The N-terminal portion of growth inhibitory factor is sufficient for biological activity

The Function of Transthyretin Complexes with Metallothionein in Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most frequently diagnosed types of dementia in the elderly. An important pathological feature in AD is the aggregation and deposition of the β-amyloid (Aβ) in extracellular plaques. Transthyretin (TTR) can cleave Aβ, resulting in the formation of short peptides with less activity of amyloid plaques formation, as well as being able to degrade Aβ peptides that have already been aggregated. In the presence of TTR, Aβ aggregation decreases and toxicity of Aβ is abolished. This may prevent amyloidosis but the malfunction of this process leads to the development of AD. In the context of Aβplaque formation in AD, we discuss metallothionein (MT) interaction with TTR, the effects of which depend on the type of MT isoform. In the brains of patients with AD, the loss of MT-3 occurs. On the contrary, MT-1/2 level has been consistently reported to be increased. Through interaction with TTR, MT-2 reduces the ability of TTR to bind to Aβ, while MT-3 causes the opposite effect. It increases TTR-Aβ binding, providing inhibition of Aβ aggregation. The protective effect, assigned to MT-3 against the deposition of Aβ, relies also on this mechanism. Additionally, both Zn₇MT-2 and Zn₇MT-3, decrease Aβ neurotoxicity in cultured cortical neurons probably because of a metal swap between Zn₇MT and Cu(II)Aβ. Understanding the molecular mechanism of metals transfer between MT and other proteins as well as cognition of the significance of TTR interaction with different MT isoforms can help in AD treatment and prevention.

Metal-dependent interactions of metallothionein-3 β-domain with amyloid-β peptide and related physiological implications

Aberrant interactions of metal ions with amyloid-β peptide (Aβ) can potentiate Alzheimer's disease (AD) by participating in the aggregation process of Aβ and in the generation of reactive oxygen species (ROS). Metallothionein-3 (MT3), which is aberrantly expressed in AD brains, is believed to play an important role in the AD progression due to its ability of maintaining metal homeostasis and scavenging ROS. However, the related molecular mechanism is not clear. In this work, the metal-dependent interactions of MT3 β-domain (βMT3) with amyloid-β peptide (Aβ) were systematically studied. The results showed that Zn₃-βMT3 has a higher affinity to Aβ (K_d: ~0.7 μM) than Cu₄-βMT3 (K_d: ~22 μM). In Zn₃-βMT3, both Pro⁷ and Pro⁹ face outwards with their five-member rings in parallel, favoring their binding with aromatic residues via CH/π interactions. Two aromatic residues (Phe⁴ and Tyr¹⁰) in Aβ were identified as the specific binding sites for βMT3. Based on these, we posit a characteristic in-situ protection role of Zn-MT3 in inhibiting the Cu²⁺-induced Aβ neurotoxicity, in which stable Zn-MT3/Aβ complex forms via the Zn₃-βMT3/Aβ interaction and effectively prevents the formation of Cu-Aβ in high viscosity physiological fluids. Our results provide the mechanistic pathway and the specific roles of βMT3 in its protective bioactivity against AD progression, which means significant for elucidating the function of MT3 in AD neuropathology and for designing a MT3-related therapeutic strategy for AD.

Metallothionein-3 Is a Component of a Multiprotein Complex in the Mouse Brain

Metallothlonein (MT)-3, originally called growth inhibitory factor (GIF), was initially identified through its ability to Inhibit the growth of neuronal cells in the presence of brain extract. MT-3 is the brain specific isoform of the MT family whose specific biological activity associates it with neurological disorders. Indeed, studies report that MT-3 is decreased by ~30% in brains of patients with Alzheimer disease (AD). Furthermore, many lines of evidence suggest that MT-3 engages in specific protein interactions. To address this, we conducted Immunoaffinity chromatography experiments using an immobilized anti-mouse MT-3 antibody. We identified five associated proteins from the pool of sixteen recovered using mass spectrometry and tandem mass spectrometry after in-gel trypsin digestion of bands from the affinity chromatography. The proteins identified were: heat shock protein 84 (HSP84), heat shock protein 70 (HSP70), dihydropyrimidinase-like protein-2 (DRP-2), creatine kinase (CK) and β-actin. Coimmunoprecipitation experiments, also conducted on whole mouse brain extract using the anti-mouse MT-3 antibody along with commercially available antibodies against HSP84 and CK, confirmed that these three proteins were in a single protein complex. Immunohistochemical experiments were then conducted on the perfused mouse brain that confirmed the in situ colocallzation of CK and MT-3 in the hippocampus region. These data provide new Insights into the involvement of MT-3 in a multiprotein complex, which will be used to understand the biological activity of MT-3 and its role in neurological disease.

