Spectroscopic characterization of metallothionein from the terrestrial snail, Helix pomatia

Metals and metallothionein evolution in snails: a contribution to the concept of metal-specific functionality from an animal model group

This is a critical review of what we know so far about the evolution of metallothioneins (MTs) in Gastropoda (snails, whelks, limpets and slugs), an important class of molluscs with over 90,000 known species. Particular attention will be paid to the evolution of snail MTs in relation to the role of some metallic trace elements (cadmium, zinc and copper) and their interaction with MTs, also compared to MTs from other animal phyla. The article also highlights the important distinction, yet close relationship, between the structural and metal-selective binding properties of gastropod MTs and their physiological functionality in the living organism. It appears that in the course of the evolution of Gastropoda, the trace metal cadmium (Cd) must have played an essential role in the development of Cd-selective MT variants. It is shown how the structures and Cd-selective binding properties in the basal gastropod clades have evolved by testing and optimizing different combinations of ancestral and novel MT domains, and how some of these domains have become established in modern and recent gastropod clades. In this context, the question of how adaptation to new habitats and lifestyles has affected the original MT traits in different gastropod lineages will also be addressed. The 3D structures and their metal binding preferences will be highlighted exemplarily in MTs of modern littorinid and helicid snails. Finally, the importance of the different metal requirements and pathways in snail tissues and cells for the shaping and functionality of the respective MT isoforms will be shown.

Functional characterization of rice metallothionein OsMT-I-Id: Insights into metal binding and heavy metal tolerance mechanisms

Metallothioneins (MTs) are cysteine-rich proteins known for their strong metal-binding capabilities, making them effective in detoxifying heavy metals (HMs). This study focuses on characterizing the functional properties of OsMT-I-Id, a type-I Metallothionein found in rice. Using a HM-responsive yeast cup1Δ (DTY4), ycf1∆ (for cadmium), and acr3∆ mutants (for trivalent arsenic), we assessed the impact of OsMT-I-Id on metal accumulation and cellular resilience. Our results demonstrated that yeast cells expressing OsMT-I-Id showed increased tolerance and accumulated higher levels of copper (Cu), arsenic (As), and cadmium (Cd), compared to control cells. This can be attributed to the protein's ability to chelate and bind HMs. Site-directed mutagenesis was employed to investigate the specific contributions of cysteine residues. The study revealed that yeast cells with a mutated C-domain displayed heightened HM sensitivity, while cells with a mutated N-domain exhibited reduced sensitivity. This underscores the critical role of C-cysteine-rich domains in metal binding and tolerance of type-I rice MTs. Furthermore, the study identified the significance of the 12th cysteine position at the N-domain and the 68th and 72nd cysteine positions at the C-domain in influencing OsMT-I-Id metal-binding capacity. This research provides novel insights into the structure-function relationship and metal binding properties of type-I plant MTs.

Metalation of a rice type 1 metallothionein isoform (OsMTI-1b)

The simultaneously functions of Metallothioneins (MTs) are relied on their metalation mechanisms that can be divided into non-cooperative, weakly cooperative and strongly cooperative mechanisms. In this study, we recombinantly synthesized OsMTI-1b, N- and C-terminal Cys-rich regions as glutathione-S-transferase (GST)-fusion proteins in E. coli. In comparison with control strains (The E. coli cells containing pET41a without gene), transgenic E. coli cells showed more tolerance against Cd²⁺ and Zn²⁺. The recombinant GST-proteins were purified using affinity chromatography. According to in vitro assays, the recombinant proteins showed a higher binding ability to Cd²⁺ and Zn²⁺. However, the affinity of apo-proteins to Cu²⁺ ions were very low. The coordination of Cd²⁺ ions in OsMTI-1b demonstrates a strongly cooperative mechanism with a priority for the C-terminal Cys-rich region that indicates the detoxifying of heavy metals as main role of P1 subfamily of MTs. While the metalation with Zn²⁺ conformed to a weakly cooperative mechanism with a specificity to N-terminal Cys-rich region. It implies the specific function of OsMTI-1b is involved in zinc homeostasis. Nevertheless, a non-cooperative metalation mechanism was perceived for Cu²⁺ that suggests the fully metalation does not occur and OsMTI-1b cannot play a significant role in dealing with Cu²⁺ ions.

