Aluminum, chemical physiology, and Alzheimer's disease

Functional roles of transferrin in the brain

BACKGROUND

Transferrin is synthesized in the brain by choroid plexus and oligodendrocytes, but only that in the choroid plexus is secreted. Transferrin is a major iron delivery protein to the brain, but the amount transcytosed across the brain microvasculature is minimal. Transferrin is the major source of iron delivery to neurons. It may deliver iron to immature oligodendrocytes but this trophic effect declines over time while iron requirements for maintaining myelination continue. Finally, transferrin may play an important role in neurodegenerative diseases through its ability to mobilize iron.

SCOPE OF REVIEW

The role of transferrin in maintaining brain iron homeostasis and the mechanism by which it enters the brain and delivers iron will be discussed. Its relevance to neurological disorders will also be addressed.

MAJOR CONCLUSIONS

Transferrin is the major iron delivery protein for neurons and the microvasculature, but has a limited role for glial cells. The main source of transferrin in the brain is likely from the choroid plexus although the concentration of transferrin at any given time in the brain includes that synthesized in oligodendrocytes. Little is known about brain iron egress or the role of transferrin in this process.

GENERAL SIGNIFICANCE

Neuron survival requires iron, which is predominantly delivered by transferrin. The concentration of transferrin in the cerebrospinal fluid is reflective of brain iron availability and can function as a biomarker in disease. Accumulation of iron in the brain contributes to neurodegenerative processes, thus an understanding of the role that transferrin plays in regulating brain iron homeostasis is essential. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The interrelationship between silicon and aluminium in the biological effects of aluminium

It is well established that aluminium is toxic at the cellular level and that pathological symptoms follow its entry into organisms (plants, fish, humans) when the normal exclusion mechanisms fail or are bypassed, as for example in renal dialysis. The present debate concerns the availability of environmental aluminium and the possible impact of its slow and insidious absorption and accumulation in vulnerable individuals. Silicon is considered as essential element but the mechanisms underlying its essentiality remain unknown and binding of the element (through oxygen) with biomolecules has not been demonstrated. There is, however, a unique affinity between aluminium and silicon, not only in solid state chemistry ([AlO4]5- and [SiO4]4- are isostructural), but also in aqueous solution chemistry as illustrated by the synthesis of zeolite from aluminate and silicate anions at high pH and under hydrothermal conditions. This affinity exists also in very dilute solution (< 10(-5) M) at near-neutral pH when hydroxyalumino-silicate species form. These species mediate the bioavailability and cellular toxicity of aluminium. The observed effects of silicon deficiency can be attributed to consequential aluminium availability. There are important implications for the epidemiology and biochemistry of aluminium-induced disorders and any consideration of one element must include the other.

Intracellular effects of aluminium on receptor-activated cytoplasmic Ca2+ signals in pancreatic acinar cells

The hypothesis that intracellular aluminium may interfere with cytoplasmic Ca2+ signals evoked by the activation of receptors linked to inositol lipid hydrolysis has been tested. Single mouse pancreatic acinar cells were used, because there is much information in this system on the mechanism by which acetylcholine (ACh) evokes cytoplasmic Ca2+ oscillations (spiking) and these spikes can be monitored in internally perfused cells by measuring the Ca(2+)-dependent chloride current. ACh normally evokes repetitive Ca2+ spikes, but when aluminium (1 microM-1 mM) is present in the internal perfusion solution the responses are reduced or absent. When aluminium is acutely infused into the internal perfusion solution the ACh-evoked Ca2+ signals quickly disappear. Aluminium also inhibits Ca2+ signals evoked by the Ca2+ releasing agent caffeine. Preliminary results suggest that silicic acid may protect against the toxic effects of aluminium. Silicic acid and citrate, in the absence of added Al3+, have the effect of enhancing the ACh-evoked Ca2+ signals. This could be due to binding of traces of Al3+ in the solutions. We conclude that aluminium can disrupt receptor-activated cytosolic Ca2+ signals when present inside cells.

Prognostic significance of low serum levels of Clara cell phospholipid-binding protein in occupational aluminium neurotoxicity

The relationship between respiratory and neurological effects of exposure to aluminium (Al) was investigated in a group of foundry workers exposed to Al at concentrations below the threshold limit value (TLV) binding in Poland (2.0 mg Al2O3 m(-3)). Neurological and neurophysiological parameters indicated subclinical effects of Al exposure on the nervous system. The measurement of serum anti-inflammatory Clara cell protein (CC16) was employed as a peripheral marker of the lung epithelium function. There was a strong inverse relationship between serum Al (Al-S) and CC16 concentrations (p = 0.006). The lowest CC16 concentrations were found in serum of workers characterised by subjective symptoms of the central nervous system (CNS) and abnormal results of neurophysiological examinations (EEG and VEP). Low serum CC16 concentrations and enhanced Al and iron (Fe) levels were also observed in the younger age group of workers with the subjective CNS symptoms and abnormal VEP results, which suggests that Fe is implicated in strengthening of the neurotoxic Al potential. The results of our study support the hypothesis that subclinical neurological symptoms (especially abnormal VEP) are most likely associated with internalisation of Al ions with lipid fractions of the lung epithelium, which in turn may help Al ions overcome the blood-brain barrier. Low serum CC16 concentrations (<10 microg L(-1)) were noted in workers with the abnormal results of neurological (CNS) and neurophysiological (EEG and VEP) examinations as well as with Al body burden manifested by urinary excretion (Al-U) below 60 microg L(-1) and Al-S concentration of 2 microg L(-1). This concentration may be considered as a threshold allowable biological concentration of aluminium.

