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.

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.

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.

The INCREASE project: Intelligent Collections of food-legume genetic resources for European agrofood systems.

Food legumes are crucial for all agriculture-related societal challenges, including climate change mitigation, agrobiodiversity conservation, sustainable agriculture, food security and human health. The transition to plant-based diets, largely based on food legumes, could present major opportunities for adaptation and mitigation, generating significant co-benefits for human health. The characterization, maintenance and exploitation of food-legume genetic resources, to date largely unexploited, form the core development of both sustainable agriculture and a healthy food system. INCREASE will implement, on chickpea (Cicer arietinum), common bean (Phaseolus vulgaris), lentil (Lens culinaris) and lupin (Lupinus albus and L. mutabilis), a new approach to conserve, manage and characterize genetic resources. Intelligent Collections, consisting of nested core collections composed of single-seed descent-purified accessions (i.e., inbred lines), will be developed, exploiting germplasm available both from genebanks and on-farm and subjected to different levels of genotypic and phenotypic characterization. Phenotyping and gene discovery activities will meet, via a participatory approach, the needs of various actors, including breeders, scientists, farmers and agri-food and non-food industries, exploiting also the power of massive metabolomics and transcriptomics and of artificial intelligence and smart tools. Moreover, INCREASE will test, with a citizen science experiment, an innovative system of conservation and use of genetic resources based on a decentralized approach for data management and dynamic conservation. By promoting the use of food legumes, improving their quality, adaptation and yield and boosting the competitiveness of the agriculture and food sector, the INCREASE strategy will have a major impact on economy and society and represents a case study of integrative and participatory approaches towards conservation and exploitation of crop genetic resources.

A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction.

KEY POINTS: Accumulation of inorganic phosphate (Pi ) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of Pi into SR are fully established.  We showed that Pi promoted Ca2+ release at concentrations below 10 mm and decreased it at higher concentrations. This decrease correlated well with that of [Ca2+ ]SR .  Pre-treatment of permeabilized myofibres with 2 mm Cl- channel blocker 9-anthracenecarboxylic acid (9AC) inhibited both effects of Pi .  The biphasic dependence of Ca2+ release on [Pi ] is explained by a direct effect of Pi acting on the SR Ca2+ release channel, combined with the intra-SR precipitation of Ca2+ salts. The effects of 9AC demonstrate that Pi enters the SR via a Cl- pathway of an as-yet-undefined molecular nature. ABSTRACT: Fatiguing exercise causes hydrolysis of phosphocreatine, increasing the intracellular concentration of inorganic phosphate (Pi ). Pi diffuses into the sarcoplasmic reticulum (SR) where it is believed to form insoluble Ca2+ salts, thus contributing to the impairment of Ca2+ release. Information on the Pi entrance pathway is still lacking. In amphibian muscles endowed with isoform 3 of the RyR channel, Ca2+ spark frequency is correlated with the Ca2+ load of the SR and can be used to monitor this variable. We studied the effects of Pi on Ca2+ sparks in permeabilized fibres of the frog. Relative event frequency (f/fref ) rose with increasing [Pi ], reaching 2.54 ± 1.6 at 5 mm, and then decreased monotonically, reaching 0.09 ± 0.03 at [Pi ] = 80 mm. Measurement of [Ca2+ ]SR confirmed a decrease correlated with spark frequency at high [Pi ]. A large [Ca2+ ]SR surge was observed upon Pi removal. Anion channels are a putative path for Pi into the SR. We tested the effect of the chloride channel blocker 9-anthracenecarboxylic acid (9AC) on Pi entrance. 9AC (400 µm) applied to the cytoplasm produced a non-significant increase in spark frequency and reduced the Pi effects on this parameter. Fibre treatment with 2 mm 9AC in the presence of high cytoplasmic Mg2+ suppressed the effects of Pi on [Ca2+ ]SR and spark frequency up to 55 mm [Pi ]. These results suggest that chloride channels (or transporters) provide the main pathway of inorganic phosphate into the SR and confirm that Pi impairs Ca2+ release by accumulating and precipitating with Ca2+ inside the SR, thus contributing to myogenic fatigue.

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.

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.

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.

Meta-QTL for resistance to white mold in common bean.

White mold, caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary, is a major disease that limits common bean production and quality worldwide. The host-pathogen interaction is complex, with partial resistance in the host inherited as a quantitative trait with low to moderate heritability. Our objective was to identify meta-QTL conditioning partial resistance to white mold from individual QTL identified across multiple populations and environments. The physical positions for 37 individual QTL were identified across 14 recombinant inbred bi-parental populations (six new, three re-genotyped, and five from the literature). A meta-QTL analysis of the 37 QTL was conducted using the genetic linkage map of Stampede x Red Hawk population as the reference. The 37 QTL condensed into 17 named loci (12 previously named and five new) of which nine were defined as meta-QTL WM1.1, WM2.2, WM3.1, WM5.4, WM6.2, WM7.1, WM7.4, WM7.5, and WM8.3. The nine meta-QTL had confidence intervals ranging from 0.65 to 9.41 Mb. Candidate genes shown to express under S. sclerotiorum infection in other studies, including cell wall receptor kinase, COI1, ethylene responsive transcription factor, peroxidase, and MYB transcription factor, were found within the confidence interval for five of the meta-QTL. The nine meta-QTL are recommended as potential targets for MAS for partial resistance to white mold in common bean.

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

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