Soluble oligomers of beta amyloid (1-42) inhibit long-term potentiation but not long-term depression in rat dentate gyrus

Beta-amyloid protein structure determines the nature of cytokine release from rat microglia

Activated microglia represent a major source of inflammatory factors in Alzheimer's disease and a possible source of cytotoxic factors. beta-Amyloid (Abeta) peptide, the predominant component in amyloid plaques, has been shown to activate microglia and stimulate their production of inflammatory factors. The present study was performed to analyze the responses of microglia to different forms of Abeta, with regard to release of the proinflammatory cytokines interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), IL-6, and interferon-gamma (IFN-gamma), as well as the IL-1 receptor antagonist (IL-1ra). Primary cultures of microglia from rat neonatal cerebral cortex were incubated with freshly dissolved Abeta1-40 or Abeta1-42, Abeta1-40 fibrils, Abeta1-40 betaamy balls, or vehicle. Abeta1-40 fibrils did not significantly stimulate any of these cytokines. Freshly dissolved Abeta1-40 resulted in a marked increase in the release of IL-1beta, and freshly dissolved Abeta1-42 significantly stimulated both IL-1alpha and IFN-gamma secretion. The Abeta1-40 betaamy balls stimulated the secretion of IL-1alpha and IL-1beta. Incubation with Abeta peptides did not affect the secretion of IL-1ra, IL-6, or TNF-alpha. In the case of IL-1beta, the response is correlated with the presence of Abeta peptide as monomers and oligomers.

Calcium-regulated signaling pathways: role in amyloid beta-induced synaptic dysfunction

Amyloid beta (Abeta) peptides have been shown to impair synaptic function, especially long-term synaptic plasticity, in transgenic mouse models of Alzheimer's disease (AD) and in acute hippocampal preparations. In the transgenic mice overexpressing mutant forms of human amyloid precursor protein (APP), the deficits in hippocampal long-term potentiation (LTP) occur prior to synaptic loss and cell death, suggesting early functional changes at these synapses. Recent studies demonstrate that Abeta-induced synaptic dysfunction is linked with altered Ca2+ signaling in hippocampal neurons. While reducing Ca2+ influx through NMDA receptors, Abeta peptides elevate intracellular Ca2+ concentration by enhancing Ca2+ influx from voltage-gated Ca2+ channels or nonselective cation channels, or by stimulating Ca2+ release from intracellular stores. Interestingly, acute application of Abeta or APP overexpression inhibits activity-dependent regulation of several protein kinase pathways that require Ca2+ influx via NMDA receptors for activation, including Ca2+/calmodulin-dependent protein kinase II, protein kinase A, and extracellular regulated kinases (Erk). On the other hand, activation of Ca2+-dependent protein phosphatase 2B (calcineurin) is implicated in Abeta inhibition of LTP. Thus, multiple Ca2+-regulated signaling pathways are involved in the synaptic action of Abeta, and malfunction of these pathways may underlie the synaptic dysfunction in early AD.

Membrane Permeabilization: A Common Mechanism in Protein-Misfolding Diseases

Protein aggregation--and, more specifically, amyloid fibril formation--has been implicated as a primary cause of neurodegeneration in Alzheimer's disease, Parkinson's disease, and related disorders, but the mechanism by which this process triggers neuronal death is unknown. Mounting evidence from in vitro studies, cell culture, and animal models of these diseases supports the hypothesis that a structural intermediate on the pathway to fibril formation, rather than amyloid fibrils themselves, may be the pathogenic species. Characterization of these intermediates in solution or upon interactions with membranes indicate that these intermediates form pores and suggests that neurons could be killed by unregulated membrane permeabilization caused by such "amyloid pores."

Nucleation-dependent polymerization is an essential component of amyloid-mediated neuronal cell death

Accumulating evidence suggests that amyloid protein aggregation is pathogenic in many diseases, including Alzheimer's disease. However, the mechanisms by which protein aggregation mediates cellular dysfunction and overt cell death are unknown. Recent reports have focused on the potential role of amyloid oligomers or protofibrils as a neurotoxic form of amyloid-beta (Abeta) and related amyloid aggregates. Here we describe studies indicating that overt neuronal cell death mediated by Abeta(1-40) is critically dependent on ongoing Abeta(1-40) polymerization and is not mediated by a single stable species of neurotoxic aggregate. The extent and rate of neuronal cell death can be controlled by conditions that alter the rate of Abeta polymerization. The results presented here indicate that protofibrils and oligomeric forms of Abeta most likely generate neuronal cell death through a nucleation-dependent process rather than acting as direct neurotoxic ligands. These findings bring into question the use of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide formazan assay (MTT assay) as a reporter of Abeta-mediated neuronal cell death and suggest that diffusible Abeta protofibrils and oligomers more likely mediate subtle alterations of synaptic function and long-term potentiation rather than overt neuronal cell death. These results have been extended to Abeta(1-42), the non-Abeta component of Alzheimer's disease amyloid plaques, and human amylin, suggesting that nucleation-dependent polymerization is a common mechanism of amyloid-mediated neuronal cell death. Our findings indicate that ongoing amyloid fibrillogenesis may be an essential mechanistic process underlying the pathogenesis associated with protein aggregation in amyloid disorders.

Globular amyloid beta-peptide oligomer - a homogenous and stable neuropathological protein in Alzheimer's disease

Amyloid beta-peptide (Abeta)(1-42) oligomers have recently been discussed as intermediate toxic species in Alzheimer's disease (AD) pathology. Here we describe a new and highly stable Abeta(1-42) oligomer species which can easily be prepared in vitro and is present in the brains of patients with AD and Abeta(1-42)-overproducing transgenic mice. Physicochemical characterization reveals a pure, highly water-soluble globular 60-kDa oligomer which we named 'Abeta(1-42) globulomer'. Our data indicate that Abeta(1-42) globulomer is a persistent structural entity formed independently of the fibrillar aggregation pathway. It is a potent antigen in mice and rabbits eliciting generation of Abeta(1-42) globulomer-specific antibodies that do not cross-react with amyloid precursor protein, Abeta(1-40) and Abeta(1-42) monomers and Abeta fibrils. Abeta(1-42) globulomer binds specifically to dendritic processes of neurons but not glia in hippocampal cell cultures and completely blocks long-term potentiation in rat hippocampal slices. Our data suggest that Abeta(1-42) globulomer represents a basic pathogenic structural principle also present to a minor extent in previously described oligomer preparations and that its formation is an early pathological event in AD. Selective neutralization of the Abeta globulomer structure epitope is expected to have a high potential for treatment of AD.

Metabolism of amyloid-beta peptide and Alzheimer's disease

The accumulation of amyloid-beta peptide (Abeta), a physiological peptide, in the brain is a triggering event leading to the pathological cascade of Alzheimer's disease (AD) and appears to be caused by an increase in the anabolic activity, as seen in familial AD cases or by a decrease in catabolic activity. Neprilysin is a rate-limiting peptidase involved in the physiological degradation of Abeta in the brain. As demonstrated by reverse genetics studies, disruption of the neprilysin gene causes elevation of endogenous Abeta levels in mouse brain in a gene-dose-dependent manner. Thus, the reduction of neprilysin activity will contribute to Abeta accumulation and consequently to AD development. Evidence that neprilysin in the hippocampus and cerebral cortex is down-regulated with aging and from an early stage of AD development supports a close association of neprilysin with the etiology and pathogenesis of AD. Therefore, the up-regulation of neprilysin represents a promising strategy for therapy and prevention. Recently, somatostatin, which acts via a G-protein-coupled receptor (GPCR), has been identified as a modulator that increases brain neprilysin activity, resulting in a decrease of Abeta levels. Thus, it may be possible to pharmacologically control brain Abeta levels with somatostatin receptor agonists.

