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

From zinc homeostasis to disease progression: Unveiling the neurodegenerative puzzle

Zinc is a crucial trace element in the human body, playing a role in various physiological processes such as oxidative stress, neurotransmission, protein synthesis, and DNA repair. The zinc transporters (ZnTs) family members are responsible for exporting intracellular zinc, while Zrt- and Irt-like proteins (ZIPs) are involved in importing extracellular zinc. These processes are essential for maintaining cellular zinc homeostasis. Imbalances in zinc metabolism have been linked to the development of neurodegenerative diseases. Disruptions in zinc levels can impact the survival and activity of neurons, thereby contributing to the progression of neurodegenerative diseases through mechanisms like cell apoptosis regulation, protein phase separation, ferroptosis, oxidative stress, and neuroinflammation. Therefore, conducting a systematic review of the regulatory network of zinc and investigating the relationship between zinc dysmetabolism and neurodegenerative diseases can enhance our understanding of the pathogenesis of these diseases. Additionally, it may offer new insights and approaches for the treatment of neurodegenerative diseases.

Zinc in Regulating Protein Kinases and Phosphatases in Neurodegenerative Diseases

Zinc is essential for human growth and development. As a trace nutrient, zinc plays important roles in numerous signal transduction pathways involved in distinct physiologic or pathologic processes. Protein phosphorylation is a posttranslational modification which regulates protein activity, degradation, and interaction with other molecules. Protein kinases (PKs) and phosphatases (PPs), with their effects of adding phosphate to or removing phosphate from certain substrates, are master regulators in controlling the phosphorylation of proteins. In this review, we summarize the disturbance of zinc homeostasis and role of zinc disturbance in regulating protein kinases and protein phosphatases in neurodegenerative diseases, with the focus of that in Alzheimer’s disease, providing a new perspective for understanding the mechanisms of these neurologic diseases.

Transcriptomic and functional analysis of Aβ1-42 oligomer-stimulated human monocyte-derived microglia-like cells

Dysregulation of microglial function contributes to Alzheimer's disease (AD) pathogenesis. Several genetic and transcriptome studies have revealed microglia specific genetic risk factors, and changes in microglia expression profiles in AD pathogenesis, viz. the human-Alzheimer's microglia/myeloid (HAM) profile in AD patients and the disease-associated microglia profile (DAM) in AD mouse models. The transcriptional changes involve genes in immune and inflammatory pathways, and in pathways associated with Aβ clearance. Aβ oligomers have been suggested to be the initial trigger of microglia activation in AD. To study the direct response to Aβ oligomers exposure, we assessed changes in gene expression in an in vitro model for microglia, the human monocyte-derived microglial-like (MDMi) cells. We confirmed the initiation of an inflammatory profile following LPS stimulation, based on increased expression of IL1B, IL6, and TNFα. In contrast, the Aβ_1-42 oligomers did not induce an inflammatory profile or a classical HAM profile. Interestingly, we observed a specific increase in the expression of metallothioneins in the Aβ_1-42 oligomer treated MDMi cells. Metallothioneins are involved in metal ion regulation, protection against reactive oxygen species, and have anti-inflammatory properties. In conclusion, our data suggests that exposure to Aβ_1-42 oligomers may initially trigger a protective response in vitro.

Systematic review of human post-mortem immunohistochemical studies and bioinformatics analyses unveil the complexity of astrocyte reaction in Alzheimer's disease

AIMS

Reactive astrocytes in Alzheimer's disease (AD) have traditionally been demonstrated by increased glial fibrillary acidic protein (GFAP) immunoreactivity; however, astrocyte reaction is a complex and heterogeneous phenomenon involving multiple astrocyte functions beyond cytoskeletal remodelling. To better understand astrocyte reaction in AD, we conducted a systematic review of astrocyte immunohistochemical studies in post-mortem AD brains followed by bioinformatics analyses on the extracted reactive astrocyte markers.

METHODS

NCBI PubMed, APA PsycInfo and WoS-SCIE databases were interrogated for original English research articles with the search terms 'Alzheimer's disease' AND 'astrocytes.' Bioinformatics analyses included protein-protein interaction network analysis, pathway enrichment, and transcription factor enrichment, as well as comparison with public human -omics datasets.

RESULTS

A total of 306 articles meeting eligibility criteria rendered 196 proteins, most of which were reported to be upregulated in AD vs control brains. Besides cytoskeletal remodelling (e.g., GFAP), bioinformatics analyses revealed a wide range of functional alterations including neuroinflammation (e.g., IL6, MAPK1/3/8 and TNF), oxidative stress and antioxidant defence (e.g., MT1A/2A, NFE2L2, NOS1/2/3, PRDX6 and SOD1/2), lipid metabolism (e.g., APOE, CLU and LRP1), proteostasis (e.g., cathepsins, CRYAB and HSPB1/2/6/8), extracellular matrix organisation (e.g., CD44, MMP1/3 and SERPINA3), and neurotransmission (e.g., CHRNA7, GABA, GLUL, GRM5, MAOB and SLC1A2), among others. CTCF and ESR1 emerged as potential transcription factors driving these changes. Comparison with published -omics datasets validated our results, demonstrating a significant overlap with reported transcriptomic and proteomic changes in AD brains and/or CSF.

CONCLUSIONS

Our systematic review of the neuropathological literature reveals the complexity of AD reactive astrogliosis. We have shared these findings as an online resource available at www.astrocyteatlas.org.

