Selenoproteins in mammalian spermatogenesis:role of the nuclear GPx4

The selenoprotein Phospholipid Hydroperoxide Glutathione Peroxidase (PHGPx/GPx4) is an enzyme unique among the various GPxs, because it is able to use protein thiols, beside glutathione, The GPx4 gene encodes for three isoforms having different subcellular localization, being located in the mitochondria (mGPx4), in the cytosol (cGPx4) and in the nucleus (nGPx4), each having distinct functions. The mGPx4 is important to male fertility, as proven by the structural abnormalities of sperm tails from KO mice specifically lacking this isoform, which make these mutants infertile. As for the nuclear isoform, nGPx4-KO mice are fertile but show impaired nuclear condensation of sperm isolated from the caput epididymis, suggesting a role in chromatin stability. To gain more insight into the functions of nGPx4, we first investigated the subnuclear localization of this form in both COS-1 cells overexpressing nGPx4 and mouse male germ cells at different steps of maturation (round spermatids and epididymal spermatozoa). We performed both biochemical and morphological analyses and found that nGPx4 was localized at the level of the nuclear matrix. To test the functional role in chromatin dynamics sperm isolated from the caput and the cauda epididymides from WT and nGPx4-KO mice were subjected to an in vitro chromatin decondensation assay. Our results show that nGPx4-KO mice sperm decondensed earlier than those from WT at all stages of epididymal maturation, providing conclusive evidence that nGPx4 is required for a correct sperm chromatin remodelling. We next addressed the issue whether the sperm nuclear structure instability caused by the lack of nGPx4 might impact on the early events occurring after fertilization. In "in vitro" fertilization experiments, we revealed that, compared to WT, nGPx4-KO mice showed an acceleration of sperm nuclear decondensation, confirming the results previously obtained.