Amyloid beta oligomers induce impairment of neuronal insulin receptors

Elevation of plasma beta-amyloid level by glucose loading in Alzheimer mouse models

With the emergence of a promising approach to treat Alzheimer disease (AD) targeting the beta-amyloid (Abeta) pathway, it is necessary to establish new diagnostic biomarkers that enable the antemortem diagnosis of AD. Although plasma Abeta has been suggested as a non-invasive biomarker, its significance has been inconclusive. Thus, it is important to improve the diagnostic potential of plasma Abeta. One of the potential approaches is to modify plasma Abeta level using various modulators. In this study, we evaluated the influence of glucometabolic status on plasma Abeta level in two lines of AD transgenic mouse. The present study demonstrated that plasma Abeta level rapidly increased after glucose loading. More importantly, the magnitude of the increase in plasma Abeta was significantly larger in AD transgenic mice than in wild-type littermates. These findings might provide a novel diagnostic tool for AD using the elevation of plasma Abeta level after glucose loading.

The role of metabolic derangements and glucocorticoid excess in the aetiology of cognitive impairment in type 2 diabetes. Implications for future therapeutic strategies

Dementia is becoming increasingly common in western societies and carries with it a substantial clinical, social and economic burden. It is now well established that type 2 diabetes is a risk factor for dementia and it is likely that this association has a multifactorial aetiology. There is a relative paucity of data on interventions to improve cognitive function in people with type 2 diabetes. Two small randomized controlled trials have suggested that better glycaemic control, over a relatively short time period, can improve or prevent decline in cognitive function. There is also increasing interest in the link between intracerebral insulin and cognitive impairment. Several studies have suggested that relative and/or absolute deficiency of insulin may occur in Alzheimer's dementia and, although one small randomized trial was essentially negative, randomized trials are currently underway to investigate the impact of thiazolidinediones on cognitive function in dementia. The hypothalamic-pituitary-adrenal axis is also activated in people with type 2 diabetes and there are data linking increased cortisol concentrations with cognitive impairment. Inhibition of the 11 beta-hydroxysteroid dehydrogenase type 1 enzyme, which generates cortisol from inactive cortisone in many tissues including the brain, is an attractive therapeutic target to enhance cognition. Large-scale epidemiological and intervention studies are now underway, which should enhance our understanding and management of cognitive impairment in type 2 diabetes.

A role for synaptic zinc in activity-dependent Abeta oligomer formation and accumulation at excitatory synapses

Soluble amyloid beta oligomers (AbetaOs) interfere with synaptic function and bind with high affinity to synapses, but the mechanism underlying AbetaO synaptic targeting is not known. Here, we show that the accumulation of synthetic or native Alzheimer's disease (AD)-brain oligomers at synapses is regulated by synaptic activity. Electrical or chemical stimulation increased AbetaO synaptic localization and enhanced oligomer formation at synaptic terminals, whereas inhibition with TTX blocked AbetaO synaptic localization and reduced AbetaO synaptic load. The zinc-binding 8-OH-quinoline clioquinol markedly reduced AbetaO synaptic targeting, which was also reduced in brain sections of animals deficient in the synaptic vesicle zinc transporter ZnT3, indicating that vesicular zinc released during neurotransmission is critical for AbetaO synaptic targeting. Oligomers were not internalized in recycled vesicles but remained at the cell surface, where they colocalized with NR2B NMDA receptor subunits. Furthermore, NMDA antagonists blocked AbetaO synaptic targeting, implicating excitatory receptor activity in oligomer formation and accumulation at synapses. In AD brains, oligomers of different size colocalized with synaptic markers in hippocampus and cortex, where oligomer synaptic accumulation correlated with synaptic loss.

