Context: Male infertility has been linked with the excessive generation of reactive oxygen species (ROS) by defective spermatozoa. However, the subcellular origins of this activity are unclear. Objective: The objective of this study was to determine the importance of sperm mitochondria in creating the oxidative stress associated with defective sperm function. Method: Intracellular measurement of mitochondrial ROS generation and lipid peroxidation was performed using the fluorescent probes MitoSOX red and BODIPY C₁₁ in conjunction with flow cytometry. Effects on sperm movement were measured by computer-assisted sperm analysis. Results: Disruption of mitochondrial electron transport flow in human spermatozoa resulted in generation of ROS from complex I (rotenone sensitive) or III (myxothiazol, antimycin A sensitive) via mechanisms that were independent of mitochondrial membrane potential. Activation of ROS generation at complex III led to the rapid release of hydrogen peroxide into the extracellular space, but no detectable peroxidative damage. Conversely, the induction of ROS on the matrix side of the inner mitochondrial membrane at complex I resulted in peroxidative damage to the midpiece and a loss of sperm movement that could be prevented by the concomitant presence of α-tocopherol. Defective human spermatozoa spontaneously generated mitochondrial ROS in a manner that was negatively correlated with motility. Simultaneous measurement of general cellular ROS generation with dihydroethidium indicated that 68% of the variability in such measurements could be explained by differences in mitochondrial ROS production. Conclusion: We conclude that the sperm mitochondria make a significant contribution to the oxidative stress experienced by defective human spermatozoa.

One of the major causes of defective sperm function is oxidative stress, which not only disrupts the integrity of sperm DNA but also limits the fertilizing potential of these cells as a result of collateral damage to proteins and lipids in the sperm plasma membrane. The origins of such oxidative stress appear to involve the sperm mitochondria, which have a tendency to generate high levels of superoxide anion as a prelude to entering the intrinsic apoptotic cascade. Unfortunately, these cells have very little capacity to respond to such an attack because they only possess the first enzyme in the base excision repair (BER) pathway, 8-oxoguanine glycosylase 1 (OGG1). The latter successfully creates an abasic site, but the spermatozoa cannot process the oxidative lesion further because they lack the downstream proteins (APE1, XRCC1) needed to complete the repair process. It is the responsibility of the oocyte to continue the BER pathway prior to initiation of S-phase of the first mitotic division. If a mistake is made by the oocyte at this stage of development, a mutation will be created that will be represented in every cell in the body. Such mechanisms may explain the increase in childhood cancers and other diseases observed in the offspring of males who have suffered oxidative stress in their germ line as a consequence of age, environmental or lifestyle factors. The high prevalence of oxidative DNA damage in the spermatozoa of male infertility patients may have implications for the health of children conceivedin vitro and serves as a driver for current research into the origins of free radical generation in the germ line.

DNA damage in the male germ line has been linked with a variety of adverse clinical outcomes including impaired fertility, an increased incidence of miscarriage and an enhanced risk of disease in the offspring. The origins of this DNA damage could, in principle, involve: (i) abortive apoptosis initiated post meiotically when the ability to drive this process to completion is in decline (ii) unresolved strand breaks created during spermiogenesis to relieve the torsional stresses associated with chromatin remodelling and (iii) oxidative stress. In this article, we present a two-step hypothesis for the origins of DNA damage in human spermatozoa that highlights the significance of oxidative stress acting on vulnerable, poorly protaminated cells generated as a result of defective spermiogenesis. We further propose that these defective cells are characterized by several hallmarks of ‘dysmaturity’ including the retention of excess residual cytoplasm, persistent nuclear histones, poor zona binding and disrupted chaperone content. The oxidative stress experienced by these cells may originate from infiltrating leukocytes or, possibly, the entry of spermatozoa into an apoptosis-like cascade characterized by the mitochondrial generation of reactive oxygen species. This oxidative stress may be exacerbated by a decline in local antioxidant protection, particularly during epididymal maturation. Finally, if oxidative stress is a major cause of sperm DNA damage then antioxidants should have an important therapeutic role to play in the clinical management of male infertility. Carefully controlled studies are now needed to critically examine this possibility.

