Selenocysteine-containing thioredoxin reductase in C. elegans

Functional expression, localization, and biochemical characterization of thioredoxin glutathione reductase from air-breathing magur catfish, Clarias magur

The glutathione (GSH) and thioredoxin (Trx) systems regulate cellular redox homeostasis and maintain antioxidant defense in most eukaryotes. We earlier reported the absence of gene coding for the glutathione reductase (GR) enzyme of the GSH system in the facultative air-breathing catfish, Clarias magur. Here, we identified three thioredoxin reductase (TrxR) genes, one of which was later confirmed as a thioredoxin glutathione reductase (TGR). We then characterized the novel recombinant TGR enzyme of C. magur (CmTGR). The tissue-specific expression of the txnrd genes and the tissue-specific activity of the TrxR enzyme were analyzed. The recombinant CmTGR is a dimer of ~133 kDa. The protein showed TrxR activity with 5,5'-diothiobis (2-nitrobenzoic acid) reduction assay with a K_m of 304.40 μM and GR activity with a K_m of 58.91 μM. Phylogenetic analysis showed that the CmTGR was related to the TrxRs of fishes and distantly related to the TGRs of platyhelminth parasites. The structural analysis revealed the conserved glutaredoxin active site and FAD- and NADPH-binding sites. To our knowledge, this is the first report of the presence of a TGR in any fish. This unusual presence of TGR in C. magur is crucial as it helps maintain redox homeostasis under environmental stressors-induced oxidative stress.

Treatment of Caenorhabditis elegans with Small Selenium Species Enhances Antioxidant Defense Systems

SCOPE

Small selenium (Se) species play a key role in Se metabolism and act as dietary sources of the essential trace element. However, they are redox-active and trigger pro- and antioxidant responses. As health outcomes are strongly species-dependent, species-specific characteristics of Se compounds are tested in vivo.

METHODS AND RESULTS

In the model organism Caenorhabditis elegans (C. elegans), immediate and sustained effects of selenite, selenomethionine (SeMet), and Se-methylselenocysteine (MeSeCys) are studied regarding their bioavailability, incorporation into proteins, as well as modulation of the cellular redox status. While all tested Se compounds are bioavailable, only SeMet persistently accumulates and is non-specifically incorporated into proteins. However, the protection toward chemically-induced formation of reactive species is independent of the applied Se compound. Increased thioredoxin reductase (TXNRD) activity and changes in mRNA expression levels of antioxidant proteins indicate the activation of cellular defense mechanisms. However, in txnrd-1 deletion mutants, no protective effects of the Se species are observed anymore, which is also reflected by differential gene expression data.

CONCLUSION

Se species protect against chemically-induced reactive species formation. The identified immediate and sustained systemic effects of Se species give rise to speculations on possible benefits facing subsequent periods of inadequate Se intake.

Selenium species-dependent toxicity, bioavailability and metabolic transformations in Caenorhabditis elegans

The essential micronutrient selenium (Se) is required for various systemic functions, but its beneficial range is narrow and overexposure may result in adverse health effects. Additionally, the chemical form of the ingested selenium contributes crucially to its health effects. While small Se species play a major role in Se metabolism, their toxicological effects, bioavailability and metabolic transformations following elevated uptake are poorly understood. Utilizing the tractable invertebrate Caenorhabditis elegans allowed for an alternative approach to study species-specific characteristics of organic and inorganic Se forms in vivo, revealing remarkable species-dependent differences in the toxicity and bioavailability of selenite, selenomethionine (SeMet) and Se-methylselenocysteine (MeSeCys). An inverse relationship was found between toxicity and bioavailability of the Se species, with the organic species displaying a higher bioavailability than the inorganic form, yet being less toxic. Quantitative Se speciation analysis with HPLC/mass spectrometry revealed a partial metabolism of SeMet and MeSeCys. In SeMet exposed worms, identified metabolites were Se-adenosylselenomethionine (AdoSeMet) and Se-adenosylselenohomocysteine (AdoSeHcy), while worms exposed to MeSeCys produced Se-methylselenoglutathione (MeSeGSH) and γ-glutamyl-MeSeCys (γ-Glu-MeSeCys). Moreover, the possible role of the sole selenoprotein in the nematode, thioredoxin reductase-1 (TrxR-1), was studied comparing wildtype and trxr-1 deletion mutants. Although a lower basal Se level was detected in trxr-1 mutants, Se toxicity and bioavailability following acute exposure was indistinguishable from wildtype worms. Altogether, the current study demonstrates the suitability of C. elegans as a model for Se species dependent toxicity and metabolism, while further research is needed to elucidate TrxR-1 function in the nematode.

The glutathione system and the related thiol network in Caenorhabditis elegans

Advances in the field of redox biology have contributed to the understanding of the complexity of the thiol-based system in mediating signal transduction. The redox environment is the overall spatiotemporal balance of oxidation-reduction systems within the integrated compartments of the cell, tissues and whole organisms. The ratio of the reduced to disulfide glutathione redox couple (GSH:GSSG) is a key indicator of the redox environment and its associated cellular health. The reaction mechanisms of glutathione-dependent and related thiol-based enzymes play a fundamental role in the function of GSH as a redox regulator. Glutathione homeostasis is maintained by the balance of GSH synthesis (de novo and salvage pathways) and its utilization through its detoxification, thiol signalling, and antioxidant defence functions via GSH-dependent enzymes and free radical scavenging. As such, GSH acts in concert with the entire redox network to maintain reducing conditions in the cell. Caenorhabditis elegans offers a simple model to facilitate further understanding at the multicellular level of the physiological functions of GSH and the GSH-dependent redox network. This review discusses the C. elegans studies that have investigated glutathione and related systems of the redox network including; orthologs to the protein-encoding genes of GSH synthesis; glutathione peroxidases; glutathione-S-transferases; and the glutaredoxin, thioredoxin and peroxiredoxin systems.

Radioactive 75Se Labeling and Detection of Selenoproteins

The trace element selenium (Se) is incorporated into proteins as the amino acid selenocysteine (Sec), which is cotranslationally inserted into specific proteins in response to a UGA codon. Proteins containing Sec at these specific positions are called selenoproteins. Most selenoproteins function as oxidoreductases, while some serve other important functions. There are 25 known selenoprotein genes in humans and 24 in mice. The use of Sec allows selenoproteins to be detected by a convenient method involving metabolic labeling with ⁷⁵Se. Labeling of cells and whole animals are used for the examination of selenoprotein expression profiles and the investigation of selenoprotein functions. In mammals, nonspecific ⁷⁵Se insertion is very low, and sensitivity and specificity of selenoprotein detection approaches that of Western blotting. This method allows for the examination of selenoprotein expression and Se metabolism in model and non-model organisms. Herein, we describe experimental protocols for analyzing selenoproteins by metabolic labeling with ⁷⁵Se both in vitro and in vivo. As an example, the procedure for metabolic labeling of HEK293T human embryonic kidney cells is described in detail. This approach remains a method of choice for the detection of selenoproteins in diverse settings.