The interaction of zinc with the multi-functional plasma thyroid hormone distributor protein, transthyretin: evolutionary and cross-species comparative aspects

A considerable body of evidence has been accumulated showing the interrelationship between zinc and the plasma thyroid hormone (TH) distributor protein, transthyretin (TTR). TTR is a multi-functional protein, which emerged from 5-hydroxyisourate hydrolase (HIUHase) by neo-functionalization after gene duplication during early chordate evolution. HIUHase is also a zinc-binding protein. Most biochemical and molecular biological findings have been obtained from mammalian studies. However, in the past two decades, it has become clear that fish TTR displays zinc-dependent TH binding. After a brief introduction on plasma zinc, THs and their binding proteins, this review will focus on the role of zinc in TTR functions of various vertebrates. In particular primitive fish TTR has an extremely high zinc content, with an increased number of histidine residues which are involved in TH binding. However, zinc-dependent TH binding may have been gradually lost from TTRs during higher vertebrate evolution. Although human TTR has a low zinc content, zinc plays an essential role in TTR functions other than TH binding: the stability of TTR-holo retinol binding protein 4 (holoRBP4) complex, TTR amyloidogenesis, the sequestration of amyloid β (Aβ) fibrils and cryptic proteolytic activity. The interaction of TTR with metallothioneins may be a critical step in the exertion of some of these functions. Evolutionary and physiological insights on zinc-dependent functions of TTRs are also discussed.

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.

Zinc transporters in Alzheimer's disease

Alzheimer’s disease (AD) is the most devastating neurodegenerative disorder. Due to the increase in population and longevity, incidence will triple by the middle of the twenty-first century. So far, no treatment has prevented or reversed the disease. More than 20 years of multidisciplinary studies have shown that brain zinc dyshomeostasis may play a critical role in AD progression, which provides encouraging clues for metal-targeted therapies in the treatment of AD. Unfortunately, the pilot clinical application of zinc chelator and/or ionophore strategy, such as the use of quinoline-based compounds, namely clioquinol and PBT2, has not yet been successful. The emerging findings revealed a list of key zinc transporters whose mRNA or protein levels were abnormally altered at different stages of AD brains. Furthermore, specifically modulating the expression of some of the zinc transporters in the central nervous system through genetic methods slowed down or prevented AD progression in animal models, resulting in significantly improved cognitive performance, movement, and prolonged lifespan. Although the underlying molecular mechanisms are not yet fully understood, it shed new light on the treatment or prevention of the disease. This review considers recent advances regarding AD, zinc and zinc transporters, recapitulating their relationships in extending our current understanding of the disease amelioration effects of zinc transport proteins as potential therapeutic targets to cure AD, and it may also provide new insights to identify novel therapeutic strategies for ageing and other neurodegenerative diseases, such as Huntington’s and Parkinson’s disease.

CuII(atsm) Attenuates Neuroinflammation

Background: Neuroinflammation and biometal dyshomeostasis are key pathological features of several neurodegenerative diseases, including Alzheimer’s disease (AD). Inflammation and biometals are linked at the molecular level through regulation of metal buffering proteins such as the metallothioneins. Even though the molecular connections between metals and inflammation have been demonstrated, little information exists on the effect of copper modulation on brain inflammation.

Methods: We demonstrate the immunomodulatory potential of the copper bis(thiosemicarbazone) complex Cu^II(atsm) in an neuroinflammatory model in vivo and describe its anti-inflammatory effects on microglia and astrocytes in vitro.

Results: By using a sophisticated in vivo magnetic resonance imaging (MRI) approach, we report the efficacy of Cu^II(atsm) in reducing acute cerebrovascular inflammation caused by peripheral administration of bacterial lipopolysaccharide (LPS). Cu^II(atsm) also induced anti-inflammatory outcomes in primary microglia [significant reductions in nitric oxide (NO), monocyte chemoattractant protein 1 (MCP-1), and tumor necrosis factor (TNF)] and astrocytes [significantly reduced NO, MCP-1, and interleukin 6 (IL-6)] in vitro. These anti-inflammatory actions were associated with increased cellular copper levels and increased the neuroprotective protein metallothionein-1 (MT1) in microglia and astrocytes.

Conclusion: The beneficial effects of Cu^II(atsm) on the neuroimmune system suggest copper complexes are potential therapeutics for the treatment of neuroinflammatory conditions.

Application of Weighted Gene Co-Expression Network Analysis to Explore the Key Genes in Alzheimer's Disease

BACKGROUND

Weighted co-expression network analysis (WGCNA) is a powerful systems biology method to describe the correlation of gene expression based on the microarray database, which can be used to facilitate the discovery of therapeutic targets or candidate biomarkers in diseases.

OBJECTIVE

To explore the key genes in the development of Alzheimer's disease (AD) by using WGCNA.

METHODS

The whole gene expression data GSE1297 from AD and control human hippocampus was obtained from the GEO database in NCBI. Co-expressed genes were clustered into different modules. Modules of interest were identified through calculating the correlation coefficient between the module and phenotypic traits. GO and pathway enrichment analyses were conducted, and the central players (key hub genes) within the modules of interest were identified through network analysis. The expression of the identified key genes was confirmed in AD transgenic mice through using qRT-PCR.

RESULTS

Two modules were found to be associated with AD clinical severity, which functioning mainly in mineral absorption, NF-κB signaling, and cGMP-PKG signaling pathways. Through analysis of the two modules, we found that metallothionein (MT), Notch2, MSX1, ADD3, and RAB31 were highly correlated with AD phenotype. Increase in expression of these genes was confirmed in aged AD transgenic mice.

CONCLUSION

WGCNA analysis can be used to analyze and predict the key genes in AD. MT1, MT2, MSX1, NOTCH2, ADD3, and RAB31 are identified to be the most relevant genes, which may be potential targets for AD therapy.