A review of the possible relevance of inositol and the phosphatidylinositol second messenger system (PI-cycle) to psychiatric disorders--focus on magnetic resonance spectroscopy (MRS) studies

Myo-inositol is an important part of the phosphatidylinositol second messenger system (PI-cycle). Abnormalities in nerve cell myo-inositol levels and/or PI-cycle regulation has been suggested as being involved in the pathophysiology and/or treatment of many psychiatric disorders including bipolar disorder, major depressive disorder, panic disorder, obsessive-compulsive disorder, eating disorders and schizophrenia. This review examines the metabolism and biochemical importance of myo-inositol and the PI-cycle. It relates this to the current in vivo evidence for myo-inositol and PI-cycle involvement in these psychiatric disorders, particularly focusing upon the magnetic resonance spectroscopy (MRS) findings in patient studies to date. From this review it is concluded that while the evidence suggests probable relevance to the pathophysiology and/or treatment of bipolar disorder, there is much less support for a significant role for the PI-cycle or myo-inositol in any other psychiatric disorder. More definitive investigation is required before PI-cycle dysfunction can be considered specific to bipolar disorder.

Effects of aluminum chloride on sodium current, transient outward potassium current and delayed rectifier potassium current in acutely isolated rat hippocampal CA1 neurons

The effects of aluminum chloride (AlCl3) on sodium current (INa), the transient outward potassium (IA) and delayed rectifier potassium currents (IK) in hippocampal CA1 neurons of rats were studied using the whole cell patch-clamp technique. AlCl3 decreased INa, IA, and IK in a partly reversible, dose and voltage-dependent manner. AlCl3 prolonged the time to peak of INa, and increased the inactivation time constants of INa and IA . In addition, 1000 microM AlCl3 shifted the voltage dependence of steady-state activation of INa, IA and IK toward positive potential, and the voltage dependence of steady-state inactivation of INa, IA toward negative potential. These results imply that AlCl3 could affect the activation and inactivation courses of sodium current and potassium current of rat hippocampal CA1 neurons, which may contribute to damage of the central nervous system by aluminum.

The role of metals in neurodegenerative processes: aluminum, manganese, and zinc

Until the last decade, little attention was given by the neuroscience community to the neurometabolism of metals. However, the neurobiology of heavy metals is now receiving growing interest, since it has been linked to major neurodegenerative diseases. In the present review some metals that could possibly be involved in neurodegeneration are discussed. Two of them, manganese and zinc, are essential metals while aluminum is non-essential. Aluminum has long been known as a neurotoxic agent. It is an etiopathogenic factor in diseases related to long-term dialysis treatment, and it has been controversially invoked as an aggravating factor or cofactor in Alzheimer's disease as well as in other neurodegenerative diseases. Manganese exposure can play an important role in causing Parkinsonian disturbances, possibly enhancing physiological aging of the brain in conjunction with genetic predisposition. An increased environmental burden of manganese may have deleterious effects on more sensitive subgroups of the population, with sub-threshold neurodegeneration in the basal ganglia, generating a pre-Parkinsonian condition. In the case of zinc, there has as yet been no evidence that it is involved in the etiology of neurodegenerative diseases in humans. Zinc is redox-inactive and, as a result of efficient homeostatic control, does not accumulate in excess. However, adverse symptoms in humans are observed on inhalation of zinc fumes, or accidental ingestion of unusually large amounts of zinc. Also, high concentrations of zinc have been found to kill bacteria, viruses, and cultured cells. Some of the possible mechanisms for cell death are reviewed.

Effects of 60 Hz electromagnetic field exposure on APP695 transcription levels in differentiating human neuroblastoma cells

Epidemiological studies have suggested that workers with primary occupation that are likely to have resulted in the medium-to-high extremely low frequency (ELF) electromagnetic field (EMF) exposure are at increased risk of Alzheimer's disease (AD) pathogenesis. As a first step in investigating the possibility of an association between the ELF-EMF exposure and AD at the cellular level, we have used the differentiating IMR-32 neuroblastoma cells. In double-blind experiments, IMR-32 cells were exposed to the magnetic field intensities of 50, 100, and 200 microT at a frequency of 60 Hz for a period of 4 h at the three ages of differentiation (2, 10, and 16 days after incubation in differentiation medium). We used a custom-made Helmholtz coil setup driven by a 60-Hz sinusoidal signal from a function generator and an in-house built power amplifier. Total RNA extracted from the exposed cells was separated by the agarose gel electrophoresis and transferred to a nylon membrane for the northern hybridization. Digoxygenin-labeled APP695 RNA probes were used to detect changes in the APP695 mRNA levels in response to the ELF-EMF exposure. The results reported herein provided no support for any relationship between the APP695 gene transcription and IMR-32 differentiation age, as well as the magnetic field exposure. This study constitutes the first step towards investigating the possibility of an association between the ELF-EMF exposure and AD manifestations at the cellular level.

Immunostaining of calmodulin and aluminium in Alzheimer's disease-affected brains

Previous in vitro studies have shown that Al(3+) binds to calmodulin, inducing alterations in its capability to interact with target proteins, accompanied by loss of immunological recognition by its conformational specific monoclonal antibody CAM1. In spite of the wealth of data of calmodulin action in vitro, little information is available on the possible involvement of this protein in the pathology typical of Alzheimer's disease. In the present study, we investigated calmodulin immunoreactivity in post-mortem human brains affected by Alzheimer's disease, compared with age-matched control brains. Conformational monoclonal antibodies raised against Ca(2+)-calmodulin, namely CAM1 and CAM4, were used in this study for the characterization of calmodulin. Calmodulin immunorecognition by monoclonal antibody CAM1 was found to be lost in cortical tissue sample from brains affected by Alzheimer's disease. This finding leads to the hypothesis of a new, possibly inactive, conformation of the molecule during the disease. On the other hand, CAM4 immunoreactivity was decreased in neurons of brains affected by Alzheimer's disease. Anti-Al(3+) monoclonal antibodies revealed instead more marked aluminium immunoreactivity in the affected brains compared to normal ones. The loss of CAM1 immunoreactivity and the occurrence of large amounts of aluminium suggest an alteration of the active conformation of calmodulin in disease-affected brains. These alterations could be involved in the development of Alzheimer's disease pathology.

