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

Membrane potential hyperpolarization: a critical factor in acrosomal exocytosis and fertilization in sperm within the female reproductive tract

Hyperpolarization of the membrane potential (Em), a phenomenon regulated by SLO3 channels, stands as a central feature in sperm capacitation-a crucial process conferring upon sperm the ability to fertilize the oocyte. In vitro studies demonstrated that Em hyperpolarization plays a pivotal role in facilitating the mechanisms necessary for the development of hyperactivated motility (HA) and acrosomal exocytosis (AE) occurrence. Nevertheless, the physiological significance of sperm Em within the female reproductive tract remains unexplored. As an approach to this question, we studied sperm migration and AE incidence within the oviduct in the absence of Em hyperpolarization using a novel mouse model established by crossbreeding of SLO3 knock-out (KO) mice with EGFP/DsRed2 mice. Sperm from this model displays impaired HA and AE in vitro. Interestingly, examination of the female reproductive tract shows that SLO3 KO sperm can reach the ampulla, mirroring the quantity of sperm observed in wild-type (WT) counterparts, supporting that the HA needed to reach the fertilization site is not affected. However, a noteworthy distinction emerges-unlike WT sperm, the majority of SLO3 KO sperm arrive at the ampulla with their acrosomes still intact. Of the few SLO3 KO sperm that do manage to reach the oocytes within this location, fertilization does not occur, as indicated by the absence of sperm pronuclei in the MII-oocytes recovered post-mating. In vitro, SLO3 KO sperm fail to penetrate the ZP and fuse with the oocytes. Collectively, these results underscore the vital role of Em hyperpolarization in AE and fertilization within their physiological context, while also revealing that Em is not a prerequisite for the development of the HA motility, essential for sperm migration through the female tract to the ampulla.

LRRC52 is likely a functional component of human KSper†

Completion of fertilization is orchestrated by various ion channels in sperm membrane. Hyperpolarization of membrane potential, an indispensable event during the capacitation process, is dominated by sperm potassium channel (KSper). In addition to sperm-specific SLO3, which forms the channel pore, the auxiliary subunit leucine-rich-repeat-containing protein 52 (LRRC52) is required to form mKSper to function under physiological conditions. However, in human sperm, although most evidence supports that hSLO3 is the pore-forming subunit, whether hLRRC52 contributes to hKSper conductance and modulates sperm function remains to be understood. Here, using an extracellular segment that is homologous between mice and humans as an antigen, we developed a polyclonal antibody designed as LID1 that specifically detected mLRRC52 and performed co-immunoprecipitation with mSLO3. Additionally, patch-clamp recordings of mouse sperm showed that, physiological activation of mKSper and sperm functions were dramatically attenuated after treatment with LID1, indicating that LID1 functionally disrupted the regulation of mLRRC52 on mKSper. Next, LID1 was used to investigate the significance of hLRRC52 for hKSper activation. As a result, hLRRC52 was expressed in human sperm and might be assembled with hSLO3. More importantly, LID1 inhibited hKSper currents and depolarized sperm membrane potential, supporting essential modulation of hLRRC52 in hKSper. Ca2+ signaling of human sperm was also compromised in the presence of LID1, which impaired sperm motility and acrosome reaction. Because LID1 specifically inhibited both mKSper and hKSper but not mCatSper or hCatSper, our results suggest that hLRRC52 functions as an important component of hKSper and regulates sperm physiological functions.

Na+/H+ Exchangers (NHEs) in Mammalian Sperm: Essential Contributors to Male Fertility

