beta-Amyloid peptide-derived, oxygen-dependent free radicals inhibit glutamate uptake in cultured astrocytes: implications for Alzheimer's disease

Alzheimer’s disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression

The synapse has consistently been considered a vulnerable and critical target within Alzheimer’s disease, and synapse loss is, to date, one of the main biological correlates of cognitive decline within Alzheimer’s disease. This occurs prior to neuronal loss with ample evidence that synaptic dysfunction precedes this, in support of the idea that synaptic failure is a crucial stage within disease pathogenesis. The two main pathological hallmarks of Alzheimer’s disease, abnormal aggregates of amyloid or tau proteins, have had demonstrable effects on synaptic physiology in animal and cellular models of Alzheimer’s disease. There is also growing evidence that these two proteins may have a synergistic effect on neurophysiological dysfunction. Here, we review some of the main findings of synaptic alterations in Alzheimer’s disease, and what we know from Alzheimer’s disease animal and cellular models. First, we briefly summarize some of the human evidence to suggest that synapses are altered, including how this relates to network activity. Subsequently, animal and cellular models of Alzheimer’s disease are considered, highlighting mouse models of amyloid and tau pathology and the role these proteins may play in synaptic dysfunction, either in isolation or examining how the two pathologies may interact in dysfunction. This specifically focuses on neurophysiological function and dysfunction observed within these animal models, typically measured using electrophysiology or calcium imaging. Following synaptic dysfunction and loss, it would be impossible to imagine that this would not alter oscillatory activity within the brain. Therefore, this review also discusses how this may underpin some of the aberrant oscillatory patterns seen in animal models of Alzheimer’s disease and human patients. Finally, an overview of some key directions and considerations in the field of synaptic dysfunction in Alzheimer’s disease is covered. This includes current therapeutics that are targeted specifically at synaptic dysfunction, but also methods that modulate activity to rescue aberrant oscillatory patterns. Other important future avenues of note in this field include the role of non-neuronal cell types such as astrocytes and microglia, and mechanisms of dysfunction independent of amyloid and tau in Alzheimer’s disease. The synapse will certainly continue to be an important target within Alzheimer’s disease for the foreseeable future.

Oxidative Stress, Amyloid-β Peptide, and Altered Key Molecular Pathways in the Pathogenesis and Progression of Alzheimer's Disease

Oxidative stress is implicated in the pathogenesis and progression of Alzheimer's disease (AD) and its earlier stage, amnestic mild cognitive impairment (aMCI). One source of oxidative stress in AD and aMCI brains is that associated with amyloid-β peptide, Aβ1-42 oligomers. Our laboratory first showed in AD elevated oxidative stress occurred in brain regions rich in Aβ1-42, but not in Aβ1-42-poor regions, and was among the first to demonstrate Aβ peptides led to lipid peroxidation (indexed by HNE) in AD and aMCI brains. Oxidatively modified proteins have decreased function and contribute to damaged key biochemical and metabolic pathways in which these proteins normally play a role. Identification of oxidatively modified brain proteins by the methods of redox proteomics was pioneered in the Butterfield laboratory. Four recurring altered pathways secondary to oxidative damage in brain from persons with AD, aMCI, or Down syndrome with AD are interrelated and contribute to neuronal death. This "Quadrilateral of Neuronal Death" includes altered: glucose metabolism, mTOR activation, proteostasis network, and protein phosphorylation. Some of these pathways are altered even in brains of persons with preclinical AD. We opine that targeting these pathways pharmacologically and with lifestyle changes potentially may provide strategies to slow or perhaps one day, prevent, progression or development of this devastating dementing disorder. This invited review outlines both in vitro and in vivo studies from the Butterfield laboratory related to Aβ1-42 and AD and discusses the importance and implications of some of the major achievements of the Butterfield laboratory in AD research.

Synaptic dysfunction in Alzheimer's disease: the effects of amyloid beta on synaptic vesicle dynamics as a novel target for therapeutic intervention

The most prevalent form of dementia in the elderly is Alzheimer's disease. A significant contributing factor to the progression of the disease appears to be the progressive accumulation of amyloid-β42 (Aβ42), a small hydrophobic peptide. Unfortunately, attempts to develop therapies targeting the accumulation of Aβ42 have not been successful to treat or even slow down the disease. It is possible that this failure is an indication that targeting downstream effects rather than the accumulation of the peptide itself might be a more effective approach. The accumulation of Aβ42 seems to affect various aspects of physiological cell functions. In this review, we provide an overview of the evidence that implicates Aβ42 in synaptic dysfunction, with a focus on how it contributes to defects in synaptic vesicle dynamics and neurotransmitter release. We discuss data that provide new insights on the Aβ42 induced pathology of Alzheimer's disease and a more detailed understanding of its contribution to the synaptic deficiencies that are associated with the early stages of the disease. Although the precise mechanisms that trigger synaptic dysfunction are still under investigation, the available data so far has enabled us to put forward a model that could be used as a guide to generate new therapeutic targets for pharmaceutical intervention.

Computational investigation of Amyloid-β-induced location- and subunit-specific disturbances of NMDAR at hippocampal dendritic spine in Alzheimer's disease

In Alzheimer’s disease (AD), dysregulation of intracellular Ca²⁺ signalling has been observed as an early event prior to the presence of clinical symptoms and is believed to be a crucial factor contributing to AD pathogenesis. Amyloid-β oligomers (AβOs) disturb the N-methyl-D-aspartate receptor (NMDAR)-mediated postsynaptic Ca²⁺ signalling in response to presynaptic stimulation by increasing the availability of extracellular glutamate as well as directly disturbing the NMDARs. The abnormal Ca²⁺ response can further lead to impairments in long-term potentiation (LTP), an important process in memory formation. In this study, we develop a mathematical model of a CA1 pyramidal dendritic spine and conduct computational experiments. We use this model to mimic alterations by AβOs under AD conditions to investigate how they are involved in the Ca²⁺ dysregulation in the dendritic spine. The alterations in glutamate availability, as well as NMDAR availability and activity, are studied both individually and globally. The simulation results suggest that alterations in glutamate availability mostly affect the synaptic response and have limited effects on the extrasynaptic receptors. Moreover, overactivation of extrasynaptic NMDARs in AD is unlikely to be induced by presynaptic stimulation, but by upregulation of the resting level of glutamate, possibly resulting from these alterations. Furthermore, internalisation of synaptic NR2A-NMDAR shows greater damage to the postsynaptic Ca²⁺ response in comparison with the internalisation of NR2B-NMDARs; thus, the suggested neuroprotective role of the latter is very limited during synaptic transmission in AD. We integrate a CaMKII state transition model with the Ca²⁺ model to further study the effects of alterations of NMDARs in the CaMKII state transition, an important downstream event in the early phase of LTP. The model reveals that cooperation between NR2A- and NR2B-NMDAR is required for LTP induction. Under AD conditions, internalisation of membrane NMDARs is suggested to be the cause of the loss of synapse numbers by disrupting CaMKII-NMDAR formation.

