Sperm surface changes and their consequences for sperm transit through the female reproductive tract

How sperm protects itself: A journey in the female reproductive system

Sperm must pass a complex route in the female reproductive tract (FRT) to reach the fertilization site and join the oocyte. Thus, it should employ several mechanisms to survive against the female immune system, fertilize the oocyte, and successfully transmit paternal genes to the next generation. In addition to self-protection, sperm may be involved in the immune tolerance to the developing embryo and regulating the FRT for embryo implantation and subsequent pregnancy. Hence, this review intends to summarize the mechanisms that protect sperm in the FRT: including immunomodulatory factors that are carried by seminal plasma, cell-to-cell and molecular interaction of sperm with epithelial and immune cells of the FRT, high regulated secretions of inflammatory factors such as cytokines, chemokines, and growth factors, inducing immune tolerance to paternal antigens, and specialized expression of cell receptors and binding proteins. In most of these events sperm induces the FRT to protect itself by modulating immune responses for its own benefit. However, not all sperm in the semen are able to trigger the survival mechanisms and only high-quality sperm will overcome this challenge. A clear understanding of the molecular mechanisms that maintain sperm viability and function in the FRT can lead to new knowledge about infertility etiology and a new approach in assisted reproductive technologies for the preparation and selection of the best sperm based on the criteria that physiologically happen in-vivo.

Beta-defensins as marker for male fertility: a comprehensive review†

Bovine male fertility in animals has a direct impact on the productivity of dairy herds. The epididymal sperm maturations involve extensive sperm surface modifications to gain the fertilizing ability, especially by absorptions of the plethora of biomolecules, including glycoprotein beta-defensins (BDs), enzymes, organic ions, protein, and phospholipids. Defensins are broad-range nonspecific antimicrobial peptides that exhibit strong relations with innate and adaptive immunity, but their roles in male fertility are relatively recently identified. In the course of evolution, BD genes give rise to different clusters with specific functions, especially reproductive functions, by undergoing duplications and nonsynonymous mutations. BD polymorphisms have been reported with milk compositions, disease resistance, and antimicrobial activities. However, in recent decades, the link of BD polymorphisms with fertility has emerged as an appealing improvement of reproductive performance such as sperm motility, membrane integrity, cervical mucus penetration, evading of uterus immunosurveillance, oviduct cell attachment, and egg recognition. The reproductive-specific glycosylated BD class-A BDs (CA-BDs) have shown age- and sex-specific expressions in male reproductive organs, signifying their physiological pleiotropism, especially in the sperm maturation and sperm transport in the female reproductive tract. By considering adult male reproductive organ-specific BD expressions, importance in sperm functionalities, and bioinformatic analysis, we have selected two bovine BBD126 and BBD129 genes as novel potential biomarkers of bovine male fertility. Despite the importance of BDs, however, genomic characterization of most BD genes across most livestock and nonmodel organisms remains predictive/incomplete. The current review discusses our understanding of BD pleiotropic functions, polymorphism, and genomic structural attributes concerning the fertilizability of the male gamete in dairy animals.

Bacterial killing activity and lysozymes: A stable defence mechanism in stallion seminal plasma?

During insemination, bacterial contamination of the ejaculate can lead to reduced sperm quality and transmission of pathogens to the female, thus should be avoided. The semen of a variety of animal taxa possess antimicrobial properties against a wide range of bacterial species through antimicrobial molecules, such as lysozyme, but their variance and the factors influencing it are unknown for most species. In this study, the antibacterial defence (bacterial killing activity (BKA) against Escherichia (E.) coli and Staphylococcus (S.) aureus as well as lysozyme concentration) was studied in seminal fluid from two consecutive ejaculates of 18 stallions. All ejaculates showed BKA against the tested bacteria, which correlated between the two consecutive ejaculates (r_S  = 0.526, p = .025 for E. coli and r_S  = 0.656, p = .003 for S. aureus) and appeared to be stable over the tested period. The lysozyme concentration (LC) showed no significant correlation between the consecutive ejaculates (r_S  = 0.161, p = .681). However, LC had a positive correlation to the ratio of apoptotic spermatozoa within the ejaculates (r_S  = 0.426, p = .019). In contrast to other livestock (e.g., boar, bull), the BKA in stallion semen did not correlate significantly with the age of the animal nor sperm quality characteristics.