Na+/H+ exchangers (NHEs) are known to be important regulators of pH in multiple intracellular compartments of eukaryotic cells. Sperm function is especially dependent on changes in pH and thus it has been postulated that NHEs play important roles in regulating the intracellular pH of these cells. For example, in order to achieve fertilization, mature sperm must maintain a basal pH in the male reproductive tract and then alkalize in response to specific signals in the female reproductive tract during the capacitation process. Eight NHE isoforms are expressed in mammalian testis/sperm: NHE1, NHE3, NHE5, NHE8, NHA1, NHA2, NHE10, and NHE11. These NHE isoforms are expressed at varying times during spermatogenesis and localize to different subcellular structures in developing and mature sperm where they contribute to multiple aspects of sperm physiology and male fertility including proper sperm development/morphogenesis, motility, capacitation, and the acrosome reaction. Previous work has provided evidence for NHE3, NHE8, NHA1, NHA2, and NHE10 being critical for male fertility in mice and NHE10 has recently been shown to be essential for male fertility in humans. In this article we review what is known about each NHE isoform expressed in mammalian sperm and discuss the physiological significance of each NHE isoform with respect to male fertility.

Mitochondrial uncouplers impair human sperm motility without altering ATP content†

In human spermatozoa, the electrochemical potentials across the mitochondrial and plasma membranes are related to sperm functionality and fertility, but the exact role of each potential has yet to be clarified. Impairing sperm mitochondrial function has been considered as an approach to creating male or unisex contraceptives, but it has yet to be shown whether this approach would ultimately block the ability of sperm to reach or fertilize an egg. To investigate whether the mitochondrial and plasma membrane potentials are necessary for sperm fertility, human sperm were treated with two small-molecule mitochondrial uncouplers (niclosamide ethanolamine and BAM15) that depolarize membranes by inducing passive proton flow, and evaluated the effects on a variety of sperm physiological processes. BAM15 specifically uncoupled human sperm mitochondria while niclosamide ethanolamine induced proton current in the plasma membrane in addition to depolarizing the mitochondria. In addition, both compounds significantly decreased sperm progressive motility with niclosamide ethanolamine having a more robust effect. However, these uncouplers did not reduce sperm adenosine triphosphate (ATP) content or impair other physiological processes, suggesting that human sperm can rely on glycolysis for ATP production if mitochondria are impaired. Thus, systemically delivered contraceptives that target sperm mitochondria to reduce their ATP production would likely need to be paired with sperm-specific glycolysis inhibitors. However, since niclosamide ethanolamine impairs sperm motility through an ATP-independent mechanism, and niclosamide is FDA approved and not absorbed through mucosal membranes, it could be a useful ingredient in on-demand, vaginally applied contraceptives.

SLO3: A Conserved Regulator of Sperm Membrane Potential

Sperm cells must undergo a complex maturation process after ejaculation to be able to fertilize an egg. One component of this maturation is hyperpolarization of the membrane potential to a more negative value. The ion channel responsible for this hyperpolarization, SLO3, was first cloned in 1998, and since then much progress has been made to determine how the channel is regulated and how its function intertwines with various signaling pathways involved in sperm maturation. Although Slo3 was originally thought to be present only in the sperm of mammals, recent evidence suggests that a primordial form of the gene is more widely expressed in some fish species. Slo3, like many reproductive genes, is rapidly evolving with low conservation between closely related species and different regulatory and pharmacological profiles. Despite these differences, SLO3 appears to have a conserved role in regulating sperm membrane potential and driving large changes in response to stimuli. The effect of this hyperpolarization of the membrane potential may vary among mammalian species just as the regulation of the channel does. Recent discoveries have elucidated the role of SLO3 in these processes in human sperm and provided tools to target the channel to affect human fertility.

The SLC9C2 Gene Product (Na+/H+ Exchanger Isoform 11; NHE11) Is a Testis-Specific Protein Localized to the Head of Mature Mammalian Sperm

