Protection against amyloid beta peptide toxicity by zinc

Metallothionein-3 and neuronal nitric oxide synthase levels in brains from the Tg2576 mouse model of Alzheimer's disease

Using antiserum against the recombinant isoform 3 of mouse brain metallothionein (MT3), the amount of MT3 protein was determined in whole brain homogenates from the Tg2576 transgenic mouse model of Alzheimer's Disease. Twenty-two month old transgenic positive mice showed a 27% decrease of MT3 normalized to the total protein in the extracts compared to same age, control transgenic negative mice. Metallothioneins bind seven molar equivalents of divalent metal ions per mole of protein so metal levels also were measured in these whole brain extracts using inductively coupled plasma atomic absorption (ICP-AA) spectrometry. No significant difference was observed for any metal assayed. Because neuronal nitric oxide synthase (nNOS) is involved in neurodegenerative disease and nitric oxide specifically interacts with MT3, the concentration and total nNOS activity also were evaluated. The transgenic positive mice showed a decrease of 28% in nNOS protein compared to the same age transgenic negative mice. Normalized to the amount of nNOS protein, total NOS activity was higher in the transgenic positive mice. These data showed that protein levels of both MT3 and nNOS were reduced in transgenic positive mice that show many characteristics of Alzheimer's Disease. In vitro studies suggested that MT3 was not a likely candidate for directly affecting nNOS activity in the brain.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Metals and amyloid-beta in Alzheimer's disease

Mounting evidence is demonstrating roles for the amyloid precursor protein (APP) and its proteolytic product Abeta in metal homeostasis. Furthermore, aberrant metal homeostasis is observed in patients with Alzheimer's disease (AD), and this may contribute to AD pathogenesis, by enhancing the formation of reactive oxygen species and toxic Abeta oligomers and facilitating the formation of the hallmark amyloid deposits in AD brain. Indeed, zinc released from synaptic activity has been shown to induce parenchymal and cerebrovascular amyloid in transgenic mice. On the other hand, abnormal metabolism of APP and Abeta may impair brain metal homeostasis as part of the AD pathogenic process. Abeta and APP expression have both been shown to decrease brain copper (Cu) levels, whereas increasing brain Cu availability results in decreased levels of Abeta and amyloid plaque formation in transgenic mice. Lowering Cu concentrations can downregulate the transcription of APP, strengthening the hypothesis that APP and Abeta form part of the Cu homeostatic machinery in the brain. This is a complex pathway, and it appears that when the sensitive metal balance in the brain is sufficiently disrupted, it can lead to the self-perpetuating pathogenesis of AD. Clinical trials are currently studying agents that can remedy abnormal Abeta-metal interactions.

The amyloid paradox: amyloid-beta-metal complexes can be neurotoxic and neuroprotective

Senile plaques in the brains of people with Alzheimer disease (AD) are primarily composed of the amyloid-beta (Abeta) peptide and contain substantially elevated levels of iron, copper and zinc. These metals bind to Abeta and have been reported to increase the toxicity of Abeta to cultured neurones. Other reports have demonstrated that Abeta can reduce the neurotoxicity of metal ions, suggesting that the interaction can, under some circumstances, be protective. To investigate these apparently conflicting results, human Abeta1-42 was co-injected with iron, copper or zinc (at the concentrations found in plaques) into rat cerebral cortex, and the resulting numbers of dying neurones were compared. It was found that Abeta complexed with either iron or zinc was more toxic than Abeta alone. In contrast, Abeta-copper complexes were not neurotoxic. Surprisingly, we observed that when iron or copper were combined with Abeta, the neurotoxicity of these metals was substantially reduced, suggesting that Abeta may help to limit the toxicity of redox-active metal ions, thereby assisting the antioxidant defence of the brain. Thus paradoxical effects occur when Abeta complexes with metal ions, where Abeta-metal complexes are capable of being neurotoxic and neuroprotective.

