Increased vulnerability of hippocampal neurons from presenilin-1 mutant knock-in mice to amyloid beta-peptide toxicity: central roles of superoxide production and caspase activation

Selective vulnerability of neurons in primary cultures and in neurodegenerative diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

Mitochondrial dysfunction: the first domino in brain aging and Alzheimer's disease?

With the increasing average life span of humans and with decreasing cognitive function in elderly individuals, age-related cognitive disorders including dementia have become a major health problem in society. Aging-related mitochondrial dysfunction underlies many common neurodegenerative disorders diseases, including Alzheimer's disease (AD). AD is characterized by two major histopathological hallmarks, initially intracellular and with the progression of the disease extracellular accumulation of oligomeric and fibrillar beta-amyloid (Abeta) peptides and intracellular neurofibrillary tangles (NFT) composed of hyperphosphorylated tau protein. In this review, the authors focus on the latest findings in AD animal models indicating that these histopathological alterations induce deficits in the function of the complexes of the respiratory chain and therefore consecutively result in mitochondrial dysfunction. This parameter is intrinsically tied to oxidative stress. Both are early events in aging and especially in the pathogenesis of aging-related severe neurodegeneration. Ginkgo biloba extract seems to be of therapeutic benefit in the treatment of mild to moderate dementia of different etiology, although the data are quite heterogeneous. Herein, the authors suggest that mitochondrial protection and subsequent reduction of oxidative stress are important components of the neuroprotective activity of Ginkgo biloba extract.

Soluble neuroprotective antioxidant uric acid analogs ameliorate ischemic brain injury in mice

Uric acid is a major antioxidant in the blood of humans that can protect cultured neurons against oxidative and metabolic insults. However, uric acid has a very low solubility which compromises its potential clinical use for neurodegenerative disorders. Here we describe the synthesis, characterization and preclinical development of neuroprotective methyl- and sulfur-containing analogs of uric acid with increased solubility. In vitro and cell culture screening identified 1,7-dimethyluric acid (mUA2) and 6,8-dithiouric acid (sUA2) as two analogs with high antioxidant and neuroprotective activities. When administered intravenously in mice, uric acid analogs mUA2 and sUA2 lessened damage to the brain and improved functional outcome in an ischemia-reperfusion mouse model of stroke. Analogs sUA2 and mUA2 were also effective in reducing damage to the cerebral cortex when administered up to 4 h after stroke onset in a permanent middle cerebral artery occlusion mouse model. These findings suggest a therapeutic potential for soluble analogs of uric acid in the treatment of stroke and related neurodegenerative conditions.

Effects of fluoride on the expression of NCAM, oxidative stress, and apoptosis in primary cultured hippocampal neurons

The mechanisms underlying the neurotoxicity of endemic fluorosis still remain unknown. To investigate the expression level of neural cell adhesion molecules (NCAM), oxidative stress, and apoptosis induced by fluoride, the primary rat hippocampal neurons were incubated with 20, 40, and 80 mg/l sodium fluoride for 24 h in vitro. The results showed that the cell survival rate in the 80 mg/l fluoride-treated group was significantly lower than that of the control group. Forty and 80 mg/l of fluoride induced significantly increased lactate dehydrogenase release, intracellular reactive oxygen species, and the percentage of apoptosis. Compared with control group, the malondialdehyde levels were significantly elevated while glutathione levels and glutathione peroxidase activities were decreased in all fluoride-treated groups, accompanied by the markedly reduced superoxide dismutase activity in 80 mg/l fluoride-treated group. With respect to NCAM mRNA expression levels, a significant dose-dependent decrease was observed in 40 and 80 mg/l fluoride-treated groups against the control group. In addition, as compared to the control group, the protein expression levels of NCAM-180 in 40 and 80 mg/l fluoride-treated groups, NCAM-140 in all fluoride-treated groups, and NCAM-120 in the 80 mg/l fluoride-treated group were significantly decreased. Our study herein suggested that fluoride could cause oxidative stress, apoptosis, and decreased mRNA and protein expression levels of NCAM in rat hippocampal neurons, contributing to the neurotoxicity induced by fluoride.

Reduction in CHT1-mediated choline uptake in primary neurons from presenilin-1 M146V mutant knock-in mice

The memory loss in Alzheimer's disease (AD) has been linked to cholinergic hypoactivity. Mutations in presenilin-1 (PS-1) may regulate cholinergic signaling, although their precise roles in cholinergic neurotransmission in AD are unsettled. Neuronal uptake of choline via the high affinity choline transporter (CHT1) is essential for cholinergic neurotransmission. CHT1 is a Na+-dependent, hemicholinium-3 (HC-3)-sensitive choline transporter. Although cholinergic neurons in the nucleus basalis of Meynert are a major source of cholinergic projections for the cerebral cortex, it is unclear whether cortical neurons exhibit intrinsic CHT1 activity that is altered in AD. We now report that primary cortical neurons express intrinsic and biologically active CHT1, and that, in these neurons, CHT1-mediated choline uptake activity is significantly reduced in PS-1 M146V mutant knock-in mice. Further kinetic studies using HC-3 binding and cell surface biotinylation assays showed that the PS-1 mutation inhibits CHT1 mediated choline uptake by reducing the ligand binding affinity of CHT1 without significantly altering levels of CHT1 expression in the plasma membrane. Since human neocortex has recently been shown to possess intrinsic cholinergic innervation, our results indicate that alterations in CHT1-mediated high affinity choline uptake in cortical neurons may contribute to Alzheimer's dementia.

Neuronal life-and-death signaling, apoptosis, and neurodegenerative disorders

When subjected to excessive oxidative stress, neurons may respond adaptively to overcome the stress, or they may activate a programmed cell death pathway called apoptosis. Apoptosis is characterized by alterations in mitochondria and the endoplasmic reticulum and activation of cysteine proteases called caspases. Increasing evidence suggests that apoptotic biochemical cascades are involved in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. Studies of normal aging, of genetic mutations that cause disease, and of environmental factors that affect disease risk are revealing cellular and molecular alterations that may cause excessive oxidative stress and trigger neuronal apoptosis. Accumulation of self-aggregating proteins such as amyloid beta-peptide, tau, alpha-synuclein, and huntingtin may be involved in apoptosis both upstream and downstream of oxidative stress. Membrane-associated oxidative stress resulting in perturbed lipid metabolism and disruption of cellular calcium homeostasis may trigger apoptosis in several different neurodegenerative disorders. Counteracting neurodegenerative processes are an array of mechanisms including neurotrophic factor signaling, antioxidant enzymes, protein chaperones, antiapoptotic proteins, and ionostatic systems. Emerging findings suggest that the resistance of neurons to death during aging can be enhanced by modifications of diet and lifestyle.

Presenilin Function in Caenorhabditis elegans

The genome of the nematode Caenorhabditis elegans contains homologs of several genes associated with familial Alzheimer's disease in humans. apl-1 encodes a transmembrane protein belonging to the amyloid precursor protein family, sel-12 and hop-1 are the two somatically expressed presenilin genes that resemble PS1 and PS2 on both a structural and a functional level. Mutations in the sel-12-encoded presenilin gene cause defective Notch/lin-12 signaling and result in reduced egg-laying, caused by cell specification and cell attachment defects. spr-1, spr-3, spr-4 and spr-5 were identified as the suppressors of the egg-laying defect of presenilin/sel-12 loss of function mutants in genetic suppressor screens. The corresponding proteins are C. elegans homologs of human REST, CoREST and LSD1, respectively. REST/NSRF (Re1 silencing transcription factor/neural-restrictive silencing factor) is a transcriptional repressor that blocks the expression of neuronal genes in non-neuronal tissues in vertebrates. CoREST is a conserved histone deacetylase and demethylase-containing co-repressor complex possessing a potential chromatin-modifying activity. It is recruited to the promoter via REST-mediated DNA binding. LSD1 is a flavin-dependent demethylase of histone H3. Mutations in spr-1, spr-3, spr-4 and spr-5 genes suppress the egg-laying phenotype of sel-12 loss of function mutants by derepressing the expression of the second C. elegans presenilin gene, hop-1.

