Alzheimer's disease and Abeta toxicity: from top to bottom

Surface plasmon resonance and nuclear magnetic resonance studies of ABAD-Abeta interaction

Abeta binding alcohol dehydrogenase (ABAD) is an NAD-dependent mitochondrial dehydrogenase. The binding between ABAD and Abeta is likely a direct link between Abeta and mitochondrial toxicity in Alzheimer's disease. In this study, surface plasmon resonance (SPR) was employed to determine the temperature dependence of the affinity of the ABAD-Abeta interaction. A van't Hoff analysis revealed that the ABAD-Abeta association is driven by a favorable entropic change (DeltaS = 300 +/- 30 J mol-1 K-1) which overcomes an unfavorable enthalpy change (DeltaH = 49 +/- 7 kJ/mol). Therefore, hydrophobic interactions and changes in protein dynamics are the dominant driving forces of the ABAD-Abeta interaction. This is the first dissection of the entropic and enthalpic contribution to the energetics of a protein-protein interaction involving Abeta. SPR confirmed the conformational changes in the ABAD-Abeta complex after Abeta binding, consistent with differences seen in the crystal structures of free ABAD and the ABAD-Abeta complex. Saturation transfer difference (STD) NMR experiments directly and unambiguously demonstrated the inhibitory effect of Abeta on the ABAD-NAD interaction. Conversely, NAD inhibits the Abeta-ABAD interaction. Binding of Abeta and binding of NAD to ABAD are likely mutually exclusive. Thus, Abeta binding induces conformational and subsequently functional changes in ABAD, which may have a role in the mechanism of Abeta toxicity in Alzheimer's disease.

AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss

Beta amyloid (Abeta), a peptide generated from the amyloid precursor protein (APP) by neurons, is widely believed to underlie the pathophysiology of Alzheimer's disease. Recent studies indicate that this peptide can drive loss of surface AMPA and NMDA type glutamate receptors. We now show that Abeta employs signaling pathways of long-term depression (LTD) to drive endocytosis of synaptic AMPA receptors. Synaptic removal of AMPA receptors is necessary and sufficient to produce loss of dendritic spines and synaptic NMDA responses. Our studies indicate the central role played by AMPA receptor trafficking in Abeta-induced modification of synaptic structure and function.

Transthyretin oligomers induce calcium influx via voltage-gated calcium channels

The deposition of transthyretin (TTR) amyloid in the PNS is a major pathological feature of familial amyloidotic polyneuropathy. The aim of the present study was to examine whether TTR could disrupt cytoplasmic Ca(2+) homeostasis and to determine the role of TTR aggregation in this process. The aggregation of amyloidogenic TTR was examined by solution turbidity, dynamic light scattering and atomic force microscopy. A nucleation-dependent polymerization process was observed in which TTR formed low molecular weight aggregates (oligomers < 100 nm in diameter) before the appearance of mature fibrils. TTR rapidly induced an increase in the concentration of intracellular Ca(2+) ([Ca(2+)](i)) when applied to SH-SY5Y human neuroblastoma cells. The greatest effect on [Ca(2+)](i) was induced by a preparation that contained the highest concentration of TTR oligomers. The TTR-induced increase in [Ca(2+)](i) was due to an influx of extracellular Ca(2+), mainly via L- and N-type voltage-gated calcium channels (VGCCs). These results suggest that increasing [Ca(2+)](i) via VGCCs may be an important early event which contributes to TTR-induced cytotoxicity, and that TTR oligomers, rather than mature fibrils, may be the major cytotoxic form of TTR.

Amyloid precursor protein overexpression depresses excitatory transmission through both presynaptic and postsynaptic mechanisms

Overexpression of the amyloid precursor protein (APP) in hippocampal neurons leads to elevated beta-amyloid peptide (Abeta) production and consequent depression of excitatory transmission. The precise mechanisms underlying APP-induced synaptic depression are poorly understood. Uncovering these mechanisms could provide insight into how neuronal function is compromised before cell death during the early stages of Alzheimer's disease. Here we verify that APP up-regulation leads to depression of transmission in cultured hippocampal autapses; and we perform whole-cell recording, FM imaging, and immunocytochemistry to identify the specific mechanisms accounting for this depression. We find that APP overexpression leads to postsynaptic silencing through a selective reduction of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents. This effect is likely mediated by Abeta because expression of mutant APP incapable of producing Abeta did not depress transmission. In addition, although we eliminate presynaptic silencing as a mechanism underlying APP-mediated inhibition of transmission, we did observe an Abeta-induced presynaptic deficit in vesicle recycling with sustained stimulation. These findings demonstrate that APP elevation disrupts both presynaptic and postsynaptic compartments.

Disease modifying therapy for AD?

Alzheimer's disease (AD) is the most common form of dementia in industrialized nations. If more effective therapies are not developed that either prevent AD or block progression of the disease in its very early stages, the economic and societal cost of caring for AD patients will be devastating. Only two types of drugs are currently approved for the treatment of AD: inhibitors of acetyl cholinesterase, which symptomatically enhance cognitive state to some degree but are not disease modifying; and the adamantane derivative, memantine. Memantine preferentially blocks excessive NMDA receptor activity without disrupting normal receptor activity and is thought to be a neuroprotective agent that blocks excitotoxicty. Memantine therefore may have a potentially disease modifying effect in multiple neurodegenerative conditions. An improved understanding of the pathogeneses of AD has now led to the identification of numerous therapeutic targets designed to alter amyloid beta protein (Abeta) or tau accumulation. Therapies that alter Abeta and tau through these various targets are likely to have significant disease modifying effects. Many of these targets have been validated in proof of concept studies in preclinical animal models, and some potentially disease modifying therapies targeting Abeta or tau are being tested in the clinic. This review will highlight both the promise of and the obstacles to developing such disease modifying AD therapies.

Distinct destructive signal pathways of neuronal death in Alzheimer's disease

Abundant neuron loss is a major feature of Alzheimer's disease (AD). Hypotheses for this loss include abnormal amyloid precursor protein processing (i.e. excess Abeta production, protein aggregation or misfolding), oxidative stress, excitotoxicity and inflammation. Neuron loss is a major cause of dementia in AD; however, it seems that there is no definitive pathway that causes cell death in the AD brain. Here, we examine the hypotheses for neuron loss in AD and pose the argument that the means by which neurons degenerate is irrelevant for cognitive decline. The best treatment for cognitive decline is to prevent the toxicity that first sets the neuron on its path to destruction, which is the production of Abeta peptide.