Copper-dependent inhibition of human cytochrome c oxidase by a dimeric conformer of amyloid-beta1-42

In studies of Alzheimer's disease pathogenesis there is an increasing focus on mechanisms of intracellular amyloid-beta (Abeta) generation and toxicity. Here we investigated the inhibitory potential of the 42 amino acid Abeta peptide (Abeta1-42) on activity of electron transport chain enzyme complexes in human mitochondria. We found that synthetic Abeta1-42 specifically inhibited the terminal complex cytochrome c oxidase (COX) in a dose-dependent manner that was dependent on the presence of Cu2+ and specific "aging" of the Abeta1-42 solution. Maximal COX inhibition occurred when using Abeta1-42 solutions aged for 3-6 h at 30 degrees C. The level of Abeta1-42-mediated COX inhibition increased with aging time up to approximately 6 h and then declined progressively with continued aging to 48 h. Photo-induced cross-linking of unmodified proteins followed by SDS-PAGE analysis revealed dimeric Abeta as the only Abeta species to provide significant temporal correlation with the observed COX inhibition. Analysis of brain and liver from an Alzheimer's model mouse (Tg2576) revealed abundant Abeta immunoreactivity within the brain mitochondria fraction. Our data indicate that endogenous Abeta is associated with brain mitochondria and that Abeta1-42, possibly in its dimeric conformation, is a potent inhibitor of COX, but only when in the presence of Cu2+. We conclude that Cu2+-dependent Abeta-mediated inhibition of COX may be an important contributor to the neurodegeneration process in Alzheimer's disease.

Behavioral phenotypes of amyloid-based genetically modified mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative affliction of the elderly, presenting with progressive memory loss and dementia and terminating with death. There have been significant advances in understanding the biology and subsequent diagnosis of AD; however, the furious pace of research has not yet translated into a disease-modifying treatment. While scientific inquiry in AD is largely centered on identifying biological players and pathological mechanisms, the day-to-day realities of AD patients and their caregivers revolve around their steady and heartbreaking cognitive decline. In the past decade, AD research has been fundamentally transformed by the development of genetically modified animal models of amyloid-driven neurodegeneration. These important in vivo models not only replicate some of the hallmark pathology of the disease, such as plaque-like amyloid accumulations and astrocytic inflammation, but also some of the cognitive impairments relevant to AD. In this article, we will provide a detailed review of the behavioral and cognitive deficits present in several transgenic mouse models of AD and discuss their functional changes in response to experimental treatments.

Synaptic targeting by Alzheimer's-related amyloid beta oligomers

The cognitive hallmark of early Alzheimer's disease (AD) is an extraordinary inability to form new memories. For many years, this dementia was attributed to nerve-cell death induced by deposits of fibrillar amyloid beta (Abeta). A newer hypothesis has emerged, however, in which early memory loss is considered a synapse failure caused by soluble Abeta oligomers. Such oligomers rapidly block long-term potentiation, a classic experimental paradigm for synaptic plasticity, and they are strikingly elevated in AD brain tissue and transgenic-mouse AD models. The current work characterizes the manner in which Abeta oligomers attack neurons. Antibodies raised against synthetic oligomers applied to AD brain sections were found to give diffuse stain around neuronal cell bodies, suggestive of a dendritic pattern, whereas soluble brain extracts showed robust AD-dependent reactivity in dot immunoblots. Antigens in unfractionated AD extracts attached with specificity to cultured rat hippocampal neurons, binding within dendritic arbors at discrete puncta. Crude fractionation showed ligand size to be between 10 and 100 kDa. Synthetic Abeta oligomers of the same size gave identical punctate binding, which was highly selective for particular neurons. Image analysis by confocal double-label immunofluorescence established that >90% of the punctate oligomer binding sites colocalized with the synaptic marker PSD-95 (postsynaptic density protein 95). Synaptic binding was accompanied by ectopic induction of Arc, a synaptic immediate-early gene, the overexpression of which has been linked to dysfunctional learning. Results suggest the hypothesis that targeting and functional disruption of particular synapses by Abeta oligomers may provide a molecular basis for the specific loss of memory function in early AD.

ApoE isoform-specific effects on LTP: blockade by oligomeric amyloid-beta1-42

Amyloid-beta1-42 (Abeta1-42) is crucial to Alzheimer disease (AD) pathogenesis but the conformation of the toxic Abeta species remains uncertain. AD risk is increased by apolipoprotein E4 (apoE4) and decreased by apoE2 compared with the apoE3 isoform, but whether inheritance of apoE4 represents a gain of negative or a loss of protective function is also unresolved. Using hippocampal slices from apoE knockout (apoE-KO) and human apoE2, E3, and E4 targeted replacement (apoE-TR) mice, we found that oligomeric Abeta1-42 inhibited long-term potentiation (LTP) with a hierarchy of susceptibility mirroring clinical AD risk (apoE4-TR > apoE3-TR = apoE-KO > apoE2-TR), and that comparable doses of unaggregated Abeta1-42 did not affect LTP. These data provide a novel link among apoE isoform, Abeta1-42, and a functional cellular model of memory. In this model, apoE4 confers a gain of negative function synergistic with Abeta1-42, apoE2 is protective, and the apoE-Abeta interaction is specific to oligomeric Abeta1-42.

The role of Alzheimer Abeta peptides in ion transport across cell membranes

Accumulation of beta amyloid (Abeta) fibrils in senile plaques and cerebral blood vessel walls is characteristic of Alzheimer's disease (AD). We discuss several models that seek to explain the neurotoxic consequences, in particular the manner in which the neurotoxicity promotes cell dysfunction and cell death by an increase in cytosolic calcium ion concentration. To base correctly a new therapy on in vitro experiments, one must choose the right model mechanism.

Targeting the neurotoxic species in Alzheimer's disease: inhibitors of Abeta oligomerization

In the past two decades, a large body of evidence has established a causative role for the beta-amyloid peptide (Abeta) in Alzheimer's disease (AD). However, recent debate has focused on whether amyloid fibrils or soluble oligomers of Abeta are the main neurotoxic species that contribute to neurodegeneration and dementia. Considerable early evidence has indicated that amyloid fibrils are toxic, but some recent studies support the notion that Abeta oligomers are the primary neurotoxins. While this crucial aspect of AD pathogenesis remains controversial, effective therapeutic strategies should ideally target both oligomeric and fibrillar species of Abeta. Here, we describe the anti-amyloidogenic and neuroprotective actions of some di- and tri-substituted aromatic compounds. Inhibition of the formation of soluble Abeta oligomers was monitored using a specific antibody-based assay that discriminates between Abeta oligomers and monomers. Thioflavin T and electron microscopy were used to screen for inhibitors of fibril formation. Taken together, these results led to the identification of compounds that more effectively block Abeta oligomerization than fibrillization. It is significant that such compounds completely blocked the neurotoxicity of Abeta to rat hippocampal neurons in culture. These findings provide a basis for the development of novel small molecule Abeta inhibitors with potential applications in AD.

