Metallothioneins in brain--the role in physiology and pathology

Reduction of metallothioneins promotes the disease expression of familial amyotrophic lateral sclerosis mice in a dose-dependent manner

We previously reported that abnormal copper release from mutated Cu, Zn-superoxide dismutase (SOD1) proteins might be a common toxic gain-of-function in the pathogenesis of familial amyotrophic lateral sclerosis (FALS) [Ogawa et al. (1997) Biochem. Biophys. Res. Commun., 241, 251-257.]. In the present study, we first examined metallothioneins (MTs), known to bind copper ions and decrease oxidative toxicity, and found a twofold increase in MTs in the spinal cord of the SOD1 transgenic mice with a FALS-linked mutation (G93A), but not in the spinal cord of wild-type SOD1 transgenic mice. We then investigated whether the clinical course of FALS mice could be modified by the reduced expression of MTs, by crossing the FALS mice with MT-I- and MT-II-deficient mice. FALS mice clearly reached the onset of clinical signs and death significantly earlier in response to the reduction of protein expression. These results indicated that the copper-mediated free radical generation derived from mutant SOD1 might be related to the degeneration of motor neurons in FALS and that MTs might play a protective role against the expression of the disease.

Metallothionein III is reduced in Alzheimer's disease

Metallothionein III (MT-III) is a functionally distinct member of the metallothionein family that displays neuroinhibitory activity and is involved in the repair of neuronal damage. Altered expression levels of MT-III have been observed in Alzheimer's disease (AD) which has led to suggestions that it could be a mitigating factor in AD-related neuronal dysfunction. However, conflicting results have been reported on this issue which may be due to methodological differences and/or sampling size. In the current study, we have assessed MT-III expression in a large number of AD cases through the quantification of mRNA as well as by immunohistochemistry and Western blotting using an MT-III specific antibody. The results of this comprehensive study indicate that the mononucleosome DNA encoding MT-III is occluded preventing transcription and that message levels are reduced by approximately 30%. In addition, protein levels were specifically decreased by approximately 55% in temporal cortex. These data support the conclusion that MT-III is significantly downregulated in AD and may contribute to the loss of its protective effects and/or repair functions that lead to an exacerbation of the pathogenic processes.

Regulation of metallothionein gene expression

The rapid and robust induction of metallothioneins (MT)-I and II by a variety of inducers that include heavy toxic metals, reactive oxygen species, and different types of stress provide a useful system to study the molecular mechanisms of this unique induction process. The specific expression of MT-III in the brain and of MT-IV in the squamous epithelium of skin and tongue offers a unique opportunity to identify and characterize the tissue-specific factors involved in their expression. Studies using transgenic mice that overexpress MTs or MT null mice have revealed the role of MT in the protection of cells against numerous tissue-damaging agents such as reactive oxygen species. The primary physiological function of these proteins, however, remains an enigma. Considerable advances have been made in the identification of the cis-acting elements that are involved in the constitutive and induced expression of MT-I and MT-II. By contrast, only one key trans-activating factor, namely MTF-1, has been extensively characterized. Studies on the epigenetic silencing of MT-I and MT-II by promoter hypermethylation in some cancer cells have posed interesting questions concerning the functional relevance of MT gene silencing, the molecular mechanisms of MT suppression in these cells, particularly chromatin modifications, and the characteristics of the repressors.

Differential expression of metallothioneins in the CNS of mice with experimental autoimmune encephalomyelitis

Multiple sclerosis is an inflammatory, demyelinating disease of the CNS. Metallothioneins-I+II are antioxidant proteins induced in the CNS by immobilisation stress, trauma or degenerative diseases which have been postulated to play a neuroprotective role, while the CNS isoform metallothionein-III has been related to Alzheimer's disease. We have analysed metallothioneins-I-III expression in the CNS of mice with experimental autoimmune encephalomyelitis. Moreover, we have examined the putative role of interferon-gamma, a pro-inflammatory cytokine, in the control of metallothioneins expression during experimental autoimmune encephalomyelitis in interferon-gamma receptor knockout mice with two different genetic backgrounds: 129/Sv and C57BL/6x129/Sv. Mice with experimental autoimmune encephalomyelitis showed a significant induction of metallothioneins-I+II in the spinal cord white matter, and to a lower extent in the brain. Interferon-gamma receptor knockout mice suffered from a more severe experimental autoimmune encephalomyelitis, and interestingly showed a higher metallothioneins-I+II induction in both white and grey matter of the spinal cord and in the brain. In contrast to the metallothioneins-I+II isoforms, metallothionein-III expression remained essentially unaltered during experimental autoimmune encephalomyelitis; interferon-gamma receptor knockout mice showed an altered metallothionein-III expression (a slight increase in the spinal cord white matter) only in the C57BL/6x129/Sv background. Metallothioneins-I+II proteins were prominent in areas of induced cellular infiltrates. Reactive astrocytes and activated monocytes/macrophages were the sources of metallothioneins-I+II proteins. From these results we suggest that metallothioneins-I+II but not metallothionein-III may play an important role during experimental autoimmune encephalomyelitis, and indicate that the pro-inflammatory cytokine interferon-gamma is unlikely an important factor in this response.

