Uptake of aluminium by human neuroblastoma cells

Aluminum enhances iron uptake and expression of neurofibrillary tangle protein in neuroblastoma cells

Aluminum (Al) and iron (Fe) have been implicated as playing a toxic role in the pathologic lesions of Alzheimer's disease. In the following report we describe the uptake and toxicity of Al, the effect of Al on Fe uptake, and the expression of neurofibrillary tangle (NFT) protein in murine neuroblastoma cells (Neuro 2A). Significant cell Al uptake and inhibition of cell growth were seen in Neuro 2A cells at 24, 48, 72, and 96 h after plating in medium containing Al transferrin (Al-Tf) and Al citrate. Al-loaded Neuro 2A cells showed increased rates of 59Fe and 125I-Tf uptake and total cellular Fe content at 24, 48, 72, and 96 h after plating compared with control cultures. Significant increases in NFT protein staining were detected in Al-exposed cells at 72 and 96 h in culture compared with controls. The intensity of NFT staining in Al-loaded cells was directly proportional to the time in culture. There was no difference in malonyldialdehyde levels measured in control versus Al-loaded Neuro 2A cells. These results suggest that the accumulation of Al in Neuro 2A cells resulted in increased uptake of Fe, inhibition of cell growth, and expression of NFT protein, partially mimicking the pathological hallmarks of Alzheimer's disease. This model system may also be applicable for Al-induced dialysis dementia, because the Al concentrations at which cell toxicity occurred can be found in dialysis patients.

A glutamatergic mechanism for aluminum toxicity in astrocytes

The effect of aluminum on the metabolism of glutamate and glutamine in astrocytes was studied to provide information about a possible biochemical mechanism for aluminum neurotoxicity and its potential contribution to neurodegenerative disease. Exposure of cultured rat brain astrocytes for 3-4 d to 5-7.5 mM aluminum lactate increased glutamine synthetase activity by 100-300% and diminished glutaminase activity by 50-85%. Increased glutamine synthetase enzyme activity was accompanied by an elevated level of glutamine synthetase mRNA. Alterations in glutaminase and glutamine synthetase following aluminum exposure caused increased intracellular glutamine levels, decreased intracellular glutamate levels, and increased conversion of glutamate to glutamine and the release of the latter into the extracellular space. The results of these changes may alter the availability of neurotransmitter glutamate in vivo and may be a mechanism for the aluminum neurotoxicity observed in individuals exposed to the metal during dialysis procedures and other situations.

The cellular toxicity of aluminium

Aluminium is a serious environmental toxicant and is inimical to biota. Omnipresent, it is linked with a number of disorders in man including Alzheimer's disease, Parkinson's dementia and osteomalacia. Evidence supporting aluminium as an aetiological agent in such disorders is not conclusive and suffers principally from a lack of consensus with respect to aluminium's toxic mode of action. Obligatory to the elucidation of toxic mechanisms is an understanding of the biological availability of aluminium. This describes the fate of and response to aluminium in any biological system and is thus an important influence of the toxicity of aluminium. A general theme in much aluminium toxicity is an accelerated cell death. Herein mechanisms are described to account for cell death from both acute and chronic aluminium challenges. Aluminium associations with both extracellular surfaces and intracellular ligands are implicated. The cellular response to aluminium is found to be biphasic having both stimulatory and inhibitory components. In either case the disruption of second messenger systems is observed and GTPase cycles are potential target sites. Specific ligands for aluminium at these sites are unknown though are likely to be proteins upon which oxygen-based functional groups are orientated to give exceptionally strong binding with the free aluminium ion.

The puzzle of Alzheimer's disease (AD)

A compromised defensive system of brain cells against aluminium, together with local defects in glucose metabolism, causes AD. Lack of citrate is a driving force and free cis-aconitate or glutamate are potential carriers, which enable the exotoxin to cross lipid membranes. Only a few aluminium ions replace magnesium in key positions. They block the reversibility of phosphorylation reactions, which are important for short term memory: sensitization of the insulin receptor and protein phosphorylations. Due to disturbed phosphorylation of the cytoskeleton, protein synthesis runs out of balance. Efforts to restore the disturbed reactions result in AD specific deposits. Aluminium ions are the common cause for the induction of AD pathogenesis in patients with genetic defects, with mechanical brain lesions or with minor infarcts, as well as with changes in the relation between numbers of neurons and neuron nursing glia cells due to age.

Aluminium toxicity and binding to Escherichia coli

The toxicity and binding of aluminium to Escherichia coli has been studied. Inhibition of growth by aluminium nitrate was markedly dependent on pH; growth in medium buffered to pH 5.4 was more sensitive to 0.9 mM or 2.25 mM aluminium than was growth at pH 6.6-6.8. In medium buffered with 2-(N-morpholino)ethanesulphonic acid (MES), aluminium toxicity was enhanced by omission of iron from the medium or by use of exponential phase starter cultures. Analysis of bound aluminium by atomic absorption spectroscopy showed that aluminium was bound intracellularly at one type of site with a Km of 0.4 mM and a capacity of 0.13 mol (g dry wt)-1. In contrast, binding of aluminium at the cell surface occurred at two or more sites with evidence of cooperativity. Addition of aluminium nitrate to a weakly buffered cell suspension caused acidification of the medium attributable to displacement of protons from cell surfaces by metal cations. It is concluded that aluminium toxicity is related to pH-dependent speciation [with Al(H2O)6(3+) probably being the active species] and chelation of aluminium in the medium. Aluminium transport to intracellular binding sites may involve Fe(III) transport pathways.

Human neuroblastoma cells treated with aluminium express an epitope associated with Alzheimer's disease neurofibrillary tangles

A number of studies have implicated aluminium as a possible factor in the pathogenesis of Alzheimer's disease (AD). Following an examination of the uptake of aluminium by human neuroblastoma cells in culture, treated with a range of concentrations of aluminium complexed with ethylene-diaminetetra-acetic acid (EDTA), we have now carried out an immunocytochemical study. Using an antibody to phosphorylated tau protein, which reacts specifically with AD neurofibrillary tangles (NFT), we have found that after treatment periods of 16 days to 8 weeks with aluminium-EDTA, the cells show positive staining with this antibody. No such reaction was detected in cells grown in medium alone, nor in aluminium-EDTA-treated cells subjected to the same immunocytochemical procedure but without added primary antibody. Cells grown in medium plus EDTA, which contains a low level of aluminium contamination, showed a slight reaction. Our system may provide a suitable model for studying the early changes which lead to NFT formation.