N-γ-(L-Glutamyl)-L-selenomethionine enhances stress resistance and ameliorates aging indicators via the selenoprotein TRXR-1 in Caenorhabditis elegans

SCOPE

Selenium is an essential trace nutrient for human health. This study investigates the organic form of selenium, N-γ-(L-Glutamyl)-L-selenomethionine (Glu-SeMet), for its effects on aging indicators and stress resistance. The role of the selenoprotein TRXR-1 was also evaluated in Caenorhabditis elegans.

METHODS AND RESULTS

Glu-SeMet-treated wild-type N2 worms showed increased survival upon oxidative and thermal stress challenges. However, Glu-SeMet treatment did not extend the lifespan of wild-type N2 C. elegans under normal conditions (p = 0.128 for 0.01 μM and p = 0.799 for 10 μM Glu-SeMet). Under stress conditions, Glu-SeMet significantly increased the survival of wild-type N2 C. elegans, but the phenomenon was absent from trxr-1 null mutant worms. Furthermore, Glu-SeMet treatments significantly ameliorated aging indicators, including body bends, pumping rate, defecation duration, and lipofuscin accumulation in wild-type N2 nematodes. Nevertheless, the ameliorative effects by Glu-SeMet were absent in the trxr-1 null mutant worms.

CONCLUSION

The findings indicate that enhanced stress resistance and improved aging indicators by Glu-SeMet in C. elegans are mediated by the selenoprotein TRXR-1. Glu-SeMet has potential for improving health and also provides new insights into selenium's regulatory mechanisms in intact organisms.

Antagonistic Growth Effects of Mercury and Selenium in Caenorhabditis elegans Are Chemical-Species-Dependent and Do Not Depend on Internal Hg/Se Ratios

The relationship between mercury (Hg) and selenium (Se) toxicity is complex, with coexposure reported to reduce, increase, and have no effect on toxicity. Different interactions may be related to chemical compound, but this has not been systematically examined. Our goal was to assess the interactive effects between the two elements on growth in the nematode Caenorhabditis elegans, focusing on inorganic and organic Hg (HgCl2 and MeHgCl) and Se (selenomethionine, sodium selenite, and sodium selenate) compounds. We utilized aqueous Hg/Se dosing molar ratios that were either above, below, or equal to 1 and measured the internal nematode total Hg and Se concentrations for the highest concentrations of each Se compound. Observed interactions were complicated, differed between Se and Hg compounds, and included greater-than-additive, additive, and less-than-additive growth impacts. Biologically significant interactions were only observed when the dosing Se solution concentration was 100-25,000 times greater than the dosing Hg concentration. Mitigation of growth impacts was not predictable on the basis of internal Hg/Se molar ratio; improved growth was observed at some internal Hg/Se molar ratios both above and below 1. These findings suggest that future assessments of the Hg and Se relationship should incorporate chemical compound into the evaluation.

Adjustments, extinction, and remains of selenocysteine incorporation machinery in the nematode lineage

Selenocysteine (Sec) is encoded by an UGA codon with the help of a SECIS element present in selenoprotein mRNAs. SECIS-binding protein (SBP2/SCBP-2) mediates Sec insertion, but the roles of its domains and the impact of its deficiency on Sec insertion are not fully understood. We used Caenorhabditis elegans to examine SBP2 function since it possesses a single selenoprotein, thioredoxin reductase-1 (TRXR-1). All SBP2 described so far have an RNA-binding domain (RBD) and a Sec-incorporation domain (SID). Surprisingly, C. elegans SBP2 lacks SID and consists only of an RBD. An sbp2 deletion mutant strain ablated Sec incorporation demonstrating SBP2 essentiality for Sec incorporation. Further in silico analyses of nematode genomes revealed conservation of SBP2 lacking SID and maintenance of Sec incorporation linked to TRXR-1. Remarkably, parasitic plant nematodes lost the ability to incorporate Sec, but retained SecP43, a gene associated with Sec incorporation. Interestingly, both selenophosphate synthetase (SPS) genes are absent in plant parasitic nematodes, while only Cys-containing SPS2 is present in Sec-incorporating nematodes. Our results indicate that C. elegans and the nematode lineage provide key insights into Sec incorporation and the evolution of Sec utilization trait, selenoproteomes, selenoproteins, and Sec residues. Finally, our study provides evidence of noncanonical translation initiation in C. elegans, not previously known for this well-established animal model.

Toxic-selenium and low-selenium transcriptomes in Caenorhabditis elegans: toxic selenium up-regulates oxidoreductase and down-regulates cuticle-associated genes

Selenium (Se) is an element that in trace quantities is both essential in mammals but also toxic to bacteria, yeast, plants and animals, including C. elegans. Our previous studies showed that selenite was four times as toxic as selenate to C. elegans, but that deletion of thioredoxin reductase did not modulate Se toxicity. To characterize Se regulation of the full transcriptome, we conducted a microarray study in C. elegans cultured in axenic media supplemented with 0, 0.05, 0.1, 0.2, and 0.4 mM Se as selenite. C. elegans cultured in 0.2 and 0.4 mM Se displayed a significant delay in growth as compared to 0, 0.05, or 0.1 mM Se, indicating Se-induced toxicity, so worms were staged to mid-L4 larval stage for these studies. Relative to 0.1 mM Se treatment, culturing C. elegans at these Se concentrations resulted in 1.9, 9.7, 5.5, and 2.3%, respectively, of the transcriptome being altered by at least 2-fold. This toxicity altered the expression of 295 overlapping transcripts, which when filtered against gene sets for sulfur and cadmium toxicity, identified a dataset of 182 toxic-Se specific genes that were significantly enriched in functions related to oxidoreductase activity, and significantly depleted in genes related to structural components of collagen and the cuticle. Worms cultured in low Se (0 mM Se) exhibited no signs of deficiency, but low Se was accompanied by a transcriptional response of 59 genes changed ≥2-fold when compared to all other Se concentrations, perhaps due to decreases in Se-dependent TRXR-1 activity. Overall, these results suggest that Se toxicity in C. elegans causes an increase in ROS and stress responses, marked by increased expression of oxidoreductases and reduced expression of cuticle-associated genes, which together underlie the impaired growth observed in these studies.

A mechanistic investigation of the C-terminal redox motif of thioredoxin reductase from Plasmodium falciparum

