How selenium has altered our understanding of the genetic code

Ribosomal protein L30 is a component of the UGA-selenocysteine recoding machinery in eukaryotes

The translational recoding of UGA as selenocysteine (Sec) is directed by a SECIS element in the 3' untranslated region (UTR) of eukaryotic selenoprotein mRNAs. The selenocysteine insertion sequence (SECIS) contains two essential tandem sheared G.A pairs that bind SECIS-binding protein 2 (SBP2), which recruits a selenocysteine-specific elongation factor and Sec-tRNA(Sec) to the ribosome. Here we show that ribosomal protein L30 is a component of the eukaryotic selenocysteine recoding machinery. L30 binds SECIS elements in vitro and in vivo, stimulates UGA recoding in transfected cells and competes with SBP2 for SECIS binding. Magnesium, known to induce a kink-turn in RNAs that contain two tandem G.A pairs, decreases the SBP2-SECIS complex in favor of the L30-SECIS interaction. We propose a model in which SBP2 and L30 carry out different functions in the UGA recoding mechanism, with the SECIS acting as a molecular switch upon protein binding.

The microbial selenoproteome of the Sargasso Sea

An analysis of the selenoproteome of the largest microbial sequence dataset, the Sargasso Sea environmental genome sequences, identified 310 selenoprotein genes that clustered into 25 families. This included 101 new selenoprotein genes that belonged to 15 families, doubling the number of prokaryotic selenoprotein families.

Background

Selenocysteine (Sec) is a rare amino acid which occurs in proteins in major domains of life. It is encoded by TGA, which also serves as the signal for termination of translation, precluding identification of selenoprotein genes by available annotation tools. Information on full sets of selenoproteins (selenoproteomes) is essential for understanding the biology of selenium. Herein, we characterized the selenoproteome of the largest microbial sequence dataset, the Sargasso Sea environmental genome project.

Results

We identified 310 selenoprotein genes that clustered into 25 families, including 101 new selenoprotein genes that belonged to 15 families. Most of these proteins were predicted redox proteins containing catalytic selenocysteines. Several bacterial selenoproteins previously thought to be restricted to eukaryotes were detected by analyzing eukaryotic and bacterial SECIS elements, suggesting that eukaryotic and bacterial selenoprotein sets partially overlapped. The Sargasso Sea microbial selenoproteome was rich in selenoproteins and its composition was different from that observed in the combined set of completely sequenced genomes, suggesting that these genomes do not accurately represent the microbial selenoproteome. Most detected selenoproteins occurred sporadically compared to the widespread presence of their cysteine homologs, suggesting that many selenoproteins recently evolved from cysteine-containing homologs.

Conclusions

This study yielded the largest selenoprotein dataset to date, doubled the number of prokaryotic selenoprotein families and provided insights into forces that drive selenocysteine evolution.

An algorithm for identification of bacterial selenocysteine insertion sequence elements and selenoprotein genes

MOTIVATION

Incorporation of selenocysteine (Sec) into proteins in response to UGA codons requires a cis-acting RNA structure, Sec insertion sequence (SECIS) element. Whereas SECIS elements in Escherichia coli are well characterized, a bacterial SECIS consensus structure is lacking.

RESULTS

We developed a bacterial SECIS consensus model, the key feature of which is a conserved guanosine in a small apical loop of the properly positioned structure. This consensus was used to build a computational tool, bSECISearch, for detection of bacterial SECIS elements and selenoprotein genes in sequence databases. The program identified 96.5% of known selenoprotein genes in completely sequenced bacterial genomes and predicted several new selenoprotein genes. Further analysis revealed that the size of bacterial selenoproteomes varied from 1 to 11 selenoproteins. Formate dehydrogenase was present in most selenoproteomes, often as the only selenoprotein family, whereas the occurrence of other selenoproteins was limited. The availability of the bacterial SECIS consensus and the tool for identification of these structures should help in correct annotation of selenoprotein genes and characterization of bacterial selenoproteomes.

Recoding elements located adjacent to a subset of eukaryal selenocysteine-specifying UGA codons

Incorporation of the 21st amino acid, selenocysteine, into proteins is specified in all three domains of life by dynamic translational redefinition of UGA codons. In eukarya and archaea, selenocysteine insertion requires a cis-acting selenocysteine insertion sequence (SECIS) usually located in the 3'UTR of selenoprotein mRNAs. Here we present comparative sequence analysis and experimental data supporting the presence of a second stop codon redefinition element located adjacent to a selenocysteine-encoding UGA codon in the eukaryal gene, SEPN1. This element is sufficient to stimulate high-level (6%) translational redefinition of the SEPN1 UGA codon in human cells. Readthrough levels further increased to 12% when tested in the presence of the SEPN1 3'UTR SECIS. Directed mutagenesis and phylogeny of the sequence context strongly supports the importance of a stem loop starting six nucleotides 3' of the UGA codon. Sequences capable of forming strong RNA structures were also identified 3' adjacent to, or near, selenocysteine-encoding UGA codons in the Sps2, SelH, SelO, and SelT selenoprotein genes.