Localization and Spectroscopic Analysis of the Cu(I) Binding Site in Wheat Metallothionein Ec-1

The early cysteine-labeled metallothionein (MT) from Triticum aestivum (common wheat), denoted Ec-1, features two structurally well-defined domains, γ and βE, coordinating two and four Zn(II) ions, respectively. While the protein is currently assumed to function mainly in zinc homeostasis, a low amount of copper ions was also recently detected in a native Ec-1 sample. To evaluate the observed copper binding in more detail, the recombinant Zn₆Ec-1 form was exposed to different amounts of Cu(I) ions and the resulting species characterized with spectroscopic methods. Data reveal that the first Cu(I) equivalent coordinates exclusively to the N-terminal γ-domain of the protein and replaces one Zn(II) ion. To analyze the ability of the γ-domain for coordination of monovalent metal ions in more detail, the γ-Ec-1 peptide fragment was incubated with increasing amounts of Cu(I) and the process monitored with UV-VIS, circular dichroism, and luminescence spectroscopy. Closely similar spectra are observed regardless if the apo- or the metal ion-loaded and, hence, pre-folded forms, were used for the titration experiments with Cu(I). The results indicate that low amounts of Cu(I) ions displace the two metal ions subsequently and stoichiometrically, despite the different coordination geometry requirements of Cu(I) and Zn(II).

Role of zinc metallothionein-3 (ZnMt3) in epidermal growth factor (EGF)-induced c-Abl protein activation and actin polymerization in cultured astrocytes

Recent evidence indicates that zinc plays a major role in neurochemistry. Of the many zinc-binding proteins, metallothionein-3 (Mt3) is regarded as one of the major regulators of cellular zinc in the brain. However, biological functions of Mt3 are not yet well characterized. Recently, we found that lysosomal dysfunction in metallothionein-3 (Mt3)-null astrocytes involves down-regulation of c-Abl. In this study, we investigated the role of Mt3 in c-Abl activation and actin polymerization in cultured astrocytes following treatment with epidermal growth factor (EGF). Compared with wild-type (WT) astrocytes, Mt3-null cells exhibited a substantial reduction in the activation of c-Abl upon treatment with EGF. Consistent with previous studies, activation of c-Abl by EGF induced dissociation of c-Abl from F-actin. Mt3 added to astrocytic cell lysates bound F-actin, augmented F-actin polymerization, and promoted the dissociation of c-Abl from F-actin, suggesting a possible role for Mt3 in this process. Conversely, Mt3-deficient astrocytes showed significantly reduced dissociation of c-Abl from F-actin following EGF treatment. Experiments using various peptide fragments of Mt3 showed that a fragment containing the N-terminal TCPCP motif (peptide 1) is sufficient for this effect. Removal of zinc from Mt3 or pep1 with tetrakis(2-pyridylmethyl)ethylenediamine abrogated the effect of Mt3 on the association of c-Abl and F-actin, indicating that zinc binding is necessary for this action. These results suggest that ZnMt3 in cultured astrocytes may be a normal component of c-Abl activation in EGF receptor signaling. Hence, modulation of Mt3 levels or distribution may prove to be a useful strategy for controlling cytoskeletal mobilization following EGF stimulation in brain cells.

New proteins found interacting with brain metallothionein-3 are linked to secretion

Metallothionein 3 (MT-3), also known as growth inhibitory factor (GIF), exhibits a neuroinhibitory activity. Our lab and others have previously shown that this biological activity involves interacting protein partners in the brain. However, nothing specific is yet known about which of these interactions is responsible for the GIF activity. In this paper, we are reporting upon new proteins found interacting with MT-3 as determined through immunoaffinity chromatography and mass spectrometry. These new partner proteins-Exo84p, 14-3-3 Zeta, α and β Enolase, Aldolase C, Malate dehydrogenase, ATP synthase, and Pyruvate kinase-along with those previously identified have now been classified into three functional groups: transport and signaling, chaperoning and scaffolding, and glycolytic metabolism. When viewed together, these interactions support a proposed model for the regulation of the GIF activity of MT-3.