High-molecular mass thioredoxin reductases (TRs) are pyridine nucleotide disulfide oxidoreductases that catalyze the reduction of the disulfide bond of thioredoxin (Trx). Trx is responsible for reducing multiple protein disulfide targets in the cell. TRs utilize reduced β-nicotinamide adenine dinucleotide phosphate to reduce a bound flavin prosthetic group, which in turn reduces an N-terminal redox center that has the conserved sequence C_ICVNVGC_CT, where C_IC is denoted as the interchange thiol while the thiol involved in charge-transfer complexation is denoted as C_CT. The reduced N-terminal redox center reduces a C-terminal redox center on the opposite subunit of the head-to-tail homodimer, the C-terminal redox center that catalyzes the reduction of the Trx-disulfide. Variations in the amino acid sequence of the C-terminal redox center differentiate high-molecular mass TRs into different types. Type Ia TRs have tetrapeptide C-terminal redox centers of with a GCUG sequence, where U is the rare amino acid selenocysteine (Sec), while the tetrapeptide sequence in type Ib TRs has its Sec residue replaced with a conventional cysteine (Cys) residue and can use small polar amino acids such as serine and threonine in place of the flanking glycine residues. The TR from Plasmodium falciparum (PfTR) is similar in structure and mechanism to type Ia and type Ib TRs except that the C-terminal redox center is different in its amino acid sequence. The C-terminal redox center of PfTR has the sequence G₅₃₄CGGGKCG₅₄₁, and we classify it as a type II high-molecular mass TR. The oxidized type II redox motif will form a 20-membered disulfide ring, whereas the absence of spacer amino acids in the type I motif results in the formation of a rare eight-membered ring. We used site-directed mutagenesis and protein semisynthesis to investigate features of the distinctive type II C-terminal redox motif that help it perform catalysis. Deletion of Gly541 reduces thioredoxin reductase activity by ∼50-fold, most likely because of disruption of an important hydrogen bond between the amide NH group of Gly541 and the carbonyl of Gly534 that helps to stabilize the β–turn−β motif. Alterations of the 20-membered disulfide ring either by amino acid deletion or by substitution resulted in impaired catalytic activity. Subtle changes in the ring structure and size caused by using semisynthesis to substitute homocysteine for cysteine also caused significant reductions in catalytic activity, demonstrating the importance of the disulfide ring’s geometry in making the C-terminal redox center reactive for thiol–disulfide exchange. The data suggested to us that the transfer of electrons from the N-terminal redox center to the C-terminal redox center may be rate-limiting. We propose that the transfer of electrons from the N-terminal redox center in PfTR to the type II C-terminal disulfide is accelerated by the use of an “electrophilic activation” mechanism. In this mechanism, the type II C-terminal disulfide is polarized, making the sulfur atom of Cys540 electron deficient, highly electrophilic, and activated for thiol–disulfide exchange with the N-terminal redox center. This hypothesis was investigated by constructing chimeric PfTR mutant enzymes containing C-terminal type I sequences GCCG and GCUG, respectively. The PfTR-GCCG chimera had 500-fold less thioredoxin reductase activity than the native enzyme but still reduced selenocystine and lipoic acid efficiently. The PfTR-GCUG chimera had higher catalytic activity than the native enzyme with Trx, selenocystine, and lipoic acid as substrates. The results suggested to us that (i) Sec in the mutant enzyme accelerated the rate of thiol–disulfide exchange between the N- and C-terminal redox centers, (ii) the type II redox center evolved for efficient catalysis utilizing Cys instead of Sec, and (iii) the type II redox center of PfTR is partly responsible for substrate recognition of the cognate PfTrx substrate relative to noncognate thioredoxins.

Why is mammalian thioredoxin reductase 1 so dependent upon the use of selenium?

Cytosolic thioredoxin reductase 1 (TR1) is the best characterized of the class of high-molecular weight (Mr) thioredoxin reductases (TRs). TR1 is highly dependent upon the rare amino acid selenocysteine (Sec) for the reduction of thioredoxin (Trx) and a host of small molecule substrates, as mutation of Sec to cysteine (Cys) results in a large decrease in catalytic activity for all substrate types. Previous work in our lab and others has shown that the mitochondrial TR (TR3) is much less dependent upon the use of Sec for the reduction of small molecules. The Sec-dependent substrate utilization behavior of TR1 may be the exception and not the rule as we show that a variety of high-Mr TRs from other organisms, including Drosophila melanogaster, Caenorhabditis elegans, and Plasmodium falciparum, do not require Sec to reduce small molecule substrates, including 5,5'-dithiobis(2-nitrobenzoic acid), lipoic acid, selenite, and selenocystine. The data show that high-Mr TRs can be divided into two groups based upon substrate utilization patterns: a TR1 group and a TR3-like group. We have constructed mutants of TR3-like enzymes from mouse, D. melanogaster, C. elegans, and P. falciparum, and the kinetic data from these mutants show that these enzymes are less dependent upon the use of Sec for the reduction of substrates. We posit that the mechanistic differences between TR1 and the TR3-like enzymes in this study are due to the presence of a "guiding bar", amino acids 407-422, found in TR1, but not TR3-like enzymes. The guiding bar, proposed by Becker and co-workers [Fritz-Wolf, K., Urig, S., and Becker, K. (2007) The structure of human thioredoxin reductase 1 provides insights into C-terminal rearrangements during catalysis. J. Mol. Biol. 370, 116-127], restricts the motion of the C-terminal tail containing the C-terminal Gly-Cys-Sec-Gly, redox active tetrapeptide so that only this C-terminal redox center can be reduced by the N-terminal redox center, with the exclusion of most other substrates. This makes TR1 highly dependent upon the use of Sec because the selenium atom is responsible for both accepting electrons from the N-terminal redox center and donating them to the substrate in this model. Loss of both Se-electrophilicity and Se-nucleophilicity in the Sec → Cys mutant of TR1 greatly reduces catalytic activity. TR3-like enzymes, in contrast, are less dependent upon the use of Sec because the absence of the guiding bar in these enzymes allows for greater access of the substrate to the N-terminal redox center and because they can make use of alternative mechanistic pathways that are not available to TR1.

The neurodegenerative effects of selenium are inhibited by FOXO and PINK1/PTEN regulation of insulin/insulin-like growth factor signaling in Caenorhabditis elegans

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Insulin/insulin-like signaling reduction alters selenium-induced neurodegeneration.

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Selenium induces nuclear translocation of DAF-16/FOXO3a.

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DAF-16 overexpression decreases GABAergic and cholinergic motor neuron degeneration.

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Loss of DAF-18/PTEN increases sensitivity to selenium-induced movement deficits.

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Glutathione requires DAF-18/PINK-1 to improve selenium-induced movement deficits.

Exposures to high levels of environmental selenium have been associated with motor neuron disease in both animals and humans and high levels of selenite have been identified in the cerebrospinal fluid of patients with amyotrophic lateral sclerosis (ALS). We have shown previously that exposures to high levels of sodium selenite in the environment of Caenorhabditis elegans adult animals can induce neurodegeneration and cell loss resulting in motor deficits and death and that this is at least partially caused by a reduction in cholinergic signaling across the neuromuscular junction. Here we provide evidence that reduction in insulin/insulin-like (IIS) signaling alters response to high dose levels of environmental selenium which in turn can regulate the IIS pathway. Most specifically we show that nuclear localization and thus activation of the DAF-16/forkhead box transcription factor occurs in response to selenium exposure although this was not observed in motor neurons of the ventral cord. Yet, tissue specific expression and generalized overexpression of DAF-16 can partially rescue the neurodegenerative and behavioral deficits observed with high dose selenium exposures in not only the cholinergic, but also the GABAergic motor neurons. In addition, two modifiers of IIS signaling, PTEN (phosphatase and tensin homolog, deleted on chromosome 10) and PINK1 (PTEN-induced putative kinase 1) are required for the cellular antioxidant reduced glutathione to mitigate the selenium-induced movement deficits. Studies have suggested that environmental exposures can lead to ALS or other neurological diseases and this model of selenium-induced neurodegeneration developed in a genetically tractable organism provides a tool for examining the combined roles of genetics and environment in the neuro-pathologic disease process.