A novel eukaryotic selenoprotein in the haptophyte alga Emiliania huxleyi

The diversity of selenoproteins raises the question of why many life forms require selenium. Especially in photosynthetic organisms, the biochemical basis for the requirement for selenium is unclear because there is little information on selenoproteins. We found six selenium-containing proteins in a haptophyte alga, Emiliania huxleyi, which requires selenium for growth. The 27-kDa protein EhSEP2 was isolated, and its cDNA was cloned. The deduced amino acid sequence revealed that EhSEP2 is homologous to protein disulfide isomerase (PDI) and contains a highly conserved thioredoxin domain. The nucleotide sequence contains an in-frame TGA codon encoding selenocysteine at the position corresponding to the cysteine residue in the reaction center of known PDIs. However, no typical selenocysteine insertion sequence was found in the EhSEP2 cDNA. The EhSEP2 mRNA level was related to the abundance of selenium. E. huxleyi possesses a novel PDI-like selenoprotein and may have a novel type of selenocysteine insertion machinery.

Selective Rescue of Selenoprotein Expression in Mice Lacking a Highly Specialized Methyl Group in Selenocysteine tRNA

Selenocysteine (Sec) is the 21st amino acid in the genetic code. Its tRNA is variably methylated on the 2'-O-hydroxyl site of the ribosyl moiety at position 34 (Um34). Herein, we identified a role of Um34 in regulating the expression of some, but not all, selenoproteins. A strain of knock-out transgenic mice was generated, wherein the Sec tRNA gene was replaced with either wild type or mutant Sec tRNA transgenes. The mutant transgene yielded a tRNA that lacked two base modifications, N(6)-isopentenyladenosine at position 37 (i(6)A37) and Um34. Several selenoproteins, including glutathione peroxidases 1 and 3, SelR, and SelT, were not detected in mice rescued with the mutant transgene, whereas other selenoproteins, including thioredoxin reductases 1 and 3 and glutathione peroxidase 4, were expressed in normal or reduced levels. Northern blot analysis suggested that other selenoproteins (e.g. SelW) were also poorly expressed. This novel regulation of protein expression occurred at the level of translation and manifested a tissue-specific pattern. The available data suggest that the Um34 modification has greater influence than the i(6)A37 modification in regulating the expression of various mammalian selenoproteins and Um34 is required for synthesis of several members of this protein class. Many proteins that were poorly rescued appear to be involved in responses to stress, and their expression is also highly dependent on selenium in the diet. Furthermore, their mRNA levels are regulated by selenium and are subject to nonsense-mediated decay. Overall, this study described a novel mechanism of regulation of protein expression by tRNA modification that is in turn regulated by levels of the trace element, selenium.

A selenocysteine tRNA and SECIS element in Plasmodium falciparum

The molecular machinery for incorporating selenocysteine into proteins is present in both prokaryotes and eukaryotes. Although selenocysteine insertion has been reported in animals, plants, and protozoans, known eukaryotic selenocysteine tRNA sequences and selenocysteine insertion sequences are limited to animals and plants. Here we present clear indications of the presence of selenocysteine-tRNA and a selenocysteine insertion sequence in Plasmodium falciparum. To our knowledge, this is the first report of an identification of protozoan selenocysteine insertion machinery at the sequence level.

Increase in selenium requirements with physical activity loads in well-trained athletes is not linear

Selenium requirements in athletes are supposed to be increased with energy expenditure (EE) to preserve selenium status and an optimal antioxidant balance. The question of whether selenium intakes are related to EE and whether plasma selenium status induces up-regulation in erythrocyte endogenous antioxidant defense and decreases plasma oxidative damage markers in athletes was addressed. 118 well-trained athletes completed 7 d food and activities records in a cross-sectional study. Blood was sampled on day 8. Among the athletes, 23% of the males and 66% of the females had selenium intakes below two-third of the French RDA. Plasma selenium concentrations in most of less trained athletes were lower than the postulated concentration to be required to maximize erythrocyte GSH-Px activity. Athletes with the highest daily EE had the highest selenium intakes, percentage of vegetal protein intakes and plasma selenium concentrations. Only 2.6% of the athletes exhibited low plasma selenium concentrations (< 0.75 micromol/l). The relation between plasma selenium and EE was polynomial (r = 0.50; P < 0.005). Erythrocyte GSH-Px activity in athletes was not linked to selenium status. Selenium requirements are increased in athletes without being linearly related to EE.

A pseudo-SECIS element in Methanococcus voltae documents evolution of a selenoprotein into a sulphur-containing homologue

Methanococcus maripaludis possesses two sets of F(420)-non-reducing hydrogenases which are differentially expressed in response to the selenium content of the medium. One of the subunits of the selenium-containing hydrogenase, VhuD, contains two selenocysteine residues, whereas the homologue of M. voltae possesses cysteine residues in the equivalent positions. Analysis of the 3' non-translated region of the M. voltae vhuD mRNA revealed the existence of a structure resembling the consensus of archaeal SECIS elements but with deviations rendering it non-functional in determining selenocysteine insertion. The presence of a pseudo-SECIS element in the 3' non-translated region of the vhuD mRNA from M. voltae suggests that VhuD from this organism has developed from a selenocysteine-containing ancestor. The 3' non-translated region from the VhcD homologues neither contained a SECIS nor a pseudo SECIS element.

Domain structure of bi-functional selenoprotein P

Human selenoprotein P (SeP), a selenium-rich plasma glycoprotein, is presumed to contain ten selenocysteine residues; one of which is located at the 40th residue in the N-terminal region and the remaining nine localized in the C-terminal third part. We have shown that SeP not only catalyses the reduction of phosphatidylcholine hydroperoxide by glutathione [Saito, Hayashi, Tanaka, Watanabe, Suzuki, Saito and Takahashi (1999) J. Biol. Chem. 274, 2866-2871], but also supplies its selenium to proliferating cells [Saito and Takahashi (2002) Eur. J. Biochem. 269, 5746-5751]. Treatment of SeP with plasma kallikrein resulted in a sequential limited proteolysis (Arg-235-Gln-236 and Arg-242-Asp-243). The N-terminal (residues 1-235) and C-terminal (residues 243-361) fragments exhibited enzyme activity and selenium-supply activity respectively. These results confirm that SeP is a bi-functional protein and suggest that the first selenocysteine residue is the active site of the enzyme and the remaining nine residues function as a selenium supplier.