The Delta33-35 Mutant alpha-Domain Containing beta-Domain-Like M(3)S(9) Cluster Exhibits the Function of alpha-Domain with M(4)S(11) Cluster in Human Growth Inhibitory Factor

Neuronal growth inhibitory factor (GIF), also known as metallothionein (metallothionein-3), impairs the survival and neurite formation of cultured neurons. It is known that the α-β domain-domain interaction of hGIF is crucial to the neuron growth inhibitory bioactivity although the exact mechanism is not clear. Herein, the β(MT3)-β(MT3) mutant and the hGIF-truncated Δ33-35 mutant were constructed, and their biochemical properties were characterized by pH titration, EDTA, and DTNB reactions. Their inhibitory activity toward neuron survival and neurite extension was also examined. We found that the Δ33-35 mutant α-domain containing β-domain-like M₃S₉ cluster exhibits the function of α-domain with M₄S₁₁ cluster in hGIF. These results showed that the stability and solvent accessibility of the metal-thiolate cluster in β-domain is very significant to the neuronal growth inhibitory activity of hGIF and also indicated that the particular primary structure of α-domain is pivotal to domain-domain interaction in hGIF.

Structural prediction of the beta-domain of metallothionein-3 by molecular dynamics simulation

The beta-domain of metallothionein-3 (MT3) has been reported to be crucial to the neuron growth inhibitory bioactivity. Little detailed three-dimensional structural information is available to present a reliable basis for elucidation on structure-property-function relationships of this unique protein by experimental techniques. So, molecular dynamics simulation is adopted to study the structure of beta-domain of MT3. In this article, a 3D structural model of beta-domain of MT3 was generated. The molecular simulations provide detailed protein structural information of MT3. As compared with MT2, we found a characteristic conformation formed in the fragment (residue 1-13) at the N-terminus of MT3 owing to the constraint induced by 5TCPCP9, in which Pro7 and Pro9 residues are on the same side of the protein, both facing outward and the two 5-member rings of prolines are arranged almost in parallel, while Thr5 is on the opposite side. Thr5 in MT3 is also found to make the first four residues relatively far from the fragment (residue 23-26) as compared with MT2. The simulated structure of beta-domain of MT3 is looser than that of MT2. The higher energy of MT3 than that of MT2 calculated supports these conclusions. Simulation on the four isomer arising from the cis- or trans-configuration of 6CPCP9 show that the trans-/trans-isomer is energetic favorable. The partially unfolding structure of beta-domain of MT3 is also simulated and the results show the influence of 6CPCP9 sequence on the correct folding of this domain. The correlations between the bioactivity of MT3 and the simulated structure as well as the folding of beta-domain of MT3 are discussed based on our simulation and previous results.

Effect of α-domain substitution on the structure, property and function of human neuronal growth inhibitory factor

Human metallothionein-3 (hMT3), also named human neuronal growth inhibitory factor (hGIF), is attractive due to its distinct neuronal growth inhibitory activity, which is not shown by other human MT isoforms. It has been reported that the neuronal growth inhibitory activity arises from the N-terminal beta-domain rather than its C-terminal alpha-domain. However, previous bioassay results have shown that the single beta-domain is less effective at inhibiting the neuron growth than that in intact hMT3 on a molar basis, which suggests that the alpha-domain is indispensable to the neuronal growth inhibitory activity of hMT3. In order to confirm this assumption, we constructed two domain-hybrid mutants, the beta(MT3)-beta(MT3) mutant and the beta(MT3)-alpha(MT1) mutant, and investigated their structural and metal binding properties by UV-vis spectroscopy, CD spectroscopy, pH titration, DTNB reaction, EDTA reaction, etc. The results showed that stability of the Cd(3)S(9) cluster of the beta(MT3)-beta(MT3) mutant decreased significantly while the Cd(3)S(9) cluster of the beta(MT3)-alpha(MT1) mutant had a similar stability and solvent accessibility to that of hMT3. Interestingly, the bioassay results showed that the neuronal growth inhibitory activity of the beta(MT3)-beta(MT3) mutant decreased significantly, while the beta(MT3)-alpha(MT1) mutant showed similar inhibitory activity to hMT3. Based on these results, we conclude that the alpha-domain is indispensable and plays an important role in modulating the stability of the metal cluster in the beta-domain by domain-domain interactions, thus influencing the bioactivity of hMT3.

