Acetylcholinesterase accelerates assembly of amyloid-beta-peptides into Alzheimer's fibrils: possible role of the peripheral site of the enzyme

Medicinal chemistry approaches for the treatment and prevention of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, which is characterised by progressive deterioration of memory and higher cortical functions that ultimately result in total degradation of intellectual and mental activities. Modern strategies in the search of new therapeutic approaches are based on the morphological and biochemical characteristics of AD, and focused on following directions: agents that compensate the hypofunction of cholinergic system, agents that interfere with the metabolism of beta-amyloid peptide, agents that protect nerve cells from toxic metabolites formed in neurodegenerative processes, agents that activate other neurotransmitter systems that indirectly compensate for the deficit of cholinergic functions, agents that affect the process of the formation of neurofibrillary tangles, anti-inflammatory agents that prevent the negative response of nerve cells to the pathological process. The goal of the present review is the validation and an analysis from the point of view of medicinal chemistry of the principles of the directed search of drugs for the treatment and prevention of AD and related neurodegenerative disorders. It is based on systematization of the data on biochemical and structural similarities in the interaction between physiologically active compounds and their biological targets related to the development of such pathologies. The main emphasis is on cholinomimetic, anti-amyloid and anti-metabolic agents, using the data that were published during the last 3 to 4 years, as well as the results of clinical trials presented on corresponding websites.

Parkinson's disease, Alzheimer's disease and motor neurone disease: identifying a common mechanism

Although Alzheimer's disease, Parkinson's disease, and motor neurone disease are distinct disorders, there could be a common neurodegenerative mechanism that characterises the death of selective neurone populations in each case. We propose that this mechanism could be an aberrantly activated, developmental process involving a non-classical, non-enzymatic action of acetylcholinesterase mediated via a short linear motif near the C-terminal end of the molecule. Since this motif has a highly conserved homology with part of the amyloid precursor protein, it may be particularly attractive as a target for novel therapeutic strategies in neurodegeneration.

Catalytic antibodies with acetylcholinesterase activity

We describe three catalytic cholinesterase-like catalytic antibodies (Ab1), as well as anti-idiotypic (Ab2) and idiotypic (Ab3) antibodies, to one of the Ab1s. The Ab1s were raised against the human erythrocyte acetylcholinesterase (AChE), and are unusual in that they both recognise and resemble acetylcholinesterase in their catalytic activity. No contamination of the antibody preparations with either acetylcholinesterase or butyrylcholinesterase (BChE) was found. None of the Ab2s showed catalytic activity, whereas four Ab3s did (an incidence of 1.26% of all Ab3s). Although there is considerable resemblance between Ab1s and Ab3s, there are significant differences between the two groups. All the antibodies were inhibited by phenylmethylsulphonyl fluoride (PMSF), indicating the presence of a serine residue in their active sites, and were inhibited by the cholinesterase active site inhibitors iso-OMPA and pyridostigmine, suggesting the similarity of the sites to those of cholinesterases. The Ab3s resemble the Ab1s in their ability to hydrolyse both acetyl and butyrylthiocholine (BTCh). However, the Ab3s appear to be better catalysts, having significantly reduced K(m) values (for acetyl, but not for butyrylthiocholine) and increased turnover numbers (K(cat)), rate enhancements (K(cat)/K(uncat)) and K(cat)/K(m) ratios, for both substrates, although these values by no means approach those of the natural enzymes. The Ab1s appear to have structures resembling the anionic sites of cholinesterases, as shown by their reaction with the anionic site inhibitors (edrophonium and tetramethylammonium). No such reactions were observed in the Ab3s. None of the antibodies show evidence of the sites resembling the peripheral anionic site (PAS) of acetylcholinesterase. All the antibodies recognise, to varying degrees, the peripheral anionic site of acetylcholinesterase. This was shown by their ability to inhibit acetylcholinesterase, to compete with peripheral site inhibitors, and to block acetylcholinesterase-mediated cell adhesion, a property of this site. The results indicate idiotypic mimicry of a catalytic antibody's active site, and suggest that the development of the catalytic activity in the anti-acetylcholinesterase antibodies may be related to the structural features of the peripheral anionic site of acetylcholinesterase.

Novel cognitive improving and neuroprotective activities of Polygala tenuifolia Willdenow extract, BT-11

We carried out this study to search a new active constituent that had cognitive enhancing activity and low side effects from natural source. We found that the extract of dried root of Polygala tenuifolia Willdenow (BT-11, 10 mg/kg, i.p.) could significantly reverse scopolamine-induced cognitive impairments in rat, using a passive avoidance and a water maze test. We also investigated the effects of BT-11 on neurotoxicity induced by glutamate (Glu) and toxic metabolites of amyloid precursor protein (APP) such as amyloid beta protein (A beta) and C-terminal fragment of APP (CT) in primary cultured neurons of rat. The pretreatment of BT-11 (0.5, 3, and 5 micro g/ml) significantly reduced cell death induced by Glu (1 mM), A beta (10 micro M) and CT105 (10 micro M) in a dose-dependent manner. In addition, BT-11 inhibited acetylcholinesterase (AChE) activity in a dose-dependent and non-competitive manner (IC(50) value; 263.7 micro g/ml). Our novel findings suggest the possibility that this extract may have some protective effects against neuronal death and cognitive impairments in Alzheimer's disease (AD), or other neurodegenerative diseases related to excitotoxicity and central cholinergic dysfunction.

Idiotypic mimicry of a catalytic antibody active site

We have previously described three catalytic antibodies (Ab1s) raised against human erythrocyte acetylcholinesterase (AChE). These antibodies both recognise and resemble AChE in their reaction with substrates and appear with a relatively high frequency. We do not know, however, why catalytic activity should have developed in response to a ground state antigen. This question has implication for autoimmune disorders, which are frequently characterised by the presence of catalytic antibodies, many of which have cytotoxic effects. In this study, we raised anti-idiotypic (Ab2) and anti-anti-idiotypic (Ab3) antibodies to a catalytic Ab1 and examined their properties. None of the Ab2s showed catalytic activity, whereas four of the Ab3s did, an incidence of 1.26%. No contamination of antibody preparations with either AChE or butyrylcholinesterase (BChE) was found. Immunisation of mice with AChE, as well as AChE complexed with various inhibitors, resulted in a significant increase in catalytic immunoglobulins in the serum, compared with non-immunised mice and mice immunised with the Ab1. There appears to be considerable resemblance between Ab1s and Ab3s, but there are also significant differences between the two groups. All the antibodies were inhibited by phenylmethylsulphonyl fluoride (PMSF), indicating the presence of a serine residue in their active sites and were inhibited by the cholinesterase active site inhibitors tetraisopropyl pyrophosphoramide (iso-OMPA) and pyridostigmine. The Ab3s resembled the Ab1s in their ability to hydrolyse both acetylthiocholine (ATCh) and butyrylthiocholine (BTCh). However, the Ab3s appear to be better catalysts, having significantly reduced K(M) values (for ATCh but not BTCh) and increased turnover numbers (K(cat)), rate enhancements (K(cat)/K(uncat)) and K(cat)/K(M) ratios. The Ab3s also had reduced affinities for cholinesterase anionic site inhibitors (edrophonium, tetramethylammonium and BW284c51) and no affinity at all for the AChE peripheral anionic site (PAS) inhibitor fasciculin. All the antibodies recognise, to some degree, the PAS of AChE, shown by their ability to inhibit AChE, to compete with peripheral site inhibitors and to block AChE-mediated cell adhesion, a property of the site. These results indicate idiotypic mimicry of the catalytic antibody's active site, suggesting that the catalytic activity is due to affinity maturation of immunoglobulin genes in response to a specific antigen, namely, the PAS of AChE. Further studies are required to determine the structural features of this ground state antigen responsible for the development of catalytic activity.

