A sustained increase in intracellular Ca(2+) is required for the acrosome reaction in sea urchin sperm

Three-Dimensional Ultrastructural and Volume Analysis of the Redundant Nuclear Envelope of Developing and Matured Sperm in Mice

The ultrastructure of the nuclear envelope (NE) and redundant NE (RNE) of the spermatozoon cannot be observed in detail using conventional electron microscopy. Thus, this study aimed to employ transmission electron microscopy (TEM) and focused ion beam/scanning electron microscopy (FIB/SEM) tomography to fill this research gap. Male mice aged 13 weeks were deeply anesthetized, and the testes and vas deferens were extracted and processed for electron microscopy. In round spermatids, the acrosomal vesicle compressed the nucleus, and the acrosomal center was depressed. The nucleoli concentrated on the contralateral side of the acrosome formation site. In mature spermatozoa, the RNE accumulated in the neck with the residual bodies. The NE pores exhibited a hexagonal pattern. The body surface area and volume of the nuclei of spermatids and spermatozoa in each maturation phase were analyzed using FIB/SEM tomography. The body surface area and volume of the nuclei decreased during spermatid maturation into spermatozoa. The RNE converged at the sperm neck and possessed a honeycomb structure. The method used revealed that the nuclei of spermatids gradually condense as they mature into spermatozoa. This method may be used to analyze small tissues, such as RNE, and detect morphological abnormalities in microtissues, such as spermatozoa.

Ca2+ signaling in mammalian spermatozoa

Calcium is an essential ion which regulates sperm motility, capacitation and the acrosome reaction (AR), three processes necessary for successful fertilization. The AR enables the spermatozoon to penetrate into the egg. In order to undergo the AR, the spermatozoon must reside in the female reproductive tract for several hours, during which a series of biochemical transformations takes place, collectively called capacitation. An early event in capacitation is relatively small elevation of intracellular Ca²⁺ (in the nM range) and bicarbonate, which collectively activate the soluble adenylyl cyclase to produce cyclic-AMP; c-AMP activates protein kinase A (PKA), leading to indirect tyrosine phosphorylation of proteins. During capacitation, there is an increase in the membrane-bound phospholipase C (PLC) which is activated prior to the AR by relatively high increase in intracellular Ca²⁺ (in the μM range). PLC catalyzes the hydrolysis of phosphatidyl-inositol-4,5-bisphosphate (PIP₂) to diacylglycerol and inositol-trisphosphate (IP₃), leading to activation of protein kinase C (PKC) and the IP₃-receptor. PKC activates a Ca²⁺- channel in the plasma membrane, and IP₃ activates the Ca²⁺- channel in the outer acrosomal membrane, leading to Ca²⁺ depletion from the acrosome. As a result, the plasma-membrane store-operated Ca²⁺ channel (SOCC) is activated to increase cytosolic Ca²⁺ concentration, enabling completion of the acrosome reaction. The hydrolysis of PIP₂ by PLC results in the release and activation of PIP₂-bound gelsolin, leading to F-actin dispersion, an essential step prior to the AR. Ca²⁺ is also involved in the regulation of sperm motility. During capacitation, the sperm develops a unique motility pattern called hyper-activated motility (HAM) which is essential for successful fertilization. The main Ca²⁺-channel that mediates HAM is the sperm-specific CatSper located in the sperm tail.

Differences and Similarities: The Richness of Comparative Sperm Physiology

Species preservation depends on the success of fertilization. Sperm are uniquely equipped to fulfill this task, and, although several mechanisms are conserved among species, striking functional differences have evolved to contend with particular sperm-egg environmental characteristics. This review highlights similarities and differences in sperm strategies, with examples within internal and external fertilizers, pointing out unresolved issues.

Ca2+ efflux via plasma membrane Ca2+-ATPase mediates chemotaxis in ascidian sperm

