Geoffry N. De Iuliis

Significance of Mitochondrial Reactive Oxygen Species in the Generation of Oxidative Stress in Spermatozoa

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 C(11) 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 alpha-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.

ORIGINS AND CONSEQUENCES OF DNA DAMAGE IN MALE GERM CELLS

DNA damage in the male germline is associated with poor fertilization rates following IVF, defective preimplantation embryonic development, and high rates of miscarriage and morbidity in the offspring, including childhood cancer. This damage is poorly characterized, but is known to involve hypomethylation of key genes, oxidative base damage, endonuclease-mediated cleavage and the formation of adducts with xenobiotics and the products of lipid peroxidation. There are many possible causes of such DNA damage, including abortive apoptosis, the oxidative stress associated with male genital tract infection, exposure to redox cycling chemicals, and defects of spermiogenesis associated with the retention of excess residual cytoplasm. Physical factors such as exposure to radiofrequency electromagnetic radiation or mild scrotal heating can also induce DNA damage in mammalian spermatozoa, although the underlying mechanisms are unclear. Ultimately, resolving the precise nature of the DNA lesions present in the spermatozoa of infertile men will be an important step towards uncovering the aetiology of this damage and developing strategies for its clinical management.

Mobile Phone Radiation Induces Reactive Oxygen Species Production and DNA Damage in Human Spermatozoa In Vitro

Background 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. Principal Findings 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. Conclusions 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.

BIOLOGICAL AND CLINICAL SIGNIFICANCE OF DNA DAMAGE IN THE MALE GERM LINE

DNA damage is a common feature of human spermatozoa with purported links to poor rates of conception, impaired embryonic development, an increased incidence of miscarriage and the appearance of various kinds of morbidity in the offspring including childhood cancer. However, difficulties in interpretation arise, because these associations are not consistently observed across all data sets. Such inconsistency reflects the inherent complexity of the reproductive process, large variations in sample size, differences in patient selection, inadequate study design as well as inter-individual differences in the type of DNA damage being detected and the effectiveness of repair mechanisms in the oocyte. This review considers the type, source and measurement of DNA damage in human spermatozoa. It also addresses the clinical utility of the information generated in such studies, and highlights areas where further research is needed to bridge the gap between an intriguing biological phenomenon and the evidence-based clinical management of male patients characterized by high levels of DNA damage in their spermatozoa.

DNA Damage in Human Spermatozoa Is Highly Correlated with the Efficiency of Chromatin Remodeling and the Formation of 8-Hydroxy-2′-Deoxyguanosine, a Marker of Oxidative Stress

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.

Electrophilic aldehydes generated by sperm metabolism activate mitochondrial reactive oxygen species generation and apoptosis by targeting succinate dehydrogenase

Background: The factors responsible for pathological levels of superoxide generation by sperm mitochondria in cases of male infertility are unknown. Results: Electrophilic aldehydes activate mitochondrial superoxide production by forming adducts with succinate dehydrogenase; nucleophiles counteract this effect and promote sperm survival. Conclusion: Products of lipid peroxidation activate mitochondrial superoxide generation. Significance: These findings clarify the causes of oxidative stress in human spermatozoa. Oxidative stress is a major cause of defective sperm function in cases of male infertility. Such stress is known to be associated with high levels of superoxide production by the sperm mitochondria; however, the causes of this aberrant activity are unknown. Here we show that electrophilic aldehydes such as 4-hydroxynonenal (4HNE) and acrolein, generated as a result of lipid peroxidation, target the mitochondria of human spermatozoa and stimulate mitochondrial superoxide generation in a dose- and time-dependent manner. The activation of mitochondrial electron leakage by 4HNE is shown to involve the disruption of succinate dehydrogenase activity and subsequent activation of an intrinsic apoptotic cascade beginning with a loss of mitochondrial membrane potential and terminating in oxidative DNA adduct formation, DNA strand breakage, and cell death. A tight correlation between spontaneous mitochondrial superoxide generation and 4HNE content (R2 = 0.89) in untreated populations of human spermatozoa emphasized the pathophysiological significance of these findings. The latter also provide a biochemical explanation for the self-perpetuating nature of oxidative stress in the male germ line, with the products of lipid peroxidation stimulating free radical generation by the sperm mitochondria in a positive feedback loop.

