Aluminum and alzheimer's disease

Neurobiology qlAcing, Vol. 7, pp. 525-532, 1986. e Ankho Internat ional Inc. Pr inted in the U .S .A . 0197-4580/86 $3.00 + .00

REVIEW

Aluminum and Alzheimer's Disease

D. R. C R A P P E R M c L A C H L A N

Department of Physiology. 3318 Medical Sciences Building University of Toronto, Toronto, Canada

R e c e i v e d 25 April 1986

CRAPPER McLACHLAN, D. R. Aluminum and Alzheimer's disease. NEUROBIOL AGING 7(6) 525-532, 1986.--There is now substantial evidence indicating that an accumulation of aluminum occurs in grey matter in diseases associated with A Izheimer neurofibrillary degeneration. Four principle sites of aluminum accumulation have been identified in Alzheimer's disease: DNA containing structures of the nucleus, the protein moieties of neurofibrillary tangles, the amyloid cores of senile plaques and cerebral ferritin. Consideration of the extensive intormation now available on the toxic effects of aluminum in these four loci strengthens the hypothesis that aluminum could be important in the pathogenesis of this neurodegenerative process. The evidence, however, does not support an etiological role for aluminum in Alzheimer's disease. The primary pathogenic events responsible for Alzheimer's disease are presumed to have affected the genetically determined barriers to aluminum resulting in increased amounts of this toxic element to vulnerable target sites.

Alzheimer's disease Aluminum

FOLLOWING the first reports that neurotoxic concentrations of aluminum were present in the cerebral cortex of patients with Alzheimer's disease [11,13], a controversy arose concerning the significance and universal reproduc- ibility of these findings. Improved technology, particularly instruments capable of performing elemental analysis upon small tissue samples and specific intracellular loci, have now confirmed that elevated concentrations of aluminum occur in this disease. As summarized in Table I, at least 9 independ- ent laboratories employing 4 different techniques have reported elevated aluminum concentrations associated with Alzheimer's disease. Alzheimer affected brain tissues ob- tained from several geographic regions within North America, Britain, France, Germany, Australia and Japan have elevated aluminum content. Furthermore, elevated aluminum concentrations occur in neurodegenerative diseases associated with Alzheimer type neurofibrillary degeneration including neurofibrillary degeneration adjacent to a hamartoma [44], Down's syndrome [ 13,53], the parkinsonism-dementia and amyotrophic lateral sclerosis complexes of Guam [29, 54, 78, 82] and the Kii peninsula of Japan [77, 78, 81, 82] (Table 1). Possible explanations for the failure to detect elevated aluminum concentrations in association with Alzheimer histopathology have been reviewed elsewhere [17,41] and are probably related to differences in analytical techniques and case selection.

Four principle loci within Alzheimer affected tissues ex- hibit elevated concentrations of aluminum: DNA containing structures of the nucleus, the protein moieties of the neurofibrillary tangle, the amyloid cores of the senile plaque and cerebral ferritin (Table l).

The central question concerning aluminum is the role this common environmental agent may play in either the etiology or the pathogenesis of Alzheimer 's disease. A definitive evaluation of the consequences of aluminum accumulation in the Alzheimer degenerative process cannot be given at this time because sufficient understanding of the molecular events responsible for both the neurotoxic effects of aluminum and Alzheimer's disease is lacking. At present, there are two opposing points of view on the functional significance of aluminum in Alzheimer's disease: (1) aluminum merely accumulates passively in neurons compromised by the Alzheimer degenerative process and the accumulation is of no significance to the mechanisms of the disease; or alternatively, (2) aluminum is a plausible candidate for a neurotoxic environmental factor acting in the pathogenesis of neurodegenerative processes associated with Alzheimer type neurofibrillary degeneration.

