International Journal Of Biology and Biological Sciences Vol. 2(9), pp. 129-135, September 2013
Available online at /journal/ijbbs
ISSN 2327-3062 ©2013 Academe Research Journals
Review
Effects of metallothionein on nervous system and
neurological diseases
Pingping ZHANG1, Ping SONG1, Xuewen TIAN2, Ye DOU1, Hongda CHENG1 and
reactive materials studiesQinglu WANG1*
1Key Laboratory of Biomedical Engineering and Technology of Shandong High School,
Shandong Wanjie Medical College, Zibo, 255213, China.
2Sports Science Research Center of Shandong Province, Jinan, 250102, China.
Accepted 9 September, 2013
Many experimental data show that metallothionein (MT), which is a metal-binding protein, has a close relation with neuroprotection and neurological diseases in mammals. This study aims to clarify the effect of metallothionein on nervous system and neurological diseases. Random control trials that investigated the association of metallothionein and neurological diseases were included in the review by researching MEDLINE, EMBASE and PubMed up to April 2013. Although single α- and β-domain all have important function of neuroprotection, each of them is indispensable in conducting its bioactivity. Many experiments showed that some drugs/compounds exert neuron protective effects mainly via up-regulation of MT. Regardless of the endogenous or exogenous MT, they all have the function of neuroprotective in rats after spinal cord injury. MT has a high potential for the neuroprotection and treatment of neurodegenerative diseases such as diabetic neuropathy, alzheimer’s disease, parkinson’s disease, huntington’s disease, cerebral ischemia, and muscle function owing t o its various functions including anti-oxidant properties and modulators not only for Zn but for Cu in the extra- and intracellular spaces.
Key words: Metallothionein, nervous system, neurological diseases, zinc ion, oxidative stress. INTRODUCTION
Metallothioneins (MTs) belong to    a superfamily of intracellular metal-binding proteins, present in virtually all living organisms. Typically, MTs have low molecular weight (<7000 Da), high metal content comprising predominantly Zn, Cu or Cd, highly conserved 18-23 cysteine residues and no aromatic amino acids or histidine (Wang et al., 2011). In mammals, MTs are thought to be primarily involved in copper sequestration and in the protection from reactive oxygen species.
MT-I and MT-II are coordinately regulated, and are preferentially expressed in astrocytes and activated microglia/macrophages, where they can be upregulated by many factors, such as heavy metal ions, oxygen radical and hormones, etc. MT-III is known to be a growth inhibitory factor (GIF), which is a brain-specific member of the MT family (Coyle et al., 2002). Several studies have shown that MT-III is abundantly distributed in the astrocytes of the human brain, however, its expression levels  were  reduced  in  the  astrocytes  of  people  with neuronal disease such as Alzheimer’s diseases (Uchida et al., 1991; Tsuji et al., 1992; Yu et al., 2001), and overexpression of MT-III prevents neuronal cell death in an animal model of brain damage (Hozumi et al., 1995; Yamada et al., 1996). Although the function of MT-III remains unclear with respect to ischemia, some studies suggest that increasing the concentration of MT-III protein  *Corresponding author. E-mail: wqlzcq@gmail. Fax: 86 533 4619562.
ABBREVIATIONS: MT, metallothionein; AD,alzheimer‘s disease; ALS,amyotrophic lateral sclerosis; PD, parkinson‘s disease; CNS,central nervous system; SCI, spinal cord injury; DRG, dorsal root ganglia; DN, diabetic neuropathy; HD,Huntington‘s disease; polyQ, polyglutamine; SCT, spinal cord transection.
Zhang et al.          130
may play a role in reducing the effects of ischemia and oxidative stress (Yuguchi et al., 1997; Koumura et al., 2009).
Although MTs was found to work as a modulator of Zn and induce anti-oxidant reaction, the precise functions and its functional mechanisms remain to be elucidated. Over the years, a new isoform of MT, MT-III (GIF), has been found in the brain, which was markedly diminished in the brain of Alzheimer’s disease (AD). Many new findings on MT have been discovered in neurodegenerative diseases other than AD such as amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), prion disease, brain trauma, brain ischemia, and psychiatric diseases. MT has a high potential for the treatment of neurodegenerative diseases such as ALS, AD, and PD owing to its various functions including anti-oxidant properties and modulators not only for Zn but for Cu in the extra- and intracellular spaces. On t
he other hand, there are still various problems on MT to be elucidated in detail, including their binding proteins and functional mechanisms (Hozumi, 2013).
