Nonsense-mediated decay factors are involved in the regulation of selenoprotein mRNA levels during selenium deficiency

Insights from selenoprotein I mouse models for understanding biological roles of this enzyme

Selenoprotein I (selenoi) is a metabolic enzyme expressed in a wide variety of tissues that catalyzes the transfer of the ethanolamine phosphate group from CDP-ethanolamine to lipid acceptors to generate ethanolamine phospholipids. It is a member of the selenoprotein family, a class of proteins that mostly play fundamental roles in redox homeostasis and are defined by the co-translational incorporation of selenium in the form of selenocysteine. Loss-of-function mutations in the human SELENOI gene have been found in rare cases leading to a complex form of hereditary spastic paraplegia. Understanding the roles of this selenoprotein and its phospholipid products in different cell types has benefited from the development of mouse models. In particular, global and conditional knockout (KO) of the Selenoi gene in mice has enabled a more complete picture to emerge of how this important selenoprotein is integrated into metabolic pathways. These data have revealed how Selenoi loss-of-function affects embryogenesis, neurodevelopment, the immune system and liver physiology. This review summarizes the insights gained through mouse model experiments and the current understanding the different physiological roles played by this selenoprotein.

Selenoprotein-mediated redox regulation shapes the cell fate of HSCs and mature lineages

ABSTRACT

The maintenance of cellular redox balance is crucial for cell survival and homeostasis and is disrupted with aging. Selenoproteins, comprising essential antioxidant enzymes, raise intriguing questions about their involvement in hematopoietic aging and potential reversibility. Motivated by our observation of messenger RNA downregulation of key antioxidant selenoproteins in aged human hematopoietic stem cells (HSCs) and previous findings of increased lipid peroxidation in aged hematopoiesis, we used selenocysteine transfer RNA (tRNASec) gene (Trsp) knockout (KO) mouse model to simulate disrupted selenoprotein synthesis. This revealed insights into the protective roles of selenoproteins in preserving HSC stemness and B-lineage maturation, despite negligible effects on myeloid cells. Notably, Trsp KO exhibited B lymphocytopenia and reduced HSCs' self-renewal capacity, recapitulating certain aspects of aged phenotypes, along with the upregulation of aging-related genes in both HSCs and pre-B cells. Although Trsp KO activated an antioxidant response transcription factor NRF2, we delineated a lineage-dependent phenotype driven by lipid peroxidation, which was exacerbated with aging yet ameliorated by ferroptosis inhibitors such as vitamin E. Interestingly, the myeloid genes were ectopically expressed in pre-B cells of Trsp KO mice, and KO pro-B/pre-B cells displayed differentiation potential toward functional CD11b+ fraction in the transplant model, suggesting that disrupted selenoprotein synthesis induces the potential of B-to-myeloid switch. Given the similarities between the KO model and aged wild-type mice, including ferroptosis vulnerability, impaired HSC self-renewal and B-lineage maturation, and characteristic lineage switch, our findings underscore the critical role of selenoprotein-mediated redox regulation in maintaining balanced hematopoiesis and suggest the preventive potential of selenoproteins against aging-related alterations.

Overcoming Challenges with Biochemical Studies of Selenocysteine and Selenoproteins

Selenocysteine (Sec) is an essential amino acid that distinguishes itself from cysteine by a selenium atom in place of a sulfur atom. This single change imparts distinct chemical properties to Sec which are crucial for selenoprotein (Sec-containing protein) function. These properties include a lower pKa, enhanced nucleophilicity, and reversible oxidation. However, studying Sec incorporation in proteins is a complex process. While we find Sec in all domains of life, each domain has distinct translation mechanisms. These mechanisms are unique to canonical translation and are composed of Sec-specific enzymes and an mRNA hairpin to drive recoding of the UGA stop codon with Sec. In this review, we highlight the obstacles that arise when investigating Sec insertion, and the role that Sec has in proteins. We discuss the strategic methods implemented in this field to address these challenges. Though the Sec translation system is complex, a remarkable amount of information has been obtained and specialized tools have been developed. Continued studies in this area will provide a deeper understanding on the role of Sec in the context of proteins, and the necessity that we have for maintaining this complex translation machinery to make selenoproteins.

Ferroptosis in health and disease

Ferroptosis is a pervasive non-apoptotic form of cell death highly relevant in various degenerative diseases and malignancies. The hallmark of ferroptosis is uncontrolled and overwhelming peroxidation of polyunsaturated fatty acids contained in membrane phospholipids, which eventually leads to rupture of the plasma membrane. Ferroptosis is unique in that it is essentially a spontaneous, uncatalyzed chemical process based on perturbed iron and redox homeostasis contributing to the cell death process, but that it is nonetheless modulated by many metabolic nodes that impinge on the cells' susceptibility to ferroptosis. Among the various nodes affecting ferroptosis sensitivity, several have emerged as promising candidates for pharmacological intervention, rendering ferroptosis-related proteins attractive targets for the treatment of numerous currently incurable diseases. Herein, the current members of a Germany-wide research consortium focusing on ferroptosis research, as well as key external experts in ferroptosis who have made seminal contributions to this rapidly growing and exciting field of research, have gathered to provide a comprehensive, state-of-the-art review on ferroptosis. Specific topics include: basic mechanisms, in vivo relevance, specialized methodologies, chemical and pharmacological tools, and the potential contribution of ferroptosis to disease etiopathology and progression. We hope that this article will not only provide established scientists and newcomers to the field with an overview of the multiple facets of ferroptosis, but also encourage additional efforts to characterize further molecular pathways modulating ferroptosis, with the ultimate goal to develop novel pharmacotherapies to tackle the various diseases associated with - or caused by - ferroptosis.

Selenoprotein P expression in glioblastoma as a regulator of ferroptosis sensitivity: preservation of GPX4 via the cycling-selenium storage

Glioblastoma (GBM) is one of the most aggressive and deadly brain tumors; however, its current therapeutic strategies are limited. Selenoprotein P (SeP; SELENOP, encoded by the SELENOP gene) is a unique selenium-containing protein that exhibits high expression levels in astroglia. SeP is thought to be associated with ferroptosis sensitivity through the induction of glutathione peroxidase 4 (GPX4) via selenium supplementation. In this study, to elucidate the role of SeP in GBM, we analyzed its expression in GBM patients and found that SeP expression levels were significantly higher when compared to healthy subjects. Knock down of SeP in cultured GBM cells resulted in a decrease in GPX1 and GPX4 protein levels. Under the same conditions, cell death caused by RSL3, a ferroptosis inducer, was enhanced, however this enhancement was canceled by supplementation of selenite. These results indicate that SeP expression contributes to preserving GPX and selenium levels in an autocrine/paracrine manner, i.e., SeP regulates a dynamic cycling-selenium storage system in GBM. We also confirmed the role of SeP expression in ferroptosis sensitivity using patient-derived primary GBM cells. These findings indicate that expression of SeP in GBM can be a significant therapeutic target to overcome anticancer drug resistance.