Whereas studies have revealed that the cryopreservation of human semen increases sperm DNA fragmentation, the mechanisms involved in this type of cryo-injury are largely unknown. Elucidation of these mechanisms may provide insight into preventing such injury. We obtained 60 semen samples from 60 men and conducted experiments to determine the cause of cryopreservation-induced DNA fragmentation using 8-oxo-7,8-dihydro-2′deoxyguanosine (8OHdG) as a biomarker of oxidative stress, percentage caspase positive cells as an indicator of apoptosis, the potential antioxidant genistein and the caspase inhibitor Z-VAD(OMe)-FMK. Cryopreservation led to a significant increase in percentage DNA fragmentation, percentage 8OHdG and percentage caspase positive cells (P < 0.001). Percentage DNA fragmentation was positively correlated with percentage 8OHdG before (r = 0.756, P < 0.001) and after cryopreservation (r = 0.528, P = 0.017). The addition of 50 and 100 µM genistein to the cryoprotectant had a significant protective effect on sperm DNA (P < 0.001) although the caspase inhibitor demonstrated no difference to the control. Human sperm DNA fragmentation is associated with an increase in oxidative stress during cryopreservation, rather than the activation of caspases and apoptosis. The estrogenic compound genistein may be useful in reducing this effect but larger trials are needed to confirm this.

DNA damage in human spermatozoa has been associated with a range of adverse clinical outcomes, including infertility, abortion, and disease in the offspring. We have advanced a two-step hypothesis to explain this damage involving impaired chromatin remodeling during spermiogenesis followed by a free radical attack to induce DNA strand breakage. The objective of the present study was to test this hypothesis by determining whether impaired chromatin protamination is correlated with oxidative base damage and DNA fragmentation in human spermatozoa. DNA fragmentation, chromatin protamination, mitochondrial membrane potential, and formation of the oxidative base adduct, 8-hydroxy-2′-deoxyguanosine (8OHdG), were monitored by flow cytometry/fluorescence microscopy. Impairment of DNA protamination during late spermatogenesis was highly correlated (P < 0.001) with DNA damage in human spermatozoa. The disruption of chromatin remodeling also was associated with a significant elevation in the levels of 8OHdG (P < 0.001), and the latter was itself highly correlated with DNA fragmentation (P < 0.001). The significance of oxidative stress in 8OHdG formation was demonstrated experimentally using H₂O₂/Fe²⁺ and by the correlation observed between this base adduct and superoxide generation (P < 0.001). That 8OHdG formation was inversely associated with mitochondrial membrane potential (P < 0.001) suggested a possible role for these organelles in the creation of oxidative stress. These results clearly highlight the importance of oxidative stress in the induction of sperm DNA damage and carry significant implications for the clinical management of this condition.

In recent times there has been some controversy over the impact of electromagnetic radiation on human health. The significance of mobile phone radiation on male reproduction is a key element of this debate since several studies have suggested a relationship between mobile phone use and semen quality. The potential mechanisms involved have not been established, however, human spermatozoa are known to be particularly vulnerable to oxidative stress by virtue of the abundant availability of substrates for free radical attack and the lack of cytoplasmic space to accommodate antioxidant enzymes. Moreover, the induction of oxidative stress in these cells not only perturbs their capacity for fertilization but also contributes to sperm DNA damage. The latter has, in turn, been linked with poor fertility, an increased incidence of miscarriage and morbidity in the offspring, including childhood cancer. In light of these associations, we have analyzed the influence of RF-EMR on the cell biology of human spermatozoa in vitro. Purified human spermatozoa were exposed to radio-frequency electromagnetic radiation (RF-EMR) tuned to 1.8 GHz and covering a range of specific absorption rates (SAR) from 0.4 W/kg to 27.5 W/kg. In step with increasing SAR, motility and vitality were significantly reduced after RF-EMR exposure, while the mitochondrial generation of reactive oxygen species and DNA fragmentation were significantly elevated (P<0.001). Furthermore, we also observed highly significant relationships between SAR, the oxidative DNA damage bio-marker, 8-OH-dG, and DNA fragmentation after RF-EMR exposure. RF-EMR in both the power density and frequency range of mobile phones enhances mitochondrial reactive oxygen species generation by human spermatozoa, decreasing the motility and vitality of these cells while stimulating DNA base adduct formation and, ultimately DNA fragmentation. These findings have clear implications for the safety of extensive mobile phone use by males of reproductive age, potentially affecting both their fertility and the health and wellbeing of their offspring.