Synapsin I phosphorylation is dysregulated by beta-amyloid oligomers and restored by valproic acid

Alzheimer's disease is the most prevalent form of dementia in the elderly but the precise causal mechanisms are still not fully understood. Growing evidence supports a significant role for Aβ42 oligomers in the development and progression of Alzheimer's. For example, intracellular soluble Aβ oligomers are thought to contribute to the early synaptic dysfunction associated with Alzheimer's disease, but the molecular mechanisms underlying this effect are still unclear. Here, we identify a novel mechanism that contributes to our understanding of the reported synaptic dysfunction. Using primary rat hippocampal neurons exposed for a short period of time to Aβ42 oligomers, we show a disruption in the activity-dependent phosphorylation cycle of SynapsinI at Ser9. SynapsinI is a pre-synaptic protein that responds to neuronal activity and regulates the availability of synaptic vesicles to participate in neurotransmitter release. Phosphorylation of SynapsinI at Ser9, modulates its distribution and interaction with synaptic vesicles. Our results show that in neurons exposed to Aβ42 oligomers, the levels of phosphorylated Ser9 of SynapsinI remain elevated during the recovery period following neuronal activity. We then investigated if this effect could be targeted by a putative therapeutic regime using valproic acid (a short branch-chained fatty acid) that has been proposed as a treatment for Alzheimer's disease. Exposure of Aβ42 treated neurons to valproic acid, showed that it restores the physiological regulation of SynapsinI after depolarisation. Our data provide a new insight on Aβ42-mediated pathology in Alzheimer's disease and supports the use of Valproic acid as a possible pharmaceutical intervention for the treatment of Alzheimer's disease.

Possible role of P-glycoprotein in the neuroprotective mechanism of berberine in intracerebroventricular streptozotocin-induced cognitive dysfunction

RATIONALE

The therapeutic potential of berberine has been well documented in various neurological problems. However, the neurological mechanism of berberine remains untapped in the light of its P-glycoprotein (P-gp)-mediated gut efflux properties responsible for reduced bioavailability. Verapamil, a well known L-type calcium channel blocker, has additional inhibitory activity against P-gp efflux pump. Thus, there is a strong scientific rationale to explore the interaction of berberine with verapamil as a possible neuroprotective strategy.

OBJECTIVE

The present study was designed to evaluate the effect of berberine, verapamil, and their combination on behavioral alterations, oxidative stress, mitochondrial dysfunction, neuroinflammation, and histopathological modifications in intracerebroventricular streptozocin (ICV-STZ)-induced sporadic dementia of Alzheimer's type in rats.

METHODS

Single bilateral ICV-STZ (3 mg/kg) administration was used as an experimental model of sporadic dementia of Alzheimer's type.

RESULTS

Berberine (25, 50, and 100 mg/kg, oral gavage) or verapamil (2.5 and 5 mg/kg, intraperitoneally) were used as treatment drugs, and memantine (5 mg/kg, intraperitoneally) was used as a standard. Berberine and verapamil significantly attenuated behavioral, biochemical, cellular, and histological alterations, suggesting their neuroprotective potential. Further, treatment of berberine (25 and 50 mg/kg) with verapamil (2.5 and 5.0 mg/kg) combinations respectively significantly potentiated their neuroprotective effect which was significant as compared to their effect per se in ICV-STZ-treated animals.

CONCLUSION

The augmentative outcome of verapamil on the neuroprotective effect of berberine can be speculated due to the inhibition of P-gp efflux mechanism and the prevention of calcium homeostasis alteration. Additionally, anti-inflammatory and antioxidant effects of both berberine and verapamil could also contribute in their protective effect.

The Essential Role of Soluble Aβ Oligomers in Alzheimer's Disease

Alzheimer's disease (AD) is a common neurodegenerative disease characterized by amyloid plaque and neurofibrillary tangles (NFT). With the finding that soluble nonfibrillar Aβ levels actually correlate strongly with the severity of the disease, the initial focus on amyloid plaques shifted to the contemporary concept that AD memory failure is caused by soluble Aβ oligomers. The soluble Aβ are known to be more neurotoxicthan fibrillar Aβ species. In this paper, we summarize the essential role of soluble Aβ oligomers in AD and discuss therapeutic strategies that target soluble Aβ oligomers.

The 2013 SFRBM discovery award: selected discoveries from the butterfield laboratory of oxidative stress and its sequela in brain in cognitive disorders exemplified by Alzheimer disease and chemotherapy induced cognitive impairment

This retrospective review on discoveries of the roles of oxidative stress in brain of subjects with Alzheimer disease (AD) and animal models thereof as well as brain from animal models of chemotherapy-induced cognitive impairment (CICI) results from the author receiving the 2013 Discovery Award from the Society for Free Radical Biology and Medicine. The paper reviews our laboratory's discovery of protein oxidation and lipid peroxidation in AD brain regions rich in amyloid β-peptide (Aβ) but not in Aβ-poor cerebellum; redox proteomics as a means to identify oxidatively modified brain proteins in AD and its earlier forms that are consistent with the pathology, biochemistry, and clinical presentation of these disorders; how Aβ in in vivo, ex vivo, and in vitro studies can lead to oxidative modification of key proteins that also are oxidatively modified in AD brain; the role of the single methionine residue of Aβ(1-42) in these processes; and some of the potential mechanisms in the pathogenesis and progression of AD. CICI affects a significant fraction of the 14 million American cancer survivors, and due to diminished cognitive function, reduced quality of life of the persons with CICI (called "chemobrain" by patients) often results. A proposed mechanism for CICI employed the prototypical ROS-generating and non-blood brain barrier (BBB)-penetrating chemotherapeutic agent doxorubicin (Dox, also called adriamycin, ADR). Because of the quinone moiety within the structure of Dox, this agent undergoes redox cycling to produce superoxide free radical peripherally. This, in turn, leads to oxidative modification of the key plasma protein, apolipoprotein A1 (ApoA1). Oxidized ApoA1 leads to elevated peripheral TNFα, a proinflammatory cytokine that crosses the BBB to induce oxidative stress in brain parenchyma that affects negatively brain mitochondria. This subsequently leads to apoptotic cell death resulting in CICI. This review outlines aspects of CICI consistent with the clinical presentation, biochemistry, and pathology of this disorder. To the author's knowledge this is the only plausible and self-consistent mechanism to explain CICI. These two different disorders of the CNS affect millions of persons worldwide. Both AD and CICI share free radical-mediated oxidative stress in brain, but the source of oxidative stress is not the same. Continued research is necessary to better understand both AD and CICI. The discoveries about these disorders from the Butterfield Laboratory that led to the 2013 Discovery Award from the Society of Free Radical and Medicine provide a significant foundation from which this future research can be launched.