Laminin affects polymerization, depolymerization and neurotoxicity of Abeta peptide

Amyloid deposition in Alzheimer fibrils forms neurotoxic senile plaques in a process that may be modulated by associated proteins. In this work we demonstrate the ability of laminin-1 and laminin-2 to inhibit fibril formation and toxicity on cultured rat hippocampal neurons. We confirm that the laminin-1-derived peptide YFQRYLI inhibits efficiently both fibril formation and neurotoxicity and show that the IKVAV peptide inhibits amyloid neurotoxicity despite its slight inhibition of fibril formation. On other hand, laminin-1 induces disaggregation of preformed fibrils in vitro, characterized as a progressive disassembly of fibrils into protofibrils and further clearance of these latter species, leading to a continual inhibition of amyloid neurotoxicity.

Changes in molecular isoform distribution of acetylcholinesterase in rat cortex and cerebrospinal fluid after intracerebroventricular administration of amyloid beta-peptide

Previous studies have shown that an abnormal salt-soluble form of G(1) acetylcholinesterase (AChE) is increased in the Alzheimer's disease (AD) brain. The aim of the present study was to examine changes in AChE activity in an in vivo model of beta-amyloid peptide (A beta) administration. Rats received intracerebroventricular injections of A beta(25-35) (20 microg/day for seven days). Levels of AChE were measured in cerebral cortex and cerebrospinal fluid (CSF) after two months. A beta(25-35) administration did not alter total AChE activity in the cerebral cortex or CSF. However, analysis of salt-extractable AChE isoforms revealed an increase in the proportion of G(1) in both cortex and CSF, similar to that previously observed in AD patients. The results support the view that changes in AChE isoform pattern in the AD brain are a direct consequence of A beta accumulation.

Determination of the dissociation constants (pKa) of basic acetylcholinesterase inhibitors by reversed-phase liquid chromatography

An RP-HPLC study for the pKa determination of a series of basic compounds related to caproctamine, a dibenzylaminediamide reversible inhibitor of acetylcholinesterase, is reported. The 2-substituted analogues, bearing substituents with different electronegativity, were analysed by RP-HPLC by using C18 C4 stationary phases with a mobile phase consisting of mixture of acetonitrile and triethylamine phosphate buffer (pH range comprised between 4 and 10). Typical sigmoidal curves were obtained, showing the dependence of the capacity factors upon pH. In general, the retention of the investigated basic analytes increased with increasing of the pH. The inflection point of the pH sigmoidal dependence was used for the dissociation constant determination at a fixed acetonitrile percentage. When plotting pKa vs. percent of acetonitrile in the mobile phase for two representative compounds, linear regression were obtained: the y intercept gave the aqueous pKa(w). The pKa estimation by HPLC method was found to be useful to underline the difference of benzylamine basicity produced by the ortho aromatic substituents. The variation of pKa values (6.15-7.80) within the series of compounds was correlated with the electronic properties of the ortho-substituents through the Hammett sigma parameter, whereas the ability of substituents to accept H-bond was found to play a role in determining the conformational behavior of the molecules.

Cholinergic changes in the APP23 transgenic mouse model of cerebral amyloidosis

Alzheimer's Disease (AD) is a neurodegenerative disorder that is characterized by extracellular deposits of amyloid-beta peptide (Abeta) and a severe depletion of the cholinergic system, although the relationship between these two events is poorly understood. In the neocortex, there is a loss of cholinergic fibers and receptors and a decrease of both choline acetyltransferase (ChAT) and acetylcholinesterase enzyme activities. The nucleus basalis of Meynert (NBM), which provides the major cholinergic input to the neocortex, undergoes profound neuron loss in AD. In the present study, we have examined the cholinergic alterations in amyloid precursor protein transgenic mice (APP23), a mouse model of cerebral beta-amyloidosis. In aged APP23 mice, our results reveal modest decreases in cortical cholinergic enzyme activity compared with age-matched wild-type mice. Total cholinergic fiber length was more severely affected, with 29 and 35% decreases in the neocortex of aged APP23 mice compared with age-matched wild-type mice and young transgenic mice, respectively. However, there was no loss of cholinergic basal forebrain neurons in these aged APP23 mice, suggesting that the cortical cholinergic deficit in APP23 mice is locally induced by the deposition of amyloid and is not caused by a loss of cholinergic basal forebrain neurons. To study the impact of cholinergic basal forebrain degeneration on cortical amyloid deposition, we performed unilateral NBM lesions in adult APP23 mice. Three to 8 months after lesioning, a 38% reduction in ChAT activity and significant cholinergic fiber loss were observed in the ipsilateral frontal cortex. There was a 19% decrease in Abeta levels of the ipsilateral compared with contralateral frontal cortex with no change in the ratio of Abeta40 to Abeta42. We conclude that the severe cholinergic deficit in AD is caused by both the loss of cholinergic basal forebrain neurons and locally by cerebral amyloidosis in the neocortex. Moreover, our results suggest that disruption of the basal cholinergic forebrain system does not promote cerebral amyloidosis in APP23 transgenic mice.

Vitamin E but not 17beta-estradiol protects against vascular toxicity induced by beta-amyloid wild type and the Dutch amyloid variant

Amyloid beta-peptide (Abeta) fibril deposition on cerebral vessels produces cerebral amyloid angiopathy that appears in the majority of Alzheimer's disease patients. An early onset of a cerebral amyloid angiopathy variant called hereditary cerebral hemorrhage with amyloidosis of the Dutch type is caused by a point mutation in Abeta yielding Abeta(Glu22-->Gln). The present study addresses the effect of amyloid fibrils from both wild-type and mutated Abeta on vascular cells, as well as the putative protective role of antioxidants on amyloid angiopathy. For this purpose, we studied the cytotoxicity induced by Abeta(1-40 Glu22-->Gln) and Abeta(1-40 wild-type) fibrils on human venule endothelial cells and rat aorta smooth muscle cells. We observed that Abeta(Glu22-->Gln) fibrils are more toxic for vascular cells than the wild-type fibrils. We also evaluated the cytotoxicity of Abeta fibrils bound with acetylcholinesterase (AChE), a common component of amyloid deposits. Abeta(1-40 wild-type)-AChE fibrillar complexes, similar to neuronal cells, resulted in an increased toxicity on vascular cells. Previous reports showing that antioxidants are able to reduce the toxicity of Abeta fibrils on neuronal cells prompted us to test the effect of vitamin E, vitamin C, and 17beta-estradiol on vascular damage induced by Abeta(wild-type) and Abeta(Glu22-->Gln). Our data indicate that vitamin E attenuated significantly the Abeta-mediated cytotoxicity on vascular cells, although 17beta-estradiol and vitamin C failed to inhibit the cytotoxicity induced by Abeta fibrils.

