BACE1 in neuroblastoma cells as well as rat primary cortical neurons expressing BACE1 via recombinant adenoviruses. In primary neurons, expression of wild-type and mutant BACE1 enhanced production of Aβ from endogenous or overexpressed APP to similar extents with the β-C-terminal fragment (β-CTF) of APP mainly distributed in nonraft fractions. Similarly, β-CTF was recovered mainly in nonraft fractions of neurons expressing Swedish mutant APP only. These results show that raft association of BACE1 does not influence β-cleavage of APP and Aβ production in neurons, and support the view that BACE1 cleaves APP mainly in nonraft domains. Thus, we propose a model of neuronal Aβ generation involving mobilization of β-CTF from nonraft to raft domains. Additionally, we obtained data indicating that palmitoylation plays a role in BACE1 shedding but not dimerization.

Abnormal cross-talk between mutant presenilin 1 (I143T, G384A) and glycosphingolipid biosynthesis

Mutations in the presenilin 1 (PS1) gene are associated with early onset familial Alzheimer's disease (FAD). In this study, we found that the expression of mutant-PS1 in stable transfectants of SH-SY5Y neuroblastoma cells results in a reduction of the biosynthesis and steady-state levels of glucosylceramide. As an in vivo corroboration of these data, there was a significant reduction of brain glucosylceramide and gangliosides in an animal model of FAD. In mutant-PS1-transfectants (I143T, G384A), immunocytochemistry disclosed a remarkable reduction of glucosylceramide synthase (GlcT-1)-like immunoreactivity in the cells when compared with those of mock- and wild-PS1 transfectants. Immunoprecipitation of GlcT-1 protein from mutant-PS1 transfectants demonstrated a marked reduction in GlcT-1 protein, but there was no reduction in the levels of GlcT-1 mRNA. Both coprecipitation and γ-secretase inhibition experiments suggest that mutant-PS1 seems to form a complex with GlcT-1 protein and to be involved in GlcT-1 degradation, which was never found in other cell types. Thus, mutations in the PS1 gene result in profound glycosphingolipids abnormalities by abnormal molecular interaction with GlcT-1.

Autoreactive-Aβ antibodies promote APP β-secretase processing

Several prior investigations of Alzheimer's disease (AD) patients have indicated naturally occurring autoantibodies against amyloid-β (Aβ) species are produced. Although many studies have focused on the relative concentrations or binding affinities of autoantibodies against Aβ-related proteins in AD and aging, data regarding their functional properties are limited. It is generally believed that these antibodies act to aid in clearance of Aβ. However, as antibodies which bind to Aβ also typically bind to the parent amyloid precursor protein (APP), we reasoned that certain Aβ-targeting autoantibodies may bind to APP thereby altering its conformation and processing. Here we show for the first time, that naturally occurring Aβ-reactive autoantibodies isolated from AD patients, but not from healthy controls, promote β-secretase activity in cultured cells. Furthermore, using monoclonal antibodies to various regions of Aβ, we found that antibodies generated against the N-terminal region, especially Aβ(1-17) , dose dependently promoted amyloidogenic processing of APP viaβ-secretase activation. Thus, this property of certain autoantibodies in driving Aβ generation could be of etiological importance in the development of sporadic forms of AD. Furthermore, future passive or active anti-Aβ immunotherapies must consider potential off-target effects resulting from antibodies targeting the N-terminus of Aβ, as co-binding to the corresponding region of APP may actually enhance Aβ generation.

ER-stress-inducible Herp, facilitates the degradation of immature nicastrin

BACKGROUND

Herp is an endoplasmic reticulum (ER)-stress-inducible membrane protein harboring an ubiquitin-like domain (ULD). However, its biological functions are not fully understood. Here, we examined the role of Herp in the degradation of γ-secretase components.

METHODS

Effects of ULD-lacking Herp (ΔUb-Herp) expression on the degradation of γ-secretase components were analyzed.

