Seminal lipid profiling and antioxidant capacity: A species comparison

Intra-individual variability of the human seminal plasma metabolome

ABSTRACT

In contrast to the conventional spermiogram, metabolomics approaches give insights into the molecular composition of semen and may provide more detailed information on the fertility status of the respective donor. Given the intra-individual variability of spermiogram parameters between two donations, this study sought to elucidate the biological variability of the seminal plasma metabolome over an average period of 8 weeks. Two time-shifted semen samples from 15 healthy donors were compared by a targeted metabolomics approach utilizing the Biocrates AbsoluteIDQ p180 kit. Next to intraclass correlation coefficients (ICC), which represent a measure of reliability, coefficients of variation within individuals (CVW) and coefficients of variation between individuals (CVB) were calculated for each metabolite to demonstrate its stability. Furthermore, men were divided into two cohorts, a similar sperm concentration (SSC) and a differing sperm concentration (DSC) cohort, based on the observed variance in sperm concentration between the two semen donations. The ICC was higher in the SSC compared to the DSC cohort. The levels of 18 metabolites, primarily acylcarnitines, varied between the initial and subsequent donations. After subdivision into subgroups, only ornithine and phosphatidylcholine 40:5 exhibited differential levels between the two donations in the SSC group, compared to 14 metabolites in the DSC group. CVB was higher than CVW but both differed between the metabolite subclasses. Biogenic amines were identified as the least reliable analytes over time, exhibiting the highest CVW, compared to sphingomyelins, which demonstrated the highest reliability with the lowest variation. CVB was the highest for ether-bound glycerophosphatidylcholines and the lowest for amino acids.

The assessment of oxidative stress in human semen: chaos and confusion in pursuit of diagnostic precision

The importance of oxidative stress in the aetiology of male infertility has occasioned numerous clinical trials designed to assess the potential of antioxidants for treating this condition. These trials have not returned definitive results, probably because they have never selected participants on the basis of oxidative stress. Clearly, if a moderate to severe state of oxidative stress does not exist in semen, antioxidants can hardly be expected to improve fertility. To resolve this issue, robust, user-friendly point-of-care assays need to be developed that will enable clinicians to quickly diagnose and monitor oxidative stress in patients' semen. Traditional assays of total antioxidant capacity do not fulfil this role, because they are time consuming, expensive and laboratory based. The introduction of an alternative electrochemical system (MiOXSYS®) was designed to address this problem. This assay records the static oxido-reductive potential of semen and then creates an index by dividing this measurement by the sperm concentration. The creation of such an index is flawed and undermines many of the data generated to date. This commentary explains the nature of this problem and emphasizes the continuing unmet need for effective diagnostic procedures capable of detecting seminal oxidative stress in the patient population.

A Comparative Analysis of the Antioxidant Profiles Generated by the RoXstaTM System for Diverse Biological Fluids Highlights the Powerful Protective Role of Human Seminal Plasma

(1) Background: The RoXstaTM system has been developed as a rapid, effective means of profiling different types of antioxidant activity. The purpose of this study was to examine its performance utilizing a diverse array of biological fluids including semen, blood plasma, serum, urine, saliva, follicular fluid and plant extracts. (2) Methods: The RoXstaTM system was used to assess the ability of different fluids to suppress free radical formation as well as scavenge a variety of toxic oxygen metabolites including free radicals and both hydrogen and organic peroxides. (3) Results: Human semen was shown to have significantly (p < 0.001) more peroxide scavenging power than any other fluid tested (10-14 mM vitamin C equivalent compared with 1-2 mM for blood serum or plasma), while urine was particularly effective in scavenging free radicals and preventing free radical formation (p < 0.001). The powerful antioxidant properties of human semen were shown to reside within the seminal plasma (SP) fraction, rather than the spermatozoa, and to be resistant to snap freezing in liquid nitrogen. Moreover, comparative studies demonstrated that human SP exhibited significantly (p < 0.001) higher levels of antioxidant potential than any other species examined (stallion, bull, dog) and that this intense activity reflected the relative vulnerability of human spermatozoa to peroxide attack. (4) Conclusions: The RoXstaTM system provides valuable information on the antioxidant profile of complex biological fluids, supporting its diagnostic role in conditions associated with oxidative stress. Based on the results secured in this study, human semen is identified as a particularly rich source of antioxidants capable of scavenging both hydrogen and organic peroxides, in keeping with the high susceptibility of human spermatozoa to peroxide-mediated damage.

