Hypoxia-inducible factor 1alpha (HIF-1alpha)-mediated hypoxia increases BACE1 expression and beta-amyloid generation

The aging brain and cognition: contribution of vascular injury and aβ to mild cognitive dysfunction

IMPORTANCE

β-Amyloid (Aβ) deposition and vascular brain injury (VBI) frequently co-occur and are both associated with cognitive decline in aging. Determining whether a direct relationship exists between them has been challenging. We sought to understand VBI's influence on cognition and clinical impairment, separate from and in conjunction with pathologic changes associated with Alzheimer disease (AD).

OBJECTIVE

To examine the relationship between neuroimaging measures of VBI and brain Aβ deposition and their associations with cognition.

DESIGN AND SETTING

A cross-sectional study in a community- and clinic-based sample recruited for elevated vascular disease risk factors.

PARTICIPANTS

Clinically normal (mean age, 77.1 years [N = 30]), cognitively impaired (mean age, 78.0 years [N = 24]), and mildly demented (mean age, 79.8 years [N = 7]) participants.

INTERVENTIONS

Magnetic resonance imaging, Aβ (Pittsburgh Compound B-positron emission tomographic [PiB-PET]) imaging, and cognitive testing.

MAIN OUTCOME MEASURES

Magnetic resonance images were rated for the presence and location of infarct (34 infarct-positive participants, 27 infarct-negative participants) and were used to quantify white matter lesion volume. The PiB-PET uptake ratios were used to create a PiB index by averaging uptake across regions vulnerable to early Aβ deposition; PiB positivity (29 PiB-positive participants, 32 PiB-negative participants) was determined from a data-derived threshold. Standardized composite cognitive measures included executive function and verbal and nonverbal memory.

RESULTS

Vascular brain injury and Aβ were independent in both cognitively normal and impaired participants. Infarction, particularly in cortical and subcortical gray matter, was associated with lower cognitive performance in all domains (P < .05 for all comparisons). Pittsburgh Compound B positivity was neither a significant predictor of cognition nor interacted with VBI.

CONCLUSIONS AND RELEVANCE

In this elderly sample with normal cognition to mild dementia, enriched for vascular disease, VBI was more influential than Aβ in contemporaneous cognitive function and remained predictive after including the possible influence of Aβ. There was no evidence that VBI increases the likelihood of Aβ deposition. This finding highlights the importance of VBI in mild cognitive impairment and suggests that the impact of cerebrovascular disease should be considered with respect to defining the etiology of mild cognitive impairment.

Mild oxidative stress induces redistribution of BACE1 in non-apoptotic conditions and promotes the amyloidogenic processing of Alzheimer's disease amyloid precursor protein

BACE1 is responsible for β-secretase cleavage of the amyloid precursor protein (APP), which represents the first step in the production of amyloid β (Aβ) peptides. Previous reports, by us and others, have indicated that the levels of BACE1 protein and activity are increased in the brain cortex of patients with Alzheimer's disease (AD). The association between oxidative stress (OS) and AD has prompted investigations that support the potentiation of BACE1 expression and enzymatic activity by OS. Here, we have established conditions to analyse the effects of mild, non-lethal OS on BACE1 in primary neuronal cultures, independently from apoptotic mechanisms that were shown to impair BACE1 turnover. Six-hour treatment of mouse primary cortical cells with 10-40 µM hydrogen peroxide did not significantly compromise cell viability but it did produce mild oxidative stress (mOS), as shown by the increased levels of reactive radical species and activation of p38 stress kinase. The endogenous levels of BACE1 mRNA and protein were not significantly altered in these conditions, whereas a toxic H2O2 concentration (100 µM) caused an increase in BACE1 protein levels. Notably, mOS conditions resulted in increased levels of the BACE1 C-terminal cleavage product of APP, β-CTF. Subcellular fractionation techniques showed that mOS caused a major rearrangement of BACE1 localization from light to denser fractions, resulting in an increased distribution of BACE1 in fractions containing APP and markers for trans-Golgi network and early endosomes. Collectively, these data demonstrate that mOS does not modify BACE1 expression but alters BACE1 subcellular compartmentalization to favour the amyloidogenic processing of APP, and thus offer new insight in the early molecular events of AD pathogenesis.

Dysregulation of hypoxia-inducible factor by presenilin/γ-secretase loss-of-function mutations

Presenilin (PSEN) 1 and 2 are the catalytic components of the γ-secretase complex, which cleaves a variety of proteins, including the amyloid precursor protein (APP). Proteolysis of APP leads to the formation of the APP intracellular domain (AICD) and amyloid β that is crucially involved in the pathogenesis of Alzheimer's disease. Prolyl-4-hydroxylase-domain (PHD) proteins regulate the hypoxia-inducible factors (HIFs), the master regulators of the hypoxic response. We previously identified the FK506 binding protein 38 (FKBP38) as a negative regulator of PHD2. Genetic ablation of PSEN1/2 has been shown to increase FKBP38 protein levels. Therefore, we investigated the role of PSEN1/2 in the oxygen sensing pathway using a variety of genetically modified cell and mouse lines. Increased FKBP38 protein levels and decreased PHD2 protein levels were found in PSEN1/2-deficient mouse embryonic fibroblasts and in the cortex of forebrain-specific PSEN1/2 conditional double knock-out mice. Hypoxic HIF-1α protein accumulation and transcriptional activity were decreased, despite reduced PHD2 protein levels. Proteolytic γ-secretase function of PSEN1/2 was needed for proper HIF activation. Intriguingly, PSEN1/2 mutations identified in Alzheimer patients differentially affected the hypoxic response, involving the generation of AICD. Together, our results suggest a direct role for PSEN in the regulation of the oxygen sensing pathway via the APP/AICD cleavage cascade.

Exploring the contribution of estrogen to amyloid-Beta regulation: a novel multifactorial computational modeling approach

According to the amyloid hypothesis, Alzheimer Disease results from the accumulation beyond normative levels of the peptide amyloid-β (Aβ). Perhaps because of its pathological potential, Aβ and the enzymes that produce it are heavily regulated by the molecular interactions occurring within cells, including neurons. This regulation involves a highly complex system of intertwined normative and pathological processes, and the sex hormone estrogen contributes to it by influencing the Aβ-regulation system at many different points. Owing to its high complexity, Aβ regulation and the contribution of estrogen are very difficult to reason about. This report describes a computational model of the contribution of estrogen to Aβ regulation that provides new insights and generates experimentally testable and therapeutically relevant predictions. The computational model is written in the declarative programming language known as Maude, which allows not only simulation but also analysis of the system using temporal-logic. The model illustrates how the various effects of estrogen could work together to reduce Aβ levels, or prevent them from rising, in the presence of pathological triggers. The model predicts that estrogen itself should be more effective in reducing Aβ than agonists of estrogen receptor α (ERα), and that agonists of ERβ should be ineffective. The model shows how estrogen itself could dramatically reduce Aβ, and predicts that non-steroidal anti-inflammatory drugs should provide a small additional benefit. It also predicts that certain compounds, but not others, could augment the reduction in Aβ due to estrogen. The model is intended as a starting point for a computational/experimental interaction in which model predictions are tested experimentally, the results are used to confirm, correct, and expand the model, new predictions are generated, and the process continues, producing a model of ever increasing explanatory power and predictive value.