Na+/H+ exchangers (NHEs) are a family of ion transporters that regulate the pH of various cell compartments across an array of cell types. In eukaryotes, NHEs are encoded by the SLC9 gene family comprising 13 genes. SLC9C2, which encodes the NHE11 protein, is the only one of the SLC9 genes that is essentially uncharacterized. Here, we show that SLC9C2 exhibits testis/sperm-restricted expression in rats and humans, akin to its paralog SLC9C1 (NHE10). Similar to NHE10, NHE11 is predicted to contain an NHE domain, a voltage sensing domain, and finally an intracellular cyclic nucleotide binding domain. An immunofluorescence analysis of testis sections reveals that NHE11 localizes with developing acrosomal granules in spermiogenic cells in both rat and human testes. Most interestingly, NHE11 localizes to the sperm head, likely the plasma membrane overlaying the acrosome, in mature sperm from rats and humans. Therefore, NHE11 is the only known NHE to localize to the acrosomal region of the head in mature sperm cells. The physiological role of NHE11 has yet to be demonstrated but its predicted functional domains and unique localization suggests that it could modulate intracellular pH of the sperm head in response to changes in membrane potential and cyclic nucleotide concentrations that are a result of sperm capacitation events. If NHE11 is shown to be important for male fertility, it will be an attractive target for male contraceptive drugs due to its exclusive testis/sperm-specific expression.

A selective inhibitor of the sperm-specific potassium channel SLO3 impairs human sperm function

To fertilize an oocyte, the membrane potential of both mouse and human sperm must hyperpolarize (become more negative inside). Determining the molecular mechanisms underlying this hyperpolarization is vital for developing new contraceptive methods and detecting causes of idiopathic male infertility. In mouse sperm, hyperpolarization is caused by activation of the sperm-specific potassium (K+) channel SLO3 [C. M. Santi et al., FEBS Lett. 584, 1041-1046 (2010)]. In human sperm, it has long been unclear whether hyperpolarization depends on SLO3 or the ubiquitous K+ channel SLO1 [N. Mannowetz, N. M. Naidoo, S. A. S. Choo, J. F. Smith, P. V. Lishko, Elife 2, e01009 (2013), C. Brenker et al., Elife 3, e01438 (2014), and S. A. Mansell, S. J. Publicover, C. L. R. Barratt, S. M. Wilson, Mol. Hum. Reprod. 20, 392-408 (2014)]. In this work, we identified the first selective inhibitor for human SLO3-VU0546110-and showed that it completely blocked heterologous SLO3 currents and endogenous K+ currents in human sperm. This compound also prevented sperm from hyperpolarizing and undergoing hyperactivated motility and induced acrosome reaction, which are necessary to fertilize an egg. We conclude that SLO3 is the sole K+ channel responsible for hyperpolarization and significantly contributes to the fertilizing ability of human sperm. Moreover, SLO3 is a good candidate for contraceptive development, and mutation of this gene is a possible cause of idiopathic male infertility.

Sperm calcium flux and membrane potential hyperpolarization observed in the Mexican big-eared bat Corynorhinus mexicanus

Mammalian sperm capacitation involves biochemical and physiological changes, such as an increase in intracellular calcium ion concentration ([Ca2+]i), hyperpolarization of the plasma membrane potential and sperm hyperactivation, among others. These changes provide sperm with the ability to fertilize. In the bat Corynorhinus mexicanus, there is an asynchrony between spermatogenesis and sperm storage in the male with the receptivity of the female. For instance, in C. mexicanus, spermatogenesis occurs before the reproductive season. During the reproductive period, sperm are stored in the epididymis for a few months and the testis undergoes a regression, indicating low or almost null sperm production. Therefore, it is unclear whether the elements necessary for sperm fertilization success undergo maturation or preparation during epididymis storage. Here, we characterized pH-sensitive motility hyperactivation and Ca2+ influx in sperm, regulated by alkalinization and progesterone. In addition, by electrophysiological recordings, we registered currents that were stimulated by alkalinization and inhibited by RU1968 (a CatSper-specific inhibitor), strongly suggesting that these currents were evoked via CatSper, a sperm Ca2+-specific channel indispensable for mammalian fertilization. We also found hyperpolarization of the membrane potential, such as in other mammalian species, which increased according to the month of capture, reaching the biggest hyperpolarization during the mating season. In conclusion, our results suggest that C. mexicanus sperm have functional CatSper and undergo a capacitation-like process such as in other mammals, particularly Ca2+ influx and membrane potential hyperpolarization.