4-Hydroxyhexenal (HHE) impairs glutamate transport in astrocyte cultures

Multiple studies show elevations of α,β-unsaturated aldehydic by-products of lipid peroxidation including 4-hydroxynonenal and acrolein in vulnerable brain regions of subjects throughout the progression of Alzheimer's disease (AD). More recently 4-hydroxyhexenal (HHE), a diffusible α,β-unsaturated aldehyde resulting from peroxidation of ω-3 polyunsaturated fatty acids, was shown to be elevated in the hippocampus/parahippocampal gyrus (HPG) of subjects with preclinical AD (PCAD) and in late stage AD (LAD). HHE treatment of primary rat cortical neuron cultures led to a time- and concentration-dependent decrease in survival and glucose uptake. To determine if HHE also impairs glutamate uptake, primary rat astrocyte cultures were exposed to HHE for 4 hours and glutamate transport measured. Results show subtoxic (2.5 μM) HHE concentrations significantly (p < 0.05) impair glutamate uptake in primary astrocytes. Immunoprecipitation of excitatory amino acid transporter-2 (EAAT-2), the primary glutamate transporter in brain, from normal control, mild cognitive impairment (MCI), PCAD, and LAD HPG followed by quantification of HHE immunolabeling showed a significant increase in HHE positive EAAT-2 in MCI and LAD HPG. Together these data suggest HHE can significantly impair glutamate uptake and may play a role in the pathogenesis of AD.

Alzheimer's disease, β-amyloid, glutamate, NMDA receptors and memantine--searching for the connections

β-amyloid (Aβ) is widely accepted to be one of the major pathomechanisms underlying Alzheimer's disease (AD), although there is presently lively debate regarding the relative roles of particular species/forms of this peptide. Most recent evidence indicates that soluble oligomers rather than plaques are the major cause of synaptic dysfunction and ultimately neurodegeneration. Soluble oligomeric Aβ has been shown to interact with several proteins, for example glutamatergic receptors of the NMDA type and proteins responsible for maintaining glutamate homeostasis such as uptake and release. As NMDA receptors are critically involved in neuronal plasticity including learning and memory, we felt that it would be valuable to provide an up to date review of the evidence connecting Aβ to these receptors and related neuronal plasticity. Strong support for the clinical relevance of such interactions is provided by the NMDA receptor antagonist memantine. This substance is the only NMDA receptor antagonist used clinically in the treatment of AD and therefore offers an excellent tool to facilitate translational extrapolations from in vitro studies through in vivo animal experiments to its ultimate clinical utility.

Amyloid-β acts as a regulator of neurotransmitter release disrupting the interaction between synaptophysin and VAMP2

BACKGROUND

It is becoming increasingly evident that deficits in the cortex and hippocampus at early stages of dementia in Alzheimer's disease (AD) are associated with synaptic damage caused by oligomers of the toxic amyloid-β peptide (Aβ42). However, the underlying molecular and cellular mechanisms behind these deficits are not fully understood. Here we provide evidence of a mechanism by which Aβ42 affects synaptic transmission regulating neurotransmitter release.

METHODOLOGY/FINDINGS

We first showed that application of 50 nM Aβ42 in cultured neurones is followed by its internalisation and translocation to synaptic contacts. Interestingly, our results demonstrate that with time, Aβ42 can be detected at the presynaptic terminals where it interacts with Synaptophysin. Furthermore, data from dissociated hippocampal neurons as well as biochemical data provide evidence that Aβ42 disrupts the complex formed between Synaptophysin and VAMP2 increasing the amount of primed vesicles and exocytosis. Finally, electrophysiology recordings in brain slices confirmed that Aβ42 affects baseline transmission.

CONCLUSIONS/SIGNIFICANCE

Our observations provide a necessary and timely insight into cellular mechanisms that underlie the initial pathological events that lead to synaptic dysfunction in Alzheimer's disease. Our results demonstrate a new mechanism by which Aβ42 affects synaptic activity.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Nutrition and Alzheimer's disease: the detrimental role of a high carbohydrate diet

Alzheimer's disease is a devastating disease whose recent increase in incidence rates has broad implications for rising health care costs. Huge amounts of research money are currently being invested in seeking the underlying cause, with corresponding progress in understanding the disease progression. In this paper, we highlight how an excess of dietary carbohydrates, particularly fructose, alongside a relative deficiency in dietary fats and cholesterol, may lead to the development of Alzheimer's disease. A first step in the pathophysiology of the disease is represented by advanced glycation end-products in crucial plasma proteins concerned with fat, cholesterol, and oxygen transport. This leads to cholesterol deficiency in neurons, which significantly impairs their ability to function. Over time, a cascade response leads to impaired glutamate signaling, increased oxidative damage, mitochondrial and lysosomal dysfunction, increased risk to microbial infection, and, ultimately, apoptosis. Other neurodegenerative diseases share many properties with Alzheimer's disease, and may also be due in large part to this same underlying cause.

Differential roles of phospholipases A2 in neuronal death and neurogenesis: implications for Alzheimer disease

The involvement of phospholipase A(2) (PLA(2)) in Alzheimer disease (AD) was first investigated nearly 15 years ago. Over the years, several PLA(2) isoforms have been detected in brain tissue: calcium-dependent secreted PLA(2) or sPLA(2) (IIA, IIC, IIE, V, X, and XII), calcium-dependent cytosolic PLA(2) or cPLA(2) (IVA, IVB, and IVC), and calcium-independent PLA(2) or iPLA(2) (VIA and VIB). Additionally, numerous in vivo and in vitro studies have suggested the role of different brain PLA(2) in both physiological and pathological events. This review aimed to summarize the findings in the literature relating the different brain PLA(2) isoforms with alterations found in AD, such as neuronal cell death and impaired neurogenesis process. The review showed that sPLA(2)-IIA, sPLA(2)-V and cPLA(2)-IVA are involved in neuronal death, whereas sPLA(2)-III and sPLA(2)-X are related to the process of neurogenesis, and that the cPLA(2) and iPLA(2) groups can be involved in both neuronal death and neurogenesis. In AD, there are reports of reduced activity of the cPLA(2) and iPLA(2) groups and increased expression of sPLA(2)-IIA and cPLA(2)-IVA. The findings suggest that the inhibition of cPLA(2) and iPLA(2) isoforms (yet to be determined) might contribute to impaired neurogenesis, whereas stimulation of sPLA(2)-IIA and cPLA(2)-IVA might contribute to neurodegeneration in AD.