Kinetic Stabilization of the Native State by Protein Engineering: Implications for Inhibition of Transthyretin Amyloidogenesis

The amyloidogenic homotetrameric protein transthyretin (TTR) must undergo rate-limiting dissociation to partially denatured monomers in order to aggregate. TTR contains two distinct quaternary interfaces, one of which defines the binding sites for thyroxine and small-molecule amyloidogenesis inhibitors. Kinetic stabilization of the tetramer can be accomplished either by the binding of amyloidogenesis inhibitors selectively to the native state over the dissociative transition state or by the introduction of trans-suppressor subunits (T119M) into heterotetramers to destabilize the dissociative transition state. In each case, increasing the dissociation activation barrier prevents tetramer dissociation. Herein, we demonstrate that tethering two subunits whose quaternary interface defines the thyroxine binding site also dramatically increases the barrier for tetramer dissociation, apparently by destabilization of the dissociative transition state. The tethered construct (TTR-L-TTR)2 is structurally and functionally equivalent to wild-type TTR. Urea is unable to denature (TTR-L-TTR)2, yet it is able to maintain the denatured state once denaturation is achieved by GdnHCl treatment, suggesting that (TTR-L-TTR)2 is kinetically rather than thermodynamically stabilized, consistent with the identical wild-type TTR and (TTR-L-TTR)2 GdnHCl denaturation curves. Studies focused on a construct containing a single TTR-L-TTR chain and two normal monomer subunits establish that alteration of only one quaternary structural interface is sufficient to impose kinetic stabilization on the entire quaternary structure.

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.

Synaptic transmission and synchronous activity is disrupted in hippocampal slices taken from aged TAS10 mice

Synaptic transmission was studied in hippocampal slices from aged (12-14 months of age) TAS10 mice overexpressing the human form of the amyloid precursor protein harboring the Swedish mutation. A significant deficit in the input-output relationship of glutamatergic synapses in the CA3-CA1 Schaffer collateral pathway was observed, while synaptic transmission in the medial perforant pathway of the dentate gyrus was comparatively preserved. Despite this deficit, relative levels of short- and long-term synaptic plasticity in the CA1 region were similar to those observed in wildtype slices. Specifically, paired pulse facilitation, frequency facilitation (at frequencies of 1, 5, and 10 Hz), and long-term potentiation induced by a theta burst stimulation paradigm were all normal in the CA3-CA1 synapses of TAS10 hippocampal slices. However, synchronized network activity induced by bath application of 4-aminopyridine (4-AP) was compromised. Thus, the frequency of synchronous events induced by 100 microM 4-AP was significantly lower in TAS10 hippocampal slices (inter-event interval: WT, 2.4+/-0.6 s; TAS10, 6.9+/-1.7 s). To study gamma-aminobutyric acid (GABA)ergic synaptic transmission NBQX (20 microM) and D-AP5 (50 microM) were added in order to isolate bicuculline-sensitive GABA-mediated synchronous network activity. The GABAergic network activity was not significantly different from wildtype in terms of frequency. This study suggests that the deficit in glutamatergic synaptic transmission observed in the TAS10 hippocampal slices, may be coupled with alterations in synchronous network activity, which in turn would lead to deficient information processing.

A humanin derivative, S14G-HN, prevents amyloid-?-induced memory impairment in mice

Humanin (HN) is a 24-amino acid peptide that protects neuronal cells from death caused by Alzheimer's disease (AD)-related genes and amyloid-beta (Abeta). Multiple studies have revealed its biochemical and neuroprotective characteristics in vitro; however, little has been known regarding whether HN is effective in vivo in AD model systems. We examined the effect of S14G-HN, a 1,000-fold more potent derivative of HN in vitro, on amnesia induced by Abeta25-35 in mice. The Y-maze test revealed that at least 50 pmol of S14G-HN by intracerebroventricular injection prevented Abeta-induced impairment of short-term/spatial working memory; however, 5 nmol of S14A-HN, a neuroprotection-defective mutant in vitro, did not prevent Abeta-induced amnesia. These results are in agreement with the structure-function correlation shown previously in vitro. In the water-finding task, S14G-HN prevented prolongation of finding latency (the time to find water) observed in Abeta-amnesic mice, indicating that S14G-HN also blocked Abeta-induced impairment of latent learning. In accordance with these observations, immunohistochemical analysis showed that S14G-HN sustained the number of cholinergic neurons in the basal forebrain and the striata nearly to the normal level. Furthermore, genistein, a specific inhibitor of tyrosine kinases, blocked recovery from scopolamine-induced amnesia by S14G-HN, suggesting that certain tyrosine kinase(s) are involved in the inhibitory function of S14G-HN in vivo. Taking these findings together, we conclude that S14G-HN has rescue activity against memory impairment caused by AD-related insults in vivo by activating the same intracellular neuroprotective machinery as elucidated previously in vitro.

Cognitive impairment in PDAPP mice depends on ApoE and ACT-catalyzed amyloid formation

Biochemical and genetic studies indicate that the inflammatory proteins, apolipoprotein E (ApoE) and alpha(1)-antichymotrypsin (ACT) are important in the pathogenesis of Alzheimer's disease (AD). Using several lines of multiply transgenic/knockout mice we show here that murine ApoE and human ACT separately and synergistically facilitate both diffuse A beta immunoreactive and fibrillar amyloid deposition and thus also promote cognitive impairment in aged PDAPP(V717F) mice. The degree of cognitive impairment is highly correlated with the ApoE- and ACT-dependent hippocampal amyloid burden, with PDAPP mice lacking ApoE and ACT having little amyloid and little learning disability. A analysis of young mice before the onset of amyloid formation shows that steady-state levels of monomeric A beta peptide are unchanged by ApoE or ACT. These data suggest that the process or product of amyloid formation is more critical than monomeric A beta for the neurological decline in AD, and that the risk factors ApoE and ACT participate primarily in disease processes downstream of APP processing.

Natural oligomers of the amyloid-beta protein specifically disrupt cognitive function

A central unresolved problem in research on Alzheimer disease is the nature of the molecular entity causing dementia. Here we provide the first direct experimental evidence that a defined molecular species of the amyloid-beta protein interferes with cognitive function. Soluble oligomeric forms of amyloid-beta, including trimers and dimers, were both necessary and sufficient to disrupt learned behavior in a manner that was rapid, potent and transient; they produced impaired cognitive function without inducing permanent neurological deficits. Although beta-amyloidosis has long been hypothesized to affect cognition, the abnormally folded protein species associated with this or any other neurodegenerative disease has not previously been isolated, defined biochemically and then specifically characterized with regard to its effects on cognitive function. The biochemical isolation of discrete amyloid-beta moieties with pathophysiological properties sets the stage for a new approach to studying the molecular mechanisms of cognitive impairment in Alzheimer disease and related neurodegenerative disorders.

Deciphering the molecular basis of memory failure in Alzheimer's disease

Acutely developing lesions of the brain have been highly instructive in elucidating the neural systems underlying memory in humans and animal models. Much less has been learned from chronic neurodegenerative disorders that insidiously impair memory. But the advent of a detailed molecular hypothesis for the development of Alzheimer's disease and the creation of compelling mouse models thereof have begun to change this situation. Experiments in rodents suggest that soluble oligomers of the amyloid beta protein (Abeta) may discretely interfere with synaptic mechanisms mediating aspects of learning and memory, including long-term potentiation. In humans, memory impairment correlates strongly with cortical levels of soluble Abeta species, which include oligomers. Local inflammatory changes, neurofibrillary degeneration, and neurotransmitter deficits all contribute to memory impairment, but available evidence suggests that these develop as a consequence of early Abeta accumulation. Accordingly, attempts to slow memory and cognitive loss by decreasing cerebral Abeta levels have entered human trials.