When a spermatozoon shows chemotactic behavior, transient [Ca2+]i increases in the spermatozoon are induced by an attractant gradient. The [Ca2+]i increase triggers a series of stereotypic responses of flagellar waveforms that comprise turning and straight-swimming. However, the molecular mechanism of [Ca2+]i modulation controlled by the attractants is not well defined. Here, we examined receptive mechanisms for the sperm attractant, SAAF, in the ascidian, Ciona intestinalis, and identified a plasma membrane Ca2+-ATPase (PMCA) as a SAAF-binding protein. PMCA is localized in sperm flagella membranes and seems to interact with SAAF through basic amino acids located in the second and third extracellular loops. ATPase activity of PMCA was enhanced by SAAF, and PMCA inhibitors, 5(6)-Carboxyeosin diacetate and Caloxin 2A1, inhibited chemotactic behavior of the sperm. Furthermore, Caloxin 2A1 seemed to inhibit efflux of [Ca2+]i in the sperm, and SAAF seemed to competitively reduce the effect of Caloxin 2A1. On the other hand, chemotactic behavior of the sperm was disordered not only at low-Ca2+, but also at high-Ca2+ conditions. Thus, PMCA is a potent candidate for the SAAF receptor, and direct control of Ca2+ efflux via PMCA is a fundamental mechanism to mediate chemotactic behavior in the ascidian spermatozoa.

Toxic effects of Pb2+ entering sperm through Ca2+ channels in the freshwater crab Sinopotamon henanense

Lead (Pb) is a heavy metal that can damage animal sperm. To study the effects of Pb on calcium homeostasis and calcium channel in the sperm of freshwater crab Sinopotamon henanense, the induction of acrosome reaction (AR) and acrosin activity were investigated when crabs were exposed to different Pb concentrations (0, 3.675, 7.35, 14.7, 29.4 and 58.8mg/L) for 3, 5 and 7 d separately. Fluorescent probe Fluo-3/AM was loaded into the sperm, and [Ca²⁺] in the sperm was measured by fluorescence microscopy and using microplate reader. The calmodulin (CaM) concentration was measured by ELISA method. Verapamil (VRP), a calcium channel blocker, was used to evaluate whether Pb can enter the sperm through calcium channels leading to sperm damage. After sperm were exposed at 50μg/L VRP, 100μg/L Pb, 50μg/L VRP+100μg/L Pb, 1000μg/L Pb and 50μg/L VRP+1000μg/L Pb for 1h in vitro，sperm quality parameters (sperm survival and sperm DNA integrity) and levels of parameters indicating oxidative stress (protein carbonylation [PCO] and malondialdehyde [MDA]) were measured. Our data showed that Pb reduced the induction of acrosome reaction (AR), down-regulated the acrosin activity, decreased the intracellular concentration of Ca²⁺ and elevated CaM concentration. Compared to controls, Pb alone induced significant stress, as reflected by decreasing sperm survival and sperm DNA integrity, and increasing PCO and MDA contents. In the presence of VRP, 100μg/L Pb-induced stresses were reduced, all the measured parameters in the sperm exposed at 100μg/L Pb returned to control levels. Our results indicate that Pb enters the sperm of the crab S. henanense through calcium channels, the inhibition of which blocks Pb-induced stresses such as sperm quality decline and oxidative damage.

Optimized protocols to analyze sphingosine-1-phosphate signal transduction pathways during acrosomal exocytosis in human sperm

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. We have recently reported that sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis (Suhaiman L et al., J Biol Chem 285:1630-16314, 2010). Acrosomal exocytosis in mammalian sperm is a regulated secretion with unusual characteristics. We therefore employed biochemical functional assays to assess the sphingolipid signaling in both permeabilized and nonpermeabilized sperm. The exocytosis of the acrosomal content is regulated by Ca(2+). During exocytosis, changes in [Ca(2+)]i occur induced by either Ca(2+)-influx or Ca(2+)-mobilization from intracellular stores. By using single cell [Ca(2+)] measurements, we detected intracellular Ca(2+) changes after sphingosine 1-phosphate treatment. Additionally, measuring sphingosine kinase activity, we determined that sphingosine 1-phosphate levels increase after an exocytotic stimulus.This chapter is designed to provide the user with sufficient background to analyze sphingosine 1--phosphate signal transduction pathways during acrosomal exocytosis in human sperm.

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.

Effect of oviductal secretion components on the fertilizing capacity of amphibian sperm: biological and ultrastructural studies

