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.

Oxidative stress and male reproductive health

The ability of spermatozoa to generate reactive oxygen species (ROS) has been appreciated since the 1940s. It is a universal property of mature spermatozoa from all mammalian species and a major contributor to the oxidative stress responsible for defective sperm function. The mechanisms by which oxidative stress limits the functional competence of mammalian spermatozoa involve the peroxidation of lipids, the induction of oxidative DNA damage, and the formation of protein adducts. ROS production in these cells involves electron leakage from the sperm mitochondria, triggered by a multitude of factors that impede electron flow along the electron transport chain. The net result of mitochondrial ROS generation is to damage these organelles and initiate an intrinsic apoptotic cascade, as a consequence of which spermatozoa lose their motility, DNA integrity, and vitality. This pathway of programmed senescence also results in the exteriorization of phosphatidylserine, which may facilitate the silent phagocytosis of these cells in the aftermath of insemination, in turn influencing the female tract immune response to sperm antigens and future fertility. Despite the vulnerability of sperm to oxidative stress, it is also clear that normal sperm function depends on low levels of ROS generation in order to promote the signal transduction pathways associated with capacitation. Modulators of ROS generation by spermatozoa may therefore have clinical utility in regulating the fertilizing capacity of these cells and preventing the development of antisperm immunity. Achievement of these objectives will require a systematic evaluation of pro- and antioxidant strategies in vivo and in vitro.

https://onlinelibrary.wiley.com/doi/pdf/10.2164/jandrol.112.016535

Reactive Oxygen Species and Sperm Function—In Sickness and In Health

Sperm cryopreservation update: Cryodamage, markers, and factors affecting the sperm freezability in pigs.

Lipid profiles of sperm and seminal plasma from boars having normal or low sperm motility

Effects of cryopreservation on phosphatidylserine translocation, intracellular hydrogen peroxide, and DNA integrity in canine sperm

Boar spermatozoa are very susceptible to reactive oxygen species (ROS), but ROS involvement in damage and/or capacitation is unclear. The impact of exposing fresh boar spermatozoa to an ROS-generating system (xanthine/xanthine oxidase; XA/XO) on sperm ROS content, membrane lipid peroxidation, phospholipase (PL) A activity, and motility, viability, and capacitation was contrasted to ROS content and sperm function after cryopreservation. Exposing boar sperm (n = 4-5 ejaculates) to the ROS-generating system for 30 min rapidly increased hydrogen peroxide (H2O2) and lipid peroxidation in all sperm, increased PLA in dead sperm, and did not affect intracellular O2- (flow cytometry of sperm labeled with 2',7'-dichlorodihydrofluorscein diacetate, BODIPY 581/591 C11, bis-BODIPY-FL C11, hydroethidine, respectively; counterstained for viability). Sperm viability remained high, but sperm became immotile. Cryopreservation decreased sperm motility, viability, and intracellular O2- significantly, but did not affect H2O2. As expected, more sperm incubated in capacitating media than Beltsville thawing solution buffer underwent acrosome reactions and protein tyrosine phosphorylation (four proteins, 58-174 kDa); which proteins were tyrosine phosphorylated was pH dependent. Pre-exposing sperm to the ROS-generating system increased the percentage of sperm that underwent acrosome reactions after incubation in capacitating conditions (P < 0.025), and decreased capacitation-dependent increases in two tyrosine-phosphorylated proteins (P < or = 0.035). In summary, H2O2 is the major free radical mediating direct ROS effects, but not cryopreservation changes, on boar sperm. Boar sperm motility, acrosome integrity, and lipid peroxidation are more sensitive indicators of oxidative stress than viability and PLA activity. ROS may stimulate the acrosome reaction in boar sperm through membrane lipid peroxidation and PLA activation.

Meghan Mackenzie-Bell

Papers

Reactive Oxygen Species and Boar Sperm Function