Sodium and epithelial sodium channels participate in the regulation of the capacitation-associated hyperpolarization in mouse sperm

Effect of tyrode albumin lactate pyruvate and modified Krebs Ringers broth media on in vitro capacitation of HD-K75 boar spermatozoa at different period of incubation

In vitro capacitation allows for a greater understanding of the mechanisms underlying fertilization and the development of improved reproductive techniques for improving fertility rates in porcine. Tyrodes albumin lactate pyruvate (TALP) and modified Krebs Ringers Broth (m-KRB) are two medias that are commonly used in research experiments to induce capacitation in boar spermatozoa (Cañón-Beltrán et al., Theriogenology, 198, 2023 and 231; Oberlender et al., Archivos de Medicina Veterinaria, 44, 2012 and 201; Sahoo et al., International Journal of Biological Macromolecules, 241, 2023 and 124502). Moreover, understanding the morphological and functional changes in boar spermatozoa at different hours of capacitation periods might aid in the development of novel techniques for improving sperm quality and increasing the litter size. This study was carried out to investigate the effect of Tyrode albumin lactate pyruvate and modified Krebs Ringers Broth media on in vitro capacitation of HD-K75 boar spermatozoa at three different periods of incubation. A total of 24 ejaculate from four clinically healthy, 10-12 months aged HD-K75 boars, maintained at ICAR-All India Coordinated Research Project (AICRP) on pig were selected. Semen was collected by 'Simple fist' method using a portable dummy. The semen samples having 200 mL volume, 103 × 106 spermatozoa/ml concentration and 70% initial motility were selected and split into two parts and suspended in TALP and m-KRB media, respectively, and incubated for 5 h at 37°C. Seminal parameters viz. sperm viability, plasma membrane integrity and acrosomal integrity were estimated in the samples at 0, 3 and 5 h of incubation. This study revealed that there was significant variation between media in live acrosome-reacted (p < .05) and HOST-reacted (p < .01) spermatozoa, while between capacitation periods significant (p < .01) variation was observed in hyperactivated spermatozoa, live acrosome-reacted spermatozoa, HOST-reacted spermatozoa, FITC-labelled PSA, extracellular protein and sperm cholesterol. Non-significant variation was observed in total phospholipid. TALP showed overall better consequence on sperm viability, plasma membrane and acrosomal integrity of boar spermatozoa. From this study, it could be concluded that both TALP and m-KRB media were virtuous to induce capacitation in HD-K75 boar spermatozoa. TALP media, however, had a better effect on sperm viability, plasma membrane and acrosomal integrity of boar spermatozoa. Out of the three different periods, 3 h capacitation period resulted in significantly (p < .01) higher incidence of sperm viability, plasma membrane and acrosomal integrity in HD-K75 boar spermatozoa.

Signaling Roleplay between Ion Channels during Mammalian Sperm Capacitation

In order to accomplish their primary goal, mammalian spermatozoa must undergo a series of physiological, biochemical, and functional changes crucial for the acquisition of fertilization ability. Spermatozoa are highly polarized cells, which must swiftly respond to ionic changes on their passage through the female reproductive tract, and which are necessary for male gametes to acquire their functional competence. This review summarizes the current knowledge about specific ion channels and transporters located in the mammalian sperm plasma membrane, which are intricately involved in the initiation of changes within the ionic milieu of the sperm cell, leading to variations in the sperm membrane potential, membrane depolarization and hyperpolarization, changes in sperm motility and capacitation to further lead to the acrosome reaction and sperm-egg fusion. We also discuss the functionality of selected ion channels in male reproductive health and/or disease since these may become promising targets for clinical management of infertility in the future.

Control of intracellular pH and bicarbonate by CO2 diffusion into human sperm

The reaction of CO2 with H2O to form bicarbonate (HCO3-) and H+ controls sperm motility and fertilization via HCO3--stimulated cAMP synthesis. A complex network of signaling proteins participates in this reaction. Here, we identify key players that regulate intracellular pH (pHi) and HCO3- in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pHi is set by amiloride-sensitive Na+/H+ exchange. The sperm-specific putative Na+/H+ exchanger SLC9C1, unlike its sea urchin homologue, is not gated by voltage or cAMP. Transporters and channels implied in HCO3- transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO3- is produced by diffusion of CO2 into cells and readjustment of the CO2/HCO3-/H+ equilibrium. The proton channel Hv1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO3- synthesis. This work provides a new framework for the study of male infertility.

Research progress on ion channels and their molecular regulatory mechanisms in the human sperm flagellum

The ion channels in sperm tail play an important role in triggering key physiological reactions, e.g., progressive motility, hyperactivation, required for successful fertilization. Among them, CatSper and KSper have been shown to be important ion channels for the transport of Ca2+ and K+ . Moreover, the voltage-gated proton channel Hv1, the sperm-specific sodium-hydrogen exchanger (sNHE), the epithelial sodium channel (ENaC), members of the temperature-sensitive TRP channel family, and the cystic fibrosis transmembrane regulator (CFTR) are also found in the flagellum. This review focuses on the latest advances in ion channels located at the flagellum, describes how they affect sperm physiological function, and summarizes some primary mutual regulation mechanism between ion channels, including PH, membrane potential, and cAMP. These ion channels may be promising targets for clinical application in infertility.

The Role of Sperm Membrane Potential and Ion Channels in Regulating Sperm Function

During the last seventy years, studies on mammalian sperm cells have demonstrated the essential role of capacitation, hyperactivation and the acrosome reaction in the acquisition of fertilization ability. These studies revealed the important biochemical and physiological changes that sperm undergo in their travel throughout the female genital tract, including changes in membrane fluidity, the activation of soluble adenylate cyclase, increases in intracellular pH and Ca2+ and the development of motility. Sperm are highly polarized cells, with a resting membrane potential of about -40 mV, which must rapidly adapt to the ionic changes occurring through the sperm membrane. This review summarizes the current knowledge about the relationship between variations in the sperm potential membrane, including depolarization and hyperpolarization, and their correlation with changes in sperm motility and capacitation to further lead to the acrosome reaction, a calcium-dependent exocytosis process. We also review the functionality of different ion channels that are present in spermatozoa in order to understand their association with human infertility.