A network dysfunction perspective on neurodegenerative diseases

Patients with Alzheimer's disease or other neurodegenerative disorders show remarkable fluctuations in neurological functions, even during the same day. These fluctuations cannot be caused by sudden loss or gain of nerve cells. Instead, it is likely that they reflect variations in the activity of neural networks and, perhaps, chronic intoxication by abnormal proteins that the brain is temporarily able to overcome. These ideas have far-reaching therapeutic implications.

Alois Alzheimer and Alzheimer's disease: a centennial perspective

The year 2006 is the centenary of the famous presentation of Alois Alzheimer which first described the neuropathology of Alzheimer's disease (AD). Since this presentation, enormous progress has been made in understanding the biology of AD. The central role of the beta-amyloid protein (Abeta) in the pathogenesis of AD and the relationship between plaque and tangle pathology is now much better understood. In this article, we review the current status of the amyloid hypothesis of AD and its role in the development of future therapy.

“The seven sins” of the Hebbian synapse: Can the hypothesis of synaptic plasticity explain long-term memory consolidation?

Memorizing new facts and events means that entering information produces specific physical changes within the brain. According to the commonly accepted view, traces of memory are stored through the structural modifications of synaptic connections, which result in changes of synaptic efficiency and, therefore, in formations of new patterns of neural activity (the hypothesis of synaptic plasticity). Most of the current knowledge on learning and initial stages of memory consolidation ("synaptic consolidation") is based on this hypothesis. However, the hypothesis of synaptic plasticity faces a number of conceptual and experimental difficulties when it deals with potentially permanent consolidation of declarative memory ("system consolidation"). These difficulties are rooted in the major intrinsic self-contradiction of the hypothesis: stable declarative memory is unlikely to be based on such a non-stable foundation as synaptic plasticity. Memory that can last throughout an entire lifespan should be "etched in stone." The only "stone-like" molecules within living cells are DNA molecules. Therefore, I advocate an alternative, genomic hypothesis of memory, which suggests that acquired information is persistently stored within individual neurons through modifications of DNA, and that these modifications serve as the carriers of elementary memory traces.

Plasmin deficiency in Alzheimer's disease brains: causal or casual?

Substantial recent evidence suggests that defects in amyloid peptide degradation can be at the base of cases of sporadic Alzheimer's disease (AD). Among the discovered brain enzymes with the capacity to degrade amyloid peptide, the serine protease plasmin acquires special physiological relevance because of its low levels in areas of AD human brains with a high susceptibility to amyloid plaque accumulation. In this article we comment on a series of observations supporting the fact that plasmin paucity in the brain is not simply a secondary event in the disease but rather a primary defect in certain cases of sporadic AD. We also refer to recent data pointing to alterations in raft membrane domains and diminished membrane cholesterol as the underlying cause. Finally, we discuss the possibility that plasmin deficiency in the brain could lead to AD symptomatology because of amyloid aggregation and the triggering of cell death signaling cascades.

Enriched environments, experience-dependent plasticity and disorders of the nervous system

Behavioural, cellular and molecular studies have revealed significant effects of enriched environments on rodents and other species, and provided new insights into mechanisms of experience-dependent plasticity, including adult neurogenesis and synaptic plasticity. The demonstration that the onset and progression of Huntington's disease in transgenic mice is delayed by environmental enrichment has emphasized the importance of understanding both genetic and environmental factors in nervous system disorders, including those with Mendelian inheritance patterns. A range of rodent models of other brain disorders, including Alzheimer's disease and Parkinson's disease, fragile X and Down syndrome, as well as various forms of brain injury, have now been compared under enriched and standard housing conditions. Here, we review these findings on the environmental modulators of pathogenesis and gene-environment interactions in CNS disorders, and discuss their therapeutic implications.

Nicotinic receptor agonists as neuroprotective/neurotrophic drugs. Progress in molecular mechanisms

In the present work we reviewed recent advances concerning neuroprotective/neurotrophic effects of acute or chronic nicotine exposure, and the signalling pathways mediating these effects, including mechanisms implicated in nicotine addiction and nAChR desensitization. Experimental and clinical data largely indicate long-lasting effects of nicotine and nicotinic agonists that imply a neuroprotective/neurotrophic role of nAChR activation, involving mainly alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR agonists. Compounds interacting with neuronal nAChRs have the potential to be neuroprotective and treatment with nAChR agonists elicits long-lasting neurotrophic effects, e.g. improvement of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Nicotine addiction, which is mediated by interaction with nACh receptors, is believed to involve the modification of signalling cascades that modulate synaptic plasticity and gene expression. Desensitization, in addition to protecting cells from uncontrolled excitation, is recently considered as a form of signal plasticity. nAChR can generate these longe-lasting effects by elaboration of complex intracellular signals that mediate medium to long-term events crucial for neuronal maintenance, survival and regeneration. Although a comprehensive survey of the gene-based molecular mechanisms that underlie nicotine effects has yet not been performed a growing amount of data is beginning to improve our understanding of signalling mechanisms that lead to neurotrophic/neuroprotective responses. Evidence for an involvement of the fibroblast growth factor-2 gene in nAChR mechanisms mediating neuronal survival, trophism and plasticity has been obtained. However, more work is needed to establish the mechanisms involved in the effects of nicotinic receptor subtype activation from cognition-enhancing and neurotrophic effects to smoking behaviour and to determine more precisely the therapeutic objectives in potential nicotinic drug treatments of neurodegenerative diseases.

Fully flexible docking models of the complex between α7 nicotinic receptor and a potent heptapeptide inhibitor of the β-amyloid peptide binding

The heptapeptide IQTTWSR (IQ), recently reported as inhibitor of the beta-amyloid (Abeta) binding to nicotinic acetylcholine receptors (nAChrs), was docked to the homology model of the alpha7 nicotinic acetylcholine receptor. The most representative models were further subjected to molecular dynamics simulations. The data obtained here suggest that Abeta needs highly specific structural motifs to bind to the alpha7nAChR. These structural motifs are located principally in the upper and lower surroundings of loop C, including loop F and sheets beta1, beta2, beta6, beta9, and beta10 of the receptor. Overall, these results suggest that IQ can be mimicked by more bioavailable, stable compounds that would be helpful for the understanding of the Abeta binding site and its dynamics, and for the design of novel agents to be used as an effective alternative against Alzheimer's disease.

Amyloid excess in Alzheimer's disease: What is cholesterol to be blamed for?