The role of Thr5 in human neuron growth inhibitory factor

GIF, a member of the metallothionein (MT) family (assigned as MT3), is a neuron growth inhibitory factor that inhibits neuron outgrowth in Alzheimer's disease. The conserved Thr5 is one of the main differences between GIF and other members in the MT family. However, natural sheep GIF has an unusual Ala5, casting doubt on the role of common Thr5. We constructed a series of human GIF mutants at site 5, and characterized their biochemical properties by UV spectroscopy, circular dichroism spectroscopy, EDTA reaction, 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) reaction, and pH titration. Their inhibitory activity toward neuron survival and neurite extension was also examined. Interestingly, the T5A mutant exhibited distinct metal thiolate activity in the EDTA and DTNB reactions, and also lost its bioactivity. Meanwhile, the T5S mutant had similar biochemical properties and biological activity as wild-type human GIF, indicating the hydroxyl group on the Thr5 was critical to the bioactivity of human GIF. We suggest the hydroxyl group in human GIF may help stabilize the biologically active conformation. On the other hand, lack of the hydroxyl group in sheep GIF may be partially compensated by its abnormal structure.

Studies on the epitope of neuronal growth inhibitory factor (GIF) with using of the specific antibody

Human neuronal growth inhibitory factor (GIF), a metalloprotein classified as metallothionein-3, is specifically expressed in mammal central nervous system (CNS). In these Studies the specific antibody to human GIF was prepared and used to search the epitope of human GIF by enzyme-linked immunosorbent assay (ELISA) and sequence comparison. The result of ELISA showed the epitope of human GIF may locate on a octapeptide (EAAEAEAE) in the alpha-domain of human GIF, and the result of nerve cell culture indicated that the biological activity of GIF may be affected by the specific antibody.

Identification of mouse brain proteins associated with isoform 3 of metallothionein

Using immunological approaches and mass spectrometry, five proteins associated with metallothionein-3 in mouse brains have been identified. Metallothionein-3 and associated proteins were isolated using immunoaffinity chromatography over immobilized anti-mouse brain MT3 antibody. Proteins in the recovered pool were separated by SDS-polyacrylamide gel electrophoresis, and distinct bands were excised and the proteins digested using trypsin. Peptides were extracted and analyzed using electrospray ionization mass spectrometry. Initial identification was done comparing the identified peptide mass:charge ratios to the MASCOT database. Confirmation of proteins was accomplished by sequencing of selected peptides using tandem mass spectrometry and comparison to the MASCOT database. The proteins were heat-shock protein 84 (mouse variant of heat-shock protein 90), heat-shock protein 70, dihydropyrimidinase-like protein 2, creatine kinase, and beta actin. Independently using antibodies against metallothionein-3, creatine kinase, and heat-shock protein 84 showed that all three proteins were coimmunoprecipitated from whole mouse brain homogenates with each of the three antibodies. Mixing purified samples of metallothionein and human brain creatine kinase also generated a complex that could be immunoprecipitated either by anti-metallothionein-3 or anticreatine kinase antibody. These data are consistent with metallothionein-3 being present in the mouse brain as part of a multiprotein complex providing new functional information for understanding the role of metallothionein-3 in neuronal physiology.

