beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

The ?-amyloid protein of Alzheimer?s disease binds to membrane lipids but does not bind to the ?7 nicotinic acetylcholine receptor

Accumulation of the amyloid protein (Abeta) in the brain is an important step in the pathogenesis of Alzheimer's disease. However, the mechanism by which Abeta exerts its neurotoxic effect is largely unknown. It has been suggested that the peptide can bind to the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). In this study, we examined the binding of Abeta1-42 to endogenous and recombinantly expressed alpha7nAChRs. Abeta1-42 did neither inhibit the specific binding of alpha7nAChR ligands to rat brain homogenate or slice preparations, nor did it influence the activity of alpha7nAChRs expressed in Xenopus oocytes. Similarly, Abeta1-42 did not compete for alpha-bungarotoxin-binding sites on SH-SY5Y cells stably expressing alpha7nAChRs. The effect of the Abeta1-42 on tau phosphorylation was also examined. Although Abeta1-42 altered tau phosphorylation in alpha7nAChR-transfected SH-SY5Y cells, the effect of the peptide was unrelated to alpha7nAChR expression or activity. Binding studies using surface plasmon resonance indicated that the majority of the Abeta bound to membrane lipid, rather than to a protein component. Fluorescence anisotropy experiments indicated that Abeta may disrupt membrane lipid structure or fluidity. We conclude that the effects of Abeta are unlikely to be mediated by direct binding to the alpha7nAChR. Instead, we speculate that Abeta may exert its effects by altering the packing of lipids within the plasma membrane, which could, in turn, influence the function of a variety of receptors and channels on the cell surface.

Application of nicotine enantiomers, derivatives and analogues in therapy of neurodegenerative disorders

This review gives a brief overview over the major aspects of application of the nicotine alkaloid and its close derivatives in the therapy of some neurodegenerative disorders and diseases (e.g. Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia etc.). The issues concerning methods of nicotine analysis and isolation, and some molecular aspects of nicotine pharmacology are included. The natural and synthetic analogues of nicotine that are considered for medical practice are also mentioned. The molecular properties of two naturally occurring nicotine enantiomers are compared--the less-common but less-toxic (R)-nicotine is suggested as a natural compound that may find its place in pharmaceutical practice.

The alpha7 nicotinic acetylcholine receptor as a pharmacological target for inflammation

The physiological regulation of the immune system encompasses comprehensive anti-inflammatory mechanisms that can be harnessed for the treatment of infectious and inflammatory disorders. Recent studies indicate that the vagal nerve, involved in control of heart rate, hormone secretion and gastrointestinal motility, is also an immunomodulator. In experimental models of inflammatory diseases, vagal nerve stimulation attenuates the production of proinflammatory cytokines and inhibits the inflammatory process. Acetylcholine, the principal neurotransmitter of the vagal nerve, controls immune cell functions via the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). From a pharmacological perspective, nicotinic agonists are more efficient than acetylcholine at inhibiting the inflammatory signaling and the production of proinflammatory cytokines. This 'nicotinic anti-inflammatory pathway' may have clinical implications as treatment with nicotinic agonists can modulate the production of proinflammatory cytokines from immune cells. Nicotine has been tested in clinical trials as a treatment for inflammatory diseases such as ulcerative colitis, but the therapeutic potential of this mechanism is limited by the collateral toxicity of nicotine. Here, we review the recent advances that support the design of more specific receptor-selective nicotinic agonists that have anti-inflammatory effects while eluding its collateral toxicity.

Glutamatergic synaptic depression by synthetic amyloid beta-peptide in the medial septum

The medial septum/diagonal band region, which participates in learning and memory processes via its cholinergic and GABAergic projection to the hippocampus, is one of the structures affected by beta amyloid (betaA) deposition in Alzheimer's disease (AD). The acute effects of betaA (25-35 and 1-40) on action potential generation and glutamatergic synaptic transmission in slices of the medial septal area of the rat brain were studied using current and patch-clamp techniques. The betaA mechanism of action through M1 muscarinic receptors and voltage-dependent calcium channels was also addressed. Excitatory evoked responses decreased (30-60%) in amplitude after betaA (2 microM) perfusion in 70% of recorded cells. However, the firing properties were unaltered at the same concentration. This depression was irreversible in most cases, and was not prevented or reversed by nicotine (5 microM). In addition, the results obtained using a paired-pulse protocol support pre- and postsynaptic actions of the peptide. The betaA effect was blocked by calcicludine (50 nM), a selective antagonist of L-type calcium channels, and also by blocking muscarinic receptors with atropine (5 muM) or pirenzepine (1 microM), a more specific M1-receptor blocker. We show that in the medial septal area this oligomeric peptide acts through calcium channels and muscarinic receptors. As blocking any of these pathways blocks the betaA effects, we propose a joint action through both mechanisms. These results may contribute to a better understanding of the pathophysiology at the onset of AD. This understanding will be required for the development of new therapeutic agents.

Nicotinic acetylcholine receptors and nicotinic cholinergic mechanisms of the central nervous system

Subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) are constructed from numerous subunit combinations that compose channel-receptor complexes with varied functional and pharmacological characteristics. Structural and functional diversity and the broad presynaptic, postsynaptic, and nonsynaptic locations of nAChRs underlie their mainly modulatory roles throughout the mammalian brain. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, postsynaptic nAChRs contribute a small minority of fast excitatory transmission, and nonsynaptic nAChRs modulate many neurotransmitter systems by influencing neuronal excitability. Nicotinic receptors have roles in development and synaptic plasticity, and nicotinic mechanisms participate in learning, memory, and attention. Decline, disruption, or alterations of nicotinic cholinergic mechanisms contribute to dysfunctions such as epilepsy, schizophrenia, Parkinson's disease, autism, dementia with Lewy bodies, Alzheimer's disease, and addiction.

High content screen microscopy analysis of A beta 1-42-induced neurite outgrowth reduction in rat primary cortical neurons: neuroprotective effects of alpha 7 neuronal nicotinic acetylcholine receptor ligands

beta-Amyloid peptide 1-42 (A beta(1-42)) is generated from amyloid precursor protein (APP) and associated with neurodegeneration in Alzheimer's disease (AD). A beta(1-42) has been shown to be cytotoxic when incubated with cultured neurons. However, APP transgenic mice over-expressing A beta(1-42) do not show substantial loss of neurons, despite deficits in learning and memory. It is thus emerging that A beta(1-42)-induced memory deficits may involve subtler neuronal alternations leading to synaptic deficits, prior to frank neurodegeneration in AD brains. In this study, high content screen (HCS) microscopy, an advanced high-throughput cellular image processing and analysis technique, was utilized in establishing an in vitro model of A beta(1-42)-induced neurotoxicity utilizing rat neonatal primary cortical cells. Neurite outgrowth was found to be significantly reduced by A beta(1-42) (300 nM to 30 microM), but not by the scrambled control peptide control, in a time- and concentration-dependent manner. In contrast, no reduction in the total number of neurons was observed. The A beta(1-42)-induced reduction of neurite outgrowth was attenuated by the NMDA receptor antagonist memantine and the alpha 7 nicotinic acetylcholine receptor (nAChR) selective agonist PNU-282987. Interestingly, the alpha 7 nAChR antagonist methyllycaconitine also significantly prevented reduction in A beta(1-42)-induced neurite outgrowth. The observed neuroprotective effects could arise either from interference of A beta(1-42) interactions with alpha 7 nAChRs or by modification of receptor-mediated signaling pathways. Our studies demonstrate that reduction of neurite outgrowth may serve as a model representing A beta(1-42)-mediated neuritic and synaptic toxicity, which, in combination of HCS, provides a high-throughput cell-based assay that can be used to evaluate compounds with neuroprotective properties in neurons.

