The glutamatergic system and Alzheimer's disease: therapeutic implications

Functional interaction of metabotropic glutamate receptor 5 and NMDA-receptor by a metabotropic glutamate receptor 5 positive allosteric modulator

The NMDA (N-methyl-D-aspartate)-receptor is fundamentally involved in cognitive functions. Recent studies demonstrated a functional interaction between the metabotropic glutamate receptor 5 (mGlu(5) receptor) and the NMDA-receptor in neurons. In rat hippocampal slices, it was shown that activation of mGlu(5) receptor by a positive modulator in the presence of a subthreshold agonist concentration potentiated NMDA-receptor mediated currents and phosphorylation of intracellular signalling proteins. In the present study, we investigated the functional interaction of mGlu(5) receptor and NMDA-receptor by the selective mGlu(5) receptor positive modulator ADX-47273 in-vitro and in-vivo. In rat primary neurons, this compound potentiated Ca(2+) mobilization in the presence of a subthreshold concentration of the mGluR(1/5) agonist DHPG (0.3 microM) with an EC(50) of 0.28+/-0.05 microM. NMDA-induced Ca(2+)-mobilization in primary neurons could be potentiated when neurons were pre-stimulated with 1 microM ADX-47273 in the presence of 0.3 microM DHPG. The specific mGlu(5) receptor antagonist MPEP and the Src-family kinase inhibitor PP2 blocked this potentiation demonstrating the functional interaction of the NMDA-receptor and mGlu(5) receptor in neurons. Furthermore, ADX-47273 elicited an enhancement of NMDA-receptor dependent long-term potentiation in rat hippocampal slices that could be reversed by MPEP. After intraperitoneal administration to rats, ADX-47273 showed a dose-dependent reduction of NMDA-receptor antagonist (ketamine) induced hyperlocomotion, supporting the mechanistic interaction of the NMDA-receptor and mGlu(5) receptor in-vivo. In conclusion, these findings further support the idea of a functional interaction between the mGlu(5) receptor and NMDA-receptor, which may provide a pharmacological strategy for addressing CNS diseases with cognitive impairments linked to NMDA-receptor hypofunction.

Amyloid beta oligomers induce Ca2+ dysregulation and neuronal death through activation of ionotropic glutamate receptors

Amyloid beta (Abeta) oligomers accumulate in brain tissue of Alzheimer disease patients and are related to pathogenesis. The precise mechanisms by which Abeta oligomers cause neurotoxicity remain unresolved. In this study, we investigated the role of ionotropic glutamate receptors on the intracellular Ca2+ overload caused by Abeta. Using rat cortical neurons in culture and entorhinal-hippocampal organotypic slices, we found that Abeta oligomers significantly induced inward currents, intracellular Ca2+ increases and apoptotic cell death through a mechanism requiring NMDA and AMPA receptor activation. The massive entry of Ca2+ through NMDA and AMPA receptors induced by Abeta oligomers caused mitochondrial dysfunction as indicated by mitochondrial Ca2+ overload, oxidative stress and mitochondrial membrane depolarization. Importantly, chronic treatment with nanomolar concentration of Abeta oligomers also induced NMDA- and AMPA receptor-dependent cell death in entorhinal cortex and hippocampal slice cultures. Together, these results indicate that overactivation of NMDA and AMPA receptor, mitochondrial Ca2+ overload and mitochondrial damage underlie the neurotoxicity induced by Abeta oligomers. Hence, drugs that modulate these events can prevent from Abeta damage to neurons in Alzheimer's disease.

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.

Transgenic Alzheimer mice have a larger minimum alveolar anesthetic concentration of isoflurane than their nontransgenic littermates

BACKGROUND

More than 12% of all people older than 65 yr have Alzheimer's disease. Because nothing is known about changes in demand of volatile anesthetics in this disease, we determined minimum alveolar anesthetic concentration (MAC) values of isoflurane in young and aged transgenic mice at risk of developing Alzheimer's disease (heterozygote APP23 mice with the "Swedish double mutation"). To differentiate between unspecific effects of the transgenic model and specific Alzheimer effects, we additionally evaluated MAC values in mice with the same genetic construct but without the Alzheimer's disease-causing Swedish double mutation (heterozygote APP51/16 mice).

METHODS

MAC was determined in 60 mice (10 per group): heterozygote APP23 mice and their wild type littermates at the age of 4 and 18 mo, respectively, and heterozygote APP51/16 mice and their wild type littermates at the age of 18 mo. Anesthesia was induced with isoflurane in oxygen/air. The concentration of inhaled isoflurane varied between 1.0 and 2.0 Vol%, and the motor reaction to toeclamping was recorded. Means of the MAC values were compared with an unpaired t-test.