Modulation of memory by insulin and glucose: neuropsychological observations in Alzheimer's disease

Converging evidence has identified a potential association among Alzheimer's disease, glucose metabolism, insulin activity, and memory. Notably, type 2 diabetes, which is characterized by insulin resistance, may modulate the risk of Alzheimer's disease, and patients with Alzheimer's disease may have a greater risk for glucoregulatory impairments than do healthy older adults. In animal studies, it has been shown that raising blood glucose levels acutely can facilitate memory, in part, by increasing cholinergic activity, which is greatly diminished in patients with Alzheimer's disease. Other studies have confirmed that glucose administration can facilitate memory in healthy humans and in patients with Alzheimer's disease. Interestingly, glucose effects on memory appear to be modulated by insulin sensitivity (efficiency of insulin-mediated glucose disposal). Of course, the acute effects of glucose administration should be distinguished from the effects of chronic hyperglycemia (diabetes), which has been associated with cognitive impairments, at least in older adults. The relationship of insulin and memory has been more difficult to characterize. In animals, systemic insulin administration has been associated with memory deficits, likely due, in part, to hypoglycemia that occurs when exogenous insulin is not supplemented with glucose to maintain euglycemia. In healthy adults and patients with Alzheimer's disease, raising plasma insulin levels while maintaining euglycemia can improve memory; however, raising plasma glucose while suppressing endogenous insulin secretion may not improve memory, suggesting that adequate levels of insulin and glucose are necessary for memory facilitation. Clinical studies have corroborated findings that patients with Alzheimer's disease are more likely than healthy older adults to have reduced insulin sensitivity, and further suggest that apolipoprotein E genotype may modulate the effects of insulin on glucose disposal, memory facilitation, and amyloid precursor protein processing. Collectively, these findings support an association among Alzheimer's disease, impaired glucose metabolism, and reduced insulin sensitivity.

Small assemblies of unmodified amyloid beta-protein are the proximate neurotoxin in Alzheimer's disease

Pioneering work in the 1950s by Christian Anfinsen on the folding of ribonuclease has shown that the primary structure of a protein "encodes" all of the information necessary for a nascent polypeptide to fold into its native, physiologically active, three-dimensional conformation (for his classic review, see [Science 181 (1973) 223]). In Alzheimer's disease (AD), the amyloid beta-protein (Abeta) appears to play a seminal role in neuronal injury and death. Recent data have suggested that the proximate effectors of neurotoxicity are oligomeric Abeta assemblies. A fundamental question, of relevance both to the development of therapeutic strategies for AD and to understanding basic laws of protein folding, is how Abeta assembly state correlates with biological activity. Evidence suggests, as argued by Anfinsen, that the formation of toxic Abeta structures is an intrinsic feature of the peptide's amino acid sequence-one requiring no post-translational modification or invocation of peptide-associated enzymatic activity.

Systemic catabolism of Alzheimer's Abeta40 and Abeta42

To better understand the physiologic excretion and/or catabolism of circulating peripheral amyloid beta (Abeta), we labeled human Abeta40 (monomeric, with predominant unordered structure) and Abeta42 (mixture of monomers and oligomers in approximately 50:50 ratio, rich in beta-sheet conformation) with either Na(125)I or (125)I-tyramine cellobiose, also known as the cell-trapping ligand procedure, testing their blood clearance and organ uptake in B6SJLF1/J mice. Irrespective of the labeling protocol, the peptide conformation, and the degree of oligomerization, both Abeta40 and Abeta42 showed a short half-life of 2.5-3.0 min. The liver was the major organ responsible for plasma clearance, accounting for >60% of the peptide uptake, followed by the kidney. In vivo, hepatocytes captured >90% of the radiolabeled peptides which, after endocytosis, were preferentially catabolized and excreted into the bile. Biliary excretion of intact as well as partially degraded Abeta species became obviously relevant at doses above 10 microg. The use of biotin-labeled Abeta allowed the visualization of the interaction with HepG2 cells in culture, whereas competitive inhibition experiments with unlabeled Abeta demonstrated the specificity of the binding. The capability of the liver to uptake, catabolize, and excrete large doses of Abeta, several orders of magnitude above its physiologic concentration, may explain not only the femtomolar plasma levels of Abeta but the little fluctuation observed with age and disease stages.

Age-dependent neurodegeneration and Alzheimer-amyloid plaque formation in transgenic Drosophila

Beta-amyloid peptides that are cleaved from the amyloid precursor protein (APP) play a critical role in Alzheimer's disease (AD) pathophysiology. Here, we show that in Drosophila, the targeted expression of the key genes of AD, APP, the beta-site APP-cleaving enzyme BACE, and the presenilins led to the generation of beta-amyloid plaques and age-dependent neurodegeneration as well as to semilethality, a shortened life span, and defects in wing vein development. Genetic manipulations or pharmacological treatments with secretase inhibitors influenced the activity of the APP-processing proteases and modulated the severity of the phenotypes. This invertebrate model of amyloid plaque pathology demonstrates Abeta-induced neurodegeneration as a basic biological principle and may allow additional genetic analyses of the underlying molecular pathways.

Amyloid β-protein induced electrophysiological changes are dependent on aggregation state: N-methyl-d-aspartate (NMDA) versus non-NMDA receptor/channel activation

Alzheimer's disease (AD) is a progressive neurodegenerative disease, however, the underlying mechanism driving this condition is unknown. Unexplored is the possibility that the time-dependent generation of different Abeta assemblies may underlie the pathogenic cascade with biophysically distinct structures interacting with unique biological targets. Thus, the presence of subtle alterations in synaptic function during the earliest clinical phase of AD may be mediated by diffusible assemblies of the amyloid beta-protein (Abeta). Using primary neocortical cultures, here we compare the synaptic responses induced by two different Abeta assemblies, protofibrils (PFs) and fibrils (FBs), and demonstrate for the first time that neuronal activation was selectively dependent on the assembly state of Abeta. PF-induced activity was specifically attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, D-APV. In contrast, the non-NMDA glutamate receptor antagonist, NBQX, preferentially reduced FB-induced activity. In support, removal of Mg(2+) from the medium, which enhances NMDA channels, increased both PF- or FB-induced activation, but D-APV was more effective in attenuating PF-induced excitatory activity. These findings suggest that PFs may activate neurons differently than fibrils and lend support to the hypothesis that pre-fibrillar assemblies of Abeta may play an important role in the development of AD-type synaptic deficits.

Soluble Arctic amyloid beta protein inhibits hippocampal long-term potentiation in vivo

Mutations in the amyloid precursor protein that result in substitutions of glutamic acid at residue 22 of the amyloid beta protein (A beta) with glutamine (Q22, Dutch) or glycine (G22, Arctic) cause aggressive familial neurological diseases characterized by cerebrovascular haemorrhages or Alzheimer's-type dementia, respectively. The present study compared the ability of these peptides to block long-term potentiation (LTP) of glutamatergic transmission in the hippocampus in vivo. The effects of intracerebroventricular injection of wild-type, Q22 and G22 A beta(1-40) peptides were examined in the CA1 area of urethane-anaesthetized rats. Both mutant peptides were approximately 100-fold more potent than wild-type A beta at inhibiting LTP induced by high-frequency stimulation when solutions of A beta were freshly prepared. Fibrillar material, as determined by electron microscopy, was obvious in all these peptide solutions and exhibited appreciable Congo Red binding, particularly for A beta(1-40)G22 and A beta(1-40)Q22. A soluble fraction of A beta(1-40)G22, obtained following high-speed centrifugation, retained full activity of the peptide solution to inhibit LTP, providing strong evidence that in the case of the Arctic disease a soluble nonfibrillar form of A beta may represent the primary mediator of A beta-related cognitive deficits, particularly early in the disease. In contrast, nonfibrillar soluble A beta(1-40)Q22 supernatant solution was approximately 10-fold less potent at inhibiting LTP than A beta(1-40)G22, a finding consistent with fibrillar A beta contributing to the inhibition of LTP by the Dutch peptide.