Abeta(1-42) modulation of Akt phosphorylation via alpha7 nAChR and NMDA receptors

Elevated Abeta and its deposition as senile plaques are pathogenic features of Alzheimer's disease. Abeta has been shown to be toxic to neurons and to inhibit long-term potentiation yet, the intracellular signalling pathways underlying these actions are unknown. We report for the first time that acute exposure of primary mouse neurons to 400nM Abeta(1-42) increased Akt phosphorylation in an alpha(7) nicotinic receptor and NMDA receptor dependant manner. However, prolonged incubation resulted in Akt phosphorylation returning to baseline consistent with the action of a physiological regulator. Analysis of an APP transgenic mouse (TAS10) revealed a significant deficit in hippocampal Akt phosphorylation at 13 months. This time point corresponds to the emergence of plaque formation and memory impairments in these mice. The present study suggests that Abeta(1-42) regulates Akt phosphorylation in a complex manner. Acutely, Abeta(1-42) stimulates Akt phosphorylation however, chronic exposure to Abeta in TAS10 mice resulted in a downregulation of Akt phosphorylation consistent with abnormalities in excitatory neurotransmission in these mice and with recent reports of Abeta(1-42) driven internalisation of NMDA receptors.

Abeta peptides can enter the brain through a defective blood-brain barrier and bind selectively to neurons

We have investigated the possibility that soluble, blood-borne amyloid beta (Abeta) peptides can cross a defective blood-brain barrier (BBB) and interact with neurons in the brain. Immunohistochemical analyses revealed extravasated plasma components, including Abeta42 in 19 of 21 AD brains, but in only 3 of 13 age-matched control brains, suggesting that a defective BBB is common in AD. To more directly test whether blood-borne Abeta peptides can cross a defective BBB, we tracked the fate of fluorescein isothiocyanate (FITC)-labeled Abeta42 and Abeta40 introduced via tail vein injection into mice with a BBB rendered permeable by treatment with pertussis toxin. Both Abeta40 and Abeta42 readily crossed the permeabilized BBB and bound selectively to certain neuronal subtypes, but not glial cells. By 48 h post-injection, Abeta42-positive neurons were widespread in the brain. In the cerebral cortex, small fluorescent, Abeta42-positive granules were found in the perinuclear cytoplasm of pyramidal neurons, suggesting that these cells can internalize exogenous Abeta42. An intact BBB (saline-injected controls) blocked entry of blood-borne Abeta peptides into the brain. The neuronal subtype selectivity of Abeta42 and Abeta40 was most evident in mouse brains subjected to direct intracranial stereotaxic injection into the hippocampal region, thereby bypassing the BBB. Abeta40 was found to preferentially bind to a distinct subset of neurons positioned at the inner face of the dentate gyrus, whereas Abeta42 bound selectively to the population of large neurons in the hilus region of the dentate gyrus. Our results suggest that the blood may serve as a major, chronic source of soluble, exogenous Abeta peptides that can bind selectively to certain subtypes of neurons and accumulate within these cells.

Pathologies involving the S100 proteins and RAGE

The S100 proteins are exclusively expressed in vertebrates and are the largest subgroup within the superfamily of EF-hand Ca2(+)-binding proteins Generally, S100 proteins are organized as tight homodimers (some as heterodimers). Each subunit is composed of a C-terminal, 'canonical' EF-hand, common to all EF-hand proteins, and a N-terminal, 'pseudo' EF-hand, characteristic of S100 proteins. Upon Ca2(+)-binding, the C-terminal EF-hand undergoes a large conformational change resulting in the exposure of a hydrophobic surface responsible for target binding A unique feature of this protein family is that some members are secreted from cells upon stimulation, exerting cytokine- and chemokine-like extracellular activities via the Receptor for Advanced Glycation Endproducts, RAGE. Recently, larger assemblies of some S100 proteins (hexamers, tetramers, octamers) have been also observed and are suggested to be the active extracellular species required for receptor binding and activation through receptor multimerization Most S100 genes are located in a gene cluster on human chromosome 1q21, a region frequently rearranged in human cancer The functional diversification of S100 proteins is achieved by their specific cell- and tissue-expression patterns, structural variations, different metal ion binding properties (Ca2+, Zn2+ and Cu2+) as well as their ability to form homo-, hetero- and oligomeric assemblies Here, we review the most recent developments focussing on the biological functions of the S100 proteins and we discuss the presently available S100-specific mouse models and their possible use as human disease models In addition, the S100-RAGE interaction and the activation of various cellular pathways will be discussed. Finally, the close association of S100 proteins with cardiomyopathy, cancer, inflammation and brain diseases is summarized as well as their use in diagnosis and their potential as drug targets to improve therapies in the future.

Modeling behavioral and neuronal symptoms of Alzheimer's disease in mice: A role for intraneuronal amyloid

The amyloid Abeta-peptide (Abeta) is suspected to play a critical role in the cascade leading to AD as the pathogen that causes neuronal and synaptic dysfunction and, eventually, cell death. Therefore, it has been the subject of a huge number of clinical and basic research studies on this disease. Abeta is typically found aggregated in extracellular amyloid plaques that occur in specific brain regions enriched in nAChRs in Alzheimer's disease (AD) and Down syndrome (DS) brains. Advances in the genetics of its familiar and sporadic forms, together with those in gene transfer technology, have provided valuable animal models that complement the traditional cholinergic approaches, although modeling the neuronal and behavioral deficits of AD in these models has been challenging. More recently, emerging evidence indicates that intraneuronal accumulation of Abeta may also contribute to the cascade of neurodegenerative events and strongly suggest that it is an early, pathological biomarker for the onset of AD and associated cognitive and other behavioral deficits. The present review covers these studies in humans, in in vitro and in transgenic models, also providing more evidence that adult 3xTg-AD mice harboring PS1M146V, APPSwe, tauP301L transgenes, and mimicking many critical hallmarks of AD, show cognitive deficits and other behavioral alterations at ages when overt neuropathology is not yet observed, but when intraneuronal Abeta, synaptic and cholinergic deficits can already be described.

Amyloid β-peptide Aβ1–42 but not Aβ1–40 attenuates synaptic AMPA receptor function

The brains of Alzheimer's disease (AD) patients have large numbers of plaques that contain amyloid beta (Abeta) peptides which are believed to play a pivotal role in AD pathology. Several lines of evidence have established the inhibitory role of Abeta peptides on hippocampal memory encoding, a process that relies heavily on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function. In this study the modulatory effects of the two major Abeta peptides, Abeta(1-40) and Abeta(1-42), on synaptic AMPA receptor function was investigated utilizing the whole cell patch clamp technique and analyses of single channel properties of synaptic AMPA receptors. Bath application of Abeta(1-42) but not Abeta(1-40) reduced both the amplitude and frequency of AMPA receptor mediated excitatory postsynaptic currents in hippocampal CA1 pyramidal neurons by approximately 60% and approximately 45%, respectively, in hippocampal CA1 pyramidal neurons. Furthermore, experiments with single synaptic AMPA receptors reconstituted in artificial lipid bilayers showed that Abeta(1-42) reduced the channel open probability by approximately 42% and channel open time by approximately 65% and increased the close times by several fold. Abeta(1-40), however, did not show such inhibitory effects on single channel properties. Application of the reverse sequence peptide Abeta(42-1) also did not alter the mEPSC or single channel properties. These results suggest that Abeta(1-42) but not Abeta(1-40) closely interacts with and exhibits inhibitory effects on synaptic AMPA receptors and may contribute to the memory impairment observed in AD.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.