L-type Ca(2+) channel-mediated Zn(2+) toxicity and modulation by ZnT-1 in PC12 cells

In view of evidence that Zn(2+) neurotoxicity contributes to some forms of pathological neuronal death, we developed a model of Zn(2+) neurotoxicity in a cell line amenable to genetic manipulations. Exposure to 500 microM ZnCl(2) for 15 min under depolarizing conditions resulted in modest levels of PC12 cell death, that was reduced by the L-type Ca(2+) channel antagonist, nimodipine, and increased by the L-type Ca(2+) channel opener, S(-)-Bay K 8644. At lower insult levels (200 micrometer Zn(2+)+Bay K 8644), Zn(2+)-induced death appeared apoptotic under electron microscopy and was sensitive to the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-CH(2)F (Z-VAD); at higher insult levels (1000 microM+Bay K 8644), cells underwent necrosis insensitive to Z-VAD. To test the hypothesis that the plasma membrane transporter, ZnT-1, modulates Zn(2+) neurotoxicity, we generated stable PC12 cell lines overexpressing wild type or dominant negative forms of rat ZnT-1 (rZnT-1). Clones T9 and T23 overexpressing wild type rZnT-1 exhibited enhanced Zn(2+) efflux and reduced vulnerability to Zn(2+)-induced death compared to the parental line, whereas clones D5 and D16 expressing dominant negative rZnT-1 exhibited the opposite characteristics.

Zn(2+): a novel ionic mediator of neural injury in brain disease

Zn(2+) is the second most prevalent trace element in the body and is present in particularly large concentrations in the mammalian brain. Although Zn(2+) is a cofactor for many enzymes in all tissues, a unique feature of brain Zn(2+) is its vesicular localization in presynaptic terminals, where its release is dependent on neural activity. Although the physiological significance of synaptic Zn(2+) release is little understood, it probably plays a modulatory role in synaptic transmission. Furthermore, several lines of evidence support the idea that, upon excessive synaptic Zn(2+) release, its accumulation in postsynaptic neurons contributes to the selective neuronal loss that is associated with certain acute conditions, including epilepsy and transient global ischaemia. More speculatively, Zn(2+) dis-homeostasis might also contribute to some degenerative conditions, including Alzheimer's disease. Further elucidation of the pathological actions of Zn(2+) in the brain should result in new therapeutic approaches to these conditions.

Altered central nervous system cytokine-growth factor expression profiles and angiogenesis in metallothionein-I+II deficient mice

To study the importance of metallothionein-I and -II (MT-I+II) for brain inflammation and regeneration, the authors examined normal and MT-I+II knock-out (MT-KO) mice subjected to a cortical freeze injury. Normal mice showed profound neurodegeneration, inflammation, and gliosis around the injury, which was repaired by 20 days postlesion (dpl). However, in MT-KO mice the lesion-associated inflammation was still present as late as 90 dpl. Scanning electron microscopy demonstrated that the number of capillaries was lower, and ultrastructural preservation of the lesioned parenchyma was poorer in MT-KO mice, suggesting an altered angiogenesis. To gain insight into the mechanisms involved, a number of cytokines and growth factors were evaluated. The number of cells expressing the proinflammatory cytokines IL-1beta, IL-6, and TNF-alpha was higher in MT-KO mice than in normal mice, which was confirmed by RNase protection analysis, whereas the number of cells expressing the growth factors bFGF, TGFbeta1, VEGF, and NT-3 was lower. Increased expression of proinflammatory cytokines could be involved in the sustained recruitment of CD-14+ and CD-34+ inflammatory cells and their altered functions observed in MT-KO mice. Decreases in trophic factors bFGF, TGFbeta1, and VEGF could mediate the decreased angiogenesis and regeneration observed in MT-KO mice after the freeze lesion. A role for MT-I+II in angiogenesis was also observed in transgenic mice expressing IL-6 under the control of the promoter of glial fibrillary acidic protein gene (GFAP-IL6 mice) because MT-I+II deficiency dramatically decreased the IL-6-induced angiogenesis of the GFAP-IL6 mice. In situ hybridization analysis indicated that the MT-III expression was not altered by MT-I+II deficiency. These results suggest that the MT-I+II isoforms have major regulatory functions in the brain inflammatory response to injury, especially in the angiogenesis process.

Enhanced seizures and hippocampal neurodegeneration following kainic acid-induced seizures in metallothionein-I + II-deficient mice

Metallothioneins (MTs) are major zinc binding proteins in the CNS that could be involved in the control of zinc metabolism as well as in protection against oxidative stress. Mice lacking MT-I and MT-II (MT-I + II deficient) because of targeted gene inactivation were injected with kainic acid (KA), a potent convulsive agent, to examine the neurobiological importance of these MT isoforms. At 35 mg/kg KA, MT-I + II deficient male mice showed a higher number of convulsions and a longer convulsion time than control mice. Three days later, KA-injected mice showed gliosis and neuronal injury in the hippocampus. MT-I + II deficiency decreased both astrogliosis and microgliosis and potentiated neuronal injury and apoptosis as shown by terminal deoxynucleotidyl transferase-mediated in situ end labelling (TUNEL), detection of single stranded DNA (ssDNA) and by increased interleukin-1beta-converting enzyme (ICE) and caspase-3 levels. Histochemically reactive zinc in the hippocampus was increased by KA to a greater extent in MT-I + II-deficient compared with control mice. KA-induced seizures also caused increased oxidative stress, as suggested by the malondialdehyde (MDA) and protein tyrosine nitration (NITT) levels and by the expression of MT-I + II, nuclear factor-kappaB (NF-kappaB), and Cu/Zn-superoxide dismutase (Cu/Zn-SOD). MT-I + II deficiency potentiated the oxidative stress caused by KA. Both KA and MT-I + II deficiency significantly affected the expression of MT-III, granulocyte-macrophage colony stimulating factor (GM-CSF) and its receptor (GM-CSFr). The present results indicate MT-I + II as important for neuron survival during KA-induced seizures, and suggest that both impaired zinc regulation and compromised antioxidant activity contribute to the observed neuropathology of the MT-I + II-deficient mice.