Expression of selenocysteine-containing glutathione S-transferase in Escherichia coli

Evolution of a probable 'glutathione-binding ancestor' resulting in a common thioredoxin-fold for glutathione S-transferases and glutathione peroxidases may possibly suggest that a glutathione S-transferase could be engineered into a selenium-containing glutathione S-transferase (seleno-GST), having glutathione peroxidase (GPX) activity. Here, we addressed this question by production of such protein. In order to obtain a recombinant seleno-GST produced in Escherichia coli, we introduced a variant bacterial-type selenocysteine insertion sequence (SECIS) element which afforded substitution with selenocysteine for the catalytic Tyr residue in the active site of GST from Schistosoma japonica. Utilizing coexpression with the bacterial selA, selB, and selC genes (encoding selenocysteine synthase, SelB, and tRNA(Sec), respectively) the yield of recombinant seleno-GST was about 2.9 mg/L bacterial culture, concomitant with formation of approximately 85% truncation product as a result of termination of translation at the selenocysteine-encoding UGA codon. The mutations inferred as a result of the introduction of a SECIS element did not affect the glutathione-binding capacity (Km = 53 microM for glutathione as compared to 63 microM for the wild-type enzyme) nor the GST activity (kcat = 14.3 s(-1) vs. 16.6 s(-1)), provided that the catalytic Tyr residue was intact. When this residue was changed to selenocysteine, however, the resulting seleno-GST lost the GST activity. It also failed to display any novel GPX activity towards three standard peroxide substrates (hydrogen peroxide, butyl hydroperoxide or cumene hydroperoxide). These results show that recombinant selenoproteins with internal selenocysteine residues may be heterologously produced in E. coli at sufficient amounts for purification. We also conclude that introduction of a selenocysteine residue into the catalytic site of a glutathione S-transferase is not sufficient to induce GPX activity in spite of a maintained glutathione-binding capacity.

The link between selenium and chemoprevention: a case for selenoproteins

Selenium is effective in reducing cancer incidence in animal models, and epidemiologic data, as well as supplementation trials, have indicated that selenium is likely to be effective in humans. The mechanism by which selenium prevents cancer remains unknown. The mammalian genome encodes 25 selenoprotein genes, each containing one or more molecules of selenium in the form of the amino acid selenocysteine, translationally inserted into the growing peptide in response to the UGA codon. There is evidence that several of these proteins may be involved with the mechanism by which selenium provides its anticancer effects. Data are reviewed indicating that genetic variants of the cytosolic glutathione peroxidase are associated with increased cancer risk, and that loss of one of the copies of this same gene may be involved with malignant progression. Similarly, allelic differences in the gene for a second selenoprotein, Sep15, may be relevant to the protection provided by selenium, and allelic loss at this locus have been reported as well. These data, along with the differential expression patterns reported for other selenoproteins in tumor vs. normal tissues, support the role of selenoproteins in the chemoprotection by selenium.

Selenoproteins and the metabolic features of the archaeal ancestor of eukaryotes

In all three branches of life, some organisms incorporate the rare amino acid selenocysteine. Selenoproteins are relevant to the controversy over the metabolic features of the archaeal ancestor of eukaryotes because among archaea, several known selenoproteins are involved in methanogenesis and autotrophic growth. Although the eukaryotic selenocysteine-specific translation apparatus and at least one selenoprotein appear to be of archaeal origin, selenoproteins have not been identified among sulfur-metabolizing crenarchaeotes. In this regard, both the phylogeny and function of archaeal selenoproteins are consistent with the argument that the archaeal ancestor was a methanogen. Selenium, however, is abundant in sulfur-rich environments, and some anaerobic bacteria reduce sulfur and have selenoproteins similar to those in archaea. As additional archaeal sequence data becomes available, it will be important to determine whether selenoproteins are present in nonmethanogenic archaea, especially the sulfur-metabolizing crenarchaeotes.

Selenoproteins, Cholesterol-Lowering Drugs, and the Consequences Revisiting of the Mevalonate Pathway

3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) and peroxisome proliferator-activated receptor alpha activators (fibrates) are the backbone of pharmacologic hypercholesterolemia and dyslipidemia treatment. Many of their clinical effects, however, are still enigmatic. This article describes how a side road of the mevalonate pathway, characterized in recent years, can rationalize a major fraction of these unexplained observations. This side road is the enzymatic isopentenylation of selenocysteine-tRNA([Ser]Sec) (Sec-tRNA), the singular tRNA to decode the unusual amino acid selenocysteine. The functionally indispensable isopentenylation of Sec-tRNA requires a unique intermediate from the mevalonate pathway, isopentenyl pyrophosphate, which concomitantly constitutes the central building block for cholesterol biosynthesis, and whose formation is suppressed by statins and fibrates. The resultant inhibition of Sec-tRNA isopentenylation profoundly decreases selenoprotein expression. This effect might seamlessly explain the immunosuppressive, redox, endothelial, sympatholytic, and thyroidal effects of statins and fibrates as well as their common side effects and drug interactions.