RESULTS

The cellular expression of ΔUb-Herp was found to inhibit the degradation of overexpressed immature nicastrin and full-length presenilin. The mechanisms underlying Herp-mediated nicastrin degradation was further analyzed. We found that immature nicastrin accumulates in the ER of ΔUb-Herp overexpressing cells or Herp-deficient cells more than that in the ER of wild-type cells. Further, ΔUb-Herp expression inhibited nicastrin ubiquitination, suggesting that the ULD of Herp is likely involved in nicastrin ubiquitination. Co-immunoprecipitation study showed that Herp as well as ΔUb-Herp potentially interacts with nicastrin, mediating nicastrin interaction with p97, which functions in retranslocation of misfolded proteins from the ER to the cytosol.

CONCLUSIONS

Thus, Herp is likely involved in degradation of immature nicastrin by facilitating p97-dependent nicastrin retranslocation and ubiquitination.

GENERAL SIGNIFICANCE

We suggest that Herp could play a role in the elimination of the excess unassembled components of a multimeric complex.

Oxidative stress up-regulates presenilin 1 in lipid rafts in neuronal cells

Oxidative stress is associated with beta-amyloid peptide (A beta) accumulation in the brains of Alzheimer's disease patients. A beta is generated upon the sequential proteolytic cleavage of transmembrane amyloid precursor protein (APP) by two membrane-bound proteases, beta-secretase (BACE1) and the gamma-secretase complex comprising presenilin 1 (PS1), nicastrin, APH-1 and PEN-2. Recent evidence suggests that significant amounts of BACE1 and gamma-secretase components localize in the cholesterol-rich region of membranes known as lipid rafts, where A beta production occurs preferentially. In this study, we investigated the effects of oxidative stress on the BACE1 and gamma-secretase components in lipid rafts using human neuroblastoma SH-SY5Y cells exposed to ethacrynic acid (EA), a compound that induces cellular glutathione depletion. Following exposure of cells to EA, heme oxygenase-1, a marker protein of oxidative stress, was strongly induced. Moreover, treatment with EA resulted in a significant increase in PS1 protein levels, but not those of nicastrin, APH-1, PEN-2 or BACE1, in both cell lysates and the lipid raft fraction. This increase in PS1 protein expression was prevented by co-treatment with an antioxidant, N-acetylcysteine (NAC). EA additionally induced a significant increase in PS1 mRNA expression, which was inhibited by NAC. Finally, EA treatment was found to promote A beta secretion from cells expressing Swedish mutant APP. It appears that in our cell culture model, oxidative stress enhances PS1 protein levels in lipid rafts via up-regulation of PS1 transcription, which may constitute the mechanism underlying the oxidative stress-associated promotion of A beta production.

Human CRB2 inhibits gamma-secretase cleavage of amyloid precursor protein by binding to the presenilin complex

Drosophila Crumbs has been reported to attenuate Notch signaling by inhibition of gamma-secretase cleavage at the wing margins. gamma-Secretase is an intramembrane protease that is responsible for the generation of amyloid-beta (Abeta) peptides from the beta-amyloid precursor protein (APP). Here, we re-examined gamma-secretase inhibition by human CRB2, which is the most abundant Crumbs ortholog in the brain. Transfected CRB2 inhibited proteolytic production of Abeta and APP intracellular domains from APP C-terminal fragments in HEK293 and SH-SY5Y cells. Conversely, knockdown of endogenous CRB2 increased gamma-secretase cleavage products in SH-SY5Y cells. CRB2 inhibition of gamma-cleavage was also detected in cell-free assays. CRB2 interacted with the gamma-secretase complex, but was not a competitive substrate for gamma-cleavage. The transmembrane domain of CRB2 was indispensable for inhibition of Abeta generation and mediated CRB2 binding with the gamma-secretase complex. In addition, the cytoplasmic domain appeared to play a supportive role in gamma-secretase inhibition, whereas mutational disruption of the two protein-binding motifs involved in the formation of cell adhesion complexes did not affect gamma-secretase inhibition. Co-overexpression of presenilin-1 or APH-1 abrogated gamma-secretase inhibition probably through prevention of the incorporation of CRB2 into the gamma-secretase complex. Our results suggest that CRB2 functions as an inhibitory binding protein that is involved in the formation of a mature but inactive pool of the gamma-secretase complex.

