Addition of the divalent cation ionophore, A23187, to washed populations of human spermatozoa resulted in a sudden burst of production of reactive oxygen species which peaked within 3-5 min. This activity was dependent upon the presence of calcium in the external medium and was unaffected by the mitochondrial inhibitors, oligomycin, antimycin and rotenone. Studies with scavengers of reactive oxygen species revealed that, while reagents directed against singlet oxygen and the hydroxyl radical were without effect, cytochrome C reduced the response to A23187 by about 50%, suggesting that the superoxide anion radical is a major product of the activated human spermatozoon. The clinical implications of these studies stem from the considerable variation observed between individuals in the levels of reactive oxygen species produced by the spermatozoa. This variability was shown to be inversely related to the ability of the spermatozoa to exhibit sperm-oocyte fusion on exposure to A23187; defective samples exhibited a basal level of reactive oxygen species production which was 40 times that observed with normal functional cells.

In Westernized societies, average consumption of n-6 polyunsaturated fatty acids (PUFAs) far exceeds nutritional requirements. The ratio of n-6 to n-3 PUFAs is generally >10:1 whereas on a primitive human diet it was closer to 1:1. Diets fed to intensively farmed livestock have followed a similar trend. Both n-6 and n-3 PUFAs can influence reproductive processes through a variety of mechanisms. They provide the precursors for prostaglandin synthesis and can modulate the expression patterns of many key enzymes involved in both prostaglandin and steroid metabolism. They are essential components of all cell membranes. The proportions of different PUFAs in tissues of the reproductive tract reflect dietary consumption. PUFA supplements (particularly n-3 PUFAs in fish oil) are promoted for general health reasons. Fish oils may also benefit fertility in cattle and reduce the risk of preterm labor in women, but in both cases current evidence to support this is inconclusive. Gamma-linolenic acid containing oils can alter the types of prostaglandins produced by cells in vitro, but published data to support claims relating to effects on reproductive health are lacking. Spermatozoa require a high PUFA content to provide the plasma membrane with the fluidity essential at fertilization. However, this makes spermatozoa particularly vulnerable to attack by reactive oxygen species, and lifestyle factors promoting oxidative stress have clear associations with reduced fertility. Adequately powered trials that control for the ratios of different PUFAs consumed are required to determine the extent to which this aspect of our diets does influence our fertility.

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We conclude that the sperm mitochondria make a significant contribution to the oxidative stress experienced by defective human spermatozoa.

Reactive oxygen metabolites are known to disrupt sperm-oocyte fusion, sperm movement, and DNA integrity; however, the relative sensitivities of these elements to oxidative stress are unknown. In this study these factors were assessed in human spermatozoa exposed to increasing levels of oxidative stress achieved through the stimulation of endogenous oxidant generation with NADPH or direct exposure to hydrogen peroxide. At low levels of oxidative stress, DNA fragmentation was significantly reduced while the rates of sperm-oocyte fusion were significantly enhanced. As the level of oxidative stress increased, the spermatozoa exhibited significantly elevated levels of DNA damage (p < 0.001) and yet continued to express an enhanced capacity for sperm-oocyte fusion. At the highest levels of oxidative stress, extremely high rates of DNA fragmentation were observed but the spermatozoa exhibited a parallel loss in their capacities for movement and oocyte fusion. These studies emphasize how redox mechanisms can either enhance or disrupt the functional and genomic integrity of human spermatozoa depending on the intensity of the oxidative stimulus. Because these qualities are affected at different rates, spermatozoa exhibiting significant DNA damage are still capable of fertilizing the oocyte. These results may have long-term implications for the safety of assisted conception procedures in cases associated with oxidative stress.

Oxidative stress plays a major role in the life and death of mammalian spermatozoa.These gametes are professional generators of reactive oxygen species (ROS), which appear to derive from three potential sources: sperm mitochondria, cytosolic L-amino acid oxidases, and plasma membrane Nicotinamide adenine dinucleotide phosphate oxidases. The oxidative stress created via these sources appears to play a significant role in driving the physiological changes associated with sperm capacitation through the stimulation of a cyclic adenosine monophosphate/Protein kinase A phosphorylation cascade, including the activation of Extracellular signal regulated kinase-like proteins, massive up-regulation of tyrosine phosphorylation in the sperm tail, as well as the induction of sterol oxidation. When generated in excess, however, ROS can induce lipid peroxidation that, in turn, disrupts membrane characteristics that are critical for the maintenance of sperm function, including the capacity to fertilize an egg.Furthermore, the lipid aldehydes generated as a consequence of lipid peroxidation bind to proteins in the mitochondrial electron transport chain, triggering yet more ROS generation in a self-perpetuating cycle. The high levels of oxidative stress created as a result of this process ultimately damage the DNA in the sperm nucleus; indeed, DNA damage in the male germ line appears to be predominantly induced oxidatively, reflecting the vulnerability of these cells to such stress. Extensive evaluation of antioxidants that protect the spermatozoa against oxidative stress while permitting the normal reduction-oxidation regulation of sperm capacitation is therefore currently being undertaken, and has already proven efficacious in animal models. | INTRODUCTIONCapacitation is a key feature of mammalian sperm cell biology, and was independently reported by Chang (1951) and Austin (1951); its discovery was a sentinel for unwrapping the mysteries of mammalian fertilization.Completion of capacitation provides sperm with a series of behaviours that are essential for the achievement of fertilization, including hyperactivated motility, sperm-zona pellucida recognition, and acrosome exocytosis. Despite being aware of capacitation for more than half a century, the molecular mechanisms that underpin this process at a physiological level are still far from resolved (Aitken & Nixon, 2013). We do know that capacitation involves an efflux of cholesterol from the sperm plasma membrane (Davis, 1981) and a global increase in tyrosine phosphorylation (Visconti et al., 1995). Yet, our understanding of how these elements are controlled is still quite embryonic. In this context, one insight that has emerged in the relatively recent past is that sperm capacitation is a reduction-oxidation (redox)-regulated event, promoted by the de novo generation of low levels of reactive oxygen species (ROS). One of the first scientists to discover this association was the late Claude Gagnon, who published an important paper in 1993 citing the ability of superoxide to...

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 conceived in vitro and serves as a driver for current research into the origins of free radical generation in the germ line.

The cellular generation of reactive oxygen species was first observed in mammalian spermatozoa in the late 1940s. The field then remained dormant for 30 years until Thaddeus Mann and Roy Jones published a series of landmark papers in the 1970s in which the importance of lipid peroxidation as a mechanism for damaging mammalian spermatozoa was first intimated. The subsequent demonstration that human spermatozoa produce reactive oxygen species and are susceptible to peroxidative damage has triggered intense interest in the role of oxidative stress in the aetiology of male infertility. Moreover, data have recently been obtained to indicate that, although excessive exposure to reactive oxygen species may be harmful to spermatozoa, in physiological amounts these molecules are of importance in the control of normal sperm function. This review considers the dualistic role of reactive oxygen species and sets out the current understanding of the importance of oxidative processes in both the physiology and the pathology of the human spermatozoon.