β-Secretase: its biology as a therapeutic target in diseases

β-Secretase (BACE1, β-site APP cleaving enzyme 1) is an aspartic proteinase that has multiple functions in various physiological processes, such as cell differentiation, immunoregulation, and cell death. There is increasing evidence that changes in BACE1 activity are involved in many diseases, such as Alzheimer's disease (AD), schizophrenia, epileptic behavior, and others. However, a deeper understanding of the molecular biology of BACE1 is necessary for further exploration of cell development, immunological regulation, and disease pathogenesis. Here, we review the molecular and cellular biology of BACE1, including its enzymatic properties, structure, biosynthesis, and physiological functions to provide a new perspective and rational assessment of drugability. Lastly, we discuss proposed strategies to control BACE1 activity for possible therapeutic application.

Death by a thousand cuts in Alzheimer's disease: hypoxia--the prodrome

A wide range of clinical consequences may be associated with obstructive sleep apnea (OSA) including systemic hypertension, cardiovascular disease, pulmonary hypertension, congestive heart failure, cerebrovascular disease, glucose intolerance, impotence, gastroesophageal reflux, and obesity, to name a few. Despite this, 82 % of men and 93 % of women with OSA remain undiagnosed. OSA affects many body systems, and induces major alterations in metabolic, autonomic, and cerebral functions. Typically, OSA is characterized by recurrent chronic intermittent hypoxia (CIH), hypercapnia, hypoventilation, sleep fragmentation, peripheral and central inflammation, cerebral hypoperfusion, and cerebral glucose hypometabolism. Upregulation of oxidative stress in OSA plays an important pathogenic role in the milieu of hypoxia-induced cerebral and cardiovascular dysfunctions. Strong evidence underscores that cerebral amyloidogenesis and tau phosphorylation--two cardinal features of Alzheimer's disease (AD), are triggered by hypoxia. Mice subjected to hypoxic conditions unambiguously demonstrated upregulation in cerebral amyloid plaque formation and tau phosphorylation, as well as memory deficit. Hypoxia triggers neuronal degeneration and axonal dysfunction in both cortex and brainstem. Consequently, neurocognitive impairment in apneic/hypoxic patients is attributable to a complex interplay between CIH and stimulation of several pathological trajectories. The framework presented here helps delineate the emergence and progression of cognitive decline, and may yield insight into AD neuropathogenesis. The global impact of CIH should provide a strong rationale for treating OSA and snoring clinically, in order to ameliorate neurocognitive impairment in aged/AD patients.

A review of the molecular aspects of melatonin's anti-inflammatory actions: recent insights and new perspectives

Melatonin is a highly evolutionary conserved endogenous molecule that is mainly produced by the pineal gland, but also by other nonendocrine organs, of most mammals including man. In the recent years, a variety of anti-inflammatory and antioxidant effects have been observed when melatonin is applied exogenously under both in vivo and in vitro conditions. A number of studies suggest that this indole may exert its anti-inflammatory effects through the regulation of different molecular pathways. It has been documented that melatonin inhibits the expression of the isoforms of inducible nitric oxide synthase and cyclooxygenase and limits the production of excessive amounts of nitric oxide, prostanoids, and leukotrienes, as well as other mediators of the inflammatory process such as cytokines, chemokines, and adhesion molecules. Melatonin's anti-inflammatory effects are related to the modulation of a number of transcription factors such as nuclear factor kappa B, hypoxia-inducible factor, nuclear factor erythroid 2-related factor 2, and others. Melatonin's effects on the DNA-binding capacity of transcription factors may be regulated through the inhibition of protein kinases involved in signal transduction, such as mitogen-activated protein kinases. This review summarizes recent research data focusing on the modulation of the expression of different inflammatory mediators by melatonin and the effects on cell signaling pathways responsible for the indole's anti-inflammatory activity. Although there are a numerous published reports that have analyzed melatonin's anti-inflammatory properties, further studies are necessary to elucidate its complex regulatory mechanisms in different cellular types and tissues.

Alzheimer's β-secretase (BACE1) regulates the cAMP/PKA/CREB pathway independently of β-amyloid

β-Amyloid protein (Aβ), the major component of neuritic plaques in Alzheimer's disease (AD), is derived from proteolytic cleavages of the amyloid precursor protein (APP) by β-site APP-cleaving enzyme 1 (BACE1) and the γ-secretase complex. BACE1 is the rate-limiting enzyme for Aβ production, and an increase in BACE1 level/activity contributes to the pathogenesis of sporadic AD. In addition to cleaving APP for Aβ generation, BACE1 plays multiple physiological roles including the regulation of synaptic functions. Here, we found that overexpression of BACE1 reduces cAMP response element binding protein (CREB) phosphorylation, protein kinase A (PKA) activity, and cAMP levels, whereas downregulation of BACE1 has the opposite effect. We showed that BACE1's effect is independent of its activity for Aβ production, which is corroborated by the observation that BACE1 transgenic mice have impaired learning/memory in the absence of neurotoxic human Aβ. Furthermore, we demonstrated that BACE1 interacts via its transmembrane domain with adenylate cyclase, resulting in reduction of cellular cAMP levels and thus PKA inactivation and reduced CREB phosphorylation. Our study suggests that in addition to its function as the β-secretase to produce Aβ, BACE1 may contribute to the memory and cognitive deficits typical of AD by regulating the cAMP/PKA/CREB pathway, which is important for memory functions.

BACE1 levels are elevated in congestive heart failure

Cardiovascular (CV) diseases are known to have a negative impact on the brain and neurocognition, and contribute to the development of vascular dementia and neurodegenerative diseases such as Alzheimer's disease (AD). Among CV diseases, congestive heart failure (CHF) after myocardial infarction (MI) is a condition where the ability of the left ventricle to eject blood to the circulation is impaired. As a consequence, CHF triggers inflammation and results in reduced cerebral blood flow which are considered among the risk factors for development of AD. However, biochemical alterations in the brain following MI and CHF remain unknown. To address this issue, we investigated microglia activation; levels of BACE1, the key rate-limiting enzyme involved in the pathogenesis of AD; and VEGF levels in the hippocampus and cortex following MI. We created MI by the ligation of the left anterior descending coronary artery in Sprague-Dawley male rats and collected brains either 3 days after MI (AMI) or 21 days after MI (CHF). We investigated microglia activation in AMI and CHF brains by immunohistochemistry and immunoblotting using macrophage/microglia marker Ionized calcium binding adaptor molecule 1 (Iba-1), and observed activated morphology of microglia in the cortex of rats in both AMI and CHF. We also showed the levels of BACE1 were increased in the cortex and hippocampus of CHF rats. To determine whether hypoxia occurs in the CHF brain, we assessed levels of VEGF in the hippocampus and cortex. Western blotting analysis showed up-regulation of VEGF in the hippocampus of CHF brains. These results suggest that neuroinflammation takes place secondary to myocardial infarction. In addition, CHF-induced hypoxia might play a role in the elevation of BACE1 and VEGF levels.

