Effect of steroid hormones and capacitation on membrane potential of human spermatozoa

Complex combined steroid mix of the female tract modulates human sperm

Human spermatozoa interact with a complex biochemical environment in the female reproductive tract en route to the site of fertilisation. Ovarian follicular fluid contributes to this complex milieu and is known to contain steroids such as progesterone, whose effects on sperm physiology have been widely characterised. We have previously reported that progesterone stimulates intracellular calcium concentration ([Ca²⁺]_i) signalling and acrosome reaction in human spermatozoa. To characterise the effects of the unified complete follicular fluid steroid hormone complement on human spermatozoa, a comprehensive, data-based, 'physiological standard' steroid hormone balance of follicular fluid (shFF) was created from individual constituents. shFF induced a rapid biphasic [Ca²⁺]_i elevation in human spermatozoa. Using population fluorimetry, we compared [Ca²⁺]_i signal amplitude in cells exposed to serial applications of shFF (6 steps from 10^-5X up to 1X shFF) with responses to the equivalent progesterone component alone (6 steps from 135 pM - 13.5μM). Threshold for the response to shFF was right-shifted (≈10-fold) compared to progesterone alone, but the maximum response to shFF was greatly enhanced. An acrosome reaction assay was used to assess functional effects of shFF-induced sperm calcium signalling. shFF as well as progesterone-treated spermatozoa showed a significant increase in % acrosome reaction (P < 0.01). All of this evidence suggests the modulation of progesterone-mediated responses by other follicular fluid steroids.

Revisiting the action of steroids and triterpenoids on the human sperm Ca2+ channel CatSper

The sperm-specific Ca2+ channel CatSper (cation channel of sperm) is vital for male fertility. Contradictory findings have been published on the regulation of human CatSper by the endogenous steroids estradiol, testosterone and hydrocortisone, as well as the plant triterpenoids, lupeol and pristimerin. The aim of this study was to elucidate this controversy by investigating the action of these steroids and plant triterpenoids on human CatSper using population-based Ca2+-fluorimetric measurements, the specific CatSper-inhibitor RU1968 and a functional test assessing the CatSper-dependent penetration of human sperm cells into methylcellulose. Estradiol, testosterone and hydrocortisone were found to induce Ca2+-signals in human sperm cells with EC50 values in the lower μM range. By employing the specific CatSper-inhibitor RU1968, all three steroids were shown to induce Ca2+-signals through an action on CatSper, similar to progesterone. The steroids were found to dose-dependently inhibit subsequent progesterone-induced Ca2+-signals with IC50 values in the lower μM range. Additionally, the three steroids were found to significantly increase the penetration of human sperm cells into methylcellulose, similar to the effect of progesterone. The two plant triterpenoids, lupeol and pristimerin, were unable to inhibit progesterone-induced Ca2+-signals, whereas the CatSper-inhibitor RU1968 strongly inhibited progesterone-induced Ca2+-signals. In conclusion, this study supports the claim that the steroids estradiol, testosterone and hydrocortisone act agonistically on CatSper in human sperm cells, thereby mimicking the effect of progesterone, and that lupeol and pristimerin do not act as inhibitors of human CatSper.

Relationship between the seasonal changes in plasma testosterone and thyroxine concentrations with sperm cryoresistance in Gabon bucks

Gabon buck is a breed with little marked seasonality in our latitude (Uruguay, 35° SL). The role of thyroid hormones on the regulation of their seasonal reproductive activity and sperm cryoresistance is unknown. Seasonal changes in testosterone concentration can affect sperm variables, but the influence of testosterone changes on sperm cryoresistance in other species determines that the recommended time for freezing sperm does not coincide with the period with greater sperm fresh quality. The objectives of the present work were to (i) describe the thyroxine seasonal pattern in bucks in a subtropical area, and its association with annual changes in sperm variables; (ii) relate the seasonal changes of testosterone and thyroxine concentrations with the sperm cryoresistance. For one year, semen of 10 adult Gabon bucks was collected by electroejaculation every two weeks. After sperm selection, the sample was frozen. Testosterone and thyroxine concentrations varied according to the month (P < 0.0001). Testosterone reached the greatest values in April (P < 0.0001) and May (P < 0.0001) and thyroxine reached minimum values (P < 0.0001) in the same months. During these months, a negative correlation ratio (CR) was found between testosterone concentration and CR-functional membrane (R =  - 0.50; P < 0.0001). CR values for most sperm variables decreased during March-May, coinciding with the presence of maximum testosterone concentrations. In conclusion, high testosterone levels are associated with the worst sperm response to freezing-thawing process. Thyroxine concentrations have a strong seasonal pattern, but there was no relationship to sperm cryoresistance.