The PI3K-Akt-mTOR pathway regulates Abeta oligomer induced neuronal cell cycle events

Accumulating evidence suggests that neurons prone to degeneration in Alzheimer's Disease (AD) exhibit evidence of re-entry into an aberrant mitotic cell cycle. Our laboratory recently demonstrated that, in a genomic amyloid precursor protein (APP) mouse model of AD (R1.40), neuronal cell cycle events (CCEs) occur in the absence of beta-amyloid (Aβ) deposition and are still dependent upon the amyloidogenic processing of the amyloid precursor protein (APP). These data suggested that soluble Aβ species might play a direct role in the induction of neuronal CCEs. Here, we show that exposure of non-transgenic primary cortical neurons to Aβ oligomers, but not monomers or fibrils, results in the retraction of neuronal processes, and induction of CCEs in a concentration dependent manner. Retraction of neuronal processes correlated with the induction of CCEs and the Aβ monomer or Aβ fibrils showed only minimal effects. In addition, we provide evidence that induction of neuronal CCEs are autonomous to primary neurons cultured from the R1.40 mice. Finally, our results also demonstrate that Aβ oligomer treated neurons exhibit elevated levels of activated Akt and mTOR (mammalian Target Of Rapamycin) and that PI3K, Akt or mTOR inhibitors blocked Aβ oligomer-induced neuronal CCEs. Taken together, these results demonstrate that Aβ oligomer-based induction of neuronal CCEs involve the PI3K-Akt-mTOR pathway.

The insulin/Akt signaling pathway is targeted by intracellular beta-amyloid

Intraneuronal beta-amyloid (Abeta(i)) accumulates early in Alzheimer's disease (AD) and inclusion body myositis. Several organelles, receptor molecules, homeostatic processes, and signal transduction components have been identified as sensitive to Abeta. Although prior studies implicate the insulin-PI3K-Akt signaling cascade, a specific step within this or any essential metabolic or survival pathway has not emerged as a molecular target. We tested the effect of Abeta42 on each component of this cascade. In AD brain, the association between PDK and Akt, phospho-Akt levels and its activity were all decreased relative to control. In cell culture, Abeta(i) expression inhibited both insulin-induced Akt phosphorylation and activity. In vitro experiments identified that beta-amyloid (Abeta), especially oligomer preparations, specifically interrupted the PDK-dependent activation of Akt. Abeta(i) also blocked the association between PDK and Akt in cell-based and in vitro experiments. Importantly, Abeta did not interrupt Akt or PI3K activities (once stimulated) nor did it affect more proximal signal events. These results offer a novel therapeutic strategy to neutralize Abeta-induced energy failure and neuronal death.

Antidiabetic drug metformin (GlucophageR) increases biogenesis of Alzheimer's amyloid peptides via up-regulating BACE1 transcription

Epidemiological, clinical and experimental evidence suggests a link between type 2 diabetes and Alzheimer's disease (AD). Insulin modulates metabolism of beta-amyloid precursor protein (APP) in neurons, decreasing the intracellular accumulation of beta-amyloid (Abeta) peptides, which are pivotal in AD pathogenesis. The present study investigates whether the widely prescribed insulin-sensitizing drug, metformin (Glucophage(R)), affects APP metabolism and Abeta generation in various cell models. We demonstrate that metformin, at doses that lead to activation of the AMP-activated protein kinase (AMPK), significantly increases the generation of both intracellular and extracellular Abeta species. Furthermore, the effect of metformin on Abeta generation is mediated by transcriptional up-regulation of beta-secretase (BACE1), which results in an elevated protein level and increased enzymatic activity. Unlike insulin, metformin exerts no effect on Abeta degradation. In addition, we found that glucose deprivation and various tyrphostins, known inhibitors of insulin-like growth factors/insulin receptor tyrosine kinases, do not modulate the effect of metformin on Abeta. Finally, inhibition of AMP-activated protein kinase (AMPK) by the pharmacological inhibitor Compound C largely suppresses metformin's effect on Abeta generation and BACE1 transcription, suggesting an AMPK-dependent mechanism. Although insulin and metformin display opposing effects on Abeta generation, in combined use, metformin enhances insulin's effect in reducing Abeta levels. Our findings suggest a potentially harmful consequence of this widely prescribed antidiabetic drug when used as a monotherapy in elderly diabetic patients.