Metallation pathway of a plant metallothionein: Cicer arietinum MT2

The plant metallothionein 2 protein from Cicer arietinum (cicMT2) is a typical member of the plant MT subfamily p2 that is characterized by an N- and C-terminal cysteine- (Cys-)rich, metal binding sequence connected by a long cysteine-free linker region. cicMT2 coordinates up to five Zn^II or Cd^II ions by its 14 cysteine thiolate groups forming a single metal-thiolate cluster. While MTs from other phyla are considerably well-studied, many details about plant MTs are missing. In this study the metallation pathway of cicMT2 is investigated using mass spectrometry. To evaluate the influence of the linker region as well as the interplay of the two Cys-rich stretches, the full-length cicMT2 protein as well as the individual Cys-rich domains with and without the linker region were analysed. Up to three Cd^II ions can be coordinated by the eight Cys residues of the N-terminal part and up to two Cd^II ions by the six Cys residues of the C-terminal sequence. However, no preferential binding to either of the two sequences is observed, which is in-line with the closely similar apparent binding constants of the individual domains obtained from competition reactions with the chelator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid. The combination of limited proteolytic digestion, mass spectrometry, dynamic light scattering, size-exclusion chromatography, and ¹⁹F NMR spectroscopy enables us to draw conclusions about the overall protein-fold and the cluster formation process.

Differential response to Cadmium exposure by expression of a two and a three-domain metallothionein isoform in the land winkle Pomatias elegans: Valuating the marine heritage of a land snail

Through evolution, marine snails have adapted several times independently to terrestrial life. A prime example for such transitions is the adaptation to terrestrial conditions in members of the gastropod clade of Littorinoidea (Caenogastropoda). Some species of this lineage like the periwinkle (Littorina littorea), live in intertidal habitats, where they are intermittently exposed to semi-terrestrial conditions. Pomatias elegans is a close relative of Littorina littorea that has successfully colonized terrestrial habitats. Evolutionary transitions from marine to terrestrial conditions have often been fostered in marine ancestors by acquisition of physiological pre-adaptations to terrestrial life. Such pre-adaptations are based, among others, on the optimization of a wide repertoire of stress resistance mechanisms, such as the expression of metal inactivating metallothioneins (MTs). The objective of our study was to explore the Cd handling strategy in the terrestrial snail Pomatias elegans in comparison to that observed previously in Littorina littorea. After Cd exposure, the metal is accumulated mainly in the midgut gland of Pomatias elegans, in a similar way as in its marine relative. Upon Cd exposure, Pomatias elegans expresses Cd-specific MTs, as also described from Littorina littorea. In contrast to the latter species, however, the detoxification of Cd in Pomatias elegans is mediated by two different MT isoforms, one two-domain and one three-domain MT. Although the MT proteins of both species are homologous and clearly originate from one common ancestor, the three-domain MT isoform of Pomatias elegans has evolved independently from the three-domain MT of its marine counterpart, probably by addition of a third domain to the pre-existing two-domain MT. Obviously, the occurrence of homologous MT structures in both species is a hereditary character, whereas the differentiation into two distinct MT isoforms with different upregulation capacities in Pomatias elegans is an adaptive feature that probably emerged upon transition to life on land.

Cyamopsis tetragonoloba type 1 metallothionein (CtMT1) gene is upregulated under drought stress and its protein product has an additional C-X-C motif and unique metal binding pattern

Metallothioneins (MTs) are involved in cellular homeostasis of essential metal ions and detoxification of nonessential metal ions. We report here the identification of four MT genes, CtMT1, CtMT2, CtMT3 and CtMT4, encoding CtMT1, CtMT2, CtMT3 and CtMT4 proteins, respectively, from the industrial guar crop. The primary structures of last three proteins were similar to those of respective MT proteins of other plants but the CtMT1 protein primary structure was different from the other plant MT1 proteins in having an additional C-X-C motif. The four MT genes showed tissue specific expression patterns suggesting their specific roles in different tissues. High expression of CtMT1 gene was observed in roots and nodules whereas CtMT2 and CtMT3 genes showed high expression in leaves. The expression of CtMT4 gene was high in seeds. The qRT-PCR studies revealed upregulation in expression of CtMT1 gene under drought stress. Recombinant CtMT1 protein was produced in E. coli Rosetta cells and purified by metal affinity chromatography. The purified protein showed antioxidant property and the order of its metal ion binding affinities was Cu²⁺ > Zn²⁺ > Fe²⁺ > Cd²⁺. This information about CtMT1 protein is expected to be useful in understanding its role in drought tolerance and other physiological processes of guar.