Increased expression of intranuclear AChE involved in apoptosis of SK-N-SH cells

Programmed cell death plays an integral role in neurodegenerative diseases such as Alzheimer's disease (AD). Acetylcholinesterase (AChE) was suggested to be neurotoxic in vivo and in vitro and accelerate assembly of amyloid peptide into Alzheimer's fibrils. In our experiments, we found increased AChE expression in apoptotic neuroblastoma SK-N-SH cells after long-term culture. Our results first showed that in apoptotic SK-N-SH cells, AChE aggregated in the nucleus and suppression of AChE expression with antisense oligonucleotide could save the cells from apoptosis. It was also found that caspase-3 activity was parallel with AChE activation in apoptotic SK-N-SH cells. These results suggest that AChE plays an important role in the procession of neuroblastoma cell apoptosis and favor the association between AChE and neuronal apoptosis in AD.

Do cholinesterase inhibitors have disease-modifying effects in Alzheimer's disease?

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer disease (AD) has resulted in drugs being registered for the first time in the US and Europe for this specific indication. The 3 agents registered are cholinesterase inhibitors (ChEIs). The major therapeutic effect of ChEIs in patients with AD is the maintenance of cognitive function, as compared with placebo, during a 6-month to 1-year period of treatment. Additional drug effects that may occur are the slowing of cognitive deterioration and improvement of behaviour and daily living activities. Comparison of clinical effects of 6 ChEIs demonstrates a rather similar magnitude of improvement in cognitive outcome measures. For some drugs, this level may represent an upper limit, while for others it may be possible to increase the benefit further. In order to maximise and prolong positive drug effects it is important to start treatment early and adjust the dosage during treatment. Recent studies that used this administration strategy have shown that in many patients, the stabilisation effect produced by ChEIs can be prolonged for as long as 36 months. This long-lasting effect suggests mechanisms of action other than symptomatic ones. In this article, the effects of ChEIs on beta-amyloid metabolism are postulated to explain the stabilising (i.e. disease-modifying) effects of the drugs. Evidence for such a mechanism is available at the experimental but not yet at the clinical level.

Acetylcholinesterase inhibitors: SAR and kinetic studies on omega-[N-methyl-N-(3-alkylcarbamoyloxyphenyl)methyl]aminoalkoxyaryl derivatives

In this work, we further investigated a class of carbamic cholinesterase inhibitors introduced in a previous paper (Rampa et al. J. Med. Chem. 1998, 41, 3976). Some new omega-[N-methyl-N-(3-alkylcarbamoyloxyphenyl)methyl]aminoalkoxyaryl analogues were designed, synthesized, and evaluated for their inhibitory activity against both acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). The structure of the lead compound (xanthostigmine) was systematically varied with the aim to optimize the different parts of the molecule. Moreover, such a structure-activity relationships (SAR) study was integrated with a kinetic analysis of the mechanism of AChE inhibition for two representative compounds. The structural modifications lead to a compound (12b) showing an IC(50) value for the AChE inhibition of 0.32 +/- 0.09 nM and to a group of BuChE inhibitors also active at the nanomolar level, the most potent of which (15d) was characterized by an IC(50) value of 3.3 +/- 0.4 nM. The kinetic analysis allowed for clarification of the role played by different molecular moieties with regard to the rate of AChE carbamoylation and the duration of inhibition. On the basis of the results presented here, it was concluded that the cholinesterase inhibitors of this class possess promising characteristics in view of a potential development as drugs for the treatment of Alzheimer's disease.

Cerebrospinal fluid acetylcholinesterase activity after long-term treatment with donepezil and rivastigmina

At present acetylcholinesterase (AChE) inhibitors (AChEIs) represent the only reliable therapeutic resource for symptomatic treatment of Alzheimer Disease (AD). This study was designed to assess the effects of 6-12 month treatment with AChEIs donepezil and rivastigmine on cerebrospinal fluid (CSF) AChE and butyrylcholinesterase (BuChE) activity in AD patients. The pattern of AChE isoforms (G4, G1, G2) before and after treatment was investigated as well. In AD patients treated with donepezil a significant increase of CFS AChE activity was observed, whereas treatment with rivastigmine induced a significant decrease of AChE activity. Both drugs did not change BuChE activity and tended to restore the physiological pattern of AChE isoform. The possible significance of the influence of AChEIs on CSF AChE activity and isoforms is discussed.

Acetylcholinesterase in Alzheimer's disease

Since the discovery of the cholinergic deficit in Alzheimer disease (AD), acetylcholinesterase (AChE) has been widely investigated in tissues involved in the disease. These studies showed modifications in AChE activity and changes in its polymorphism in brain as well as in cerebro-spinal fluid (CSF) and blood. The co-localization of the enzyme in the senile plaque provided evidence of its anomalous features. It has been also shown that AChE forms a stable complex with senile plaque components through its peripheral anionic site. Moreover, the neurotoxicity of amyloid components is increased by the presence of AChE. The occurrence of an altered glycosylation of some AChE forms in AD is closely related to the presence of amyloid formations. Literature on expression, relationships and modifications in the molecular polymorphism of AChE, in brain, CSF and blood in AD is reviewed.

Coumarins derivatives as dual inhibitors of acetylcholinesterase and monoamine oxidase

A set of 17 coumarin and 2 chromone derivatives with known inhibitory activity toward monoamine oxidase (MAO) A and B were tested as acetylcholinesterase (AChE) inhibitors. All compounds inhibited AChE with values in the micromolar range (3-100 microM). A kinetic study showed that most compounds acted as noncompetitive AChE inhibitors. This finding may be of interest in the context of Alzheimer's disease because recent observations suggest that MAO and AChE inhibition might decrease beta-amyloid deposition.

Anticholinesterase treatment of chicken retinal cells increases acetylcholinesterase protein independently of protein kinase C

It has been reported that anticholinesterase exposure, e.g. by environmental toxins or nerve gases, can increase acetylcholinesterase (AChE) protein, possibly as an autoregulatory stress response. We earlier have transfected retinal cells of the chick embryo with a pSVK3-AChE(rab)-cDNA vector to heterologously express rabbit AChE, which concomitantly also increased AChE protein from chick. To analyse further the cell-internal pathways of these different paradigms (anticholinesterase treatment vs. AChE transfection) which both lead to an AChE increase, we here show that AChE overexpression by transfection leads to an increase in protein kinase C (PKC). Most remarkably, when cells independently of, or in addition to their transfection are treated with 10 microM of the AChE inhibitor BW284c51, AChE protein levels are much more dramatically increased up to 20-fold. This treatment, however, does not affect PKC. These data show that (i) retinal cells respond to anticholinesterase insult by a massive increase of AChE protein; (ii) the response to BW284c51 is not PKC-mediated; and (iii) both strategies of AChE increase follow different cell-internal pathways, their effects being additive. The ecological and biomedical implications of these findings are briefly discussed.

