No abstract

Spermatozoa are highly vulnerable to oxidative attack because they lack significant antioxidant protection due to the limited volume and restricted distribution of cytoplasmic space in which to house an appropriate armoury of defensive enzymes. In particular, sperm membrane lipids are susceptible to oxidative stress because they abound in significant amounts of polyunsaturated fatty acids. Susceptibility to oxidative attack is further exacerbated by the fact that these cells actively generate reactive oxygen species (ROS) in order to drive the increase in tyrosine phosphorylation associated with sperm capacitation. However, this positive role for ROS is reversed when spermatozoa are stressed. Under these conditions, they default to an intrinsic apoptotic pathway characterised by mitochondrial ROS generation, loss of mitochondrial membrane potential, caspase activation, phosphatidylserine exposure and oxidative DNA damage. In responding to oxidative stress, spermatozoa only possess the first enzyme in the base excision repair pathway, 8-oxoguanine DNA glycosylase. This enzyme catalyses the formation of abasic sites, thereby destabilising the DNA backbone and generating strand breaks. Because oxidative damage to sperm DNA is associated with both miscarriage and developmental abnormalities in the offspring, strategies for the amelioration of such stress, including the development of effective antioxidant formulations, are becoming increasingly urgent.

We have isolated vesicular structures from mouse epididymal fluid, referred to as epididymosomes. Epididymosomes have a roughly spherical aspect and a bilayer membrane, and they are heterogeneous in size and content. They originate from the epididymal epithelium, notably from the caput region, and are emitted in the epididymal lumen by way of apocrine secretion. We characterized their membranous lipid profiles in caput and cauda epididymidal fluid samples and found that epididymosomes were particularly rich in sphingomyelin (SM) and arachidonic acid. The proportion of SM increased markedly during epididymal transit and represented half the total phospholipids in cauda epididymidal epididymosomes. The cholesterol:phospholipid ratio increased from 0.26 in the caput to 0.48 in the cauda epididymidis. Measures of epididymosomal membrane anisotropy revealed that epididymosomes became more rigid during epididymal transit, in agreement with their lipid composition. In addition, we have characterized the membrane lipid pattern of murine epididymal spermatozoa during their maturation. Here, we have shown that mouse epididymal spermatozoa were distinguished by high percentages of SM and polyunsaturated membranous fatty acids (PUFAs), principally represented by arachidonic, docosapentanoic, and docosahexanoic acids. Both SM and PUFA increased throughout the epididymal tract. In particular, we observed a threefold rise in the ratio of docosapentanoic acid. Epididymal spermatozoa had a constant cholesterol:phospholipid ratio (average, 0.30) during epididymal transit. These data suggest that in contrast with epididymosomes, spermatozoal membranes seem to become more fluid during epididymal maturation.

The mammalian epididymis provides sperm with an environment that promotes their maturation and protects them from external stresses. For example, it harbors an array of antioxidants, including non-conventional glutathione peroxidase 5 (GPX5), to protect them from oxidative stress. To explore the role of GPX5 in the epididymis, we generated mice that lack epididymal expression of the enzyme. Histological analyses of Gpx5 -/-epididymides and sperm cells revealed no obvious defects. Furthermore, there were no apparent differences in the fertilization rate of sexually mature Gpx5 -/-male mice compared with WT male mice. However, a higher incidence of miscarriages and developmental defects were observed when WT female mice were mated with Gpx5-deficient males over 1 year old compared with WT males of the same age. Flow cytometric analysis of spermatozoa recovered from Gpx5-null and WT male mice revealed that sperm DNA compaction was substantially lower in the cauda epididymides of Gpx5-null animals and that they suffered from DNA oxidative attacks. Real-time PCR analysis of enzymatic scavengers expressed in the mouse epididymis indicated that the cauda epididymidis epithelium of Gpx5-null male mice mounted an antioxidant response to cope with an excess of ROS. These observations suggest that GPX5 is a potent antioxidant scavenger in the luminal compartment of the mouse cauda epididymidis that protects spermatozoa from oxidative injuries that could compromise their integrity and, consequently, embryo viability.

No abstract

This study investigated the enzymatic function of two putative plant GPXs, GPXle1 from Lycopersicon esculentum and GPXha2 from Helianthus annuus, which show sequence identities with the mammalian phospholipid hydroperoxide glutathione peroxidase (PHGPX). Both purified recombinant proteins expressed in Escherichia coli show PHGPX activity by reducing alkyl, fatty acid and phospholipid hydroperoxides but not hydrogen peroxide in the presence of glutathione. Interestingly, both recombinant GPXle1 and GPXha2 proteins also reduce alkyl, fatty acid and phospholipid hydroperoxides as well as hydrogen peroxide using thioredoxin as reducing substrate. Moreover, thioredoxin peroxidase (TPX) activities were found to be higher than PHGPX activities in terms of efficiency and substrate affinities, as revealed by their respective V max and K m values. We therefore conclude that these two plant GPX-like proteins are antioxidant enzymes showing PHGPX and TPX activities.

The past decade has seen a tremendous increase in interest and progress in the field of sperm epigenetics. Studies have shown that chromatin regulation during male germline development is multiple and complex, and that the spermatozoon possesses a unique epigenome. Its DNA methylation profile, DNA-associated proteins, nucleo-protamine distribution pattern and non-coding RNA set up a unique epigenetic landscape which is delivered, along with its haploid genome, to the oocyte upon fertilization, and therefore can contribute to embryogenesis and to the offspring health. An emerging body of compelling data demonstrates that environmental exposures and paternal lifestyle can change the sperm epigenome and, consequently, may affect both the embryonic developmental program and the health of future generations. This short review will attempt to provide an overview of what is currently known about sperm epigenome and the existence of transgenerational epigenetic inheritance of paternally acquired traits that may contribute to the offspring phenotype.

This article addresses the importance of oxidative processes in both the generation of functional gametes and the aetiology of defective sperm function. Functionally, sperm capacitation is recognized as a redox-regulated process, wherein a low level of reactive oxygen species (ROS) generation is intimately involved in driving such events as the stimulation of tyrosine phosphorylation, the facilitation of cholesterol efflux and the promotion of cAMP generation. However, the continuous generation of ROS ultimately creates problems for spermatozoa because their unique physical architecture and unusual biochemical composition means that they are vulnerable to oxidative stress. As a consequence, they are heavily dependent on the antioxidant protection afforded by the fluids in the male and female reproductive tracts and, during the precarious process of insemination, seminal plasma. If this antioxidant protection should be compromised for any reason, then the spermatozoa experience pathological oxidative damage. In addition, situations may prevail that cause the spermatozoa to become exposed to high levels of ROS emanating either from other cells in the immediate vicinity (particularly neutrophils) or from the spermatozoa themselves. The environmental and lifestyle factors that promote ROS generation by the spermatozoa are reviewed in this article, as are the techniques that might be used in a diagnostic context to identify patients whose reproductive capacity is under oxidative threat. Understanding the strengths and weaknesses of ROS-monitoring methodologies is critical if we are to effectively identify those patients for whom treatment with antioxidants might be considered a rational management strategy.