Metallothionein-III prevents glutamate and nitric oxide neurotoxicity in primary cultures of cerebellar neurons

Metallothionein (MT)-III, a member of the MT family of metal-binding proteins, is mainly expressed in the CNS and is abundant in glutamatergic neurons. Results in genetically altered mice indicate that MT-III may play neuroprotective roles in the brain, but the mechanisms through which this protein functions have not been elucidated. The aim of this work was to assess whether MT-III is able to prevent glutamate neurotoxicity and to identify the step of the neurotoxic process interfered with by MT-III. Glutamate neurotoxicity in cerebellar neurons in culture is mediated by excessive activation of glutamate receptors, increased intracellular calcium, and increased nitric oxide. It is shown that MT-III prevented glutamate- and nitric oxide-induced neurotoxicity in a dose-dependent manner, with nearly complete protection at 0.3-1 microgram/ml. MT-III did not prevent the glutamate-induced rise of intracellular calcium level but reduced significantly the nitric oxide-induced formation of cyclic GMP. Circular dichroism analysis revealed that nitric oxide triggers the release of the metals coordinated to the cysteine residues of MT-III, indicative of the S(Cys)-nitrosylation of the protein. Therefore, the present results indicate that MT-III can quench pathological levels of nitric oxide, thus preventing glutamate and nitric oxide neurotoxicity.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

IL-6 deficiency leads to reduced metallothionein-I+II expression and increased oxidative stress in the brain stem after 6-aminonicotinamide treatment

We examined the effects of interleukin-6 (IL-6) deficiency on brain inflammation and the accompanying bone marrow (BM) leukopoiesis and spleen immune reaction after systemic administration of a niacin antagonist, 6-aminonicotinamide (6-AN), which causes both astroglial degeneration/cell death in brain stem gray matter areas and BM toxicity. In both normal and genetically IL-6-deficient mice (IL-6 knockout (IL-6KO) mice), the extent of astroglial degeneration/cell death in the brain stem was similar as determined from disappearance of GFAP immunoreactivity. In 6-AN-injected normal mice reactive astrocytosis encircled gray matter areas containing astroglial degeneration/cell death, which were infiltrated by several macrophages and some T-lymphocytes. Reactive astrocytes and a few macrophages increased significantly the antioxidants metallothionein-I+II (MT-I+II) and moderately the MT-III isoform. In 6-AN-injected IL-6KO mice reactive astrocytosis and recruitment of macrophages and T-lymphocytes were clearly reduced, as were BM leukopoiesis and spleen immune reaction. Expression of MT-I+II was significantly reduced while MT-III was increased. Oxidative stress, as determined by measuring nitrated tyrosine and malondialdehyde, was increased by 6-AN to a greater extent in IL-6KO mice. The blood-brain barrier to albumin was only disrupted in 6-AN-injected normal mice, which likely is due to the substantial migration of blood-derived inflammatory cells into the CNS. The present results demonstrate that inflammation in CNS is clearly reduced during IL-6 deficiency and this effect is likely due to significant inhibition of BM leukopoiesis. We also show that IL-6 deficiency reduces the levels of neuroprotective antioxidants MT-I+II followed by an increased oxidative stress during CNS inflammation.

Metallothioneins are upregulated in symptomatic mice with astrocyte-targeted expression of tumor necrosis factor-alpha

Transgenic mice expressing TNF-alpha under the regulatory control of the GFAP gene promoter (GFAP-TNFalpha mice) exhibit a unique, late-onset chronic-progressive neurological disorder with meningoencephalomyelitis, neurodegeneration, and demyelination with paralysis. Here we show that the metallothionein-I + II (MT-I + II) isoforms were dramatically upregulated in the brain of symptomatic but not presymptomatic GFAP-TNFalpha mice despite TNF-alpha expression being present in both cases. In situ hybridization analysis for MT-I RNA and radioimmunoassay results for MT-I + II protein revealed that the induction was observed in the cerebellum but not in other brain areas. Increased MT-I RNA levels occurred in the Purkinje and granular neuronal layers of the cerebellum but also in the molecular layer. Reactive astrocytes, activated rod-like microglia, and macrophages, but not the infiltrating lymphocytes, were identified as the cellular sources of the MT-I + II proteins. In situ hybridization for MT-III RNA revealed a modest increase in the white matter of the cerebellum, which was confirmed by immunocytochemistry. MT-III immunoreactivity was present in cells which were mainly round or amoeboid monocytes/macrophages. The pattern of expression of the different MT isoforms in the GFAP-TNFalpha mice differed substantially from that described previously in GFAP-IL6 mice, demonstrating unique effects associated with the expression of each cytokine. The results suggest that the MT expression in the CNS reflects the inflammatory response and associated damage rather than a direct role of the TNF-alpha in their regulation and support a major role of these proteins during CNS injury.

