Conclusions

The data presented in this review clearly illustrate the ambiguous situation existing in maturing epididymal spermatozoa. Firstly, during the epididymal journey, spermatozoa use ROS to mediate disulfide-bridging events that are necessary for the completion of their structural modifications. Sperm DNA compaction is one of these structural changes that are not completed when spermatozoa enter the epididymis tubule. The increased sperm DNA compaction ensured by protamine oxidation in the epididymis is a crucial phenomenon that serves both to protect paternal DNA from mutational effects and to reduce the volume of the sperm head allowing an optimum velocity of mature spermatozoa, both being critical for the success of fertilization. Secondly, however, these ROS-mediated oxidation events have to be particularly well balanced, since spermatozoa are very susceptible to oxidative insults that may have dramatic effects on their integrity and consequently on their fertilizing potential. Although the epididymal lumen contains several non-enzymatic antioxidants, GPx proteins appear to be the masters in controlling this fine equilibrium, acting either as disulfide isomerases or true GPx H2O2 scavenger. Whereas nGPx4 uses H2O2 or other organic hydroperoxides to perform oxidation of protamines which further compacts the sperm nucleus and locks it in this condensed state, the luminal GPx5 protein controls the amount of luminal H2O2 available for optimal oxidation and also protects maturing spermatozoa against H2 O2-mediated damage. PRDX that have recently been shown to be present on spermatozoa may also participate in this fine regulation of beneficial vs. detrimental effects of ROS on spermatozoa. The precise interplay between GPx proteins and H2O2 during the last steps in the generation of fully competent spermatozoa in the male genital tract may explain why oxidative stress is a parameter so frequently associated with male infertility whether it comes from infectious situations and infiltrating leucocytes, environmental toxicants, metabolic syndromes, or aging, all situations that are known to lead to excessive generation of ROS or alteration of ROS scavengers.

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