Protein oxidation and lipid peroxidation in brain of subjects with Alzheimer's disease: insights into mechanism of neurodegeneration from redox proteomics

Alzheimer's disease (AD), the leading cause of dementia, involves regionalized neuronal death, synaptic loss, and an accumulation of intraneuronal, neurofibrillary tangles and extracellular senile plaques. Although the initiating causes leading to AD are unknown, a number of previous studies reported the role of oxidative stress in AD brain. Postmortem analysis of AD brain showed elevated markers of oxidative stress including protein nitrotyrosine, carbonyls in proteins, lipid oxidation products, and oxidized DNA bases. In this review, we focus our attention on the role of protein oxidation and lipid peroxidation in the pathogenesis of AD. Particular attention is given to the current knowledge about the redox proteomics identification of oxidatively modified proteins in AD brain.

Synthesis, nicotinic acetylcholine receptor binding, antinociceptive and seizure properties of methyllycaconitine analogs

A series of methyllycaconitine (1a, MLA) analogs was synthesized where the (S)-2-methylsuccinimidobenzoyl group in MLA was replaced with a (R)-2-methyl, 2,2-dimethyl-, 2,3-dimethyl, 2-phenyl-, and 2-cyclohexylsuccinimidobenzoyl (1b-f) group. The analogs 1b-f were evaluated for their inhibition of [(125)I]iodo-MLA binding at rat brain alpha7 nicotinic acetylcholine receptors (nAChR). In order to determine selectivity, MLA and the analogs 1b-f were evaluated for inhibition of binding to rat brain alpha,beta nAChR using [(3)H]epibatidine. At the alpha7 nAChR, MLA showed a K(i) value of 0.87 nM, analogs 1b-e possessed K(i) values of 1.67-2.16 nM, and 1f showed a K(i) value of 26.8 nM. Surprisingly, the analog 1e containing the large phenyl substituent (K(i)=1.67 nM) possessed the highest affinity. None of the compounds possessed appreciable affinity for alpha,beta nAChRs. MLA antagonized nicotine-induced seizures with an AD(50)=2 mg/kg. None of the MLA analogs were as potent as MLA in this assay. MLA and all of the MLA analogs, with the exception of 1b, antagonized nicotine's antinociceptive effects in the tail-flick assay. Compound 1c (K(i)=1.78 nM at alpha7 nAChR) with an AD(50) value of 1.8 mg/kg was 6.7 times more potent than MLA (AD(50)=12 mg/kg) in antagonizing nicotine's antinociceptive effects but was 5-fold less potent than MLA in blocking nicotine-induced seizures. Since MLA has been reported to show neuroprotection against beta-amyloid(1-42), these new analogs which have high alpha7 nAChR affinity and good selectivity relative to alpha,beta nAChRs will be useful biological tools for studying the effects of alpha7 nAChR antagonist and neuroprotection.

The significance of the cholinergic system in the brain during aging and in Alzheimer's disease

Acetylcholine is widely distributed in the nervous system and has been implicated to play a critical role in cerebral cortical development, cortical activity, controlling cerebral blood flow and sleep-wake cycle as well as in modulating cognitive performances and learning and memory processes. Cholinergic neurons of the basal forebrain complex have been described to undergo moderate degenerative changes during aging, resulting in cholinergic hypofunction that has been related to the progressing memory deficits with aging. Basal forebrain cholinergic cell loss is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed, and has led to the formulation of the cholinergic hypotheses of geriatric memory dysfunction. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology and increase phosphorylation of tau protein the main component of neurofibrillar tangles in Alzheimer's disease. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, and tau pathology in Alzheimer's disease, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Neuronal nicotinic acetylcholine receptors serve as sensitive targets that mediate beta-amyloid neurotoxicity

Alzheimer's disease (AD) is the most common form of brain dementia characterized by the accumulation of beta-amyloid peptides (Abeta) and loss of forebrain cholinergic neurons. Abeta accumulation and aggregation are thought to contribute to cholinergic neuronal degeneration, in turn causing learning and memory deficits, but the specific targets that mediate Abeta neurotoxicity remain elusive. Recently, accumulating lines of evidence have demonstrated that Abeta directly modulates the function of neuronal nicotinic acetylcholine receptors (nAChRs), which leads to the new hypothesis that neuronal nAChRs may serve as important targets that mediate Abeta neurotoxicity. In this review, we summarize current studies performed in our laboratory and in others to address the question of how Abeta modulates neuronal nAChRs, especially nAChR subunit function.

The induction of surface β-amyloid binding proteins and enhanced cytotoxicity in cultured PC-12 and IMR-32 cells by advanced glycation end products

During aging the non-enzymatic glycation of proteins and other molecules increases significantly, leading to the accumulation of advanced glycation end-products (AGEs). These AGEs enhance inflammatory and autoimmune reactions with resultant cytotoxicity. We noted in an earlier study that individuals with Alzheimer's disease exhibit enhanced expression of the receptor for advanced glycation end-products (RAGE) on the surface of their leukocytes. In order to better understand the relationship between AGEs and the cell surface binding of amyloid-beta protein (Abeta) 42 we studied the effect of two AGEs: glycated bovine serum albumin (BSA), and epsilon-carboxymethyllysine-BSA (CML), a glycoxidation product, on the binding of Abeta42 to rat PC-12 and IMR-32 cells. We measured the expression of three potential cell surface receptors binding Abeta42: RAGE, beta-amyloid precursor protein (beta-APP), and the alpha7 subtype of the nicotinic acetylcholine receptor (alpha7nAChR) by using specific antibody probes. Incubation of PC-12 or IMR-32 cells with bovine serum albumin-advanced glycation end-product (BSA-AGE) or with CML induced small but significant concentration-dependent increases in the expression of beta-APP, RAGE, and alpha7nAChRs as measured by flow cytometry or by ELISA. Incubation of the cells with 48 microM of either of the AGEs combined with varying concentrations (138-1100 nM) of Abeta42 resulted in the enhanced binding of the Abeta42 to the cell surface as compared with cells not exposed to the AGE co-treatment. The combination of AGE and Abeta treatment also resulted in the heightened expression of all three potential Abeta binding sites as well as their gene precursors. Exposure of cells to the same regimen of AGE plus Abeta resulted in the production of reactive oxygen species and mitochondrial toxicity. These results are consistent with the ability of AGEs to enhance the cell surface expression of diverse Abeta42 binding sites, a factor that can lead to the enhanced binding of amyloid and subsequent cell death.

Partial agonists as therapeutic agents at neuronal nicotinic acetylcholine receptors

Improved understanding of how brain function is altered in neurodegenerative disease states, pain and conditions, such as schizophrenia and attention deficit disorder, has highlighted the role of nicotinic acetylcholine receptors (nAChRs) in these conditions and identified them as promising therapeutic targets. nAChRs are widely expressed throughout the peripheral and central nervous system, and this widespread nature underlines the need for new ligands with different selectivities and pharmacological profiles if we are to avoid the adverse side effects associated with many of the nAChR modulators currently identified. Partial agonists have the unique property of being able to act both as agonists or antagonists depending on the concentration of endogenous neurotransmitter. Moreover, the agonist action of partial agonists has a 'ceiling' effect, giving them a large safety margin and making them an attractive proposition for therapeutic molecules. Partial agonists of nAChRs are currently being developed as a nicotine replacement therapy for smoking cessation and for the treatment of a number of neurological diseases associated with a loss of cholinergic function. This commentary will discuss the pharmacological properties of partial agonists and review recent research developments in the field of partial agonists acting at nicotinic receptors.