Importance of zinc in the central nervous system: the zinc-containing neuron

Zinc is essential to the structure and function of myriad proteins, including regulatory, structural and enzymatic. It is estimated that up to 1% of the human genome codes for zinc finger proteins. In the central nervous system, zinc has an additional role as a neurosecretory product or cofactor. In this role, zinc is highly concentrated in the synaptic vesicles of a specific contingent of neurons, called "zinc-containing" neurons. Zinc-containing neurons are a subset of glutamatergic neurons. The zinc in the vesicles probably exceeds 1 mmol/L in concentration and is only weakly coordinated with any endogenous ligand. Zinc-containing neurons are found almost exclusively in the forebrain, where in mammals they have evolved into a complex and elaborate associational network that interconnects most of the cerebral cortices and limbic structures. Indeed, one of the intriguing aspects of these neurons is that they compose somewhat of a chemospecific "private line" of the mammalian cerebral cortex. The present review outlines (1) the methods used to discover, define and describe zinc-containing neurons; (2) the neuroarchitecture and synaptology of zinc-containing neural circuits; (3) the physiology of regulated vesicular zinc release; (4) the "life cycle" and molecular biology of vesicular zinc; (5) the importance of synaptically released zinc in the normal and pathological processes of the cerebral cortex; and (6) the role of specific and nonspecific stressors in the release of zinc.

Characterization of cis-acting elements in the promoter of the mouse metallothionein-3 gene. Activation of gene expression during neuronal differentiation of P19 embryonal carcinoma cells

The metallothionein (MT)3 gene is expressed predominantly in the brain and the organs of the reproductive system, and fails to respond to metal ions in vivo. A CTG repeat was proposed to function as a potential repressor element in nonpermissive cells, and a sequence similar to the JC virus silencer element was found to function as a negative element in permissive primary astrocytes. The objective of this study was to characterize further the mechanisms governing cell-type specific MT-3 gene transcription. We searched for a suitable cell line expressing the MT-3 gene to be used for determination of MT-3 promoter tissue specificity, and showed that MT-3 expression is activated during neuroectodermal differentiation of P19 cells induced by retinoic acid to levels similar to those found in whole brain. Deletion of the CTG repeat or of the JC virus silencer did not promote MT-3 promoter activity in nonpermissive cells, or enhance expression in permissive cells. We identified MT-3 promoter sequences interacting with liver and brain nuclear proteins, as assayed by DNase I footprinting analyses and electrophoretic mobility shift assay, and assessed the role of these sequences in the regulation of MT-3 expression by cotransfection experiments. We generated stable transfectants in permissive C6 and nonpermissive NIH-3T3 cells, and analysed the methylation status of the MT-3 gene. These studies show that regulation of tissue-specific MT-3 gene expression does not appear to involve a repressor, and suggest that other mechanisms such as chromatin organization and epigenetic modifications could account for the absence of MT-3 gene transcription in nonpermissive cells.

Foreign metallothionein-I expression by transient transfection in MT-I and MT-II null astrocytes confers increased protection against acute methylmercury cytotoxicity

The mechanisms associated with metallothionein (MT) gene regulation are complex and poorly understood. Only a modest increase in brain MT expression levels is attained by exposure to metals, MT gene transfection, and MT gene knock-in techniques. Accordingly, in the present study, MT null astrocytes isolated from transgenic mice deficient in MT-I and MT-II genes were introduced as a zero background model of MT expression. MT protein levels were determined by western blot analysis. MT proteins in MT-I and MT-II null astrocytes were undetectable. Transient MT-I gene transfection increased the levels of foreign MT expression in MT-I and MT-II null astrocytes by 2.3-fold above basal levels in wild-type astrocytes. Intracellular Na(2)51CrO(4) efflux and D-[2,3-3H]aspartate uptake were studied as indices of acute methylmercury (MeHg) (5 microM) cytotoxicity. In MT-I and MT-II knockout astrocytes MeHg led to significant (p<0.01) increase in Na(2)51CrO(4) efflux and a significant (p<0.05) decrease in the initial rate (1 min) of D-[2, 3-3H]aspartate uptake compared to MT-I and MT-II knockout controls. Transfection of the MT-I gene in MT-I and MT-II null mice significantly (p<0.01) decreased the effect of MeHg on Na(2)51CrO(4) efflux in MT null, as well as wild-type astrocytes. MT-I gene transfection in MT-I and MT-II null astrocytes reversed the inhibitory effect of MeHg on D-[2,3-3H]aspartate uptake, such that initial rates of uptake in MT-I transfected cells in the presence and absence of MeHg (5 microM) were indistinguishable. These results demonstrate that: (1) astrocytes lacking MTs are more sensitive to MeHg than those with basal MT protein levels, (2) the MT-I gene can be overexpressed in MT-I and MT-II null astrocytes by transient MT-I gene transfection, and (3) that foreign MT expression endows astrocytes with increased resistance to MeHg.