Disturbance of cerebral function in people exposed to drinking water contaminated with aluminium sulphate: retrospective study of the Camelford water incident

OBJECTIVE

To establish whether people exposed to drinking water contaminated with 20 tonnes of aluminium sulphate in the Camelford area of Cornwall in the south west of England in July 1988 had suffered organic brain damage as opposed to psychological trauma only.

DESIGN

Retrospective study of affected people.

PARTICIPANTS

55 affected people and 15 siblings nearest in age to one of the group but who had not been exposed to the contaminated water were studied.

MAIN OUTCOME MEASURES

Various clinical and psychological tests to determine medical condition and anxiety levels in affected people. Assessment of premorbid IQ (pFSIQ) with the national adult reading test, a computerised battery of psychomotor testing, and measurement of the difference in latencies between the flash and pattern visual evoked potentials in all participants.

RESULTS

The mean (SE) pFSIQ was above average at 114.4 (1.1). The most sensitive of the psychomotor tests for organic brain disease was the symbol digit coding (SDC) test (normal score 100, abnormal <85). PARTICIPANTS performed less well on this test (54.5 (6.0)) than expected from their pFSIQ (P<0.0001) and a little less poorly on the averaged less discriminating tests within the battery (86.1 (2.5), P<0.0001). In a comparison with the 15 sibling pairs (affected people's age 41.0 (3.3) years v sibling age of 42.7 (3.1) years (P=0.36) the exposed people had similar pFSIQ (114.7 (2.1)) to their siblings (116.3 (2.1), (P=0.59) but performed badly on the symbol digit coding test (51.8 (16.6)) v (87.5 (4.9) for siblings, P=0.03). The flash-pattern differences in exposed people were greater than in 42 unrelated control subjects of similar age (27.33 (1.64) ms v 18. 57 (1.47) ms, P=0.0002). The 15 unexposed siblings had significantly better flash-pattern differences than their affected siblings (13.4 (2.4) ms v 29.6 (2.9) ms, P=0.0002). No effect of anxiety could be shown on these measurements from the analysis of the anxiety scores of exposed people.

CONCLUSION

People who were exposed to the contaminated water at Camelford suffered considerable damage to cerebral function, which was not related to anxiety. Follow up studies would be required to determine the longer term prognosis for affected individuals.

Interaction of aluminum with PHFtau in Alzheimer's disease neurofibrillary degeneration evidenced by desferrioxamine-assisted chelating autoclave method

To demonstrate that aluminum III (Al) interacts with PHFtau in neurofibrillary degeneration (NFD) of Alzheimer's disease (AD) brain, we developed a "chelating autoclave method" that allows Al chelation by using trivalent-cationic chelator desferrioxamine. Its application to AD brain sections before Morin histochemistry for Al attenuated the positive fluorescence of neurofibrillary tangles, indicating Al removal from them. This method, applied for immunostaining with phosphorylation-dependent anti-tau antibodies, significantly enhanced the PHFtau immunoreactivity of the NFD. These results suggest that each of the phosphorylated epitopes in PHFtau are partially masked by Al binding. Incubation of AD sections with AlCl(3) before Morin staining revealed Al accumulation with association to neurofibrillary tangles. Such incubation before immunostaining with the phosphorylation-dependent anti-tau antibodies abolished the immunolabeling of the NFD and this abolition was reversed by the Al chelation. These findings indicate cumulative Al binding to and thereby antigenic masking of the phosphorylated epitopes of PHFtau. Al binding was further documented for electrophoretically-resolved PHFtau on immunoblots, indicating direct Al binding to PHFtau. In vitro aggregation by AlCl(3) was observed for PHFtau but was lost on dephosphorylation of PHFtau. Taken together, phosphorylation-dependent and direct PHFtau-Al interaction occurs in the NFD of the AD brain.

Aluminum-induced degeneration of astrocytes occurs via apoptosis and results in neuronal death

The mechanisms by which aluminum interacts with the nervous system are only partly understood. In this study, we used cultured astrocytes and neurons to investigate the effects of long exposures to aluminum (1 mM). We found that aluminum accumulated both in neurons and astrocytes. After 8-12 days exposure, aluminum caused strong changes in the morphology of astrocytes including shrinkage of cell bodies and retraction of processes. Exposures over 15-18 days reduced astrocytes viability by 50%. Aluminum-induced degeneration of astrocytes involved the DNA fragmentation characteristic of apoptosis, and staining of aluminum-treated astrocytes with the DNA-binding fluorochrome Hoeschst 33258 revealed the typical apoptotic condensation and fragmentation of chromatin. Aluminum was also found to be neurotoxic, causing first (4-6 days) abnormal clustering and aggregation, and later (8-12 days) neuronal death. Interestingly, aluminum neurotoxicity occurred in neuroglial cultures containing approximately 10% astrocytes but not in near-pure neuronal cultures containing only 1% astrocytes. Staining of co-cultured cells with Hoeschst 33258 showed apoptotic condensation and fragmentation of chromatin in aluminum-treated astrocytes but not in co-cultured neurons. Our study demonstrates that aluminum can induce the apoptotic degeneration of astrocytes, and that this toxicity is critical in determining neuronal degeneration and death. Aluminum-mediated apoptosis of cultured astrocytes may be also a valuable model system to study the mechanisms underlying apoptosis in glial cells.