Sulforaphane acts through the NFE2L2/AMPK signaling pathway to protect boar spermatozoa from cryoinjury by activating antioxidant defenses

During cryopreservation, a substantial portion of spermatozoa undergoes apoptosis due to cryoinjury, resulting in decreased fertility. Boar spermatozoa are highly sensitive to temperature, with low temperature triggering reactive oxygen species (ROS) generation, leading to oxidative stress and apoptosis. Sulforaphane (SFN), a potent natural compound found in cruciferous vegetables, is efficacious in mitigating oxidative stress. We here supplemented different SFN concentrations (0, 1.25, 2.5, 5, 10, and 20 μM) into the freezing extender to explore its effect on boar sperm during cryopreservation and determine the optimal SFN concentration. Supplementation of 5 μM SFN exhibited the highest sperm motility, motion performance, plasma membrane integrity, acrosome integrity, and antioxidant properties (total antioxidant capacity (T-AOC) and antioxidant enzyme activity) after freezing and thawing. Then, RT group, C group and C + SFN group were established to explore the effect of SFN on the cryopreservation-induced sperm apoptosis level and fertilizing capacity of post-thawed sperms. SFN effectively rescued the apoptosis and fertilizing capacity of post-thawed sperms. Mechanistically, SFN activated the redox-sensitive nuclear factor erythroid 2-related factor 2 (NRF2/NFE2L2) by promoting adenosine monophosphate-activated protein kinase (AMPK) phosphorylation. This activation improved antioxidant defenses, ultimately improving cryoinjury in boar spermatozoa. In summary, SFN suppressed cryopreservation-induced apoptosis of spermatozoa by activating antioxidant defenses and the AMPK/NFE2L2 signaling pathway. These findings suggest a novel approach for augmenting the cryoprotective efficiency and spermatozoa fertility after cryopreservation.

Phospholipid-derived lysophospholipids in (patho)physiology

Phospholipids (PL) are major components of cellular membranes and changes in PL metabolism have been associated with the pathogenesis of numerous diseases. Lysophosphatidylcholine (LPC) in particular, is a comparably abundant component of oxidatively damaged tissues. LPC originates from the cleavage of phosphatidylcholine (PC) by phospholipase A2 or the reaction of lipids with reactive oxygen species (ROS) such as HOCl. Another explanation of increased LPC concentration is the decreased re-acylation of LPC into PC. While there are also several other lysophospholipids, LPC is the most abundant lysophospholipid in mammals and will therefore be the focus of this review. LPC is involved in many physiological processes. It induces the migration of lymphocytes, fostering the production of pro-inflammatory compounds by inducing oxidative stress. LPC also "signals" via G protein-coupled and Toll-like receptors and has been implicated in the development of different diseases. However, LPCs are not purely "bad": this is reflected by the fact that the concentration and fatty acyl composition of LPC varies under different conditions, in plasma of healthy and diseased individuals, in tissues and different tumors. Targeting LPC and lipid metabolism and restoring homeostasis might be a potential therapeutic method for inflammation-related diseases.

Ether lipids and a peroxisomal riddle in sperm

Sperm are terminally differentiated cells that lack most of the membranous organelles, resulting in a high abundance of ether glycerolipids found across different species. Ether lipids include plasmalogens, platelet activating factor, GPI-anchors and seminolipid. These lipids play important roles in sperm function and performance, and thus are of special interest as potential fertility markers and therapeutic targets. In the present article, we first review the existing knowledge on the relevance of the different types of ether lipids for sperm production, maturation and function. To further understand ether-lipid metabolism in sperm, we then query available proteomic data from highly purified sperm, and produce a map of metabolic steps retained in these cells. Our analysis pinpoints the presence of a truncated ether lipid biosynthetic pathway that would be competent for the production of precursors through the initial peroxisomal core steps, but devoid of subsequent microsomal enzymes responsible for the final synthesis of all complex ether-lipids. Despite the widely accepted notion that sperm lack peroxisomes, the thorough analysis of published data conducted herein identifies nearly 70% of all known peroxisomal resident proteins as part of the sperm proteome. In view of this, we highlight open questions related to lipid metabolism and possible peroxisomal functions in sperm. We propose a repurposed role for the truncated peroxisomal ether-lipid pathway in detoxification of products from oxidative stress, which is known to critically influence sperm function. The likely presence of a peroxisomal-derived remnant compartment that could act as a sink for toxic fatty alcohols and fatty aldehydes generated by mitochondrial activity is discussed. With this perspective, our review provides a comprehensive metabolic map associated with ether-lipids and peroxisomal-related functions in sperm and offers new insights into potentially relevant antioxidant mechanisms that warrant further research.

Lipid alterations by oxidative stress increase detached acrosomes after cryopreservation of semen in Holstein bulls