Antioxidants protect against dopamine-induced metallothionein-III (GIF) mRNA expression in mouse glial cell line (VR-2g)

Metallothionein (MT)-III, originally discovered as a growth inhibitory factor (GIF), is a brain specific isomer of MTs and is markedly reduced in the brain of patients with Alzheimer's disease (AD) or other neurodegenerative diseases. We analyzed the level and regulation of mRNA expression of MT-III in immortalized fetal mouse brain glial cells (VR-2g) by reverse transcriptase-polymerase chain reaction (RT-PCR). We have recently reported that dopamine (DA) increases the expression of MT-III mRNA in vitro. In this study, we investigated the mechanism of such increase by examining the effects of DA agonists (SKF38393 or bromocriptine) and DA antagonists (SCH23390 or sulpiride) on the expression of MT-III mRNA. MT-III mRNA did not change by either agonist and DA-increased MT-III mRNA was not inhibited by either antagonist. These results suggested that the induction of MT-III mRNA by DA was not mediated by stimulation of DA receptors. On the other hand, DA-induced MT-III mRNA expression was strongly inhibited by the addition of antioxidants (glutathione, vitamin E or ascorbic acid), indicating that DA-enhanced MT-III mRNA was mediated by reactive oxygen species. Our results suggest that oxidative stress may be one of the principle factors that modulate MT-III mRNA expression.

Suppression of metallothionein gene expression in a rat hepatoma because of promoter-specific DNA methylation

Metallothionein I can be induced in response to a variety of agents that include heavy metals and oxidative stress. On the contrary, its induction was suppressed in some lymphoid-derived cancer cells. The mechanism of this repression has not been elucidated. Here, we show silencing of MT-I gene in a solid transplanted rat tumor as a result of promoter methylation at all the 21 CpG dinucleotides that span the region from -225 bp to +1 bp. By contrast, none of these CpG dinucleotides were methylated in the livers from the rats bearing the tumor, which was consistent with the efficient induction of the gene in this tissue by zinc sulfate. Genomic footprinting revealed lack of access of the transcriptional activators to the respective cis-acting elements of the methylated MT-I promoter in the hepatoma. The absence of footprinting was not due to inactivation of the metal regulatory transcription factor MTF-1, because it was highly active in the hepatoma. Treatment of the hepatoma bearing rats with 5-azacytidine, a demethylating agent, induced basal as well as heavy metal-activated MT-I gene expression in the hepatoma, implying that methylation was indeed responsible for silencing the gene. Bisulfite genomic sequencing showed significant (>90%) demethylation of CpG dinucleotides spanning MT-I promoter in the hepatoma following treatment with 5-AzaC. The hypermethylation of MT-I promoter was probably caused by significantly higher (as much as 7-fold) level of DNA methyl transferase activity as well as enhanced expression of its gene in the hepatoma relative to the host liver. These data elucidated for the first time the molecular mechanism for the silencing of a highly inducible gene in a solid tumor transplanted in an animal, as compared with the robust induction in the corresponding parental tissue and have discussed the probable reasons for the suppression of this gene in some tumors.

Silencing of metallothionein-I gene in mouse lymphosarcoma cells by methylation

Metallothionein-I (MT-I) gene is silenced by methylation of CpG islands in mouse lymphosarcoma P1798 cells but not in the thymus, the cell type from which the tumor was derived. Bisulfite genomic sequencing revealed that all 21 CpG dinucleotides present within -216 bp to +1 bp with respect to transcription start site are methylated in the tumor cell line, but none is methylated in the thymus. The lymphosarcoma cells induced MT-I in response to heavy metals only after demethylation with 5-azacytidine (5-AsaC). The electrophoretic mobility shift assay using specific oligonucleotide probes showed that the key transcription factors regulating MT-I gene (e.g., MTF-1, Sp 1 and MLTF/USF) are active in P1798 cells. In vivo footprinting of the proximal promoter region showed that none of the metal regulatory elements (MREs) or MLTF/USF are occupied in response to heavy metals. Demethylation of the lymphosarcoma cells with 5-AzaC resulted in constitutive footprinting at MLTF/ARE, and zinc-inducible footprinting at MRE-c, MRE-d and MRE-e sites. Demethylation of just 10-20% of the CpG islands was sufficient to render the gene inducible by cadmium or zinc. The MT-I induction persisted in the cancer cells for several generations even after withdrawal of 5-AzaC from the culture medium.

Regional distribution of metallothionein, zinc, and copper in the brain of different strains of rats

The regional brain distribution of metallothionein (MT), zinc, and copper in the brain was determined in nine anatomical regions (olfactory bulb, cortex, corpus striatum, hippocampus, thalamus plus hypothalamus, pons plus medulla oblongata, cerebellum, midbrain, and white matter) and was compared between two different strains of rat (Sprague-Dawley [SD] and Lewis). No significant difference was observed in the whole-brain MT level between the two strains (17.8 +/- 3.4 microg/g in SD rats and 20.3 +/- 2.3 microg/g in Lewis rats). In SD rats, however, MT was more highly expressed in the white matter than in the other regions studied. In contrast, MT concentration was highest in the cortex and lowest in the olfactory bulb in Lewis rats. The MT levels in the cortex, corpus striatum, hippocampus, and thalamus plus hypothalamus were significantly lower in SD rats than in Lewis rats. In both strains, the olfactory bulb contained markedly higher levels of both zinc and copper than the other regions (27.9 +/- 6.8 microg/g zinc in SD rats and 27.6 +/- 6.9 microg/g zinc in Lewis rats, and 5.2 +/- 1.5 microg/g copper in SD rats and 11.1 +/- 4.8 microg/g copper in Lewis rats). The next highest zinc levels were seen in the hippocampus, whereas the next highest copper levels were in the corpus striatum in both SD and Lewis rats. The high levels of zinc and copper in the olfactory bulb were not accompanied by concomitant high MT concentrations. These results indicate that the strain of rat as well as the anatomical brain region should be taken into account in MT and metal distribution studies. However, the highest concentrations of zinc and copper in olfactory bulb were common to both SD and Lewis rats. The discrepancy between MT and the metal levels in olfactory bulb suggests a role for other proteins in addition to MT in the homeostatic control of zinc and copper.