Influence of Transgenic Metallothionein-1 on Gliosis, CA1 Neuronal Loss, and Brain Metal Levels of the Tg2576 Mouse Model of Alzheimer's Disease

The mouse model of Alzheimer’s disease (AD), Tg2576 mice (APP), has provided valuable information, such as the role of the metallothionein (MT) family in their behavioral and amyloidosis phenotypes. In this study, we further characterize the role of MT-1 by crossing Mt1-overexpressing mice with Tg2576 mice (APPTgMT). In 14-month-old mice, MT-1(/2) protein levels were dramatically increased by Mt1 overexpression throughout the cortex (Cx), which showed a prominent caudal-rostral gradient, and the hippocampus (HC). There was a trend for MT-1(/2) immunostaining to be increased in the areas surrounding the amyloid plaques in control male mice but not in Mt1-overexpressing mice. Gliosis was elicited by the amyloid plaques, but the effects of Mt1 overexpression were modest. However, in hippocampal western blots the microglial marker Iba-1 was increased in old male APPTgMT mice compared to APP-wild type (APPWT) mice, and the opposite was observed in young mice. Hippocampal CA1 neuronal loss was observed in Tg2576 mice, but was unaffected by Mt1 overexpression. Aging increased Zn and Cu levels differently depending on brain area, sex, and genotype. Thus, the effects of Mt1 overexpression on the phenotype of Tg2576 mice here studied are modest.

Expression of Metallothionein-I, -II, and -III in Alzheimer Disease and Animal Models of Neuroinflammation

In recent years it has become increasingly clear that the metallothionein (MT) family of proteins is important in neurobiology. MT-I and MT-II are normally dramatically up-regulated by neuroinflammation. Results for MT-III are less clear. MTs could also be relevant in human neuropathology. In Alzheimer disease (AD), a major neurodegenerative disease, clear signs of inflammation and oxidative stress were detected associated with amyloid plaques. Furthermore, the number of cells expressing apoptotic markers was also significantly increased in these plaques. As expected, MT-I and MT-II immunostaining was dramatically increased in cells surrounding the plaques, consistent with astrocytosis and microgliosis, as well as the increased oxidative stress elicited by the amyloid deposits. MT-III, In contrast, remained essentially unaltered, which agrees with some but not all studies, of AD. In situ hybridization results in a transgenic mouse model of AD amyloid deposits, the Tg2576 mouse, which expresses human Aβ precursor protein harboring the Swedish K670N/M671L mutations, are in accordance with results in human brains. Overall, these and other studies strongly suggest specific roles for MT-I, MT-II, and MT-III in brain physiology.

Multi-Target Directed Donepezil-Like Ligands for Alzheimer's Disease

HIGHLIGHTS ASS234 is a MTDL compound containing a moiety from Donepezil and the propargyl group from the PF 9601N, a potent and selective MAO B inhibitor. This compound is the most advanced anti-Alzheimer agent for preclinical studies identified in our laboratory.Derived from ASS234 both multipotent donepezil-indolyl (MTDL-1) and donepezil-pyridyl hybrids (MTDL-2) were designed and evaluated as inhibitors of AChE/BuChE and both MAO isoforms. MTDL-2 showed more high affinity toward the four enzymes than MTDL-1.MTDL-3 and MTDL-4, were designed containing the N-benzylpiperidinium moiety from Donepezil, a metal- chelating 8-hydroxyquinoline group and linked to a N-propargyl core and they were pharmacologically evaluated.The presence of the cyano group in MTDL-3, enhanced binding to AChE, BuChE and MAO A. It showed antioxidant behavior and it was able to strongly complex Cu(II), Zn(II) and Fe(III).MTDL-4 showed higher affinity toward AChE, BuChE.MTDL-3 exhibited good brain penetration capacity (ADMET) and less toxicity than Donepezil. Memory deficits in scopolamine-lesioned animals were restored by MTDL-3.MTDL-3 particularly emerged as a ligand showing remarkable potential benefits for its use in AD therapy. Alzheimer's disease (AD), the most common form of adult onset dementia, is an age-related neurodegenerative disorder characterized by progressive memory loss, decline in language skills, and other cognitive impairments. Although its etiology is not completely known, several factors including deficits of acetylcholine, β-amyloid deposits, τ-protein phosphorylation, oxidative stress, and neuroinflammation are considered to play significant roles in the pathophysiology of this disease. For a long time, AD patients have been treated with acetylcholinesterase inhibitors such as donepezil (Aricept®) but with limited therapeutic success. This might be due to the complex multifactorial nature of AD, a fact that has prompted the design of new Multi-Target-Directed Ligands (MTDL) based on the "one molecule, multiple targets" paradigm. Thus, in this context, different series of novel multifunctional molecules with antioxidant, anti-amyloid, anti-inflammatory, and metal-chelating properties able to interact with multiple enzymes of therapeutic interest in AD pathology including acetylcholinesterase, butyrylcholinesterase, and monoamine oxidases A and B have been designed and assessed biologically. This review describes the multiple targets, the design rationale and an in-house MTDL library, bearing the N-benzylpiperidine motif present in donepezil, linked to different heterocyclic ring systems (indole, pyridine, or 8-hydroxyquinoline) with special emphasis on compound ASS234, an N-propargylindole derivative. The description of the in vitro biological properties of the compounds and discussion of the corresponding structure-activity-relationships allows us to highlight new issues for the identification of more efficient MTDL for use in AD therapy.

Metallostasis in Alzheimer's disease

2012 has been another year in which multiple large-scale clinical trials for Alzheimer's disease (AD) have failed to meet their clinical endpoints. With the social and financial burden of this disease increasing every year, the onus is now on the field of AD researchers to investigate alternative ideas to deliver outcomes for patients. Although several major clinical trials targeting Aβ have failed, three smaller clinical trials targeting metal interactions with Aβ have all shown benefit for patients. Here we review the genetic, pathological, biochemical, and pharmacological evidence that underlies the metal hypothesis of AD. The AD-affected brain suffers from metallostasis, or fatigue of metal trafficking, resulting in redistribution of metals into inappropriate compartments. The metal hypothesis is built upon a triad of transition elements: iron, copper, and zinc. The hypothesis has matured from early investigations showing amyloidogenic and oxidative stress consequences of these metals; recently, disease-related proteins, APP, tau, and presenilin, have been shown to have major roles in metal regulation, which provides insight into the pathway of neurodegeneration in AD and illuminates potential new therapeutic avenues.

