Intracellular sodium changes during the speract response and the acrosome reaction in sea urchin sperm

Reviewing mathematical models of sperm signaling networks

Dave Garbers' work significantly contributed to our understanding of sperm's regulated motility, capacitation, and the acrosome reaction. These key sperm functions involve complex multistep signaling pathways engaging numerous finely orchestrated elements. Despite significant progress, many parameters and interactions among these elements remain elusive. Mathematical modeling emerges as a potent tool to study sperm physiology, providing a framework to integrate experimental results and capture functional dynamics considering biochemical, biophysical, and cellular elements. Depending on research objectives, different modeling strategies, broadly categorized into continuous and discrete approaches, reveal valuable insights into cell function. These models allow the exploration of hypotheses regarding molecules, conditions, and pathways, whenever they become challenging to evaluate experimentally. This review presents an overview of current theoretical and experimental efforts to understand sperm motility regulation, capacitation, and the acrosome reaction. We discuss the strengths and weaknesses of different modeling strategies and highlight key findings and unresolved questions. Notable discoveries include the importance of specific ion channels, the role of intracellular molecular heterogeneity in capacitation and the acrosome reaction, and the impact of pH changes on acrosomal exocytosis. Ultimately, this review underscores the crucial importance of mathematical frameworks in advancing our understanding of sperm physiology and guiding future experimental investigations.

Modular analysis of the control of flagellar Ca2+-spike trains produced by CatSper and CaV channels in sea urchin sperm

Intracellular calcium ([Ca²⁺]_i) is a basic and ubiquitous cellular signal controlling a wide variety of biological processes. A remarkable example is the steering of sea urchin spermatozoa towards the conspecific egg by a spatially and temporally orchestrated series of [Ca²⁺]_i spikes. Although this process has been an experimental paradigm for reproduction and sperm chemotaxis studies, the composition and regulation of the signalling network underlying the cytosolic calcium fluctuations are hitherto not fully understood. Here, we used a differential equations model of the signalling network to assess which set of channels can explain the characteristic envelope and temporal organisation of the [Ca²⁺]_i-spike trains. The signalling network comprises an initial membrane hyperpolarisation produced by an Upstream module triggered by the egg-released chemoattractant peptide, via receptor activation, cGMP synthesis and decay. Followed by downstream modules leading to intraflagellar pH (pH_i), voltage and [Ca²⁺]_i fluctuations. The Upstream module outputs were fitted to kinetic data on cGMP activity and early membrane potential changes measured in bulk cell populations. Two candidate modules featuring voltage-dependent Ca²⁺-channels link these outputs to the downstream dynamics and can independently explain the typical decaying envelope and the progressive spacing of the spikes. In the first module, [Ca²⁺]_i-spike trains require the concerted action of a classical Ca_V-like channel and a potassium channel, BK (Slo1), whereas the second module relies on pH_i-dependent CatSper dynamics articulated with voltage-dependent neutral sodium-proton exchanger (NHE). We analysed the dynamics of these two modules alone and in mixed scenarios. We show that the [Ca²⁺]_i dynamics observed experimentally after sustained alkalinisation can be reproduced by a model featuring the CatSper and NHE module but not by those including the pH-independent Ca_V and BK module or proportionate mixed scenarios. We conclude in favour of the module containing CatSper and NHE and highlight experimentally testable predictions that would corroborate this conclusion.

Fertilisation in marine invertebrates, such as the sea urchin, occurs during broadcast spawning events in which males and females of co-localised species ejaculate sperm and spawn eggs synchronously. During these events, spermatozoa have to find and fertilise conspecific eggs in the midst of all the other ones, which is a remarkable navigation and mating choice achievement. Sperm cells do this by navigating towards the source of species-specific peptides released by the egg, steered by spatial and temporally orchestrated peaks in intracellular calcium concentration that trigger sudden reorientations. How these calcium spikes are regulated and timed remains elusive. Different calcium channels have been implicated by indirect experimental evidence giving rise to a complex network of putative interacting components. We gained insight into the structure and function of this network by modelling it as a set of candidate modules that could be studied separately. By using this ‘divide and conquer’ approach to the complexity of the network, we could characterise the potential dynamics of each module and confront these dynamics with specific quantitative data. Our results indicate that the channel mediating calcium signals in sea urchin sperm is likely CatSper, a calcium channel necessary for human male fertility.

