Sodium influx induced by external calcium chelation decreases human sperm motility

Trace and essential elements as vital components to improve the performance of the male reproductive system: Implications in cell signaling pathways

Successful male fertilization requires the main processes such as normal spermatogenesis, sperm capacitation, hyperactivation, and acrosome reaction. The progress of these processes depends on some endogenous and exogenous factors. So, the optimal level of ions and essential and rare elements such as selenium, zinc, copper, iron, manganese, calcium, and so on in various types of cells of the reproductive system could affect conception and male fertility rates. The function of trace elements in the male reproductive system could be exerted through some cellular and molecular processes, such as the management of active oxygen species, involvement in the action of membrane channels, regulation of enzyme activity, regulation of gene expression and hormone levels, and modulation of signaling cascades. In this review, we aim to summarize the available evidence on the role of trace elements in improving male reproductive performance. Also, special attention is paid to the cellular aspects and the involved molecular signaling cascades.

Human fertilization in vivo and in vitro requires the CatSper channel to initiate sperm hyperactivation

The infertility of many couples rests on an enigmatic dysfunction of the man's sperm. To gain insight into the underlying pathomechanisms, we assessed the function of the sperm-specific multisubunit CatSper-channel complex in the sperm of almost 2,300 men undergoing a fertility workup, using a simple motility-based test. We identified a group of men with normal semen parameters but defective CatSper function. These men or couples failed to conceive naturally and upon medically assisted reproduction via intrauterine insemination and in vitro fertilization. Intracytoplasmic sperm injection (ICSI) was, ultimately, required to conceive a child. We revealed that the defective CatSper function was caused by variations in CATSPER genes. Moreover, we unveiled that CatSper-deficient human sperm were unable to undergo hyperactive motility and, therefore, failed to penetrate the egg coat. Thus, our study provides the experimental evidence that sperm hyperactivation is required for human fertilization, explaining the infertility of CatSper-deficient men and the need of ICSI for medically assisted reproduction. Finally, our study also revealed that defective CatSper function and ensuing failure to hyperactivate represents the most common cause of unexplained male infertility known thus far and that this sperm channelopathy can readily be diagnosed, enabling future evidence-based treatment of affected couples.

Elusive physiological role of prostatic acid phosphatase (PAP): generation of choline for sperm motility via auto-and paracrine cholinergic signaling

Prostatic acid phosphatase (PAP) exists as two splice variants, secreted PAP and transmembrane PAP, the latter of which is implicated in antinociceptive signaling in dorsal root ganglia. However, PAP is predominantly expressed in the prostate gland and the physiological role of seminal PAP, first identified in 1938, is largely unknown. Here, the author proposes that PAP, following ejaculation, functions to hydrolyze phosphocholine (PC) in seminal fluid and generate choline, which is imported by sperm via a choline transporter and converted to acetylcholine (ACh) by choline acetyltransferase. Auto- and paracrine cholinergic signaling, or choline directly, may subsequently stimulate sperm motility via α7 nicotinic ACh receptors (nAChRs) and contractility of the female reproductive tract through muscarinic ACh receptors (mAChRs). Consistent with a role of PAP in cholinergic signaling, 1) seminal vesicles secrete PC, 2) the prostate gland secretes PAP, 3) PAP specifically catalyzes the hydrolysis of PC into inorganic phosphate and choline, 4) seminal choline levels increase post-ejaculation, 5) pharmacological inhibition of choline acetyltransferase inhibits sperm motility, 6) inhibition or genetic deletion of α7 nAChRs impairs sperm motility, and 7) mAChRs are expressed in the uterus and oviduct (fallopian tube). Notably, PAP does not degrade glycerophosphocholine (GPC), the predominant choline source in the semen of rats and other mammals. Instead, uterine GPC phosphodiesterases may liberate choline from seminal GPC. In summary, the author deduces that PAP in humans, and uterine GPC phosphodiesterases in other mammals, function to generate choline for sperm cholinergic signaling, which promotes sperm motility and possibly contractility of the female reproductive tract.

