Efficient incorporation of multiple selenocysteines involves an inefficient decoding step serving as a potential translational checkpoint and ribosome bottleneck

The selenoprotein P 3' untranslated region is an RNA binding protein platform that fine tunes selenocysteine incorporation

Selenoproteins contain the 21st amino acid, selenocysteine (Sec), which is incorporated at select UGA codons when a specialized hairpin sequence, the Sec insertion sequence (SECIS) element, is present in the 3' UTR. Aside from the SECIS, selenoprotein mRNA 3' UTRs are not conserved between different selenoproteins within a species. In contrast, the 3'-UTR of a given selenoprotein is often conserved across species, which supports the hypothesis that cis-acting elements in the 3'-UTR other than the SECIS exert post-transcriptional control on selenoprotein expression. In order to determine the function of one such SECIS context, we chose to focus on the plasma selenoprotein, SELENOP, which is required to maintain selenium homeostasis as a selenium transport protein that contains 10 Sec residues. It is unique in that its mRNA contains two SECIS elements in the context of a highly conserved 843-nucleotide 3' UTR. Here we have used RNA affinity chromatography and identified PTBP1 as the major RNA binding protein that specifically interacts with the sequence between the two SECIS elements. We then used CRISPR/Cas9 genome editing to delete two regions surrounding the first SECIS element. We found that these sequences are involved in regulating SELENOP mRNA and protein levels, which are inversely altered as a function of selenium concentrations.

Ribosome Fate during Decoding of UGA-Sec Codons

Decoding of genetic information into polypeptides occurs during translation, generally following the codon assignment rules of the organism's genetic code. However, recoding signals in certain mRNAs can overwrite the normal rules of translation. An exquisite example of this occurs during translation of selenoprotein mRNAs, wherein UGA codons are reassigned to encode for the 21st proteogenic amino acid, selenocysteine. In this review, we will examine what is known about the mechanisms of UGA recoding and discuss the fate of ribosomes that fail to incorporate selenocysteine.

Mechanisms Affecting the Biosynthesis and Incorporation Rate of Selenocysteine

Selenocysteine (Sec) is the 21st non-standard proteinogenic amino acid. Due to the particularity of the codon encoding Sec, the selenoprotein synthesis needs to be completed by unique mechanisms in specific biological systems. In this paper, the underlying mechanisms for the biosynthesis and incorporation of Sec into selenoprotein were comprehensively reviewed on five aspects: (i) the specific biosynthesis mechanism of Sec and the role of its internal influencing factors (SelA, SelB, SelC, SelD, SPS2 and PSTK); (ii) the elements (SECIS, PSL, SPUR and RF) on mRNA and their functional mechanisms; (iii) the specificity (either translation termination or translation into Sec) of UGA; (iv) the structure–activity relationship and action mechanism of SelA, SelB, SelC and SelD; and (v) the operating mechanism of two key enzyme systems for inorganic selenium source flow before Sec synthesis. Lastly, the size of the translation initiation interval, other action modes of SECIS and effects of REPS (Repetitive Extragenic Palindromic Sequences) that affect the incorporation efficiency of Sec was also discussed to provide scientific basis for the large-scale industrial fermentation for the production of selenoprotein.

New Directions for Understanding the Codon Redefinition Required for Selenocysteine Incorporation

The fact that selenocysteine (Sec) is delivered to the elongating ribosome by a tRNA that recognizes a UGA stop codon makes it unique and a thorn in the side of what was originally thought to be a universal genetic code. The mechanism by which this redefinition occurs has been slowly coming to light over the past 30 years, but key questions remain. This review seeks to highlight the prominent mechanistic questions that will guide the direction of work in the near future. These questions arise from two major aspects of Sec incorporation: (1) novel functions for the Sec insertion sequence (SECIS) that resides in all selenoprotein mRNAs and (2) the myriad of RNA-binding proteins, both known and yet to be discovered, that act in concert to modify the translation elongation process to allow Sec incorporation.

Processive Recoding and Metazoan Evolution of Selenoprotein P: Up to 132 UGAs in Molluscs

Selenoproteins typically contain a single selenocysteine, the 21st amino acid, encoded by a context-redefined UGA. However, human selenoprotein P (SelenoP) has a redox-functioning selenocysteine in its N-terminal domain and nine selenium transporter-functioning selenocysteines in its C-terminal domain. Here we show that diverse SelenoP genes are present across metazoa with highly variable numbers of Sec-UGAs, ranging from a single UGA in certain insects, to 9 in common spider, and up to 132 in bivalve molluscs. SelenoP genes were shaped by a dynamic evolutionary process linked to selenium usage. Gene evolution featured modular expansions of an ancestral multi-Sec domain, which led to particularly Sec-rich SelenoP proteins in many aquatic organisms. We focused on molluscs, and chose Pacific oyster Magallana gigas as experimental model. We show that oyster SelenoP mRNA with 46 UGAs is translated full-length in vivo. Ribosome profiling indicates that selenocysteine specification occurs with ∼5% efficiency at UGA1 and approaches 100% efficiency at distal 3' UGAs. We report genetic elements relevant to its expression, including a leader open reading frame and an RNA structure overlapping the initiation codon that modulates ribosome progression in a selenium-dependent manner. Unlike their mammalian counterparts, the two SECIS elements in oyster SelenoP (3'UTR recoding elements) do not show functional differentiation in vitro. Oysters can increase their tissue selenium level up to 50-fold upon supplementation, which also results in extensive changes in selenoprotein expression.

Induction of Selenoprotein P mRNA during Hepatitis C Virus Infection Inhibits RIG-I-Mediated Antiviral Immunity

Patients infected with hepatitis C virus (HCV) have an increased risk of developing type 2 diabetes. HCV infection is linked to various liver abnormalities, potentially contributing to this association. We show that HCV infection increases the levels of hepatic selenoprotein P (SeP) mRNA (SEPP1 mRNA) and serum SeP, a hepatokine linked to insulin resistance. SEPP1 mRNA inhibits type I interferon responses by limiting the function of retinoic-acid-inducible gene I (RIG-I), a sensor of viral RNA. SEPP1 mRNA binds directly to RIG-I and inhibits its activity. SEPP1 mRNA knockdown in hepatocytes causes a robust induction of interferon-stimulated genes and decreases HCV replication. Clinically, high SeP serum levels are significantly associated with treatment failure of direct-acting antivirals in HCV-infected patients. Thus, SeP regulates insulin resistance and innate immunity, possibly inducing immune tolerance in the liver, and its upregulation may explain the increased risk of type 2 diabetes in HCV-infected patients.