Long-term exposure to the new nicotinic antagonist 1,2-bisN-cytisinylethane upregulates nicotinic receptor subtypes of SH-SY5Y human neuroblastoma cells

Nicotinic drug treatment can affect the expression of neuronal nicotinic acetylcholine receptors (nAChR) both in vivo and in vitro through molecular mechanisms not fully understood. The present study investigated the effect of the novel cytisine dimer 1,2-bisN-cytisinylethane (CC4) on nAChR natively expressed by SH-SY5Y neuroblastoma cells in culture. CC4 lacked the agonist properties of cytisine and was a potent antagonist (IC50=220 nM) on nAChRs. Chronic treatment of SH-SY5Y cells with 1 mM CC4 for 48 h increased the expression of 3H-epibatidine (3H-Epi; 3-4-fold) or 125I-alpha-bungarotoxin (125I-alphaBgtx; 1.2-fold) sensitive receptors present on the cell membrane and in the intracellular pool. Comparable data were obtained with nicotine or cytisine, but not with carbamylcholine, d-tubocurarine, di-hydro-beta-erythroidine or hexametonium. Immunoprecipitation and immunopurification studies showed that the increase in 3H-Epi-binding receptors was due to the enhanced expression of alpha3beta2 and alpha3beta2beta4 subtypes without changes in subunit mRNA transcription or receptor half-life. The upregulation was not dependent on agonist/antagonist properties of the drugs, and did not concern muscarinic or serotonin receptors. Whole-cell patch clamp analysis of CC4-treated cells demonstrated larger nicotine-evoked inward currents with augmented sensitivity to the blockers alpha-conotoxin MII or methyllycaconitine. In conclusion, chronic treatment with CC4 increased the number of nAChRs containing beta2 and alpha7 subunits on the plasma membrane, where they were functionally active. In the case of beta2-containing receptors, we propose that CC4, by binding to intracellular receptors, triggered a conformational reorganisation of intracellular subunits that stimulated preferential assembly and membrane-directed trafficking of beta2-containing receptor subtypes..

Failure of nicotine-dependent enhancement of synaptic efficacy at Schaffer-collateral CA1 synapses of AD11 anti-nerve growth factor transgenic mice

Alzheimer's disease is a neurodegenerative disorder characterized by neuronal loss associated with a progressive impairment of cognitive functions. Early consequences of Alzheimer's disease include deficit of cholinergic signalling in particular regions controlling memory processes, such as the cortex and hippocampus, and accumulation of beta-amyloid (Abeta) peptide in neuritic plaques. The cholinergic system depends for its integrity and function on nerve growth factor. Chronic nerve growth factor deprivation in transgenic mice (AD11) engineered to produce recombinant neutralizing anti-nerve growth factor antibodies leads to progressive age-dependent Alzheimer's-like neurodegenerative pathology similar to that found in patients with Alzheimer's disease, associated with a selective loss of cholinergic neurones in the basal forebrain. Here we show that in the hippocampus of 6-month-old AD11 mice, Abeta aggregates started appearing in the CA1 region. The accumulation of Abeta was associated with a loss of cholinergic function at CA3-CA1 synapses. Whereas in wild-type mice nicotine induced a persistent increase of synaptic efficacy via alpha7 nicotine acetylcholine receptors, in AD11 mice this alkaloid failed to modify synaptic strength. Moreover, nicotine failed to transiently enhance the frequency of spontaneous miniature glutamatergic currents (miniature excitatory postsynaptic currents) recorded from CA1 but not from CA3 pyramidal neurones of AD11 mice. However, in CA3 principal cells of AD11 mice, the potentiating effect of nicotine on miniature excitatory postsynaptic currents was prevented when Abeta peptide 1-42 was added to the extracellular solution. These data suggest that in AD11 mice, Abeta interferes with nicotine acetylcholine receptors at the level of presynaptic glutamatergic terminals, inhibiting their function possibly through calcium signalling via presynaptic alpha7 nicotine acetylcholine receptors.

Early diagnostics and therapeutics for Alzheimer's disease--how early can we get there?

Alzheimer's disease (AD) is a major threat for the rapidly aging world population. AD is the leading cause of dementia and a major cause of death in developed countries. The disease puts a tremendous practical, emotional and financial burden on individuals and governments. Clinicians and researchers in the AD field face great challenges: the pathophysiological processes that cause AD are not well understood, definite diagnosis of AD requires autopsy, and therapeutic options are limited to treating the symptoms rather than the cause of the disease. Nevertheless, new insights into the earliest events that lead to development of AD increase hope that reliable diagnostics and efficacious therapies may emerge.

Neuroprotective effects of galanthamine and tacrine against glutamate neurotoxicity

We examined the mechanisms of the neuroprotective effects of two central-type acetylcholinesterase inhibitors, galanthamine and tacrine, on nitric oxide-mediated glutamate neurotoxicity using primary cultures from the cerebral cortex of fetal rats. Galanthamine and tacrine showed prominent protective effects against glutamate neurotoxicity. Mecamylamine, a nicotinic acetylcholine receptor antagonist, but not scopolamine, a muscarinic acetylcholine receptor antagonist, inhibited the protective effects of these inhibitors on glutamate neurotoxicity. Furthermore, dihydro-beta-erythroidine, an alpha4-nicotinic receptor antagonist, and methyllycaconitine, an alpha7-nicotinic receptor antagonist, inhibited the neuroprotective effects of galanthamine but not tacrine. Next, we investigated the site of action where galanthamine and tacrine prevent glutamate neurotoxicity. Both these acetylcholinesterase inhibitors prevented glutamate- and ionomycin-induced neurotoxicity, but only tacrine prevented S-nitrosocysteine-induced neurotoxicity. These results suggest that galanthamine and tacrine protect cortical neurons from glutamate neurotoxicity via different mechanisms.

Synapse formation and function is modulated by the amyloid precursor protein

The amyloid precursor protein (APP) is critical in the pathogenesis of Alzheimer's disease. The question of its normal biological function in neurons, in which it is predominantly located at synapses, is still unclear. Using autaptic cultures of hippocampal neurons, we demonstrate that hippocampal neurons lacking APP show significantly enhanced amplitudes of evoked AMPA- and NMDA-receptor-mediated EPSCs. The size of the readily releasable synaptic vesicle pool was also increased in neurons lacking APP, whereas the release probability was not affected. In addition, the analysis of spontaneous miniature synaptic currents revealed an augmented frequency in neurons lacking APP, whereas the amplitude of miniature synaptic currents was not found to be altered. Together, these findings strongly indicate that lack of APP increases the number of functional synapses. This hypothesis is further supported by morphometric immunohistochemical analysis revealing an increase of synaptophysin-positive puncta per cultured APP knock-out neuron. In conclusion, lack of APP affects synapse formation and transmission in cultured hippocampal neurons.