A link between alterations in cholesterol homeostasis and Alzheimer's disease (AD) is nowadays widely accepted. However, the molecular mechanism/s underlying such link remain unclear. Numerous experimental evidences support the view that changes in neuronal membrane cholesterol levels and/or subcellular distribution determine the aberrant accumulation of the amyloid peptide in the disease. Still, this view comes from rather contradictory data supporting the existence of either high or low brain cholesterol content. This is of particular concern considering that therapeutical strategies aimed to reduce cholesterol levels are already being tested in humans. Here, we review the molecular mechanisms proposed and discuss the perspectives they open.

Non-fibrillar β-amyloid abates spike-timing-dependent synaptic potentiation at excitatory synapses in layer 2/3 of the neocortex by targeting postsynaptic AMPA receptors

Cognitive decline in Alzheimer's disease (AD) stems from the progressive dysfunction of synaptic connections within cortical neuronal microcircuits. Recently, soluble amyloid beta protein oligomers (Abeta(ol)s) have been identified as critical triggers for early synaptic disorganization. However, it remains unknown whether a deficit of Hebbian-related synaptic plasticity occurs during the early phase of AD. Therefore, we studied whether age-dependent Abeta accumulation affects the induction of spike-timing-dependent synaptic potentiation at excitatory synapses on neocortical layer 2/3 (L2/3) pyramidal cells in the APPswe/PS1dE9 transgenic mouse model of AD. Synaptic potentiation at excitatory synapses onto L2/3 pyramidal cells was significantly reduced at the onset of Abeta pathology and was virtually absent in mice with advanced Abeta burden. A decreased alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/N-methyl-D-aspartate (NMDA) receptor-mediated current ratio implicated postsynaptic mechanisms underlying Abeta synaptotoxicity. The integral role of Abeta(ol)s in these processes was verified by showing that pretreatment of cortical slices with Abeta((25-35)ol)s disrupted spike-timing-dependent synaptic potentiation at unitary connections between L2/3 pyramidal cells, and reduced the amplitude of miniature excitatory postsynaptic currents therein. A robust decrement of AMPA, but not NMDA, receptor-mediated currents in nucleated patches from L2/3 pyramidal cells confirmed that Abeta(ol)s perturb basal glutamatergic synaptic transmission by affecting postsynaptic AMPA receptors. Inhibition of AMPA receptor desensitization by cyclothiazide significantly increased the amplitude of excitatory postsynaptic potentials evoked by afferent stimulation, and rescued synaptic plasticity even in mice with pronounced Abeta pathology. We propose that soluble Abeta(ol)s trigger the diminution of synaptic plasticity in neocortical pyramidal cell networks during early stages of AD pathogenesis by preferentially targeting postsynaptic AMPA receptors.

The role of beta-amyloid protein in synaptic function: implications for Alzheimer's disease therapy

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive and irreversible loss of memory and other cognitive functions. Substantial evidence based on genetic, neuropathological and biochemical data has established the central role of beta-amyloid protein (betaAP) in this pathology. Although the precise etiology of AD is not well understood yet, strong evidence for some of the molecular events that lead to progressive brain dysfunction and neurodegeneration in AD has been afforded by identification of biochemical pathways implicated in the generation of betaAP, development of transgenic models exhibiting progressive disease pathology and by data on the effects of betaAP at the neuronal network level. However, the mechanisms by which betaAP causes cognitive decline have not been determined, nor is it clear if the degree of dementia correlates in time with the degree of neuronal loss. Hence, it is of interest to understand the biochemical processes involved in the mechanisms of betaAP-induced neurotoxicity and the mechanisms involved in electrophysiological effects of this protein on different parameters of synaptic transmission and on neuronal firing properties. In this review we analyze recent evidence suggesting a complex role of betaAP in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in AD as well as the therapeutic implications based on betaAP metabolism inhibition.

High resolution scanning tunnelling microscopy of the beta-amyloid protein (Abeta1-40) of Alzheimer's disease suggests a novel mechanism of oligomer assembly

The aggregation of the beta-amyloid protein (Abeta) is an important step in the pathogenesis of Alzheimer's disease. There is increasing evidence that lower molecular weight oligomeric forms of Abeta may be the most toxic species in vivo. However, little is known about the structure of Abeta oligomers. In this study, scanning tunnelling microscopy (STM) was used to examine the structure of Abeta monomers, dimers and oligomers. Abeta1-40 was visualised by STM on a surface of atomically flat gold. At low concentrations (0.5 microM) small globular structures were observed. High resolution STM of these structures revealed them to be monomers of Abeta. The monomers measured approximately 3-4 nm in diameter. Internal structure was seen in many of the monomers consistent with a conformation in which the polypeptide chain is folded into 3 or 4 domains. Oligomers were seen after ageing the Abeta solution for 24 h. The oligomers were also 3-4 nm in width and appeared to be formed by the end-to-end association of monomers with the polypeptide chain oriented at 90 degrees to the axis of the oligomer. The results suggest that the oligomer formation can proceed through a mechanism involving the linear association of monomers.

AMPA receptor downscaling at the onset of Alzheimer's disease pathology in double knockin mice

It is widely thought that Alzheimer's disease (AD) begins as a malfunction of synapses, eventually leading to cognitive impairment and dementia. Homeostatic synaptic scaling is a mechanism that could be crucial at the onset of AD but has not been examined experimentally. In this process, the synaptic strength of a neuron is modified so that the overall excitability of the cell is maintained. Here, we investigate whether synaptic scaling mediated by l-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) contributes to pathology in double knockin (2 x KI) mice carrying human mutations in the genes for amyloid precursor protein and presenilin-1. By using whole-cell recordings, we show that 2 x KI mice exhibit age-related downscaling of AMPAR-mediated evoked currents and spontaneous, miniature currents. Electron microscopic analysis further corroborates the synaptic AMPAR decrease. Additionally, 2 x KI mice show age-related deficits in bidirectional plasticity (long-term potentiation and long-term depression) and memory flexibility. These results suggest that AMPARs are important synaptic targets for AD and provide evidence that cognitive impairment may involve downscaling of postsynaptic AMPAR function.

GENE–ENVIRONMENT INTERACTIONS, NEURONAL DYSFUNCTION AND PATHOLOGICAL PLASTICITY IN HUNTINGTON'S DISEASE

Huntington's disease (HD) is a fatal autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. The dynamic mutation that causes the disease is common to numerous other brain disorders, which may share similar pathogenic mechanisms. Much progress has been made in the past decade in understanding how a trinucleotide (CAG) repeat expansion, encoding an expanded polyglutamine tract in the huntingtin protein, induces dysfunction at molecular and cellular levels. The present review integrates various lines of experimental evidence in an attempt to move towards a unifying mechanistic framework, which may explain the pathogenesis of HD, from molecular through to neuronal network and behavioural levels. Recent evidence, using transgenic mouse models, also suggests that environmental factors can modify the onset and progression of HD. The effects of specific environmental manipulations are discussed in the context of gene-environment interactions and experience-dependent plasticity in the healthy and diseased brain, particularly the cerebral cortex.