The two-hydrophobic domain tertiary structure of reticulon proteins is critical for modulation of beta-secretase BACE1

beta-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is a membrane-bound protease that is essential for the production of beta-amyloid protein (Abeta). Given the crucial role of Abeta accumulation in Alzheimer's disease (AD), inhibition of BACE1 activity may represent a feasible therapeutic strategy in the treatment of AD. Recently, we and others identified reticulon 3 (RTN3) and reticulon 4-B/C (RTN4-B/C or Nogo-B/C) as membrane proteins that interact with BACE1 and inhibit its ability to produce Abeta. In this study, we employed various mutants of RTN3 and RTN4-C and C. elegans RTN to investigate the molecular mechanisms by which RTNs regulate BACE1. We found that RTN3 mutants lacking the N-terminal or C-terminal or loop domain as well as a RTN4-C mutant lacking the C-terminal domain bound to BACE1 comparably to wild-type RTN3 and RTN4-C. Furthermore, overexpression of wild-type RTN3, RTN4-C, and these RTN mutants similarly reduced Abeta40 and Abeta42 secretion by cells expressing Swedish mutant APP. C. elegans RTN, which has low homology to human RTNs, also interacted with BACE1 and inhibited Abeta secretion. In contrast, two RTN3 mutants containing deletions of the first or second potential transmembrane domains and an RTN3 swap mutant of the second transmembrane domain bound BACE1 but failed to inhibit Abeta secretion. Collectively, these results suggest that the two-transmembrane-domain tertiary structure of RTN proteins is critical for the ability of RTNs to modulate BACE1 activity, whereas N-terminal, C-terminal and loop regions are not essential for this function.

An E3 ubiquitin ligase, Synoviolin, is involved in the degradation of immature nicastrin, and regulates the production of amyloid beta-protein

The presenilin complex, consisting of presenilin, nicastrin, anterior pharynx defective-1 and presenilin enhancer-2, constitutes gamma-secretase, which is required for the generation of amyloid beta-protein. In this article, we show that Synoviolin (also called Hrd1), which is an E3 ubiquitin ligase implicated in endoplasmic reticulum-associated degradation, is involved in the degradation of endogenous immature nicastrin, and affects amyloid beta-protein generation. It was found that the level of immature nicastrin was dramatically increased in synoviolin-null cells as a result of the inhibition of degradation, but the accumulation of endogenous presenilin, anterior pharynx defective-1 and presenilin enhancer-2 was not changed. This was abolished by the transfection of exogenous Synoviolin. Moreover, nicastrin was co-immunoprecipitated with Synoviolin, strongly suggesting that nicastrin is the substrate of Synoviolin. Interestingly, amyloid beta-protein generation was increased by the overexpression of Synoviolin, although the nicastrin level was decreased. Thus, Synoviolin-mediated ubiquitination is involved in the degradation of immature nicastrin, and probably regulates amyloid beta-protein generation. Structured digital abstract: * MINT-7255352: Synoviolin (uniprotkb:Q9DBY1) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti tag coimmunoprecipitation (MI:0007) * MINT-7255377: Ubiquitin (uniprotkb:P62991) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti bait coimmunoprecipitation (MI:0006) * MINT-7255363: NCT (uniprotkb:P57716) physically interacts (MI:0915) with Synoviolin (uniprotkb:Q9DBY1) by anti bait coimmunoprecipitation (MI:0006).

Expression of reticulon 3 in Alzheimer's disease brain

AIMS

Reticulon 3 (RTN3), a member of the reticulon family of proteins, interacts with the beta-secretase, beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1), and inhibits its activity to produce beta-amyloid protein. The aim of the present study was to clarify the biological role of RTN3 in the brain and its potential involvement in the neuropathology of Alzheimer's disease (AD).

METHODS

We performed immunohistochemical and biochemical analyses using a specific antibody against RTN3 to investigate the expression and subcellular localization of RTN3 in control and AD brain tissue samples.