RESULTS

The MAC of 18-mo-old heterozygote APP23 mice was 1.67 +/- 0.09, i.e., 9% larger than the MAC of their wild type littermates (1.53 +/- 0.14; P = 0.020). Heterozygote APP51/16 mice had a lower MAC than their wild type littermates (1.32 +/- 0.14 vs 1.48 +/- 0.13; P = 0.037). All wild type groups and young heterozygote APP23 mice had comparable MAC values.

CONCLUSIONS

The increased MAC value in aged heterozygote APP23 mice seems to be attributable to changes related to Alzheimer's disease.

Neuroprotective effects of Dioscorea opposita on scopolamine-induced memory impairment in in vivo behavioral tests and in vitro assays

ETHNOPHARMACOLOGICAL RELEVANCE

Plants belong to the genus Dioscorea have long been used as edible tuber crops in many tropical and subtropical areas and as a traditional herbal medicine in oriental countries including China, Japan and Korea.

AIM OF THE STUDY

In this study, in vivo and in vitro tests were carried out to evaluate the cognitive enhancing effects of CHCl(3)-soluble extract from Dioscorea opposita against scopolamine-induced amnesic mice and glutamate- and H(2)O(2)-treated cortical neurons of rats.

MATERIALS, METHODS AND RESULTS

Acute treatment (200 mg/kg body weight, p.o.) and 10 days' daily administration (50 mg/kg body weight, p.o.) of CHCl(3)-soluble extract showed significant spatial learning and memory improvement on mice. Furthermore, the neuroprotective effects on glutamate- and H(2)O(2)-induced neurotoxicity in primary cultured cortical neurons of rats were assessed. Pretreatment with the extract was found to impart significant protection against neurotoxicity.

CONCLUSIONS

These in vivo and in vitro results suggest that the Dioscorea opposita has neuroprotective effects on memory impairment related neurodegenerative diseases.

Neuronal viability is controlled by a functional relation between synaptic and extrasynaptic NMDA receptors

N-methyl-D-aspartate receptors (NMDARs) are critical for synaptic plasticity that underlies learning and memory. But, they have also been described as a common source of neuronal damage during stroke and neurodegenerative diseases. Several studies have suggested that cellular location of NMDARs (synaptic or extrasynaptic) is a key parameter controlling their effect on neuronal viability. The aim of the study was to understand the relation between these two pools of receptors and to determine their implication in both beneficial and/or deleterious events related to NMDAR activation. We demonstrated that selective extrasynaptic NMDAR activation, as well as NMDA bath application, does not activate extracellular signal-regulated kinase (ERK) pathways, but induces mitochondrial membrane potential breakdown and triggers cell body and dendrite damages, whereas synaptic NMDAR activation is innocuous and induces a sustained ERK activation. The functional dichotomy between these two NMDAR pools is tightly controlled by glutamate uptake systems. Finally, we demonstrated that the only clinically approved NMDAR antagonist, memantine, preferentially antagonizes extrasynaptic NMDARs. Together, these results suggest that extrasynaptic NMDAR activation contributes to excitotoxicity and that a selective targeting of the extrasynaptic NMDARs represents a promising therapeutic strategy for brain injuries.

Amyloid beta peptide and NMDA induce ROS from NADPH oxidase and AA release from cytosolic phospholipase A2 in cortical neurons

Increase in oxidative stress has been postulated to play an important role in the pathogenesis of a number of neurodegenerative diseases including Alzheimer's disease. There is evidence for involvement of amyloid-beta peptide (Abeta) in mediating the oxidative damage to neurons. Despite yet unknown mechanism, Abeta appears to exert action on the ionotropic glutamate receptors, especially the N-methyl-D-aspartic acid (NMDA) receptor subtypes. In this study, we showed that NMDA and oligomeric Abeta(1-42) could induce reactive oxygen species (ROS) production from cortical neurons through activation of NADPH oxidase. ROS derived from NADPH oxidase led to activation of extracellular signal-regulated kinase 1/2, phosphorylation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha), and arachidonic acid (AA) release. In addition, Abeta(1-42)-induced AA release was inhibited by d(-)-2-amino-5-phosphonopentanoic acid and memantine, two different NMDA receptor antagonists, suggesting action of Abeta through the NMDA receptor. Besides serving as a precursor for eicosanoids, AA is also regarded as a retrograde messenger and plays a role in modulating synaptic plasticity. Other phospholipase A(2) products such as lysophospholipids can perturb membrane phospholipids. These results suggest an oxidative-degradative mechanism for oligomeric Abeta(1-42) to induce ROS production and stimulate AA release through the NMDA receptors. This novel mechanism may contribute to the oxidative stress hypothesis and synaptic failure that underline the pathogenesis of Alzheimer's disease.

