Biochemical studies on rabbits with aluminium induced neurofilament accumulations

Memory performance, brain excitatory amino acid and acetylcholinesterase activity of chronically aluminum exposed mice in response to soy isoflavones treatment

Memory performance, brain excitatory amino acid and acetylcholinesterase activity of chronically aluminum (Al) exposed mice in response to soy isoflavones (SI) treatment was investigated in the study. Forty eight mice were allotted randomly into a control group, an Al exposed group (100 mg/kg Al) and an Al exposed group treated with SI (100 mg/kg Al + 60 mg/kg SI) for 60 days. Chronic Al exposure significantly impaired long memory performance in mice as assessed using a passive avoidance task test (χ(2) analysis, p < 0.05). Interestingly, SI treatment markedly improved the memory performance score in the Al exposed mice. This improvement was associated with a total reversal of Al-induced increases in acetylcholinesterase activity in the cerebral cortex and hippocampus of mice. The Al exposure also led to significant decreases in brain levels of aspartic and glutamic acids, two excitatory amino acid neurotransmitters; whereas SI treatment partially reversed the decreased aspartic and glutamic acid contents in the hippocampus. The results suggest that SI can improve long memory performance in the Al exposed mice, possibly by modulating the metabolism of brain acetylcholine and amino acid neurotransmitters.

Effect of long-term exposure to aluminum on the acetylcholinesterase activity in the central nervous system and erythrocytes

Aluminum (Al), a neurotoxic agent, has been associated with Alzheimer's disease (AD), which is characterized by cholinergic dysfunction in the central nervous system. In this study, we evaluated the effect of long-term exposure to aluminum on acetylcholinesterase (AChE) activity in the central nervous system in different brain regions, in synaptosomes of the cerebral cortex and in erythrocytes. The animals were loaded by gavage with AlCl(3) 50 mg/kg/day, 5 days per week, totalizing 60 administrations. Rats were divided into four groups: (1) control (C); (2) 50 mg/kg of citrate solution (Ci); (3) 50 mg/kg of Al plus citrate (Al + Ci), and (4) 50 mg/kg of Al (Al). AChE activity in striatum was increased by 15% for Ci, 19% for Al + Ci and 30% for Al, when compared to control (P < 0.05). The activity in hypothalamus increased 23% for Ci, 26% for Al + Ci and 28% for Al, when compared to control (P < 0.05). AChE activity in cerebellum, hippocampus and cerebral cortex was decreased by 11%, 23% and 21% respectively, for Al, when compared to the respective controls (P < 0.05). AChE activity in synaptosomes was increased by 14% for Al, when compared to control (P < 0.05). Erythrocyte AChE activity was increased by 17% for Al + Ci and 11% for Al, when compared to control (P < 0.05). These results indicate that Al affects at the same way AChE activity in the central nervous system and erythrocyte. AChE activity in erythrocytes may be considered a marker of easy access of the central cholinergic status.

Does neurotransmission impairment accompany aluminium neurotoxicity?

Neurobehavioral disorders, except their most overt form, tend to lie beyond the reach of clinicians. Presently, the use of molecular data in the decision-making processes is limited. However, as details of the mechanisms of neurotoxic action of aluminium become clearer, a more complete picture of possible molecular targets of aluminium can be anticipated, which promises better prediction of the neurotoxicological potential of aluminium exposure. In practical terms, a critical analysis of current data on the effects of aluminium on neurotransmission can be of great benefit due to the rapidly expanding knowledge of the neurotoxicological potential of aluminium. This review concludes that impairment of neurotransmission is a strong predictor of outcome in neurobehavioral disorders. Key questions and challenges for future research into aluminium neurotoxicity are also identified.

Disruption of Neuronal Calcium Homeostasis after Chronic Aluminium Toxicity in Rats

The present study was designed to investigate the possible effects of chronic aluminium exposure on the various aspects of calcium homeostasis in rat brain. Chronic aluminium administration caused significant rise in the intrasynaptosomal calcium levels. The activity of major calcium-expelling enzyme, i.e. Ca2+ ATPase was found to be lowered. Also, the calcium uptake via voltage-operated calcium channels increased significantly. Similar to the increase in intrasynaptosomal calcium, calpain activity was found to be increased. The results presented here, indicate that the toxic effects of aluminium could be mediated through modifications in the intracellular calcium homeostasis, which may lead to impaired neuronal function.