RESULTS

Western blot analysis revealed no significant differences in the RTN3 levels between control and AD brains. Immunohistochemical staining showed that RTN3 immunoreactivity was predominantly localized in pyramidal neurones of the cerebral cortex. The patterns of RTN3 immunostaining were similar in control and AD cerebral cortices, and senile plaques were generally negative for RTN3. Biochemical subcellular fractionation disclosed that RTN3 colocalized with BACE1 in various fractions, including the endoplasmic reticulum and the Golgi apparatus. Double-immunofluorescence staining additionally indicated that RTN3 was localized in both endoplasmic reticulum and Golgi compartments in neurones.

CONCLUSIONS

These results show that RTN3 is primarily expressed in pyramidal neurones of the human cerebral cortex and that no clear difference of RTN3 immunoreactivity is observable between control and AD brains. Our data also suggest that there is considerable colocalization of RTN3 with BACE1 at a subcellular level.

Prostaglandin E2 stimulates the production of amyloid-beta peptides through internalization of the EP4 receptor

Amyloid-beta (Abeta) peptides, generated by the proteolysis of beta-amyloid precursor protein by beta- and gamma-secretases, play an important role in the pathogenesis of Alzheimer disease. Inflammation is also important. We recently reported that prostaglandin E(2) (PGE(2)), a strong inducer of inflammation, stimulates the production of Abeta through EP(2) and EP(4) receptors, and here we have examined the molecular mechanism. Activation of EP(2) and EP(4) receptors is coupled to an increase in cellular cAMP levels and activation of protein kinase A (PKA). We found that inhibitors of adenylate cyclase and PKA suppress EP(2), but not EP(4), receptor-mediated stimulation of the Abeta production. In contrast, inhibitors of endocytosis suppressed EP(4), but not EP(2), receptor-mediated stimulation. Activation of gamma-secretase was observed with the activation of EP(4) receptors but not EP(2) receptors. PGE(2)-dependent internalization of the EP(4) receptor was observed, and cells expressing a mutant EP(4) receptor lacking the internalization activity did not exhibit PGE(2)-stimulated production of Abeta. A physical interaction between the EP(4) receptor and PS-1, a catalytic subunit of gamma-secretases, was revealed by immunoprecipitation assays. PGE(2)-induced internalization of PS-1 and co-localization of EP(4), PS-1, and Rab7 (a marker of late endosomes and lysosomes) was observed. Co-localization of PS-1 and Rab7 was also observed in the brain of wild-type mice but not of EP(4) receptor null mice. These results suggest that PGE(2)-stimulated production of Abeta involves EP(4) receptor-mediated endocytosis of PS-1 followed by activation of the gamma-secretase, as well as EP(2) receptor-dependent activation of adenylate cyclase and PKA, both of which are important in the inflammation-mediated progression of Alzheimer disease.

IGF-1 promotes beta-amyloid production by a secretase-independent mechanism

Beta-amyloid peptide (Abeta) is generated via the sequential proteolysis of beta-amyloid precursor protein (APP) by beta- and gamma-secretases, and plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Here, we sought to clarify the role of insulin-like growth factor-1 (IGF-1), implicated in the AD pathomechanism, in the generation of Abeta. Treatment of neuroblastoma SH-SY5Y cells expressing AD-associated Swedish mutant APP with IGF-1 did not alter cellular levels of APP, but significantly increased those of beta-C-terminal fragment (beta-CTF) and secreted Abeta. IGF-1 also enhanced APP phosphorylation at Thr668. Treatment of beta-CTF-expressing cells with IGF-1 increased the levels of beta-CTF and secreted Abeta. The IGF-1-induced augmentation of beta-CTF was observed in the presence of gamma-secretase inhibitors, but not in cells expressing beta-CTF with a Thr668 to alanine substitution. These results suggest that IGF-1 promotes Abeta production through a secretase-independent mechanism involving APP phosphorylation.