Selenium and brain function: a poorly recognized liaison

Molecular biology has recently contributed significantly to the recognition of selenium (Se)2 and Se-dependent enzymes as modulators of brain function. Increased oxidative stress has been proposed as a pathomechanism in neurodegenerative diseases including, among others, Parkinson's disease, stroke, and epilepsy. Glutathione peroxidases (GPx), thioredoxin reductases, and one methionine-sulfoxide-reductase are selenium-dependent enzymes involved in antioxidant defense and intracellular redox regulation and modulation. Selenium depletion in animals is associated with decreased activities of Se-dependent enzymes and leads to enhanced cell loss in models of neurodegenerative disease. Genetic inactivation of cellular GPx increases the sensitivity towards neurotoxins and brain ischemia. Conversely, increased GPx activity as a result of increased Se supply or overexpression ameliorates the outcome in the same models of disease. Genetic inactivation of selenoprotein P leads to a marked reduction of brain Se content, which has not been achieved by dietary Se depletion, and to a movement disorder and spontaneous seizures. Here we review the role of Se for the brain under physiological as well as pathophysiological conditions and highlight recent findings which open new vistas on an old essential trace element.

Identification and characterization of phosphoseryl-tRNA[Ser]Sec kinase

In 1970, a kinase activity that phosphorylated a minor species of seryl-tRNA to form phosphoseryl-tRNA was found in rooster liver [Maenpaa, P. H. & Bernfield, M. R. (1970) Proc. Natl. Acad. Sci. USA 67, 688-695], and a minor seryl-tRNA that decoded the nonsense UGA was detected in bovine liver. The phosphoseryl-tRNA and the minor UGA-decoding seryl-tRNA were subsequently identified as selenocysteine (Sec) tRNA[Ser]Sec, but the kinase activity remained elusive. Herein, by using a comparative genomics approach that searched completely sequenced archaeal genomes for a kinase-like protein with a pattern of occurrence similar to that of components of Sec insertion machinery, we detected a candidate gene for mammalian phosphoseryl-tRNA[Ser]Sec kinase (pstk). Mouse pstk was cloned, and the gene product (PSTK) was expressed and characterized. PSTK specifically phosphorylated the seryl moiety on seryl-tRNA[Ser]Sec and, in addition, had a requirement for ATP and Mg2+. Proteins with homology to mammalian PSTK occur in Drosophila, Caenorhabditis elegans, Methanopyrus kandleri, and Methanococcus jannaschii, suggesting a conservation of its function across archaea and eukaryotes that synthesize selenoproteins and the absence of this function in bacteria, plants, and yeast. The fact that PSTK has been highly conserved in evolution suggests that it plays an important role in selenoprotein biosynthesis and/or regulation.

GPx-1 modulates Akt and P70S6K phosphorylation and Gadd45 levels in MCF-7 cells

Selenium has been shown to prevent cancer in animal models, and recent data indicate it is likely to be effective in humans as well. One selenium-containing protein, the cytoplasmic form of glutathione peroxidase (GPx-1), has been implicated in cancer risk and development by genetic studies identifying at-risk alleles and loss of heterozygosity in tumors. In order to evaluate the biological consequences of GPx-1 overexpression, human MCF-7 cells were stably transfected with a GPx-1 expression construct and the effects of GPx-1 on protein kinases associated with stress responses were determined. GPx-1 overexpression affected phosphorylation of p70S6K, whereas Erk1/2 and p38 MAPK were not affected. Site-specific phosphorylation of Akt declined and the levels of Gadd45, a DNA damage response protein, increased significantly as a consequence of elevated GPx-1 expression. Effects on p70S6K and Gadd45 after selenium supplementation have been reported, and given previous data demonstrating a role for GPx-1 in cancer etiology, these results support the concept that the chemopreventive properties of selenium may be due, at least in part, to its role in regulating GPx-1.

IDENTIFICATION OF TRACE ELEMENT–CONTAINING PROTEINS IN GENOMIC DATABASES

Development of bioinformatics tools provided researchers with the ability to identify full sets of trace element-containing proteins in organisms for which complete genomic sequences are available. Recently, independent bioinformatics methods were used to identify all, or almost all, genes encoding selenocysteine-containing proteins in human, mouse, and Drosophila genomes, characterizing entire selenoproteomes in these organisms. It also should be possible to search for entire sets of other trace element-associated proteins, such as metal-containing proteins, although methods for their identification are still in development.

Linked thioredoxin-glutathione systems in platyhelminths

The thioredoxin and glutathione systems play a central role in thiol-disulfide redox homeostasis in many organisms by providing electrons to essential enzymes, and defence against oxidative stress. These systems have recently been characterized in platyhelminth parasites, and the emerging biochemical scenario is the existence of linked processes with the enzyme thioredoxin glutathione reductase supplying reducing equivalents to both pathways. In contrast to their hosts, conventional thioredoxin reductase and glutathione reductase enzymes appear to be absent. Analysis of published data and expressed-sequence tag databases indicates the presence of linked thioredoxin-glutathione systems in the cytosolic and mitochondrial compartments of these parasites.