Processive incorporation of multiple selenocysteine residues is driven by a novel feature of the selenocysteine insertion sequence

RNA stem loop structures have been frequently shown to regulate essential cellular processes. The selenocysteine insertion sequence (SECIS) element, found in the 3' UTRs of all selenoprotein mRNAs, is an example of such a structure, as it is required for the incorporation of the 21st amino acid, selenocysteine (Sec). Selenoprotein synthesis poses a mechanistic challenge because Sec is incorporated during translation in response to a stop codon (UGA). Although it is known that a SECIS-binding protein (SBP2) is required for Sec insertion, the mechanism of action remains elusive. Additional complexity is present in the synthesis of selenoprotein P (SELENOP), which is the only selenoprotein that contains multiple UGA codons and possesses two SECIS elements in its 3' UTR. Thus, full-length SELENOP synthesis requires processive Sec incorporation. Using zebrafish Selenop, in vitro translation assays, and ⁷⁵Se labeling in HEK293 cells, we found here that processive Sec incorporation is an intrinsic property of the SECIS elements. Specifically, we identified critical features of SECIS elements that are required for processive Sec incorporation. A screen of the human SECIS elements revealed that most of these elements support processive Sec incorporation in vitro; however, we also found that the processivity of Sec incorporation into Selenop in cells is tightly regulated. We propose a model for processive Sec incorporation that involves differential recruitment of SECIS-binding proteins.

The Selenium Transport Protein, Selenoprotein P, Requires Coding Sequence Determinants to Promote Efficient Selenocysteine Incorporation

Selenoproteins are an essential and unique group of proteins in which selenocysteine (Sec) is incorporated in response to a stop codon (UGA). Reprograming of UGA for Sec insertion in eukaryotes requires a cis-acting stem-loop structure in the 3' untranslated region of selenoprotein mRNA and several trans-acting factors. Together these factors are sufficient for Sec incorporation in vitro, but the process is highly inefficient. An additional challenge is the synthesis of selenoprotein P (SELENOP), which uniquely contains multiple UGA codons. Full-length SELENOP expression requires processive Sec incorporation, the mechanism for which is not understood. In this study, we identify core coding region sequence determinants within the SELENOP mRNA that govern SELENOP synthesis. Using ⁷⁵Se labeling in cells, we determined that the N-terminal coding sequence (upstream of the second UGA) and C-terminal coding sequence context are two independent determinants for efficient synthesis of full-length SELENOP. In addition, the distance between the first UGA and the consensus signal peptide is also critical for efficiency.

Molecular mechanism of selenoprotein P synthesis

BACKGROUND

Selenoprotein synthesis requires the reinterpretation of a UGA stop codon as one that encodes selenocysteine (Sec), a process that requires a set of dedicated translation factors. Among the mammalian selenoproteins, Selenoprotein P (SELENOP) is unique as it contains a selenocysteine-rich domain that requires multiple Sec incorporation events.

SCOPE OF REVIEW

In this review we elaborate on new data and current models that provide insight into how SELENOP is made.

MAJOR CONCLUSIONS

SELENOP synthesis requires a specific set of factors and conditions.

GENERAL SIGNIFICANCE

As the key protein required for proper selenium distribution, SELENOP stands out as a lynchpin selenoprotein that is essential for male fertility, proper neurologic function and selenium metabolism.

The A to Z of modulated cell patterning by mammalian thioredoxin reductases

Mammalian thioredoxin reductases (TrxRs) are selenocysteine-containing proteins (selenoproteins) that propel a large number of functions through reduction of several substrates including the active site disulfide of thioredoxins (Trxs). Well-known enzymatic systems that in turn are supported by Trxs and TrxRs include deoxyribonucleotide synthesis through ribonucleotide reductase, antioxidant defense through peroxiredoxins and methionine sulfoxide reductases, and redox modulation of a number of transcription factors. Although these functions may be essential for cells due to crucial roles in maintenance of cell viability and proliferation, findings during the last decade reveal that mammals have major redundancy in their cellular reductive systems. The synthesis of glutathione (GSH) and reductive functions of GSH-dependent pathways typically act in parallel with Trx-dependent pathways, with only one of these systems often being sufficient to support viability. Importantly, this does not imply that a modulation of the Trx system will remain without consequences, even when GSH-dependent pathways remain functional. As suggested by several recent findings, the Trx system in general and the TrxRs in particular, function as key regulators of signaling pathways. In this review article we will discuss findings that collectively suggest that modulation in mammalian systems of cytosolic TrxR1 (TXNRD1) or mitochondrial TrxR2 (TXNRD2) influence cell patterning and cellular stress responses. Effects of lower activities include increased adipogenesis, insulin responsiveness, glycogen accumulation, hyperproliferation, and distorted embryonic development, while increased activities correlate with decreased proliferation and extended lifespan, as well as worse cancer prognosis. The molecular mechanisms that underlie these diverse effects, involving regulation of protein phosphorylation cascades and of key transcription factors that guide cellular differentiation pathways, will be discussed. We conclude that the selenium-dependent oxidoreductases TrxR1 and TrxR2 should be considered as key components of signaling pathways that control cell differentiation and cellular stress responses.

Multiple RNA structures affect translation initiation and UGA redefinition efficiency during synthesis of selenoprotein P

Gene-specific expansion of the genetic code allows for UGA codons to specify the amino acid selenocysteine (Sec). A striking example of UGA redefinition occurs during translation of the mRNA coding for the selenium transport protein, selenoprotein P (SELENOP), which in vertebrates may contain up to 22 in-frame UGA codons. Sec incorporation at the first and downstream UGA codons occurs with variable efficiencies to control synthesis of full-length and truncated SELENOP isoforms. To address how the Selenop mRNA can direct dynamic codon redefinition in different regions of the same mRNA, we undertook a comprehensive search for phylogenetically conserved RNA structures and examined the function of these structures using cell-based assays, in vitro translation systems, and in vivo ribosome profiling of liver tissue from mice carrying genomic deletions of 3′ UTR selenocysteine-insertion-sequences (SECIS1 and SECIS2). The data support a novel RNA structure near the start codon that impacts translation initiation, structures located adjacent to UGA codons, additional coding sequence regions necessary for efficient production of full-length SELENOP, and distinct roles for SECIS1 and SECIS2 at UGA codons. Our results uncover a remarkable diversity of RNA elements conducting multiple occurrences of UGA redefinition to control the synthesis of full-length and truncated SELENOP isoforms.

