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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.

Background— Excessive formation of reactive oxygen species contributes to tissue injury and functional deterioration after myocardial ischemia/reperfusion. Especially, mitochondrial reactive oxygen species are capable of opening the mitochondrial permeability transition pore, a harmful event in cardiac ischemia/reperfusion. Thioredoxins are key players in the cardiac defense against oxidative stress. Mutations in the mitochondrial thioredoxin reductase (thioredoxin reductase-2, Txnrd2) gene have been recently identified to cause dilated cardiomyopathy in patients. Here, we investigated whether mitochondrial thioredoxin reductase is protective against myocardial ischemia/reperfusion injury. Methods and Results— In mice, α-MHC-restricted Cre-mediated Txnrd2 deficiency, induced by tamoxifen ( Txnrd2-/-ic ), aggravated systolic dysfunction and cardiomyocyte cell death after ischemia (90 minutes) and reperfusion (24 hours). Txnrd2-/-ic was accompanied by a loss of mitochondrial integrity and function, which was resolved on pretreatment with the reactive oxygen species scavenger N-acetylcysteine and the mitochondrial permeability transition pore blocker cyclosporin A. Likewise, Txnrd2 deletion in embryonic endothelial precursor cells and embryonic stem cell-derived cardiomyocytes, as well as introduction of Txnrd2-shRNA into adult HL-1 cardiomyocytes, increased cell death on hypoxia and reoxygenation, unless N-acetylcysteine was coadministered. Conclusions— We report that Txnrd2 exerts a crucial function during postischemic reperfusion via thiol regeneration. The efficacy of cyclosporin A in cardiac Txnrd2 deficiency may indicate a role for Txnrd2 in reducing mitochondrial reactive oxygen species, thereby preventing opening of the mitochondrial permeability transition pore.

The increasing interest in biomedical applications of semiconductor quantum dots (QDs) is closely linked to the use of surface modifications to target specific sites of the body. The immense surface area of vascular endothelium is a possible interaction platform with systemically administered QDs. Therefore, the aim of this study was to investigate the microvascular distribution of neutral, cationic, and anionic QDs in vivo. QDs with carboxyl-, amine- and polyethylene glycol surface coatings were injected into the blood circulation of mice. In vivo microscopy of the cremaster muscle, two-photon microscopy of skeletal and heart muscle, as well as quantitative fluorescence measurements of blood, excreta, and tissue samples were performed. Transmission electron microscopy was used to detect QDs at the cellular level. The in vitro association of QDs with cultured endothelial cells was investigated by flow cytometry and confocal microscopy. Anionic QDs exhibited a very low residence time in the blood stream, preferably accumulated in organs with a prominent mononuclear phagocytic component, but were also found in other tissues with low phagocytic properties where they were predominantly associated with capillary endothelium. This deposition behavior was identified as a new, phagocyte-independent principle contributing to the rapid clearance of anionic QDs from the circulation.