Immuno-detection of aluminium and aluminium induced conformational changes in calmodulin--implications in Alzheimer's disease

Binding of calcium to calmodulin (CAM) induces specific structural rearrangements in the whole protein molecule. Ca2+ organizes and stabilizes the four-domains structure of calmodulin in a helical, active conformation that can bind to its target proteins; the central helix remaining flexible is an essential condition for their bio-recognition. The conformation of calmodulin, and its efficacy to interact with target proteins, is profoundly altered when bound to metal ions other than calcium. As recently reported, the local structural changes of CaM, which occur upon aluminium binding, lead to the impairment of protein flexibility and to the loss of its ability to interact with several other proteins, which may decrease or inhibit the regulatory character of calmodulin. In this study we followed conformational changes occurring in the calmodulin molecule after aluminium binding using highly specific monoclonal antibodies (mAbs) able to differentiate between the conformational states of calmodulin, as well as mAbs which recognize aluminium free or bound to proteins. Under the same experimental conditions, mAb CAM-1, a Ca2+ conformation sensitive antibody raised against calmodulin, fails to recognize the calmodulin-aluminium complex, despite the presence of Ca2+, while the anti-Al antibodies show a maximal binding pattern towards their antigen. These data suggest that Al3+ ions bind to calmodulin in the presence of Ca2+ ions, leading to an inactive, reversible conformation, instead of its physiological active form. Alteration of the conformation of calmodulin imposed by Al binding may have possible implications in the neurotoxicity mechanism related to Alzheimer's disease.

Phosphorylation sensitizes microtubule-associated protein tau to Al(3+)-induced aggregation

In Alzheimer's disease the microtubule-associated protein tau becomes hyperphosphorylated and aggregates into paired helical filaments (PHFs). Although the biochemical basis of the aggregation of tau into PHFs is not very clear, Al3+ has been suggested to play some role. Previous studies have shown that Al3+ alters the phosphorylation state and causes aggregation of tau in experimental animals and cultured neurons. In this study Al3+ inhibited phosphorylation of tau by neuronal cdc2-like kinase and dephosphorylation of phosphorylated tau by phosphatase 2B. These inhibitions are very likely due to Al(3+)-induced aggregations of various proteins present in phosphorylation/dephosphorylation assay mixtures since Al3+ caused aggregations of all proteins examined. Furthermore, compared to other proteins, tau displayed only an average sensitivity towards Al(3+)-induced aggregation. However upon phosphorylation, tau's sensitivity towards Al3+ increased 3.5 fold. In the presence of the metal chelator EDTA, Al(3+)-induced aggregates of tau became soluble, whereas Al(3+)-induced phosphorylated tau aggregates were insoluble in the buffer containing EDTA and remained insensitive to proteolysis. Our data suggest that phosphorylation sensitizes tau to Al3+ and phosphorylated tau transforms irreversibly into a phosphatase and protease resistant aggregate in presence of this metal ion.

Specificity of an anti-aluminium monoclonal antibody toward free and protein-bound aluminium

Anti-aluminium monoclonal antibodies (mAbs) were prepared using aluminium chloride-bovine serum albumin complex (Al-BSA) as immunogen. Competitive enzyme-linked immunosorbant assay (ELISA), using an Al-BSA coated immunoplate, demonstrated that mice immune sera showed stronger reactivity to AlCl3 than to BSA. Supernatants from hybridomas prepared from cloned anti-Al antibody-producing cells reacted in ELISA assays whether the metal was bound to proteins like calmodulin (CaM) and S100b protein or to immunogen BSA. Moreover, addition of citrate, a potent ligand for trivalent cations, resulted in a significant withdrawal in mAb recognition of aluminium which was previously bound to either CaM or S100b proteins. The anti-Al mAbs also reacted with aluminosilicate complexes formed from aluminium chloride and silicic acid. The results indicate that the monoclonal antibodies recognized aluminium alone, aluminium bound to silicate, or aluminium bound to a protein core and thus may be used as an immunologic tool for identifying aluminium in both in vitro and in vivo systems.

Neurodegenerative disorders in humans: the role of glutathione in oxidative stress-mediated neuronal death

Oxidative stress has been implicated in both normal aging and in various neurodegenerative disorders and may be a common mechanism underlying various forms of cell death including necrosis, apoptosis, and excitotoxicity. In this review, we develop the hypothesis that oxidative stress-mediated neuronal loss may be initiated by a decline in the antioxidant molecule glutathione (GSH). GSH plays multiple roles in the nervous system including free radical scavenger, redox modulator of ionotropic receptor activity, and possible neurotransmitter. GSH depletion can enhance oxidative stress and may also increase the levels of excitotoxic molecules; both types of action can initiate cell death in distinct neuronal populations. Evidence for a role of oxidative stress and diminished GSH status is presented for Lou Gehrig's disease (ALS), Parkinson's disease, and Alzheimer's disease. Potential links to the Guamanian variant of these diseases (ALS-PD complex) are discussed. In context to the above, we provide a GSH-depletion model of neurodegenerative disorders, suggest experimental verifications of this model, and propose potential therapeutic approaches for preventing or halting these diseases.