Seleno- and telluro-xylofuranosides attenuate Mn-induced toxicity in C. elegans via the DAF-16/FOXO pathway

Organochalcogens are promising pharmacological agents that possess significant biological activities. Nevertheless, because of the complexity of mammalian models, it has been difficult to determine the molecular pathways and specific proteins that are modulated in response to treatments with these compounds. The nematode worm Caenorhabditis elegans is an alternative experimental model that affords easy genetic manipulations, green fluorescent protein tagging and in vivo live analysis of toxicity. Abundant evidence points to oxidative stress in mediating manganese (Mn)-induced toxicity. In this study we challenged worms with Mn, and investigated the efficacy of inedited selenium- and tellurium-xylofuranosides in reversing and/or protecting the worms from Mn-induced toxicity. In addition, we investigated their putative mechanism of action. First, we determined the lethal dose 50% (LD50) and the effects of the xylofuranosides on various toxic parameters. This was followed by studies on the ability of xylofuranosides to afford protection against Mn-induced toxicity. Both Se- and Te-xylofuranosides increased the expression of superoxide dismutase (SOD-3). Furthermore, we observed that the xylofuranosides induced nuclear translocation of the transcription factor DAF-16/FOXO, which in the worm is known to regulate stress responsiveness, aging and metabolism. These findings suggest that xylofuranosides attenuate toxicity Mn-induced, by regulating the DAF-16/FOXO signaling pathway.

Deletion of thioredoxin reductase and effects of selenite and selenate toxicity in Caenorhabditis elegans

Thioredoxin reductase-1 (TRXR-1) is the sole selenoprotein in C. elegans, and selenite is a substrate for thioredoxin reductase, so TRXR-1 may play a role in metabolism of selenium (Se) to toxic forms. To study the role of TRXR in Se toxicity, we cultured C. elegans with deletions of trxr-1, trxr-2, and both in axenic media with increasing concentrations of inorganic Se. Wild-type C. elegans cultured for 12 days in Se-deficient axenic media grow and reproduce equivalent to Se-supplemented media. Supplementation with 0-2 mM Se as selenite results in inverse, sigmoidal response curves with an LC50 of 0.20 mM Se, due to impaired growth rather than reproduction. Deletion of trxr-1, trxr-2 or both does not modulate growth or Se toxicity in C. elegans grown axenically, and (75)Se labeling showed that TRXR-1 arises from the trxr-1 gene and not from bacterial genes. Se response curves for selenide (LC50 0.23 mM Se) were identical to selenite, but selenate was 1/4(th) as toxic (LC50 0.95 mM Se) as selenite and not modulated by TRXR deletion. These nutritional and genetic studies in axenic media show that Se and TRXR are not essential for C. elegans, and that TRXR alone is not essential for metabolism of inorganic Se to toxic species.

Protective efficacy of selenite against lead-induced neurotoxicity in Caenorhabditis elegans

Background

Selenium is an essential micronutrient that has a narrow exposure window between its beneficial and toxic effects. This study investigated the protective potential of selenite (IV) against lead (Pb(II))-induced neurotoxicity in Caenorhabditis elegans.

Principal Findings

The results showed that Se(IV) (0.01 µM) pretreatment ameliorated the decline of locomotion behaviors (frequencies of body bends, head thrashes, and reversal ) of C. elegans that are damaged by Pb(II) (100 µM) exposure. The intracellular ROS level of C. elegans induced by Pb(II) exposure was significantly lowered by Se(IV) supplementation prior to Pb(II) exposure. Finally, Se(IV) protects AFD sensory neurons from Pb(II)-induced toxicity.

Conclusions

Our study suggests that Se(IV) has protective activities against Pb(II)-induced neurotoxicity through its antioxidant property.

Identification of GR and TrxR systems in Setaria cervi: Purification and characterization of glutathione reductase

The glutathione reductase (GR) and thioredoxin reductase (TrxR) are important enzymes of the redox system that aid parasites to maintain an adequate intracellular redox environment. In the present study, the enzyme activity of GR and TrxR was investigated in Setaria cervi (S. cervi). Significant activity of both enzymes was detected in the somatic extract of adult and microfilariae stages of S. cervi. Both GR and TrxR were separated by partial purification using ammonium sulfate fractionation and DEAE ion exchange chromatography suggesting the presence of both glutathione and thioredoxin systems in S. cervi. The enzyme glutathione reductase (ScGR) was purified to homogeneity using affinity and ion exchange chromatography that resulted in 90 fold purification with a yield of 11.54%. The specific activity of the ScGR was 643U/mg that migrated as a single band on SDS-PAGE. The subunit molecular mass was determined to be ~50kDa while the optimum pH and temperature were found to be 7.0 and 35°C respectively. The activation energy (Ea) was calculated from the slope of Arrhenius plot as 16.29±1.40kcal/mol. The Km and Vmax were determined to be 0.27±0.045mM; 30.30±1.30U/ml with NADPH and 0.59±0.060mM; 4.16±0.095U/ml with GSSG respectively. DHBA, a specific inhibitor for GR has completely inhibited the enzyme activity at 1μM concentration. The inhibition of ScGR activity with NAI (IC50 0.71mM), NEM (IC50 0.50mM) and DEPC (IC50 0.27mM) suggested the presence of tyrosine, cysteine and histidine residues at its active site. Further studies on characterization and understanding of these antioxidant enzymes may lead to designing of an effective drug against lymphatic filariasis.

Two thioredoxin reductases, trxr-1 and trxr-2, have differential physiological roles in Caenorhabditis elegans

Thioredoxin reductase (TrxR) is a member of the pyridine nucleotide-disulfide reductase family, which mainly functions in the thioredoxin system. TrxR is found in all living organisms and exists in two major ubiquitous isoenzymes in higher eukaryotic cells; One is cytosolic and the other mitochondrial. Mitochondrial TrxR functions to protect mitochondria from oxidative stress, where reactive oxidative species are mainly generated, while cytosolic TrxR plays a role to maintain optimal oxido-reductive status in cytosol. In this study, we report differential physiological functions of these two TrxRs in C. elegans. trxr-1, the cytosolic TrxR, is highly expressed in pharynx, vulva and intestine, whereas trxr-2, the mitochondrial TrxR, is mainly expressed in pharyngeal and body wall muscles. Deficiency of the non-selenoprotein trxr-2 caused defects in longevity and delayed development under stress conditions, while deletion mutation of the selenoprotein trxr-1 resulted in interference in acidification of lysosomal compartment in intestine. Interestingly, the acidification defect of trxr-1(jh143) deletion mutant was rescued, not only by selenocystein-containing wild type TRXR-1, but also cysteine-substituted mutant TRXR-1. Both trxr-1 and trxr-2 were up-regulated when worms were challenged by environmental stress such as heat shock. These results suggest that trxr-1 and trxr-2 function differently at organismal level presumably by their differential sub-cellular localization in C. elegans.