Functional comparision between truncated MTT1 and truncated MTT2 from Tetrahyemna thermophila

Metallothioneins (MTs) are low-molecular-weight proteins with high Cys content and high metal-chelating ability. CdMT and CuMT subfamilies present different characteristics in Tetrahymena. To explore the effect of the cysteine arrangement and sequence length of MTs for binding different metal ions, MTT1, truncated MTT1 (TM1), MTT2, and truncated MTT2 (TM2) were expressed in E. coli. The half-maximal inhibiting concentrations (IC50) of Cd2+ and Cu+ for the recombinant strains were different. Furthermore, E. coli cells expressing MTT1 and TM1 exhibited higher accumulating ability for Cd2+ than cells expressing MTT2 and TM2. However, the opposite is true for Cu+. The binding ability of the different recombinant proteins to Cd2+ and Cu+ were also different. MTT1 and truncated mutant TM1 were the preference for Cd2+, whereas MTT2 and truncated mutant TM2 were the preference for Cu+ coordination. These results showed that metal ion tolerance and accumulation ability not only depended on cysteine arrangement pattern but also on sequence length of MT in Tetrahymena.

Successive Onset of Molecular, Cellular and Tissue-Specific Responses in Midgut Gland of Littorina littorea Exposed to Sub-Lethal Cadmium Concentrations

Cadmium (Cd) is one of the most harmful metals, being toxic to most animal species, including marine invertebrates. Among marine gastropods, the periwinkle (Littorina littorea) in particular can accumulate high amounts of Cd in its midgut gland. In this organ, the metal can elicit extensive cytological and tissue-specific alterations that may reach, depending on the intensity of Cd exposure, from reversible lesions to pathological cellular disruptions. At the same time, Littorina littorea expresses a Cd-specific metallothionein (MT) that, due to its molecular features, expectedly exerts a protective function against the adverse intracellular effects of this metal. The aim of the present study was, therefore, to assess the time course of MT induction in the periwinkle’s midgut gland on the one hand, and cellular and tissue-specific alterations in the digestive organ complex (midgut gland and digestive tract) on the other, upon exposure to sub-lethal Cd concentrations (0.25 and 1 mg Cd/L) over 21 days. Depending on the Cd concentrations applied, the beginning of alterations of the assessed parameters followed distinct concentration-dependent and time-dependent patterns, where the timeframe for the onset of the different response reactions became narrower at higher Cd concentrations compared to lower exposure concentrations.

Biomphalaria glabrata Metallothionein: Lacking Metal Specificity of the Protein and Missing Gene Upregulation Suggest Metal Sequestration by Exchange Instead of through Selective Binding

The wild-type metallothionein (MT) of the freshwater snail Biomphalaria glabrata and a natural allelic mutant of it in which a lysine residue was replaced by an asparagine residue, were recombinantly expressed and analyzed for their metal-binding features with respect to Cd²⁺, Zn²⁺ and Cu⁺, applying spectroscopic and mass-spectrometric methods. In addition, the upregulation of the Biomphalaria glabrataMT gene was assessed by quantitative real-time detection PCR. The two recombinant proteins revealed to be very similar in most of their metal binding features. They lacked a clear metal-binding preference for any of the three metal ions assayed-which, to this degree, is clearly unprecedented in the world of Gastropoda MTs. There were, however, slight differences in copper-binding abilities between the two allelic variants. Overall, the missing metal specificity of the two recombinant MTs goes hand in hand with lacking upregulation of the respective MT gene. This suggests that in vivo, the Biomphalaria glabrata MT may be more important for metal replacement reactions through a constitutively abundant form, rather than for metal sequestration by high binding specificity. There are indications that the MT of Biomphalaria glabrata may share its unspecific features with MTs from other freshwater snails of the Hygrophila family.

Copper metallothioneins

Metallothioneins (MTs) are a class of low molecular weight and cysteine-rich metal binding proteins present in all the branches of the tree of life. MTs efficiently bind with high affinity several essential and toxic divalent and monovalent transition metals by forming characteristic polynuclear metal-thiolate clusters within their structure. MTs fulfil multiple biological functions related to their metal binding properties, with essential roles in both Zn(II) and Cu(I) homeostasis as well as metal detoxification. Depending on the organism considered, the primary sequence, and the specific physiological and metabolic status, Cu(I)-bound MT isoforms have been isolated, and their chemistry and biology characterized. Besides the recognized role in the biochemistry of divalent metals, it is becoming evident that unique biological functions in selectively controlling copper levels, its reactivity as well as copper-mediated biochemical processes have evolved in some members of the MT superfamily. Selected examples are reviewed to highlight the peculiar chemical properties and biological functions of copper MTs. © 2016 IUBMB Life, 69(4):236-245, 2017.