DIAP1 suppresses ROS-induced apoptosis caused by impairment of the selD/sps1 homolog in Drosophila

The cellular antioxidant defense systems neutralize the cytotoxic by-products referred to as reactive oxygen species (ROS). Among them, selenoproteins have important antioxidant and detoxification functions. The interference in selenoprotein biosynthesis results in accumulation of ROS and consequently in a toxic intracellular environment. The resulting ROS imbalance can trigger apoptosis to eliminate the deleterious cells. In Drosophila, a null mutation in the selD gene (homologous to the human selenophosphate synthetase type 1) causes an impairment of selenoprotein biosynthesis, a ROS burst and lethality. We propose this mutation (known as selDptuf) as a tool to understand the link between ROS accumulation and cell death. To this aim we have analyzed the mechanism by which selDptuf mutant cells become apoptotic in Drosophila imaginal discs. The apoptotic effect of selDptuf does not require the activity of the Ras/MAPK-dependent proapoptotic gene hid, but results in stabilization of the tumor suppressor protein Dmp53 and transcription of the Drosophila pro-apoptotic gene reaper (rpr). We also provide genetic evidence that the initiator caspase DRONC is activated and that the effector caspase DRICE is processed to commit selDptuf mutant cells to death. Moreover, the ectopic expression of the inhibitor of apoptosis DIAP1 rescues the cellular viability of selDptuf mutant cells. These observations indicate that selDptuf ROS-induced apoptosis in Drosophila is mainly driven by the caspase-dependent Dmp53/Rpr pathway.

Regulation of the selenoprotein SelS by glucose deprivation and endoplasmic reticulum stress - SelS is a novel glucose-regulated protein

SelS is a newly identified selenoprotein and its gene expression is up-regulated in the liver of Psammomys obesus after fasting. We have examined whether SelS is regulated by glucose deprivation and endoplasmic reticulum (ER) stress in HepG2 cells. Glucose deprivation and the ER stress inducers tunicamycin and thapsigargin increased SelS gene expression and protein content several-fold in parallel with glucose-regulated protein 78. The overexpression of SelS increased Min6 cell resistance to oxidative stress-induced toxicity. These results indicate that SelS is a novel member of the glucose-regulated protein family and its function is related to the regulation of cellular redox balance.

The prokaryotic selenoproteome

In the genetic code, the UGA codon has a dual function as it encodes selenocysteine (Sec) and serves as a stop signal. However, only the translation terminator function is used in gene annotation programs, resulting in misannotation of selenoprotein genes. Here, we applied two independent bioinformatics approaches to characterize a selenoprotein set in prokaryotic genomes. One method searched for selenoprotein genes by identifying RNA stem-loop structures, selenocysteine insertion sequence elements; the second approach identified Sec/Cys pairs in homologous sequences. These analyses identified all or almost all selenoproteins in completely sequenced bacterial and archaeal genomes and provided a view on the distribution and composition of prokaryotic selenoproteomes. In addition, lineage-specific and core selenoproteins were detected, which provided insights into the mechanisms of selenoprotein evolution. Characterization of selenoproteomes allows interpretation of other UGA codons in completed genomes of prokaryotes as terminators, addressing the UGA dual-function problem.

Efficient selenium transfer from mother to offspring in selenoprotein-P-deficient mice enables dose-dependent rescue of phenotypes associated with selenium deficiency

Mice deficient in selenoprotein P exhibit a disturbed selenium distribution and reduced activities of other selenoenzymes and display defects in growth and motor co-ordination. We have normalized selenoenzyme activities and rescued the phenotype of mutant mice by supplementing their nursing mothers with sodium selenite. Our results indicate that selenium from inorganic sources can be transferred efficiently via mother's milk to the developing offspring in a form that is both highly bioavailable by target tissues and yet sufficiently safe to prevent overdosages.

The neurobiology of selenium: lessons from transgenic mice

The brain represents a privileged organ with respect to selenium (Se) supply and retention. It contains high amounts of this essential trace element, which is efficiently retained even in conditions of Se deficiency. Accordingly, no severe neurological phenotype has been reported for animals exposed to Se-depleted diets. They are, however, more susceptible to neuropathological challenges. Recently, gene disruption experiments supported a pivotal role for different selenoproteins in brain function. Using these and other transgenic models, longstanding questions concerning the preferential supply of Se to the brain and the hierarchy among the different selenoproteins are readdressed. Given that genes for at least 25 selenoproteins have been identified in the human genome, and most of these are expressed in the brain, their specific roles for normal brain function and neurological diseases remain to be elucidated.

Selenoprotein synthesis and side-effects of statins

Statins are possibly the most effective drugs for the prevention and treatment of hypercholesterolaemia and coronary heart disease. They are generally well tolerated, however, they do cause some unusual side-effects with potentially severe consequences, most prominently myopathy or rhabdomyolysis and polyneuropathy. We noted that the pattern of side-effects associated with statins resembles the pathology of selenium deficiency, and postulated that the mechanism lay in a well established, but often overlooked, biochemical pathway--the isopentenylation of selenocysteine-tRNA([Ser]Sec). A negative effect of statins on selenoprotein synthesis does seem to explain many of the enigmatic effects and side-effects of statins, in particular, statin-induced myopathy.

Glutathione peroxidases of the potato cyst nematode Globodera Rostochiensis

We report the cloning and characterisation of full-length DNAs complementary to RNA (cDNAs) encoding two glutathione peroxidases (GpXs) from a plant parasitic nematode, the potato cyst nematode (PCN) Globodera rostochiensis. One protein has a functional signal peptide that targets the protein for secretion from animal cells while the other is predicted to be intracellular. Both genes are expressed in all parasite stages tested. The mRNA encoding the intracellular GpX is present throughout the nematode second stage juvenile and is particularly abundant in metabolically active tissues including the genital primordia. The mRNA encoding the secreted GpX is restricted to the hypodermis, the outermost cellular layer of the nematode, a location from which it is likely to be secreted to the parasite surface. Biochemical studies confirmed the secreted protein as a functional GpX and showed that, like secreted GpXs of other parasitic nematodes, it does not metabolise hydrogen peroxide but has a preference for larger hydroperoxide substrates. The intracellular protein is likely to have a role in metabolism of active oxygen species derived from internal body metabolism while the secreted protein may protect the parasite from host defences. Other functional roles for this protein are discussed.