Progesterone, spermatozoa and reproduction: An updated review

The rapid effects of steroids on spermatozoa have been demonstrated for the first time more than three decades ago. Progesterone (P), which is present throughout the female genital tract with peaks of levels in the cumulus matrix surrounding the oocyte, has been shown to stimulate several sperm functions in vitro, including capacitation, hyperactivation, chemotaxis and acrosome reaction (AR). Besides an increase of intracellular calcium, P has been shown to activate other sperm signalling pathways including tyrosine phosphorylation of several sperm proteins. All these effects are mediated by extra-nuclear pathways likely involving interaction with molecules present on the sperm surface. In particular, the increase in intracellular calcium ([Ca²⁺]_i) in spermatozoa from human and several other mammalian species is mediated by the sperm specific calcium channel CatSper, whose expression and function are required for sperm hyperactive motility. P-mediated CatSper activation is indeed involved in promoting sperm hyperactivation, but the involvement of this channel in other P-stimulated sperm functions, such as AR and chemotaxis, is less clear and further studies are required to disclose all the involved pathways. In human spermatozoa, responsiveness to P in terms of [Ca²⁺]_i increase and AR is highly related to sperm fertilizing ability in vitro, suggesting that the steroid is a physiological inducer of AR during in vitro fertilization. In view of their physiological relevance, P-stimulated sperm functions are currently investigated to develop new tools to select highly performant spermatozoa for assisted reproduction.

Positive effect of progesterone on motility and velocity of fresh, vitrified without permeable cryoprotectants and frozen with permeable cryoprotectants human spermatozoa

Steroid hormone progesterone has been found to play an important role in the migration of spermatozoa through the reproductive tract, as well as to induce hyperactive motility and increase sperm velocity. The aim of this study was to examine whether progesterone could induce beneficial effects in vitrified and slow-frozen spermatozoa. During the research process, 50 semen samples were divided into three treatment groups; noncryopreserved, slow-freezing and vitrification. After thawing and an incubation period of 2 hr to induce capacitation, semen samples from each treatment group were treated with 50 nM, 25 nM progesterone and a control solution for 30 min. Thereafter, the sperm suspensions were examined manually to assess the proportion of viable and motile spermatozoa, as well as using the CASA to evaluate the velocity parameters. The results indicated a higher proportion of progressively motile spermatozoa in vitrified teratozoospermic samples and improved velocity parameters in slow-frozen normozoospermic and teratozoospermic samples. The main conclusion of this research was that the used progesterone concentration of 50 nM was sufficient to significantly improve the motility of vitrified teratozoospermic samples and velocity parameters of cryopreserved sperm samples. The present findings might have important implications in determining ways of improving the current low rates of motility in cryopreserved spermatozoa.

Modulation of Human Sperm Capacitation by Progesterone, Estradiol, and Luteinizing Hormone

Sperm residency in female reproductive tract is essential to undergo functional changes that allow the cell to encounter the oocyte and fertilize it. Those changes, known as capacitation, are modulated by molecules located in the uterotubal surface and fluids. During the fertile window, there is a notable increase in some reproductive hormones such as progesterone, estradiol, and luteinizing hormone in the female reproductive tract, so spermatozoa are exposed to these hormones in an environment that must favor gamete encountering and fusion. This spatiotemporal coincidence suggests that they are suitable candidates to modulate sperm function in order to synchronize the events that ultimately allow the success of fertilization. The presence of receptors for these hormones in the human sperm has been described, but their physiological relevance and mechanisms of action have been either subject of controversy or not properly investigated. This review intends to summarize the evidence that support the participation of these hormones in the regulation of sperm capacitation.