Cardiovascular risk factors, cortisol, and amyloid-β deposition in Alzheimer's Disease Neuroimaging Initiative

BACKGROUND

There is epidemiological evidence that cardiovascular risk factors (CVRF) also are risk factors for Alzheimer's disease, but there is limited information on this from neuropathological studies, and even less from in vivo studies. Therefore, we examined the relationship between CVRF and amyloid-β (Aβ) brain burden measured by Pittsburgh Compound B-positron emission tomography (PiB-PET) studies in the Alzheimer's Disease Neuroimaging Initiative.

METHODS

Ninety-nine subjects from the Alzheimer's Disease Neuroimaging Initiative cohort who had a PiB-PET study measure, apolipoprotein E genotyping data, and information available on CVRF (body mass index [BMI], systolic blood pressure, diastolic blood pressure [DBP], and cholesterol and fasting glucose test results) were included. Eighty-one subjects also had plasma cortisol, C-reactive protein, and superoxide dismutase 1 measurements. Stepwise regression models were used to assess the relation between the CVRF and the composite PiB-PET score.

RESULTS

The first model included the following as baseline variables: age, clinical diagnosis, number of apolipoprotein ɛ4 alleles, BMI (P = .023), and DBP (P = .012). BMI showed an inverse relation with PiB-PET score, and DBP had a positive relation with PiB-PET score. In the second adjusted model, cortisol plasma levels were also associated with PiB-PET score (P = .004). Systolic blood pressure, cholesterol, or impaired fasting glucose were not found to be associated with PiB-PET values.

CONCLUSION

In this cross-sectional study, we found an association between Aβ brain burden measured in vivo and DBP and cortisol, indicating a possible link between these CVRF and Aβ burden measured by PiB-PET. These findings highlight the utility of biomarkers to explore potential pathways linking diverse Alzheimer's disease risk factors.

Brain ischemia activates β- and γ-secretase cleavage of amyloid precursor protein: significance in sporadic Alzheimer's disease

Amyloid precursor protein cleavage through β- and γ-secretases produces β-amyloid peptide, which is believed to be responsible for death of neurons and dementia in Alzheimer’s disease. Levels of β- and γ-secretase are increased in sensitive areas of the Alzheimer’s disease brain, but the mechanism of this process is unknown. In this review, we prove that brain ischemia generates expression and activity of both β- and γ-secretases. These secretases are induced in association with oxidative stress following brain ischemia. Data suggest that ischemia promotes overproduction and aggregation of β-amyloid peptide in brain, which is toxic for ischemic neuronal cells. In our review, we demonstrated the role of brain ischemia as a molecular link between the β- and the γ-secretase activities and provided a molecular explanation of the possible neuropathogenesis of sporadic Alzheimer’s disease.

BACE1 is at the crossroad of a toxic vicious cycle involving cellular stress and β-amyloid production in Alzheimer's disease

Alzheimer’s disease (AD) is a complex age-related pathology, the etiology of which has not been firmly delineated. Among various histological stigmata, AD-affected brains display several cellular dysfunctions reflecting enhanced oxidative stress, inflammation process and calcium homeostasis disturbance. Most of these alterations are directly or indirectly linked to amyloid β-peptides (Aβ), the production, molecular nature and biophysical properties of which likely conditions the degenerative process. It is particularly noticeable that, in a reverse control process, the above-described cellular dysfunctions alter Aβ peptides levels. β-secretase βAPP-cleaving enzyme 1 (BACE1) is a key molecular contributor of this cross-talk. This enzyme is responsible for the primary cleavage generating the N-terminus of “full length” Aβ peptides and is also transcriptionally induced by several cellular stresses. This review summarizes data linking brain insults to AD-like pathology and documents the key role of BACE1 at the cross-road of a vicious cycle contributing to Aβ production.

Vascular factors and epigenetic modifications in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a debilitating illness with no known cure. Nowadays accumulating evidence suggested that the vascular endothelium and chronic hypoperfusion may play important role in pathobiology of AD. The vascular endothelium which regulates the passage of macromolecules and circulating cells from blood to tissue, is a major target of oxidative stress, playing a critical role in the pathophysiology of vascular diseases. Since the vascular endothelium, neurons and glia are all able to synthesize, store and release reactive oxygen species (ROS) and vascular active substances in response to certain stimuli, their contribution to the pathophysiology of AD can be very important. New evidence indicates that continuous formation of free ROS induces cellular damage and decreases antioxidant defenses. Specifically, oxidative stress increases vascular endothelial permeability and promotes leukocyte adhesion. We summarize the reports that sporadic, late-onset of AD results from vascular etiology. Recently an involvement of epigenetic alterations in the etiology of AD is also intensively investigated. Gaining a more complete understanding of the essential components and underlying mechanisms involved in epigenetic regulation could lead to novel treatments for a number of neurological and psychiatric conditions.

Mutant amyloid precursor protein differentially alters adipose biology under obesogenic and non-obesogenic conditions

Mutations in amyloid precursor protein (APP) have been most intensely studied in brain tissue for their link to Alzheimer’s disease (AD) pathology. However, APP is highly expressed in a variety of tissues including adipose tissue, where APP is also known to exhibit increased expression in response to obesity. In our current study, we analyzed the effects of mutant APP (E693Q, D694N, K670N/M671L) expression toward multiple aspects of adipose tissue homeostasis. These data reveal significant hypoleptinemia, decreased adiposity, and reduced adipocyte size in response to mutant APP, and this was fully reversed upon high fat diet administration. Additionally, mutant APP was observed to significantly exacerbate insulin resistance, triglyceride elevations, and macrophage infiltration of adipose tissue in response to a high fat diet. Taken together, these data have significant implications for linking mutant APP expression to adipose tissue dysfunction and global changes in endocrine and metabolic function under both obesogenic and non-obesogenic conditions.

From mitochondrial dysfunction to amyloid beta formation: novel insights into the pathogenesis of Alzheimer's disease

The non-Mendelian sporadic Alzheimer's disease (AD) is the most frequent form of dementia diagnosed worldwide. The most important risk factor to develop sporadic AD is aging itself. Next to hyperphosphorylated Tau, intracellular amyloid beta (Aß) oligomers are known to initiate a cascade of pathological events ranging from mitochondrial dysfunction, synaptic dysfunction, oxidative stress, and loss of calcium regulation, to inflammation. All these events are considered to play an important role in the progressive loss of neurons. The molecular mechanisms determining the balance between Aß production and clearance during the progression of the disease are not well understood. Furthermore, there is cumulating evidence that Aß formation impairs mitochondrial function and that mitochondrial dysfunction is an early event in the pathogenesis of AD. On the other hand, mitochondrial dysfunction, in particular increased formation of mitochondrially derived reactive oxygen species, promote Aß formation. Here, we review these latest findings linking mitochondrial dysfunction and Aß formation. We propose that mitochondrial dysfunction, which is well-known to increase with age, is an initial trigger for Aß production. As Aß itself further accelerates mitochondrial dysfunction and oxidative stress, its formation is self-stimulated. Taken together, a vicious cycle is initiated that originates from mitochondrial dysfunction, implying that AD can be viewed as an age-associated mitochondrial disorder. The proposed mechanism sheds new light on the pathophysiological changes taking place during the progression of AD as well as in the aging process.