A family of membrane proteins associated with presenilin expression and gamma-secretase function

Presenilin 1 (PS1) forms the gamma-secretase complex with at least three components: nicastrin, APH-1, and PEN-2. This complex mediates intramembrane cleavage of amyloid precursor protein (APP) to generate beta-amyloid protein (Abeta) as well as other type 1 transmembrane proteins. Although PS1 mutations linked to familial Alzheimer's disease influence these cleavages, their biological consequences have not been fully understood. In this study, we used mRNA differential display analysis to identify a gene, denoted adoplin-1/ORMDL-1, which displays significantly reduced expression in association with PS1 mutations. Adoplin-1 and two highly homologous genes (adoplin-2, -3) constitute a gene family that encodes transmembrane proteins. The mRNA and protein levels of adoplins (particularly adoplin-1, -2) were markedly elevated in PS-deficient fibroblasts, compared to wild-type cells. Moreover, knockdown of the three adoplins by RNA interference affected maturation of nicastrin and its association with PS1. Adoplin knockdown additionally resulted in elevated levels of APP C-terminal fragments and decreased Abeta production, suggestive of reduced gamma-secretase activity. Our data collectively indicate that adoplins are unique molecules with PS-related expression and functions that may play important role(s) in the maturation and activity of the gamma-secretase complex.

Involvement of prostaglandin E2 in production of amyloid-beta peptides both in vitro and in vivo

Amyloid-beta peptides (Abeta), generated by proteolysis of the beta-amyloid precursor protein (APP) by beta- and gamma-secretases, play an important role in the pathogenesis of Alzheimer disease (AD). Inflammation is also believed to be integral to the pathogenesis of AD. Here we show that prostaglandin E(2) (PGE(2)), a strong inducer of inflammation, stimulates the production of Abeta in cultured human embryonic kidney (HEK) 293 or human neuroblastoma (SH-SY5Y) cells, both of which express a mutant type of APP. We have demonstrated using subtype-specific agonists that, of the four main subtypes of PGE(2) receptors (EP(1-4)), EP(4) receptors alone or EP(2) and EP(4) receptors together are responsible for this PGE(2)-stimulated production of Abeta in HEK293 or SH-SY5Y cells, respectively. An EP(4) receptor antagonist suppressed the PGE(2)-stimulated production of Abeta in HEK293 cells. This stimulation was accompanied by an increase in cellular cAMP levels, and an analogue of cAMP stimulated the production of Abeta, demonstrating that increases in the cellular level of cAMP are responsible for the PGE(2)-stimulated production of Abeta. Immunoblotting experiments and direct measurement of gamma-secretase activity suggested that PGE(2)-stimulated production of Abeta is mediated by activation ofgamma-secretase but not of beta-secretase. Transgenic mice expressing the mutant type of APP showed lower levels of Abeta in the brain, when they were crossed with mice lacking either EP(2) or EP(4) receptors, suggesting that PGE(2)-mediated activation of EP(2) and EP(4) receptors is involved in the production of Abeta in vivo and in the pathogenesis of AD.

Endoplasmic reticulum chaperones inhibit the production of amyloid-beta peptides

Abeta (amyloid-beta peptides) generated by proteolysis of APP (beta-amyloid precursor protein), play an important role in the pathogenesis of AD (Alzheimer's disease). ER (endoplasmic reticulum) chaperones, such as GRP78 (glucose-regulated protein 78), make a major contribution to protein quality control in the ER. In the present study, we examined the effect of overexpression of various ER chaperones on the production of Abeta in cultured cells, which produce a mutant type of APP (APPsw). Overexpression of GRP78 or inhibition of its basal expression, decreased and increased respectively the level of Abeta40 and Abeta42 in conditioned medium. Co-expression of GRP78's co-chaperones ERdj3 or ERdj4 stimulated this inhibitory effect of GRP78. In the case of the other ER chaperones, overexpression of some (150 kDa oxygen-regulated protein and calnexin) but not others (GRP94 and calreticulin) suppressed the production of Abeta. These results indicate that certain ER chaperones are effective suppressors of Abeta production and that non-toxic inducers of ER chaperones may be therapeutically beneficial for AD treatment. GRP78 was co-immunoprecipitated with APP and overexpression of GRP78 inhibited the maturation of APP, suggesting that GRP78 binds directly to APP and inhibits its maturation, resulting in suppression of the proteolysis of APP. On the other hand, overproduction of APPsw or addition of synthetic Abeta42 caused up-regulation of the mRNA of various ER chaperones in cells. Furthermore, in the cortex and hippocampus of transgenic mice expressing APPsw, the mRNA of some ER chaperones was up-regulated in comparison with wild-type mice. We consider that this up-regulation is a cellular protective response against Abeta.