Serum zinc, copper, insulin and lipids in Alzheimer's disease epsilon 4 apolipoprotein E allele carriers

BACKGROUND

Copper (Cu) and zinc (Zn) have been implicated in the development of Alzheimer's disease (AD) and, in this regard, Cu and Zn serum concentrations have been analysed but with inconclusive results. Serum insulin, glucose and cholesterol concentrations have been related to the apolipoprotein E genotype in non-AD populations.

DESIGN

In this study, we have analysed the relationship between serum Cu, Zn, insulin, glucose and lipid parameters (cholesterol, triglycerides, apoA and apoB apolipoproteins) in AD and AD epsilon 4 apolipoprotein E carriers by multivariate analysis using logistic regression, including the variables that showed a significance of P < 0.05 in the bivariate analysis.

RESULTS

The results obtained show that epsilon 4 apoE allele is an independent AD risk factor (OR = 6. 67, 95% CI = 2.59-17.16). In AD epsilon 4 apoE allele carriers, we found significantly higher Zn, Cu and insulin serum concentrations. Non-demented control subjects with at least one epsilon 4 apoE allele had the lowest serum insulin concentrations. There was no significant association between epsilon 4 apolipoprotein E allele and lipid parameters in the sample studied.

CONCLUSIONS

In AD we have found a significant association between higher serum Zn, Cu and insulin concentrations and the presence of an epsilon 4 apoE allele, but only greater serum Zn concentration appears to be an independent risk factor associated with the development of AD.

Metallothionein: an intracellular protein to protect against cadmium toxicity

Metallothioneins (MT) are low-molecular-weight, cysteine-rich, metal-binding proteins. MT genes are readily induced by various physiologic and toxicologic stimuli. Because the cysteines in MT are absolutely conserved across species, it was suspected that the cysteines are necessary for function and MT is essential for life. In attempts to determine the function(s) of MT, studies have been performed using four different experimental paradigms: (a) animals injected with chemicals known to induce MT; (b) cells adapted to survive and grow in high concentrations of MT-inducing toxicants; (c) cells transfected with the MT gene; and (d) MT-transgenic and MT-null mice. Most often, results from studies using the first three approaches have indicated multiple functions of MT in cell biology: MT (a) is a "storehouse" for zinc, (b) is a free-radical scavenger, and (c) protects against cadmium (Cd) toxicity. However, studies using MT-transgenic and null mice have not strongly supported the first two proposed functions but strongly support its function in protecting against Cd toxicity. Repeated administration of Cd to MT-null mice results in nephrotoxicity at one tenth the dose that produces nephrotoxicity in control mice. Human studies indicate that 7% of the general population have renal dysfunction from Cd exposure. Therefore, if humans did not have MT, "normal" Cd exposure would be nephrotoxic to humans. Thus, it appears that during evolution, the ability of MT to protect against Cd toxicity might have taken a more pivotal role in the maintenance of life processes, as compared with its other proposed functions (i.e. storehouse for zinc and free radical scavenger).

CNS wound healing is severely depressed in metallothionein I- and II-deficient mice

To characterize the physiological role of metallothioneins I and II (MT-I+II) in the brain, we have examined the chronological effects of a freeze injury to the cortex in normal and MT-I+II null mice. In normal mice, microglia/macrophage activation and astrocytosis were observed in the areas surrounding the lesion site, peaking at approximately 1 and 3 d postlesion (dpl), respectively. At 20 dpl, the parenchyma had regenerated. Both brain macrophages and astrocytes surrounding the lesion increased the MT-I+II immunoreactivity, peaking at approximately 3 dpl, and at 20 dpl it was similar to that of unlesioned mice. In situ hybridization analysis indicates that MT-I+II immunoreactivity reflects changes in the messenger levels. In MT-I+II null mice, microglia/macrophages infiltrated the lesion heavily, and at 20 dpl they were still present. Reactive astrocytosis was delayed and persisted at 20 dpl. In contrast to normal mice, at 20 dpl no wound healing had occurred. The rate of apoptosis, as determined by using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling, was drastically increased in neurons of ipsilateral cortex of the MT-I+II null mice. Our results demonstrate that MT-I+II are essential for a normal wound repair in the CNS, and that their deficiency impairs neuronal survival.

Ischemia induces metallothionein III expression in neurons of rat brain

Metallothionein III (MT-III) is a brain-specific member of the metallothionein family and binds zinc in vivo. In order to confirm the precise localization of MT-III in normal rat brain and the change of MT-III expression after transient whole brain ischemia, we raised a high affinity phagemid-antibody specific for rat MT-III. Immunohistochemical analysis revealed that MT-III in normal brain is localized abundantly in neuronal cell bodies in CA1-3 regions of hippocampus, dentate gyrus, cerebral cortex, olfactory bulb and Purkinje cells in cerebellum. This expression pattern of MT-III was similar to that of MT-III mRNA observed by in situ hybridization studies. ELISA and Northern blot analysis revealed that MT-III protein as well as mRNA levels were up-regulated in cerebrum soon after ischemic stress. Immunohistochemical analysis also demonstrated intense staining in neurons in injured brain after ischemia, which distributed in the same regions as in normal brain. These results suggest that MT-III plays an important role in protecting neurons from ischemic insult by reducing neurotoxic zinc levels and inhibits uncontrolled growth of neurites after ischemia.

