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.

The selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx) changes its physical characteristics and biological functions during sperm maturation. PHGPx exists as a soluble peroxidase in spermatids but persists in mature spermatozoa as an enzymatically inactive, oxidatively cross-linked, insoluble protein. In the midpiece of mature spermatozoa, PHGPx protein represents at least 50 percent of the capsule material that embeds the helix of mitochondria. The role of PHGPx as a structural protein may explain the mechanical instability of the mitochondrial midpiece that is observed in selenium deficiency.

Ferroptosis is a form of cell death primed by iron and lipid hydroperoxides and prevented by GPx4. Ferrostatin-1 (fer-1) inhibits ferroptosis much more efficiently than phenolic antioxidants. Previous studies on the antioxidant efficiency of fer-1 adopted kinetic tests where a diazo compound generates the hydroperoxyl radical scavenged by the antioxidant. However, this reaction, accounting for a chain breaking effect, is only minimally useful for the description of the inhibition of ferrous iron and lipid hydroperoxide dependent peroxidation. Scavenging lipid hydroperoxyl radicals, indeed, generates lipid hydroperoxides from which ferrous iron initiates a new peroxidative chain reaction. We show that when fer-1 inhibits peroxidation, initiated by iron and traces of lipid hydroperoxides in liposomes, the pattern of oxidized species produced from traces of pre-existing hydroperoxides is practically identical to that observed following exhaustive peroxidation in the absence of the antioxidant. This supported the notion that the anti-ferroptotic activity of fer-1 is actually due to the scavenging of initiating alkoxyl radicals produced, together with other rearrangement products, by ferrous iron from lipid hydroperoxides. Notably, fer-1 is not consumed while inhibiting iron dependent lipid peroxidation. The emerging concept is that it is ferrous iron itself that reduces fer-1 radical. This was supported by electroanalytical evidence that fer-1 forms a complex with iron and further confirmed in cells by fluorescence of calcein, indicating a decrease of labile iron in the presence of fer-1. The notion of such as pseudo-catalytic cycle of the ferrostatin-iron complex was also investigated by means of quantum mechanics calculations, which confirmed the reduction of an alkoxyl radical model by fer-1 and the reduction of fer-1 radical by ferrous iron. In summary, GPx4 and fer-1 in the presence of ferrous iron, produces, by distinct mechanism, the most relevant anti-ferroptotic effect, i.e the disappearance of initiating lipid hydroperoxides.

Selenium is linked to male fertility. Glutathione peroxidase 4 (GPx4), first described as an antioxidant enzyme, is the predominant selenoenzyme in testis and has been suspected of being vital for spermatogenesis. Cytosolic, mitochondrial, and nuclear isoforms are all encoded by the same gene. While disruption of entire GPx4 causes early embryonic lethality in mice, inactivation of nuclear GPx4 does not impair embryonic development or fertility. Here, we show that deletion of mitochondrial GPx4 (mGPx4) allows both normal embryogenesis and postnatal development, but causes male infertility. Infertility was associated with impaired sperm quality and severe structural abnormalities in the midpiece of spermatozoa. Knockout sperm display higher protein thiol content and recapitulate features typical of severe selenodeficiency. Interestingly, male infertility induced by mGPx4 depletion could be bypassed by intracytoplasmic sperm injection. We also show for the first time that mGPx4 is the prevailing GPx4 product in male germ cells and that mGPx4 disruption has no effect on proliferation or apoptosis of germinal or somatic tissue. Our study finally establishes that mitochondrial GPx4 confers the vital role of selenium in mammalian male fertility and identifies cytosolic GPx4 as the only GPx4 isoform being essential for embryonic development and apoptosis regulation.

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPx) is regarded as the major molecular target of selenodeficiency in rodents, accounting for most of the histopathological and structural abnormalities of testicular tissue and male germ cells. PHGPx exists as a cytosolic form, mitochondrial form, and nuclear form (nPHGPx) predominantly expressed in late spermatids and spermatozoa. Here, we demonstrate that mice with a targeted deletion of the nPHGPx gene were, unlike mice with the full knockout (KO) of PHGPx, not only viable but also, surprisingly, fully fertile. While both morphological analysis of testis and epididymis and sperm parameter measurements did not show any apparent abnormality, toluidine blue and acridine orange stainings of spermatozoa indicated defective chromatin condensation in the KO sperm isolated from the caput epididymis. Furthermore, upon drying and hydrating, KO sperm exhibited a significant proportion of morphologically abnormal heads. Monobromobimane labeling and protein-free thiol titration revealed significantly less extensive oxidation in the cauda epididymis when compared to that in the wild type. We conclude that nPHGPx, by acting as a protein thiol peroxidase in vivo, contributes to the structural stability of sperm chromatin.Sperm chromatin condensation during the final steps of spermatogenesis in mammals is a multistep process that includes the sequential replacement of the majority of histones by transition proteins and protamines in testis (6, 7). During epididymal transit of spermatozoa, protamine thiol oxidation is completed and intra-and intermolecular cross-links are formed. Hence, a transcriptionally inactive and tightly packed haploid genome is generated rendering sperm nuclei more resistant to mechanical and chemical insults (2). Recently, Cho and colleagues showed that chimeric mice hemizygous for protamine 1 or 2 fail to transmit the targeted allele to the germ line (8).Selenium depletion studies of rodents clearly demonstrated the importance of this trace element in male fertility. Third generation selenium deficiency is associated with structural abnormalities, such as broken midpieces of sperm tails, giant heads, and reversible testicular atrophy (5, 34). Due to its particular high expression in mammalian testis (21) and its resistance to selenium deprivation in testis, the selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPx) is thought to account for most of the defects associated with severe selenium deficiency.PHGPx was initially characterized as a lipid peroxidationinhibiting protein (33) and was later shown to be an unusual member of the glutathione peroxidase family, in particular for its scarce specificity for both the oxidizing and reducing substrates (32). Most relevant in this respect was the observation that, in the presence of low glutathione (GSH) concentration, specific protein -SH groups may act as a reductant in the catalytic cycle with a stoichiometry of 2 equivalents of thiol per mole of hydroperoxide (13,22,...