Repair mechanism of Wuwei Fuzheng Yijing formula in di-2-ethylhexyl phthalate-induced sperm DNA fragmentation in mice

CONTEXT

Di-2-ethylhexyl phthalate (DEHP), a known persistent organic pollutant, can increase the sperm DNA fragmentation index (DFI).

OBJECTIVE

To investigate the mechanism underlying the repair of DEHP-induced sperm DNA damage in mice by Wuwei Fuzheng Yijing (WFY) formula.

MATERIALS AND METHODS

The potential targets of WFY and sperm DNA fragment (SDF) were obtained from the TCMSP, BATMAN-TCM, OMIM and GeneCards. The protein-protein interaction (PPI) network, GO and KEGG pathway analyses of WFY-SDF were constructed. An animal model of DEHP-induced sperm DNA damage was replicated by gavage of SPF ICR (CD1) mice DEHP at 1 g/kg/d and treated with WFY at 8.92, 17.84 and 35.67 g/kg, respectively, for 60 d. Sperm DFI of each group was detected and compared. The target genes of WFY identified by transcriptomic and proteomic analyses were validated by qRT-PCR and Western blotting.

RESULTS

Network pharmacology pathway analysis indicated that PI3K/Akt was the potential target of WFY on SDF. The DFI of the DEHP group (25.48%) was significantly higher than that of the control group (4.02%). The high-dose WFY group (19.05%) exhibited the most significant repairing effect. The related pathways were PI3K/Akt and metabolic. Aass, Aldh1a7, GSTA3, betaine homocysteine S-methyltransferase (Bhmt), Mug2 and Svs1 were screened and Bhmt was validated.

DISCUSSION AND CONCLUSIONS

WFY can repair sperm DNA damage caused by DEHP, and the mechanism may be related to PI3K/Akt and metabolic pathways, and Bhmt. This provides a new direction for using traditional Chinese medicine to prevent and repair reproductive system injury caused by pollutants.

Elevated and Sustained Intracellular Calcium Signalling Is Necessary for Efficacious Induction of the Human Sperm Acrosome Reaction

Progesterone and prostaglandin E1 are postulated to trigger the human sperm acrosome reaction (AR). However, their reported efficacy is very variable which likely, in part, reflects the plethora of experimental conditions and methodologies used to detect this physiologically relevant event. The purpose of this study was to develop an assay for the robust induction and objective measurement of the complete AR. Sperm from healthy volunteers or patients undertaking IVF were treated with a variety of ligands (progesterone, prostaglandin E1 or NH4Cl, alone or in combinations). AR, motility and intracellular calcium measurements were measured using flow cytometry, computer-assisted sperm analysis (CASA) and fluorimetry, respectively. The AR was significantly increased by the simultaneous application of progesterone, prostaglandin E1 and NH4Cl, following an elevated and sustained intracellular calcium concentration. However, we observed notable inter- and intra-donor sample heterogeneity of the AR induction. When studying the patient samples, we found no relationship between the IVF fertilization rate and the AR. We conclude that progesterone and prostaglandin E1 alone do not significantly increase the percentage of live acrosome-reacted sperm. This assay has utility for drug discovery and sperm toxicology studies but is not predictive for IVF success.

Transcriptomics and proteomics analysis to explore the mechanism of Yishen Tongluo formula repairing sperm DNA damage in rats

The sperm DNA fragmentation index (DFI) is an objective indicator of male fertility. Currently, effective treatments for high sperm DFI are limited and traditional Chinese medicine (TCM) has certain advantages in this aspect. Yishen Tongluo formula (YSTL), a TCM formula, has been found to reduce DFI in patients. To better understand the mechanisms underlying its activity, we used transcriptomics and proteomics to analyse the potential target gene YSTL repairing tripterygium glycosides (TGs)-mediated sperm DNA damage in rats, followed by validation analyses using RT-qPCR and western blotting, which showed that relative to the control group, DFI was markedly elevated in the TGs group, but markedly lower in the YSTL group relative to the TGs group. KEGG pathway analysis of 119 differentially expressed genes and 158 DEPs identified using trend analysis revealed that they were enriched for apoptosis and base excision repair at the transcriptomic level and for microRNAs in cancer and complement and coagulation cascades at the proteomic level. Ttr and Pnpla2 were identified as potential target genes for YSTL. Our data show that YSTL can protect rat sperm DNA from TGs-induced damage, which may be related to apoptosis, DNA repair and other pathways, and the possible target genes are Ttr and Pnpla2.

