Effect of exogenous sperm capacitation inducers on stallion sperm

Although under appropriate laboratory conditions, sperm from different mammalian species can be capacitated in vitro, the optimal conditions for sperm capacitation in the stallion have been elusive. This study evaluated the effect of different capacitating inducers in Whitten and Tyrode media and assessed their impact on capacitation-related factors. Stallion sperm were incubated with different combinations of capacitating inducers at 38.5 °C in an air atmosphere. Sperm quality variables such as motility, mitochondrial membrane potential, and lipid peroxidation were assessed. Membrane fluidity and intracellular calcium levels were evaluated as early markers of capacitation, while tyrosine phosphorylation events and the sperm's ability to perform acrosomal exocytosis were used as late capacitation markers. Finally, these sperm were evaluated using a heterologous zona pellucida binding assay. The findings confirm that capacitating conditions evaluated increase intracellular calcium levels and membrane fluidity in both media. Similarly, including 2 or 3 inducers in both media increased tyrosine phosphorylation levels and acrosomal exocytosis after exposure to progesterone, confirming that stallion sperm incubated in these conditions shows cellular and molecular changes consistent with sperm capacitation. Furthermore, the zona pellucida binding assay confirmed the binding capacity of sperm incubated in capacitation conditions, a key step for stallion in vitro fertilization success. Further studies are needed to evaluate the effect of these conditions on in vitro fertilization in the horse.

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.

DHCR24-mediated sterol homeostasis during spermatogenesis is required for sperm mitochondrial sheath formation and impacts male fertility over time

Desmosterol and cholesterol are essential lipid components of the sperm plasma membrane. Cholesterol efflux is required for capacitation, a process through which sperm acquire fertilizing ability. In this study, using a transgenic mouse model overexpressing 24-dehydrocholesterol reductase (DHCR24), an enzyme in the sterol biosynthesis pathway responsible for the conversion of desmosterol to cholesterol, we show that disruption of sterol homeostasis during spermatogenesis led to defective sperm morphology characterized by incomplete mitochondrial packing in the midpiece, reduced sperm count and motility, and a decline in male fertility with increasing paternal age, without changes in body fat composition. Sperm depleted of desmosterol exhibit inefficiency in the acrosome reaction, metabolic dysfunction, and an inability to fertilize the egg. These findings provide molecular insights into sterol homeostasis for sperm capacitation and its impact on male fertility.

Aquaporin-7-Mediated Glycerol Permeability Is Linked to Human Sperm Motility in Asthenozoospermia and during Sperm Capacitation

Osmoregulation plays a vital role in sperm function, encompassing spermatogenesis, maturation, and fertilization. Aquaglyceroporins, a subclass of aquaporins (AQPs), facilitate the transport of water and glycerol across the sperm membrane, with glycerol serving as an important substrate for sperm bioenergetics. This study aimed to elucidate the significance of AQP-mediated glycerol permeability in sperm motility. The presence and localization of AQP3 and AQP7 in human sperm were assessed using immunofluorescence. Subsequently, the glycerol permeability of spermatozoa obtained from normozoospermic individuals (n = 30) was measured, using stopped-flow light scattering, after incubation with specific aquaporin inhibitors targeting AQP3 (DFP00173), AQP7 (Z433927330), or general aquaglyceroporin (phloretin). Sperm from asthenozoospermic men (n = 30) were utilized to evaluate the AQP7-mediated glycerol permeability, and to compare it with that of normozoospermic men. Furthermore, hypermotile capacitated sperm cells were examined, to determine the AQP7 expression and membrane glycerol permeability. AQP3 was predominantly observed in the tail region, while AQP7 was present in the head, midpiece, and tail of human sperm. Our findings indicate that AQP7 plays a key role in glycerol permeability, as the inhibition of AQP7 resulted in a 55% decrease in glycerol diffusion across the sperm membrane. Importantly, this glycerol permeability impairment was evident in spermatozoa from asthenozoospermic individuals, suggesting the dysregulation of AQP7-mediated glycerol transport, despite similar AQP7 levels. Conversely, the AQP7 expression increased in capacitated sperm, compared to non-capacitated sperm. Hence, AQP7-mediated permeability may serve as a valuable indicator of sperm motility, and be crucial in sperm function.

Proteomic Landscape of Human Sperm in Patients with Different Spermatogenic Impairments

Although the proteome of sperm has been characterized, there is still a lack of high-throughput studies on dysregulated proteins in sperm from subfertile men, with only a few studies on the sperm proteome in asthenozoospermic and oligoasthenozoospermic men. Using liquid chromatography–mass spectrometry (LC-MS/MS) along with bioinformatics analyses, we investigated the proteomic landscape of sperm collected from subfertile men (n = 22), i.e., asthenozoospermic men (n = 13), oligoasthenozoospermic men (n = 9) and normozoospermic controls (n = 31). We identified 4412 proteins in human sperm. Out of these, 1336 differentially abundant proteins were identified in 70% of the samples. In subfertile men, 32 proteins showed a lower abundance level and 34 showed a higher abundance level when compared with normozoospermic men. Compared to normozoospermic controls, 95 and 8 proteins showed a lower abundance level, and 86 and 1 proteins showed a higher abundance level in asthenozoospermic and oligoasthenozoospermic men, respectively. Sperm motility and count were negatively correlated with 13 and 35 and positively correlated with 37 and 20 differentially abundant proteins in asthenozoospermic and oligoasthenozoospermic men, respectively. The combination of the proteins APCS, APOE, and FLOT1 discriminates subfertile males from normozoospermic controls with an AUC value of 0.95. Combined APOE and FN1 proteins discriminate asthenozoospermic men form controls with an AUC of 1, and combined RUVBL1 and TFKC oligoasthenozoospermic men with an AUC of 0.93. Using a proteomic approach, we revealed the proteomic landscape of sperm collected from asthenozoospermic or oligoasthenozoospermic men. Identified abundance changes of several specific proteins are likely to impact sperm function leading to subfertility. The data also provide evidence for the usefulness of specific proteins or protein combinations to support future diagnosis of male subfertility.

Male infertility and somatic health - insights into lipid damage as a mechanistic link

Over the past decade, mounting evidence has shown an alarming association between male subfertility and poor somatic health, with substantial evidence supporting the increased incidence of oncological disease, cardiovascular disease, metabolic disorders and autoimmune diseases in men who have previously received a subfertility diagnosis. This paradigm is concerning, but might also provide a novel window for a crucial health reform in which the infertile phenotype could serve as an indication of potential pathological conditions. One of the major limiting factors in this association is the poor understanding of the molecular features that link infertility with comorbidities across the life course. Enzymes involved in the lipid oxidation process might provide novel clues to reconcile the mechanistic basis of infertility with incident pathological conditions. Building research capacity in this area is essential to enhance the early detection of disease states and provide crucial information about the disease risk of offspring conceived through assisted reproduction.

A stallion spermatozoon's journey through the mare's genital tract: In vivo and in vitro aspects of sperm capacitation

Conventional in vitro fertilization is not efficacious when working with equine gametes. Although stallion spermatozoa bind to the zona pellucida in vitro, these gametes fail to initiate the acrosome reaction in the vicinity of the oocyte and cannot, therefore, penetrate into the perivitelline space. Failure of sperm penetration most likely relates to the absence of optimized in vitro fertilization media containing molecules essential to support stallion sperm capacitation. In vivo, the female reproductive tract, especially the oviductal lumen, provides an environmental milieu that appropriately regulates interactions between the gametes and promotes fertilization. Identifying these 'fertilization supporting factors' would be a great contribution for development of equine in vitro fertilization media. In this review, a description of the current understanding of the interactions stallion spermatozoa undergo during passage through the female genital tract, and related specific molecular changes that occur at the sperm plasma membrane is provided. Understanding these molecular changes may hold essential clues to achieving successful in vitro fertilization with equine gametes.