Sonomechanobiology: Vibrational stimulation of cells and its therapeutic implications

All cells possess an innate ability to respond to a range of mechanical stimuli through their complex internal machinery. This comprises various mechanosensory elements that detect these mechanical cues and diverse cytoskeletal structures that transmit the force to different parts of the cell, where they are transcribed into complex transcriptomic and signaling events that determine their response and fate. In contrast to static (or steady) mechanostimuli primarily involving constant-force loading such as compression, tension, and shear (or forces applied at very low oscillatory frequencies (≤ 1 Hz) that essentially render their effects quasi-static), dynamic mechanostimuli comprising more complex vibrational forms (e.g., time-dependent, i.e., periodic, forcing) at higher frequencies are less well understood in comparison. We review the mechanotransductive processes associated with such acoustic forcing, typically at ultrasonic frequencies (> 20 kHz), and discuss the various applications that arise from the cellular responses that are generated, particularly for regenerative therapeutics, such as exosome biogenesis, stem cell differentiation, and endothelial barrier modulation. Finally, we offer perspectives on the possible existence of a universal mechanism that is common across all forms of acoustically driven mechanostimuli that underscores the central role of the cell membrane as the key effector, and calcium as the dominant second messenger, in the mechanotransduction process.

High-throughput screening method for discovering CatSper inhibitors using membrane depolarization caused by external calcium chelation and fluorescent cell barcoding

The exclusive expression of CatSper in sperm and its critical role in sperm function makes this channel an attractive target for contraception. The strategy of blocking CatSper as a male, non-hormonal contraceptive has not been fully explored due to the lack of robust screening methods to discover novel and specific inhibitors. The reason for this lack of appropriate methodology is the structural and functional complexity of this channel. We have developed a high-throughput method to screen drugs with the capacity to block CatSper in mammalian sperm. The assay is based on removing external free divalent cations by chelation, inducing CatSper to efficiently conduct monovalent cations. Since Na⁺ is highly concentrated in the extracellular milieu, a sudden influx depolarizes the cell. Using CatSper1 KO sperm we demonstrated that this depolarization depends on CatSper function. A membrane potential (Em) assay was combined with fluorescent cell barcoding (FCB), enabling higher throughput flow cytometry based on unique fluorescent signatures of different sperm samples. These differentially labeled samples incubated in distinct experimental conditions can be combined into one tube for simultaneous acquisition. In this way, acquisition times are highly reduced, which is essential to perform larger screening experiments for drug discovery using live cells. Altogether, a simple strategy for assessing CatSper was validated, and this assay was used to develop a high-throughput drug screening for new CatSper blockers.

Cdc42 localized in the CatSper signaling complex regulates cAMP-dependent pathways in mouse sperm

Sperm acquire the ability to fertilize in a process called capacitation and undergo hyperactivation, a change in the motility pattern, which depends on Ca²⁺ transport by CatSper channels. CatSper is essential for fertilization and it is subjected to a complex regulation that is not fully understood. Here, we report that similar to CatSper, Cdc42 distribution in the principal piece is confined to four linear domains and this localization is disrupted in CatSper1-null sperm. Cdc42 inhibition impaired CatSper activity and other Ca²⁺ -dependent downstream events resulting in a severe compromise of the sperm fertilizing potential. We also demonstrate that Cdc42 is essential for CatSper function by modulating cAMP production by soluble adenylate cyclase (sAC), providing a new regulatory mechanism for the stimulation of CatSper by the cAMP-dependent pathway. These results reveal a broad mechanistic insight into the regulation of Ca²⁺ in mammalian sperm, a matter of critical importance in male infertility as well as in contraception.

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.