Zinc homeostasis and neurodegenerative disorders

Zinc is an essential trace element, whose importance to the function of the central nervous system (CNS) is increasingly being appreciated. Alterations in zinc dyshomeostasis has been suggested as a key factor in the development of several neuropsychiatric disorders. In the CNS, zinc occurs in two forms: the first being tightly bound to proteins and, secondly, the free, cytoplasmic, or extracellular form found in presynaptic vesicles. Under normal conditions, zinc released from the synaptic vesicles modulates both ionotropic and metabotropic post-synaptic receptors. While under clinical conditions such as traumatic brain injury, stroke or epilepsy, the excess influx of zinc into neurons has been found to result in neurotoxicity and damage to postsynaptic neurons. On the other hand, a growing body of evidence suggests that a deficiency, rather than an excess, of zinc leads to an increased risk for the development of neurological disorders. Indeed, zinc deficiency has been shown to affect neurogenesis and increase neuronal apoptosis, which can lead to learning and memory deficits. Altered zinc homeostasis is also suggested as a risk factor for depression, Alzheimer's disease (AD), aging, and other neurodegenerative disorders. Under normal CNS physiology, homeostatic controls are put in place to avoid the accumulation of excess zinc or its deficiency. This cellular zinc homeostasis results from the actions of a coordinated regulation effected by different proteins involved in the uptake, excretion and intracellular storage/trafficking of zinc. These proteins include membranous transporters (ZnT and Zip) and metallothioneins (MT) which control intracellular zinc levels. Interestingly, alterations in ZnT and MT have been recently reported in both aging and AD. This paper provides an overview of both clinical and experimental evidence that implicates a dysfunction in zinc homeostasis in the pathophysiology of depression, AD, and aging.

Genes and pathways underlying regional and cell type changes in Alzheimer's disease

Background

Transcriptional studies suggest Alzheimer's disease (AD) involves dysfunction of many cellular pathways, including synaptic transmission, cytoskeletal dynamics, energetics, and apoptosis. Despite known progression of AD pathologies, it is unclear how such striking regional vulnerability occurs, or which genes play causative roles in disease progression.

Methods

To address these issues, we performed a large-scale transcriptional analysis in the CA1 and relatively less vulnerable CA3 brain regions of individuals with advanced AD and nondemented controls. In our study, we assessed differential gene expression across region and disease status, compared our results to previous studies of similar design, and performed an unbiased co-expression analysis using weighted gene co-expression network analysis (WGCNA). Several disease genes were identified and validated using qRT-PCR.

Results

We find disease signatures consistent with several previous microarray studies, then extend these results to show a relationship between disease status and brain region. Specifically, genes showing decreased expression with AD progression tend to show enrichment in CA3 (and vice versa), suggesting transcription levels may reflect a region's vulnerability to disease. Additionally, we find several candidate vulnerability (ABCA1, MT1H, PDK4, RHOBTB3) and protection (FAM13A1, LINGO2, UNC13C) genes based on expression patterns. Finally, we use a systems-biology approach based on WGCNA to uncover disease-relevant expression patterns for major cell types, including pathways consistent with a key role for early microglial activation in AD.

Conclusions

These results paint a picture of AD as a multifaceted disease involving slight transcriptional changes in many genes between regions, coupled with a systemic immune response, gliosis, and neurodegeneration. Despite this complexity, we find that a consistent picture of gene expression in AD is emerging.

Metallothioneins and brain injury: What transgenic mice tell us

In rodents, the metallothionein (MT) family is composed of four members, MT-1 to MT-4. MT-1&2 are expressed in virtually all tissues including those of the Central Nervous System (CNS), while MT-3 (also called Growth Inhibitory Factor) and MT-4 are expressed prominently in the brain and in keratinizing epithelia, respectively. For the understanding of the physiological functions of these proteins in the brain, the use of transgenic mice has provided essential information. Results obtained inMT-1&2-null mice and in MT-1-overexpressing mice strongly suggeset that these MT isoforms are important antioxidant, anti-inflammatory and antiapoptotic proteins in the brain. Results inMT-3-null mice show a very different pattern, with no support for MT-1&2-like functions. Rather, MT-3 could be involved in neuronal sprouting and survival. Results obtained in a model of peripheral nervous system injury also suggest that MT-3 could be involved in the control of nerve growth.

Metallothionein and brain inflammation

Since the seminal discoveries of Bert Vallee regarding zinc and metallothioneins (MTs) more than 50 years ago, thousands of studies have been published concerning this fascinating story. One of the most active areas of research is the involvement of these proteins in the inflammatory response in general, and in neuroinflammation in particular. We describe the general aspects of the inflammatory response, highlighting the essential role of the major cytokine interleukin-6, and review briefly the expression and function of MTs in the central nervous system in the context of neuroinflammation. Particular attention is paid to the Tg2576 Alzheimer disease mouse model and the preliminary results obtained in mice into which human Zn(7)MT-2A was injected, which suggest a reversal of the behavioral deficits while enhancing amyloid plaque load and gliosis.