Indirect sexual selection drives rapid sperm protein evolution in abalone

Sexual selection can explain the rapid evolution of fertilization proteins, yet sperm proteins evolve rapidly even if not directly involved in fertilization. In the marine mollusk abalone, sperm secrete enormous quantities of two rapidly evolving proteins, lysin and sp18, that are stored at nearly molar concentrations. We demonstrate that this extraordinary packaging is achieved by associating into Fuzzy Interacting Transient Zwitterion (FITZ) complexes upon binding the intrinsically disordered FITZ Anionic Partner (FITZAP). FITZ complexes form at intracellular ionic strengths and, upon exocytosis into seawater, lysin and sp18 are dispersed to drive fertilization. NMR analyses revealed that lysin uses a common molecular interface to bind both FITZAP and its egg receptor VERL. As sexual selection alters the lysin-VERL interface, FITZAP coevolves rapidly to maintain lysin binding. FITZAP-lysin interactions exhibit a similar species-specificity as lysin-VERL interactions. Thus, tethered molecular arms races driven by sexual selection can generally explain rapid sperm protein evolution.

Network model predicts that CatSper is the main Ca2+ channel in the regulation of sea urchin sperm motility

Spermatozoa sea urchin swimming behaviour is regulated by small peptides from the egg outer envelope. Speract, such a peptide, after binding to its receptor in Strongylocentrotus purpuratus sperm flagella, triggers a signaling pathway that culminates with a train of intracellular calcium oscillations, correlated with changes in sperm swimming pattern. This pathway has been widely studied but not fully characterized. Recent work on Arbacia punctulata sea urchin spermatozoa has documented the presence of the Ca²⁺ CatSper channel in their flagella and its involvement in chemotaxis. However, if other calcium channels participate in chemotaxis remains unclear. Here, based on an experimentally-backed logical network model, we conclude that CatSper is fundamental in the S. purpuratus speract-activated sea urchin sperm signaling cascade, although other Ca²⁺ channels could still be relevant. We also present for the first time experimental corroboration of its active presence in S. purpuratus sperm flagella. We argue, prompted by in silico knock-out calculations, that CatSper is the main generator of calcium oscillations in the signaling pathway and that other calcium channels, if present, have a complementary role. The approach adopted here allows us to unveil processes, which are hard to detect exclusively by experimental procedures.

Sodium affects the sperm motility in the European eel

The role of seminal plasma sodium and activation media sodium on sperm motility was examined by selectively removing the element from these two media, in European eel sperm. Sperm size (sperm head area) was also measured using an ASMA (Automated Sperm Morphometry Analyses) system, in the different conditions. Intracellular sodium [Na(+)]i was quantitatively analyzed by first time in the spermatozoa from a marine fish species. Measurement of [Na(+)]i was done before and after motility activation, by Flow Cytometry, using CoroNa Green AM as a dye. Sperm motility activation induced an increase in [Na(+)]i, from 96.72mM in quiescent stage to 152.21mM post-activation in seawater. A significant decrease in sperm head area was observed post-activation in seawater. There was a notable reduction in sperm motility when sodium was removed from the seminal plasma, but not when it was removed from the activation media. Sodium removal was also linked to a significant reduction in sperm head area in comparison to the controls. Our results indicate that the presence of the ion Na(+) in the seminal plasma (or in the extender medium) is necessary for the preservation of sperm motility in European eel, probably because it plays a role in maintaining an appropriate sperm cell volume in the quiescent stage of the spermatozoa.

Speract, a sea urchin egg peptide that regulates sperm motility, also stimulates sperm mitochondrial metabolism

