SLO2.1/NALCN Functional Complex Activity in Mouse Myometrial Smooth Muscle Cells During Pregnancy.

At the end of pregnancy, the uterus transitions from a quiescent to a highly contractile state. This is partly due to depolarization of the resting membrane potential in uterine (myometrial) smooth muscle cells (MSMCs). Experiments with human MSMCs showed that the membrane potential is regulated by a functional complex between the sodium (Na+)-activated potassium (K+) channel SLO2.1 and the Na+ Leak Channel Non-Selective (NALCN). In human MSMCs, Na+ entering through NALCN activates SLO2.1, leading to K+ efflux, membrane hyperpolarization (cells become more negative inside), and reduced contractility. Decreased SLO2.1/NALCN activity results in reduced K+ efflux, leading to membrane depolarization, Ca2+ influx via voltage-dependent calcium channels, and increased MSMC contractility. However, all of these experiments were performed with MSMCs isolated from women at term, so the role of the SLO2.1/NALCN complex early in pregnancy was speculative. To address this question here, we examined the role of the SLO2.1/NALCN complex in regulating mouse MSMC membrane potential across pregnancy. We report that Slo2.1 and Nalcn expression change along pregnancy, being more highly expressed in MSMCs from non-pregnant and early pregnant mice than in those from late-pregnant mice. Functional studies revealed that SLO2.1 channels mediate a significant portion of the K+ current in mouse MSMCs, particularly in cells from non-pregnant and early pregnant mice. Activation of SLO2.1 by Na+ influx through NALCN led to membrane hyperpolarization in MSMCs from early pregnancy but not in MSMCs from later pregnancy. Moreover, we found that the NALCN/SLO2.1 complex regulates intracellular Ca2+ responses more in MSMCs from non-pregnant and early pregnancy mice than in MSMCs from late pregnancy. Together, these findings reveal that the SLO2.1/NALCN functional complex is conserved between mouse and humans and functions throughout pregnancy. This work could open avenues for targeted pharmacological interventions in pregnancy-related complications.

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.

The sodium-proton exchangers sNHE and NHE1 control plasma membrane hyperpolarization in mouse sperm.

Sperm capacitation, crucial for fertilization, occurs in the female reproductive tract and can be replicated in vitro using a medium rich in bicarbonate, calcium, and albumin. These components trigger the cAMP-PKA signaling cascade, proposed to promote hyperpolarization of the mouse sperm plasma membrane through activation of SLO3 K+ channel. Hyperpolarization is a hallmark of capacitation: proper membrane hyperpolarization renders higher in vitro fertilizing ability, while Slo3 KO mice are infertile. However, the precise regulation of SLO3 opening remains elusive. Our study challenges the involvement of PKA in this event and reveals the role of Na+/H+ exchangers. During capacitation, calcium increase through CatSper channels activates NHE1, while cAMP directly stimulates the sperm-specific NHE, collectively promoting the alkalinization threshold needed for SLO3 opening. Hyperpolarization then feeds back Na+/H+ activity. Our work is supported by pharmacology, and a plethora of KO mouse models, and proposes a novel pathway leading to hyperpolarization.

Differential role of bovine serum albumin and HCO3- in the regulation of GSK3 alpha during mouse sperm capacitation.