Hippocampal spatial memory impairments caused by the familial Alzheimer's disease-linked presenilin 1 M146V mutation

Mutations in presenilins (PS) 1 and 2 are the major cause of familial Alzheimer's disease. Conditional inactivation of PS1 in the mouse postnatal forebrain leads to mild deficits in spatial learning and memory, whereas inactivation of both PS1 and PS2 results in severe memory and synaptic plasticity impairments, followed by progressive and substantial neurodegeneration. Here we investigate the effect of a familial Alzheimer's disease-linked PS1 missense mutation using knock-in (KI) mice, in which the wild-type PS1 allele is replaced with the M146V mutant allele. In the Morris water maze task, PS1 KI mice at 3 months of age exhibit reduced quadrant occupancy and platform crossing in the probe trial after 6 days of training, though their performance was normal in the probe trial after 12 days of training. By the age of 9 months, even after 12 days of training, PS1 homozygous KI mice still exhibit reduced platform crossing in the post-training probe trial. ELISA analysis revealed a selective increase in cortical levels of beta-amyloid 42 in PS1 KI mice, whereas production of beta-amyloid 40 was normal. Histological and quantitative real-time RT-PCR analyses showed normal gross hippocampal morphology and unaltered expression of three genes involved in inflammatory responses in PS1 KI mice. These results show hippocampal spatial memory impairments caused by the PS1 M146V mutation and age-related deterioration of the memory impairment, suggesting that PS1 KI mice are a valuable model system for the study of memory loss in AD.

Hypocapnia induces caspase-3 activation and increases Abeta production

BACKGROUND

At least half of all cases of early onset (<60) familial Alzheimer's disease (FAD) are caused by any of over 150 mutations in three genes: the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). Mutant forms of PS1 have been shown to sensitize cells to apoptotic cell death.

OBJECTIVE

We investigated the effects of hypocapnia, a risk factor for both cognitive and neurodevelopment deficits, on caspase-3 activation, apoptosis, and amyloid beta-protein (Abeta) production, and assessed the influence of the PS1Delta9 FAD mutation on these effects.

METHOD

For this purpose, we exposed stably transfected H4 human neuroglioma cells to conditions consistent with hypocapnia (PCO2<40 mm Hg) and hypocapnia plus hypoxia (PO2<21%).

RESULTS

Hypocapnia (20 mm Hg CO2 for 6 h) induced caspase-3 activation and apoptosis; the PS1Delta9 FAD mutation significantly potentiated these effects. Moreover, the combination of hypocapnia (20 mm Hg CO2) and hypoxia (5%O2) induced caspase-3 activation and apoptosis in a synergistic manner. Hypocapnia (5 and 20 mm Hg CO2 for 6 h) also led to an increased Abeta production.

CONCLUSION

The findings suggest that hypocapnia (e.g. during general anesthesia) could exacerbate AD neuropathogenesis.

Involvement of oxidative stress in Alzheimer disease

Genetic and lifestyle-related risk factors for Alzheimer disease (AD) are associated with an increase in oxidative stress, suggesting that oxidative stress is involved at an early stage of the pathologic cascade. Moreover, oxidative stress is mechanistically and chronologically associated with other key features of AD, namely, metabolic, mitochondrial, metal, and cell-cycle abnormalities. Contrary to the commonly held notion that pathologic hallmarks of AD signify etiology, several lines of evidence now indicate that aggregation of amyloid-beta and tau is a compensatory response to underlying oxidative stress. Therefore, removal of proteinaceous accumulations may treat the epiphenomenon rather than the disease and may actually enhance oxidative damage. Although some antioxidants have been shown to reduce the incidence of AD, the magnitude of the effect may be modified by individual factors such as genetic predisposition (e.g. apolipoprotein E genotype) and habitual behaviors. Because caloric restriction, exercise, and intellectual activity have been experimentally shown to promote neuronal survival through enhancement of endogenous antioxidant defenses, a combination of dietary regimen of low total calorie and rich antioxidant nutrients and maintaining physical and intellectual activities may ultimately prove to be one of the most efficacious strategies for AD prevention.

Salvianolic acid B, an antioxidant from Salvia miltiorrhiza, prevents Abeta(25-35)-induced reduction in BPRP in PC12 cells

Several lines of evidence support that beta-amyloid (Abeta)-induced neurotoxicity is mediated through the generation of reactive oxygen species (ROS) and elevation of intracellular calcium. Salvianolic acid B (Sal B), the major and most active anti-oxidant from Salvia miltiorrhiza, protects diverse kinds of cells from damage caused by a variety of toxic stimuli. In the present study, we investigated the effects of Sal B against beta-amyloid peptide 25-35 (Abeta(25-35))-induced neurotoxicity, focused mainly on the neurotoxic effects of Abeta(25-35) and the neuroprotective effects of Sal B on the expression of brain-pancreas relative protein (BPRP), which is a new protein and mainly expressed in brain and pancreas. Following exposure of PC12 cells to 20 microM Abeta(25-35), a marked reduction in the expression of BPRP was observed, accompanied with decreased cell viability and increased cell apoptosis, as well as increased ROS production and calcium influx. Treatment of the PC12 cells with Sal B significantly reversed the expression of BPRP and cell viability while it decreased ROS production and intracellular calcium. These data indicate that Abeta(25-35) decreases the expression of BPRP via enhanced formation of intracellular ROS and increased intracellular calcium, and that Sal B, as an anti-oxidant, protects against Abeta(25-35)-induced reduction in expression of BPRP through its effects on suppressing the production of ROS, calcium flux, and apoptosis. However, the role(s) of BPRP in AD and the definite mechanisms by which Sal B protects against Abeta(25-35)-induced reduction in the expression of BPRP require further study.

George-Hyslop P, Checler F. Presenilin-dependent gamma-secretase-mediated control of p53-associated cell death in Alzheimer's disease

Presenilins (PSs) are part of the gamma-secretase complex that produces the amyloid beta-peptide (Abeta) from its precursor [beta-amyloid precursor protein (betaAPP)]. Mutations in PS that cause familial Alzheimer's disease (FAD) increase Abeta production and trigger p53-dependent cell death. We demonstrate that PS deficiency, catalytically inactive PS mutants, gamma-secretase inhibitors, and betaAPP or amyloid precursor protein-like protein 2 (APLP2) depletion all reduce the expression and activity of p53 and lower the transactivation of its promoter and mRNA expression. p53 expression also is diminished in the brains of PS- or betaAPP-deficient mice. The gamma- and epsilon-secretase-derived amyloid intracellular C-terminal domain (AICD) fragments (AICDC59 and AICDC50, respectively) of betaAPP trigger p53-dependent cell death and increase p53 activity and mRNA. Finally, PS1 mutations enhance p53 activity in human embryonic kidney 293 cells and p53 expression in FAD-affected brains. Thus our study shows that AICDs control p53 at a transcriptional level, in vitro and in vivo, and that FAD mutations increase p53 expression and activity in cells and human brains.

Loss of nicastrin elicits an apoptotic phenotype in mouse embryos

Nicastrin is a member of the high molecular weight presenilin complex that plays a central role in gamma-secretase cleavage of numerous type-1 membrane-associated proteins required for cell signaling, proliferation and lineage development. We have generated a nicastrin-null mouse line by disruption of exon 3. Similar to previously described nicastrin-null mice, these animals demonstrate severe growth retardation, mortality beginning at embryonic age 10.5 days, and marked developmental abnormalities indicative of a severe Notch phenotype. Preceding their mortality, 10.5-day-old nicastrin-null embryos were found to also exhibit specific apoptosis within regions showing profound deformities, particularly in the developing heart and brain. This result suggests that complete disruption of presenilin complexes elicits programmed cell death, in addition to a Notch phenotype, which may contribute to the developmental abnormalities and embryonic mortality of nicastrin-null mice and possibly neurodegeneration in Alzheimer's disease.