Design, synthesis, and structure-activity relationships of a series of 3-[2-(1-benzylpiperidin-4-yl)ethylamino]pyridazine derivatives as acetylcholinesterase inhibitors

Starting from the 3-[2-(1-benzylpiperidin-4-yl)ethylamino]-6-phenylpyridazine 1, we performed the design, the synthesis, and the structure-activity relationships of a series of pyridazine analogues acting as AChE inhibitors. Structural modifications were achieved on four different parts of compound 1 and led to the following observations: (i) introduction of a lipophilic environment in the C-5 position of the pyridazine ring is favorable for the AChE-inhibitory activity and the AChE/BuChE selectivity; (ii) substitution and various replacements of the C-6 phenyl group are possible and led to equivalent or slightly more active derivatives; (iii) isosteric replacements or modifications of the benzylpiperidine moiety are detrimental to the activity. Among all derivatives prepared, the indenopyridazine derivative 4g was found to be the more potent inhibitor with an IC(50) of 10 nM on electric eel AChE. Compared to compound 1, this represents a 12-fold increase in potency. Moreover, 3-[2-(1-benzylpiperidin-4-yl)ethylamino]-5-methyl-6-phenylpyridazine 4c, which showed an IC(50) of 21 nM, is 100-times more selective for human AChE (human BuChE/AChE ratio of 24) than the reference compound tacrine.

Effects of chronic metrifonate treatment on cholinergic enzymes and the blood-brain barrier

After an acute (4 h) treatment with an irreversible cholinesterase inhibitor organophosphate, metrifonate (100 mg/kg i.p.), the activities of both acetyl- and butyrylcholinesterase were inhibited (66.0-70.7% of the control level) in the rat brain cortex and hippocampus. There were no significant changes in the acetyl- and butyrylcholinesterase activities in the olfactory bulb, or in the choline acetyltransferase activity in all three brain areas. After chronic (2 or 5 week) metrifonate treatment (100 mg/kg daily i.p.), the activities of both cholinesterases were substantially inhibited in the rat brain cortex and hippocampus (15.8-31.8% of the control levels), but there was no inhibition of the choline acetyltransferase activity. Moreover, chronic metrifonate treatment did not have any effect on the distribution of the acetylcholinesterase molecular forms. In vitro, metrifonate proved to be a more potent inhibitor of butyryl- than of acetylcholinesterase in both the cortex and the hippocampus. In the hippocampus, the butyrylcholinesterase activity was twice as sensitive to metrifonate inhibition as that in the cortex (IC50 values 0.22 and 0.46 microM, respectively). The effects of chronic (5 week) metrifonate treatment on the blood-brain barrier of the adult rat were examined. The damage to the blood-brain barrier was judged by the extravasation of Evans' blue dye in three brain regions: the cerebral cortex, the hippocampus, and the striatum. No extravasation of Evans' blue dye was found in the brain by fluorometric quantitation. These data indicate that chronic metrifonate treatment may increase the extracellular acetylcholine level via cholinesterase inhibition, but it does not have any effects on the blood-brain barrier. Therefore, it appears reasonable to hypothesize that cholinesterase activities do not play a role in the blood-brain barrier permeability.

Is anti-cholinesterase therapy of Alzheimer's disease delaying progression?

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer's disease (AD) resulted in three drugs being registered for the first time in the US and Europe. All three compounds are cholinesterase inhibitors (ChEI). The major therapeutic effect of ChEI on AD patients is to maintain cognitive function at a stable level during a 6-month to 1-year period of treatment, as compared to placebo. Additional drug effects are to slow down cognitive deterioration and improve behavioral and daily living activity. Recent studies show that in many patients the cognitive stabilization effect can be prolonged up to 24 months. This long-lasting effect suggests a mechanism of action other than symptomatic, and directly cholinergic. In vitro and in vivo studies have consistently demonstrated a link between cholinergic activation and amyloid precursor protein (APP) metabolism. Lesions of cholinergic nuclei cause a rapid increase in cortical APP and cholinergic synaptic function; the effect of such lesions can be reversed by ChEI treatment. A reduction in cholinergic neurotransmission, experimental or pathological, leads to amyloidogenic metabolism and contributes to the development of neuropathology and cognitive dysfunction. To explain the long-term effect of ChEI, for which evidence is available on an experimental as well as clinical level, a mechanism based on beta-amyloid metabolism is postulated. The question whether cholinergic stabilization implies simply slowing down progression of disability or also involves delay of disease progression is discussed.

Acetylcholinesterase-amyloid-beta-peptide interaction and Wnt signaling involvement in Abeta neurotoxicity

Previous studies have indicated that acetylcholinesterase (AChE) promotes amyloid-beta-peptide (Abeta) fibril formation and AChE-Abeta complexes increase Abeta-dependent neurotoxicity. Here we present evidence for the: i) identification of the AChE motif that promotes amyloid formation, ii) in vivo effect of AChE on brain plaque formation, and iii) connection between AChE-Abeta neurotoxicity and the Wnt signal transduction pathway. Computer modeling, stereotaxic infusions and cell biological techniques were used to study the above problems. Results indicated that a 3.4 kDa AChE peptide promotes Abeta fibril formation. AChE infusion into rat hippocampus determines the appearance of anti-Abeta and thioflavine-S positive plaques, and AChE-Abeta toxicity on hippocampal cultures was blocked by lithium, an activator of the Wnt cascade. We suggest that AChE-Abeta/Abeta dependent neurotoxicity may result in loss of function of Wnt signaling components, and open the possibility that lithium may be considered as a candidate for therapeutic intervention in Alzheimer's disease pathology.

Structure-based 3D QSAR and design of novel acetylcholinesterase inhibitors

The paper describes the construction, validation and application of a structure-based 3D QSAR model of novel acetylcholinesterase (AChE) inhibitors. Initial use was made of four X-ray structures of AChE complexed with small, non-specific inhibitors to create a model of the binding of recently developed aminopyridazine derivatives. Combined automated and manual docking methods were applied to dock the co-crystallized inhibitors into the binding pocket. Validation of the modelling process was achieved by comparing the predicted enzyme-bound conformation with the known conformation in the X-ray structure. The successful prediction of the binding conformation of the known inhibitors gave confidence that we could use our model to evaluate the binding conformation of the aminopyridazine compounds. The alignment of 42 aminopyridazine compounds derived by the docking procedure was taken as the basis for a 3D QSAR analysis applying the GRID/GOLPE method. A model of high quality was obtained using the GRID water probe, as confirmed by the cross-validation method (q2LOO = 0.937, q2L50%O = 0.910). The validated model, together with the information obtained from the calculated AChE-inhibitor complexes, were considered for the design of novel compounds. Seven designed inhibitors which were synthesized and tested were shown to be highly active. After performing our modelling study the X-ray structure of AChE complexed with donepezil, an inhibitor structurally related to the developed aminopyirdazines, has been made available. The good agreement found between the predicted binding conformation of the aminopyridazines and the one observed for donepezil in the crystal structure further supports our developed model.

Treatment of alzheimer's disease with cholinesterase inhibitors

The cholinergic system is the most severely affected neurotransmitter system in patients with Alzheimer's disease, and therapeutic strategies have been developed to restore cholinergic function in these patients. This article reviews the present status of cholinesterase inhibitors for the treatment of Alzheimer's disease. It then discusses treatment approaches and current and future issues regarding efficacy and safety.