Protective effect of zinc on amyloid-beta 25-35 and 1-40 mediated toxicity

Amyloid beta-peptide (Abeta) is widely held to be associated with Alzheimer's disease, the insoluble aggregates of the peptide being the major constituents of senile plaques. In this study, we evaluated the effect of Zn(2+) (5, 50 and 200 microM) on Abeta induced toxicity using the human teratocarcinome (NT2) cell line. Our results proved that 50 and 200 microM Zn(2+) protected NT2 cells from Abeta 25-35 toxicity. Zinc was also shown to be effective by preventing the loss of mitochondrial membrane potential (DeltaPsi(m)) induced by Abeta 25-35, not allowing cytochrome c release from mitochondria, and subsequently, caspase 3 activation. However, when the cells were treated with Abeta 1-40, only Zn(2+) 5 microM had a protective effect. We have further observed that 5 microM Zn(2+) prevented Abeta 1-40 aggregation into a beta-sheet structure. Considering the results presented, we argue that Zn(2+) has a concentration-dependent protective effect.

A mechanism for zinc toxicity in neuroblastoma cells

Zinc is an important component of proteins essential for normal functioning of the brain. However, it has been shown in vitro that this metal, at elevated levels, can be toxic to cells leading to their death. We investigated possible mechanisms of cell death caused by zinc: firstly, generation of reactive oxygen species, and secondly, the activation of the MAP-kinase pathway. Cell viability was assessed by means of the methyl-thiazolyl tetrazolium salt (MTT) assay and confirmed by tetramethylrhodamine methyl ester (TMRM) staining. We measured the phosphorylation status of Erk and p38 as indicators of MAP-kinase activity, using Western Blot techniques. A time curve was established when neuroblastoma (N2alpha) cells were exposed to 100 microM of zinc for 4, 12, and 24 h. Zinc caused a significant reduction in cell viability as early as 4 h, and indirectly stimulated the accumulation of reactive oxygen species as determined by 2.7 dichlorodihydrofluorescein diacetate (DCDHF) staining and confocal microscopy. Investigation of the MAP-kinase pathway indicated that Erk was downregulated, while p38 was stimulated. Our results therefore led us to conclude that in vitro, zinc toxicity involved the generation of reactive oxygen species and the activation of the MAP-kinase pathway.

Zinc and energy requirements in induction of oxidative stress to retinal pigmented epithelial cells

In age-related macular degeneration (AMD), retinal pigmented epithelium (RPE) cells are believed to be detrimentally affected. It is thought that zinc may play a part in this process. In the past, therefore, zinc supplementation has been suggested as a treatment for AMD. Experimental data shown here confound this view by indicating that whereas low amounts of zinc do protect RPE cells in culture from stress-induced effects, greater amounts of zinc have the opposite influence. These effects are partly dependent upon the "health status" of the cells. Experimental data presented herein also show that zinc-induced death of RPE cells can, however, be attenuated by compounds such as antioxidants (alpha-tocopherol, trolox, and metipranolol), or cellular energy substrates (pyruvate and oxaloacetate). It is therefore concluded that a combination of zinc and antioxidants or energy substrates rather that zinc alone should provide a safer and more effective way to treat a disease such as AMD.

The metallobiology of Alzheimer's disease

The cause of Alzheimer's disease (AD) is closely related to the aggregation of a normal protein, beta-amyloid (Abeta), within the neocortex. Recently, evidence has been gathered to suggest that Abeta precipitation and toxicity in AD are caused by abnormal interactions with neocortical metal ions, especially Zn, Cu and Fe. However, Abeta might also participate in normal metal-ion homeostasis. An inevitable, age-dependent rise in brain Cu and Fe might hypermetallate the Abeta peptide, causing the catalysis of H(2)O(2) production that mediates the toxicity and auto-oxidation of Abeta. The greater incidence of AD in females could be due to greater constitutive activity of the synaptic Zn transporter ZnT3, and attenuated binding of metal ions to the rodent homologue of Abeta might explain why these animals are spared Alzheimer's pathology. Compounds that interdict metal-ion binding to Abeta dissolve brain deposits in vitro and one such compound, clioquinol, inhibits Abeta deposition in the Tg2576 mouse model for AD and could be useful clinically. These insights could also apply to other degenerative disorders in which metal-ion-protein interactions have been implicated.

Zinc takes the center stage: its paradoxical role in Alzheimer's disease

There is compelling evidence that the etiology of Alzheimer's disease (AD) involves characteristic amyloid-beta (Abeta) deposition, oxidative stress, and anomalous metal-Abeta protein interaction. New studies have implicated redox active metals such as copper, iron, and zinc as key mediating factors in the pathophysiology of Alzheimer's disease. There is also evidence that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. This paper reviews current observations on the etiologic role of zinc in AD. We also discuss the interactions of zinc and copper with Abeta, a factor that purportedly facilitates disease processes. Finally, we review the protective role of zinc against Abeta cytotoxicity and hypothesize how the apparent effect of zinc on AD pathology may be paradoxical, The Zinc Paradox. Indeed, complex pathologic stressors inherent to the Alzheimer's diseased brain dictate whether or not zinc will be neuroprotective or neurodegenerative. Further research on the zinc paradox in AD is needed in order to elucidate the exact role zinc plays in AD pathogenesis.