H Y P O T H E S I S 1: A L U M I N U M AS A T R I V I A l , D I S E A S E M A R K E R

Neurofibrillary degeneration was discovered by A. AIz- heimer [1] with the aid of the Bielschowsky stain, a technique now known to depend upon the deposition of silver upon the highly acidic COOH-terminal tail segments of neurofilaments [28]. By analogy, the accumulation of aluminum in neurons exhibiting neurofibrillary tangles could be a passive process resulting from a marked increase in high affinity intraneuronal aluminum binding sites which occur as a secondary consequence of the degenerative process such as the formation of neurofibrillary tangles. Alternatively, aluminum might non-specifically "leak'" into neurons ex-

525

526 CRAPPER M(I~ACI-tI.AN

TABLE I

Aluminum Elevated in Alzheimers Disease

Tissue Author Year Technique Site Source

1. Crapper et al. [11,13] 1973, 1976 AA Total Tissue Canada 2. Duckett and GaUe [25] 1976 XMA Neuritic Plaque Cores France 3. Trapp et al. [71] 1978 AA Total Tissue Central USA 4. Crapperet al. [15] 1980 AA Nucleus Canada 5. Perl and Brody [53] 1980 XMA Nucleus N.E, USA 6. Candy et al. [8] 1985 XMA Amyloid of Neuritic England

Plaque Core 7. Masters et al. [46] 1985 AA Tangle and Plaque Cores Australia 8. Yoshimasu et al. [82] 1985 INA Total Tissue Japan 9. Joshi et al. [36] 1985 AA Ferritin S.E. USA

Aluminum Elevated in Tangle Bearing Neurons

Author Year Technique Tissue Source Disease

1. Liss et al. [44] 1979 AA Central USA

2. Yase [78] 1980 INA Guam and Kii Peninsula, Japan

3. Perl and Brody [53] 1982 XMA Guam ALS/PD 4. Garruto et al. [29] 1984 XMA Guam ALS/PD 5. Yoshimasu et al. [82] 1985 INA Guam ALS/PD

Neocortex adjacent to hamartoma

ALS/PD

Abbreviations for analytical techniques: AA: atomic absorption spectrophotometry; INA: instrumental neutron activation; XMA: x-ray microanalysis and wavelength dispersive spectrometry: and ALS/PD: amyotrophic lateral sclerosis and parkinsonism-dementia complex of Guam.

hibiting neurofibrillary degeneration as a secondary consequence of a dying cell. A third possibility would be that neurons undergoing neurofibrillary degeneration lack the mechanisms which extrude aluminum. At present, there is insufficient understanding of the pathogenesis of the Alz- heimer's disease to exclude any of these explanations.

HYPOTHESIS II: ALUMINUM AS A SIGNIFICANT DISEASE MARKER

A satisfactory model for the sequence of molecular events responsible for Alzheimer's disease is not yet available. Aluminum has been implicated in the pathogenesis of this disease on circumstantial evidence only. Direct cause and effect observations relevant to the etiology or pathogenesis of Alzheimer's disease for any agent or hypothesis are likely to be difficult to obtain because no animal species other than man undergoes an analogous neurodegenerative process. In View of the present lack of understanding of the pathogenesls of diseases associated with Alzheimer neurofibrillary degeneration, the known neurotoxic effects of aluminum in each of the four compartments listed in Table 1 require that aluminum be examined for a possible role in the degenerative process. However, considerable further work will be required to evaluate the relative importance of any of the known toxic effects of aluminum in the pathogenesis of Alz- heimer's disease.

N u c l e a r EJf i , c t s o f A l u m i n u m

The nucleus is a major site of accumulation of aluminum in the brains of susceptible experimental animals, normal human brain and in Alzheimer's disease [15,53]. In control human cerebral cortex, approximately 95% of total tissue aluminum occurs in the nuclear compartment. In aluminum sensitive animals, such as the cat and rabbit, approximately 81Y~ of the total tissue aluminum content, measured in the latter stages of the aluminum induced encephalopathy, occurs in the nucleus whereas only 20 to 30% of the total aluminum occurs in the nucleus of aluminum reststant animals such as the rat [15].