In vivo and in vitro effects of aluminum on the activity of mouse brain acetylcholinesterase

Cholinesterases are a large family of enzymatic proteins widely distributed throughout both neuronal and non-neuronal tissues. In Alzheimer's disease (AD), analytical as well as epidemiological studies suggest an implication of an abnormal focal accumulation of aluminum in the brain. In this devastating disease, aluminum may interfere with various biochemical processes including acetylcholine metabolism, and can thus act as a possible etiopathogenic cofactor. Acetylcholinesterase (AChE) exists in several molecular forms that differ in solubility and mode of membrane attachment rather than in catalytic activity. Mice were treated orally with aluminum chloride or aluminum lactate (Al(lac)(3)), and AChE activity in their brain homogenates was then assayed. Results showed that this in vivo treatment augmented the activity of the enzyme. An activating effect was also observed in vitro, when the aluminum compounds were added directly to mouse brain homogenates. However, the activating effect observed in vivo was much more marked than that observed in vitro. In addition, the activation produced by Al(lac)(3) was higher than that obtained after aluminum chloride treatment. Kinetics measurements of AChE activity in the absence and presence of treatment with aluminum both in vivo and in vitro are reported. The influence of the metal speciation on enzymatic activity is discussed in relation to a possible implication of aluminum in some neurodegenerative diseases.

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.

Possible factors in the etiology of Alzheimer's disease

Inherited cases of Alzheimer's disease (AD) comprise only a very small proportion of the total. The remainder are of unknown etiopathogenesis, but they are very probably multifactorial in origin. This article describes studies on four possible factors: aluminum; viruses--in particular, herpes simplex type I virus (HSV1); defective DNA repair; and head trauma. Specific problems associated with aluminum, such as inadvertent contamination and its insolubility, have led to some controversy over its usage. Nonetheless, the effects of aluminum on animals and neuronal cells in culture have been studied intensively. Changes in protein structure and location in the cell are described, including the finding in this laboratory of a change in tau resembling that in AD neurofibrillary tangles, and also the lack of appreciable binding of aluminum to DNA. As for HSV1, there has previously been uncertainty about whether HSV1 DNA is present in human brain. Work in this laboratory using polymerase chain reaction has shown that HSV1 DNA is present in many normal aged brains and AD brains, but is absent in brains from younger people. Studies on DNA damage and repair in AD and normal cells are described, and finally, the possible involvement of head trauma is discussed.

Cholinotoxic effects of aluminum in rat brain

The in vivo and in vitro effects of Al on the cholinergic system of rat brain were studied. The amount of Al accumulated after the chronic, intraperitoneal administration of aluminium gluconate (Al-G) or AlCl3, both at a dose of 1 mg/ml/100 g of body weight, increased in the frontal and parietal cortices, the hippocampus, and the striatum. Significantly decreased choline acetyltransferase activities after chronic Al treatment were measured in the parietal cortex, the hippocampus, and the striatum, but not in the frontal cortex. The acetylcholinesterase activity was not changed significantly in any brain area investigated. Both Al-G and AlCl3 administrations resulted in a general decrease (to 40-70% of the control values) in the specific l-[3H]nicotine binding, involving all brain areas studied. The specific (-)-[3H]quinuclidinyl benzilate binding was reduced (to 40-60% of the control values) only after 25 days of Al treatment. Al-G and AlCl3 were equivalent in eliciting these reductions in vitro studies revealed different alterations of the cholinergic system in response to Al treatment. No changes were observed either in choline acetyltransferase activity or in cholinergic receptor bindings. Both Al-G and Al2(SO4)3 treatments, however, exhibited a biphasic effect on the acetylcholinesterase activity. At low Al concentrations (10(-8)-10(-6) M), the activity was slightly increased, whereas at higher concentrations (10(-6)-10(-4) M), it was inhibited by a maximum of 25% as compared to the controls. Thus, these cholinotoxic effects are probably due not to a direct interaction between the metal and the cholinergic marker proteins, but rather to a manifestation and consequence of its neurodegenerative effects.

