Sperm Mitochondria, the Driving Force Behind Human Spermatozoa Activities: Its Functions and Dysfunctions - A Narrative Review

The Zebrafish Sperm Proteome

One of the key processes that forms the basis of fertilisation is the tight interaction between sperm and egg. Both sperm and egg proteomes are known to evolve and diverge rapidly even between closely related species. Understanding the sperm proteome therefore provides key insights into the proteins that underpin the mechanisms involved during fertilisation and the fusion between sperm and egg, and how they can differ across individuals of the same species. Despite being a commonly used model organism for reproductive research, little is currently understood about the zebrafish Danio rerio sperm proteome. We performed nanoLC-MS/MS proteomics analysis after off-line sample fractionation with six pooled samples containing sperm from ten males each. We confidently identified 5410 proteins, from which a total of 3900 GeneIDs were generated leading to 1720 Gene Ontology terms.

Di (2-ethylhexyl) phthalate reduces sperm motility by decreasing sperm tail energy supply

Di (2-ethylhexyl) phthalate (DEHP) is widespread in the environment. It can impair sperm function through damaging the sperm development process. However, few studies have focused on the sperm tail that is directly related to sperm motility. In this study, we demonstrate that DEHP impedes the conversion of riboflavin in the mitochondrial sheath in the midsection of the sperm tail, resulting in reduced ATP (adenosine triphosphate) synthesis, thereby inhibiting sperm motility. The results of HPLC-MS/MS showed that DEHP metabolites were transported to the testes and epididium, indicating that MEHP (Mono-2-ethylexyl phthalate) could directly affect the espermatozoa of mature. Sperm motility analysis determined that sperm motility decreased with increasing DEHP concentration. The movement of sperm is mainly dependent on the tail motility, which is largely determined by tail structure and energy supply. Electron microscopy images illustrate that there are no observable changes in the basic kinematic structure of the sperm tail. However, DEHP causes a decrease in complex II activity of mitochondrial respiratory chain by interfering with the synthesis of the cofactor FAD (Flavin adenine dinucleotide), which leads to a decrease in ATP concentration. Therefore, DEHP exposure can reduce sperm motility by decreasing sperm tail energy supply. This study exemplifies the importance of the sperm tail in sperm dysfunction caused by environmental pollutants.

Successful pregnancy after preimplantation genetic testing for structural rearrangements in a couple with complex chromosome rearrangement and recurrent in vitro fertilization failures: a case report

Objective

To present a case of a couple with 20 years of infertility and 10 recurrent in vitro fertilization (IVF) failures, identifying a paternal complex chromosome rearrangement using high-resolution karyotype together with preimplantation genetic testing for structural rearrangements (PGT-SR) and utilizing IVF-intracytoplasmic sperm injection to achieve a successful pregnancy.

Design

Case report.

Setting

Al Ain Fertility Center, Abu Dhabi, United Arab Emirates.

Patients

A 40-year-old male patient and a 37-year-old female patient with a history of infertility and recurrent IVF failures.

Exposure

In vitro fertilization-intracytoplasmic sperm injection with high-resolution karyotype and PGT-SR.

Main Outcome Measures

Identification of chromosomal abnormalities, successful embryo development, pregnancy outcome, and newborn karyotyping.

Results

Karyotyping revealed a paternal complex chromosome rearrangement, t(3;4;12) (q21;q33;q21), and a chromosomal polymorphism in the female (1qh+). In vitro fertilization-intracytoplasmic sperm injection with PGT-SR produced one euploid/balanced female embryo from 20 embryos across 8 cycles. The patient conceived after hormone replacement therapy and frozen embryo transfer, resulting in an uneventful, full-term pregnancy and delivery of a healthy baby via C-section. Newborn karyotyping was normal (46,XX).

Conclusion

High-resolution karyotype and PGT-SR should be offered to patients undergoing IVF, especially those with severe male factors, recurrent IVF failures, implantation failures, or recurrent pregnancy losses, to enhance the chances of a successful pregnancy.