Human selenoprotein P and S variant mRNAs with different numbers of SECIS elements and inferences from mutant mice of the roles of multiple SECIS elements

Dynamic redefinition of the 10 UGAs in human and mouse selenoprotein P (Sepp1) mRNAs to specify selenocysteine instead of termination involves two 3' UTR structural elements (SECIS) and is regulated by selenium availability. In addition to the previously known human Sepp1 mRNA poly(A) addition site just 3' of SECIS 2, two further sites were identified with one resulting in 10-25% of the mRNA lacking SECIS 2. To address function, mutant mice were generated with either SECIS 1 or SECIS 2 deleted or with the first UGA substituted with a serine codon. They were fed on either high or selenium-deficient diets. The mutants had very different effects on the proportions of shorter and longer product Sepp1 protein isoforms isolated from plasma, and on viability. Spatially and functionally distinctive effects of the two SECIS elements on UGA decoding were inferred. We also bioinformatically identify two selenoprotein S mRNAs with different 5' sequences predicted to yield products with different N-termini. These results provide insights into SECIS function and mRNA processing in selenoprotein isoform diversity.

The RNA-binding protein Secisbp2 differentially modulates UGA codon reassignment and RNA decay

Dual-assignment of codons as termination and elongation codons is used to expand the genetic code. In mammals, UGA can be reassigned to selenocysteine during translation of selenoproteins by a mechanism involving a 3΄ untranslated region (UTR) selenocysteine insertion sequence (SECIS) and the SECIS-binding protein Secisbp2. Here, we present data from ribosome profiling, RNA-Seq and mRNA half-life measurements that support distinct roles for Secisbp2 in UGA-redefinition and mRNA stability. Conditional deletions of the Secisbp2 and Trsp (tRNA^Sec) genes in mouse liver were compared to determine if the effects of Secisbp2 loss on selenoprotein synthesis could be attributed entirely to the inability to incorporate Sec. As expected, tRNA^Sec depletion resulted in loss of ribosome density downstream of all UGA-Sec codons. In contrast, the absence of Secisbp2 resulted in variable effects on ribosome density downstream of UGA-Sec codons that demonstrate gene-specific differences in Sec incorporation. For several selenoproteins in which loss of Secisbp2 resulted in greatly diminished mRNA levels, translational activity and Sec incorporation efficiency were shown to be unaffected on the remaining RNA. Collectively, these results demonstrate that Secisbp2 is not strictly required for Sec incorporation and has a distinct role in stabilizing mRNAs that can be separated from its effects on UGA-redefinition.

Aminoglycoside-driven biosynthesis of selenium-deficient Selenoprotein P

Selenoprotein biosynthesis relies on the co-translational insertion of selenocysteine in response to UGA codons. Aminoglycoside antibiotics interfere with ribosomal function and may cause codon misreading. We hypothesized that biosynthesis of the selenium (Se) transporter selenoprotein P (SELENOP) is particularly sensitive to antibiotics due to its ten in frame UGA codons. As liver regulates Se metabolism, we tested the aminoglycosides G418 and gentamicin in hepatoma cell lines (HepG2, Hep3B and Hepa1-6) and in experimental mice. In vitro, SELENOP levels increased strongly in response to G418, whereas expression of the glutathione peroxidases GPX1 and GPX2 was marginally affected. Se content of G418-induced SELENOP was dependent on Se availability, and was completely suppressed by G418 under Se-poor conditions. Selenocysteine residues were replaced mainly by cysteine, tryptophan and arginine in a codon-specific manner. Interestingly, in young healthy mice, antibiotic treatment failed to affect Selenop biosynthesis to a detectable degree. These findings suggest that the interfering activity of aminoglycosides on selenoprotein biosynthesis can be severe, but depend on the Se status, and other parameters likely including age and general health. Focused analyses with aminoglycoside-treated patients are needed next to evaluate a possible interference of selenoprotein biosynthesis by the antibiotics and elucidate potential side effects.

Factors impacting the aminoglycoside-induced UGA stop codon readthrough in selenoprotein translation

Aminoglycosides (AG) are oligosaccharide antibiotics that interfere with the small ribosomal subunit in aerobic, Gram-negative bacteria, causing pathogen-destructing error rates in their protein biosynthesis. Aminoglycosides also induce mRNA misinterpretation in eukaryotic cells, especially of the UGA (Opal)-stop codon, albeit to a lower extent. UGA recoding is essentially required for the incorporation of selenocysteine (Sec) into growing selenoproteins during translation. Selenocysteine incorporation requires the presence of a selenoprotein-specific stem-loop structure within the 3'-untranslated region of the mRNA, the so-called Sec-insertion sequence (SECIS) element. Interestingly, selenoprotein genes differ in their SECIS-element sequence and in their UGA base context. We hypothesized that the SECIS-element and the specific codon context synergize in controlling the effects of AG on stop codon readthrough. To this end, the SECIS-elements of glutathione peroxidase 1, glutathione peroxidase 4 and selenoprotein P transcripts were cloned into a reporter system and analyzed in combination with different UGA codon contexts. Our results indicate that a cytosine in position 4 (directly downstream of UGA) confers strongest effects on both the Se- and AG-dependent readthrough. Overall selenoprotein biosynthesis rate depends on the Se-status, AG concentration and the specific SECIS-element present in the transcript. These findings help to get a better understanding for the susceptibility of different transcripts towards AG-mediated interference with the biosynthesis of functional Se-containing selenoproteins, and highlight the importance of the Se-status for successful selenoprotein biosynthesis under antibiotic therapy.

Update on selenoprotein biosynthesis

SIGNIFICANCE

Selenium is an essential trace element that is incorporated in the small but vital family of proteins, namely the selenoproteins, as the selenocysteine amino acid residue. In humans, 25 selenoprotein genes have been characterized. The most remarkable trait of selenoprotein biosynthesis is the cotranslational insertion of selenocysteine by the recoding of a UGA codon, normally decoded as a stop signal.

RECENT ADVANCES

In eukaryotes, a set of dedicated cis- and trans-acting factors have been identified as well as a variety of regulatory mechanisms, factors, or elements that control the selenoprotein expression at the level of the UGA-selenocysteine recoding process, offering a fascinating playground in the field of translational control. It appeared that the central players are two RNA molecules: the selenocysteine insertion sequence (SECIS) element within selenoprotein mRNA and the selenocysteine-tRNA([Ser]Sec); and their interacting partners.

CRITICAL ISSUES

After a couple of decades, despite many advances in the field and the discovery of many essential and regulatory components, the precise mechanism of UGA-selenocysteine recoding remains elusive and more complex than anticipated, with many layers of control. This review offers an update of selenoproteome biosynthesis and regulation in eukaryotes.