Aberrant detergent-insoluble excitatory amino acid transporter 2 accumulates in Alzheimer disease

Alzheimer disease (AD) is characterized by deposition of amyloid-beta, tau, and other specific proteins that accumulate in the brain in detergent-insoluble complexes. Alzheimer disease also involves glutamatergic neurotransmitter system disturbances. Excitatory amino acid transporter 2 (EAAT2) is the dominant glutamate transporter in cerebral cortex and hippocampus. We investigated whether accumulation of detergent-insoluble EAAT2 is related to cognitive impairment and neuropathologic changes in AD by quantifying detergent-insoluble EAAT2 levels in hippocampus and frontal cortex of cognitively normal patients, patients with clinical dementia rating of 0.5 (mildly impaired), and AD patients. Parkinson disease patients served as neurodegenerative disease controls. We found that Triton X-100-insoluble EAAT2 levels were significantly increased in patients with AD compared with controls, whereas Triton X-100-insoluble EAAT2 levels inpatients with clinical dementia rating of 0.5 were intermediately elevated between control and AD subjects. Detergent insolubility of presenilin-1, a structurally similar protein, did not differ among the groups, thus arguing that EAAT2 detergent insolubility was not caused by nonspecific cellular injury. These findings demonstrate that detergent-insoluble EAAT2 accumulation is a progressive biochemical lesion that correlates with cognitive impairment and neuropathologic changes in AD. These findings lend further support to the idea that dysregulation of the glutamatergic system may play a significant role in AD pathogenesis.

Cognitive decline in Alzheimer's disease is associated with selective changes in calcineurin/NFAT signaling

Upon activation by calcineurin, the nuclear factor of activated T-cells (NFAT) translocates to the nucleus and guides the transcription of numerous molecules involved in inflammation and Ca(2+) dysregulation, both of which are prominent features of Alzheimer's disease (AD). However, NFAT signaling in AD remains relatively uninvestigated. Using isolated cytosolic and nuclear fractions prepared from rapid-autopsy postmortem human brain tissue, we show that NFATs 1 and 3 shifted to nuclear compartments in the hippocampus at different stages of neuropathology and cognitive decline, whereas NFAT2 remained unchanged. NFAT1 exhibited greater association with isolated nuclear fractions in subjects with mild cognitive impairment (MCI), whereas NFAT3 showed a strong nuclear bias in subjects with severe dementia and AD. Similar to NFAT1, calcineurin-Aalpha also exhibited a nuclear bias in the early stages of cognitive decline. But, unlike NFAT1 and similar to NFAT3, the nuclear bias for calcineurin became more pronounced as cognition worsened. Changes in calcineurin/NFAT3 were directly correlated to soluble amyloid-beta (Abeta((1-42))) levels in postmortem hippocampus, and oligomeric Abeta, in particular, robustly stimulated NFAT activation in primary rat astrocyte cultures. Oligomeric Abeta also caused a significant reduction in excitatory amino acid transporter 2 (EAAT2) protein levels in astrocyte cultures, which was blocked by NFAT inhibition. Moreover, inhibition of astrocytic NFAT activity in mixed cultures ameliorated Abeta-dependent elevations in glutamate and neuronal death. The results suggest that NFAT signaling is selectively altered in AD and may play an important role in driving Abeta-mediated neurodegeneration.

Synaptic depression and aberrant excitatory network activity in Alzheimer's disease: two faces of the same coin?

Neurodegenerative diseases, including Alzheimer's disease (AD), target specific and functionally connected neuronal networks, raising the possibility that neurodegeneration may spread through abnormal patterns of neural network activity. AD is associated with high levels of amyloid-beta (A beta) peptides in the brain, synaptic depression, aberrant excitatory neuronal activity, and cognitive decline. However, the relationships among these alterations and their underlying mechanisms are poorly understood. In experimental models of AD, high concentrations of pathogenic A beta assemblies reduce glutamatergic transmission and enhance long-term depression at the synaptic level. At the network level, they cause dysrhythmias, including neuronal synchronization, epileptiform activity, seizures, and postictal suppression. Both synaptic depression and aberrant network synchronization likely interfere with activity-dependent synaptic regulation, which is critical for learning and memory. Abnormal patterns of neuronal activity across functionally connected brain regions may also trigger and perpetuate trans-synaptic mechanisms of neurodegeneration. It remains to be determined if synaptic depression and network dysrhythmias are mechanistically related, which of them is primary or secondary, and whether normalization of one will prevent the other as well as cognitive dysfunction in AD.

Alzheimer's disease amyloid beta-protein and synaptic function

Alzheimer's disease (AD) is characterized neuropathologically by the deposition of different forms of amyloid beta-protein (A beta) including variable amounts of soluble species that correlate with severity of dementia. The extent of synaptic loss in the brain provides the best morphological correlate of cognitive impairment in clinical AD. Animal research on the pathophysiology of AD has therefore focussed on how soluble A beta disrupts synaptic mechanisms in vulnerable brain regions such as the hippocampus. Synaptic plasticity in the form of persistent activity-dependent increases or decreases in synaptic strength provide a neurophysiological substrate for hippocampal-dependent learning and memory. Acute treatment with human-derived or chemically prepared soluble A beta that contains certain oligomeric assemblies, potently and selectively disrupts synaptic plasticity causing inhibition of long-term potentiation (LTP) and enhancement of long-term depression (LTD) of glutamatergic transmission. Over time these and related actions of A beta have been implicated in reducing synaptic integrity. This review addresses the involvement of neurotransmitter intercellular signaling in mediating or modulating the synaptic plasticity disrupting actions of soluble A beta, with particular emphasis on the different roles of glutamatergic and cholinergic mechanisms. There is growing evidence to support the view that NMDA and possibly nicotinic receptors are critically involved in mediating the disruptive effect of A beta and that targeting muscarinic receptors can indirectly modulate A beta's actions. Such studies should help inform ongoing and future clinical trials of drugs acting through the glutamatergic and cholinergic systems.

Region-specific neuroprotective effect of ZM 241385 towards glutamate uptake inhibition in cultured neurons

Active uptake by neurons and glial cells is the main mechanism for maintaining extracellular glutamate at low, non-toxic concentrations. Adenosine A(2A) receptors regulate extracellular glutamate levels by acting on both the release and the uptake of glutamate. The aim of this study was to evaluate whether the inhibition of the effects of glutamate uptake blockers by adenosine A(2A) receptor antagonists resulted in neuroprotection. In cortical and striatal neuronal cultures, the application of l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, a transportable competitive inhibitor of glutamate uptake), induced a dose-dependent increase in lactate dehydrogenase (LDH) levels, an index of cytotoxicity. Such an effect of PDC was significantly reduced by pre-treatment with the adenosine A(2A) receptor antagonist ZM 241385 (50 nM) in striatal, but not cortical, cultures. The protective effects of ZM 241385 were specifically due to a counteraction of PDC effects, since ZM 241385 was totally ineffective in preventing the cytotoxicity induced by direct application of glutamate to cultures. These results indicate that adenosine A(2A) receptor antagonists prevent the toxic effects induced by a transportable competitive inhibitor of glutamate uptake, that such an effect specifically occurs in the striatum and that it does not depend on a direct blockade of glutamate-induced toxicity.

