NMDA receptor activation inhibits alpha-secretase and promotes neuronal amyloid-beta production

N-cadherin enhances APP dimerization at the extracellular domain and modulates Aβ production

Sequential processing of amyloid precursor protein (APP) by β- and γ-secretase leads to the generation of amyloid-β (Aβ) peptides, which plays a central role in Alzheimer's disease pathogenesis. APP is capable of forming a homodimer through its extracellular domain as well as transmembrane GXXXG motifs. A number of reports have shown that dimerization of APP modulates Aβ production. On the other hand, we have previously reported that N-cadherin-based synaptic contact is tightly linked to Aβ production. In the present report, we investigated the effect of N-cadherin expression on APP dimerization and metabolism. Here, we demonstrate that N-cadherin expression facilitates cis-dimerization of APP. Moreover, N-cadherin expression led to increased production of Aβ as well as soluble APPβ, indicating that β-secretase-mediated cleavage of APP is enhanced. Interestingly, N-cadherin expression affected neither dimerization of C99 nor Aβ production from C99, suggesting that the effect of N-cadherin on APP metabolism is mediated through APP extracellular domain. We confirmed that N-cadherin enhances APP dimerization by a novel luciferase-complementation assay, which could be a platform for drug screening on a high-throughput basis. Taken together, our results suggest that modulation of APP dimerization state could be one of mechanisms, which links synaptic contact and Aβ production.

Amyloid-β as a modulator of synaptic plasticity

Alzheimer's disease is associated with synapse loss, memory dysfunction, and pathological accumulation of amyloid-β (Aβ) in plaques. However, an exclusively pathological role for Aβ is being challenged by new evidence for an essential function of Aβ at the synapse. Aβ protein exists in different assembly states in the central nervous system and plays distinct roles ranging from synapse and memory formation to memory loss and neuronal cell death. Aβ is present in the brain of symptom-free people where it likely performs important physiological roles. New evidence indicates that synaptic activity directly evokes the release of Aβ at the synapse. At physiological levels, Aβ is a normal, soluble product of neuronal metabolism that regulates synaptic function beginning early in life. Monomeric Aβ40 and Aβ42 are the predominant forms required for synaptic plasticity and neuronal survival. With age, some assemblies of Aβ are associated with synaptic failure and Alzheimer's disease pathology, possibly targeting the N-methyl-D-aspartic acid receptor through the nicotinic acetylcholine receptor, mitochondrial Aβ alcohol dehydrogenase, and cyclophilin D. But emerging data suggests a distinction between age effects on the target response in contrast to the assembly state or the accumulation of the peptide. Both aging and Aβ independently decrease neuronal plasticity. Our laboratory has reported that Aβ, glutamate, and lactic acid are each increasingly toxic with neuron age. The basis of the age-related toxicity partly resides in age-related mitochondrial dysfunction and an oxidative shift in mitochondrial and cytoplasmic redox potential. In turn, signaling through phosphorylated extracellular signal-regulated protein kinases is affected along with an age-independent increase in phosphorylated cAMP response element-binding protein. This review examines the long-awaited functional impact of Aβ on synaptic plasticity.

Synapses, NMDA receptor activity and neuronal Aβ production in Alzheimer's disease

A direct relationship has been established between synaptic activity and amyloid-β secretion. Dysregulation of neuronal calcium homeostasis was shown to increase production of amyloid-β, contributing to the initiation of Alzheimer's disease. Among the different routes of Ca(2+) entry, N-methyl-d-aspartate (NMDA) receptors, a subtype of ionotropic glutamate receptors, are especially involved in this process because of their ability to gate high levels of Ca(2+) influx. These receptors have been extensively studied for their crucial roles in synaptic plasticity that underlies learning and memory but also in neurotoxicity occurring during acute brain injuries and neurodegenerative diseases. For one decade, several studies provided evidence that NMDA receptor activation could have distinct consequences on neuronal fate, depending on their location. Synaptic NMDA receptor activation is neuroprotective, whereas extrasynaptic NMDA receptors trigger neuronal death and/or neurodegenerative processes. Recent data suggest that chronic activation of extrasynaptic NMDA receptors leads to a sustained neuronal amyloid-β release and could be involved in the pathogenesis of Alzheimer's disease. Thus, as for other neurological diseases, therapeutic targeting of extrasynaptic NMDA receptors could be a promising strategy. Following this concept, memantine, unlike other NMDA receptor antagonists was shown, to preferentially target the extrasynaptic NMDA receptor signaling pathways, while relatively sparing normal synaptic activity. This molecular mechanism could therefore explain why memantine is, to date, the only clinically approved NMDA receptor antagonist for the treatment of dementia.

Endogenous amyloidogenesis in long-term rat hippocampal cell cultures

BACKGROUND

Long-term primary neuronal cultures are a useful tool for the investigation of biochemical processes associated with neuronal senescence. Improvements in available technology make it possible to observe maturation of neural cells isolated from different regions of the rodent brain over a prolonged period in vitro. Existing experimental evidence suggests that cellular aging occurs in mature, long-term, primary neuronal cell cultures. However, detailed studies of neuronal development in vitro are needed to demonstrate the validity of long-term cell culture-based models for investigation of the biochemical mechanisms of in vitro neuronal development and senescence.

RESULTS

In the current study, neuron-enriched hippocampal cell cultures were used to analyze the differentiation and degeneration of hippocampal neurons over a two month time period. The expression of different neuronal and astroglial biomarkers was used to determine the cytochemical characteristics of hippocampal cells in long-term cultures of varying ages. It was observed that the expression of the intermediate filament nestin was absent from cultures older than 21 days in vitro (DIV), and the expression of neuronal or astrocytic markers appeared to replace nestin. Additionally, morphological evaluations of neuronal integrity and Hoescht staining were used to assess the cellular conditions in the process of hippocampal culture development and aging. It was found that there was an increase in endogenous production of Aβ(1-42) and an increase in the accumulation of Congo Red-binding amyloidal aggregates associated with the aging of neurons in primary culture. In vitro changes in the morphology of co-existing astrocytes and cell culture age-dependent degeneration of neurodendritic network resemble features of in vivo brain aging at the cellular level.