Flow cytometry: past and future

Flow cytometry is a single-cell technology that measures scatter and fluorescence to establish a set of unique cellular properties. Flow cytometry is used in many areas of science, in particular biotechnology and medicine, but also in industrial applications. Flow cytometry can identify multiple phenotypic subsets from a mixture, select a single cell and even isolate that cell by a process called cell sorting. The field is currently undergoing dramatic changes. We are moving rapidly from the polychromic flow cytometry that has been the go-to technology for 45 years to spectral flow cytometry, which is now the most significant change in nearly half a century of flow cytometry. With change comes opportunity. Even spectral flow cytometry will morph into second-generation spectral flow cytometry within 5 years. New, exciting features will open up molecular diagnostics and physiology to flow cytometry.

A genetically targeted sensor reveals spatial and temporal dynamics of acrosomal calcium and sperm acrosome exocytosis

Secretion of the acrosome, a single vesicle located rostrally in the head of a mammalian sperm, through a process known as "acrosome exocytosis" (AE), is essential for fertilization. However, the mechanisms leading to and regulating this complex process are controversial. In particular, poor understanding of Ca2+ dynamics between sperm subcellular compartments and regulation of membrane fusion mechanisms have led to competing models of AE. Here, we developed a transgenic mouse expressing an Acrosome-targeted Sensor for Exocytosis (AcroSensE) to investigate the spatial and temporal Ca2+ dynamics in AE in live sperm. AcroSensE combines a genetically encoded Ca2+ indicator (GCaMP) fused with an mCherry indicator to spatiotemporally resolve acrosomal Ca2+ rise (ACR) and membrane fusion events, enabling real-time study of AE. We found that ACR is dependent on extracellular Ca2+ and that ACR precedes AE. In addition, we show that there are intermediate steps in ACR and that AE correlates better with the ACR rate rather than absolute Ca2+ amount. Finally, we demonstrate that ACR and membrane fusion progression kinetics and spatial patterns differ with different stimuli and that sites of initiation of ACR and sites of membrane fusion do not always correspond. These findings support a model involving functionally redundant pathways that enable a highly regulated, multistep AE in heterogeneous sperm populations, unlike the previously proposed "acrosome reaction" model.

SLO2.1/NALCN a sodium signaling complex that regulates uterine activity

Depolarization of the myometrial smooth muscle cell (MSMC) resting membrane potential is necessary for the uterus to transition from a quiescent state to a contractile state. The molecular mechanisms involved in this transition are not completely understood. Here, we report that a coupled system between the Na+-activated K+ channel (SLO2.1) and the non-selective Na+ leak channel (NALCN) determines the MSMC membrane potential. Our data indicate that Na+ entering through NALCN acts as an intracellular signaling molecule that activates SLO2.1. Potassium efflux through SLO2.1 hyperpolarizes the membrane. A decrease in SLO2.1/NALCN activity induces membrane depolarization, triggering Ca2+ entry through voltage-dependent Ca2+ channels and promoting contraction. Consistent with functional coupling, our data show that NALCN and SLO2.1 are in close proximity in human MSMCs. We propose that these arrangements of SLO2.1 and NALCN permit these channels to functionally regulate MSMC membrane potential and cell excitability and modulate uterine contractility.

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.

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.