Redox labile site in a Zn4 cluster of Cu4,Zn4-metallothionein-3

Human metallothionein-3 (MT-3) is a neuronal inhibitory factor mainly expressed in brain and downregulated in Alzheimer's disease. The neuroinhibitory activity has been established for native Cu(4),Zn(3)-MT-3 and recombinant Zn(7)-MT-3. However, there is only limited knowledge about the structure and properties of the former metalloform. We have now generated native-like MT-3 through direct Cu(I) and Zn(II) incorporation into the recombinant apoprotein. Its characterization revealed monomeric Cu(4),Zn(4)-MT-3 containing metal-thiolate clusters located in two mutually interacting protein domains, a Cu(4) cluster in the beta-domain and a Zn(4) cluster in the alpha-domain. Using the PC12 cell line, the nontoxic nature of the protein was demonstrated. The results of electronic absorption and Cu(I) luminescence at 77 K showed that the Cu(4) cluster possesses an unprecedented stability in air. In contrast, the Zn(4) cluster is air sensitive. Its oxidation results in the release of one Zn(II) and the formation of a Zn(3) cluster, i.e., Cu(4),Zn(3)-MT-3. This process can be prevented or reversed under reducing conditions. The determined apparent stability constant for the Zn(4) cluster of 2.4 x 10(11) M(-1) is similar to that obtained for other zinc-containing MTs. This suggests that a substantially increased nucleophilic reactivity of specific thiolate ligands is responsible for this effect. Thus, the Zn(4) cluster in MT-3 may play a redox-dependent regulatory role.

Role of metallothionein-III following central nervous system damage

We evaluated the physiological relevance of metallothionein-III (MT-III) in the central nervous system following damage caused by a focal cryolesion onto the cortex by studying Mt3-null mice. In normal mice, dramatic astrogliosis and microgliosis and T-cell infiltration were observed in the area surrounding the lesioned tissue, along with signs of increased oxidative stress and apoptosis. There was also significant upregulation of cytokines/growth factors such as tumor necrosis factor-alpha, interleukin (IL)-1 alpha/beta, and IL-6 as measured by ribonuclease protection assay. Mt3-null mice did not differ from control mice in these responses, in sharp contrast to results obtained in Mt1- Mt2-null mice. In contrast, Mt3-null mice showed increased expression of several neurotrophins as well as of the neuronal sprouting factor GAP-43. Thus, unlike MT-I and MT-II, MT-III does not affect the inflammatory response elicited in the central nervous system by a cryoinjury, nor does it serve an important antioxidant role, but it may influence neuronal regeneration during the recovery process.

Anti-amyloid beta activity of metallothionein-III is different from its neuronal growth inhibitory activity: structure-activity studies

Recent studies have shown that metallothionein-III (MT-III), but not MT-I or -II, antagonizes both the neurotrophic and neurotoxic effects of amyloid beta peptides (Abetas). Further, its anti-Abeta-toxicity effect was attributed to the fact that it inhibits the formation of fibrillar Abeta. MT-III alone also affects neuron survival in culture-promoting at low but inhibiting at high concentrations. To characterize these biological activities of MT-III in relation to its neuronal growth inhibitory activity discovered by Uchida et al. [Neuron 7 (1991) 337-347], we here studied effects of the P7S/P9A double mutant, and the N- and C-terminal domains of MT-III on primary cultures of rat embryonic cortical neurons in the presence and absence of Abeta. Results show that (i). only the wild-type MT-III inhibited the formation of SDS-resistant Abeta aggregates and protected cortical neurons from the toxic effect of Abeta, and (ii). both the wild type and the N-terminal domain of MT-III promote neuron survival at low concentrations but inhibited it at high concentrations. On the basis of these findings, we conclude that the anti-Abeta activity of MT-III is different from its neuronal growth inhibitory activity and suggest that the increased trophic activity of AD brain extracts could be attributed to its low MT-III content.

Engineering of metallothionein-3 neuroinhibitory activity into the inactive isoform metallothionein-1

The third isoform of mammalian metallothioneins (MT-3), mainly expressed in brain and down-regulated in Alzheimer's disease, exhibits neuroinhibitory activity in vitro and a highly flexible structure that distinguishes it from the widely expressed MT-1/-2 isoforms. Previously, we showed that two conserved prolyl residues of MT-3 are crucial for both the bioactivity and cluster dynamics of this isoform. We have now used genetic engineering to introduce these residues into mouse MT-1. The S6P,S8P MT-1 mutant is inactive in neuronal survival assays. However, the additional introduction of the unique Thr5 insert of MT-3 resulted in a bioactive MT-1 form. Temperature-dependent and saturation transfer (113)Cd NMR experiments performed on the (113)Cd-reconstituted wild-type and mutant Cd(7)-MT-1 forms revealed that the gain of MT-3-like neuronal inhibitory activity is paralleled by an increase in conformational flexibility and intersite metal exchange in the N-terminal Cd(3)-thiolate cluster. The observed correlation suggests that structure/cluster dynamics are critical for the biological activity of MT-3. We propose that the interplay between the specific Pro-induced conformational requirements and those of the metal-thiolate bonds gives rise to an alternate and highly fluctuating cluster ensemble kinetically trapped by the presence of the (5)TCPCP(9) motif. The functional significance of such heterogeneous cluster ensemble is discussed.