The characterization of the Caenorhabditis elegans mitochondrial thioredoxin system uncovers an unexpected protective role of thioredoxin reductase 2 in β-amyloid peptide toxicity

AIM

Functional in vivo studies on the mitochondrial thioredoxin system are hampered by the embryonic or larval lethal phenotypes displayed by murine or Drosophila knock-out models. Thus, the access to alternative metazoan knock-out models for the mitochondrial thioredoxin system is of critical importance.

RESULTS

We report here the characterization of the mitochondrial thioredoxin system of Caenorhabditis elegans that is composed of the genes trx-2 and trxr-2. We demonstrate that the proteins thioredoxin 2 (TRX-2) and thioredoxin reductase 2 (TRXR-2) localize to the mitochondria of several cells and tissues of the nematode and that trx-2 and trxr-2 are upregulated upon induction of the mitochondrial unfolded protein response. Surprisingly, C. elegans trx-2 (lof ) and trxr-2 (null) single and double mutants are viable and display similar growth rates as wild-type controls. Moreover, the lack of the mitochondrial thioredoxin system does not affect longevity, reactive oxygen species production or the apoptotic program. Interestingly, we found a protective role of TRXR-2 in a transgenic nematode model of Alzheimer's disease (AD) that expresses human β-amyloid peptide and causes an age-dependent progressive paralysis. Hence, trxr-2 downregulation enhanced the paralysis phenotype, while a strong decrease of β-amyloid peptide and amyloid deposits occurred when TRXR-2 was overexpressed.

INNOVATION

C. elegans provides the first viable metazoan knock-out model for the mitochondrial thioredoxin system and identifies a novel role of this system in β-amyloid peptide toxicity and AD.

CONCLUSION

The nematode strains characterized in this work make C. elegans an ideal model organism to study the pathophysiology of the mitochondrial thioredoxin system at the level of a complete organism.

Measurement of thioredoxin and thioredoxin reductase

The thioredoxin system is ubiquitous, providing reducing equivalents to essential biosynthetic enzymes like ribonucleotide reductase. It is essential for cellular redox regulation, control of oxidative stress, and protection against oxidative damage. This unit includes protocols for measuring thioredoxin or thioredoxin reductase in biological preparations or as purified enzymes.

Selenoprotein TRXR-1 and GSR-1 are essential for removal of old cuticle during molting in Caenorhabditis elegans

Selenoproteins, in particular thioredoxin reductase, have been implicated in countering oxidative damage occurring during aging but the molecular functions of these proteins have not been extensively investigated in different animal models. Here we demonstrate that TRXR-1 thioredoxin reductase, the sole selenoprotein in Caenorhabditis elegans, does not protect against acute oxidative stress but functions instead together with GSR-1 glutathione reductase to promote the removal of old cuticle during molting. We show that the oxidation state of disulfide groups in the cuticle is tightly regulated during the molting cycle, and that when trxr-1 and gsr-1 function is reduced, disulfide groups in the cuticle remain oxidized. A selenocysteine-to-cysteine TRXR-1 mutant fails to rescue molting defects. Furthermore, worms lacking SELB-1, the C. elegans homolog of Escherichia coli SelB or mammalian EFsec, a translation elongation factor known to be specific for selenocysteine in E. coli, fail to incorporate selenocysteine, and display the same phenotype as those lacking trxr-1. Thus, TRXR-1 function in the reduction of old cuticle is strictly selenocysteine dependent in the nematode. Exogenously supplied reduced glutathione reduces disulfide groups in the cuticle and induces apolysis, the separation of old and new cuticle, strongly suggesting that molting involves the regulated reduction of cuticle components driven by TRXR-1 and GSR-1. Using dauer larvae, we demonstrate that aged worms have a decreased capacity to molt, and decreased expression of GSR-1. Together, our results establish a function for the selenoprotein TRXR-1 and GSR-1 in the removal of old cuticle from the surface of epidermal cells.

Reactive Oxygen Species and Aging in Caenorhabditis elegans: Causal or Casual Relationship?

The free radical theory of aging proposes a causal relationship between reactive oxygen species (ROS) and aging. While it is clear that oxidative damage increases with age, its role in the aging process is uncertain. Testing the free radical theory of aging requires experimentally manipulating ROS production or detoxification and examining the resulting effects on lifespan. In this review, we examine the relationship between ROS and aging in the genetic model organism Caenorhabditis elegans, summarizing experiments using long-lived mutants, mutants with altered mitochondrial function, mutants with decreased antioxidant defenses, worms treated with antioxidant compounds, and worms exposed to different environmental conditions. While there is frequently a negative correlation between oxidative damage and lifespan, there are many examples in which they are uncoupled. Neither is resistance to oxidative stress sufficient for a long life nor are all long-lived mutants more resistant to oxidative stress. Similarly, sensitivity to oxidative stress does not necessarily shorten lifespan and is in fact compatible with long life. Overall, the data in C. elegans indicate that oxidative damage can be dissociated from aging in experimental situations.

The ameliorative and toxic effects of selenite on Caenorhabditis elegans

Selenium is an essential trace nutrient that has a narrow exposure window between its beneficial and detrimental effects. We investigated how selenium affected the development, fertility, and cholinergic signaling of the nematode, Caenorhabditis elegans. Our results showed that selenite supplementation at 0.01 and 0.05 μM accelerated development and increased the brood size, while the addition of 20 μM selenite retarded the developmental rate and decreased the brood size. We also showed that the 0.01 μM selenite-pretreated nematodes were more resistant to paralysis induced by an acetylcholinesterase inhibitor, aldicarb, and a nicotinic acetylcholine receptor agonist, levamisole, compared to untreated worms. In contrast, 20 μM selenite-pretreated animals were more sensitive to aldicarb- and levamisole-induced paralysis compared to untreated worms. We measured the internal selenium in supplemented worms using inductively coupled plasma atomic emission spectroscopy, and the data obtained suggested that selenite added to growth medium was taken up by the worms. Taken together, these results suggest that selenite exerts both ameliorative and toxic effects on C.elegans, depending on the amount. Our investigations here thus reinforce our understanding of the ameliorative and toxic effects of selenium on development, reproduction, and cholinergic signaling.

The glutaredoxin GLRX-21 functions to prevent selenium-induced oxidative stress in Caenorhabditis elegans

Selenium is an essential micronutrient that functions as an antioxidant. Yet, at higher concentrations, selenium is pro-oxidant and toxic. In extreme cases, exposures to excess selenium can lead to death or selenosis, a syndrome characterized by teeth, hair and nail loss, and nervous system alterations. Recent interest in selenium as an anti- tumorigenic agent has reemphasized the need to understand the mechanisms underlying the cellular consequences of increased selenium exposure. We show here, that in the nematode, Caenorhabditis elegans, selenium has a concentration range in which it functions as an antioxidant, but beyond this range it exhibits a dose- and time-dependent lethality. Oxidation-induced fluorescence emitted by the dye, carboxy-H(2)DCFDA, indicative of reactive oxygen species formation was significantly higher in animals after a brief exposure to 5mM sodium selenite. Longer-term exposures lead to a progressive selenium-induced motility impairment that could be partially prevented by coincident exposure to the cellular antioxidant-reduced glutathione. The C elegans glrx-21 gene belongs to the family of glutaredoxins (glutathione-dependent oxidoreductases) and the glrx-21(tm2921) allele is a null mutation that renders animals hypersensitive for the selenium-induced motility impairment, but not lethality. In addition, the lethality of animals with the tm2921 mutation exposed to selenium was unaffected by the addition of reduced glutathione, suggesting that GLRX-21 is required for glutathione to moderate this selenium-induced lethality. Our findings provide the first description of selenium-induced toxicity in C elegans and support its use as a model for elucidating the mechanisms of selenium toxicity.