The role of seladin-1/DHCR24 in cholesterol biosynthesis, APP processing and Abeta generation in vivo

The cholesterol-synthesizing enzyme seladin-1, encoded by the Dhcr24 gene, is a flavin adenine dinucleotide-dependent oxidoreductase and regulates responses to oncogenic and oxidative stimuli. It has a role in neuroprotection and is downregulated in affected neurons in Alzheimer's disease (AD). Here we show that seladin-1-deficient mouse brains had reduced levels of cholesterol and disorganized cholesterol-rich detergent-resistant membrane domains (DRMs). This was associated with inefficient plasminogen binding and plasmin activation, the displacement of beta-secretase (BACE) from DRMs to APP-containing membrane fractions, increased beta-cleavage of APP and high levels of Abeta peptides. In contrast, overexpression of seladin-1 increased both cholesterol and the recruitment of DRM components into DRM fractions, induced plasmin activation and reduced both BACE processing of APP and Abeta formation. These results establish a role of seladin-1 in the formation of DRMs and suggest that seladin-1-dependent cholesterol synthesis is involved in lowering Abeta levels. Pharmacological enhancement of seladin-1 activity may be a novel Abeta-lowering approach for the treatment of AD.

The beta-amyloid peptide of Alzheimer's disease decreases adhesion of vascular smooth muscle cells to the basement membrane

Cerebral amyloid angiopathy (CAA) is a major feature of Alzheimer's disease pathology. In CAA, degeneration of vascular smooth muscle cells (VSMCs) occurs close to regions of the basement membrane where the amyloid protein (Abeta) builds up. In this study, the possibility that Abeta disrupts adhesive interactions between VSMCs and the basement membrane was examined. VSMCs were cultured on a commercial basement membrane substrate (Matrigel). The presence of Abeta in the Matrigel decreased cell-substrate adhesion and cell viability. Full-length oligomeric Abeta was required for the effect, as N- and C-terminally truncated peptide analogues did not inhibit adhesion. Abeta that was fluorescently labelled at the N-terminus (fluo-Abeta) bound to Matrigel as well as to the basement membrane heparan sulfate proteoglycan (HSPG) perlecan and laminin. Adhesion of VSMCs to perlecan or laminin was decreased by Abeta. As perlecan influences VSMC viability through the extracellular signal-regulated kinase (ERK)1/2 signalling pathway, the effect of Abeta1-40 on ERK1/2 phosphorylation was examined. The level of phospho-ERK1/2 was decreased in cells following Abeta treatment. An inhibitor of ERK1/2 phosphorylation enhanced the effect of Abeta on cell adhesion. The studies suggest that Abeta can decrease VSMC viability by disrupting VSMC-extracellular matrix (ECM) adhesion.

β-Amyloid fibril formation is promoted by step edges of highly oriented pyrolytic graphite

The aggregation of the amyloid-beta-protein (Abeta) is an important step in the pathogenesis of Alzheimer's disease. As Abeta fibrils are not found in all brain regions, endogenous factors may influence Abeta fibril formation. In this study, atomic force microscopy was used to investigate the role of surface phenomena in directing amyloid aggregation. Abeta1-40 was applied to a surface of highly oriented pyrolytic graphite at a concentration of 0.5 microM. Steps formed by edge-plane surface defects on the graphite were found to act as a template to promote the assembly of Abeta into fibrils. Initially, after being deposited on the graphite surface, Abeta had a uniform beaded morphology. However, after incubating (aging) the Abeta on the surface for several hours, the Abeta assembled along step edges to form linear aggregates. After more prolonged incubation, the linear Abeta aggregates fused to form mature fibrils with a distinctive helical morphology. The results demonstrate that surface interactions can promote the aggregation of Abeta into amyloid fibrils and they suggest that similar interactions could promote amyloid aggregation in vivo.

Nanoparticle-mediated local and remote manipulation of protein aggregation

The local heat delivered by metallic nanoparticles selectively attached to their target can be used as a molecular surgery to safely remove toxic and clogging aggregates. We apply this principle to protein aggregates, in particular to the amyloid beta protein (Abeta) involved in Alzheimer's disease (AD), a neurodegenerative disease where unnaturally folded Abeta proteins self-assemble and deposit forming amyloid fibrils and plaques. We show the possibility to remotely redissolve these deposits and to interfere with their growth, using the local heat dissipated by gold nanoparticles (AuNP) selectively attached to the aggregates and irradiated with low gigahertz electromagnetic fields. Simultaneous tagging and manipulation by AuNP of Abeta at different stages of aggregation allow both, noninvasive exploration and dissolution of molecular aggregates.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Protein aggregation as a cause for disease

The ability of proteins to fold into a defined and functional conformation is one of the most fundamental processes in biology. Certain conditions, however, initiate misfolding or unfolding of proteins. This leads to the loss of functional protein or it can result in a wide range of diseases. One group of diseases, which includes Alzheimer's, Parkinson's, Huntington's disease, and the transmissible spongiform encephalopathies (prion diseases), involves deposition of aggregated proteins. Normally, such protein aggregates are not found in properly functioning biological systems, because a variety of mechanisms inhibit their formation. Understanding the nature of these protective mechanisms together with the understanding of factors reducing or deactivating the natural protection machinery will be crucial for developing strategies to prevent and treat these disastrous diseases.

β-Amyloid increases dendritic Ca2+ influx by inhibiting the A-type K+ current in hippocampal CA1 pyramidal neurons

Accumulation of the beta-amyloid peptide (Abeta) is a primary event in the pathogenesis of Alzheimer's disease (AD). However, the mechanisms by which Abeta mediates neurotoxicity and initiates the degenerative processes of AD are still not clear. Recent evidence shows that voltage-gated K+ channels may be involved in Abeta-induced neurodegenerative processes. In particular, a transient A-type K+ current, with a linear increase in its density with distance from soma to distal dendrites in hippocampal CA1 pyramidal neurons, has been shown to contribute to dendritic membrane excitability. Here, I report that Abeta (1-42) inhibits the dendritic A-type K+ current in hippocampal CA1 pyramidal neurons, and this inhibition causes increases in back-propagating dendritic action potential amplitude and associated Ca2+ influx. These results suggest that the persistent inhibition of the A-type K+ current resulting from deposition of Abeta in dendritic arborization will induce a sustained increase in dendritic Ca2+ influx and lead to loss of Ca2+ homeostasis. This may be a component of the events that cause synaptic failure and initiate neuronal degenerative processes in the hippocampus.