A novel monoclonal antibody specific for the amino-truncated beta-amyloid Abeta5-40/42 produced from caspase-cleaved amyloid precursor protein

Deposition of beta-amyloid peptide (Abeta) as senile plaques and amyloid angiopathy are the major neuropathological features of Alzheimer's disease (AD). Heterogeneity is observed in the N- and C-termini of the deposited Abeta species. Recent evidence implicates caspase activation and apoptosis in AD neurodegeneration. We previously reported that a distinct N-terminally truncated Abeta species, Abeta5-40/42 is preferentially produced from the caspase-cleaved form of amyloid precursor protein (APP) lacking its C-terminal 31 amino acids and that it is deposited in AD brain tissues. Here, we generated a novel monoclonal antibody specific to the N-terminal end of Abeta5-40/42. Western blotting confirmed that this antibody recognizes Abeta5-40 but not Abeta1-40. We also showed that the antibody is able to immunoprecipitate Abeta5-40 but not Abeta1-40. Immunoprecipitation with the antibody followed by mass spectrometric analysis further detected Abeta5-40 in the conditioned media from neuroblastoma cells expressing the caspase-cleaved APP. The antibody reacted weakly with Abeta derived from AD brains. These results suggest that our novel monoclonal antibody is useful for detecting the N-terminally truncated Abeta produced in conjunction with caspase activation.

Reticulons RTN3 and RTN4-B/C interact with BACE1 and inhibit its ability to produce amyloid β-protein

Beta-secretase beta-site APP cleaving enzyme 1 (BACE1), is a membrane-bound aspartyl protease necessary for the generation of amyloid beta-protein (Abeta), which accumulates in the brains of individuals with Alzheimer's disease (AD). To gain insight into the mechanisms by which BACE1 activity is regulated, we used proteomic methods to search for BACE1-interacting proteins in human neuroblastoma SH-SY5Y cells, which overexpress BACE1. We identified reticulon 4-B (RTN4-B; Nogo-B) as a BACE1-associated membrane protein. Co-immunoprecipitation experiments confirmed a physical association between BACE1 and RTN4-B, RTN4-C (the shortest isoform of RTN-4), and their homologue reticulon 3 (RTN3), both in SH-SY5Y cells and in transfected human embryonic kidney (HEK) 293 cells. Overexpression of these reticulons (RTNs) resulted in a 30-50% reduction in the secretion of both Abeta40 and Abeta42 from HEK293 cells expressing the AD-associated Swedish mutant amyloid precursor protein (APP), but did not affect Abeta secretion from cells expressing the APP beta-C-terminal fragment (beta-CTF), indicating that these RTNs can inhibit BACE1 activity. Furthermore, a BACE1 mutant lacking most of the N-terminal ectodomain also interacted with these RTNs, suggesting that the transmembrane region of BACE1 is critical for the interaction. We also observed a similar interaction between these RTNs and the BACE1 homologue BACE2. Because RTN3 and RTN4-B/C are substantially expressed in neural tissues, our findings suggest that they play important roles in the regulation of BACE1 function and Abeta production in the brain.

Reconstitution of gamma-secretase by truncated presenilin (PS) fragments revealed that PS C-terminal transmembrane domain is critical for formation of gamma-secretase complex