FUTURE DIRECTIONS

The regulation of selenoproteins in response to a variety of pathophysiological conditions and cellular stressors, including selenium levels, oxidative stress, replicative senescence, or cancer, awaits further detailed investigation. Clearly, the efficiency of UGA-selenocysteine recoding is the limiting stage of selenoprotein synthesis. The sequence of events leading Sec-tRNA([Ser]Sec) delivery to ribosomal A site awaits further analysis, notably at the level of a three-dimensional structure.

The molecular biology of selenocysteine

Selenium is an essential trace element that is incorporated into 25 human proteins as the amino acid selenocysteine (Sec). The incorporation of this amino acid turns out to be a fascinating problem in molecular biology because Sec is encoded by a stop codon, UGA. Layered on top of the canonical translation elongation machinery is a set of factors that exist solely to incorporate this important amino acid. The mechanism by which this process occurs, put into the context of selenoprotein biology, is the focus of this review.

Regulation of selenocysteine incorporation into the selenium transport protein, selenoprotein P

Selenoproteins are unique as they contain selenium in their active site in the form of the 21st amino acid selenocysteine (Sec), which is encoded by an in-frame UGA stop codon. Sec incorporation requires both cis- and trans-acting factors, which are known to be sufficient for Sec incorporation in vitro, albeit with low efficiency. However, the abundance of the naturally occurring selenoprotein that contains 10 Sec residues (SEPP1) suggests that processive and efficient Sec incorporation occurs in vivo. Here, we set out to study native SEPP1 synthesis in vitro to identify factors that regulate processivity and efficiency. Deletion analysis of the long and conserved 3'-UTR has revealed that the incorporation of multiple Sec residues is inherently processive requiring only the SECIS elements but surprisingly responsive to the selenium concentration. We provide evidence that processive Sec incorporation is linked to selenium utilization and that reconstitution of known Sec incorporation factors in a wheat germ lysate does not permit multiple Sec incorporation events, thus suggesting a role for yet unidentified mammalian-specific processes or factors. The relationship between our findings and the channeling theory of translational efficiency is discussed.

Effects of acclimation salinity on the expression of selenoproteins in the tilapia, Oreochromis mossambicus

Selenoproteins are ubiquitously expressed, act on a variety of physiological redox-related processes, and are mostly regulated by selenium levels in animals. To date, the expression of most selenoproteins has not been verified in euryhaline fish models. The Mozambique tilapia, Oreochromis mossambicus, a euryhaline cichlid fish, has a high tolerance for changes in salinity and survives in fresh water (FW) and seawater (SW) environments which differ greatly in selenium availability. In the present study, we searched EST databases for cichlid selenoprotein mRNAs and screened for their differential expression in FW and SW-acclimated tilapia. The expression of mRNAs encoding iodothyronine deiodinases 1, 2 and 3 (Dio1, Dio2, Dio3), Fep15, glutathione peroxidase 2, selenoproteins J, K, L, M, P, S, and W, was measured in the brain, eye, gill, kidney, liver, pituitary, muscle, and intraperitoneal white adipose tissue. Gene expression of selenophosphate synthetase 1, Secp43, and selenocysteine lyase, factors involved in selenoprotein synthesis or in selenium metabolism, were also measured. The highest variation in selenoprotein and synthesis factor mRNA expression between FW- and SW-acclimated fish was found in gill and kidney. While the branchial expression of Dio3 was increased upon transferring tilapia from SW to FW, the inverse effect was observed when fish were transferred from FW to SW. Protein content of Dio3 was higher in fish acclimated to FW than in those acclimated to SW. Together, these results outline tissue distribution of selenoproteins in FW and SW-acclimated tilapia, and indicate that at least Dio3 expression is regulated by environmental salinity.

Sepp1(UF) forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1

Mouse selenoprotein P (Sepp1) consists of an N-terminal domain (residues 1-239) that contains one selenocysteine (U) as residue 40 in a proposed redox-active motif (-UYLC-) and a C-terminal domain (residues 240-361) that contains nine selenocysteines. Sepp1 transports selenium from the liver to other tissues by receptor-mediated endocytosis. It also reduces oxidative stress in vivo by an unknown mechanism. A previously uncharacterized plasma form of Sepp1 is filtered in the glomerulus and taken up by renal proximal convoluted tubule (PCT) cells via megalin-mediated endocytosis. We purified Sepp1 forms from the urine of megalin(-/-) mice using a monoclonal antibody to the N-terminal domain. Mass spectrometry revealed that the purified urinary Sepp1 consisted of N-terminal fragments terminating at 11 sites between residues 183 and 208. They were therefore designated Sepp1(UF). Because the N-terminal domain of Sepp1 has a thioredoxin fold, Sepp1(UF) were compared with full-length Sepp1, Sepp1(Δ240-361), and Sepp1(U40S) as a substrate of thioredoxin reductase-1 (TrxR1). All forms of Sepp1 except Sepp1(U40S), which contains serine in place of the selenocysteine, were TrxR1 substrates, catalyzing NADPH oxidation when coupled with H2O2 or tert-butylhydroperoxide as the terminal electron acceptor. These results are compatible with proteolytic cleavage freeing Sepp1(UF) from full-length Sepp1, the form that has the role of selenium transport, allowing Sepp1(UF) to function by itself as a peroxidase. Ultimately, plasma Sepp1(UF) and small selenium-containing proteins are filtered by the glomerulus and taken up by PCT cells via megalin-mediated endocytosis, preventing loss of selenium in the urine and providing selenium for the synthesis of glutathione peroxidase-3.

Translational regulation of GPx-1 and GPx-4 by the mTOR pathway

Glutathione peroxidase activity was previously determined to be elevated in lymphocytes obtained from patients treated with the Bcr-Abl kinase inhibitor imatinib mesylate. In order to expand upon this observation, the established chronic myelogenous leukemia cell lines KU812 and MEG-01 were treated with imatinib and the effect on several anti-oxidant proteins was determined. The levels of GPx-1 were significantly increased following treatment with imatinib. This increase was not due to altered steady-state mRNA levels, and appeared to be dependent on the expression of Bcr-Abl, as no increases were observed following imatinib treatment of cells that did not express the fusion protein. The nutrient-sensing signaling protein, mammalian target of rapamycin (mTOR), can be activated by Bcr-Abl and its activity regulates the translation of many different proteins. Treatment of those same cells used in the imatinib studies with rapamycin, an inhibitor of mTOR, resulted in elevated GPx-1 and GPx-4 protein levels independent of Bcr-Abl expression. These proteins all belong to the selenoprotein family of peptides that contain the UGA-encoded amino acid selenocysteine. Collectively, these data provide evidence of a novel means of regulating anti-oxidants of the selenoprotein family via the mTOR pathway.