Efficacy of feeding hydroxy-selenomethionine on plasma and milk selenium in mid-lactation dairy cows

In this study, we aimed to determine the amount of Se transferred to milk and blood of mid- to late-lactation dairy cows when supplemental Se from hydroxy-selenomethionine (OH-SeMet) was fed compared with an unsupplemented group and a group supplemented with a seleno-yeast (SY). Twenty-four lactating Holstein cows (178 ± 43 d in milk) were used in a complete randomized block design for 91 d (7-d covariate period and 84-d treatment period). Treatments were (1) basal diet with an analyzed Se background of 0.2 mg of Se per kg as-fed (control); (2) basal diet + 0.3 mg of Se/kg as-fed from SY (SY-0.3); (3) basal diet + 0.1 mg of Se/kg as-fed from OH-SeMet (OH-SeMet-0.1); and (4) basal diet + 0.3 mg of Se/kg as-fed from OH-SeMet (OH-SeMet-0.3). During the trial, plasma and milk were analyzed for total Se, and plasma was analyzed for glutathione peroxidase activity. The mean plasma and milk Se concentrations exhibited the same relationship, where OH-SeMet-0.3 resulted in the highest values (142 µg/L of plasma and 104 µg/kg of milk), followed by SY-0.3 (134 µg/L and 85 µg/kg), OH-SeMet-0.1 (122 µg/L and 67 µg/kg), and the control group had the lowest values (120 µg/L and 50 µg/kg). The increment of Se in milk induced by OH-SeMet-0.3 (+54 µg/kg) was 54% higher than that induced by SY-0.3 (+35 µg/kg). Additionally, dietary supplementation of 0.2 mg/kg Se from OH-SeMet in the total mixed ration was estimated to be similar to 0.3 mg/kg Se from SY in the total mixed ration when considering the level of Se in the milk. There was no difference in plasma glutathione peroxidase activity between groups; however, OH-SeMet-0.3 significantly decreased somatic cell count. The results confirmed that supplementation with organic Se increases milk and plasma Se concentrations. Moreover, when administered at the same level of supplementation, OH-SeMet was shown to be more efficient than SY in improving milk quality by increasing Se content and decreasing milk somatic cell count.

Selenium in Bodily Homeostasis: Hypothalamus, Hormones, and Highways of Communication

The ability of the body to maintain homeostasis requires constant communication between the brain and peripheral tissues. Different organs produce signals, often in the form of hormones, which are detected by the hypothalamus. In response, the hypothalamus alters its regulation of bodily processes, which is achieved through its own pathways of hormonal communication. The generation and transmission of the molecules involved in these bi-directional axes can be affected by redox balance. The essential trace element selenium is known to influence numerous physiological processes, including energy homeostasis, through its various redox functions. Selenium must be obtained through the diet and is used to synthesize selenoproteins, a family of proteins with mainly antioxidant functions. Alterations in selenium status have been correlated with homeostatic disturbances in humans and studies with animal models of selenoprotein dysfunction indicate a strong influence on energy balance. The relationship between selenium and energy metabolism is complicated, however, as selenium has been shown to participate in multiple levels of homeostatic communication. This review discusses the role of selenium in the various pathways of communication between the body and the brain that are essential for maintaining homeostasis.

The health benefits of selenium in food animals: a review

Selenium is an essential trace mineral important for the maintenance of homeostasis in animals and humans. It evinces a strong antioxidant, anti-inflammatory and potential antimicrobial capacity. Selenium biological function is primarily achieved by its presence in selenoproteins as a form of selenocysteine. Selenium deficiency may result in an array of health disorders, affecting many organs and systems; to prevent this, dietary supplementation, mainly in the forms of organic (i.e., selenomethionine and selenocysteine) inorganic (i.e., selenate and selenite) sources is used. In pigs as well as other food animals, dietary selenium supplementation has been used for improving growth performance, immune function, and meat quality. A substantial body of knowledge demonstrates that dietary selenium supplementation is positively associated with overall animal health especially due to its immunomodulatory activity and protection from oxidative damage. Selenium also possesses potential antiviral activity and this is achieved by protecting immune cells against oxidative damage and decreasing viral replication. In this review we endeavor to combine established and novel knowledge on the beneficial effects of dietary selenium supplementation, its antioxidant and immunomodulatory actions, and the putative antimicrobial effect thereof. Furthermore, our review demonstrates the gaps in knowledge pertaining to the use of selenium as an antiviral, underscoring the need for further in vivo and in vitro studies, particularly in pigs.

Regulation of A-to-I RNA editing and stop codon recoding to control selenoprotein expression during skeletal myogenesis

Selenoprotein N (SELENON), a selenocysteine (Sec)-containing protein with high reductive activity, maintains redox homeostasis, thereby contributing to skeletal muscle differentiation and function. Loss-of-function mutations in SELENON cause severe neuromuscular disorders. In the early-to-middle stage of myoblast differentiation, SELENON maintains redox homeostasis and modulates endoplasmic reticulum (ER) Ca2+ concentration, resulting in a gradual reduction from the middle-to-late stages due to unknown mechanisms. The present study describes post-transcriptional mechanisms that regulate SELENON expression during myoblast differentiation. Part of an Alu element in the second intron of SELENON pre-mRNA is frequently exonized during splicing, resulting in an aberrant mRNA that is degraded by nonsense-mediated mRNA decay (NMD). In the middle stage of myoblast differentiation, ADAR1-mediated A-to-I RNA editing occurs in the U1 snRNA binding site at 5' splice site, preventing Alu exonization and producing mature mRNA. In the middle-to-late stage of myoblast differentiation, the level of Sec-charged tRNASec decreases due to downregulation of essential recoding factors for Sec insertion, thereby generating a premature termination codon in SELENON mRNA, which is targeted by NMD.

Inherited Disorders of Thyroid Hormone Metabolism Defect Caused by the Dysregulation of Selenoprotein Expression

Consistent activation and functioning of thyroid hormones are essential to the human body as a whole, especially in controlling the metabolic rate of all organs and systems. Impaired sensitivity to thyroid hormones describes any process that interferes with the effectiveness of thyroid hormones. The genetic origin of inherited thyroid hormone defects and the investigation of genetic defects upon the processing of thyroid hormones are of utmost importance. Impaired sensitivity to thyroid hormone can be categorized into three conditions: thyroid hormone cell membrane transport defect (THCMTD), thyroid hormone metabolism defect (THMD), and thyroid hormone action defect (THAD). THMD is caused by defects in the synthesis and processing of deiodinases that convert the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3). Deiodinase, a selenoprotein, requires unique translation machinery that is collectively composed of the selenocysteine (Sec) insertion sequence (SECIS) elements, Sec-insertion sequence-binding protein 2 (SECISBP2), Sec-specific eukaryotic elongation factor (EEFSEC), and Sec-specific tRNA (TRU-TCA1-1), which leads to the recognition of the UGA codon as a Sec codon for translation into the growing polypeptide. In addition, THMD could be expanded to the defects of enzymes that are involved in thyroid hormone conjugation, such as glucuronidation and sulphation. Paucity of inherited disorders in this category leaves them beyond the scope of this review. This review attempts to specifically explore the genomic causes and effects that result in a significant deficiency of T3 hormones due to inadequate function of deiodinases. Moreover, along with SECISBP2, TRU-TCA1-1, and deiodinase type-1 (DIO1) mutations, this review describes the variants in DIO2 single nucleotide polymorphism (SNP) and thyroid stimulating hormone receptor (TSHR) that result in the reduced activity of DIO2 and subsequent abnormal conversion of T3 from T4. Finally, this review provides additional insight into the general functionality of selenium supplementation and T3/T4 combination treatment in patients with hypothyroidism, suggesting the steps that need to be taken in the future.

