NOX5 in human spermatozoa: expression, function, and regulation

A comprehensive review on potential role of selenium, selenoproteins and selenium nanoparticles in male fertility

Selenium (Se), a component of selenoproteins and selenocompounds in the human body, is crucial for the development of male reproductive organs, DNA synthesis, thyroid hormone, metabolism, and defence against infections and oxidative damage. In the testis, it must exceed a desirable level since either a shortage or an overabundance causes aberrant growth. The antioxidant properties of selenium are essential for preserving human reproductive health. Selenoproteins, which have important structural and enzymatic properties, control the biological activities of Se primarily. These proteins specifically have a role in metabolism and a variety of cellular processes, such as the control of selenium transport, thyroid hormone metabolism, immunity, and redox balance. Selenium nanoparticles (SeNPs) are less hazardous than selenium-based inorganic and organic materials. Upon being functionalized with active targeting ligands, they are both biocompatible and capable of efficiently delivering combinations of payloads to particular cells. In this review, we discuss briefly the chemistry, structure and functions of selenium and milestones of selenium and selenoproteins. Next we discuss the various factors influences male infertility, biological functions of selenium and selenoproteins, and role of selenium and selenoproteins in spermatogenesis and male fertility. Furthermore, we discuss the molecular mechanism of selenium transport and protective effects of selenium on oxidative stress, apoptosis and inflammation. We also highlight critical contribution of selenium nanoparticles on male fertility and spermatogenesis. Finally ends with conclusion and future perspectives.

Effect of Trehalose Supplementation in Egg-Yolk-Free Extender on Conventional Parameters and Gene Expression Related to Reactive Oxygen Species, Apoptosis, and Motility of Frozen Dog Spermatozoa

The present study was conducted to evaluate the effects of trehalose supplementation in egg-yolk (EY)-free tris extender on dog spermatozoa. Pooled spermatozoa were diluted with extender 1 (EY-free tris extender supplemented with 0, 10, 15, 20, or 30 mM trehalose) and cooled (2 × 108 sperm/mL) for 1 hour at 4°C. After that, extender 2 (extender 1 containing 1 M glycerol) was added (v:v) to the diluted sperm, loaded in 0.5-mL straws (1 × 108 sperm/mL), and incubated at 4°C for 30 minutes. The sperm straws were frozen over liquid nitrogen (LN2) vapor for 20 minutes and then plunged directly into LN2. After thawing at 37°C for 25 seconds, sperm progressive motility (CASA), viability (SYBR-14/PI), apoptosis (Annexin V/PI), and reactive oxygen species (ROS; H2DCFDA/PI) were evaluated. Thereafter, the optimal concentrations of trehalose were selected, and the gene expression of BAX, BCL2, NOX5, SMOX, OGG1, and ROMO1 was evaluated after freeze-thawing. Supplementation with 20 and 30 mM trehalose significantly increased sperm progressive motility and viability compared to the control. However, trehalose had no significant effect on sperm ROS or phosphatidylserine translocation index. There were minor numerical increases and decreases in gene expression when the selected optimal concentrations of trehalose (20 and 30 mM) were compared to the control. However, there were no significant differences. We conclude that the addition of trehalose (20 and 30 mM) in EY-free extender could improve sperm motility and viability without significant effects on ROS, apoptosis, or gene expression.

The impact of mitochondrial impairments on sperm function and male fertility: a systematic review

BACKGROUND

Besides adenine triphosphate (ATP) production for sustaining motility, the mitochondria of sperm also host other critical cellular functions during germ cell development and fertilization including calcium homeostasis, generation of reactive oxygen species (ROS), apoptosis, and in some cases steroid hormone biosynthesis. Normal mitochondrial membrane potential with optimal mitochondrial performance is essential for sperm motility, capacitation, acrosome reaction, and DNA integrity.

RESULTS

Defects in the sperm mitochondrial function can severely harm the fertility potential of males. The role of sperm mitochondria in fertilization and its final fate after fertilization is still controversial. Here, we review the current knowledge on human sperm mitochondria characteristics and their physiological and pathological conditions, paying special attention to improvements in assistant reproductive technology and available treatments to ameliorate male infertility.

CONCLUSION

Although mitochondrial variants associated with male infertility have potential clinical use, research is limited. Further understanding is needed to determine how these characteristics lead to adverse pregnancy outcomes and affect male fertility potential.

The Impact of NADPH Oxidase 5 Activity and Reactive Oxygen Species on Capacitated Human Sperm

Background

Progesterone (P4) activates sperm calcium channels (CatSper), allowing calcium to enter the cell, which activates NADPH Oxidase-5 (NOX5) and produces reactive oxygen species (ROS). While calcium and ROS are essential for sperm capacitation, the role of NOX5 in capacitated sperm is unclear. This study investigated NOX5 activity in capacitated human sperm.

Methods

Forty semen samples from fertile men were processed, with motile sperm separated and divided into nine groups: control (Ham's F-10), solvent (DMSO), progesterone, diphenyleneiodonium chloride (DPI, NOX5 inhibitor), phorbol-12-myristate 13-acetate (PMA, NOX5 activator), P4+DPI, P4+PMA, Trolox, and P4+ Trolox. Sperm kinematics, membrane integrity, survival rate, and ROS production was evaluated. Data were analyzed using ANOVA and Kruskal-Wallis tests, p≤ 0.05 considered statistically significant.

Results

Progressive motility significantly decreased with DPI (56.2±2.1%) and PMA (56.5±2.1%), both alone and combined with progesterone (58.0±2.0% and 57.4±2.2%), compared to the progesterone group (66.0±1.9%). No significant change was observed in the Trolox groups. Progesterone, alone or combined with DPI, PMA, and Trolox, significantly reduced sperm linearity from 0.6±0.01 to 0.5±0.01%. Straight-line velocity decreased in P4+PMA and P4+Trolox groups (88.2±4.4 and 89.7±3.9 μm/s) compared to the control group (105.0±5.5 μm/s). Trolox reduced ROS content, while other treatments had no effect on ROS levels.

Conclusion

NOX5 does not play a prominent role in capacitated sperm. The negative effects of PMA and DPI on sperm motility appear independent of their actions on NOX5 and ROS production. Trolox did not affect sperm motility and survival, indicating that capacitated sperm require little or no ROS.

Association of NOX5 Expression with Sperm Activity and Motility in Pathospermic Infertile Men

Background

The newest NOX isoform, NOX5, has been found in mammalian spermatozoa. Many physiological and pathological situations in spermatozoa are mediated by reactive oxygen species (ROS). NOX5 is the main source of ROS in spermatozoa. Our purpose was to investigate the changes in NOX5 expression and the effect of NOX5 expression on sperm motility, chromatin integrity, and oxidative status in oligoasthenozoospermic compared to normozoospermic men.

Methods

Semen samples were collected from 30 normozoospermic (NS) and 30 oligoasthenozoospermic (OAS) men. NOX5 protein expression in sperm samples was evaluated by immunohistochemistry and western blot. Oxidative stress status was evaluated by total antioxidant capacity (TAC), total oxidant capacity (TOC), and oxidative stress index (OSI) parameters. Chromatin integrity in spermatozoa was evaluated by toluidine blue staining.

Results

NOX5 expression levels were significantly higher in OAS group than in NS group (p<0.001). In addition, chromatin integrity was significantly higher in the OAS group in comparison to NS group (p<0.001). TAC levels were higher in the NS group, but OSI and TOC levels were significantly higher in OAS group (p<0.001). It was found that NOX5 protein expression was positively correlated with oxidative stress and chromatin integrity and negatively correlated with motility (p<0.01).

Conclusion

These results suggest that overexpression of NOX5 may be the source of excessive ROS production and oxidative stress injuries in oligoasthenozoospermic men. Considering that NOX5 expression is positively correlated with oxidative stress and chromatin integrity but negatively correlated with motility, it can be considered a biomarker to be used in assisted reproductive procedures.