Aluminum, altered transcription, and the pathogenesis of Alzheimer's disease

The etiology of some, if not all, cases of Alzheimer's disease is linked to a mutation in the proximal portion of the long arm of chromosome 21∶21q11.2 → 21q22.2. While the functional consequences of the mutation are unknown, we speculate that one consequence of the mutation is loss of the natural barriers and intracellular ligands for aluminum. As a result, aluminum gains access to several brain sites including the nuclear compartment in certain neurons of the central nervous system.Both sporadic and familial Alzheimer's disease are associated with an increased compaction of DNA within chromatin as measured by physical shearing and resistance to digestion by micrococcal nuclease and DNase I. There is also an increase in linker histone Hl(o) content on dinucleosomes released by light (3-5% ASN) micrococcal nuclease digestion, and an increase in the affinity of histone Hl(o) for DNA as measured by a salt elution technique. The change in enzyme accessibility to chromatin also involves the 5' promoter region of at least one physiologically important gene: the gene which codes for the low molecular weight moiety of neurofilament (NF-L). The conformation change involving the 5' regulator region probably reduces transcription because the pool size of the mRNA coding for NF-L is reduced to 14% of age matched control in cerebral grey matter. Reduced transcription may account for many disorders in cellular metabolic processes including the regulation of phosphorylation, calcium homeostasis, free radical metabolism, proteolysis and neurotransmitter metabolism.The experimental evidence indicates that one important toxic action of aluminum in Alzheimer's disease neocortex is to increase the binding of histones, particularly Hl(o), to DNA which results in increased compaction of chromatin and reduced transcription. The supporting evidence includes: (1) A statistically reliable correlation between the aluminum to DNA ratio on intermediate euchromatin and the amount of highly condensed heterochromatin found in a given preparation from Alzheimer affected neocortex (Crapperet al., 1980). (2) A nine-fold increase in aluminum content in Alzheimer's disease in the di- and tri- nucleosome fraction released by light micrococcal nuclease digestion of nuclei from cerebral grey matter compared to age matched controls. Compared to age matched control dinucleosomes, the Alzheimer affected dinucleosomes contain an increased abundance of the linker histone Hl(o) and an increased proportion of DNA containing the promoter region of the gene coding for NF-L. (3) A reduction in abundance to 14% of control mRNA coding for NF-L in Alzheimer affected neocortex (Crapper McLachlanet al., 1988). (4) In vitro evidence that Alzheimer linker histones bind more tightly to DNA than control and that aluminum added to nuclei,in vitro, extracted from normal control brain, enhances DNA-protein binding of Hl and Hl(o) at concentrations found in the Alzheimer affected chromatin (Lukiwet al., 1987). (5) Application of a band retardation assay indicates that aluminum,in vitro, selectively binds human Hl(o) to a 300 bp human ALU DNA fragment from a crude extract of 5% per chloric acid soluble proteins. (6) Aluminum experimentally applied to rabbit CNS induces a marked reduction in NF-L mRNA in anterior horn cells (Mumaet al., 1988). We therefore conclude that aluminum plays a major role in the pathogenesis of Alzheimer's disease. Further understanding of the role of aluminum in Alzheimer's disease requires a detailed investigation of the precise sites of co-ordination of this trivalent metal within chromatin.

Aluminum neurotoxicity in mammals

Although aluminum comprises a large percentage of the Earth's crust, it is excluded from body tissues, and especially from the central nervous system. When aluminum is experimentally introduced to the central nervous system, several neurotoxic effects are observed:i.e. neurofibrillary changes, behavioral and cognitive deficits and enzymatic and neurotransmitter changes, as well as certain types of epileptic seizures.The localization of relatively high levels of aluminum in Alzheimer disease, Guamanian amyotrophic lateral sclerosis and Parkinsonism-dementia has led to the implication of aluminum as a pathogenic factor in these diseases. Recent studies have shown that microtubule-associated proteins are part of the paired helical filaments which make up the intraneuronal neurofibrillary tangle. Other studies have identified the protein making the vascular and neuritic (senile) plaque amyloid and located the gene responsible for this protein to chromosome 21.Our electron microprobe analysis studies have not found the levels of aluminum or silicon in either the neurofibrillary tangles or amyloid cores reported elsewhere, nor have the levels of aluminum been elevated in approximately one half of the tangles and plaque cores examined to date.