To become fertile, mammalian sperm are required to undergo capacitation in the female tract or in vitro in defined media containing ions (e.g. HCO3 -, Ca2+, Na+, and Cl-), energy sources (e.g. glucose, pyruvate) and serum albumin (e.g. bovine serum albumin (BSA)). These different molecules initiate sequential and concomitant signaling pathways, leading to capacitation. Physiologically, capacitation induces changes in the sperm motility pattern (e.g. hyperactivation) and prepares sperm for the acrosomal reaction (AR), two events required for fertilization. Molecularly, HCO3 - activates the atypical adenylyl cyclase Adcy10 (aka sAC), increasing cAMP and downstream cAMP-dependent pathways. BSA, on the other hand, induces sperm cholesterol release as well as other signaling pathways. How these signaling events, occurring in different sperm compartments and with different kinetics, coordinate among themselves is not well established. Regarding the AR, recent work has proposed a role for glycogen synthase kinases (GSK3α and GSK3ß). GSK3α and GSK3ß are inactivated by phosphorylation of residues Ser21 and Ser9, respectively, in their N-terminal domain. Here, we present evidence that GSK3α (but not GSK3ß) is present in the anterior head and that it is regulated during capacitation. Interestingly, BSA and HCO3 - regulate GSK3α in opposite directions. While BSA induces a fast GSK3α Ser21 phosphorylation, HCO3 - and cAMP-dependent pathways dephosphorylate this residue. We also show that the HCO3--induced Ser21 dephosphorylation is mediated by hyperpolarization of the sperm plasma membrane potential (Em) and by intracellular pH alkalinization. Previous reports indicate that GSK3 kinases mediate the progesterone-induced AR. Here, we show that GSK3 inhibition also blocks the Ca2+ ionophore ionomycin-induced AR, suggesting a role for GSK3 kinases downstream of the increase in intracellular Ca2+ needed for this exocytotic event. Altogether, our data indicate a temporal and biphasic GSK3α regulation with opposite actions of BSA and HCO3 -. Our results also suggest that this regulation is needed to orchestrate the AR during sperm capacitation.

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 effect of Dnaaf5 gene dosage on primary ciliary dyskinesia phenotypes.

DNAAF5 is a dynein motor assembly factor associated with the autosomal heterogenic recessive condition of motile cilia, primary ciliary dyskinesia (PCD). The effects of allele heterozygosity on motile cilia function are unknown. We used CRISPR-Cas9 genome editing in mice to recreate a human missense variant identified in patients with mild PCD and a second, frameshift-null deletion in Dnaaf5. Litters with Dnaaf5 heteroallelic variants showed distinct missense and null gene dosage effects. Homozygosity for the null Dnaaf5 alleles was embryonic lethal. Compound heterozygous animals with the missense and null alleles showed severe disease manifesting as hydrocephalus and early lethality. However, animals homozygous for the missense mutation had improved survival, with partially preserved cilia function and motor assembly observed by ultrastructure analysis. Notably, the same variant alleles exhibited divergent cilia function across different multiciliated tissues. Proteomic analysis of isolated airway cilia from mutant mice revealed reduction in some axonemal regulatory and structural proteins not previously reported in DNAAF5 variants. Transcriptional analysis of mouse and human mutant cells showed increased expression of genes coding for axonemal proteins. These findings suggest allele-specific and tissue-specific molecular requirements for cilia motor assembly that may affect disease phenotypes and clinical trajectory in motile ciliopathies.

The effect of Dnaaf5 gene dosage on primary ciliary dyskinesia phenotypes.

DNAAF5 is a dynein motor assembly factor associated with the autosomal heterogenic recessive condition of motile cilia, primary ciliary dyskinesia (PCD). The effects of allele heterozygosity on motile cilia function are unknown. We used CRISPR-Cas9 genome editing in mice to recreate a human missense variant identified in patients with mild PCD and a second, frameshift null deletion in Dnaaf5 . Litters with Dnaaf5 heteroallelic variants showed distinct missense and null gene dosage effects. Homozygosity for the null Dnaaf5 alleles was embryonic lethal. Compound heterozygous animals with the missense and null alleles showed severe disease manifesting as hydrocephalus and early lethality. However, animals homozygous for the missense mutation had improved survival, with partial preserved cilia function and motor assembly observed by ultrastructure analysis. Notably, the same variant alleles exhibited divergent cilia function across different multiciliated tissues. Proteomic analysis of isolated airway cilia from mutant mice revealed reduction in some axonemal regulatory and structural proteins not previously reported in DNAAF5 variants. While transcriptional analysis of mouse and human mutant cells showed increased expression of genes coding for axonemal proteins. Together, these findings suggest allele-specific and tissue-specific molecular requirements for cilia motor assembly that may affect disease phenotypes and clinical trajectory in motile ciliopathies. Brief Summary: A mouse model of human DNAAF5 primary ciliary dyskinesia variants reveals gene dosage effects of mutant alleles and tissue-specific molecular requirements for cilia motor assembly.