Extremely low-frequency electromagnetic field (ELF-EMF) does not affect the expression of α3, α5 and α7 nicotinic receptor subunit genes in SH-SY5Y neuroblastoma cell line

Neuronal nicotinic acetylcholine receptors (nAChRs) are involved in a number of functional processes, including cognition, learning and memory, and alterations in their expression and/or activity have been implicated in various neurological disorders such as Alzheimer's disease (AD), Parkinson's disease and schizophrenia. Epidemiological studies have shown that exposure to electromagnetic fields (EMF) may contribute to the pathogenesis of neurodegenerative diseases such as Alzheimer's disease. Given the role of nAChRs in physiological and pathological conditions, we wondered whether an extremely low-frequency electromagnetic field (ELF-EMF) may affect the expression of the molecules involved in neurodegenerative processes. In order to investigate this possibility, we studied the expression of alpha3, alpha5 and alpha7 nicotinic subunits upon exposure of the SH-SY5Y human neuroblastoma cell line to a 50 Hz power-line magnetic field in a "blind trial" system; various magnetic flux densities and exposure times were applied. Our studies show that the expression of some relevant components of the cholinergic nicotinic system, which is one of the most affected neurotransmission systems in AD, did not undergo any change at molecular level by environmental exposure to ELF-EMF.

Fully flexible docking models of the complex between α7 nicotinic receptor and a potent heptapeptide inhibitor of the β-amyloid peptide binding

The heptapeptide IQTTWSR (IQ), recently reported as inhibitor of the beta-amyloid (Abeta) binding to nicotinic acetylcholine receptors (nAChrs), was docked to the homology model of the alpha7 nicotinic acetylcholine receptor. The most representative models were further subjected to molecular dynamics simulations. The data obtained here suggest that Abeta needs highly specific structural motifs to bind to the alpha7nAChR. These structural motifs are located principally in the upper and lower surroundings of loop C, including loop F and sheets beta1, beta2, beta6, beta9, and beta10 of the receptor. Overall, these results suggest that IQ can be mimicked by more bioavailable, stable compounds that would be helpful for the understanding of the Abeta binding site and its dynamics, and for the design of novel agents to be used as an effective alternative against Alzheimer's disease.

Acetylcholinesterase inhibitors used in treatment of Alzheimer's disease prevent glutamate neurotoxicity via nicotinic acetylcholine receptors and phosphatidylinositol 3-kinase cascade

We show here that donepezil, galanathamine and tacrine, therapeutic acetylcholinesterase inhibitors currently being used for treatment of Alzheimer's disease, protect neuronal cells in a time- and concentration-dependent manner from glutamate neurotoxicity that involves apoptosis. The neuroprotective effects were antagonized by mecamylamine, an inhibitor of nicotinic acetylcholine receptors (nAChRs). Dihydro-beta-erythroidine and methyllycaconitine, antagonists for alpha4-nAChR and alpha7-nAChR, respectively, antagonized the protective effect of donepezil and galanthamine, but not that of tacrine. Previous reports suggest the involvement of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway in the nicotine-induced neuroprotection. Inhibitors for a non-receptor type tyrosine kinase, Fyn, and janus-activated kinase 2, suppressed the neuroprotective effect of donepezil and galanthamine, but not that of tacrine. Furthermore, LY294002, a PI3K inhibitor, also suppressed the neuroprotective effect of donepezil and galanthamine, but not that of tacrine. The phosphorylation of Akt, an effector of PI3K, and the expression level of Bcl-2, an anti-apoptotic protein, increased with donepezil and galanthamine treatment, but not with tacrine treatment. These results suggest that donepezil and galanthamine prevent glutamate neurotoxicity through alpha4- and alpha7-nAChRs, followed by the PI3K-Akt pathway, and that tacrine protects neuronal cells through a different pathway.

Cerebral ischemia combined with β-amyloid impairs spatial memory in the eight-arm radial maze task in rats

beta-Amyloid (Abeta), a major component of senile plaques in Alzheimer's disease, has been implicated in neuronal cell death, a characteristic feature of this condition. In our previous experiments using primary cultures of hippocampal neurons, Abeta treatment induced neuronal cell death, displaying morphological characteristics of apoptosis that was significantly enhanced by hypoxia. Based on these results, we developed a simple in vivo rat model of Alzheimer's disease using cerebral ischemia, instead of hypoxia, combined with continuous intracerebroventricular administration of Abeta. The combination of cerebral ischemia and Abeta administration, but not either treatment alone, significantly impaired spatial memory in an eight-arm radial maze. A microdialysis study showed that spontaneous release of acetylcholine (ACh) from the dorsal hippocampus had a tendency to decrease in response to Abeta treatment alone or the combination of ischemia and Abeta. High K(+)-evoked increase in ACh release had a tendency to be inhibited by either ischemia or Abeta treatment alone and was significantly inhibited by the combination of both. Moreover, combination of ischemia and Abeta induced apoptosis of pyramidal neurons in the CA1 region of the hippocampus. Donepezil, a drug currently in clinical use for Alzheimer's disease, improved the impairment of spatial memory induced by cerebral ischemia combined with Abeta. These findings suggest that ischemia is an important factor facilitating the symptoms of Alzheimer's disease, and this model may be useful for developing new drugs for the treatment of Alzheimer's disease.

Excitation of rat hippocampal interneurons via modulation of endogenous agonist activity at the alpha7 nicotinic ACh receptor

The alpha7 subtype of the nicotinic acetylcholine receptor (alpha7 nAChR) is prominently expressed in the hippocampus where it is thought to play a role in the regulation of cognitive function. In this study, we have investigated the effects of 5-hydroxyindole (5-HI), a positive modulator of the alpha7 nAChR, on GABAergic activity in hippocampal CA1 stratum radiatum interneurons in acute rat brain slices. Superfusion of 5-HI (100 microM) increased the mean frequency and amplitude of spontaneous IPSCs (sIPSCs). The potentiation was occluded by pretreatment of slices with: (1) a high concentration of the broad-spectrum agonist nicotine to desensitize the alpha7 receptor, (2) an alpha7 nAChR antagonist, and (3) tetrodotoxin to block action potential firing. These results indicate that facilitation by 5-HI was mediated by the alpha7 nAChR and required neuronal excitation. In contrast, 5-HI had no effect on sIPSCs recorded in hippocampal slices from younger animals, even though the expression of functional alpha7 nAChRs was confirmed by agonist application experiments. In these slices, 5-HI only enhanced sIPSCs after pretreatment with the acetylcholinesterase inhibitor Bw284c51. Taken together, our results suggest that 5-HI facilitates GABAergic transmission via excitation of the alpha7 nAChR, and that this effect requires the presence of the endogenous agonist ACh in the extracellular environment of the receptor.

Novel neuroprotective effects of the aqueous extracts from Verbena officinalis Linn

Verbena officinialis Linn. (Verbenaceae) is a perennial plant which has been used as herbal medicine or health supplement in both Western and Eastern countries for centuries. It has been used to treat acute dysentery, enteritis, amenorrhea and depression. In view of its wide array of biological effects, we hypothesized that V. officinalis can exert cytoprotective effects on cells of the central nervous system. Pre-treatment of aqueous extracts of V. officinalis significantly attenuated the toxicity of beta-amyloid (Abeta) peptide and reducing agent dithiothreitol in primary cultures of cortical neurons. As extracellular accumulation of Abeta peptide is an important cytotoxic factor involved in Alzheimer's disease (AD), we have further explored its neuroprotective effect against Abeta. Treatment of V. officinalis attenuated Abeta-triggered DEVD- and VDVAD-cleavage activities in a dose-dependent manner. Further studies elucidated that phosphorylation of both interferon-inducing protein kinase (PKR) and c-Jun N-terminal kinase (JNK) was attenuated in Abeta-treated neurons. Taken together, we have proved our hypothesis by showing the novel neuroprotective effects of V. officinalis. As V. officinalis has long been used for many years to be a folk medicine, our study may provide a lead for its potential to be a neuroprotective agent against neuronal loss in AD.