NMDA receptor modulation by D-cycloserine promotes episodic-like memory in mice

RATIONALE

NMDA-R (N-methyl-D-aspartate receptors) have been implicated in synaptic plasticity underlying one-trial learning of event-place associations. In rodents, episodic-like memory (ELM) of personally experienced events can be inferred from behavior that reflects the remembrance of the content (what kind of object was presented), place (where was this object placed), and temporal context (when was the object presented). We have previously shown that that D-cycloserine (DCS), an NMDA-R agonist, ameliorates stress-induced deficits in ELM.

OBJECTIVES

In this study, we used an experimental protocol designed to detect promnestic drug effects and investigated whether DCS, which is known to enhance learning and memory, can induce ELM under conditions where mice normally do not show ELM.

RESULTS

Mice that have been treated i.p. with DCS (20 mg/kg) both remembered the temporal order in which two different objects had been encountered during two consecutive sample trials, as well as their spatial position during the sample trials. Most importantly, the test trial performance of these mice is compatible with ELM in terms of an integrated memory for unique experiences comprising "what", "where", and "when" information. In contrast, mice that have received either a saline injection or lower doses of DCS (0.2 and 2.0 mg/kg) did not show such an integrated ELM.

CONCLUSIONS

To our knowledge, this is the first report showing that DCS can promote ELM in mice.

Deficit of NMDA receptor activation in CA1 hippocampal area of aged rats is rescued by D-cycloserine

Activation of the glycine modulatory site of the N-methyl-D-aspartate glutamate receptor (NMDAR) may reduce cognitive impairments associated with normal ageing. In order to test this hypothesis, we assessed the effects of the partial agonist D-cycloserine (DCS) on cellular activities involved in memory formation. This was performed in CA1 cellular networks of adult and aged Sprague-Dawley rat hippocampal slices using extracellular field excitatory postsynaptic potential recordings. Synaptic potentials specifically mediated by NMDAR were significantly reduced in aged animals. DCS increased the magnitude of these responses in both adult and old rats but this effect was significantly higher in the latter, thus reversing the age-related decrease in NMDAR synaptic potentials. NMDAR-mediated theta burst long-term potentiation (TBS-LTP) as well as long-term depression (LTD) of synaptic transmission, prominent models for the cellular basis of learning and memory, were also weakened in aged animals. Age-related alterations of both forms of synaptic plasticity were rescued by DCS. In addition, the DCS-induced decrease in basal fast glutamatergic neurotransmission involving the activation of inhibitory glycinergic receptors, previously reported in young rats (Rouaud & Billard, 2003), was severely attenuated in aged animals. In summary, our results indicate that the facilitation of NMDAR activation through its glycine-binding site rescues the age-related deficit of cellular mechanisms of learning and memory. Such physiological evidences suggest that this modulation site of NMDAR represents an important target to alleviate cognitive deficits associated with normal ageing.

Selective increase in the extracellular D-serine contents by D-cycloserine in the rat medial frontal cortex

A partial agonist of the N-methyl-D-aspartate (NMDA) receptor, D-cycloserine, acting at its glycine modulatory site, ameliorates the neuropsychiatric symptoms that are mimicked by NMDA antagonists and include cognitive disturbances, antipsychotic-resistant schizophrenic symptoms and cerebellar ataxia. To obtain a further insight into the mechanisms of the therapeutic efficacies of D-cycloserine, we investigated the effects of the systemic administration of D-cycloserine on the extracellular contents of an endogenous NMDA co-agonist, D-serine, in the medial frontal cortex of the rat using an in vivo dialysis technique. An acute intraperitoneal injection of D-cycloserine (50 and 100 mg/kg) caused an increase in extracellular concentrations of D-serine without significant effects on those of L-serine, glycine, L-glutamate, L-aspartate, L-glutamine, L-asparagine, L-alanine, L-threonine and taurine in the medial frontal cortex. The selective increase in the extracellular D-serine contents may, at least partially, be associated with the facilitating effects of D-cycloserine on the NMDA receptor functions in addition to its direct stimulation of the NMDA receptor glycine site.