Chemistry and biology of mammalian metallothioneins

Metallothioneins (MTs) are a class of ubiquitously occurring low molecular mass, cysteine- and metal-rich proteins containing sulfur-based metal clusters formed with Zn(II), Cd(II), and Cu(I) ions. In mammals, four distinct MT isoforms designated MT-1 through MT-4 exist. The first discovered MT-1/MT-2 are widely expressed isoforms, whose biosynthesis is inducible by a wide range of stimuli, including metals, drugs, and inflammatory mediators. In contrast, MT-3 and MT-4 are noninducible proteins, with their expression primarily confined to the central nervous system and certain squamous epithelia, respectively. MT-1 through MT-3 have been reported to be secreted, suggesting that they may play different biological roles in the intracellular and extracellular space. Recent reports established that these isoforms play an important protective role in brain injury and metal-linked neurodegenerative diseases. In the postgenomic era, it is becoming increasingly clear that MTs fulfill multiple functions, including the involvement in zinc and copper homeostasis, protection against heavy metal toxicity, and oxidative damage. All mammalian MTs are monomeric proteins, containing two metal-thiolate clusters. In this review, after a brief summary of the historical milestones of the MT-1/MT-2 research, the recent advances in the structure, chemistry, and biological function of MT-3 and MT-4 are discussed.

The native copper- and zinc-binding protein metallothionein blocks copper-mediated Abeta aggregation and toxicity in rat cortical neurons

Background

A major pathological hallmark of AD is the deposition of insoluble extracellular β-amyloid (Aβ) plaques. There are compelling data suggesting that Aβ aggregation is catalysed by reaction with the metals zinc and copper.

Methodology/Principal Findings

We now report that the major human-expressed metallothionein (MT) subtype, MT-2A, is capable of preventing the in vitro copper-mediated aggregation of Aβ_1–40 and Aβ_1–42. This action of MT-2A appears to involve a metal-swap between Zn₇MT-2A and Cu(II)-Aβ, since neither Cu₁₀MT-2A or carboxymethylated MT-2A blocked Cu(II)-Aβ aggregation. Furthermore, Zn₇MT-2A blocked Cu(II)-Aβ induced changes in ionic homeostasis and subsequent neurotoxicity of cultured cortical neurons.

Conclusions/Significance

These results indicate that MTs of the type represented by MT-2A are capable of protecting against Aβ aggregation and toxicity. Given the recent interest in metal-chelation therapies for AD that remove metal from Aβ leaving a metal-free Aβ that can readily bind metals again, we believe that MT-2A might represent a different therapeutic approach as the metal exchange between MT and Aβ leaves the Aβ in a Zn-bound, relatively inert form.

Human metallothioneins 2 and 3 differentially affect amyloid-beta binding by transthyretin

Transthyretin (TTR), an amyloid-beta (Abeta) scavenger protein, and metallothioneins 2 and 3 (MT2 and MT3), low molecular weight metal-binding proteins, have recognized impacts in Abeta metabolism. Because TTR binds MT2, an ubiquitous isoform of the MTs, we investigated whether it also interacts with MT3, an isoform of the MTs predominantly expressed in the brain, and studied the role of MT2 and MT3 in human TTR-Abeta binding. The TTR-MT3 interaction was characterized by yeast two-hybrid assays, saturation-binding assays, co-immunolocalization and co-immunoprecipitation. The effect of MT2 and MT3 on TTR-Abeta binding was assessed by competition-binding assays. The results obtained clearly demonstrate that TTR interacts with MT3 with a K(d) of 373.7 +/- 60.2 nm. Competition-binding assays demonstrated that MT2 diminishes TTR-Abeta binding, whereas MT3 has the opposite effect. In addition to identifying a novel ligand for TTR that improves human TTR-Abeta binding, the present study highlights the need to clarify whether the effects of MT2 and MT3 in human TTR-Abeta binding observed in vitro have a relevant impact on Abeta deposition in animal models of Alzheimer's disease.

The link between iron, metabolic syndrome, and Alzheimer's disease

Both Alzheimer's disease (AD), the most common form of dementia, and type-2 diabetes mellitus (T2DM), a disease associated with metabolic syndrome (MetS), affect a great number of the world population and both have increased prevalence with age. Recently, many studies demonstrated that pre-diabetes, MetS, and T2DM are risk factors in the development of AD and have many common mechanisms. The main focus of studies is the insulin resistance outcome found both in MetS as well as in brains of AD subjects. However, oxidative stress (OS)-related mechanisms, which are well known to be involved in AD, including mitochondrial dysfunction, elevated iron concentration, reactive oxygen species (ROS), and stress-related enzyme or proteins (e.g. heme oxygenase-1, transferrin, etc.), have not been elucidated in MetS or T2DM brains although OS and iron are involved in the degeneration of the pancreatic islet β cells. Therefore, this review sets to cover the current literature regarding OS and iron in MetS and T2DM and the similarities to mechanisms in AD both in human subjects as well as in animal models.

Metallothioneins and copper metabolism are candidate therapeutic targets in Huntington's disease

HD (Huntington's disease) is caused by a polyQ (polyglutamine) expansion in the huntingtin protein, which leads to protein misfolding and aggregation of this protein. Abnormal copper accumulation in the HD brain was first reported more than 15 years ago. Recent findings show that copper-regulatory genes are induced during HD and copper binds to an N-terminal fragment of huntingtin, supporting the involvement of abnormal copper metabolism in HD. We have demonstrated that in vitro copper accelerates the fibrillization of an N-terminal fragment of huntingtin with an expanded polyQ stretch (httExon1). As we found that copper also increases polyQ aggregation and toxicity in mammalian cells expressing httExon1, we investigated further whether overexpression of genes involved in copper metabolism, notably MTs (metallothioneins) known to bind copper, protect against httExon1 toxicity. Using a yeast model of HD, we have shown that overexpression of several genes involved in copper metabolism reduces polyQ-mediated toxicity. Overexpression of MT-3 in mammalian cells significantly reduced polyQ aggregation and toxicity. We propose that copper-binding and/or -chaperoning proteins, especially MTs, are potential therapeutic targets for HD.