The Stallion Spermatozoa: A Valuable Model to Help Understand the Interplay Between Metabolism and Redox (De)regulation in Sperm Cells

Significance: Proper functionality of the spermatozoa depends on the tight regulation of their redox status; at the same time these cells are highly energy demanding and in the energetic metabolism, principally in the electron transport chain in the mitochondria, reactive oxygen species are continuously produced, in addition to that observed in the Krebs cycle and during the β-oxidation of fatty acids. Recent Advances: In addition, in glycolysis, elimination of phosphate groups from glyceraldehyde 3-phosphate and dihydroxyacetone phosphate results in the byproducts glyoxal (G) and methylglyoxal (MG); these products are 2-oxoaldehydes. The presence of adjacent carbonyl groups makes them strong electrophiles that react with nucleophiles in proteins, lipids, and DNA, forming advanced glycation end products. Critical Issues: This mechanism is behind subfertility in diabetic patients; in the animal breeding industry, commercial extenders for stallion semen contain a supraphysiological concentration of glucose that promotes MG production, constituting a potential model of interest. Future Directions: Increasing our knowledge of sperm metabolism and its interactions with redox regulation may improve current sperm technologies in use, and shall provide new clues to understanding infertility in males. Moreover, stallion spermatozoa due to its accessibility, intense metabolism, and suitability for proteomics/metabolomic studies may constitute a suitable model for studying regulation of metabolism and interactions between metabolism and redox homeostasis. Antioxid. Redox Signal. 37, 521-537.

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.

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.

Measuring Reactive Oxygen Species in Semen for Male Preconception Care: A Scientist Perspective

Oxidative stress and elevated levels of seminal and sperm reactive oxygen species (ROS) may contribute to up to 80% of male infertility diagnosis, with sperm ROS concentrations at fertilization important in the development of a healthy fetus and child. The evaluation of ROS in semen seems promising as a potential diagnostic tool for male infertility and male preconception care with a number of clinically available tests on the market (MiOXSYS, luminol chemiluminescence and OxiSperm). While some of these tests show promise for clinical use, discrepancies in documented decision limits and lack of cohort studies/clinical trials assessing their benefits on fertilization rates, embryo development, pregnancy and live birth rates limit their current clinical utility. In this review, we provide an update on the current techniques used for analyzing semen ROS concentrations clinically, the potential to use of ROS research tools for improving clinical ROS detection in sperm and describe why we believe we are likely still a long way away before semen ROS concentrations might become a mainstream preconception diagnostic test in men.

Bicarbonate-Stimulated Membrane Reorganization in Stallion Spermatozoa

Classical in vitro fertilization (IVF) is still poorly successful in horses. This lack of success is thought to be due primarily to inadequate capacitation of stallion spermatozoa under in vitro conditions. In species in which IVF is successful, bicarbonate, calcium, and albumin are considered the key components that enable a gradual reorganization of the sperm plasma membrane that allows the spermatozoa to undergo an acrosome reaction and fertilize the oocyte. The aim of this work was to comprehensively examine contributors to stallion sperm capacitation by investigating bicarbonate-induced membrane remodelling steps, and elucidating the contribution of cAMP signalling to these events. In the presence of capacitating media containing bicarbonate, a significant increase in plasma membrane fluidity was readily detected using merocyanine 540 staining in the majority of viable spermatozoa within 15 min of bicarbonate exposure. Specific inhibition of soluble adenylyl cyclase (sAC) in the presence of bicarbonate by LRE1 significantly reduced the number of viable sperm with high membrane fluidity. This suggests a vital role for sAC-mediated cAMP production in the regulation of membrane fluidity. Cryo-electron tomography of viable cells with high membrane fluidity revealed a range of membrane remodelling intermediates, including destabilized membranes and zones with close apposition of the plasma membrane and the outer acrosomal membrane. However, lipidomic analysis of equivalent viable spermatozoa with high membrane fluidity demonstrated that this phenomenon was neither accompanied by a gross change in the phospholipid composition of stallion sperm membranes nor detectable sterol efflux (p > 0.05). After an early increase in membrane fluidity, a significant and cAMP-dependent increase in viable sperm with phosphatidylserine (PS), but not phosphatidylethanolamine (PE) exposure was noted. While the events observed partly resemble findings from the in vitro capacitation of sperm from other mammalian species, the lack of cholesterol removal appears to be an equine-specific phenomenon. This research will assist in the development of a defined medium for the capacitation of stallion sperm and will facilitate progress toward a functional IVF protocol for horse gametes.

Parkinson Disease Protein 7 (PARK7) Is Related to the Ability of Mammalian Sperm to Undergo In Vitro Capacitation

Parkinson disease protein 7 (PARK7) is a multifunctional protein known to be involved in the regulation of sperm motility, mitochondrial function, and oxidative stress response in mammalian sperm. While ROS generation is needed to activate the downstream signaling pathways required for sperm to undergo capacitation, oxidative stress has detrimental effects for sperm cells and a precise balance between ROS levels and antioxidant activity is needed. Considering the putative antioxidant role of PARK7, the present work sought to determine whether this protein is related to the sperm ability to withstand in vitro capacitation. To this end, and using the pig as a model, semen samples were incubated in capacitation medium for 300 min; the acrosomal exocytosis was triggered by the addition of progesterone after 240 min of incubation. At each relevant time point (0, 120, 240, 250, and 300 min), sperm motility, acrosome and plasma membrane integrity, membrane lipid disorder, mitochondrial membrane potential, intracellular calcium and ROS were evaluated. In addition, localization and protein levels of PARK7 were also assessed through immunofluorescence and immunoblotting. Based on the relative content of PARK7, two groups of samples were set. As early as 120 min of incubation, sperm samples with larger PARK7 content showed higher percentages of viable and acrosome-intact sperm, lipid disorder and superoxide levels, and lower intracellular calcium levels when compared to sperm samples with lower PARK7. These data suggest that PARK7 could play a role in preventing sperm from undergoing premature capacitation, maintaining sperm viability and providing a better ability to keep ROS homeostasis, which is needed to elicit sperm capacitation. Further studies are required to elucidate the antioxidant properties of PARK7 during in vitro capacitation and acrosomal exocytosis of mammalian sperm, and the relationship between PARK7 and sperm motility.

Flow-cytometric analysis of membrane integrity of stallion sperm in the face of agglutination: the "zombie sperm" dilemma

PURPOSE

To define the effect of sperm agglutination, associated with incubation under capacitating conditions, on accuracy of membrane assessment via flow cytometry and to develop methods to mitigate that effect.

METHODS

Sperm motility was measured by CASA. Sperm were stained with PI-PSA or a novel method, LD-PSA, using fixable live/dead stain and cell dissociation treatment, before flow-cytometric analysis. Using LD-PSA, acrosome reaction and plasma membrane status were determined in equine sperm treated with 10 μm A23187 for 10 min, followed by 0, 1, or 2 h incubation in capacitating conditions.