Quantitative phosphoproteomics analyses reveal the regulatory mechanisms related to frozen-thawed sperm capacitation and acrosome reaction in yak (Bos grunniens)

Mammalian spermatozoa are not mature after ejaculation and must undergo additional functional and structural changes within female reproductive tracts to achieve subsequent fertilization, including both capacitation and acrosome reaction (AR), which are dominated by post-translational modifications (PTMs), especially phosphorylation. However, the mechanism of protein phosphorylation during frozen-thawed sperm capacitation and AR has not been well studied. In this study, the phosphoproteomics approach was employed based on tandem mass tag (TMT) labeling combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) strategy to analyze frozen-thawed sperm in Ashidan yak under three sequential conditions (density gradient centrifugation-based purification, incubation in the capacitation medium and induction of AR processes by the calcium ionophore A23187 treatment). The identification of 1,377 proteins with 5,509 phosphorylation sites revealed changes in phosphorylation levels of sperm-specific proteins involved in regulation of spermatogenesis, sperm motility, energy metabolism, cilium movement, capacitation and AR. Some phosphorylated proteins, such as AKAP3, AKAP4, SPA17, PDMD11, CABYR, PRKAR1A, and PRKAR2A were found to regulate yak sperm capacitation and AR though the cAMP/PKA signaling pathway cascades. Notably, the phosphorylation level of SPA17 at Y156 increased in capacitated sperm, suggesting that it is also a novel functional protein besides AKAPs during sperm capacitation. Furthermore, the results of this study suggested that the phosphorylation of PRKAR1A and PRKAR2A, and the dephosphorylation of CABYR both play key regulatory role in yak sperm AR process. Protein-protein interaction analysis revealed that differentially phosphorylated proteins (AKAP3, AKAP4, FSIP2, PSMD11, CABYR, and TPPP2) related to capacitation and AR process played a key role in protein kinase A binding, sperm motility, reproductive process, cytoskeleton and sperm flagella function. Taken together, these data provide not only a solid foundation for further exploring phosphoproteome of sperm in yak, but an efficient way to identify sperm fertility-related marker phosphorylated proteins.

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.

Mathematical model reveals that heterogeneity in the number of ion transporters regulates the fraction of mouse sperm capacitation

Capacitation is a complex maturation process mammalian sperm must undergo in the female genital tract to be able to fertilize an egg. This process involves, amongst others, physiological changes in flagellar beating pattern, membrane potential, intracellular ion concentrations and protein phosphorylation. Typically, in a capacitation medium, only a fraction of sperm achieve this state. The cause for this heterogeneous response is still not well understood and remains an open question. Here, one of our principal results is to develop a discrete regulatory network, with mostly deterministic dynamics in conjunction with some stochastic elements, for the main biochemical and biophysical processes involved in the early events of capacitation. The model criterion for capacitation requires the convergence of specific levels of a select set of nodes. Besides reproducing several experimental results and providing some insight on the network interrelations, the main contribution of the model is the suggestion that the degree of variability in the total amount and individual number of ion transporters among spermatozoa regulates the fraction of capacitated spermatozoa. This conclusion is consistent with recently reported experimental results. Based on this mathematical analysis, experimental clues are proposed for the control of capacitation levels. Furthermore, cooperative and interference traits that become apparent in the modelling among some components also call for future theoretical and experimental studies.

Kolaviron abates busulfan-induced episodic memory deficit and testicular dysfunction in rats: The implications for neuroendopathobiological changes during chemotherapy

Busulfan is a popular antileukemia chemotherapeutic alkylating agent widely known to induce variety of serious adverse effects including chemobrain-related cognitive impairments and dysfunction in male reproductive system. Whether kolaviron, a neuro- and repro-active compound obtained from Garcinia kola, with neuroprotective and reproductive-promoting activities, mitigates busulfan-induced cognitive and male reproductive impairments remain unknown. Hence, we investigated the reversal effects of kolaviron on busulfan-induced episodic memory deficit and testicular dysfunction, and its underlying mechanisms in male rats. In the treatment-protocol, rats in groups 1 and 2 received saline (10 mL/kg/p.o./day) and DMSO (10 mL/kg/p.o./day) respectively, group 3 was given kolaviron (200 mg/kg/p.o./day), group 4 received busulfan (50 mg/kg/p.o./day) and group 5 was pretreated with busulfan (50 mg/kg/p.o./day) consecutively for 56 days prior to kolaviron treatment (200 mg/kg/p.o./day) from days 29-56. Episodic memory deficit was assessed using passive avoidance task (PAT). Following euthanization, blood samples, epididymal sperm, testes and brain were harvested and hormonal and neurochemical contents and their metabolizing enzymes were assayed. Kolaviron reversed busulfan-induced episodic cognitive deficit in the PAT. The reduced serotonin, dopamine, noradrenaline concentrations, elevated glutamate levels, acetylcholinesterase, monoamine oxidase-A and B activities were normalized by kolaviron. Kolaviron also reversed the busulfan-induced decreased testicular/body weights and spermatogenesis. Kolaviron abated busulfan-induced changes in androgenic hormones (testosterone, FSH, LH), dehydrogenase enzymes (3ß-HSD and 17ß-HSD), altered sperm-chromatin, sperm-membrane integrity and sperm-acrosomal reaction and capacitation impairments. Our findings suggest that kolaviron could mitigate busulfan-induced episodic memory deficit and dysfunction in male reproductive system via neurochemical modulations and increase testicular androgenic hormones/enzymes in rats.

Membrane hyperpolarization abolishes calcium oscillations that prevent induced acrosomal exocytosis in human sperm

Sperm capacitation is essential to gain fertilizing capacity. During this process, a series of biochemical and physiological modifications occur that allow sperm to undergo acrosomal exocytosis (AE). At the molecular level, hyperpolarization of the sperm membrane potential (Em) takes place during capacitation. This study shows that human sperm incubated under conditions that do not support capacitation (NC) can become ready for an agonist stimulated AE by pharmacologically inducing Em hyperpolarization with Valinomycin or Amiloride. To investigate how Em hyperpolarization promotes human sperm's ability to undergo AE, live single-cell imaging experiments were performed to simultaneously monitor changes in [Ca²⁺ ]_i and the occurrence of AE. Em hyperpolarization turned [Ca²⁺ ]_i dynamics in NC sperm from spontaneously oscillating into a sustained slow [Ca²⁺ ]_i increase. The addition of progesterone (P4) or K⁺ to Valinomycin-treated sperm promoted that a significant number of cells displayed a transitory rise in [Ca²⁺ ]_i which then underwent AE. Altogether, our results demonstrate that Em hyperpolarization is necessary and sufficient to prepare human sperm for the AE. Furthermore, this Em change decreased Ca²⁺ oscillations that block the occurrence of AE, providing strong experimental evidence of the molecular mechanism that drives the acquisition of acrosomal responsiveness.