Male Decapacitation Factor SPINK3 Blocks Membrane Hyperpolarization and Calcium Entry in Mouse Sperm

Mammalian sperm acquire ability to fertilize through a process called capacitation, occurring after ejaculation and regulated by both female molecules and male decapacitation factors. Bicarbonate and calcium present in the female reproductive tract trigger capacitation in sperm, leading to acrosomal responsiveness and hyperactivated motility. Male decapacitating factors present in the semen avert premature capacitation, until detached from the sperm surface. However, their mechanism of action remains elusive. Here we describe for the first time the molecular basis for the decapacitating action of the seminal protein SPINK3 in mouse sperm. When present in the capacitating medium, SPINK3 inhibited Src kinase, a modulator of the potassium channel responsible for plasma membrane hyperpolarization. Lack of hyperpolarization affected calcium channels activity, impairing the acquisition of acrosomal responsiveness and blocking hyperactivation. Interestingly, SPINK3 acted only on non-capacitated sperm, as it did not bind to capacitated cells. Binding selectivity allows its decapacitating action only in non-capacitated sperm, without affecting capacitated cells.

The Effects of EGTA on the Quality of Fresh and Cryopreserved-Thawed Human Spermatozoa

Background:

Sperm cryopreservation-thawing process has damaging effects on the structure and function of sperm, namely cryoinjury. Calcium overload has been reported as a postulated mechanism for sperm damage during the first steps after thawing. This study was designed to assess the intracellular calcium (Ca²⁺_i) after cryopreservation and to clarify the role of a calcium chelator ethylene glycol-bis (2-aminoethyl ether)-N, N, N′, N′-tetraacetic acid (EGTA) on human sperm quality.

Methods:

Forty semen samples were obtained from fertile men (March 2017 to 2018). The samples were randomly divided into fresh (F) and cryopreserved-thawed (CT) groups. The F and CT samples were divided into control and 1 mM EGTA-treated groups. Sperm kinematics and membrane integrity were assessed. The reactive oxygen species (ROS) and adenosine triphosphate (ATP) were measured by luminescent methods. Ca²⁺_i, apoptotic rate, and mitochondrial membrane potential (MMP) were evaluated using flow cytometric methods. Data were compared using SPSS software, version 16.0 by ANOVA and Kruskal-Wallis test. P<0.05 was considered as significant.

Results:

Cryopreservation decreased sperm motility, viability, membrane integrity, Ca²⁺_i, MMP, and induced cell apoptosis and ROS production. EGTA could not protect the cryopreserved sperm from cryoinjury. It was found to have destructive effects on fresh sperm motility and viability (P=0.009) relative to cryopreserved sperm. ATP was reduced (P=0.02) and ROS production (P=0.0001) was increased in the EGTA-treated F and CT sperms.

Conclusion:

Despite Ca²⁺_i reduction by EGTA, it had no protective effects on fresh or cryopreserved sperm. We concluded that sperm cryoinjury was not dependent on calcium overload, and it was suggested that cryoinjury was mainly related to cell membranes damage.

Rotational motion and rheotaxis of human sperm do not require functional CatSper channels and transmembrane Ca2+ signaling

Navigation of sperm in fluid flow, called rheotaxis, provides long‐range guidance in the mammalian oviduct. The rotation of sperm around their longitudinal axis (rolling) promotes rheotaxis. Whether sperm rolling and rheotaxis require calcium (Ca²⁺) influx via the sperm‐specific Ca²⁺ channel CatSper, or rather represent passive biomechanical and hydrodynamic processes, has remained controversial. Here, we study the swimming behavior of sperm from healthy donors and from infertile patients that lack functional CatSper channels, using dark‐field microscopy, optical tweezers, and microfluidics. We demonstrate that rolling and rheotaxis persist in CatSper‐deficient human sperm. Furthermore, human sperm undergo rolling and rheotaxis even when Ca²⁺ influx is prevented. Finally, we show that rolling and rheotaxis also persist in mouse sperm deficient in both CatSper and flagellar Ca²⁺‐signaling domains. Our results strongly support the concept that passive biomechanical and hydrodynamic processes enable sperm rolling and rheotaxis, rather than calcium signaling mediated by CatSper or other mechanisms controlling transmembrane Ca²⁺ flux.