Independent metal-thiolate cluster formation in C-terminal Cys-rich region of a rice type 1 metallothionein isoform

In this study we examined the independent self assembly of metal-binding in C-terminal Cys- rich region of a type 1 metallothionein (MT) isoform from rice (OsMTI-1b). To this end the N-terminal of OsMTI-1b (C-OsMTI-1b) was heterologously expressed in Escherichia coli as fusion protein with glutathione-S-transferase (GST). As compared with control (The E. coli cells containing pET41a without gene), transgenic E. coli cells expressing GST-C-OsMTI-1b accumulated more Ni²⁺, Cd²⁺, and Zn²⁺ from culture medium and showed increased tolerance against these metals. The recombinant GST-C-OsMTI-1b was purified using affinity chromatography. According to in vitro assays the protein GST-C-OsMTI-1b was able to form complexes with Ni²⁺, Cd²⁺ and Zn²⁺. These results demonstrate the formation of independent metal-thiolate cluster at C-terminal Cys-rich region of OsMTI-1b without participation of N-terminal Cys-rich region.

Physiological, Diurnal and Stress-Related Variability of Cadmium-Metallothionein Gene Expression in Land Snails

The terrestrial Roman snail Helix pomatia has successfully adapted to strongly fluctuating conditions in its natural soil habitat. Part of the snail’s stress defense strategy is its ability to express Metallothioneins (MTs). These are multifunctional, cysteine-rich proteins that bind and inactivate transition metal ions (Cd²⁺, Zn²⁺, Cu⁺) with high affinity. In Helix pomatia a Cadmium (Cd)-selective, inducible Metallothionein Isoform (CdMT) is mainly involved in detoxification of this harmful metal. In addition, the snail CdMT has been shown to also respond to certain physiological stressors. The aim of the present study was to investigate the physiological and diurnal variability of CdMT gene expression in snails exposed to Cd and non-metallic stressors such as desiccation and oxygen depletion. CdMT gene expression was upregulated by Cd exposure and desiccation, whereas no significant impact on the expression of CdMT was measured due to oxygen depletion. Overall, Cd was clearly more effective as an inducer of the CdMT gene expression compared to the applied non-metallic stressors. In unexposed snails, diurnal rhythmicity of CdMT gene expression was observed with higher mRNA concentrations at night compared to daytime. This rhythmicity was severely disrupted in Cd-exposed snails which exhibited highest CdMT gene transcription rates in the morning. Apart from diurnal rhythmicity, feeding activity also had a strong impact on CdMT gene expression. Although underlying mechanisms are not completely understood, it is clear that factors increasing MT expression variability have to be considered when using MT mRNA quantification as a biomarker for environmental stressors.

Does Variation of the Inter-Domain Linker Sequence Modulate the Metal Binding Behaviour of Helix pomatia Cd-Metallothionein?

Snail metallothioneins (MTs) constitute an ideal model to study structure/function relationships in these metal-binding polypeptides. Helix pomatia harbours three MT isoforms: the highly specific CdMT and CuMT, and an unspecific Cd/CuMT, which represent paralogous proteins with extremely different metal binding preferences while sharing high sequence similarity. Preceding work allowed assessing that, although, the Cys residues are responsible for metal ion coordination, metal specificity or preference is achieved by diversification of the amino acids interspersed between them. The metal-specific MT polypeptides fold into unique, energetically-optimized complexes of defined metal content, when binding their cognate metal ions, while they produce a mixture of complexes, none of them representing a clear energy minimum, with non-cognate metal ions. Another critical, and so far mostly unexplored, region is the stretch linking the individual MT domains, each of which represents an independent metal cluster. In this work, we have designed and analyzed two HpCdMT constructs with substituted linker segments, and determined their coordination behavior when exposed to both cognate and non-cognate metal ions. Results unequivocally show that neither length nor composition of the inter-domain linker alter the features of the Zn(II)- and Cd(II)-complexes, but surprisingly that they influence their ability to bind Cu(I), the non-cognate metal ion.