Discrete Dynamic Model of the Mammalian Sperm Acrosome Reaction: The Influence of Acrosomal pH and Physiological Heterogeneity

The acrosome reaction (AR) is an exocytotic process essential for mammalian fertilization. It involves diverse physiological changes (biochemical, biophysical, and morphological) that culminate in the release of the acrosomal content to the extracellular medium as well as a reorganization of the plasma membrane (PM) that allows sperm to interact and fuse with the egg. In spite of many efforts, there are still important pending questions regarding the molecular mechanism regulating the AR. Particularly, the contribution of acrosomal alkalinization to AR triggering physiological conditions is not well understood. Also, the dependence of the proportion of sperm capable of undergoing AR on the physiological heterogeneity within a sperm population has not been studied. Here, we present a discrete mathematical model for the human sperm AR based on the physiological interactions among some of the main components of this complex exocytotic process. We show that this model can qualitatively reproduce diverse experimental results, and that it can be used to analyze how acrosomal pH (pH a ) and cell heterogeneity regulate AR. Our results confirm that a pH a increase can on its own trigger AR in a subpopulation of sperm, and furthermore, it indicates that this is a necessary step to trigger acrosomal exocytosis through progesterone, a known natural inducer of AR. Most importantly, we show that the proportion of sperm undergoing AR is directly related to the detailed structure of the population physiological heterogeneity.

high-throughput droplet vitrification of stallion sperm using permeating cryoprotective agents

Stallion sperm is typically cryopreserved using low cooling rates and low concentrations of cryoprotective agents (CPAs). The inevitable water-to-ice phase transition during cryopreservation is damaging and can be prevented using vitrification. Vitrification requires high cooling rates and high CPA concentrations. In this study, the feasibility of stallion sperm vitrification was investigated. A dual-syringe pump system was used to mix sperm equilibrated in a solution with a low concentration of CPAs, with a solution containing a high CPA concentration, and to generate droplets of a defined size (i.e., ~20 μL) that were subsequently cooled by depositing on an aluminum alloy block placed in liquid nitrogen. Mathematical modeling was performed to compute the heat transfer and rate of cooling. The minimum CPA concentration needed for vitrification was determined for various CPAs (glycerol, ethylene glycol, propylene glycol, dimethyl sulfoxide) and combinations thereof, while effects of droplet size and carrier solution were also identified. Sperm vitrification was eventually done using a glycerol/propylene glycol (1/1) mixture at a final concentration of 45% in buffered saline supplemented with 3% albumin and polyvinylpyrrolidon, while warming was done in standard diluent supplemented with 100 mM sucrose. The sperm concentration was found to greatly affect sperm membrane integrity after vitrification-and-warming, i.e., was found to be 21 ± 12% for 10 × 10⁶ sperm mL^-1 and 54 ± 8% for 1 × 10⁶ sperm mL^-1. However, an almost complete loss of sperm motility was observed. In conclusion, successful sperm vitrification requires establishing the narrow balance between droplet size, sperm concentration, CPA type and concentration, and exposure time.

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.

Machine-learning algorithm incorporating capacitated sperm intracellular pH predicts conventional in vitro fertilization success in normospermic patients

OBJECTIVE

To measure human sperm intracellular pH (pH_i) and develop a machine-learning algorithm to predict successful conventional in vitro fertilization (IVF) in normospermic patients.

DESIGN

Spermatozoa from 76 IVF patients were capacitated in vitro. Flow cytometry was used to measure sperm pH_i, and computer-assisted semen analysis was used to measure hyperactivated motility. A gradient-boosted machine-learning algorithm was trained on clinical data and sperm pH_i and membrane potential from 58 patients to predict successful conventional IVF, defined as a fertilization ratio (number of fertilized oocytes [2 pronuclei]/number of mature oocytes) greater than 0.66. The algorithm was validated on an independent set of data from 18 patients.

SETTING

Academic medical center.

PATIENT(S)

Normospermic men undergoing IVF. Patients were excluded if they used frozen sperm, had known male factor infertility, or used intracytoplasmic sperm injection only.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Successful conventional IVF.

RESULT(S)

Sperm pH_i positively correlated with hyperactivated motility and with conventional IVF ratio (n = 76) but not with intracytoplasmic sperm injection fertilization ratio (n = 38). In receiver operating curve analysis of data from the test set (n = 58), the machine-learning algorithm predicted successful conventional IVF with a mean accuracy of 0.72 (n = 18), a mean area under the curve of 0.81, a mean sensitivity of 0.65, and a mean specificity of 0.80.