Reactive oxygen species: A radical role in development?

Reactive oxygen species (ROS), mostly derived from mitochondrial activity, can damage various macromolecules and consequently cause cell death. This ROS activity has been characterized in vitro, and correlative evidence suggests a role in various pathological conditions. In addition to this passive ROS activity, ROS also participate in cell signaling processes, though the relevance of this function in vivo is poorly understood. Throughout development, elevated cell activity is probably accompanied by highly active metabolism and, consequently, the production of large amounts of ROS. To allow proper development, cells must protect themselves from these potentially damaging ROS. However, to what degree ROS could participate as signaling molecules controlling fundamental and developmentally relevant cellular processes such as proliferation, differentiation, and death is an open question. Here we discuss why available data do not yet provide conclusive evidence on the role of ROS in development, and we review recent methods to detect ROS in vivo and genetic strategies that can be exploited specifically to resolve these uncertainties.

Nuclear factor-kappa B as a promising target for selenium chemoprevention in rat hepatocarcinogenesis

BACKGROUND AND AIMS

Selenium's molecular mechanism for cancer chemoprevention remains unknown. We aimed to study the gene expression of nuclear factor-kappaB (NF-kappaB), tumor growth factor-alpha (TGF-alpha) and cyclin D1 and the effects of sodium selenite using preventive and therapeutic approaches in chemically-induced hepatocarcinogenesis in rats.

METHODS

Rats were divided randomly into six groups: negative control, positive control (diethyl nitrosamine [DEN] + 2-acetylaminofluorene [2-AAF]), preventive group, preventive control (respective control for preventive group), therapeutic group and therapeutic control (respective control for therapeutic group). The relative gene expression of NF-kappaB, TGF-alpha and cyclin D1 in liver tissues were measured using real-time polymerase chain reaction.

RESULTS

The findings showed that the gene expression of NF-kappaB in the preventive group and its respective control was significantly lower (P < 0.05) when compared with both the negative and positive controls. However, the expression of NF-kappaB in the positive controls and therapeutic group was significantly higher (P < 0.05) when compared with the negative controls. The expression of TGF-alpha and cyclin D1 was insignificant in all groups.

CONCLUSION

The inhibition of the NF-kappaB pathway in the initiation phase of hepatocarcinogenesis could be a promising target for selenium chemoprevention. However, further studies are required.

Differing views of the role of selenium in thioredoxin reductase

This review covers three different chemical explanations that could account for the requirement of selenium in the form of selenocysteine in the active site of mammalian thioredoxin reductase. These views are the following: (1) the traditional view of selenocysteine as a superior nucleophile relative to cysteine, (2) the superior leaving group ability of a selenol relative to a thiol due to its significantly lower pK (a) and, (3) the superior ability of selenium to accept electrons (electrophilicity) relative to sulfur. We term these chemical explanations as the "chemico-enzymatic" function of selenium in an enzyme. We formally define the chemico-enzymatic function of selenium as its specific chemical property that allows a selenoenzyme to catalyze its individual reaction. However we, and others, question whether selenocysteine is chemically necessary to catalyze an enzymatic reaction since cysteine-homologs of selenocysteine-containing enzymes catalyze their specific enzymatic reactions with high catalytic efficiency. There must be a unique chemical reason for the presence of selenocysteine in enzymes that explains the biological pressure on the genome to maintain the complex selenocysteine-insertion machinery. We term this biological pressure the "chemico-biological" function of selenocysteine. We discuss evidence that this chemico-biological function is the ability of selenoenzymes to resist inactivation by irreversible oxidation. The way in which selenocysteine confers resistance to oxidation could be due to the superior ability of the oxidized form of selenocysteine (Sec-SeO(2)(-), seleninic acid) to be recycled back to its parent form (Sec-SeH, selenocysteine) in comparison to the same cycling of cysteine-sulfinic acid to cysteine (Cys-SO(2)(-) to Cys-SH).

Eukaryotic selenoproteins and selenoproteomes

Selenium is an essential trace element for which both beneficial and toxic effects in human health have been described. It is now clear that the importance of having adequate amounts of this micronutrient in the diet is primarily due to the fact that selenium is required for biosynthesis of selenocysteine, the twenty first naturally occurring amino acid in protein. In this review, we provide an overview of eukaryotic selenoproteins and selenoproteomes, which are sets of selenoproteins in these organisms. In eukaryotes, selenoproteins show a mosaic occurrence, with some organisms, such as vertebrates and algae, having dozens of these proteins, while other organisms, such as higher plants and fungi, having lost all selenoproteins during evolution. We also discuss selenoprotein functions and evolutionary trends in the use of these proteins in eukaryotes. Functional analysis of selenoproteins is critical for better understanding of the role of selenium in human health and disease.

Refined annotation and assembly of the Tetrahymena thermophila genome sequence through EST analysis, comparative genomic hybridization, and targeted gap closure

BACKGROUND

Tetrahymena thermophila, a widely studied model for cellular and molecular biology, is a binucleated single-celled organism with a germline micronucleus (MIC) and somatic macronucleus (MAC). The recent draft MAC genome assembly revealed low sequence repetitiveness, a result of the epigenetic removal of invasive DNA elements found only in the MIC genome. Such low repetitiveness makes complete closure of the MAC genome a feasible goal, which to achieve would require standard closure methods as well as removal of minor MIC contamination of the MAC genome assembly. Highly accurate preliminary annotation of Tetrahymena's coding potential was hindered by the lack of both comparative genomic sequence information from close relatives and significant amounts of cDNA evidence, thus limiting the value of the genomic information and also leaving unanswered certain questions, such as the frequency of alternative splicing.

RESULTS

We addressed the problem of MIC contamination using comparative genomic hybridization with purified MIC and MAC DNA probes against a whole genome oligonucleotide microarray, allowing the identification of 763 genome scaffolds likely to contain MIC-limited DNA sequences. We also employed standard genome closure methods to essentially finish over 60% of the MAC genome. For the improvement of annotation, we have sequenced and analyzed over 60,000 verified EST reads from a variety of cellular growth and development conditions. Using this EST evidence, a combination of automated and manual reannotation efforts led to updates that affect 16% of the current protein-coding gene models. By comparing EST abundance, many genes showing apparent differential expression between these conditions were identified. Rare instances of alternative splicing and uses of the non-standard amino acid selenocysteine were also identified.

CONCLUSION

We report here significant progress in genome closure and reannotation of Tetrahymena thermophila. Our experience to date suggests that complete closure of the MAC genome is attainable. Using the new EST evidence, automated and manual curation has resulted in substantial improvements to the over 24,000 gene models, which will be valuable to researchers studying this model organism as well as for comparative genomics purposes.