Prevention and reversal of chlorpromazine induced testicular dysfunction in rats by synergistic testicle-active flavonoids, taurine and coenzyme-10

Evidences have shown that alterations in testicular dehydrogenase and ionic-ATPase activities have important implications in spermatogenesis and sperm capacitation, a penultimate biochemical change required for fertilization. Previous studies have revealed that taurine and coenzyme-Q10 (COQ-10), which are synergistic testicle-active bioflavonoids, with proven gonadotropin-enhancing properties reduce testicular damage in rats. Hence, this study investigated the effects of taurine and COQ-10 or their combination alone, and in the preventive and reversal of chlorpromazine-induced inhibition of testicular dehydrogenase enzymes, electrogenic pumps, sperm capacitation and acrosomal-reaction in male Wister rats. In the drug-treatment alone or preventive-protocol, rats received oral treatment of saline (10 mL/kg), taurine (150 mg/kg/day), COQ-10 (10 mg/kg/day) or both alone repeatedly for 56 days, or in combination with chlorpromazine (30 mg/kg/p.o./day) from days 29-56. In the reversal-protocol, the animals received chlorpromazine for 56 days prior to saline, taurine, COQ-10 or the combination from days 29-56. Thereafter, spermatogenesis (sperm count, viability, motility and morphology), testicular dehydrogenase [3beta-hydroxysteroid dehydrogenase (3ß-HSD), 17beta-hydroxysteroid dehydrogenase (17ß-HSD), glucose-6-phosphate dehydrogenase (G6PDH), lactate dehydrogenase-X (LDH-X)], ATPase (Na⁺/K⁺, Ca²⁺, Mg²⁺, H⁺) activities, sperm capacitation and acrosomal reaction were evaluated. Taurine and COQ-10 or their combination increased spermatogenesis, testicular 3ß-HSD, 17ß-HSD, G6PDH and LDH-X enzymes of naïve and chlorpromazine-treated rats. Both taurine and COQ-10 increased Na⁺/K⁺, Ca²⁺, Mg²⁺ and H⁺-ATPase activities. Also, taurine and COQ-10 or their combination prevented and reversed chlorpromazine-induced inhibition of sperm capacitation and acrosomal-reaction. The study showed that taurine and COQ-10 prevent and reverse chlorpromazine-induced inhibition of spermatogenesis, epididymal sperm capacitation and acrosomal reaction in rats through increased testicular dehydrogenases and electrogenic pump activities.

Na+/H+ Exchangers Involve in Regulating the pH-Sensitive Ion Channels in Mouse Sperm

Sperm-specific K⁺ ion channel (KSper) and Ca²⁺ ion channel (CatSper), whose elimination causes male infertility in mice, determine the membrane potential and Ca²⁺ influx, respectively. KSper and CatSper can be activated by cytosolic alkalization, which occurs during sperm going through the alkaline environment of the female reproductive tract. However, which intracellular pH (pH_i) regulator functionally couples to the activation of KSper/CatSper remains obscure. Although Na⁺/H⁺ exchangers (NHEs) have been implicated to mediate pH_i in sperm, there is a lack of direct evidence confirming the functional coupling between NHEs and KSper/CatSper. Here, 5-(N,N-dimethyl)-amiloride (DMA), an NHEs inhibitor that firstly proved not to affect KSper/CatSper directly, was chosen to examine NHEs function on KSper/CatSper in mouse sperm. The results of patch clamping recordings showed that, when extracellular pH was at the physiological level of 7.4, DMA application caused KSper inhibition and the depolarization of membrane potential when pipette solutions were not pH-buffered. In contrast, these effects were minimized when pipette solutions were pH-buffered, indicating that they solely resulted from pH_i acidification caused by NHEs inhibition. Similarly, DMA treatment reduced CatSper current and intracellular Ca²⁺, effects also dependent on the buffer capacity of pH in pipette solutions. The impairment of sperm motility was also observed after DMA incubation. These results manifested that NHEs activity is coupled to the activation of KSper/CatSper under physiological conditions.

Polyamines Influence Mouse Sperm Channels Activity

Polyamines are ubiquitous polycationic compounds that are highly charged at physiological pH. While passing through the epididymis, sperm lose their capacity to synthesize the polyamines and, upon ejaculation, again come into contact with the polyamines contained in the seminal fluid, unleashing physiological events that improve sperm motility and capacitation. In the present work, we hypothesize about the influence of polyamines, namely, spermine, spermidine, and putrescine, on the activity of sperm channels, evaluating the intracellular concentrations of chloride [Cl⁻]i, calcium [Ca²⁺]i, sodium [Na⁺]i, potassium [K⁺]i, the membrane V_m, and pHi. The aim of this is to identify the possible regulatory mechanisms mediated by the polyamines on sperm-specific channels under capacitation and non-capacitation conditions. The results showed that the presence of polyamines did not directly influence the activity of calcium and chloride channels. However, the results suggested an interaction of polyamines with sodium and potassium channels, which may contribute to the membrane V_m during capacitation. In addition, alkalization of the pHi revealed the possible activation of sperm-specific Na⁺/H⁺ exchangers (NHEs) by the increased levels of cyclic AMP (cAMP), which were produced by soluble adenylate cyclase (sAC) and interact with the polyamines, evidence that is supported by in silico analysis.

Everything you ever wanted to know about PKA regulation and its involvement in mammalian sperm capacitation

The 3', 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) is a tetrameric holoenzyme comprising a set of two regulatory subunits (PKA-R) and two catalytic (PKA-C) subunits. The PKA-R subunits act as sensors of cAMP and allow PKA-C activity. One of the first signaling events observed during mammalian sperm capacitation is PKA activation. Thus, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. It is widely accepted that PKA specificity depends on several levels of regulation. Anchoring proteins play a pivotal role in achieving proper localization signaling, subcellular targeting and cAMP microdomains. These multi-factorial regulation steps are necessary for a precise spatio-temporal activation of PKA. Here we discuss recent understanding of regulatory mechanisms of PKA in mammalian sperm, such as post-translational modifications, in the context of its role as the master orchestrator of molecular events conducive to capacitation.

Oviductal high concentration of K+ suppresses hyperpolarization but does not prevent hyperactivation, acrosome reaction and in vitro fertilization in hamsters

Mammalian sperm have to undergo capacitation to be fertilization competent. Capacitated sperm in vitro show hyperpolarization of the membrane potential. It has been reported that in mouse membrane hyperpolarization is necessary for the acrosome reaction. We recently found that the fluid of the hamster oviduct, where fertilization occurs, contained a high potassium (K+) concentration (~20 mEq/l). This high K+ concentration could depolarize the membrane potential and prevent the acrosome reaction/fertilization. Conversely, some beneficial effects on capacitation of high K+ concentration or a high K/Na ratio were also reported. In the present study, we investigated the effects of oviduct high K+ concentration on hamster sperm capacitation-associated events and fertilization. The present study confirmed that, in hamster sperm, membrane potential was hyperpolarized upon in vitro capacitation, indicating that capacitation-associated hyperpolarization is a universal phenomenon among mammalian species. An increase in KCl concentration in the medium to 20 mM significantly depolarized the membrane potential and suppressed hyperpolarization when in the presence of >101 mM NaCl. However, an increase in the KCl concentration to 20 mM did not significantly affect the percentage of motile sperm, hyperactivation or the acrosome reaction. No effect of 20 mM KCl on in vitro fertilization was observed. In addition, no correlative changes in hyperactivation and the acrosome reaction with K/Na ratio were observed. These results suggested that in hamsters the oviduct K+ concentration suppressed hyperpolarization but had no effect on capacitation and in vitro fertilization. Our results raised a question over the physiological significance of capacitation-associated hyperpolarization.