Protection of synapses against Alzheimer's-linked toxins: insulin signaling prevents the pathogenic binding of Abeta oligomers

Synapse deterioration underlying severe memory loss in early Alzheimer's disease (AD) is thought to be caused by soluble amyloid beta (Abeta) oligomers. Mechanistically, soluble Abeta oligomers, also referred to as Abeta-derived diffusible ligands (ADDLs), act as highly specific pathogenic ligands, binding to sites localized at particular synapses. This binding triggers oxidative stress, loss of synaptic spines, and ectopic redistribution of receptors critical to plasticity and memory. We report here the existence of a protective mechanism that naturally shields synapses against ADDL-induced deterioration. Synapse pathology was investigated in mature cultures of hippocampal neurons. Before spine loss, ADDLs caused major downregulation of plasma membrane insulin receptors (IRs), via a mechanism sensitive to calcium calmodulin-dependent kinase II (CaMKII) and casein kinase II (CK2) inhibition. Most significantly, this loss of surface IRs, and ADDL-induced oxidative stress and synaptic spine deterioration, could be completely prevented by insulin. At submaximal insulin doses, protection was potentiated by rosiglitazone, an insulin-sensitizing drug used to treat type 2 diabetes. The mechanism of insulin protection entailed a marked reduction in pathogenic ADDL binding. Surprisingly, insulin failed to block ADDL binding when IR tyrosine kinase activity was inhibited; in fact, a significant increase in binding was caused by IR inhibition. The protective role of insulin thus derives from IR signaling-dependent downregulation of ADDL binding sites rather than ligand competition. The finding that synapse vulnerability to ADDLs can be mitigated by insulin suggests that bolstering brain insulin signaling, which can decline with aging and diabetes, could have significant potential to slow or deter AD pathogenesis.

Beta amyloid oligomers and fibrils stimulate differential activation of primary microglia

Background

Beta amyloid (Aβ) peptides are the major constituents of the senile plaques present in Alzheimer's diseased brain. Pathogenesis has been associated with the aggregated form of the peptide as these fibrils are the conformation readily found in the plaques. However, recent studies have shown that the nonaggregated, soluble assemblies of Aβ have the potential to stimulate neuronal dysfunction and may play a prominent role in the pathogenesis of Alzheimer's disease.

Methods

Soluble, synthetic Aβ1–42 oligomers were prepared producing mainly dimer-trimer conformations as assessed by SDS-PAGE. Similar analysis demonstrated fibril preparations to produce large insoluble aggregates unable to migrate out of the stacking portion of the gels. These peptide preparations were used to stimulate primary murine microglia and cortical neuron cultures. Microglia were analyzed for changes in signaling response and secretory phenotype via Western analysis and ELISA. Viability was examined by quantifying lactate dehydrogenase release from the cultures.

Results

Aβ oligomers and fibrils were used to stimulate microglia for comparison. Both the oligomers and fibrils stimulated proinflammatory activation of primary microglia but the specific conformation of the peptide determined the activation profile. Oligomers stimulated increased levels of active, phosphorylated Lyn and Syk kinase as well as p38 MAP kinase compared to fibrils. Moreover, oligomers stimulated a differential secretory profile for interleukin 6, monocyte chemoattractant protein-1 and keratinocyte chemoattractant when compared to fibrils. Finally, soluble oligomers stimulated death of cultured cortical neurons that was exacerbated by the presence of microglia.

Conclusion

These data suggest that fibrils and oligomers stimulate unique signaling responses in microglia leading to discrete secretory changes and effects on neuron survival. This suggests that inflammation changes during disease may be the consequence of unique peptide-stimulated events and each conformation may represent an individual anti-inflammatory therapeutic target.

Abeta oligomers induce neuronal cell cycle events in Alzheimer's disease

Neurons subject to degeneration in Alzheimer's disease (AD) exhibit evidence of re-entry into a mitotic cell cycle even before the development of substantial AD brain pathology. In efforts to identify the initiating factors underlying these cell cycle events (CCEs), we have characterized the appearance of the neuronal CCEs in the genomic-based R1.40 transgenic mouse model of AD. Notably, R1.40 mice exhibit neuronal CCEs in a reproducible temporal and spatial pattern that recapitulates the neuronal vulnerability seen in human AD. Neuronal CCEs first appear at 6 months in the frontal cortex layers II/III. This is 6-8 months before detectable amyloid beta (Abeta) deposition, suggesting that specific amyloid precursor protein (APP) processing products are responsible for the induction of neuronal CCEs. Furthermore, a reduction in the levels of Abeta (achieved by shifting the genetic background from C57BL/6 to the DBA/2 mouse strain) dramatically delays the appearance of neuronal CCEs. More significantly, elimination of beta-secretase activity blocks the appearance of CCEs, providing direct genetic evidence that the amyloidogenic processing of APP is required for the induction of CCEs. Finally, in vitro preparations of oligomeric, but not monomeric, Abeta induce DNA synthesis in dissociated cortical neurons, and this response is blocked by antioligomer specific antibodies. Together, our data suggest that low molecular weight aggregates of Abeta induce neuronal cell cycle re-entry in mouse models of Alzheimer's disease.