Antioxidant strategies for Alzheimer's disease

Oxidative damage is present within the brains of patients with Alzheimer's disease (AD), and is observed within every class of biomolecule, including nucleic acids, proteins, lipids and carbohydrates. Oxidative injury may develop secondary to excessive oxidative stress resulting from beta-amyloid-induced free radicals, mitochondrial abnormalities, inadequate energy supply, inflammation or altered antioxidant defences. Treatment with antioxidants is a promising approach for slowing disease progression to the extent that oxidative damage may be responsible for the cognitive and functional decline observed in AD. Although not a uniformly consistent observation, a number of epidemiological studies have found a link between antioxidant intake and a reduced incidence of dementia, AD and cognitive decline in elderly populations. In AD clinical trials molecules with antioxidant properties such as vitamin E and Ginkgo biloba extract have shown modest benefit. A clinical trial with vitamin E is currently ongoing to determine if it can delay progression to AD in individuals with mild cognitive impairment. Combinations of antioxidants might be of even greater potential benefit for AD, especially if the agents worked in different cellular compartments or had complementary activity (e.g. vitamins E, C and ubiquinone). Naturally-occurring compounds with antioxidant capacity are available and widely marketed (e.g. vitamin C, ubiquinone, lipoic acid, beta-carotene, creatine, melatonin, curcumin) and synthetic compounds are under development by industry. Nevertheless, the clinical value of these agents for AD prevention and treatment is ambiguous, and will remain so until properly designed human trials have been performed.

Determination of cadmium and zinc in Alzheimer's brain tissue using inductively coupled plasma mass spectrometry

In this work, brain tissue was taken from Alzheimer's Disease (AD) subjects (n=11), 'normal' subjects (n=10) and from subjects with senile involutive cortical changes (SICC) (n=6). Concentrations of Cd and Zn were determined in all samples, using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The brain tissue was selected and obtained from the Netherlands Brain Bank. Samples were taken in each case, from both hemispheres of the superior frontal gyrus, the superior parietal gyrus, the medial temporal gyrus, the hippocampus and the thalamus of the same brain.Cd which is known to have no essential role in the brain was found to follow, as expected, a lognormal distribution of concentrations in 'normal' subjects (Shapiro-Wilk's test (0.98) (p<0.18)). For the Alzheimer's Disease subjects and SICC subjects, the data tends to follow a lognormal distribution, rather than a normal distribution, but is still significantly different from it (Shapiro-Wilk's test (0.97) (p<0.03); (0.93) (p<0.0067), respectively)). In the case of Zn concentrations, the data tends to follow a normal distribution for the 'normal' subject group, even though the data is significantly different from it (Shapiro-Wilk's test (0.95) (p<0.001)). Whereas in the Alzheimer's Disease and SICC subject groups, the data follows a normal distribution (Shapiro-Wilk's test (0.98) (p<0.21); (0.97) (p<0.2002), respectively)). When comparing age-matched groups, for all regions and both hemispheres, no significant differences (p>0.1) for Cd were found between 'normals' and Alzheimer's Disease subjects and Alzheimer's Disease subjects and SICC but at a low level of significance, lower concentrations of Cd were found in the SICC group compared to the 'normals'. For all regions and both hemispheres, Zn was found to be significantly decreased in the Alzheimer's Disease group, compared to the 'normal' and SICC groups. Zn concentrations were also found to be significantly decreased in the 'normals' compared to the SICC group. It is also of interest that Cd negatively correlates with the scale of tangles in both 'normals' (p<0.001) and Alzheimer's Disease subjects (p<0.01). In the SICC subjects Cd correlates negatively with the tangles but not significantly so (p>0.1).

Carbonic anhydrase gating of attention: memory therapy and enhancement

Enhancement of memory acquisition and recall represents an important pharmacological goal in the treatment of cognitive disorders. In addition to its involvement in pH regulation, HCO3- reabsorption and CO2 expiration, carbonic anhydrase plays a crucial role in signal processing, long-term synaptic transformation and attentional gating of memory storage. Carbonic anhydrase dysfunction impairs cognition and is associated with mental retardation, Alzheimer's disease and aging. The pharmacological profile of carbonic anhydrase has been refined and specific activators have been developed. In this article, an integrated view of the involvement of carbonic anhydrase activity in synaptic plasticity and cognition will be presented, with particular focus on attentional gating of spatial learning and memory.