Apoptotic and anti-apoptotic synaptic signaling mechanisms

Although several prominent morphological features of apoptosis are evident in the cell body (e.g., cell shrinkage, membrane blebbing, and nuclear DNA condensation and fragmentation) the biochemical and molecular cascades that constitute the cell death machinery can be engaged in synaptic terminals and neurites. Initiating events such as oxyradical production and calcium influx, and effector processes such as Par-4 production, mitochondrial alterations and caspase activation, can be induced in synapses and neurites. Several prominent signal transduction pathways in synaptic terminals play important roles in either promoting or preventing neuronal death in physiological and pathological conditions. For example, activation of glutamate receptors in postsynaptic spines can induce neuronal apoptosis, whereas local activation of neurotrophic factor receptors in presynaptic terminals can prevent neuronal death. Factors capable of inducing nuclear chromatin condensation and fragmentation can be produced locally in synaptic terminals and neurites, and may propogate to the cell body. Recent findings suggest that, beyond their roles in inducing or preventing cell death, apoptotic and anti-apoptotic cascades play roles in synaptic plasticity (structural remodelling and long-term functional changes). For example, caspase activation results in proteolysis of glutamate receptor (AMPA) subunits, which results in altered neuronal responsivity to glutamate. Activation of neurotrophic factor receptors in synaptic terminals can result in local changes in energy metabolism and calcium homeostasis, and can induce long-term changes in synaptic transmission. The emerging data therefore suggest that synapses can be considered as autonomous compartments in which both pro- and anti-apoptotic signaling pathways are activated resulting in structural and functional changes in neuronal circuits. A better understanding of such synaptic signaling mechanisms may reveal novel approaches for preventing and treating an array of neurodegenerative conditions that are initiated by perturbed synaptic homeostasis.

Alzheimer's disease and post-operative cognitive dysfunction

Alzheimer's disease (AD), an insidious and progressive neurodegenerative disorder accounting for the vast majority of dementia, is characterized by global cognitive decline and the robust accumulation of amyloid deposits and neurofibrillary tangles in the brain. This review article is based on the currently published literature regarding molecular studies of AD and the potential involvement of AD neuropathogenesis in post-operative cognitive dysfunction (POCD). Genetic evidence, confirmed by neuropathological and biochemical studies, indicates that excessive beta-amyloid protein (Abeta) generated from amyloidogenic processing of the beta-amyloid precursor protein (APP) plays a fundamental role in the AD neuropathogenesis. Abeta is produced from APP by beta-secretase, and then gamma-secretase complex, consisting of presenilins, nicastrin (NCSTN), APH-1 and PEN-2. Additionally, Abeta clearance and APP adaptor proteins can contribute to AD neuropathogenesis via affecting Abeta levels. Finally, cellular apoptosis may also be involved in AD neuropathogenesis. Surgery and anesthesia can cause cognitive disorders, especially in elderly patients. Even the molecular mechanisms underlying these disorders are largely unknown; several perioperative factors such as hypoxia, hypocapnia and anesthetics may be associated with AD and render POCD via trigging AD neuropathogenesis. More studies to assess the potential relationship between anesthesia/surgery and AD dementia are, therefore, urgently needed.

Aging sensitizes toward ROS formation and lipid peroxidation in PS1M146L transgenic mice

Mutations in the presenilins (PS) account for the majority of familial Alzheimer disease (FAD) cases. To test the hypothesis that oxidative stress can underlie the deleterious effects of presenilin mutations, we analyzed lipid peroxidation products (4-hydroxynonenal (HNE) and malondialdehyde) and antioxidant defenses in brain tissue and levels of reactive oxygen species (ROS) in splenic lymphocytes from transgenic mice bearing human PS1 with the M146L mutation (PS1M146L) compared to those from mice transgenic for wild-type human PS1 (PS1wt) and nontransgenic littermate control mice. In brain tissue, HNE levels were increased only in aged (19-22 months) PS1M146L transgenic animals compared to PS1wt mice and not in young (3-4 months) or middle-aged mice (13-15 months). Similarly, in splenic lymphocytes expressing the transgenic PS1 proteins, mitochondrial and cytosolic ROS levels were elevated to 142.1 and 120.5% relative to controls only in cells from aged PS1M146L animals. Additionally, brain tissue HNE levels were positively correlated with mitochondrial ROS levels in splenic lymphocytes, indicating that oxidative stress can be detected in different tissues of PS1 transgenic mice. Antioxidant defenses (activities of antioxidant enzymes Cu/Zn-SOD, GPx, or GR) or susceptibility to in vitro oxidative stimulation was unaltered. In summary, these results demonstrate that the PS1M146L mutation increases mitochondrial ROS formation and oxidative damage in aged mice. Hence, oxidative stress caused by the combined effects of aging and PS1 mutations may be causative for triggering neurodegenerative events in FAD patients.

Protein folding in the endoplasmic reticulum and the unfolded protein response

In all eukaryotic cells, the endoplasmic reticulum (ER) is an intracellular organelle where folding and assembly occurs for proteins destined to the extracellular space, plasma membrane, and the exo/endocytic compartments (Kaufman 1999). As a protein-folding compartment, the ER is exquisitely sensitive to alterations in homeostasis, and provides stringent quality control systems to ensure that only correctly folded proteins transit to the Golgi and unfolded or misfolded proteins are retained and ultimately degraded. A number of biochemical and physiological stimuli, such as perturbation in calcium homeostasis or redox status, elevated secretory protein synthesis, expression of misfolded proteins, sugar/glucose deprivation, altered glycosylation, and overloading of cholesterol can disrupt ER homeostasis, impose stress to the ER, and subsequently lead to accumulation of unfolded or misfolded proteins in the ER lumen. The ER has evolved highly specific signaling pathways called the unfolded protein response (UPR) to cope with the accumulation of unfolded or misfolded proteins. Elucidation of the molecular mechanisms by which accumulation of unfolded proteins in the ER transmits a signal to the cytoplasm and nucleus has led to major new insights into the diverse cellular and physiological processes that are regulated by the UPR. This chapter summarizes how cells respond to the accumulation of unfolded proteins in the cell and the relevance of these signaling pathways to human physiology and disease.

Formation of tau inclusions in knock-in mice with familial Alzheimer disease (FAD) mutation of presenilin 1 (PS1)

Mutations in the presenilin 1 (PS1) gene are responsible for the early onset of familial Alzheimer disease (FAD). Accumulating evidence shows that PS1 is involved in gamma-secretase activity and that FAD-associated mutations of PS1 commonly accelerate Abeta(1-42) production, which causes Alzheimer disease (AD). Recent studies suggest, however, that PS1 is involved not only in Abeta production but also in other processes that lead to neurodegeneration. To better understand the causes of neurodegeneration linked to the PS1 mutation, we analyzed the development of tau pathology, another key feature of AD, in PS1 knock-in mice. Hippocampal samples taken from FAD mutant (I213T) PS1 knock-in mice contained hyperphosphorylated tau that reacted with various phosphodependent tau antibodies and with Alz50, which recognizes the conformational change of PHF tau. Some neurons exhibited Congo red birefringence and Thioflavin T reactivity, both of which are histological criteria for neurofibrillary tangles (NFTs). Biochemical analysis of the samples revealed SDS-insoluble tau, which under electron microscopy examination, resembled tau fibrils. These results indicate that our mutant PS1 knock-in mice exhibited NFT-like tau pathology in the absence of Abeta deposition, suggesting that PS1 mutations contribute to the onset of AD not only by enhancing Abeta(1-42) production but by also accelerating the formation and accumulation of filamentous tau.