Structural basis of human NOX5 activation

NADPH oxidase 5 (NOX5) catalyzes the production of superoxide free radicals and regulates physiological processes from sperm motility to cardiac rhythm. Overexpression of NOX5 leads to cancers, diabetes, and cardiovascular diseases. NOX5 is activated by intracellular calcium signaling, but the underlying molecular mechanism of which - in particular, how calcium triggers electron transfer from NADPH to FAD - is still unclear. Here we capture motions of full-length human NOX5 upon calcium binding using single-particle cryogenic electron microscopy (cryo-EM). By combining biochemistry, mutagenesis analyses, and molecular dynamics (MD) simulations, we decode the molecular basis of NOX5 activation and electron transfer. We find that calcium binding to the EF-hand domain increases NADPH dynamics, permitting electron transfer between NADPH and FAD and superoxide production. Our structural findings also uncover a zinc-binding motif that is important for NOX5 stability and enzymatic activity, revealing modulation mechanisms of reactive oxygen species (ROS) production.

Effects of Reactive Oxygen and Nitrogen Species on Male Fertility

Significance: In recent decades, male fertility has been severely reduced worldwide. The causes underlying this decline are multifactorial, and include, among others, genetic alterations, changes in the microbiome, and the impact of environmental pollutants. Such factors can dysregulate the physiological levels of reactive species of oxygen (ROS) and nitrogen (RNS) in the patient, generating oxidative and nitrosative stress that impairs fertility. Recent Advances: Recent studies have delved into other factors involved in the dysregulation of ROS and RNS levels, such as diet, obesity, persistent infections, environmental pollutants, and gut microbiota, thus leading to new strategies to solve male fertility problems, such as consuming prebiotics to regulate gut flora or treating psychological conditions. Critical Issues: The pathways where ROS or RNS may be involved as modulators are still under investigation. Moreover, the extent to which treatments can rescue male infertility as well as whether they may have side effects remains, in most cases, to be elucidated. For example, it is known that prescription of antioxidants to treat nitrosative stress can alter sperm chromatin condensation, which makes DNA more exposed to ROS and RNS, and may thus affect fertilization and early embryo development. Future Directions: The involvement of extracellular vesicles, which might play a crucial role in cell communication during spermatogenesis and epididymal maturation, and the relevance of other factors such as sperm epigenetic signatures should be envisaged in the future.

X-ray structure and enzymatic study of a bacterial NADPH oxidase highlight the activation mechanism of eukaryotic NOX

NADPH oxidases (NOX) are transmembrane proteins, widely spread in eukaryotes and prokaryotes, that produce reactive oxygen species (ROS). Eukaryotes use the ROS products for innate immune defense and signaling in critical (patho)physiological processes. Despite the recent structures of human NOX isoforms, the activation of electron transfer remains incompletely understood. SpNOX, a homolog from Streptococcus pneumoniae, can serves as a robust model for exploring electron transfers in the NOX family thanks to its constitutive activity. Crystal structures of SpNOX full-length and dehydrogenase (DH) domain constructs are revealed here. The isolated DH domain acts as a flavin reductase, and both constructs use either NADPH or NADH as substrate. Our findings suggest that hydride transfer from NAD(P)H to FAD is the rate-limiting step in electron transfer. We identify significance of F397 in nicotinamide access to flavin isoalloxazine and confirm flavin binding contributions from both DH and Transmembrane (TM) domains. Comparison with related enzymes suggests that distal access to heme may influence the final electron acceptor, while the relative position of DH and TM does not necessarily correlate with activity, contrary to previous suggestions. It rather suggests requirement of an internal rearrangement, within the DH domain, to switch from a resting to an active state. Thus, SpNOX appears to be a good model of active NOX2, which allows us to propose an explanation for NOX2's requirement for activation.

Mitochondrial function and reactive oxygen species production during human sperm capacitation: Unraveling key players

Sperm capacitation is a critical process for male fertility. It involves a series of biochemical and physiological changes that occur in the female reproductive tract, rendering the sperm competent for successful fertilization. The precise mechanisms and, specifically, the role of mitochondria, in sperm capacitation remain incompletely understood. Previously, we revealed that in mouse sperm mitochondrial activity (e.g., oxygen consumption, membrane potential, ATP/ADP exchange, and mitochondrial Ca2+ ) increases during capacitation. Herein, we studied mitochondrial function by high-resolution respirometry (HRR) and reactive oxygen species production in capacitated (CAP) and non-capacitated (NC) human spermatozoa. We found that in capacitated sperm from normozoospermic donors, the respiratory control ratio increased by 36%, accompanied by a double oxygen consumption rate (OCR) in the presence of antimycin A. Extracellular hydrogen peroxide (H2 O2 ) detection was three times higher in CAP than in NC sperm cells. To confirm that H2 O2 production depends on mitochondrial superoxide (O 2 · - $$ {\mathrm{O}}_2^{\cdotp -} $$ ) formation, we evaluated mitochondrial aconitase (ACO2) amount, activity, and role in the metabolic flux from the sperm tricarboxylic acid cycle. We estimated that CAP cells produce, on average by individual, (59 ± 22)% moreO 2 · - $$ {\mathrm{O}}_2^{\cdotp -} $$ in the steady-state compared to NC cells. Finally, we analyzed two targets of oxidative stress: lipid peroxidation by western blot against 4-hydroxynonenal and succinate dehydrogenase (SDH) activity by HRR. We did not observe modifications in lipoperoxidation nor the activity of SDH, suggesting that during capacitation, the increase in mitochondrial H2 O2 production does not damage sperm and it is necessary for the normal CAP process.

Transcendent Aspects of Proton Channels

A handful of biological proton-selective ion channels exist. Some open at positive or negative membrane potentials, others open at low or high pH, and some are light activated. This review focuses on common features that result from the unique properties of protons. Proton conduction through water or proteins differs qualitatively from that of all other ions. Extraordinary proton selectivity is needed to ensure that protons permeate and other ions do not. Proton selectivity arises from a proton pathway comprising a hydrogen-bonded chain that typically includes at least one titratable amino acid side chain. The enormously diverse functions of proton channels in disparate regions of the phylogenetic tree can be summarized by considering the chemical and electrical consequences of proton flux across membranes. This review discusses examples of cells in which proton efflux serves to increase pHi, decrease pHo, control the membrane potential, generate action potentials, or compensate transmembrane movement of electrical charge.

Reactive oxygen species in biological systems: Pathways, associated diseases, and potential inhibitors-A review

Reactive oxygen species (ROS) are produced under normal physiological conditions and may have beneficial and harmful effects on biological systems. ROS are involved in many physiological processes such as differentiation, proliferation, necrosis, autophagy, and apoptosis by acting as signaling molecules or regulators of transcription factors. In this case, maintaining proper cellular ROS levels is known as redox homeostasis. Oxidative stress occurs because of the imbalance between the production of ROS and antioxidant defenses. Sources of ROS include the mitochondria, auto-oxidation of glucose, and enzymatic pathways such as nicotinamide adenine dinucleotide phosphate reduced (NAD[P]H) oxidase. The possible ROS pathways are NF-κB, MAPKs, PI3K-Akt, and the Keap1-Nrf2-ARE signaling pathway. This review covers the literature pertaining to the possible ROS pathways and strategies to inhibit them. Additionally, this review summarizes the literature related to finding ROS inhibitors.

Molecular mechanisms of mammalian sperm capacitation, and its regulation by sodium-dependent secondary active transporters

Background

Mammalian spermatozoa have to be "capacitated" to be fertilization-competent. Capacitation is a collective term for the physiological and biochemical changes in spermatozoa that occur within the female body. However, the regulatory mechanisms underlying capacitation have not been fully elucidated.