Here, the association studies of MT and neuron/neural disease were summarized, and then a brief overview was given. The focus of this review is on mammalian MT (MT-I, MT-II and MT-III) with emphasis on findings from the neuroprotection of MT and expression change of MT in neural disease.
NEUROPROTECTION OF METALLOTHIONEIN
Effect of single α-domain and β-domain on neuroprotection
The central nervous system (CNS) is one of the organizations of MT-I, -II and -III gene expression. Of major significance is the result that exogenous MT can exert potent anti-inflammatory and neuroprotection effects in a number of animal models (Hidalgo et al., 2001; Chung et al., 2008a). While this has been known for some years, there was no clue of the putative mechanisms involved until it has been convincingly demonstrated that neurons can bind and incorporate MTs through members of the low-density lipoprotein receptor family (Ambjorn et al., 2008; Chung et al., 2008b). Ambjorn et al. (2008) demonstrated that a peptide modeled after the β-domain of MT, EmtinB, had effects on cultured neurons comparable to those of the native full protein despite major differences in the primary structur
e. Similar peptides modeled after the α-domain has also recently been described (Asmussen et al., 2009). Manso et al. (2010) reported the results that compared the full mouse MT-I with the independent α- and β- domains, alone or together, in a cryoinjury mouse model. The lesion of the cortex caused the mice to perform worse in the horizontal ladder beam and the rota-rod tests; they found out that all the proteins showed a modest effect in the  horizontal  ladder  beam  test,  while  only  full  MT-I  improved the performance of animals in the rota-rod, and the α-domain showed a rather detrimental effect. So their conclusion was that splitting the two MT-I domains do not seem to eliminate all MT functions but certainly modifies them, and different motifs seem to be present in the protein underlying such functions (Manso et al., 2010).
In the study of Luo et al. (2013), the α and β single-domain proteins, heterozygous β(MT3)-α(MT1), and a linker-truncated mutant D31-34 were prepared and characterized. They confirmed the two domains through the linker Lys-Lys-Ser from a cooperative unit, and each of them is indispensable in conducting its bioactivity. The α domain plays an important role in modulating the stability of the metalthiolate cluster, and the α-β domain-domain interaction through the linker is critical for its protective role in the brain (Luo et al., 2013). Neuroprotection of full metallothionein
Zinc is a trace element essential for the development of the CNS. The largest concentration of zinc in t
he brain is found in the hippocampus where it has been studied for its possible role in memory and neurotoxicity (Xie and Smart, 1994). Zinc is sequestered and released from hippocampal neurons, suggesting a possible modulatory role. MT-III may play a role in the availability of zinc because it has been found in areas of the brain capable of concentrating and releasing zinc, and it is expressed most abundantly in large neuronal cell bodies in the brain (Masters et al., 1994).
Many experiments showed that some drugs/compounds exert neuron protective effects mainly via up-regulation of metallothionein. The protective effects of apomorphine on cortical neurons are regulated, at least in part, by its oxidized products, and prevention of intracellular accumulation of Zn2+contributes to apomorphine protection against Zn2+neurotoxicity (Hara et al., 2013). Exogenous H2O2enhances cytosolic zinc content in a MT-III-requiring manner. Both exogenous H2O2and endogenous ROS mediated transactivation of tropomyosin receptor kinase    B (TrkB) requires intracellular zinc and MT-III. Thus, a molecular signaling event was proposed whereby ROS induces release of zinc from cytosolic MT-III. The increased cytosolic zinc transactivates TrkB, and the enhanced Shc signaling downstream from TrkB promote prosurvival effects. Such neuroprotective effects mediated by ROS are operative in diverse acute and chronic neurological disorders (Huang and McNamara, 2012).