Involvement of glutaredoxin-1 and thioredoxin-1 in beta-amyloid toxicity and Alzheimer's disease

Strong evidence indicates oxidative stress in the pathogenesis of Alzheimer's disease (AD). Amyloid beta (Abeta) has been implicated in both oxidative stress mechanisms and in neuronal apoptosis. Glutaredoxin-1 (GRX1) and thioredoxin-1 (TRX1) are antioxidants that can inhibit apoptosis signal-regulating kinase (ASK1). We examined levels of GRX1 and TRX1 in AD brain as well as their effects on Abeta neurotoxicity. We show an increase in GRX1 and a decrease in neuronal TRX1 in AD brains. Using SH-SY5Y cells, we demonstrate that Abeta causes an oxidation of both GRX1 and TRX1, and nuclear export of Daxx, a protein downstream of ASK1. Abeta toxicity was inhibited by insulin-like growth factor-I (IGF-I) and by overexpressing GRX1 or TRX1. Thus, Abeta neurotoxicity might be mediated by oxidation of GRX1 or TRX1 and subsequent activation of the ASK1 cascade. Deregulation of GRX1 and TRX1 antioxidant systems could be important events in AD pathogenesis.

Are mitochondria critical in the pathogenesis of Alzheimer's disease?

This review summarizes recent findings that suggest a causal connection between mitochondrial abnormalities and sporadic Alzheimer's disease (AD). Genetic causes of AD are known only for a small proportion of familial AD patients, but for a majority of sporadic AD patients, genetic causal factors are still unknown. Currently, there are no early detectable biomarkers for sporadic AD, and there is a lack of understanding of the pathophysiology of the disease. Findings from recent genetic studies of AD pathogenesis suggest that mitochondrial defects may play an important role in sporadic AD progression, and that mitochondrial abnormalities and oxidative damage may play a significant role in the progression of familial AD. Findings from biochemical studies, in vitro studies, gene expression studies, and animal model studies of AD are reviewed, and the possible contribution of mitochondrial mutations to late-onset sporadic AD is discussed.

Acetylcholinesterase inhibitors for the treatment of dementia in Alzheimer’s disease: do we need new inhibitors?

Acetylcholinesterase inhibitors (AChEIs) have been shown to produce a small, but significant, improvement in cognition in patients with Alzheimer's disease. However, not all patients respond equally, and cognitive benefit may be of limited duration. Although new AChEIs continue to be developed, more recent studies have been aimed at developing inhibitors that have additional actions separate from AChE inhibition. Importantly, new treatments that target the underlying pathogenic mechanism of Alzheimer's disease (statins, secretase inhibitors, vaccination) may eventually emerge. These new treatments could make AChEI therapy less relevant for treatment of Alzheimer's disease.

Amyloid-β fibril formation is not necessarily required for microglial activation by the peptides

There is increasing evidence that microglial activation has pathogenic influence on Alzheimer's disease. According to in vitro studies, microglia activated by amyloid-beta (Abeta) peptides have been reported to damage or kill neurons by the release of neurotoxic molecules such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta, nitric oxide or reactive oxygen species. Although the relationship between the aggregational state of Abeta peptides and their neurotoxic activities has been well investigated, little is known about the relationship between the aggregational state of Abeta peptides and their ability to induce microglial activation. In the present study, we thus performed both structural and biochemical studies to clarify the relationship between the aggregational state of Abeta peptides and their ability to activate microglia. Our results have shown that, in the presence of interferon-gamma, the Abeta25-35(M(35)Nle) peptide had almost the same potency of activating microglia and producing TNF-alpha as the Abeta25-35 peptide on both protein and mRNA levels, in spite of the fact that former peptide represented much less amyloid fibril formation than the latter in a thioflavine-T fluorometric assay. These results suggest that Abeta fibril formation is not necessarily required for microglial activation by the peptides.

Mapping cellular transcriptosomes in autopsied Alzheimer's disease subjects and relevant animal models

Alzheimer's disease (AD) is a late-onset and progressive neurodegenerative disorder characterized clinically by memory loss, impairment of other cognitive functions, and changes in behavior and personality. The overall aim of this review is to summarize recent advances in studies of AD progression and the use of animal models in gene expression studies of AD progression. Genetic causes of AD are known only for early-onset AD patients. For a majority of late-onset AD patients, causal factors are still unknown. Currently, there are no early detectable biomarkers for late-onset AD, and there is a lack of understanding of AD pathophysiology, particularly at the early stages of disease progression, before pathology develops. Human histopathological and biochemical studies provide valuable information regarding the last stages of AD pathogenesis. However, to understand early cellular changes in AD progression before symptoms develop, animal models are still our only alternative. Several research groups have created genetically engineered animal models, particularly models of the mouse, rat, fly, and worm, which have allowed us to better, understand the initiating events of AD progression. Recently, state-of-the-art methods have helped elucidate gene expression changes in affected and unaffected tissues from postmortem AD brains and from animal models developed for AD studies. These methods allow the investigation of mRNA-based transcriptosomal profiles of brain specimens from AD humans and transgenic animals. The major finding from these studies is that AD progression and pathogenesis involve multiple cellular pathways, which suggests that AD is a complex and heterogeneous disease.

Age-related loss of synaptophysin immunoreactive presynaptic boutons within the hippocampus of APP751SL, PS1M146L, and APP751SL/PS1M146L transgenic mice

Neuron and synapse loss are important features of the neuropathology of Alzheimer's disease (AD). Recently, we observed substantial age-related hippocampal neuron loss in APP751SL/PS1M146L transgenic mice but not in PS1M146L mice. Here, we investigated APP751SL mice, PS1M146L mice, and APP751SL/PS1M146L mice for age-related alterations in synaptic integrity within hippocampal stratum moleculare of the dentate gyrus (SM), stratum lucidum of area CA3 (SL), and stratum radiatum of area CA1-2 (SR) by analyzing densities and numbers of synaptophysin-immunoreactive presynaptic boutons (SIPBs). Wild-type mice, APP751SL mice and PS1M146L mice showed similar amounts of age-related SIPB loss within SM, and no SIPB loss within SL. Both APP751SL mice and PS1M146L mice showed age-related SIPB loss within SR. Importantly, APP751SL/PS1M146L) mice displayed the severest age-related SIPB loss within SM, SL, and SR, even in regions free of extracellular Abeta deposits. Together, these mouse models offer a unique framework to study the impact of several molecular and cellular events caused by mutant APP and/or mutant PS1 on age-related alterations in synaptic integrity. The observation of age-related SIPB loss within SR of PS1M146L mice supports a role of mutant PS1 in neurodegeneration apart from its contribution to alterations in Abeta generation.