Methods

Previously published papers on capacitation, especially from the perspective of ions/channels/transporters, were extracted and summarized.

Results

Capacitation can be divided into two processes: earlier events (membrane potential hyperpolarization, intracellular pH rise, intracellular Ca2+ rise, etc.) and two major later events: hyperactivation and the acrosome reaction. Earlier events are closely interconnected with each other. Various channels/transporters are involved in the regulation of them, which ultimately lead to the later events. Manipulating the extracellular K+ concentration based on the oviductal concentration modifies membrane potential; however, the later events and fertilization are not affected, suggesting the uninvolvement of membrane potential in capacitation. Hyperpolarization is a highly conserved phenomenon among mammalian species, indicating its importance in capacitation. Therefore, the physiological importance of hyperpolarization apart from membrane potential is suggested.

Conclusion

The hypotheses are (1) hyperpolarizing Na+ dynamics (decrease in intracellular Na+) and Na+-driven secondary active transporters play a vital role in capacitation and (2) the sperm-specific potassium channel Slo3 is involved in volume and/or morphological regulation.

A novel monoclonal antibody reveals the enrichment of NADPH oxidase 5 in human splenic endothelial cells

Members of the NOX/DUOX family of NADPH oxidases are responsible for regulated ROS production in diverse cells and tissues. Detection of NOX/DUOX proteins at the protein level remains an important challenge in the field. Here we report the development and characterization of a novel anti-NOX5 monoclonal antibody, which recognizes the human NOX5 protein in both Western blot, immunocytochemistry, and histochemistry applications. With the help of the antibody we could successfully detect both heterologously and endogenously expressed NOX5 in mammalian cells. Furthermore, we could also detect NOX5 protein in the human spleen, testis, and ovary. Immunohistochemical studies on human testis revealed that NOX5 localized to spermatogenic cells. This expression pattern was also supported by the result of in silico analysis of single-cell RNA sequencing data that indicated that NOX5 protein is present in developing spermatids and spermatocytes. Mature spermatozoa, however, did not contain detectable NOX5. In the human ovary, both immunostaining and single-cell RNA sequencing suggest that NOX5 is expressed in interstitial fibroblasts and theca cells. We also analyzed vascular cells for the presence of NOX5 and we found that NOX5 expression is a fairly specific feature of splenic endothelial cells.

Role of the Voltage-Gated Proton Channel Hv1 in Nervous Systems

Hv1 is the only voltage-gated proton-selective channel in mammalian cells. It contains a conserved voltage-sensor domain, shared by a large class of voltage-gated ion channels, but lacks a pore domain. Its primary role is to extrude protons from the cytoplasm upon pH reduction and membrane depolarization. The best-known function of Hv1 is the regulation of cytosolic pH and the nicotinamide adenine dinucleotide phosphate oxidase-dependent production of reactive oxygen species. Accumulating evidence indicates that Hv1 is expressed in nervous systems, in addition to immune cells and others. Here, we summarize the molecular properties, distribution, and physiological functions of Hv1 in the peripheral and central nervous systems. We describe the recently discovered functions of Hv1 in various neurological diseases, including brain or spinal cord injury, ischemic stroke, demyelinating diseases, and pain. We also summarize the current advances in the discovery and application of Hv1-targeted small molecules in neurological diseases. Finally, we discuss the current limitations of our understanding of Hv1 and suggest future research directions.

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Structure, regulation, and physiological functions of NADPH oxidase 5 (NOX5)

NOX5 is the last member of the NADPH oxidase (NOXs) family to be identified and presents some specific characteristics differing from the rest of the NOXs. It contains four Ca²⁺ binding domains at the N-terminus and its activity is regulated by the intracellular concentration of Ca²⁺. NOX5 generates superoxide (O₂^•-) using NADPH as a substrate, and it modulates functions related to processes in which reactive oxygen species (ROS) are involved. Those functions appear to be detrimental or beneficial depending on the level of ROS produced. For example, the increase in NOX5 activity is related to the development of various oxidative stress-related pathologies such as cancer, cardiovascular, and renal diseases. In this context, pancreatic expression of NOX5 can negatively alter insulin action in high-fat diet-fed transgenic mice. This is consistent with the idea that the expression of NOX5 tends to increase in response to a stimulus or a stressful situation, generally causing a worsening of the pathology. On the other hand, it has also been suggested that it might have a positive role in preparing the body for metabolic stress, for example, by inducing a protective adipose tissue adaptation to the excess of nutrients supplied by a high-fat diet. In this line, its endothelial overexpression can delay lipid accumulation and insulin resistance development in obese transgenic mice by inducing the secretion of IL-6 followed by the expression of thermogenic and lipolytic genes. However, as NOX5 gene is not present in rodents and human NOX5 protein has not been crystallized, its function is still poorly characterized and further extensive research is required.

Calpain Regulates Reactive Oxygen Species Production during Capacitation through the Activation of NOX2 and NOX4

Capacitation is a series of physiological, biochemical, and metabolic changes experienced by mammalian spermatozoa. These changes enable them to fertilize eggs. The capacitation prepares the spermatozoa to undergo the acrosomal reaction and hyperactivated motility. Several mechanisms that regulate capacitation are known, although they have not been fully disclosed; among them, reactive oxygen species (ROS) play an essential role in the normal development of capacitation. NADPH oxidases (NOXs) are a family of enzymes responsible for ROS production. Although their presence in mammalian sperm is known, little is known about their participation in sperm physiology. This work aimed to identify the NOXs related to the production of ROS in guinea pig and mouse spermatozoa and define their participation in capacitation, acrosomal reaction, and motility. Additionally, a mechanism for NOXs' activation during capacitation was established. The results show that guinea pig and mouse spermatozoa express NOX2 and NOX4, which initiate ROS production during capacitation. NOXs inhibition by VAS2870 led to an early increase in the capacitation and intracellular concentration of Ca²⁺ in such a way that the spermatozoa also presented an early acrosome reaction. In addition, the inhibition of NOX2 and NOX4 reduced progressive motility and hyperactive motility. NOX2 and NOX4 were found to interact with each other prior to capacitation. This interaction was interrupted during capacitation and correlated with the increase in ROS. Interestingly, the association between NOX2-NOX4 and their activation depends on calpain activation, since the inhibition of this Ca²⁺-dependent protease prevents NOX2-NOX4 from dissociating and ROS production. The results indicate that NOX2 and NOX4 could be the most important ROS producers during guinea pig and mouse sperm capacitation and that their activation depends on calpain.

NADPH Oxidase-Mediated Testicular Oxidative Imbalance Regulates the TXNIP/NLRP3 Inflammasome Axis Activation after Ischemia Reperfusion Injury

Oxidative stress, inflammation and germ cell death are the main characteristics of testicular ischemia reperfusion injury (tIRI), which is considered as the underlying mechanism for testicular torsion and detorsion. The study aimed to examine the effect of tIRI-activated NADPH oxidase (NOX) on the expression of the NLRP3 inflammasome pathway components. Three groups of male Sprague-Dawley rats (n = 12 each) were studied: sham, unilateral tIRI only and tIRI treated with apocynin, a NOX-specific inhibitor. The tIRI rat model was subjected to 1 h of ischemia followed by 4 h of reperfusion. H&E staining, real time PCR, biochemical assays, and Western blot were utilized to evaluate spermatogenic damage, gene expression, oxidative stress markers, and NLRP3 pathway components, respectively. As a result of tIRI, decreased total antioxidant capacity and suppressed activities of superoxide dismutase and catalase were associated with spermatogenic arrest. The components of the NLRP3 inflammasome pathway (TXNIP, NLRP3, ASC, caspase-1, GSDMD, MMP-9) were upregulated transcriptionally and post-transcriptionally during tIRI. In parallel, tissue inflammation was demonstrated by a marked increase in the concentrations of myeloperoxidase, IL-1β, and IL-18. Apocynin treatment prevented testicular oxidative stress and inflammation. Thus, NOX inhibition by apocynin prevented ROS accumulation, proinflammatory cytokine overexpression and NLRP3 inflammasome activation during tIRI.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: The protection of mammalian spermatozoa against oxidative stress

In brief

This review focuses on the enzymatic antioxidant mechanisms to fight oxidative stress by spermatozoa, highlighting the differences among mammalian species. We discuss recent evidence about players that promote and fight oxidative stress and the need for novel strategies to diagnose and treat cases of male infertility associated with oxidative damage of the spermatozoon.