Determination of inhibitors' potency (IC50) by a direct high-performance liquid chromatographic method on an immobilised acetylcholinesterase column

An immobilised acetylcholinesterase (AChE) stationary phase was prepared by using an in situ AChE immobilisation procedure. A stainless steel column packed with epoxide silica was connected to the HPLC system and the enzyme solution at pH 5.8 was recycled through the column at a flow-rate of 0.5 ml/min for 24 h. The activity of the immobilised AChE was determined by injecting the substrate acetylthiocholine, using as mobile phase 0.1 M phosphate buffer (pH 7.4) containing Ellman's reagent [5,5'-dithio-bis(2-nitrobenzoic acid)] and measuring the area of the obtained peak with UV detection at 412 nm. The effect of AChE inhibitors tacrine, edrophonium and donepezil were evaluated by the simultaneous injection of each inhibitor with the substrate. The resulting decrease in the AChE activity, as expressed by the decrease of the peak area detected at 412 nm, was related to the concentration and potency of the solutes. The obtained IC50 values were compared with those derived by the conventional spectrophotometric method. This immobilized enzyme reactor, included in a chromatographic system, can be used for the rapid screening for new inhibitors allowing for the on-line determination of a compound's inhibitory potency. The advantages over the conventional methods are the increased enzyme stability and system automation which allows a large number of compounds to be analysed continuously.

Promotion of formation of amyloid fibrils by aluminium adenosine triphosphate (AlATP)

The formation of amyloid fibrils is considered to be an important step in the aetiology of Alzheimer's disease and other amyloidoses. Fibril formation in vitro has been shown to depend on many different factors including modifications to the amino acid profile of fibrillogenic peptides and interactions with both large and small molecules of physiological significance. How these factors might contribute to amyloid fibril formation in vivo is not clear as very little is known about the promotion of fibril formation in undersaturated solutions of amyloidogenic peptides. We have used thioflavin T fluorescence and reverse phase high performance liquid chromatography to show that ATP, and in particular AlATP, promoted the formation of thioflavin T-reactive fibrils of beta amyloid and, an unrelated amyloidogenic peptide, amylin. Evidence is presented that induction of fibril formation followed the complexation of AIATP by one or more monomers of the respective peptide. However, the complex formed could not be identified directly and it is suggested that AlATP might be acting as a chaperone in the assembly of amyloid fibrils. The effect of AlATP was not mimicked by either AlADP or AlAMP. However, it was blocked by suramin, a P2 ATP receptor antagonist, and this has prompted us to speculate that the precursor proteins to beta amyloid and amylin may be substrates or receptors for ATP in vivo.

Acetylcholinesterase--new roles for an old actor

The discovery of the first neurotransmitter--acetylcholine--was soon followed by the discovery of its hydrolysing enzyme, acetylcholinesterase. The role of acetylcholinesterase in terminating acetylcholine-mediated neurotransmission made it the focus of intense research for much of the past century. But the complexity of acetylcholinesterase gene regulation and recent evidence for some of the long-suspected 'non-classical' actions of this enzyme have more recently driven a profound revolution in acetylcholinesterase research. Although our understanding of the additional roles of acetylcholinesterase is incomplete, the time is ripe to summarize the evidence on a remarkable diversity of acetylcholinesterase functions.

Models of amyloid seeding in Alzheimer's disease and scrapie: mechanistic truths and physiological consequences of the time-dependent solubility of amyloid proteins

Ordered protein aggregation in the brain is a hallmark of Alzheimer's disease and scrapie. The disease-specific amyloid fibrils comprise primarily a single protein, amyloid beta, in Alzheimer's disease, and the prion protein in scrapie. These proteins can be induced to form aggregates in vitro that are indistinguishable from brain-derived fibrils. Consequently, much effort has been invested in the development of in vitro model systems to study the details of the aggregation processes and the effects of endogenous molecules that have been implicated in disease. Selected studies of this type are reviewed herein. A simple mechanistic model has emerged for both processes that involves a nucleation-dependent polymerization. This mechanism dictates that aggregation is dependent on protein concentration and time. Furthermore, amyloid formation can be seeded by a preformed fibril. The physiological consequences of this mechanism are discussed.

Cholinesterase inhibitors stabilize Alzheimer's disease

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer disease (AD) has resulted into three drugs being registered for the first time in the USA and Europe for this specific indication. All three are cholinesterase inhibitors (ChEI). The major therapeutic effect of ChEI on AD patients is to maintain cognitive function at a constant level during a six-month to one-year period of treatment, as compared to placebo. Additional drug effects might slow cognitive deterioration and improve behavioral and daily living conditions. Comparison of clinical effects of six ChEI demonstrates a rather similar magnitude of improvement in cognitive measures. For some drugs, this may represent an upper limit, whereas for other it may still be possible to further increase the benefit. In order to maximize and prolong positive drug effects, it is important to start early and adjust dosage during the treatment. Recent studies show that in many patients the stabilization effect produced by ChEI can be prolonged for as long as a 24-month period. In order to explain the stabilizing effect of ChEI, a mechanism other than AChE inhibition, based on beta-amyloid metabolism, is postulated.

Effects of mutations of active site residues and amino acids interacting with the Omega loop on substrate activation of butyrylcholinesterase

The peripheral anionic site (PAS) of human butyrylcholinesterase is involved in the mechanism of substrate activation by positively charged substrates and ligands. Two substrate binding loci, D70 in the PAS and W82 in the active site, are connected by the Omega loop. To determine whether the Omega loop plays a role in the signal transduction between the PAS and the active site, residues involved in stabilization of the loop, N83, K339 and W430, were mutated. Mutations N83A and N83Q caused loss of substrate activation, suggesting that N83 which interacts with the D70 backbone may be an element of the transducing system. The K339M and W430A mutant enzymes retained substrate activation. Residues W82, E197, and A328 in the active site gorge have been reported to be involved in substrate activation. At butyrylthiocholine concentrations greater then 2 mM, W82A showed apparent substrate activation. Mutations E197Q and E197G strongly reduced substrate activation, while mutation E197D caused a moderate effect, suggesting that the carboxylate of residue E197 is involved in substrate activation. Mutations A328F and A328Y showed no substrate activation, whereas A328G retained substrate activation. Substrate activation can result from an allosteric effect due to binding of the second substrate molecule on the PAS. Mutation W430A was of special interest because this residue hydrogen bonds to W82 and Y332. W430A had strongly reduced affinity for tetramethylammonium. The bimolecular rate constant for reaction with diisopropyl fluorophosphate was reduced 10000-fold, indicating severe alteration in the binding area in W430A. The kcat values for butyrylthiocholine, o-nitrophenyl butyrate, and succinyldithiocholine were lower. This suggested that the mutation had caused misfolding of the active site gorge without altering the Omega loop conformation/dynamics. W430 as well as W231 and W82 appear to form the wall of the active site gorge. Mutation of any of these tryptophans disrupts the architecture of the active site.