Sea urchin sperm have only one mitochondrion, that in addition to being the main source of energy, may modulate intracellular Ca(2+) concentration ([Ca(2+)]i) to regulate their motility and possibly the acrosome reaction. Speract is a decapeptide from the outer jelly layer of the Strongylocentrotus purpuratus egg that upon binding to its receptor in the sperm, stimulates sperm motility, respiration and ion fluxes, among other physiological events. Altering the sea urchin sperm mitochondrial function with specific inhibitors of this organelle, increases [Ca(2+)]i in an external Ca(2+) concentration ([Ca(2+)]ext)-dependent manner (Ardón, et al., 2009. BBActa 1787: 15), suggesting that the mitochondrion is involved in sperm [Ca(2+)]i homeostasis. To further understand the interrelationship between the mitochondrion and the speract responses, we measured mitochondrial membrane potential (ΔΨ) and NADH levels. We found that the stimulation of sperm with speract depolarizes the mitochondrion and increases the levels of NADH. Surprisingly, these responses are independent of external Ca(2+) and are due to the increase in intracellular pH (pHi) induced by speract. Our findings indicate that speract, by regulating pHi, in addition to [Ca(2+)]i, may finely modulate mitochondrial metabolism to control motility and ensure that sperm reach the egg and fertilize it.

Zn(2+) induces hyperpolarization by activation of a K(+) channel and increases intracellular Ca(2+) and pH in sea urchin spermatozoa

Zinc (Zn(2+)) has been recently recognized as a crucial element for male gamete function in many species although its detailed mechanism of action is poorly understood. In sea urchin spermatozoa, Zn(2+) was reported as an essential trace ion for efficient sperm motility initiation and the acrosome reaction by modulating intracellular pH (pHi). In this study we found that submicromolar concentrations of free Zn(2+) change membrane potential (Em) and increase the concentration of intracellular Ca(2+) ([Ca(2+)]i) and cAMP in Lytechinus pictus sperm. Our results indicate that the Zn(2+) response in sperm of this species mainly involves an Em hyperpolarization caused by K(+) channel activation. The pharmacological profile of the Zn(2+)-induced hyperpolarization indicates that the cGMP-gated K(+) selective channel (tetraKCNG/CNGK), which is crucial for speract signaling, is likely a main target for Zn(2+). Considering that Zn(2+) also induces [Ca(2+)]i fluctuations, our observations suggest that Zn(2+) activates the signaling cascade of speract, except for an increase in cGMP, and facilitates sperm motility initiation upon spawning. These findings provide new insights about the role of Zn(2+) in male gamete function.

In silico determination of the effect of multi-target drugs on calcium dynamics signaling network underlying sea urchin spermatozoa motility

The motility of spermatozoa of both Lytechinus pictus and Strongylocentrotus purpuratus sea urchin species is modulated by the egg-derived decapeptide speract via an oscillatory [Ca2+]-dependent signaling pathway. Comprehension of this pathway is hence directly related to the understanding of regulated sperm swimming. Niflumic acid (NFA), a nonsteroidal anti-inflammatory drug alters several ion channels. Though unspecific, NFA profoundly affects how sea urchin sperm respond to speract, increasing the [Ca2+]i oscillation period, amplitude, peak and average level values of the responses in immobilized and swimming cells. A previous logical network model we developed for the [Ca2+] dynamics of speract signaling cascade in sea urchin sperm allows integrated dissection of individual and multiple actions of NFA. Among the channels affected by NFA are: hyperpolarization-activated and cyclic nucleotide gated Na+ channels (HCN), [Ca2+]-dependent Cl- channels (CaCC) and [Ca2+]-dependent K+ channels (CaKC), all present in the sea urchin genome. Here, using our model we investigated the effect of blocking in silico HCN and CaCC channels suggested by experiments. Regarding CaKC channels, arguments can be provided for either their blockage or activation by NFA. Our study yielded two scenarios compliant with experimental observations: i) under CaKC inhibition, this [Ca2+]-dependent K+ channel should be different from the Slo1 channel and ii) under activation of the CaKC channel, another [Ca2+] channel not considered previously in the network is required, such as the pH-dependent CatSper channel. Additionally, our findings predict cause-effect relations resulting from a selective inhibition of those channels. Knowledge of these relations may be of consequence for a variety of electrophysiological studies and have an impact on drug related investigations. Our study contributes to a better grasp of the network dynamics and suggests further experimental work.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Discrete dynamics model for the speract-activated Ca2+ signaling network relevant to sperm motility

