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The selenium (Se)-containing antioxidant selenoneine (2-selenyl-Nα,Nα,Nα-trimethyl-l-histidine) has recently been discovered to be the predominant form of organic Se in tuna blood. Although dietary intake of fish Se has been suggested to reduce methylmercury (MeHg) toxicity, the molecular mechanism of MeHg detoxification by Se has not yet been determined. Here, we report evidence that selenoneine accelerates the excretion and demethylation of MeHg, mediated by a selenoneine-specific transporter, organic cations/carnitine transporter-1 (OCTN1). Selenoneine was incorporated into human embryonic kidney HEK293 cells transiently overexpressing OCTN1 and zebrafish blood cells by OCTN1. The Km for selenoneine uptake was 13.0 μM in OCTN1-overexpressing HEK293 cells and 9.5 μM in zebrafish blood cells, indicating high affinity of OCTN1 for selenoneine in human and zebrafish cells. When such OCTN1-expressing cells and embryos were exposed to MeHg–cysteine (MeHgCys), MeHg accumulation was decreased and the excretion and demethylation of MeHg were enhanced by selenoneine. In addition, exosomal secretion vesicles were detected in the culture water of embryos that had been microinjected with MeHgCys, suggesting that these may be responsible for MeHg excretion and demethylation. In contrast, OCTN1-deficient embryos accumulated MeHg, and MeHg excretion and demethylation were decreased. Furthermore, Hg accumulation was decreased in OCTN1-overexpressing HEK293 cells, but not in mock vector-transfected cells, indicating that selenoneine and OCTN1 can regulate MeHg detoxification in human cells. Thus, the selenoneine-mediated OCTN1 system regulates secretory lysosomal vesicle formation and MeHg demethylation.Electronic supplementary materialThe online version of this article (doi:10.1007/s10126-013-9508-1) contains supplementary material, which is available to authorized users.

Levels of the selenium-containing imidazole compound selenoneine and overall organic selenium were measured in the muscle of fishes by speciation analysis.

Kilometer‐scale fine structures of midlatitude sporadic E (Es) plasma patches have been directly imaged for the first time by an interferogram derived from L band Advanced Land Observation Satellite/Phased Array‐type L band Synthetic Aperture Radar data obtained over southwestern Japan. The synthetic aperture radar interferogram captured the eastern part of a large‐scale frontal structure of daytime midlatitude Es which spans over 250 km in the east‐northeast to west‐southwest direction. Fine structures are characterized by frontal and disc‐shaped patches which are elongated in the same direction as the large‐scale frontal structure. Length and width of the disc‐shaped patches are 10–20 km and 5–10 km, respectively, and they are quasi‐periodically located with a typical separation of 10–15 km. The Kelvin‐Helmholtz instability with the vertical shear of zonal winds is considered to be the most likely candidate for the generation mechanism of the frontal patch and disc‐shaped patches aligned in the zonal direction.