Synthesis and evaluation of [125I]I-TSA as a brain nicotinic acetylcholine receptor α7 subtype imaging agent

INTRODUCTION

Some in vitro investigations have suggested that the nicotinic acetylcholine receptor (nAChR) alpha7 subtype is implicated in Alzheimer's disease, schizophrenia and others. Recently, we developed (R)-3'-(5-bromothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one (Br-TSA), which has a high affinity and selectivity for alpha7 nAChRs. Therefore we synthesized (R)-3'-(5-[125I]iodothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one ([125I]I-TSA) and evaluated its potential for the in vivo detection of alpha7 nAChR in brain.

METHODS

In vitro binding affinity of I-TSA was measured in rat brain homogenates. Radioiodination was accomplished by a Br-I exchange reaction. Biodistribution studies were undertaken in mice by tail vein injection of [(125)I]I-TSA. In vivo receptor blocking studies were carried out by treating mice with methyllycaconitine (MLA; 5 nmol/5 mul, i.c.v.) or nonradioactive I-TSA (50 micromol/kg, i.v.).

RESULTS

I-TSA exhibited a high affinity and selectivity for the alpha7 nAChR (K(i) for alpha7 nAChR = 0.54 nM). Initial uptake in the brain was high (4.42 %dose/g at 5 min), and the clearance of radioactivity was relatively slow in the hippocampus (alpha7 nAChR-rich region) and was rather rapid in the cerebellum (alpha7 nAChR poor region). The hippocampus to cerebellum uptake ratio was 0.9 at 5 min postinjection, but it was increased to 1.8 at 60 min postinjection. Although the effect was not statistically significant, administration of I-TSA and MLA decreased the accumulation of radioactivity in hippocampus.

CONCLUSION

Despite its high affinity and selectivity, [125I]I-TSA does not appear to be a suitable tracer for in vivo alpha7 nAChR receptor imaging studies due to its high nonspecific binding. Further structural optimization is needed.

Identification of amyloid-beta 1–42 binding protein fragments by screening of a human brain cDNA library

Extracellular and intraneuronal formation of amyloid-beta (Abeta) deposits have been demonstrated to be involved in the pathogenesis of Alzheimer's disease (AD). However, the precise mechanism of Abeta neurotoxicity is not completely understood. Previous studies suggest that binding of Abeta with a number of targets have deleterious effects on cellular functions. It has been shown that Abeta directly interacted with intracellular protein ERAB (endoplasmic reticulum amyloid beta-peptide-binding protein) also known as ABAD (Abeta-binding alcohol dehydrogenase) resulting in mitochondrial dysfunction and cell death. In the present study we have identified another mitochondrial enzyme, ND3 of the human complex I, that binds to Abeta1-42 by the screening of a human brain cDNA library expressed on M13 phage. Our results indicated a strong interaction between Abeta and a phage-displayed 25 amino acid long peptide TTNLPLMVMSSLLLIIILALSLAYE corresponding to C-terminal peptide domain of NADH dehydrogenase, subunit 3 (MTND3) encoded by mitochondrial DNA (mtDNA). This interaction may explain, in part, the inhibition of complex I activity in astrocytes and neurons in the presence of Abeta, described recently. To our knowledge, the present study is the first demonstration of interaction between Abeta and one of the subunits of the human complex I.

Effect of subchronic treatment of memantine, galantamine, and nicotine in the brain of Tg2576 (APPswe) transgenic mice

An increasing number of studies suggest that the present clinical therapy used in Alzheimer's disease (AD), in addition to having a symptomatic effect, also may interact with the ongoing neuropathological processes in the brain. The aim of this study was to investigate the effect of the cholinesterase inhibitor galantamine and the N-methyl-d-aspartate (NMDA) antagonist memantine in comparison to nicotine on the neuropathology of Tg2576 transgenic mice (APPswe). Nontransgenic and APPswe mice at 10 months of age were treated subcutaneously with saline, memantine, galantamine, or nicotine for 10 days. Nicotine reduced the guanidinium-soluble amyloid-beta peptide (Abeta) levels by 46 to 66%, whereas the intracellular Abeta levels remained unchanged. Treatment with nicotine also resulted in less glial fibrillary acidic protein immunoreactive astrocytes around the plaques, increased levels of synaptophysin, and increased number of alpha7 nicotinic acetylcholine receptors (nAChRs) in the cortex of APPswe transgenic mice. Galantamine treatment caused an increase in the cortical levels of synaptophysin in the APPswe mice. Memantine treatment reduced the total cortical levels of membrane-bound amyloid precursor protein (45-55%) in both transgenic and nontransgenic mice, which eventually may decrease the level of Abeta. In conclusion, galantamine, memantine, and nicotine have different interactions with Abeta processes, alpha7 nAChRs, and NMDA receptors in APPswe mice. These different effects might have therapeutic relevance, and this knowledge might be applicable to the development of new effective therapeutic strategies for AD.

Expression of the α7, α4 and α3 nicotinic receptor subtype in the brain and adrenal medulla of transgenic mice carrying genes coding for human AChE and β‐amyloid

Human AChE-enzyme (hAChE) enhances the over-expression of beta-amyloid (Abeta) containing plaques in the brain of transgenic mice (APP(SWE)/hAChE-Tg) carrying mutated genes for human amyloid precursor protein (APP(SWE)) and hAChE. In this study, we showed that interaction of hAChE with Abeta affects the plasticity of the alpha7 nicotinic acetylcholine receptors (nAChRs) both in the brain and adrenal medulla. An age-related increase in the (125)I-alphabungarotoxin ((125)I-alphaBTX) binding (specific to alpha7 nAChRs) was observed in the adrenal medulla of 3, 7 and 10 months old control mice. In contrast, a significant decrease in (125)I-alphaBTX binding was detected in the adrenal medulla of 10 months old APP(SWE)/hAChE-Tg. A significantly higher alpha7 nAChR mRNA level was observed in the brain of APP(SWE)/hAChE-Tg at 3 and 7 months of age and in the adrenal medulla at 3 and 10 months of age compared to those of the control mice. The alpha3 nAChR mRNA level was significantly higher in the brain of APP(SWE)/hAChE-Tg at 3 months of age and in the adrenal medulla at 10 months of age. The alpha4 nAChR mRNA level remained unchanged in the brain and adrenal medulla of APP(SWE)/hAChE-Tg for all age groups. Based on these observations, we conclude that a high load of Abeta and an over-expression of hAChE induce differences in the expression of the nAChR subtypes at various ages in the brain and in the adrenal medulla of hAChE/APP(SWE)Tg mice. The findings may have implications for a better understanding the underlying mechanism for AD-related pathogenesis.

The role of nicotinic acetylcholine receptors in Alzheimer’s disease

The two hallmark lesions of Alzheimer's disease (AD) are extracellular amyloid plaques, mainly formed by a small peptide called amyloid-beta (Abeta), and neurofibrillary tangles, which are intracellular inclusions formed by aggregates of hyperphosphorylated tau protein. One of the major neurochemical features of AD is the marked reduction of nicotinic acetylcholine receptors in disease-relevant brain regions such as the cerebral cortex and hippocampus. This loss is further compounded by the loss of cholinergic cells, which contributes to the cognitive dysfunction. This observation has had a major impact on therapeutic treatments, as efforts to restore cholinergic function such as the administration of acetylcholinesterase inhibitors have been, until recently, the major treatment options available for AD. Understanding the relationship of these hallmark lesions with the plethora of other changes that occur in the AD brain has proven to be a difficult challenge to resolve. The utilization of transgenic mouse models, that recapitulate one or more neuropathological and neurochemical features of the AD brain is providing some inroads, as they offer a means to gain mechanistic insights into the disease process in an in vivo setting. In this review, we consider the role of nicotinic acetylcholine receptors in transgenic models and in AD.

