Extreme metal adapted, knockout and knockdown strains reveal a coordinated gene expression among different Tetrahymena thermophila metallothionein isoforms

Cellular Response of Adapted and Non-Adapted Tetrahymena thermophila Strains to Europium Eu(III) Compounds

Europium is one of the most reactive lanthanides and humans use it in many different applications, but we still know little about its potential toxicity and cellular response to its exposure. Two strains of the eukaryotic microorganism model Tetrahymena thermophila were adapted to high concentrations of two Eu(III) compounds (EuCl3 or Eu2O3) and compared to a control strain and cultures treated with both compounds. In this ciliate, EuCl3 is more toxic than Eu2O3. LC50 values show that this microorganism is more resistant to these Eu(III) compounds than other microorganisms. Oxidative stress originated mainly by Eu2O3 is minimized by overexpression of genes encoding important antioxidant enzymes. The overexpression of metallothionein genes under treatment with Eu(III) compounds supports the possibility that this lanthanide may interact with the -SH groups of the cysteine residues from metallothioneins and/or displace essential cations of these proteins during their homeostatic function. Both lipid metabolism (lipid droplets fusing with europium-containing vacuoles) and autophagy are involved in the cellular response to europium stress. Bioaccumulation, together with a possible biomineralization to europium phosphate, seems to be the main mechanism of Eu(III) detoxification in these cells.

Autophagy and lipid droplets are a defense mechanism against toxic copper oxide nanotubes in the eukaryotic microbial model Tetrahymena thermophila

The widespread use of inorganic nanomaterials of anthropogenic origin has significantly increased in the last decade, being now considered as emerging pollutants. This makes it necessary to carry out studies to further understand their toxicity and interactions with cells. In the present work we analyzed the toxicity of CuO nanotubes (CuONT) in the ciliate Tetrahymena thermophila, a eukaryotic unicellular model with animal biology. CuONT exposure rapidly induced ROS generation in the cell leading to oxidative stress and upregulation of genes encoding antioxidant enzymes (catalase, superoxide dismutase, glutathione peroxidase), metal-chelating metallothioneins and cytochrome P450 monooxygenases. Comet assays and overexpression of genes involved in DNA repair confirmed oxidative DNA damage in CuONT-treated cells. Remarkably, both electron and fluorescent microscopy revealed numerous lipid droplets and autophagosomes containing CuONT aggregates and damaged mitochondria, indicating activation of macroautophagy, which was further confirmed by a dramatic upregulation of ATG (AuTophaGy related) genes. Treatment with autophagy inhibitors significantly increased CuONT toxicity, evidencing the protective role of autophagy towards CuONT-induced damage. Moreover, increased formation of lipid droplets appears as an additional mechanism of CuONT detoxification. Based on these results, we present a hypothetical scenario summarizing how T. thermophila responds to CuONT toxicity. This study corroborates the use of this ciliate as an excellent eukaryotic microbial model for analyzing the cellular response to stress caused by toxic metal nanoparticles.

Arsenate and arsenite differential toxicity in Tetrahymena thermophila

A comparative analysis of toxicities of both arsenic forms (arsenite and arsenate) in the model eukaryotic microorganism Tetrahymena thermophila (ciliate protozoa) has shown the presence of various detoxification mechanisms and cellular effects comparable to those of animal cells under arsenic stress. In the wild type strain SB1969 arsenate is almost 2.5 times more toxic than arsenite. According to the concentration addition model used in binary metallic mixtures their toxicities show an additive effect. Using fluorescent assays and flow cytometry, it has been detected that As(V) generates elevated levels of ROS/RNS compared to As(III). Both produce the same levels of superoxide anion, but As(V) also causes greater increases in hydrogen peroxide and peroxynitrite. The mitochondrial membrane potential is affected by both As(V) and As(III), and electron microscopy has also revealed that mitochondria are the main target of both arsenic ionic forms. Fusion/fission and swelling mitochondrial and mitophagy, together with macroautophagy, vacuolization and mucocyst extruction are mainly associated to As(V) toxicity, while As(III) induces an extensive lipid metabolism dysfunction (adipotropic effect). Quantitative RT-PCR analysis of some genes encoding antioxidant proteins or enzymes has shown that glutathione and thioredoxin metabolisms are involved in the response to arsenic stress. Likewise, the function of metallothioneins seems to be crucial in arsenic detoxification processes, after using both metallothionein knockout and knockdown strains and cells overexpressing metallothionein genes from this ciliate. The analysis of the differential toxicity of As(III) and As(V) shown in this study provides cytological and molecular tools to be used as biomarkers for each of the two arsenic ionic forms.