CONCLUSION

In conclusion, this study suggests that long-term primary CNS culture is a viable model for the study of basic mechanisms and effective methods to decelerate the process of neuronal senescence.

The KATP channel activator diazoxide ameliorates amyloid-β and tau pathologies and improves memory in the 3xTgAD mouse model of Alzheimer's disease

Compromised cellular energy metabolism, cerebral hypoperfusion, and neuronal calcium dysregulation are involved in the pathological process of Alzheimer's disease (AD). ATP-sensitive potassium (KATP) channels in plasma membrane and inner mitochondrial membrane play important roles in modulating neuronal excitability, cell survival, and cerebral vascular tone. To investigate the therapeutic potential of drugs that activate KATP channels in AD, we first characterized the effects of the KATP channel opener diazoxide on cultured neurons, and then determined its ability to modify the disease process in the 3xTgAD mouse model of AD. Plasma and mitochondrial membrane potentials, cell excitability, intracellular Ca2+ levels and bioenergetics were measured in cultured cerebral cortical neurons exposed to diazoxide. Diazoxide hyperpolarized neurons, reduced the frequency of action potentials, attenuated Ca2+ influx through NMDA receptor channels, and reduced oxidative stress. 3xTgAD mice treated with diazoxide for 8 months exhibited improved performance in a learning and memory test, reduced levels of anxiety, decreased accumulation of Aβ oligomers and hyperphosphorylated tau in the cortex and hippocampus, and increased cerebral blood flow. Our findings show that diazoxide can ameliorate molecular, cytopathological, and behavioral alterations in a mouse model of AD suggesting a therapeutic potential for drugs that activate KATP channels in the treatment of AD.

Post-synaptic scaffolding protein interactions with glutamate receptors in synaptic dysfunction and Alzheimer's disease

Alzheimer's disease (AD) is characterized clinically by an insidious decline in cognition. Much attention has been focused on proposed pathogenic mechanisms that relate Aβ plaque and neurofibrillary tangle pathology to cognitive symptoms, but compelling evidence now identifies early synaptic loss and dysfunction, which precede plaque and tangle formation, as the more probable initiators of cognitive impairment. Glutamate-mediated transmission is severely altered in AD. Glutamate receptor expression is most markedly altered in regions of the AD brain that show the greatest pathological changes. Signaling via glutamate receptors controls synaptic strength and plasticity, and changes in these parameters are likely to contribute to memory and cognitive deficits in AD. Glutamate receptor expression and activity are modulated by interactions with post-synaptic scaffolding proteins that augment the strength and direction of signal cascades initiated by glutamate receptor activity. Scaffold proteins offer promising targets for more focused and effective drug therapy. In consequence, interest is developing into the roles these proteins play in neurological disease. In this review we discuss disruptions to excitatory neurotransmission at the level of glutamate receptor-post-synaptic scaffolding protein interactions that may contribute to synaptic dysfunction in AD.

A perfect storm: Converging paths of epilepsy and Alzheimer's dementia intersect in the hippocampal formation

Seizures in the human temporal lobe transiently impair cognition and steadily damage hippocampal circuitry, leading to progressive memory loss. Similarly, the toxic accumulation of Aβ peptides underlying Alzheimer's disease (AD) triggers synaptic degeneration, circuit remodeling, and abnormal synchronization within the same networks. Because neuronal hyperexcitability amplifies the synaptic release of Aβ, seizures create a vicious spiral that accelerates cell death and cognitive decline in the AD brain. The confluence of hyperexcitability and excitotoxicity, combined with the challenge of seizure detection in the human hippocampus, make epilepsy in these individuals extremely important to correctly diagnose and treat. Emerging clinical evidence reveals an elevated comorbidity of epilepsy in AD, particularly when linked to mutations in the APP/Aβ gene pathway. Experimental models in genetically engineered mice confirm and extend these findings, highlighting the presence of subclinical seizures and overlapping pathophysiologic cascades. There is an urgent need for more clinical and basic investigation to improve the early recognition of hippocampal seizures arising during the course of dementing disorders, and to validate molecular blockers of Aβ-induced aberrant excitability that can slow and potentially reverse the progression of cognitive decline.

Activation of extrasynaptic, but not synaptic, NMDA receptors modifies amyloid precursor protein expression pattern and increases amyloid-ß production

Calcium is a key mediator controlling essential neuronal functions depending on electrical activity. Altered neuronal calcium homeostasis affects metabolism of amyloid precursor protein (APP), leading to increased production of β-amyloid (Aβ), and contributing to the initiation of Alzheimer's disease (AD). A linkage between excessive glutamate receptor activation and neuronal Aβ release was established, and recent reports suggest that synaptic and extrasynaptic NMDA receptor (NMDAR) activation may have distinct consequences in plasticity, gene regulation, and neuronal death. Here, we report for the first time that prolonged activation of extrasynaptic NMDAR, but not synaptic NMDAR, dramatically increased the neuronal production of Aβ. This effect was preceded by a shift from APP695 to Kunitz protease inhibitory domain (KPI) containing APPs (KPI-APPs), isoforms exhibiting an important amyloidogenic potential. Conversely, after synaptic NMDAR activation, we failed to detect any KPI-APP expression and neuronal Aβ production was not modified. Calcium imaging data showed that intracellular calcium concentration after extrasynaptic NMDAR stimulation was lower than after synaptic activation. This suggests distinct signaling pathways for each pool of receptors. We found that modification of neuronal APP expression pattern triggered by extrasynaptic NMDAR activation was regulated at an alternative splicing level involving calcium-/calmodulin-dependent protein kinase IV, but overall APP expression remained identical. Finally, memantine dose-dependently inhibited extrasynaptic NMDAR-induced KPI-APPs expression as well as neuronal Aβ release. Altogether, these data suggest that a chronic activation of extrasynaptic NMDAR promotes amyloidogenic KPI-APP expression leading to neuronal Aβ release, representing a causal risk factor for developing AD.