Hexahydrochromeno[4,3-b]pyrrole derivatives as acetylcholinesterase inhibitors

In a search for less flexible analogues of caproctamine (1), a diamine diamide endowed with an interesting AChE affinity profile, we discovered compound 2, in which the terminal 2-methoxybenzyl groups of 1 have been incorporated into a tricyclic system. Since this compound retains good AChE inhibitory activity and its hexahydrochromeno[4,3-b]pyrrole moiety is reminiscent of the hexahydropyrrolo[2,3-b]indole of physostigmine (3), we have designed and synthesized carbamates 4-6, and their biological evaluation has been assessed in vitro against human AChE and BChE. The 6-carbamate 4 was almost as potent as physostigmine and was 60- and 550-fold more potent than the 7-carbamate 5 and the 8-carbamate 6, respectively. The two enantiomers of 4, (-)-4 and (+)-4, did not show a marked enantioselectivity. Finally, a similar time-dependent pattern of inhibition of AChE was observed for 3 and 4.

Cholinesterases modulate cell adhesion in human neuroblastoma cells in vitro

Cholinesterases are expressed non-synaptically during embryonic development, neoplasia and neurodegeneration. We have investigated the effects of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) and, conversely, anti-AChE and -BChE antibodies and inhibitors on cell adhesion and neurite outgrowth in human neuroblastoma cells. Analysis of cholinesterase levels and isoforms in undifferentiated and differentiated cells indicated a significant rise in AChE levels on differentiation. This increase was related to both cell-associated and secreted enzyme, and was predominantly the G4 isoform. BChE levels and isoforms, on the other hand, showed no significant variation. Coating the tissue culture plate with AChE stimulated neurite outgrowth, while BChE had an anti-adhesive effect. Cell adhesion was affected by the BChE inhibitor, ethopropazine, and the AChE peripheral site inhibitor, BW284c51, but not by eserine which binds to the active site. This indicates that the adhesion function is non-cholinergic, a finding supported by the lack of effect of AE-2, a monoclonal antibody that inhibits AChE, on cell adhesion. Four out of a panel of nine anti-AChE antibodies inhibited adhesion to varying degrees. Of these antibodies, two are catalytic, with epitopes associated with the peripheral anionic site of AChE, and the remaining two have epitopes overlapping this site. Neither of the two anti-BChE antibodies used had any effect on adhesion. These results indicate the importance of AChE in neuroblastoma cell adhesion and neurite outgrowth, and suggest that the peripheral anionic site may be involved in these processes.

Animal models of Alzheimer's disease and evaluation of anti-dementia drugs

Alzheimer's disease (AD) is the most common cause of progressive decline of cognitive function in aged humans, and is characterized by the presence of numerous senile plaques and neurofibrillary tangles accompanied by neuronal loss. Some, but not all, of the neuropathological alterations and cognitive impairment in AD can be reproduced genetically and pharmacologically in animals. It should be possible to discover novel drugs that slow the progress or alleviate the clinical symptoms of AD by using these animal models. We review the recent progress in the development of animal models of AD and discuss how to use these model animals to evaluate novel anti-dementia drugs.

Cholinesterase inhibitor therapy stabilizes symptoms of Alzheimer disease

Cholinesterase inhibitors tested in clinical trials in Europe, the United States, and Japan include fewer than 10 drugs; however, most of these compounds have advanced to clinical phase III trials. Based on results related to a population of more than 8,000 patients, we conclude that several of these compounds have shown significant clinical efficacy and safety in the treatment of Alzheimer disease. There are, however, differences with regard to side effects. The major clinical effect is stabilization of cognitive function during a 6- to 12-month period with an improvement of behavioral symptoms. The long-term effect of cholinesterase inhibitors extending to a 2-year period was reported. Future applications of these drugs include treatment of other types of dementias such as Lewy bodies dementia, vascular dementia, and Down syndrome dementia. The combination of cholinesterase inhibitors with estrogens, antioxidants, and anti-inflammatories may represent a further improvement of the therapy. From an economical point of view, treatment with cholinesterase inhibitors is not cost neutral.

Cholinesterase inhibitors stabilize Alzheimer disease

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer disease (AD) has resulted into three drugs being registered for the first time in USA and Europe for this specific indication. All three are cholinesterase inhibitors (ChEI). The major therapeutic effect of ChEI on AD patients is to maintain cognitive function at a constant level during a 6 months to one year period of treatment as compared to placebo. Additional drug effects might be slowing cognitive deterioration and improving behavioral and daily living conditions. Comparison of clinical effects of 6 ChEI demonstrates a rather similar magnitude of improvement in cognitive measures. For some drugs. this may represent an upper limit while for other it may still be possible to increase further the benefit. In order to maximize and prolong positive drug effects it is important to start early and adjust dosage during the treatment. Recent studies show that in many patients the stabilization effect produced by ChEI can be prolonged for as long as a 24 month period. In order to explain the stabilizing effect of ChEI, a mechanism other than AChE inhibition, based on beta-amyloid metabolism, is postulated.

Wnt signaling function in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease with progressive dementia accompanied by three main structural changes in the brain: diffuse loss of neurons; intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular protein deposits termed amyloid or senile plaques, surrounded by dystrophic neurites. Two major hypotheses have been proposed in order to explain the molecular hallmarks of the disease: The 'amyloid cascade' hypothesis and the 'neuronal cytoskeletal degeneration' hypothesis. While the former is supported by genetic studies of the early-onset familial forms of AD (FAD), the latter revolves around the observation in vivo that cytoskeletal changes - including the abnormal phosphorylation state of the microtubule associated protein tau - may precede the deposition of senile plaques. Recent studies have suggested that the trafficking process of membrane associated proteins is modulated by the FAD-linked presenilin (PS) proteins, and that amyloid beta-peptide deposition may be initiated intracellularly, through the secretory pathway. Current hypotheses concerning presenilin function are based upon its cellular localization and its putative interaction as macromolecular complexes with the cell-adhesion/signaling beta-catenin molecule and the glycogen synthase kinase 3beta (GSK-3beta) enzyme. Developmental studies have shown that PS proteins function as components in the Notch signal transduction cascade and that beta-catenin and GSK-3beta are transducers of the Wnt signaling pathway. Both pathways are thought to have an important role in brain development, and they have been connected through Dishevelled (Dvl) protein, a known transducer of the Wnt pathway. In addition to a review of the current state of research on the subject, we present a cell signaling model in which a sustained loss of function of Wnt signaling components would trigger a series of misrecognition events, determining the onset and development of AD.

Review: modulating factors in amyloid-beta fibril formation

Amyloid formation is a key pathological feature of Alzheimer's disease and is considered to be a major contributing factor to neurodegeneration and clinical dementia. Amyloid is found as both diffuse and senile plaques in the parenchyma of the brain and is composed primarily of the 40- to 42-residue amyloid-beta (Abeta) peptides. The characteristic amyloid fiber exhibits a high beta-sheet content and may be generated in vitro by the nucleation-dependent self-association of the Abeta peptide and an associated conformational transition from random to beta-conformation. Growth of the fibrils occurs by assembly of the Abeta seeds into intermediate protofibrils, which in turn self-associate to form mature fibers. This multistep process may be influenced at various stages by factors that either promote or inhibit Abeta fiber formation and aggregation. Identification of these factors and understanding the driving forces behind these interactions as well as the structural motifs necessary for these interactions will help to elucidate potential sites that may be targeted to prevent amyloid formation and its associated toxicity. This review will discuss some of the modulating factors that have been identified to date and their role in fibrillogenesis.