The sodium-glucose cotransporter isoform 1 (SGLT-1) is important for sperm energetics, motility, and fertility†

Glucose is a key substrate for supporting sperm energy production and function. Previous studies have demonstrated that sperm glucose uptake is facilitated by several isoforms of the glucose transporters (GLUT). Here, we report that sperm also expresses the Na+-dependent sodium glucose cotransporter (SGLT). This was first suggested by our observation that genetic deletion of the testis-specific Na,K-ATPase α4, which impairs the sperm plasma membrane Na+ gradient, reduces glucose uptake and ATP production. Immunoblot analysis revealed the presence of an SGLT in sperm, with specific expression of isoform 1 (SGLT-1), but not of isoform 2 (SGLT-2). Immunocytochemistry identified SGLT-1 in the mid- and principal piece of the sperm flagellum. Inhibition of SGLT-1 with the isotype-selective inhibitor phlorizin significantly reduced glucose uptake, glycolytic activity, and ATP production in noncapacitated and capacitated sperm from wild-type mice. Phlorizin also decreased total sperm motility, as well as other parameters of sperm movement. In contrast, inhibition of SGLT-1 had no significant effect on sperm hyperactivation, protein tyrosine phosphorylation, or acrosomal reaction. Importantly, phlorizin treatment impaired the fertilizing capacity of sperm. Altogether, these results demonstrate that mouse sperm express a functional SGLT transport system that is important for supporting sperm energy production, motility, and fertility.

Sperm-oviduct interaction: Differential gene expression of growth factors induced by sperm DNA fragmentation

The present study investigated the effects of DNA fragmentation of spermatozoa on the growth factors expression by a human oviduct epithelial cell line (OE-E6/E7). Two separate groups were examined in this study. The cell line was cultured in the presence of spermatozoa with normal DNA fragmentation index (DFI) or abnormal DFI. Total RNA from the cell line in each group was isolated, and relative expression of objective genes was analysed using PCR array. Also, the concentration of VEGF, BMP-2, BMP-7 and MSTN in the supernatant of cell culture was analysed by the ELISA method. The PCR array analysis revealed that most of the growth factors had been upregulated in the abnormal group. However, the differences between groups were statistically significant (p < 0.05) for five genes, including VEGF-A, BMP-2, BMP-6, BMP-7 and OSM. Furthermore, MSTN was the only gene that down-regulated significantly under the influence of the spermatozoa with abnormal DFI. Moreover, the results of ELISA analysis were in agreement with the data of the PCR array. It has been concluded that DNA fragmentation in human spermatozoa can probably change regular events throughout the oviducts. Consequently, the genes of interest may change sperm function and probably its fate in the female reproductive tract.

Seminal lipid profiling and antioxidant capacity: A species comparison

On their way to the oocyte, sperm cells are subjected to oxidative stress, which may trigger the oxidation of phospholipids (PL). Applying MALDI-TOF MS, HPTLC and ESI-IT MS, we comparatively analyzed the PL compositions of semen and blood of species differing in their reproductive systems and types of nutrition (bull, boar, stallion, lion and man) with regard to the sensitivity to oxidation as well as the accumulation of harmful lyso-PL (LPL), transient products of lipid oxidation. In addition, the protective capacity of seminal fluid (SF) was also examined. The PL composition of erythrocytes and blood plasma is similar across the species, while pronounced differences exist for sperm and SF. Since the blood function is largely conserved across mammalian species, but the reproductive systems may vary in many aspects, the obtained results suggest that the PL composition is not determined by the type of nutrition, but by the relatedness of species and by functional requirements of cell membranes such as fluidity. Sperm motion and fertilization of oocytes require a rather flexible membrane, which is accomplished by significant moieties of unsaturated fatty acyl residues in sperm lipids of most species, but implies a higher risk of oxidation. Due to a high content of plasmalogens (alkenyl ether lipids), bull sperm are most susceptible to oxidation. Our data indicate that bull sperm possess the most effective protective power in SF. Obviously, a co-evolution of PL composition and protective mechanisms has occurred in semen and is related to the reproductive characteristics. Although the protective capacity in human SF seems well developed, we recorded the most pronounced individual contaminations with LPL in human semen. Probably, massive oxidative challenges related to lifestyle factors interfere with natural conditions.