The hippocampus plays a critical role in memory formation, organization and storage. Propofol was shown to attenuate hippocampal neuron death and promote neurogenesis after oxygen deprivation or cerebral ischemia (Lasarzik et al., 2009). MT-III attenuates apoptotic neuron death in hippocampus and it preserves cognitive function (Ma et al., 2011). In addition to these mechanistic  explanations, the researchers  deduced that
propofol enhanced MT-III mRNA and protein expression, so they thought MT-III is the target protein of propofol, which provides new insight into propofol’s neuroprotective effects (He et al., 2013).
METALLOTHIONEIN AND SPINAL CORD INJURY Spinal cord injury (SCI) is characterized by two chronological events: the primary injury and the secondary injury. The primary injury is directly caused by the mechanical trauma and leads to a complex cascade of pathophysiological processes known as secondary injury, expanding the site of damage soon after the injury and for    a long time later, resulting in greater neurodegeneration and neurological dysfunction (Carmel et al., 2004). Subsequently, auto-destructive mechanisms such as excitotoxicity, inflammation, oxidative stress and cellular death by apoptosis or necrosis are activated during secondary injury (Carmel et al., 2004). Oxidative stress is an important mechanism involved in this process, where the antioxidant defenses are up-regulated in order to control the ROS. In this context, MT plays a role as antioxidant thiol-defen
ce in the acute phase of SCI. MT functions include the transport and storage of essential transition metals and detoxification and protection against ROS (Coyle et al., 2002), which are important mechanisms for the host defense response, immunoregulation, cell survival and brain repair (Penkowa and Hidalgo, 2003). MT-III mRNA, measured using RT-PCR, and MT-III immunoreactivity, are both present in the spinal cord as well as the thoracic and lumbar dorsal root ganglia (DRG) (Velazquez et al., 1999), while MT-I and MT-II are expressed in astrocytes and microglia (Arellano-Ruiz et al., 2012).
After SCI, a complex cascade of pathophysiological processes rapidly damages the nervous tissue. In the latest study, it has been confirmed that despite the endogenous or exogenous MT, they all have the function of neuroprotective in rats after SCI (Arellano-Ruiz et al., 2012). However, the participation of MT in neuroprotective processes after SCI is still unknown. METALLOTHIONEIN AND NEUROLOGICAL DISEASES
Diabetic neuropathy
Diabetic neuropathy (DN) is a serious complication in 60-70% of diabetic patients (both Type 2 and Type 1). However, the mechanisms responsible for this complication are unclear. MT is an antioxidant that is very efficient in scavenging various free radicals but has limited ability to cross lipid bilayers (Par
k et al., 2011). MT has the potential to protect cells and tissues against diabetes and diabetic complications due to its antiapoptotic and antioxidant effects. Min et al. (2012) made Tat-MT and Tat-SOD constructs and then  delivered
Int. J. Biol. Biol. Sci.          131 them to PC12 cells. The results confirmed that Tat-MT and Tat-SOD decreased the expression of both mitochondrial- and endoplasmic reticulum (ER)-mediated apoptosis proteins in PC12 cells. Tat-MT and Tat-SOD protect against high glucose-, hypoxia-, and ROS-induced cell death and improved myelination of sciatic nerves and delayed the clinical development of DN (Min et al., 2012).
Alzheimer’s disease
Among the dementias, Alzheimer’s disease (AD) is the most commonly diagnosed, but there are still no effective drugs available for its treatment. Significant alterations of tissue metal levels have been reported in AD. The data of Lui et al. (1990) further suggested that the metabolism of cadmium and zinc is altered in AD. It has been suggested that MT-III could be somehow involved in the etiology of AD, and in fact very promising results have been found in in vitro studies, but the role of MT-III in vivo needs further analysis (Manso et al., 2012).
As a pathological hallmark of AD, the cerebral amyloid deposits are composed of a 39-43 residue amyloid peptide (Aβ), which is derived from the parental amyloid precursor protein by sequential cleavage with two aspartyl proteases termed β-secretase and γ-secretase (Kang et al., 1987). The most preva lent forms of Aβ are Aβ1-40and Aβ1-42, with an approximate ratio of 9:1 in biological fluids of humans (Hardy, 1997). In senile plaques, Aβ is present in a β-sheet, probably a parallel β-helix conformation, and Cu2+and Zn2+are bound to histidine side chains within this structure (Dong et al., 2003). Transition metal ions, such as copper, zinc, and iron, exhibit dyshomeostasis in the brain of AD patient (Smith et al., 2007). MT-III antagonizes the neurotoxic effects of Aβ peptides mainly by abolishing the formation of toxic aggregates of Aβ peptides (Irie and Keung, 2003). The metal-exchange process between Zn7MT-III and Aβ1-40Cu2+was recently found, by which the production of ROS or the related cellular toxicity was quenched (Pedersen et al., 2012).
