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.

Recent evidence has shown that the sperm epigenome is vulnerable to dynamic modifications arising from a variety of paternal environment exposures and that this legacy can serve as an important determinant of intergenerational inheritance. It has been postulated that such exchange is communicated to maturing spermatozoa via the transfer of small non-protein-coding RNAs (sRNAs) in a mechanism mediated by epididymosomes; small membrane bound vesicles released by the soma of the male reproductive tract (epididymis). Here we confirm that mouse epididymosomes encapsulate an impressive cargo of >350 microRNAs (miRNAs), a developmentally important sRNA class, the majority (~60%) of which are also represented by the miRNA signature of spermatozoa. This includes >50 miRNAs that were found exclusively in epididymal sperm and epididymosomes, but not in the surrounding soma. We also documented substantial changes in the epididymosome miRNA cargo, including significant fold changes in almost half of the miRNAs along the length of the epididymis. Finally, we provide the first direct evidence for the transfer of several prominent miRNA species between mouse epididymosomes and spermatozoa to afford novel insight into a mechanism of intercellular communication by which the sRNA payload of sperm can be selectively modified during their post-testicular maturation.

In recent years considerable effort has been devoted to understanding the epigenetic control of sperm development leading to an increased appreciation of the importance of RNA interference pathways, and in particular microRNAs (miRNAs), as key regulators of spermatogenesis and epididymal maturation. It has also been shown that sperm are endowed with an impressive array of miRNA that have been implicated in various aspects of fertilization and embryo development. However, to date there have been no reports on whether the sperm miRNA signature is static or whether it is influenced by their prolonged maturation within the male reproductive tract. To investigate this phenomenon we employed next generation sequencing to systematically profile the miRNA signature of maturing mouse spermatozoa. In so doing we have provided the first evidence for the post-testicular modification of the sperm miRNA profile under normal physiological conditions. Such modifications include the apparent loss and acquisition of an impressive cohort of some 113 and 115 miRNAs, respectively between the proximal and distal epididymal segments. Interestingly, the majority of these changes occur late in maturation and include the uptake of novel miRNA species in addition to a significant increase in many miRNAs natively expressed in immature sperm. Since sperm are not capable of de novo transcription these findings identify the epididymis as an important site in establishing the sperm epigenome with the potential to influence the peri-conceptual environment of the female reproductive tract, contribute to the inheritance of acquired characteristics, and/or alter the developmental trajectory of the resulting offspring.

In women, the non-growing population of follicles that comprise the ovarian reserve is determined at birth and serves as the reservoir for future fertility. This reserve of dormant, primordial follicles and the mechanisms controlling their selective activation which constitute the committing step into folliculogenesis are essential for determining fertility outcomes in women. Much of the available data on the mechanisms responsible for primordial follicle activation focuses on a selection of key molecular pathways, studied primarily in animal models, with findings often not synonymous in humans. The excessive induction of primordial follicle activation may cause the development of premature ovarian insufficiency (POI), a condition characterised by menopause before age 40 years. POI affects 1–2% of all women and is accompanied by additional health risks. Therefore, it is critical to further our understanding of primordial follicle activation in order to diagnose, treat and prevent premature infertility. Research in primordial follicle activation has focused on connecting new molecules to already established key signalling pathways, such as phosphatidylinositol 3-Kinase (PI3K) and mammalian target of rapamycin (mTOR). Additionally, other aspects of the ovarian environment, such as the function of the extracellular matrix, in contributing to primordial follicle activation have gained traction. Clinical applications are examining replication of this extracellular environment through the construction of biological matrices mimicking the 3D ovary, to support follicular growth through to ovulation. This review outlines the importance of the events leading to the establishment of the ovarian reserve and highlights the fundamental factors known to influence primordial follicle activation in humans presenting new horizons for female infertility treatment.

Lipid peroxidation is known to be a major factor in the aetiology of defective sperm function. Although biochemical assays for this process exist, they are relatively insensitive and require large numbers of spermatozoa; a condition that cannot be met with many infertility specimens. Recently, a new approach for monitoring peroxidative damage has been introduced, involving the probe BODIPY (581/591) C₁₁, which readily incorporates into cells and undergoes a spectral emission shift when attacked by reactive oxygen metabolites. We have examined the applicability of this probe as an indicator of oxidative stress in human sperm populations using flow cytometry as an end point. The measurement of peroxidation with BODIPY C₁₁ demonstrated significant dependence on the presence of a ferrous ion promoter (P < 0.001), which was significantly enhanced in sperm recovered from low-density Percoll fractions (P < 0.05) and was particularly damaging to the sperm midpiece. Iron–induced radical formation was suppressed by ascorbate in a dose-dependent manner (P < 0.001) and could only be promoted by Fe(II) and Cu(II); nickel, zinc and Fe(III) were ineffective. The Fe(II)-promoted BODIPY C₁₁ signal was significantly correlated with the measurement of reactive oxygen species generation with dihydroethidium. We conclude that BODIPY C₁₁ is an extremely useful probe for indexing peroxidative damage in human spermatozoa.

Mammalian spermatozoa undergo a series of molecular and biochemical changes collectively termed capacitation prior to acquiring the ability to fertilise the oocyte. Although phosphorylation of sperm proteins on tyrosine residues has been recognised as an important component of this process, the precise relationship between the phosphorylation status of mammalian spermatozoa and their capacity for fertilisation has remained unclear. In this study we demonstrate a causal relationship between tyrosine phosphorylation in spermatozoa and sperm-zona interaction. The phosphotyrosine expression associated with sperm capacitation localised to internal flagellar structures in permeabilised cells but could also be detected on the exterior surface of the sperm head in live cells. Importantly, almost all spermatozoa bound to the zona pellucida demonstrated this pattern of phosphoprotein localisation, compared to fewer than 15% of the free-swimming population. These data suggest that tyrosine phosphorylation plays a significant role in remodelling the sperm surface, so that these cells are able to recognise the zona pellucida. Phosphoproteome analysis yielded the first evidence of molecular chaperones, endoplasmin (erp99) and heat shock protein 60 (hsp60), as targets for phosphorylation on the surface of mouse spermatozoa, whereas immunofluorescence localised these proteins to the precise region of the sperm head that participates in zona recognition. Based on these results, we propose a novel mechanism for mammalian gamete interaction whereby the activation of sperm-surface chaperones by tyrosine phosphorylation during capacitation may trigger conformational changes facilitating the formation of a functional zona pellucida receptor complex on the surface of mammalian spermatozoa.