Presenilin 1 deficiency alters the activity of voltage-gated Ca2+ channels in cultured cortical neurons

Presenilins 1 and 2 (PS1 and PS2, respectively) play a critical role in mediating gamma-secretase cleavage of the amyloid precursor protein (APP). Numerous mutations in the presenilins are known to cause early-onset familial Alzheimer's disease (FAD). In addition, it is well established that PS1 deficiency leads to altered intracellular Ca(2+) homeostasis involving endoplasmic reticulum Ca(2+) stores. However, there has been little evidence suggesting Ca(2+) signals from extracellular sources are influenced by PS1. Here we report that the Ca(2+) currents carried by voltage-dependent Ca(2+) channels are increased in PS1-deficient cortical neurons. This increase is mediated by a significant increase in the contributions of L- and P-type Ca(2+) channels to the total voltage-mediated Ca(2+) conductance in PS1 (-/-) neurons. In addition, chelating intracellular Ca(2+) with 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) produced an increase in Ca(2+) current amplitude that was comparable to the increase caused by PS1 deficiency. In contrast to this, BAPTA had no effect on voltage-dependent Ca(2+) conductances in PS1-deficient neurons. These data suggest that PS1 deficiency may influence voltage-gated Ca(2+) channel function by means that involve intracellular Ca(2+) signaling. These findings reveal that PS1 functions at multiple levels to regulate and stabilize intracellular Ca(2+) levels that ultimately control neuronal firing behavior and influence synaptic transmission.

Calcium dysregulation in Alzheimer's disease: Recent advances gained from genetically modified animals

Alzheimer's disease is a progressive and irreversible neurodegenerative disorder that leads to cognitive, memory and behavioural impairments. Two decades of research have implicated disturbances of intracellular calcium homeostasis as playing a proximal pathological role in the neurodegeneration associated with Alzheimer's disease. A large preponderance of evidence has been gained from the use of a diverse range of cell lines. Whilst useful in understanding the principal mechanism of neurotoxicity associated with Alzheimer's disease, technical differences, such as cell type or even the form of amyloid-beta used often underlie conflicting results. In this review, we discuss recent contributions that transgenic technology has brought to this field. For example, the triple transgenic mouse model of Alzheimer's disease has implicated intraneuronal accumulation of the amyloid-beta peptide as an initiating factor in synaptic dysfunction and behavioural deficits. Importantly, this synaptic dysfunction occurs prior to cell loss or extracellular amyloid plaque accumulation. The cause of synaptic dysfunction is unknown but it is likely that amyloid-beta and its ability to disrupt intracellular calcium homeostasis plays a key role in this process.

PS1 knockin mice with the Japanese I213T mutation: Effects on exploratory activity, motor coordination, and spatial learning

Knockin (KI) mice with a PS1/I213T mutation were compared to wild-type controls on the SHIRPA primary screening battery and for exploratory activity, motor coordination, and spatial learning. By comparison to non-transgenic controls, PS1/I213T KI mice had retarded acquisition of place learning in the Morris water maze without being impaired in the probe trial and in the visible platform subtest. PS1/I213T KI mice were more likely to display whole-body startle to an auditory stimulus and a tighter grip on a horizontal grid. PS1/I213T KI mice also had fewer enclosed arm entries in the elevated plus-maze, but did not differ from controls in open-field, photocell actimeter, and T-maze spontaneous alternation tests. No intergroup difference was seen in three motor coordination tests. The dissociation between hidden and visible platform versions in the water maze is consistent with the hypothesis that elevated Abeta42 concentrations cause cognitive disturbances.

Interaction of presenilins with FKBP38 promotes apoptosis by reducing mitochondrial Bcl-2

Presenilins 1 and 2 (PS1/2), causative molecules for familial Alzheimer's disease (FAD), are multipass transmembrane proteins localized predominantly in the endoplasmic reticulum (ER) and Golgi apparatus. Heteromeric protein complexes containing PS1/2 are thought to participate in several functions, including intramembrane proteolysis mediated by their gamma-secretase activities. Previous studies have shown that PS1/2 are also involved in the regulation of apoptotic cell death, although the underlying mechanism remains unknown. Here, we demonstrate that FKBP38, an immunophilin family member residing in the mitochondrial membrane, is an authentic PS1/2-interacting protein. PS1/2 and FKBP38 form macromolecular complexes together with anti-apoptotic Bcl-2. PS1/2 promote the degradation of FKBP38 and Bcl-2 and sequester these proteins in the ER/Golgi compartments, thereby inhibiting FKBP38-mediated mitochondrial targeting of Bcl-2 via a gamma-secretase-independent mechanism. Thus, PS1/2 increase the susceptibility to apoptosis by antagonizing the anti-apoptotic function of FKBP38. In contrast, C-terminal fragments of caspase-processed PS1/2 redistribute Bcl-2 to the mitochondria by abrogating the activity of full-length PS1/2, resulting in a dominant-negative anti-apoptotic effect. In cultured cells and mutant PS1-knockin mice brains, FAD-linked PS1/2 mutants enhance the pro-apoptotic activity by causing a more efficient reduction in mitochondrial Bcl-2 than wild-type PS1/2. These results suggest a novel molecular mechanism for the regulation of mitochondria-mediated apoptosis by competition between PS1/2 and FKBP38 for subcellular targeting of Bcl-2. Excessive pro-apoptotic activity of PS1/2 may play a role in the pathogenesis of FAD.

Biomarkers for apoptosis in Alzheimer's disease

Alzheimer's disease (AD) affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. Today there is no cure for this devastating and always lethal disease and therefore it is of great interest for patients, relatives and societies to find new drugs that can hinder the disease process. During the progression of AD a substantial amount of neurons degenerate in the brain. The mechanisms of cell death involved in AD have not been fully elucidated. However, there are several reports showing that neurons die partly by apoptosis in the AD brain. Drugs blocking apoptosis could therefore be potentially useful for early prevention of neuronal cell death. Biomarkers for apoptosis should be important tools in the evaluation of drug effects and in the diagnostics of AD. Here we review the current knowledge in the field and discuss potential biomarkers for apoptosis in AD.

Anti-oxidant gene expression imbalance, aging and Down syndrome

The expression of copper zinc superoxide dismutase (SOD1), manganese superoxide dismutase (SOD2), glutathione peroxidase (GPx), and catalase (CAT) genes have been detected in human skin fibroblast cells for 2 year normal child (control), 50 year old normal male and female and a 1 year old Down Syndrome (DS) male and female with established trisomy karyotype using the RT-PCR technique. Differential expression of these genes is quantified individually against a beta-Actin gene that has been employed as an internal control. The immunoblotting of cell lysate proteins with polyclonal antibodies exhibit SOD1 (16 kD), SOD2 (40 kD), GPx (23 and 92 kD), CAT (64 kD), and Actin (43 kD) as translational products. The results demonstrate that the enhancement in the level of mRNAs encoding SOD1 in DS male and female, as well as aged male and female are 51, 21, 31 and 50% respectively compared to the normal child (control). In SOD2, DS male and female display higher (176%) and lower (26%) levels of expression whereas aged male and female exhibit enhanced levels of expression (66 and 119%) respectively compared to the control. This study demonstrates that DS affects the female less than the male whereas in the aging process, the female is more prone to oxidative damage than the male. These results not only indicate that the level of GPx mRNA is constant except in DS male, which shows a downward regulation but that even CAT mRNA is upward regulated in aged as well as in DS males and females. These disproportionate changes in anti-oxidant genes, which are incapable of coping with over expressed genes, may contribute towards the aging process, dementia and Down syndrome.

Increased caspase activation in peripheral blood mononuclear cells of patients with Alzheimer's disease

In this study, we investigated whether alterations in the pattern of caspase activation could be found at the level of peripheral blood mononuclear cells (PBMCs) in patients with Alzheimer's disease (AD). The results showed that in experimental conditions resembling a physiological stimulation, there was a statistically significant increase in the enzymatic activity of caspase-3, caspase-8, and caspase-9 in PBMCs from a small, but well-characterized, cohort of sporadic AD patients compared to those from a comparable control group of aged adults (AA). This was accompanied by a parallel, early increase in the cleavage activity of the same caspases. The higher level of caspase activity in PBMCs from AD compared to AA was not associated with quantitative differences in cell subset profiles. Moreover, no increase in apoptosis level, in the same experimental conditions, was found in PBMCs from this cohort of AD patients compared to those from AA. Conversely, the higher proneness to caspase activation in PBMCs from AD patients in comparison with that from AA was associated with a higher proliferative response to PHA or CD3. These data show a new dysfunction in AD patients at the PBMCs level and suggest that increased proneness to caspase activation in lymphocytes could reflect an ongoing systemic response in neurodegenerative disease with pathogenetic implications.