Roles for Selenoprotein I and Ethanolamine Phospholipid Synthesis in T Cell Activation

The selenoprotein family includes 25 members, many of which are antioxidant or redox regulating enzymes. A unique member of this family is Selenoprotein I (SELENOI), which does not catalyze redox reactions, but instead is an ethanolamine phosphotransferase (Ept). In fact, the characteristic selenocysteine residue that defines selenoproteins lies far outside of the catalytic domain of SELENOI. Furthermore, data using recombinant SELENOI lacking the selenocysteine residue have suggested that the selenocysteine amino acid is not directly involved in the Ept reaction. SELENOI is involved in two different pathways for the synthesis of phosphatidylethanolamine (PE) and plasmenyl PE, which are constituents of cellular membranes. Ethanolamine phospholipid synthesis has emerged as an important process for metabolic reprogramming that occurs in pluripotent stem cells and proliferating tumor cells, and this review discusses roles for upregulation of SELENOI during T cell activation, proliferation, and differentiation. SELENOI deficiency lowers but does not completely diminish de novo synthesis of PE and plasmenyl PE during T cell activation. Interestingly, metabolic reprogramming in activated SELENOI deficient T cells is impaired and this reduces proliferative capacity while favoring tolerogenic to pathogenic phenotypes that arise from differentiation. The implications of these findings are discussed related to vaccine responses, autoimmunity, and cell-based therapeutic approaches.

Hydroxy-Selenomethionine Improves the Selenium Status and Helps to Maintain Broiler Performances under a High Stocking Density and Heat Stress Conditions through a Better Redox and Immune Response

This study has determined whether hydroxy-selenomethionine (OH-SeMet) exerts a better protective action on broilers against environmental stress than sodium selenite (SS) or seleno-yeast (SY). Day-old male Cobb 500 broilers (12 cages/diet, 9 broilers/cage) were fed a selenium (Se)-deficient diet (0.047 mg/kg) supplemented with SS, SY or OH-SeMet at 0.3 mg Se/kg under a high stocking density and heat stress condition for six weeks. OH-SeMet improved the FCR and Se concentration in the tissues than SS and SY. SY and OH-SeMet both reduced the serum cortisol, T3, IL-6, IgA, IgM and LPS, more than SS, while only OH-SeMet further increased IL-10 and IgG. SY and OH-SeMet improved the intestinal morphology and increased the T-AOC, TXRND, SELENON and OCCLUDIN activities but decreased CLAUDIN2 in the jejunum than SS, while OH-SeMet further improved these values than SY. SY and OH-SeMet both increased SELENOS and TXNRD2 in the muscles than SS, and OH-SeMet further raised T-AOC, GPX4, SELENOP, SELENOW and TXNRD1, and reduced malondialdehyde and protein carbonyl in the muscles than SS and SY. OH-SeMet showed a better ability to maintain the performance and the redox and immune status of broilers under a high stocking density and heat stress challenge than SS and SY.

Identification of Selenoprotein H Isoforms and Impact of Selenoprotein H Overexpression on Protein But Not mRNA Levels of 2 Other Selenoproteins in 293T Cells

BACKGROUND

Selenoprotein H (SELONOH), a member of the thioredoxin-like family proteins, is prioritized to degradation in selenium (Se) insufficiency. Recent studies implicate protective roles of SELENOH in oxidative stress, cellular senescence, and intestinal tumorigenesis. Although the nonselenoprotein H0YE28 is suggested as shortened SELENOH according to genomic and proteomic data repositories, this variant has not been verified biochemically.

OBJECTIVES

We sought to identify SELENOH isoforms and explore the impact of Se flux on selenoprotein expression in SELENOH-overexpressing cells.

METHODS

A vector expressing a FLAG (the DYKDDDDK sequence) tag on the N-terminal end of wild-type SELENOH was constructed and transiently transfected into 293T cells incubated with graded concentrations of Na2SeO3 (0-200 nM). Cells were subjected to immunoprecipitation, LC-MS/MS protein analysis, immunoblotting, qRT-PCR, and senescence assays. Data were analyzed by 1-way or 2-way ANOVA.

RESULTS

Results of anti-FLAG immunoblotting showed that FLAG-SELENOH transfection increased (3.7-fold; P < 0.05) protein levels of the long, but not the short, SELENOH variants in the presence of Na2SeO3 (100 nM). By contrast, SELENOH mRNA levels were increased by 53-fold upon FLAG-SELENOH transfection but were comparable with or without supplemental Se (100 nM). LC-MS/MS analyses of anti-FLAG immunoprecipitates designated both anti-FLAG bands as SELENOH and co-identified three 60S ribosomal and 9 other proteins. Overexpression of FLAG-SELENOH 1) reduced glutathione peroxidase 1 and thioredoxin reductase 1 expression at the protein rather than the mRNA level in the absence but not presence of supplemental Se (100 nM; P < 0.05); 2) increased mRNA levels of 3 heat shock proteins (HSP27, HSP70-1A, and HSP70-1B; P < 0.05); and 3) reduced senescence induced by H2O2 (20 μM, 4 hours; P < 0.05).

CONCLUSIONS

These cellular studies demonstrate a Se-independent, shortened SELENOH variant and suggest competition of overexpressed FLAG-SELENOH with 2 other selenoproteins for the expression at the protein but not the mRNA level in Se insufficiency.

Juvenile Selenium Deficiency Impairs Cognition, Sensorimotor Gating, and Energy Homeostasis in Mice

Selenium (Se) is an essential micronutrient of critical importance to mammalian life. Its biological effects are primarily mediated via co-translational incorporation into selenoproteins, as the unique amino acid, selenocysteine. These proteins play fundamental roles in redox signaling and includes the glutathione peroxidases and thioredoxin reductases. Environmental distribution of Se varies considerably worldwide, with concomitant effects on Se status in humans and animals. Dietary Se intake within a narrow range optimizes the activity of Se-dependent antioxidant enzymes, whereas both Se-deficiency and Se-excess can adversely impact health. Se-deficiency affects a significant proportion of the world's population, with hypothyroidism, cardiomyopathy, reduced immunity, and impaired cognition being common symptoms. Although relatively less prevalent, Se-excess can also have detrimental consequences and has been implicated in promoting both metabolic and neurodegenerative disease in humans. Herein, we sought to comprehensively assess the developmental effects of both Se-deficiency and Se-excess on a battery of neurobehavioral and metabolic tests in mice. Se-deficiency elicited deficits in cognition, altered sensorimotor gating, and increased adiposity, while Se-excess was surprisingly beneficial.

Selenium and viral infection: are there lessons for COVID-19?

Se is a micronutrient essential for human health. Sub-optimal Se status is common, occurring in a significant proportion of the population across the world including parts of Europe and China. Human and animal studies have shown that Se status is a key determinant of the host response to viral infections. In this review, we address the question whether Se intake is a factor in determining the severity of response to coronavirus disease 2019 (COVID-19). Emphasis is placed on epidemiological and animal studies which suggest that Se affects host response to RNA viruses and on the molecular mechanisms by which Se and selenoproteins modulate the inter-linked redox homeostasis, stress response and inflammatory response. Together these studies indicate that Se status is an important factor in determining the host response to viral infections. Therefore, we conclude that Se status is likely to influence human response to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and that Se status is one (of several) risk factors which may impact on the outcome of SARS-CoV-2 infection, particularly in populations where Se intake is sub-optimal or low. We suggest the use of appropriate markers to assess the Se status of COVID-19 patients and possible supplementation may be beneficial in limiting the severity of symptoms, especially in countries where Se status is regarded as sub-optimal.