Protection against Aβ-mediated rapid disruption of synaptic plasticity and memory by memantine

Soluble amyloid-β protein (Aβ) may cause cognitive impairment in Alzheimer's disease in the absence of significant neurodegeneration. Here, the ability of the NMDA receptor (NMDAR) antagonist memantine to prevent synthetic Aβ-mediated rapid functional deficits in learned behavior and synaptic plasticity was assessed in the rat. In vitro, pretreatment with a clinically relevant, NMDAR blocking concentration of memantine partially inhibited the induction of long-term potentiation (LTP) in the dentate gyrus and prevented further inhibition caused by exposure to Aβ(1-42). Whereas systemic injection with memantine alone inhibited LTP in the CA1 area in vivo, a subthreshold dose partially abrogated the inhibition of LTP by intracerebroventricular soluble Aβ(1-42). Similarly, systemic treatment with memantine alone impaired performance of an operant learning task and a subthreshold dose prevented the Aβ(1-42)-mediated increase in perseveration errors. The acute protection afforded by memantine, albeit in a narrow dose range, against the rapid disruptive effects of soluble Aβ(1-42) on synaptic plasticity and learned behavior strongly implicate NMDAR-dependent reversible dysfunction of synaptic mechanisms in Aβ-mediated cognitive impairment.

Neuronal expression of splice variants of "glial" glutamate transporters in brains afflicted by Alzheimer's disease: unmasking an intrinsic neuronal property

Anomalies in glutamate homeostasis may contribute to the pathological processes involved in Alzheimer's disease (AD). Glutamate released from neurons or glial cells is normally rapidly cleared by glutamate transporters, most of which are expressed at the protein level by glial cells. However, in some patho-physiological situations, expression of glutamate transporters that are normally considered to be glial types, appears to be evoked in populations of distressed neurons. This study analysed the expression of exon-skipping forms of the three predominant excitatory amino acid (glutamate) transporters (EAATs1-3) in brains afflicted with AD. We demonstrate by immunocytochemistry in temporal cortex, the expression of these proteins particularly in limited subsets of neurons, some of which appeared to be dys-morphic. Whilst the neuronal expression of the "glial" glutamate transporters EAAT1 and EAAT2 is frequently considered to represent the abnormal and ectopic expression of such transporters, we suggest this may be a misinterpretation, since neurons such as cortical pyramidal cells normally express abundant mRNA for these EAATs (but little if any EAAT protein expression). We hypothesize instead that distressed neurons in the AD brain can turn on the translation of pre-existent mRNA pools, or suppress the degradation of alternately spliced glutamate transporter protein, leading to the "unmasking" of, rather than evoked expression of "glial" glutamate transporters in stressed neurons.

Detergent-insoluble EAAC1/EAAT3 aberrantly accumulates in hippocampal neurons of Alzheimer's disease patients

Disturbed glutamate homeostasis may contribute to the pathological processes involved in Alzheimer's disease (AD). Once glutamate is released from synapses or from other intracellular sources, it is rapidly cleared by glutamate transporters. EAAC1 (also called EAAT3 or SLC1A1) is the primary glutamate transporter in forebrain neurons. In addition to transporting glutamate, EAAC1 plays other roles in regulating GABA synthesis, reducing oxidative stress in neurons, and is important in supporting neuron viability. Currently, little is known about EAAC1 in AD. To address whether EAAC1 is disturbed in AD, immunohistochemistry was performed on tissue from hippocampus and frontal cortex of AD and normal control subjects matched for age and gender. While EAAC1 immunostaining in cortex appeared comparable to controls, in the hippocampus, EAAC1 aberrantly accumulated in the cell bodies and proximal neuritic processes of CA2-CA3 pyramidal neurons in AD patients. Biochemical analyses showed that Triton X-100-insoluble EAAC1 was significantly increased in the hippocampus of AD patients compared to both controls and Parkinson's disease patients. These findings suggest that aberrant glutamate transporter expression is associated with AD-related neuropathology and that intracellular accumulation of detergent-insoluble EAAC1 is a feature of the complex biochemical lesions in AD that include altered protein solubility.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Amyloid peptides and proteins in review

Amyloids are filamentous protein deposits ranging in size from nanometres to microns and composed of aggregated peptide beta-sheets formed from parallel or anti-parallel alignments of peptide beta-strands. Amyloid-forming proteins have attracted a great deal of recent attention because of their association with over 30 diseases, notably neurodegenerative conditions like Alzheimer's, Huntington's, Parkinson's, Creutzfeldt-Jacob and prion disorders, but also systemic diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease) and type II diabetes. These diseases are all thought to involve important conformational changes in proteins, sometimes termed misfolding, that usually produce beta-sheet structures with a strong tendency to aggregate into water-insoluble fibrous polymers. Reasons for such conformational changes in vivo are still unclear. Intermediate aggregated state(s), rather than precipitated insoluble polymeric aggregates, have recently been implicated in cellular toxicity and may be the source of aberrant pathology in amyloid diseases. Numerous in vitro studies of short and medium length peptides that form amyloids have provided some clues to amyloid formation, with an alpha-helix to beta-sheet folding transition sometimes implicated as an intermediary step leading to amyloid formation. More recently, quite a few non-pathological amyloidogenic proteins have also been identified and physiological properties have been ascribed, challenging previous implications that amyloids were always disease causing. This article summarises a great deal of current knowledge on the occurrence, structure, folding pathways, chemistry and biology associated with amyloidogenic peptides and proteins and highlights some key factors that have been found to influence amyloidogenesis.

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.