A number of in vitro effects of aluminum on nuclear processes have been reported. These include the formation of at least three complexes with DNA [37], a 50% reduction in DNA template accessibility of chromatin spreads to E, coli RNA polymerase [47,62], an inhibition of polytene chromosome puffing in response to a steroid hormone, ecdysterone [61], and a 25% decrease in vitro in 3H-corticosterone binding of rabbit hippocampal nuclei extracted in the early stages of an aluminum induced encephalopathy [60]. In these latter experiments the nuclei were extracted at a stage when hippocampal slices from litter mates exhibited deficits in long- term electrical potentiation [26] and the intact animals demonstrated deficits in the acquisition of learning and mem-

ALUMINUM AND ALZHE1MER'S DISEASE 527

ory tasks [56,57]. Aluminum also induces a 34% decrease in sister chromatid exchange in cultured lymphocytes and an increase in unscheduled DNA synthesis [22]. Aluminum inhibits ADP-ribosylation in vivo and in vitro, a mechanism considered important in DNA repair [18].

The in vivo effects of aluminum upon transcription processes have been assessed by comparing control and aluminum treated rabbit brain total RNA and poly(A) RNA yields at various stages of the encephalopathy. Aluminum effects are complex. The yield of total RNA was not altered significantly (p>0.05) during the encephalopathy [74], although a developmentally programmed increase in total RNA seen at 45 days post-natal development in control animals was inhibited at the p<0.08 level in the aluminum encephalopathy. Total poly(A) RNA was significantly elevated at the end of the asymptomatic period of the aluminum encephalopathy but decreased in the latter stages. Work in progess in this laboratory indicates that aluminum affects the pool size for specific RNAs in a complex manner. The yield of calmodulin messenger, an important calcium receptor molecule and regulator of intracellular calcium concentration, was reduced by about 30eX within 24 hours of aluminum injection and remained significantly depressed throughout the asymptomatic stages of encephalopathy in rabbit forebrain. The yield of total messenger RNA increased as did the pool size for a-actin whereas a-tubulin and neurofilament 68 KD mRNA was not affected.

Recent work in this laboratory also indicates that in neocortex affected by Alzheimer's disease both the yield of calmodulin, the peptide, and the messenger RNA encoding for calmodulin are markedly reduced compared to neurologically normal and non-Alzheimer neurologically damaged age and postmortem interval matched control brains. Whether the molecular mechanisms responsible for reduced calmodulin mRNA pool size are similar in the human disease and the experimental encephalopathy requires further work.

A change in chromatin structure towards a more condensed state also occurs in Alzheimer affected neocortex [19]. Compared to dialysis encephalopathy, multi-infarct dementia and other non-Alzheimer neurodegenerative diseases, there is a shift in the proportion of DNA to the highly condensed conformation of chromatin when probed with either ultrasound [14] or the enzyme micrococcal nuclease [42]. Dinucleosomes released during micrococcal nuclease digestion revealed an increase in the proportion of the linker histone, HI". In experiments in which the binding of linker histones to DNA were measured by elution with increasing concentrations of salt. Alzheimer affected nuclei demonstrated a marked increase in binding affinity compared to control [19]. Importantly. this effect is reproduced, in vitro, by the addition of aluminum ion in the ratio of approximately one aluminum atom per 20 base pairs in human cerebral nuclei but not in those extracted from rabbit or rat (un- published, this laboratoryl. Considerable species variations in linker histones occur and the strong possibility exists that aluminum may have an effect upon human linker histones which does not occur in laboratory animals. Changes in chromatin structure have also not been documented in nuclei extracted from the forebrain of rabbit during the aluminum encephalopathy. In the human disease, the amount of aluminum detected in intermediate euchromatin fractions prepared from Alzheimer affected neocortex was a significant predictor for the amount of highly condensed chromatin re- covered in the same preparation [15]. In summary, the pre- liminary evidence indicates that aluminum has specific ef-

fects upon messenger RNA pool size in laboratory animals and a species specific, in vitro, effect upon chromatin structure which is unique to man. Nevertheless, no molecular mechanism known to be disturbed in Alzheimer's disease has been unequivocally related to the presence of aluminum. A challenge for future investigation will be the identification of Alzheimer specific alterations in gene expression, the sub- sequent demonstration of the presence of aluminum at the appropriate site and the documentation of an aluminum effect upon gene expression which reproduces that found in Alzheimer's disease.