Experimental Evolution in Tetrahymena

Experimental evolution has provided novel insight into a wide array of biological processes. Species in the genus Tetrahymena are proving to be a highly useful system for studying a range of questions using experimental evolution. Their unusual genomic architecture, diversity of life history traits, importance as both predator and prey, and amenability to laboratory culture allow them to be studied in a variety of contexts. In this paper, we review what we are learning from experimental evolution with Tetrahymena about mutation, adaptation, and eco-evolutionary dynamics. We predict that future experimental evolution studies using Tetrahyemena will continue to shed new light on these processes.

Tunicates Illuminate the Enigmatic Evolution of Chordate Metallothioneins by Gene Gains and Losses, Independent Modular Expansions, and Functional Convergences

To investigate novel patterns and processes of protein evolution, we have focused in the metallothioneins (MTs), a singular group of metal-binding, cysteine-rich proteins that, due to their high degree of sequence diversity, still represents a "black hole" in Evolutionary Biology. We have identified and analyzed more than 160 new MTs in nonvertebrate chordates (especially in 37 species of ascidians, 4 thaliaceans, and 3 appendicularians) showing that prototypic tunicate MTs are mono-modular proteins with a pervasive preference for cadmium ions, whereas vertebrate and cephalochordate MTs are bimodular proteins with diverse metal preferences. These structural and functional differences imply a complex evolutionary history of chordate MTs-including de novo emergence of genes and domains, processes of convergent evolution, events of gene gains and losses, and recurrent amplifications of functional domains-that would stand for an unprecedented case in the field of protein evolution.

Modularity in Protein Evolution: Modular Organization and De Novo Domain Evolution in Mollusk Metallothioneins

Metallothioneins (MTs) are proteins devoted to the control of metal homeostasis and detoxification, and therefore, MTs have been crucial for the adaptation of the living beings to variable situations of metal bioavailability. The evolution of MTs is, however, not yet fully understood, and to provide new insights into it, we have investigated the MTs in the diverse classes of Mollusks. We have shown that most molluskan MTs are bimodular proteins that combine six domains—α, β1, β2, β3, γ, and δ—in a lineage-specific manner. We have functionally characterized the Neritimorpha β₃β₁ and the Patellogastropoda γβ₁ MTs, demonstrating the metal-binding capacity of the new γ domain. Our results have revealed a modular organization of mollusk MT, whose evolution has been impacted by duplication, loss, and de novo emergence of domains. MTs represent a paradigmatic example of modular evolution probably driven by the structural and functional requirements of metal binding.

The Enigmatic Metallothioneins: A Case of Upward-Looking Research

In the mid-1950s, Bert Lester Vallee and his colleague Marvin Margoshes discovered a molecule referred to today as metallothionein (MT). Meanwhile, MTs have been shown to be common in many biological organisms. Despite their prevalence, however, it remains unclear to date what exactly MTs do and how they contribute to the biological function of an organism or organ. We investigate why biochemical research has not yet been able to pinpoint the function(s) of MTs. We shall systematically examine both the discovery of and recent research on Dr. Vallee's beloved family of MT proteins utilizing tools from philosophy of science. Our analysis highlights that Vallee's initial work exhibited features prototypical of a developing research tradition: it was upward-looking, exploratory, and utilized mere interactions. Since the 1960s, MT research has increasingly become intervention- and hypothesis-based while it remained largely upward-looking in character. Whilst there is no reason to think that upward-looking research cannot successfully yield structure-function mappings, it has not yet been successful in the case of MTs. Thus, we suggest it might be time to change track and consider other research strategies looking into the evolution of MTs. Recent studies in mollusks render research in this direction worthy of pursuit.