Relationships between the amyloid precursor protein and its various proteolytic fragments and neuronal systems

Alzheimer's disease (AD) is a progressive neurodegenerative disease and in its familial form is associated with mutations in the amyloid precursor protein (APP) and the presenilins (PSs). Much data regarding the interactions of APP, its proteolytic fragments and PS have been generated, expanding our understanding of the roles of these proteins in mechanisms underlying cognitive function and revealing many complex relationships with wide ranging cellular systems. In this review, we examine the multiple interactions of APP and its proteolytic fragments with other neuronal systems in terms of feedback loops and use these relationships to build a map. We highlight the complexity involved in the APP proteolytic system and discuss alternative perspectives on the roles of APP and its proteolytic fragments in dynamic processes associated with disease progression in AD. We highlight areas where data are missing and suggest potential confounding factors. We suggest that a systems biology approach enhances representations of the data and may be more useful in modelling both normal cognition and disease processes.

Mitochondria-targeted catalase reduces abnormal APP processing, amyloid β production and BACE1 in a mouse model of Alzheimer's disease: implications for neuroprotection and lifespan extension

The purpose of this study was to investigate the protective effects of the mitochondria-targeted antioxidant catalase (MCAT) and lifespan extension in mice that express amyloid beta (Aβ). Using immunoblotting and immunostaining analyses, we measured the production of full-length amyloid precursor protein (APP), soluble APPα, C-terminal fragments CTF99 and CTF83, monomeric and oligomeric Aβ, Aβ deposits and beta site amyloid precursor protein cleaving enzyme 1 (BACE1), in different stages of disease progression in MCAT/AβPP and AβPP mice. Using quantitative reverse transcriptase polymerase chain reaction and immunostaining analyses, we studied the expression of catalase, BACE1, the Alzheimer's disease (AD) markers, synaptophysin, APP, neprilysin, insulin-degrading enzyme and transthyretin in MCAT, AβPP, MCAT/AβPP and wild-type (WT) mice. Using the high pressure liquid chromatography analysis of 8-hydroxy-2-deoxyguanosine, we measured oxidative DNA damage in the cerebral cortical tissues from MCAT, AβPP, MCAT/AβPP and WT mice. We found that the AβPP transgenic mice that carried the human MCAT gene lived 5 months longer than did the AβPP mice. We also found that the overexpression of MCAT in the brain sections from the MCAT/AβPP transgenic mice significantly correlated with a reduction in the levels of full-length APP, CTF99, BACE1, Aβ levels (40 and 42), Aβ deposits and oxidative DNA damage relative to the brain sections from the AβPP mice. Interestingly, we found significantly increased levels of soluble APPα and CTF83 in the MCAT/AβPP mice, relative to the AβPP mice. These data provide direct evidence that oxidative stress plays a primary role in AD etiopathology and that in MCAT mice express Aβ, MCAT prevents abnormal APP processing, reduces Aβ levels and enhances Aβ-degrading enzymes in mice at different ages, corresponding to different stages of disease progression. These findings indicate that mitochondria-targeted molecules may be an effective therapeutic approach to treat patients with AD.

Natural polyphenols against neurodegenerative disorders: potentials and pitfalls

Within the last years, a rapidly growing number of polyphenolic compounds with neuroprotective effects have been described. Many efforts have been made to explore the mechanisms behind the neuroprotective action of polyphenols. However, many pathways and mechanisms considered for mediating these effects are rather general than specific. Moreover, despite the beneficial effects of polyphenols in experimental treatment of neurodegeneration, little has been achieved in bringing them into routine clinical applications. In this review, we have summarized the protective effects of polyphenols against neurodegeneration, and we have also discussed some of the barricades in translating these biochemical compounds, into relevant therapeutics for neurodegenerative diseases.

Inhibitory effect of ethanol extract of Magnolia officinalis on memory impairment and amyloidogenesis in a transgenic mouse model of Alzheimer's disease via regulating β-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized by deposition of amyloid beta (Aβ) in the brain. The components of the herb Magnolia officinalis are known to have antiinflammatory, antioxidative and neuroprotective activities. In this study we investigated the effects of ethanol extract of M. officinalis on memory dysfunction and amyloidogenesis in a transgenic mouse model of AD. Oral pretreatment of ethanol extract of M. officinalis (10 mg/kg in 0.05% ethanol) into drinking water for 3 months inhibited memory impairment and Aβ deposition in the brain of Tg2576 mice. Ethanol extract of M. officinalis also decreased activity of β-secretase, cleaving Aβ from amyloid precursor protein (APP), and expression of β-site APP cleaving enzyme 1 (BACE1), APP and its product, C99. Our results showed that ethanol extract of M. officinalis effectively prevented memory impairment via down-regulating β-secretase activity.

Ginkgo biloba protects against intermittent hypoxia-induced memory deficits and hippocampal DNA damage in rats

The aim of the present study was to explore the potential protective effect of Ginkgo biloba extract (EGb 761) on intermittent hypoxia (IH)-induced memory deficits and oxidative stress in rats.

METHODS

The passive avoidance reflex (PAR) test was employed to assess the effect of concurrent EGb 761 treatment in different dose levels on the memory deficits that were induced by concurrent long-term exposure to IH (21 days). The levels of hippocampal malondialdehyde (MDA), nitric oxide (NO), and intracellular glutathione (GSH) and the activity of glutathione peroxidase (GSH-Px) were estimated. In addition, serum and hippocampal 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels were assessed to study the effect of EGb 761 on hippocampal oxidative DNA damage induced by IH.

RESULTS

Exposure to long-term IH in rats induced marked memory impairment that was indicated by a significant decrease in the retention latency in the PAR test. This effect was accompanied by a significant increase in hippocampal oxidative stress and DNA damage. EGb 761 that was administered in either 50- or 100-mg/kg doses per day reversed IH-induced memory deficits, an effect that was accompanied by a significant decrease in hippocampal MDA and NO levels. The antioxidant defence (GSH and GSH-Px) that was depressed by IH was significantly reactivated by EGb 761. Furthermore, serum and hippocampal levels of 8-OHdG that were elevated by IH were significantly reduced.

CONCLUSIONS

EGb 761 can protect against IH-induced memory impairment, oxidative stress and neuronal DNA damage, possibly through multiple mechanisms involving its potential anti-oxidative effect.