RESULTS

Using PI-PSA, measured membrane integrity (MI; live sperm) was dramatically lower than was total motility (TMOT), indicating spurious results ("zombie sperm"). Sperm aggregates were largely of motile sperm. Loss of motility after A23187 treatment was associated with disaggregation and increased MI. On disaggregation using LD-PSA, MI rose, and MI then corresponded with TMOT. In equine sperm incubated after A23187 treatment, as the percentage of live acrosome-reacted sperm increased, TMOT decreased to near 0.

CONCLUSION

Flow cytometry assesses only individualized sperm; thus, agglutination of viable sperm alters recorded membrane integrity. As viable sperm become immotile, they individualize; therefore, factors that decrease motility, such as A23187, result in increased measured MI. Disaggregation before assessment allows more accurate determination of sperm membrane status; in this case we documented a mismatch between motility and live acrosome-reacted equine sperm that may relate to the poor repeatability of A23187 treatment for equine IVF. These findings are of profound value to future studies on sperm capacitation.

Graphene Oxide Improves in vitro Fertilization in Mice With No Impact on Embryo Development and Preserves the Membrane Microdomains Architecture

During the latest years, human infertility worsened all over the world and is nowadays reputed as a global public health issue. As a consequence, the adoption of Assisted Reproductive Technologies (ARTs) such as In Vitro Fertilization (IVF) is undergoing an impressive increase. In this context, one of the most promising strategies is the innovative adoption of extra-physiological materials for advanced sperm preparation methods. Here, by using a murine model, the addition of Graphene Oxide (GO) at a specific concentration has demonstrated to increase the spermatozoa fertilizing ability in an IVF assay, finding that 0.5 μg/ml GO addition to sperm suspensions before IVF is able to increase both the number of fertilized oocytes and embryos created with a healthy offspring given by Embryo Transplantation (ET). In addition, GO treatment has been found more effective than that carried out with methyl-β-cyclodextrin, which represents the gold standard in promoting in vitro fertility of mice spermatozoa. Subsequent biochemical characterization of its interaction with male gametes has been additionally performed. As a result, it was found that GO exerts its positive effect by extracting cholesterol from membranes, without affecting the integrity of microdomains and thus preserving the sperm functions. In conclusion, GO improves IVF outcomes in vitro and in vivo, defining new perspectives for innovative strategies in the treatment of human infertility.

Male Infertility: Shining a Light on Lipids and Lipid-Modulating Enzymes in the Male Germline

Despite the prevalence of male factor infertility, most cases are defined as idiopathic, thus limiting treatment options and driving increased rates of recourse to assisted reproductive technologies (ARTs). Regrettably, our current armory of ARTs does not constitute therapeutic treatments for male infertility, thus highlighting an urgent need for novel intervention strategies. In our attempts to fill this void, we have come to appreciate that the production of pathological levels of oxygen radicals within the male germline are a defining etiology of many idiopathic infertility cases. Indeed, an imbalance of reactive oxygen species can precipitate a cascade of deleterious sequelae, beginning with the peroxidation of membrane lipids and culminating in cellular dysfunction and death. Here, we shine light on the importance of lipid homeostasis, and the impact of lipid stress in the demise of the male germ cell. We also seek to highlight the utility of emerging lipidomic technologies to enhance our understanding of the diverse roles that lipids play in sperm function, and to identify biomarkers capable of tracking infertility in patient cohorts. Such information should improve our fundamental understanding of the mechanistic causes of male infertility and find application in the development of efficacious treatment options.

pH-dependent effects of procaine on equine gamete activation†

Procaine directly triggers pH-dependent cytokinesis in equine oocytes and induces hypermotility in stallion spermatozoa, an important event during capacitation. However, procaine-induced hyperactivated motility is abolished when sperm is washed to remove the procaine prior to sperm-oocyte co-incubation. To understand how procaine exerts its effects, the external Ca2+ and Na+ and weak base activity dependency of procaine-induced hyperactivation in stallion spermatozoa was assessed using computer-assisted sperm analysis. Percoll-washed stallion spermatozoa exposed to Ca2+-depleted (+2 mM EGTA) procaine-supplemented capacitating medium (CM) still demonstrated hyperactivated motility, whereas CM without NaCl or Na+ did not. Both procaine and NH4Cl, another weak base, were shown to trigger a cytoplasmic pH increase (BCECF-acetoxymethyl (AM)), which is primarily induced by a pH rise in acidic cell organelles (Lysosensor green dnd-189), accompanied by hypermotility in stallion sperm. As for procaine, 25 mM NH4Cl also induced oocyte cytokinesis. Interestingly, hyperactivated motility was reliably induced by 2.5-10 mM procaine, whereas a significant cytoplasmic cAMP increase and tail-associated protein tyrosine phosphorylation were only observed at 10 mM. Moreover, 25 mM NH4Cl did not support the latter capacitation characteristics. Additionally, cAMP levels were more than 10× higher in boar than stallion sperm incubated under similar capacitating conditions. Finally, stallion sperm preincubated with 10 mM procaine did not fertilize equine oocytes. In conclusion, 10 mM procaine causes a cytoplasmic and acidic sperm cell organelle pH rise that simultaneously induces hyperactivated motility, increased levels of cAMP and tail-associated protein tyrosine phosphorylation in stallion spermatozoa. However, procaine-induced hypermotility is independent of the cAMP/protein tyrosine phosphorylation pathway.

Redox Regulation and Oxidative Stress: The Particular Case of the Stallion Spermatozoa

Redox regulation and oxidative stress have become areas of major interest in spermatology. Alteration of redox homeostasis is recognized as a significant cause of male factor infertility and is behind the damage that spermatozoa experience after freezing and thawing or conservation in a liquid state. While for a long time, oxidative stress was just considered an overproduction of reactive oxygen species, nowadays it is considered as a consequence of redox deregulation. Many essential aspects of spermatozoa functionality are redox regulated, with reversible oxidation of thiols in cysteine residues of key proteins acting as an “on–off” switch controlling sperm function. However, if deregulation occurs, these residues may experience irreversible oxidation and oxidative stress, leading to malfunction and ultimately death of the spermatozoa. Stallion spermatozoa are “professional producers” of reactive oxygen species due to their intense mitochondrial activity, and thus sophisticated systems to control redox homeostasis are also characteristic of the spermatozoa in the horse. As a result, and combined with the fact that embryos can easily be collected in this species, horses are a good model for the study of redox biology in the spermatozoa and its impact on the embryo.

Impairment of sperm maturation and capacitation due to diet-dependent cholesterol overload

BACKGROUND

Lipid metabolic disorders (dyslipidemia) are constantly increasing in occidental societies and lead to the development of pathologies such as obesity, diabetes, and metabolic syndrome. It has been demonstrated that dyslipidemia can alter the reproductive function. Animal models have recently been used to show that the offspring of dyslipidemic males could also develop such pathologies and that the transgenerational transmission involved post-testicular sperm maturation. These data targeted the essential role of male gamete epididymal maturation and its importance for the health of the offspring.

OBJECTIVES

This publication summarizes in the first place experimental data obtained using a mouse model of dyslipidemia-induced post-testicular infertility, knockout mice for the two isoforms of the 'Liver X Receptors' (Lxrα;β^-/- ), the major regulators of cholesterol homeostasis. The impact of a high cholesterol diet (HCD) on the protein YWHAZ (14-3-3 ζ or tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein Zeta) was also investigated in our model.

MATERIALS AND METHODS

In our mouse model, when young fertile Lxrα;β^-/- males aged three months were fed four weeks with a HCD, they developed an epididymal phenotype leading to infertility. The level of sperm YWHAZ was evaluated by Western blot and its tyrosine phosphorylation state by immunoprecipitation followed by Western blot.