Role of human Hv1 channels in sperm capacitation and white blood cell respiratory burst established by a designed peptide inhibitor

Using a de novo peptide inhibitor, Corza6 (C6), we demonstrate that the human voltage-gated proton channel (hHv1) is the main pathway for H⁺ efflux that allows capacitation in sperm and permits sustained reactive oxygen species (ROS) production in white blood cells (WBCs). C6 was identified by a phage-display strategy whereby ∼1 million novel peptides were fabricated on an inhibitor cysteine knot (ICK) scaffold and sorting on purified hHv1 protein. Two C6 peptides bind to each dimeric channel, one on the S3-S4 loop of each voltage sensor domain (VSD). Binding is cooperative with an equilibrium affinity (K _d) of ∼1 nM at -50 mV. As expected for a VSD-directed toxin, C6 inhibits by shifting hHv1 activation to more positive voltages, slowing opening and speeding closure, effects that diminish with membrane depolarization.

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.

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

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

Lysine acetylation modulates mouse sperm capacitation

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. To gain fertilization competence, they need to undergo a series of modifications inside the female reproductive tract, known as capacitation. Capacitation involves several molecular events such as phosphorylation cascades, hyperpolarization of the plasma membrane and intracellular Ca²⁺ changes, which prepare the sperm to develop two essential features for fertilization competence: hyperactivation and acrosome reaction. Since sperm cells lack new protein biosynthesis, post-translational modification of existing proteins plays a crucial role to obtain full functionality. Here, we show the presence of acetylated proteins in murine sperm, which increase during capacitation. Pharmacological hyperacetylation of lysine residues in non-capacitated sperm induces activation of PKA, hyperpolarization of the sperm plasma membrane, CatSper opening and Ca²⁺ influx, all capacitation-associated molecular events. Furthermore, hyperacetylation of non-capacitated sperm promotes hyperactivation and prepares the sperm to undergo acrosome reaction. Together, these results indicate that acetylation could be involved in the acquisition of fertilization competence of mammalian sperm.

Molecular Basis of Human Sperm Capacitation

In the early 1950s, Austin and Chang independently described the changes that are required for the sperm to fertilize oocytes in vivo. These changes were originally grouped under name of “capacitation” and were the first step in the development of in vitro fertilization (IVF) in humans. Following these initial and fundamental findings, a remarkable number of observations led to characterization of the molecular steps behind this process. The discovery of certain sperm-specific molecules and the possibility to record ion currents through patch-clamp approaches helped to integrate the initial biochemical observation with the activity of ion channels. This is of particular importance in the male gamete due to the fact that sperm are transcriptionally inactive. Therefore, sperm must control all these changes that occur during their transit through the male and female reproductive tracts by complex signaling cascades that include post-translational modifications. This review is focused on the principal molecular mechanisms that govern human sperm capacitation with particular emphasis on comparing all the reported pieces of evidence with the mouse model.

Only a subpopulation of mouse sperm displays a rapid increase in intracellular calcium during capacitation

Mammalian sperm must undergo a functionally defined process called capacitation to be able to fertilize oocytes. They become capacitated in vivo by interacting with the female reproductive tract or in vitro in a defined capacitation medium that contains bovine serum albumin, calcium (Ca²⁺ ), and bicarbonate (HCO₃ ^- ). In this work, sperm were double stained with propidium iodide and the Ca²⁺ dye Fluo-4 AM and analyzed by flow cytometry to determine changes in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in individual live sperm. An increase in [Ca²⁺ ]_i was observed in a subpopulation of capacitated live sperm when compared with noncapacitated ones. Sperm exposed to the capacitating medium displayed a rapid increase in [Ca²⁺ ]_i within 1 min of incubation, which remained sustained for 90 min. These rise in [Ca²⁺ ]_i after 90 min of incubation in the capacitating medium was evidenced by an increase in the normalized median fluorescence intensity. This increase was dependent on the presence of extracellular Ca²⁺ and, at least in part, reflected the contribution of a new subpopulation of sperm with higher [Ca²⁺ ]_i . In addition, it was determined that the capacitation-associated [Ca²⁺ ]_i increase was dependent of CatSper channels, as sperm derived from CatSper knockout (CatSper KO) or incubated in the presence of CatSper inhibitors failed to increase [Ca²⁺ ]_i . Surprisingly, a minimum increase in [Ca²⁺ ]_i was also observed in CatSper KO sperm suggesting the existence of other Ca²⁺ transport systems. Altogether, these results indicate that a subpopulation of sperm increases [Ca²⁺ ]_i very rapidly during capacitation mainly due to a CatSper-mediated influx of extracellular Ca²⁺ .

