A luminescence probe for metallothionein in liver tissue: emission intensity measured directly from copper metallothionein induced in rat liver

Unravelling the mechanistic details of metal binding to mammalian metallothioneins from stoichiometric, kinetic, and binding affinity data

Metallothioneins (MTs) are small, cysteine-rich proteins, found throughout Nature. Their ability to bind a number of different metals with a range of stoichiometric ratios means that this protein family is critically important for essential metal (Zn²⁺ and Cu⁺) homeostasis, metal storage, metal donation to nascent metalloenzymes as well as heavy metal detoxification. With its 20 cysteines, metallothionein is also considered to protect cells against oxidative stress. MT has been studied by a large number of researchers over the last 6 decades using a variety of spectroscopic techniques. The lack of distinguishing chromophores for the multitude of binding sites has made the evaluation of stoichiometric properties for different metals challenging. Initially, only ¹¹³Cd-NMR spectroscopy could provide strong evidence for the proposed cluster formation of Cd-MT. The extraordinary development of electrospray ionization mass spectrometry (ESI-MS), where all coexisting species in solution are observed, revolutionized MT research. Prior to the use of ESI-MS data, a range of "magic numbers" representing metal-to-MT molar ratios were reported from optical spectroscopic studies. The availability of ESI mass spectral data led to (i) the confirmation of cluster formation, (ii) a conceptual understanding of the cooperativity involved in multiple metal binding events, (iii) the presence of domain specificity between regions of the protein and (iv) mechanistic details involving both binding affinities and rate constants. The kinetic experiments identified the presence of multiple individual binding sites, each with a unique rate constant and an analogous binding affinity. The almost linear trend in rate constants as a function of bound As³⁺ provided a unique insight that became a critical step in the complete understanding of the mechanistic details of the metalation of MT. To fully define the biological function of this sulfur-rich protein it is necessary to determine kinetic rate constants and binding affinities for the essential metals. Recently, Zn²⁺ competition experiments between both of the isolated fragments (α and β) and the full-length protein (βα-MT 1a) as well as Zn²⁺ competition between βα-MT 1a and carbonic anhydrase were reported. From these data, the trend in binding affinities and the values of the K_f of the 7 bimolecular reactions involved in metalation were determined. From the analysis of ESI-MS data for Cu⁺ binding to βα-MT 1a at different pH-values, a trend in the 20 binding affinities for the complete metalation mechanism was reported. This review details a personal view of the historical development of the determination of stoichiometry for metal binding, the structure of the binding sites, the rates of the metalation reactions and the underlying binding affinities for each metalation step. We have attempted to summarize the experimental developments that led to the publication in May 2017 of the experimental determination of the 20 binding constants for the 20 sequential bimolecular reactions for Cu⁺ binding to the 20 Cys of apoMT as a function of pH that show the appearance and disappearance of clusters. We report both published data and in a series of tables an assembly of stoichiometries, and equilibrium constants for Zn²⁺ and Cu⁺ for many different metallothioneins.

On the nature of the Cu-rich aggregates in brain astrocytes

Fulfilling a bevy of biological roles, copper is an essential metal for healthy brain function. Cu dyshomeostasis has been demonstrated to be involved in some neurological conditions including Menkes and Alzheimer's diseases. We have previously reported localized Cu-rich aggregates in astrocytes of the subventricular zone (SVZ) in rodent brains with Cu concentrations in the hundreds of millimolar. Metallothionein, a cysteine-rich protein critical to metal homeostasis and known to participate in a variety of neuroprotective and neuroregenerative processes, was proposed as a binding protein. Here, we present an analysis of metallothionein(1,2) knockout (MTKO) mice and age-matched controls using X-ray fluorescence microscopy. In large structures such as the corpus callosum, cortex, and striatum, there is no significant difference in Cu, Fe, or Zn concentrations in MTKO mice compared to age-matched controls. In the astrocyte-rich subventricular zone where Cu-rich aggregates reside, approximately 1/3 as many Cu-rich aggregates persist in MTKO mice resulting in a decrease in periventricular Cu concentration. Aggregates in both wild-type and MTKO mice show XANES spectra characteristic of CuxSy multimetallic clusters and have similar [S]/[Cu] ratios. Consistent with assignment as a CuxSy multimetallic cluster, the astrocyte-rich SVZ of both MTKO and wild-type mice exhibit autofluorescent bodies, though MTKO mice exhibit fewer. Furthermore, XRF imaging of Au-labeled lysosomes and ubiquitin demonstrates a lack of co-localization with Cu-rich aggregates suggesting they are not involved in a degradation pathway. Overall, these data suggest that Cu in aggregates is bound by either metallothionein-3 or a yet unknown protein similar to metallothionein.