Alumi#u~m and the Cvto,skeleto#t

An important marker of aluminum neurotoxicity is the induction of neurofibrillary degeneration. The soluble salts of aluminum, applied either intracerebrally [10, 12, 39, 70] or systemically [21], induce in several classes of neurons, including the large pyramidal shaped neurons of the neo- and pyriform cortex and anterior horn cells of the spinal cord, a marked accumulation of dense bundles of 10 nm single filaments, apparently neurofilaments [66]. The neurofilaments of the aluminum induced tangles have epitopes indicative of hyperphosphorylation [72] which normally occurs beyond the soma and proximal processes. A recent hypothesis attri- butes the accumulation of neurofilaments in the soma and proximal processes in the aluminum encephalopathy to a failure of anterograde movement of the neurofilaments into axons [6i, possibly because hyperphosphorylation has altered the mechanisms responsible for the transport of neurofilaments [68]. Alternatively, the accumulation of dense bundles of hyperphosphorylated neurofilaments form- ing neurofibrillary tangles in the perisomal region could have resulted from an aluminum induced alteration in the struc- tural tie points for filaments already phosphorylated in the distal processes and extruded in a reverse direction into the perisomal and proximal dendritic regions of the neurons.

The neurofibrillary tangles of Alzheimer's disease differ morphologically from the aluminum induced tangle in laboratory animals. The Alzheimer tangle is composed of paired 10 nm filaments wound in a helix (PHFs). The chemical composition of the PHFs is uncertain at present. There are two principle concepts concerning Alzheimer type PHFs: in the most widely held model, PHFs are considered to be composed of modified cytoskeletal proteins. Epitopes for neurofilaments [55,58], microtubule associated proteins (MAP 2 proteins [50]; and Tau protein [40,51]) and vimentin [79] have all been reported in association with the AIz- heimer tangle. A unique antigenic site in Alzheimer tangles gives origin to the " P H F " antibody which reacts im- munologically with Alzheimer's tangles, Pick bodies [59] and the tangles of supranuclear palsy [24] but does not recognize normal proteins [35]. Alzheimer tangles, despite their location in neuronal perikarya, also react immunohistochemi- cally with monoclonal antibodies to phosphorylated epitopes of neurofilaments normally found in more distal processes [68]. The phosphorylated epitopes in human PHFs and ahlmi- num induced tangles in rabbit appear to differ. However, despite considerable new information about PHFs, a satisfactory model of how the various peptides, putatively identified by immunological means, are assembled into PHFs remains to be achieved.

The second model of PHFs has been offered by a group of workers who have presented evidence that PHFs arise fiom a unique protein not known to occur in normal brain tissue.

52S CRAPtq.: R M¢I ,A~,H I.A :'~

Masters et al. [45,461 reported that the amino acid sequence of purified cerebral amyloid and purified neurofibrillary tangles are identical and postulate that cerebral amyloid may be of neuronal origin. The monomeric form of these proteins appears to be about 3.5 kD and also reacts with the antibody raised to PHFs. The amino acid sequence of the 3.5 kD protein reported by Masters et al. [45,46] is unrelated to any known protein except amyloid found in Alzheimer's disease [30]. Resolution of the controversy concerning the chemical composition of PHFs must await further investigations.

