Acetylcholinesterase provides deeper insights into Alzheimer's disease

The effects of rivastigmine plus selegiline on brain acetylcholinesterase, (Na, K)-, Mg-ATPase activities, antioxidant status, and learning performance of aged rats

We investigated the effects of rivastigmine (a cholinesterase inhibitor) and selegiline ((-)deprenyl, an irreversible inhibitor of monoamineoxidase-B), alone and in combination, on brain acetylcholinesterase (AChE), (Na(+), K(+))-, Mg(2+)-ATPase activities, total antioxidant status (TAS), and learning performance, after long-term drug administration in aged male rats. The possible relationship between the biochemical and behavioral parameters was evaluated.

METHODS

Aged rats were treated (for 36 days) with rivastigmine (0.3 mg/kg rat/day ip), selegiline (0.25 mg/kg rat/day im), rivastigmine plus selegiline in the same doses and way of administration as separately. Aged and adult control groups received NaCl 0.9% 0.5 ml ip.

RESULTS

TAS was lower in aged than in adult rats, rivastigmine alone does not affect TAS, decreases AChE activity, increases (Na(+), K(+))-ATPase and Mg(2+)-ATPase activity of aged rat brain and improves cognitive performance. Selegiline alone decreases free radical production and increases AChE activity and (Na(+), K(+))-ATPase activity, improving cognitive performance as well. In the combination: rivastigmine seems to cancel selegiline action on TAS and AChE activity, while it has additive effect on (Na(+), K(+))-ATPase activity. In the case of Mg(2+)-ATPase selegiline appears to attenuate rivastigmine activity. No statistically significant difference was observed in the cognitive performance.

CONCLUSION

Reduced TAS, AChE activity and learning performance was observed in old rats. Both rivastigmine and selesiline alone improved performance, although they influenced the biochemical parameters in a different way. The combination of the two drugs did not affect learning performance.

Brain cholinesterases: I. The clinico-histopathological and biochemical basis of Alzheimer's disease

Substantial evidence is presented demonstrating that it is the cholinesterases (ChEs) that constitute the organizer, the connector and the safeguard for multiple neurochemical functions and mature anatomical architecture of the brain. In Alzheimer's disease (AD), the histopathological characteristics are initially and primarily associated with the degeneration of the acetylcholinesterase (AChE) system in various brain regions. Multiple classic and/or putative neurotransmitters and neuromodulators, virtually all the peptide hormones of the endocrine and neuroendocrine systems in the brain, their specific synthesizing and hydrolyzing marker enzymes and associated uptake processes (transporters), and receptors, do not actually participate in the formation of senile plaques and neurofibrillary tangles in the brains of patients suffering from AD. The massive perturbation in different neurochemicals seen in AD is essentially caused by the ChEs-associated pathology. The graded patterns of brain ChEs expression affect the preferential vulnerability and severity of the AD clinico-pathologic presentation. It seems that the common law in nature may also dominate the destiny of brain ChEs system, i.e., the weaker the cells express AChE, the more susceptible the cells are to AD degeneration, and vice versa.

Brain cholinesterases: II. The molecular and cellular basis of Alzheimer's disease

Currently available evidence demonstrates that cholinesterases (ChEs), owing to their powerful enzymatic and non-catalytic actions, unusually strong electrostatics, and exceptionally ubiquitous presence and redundancy in their capacity as the connector, the organizer and the safeguard of the brain, play fundamental role(s) in the well-being of cells, tissues, animal and human lives, while they present themselves adequately in quality and quantity. The widespread intracellular and extracellular membrane networks of ChEs in the brain are also subject to various insults, such as aging, gene anomalies, environmental hazards, head trauma, excessive oxidative stress, imbalances and/or deficits of organic constituents. The loss and the alteration of ChEs on the outer surface membranous network may initiate the formation of extracellular senile plaques and induce an outside-in cascade of Alzheimer's disease (AD). The alteration in ChEs on the intracellular compartments membranous network may give rise to the development of intracellular neurofibrillary tangles and induce an inside-out cascade of AD. The abnormal patterns of glycosylation and configuration changes in ChEs may be reflecting their impaired metabolism at the molecular and cellular level and causing the enzymatic and pharmacodynamical modifications and neurotoxicity detected in brain tissue and/or CSF of patients with AD and in specimens in laboratory experiments. The inflammatory reactions mainly arising from ChEs-containing neuroglial cells may facilitate the pathophysiologic process of AD. It is proposed that brain ChEs may serve as a central point rallying various hypotheses regarding the etio-pathogenesis of AD.