Adaptive Thermogenesis in a Mouse Model Lacking Selenoprotein Biosynthesis in Brown Adipocytes

Selenoproteins are a class of proteins with the selenium-containing amino acid selenocysteine (Sec) in their primary structure. Sec is incorporated into selenoproteins via recoding of the stop codon UGA, with specific cis and trans factors required during translation to avoid UGA recognition as a stop codon, including a Sec-specific tRNA, tRNA[Ser]Sec, encoded in mice by the gene Trsp. Whole-body deletion of Trsp in mouse is embryonically lethal, while targeted deletion of Trsp in mice has been used to understand the role of selenoproteins in the health and physiology of various tissues. We developed a mouse model with the targeted deletion of Trsp in brown adipocytes (Trspf/f-Ucp1-Cre+/-), a cell type predominant in brown adipose tissue (BAT) controlling energy expenditure via activation of adaptive thermogenesis, mostly using uncoupling protein 1 (Ucp1). At room temperature, Trspf/f-Ucp1-Cre+/- mice maintain oxygen consumption and Ucp1 expression, with male Trspf/f-Ucp1-Cre+/- mice accumulating more triglycerides in BAT than both female Trspf/f-Ucp1-Cre+/- mice or Trspf/f controls. Acute cold exposure neither reduced core body temperature nor changed the expression of selenoprotein iodothyronine deiodinase type II (Dio2), a marker of adaptive thermogenesis, in Trspf/f-Ucp1-Cre+/- mice. Microarray analysis of BAT from Trspf/f-Ucp1-Cre+/- mice revealed glutathione S-transferase alpha 3 (Gsta3) and ELMO domain containing 2 (Elmod2) as the transcripts most affected by the loss of Trsp. Male Trspf/f-Ucp1-Cre+/- mice showed mild hypothyroidism while downregulating thyroid hormone-responsive genes Thrsp and Tshr in their BATs. In summary, modest changes in the BAT of Trspf/f-Ucp1-Cre +/- mice implicate a mild thyroid hormone dysfunction in brown adipocytes.

Effects of selenium supplementation on diet-induced obesity in mice with a disruption of the selenocysteine lyase gene

BACKGROUND

The amino acid selenocysteine (Sec) is an integral part of selenoproteins, a class of proteins mostly involved in strong redox reactions. The enzyme Sec lyase (SCLY) decomposes Sec into selenide allowing for the recycling of the selenium (Se) atom via the selenoprotein synthesis machinery. We previously demonstrated that disruption of the Scly gene (Scly KO) in mice leads to the development of obesity and metabolic syndrome, with effects on glucose homeostasis, worsened by Se deficiency or a high-fat diet, and exacerbated in male mice. Our objective was to determine whether Se supplementation could ameliorate obesity and restore glucose homeostasis in the Scly KO mice.

METHODS

Three-weeks old male and female Scly KO mice were fed in separate experiments a diet containing 45 % kcal fat and either sodium selenite or a mixture of sodium selenite and selenomethionine (selenite/SeMet) at moderate (0.25 ppm) or high (0.5-1 ppm) levels for 9 weeks, and assessed for metabolic parameters, oxidative stress and expression of selenoproteins.

RESULTS

Se supplementation was unable to prevent obesity and elevated epididymal white adipose tissue weights in male Scly KO mice. Serum glutathione peroxidase activity in Scly KO mice was unchanged regardless of sex or dietary Se intake; however, supplementation with a mixture of selenite/SeMet improved oxidative stress biomarkers in the male Scly KO mice.

CONCLUSION

These results unveil sex- and selenocompound-specific regulation of energy metabolism after the loss of Scly, pointing to a role of this enzyme in the control of whole-body energy metabolism regardless of Se levels.

Selenoproteins as regulators of T cell proliferation, differentiation, and metabolism

Selenium (Se) is an essential micronutrient that plays a key role in regulating the immune system. T cells are of particular interest due to their important role in promoting adaptive immunity against pathogens and cancer as well as regulating tolerance, all of which are influenced by dietary Se levels. The biological effects of Se are mainly exerted through the actions of the proteins into which it is inserted, i.e. selenoproteins. Thus, the roles that selenoproteins play in regulating T cell biology and molecular mechanisms involved have emerged as important areas of research for understanding how selenium affects immunity. Members of this diverse family of proteins exhibit a wide variety of functions within T cells that include regulating calcium flux induced by T cell receptor (TCR) engagement, shaping the redox tone of T cells before, during, and after activation, and linking TCR-induced activation to metabolic reprogramming required for T cell proliferation and differentiation. This review summarizes recent insights into the roles that selenoproteins play in these processes and their implications in understanding how Se may influence immunity.

Nutrient Control of mRNA Translation

The emergence of genome-wide analyses to interrogate cellular DNA, RNA, and protein content has revolutionized the study of control networks that mediate cellular homeostasis. mRNA translation represents the last step of genetic flow and primarily defines the proteome. Translational regulation is thus critical for gene expression, in particular under nutrient excess or deficiency. Until recently, it was unclear how the global effects of translational control are orchestrated by nutrient signaling pathways. An emerging concept of translational reprogramming addresses how to maintain the expression of specific proteins during nutrient stress by translation of selective mRNAs. In this review, we describe recent advances in our understanding of translational control principles; nutrient-sensing mechanisms; and their dysregulation in human diseases such as diabetes, cancer, and aging. The mechanistic understanding of translational regulation in response to different nutrient conditions may help identify potential dietary and therapeutic targets to improve human health.

Mechanisms and Regulation of Nonsense-Mediated mRNA Decay and Nonsense-Associated Altered Splicing in Lymphocytes

The presence of premature termination codons (PTCs) in transcripts is dangerous for the cell as they encode potentially deleterious truncated proteins that can act with dominant-negative or gain-of-function effects. To avoid the synthesis of these shortened polypeptides, several RNA surveillance systems can be activated to decrease the level of PTC-containing mRNAs. Nonsense-mediated mRNA decay (NMD) ensures an accelerated degradation of mRNAs harboring PTCs by using several key NMD factors such as up-frameshift (UPF) proteins. Another pathway called nonsense-associated altered splicing (NAS) upregulates transcripts that have skipped disturbing PTCs by alternative splicing. Thus, these RNA quality control processes eliminate abnormal PTC-containing mRNAs from the cells by using positive and negative responses. In this review, we describe the general mechanisms of NMD and NAS and their respective involvement in the decay of aberrant immunoglobulin and TCR transcripts in lymphocytes.

2-Hydroxy-(4-methylseleno)butanoic Acid Is Used by Intestinal Caco-2 Cells as a Source of Selenium and Protects against Oxidative Stress

BACKGROUND

Selenium (Se) participates in different functions in humans and other animals through its incorporation into selenoproteins as selenocysteine. Inadequate dietary Se is considered a risk factor for several chronic diseases associated with oxidative stress.

OBJECTIVE

The role of 2-hydroxy-(4-methylseleno)butanoic acid (HMSeBA), an organic form of Se used in animal nutrition, in supporting selenoprotein synthesis and protecting against oxidative stress was investigated in an in vitro model of intestinal Caco-2 cells.

METHODS

Glutathione peroxidase (GPX) and thioredoxin reductase (TXNRD) activities, selenoprotein P1 protein (SELENOP) and gene (SELENOP) expression, and GPX1 and GPX2 gene expression were studied in Se-deprived (FBS removal) and further HMSeBA-supplemented (0.1-625 μM, 72 h) cultures. The effect of HMSeBA supplementation (12.5 and 625 μM, 24 h) on oxidative stress induced by H2O2 (1 mM) was evaluated by the production of reactive oxygen species (ROS), 4-hydroxy-2-nonenal (4-HNE) adducts, and protein carbonyl residues compared with a sodium selenite control (SS, 5 μM).