Snails from heavy-metal polluted environments have reduced sensitivity to carbon dioxide-induced acidity

Anthropogenic atmospheric CO₂ reacts with water to form carbonic acid (H₂CO₃) which increases water acidity. While marine acidification has received recent consideration, less attention has been paid to the effects of atmospheric carbon dioxide on freshwater systems—systems that often have low buffering potential. Since many aquatic systems are already impacted by pollutants such as heavy metals, we wondered about the added effect of rising atmospheric CO₂ on freshwater organisms. We studied aquatic pulmonate snails (Physella columbiana) from both a heavy-metal polluted watershed and snails from a reference watershed that has not experienced mining pollution. We used gaseous CO₂ to increase water acidity and we then measured changes in antipredatory behavior and also survival. We predicted a simple negative additive effect of low pH. We hypothesized that snails from metal-polluted environments would be physiologically stressed and impaired due to defense responses against heavy metals. Instead, snails from populations that acclimated or evolved in the presence of heavy metal mining pollution were more robust to acidic conditions than were snails from reference habitats. Snails from mining polluted sites seemed to be preadapted to a low pH environment. Their short-term survival in acidic conditions was better than snails from reference sites that lacked metal pollution. In fact, the 48 h survival of snails from polluted sites was so high that it did not significantly differ from the 24 h survival of snails from control sites. This suggests that the response of organisms to a world with rising anthropogenic carbon dioxide levels may be complex and difficult to predict. Snails had a weaker behavioral response to stressful stimuli if kept for 1 month at a pH that differed from their lake of origin. We found that snails raised at a pH of 5.5 had a weaker response (less of a decrease in activity) to concentrated heavy metals than did snails raised at their natal pH of 6.5. Furthermore, snails raised a pH of 5.5, 6.0, and 7.0 all had a weaker antipredatory response to an extract of crushed snail cells than did the pH 6.5 treatment snails.

Discrimination between two rice metallothionein isoforms belonging to type 1 and type 4 in metal-binding ability

Metallothioneins (MTs) are a superfamily of low-molecular-weight, cysteine (Cys)-rich proteins that are believed to play important roles in protection against metal toxicity and oxidative stress. Plants have several MT isoforms, which are classified into four types based on the arrangement of Cys residues. In this study, two rice (Oryza sativa) MT isoforms, OsMTI-1b and OsMTII-1a from type 1 and type 4, respectively, were heterologously expressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). Transformed cells expressing GST-OsMTI-1b showed increased tolerance to Ni(2+) , Cd(2+) , and Zn(2+) and accumulated more metal ions compared with cells expressing GST alone. However, heterologous expression of GST-OsMTII-1a had no significant effects on metal tolerance or ion accumulation. The UV absorption spectra and competitive reactions of in vitro Cd-incubated proteins with 5-5'-dithiobis(2-nitrobenzoic) acid revealed that GST-OsMTI-1b, but not GST-OsMTII-1a, is able to form Cd-thiolate clusters. Furthermore, heterologous expression of both GST-OsMTI-1b and GST-OsMTII-1a conferred H2 O2 tolerance to E. coli cells. Taken together, the results presented here show that two different rice MT isoforms belonging to type 1 and type 4 differ in Ni(2+) , Cd(2+) , and Zn(2+) binding abilities, but they may have overlapping function in protection of cells against oxidative stress.

Heterologous expression and metal-binding characterization of a type 1 metallothionein isoform (OsMTI-1b) from rice (Oryza sativa)

Metallothioneins (MTs) are ubiquitous, low molecular mass and cysteine-rich proteins that play important roles in maintaining intracellular metal homeostasis, eliminating metal toxification and protecting the cells against oxidative damages. MTs are able to bind metal ions through the thiol groups of their cysteine residues. Plants have several MT isoforms which are classified into four types based on the arrangement of cysteine residues. In the present study, a rice (Oryza sativa) gene encoding type 1 MT isoform, OsMTI-1b, was inserted in vector pET41a and overexpressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). The recombinant protein GST-OsMTI-1b was purified using affinity chromatography and its ability to bind with Ni(2+), Cd(2+), Zn(2+) and Cu(2+) ions was analyzed. The results demonstrated that this isoform has ability to bind Ni(2+), Cd(2+) and Zn(2+) ions in vitro, whereas it has no substantial ability to bind Cu(2+) ions. From competitive reaction with 5,5'-dithiobis(2-nitrobenzoic acid), DTNB, the affinity of metal ions for recombinant form of GST-OsMTI-1b was as follows: Ni(2+)/Cd(2+) > Zn(2+) > Cu(2+).