Acetylcholinesterase induces neuronal cell loss, astrocyte hypertrophy and behavioral deficits in mammalian hippocampus

Previous studies have demonstrated that acetylcholinesterase (AChE) promotes the assembly of amyloid-beta-peptides into neurotoxic amyloid fibrils and is toxic for chick retina neuronal cultures and neuroblastoma cells. Moreover, AChE is present in senile plaques in Alzheimer's disease (AD) brains. Here we have studied the effect of AChE on astrocytes and hippocampal neurons in vivo. Morphological as well as behavioral disturbances were analyzed after intrahippocampal injection of AChE. Rats were trained in the Morris water maze and assayed for behavioral parameters. Neuronal cell loss was found in the upper leaf of the dentate gyrus in rats injected with AChE in comparison with control animals. Glial fibrillary acidic protein immunoreactivity showed astrocytic hypertrophy and the magnitude of the response was associated with neuronal cell loss. Behavioral results show that injection of AChE produces cognitive impairment demonstrated by an altered water maze performance including (i) a higher escape latency score, (ii) a decreased spatial acuity and (iii) a shorter time of swimming in the platform quadrant. These findings indicate that a local increment in neuronal AChE concentration at the mammalian hippocampus, such as those present in amyloid deposits, may play a role in triggering neuropathological and behavioral changes such as those observed in AD brains.

Effect of long-term aluminum feeding on kinetics attributes of tissue cholinesterases

Aluminum (Al) is considered a potential etiological factor in Alzheimer's disease (AD). Neurotoxicity from excess brain exposure to Al is documented from both clinical observations and animal experiments. A key role of the acetylcholine system in memory disturbances that characterize AD has been reported. On this basis, we studied the effect of long-term Al feeding on kinetic properties of cholinesterases employing the rat as experimental model. Animals were given prolonged treatment with soluble salts of Al (100mg AlCl(3)/kg body weight mixed with food for 100-115 days), and the kinetic properties of cholinesterases (acetylcholinesterase, AChE, and butyrylcholinesterase, BChE) were determined in different tissues. Prolonged treatment with Al had no effect on the K(m) values of the soluble and membrane-bound forms of AChE in the brain, but V(max) was instead decreased in all the components of soluble and membrane-bound forms of AChE in the brain. In addition, the Al treatment resulted in complete loss of the component II of erythrocyte membrane AChE. Surprisingly, after prolonged treatment with Al, higher V(max) was observed in all the components of soluble and membrane-bound forms of BChE in the heart and liver. Variable effects of Al exposure were observed on temperature kinetic properties of cholinesterases. Altogether these findings indicate that long-term Al feeding results in inhibition of AChE, while an opposite effect is observed on BChE. Decreased V(max) of the brain AChE could represent the mode of action through which Al may contribute to pathological processes in Al-induced neurotoxicity.

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.

Neurotransmitters, peptides, and neural cell adhesion molecules in the cortices of normal elderly humans and Alzheimer patients: a comparison

Immunocytochemical techniques was used to compare the proportion of neurons expressing various neurotransmitters (tyrosine hydroxylase, choline acetyltransferase and gamma-aminobutyric acid), neuropeptides (Leu-enkephalin and substance P) and neural cell adhesion molecules (NCAM) in the hippocampus, frontal (area 10) and occipital (area 17) cortices of neurologically normal elderly humans to that of age-matched Alzheimer disease (AD) patients. There was no difference in the proportion of GABAergic and cholinergic cells between the normal and AD groups in all three brain regions studied. However, the catecholaminergic cells in the frontal cortex of the AD patients revealed a significant decrease. The catecholaminergic cells present in the cortex were both neurons and astrocytes, as revealed by a double immunostaining of tyrosine hydroxylase and glial fibrillary acid protein (GFAP). Furthermore, the difference in the proportion of cells expressing Substance P and Leu-enkephalin was minimal between the two groups studied. Although there was little difference in the levels of NCAM in the occipital cortex and hippocampus of the two groups, there were significantly fewer positive NCAM neurons in the frontal cortex of AD than normal aging individuals.