Beta-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide

The mechanisms underlying the inhibition of long-term potentiation (LTP) induction by amyloidbeta-peptide (Abeta) were investigated in the medial perforant path of the rat and mouse dentate gyrus in vitro. Evidence is presented in this study that the Abeta-mediated inhibition of LTP induction involves activation of microglia and production of reactive oxygen and nitrogen species. In control slices, Abeta strongly inhibited induction of NMDA receptor-dependent (NMDAR-dependent) LTP, although not induction of NMDAR-independent LTP or long-term depression (LTD). The inhibition of NMDAR-dependent LTP was prevented by minocycline, an agent that prevents activation of microglia. The involvement of inducible nitric oxide synthase (iNOS) was shown by the inability of Abeta to inhibit LTP induction in iNOS knock-out mice and also by the ability of two iNOS inhibitors, aminoguanidine and 1400W, to prevent the Abeta-mediated inhibition of LTP induction. The Abeta-mediated inhibition of LTP induction also was prevented by the superoxide scavenger superoxide dismutase applied together with catalase. Evidence for involvement of superoxide in the action of Abeta on LTP induction was shown by the ability of an inhibitor of NADPH oxidase to prevent the Abeta-mediated inhibition of LTP induction. The study thus provides evidence that the Abeta-mediated inhibition of LTP induction involves an inflammatory-type reaction in which activation of microglia results in the production of nitric oxide and superoxide and thence possibly peroxynitrite, a highly reactive oxidant.

Amyloid beta prevents activation of calcium/calmodulin-dependent protein kinase II and AMPA receptor phosphorylation during hippocampal long-term potentiation

Accumulation of amyloid beta-peptides (Abeta) in the brain has been linked with memory loss in Alzheimer's disease and its animal models. However, the synaptic mechanism by which Abeta causes memory deficits remains unclear. We previously showed that acute application of Abeta inhibited long-term potentiation (LTP) in the hippocampal perforant path via activation of calcineurin, a Ca2+ -dependent protein phosphatase. This study examined whether Abeta could also inhibit Ca2+/calmodulin dependent protein kinase II (CaMKII), further disrupting the dynamic balance between protein kinase and phosphatase during synaptic plasticity. Immunoblot analysis was conducted to measure autophosphorylation of CaMKII at Thr286 and phosphorylation of the GluR1 subunit of AMPA receptors in single rat hippocampal slices. A high-frequency tetanus applied to the perforant path significantly increased CaMKII autophosphorylation and subsequent phosphorylation of GluR1 at Ser831, a CaMKII-dependent site, in the dentate area. Acute application of Abeta1-42 inhibited dentate LTP and associated phosphorylation processes, but was without effect on phosphorylation of GluR1 at Ser845, a protein kinase A-dependent site. These results suggest that activity-dependent CaMKII autophosphorylation and AMPA receptor phosphorylation are essential for dentate LTP. Disruption of such mechanisms could directly contribute to Abeta-induced deficits in hippocampal synaptic plasticity and memory.

Presynaptic localization of neprilysin contributes to efficient clearance of amyloid-beta peptide in mouse brain

A local increase in amyloid-beta peptide (Abeta) is closely associated with synaptic dysfunction in the brain in Alzheimer's disease. Here, we report on the catabolic mechanism of Abeta at the presynaptic sites. Neprilysin, an Abeta-degrading enzyme, expressed by recombinant adeno-associated viral vector-mediated gene transfer, was axonally transported to presynaptic sites through afferent projections of neuronal circuits. This gene transfer abolished the increase in Abeta levels in the hippocampal formations of neprilysin-deficient mice and also reduced the increase in young mutant amyloid precursor protein transgenic mice. In the latter case, Abeta levels in the hippocampal formation contralateral to the vector-injected side were also significantly reduced as a result of transport of neprilysin from the ipsilateral side, and in both sides soluble Abeta was degraded more efficiently than insoluble Abeta. Furthermore, amyloid deposition in aged mutant amyloid precursor protein transgenic mice was remarkably decelerated. Thus, presynaptic neprilysin has been demonstrated to degrade Abeta efficiently and to retard development of amyloid pathology.

Insulin and neurodegenerative disease: shared and specific mechanisms

Insulin has functions in the brain and dysregulation of these functions may contribute to the expression of late-life neurodegenerative disease. We provide a brief summary of research on the influence of insulin on normal brain function. We then review evidence that perturbation of this role may contribute to the symptoms and pathogenesis of various neurodegenerative disorders, such as Alzheimer's disease, vascular dementia, Parkinson's disease, and Huntington's disease. We conclude by considering whether insulin dysregulation contributes to neurodegenerative disorders through disease-specific or general mechanisms.

The role of c-Jun N-terminal kinase in the Aβ-mediated impairment of LTP and regulation of synaptic transmission in the hippocampus

The effects of the beta-amyloid peptide (Abeta) fragment 25-35 were investigated on hippocampal synaptic transmission and long-term potentiation (LTP) in vitro. Abeta([25-35]) was found to impair both post-tetanic potentiation (PTP) and LTP in the hippocampal CA1. The anthra[1,9-cd]pyrazol-6(2H)-one, SP600125, was used to inhibit c-Jun N-terminal kinase (JNK) activity, which is believed to mediate cell death. Prior application of SP600125 attenuated the Abeta([25-35])-mediated impairment of PTP and LTP, when measured from the pre-drug baseline. In the presence of SP600125 alone, we observed an increase in baseline synaptic transmission and reduction in paired-pulse facilitation, consistent with an increase in synaptic transmission. There was no alteration in the level of PTP and LTP obtained, when measured from the pre-drug baseline. In the presence of both SP600125 and Abeta, however, PTP was greatly enhanced compared with controls. We therefore suggest that the activation of the JNK signalling pathway mediates the effects of Abeta on synaptic plasticity. Our data also indicate that endogenous JNK activity may regulate neurotransmitter release in the hippocampal CA1 in vitro.

Metabolite-initiated protein misfolding may trigger Alzheimer's disease

Anfinsen showed that a protein's fold is specified by its sequence. Although it is clear why mutant proteins form amyloid, it is harder to rationalize why a wild-type protein adopts a native conformation in most individuals, but it misfolds in a minority of others, in what should be a common extracellular environment. This discrepancy suggests that another event likely triggers misfolding in sporadic amyloid disease. One possibility is that an abnormal metabolite, generated only in some individuals, covalently modifies the protein or peptide and causes it to misfold, but evidence for this is sparse. Candidate metabolites are suggested by the recently appreciated links between Alzheimer's disease (AD) and atherosclerosis, known chronic inflammatory metabolites, and the newly discovered generation of ozone during inflammation. Here we report detection of cholesterol ozonolysis products in human brains. These products and a related, lipid-derived aldehyde covalently modify Abeta, dramatically accelerating its amyloidogenesis in vitro, providing a possible chemical link between hypercholesterolemia, inflammation, atherosclerosis, and sporadic AD.