Energy metabolism and spermatogenesis

Infertility has become a significant health burden around the globe as it is believed that 15 % of married couples struggle with infertility, with half of the problem accrued to the male. The issue of male infertility could be traced to insufficient or absence of spermatozoa. Glucose metabolism is essential for continued spermatogenesis and for the reproductive potential of sperm cells. Appropriate nutrition is critical in maintaining reproductive function as caloric restriction along with weight reduction, excessive food consumption and obesity are harmful to reproductive function. The link between metabolism and reproduction is tied to metabolic hormones like insulin, leptin and thyroid, extracellular environment, mitochondria function, nutrient substrate, availability, and environmental stressors. Although matured spermatozoa utilize glucose directly, it is not the preferred energy substrate for germ cells as they rely on Sertoli cells to supply lactate. The reproductive potential of sperm cells depends on certain modifications like hyperactivated motility, which is mainly dependent on glucose metabolism. Without other energy sources, spermatozoa utilize their internal lipid stores. The uptake and metabolism of glucose by sperm are essential endpoints for determining the potential fertility of male individuals. The biological energy in sperm cells fuels all the physiological processes they engage in, from their deposition in the female reproductive tract to the point where they fertilize an egg. This article thus reviews facts pertinent to the energy metabolism of male germ cells and Sertoli cells.

Damage to Mitochondria During the Cryopreservation, Causing ROS Leakage, Leading to Oxidative Stress and Decreased Quality of Ram Sperm

Semen cryopreservation can achieve long-term preservation of sperm. Ice crystal damage, as well as oxidative stress, result in mitochondrial dysfunction and a reduction in sperm motility after thawing. However, limited information exists regarding the impact of reactive oxygen species (ROS) and mitochondria on the cryopreservation of ram sperm. The primary objective of this study was to investigate the relationship between ROS and mitochondria concerning sperm quality during the cryopreservation of ram sperm. This investigation assessed sperm motility, kinematic characteristics, membrane integrity, acrosome integrity, mitochondrial membrane potential (MMP), adenosine triphosphate (ATP) levels, expression of mitochondrial respiratory genes (NDUFV2, SDHA, CYC1, and COXIV), ROS levels, malondialdehyde (MDA) content, phosphatidylserine externalisation rate, sperm ultrastructure, mtDNA copy number, expression of apoptosis-related genes (Bax, Caspase-3, and Caspase-8), Cytochrome C, and Caspase-3 content. The results showed the cryopreservation significantly (p < 0.05) decreased motility, kinetic parameters, membrane integrity, acrosome integrity, MMP, ATP, mRNA expression levels of mitochondrial respiratory-related genes, and significantly (p < 0.05) increased ROS levels, MDA content, phosphatidylserine externalisation rate, damage of sperm ultrastructure, mtDNA copy number, mRNA expression levels of apoptosis-related genes, Cytochrome C and Caspase-3 content compared to the fresh semen group. In conclusion, the cryopreservation causes damage to mitochondria, leading to increased ROS and subsequent oxidative stress. This process also initiates mitochondrial dysfunction and interferes with the electron transport chain, ultimately resulting in decreased MMP and ATP production. Furthermore, the liberation of Cytochrome C prompted the increase in Caspase-3 expression and subsequent sperm apoptosis occurred, ultimately leading to a deterioration in sperm quality after thawing.

Human sperm mitochondrial DNA copy numbers and deletion rates: Comparing persons living in two urban industrial agglomerations differing in sources of air pollution

Persons living in industrial environments are exposed to levels of air pollution that can affect their health and fertility. The Czech capital city, Prague, and the Ostrava industrial agglomeration differ in their major sources of air pollution. In Prague, heavy traffic produces high levels of nitrogen oxides throughout the year. In the Ostrava region, an iron industry and local heating are sources of particulate matter (PM) and benzo[a]pyrene (B[a]P), especially in the winter. We evaluated the effects of air pollution on human sperm mitochondrial DNA (mtDNA). Using real-time PCR, we analysed sperm mtDNA copy number and deletion rate in Prague city policemen in two seasons (spring and autumn) and compared the results with those from Ostrava. In Prague, the sperm mtDNA deletion rate was significantly higher in autumn than in spring, which is the opposite of the results from Ostrava. The sperm mtDNA copy number did not show any seasonal differences in either of the cities; it was correlated negatively with sperm concentration, motility, and viability, and with sperm chromatin integrity (assessed with the Sperm Chromatin Structure Assay). The comparison between the two cities showed that the sperm mtDNA deletion rate in spring and the sperm mtDNA copy number in autumn were significantly lower in Prague vs. Ostrava. Our study supports the hypothesis that sperm mtDNA deletion rate is affected by the composition of air pollution. Sperm mtDNA abundance is closely associated with chromatin damage and standard semen characteristics.