New insights on how metals disrupt amyloid beta-aggregation and their effects on amyloid-beta cytotoxicity

Amyloid-beta protein (A beta) aggregates in the brain to form senile plaques. By using thioflavin T, a dye that specifically binds to fibrillar structures, we found that metals such as Zn(II) and Cu(II) normally inhibit amyloid beta-aggregation. Another method for detecting A beta, which does not distinguish the types of aggregates, showed that these metals induce a non-beta-sheeted aggregation, as reported previously. Secondary structural analysis and microscopic studies revealed that metals induced A beta to make non-fibrillar aggregates by disrupting beta-sheet formation. These non-fibrillar A beta aggregates displayed much weaker Congo Red birefringence, and in separate cell culture experiments, were less toxic than self beta-aggregates, as demonstrated by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay. The toxicity of soluble A beta was enhanced in the presence of Cu(II), which suggests the previously hypothesized role of A beta in generating oxidative stress. Finally, under an acidic condition, similar to that in the inflammation associated with senile plaques, beta-aggregation was robustly facilitated at one specific concentration of Zn(II) in the presence of heparin. However, because a higher concentration of Zn(II) virtually abolished this abnormal phenomenon, and at normal pH any concentrations strongly inhibit beta-aggregation and its associated cytotoxicity, including its anti-oxidative nature we suggest that Zn(II) has an overall protective effect against beta-amyloid toxicity.

Mechanisms of amyloid beta protein-induced modification in ion transport systems: implications for neurodegenerative diseases

1. Alzheimer's disease (AD) is a neurodegenerative disorder that affects the cognitive function of the brain. Pathological changes in AD are characterized by the formation of amyloid plaques and neurofibrillary tangles as well as extensive neuronal loss. Abnormal proteolytic processing of amyloid precursor protein (APP) is the central step that leads to formation of amyloid plaque, neurofibrillary tangles, and neuronal loss. 2. The plaques, which accumulate extracellularly in the brain, are composed of aggregates and cause direct neurotoxic effects and/or increase neuronal vulnerability to excitotoxic insults. The aggregates consist of soluble pathologic amyloid beta peptides AbetaP[1-42] and AbetaP[1-43] and soluble nonpathologic AbetaP[1-40]. Both APP and AbetaP interact with ion transport systems. AbetaP induces a wide range of effects as the result of activating a cascade of mechanisms. 3. The major mechanisms proposed for AbetaP-induced cytotoxicity involve the loss of Ca2+ homeostasis and the generation of reactive oxygen species (ROS). The changes in Ca2+ homeostasis could be the result of (1) changes in endogenous ion transport systems, e.g. Ca2+ and K+ channels and Na+/K+-ATPase, and membrane receptor proteins, such as ligand-driven ion channels and G-protein-driven releases of second messengers, and (2) formation of heterogeneous ion channels. 4. The consequences of changes in Ca2+-homeostasis-induced generation of ROS are (a) direct modification of intrinsic ion transport systems and their regulatory mechanisms, and (b) indirect effects on ion transport systems via peroxidation of phospholipids in the membrane, inhibition of phosphorylation, and reduction of ATP levels and cytoplasmic pH. 5. We propose that in AD, AbetaP with its different conformations alters cell regulation by modifying several ion transport systems and also by forming heterogeneous ion channels. The changes in membrane transport systems are proposed as early steps in impairing neuronal function preceding plaque formation. We conclude that these changes damage the membrane by compromising its integrity and increasing its ion permeability. This mechanism of membrane damage is not only central for AD but also may explain other malfunctioned protein-processing-related pathologies.