Comparative transcriptome analysis uncovers roles of hydrogen sulfide for alleviating cadmium toxicity in Tetrahymena thermophila

Background

Cadmium (Cd) is a nonessential heavy metal with potentially deleterious effects on different organisms. The organisms have evolved sophisticated defense system to alleviate heavy metal toxicity. Hydrogen sulfide (H₂S) effectively alleviates heavy metal toxicity in plants and reduces oxidative stress in mammals. However, the function of H₂S for alleviating heavy metal toxicity in aquatic organisms remains less clear. Tetrahymena thermophila is an important model organism to evaluate toxic contaminants in an aquatic environment. In this study, the molecular roles of exogenously H₂S application were explored by RNA sequencing under Cd stress in T. thermophila.

Results

The exposure of 30 μM Cd resulted in T. thermophila growth inhibition, cell nigrescence, and malondialdehyde (MDA) content considerably increase. However, exogenous NaHS (donor of H₂S, 70 μM) significantly alleviated the Cd-induced toxicity by inhibiting Cd absorbtion, promoting CdS nanoparticles formation and improving antioxidant system. Comparative transcriptome analysis showed that the expression levels of 9152 genes changed under Cd stress (4658 upregulated and 4494 downregulated). However, only 1359 genes were differentially expressed with NaHS treatment under Cd stress (1087 upregulated and 272 downregulated). The functional categories of the differentially expressed genes (DEGs) by gene ontology (GO) revealed that the transcripts involved in the oxidation–reduction process, oxidoreductase activity, glutathione peroxidase activity, and cell redox homeostasis were the considerable enrichments between Cd stress and NaHS treatment under Cd stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) indicated that the carbon metabolism, glutathione metabolism, metabolism of xenobiotics by cytochrome P450, and ABC transporters were significantly differentially expressed components between Cd stress and NaHS treatment under Cd stress in T. thermophila. The relative expression levels of six DEGs were further confirmed through quantitative real-time polymerase chain reaction (qRT-PCR).

Conclusion

NaHS alleviated Cd stress mainly through inhibiting Cd absorbtion, promoting CdS nanoparticles formation, increasing oxidation resistance, and regulation of transport in free-living unicellular T. thermophila. These findings will expand our understanding for H₂S functions in the freshwater protozoa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07337-9.

Impact of metallothionein-knockdown on cisplatin resistance in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is a rare, but aggressive tumor with dismal prognosis. Platinum-based chemotherapy is regularly used as part of multimodality therapy. The expression of metallothioneins (MT) has been identified as a reason for cisplatin resistance, which often leads to early therapy failure or relapse. Thus, knockdown of MT expression may improve response to cisplatin treatment. The MT gene- and protein expression of the MPM-cell lines MSTO-211H, NCI-H2052 and NCI-H2452 and the human fibroblast cell line MRC-5, as well as their sensitivity to cisplatin treatment have been evaluated. Knockdown of MT1A, 1B and 2A expression was induced by RNA interference. MT expression was measured using quantitative real-time PCR. An in vitro Assay based on enzyme activity was used to detect cell viability, necrosis and apoptosis before and after incubation with cisplatin. MT2A gene expression could be detected in all MPM cell lines, showing the highest expression in NCI-H2452 and NCI-H2052, whereas gene expression levels of MT1A and MT1B were low or absent. The immunohistochemically protein expression of MT-I/II reflect MT2A gene expression levels. Especially for MSTO-211H cell presenting low initial MT2A levels, a strong induction of MT2A expression could be observed during cisplatin treatment, indicating a cell line-specific and platin-dependent adaption mechanism. Additionally, a MT2A-dependent cellular evasion of apoptosis during cisplatin could be observed, leading to three different MT based phenotypes. MSTO-211H cells showed lower apoptosis rates at an increased expression level of MT2A after cisplatin treatment (from sixfold to fourfold). NCI-H2052 cells showed no changes in MT2A expression, while apoptosis rate is the highest (8-12-fold). NCI-H2452 cells showed neither changes in alteration rate of MT2A expression nor changes in apoptosis rates, indicating an MT2A-independent resistance mechanism. Knockdown of MT2A expression levels resulted in significantly induced apoptotic rates during cisplatin treatment with strongest induction of apoptosis in each of the MPM cell lines, but in different markedness. A therapeutic meaningful effect of MT2A knockdown and subsequent cisplatin treatment could be observed in MSTO-211H cells. The present study showed MT2A to be part of the underlying mechanism of cisplatin resistance in MPM. Especially in MSTO-211H cells we could demonstrate major effects by knockdown of MT2A expression, verifying our hypothesis of an MT driven resistance mechanism. We could prove the inhibition of MT2A as a powerful tool to boost response rates to cisplatin-based therapy in vitro. These data carry the potential to enhance the clinical outcome and management of MPM in the future.