CONCLUSION(S)

Sperm pH_i correlates with conventional fertilization outcomes in normospermic patients undergoing IVF. A machine-learning algorithm can use clinical parameters and markers of capacitation to accurately predict successful fertilization in normospermic men undergoing conventional IVF.

Time-Lapse Flow Cytometry: A Robust Tool to Assess Physiological Parameters Related to the Fertilizing Capability of Human Sperm

Plasma membrane (PM) hyperpolarization, increased intracellular pH (pH_i), and changes in intracellular calcium concentration ([Ca²⁺]_i) are physiological events that occur during human sperm capacitation. These parameters are potential predictors of successful outcomes for men undergoing artificial reproduction techniques (ARTs), but methods currently available for their determination pose various technical challenges and limitations. Here, we developed a novel strategy employing time-lapse flow cytometry (TLFC) to determine capacitation-related membrane potential (E_m) and pH_i changes, and progesterone-induced [Ca²⁺]_i increases. Our results show that TLFC is a robust method to measure absolute E_m and pH_i values and to qualitatively evaluate [Ca²⁺]_i changes. To support the usefulness of our methodology, we used sperm from two types of normozoospermic donors: known paternity (subjects with self-reported paternity) and no-known paternity (subjects without self-reported paternity and no known fertility problems). We found relevant differences between them. The incidences of membrane hyperpolarization, pH_i alkalinization, and increased [Ca²⁺]_i were consistently high among known paternity samples (100%, 100%, and 86%, respectively), while they varied widely among no-known paternity samples (44%, 17%, and 45%, respectively). Our results indicate that TLFC is a powerful tool to analyze key physiological parameters of human sperm, which pending clinical validation, could potentially be employed as fertility predictors.

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.

Sperm ion channels and transporters in male fertility and infertility

Mammalian sperm cells must respond to cues originating from along the female reproductive tract and from the layers of the egg in order to complete their fertilization journey. Dynamic regulation of ion signalling is, therefore, essential for sperm cells to adapt to their constantly changing environment. Over the past 15 years, direct electrophysiological recordings together with genetically modified mouse models and human genetics have confirmed the importance of ion channels, including the principal Ca²⁺-selective plasma membrane ion channel CatSper, for sperm activity. Sperm ion channels and membrane receptors are attractive targets for both the development of contraceptives and infertility treatment drugs. Furthermore, in this era of assisted reproductive technologies, understanding the signalling processes implicated in defective sperm function, particularly those arising from genetic abnormalities, is of the utmost importance not only for the development of infertility treatments but also to assess the overall health of a patient and his children. Future studies to improve reproductive health care and overall health care as a function of the ability to reproduce should include identification and analyses of gene variants that underlie human infertility and research into fertility-related molecules.

Membrane Potential Assessment by Fluorimetry as a Predictor Tool of Human Sperm Fertilizing Capacity

Mammalian sperm acquire the ability to fertilize eggs by undergoing a process known as capacitation. Capacitation is triggered as the sperm travels through the female reproductive tract. This process involves specific physiological changes such as rearrangement of the cell plasma membrane, post-translational modifications of certain proteins, and changes in the cellular permeability to ions - with the subsequent impact on the plasma membrane potential (Em). Capacitation-associated Em hyperpolarization has been well studied in mouse sperm, and shown to be both necessary and sufficient to promote the acrosome reaction (AR) and fertilize the egg. However, the relevance of the sperm Em upon capacitation on human fertility has not been thoroughly characterized. Here, we performed an extensive study of the Em change during capacitation in human sperm samples using a potentiometric dye in a fluorimetric assay. Normospermic donors showed significant Em hyperpolarization after capacitation. Em values from capacitated samples correlated significantly with the sperm ability to undergo induced AR, highlighting the role of hyperpolarization in acrosomal responsiveness, and with successful in vitro fertilization (IVF) rates. These results show that Em hyperpolarization could be an indicator of human sperm fertilizing capacity, setting the basis for the use of Em values as a robust predictor of the success rate of IVF.