RESULTS

Se deprivation induced a reduction (P < 0.05) in GPX activity (62%), GPX1 expression, and both SELENOP (33%) and SELENOP expression. In contrast, an increase (P < 0.05) in GPX2 expression and no effect in TXNRD activity (P = 0.09) were observed. HMSeBA supplementation increased (P < 0.05) GPX activity (12.5-625 μM, 1.68-1.82-fold) and SELENOP protein expression (250 and 625 μM, 1.87- and 2.04-fold). Moreover, HMSeBA supplementation increased (P < 0.05) GPX1 (12.5 and 625 μM), GPX2 (625 μM), and SELENOP (12.5 and 625 μM) expression. HMSeBA (625 μM) was capable of decreasing (P < 0.05) ROS (32%), 4-HNE adduct (49%), and protein carbonyl residue (75%) production after H2O2 treatment.

CONCLUSION

Caco-2 cells can use HMSeBA as an Se source for selenoprotein synthesis, resulting in protection against oxidative stress.

Molecular Mechanisms by Which Selenoprotein K Regulates Immunity and Cancer

Many of the 25 members of the selenoprotein family function as enzymes that utilize their selenocysteine (Sec) residues to catalyze redox-based reactions. However, some selenoproteins likely do not exert enzymatic activity by themselves and selenoprotein K (SELENOK) is one such selenoprotein family member that uses its Sec residue in an alternative manner. SELENOK is an endoplasmic reticulum (ER) transmembrane protein that has been shown to be important for ER stress and for calcium-dependent signaling. Molecular mechanisms for the latter have recently been elucidated using knockout mice and genetically manipulated cell lines. These studies have shown that SELENOK interacts with an enzyme in the ER membrane, DHHC6 (letters represent the amino acids aspartic acid, histidine, histidine, and cysteine in the catalytic domain), and the SELENOK/DHHC6 complex catalyzes the transfer of acyl groups such as palmitate to cysteine residues in target proteins, i.e., palmitoylation. One protein palmitoylated by SELENOK/DHHC6 is the calcium channel protein, the inositol 1,4,5-trisphosphate receptor (IP3R), which is acylated as a means for stabilizing the tetrameric calcium channel in the ER membrane. Factors that lower SELENOK levels or function impair IP3R-driven calcium flux. This role for SELENOK is important for the activation and proliferation of immune cells, and recently, a critical role for SELENOK in promoting calcium flux for the progression of melanoma has been demonstrated. This review provides a summary of these findings and their implications in terms of designing new therapeutic interventions that target SELENOK for treating cancers like melanoma.

Bioefficacy of hydroxy-selenomethionine as a selenium supplement in pregnant dairy heifers and on the selenium status of their calves

This study aimed to determine the effects of supplementing pregnant heifers with the organic selenium (Se) source 2-hydroxy-4-methylselenobutanoic acid (HMSeBA) during the last 8 wk of pregnancy on dam and calf Se status. A total of 42 in-calf heifers were recruited to the study and randomly allocated to 1 of 3 treatments; a negative control (Con), sodium selenite (NaSe), or HMSeBA. Animals were blocked by body weight, body condition score, and expected calving date before treatment allocation. Following enrollment, all animals underwent a 7-wk wash-out period, after which they received their respective supplements, top-dressed daily onto a basal diet for the last 8 wk of pregnancy. Heifer blood samples were taken at weekly intervals from enrollment until 2 wk before expected calving date and as soon as possible after calving for determination of whole-blood glutathione peroxidase activity (GSH-Px) and plasma Se and malondialdehyde (MDA) concentrations. Selenized AA were determined in plasma samples taken at 3 wk precalving. A colostrum sample was taken as close to parturition as possible for determination of colostrum total Se, selenized AA, and IgG concentration. Calves were blood sampled as close to birth as possible for determination of whole-blood GSH-Px activity and plasma Se and MDA concentrations. Differences in whole-blood GSH-Px activity did not become apparent until calving; GSH-Px activity was lowest in Con heifers but similar between NaSe and HMSeBA heifers. Plasma Se was lowest in unsupplemented heifers and greatest in those supplemented with HMSeBA; this was attributable to greater selenomethionine concentrations in the plasma of HMSeBA heifers. Colostrum Se was lowest in Con heifers and greatest in HMSeBA heifers. The greater Se concentration of HMSeBA heifers was attributable to a greater proportion of total Se comprising selenocysteine; the reason for this is not known. There was no effect of supplementation on colostrum IgG concentration. Plasma Se was lowest in calves born to Con heifers and greatest in those born to HMSeBA heifers. There were no effects of treatment on calf whole-blood GSH-Px activity or plasma MDA concentration. The enhanced Se status associated with HMSeBA supplementation is likely a consequence of selenomethionine supply and may confer benefits to both the dam and her calf postpartum.

Gene ontology analysis of expanded porcine blastocysts from gilts fed organic or inorganic selenium combined with pyridoxine

Background

Gene ontology analysis using the microarray database generated in a previous study by this laboratory was used to further evaluate how maternal dietary supplementation with pyridoxine combined with different sources of selenium (Se) affected global gene expression of expanded porcine blastocysts. Data were generated from 18 gilts randomly assigned to one of three experimental diets (n = 6 per treatment): i) basal diet without supplemental Se or pyridoxine (CONT); ii) CONT + 0.3 mg/kg of Na-selenite and 10 mg/kg of HCl-pyridoxine (MSeB₆10); and iii) CONT + 0.3 mg/kg of Se-enriched yeast and 10 mg/kg of HCl-pyridoxine (OSeB₆10). All gilts were inseminated at their fifth post-pubertal estrus and euthanized 5 days later for embryo harvesting. Differential gene expression between MSeB₆10 vs CONT, OSeB₆10 vs CONT and OSeB₆10 vs MSeB₆10 was performed using a porcine embryo-specific microarray.

Results

There were 559, 2458, and 1547 differentially expressed genes for MSeB₆10 vs CONT, OSeB₆10 vs CONT and OSeB₆10 vs MSeB₆10, respectively. MSeB₆10 vs CONT stimulated 13 biological processes with a strict effect on RNA binding and translation initiation. OSeB₆10 vs CONT and OSeB₆10 vs MSeB₆10 impacted 188 and 66 biological processes, respectively, with very similar effects on genome stability, ceramide biosynthesis, protein trafficking and epigenetic events. The stimulation of genes related with these processes was confirmed by quantitative real-time RT-PCR.

Conclusions

Gene expression of embryos from OSeB₆10 supplemented gilts was more impacted than those from MSeB₆10 supplemented gilts. Whereas maternal OSeB₆10 supplementation influenced crucial aspects of embryo development, maternal MSeB₆10 supplementation was restricted to binding activity.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5237-1) contains supplementary material, which is available to authorized users.

Roles for selenium and selenoprotein P in the development, progression, and prevention of intestinal disease

Selenium (Se) is a micronutrient essential to human health, the function of which is mediated in part by incorporation into a class of proteins known as selenoproteins (SePs). As many SePs serve antioxidant functions, Se has long been postulated to protect against inflammation and cancer development in the gut by attenuating oxidative stress. Indeed, numerous studies over the years have correlated Se levels with incidence and severity of intestinal diseases such as inflammatory bowel disease (IBD) and colorectal cancer (CRC). Similar results have been obtained with the Se transport protein, selenoprotein P (SELENOP), which is decreased in the plasma of both IBD and CRC patients. While animal models further suggest that decreases in Se or SELENOP augment colitis and intestinal tumorigenesis, large-scale clinical trials have yet to show a protective effect in patient populations. In this review, we discuss the function of Se and SELENOP in intestinal diseases and how research into these mechanisms may impact patient treatment.