RESULTS

Our data revealed that sperm lipid composition and structure were altered, leading to defects of the capacitation-associated signaling pathway. They also showed that both the level and the tyrosine phosphorylation state of YWHAZ were affected by the HCD in sperm cells from Lxrα;β^-/- males.

DISCUSSION AND CONCLUSION

YWHAZ could be a new important regulator of capacitation-associated tyrosine phosphorylation and a marker of dyslipidemia-induced infertility.

Graphene Oxide increases mammalian spermatozoa fertilizing ability by extracting cholesterol from their membranes and promoting capacitation

Graphene Oxide (GO) is a widely used biomaterial with an amazing variety of applications in biology and medicine. Recently, we reported the ability of GO to improve the in vitro fertilization (IVF) outcomes in swine, a validated animal model with a high predictive value for human fertility. For that reason, here we characterized the mechanisms involved in this positive interaction by adopting an experimental approach combining biological methods (confocal microscopy analysis on single cell, flow cytometry on cell populations and co-incubation with epithelial oviductal cells), physical-chemical techniques (Differential Scanning Calorimetry and Thermogravimetric Analysis), and chemical methods (mass spectrometry and lipid measurement). As a result, we propose a model in which GO is able to extract cholesterol from the spermatozoa membrane without causing any detrimental effect. In this way, the cholesterol extraction promotes a change in membrane chemical-physical properties that could positively affect male gamete function, modulating sperm signalling function and increasing in this way the fertilizing potential, without losing the ability to physiologically interact with the female environment. In conclusion, these data seem to suggest new intriguing possibilities in engineering sperm membrane for improving assisted reproduction technologies outcomes, even in human medicine.

Oviductal extracellular vesicles (oviductosomes, OVS) are conserved in humans: murine OVS play a pivotal role in sperm capacitation and fertility

STUDY QUESTIONS

Are extracellular vesicles (EVs) in the murine oviduct (oviductosomes, OVS) conserved in humans and do they play a role in the fertility of Pmca4-/- females?

SUMMARY ANSWER

OVS and their fertility-modulating proteins are conserved in humans, arise via the apocrine pathway, and mediate a compensatory upregulation of PMCA1 (plasma membrane Ca2+-ATPase 1) in Pmca4-/- female mice during proestrus/estrus, to account for their fertility.

WHAT IS KNOWN ALREADY

Recently murine OVS were identified and shown during proestrus/estrus to express elevated levels of PMCA4 which they can deliver to sperm. PMCA4 is the major Ca2+ efflux pump in murine sperm and Pmca4 deletion leads to loss of sperm motility and male infertility as there is no compensatory upregulation of the remaining Ca2+ pump, PMCA1. Of the four family members of PMCAs (PMCA1-4), PMCA1 and PMCA4 are ubiquitous, and to date there have been no reports of one isoform being upregulated to compensate for another in any organ/tissue. Since Pmca4-/- females are fertile, despite the abundant expression of PMCA4 in wild-type (WT) OVS, we propose that OVS serve a role of packaging and delivering to sperm elevated levels of PMCA1 in Pmca4-/- during proestrus/estrus to compensate for PMCA4's absence.

STUDY DESIGN, SIZE, DURATION

Fallopian tubes from pre-menopausal women undergoing hysterectomy were used to study EVs in the luminal fluid. Oviducts from sexually mature WT mice were sectioned after perfusion fixation to detect EVs in situ. Oviducts were recovered from WT and Pmca4-/- after hormonally induced estrus and sectioned for PMCA1 immunofluorescence (IF) (detected with confocal microscopy) and hematoxylin and eosin staining. Reproductive tissues, luminal fluids and EVs were recovered after induced estrus and after natural cycling for western blot analysis of PMCA1 and qRT-PCR of Pmca1 to compare expression levels in WT and Pmca4-/-. OVS, uterosomes, and epididymal luminal fluid were included in the comparisons. WT and Pmca4-/- OVS were analyzed for the presence of known PMCA4 partners in sperm and their ability to interact with PMCA1, via co-immunoprecipitation. In vitro uptake of PMCA1 from OVS was analyzed in capacitated and uncapacitated sperm via quantitative western blot analysis, IF localization and flow cytometry. Caudal sperm were also assayed for uptake of tyrosine-phosphorylated proteins which were shown to be present in OVS. Finally, PMCA1 and PMCA4 in OVS and that delivered to sperm were assayed for enzymatic activity.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human fallopian tubes were flushed to recover luminal fluid which was processed for OVS via ultracentrifugation. Human OVS were negatively stained for transmission electron microscopy (TEM) and subjected to immunogold labeling, to detect PMCA4. Western analysis was used to detect HSC70 (an EV biomarker), PMCA1 and endothelial nitric oxide synthase (eNOS) which is a fertility-modulating protein delivered to human sperm by prostasomes. Oviducts of sexually mature female mice were sectioned after perfusion fixation for TEM tomography to obtain 3D information and to distinguish cross-sections of EVs from those of microvilli and cilia. Murine tissues, luminal fluids and EVs were assayed for PMCA1 (IF and western blot) or qRT-PCR. PMCA1 levels from western blots were quantified, using band densities and compared in WT and Pmca4-/- after induced estrus and in proestrus/estrus and metestrus/diestrus in cycling females. In vitro uptake of PMCA1 and tyrosine-phosphorylated proteins was quantified with flow cytometry and/or quantitative western blot. Ca2+-ATPase activity in OVS and sperm before and after PMCA1 and PMCA4 uptake was assayed, via the enzymatic hydrolysis rate of ATP.

MAIN RESULTS AND THE ROLE OF CHANCE

TEM revealed that human oviducts contain EVs (exosomal and microvesicular). These EVs contain PMCA4 (immunolabeling), eNOS and PMCA1 (western blot) in their cargo. TEM tomography showed the murine oviduct with EV-containing blebs which typify the apocrine pathway for EV biogenesis. Western blots revealed that during proestrus/estrus PMCA1 was significantly elevated in the oviductal luminal fluid (OLF) (P = 0.02) and in OVS (P = 0.03) of Pmca4-/-, compared to WT. Further, while PMCA1 levels did not fluctuate in OLF during the cycle in WT, they were significantly (P = 0.02) higher in proestrus/estrus than at metestrus/diestrus in Pmca4-/-. The elevated levels of PMCA1 in proestrus/estrus, which mimics PMCA4 in WT, is OLF/OVS-specific, and is not seen in oviductal tissues, uterosomes or epididymal luminal fluid of Pmca4-/-. However, qRT-PCR revealed significantly elevated levels of Pmca1 transcript in Pmca4-/- oviductal tissues, compared to WT. PMCA1 could be transferred from OVS to sperm and the levels were significantly higher for capacitated vs uncapacitated sperm, as assessed by flow cytometry (P = 0.001) after 3 h co-incubation, quantitative western blot (P < 0.05) and the frequency of immuno-labeled sperm (P < 0.001) after 30 min co-incubation. Tyrosine phosphorylated proteins were discovered in murine OVS and could be delivered to sperm after their co-incubation with OVS, as detected by western, immunofluorescence localization, and flow cytometry. PMCA1 and PMCA4 in OVS were shown to be enzymatically active and this activity increased in sperm after OVS interaction.

LARGE SCALE DATA

None.