The presenilin (PS) complex, including PS, nicastrin (NCT), APH-1 and PEN-2, is essential for gamma-secretase activity. Previously, the PS C-terminal tail was shown to be essential for gamma-secretase activity. Here, to further understand the precise mechanism underlying the activation of gamma-secretase regulated by PS cofactors, we focused on the role of the PS1 C-terminal region including transmembrane domain (TM) 8 in gamma-secretase activity. For this purpose, we co-expressed C-terminally truncated PS1 (PS1DeltaC) completely lacking gamma-secretase activity and the PS1 C-terminal short fragment in PS-null cells, because the successful reconstitution of gamma-secretase activity in PS-null cells by the co-expression of PS1DeltaC and the PS1 C-terminal short fragment would allow us to investigate the role of the PS1 C-terminal region in gamma-secretase activity. We found that the exogenous expression of the PS1 C-terminal short fragment with NCT and APH-1 completely rescued a defect of the gamma-secretase activity of PS1DeltaC in PS-null cells. With this reconstitution system, we demonstrate that both TM8 and the PS1 C-terminal seven-amino-acid-residue tail are involved in the formation of the active gamma-secretase complex via the assembly of PS1 with NCT and APH-1.

Altered localization of amyloid precursor protein under endoplasmic reticulum stress

Recent reports have shown that the endoplasmic reticulum (ER) stress is relevant to the pathogenesis of Alzheimer disease. Following the amyloid cascade hypothesis, we therefore attempted to investigate the effects of ER stress on amyloid-beta peptide (Abeta) generation. In this study, we found that ER stress altered the localization of amyloid precursor protein (APP) from late compartments to early compartments of the secretory pathway, and decreased the level of Abeta 40 and Abeta 42 release by beta- and gamma-cutting. Transient transfection with BiP/GRP78 also caused a shift of APP and a reduction in Abeta secretion. It was revealed that the ER stress response facilitated binding of BiP/GRP78 to APP, thereby causing it to be retained in the early compartments apart from a location suitable for the cleavages of Abeta. These findings suggest that induction of BiP/GRP78 during ER stress may be one of the regulatory mechanisms of Abeta generation.

Characterization of APH-1 Mutants With a Disrupted Transmembrane GxxxG Motif

APH-1 is one of the four essential components of presenilin (PS)-gamma-secretase complexes. There are three major isoforms of APH-1 in humans: APH-1aS, APH-1aL, and APH-1b. To gain insight into the functional role of APH-1 in gamma-secretase complexes, we analyzed the relationship between the three APH-1 forms and characterized APH-1 mutants with a disrupted transmembrane GxxxG motif. We found that overexpression of APH-1aS or APH-1b in human cells significantly reduced the levels of endogenous APH-1aL protein. However, this displacement was not observed in PS-deficient cells, suggesting that it is dependent on PS. In transiently transfected cells, the levels of APH-1aL with G122D or L123D mutations were much lower than wild-type APH-1aL. Also, cycloheximide treatment of stable transfectants revealed that the mutant proteins are much less stable than the wild type. Furthermore, coimmunoprecipitation analysis showed that wild-type but not the mutant APH-1aL is incorporated into PS1 complexes, displacing endogenous APH-1aS. These results collectively indicate that the three forms of APH-1 can replace each other in PS complexes and that the transmembrane GxxxG region is essential for the stability of the APH-1 protein as well as the assembly of PS complexes.

Extracellular release of BACE1 holoproteins from human neuronal cells

BACE1 is a membrane-bound aspartyl protease involved in production of the Alzheimer's amyloid beta-protein. The BACE1 ectodomain is partially cleaved to generate soluble BACE1, but the physiological significance of this event is unclear. During our characterization of BACE1 shedding from human neuroblastoma SH-SY5Y cells stably expressing BACE1, we unexpectedly found that detectable amounts of BACE1 holoproteins were released extracellularly along with soluble BACE1. Treatment with the metalloprotease inhibitor, TAPI-1, inhibited BACE1 shedding but increased BACE1 holoprotein release. Soluble and full-length BACE1 were released in parallel, at least partly originating from the plasma membrane. Furthermore, the release of soluble BACE1, but not full-length BACE1, was increased by deletion of the C-terminal dileucine motif, indicating that dysregulated BACE1 sorting affects BACE1 shedding. These findings suggest that the release of BACE1 holoproteins may be a physiologically relevant cellular process.