Alzheimer's disease: pathological mechanisms and the beneficial role of melatonin

Alzheimer's disease (AD) is a highly complex neurodegenerative disorder of the aged that has multiple factors which contribute to its etiology in terms of initiation and progression. This review summarizes these diverse aspects of this form of dementia. Several hypotheses, often with overlapping features, have been formulated to explain this debilitating condition. Perhaps the best-known hypothesis to explain AD is that which involves the role of the accumulation of amyloid-β peptide in the brain. Other theories that have been invoked to explain AD and summarized in this review include the cholinergic hypothesis, the role of neuroinflammation, the calcium hypothesis, the insulin resistance hypothesis, and the association of AD with peroxidation of brain lipids. In addition to summarizing each of the theories that have been used to explain the structural neural changes and the pathophysiology of AD, the potential role of melatonin in influencing each of the theoretical processes involved is discussed. Melatonin is an endogenously produced and multifunctioning molecule that could theoretically intervene at any of a number of sites to abate the changes associated with the development of AD. Production of this indoleamine diminishes with increasing age, coincident with the onset of AD. In addition to its potent antioxidant and anti-inflammatory activities, melatonin has a multitude of other functions that could assist in explaining each of the hypotheses summarized above. The intent of this review is to stimulate interest in melatonin as a potentially useful agent in attenuating and/or delaying AD.

Traumatic brain injury: a risk factor for Alzheimer's disease

Traumatic brain injury (TBI) constitutes a major global health and socio-economic problem with neurobehavioral sequelae contributing to long-term disability. It causes brain swelling, axonal injury and hypoxia, disrupts blood brain barrier function and increases inflammatory responses, oxidative stress, neurodegeneration and leads to cognitive impairment. Epidemiological studies show that 30% of patients, who die of TBI, have Aβ plaques which are pathological features of Alzheimer's disease (AD). Thus TBI acts as an important epigenetic risk factor for AD. This review focuses on AD related genes which are expressed during TBI and its relevance to progression of the disease. Such understanding will help to diagnose the risk of TBI patients to develop AD and design therapeutic interventions.

The Membrane-Bound Aspartyl Protease BACE1: Molecular and Functional Properties in Alzheimer's Disease and Beyond

The β-site APP cleaving enzyme 1 (BACE1) is a transmembrane aspartyl protease involved in Alzheimer’s disease (AD) pathogenesis and in myelination. BACE1 initiates the generation of the pathogenic amyloid β-peptide, which makes BACE1 a major drug target for AD. BACE1 also cleaves and activates neuregulin 1, thereby contributing to postnatal myelination, in particular in the peripheral nervous system. Additional proteins are also cleaved by BACE1, but less is known about the physiological consequences of their cleavage. Recently, new phenotypes were described in BACE1-deficient mice. Although it remains unclear through which BACE1 substrates they are mediated, the phenotypes suggest a versatile role of this protease for diverse physiological processes. This review summarizes the enzymatic and cellular properties of BACE1 as well as its regulation by lipids, by transcriptional, and by translational mechanisms. The main focus will be on the recent progress in understanding BACE1 function and its implication for potential mechanism-based side effects upon therapeutic inhibition.

Forebrain-dominant deficit in cerebrovascular reactivity in Alzheimer's disease

Epidemiologic evidence and postmortem studies of cerebral amyloid angiopathy suggest that vascular dysfunction may play an important role in the pathogenesis of Alzheimer's disease (AD). However, alterations in vascular function under in vivo conditions are poorly understood. In this study, we assessed cerebrovascular-reactivity (CVR) in AD patients and age-matched controls using CO(2)-inhalation while simultaneously acquiring Blood-Oxygenation-Level-Dependent (BOLD) MR images. Compared with controls, AD patients had widespread reduction in CVR in the rostral brain including prefrontal, anterior cingulate, and insular cortex (p < 0.01). The deficits could not be explained by cardiovascular risk factors. The spatial distribution of the CVR deficits differed drastically from the regions of cerebral blood flow (CBF) deficits, which were found in temporal and parietal cortices. Individuals with greater CVR deficit tended to have a greater volume of leukoaraiosis as seen on FLAIR MRI (p = 0.004). Our data suggest that early AD subjects have evidence of significant forebrain vascular contractility deficits. The localization, while differing from CBF findings, appears to be spatially similar to PIB amyloid imaging findings.

Road to Alzheimer's disease: the pathomechanism underlying

Alzheimer's disease (AD), the most common cause of dementia, results from the interplay of various deregulated mechanisms triggering a complex pathophysiology. The neurons suffer from and slowly succumb to multiple irreversible damages, resulting in cell death and thus memory deficits that characterize AD. In spite of our vast knowledge, it is still unclear as to when the disease process starts and how long the perturbations continue before the disease manifests. Recent studies provide sufficient evidence to prove amyloid β (Aβ) as the primary cause initiating secondary events, but Aβ is also known to be produced under normal conditions and to possess physiological roles, hence, the questions that remain are: What are the factors that lead to abnormal Aβ production? When does Aβ turn into a pathological molecule? What is the chain of events that follows Aβ? The answers are still under debate, and further insight may help us in creating better diagnostic and therapeutic options in AD. The present article attempts to review the current literature regarding AD pathophysiology and proposes a pathophysiologic cascade in AD.

Regulation of β-site APP-cleaving enzyme 1 gene expression and its role in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder leading to dementia. Neuritic plaques are the hallmark neuropathology in AD brains. Proteolytic processing of amyloid-β precursor protein at the β site by beta-site amyloid-β precursor protein-cleaving enzyme 1 (BACE1) is essential to generate Aβ, a central component of the neuritic plaques. BACE1 is increased in some sporadic AD brains, and dysregulation of BACE1 gene expression plays an important role in AD pathogenesis. This review will focus on the regulation of BACE1 gene expression at the transcriptional, post-transcriptional, translation initiation, translational and post-translational levels, and its role in AD pathogenesis. Further studies on BACE1 gene expression regulation will greatly contribute to our understanding of AD pathogenesis and reveal potential novel approaches for AD prevention and drug development.

Neurovascular pathways to neurodegeneration in Alzheimer's disease and other disorders

The neurovascular unit (NVU) comprises brain endothelial cells, pericytes or vascular smooth muscle cells, glia and neurons. The NVU controls blood-brain barrier (BBB) permeability and cerebral blood flow, and maintains the chemical composition of the neuronal 'milieu', which is required for proper functioning of neuronal circuits. Recent evidence indicates that BBB dysfunction is associated with the accumulation of several vasculotoxic and neurotoxic molecules within brain parenchyma, a reduction in cerebral blood flow, and hypoxia. Together, these vascular-derived insults might initiate and/or contribute to neuronal degeneration. This article examines mechanisms of BBB dysfunction in neurodegenerative disorders, notably Alzheimer's disease, and highlights therapeutic opportunities relating to these neurovascular deficits.