LIMITATIONS REASONS FOR CAUTION

Although oviductal tissues of WT and Pmca4-/- showed no significant difference in PMCA1 levels, Pmca4-/- levels of OVS/OLF during proestrus/estrus were significantly higher than in WT. We have attributed this enrichment or upregulation of PMCA1 in Pmca4-/- partly to selective packaging in OVS to compensate for the lack of PMCA4. However, in the absence of a difference between WT and Pmca4-/- in the PMCA1 levels in oviductal tissues as a whole, we cannot rule out significantly higher PMCA1 expression in the oviductal epithelium that gives rise to the OVS as significantly higher Pmca1 transcripts were detected in Pmca4-/-.

WIDER IMPLICATIONS OF THE FINDINGS

Since OVS and fertility-modulating cargo components are conserved in humans, it suggests that murine OVS role in regulating the expression of proteins required for capacitation and fertility is also conserved. Secondly, OVS may explain some of the differences in in vivo and in vitro fertilization for mouse mutants, as seen in mice lacking the gene for FER which is the enzyme required for sperm protein tyrosine phosphorylation. Our observation that murine OVS carry and can modulate sperm protein tyrosine phosphorylation by delivering them to sperm provides an explanation for the in vivo fertility of Fer mutants, not seen in vitro. Finally, our findings have implications for infertility treatment and exosome therapeutics.

STUDY FUNDING AND COMPETING INTEREST(S)

The work was supported by National Institute of Health (RO3HD073523 and 5P20RR015588) grants to P.A.M.-D. There are no conflicts of interests.

Molecular Basis of Human Sperm Capacitation

In the early 1950s, Austin and Chang independently described the changes that are required for the sperm to fertilize oocytes in vivo. These changes were originally grouped under name of “capacitation” and were the first step in the development of in vitro fertilization (IVF) in humans. Following these initial and fundamental findings, a remarkable number of observations led to characterization of the molecular steps behind this process. The discovery of certain sperm-specific molecules and the possibility to record ion currents through patch-clamp approaches helped to integrate the initial biochemical observation with the activity of ion channels. This is of particular importance in the male gamete due to the fact that sperm are transcriptionally inactive. Therefore, sperm must control all these changes that occur during their transit through the male and female reproductive tracts by complex signaling cascades that include post-translational modifications. This review is focused on the principal molecular mechanisms that govern human sperm capacitation with particular emphasis on comparing all the reported pieces of evidence with the mouse model.

Factors and pathways involved in capacitation: how are they regulated?

In mammals, fertilization occurs via a comprehensive progression of events. Freshly ejaculated sperm have yet to acquire progressive motility or fertilization ability. They must first undergo a series of biochemical and physiological changes, collectively known as capacitation. Capacitation is a significant prerequisite to fertilization. During the process of capacitation, changes in membrane properties, intracellular ion concentration and the activities of enzymes, together with other protein modifications, induce multiple signaling events and pathways in defined media in vitro or in the female reproductive tract in vivo. These, in turn, stimulate the acrosome reaction and prepare spermatozoa for penetration of the egg zona pellucida prior to fertilization. In the present review, we conclude all mainstream factors and pathways regulate capacitation and highlight their crosstalk. We also summarize the relationship between capacitation and assisted reproductive technology or human disease. In the end, we sum up the open questions and future avenues in this field.

Cholesterol: A Gatekeeper of Male Fertility?

Cholesterol is essential for mammalian cell functions and integrity. It is an important structural component maintaining the permeability and fluidity of the cell membrane. The balance between synthesis and catabolism of cholesterol should be tightly regulated to ensure normal cellular processes. Male reproductive function has been demonstrated to be dependent on cholesterol homeostasis. Here we review data highlighting the impacts of cholesterol homeostasis on male fertility and the molecular mechanisms implicated through the signaling pathways of some nuclear receptors.

Aminopurvalanol A, a Potent, Selective, and Cell Permeable Inhibitor of Cyclins/Cdk Complexes, Causes the Reduction of in Vitro Fertilizing Ability of Boar Spermatozoa, by Negatively Affecting the Capacitation-Dependent Actin Polymerization

The adoption of high-througput technologies demonstrated that in mature spermatozoa are present proteins that are thought to be not present or active in sperm cells, such as those involved in control of cell cycle. Here, by using an in silico approach based on the application of networks theory, we found that Cyclins/Cdk complexes could play a central role in signal transduction active during capacitation. Then, we tested this hypothesis in the vitro model. With this approach, spermatozoa were incubated under capacitating conditions in control conditions (CTRL) or in the presence of Aminopurvalanol A a potent, selective and cell permeable inhibitor of Cyclins/Cdk complexes at different concentrations (2, 10, and 20 μM). We found that this treatment caused dose-dependent inhibition of sperm fertilizing ability. We attribute this event to the loss of acrosome integrity due to the inhibition of physiological capacitation-dependent actin polymerization, rather than to a detrimental effect on membrane lipid remodeling or on other signaling pathways such as tubulin reorganization or MAPKs activation. In our opinion, these data could revamp the knowledge on biochemistry of sperm capacitation and could suggest new perspectives in studying male infertility.

Dyslipidemia alters sperm maturation and capacitation in LXR-null mice

Lipid metabolism disorders (dyslipidemia) are causes of male infertility, but little is known about their impact on male gametes when considering post-testicular maturation events, given that studies concentrate most often on endocrine dysfunctions and testicular consequences. In this study, three-month-old wild-type (wt) and Liver-X-Receptors knock out (Lxrα;β−/−) males were fed four weeks with a control or a lipid-enriched diet containing 1.25% cholesterol (high cholesterol diet (HCD)). The HCD triggered a dyslipidemia leading to sperm post-testicular alterations and infertility. Sperm lipids were analyzed by LC–MS and those fromLxrα;β−/−males fed the HCD showed higher chol/PL and PC/PE ratios compared towt-HCD (P < 0.05) and lower oxysterol contents compared to wt (P < 0.05) orLxrα;β−/−(P < 0.05). These modifications impaired membrane-associated events triggering the tyrosine phosphorylation normally occurring during the capacitation process, as shown by phosphotyrosine Western blots. Using flow cytometry, we showed that a smaller subpopulation of spermatozoa fromLxrα;β−/−-HCD males could raise their membrane fluidity during capacitation (P < 0.05 vswtorwt-HCD) as well as their intracellular calcium concentration (P < 0.05 vsLxrα;β−/−andP < 0.001 vswt). The accumulation of the major sperm calcium efflux pump (PMCA4) was decreased inLxrα;β−/−males fed the HCD (P < 0.05 vsLxrα;β−/−andP < 0.001 vswt). This study is the first showing an impact of dyslipidemia on post-testicular sperm maturation with consequences on the capacitation signaling cascade. It may lead to the identification of fertility prognostic markers in this pathophysiological situation, which could help clinicians to better understand male infertilities which are thus far classified as idiopathic.