NMDA and non-NMDA receptor-mediated differential Ca2+ load and greater vulnerability of motor neurons in spinal cord cultures

Glutamate receptor activated neuronal cell death has been implicated in the pathogenesis of motor neuron disease but the molecular mechanism responsible for neuronal dysfunction needs to be elucidated. In the present study, we examined the contribution of NMDA and non-NMDA sub-types of glutamate receptors in selective vulnerability of motor neurons. Glutamate receptor activated Ca2+ signaling, mitochondrial functions and neurotoxicity in motor neurons and other spinal neurons were studied in mixed spinal cord primary cultures. Exposure of cells to glutamate receptor agonists glutamate, NMDA and AMPA elevated the intracellular Ca2+, mitochondrial Ca2+ and caused mitochondrial depolarization and cytotoxicity in both motor neurons and other spinal neurons but a striking difference was observed in the magnitude and temporal patterns of the [Ca2+]i responses between the two neuronal cell types. The motor neurons elicited higher Ca2+ load than the other spinal neurons and the [Ca2+]i levels were elevated for a longer duration in motor neurons. AMPA receptor stimulation was more effective than NMDA. Both the NMDA and non-NMDA receptor antagonists APV and NBQX inhibited the Ca2+ entry and decreased the cell death significantly; however, NBQX was more potent than APV. Our results demonstrate that both NMDA and non-NMDA sub-types of glutamate receptors contribute to glutamate-mediated motor neuron damage but AMPA receptors play the major role. AMPA receptor-mediated excessive Ca2+ load and differential handling/regulation of Ca2+ buffering by mitochondria in motor neurons could be central in their selective vulnerability to excitotoxicity.

Genetic basis of Alzheimer's dementia: role of mtDNA mutations

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated to dementia in late adulthood. Amyloid precursor protein, presenilin 1 and presenilin 2 genes have been identified as causative genes for familial AD, whereas apolipoprotein E epsilon4 allele has been associated to the risk for late onset AD. However, mutations on these genes do not explain the majority of cases. Mitochondrial respiratory chain (MRC) impairment has been detected in brain, muscle, fibroblasts and platelets of Alzheimer's patients, indicating a possible involvement of mitochondrial DNA (mtDNA) in the aetiology of the disease. Several reports have identified mtDNA mutations in Alzheimer's patients, suggesting the existence of related causal factors probably of mtDNA origin, thus pointing to the involvement of mtDNA in the risk contributing to dementia, but there is no consensual opinion in finding the cause for impairment. However, mtDNA mutations might modify age of onset, contributing to the neurodegenerative process, probably due to an impairment of MRC and/or translation mechanisms.

Chronic dietary alpha-lipoic acid reduces deficits in hippocampal memory of aged Tg2576 mice

Oxidative stress may play a key role in Alzheimer's disease (AD) neuropathology. Here, the effects of the antioxidant, alpha-lipoic acid (ALA) were tested on the Tg2576 mouse, a transgenic model of cerebral amyloidosis associated with AD. Ten-month old Tg2576 and wild type mice were fed an ALA-containing diet (0.1%) or control diet for 6 months and then assessed for the influence of diet on memory and neuropathology. ALA-treated Tg2576 mice exhibited significantly improved learning, and memory retention in the Morris water maze task compared to untreated Tg2576 mice. Twenty-four hours after contextual fear conditioning, untreated Tg2576 mice exhibited significantly impaired context-dependent freezing. ALA-treated Tg2576 mice exhibited significantly more context freezing than the untreated Tg2576 mice. Assessment of brain soluble and insoluble beta-amyloid levels revealed no differences between ALA-treated and untreated Tg2576 mice. Brain levels of nitrotyrosine, a marker of nitrative stress, were elevated in Tg2576 mice, while F2 isoprostanes and neuroprostanes, oxidative stress markers, were not elevated in the Tg2576 mice relative to wild type. These data indicate that chronic dietary ALA can reduce hippocampal-dependent memory deficits of Tg2576 mice without affecting beta-amyloid levels or plaque deposition.

Neuroprotective strategies in Alzheimer's disease

In addition to strategies designed to decrease amyloid beta (A beta) levels, it is likely that successful Alzheimer's disease (AD) therapeutic regimens will require the concomitant application of neuroprotective agents. Elucidation of pathophysiological processes occurring in AD and identification of the molecular targets mediating these processes point to potential high-yield neuroprotective strategies. Candidate neuroprotective agents include those that interact specifically with neuronal targets to inhibit deleterious intraneuronal mechanisms triggered by A beta and other toxic stimuli. Strategies include creating small molecules that block A beta interactions with cell surface and intracellular targets, down-regulate stress kinase signaling cascades, block activation of caspases and expression of pro-apoptotic proteins, and inhibit enzymes mediating excessive tau protein phosphorylation. Additional potential neuroprotective compounds include those that counteract loss of cholinergic function, promote the trophic state and plasticity of neurons, inhibit accumulation of reactive oxygen species, and block excitotoxicity. Certain categories of compounds, such as neurotrophins or neurotrophin small molecule mimetics, have the potential to alter neuronal signaling patterns such that several of these target actions might be achieved by a single agent.

Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer's disease

Extracellular amyloid plaques, intracellular neurofibrillary tangles, and loss of basal forebrain cholinergic neurons in the brains of Alzheimer's disease (AD) patients may be the end result of abnormalities in lipid metabolism and peroxidation that may be caused, or exacerbated, by beta-amyloid peptide (Abeta). Apolipoprotein E (apoE) is a major apolipoprotein in the brain, mediating the transport and clearance of lipids and Abeta. ApoE-dependent dendritic and synaptic regeneration may be less efficient with apoE4, and this may result in, or unmask, age-related neurodegenerative changes. The increased risk of AD associated with apoE4 may be modulated by diet, vascular risk factors, and genetic polymorphisms that affect the function of other transporter proteins and enzymes involved in brain lipid homeostasis. Diet and apoE lipoproteins influence membrane lipid raft composition and the properties of enzymes, transporter proteins, and receptors mediating Abeta production and degradation, tau phosphorylation, glutamate and glucose uptake, and neuronal signal transduction. The level and isoform of apoE may influence whether Abeta is likely to be metabolized or deposited. This review examines the current evidence for diet, lipid homeostasis, and apoE in the pathogenesis of AD. Effects on the cholinergic system and response to cholinesterase inhibitors by APOE allele carrier status are discussed briefly.

Xanthine oxidase, nitric oxide synthase and phospholipase A2 produce reactive oxygen species via mitochondria

The formation of reactive oxygen species (ROS) has been suggested to be associated with excitotoxicity but the involvement of cytoplasmic enzymes in ROS formation is not clearly known. In the present study, we examined the role of xanthine oxidase (XO), nitric oxide synthase (NOS) and phospholipase A(2) (PLA(2)) in glutamate-induced oxidative stress in rat cortical slices. Glutamate-induced ROS formation and mitochondrial depolarization were measured in rat cortical slices in presence of allopurinol, L-NAME and 4-bromophenacylbromide, the specific inhibitors of XO, NOS and PLA(2), respectively. Upon stimulation of slices with glutamate, a significant increase in ROS formation and mitochondrial depolarization was observed. However, pretreatment of slices with allopurinol, L-NAME and 4-bromophenacylbromide inhibited the glutamate-induced ROS formation and mitochondrial depolarization. The glutamate-induced ROS formation was dependent on the concentration of these inhibitors and also on the duration of the treatment. Allopurinol was found to be less effective as compared to L-NAME and 4-bromophenacylbromide. The combined treatment of slices with these enzyme inhibitors showed further inhibition in ROS formation and mitochondrial depolarization. The inhibition in ROS formation as well as mitochondrial depolarization by allopurinol, L-NAME and 4-bromophenacylbromide clearly suggests that the activation of XO, NOS and PLA(2) by calcium during glutamate receptor stimulation may release some chemicals which depolarize mitochondria resulting in ROS formation.