Cerebrovascular disease, β-amyloid, and cognition in aging

The present study evaluated cerebrovascular disease (CVD), β-amyloid (Aβ), and cognition in clinically normal elderly adults. Fifty-four participants underwent magnetic resonance imaging (MRI), Pittsburgh compound B (PIB)-positron emission tomography (PET) imaging, and neuropsychological evaluation. High white matter hyperintensity burden and/or presence of infarct defined CVD status (CVD-: n = 27; CVD+: n = 27). PIB-positron emission tomography ratios of Aβ deposition were extracted using Logan plotting (cerebellar reference). Presence of high levels of Aβ in prespecified regions determined PIB status (PIB-: n = 33; PIB+: n = 21). Executive functioning and episodic memory were measured using composite scales. CVD and Aβ, defined as dichotomous or continuous variables, were unrelated to one another. CVD+ participants showed lower executive functioning (p = 0.001) when compared with CVD- individuals. Neither PIB status nor amount of Aβ affected cognition (ps ≥ 0.45), and there was no statistical interaction between CVD and PIB on either cognitive measure. Within this spectrum of normal aging CVD and Aβ aggregation appear to be independent processes with CVD primarily affecting cognition.

Neurobiology of vascular dementia

Vascular dementia is, in its current conceptual form, a distinct type of dementia with a spectrum of specific clinical and pathophysiological features. However, in a very large majority of cases, these alterations occur in an already aged brain, characterized by a milieu of cellular and molecular events common for different neurodegenerative diseases. The cell signaling defects and molecular dyshomeostasis might lead to neuronal malfunction prior to the death of neurons and the alteration of neuronal networks. In the present paper, we explore some of the molecular mechanisms underlying brain malfunction triggered by cerebrovascular disease and risk factors. We suggest that, in the age of genetic investigation and molecular diagnosis, the concept of vascular dementia needs a new approach.

Regulation of β-amyloid level in the brain of rats with cerebrovascular hypoperfusion

Cerebrovascular hypoperfusion occurs prior to clinical symptoms of Alzheimer's disease (AD) and represents the most accurate indicator predicting whether an individual develops AD in a future time. In order to explore the contribution of cerebrovascular hypoperfusion to AD, cerebrovascular hypoperfusion induced by bilateral carotid occlusion surgery in adult rats was used to investigate its impacts on spatial memory, amyloid-β protein (Aβ) production and clearance in the brain. The progressive spatial memory deficits were observed through Morris water maze test of the rats with cerebrovascular hypoperfusion induced by occlusion surgery. The memory deficits were accompanied with the increase of brain Aβ associated with Aβ overproduction due to the increased expression of β-amyloid precursor protein (APP) and enhanced activities of amyloid precursor protein cleavage enzymes such as β- and γ-secretases. Western blot and immunohistochemisty studies further revealed that cerebrovascular hypoperfusion could induce abnormal expression of β-amyloid receptor proteins including the receptor for advanced glycation end products (RAGE) and low-density lipoprotein receptor-related protein-1 (LRP-1), and result in a shift of immunoreactivity between neurons and vasculatures. Taken together, our results suggested that chronic cerebrovascular hypoperfusion could cause memory impairment and Aβ accumulation in brain associated with increased generation and impaired clearance of Aβ. Cerebrovascular hypoperfusion plays an important role in the pathogenesis and development of AD.

Regulation of β cleavage of amyloid precursor protein

Alzheimer's disease ranks the first cause for senile dementia. The amyloid cascade is proposed to contribute to the pathogenesis of this disease. In this cascade, amyloid β peptide (Aβ) is produced through a sequential cleavage of amyloid precursor protein (APP) by β and γ secretases, while its cleavage by α secretase precludes Aβ production and generates neurotrophic sAPPα. Thus, enhancing α secretase activity or suppressing β and γ cleavage may reduce Aβ formation and ameliorate the pathological process of the disease. Several regulatory mechanisms of APP cleavage have been established. The present review mainly summarizes the signaling pathways pertinent to the regulation of APP β cleavage.

The Basic Biology of BACE1: A Key Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.

Activation of PKR causes amyloid ß-peptide accumulation via de-repression of BACE1 expression

BACE1 is a key enzyme involved in the production of amyloid ß-peptide (Aß) in Alzheimer's disease (AD) brains. Normally, its expression is constitutively inhibited due to the presence of the 5′untranslated region (5′UTR) in the BACE1 promoter. BACE1 expression is activated by phosphorylation of the eukaryotic initiation factor (eIF)2-alpha, which reverses the inhibitory effect exerted by BACE1 5′UTR. There are four kinases associated with different types of stress that could phosphorylate eIF2-alpha. Here we focus on the double-stranded (ds) RNA-activated protein kinase (PKR). PKR is activated during viral infection, including that of herpes simplex virus type 1 (HSV1), a virus suggested to be implicated in the development of AD, acting when present in brains of carriers of the type 4 allele of the apolipoprotein E gene. HSV1 is a dsDNA virus but it has genes on both strands of the genome, and from these genes complementary RNA molecules are transcribed. These could activate BACE1 expression by the PKR pathway. Here we demonstrate in HSV1-infected neuroblastoma cells, and in peripheral nervous tissue from HSV1-infected mice, that HSV1 activates PKR. Cloning BACE1 5′UTR upstream of a luciferase (luc) gene confirmed its inhibitory effect, which can be prevented by salubrinal, an inhibitor of the eIF2-alpha phosphatase PP1c. Treatment with the dsRNA analog poly (I∶C) mimicked the stimulatory effect exerted by salubrinal over BACE1 translation in the 5′UTR-luc construct and increased Aß production in HEK-APPsw cells. Summarizing, our data suggest that PKR activated in brain by HSV1 could play an important role in the development of AD.

AMP-activated protein kinase: a potential player in Alzheimer's disease

AMP-activated protein kinase (AMPK) stimulates energy production via glucose and lipid metabolism, whereas it inhibits energy consuming functions, such as protein and cholesterol synthesis. Increased cytoplasmic AMP and Ca(2+) levels are the major activators of neuronal AMPK signaling. Interestingly, Alzheimer's disease (AD) is associated with several abnormalities in neuronal energy metabolism, for example, decline in glucose uptake, mitochondrial dysfunctions and defects in cholesterol metabolism, and in addition, with problems in maintaining Ca(2+) homeostasis. Epidemiological studies have also revealed that many metabolic and cardiovascular diseases are risk factors for cognitive impairment and sporadic AD. Emerging studies indicate that AMPK signaling can regulate tau protein phosphorylation and amyloidogenesis, the major hallmarks of AD. AMPK is also a potent activator of autophagic degradation which seems to be suppressed in AD. All these observations imply that AMPK is involved in the pathogenesis of AD. However, the responses of AMPK activation are dependent on stimulation and the extent of activating stress. Evidently, AMPK signaling can repress and delay the appearance of AD pathology but later on, with increasing neuronal stress, it can trigger detrimental effects that augment AD pathogenesis. We will outline the potential role of AMPK function in respect to various aspects affecting AD pathogenesis.