Isoform-specific binding of selenoprotein P to the β-propeller domain of apolipoprotein E receptor 2 mediates selenium supply

Sepp1 supplies selenium to tissues via receptor-mediated endocytosis. Mice, rats, and humans have 10 selenocysteines in Sepp1, which are incorporated via recoding of the stop codon, UGA. Four isoforms of rat Sepp1 have been identified, including full-length Sepp1 and three others, which terminate at the second, third, and seventh UGA codons. Previous studies have shown that the longer Sepp1 isoforms bind to the low density lipoprotein receptor apoER2, but the mechanism remains unclear. To identify the essential residues for apoER2 binding, an in vitro Sepp1 binding assay was developed using different Sec to Cys substituted variants of Sepp1 produced in HEK293T cells. ApoER2 was found to bind the two longest isoforms. These results suggest that Sepp1 isoforms with six or more selenocysteines are taken up by apoER2. Furthermore, the C-terminal domain of Sepp1 alone can bind to apoER2. These results indicate that apoER2 binds to the Sepp1 C-terminal domain and does not require the heparin-binding site, which is located in the N-terminal domain. Site-directed mutagenesis identified three residues of Sepp1 that are necessary for apoER2 binding. Sequential deletion of extracellular domains of apoER2 surprisingly identified the YWTD β-propeller domain as the Sepp1 binding site. Finally, we show that apoER2 missing the ligand-binding repeat region, which can result from cleavage at a furin cleavage site present in some apoER2 isoforms, can act as a receptor for Sepp1. Thus, longer isoforms of Sepp1 with high selenium content interact with a binding site distinct from the ligand-binding domain of apoER2 for selenium delivery.

A novel insight into the mechanism of mammalian selenoprotein synthesis

The amino acid selenocysteine is encoded by UGA, usually a stop codon, thus requiring a specialized machinery to enable its incorporation into selenoproteins. The machinery comprises the tRNA(Sec), a 3'-UTR mRNA stem-loop termed SElenoCysteine Insertion Sequence (SECIS), which is mandatory for recoding UGA as a Sec codon, the SECIS Binding Protein 2 (SBP2), and other proteins. Little is known about the molecular mechanism and, in particular, when, where, and how the SECIS and SBP2 contact the ribosome. Previous work by others used the isolated SECIS RNA to address this question. Here, we developed a novel approach using instead engineered minimal selenoprotein mRNAs containing SECIS elements derivatized with photoreactive groups. By cross-linking experiments in rabbit reticulocyte lysate, new information could be gained about the SBP2 and SECIS contacts with components of the translation machinery at various translation steps. In particular, we found that SBP2 was bound only to the SECIS in 48S pre-initiation and 80S pretranslocation complexes. In the complex where the Sec-tRNA(Sec) was accommodated to the A site but transpeptidation was blocked, SBP2 bound the ribosome and possibly the SECIS element as well, and the SECIS had flexible contacts with the 60S ribosomal subunit involving several ribosomal proteins. Altogether, our findings led to broadening our understanding about the unique mechanism of selenocysteine incorporation in mammals.

UGA codon position-dependent incorporation of selenocysteine into mammalian selenoproteins

It is thought that the SelenoCysteine Insertion Sequence (SECIS) element and UGA codon are sufficient for selenocysteine (Sec) insertion. However, we found that UGA supported Sec insertion only at its natural position or in its close proximity in mammalian thioredoxin reductase 1 (TR1). In contrast, Sec could be inserted at any tested position in mammalian TR3. Replacement of the 3′-UTR of TR3 with the corresponding segment of a Euplotes crassus TR restricted Sec insertion into the C-terminal region, whereas the 3′-UTR of TR3 conferred unrestricted Sec insertion into E. crassus TR, in which Sec insertion is normally limited to the C-terminal region. Exchanges of 3′-UTRs between mammalian TR1 and E. crassus TR had no effect, as both proteins restricted Sec insertion. We further found that these effects could be explained by the use of selenoprotein-specific SECIS elements. Examination of Sec insertion into other selenoproteins was consistent with this model. The data indicate that mammals evolved the ability to limit Sec insertion into natural positions within selenoproteins, but do so in a selenoprotein-specific manner, and that this process is controlled by the SECIS element in the 3′-UTR.

Translational redefinition of UGA codons is regulated by selenium availability

Incorporation of selenium into ~25 mammalian selenoproteins occurs by translational recoding whereby in-frame UGA codons are redefined to encode the selenium containing amino acid, selenocysteine (Sec). Here we applied ribosome profiling to examine the effect of dietary selenium levels on the translational mechanisms controlling selenoprotein synthesis in mouse liver. Dietary selenium levels were shown to control gene-specific selenoprotein expression primarily at the translation level by differential regulation of UGA redefinition and Sec incorporation efficiency, although effects on translation initiation and mRNA abundance were also observed. Direct evidence is presented that increasing dietary selenium causes a vast increase in ribosome density downstream of UGA-Sec codons for a subset of selenoprotein mRNAs and that the selenium-dependent effects on Sec incorporation efficiency are mediated in part by the degree of Sec-tRNA([Ser]Sec) Um34 methylation. Furthermore, we find evidence for translation in the 5'-UTRs for a subset of selenoproteins and for ribosome pausing near the UGA-Sec codon in those mRNAs encoding the selenoproteins most affected by selenium availability. These data illustrate how dietary levels of the trace element selenium can alter the readout of the genetic code to affect the expression of an entire class of proteins.

Selenium. Role of the essential metalloid in health

Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. It is a non-metal with properties that are intermediate between the chalcogen elements sulfur and tellurium. Selenium exerts its biological functions through selenoproteins. Selenoproteins contain selenium in the form of the 21st amino acid, selenocysteine (Sec), which is an analog of cysteine with the sulfur-containing side chain replaced by a Se-containing side chain. Sec is encoded by the codon UGA, which is one of three termination codons for mRNA translation in non-selenoprotein genes. Recognition of the UGA codon as a Sec insertion site instead of stop requires a Sec insertion sequence (SECIS) element in selenoprotein mRNAs and a unique selenocysteyl-tRNA, both of which are recognized by specialized protein factors. Unlike the 20 standard amino acids, Sec is biosynthesized from serine on its tRNA. Twenty-five selenoproteins are encoded in the human genome. Most of the selenoprotein genes were discovered by bioinformatics approaches, searching for SECIS elements downstream of in-frame UGA codons. Sec has been described as having stronger nucleophilic and electrophilic properties than cysteine, and Sec is present in the catalytic site of all selenoenzymes. Most selenoproteins, whose functions are known, are involved in redox systems and signaling pathways. However, several selenoproteins are not well characterized in terms of their function. The selenium field has grown dramatically in the last few decades, and research on selenium biology is providing extensive new information regarding its importance for human health.