Challenging conventional wisdom: single domain metallothioneins

Metallation studies of human metallothioneins support the role of single metal-binding-domains as commonplace with the typical two-domain-cluster structure as exceptional.

Dysregulation of intracellular copper homeostasis is common to transgenic mice expressing human mutant superoxide dismutase-1s regardless of their copper-binding abilities

Over 170 mutations in superoxide dismutase-1 (SOD1) have been linked to amyotrophic lateral sclerosis (ALS). The properties of SOD1 mutants differ considerably including copper-binding abilities. Nevertheless, they cause the same disease phenotype, suggesting a common neurotoxic pathway. We have previously reported that copper homeostasis is disturbed in spinal cords of SOD1(G93A) mice. However, it is unknown whether copper dyshomeostasis is induced by other SOD1 mutants. Using the additional mouse strains SOD1(G127insTGGG), SOD1(G85R), and SOD1(D90A), which express SOD1 mutants with different copper-binding abilities, we show that copper dyshomeostasis is common to SOD1 mutants. The SOD1 mutants shifted the copper trafficking systems toward copper accumulation in spinal cords of the mice. Copper contents bound to the SOD1 active site varied considerably between SOD1 mutants. Still, copper bound to other ligands in the spinal cord were markedly increased in all. Zinc was also increased, whereas there were no changes in magnesium, calcium, aluminum, manganese and iron. Further support for a role of copper dyshomeostasis in ALS was gained from results of pharmacological intervention. Ammonium tetrathiomolybdate (TTM), a copper chelating agent, prolonged survival and slowed the disease progression of SOD1(G93A) mice, even when the treatment was started after the disease onset. TTM markedly attenuated pathology, including the loss of motor neurons and axons, and atrophy of skeletal muscles. Additionally, TTM decreased amounts of SOD1 aggregates. We propose that pharmacological agents that are capable of modulating copper dyshomeostasis, such as TTM, might be beneficial for the treatment of ALS caused by SOD1 mutations.

The "magic numbers" of metallothionein

Metallothioneins (MT) are a family of small cysteine rich proteins, which since their discovery in 1957, have been implicated in a range of roles including toxic metal detoxification, protection against oxidative stress, and as a metallochaperone involved in the homeostasis of both zinc and copper. The most well studied member of the family is the mammalian metallothionein, which consists of two domains: a β-domain with 9 cysteine residues, which sequesters 3 Cd(2+) or Zn(2+) or 6 Cu(+) ions, and an α-domain with 11 cysteine residues and, which sequesters 4 Cd(2+) or Zn(2+) or 6 Cu(+) ions. Despite over half a century of research, the exact functions of MT are still unknown. Much of current research aims to elucidate the mechanism of metal binding, as well as to isolate intermediates in metal exchange reactions; reactions necessary to maintain homeostatic equilibrium. These studies further our understanding of the role(s) of this remarkable and ubiquitous protein. Recently, supermetallated forms of the protein, where supermetallation describes metallation in excess of traditional levels, have been reported. These species may potentially be the metal exchange intermediates necessary to maintain homeostatic equilibrium. This review focuses on recent advances in the understanding of the mechanistic properties of metal binding, the implications for the metal induced protein folding reactions proposed for metallothionein metallation, the value of "magic numbers", which we informally define as the commonly determined metal-to-protein stoichiometric ratios and the significance of the new supermetallated states of the protein and the possible interpretation of the structural properties of this new metallation status. Together we provide a commentary on current experimental and theoretical advances and frame our consideration in terms of the possible functions of MT.