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

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

Acrosomal alkalization triggers Ca2+ release and acrosome reaction in mammalian spermatozoa

The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca²⁺ permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca²⁺ store whose luminal pH is acidic, contains hydrolytic enzymes. It is known that acrosomal pH (pH_acr ) increases during capacitation and this correlates with spontaneous AR. Some AR inducers increase intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) through Ca²⁺ release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca²⁺ channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55-0396, two CatSper blockers, themselves elevate [Ca²⁺ ]_i by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pH_acr , trigger Ca²⁺ release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, μM concentrations of NNC55-0396 induced AR even in nominally Ca²⁺ free media. Our findings suggest that alkalization of the acrosome is critical step for Ca²⁺ release from the acrosome that leads to the acrosome reaction.

Na+ influx via Orai1 inhibits intracellular ATP-induced mTORC2 signaling to disrupt CD4 T cell gene expression and differentiation

T cell effector functions require sustained calcium influx. However, the signaling and phenotypic consequences of non-specific sodium permeation via calcium channels remain unknown. α-SNAP is a crucial component of Orai1 channels, and its depletion disrupts the functional assembly of Orai1 multimers. Here we show that α-SNAP hypomorph, hydrocephalus with hopping gait, Napa^hyh/hyh mice harbor significant defects in CD4 T cell gene expression and Foxp3 regulatory T cell (Treg) differentiation. Mechanistically, TCR stimulation induced rapid sodium influx in Napa^hyh/hyh CD4 T cells, which reduced intracellular ATP, [ATP]_i. Depletion of [ATP]_i inhibited mTORC2 dependent NFκB activation in Napa^hyh/hyh cells but ablation of Orai1 restored it. Remarkably, TCR stimulation in the presence of monensin phenocopied the defects in Napa^hyh/hyh signaling and Treg differentiation, but not IL-2 expression. Thus, non-specific sodium influx via bonafide calcium channels disrupts unexpected signaling nodes and may provide mechanistic insights into some divergent phenotypes associated with Orai1 function.

DOI:http://dx.doi.org/10.7554/eLife.25155.001

CRISP1 as a novel CatSper regulator that modulates sperm motility and orientation during fertilization

Ca(2+)-dependent mechanisms are critical for successful completion of fertilization. Here, we demonstrate that CRISP1, a sperm protein involved in mammalian fertilization, is also present in the female gamete and capable of modulating key sperm Ca(2+) channels. Specifically, we show that CRISP1 is expressed by the cumulus cells that surround the egg and that fertilization of cumulus-oocyte complexes from CRISP1 knockout females is impaired because of a failure of sperm to penetrate the cumulus. We provide evidence that CRISP1 stimulates sperm orientation by modulating sperm hyperactivation, a vigorous motility required for penetration of the egg vestments. Moreover, patch clamping of sperm revealed that CRISP1 has the ability to regulate CatSper, the principal sperm Ca(2+) channel involved in hyperactivation and essential for fertility. Given the critical role of Ca(2+) for sperm motility, we propose a novel CRISP1-mediated fine-tuning mechanism to regulate sperm hyperactivation and orientation for successful penetration of the cumulus during fertilization.

