Sex-Specific Metabolic Impairments in a Mouse Model of Disrupted Selenium Utilization

The essential micronutrient selenium (Se) provides antioxidant defense and supports numerous biological functions. Obtained through dietary intake, Se is incorporated into selenoproteins via the amino acid, selenocysteine (Sec). Mice with genetic deletion of the Se carrier, selenoprotein P (SELENOP), and the Se recycling enzyme selenocysteine lyase (SCLY), suffer from sexually dimorphic neurological deficits and require Se supplementation for viability. These impairments are more pronounced in males and are exacerbated by dietary Se restriction. We report here that, by 10 weeks of age, female Selenop/Scly double knockout (DKO) mice supplemented with 1 mg/ml sodium selenite in drinking water develop signs of hyper-adiposity not seen in male DKO mice. Unexpectedly, this metabolic phenotype can be reversed by removing Se from the drinking water at post-natal day 22, just prior to puberty. Restricting access to Se at this age prevents excess body weight gain and restriction from either post-natal day 22 or 37 reduces gonadal fat deposits. These results provide new insight into the sex-dependent relationship between Se and metabolic homeostasis.

Effect of Statin Treatment on Obese Mice Lacking Selenocysteine Lyase

Objectives Americans are a selenium-replete population with high use of statins, a drug commonly prescribed to ameliorate hypercholesterolemia. Statins act by inhibiting the first step of cholesterol biosynthesis in the liver, the mevalonate pathway. This pathway is also important for the maturation of the selenocysteine tRNA, responsible for the expression of selenoproteins. Selenoprotein deficits were suggested as responsible for side effects of statins, such as increased oxidative stress leading to muscle cramps and myopathies. Selenium for selenoprotein synthesis can be provided by diet or by the actions of the selenium recycling enzyme selenocysteine lyase (Scly). Our objective was to investigate the effects of the use of statins in an animal model lacking the enzyme Scly and fed a high-fat, Se-supplemented diet. Methods Age-matched homozygous littermates wild-type C57BL/6 N (WT) and Scly knockout (Scly KO) mice were fed a high-fat diet containing a 1 ppm blend of selenite plus selenomethionine, and were daily treated with simvastatin (5 mg/kg of body weight) for 21 days. We analyzed serum cholesterol levels, oxidative stress and creatine kinase (CK) activity, and liver and skeletal muscle gene expression of selenoproteins and enzymes involved in creatine metabolism. Results Obese Scly KO mice had a sex-dependent response statin treatment. While female Scly KO mice improved their body weight after statin treatment, male Scly KO mice further gained weight. Statin treatment improved serum oxidative stress in the Scly KO mice, despite sharp elevation of CK activity. Statin treatment did not affect hepatic selenoprotein gene expression of Scly KO mice, but impaired glutathione peroxidase 1 expression in the muscle of male Scly KO mice, while enhancing it in females. Conclusions Responses to statin treatment in Scly KO mice fed a high-fat, Se-supplemented diet vary according to sex, benefiting more female than male mice, and are mostly independent of selenoprotein expression. Funding Sources National Institutes of Health (NIH) grants U54MD007601 Ola Hawaii (subproject 5544) and R01DK47320, and Fundação de Amparo à Pesquisa do Estado de São Paulo fellowship 2018/0,9478–4. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

Exploring the selenium-over-sulfur substrate specificity and kinetics of a bacterial selenocysteine lyase

ABSTRACTSelenium is a vital micronutrient in many organisms. While traces are required for utilization by the microbe, excess amounts are toxic; thus, selenium can be regarded as a biological “double-edged sword”. Selenium is chemically similar to the essential element sulfur, but curiously, evolution has selected the former over the latter for a subset of oxidoreductases. Enzymes involved in sulfur metabolism are less discriminate in terms of preventing selenium incorporation; however, its specific incorporation into selenoproteins reveals a highly discriminate process that is not completely understood. We have identified SclA, a selenocysteine lyase in the nosocomial pathogen, Enterococcus faecalis, and characterized its enzymatic activity and specificity for L-selenocysteine over L-cysteine. It is known that a single residue in the human selenocysteine lyase, D146, is considered to control selenocysteine specificity. Thus, using computational biology, we identified H100, a D146 ortholog in SclA, and generated mutant enzymes with site-directed mutagenesis. The proteins were overexpressed, purified, and characterized for their biochemical properties. All mutants exhibited varying levels of activity towards L-selenocysteine, suggesting a catalytic role for H100. Additionally, L-cysteine acted as a competitive inhibitor towards all enzymes with higher affinity than L-selenocysteine. Our findings offer key insight into the molecular mechanisms behind selenium-over-sulfur specificity and may further elucidate the role of selenocysteine lyases in vivo.