Relationship between semen quality and seminal plasma cholesterol, triacylglycerols and proteins in the domestic cat

OBJECTIVES

The current study aimed to evaluate the relationship between specific seminal plasma components - cholesterol (CHOL), triacylglycerols (TAG) and total protein (PROT) concentrations - and semen quality in cats. A further aim was to determine the relationship between specific seminal protein bands and semen quality.

METHODS

Thirteen toms, 2-5 years of age, were included. Semen collection was performed by electroejaculation every 4 weeks. Fifty-eight ejaculates were assessed for motility, velocity, volume, sperm concentration, total sperm count, viability, acrosome integrity, plasma membrane integrity and sperm morphology. Samples were divided into two groups: good semen quality (GSQ) and poor semen quality (PSQ). After evaluation, seminal plasma was separated from the sperm by centrifugation and stored at -20°C. CHOL, TAG and PROT concentrations were then assessed and seminal plasma protein profile was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

Seminal plasma CHOL and TAG concentrations, motility, velocity, sperm concentration, total sperm count and sperm morphology were significantly higher in GSQ cats compared with PSQ cats (P <0.01). Moreover, seminal plasma SDS-PAGE analysis showed an identifiable extra band exclusively in the GSQ group.

CONCLUSIONS AND RELEVANCE

Data obtained in this study showed that seminal plasma CHOL and TAG concentrations and specific protein bands could be used to improve semen evaluation in toms. In this sense, the 14 kDa protein band could be a valuable marker for semen quality in the cat and should be further investigated. However, more studies are necessary to determine its relationship with fertility.

Adhesion to oviduct glycans regulates porcine sperm Ca2+ influx and viability

Before fertilization, sperm bind to epithelial cells of the oviduct isthmus to form a reservoir that regulates sperm viability and capacitation. The sperm reservoir maintains optimum fertility in species, like swine, in which semen deposition and ovulation may not be well synchronized. We demonstrated previously that porcine sperm bind to two oviductal glycan motifs, a biantennary 6-sialylated N-acetyllactosamine (bi-SiaLN) oligosaccharide and 3-O-sulfated Lewis X trisaccharide (suLe^X). Here, we assessed the ability of these glycans to regulate sperm Ca²⁺ influx, capacitation and affect sperm lifespan. After 24 h, the viability of sperm bound to immobilized bi-SiaLN and suLe^X was higher (46% and 41% respectively) compared to viability of free-swimming sperm (10–12%). Ca²⁺ is a central regulator of sperm function so we assessed whether oviduct glycans could affect the Ca²⁺ influx that occurs during capacitation. Using a fluorescent intracellular Ca²⁺ probe, we observed that both oviduct glycans suppressed the Ca²⁺ increase that occurs during capacitation. Thus, specific oviduct glycans can regulate intracellular Ca²⁺. Because the increase in intracellular Ca²⁺ was suppressed by oviduct glycans, we examined whether glycans affected capacitation, as determined by protein tyrosine phosphorylation and the ability to undergo a Ca²⁺ ionophore-induced acrosome reaction. We found no discernable suppression of capacitation in sperm bound to oviduct glycans. We also detected no effect of oviduct glycans on sperm motility during capacitation. In summary, Le^X and bi-SiaLN glycan motifs found on oviduct oligosaccharides suppress the Ca²⁺ influx that occurs during capacitation and extend sperm lifespan but do not affect sperm capacitation or motility.

How defective spermatogenesis affects sperm DNA integrity

Spermatogenesis is the essential process to maintain and promote male fertility. It is extraordinarily complex with many regulatory elements and numerous steps. The process involves several cell types, regulatory molecules, repair mechanisms and epigenetic regulators. Evidence has shown that fertility can be negatively impacted by reduced sperm DNA integrity. Sources of sperm DNA damage include replication errors and causes of DNA fragmentation which include abortive apoptosis, defective maturation and oxidative stress. This review outlines the process of spermatogenesis, spermatogonial regulation and sperm differentiation; additionally, DNA damage and currently studied DNA repair mechanisms in spermatozoon are also covered.

Membrane Potential Determined by Flow Cytometry Predicts Fertilizing Ability of Human Sperm

Infertility affects 10 to 15% of couples worldwide, with a male factor contributing up to 50% of these cases. The primary tool for diagnosing male infertility is traditional semen analysis, which reveals sperm concentration, morphology, and motility. However, 25% of infertile men are diagnosed as normozoospermic, meaning that, in many cases, normal-appearing sperm fail to fertilize an egg. Thus, new information regarding the mechanisms by which sperm acquire fertilizing ability is needed to develop a clinically feasible test that can predict sperm function failure. An important feature of sperm fertilization capability in many species is plasma membrane hyperpolarization (membrane potential becoming more negative inside) in response to signals from the egg or female genital tract. In mice, this hyperpolarization is necessary for sperm to undergo the changes in motility (hyperactivation) and acrosomal exocytosis required to fertilize an egg. Human sperm also hyperpolarize during capacitation, but the physiological relevance of this event has not been determined. Here, we used flow cytometry combined with a voltage-sensitive fluorescent probe to measure absolute values of human sperm membrane potential. We found that hyperpolarization of human sperm plasma membrane correlated positively with fertilizing ability. Hyperpolarized human sperm had higher in vitro fertilization (IVF) ratios and higher percentages of acrosomal exocytosis and hyperactivated motility than depolarized sperm. We propose that measurements of human sperm membrane potential could be used to diagnose men with idiopathic infertility and predict IVF success in normozoospermic infertile patients. Patients with depolarized values could be guided toward intracytoplasmic sperm injection, preventing unnecessary cycles of intrauterine insemination or IVF. Conversely, patients with hyperpolarized values of sperm membrane potential could undergo only conventional IVF, avoiding the risks and costs associated with intracytoplasmic sperm injection.

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.

Determination of a Robust Assay for Human Sperm Membrane Potential Analysis

Mammalian sperm must undergo a complex process called capacitation in order to fertilize the egg. During this process, hyperpolarization of the sperm plasma membrane has been mostly studied in mouse, and associated to its importance in the preparation to undergo the acrosome reaction (AR). However, despite the increasing evidence of membrane hyperpolarization in human sperm capacitation, no reliable techniques have been set up for its determination. In this report we describe human sperm membrane potential (Em) measurements by a fluorimetric population assay, establishing optimal conditions for Em determination. In addition, we have conducted parallel measurements of the same human sperm samples by flow cytometry and population fluorimetry, before and after capacitation, to conclusively address their reliability. This integrative analysis sets the basis for the study of Em in human sperm allowing future work aiming to understand its role in human sperm capacitation.