Increased oxidative stress and astrogliosis responses in conditional double-knockout mice of Alzheimer-like presenilin-1 and presenilin-2

Conditional presenilin 1 and presenilin 2 double knockout causes memory dysfunction and reproduces neurodegenerative phenotypes of Alzheimer disease (AD) in mice. Oxidative stress has been long implicated predominantly in amyloidosis-mediated AD pathologies; however, its role in response to the loss-of-function pathogenic mechanism of AD remains unclear. In this study, we examined the oxidative stress status in PS1 and PS2 double-knockout (PS cDKO) mice using F(2)-isoprostanes (iPF(2alpha)-III) as the marker of lipid peroxidation. Lipid peroxidation was enhanced in a gender- and age-related manner in the PS cDKO mice independent of brain Abeta deposition. Such oxidative abnormalities predominantly in cerebral cortex at 2-4 months of age preceded the onset of many pronounced AD neuropathologies, suggesting that increased lipid peroxidation is not only an early pathophysiological response to PS inactivation, but also a potential culprit responsible for the AD-like neurodegenerative pathologies in the PS cDKO mice. Western blot analysis of cortical glial fibrillary acidic protein demonstrated an increased astrogliosis response to PS inactivation, in particular in the PS cDKO mice at as young as 2 months of age, suggesting that lipid peroxidation and neuronal injury may be closely associated with the loss-of-function neuropathogenic mechanism of AD.

Metallothionein in the central nervous system: Roles in protection, regeneration and cognition

Metallothionein (MT) is an enigmatic protein, and its physiological role remains a matter of intense study and debate 50 years after its discovery. This is particularly true of its function in the central nervous system (CNS), where the challenge remains to link its known biochemical properties of metal binding and free radical scavenging to the intricate workings of brain. In this compilation of four reports, first delivered at the 11th International Neurotoxicology Association (INA-11) Meeting, June 2007, the authors present the work of their laboratories, each of which gives an important insight into the actions of MT in the brain. What emerges is that MT has the potential to contribute to a variety of processes, including neuroprotection, regeneration, and even cognitive functions. In this article, the properties and CNS expression of MT are briefly reviewed before Dr Hidalgo describes his pioneering work using transgenic models of MT expression to demonstrate how this protein plays a major role in the defence of the CNS against neurodegenerative disorders and other CNS injuries. His group's work leads to two further questions, what are the mechanisms at the cellular level by which MT acts, and does this protein influence higher order issues of architecture and cognition? These topics are addressed in the second and third sections of this review by Dr West, and Dr Levin and Dr Eddins, respectively. Finally, Dr Aschner examines the ability of MT to protect against a specific toxicant, methylmercury, in the CNS.

Metallothionein-I and -III expression in animal models of Alzheimer disease

Previous studies have described altered expression of metallothioneins (MTs) in neurodegenerative diseases like multiple sclerosis (MS), Down syndrome, and Alzheimer's disease (AD). In order to gain insight into the possible role of MTs in neurodegenerative processes and especially in human diseases, the use of animal models is a valuable tool. Several transgenic mouse models of AD amyloid deposits are currently available. These models express human beta-amyloid precursor protein (AbetaPP) carrying different mutations that subsequently result in a varied pattern of beta-amyloid (Abeta) deposition within the brain. We have evaluated the expression of MT-I and MT-III mRNA by in situ hybridization in three different transgenic mice models of AD: Tg2576 (carrying AbetaPP harboring the Swedish K670N/M671L mutations), TgCRND8 (Swedish and the Indiana V717F mutations), and Tg-SwDI (Swedish and Dutch/Iowa E693Q/D694N mutations). MT-I mRNA levels were induced in all transgenic lines studied, although the pattern of induction differed between the models. In the Tg2576 mice MT-I was weakly upregulated in cells surrounding Congo Red-positive plaques in the cortex and hippocampus. A more potent induction of MT-I was observed in the cortex and hippocampus of the TgCRND8 mice, likely reflecting their higher amyloid plaques content. MT-I upregulation was also more significant in Tg-SwDI mice, especially in the subiculum and hippocampus CA1 area. Immunofluorescence stainings demonstrate that astrocytes and microglia/macrophages surrounding the plaques express MT-I&II. In general, MT-I regulation follows a similar but less potent response than glial fibrillary acidic protein (GFAP) expression. In contrast to MT-I, MT-III mRNA expression was not significantly altered in any of the models examined suggesting that the various MT isoforms may have different roles in these experimental systems, and perhaps also in human AD.

The role of methallothioneins in experimental autoimmune encephalomyelitis and multiple sclerosis

Multiple sclerosis is a chronic inflammatory and demyelinating disease of the central nervous system (CNS) in which oxidative stress plays a pathogenic role. Metallothioneins are antioxidant proteins induced in the CNS under conditions where oxidative stress has taken place, such as tissue injury, stress, and some neurodegenerative diseases, which have been postulated to play a neuroprotective role. In this review we summarize recent progress in understanding the regulation and function of methallothioneins during experimental autoimmune encephalomyelitis and multiple sclerosis.

Neuronal apoptosis, metallothionein expression and proinflammatory responses during cerebral malaria in mice

BACKGROUND

Cerebral malaria (CM) is an acute encephalopathy in humans due to the infection with Plasmodium falciparum. Neuro-cognitive impairment following CM occurs in about 10% of the treated survivors, while the precise pathophysiological mechanism remains unknown. Metallothionein I + II (MT-I + II) are increased during CNS pathology and disorders. As previously shown, MT-I + II are neuroprotective through anti-inflammatory, antioxidant and antiapoptotic functions. We have analyzed neuronal apoptosis and MT-I + II expression in brains of mice with experimental CM.

METHODS

C57BL/6j mice, infected with Plasmodium berghei ANKA, were studied on day 7, day 9, and when presenting signs of CM on days 10-12. We investigated brain histopathology by immunohistochemistry and TUNEL (Terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-digoxigenin nick end labeling). For statistics, we used quantitation (cellular counts) of the analyzed variables.