The present study was carried out to analyze the fertilization-supporting activity of Rhinella arenarum egg-jelly components. Spermatozoa were treated with diffusible factor (DF) constituted by the components released from the jelly coat into deionized water or with full jelly (FJ) containing all the components secreted by the oviductal pars convoluta (PC) during the transit of the oocytes through the duct, or with washed jelly (WJ) constituted only by structural components. Both jellies were solubilized by ultraviolet irradiation. These gametes were used for ultrastructural and biological studies in order to determine the acrosome state and the fertilizing capacity. Additional experiments were performed by using Ca(2+), a diffusible cation present in R. arenarum jelly envelopes. Results demonstrated a marked increase in the acrosome reaction (AR) of sperm treated with FJ or DF compared to the controls (Ringer's solution), no significant differences being observed between both treatments, while WJ showed low AR percentages similar to the ones obtained with the controls. The addition of Ca(2+) induced an increase in this parameter in a dose-dependent manner, although the values reached with FJ or DF were not attained. The results of the "in vitro" fertilization show a strong inverse association to the acrosome reaction (AR) rate. Treatment with Ca(2+) at the concentration present in the jelly (6.3 ± 0.9 mM) inhibited the fertilizing capacity as a function of incubation time, showing that at 2 min there was a decrease in the fertilization percentages compared to 10% Ringer's. Data indicated that Ca(2+) present in jelly is involved in the AR induction but suggests the possible participation of other diffusible and/or structural components of the oviductal secretion in this process.

Flow-cytometric analyses of viability biomarkers in pesticide-exposed sperm of three aquatic invertebrates

Toxicity studies on sperm often use fertilization success as the end point. This type of assay can be affected by sperm density, egg quality, and sperm-egg compatibility. Testing sperm viability biomarkers with flow cytometry is a fast, high-throughput technique for seminal analysis. In this study, we detected sperm viability biomarkers with several fluorescent reporter dyes using flow cytometry in three aquatic invertebrates (Crassostrea virginica, Dreissena polymorpha, and Lytechinus variegatus) after exposure to a pesticide and herbicide. The pesticide, Bayluscide, appeared to affect mitochondrial membrane potential in the sperm of all three species, as measured with MitoTracker Red CMXRos. A decrease in the percentage of sperm stained with SYBR-14 (indicating uncompromised plasma membrane) was observed in C. virginica and D. polymorpha sperm exposed to Bayluscide, but propidium iodide staining (indicating compromised plasma membranes) appeared to be inhibited by Bayluscide. Acrosome-reacted sperm, as measured by FITC-PNA, decreased after Bayluscide exposure in C. virginica and D. polymorpha sperm. The herbicide, Roundup Ready To-Use-Plus, did not affect the overall percentages of sperm stained with MitoTracker but did cause an increase in MitoTracker fluorescence intensity at 16 mg/L in D. polymorpha. Roundup also caused significant decreases in SYBR-14 staining, significant increases in propidium iodide staining, and significant increases in FITC-PNA staining in D. polymorpha sperm. By not having to rely on egg availability and optimal sperm density, sperm toxicity can be more accurately assessed with flow cytometry as being directly correlated to sperm viability rather than the possibility of altered toxicity results due to sperm-to-egg compatibility.

The calcium-mobilizing messenger nicotinic acid adenine dinucleotide phosphate participates in sperm activation by mediating the acrosome reaction

Before a sperm can fertilize an egg it must undergo a final activation step induced by the egg termed the acrosome reaction. During the acrosome reaction a lysosome-related organelle, the acrosome, fuses with the plasma membrane to release hydrolytic enzymes and expose an egg-binding protein. Because NAADP (nicotinic acid adenine dinucleotide phosphate) releases Ca²⁺ from acidic lysosome-related organelles in other cell types, we investigated a possible role for NAADP in mediating the acrosome reaction. We report that NAADP binds with high affinity to permeabilized sea urchin sperm. Moreover, we used Mn²⁺ quenching of luminal fura-2 and ⁴⁵Ca²⁺ to directly demonstrate NAADP regulation of a cation channel on the acrosome. Additionally, we show that NAADP synthesis occurs through base exchange and is driven by an increase in Ca²⁺. We propose a new model for acrosome reaction signaling in which Ca²⁺ influx initiated by egg jelly stimulates NAADP synthesis and that this NAADP acts on its receptor/channel on the acrosome to release Ca²⁺ to drive acrosomal exocytosis.

Sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. Here we report that this lipid triggers acrosomal exocytosis in human sperm by a mechanism involving a G(i)-coupled receptor. Real-time imaging showed a remarkable increase of cytosolic calcium upon activation with sphingosine 1-phosphate and pharmacological experiments indicate that the process requires extracellular calcium influx through voltage and store-operated calcium channels and efflux from intracellular stores through inositol 1,4,5-trisphosphate-sensitive calcium channels. Sphingosine 1-phosphate-induced exocytosis requires phospholipase C and protein kinase C activation. We investigated possible sources of the lipid. Western blot indicates that sphingosine kinase 1 is present in spermatozoa. Indirect immunofluorescence showed that phorbol ester, a potent protein kinase C activator that can also trigger acrosomal exocytosis, redistributes sphingosine kinase 1 to the acrosomal region. Functional assays showed that phorbol ester-induced exocytosis depends on the activation of sphingosine kinase 1. Furthermore, incorporation of (32)P to sphingosine demonstrates that cells treated with the phorbol ester increase their sphingosine kinase activity that yields sphingosine 1-phosphate. We present here the first evidence indicating that human spermatozoa produce sphingosine 1-phosphate when challenged with an exocytic stimulus. These observations point to a new role of sphingosine 1-phosphate in a signaling cascade that facilitates acrosome reaction providing some clues about novel lipid molecules involved in exocytosis.