Review: disruption of the postsynaptic density in Alzheimer's disease and other neurodegenerative dementias

The most common causes of neurodegenerative dementia include Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD). We believe that, in all 3, aggregates of pathogenic proteins are pathological substrates which are associated with a loss of synaptic function/plasticity. The synaptic plasticity relies on the normal integration of glutamate receptors at the postsynaptic density (PSD). The PSD organizes synaptic proteins to mediate the functional and structural plasticity of the excitatory synapse and to maintain synaptic homeostasis. Here, we will discuss the relevant disruption of the protein network at the PSD in these dementias and the accumulation of the pathological changes at the PSD years before clinical symptoms. We suggest that the functional and structural plasticity changes of the PSD may contribute to the loss of molecular homeostasis within the synapse (and contribute to early symptoms) in these dementias.

An overview of APP processing enzymes and products

The generation of amyloid beta-peptide (A beta) by enzymatic cleavages of the beta-amyloid precursor protein (APP) has been at the center of Alzheimer's disease (AD) research. While the basic process of beta- and gamma-secretase-mediated generation of A beta is text book knowledge, new aspects of A beta and other cleavage products have emerged in recent years. Also our understanding of the enzymes involved in APP proteolysis has increased dramatically. All of these discoveries contribute to a more complete understanding of APP processing and the physiologic and pathologic roles of its secreted and intracellular protein products. Understanding APP processing is important for any therapeutic strategy aimed at reducing A beta levels in AD. In this review, we provide a concise description of the current state of understanding the enzymes involved in APP processing, the cleavage products generated by different processing patterns, and the potential functions of those cleavage products.

HIV-1 protein-mediated amyloidogenesis in rat hippocampal cell cultures

Since the beginning of the highly active antiretroviral therapy (HAART) era, epidemiological evidence indicates an increasing incidence of Alzheimer's (AD)-like brain pathology in aging HIV patients. Emerging evidence warns of potential convergent mechanisms underlying HIV- and Abeta-mediated neurodegeneration. We found that HIV-1 Tat B and gp120 promote the secretion of Abeta 1-42 in primary rat fetal hippocampal cell cultures. Our results demonstrate that the variant of Tat expressed by the neurotropic subtype of HIV-1 virus (HIV-1 clade B) specifically induces both the release of amyloidogenic Abeta 1-42 and the accumulation of cell-bound amyloid aggregates. The results of the research rationalize testing of the ability of beta-amyloid aggregation inhibitors to attenuate HIV protein-mediated cognitive deficits in animal models of NeuroAIDS. The long-term goal of the study is to evaluate the potential benefits of anti-amyloidogenic therapies for management of cognitive dysfunction in aging HIV-1 patients.

TRPM2, calcium and neurodegenerative diseases

NMDA receptor overactivation triggers intracellular Ca(2+) dysregulation, which has long been thought to be critical for initiating excitotoxic cell death cascades associated with stroke and neurodegenerative disease. The inability of NMDA receptor antagonists to afford neuroprotection in clinical stroke trials has led to a re-evaluation of excitotoxic models of cell death and has focused research efforts towards identifying additional Ca(2+) influx pathways. Recent studies indicate that TRPM2, a member of the TRPM subfamily of Ca(2+)-permeant, non-selective cation channel, plays an important role in mediating cellular responses to a wide range of stimuli that, under certain situations, can induce cell death. These include reactive oxygen and nitrogen species, tumour necrosis factor as well as soluble oli-gomers of amyloid beta. However, the molecular basis of TRPM2 channel involvement in these processes is not fully understood. In this review, we summarize recent studies about the regulation of TRPM2, its interaction with calcium and the possible implications for neurodegenerative diseases.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Interactions between chemokines: regulation of fractalkine/CX3CL1 homeostasis by SDF/CXCL12 in cortical neurons

The soluble form of the chemokine fractalkine/CX(3)CL1 regulates microglia activation in the central nervous system (CNS), ultimately affecting neuronal survival. This study aims to determine whether CXCL12, another chemokine constitutively expressed in the CNS (known as stromal cell-derived factor 1; SDF-1), regulates cleavage of fractalkine from neurons. To this end, ELISA was used to measure protein levels of soluble fractalkine in the medium of rat neuronal cultures exposed to SDF-1. Gene arrays, quantitative RT-PCR, and Western blot were used to measure overall fractalkine expression in neurons. The data show that the rate of fractalkine shedding in healthy cultures positively correlates with in vitro differentiation and survival. In analogy to non-neuronal cells, metalloproteinases (ADAM10/17) are involved in cleavage of neuronal fractalkine as indicated by studies with pharmacologic inhibitors. Moreover, treatment of the neuronal cultures with SDF-1 stimulates expression of the inducible metalloproteinase ADAM17 and increases soluble fractalkine content in culture medium. The effect of SDF-1 is blocked by an inhibitor of both ADAM10 and -17, but only partially affected by a more specific inhibitor of ADAM10. In addition, SDF-1 also up-regulates expression of the fractalkine gene. Conversely, exposure of neurons to an excitotoxic stimulus (i.e. NMDA) inhibits alpha-secretase activity and markedly diminishes soluble fractalkine levels, leading to cell death. These results, along with previous findings on the neuroprotective role of both SDF-1 and fractalkine, suggest that this novel interaction between the two chemokines may contribute to in vivo regulation of neuronal survival by modulating microglial neurotoxic properties.

Memantine lowers amyloid-beta peptide levels in neuronal cultures and in APP/PS1 transgenic mice

Memantine is a moderate-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist that stabilizes cognitive, functional, and behavioral decline in patients with moderate to severe Alzheimer's disease (AD). In AD, the extracellular deposition of fibrillogenic amyloid-beta peptides (Abeta) occurs as a result of aberrant processing of the full-length Abeta precursor protein (APP). Memantine protects neurons from the neurotoxic effects of Abeta and improves cognition in transgenic mice with high brain levels of Abeta. However, it is unknown how memantine protects cells against neurodegeneration and affects APP processing and Abeta production. We report the effects of memantine in three different systems. In human neuroblastoma cells, memantine, at therapeutically relevant concentrations (1-4 muM), decreased levels of secreted APP and Abeta(1-40). Levels of the potentially amylodogenic Abeta(1-42) were undetectable in these cells. In primary rat cortical neuronal cultures, memantine treatment lowered Abeta(1-42) secretion. At the concentrations used, memantine treatment was not toxic to neuroblastoma or primary cultures and increased cell viability and/or metabolic activity under certain conditions. In APP/presenilin-1 (PS1) transgenic mice exhibiting high brain levels of Abeta(1-42), oral dosing of memantine (20 mg/kg/day for 8 days) produced a plasma drug concentration of 0.96 microM and significantly reduced the cortical levels of soluble Abeta(1-42). The ratio of Abeta(1-40)/Abeta(1-42) increased in treated mice, suggesting effects on the gamma-secretase complex. Thus, memantine reduces the levels of Abeta peptides at therapeutic concentrations and may inhibit the accumulation of fibrillogenic Abeta in mammalian brains. Memantine's ability to preserve neuronal cells against neurodegeneration, to increase metabolic activity, and to lower Abeta level has therapeutic implications for neurodegenerative disorders.