Abstract

The spermatozoon is very sensitive to high reactive oxygen species (ROS) levels due to its limited antioxidant system. A consortium of antioxidant enzymes, including superoxide dismutase (SOD), glutathione peroxidases (GPXs), peroxiredoxins (PRDXs), thioredoxins, and glutathione-S-transferases, is necessary to produce healthy spermatozoa and to maintain sperm quality to ensure motility, capacitation, and DNA integrity. A delicate balance between ROS production and antioxidant enzymes is needed to ensure ROS-dependent sperm capacitation. GPX4 is an essential component of the mitochondrial sheath in mammalian spermatozoa, and GPX5 is a crucial antioxidant defence in the mouse epididymis to protect the sperm genome during the maturation of the spermatozoon. The mitochondrial superoxide (O2·–) production is controlled by SOD2, and the hydrogen peroxide (H2O2) generated by SOD2 activity and peroxynitrite (ONOO–) are scavenged mainly by PRDXs in human spermatozoa. PRDXs regulate the redox signalling necessary for sperm motility and capacitation, particularly by PRDX6. This enzyme is the first line of defence against oxidative stress to prevent lipid peroxidation and DNA oxidation by scavenging H2O2 and ONOO– through its peroxidase activity and repairing oxidized membranes by its calcium-independent phospholipase A2 activity. The success of antioxidant therapy in treating infertility resides in the proper diagnosis of the presence of oxidative stress and which type of ROS are produced. Thus, more research on the molecular mechanisms affected by oxidative stress, the development of novel diagnostic tools to identify infertile patients with oxidative stress, and randomized controlled trials are of paramount importance to generate personalized antioxidant therapy to restore male fertility.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: The impact of oxidative stress on reproduction: a focus on gametogenesis and fertilization

In brief

Many aspects of the reproductive process are impacted by oxidative stress. This article summarizes the chemical nature of reactive oxygen species and their role in both the physiological regulation of reproductive processes and the pathophysiology of infertility.

Abstract

This article lays out the fundamental principles of oxidative stress. It describes the nature of reactive oxygen species (ROS), the way in which these potentially toxic metabolites interact with cells and how they impact both cellular function and genetic integrity. The mechanisms by which ROS generation is enhanced to the point that the cells' antioxidant defence mechanisms are overwhelmed are also reviewed taking examples from both the male and female reproductive system, with a focus on gametogenesis and fertilization. The important role of external factors in exacerbating oxidative stress and impairing reproductive competence is also examined in terms of their ability to disrupt the physiological redox regulation of reproductive processes. Developing diagnostic and therapeutic strategies to cope with oxidative stress within the reproductive system will depend on the development of a deeper understanding of the nature, source, magnitude, and location of such stress in order to fashion personalized treatments that meet a given patient's clinical needs.

Mitochondria-Targeted Compounds to Assess and Improve Human Sperm Function

Significance: Currently 10%-15% of couples in reproductive age face infertility issues. More importantly, male factor contributes to 50% of these cases (either alone or in combination with female causes). Among various reasons, impaired sperm function is the main cause for male infertility. Furthermore, mitochondrial dysfunction and oxidative stress due to increased reactive oxygen species (ROS) production, particularly of mitochondrial origin, are believed to be the main contributors. Recent Advances: Mitochondrial dysfunction, particularly due to increased ROS production, has often been linked to impaired sperm function/quality. For decades, different methods and approaches have been developed to assess mitochondrial features that might correlate with sperm functionality. This connection is now completely accepted, with mitochondrial functionality assessment used more commonly as a readout of sperm functionality. More recently, mitochondria-targeted compounds are on the frontline for both assessment and therapeutic approaches. Critical Issues: In this review, we summarize the current methods for assessing key mitochondrial parameters known to reflect sperm quality as well as therapeutic strategies using mitochondria-targeted antioxidants aiming to improve sperm function in various situations, particularly after sperm cryopreservation. Future Directions: Although more systematic research is needed, mitochondria-targeted compounds definitely represent a promising tool to assess as well as to protect and improve sperm function. Antioxid. Redox Signal. 37, 451-480.

Redox Regulation to Modulate Phosphorylation Events in Human Spermatozoa

Significance: Spermatozoa are complex and compartmentalized cells that undergo capacitation, a series of biochemical and morphological changes to acquire the ability to fertilize oocytes. Reactive oxygen species (ROS) have a prominent dual role in capacitation. At physiological levels, ROS regulate numerous cellular processes, including increases of cyclic adenosine monophosphate, calcium, and activation of phosphorylation events needed for capacitation. On the contrary, at high concentrations that do not impair sperm viability, ROS can cause loss of motility and inhibition of capacitation. Higher ROS concentrations promote oxidation of lipids, proteins, and DNA leading to cell death, and these damages have been associated with male infertility. Critical Issues: When incubated under specific conditions, spermatozoa can produce low and controlled amounts of ROS that are not harmful but instead regulate numerous cellular processes, including the phosphorylation of tyrosine, serine, and threonine residues in critical proteins needed for sperm capacitation. Here, we outline the complex redox signaling in human spermatozoa needed to achieve fertility and the role of ROS as physiological mediators that trigger phosphorylation cascades. Moreover, we illustrate the importance of various phosphoproteins in spermatozoa capacitation, viability, and hyperactive motility. Future Directions: Further studies to elucidate the different phosphorylation players during sperm capacitation and acrosome reaction (the regulated exocytotic event that releases proteolytic enzymes allowing the spermatozoon to penetrate the zona pellucida and fertilize the oocyte) are essential to understand how the spermatozoon acquires the fertilizing ability to fertilize the oocyte. This knowledge will serve to develop novel diagnostic tools and therapy for male infertility. Antioxid. Redox Signal. 37, 437-450.

Exploring the Role of Oxidative Stress in Sperm Motility: A Proteomic Network Approach

Significance: Infertility is a major global health problem, with nearly half of the cases being associated with male factors. Although reactive oxygen species (ROS) are crucial for sperm cell normal physiological processes, an imbalance between ROS production and antioxidants can lead to oxidative stress that can impair sperm function. Indeed, high semen ROS levels are reported in 30%-80% of infertile men. Recent Advances: Male oxidative stress infertility is an uprising classification for idiopathic infertility. Proteomic approaches, including quantitative mass spectrometry (MS)-based proteomics, are being utilized to explore the molecular mechanisms associated with oxidative stress in male infertility. Critical Issues: In this review, proteome data were collected from articles available on PubMed centered on MS-based proteomic studies, performed in seminal plasma and sperm cell samples, and enrolling men with impaired semen parameters. The bioinformatic analysis of proteome data with Cytoscape (ClueGO+CluePedia) and STRING tools allowed the identification of the biological processes more prevalent in asthenozoospermia, with focus on the ones related to oxidative stress. Future Directions: The identification of the antioxidant proteins in seminal plasma and sperm cells that can protect sperm cells from oxidative stress is crucial not only for a better understanding of the molecular mechanisms associated with male infertility but specially to guide new therapeutic possibilities. Antioxid. Redox Signal. 37, 501-520.