High throughput small RNA and transcriptome sequencing reveal capacitation-related microRNAs and mRNA in boar sperm

Background

Capacitation, a prerequisite for oocyte fertilization, is a complex process involving series of structural and functional changes in sperms such as membrane modifications, modulation of enzyme activities, and protein phosphorylation. In order to penetrate and fertilize an oocyte, mammalian sperms must undergo capacitation. Nevertheless, the process of sperm capacitation remains poorly understood and requires further elucidation. In the current study, via high throughput sequencing, we identified and explored the differentially expressed microRNAs (miRNAs) and mRNAs involved in boar sperm capacitation.

Results

We identified a total of 5342 mRNAs and 204 miRNAs that were differentially expressed in fresh and capacitated boar sperms. From these, 12 miRNAs (8 known and 4 newly identified miRNAs) and their differentially expressed target mRNAs were found to be involved in sperm capacitation-related PI3K-Akt, MAPK, cAMP-PKA and Ca²⁺signaling pathways.

Conclusions

Our study is first to provide the complete miRNA and transcriptome profiles of boar sperm. Our findings provide important insights for the understanding of the RNA profile in boar sperm and future elucidation of the underlying molecular mechanism relevant to mammalian sperm capacitation.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5132-9) contains supplementary material, which is available to authorized users.

Tyrosine phosphorylation signaling regulates Ca2+ entry by affecting intracellular pH during human sperm capacitation

Capacitation is a mandatory process for the acquisition of mammalian sperm fertilization competence and involves the activation of a complex and still not fully understood system of signaling pathways. Under in vitro conditions, there is an increase in both protein tyrosine phosphorylation (pTyr) and intracellular Ca²⁺ levels in several species. In human sperm, results from our group revealed that pTyr signaling can be blocked by inhibiting proline-rich tyrosine kinase 2 (PYK2). Based on the role of PYK2 in other cell types, we investigated whether the PYK2-dependent pTyr cascade serves as a sensor for Ca ²⁺ signaling during human sperm capacitation. Flow cytometry studies showed that exposure of sperm to the PYK2 inhibitor N-[2-[[[2-[(2,3-dihydro-2-oxo-1 H-indol-5-yl)amino]-5-(trifluoromethyl)-4-pyrimidinyl]amino]methyl]phenyl]- N-methyl-methanesulfonamide hydrate (PF431396) produced a significant and concentration-dependent reduction in intracellular Ca ²⁺ levels during capacitation. Further studies revealed that PF431396-treated sperm exhibited a decrease in the activity of CatSper, a key sperm Ca ²⁺ channel. In addition, time course studies during capacitation in the presence of PF431396 showed a significant and sustained decrease in both intracellular Ca ²⁺ and pH levels after 2 hr of incubation, temporarily coincident with the activation of PYK2 during capacitation. Interestingly, decreases in Ca ²⁺ levels and progressive motility caused by PF431396 were reverted by inducing intracellular alkalinization with NH ₄ Cl, without affecting the pTyr blockage. Altogether, these observations support pTyr as an intracellular sensor for Ca ²⁺ entry in human sperm through regulation of cytoplasmic pH. These results contribute to a better understanding of the modulation of the polymodal CatSper and signaling pathways involved in human sperm capacitation.

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.

Role of the Na+/K+-ATPase ion pump in male reproduction and embryo development

Na⁺/K⁺-ATPase was one of the first ion pumps studied because of its importance in maintaining osmotic and ionic balances between intracellular and extracellular environments, through the exchange of three Na⁺ ions out and two K⁺ ions into a cell. This enzyme, which comprises two main subunits (α and β), with or without an auxiliary polypeptide (γ), can have specific biochemical properties depending on the expression of associated isoforms (α1β1 and/or α2β1) in the cell. In addition to the importance of Na⁺/K⁺-ATPase in ensuring the function of many tissues (e.g. brain, heart and kidney), in the reproductive tract this protein is essential for embryo development because of its roles in blastocoel formation and embryo hatching. In the context of male reproduction, the discovery of a very specific subunit (α4), apparently restricted to male germ cells, only expressed after puberty and able to influence sperm function (e.g. motility and capacitation), opened a remarkable field for further investigations regarding sperm biology. Therefore, the present review focuses on the importance of Na⁺/K⁺-ATPase on male reproduction and embryo development.

CFTR/ENaC-dependent regulation of membrane potential during human sperm capacitation is initiated by bicarbonate uptake through NBC

To fertilize an egg, sperm must reside in the female reproductive tract to undergo several maturational changes that are collectively referred to as capacitation. From a molecular point of view, the HCO₃^--dependent activation of the atypical soluble adenylyl cyclase (ADCY10) is one of the first events that occurs during capacitation and leads to the subsequent cAMP-dependent activation of protein kinase A (PKA). Capacitation is also accompanied by hyperpolarization of the sperm plasma membrane. We previously reported that PKA activation is necessary for CFTR (cystic fibrosis transmembrane conductance regulator channel) activity and for the modulation of membrane potential (Em). However, the main HCO₃^- transporters involved in the initial transport and the PKA-dependent Em changes are not well known nor characterized. Here, we analyzed how the activity of CFTR regulates Em during capacitation and examined its relationship with an electrogenic Na⁺/HCO₃^- cotransporter (NBC) and epithelial Na⁺ channels (ENaCs). We observed that inhibition of both CFTR and NBC decreased HCO₃^- influx, resulting in lower PKA activity, and that events downstream of the cAMP activation of PKA are essential for the regulation of Em. Addition of a permeable cAMP analog partially rescued the inhibitory effects caused by these inhibitors. HCO₃^- also produced a rapid membrane hyperpolarization mediated by ENaC channels, which contribute to the regulation of Em during capacitation. Altogether, we demonstrate for the first time, that NBC cotransporters and ENaC channels are essential in the CFTR-dependent activation of the cAMP/PKA signaling pathway and Em regulation during human sperm capacitation.

Disruption of protein kinase A localization induces acrosomal exocytosis in capacitated mouse sperm

Protein kinase A (PKA) is a broad-spectrum Ser/Thr kinase involved in the regulation of several cellular activities. Thus, its precise activation relies on being localized at specific subcellular places known as discrete PKA signalosomes. A-Kinase anchoring proteins (AKAPs) form scaffolding assemblies that play a pivotal role in PKA regulation by restricting its activity to specific microdomains. Because one of the first signaling events observed during mammalian sperm capacitation is PKA activation, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. Here, we demonstrate that the anchoring of PKA to AKAP is not only necessary but also actively regulated during sperm capacitation. However, we find that once capacitated, the release of PKA from AKAP promotes a sudden Ca²⁺ influx through the sperm-specific Ca²⁺ channel CatSper, starting a tail-to-head Ca²⁺ propagation that triggers the acrosome reaction. Three-dimensional super-resolution imaging confirmed a redistribution of PKA within the flagellar structure throughout the capacitation process, which depends on anchoring to AKAP. These results represent a new signaling event that involves CatSper Ca²⁺ channels in the acrosome reaction, sensitive to PKA stimulation upon release from AKAP.