Role of Glycogen Synthase Kinase-3β in APP Hyperphosphorylation Induced by NMDA Stimulation in Cortical Neurons

The phosphorylation of Amyloid Precursor Protein (APP) at Thr⁶⁶⁸ plays a key role in APP metabolism that is highly relevant to AD. The c-Jun-N-terminal kinase (JNK), glycogen synthase kinase-3β (GSK-3β) and cyclin-dependent kinase 5 (Cdk5) can all be responsible for this phosphorylation. These kinases are activated by excitotoxic stimuli fundamental hallmarks of AD. The exposure of cortical neurons to a high dose of NMDA (100 μM) for 30’-45’ led to an increase of P-APP Thr⁶⁶⁸. During NMDA stimulation APP hyperphosphorylation has to be assigned to GSK-3β activity, since addition of L803-mts, a substrate competitive inhibitor of GSK-3β reduced APP phosphorylation induced by NMDA. On the contrary, inhibition of JNK and Cdk5 with D-JNKI1 and Roscovitine respectively did not prevent NMDA-induced P-APP increase. These data show a tight connection, in excitotoxic conditions, between APP metabolism and the GSK-3β signaling pathway.

Amyloid precursor protein 695 associates with assembled NR2A- and NR2B-containing NMDA receptors to result in the enhancement of their cell surface delivery

This is a study of the interaction between the two NMDA neurotransmitter receptor subtypes, NR1/NR2A and NR1/NR2B, and amyloid precursor protein (APP) 695, the major APP variant expressed in neurones. APP695 co-immunoprecipitated with assembled NR1-1a/NR2A and NR1-1a/NR2B NMDA receptors following expression in mammalian cells. Single NR1-1a, NR1-2a, NR1-4b(c-Myc), or NR2 subunit transfections revealed that co-association of APP695 with assembled NMDA receptors was mediated via the NR1 subunit; it was independent of the NR1 C1, C2, and C2' cassettes and, the use of an NR1-2a(c-Myc)-trafficking mutant suggested that interaction between the two proteins occurs in the endoplasmic reticulum. The use of antibodies directed against extracellular and intracellular NR2 subunit epitopes for immunoprecipitations suggested that APP/NMDA receptor association was mediated via N-terminal domains. Anti-APP antibodies immunoprecipitated NR1, NR2A, and NR2B immunoreactive bands from detergent extracts of mammalian brain; reciprocally, anti-NR1 or anti-NR2A antibodies co-immunoprecipitated APP immunoreactivity. Immune pellets from brain were sensitive to endoglycosidase H suggesting that, as for heterologous expression, APP and NMDA receptor association occurs in the endoplasmic reticulum. Co-expression of APP695 in mammalian cells resulted in enhanced cell surface expression of both NR1-1a/NR2A and NR1-1a/NR2B NMDA receptors with no increase in total subunit expression. These findings are further evidence for a role of APP in intracellular trafficking mechanisms. Further, they provide a link between two major brain proteins that have both been implicated in Alzheimer's disease.

Subcellular and metabolic examination of amyloid-beta peptides in Alzheimer disease pathogenesis: evidence for Abeta(25-35)

Amyloid-beta peptide (Abeta) is a central player in the pathogenesis and diagnosis of Alzheimer disease. It aggregates to form the core of Alzheimer disease-associated plaques found in coordination with tau deposits in diseased individuals. Despite this clinical relevance, no single hypothesis satisfies and explicates the role of Abeta in toxicity and progression of the disease. To explore this area, investigators have focused on mechanisms of cellular dysfunction, aggregation, and maladaptive responses. Extensive research has been conducted using various methodologies to investigate Abeta peptides and oligomers, and these multiple facets have provided a wealth of data from specific models. Notably, the utility of each experiment must be considered in regards to the brain environment. The use of Abeta(25-35) in studies of cellular dysfunction has provided data indicating that the peptide is indeed responsible for multiple disturbances to cellular integrity. We will review how Abeta peptide induces oxidative stress and calcium homeostasis, and how multiple enzymes are deleteriously impacted by Abeta(25-35). Understanding and discussing the origin and properties of Abeta peptides is essential to evaluating their effects on various intracellular metabolic processes. Attention will also be specifically directed to metabolic compartmentation in affected brain cells, including mitochondrial, cytosolic, nuclear, and lysosomal enzymes.

In vivo [(123)I]CNS-1261 binding to D-serine-activated and MK801-blocked NMDA receptors: A storage phosphor imaging study in rats

Disturbances of activity of the glutamatergic neurotransmitter system in the brain are present in many neuropsychiatric disorders. The N-methyl-D-aspartate (NMDA) receptor is the most abundant receptor of the glutamatergic system. In the neurodegenerative events of Alzheimer's disease, excessive activation of NMDA receptors may contribute to neuronal death. Inhibition of NMDA receptor activation may have neuroprotective effects and (semi)quantitative imaging of the activated system may help in the selection of patients for such inhibition therapies. In this study we evaluated [(123)I]CNS-1261 binding in the rat brain. This radiotracer binds in vivo to the MK801 binding site of activated NMDA receptors. To determine the optimal time point for ex vivo assessments after bolus injection [(123)I]CNS-1261 binding in rats, we performed a time course biodistribution study using dissection techniques. [(123)I]CNS-1261 binding was also studied in the rat brain using autoradiography by means of storage phosphor imaging, with prior facilitation of NMDA receptor activation by injection of the potent coagonist D-serine and after blocking of the NMDA receptor binding site by MK801 injection in D-serine pretreated rats. Measurements of [(123)I]CNS-1261 uptake matched the distribution of similar tracers for the MK801 binding site of the NMDA receptor and revealed an optimal time point of 2 h post injection for the assessment of tracer distribution in the rat brain. The blocking experiments indicated specific binding of [(123)I]CNS-1261 to NMDA receptors but also a considerable amount of nonspecific binding. Facilitation of NMDA receptor activation by D-serine did not result in an enhancement of binding of the radiotracer in the NMDA receptor-rich rat hippocampus compared to the untreated group, as measured by autoradiography. In conclusion, our study has shown that [(123)I]CNS-1261 binding is influenced by NMDA receptor availability. However, high nonspecific binding limits quantification and small changes in receptor availability are unlikely to be detected.