Ovariectomy combined with amyloid beta(1-42) impairs memory by decreasing acetylcholine release and alpha 7nAChR expression without induction of apoptosis in the hippocampus CA1 neurons of rats

In this study, the effect of ovariectomy and amyloid Beta(1-42) (ABeta(1-42))on eight-armed radial maze performance, acetylcholine (ACh) release, Alpha7nACh receptor (Alpha7nAChr), glyceraldehyde 3-phosphate dehydrogenase (GAPDH) expression, and apoptosis of CA1 neurons in the dorsal hippocampus were investigated in rat. The results showed that the dorsal hippocampus of sham rats contains 136.7 -/+ 16.7 to 160.4 -/+ 21.1 fmol/microl ACh, and respective 201 -/+ 22.9 and 416.6 -/+ 66.3 expression of mRNA for a7nAChR and GAPDH. Ovariectomy alone, after 4 weeks, did not impair memory, and neither induced apoptosis nor changed the basal ACh release. On the other hand, ABeta(1-42) (600 pmol/10 microl/body/day i.c.v. for 7 days) impaired memory, an effect characterized by increased error choices and reduced (50-59%) ACh release, but only with slight apoptosis. Moreover, ovariectomy combined with ABeta(1-42) induced memory impairment characterized by decreased numbers of correct choices and increased numbers of errors. This effect was accompanied by a decrease of the basal ACh level (67%), a7nAChR mRNA expression (52%) and a7nAChR/GAPDH ratio (44%) without induction of apoptosis in the dorsal hippocampus. The high K+-evoked ACh release was not altered in ovariectomized rats, but was decreased by ABeta(1-42) (43%) and ovariectomy + ABeta(1-42) (80%). These results suggest that ovariectomy-induced hormonal deprivation after 4 weeks, when accompanied by ABeta(1-42) accumulation in the dorsal hippocampus, could impair memory by decreasing ACh release and a7nAChR expression without inducing apoptosis in the CA1 field of the dorsal hippocampus.

Elucidation of primary structure elements controlling early amyloid beta-protein oligomerization

Assembly of monomeric amyloid beta-protein (A beta) into oligomeric structures is an important pathogenetic feature of Alzheimer's disease. The oligomer size distributions of aggregate-free, low molecular weight A beta 40 and A beta 42 can be assessed quantitatively using the technique of photo-induced cross-linking of unmodified proteins. This approach revealed that low molecular weight A beta 40 is a mixture of monomer, dimer, trimer, and tetramer, in rapid equilibrium, whereas low molecular weight A beta 42 preferentially exists as pentamer/hexamer units (paranuclei), which self-associate to form larger oligomers. Here, photo-induced cross-linking of unmodified proteins was used to evaluate systematically the oligomerization of 34 physiologically relevant A beta alloforms, including those containing familial Alzheimer's disease-linked amino acid substitutions, naturally occurring N-terminal truncations, and modifications altering the charge, the hydrophobicity, or the conformation of the peptide. The most important structural feature controlling early oligomerization was the length of the C terminus. Specifically, the side-chain of residue 41 in A beta 42 was important both for effective formation of paranuclei and for self-association of paranuclei into larger oligomers. The side-chain of residue 42, and the C-terminal carboxyl group, affected paranucleus self-association. A beta 40 oligomerization was particularly sensitive to substitutions of Glu22 or Asp23 and to truncation of the N terminus, but not to substitutions of Phe19 or Ala21. A beta 42 oligomerization, in contrast, was largely unaffected by substitutions at positions 22 or 23 or by N-terminal truncations, but was affected significantly by substitutions of Phe19 or Ala21. These results reveal how specific regions and residues control A beta oligomerization and show that these controlling elements differ between A beta 40 and A beta 42.

Alzheimer's disease-affected brain: presence of oligomeric A beta ligands (ADDLs) suggests a molecular basis for reversible memory loss

A molecular basis for memory failure in Alzheimer's disease (AD) has been recently hypothesized, in which a significant role is attributed to small, soluble oligomers of amyloid beta-peptide (A beta). A beta oligomeric ligands (also known as ADDLs) are known to be potent inhibitors of hippocampal long-term potentiation, which is a paradigm for synaptic plasticity, and have been linked to synapse loss and reversible memory failure in transgenic mouse AD models. If such oligomers were to build up in human brain, their neurological impact could provide the missing link that accounts for the poor correlation between AD dementia and amyloid plaques. This article, using antibodies raised against synthetic A beta oligomers, verifies the predicted accumulation of soluble oligomers in AD frontal cortex. Oligomers in AD reach levels up to 70-fold over control brains. Brain-derived and synthetic oligomers show structural equivalence with respect to mass, isoelectric point, and recognition by conformation-sensitive antibodies. Both oligomers, moreover, exhibit the same striking patterns of attachment to cultured hippocampal neurons, binding on dendrite surfaces in small clusters with ligand-like specificity. Binding assays using solubilized membranes show oligomers to be high-affinity ligands for a small number of nonabundant proteins. Current results confirm the prediction that soluble oligomeric A beta ligands are intrinsic to AD pathology, and validate their use in new approaches to therapeutic AD drugs and vaccines.

Protofibrils of amyloid beta-protein inhibit specific K+ currents in neocortical cultures

Protofibrils (PFs) are recently described intermediate assemblies formed during the fibrillogenesis of amyloidogenic proteins and may play an important pathogenic role in Parkinson's and Alzheimer's disease (AD). Here we show for the first time that amyloid beta-protein (Abeta) attenuation of specific K(+) currents is dependent on the aggregation state; PFs inhibit K(+) currents, whereas low-molecular-weight assemblies have no effect. Using patch clamp analysis in whole cell current-clamp mode, we showed that at low nanomolar concentrations Abeta(1-42) PFs induce reversible, Ca(2+)-dependent increases in spontaneous action potentials and membrane depolarizations. The low nanomolar PF concentrations used, the instantaneous responses observed, and the reversibility of the effect all suggest that PFs may bind to specific channels or membrane proteins. Switching to voltage-clamp mode, we found that PFs at 1-2 microM can inhibit specifically the 4AP-sensitive K(+) currents, A-type and D-type, but not other outward or inward rectifying K(+) channels. Finally, we show that a consequence of PF-induced membrane activity is an increase in intracellular Ca(2+) spikes that are dependent on synaptic connections in the neural network formed in culture. Our data strongly support the concept that PFs can induce subtle synaptic alterations that may underlie early symptoms of AD.

Activation of the c-Jun N-terminal kinase signaling cascade mediates the effect of amyloid-beta on long term potentiation and cell death in hippocampus: a role for interleukin-1beta?

Amyloid-beta (Abeta) is a major constituent of the neuritic plaque found in the brain of Alzheimer's disease patients, and a great deal of evidence suggests that the neuronal loss that is associated with the disease is a consequence of the actions of Abeta. In the past few years, it has become apparent that activation of c-Jun N-terminal kinase (JNK) mediates some of the effects of Abeta on cultured cells; in particular, the evidence suggests that Abeta-triggered JNK activation leads to cell death. In this study, we investigated the effect of intracerebroventricular injection of Abeta(1-40) on signaling events in the hippocampus and on long term potentiation in Schaffer collateral CA1 pyramidal cell synapses in vivo. We report that Abeta(1-40) induced activation of JNK in CA1 and that this was coupled with expression of the proapoptotic protein, Bax, cytosolic cytochrome c, poly-(ADP-ribose) polymerase cleavage, and Fas ligand expression in the hippocampus. These data indicate that Abeta(1-40) inhibited expression of long term potentiation, and this effect was abrogated by administration of the JNK inhibitor peptide, D-JNKI1. In parallel with these findings, we observed that Abeta-induced changes in caspase-3 activation and TdT-mediated dUTP nick-end labeling staining in neuronal cultured cells were inhibited by D-JNKI1. We present evidence suggesting that interleukin (IL)-1beta plays a significant role in mediating the effects of Abeta(1-40) because Abeta(1-40) increased hippocampal IL-1beta and because several effects of Abeta(1-40) were inhibited by the caspase-1 inhibitor Ac-YVAD-CMK. On the basis of our findings, we propose that Abeta-induced changes in hippocampal plasticity are likely to be dependent upon IL-1beta-triggered activation of JNK.