Regulation mechanisms and implications of sperm membrane hyperpolarization

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. In order to gain fertilization competence, they need to undergo a series of biochemical and physiological modifications inside the female reproductive tract, known as capacitation. Capacitation correlates with two essential events for fertilization: hyperactivation, an asymmetric and vigorous flagellar motility, and the ability to undergo the acrosome reaction. At a molecular level, capacitation is associated to: phosphorylation cascades, modification of membrane lipids, alkalinization of the intracellular pH, increase in the intracellular Ca²⁺ concentration and hyperpolarization of the sperm plasma membrane potential. Hyperpolarization is a crucial event in capacitation since it primes the sperm to undergo the exocytosis of the acrosome content, essential to achieve fertilization of the oocyte.

Localization of epithelial sodium channel (ENaC) and CFTR in the germinal epithelium of the testis, Sertoli cells, and spermatozoa

Spermatogenesis starts within the seminiferous tubules of the testis by mitotic division of spermatogonia that produces spermatocytes. Meiotic division of these spermatocytes produces haploid spermatids that differentiate into spermatozoa. In this study, we examined the expression of ENaC and CFTR (a Cl^- channel) in rat testicular sections using confocal microscopic immunofluorescence. The structural integrity of the seminiferous tubule sections was verified by precise phalloidin staining of the actin fibers located abundantly at both basal and adluminal tight junctions. The acrosome forming regions in the round spermatids were stained using an FITC coupled lectin (wheat germ agglutinin). In all phases of the germ cells (spermatogonia, spermatocytes, and spermatids) ENaC was localized in cytoplasmic pools. Prior to spermiation, ENaC immunofluorescence appeared along the tails of the spermatids. In spermatozoa isolated from the epididymis, ENaC was localized at the acrosome and a central region of the sperm flagellum. The mature sperm are transcriptionally silent. Hence, we suggest that ENaC subunits in cytoplasmic pools in germ cells serve as the source of ENaC subunits located along the tail of spermatozoa. The locations of ENaC is compatible with a possible role in the acrosomal reaction and sperm mobility. In contrast to ENaC, CFTR immunofluorescence was most strongly observed specifically within the Sertoli cell nuclei. Based on the nuclear localization of CFTR we suggest that, in addition to its role as an ion channel, CFTR may have an independent role in gene regulation within the nuclei.

Design, Synthesis, and in Vitro and in Vivo Evaluation of Ouabain Analogues as Potent and Selective Na,K-ATPase α4 Isoform Inhibitors for Male Contraception

Na,K-ATPase α4 is a testis-specific plasma membrane Na⁺ and K⁺ transporter expressed in sperm flagellum. Deletion of Na,K-ATPase α4 in male mice results in complete infertility, making it an attractive target for male contraception. Na,K-ATPase α4 is characterized by a high affinity for the cardiac glycoside ouabain. With the goal of discovering selective inhibitors of the Na,K-ATPase α4 and of sperm function, ouabain derivatives were modified at the glycone (C3) and the lactone (C17) domains. Ouabagenin analogue 25, carrying a benzyltriazole moiety at C17, is a picomolar inhibitor of Na,K-ATPase α4, with an outstanding α4 isoform selectivity profile. Moreover, compound 25 decreased sperm motility in vitro and in vivo and affected sperm membrane potential, intracellular Ca²⁺, pH, and hypermotility. These results proved that the new ouabagenin triazole analogue is an effective and selective inhibitor of Na,K-ATPase α4 and sperm function.

Veratridine-sensitive Na+ channels regulate human sperm fertilization capacity

AIMS

The sperm plasma membrane contains specific ion channels and transporters that initiate changes in Ca²⁺, Na⁺, K⁺ and H⁺ ions in the sperm cytoplasm. Ion channels are key regulators of the sperm membrane potential, cytoplasmic Ca²⁺ and intracellular pH (pH_i), which leads to regulate motility, capacitation, acrosome reaction and other physiological processes crucial for successful fertilization. Expression of epithelial sodium channels (ENaC) and voltage-gated sodium channels (Na_v) in human spermatozoa has been reported, but the role of Na⁺ fluxes sodium channels in the regulation of sperm cell function remains poorly understood. In this context, we aimed to analyze the physiological role of Na_v channels in human sperm.

MAIN METHODS

Motility and hyperactivation analysis was conducted by CASA analysis. Flow cytometry and spectrophotometry approaches were carried out to measure Capacitation, Acrosome reaction, immunohistochemistry for Tyr-residues phosporylation, [Ca²⁺]_i levels and membrane potential.

KEY FINDINGS

Functional studies showed that veratridine, a voltage-gated sodium channel activator, increased sperm progressive motility without producing hyperactivation while the Na_v antagonist lidocaine did induce hyperactivated motility. Veratridine increased protein tyrosine phosphorylation, an event occurring during capacitation, and its effects were inhibited in the presence of lidocaine and tetrodotoxin. Veratridine had no effect on the acrosome reaction by itself, but was able to block the progesterone-induced acrosome reaction. Moreover, veratridine caused a membrane depolarization and modified the effect of progesterone on [Ca²⁺]_i and sperm membrane potential.

SIGNIFICANCE

Our results suggest that veratridine-sensitive Na_v channels are involved on human sperm fertility acquisition regulating motility, capacitation and the progesterone-induced acrosome reaction in human sperm.