Kinetics of aluminum-induced inhibition of delta-aminolevulinic acid dehydratase in vitro

Anemia, one consequence of aluminum toxicity, may be due to inhibition of enzymes in the heme biosynthetic pathway. In this study, the in vitro effect of aluminum on rat liver and erythrocyte delta-aminolevulinic acid dehydratase (delta-ALA dehydratase), an enzyme that is sensitive to a number of metal ions, was investigated. The presence of 1-10 microM AlCl3 caused a concentration-dependent inhibition of liver delta-ALA dehydratase activity. The Ki for AlCl3-induced inhibition of delta-ALA dehydratase was 4.1 microM, and 10 microM AlCl3 virtually abolished delta-ALA dehydratase activity (99% inhibition). Erythrocyte delta-ALA dehydratase was also inhibited by similar concentrations of AlCl3 and displayed a Ki of 1.1 microM. AlCl3 (5 microM) decreased the Vmax by 50% but did not change the Km, suggestive of reversible, noncompetitive inhibition. Sodium citrate (50 microM) when added with AlCl3 completely restored delta-ALA dehydratase activity to basal levels. Thus, disruption of delta-ALA dehydratase occurred at low micromolar levels of AlCl3 in vitro, which may help to explain abnormalities in the heme pathway in cases of aluminum poisoning.

Effects of aluminum on neuronal signal transduction: mechanisms underlying disruption of phosphoinositide hydrolysis

1. Aluminum is neurotoxic in humans and animals and alters formation of inositol phosphate (IP) second messengers following in vivo or in vitro exposure. 2. Several components of the IP signalling system including G-proteins, phosphatidylinositol-specific phospholipase C (PI-PLC), protein kinase C (PKC) and Ca2+ homeostasis are susceptible to inhibition/disruption by aluminum compounds. 3. Recent evidence suggests that, despite its effects on other components, competitive inhibition by aluminum of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis by PI-PLC underlies its effects on agonist-stimulated IP generation.

Aggregation of amyloid precursor proteins by aluminum in vitro

The effect of various metal ions on aggregation of human recombinant amyloid precursor protein (APP) in vitro was investigated based on characterizations of altered migration on SDS-PAGE or immunoblots. Most biological metal ions tested had no significant effect on aggregation of APP. In contrast, AlCl3 in particular promoted aggregation of APP or APP-CT105 in a dose dependent manner. This effect of AlCl3 on APP mobility shift was prevented or reversed by the metal chelator, EDTA. Amorphous aggregates were observed in AlCl3 treated APP when examined by EM. These results suggest that aluminum may play a role in the pathogenesis of AD by directly promoting aggregation of APP.

Determination of serum aluminium using an ion-pair reversed-phase high-performance liquid chromatographic-fluorimetric system with lumogallion

An ion-pair reversed-phase high-performance liquid chromatographic method with fluorimetric detection, using lumogallion [4-chloro-3-(2,4-dihydroxyphenylazo)-2-hydroxybenzene-1-sulphonic acid] as a ligand, has been successfully applied to the determination of aluminium in human serum. The highly fluorescent aluminium-lumogallion complex (lambda ex 505 nm, lambda em 574 nm) was separated on a LiChrosorb RP-18 column with an eluent consisting of 30% acetonitrile, 70% 0.02 M potassium hydrogen phthalate and 10 microM lumogallion. The proposed system offers a simple, rapid, selective and sensitive method for the determination of aluminium in serum. The detection limit for aluminium was 0.05 microgram/l in aqueous solution and the limit of determination was 2.2 micrograms/l in serum. The recovery of the method is generally over 90%.

Effects of aluminium on the hepatic inositol polyphosphate phosphatase

There is speculation that some of the toxic effects of Al3+ may originate from it perturbing inositol phosphate/Ca2+ signalling. For example, in permeabilized L1210 mouse lymphoma cells, 10-50 microM Al3+ activated Ins(1,3,4,5)P4-dependent Ca2+ mobilization and Ins(1,3,4,5)P4 3-phosphatase activity [Loomis-Husselbee, Cullen, Irvine and Dawson (1991) Biochem. J. 277, 883-885]. Ins(1,3,4,5)P4 3-phosphatase activity is performed by a multiple inositol polyphosphate phosphatase (MIPP) that also attacks Ins(1,3,4,5,6)P5 and InsP6 [Craxton, Ali and Shears (1995) Biochem. J. 305, 491-498]: 5-50 microM Al3+ increased MIPP activity towards both Ins(1,3,4,5)P4 (by 30%) and Ins(1,3,4,5,6)P5 (by up to 500%), without affecting metabolism of InsP6. Higher concentrations of Al3+ inhibited metabolism of all three substrates, and in the case of InsP6, Al3+ altered the pattern of accumulating products. When 1-50 microM Al3+ was present, InsP6 became a less effective inhibitor of Ins(1,3,4,5)P4 3-phosphatase activity; this effect did not depend on the presence of cellular membranes, contrary to a previous proposal. The latter phenomenon largely explains how, in a cell-free system where Ins(1,3,4,5)P4 3-phosphatase is inhibited by endogenous InsP6, the addition of Al3+ can apparently increase the enzyme activity. However, there was no effect of either 10 or 25 microM Al3+ (in either the presence or absence of apotransferrin) on inositol phosphate profiles in either Jurkat E6-1 lymphoma cells or AR4-2J pancreatoma cells.