The comorbidity of HIV-associated neurocognitive disorders and Alzheimer's disease: a foreseeable medical challenge in post-HAART era

Although the introduction of highly active antiretroviral therapy (HAART) has led to a strong reduction of HIV-associated dementia (HAD) incidence, the prevalence of minor HIV-1-associated neurocognitive disorder (HAND) is rising among AIDS patients. HAART medication has shifted neuropathology from a subacute encephalitic condition to a subtle neurodegenerative process involving synaptic and dendritic degeneration, particularly of hippocampal neurons that are spared prior to HAART medication. Considerable neuroinflammation coupled with mononuclear phagocyte activation is present in HAART-medicated brains, particularly in the hippocampus. Accumulating evidence suggests that the resultant elevated secretion of pro-inflammatory cytokines such as interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta can increase amyloid-beta peptide (Abeta) generation and reduce Abeta clearance. Recent advancements in Alzheimer's disease (AD) research identified Abeta biogenesis and clearance venues that are potentially influenced by HIV viral infection, providing new insights into beta-amyloidosis segregation in HIV patients. Our study suggests enhanced beta-amyloidosis in ART-treated HAD and HIV-associated encephalitis brains and suppression of Abeta clearance by viral infection of human primary macrophages. A growing awareness of potential convergent mechanisms leading to neurodegeneration shared by HIV and Abeta points to a significant chance of comorbidity of AD and HAND in senile HIV patients, which calls for a need of basic studies.

Synaptic NMDA receptor activation stimulates alpha-secretase amyloid precursor protein processing and inhibits amyloid-beta production

Altered amyloid precursor protein (APP) processing leading to increased production and oligomerization of Abeta may contribute to Alzheimer's disease (AD). Understanding how APP processing is regulated under physiological conditions may provide new insights into AD pathogenesis. Recent reports demonstrate that excitatory neural activity regulates APP metabolism and Abeta levels, although understanding of the molecular mechanisms involved is incomplete. We have investigated whether NMDA receptor activity regulates APP metabolism in primary cultured cortical neurons. We report that a pool of APP is localized to the postsynaptic compartment in cortical neurons and observed partial overlap of APP with both NR1 and PSD-95. NMDA receptor stimulation increased nonamyloidogenic alpha-secretase-mediated APP processing, as measured by a 2.5-fold increase in cellular alpha-C-terminal fragment (C83) levels after glutamate or NMDA treatment. This increase was blocked by the NMDA receptor antagonists d-AP5 and MK801 but not by the AMPA receptor antagonist CNQX or the L-type calcium channel blocker nifedipine, was prevented by chelation of extracellular calcium, and was blocked by the alpha-secretase inhibitor TAPI-1. Cotreatment of cortical neurons with bicuculline and 4-AP, which stimulates glutamate release and activates synaptic NMDA receptors, evoked an MK801-sensitive increase in C83 levels. Furthermore, NMDA receptor stimulation caused a twofold increase in the amount of soluble APP detected in the neuronal culture medium. Finally, NMDA receptor activity inhibited both Abeta1-40 release and Gal4-dependent luciferase activity induced by beta-gamma-secretase-mediated cleavage of an APP-Gal4 fusion protein. Altogether, these data suggest that calcium influx through synaptic NMDA receptors promotes nonamyloidogenic alpha-secretase-mediated APP processing.

The effects of amyloid precursor protein on postsynaptic composition and activity

The amyloid precursor protein (APP) is cleaved to produce the Alzheimer disease-associated peptide Abeta, but the normal functions of uncleaved APP in the brain are unknown. We found that APP was present in the postsynaptic density of central excitatory synapses and coimmunoprecipitated with N-methyl-d-aspartate receptors (NMDARs). The presence of APP in the postsynaptic density was supported by the observation that NMDARs regulated trafficking and processing of APP; overexpression of the NR1 subunit increased surface levels of APP, whereas activation of NMDARs decreased surface APP and promoted production of Abeta. We transfected APP or APP RNA interference into primary neurons and used electrophysiological techniques to explore the effects of APP on postsynaptic function. Reduction of APP decreased (and overexpression of APP increased) NMDAR whole cell current density and peak amplitude of spontaneous miniature excitatory postsynaptic currents. The increase in NMDAR current by APP was due to specific recruitment of additional NR2B-containing receptors. Consistent with these findings, immunohistochemical experiments demonstrated that APP increased the surface levels and decreased internalization of NR2B subunits. These results demonstrate a novel physiological role of postsynaptic APP in enhancing NMDAR function.

Memantine leads to behavioral improvement and amyloid reduction in Alzheimer's-disease-model transgenic mice shown as by micromagnetic resonance imaging

Memantine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has been shown to improve learning and memory in several preclinical models of Alzheimer's disease (AD). Memantine has also been shown to reduce the levels of amyloid beta (A beta) peptides in human neuroblastoma cells as well as to inhibit A beta oligomer-induced synaptic loss. In this study, we assessed whether NMDA receptor inhibition by memantine in transgenic mice expressing human amyloid-beta precursor protein (APP) and presenilin 1 (PS1) is associated with cognitive benefit and amyloid burden reduction by using object recognition, micromagnetic resonance imaging (micro MRI), and histology. APP/PS1 Tg mice were treated either with memantine or with vehicle for a period of 4 months starting at 3 months of age. After treatment, the mice were subjected to an object recognition test and analyzed by ex vivo micro MRI, and histological examination of amyloid burden. micro MRI was performed following injection with gadolinium-DTPA-A beta(1-40). We found that memantine-treated Tg mice performed the same as wild-type control mice, whereas the performance of vehicle-treated Tg mice was significantly impaired (P = 0.0081, one-way ANOVA). Compared with vehicle-treated animals, memantine-treated Tg mice had a reduced plaque burden, as determined both histologically and by micro MRI. This reduction in amyloid burden correlates with an improvement in cognitive performance. Thus, our findings provide further evidence of the potential role of NMDA receptor antagonists in ameliorating AD-related pathology. In addition, our study shows, for the first time, the utility of micro MRI in conjunction with gadolinium-labeled A beta labeling agents to monitor the therapeutic response to amyloid-reducing agents.