Parkinson’s disease
Parkinson’s disease (PD) is characterized by    a progressive loss of dopaminergic neurons in the substantia nigra zona compacta, and in other sub-cortical nuclei associated with a widespread occurrence of Lewy bodies. The cause of cell death in PD is still poorly understood, but    a defect in mitochondrial oxidative phosphorylation and enhanced oxidative and nitrative stresses have been prop
osed (Ebadi et al., 1991).
Ebadi et al. (1991) in their study found an interesting relation between MT and oxidation reactions in PD, but in 2005 other people through mice experiment obtained some data. These data are interpreted to suggest that peroxynitrite ions are involved in  the  etiopathogenesis of
Zhang et al.          132
PD, and MT-mediated coenzyme Q10 synthesis may pro-vide neuroprotection (Ebadi et al., 2005). Furthermore, MT provides ubiquinone-mediated neuroprotection in PD (Ebadi et al., 2002). Latest findings demonstrated that MT increased the expression by reactive astrocytes in PD nigra and support a neuroprotective role for glial cells (Michael et al., 2011).
Huntington’s disease
Huntington's disease (HD) is caused by a polyglutamine (polyQ) expansion in the huntingtin protein, which leads to protein misfolding and aggregation of this protein. Abnormal copper accumulation in the HD brain has been reported, but the role of these metals in HD pathogenesis is unknown (Dexter et al., 1991). Recent findings show that copper-regulatory genes are induced during HD and copper bind
s to an N-terminal fragment of huntingtin, supporting the involvement of abnormal copper metabolism in HD. Ocampo and Barrientos (2008) in their study have constructed yeast models of Huntington’s disease based on the expression, under the control of different promoters, of the first exon of the huntingtin-containing polyglutamine tracts of both wild-type and mutant lengths. Using this yeast model of HD, Hands et al. (2010) showed that overexpression of MT-III involved in copper metabolism reduces polyQ-mediated toxicity. Overexpression of MT-III in mammalian cells significantly reduced polyQ aggregation and toxicity. So they proposed that MTs are potential therapeutic targets for HD.
Cerebral ischemia
Association of cerebral ischemia and metallothionein was first found in 1996 (Inuzuka et al., 1996). Recently, accumulating studies have suggested that MTs are an important neuroprotective substance for cerebral ischemia and retinal diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), which are characterized by a progressive retinal degeneration (Ito et al., 2013). Oxidative stress and/or zinc toxicity has been implicated as part of the common pathway in these diseases. In MT-III knockout mice, the neuronal damage was aggravated after transient focal cerebral ischemia (Koumura et al., 2009). Serial analysis of gene expression also identified MT-II as major neuroprotective gene in mouse focal cerebral ischemia (Trendelenburg et al., 2002).
Park et al. (2013) investigated the effect of hypothermia on MT expression and the underlying mechanisms. It was observed that hypothermia augmented MT levels. They demonstrated that hypothermia was a potent inducer of MT gene transcription in brain endothelial cells, and enhanced MT expression might contribute to protection against ischemia. MT gene expression is induced by hypothermia  mainly  through  the  signal  transducer  and  activator of transcription    3 (STAT3) pathway. DNA methylation may contribute to MT gene regulation under ischemic or hypothermic conditions.
MUSCLE FUNCTION AND METALLOTHIONEIN Soleus muscle contractile dysfunction
MT-1/MT-2 null (MT(-/-)) mice would display exacerbated soleus muscle atrophy, oxidative injury, and contractile dysfunction compared with the response of wild-type (WT) mice following acute spinal cord transection (SCT). MT deficiency did not impact soleus muscle mass loss, but resulted in contractile dysfunction and increased lipid peroxidation following acute SCT. The role of MT in mediating protective adaptations in skeletal muscle follows disuse mediated by spinal cord injury (DeRuisseau et al., 2009).