Acrosome reaction of sperm in the mud crab Scylla serrata as a sensitive toxicity test for metal exposures

In order to test the sensitivity of the sperm cell of the mud crab Scylla serrata to heavy metals, the toxic effects of Ag+, Cd2+, Cu2+, and Zn2+ on the acrosome reaction (AR) were studied by artificially inducing the AR of sperm exposed to heavy metals, counting the AR rates by light microscopy, and observing structural changes in sperm by transmission electron microscopy. The AR in S. serrata occurs at two stages. The first stage (ARI) is the eversion of the subacrosomal material. The second stage (ARII) is the ejection of the acrosomal filament. The results showed the EC50 values of the AR based on (ARI + ARII)% for Ag+, Cd2+, Cu2+, and Zn2+ were 10.02, 2.14, 13.69, and 2.21 microg/L, and the EC50 values based on ARII % of Ag+, Cd2+, Cu2+, and Zn2+ were 1.96, 0.20, 1.46, and 0.34 microg/L. The order of toxicity is Cd2+ > Zn2+ > Cu2+ > Ag+ based on the percentage of reacted sperm at the second stage. Sperm cells exposed to heavy metals showed an increased rate of swelling, shape irregularities, and the acrosomal filament of some sperm cells was, crooked, ruptured, and even dissolved. The AR of the sperm cell from S. serrata is more sensitive to the tested heavy metals compared to sea urchin sperm cell toxicity tests.

Ca2+-stores in sperm: their identities and functions

Intracellular Ca2+ stores play a central role in the regulation of cellular [Ca2+](i) and the generation of complex [Ca2+] signals such as oscillations and waves. Ca2+ signalling is of particular significance in sperm cells, where it is a central regulator in many key activities (including capacitation, hyperactivation, chemotaxis and acrosome reaction) yet mature sperm lack endoplasmic reticulum and several other organelles that serve as Ca2+ stores in somatic cells. Here, we review i) the evidence for the expression in sperm of the molecular components (pumps and channels) which are functionally significant in the activity of Ca2+ stores of somatic cells and ii) the evidence for the existence of functional Ca2+ stores in sperm. This evidence supports the existence of at least two storage organelles in mammalian sperm, one in the acrosomal region and another in the region of the sperm neck and midpiece. We then go on to discuss the probable identity of these organelles and their discrete functions: regulation by the acrosome of its own secretion and regulation by membranous organelles at the sperm neck (and possibly by the mitochondria) of flagellar activity and hyperactivation. Finally, we consider the ability of the sperm discretely to control mobilisation of these stores and the functional interaction of stored Ca2+ at the sperm neck/midpiece with CatSper channels in the principal piece in regulation of the activities of mammalian sperm.

Egg water from the amphibian Bufo arenarum modulates the ability of homologous sperm to undergo the acrosome reaction in the presence of the vitelline envelope

Sperm from the toad Bufo arenarum must penetrate the egg jelly before reaching the vitelline envelope (VE), where the acrosome reaction is triggered. When the jelly coat is removed, sperm still bind to the VE, but acrosomal exocytosis is not promoted. Our previous work demonstrated that diffusible substances of the jelly coat, termed "egg water" (EW), triggered capacitation-like changes in B. arenarum sperm, promoting the acquisition of a transient fertilizing capacity. In the present work, we correlated this fertilizing capacity with the ability of the sperm to undergo the acrosome reaction, further substantiating the role of the jelly coat in fertilization. When sperm were exposed to the VE, only those preincubated in EW for 5 or 8 min underwent an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)), which led to acrosomal exocytosis. Responsiveness to the VE was not acquired on preincubation in EW for 2 or 15 min or in Ringer solution regardless of the preincubation time. In contrast, depletion of intracellular Ca(2+) stores (induced by thapsigargin) promoted [Ca(2+)](i) rise and the acrosome reaction even in sperm that were not exposed to EW. Acrosomal exocytosis was blocked by the presence of Ca(2+) chelators independent of whether a physiological or pharmacological stimulus was used. However, Ni(2+) and mibefradil prevented [Ca(2+)](i) rise and the acrosome reaction of sperm exposed to the VE but not of sperm exposed to thapsigargin. These data suggest that the acrosomal responsiveness of B. arenarum sperm, present during a narrow period, is acquired during EW incubation and involves the modulation of a voltage-dependent Ca(2+) channel.