RESULTS

During CM, we observed significant inflammatory responses of F4/80+ microglia/macrophages and GFAP+ reactive astrocytes and increased immunoreactivity of 8-oxoguanine (marker of oxidative stress). As novel findings, we show: (1) a localized CM-induced neuronal apoptosis (detected by TUNEL) indicating severe and irreversible pathology. (2) A significant increase in MT-I + II expression in reactive astrocytes, macrophages/microglia and vascular endothelium.

INTERPRETATION

This is the first report showing apoptosis of neurons in CM by TUNEL, pointing out a possible pathophysiological mechanism leading to persisting brain damage. The possible neuroprotective role of MT-I + II during CM deserves further attention.

Proteasome inhibition by lactacystin in primary neuronal cells induces both potentially neuroprotective and pro-apoptotic transcriptional responses: a microarray analysis

Although inhibition of the ubiquitin proteasome system has been postulated to play a key role in the pathogenesis of neurodegenerative diseases, studies have also shown that proteasome inhibition can induce increased expression of neuroprotective heat-shock proteins (HSPs). The global gene expression of primary neurons in response to treatment with the proteasome inhibitor lactacystin was studied to identify the widest range of possible pathways affected. Our results showed changes in mRNA abundance, both at different time points after lactacystin treatment and at different lactacystin concentrations. Genes that were differentially up-regulated at the early time point but not when most cells were undergoing apoptosis might be involved in an attempt to reverse proteasome inhibitor-mediated apoptosis and include HSP70, HSP22 and cell cycle inhibitors. The up-regulation of HSP70 and HSP22 appeared specific towards proteasome inhibitor-mediated cell death. Overexpression of HSP22 was found to protect against proteasome inhibitor-mediated loss of viability by up to 25%. Genes involved in oxidative stress and the inflammatory response were also up-regulated. These data suggest an initial neuroprotective pathway involving HSPs, antioxidants and cell cycle inhibitors, followed by a pro-apoptotic response possibly mediated by inflammation, oxidative stress and aberrant activation of cell cycle proteins.

Metallothionein biology in the ageing and neurodegenerative brain

In recent years metallothionein (MT) biology has moved from investigation of its ability to protect against environmental heavy metals to a wider appreciation of its role in responding to cellular stress, whether as a consequence of normal function, or following injury and disease. This is exemplified by recent investigation of MT in the mammalian brain where plausible roles for MT action have been described, including zinc metabolism, free radical scavenging, and protection and regeneration following neurological injury. Along with other laboratories we have used several models of central nervous system (CNS) injury to investigate possible parallels between injury-dependent changes in MT expression and those observed in the ageing and/or degenerating brain. Therefore, this brief review aims to summarise existing information on MT expression during CNS ageing, and to examine the possible involvement of this protein in the course of human neurodegenerative disease, as exemplified by Alzheimer's disease.

A role for extracellular metallothioneins in CNS injury and repair

For many years, research focus on metallothioneins, small zinc binding proteins found predominantly within astrocytes in the brain, has centred on their ability to indirectly protect neurons from oxygen free radicals and heavy metal-induced neurotoxicity. However, in recent years it has been demonstrated that these proteins have previously unsuspected roles within the cellular response to brain injury. The aim of this commentary is to provide an overview of the exciting recent experimental evidence from several laboratories including our own suggesting a possible extracellular role for these proteins, and to present a hypothetical model explaining the newly identified function of extracellular metallothioneins in CNS injury and repair.

Metallothionein-IIA promotes initial neurite elongation and postinjury reactive neurite growth and facilitates healing after focal cortical brain injury

Metallothioneins (MTs) are small, cysteine-rich, metal binding proteins. Their function has often been considered as stress-related proteins capable of protecting cells from heavy metal toxicity and oxidative free radicals. However, recent interest has focused on the brain-specific MT-III isoform, which has neurite-inhibitory properties. To investigate the effect of another MT isoform, human MT-IIA, on neurite growth, we used rat cortical neuron cultures. MT-IIA promoted a significant increase in the rate of initial neurite elongation of individually plated neurons. We also investigated the effect of MT-IIA on the neuronal response to axonal transection in vitro. MT-IIA promoted reactive axonal growth after injury, and, by 18 hr after transection, MT-IIA had promoted axonal growth across the injury tract. Exogenous application of MT-IIA after cortical brain injury promoted wound healing, as observed by a significant decrease in cellular degradation at 4 d after injury. Furthermore, MT-IIA-treated rats exhibited numerous SMI-312-immunoreactive axonal processes within the injury tract. This was in contrast to vehicle-treated animals, in which few axonal sprouts were observed. By 7 d after injury, MT-IIA treatment resulted in a total closing over of the injury tract by microglia, astrocytes, and reactive axonal processes. However, although some reactive axonal processes were observed within the injury tract of vehicle-treated rats, the tract itself was almost never entirely enclosed. These results are discussed in relation to a possible physiological role of metallothioneins in the brain, as well as in a therapeutic context.

Beta-amyloid peptides decrease soluble guanylyl cyclase expression in astroglial cells

In astroglial cells beta-amyloid peptides (betaA) induce a reactive phenotype and increase expression of NO synthase. Here we show that treatment of rat brain astrocytes with betaA decreases their capacity to accumulate cyclic GMP (cGMP) in response to NO as a result of a decreased expression of soluble guanylyl cyclase (sGC) at the protein and mRNA levels. Potentiation of betaA-induced NO formation by interferon-gamma did not result in a larger decrease in cGMP formation and inhibition of NO synthase failed to reverse down-regulation of sGC, indicating that NO is not involved. The betaA effect was prevented by the protein synthesis inhibitor cycloheximide. Intracerebral betaA injection also decreased sGC beta1 subunit mRNA levels in adult rat hippocampus and cerebellum. A loss of sGC in reactive astrocytes surrounding beta-amyloid plaques could be a mechanism to prevent excess signalling via cGMP at sites of high NO production.