Insulin resistance and amyloidogenesis as common molecular foundation for type 2 diabetes and Alzheimer's disease

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Abeta), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Abeta aggregation may not only be the consequence of excitotoxicity, aberrant Ca(2+) signals, and proinflammatory cytokines such as TNF-alpha, but may also promote these pathological effectors. At the molecular level, insulin resistance and Abeta disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

Insulin in combination with other diabetes medication is associated with less Alzheimer neuropathology

OBJECTIVE

To examine the association between treatment for diabetes and Alzheimer disease (AD) neuropathology.

METHODS

This postmortem study matched 124 subjects with diabetes to 124 without diabetes from the Mount Sinai School of Medicine Brain Bank, on age (mean = 81.2 + 9.3), sex (57.3% F), and severity of dementia (Clinical Dementia Rating [CDR] 2.4 + 1.7). Densities of neuritic plaques (NPs) and of neurofibrillary tangles (NFTs) were assessed in several neocortical regions and in the hippocampus, entorhinal cortex, and amygdala. Diabetic subjects were classified according to their recorded lifetime antidiabetic medications: none (n = 29), insulin only (n = 49), diabetes medications other than insulin only (n = 28), or concomitant use of both insulin and any oral antidiabetic medications (n = 18). For each dependent variable, analysis of covariance controlling for age at death, sex, and CDR distinguished among the nondiabetic patients and four diabetic subgroups.

RESULTS

There were differences among the five groups for NP ratings in the entorhinal cortex (p = 0.003), amygdala (p = 0.009), and overall NP (p = 0.014) as well as counts of NPs in all regions examined (p values ranging from 0.009 to 0.04). NP ratings in the hippocampus (p = 0.057) and the combined neocortical measure (p = 0.052) approached significance. In each analysis, the concomitant medication group had significantly fewer NPs (approximately 20%) than any of the other groups, which were relatively similar. No significant NFT differences were found.

CONCLUSION

The results of this study suggest that the combination of insulin with other diabetes medication is associated with substantially lower neuritic plaque density consistent with the effects of both on the neurobiology of insulin.

New roles for insulin-like hormones in neuronal signalling and protection: new hopes for novel treatments of Alzheimer's disease?

Type 2 diabetes has been identified as a risk factor for Alzheimer's disease (AD). This is most likely due to the desensitisation of insulin receptors in the brain. Insulin acts as a growth factor and supports neuronal repair, dendritic sprouting, and differentiation. This review discusses the potential role that insulin-like hormones could play in ameliorating the reduced growth factor signalling in the brains of people with AD. The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) have very similar properties in protecting neurons from toxic effects, and are capable of reversing the detrimental effects that beta-amyloid fragments have on synaptic plasticity. Therefore, incretins show great promise as a novel treatment for reducing degenerative processes in AD.

Abeta(1-42) reduces synapse number and inhibits neurite outgrowth in primary cortical and hippocampal neurons: a quantitative analysis

Synaptic loss represents one of the earliest signs of neuronal damage and is observed within both Alzheimer's disease patients and transgenic mouse models of the disease. We have developed a novel in vitro assay using high content screening technology to measure changes in a number of cell physiological parameters simultaneously within a neuronal population. Using Hoechst-33342 to label nuclei, betaIII-tubulin as a neuron-specific marker, and synapsin-I as an indicator of pre-synaptic sites, we have designed software to interrogate triple-labelled images, counting only those synaptic puncta associated with tubulin-positive structures. Here we demonstrate that addition of amyloid beta peptide (Abeta(1-42)), to either primary hippocampal or cortical neurons for 4 days in vitro has deleterious effects upon synapse formation, neurite outgrowth and arborisation in a concentration-dependent manner. Control reverse peptide showed no effect over the same concentration range. The effects of Abeta(1-42) were inhibited by D-KLVFFA, which contains residues 16-20 of Abeta that function as a self-recognition element during Abeta assembly and bind to the homologous region of Abeta and block its oligomerisation. These effects of Abeta(1-42) on synapse number and neurite outgrowth are similar to those described within AD patient pathology and transgenic mouse models.