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.

[Ca2+]i signalling in sperm — making the most of what you've got

Thanks to a worrying decrease in male fertility, understanding how sperm 'work' is a matter both of interest and great importance. Sperm of all animals detect various environmental cues. The 'behavioural' and physiological responses of sperm must be specific, appropriate and correctly timed. Strangely, in a cell with few organelles and minimal cytoplasmic volume, internal Ca(2+) concentration, [Ca(2+)](i), regulates almost all these activities. How does such a simple cell achieve this - and is it as simple as it seems?

Patterns of [Ca2+]i mobilization and cell response in human spermatozoa exposed to progesterone

Human spermatozoa stimulated with progesterone (a product of the cumulus and thus encountered by sperm prior to fertilization in vivo) apparently mobilize Ca(2+) and respond very differently according to the way in which the steroid is presented. A progesterone concentration ramp (0-3 microM) induces [Ca(2+)](i) oscillations (repetitive store mobilization) which modify flagellar beating, whereas bolus application of micromolar progesterone causes a single large transient (causing acrosome reaction) which is apparently dependent upon Ca(2+) influx. We have investigated Ca(2+)-mobilization and functional responses in human sperm exposed to 3 muM progesterone. The [Ca(2+)](i) response to progesterone was abolished by 4 min incubation in 0 Ca(2+) medium (2 mM EGTA) but in nominally Ca(2+)-free medium (no added Ca(2+); 0 EGTA) a smaller, slow response occurred. Single cell imaging showed a similar effect of nominally Ca(2+)-free medium and approximately 5% of cells generated a small transient even in the presence of EGTA. When cells were exposed to EGTA-containing saline (5 min) and then returned to nominally Ca(2+)-free medium before stimulation, the [Ca(2+)](i) transient was greatly delayed (approximately 50 s) and rise time was doubled in comparison to cells not subjected to EGTA pre-treatment. We conclude that mobilization of stored Ca(2+) contributes a 'slow' component to the progesterone-induced [Ca(2+)](i) transient and that incubation in EGTA-buffered saline is able rapidly to deplete this store. Analysis of flagellar activity induced by 3 muM progesterone showed an effect (modified beating) associated with the [Ca(2+)](i) transient, in >80% of cells bathed in nominally Ca(2+)-free medium. This was reduced greatly in cells subjected to 5 min EGTA pre-treatment. The store-mediated transient showed a pharmacological sensitivity similar to that of progesterone-induced [Ca(2+)](i) oscillations (consistent with filling of the store by an SPCA) suggesting that the transient induced by micromolar progesterone is a 'single shot' activation of the same store that generates Ca(2+) oscillations.

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.

Mouse uterine 24p3 protein as a suppressor of sperm acrosome reaction

Under in vitro conditions, incubation with 0.3% bovine serum albumin (BSA) and 1.8 mM CaCl(2) induces mouse sperm capacitation and increases the consequential acrosome-reaction. The effect of mouse uterine 24p3 protein on such stimulated sperm has been investigated to understand the biological function of the 24p3 protein. Variations in the intracellular pH (pHi), calcium concentration, cAMP levels and tyrosine phosphorylation in cytosol were determined and on in vitro mouse fertilization was evaluated. The presence of 24p3 protein reduced the response of sperm to BSA and calcium by suppressing the elevation of intracellular pH, calcium uptake, cAMP accumulation and protein tyrosine phosphorylation of BSA/calcium-stimulated sperm and showed inhibitory effect on mouse in vitro fertilization. The results indicated the inhibition of the BSA-stimulated sperm acrosome reaction by 24p3 protein then suppressed sperm fertilization. We suggested that the 24p3 protein acts as an in vitro inhibitor of the acrosome reaction in BSA stimulated sperm and this might be an anti-fertilization factor in vitro.