Understanding how spermatozoa approach the egg is a central biological issue. Recently a considerable amount of experimental evidence has accumulated on the relation between oscillations in intracellular calcium ion concentration ([Ca]) in the sea urchin sperm flagellum, triggered by peptides secreted from the egg, and sperm motility. Determination of the structure and dynamics of the signaling pathway leading to these oscillations is a fundamental problem. However, a biochemically based formulation for the comprehension of the molecular mechanisms operating in the axoneme as a response to external stimulus is still lacking. Based on experiments on the S. purpuratus sea urchin spermatozoa, we propose a signaling network model where nodes are discrete variables corresponding to the pathway elements and the signal transmission takes place at discrete time intervals according to logical rules. The validity of this model is corroborated by reproducing previous empirically determined signaling features. Prompted by the model predictions we performed experiments which identified novel characteristics of the signaling pathway. We uncovered the role of a high voltage-activated channel as a regulator of the delay in the onset of fluctuations after activation of the signaling cascade. This delay time has recently been shown to be an important regulatory factor for sea urchin sperm reorientation. Another finding is the participation of a voltage-dependent calcium-activated channel in the determination of the period of the fluctuations. Furthermore, by analyzing the spread of network perturbations we find that it operates in a dynamically critical regime. Our work demonstrates that a coarse-grained approach to the dynamics of the signaling pathway is capable of revealing regulatory sperm navigation elements and provides insight, in terms of criticality, on the concurrence of the high robustness and adaptability that the reproduction processes are predicted to have developed throughout evolution.

Mechanism regulating Ca2+-dependent mechanosensory behaviour in sea urchin spermatozoa

Flagellar movement of the sea urchin sperm is regulated by intracellular Ca(2+). Flagellasialin, a polysialic acid-containing glycoprotein, as well as other membrane proteins seems responsible for the Ca(2+) control. To elucidate the mechanism of Ca(2+) dynamics underlying flagellar movement, we analysed the sperm's mechanosensory behavioural responses by using microtechniques. In sea water containing 10 mM Ca(2+), the sperm swim in circular paths. When a mechanical stimulus was applied to the sperm head with a glass microstylus, the sperm showed a series of flagellar responses, consisting of a stoppage of beating (quiescence) and a recovery of swimming in a straight path, followed by swimming in a circular path again; as the result the sperm avoided the obstacle. Ca(2+)-imaging with Fluo-4 showed that the intracellular Ca(2+) was high in the quiescence and gradually decreased after that. The effects of blockers and antibodies against candidate components revealed that the Ca(2+) influx was induced by Ca(2+) channels and the Ca(2+) efflux was induced by a flagellasialin-related Ca(2+)-efflux system, plasma membrane Ca(2+)-ATPases and the K(+)-dependent Na(+)/Ca(2+) exchanger. The results show that the Ca(2+)-dependent mechanosensory behaviour of the sea urchin sperm is regulated by organized functioning of the membrane environment including the plasma membrane proteins and flagellasialin.

FRAP analysis of molecular diffusion inside sea-urchin spermatozoa

In sea-urchin spermatozoa, energy required for flagellar motility is provided by ATP diffusion from mitochondria located at the proximal ends of the flagella along with the creatine shuttle system. However, no direct analysis of the diffusion rates inside flagella has been carried out thus far. Using a FRAP (fluorescence recovery after photobleaching) technique, we determined the diffusion coefficients of fluorescein-derivatives (calcein, carboxyfluorescein and Oregon Green) to be 63-64 microm2 s(-1). Although these values are about one third of the estimates that were previously used for theoretical calculations, we concluded that the rate of ATP diffusion inside spermatozoa was high enough to support the continuous motility of sea-urchin sperm flagella if the creatine shuttle system is working. We also investigated the diffusion rate through the ;neck' region between the head and tail. When the head region of a calcein-loaded spermatozoon was photobleached, slow recovery of head fluorescence along with the decrease of fluorescence signal in the tail region was observed. It suggests that small molecules such as calcein (Mr, 622.54) can move beyond the boundary between the head and the flagellum. We expect that these findings regarding the diffusion properties inside spermatozoa will provide us with more general insights into the energy equilibrium and material transportation by passive diffusion inside eukaryotic cilia and flagella.