3-Nitropropionic acid toxicity in hippocampus: protection through N-methyl-D-aspartate receptor antagonism

The over-activation of glutamate receptors can lead to excitotoxic cell death and is believed to be involved in the progression of neurodegenerative events in the vulnerable hippocampus. Here, we used an in vitro slice model to study toxicity produced in the hippocampus by the mitochondrial toxin 3-nitropropionic acid (3-NP). The organotypic slice cultures exhibit native cellular organization as well as dense arborization of neuronal processes and synaptic contacts. The hippocampal slices were exposed to 3-NP for 2-20 days, causing calpain-mediated breakdown of the spectrin cytoskeleton, a loss of pre- and postsynaptic markers, and neuronal atrophy. The N-methyl-D-aspartate (NMDA) receptor antagonist memantine reduced both the cytoskeletal damage and synaptic decline in a dose-dependent manner. 3-NP-induced cytotoxicity, as determined by the release of lactate dehydrogenase, was also reduced by memantine with EC50 values from 1.7 to 2.3 microM. Propidium iodide fluorescence and phase contrast microscopy confirmed memantine neuroprotection against the chronic toxin exposure. In addition, the protected tissue exhibited normal neuronal morphology in the major hippocampal subfields. These results indicate that antagonists of NMDA-type glutamate receptors are protective during the toxic outcome associated with mitochondrial dysfunction. They also provide further evidence of memantine's therapeutic potential against neurodegenerative diseases.

Oxidative stress in Alzheimer's disease brain: New insights from redox proteomics

Alzheimer's disease, an age-related neurodegenerative disorder, is characterized clinically by a progressive loss of memory and cognitive functions. Neuropathologically, Alzheimer's disease is defined by the accumulation of extracellular amyloid protein deposited senile plaques and intracellular neurofibrillary tangles made of abnormal and hyperphosphorylated tau protein, regionalized neuronal death, and loss of synaptic connections within selective brain regions. Evidence has suggested a critical role for amyloid-beta peptide metabolism and oxidative stress in Alzheimer's disease pathogenesis and progression. Among the other indices of oxidative stress in Alzheimer's disease brain are protein carbonyls and 3-nitrotyrosine, which are the markers of protein oxidation. Thus, in this review, we discuss the application of redox proteomics for the identification of oxidatively modified proteins in Alzheimer's disease brain and also discuss the functions associated with the identified oxidized proteins in relation to Alzheimer's disease pathology. The information obtained from proteomics may be helpful in understanding the molecular mechanisms involved in the development and progression of Alzheimer's disease as well as of other neurodegenerative disorders. Further, redox proteomics may provide potential targets for drug therapy in Alzheimer's disease.

Alzheimer' s disease, oxidative stress and gammahydroxybutyrate

Although the cause of Alzheimer's disease is unknown, oxidative stress, energy depletion, excitotoxicity and vascular endothelial pathology are all considered to play a part in its pathogenesis. In reaction to these adverse events, the Alzheimer brain appears to deploy a highly conserved biological response to tissue stress. Oxidative metabolism is turned down, the expression of antioxidative enzymes is increased and intermediary metabolism is shifted in the direction of the pentose phosphate shunt to promote reductive detoxification, repair and biosynthesis. Gathering evidence suggests that the release of beta-amyloid and the formation of neurofibrillary tangles, the two hallmarks of Alzheimer's disease, are components of this protective response. Gammahydroxybutyrate (GHB), an endogenous short chain fatty acid, may be able to buttress this response. GHB can reduce glucose utilization, shift intermediary metabolism in the direction the pentose phosphate shunt and generate NADPH, a key cofactor in the activity of many antioxidative and reductive enzymes. GHB has been shown to spare cerebral energy utilization, block excitotoxicity and maintain vascular integrity in the face of impaired perfusion. Most important, GHB has repeatedly been shown to prevent the tissue damaging effects of oxidative stress. It may therefore be possible to utilize GHB to strengthen the brain's innate defences against the pathological processes operating in the Alzheimer brain and, in this way, stem the advance of Alzheimer's disease.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Group I metabotropic glutamate receptors activate the p70S6 kinase via both mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase (ERK 1/2) signaling pathways in rat striatal and hippocampal synaptoneurosomes

Group I metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity via a rapamycin-sensitive mRNA translation signaling pathway. Various growth factors can stimulate this pathway, leading to the phosphorylation and activation of mammalian target of rapamycin (mTOR), a serine/threonine protein kinase that modulates the activity of several translation regulatory factors, such as p70S6 kinase. However, little is known about the cellular and molecular mechanisms that bring the plastic changes of synaptic transmission after stimulation of group I mGluRs. Here, we investigated the role of the mTOR-p70S6K and the ERK1/2-p70S6K pathways in rat striatal and hippocampal synaptoneurosomes after group I mGluR stimulation. Our findings show that (S)-3,5-dihydroxyphenylglycine (DHPG) increases significantly the activation of mTOR and p70S6K (Thr389, controlled by mTOR) in both brain areas. The mTOR activation is dose-dependent and requires the stimulation of mGluR1 subtype receptors as for the p70S6K activation observed in striatum and hippocampus. In addition, the p70S6K (Thr421/Ser424) activation via the ERK1/2 activation is increased and involved also mGluR1 receptors. These results demonstrate that group I mGluRs are coupled to mTOR-p70S6K and ERK1/2-p70S6K pathways in striatal and hippocampal synaptoneurosomes. The translational factor p70S6K could be involved in the group I mGluRs-modulated synaptic efficacy.