Kinetics of the progesterone-induced acrosome reaction and its relation to intracellular calcium responses in individual human spermatozoa

Progesterone at 3 microM triggers a biphasic (transient and sustained) increase in intracellular calcium ([Ca(2+)](i)) in human sperm, which is believed to be a prerequisite for progesterone-induced acrosome reaction (AR). As very little is known about how AR occurrence, latency, and completion relate to the characteristics of the progesterone-induced [Ca(2+)](i) signal, we examined these events using fluorescence microscopy of individual living human sperm. Direct assessment of acrosomal status after calcium imaging showed no differences in kinetics or amplitude of the preceding progesterone-induced calcium responses in acrosome-reacted and acrosome-intact cells, which indicates that the amplitude of the [Ca(2+)](i) signal is not the critical determinant of AR. Chelation of extracellular calcium to arrest AR at varying times after progesterone stimulation revealed that maximal AR occurred immediately following progesterone stimulation, during the initial transient calcium influx rather than during the sustained calcium response. Attempts to follow acrosomal dispersal in real-time by staining with the acidic organelle probes LysoTracker DND-99 and dapoxyl (2-aminoethyl) sulphonamide (DAES) proved inconclusive due to heterogeneous labeling of the cell population. Surprisingly, the dye was often not confined to the acrosome but stained the whole sperm head, which suggests that only a subpopulation of human sperm cells contains a sufficiently acidic acrosome.

Evidence for a secretory pathway Ca2+-ATPase in sea urchin spermatozoa

Plasma membrane, sarco-endoplasmic reticulum and secretory pathway Ca2+-ATPases (designated PMCA, SERCA and SPCA) regulate intracellular Ca2+ in animal cells. The presence of PMCA, and the absence of SERCA, in sea urchin sperm is known. By using inhibitors of Ca2+-ATPases, we now show the presence of SPCA and Ca2+ store in sea urchin sperm, which refills by SPCA-type pumps. Immunofluorescence shows SPCA localizes to the mitochondrion. Ca2+ measurements reveal that approximately 75% of Ca2+ extrusion is by Ca2+ ATPases and 25% by Na+ dependent Ca2+ exchanger/s. Bisphenol, a Ca2+ ATPase inhibitor, completely blocks the acrosome reaction, indicating the importance of Ca2+-ATPases in fertilization.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

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.

Tandem mass spectrometry identifies proteins phosphorylated by cyclic AMP-dependent protein kinase when sea urchin sperm undergo the acrosome reaction

The exocytotic acrosome reaction (AR), which is required for fertilization, occurs when sea urchin sperm contact the egg jelly (EJ) layer. Among other physiological changes, increases in adenylyl cyclase activity, cAMP and cAMP-dependent protein kinase (PKA) activity occur coincident with the AR. By using inhibitors of PKA, a permeable analog of cAMP and the phosphodiesterase inhibitor IBMX, we show that PKA activity is required for AR induction by EJ. A minimum of six sperm proteins are phosphorylated by PKA upon exposure to EJ, as detected by a PKA substrate-specific antibody. The phosphorylation of these proteins and the percentage of acrosome reacted sperm can be regulated by PKA modulators. The fucose sulfate polymer (FSP), a major component of EJ, is the molecule that triggers sperm PKA activation. Extracellular Ca(2+) is required for PKA activation. Six sperm proteins phosphorylated by PKA were identified by tandem mass spectrometry (MS/MS) utilizing the emerging sea urchin genome. Based on their identities and localizations in sperm head and flagellum, the putative functions of these proteins in sperm physiology and AR induction are discussed.

Catsper3 and catsper4 encode two cation channel-like proteins exclusively expressed in the testis

CATSPER1 and CATSPER2 are two cation channel-like proteins exclusively expressed in the testis and essential for normal sperm motility and male fertility. Using in silico subtraction and database mining, we identified expressed sequence tags encoding two previously uncharacterized cation channel-like proteins structurally homologous to CATSPER1 and CATSPER2. Similar to CATSPER1 and CATSPER2, these two proteins contain a single-ion transport domain comprised of six transmembrane spanning regions, in which the fourth transmembrane region resembles a voltage sensor and a pore-forming region lies between transmembrane regions 5 and 6. The pore contains the consensus sequence T x D x W, which is indicative of a potential calcium-selective channel. The mRNAs for Catsper3 and Catsper4 were detected exclusively in the testis using multitissue Northern blot and RT-PCR analyses. The onsets of both genes coincide with the first appearance of spermatids during testicular development. In situ hybridization analyses revealed that Catsper3 and Catsper4 mRNAs displayed identical localization patterns and were confined to spermatids of steps 1-8. Immunofluorescence and immunohistochemistry analyses demonstrated that these two proteins were expressed within the acrosome of late spermatids and spermatozoa. Our data suggest that CATSPER3 and CATSPER4 are two cation-channel proteins and have roles in acrosome reaction and male fertility.