Impact of calcium and reactive oxygen species on human sperm function: Role of NOX5

NOX5 is introduced as a new therapeutic target for infertility treatment. This study aimed to compare the basal and stimulated reactive oxygen species (ROS) production and sperm function in human teratozoospermic (n = 15) and normozoospermic (n = 17) semen samples following calcium overload and NOX5 activation. Washed spermatozoa incubated for 1 h under five various conditions: control group, adding a calcium ionophore A23187, phorbol myristate acetate (PMA), A23187 + PMA, and diphenylene iodonium (DPI) + A23187 + PMA. ROS generation was measured immediately after treatment for 30 min. Motility, viability, acrosome reaction, and apoptosis were evaluated after 1-h incubation. ROS production significantly increased when A23187 or PMA was added to the sperm medium. DPI had suppressive effects on ROS generation. Progressive and total motility significantly decreased following calcium elevation and NOX5 activation, which was somewhat returned by DPI. Necrotic and live cells in teratozoospermia was, respectively, higher and lower than normozoospermia samples. Incubation with A23187 significantly increased the percentage of early and late apoptosis. Teratozoosperm are more vulnerable than normal spermatozoa, and produce more basal and stimulated ROS. It seems that calcium overload induces apoptosis in spermatozoa and loss of viability through MPT pore opening and increased intracellular ROS.

Male Infertility and Oxidative Stress: A Focus on the Underlying Mechanisms

Reactive oxygen species (ROS) play a critical role in defining the functional competence of human spermatozoa. When generated in moderate amounts, ROS promote sperm capacitation by facilitating cholesterol efflux from the plasma membrane, enhancing cAMP generation, inducing cytoplasmic alkalinization, increasing intracellular calcium levels, and stimulating the protein phosphorylation events that drive the attainment of a capacitated state. However, when ROS generation is excessive and/or the antioxidant defences of the reproductive system are compromised, a state of oxidative stress may be induced that disrupts the fertilizing capacity of the spermatozoa and the structural integrity of their DNA. This article focusses on the sources of ROS within this system and examines the circumstances under which the adequacy of antioxidant protection might become a limiting factor. Seminal leukocyte contamination can contribute to oxidative stress in the ejaculate while, in the germ line, the dysregulation of electron transport in the sperm mitochondria, elevated NADPH oxidase activity, or the excessive stimulation of amino acid oxidase action are all potential contributors to oxidative stress. A knowledge of the mechanisms responsible for creating such stress within the human ejaculate is essential in order to develop better antioxidant strategies that avoid the unintentional creation of its reductive counterpart.

Physiological role of reactive oxygen species in testis and epididymal spermatozoa

The reactive oxygen species (ROS) play an important role in various aspects of male reproductive function, for spermatozoa to acquire the ability to fertilize. However, the increase in ROS generation, both due to internal and external factors, can induce oxidative stress, causing alterations in the structure and function of phospholipids and proteins. In the nucleus, ROS attack DNA, causing its fragmentation and activation of apoptosis, thus altering gene and protein expression. Accumulating evidence also reveals that endogenously produced ROS can act as second messengers in regulating cell signalling pathways and in the transduction of signals that are responsible for regulating spermatogonia self-renewal and proliferation. In the epididymis, they actively participate in the formation of disulphide bridges required for the final condensation of chromatin, as well as in the phosphorylation and dephosphorylation of proteins contained in the fibrous sheath of the flagellum, stimulating the activation of progressive motility in epididymal spermatozoa. In this review, the role of small amounts of ROS during spermatogenesis and epididymal sperm maturation was discussed.

Roles of Oxidative Stress in the Male Reproductive System: Potential of Antioxidant Supplementation for Infertility Treatment

The decline of fertility in modern society is a serious worldwide concern, and the reasons behind it are complex and difficult to unveil. The fact that a big percentage of infertility cases remain diagnosed as idiopathic, turn the strategies to treat such conditions very limited. Nevertheless, one must agree that keeping the oxidative balance of the reproductive tissues should be one of the first lines of treatment for infertile patients. As reported, 30-80% of male infertile individuals present high levels of prooxidant species in the seminal fluid. Thus, antioxidant therapies, which consist of dietary supplementation therapy with one or more antioxidant compound, remain the first step in the treatment of male infertility. Nevertheless, the efficacy of such therapies is variable between individuals. The most common prescribed antioxidants are carnitines and vitamins C and E, but recently phytochemical quercetin has emerged as a potential compound for the treatment of oxidative stress in the male reproductive system. Although there are several animals' evidence about the great potential of quercetin for the treatment of infertility, clinical trials on this subject remain scarce.

Blocking NHE Channels Reduces the Ability of In Vitro Capacitated Mammalian Sperm to Respond to Progesterone Stimulus

Few data exist about the presence and physiological role of Na+/H+ exchangers (NHEs) in the plasma membrane of mammalian sperm. In addition, the involvement of these channels in the ability of sperm to undergo capacitation and acrosomal reaction has not been investigated in any mammalian species. In the present study, we addressed whether these channels are implicated in these two sperm events using the pig as a model. We also confirmed the presence of NHE1 channels in the plasma membrane of ejaculated sperm by immunofluorescence and immunoblotting. The function of NHE channels during in vitro capacitation was analyzed by incubating sperm samples in capacitating medium for 300 min in the absence or presence of a specific blocker (DMA; 5-(N,N-dimethyl)-amiloride) at different concentrations (1, 5, and 10 µM); acrosome exocytosis was triggered by adding progesterone after 240 min of incubation. Sperm motility and kinematics, integrity of plasma and acrosome membranes, membrane lipid disorder, intracellular calcium and reactive oxygen species (ROS) levels, and mitochondrial membrane potential (MMP) were evaluated after 0, 60, 120, 180, 240, 250, 270, and 300 min of incubation. NHE1 localized in the connecting and terminal pieces of the flagellum and in the equatorial region of the sperm head and was found to have a molecular weight of 75 kDa. During the first 240 min of incubation, i.e., before the addition of progesterone, blocked and control samples did not differ significantly in any of the parameters analyzed. However, from 250 min of incubation, samples treated with DMA showed significant alterations in total motility and the amplitude of lateral head displacement (ALH), acrosomal integrity, membrane lipid disorder, and MMP. In conclusion, while NHE channels are not involved in the sperm ability to undergo capacitation, they could be essential for triggering acrosome exocytosis and hypermotility after progesterone stimulus.

Effects of Inositol Supplementation in Sperm Extender on the Quality of Cryopreserved Mesopotamian Catfish (Silurus triostegus, H. 1843) Sperm

In this study, the effects of supplemented inositol on sperm extenders were examined on the spermatozoa motility rate and duration, total antioxidant and oxidant status, apoptotic spermatozoa and DNA damage, during the sperm post-thaw process of Mesopotamian Catfish (Silurus triostegus, H. 1843). The semen was frozen in diluents containing different inositol concentrations (5, 10, 20 and 40 mg). Increasing levels of inositol linearly improved the spermatozoa motility rate and duration significantly (p < 0.05). MDA and TOS were linearly decreased, however, TAS and GSH linearly increased (p < 0.05). The increasing inositol levels resulted in a linear and quadratic decrease in DNA damage in the comet assay, 8-hydroxydeoxyguanosine and the determined percentage of apoptotic spermatozoa (p < 0.05). These results suggest that there are many positive effects of the use of supplemental inositol on enhancing sperm cryopreservation efficiency in Silurus triostegus.

Nitric oxide and heme-NO stimulate superoxide production by NADPH oxidase 5

Nitric oxide (NO) is a ubiquitous cell signaling molecule which mediates widespread and diverse processes in the cell. These NO dependent effects often involve activation (e.g. NO binding to the heme group of soluble guanylyl cyclase for cGMP production) or inactivation (e.g. S-nitrosation) of protein targets. We studied the effect of NO and heme-NO on the transmembrane signaling enzyme NADPH oxidase 5 (NOX5), a heme protein which produces superoxide in response to increases in intracellular calcium. We found that treatment with NO donors increases NOX5 activity through heme-dependent effects, and that this effect could be recapitulated by the addition of heme-NO. This work adds to our understanding of NOX5 regulation in the cell but also provides a framework for understanding how NO could cause widespread changes in hemeprotein activity based on different affinities for heme v. heme-NO, and helps explain the opposing roles NO plays in activation and inactivation of hemeprotein targets.