Binding of Amyloidogenic Transthyretin to the Plasma Membrane Alters Membrane Fluidity and Induces Neurotoxicity

Transthyretin (TTR) can deposit as amyloid in the peripheral nervous system and induce a peripheral neuropathy. We examined the mechanism of TTR amyloid neurotoxicity on SH-SY5Y neuroblastoma cells. Wild-type (WT) TTR and two amyloidogenic mutants (V30M and L55P) were expressed in Escherichia coli. Incubation (aging) of WT TTR at 37 degrees C for 1 week caused no significant aggregation. However, there was a significant increase in the extent of amyloid fibril formation after the amyloidogenic mutants had been aged. L55P TTR aggregated more readily than V30M TTR. Both amyloidogenic mutants were neurotoxic after aging. The order of neurotoxicity was as follows: L55P > V30M > WT. As binding of amyloid proteins to the plasma membrane may cause cytotoxicity, we studied the binding of TTR to a plasma membrane-enriched preparation from SH-SY5Y cells by surface plasmon resonance. All three forms bound to the plasma membrane through electrostatic interactions. The binding of the amyloidogenic mutants was increased by aging. The amount of binding correlated closely with the amount of aggregation and with the cytotoxicity of each form. As membrane fluidity can influence cell viability, we also examined the effect of TTR on membrane fluidity using a fluorescence anisotropy method. Binding of the amyloidogenic TTR mutants increased membrane fluidity, and once again, the order of potency was as follows: L55P > V30M > WT. These results demonstrate that TTR can bind to the plasma membrane and cause a change in membrane fluidity. Altered membrane fluidity may be the cause of the neurotoxicity.

Single particle detection and characterization of synuclein co-aggregation

Protein aggregation is the key event in a number of human diseases such as Alzheimer's and Parkinson's disease. We present a general method to quantify and characterize protein aggregates by dual-colour scanning for intensely fluorescent targets (SIFT). In addition to high sensitivity, this approach offers a unique opportunity to study co-aggregation processes. As the ratio of two fluorescently labelled components can be analysed for each aggregate separately in a homogeneous assay, the molecular composition of aggregates can be studied even in samples containing a mixture of different types of aggregates. Using this method, we could show that wild-type alpha-synuclein forms co-aggregates with a mutant variant found in familial Parkinson's disease. Moreover, we found a striking increase in aggregate formation at non-equimolar mixing ratios, which may have important therapeutic implications, as lowering the relative amount of aberrant protein may cause an increase of protein aggregation leading to adverse effects.

Nature, nurture and neurology: gene-environment interactions in neurodegenerative disease. FEBS Anniversary Prize Lecture delivered on 27 June 2004 at the 29th FEBS Congress in Warsaw

Neurodegenerative disorders, such as Huntington's, Alzheimer's, and Parkinson's diseases, affect millions of people worldwide and currently there are few effective treatments and no cures for these diseases. Transgenic mice expressing human transgenes for huntingtin, amyloid precursor protein, and other genes associated with familial forms of neurodegenerative disease in humans provide remarkable tools for studying neurodegeneration because they mimic many of the pathological and behavioural features of the human conditions. One of the recurring themes revealed by these various transgenic models is that different diseases may share similar molecular and cellular mechanisms of pathogenesis. Cellular mechanisms known to be disrupted at early stages in multiple neurodegenerative disorders include gene expression, protein interactions (manifesting as pathological protein aggregation and disrupted signaling), synaptic function and plasticity. Recent work in mouse models of Huntington's disease has shown that enriching the environment of transgenic animals delays the onset and slows the progression of Huntington's disease-associated motor and cognitive symptoms. Environmental enrichment is known to induce various molecular and cellular changes in specific brain regions of wild-type animals, including altered gene expression profiles, enhanced neurogenesis and synaptic plasticity. The promising effects of environmental stimulation, demonstrated recently in models of neurodegenerative disease, suggest that therapy based on the principles of environmental enrichment might benefit disease sufferers and provide insight into possible mechanisms of neurodegeneration and subsequent identification of novel therapeutic targets. Here, we review the studies of environmental enrichment relevant to some major neurodegenerative diseases and discuss their research and clinical implications.

Correlation between dopaminergic neurons, acetylcholinesterase and nicotinic acetylcholine receptors containing the α3- or α5-subunit in the rat substantia nigra

The aim of this study was to investigate the relationship between the cells possessing the alpha3 or alpha5 nicotinic acetylcholine receptor subunits and the enzyme acetylcholinesterase, with respect to tyrosine hydroxylase immunoreactive dopaminergic neurons in the rat substantia nigra. Most, but certainly not all, acetylcholinesterase immunoreactive cells were located in the pars compacta. In the substantia nigra pars compacta there were in turn two populations of acetylcholinesterase containing neurons: those that were tyrosine hydroxylase reactive and those that were not. Double label studies, that included an antibody immunoreactive against a common immunogen on alpha1 of muscle and alpha3 and alpha5 neuronal nicotinic acetylcholine receptor subunits, revealed that nearly all nicotinic receptor positive cells were also tyrosine hydroxylase neurons. However, a minority non-tyrosine hydroxylase population was alpha3- and/or alpha5-nAChR positive and these were always AChE-immunoreactive. In summary, there appears to be a close correlation between nicotinic receptors and acetylcholinesterase in the substantia nigra, irrespective of the transmitter phenotype in different neuronal subpopulations.

Controlling neural stem cell division within the adult subventricular zone: an APPealing job

For years, scientists investigating amyloid precursor protein (APP) have focused on its pathogenetic role in the brains of Alzheimer's disease patients. Now, a study by Caille et al. adds new sites of action and new physiological functions for APP. They show that there are binding sites for secreted N-terminal nonamyloidogenic APP (sAPP) on epidermal growth factor (EGF)-responsive neural stem cells in the subventricular zone of the adult brain, where sAPP acts as an EGF cofactor to stimulate proliferation of these cells. This result opens the hypothesis that changes in the levels of sAPP could influence activity of the neurogenic regions of the adult brain in normal and pathological conditions.