A beta 25-35-induced depression of long-term potentiation in area CA1 in vivo and in vitro is attenuated by verapamil

The effect of intracerebroventricular (icv) injection of A beta 25-35 and/or intraperitoneal (ip) application of the L-type calcium channel (VDCC) blockers verapamil or diltiazem were examined in vivo. To by-pass possible systemic actions of these agents, their effects on long-term potentiation (LTP) in the CA1 region of the in vitro hippocampal slice preparation were also examined. Application of A beta 25-35 (10 nmol in 5 microl, i.c.v.) significantly impaired LTP in vivo, as did IP injection of verapamil (1 or 10 mg/kg) or diltiazem (1 or 10 mg/kg). In the in vitro slice preparation, LTP was also depressed by prior application of A beta 25-35 (500 nmol), verapamil (20 microM), or diltiazem (50 microM). Combined application of A beta 25-35 and verapamil in either the in vivo or in vitro preparation resulted in a significant reversal of the LTP depression observed in the presence of either agent alone. However, co-application of diltiazem and A beta 25-35 failed to attenuate the depression of LTP observed in the presence of either agent alone in vivo or in vitro. Since LTP is a cellular correlate of memory and A beta is known to be involved in Alzheimer's disease (AD), these results indicate that verapamil, a phenylalkylamine, may be useful in the treatment of cognitive deficits associated with AD.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

Synaptic plasticity in animal models of early Alzheimer's disease

Amyloid beta-protein (Abeta) is believed to be a primary cause of Alzheimer's disease (AD). Recent research has examined the potential importance of soluble species of Abeta in synaptic dysfunction, long before fibrillary Abeta is deposited and neurodegenerative changes occur. Hippocampal excitatory synaptic transmission and plasticity are disrupted in transgenic mice overexpressing human amyloid precursor protein with early onset familial AD mutations, and in rats after exogenous application of synthetic Abeta both in vitro and in vivo. Recently, naturally produced soluble Abeta was shown to block the persistence of long-term potentiation (LTP) in the intact hippocampus. Sub-nanomolar concentrations of oligomeric Abeta were sufficient to inhibit late LTP, pointing to a possible reason for the sensitivity of hippocampus-dependent memory to impairment in the early preclinical stages of AD. Having identified the active species of Abeta that can play havoc with synaptic plasticity, it is hoped that new ways of targeting early AD can be developed.

Amyloid deposition in the hippocampus and entorhinal cortex: quantitative analysis of a transgenic mouse model

Various transgenic mouse models of Alzheimer's disease (AD) have been developed that overexpress mutant forms of amyloid precursor protein in an effort to elucidate more fully the potential role of beta-amyloid (A beta) in the etiopathogenesis of the disease. The present study represents the first complete 3D reconstruction of A beta in the hippocampus and entorhinal cortex of PDAPP transgenic mice. A beta deposits were detected by immunostaining and thioflavin fluorescence, and quantified by using high-throughput digital image acquisition and analysis. Quantitative analysis of amyloid load in hippocampal subfields showed a dramatic increase between 12 and 15 months of age, with little or no earlier detectable deposition. Three-dimensional reconstruction in the oldest brains visualized previously unrecognized sheets of A beta coursing through the hippocampus and cerebral cortex. In contrast with previous hypotheses, compact plaques form before significant deposition of diffuse A beta, suggesting that different mechanisms are involved in the deposition of diffuse amyloid and the aggregation into plaques. The dentate gyrus was the hippocampal subfield with the greatest amyloid burden. Sublaminar distribution of A beta in the dentate gyrus correlated most closely with the termination of afferent projections from the lateral entorhinal cortex, mirroring the selective vulnerability of this circuit in human AD. This detailed temporal and spatial analysis of A beta and compact amyloid deposition suggests that specific corticocortical circuits express selective, but late, vulnerability to the pathognomonic markers of amyloid deposition, and can provide a basis for detecting prior vulnerability factors.

Guinea pigs as a nontransgenic model for APP processing in vitro and in vivo

Alzheimer's disease (AD) is characterized, amongst others, by the appearance of vascular and parenchymal beta-amyloid deposits in brain. Such aggregates are mainly composed of beta-amyloid peptides, which are derived by proteolytic processing of a larger amyloid precursor protein (APP). APP is highly conserved among mammalian species, but experimental studies in rodents are often hampered by the humble APP-processing in the amyloidogenic pathway and by the inability of rodent beta-amyloid peptides to form higher molecular aggregates such as soluble oligomers and insoluble beta-amyloid plaques. Thus, there is need for in vitro and in vivo model systems that allow identification of factors that increase amyloidogenic APP processing and accelerate beta-amyloid plaque formation and testing the potency of pharmacological manipulations to ameliorate beta-amyloid load in brain. Transgenic mice that overexpress human APP containing AD-associated mutations that favor the amyloidogenic pathway of APP processing represent such a model. However, mutations of the APP gene are not frequent in AD and, therefore, the mechanisms of beta-amyloid plaque formation, the composition of beta-amyloid plaques, and the accompanying tissue response in brain of these animals may be different from that in AD. In contrast, guinea pigs express beta-amyloid peptides of the human sequence and appear to represent a more physiological model to examine the long-term effects of experimental manipulations on APP processing and beta-amyloid plaque formation in vivo. Additionally, APP processing in guinea pig primary neuronal cultures has been shown to be similar to cultures of human origin. In this article we highlight the advantages and limitations of using guinea pigs as experimental models to study APP processing.

In vitro characterization of conditions for amyloid-beta peptide oligomerization and fibrillogenesis

Extensive research causally links amyloid-beta peptide (A beta) to Alzheimer's disease, although the pathologically relevant A beta conformation remains unclear. A beta spontaneously aggregates into the fibrils that deposit in senile plaques. However, recent in vivo and in vitro reports describe a potent biological activity for oligomeric assemblies of A beta. To consistently prepare in vitro oligomeric and fibrillar forms of A beta 1-42, a detailed knowledge of how solution parameters influence structure is required. This manuscript represents the first study using a single chemically and structurally homogeneous unaggregated starting material to demonstrate that the formation of oligomers, fibrils, and fibrillar aggregates is determined by time, concentration, temperature, pH, ionic strength, and A beta species. We recently reported that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta 1-42-induced neurotoxicity significant at 10 nm. In addition, we were able to differentiate by structure and neurotoxic activity wild-type A beta1-42 from isoforms containing familial mutations (Dahlgren, K. N., Manelli, A. M., Stine, W. B., Jr., Baker, L. K., Krafft, G. A., and LaDu, M. J. (2002) J. Biol. Chem. 277, 32046-32053). Understanding the biological role of specific A beta conformations may define the link between A beta and Alzheimer's disease, re-focusing therapeutic approaches by identifying the pernicious species of A beta ultimately responsible for the cognitive dysfunction that defines the disease.