Glutamate metabolizing enzymes in prefrontal cortex of Alzheimer's disease patients

Amounts of glutamate metabolizing enzymes such as glutamate dehydrogenase (GDH), glutamine synthetase (GS), GS-like protein (GSLP), and phosphate-activated glutaminase (PAG) were compared in prefrontal cortex of control subjects and patients with Alzheimer disease (AD). The target proteins were quantified by ECL-Western immunoblotting in extracts from brain tissue prepared by two different techniques separating enzymes preferentially associated with cytoplasm (GDH I and II isoenzymes, GS, and partially GSLP) and membrane (GDH III, PAG, and partially GSLP) fractions. Amounts of all listed enzymes were found significantly increased in the patient group compared with controls. Some links between the measured values were observed in the control, but not in the AD patient group. The results may suggest for the pathological interruption of regulatory relations between distinct enzymes of glutamate metabolism in brain of AD patients.

Effects of the NMDA receptor antagonist MK-801 on short-interval timing in rats

Effects of MK-801, an N-methyl-D-aspartate antagonist, on short-interval timing were examined using the peak-interval (PI) and PI-gap procedures. Fisher 344 rats were given daily injections of 0.025 mg/kg, 0.05 mg/kg, and 0.2 mg/kg MK-801. The main results were (a) 0.2 mg/kg MK-801 produced an immediate overestimation of the criterion time; (b) MK-801 increased peak rate of responding; (c) 0.2 mg/kg MK-801 produced an increase in variability; (d) during the PI-gap procedure, a reset pattern was observed for all rats (MK-801 and saline). Results suggest that MK-801 has at least 2 effects. First, MK-801 interferes with short-interval timing by producing an overestimation of time and a nonscalar increase in variability. Second, MK-801 increases response rate, suggesting a decrease in response inhibition.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

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.

N-methyl-D-aspartate attenuates CXCR2-mediated neuroprotection through enhancing the receptor phosphorylation and blocking the receptor recycling

Abnormal extracellular accumulations of beta-amyloid, a major component of the senile plaques, and of the excitatory amino acid glutamate are both believed to be associated with degeneration of nerve cells in the central nervous system of patients with Alzheimer's disease. The chemokine receptor CXCR2 has been shown to play a role in protecting neurons against beta-amyloid-induced injury in vitro, but it remains unclear whether CXCR2-mediated neuroprotection is affected by glutamate. We demonstrated that pretreatment of hippocampal neurons with a sublethal concentration of N-methyl-d-aspartate (NMDA) attenuated the macrophage inflammatory protein 2 (MIP2)-induced protection against beta-amyloid-induced neuronal death. The NMDA induced inhibition was blocked by (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), a noncompetitive NMDA receptor antagonist, indicating the involvement of NMDA receptors in this process. A sublethal dose of NMDA pretreatment induced CXCR2 phosphorylation, although to a lesser extent than the receptor phosphorylation induced by MIP2, and differential serine residues were involved in NMDA- and MIP2-induced CXCR2 phosphorylation. Moreover, NMDA treatment reduced the CXCR2-mediated Ca(2+) mobilization, suggesting that NMDA induces cross-desensitization of CXCR2. CXCR2 underwent dephosphorylation after removal of the extracellular ligand, but the dephosphorylation rate was significantly reduced in the cells pretreated with NMDA. Treatment of the neuronal cells with NMDA retarded the recycling of CXCR2. In view of the critical role of receptor phosphorylation and recycling in the functional responsiveness of the chemokine receptor, these observations indicate a novel pathway through which glutamate may interfere with the neuroprotective function of chemokines.

Alpha-tocopherol: roles in prevention and therapy of human disease

Alpha-tocopherol, one of the eight isoforms of vitamin E, is the most potent fat-soluble antioxidant known in nature. For years, it was thought that alpha-tocopherol only functioned as a scavenger of lipid peroxyl radicals, specifically, oxidized low-density lipoprotein (oxLDL), thereby serving as a chief antioxidant for the prevention of atherosclerosis. In recent years, the many roles of alpha-tocopherol have been uncovered, and include not only antioxidant functions, but also pro-oxidant, cell signaling and gene regulatory functions. Decades of clinical and preclinical studies have broadened our understanding of the antioxidant vitamin E and its utility in a number of chronic, oxidative stress-induced pathologies. The results of these studies have shown promising, albeit mixed reviews on the efficacy of alpha-tocopherol in the prevention and treatment of heart disease, cancer and Alzheimer's disease. Future studies to uncover cellular and systemic mechanisms may help guide appropriate clinical treatment strategies using vitamin E across a diverse population of aging individuals.