Neuroprotection by hypothalamic peptide proline-rich peptide-1 in Abeta25-35 model of Alzheimer's disease

BACKGROUND

This work sought to determine the effects of hypothalamic proline-rich peptide (PRP)-1 in a rat model of Alzheimer's disease.

METHODS

Complex histochemical, electrophysiologic, and behavioral analyses were performed on intact or diseased Wistar rats (n = 28). Pathologic conditions were induced by bilateral intracerebroventricular injection of amyloid peptide Abeta25-35. The diseased rats received systemic administration of PRP-1 or placebo control.

RESULTS

Abeta25-35 caused cellular neurodegeneration with marked glial reaction in the hippocampal complex and almost full destruction of the dentate fascia, which was not observed in conditions of PRP-1 administration after Abeta25-35 injection. Hippocampal neurons of intact animals responded to high-frequency (tetanic) stimulation of entorhinal cortex of ipsilateral cerebral hemisphere by tetanic and posttetanic potentiation of a different intensity and duration, which was accompanied by posttetanic depression. Abeta25-35 led to significant changes in the level and pattern of hippocampal neuronal activity, indicating the absence of both tetanic and posttetanic activity. Poststimulus activity manifestations rarely occurred and rapidly decreased after repeated trials. This indicated the focal character of lesion. Regular administration of PRP-1 for 4 weeks resulted in optimal restoration of electrophysiologic parameters. PRP-1 maintained the initial learning level achieved in a behavioral study in a Morris water maze.

CONCLUSIONS

Systemic administration of PRP-1 possesses neuroprotective effects and can prevent the neurodegeneration in hippocampus induced by Abeta25-35. This suggests that PRP-1 could be a potential therapeutic agent for specific neurodegenerative diseases.

Cathepsins B and L differentially regulate amyloid precursor protein processing

Previous studies have shown that cathepsins control amyloid beta (Abeta) levels in chromaffin cells via a regulated secretory pathway. In the present study, this concept was extended to investigations in primary hippocampal neurons to test whether Abeta release was coregulated by cathepsins and electrical activity, proposed components of a regulated secretory pathway. Inhibition of cathepsin B (catB) activity with CA074Me or attenuation of catB expression through small interfering RNA produced decreases in Abeta release, similar to levels produced with suppression of beta-site APP-cleaving enzyme 1 (BACE1) expression. To test whether the catB-dependent release of Abeta was linked to ongoing electrical activity, neurons were treated with tetrodotoxin (TTX) and CA074Me. These comparisons demonstrated no additivity between decreases in Abeta release produced by TTX and CA074Me. In contrast, pharmacological inhibition of cathepsin L (catL) selectively elevated Abeta42 levels but not Abeta40 or total Abeta. Mechanistic studies measuring C-terminal fragments of amyloid precursor protein (APP) suggested that catL elevated alpha-secretase activity, thereby suppressing Abeta42 levels. The mechanism of catB-mediated regulation of Abeta release remains unclear but may involve elevation of beta-secretase. In summary, these studies provide evidence for a significant alternative pathway for APP processing that involves catB and activity-dependent release of Abeta in a regulated secretory pathway for primary neurons.

N-cadherin-based adhesion enhances Abeta release and decreases Abeta42/40 ratio

In neurons, Presenilin 1(PS1)/gamma-secretase is located at the synapses, bound to N-cadherin. We have previously reported that N-cadherin-mediated cell-cell contact promotes cell-surface expression of PS1/gamma-secretase. We postulated that N-cadherin-mediated trafficking of PS1 might impact synaptic PS1-amyloid precursor protein interactions and Abeta generation. In the present report, we evaluate the effect of N-cadherin-based contacts on Abeta production. We demonstrate that stable expression of N-cadherin in Chinese hamster ovary cells, expressing the Swedish mutant of human amyloid precursor protein leads to enhanced secretion of Abeta in the medium. Moreover, N-cadherin expression decreased Abeta(42/40) ratio. The effect of N-cadherin expression on Abeta production was accompanied by the enhanced accessibility of PS1/gamma-secretase to amyloid precursor protein as well as a conformational change of PS1, as demonstrated by the fluorescence lifetime imaging technique. These results indicate that N-cadherin-mediated synaptic adhesion may modulate Abeta secretion as well as the Abeta(42/40) ratio via PS1/N-cadherin interactions.

Peripheral biomarkers in Autism: secreted amyloid precursor protein-alpha as a probable key player in early diagnosis

Autism is a pervasive developmental disorder characterized by impairments in socialization and communication. There is currently no single molecular marker or laboratory tool capable of diagnosing autism at an early age. The purpose of this study is to explore the plausible use of peripheral biomarkers in the early diagnosis of autism via a sensitive ELISA. Here, we measured plasma secreted amyloid precursor protein alpha (sAPP-alpha) levels in autistic and aged-matched control blood samples and found a significantly increased level of sAPP-alpha in 60% of the known autistic children. We then tested 150 human umbilical cord blood (HUCB) samples and found significantly elevated levels of plasma sAPP-alpha in 10 of 150 samples. As an additional confirmatory measure, we performed Western blot analysis on these samples which consistently showed increased sAPP-alpha levels in autistic children and 10 of 150 HUCB samples; suggesting a group of autistic patients which could be identified in early childhood by levels of sAPP-alpha. While there is need for further studies of this concept, the measurement of sAPP-alpha levels in serum and human umbilical cord blood by ELISA is a potential tool for early diagnosis of autism.