Pyridostigmine bromide and Gulf War syndrome

Gulf War Syndrome has become a growing concern of US government, military Gulf war veterans and their families. It is suggested that research on genotype/phenotype of acetylcholinesterase and butyrylcholinesterase may help to discover the role of pyridostigmine bromide in the cause of Gulf War Syndrome.

Stable complexes involving acetylcholinesterase and amyloid-beta peptide change the biochemical properties of the enzyme and increase the neurotoxicity of Alzheimer's fibrils

Brain acetylcholinesterase (AChE) forms stable complexes with amyloid-beta peptide (Abeta) during its assembly into filaments, in agreement with its colocalization with the Abeta deposits of Alzheimer's brain. The association of the enzyme with nascent Abeta aggregates occurs as early as after 30 min of incubation. Analysis of the catalytic activity of the AChE incorporated into these complexes shows an anomalous behavior reminiscent of the AChE associated with senile plaques, which includes a resistance to low pH, high substrate concentrations, and lower sensitivity to AChE inhibitors. Furthermore, the toxicity of the AChE-amyloid complexes is higher than that of the Abeta aggregates alone. Thus, in addition to its possible role as a heterogeneous nucleator during amyloid formation, AChE, by forming such stable complexes, may increase the neurotoxicity of Abeta fibrils and thus may determine the selective neuronal loss observed in Alzheimer's brain.

CSF cholinesterase activity in demented and non-demented subjects

Samples of cerebrospinal fluid (CSF) were examined, looking mainly at total cholinesterase (ChE) and acetyl-cholinesterase (AChE) levels from 139 living subjects. At the completion of the study, 35 of the 139 patients had died and pathological confirmation of the presence of dementia had been obtained. These results, together with results from other laboratories, provide evidence that a low CSF ChE level presenting in demented patients may indicate a depletion of the brain AChE system, and this may confirm a clinical diagnosis of AD as well as other types of dementia which are associated with an alteration of the brain AChE system. The overlap in the levels of CSF biochemical markers between demented and non-demented subjects which has led to many conflicting reports has always disappointed investigators. It is suggested that some 'control' subjects with CSF ChE activity indistinguishable from that in AD patients may have an abnormal ageing process in their brains (brain at risk), although the symptoms of dementia have not yet been detected. Recognition of a pre-clinical or incubation period is very beneficial for explaining discrepancies in biochemistry and pathology in the literature, and must be considered for both the treatment and the prevention of dementia. The long used treatment, which was designed to inhibit AChE, should no longer be used: treatment must be designed to enhance the activity of the neuronal AChE system, or slow its degeneration.

An CSF anamalous molecular form of acetylcholinesterase in demented and non-demented subjects

Analysis of 139 CSF samples from living subjects, using iso-electric focusing in polyacrylamide gels, demonstrated an anomalous molecular form of acetylcholinesterase (AChE). This form was present in 84 of 87 patients with a clinical diagnosis of Alzheimer's disease (AD), 28 of whom have died and in whom histopathological confirmation of AD was obtained. The abnormal AChE form was also present in 22 of 23 patients with clinical dementia not regarded as AD type. In the six patients who died this abnormal AChE form was found in three cases of multi-infarct dementia, one with cerebral glioma with dementia and one with clinical dementia, but no pathology was found based on the Khachaturian criteria for AD. One patient with normal pressure hydrocephalus was negative when tested for the abnormal AChE form. This evidence indicates that the anomalous molecular form of AChE may not be specific for AD, and may possibly be a common indicator for organic dementia. The discovery of this form in 27 of 29 age-matched non-demented controls may indicate that the anomalous molecular form of AChE may not only exist in patients with clinically detectable dementia, but is probably present for a period before the onset of dementia. Recognizing and understanding the existence of pre-clinical dementia would be beneficial in designing a strategy for both the prevention and the treatment of dementia.