Inorganic mercury pre-exposures protect against methyl mercury toxicity in NSC-34 (neuron x spinal cord hybrid) cells

A neuron spinal chord x hybrid (NSC-34) cell culture derived from neonatal mouse was characterized for studies on mercury toxicity. Exposure of NSC-34 cells to methyl mercury chloride (MeHgCl) (0-16 microM) resulted in significant dose-dependent cell damage and death (P < 0.05). MeHgCl was more toxic than inorganic mercury (Hg2+) for both the NSC-34 cells and its parent neuroblastoma cell line N18TG-2 (P < 0.05). Hg2+, but not ZnCl2 or MeHg exposure induced metallothionein (MT) (P < 0.05). To mimic the increase in Hg2+ in the mammalian brain with long term MeHg exposure, the cells were treated with 1 microM mercuric chloride (HgCl2) for five passages before exposure to MeHgCl (1-16 microM) for 48 h. MeHgCl toxicity was measured by trypan blue exclusion, reduction of resazurin dye and acid phosphatase activity. Pre-exposure to HgCl2 lessened the toxicity as shown by trypan blue exclusion (P = 0.0559) and reduction of resazurin (P = 0.0001). Pre-exposure to HgCl2 also resulted in induction of MT (P = 0.0066) and lessened the decrease of reduced glutathione (GSH) (P = 0.0013). These results suggest that MT and GSH may play a protective role in methyl mercury induced neurotoxicity of neuron spinal chord cells. The NSC-34 hybrid cell line can be a useful model for the study of MeHg neurotoxicity.

Transfection and overexpression of metallothionein-I in neonatal rat primary astrocyte cultures and in astrocytoma cells increases their resistance to methylmercury-induced cytotoxicity

Metallothionein-I (MT-I) was expressed in neonatal rat primary astrocyte cultures and an astrocytoma cell line by pGFAP-MT-I plasmid transfection under the control of the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter. Following transient transfection of the pGFAP-MT-I plasmid, MT-I mRNA and MT-I protein levels were determined by northern blot and immunoprecipitation analyses, respectively. The ability of cells over-expressing MT-I to withstand acute methylmercury (MeHg) treatment was measured by the release of preloaded Na251CrO4, an indicator of membrane integrity. Transfection with the pGFAP-MT-I plasmid led to increased mRNA (2. 5-fold in astrocytes and 7.4-fold in astrocytomas) and MT-I protein (2.4-fold in astrocytes and 4.0-fold in astrocytomas) levels compared with their respective controls. Increased expression of MT-I was associated with attenuated release of Na251CrO4 upon MeHg (5 microM) treatment. These results demonstrate that MT-I can be highly expressed both in primary astrocyte cultures and astrocytomas by pGFAP-MT-I plasmid transfection, and lend credence to the hypothesis that increased expression of MT-I affords protection against the cytotoxic effects of MeHg. Taken together, the data suggest that MT offer effective cellular adaptation to MeHg cytotoxicity.

Increased density of metallothionein I/II-immunopositive cortical glial cells in the early stages of Alzheimer's disease

We have examined the possible role of metallothionein I/II (MT I/II) in Alzheimer's disease (AD), with a focus on the cellular localization of MT I/II relative to the astrocyte marker, glial fibrillary acidic protein (GFAP). In AD and preclinical AD cases, MT I/II immunolabeling was present in glial cells and did not show a spatial relationship with beta-amyloid plaques or neurofibrillary pathology. There was a six- to sevenfold increase in both MT I/II- and GFAP-labeled cells in the gray matter of AD cases, relative to non-AD cases. However, there was a threefold increase in MT I/II-immunoreactive cells, but not GFAP-labeled cells, in the gray matter of preclinical AD cases compared to non-AD cases. Therefore, the specific increase in MT I/II is associated with the initial stages of the disease process, perhaps due to oxidative stress or the mismetabolism of heavy metals.

Methylmercury intoxication and histochemical demonstration of NADPH-diaphorase activity in the striate cortex of adult cats

The effects of methylmercury (MeHg) on histochemical demonstration of the NADPH-diaphorase (NADPH-d) activity in the striate cortex were studied in 4 adult cats. Two animals were used as control. The contaminated animals received 50 ml milk containing 0.42 microgram MeHg and 100 g fish containing 0.03 microgram MeHg daily for 2 months. The level of MeHg in area 17 of intoxicated animals was 3.2 micrograms/g wet weight brain tissue. Two cats were perfused 24 h after the last dose (group 1) and the other animals were perfused 6 months later (group 2). After microtomy, sections were processed for NADPHd histochemistry procedures using the malic enzyme method. Dendritic branch counts were performed from camera lucida drawings for control and intoxicated animals (N = 80). Average, standard deviation and Student t-test were calculated for each data group. The concentrations of mercury (Hg) in milk, fish and brain tissue were measured by acid digestion of samples, followed by reduction of total Hg in the digested sample to metallic Hg using stannous chloride followed by atomic fluorescence analysis. Only group 2 revealed a reduction of the neuropil enzyme activity and morphometric analysis showed a reduction in dendritic field area and in the number of distal dendrite branches of the NADPHd neurons in the white matter (P < 0.05). These results suggest that NADPHd neurons in the white matter are more vulnerable to the long-term effects of MeHg than NADPHd neurons in the gray matter.