Thalamic distribution of zinc-rich terminal fields and neurons of origin in the rat

Several cortico-cortical and limbic-related circuits are enriched in zinc, which is considered as an important modulator of glutamatergic transmission. While heavy metals have been detected in the thalamus, the specific presence of zinc has not been examined in this region. We have used two highly sensitive variations of the Timm method to study the zinc-rich innervation in the rat thalamus, which was compared to the distribution of acetylcholinesterase activity. The origin of some of these zinc-rich projections was also investigated by means of retrograde transport after intracerebral infusions of sodium selenium (Na2SeO3). The overall zinc staining in the thalamus was much lower than in the neocortex, striatum or basal forebrain; however, densely stained terminal fields were observed in the dorsal tip of the reticular thalamic nucleus, the anterodorsal and lateral dorsal thalamic nuclei and the zona incerta. In addition, moderately stained zinc-rich terminal fields were found in the rostral intralaminar nuclei, nucleus reuniens and lateral habenula. Intracerebral infusions of Na2SeO3 in the lateral dorsal nucleus resulted in retrogradely labeled neurons that were located in the postsubiculum, and also in the pre- and parasubiculum. These results are the first to establish the existence of a zinc-rich subicular-thalamic projection. Similar infusions in either the intralaminar nuclei or the zona incerta resulted in labeling of neurons in several brainstem structures related to the reticular formation. Our results provide morphological evidence for zinc modulation of glutamatergic inputs to highly selective thalamic nuclei, arising differentially from either cortical limbic areas or from brainstem ascending activation systems.

Evidence that the beta-amyloid plaques of Alzheimer's disease represent the redox-silencing and entombment of abeta by zinc

Abeta binds Zn(2+), Cu(2+), and Fe(3+) in vitro, and these metals are markedly elevated in the neocortex and especially enriched in amyloid plaque deposits of individuals with Alzheimer's disease (AD). Zn(2+) precipitates Abeta in vitro, and Cu(2+) interaction with Abeta promotes its neurotoxicity, correlating with metal reduction and the cell-free generation of H(2)O(2) (Abeta1-42 > Abeta1-40 > ratAbeta1-40). Because Zn(2+) is redox-inert, we studied the possibility that it may play an inhibitory role in H(2)O(2)-mediated Abeta toxicity. In competition to the cytotoxic potentiation caused by coincubation with Cu(2+), Zn(2+) rescued primary cortical and human embryonic kidney 293 cells that were exposed to Abeta1-42, correlating with the effect of Zn(2+) in suppressing Cu(2+)-dependent H(2)O(2) formation from Abeta1-42. Since plaques contain exceptionally high concentrations of Zn(2+), we examined the relationship between oxidation (8-OH guanosine) levels in AD-affected tissue and histological amyloid burden and found a significant negative correlation. These data suggest a protective role for Zn(2+) in AD, where plaques form as the result of a more robust Zn(2+) antioxidant response to the underlying oxidative attack.

Histochemically-reactive zinc in amyloid plaques, angiopathy, and degenerating neurons of Alzheimer's diseased brains

Excess brain zinc has been implicated in Alzheimer's neuropathology. Here we evaluated that hypothesis by searching the brains of Alzheimer's patients for abnormal zinc deposits. Using histochemical methods, we found vivid Zn2+ staining in the amyloid deposits of dense-core (senile) plaques, in the amyloid angiopathy surrounding diseased blood vessels, and in the somata and dendrites of neurons showing the characteristic neurofibrillary tangles (NFT) of Alzheimer's. In contrast, brains from age-matched, non-demented subjects showed only occasional staining for Zn2+ in scattered neurons and possible plaques. A role of abnormal zinc metabolism in Alzheimer's neuropathology is suggested.

Effect of zinc ions on the cytotoxicity induced by the amyloid beta-peptide

Insoluble aggregates of the amyloid beta-peptide (A beta) are a major constituent of senile plaques found in brains of Alzheimer's disease patients. The beta-amyloid fragment A beta(1-40) is toxic to rat pheochromocytoma PC12 cells, leading to a concentration-dependent decrease in the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). The detrimental effects of A beta(1-40) are enhanced in the presence of 1 mM zinc, whereas 50 microM zinc exerts a protective effect against A beta(1-40)-induced toxicity. Exposure of PC12 cells to low zinc concentrations (50 microM) affords a decrease (1.4-fold) in the extent of lipid peroxidation, a decrement in protein oxidation (1.1-fold), and an increase in ATP levels (1.2-fold), although the differences were not statistically significant. However, treatment of cells with high concentrations of zinc (1 mM) led to significant increases in lipid peroxidation (3.7-fold) and protein oxidation (1.5-fold) and to depletion of the ATP pool (21-fold). These data suggest that zinc has a concentration-dependent dual effect, protective and toxic, thus playing an important role in the pathogenesis of Alzheimer's disease.