Actions of aluminum on voltage-activated calcium channel currents

1. Extracellular and intracellular effects of aluminum (Al) on voltage-activated calcium channel currents (VACCCs) of cultured rat dorsal root ganglion (DRG) neurons were investigated. Al (0.54 to 5.4 micrograms/ml = 20-200 microM) applied extracellularly reduces VACCCs in a concentration-dependent manner. The IC50 was calculated to be 2.3 micrograms/ml (83 microM). All types of VACCCs were similarly reduced by Al treatment. A slight shift of the current-voltage relation to depolarized potentials was observed for higher Al concentrations (> 2 micrograms/ml). The action of Al was found to be use dependent, with little recovery (max. 20%) after wash. 2. The effect of Al was highly pH dependent in the investigated range (pH 6.4 to 7.8). We observed a rightward shift of the concentration-response curve at pH 7.7 (IC50:3.1 micrograms/ml) and a leftward shift at pH 6.4 (IC50:0.56 microgram/ml) compared to the concentration-response curve at pH 7.3. 3. The VACCC declined when 2.7 micrograms/ml Al was added to the internal solution. A steady state was reached within a few minutes. Additional extracellular application of the same concentration lead to an additional decrease of the current. These observations strongly suggest the existence of both intracellular and extracellular accessible binding sites for Al on voltage-activated calcium channels (VACCs). 4. The special characteristics of the action of Al on VACCCs, i.e., the irreversibility, use dependence, and pH dependence, as well as the additional internal binding site may contribute to its neurotoxicity.

The effects of Al on the calcium currents in Helix neurons

1. The effects of aluminium (Al) on calcium (Ca) currents were investigated by using the conventional two-electrode voltage clamp technique in Helix pomatia neurons. The peak amplitude, kinetics, and voltage dependence of activation and inactivation of the Ca currents were studied in the presence of 10(-5)-10(-3) M AlCl3, at pH 6. 2. Al prolonged the rising phase of the Ca currents and therefore increased the time to peak at each command voltage step used. 3. There was no significant influence of Al on the peak amplitude of the Ca currents, but the voltage dependence of the time to peak, activation, and inactivation of the Ca currents shifted to more positive potentials as a consequence of Al treatment. 4. The leak currents were not influenced by Al up to 1 mM, which was the maximal dose applied. 5. The results support the suggestion that Al may modify the Ca homeostasis and that it exerts a neurotoxic effect, at least in part, by modulation of the Ca current of the neuronal membrane.

Aluminium inhibits muscarinic agonist-induced inositol 1,4,5-trisphosphate production and calcium mobilization in permeabilized SH-SY5Y human neuroblastoma cells

The effects of aluminium (as Al3+) on carbachol-induced inositol 1,4,5-trisphosphate (InsP3) production and Ca2+ mobilisation were assessed in electropermeabilised human SH-SY5Y neuroblastoma cells. Al3+ had no effect on InsP3-induced Ca2+ release but appreciably reduced carbachol-induced Ca2+ release (IC50 of approximately 90 microM). Al3+ also inhibited InsP3 production (IC50 of approximately 15 microM). Dimethyl hydroxypyridin-4-one, a potent Al3+ chelator (Ks = 31), at 100 microM was able to abort and reverse the effects of Al3+ on both Ca2+ release and InsP3 production. These data suggest that, in permeabilised cells, the effect of Al3+ on the phosphoinositide-mediated signalling pathway is at the level of phosphatidylinositol 4,5-bisphosphate hydrolysis. This may reflect interference with receptor-G protein-phospholipase C coupling or an interaction with phosphatidylinositol 4,5-bisphosphate.

Aluminum interaction with phosphoinositide-associated signal transduction

Concerning molecular and cellular mechanisms of aluminum toxicity, recent studies support the hypothesis that interactions of aluminum ions with elements of signal transduction pathways are apparently primary events in cells. In the case of the phosphoinositide-associated signalling pathway of neuroblastoma cells, guanine nucleotide-binding proteins (G proteins) and a phosphatidylinositol-4,5-diphosphate (PIP2)-specific phospholipase C are probable interaction sites for inhibitory actions of aluminum ions. Following interiorization of aluminum by the cell, metal interactions decrease the accumulation of inositol phosphates, especially that of inositol-1,4,5-triphosphate (IP3), concomitant with derangements of intracellular Ca2+ homeostasis. In the presence of high concentrations of Ca2+, formation of IP3 is also diminished in aluminum-pretreated cells, presumably involving a process not requiring Mg(2+)-dependent G proteins. At higher aluminum doses, metal-induced changes in the lipid milieu of the membrane-bound phospholipase may play a role. These types of primary interactions of aluminum ions with elements of cellular communication channels are probably crucial in the manifestation of the multifacetted aluminum toxicity syndrome. If present as a phosphate-like fluoro-aluminate, a stimulatory role of aluminum ions is displayed in G protein-coupled transmembrane signalling.

Increased iron in the substantia nigra compacta of the MPTP-lesioned hemiparkinsonian African green monkey: evidence from proton microprobe elemental microanalysis

The association of free radicals and particularly free iron in the pathogenesis of idiopathic Parkinson's disease and MPTP-induced parkinsonism remains controversial. Whereas the actual cause of dopamine cell death in the substantia nigra compacta (SNc) remains unknown, disturbances in lipid peroxidation and subsequent mitochondrial and cell membrane disruption has been demonstrated. In a genetically susceptible host, abnormal elimination of oxygen and trace metal free radicals may further damage dopamine cells. Using a unilaterally MPTP-treated African Green monkey, which showed obvious contralateral hemiparkinsonism, the total free iron concentration was measured. Iron, Fe2+ and Fe3+, but not other trace elements, was significantly elevated in the SNc compared with the opposite unlesioned side, which was similar to separate control animals. Iron content in the SNc, periaqueductal gray area, and crus cerebri was 228-270 ppm. Normal control SNc was 285 (+/- 59) ppm, whereas iron levels of 532 (+/- 151) ppm were found in the MPTP-lesioned SNc. These animals were drug naive and not on long-term levodopa maintenance. Proton microprobe elemental analysis was matched against adjacent immunocytochemically stained tissue slices to ensure the cells studied were in the SNc. Iron was found not only in the degenerating dopamine cells themselves but also in the surrounding matrix and glial cells. Whether free iron that is not bound to neuromelanin is responsible for dopamine cell death as suggested by these experiments remains to be proved.