BACE1 gene deletion: Impact on behavioral function in a model of Alzheimer's disease

Accumulation of cerebral amyloid-beta (Abeta) has been implicated as a putative causal factor in the development of Alzheimer's disease (AD). Transgenic mice like the PDAPP line overexpress human mutant Amyloid Precursor Protein (hAPP) and recapitulate many features of AD, including amyloid neuropathology and cognitive deficits. Inhibition of the beta-site aspartyl cleaving enzyme (BACE1) enzyme responsible for the first proteolytic cleavage that ultimately generates Abeta has been proposed as a strategy for AD therapy. To assess the theoretical repercussions of beta-secretase activity reduction in an in vivo model of AD, BACE1(-/-) mice bred to the PDAPP line were examined in a series of behavioral tasks. Although BACE1 gene ablation abolished hAbeta accumulation, BACE1(-/-) mice had unexpected sensorimotor impairments, spatial memory deficits, and displayed seizures, phenotypes which were severe on the PDAPP background. These results suggest that while excess Abeta is functionally pathological, BACE1-mediated processing of APP and other substrates play a role in "normal" learning, memory and sensorimotor processes.

Endocytosis is required for synaptic activity-dependent release of amyloid-beta in vivo

Aggregation of amyloid-beta (Abeta) peptide into soluble and insoluble forms within the brain extracellular space is central to the pathogenesis of Alzheimer's disease. Full-length amyloid precursor protein (APP) is endocytosed from the cell surface into endosomes where it is cleaved to produce Abeta. Abeta is subsequently released into the brain interstitial fluid (ISF). We hypothesized that synaptic transmission results in more APP endocytosis, thereby increasing Abeta generation and release into the ISF. We found that inhibition of clathrin-mediated endocytosis immediately lowers ISF Abeta levels in vivo. Two distinct methods that increased synaptic transmission resulted in an elevation of ISF Abeta levels. Inhibition of endocytosis, however, prevented the activity-dependent increase in Abeta. We estimate that approximately 70% of ISF Abeta arises from endocytosis-associated mechanisms, with the vast majority of this pool also dependent on synaptic activity. These findings have implications for AD pathogenesis and may provide insights into therapeutic intervention.

Endogenous secreted amyloid precursor protein-alpha regulates hippocampal NMDA receptor function, long-term potentiation and spatial memory

Secreted amyloid precursor protein-alpha (sAPP alpha) levels are reduced during the pathogenesis of Alzheimer's disease, but the significance of this for neural function is not well understood. Here, we show that intrahippocampal infusion of antibodies targeted to endogenous sAPP alpha reduced long-term potentiation (LTP) in the dentate gyrus of adult rats by approximately 50%. Conversely, infusion of recombinant sAPP alpha dose-dependently increased LTP and facilitated in vitro tetanically evoked NMDA receptor-mediated currents. Pharmacological inhibition of alpha-secretase and other a-disintegrin-and-metalloproteases by TAPI-1 reduced both LTP and tetanus-evoked NMDA receptor-mediated currents in dentate granule cells. Both effects were prevented by co-application of exogenous recombinant sAPP alpha. Similarly, spatial memory was inhibited by intrahippocampal TAPI-1, an effect that was prevented by co-application of recombinant sAPP alpha. Together these findings indicate that endogenous sAPP alpha is a key contributor to synaptic plasticity and spatial memory. Its reduced production in Alzheimer's disease may thus contribute to the clinical memory deficits.

The involvement of astrocytes and kynurenine pathway in Alzheimer's disease

The kynurenine pathway (KP) and several of its neuroactive products, especially quinolinic acid (QUIN), are considered to be involved in the neuropathogenesis of Alzheimer's disease (AD). There is growing evidence suggesting that astrocytes play a critical role in the regulation of the excitotoxicity and inflammatory processes that occur during the evolution of AD. This review focuses on the role of astrocytes through their relation with the KP to the different features associated with AD including cytokine, chemokine and adhesion molecule production, cytoskeletal changes, astrogliosis, excitotoxicity, apoptosis and neurodegeneration.

Beta-site amyloid precursor protein-cleaving enzyme-1 (BACE1)-mediated changes of endogenous amyloid beta in wild-type and transgenic mice in vivo

Beta-site amyloid precursor protein-cleaving enzyme-1 (BACE1) initiates generation of amyloid beta (Abeta), a pathological hallmark of Alzheimer's disease. We investigated the impact of BACE1 protein level on endogenous Abeta. Endogenous Abeta and BACE1 protein levels were concurrently and significantly reduced during early life. However, Abeta levels were similar between BACE1 transgenic and wildtype mice. This suggests that BACE1 protein level has a minimal effect on the level of endogenous Abeta. Consequently, other factors must be involved in modulation of Abeta production in adult and ageing brain and investigation of such factors may yield therapeutic targets. Further, these results suggest that substantial inhibition of BACE1 in brain may be required for clinical benefit in Alzheimer's disease.

Amyloid beta peptides and glutamatergic synaptic dysregulation

Alzheimer's disease (AD) is a major neurodegenerative disorder in which overproduction and accumulation of amyloid beta (Abeta) peptides result in synaptic dysfunction. Recent reports strongly suggest that in the initial stages of AD glutamate receptors are dysregulated by Abeta accumulation resulting in disruption of glutamatergic synaptic transmission which parallels early cognitive deficits. In the presence of Abeta, 2-amino-3-(3-hydoxy-5-methylisoxazol-4-yl) propionic acid (AMPA) glutamate receptor function is disrupted and the surface expression is reduced. Abeta has also been shown to modulate N-methyl-d-aspartate receptors (NMDARs) and metabotropic glutamate receptors. The Abeta mediated glutamate receptor modifications can lead to synaptic dysfunction resulting in excitotoxic neurodegeneration during the progression of AD. This review discusses the recent findings that glutamatergic signaling could be compromised by Abeta induced modulation of synaptic glutamate receptors in specific brain regions.