One-Year Impact of Occupational Exposure to Polycyclic Aromatic Hydrocarbons on Sperm Quality

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) have toxic potential, especially as carcinogens, neurotoxins, and endocrine disruptors. The objective of this study is to know the impact of exposure to PAHs on the reproductive health of male workers who operate in solar thermal plants.

METHODS

Case-control study. A total of 61 men were included: 32 workers exposed to PAH at a solar thermal plant and 29 unexposed people. Seminal quality was studied both at the cellular level (quantity and quality of sperm) and at the biochemical level (magnitudes of oxidative stress in seminal plasma).

RESULTS

In exposure to PAHs, a significantly higher seminal leukocyte infiltration was observed, as well as lower activity in seminal plasma of superoxide dismutase (SOD) and a reduced glutathione/oxidised glutathione (GSH/GSSG) ratio. The oxidative stress parameters of seminal plasma did not show a relationship with sperm cellularity, neither in those exposed nor in those not exposed to PAH.

CONCLUSION

One year of exposure to PAH in a solar thermal plant does not have a negative impact on the sperm cellularity of the worker, either quantitatively (sperm count) or qualitatively (motility, vitality, morphology, or cellular DNA fragmentation). However, PAH exposure is associated with lower antioxidant capacity and higher leukocyte infiltration in seminal plasma.

Construction of Near-Infrared Probes with Remarkable Large Stokes Shift Based on a Novel Purine Platform for the Visualization of mtG4 Upregulation during Mitochondrial Disorder in Somatic Cells and Human Sperms

G-quadruplex structures within the nuclear genome (nG4) is an important regulatory factor, while the function of G4 in the mitochondrial genome (mtG4) still needs to be explored, especially in human sperms. To gain a better understanding of the relationship between mtG4 and mitochondrial function, it is crucial to develop excellent probes that can selectively visualize and track mtG4 in both somatic cells and sperms. Herein, based on our previous research on purine frameworks, we attempted for the first time to extend the conjugated structure from the C-8 site of purine skeleton and discovered that the purine derivative modified by the C-8 aldehyde group is an ideal platform for constructing near-infrared probes with extremely large Stokes shift (>220 nm). Compared with the compound substituted with methylpyridine (PAP), the molecule substituted with methylthiazole orange (PATO) showed better G4 recognition ability, including longer emission (∼720 nm), more significant fluorescent enhancement (∼67-fold), lower background, and excellent photostability. PATO exhibited a sensitive response to mtG4 variation in both somatic cells and human sperms. Most importantly, PATO helped us to discover that mtG4 was significantly increased in cells with mitochondrial respiratory chain damage caused by complex I inhibitors (6-OHDA and rotenone), as well as in human sperms that suffer from oxidative stress. Altogether, our study not only provides a novel ideal molecular platform for constructing high-performance probes but also develops an effective tool for studying the relationship between mtG4 and mitochondrial function in both somatic cells and human sperms.