The nearly exclusive use of cryopreserved semen in cattle breeding enables long shipping distances, higher storage times, quarantine to avoid germ transmission and easy dispersal of high genetic value bulls. Spermatozoa from bulls are well freezable and improvement of cryopreservation protocols over decades has led to high semen quality. However, there is still some loss of spermatozoa in each semen dose due to detached acrosomes after thawing. There are even individual bulls with extremely high numbers of detached acrosomes after cryopreservation, called "bad freezers". This study screened 1092 ejaculates from 59 Holstein bulls for the difference in detached acrosomes before and after cryopreservation (ΔAC). The individual bull influenced ΔAC (P < 0.001) and allowed selection for individuals with repeatedly low ΔAC (good freezers) or high ΔAC (bad freezers). Good freezers were superior to bad freezers in a thermo-resistance test (78.2% vs. 33.6% total motility, respectively, P = 0.047) and had higher non-return rates (NRR: 46.8% vs. 40.8%, respectively, P = 0.016). Since oxidative stress is one possible explanation for premature acrosome reaction, the radical reduction capacity of the seminal fluid was measured, finding that this parameter was reduced in bad freezer bulls during cryopreservation (P = 0.043). Analysis of lipid species in sperm cells by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) showed a reduction of ether lipids and plasmalogens as well as an increase in formyl-lysophosphatidylcholines only within the bad freezers during cryopreservation (P = 0.043). In conclusion these findings show, that lipid alteration caused by oxidative stress is one essential reason for highly augmented acrosome reacted spermatozoa in bad freezer bulls. Therefore, increased use of antioxidants in the extender could be a possible starting point for developing individualized extenders for bad freezer bulls of high genetic value, in order to raise sperm quality after cryopreservation even in those bulls.

Integrating sperm cell transcriptome and seminal plasma metabolome to analyze the molecular regulatory mechanism of sperm motility in Holstein stud bulls

Considering that artificial insemination is the most widely used assisted reproductive technique in the dairy industry, the semen quality of bulls is very important for selecting excellent stud bulls. Sperm motility is one of the important traits of semen quality, and related genes may be regulated by environmental factors. Seminal plasma can affect sperm cell transcriptome and further affect sperm motility through exosome or other processes. However, the molecular regulation mechanism of bull sperm motility has not been studied by combining the sperm cell transcriptome with seminal plasma metabolome. The number of motile sperm per ejaculate (NMSPE) is an integrated indicator for assessing sperm motility in stud bulls. In the present study, we selected 7 bulls with higher NMSPE (5,698.55 million +/- 945.40 million) as group H and 7 bulls with lower NMSPE (2,279.76 million +/- 1,305.69 million) as group L from 53 Holstein stud bulls. The differentially expressed genes (DEGs) in sperm cells were evaluated between the two groups (H vs. L). We conducted gene co-expression network analysis (WGCNA) on H and L groups of bulls, as well as two monozygotic twin Holstein bulls with different NMSPE values, to screen candidate genes for NMSPE. The regulatory effect of seminal plasma metabolome on the candidate genes of NMSPE was also investigated. A total of 1,099 DEGs were identified in the sperm cells of H and L groups. These DEGs were primarily concentrated in energy metabolism and sperm cell transcription. The significantly enriched Kyoto encyclopedia of genes and genomes (KEGG) pathways of the 57 differential metabolites were the aminoacyl-tRNA biosynthesis pathway and vitamin B6 metabolism pathway. Our study discovered 14 genes as the potential candidate markers for sperm motility, including FBXO39. We observed a broad correlation between transcriptome of sperm cells and seminal plasma metabolome, such as three metabolites, namely, mesaconic acid, 2-coumaric acid, and 4-formylaminoantipyrine, might regulate FBXO39 expression through potential pathways. The genes related to seminal plasma metabolites expressed in sperm cells are not only located near the quantitative trait loci of reproductive traits, but also enriched in the genome-wide association study signal of sire conception rate. Collectively, this study was the first to investigate the interplays among transcriptome of sperm cells and seminal plasma metabolome from Holstein stud bulls with different sperm motility.

Live cells are not affected by dead sperm in liquid boar semen: new insights based on a thermo-resistance test

This study evaluated the effect of different proportions of dead spermatozoa on the quality of liquid boar semen during a thermo-resistance test (TRT). After three days of storage (17°C), 54 conventional AI semen doses (~ 23 × 10⁶ sperm/mL in ~ 88 mL of BTS) were split into three 15 mL-treatments (25%, 50% and 75% dead sperm cells) by mixing two subsamples containing 75% (I) and 0% (II) of live cells. Spermatozoa were evaluated after TRT at 30 (on-test) and 300 min (off-test) incubation at 38°C. At the on-test, treatments 25%, 50% and 75% dead sperm cells showed medians for total sperm motility of 77.6%, 50.2% and 25.6%, respectively. Considering the absolute variation of sperm motility during TRT, doses with 25% dead sperm lost more percentage points (pp) (-9.4 pp) compared to doses containing 50% (-8.2 pp) and 75% dead sperm (-4.5 pp). The lowest loss was observed for doses with 75% dead sperm (P < 0.01). However, data showed that treatments lost similar proportion of motile cells over the TRT: 25% dead sperm = -11.9%, 50% dead sperm = -16.0% and 75% dead sperm = -17.5% (P = 0.31). Regarding the flow cytometry parameters (plasma and acrosomal membrane integrity, mitochondrial activity of cells with intact plasma membrane, high degree of lipid disorder and apoptotic cells), the absolute variations did not surpass values of -1.8 pp, 3.4 pp, -5.4 pp and 4.7 pp, respectively. Moreover, the relative variation suggested that dead sperm did not substantially change their values over the TRT. In conclusion, dead sperm cells did not influence the quality of contemporary live cells during the period and in conditions of a TRT.