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.

Discovery and Characterization of Multiple Classes of Human CatSper Blockers.

The cation channel of sperm (CatSper) is a validated target for nonhormonal male contraception, but it lacks selective blockers, hindering studies to establish its role in both motility and capacitation. Via an innovative calcium uptake assay utilizing human sperm we discovered novel inhibitors of CatSper function from a high-throughput screening campaign of 72,000 compounds. Preliminary SAR was established for seven hit series. HTS hits or their more potent analogs blocked potassium-induced depolarization and noncompetitively inhibited progesterone-induced CatSper activation. CatSper channel blockade was confirmed by patch clamp electrophysiology and these compounds inhibited progesterone- and prostaglandin E1-induced hyperactivated sperm motility. One of the hit compounds is a potent CatSper inhibitor with high selectivity for CatSper over hCav1.2, hNav1.5, moderate selectivity over hSlo3 and hERG, and low cytotoxicity and is therefore the most promising inhibitor identified in this study. These new CatSper blockers serve as useful starting points for chemical probe development and drug discovery efforts.

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.

Increased mitochondrial activity upon CatSper channel activation is required for mouse sperm capacitation.

To fertilize an oocyte, sperm must undergo several biochemical and functional changes known as capacitation. A key event in capacitation is calcium influx through the cation channel of sperm (CatSper). However, the molecular mechanisms of capacitation downstream of this calcium influx are not completely understood. Capacitation is also associated with an increase in mitochondrial oxygen consumption, and several lines of evidence indicate that regulated calcium entry into mitochondria increases the efficiency of oxidative respiration. Thus, we hypothesized that calcium influx through CatSper during capacitation increases mitochondrial calcium concentration and mitochondrial efficiency and thereby contributes to sperm hyperactivation and fertilization capacity. To test this hypothesis, we used high-resolution respirometry to measure mouse sperm mitochondrial activity. We also measured mitochondrial membrane potential, ATP/ADP exchange during capacitation, and mitochondrial calcium concentration in sperm from wild-type and CatSper knockout mice. We show that the increase in mitochondrial activity in capacitated wild-type sperm parallels the increase in mitochondrial calcium concentration. This effect is blunted in sperm from CatSper knockout mice. Importantly, these mechanisms are needed for optimal hyperactivation and fertilization in wild-type mice, as confirmed by using mitochondrial inhibitors. Thus, we describe a novel mechanism of sperm capacitation. This work contributes to our understanding of the role of mitochondria in sperm physiology and opens the possibility of new molecular targets for fertility treatments and male contraception.

Conserved Mechanism of Bicarbonate-Induced Sensitization of CatSper Channels in Human and Mouse Sperm.

To fertilize an egg, mammalian sperm must undergo capacitation in the female genital tract. A key contributor to capacitation is the calcium (Ca2+) channel CatSper, which is activated by membrane depolarization and intracellular alkalinization. In mouse epididymal sperm, membrane depolarization by exposure to high KCl triggers Ca2+ entry through CatSper only in alkaline conditions (pH 8.6) or after in vitro incubation with bicarbonate (HCO3 -) and bovine serum albumin (capacitating conditions). However, in ejaculated human sperm, membrane depolarization triggers Ca2+ entry through CatSper in non-capacitating conditions and at lower pH (< pH 7.4) than is required in mouse sperm. Here, we aimed to determine the mechanism(s) by which CatSper is activated in mouse and human sperm. We exposed ejaculated mouse and human sperm to high KCl to depolarize the membrane and found that intracellular Ca2+ concentration increased at pH 7.4 in sperm from both species. Conversely, intracellular Ca2+ concentration did not increase under these conditions in mouse epididymal or human epididymal sperm. Furthermore, pre-incubation with HCO3 - triggered an intracellular Ca2+ concentration increase in response to KCl in human epididymal sperm. Treatment with protein kinase A (PKA) inhibitors during exposure to HCO3 - inhibited Ca2+ concentration increases in mouse epididymal sperm and in both mouse and human ejaculated sperm. Finally, we show that soluble adenylyl cyclase and increased intracellular pH are required for the intracellular Ca2+ concentration increase in both human and mouse sperm. In summary, our results suggest that a conserved mechanism of activation of CatSper channels is present in both human and mouse sperm. In this mechanism, HCO3 - in semen activates the soluble adenylyl cyclase/protein kinase A pathway, which leads to increased intracellular pH and sensitizes CatSper channels to respond to membrane depolarization to allow Ca2+ influx. This indirect mechanism of CatSper sensitization might be an early event capacitation that occurs as soon as the sperm contact the semen.