Different protective roles in vitro of alpha- and beta-domains of growth inhibitory factor (GIF) on neuron injuries caused by oxygen free radicals

It was well known that beta-amyloid (Abeta) and tau protein play an important role in pathological procedure of Alzheimer's disease (AD), a senile dementia. The growth inhibitory factor (GIF, also named metallothionein-3, MT-3) had been demonstrated to inhibit the outgrowth of cortex neurons in the medium with extract of the AD patient brain. In our experiments, it was found that the neurons of cortex and the PC12 (pheochromocytoma) cells could be protected from the cytotoxicity of beta-amyloid 25-35 in presence of GIF and its domains. Additionally, GIF can scavenge the hydroxyl radical efficiently in CytC-VitC radical producing system and its alpha-domain shown more effective potentials than its beta-domain. The electron paramagnetic resonance spectra also show that the alpha-domain has more potential ability for eliminating reactive oxygen free radicals than its beta-domain. The results suggest that GIF could act as an efficient scavenger against free radicals in vitro and the alpha-domain in GIF molecule shows more potential in protecting against reactive oxygen species injury than the beta-domain.

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.

Metal-thiolate clusters in neuronal growth inhibitory factor (GIF)

Human neuronal growth inhibitory factor (GIF) is a metallothionein-like protein specific to the central nervous system, which has been linked to Alzheimer's disease. In this article a short overview of the biological and structural properties of native Cu4,Zn3-GIF are described. Moreover, metal-thiolate clusters formed in the synthetic beta-domain (residues 1-32) and the alpha-domain (residues 32-68) both with native CuI and ZnII, and as a spectroscopic probe also with Cd(II) are discussed. The cluster formation was followed by electronic absorption, circular dichroism (CD), magnetic circular dichroism (MCD) and 113Cd NMR spectroscopy and, in the special case of Cu(I) complexes, by luminescence spectroscopy at 77 K. These structural features are compared with those of recombinant Zn7- and 113Cd7-GIF. The structural studies suggest the existence of distinct MeII4S11 and MeII3S9 clusters located in the mutually interacting alpha- and beta-domains, respectively, of Cd7-GIF. In addition, evidence for a highly dynamic and flexible structure of this protein is presented.

Structural and functional analysis of the rat metallothionein III genomic locus

Metallothionein III (MT III) has been reported to suppress neuronal growth in a rat in vitro model system. The protein and its specific mRNA are detected predominantly in the brain, differentiating MT III from the well-characterised archetypal metallothioneins. Isolation, sequencing and functional analysis of the rat MT III genomic locus indicated that, although the organisation of the gene was conserved between MT III and the more conventional metallothioneins, the 5' flanking region of the MT III gene was distinct. Within this region, a number of putative regulatory elements were identified, including the metal regulatory elements (MREs) characteristic of metallothionein promoters. However, despite their conservation in sequence with active elements, the MREs of MT III were unresponsive to zinc. A 'silencing element' was revealed within a 250 bp section of the MT III promoter which suppressed gene expression in two brain cell lines. The operation of this silencing region in conjunction with the inactive MREs may explain the distinct expression profile observed for MT III within the central nervous system and during neuronal development.

Regulation of metallothionein-3 mRNA by thyroid hormone in developing rat brain and primary cultures of rat astrocytes and neurons