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis (ALS) affects anterior horn cells of the spinal cord causing an indolent slow and steady deterioration of muscle strength leading inevitably to death in respiratory failure. ALS is a model condition for neurodegenerative disorders (Roos and Dencker, 2012). Defect of MT-III has been reported to be a contributor to the progression of ALS. Latest, the expression and effects of MT-III on the motor neurons of spinal cords of ALS model mice [G93A Cu/Zn superoxide dismutase (SOD-1) mutant-transgenic (Tg) mice] were investigated using a retrograde viral delivery system. The results demonstrated that MT-III prevents the loss of motor neurons of ALS model mice and prolongs the life span, even when the administration is started at the time of onset (Hashimoto et al., 2011).
Other studies have different results when compared with the above studies. MT-I mRNA expression was already significantly upregulated in the region of the spinal cord responsible for motor paralysis. The expression of another isoform, MT-III, was not changed. So these results indicated that the MT-I isoform, but not the MT-III isoform, is associated with motor neuron death in ALS and suggested that the disease might be a systemic disorder to which the spinal cord is particularly susceptible (Ono et al., 2006).
Although some studies have shown differences in previous studies, ALS tissue remains capable of expressing all the major MT genes. MT, present in protoplasmic glia, arises locally and is not secondar
y to increases of hepatic or renal MT. Because MT-III is also expressed by the normal and ALS spinal cord, it is a central nervous system-specific and not only a brain-specific protein. Thus, the excess of MT in ALS liver seems to be an effect of slower catabolism rather than faster synthesis of protein (Blaauwgeers et al., 1996). MTs may serve an early and important protective function
Int. J. Biol. Biol. Sci.          133 Figure 1. Metallothionein and neurological diseases.
in family ALS (Gong and Elliott, 2000).
CONCLUSIONS
MT is a protein with redox and metal-binding properties that endow it with wide-ranging functional capabilities in biosystems. Redox control of the metabolic influence of Zn has been proposed as the core function of MT (Coyle et al., 2002). Based on the analysis of the above nervous system and neurological diseases under MT effect, this study concluded that the function of MT in the neuroprotection and recovery of neurological diseases is largely due to its ability of regulating zinc ion concentration and reducing the effects of oxidative stress (Figure 1).
ACKNOWLEDGMENTS
This work was supported by grants from the Program of Science and Technology Development of Shandong Province (2012YD19015) and the Program of Nature and Science of Shandong Wanjie Medical College (X12ZK01).
REFERENCES
Ambjorn M, Asmussen JW, Lindstam M, Gotfryd K, Jacobsen C, Kiselyov VV, Moestrup SK, Penkowa M, Bock  E,  Berezin  V  (2008).  Metallothionein  and  a
peptide modeled after metallothionein, emtinb, induce neuronal differentiation and survival through binding to receptors of the low-density lipoprotein receptor family. J. Neurochem., 104(1): 21-37.
Arellano-Ruiz S, Rios C, Salgado-Ceballos H, Mendez-Armenta M, Valle-Mondragon Del L, Nava-Ruiz C, Altagracia-Martinez M, Diaz-Ruiz    A (2012). Metallothionein-ii improves motor function recovery and increases spared tissue after spinal cord injury in rats. Neurosci. Lett., 514(1): 102-105.
Asmussen JW, Ambjorn M, Bock E, Berezin V (2009). Peptides modeled after the alpha-domain of metallothionein induce neurite outgrowth and promote survival of cerebellar granule neurons. Eur. J. Cell Biol., 88(8): 433-443.
Blaauwgeers HG, Chand Anwar M, Berg van den FM, Jong Vianney de JM, Troost D (1996). Expression of different metallothionein messenger ribonucleic acids in motor cortex, spinal cord and liver from patients with amyotrophic lateral sclerosis. J. Neurol. Sci., 142(1-2): 39-44.
Carmel JB, Kakinohana O, Mestril R, Young W, Marsala M, Hart RP (2004). Mediators of ischemic preconditioning identified by microarray analysis of rat spinal cord. Exp. Neurol., 185(1): 81-96.
Chung RS, Hidalgo J, West AK (2008a). New insight into the molecular pathways of metallothionein-mediated neuroprotection and regeneration. J. Neurochem.,
104(1): 14-20.

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