Progesterone and estrogen regulate NALCN expression in human myometrial smooth muscle cells.

During pregnancy, the uterus transitions from a quiescent state to an excitable, highly contractile state to deliver the fetus. Two important contributors essential for this transition are hormones and ion channels, both of which modulate myometrial smooth muscle cell (MSMC) excitability. Recently, the sodium (Na+) leak channel, nonselective (NALCN), was shown to contribute to a Na+ leak current in human MSMCs, and mice lacking NALCN in the uterus had dysfunctional labor. Microarray data suggested that the proquiescent hormone progesterone (P4) and the procontractile hormone estrogen (E2) regulated this channel. Here, we sought to determine whether P4 and E2 directly regulate NALCN. In human MSMCs, we found that NALCN mRNA expression decreased by 2.3-fold in the presence of E2 and increased by 5.6-fold in the presence of P4. Similarly, E2 treatment decreased, and P4 treatment restored NALCN protein expression. Additionally, E2 significantly inhibited, and P4 significantly enhanced an NALCN-dependent leak current in MSMCs. Finally, we identified estrogen response and progesterone response elements (EREs and PREs) in the NALCN promoter. With the use of luciferase assays, we showed that the PREs, but not the ERE, contributed to regulation of NALCN expression. Our findings reveal a new mechanism by which NALCN is regulated in the myometrium and suggest a novel role for NALCN in pregnancy.

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.

Oxytocin can regulate myometrial smooth muscle excitability by inhibiting the Na+ -activated K+ channel, Slo2.1.

KEY POINTS: At the end of pregnancy, the uterus transitions from a quiescent state to a highly contractile state. This transition requires that the uterine (myometrial) smooth muscle cells increase their excitability, although how this occurs is not fully understood. We identified SLO2.1, a potassium channel previously unknown in uterine smooth muscle, as a potential significant contributor to the electrical excitability of myometrial smooth muscle cells. We found that activity of the SLO2.1 channel is negatively regulated by oxytocin via Gαq-protein-coupled receptor activation of protein kinase C. This results in depolarization of the uterine smooth muscle cells and calcium entry, which may contribute to uterine contraction. These findings provide novel insights into a previously unknown mechanism by which oxytocin may act to modulate myometrial smooth muscle cell excitability. Our findings also reveal a new potential pharmacological target for modulating uterine excitability. ABSTRACT: During pregnancy, the uterus transitions from a quiescent state to a more excitable contractile state. This is considered to be at least partly a result of changes in the myometrial smooth muscle cell (MSMC) resting membrane potential. However, the ion channels controlling the myometrial resting membrane potential and the mechanism of transition to a more excitable state have not been fully clarified. In the present study, we show that the sodium-activated, high-conductance, potassium leak channel, SLO2.1, is expressed and active at the resting membrane potential in MSMCs. Additionally, we report that SLO2.1 is inhibited by oxytocin binding to the oxytocin receptor. Inhibition of SLO2.1 leads to membrane depolarization and activation of voltage-dependent calcium channels, resulting in calcium influx. The results of the present study reveal that oxytocin may modulate MSMC electrical activity by inhibiting SLO2.1 potassium channels.

CatSper channels are regulated by protein kinase A.