Serpins show structural basis for oligomer toxicity and amyloid ubiquity

Many disorders, including Alzheimer’s, the prion encephalopathies and other neurodegenerative diseases, result from aberrant protein aggregation. Surprisingly, cellular toxicity is often due not to the highly-ordered aggregates but to the oligomers that precede their formation. Using serpins as a paradigm, we show how the active and infective interface of oligomers is inherently toxic and can promiscuously bind to unrelated peptides, including neurotransmitters. Extension of the oligomer and its eventual sequestration as amyloid can thus be seen as a protective response to block the toxic interface. We illustrate how the preferential self-association that gives this protection has been selectively favoured.

Why is the amyloid beta peptide of Alzheimer's disease neurotoxic?

In this article, we support the case that the neurotoxic agent in Alzheimer's disease is a soluble aggregated form of the amyloid beta peptide (Abeta), probably complexed with divalent copper. The structure and chemical properties of the monomeric peptide and its Cu(ii) complex are discussed, as well as what little is known about the oligomeric species. Abeta oligomers are neurotoxic by a variety of mechanisms. They adhere to plasma and intracellular membranes and cause lesions by a combination of radical-initiated lipid peroxidation and formation of ion-permeable pores. In endothelial cells this damage leads to loss of integrity of the blood-brain barrier and loss of blood flow to the brain. At synapses, the oligomers close neuronal insulin receptors, mirroring the effects of Type II diabetes. In intracellular membranes, the most damaging effect is loss of calcium homeostasis. The oligomers also bind to a variety of substances, mostly with deleterious effects. Binding to cholesterol is accompanied by its oxidation to products that are themselves neurotoxic. Possibly most damaging is the binding to tau, and to several kinases, that results in the hyperphosphorylation of the tau and abrogation of its microtubule-supporting role in maintaining axon structure, leading to diseased synapses and ultimately the death of neurons. Several strategies are presented and discussed for the development of compounds that prevent the oligomerization of Abeta into the neurotoxic species.

Protease-resistant glucose-dependent insulinotropic polypeptide agonists facilitate hippocampal LTP and reverse the impairment of LTP induced by beta-amyloid

Type 2 diabetes has been identified as a risk factor for Alzheimer's disease (AD). Insulin signaling is often impaired in AD, contributing to the neurodegeneration observed in AD patients. One potential strategy to overcome this impairment is to normalize insulin signaling in the brain. In the present study, we have examined the effects of an enzyme-resistant analogue of glucose-dependent insulinotropic polypeptide (GIP), N-AcGIP, on synaptic plasticity. N-AcGIP is a stable, long-acting peptide hormone that regulates glucose homeostasis and insulin release. We tested the effects of native GIP and the agonist N-AcGIP on synaptic plasticity [long-term potentiation (LTP)] in the hippocampus [15 nmol, administered intracerebroventricularly (icv)] and report for the first time that both peptides have enhancing effects on LTP. In contrast, the antagonist of GIP, Pro(3)GIP (15 nmol icv), reduced LTP. Injection of beta-amyloid(25-35) (100 nmol), a peptide that aggregates in brains of AD patients, also impaired LTP. The injection of N-AcGIP (15 nmol icv) 30 min prior to injection of amyloid(25-35) (100 nmol icv) fully reversed the impairment of LTP induced by beta-amyloid. The results demonstrate for the first time that GIP (particularly enzyme-resistant forms) not only directly modulates neurotransmitter release and LTP formation, but also protects synapses from the detrimental effects of beta-amyloid fragments on LTP formation. The use of enzyme-resistant analogues of GIP show great promise as a potential novel treatment for preventing neurodegenerative processes in AD and other related disorders.

The relationship between type 2 diabetes and dementia

INTRODUCTION

The prevalence of type 2 diabetes and dementia are set to rise inexorably over the next 30-40 years. There are now substantial data to suggest that type 2 diabetes is associated with an increased risk of dementia.