Expression and regulation of mouse selenoprotein P transcript variants differing in non-coding RNA

Selenoprotein P (Sepp1), a glycoprotein rich in selenium, is thought to function in selenium transport throughout the body. The sepp1 gene locus potentially produces three alternative transcripts that differ only in their 5' untranslated regions (5'UTRs) and not in their protein coding regions, as indicated by transcript information in genomic databases. Here we investigated the distribution, relative expression, and biological significance of these transcript variants. We confirmed the expression of Sepp1 transcript variants using PCR and sequencing. Using 5'-RACE, we identified multiple 5'-termini upstream from three different splice donor sites, and a single splice acceptor site for exon 2. We found regional and temporal changes in variant expression in select adult and neonate murine tissue and brain regions. Distribution of variants in heart and kidney varied with stage of development. Notably, the Sepp1b variant was localized specifically to the hippocampus in brain. Targeted silencing of individual variants using RNAi demonstrated the biological importance for all transcript variants in cell viability. Additionally, we determined that the Sepp1b variant is a specific target for the miR-7 microRNA by means of its unique 5'UTR structure. Our results emphasize the importance of non-coding transcript variations as a regulatory means for Sepp1 expression in different tissues and stages of development. The presence of a variant localized in the hippocampus and regulated by a microRNA may have implications for the known deficits in synaptic function caused by genetic deletion of Sepp1.

SeP, ApoER2 and megalin as necessary factors to maintain Se homeostasis in mammals

Selenoprotein P (SeP) is an extracellular protein containing ten selenium atoms in the form of selenocysteine, secreted mainly from the liver. About 60% of the whole plasma selenium level is present in SeP, which makes it a useful biomarker of selenium nutritional status. The main functions of SeP are transport and storage of selenium in plasma. It is especially an important protein for the brain, testes and kidneys where the supplementation of the proper amount of Se ensures the synthesis of selenoenzymes with antioxidant properties.Recently, it has been found that SeP uptake in kidneys, testes and brain depends on the apolipoprotein receptor 2 (ApoER2) and lipoprotein megalin receptor (Lrp2). Megalin receptor represents a physiological SeP receptor in kidneys, mediating the re-uptake of secreted SeP from the primary urine. The absence of a functional megalin receptor causes a significant reduction of plasma selenium and the SeP levels as a result of Se excretion. ApoER2 is a SeP receptor in the brain and testes which uptakes Se from the extracellular fluid. Deletion of ApoER2 in mice leads to a lowered selenium level in the brain and testes, neurological dysfunction, production of abnormal spermatozoa, infertility and even death when the subjects are fed a low-selenium diet.

Processive selenocysteine incorporation during synthesis of eukaryotic selenoproteins

Selenoproteins are a family of proteins that share the common feature of containing selenocysteine, the "twenty-first" amino acid. Selenocysteine incorporation occurs during translation of selenoprotein messages by redefinition of UGA codons, which normally specify termination of translation. Studies of the eukaryotic selenocysteine incorporation mechanism suggest that selenocysteine insertion is inefficient compared with termination. Nevertheless, selenoprotein P and several other selenoproteins are known to contain multiple selenocysteines. The production of full-length (FL) protein from these messages would seem to demand highly efficient selenocysteine incorporation due to the compounding effect of termination at each UGA codon. We present data demonstrating that efficient incorporation of multiple selenocysteines can be reconstituted in rabbit reticulocyte lysate translation reactions. Selenocysteine incorporation at the first UGA codon is inefficient but increases by approximately 10-fold at subsequent downstream UGA codons. We found that ribosomes in the "processive" phase of selenocysteine incorporation (i.e., after decoding the first UGA codon as selenocysteine) are fully competent to terminate translation at UAG and UAA codons, that ribosomes become less efficient at selenocysteine incorporation as the distance between UGA codons is increased, and that efficient selenocysteine incorporation is not dependent on cis-acting elements unique to selenoprotein P. Furthermore, we found that the percentage of ribosomes decoding a UGA codon as selenocysteine rather than termination can be increased by 3- to 5-fold by placing the murine leukemia virus UAG read-through element upstream of the first UGA codon or by providing a competing messenger RNA in trans. The mechanisms of selenocysteine incorporation and selenoprotein synthesis are discussed in light of these results.

Threading the needle: getting selenocysteine into proteins

The co-translational incorporation of selenocysteine (Sec) requires that UGA be recognized as a sense rather than a nonsense codon. This is accomplished by the concerted action of a Sec insertion sequence (SECIS) element, SECIS binding protein 2, and a ternary complex of the Sec specific elongation factor, Sec-tRNA(Sec), and GTP. The mechanism by which they alter the canonical protein synthesis reaction has been elusive. Here we present an overview of the mechanistic perspective on Sec incorporation, highlighting recent advances in the field.

Novel structural determinants in human SECIS elements modulate the translational recoding of UGA as selenocysteine

The selenocysteine insertion sequence (SECIS) element directs the translational recoding of UGA as selenocysteine. In eukaryotes, the SECIS is located downstream of the UGA codon in the 3'-UTR of the selenoprotein mRNA. Despite poor sequence conservation, all SECIS elements form a similar stem-loop structure containing a putative kink-turn motif. We functionally characterized the 26 SECIS elements encoded in the human genome. Surprisingly, the SECIS elements displayed a wide range of UGA recoding activities, spanning several 1000-fold in vivo and several 100-fold in vitro. The difference in activity between a representative strong and weak SECIS element was not explained by differential binding affinity of SECIS binding Protein 2, a limiting factor for selenocysteine incorporation. Using chimeric SECIS molecules, we identified the internal loop and helix 2, which flank the kink-turn motif, as critical determinants of UGA recoding activity. The simultaneous presence of a GC base pair in helix 2 and a U in the 5'-side of the internal loop was a statistically significant predictor of weak recoding activity. Thus, the SECIS contains intrinsic information that modulates selenocysteine incorporation efficiency.