Beneficial and paradoxical roles of selenium at nutritional levels of intake in healthspan and longevity

Accumulation of genome and macromolecule damage is a hallmark of aging, age-associated degeneration, and genome instability syndromes. Although processes of aging are irreversible, they can be modulated by genome maintenance pathways and environmental factors such as diet. Selenium (Se) confers its physiological functions mainly through selenoproteins, but Se compounds and other proteins that incorporate Se nonspecifically also impact optimal health. Bruce Ames proposed that the aging process could be mitigated by a subset of low-hierarchy selenoproteins whose levels are preferentially reduced in response to Se deficiency. Consistent with this notion, results from two selenotranscriptomic studies collectively implicate three low-hierarchy selenoproteins in age or senescence. Experimental evidence generally supports beneficial roles of selenoproteins in the protection against damage accumulation and redox imbalance, but some selenoproteins have also been reported to unexpectedly display harmful functions under sporadic conditions. While longevity and healthspan are usually thought to be projected in parallel, emerging evidence suggests a trade-off between longevity promotion and healthspan deterioration with damage accumulation. We propose that longevity promotion under conditions of Se deficiency may be attributed to 1) stress-response hormesis, an advantageous event of resistance to toxic chemicals at low doses; 2) reduced expression of selenoproteins with paradoxical functions to a lesser extent. In particular, selenoprotein H is an evolutionally conserved nuclear selenoprotein postulated to confer Se functions in redox regulation, genome maintenance, and senescence. This review highlights the need to pinpoint roles of specific selenoproteins and Se compounds in healthspan and lifespan for a better understanding of Se contribution at nutritional levels of intake to healthy aging.

Selenium-Related Transcriptional Regulation of Gene Expression

The selenium content of the body is known to control the expression levels of numerous genes, both so-called selenoproteins and non-selenoproteins. Selenium is a trace element essential to human health, and its deficiency is related to, for instance, cardiovascular and myodegenerative diseases, infertility and osteochondropathy called Kashin–Beck disease. It is incorporated as selenocysteine to the selenoproteins, which protect against reactive oxygen and nitrogen species. They also participate in the activation of the thyroid hormone, and play a role in immune system functioning. The synthesis and incorporation of selenocysteine occurs via a special mechanism, which differs from the one used for standard amino acids. The codon for selenocysteine is a regular in-frame stop codon, which can be passed by a specific complex machinery participating in translation elongation and termination. This includes a presence of selenocysteine insertion sequence (SECIS) in the 3′-untranslated part of the selenoprotein mRNAs. Nonsense-mediated decay is involved in the regulation of the selenoprotein mRNA levels, but other mechanisms are also possible. Recent transcriptional analyses of messenger RNAs, microRNAs and long non-coding RNAs combined with proteomic data of samples from Keshan and Kashin–Beck disease patients have identified new possible cellular pathways related to transcriptional regulation by selenium.

Selenium, Selenoproteins, and Immunity

Selenium is an essential micronutrient that plays a crucial role in development and a wide variety of physiological processes including effect immune responses. The immune system relies on adequate dietary selenium intake and this nutrient exerts its biological effects mostly through its incorporation into selenoproteins. The selenoproteome contains 25 members in humans that exhibit a wide variety of functions. The development of high-throughput omic approaches and novel bioinformatics tools has led to new insights regarding the effects of selenium and selenoproteins in human immuno-biology. Equally important are the innovative experimental systems that have emerged to interrogate molecular mechanisms underlying those effects. This review presents a summary of the current understanding of the role of selenium and selenoproteins in regulating immune cell functions and how dysregulation of these processes may lead to inflammation or immune-related diseases.

The influence of selenium and selenoproteins on immune responses of poultry and pigs

Selenium is an essential nutrient for poultry and pigs, and is important for a number of physiological processes including regulation and function of the immune system. Through its incorporation into selenoproteins, Se is involved in the regulation of oxidative stress, redox mechanisms, and other crucial cellular processes involved in innate and adaptive immune response. This review provides current knowledge on the mechanisms by which selenium can modulate the resilience to infectious diseases, and how this micronutrient can influence the capacity of the bird or the pig to maintain its productivity during an infectious challenge. In relation to the most frequent and economically important infectious diseases in poultry and pig production, the present paper considers the influence of different selenium sources (organic vs. inorganic Se) as well as dietary concentrations on the immune responses of poultry and pigs with major emphasis on the potential beneficial impact on animal resilience to common infectious diseases.

Selenium requirements based on muscle and kidney selenoprotein enzyme activity and transcript expression in the turkey poult (Meleagris gallopavo)

The current NRC selenium (Se) requirement for turkeys is 0.2 μg Se/g diet. We previously fed turkey poults a Se-deficient diet (0.005 μg Se/g) supplemented with 10 graded levels of Se (0, 0.025, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0 μg Se/g as Na2SeO3, 5/treatment) for 4 wk, and found that the minimum dietary Se requirement was 0.3 μg Se/g based on selenoprotein enzyme activity in blood, liver, gizzard and pancreas. Because the turkey is primarily a production animal, we expanded this analysis to kidney, heart, breast and thigh. Se concentrations in Se-deficient poults were 5.0, 9.8, 33, and 15% of levels in poults fed 0.4 μg Se/g in liver, kidney, thigh and breast, respectively. Increasing Se supplementation resulted in hyperbolic response curves for all tissues; breakpoint analysis indicated minimum Se requirements of 0.34-0.36 μg Se/g based on tissue Se levels in liver, kidney and thigh. Similarly, GPX1 activity in muscle tissues and kidney responded hyperbolically to increasing dietary Se, reaching well-defined plateaus with breakpoints at 0.30-0.36 μg Se/g. Minimum Se requirements based on GPX4 activity were 0.30-0.32 μg Se/g for breast and thigh. Selenoprotein transcript expression decreased significantly in Se deficiency for only 2, 3, 5, and 6 mRNA in breast, thigh, heart, and kidney, respectively, out of 24 known avian selenoproteins. Se response curves for regulated selenoprotein transcripts were hyperbolic, and reached well-defined plateaus with breakpoints in a narrow range of 0.08-0.19 μg Se/g. No selenoprotein transcript was altered by supernutritional Se. In summary, these results clearly indicate that the NRC dietary Se requirement should be raised to 0.4 μg Se/g, at least for poults, to meet the nutritional needs of the young turkey. The Se response curve plateaus further show that limits for turkey supplementation with selenite could safely be raised to 0.5 μg Se/g diet.

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.