Mammalian sperm must undergo capacitation as a preparation for entering into hyperactivated motility, undergoing the acrosome reaction, and acquiring fertilizing ability. One of the initial capacitation events occurs when sperm encounter an elevated HCO3- concentration. This anion activates the atypical adenylyl cyclase Adcy10, increases intracellular cAMP, and stimulates protein kinase A (PKA). Moreover, an increase in intracellular Ca2+ concentration ([Ca2+] i ) is essential for sperm capacitation. Although a cross-talk between cAMP-dependent pathways and Ca2+ clearly plays an essential role in sperm capacitation, the connection between these signaling events is incompletely understood. Here, using three different approaches, we found that CatSper, the main sperm Ca2+ channel characterized to date, is up-regulated by a cAMP-dependent activation of PKA in mouse sperm. First, HCO3- and the PKA-activating permeable compound 8-Br-cAMP induced an increase in [Ca2+] i , which was blocked by the PKA peptide inhibitor PKI, and H89, another PKA inhibitor, also abrogated the 8-Br-cAMP response. Second, HCO3- increased the membrane depolarization induced upon divalent cation removal by promoting influx of monovalent cations through CatSper channels, which was inhibited by PKI, H89, and the CatSper blocker HC-056456. Third, electrophysiological patch clamp, whole-cell recordings revealed that CatSper activity is up-regulated by HCO3- and by direct cAMP injection through the patch-clamp pipette. The activation by HCO3- and cAMP was also blocked by PKI, H89, Rp-cAMPS, and HC-056456, and electrophysiological recordings in sperm from CatSper-KO mice confirmed CatSper's role in these activation modes. Our results strongly suggest that PKA-dependent phosphorylation regulates [Ca2+] i homeostasis by activating CatSper channel complexes.

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.

A genetic variant of the sperm-specific SLO3 K+ channel has altered pH and Ca2+ sensitivities.

To fertilize an oocyte, sperm must first undergo capacitation in which the sperm plasma membrane becomes hyperpolarized via activation of potassium (K+) channels and resultant K+ efflux. Sperm-specific SLO3 K+ channels are responsible for these membrane potential changes critical for fertilization in mouse sperm, and they are only sensitive to pH i However, in human sperm, the major K+ conductance is both Ca2+- and pH i -sensitive. It has been debated whether Ca2+-sensitive SLO1 channels substitute for human SLO3 (hSLO3) in human sperm or whether human SLO3 channels have acquired Ca2+ sensitivity. Here we show that hSLO3 is rapidly evolving and reveal a natural structural variant with enhanced apparent Ca2+ and pH sensitivities. This variant allele (C382R) alters an amino acid side chain at a principal interface between the intramembrane-gated pore and the cytoplasmic gating ring of the channel. Because the gating ring contains sensors to intracellular factors such as pH and Ca2+, the effectiveness of transduction between the gating ring and the pore domain appears to be enhanced. Our results suggest that sperm-specific genes can evolve rapidly and that natural genetic variation may have led to a SLO3 variant that differs from wild type in both pH and intracellular Ca2+ sensitivities. Whether this physiological variation confers differences in fertility among males remains to be established.

Transient exposure to calcium ionophore enables in vitro fertilization in sterile mouse models.

Mammalian sperm acquire fertilizing capacity in the female tract in a process called capacitation. At the molecular level, capacitation requires protein kinase A activation, changes in membrane potential and an increase in intracellular calcium. Inhibition of these pathways results in loss of fertilizing ability in vivo and in vitro. We demonstrated that transient incubation of mouse sperm with Ca(2+) ionophore accelerated capacitation and rescued fertilizing capacity in sperm with inactivated PKA function. We now show that a pulse of Ca(2+) ionophore induces fertilizing capacity in sperm from infertile CatSper1 (Ca(2+) channel), Adcy10 (soluble adenylyl cyclase) and Slo3 (K(+) channel) KO mice. In contrast, sperm from infertile mice lacking the Ca(2+) efflux pump PMACA4 were not rescued. These results indicate that a transient increase in intracellular Ca(2+) can overcome genetic infertility in mice and suggest this approach may prove adaptable to rescue sperm function in certain cases of human male infertility.