Steroid receptors and their ligands: effects on male gamete functions

In recent years a new picture of human sperm biology is emerging. It is now widely recognized that sperm contain nuclear encoded mRNA, mitochondrial encoded RNA and different transcription factors including steroid receptors, while in the past sperm were considered incapable of transcription and translation. One of the main targets of steroid hormones and their receptors is reproductive function. Expression studies on Progesterone Receptor, estrogen receptor, androgen receptor and their specific ligands, demonstrate the presence of these systems in mature spermatozoa as surface but also as nuclear conventional receptors, suggesting that both systemic and local steroid hormones, through sperm receptors, may influence male reproduction. However, the relationship between the signaling events modulated by steroid hormones and sperm fertilization potential as well as the possible involvement of the specific receptors are still controversial issues. The main line of this review highlights the current research in human sperm biology examining new molecular systems of response to the hormones as well as specific regulatory pathways controlling sperm cell fate and biological functions. Most significant studies regarding the identification of steroid receptors are reported and the mechanistic insights relative to signaling pathways, together with the change in sperm metabolism energy influenced by steroid hormones are discussed.The reviewed evidences suggest important effects of Progesterone, Estrogen and Testosterone and their receptors on spermatozoa and implicate the involvement of both systemic and local steroid action in the regulation of male fertility potential.

Nongenomic activation of spermatozoa by steroid hormones: facts and fictions

The rapid effects of steroids on spermatozoa have been demonstrated for the first time two decades ago. Progesterone (P), which is present throughout the female genital tract with peaks of levels in the cumulus matrix surrounding the oocyte, stimulates several sperm functions, including hyperactivation and acrosome reaction. These effects are mediated by an extranuclear pathway, as P stimulates an influx of calcium, the tyrosine phosphorylation of sperm proteins and other signalling cascades in a rapid manner. Whether these effects are receptor mediated and which receptors mediate these effects are still a matter of discussion despite all the efforts of the scientific community aimed at identifying them during the last 20 years. Although responsiveness to P is related to sperm fertilizing ability, the physiological role of P during the process of fertilization is discussed, and recent evidence points for a role of the steroid as a chemotactic agent for sperm. A similar situation applies for estrogens (E), which have been shown to induce direct effects on sperm by an extranuclear pathway. In particular, E appear to decrease acrosome reaction in response to P, exerting a role in ensuring an appropriate timing for sperm exocytosis during the process of fertilization.

Correlation of seminal parameters with serum lipid profile and sex hormones

The phospholipids and fatty acid composition of sperm are altered under the influence of serum lipids on impaired seminal parameters in infertile men. We studied the correlation of the serum lipid profile and sex hormone results of a group of infertile men with sperm characteristics of 18 infertile men. Semen samples were collected and analyzed. Serum fasted and post-meal glucose levels, insulin, total cholesterol, HDL, LDL, VLDL, triglyceride, FSH, LH, and testosterone measurements were performed. The increased serum VLDL, total triglyceride, and testosterone values were significantly correlated with decreased sperm motility. Serum FSH values were also inversely correlated with normal sperm morphology. The increased VLDL impaired seminal parameters; additionally, increased triglycerides may have deleterious effects on spermatogenesis. Deterioration is related with increased serum VLDL and triglyceride levels.

Progesterone induces hyperpolarization after a transient depolarization phase in human spermatozoa

Progesterone (P4) induces a membrane depolarization and various ion fluxes (chloride efflux, sodium and calcium influxes), which are required for the human sperm acrosome reaction (AR). By use of the potentiometric fluorescent dye DiSC3(5) and two different technical approaches, the present study aimed to quantify and further analyze P4-induced modifications in membrane potential in capacitated human spermatozoa. Spectrofluorimetric analysis revealed that the mean resting membrane potential of sperm was -58 +/- 2 mV (n = 12). When 10 microM P4 was added, the sperm membrane depolarized by approximately +15 mV, partly driven by a Cl- efflux. It subsequently repolarized to reach a significant lower potential than the initial resting potential in two thirds of the tested samples. The flow cytometry analysis showed a heterogeneous resting membrane potential and revealed that the depolarization-hyperpolarization events concerned only subpopulations, between 3% and 40% of the sperm cells according to the samples (n = 7). We hypothesize that P4 has a beneficial effect on the ability of zona pellucida to promote the AR in a sperm subpopulation by increasing the number of hyperpolarized cells presenting a membrane potential that is compatible with the opening of T-type calcium channels by subsequent zona pellucida-induced depolarization.