Dentate gyrus volume is reduced before onset of plaque formation in PDAPP mice: a magnetic resonance microscopy and stereologic analysis

High-resolution magnetic resonance microscopy (MRM) was used to determine regional brain volumetric changes in a mouse model of Alzheimer's disease. These transgenic (Tg) mice overexpress human mutant amyloid precursor protein (APP) V717F under control of platelet-derived growth factor promoter (PDAPP mice), and cortical and hippocampal beta-amyloid (Abeta) deposits accumulate in heterozygotes after 8-10 mos. We used MRM to obtain 3D volumetric data on mouse brains imaged in their skulls to define genotype- and age-related changes. Hippocampal, cerebellar, and brain volumes and corpus callosum length were quantified in 40-, 100-, 365-, and 630-day-old mice. Measurements taken at age 100 days, before Abeta deposition, revealed a 12.3% reduction of hippocampus volume in Tg mice compared with WT controls. This reduction persisted without progression to age 21 mos. A significant 18% increase in hippocampal volume occurred between 40 and 630 days in WT mice, and no corresponding significant increase occurred in Tg mice. Cavalieri volume estimates of hippocampal subfields from 100-day-old Tg mice further localized a 28% volume deficit in the dentate gyrus. In addition, corpus callosum length was reduced by approximately 25% in Tg mice at all ages analyzed. In summary, reduced hippocampal volume and corpus callosum length can be detected by MRM before Abeta deposition. We conclude that overexpression of APP and amyloid may initiate pathologic changes before the appearance of plaques, suggesting novel targets for the treatment of Alzheimer's disease and further reinforcing the need for early diagnosis and treatment.

Selective neuronal degeneration induced by soluble oligomeric amyloid beta protein

The prevailing amyloid hypothesis for Alzheimer's disease (AD) holds that amyloid beta-protein (Abeta) causes neuronal degeneration by forming neurotoxic fibrillar structures. Yet, many aspects of AD pathology and symptoms are not well explained by this hypothesis. Here, we present evidence that neurotoxicity of soluble oligomeric Abeta closely corresponds to the selective neurodegeneration so distinctly manifest in AD. Selectivity was first observed in vitro, where only the human central nervous system neuronal cells were susceptible to soluble oligomeric Abeta. Furthermore, in mouse cerebral slice treated with soluble oligomeric Abeta, selective regiospecific toxicity was evident in the hippocampal CA1, a division important for memory, but not in the CA3 subfield. The fibrillar Abeta, however, killed neurons in all regions of the cerebral slice cultures and also in cerebellar slices. Remarkably, even at the highest soluble oligomeric Abeta concentrations, cerebellar neurons were completely spared, consistent with one of the hallmark features of AD pathology. Our observation of the selective neurodegeneration of soluble oligomeric Abeta to neurons involved in cognitive function may provide a new opportunity for the development of an effective AD therapy as well as elucidating the pathological mechanism of AD.

Abeta toxicity in Alzheimer's disease: globular oligomers (ADDLs) as new vaccine and drug targets

Over the past several years, experiments with synthetic amyloid-beta peptide (Abeta) and animal models have strongly suggested that pathogenesis of Alzheimer's disease (AD) involves soluble assemblies of Abeta peptides (Trends Neurosci. 24 (2001) 219). These soluble neurotoxins (known as ADDLs and protofibrils) seem likely to account for the imperfect correlation between insoluble fibrillar amyloid deposits and AD progression. Recent experiments have detected the presence of ADDLs in AD-afflicted brain tissue and in transgenic-mice models of AD. The presence of high affinity ADDL binding proteins in hippocampus and frontal cortex but not cerebellum parallels the regional specificity of AD pathology and suggests involvement of a toxin receptor-mediated mechanism. The properties of ADDLs and their presence in AD-afflicted brain are consistent with their putative role even in the earliest stages of AD, including forms of mild cognitive impairment.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Paradigm shifts in Alzheimer's disease and other neurodegenerative disorders: the emerging role of oligomeric assemblies

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder characterized by amyloid deposition in the cerebral neuropil and vasculature. These amyloid deposits comprise predominantly fragments and full-length (40 or 42 residue) forms of the amyloid beta-protein (Abeta) organized into fibrillar assemblies. Compelling evidence indicates that factors that increase overall Abeta production or the ratio of longer to shorter forms, or which facilitate deposition or inhibit elimination of amyloid deposits, cause AD or are risk factors for the disease. In vitro studies have demonstrated that fibrillar Abeta has potent neurotoxic effects on cultured neurons. In vivo experiments in non-human primates have demonstrated that Abeta fibrils directly cause pathologic changes, including tau hyperphosphorylation. In concert with histologic studies revealing a lack of tissue injury in areas of the neuropil in which non-fibrillar deposits were found, these data suggested that fibril assembly was a prerequisite for Abeta-mediated neurotoxicity in vivo. Recently, however, both in vitro and in vivo studies have revealed that soluble, oligomeric forms of Abeta also have potent neurotoxic activities, and in fact, may be the proximate effectors of the neuronal injury and death occurring in AD. A paradigm shift is thus emerging that necessitates the reevaluation of the relative importance of polymeric (fibrillar) vs. oligomeric assemblies in the pathobiology of AD. In addition to AD, an increasing number of neurodegenerative disorders, including Parkinson's disease, familial British dementia, familial amyloid polyneuropathy, amyotrophic lateral sclerosis, and prion diseases, are associated with abnormal protein assembly processes. The archetypal features of the assembly-dependent neuropathogenetic effects of Abeta may thus be of relevance not only to AD but to these other disorders as well.

Oligomeric and fibrillar species of amyloid-beta peptides differentially affect neuronal viability

Genetic evidence predicts a causative role for amyloid-beta (A beta) in Alzheimer's disease. Recent debate has focused on whether fibrils (amyloid) or soluble oligomers of A beta are the active species that contribute to neurodegeneration and dementia. We developed two aggregation protocols for the consistent production of stable oligomeric or fibrillar preparations of A beta-(1-42). Here we report that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta-(1-42)-induced inhibition significant at 10 nm. Under A beta-(1-42) oligomer- and fibril-forming conditions, A beta-(1-40) remains predominantly as unassembled monomer and had significantly less effect on neuronal viability than preparations of A beta-(1-42). We applied the aggregation protocols developed for wild type A beta-(1-42) to A beta-(1-42) with the Dutch (E22Q) or Arctic (E22G) mutations. Oligomeric preparations of the mutations exhibited extensive protofibril and fibril formation, respectively, but were not consistently different from wild type A beta-(1-42) in terms of inhibition of neuronal viability. However, fibrillar preparations of the mutants appeared larger and induced significantly more inhibition of neuronal viability than wild type A beta-(1-42) fibril preparations. These data demonstrate that protocols developed to produce oligomeric and fibrillar A beta-(1-42) are useful in distinguishing the structural and functional differences between A beta-(1-42) and A beta-(1-40) and genetic mutations of A beta-(1-42).

Clusterin promotes amyloid plaque formation and is critical for neuritic toxicity in a mouse model of Alzheimer's disease

Studies have shown that clusterin (also called apolipoprotein J) can influence the structure and toxicity of amyloid-beta (Abeta) in vitro. To determine whether endogenous clusterin plays a role in influencing Abeta deposition, structure, and toxicity in vivo, we bred PDAPP mice, a transgenic mouse model of Alzheimer's disease, to clusterin(-/-) mice. By 12 months of age, PDAPP, clusterin(-/-) mice had similar levels of brain Abeta deposition as did PDAPP, clusterin(+/+) mice. Although Abeta deposition was similar, PDAPP, clusterin(-/-) mice had significantly fewer fibrillar Abeta (amyloid) deposits than PDAPP mice expressing clusterin. In the absence of clusterin, neuritic dystrophy associated with the deposited amyloid was markedly reduced, resulting in a dissociation between fibrillar amyloid formation and neuritic dystrophy. These findings demonstrate that clusterin markedly influences Abeta structure and neuritic toxicity in vivo and is likely to play an important role in Alzheimer's disease pathogenesis.