Real-time analysis of the role of Ca(2+) in flagellar movement and motility in single sea urchin sperm

Eggs of many marine and mammalian species attract sperm by releasing chemoattractants that modify the bending properties of flagella to redirect sperm paths toward the egg. This process, called chemotaxis, is dependent on extracellular Ca(2+). We used stroboscopic fluorescence imaging to measure intracellular Ca(2+) concentration ([Ca(2+)]i) in the flagella of swimming sea urchin sperm. Uncaging of cyclic GMP induced Ca(2+) entry via at least two distinct pathways, and we identified a nimodipine-sensitive pathway, compartmentalized in the flagella, as a key regulator of flagellar bending and directed motility changes. We found that, contrary to current models, the degree of flagellar bending does not vary in proportion to the overall [Ca(2+)]i. Instead we propose a new model whereby flagella bending is increased by Ca(2+) flux through the nimodipine-sensitive pathway, and is unaffected by [Ca(2+)]i increases through alternative pathways.

Effects of urocortin on T-type calcium currents in mouse spermatogenic cells

Urocortin (UCN), a newly isolated peptide, has been found to play an important role mainly in female reproductive system. In order to investigate the effect of UCN on T-type calcium currents (I(Ca,T)), exploring the mechanisms of UCN's role in male reproductive system, especially in acrosome reaction, we directly measured the I(Ca,T) in mouse spermatogenic cells exposed to UCN using standard whole-cell patch-clamp recording technique. Our results showed that UCN reversibly inhibited the T-type Ca(2+) currents in the cells in a concentration-dependent manner. The current density was inhibited by about 19% after exposure of the cells to UCN (0.1 microM) for 5 min, from the control value of 6.75+/-1.17 to 5.26+/-0.82pA/pF. UCN up-shifted the current-voltage (I-V) curve. Frequency-dependence of UCN's effects on I(Ca,T) was also observed. Moreover, UCN at 0.1 microM did not markedly affect the activation of I(Ca,T) but shifted the inactivation curve of I(Ca,T) to the left. The inhibitory effect of UCN on the T-type Ca(2+) current was not affected by Astressin, the CRF receptor blocker. Since T-type calcium channels are a key component in acrosome reaction, our data suggest that UCN might be a significant factor in male reproductive action and a potential contraceptive agent.

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.

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.

Store-operated calcium channels trigger exocytosis of the sea urchin sperm acrosomal vesicle

The acrosome reaction (AR) of sperm is a prerequisite for fusion with the egg. In sea urchins, the complete AR (CAR) consists of exocytosis of the acrosomal vesicle (AV) and polymerization of acrosomal actin to form the approximately 1 micro m long acrosomal process. The fucose sulfate polymer (FSP) of egg jelly stimulates Ca(2+) entry through two distinct Ca(2+) channels and induces the CAR. Here we report that the second channel is blocked by SKF96365 (SKF), an inhibitor of store-operated channels. SKF also blocks the thapsigargin (TG), trifluoperazine (TFP), and calmidizolium (CMZ) stimulated Ca(2+) entry into sperm. These data indicate that the second Ca(2+) channel is a store-operated channel (SOC) that may be regulated by calmodulin. The TG, TFP, and CMZ-induced intracellular Ca(2+) elevations are similar to those induced by FSP, but the sperm acrosomal process does not polymerize. An antibody to bindin, the major protein of the AV, showed that in a significant percentage of these drug-treated sperm, the AV had undergone exocytosis. When NH(4)Cl was added to increase intracellular pH, the TG-treated sperm polymerized actin to form the acrosomal process. We conclude that the second Ca(2+) channel of sea urchin sperm is a SOC that triggers AV exocytosis.

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.

Store-operated calcium channel regulates the chemotactic behavior of ascidian sperm

The sperm-activating and -attracting factor released from the eggs of the ascidians Ciona intestinalis and Ciona savignyi requires extracellular Ca(2+) for activating sperm motility and eliciting chemotactic behavior of the activated sperm toward the egg. Here, we show that modulators of the store-operated Ca(2+) channel, SK&F96365, Ni(2+), 2-aminoethoxydiphenylborane, and thapsigargin inhibit the chemotactic behavior of the ascidian sperm; on the other hand, blockers of voltage-dependent Ca(2+) channels did not inhibit the chemotaxis, even though they inhibited the sperm activation operated by voltage-dependent Ca(2+) channels. The blockers of store-operated Ca(2+) channels also inhibited the asymmetrical flagellar beating and turning movements of the ascidian sperm, which are typical signs of sperm chemotaxis. Depletion of internal Ca(2+) stores by thapsigargin induced capacitative Ca(2+) entry into the sperm, which was blocked by SK&F96365. These results suggest that the intracellular Ca(2+) concentration increase through the store-operated Ca(2+) channels induces asymmetrical flagellar movements to establish the chemotactic behavior of the sperm.