Fluorescent probes for the detection of reactive oxygen species in human spermatozoa

Reactive oxygen species (ROS) production is a by-product of mitochondrial activity and is necessary for the acquisition of the capacitated state, a requirement for functional spermatozoa. However, an increase in oxidative stress, due to an abnormal production of ROS, has been shown to be related to loss of sperm function, highlighting the importance of an accurate detection of sperm ROS, given the specific nature of this cell. In this work, we tested a variety of commercially available fluorescent probes to detect ROS and reactive nitrogen species (RNS) in human sperm, to define their specificity. Using both flow cytometry (FC) and fluorescence microscopy (FM), we confirmed that MitoSOX™ Red and dihydroethidium (DHE) detect superoxide anion (as determined using antimycin A as a positive control), while DAF-2A detects reactive nitrogen species (namely, nitric oxide). For the first time, we also report that RedoxSensor™ Red CC-1, CellROX^® Orange Reagent, and MitoPY1 seem to be mostly sensitive to hydrogen peroxide, but not superoxide. Furthermore, mean fluorescence intensity (and not percentage of labeled cells) is the main parameter that can be reproducibly monitored using this type of methodology.

Phycobiliproteins Ameliorate Gonadal Toxicity in Male Mice Treated with Cyclophosphamide

Cyclophosphamide (CP)-which is used to treat autoimmune diseases and cancer-is related to gonadotoxicity attributed to oxidative stress. As phycobiliproteins (PBPs) are strong antioxidants that are unexplored as protective agents against male gonadotoxicity, our work aimed to investigate the effects of PBP crude extract on testicular damage and sperm parameter alterations caused by CP in mice. Three doses of PBP (50, 100, and 200 mg/kg) were tested in the experimental groups (n = 8 per group), administered concomitantly with 100 mg/kg CP. After 42 days receiving PBP daily and CP weekly, body and relative testicular weights, serum testosterone levels, testicular lipoperoxidation and antioxidant enzyme activity levels, and testicular histology and sperm parameter alterations were assessed. The results showed that PBP crude extract at 200 mg/kg prevented testosterone serum reduction, body weight loss, lipoperoxidation and enzyme activity increments, and sperm parameter alterations and partially ameliorated relative testicular weight reductions and histological damage in CP-treated mice. In conclusion, we showed that PBP crude extract (200 mg/kg) mitigated oxidative damage in the testes and ameliorated alterations in sperm parameters in mice treated with CP (100 mg/kg); therefore, PBP extract could be considered as a potential protective agent against CP toxicity.

HIFs: New arginine mimic inhibitors of the Hv1 channel with improved VSD-ligand interactions

The human voltage-gated proton channel Hv1 is a drug target for cancer, ischemic stroke, and neuroinflammation. It resides on the plasma membrane and endocytic compartments of a variety of cell types, where it mediates outward proton movement and regulates the activity of NOX enzymes. Its voltage-sensing domain (VSD) contains a gated and proton-selective conduction pathway, which can be blocked by aromatic guanidine derivatives such as 2-guanidinobenzimidazole (2GBI). Mutation of Hv1 residue F150 to alanine (F150A) was previously found to increase 2GBI apparent binding affinity more than two orders of magnitude. Here, we explore the contribution of aromatic interactions between the inhibitor and the channel in the presence and absence of the F150A mutation, using a combination of electrophysiological recordings, classic mutagenesis, and site-specific incorporation of fluorinated phenylalanines via nonsense suppression methodology. Our data suggest that the increase in apparent binding affinity is due to a rearrangement of the binding site allowed by the smaller residue at position 150. We used this information to design new arginine mimics with improved affinity for the nonrearranged binding site of the wild-type channel. The new compounds, named “Hv1 Inhibitor Flexibles” (HIFs), consist of two “prongs,” an aminoimidazole ring, and an aromatic group connected by extended flexible linkers. Some HIF compounds display inhibitory properties that are superior to those of 2GBI, thus providing a promising scaffold for further development of high-affinity Hv1 inhibitors.

Sperm Oxidative Stress during In Vitro Manipulation and Its Effects on Sperm Function and Embryo Development

Reactive oxygen species (ROS) generated at low levels during mitochondrial respiration have key roles in several signaling pathways. Oxidative stress (OS) arises when the generation of ROS exceeds the cell's antioxidant scavenging ability and leads to cell damage. Physiological ROS production in spermatozoa regulates essential functional characteristics such as motility, capacitation, acrosome reaction, hyperactivation, and sperm-oocyte fusion. OS can have detrimental effects on sperm function through lipid peroxidation, protein damage, and DNA strand breakage, which can eventually affect the fertility of an individual. Substantial evidence in the literature indicates that spermatozoa experiencing OS during in vitro manipulation procedures in human- and animal-assisted reproduction are increasingly associated with iatrogenic ROS production and eventual impairment of sperm function. Although a direct association between sperm OS and human assisted reproductive techniques (ART) outcomes after in vitro fertilization (IVF) and/or intracytoplasmic sperm injection (ICSI) is still a matter of debate, studies in animal models provide enough evidence on the adverse effects of sperm OS in vitro and defective fertilization and embryo development. This review summarized the literature on sperm OS in vitro, its effects on functional ability and embryo development, and the approaches that have been proposed to reduce iatrogenic sperm damage and altered embryonic development.

Effect of glucose concentration and cryopreservation on mitochondrial functions of bull spermatozoa and relationship with sire conception rate

Mitochondrial function is essential for sperm viability, not only from a sperm metabolism perspective, but also for improvement of sperm storage in liquid and frozen states. Bull sperm have notable metabolic variability with energy production for motility and subsequently for fertilizing capacity resulting from both glycolysis and oxidative phosphorylation. The objective of this study was to determine mitochondrial function of sperm using high-throughput Seahorse Analyzer technology in fresh semen and subsequent to freezing-thawing when there was incubation in media commonly used for sperm storage (relatively large glucose concentration) and female tract (relatively small glucose concentration). Additionally, there were determinations whether there were differences in values for fertility variables by regressing sire conception rate on values for mitochondrial variables when there was evaluation of semen from bulls with varying fertility. Media with larger concentrations of glucose inhibited mitochondrial function in fresh sperm, as indicated by less maximal oxygen consumption, spare respiratory capacity and coupling efficiency when compared to sperm in the media containing less glucose. Furthermore, there was greater (P <  0.05) mitochondrial function in cryopreserved-thawed compared to fresh samples with there being no effect of incubation media. These results indicate that mitochondrial damage from cryopreservation cannot be simply overcome post-thawing with glucose supplementation of bull semen incubation media. The increase in mitochondrial function is likely due to "non-productive" oxygen consumption to maintain the mitochondrial proton gradient. Furthermore, there was a negative association of mitochondrial proton leakage with sire conception rate indicating this could be a potential biomarker of bull fertility.