Adenosine A 2A receptor antagonists prevent the increase in striatal glutamate levels induced by glutamate uptake inhibitors

Active uptake by neurons and glial cells is the main mechanism for maintaining extracellular glutamate at low, non-toxic concentrations. Activation of adenosine A(2A) receptors increases extracellular glutamate levels, while A(2A) receptor antagonists reduce stimulated glutamate outflow. Whether a modulation of the glutamate uptake system is involved in the effects elicited by A(2A) receptor blockers has never been investigated. This study examined the ability of adenosine A(2A) receptor antagonists to prevent the increase in glutamate levels induced by blockade of the glutamate uptake. In rats implanted with a microdialysis probe in the dorsal striatum, perfusion with 4 mm l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, a transportable competitive inhibitor of glutamate uptake), or 10 mm dihydrokainic acid (DHK, a non-transportable competitive inhibitor that mainly blocks the glial glutamate transporter GLT-1), significantly increased extracellular glutamate levels. The effects of PDC and DHK were completely prevented by the adenosine A(2A) receptor antagonists SCH 58261 (0.01 mg/kg i.p.) and/or ZM 241385 (5 nm via probe). Since an impairment in glutamate transporter function is thought to play a major role in neurodegenerative disorders, the regulation of glutamate uptake may be one of the mechanisms of the neuroprotective effects of A(2A) receptor antagonists.

β-Amyloid25-35 inhibits glutamate uptake in cultured neurons and astrocytes: modulation of uptake as a survival mechanism

Glutamate transporters are vulnerable to oxidants resulting in reduced uptake function. We have studied the effects of beta-amyloid(25-35) (beta A(25-35)) on [(3)H]-glutamate uptake on cortical neuron or astrocyte cultures in comparison with a scrambled peptide (SCR) and dihydrokainic acid (DHK), a prototypic uptake inhibitor. beta A(25-35) was more potent than DHK in inhibiting glutamate uptake and the effects of both were more marked on astrocytes than on neurons. At 24 h, beta A(25-35) dose-dependently (0.5-15 microM) increased glutamate levels in media from neuron cultures. DHK only enhanced extracellular glutamate at the highest concentration tested (2500 microM). beta A(25-35) induced gradual neurotoxicity (0.1-50 microM) over time. Exposure to beta A(25-35) resulted in increased uptake in astrocytes (0.25-5 microM) and neurons (0.5-15 microM) surviving its toxic effects. However, exposure to DHK (2.5-2500 microM) did not induce neurotoxicity nor modulated uptake. These results indicate that, while inhibition of glutamate uptake may be involved in the neurotoxic effects of beta A(25-35), enhancement of uptake may be a survival mechanism following exposure to beta A(25-35).

The NMDA receptor antagonist memantine as a symptomatological and neuroprotective treatment for Alzheimer's disease: preclinical evidence

There is increasing evidence for the involvement of glutamate-mediated neurotoxicity in the pathogenesis of Alzheimer's disease (AD). We suggest that glutamate receptors of the N-methyl-D-aspartate (NMDA) type are overactivated in a tonic rather than a phasic manner in this disorder. This continuous mild activation may lead to neuronal damage and impairment of synaptic plasticity (learning). It is likely that under such conditions Mg(2+) ions, which block NMDA receptors under normal resting conditions, can no longer do so. We found that overactivation of NMDA receptors using a direct agonist or a decrease in Mg(2+) concentration produced deficits in synaptic plasticity (in vivo: passive avoidance test and/or in vitro: LTP in the CA1 region). In both cases, memantine-an uncompetitive NMDA receptor antagonists with features of an 'improved' Mg(2+) (voltage-dependency, kinetics, affinity)-attenuated this deficit. Synaptic plasticity was restored by therapeutically-relevant concentrations of memantine (1 microM). Moreover, doses leading to similar brain/serum levels provided neuroprotection in animal models relevant for neurodegeneration in AD such as neurotoxicity produced by inflammation in the NBM or beta-amyloid injection to the hippocampus. As such, if overactivation of NMDA receptors is present in AD, memantine would be expected to improve both symptoms (cognition) and to slow down disease progression because it takes over the physiological function of magnesium.

Amyloid-beta induces Smac release via AP-1/Bim activation in cerebral endothelial cells

Insoluble fibrils of amyloid-beta peptide (Abeta) are the major component of senile and vascular plaques found in the brains of Alzheimer's disease (AD) patients. Abeta has been implicated in neuronal and vascular degeneration because of its toxicity to neurons and endothelial cells in vitro; some of these cells die with characteristic features of apoptosis. We used primary cultures of murine cerebral endothelial cells (CECs) to explore the mechanisms involved in Abeta-induced cell death. We report here that Abeta(25-35), a cytotoxic fragment of Abeta, induced translocation of the apoptosis regulator termed second-mitochondria-derived activator of caspase (Smac) from the intramembranous compartment of the mitochondria to the cytosol 24 hr after exposure. In addition, we demonstrated that X chromosome-linked inhibitor-of-apoptosis protein (XIAP) coimmunoprecipitated with Smac, suggesting that the two proteins bound to one another subsequent to the release of Smac from the mitochondria. Abeta(25-35) treatment also led to rapid AP-1 activation and subsequent expression of Bim, a member of the BH3-only family of proapoptotic proteins. Bim knockdown using an antisense oligonucleotide strategy suppressed Abeta(25-35)-induced Smac release and resulted in attenuation of CEC death. Furthermore, AP-1 inhibition, with curcumin or c-fos antisense oligonucleotide, reduced bim expression. These results suggest that Abeta activates an apoptotic cascade involving AP-1 DNA binding, subsequent bim induction, followed by Smac release and binding to XIAP, resulting in CEC death.

Paraoxonase 1 activity: a new vascular marker of dementia?

