Cysteine mutant of mammalian GPx4 rescues cell death induced by disruption of the wild-type selenoenzyme.

Selenoproteins are expressed in many organisms, including bacteria, insects, fish, and mammals. Yet, it has remained obscure why some organisms rely on selenoproteins while others, like yeast and plants, express Cys-containing homologues. This study addressed the possible advantage of selenocysteine (Sec) vs. Cys in the essential selenoprotein glutathione peroxidase 4 (GPx4), using 4-hydroxy-tamoxifen-inducible Cre-excision of loxP-flanked GPx4 alleles in murine cells. Previously, it was shown that GPx4 disruption caused rapid cell death, which was prevented by α-tocopherol. Results presented herein demonstrate that the expression of wild-type (WT) GPx4 and its Sec/Cys (U46C) mutant rescued cell death of GPx4(-/-) cells, whereas the Sec/Ser (U46S) mutant failed. Notably, the specific activity of U46C was decreased by ∼90% and was indistinguishable from U46S-expressing and mock-transfected cells. Hence, the U46C mutant prevented apoptosis despite hardly measurable in vitro activity. Doxycycline-inducible expression revealed that minute amounts of either U46C or WT GPx4 prevented cell death, albeit WT GPx4 was more efficient. Interestingly, at the same expression level, proliferation was promoted in U46C-expressing cells but attenuated in WT-expressing cells. In summary, both catalytic efficiency and the expression level of GPx4 control the balance between cell survival and proliferation.

Mutations in the mitochondrial thioredoxin reductase gene TXNRD2 cause dilated cardiomyopathy.

AIMS: Cardiac energy requirement is met to a large extent by oxidative phosphorylation in mitochondria that are highly abundant in cardiac myocytes. Human mitochondrial thioredoxin reductase (TXNRD2) is a selenocysteine-containing enzyme essential for mitochondrial oxygen radical scavenging. Cardiac-specific deletion of Txnrd2 in mice results in dilated cardiomyopathy (DCM). The aim of this study was to investigate whether TXNRD2 mutations explain a fraction of monogenic DCM cases. METHODS AND RESULTS: Sequencing and subsequent genotyping of TXNRD2 in patients diagnosed with DCM (n = 227) and in DCM-free (n = 683) individuals from the general population sample KORA S4 was performed. The functional impact of observed mutations on Txnrd2 function was tested in mouse fibroblasts. We identified two novel amino acid residue-altering TXNRD2 mutations [175G > A (Ala59Thr) and 1124G > A (Gly375Arg)] in three heterozygous carriers among 227 patients that were not observed in the 683 DCM-free individuals. Both DCM-associated mutations result in amino acid substitutions of highly conserved residues in helices contributing to the flavin-adenine dinucleotide (FAD)-binding domain of TXNRD2. Functional analysis of both mutations in Txnrd2(-/-) mouse fibroblasts revealed that contrasting to wild-type (wt) Txnrd2, neither mutant did restore Txnrd2 function. Mutants even impaired the survival of Txnrd2 wt cells under oxidative stress by a dominant-negative mechanism. CONCLUSION: For the first time, we describe mutations in DCM patients in a gene involved in the regulation of cellular redox state. TXNRD2 mutations may explain a fraction of human DCM disease burden.

Three-dimensional positioning of genes in mouse cell nuclei.

To understand the regulation of the genome, it is necessary to understand its three-dimensional organization in the nucleus. We investigated the positioning of eight gene loci on four different chromosomes, including the beta-globin gene, in mouse embryonic stem cells and in in vitro differentiated macrophages by fluorescence in situ hybridization on structurally preserved nuclei, confocal microscopy, and 3D quantitative image analysis. We found that gene loci on the same chromosome can significantly differ from each other and from their chromosome territory in their average radial nuclear position. Radial distribution of a given gene locus can change significantly between cell types, excluding the possibility that positioning is determined solely by the DNA sequence. For the set of investigated gene loci, we found no relationship between radial distribution and local gene density, as it was described for human cell nuclei. We did find, however, correlation with other genomic properties such as GC content and certain repetitive elements such as long terminal repeats or long interspersed nuclear elements. Our results suggest that gene density itself is not a driving force in nuclear positioning. Instead, we propose that other genomic properties play a role in determining nuclear chromatin distribution.