Analyses of Selenotranscriptomes and Selenium Concentrations in Response to Dietary Selenium Deficiency and Age Reveal Common and Distinct Patterns by Tissue and Sex in Telomere-Dysfunctional Mice

Background: The hierarchies of tissue selenium distribution and selenotranscriptomes are thought to critically affect healthspan and longevity.Objective: We determined selenium status and selenotranscriptomes in response to long-term dietary selenium deficiency and age in tissues of male and female mice.Methods: Weanling telomerase RNA component knockout C57BL/6 mice were fed a selenium-deficient (0.03 mg Se/kg) Torula yeast-based AIN-93G diet or a diet supplemented with sodium selenate (0.15 mg Se/kg) until age 18 or 24 mo. Plasma, hearts, kidneys, livers, and testes were collected to assay for selenotranscriptomes, selected selenoproteins, and tissue selenium concentrations. Data were analyzed with the use of 2-factor ANOVA (diet × age) in both sexes.Results: Dietary selenium deficiency decreased (P ≤ 0.05) selenium concentrations (65-72%) and glutathione peroxidase (GPX) 3 (82-94%) and selenoprotein P (SELENOP) (17-41%) levels in the plasma of both sexes of mice and mRNA levels (9-68%) of 4, 4, and 12 selenoproteins in the heart, kidney, and liver of males, respectively, and 5, 16, and 14 selenoproteins, respectively, in females. Age increased selenium concentrations and SELENOP levels (27% and 30%, respectively; P ≤ 0.05) in the plasma of males only but decreased (12-46%; P < 0.05) mRNA levels of 1, 5, and 13 selenoproteins in the heart, kidney, and liver of males, respectively, and 6, 5, and 0 selenoproteins, respectively, in females. Among these mRNAs, selenoprotein H (Selenoh), selenoprotein M (Selenom), selenoprotein W (Selenow), methionine-R-sulfoxide reductase 1 (MsrB1), Gpx1, Gpx3, thioredoxin reductase 1 (Txnrd1), Txnrd2, selenoprotein S (Selenos), selenoprotein F (Selenof), and selenoprotein O (Selenoo) responded in parallel to dietary selenium deficiency and age in ≥1 tissue or sex, or both. Dietary selenium deficiency upregulated (40-160%; P ≤ 0.05) iodothyronine deiodinase 2 (Dio2) and selenoprotein N (Selenon) in the kidneys of males. Age upregulated (11-44%; P < 0.05) Selenon in the kidneys of males, selenoprotein K (Selenok) and selenoprotein I (Selenoi) in the kidneys of females, and Selenof and Selenok in the testes.Conclusions: These results illustrate tissue-specific sexual dimorphisms of selenium status and selenotranscriptomes because of dietary selenium deficiency and age.

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.

Control of gene expression through the nonsense-mediated RNA decay pathway

Nonsense-mediated RNA decay (NMD) was originally discovered as a cellular surveillance pathway that safeguards the quality of mRNA transcripts in eukaryotic cells. In its canonical function, NMD prevents translation of mutant mRNAs harboring premature termination codons (PTCs) by targeting them for degradation. However, recent studies have shown that NMD has a much broader role in gene expression by regulating the stability of many normal transcripts. In this review, we discuss the function of NMD in normal physiological processes, its dynamic regulation by developmental and environmental cues, and its association with human disease.

Opposing impacts on healthspan and longevity by limiting dietary selenium in telomere dysfunctional mice

Selenium (Se) is a trace metalloid essential for life, but its nutritional and physiological roles during the aging process remain elusive. While telomere attrition contributes to replicative senescence mainly through persistent DNA damage response, such an aging process is mitigated in mice with inherently long telomeres. Here, weanling third generation telomerase RNA component knockout mice carrying short telomeres were fed a Se‐deficient basal diet or the diet supplemented with 0.15 ppm Se as sodium selenate to be nutritionally sufficient throughout their life. Dietary Se deprivation delayed wound healing and accelerated incidence of osteoporosis, gray hair, alopecia, and cataract, but surprisingly promoted longevity. Plasma microRNA profiling revealed a circulating signature of Se deprivation, and subsequent ontological analyses predicted dominant changes in metabolism. Consistent with this observation, dietary Se deprivation accelerated age‐dependent declines in glucose tolerance, insulin sensitivity, and glucose‐stimulated insulin production in the mice. Moreover, DNA damage and senescence responses were enhanced and Pdx1 and MafA mRNA expression were reduced in pancreas of the Se‐deficient mice. Altogether, these results suggest a novel model of aging with conceptual advances, whereby Se at low levels may be considered a hormetic chemical and decouple healthspan and longevity.

Selenoproteins in Tumorigenesis and Cancer Progression

Selenium is a micronutrient essential to human health and has long been associated with cancer prevention. Functionally, these effects are thought to be mediated by a class of selenium-containing proteins known as selenoproteins. Indeed, many selenoproteins have antioxidant activity which can attenuate cancer development by minimizing oxidative insult and resultant DNA damage. However, oxidative stress is increasingly being recognized for its "double-edged sword" effect in tumorigenesis, whereby it can mediate both negative and positive effects on tumor growth depending on the cellular context. In addition to their roles in redox homeostasis, recent work has also implicated selenoproteins in key oncogenic and tumor-suppressive pathways. Together, these data suggest that the overall contribution of selenoproteins to tumorigenesis is complicated and may be affected by a variety of factors. In this review, we discuss what is currently known about selenoproteins in tumorigenesis with a focus on their contextual roles in cancer development, growth, and progression.

Modeling and gene knockdown to assess the contribution of nonsense-mediated decay, premature termination, and selenocysteine insertion to the selenoprotein hierarchy

The expression of selenoproteins, a specific group of proteins that incorporates selenocysteine, is hierarchically regulated by the availability of Se, with some, but not all selenoprotein mRNA transcripts decreasing in abundance with decreasing Se. Selenocysteine insertion into the peptide chain occurs during translation following recoding of an internal UGA stop codon. There is increasing evidence that this UGA recoding competes with premature translation termination, which is followed by nonsense-mediated decay (NMD) of the transcript. In this study, we tested the hypothesis that the susceptibility of different selenoprotein mRNAs to premature termination during translation and differential sensitivity of selenoprotein transcripts to NMD are major factors in the selenoprotein hierarchy. Selenoprotein transcript abundance was measured in Caco-2 cells using real-time PCR under different Se conditions and the data obtained fitted to mathematical models of selenoprotein translation. A calibrated model that included a combination of differential sensitivity of selenoprotein transcripts to NMD and different frequency of non-NMD related premature translation termination was able to fit all the measurements. The model predictions were tested using SiRNA to knock down expression of the crucial NMD factor UPF1 (up-frameshift protein 1) and selenoprotein mRNA expression. The calibrated model was able to predict the effect of UPF1 knockdown on gene expression for all tested selenoproteins, except SPS2 (selenophosphate synthetase), which itself is essential for selenoprotein synthesis. These results indicate an important role for NMD in the hierarchical regulation of selenoprotein mRNAs, with the exception of SPS2 whose expression is likely regulated by a different mechanism.

Selenium Deficiency Affects the mRNA Expression of Inflammatory Factors and Selenoprotein Genes in the Kidneys of Broiler Chicks

The aim of this study was to investigate the influence of Se deficiency on the transcription of inflammatory factors and selenoprotein genes in the kidneys of broiler chicks. One hundred fifty 1-day-old broiler chicks were randomly assigned to two groups fed with either a low-Se diet (L group, 0.033 mg/kg Se) or an adequate Se diet (C group, 0.2 mg/kg Se). The levels of uric acid (UA) and creatinine (Cr) in the serum and the mRNA levels of 6 inflammatory factors and 25 selenoprotein genes in the kidneys were measured as the clinical signs of Se deficiency occurred at 20 days old. The results indicated that the contents of UA and Cr in the serum increased in L group (p < 0.05), and the mRNA levels of the inflammatory factors (NF-κB, iNOS, COX-2, and TNF-α) increased in L group (p < 0.05). Meanwhile, the mRNA levels of PTGEs and HO-1 were not changed. In addition, 25 selenoprotein transcripts displayed ubiquitous expression in the kidneys of the chicks. The mRNA levels of 14 selenoprotein genes (Dio1, Dio2, GPx3, Sepp1, SelH, SelI, SelK, Sepn1, SelO, SelW, Sep15, SelT, SelU, and SelS) decreased, and 9 selenoprotein genes (GPx1, GPx2, GPx4, SelPb, Txnrd1, Txnrd2, Txnrd3, SPS2, and SelM) increased in L group (p < 0.05), but the Dio3 and Sepx1 mRNA levels did not change. The results indicated that Se deficiency resulted in kidney dysfunction, activation of the NF-κB pathway, and a change in selenoprotein gene expression. The changes of inflammatory factor and selenoprotein gene expression levels were directly related to the abnormal renal functions induced by Se deficiency.