The Alzheimer's amyloid β-peptide (Aβ) binds a specific DNA Aβ-interacting domain (AβID) in the APP, BACE1, and APOE promoters in a sequence-specific manner: characterizing a new regulatory motif

Deposition of extracellular plaques, primarily consisting of amyloid β peptide (Aβ), in the brain is the confirmatory diagnostic of Alzheimer's disease (AD); however, the physiological and pathological role of Aβ is not fully understood. Herein, we demonstrate novel Aβ activity as a putative transcription factor upon AD-associated genes. We used oligomers from 5'-flanking regions of the apolipoprotein E (APOE), Aβ-precursor protein (APP) and β-amyloid site cleaving enzyme-1 (BACE1) genes for electrophoretic mobility shift assay (EMSA) with different fragments of the Aβ peptide. Our results suggest that Aβ bound to an Aβ-interacting domain (AβID) with a consensus of "KGGRKTGGGG". This peptide-DNA interaction was sequence specific, and mutation of the first "G" of the decamer's terminal "GGGG" eliminated peptide-DNA interaction. Furthermore, the cytotoxic Aβ25-35 fragment had greatest DNA affinity. Such specificity of binding suggests that the AβID is worth of further investigation as a site wherein the Aβ peptide may act as a transcription factor.

Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system

β-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is an aspartyl protease best known for its role in generating the amyloid-β peptides that are present in plaques of Alzheimer's disease. BACE1 has been an attractive target for drug development. In cultured embryonic neurons, BACE1-cleaved N-terminal APP is further processed to generate a fragment that can trigger axonal degeneration, suggesting a vital role for BACE1 in axonal health. In addition, BACE1 cleaves neuregulin 1 type III, a protein critical for myelination of peripheral axons by Schwann cells during development. Here, we asked whether axonal degeneration or axonal regeneration in adult nerves might be affected by inhibition or elimination of BACE1. We report that BACE1 knock-out and wild-type nerves degenerated at a similar rate after axotomy and to a similar extent in the experimental neuropathies produced by administration of paclitaxel and acrylamide. These data indicate N-APP is not the sole culprit in axonal degeneration in adult nerves. Unexpectedly, however, we observed that BACE1 knock-out mice had markedly enhanced clearance of axonal and myelin debris from degenerated fibers, accelerated axonal regeneration, and earlier reinnervation of neuromuscular junctions, compared with littermate controls. These observations were reproduced in part by pharmacological inhibition of BACE1. These data suggest BACE1 inhibition as a therapeutic approach to accelerate regeneration and recovery after peripheral nerve damage.

Hypoxia-induced down-regulation of neprilysin by histone modification in mouse primary cortical and hippocampal neurons

Amyloid β-peptide (Aβ) accumulation leads to neurodegeneration and Alzheimer's disease (AD). Aβ metabolism is a dynamic process in the Aβ production and clearance that requires neprilysin (NEP) and other enzymes to degrade Aβ. It has been reported that NEP expression is significantly decreased in the brain of AD patients. Previously we have documented hypoxia is a risk factor for Aβ generation in vivo and in vitro through increasing Aβ generation by altering β-cleavage and γ-cleavage of APP and down-regulating NEP, and causing tau hyperphosphorylation. Here, we investigated the molecular mechanisms of hypoxia-induced down-regulation of NEP. We found a significant decrease in NEP expression at the mRNA and protein levels after hypoxic treatment in mouse primary cortical and hippocampal neurons. Chromatin immunoprecipitation (ChIP) assays and relative quantitative PCR (q-PCR) revealed an increase of histone H3-lysine9 demethylation (H3K9me2) and a decrease of H3 acetylation (H3-Ace) in the NEP promoter regions following hypoxia. In addition, we found that hypoxia caused up-regulation of histone methyl transferase (HMT) G9a and histone deacetylases (HDACs) HDAC-1. Decreased expression of NEP during hypoxia can be prevented by application with the epigenetic regulators 5-Aza-2′-deoxycytidine (5-Aza), HDACs inhibitor sodium valproate (VA), and siRNA-mediated knockdown of G9a or HDAC1. DNA methylation PCR data do not support that hypoxia affects the methylation of NEP promoters. This study suggests that hypoxia may down-regulate NEP by increasing H3K9me2 and decreasing H3-Ace modulation.

Amyloid beta resistance in nerve cell lines is mediated by the Warburg effect

Amyloid beta (Aβ) peptide accumulation in the brains of patients with Alzheimer's disease (AD) is closely associated with increased nerve cell death. However, many cells survive and it is important to understand the mechanisms involved in this survival response. Recent studies have shown that an anti-apoptotic mechanism in cancer cells is mediated by aerobic glycolysis, also known as the Warburg effect. One of the major regulators of aerobic glycolysis is pyruvate dehydrogenase kinase (PDK), an enzyme which represses mitochondrial respiration and forces the cell to rely heavily on glycolysis, even in the presence of oxygen. Recent neuroimaging studies have shown that the spatial distribution of aerobic glycolysis in the brains of AD patients strongly correlates with Aβ deposition. Interestingly, clonal nerve cell lines selected for resistance to Aβ exhibit increased glycolysis as a result of activation of the transcription factor hypoxia inducible factor 1. Here we show that Aβ resistant nerve cell lines upregulate Warburg effect enzymes in a manner reminiscent of cancer cells. In particular, Aβ resistant nerve cell lines showed elevated PDK1 expression in addition to an increase in lactate dehydrogenase A (LDHA) activity and lactate production when compared to control cells. In addition, mitochondrial derived reactive oxygen species (ROS) were markedly diminished in resistant but not sensitive cells. Chemically or genetically inhibiting LDHA or PDK1 re-sensitized resistant cells to Aβ toxicity. These findings suggest that the Warburg effect may contribute to apoptotic-resistance mechanisms in the surviving neurons of the AD brain. Loss of the adaptive advantage afforded by aerobic glycolysis may exacerbate the pathophysiological processes associated with AD.

The potential dual effects of anesthetic isoflurane on hypoxia-induced caspase-3 activation and increases in β-site amyloid precursor protein-cleaving enzyme levels

BACKGROUND

β-Amyloid protein (Aβ) accumulation, caspase activation, apoptosis, and hypoxia-induced neurotoxicity have been suggested to be involved in Alzheimer disease neuropathogenesis. Aβ is produced from amyloid precursor protein through proteolytic processing by the aspartyl protease β-site amyloid precursor protein-cleaving enzyme (BACE) and γ-secretase. Inhaled anesthetics have long been considered to protect against neurotoxicity. However, recent studies have suggested that the inhaled anesthetic isoflurane may promote neurotoxicity by inducing caspase activation and apoptosis, and by increasing levels of BACE and Aβ. We therefore sought to determine whether isoflurane can induce concentration-dependent dual effects on hypoxia-induced caspase-3 activation and increases in BACE levels: protection versus promotion.