Dietary n-3 polyunsaturated fatty acid depletion activates caspases and decreases NMDA receptors in the brain of a transgenic mouse model of Alzheimer's disease

Epidemiological data indicate that low n-3 polyunsaturated fatty acids (PFA) intake is a readily manipulated dietary risk factor for Alzheimer's disease (AD). Studies in animals confirm the deleterious effect of n-3 PFA depletion on cognition and on dendritic scaffold proteins. Here, we show that in transgenic mice overexpressing the human AD gene APPswe (Tg2576), safflower oil-induced n-3 PFA deficiency caused a decrease in N-methyl-D-aspartate (NMDA) receptor subunits, NR2A and NR2B, in the cortex and hippocampus with no loss of the presynaptic markers, synaptophysin and synaptosomal-associated protein 25 (SNAP-25). n-3 PFA depletion also decreased the NR1 subunit in the hippocampus and Ca2+/calmodulin-dependent protein kinase (CaMKII) in the cortex of Tg2576 mice. These effects of dietary n-3 PFA deficiency were greatly amplified in Tg2576 mice compared to nontransgenic mice. Loss of the NR2B receptor subunit was not explained by changes in mRNA expression, but correlated with p85alpha phosphatidylinositol 3-kinase levels. Most interestingly, n-3 PFA deficiency dramatically increased levels of protein fragments, corresponding to caspase/calpain-cleaved fodrin and gelsolin in Tg2576 mice. This effect was minimal in nontransgenic mice suggesting that n-3 PFA depletion potentiated caspase activation in the Tg2576 mouse model of AD. Dietary supplementation with docosahexaenoic acid (DHA; 22 : 6n-3) partly protected from NMDA receptor subunit loss and accumulation of fodrin and gelsolin fragments but fully prevented CaMKII decrease. The marked effect of dietary n-3 PFA on NMDA receptors and caspase/calpain activation in the cortex of an animal model of AD provide new insights into how dietary essential fatty acids may influence cognition and AD risk.

Treatments for Alzheimer Disease

As our population ages, the incidence and prevalence of Alzheimer disease (AD) will increase dramatically. A number of therapies have been investigated for the treatment and prevention of AD. Clinicians should be prepared to provide evidence-based answers to inquiries regarding AD treatment. There is insufficient evidence to recommend ginkgo biloba, estrogen, statins, or nonsteroidal anti-inflammatory drugs for the prevention or treatment of AD. The use of vitamin E is supported by a single randomized controlled trial, while data on other antioxidants is mixed. There is good evidence that cholinesterase inhibitors and memantine are modestly effective in the treatment of AD. Cholinesterase inhibitors appear to be effective throughout the spectrum of AD, while memantine, alone or in combination with cholinesterase inhibitors, is effective in late stage disease. There is insufficient evidence to suggest superiority of one cholinesterase over another.

The pharmacological stimulation of NMDA receptors via co-agonist site: an fMRI study in the rat brain

d-Serine has been proposed as an endogenous modulator at the co-agonist glycine-binding site of N-methyl-d-aspartate (NMDA) receptors. There is still some debate as to whether this site is saturated in vivo, but it seems likely that this depends on regional differences in local glycine or d-serine concentrations. In order to identify areas where the co-agonist site was not fully activated in vivo, we studied the effect of intraperitoneal d-serine administration in the rat brain using functional magnetic resonance imaging (fMRI). Using contrast agent injection, the variations in the relative cerebral blood volume (CBVrel) in several regions of interest were evaluated. d-Serine (50 mg/kg) elicited a significant statistical increase in the CBVrel in the hippocampus. This effect was inhibited by the specific full antagonist of the co-agonist glycine site L-701,324 indicating that the hippocampal activation occurred through the binding of the agonist d-serine to the glycine-binding site of NMDA receptors. This result demonstrates that in the hippocampus, the co-agonist sites of NMDA receptors are not endogenously saturated under our experimental conditions, suggesting an important role of d-serine in the modulation of receptor function in the hippocampus.

Biomarkers for apoptosis in Alzheimer's disease

Alzheimer's disease (AD) affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. Today there is no cure for this devastating and always lethal disease and therefore it is of great interest for patients, relatives and societies to find new drugs that can hinder the disease process. During the progression of AD a substantial amount of neurons degenerate in the brain. The mechanisms of cell death involved in AD have not been fully elucidated. However, there are several reports showing that neurons die partly by apoptosis in the AD brain. Drugs blocking apoptosis could therefore be potentially useful for early prevention of neuronal cell death. Biomarkers for apoptosis should be important tools in the evaluation of drug effects and in the diagnostics of AD. Here we review the current knowledge in the field and discuss potential biomarkers for apoptosis in AD.