Acetylcholinesterase promotes the aggregation of amyloid-beta-peptide fragments by forming a complex with the growing fibrils

Acetylcholinesterase (AChE), an enzyme involved in the hydrolysis of the neurotransmitter acetylcholine, consistently colocalizes with the amyloid deposits characteristic of Alzheimer's disease and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. In order to identify the structural domains of the amyloid-beta-peptide (Abeta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on Abeta peptide fragments of different sizes. AChE enhanced the aggregation of the Abeta(12-28) and Abeta(25-35) peptides but not of the Abeta(1-16) fragment. The inductive effect of AChE on the aggregation of Abeta(12-28) was abolished by the presence of either Abeta(1-16) or Abeta(9-21). The effect of the enzyme was also analysed using two different mutant fragments, possessing a low and the other a high capacity for fibrillogenesis. The fragments used were Abeta(12-28)Val18-->Ala and Abeta(12-28)Glu22-->Gln, respectively. AChE was able to promote the aggregation of these fragments in a very specific way and both mutant peptides were able to form amyloid fibrils, as revealed by negative staining under the electron microscope. Binding assays indicated that AChE was bound to Abeta(12-28), as well as to the Abeta(1-16) peptide. AChE was seen to form strong complexes with the Abeta(12-28) fibrils as such complexes stained positively for both thioflavine-T and AChE activity, were resistant to high ionic strength treatment, and were partially sensitive to detergents, suggesting that hydrophobic interactions may play a role in the stabilization of the AChE-Abeta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of Abeta peptides. This is consistent with the fact that AChE is known to be present within Abeta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain.

The cholinergic system in Alzheimer's disease

The past decade has witnessed an enormous increase in our knowledge of the variety and complexity of neuropathological and neurochemical changes in Alzheimer's disease. Although the disease is characterized by multiple deficits of neurotransmitters in the brain, this overview emphasizes the structural and neurochemical localization of the elements of the acetylcholine system (choline acetyltransferase, acetylcholinesterase, and muscarinic and nicotinic acetylcholine receptors) in the non-demented brain and in Alzheimer's disease brain samples. The results demonstrate a great variation in the distribution of acetylcholinesterase, choline acetyltransferase, and the nicotinic and muscarinic acetylcholine receptors in the different brain areas, nuclei and subnuclei. When stratification is present in certain brain regions (olfactory bulb, cortex, hippocampus, etc.), differences can be detected as regards the laminar distribution of the elements of the acetylcholine system. Alzheimer's disease involves a substantial loss of the elements of the cholinergic system. There is evidence that the most affected areas include the cortex, the entorhinal area, the hippocampus, the ventral striatum and the basal part of the forebrain. Other brain areas are less affected. The fact that the acetylcholine system, which plays a significant role in the memory function, is seriously impaired in Alzheimer's disease has accelerated work on the development of new drugs for treatment of the disease of the 20th century.

The significance of the activity of CSF cholinesterases in dementias

This article reviews the significance of changes in the level of cerebrospinal fluid acetylcholinesterase or cholinesterase in patients with Alzheimer's disease or other dementias. Evidence has shown that the methodology of assaying cerebrospinal fluid acetylcholinesterase or cholinesterase is reliable and the activity of the enzyme is stable. Low acetylcholinesterase or cholinesterase levels presenting in cerebrospinal fluid of a demented individual may confirm the clinical diagnosis of Alzheimer's disease or other organic dementia. A low activity of acetylcholinesterase or cholinesterase existing in cerebrospinal fluid of a non-demented individual may indicate a brain at risk, or that the person is in the preclinical stage of dementia. Recognition of the presence of the preclinical stage may be very beneficial for explaining the real meaning of the 'overlap' in the biochemistry and pathology between dementia and non-dementia, and also very important for prevention and treatment. Therefore, the strategy of prevention and of treatment should no longer be designed to inhibit acetylcholinesterase activity. In contrast, it should be designed to enhance the neuronal acetylcholinesterase activity or to delay the degeneration of brain acetylcholinesterase system.

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

Acetylcholinesterase (AChE), an important component of cholinergic synapses, colocalizes with amyloid-beta peptide (A beta) deposits of Alzheimer's brain. We report here that bovine brain AChE, as well as the human and mouse recombinant enzyme, accelerates amyloid formation from wild-type A beta and a mutant A beta peptide, which alone produces few amyloid-like fibrils. The action of AChE was independent of the subunit array of the enzyme, was not affected by edrophonium, an active site inhibitor, but it was affected by propidium, a peripheral anionic binding site ligand. Butyrylcholinesterase, an enzyme that lacks the peripheral site, did not affect amyloid formation. Furthermore, AChE is a potent amyloid-promoting factor when compared with other A beta-associated proteins. Thus, in addition to its role in cholinergic synapses, AChE may function by accelerating A beta formation and could play a role during amyloid deposition in Alzheimer's brain.