Evaluation of biotargeting and ecotoxicity of Co²⁺-containing nanoscale polymeric complex by applying multi-marker approach in bivalve mollusk Anodonta cygnea

Cobalt (Co(2+)) is present in many nanoscaled materials created for various applications. The key goal of our study was to develop sensitive approaches for assessing the bio-risks associated with using novel Co(2+)-containing nanoscaled polymeric complex (Co-NC). Freshwater bivalve mollusk Anodonta cygnea (Unionidae) was subjected to 14 d action of the developed Co-NC, as well as of Co(2+) applied in the corresponding concentration (50 μg L(-1)) or polymeric substance (PS). All experimental groups under study have demonstrated signs of toxic targeting, notably changes in DNA characteristics, oxidative stress (with particularities in each exposed group) and activation of anaerobiosis (Co(2+) and Co-NC). However, the group exposed to Co-NC showed some advantages that can be related to the activation of metallothionein (MT) function (increase in the level of MT-related SH-groups (MT-SH)): low level of oxyradical formation, no increase in protein carbonylation and vitellogenin-like proteins concentration unlike in Co(2+) and PS exposed groups. On the other hand, Co(2+) increased metal (Co, Cu, Zn and Cd) binding to MT (MT-Me) without changes in MT-SH level jointly with activation of oxyradical formation and apoptosis and decreasing of lysosomal membrane stability. PS per se initiated unbalanced changes in activities of the biotransformation enzymes ethoxyresorufin-O-deethylase and glutathione-S-transferase. Thus, Co(2+) complexing with the developed PS prevented bio-toxic effects of free Co(2+) ions and PS per se, at least in the studied hydrobiont. The MT-SH was the main distinguishing index of Co-NC group selected by classification and regression tree analysis.

Nematode and snail metallothioneins

Metallobiologists have, at large, neglected soil dwelling invertebrates; exceptions are the nematode (Caenorhabditis elegans) and snails (Helix pomatia and Cantareus aspersus). This review aims to compare and contrast the molecular, protein and cellular mechanisms of the multifunctional nematode and snail metallothioneins (MTs). The C. elegans genome contains two MT genes, mtl-1, which is constitutively expressed in the pharynx and likely to act as an essential and/or toxic metal sensor, and mtl-2, which plays a negligible role under normal physiological conditions but is strongly induced (as mtl-1) in intestinal cells upon metal exposure. It has been possible to follow the intricate phenotypic responses upon the knockdown/knockout of single and multiple MT isoforms and we have started to decipher the multifunctional role of C. elegans MTs. The snails have contributed to our understanding regarding MT evolution and diversity, structure and metal-specific functionality. The H. pomatia and C. aspersus genomes contain at least three MT isoform genes. CdMT is responsible for cadmium detoxification, CuMT is involved in copper homeostasis and Cd/CuMT is a putative ancestral MT possibly only of minor importance in metal metabolism. Further investigations of nematode, snail and other invertebrate MTs will allow the development of alternative biomarker approaches and lead to an improved understanding of metallobiology, protein evolution and toxicogenomics.

Shaping mechanisms of metal specificity in a family of metazoan metallothioneins: evolutionary differentiation of mollusc metallothioneins

Background

The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu⁺ or Cd²⁺. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order to elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion.

Results

HpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd²⁺, Zn²⁺ or Cu²⁺ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd₆-HpCdMT and Cu₁₂-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions.

Conclusion

The Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.

Cadmium stress stimulates tissue turnover in Helix pomatia: increasing cell proliferation from metal tolerance to exhaustion in molluscan midgut gland

In terrestrial pulmonate snails, cadmium (Cd) uptake leads to the induction of a Cd-specific metallothionein isoform (Cd-MT) that protects against adverse interactions of this toxic metal ion. Increasing concentrations of Cd cause increased individual mortality possibly linked to pathological alterations in the snail midgut gland. Histological, immuno-histochemical, and electron-microscopic methods in combination with tissue metal analyses and quantification of MT induction parameters were applied to the midgut gland of Cd-exposed Roman snails (Helix pomatia). Conspicuous concentration-dependent alterations occurred in this organ, including the metal-induced increase of Cd-MT concentration and manifestation of Cd-MT mRNA precipitations in all midgut gland cell types. The most evident alteration was an increase of cellular turnover reflected by enhanced cell proliferation. Intensified vesiculation of endoplasmic reticulum was noted in basophilic cells and an increasing formation of lipofuscin granules in excretory cells. At the highest Cd concentrations, mitochondrial membranes were disrupted in basophilic cells, and lipofuscin granules were released from excretory cells into the midgut gland tubular system. Some of these alterations (e.g., increased cell proliferation rate, vesiculation of endoplasmic reticulum) detected at low Cd concentrations were interpreted as adaptive response processes enhancing the tolerance of exposed individuals to metal stress. Cellular alterations at higher Cd concentrations (e.g., mitochondrial structural damage) clearly represented ongoing irreversible cellular disruption. Combined evaluation of cellular biomarkers and MT saturation levels indicated that the transition from stress resistance to depletion of resistance capacity occurred above a threshold of 0.8 micromol Cd/g dry weight in the midgut gland of H. pomatia. At these Cd concentrations, Cd-MT was saturated with Cd(2+) ions, whereas at the cellular level, structural alterations turned into pathological deterioration.