Receptors and channels regulating acrosome reactions

Prospective clinical studies informed by cloning and sequencing of sperm surface receptors and metal ion channels have elucidated critical early steps in the acrosome reaction that explain aspects of metal ion-related male infertility. Induction of the acrosome reaction is proposed to include non-nuclear progesterone receptor activation of Shaker-related sperm head voltage-gated potassium ion channels (VGKC). Men express VGKC isoforms with differing sensitivities to lead (Pb(2+)) inhibition, thus explaining interindividual variabilities in Pb(2+)-related male infertility. VGKC opening induces calcium (Ca(2+)) transients, and a signalling cascade induced by zona receptor aggregation requires an actin cytoskeleton created by the VGKC-induced Ca(2+) transients. Actin polymerization and stabilization, favoured by zinc (Zn(2+)) and depolymerized by cadmium (Cd(2+)), may mediate low Zn(2+) and high Cd(2+) infertile states. Zona receptor aggregation induces phosphotyrosine signals at sites, including sperm voltage-dependent Ca(2+) channels (VDCC), intermediate in electrophysiology between T- and L-type channels. Sperm surface VDCC localize at the sperm equatorial segment, the terminus of zona receptor translocation. Opening of VDCC admits a second Ca(2+) wave that activates phospholipase C phosphorylated in the zona receptor cascade. Phospholipase C induces fusogenic lipids and activates actin-severing proteins, depolymerizing the actin cytoskeleton and permitting apposition and fusion of acrosomal and plasma membranes.

Ion channels in sperm physiology

Fertilization is a matter of life or death. In animals of sexual reproduction, the appropriate communication between mature and competent male and female gametes determines the generation of a new individual. Ion channels are key elements in the dialogue between sperm, its environment, and the egg. Components from the outer layer of the egg induce ion permeability changes in sperm that regulate sperm motility, chemotaxis, and the acrosome reaction. Sperm are tiny differentiated terminal cells unable to synthesize protein and difficult to study electrophysiologically. Thus understanding how sperm ion channels participate in fertilization requires combining planar bilayer techniques, in vivo measurements of membrane potential, intracellular Ca2+ and intracellular pH using fluorescent probes, patch-clamp recordings, and molecular cloning and heterologous expression. Spermatogenic cells are larger than sperm and synthesize the ion channels that will end up in mature sperm. Correlating the presence and cellular distribution of various ion channels with their functional status at different stages of spermatogenesis is contributing to understand their participation in differentiation and in sperm physiology. The multi-faceted approach being used to unravel sperm ion channel function and regulation is yielding valuable information about the finely orchestrated events that lead to sperm activation, induction of the acrosome reaction, and in the end to the miracle of life.

Progesterone-mediated calcium influx and acrosome reaction of human spermatozoa: pharmacological investigation of T-type calcium channels

The mechanisms of the progesterone (P4)-activated Ca2+ influx and the relationship between the intracellular free Ca2+ concentration ([Ca2+]i) and the acrosome reaction (AR) were investigated in this study. We compared the [Ca2+]i of uncapacitated and capacitated human sperm populations in response to P4 stimulation; characterized the effects of the pharmacological agents pimozide and mibefradil, inhibitors of T-type voltage-operated calcium channels (VOCCT), on the P4-activated Ca2+ influx; and determined the effects of these drugs on the P4-initiated AR. Since pimozide can also inhibit calmodulin-dependent enzymes, we examined the effects of the calmodulin antagonist, calmidazolium, on the above-mentioned events. The basal [Ca2+]i and the amplitude of the P4-activated Ca2+ influx were significantly (p < 0.05) higher in capacitated sperm populations. Also, in capacitated sperm populations, all three pharmacological agents significantly (p < 0.05) inhibited the P4-activated Ca2+ influx (IC50): calmidazolium (0.7 microM) > pimozide (8 microM) > mibefradil (11 microM). By contrast, the effects of these drugs on the P4-initiated AR were varied: pimozide (10 and 20 microM) significantly (p < 0.05) increased the percentage of AR spermatozoa, calmidazolium was without effect, and mibefradil (20 microM) significantly (p < 0.05) inhibited the AR. These disparate results do not allow us to reach any definitive conclusion concerning the role of a sperm VOCCT in the mechanism of the P4-initiated AR. However, the differences between the [Ca2+]i and AR effects, in particular the inverse relationship in the case of pimozide, suggest a dissociation between the amplitude of the P4-stimulated Ca2+ signal and the downstream biological effect of that signal, the AR.