Nicastrin, Presenilin, APH-1, and PEN-2 Form Active γ-Secretase Complexes in Mitochondria

Mitochondria are central in the regulation of cell death. Apart from providing the cell with ATP, mitochondria also harbor several death factors that are released upon apoptotic stimuli. Alterations in mitochondrial functions, increased oxidative stress, and neurons dying by apoptosis have been detected in Alzheimer's disease patients. These findings suggest that mitochondria may trigger the abnormal onset of neuronal cell death in Alzheimer's disease. We previously reported that presenilin 1 (PS1), which is often mutated in familial forms of Alzheimer's disease, is located in mitochondria and hypothesized that presenilin mutations may sensitize cells to apoptotic stimuli at the mitochondrial level. Presenilin forms an active gamma-secretase complex together with Nicastrin (NCT), APH-1, and PEN-2, which among other substrates cleaves the beta-amyloid precursor protein (beta-APP) generating the amyloid beta-peptide and the beta-APP intracellular domain. Here we have identified dual targeting sequences (for endoplasmic reticulum and mitochondria) in NCT and showed expression of NCT in mitochondria by immunoelectron microscopy. We also showed that NCT together with APH-1, PEN-2, and PS1 form a high molecular weight complex located in mitochondria. gamma-secretase activity in isolated mitochondria was demonstrated using C83 (alpha-secretase-cleaved C-terminal 83-residue beta-APP fragment from BD8 cells lacking presenilin and thus gamma-secretase activity) or recombinant C100-Flag (C-terminal 100-residue beta-APP fragment) as substrates. Both systems generated an APP intracellular domain, and the activity was inhibited by the gamma-secretase inhibitors l-685,458 or Compound E. This novel localization of NCT, PS1, APH-1, and PEN-2 expands the role and importance of gamma-secretase activity to mitochondria.

Efficacy and tolerability of memantine in the treatment of dementia

BACKGROUND

Until recently, acetylcholinesterase inhibitors were the only approved agents for the treatment of Alzheimer's disease (AD). These medications have also been used in the treatment of vascular dementia (VD). Memantine, the first N-methyl-D-aspartate (NMDA)-receptor antagonist to be well tolerated, has been approved for the treatment of moderate to severe AD.

OBJECTIVE

The aim of this study was to review the current literature on the efficacy and tolerability of memantine in the treatment of AD and VD.

METHODS

A MEDLINE search of the English-language literature from January 1970 to March 2004 was conducted to identify randomized, double-blind, placebo-controlled, parallel-group trials in which memantine was administered to patients with VD or AD. The search terms were memantine, NMDA inhibitor, and NMDA antagonist.

RESULTS

Excessive glutamate, the brain's major excitatory neurotransmitter, can cause excitotoxicity by allowing too much calcium to enter neuronal cells. Moderate-affinity NMDA-receptor antagonists such as memantine block pathologic activity of glutamate while allowing physiologic activity. Use of memantine has been associated with significant improvements in measures of cognition, function, and behavior in both VD and AD. Adverse events associated with memantine have been comparable to those with placebo, with the exception of an increased incidence of dizziness, constipation, cataracts, nausea, dyspnea, confusion, headache, and urinary incontinence.

CONCLUSIONS

Memantine seems to be promising and well tolerated in the treatment of moderate to severe VD or AD, either as monotherapy or in combination with donepezil. It appears to be particularly effective in improving cognitive, functional, and global outcomes in moderate to severe AD and in improving cognitive end points in mild to moderate VD. More research is needed on important clinical questions, including whether memantine can prolong patients' ability to provide self-care and delay institutional placement.

Antisense directed at the Abeta region of APP decreases brain oxidative markers in aged senescence accelerated mice

Amyloid beta-peptide (Abeta) is known to induce free radical-mediated oxidative stress in the brain. Free radical-mediated damage to the neuronal membrane components has been implicated in the etiology of Alzheimer's disease (AD). Abeta is produced by proteolytic processing of the amyloid precursor protein (APP). The senescence accelerated mouse prone 8 (SAMP8) strain was developed by phenotypic selection from a common genetic pool. The SAMP8 strain exhibits age-related deterioration in memory and learning as well as Abeta accumulation, and it is considered an effective model for studying brain aging in accelerated senescence. Previous research has shown that a phosphorothiolated antisense oligonucleotide directed against the Abeta region of APP decreases the expression of APP and reverses deficits in learning and memory in aged SAMP8 mice. Consistent with other reports, our previous study showed that 12-month-old SAMP8 mice have increased levels of oxidative stress markers in the brain compared with that in brains from 4-month-old SAMP8 mice. In the current study, 12-month-old SAMP8 mice were treated with antisense oligonucleotide directed against the Abeta region of APP, and the oxidative markers in brain were decreased significantly. Therefore, we conclude that Abeta may contribute to the oxidative stress found in aged SAMP8 mice that have learning and memory impairments. These results are discussed in reference to AD.