Voltage gated calcium channel currents of rat dorsal root ganglion (DRG) cells are blocked by Al3+

The effects of the trivalent cation aluminum (Al3+) on voltage activated calcium channel currents were examined. Al3+ blocks sustained and transient components of voltage activated calcium channel currents of cultured rat dorsal root ganglion (DRG) cells. Currents were elicited by voltage jumps from -80 to 0 mV. The channel block was use dependent. Steady state blockade occurred after 1 to 5 min, when opening the channel every 10 s. There was little or no recovery after washing. Threshold concentration was about 20 microM Al3+ and total blockade (> 80%) was obtained at 200 microM Al3+; the IC50 was 83 microM and the Hill number was around 3. The degree of blockade was pH dependent, and increased with H+ concentration. The current-voltage relation frequently shifted to depolarised voltages after applying Al3+. The degree of the shift was a function of Al3+ concentration, but the magnitude differed from cell to cell. In the effective concentration range (< 200 microM Al3+) the effect was quite specific to voltage activated calcium channel currents. Voltage activated potassium and sodium channels were reduced less than 15% by 200 microM Al3+. We conclude that Al3+ is a potent and irreversible blocker of voltage activated calcium channel currents in mammalian neurons.

Aluminum silicate toxicity in cell cultures

To assess the cytotoxicity of four clays containing an aluminum silicate--montmorillonite, bentonite, kaolinite and erionite--we used human umbilical vein endothelial, N1E-115 neuroblastoma, and ROC-1 oligodendroglial cells. Morphological examination, lactate dehydrogenase release and fatty acid release were used as indices of trauma. The clays were added in suspension to the cell cultures at concentrations of 0.1, 0.03 and 0.01 mg/ml of medium and the cells were incubated for 1, 6 and 24 h. The clays did not lyse ROC-1 and N1E-115 cells and did not cause a dose-dependent increase in fatty acid levels at 24 h. There were no significant increases in lactate dehydrogenase activity in N1E-115 neuroblastoma or ROC-1 oligodendroglial cells. In human umbilical vein endothelial cells, montmorillonite, kaolinite and bentonite caused a dose-dependent increase in fatty acids at 24 h. All three clays caused cell lysis. We postulate that the cytotoxicity of the clays containing an aluminum silicate towards endothelial cells may disrupt the blood-brain barrier in the affected areas, allowing the entry of the clay particle into the brain. Aluminum silicate clays caused a dose-dependent release of fatty acids in human umbilical vein endothelial cells. The clays also caused lysis of these cells. ROC-1 oligodendroglia and N1E-115 neuroblastoma cells were not lysed by the clays, suggesting that this is not a general phenomenon.

The reduction of the inhibitory effect of aluminum on Na+ efflux in barnacle muscles fibers by preinjecting phosphate

The object of the present study was to test the hypothesis that the pre-enrichment of single muscle fibers from the barnacle Balanus nubilus with inorganic phosphate may protect the basal Na efflux from the inhibitory effect of Al injection. This approach was adopted in the light of evidence that the preinjection of ATP fails to stop the Na efflux in unpoisoned fibers from falling following the injection of Al. The results of the experiments are as follows: (i) Preinjection of K2HPO4 into unpoisoned fibers reduces the magnitude of the inhibitory effect on the basal Na efflux of injected Al in a dose-dependent manner but fails to completely stop it from occurring. (ii) Injection of K2HPO4 following Al into unpoisoned fibers fails to arrest or reverse the inhibitory effect of injected Al. (iii) Injection of K2HPO4 in a concentration as high as 0.5 M is without effect on the course of the basal Na efflux. (iv) Injection of K2HPO4 into ouabain-poisoned fibers fails to stop Al from stimulating the ouabain-insensitive Na efflux. Injection of K2HPO4 following peak stimulation by injecting Al is also without effect. (v) Injection of K2HPO4 in a concentration as high as 0.5 M is without effect on the course of the ouabain-insensitive Na efflux. Collectively, the results obtained with unpoisoned 'hypersensitive' fibers are consistent with the view that a significant fraction of the injected inorganic phosphate binds Al3+, and hence protects the basal Na efflux from the untoward action of Al3+.

Reactive oxygen species and the central nervous system

Radicals are species containing one or more unpaired electrons, such as nitric oxide (NO.). The oxygen radical superoxide (O2.-) and the nonradical hydrogen peroxide (H2O2) are produced during normal metabolism and perform several useful functions. Excessive production of O2.- and H2O2 can result in tissue damage, which often involves generation of highly reactive hydroxyl radical (.OH) and other oxidants in the presence of "catalytic" iron or copper ions. An important form of antioxidant defense is the storage and transport of iron and copper ions in forms that will not catalyze formation of reactive radicals. Tissue injury, e.g., by ischemia or trauma, can cause increased metal ion availability and accelerate free radical reactions. This may be especially important in the brain because areas of this organ are rich in iron and CSF cannot bind released iron ions. Oxidative stress on nervous tissue can produce damage by several interacting mechanisms, including increases in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that free radical reactions are involved in the neurotoxicity of aluminum and in damage to the substantia nigra in patients with Parkinson's disease are reviewed. Finally, the nature of antioxidants is discussed, it being suggested that antioxidant enzymes and chelators of transition metal ions may be more generally useful protective agents than chain-breaking antioxidants. Careful precautions must be used in the design of antioxidants for therapeutic use.