Metallothionein-3 (MT-3) is a brain specific member of the MT family. Unlike other members of this family, MT-3 has been shown to act as a neuronal growth inhibitory factor. MT-3 mRNA abundance increases throughout the developmental period, reaching adult levels by postnatal day 21. The role of thyroid hormone in the developmental regulation of MT-3 mRNA was tested because thyroid hormone is known to regulate brain gene expression. Furthermore, gestational hypothyroidism results in developmental brain abnormalities. Hypothyroidism was induced in pregnant dams by the administration of PTU from gestational day 7, resulting in a 4- to 6-fold increase in pup MT-3 mRNA abundance on the day of birth (day 0) and on postnatal day 3. Normal pups did not reach this level of brain MT-3 mRNA until postnatal day 21. Administration of thyroxine (T(4), 2 microg/g) to pups on postnatal day 1 or day 20 resulted in a decrease in MT-3 mRNA abundance on postnatal day 21, regardless of when the injection was given. Furthermore, addition of T(4) to primary cultures of brain (olfactory bulb) astrocytes and neurons from 4-day-old rats resulted in a significant decrease in MT-3 mRNA in 24 h. Given the neuronal growth inhibitory function of MT-3, these data suggest that MT-3 may play a role in the CNS-related consequences of hypo- and hyperthyroidism during development.

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.

Growth inhibitory factor and zinc affect neural cell cultures in a tissue specific manner

Deficiency of neuronal growth inhibitory factor (GIF) and abnormalities in zinc homeostasis have been suggested to play a role in the neuropathogenesis of Alzheimer's disease. We report here that embryonic chick cerebral cell cultures zinc and copper containing GIF in the presence of marmoset hippocampal extract reduces significantly and concentration dependently mitochondrial succinate dehydrogenase activity (MTT) and cell mass. In contrast, no indications could be found that GIF affected neural retina cell cultures. Our results suggest that the observed effects of GIF are not elicited by zinc.

Expression of the gene encoding metallothionein-3 in organs of the reproductive system

Metallothionein-3 (MT-3) is a new MT gene-family member that inhibits survival of rat neurons cultured in presence of brain extracts. Contrary to other MT genes, which are expressed in most tissues and which are highly inducible by metals, MT-3 expression was reported to be mainly in the brain, and it failed to respond to metals in vivo. We show here that MT-3 mRNA is present in several organs other than the brain, as assayed by Northern analyses. In the rat, MT-3 mRNA was detected in the testis, prostate, epididymis, tongue, ovary, uterus, stomach, heart, and seminal vesicles. The MT-3 mRNA levels in the testis, epididymis, prostate, and tongue were 22% of those in brain, while in ovary, uterus, and stomach, they were 4% of the brain level, and they were lower still in the other organs. The MT-3 gene was not inducible by CdCl2 or lipopolysaccharide in rat testis and prostate. In the mouse and the human, relative MT-3 mRNA levels were lower than those found in the rat when compared with those present in brain. Testicular MT-3 transcript levels remained quite constant during rat postnatal development in animals aged from 6 to 43 days. In situ hybridization analyses on human testis sections showed that MT-3 mRNA was present at different levels in both the Leydig cells and the seminiferous tubules. In orchiectomized rats, prostatic MT-3 mRNA was decreased by 75%, and injections of dihydrotestosterone restored MT-3 mRNA levels to control values. Overall, these results show that MT-3 tissue-specific gene expression is broader than previously reported and provide new experimental systems to study the function and mechanism of action of the MT-3 protein.

Brain injury and growth inhibitory factor (GIF)--a minireview

Growth inhibitory factor (GIF) is a small (7 kDa), heat-stable, acidic, hydrophilic metallothionein (MT)-like protein. GIF inhibits the neurotrophic activity in Alzheimer's disease (AD) brain extracts on neonatal rat cortical neurons in culture. GIF has been shown to be drastically reduced and down-regulated in AD brains. In neurodegenerative diseases in humans, GIF expression levels are reduced whereas GFAP expression levels are markedly induced in reactive astrocytes. Both GIF and GIF mRNA are present at high levels in reactive astrocytes following acute experimental brain injury. In chronological observations the level of GIF was found to increase more slowly and remain elevated for longer periods than that of glial fibrillary acidic protein (GFAP). These differential patterns and distribution of GIF and GFAP seem to be important in understanding the mechanism of brain tissue repair. The most important point concerning GIF in AD is not simply the decrease in the level of expression throughout the brain, but the drastic decrease in the level of expression in reactive astrocytes around senile plaques in AD. Although what makes the level of GIF decrease drastically in reactive astrocytes in AD is still unknown, supplements of GIF may be effective for AD, based on a review of current evidence. The processes of tissue repair following acute brain injury are considered to be different from those in AD from the viewpoint of reactive astrocytes.