Induction of neuronal death by ER stress in Alzheimer's disease

Recent studies have suggested that neuronal death in Alzheimer's disease (AD) or ischemia could arise from dysfunction of the endoplasmic reticulum (ER). Inhibition of protein glycosylation, perturbation of calcium homeostasis, and reduction of disulfide bonds provoke accumulation of unfolded protein in the ER, and are called 'ER stress'. Normal cells respond to ER stress by increasing transcription of genes encoding ER-resident chaperones such as GRP78/BiP, to facilitate protein folding or by suppressing the mRNA translation to synthesize proteins. These systems are termed the unfolded protein response (UPR). Familial Alzheimer's disease-linked presenilin-1 (PS1) mutation downregulates the unfolded protein response and leads to vulnerability to ER stress. The mechanisms by which mutant PS1 affects the ER stress response are attributed to the inhibited activation of ER stress transducers such as IRE1, PERK and ATF6. On the other hand, in sporadic Alzheimer's disease (sAD), we found the aberrant splicing isoform (PS2V), generated by exon 5 skipping of the Presenilin-2 (PS2) gene transcript, responsible for induction of high mobility group A1a protein (HMGA1a). The PS2V also downregulates the signaling pathway of the UPR, in a similar fashion to that reported for mutants of PS1 linked to familial AD. It was clarified what molecules related to cell death are activated in the case of AD and we discovered that caspase-4 plays a key role in ER stress-induced apoptosis. Caspase-4 also seems to act upstream of the beta-amyloid-induced ER stress pathway, suggesting that activation of caspase-4 might mediate neuronal cell death in AD.

Neuronal RNA oxidation is a prominent feature of familial Alzheimer's disease

An in situ approach was used to identify the oxidized RNA nucleoside 8-hydroxyguanosine (8OHG) in the frontal cortex of familial Alzheimer's disease (FAD) with a mutation in presenilin-1 (PS-1) or amyloid beta protein precursor (AbetaPP) gene (n = 13, age 47-81 years). Neurons with marked 8OHG immunoreaction in the cytoplasm were widely distributed in the superior/middle frontal gyrus of FAD. Relative intensity measurements of neuronal 8OHG immunoreactivity showed that there was a significant increase in FAD compared with controls (n = 15, age 59-81 years), while there was no difference in relative 8OHG between the PS-1 and the AbetaPP FAD. Interestingly, a presymptomatic case carrying a PS-1 mutation showed a considerable level of relative 8OHG, and the increased levels of neuronal 8OHG in FAD were more prominent in cases with a lower percentage area of Abeta42 burden. These results suggest that oxidative stress is an early event involved in the pathological cascade of FAD.

alpha-keto-beta-methyl-n-valeric acid diminishes reactive oxygen species and alters endoplasmic reticulum Ca(2+) stores

Mitochondrial dysfunction and oxidative stress occur in neurodegenerative diseases. Other results show that bombesin-releasable calcium stores (BRCS) from the endoplasmic reticulum (ER) are exaggerated in fibroblasts from patients with Alzheimer's disease (AD) compared with controls and in fibroblasts from a young control treated with H(2)O(2). We hypothesize that alterations in oxidative stress underlie the exaggeration in BRCS in AD, and that appropriate antioxidants may be useful in treating this abnormality. Two indicators of different oxidant species were used to determine the effects of select oxidants on cellular oxidation status: carboxydichlorofluorescein (c-DCF) to detect reactive oxygen species (ROS), and 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF) to detect nitric oxide (NO(.-)). Various conditions that induce ROS, including H(2)O(2), oxygen/glucose deprivation, and 3-morpholinosyndnonimine (SIN-1), were used to test the ability of alpha-keto-ss-methyl-n-valeric acid (KMV) to scavenge ROS. KMV diminished c-DCF-detectable ROS that were induced by H(2)O(2), oxygen/glucose deprivation, or SIN-1 in PC12 cells, primary neuronal cultures, or fibroblasts. Furthermore, KMV reduced the H(2)O(2)-induced increase in BRCS and diminished the elevation in BRCS in cells from AD patients to control levels. On the other hand, DAF-detectable NO(.-) induced by SIN-1 was not scavenged by KMV and did not exaggerate BRCS. The results indicate that KMV is an effective antioxidant of c-DCF-detectable ROS. The effects of KMV are not cell type specific, but are ROS specific. The same H(2)O(2)-induced ROS that reacts with KMV may also underlie the changes in BRCS related to AD. Thus, KMV ameliorates the effects of ROS on calcium homeostasis related to oxidative stress and to AD.

Intracellularly generated amyloid-β peptide counteracts the antiapoptotic function of its precursor protein and primes proapoptotic pathways for activation by other insults in neuroblastoma cells

Most mutations in amyloid precursor proteins (APPs) linked to early onset familial Alzheimer's disease (FAD) increase the production of amyloid-beta peptides ending at residue 42 (Abeta42), which are released from APP by beta- and gamma-secretase cleavage. Stably transfected cells expressing wild-type human APP (APP(WT)) were more resistant to apoptosis-inducing treatments than cells expressing FAD-mutant human APP (APP(FAD)). Preventing Abeta42 production with an M596I mutation (beta-), which blocks beta-secretase cleavage of APP, or by treatment with a gamma-secretase inhibitor increased the resistance of APP(FAD)-expressing cells to apoptosis. Exposing hAPP(FAD/beta-) cells to exogenous Abeta42 or conditioned medium from Abeta42-producing APP(FAD) cells did not diminish their resistance to apoptosis. Preventing APP from entering the distal secretory pathway, where most Abeta peptides are generated, by retaining APP in the endoplasmic reticulum (ER)/intermediate compartment (IC) increased the resistance of APP(FAD)-expressing cells to apoptosis and did not alter the resistance of APP(WT)-expressing cells. p53-mediated gene transactivation after apoptosis-inducing treatments was much stronger in APP(FAD) cells than in hAPP(WT) or hAPP(FAD/beta-) cells. In contrast, upon induction of ER stress, cells expressing APP(FAD), hAPP(FAD/beta-), or APP(WT) had comparable levels of glucose-regulated protein-78 mRNA, an unfolded protein response indicator. We conclude that Abeta, especially intracellular Abeta, counteracts the antiapoptotic function of its precursor protein and predisposes cells to p53-mediated, and possibly other, proapoptotic pathways.

Effects of RNA Interference-mediated Silencing of γ-Secretase Complex Components on Cell Sensitivity to Caspase-3 Activation

Familial Alzheimer's disease mutations in the presenilin 1 gene (PSEN1) have been previously shown to potentiate caspase activation and apoptosis in transfected cells and transgenic mice. However, the mechanism underlying this effect is not known. We set out to determine whether cellular sensitivity to caspase activation could be affected by modulating presenilin 1 (PS1) processing. PS1 processing was altered using RNA interference (RNAi) aimed at silencing the expression of the genes encoding the four components of the gamma-secretase complex, PSEN1, APH-1, PEN-2, and nicastrin. RNAi for these genes was carried out in naive H4 human neuroglioma cells, as well as H4 cell lines overexpressing either wild-type PSEN1 or the Familial Alzheimer's disease mutant PSEN1-Delta9 (PS1-mutant), that were induced to undergo apoptosis. In wild-type PSEN1 cells, RNAi for PEN-2, as expected, increased levels of full-length PS1 (PS1-FL) and decreased PS1 endoproteolysis. This was accompanied by potentiated caspase-3 activation in response to an apoptotic stimulus. In contrast, nicastrin RNAi, which only decreased levels of PS1-amino-terminal fragment and did not affect PS1-FL levels, had no effect on caspase-3 activation during apoptosis. Surprisingly, in the PS1-mutant cells, RNAi for PEN-2 (and APH-1) did not increase but instead reduced the levels of PS1-FL deleted for exon 9. In turn, this was accompanied by attenuated caspase-3 activation in response to an apoptotic stimulus. Finally, in naive H4 cells, PSEN1 RNAi also attenuated caspase-3 activation in response to an apoptotic stimulus. Collectively, these findings indicate that cellular sensitivity to caspase activation correlates with overall PS1 protein levels, particularly with levels of FL-PS1.