Diversity, structure and regulation of microbial metallothionein: metal resistance and possible applications in sequestration of toxic metals

Metallothioneins (MTs) are a group of cysteine-rich, universal, low molecular weight proteins distributed widely in almost all major taxonomic groups ranging from tiny microbes to highly organized vertebrates. The primary function of this protein is storage, transportation and binding of metals, which enable microorganisms to detoxify heavy metals. In the microbial world, these peptides were first identified in a cyanobacterium Synechococcus as the SmtA protein which exhibits high affinity towards rising level of zinc and cadmium to preserve metal homeostasis in a cell. In yeast, MTs aid in reserving copper and confer protection against copper toxicity by chelating excess copper ions in a cell. Two MTs, CUP1 and Crs5, originating from Saccharomyces cerevisiae predominantly bind to copper though are capable of binding with zinc and cadmium ions. MT superfamily 7 is found in ciliated protozoa which show high affinity towards copper and cadmium. Several tools and techniques, such as western blot, capillary electrophoresis, inductively coupled plasma, atomic emission spectroscopy and high performance liquid chromatography, have been extensively utilized for the detection and quantification of microbial MTs which are utilized for the efficient remediation and sequestration of heavy metals from a contaminated environment.

Toxicity, Physiological, and Ultrastructural Effects of Arsenic and Cadmium on the Extremophilic Microalga Chlamydomonas acidophila

The cytotoxicity of cadmium (Cd), arsenate (As(V)), and arsenite (As(III)) on a strain of Chlamydomonas acidophila, isolated from the Rio Tinto, an acidic environment containing high metal(l)oid concentrations, was analyzed. We used a broad array of methods to produce complementary information: cell viability and reactive oxygen species (ROS) generation measures, ultrastructural observations, transmission electron microscopy energy dispersive x-ray microanalysis (TEM-XEDS), and gene expression. This acidophilic microorganism was affected differently by the tested metal/metalloid: It showed high resistance to arsenic while Cd was the most toxic heavy metal, showing an LC₅₀ = 1.94 µM. Arsenite was almost four-fold more toxic (LC₅₀= 10.91 mM) than arsenate (LC₅₀ = 41.63 mM). Assessment of ROS generation indicated that both arsenic oxidation states generate superoxide anions. Ultrastructural analysis of exposed cells revealed that stigma, chloroplast, nucleus, and mitochondria were the main toxicity targets. Intense vacuolization and accumulation of energy reserves (starch deposits and lipid droplets) were observed after treatments. Electron-dense intracellular nanoparticle-like formation appeared in two cellular locations: inside cytoplasmic vacuoles and entrapped into the capsule, around each cell. The chemical nature (Cd or As) of these intracellular deposits was confirmed by TEM-XEDS. Additionally, they also contained an unexpected high content in phosphorous, which might support an essential role of poly-phosphates in metal resistance.

Selenium cytotoxicity in Tetrahymena thermophila: New clues about its biological effects and cellular resistance mechanisms

Selenium is an essential micronutrient but at high concentrations can produce severe cytotoxicity and genomic damage. We have evaluated the cytotoxicity, ultrastructural and mitochondrial alterations of the two main selenium inorganic species; selenite and selenate, in the eukaryotic microorganism Tetrahymena thermophila. In this ciliate, selenite is more toxic than selenate. Their LC₅₀ values were calculated as 27.65 μM for Se(IV) and 56.88 mM for Se(VI). Significant levels of peroxides/hydroperoxides are induced under low-moderate selenite or selenate concentrations. Se(VI) exposures induce an immediate mitochondrial membrane depolarization. Selenium treated cells show an intense vacuolization and some of them present numerous discrete and small electrondense particles, probably selenium deposits. Mitochondrial fusion, an intense swelling in peripheral mitochondria and mitophagy are detected in selenium treated cells, especially in those exposed to Se (IV). qRT-PCR analysis of diverse genes, encoding relevant antioxidant enzymes or other proteins, like metallothioneins, involved in an environmental general stress response, have shown that they may be crucial against Se(IV) and/or Se (VI) cytotoxicity.