β-Amyloid peptide25–35 depresses excitatory synaptic transmission in the rat basolateral amygdala “in vitro”

The effects of beta-amyloid peptide25-35 on resting membrane potential, spontaneous and evoked action potential and synaptic activity have been studied in basolateral amygdaloid complex on slices obtained from adult rats. Intracellular recordings reveal that perfusion with beta-amyloid peptide25-35 at concentrations of 400 nM and less did not generate any effect on resting membrane potential. However, concentrations in the range of 800-1200 nM produced an unpredictable effect, depolarization and/or hyperpolarization, which were blocked by tetrodotoxin or 6-cyano-7-nitroquinoxaline-2,3-dione+D-(-)-2-amino-5-phosphonopentanoic acid together with bicuculline. Excitatory and inhibitory evoked responses mediated by glutamic acid or gamma-aminobutyric acid decreased in amplitude after beta-amyloid peptide25-35 perfusion. Additionally, results obtained using the paired-pulse protocol offer support for a presynaptic mode of action. To determine which type of receptors and/or channels are involved in the presynaptic mechanism of action, a specific blocker of alpha-7 nicotinic receptors (methyllycaconitine citrate) or L-type calcium channel blockers (calcicludine or nifedipine) were used. beta-amyloid petide25-35 decreased excitatory postsynaptic potentials amplitude in control conditions and also in slices permanently perfused with methyllycaconitine citrate. However, this effect was blocked in slices perfused with calcicludine or nifedipine suggesting the involvement of the L-type calcium channels. On the whole, these experiments provide evidence that beta-amyloid peptide25-35 affects neurotransmission in basolateral amygdala and its action is mediated through L-type calcium channels.

A case for a non-transgenic animal model of Alzheimer's disease

Alzheimer's disease (AD) is associated with an early impairment in memory and is the major cause of dementia in the elderly. beta-Amyloid (Abeta) is believed to be a primary factor in the pathogenic pathway leading to dementia. Mounting evidence suggests that this syndrome begins with subtle alterations in synaptic efficacy prior to extensive neuronal degeneration and that the synaptic dysfunction could be caused by diffusible oligomeric assemblies of Abeta. This paper reviews the findings from behavioral analysis, electrophysiology, neuropathology and nootropic drug screening studies involving exogenous administration of Abeta in normal rodent brains. This non-transgenic model of amyloid pathology in vivo is presented as a complementary alternative model to transgenic mice to study the cellular and molecular pathways induced by amyloid, which in turn may be a causal factor in the disruption of cognition. The data reviewed here confirm that the diffusible form of Abeta rapidly induces synaptic dysfunction and a secondary process involving cellular cascades induced by the fibrillar form of amyloid. The time-course of alteration in memory processes implicates at least two different mechanisms that may be targeted with selective therapies aimed at improving memory in some AD patients.

beta-Amyloid peptides impair PKC-dependent functions of metabotropic glutamate receptors in prefrontal cortical neurons

The metabotropic glutamate receptors (mGluRs) have been implicated in cognition, memory, and some neurodegenerative disorders, including the Alzheimer's disease (AD). To understand how the dysfunction of mGluRs contributes to the pathophysiology of AD, we examined the beta-amyloid peptide (Abeta)-induced alterations in the physiological functions of mGluRs in prefrontal cortical pyramidal neurons. Two potential targets of mGluR signaling involved in cognition, the GABAergic system and the N-methyl-d-aspartate (NMDA) receptor, were examined. Activation of group I mGluRs with (S)-3,5-dihydroxyphenylglycine (DHPG) significantly increased the spontaneous inhibitory postsynaptic current (sIPSC) amplitude, and this effect was protein kinase C (PKC) sensitive. Treatment with Abeta abolished the DHPG-induced enhancement of sIPSC amplitude. On the other hand, activation of group II mGluRs with (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) significantly increased the NMDA receptor (NMDAR)-mediated currents via a PKC-dependent mechanism, and Abeta treatment also diminished the APDC-induced potentiation of NMDAR currents. In Abeta-treated slices, both DHPG and APDC failed to activate PKC. These results indicate that the mGluR regulation of GABA transmission and NMDAR currents is impaired by Abeta treatment probably due to the Abeta-mediated interference of mGluR activation of PKC. This study provides a framework within which the role of mGluRs in normal cognitive functions and AD can be better understood.

Cytoplasmic domain of the ?-amyloid protein precursor of Alzheimer's disease: Function, regulation of proteolysis, and implications for drug development

The beta-amyloid protein precursor (APP) has been extensively studied for its role in amyloid production and the pathogenesis of Alzheimer's disease (AD). However, little is known about the normal function of APP and its biological interactions. In this Mini-Review, the role of the cytoplasmic domain of APP in APP trafficking and proteolysis is described. These studies suggest that proteins that bind to the cytoplasmic domain may be important targets for drug development in AD.

Surface plasmon resonance for the analysis of beta-amyloid interactions and fibril formation in Alzheimer's disease research

Alzheimer's disease (AD) is a neurodegenerative disorder characterised by the accumulation of amyloid deposits, the major component of which is a 4 kDa polypeptide known as beta-amyloid protein (ABeta). Identifying the mechanism underlying the formation of Abeta and the pathways that lead to its toxicity is crucial to understanding the mechanism of AD and addressing the urgent need for new and effective treatments for AD. The accumulation of ABeta is the result of a complex interplay between genetic and environmental factors that affect the generation, clearance and aggregation of the peptide. Because of its propensity to aggregate, ABeta builds up in the brain and assembles into amyloid fibrils, ultimately creating amyloid plaques (APs) and cerebral amyloid angiopathy (CAA). Abeta has been shown to interact with a number of intracellular and extracellular molecules, but the relative contribution of these interactions to the toxicity of Abeta is not well understood. A critical step in characterising the importance of these interactions is the ability to measure both the affinity and kinetics of these interactions. Surface plasmon resonance (SPR) spectroscopy has become a widely used technique to study molecular interactions such as antibody-antigen, DNA-DNA, DNA-protein, protein-protein, receptor-ligand and peptide- and protein-membrane interactions. This article reviews the application of SPR to the study of the molecular interactions associated with AD and how this information enhances our molecular understanding of ABeta -mediated toxicity.

Amyloid-β-stimulated plasminogen activation by tissue-type plasminogen activator results in processing of neuroendocrine factors

Alzheimer's disease brain is characterized by the abundant presence of amyloid deposits. Accumulation of the major constituent of these deposits, amyloid-beta (Abeta), has been associated with decreased neurotransmission, increased neuronal cell death, and with cognitive decline. The mechanisms underlying these phenomena have not yet been fully elucidated. We have previously shown that amyloid peptides like Abeta bind tissue-type plasminogen activator (tPA) and cause enhanced plasmin production. Here we describe the identification of five major neuronal cell-produced Abeta-associated proteins and how Abeta-stimulated plasmin formation affects their processing. These five proteins are all neuroendocrine factors (NEFs): chromogranins A, B and C; truncated chromogranin B; and VGF. Plasminogen caused processing of Abeta-bound (but not soluble) tPA, chromogranin B and VGF and the degradation products were released from Abeta. Processing of the neuroendocrine factors was dependent on tPA as it was largely abrogated in tPA-/- cells or in the presence of a specific tPA-inhibitor. If plasmin indeed produces NEF-derived peptides in vivo, some of these peptides may have biological activity, for instance in regulating neurotransmitter release that may affect the pathology of Alzheimer's disease.