Modulation of voltage-gated sodium channels induces capacitation in bull spermatozoa through phosphorylation of tyrosine containing proteins

In our previous study, we have reported the molecular presence of Na_v 1.8 in bull spermatozoa and its potential involvement in regulation of sperm functions. With the selective blocking of Nav 1.8 using A-803467, alterations in sperm functions were observed, therefore, we envisaged of investigating the involvement of Na_v in regulating sperm function and the mechanism(s) involved in it using veratridine, a selective opener of Na_v channels. Forty ejaculates were collected from four Hariana bulls and semen samples were pooled in view of the non-significant variations between the different ejaculates. Treatment of sperm cells with veratridine (6, 8, and 10 μM) resulted in concentration- and time-dependent increase in forward progressive sperm motility and it persisted up to 6 h. However, hyperactive motility was induced by veratridine at higher concentrations (8 and 10 μM) and after 2 h of incubation, which was confirmed by subjective assessment followed by chlortetracycline staining showing the increased B-pattern spermatozoa, and thereby suggesting the involvement of Na_v in regulation of capacitation in spermatozoa. To substantiate the functional study observations especially veratridine-induced capacitation, immunoblotting and indirect immune fluorescence assays were performed for detection of the tyrosine-phosphorylated proteins. The immune blot study revealed the presence of five tyrosine phosphorylated proteins, namely-p17, p30, p54, p90 and p100. The p17 protein showed the highest band intensity compared to other protein bands indicating its potential involvement in the process of capacitation. Immunolocalization study revealed positive immunoreactivity for tyrosine phosphorylated proteins in the middle piece, post acrosomal region (high fluorescence) and tail of the spermatozoa (low fluorescence). From the results of present study, it is evident that activation of Na_V by veratridine, especially at higher concentrations, induced capacitation which is evidently mediated through phosphorylation of the tyrosine containing proteins localized in the post acrosomal regions, middle piece and tail of the spermatozoa. However, further studies will help in unraveling the involvement of Na_v and other ion channels regulating different physiological functions of sperm.

Neuronal signaling repertoire in the mammalian sperm functionality

The common embryonic origin has been a recurrent explanation to understand the presence of "neural receptors" in sperm. However, this designation has conditioned a bias marked by the classical neurotransmission model, dismissing the possibility that neurotransmitters can play specific roles in the sperm function by themselves. For instance, the launching of acrosome reaction, a fundamental sperm function, includes several steps that recall the process of presynaptic secretion. Unlike of postsynaptic neuron, whose activation is mediated by molecular interaction between neurotransmitter and postsynaptic receptors, the oocyte activation is not mediated by receptors, but by cytosolic translocation of sperm phospholipase (PLCζ). Thus, the sperm has a cellular design to access and activate the oocyte and restore the ploidy of the species by an "allogenic pronuclear fusion." At subcellular level, the events controlling sperm function, particularly the capacitation process, are activated by chemical signals that trigger ion fluxes, sterol oxidation, synthesis of cyclic adenosine monophosphate, protein kinase A activation, tyrosine phosphorylations and calcium signaling, which correspond to second messengers similar to those associated with exocytosis and growth cone guidance in neurons. Classically, the sperm function associated with neural signals has been analyzed as a unidimensional approach (single ligand-receptor effect). However, the in vivo sperm are exposed to multidimensional signaling context, for example, the GABAergic, monoaminergic, purinergic, cholinergic, and melatoninergic, to name a few. The aim of this review is to present an overview of sperm functionality associated with "neuronal signaling" and possible cellular and molecular mechanisms involved in their regulation.

Focal adhesion kinase is required for actin polymerization and remodeling of the cytoskeleton during sperm capacitation

Several focal adhesion proteins are known to cooperate with integrins to link the extracellular matrix to the actin cytoskeleton; as a result, many intracellular signaling pathways are activated and several focal adhesion complexes are formed. However, how these proteins function in mammalian spermatozoa remains unknown. We confirm the presence of focal adhesion proteins in guinea pig spermatozoa, and we explore their role during capacitation and the acrosome reaction, and their relationship with the actin cytoskeleton. Our results suggest the presence of a focal adhesion complex formed by β1-integrin, focal adhesion kinase (FAK), paxillin, vinculin, talin, and α-actinin in the acrosomal region. Inhibition of FAK during capacitation affected the protein tyrosine phosphorylation associated with capacitation that occurs within the first few minutes of capacitation, which caused the acrosome reaction to become increasingly Ca²⁺ dependent and inhibited the polymerization of actin. The integration of vinculin and talin into the complex, and the activation of FAK and paxillin during capacitation, suggests that the complex assembles at this time. We identify that vinculin and α-actinin increase their interaction with F-actin while it remodels during capacitation, and that during capacitation focal adhesion complexes are structured. FAK contributes to acrosome integrity, likely by regulating the polymerization and the remodeling of the actin cytoskeleton.

Summary: We describe the role of FAK and focal adhesion proteins in capacitation, acrosome reaction, polymerization and remodeling of actin cytoskeleton, and how inhibition of FAK affects sperm physiology.

Regulation of hamster sperm hyperactivation by extracellular Na+

Mammalian sperm motility has to be hyperactivated to be fertilization-competent. Hyperactivation is regulated by extracellular environment. Osmolality of mammalian semen is higher than that in female reproductive tract; however, the effect of them on hyperactivation has not been investigated. So we investigated the effect of osmotic environment on hyperactivation using hamster spermatozoa at first. Increase in the osmolality of the media (∼370 mOsm) by increasing the concentration of NaCl (∼150 mmol/L) caused the delay of the expression of hyperactivation. When NaCl concentration varied in the same range (75–150 mmol/L) whereas the osmolality was fixed at 370 mOsm by adding mannitol, the delay of hyperactivation occurred dependent on NaCl concentration. Increase in NaCl concentration also caused suppression of curvilinear velocity, bend angle, and sliding velocity of the flagellum at the onset of incubation, suggesting that NaCl concentration affect both activation and hyperactivation in hamster spermatozoa. Hamster sperm intracellular Ca2+ concentration decreased as extracellular NaCl concentration increased, whereas membrane potential and intracellular pH were unaffected by extracellular NaCl concentration. SN-6 and SEA0400, inhibitors of Na+-Ca2+ exchanger (NCX), increased intracellular Ca2+ and accelerated hyperactivation in the presence of 150 mmol/L NaCl. Tyrosine phosphorylation on fibrous sheath proteins was unaffected by extracellular NaCl concentration. These results suggest that extracellular Na+ suppresses hamster sperm hyperactivation by reducing intracellular Ca2+ via an action of NCX in a tyrosine phosphorylation-independent manner. It seems that the removal of suppression by extracellular Na+ leads to the expression of hyperactivated motility.

Ouabain Regulates CFTR-Mediated Anion Secretion and Na,K-ATPase Transport in ADPKD Cells

Cyst enlargement in autosomal dominant polycystic kidney disease (ADPKD) requires the transepithelial secretion of fluid into the cyst lumen. We previously showed that physiological amounts of ouabain enhance cAMP-dependent fluid secretion and cyst growth of human ADPKD cyst epithelial cells in culture and formation of cyst-like dilations in metanephric kidneys from Pkd1 mutant mice. Here, we investigated the mechanisms by which ouabain promotes cAMP-dependent fluid secretion and cystogenesis. Ouabain (3 nM) enhanced cAMP-induced cyst-like dilations in embryonic kidneys from Pkd1 (m1Bei) mice, but had no effect on metanephroi from Pkd1 (m1Bei) mice that lack expression of the cystic fibrosis transmembrane conductance regulator (CFTR). Similarly, ouabain stimulation of cAMP-induced fluid secretion and in vitro cyst growth of ADPKD cells were abrogated by CFTR inhibition, showing that CFTR is required for ouabain effects on ADPKD fluid secretion. Moreover, ouabain directly enhanced the cAMP-dependent Cl(-) efflux mediated by CFTR in ADPKD monolayers. Ouabain increased the trafficking of CFTR to the plasma membrane and up-regulated the expression of the CFTR activator PDZK1. Finally, ouabain decreased plasma membrane expression and activity of the Na,K-ATPase in ADPKD cells. Altogether, these results show that ouabain enhances net fluid secretion and cyst formation by activating apical anion secretion via CFTR and decreasing basolateral Na(+) transport via Na,K-ATPase. These results provide new information on the mechanisms by which ouabain affects ADPKD cells and further highlight the importance of ouabain as a non-genomic stimulator of cystogenesis in ADPKD.