Selenoprotein P-expression, functions, and roles in mammals

Selenoprotein P (Sepp1) is a secreted protein that is made up of 2 domains. The larger N-terminal domain contains 1 selenocysteine residue in a redox motif and the smaller C-terminal domain contains the other 9 selenocysteines. Sepp1 isoforms of varying lengths occur but quantitation of them has not been achieved. Hepatic synthesis of Sepp1 affects whole-body selenium content and the liver is the source of most plasma Sepp1. ApoER2, a member of the lipoprotein receptor family, binds Sepp1 and facilitates its uptake into the testis and retention of its selenium by the brain. Megalin, another lipoprotein receptor, facilitates uptake of filtered Sepp1 into proximal tubule cells of the kidney. Thus, Sepp1 serves in homeostasis and distribution of selenium. Mice with deletion of Sepp1 suffer greater morbidity and mortality from infection with Trypanosoma congolense than do wild-type mice. Mice that express only the N-terminal domain of Sepp1 have the same severity of illness as wild-type mice, indicating that the protective function of Sepp1 against the infection resides in the N-terminal (redox) domain. Thus, Sepp1 has several functions. In addition, plasma Sepp1 concentration falls in selenium deficiency and, therefore, it can be used as an index of selenium nutritional status.

Selenoproteins in Archaea and Gram-positive bacteria

Selenium is an essential trace element for many organisms by serving important catalytic roles in the form of the 21st co-translationally inserted amino acid selenocysteine. It is mostly found in redox-active proteins in members of all three domains of life and analysis of the ever-increasing number of genome sequences has facilitated identification of the encoded selenoproteins. Available data from biochemical, sequence, and structure analyses indicate that Gram-positive bacteria synthesize and incorporate selenocysteine via the same pathway as enterobacteria. However, recent in vivo studies indicate that selenocysteine-decoding is much less stringent in Gram-positive bacteria than in Escherichia coli. For years, knowledge about the pathway of selenocysteine synthesis in Archaea and Eukarya was only fragmentary, but genetic and biochemical studies guided by analysis of genome sequences of Sec-encoding archaea has not only led to the characterization of the pathways but has also shown that they are principally identical. This review summarizes current knowledge about the metabolic pathways of Archaea and Gram-positive bacteria where selenium is involved, about the known selenoproteins, and about the respective pathways employed in selenoprotein synthesis.

Hierarchical regulation of selenoprotein expression and sex-specific effects of selenium

The expression of selenoproteins is controlled on each one of the textbook steps of protein biosynthesis, i.e., during gene transcription, RNA processing, translation and posttranslational events as well as via control of the stability of the involved intermediates and final products. Selenoproteins are unique in their dependence on the trace element Se which they incorporate as the 21st proteinogenic amino acid, selenocysteine. Higher mammals have developed unique pathways to enable a fine-tuned expression of all their different selenoproteins according to developmental stage, actual needs, and current availability of the trace element. Tightly controlled and dynamic expression patterns of selenoproteins are present in different tissues. Interestingly, these patterns display some differences in male and female individuals, and can be grossly modified during disease, e.g. in cancer, inflammation or neurodegeneration. Likewise, important health issues related to the selenium status show unexpected sexual dimorphisms. Some detailed molecular insights have recently been gained on how the hierarchical Se distribution among the different tissues is achieved, how the selenoprotein biosynthesis machinery discriminates among the individual selenoprotein transcripts and how impaired selenoprotein biosynthesis machinery becomes phenotypically evident in humans. This review tries to summarize these fascinating findings and highlights some interesting and surprising sex-specific differences.

The selenium to selenoprotein pathway in eukaryotes: more molecular partners than anticipated

The amino acid selenocysteine (Sec) is the major biological form of the trace element selenium. Sec is co-translationally incorporated in selenoproteins. There are 25 selenoprotein genes in humans, and Sec was found in the active site of those that have been attributed a function. This review will discuss how selenocysteine is synthesized and incorporated into selenoproteins in eukaryotes. Sec biosynthesis from serine on the tRNA(Sec) requires four enzymes. Incorporation of Sec in response to an in-frame UGA codon, otherwise signaling termination of translation, is achieved by a complex recoding machinery to inform the ribosomes not to stop at this position on the mRNA. A number of the molecular partners acting in this machinery have been identified but their detailed mechanism of action has not been deciphered yet. Here we provide an overview of the literature in the field. Particularly striking is the higher than originally envisaged number of factors necessary to synthesize Sec and selenoproteins. Clearly, selenoprotein synthesis is an exciting and very active field of research.

Reduced reliance on the trace element selenium during evolution of mammals

Evolution from fish to mammals was accompanied by decreased use of selenocysteine, raising questions about the need for selenium dietary supplements when pathology is not imminent.

Background

Selenium (Se) is an essential trace element that occurs in proteins in the form of selenocysteine (Sec). It is transported throughout the body in the form of Sec residues in Selenoprotein P (SelP), a plasma protein of unclear origin recently proposed as an experimental marker of dietary Se status.

Results

Here, we report that the amino-terminal domain of SelP is distantly related to ancestral bacterial thiol oxidoreductases of the thioredoxin superfamily, and that its carboxy-terminal Se transport domain may have originated in early metazoan evolution by de novo accumulation of Sec residues. Reconstruction of evolutionary changes in the Se transport domain indicates a decrease in Sec content of SelP specifically in the mammalian lineage via replacement of Sec with cysteine (Cys). Sec content of mammalian SelPs varies more than two-fold and is lowest in rodents and primates. Compared to mammals, fish show higher Sec content of SelP, larger selenoproteomes, elevated SelP gene expression, and higher levels of tissue Se. In addition, mammals replaced Sec with Cys in several proteins and lost several selenoproteins altogether, whereas such events are not found in fish.

Conclusion

These data suggest that evolution from fish to mammals was accompanied by decreased use of Sec and that analyses of SelP, selenoproteomes and Sec/Cys transitions provide a genetic marker of utilization of this trace element in vertebrates. The evolved reduced reliance on Se raises questions regarding the need to maximize selenoprotein expression by Se dietary supplements in situations when pathology is not imminent, a currently accepted practice.

SBP2 binding affinity is a major determinant in differential selenoprotein mRNA translation and sensitivity to nonsense-mediated decay

Selenoprotein mRNAs are potential targets for degradation via nonsense-mediated decay due to the presence of in-frame UGA codons that can be decoded as either selenocysteine or termination codons. When UGA decoding is inefficient, as occurs when selenium is limiting, termination occurs at these positions. Based on the predicted exon-intron structure, 14 of the 25 human selenoprotein mRNAs are predicted to be sensitive to nonsense-mediated decay. Among these, sensitivity varies widely, resulting in a hierarchy of preservation or degradation of selenoprotein mRNAs and, thus, of selenoprotein synthesis. Potential factors in dictating the hierarchy of selenoprotein synthesis are the Sec insertion sequence RNA-binding proteins, SBP2 and nucleolin. To investigate the mechanistic basis for this hierarchy and the role of these two proteins, we carried out knockdowns of SBP2 expression and assessed the effects on selenoprotein mRNA levels. We also investigated in vivo binding of selenoprotein mRNAs by SBP2 and nucleolin via immunoprecipitation of the proteins and quantitation of bound mRNAs. We report that SBP2 exhibits strong preferential binding to some selenoprotein mRNAs over others, whereas nucleolin exhibits minimal differences in binding. Thus, SBP2 is a major determinant in dictating the hierarchy of selenoprotein synthesis via differential selenoprotein mRNA translation and sensitivity to nonsense-mediated decay.