Structural membrane alterations in Alzheimer brains found to be associated with regional disease development; increased density of gangliosides GM1 and GM2 and loss of cholesterol in detergent-resistant membrane domains

The formation of neurotoxic beta-amyloid fibrils in Alzheimer's disease (AD) is suggested to involve membrane rafts and to be promoted, in vitro, by enriched concentrations of gangliosides, particularly GM1, and the cholesterol therein. In our study, the presence of rafts and their content of the major membrane lipids and gangliosides in the temporal cortex, reflecting late stages of AD pathology, and the frontal cortex, presenting earlier stages, has been investigated. Whole tissue and isolated detergent-resistant membrane fractions (DRMs) were analysed from 10 AD and 10 age-matched control autopsy brains. DRMs from the frontal cortex of AD brains contained a significantly higher concentration (micromol/micromol glycerophospholipids), of ganglioside GM1 (22.3 +/- 4.6 compared to 10.3 +/- 6.4, p <0.001) and GM2 (2.5 +/- 1.0 compared to 0.55 +/- 0.3, p <0.001). Similar increases of these gangliosides were also seen in DRMs from the temporal cortex of AD brains, which, in addition, comprised significantly lower proportions of DRMs. Moreover, these remaining rafts were depleted in cholesterol (from 1.5 +/- 0.2 to 0.6 +/- 0.3 micromol/micromol glycerophospholipids, p <0.001). In summary, we found an increased proportion of GM1 and GM2 in DRMs, and accelerating plaque formation at an early stage, which may gradually lead to membrane raft disruptions and thereby affect cellular functions associated with the presence of such membrane domains.

Signal transduction during amyloid-β-peptide neurotoxicity: role in Alzheimer disease

Alzheimer's disease (AD) is a neurodegenerative disorder with progressive dementia accompanied by two main structural changes in the brain: intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular amyloid protein deposits surrounded by dystrophic neurites that constitutes the senile plaques. Currently, it is widely accepted that amyloid beta-peptide (A beta) metabolism disbalance is crucial for AD progression. A beta deposition may be enhanced by molecular chaperones, including metals like copper and proteins like acetylcholinesterase (AChE). At the neuronal level, several AD-related proteins interact with transducers of the Wnt/beta-catenin signaling pathway, including beta-catenin and glycogen synthase kinase 3 beta (GSK-3 beta) and both in vitro and in vivo studies suggest that Wnt/beta-catenin signaling is a target for A beta toxicity. Accordingly, activation of this signaling by lithium or Wnt ligands in AD-experimental animal models or in primary hippocampal neurons attenuate A beta neurotoxicity by recovering beta-catenin levels and Wnt-target gene expression of survival genes such as bcl-2. On the other hand, peroxisomal proliferator-activated receptor gamma (PPAR gamma) and muscarinic acetylcholine receptor (mAChR) agonists also activate Wnt/beta-catenin signaling and they have neuroprotective effects on hippocampal neurons. Our studies are consistent with the idea that a sustained loss of function of Wnt signaling components would trigger a series of events, determining the onset and development of AD and that modulation of this pathway through the activation of cross-talking signaling cascades should be considered as a possible therapeutic strategy for AD treatment.

Brain ?-Amyloid Accumulation in Transgenic Mice Expressing Mutant Superoxide Dismutase 1

Oxidative stress is implicated in both the deposition and pathogenesis of beta-amyloid (Abeta) protein in Alzheimer's disease (AD). Accordingly, overexpression of the antioxidant enzyme superoxide dismutase 1 (SOD1) in neuronal cells and transgenic AD mice reduces Abeta toxicity and accumulation. In contrast, mutations in SOD1 associated with amyotrophic lateral sclerosis (ALS) confer enhanced pro-oxidative enzyme activities. We therefore examined whether ALS-linked mutant SOD1 overexpression in motor neuronal cells or transgenic ALS mice modulates Abeta toxicity or its accumulation in the brain. Aggregated, but not freshly solubilised, substrate-bound Abeta peptides induced degenerative morphology and cytotoxicity in motor neuron-like NSC-34 cells. Transfection of NSC-34 cells with human wild-type SOD1 attenuated Abeta-induced toxicity, however this neuroprotective effect was also observed for ALS-linked mutant SOD1. Analysis of the cerebral cortex, brainstem, cerebellum and olfactory bulb from transgenic SOD1G93A mice using enzyme-linked immunosorbent assay of acid-guanidine extracts revealed age-dependent elevations in Abeta levels, although not significantly different from wild-type mouse brain. In addition, brain amyloid protein precursor (APP) levels remained unaltered as a consequence of mutant SOD1 expression. We therefore conclude that mutant SOD1 overexpression promotes neither Abeta toxicity nor brain accumulation in these ALS models.

Intracellular Abeta42 activates p53 promoter: a pathway to neurodegeneration in Alzheimer's disease

The amyloid beta-protein (Abeta) ending at 42 plays a pivotal role in Alzheimer's disease (AD). We have reported previously that intracellular Abeta42 is associated with neuronal apoptosis in vitro and in vivo. Here, we show that intracellular Abeta42 directly activated the p53 promoter, resulting in p53-dependent apoptosis, and that intracellular Abeta40 had a similar but lesser effect. Moreover, oxidative DNA damage induced nuclear localization of Abeta42 with p53 mRNA elevation in guinea-pig primary neurons. Also, p53 expression was elevated in brain of sporadic AD and transgenic mice carrying mutant familial AD genes. Remarkably, accumulation of both Abeta42 and p53 was found in some degenerating-shape neurons in both transgenic mice and human AD cases. Thus, the intracellular Abeta42/p53 pathway may be directly relevant to neuronal loss in AD. Although neurotoxicity of extracellular Abeta is well known and synaptic/mitochondrial dysfunction by intracellular Abeta42 has recently been suggested, intracellular Abeta42 may cause p53-dependent neuronal apoptosis through activation of the p53 promoter; thus demonstrating an alternative pathogenesis in AD.