Paraoxonase 1 (PON1), an A-esterase with peroxidase-like activity present on the surface of HDL, decreases the peroxidation of LDL. Serum PON1 activity (PON1a) decreases with aging and in disorders associated with a high risk of adverse cardiovascular events (acute myocardial infarction, diabetes mellitus, and chronic renal failure). The implication of vascular factors in Alzheimer-type dementia (ATD) is strongly suspected. We measured PON1a by spectrophotometry using the paraoxon substrate in 180 healthy subjects (controls; mean age: 75.3 +/- 8.9 years; 98 women) and 154 patients admitted for cognitive testing. According to criteria, 45 patients had mild cognitive impairments (MCI; mean age: 75.6 +/- 9.3 years; 28 women), 60 had ATD (mean age: 75.6 +/- 8.3 years; 47 women), and 49 had vascular dementia (VaD; mean age: 77.5 +/- 7.2 years; 33 women). Mean PON1a was lower in VaD (0.25 +/- 0.1 U/mL) than in controls or ATD (both 0.41 +/- 0.2 U/mL, p < 0.01). Mean PON1a values in MCI (0.34 +/- 0.2 U/mL) and ATD (0.41 +/- 0.2 U/mL) were not significantly different. In multiple linear regression, PON1a was negatively correlated with male sex, age, and VaD, and positively correlated with ATD (each correlation p < 0.001). As shown in other high-risk cardiovascular disorders, PON1a seems to be a reliable marker of VaD. Its modification in Alzheimer's disease supports the implication of vascular risk factors in this type of dementia.

Paraoxonase 1 192/55 gene polymorphisms in Alzheimer's disease

An esterase, paraoxonase 1 (PON1), protects against organophosphate neurotoxicity and decreases lipoprotein oxidation. Two polymorphisms of PON1 [192 (R or Q) and 55 (M or L)] exist and are associated with coronary artery disease. We have previously shown that serum PON1 activity (PON1a) is lower in vascular dementia (VaD) than in Alzheimer's disease (AD), suggesting that PON1a may distinguish VaD from AD. As PON1 polymorphism modifies PON1a, we determined 192 and 55 PON1 polymorphisms by sequence-specific primer PCR in 64 healthy subjects (HS; mean age: 79.5 +/- 6.3 years; 38 women) and in 72 patients (mean age: 80.2 +/- 6.8 years; 51 women) undergoing cognitive evaluations. According to DSM-IV/NINCDS/ADRDA/NINDS/AIREN criteria, 45 patients (mean age: 80.0 +/- 7.2 years, 34 women) had AD and 27 patients (mean age: 79.8 +/- 6.6 years, 16 women) had VaD. We also measured serum PON1a by spectrophotometry. No significant differences in phenotype distributions among the three study groups were detected by chi(2) test. Among the variables, age, sex, and phenotypes 192 and 55, logistic regression selected only polymorphism 192, but not 55, as a discriminating factor between AD and VaD (p < 0.05). Substitution of serum PON1a for genotype yielded a similar result. PON1 polymorphism 192 appears to be a reliable marker to distinguish patients with AD from patients with VaD and from healthy subjects. Changes in 192 polymorphism distributions in AD and in VaD may at least partially explain the significant difference in PON1a in these two types of dementia.

Amyloid beta-peptide induces cholinergic dysfunction and cognitive deficits: a minireview

Amyloid beta-peptide (Abeta) plays a critical role in the development of Alzheimer's disease (AD). Much progress has been made in understanding this age-related neurodegenerative disorder, thus an insight into the cellular actions of Abeta and resulting functional consequences may contribute to preventive and therapeutic approaches for AD. In this review, recent evidence of Abeta-induced brain dysfunction, particularly of cholinergic impairment and memory deficits is summarized. Moreover, proposed mechanisms for Abeta-induced neurotoxicity such as oxidative stress, ion-channel formation, and Abeta-receptor interaction are discussed.

Lipid peroxidation and protein oxidation in Alzheimer's disease brain: potential causes and consequences involving amyloid beta-peptide-associated free radical oxidative stress

Amyloid beta-peptide (A(beta)) is heavily deposited in the brains of Alzheimer's disease (AD) patients, and free radical oxidative stress, particularly of neuronal lipids and proteins, is extensive. Recent research suggests that these two observations may be linked by A(beta)-induced oxidative stress in AD brain. This review summarizes current knowledge on phospholipid peroxidation and protein oxidation in AD brain, one potential cause of this oxidative stress, and consequences of A(beta)-induced lipid peroxidation and protein oxidation in AD brain.

Role of glycine-33 and methionine-35 in Alzheimer's amyloid beta-peptide 1-42-associated oxidative stress and neurotoxicity

Recent theoretical calculations predicted that Gly33 of one molecule of amyloid beta-peptide (1-42) (Abeta(1-42)) is attacked by a putative sulfur-based free radical of methionine residue 35 of an adjacent peptide. This would lead to a carbon-centered free radical on Gly33 that would immediately bind oxygen to form a peroxyl free radical. Such peroxyl free radicals could contribute to the reported Abeta(1-42)-induced lipid peroxidation, protein oxidation, and neurotoxicity, all of which are prevented by the chain-breaking antioxidant vitamin E. In the theoretical calculations, it was shown that no other amino acid, only Gly, could undergo such a reaction. To test this prediction we studied the effects of substitution of Gly33 of Abeta(1-42) on protein oxidation and neurotoxicity of hippocampal neurons and free radical formation in synaptosomes and in solution. Gly33 of Abeta(1-42) was substituted by Val (Abeta(1-42G33V)). The substituted peptide showed almost no neuronal toxicity compared to the native Abeta(1-42) as well as significantly lowered levels of oxidized proteins. In addition, synaptosomes subjected to Abeta(1-42G33V) showed considerably lower dichlorofluorescein-dependent fluorescence - a measure of reactive oxygen species (ROS) - in comparison to native Abeta(1-42) treatment. The ability of the peptides to generate ROS was also evaluated by electron paramagnetic resonance (EPR) spin trapping methods using the ultrapure spin trap N-tert-butyl-alpha-phenylnitrone (PBN). While Abeta(1-42) gave a strong mixture of four- and six-line PBN-derived spectra, the intensity of the EPR signal generated by Abeta(1-42G33V) was far less. Finally, the ability of the peptides to form fibrils was evaluated by electron microscopy. Abeta(1-42G33V) does not form fibrils nearly as well as Abeta(1-42) after 48 h of incubation. The results suggest that Gly33 may be a possible site of free radical propagation processes that are initiated on Met35 of Abeta(1-42) and that contribute to the peptide's toxicity in Alzheimer's disease brain.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.

Brain protein oxidation in age-related neurodegenerative disorders that are associated with aggregated proteins

Protein oxidation, one of a number of brain biomarkers of oxidative stress, is increased in several age-related neurodegenerative disorders or animal models thereof, including Alzheimer's disease, Huntington's disease, prion disorders, such as Creutzfeld-Jakob disease, and alpha-synuclein disorders, such as Parkinson's disease and frontotemporal dementia. Each of these neurodegenerative disorders is associated with aggregated proteins in brain. However, the relationship among protein oxidation, protein aggregation, and neurodegeneration remain unclear. The current rapid progress in elucidation of mechanisms of protein oxidation in neuronal loss should provide further insight into the importance of free radical oxidative stress in these neurodegenerative disorders.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.