Regulation of axonemal motility in demembranated equine sperm

Equine in vitro fertilization is not yet successful because equine sperm do not effectively capacitate in vitro. Results of previous studies suggest that this may be due to failure of induction of hyperactivated motility in equine sperm under standard capacitating conditions. To evaluate factors directly affecting axonemal motility in equine sperm, we developed a demembranated sperm model and analyzed motility parameters in this model under different conditions using computer-assisted sperm analysis. Treatment of ejaculated equine sperm with 0.02% Triton X-100 for 30 sec maximized both permeabilization and total motility after reactivation. The presence of ATP was required for motility of demembranated sperm after reactivation, but cAMP was not. The calculated intracellular pH of intact equine sperm was 7.14 ± 0.07. Demembranated sperm showed maximal total motility at pH 7. Neither increasing pH nor increasing calcium levels, nor any interaction of the two, induced hyperactivated motility in demembranated equine sperm. Motility of demembranated sperm was maintained at free calcium concentrations as low as 27 pM, and calcium arrested sperm motility at much lower concentrations than those reported in other species. Calcium arrest of sperm motility was not accompanied by flagellar curvature, suggesting a failure of calcium to induce the tonic bend seen in other species and thought to support hyperactivated motility. This indicated an absence, or difference in calcium sensitivity, of the related asymmetric doublet-sliding proteins. These studies show a difference in response to calcium of the equine sperm axoneme to that reported in other species that may be related to the failure of equine sperm to penetrate oocytes in vitro under standard capacitating conditions. Further work is needed to determine the factors that stimulate hyperactivated motility at the axonemal level in equine sperm.

Evidence for the involvement of proline-rich tyrosine kinase 2 in tyrosine phosphorylation downstream of protein kinase A activation during human sperm capacitation

Sperm capacitation involves an increase in intracellular Ca(2+) concentration as well as in protein kinase A (PKA)-dependent protein tyrosine (Tyr) phosphorylation. Interestingly, in humans, a decrease in extracellular Ca(2+) concentration ([Ca(2+)]e) during capacitation induces an increase in Tyr phosphorylation indicating the complexity of Ca(2+) signaling during this process. In view of this, in the present study we further investigated the Ca(2+)-mediated signaling pathways implicated in Tyr phosphorylation during human sperm capacitation. Results revealed that sperm incubation in a medium without added Ca(2+) (⊖ Ca(2+)) increased Tyr phosphorylation but did not modify PKA-mediated phosphorylation. Moreover, inhibition of either PKA or Src family kinase signaling cascades in ⊖ Ca(2+) down-regulated both PKA substrate and Tyr phosphorylations, indicating that the [Ca(2+)]e effects on Tyr phosphorylation depend on PKA targets. Inhibition of calmodulin or Ser/Thr protein phosphatase 2B also increased Tyr phosphorylation without affecting PKA-mediated phosphorylation, supporting the potential role of these Ca(2+) downstream effectors in the increase in Tyr phosphorylation observed in ⊖ Ca(2+). Experiments aimed to identify the kinase responsible for these observations revealed the presence of proline-rich tyrosine kinase 2 (PYK2), a focal adhesion kinase (FAK) family member, in human sperm, and the use of PF431396, an FAK inhibitor, supported the involvement of PYK2 in Tyr phosphorylation downstream of PKA activation. Results also showed that PYK2 was activated in ⊖ Ca(2+) as well as during capacitation and that PF431396 affected capacitated sperm motility, acrosome reaction and ability to penetrate both mouse cumulus matrix and zona-free hamster eggs. Together, our observations support PYK2 as an intermediary component of Ca(2+) signaling between PKA-mediated and Tyr phosphorylations that is required for achieving functional human sperm capacitation.

Patch clamp studies of human sperm under physiological ionic conditions reveal three functionally and pharmacologically distinct cation channels

Whilst fertilizing capacity depends upon a K⁺ conductance (G_K) that allows the spermatozoon membrane potential (V_m) to be held at a negative value, the characteristics of this conductance in human sperm are virtually unknown. We therefore studied the biophysical/pharmacological properties of the K⁺ conductance in spermatozoa from normal donors held under voltage/current clamp in the whole cell recording configuration. Our standard recording conditions were designed to maintain quasi-physiological, Na⁺, K⁺ and Cl⁻ gradients. Experiments that explored the effects of ionic substitution/ion channel blockers upon membrane current/potential showed that resting V_m was dependent upon a hyperpolarizing K⁺ current that flowed via channels that displayed only weak voltage dependence and limited (∼7-fold) K⁺ versus Na⁺ selectivity. This conductance was blocked by quinidine (0.3 mM), bupivacaine (3 mM) and clofilium (50 µM), NNC55-0396 (2 µM) and mibefradil (30 µM), but not by 4-aminopyridine (2 mM, 4-AP). Progesterone had no effect upon the hyperpolarizing K⁺ current. Repolarization after a test depolarization consistently evoked a transient inward ‘tail current’ (I_Tail) that flowed via a second population of ion channels with poor (∼3-fold) K⁺ versus Na⁺ selectivity. The activity of these channels was increased by quinidine, 4-AP and progesterone. V_m in human sperm is therefore dependent upon a hyperpolarizing K⁺ current that flows via channels that most closely resemble those encoded by Slo3. Although 0.5 µM progesterone had no effect upon these channels, this hormone did activate the pharmacologically distinct channels that mediate I_Tail. In conclusion, this study reveals three functionally and pharmacologically distinct cation channels: Ik, I_Tail, I_CatSper.