Leukocytospermia and/or Bacteriospermia: Impact on Male Infertility

Infertility is a globally underestimated public health concern affecting almost 190 million people, i.e., about 17.5% of people during their lifetime, while the prevalence of male factor infertility is about 7%. Among numerous other causes, the prevalence of male genital tract infections reportedly ranges between 10% and 35%. Leukocytospermia is found in 30% of infertile men and up to 20% in fertile men. Bacterial infections cause an inflammatory response attracting leukocytes, which produce reactive oxygen species (ROS) and release cytokines, both of which can cause damage to sperm, rendering them dysfunctional. Although leukocytospermia and bacteriospermia are both clinical conditions that can negatively affect male fertility, there is still debate about their impact on assisted reproduction outcomes and management. According to World Health Organization (WHO) guidelines, leukocytes should be determined by means of the Endtz test or with monoclonal antibodies against CD15, CD68 or CD22. The cut-off value proposed by the WHO is 1 × 106 peroxidase-positive cells/mL. For bacteria, Gram staining and semen culture are regarded as the "gold standard", while modern techniques such as PCR and next-generation sequencing (NGS) are allowing clinicians to detect a wider range of pathogens. Whereas the WHO manual does not specify a specific value as a cut-off for bacterial contamination, several studies consider semen samples with more than 103 colony-forming units (cfu)/mL as bacteriospermic. The pathogenic mechanisms leading to sperm dysfunction include direct interaction of bacteria with the male germ cells, bacterial release of spermatotoxic substances, induction of pro-inflammatory cytokines and ROS, all of which lead to oxidative stress. Clinically, bacterial infections, including "silent" infections, are treatable, with antibiotics being the treatment of choice. Yet, non-steroidal antiphlogistics or antioxidants should also be considered to alleviate inflammatory lesions and improve semen quality. In an assisted reproduction set up, sperm separation techniques significantly reduce the bacterial load in the semen. Nonetheless, contamination of the semen sample with skin commensals should be prevented by applying relevant hygiene techniques. In patients where leukocytospermia is detected, the causes (e.g. infection, inflammation, varicocele, smoking, etc.) of the leukocyte infiltration have to be identified and addressed with antibiotics, anti-inflammatories or antioxidants in cases where high oxidative stress levels are detected. However, no specific strategy is available for the management of leukocytospermia. Therefore, the relationship between bacteriospermia and leukocytospermia as well as their specific impact on functional sperm parameters and reproductive outcome variables such as fertilization or clinical pregnancy must be further investigated. The aim of this narrative review is to provide an update on the current knowledge on leukocytospermia and bacteriospermia and their impact on male fertility.

Map-1a regulates Sertoli cell BTB dynamics through the cytoskeletal organization of microtubule and F-actin

Microtubule-associated protein 1a (Map1a) is a microtubule (MT) regulatory protein that binds to the MT protofilaments in mammalian cells to promote MT stabilization. Maps work with MT cleavage proteins and other MT catastrophe-inducing proteins to confer MT dynamics to support changes in the Sertoli cell shape to sustain spermatogenesis. However, no functional studies are found in the literature to probe its role in spermatogenesis. Using an RNAi approach, coupled with the use of toxicant-induced testis (in vivo)- and Sertoli cell (in vitro)-injury models, RNA-Seq analysis, transcriptome profiling, and relevant bioinformatics analysis, immunofluorescence analysis, and pertinent biochemical assays for cytoskeletal organization, we have delineated the functional role of Map1a in Sertoli cells and testes. Map1a was shown to support MT structural organization, and its knockdown (KD) also perturbed the structural organization of actin, vimentin, and septin cytoskeletons as these cytoskeletons are intimately related, working in concert to support spermatogenesis. More importantly, cadmium-induced Sertoli cell injury that perturbed the MT structural organization across the cell cytoplasm was associated with disruptive changes in the distribution of Map1a and a surge in p-p38-MAPK (phosphorylated p38-mitogen-activated protein kinase) expression but not total p38-MAPK. These findings thus support the notion that p-p38-MAPK activation is involved in cadmium-induced Sertoli cell injury. This conclusion was supported by studies using doramapimod, a specific p38-MAPK phosphorylation (activation) inhibitor, which was capable of restoring the cadmium-induced disruptive structural organization of MTs across the Sertoli cell cytoplasm. In summary: this study provides mechanistic insights regarding restoration of toxicant-induced Sertoli cell and testis injury and male infertility.

Evaluation of Genistein as a Mitochondrial Modulator and Its Effects on Sperm Quality

Phytoestrogens, such as isoflavones, are bioactive compounds found in plants with defense and protection functions. In the human body, they simulate the behavior of the hormone estradiol and can modulate the function of the male hypothalamic-pituitary-gonadal axis. This study aims to describe the effects of genistein on sperm quality of Wistar rats (male/adult) after a short oral administration protocol (50 mg/day, for 5 days), focusing on mitochondrial function. No signs of toxicity were observed in the animals during the period. The testicular mass of rats from the genistein-treated group was lower than that from the control group. Isoflavone increased the number of viable Leydig and Sertoli cells, spermatogonia, and primary spermatocytes in the treated group. The rounded spermatid count was similar to the control group, and a decrease in elongated spermatids was observed in the treated group. Genistein treatment increased plasma testosterone levels in the treated group. To the best of our knowledge, this is the first report of an in vivo short protocol demonstrating that genistein administration stimulates the overall oxygen consumption in rat seminal samples. Therefore, genistein induced a pro-spermatogenesis effect, enhanced plasma testosterone levels, and increased oxygen consumption, improving sperm mitochondrial efficiency. Similar protocols can be explored in animal and human infertility issues.