Voltage-dependent calcium influx in human sperm assessed by simultaneous optical detection of intracellular calcium and membrane potential

There are several physiological and pharmacological evidences indicating that opening of voltage dependent calcium channels play a crucial role in the induction of the acrosome reaction in mammalian sperm. In mature sperm, physiological inductors of the acrosome reaction such as ZP3, a zona pellucida protein, and the steroid hormone progesterone, induce depolarization and calcium influx, which are required for the acrosome reaction. In this paper, we describe a voltage-dependent calcium influx present in human sperm. We report an experimental procedure that allows measurement of intracellular calcium and membrane potential simultaneously using the fluorescent dyes DiSC3(5) and Fura-2. We found that in human uncapacitated sperm, depolarization induces a nifedipine-insensitive calcium influx that, in most cases, was transient. Calcium influx was observed in the range of -60 to -15 mV (the range tested). At resting membrane potential (around -40 mV), potassium addition depolarized and induced calcium influx, but when the depolarization was preceded by a hyperpolarization (induced with valinomycin), calcium influx was remarkably enhanced, suggesting that at -40 mV, channels are in a putative inactivated state. When sperm was incubated in medium without calcium, calcium restoration caused calcium influx that depended on voltage, and decayed between 1 and 2 min after depolarization. Unlike ram, mouse or bovine sperm, in which an alkalinization is required to induce calcium influx with potassium, the voltage-dependent calcium influx observed in human sperm did not require an increase in internal or external pH. However, we observed that ammonium, which increases intracellular pH, enhanced the voltage-dependent calcium influx about 90%. Furthermore, depolarization by itself caused a small increase in intracellular pH suggesting that pH can be regulated by membrane potential in human sperm.

Defective function of membrane potential (psi) on sperm of infertile men

Membrane potential across the plasma membrane (psi) of human sperm was determined by measuring the accumulation of triphenylmethylphosphonium (TPMP+). Washed sperm of fertile or infertile men were suspended, incubated in low-K+ and high-Na+ or high-K+ and low-Na+ medium, and allowed to take up the cation TPMP+ to a steady state (20 min at 37 degrees C). Under these conditions a psi of -75 +/- 6 mV was observed in sperm cells of fertile men, while in spermatozoa of idiopathic and asthenozoospermic infertile men, the psi was of 35 +/- 5 mV and 32 +/- 5 mV, respectively. Upon depolarization of the sperm membrane by K+, the transport system Na(+)-K+ in spermatozoa of idiopathic infertile men decreased 53 +/- 3.5%, while that in sperm cells asthenozoospermic men decreased 57 +/- 3% with respect to spermatozoa of fertile men. The results of these studies indicate their possible application to the diagnosis of male infertility.

Potential assays for sperm capacitation in mammals

Sperm capacitation is an essential process in fertilization. It apparently involves a large number of processes, the common denominator of which is that they donate to sperm the potential to undergo the acrosome reaction, i.e., to release proteolytic enzymes enabling sperm penetration through the egg coat. Although the phenomenon of capacitation has been known for more than 40 years, it is far from understood, and, consequently, there is, as yet, no operational definition of it. The lack of an assay to identify capacitated spermatozoa is both the cause and the effect of this situation. Here we critically review the major changes that are thought to occur during sperm capacitation, and assess their potential use as markers for the identification of capacitated spermatozoa.

Rapid non-genomic actions of progesterone stimulate Ca2+ influx and the acrosome reaction in human sperm

This review summarizes some recent findings in human sperm which show that progesterone and 17 alpha hydroxyprogesterone are able rapidly (within seconds) to elevate [Ca2+]i and elicit the acrosome reaction (AR) via a non-genomic cell surface receptor. Progesterone promotes a transient elevation in [Ca2+]i which is blocked by extracellular La3+ and Ni2+ and removal of extracellular Ca2+ following chelation with EGTA. Some studies suggest that polyamines, trypsin-like proteases, and progesterone receptor aggregation are involved in progesterone-induced Ca2+ influx and AR. The receptor is not stimulated by the potent synthetic progestigins (e.g. promegestone, norethynodrel, megestrol acetate, cyproterone acetate) and is weakly antagonized by the genomic anti-progestins RU 486 and ZK 98.299. The sedative-hypnotic 3 alpha hydroxyl A-ring reduced pregnane steroids, which are powerful activators of the GABAA Cl- channel, are weak activators of Ca2+ influx and the AR. These data suggest that human sperm have a cell surface steroid receptor which is unlike the genomic progesterone receptor and the GABAA Cl- channel steroid receptor.