Notwithstanding the uncertainty in the composition and structure of neurofibrillary degeneration, aluminum is present in the human Alzheimer tangle when examined with the electron probe and x-ray analysis [29,54]. Masters et a/. 146] report that the insoluble residue, remaining after formic acid extraction of protein from fractions enriched in PHFs and amyloid cores, contains aluminum silicate. These authors speculate that the aluminum silicates could form a linear crystal with cationic exchange sites capable of ordering molecules in a linear array. Candy et al. [8] have also reported aluminum silicate in the amyloid cores of senile plaques from Alzheimer's disease and Down's syndrome but at present neither report has presented evidence which indicates the exact physical state of the aluminum or silicon. However, there are two aluminum silicate crystal arrays found in nature which are of particular biological interest: montmorillonite and imogolite clays. The surface chemistry of montmorillonite clay reveals that the crystal lattice offers cationic exchange sites capable of cleaving peptide bonds and organizing biological molecules [76]. Montmorillonites and their homologues are silicates with a layered structure. The individual crystals of the layers are arranged in parallel sheets of 0.9 to 1.0 nm in thickness and in an aqueous en- vironment water molecules penetrate into the interlayer region to increase the interlayer distance. The replacement of Si ~+ by AI a+ creates an excess negative charge within the layer, a Lewis base site. Lewis base and acid sites represent sites of high catalytic activity. At a given charge density of the silicate, the more peptide bonds are cleaved the lower the content of lysine and arginine. This model raises the possibility that aluminum may act as a catalyst in the cleav- age and assembly of cytoskeletal constituents into PHFs.

Imogotite clays have a characteristic electron micro- scopic configuration: smooth, curved and often branched threads varying in diameter from 2 to 20 nm and extending for several p,m in length [48]. One speculation is that an imogolite fiber could act as the cental core for the assembly of various moieties of cytoskeletal constituents into the characteristic PHF configuration and contribute to the low solubility and resistance to enzyme degradation characteristic of PHFs. Further progress in this hypothesis will depend upon establishing whether the aluminum detected in the AIz- heimer tangle is in crystalline association with silicon.

In summary, aluminum may influence PHF formation through two types of mechanisms: one which results in the cytoplasmic accumulation of neurofilaments, as in the aluminum encephalopathy, and the other by crystal formation with silicon which contributes to the regular organization of peptide moieties into the unique PHF structure.

Aluminum and Ferritin

Joshi and associates [36] have reported that ferritin exists in human brain and that ferritin extracted from Alzheimer affected brain contains 10 times more aluminum than age and

sex matched controls. Aluminum bound to ferritm alters the binding properties of this important iron storage molecule and it has been postulated by these workers thai unbound i ron{ll~ salts might contribute to free radical formation and membrane damage through peroxidation of lipid membranes~ In vitro experiments of Gutteridge e/ a/. [311 indicate that aluminum greatly accelerates peroxidation of membrane lipids stimulated by iron salts. Aluminum salts themselves do not stimulate peroxidation of ox-brain phospholipid lipo- somes, but they greatly accelerate peroxidation induced by Fe(ll) salts at acidic pH. The effect is blocked by the triva- lent metal chelator desferoxamine. Oral aluminum hydroxide given for seven days increased rat brain lipid peroxidation by 1429~ when measured 24 hours after the last dose, whereas lipid peroxidation in kidney, lung, liver and spleen were not affected [52]. Furthermore the activity of superoxide dis- mutase, an enzyme which catalyzes the dismutation of the superoxide radical, is also inhibited by aluminum. These recent experimental findings plausibly implicate aluminum in toxic actions which could lead to membrane damage and possible neuron death. An aluminum effect upon lipid peroxidation deserves further investigation as one of the factors which contribute to cell death both in Alzheimer's disease and Guam amyotrophic lateral sclerosis.

Toxic Cytoplasmic Ey,/bc/s

While a large number of toxic cytoplasmic effects of aluminum have now been identified [16,20], e!ectrophysiologi- cal studies of isolated hippocampal slices removed at various stages of the aluminum encephalopathy indicate that a prin- cipal effect upon the electrical excitability of CA I pyramidal shaped neurons involves calcium homeostasis 126,27]. Fol- lowing the intracranial application of aluminum, brain tissue calcium rises progressively. An aluminum effect upon tissue calcium metabolism must be considered in the interpretation of the association of elevated calcium and aluminum in neurons undergoing neurofibrillary degeneration, particularly prominent in Guam PD/ALS neurofibriltary degeneration [29,54].