Reactive oxygen species as mediators of sperm capacitation and pathological damage

Oxidative stress plays a major role in the life and death of mammalian spermatozoa. These gametes are professional generators of reactive oxygen species (ROS), which appear to derive from three potential sources: sperm mitochondria, cytosolic L-amino acid oxidases, and plasma membrane Nicotinamide adenine dinucleotide phosphate oxidases. The oxidative stress created via these sources appears to play a significant role in driving the physiological changes associated with sperm capacitation through the stimulation of a cyclic adenosine monophosphate/Protein kinase A phosphorylation cascade, including the activation of Extracellular signal regulated kinase-like proteins, massive up-regulation of tyrosine phosphorylation in the sperm tail, as well as the induction of sterol oxidation. When generated in excess, however, ROS can induce lipid peroxidation that, in turn, disrupts membrane characteristics that are critical for the maintenance of sperm function, including the capacity to fertilize an egg. Furthermore, the lipid aldehydes generated as a consequence of lipid peroxidation bind to proteins in the mitochondrial electron transport chain, triggering yet more ROS generation in a self-perpetuating cycle. The high levels of oxidative stress created as a result of this process ultimately damage the DNA in the sperm nucleus; indeed, DNA damage in the male germ line appears to be predominantly induced oxidatively, reflecting the vulnerability of these cells to such stress. Extensive evaluation of antioxidants that protect the spermatozoa against oxidative stress while permitting the normal reduction-oxidation regulation of sperm capacitation is therefore currently being undertaken, and has already proven efficacious in animal models.

Reproductive tract modifications of the boar sperm surface

The sperm cell has a unique, polarized, and segregated surface that is modified extensively by the changing environments in both the male and the female reproductive tracts. The sperm cannot refresh its surface, as protein translation and membrane recycling by intracellular vesicular transport have ceased upon its maturation. So, how is the sperm surface modified in the reproductive tracts and how do these processes affect fertilization? This review traces these modifications as boar sperm travels from their liberation from the Sertoli cell into the lumen of seminiferous tubules of the testis to the site of fertilization in the ampulla of the oviduct in the sow, via an artificial insemination route. The effect of sperm dilution for artificial insemination, as well as more extensive sperm processing for in vitro fertilization, cryopreservation, or sex sorting, are also discussed with respect to how these procedures affect sperm surface organization and fertilization capacity.

Capacitation-Related Lipid Remodeling of Mammalian Spermatozoa Membrane Determines the Final Fate of Male Gametes: A Computational Biology Study

To become fully fertile, mammalian spermatozoa must undergo a complex process of biochemical maturation within the female genital tract, which determines a marked lipid remodeling (LR) of membranes. Here, we represent this process as a biological network, which is a graph constituted by nodes (the molecules involved in LR) and by edges (their interactions). As a result, we found that LR network has a scale-free and small world topology. This implies that it is robust against random damage and that it allows a fast and specific transmission of information. In addition, the hubs in the network allow identification of the control mechanisms involved in membrane-related signaling, which could concur in determining the fate of ejaculated spermatozoa. Interestingly, different pathways involved in LR (maintenance of functional incompetence, reaching of fertilizing ability, apoptosis) are overlapped and some molecules take part in different signalling cascades; thus their role in sperm biology needs to be interpreted in a more large context. In addition, it was possible to differentiate, either based on their topological and biological characteristics, the molecules acting as global or local controller in LR. These findings may contribute to the understanding of capacitation-related signaling and of sperm physiopathology.

Are sperm capacitation and apoptosis the opposite ends of a continuum driven by oxidative stress?

This chapter explores the possibility that capacitation and apoptosis are linked processes joined by their common dependence on the continued generation of reactive oxygen species (ROS). According to this model capacitation is initiated in spematozoa following their release into the female reproductive tract as a consequence of intracellular ROS generation, which stimulates intracellular cAMP generation, inhibits tyrosine phosphatase activity and enhances the formation of oxysterols prior to their removal from the sperm surface by albumin. The continued generation of ROS by capacitating populations of spermatozoa eventually overwhelms the limited capacity of these cells to protect themselves from oxidative stress. As a result the over-capacitation of spermatozoa leads to a state of senescence and the activation of a truncated intrinsic apoptotic cascade characterized by enhanced mitochondrial ROS generation, lipid peroxidation, motility loss, caspase activation and phosphatidylserine externalization. The latter may be particularly important in instructing phagocytic leukocytes that the removal of senescent, moribund spermatozoa should be a silent process unaccompanied by the generation of proinflammatory cytokines. These observations reveal the central role played by redox chemistry in defining the life and death of spermatozoa. A knowledge of these mechanisms may help us to engineer novel solutions to both support and preserve the functionality of these highly specialized cells.

New insights into the regulation of cholesterol efflux from the sperm membrane

Cholesterol is an essential component of the mammalian plasma membrane because it promotes membrane stability without comprising membrane fluidity. Given this important cellular role, cholesterol levels are tightly controlled at multiple levels. It has been clearly shown that cholesterol redistribution and depletion from the sperm membrane is a key part of the spermatozoon's preparation for fertilization. Some factors that regulate these events are described (e.g., bicarbonate, calcium) but the mechanisms underlying cholesterol export are poorly understood. How does a hydrophobic cholesterol molecule inserted in the sperm plasma membrane enter the energetically unfavorable aqueous surroundings? This review will provide an overview of knowledge in this area and highlight our gaps in understanding. The overall aim is to better understand cholesterol redistribution in the sperm plasma membrane, its relation to the possible activation of a cholesterol transporter and the role of cholesterol acceptors. Armed with such knowledge, sperm handling techniques can be adapted to better prepare spermatozoa for in vitro and in vivo fertilization.

Ultrastructure and lipid composition of detergent-resistant membranes derived from mammalian sperm and two types of epithelial cells

Lipid rafts are micro-domains of ordered lipids (L_o phase) in biological membranes. The L_o phase of cellular membranes can be isolated from disordered lipids (L_d phase) after treatment with 1 % Triton  X-100 at 4 °C in which the L_o phase forms the detergent-resistant membrane (DRM) fraction. The lipid composition of DRM derived from Madin-Darby canine kidney (MDCK) cells, McArdle cells and porcine sperm is compared with that of the whole cell. Remarkably, the unsaturation and chain length degree of aliphatic chains attached to phospholipids is virtually the same between DRM and whole cells. Cholesterol and sphingomyelin were enriched in DRMs but to a cell-specific molar ratio. Sulfatides (sphingolipids from MDCK cells) were enriched in the DRM while a seminolipid (an alkylacylglycerolipid from sperm) was depleted from the DRM. Treatment with <5 mM methyl-ß-cyclodextrin (MBCD) caused cholesterol removal from the DRM without affecting the composition and amount of the phospholipid while higher levels disrupted the DRM. The substantial amount of (poly)unsaturated phospholipids in DRMs as well as a low stoichiometric amount of cholesterol suggest that lipid rafts in biological membranes are more fluid and dynamic than previously anticipated. Using negative staining, ultrastructural features of DRM were monitored and in all three cell types the DRMs appeared as multi-lamellar vesicular structures with a similar morphology. The detergent resistance is a result of protein–cholesterol and sphingolipid interactions allowing a relatively passive attraction of phospholipids to maintain the L_o phase. For this special issue, the relevance of our findings is discussed in a sperm physiological context.

The impact of oxidative stress on chaperone-mediated human sperm-egg interaction

STUDY QUESTION

How does oxidative stress impact upon human sperm-egg interaction and in particular the formation of zona pellucida-receptor complexes on the sperm surface?

SUMMARY ANSWER

Oxidative stress during human sperm capacitation resulted in the chemical alkylation of the molecular chaperone heat shock protein A2 (HSPA2), a concomitant reduction in surface expression of the zona pellucida-receptor arylsulphatase A (ARSA) and a severe loss of zona pellucida binding ability.

WHAT IS KNOWN ALREADY

An inability to bind to the zona pellucida is commonly encountered in the defective spermatozoa generated by male infertility patients; however, the underlying mechanisms remain unresolved. Recent studies have revealed that zona pellucida binding is mediated by molecular chaperones, particularly HSPA2, that facilitate the formation of multimeric zona pellucida-receptor complexes on the surface of mammalian spermatozoa during capacitation.