Metallothioneins are highly expressed in astrocytes and microcapillaries in Alzheimer's disease

One of the neuropathological characteristics of Alzheimer's disease is the presence of a large number of reactive astrocytes, often, but not always, associated with senile plaques. The factors responsible for such an activation are as yet totally unknown. Other characteristic features of this disease such as betaA4 amyloid accumulation, senile plaques and neurofibrillary tangles represent well known pathological phenomena. Some studies suggest that betaA4 plays a major role in the reactive astrocytosis characteristic of Alzheimer's disease. In the normal human brain, metallothionein isoforms I and II are expressed in astrocytes but not in neurons. In the present study, we used anti-metallothionein antibodies to detect cells expressing metallothioneins isoforms I and II in normal and Alzheimer's disease (AD) brain sections. Results showed that expression of these proteins in the cortex, cerebral white matter and cerebellum is a relevant anatomopathological characteristic of Alzheimer's disease. Analysis of Alzheimer's disease brain sections revealed high expression of metallothioneins I/II in astrocytes and microcapillaries, and in the granular but not the molecular layer of the cerebellum. Furthermore, metallothionein expression can be used as a marker to identify subtypes of astrocytes.

Dimethyl sulfoxide, but not acidosis-induced metallothionein mRNA expression in neonatal rat primary astrocyte cultures is inhibited by the bioflavonoid, quercetin

Metallothionein (MT) mRNA levels were analyzed following exposure of neonatal rat primary astrocyte cultures to physiologic pH (7.4), acidosis (pH 6.5 and 6.0), and dimethyl sulfoxide (DMSO). Treatments were carried out both in the presence and absence of the bioflavonoid, quercetin. Total RNA was probed on northern blots with [alpha32P]dCTP-labeled synthetic cDNA probes specific for rat MT isoform mRNAs. MT-I and MT-II mRNA levels in astrocytes exposed to pH 6.5 or pH 6.0 were increased compared to controls (pH 7.4). Treatment with DMSO in the presence and absence of acidosis, also increased MT-I and MT-II mRNA levels compared to controls (pH 7.4). The DMSO-induced increase in MT mRNA expression was reversed by treatment of astrocytes with quercetin, such that MT-I and MT-II mRNA levels in DMSO plus quercetin-treated astrocytes were indistinguishable from mRNA levels in their respective controls at pH 7.4, pH 6.5, and pH 6.0. These findings suggest that both acidosis and DMSO exposure are associated with increased astrocytic MT synthesis at the mRNA level, and that quercetin, effectively blocks MT mRNA induction by DMSO.

Zinc and brain injury

Zinc is an essential catalytic or structural element of many proteins, and a signaling messenger that is released by neural activity at many central excitatory synapses. Growing evidence suggests that zinc may also be a key mediator and modulator of the neuronal death associated with transient global ischemia and sustained seizures, as well as perhaps other neurological disease states. Manipulations aimed at reducing extracellular zinc accumulation, or cellular vulnerability to toxic zinc exposure, may provide a novel therapeutic approach toward ameliorating pathological neuronal death in these settings.

Localization of metallothionein-I and -II in hypertrophic astrocytes in brain lesions of dogs

To study the neurophysiological functions of metallothioneins (MTs), localization of MT-I and -II was examined immunohistochemically in a variety of brain lesions in dogs, including infarct, laminar cortical necrosis, hemorrhage, invasive growth of tumour, inflammatory lesions in granulomatous meningoencephalitis and distemper encephalitis. MT-I and -II were demonstrated in both nucleus and cytoplasm of hypertrophic astrocytes in most brain lesions examined regardless of the type, size, localization and duration of the lesions. In addition, MT expression was stronger in a population of hypertrophic astrocytes localizing inside of the surviving brain tissue rather than those localizing at the boundary between the surviving brain tissue and necrotic area, where severe inflammatory changes were developing. These results suggest that MT-I and -II may play roles not only in protection of neurons from metals and free radicals ubiquitous in the inflammatory lesions but also in repair of injured neural tissues.

Induction of metallothionein-I (MT-I) mRNA in primary astrocyte cultures is mediated by hypotonicity and not ethanol (EtOH) per se

Metallothionein (MT) mRNA was determined in rat astrocyte cultures in response to ethanol (EtOH). MT-I mRNA was significantly increased after 6 h exposure to isosmotic EtOH, but not hyperosmotic EtOH. Exposure to a hyposmotic/hypotonic solution also led to a significant increase in the expression of astrocytic MT-I mRNA. The large increase in MT-I mRNA was not due to removal of extracellular NaCl, because this effect was reversed by replacement of NaCl with N-methyl D-glucamine chloride. A significant decrease in MT-I mRNA was also noted in astrocytes exposed to an EtOH-free hyperosmotic/hypertonic solution. These results suggest (1) that EtOH per se does not directly induce MT-I mRNA expression, (2) that the induction by EtOH of MT-I mRNA is secondary to hypotonicity, and (3) that hyperosmotic/hypertonic exposure is associated with reduced expression of MT-I mRNA in astrocyte cultures.