Reactive oxygen species induce different cell death mechanisms in cultured neurons

Apoptosis is characterized by chromatin condensation, phosphatidylserine translocation, and caspase activation. Neuronal apoptotic death involves the participation of reactive oxygen species (ROS), which have also been implicated in necrotic cell death. In this study we evaluated the role of different ROS in neuronal death. Superoxide anion was produced by incubating cells with xanthine and xanthine oxidase plus catalase, singlet oxygen was generated with rose Bengal and luminic stimuli, and hydrogen peroxide was induced with the glucose and glucose oxidase. Cultured cerebellar granule neurons died with the characteristics of apoptotic death in the presence of superoxide anion or singlet oxygen. These two conditions induced caspase activation, nuclear condensation, phosphatidylserine translocation, and a decrease in intracellular calcium levels. On the other hand, hydrogen peroxide led to a necrosis-like cell death that did not induce caspase activation, phosphatidylserine translocation, or changes in calcium levels. Cell death produced by both singlet oxygen and superoxide anion, but not hydrogen peroxide, was partially reduced by an increase in intracellular calcium levels. These results suggest that formation of specific ROS can lead to different molecular cell death mechanisms (necrosis and apoptosis) and that ROS formed under different conditions could act as initiators or executioners on neuronal death.

Clinical, pathological, and biochemical spectrum of Alzheimer disease associated with PS-1 mutations

Three genes have been implicated in the etiology of early-onset autosomal-dominant Alzheimer disease (AD): the amyloid precursor protein, the presenilin-1, and presenilin-2 genes. Approximately half of autosomal-dominant AD cases are associated with mutations in the presenilin-1 (PS-1) gene on the long arm of Chromosome 14. Marked allelic heterogeneity characterizes families with PS-1 gene mutations; more than 100 different mutations have been found in independent families thus far. With the exception of age at onset, the clinical phenotype is similar to late-onset AD, although some rare specific phenotypes have been described. These mutations lead to enhanced deposition of total Abeta and Abeta42 (but not Abeta40) in the brain, compared with sporadic AD. There is a considerable heterogeneity in the histological profiles among brains from patients with different mutations, and although some lead to predominantly parenchymal deposition of Abeta in the form of diffuse and cored plaques, others show predominantly vascular deposition, with severe amyloid angiopathy. Only some mutations are associated with enhanced neurofibrillary tangle formation and increased neuronal loss compared with sporadic AD. However, there is an important clinical and pathological variability even among family members with the same mutation, which suggests the involvement of other genetic or environmental factors that modulate the clinical expression of the disease. This represents a valuable model for identifying such factors and has potential implications for the development of new therapeutic strategies for delaying disease onset.

Expression of nitric oxide system in clinically evaluated cases of Alzheimer's disease

The expression of neuronal nitric oxide (nNOS) and inducible nitric oxide (iNOS) as isoforms of the nitric oxide synthase (NOS) as well as nitrotyrosine as an end product of protein nitration was analyzed in sections of temporal cortex taken from postmortem brains of patients with Alzheimer's disease (AD). The patients were evaluated by the Clinical Dementia Rating scale (CDR0-CDR3) and studied in the Memory and Aging Project (MAP) of the Washington University Alzheimer Disease Research Center (ADCR). With the use of immunocytochemical procedures, neurons immunoreactive to nNOS were found to show large and small multipolar and pyramidal morphologies over the entire chronic AD evolution. The iNOS and nitrotyrosine immunoreactivities were also found in pyramidal-like cortical neurons and glial cells. Here, we speculate on the interaction among all specific neurodegenerative changes in AD and nitric oxide as an additional contribution to neuronal death in AD.

Dysregulated IP3 signaling in cortical neurons of knock-in mice expressing an Alzheimer's-linked mutation in presenilin1 results in exaggerated Ca2+ signals and altered membrane excitability

Disruptions in intracellular Ca2+ signaling are proposed to underlie the pathophysiology of Alzheimer's disease (AD), and it has recently been shown that AD-linked mutations in the presenilin 1 gene (PS1) enhance inositol triphosphate (IP3)-mediated Ca2+ liberation in nonexcitable cells. However, little is known of these actions in neurons, which are the principal locus of AD pathology. We therefore sought to determine how PS1 mutations affect Ca2+ signals and their subsequent downstream effector functions in cortical neurons. Using whole-cell patch-clamp recording, flash photolysis, and two-photon imaging in brain slices from 4-5-week-old mice, we show that IP3-evoked Ca2+ responses are more than threefold greater in PS1(M146V) knock-in mice relative to age-matched nontransgenic controls. Electrical excitability is thereby reduced via enhanced Ca2+ activation of K+ conductances. Action potential-evoked Ca2+ signals were unchanged, indicating that PS1(M146V) mutations specifically disrupt intracellular Ca2+ liberation rather than reduce cytosolic Ca2+ buffering or clearance. Moreover, IP3 receptor levels are not different in cortical homogenates, further suggesting that the exaggerated cytosolic Ca2+ signals may result from increased store filling and not from increased flux through additional IP3-gated channels. Even in young animals, PS1 mutations have profound effects on neuronal Ca2+ and electrical signaling: cumulatively, these disruptions may contribute to the long-term pathophysiology of AD.

AATF Inhibits Aberrant Production of Amyloid β Peptide 1-42 by Interacting Directly with Par-4

Aggregation of the neurotoxic amyloid beta peptide 1-42 (Abeta-(1-42)) in the brain is considered to be an early event in the pathogenesis of Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a leucine zipper protein that is pro-apoptotic and associated with neuronal degeneration in AD. Overexpression of Par-4 significantly increased production of Abeta-(1-42) after initiation of apoptotic cascades, indicating factors regulating apoptotic pathways may also affect processing of beta-amyloid precursor protein (APP). AATF (apoptosis-antagonizing transcription factor) was recently identified as an interaction partner of DAP-like kinase (Dlk), a member of the DAP (death-associated protein) kinase family. AATF antagonizes apoptosis induced by Par-4, suggesting that AATF might directly or indirectly participate in regulation of Par-4 activity. We now report that AATF colocalizes with Par-4 in both cytoplasmic and nuclear compartments, and it interacts directly and selectively with Par-4 via the leucine zipper domain in neural cells. Par-4 induced an aberrant production and secretion of Abeta in neuroblastoma IMR-32 cells after apoptotic cascades are initiated. Co-expression of AATF completely blocked aberrant production and secretion of Abeta-(1-42) induced by Par-4, and AATF/Par-4 complex formation was essential for the inhibitory effect of AATF on aberrant Abeta secretion. These results indicate that AATF is an endogenous antagonist of Par-4 activity and an effective inhibitor of aberrant Abeta production and secretion under apoptotic conditions.

Amyloid-beta: a chameleon walking in two worlds: a review of the trophic and toxic properties of amyloid-beta

Although much maligned, the amyloid-beta (Abeta) protein has been shown to possess a number of trophic properties that emanate from the protein's ability to bind Cu, Fe and Zn. Abeta belongs to a group of proteins that capture redox metal ions (even under mildly acidotic conditions), thereby preventing them from participating in redox cycling with other ligands. The coordination of Cu appears to be crucial for Abeta's own antioxidant activity that has been demonstrated both in vitro as well as in the brain, cerebrospinal fluid and plasma. The chelation of Cu by Abeta would therefore be predicted to dampen oxidative stress in the mildly acidotic and oxidative environment that accompanies acute brain trauma and Alzheimer's disease (AD). Given that oxidative stress promotes Abeta generation, the formation of diffuse amyloid plaques is likely to be a compensatory response to remove reactive oxygen species. This review weighs up the evidence supporting both the trophic and toxic properties of Abeta, and while evidence for direct Abeta neurotoxicity in vivo is scarce, we postulate that the product of Abeta's antioxidant activity, hydrogen peroxide (H(2)O(2)), is likely to mediate toxicity as the levels of this oxidant rise with the accumulation of Abeta in the AD brain. We propose that metal ion chelators, antioxidants, antiinflammatories and amyloid-lowering drugs that target the reduction of H(2)O(2) and/or Abeta generation may be efficacious in decreasing neurotoxicity. However, given the antioxidant activity of Abeta, we suggest that the excessive removal of Abeta may prevent adequate chelation of metal ions and removal of O(2)(-z.ccirf;), leading to enhanced, rather than reduced, neuronal oxidative stress.