Yoshimasu and co-workers [81] and Yase [77,78], employing neutron activation, reported an increase in both calcium and aluminum in the spinal cord from cases of ALS from Guam and the Kii peninsula of Japan. These workers speculated that metallic ions such as aluminum may alter calcium transport. X-ray emission studies by Perl and Brody [53], Perl el al. {54] and Garruto el al. [29] demonstrated that aluminum and calcium were markedly elevated in neurons with neurofibrillary degeneration but manganese, magnesium, silicon and iron were not elevated. Since calcium accumulation occurs in injured and degenerating tissues in all body organs, the occurrence of increased concentrations of aluminum and calcium in neurons with neurofibrillary degeneration might be considered non-specific. However, in healthy brain tissues of rabbits a single intracraniat injection of aluminum results in a delayed, progressive, rise in total brain calcium content [27]. The rise in tissue calcium correlates well with the progressive behavioral and motor abnor- malities of the encephalopathy.

Total tissue calcium content of control, age matched, rabbit prosencephalon is 263/xg/g dry weight as measured by atomic absorption or inductively coupled plasma emission spectroscopy. About 7 to 10 days after aluminum injection, or at the end of the early, asymptomatic stage of the encephalopathy, the content had risen to 294 p,g/g. At the

ALUMINUM AND ALZHE1MER'S DISEASE 529

end of the second stage, characterized by learning-memory deficits and the appearance of minor motor control difficul- ties, the calcium content was 340 p,g/g. When animals reach the third, or terminal stage of the encephalopathy, they ex- hibit major motor dysfunction and the average total tissue calcium content was found to be 550/zg/g. While the total sodium and magnesium content in the same tissue is not altered as the calcium rises, potassium concentration increased by 16% in the last stage of the encephalopathy [74].

The aluminum induced changes in calcium content corre- late strongly with alterations in neuronal electrical activity and the appearance of behavioral and neurological motor signs of the encephalopathy. In hippocampal slices removed from control and aluminum injected rabbits at various stages of the encephalopathy, the normal relation between the population EPSP and the population CA1 spike output potential was altered indicating an increase in spike generat- ing threshold ]26]. No change was noted in the absolute am- plitude of either the evoked EPSP or antidromic compound action potential. The changes were partially reversed by increasing the calcium concentration in the bathing medium [27]. Long-term potentiation was also affected in the encephalopathy. Long-term potentiation of electrical activity within the hippocampus has been postulated as the mechanism responsible for electrical changes observed in the hippocampus during classical conditioning [5]. Calcium dependent steps appear to be involved in the alterations in membrane properties which modify synaptic efficacy at presynaptic 173] and postsynaptic sites [4]. The reduction in long-term potentiation is temporally related to the learning- memory changes which develop in the early middle stage of encephalopathy [9, 56, 57, 80]. Farnell et al. [27] demonstrated that modest depressions in long-term potentiation could be restored to control values when the concentration of calcium was increased in the bathing medium. Thus, aluminum disturbs the intracellular regulation of calcium and important calcium dependent electrophysiological functions. One cytotoxic effect of aluminum may involve calmodulin. Calmodulin (CAM) is a highly conserved ubiqui- tous protein with 4 high affinity calcium binding sites [3,49]. It has been implicated in a number of cellular functions including cyclic AMP regulation via both adenylate cyclase [7] and 3',5'-cyclic nucleotide phosphodiesterase [43], regulation of calmodulin dependent kinase activity [631 and interactions with both tubulin [23] and actin [75] to regulate the cytoskeleton.

Aluminum binds to CaM with 10 times higher affinity than calcium, induces a conformational change that reduces the alpha helical content by 30c~ and increases the total hydrophobic surface area 20-fold [64]. This may alter CaM's ability to bind substrate proteins and disturb the equilibrium between cytosolic and membrane bound CaM. Both these contkwmational changes are quenched by EGTA or by pre- treatment of CaM with citric acid. However, once aluminum binds to CaM, citric acid can only partially restore CaM to its native structure [69]. It is doubtful that CaM, freed from aluminum, remains fully active because a net reduction in the alpha helical content prevails. Nevertheless, in vivo, CaM may be partially protected from aluminum toxicity by endogenous citrate.