Variation in the selenoenzyme genes and risk of advanced distal colorectal adenoma

BACKGROUND

Epidemiologic and animal studies provide evidence for a chemopreventive effect of selenium on colorectal cancer, which may be mediated by the antioxidative and anti-inflammatory properties of selenoenzymes. We therefore investigated whether genetic variants in selenoenzymes abundantly expressed in the colon are associated with advanced colorectal adenoma, a cancer precursor.

METHODS

Cases with a left-sided advanced adenoma (n = 772) and matched controls (n = 777) screen negative for polyps based on sigmoidoscopy examination were randomly selected from participants in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The underlying genetic variation was determined by resequencing. We genotyped 44 tagging single nucleotide polymorphisms (SNP) in six genes [glutathione peroxidase 1-4 (GPX1, GPX2, GPX3, and GPX4), selenoprotein P (SEPP1), and thioredoxin reductase 1 (TXNRD1)] to efficiently predict common variation across these genes.

RESULTS

Four variants in SEPP1 were significantly associated with advanced adenoma risk. A rare variant in the 5' region of SEPP1 (-4166C>G) was present in nine cases but in none of the controls (exact P = 0.002). Three SNPs located in the 3' region of SEPP1, which is overlapping with the promoter region of an antisense transcript, were significantly associated with adenoma risk: homozygotes at two SEPP1 loci (31,174 bp 3' of STP A>G and 43,881 bp 3' of STP G>A) were associated with increased adenoma risk [odds ratio (OR), 1.48; 95% confidence interval (95% CI), 1.00-2.19 and OR, 1.53; 95% CI, 1.05-2.22, respectively] and the variant SEPP1 44,321 bp 3' of STP C>T was associated with a reduced adenoma risk (CT versus CC OR, 0.85; 95% CI, 0.63-1.15). Furthermore, we observed a significant 80% reduction for advanced colorectal adenoma risk for carriers of the variant allele at TXNRD1 IVS1-181C>G (OR, 0.20; 95% CI, 0.07-0.55; P trend = 0.004). Consistent with the individual SNP results, we observed a significant overall association with adenoma risk for SEPP1 and TXNRD1 (global P = 0.02 and 0.008, respectively) but not for the four GPX genes.

CONCLUSION

Our study suggests that genetic variants at or near the SEPP1 and TXNRD1 loci may be associated with advanced colorectal adenoma. As this is the first study to comprehensively investigate this hypothesis, confirmation in independent study populations is needed.

Analysis of selenocysteine-containing proteins

Representatives of three primary life domains--bacteria, archaea, and eukaryotes--possess specific selenium-containing proteins. The majority of naturally occurring selenoproteins contain an amino acid, selenocysteine, that is incorporated into protein in response to the code word UGA. The presence of selenium in natural selenoproteins and in proteins in which this element is introduced by chemical or biological manipulations provides additional opportunities for characterizing structure, function, and mechanism of action. This unit provides an overview of known selenocysteine-containing proteins, examples of targeted incorporation of selenium into proteins, and methods specific for selenoprotein identification and characterization.

Identification of acid-base catalytic residues of high-Mr thioredoxin reductase from Plasmodium falciparum

High-M(r) thioredoxin reductase from the malaria parasite Plasmodium falciparum (PfTrxR) contains three redox active centers (FAD, Cys-88/Cys-93, and Cys-535/Cys-540) that are in redox communication. The catalytic mechanism of PfTrxR, which involves dithiol-disulfide interchanges requiring acid-base catalysis, was studied by steady-state kinetics, spectral analyses of anaerobic static titrations, and rapid kinetics analysis of wild-type enzyme and variants involving the His-509-Glu-514 dyad as the presumed acid-base catalyst. The dyad is conserved in all members of the enzyme family. Substitution of His-509 with glutamine and Glu-514 with alanine led to TrxR with only 0.5 and 7% of wild type activity, respectively, thus demonstrating the crucial roles of these residues for enzymatic activity. The H509Q variant had rate constants in both the reductive and oxidative half-reactions that were dramatically less than those of wild-type enzyme, and no thiolateflavin charge-transfer complex was observed. Glu-514 was shown to be involved in dithiol-disulfide interchange between the Cys-88/Cys-93 and Cys-535/Cys-540 pairs. In addition, Glu-514 appears to greatly enhance the role of His-509 in acid-base catalysis. It can be concluded that the His-509-Glu-514 dyad, in analogy to those in related oxidoreductases, acts as the acid-base catalyst in PfTrxR.

Characterization of mitochondrial thioredoxin reductase from C. elegans

Thioredoxin reductase catalyzes the NADPH-dependent reduction of the catalytic disulfide bond of thioredoxin. In mammals and other higher eukaryotes, thioredoxin reductases contain the rare amino acid selenocysteine at the active site. The mitochondrial enzyme from Caenorhabditis elegans, however, contains a cysteine residue in place of selenocysteine. The mitochondrial C. elegans thioredoxin reductase was cloned from an expressed sequence tag and then produced in Escherichia coli as an intein-fusion protein. The purified recombinant enzyme has a kcat of 610 min(-1) and a Km of 610 microM using E. coli thioredoxin as substrate. The reported kcat is 25% of the kcat of the mammalian enzyme and is 43-fold higher than a cysteine mutant of mammalian thioredoxin reductase. The enzyme would reduce selenocysteine, but not hydrogen peroxide or insulin. The flanking glycine residues of the GCCG motif were mutated to serine. The mutants improved substrate binding, but decreased the catalytic rate.

Thioredoxin is related to life span regulation and oxidative stress response in Caenorhabditis elegans

Thioredoxin, an oxidoreductase, is a multifunction protein. The thioredoxin system is composed of NADPH, thioredoxin reductase and thioredoxin. This enzyme is highly conserved from bacteria to humans. We have characterized TRX-1, a thioredoxin homolog in C. elegans, which has about 36% identity in amino acid sequence with human thioredoxin. By gfp reporter system, trx-1 has been shown to be restrictedly expressed in ASI and ASJ neurons and in intestine. Immunostaining confirmed the intestinal expression. Full-length cDNA of trx-1 has been isolated by cDNA library PCR and subsequently cloned and sequenced. We have shown that the encoded protein functions as a reductase in the insulin reducing assay. Moreover, we have isolated a deletion mutant by PCR-based TMP-UV mutagenesis method. Mutant animals have reduced life span and are sensitive to oxidative stress. Reintroduction of trx-1 into mutant worms fully restored the wild-type phenotype. Our results suggest that trx-1 has important functions in life span regulation and oxidative stress response in C. elegans.