Structural studies of metal-free metallothionein

We report the first molecular dynamics calculations on the structure of metal-free betaalpha recombinant human metallothionein, with comparison to the two isolated fragments, alpha-rhMT and beta-rhMT, starting from a linear synthesized strand as well as a demetallated conformation. Following a 5000 ps MM3/MD calculation, the cysteine side chains were found to populate the outside surface of the metal-free protein, regardless of the initial conformation. The polypeptide backbone adopted a random coil conformation when starting from the linear strand, however, it retained a significant amount of secondary structure when starting from the demetallated conformation. We propose that the inverted cysteinyl sulfur orientation facilitates the binding of the metal ions to form the proteolytically stable, metallated protein.

Copper speciation in the alpha and beta domains of recombinant human metallothionein by electrospray ionization mass spectrometry

ESI-MS data are reported for Cu(I) binding to the metal-free and cadmium-alpha and beta domains of recombinant human metallothionein. These data provide information on the stoichiometric ratios of copper and cadmium that bind to the 11 thiolate sulfurs in the alpha fragment and the nine thiolate sulfurs in the beta fragment. The data show the effects of the existing three-dimensional structure on the formation of different Cu(I)-thiolate clusters. Charge-state spectra are reported for a range of Cu(I) binding at low and neutral pH to the isolated alpha and beta domains. There is an uneven distribution of charge states that show that changes in the three-dimensional structure take place as a function of Cu(I) loading. Metallation of the alpha domain at low pH takes place in a series of steps with the Cu7 species dominating until at higher levels of Cu(I) the clusters become unstable resulting in increased concentrations of the metal-free being detected. We interpret this behavior as being the result of the expansion of the Cu-S domain structure to accommodate digonal co-ordination for the increased Cu(I) loading. This larger structure is unstable in the mass spectrometer and demetallation takes place. Metallation of the beta domain at low pH proceeds in steps that involve initial formation of a Cu5S9 cluster, followed by the Cu6S9 at higher concentrations of Cu(I). The charge state spectra indicate a significant change in exposure of protonatable amino acids between Cu5S9 and Cu6S9 clusters, which indicates a change in peptide conformation when the Cu6S9 cluster forms. Metallation at neutral pH follows this same trend, namely, a much greater range of copper species is found during titrations of the Cd4S11-alpha fragment compared with the number of species that form when Cu(I) is added to Cd3S9-beta. The mass spectral data indicate that at neutral pH, the presence of the tetrahedral geometry of the Cd(II) facilitates formation of mixed trigonal and digonal geometries for the incoming Cu(I) so that the most prominent species in the beta fragment is Cd1Cu5S9 which transforms into Cu7S9 at higher concentrations of Cu(I), and finally to Cu9S9 at saturation, all species involving a number of Cu(I) in digonal geometries. The observation that the metallation patterns of the alpha and beta clusters follow different pathways at both low and neutral pH's, suggests that the structures in the two domains are quite different, in agreement with previous proposals

Circular dichroism, kinetic and mass spectrometric studies of copper(I) and mercury(II) binding to metallothionein

The metalloprotein metallothionein (MT) is remarkable in its metal binding properties: for the mammalian protein, well-characterized species exist for metal to sulfur ratios of M7S20, M12S20, and M18S20, where M = Cd(II), Zn(II), Hg(II), Ag(I), Au(I), and Cu(I). Optical spectra in general, and circular dichroism (CD) and luminescence spectra in particular, provide rich detail of a complicated metal binding chemistry when metals are added directly to the metal-free or zinc-containing protein. CD spectral data unambiguously identify key metal to protein stoichiometric ratios that result in well-defined structures. Electrospray ionization-mass spectrometry data are reported for reactions in which Hg(II) binds to apo-MT 2A as previously described from CD data. Emission spectra in the 450-750 nm region have been reported for metallothioneins containing Ag(I), Au(I), and Cu(I). The luminescence of Cu-MT can also be detected directly from mammalian and yeast cells. We report both steady-state and new dynamic data for titrations of Zn-MT with Cu(I). Analysis of kinetic data for the addition of the first two Cu(I) atoms to Zn-MT indicates a first-order mechanism over a concentration range of 5-50 microM. Three-dimensional modeling was carried out using the results of the CD and EXAFS studies, model calculations for Zn7-MT, Hg7-MT, and Cu12-MT are described.