Acetylcholinesterase-positive fiber deafferentation and cell shrinkage in the septohippocampal pathway of aged amyloid precursor protein london mutant transgenic mice

Several lines of evidence implicate a cholinergic deficit in Alzheimer's disease (AD). Transgenic mice that overexpress clinical mutants of the human amyloid precursor protein (APP) have been generated that recapitulate many aspects of AD. We now analyzed the cholinergic system in aged APP/London transgenic mice. The major finding was the reorganization of acetylcholinesterase-positive fibers within the hippocampus and the reduced size of cholinergic cells in the medial septum. The reduction of acetylcholinesterase-positive fibers in the subiculum together with increased fiber density in the CA1 and in the dentate gyrus suggests a synaptic sprouting compensatory mechanism within the hippocampus. In the cortex, amyloid plaques were associated with intense acetylcholinesterase activity and surrounded by dystrophic acetylcholinesterase-positive fibers. Nevertheless, the overall pattern of cholinergic innervation was unchanged. These results demonstrate that overexpression of APP/London caused, besides amyloid plaques in aged mouse brain, also cholinergic deafferentation and cholinergic cell shrinkage.

High density lipoprotein inhibits assembly of amyloid beta-peptides into fibrils

The extracellular deposition of amyloid beta (Abeta) in senile plaques constitutes one of the defining hallmarks of Alzheimer's disease. Abeta peptides can aggregate spontaneously to highly insoluble amyloid fibrils, but several components are likely to influence the kinetics of fibrillogenesis in vivo. We report here that high density lipoprotein (HDL), the predominant lipoprotein in the human brain, reduces amyloid formation in vitro as determined by thioflavin T fluorescence and high speed sedimentation assays. The inhibition occurred in a dose dependent manner, and with concentrations of HDL above 1% resulting in more than 70% inhibition. We also examined the combined effect of apolipoprotein E (apoE) and HDL on Abeta fibrillogenesis. We found that HDL particles enriched with any of the three apoE isoforms inhibited Abeta fibrillogenesis as their native counterparts. Taken together, these findings suggest that HDL-like particles in the brain may prevent the formation of Abeta fibrils.

Acetylcholinesterase inhibitors for potential use in Alzheimer's disease: molecular modeling, synthesis and kinetic evaluation of 11H-indeno-[1,2-b]-quinolin-10-ylamine derivatives

Continuing our work on tetracyclic tacrine analogues, we synthesized a series of acetylcholinesterase (AChE) inhibitors of 11H-indeno-[1,2-b]-quinolin-10-ylaminic structure. Selected substituents were placed in synthetically accessible positions of the tetracyclic nucleus, in order to explore the structure-activity relationships (SAR) and the mode of action of this class of anticholinesterases. A molecular modeling investigation of the binding interaction of the lead compound (1a) with the AChE active site was performed, from which it resulted that, despite the rather wide and rigid structure of 1a, there may still be the possibility to introduce some small substituent in some positions of the tetracycle. However, from the examination of the experimental IC50 values, it derived that the indenoquinoline nucleus probably represents the maximum allowable molecular size for rigid compounds binding to AChE. In fact, only a fluorine atom in position 2 maintains the AChE inhibitory potency of the parent compound, and, actually, increases the AChE-selectivity with respect to the butyrylcholinesterase inhibition. By studying the kinetics of AChE inhibition for two representative compounds of the series, it resulted that the lead compound (1a) shows an inhibition of mixed type, binding to both the active and the peripheral sites, while the more sterically hindered analogue 2n seems to interact only at the external binding site of the enzyme. This finding seems particularly important in the context of Alzheimer's disease research in the light of recent observations showing that peripheral AChE inhibitors might decrease the aggregating effects of the enzyme on the beta-amyloid peptide (betaA).

Neuronal-glial interactions mediated by interleukin-1 enhance neuronal acetylcholinesterase activity and mRNA expression

Cholinergic dysfunction in Alzheimer's disease has been attributed to stress-induced increases in acetylcholinesterase (AChE) activity. Interleukin-1 (IL-1) is overexpressed in Alzheimer's disease, and stress-related changes in long-term potentiation, an ACh-related cerebral function, are triggered by interleukin-1. Microglial cultures (N9) synthesized and released IL-1 in response to conditioned media obtained from glutamate-treated primary neuron cultures or PC12 cells. This conditioned media contained elevated levels of secreted beta-amyloid precursor protein (sAPP). Naive PC12 cells cocultured with stimulated N9 cultures showed increased AChE activity and mRNA expression. These effects on AChE expression and activity could be blocked by either preincubating the glutamate-treated PC12 supernatants with anti-sAPP antibodies or preincubating naive PC12 cells with IL-1 receptor antagonist. These findings were confirmed in vivo; IL-1-containing pellets implanted into rat cortex also increased AChE mRNA levels. Neuronal stress in Alzheimer's disease may induce increases in AChE expression and activity through a molecular cascade that is mediated by sAPP-induced microglial activation and consequent overexpression of IL-1.

Present and future of Alzheimer therapy

Three major lines of drugs have been developed or are under development for the treatment of Alzheimer Disease (AD): cholinergic drugs (mainly cholinesterase inhibitors), anti-beta-amyloid drugs, estrogens and anti-inflammatories. Cholinesterase inhibitors are the only drugs presently approved in USA and Europe for the indication of AD. Cholinesterase inhibitors tested in clinical trials in Europe, USA and Japan include less than ten drugs, however most of these compounds have advanced to clinical trials III. Based on results related to a population of over 8,000 patients we conclude that several of these compounds have shown significant clinical efficacy and safety in the treatment of Alzheimer disease. There are, however, differences with regard to side effects. The major clinical effect is stabilization of cognitive function during a six- to 12-months period with a parallel improvement of behavioral symptoms. Long-term effect of cholinesterase inhibitors extending to a two year-period has been reported. Future applications of these drugs are treatment of other types dementias such as Lewy body dementia, vascular dementia and Down Syndrome dementia. Combination of cholinesterase inhibitors with estrogens, anti-oxidants and anti-inflammatories may represent a further improvement of the therapy. From the economical point of view, treatment with cholinesterase inhibitors is not cost neutral.

