Selenoproteins are rare proteins among all kingdoms of life containing the 21 amino acid, selenocysteine. Selenocysteine resembles cysteine, differing only by the substitution of selenium for sulfur. Yet the actual advantage of selenolate- versus thiolate-based catalysis has remained enigmatic, as most of the known selenoproteins also exist as cysteine-containing homologs. Here, we demonstrate that selenolate-based catalysis of the essential mammalian selenoprotein GPX4 is unexpectedly dispensable for normal embryogenesis. Yet the survival of a specific type of interneurons emerges to exclusively depend on selenocysteine-containing GPX4, thereby preventing fatal epileptic seizures. Mechanistically, selenocysteine utilization by GPX4 confers exquisite resistance to irreversible overoxidation as cells expressing a cysteine variant are highly sensitive toward peroxide-induced ferroptosis. Remarkably, concomitant deletion of all selenoproteins in Gpx4 cells revealed that selenoproteins are dispensable for cell viability provided partial GPX4 activity is retained. Conclusively, 200 years after its discovery, a specific and indispensable role for selenium is provided.

Summary Obesity and type-2 diabetes are associated with tissue-inflammation and metabolic defects in fat depots. Foxp3+regulatory T(Treg) cells mediate T-cell tolerance, thereby controlling tissue inflammation. However, the molecular underpinnings how environmental stimuli interlink T-cell tolerance with adipose tissue function remain largely unknown. Here, we report that cold exposure or beta3-adrenergic receptor (ADRB3) stimulation induces T-cell tolerance in vitro and in murine and humanized models. Tolerance induction was verified by CD4+T-cell-proteomes revealing higher protein expression of Foxp3 regulatory networks. Specifically, Ragulator-interacting protein C17orf59, which limits mTORC1 activity, was upregulated by either ADRB3-stimulation or cold-exposure, and therefore might enhance Treg induction. By loss and gain-of-function studies, including Treg depletion and transfers in vivo, we demonstrated that a T-cell-specific Stat6/Pten axis links cold-exposure or ADRB3 stimulation with Foxp3+Treg induction and adipose tissue function. Our findings open new avenues in understanding tissue-specific T-cell tolerance and the design of precision concepts toward personalized immune-metabolic health.