Female-induced selective modification of sperm protein SUMOylation-potential mechanistic insights into the non-random fertilization in humans

In many species, mate choice continues after the mating via female‐ or egg‐derived biochemical factors that induce selective changes in sperm pre‐fertilization physiology and behaviour. Recent studies have indicated that gamete‐mediated mate choice likely occurs also in humans, but the mechanistic basis of the process has remained virtually unexplored. Here, we investigated whether female‐induced modifications in sperm protein SUMOylation (post‐translational modification of the proteome) could serve as a novel mechanism for gamete‐mediated mate choice in humans. We treated the sperm of ten males with the oocyte‐surrounding bioactive liquid (follicular fluid) of five females and investigated motility, viability and global protein SUMOylation status of the sperm in all (n = 50) of these male–female combinations (full‐factorial design). All the measured sperm traits were affected by male–female combinations, and sperm protein SUMOylation status was also negatively associated with sperm motility. Furthermore, our results indicate that female‐induced sperm protein SUMOylation is selective, potentially allowing females to increase sperm motility in some males, whereas decreasing it in the others. Consequently, our findings suggest that follicular fluid may non‐randomly modify the structure and function of sperm proteome and in this way facilitate gamete‐mediated mate choice in humans and possibly many other species. However, due to the relatively low number of female subjects and their potential infertility problems, our results should be replicated with larger subset of fully fertile women.

Protein Identification of Spermatozoa and Seminal Plasma in Bottlenose Dolphin (Tursiops truncatus)

Proteins play an important role in many reproductive functions such as sperm maturation, sperm transit in the female genital tract or sperm-oocyte interaction. However, in general, little information concerning reproductive features is available in the case of aquatic animals. The present study aims to characterize the proteome of both spermatozoa and seminal plasma of bottlenose dolphins (Tursiops truncatus) as a model organism for cetaceans. Ejaculate samples were obtained from two trained dolphins housed in an aquarium. Spermatozoa and seminal plasma were analyzed by means of proteomic analyses using an LC-MS/MS, and a list with the gene symbols corresponding to each protein was submitted to the DAVID database. Of the 419 proteins identified in spermatozoa and 303 in seminal plasma, 111 proteins were shared by both. Furthermore, 70 proteins were identified as involved in reproductive processes, 39 in spermatozoa, and 31 in seminal plasma. The five most abundant proteins were also identified in these samples: AKAP3, ODF2, TUBB, GSTM3, ROPN1 for spermatozoa and CST11, LTF, ALB, HSP90B1, PIGR for seminal plasma. In conclusion, this study provides the first characterization of the proteome in cetacean sperm and seminal plasma, opening the way to future research into new biomarkers, the analysis of conservation capacity or possible additional applications in the field of assisted reproductive technologies.

Comparative Proteomic Analysis of Young and Adult Bull (Bos taurus) Cryopreserved Semen

The age of the bull is widely accepted to influence the production of sperm, affecting the amount and quality of produced semen, which in turn impacts the results of cryopreservation. However, the exact influence of the maturation process on cryopreserved sperm, as well as the underlying molecular mechanisms of this process, are not fully understood. The goal of this study was to evaluate changes in the proteome of thawed semen (spermatozoa and supernatant) collected from young and adult bulls (n = 6) using the 2D-DIGE approach. The quality of semen was assessed using a CASA system and flow cytometry. We found no significant age-related variation in semen quality, with the exception of the average path velocity of sperm movement, which was higher in adult bulls. Proteomic analysis indicated 15 spermatozoa proteins and 10 supernatant proteins with significant age-related changes. Our results suggest that semen from adult bulls is better equipped with proteins related to energy production, protection of spermatozoa against oxidative stress and fertilizing ability. Proteins increased in abundance in young bull spermatozoa were connected to the cytoskeleton and its development, which strongly suggests that developmental processes are still in progress. In conclusion, our results provide novel insight into the mechanism of the development of the male reproductive system of cattle.

Ligands and Receptors Involved in the Sperm-Zona Pellucida Interactions in Mammals

Sperm-zona pellucida (ZP) interaction, involving the binding of sperm surface ligands to complementary carbohydrates of ZP, is the first direct gamete contact event crucial for subsequent gamete fusion and successful fertilization in mammals. It is a complex process mediated by the coordinated engagement of multiple ZP receptors forming high-molecular-weight (HMW) protein complexes at the acrosomal region of the sperm surface. The present article aims to review the current understanding of sperm-ZP binding in the four most studied mammalian models, i.e., murine, porcine, bovine, and human, and summarizes the candidate ZP receptors with established ZP affinity, including their origins and the mechanisms of ZP binding. Further, it compares and contrasts the ZP structure and carbohydrate composition in the aforementioned model organisms. The comprehensive understanding of sperm-ZP interaction mechanisms is critical for the diagnosis of infertility and thus becomes an integral part of assisted reproductive therapies/technologies.