Neuroprotection in Alzheimer's disease - new strategies for treatment

Alzheimer's disease is the most common dementia disorder characterized by multiple pathological changes in the brain leading to a progressive memory loss and other cognitive symptoms producing occupational and social disabilities. Although a great deal of progress has been made in recent years in further understanding the genetic aberrations and patho-physiological processes of Alzheimer's disease there is still no cure of the disease. The transmitter replacement therapy is so far the most explored therapy. Three cholinesterase inhibitors have so far been approved and presently in clinical use in many countries. Although the cholinesterase inhibitors generally appear to produce symptomatic effects with palliative effect on existing cognitive disturbances recent data suggest that they also may have effect on progression of the disease including possible neuroprotective effects. Possible interactions between Abeta and cholinergic neurotransmission may exist. Treatment of cells with Abeta causes decreased cholinergic activity. Pretreatment of PC12 cells with cholinesterase inhibitors such as tacrine and donepezil in clinical relevant concentrations can attenuate Abeta (25-35) toxicity through mechanisms which may be mediated via nicotinic receptors. Estrogen has been shown to protect against Abeta toxicity in different cell lines and also to reduce the formation of Abeta. Its mechanism for the neuroprotective effect is however not fully clarified. A potentiation of the clinical effect of cholinesterase inhibitors in Alzheimer patients has been given together with estrogen. Experimental data suggest that the neuroprotective effect of estrogen as studied in PC12 cells was mediated at least partly via the alpha(7) nicotinic receptor. Treatment with Abeta in nanomolar concentrations for 7 days in PC12 cells significantly decreased the number of nicotinic receptor binding sites and mRNA levels. The effects by Abeta on nicotinic receptors are prevented by nicotine pretreatment. The finding suggests a possible link between Abeta and nicotinic receptor deficits in Alzheimer patients in the early course of the disease.

Tracking cholinergic pathways from psychological and chemical stressors to variable neurodeterioration paradigms

Cholinergic hyperexcitation can be induced by both acute psychological stress and exposure to acetylcholinesterase inhibitors. Both factors are known risk factors for delayed neurodeterioration processes such as Alzheimer's disease and Parkinson's disease. Recent publications on the involvement of cholinergic pathways in these and other neurodeterioration syndromes are reviewed.

A modular treatment of molecular traffic through the active site of cholinesterase

We present a model for the molecular traffic of ligands, substrates, and products through the active site of cholinesterases (ChEs). First, we describe a common treatment of the diffusion to a buried active site of cationic and neutral species. We then explain the specificity of ChEs for cationic ligands and substrates by introducing two additional components to this common treatment. The first module is a surface trap for cationic species at the entrance to the active-site gorge that operates through local, short-range electrostatic interactions and is independent of ionic strength. The second module is an ionic-strength-dependent steering mechanism generated by long-range electrostatic interactions arising from the overall distribution of charges in ChEs. Our calculations show that diffusion of charged ligands relative to neutral isosteric analogs is enhanced approximately 10-fold by the surface trap, while electrostatic steering contributes only a 1.5- to 2-fold rate enhancement at physiological salt concentration. We model clearance of cationic products from the active-site gorge as analogous to the escape of a particle from a one-dimensional well in the presence of a linear electrostatic potential. We evaluate the potential inside the gorge and provide evidence that while contributing to the steering of cationic species toward the active site, it does not appreciably retard their clearance. This optimal fine-tuning of global and local electrostatic interactions endows ChEs with maximum catalytic efficiency and specificity for a positively charged substrate, while at the same time not hindering clearance of the positively charged products.

Molecular modeling of the amyloid-beta-peptide using the homology to a fragment of triosephosphate isomerase that forms amyloid in vitro

The main component of the amyloid senile plaques found in Alzheimer's brain is the amyloid-beta-peptide (A beta), a proteolytic product of a membrane precursor protein. Previous structural studies have found different conformations for the A beta peptide depending on the solvent and pH used. In general, they have suggested an alpha-helix conformation at the N-terminal domain and a beta-sheet conformation for the C-terminal domain. The structure of the complete A beta peptide (residues 1-40) solved by NMR has revealed that only helical structure is present in A beta. However, this result cannot explain the large beta-sheet A beta aggregates known to form amyloid under physiological conditions. Therefore, we investigated the structure of A beta by molecular modeling based on extensive homology using the Smith and Waterman algorithm implemented in the MPsrch program (Blitz server). The results showed a mean value of 23% identity with selected sequences. Since these values do not allow a clear homology to be established with a reference structure in order to perform molecular modeling studies, we searched for detailed homology. A 28% identity with an alpha/beta segment of a triosephosphate isomerase (TIM) from Culex tarralis with an unsolved three-dimensional structure was obtained. Then, multiple sequence alignment was performed considering A beta, TIM from C.tarralis and another five TIM sequences with known three-dimensional structures. We found a TIM segment with secondary structure elements in agreement with previous experimental data for A beta. Moreover, when a synthetic peptide from this TIM segment was studied in vitro, it was able to aggregate and to form amyloid fibrils, as established by Congo red binding and electron microscopy. The A beta model obtained was optimized by molecular dynamics considering ionizable side chains in order to simulate A beta in a neutral pH environment. We report here the structural implications of this study.

Cysteine 144 is a key residue in the copper reduction by the beta-amyloid precursor protein

The beta-amyloid precursor protein (beta-APP) contains a copper-binding site localized between amino acids 135 and 156 (beta-APP(135-156)). We have employed synthetic beta-APP peptides to characterize their capacities to reduce Cu(II) to Cu(I). Analogues of the wild-type beta-APP(135-156) peptide, containing specific amino acid substitutions, were used to establish which residues are specifically involved in the reduction of copper by beta-APP(135-156). We report here that beta-APP's copper-binding domain reduced Cu(II) to Cu(I). The single-mutant beta-APP(His147-->Ala) and the double-mutant beta-APP(His147-->Ala/His149-->Ala) showed a small decrease in copper reduction in relation to the wild-type peptide and the beta-APP(Cys144-->Ser) mutation abolished it, suggesting that Cys144 is the key amino acid in the oxidoreduction reaction. Our results confirm that soluble beta-APP is involved in the reduction of Cu(II) to Cu(I).

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Antisense technologies have a future fighting neurodegenerative diseases

Our growing understanding of the role that unfavorable patterns of gene expression play in the etiology of neurodegenerative disease emphasizes the need for strategies to selectively block the biosynthesis of harmful proteins in the brain. Antisense technologies are ideally suited to this purpose. Tailor-designed to target specific RNA, antisense oligonucleotides and ribozymes offer tools to suppress the production of proteins mediating neurodegeneration. Although technical limitations must still be overcome, the antisense approach represents a novel and exciting strategy for intervention in diseases of the central nervous system.

Laminin 1 attenuates beta-amyloid peptide Abeta(1-40) neurotoxicity of cultured fetal rat cortical neurons

A growing amount of evidence indicates the involvement of extracellular matrix components, especially laminins, in the development of Alzheimer's disease, although their role remains unclear. In this study, we clearly demonstrate that laminin 1 inhibits beta-amyloid peptide (Abeta)-induced neuronal cell death by preventing the fibril formation and interaction of the Abeta peptide with cell membranes. The presence of laminin at a laminin/Abeta peptide molar ratio of 1:800 significantly inhibits the Abeta-induced apoptotic events, together with inhibition of amyloid fibril formation. The inhibitory effects of laminin 1 were time- and dose-dependent, whereas laminin 2 had less effect on Abeta neurotoxicity. A preincubation of laminin and Abeta was not required to observe the protective effect of laminin, suggesting a direct interaction between laminin 1 and Abeta. Moreover, laminin had no effect on the toxicity of the fibrillar Abeta peptide, suggesting an interaction of laminin with nonfibrillar species of the Abeta peptide, sequestering the peptide in a soluble form. These data extend our understanding of laminin-dependent binding of Abeta and highlight the possible modulation role of laminin regarding Abeta aggregation and neurotoxicity in vivo.