Src Kinase Is the Connecting Player between Protein Kinase A (PKA) Activation and Hyperpolarization through SLO3 Potassium Channel Regulation in Mouse Sperm

Plasma membrane hyperpolarization is crucial for mammalian sperm to acquire acrosomal responsiveness during capacitation. Among the signaling events leading to mammalian sperm capacitation, the immediate activation of protein kinase A plays a pivotal role, promoting the subsequent stimulation of protein tyrosine phosphorylation that associates with fertilizing capacity. We have shown previously that mice deficient in the tyrosine kinase cSrc are infertile and exhibit improper cauda epididymis development. It is therefore not clear whether lack of sperm functionality is due to problems in epididymal maturation or to the absence of cSrc in sperm. To further address this problem, we investigated the kinetics of cSrc activation using anti-Tyr(P)-416-cSrc antibodies that only recognize active cSrc. Our results provide evidence that cSrc is activated downstream of PKA and that inhibition of its activity blocks the capacitation-induced hyperpolarization of the sperm plasma membrane without blocking the increase in tyrosine phosphorylation that accompanies capacitation. In addition, we show that cSrc inhibition also blocks the agonist-induced acrosome reaction and that this inhibition is overcome by pharmacological hyperpolarization. Considering that capacitation-induced hyperpolarization is mediated by SLO3, we evaluated the action of cSrc inhibitors on the heterologously expressed SLO3 channel. Our results indicate that, similar to SLO1 K(+) channels, cSrc blockers significantly decreased SLO3-mediated currents. Together, these results are consistent with findings showing that hyperpolarization of the sperm plasma membrane is necessary and sufficient to prepare the sperm for the acrosome reaction and suggest that changes in sperm membrane potential are mediated by cSrc activation.

Flow cytometry analysis reveals that only a subpopulation of mouse sperm undergoes hyperpolarization during capacitation

To gain fertilizing capacity, mammalian sperm should reside in the female tract for a period of time. The physiological changes that render the sperm able to fertilize are known as capacitation. Capacitation is associated with an increase in intracellular pH, an increase in intracellular calcium, and phosphorylation of different proteins. This process is also accompanied by the hyperpolarization of the sperm plasma membrane potential (Em). In the present work, we used flow cytometry to analyze changes in sperm Em during capacitation in individual cells. Our results indicate that a subpopulation of hyperpolarized mouse sperm can be clearly distinguished by sperm flow cytometry analysis. Using sperm bearing green fluorescent protein in their acrosomes, we found that this hyperpolarized subpopulation is composed of sperm with intact acrosomes. In addition, we show that the capacitation-associated hyperpolarization is blocked by high extracellular K(+), by PKA inhibitors, and by SLO3 inhibitors in CD1 mouse sperm, and undetectable in Slo3 knockout mouse sperm. On the other hand, in sperm incubated in conditions that do not support capacitation, sperm membrane hyperpolarization can be induced by amiloride, high extracellular NaHCO3, and cAMP agonists. Altogether, our observations are consistent with a model in which sperm Em hyperpolarization is downstream of a cAMP-dependent pathway and is mediated by the activation of SLO3 K(+) channels.

Role of human Na,K-ATPase alpha 4 in sperm function, derived from studies in transgenic mice

Most of our knowledge on the biological role of the testis-specific Na,K-ATPase alpha 4 isoform derives from studies performed in non-human species. Here, we studied the function of human Na,K-ATPase alpha 4 after its expression in transgenic mice. Using a bacterial artificial chromosome (BAC) construct containing the human ATP1A4 gene locus, we obtained expression of the human α4 transgene specifically in mouse sperm testis and, in the sperm flagellum. The expressed human alpha 4 was active, and compared to wild-type sperm, those from transgenic mice displayed higher Na,K-ATPase alpha 4 activity and greater binding of fluorescently labeled ouabain, which is typical of the alpha 4 isoform. The expression and activity of endogenous alpha 4 and the other Na,K-ATPase alpha isoform present in sperm, alpha 1, remained unchanged. Male mice expressing the human ATP1A4 transgene exhibited similar testis size and morphology, normal sperm number and shape, and no changes in overall fertility compared to wild-type mice. Sperm carrying the human transgene exhibited enhanced total motility and an increase in multiple parameters of sperm movement, including higher sperm hyperactive motility. In contrast, no statistically significant changes in sperm membrane potential, protein tyrosine phosphorylation, or spontaneous acrosome reaction were found between wild-type and transgenic mice. Altogether, these results provide new genetic evidence for an important role of human Na,K-ATPase alpha 4 in sperm motility and hyperactivation, and establishes a new animal model for future studies of this isoform.

SLO3 K+ channels control calcium entry through CATSPER channels in sperm

Here we show how a sperm-specific potassium channel (SLO3) controls Ca(2+) entry into sperm through a sperm-specific Ca(2+) channel, CATSPER, in a totally unanticipated manner. The genetic deletion of either of those channels confers male infertility in mice. During sperm capacitation SLO3 hyperpolarizes the sperm, whereas CATSPER allows Ca(2+) entry. These two channels may be functionally connected, but it had not been demonstrated that SLO3-dependent hyperpolarization is required for Ca(2+) entry through CATSPER channels, nor has a functional mechanism linking the two channels been shown. In this study we show that Ca(2+) entry through CATSPER channels is deficient in Slo3 mutant sperm lacking hyperpolarization; we also present evidence supporting the hypothesis that SLO3 channels activate CATSPER channels indirectly by promoting a rise in intracellular pH through a voltage-dependent mechanism. This mechanism may work through a Na(+)/H(+) exchanger (sNHE) and/or a bicarbonate transporter, which utilizes the inward driving force of the Na(+) gradient, rendering it intrinsically voltage-dependent. In addition, the sperm-specific Na(+)/H(+) exchanger (sNHE) possess a putative voltage sensor that might be activated by membrane hyperpolarization, thus increasing the voltage sensitivity of internal alkalization.