Distinct characteristics of Ag+ and Cd2+ binding to CopZ from Bacillus subtilis

The chaperone CopZ together with the P-type ATPase transporter CopA constitute a copper-detoxification system in Bacillus subtilis that is commonly found in bacteria and higher cells. Previous studies of the regulation of the copZA operon showed that expression is significantly upregulated in response to elevated concentrations of environmental silver and cadmium, as well as copper. Here, we have used spectroscopic and bioanalytical methods to investigate in detail the capacity of CopZ to bind these metal ions (as Ag(+) and Cd(2+)). We demonstrate that Ag(+) binding mimics closely that of Cu(+): Ag(+)-mediated dimerisation of the protein occurs, and distinct Ag(+)-bound species are formed at higher Ag(+) loadings. Cd(2+) also binds to CopZ, but exhibits significantly different behaviour. Cd(2+)-mediated dimerisation is only observed at low loadings, such that at 0.5 and one Cd(2+) per CopZ the protein is present mainly in a monomeric form; and multinuclear higher-order forms of Cd(2+)-CopZ are not observed. Competition binding studies reveal that Ag(+) binds with an affinity very similar to that of Cu(+), while Cd(2+) binding is significantly weaker. These data provide support for the proposal that CopZ may be involved in the detoxification of silver and cadmium, in addition to copper.

Comparison of transfer and effects of Cd on rats exposed in a short experimental snail-rat food chain or to CdCl2 dosed food

Transfer and toxic effects of two cadmium (Cd) forms, inorganic (CdCl2 dosed rat food) or organic (contaminated snail-based rat food) were studied in Wistar rat. Cd concentrations in rat food were 0 and 2.5 microg Cd g(-1) for both inorganic and organic forms and a high concentration of 100 microg Cd g(-1) was also tested for the inorganic form. Rats were exposed for four weeks to contaminated food. Both forms of Cd were bioavailable to rats, with a percentage of transfer from food to rats of around 1% for all contaminated groups. Cd concentrations in rat tissues increased with increasing Cd concentrations in the food. Rats fed with organic form of Cd accumulated significantly more Cd in the main organ for Cd toxicity, the kidney, than those eating the inorganic form. Survival was not affected for any rat group but a decrease in growth and food consumption was observed for the inorganic form. As a defence system against Cd toxicity, rats increased their metallothionein (MT) synthesis at the highest Cd concentration in the target organs (kidney, liver and small intestine) and even did the same at low Cd concentrations (2.5 microg Cd g(-1)) in the kidney. At this low Cd concentration, MT induction was lower in the small intestine of rats ingesting organic Cd than those ingesting inorganic Cd. Bioavailability of organic and inorganic forms of Cd was similar, but subsequent Cd distribution within organs was different. This quantification of the trophic transfer of both inorganic and organic forms of a toxicant is a basis for a better assessment of the fate and effects of chemicals in food webs.

Molecular characterization and function analysis of MT-10 and MT-20 metallothionein isoforms from Mytilus galloprovincialis

Structure and function of molluscan metallothioneins (MTs) are still poorly understood. The sea mussel Mytilus galloprovincialis displays two MT isoforms which differ in both primary sequences and physiological functions. MT-10 is the constitutive isoform, whereas MT-20 is mainly induced by cadmium (Cd). Both MTs were produced as recombinant proteins and showed identical Cd content and similar Cd-binding properties. Conversely, circular dichroism disclosed marked differences in the secondary conformations of the two Cd(7)-MTs. The possible relapses of these structural differences on protein stability and function were assessed. MT-10 presented a higher thermal stability and a more compact structure than MT-20, as it was inferred by absorption and emission spectroscopy studies. Moreover, the kinetics of Cd-release clearly indicated that MT-10 is much more sensitive to oxidation than is MT-20. The observed differences between MT-10 and MT-20 are discussed in terms of the different physiological roles exerted by the two isoforms in mussel.