Membrane fluidity of platelets and erythrocytes in patients with Alzheimer's disease and the effect of small amounts of aluminium on platelet and erythrocyte membranes

The membrane fluidity of platelet and erythrocyte membranes in 10 Alzheimer's disease patients and 9 age-matched controls was studied. The platelet membranes of patients with Alzheimer's disease were found to be significantly more fluid than those of controls (p less than 0.02). However, erythrocyte membranes of Alzheimer patients were less fluid (more viscous) than those of controls (p less than 0.05). On further investigation of platelet and erythrocyte membranes obtained from healthy volunteers, the fluidity was found to change with increasing aluminium concentrations. When aluminium ammonium sulphate (0.01-10 microM) was added to membrane suspensions, the fluidity of platelet membranes was increased, whereas the fluidity of erythrocyte membranes was decreased (i.e. the microviscosity was increased).

Transferrin receptors of rat and human brain and cerebral microvessels and their status in Alzheimer's disease

We studied the regional distribution of specific [125I]transferrin binding to transferrin receptors in the brains and cerebral microvessels of humans and rats. We also assessed transferrin receptors in subjects with Alzheimer's disease. Human diferric [125I]transferrin bound to regional brain and cerebral microvessels with high affinity (dissociation constants of 1-10 nM), and the maximal binding densities ranged from 30 to 90 pmol/mg protein in the brain and were several-fold higher in cerebral microvessels. In Alzheimer's disease, transferrin receptor densities were significantly reduced in the hippocampus and the temporal and occipital cortex but were unchanged in the frontal and parietal cortex and the cerebellum. Although [125I]transferrin binding was higher in cerebral microvessels from subjects with Alzheimer's disease than in those of age-matched controls, this difference did not attain statistical significance. These results suggest that transferrin receptor density was decreased in some cortical areas including the hippocampus in Alzheimer's disease but relatively unchanged in cerebral microvessels.

Aluminium-adjuvanted vaccines transiently increase aluminium levels in murine brain tissue

Aluminium is widely used as an adjuvant in human vaccines, and children can often receive up to 3.75 mg of parenteral aluminium during the first six months of life. We show that intraperitoneal injection of aluminium adsorbed vaccines into mice causes a transient rise in brain tissue aluminium levels peaking around the second and third day after injection. This rise is not seen in the saline control group of animals or with vaccine not containing aluminium. It is likely that aluminium is transported to the brain by the iron-binding protein transferrin and enters the brain via specific transferrin receptors.

Aluminum content of source plasma and sodium citrate anticoagulant

The concentration of aluminum was determined in samples of source plasma collected by the normal plasmapheresis procedure, which involves collection in anticoagulant and immediate freezing. Samples of sodium citrate anticoagulant used in the collection of source plasma were also tested for aluminum, as were empty source plasma containers and 0.9% sodium chloride infusion (USP). Samples of source plasma were collected from a geographic cross-section of the donor population in the USA by three different manufacturers. Aliquots of these samples were mixed with Triton X-100 and sulfuric acid and analyzed for aluminum by atomic absorption spectrometry using electrothermal atomization (graphite furnace) and Zeeman background correction. The arithmetic mean and standard deviation for the aluminum content of 28 samples of source plasma were found to be 25.5 +/- 8.4 ng Al/ml. The aluminum content of the individual samples of source plasma ranged from 12 to 48 ng Al/ml. The aluminum content of 6 samples from two manufacturers of the sodium citrate anticoagulant that is used in source plasma ranged from 410 to 2,080 ng/ml. Aluminum levels found in saline for infusion and nitric acid leachates from empty source plasma containers were less than 6.9 ng/ml. The level of aluminum expected in uncontaminated human blood has been estimated to be 10 ng Al/ml or less. Comparison of this figure with the present data indicates that the sodium citrate anticoagulant contributes significantly to the aluminum load of source plasma and, therefore, to the aluminum content of products such as albumin derived from source plasma.

A cross-sectional study of plasma and urinary aluminum levels in term and preterm infants

High aluminum levels have been reported in sick and intravenously fed premature infants; however, aluminum is a ubiquitous pollutant of food. This study compares the usual aluminum levels of healthy newborns from birth to the third month of life with those of enterally fed premature infants free of renal failure. Plasma and urine concentrations were determined 66 times in full-term newborns (n = 58), 56 times in a group of preterm infants whose gestational age at birth was 28 to 32 weeks (n = 36) and 54 times in another group of preterm infants whose gestational age at birth was 33 to 36 weeks (n = 50). Daily aluminum intakes (+/- SE) of the full-term infants and the two groups of preterm infants were 0.42 +/- 0.05, 0.64 +/- 0.03, and 0.52 +/- 0.03 mumol/kg per day, respectively (p = .05). Plasma aluminum levels were 0.29 +/- 0.05, 0.49 +/- 0.06, and 0.39 +/- 0.05 mumol/L (p = .007); urine excretion levels were 0.80 +/- 0.12, 0.77 +/- 0.21, and 0.78 +/- 0.2 mumol of aluminum/mmol of creatinine (p value not significant). Although the metabolic consequences of the high aluminum intakes and blood levels we have observed in very low birth weight infants remain to be assessed, these results suggest that more attention should be paid to the aluminum status and intake of healthy premature babies.