Rho-kinase (ROCK) in sea urchin sperm: its role in regulating the intracellular pH during the acrosome reaction

Sperm must undergo the acrosome reaction (AR) in order to fertilize the egg. In sea urchins, this reaction is triggered by the egg jelly (EJ) which, upon binding to its sperm receptor, induces increases in the ion permeability of the plasma membrane and changes in protein phosphorylation. Here, we demonstrated that the sperm expresses ROCK (approximately 135kDa), which is a serine/threonine protein kinase. ROCK localized, as RhoGTPase (Rho), in the acrosomal region, midpiece and flagellum. H-1152, a ROCK antagonist, inhibited the two cellular processes defining the AR: the acrosomal exocytosis and the actin polymerization. The ionophores nigericin and A23187 reversed the AR inhibition induced by H-1152, suggesting that ROCK functions at the level of the EJ-induced ion fluxes. Accordingly, H-1152 blocked 70% the intracellular alkalinization induced by EJ. These results indicate that EJ activates a Na+-H+ exchanger (NHE) in the sperm through a Rho/ROCK-dependent signaling pathway that culminates in the AR.

Altering the speract-induced ion permeability changes that generate flagellar Ca2+ spikes regulates their kinetics and sea urchin sperm motility

Speract, an egg-derived sperm-activating peptide, induces changes in intracellular Ca2+, Na+, pH, cAMP, cGMP, and membrane potential in sperm of the sea urchin Strongylocentrotus purpuratus. Ca2+ is a key regulator of motility in all sperm and, in many marine species, is required for generating turns interspersed with straighter swimming paths that are essential for chemotaxis towards the egg. We show that speract triggers a train of increases in flagellar Ca2+, and that each individual Ca2+ fluctuation induces a transient increase in flagellar asymmetry that leads to a turn. We also find that modifying the amplitude, duration and interval between individual Ca2+ fluctuations by treating sperm with niflumic acid, an inhibitor of Ca2+-activated Cl(-) channels, correspondingly alters the properties of the sperm turns. We conclude that Ca2+ entry through a fast flagellar pathway not only induces sperm turns, but the kinetics of Ca2+ entry may shape the nature of these turns, and that these kinetics are tuned by other channels, possibly including Cl(-) channels. In addition, the speract-induced changes in sperm motility closely resemble those seen during chemotaxis in other marine organisms, yet speract is not a chemoattractant. This implies the Ca2+-induced motility changes are necessary but not sufficient for chemotaxis.

Na+ /Ca2+ exchanger contributes to asterosap-induced elevation of intracellular Ca2+ concentration in starfish spermatozoa

Asterosap, a group of equally active isoforms of sperm-activating peptides from the egg jelly of the starfish Asterias amurensis, functions as a chemotactic factor for sperm. It transiently increases the intracellular cGMP level of sperm, which in turn induces a transient elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). Using a fluorescent Ca(2+)-sensitive dye, Fluo-4 AM, we measured the changes in sperm [Ca(2+)](i) in response to asterosap. KB-R7943 (KB), a selective inhibitor of Na(+)/Ca(2+) exchanger (NCX), significantly inhibited the asterosap-induced transient elevation of [Ca(2+)](i), suggesting that asterosap influences [Ca(2+)](i) through activation of a K+-dependent NCX (NCKX). An NCKX activity of starfish sperm also shows K(+) dependency like other NCKXs. Therefore, we cloned an NCKX from the starfish testes and predicted that it codes for a 616 amino acid protein that is a member of the NCKX family. Pharmacological evidence suggests that this exchanger participates in the asterosap-induced Ca(2+) entry into sperm.

Na+/Ca2+ exchanger modulates the flagellar wave pattern for the regulation of motility activation and chemotaxis in the ascidian spermatozoa

Ion channels and ion exchangers are known to be important participants in various aspects of sperm physiology, e.g. motility activation, chemotaxis, the maintenance of motility and the acrosome reaction in the sperm. We report here on a role of the K+ -independent Na+/Ca2+ exchanger (NCX) on ascidian sperm. Reverse-transcriptase PCR reveals that the NCX is expressed in the testis while immunoblotting and immunolocalization demonstrate that the NCX exists on the sperm in the ascidian Ciona savignyi and C. intestinalis. A potent blocker of the NCX, KB-R7943 was found to block sperm-activating and -attracting factor (SAAF)-induced motility activation, sperm motility and sperm chemotaxis. We further analyzed the effects of this blocker on motility parameters such as the flagellar waveform, curvature, beat frequency, amplitude and wavelength of the sperm flagella. Inhibition of the NCX caused two distinct effects: a low concentration of KB-R7943 induced symmetric bending, whereas a high concentration of KB-R7943 resulted in asymmetric flagellar bending. These findings suggest that the NCX plays important roles in the regulation of SAAF-induced sperm chemotaxis, motility activation and motility maintenance in the ascidian. This study provides new information toward an understanding of Ca2+ transport systems in sperm motility and chemotaxis.