When aluminum binds to CaM at a 3:1 mole ratio, calmodulin is unable to stimulate phosphodiesterase in vitro [64]. Aluminum also inhibits CaM stimulation of Ca-Mg ATP- ase activity in barley root membranes at an M/CaM mole ratio of 3:1 [65]. This latter effect could result in the in vivo

accumulation of intracellular Ca [34]. Indeed, the activity of calmodulin as a calcium-calmodulin activator of the enzyme 3',5'-cyclic nucleotide phosphodiesterase declined progressively in the rabbit brain as the aluminum encephalopathy developed [27]. For hippocampal extracts, the Km for control tissue was 0.019 pM calmodulin, expressed in terms of calmodulin measured by radioimmune assay, and 0.038 pM in the middle stages of the encephalopathy. In the late stage, extracts prepared from aluminum treated hippocampi exhibited one third the calmodulin activity required to achieve the same rate of release of phosphorus as calmodulin extracted from control hippocampus, at a Km value of 0.053 pM. Since calmodulin as a major intracellular receptor of calcium and mediator of many calcium effects in eukaryotes, the effects of aluminum upon this pivotal molecule predict that the activity of calmodulin would no longer be regulated by calcium flux. In addition to the failure to maintain normal intracellular calcium concentration other calmodulin dependent biochemical processes may be altered.

Aluminum Tolerance Gene Hypothesis

Although aluminum is the third most common element in the earth's crust, 8% by weight, there is no known biological function for this element. The brain tissues of 6 common mammals have low, closely similar concentrations ranging between 1.1 and 1.9 /,~g A1/g dry weight [13]. The small crystal ionic radius and high charge (0.05 nm and val- ence 3) endow aluminum with prolonged ligand dissociation times, perhaps preventing the incorporation of this element into biological systems which require rapid, energy efficient, dissociation kinetics. Despite high bioavailability, the low cellular concentrations of aluminum implicate genetically determined regulatory mechanisms which may operate broadly in biological systems. Aluminum binds in aquatic plants and animals to the cell wall, as in certain algae and mac- rophytes, although some species do not appear to accumulate aluminum Ihydrodictyont (Hawls et al. ]33]). Crusta- ceans characteristically accumulate aluminum in salt exchange organs such as the chloride cells of Branchinecta and Daphnia. Fish accumulate aluminum on the gills. In aquatic animals that are sensitive to acidic water, aluminum accumulation on the gill system frequently results in defec- tive salt exchange 132]. Little is known about the genetic mechanisms regulating either the exclusion or tolerance to aluminum although the chromosome locations for the genes controlling aluminum tolerance in wheat, rye and triticale have been localized to loci on chromosomes 2, 3, 4, 6 and 7 [2]. Unfortunately, no information is available on the genetic loci important in the exclusion or turnover of aluminum in the mammalian brain. Nevertheless, the wide range of sen- sitivity to the neurotoxic effects of aluminum in mammals indicate genetic determinants for exclusion, detoxification and turnover. Rats and mice have considerably higher aluminum tolerance than cats and rabbits 1381. Perhaps resistant species have a surface configuration of genetically specified glycoproteins which exclude aluminum salts more effectively than susceptible species. This postulate argues that the presence of aluminum in human degenerative brain disease indicates that the primary etiological events responsible for the initiation of the disease, among other disturbances, alters either the membrane barriers or the aluminum tolerance gene permitting aluminum to accumulate in neurons. From this viewpoint, aluminum is not the cause of the disorder but may be an

530 CRAPPF, R M c l A f ' H 1 AN

important neurotoxic factor in the pathogenesis of degenera- t ive processes .