Pharmacotherapeutic approaches to the treatment of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD), a progressive degenerative disorder of the brain, is the most common cause of cognitive impairment in the elderly. The pharmacotherapy of AD is evolving rapidly. Cholinergic stabilization with cholinesterase-inhibitor (ChEI) therapy implies neuroprotection and a resultant slowing of disability and disease progression. The moderate-affinity N-methyl-d-aspartate (NMDA)-receptor antagonist memantine may block neural excitotoxicity.

OBJECTIVE

The purpose of this review was to examine the evidence for the responsiveness to pharmacotherapy of established AD; specifically, the extent to which the benefits of therapy have been proved, the extent to which currently available ChEIs support cholinergic neurotransmission, and the extent to which currently available ChEIs and memantine provide neuroprotection.

METHODS

Relevant studies were identified through a comprehensive search of MEDLINE for articles published between January 1999 and February 2004 using the terms Alzheimer's pharmacotherapy, cholinesterase inhibitor therapy, Alzheimer's disease, donepezil, rivastigmine, galantamine, glutamatergic system modifiers, and memantine; a search of the reference lists of identified articles; and a manual search of pertinent journals. Articles were selected that contained higher-level evidence, based on explicit validated criteria.

RESULTS

ChEI therapy was associated with quality-of-life improvements that included enhanced performance of activities of daily living, reduced behavioral disturbances, stabilized cognitive impairment, decreased caregiver stress, and delay in the first dementia-related nursing home placement. In large clinical trials in moderate to severe AD (a stage that is associated with distress for patients and caregiver burden, and for which other treatments are not available), memantine showed an ability to delay cognitive and functional deterioration. The combination of memantine and ChEI therapy was significantly more efficacious than ChEI therapy alone (P < 0.001) and was well tolerated.

CONCLUSIONS

The idea that AD is pharmacologically unresponsive appears to be changing. With the use of ChEI and NMDA-receptor antagonist therapy, the symptoms and outcomes of this devastating neurodegenerative disease can be improved and its course altered.

GABAergic function in Alzheimer's disease: evidence for dysfunction and potential as a therapeutic target for the treatment of behavioural and psychological symptoms of dementia

Alzheimer's disease (AD) is characterized by disruptions in multiple major neurotransmitters. While many studies have attempted to establish whether GABA is disrupted in AD patients, findings have varied. We review evidence for disruptions in GABA among patients with AD and suggest that the variable findings reflect subtypes of the disease that are possibly manifested clinically by differing behavioural symptoms. GABA, the major inhibitory neurotransmitter, has long been a target for anxiolytics, hypnotic sedatives, and anticonvulsants. We review the clinical use of GABAergic agents in treating persons with AD symptoms. While newer generation GABAergic medications are now available, they have yet to be evaluated among patients with AD.

Is pigment epithelium-derived factor level in cerebrospinal fluid a promising biomarker for early diagnosis of Alzheimer's disease?

Alzheimer's disease (AD) is the most common cause of dementia in Western countries and in Japan. Early diagnosis and treatment is needed to slow down the degenerative process and dementia in AD. The main histopathological characteristics of AD are senile plaques and neurofibrillary tangles. Based on the disease pathology, numerous blood and cerebrospinal fluid (CSF) tests have been proposed for early detection of AD. However, there is no definite clinical method to determine in which patients with mild cognitive impairment will progress to AD with dementia. Since pigment epithelium-derived factor (PEDF) has been recently shown to protect various types of cells including neuronal cells against oxidative stress- or glutamate-induced injury through its anti-oxidative properties, we examined here the expression levels of PEDF in AD's brain. PEDF was found to have the strong immunoreactivity in cortical neurons and astrocytes in the brains of AD. Further, the distribution of PEDF proteins was good concordance with RAGE proteins, one of the receptors for amyloid beta peptides, which are involved in neuronal cell death and microglial activation in AD. These results suggest that PEDF overexpression may indicate a compensation mechanism to fight against neuronal cell injury in AD. Our present observations suggest that PEDF in CSF might reflect cerebral PEDF turnover and provide a means for monitoring neuronal perturbation induced by oxidative stress in the early stage of AD. Clinical use of CSF-PEDF as a biomarker for AD might enable more effective diagnosis and treatment of patients with this disorder.