Metal ion binding properties of Tricium aestivum Ec-1 metallothionein: evidence supporting two separate metal thiolate clusters

Metallothioneins are ubiquitous low molecular mass, cysteine-rich proteins with an extraordinary high metal ion content. In contrast to the situation for the vertebrate forms, information regarding the properties of members of the plant metallothionein family is still scarce. We present the first spectroscopic investigation aiming to elucidate the metal ion binding properties and metal thiolate cluster formation of the Triticum [corrected] aestivum (common wheat) early cysteine-labeled plant metallothionein (Ec-1). For this, the protein was overexpressed recombinantly in Escherichia coli. Recombinant Ec-1 is able to bind a total of six divalent d10 metal ions in a metal thiolate cluster arrangement. The pH stability of the zinc and cadmium clusters investigated is comparable to stabilities found for mammalian metallothioneins. Using cobalt(II) as a paramagnetic probe, we were able to show the onset of cluster formation taking place with the addition of a fourth metal ion equivalent to the apo protein. Limited proteolytic digestion experiments complemented with mass spectrometry and amino acid analysis provide clear evidence for the presence of two separate metal thiolate clusters. One cluster consists of four metal ions and is made up by a part of the protein containing 11 cysteine residues, comparable to the situation found in the mammalian counterparts. The second cluster features two metal ions coordinated by six cysteine residues. The occurrence of the latter cluster is unprecedented in the metallothionein superfamily so far.

Copper in Helix pomatia (Gastropoda) is regulated by one single cell type: differently responsive metal pools in rhogocytes

Like all other animal species, terrestrial pulmonate snails require Cu as an essential trace element. On the other hand, elevated amounts of Cu can exert toxic effects on snails. The homeostatic regulation of Cu must therefore be a pivotal goal of terrestrial pulmonates to survive. Upon administration of Cu, snails accumulate the metal nearly equally in most of their organs. Quantitative studies in connection with HPLC and electrospray ionization mass spectrometry reveal that a certain fraction of Cu in snails is bound to a Cu-metallothionein (Cu-MT) isoform that occurs in most organs at constant concentrations, irrespective of whether the animals had been exposed to physiological or elevated amounts of Cu. In situ hybridization demonstrates that at the cellular level, the Cu-binding MT isoform is exclusively expressed in the so-called pore cells (or rhogocytes), which can be found in all major snail organs. The number of pore cells with Cu-MT mRNA reaction products remains unaffected by Cu exposure. Rhogocytes also are major storage sites of Cu in a granular form, the metal quickly entering the snail tissues upon elevated exposure. The number of rhogocytes with granular Cu precipitations strongly increases upon Cu administration via food. Thus, whereas Cu-MT in the rhogocytes represents a stable pool of Cu that apparently serves physiological tasks, the granular Cu precipitations form a second, quickly inducible, and more easily available pool of the metal that serves Cu regulation by responding to superphysiological metal exposure.

Cd accumulation and Cd-metallothionein as a biomarker in Cepaea hortensis (Helicidae, Pulmonata) from laboratory exposure and metal-polluted habitats

Cepaea hortensis is a widespread terrestrial pulmonate, contributing significantly to element fluxes in soil ecosystems. Due to its capacity of accumulating certain trace elements in its tissues, Cepaea hortensis can serve as a biological indicator of metal accumulation in contaminated areas. In response to Cd exposure this species and related helicid pulmonates are also able to synthesize an inducible, Cd-binding metallothionein (MT) isoform specifically serving in binding and detoxification of this metal. As shown by field-collected garden-snails from a metal-contaminated site near a zinc smelting works in Avonmouth (UK) and an unpolluted reference site in Reutte (Tyrol, Austria), Cd and Cd MT concentrations in midgut gland of C. hortensis from these sites increased with rising Cd concentrations in the soil substrate from the same contaminated sites. By combining the results of these field data with laboratory experiments it appears that midgut gland Cd-MT of Cepaea hortensis seems to fulfil the criteria of a successful biomarker in many respects. First, the synthesis of the protein can rapidly be induced by Cd exposure. Second, the level of Cd MT induction in C. hortensis directly reflects the intensity of metal exposure. Third, the induced signal of increased Cd-MT concentration in C. hortensis is persistent over extended periods of time. Fourth, the Cd-MT signal in C. hortensis seems to be very specific for Cd exposure. Regression analyses demonstrate that tissue levels of Cd and Cd MT in C. hortensis depend on Cd concentrations in the substrate which is represented by either soil or plant material on which snails normally feed. In both cases the best fit for this dependence is exhibited by a semi-logarithmic relationship, with substrate (soil or plant feed) concentrations expressed on a logarithmic scale. It is concluded that C. hortensis and other related pulmonates can successfully be used either as biological indicators of Cd accumulation, or as key species in biomonitoring studies focusing on Cd-MT induction as a biomarker for Cd exposure.