A re-evaluation of the effects of gonadal steroids on neuronal activity in the male rat

Single unit activity (SUA) was recorded from 77 cells located in the arcuate nucleus (ARC) and medial preoptic area (MPA) of anesthetized, intact male rats. Animals were administered vehicle, testosterone (T; 5 or 50 micrograms) or 17 beta-estradiol (E; 0.5 microgram) intravenously and SUA was monitored for 8-12 min. T (50 micrograms) reduced SUA in 50% of ARC units and 44% of MPA units within 2.1 +/- 0.46 and 3.3 +/- 0.92 min, respectively. Inhibition of ARC SUA was more pronounced than MPA SUA. A small percentage (9%) of ARC units were excited by T. E reduced SUA in 29% of ARC units and 27% of MPA units. Single doses of 5 micrograms T did not affect ARC activity. However, when followed within 10 min by an additional dose of 5 or 50 micrograms T, 30% and 43% of ARC units were inhibited, respectively. Doses (10 micrograms) of T produced plasma T concentrations within physiological limits, although 50 micrograms doses produced supraphysiological T levels. Neither dose affected circulating LH concentrations. We conclude that physiological and supraphysiological concentrations of T can rapidly affect SUA within the ARC.

Action of cholinergic drugs on accumulation of TPMP+ on human spermatozoa

Neurotransmitters or antagonists with nicotinic- or acetylcholine-type receptors were utilized to measure the accumulation of the radiolabeled lipophilic cation triphenylmethylphosphonium (TPMP+) on the plasma membrane of human sperm. Washed sperm incubated in the presence of TPMP+ in a low-K+ or high-K+ medium were allowed to take up the cation to a steady state (i.e., 20 min at 37 degrees C). The difference between TPMP+ in each incubation medium was inserted in the Nernst equation yielding a resting membrane potential of -69 +/- 2 mV. The addition of nicotine or acetylcholine to the low-K+ and high-K+ medium induced a hyperpolarization of 17% and 20%, respectively. In the presence of propranolol the membrane potential decreased by 44%. The association of nicotinic or acetylcholine receptor on human sperm or adrenergic beta-receptor blocking agent may alter the ionic permeability of the membrane, possibly due to hyperpolarization or depolarization.

Hyperpolarization/depolarization on human spermatozoa

The accumulation of the lipophilic cation radiolabeled triphenylmethylphosphonium (TPMP+) was utilized to determine the resting membrane potential across the plasma membrane (psi) of human sperm. Washed sperm were suspended and incubated in low-K+ and high-K+ medium and allowed to take up the cation to a steady state (20 min at 37 degrees C). By using this differential, the value obtained was inserted in the Nernst equation and the value yielded a psi of -69 +/- 2 mV. When the Na+ or K+ concentration is high in the medium, the accumulation of TPMP+ in the membrane sperm cells was increased or decreased, respectively, inducing hyperpolarization and depolarization of the membrane 20% and 85%, respectively. The presence of divalent cations Zn++ and Mg++ in the incubation medium both induced a hyperpolarization of 10% and 8.6%, respectively. The addition of specific reagents such as p-chloromercuribenzenosulfonate and ethylenediaminetetraacetic acid sodium salt both decreased the psi 35% and 58%, respectively. The agents acting on the sperm cell membrane, such as dithiothreitol and progesterone, both induced hyperpolarization and depolarization of the membrane 16% and 40%, respectively. The presence of propranolol and L-alpha-lysophosphatidylcholine, which affect the ionic gradients present across the plasma membrane, both induced a depolarization from 43% and 92%, respectively. Finally the psi was glucose-dependent. The result of these studies was that, by the use of agents causing hyperpolarization or depolarization, we obtained changes in the psi of -83.4 +/- 2.2 mV, until -6 +/- 0.6 mV changes of -76.8 +/- 2 mV translated across the sperm cell membrane.