Sperm channel diversity and functional multiplicity

Ion channels are extraordinarily efficient machines that move ions in diversely controlled manners, allowing cells to rapidly exchange information with the outside world and with other cells. Communication is the currency of fertilization, as it is of most fundamental cell signaling events. Ion channels are deeply involved in the dialogue between sperm, its surroundings, and the egg. How sperm swim, find the egg and fertilize it depend on ion permeability changes modulated by environmental cues and components of the egg outer layer. Different ion channels distinctly localized in these tiny, amazing cells perform specific decoding functions that shape the sophisticated behavior of sperm. It is not surprising that certain sperm ion channels are turning out to be unique. New strategies to characterize sperm ion transport have opened exciting possibilities to dissect sperm-egg signaling and unveil novel contraception targets.

Identification of voltage-dependent Ca2+channels in sea urchin sperm

Functional evidence indicates that voltage-dependent Ca2+ (Cav) channels participate in sea urchin sperm motility and the acrosome reaction (AR), however, their molecular identity remains unknown. We have identified transcripts for two Ca2+ channel alpha1 subunits in sea urchin testis similar in sequence to Cav1.2 and Cav2.3. Antibodies against rat Cav1.2 and Cav2.3 channels differentially label proteins in the flagella and acrosome of mature sea urchin sperm. The Cav channel antagonists nifedipine and nimodipine, which inhibit the AR, diminish the intracellular Ca2+ elevation induced by a K+-induced depolarization in valinomycin-treated sperm. These findings reveal that Cav1.2 and Cav2.3 channels could participate in motility and/or the AR in sea urchin sperm.

Fluorescence measurement of intracellular sodium concentration in single Escherichia coli cells

The energy-transducing cytoplasmic membrane of bacteria contains pumps and antiports maintaining the membrane potential and ion gradients. We have developed a method for rapid, single-cell measurement of the internal sodium concentration ([Na(+)](in)) in Escherichia coli using the sodium ion fluorescence indicator, Sodium Green. The bacterial flagellar motor is a molecular machine that couples the transmembrane flow of ions, either protons (H(+)) or sodium ions (Na(+)), to flagellar rotation. We used an E. coli strain containing a chimeric flagellar motor with H(+)- and Na(+)-driven components that functions as a sodium motor. Changing external sodium concentration ([Na(+)](ex)) in the range 1-85 mM resulted in changes in [Na(+)](in) between 5-14 mM, indicating a partial homeostasis of internal sodium concentration. There were significant intercell variations in the relationship between [Na(+)](in) and [Na(+)](ex), and the internal sodium concentration in cells not expressing chimeric flagellar motors was 2-3 times lower, indicating that the sodium flux through these motors is a significant fraction of the total sodium flux into the cell.

Sperm-Activating Peptide Induces Asymmetric Flagellar Bending in Sea Urchin Sperm