A process which could bypass the genetical ly determined barriers to a luminum uptake by neurons is the complexing of a luminum to a lipid soluble ligand. Dr. C. Orvig of the Uni- versi ty of British Columbia has synthesized compounds , such as a luminum maltol, which lower the lethal dose of applied intracranial a luminum 20-fold compared to the soluble salts o f a luminum. In a series of toxicological studies per formed in this laboratory, the aluminum brain concen- trat ion necessary to ach ieve LD50 in rabbit remained about 6 .5/zg/g dry weight o f brain, approximately the same average tissue concent ra t ion which has been est imated for o ther soluble salts of a luminum, H o w e v e r the applied intracranial dose of a luminum necessary to achieve this tissue concen t r a t ion employ ing a luminum maltol is only 5.5c~ that requi red for a soluble salt such as a luminum lactate. Maltol is found in the larch t ree , pine needles , ch icory , wood tars and oils and in roas ted malts . Malto] is also formed by alkaline hydrolysis of s t rep tomycese . Aluminum maltol is formed by heating for a few minutes an aqueous solution of a luminum and maltol at pH of 8. A systematic search in the env i ronment and food chain o f pat ients who deve lop Alzhe imer ' s disease and in geographic regions of high incidence of neurofibri l lary associated diseases such as on the island of Guam has not yet been comple ted , but toxicological study of this new class o f a luminum ligands raise the possibili ty that lipid soluble compounds could bypass the highly efficient genetical ly determined barriers for aluminum and permit the neuro tox ic express ion of this e lement .

CONCLUSION

Reconsidera t ion of the hypothesis that a luminum accumulat ion in A lzhe imer ' s disease is a trivial non-specif ic

binding of a metal appears unlikely because metals of muct~ grealer bioavailability such as iron, zinc. manganese and cobalt are not associated with the neurofibrillary process [54]. Aluminum is also e levated in brain ferritin, in thc amyloid cores o f neuritic plaques and on D N A containing structures which argues against a simple increase in cation binding sites in neurons undergoing neurofibrillary degeneration. Fur thermore , a luminum is not e levated in the cerebral spinal fluid in Alzhe imer ' s disease [67] and must gain access 1o neurons through the blood-brain-barrier which implies that not only is the membrane of the neuron with neurofibrillary degenerat ion " l e a k y , " but the astrocyt ic membranes and cytoplasmic processes may also have defect ive barriers to aluminum. If the accumulat ion of a luminum is the result of a defect ive extrusion mechanism, the defect in itself may not be of functional impor tance but the resulting high local con- centrat ion of a luminum could well be of importance in the pathogenesis of the disease.

Despite compell ing arguments that a luminum is a potential factor in the pathogenesis of Alzhe imer ' s disease, the establ ishment of a cause and effect relation demands a much greater comprehens ion of the molecular disorders responsible for Alzhe imer ' s disease and a precise documenta t ion of exact ly how aluminum acts at the molecular level in the human brain.

ACKNOWLEDGEMENT

Supported by the Ontario Mental Health Foundation.

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COMMENTARIES

No Evidence for Aluminum in Etiology and Pathogenesis of Alzheimer's Disease

H E N R Y M. W I S N I E W S K I , R O G E R C. M O R E T Z A N D K H A L I D I Q B A L

N e w York S t a t e Of f i ce o f M e n t a l R e t a r d a t i o n a n d D e v e l o p m e n t a l Disabi l i t ies In s t i t u t e f o r Bas i c R e s e a r c h in D e v e l o p m e n t a l Disabi l i t ies , S t a t e n Is land, N Y 10314

The review by Crappor McLachlan provides a one sided view in support of the possible role of aluminum in Alzheimer's disease without going into the biology, pathology and biochemistry of the disease or the disease proceSs. Aluminum has been implicated as a cause or an important factor in Alzheimer's disease, amyotrophic lateral sclerosis (ALS), ALS- parkinsonism dementia and dialysis dementia. However, outside of the dialysis dementia syndrome, to date there is no evidence that aluminum has a role in the observed pathological changes, signs and symptoms in any of these diseases.

A L Z H E I M E R ' S d i sease (AD) is a m a j o r d i so rde r a m o n g the and r e s e a r c h c o m m u n m e s . The e t iology of the d i sease and e lder ly popu la t i on and is a s ignif icant focus of the hea l th care fac tors in its p a t h o g e n e s i s are u n k n o w n , a l t hough a n u m b e r

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Aluminum and alzheimer's disease