Functional analysis of the interplay between translation termination, selenocysteine codon context, and selenocysteine insertion sequence-binding protein 2

A selenocysteine insertion sequence (SECIS) element in the 3'-untranslated region and an in-frame UGA codon are the requisite cis-acting elements for the incorporation of selenocysteine into selenoproteins. Equally important are the trans-acting factors SBP2, Sec-tRNA[Ser]Sec, and eEFSec. Multiple in-frame UGAs and two SECIS elements make the mRNA encoding selenoprotein P (Sel P) unique. To study the role of codon context in determining the efficiency of UGA readthrough at each of the 10 rat Sel P Sec codons, we individually cloned 27-nucleotide-long fragments representing each UGA codon context into a luciferase reporter construct harboring both Sel P SECIS elements. Significant differences, spanning an 8-fold range of UGA readthrough efficiency, were observed, but these differences were dramatically reduced in the presence of excess SBP2. Mutational analysis of the "fourth base" of contexts 1 and 5 revealed that only the latter followed the established rules for hierarchy of translation termination. In addition, mutations in either or both of the Sel P SECIS elements resulted in differential effects on UGA readthrough. Interestingly, even when both SECIS elements harbored a mutation of the core region required for Sec incorporation, context 5 retained a significantly higher level of readthrough than context 1. We also show that SBP2-dependent Sec incorporation is able to repress G418-induced UGA readthrough as well as eRF1-induced stimulation of termination. We conclude that a large codon context forms a cis-element that works together with Sec incorporation factors to determine readthrough efficiency.

From selenium to selenoproteins: synthesis, identity, and their role in human health

The requirement of the trace element selenium for life and its beneficial role in human health has been known for several decades. This is attributed to low molecular weight selenium compounds, as well as to its presence within at least 25 proteins, named selenoproteins, in the form of the amino acid selenocysteine (Sec). Incorporation of Sec into selenoproteins employs a unique mechanism that involves decoding of the UGA codon. This process requires multiple features such as the selenocysteine insertion sequence (SECIS) element and several protein factors including a specific elongation factor EFSec and the SECIS binding protein 2, SBP2. The function of most selenoproteins is currently unknown; however, thioredoxin reductases (TrxR), glutathione peroxidases (GPx) and thyroid hormone deiodinases (DIO) are well characterised selenoproteins involved in redox regulation of intracellular signalling, redox homeostasis and thyroid hormone metabolism. Recent evidence points to a role for selenium compounds as well as selenoproteins in the prevention of some forms of cancer. A number of clinical trials are either underway or being planned to examine the effects of selenium on cancer incidence. In this review we describe some of the recent progress in our understanding of the mechanism of selenoprotein synthesis, the role of selenoproteins in human health and disease and the therapeutic potential of some of these proteins.

Selenoprotein P mRNA expression in human hepatic tissues

AIM

To investigate the expression of selenoprotein P mRNA (SePmRNA) in tissues of normal liver, liver cirrhosis and hepatocellular carcinoma (HCC), and its relationship with HCC occurrence and development.

METHODS

The expression of SePmRNA in tissues of normal liver, liver cirrhosis and HCC were detected by in situ hybridization using a cDNA probe.

RESULTS

The enzyme digesting products of PBluescript-Human Selenoprotein P were evaluated by electrophoresis. The positive expression of SePmRNA was found in the tissues of normal liver, liver cirrhosis and HCC. The expression of SeP mRNA was found in hepatic interstitial substance, especially in endothelial cells and lymphocytes of vasculature. The positive rate of SePmRNA in normal liver tissue was 84.6% (11/13) and the positive signals appeared in the nucleus and cytoplasm, mostly in the nucleolus, and the staining granules were larger in the nucleolus and around the nucleus. The positive rate of SePmRNA in liver cirrhosis tissue was 45.0% (9/20) and the positive signals were mainly in the nucleolus and cytoplasm, being less around the nucleus and inner nucleus than that in normal liver tissue. The positive rate of SePmRNA in HCC tissue was 30.0% (9/30) and the positive signals were in the cytoplasm, but less in the nucleus.

CONCLUSION

SePmRNA expression in the tissues of normal liver and HCC is significantly different (84.6% vs 30.0%, P=0.003), suggesting that SeP might play a role in the occurrence and development of HCC.

The Selenium-rich C-terminal Domain of Mouse Selenoprotein P Is Necessary for the Supply of Selenium to Brain and Testis but Not for the Maintenance of Whole Body Selenium

Selenoprotein P (Sepp1) has two domains with respect to selenium content: the N-terminal, selenium-poor domain and the C-terminal, selenium-rich domain. To assess domain function, mice with deletion of the C-terminal domain have been produced and compared with Sepp1-/- and Sepp1+/+ mice. All mice studied were males fed a semipurified diet with defined selenium content. The Sepp1 protein in the plasma of mice with the C-terminal domain deleted was determined by mass spectrometry to terminate after serine 239 and thus was designated Sepp1Delta240-361. Plasma Sepp1 and selenium concentrations as well as glutathione peroxidase activity were determined in the three types of mice. Glutathione peroxidase and Sepp1Delta240-361 accounted for over 90% of the selenium in the plasma of Sepp1Delta240-361 mice. Calculations using results from Sepp1+/+ mice revealed that Sepp1, with a potential for containing 10 selenocysteine residues, contained an average of 5 selenium atoms per molecule, indicating that shortened and/or selenium-depleted forms of the protein were present in these wild-type mice. Sepp1Delta240-361 mice had low brain and testis selenium concentrations that were similar to those in Sepp1-/- mice but they better maintained their whole body selenium. Sepp1Delta240-361 mice had depressed fertility, even when they were fed a high selenium diet, and their spermatozoa were defective and morphologically indistinguishable from those of selenium-deficient mice. Neurological dysfunction and death occurred when Sepp1Delta240-361 mice were fed selenium-deficient diet. These phenotypes were similar to those of Sepp1-/- mice but had later onset or were less severe. The results of this study demonstrate that the C terminus of Sepp1 is critical for the maintenance of selenium in brain and testis but not for the maintenance of whole body selenium.