Reprogrammed genetic decoding in cellular gene expression

Reprogrammed genetic decoding signals in mRNAs productively overwrite the normal decoding rules of translation. These "recoding" signals are associated with sites of programmed ribosomal frameshifting, hopping, termination codon suppression, and the incorporation of the unusual amino acids selenocysteine and pyrrolysine. This review summarizes current knowledge of the structure and function of recoding signals in cellular genes, the biological importance of recoding in gene regulation, and ways to identify new recoded genes.

Specific Excision of the Selenocysteine tRNA[Ser]Sec (Trsp) Gene in Mouse Liver Demonstrates an Essential Role of Selenoproteins in Liver Function

Selenium is essential in mammalian embryonic development. However, in adults, selenoprotein levels in several organs including liver can be substantially reduced by selenium deficiency without any apparent change in phenotype. To address the role of selenoproteins in liver function, mice homozygous for a floxed allele encoding the selenocysteine (Sec) tRNA([Ser]Sec) gene were crossed with transgenic mice carrying the Cre recombinase under the control of the albumin promoter that expresses the recombinase specifically in liver. Recombination was nearly complete in mice 3 weeks of age, whereas liver selenoprotein synthesis was virtually absent, which correlated with the loss of Sec tRNA([Ser]Sec) and activities of major selenoproteins. Total liver selenium was dramatically decreased, whereas levels of low molecular weight selenocompounds were little affected. Plasma selenoprotein P levels were reduced by about 75%, suggesting that selenoprotein P is primarily exported from the liver. Glutathione S-transferase levels were elevated in the selenoprotein-deficient liver, suggesting a compensatory activation of this detoxification program. Mice appeared normal until about 24 h before death. Most animals died between 1 and 3 months of age. Death appeared to be due to severe hepatocellular degeneration and necrosis with concomitant necrosis of peritoneal and retroperitoneal fat. These studies revealed an essential role of selenoproteins in liver function.

Molecular Actions of Selenium in the Brain: Neuroprotective Mechanisms of an Essential Trace Element

In addition to acting as an essential nutrient for the immune system and overall body function, it is apparent that selenium also plays a critical role in the operation of the nervous system. Selenium itself is a constituent of selenoproteins, which are primarily involved in antioxidant function and redox status. However, apart from its covalent incorporation into these proteins, selenium also performs neuroprotective actions independent of translational processes. Furthermore, low selenium intake has detrimental effects on proper brain function, such as epileptic episodes and neuronal cell death, which have, in turn, been shown to be mitigated by higher selenium levels. Understanding the mechanisms of selenium action will be crucial to determining its potential as a preventive and therapeutic agent against excitatory brain damage.

Methionine sulfoxide reduction in mammals: characterization of methionine-R-sulfoxide reductases

Methionine residues in proteins are susceptible to oxidation by reactive oxygen species, but can be repaired via reduction of the resulting methionine sulfoxides by methionine-S-sulfoxide reductase (MsrA) and methionine-R-sulfoxide reductase (MsrB). However, the identity of all methionine sulfoxide reductases involved, their cellular locations and relative contributions to the overall pathway are poorly understood. Here, we describe a methionine-R-sulfoxide reduction system in mammals, in which two MsrB homologues were previously described. We found that human and mouse genomes possess three MsrB genes and characterized their protein products, designated MsrB1, MsrB2, and MsrB3. MsrB1 (Selenoprotein R) was present in the cytosol and nucleus and exhibited the highest methionine-R-sulfoxide reductase activity because of the presence of selenocysteine (Sec) in its active site. Other mammalian MsrBs contained cysteine in place of Sec and were less catalytically efficient. MsrB2 (CBS-1) resided in mitochondria. It had high affinity for methionine-R-sulfoxide, but was inhibited by higher concentrations of the substrate. The human MsrB3 gene gave rise to two protein forms, MsrB3A and MsrB3B. These were generated by alternative splicing that introduced contrasting N-terminal and C-terminal signals, such that MsrB3A was targeted to the endoplasmic reticulum and MsrB3B to mitochondria. We found that only mitochondrial forms of mammalian MsrBs (MsrB2 and MsrB3B) could compensate for MsrA and MsrB deficiency in yeast. All mammalian MsrBs belonged to a group of zinc-containing proteins. The multiplicity of MsrBs contrasted with the presence of a single mammalian MsrA gene as well as with the occurrence of single MsrA and MsrB genes in yeast, fruit flies, and nematodes. The data suggested that different cellular compartments in mammals maintain a system for repair of oxidized methionine residues and that this function is tuned in enzyme- and stereo-specific manner.

The Drosophila selenoprotein BthD is required for survival and has a role in salivary gland development

Selenium is implicated in many diseases, including cancer, but its function at the molecular level is poorly understood. BthD is one of three selenoproteins recently identified in Drosophila. To elucidate the function of BthD and the role of selenoproteins in cellular metabolism and health, we analyzed the developmental expression profile of this protein and used inducible RNA interference (RNAi) to ablate function. We find that BthD is dynamically expressed during Drosophila development. bthD mRNA and protein are abundant in the ovaries of female flies and are deposited into the developing oocyte. Maternally contributed protein and RNA persist during early embryonic development but decay by the onset of gastrulation. At later stages of embryogenesis, BthD is expressed highly in the developing salivary gland. We generated transgenic fly lines carrying an inducible gene-silencing construct, in which an inverted bthD genomic-cDNA hybrid is under the control of the Drosophila Gal4 upstream activation sequence system. Duplex RNAi induced from this construct targeted BthD mRNA for destruction and reduced BthD protein levels. We found that loss of BthD compromised salivary gland morphogenesis and reduced animal viability.