SOURCES OF DATA

This is a narrative review using data from individual studies and review articles known to the authors. A Medline search was also undertaken and reference lists were reviewed to identify additional relevant studies.

AREAS OF AGREEMENT

Type 2 diabetes is associated with an increased risk of both Alzheimer's and Vascular dementia, although the reality is that many affected individuals have mixed forms of dementia.

AREAS OF CONTROVERSY

The mechanisms underpinning this association remain to be clearly delineated. Type 2 diabetes is a complex disorder and so it is likely that multiple different, synergistic processes may interact to promote cognitive decrements.

GROWING POINTS

Recent data suggest that glucocorticoids excess and elevated inflammatory markers may also have a role in the aetiology of diabetes-related cognitive impairment.

AREAS TIMELY FOR DEVELOPING RESEARCH

Large-scale, prospective epidemiological studies are now required to accurately delineate the pathogenesis of cognitive impairment in people with type 2 diabetes. These are underway and randomized trials of diabetes-specific interventions are also starting to include cognitive function as an outcome measure.

Why Alzheimer's is a disease of memory: the attack on synapses by A beta oligomers (ADDLs)

Individuals with early-stage Alzheimer's disease (AD) suffer from profound failure to form new memories. A novel molecular mechanism with implications for therapeutics and diagnostics is now emerging in which the specificity of AD for memory derives from disruption of plasticity at synapses targeted by neurologically active A beta oligomers (1). We have named these oligomers "ADDLs" (for pathogenic A beta-Derived Diffusible Ligands). ADDLs constitute metastable alternatives to the disease-defining A beta fibrils deposited in amyloid plaques. In AD brain, ADDLs accumulate primarily as A beta 12mers (2) (approximately 54 kDa) and can be found in dot-like clusters distinct from senile plaques (3). Oligomers of equal mass have been reported to occur in tgmouse AD models where they emerge concomitantly with memory failure (4), consistent with ADDL inhibition of LTP (1). In cell biology studies, ADDLs act as pathogenic gain-of-function ligands that target particular synapses, binding to synaptic spines at or near NMDA receptors (5,6). Binding produces ectopic expression of the memory-linked immediate early gene Arc. Subsequent ADDL-induced abnormalities in spine morphology and synaptic receptor composition (7) are predicted consequences of Arc overexpression, a pathology associated with memory dysfunction in tg-Arc mice. Significantly, the attack on synapses provides a plausible mechanism unifying memory dysfunction with major features of AD neuropathology; recent findings show that ADDL binding instigates synapse loss, oxidative damage, and AD-type tau hyperphosphorylation. Acting as novel neurotoxins that putatively account for memory loss and neuropathology, ADDLs present significant targets for disease-modifying therapeutics in AD.

Advances on the understanding of the origins of synaptic pathology in AD

Although Alzheimer's disease (AD) was first discovered a century ago, we are still facing a lack of definitive diagnosis during the patient's lifetime and are unable to prescribe a curative treatment. However, the past 10 years have seen a "revamping" of the main hypothesis about AD pathogenesis and the hope to foresee possible treatment. AD is no longer considered an irreversible disease. A major refinement of the classic beta-amyloid cascade describing amyloid fibrils as neurotoxins has been made to integrate the key scientific evidences demonstrating that the first pathological event occurring in AD early stages affects synaptic function and maintenance. A concept fully compatible with synapse loss being the best pathological correlate of AD rather than other described neuropathological hallmarks (amyloid plaques, neurofibrillary tangles or neuronal death). The notion that synaptic alterations might be reverted, thus offering a potential curability, was confirmed by immunotherapy experiments targeting beta-amyloid protein in transgenic AD mice in which cognitive functions were improved despite no reduction in the amyloid plaques burden. The updated amyloid cascade now integrates the synapse failure triggered by soluble Abeta-oligomers. Still no consensus has been reached on the most toxic Abeta conformations, neither on their site of production nor on their extra- versus intra-cellular actions. Evidence shows that soluble Abeta oligomers or ADDLs bind selectively to neurons at their synaptic loci, and trigger major changes in synapse composition and morphology, which ultimately leads to dendritic spine loss. However, the exact mechanism is not yet fully understood but is suspected to involve some membrane receptor(s).