Cholinergic dysfunction in a mouse model of Alzheimer disease is reversed by an anti-A beta antibody

Disruption of cholinergic neurotransmission contributes to the memory impairment that characterizes Alzheimer disease (AD). Since the amyloid cascade hypothesis of AD pathogenesis postulates that amyloid beta (A beta) peptide accumulation in critical brain regions also contributes to memory impairment, we assessed cholinergic function in transgenic mice where the human A beta peptide is overexpressed. We first measured hippocampal acetylcholine (ACh) release in young, freely moving PDAPP mice, a well-characterized transgenic mouse model of AD, and found marked A beta-dependent alterations in both basal and evoked ACh release compared with WT controls. We also found that A beta could directly interact with the high-affinity choline transporter which may impair steady-state and on-demand ACh release. Treatment of PDAPP mice with the anti-A beta antibody m266 rapidly and completely restored hippocampal ACh release and high-affinity choline uptake while greatly reducing impaired habituation learning that is characteristic of these mice. Thus, soluble "cholinotoxic" species of the A beta peptide can directly impair cholinergic neurotransmission in PDAPP mice leading to memory impairment in the absence of overt neurodegeneration. Treatment with certain anti-A beta antibodies may therefore rapidly reverse this cholinergic dysfunction and relieve memory deficits associated with early AD.

Alzheimer precursor protein interaction with the Nogo-66 receptor reduces amyloid-beta plaque deposition

Pathophysiologic hypotheses for Alzheimer's disease (AD) are centered on the role of the amyloid plaque Abeta peptide and the mechanism of its derivation from the amyloid precursor protein (APP). As part of the disease process, an aberrant axonal sprouting response is known to occur near Abeta deposits. A Nogo to Nogo-66 receptor (NgR) pathway contributes to determining the ability of adult CNS axons to extend after traumatic injuries. Here, we consider the potential role of NgR mechanisms in AD. Both Nogo and NgR are mislocalized in AD brain samples. APP physically associates with the NgR. Overexpression of NgR decreases Abeta production in neuroblastoma culture, and targeted disruption of NgR expression increases transgenic mouse brain Abeta levels, Abeta plaque deposition, and dystrophic neurites. Infusion of a soluble NgR fragment reduces Abeta levels, amyloid plaque deposits, and dystrophic neurites in a mouse transgenic AD model. Changes in NgR level produce parallel changes in secreted APPalpha and Abeta, implicating NgR as a blocker of secretase processing of APP. The NgR provides a novel site for modifying the course of AD and highlights the role of axonal dysfunction in the disease.

α7 Nicotinic acetylcholine receptor as a target to rescue deficit in hippocampal LTP induction in β-amyloid infused rats

Continuous intracerebroventricular infusion of beta-amyloid peptide 1-40 (Abeta(1-40)) in animal models induces learning and memory impairment associated with dysfunction of the cholinergic neuronal system, which has been considered to be a pathological model of Alzheimer's disease [Nitta, A., Itoh, A., Hasegawa, T., Nabeshima, T., 1994. Beta-amyloid protein-induced Alzheimer's disease animal model. Neurosci. Lett. 170, 63-66.]. Here, using a real-time optical recording technique, we demonstrate that basal synaptic transmission and several forms of synaptic plasticity, including long-term potentiation (LTP), post-tetanic potentiation (PTP) and paired-pulse facilitation (PPF) are deficient at the Schaffer collateral-CA1 synapse in hippocampal slices from Abeta-infused brain. Throughout this study, an effort was made to address whether the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), which is believed to be a primary target of Abeta [Wang, H.Y., Lee, D.H., Davis, C.B., Shank, R.P., 2000a. Amyloid peptide Abeta (1-42) binds selectively and with picomolar affinity to alpha 7 nicotinic acetylcholine receptors. J. Neurochem. 75, 1155-1161.], is responsible for the deficits in synaptic plasticity observed in the Abeta-infused rats. First, we found that Abeta-infusion markedly depressed the response of alpha7nAChR to a selective alpha7nAChR agonist [3-(2,4-dimethoxybenzylidene)-anabaseine] (DMXB). Second, blockade of alpha7nAChR with either methyllycaconitine (MLA) or alpha-bungarotoxin (alpha-BTX) in control rats inhibited LTP induction, suggesting that the activation of alpha7nAChR is required for LTP induction. Finally, pre-treatment of the slices from Abeta-infused rats with 10 microM DMXB rescued CA1 synapses from the deficit in LTP and PPF. These results suggest that Abeta-impaired LTP and PPF arise as a consequence of dysfunctional alpha7nAChR, and that alpha7nAChR may be an important target to help ameliorate AD patient cognitive deficits.

Basal forebrain cholinergic dysfunction in Alzheimer's disease--interrelationship with beta-amyloid, inflammation and neurotrophin signaling

Alzheimer's disease, the most common neurodegenerative disorder of senile dementia, is characterized by two major morpho-pathological hallmarks. Deposition of extracellular neuritic, beta-amyloid peptide-containing plaques (senile plaques) in cerebral cortical regions of Alzheimer patients is accompanied by the presence of intracellular neurofibrillary tangles in cerebral pyramidal neurons. Basal forebrain cholinergic dysfunction is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed in patients with Alzheimer's disease. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology in Alzheimer's disease by affecting expression and processing of the beta-amyloid precursor protein (APP). Vice versa, low level of soluble beta-amyloid has been observed to inhibit cholinergic synaptic function. Deposition of beta-amyloid plaques in Alzheimer's disease is also accompanied by a significant plaque-associated glial up-regulation of interleukin-1, which has been attributed to affect expression and metabolism of APP and to interfere with cholinergic transmission. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, as well as local inflammatory upregulation, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Statins prevent beta-amyloid inhibition of sympathetic alpha7-nAChR-mediated nitrergic neurogenic dilation in porcine basilar arteries

The exact mechanism underlying regional cerebral hypoperfusion in the early phase of Alzheimer's disease (AD) is not understood. We have shown in isolated porcine cerebral arteries that stimulation of sympathetic alpha7-nicotinic acetylcholine receptors (alpha7-nAChRs) causes release of nitric oxide in parasympathetic nitrergic nerves and vasodilation. We therefore examined if beta-amyloid peptides (Abetas), which play a key role in pathogenesis of AD, blocked sympathetic alpha7-nAChRs leading to reduced neurogenic nitrergic dilation in isolated porcine basilar arteries, using in vitro tissue bath, calcium image, and patch clamping techniques. The results indicated that Abeta(1-40), but not Abeta(40-1), blocked relaxation of endothelium-denuded basilar arterial rings induced by nicotine (100 micromol/L) and choline (1 mmol/L) without affecting that induced by sodium nitroprusside or isoproterenol. In cultured superior cervical ganglion (SCG) cells, Abeta(1-40), but not Abeta(40-1), blocked choline- and nicotine-induced calcium influx and inward currents. The Abeta blockade of the nitrergic vasodilation and inward currents, but not that of calcium influx, was prevented by acute pretreatment with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors mevastatin and lovastatin. These results suggest that Abeta(1-40) blocks cerebral perivascular sympathetic alpha7-nAChRs, resulting in the attenuation of cerebral nitrergic neurogenic vasodilation. This effect of Abeta may be responsible in part for cerebral hypoperfusion occurred in the early phase of the AD, which may be prevented by statins most likely because of their effects independent of cholesterol lowering. Statins may offer an alternative strategy in the prevention and treatment of AD.