Memantine: a NMDA receptor antagonist that improves memory by restoration of homeostasis in the glutamatergic system--too little activation is bad, too much is even worse

The neurotransmitter glutamate activates several classes of metabotropic receptor and three major types of ionotropic receptor--alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA). The involvement of glutamate mediated neurotoxicity in the pathogenesis of Alzheimer's disease (AD) is finding increasing scientific acceptance. Central to this hypothesis is the assumption that glutamate receptors, in particular of the NMDA type, are overactivated in a tonic rather than a phasic manner. Such continuous, mild, chronic activation ultimately leads to neuronal damage/death. Additionally, impairment of synaptic plasticity (learning) may result not only from neuronal damage per se but may also be a direct consequence of this continuous, non-contingent NMDA receptor activation. Complete NMDA receptor blockade has also been shown to impair neuronal plasticity, thus, both hypo- and hyperactivity of the glutamatergic system leads to dysfunction. Memantine received marketing authorization from the EMEA (European Medicines Agency) for the treatment of moderate to severe AD in Europe and was subsequently also approved by the FDA (Food and Drug Administration) for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive NMDA receptor antagonist with strong voltage-dependency and fast kinetics. This review summarizes existing hypotheses on the mechanism of action (MOA) of memantine in an attempt to understand how the accepted interaction with NMDA receptors could allow memantine to provide both neuroprotection and reverse deficits in learning/memory by the same MOA.

Spatial relationship between synapse loss and beta-amyloid deposition in Tg2576 mice

Although there is evidence that beta-amyloid impairs synaptic function, the relationship between beta-amyloid and synapse loss is not well understood. In this study we assessed synapse density within the hippocampus and the entorhinal cortex of Tg2576 mice at 6-18 months of age using stereological methods at both the light and electron microscope levels. Under light microscopy we failed to find overall decreases in the density of synaptophysin-positive boutons in any brain areas selected, but bouton density was significantly decreased within 200 mum of compact beta-amyloid plaques in the outer molecular layer of the dentate gyrus and Layers II and III of the entorhinal cortex at 15-18 months of age in Tg 2576 mice. Under electron microscopy, we found overall decreases in synapse density in the outer molecular layer of the dentate gyrus at both 6-9 and 15-18 months of age, and in Layers II and III of the entorhinal cortex at 15-18 months of age in Tg 2576 mice. However, we did not find overall changes in synapse density in the stratum radiatum of the CA1 subfield. Furthermore, in the two former brain areas we found a correlation between lower synapse density and greater proximity to beta-amyloid plaques. These results provide the first quantitative morphological evidence at the ultrastructure level of a spatial relationship between beta-amyloid plaques and synapse loss within the hippocampus and the entorhinal cortex of Tg2576 mice.

The amyloid beta ion channel hypothesis of Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of chronic dementia in the US. Its incidence is increasing with an attendant increase in associated health care costs. Since its first description in a patient by Dr. Alois Alzheimer over a century ago, a large body of biomedical literature has established a detailed clinical and molecular profile of this disorder. Amyloid beta peptide (Abeta; a 39-42 amino acid molecule) is the major component of senile plaques, the lesions that are one of the pathologic hallmarks of AD (Wong et al 1985). Although many aspects of the biology of amyloid beta have been investigated, several fundamental questions about how this peptide causes AD neuropathology remain unanswered. The key question is: How is Abeta toxic to cerebral neurons? Because plaques are extra-neuronal deposits, it is difficult to imagine a structural basis for their toxicity. As an interesting contrast the other pathognomonic feature of AD, neurofibrillary tangles, are intra-axonal structural anomalies that are composed of the hyperphosphorylated microtubule associated (MAP) protein, tau. This review will assess the current thinking that relates to a recent hypothesis of Abeta toxicity. In 1992, Hardy and Higgins reported findings that suggested a new and intriguing possibility. These authors found that Abeta peptides disrupt Ca(2+) homeostasis in neurons and increase intracellular Ca(2+) [Ca(2+)](i). This was corroborated by Mattson and his colleagues who demonstrated that Abeta exposure to human cortical neurons raised [Ca2(+)](i) (Mattson, Cheng et al 1992); (Hardy and Higgins 1992). Finally, Nelson Arispe's group at the NIH specifically investigated the possibility that Abeta peptides might function like Ca(2+) ion channels (Arispe et al 1993). This and several subsequent studies have laid the foundation for a novel idea: "Abeta peptides are, in part, toxic to neurons because they form aberrant ion channels in neuronal membranes and thereby disrupt neuronal homeostasis". In this review we shall critically examine this theory in light of classic and contemporary literature.

APP processing and the APP-KPI domain involvement in the amyloid cascade

Alternative APP mRNA splicing can generate isoforms of APP containing a Kunitz protease inhibitor (KPI) domain. KPI is one of the main serine protease inhibitors. Protein and mRNA KPI(+)APP levels are elevated in Alzheimer's disease (AD) brain and are associated with increased amyloid beta deposition. In the last years increasing evidence on multiple points in the amyloid cascade where KPI(+)APP is involved has been accumulated, admitting an outstanding position in the pathogenesis of AD to the KPI domain. This review focuses on the APP processing, the molecular activity of KPI and its physiological and pathological roles and the KPI involvement in the amyloid cascade through the nerve growth factor, the lipoprotein receptor-related protein, the tumor necrosis factor-alpha converting enzyme and the Notch1 protein.

Physiological roles for amyloid beta peptides

Alzheimer's disease is recognized post mortem by the presence of extracellular senile plaques, made primarily of aggregation of amyloid beta peptide (Abeta). This peptide has consequently been regarded as the principal toxic factor in the neurodegeneration of Alzheimer's disease. As such, intense research effort has been directed at determining its source, activity and fate, primarily with a view to preventing its formation or its biological activity, or promoting its degradation. Clearly, much progress has been made concerning its formation by proteolytic processing of the amyloid precursor protein, and its degradation by enzymes such as neprilysin and insulin degrading enzyme. The activities of Abeta, however, are numerous and yet to be fully elucidated. What is currently emerging from such studies is a diffuse but steadily growing body of data that suggests Abeta has important physiological functions and, further, that it should only be regarded as toxic when its production and degradation are imbalanced. Here, we review these data and suggest that physiological levels of Abeta have important physiological roles, and may even be crucial for neuronal cell survival. Thus, the view of Abeta being a purely toxic peptide requires re-evaluation.