The voltage-gated sodium channel nav1.8 is expressed in human sperm

The role of Na(+) fluxes through voltage-gated sodium channels in the regulation of sperm cell function remains poorly understood. Previously, we reported that several genes encoding voltage-gated Na(+) channels were expressed in human testis and mature spermatozoa. In this study, we analyzed the presence and function of the TTX-resistant VGSC α subunit Nav1.8 in human capacitated sperm cells. Using an RT-PCR assay, we found that the mRNA of the gene SCN10A, that encode Na v1.8, was abundantly and specifically expressed in human testis and ejaculated spermatozoa. The Na v1.8 protein was detected in capacitated sperm cells using three different specific antibodies against this channel. Positive immunoreactivity was mainly located in the neck and the principal piece of the flagellum. The presence of Na v1.8 in sperm cells was confirmed by Western blot. Functional studies demonstrated that the increases in progressive motility produced by veratridine, a voltage-gated sodium channel activator, were reduced in sperm cells preincubated with TTX (10 μM), the Na v1.8 antagonist A-803467, or a specific Na v1.8 antibody. Veratridine elicited similar percentage increases in progressive motility in sperm cells maintained in Ca(2+)-containing or Ca(2+)-free solution and did not induce hyperactivation or the acrosome reaction. Veratridine caused a rise in sperm intracellular Na(+), [Na(+)]i, and the sustained phase of the response was inhibited in the presence of A-803467. These results verify that the Na(+) channel Na v1.8 is present in human sperm cells and demonstrate that this channel participates in the regulation of sperm function.

Acute slices of mice testis seminiferous tubules unveil spontaneous and synchronous Ca2+ oscillations in germ cell clusters

Spermatogenic cell differentiation involves changes in the concentration of cytoplasmic Ca(2+) ([Ca(2+)]i); however, very few studies exist on [Ca(2+)]i dynamics in these cells. Other tissues display Ca(2+) oscillations involving multicellular functional arrangements. These phenomena have been studied in acute slice preparations that preserve tissue architecture and intercellular communications. Here we report the implementation of intracellular Ca(2+) imaging in a sliced seminiferous tubule (SST) preparation to visualize [Ca(2+)]i changes of living germ cells in situ within the SST preparation. Ca(2+) imaging revealed that a subpopulation of male germ cells display spontaneous [Ca(2+)]i fluctuations resulting from Ca(2+) entry possibly throughout Ca(V)3 channels. These [Ca(2+)]i fluctuation patterns are also present in single acutely dissociated germ cells, but they differ from those recorded from germ cells in the SST preparation. Often, spontaneous Ca(2+) fluctuations of spermatogenic cells in the SST occur synchronously, so that clusters of cells can display Ca(2+) oscillations for at least 10 min. Synchronous Ca(2+) oscillations could be mediated by intercellular communication via gap junctions, although intercellular bridges could also be involved. We also observed an increase in [Ca(2+)]i after testosterone application, suggesting the presence of functional Sertoli cells in the SST. In summary, we believe that the SST preparation is suitable to explore the physiology of spermatogenic cells in their natural environment, within the seminiferous tubules, in particular Ca(2+) signaling phenomena, functional cell-cell communication, and multicellular functional arrangements.