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

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

Cerebrometabolic abnormalities in Alzheimer's disease

Extensive, replicated evidence in patients in vivo and in Alzheimer (AD) tissues in vitro indicates that impaired brain metabolism is one of the cardinal and essentially invariable events in AD. The degree of impairment in brain metabolism is proportional to the degree of clinical disability, both in vivo and in vitro. The 'cerebrometabolic lesion' cannot be attributed to 'slower thinking' or 'brain atrophy', because of quantitative considerations and because the metabolic lesion precedes the development of neuropsychological abnormalities or decreases in brain mass detectable by modern imaging techniques. The causes of the cerebrometabolic lesion in AD are not well defined. Free radicals seem likely to be involved, including free radicals generated from Alzheimer amyloid. Thus, the importance of the cerebrometabolic lesion is entirely compatible with most versions of the widely accepted 'amyloid cascade hypothesis' of AD. A variety of plausible, redundantly documented mechanisms are compatible with the proposal that the cerebrometabolic lesion is a proximate cause of the clinical disability in AD. In agreement with these findings, recent attempts to treat the cerebrometabolic lesion in AD have given encouraging preliminary results. The cerebrometabolic lesion in AD deserves further study.

Mitochondrial dysfunction, apoptotic cell death, and Alzheimer’s disease

Being major sources of reactive oxygen species (ROS), mitochondrial structures are exposed to high concentrations of ROS and might therefore be particularly susceptible to oxidative injury. Mitochondrial damage may play a pivotal role in the cell death decision. Bolstered evidence indicates that mitochondrial abnormalities might be part of the spectrum of chronic oxidative stress occurring in Alzheimer's disease (AD) finally contributing to synaptic failure and neuronal degeneration. Accumulation and oligomerization of amyloid beta (Abeta) is also thought to play a central role in the pathogenesis of this disease by probably directly leading to mitochondrial dysfunction. Moreover, numerous lines of findings indicate increased susceptibility to apoptotic cell death and increased oxidative damage as common features in neurons from sporadic AD patients but also from familial AD (FAD) cases. Here we provide a summary of recent work demonstrating some key abnormalities that may initiate and promote pathological events in AD. Finally, we emphasize a hypothetical sequence of the pathogenic steps linking sporadic AD, FAD, and Abeta production with mitochondrial dysfunction, caspase pathway, and neuronal loss.

Overexpression of wild-type presenilin 2 or its familial Alzheimer's disease-associated mutant does not induce or increase susceptibility to apoptosis in different cell lines

Programmed cell death, or apoptosis, has been implicated in Alzheimer's disease. Mutations in the presenilin (PS) genes, PS1 and PS2, are a major cause of early-onset familial Alzheimer's disease (FAD). Previous studies have suggested that the PS play a role in apoptosis. However, the mechanisms whereby presenilins affect apoptosis and the relationship of FAD-associated presenilin mutants to the apoptotic effect have not been elucidated. In the present study, in an attempt to further explore the effect of PS2 on apoptosis we examined whether overexpression of wild-type or mutant PS2 can directly induce apoptosis or increase cell susceptibility to apoptosis in various cell lines, such as N2a, CHO, and HEK 293T. Wild-type or mutant PS2 was transiently transfected into these cell lines and the viability of the transfected cells was evaluated by their morphology, DNA fragmentation and condensation, appearance of sub-G(1/0) cells, and caspase activation. We also examined the susceptibility of the PS2-transfected cells to apoptosis induced by the apoptotic inducers staurosporine and H(2)O(2). Our results showed that overexpression of either wild type or mutant PS2 in these cell lines did not directly induce apoptosis or increase the susceptibility to apoptosis induced by staurosporine or H(2)O(2). Taken together, these results suggest that overexpression of PS2 does not cause pro-apoptotic effects, at least not in the cellular systems and conditions employed in this study, and therefore it seems unlikely that apoptosis plays a prominent role in the neuropathological effects of PS2 in Alzheimer's disease.

Transgenic mice expressing the PS1-A246E mutation: effects on spatial learning, exploration, anxiety, and motor coordination

The functional consequence of the PS1-A246E mutation was assessed in transgenic mice on a background lacking the endogenous PS1 gene. These mice have elevated concentrations of A-beta protein (Abeta(42)) in the absence of plaque formation. By comparison to a mixed background strain (50% B6, 25% SJl, 25% 129Sv) controlled for age and gender, PS1-A246E transgenic mice displayed disinhibitory tendencies, as indicated by increased entries and duration in the open arms of the elevated plus-maze. Despite normal spontaneous alternation rates in a T-maze, latencies before responding were higher in PS1-A246E transgenic mice than controls. Moreover, the PS1-A246E transgenic mice fell more often from two stationary beams, but not from the coat-hanger and the rotorod. By contrast, ambulation in an automated photocell chamber and in an open-field was not affected. Nor was acquisition of place learning in the Morris water maze task. These results indicate that elevated Abeta(42) levels were insufficient for causing spatial defects but caused disinhibition, psychomotor slowing, and loss of motor skills in this model of familial Alzheimer's disease.

Propentofylline protects beta-amyloid protein-induced apoptosis in cultured rat hippocampal neurons

beta-Amyloid protein 1-42 (beta42) can induce apoptosis in the cultured hippocampal neurons, suggesting that it plays an important role in causing neurodegeneration in Alzheimer's disease. Recently, propentofylline, a synthetic xanthine derivative, has been reported to depress ischemic degeneration of hippocampal neurons in gerbils. The present study investigated whether or not propentofylline affected the beta42-induced apoptosis of hippocampal neurons, and if so, which type of signaling machinery works in the neuroprotective action of propentofylline. Addition of propentofylline markedly attenuated the beta42-induced cell death of rat hippocampal neurons. The amyloid protein certainly induced apoptosis in the cultured hippocampal cells revealed by nuclear condensation, caspase-3 activation and an increase of Bax. Intriguingly, propentofylline blocked both the apoptotic features induced by beta42 and further induced an anti-apoptotic protein, Bcl-2, during a short time of incubation. The neuroprotective action of propentofylline was comparably replaced with dibutyryl cAMP (dbcAMP) and was completely suppressed by a low concentration of specific protein kinase A (PKA) inhibitor. Taken altogether, the data strongly suggest that the protection of propentofylline on the beta42-induced neurotoxicity is caused by enhancing anti-apoptotic action through cAMP-PKA system. Propentofylline as a therapeutic agent to Alzheimer's disease is discussed.

Disruption of neurogenesis by amyloid beta-peptide, and perturbed neural progenitor cell homeostasis, in models of Alzheimer's disease

Neurogenesis occurs in the adult mammalian brain and may play roles in learning and memory processes and recovery from injury, suggesting that abnormalities in neural progenitor cells (NPC) might contribute to the pathogenesis of disorders of learning and memory in humans. The objectives of this study were to determine whether NPC proliferation, survival and neuronal differentiation are impaired in a transgenic mouse model of Alzheimer's disease (AD), and to determine the effects of the pathogenic form of amyloid beta-peptide (Abeta) on the survival and neuronal differentiation of cultured NPC. The proliferation and survival of NPC in the dentate gyrus of the hippocampus was reduced in mice transgenic for a mutated form of amyloid precursor protein that causes early onset familial AD. Abeta impaired the proliferation and neuronal differentiation of cultured human and rodent NPC, and promoted apoptosis of neuron-restricted NPC by a mechanism involving dysregulation of cellular calcium homeostasis and the activation of calpains and caspases. Adverse effects of Abeta on NPC may contribute to the depletion of neurons and cognitive impairment in AD.