Speract, a sperm-activating peptide (SAP) from sea urchin eggs, induces various sperm responses including a transient increase in the intracellular Ca2+ concentration. However, it has not been clarified how speract modulates sperm motility and whether it functions as a chemoattractant. To confirm the effect of speract on sperm motility, we observed the flagellar bending response to speract in sperm of Hemicentrotus pulcherrimus, in experiments using caged speract and a lighting system for a microscope newly developed with a power LED. We found that speract induces increases in curvature of swimming paths and changes flagellar bending shape to asymmetric. These facts show that speract directly regulates flagellar motility, and suggest that speract-induced increases in intracellular Ca2+ concentration play an actual role in regulation of the flagellar movement.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Human sperm are endowed with putative voltage-dependent calcium channels (VDCC) that produce measurable increases in intracellular calcium concentration ([Ca2+]i) in response to membrane depolarization with potassium. These channels are blocked by nickel, inactivate in 1–2 min in calcium-deprived medium, and are remarkably stimulated by NH4Cl, suggesting a role for intracellular pH (pHi). In a previous work, we showed that calcium permeability through these channels increases approximately onefold during in vitro “capacitation,” a calcium-dependent process that sperm require to fertilize eggs. In this work, we have determined the pHidependence of sperm VDCC. Simultaneous depolarization and pHialkalinization with NH4Cl induced an [Ca2+]iincrease that depended on the amount of NH4Cl added. VDCC stimulation as a function of pHishowed a sigmoid curve in the 6.6–7.2 pHirange, with a half-maximum stimulation at pH ∼7.00. At higher pHi(≥7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pHibecause the [Ca2+]iincrease induced by progesterone, which opens a calcium permeability pathway that does not involve gating of VDCC, was unaffected by ammonium. The ratio of pHi-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pHi-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pHiincreased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pHialkalinization. Additionally, a calcium permeability pathway triggered exclusively by pHialkalinization was detected.

A sea urchin egg jelly peptide induces a cGMP-mediated decrease in sperm intracellular Ca(2+) before its increase

Speract, a sperm-activating peptide (SAP) from sea urchin eggs, increases the intracellular concentration of Ca(2+) ([Ca(2+)]i) and modulates sperm motility. We measured the initial sperm response to speract using its caged analog and observed, for the first time, a small but significant decrease in sperm [Ca(2+)]i before the increase. Both directions of the [Ca(2+)]i change were completely blocked in high K(+) seawater. Using membrane-permeant caged cyclic nucleotides (cNMP), only cGMP induced the decrease in [Ca(2+)]i although both cGMP and cAMP increased the [Ca(2+)]i. The decrease in the [Ca(2+)]i induced by cGMP was more notable following a second photolytic event, once [Ca(2+)]i had been elevated by an initial flash. This pattern of [Ca(2+)]i change was confirmed in individual sperm. These results together with pharmacological evidence suggest that the initial [Ca(2+)]i decrease is due to a Na(+)/Ca(2+) exchanger activity, stimulated by hyperpolarization mediated by K(+) efflux through cGMP-regulated K(+) channels.

Ligands and receptors mediating signal transduction in sea urchin spermatozoa

Sea urchins have long been a model system for the study of fertilization. Much has been learned about how sea urchin sperm locate and fertilize the egg. Sperm and eggs are spawned simultaneously into the surrounding seawater. Sperm signaling pathways lead to downstream events that ensure fertilization. Upon spawning, sperm must acquire motility and then they must swim towards or respond to the egg in some way. Finally, they must undergo a terminal exocytotic event known as the acrosome reaction that allows the sperm to bind to the vitelline layer of the egg and then to fuse with the egg plasma membrane. Motility is stimulated by exposure to seawater, while later events are orchestrated by factors from the egg. The sperm signaling pathways are exquisitely tuned to bring the sperm to the egg, bind, and fuse the two cells as quickly as possible.

Speract induces calcium oscillations in the sperm tail

Sea urchin sperm motility is modulated by sperm-activating peptides. One such peptide, speract, induces changes in intracellular free calcium concentration ([Ca2+]i). High resolution imaging of single sperm reveals that speract-induced changes in [Ca2+]i have a complex spatiotemporal structure. [Ca2+]i increases arise in the tail as periodic oscillations; [Ca2+]i increases in the sperm head lag those in the tail and appear to result from the summation of the tail signal transduction events. The period depends on speract concentration. Infrequent spontaneous [Ca2+]i transients were also seen in the tail of unstimulated sperm, again with the head lagging the tail. Speract-induced fluctuations were sensitive to membrane potential and calcium channel blockers, and were potentiated by niflumic acid, an anion channel blocker. 3-isobutyl-1-methylxanthine, which potentiates the cGMP/cAMP-signaling pathways, abolished the [Ca2+]i fluctuations in the tail, leading to a very delayed and sustained [Ca2+]i increase in the head. These data point to a model in which a messenger generated periodically in the tail diffuses to the head. Sperm are highly polarized cells. Our results indicate that a clear understanding of the link between [Ca2+]i and sperm motility will only be gained by analysis of [Ca2+]i signals at the level of the single sperm.