NADPH Oxidase 5 and Melatonin: Involvement in Ram Sperm Capacitation

Reactive oxygen species (ROS) play an essential role in mammalian sperm capacitation. NADPH oxidase 5 (NOX5) has been described as the main source of ROS production in some mammalian spermatozoa, such as human and equine. On the other hand, melatonin can decrease cellular ROS levels and regulates NOX activity in somatic cells. Therefore, the objectives of this work were (1) to identify NOX5 in ram spermatozoa and analyze its possible changes during in vitro capacitation and (2) to investigate the effect of melatonin on NOX5 expression and localization and on superoxide levels in capacitated ram spermatozoa. Protein bands associated with NOX5 were detected by Western blot analysis. Likewise, indirect immunofluorescence (IIF) revealed six different immunotypes for NOX5, which varied throughout in vitro capacitation. Superoxide (O₂^⋅–), evaluated by DHE/Yo-Pro-1, rose after in vitro capacitation and in the presence of the calcium ionophore A23187 but decreased in the presence of the NOX inhibitor GKT136901. GKT also reduced the percentage of capacitated and acrosome-reacted spermatozoa that had increased during incubation in capacitating conditions. The presence of melatonin at micromolar concentrations avoided the increment in O₂^⋅– and the changes in NOX5 immunotypes provoked by capacitation. In conclusion, NOX5 is present in ram spermatozoa and the changes in its distribution, associated with sperm capacitation, can be prevented by melatonin. To this extent, it could imply that melatonin exerts its antioxidant role, at least in part, by modulating NOX5 activity during ram sperm capacitation.

Functions and Mechanisms of the Voltage-Gated Proton Channel Hv1 in Brain and Spinal Cord Injury

The voltage-gated proton channel Hv1 is a newly discovered ion channel that is highly conserved among species. It is known that Hv1 is not only expressed in peripheral immune cells but also one of the major ion channels expressed in tissue-resident microglia of the central nervous systems (CNS). One key role for Hv1 is its interaction with NADPH oxidase 2 (NOX2) to regulate reactive oxygen species (ROS) and cytosolic pH. Emerging data suggest that excessive ROS production increases and requires proton currents through Hv1 in the injured CNS, and manipulations that ablate Hv1 expression or induce loss of function may provide neuroprotection in CNS injury models including stroke, traumatic brain injury, and spinal cord injury. Recent data demonstrating microglial Hv1-mediated signaling in the pathophysiology of the CNS injury further supports the idea that Hv1 channel may function as a key mechanism in posttraumatic neuroinflammation and neurodegeneration. In this review, we summarize the main findings of Hv1, including its expression pattern, cellular mechanism, role in aging, and animal models of CNS injury and disease pathology. We also discuss the potential of Hv1 as a therapeutic target for CNS injury.

Mitochondria: their role in spermatozoa and in male infertility

BACKGROUND

The best-known role of spermatozoa is to fertilize the oocyte and to transmit the paternal genome to offspring. These highly specialized cells have a unique structure consisting of all the elements absolutely necessary to each stage of fertilization and to embryonic development. Mature spermatozoa are made up of a head with the nucleus, a neck, and a flagellum that allows motility and that contains a midpiece with a mitochondrial helix. Mitochondria are central to cellular energy production but they also have various other functions. Although mitochondria are recognized as essential to spermatozoa, their exact pathophysiological role and their functioning are complex. Available literature relative to mitochondria in spermatozoa is dense and contradictory in some cases. Furthermore, mitochondria are only indirectly involved in cytoplasmic heredity as their DNA, the paternal mitochondrial DNA, is not transmitted to descendants.

OBJECTIVE AND RATIONAL

This review aims to summarize available literature on mitochondria in spermatozoa, and, in particular, that with respect to humans, with the perspective of better understanding the anomalies that could be implicated in male infertility.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews pertaining to human spermatozoa and mitochondria. Searches were performed using keywords belonging to three groups: 'mitochondria' or 'mitochondrial DNA', 'spermatozoa' or 'sperm' and 'reactive oxygen species' or 'calcium' or 'apoptosis' or signaling pathways'. These keywords were combined with other relevant search phrases. References from these articles were used to obtain additional articles.

OUTCOMES

Mitochondria are central to the metabolism of spermatozoa and they are implicated in energy production, redox equilibrium and calcium regulation, as well as apoptotic pathways, all of which are necessary for flagellar motility, capacitation, acrosome reaction and gametic fusion. In numerous cases, alterations in one of the aforementioned functions could be linked to a decline in sperm quality and/or infertility. The link between the mitochondrial genome and the quality of spermatozoa appears to be more complex. Although the quantity of mtDNA, and the existence of large-scale deletions therein, are inversely correlated to sperm quality, the effects of mutations seem to be heterogeneous and particularly related to their pathogenicity.

WIDER IMPLICATIONS

The importance of the role of mitochondria in reproduction, and particularly in gamete quality, has recently emerged following numerous publications. Better understanding of male infertility is of great interest in the current context where a significant decline in sperm quality has been observed.

HVCN1 but Not Potassium Channels Are Related to Mammalian Sperm Cryotolerance

Little data exist about the physiological role of ion channels during the freeze–thaw process in mammalian sperm. Herein, we determined the relevance of potassium channels, including SLO1, and of voltage-gated proton channels (HVCN1) during mammalian sperm cryopreservation, using the pig as a model and through the addition of specific blockers (TEA: tetraethyl ammonium chloride, PAX: paxilline or 2-GBI: 2-guanidino benzimidazole) to the cryoprotective media at either 15 °C or 5 °C. Sperm quality of the control and blocked samples was performed at 30- and 240-min post-thaw, by assessing sperm motility and kinematics, plasma and acrosome membrane integrity, membrane lipid disorder, intracellular calcium levels, mitochondrial membrane potential, and intracellular O₂⁻⁻ and H₂O₂ levels. General blockade of K⁺ channels by TEA and specific blockade of SLO1 channels by PAX did not result in alterations in sperm quality after thawing as compared to control samples. In contrast, HVCN1-blocking with 2-GBI led to a significant decrease in post-thaw sperm quality as compared to the control, despite intracellular O₂⁻⁻ and H₂O₂ levels in 2-GBI blocked samples being lower than in the control and in TEA- and PAX-blocked samples. We can thus conclude that HVCN1 channels are related to mammalian sperm cryotolerance and have an essential role during cryopreservation. In contrast, potassium channels do not seem to play such an instrumental role.

NOX2-Derived Reactive Oxygen Species in Cancer

The formation of reactive oxygen species (ROS) by the myeloid cell NADPH oxidase NOX2 is critical for the destruction of engulfed microorganisms. However, recent studies imply that ROS, formed by NOX2⁺ myeloid cells in the malignant microenvironment, exert multiple actions of relevance to the growth and spread of neoplastic cells. By generating ROS, tumor-infiltrating myeloid cells and NOX2⁺ leukemic myeloid cells may thus (i) compromise the function and viability of adjacent cytotoxic lymphocytes, including natural killer (NK) cells and T cells, (ii) oxidize DNA to trigger cancer-promoting somatic mutations, and (iii) affect the redox balance in cancer cells to control their proliferation and survival. Here, we discuss the impact of NOX2-derived ROS for tumorigenesis, tumor progression, regulation of antitumor immunity, and metastasis. We propose that NOX2 may be a targetable immune checkpoint in cancer.

Voltage-gated proton channels exist in the plasma membrane of human oocytes

STUDY QUESTION

Do human oocytes express voltage-gated proton channels?

SUMMARY ANSWER

Human oocytes exhibit voltage-gated proton currents.

WHAT IS KNOWN ALREADY

Voltage-gated proton currents have been reported in human sperm, where they contribute to capacitation and motility. No such studies of human oocytes exist.

STUDY DESIGN, SIZE, DURATION

Voltage-clamp studies were undertaken using entire oocytes and vesicles derived from oocytes and in excised patches of membrane from oocytes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Frozen, thawed human metaphase II oocytes were obtained from material donated to the gamete repository at the Rush Center for Advanced Reproductive Care. Prior to patch clamping, oocytes were warmed and equilibrated. Formation of an electrically tight seal requires exposing bare oolemma. Sections of the zona pellucida (ZP) were removed using a laser, followed by repeated pipetting, to further separate the oocyte from the ZP. Patch-clamp studies were performed using the whole-cell configuration on oocytes or vesicles derived from oocytes, and using inside-out patches of membrane, under conditions optimized to detect voltage-gated proton currents.