Cloning of porcine neuron growth inhibitory factor (metallothionein III) cDNA and expression of the gene in Saccharomyces cerevisiae

Growth inhibitory factor (GIF), a member of the metallothionein (MT) family, is also known as MTIII. This protein distinguishes itself from other MT isoforms by exerting an inhibitory effect on cortical neuron growth instead of metal ion buffering. In this work, we cloned MTIII genes from a porcine brain cDNA library. Two species of clones were isolated that vary with respect to one nt in the coding sequence. This discrepancy results in the translation of two MTIII primary structures having a different amino acid at residue 46. Herein, both MTIII cDNAs were constructed into an expression vector and transformed into yeast cells, respectively. The yeast carrying either MTIII gene displayed a similar metal tolerance when cultured in a medium containing metal. The resistance to metal toxicity was attributed to the expression of MTIII gene which was confirmed by RNA and protein analyses. The characteristics of the protein stability, metal binding affinity and ultraviolet absorption spectrum of the yeast produced MTIII are also compared with those of MTII. The comparison reveals that both MTs have similar physical characteristics. Moreover, circular dichroism spectrum of Cd saturated MTIII was analyzed as well. Typical Cys-Cd bands for MTII appear in the spectrum, indicating similar metal-thiol interactions for MTIII to those for other MT isoforms.

Identification and characterization of metallothionein III (growth inhibitory factor) from porcine brain

Porcine metallothionein III (MTIII) was isolated from brain tissue by the combination of gel filtration and anion-exchange chromatographies. The identity of MTIII was confirmed by comparing the chromatographic characteristics to other MT isoforms isolated from porcine liver. Porcine MTIII has a low molecular weight and a pl of 4.1. This protein contains both zinc and copper, and the zinc can be replaced by cadmium. Using reverse transcriptase-polymerase chain reaction, a 0.2 kb DNA fragment can be amplified from the porcine brain mRNA. This DNA fragment was demonstrated to contain MTIII coding region after cloning and sequencing. The revealed DNA sequence can be translated into 68 amino acids and shows a common structural characteristic of MTIII from other species. Northern blot analysis indicated that MTIII and mRNA is expressed in every specified region of the porcine brain examined here.

Evidence for Cu(I) clusters and Zn(II) clusters in neuronal growth-inhibitory factor isolated from bovine brain

Neuronal growth-inhibitory factor (GIF), a central-nervous-system-specific metallothionein-like protein, has been isolated by means of an improved isolation procedure from bovine brain. The native protein contains 4-5 Cu+ and 2-2.5 Zn2+, which results in an overall stoichiometry of 6-7 mol metal ions/mol protein. Native Cu, ZN-GIF and the Zn2+ -substituted and Cd2+-substituted metalloforms have been characterized by means of electronic-absorption, CD, magnetic-circular-dichroism (MCD) and low-temperature (77 K) Cu(I)-luminescence spectroscopy. Analysis of the metal-induced-charge-transfer transitions below 300 nm in the electronic-absorption and CD spectra of Cu, ZN-GIF revealed spectral features characteristic of metal-thiolate coordination. The presence of formally spin-forbidden 3d --> 4s Cu(I)-cluster-centered transitions, above 300 nm in the corresponding CD and MCD spectra indicate the existence of a Cu(I) cluster. The 77-K luminescence spectrum of Cu, ZN-GIF revealed two emissive bands at approximately 420 nm and 570 nm, which were reported also for CU4 clusters in mammalian Cu8-metallothionein. By analogy with Cu8-metallothionein, we propose the presence of a Cu4 cluster with similar electronic structure in native GIF. However, the determined Cys/Cu+ ratio of approximately 2:1 in Cu, Zn-GIF is higher than the ratio found in mammalian Cu(I)-metallothionein forms (approximately 1.6:1 ), which implies that the coordination geometry of CU+-binding sites is different in the CU4 Cluster. The spectroscopic characterization of Zn2+-substituted and Cd2+-substituted GIF (6-7 metal ions/protein) showed CD and MCD features at positions identical to those reported for the well-characterized mammalian Zn7-metallothionein and Cd7-metallothionein. Therefore, it is inferred that the cluster organization in GIF with divalent metal ions is comparable to that found in mammalian metallothioneins. The effect of metal ions on the protein structure with regard to the biological function of GIF is discussed.