Homocysteic acid induces intraneuronal accumulation of neurotoxic Abeta42: implications for the pathogenesis of Alzheimer's disease

The causes of neuronal dysfunction and degeneration in Alzheimer's disease (AD) are not fully understood, but increased production of neurotoxic forms of amyloid beta-peptide-42 (Abeta42) seems of major importance. Large extracellular deposits of aggregated Abeta42 (plaques) is a diagnostic feature of AD, but Abeta42 may be particularly cytotoxic when it accumulates inside neurons. The factors that may promote the intracellular accumulation of Abeta42 in AD are unknown, but recent findings suggest that individuals with elevated homocysteine levels are at increased risk for AD. We show that homocysteic acid (HA), an oxidized metabolite of homocysteine, induces intraneuronal accumulation of a Abeta42 that is associated with cytotoxicity. The neurotoxicity of HA can be attenuated by an inhibitor of gamma-secretase, the enzyme activity that generates Abeta42, suggesting a key role for intracellular Abeta42 accumulation in the neurotoxic action of HA. Concentrations of HA in cerebrospinal fluid (CSF) were similar in AD and control subjects. CSF homocysteine levels were elevated significantly in AD patients, however, and homocysteine exacerbated HA-induced neurotoxicity, suggesting a role for HA in the pathogenic action of elevated homocysteine levels in AD. These findings suggest that the intracellular accumulation of Abeta42 plays a role in the neurotoxic action of HA, and suggest a potential therapeutic benefit of agents that modify the production and neurotoxic actions of HA and homocysteine.

Abnormal intracellular trafficking of high affinity nerve growth factor receptor, Trk, in stable transfectants expressing presenilin 1 protein

The pathogenesis of Alzheimer's disease (AD) is now thought to be tightly linked to Abeta deposition and oxidative stress, but it is still unknown how these factors result in neuronal dysfunction and cell death. Mutations of presenilin 1 (PS1) gene are the causative gene for early onset familial AD (FAD) due to the overproduction and deposition of pathogenic Abeta1-42 peptides. We report here the molecular influences of the overexpression of PS1 protein by stable transfection of PS1 cDNA into SH-SY5Y neuroblastoma cells on the function of high affinity nerve growth factor receptor, Trk, that is essential for neuronal survival and differentiation. We examined the sensitivity of these transfectants to oxidative stress and found that mutant (I143T) PS1-expressing clones showed the highest vulnerability to an oxidative stress inducer, hydrogen peroxide treatment compared with that of mock-transfected clones, whereas wild PS1-expressing cells were less vulnerable to the treatment than mutant PS1 transfectants. Because nerve growth factor (NGF) is known to protect neuronal cells from oxidative stress-induced cell death, we examined the NGF-Trk-mediated intracellular signaling pathway in these transfectants. In the wild and mutant PS1 cDNA-transfected cells, NGF did not elicit the autophosphorylation response of Trk, although their basal levels of tyrosine phosphorylation were higher than those of mock-transfected cells. Immunocytochemical and subcellular fractionation studies revealed that most of Trk proteins are abnormally located in the cytoplasm as well as in the nucleus in PS1-overexpressing clones irrespective of wild and mutant forms. These results strongly indicate that the expression level of PS1 protein has a cross talk with the Trk-dependent neuroprotective intracellular signaling pathway.

Amyloid β peptide binds a novel death‐inducing protein, AB‐DIP

Amyloid beta-peptide (Abeta) plays a central role in the pathogenesis of Alzheimer's disease (AD). It is toxic to neurons, but the mechanism for its action remains largely unknown. Here, we have identified a novel death-inducing protein, Abeta-related DIP (AB-DIP), by two-hybrid screening of the human brain cDNA library and confirmed the binding of Abeta with AB-DIP by coimmunoprecipitation. Overexpression of AB-DIP-induced cell death and coexpression of Abeta enhanced the cell death. During apoptosis, the 97-kDa AB-DIP was cleaved to a 62-kDa protein (AB-DIP p62) at the caspase cleavage site, LEKD. It is more important that cotransfection of Abeta with AB-DIP produced the AB-DIP p62 fragment. Small interfering RNA-mediated knockdown of AB-DIP protein expression significantly protected neuroblastoma cells from Abeta-induced neurotoxicity. AB-DIP may mediate the neurotoxicity of Abeta, and therefore, AB-DIP may be a potential, therapeutic target for AD.