Trypanosomatid selenophosphate synthetase structure, function and interaction with selenocysteine lyase

Early branching eukaryotes have been used as models to study the evolution of cellular molecular processes. Strikingly, human parasite of the Trypanosomatidae family (T. brucei, T. cruzi and L. major) conserve the complex machinery responsible for selenocysteine biosynthesis and incorporation in selenoproteins (SELENOK/SelK, SELENOT/SelT and SELENOTryp/SelTryp), although these proteins do not seem to be essential for parasite viability under laboratory controlled conditions. Selenophosphate synthetase (SEPHS/SPS) plays an indispensable role in selenium metabolism, being responsible for catalyzing the formation of selenophosphate, the biological selenium donor for selenocysteine synthesis. We solved the crystal structure of the L. major selenophosphate synthetase and confirmed that its dimeric organization is functionally important throughout the domains of life. We also demonstrated its interaction with selenocysteine lyase (SCLY) and showed that it is not present in other stable complexes involved in the selenocysteine pathway, namely the phosphoseryl-tRNASec kinase (PSTK)-Sec-tRNASec synthase (SEPSECS) and the tRNASec-specific elongation factor (eEFSec)-ribosome. Endoplasmic reticulum stress with ditiothreitol (DTT) or tunicamycin upon selenophosphate synthetase ablation in procyclic T. brucei cells led to a growth defect. On the other hand, only DTT presented a negative effect in bloodstream T. brucei expressing selenophosphate synthetase-RNAi. Although selenoprotein T (SELENOT) was dispensable for both forms of the parasite, SELENOT-RNAi procyclic T. brucei cells were sensitive to DTT. Together, our data suggest a role for the T. brucei selenophosphate synthetase in regulation of the parasite’s ER stress response.SynopsisSelenium is both a toxic compound and a micronutrient. As a micronutrient, it participates in the synthesis of specific proteins, selenoproteins, as the amino acid selenocysteine. The synthesis of selenocysteine is present in organisms ranging from bacteria to humans. The protozoa parasites of the Trypanosomatidae family, that cause major tropical diseases, conserve the complex machinery responsible for selenocysteine biosynthesis and incorporation in selenoproteins. However, this pathway has been considered dispensable for the protozoa cells. This has intrigued us, and lead to question that if maintained in the cell it should be under selective pressure and therefore be necessary. Also, since the intermediate products of selenocysteine synthesis are toxic to the cell, it has been proposed that these compounds need to be sequestered from the cytoplasm. Therefore, extensive and dynamic protein-protein interactions must happen to deliver those intermediates along the pathway. In this study we have investigated the molecular and structural interactions of different proteins involved in selenocystein synthesis and describe its involvement in the endoplasmic reticulum protection to oxidative stress. Our results also show how the interaction of different proteins leads to the protection of the cell against the toxic effects of seleium compounds during selenocysteine synthesis.

Combined Omics Reveals That Disruption of the Selenocysteine Lyase Gene Affects Amino Acid Pathways in Mice

Selenium is a nonmetal trace element that is critical for several redox reactions and utilized to produce the amino acid selenocysteine (Sec), which can be incorporated into selenoproteins. Selenocysteine lyase (SCL) is an enzyme which decomposes Sec into selenide and alanine, releasing the selenide to be further utilized to synthesize new selenoproteins. Disruption of the selenocysteine lyase gene (Scly) in mice (Scly−/− or Scly KO) led to obesity with dyslipidemia, hyperinsulinemia, glucose intolerance and lipid accumulation in the hepatocytes. As the liver is a central regulator of glucose and lipid homeostasis, as well as selenium metabolism, we aimed to pinpoint hepatic molecular pathways affected by the Scly gene disruption. Using RNA sequencing and metabolomics, we identified differentially expressed genes and metabolites in the livers of Scly KO mice. Integrated omics revealed that biological pathways related to amino acid metabolism, particularly alanine and glycine metabolism, were affected in the liver by disruption of Scly in mice with selenium adequacy. We further confirmed that hepatic glycine levels are elevated in male, but not in female, Scly KO mice. In conclusion, our results reveal that Scly participates in the modulation of hepatic amino acid metabolic pathways.