Lifespan decrease in a Caenorhabditis elegans mutant lacking TRX-1, a thioredoxin expressed in ASJ sensory neurons

Thioredoxins are a class of small proteins that play a key role in regulating many cellular redox processes. We report here the characterization of the first member of the thioredoxin family in metazoans that is mainly associated with neurons. The Caenorhabditis elegans gene B0228.5 encodes a thioredoxin (TRX-1) that is expressed in ASJ ciliated sensory neurons, and to some extent also in the posterior-most intestinal cells. TRX-1 is active at reducing protein disulfides in the presence of a heterologous thioredoxin reductase. A mutant worm strain carrying a null allele of the trx-1 gene displays a reproducible decrease in both mean and maximum lifespan when compared to wild-type. The identification and characterization of TRX-1 paves the way to use C. elegans as an in vivo model to study the role of thioredoxins in lifespan and nervous system physiology and pathology.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Selenoprotein synthesis: a unique translational mechanism used by a diverse family of proteins

The purpose of this review is to provide an overview of the unique mechanism by which mammalian selenoprotein synthesis occurs. Selenoprotein synthesis requires translational recoding of the UGA codon from a stop signal to a selenocysteine insertion signal (SECIS). Dedicated factors directly involved in this translation process include specific secondary structure in the mRNA (SECIS), a unique tRNA (Sec-tRNA(Sec)), an RNA binding protein (SBP2), and a specialized elongation factor (EFsec). Regulation of this process is discussed along with physiologic and clinical issues regarding selenoprotein synthesis, including the side effects associated with statin drugs.

An HIV-1 encoded peptide mimics the DNA binding loop of NF-kappaB and binds thioredoxin with high affinity

Pro-fs is a human immunodeficiency virus type 1 (HIV-l)-encoded putative selenoprotein, predicted by a theoretical analysis of the viral genome; it is potentially expressed by a -1 frameshift from the protease coding region. Pro-fs has significant sequence similarity to the DNA binding loop of nuclear factor kappa B (NF-kappaB), which is known to bind thioredoxin (Trx). We hypothesize that the putative HIV-1 pro-fs gene product functions by mimicry of NF-kappaB via binding to Trx. The hypothesis was tested in vitro by co-immunoprecipitation and GST-pull down assays, using a purified mutant pro-fs protein, in which the two potential selenocysteine residues were mutated to cysteines, in order to permit expression in bacteria. Both experiments showed that pro-fs binds to human wild type Trx (Trx-wt) with high affinity. Mutation of the two conserved cysteine residues in the Trx active site redox center to serine (Ser) (Trx-CS) weakened but failed to abolish the interaction. In pro-fs-transfected 293T cells, using confocal microscopy and fluorescence resonance energy transfer (FRET), we have observed that pro-fs localizes in cell nuclei and forms oligomers. Upon stimulation by phorbol 12-myristate 13-acetate (PMA), Trx translocates into cell nuclei. Significant FRET efficiency was detected in the nuclei of PMA-stimulated 293T cells co-expressing fluorescence-tagged pro-fs and Trx-wt or Trx-CS. These results indicate that in living cells the double cysteine mutant of pro-fs binds to both Trx and Trx-CS with high affinity, suggesting that Trx-pro-fs binding is a structurally-specific interaction, involving more of the Trx molecule than just its active site cysteine residues. These results establish the capacity for functional mimicry of the Trx binding ability of the NF-kappaB/Rel family of transcription factors by the putative HIV-1 pro-fs protein.

A 35 kDa NAD(P)H oxidase previously isolated from the archaeon Sulfolobus solfataricus is instead a thioredoxin reductase

A thioredoxin reductase (TrxR) has been identified in the hyperthermophilic archaeon Sulfolobus solfataricus (Ss). This enzyme is a homodimeric flavoprotein that was previously identified as NADH oxidase in the same micro-organism ('Biotechnol. Appl. Biochem. 23 (1996) 47'). The primary structure of SsTrxR is made of 323 amino acid residues and contains two putative betaalphabeta regions for the binding of FAD, and a NADP(H) binding consensus sequence in the proximity of a CXXC motif. These findings indicate that SsTrxR is structurally related to the class II of the pyridine nucleotide-disulphide oxidoreductases family. Moreover, the enzyme exhibits a NADP(H) dependent thioredoxin reductase activity requiring the presence of FAD. Surprisingly, the reductase activity of SsTrxR is reduced in the presence of a specific inhibitor of mammalian TrxR. This finding demonstrates that the archaeal enzyme, although structurally related to eubacterial TrxR, is functionally closer to eukaryal enzymes. Experimental evidences indicate that a disulphide bridge is required for the reductase but also for the NADH oxidase activity of the enzyme. These results are further supported by the significantly reduced activities exerted by the C147A mutant. The integrity of the CXXC motif is also involved in the stability of the enzyme.

Selenocysteine in proteins-properties and biotechnological use

Selenocysteine (Sec), the 21st amino acid, exists naturally in all kingdoms of life as the defining entity of selenoproteins. Sec is a cysteine (Cys) residue analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group in Cys. The selenium atom gives Sec quite different properties from Cys. The most obvious difference is the lower pK(a) of Sec, and Sec is also a stronger nucleophile than Cys. Proteins naturally containing Sec are often enzymes, employing the reactivity of the Sec residue during the catalytic cycle and therefore Sec is normally essential for their catalytic efficiencies. Other unique features of Sec, not shared by any of the other 20 common amino acids, derive from the atomic weight and chemical properties of selenium and the particular occurrence and properties of its stable and radioactive isotopes. Sec is, moreover, incorporated into proteins by an expansion of the genetic code as the translation of selenoproteins involves the decoding of a UGA codon, otherwise being a termination codon. In this review, we will describe the different unique properties of Sec and we will discuss the prerequisites for selenoprotein production as well as the possible use of Sec introduction into proteins for biotechnological applications. These include residue-specific radiolabeling with gamma or positron emitters, the use of Sec as a reactive handle for electophilic probes introducing fluorescence or other peptide conjugates, as the basis for affinity purification of recombinant proteins, the trapping of folding intermediates, improved phasing in X-ray crystallography, introduction of 77Se for NMR spectroscopy, or, finally, the analysis or tailoring of enzymatic reactions involving thiol or oxidoreductase (redox) selenolate chemistry.

Nematode selenoproteome: the use of the selenocysteine insertion system to decode one codon in an animal genome?

Selenocysteine (Sec) is co-translationally inserted into selenoproteins in response to codon UGA with the help of the selenocysteine insertion sequence (SECIS) element. The number of selenoproteins in animals varies, with humans having 25 and mice having 24 selenoproteins. To date, however, only one selenoprotein, thioredoxin reductase, has been detected in Caenorhabditis elegans, and this enzyme contains only one Sec. Here, we characterize the selenoproteomes of C.elegans and Caenorhabditis briggsae with three independent algorithms, one searching for pairs of homologous nematode SECIS elements, another searching for Cys- or Sec-containing homologs of potential nematode selenoprotein genes and the third identifying Sec-containing homologs of annotated nematode proteins. These methods suggest that thioredoxin reductase is the only Sec-containing protein in the C.elegans and C.briggsae genomes. In contrast, we identified additional selenoproteins in other nematodes. Assuming that Sec insertion mechanisms are conserved between nematodes and other eukaryotes, the data suggest that nematode selenoproteomes were reduced during evolution, and that in an extreme reduction case Sec insertion systems probably decode only a single UGA codon in C.elegans and C.briggsae genomes. In addition, all detected genes had a rare form of SECIS element containing a guanosine in place of a conserved adenosine present in most other SECIS structures, suggesting that in organisms with small selenoproteomes SECIS elements may change rapidly.