FeTPPS, a Peroxynitrite Decomposition Catalyst, Ameliorates Nitrosative Stress in Human Spermatozoa

Excessive levels of reactive nitrogen species (RNS), such as peroxynitrite, promote nitrosative stress, which is an important cause of impaired sperm function. The metalloporphyrin FeTPPS is highly effective in catalyzing the decomposition of peroxynitrite, reducing its toxic effects in vivo and in vitro. FeTPPS has significant therapeutic potential in peroxynitrite-related diseases; however, its effects on human spermatozoa under nitrosative stress have not been described. This work aimed to evaluate the in vitro effect of FeTPPS against peroxynitrite-mediated nitrosative stress in human spermatozoa. For this purpose, spermatozoa from normozoospermic donors were exposed to 3-morpholinosydnonimine, a molecule that generates peroxynitrite. First, the FeTPPS-mediated peroxynitrite decomposition catalysis was analyzed. Then, its individual effect on sperm quality parameters was evaluated. Finally, the effect of FeTPPS on ATP levels, motility, mitochondrial membrane potential, thiol oxidation, viability, and DNA fragmentation was analyzed in spermatozoa under nitrosative stress conditions. The results showed that FeTPPS effectively catalyzes the decomposition of peroxynitrite without affecting sperm viability at concentrations up to 50 μmol/L. Furthermore, FeTPPS mitigates the deleterious effects of nitrosative stress on all sperm parameters analyzed. These results highlight the therapeutic potential of FeTPPS in reducing the negative impact of nitrosative stress in semen samples with high RNS levels.

Mitochondrial metabolism determines the functional status of human sperm and correlates with semen parameters

The diagnosis of male infertility is based essentially on the patient’s medical history and a standard semen analysis. However, the latter rarely provides information on the causes of a possible infertility, emphasizing the need to extend the analysis of the sperm function. Mitochondrial function has been associated with sperm function and dysfunction, the latter primarily through the production of excessive amounts of reactive oxygen species (ROS). We hypothesized that analysis of sperm mitochondrial metabolism together with sperm ROS production could be an additional tool to improve routine semen analysis, after appropriate validations. To test our hypothesis, we performed several experiments using a non-routine method (high-resolution respirometry, HRR) to access mitochondrial function. First, we investigated whether mitochondrial function is related to human sperm motility and morphology. When mitochondrial metabolism was challenged, sperm motility decreased significantly. Additionally, morphological abnormalities in the sperm mid-piece and mitochondria were associated with global sperm defects evaluated by routine methods. Subsequently, sperm mitochondrial function was assessed by HRR. Respiratory control ratio (RCR) was determined and evaluated in the context of classical sperm analysis. In parallel, sperm hydrogen peroxide (H₂O₂) production and seminal plasma (SP) antioxidant capacity were measured. The percentage of sperm with progressive motility correlated positively with RCR, SP antioxidant capacity, and negatively with the concentration of extracellular H₂O₂ production ([H₂O₂]). The percentage of normal sperm morphology correlated positively with RCR and negatively with [H₂O₂]. Sperm morphology did not correlate with seminal plasma antioxidant capacity. Furthermore, Receiver Operating Characteristic curves were used for the first time to test the diagnostic ability of RCR, [H₂O₂], and SP antioxidant capacity as binary classifiers. An RCR cut off value of 3.2 was established with a sensitivity of 73% and a specificity of 61%, using reference values considered normal or abnormal in routine semen analysis. The cut off value for [H₂O₂] was 0.2 μM/10⁶ sperm (sensitivity = 65%, specificity = 60%). There were no reference values for SP antioxidant capacity that distinguished between abnormal and normal sperm samples. We conclude that sperm mitochondrial function indices in combination with [H₂O₂] may be useful tools to complement the routine semen analysis.