Neurochemical characteristics of aluminum-induced neurofibrillary degeneration in rabbits

Aluminum-induced neurofibrillary degeneration in rabbits is known to affect particular populations of neurons. The neurotransmitter alterations which accompany aluminum neurofibrillary degeneration were examined in order to assess how closely they mimic those of Alzheimer's disease. There was a significant reduction in choline acetyltransferase activity in entorhinal cortex and hippocampus as well as significant reductions in cortical concentrations of serotonin and norepinephrine in the aluminum-treated rabbits. Significant reductions in glutamate, aspartate and taurine were found in frontoparietal and posterior parietal cortex. Concentrations of GABA were unchanged in cerebral cortex. Both substance P and cholecystokinin immunoreactivity were significantly reduced in entorhinal cortex but there were no significant changes in somatostatin, neuropeptide Y and vasoactive intestinal polypeptide. The five neuropeptides were unaffected in striatum, thalamus, cerebellum and brainstem. Neurochemical changes were found in the regions with the most neurofibrillary degeneration while regions with little or no neurofibrillary degeneration were unaffected. The reductions in choline acetyltransferase activity, serotinin and noradrenaline suggest that some neuronal populations preferentially affected in Alzheimer's disease are also affected by aluminum-induced neurofibrillary degeneration; however, the cortical somatostatin deficit which is a feature of Alzheimer's disease is not replicated in the aluminum model.

Aluminum-induced decreases in choline acetyltransferase, tyrosine hydroxylase, and glutamate decarboxylase in selected regions of rabbit brain

The neuropathological and neurochemical effects of intracisternally administered aluminum-powder suspensions were studied in adult rabbits. The right half of each brain was fixed for neuropathological examination, and neurotransmitter-synthesizing enzyme activities were measured in homogenates of structures dissected from the left half of each brain. The neuropathological changes associated with aluminum-induced encephalomyelopathy, including neurofibrillary degeneration, were observed in several regions of the central nervous system of the aluminum-treated rabbits. The striatum was consistently free of changes. Decreases in choline acetyltransferase and tyrosine hydroxylase activities of more than 30% were observed in the striatum of animals within 14-21 d and at longer times after aluminum injection. The decrease in striatal choline acetyltransferase activity appears to be unrelated to pathological changes in the striatal cholinergic neurons. The decrease in tyrosine hydroxylase activity in the striatum may be unrelated to neuropathological changes in dopaminergic cell bodies in the midbrain. Significant decreases in glutamate decarboxylase activity in the cerebellum may be related to cell losses in this region, whereas choline acetyltransferase activity deficits in the whole hippocampus remain unexplained.

Aluminum and Alzheimer's disease: perspectives for a cytoskeletal mechanism

A considerable volume of literature has accumulated concerning the association of aluminum with Alzheimer's disease. The pathogenic mechanisms resulting in Alzheimer's disease remain unknown, but recent investigations have focused on cytoskeletal abnormalities as perhaps the key lesion in Alzheimer's disease and related neurological disorders. The diversity of neuronal functions that are dependent on cytoskeletal integrity suggests that subtle effects on polymerization, assembly, transcription, or processing of cytoskeletal elements may have significant and far-reaching neurological effects. That aluminum may participate in the development of neuropathological lesions characteristic of Alzheimer's disease is suggested by evidence that aluminum is a potent cytoskeletal toxin, produces cognitive deficits in laboratory animals, and can be detected within abnormal neurons isolated from brain tissue from Alzheimer's disease patients. In this review, a critical look will be taken at the enigmatic role aluminum has played in Alzheimer's disease research, the possibility of its pathogenicity, and its use as a tool for the investigation of cytoskeletal changes that may result in the biochemical and, ultimately, clinical manifestations of Alzheimer's disease.

Total and poly(A) RNA yields during an aluminum encephalopathy in rabbit brains

The yields of total and poly(A) RNA were examined in rabbit forebrains during an experimentally induced aluminum encephalopathy. Rabbits (35 day old) were injected intracranially with 13 mumole Al lactate and sacrificed 1, 3, 7, 10, or 12 days later. IRNA yields (total RNA minus transfer RNA) were not significantly altered during the encephalopathy. Poly(A) RNA yields, assayed by oligo(dT)-cellulose fractionation and by a [3H]poly(U) hybridization assay on IRNA, were increased significantly by the end of the asymptomatic stage of the encephalopathy (7 days post-Al injection). The increase in messenger RNA population may represent either a compensatory response to cell damage induced by aluminum or the accumulation of messenger RNA for proteins directly related to the expression of aluminum toxicity.