Selenium and Metabolic Disorders: An Emphasis on Type 2 Diabetes Risk

Selenium (Se) is a micronutrient that maintains biological functions through the action of Se containing proteins known as selenoproteins. Due to the known antioxidant effects of Se, supplements containing Se have been on the rise. While Se supplementation may be beneficial for Se deficient populations, few are at risk for Se deficiency due to the transportation of food from Se-rich regions and the rise of Se-enriched foods. Alarmingly, Se supplementation may have adverse effects in people who already receive an adequate Se supply. Specifically, an increased risk of type 2 diabetes has been reported in individuals with high baseline Se levels. However, this effect was restricted to males, suggesting the relationship between Se and glucose homeostasis may be sexually dimorphic. This review will discuss the current understanding of the interaction between Se and glucose homeostasis, including any sex differences that have been described.

Mechanism, factors, and physiological role of nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is a translation-dependent, multistep process that degrades irregular or faulty messenger RNAs (mRNAs). NMD mainly targets mRNAs with a truncated open reading frame (ORF) due to premature termination codons (PTCs). In addition, NMD also regulates the expression of different types of endogenous mRNA substrates. A multitude of factors are involved in the tight regulation of the NMD mechanism. In this review, we focus on the molecular mechanism of mammalian NMD. Based on the published data, we discuss the involvement of translation termination in NMD initiation. Furthermore, we provide a detailed overview of the core NMD machinery, as well as several peripheral NMD factors, and discuss their function. Finally, we present an overview of diseases associated with NMD factor mutations and summarize the current state of treatment for genetic disorders caused by nonsense mutations.

Nuclear selenoproteins and genome maintenance

Selenium is an essential metalloid required for the expression of selenoproteins. While cells are constantly challenged by clastogens of endogenous and exogenous origins, genome integrity is maintained by direct repair of DNA damage, redox balance, and epigenetic regulation. To date, only five selenoproteins are experimentally demonstrated to reside in nucleus, exclusively or partially, including selenoprotein H, methionine-R-sulfoxide reductase 1, glutathione peroxidase-4, thioredoxin reductase-1, and thioredoxin glutathione reductase. All these five selenoproteins have demonstrated or potential roles in redox regulation and genome maintenance. Selenoprotein H is known to transactivate the expression of a couple of genes against oxidative stress. The thioredoxin reductase-1b isoform delivers estrogen receptor-α and -β to the nucleus. Nuclear glutathione peroxidase-4 epigenetically and globally inhibits gene expression through the maintenance of chromatin compactness in testes. Continued studies on how these and additional nuclear selenoproteins regulate genome stability will have profound impact on advancing our understanding in selenium regulation of optimal health. © 2015 IUBMB Life, 68(1):5-12, 2016.

Nonsense-Mediated mRNA Decay: Degradation of Defective Transcripts Is Only Part of the Story

Nonsense-mediated mRNA decay (NMD) is a eukaryotic surveillance mechanism that monitors cytoplasmic mRNA translation and targets mRNAs undergoing premature translation termination for rapid degradation. From yeasts to humans, activation of NMD requires the function of the three conserved Upf factors: Upf1, Upf2, and Upf3. Here, we summarize the progress in our understanding of the molecular mechanisms of NMD in several model systems and discuss recent experiments that address the roles of Upf1, the principal regulator of NMD, in the initial targeting and final degradation of NMD-susceptible mRNAs. We propose a unified model for NMD in which the Upf factors provide several functions during premature termination, including the stimulation of release factor activity and the dissociation and recycling of ribosomal subunits. In this model, the ultimate degradation of the mRNA is the last step in a complex premature termination process.

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.

Nonsense-mediated mRNA decay: an intricate machinery that shapes transcriptomes

Nonsense-mediated mRNA decay (NMD) is probably the best characterized eukaryotic RNA degradation pathway. Through intricate steps, a set of NMD factors recognize and degrade mRNAs with translation termination codons that are positioned in abnormal contexts. However, NMD is not only part of a general cellular quality control system that prevents the production of aberrant proteins. Mammalian cells also depend on NMD to dynamically adjust their transcriptomes and their proteomes to varying physiological conditions. In this Review, we discuss how NMD targets mRNAs, the types of mRNAs that are targeted, and the roles of NMD in cellular stress, differentiation and maturation processes.

SELENOPROTEINS. CRL2 aids elimination of truncated selenoproteins produced by failed UGA/Sec decoding

Selenocysteine (Sec) is translated from the codon UGA, typically a termination signal. Codon duality extends the genetic code; however, the coexistence of two competing UGA-decoding mechanisms immediately compromises proteome fidelity. Selenium availability tunes the reassignment of UGA to Sec. We report a CRL2 ubiquitin ligase-mediated protein quality-control system that specifically eliminates truncated proteins that result from reassignment failures. Exposing the peptide immediately N-terminal to Sec, a CRL2 recognition degron, promotes protein degradation. Sec incorporation destroys the degron, protecting read-through proteins from detection by CRL2. Our findings reveal a coupling between directed translation termination and proteolysis-assisted protein quality control, as well as a cellular strategy to cope with fluctuations in organismal selenium intake.

Regulation of Selenium Metabolism and Transport

Selenium is regulated in the body to maintain vital selenoproteins and to avoid toxicity. When selenium is limiting, cells utilize it to synthesize the selenoproteins most important to them, creating a selenoprotein hierarchy in the cell. The liver is the central organ for selenium regulation and produces excretory selenium forms to regulate whole-body selenium. It responds to selenium deficiency by curtailing excretion and secreting selenoprotein P (Sepp1) into the plasma at the expense of its intracellular selenoproteins. Plasma Sepp1 is distributed to tissues in relation to their expression of the Sepp1 receptor apolipoprotein E receptor-2, creating a tissue selenium hierarchy. N-terminal Sepp1 forms are taken up in the renal proximal tubule by another receptor, megalin. Thus, the regulated whole-body pool of selenium is shifted to needy cells and then to vital selenoproteins in them to supply selenium where it is needed, creating a whole-body selenoprotein hierarchy.

Nonsense-mediated RNA decay--a switch and dial for regulating gene expression

Nonsense-mediated RNA decay (NMD) represents an established quality control checkpoint for gene expression that protects cells from consequences of gene mutations and errors during RNA biogenesis that lead to premature termination during translation. Characterization of NMD-sensitive transcriptomes has revealed, however, that NMD targets not only aberrant transcripts but also a broad array of mRNA isoforms expressed from many endogenous genes. NMD is thus emerging as a master regulator that drives both fine and coarse adjustments in steady-state RNA levels in the cell. Importantly, while NMD activity is subject to autoregulation as a means to maintain homeostasis, modulation of the pathway by external cues provides a means to reprogram gene expression and drive important biological processes. Finally, the unanticipated observation that transcripts predicted to lack protein-coding capacity are also sensitive to this translation-dependent surveillance mechanism implicates NMD in regulating RNA function in new and diverse ways.