STUDY DESIGN, SIZE, DURATION

Spermatozoa were collected from healthy normozoospermic donors (n = 15). Low levels of oxidative stress were induced in populations of non-capacitated spermatozoa by a 1 h treatment with 4-hydroxynonenal (4HNE) or hydrogen peroxide (H2O2) and then these insults were removed and cells were capacitated for 3 h.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Motility, membrane fluidity, protein tyrosine phosphorylation and lipid raft distribution were evaluated after sperm capacitation to determine the impact of oxidative stress on this process. The surface expression of ARSA and sperm adhesion molecule 1 (SPAM1) was observed using fluorescence microscopy, and the ability of treated cells to interact with homologous human zonae pellucidae was assessed through gamete co-incubation. Proximity ligation was used to evaluate the state of the HSPA2-laden zona pellucida-receptor complex and an immunoprecipitation approach was taken to establish the chemical alkylation of HSPA2 by the cytotoxic lipid aldehyde 4HNE. The validity of these findings was then tested through treatment of oxidatively stressed cells with the nucleophile penicillamine in order to scavenge lipid aldehydes and limit their ability to interact with HSPA2. All experiments were performed on samples pooled from two or more donors per replicate, with a minimum of three replicates.

MAIN RESULTS AND THE ROLE OF CHANCE

The oxidative treatments employed in this study did not influence sperm motility or capacitation-associated changes in membrane fluidity, tyrosine phosphorylation and lipid raft redistribution. However, they did significantly impair zona pellucida binding compared with the capacitated control (P < 0.01). The reduction in zona pellucida binding was associated with the impaired surface expression (P < 0.02) of a zona pellucida-receptor complex comprising HSPA2, SPAM1 and ARSA. Proximity ligation and immunoprecipitation assays demonstrated that impaired zona pellucida binding was, in turn, associated with the chemical alkylation of HSPA2 with 4HNE and the concomitant disruption of this zona pellucida-receptor complex. The use of penicillamine enabled a partial recovery of ARSA surface expression and zona pellucida adherence in H2O2-treated cells. These data suggest that the ability of low levels of oxidative stress to disrupt sperm function is mediated by the production of lipid aldehydes as a consequence of lipid peroxidation and their adduction to the molecular chaperone HSPA2 that is responsible for co-ordinating the assembly of functional zona pellucida-receptor complexes during sperm capacitation.

LIMITATIONS, REASONS FOR CAUTION

While these results extend only to one particular zona pellucida-receptor complex, we postulate that oxidative stress may more broadly impact upon sperm surface architecture. In this light, further study is required to assess the impact of oxidative stress on additional HSPA2-laden protein complexes.

WIDER IMPLICATIONS OF THE FINDINGS

These findings link low levels of oxidative stress to a severe loss of sperm function. In doing so, this work suggests a potential cause of male infertility pertaining to a loss of zona pellucida recognition ability and will contribute to the more accurate diagnosis and treatment of such conditions.

An update on post-ejaculatory remodeling of the sperm surface before mammalian fertilization

The fusion of a sperm with an oocyte to form new life is a highly regulated event. The activation-also termed capacitation-of the sperm cell is one of the key preparative steps required for this process. Ejaculated sperm has to make a journey through the female uterus and oviduct before it can approach the oocyte. The oocyte at that moment also has become prepared to facilitate monospermic fertilization and block immediately thereafter the chance for polyspermic fertilization. Interestingly, ejaculated sperm is not properly capacitated and consequently is not yet able to fertilize the oocyte. During the capacitation process, the formation of competent lipid-protein domains on the sperm head enables sperm-cumulus and zona pellucida interactions. This sperm binding allows the onset for a cascade reaction ultimately resulting in oocyte-sperm fusion. Many different lipids and proteins from the sperm surface are involved in this process. Sperm surface processing already starts when sperm are liberated from the seminiferous tubules and is followed by epididymal maturation where the sperm cell surface is modified and loaded with proteins to ensure it is prepared for its fertilization task. Although cauda epididymal sperm can fertilize the oocyte IVF, they are coated with so-called decapacitation factors during ejaculation. The seminal plasma-induced stabilization of the sperm surface permits the sperm transit through the cervix and uterus but prevents sperm capacitation and thus inhibits fertilization. For IVF purposes, sperm are washed out of seminal plasma and activated to get rid of decapacitation factors. Only after capacitation, the sperm can fertilize the oocyte. In recent years, IVF has become a widely used tool to achieve successful fertilization in both the veterinary field and human medicine. Although IVF procedures are very successful, scientific knowledge is still far from complete when identifying all the molecular players and processes during the first stages the fusion of two gametes into a new life. A concise overview in the current understanding of the process of capacitation and the sperm surface changes is provided. The gaps in knowledge of these prefertilization processes are critically discussed.

Centrifugation stress reduces the responsiveness of spermatozoa to a capacitation stimulus in in vitro-aged semen

Density gradient centrifugation of semen is commonly used in many assisted reproduction techniques. Although gradients have the potential to isolate and enrich motile and viable spermatozoa, the centrifugation force presents a stress factor to cell organelles and membranes. The objective of the study was to evaluate the impact of density gradient centrifugation stress on sperm capacitation dynamics, cell stability and the ability of spermatozoa to specifically respond to bicarbonate in extended semen undergoing in vitro ageing. Extended boar semen (n = 7) was stored for 12, 24, 72 and 120 h respectively at 17 °C before centrifugation and incubation in variations of an in vitro capacitation medium. The number of viable, acrosome intact sperm and motility parameters as assessed by computer-assisted semen analysis did not change during storage. Kinetic changes in viability (plasma membrane integrity) and intracellular calcium levels (calcium influx) during in vitro capacitation were assessed after preparation of semen samples with both, a Percoll and a sucrose gradient centrifugation, either only Percoll, only sucrose centrifugation or no centrifugation. Changes in the viable sperm population that could be specifically attributed as a response to either bicarbonate or calcium were determined. In in vitro-aged (>12 h stored) spermatozoa, centrifugation reduced the proportion of spermatozoa which specifically responded to the capacitating stimulus bicarbonate. Concomitantly, centrifugation increased the proportion of spermatozoa responding to calcium in absence of bicarbonate, thus indicating an increased sensitivity to incubation per se. Absence of centrifugation steps during semen preparation, revealed a highly conserved ability of in vitro-aged spermatozoa to specifically respond to bicarbonate. In conclusion, density gradient centrifugation alters the physiological property of spermatozoa for controlled capacitation, which may influence the success rates of centrifuged semen in assisted reproductive technologies and confound interpretation of capacitation assays.

Murine binder of sperm protein homolog 1: a new player in HDL-induced capacitation

Binder of sperm (BSP) proteins are ubiquitous among mammals and are exclusively expressed in male genital tract. The main function associated with BSP proteins is their ability to promote sperm capacitation. In mice, two proteins (BSP protein homolog 1 (BSPH1) and BSPH2) have been studied. Using recombinant strategies, BSPH1 was found to bind to epididymal sperm membranes and promote sperm capacitation in vitro. The goal of this study was to evaluate the role of native murine BSPH1 protein in sperm capacitation induced by BSA and HDLs. The effect of antibodies, antigen-binding fragments (Fabs), and F(ab′)2 specific for murine BSPH1 on BSA- and HDL-induced capacitation was tested. Results indicate that BSPH1 has no direct role in BSA-induced capacitation. However, antibodies, Fabs, and F(ab′)2 could block capacitation induced by HDLs and could inhibit the HDL-induced increase in tyrosine phosphorylation, suggesting a specific interaction between HDLs and BSPH1. Results indicate that murine BSPH1 proteins in mice could be a new important piece of the puzzle in sperm capacitation induced by HDLs. As murine BSPH1 is orthologous to human BSPH1, this study could also lead to new insights into the functions and the importance of the human protein in male fertility.

