Selective Restoration of the Selenoprotein Population in a Mouse Hepatocyte Selenoproteinless Background with Different Mutant Selenocysteine tRNAs Lacking Um34

Abstract

Novel mouse models were developed in which the hepatic selenoprotein population was targeted for removal by disrupting the selenocysteine (Sec) tRNA [Ser]Sec gene (trsp), and selenoprotein expression was then restored by introducing wild type or mutant trsp transgenes. The selenoprotein population was partially replaced in liver with mutant transgenes encoding mutations at either position 34 (34T3 A) or 37 (37A3 G) in tRNA[Ser]Sec . The A34 transgene product lacked the highly modified 5-methoxycarbonylmethyl-2-O-methyluridine, and its mutant base A was converted to I34. The G37 transgene product lacked the highly modified N 6 -isopentenyladenosine. Both mutant tRNAs lacked the 2-methylribose at position 34 (Um34), and both supported expression of housekeeping selenoproteins (e.g. thioredoxin reductase 1) in liver but not stress-related proteins (e.g. glutathione peroxidase 1). Thus, Um34 is responsible for synthesis of a select group of selenoproteins rather than the entire selenoprotein population. The ICA anticodon in the A34 mutant tRNA decoded Cys codons, UGU and UGC, as well as the Sec codon, UGA. However, metabolic labeling of A34 transgenic mice with 75 Se revealed that selenoproteins incorporated the label from the A34 mutant tRNA, whereas other proteins did not. These results suggest that the A34 mutant tRNA did not randomly insert Sec in place of Cys, but specifically targeted selected selenoproteins. High copy numbers of A34 transgene, but not G37 transgene, were not tolerated in the absence of wild type trsp, further suggesting insertion of Sec in place of Cys in selenoproteins.There are 24 known selenoproteins in rodents and 25 in humans (1). The targeted removal of specific selenoproteins has shown that some are essential in development, whereas others appear to be nonessential. For example, the loss of selenoproteins glutathione peroxidase 4 (GPx4) (2) or thioredoxin reductase 1 (TR1 or Txnrd1) (3) or 2 (TR3 or Txnrd2) (4) is embryonic lethal, whereas the loss of glutathione peroxidase 1 (GPx1) (5) or 2 (GPx2) (6) appears to be of little or no consequence. Other studies, however, suggest that those selenoproteins whose loss results in little or no phenotypic change may function in protective mechanisms against certain environmental stresses (see Ref. 6 and references therein). There are selenoproteins whose removal or mutation results in dramatic effects on health. For example, knock-out of selenoprotein P (SelP) 6 causes neurological problems (7, 8), and knock-out of type 2 iodothyronine deiodinase results in a variety of defects, including an impaired adaptive thermogenesis and hypothermia in cold-exposed mice (see Ref. 9 and references therein), retarded cochlear development and hearing loss (10), and a pituitary resistance to thyroxine (11). Mutations affecting selenoprotein N (SelN) result in several muscle disorders (12, 13).LoxP-Cre technology, which allows the removal of embryonic lethal genes in specific tissues and organs (3,4,14), has been used to examine the roles of essential sele...

Nrf2 target genes are induced under marginal selenium-deficiency

“…The two Nrf2 selenoprotein target genes, GPx2 and TrxR1, are special cases because their translation depends on selenium availability and their ranking in the hierarchy. The mRNA of both high-ranking selenoproteins GPx2 [57] and TrxR1 [10] was significantly reduced in selenium-poor colon with GPx2 being less affected than TrxR1. In contrast, mRNA of both enzymes was enhanced in seleniumpoor duodenum (Fig.…”

“…These mice did not respond with a compensatory up-regulation of phase II enzymes, indicating that a housekeeping selenoprotein must prevent Nrf2 activation [50]. Since the thioredoxin reductase family belongs to the housekeeping selenoproteins [10], a liver-specific TrxR1 knockout was analyzed for potential activation of the Nrf2 pathway. Indeed, the TrxR1 knockout resulted in both, the nuclear accumulation of Nrf2 and the induction of 21 Nrf2 target genes [51].…”

Loss of housekeeping selenoprotein expression in mouse liver modulates lipoprotein metabolism

“…Several selenoprotein mRNAs were restored in A34 and G37 transgenic mice, such as Dio1, Selk, Sepp1, and Sep15 [13], while others like TR1 and TR3 were restored partially. Although mRNA levels might not necessarily match protein expression levels, our data suggest that some of these or other restored selenoproteins are responsible for the role of selenium in ApoE and cholesterol metabolism.…”

“…In addition, we generated transgenic mice in which DTrsp mice were rescued with a mutant Trsp transgene lacking i 6 A at position 37 (generated by an A37 fi G37 mutation and the resulting transgenic mouse was designed as G37) [12] or mcm 5 U at position 34 (generated by an T34 fi A34 mutation and the resulting transgenic mouse was designed as A34) in the Sec tRNA [Ser]Sec transgene product [13]. Interestingly, housekeeping, but not stress-related selenoproteins are synthesized by A34 and G37 Sec tRNAs [Ser]Sec that are transcribed and modified from the corresponding A34 and G37 transgenes [12].…”

Selenoproteins: Molecular Pathways and Physiological Roles

“…Interestingly, housekeeping selenoproteins (e.g., thioredoxin reductases TR1 and TR3) are synthesized by the mcm 5 U isoform, while stress-related selenoproteins (e.g., glutathione peroxidases GPx1 and GPx3, and selenoproteins MsrB1, SelT, and SelW) are synthesized by mcm 5 Um (44). There are also several selenoproteins, such as GPx4 and selenoprotein P, that appear to be synthesized by both isoforms (42).…”