Sperm Motility Is Lost In Vitro As a Consequence of Mitochondrial Free Radical Production and the Generation of Electrophilic Aldehydes but Can Be Significantly Rescued by the Presence of Nucleophilic Thiols

ABSTRACT The prolonged incubation of human spermatozoa in vitro was found to induce a loss of motility associated with the activation of mitochondrial reactive oxygen species generation in the absence of any change in mitochondrial membrane potential. The increase in mitochondrial free radical production was paralleled by a loss of protein thiols and a concomitant rise in the formation of 4-hydroxynonenal, an electrophilic product of lipid peroxidation that was found to directly suppress sperm movement. These results prompted a search for nucleophiles that could counteract the action of such cytotoxic aldehydes, as a means of ensuring the long-term survival of spermatozoa in vitro. Four nucleophilic compounds were consequently assessed (penicillamine, homocysteine, N-acetylcysteine, and mercaptosuccinate) in three species (human, rat, and horse). The results of this analysis revealed drug and species specificity in the manner in which these compounds affected sperm function, with penicillamine conferring the most consistent, effective support. This prosurvival effect was achieved downstream of mitochondrial reactive oxygen species generation and was associated with the stabilization of 4-hydroxynonenal generation, the preservation of sperm thiols, and a reduction in 8-hydroxy-2′-deoxyguanosine formation. Theoretical calculations of Fe-S and Cu-S bond distances and corresponding binding energies suggested that the particular effectiveness of penicillamine may, in part, reflect the ability of this nucleophile to form stable complexes with transition metals that catalyze lipid peroxidation. The practical implications of these findings were indicated by the effective preservation of equine spermatozoa for 8 days at ambient temperature when the culture medium was supplemented with penicillamine.

New Insights into Sperm Physiology and Pathology

Infertility is a relatively common condition affecting approximately one in ten of the population. In half of these cases, a male factor is involved, making defective sperm function the largest single, defined cause of human infertility. Among other factors, recent data suggest that oxidative stress plays a major role in the etiology of this condition. Spermatozoa spontaneously produce a variety of reactive oxygen species (ROS) including the superoxide anion, hydrogen peroxide and nitric oxide. Produced in small amounts, ROS are functionally important in driving the tyrosine phosphorylation cascades associated with sperm capacitation. However, when ROS production exceeds the spermatozoas limited antioxidant defenses, a state of oxidative stress is induced characterized by peroxidative damage to the sperm plasma membrane and DNA strand breakage in the sperm nucleus. Such oxidative stress not only disrupts the fertilizing potential of human spermatozoa but also the ability of these cells to create a normal healthy embryo. As a result, DNA damage in human spermatozoa is correlated with an increased incidence of miscarriage and various kinds of morbidity in the offspring. These insights into the pathophysiology of defective sperm function have clear implications for the diagnosis and treatment of male infertility, particularly with respect to the potential importance of antioxidant therapy. These concepts may also be relevant to the design of novel approaches to male contraception that attempt to replicate the pathological situation.

Superior hydrolytic DNA cleavage by a dinuclear copper(II) N4S4-donor complex compared with a mononuclear N2S2-donor close analogue

Abstract The cleavage of plasmid DNA by a dinuclear copper(II) complex of 1,2,4,5-tetrakis(1′-amino-3′-thiabutyl)benzene is promoted significantly (rate constant of 3.3×10 −5 s −1 at pH 7.8 and 37°C) compared with the essentially inactive mononuclear copper(II) complex of the analogue 1,2-bis(1′-amino-3′-thiabutyl)benzene (rate constant −10 s −1 ). This implies that the two copper ions in close proximity and acting synergetically are important in accelerating cleavage, and separate roles for the two coppers in the dimer are proposed.

The lipid peroxidation product 4-hydroxynonenal contributes to oxidative stress-mediated deterioration of the ageing oocyte

An increase in intraovarian reactive oxygen species (ROS) has long been implicated in the decline in oocyte quality associated with maternal ageing. Oxidative stress (OS)-induced lipid peroxidation and the consequent generation of highly electrophilic aldehydes, such as 4-hydroxynonenal (4-HNE), represents a potential mechanism by which ROS can inflict damage in the ageing oocyte. In this study, we have established that aged oocytes are vulnerable to damage by 4-HNE resulting from increased cytosolic ROS production within the oocyte itself. Further, we demonstrated that the age-related induction of OS can be recapitulated by exposure of germinal vesicle (GV) oocytes to exogenous H2O2. Such treatments stimulated an increase in 4-HNE generation, which remained elevated during in vitro oocyte maturation to metaphase II. Additionally, exposure of GV oocytes to either H2O2 or 4-HNE resulted in decreased meiotic completion, increased spindle abnormalities, chromosome misalignments and aneuploidy. In seeking to account for these data, we revealed that proteins essential for oocyte health and meiotic development, namely α-, β-, and γ-tubulin are vulnerable to adduction via 4-HNE. Importantly, 4-HNE-tubulin adduction, as well as increased aneuploidy rates, were resolved by co-treatment with the antioxidant penicillamine, demonstrating a possible therapeutic mechanism to improve oocyte quality in older females.