Selenoprotein-deficient transgenic mice exhibit enhanced exercise-induced muscle growth

Dietary intake of selenium has been implicated in a wide range of health issues, including aging, heart disease and cancer. Selenium deficiency, which can reduce selenoprotein levels, has been associated with several striated muscle pathologies. To investigate the role of selenoproteins in skeletal muscle biology, we used a transgenic mouse (referred to as i6A-) that has reduced levels of selenoproteins due to the introduction and expression of a dominantly acting mutant form of selenocysteine transfer RNA (tRNA[Ser]Sec). As a consequence, each organ contains reduced levels of most selenoproteins, yet these mice are normal with regard to fertility, overall health, behavior and blood chemistries. In the present study, although skeletal muscles from i6A- mice were phenotypically indistinguishable from those of wild-type mice, plantaris muscles were approximately 50% heavier after synergist ablation, a model of exercise overload. Like muscle in wild-type mice, the enhanced growth in the i6A- mice was completely blocked by inhibition of the mammalian target of rapamycin (mTOR) pathway. Muscles of transgenic mice exhibited increased site-specific phosphorylation on both Akt and p70 ribosomal S6 kinase (p70S6k) (P < 0.05) before ablation, perhaps accounting for the enhanced response to synergist ablation. Thus, a single genetic alteration resulted in enhanced skeletal muscle adaptation after exercise, and this is likely through subtle changes in the resting phosphorylation state of growth-related kinases.

Mechanism and regulation of selenoprotein synthesis

Selenium is an essential trace element that is incorporated into proteins as selenocysteine (Sec), the twenty-first amino acid. Sec is encoded by a UGA codon in the selenoprotein mRNA. The decoding of UGA as Sec requires the reprogramming of translation because UGA is normally read as a stop codon. The translation of selenoprotein mRNAs requires cis-acting sequences in the mRNA and novel trans-acting factors dedicated to Sec incorporation. Selenoprotein synthesis in vivo is highly selenium-dependent, and there is a hierarchy of selenoprotein expression in mammals when selenium is limiting. This review describes emerging themes from studies on the mechanism, kinetics, and efficiency of Sec insertion in prokaryotes. Recent developments that provide mechanistic insight into how the eukaryotic ribosome distinguishes between UGA/Sec and UGA/stop codons are discussed. The efficiency and regulation of mammalian selenoprotein synthesis are considered in the context of current models for Sec insertion.

Cell death caused by selenium deficiency and protective effect of antioxidants

Selenium is an essential trace element and it is well known that selenium is necessary for cell culture. However, the mechanism underlying the role of selenium in cellular proliferation and survival is still unknown. The present study using Jurkat cells showed that selenium deficiency in a serum-free medium decreased the selenium-dependent enzyme activity (glutathione peroxidases and thioredoxin reductase) within cells and cell viability. To understand the mechanism of this effect of selenium, we examined the effect of other antioxidants, which act by different mechanisms. Vitamin E, a lipid-soluble radical-scavenging antioxidant, completely blocked selenium deficiency-induced cell death, although alpha-tocopherol (biologically the most active form of vitamin E) could not preserve selenium-dependent enzyme activity. Other antioxidants, such as different isoforms and derivatives of vitamin E, BO-653 and deferoxamine mesylate, also exerted an inhibitory effect. However, the water-soluble antioxidants, such as ascorbic acid, N-acetyl cysteine, and glutathione, displayed no such effect. Dichlorodihydrofluorescein (DCF) assay revealed that cellular reactive oxygen species (ROS) increased before cell death, and sodium selenite and alpha-tocopherol inhibited ROS increase in a dose-dependent manner. The generation of lipid hydroperoxides was observed by fluorescence probe diphenyl-1-pyrenylphosphine (DPPP) and HPLC chemiluminescence only in selenium-deficient cells. These results suggest that the ROS, especially lipid hydroperoxides, are involved in the cell death caused by selenium deficiency and that selenium and vitamin E cooperate in the defense against oxidative stress upon cells by detoxifying and inhibiting the formation of lipid hydroperoxides.

Cloning of human 15ku selenoprotein gene from H9 T cells

AIM: To clone human 15ku selenoprotein gene.

METHODS: H9 human T cells were cultured in RPMI1640 medium supplemented with 100 mL/L fetal calf serum. mRNA was isolated from the cells. cDNA library was constructed by RT-PCR. The human 15ku selenoprotein gene was obtained by PCR and cloned into T vector and sequenced.

RESULTS: A unique cDNA fragment about 1244 bp was obtained. Sequence analysis identified an open reading frame within the cDNA. The gene had an in-frame TGA, which encoded selenocysteine (Sec), and a 3’-UTR SECIS element, which was required for synthesis of selenoprotein. The predicted protein molecular mass was about 15ku (162 residues). The result was identical with human liver 15ku selenoprotein gene published in Genbank.

CONCLUSION: Human 15ku selenoprotein gene can be successfully obtained from T cell line.