MAIN RESULTS AND THE ROLE OF CHANCE

Proton currents are present at significant levels in human oocytes where they exhibit properties similar to those reported in other human cells, as well as those in heterologous expression systems transfected with the HVCN1 gene that codes for the voltage-gated proton channel.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Human oocytes are large cells, which limits our ability to control the intracellular solution. Subtle effects of cryopreservation by vitrification and subsequent warming on properties of HVCN1, the HVCN1 gene product, cannot be ruled out.

WIDER IMPLICATIONS OF THE FINDINGS

Possible functions for voltage-gated proton channels in human oocytes may now be contemplated.

STUDY FUNDING/COMPETING INTEREST(S)

NIH R35GM126902 (TED), Bears Care (DM). No competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Inhibiting the Activity of NADPH Oxidase in Cancer

Significance: The primary function of NADPH oxidases (NOX1-5 and dual oxidases DUOX1/2) is to produce reactive oxygen species (ROS). If inadequately regulated, NOX-associated ROS can promote oxidative stress, aberrant signaling, and genomic instability. Correspondingly, NOX isoforms are known to be overexpressed in multiple malignancies, thus constituting potential therapeutic targets in cancer. Recent Advances: Multiple genetic studies aimed at suppressing the expression of NOX proteins in cellular and animal models of cancer have provided support for the notion that NOXs play a pro-tumorigenic role. Further, large drug screens and rational design efforts have yielded inhibitor compounds, such as the diphenylene iodonium (DPI) analog series developed by our group, with increased selectivity and potency over "first generation" NOX inhibitors such as apocynin and DPI. Critical Issues: The precise role of NOX enzymes in tumor biology remains poorly defined. The tumorigenic properties of NOXs vary with cancer type, and precise tools, such as selective inhibitors, are needed to deconvolute NOX contribution to cancer development. Most NOX inhibitors developed to date are unspecific, and/or their mechanistic and pharmacological characteristics are not well defined. A lack of high-resolution crystal structures for NOX functional domains has hindered the development of potent and selective inhibitors. Future Directions: In-depth studies of NOX interactions with the tumor microenvironment (e.g., cytokines, cell-surface antigens) will help identify new approaches for NOX inhibition in cancer.

Membranous NOX5-derived ROS oxidizes and activates local Src to promote malignancy of tumor cells

Reactive oxygen species (ROS) localized at the precise subcellular compartments are essential for regulating the activity of signaling proteins. Furthermore, ROS are master regulators of tumor malignant progression that respond to a diverse set of environmental stress, especially hypoxia. NADPH oxidases (NOXs) appear to be activated within discrete subcellular compartments to facilitate local ROS production. However, the subcellular function of NOXs in hypoxic tumor is still unclear. In this study, we demonstrated that NOX5 was greatly upregulated in clinical esophageal squamous cell carcinoma (ESCC) tumors, ESCC cell lines or primary ESCC cells, and elevated NOX5 was correlated to malignancy of ESCC tumors and poor prognosis. NOX5 induced the malignant progression of ESCC by activating Src, especially under hypoxic condition. Mechanistically, we showed that hypoxia promoted the interaction between NOX5 and Pyk2 on cell membrane via facilitating Ca²⁺-mediated Pyk2 Tyr⁴⁰² site phosphorylation. Subsequently, Pyk2 acted as a scaffold for c-Abl phosphorylating the catalytic domain of NOX5 Tyr^476/478 sites, which in turn upregulated hydrogen peroxide (H₂O₂) inside the Pyk2/NOX5 complex to oxidize and activate local Src. These findings provide insights into the biological significance of NOX5 in the development of ESCC.

The NADPH Oxidase Family and Its Inhibitors

Significance: The oxidative stress, resulting from an imbalance in the production and scavenging of reactive oxygen species (ROS), is known to be involved in the development and progression of several pathologies. The excess of ROS production is often due to an overactivation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) and for this reason these enzymes became promising therapeutic targets. However, even if NOX are now well characterized, the development of new therapies is limited by the lack of highly isoform-specific inhibitors. Recent Advances: In the past decade, several groups and laboratories have screened thousands of molecules to identify new specific inhibitors with low off-target effects. These works have led to the characterization of several new potent NOX inhibitors; however, their specificity varies a lot depending on the molecules. Critical Issues: Here, we are reviewing more than 25 known NOX inhibitors, focusing mainly on the newly identified ones such as APX-115, NOS31, Phox-I1 and 2, GLX7013114, and GSK2795039. To have a better overall view of these molecules, the inhibitors were classified according to their specificity, from pan-NOX inhibitors to highly isoform-specific ones. We are also presenting the use of these compounds both in vitro and in vivo. Future Directions: Several of these new molecules are potent and very specific inhibitors that could be good candidates for the development of new drugs. Even if the results are very promising, most of these compounds were only validated in vitro or in mice models and further investigations will be required before using them as potential therapies.

NADPH oxidase 5 activation; a novel approach to human sperm cryoinjury

Sperm cryopreservation leads to various structural and functional damages, some of which induce by oxidative stress. The reactive oxygen species (ROS) generates by mitochondria and membrane NADPH oxidases (NOXs). Among the NOXs, only NOX5 has been identified in the cell membrane of human sperm. This study was designed to clarify the possible role of NOX5 on sperm cryoinjury. Forty human semen samples were washed and randomly divided into fresh and cryopreserved groups. Each group was divided into 4 subgroups containing Ham's F10 (control), 0.1% DMSO (vehicle), 100 nM of PMA (phorbol 12-myristate 13-acetate) and 1 µM of DPI (diphenyleneiodonium), as NOX5 activator and inhibitor. The samples of cryopreserved groups were preserved in liquid nitrogen for 1 month. The sperm kinematics, membrane integrity, ROS production, apoptosis rate, mitochondrial membrane potential (MMP), intracellular ATP and calcium concentration [Ca²⁺]_i were evaluated. The percent of sperm with intact membrane and motile sperm reduced significantly after thawing (p ≤ 0.01). The ROS production (p ≤ 0.01) and the apoptotic rate increased, MMP dissipated, and the percentage of live cells with high [Ca²⁺]_i decreased significantly in the cryopreserved control group relative to the fresh control group. DPI, in contrast to PMA, improved sperm progressive motility (p ≤ 0.01), membrane integrity in fresh and cryopreserved groups and reduced the ROS amount in cryopreserved group (p ≤ 0.01). Apoptotic rate, [Ca²⁺]_i, ATP, and MMP did not change with DPI and PMA in cryopreserved groups. We conclude that NOX5 activity in fresh sperm is low, and it increases during cryopreservation. NOX5 inhibition improves the cryopreserved sperm quality.

Sperm Antioxidant Biomarkers and Their Correlation with Clinical Condition and Lifestyle with Regard to Male Reproductive Potential

Measurement of sperm oxidative-antioxidant indicators is widely used in the assessment and detection of biochemical causes of male infertility The main purpose of this study was to identify biomarkers that assist in diagnostics and monitoring of male reproductive potential. We performed the assessment of oxidative-antioxidant malondialdehyde (MDA), glutathione (GSH), and total redox antioxidant potential (TRAP) indicators in seminal plasma, seminogram, clinical condition, and lifestyle of people with reproductive problems. The combined assessment of GSH and TRAP as potential biomarkers of male infertility in semen plasma was characterized by the highest total sensitivity and specificity. Furthermore, we provide evidence that male reproductive potential is significantly correlated with basic sperm parameters, sperm cell membrane integrity, their morphology, lifestyle, eating habits, occupation, and mental health. Our results provide evidence on the importance of oxidative stress and defense against free radicals in diagnosing and monitoring men with infertility that are consistent with previously conducted research. We provide an alternative approach on the possibility of interpreting the combination of the biomarkers that can bring benefits to a multi-threaded approach to the diagnosis and treatment of male infertility.