METHODS

H4 human neuroglioma cells were treated with hypoxia (3% O(2)) alone, different concentrations of isoflurane (0.5% and 2%), and the combination of hypoxia and 0.5% or 2% isoflurane. The levels of caspase-3 cleavage (activation), BACE, and Bcl-2 were determined by Western blot analysis.

RESULTS

We show for the first time that treatment with 0.5% isoflurane for 8 hours attenuated, whereas treatment with 2% isoflurane for 8 hours enhanced, hypoxia-induced caspase-3 activation and increases in BACE levels. The 2% isoflurane treatment also enhanced a hypoxia-induced decrease in Bcl-2 levels.

CONCLUSIONS

These results suggest a potential concept that isoflurane has dual effects (protection versus promotion) on hypoxia-induced toxicity, which may act through Bcl-2 family proteins. These findings could lead to more systematic studies to determine the potential dual effects of anesthetics on Alzheimer disease-associated neurotoxicity.

Apoptogenic interactions of plasmalemmal type-1 VDAC and Aβ peptides via GxxxG motifs induce Alzheimer's disease - a basic model of apoptosis?

Human type-1 porin/VDAC (voltage-dependent anion channel) carries a GxxxG motif in its N-terminal part, traversing the β-barrel, while the Alzheimer's disease (AD) relevant amyloid peptides Aβ1-42 and Aβ1-40 show a series of corresponding motifs close to their C-terminus. GxxxG motifs are established as aggregation/membrane perturbation motifs. These peptide primary structure data support a proposal I recently made on the basis of a synopsis of recent literature. Accordingly, amyloid Aβ, cut from APP by beta-secretase BACE1 and gamma-secretase, has been insinuated to induce Alzheimer's disease via apoptosis by opening type-1 porin/VDAC in cell membranes of hypometabolic neuronal cells. Considering the ubiquitous expression modus of APP, beta- and gamma-secretases and type-1 VDAC/eukaryotic porin a basic model of apoptosis might be given.

Decreased expression of GGA3 protein in Alzheimer's disease frontal cortex and increased co-distribution of BACE with the amyloid precursor protein

BACE initiates the amyloidogenic processing of the amyloid precursor protein (APP) that results in the production of Aβ peptides associated with Alzheimer's disease (AD). Previous studies have indicated that BACE is elevated in the frontal cortex of AD patients. Golgi-localized γ-ear containing ADP ribosylation factor-binding proteins (GGA) control the cellular trafficking of BACE and may alter its levels. To investigate a link between BACE and GGA expression in AD, frontal cortex samples from AD (N = 20) and healthy, age-matched controls (HC, N =17) were analyzed by immunoblotting. After normalization to the neuronal marker β-tubulin III, the data indicate an average two-fold increase of BACE protein (p = 0.01) and a 64% decrease of GGA3 in the AD group compared to the HC (p = 0.006). GGA1 levels were also decreased in AD, but a statistical significance was not achieved. qRT-PCR analysis of GGA3 mRNA showed no difference between AD and HC. There was a strong correlation between GGA1 and GGA3 in both AD and HC, but no correlation between BACE and GGA levels. Subcellular fractionation of AD cortex with low levels of GGA proteins showed an alteration of BACE distribution and extensive co-localization with APP. These data suggest that altered compartmentalization of BACE in AD promotes the amyloidogenic processing of APP.

Matrix metalloproteinase (MMP)-9 induced by Wnt signaling increases the proliferation and migration of embryonic neural stem cells at low O2 levels

Recent studies have shown that various neural and embryonic stem cells cultured in 1-8% oxygen (O(2)), levels lower than those typically used in cell culture (20.9%), displayed increased rates of proliferation; however, the molecular mechanisms underlying these changes are largely undefined. In this study, using rigorously controlled O(2) levels, we found that neural stem cells (NSCs) from embryonic day 15 rat cortex increased their rate of proliferation and migration in 1% O(2) relative to 20% O(2) without changes in viability. We sought to identify molecular changes in NSCs grown in 1% O(2) that might account for these increases. In 1% O(2), levels of the hypoxia-inducible transcription factor HIF-1α were transiently increased. Reduced adherence of NSCs in 1% O(2) to basement membrane-coated plates was observed, and quantitative RT-PCR analysis confirmed that the levels of mRNA for an assortment of cell adhesion and extracellular matrix molecules were altered. Most notable was a 5-fold increase in matrix metalloproteinase (MMP)-9 mRNA. Specific inhibition of MMP-9 activity, verified using a fluorescent substrate assay, prevented the increase in proliferation and migration in 1% O(2). The canonical Wnt pathway was recently shown to be activated in stem cells in low O(2) via HIF-1α. Inhibition of Wnt signaling by DKK-1 also prevented the increase in proliferation, migration, and MMP-9 expression. Thus, MMP-9 is a key molecular effector, downstream of HIF-1α and Wnt activation, responsible for increased rates of NSC proliferation and migration in 1% O(2).

Decrease in brain soluble amyloid precursor protein β (sAPPβ) in Alzheimer's disease cortex

Amyloid-β peptide (Aβ) is generated by sequential cleavage of the amyloid precursor protein (APP) by β-site amyloid precursor protein cleaving enzyme 1 (β-secretase, or BACE1) and γ-secretase. Several reports demonstrate increased BACE1 enzymatic activity in brain and cerebrospinal fluid (CSF) from Alzheimer's disease (AD) subjects, suggesting that an increase in BACE1-mediated cleavage of APP drives amyloid pathophysiology in AD. BACE1 cleavage of APP leads to the generation of a secreted N-terminal fragment of APP (sAPPβ). To relate BACE1 activity better to endogenous APP processing in AD and control brains, we have directly measured brain sAPPβ levels using a novel APP β-site specific enzyme-linked immunosorbent assay. We demonstrate a significant reduction in brain cortical sAPPβ levels in AD compared with control subjects. In the same brain samples, BACE1 activity was unchanged, full-length APP and sAPPα levels were significantly reduced, and Aβ peptides were significantly elevated. In conclusion, a reduction in cortical brain sAPPβ together with unchanged BACE1 activity suggests that this is due to reduced full-length APP substrate in late-stage AD subjects. These results highlight the need for multiparameter analysis of the amyloidogenic process to understand better AD pathophysiology in early vs. late-stage AD.

Long term changes in phospho-APP and tau aggregation in the 3xTg-AD mice following cerebral ischemia

The risk of Alzheimer's disease increases following cerebral hypoperfusion. We studied the long-term interaction between low blood flow to the brain and Alzheimer's disease by inducing a transient global ischemic insult in aged 3xTg-AD mice and determining the effects on AD pathology 3-months post injury. We found that global ischemia does not increase the levels of amyloid-β in these mice. However, the injury did lead to enhanced phosphorylation of the amyloid precursor protein (APP) at the Thr668 site in both the 3xTg-AD mice and wild-type controls. Furthermore, we found an increase in insoluble total tau 3-months post-injury. Together these findings further elucidate the long-term impact of cerebral hypoperfusion on Alzheimer's disease.