Effects of gelsemine on oxidative stress and DNA damage responses of Tetrahymena thermophila

Gelsemine is an important toxic substance extracted from Gelsemium elegans, which has a lot of biological functions in cells and organisms, but its toxicity has been rarely reported in Tetrahymena thermophila. In this study, we used the protozoan T. thermophila as an experimental model to investigate the potential toxicity-induced mechanism of gelsemine in the unicellular eukaryote. Our results clearly showed gelsemine inhibited T. thermophila growth in a dose-dependent manner. This exposure also resulted in oxidative stress on T. thermophila cells and antioxidant enzyme levels were significantly altered at high gelsemine levels (p < 0.05). Gelsemine produced a slight apoptotic effect at the highest (0.8 mg/mL) gelsemine level used here (p < 0.05). Furthermore, the toxin-induced DNA damage in a dose-dependent manner. The ultrastructural analysis also revealed mitophagic vacuoles at 0.4 and 0.8 mg/mL levels of gelsemine exposure. Moreover, expressions of oxidative stress-related and MAP kinase genes were significantly changed after exposure to 0.8 mg/mL level of gelsemine (p < 0.05). Altogether, our results clearly show that gelsemine from G. elegans can inhibit the growth via inducing oxidative stress and DNA damage in T. thermophila cells.

AP-1 (bZIP) Transcription Factors as Potential Regulators of Metallothionein Gene Expression in Tetrahymena thermophila

Metallothioneins (MT) are multi-stress proteins mainly involved in metal detoxification. MT gene expression is normally induced by a broad variety of stimulus and its gene expression regulation mainly occurs at a transcriptional level. Conserved motifs in the Tetrahymena thermophila MT promoters have been described. These motifs show a consensus sequence very similar to AP-1 sites, and bZIP type transcription factors might participate in the MT gene expression regulation. In this research work, we characterize four AP-1 transcription factors in each of four different analyzed Tetrahymena species, detecting a high conservation among them. Each AP-1 molecule has its counterpart in the other three Tetrahymena species. A comparative qRT-PCR analysis of these AP-1 genes have been carried out in different T. thermophila strains (including metal-adapted, knockout and/or knockdown strains among others), and under different metal-stress conditions (1 or 24 h Cd²⁺, Cu²⁺, or Pb²⁺ treatments). The possible interaction of these transcription factors with the conserved AP-1 motifs present in MT promoters has been corroborated by protein-DNA interaction experiments. Certain connection between the expression patterns of the bZIP and MT genes seems to exist. For the first time, and based on our findings, a possible gene expression regulation model including both AP-1 transcription factors and MT genes from the ciliate T. thermophila has been elaborated.

Genome plasticity in response to stress in Tetrahymena thermophila: selective and reversible chromosome amplification and paralogous expansion of metallothionein genes

Extreme stress situations can induce genetic variations including genome reorganization. In ciliates like Tetrahymena thermophila, the approximately 45-fold ploidy of the somatic macronucleus may enable adaptive responses that depend on genome plasticity. To identify potential genome-level adaptations related to metal toxicity, we isolated three Tetrahymena thermophila strains after an extended adaptation period to extreme metal concentrations (Cd²⁺ , Cu²⁺ or Pb²⁺ ). In the Cd-adapted strain, we found a approximately five-fold copy number increase of three genes located in the same macronuclear chromosome, including two metallothionein genes, MTT1 and MTT3. The apparent amplification of this macronuclear chromosome was reversible and reproducible, depending on the presence of environmental metal. We also analysed three knockout (KO) and/or knockdown (KD) strains for MTT1 and/or MTT5. In the MTT5KD strain, we found at least two new genes arising from paralogous expansion of MTT1, which encode truncated variants of MTT1. The expansion can be explained by a model based on somatic recombination between MTT1 genes on pairs of macronuclear chromosomes. At least two of the new paralogs are transcribed and upregulated in response to Cd²⁺ . Altogether, we have thus identified two distinct mechanisms, both involving genomic plasticity in the polyploid macronucleus that may represent adaptive responses to metal-related stress.