Abnormal accumulation of porphyrin derivatives in the kidneys of Long-Evans Cinnamon rats, as evidenced by microspectrophotometry

In the study described here we have revealed an abnormal accumulation of porphyrin derivatives in the kidneys of Long-Evans Cinnamon (LEC) rats, an animal model for human Wilson's disease. In addition, we have confirmed that the derivatives emitted red-orange light in renal sections under UV excitation. This renal red-orange emission has previously been identified as luminescence from cuprous metallothioneins [Cu(I)-MTs], which also accumulate in both the kidneys and liver of LEC rats. In this study, we measured the emission spectra of the luminescence in the kidneys using microspectrophotometry. The spectra of the renal red-orange emission resembled those of porphyrin derivatives rather than those of Cu(I)-MTs. We then extracted these derivatives from the kidneys. An abundance of porphyrin derivatives was established. A significant increase in the levels of the derivatives in the liver and urine of the LEC rats was also confirmed. These results provide evidence of a heme-metabolism abnormality in LEC rats.

Copper-metallothionein in the kidney of macular mice: a model for Menkes disease

Menkes disease is an X-linked disorder of copper metabolism. Excess amounts of copper in the kidney of Macular mice, a model for this disease, were found as copper-metallothionein (Cu-MT) from kidney of the mice. Histochemical studies of Cu-MT based on its autofluorescent emission properties showed that the protein was predominant in the proximal convoluted tubule (PCT) cells of the cortex. PCT cells are known to be the primary site of the nephrotoxicity caused by heavy metals. MT mRNA was also observed in the cortex, indicating that the protein was biosynthesized in this region. On the basis of these results, we suggest that biosynthesis and degradation of Cu-MT occur repeatedly in the PCT cells of the cortex. We also compared the histochemical localization of Cu-MT in Macular mice and Long-Evans cinnamon rats, a model for Wilson's disease. The significance of this comparison is discussed.

Localization of renal Cu-binding metallothionein induced by Au injection into rats

The localization of Au-induced metallothionein (MT) in kidneys is reported. Au, Cu and Zn contents in kidneys and liver increased after Au injection. Especially the Cu content in the kidney increased in comparison with the Zn content. The yellow-orange autofluorescent signals which are a marker of Cu-MT were observed predominantly in the outer stripe of the outer medulla with a ring shape in the kidneys of Au-injected rats. MT mRNA was also located in only the outer stripe of the outer medulla. Neither autofluorescence nor MT mRNA was found in the kidneys of control rats. These results indicate that MT was biosynthesized in only the outer stripe of the outer medulla and the biosynthesized MT was bound to Cu. Representative Sephadex G-75 elution profiles of the renal cytosol of rats injected with Au showed that Au, Cu and Zn contents in MT fractions increased after Au injection. Interestingly, Cu in MT fractions dramatically increased in comparison with Zn in the MT fractions in spite of Au injection into rats. Only the Cu-containing MT fractions emitted a yellow-orange autofluorescence. The accumulated Cu in the kidneys of Au-injected rat was thought to be associated with renal toxicity.

Visualization of yellowish-orange luminescence from cuprous metallothioneins in liver of Long-Evans Cinnamon rat

We describe the first use of an emission probe, based on the cuprous thiolate chromophore, for direct microscopical observation of cuprous metallothioneins located in liver of 15-week-old (just before spontaneous hepatitis) Long-Evans Cinnamon rats. The rats show remarkable accumulations of copper and cuprous metallothioneins. In the mildly fixed liver, we visualized the same yellowish-orange luminescence as the specific emission from cuprous metallothioneins, following excitation in 330-385 nm region. In liver from Long-Evans Agouti rat, a counter part of Long-Evans Cinnamon rat, no similar luminescence was found. So, it was thought that cuprous metallothioneins accumulated in the Long-Evans Cinnamon rat liver might emit the yellowish-orange light. To verify this presumption, we tentatively defined three histochemical criteria, quenching tests by oxidation, protonation and mercury treatment, based on the coordination chemical characteristics of metallothioneins. The emission completely satisfied these criteria. Furthermore, the reliability of these criteria was supported by immunocytochemical and biochemical results. Consequently, all results sufficiently indicate that the yellowish-orange luminescence in the Long-Evans Cinnamon rat liver is the emission from cuprous metallothioneins.