Central role of glycogen synthase kinase-3beta in endoplasmic reticulum stress-induced caspase-3 activation

Stress of the endoplasmic reticulum (ER), which is associated with many neurodegenerative conditions, can lead to the elimination of affected cells by apoptosis through only partially understood mechanisms. Thapsigargin, which causes ER stress by inhibiting the ER Ca(2+)-ATPase, was found to not only activate the apoptosis effector caspase-3 but also to cause a large and prolonged increase in the activity of glycogen synthase kinase-3beta (GSK3beta). Activation of GSK3beta was obligatory for thapsigargin-induced activation of caspase-3, because inhibition of GSK3beta by expression of dominant-negative GSK3beta or by the GSK3beta inhibitor lithium blocked caspase-3 activation. Thapsigargin treatment activated GSK3beta by inducing dephosphorylation of phospho-Ser-9 of GSK3beta, a phosphorylation that normally maintains GSK3beta inactivated. Caspase-3 activation induced by thapsigargin was blocked by increasing the phosphorylation of Ser-9-GSK3beta with insulin-like growth factor-1 or with the phosphatase inhibitors okadaic acid and calyculin A, but the calcineurin inhibitors FK506 and cyclosporin A were ineffective. Insulin-like growth factor-1, okadaic acid, calyculin A, and lithium also protected cells from two other inducers of ER stress, tunicamycin and brefeldin A. Thus, ER stress activates GSK3beta through dephosphorylation of phospho-Ser-9, a prerequisite for caspase-3 activation, and this process is amenable to pharmacological intervention.

Pycnogenol protects neurons from amyloid-beta peptide-induced apoptosis

Neuronal apoptosis is one of the pathological features of Alzheimer's disease (AD). Morphological pathology reveals that neuronal apoptosis is associated with senile plaques containing amyloid-beta peptide (Abeta) in AD brains. Reactive oxygen species (ROS) has been proposed to be involved in the apoptotic mechanism of Abeta-mediated neurotoxicity. In the present study, using a rat pheochromocytoma (PC12) cell line, we investigated the effect of Pycnogenol (PYC), a potent antioxidant and ROS scavenger, on Abeta(25-35)-induced apoptosis and ROS generation. We used vitamin E, a known antioxidant agent, to verify the effect of PYC. Abeta(25-35)-induced apoptosis in PC12 cells was demonstrated by: (1) a dose-dependent loss of cell viability; (2) a time- and dose-dependent increase in the apoptotic cells; (3) an induction of DNA fragmentation; and (4) an increase in caspase-3 activity and cleavage of poly (ADP-ribose) polymerase (PARP). Our data showed that a significant increase in ROS formation preceded apoptotic events after PC12 cells were exposed to Abeta(25-35). We further found that PYC not only suppressed the generation of ROS but also attenuated caspase-3 activation, DNA fragmentation, PARP cleavage, and eventually protected against Abeta-induced apoptosis. Vitamin E also suppressed cell death and caspase-3 activation induced by Abeta(25-35). Taken together, these results suggest that ROS may be involved in Abeta-induced apoptosis in PC12 cells. They further suggest that PYC can reduce apoptosis, possibly by decreasing free radical generation in PC12 cells.

Modification of brain aging and neurodegenerative disorders by genes, diet, and behavior

Multiple molecular, cellular, structural, and functional changes occur in the brain during aging. Neural cells may respond to these changes adaptively, or they may succumb to neurodegenerative cascades that result in disorders such as Alzheimer's and Parkinson's diseases. Multiple mechanisms are employed to maintain the integrity of nerve cell circuits and to facilitate responses to environmental demands and promote recovery of function after injury. The mechanisms include production of neurotrophic factors and cytokines, expression of various cell survival-promoting proteins (e.g., protein chaperones, antioxidant enzymes, Bcl-2 and inhibitor of apoptosis proteins), preservation of genomic integrity by telomerase and DNA repair proteins, and mobilization of neural stem cells to replace damaged neurons and glia. The aging process challenges such neuroprotective and neurorestorative mechanisms. Genetic and environmental factors superimposed upon the aging process can determine whether brain aging is successful or unsuccessful. Mutations in genes that cause inherited forms of Alzheimer's disease (amyloid precursor protein and presenilins), Parkinson's disease (alpha-synuclein and Parkin), and trinucleotide repeat disorders (huntingtin, androgen receptor, ataxin, and others) overwhelm endogenous neuroprotective mechanisms; other genes, such as those encoding apolipoprotein E(4), have more subtle effects on brain aging. On the other hand, neuroprotective mechanisms can be bolstered by dietary (caloric restriction and folate and antioxidant supplementation) and behavioral (intellectual and physical activities) modifications. At the cellular and molecular levels, successful brain aging can be facilitated by activating a hormesis response in which neurons increase production of neurotrophic factors and stress proteins. Neural stem cells that reside in the adult brain are also responsive to environmental demands and appear capable of replacing lost or dysfunctional neurons and glial cells, perhaps even in the aging brain. The recent application of modern methods of molecular and cellular biology to the problem of brain aging is revealing a remarkable capacity within brain cells for adaptation to aging and resistance to disease.

Hypoxia potentiates exocytosis and Ca2+ channels in PC12 cells via increased amyloid beta peptide formation and reactive oxygen species generation

Exposure of PC12 cells to chronic hypoxia (CH; 10 % O(2), 24 h) augments catecholamine secretion via formation of a Cd2+-resistant Ca2+ influx pathway, and up-regulates native L-type Ca2+ channels. These effects are mimicked by exposure of cells to Alzheimer's disease-associated amyloid beta peptides (AbetaPs). Since pathological effects of AbetaPs have been associated with increased levels of reactive oxygen species (ROS), the involvement of ROS in hypoxia-mediated up-regulation of exocytosis and Ca2+ channel activity was examined. Both melatonin and ascorbic acid (two structurally unrelated antioxidants) fully blocked the enhancement of catecholamine secretion caused by CH (as determined amperometrically). Enhanced immunofluorescence, observed in chronically hypoxic cells using a primary monoclonal antibody raised against the N-terminus of AbetaP, was also suppressed by melatonin. Ascorbic acid, melatonin and ebselen (an additional antioxidant) also fully prevented augmentation of whole-cell Ca2+ currents caused by CH (as monitored using whole-cell patch-clamp recordings). Exposure of normoxic cells to H(2)O(2) (40 microM, 24 h), like hypoxia, caused Ca2+ channel up-regulation. Importantly, AbetaP formation appeared to be an absolute requirement for the effects of hypoxia, since the ability of CH to augment exocytosis and Ca2+ channel activity was blocked by two novel inhibitors of gamma secretase, an enzyme complex required for AbetaP formation. Our results indicate that the effects of hypoxia require ROS generation from AbetaPs, and suggest that elevated levels of ROS mediate hypoxic and AbetaP-mediated pathological remodelling of Ca2+ homeostasis.

Intraneuronal advanced glycation endproducts in presenilin-1 Alzheimer's disease

The most frequently mutated gene resulting in dominantly inherited Alzheimer's disease is presenilin-1. We have used antibodies against advanced glycation endproducts (AGE) in brain tissue sections of four patients with three different presenilin I mutations. Accumulation of intracellular AGE was observed in 75-95% of pyramidal neurons in patients with presenilin-1 mutations, far exceeding the percentage of presenilin-1-, tau- or ubiquitin-positive neurons. This high level of AGE-modified proteins in vulnerable neurons is most likely explained by higher levels of their precursors (reactive (di)carbonyl products) or a slower turnover of the participating proteins. These conditions of carbonyl stress may contribute to increased neuronal dysfunction and vulnerability leading to the early disease onset.