Metallothionein-1+2 protect the CNS after a focal brain injury

We have evaluated the physiological relevance of metallothionein-1+2 (MT-1+2) in the CNS following damage caused by a focal cryolesion onto the cortex. In comparison to normal mice, transgenic mice overexpressing the MT-1 isoform (TgMTI* mice) showed a significant decrease of the number of activated microglia/macrophage and of CD3+ T lymphocytes in the area surrounding the lesion, while astrocytosis was increased. The TgMTI* mice showed a diminished peripheral macrophage but not CD3 T cell response to the cryolesion. This altered inflammatory response produced a decreased expression of the proinflammatory cytokines IL-1beta, IL-6, and TNF-alpha and an increased expression of the growth factors bFGF, TGFbeta1, and VEGF in the TgMTI* mice relative to control mice, which might be related to the increased angiogenesis and regeneration of the parenchyma of the former mice. The overexpression of MT-1 dramatically reduced the cryolesion-induced oxidative stress and neuronal apoptosis. Remarkably, these effects were also obtained by the intraperitoneal administration of MT-2 to both normal and MT-1+2 knock-out mice. These results fully support the notion that MT-1+2 are essential in the CNS for coping with focal brain injury and suggest a potential therapeutic use of these proteins.

A gene expression profile of Alzheimer's disease

Postmortem analysis of brains of patients with Alzheimer's disease (AD) has led to diverse theories about the causes of the pathology, suggesting that this complex disease involves multiple physiological changes. In an effort to better understand the variety and integration of these changes, we generated a gene expression profile for AD brain. Comparing affected and unaffected brain regions in nine controls and six AD cases, we showed that 118 of the 7050 sequences on a broadly representative cDNA microarray were differentially expressed in the amygdala and cingulate cortex, two regions affected early in the disease. The identity of these genes suggests the most prominent upregulated physiological correlates of pathology involve chronic inflammation, cell adhesion, cell proliferation, and protein synthesis (31 upregulated genes). Conversely, downregulated correlates of pathology involve signal transduction, energy metabolism, stress response, synaptic vesicle synthesis and function, calcium binding, and cytoskeleton (87 downregulated genes). The results support several separate theories of the causes of AD pathology, as well as add to the list of genes associated with AD. In addition, approximately 10 genes of unknown function were found to correlate with the pathology.

Roles of the metallothionein family of proteins in the central nervous system

Metallothioneins (MTs) constitute a family of proteins characterized by a high heavy metal [Zn(II), Cu(I)] content and also by an unusual cysteine abundance. Mammalian MTs are comprised of four major isoforms designated MT-1 trough MT-4. MT-1 and MT-2 are expressed in most tissues including the brain, whereas MT-3 (also called growth inhibitory factor) and MT-4 are expressed predominantly in the central nervous system and in keratinizing epithelia, respectively. All MT isoforms have been implicated in disparate physiological functions, such as zinc and copper metabolism, protection against reactive oxygen species, or adaptation to stress. In the case of MT-3, an additional involvement of this isoform in neuromodulatory events and in the pathogenesis of Alzheimer's disease has also been suggested. It is essential to gain insight into how MTs are regulated in the brain in order to characterize MT functions, both in normal brain physiology, as well as in pathophysiological states. The focus of this review concerns the biology of the MT family in the context of their expression and functional roles in the central nervous system.

Homocysteine potentiates copper- and amyloid beta peptide-mediated toxicity in primary neuronal cultures: possible risk factors in the Alzheimer's-type neurodegenerative pathways

Oxidative stress may have an important role in the progression of neurodegenerative disorders such as Alzheimer's disease (AD) and prion diseases. Oxidative damage could result from interactions between highly reactive transition metals such as copper (Cu) and endogenous reducing and/or oxidizing molecules in the brain. One such molecule, homocysteine, a thiol-containing amino acid, has previously been shown to modulate Cu toxicity in HeLa and endothelial cells in vitro. Due to a possible link between hyperhomocysteinemia and AD, we examined whether interaction between homocysteine and Cu could potentiate Cu neurotoxicity. Primary mouse neuronal cultures were treated with homocysteine and either Cu (II), Fe (II or III) or Zn (II). Homocysteine was shown to selectively potentiate toxicity from low micromolar concentrations of Cu. The toxicity of homocysteine/Cu coincubation was dependent on the ability of homocysteine to reduce Cu (II) as reflected by the inhibition of toxicity with the Cu (I)-specific chelator, bathocuproine disulphonate. This was supported by data showing that homocysteine reduced Cu (II) more effectively than cysteine or methionine but did not reduce Fe (III) to Fe (II). Homocysteine also generated high levels of hydrogen peroxide in the presence of Cu (II) and promoted Abeta/Cu-mediated hydrogen peroxide production and neurotoxicity. The potentiation of metal toxicity did not involve excitotoxicity as ionotropic glutamate receptor antagonists had no effect on neurotoxicity. Homocysteine alone also had no effect on neuronal glutathione levels. These studies suggest that increased copper and/or homocysteine levels in the elderly could promote significant oxidant damage to neurons and may represent additional risk factor pathways which conspire to produce AD or related neurodegenerative conditions.

Metallothionein in olfactory mucosa of Alzheimer's disease patients and apoE-deficient mice

Immunostaining for metallothionein was performed on olfactory mucosa of the superior part of the nasal septum obtained from Alzheimer's disease (AD) patients and normal age-matched controls. In general, specimens from AD patients showed increased MT immunoreactivity as indicated by more frequent and intense staining in Bowman's glands, olfactory epithelium and the underlying lamina propria. Induction of MT may result from increased levels of reactive oxygen species commonly associated with AD. Sections of the entire nasal cavity and olfactory bulb of apoE-deficient and wild-type mice revealed no difference in the pattern of staining, suggesting that MT expression is not linked to the apoE gene.