Spatial and temporal expression patterns of selenoprotein genes during embryogenesis in zebrafish

Selenium is important for embryogenesis in vertebrates but little is known about the expression patterns and biological functions of most selenoprotein genes. Taking advantage of the zebrafish model, systematic analysis of selenoprotein gene expression was performed by in situ hybridization on whole-mount embryos at different developmental stages. Twenty-one selenoprotein mRNAs were analyzed and all of them exhibited expression patterns restricted to specific tissues. Moreover, we demonstrated that highly similar selenoprotein paralogs were expressed within distinct territories. Therefore, tissue- and development-specific expression patterns provided new information for selenoproteins of unknown function.

Chlamydomonas reinhardtii selenocysteine tRNA[Ser]Sec

Eukaryotic selenocysteine (Sec) protein insertion machinery was thought to be restricted to animals, but the occurrence of both Sec-containing proteins and the Sec insertion system was recently found in Chlamydomonas reinhardtii, a member of the plant kingdom. Herein, we used RT-PCR to determine the sequence of C. reinhardtii Sec tRNA[Ser]Sec, the first non-animal eukaryotic Sec tRNA[Ser]Sec sequence. Like its animal counterpart, it is 90 nucleotides in length, is aminoacylated with serine by seryl-tRNA synthetase, and decodes specifically UGA. Evolutionary analyses of known Sec tRNAs identify the C. reinhardtii form as the most diverged eukaryotic Sec tRNA[Ser]Sec and reveal a common origin for this tRNA in bacteria, archaea, and eukaryotes.

Genomics perspective on disulfide bond formation

Disulfide bond formation, reduction, and isomerization in substrate proteins are catalyzed by designated pathways composed of thiol-dependent enzymes. Disulfides are generated in oxidizing environments, such as bacterial periplasm and eukaryotic endoplasmic reticulum (ER), but could also be formed in the cytosol. Major contributors to the formation of intramolecular disulfides in proteins are thiol/disulfide oxidoreductases containing a conserved CxxC motif (two cysteines separated by two other residues), which in turn transfer reducing equivalents to adapter or membrane-bound oxidoreductases. Disulfide bond formation is accompanied by disulfide bond reduction and isomerization processes, allowing disulfide repair and quality control. Higher eukaryotes evolved a complex network of thiol/disulfide oxidoreductases that are involved in disulfide bond formation and isomerization and thiol-dependent protein retention. Emerging evidence suggests that these ER functions might be assisted by mammalian selenocysteine-containing oxidoreductases Sep15 and SelM.

Regulation of gene expression by stop codon recoding: selenocysteine

The regulation of gene expression at the translational level not only allows for rapid changes in specific protein levels but also provides an opportunity to alter codon specificity. For the incorporation of selenocysteine (Sec) into protein, the UGA codon is transformed from one that signals translation termination to one specific for Sec. This review provides a look at Sec incorporation from the perspective of the individual steps involved in protein synthesis: initiation, elongation and termination. The roles of the factors known to be required for Sec incorporation are considered in the context of each step in translation including structural modeling of the differences between the standard elongation factor eEF1A and the Sec-specific counterpart, eEFSec.

Models for assessing the role of selenoproteins in health

Two model systems for examining the role of selenoproteins in health are discussed. One utilizes transgenic mice that carry mutant selenocysteine (Sec) tRNA transgenes that result in the reduction of selenoprotein expression in a protein- and tissue-specific manner. The other utilizes loxP-Cre technology to selectively remove the Sec tRNA gene in mammary epithelium that results in the reduction of only certain selenoproteins in this tissue. Both approaches provide important tools for examining the role of selenoproteins in health.

Characterization of mammalian selenoproteomes

In the genetic code, UGA serves as a stop signal and a selenocysteine codon, but no computational methods for identifying its coding function are available. Consequently, most selenoprotein genes are misannotated. We identified selenoprotein genes in sequenced mammalian genomes by methods that rely on identification of selenocysteine insertion RNA structures, the coding potential of UGA codons, and the presence of cysteine-containing homologs. The human selenoproteome consists of 25 selenoproteins.

Selenoprotein metabolism and function: evidence for more than one function for selenoprotein P

Biological functions of selenium are exerted by selenoproteins that contain selenocysteine in their primary structure. Selenocysteine is synthesized and inserted into proteins cotranslationally by a complex process. Families of selenoproteins include the glutathione peroxidases, the iodothyronine deiodinases and the thioredoxin reductases. These are redox enzymes that take advantage of the chemical properties of selenium to catalyze, respectively, removal of hydroperoxides by glutathione, deiodination of thyroid hormones and support of cellular processes requiring reduction of disulfides. Approximately 10 additional selenoproteins have been identified. One of them, selenoprotein P, is an extracellular protein that contains most of the selenium in plasma. It associates with endothelial cells, probably through its heparin-binding properties. Selenoprotein P has been postulated to protect against oxidative injury and to transport selenium from the liver to peripheral tissues. Selenium-dependent protection against diquat-induced liver necrosis and lipid peroxidation in the rat correlates with the presence of selenoprotein P. Recent results support a transport function. When (75)SeO(3)(2-) was administered intravenously to rats, liver tissue took up (75)Se within minutes, associated with a rapid decline in plasma (75)Se. Brain tissue did not begin accumulating (75)Se until (75)Se-labeled selenoprotein P had begun appearing in the plasma after 30 min. These results suggest that tissues like liver can take up small-molecule forms of selenium whereas presence of the element in selenoprotein P facilitates uptake by tissues like brain. Thus, there is evidence for both antioxidant and selenium transport functions of selenoprotein P.