Transgenic mice over-expressing human beta-amyloid have functional nicotinic alpha 7 receptors

A potentially major factor in the development of Alzheimer's disease is the enhanced production of soluble beta-amyloid peptide fragments amyloid beta peptide(1-40) and amyloid beta peptide(1-42). These amyloid peptides are generated by cleavage of the amyloid-precursor protein and aggregate spontaneously to form amyloid plaques, which are a classical pathological hallmark in Alzheimer's disease. Although the precise mechanisms are unknown, it is widely believed that amyloid peptides initiate the degenerative process, resulting in subsequent cognitive decline. One interaction of amyloid beta peptide that may contribute to an impairment of cognition is its high affinity binding to the alpha 7 nicotinic receptor; a receptor shown to be important for cognition in a number of studies. There is some controversy, however, whether amyloid beta peptide inhibits or activates this receptor. We have cloned and stably expressed the human alpha 7 receptor and investigated its interaction with amyloid beta peptide using patch clamp electrophysiology. Human alpha 7 was activated in a concentration-dependent fashion by nicotine, acetylcholine and choline and potently inhibited by methyllycaconitine citrate. The responses were inwardly rectifying and exhibited rapid activation, desensitization and deactivation. Amyloid beta peptide(1-42) antagonized human alpha7 responses in a partially reversible fashion; no agonist effects of amyloid beta peptide(1-42) were detected. A similar inhibition of mouse alpha 7 was also observed. In addition, we have assessed the function of native alpha 7 receptors in hippocampal slices prepared from transgenic mice that over-express human amyloid. Despite this clear inhibition of recombinant receptors, hippocampal GABAergic interneurones in slices from beta-amyloid over-expressing mice still possess alpha 7 receptor-mediated currents.

Rhesus monkey alpha7 nicotinic acetylcholine receptors: comparisons to human alpha7 receptors expressed in Xenopus oocytes

An alpha7 nicotinic acetylcholine receptor sequence was cloned from Rhesus monkey (Macaca mulatta). This clone differs from the mature human alpha7 nicotinic acetylcholine receptor in only four amino acids, two of which are in the extracellular domain. The monkey alpha7 nicotinic receptor was characterized in regard to its functional responses to acetylcholine, choline, cytisine, and the experimental alpha7-selective agonists 4OH-GTS-21, TC-1698, and AR-R17779. For all of these agonists, the EC(50) for activation of monkey receptors was uniformly higher than for human receptors. In contrast, the potencies of mecamylamine and MLA for inhibiting monkey and human alpha7 were comparable. Acetylcholine and 4OH-GTS-21 were used to probe the significance of the single point differences in the extracellular domain. Mutants with the two different amino acids in the extracellular domain of the monkey receptor changed to the corresponding sequence of the human receptor had responses to these agonists that were not significantly different in EC(50) from wild-type human alpha7 nicotinic receptors. Monkey alpha7 nicotinic receptors have a serine at residue 171, while the human receptors have an asparagine at this site. Monkey S171N mutants were more like human alpha7 nicotinic receptors, while mutations at the other site (K186R) had relatively little effect. These experiments point toward the basic utility of the monkey receptor as a model for the human alpha7 nicotinic receptor, albeit with the caveat that these receptors will vary in their agonist concentration dependency. They also point to the potential importance of a newly identified sequence element for modeling the specific amino acids involved with receptor activation.

Early changes in Aβ levels in the brain of APPswe transgenic mice—Implication on synaptic density, α7 neuronal nicotinic acetylcholine- and N-methyl-d-aspartate receptor levels

Tg 2576 (APPswe) mice develop age-related amyloid deposition as well as behavioural- and electrophysiological changes in the brain. In this study, APPswe mice were investigated from 7 to 90 days of age. We observed high Abeta levels in the cortex of APPswe mice at 7 days of age, suggesting that these mice produce Abeta from birth. A positive correlation between Abeta and synaptophysin levels, followed by changes in ERK MAPK activity, indicated that Abeta causes altered synaptic function and an increase in the number of synaptic terminals. In addition, alterations in [(125)I]alphabungarotoxin- and [(3)H]MK-801 binding sites were also observed in APPswe mice compared to controls. In conclusion, over-expression of Abeta early in life causes changes in synaptophysin levels and number of [(125)I]alphabungarotoxin- and [(3)H]MK-801 binding sites. The results may provide important information about the onset and consequences of Abeta pathology in this transgenic mouse model.

Characterization of the signaling pathway downstream p75 neurotrophin receptor involved in beta-amyloid peptide-dependent cell death

The accumulation of beta-amyloid (Abeta) peptide is a key pathogenic event in Alzheimer's disease. Previous studies have shown that Abeta peptide can damage neurons by activating the p75 neurotrophin receptor (p75NTR). However, the signaling pathway leading to neuronal cell death is not completely understood. By using a neuroblastoma cell line devoid of neurotrophin receptors and engineered to express either a full-length or a death domain (DD)-truncated form of p75NTR, we demonstrated that Abeta peptide activates the mitogen-activated protein kinases (MAPKs) p38 and c-Jun N-terminal kinase (JNK). We also found that Abeta peptide induces the translocation of nuclear factor-kappaB (NF-kappaB). These events depend on the DD of p75NTR. Beta-amyloid (Abeta) peptide was found not to be toxic when the above interactors were inhibited, indicating that they are required for Abeta-induced neuronal cell death. p75 neurotrophin receptor (p75NTR)-expressing cells became resistant to Abeta toxicity when transfected with dominant-negative mutants of MAPK kinases 3, 4, or 6 (MKK3, MKK4, or MKK6), the inhibitor of kappaBalpha, or when treated with chemical inhibitors of p38 and JNK. Furthermore, p75NTR-expressing cells became resistant to Abeta peptide upon transfection with a dominant-negative mutant of p53. These results were obtained in the presence of normal p38 and JNK activation, indicating that p53 acts downstream of p38 and JNK. Finally, we demonstrated that NF-kappaB activation is dependent on p38 and JNK activation. Therefore, our data suggest a signaling pathway in which Abeta peptide binds to p75NTR and activates p38 and JNK in a DD-dependent manner, followed by NF-kappaB translocation and p53 activation.

Regulation of NMDA receptor trafficking by amyloid-beta

Amyloid-beta peptide is elevated in the brains of patients with Alzheimer disease and is believed to be causative in the disease process. Amyloid-beta reduces glutamatergic transmission and inhibits synaptic plasticity, although the underlying mechanisms are unknown. We found that application of amyloid-beta promoted endocytosis of NMDA receptors in cortical neurons. In addition, neurons from a genetic mouse model of Alzheimer disease expressed reduced amounts of surface NMDA receptors. Reducing amyloid-beta by treating neurons with a gamma-secretase inhibitor restored surface expression of NMDA receptors. Consistent with these data, amyloid-beta application produced a rapid and persistent depression of NMDA-evoked currents in cortical neurons. Amyloid-beta-dependent endocytosis of NMDA receptors required the alpha-7 nicotinic receptor, protein phosphatase 2B (PP2B) and the tyrosine phosphatase STEP. Dephosphorylation of the NMDA receptor subunit NR2B at Tyr1472 correlated with receptor endocytosis. These data indicate a new mechanism by which amyloid-beta can cause synaptic dysfunction and contribute to Alzheimer disease pathology.