Free French abstract

A French translation of this abstract is freely available at http://www.reproduction-online.org/content/149/4/367/suppl/DC1.

Distinct segment-specific functions of calyculin A-sensitive protein phosphatases in the regulation of cAMP-triggered events in ejaculated bull spermatozoa

Livestock spermatozoa possess more tenacious suppressors of cAMP-triggered events-including capacitation-associated changes-than laboratory animal spermatozoa, leading to flagellar hyperactivation. In order to identify the suppressors, we examined effects of an inhibitor of serine/threonine protein phosphatases (calyculin A) on cAMP-triggered changes in the protein phosphorylation state, and subsequent occurrence of hyperactivation and acrosome reaction in ejaculated bull spermatozoa. Ejaculated spermatozoa were incubated in cAMP-supplemented medium, then assessed for motility, acrosome morphology, and phosphorylated protein localization. The addition of calyculin A greatly enhanced cAMP-triggered protein phosphorylation at serine/threonine and tyrosine residues in the connecting piece and induction of flagellar hyperactivation. Most hyperactivated spermatozoa exhibited extremely asymmetrical bends at the middle piece, which produced intensive twisting or figure-eight movements. In the sperm head, however, cAMP-triggered dephosphorylation of serine/threonine-phosphorylated proteins and subsequent acrosome reaction were abolished by the addition of calyculin A. Based on these results, we suggest that calyculin A-sensitive protein phosphatases in the connecting piece are suppressors of cAMP-triggered events leading to hyperactivation. By contrast, similar protein phosphatases in the sperm head accelerate cAMP-triggered events leading to the acrosome reaction. These findings are consistent with the indication that calyculin A-sensitive protein phosphatases have distinct functions in the regulation of cAMP-triggered events in different regions of ejaculated bull spermatozoa.

Effect of calcium, bicarbonate, and albumin on capacitation-related events in equine sperm

Repeatable methods for IVF have not been established in the horse, reflecting the failure of standard capacitating media to induce changes required for fertilization capacity in equine sperm. One important step in capacitation is membrane cholesterol efflux, which in other species is triggered by cholesterol oxidation and is typically enhanced using albumin as a sterol acceptor. We incubated equine sperm in the presence of calcium, BSA, and bicarbonate, alone or in combination. Bicarbonate induced an increase in reactive oxygen species (ROS) that was abolished by the addition of calcium or BSA. Bicarbonate induced protein tyrosine phosphorylation (PY), even in the presence of calcium or BSA. Incubation at high pH enhanced PY but did not increase ROS production. Notably, no combination of these factors was associated with significant cholesterol efflux, as assessed by fluorescent quantitative cholesterol assay and confirmed by filipin staining. By contrast, sperm treated with methyl-β-cyclodextrin showed a significant reduction in cholesterol levels, but no significant increase in PY or ROS. Presence of BSA increased sperm binding to bovine zonae pellucidae in all three stallions. These results show that presence of serum albumin is not associated with a reduction in membrane cholesterol levels in equine sperm, highlighting the failure of equine sperm to exhibit core capacitation-related changes in a standard capacitating medium. These data indicate an atypical relationship among cholesterol efflux, ROS production, and PY in equine sperm. Our findings may help to elucidate factors affecting failure of equine IVF under standard conditions.

Capacitation in the presence of methyl-β-cyclodextrin results in enhanced zona pellucida-binding ability of stallion spermatozoa

While IVF has been widely successful in many domesticated species, the development of a robust IVF system for the horse remains an elusive and highly valued goal. A major impediment to the development of equine IVF is the fact that optimised conditions for the capacitation of equine spermatozoa are yet to be developed. Conversely, it is known that stallion spermatozoa are particularly susceptible to damage arising as a consequence of capacitation-like changes induced prematurely in response to semen handling and transport conditions. To address these limitations, this study sought to develop an effective system to both suppress and promote thein vitrocapacitation of stallion spermatozoa. Our data indicated that the latter could be achieved in a bicarbonate-rich medium supplemented with a phosphodiesterase inhibitor, a cyclic AMP analogue, and methyl-β-cyclodextrin, an efficient cholesterol-withdrawing agent. The populations of spermatozoa generated under these conditions displayed a number of hallmarks of capacitation, including elevated levels of tyrosine phosphorylation, a reorganisation of the plasma membrane leading to lipid raft coalescence in the peri-acrosomal region of the sperm head, and a dramatic increase in their ability to interact with heterologous bovine zona pellucida (ZP) and undergo agonist-induced acrosomal exocytosis. Furthermore, this functional transformation was effectively suppressed in media devoid of bicarbonate. Collectively, these results highlight the importance of efficient cholesterol removal in priming stallion spermatozoa for ZP bindingin vitro.

Prostasomes: extracellular vesicles from the prostate

The term 'prostasomes' is generally used to classify the extracellular vesicles (EVs) released into prostatic fluid by prostate epithelial cells. However, other epithelia within the male reproductive tract also release EVs that mix with 'true' prostasomes during semen emission or ejaculation. Prostasomes have been proposed to regulate the timing of sperm cell capacitation and induction of the acrosome reaction, as well as to stimulate sperm motility where all three are prerequisite processes for spermatozoa to attain fertilising capacity. Other proposed functions of prostasomes include interfering with the destruction of spermatozoa by immune cells within the female reproductive tract. On the other hand, it is unclear whether the distinct presumed functions are performed collectively by a single type of prostasome or by separate distinct sub-populations of EVs. Moreover, the exact molecular mechanisms through which prostasomes exert their functions have not been fully resolved. Besides their physiological functions, prostasomes produced by prostate tumour cells have been suggested to support prostate cancer spread development, and prostasomes in peripheral blood plasma may prove to be valuable biomarkers for prostate cancer.

Mitochondrial hydrogen peroxide and defective cholesterol efflux prevent in vitro fertilization by cryopreserved inbred mouse sperm

Recent advances in the cryopreservation of mouse sperm have resulted in dramatically improved in vitro fertilization (IVF) rates, but the biological mechanisms underlying the techniques remain unclear. Two different classes of compounds have been widely utilized to improve the IVF rates of cryopreserved mouse sperm: antioxidants and cyclodextrins. To determine how cryopreservation reduces mouse sperm IVF and how antioxidants and cyclodextrins mitigate this effect, we examined sperm function and oxidative damage after cryopreservation, with and without treatments, in mouse strains important for biomedical research. Our investigation revealed mouse strain-specific effects on IVF by modulation of oxidative stress and cholesterol efflux of cryopreserved sperm. Antioxidants improved IVF rates of C57Bl6/J cryopreserved mouse sperm by reducing hydrogen peroxide produced by sperm mitochondria and ameliorating peroxidative damage to the sperm acrosome. Enhancing cholesterol efflux with cyclodextrin restored capacitation-dependent sperm function and IVF after cryopreservation of C57Bl/6J, C57Bl/6N, and 129X1 mouse sperm. Our results highlight two accessible pathways for continued development of IVF techniques for mouse sperm and provide novel endpoints prognostic of IVF success. These insights may improve sperm cryopreservation methods of other mouse strains and species.