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

The sperm-activating peptide speract and fucose-sulphate glycoconjugate (FSG) are sea urchin egg-envelope components that modulate sperm ion permeability. They influence motility and induce acrosomal reaction (AR), respectively. A fluorescent Na(+)-sensitive dye (Na(+)-binding benzofuran isophthalate, SBFI) was used to determine how these egg envelope components influence sperm Na(+) permeability. [Ca(2+)](i) and pH(i) were also measured to correlate their changes in response to speract and FSG with those observed in [Na(+)](i). SBFI determinations indicate that the resting [Na(+)](i) is 20 +/- 8 mM in sea urchin sperm. Saturating levels of speract increased [Na(+)](i) by approximately 15 mM, while similar levels of FSG caused a further elevation of approximately 30 mM. The kinetics of the [Na(+)](i), [Ca(2+)](i) and pH(i) changes induced by saturating levels of speract were faster than those induced by FSG. Both egg ligands appeared to activate more than one Na(+) transport system. Nifedipine, Ni(2+) and TEA(+) inhibited the ionic changes and the AR induced by FSG but, importantly, did not alter those caused by speract. Thus, there are differences in some of the ionic transport mechanisms that operate in the speract and FSG responses. ZD2788, a blocker of hyperpolarization and cyclic-nucleotide-gated (HCN) channels such as SpHCN present in sea urchin sperm, did not decrease the speract-induced [Na(+)](i) increase, but slowed its kinetics. Therefore, SpHCN does not play a major role in the uptake of Na(+) triggered by this decapeptide. KB-R7943, an inhibitor of Na(+)/Ca(2+) exchangers, decreased the resting [Na(+)](i) and did not change significantly the speract-induced [Ca(2+)](i) increase, but slowed its recovery.

The intraacrosomal calcium pool plays a direct role in acrosomal exocytosis

The acrosome reaction is a unique type of regulated exocytosis. The single secretory granule of the sperm fuses at multiple points with the overlying plasma membrane. In the past few years we have characterized several aspects of this process using streptolysin O-permeabilized human spermatozoa. Here we show that Rab3A triggers acrosomal exocytosis in the virtual absence of calcium in the cytosolic compartment. Interestingly, exocytosis is blocked when calcium is depleted from intracellular stores. By using a membrane-permeant fluorescent calcium probe, we observed that the acrosome actually behaves as a calcium store. Depleting calcium from this compartment by using a light-sensitive chelator prevents secretion promoted by Rab3A. UV inactivation of the chelator restores exocytosis. Rab3A-triggered exocytosis is blocked by calcium pump and inositol 1,4,5-trisphosphate (IP(3))-sensitive calcium channel inhibitors. Calcium measurements inside and outside the acrosome showed that Rab3A promotes a calcium efflux from the granule. Interestingly, release of calcium through IP(3)-sensitive calcium channels was necessary even when exocytosis was initiated by increasing free calcium in the extraacrosomal compartment in both permeabilized and intact spermatozoa. Our results show that a calcium efflux from the acrosome through IP(3)-sensitive channels is necessary downstream Rab3A activation during the membrane fusion process leading to acrosomal exocytosis.

Egg fucose sulfate polymer, sialoglycan, and speract all trigger the sea urchin sperm acrosome reaction

Macromolecules surrounding eggs induce the acrosome reaction (AR) of spermatozoa. In sea urchins, three egg jelly (EJ) molecules: a fucose sulfate polymer (FSP), a sialoglycan (SG), and speract mediate ionic fluxes triggering the AR. SG and speract are noninductive without FSP. Speract's role in AR induction is controversial. Here we show that speract potentiates the FSP-induced AR at pH 7.0, approximately 1 pH unit lower than natural seawater. At pH 7.0, a mixture of FSP, SG, and speract produces the intracellular pH increase necessary for maximum AR induction. Each EJ component may mediate a distinct intracellular pH control mechanism, and all three may function synergistically to increase the intracellular pH permitting AR induction. Speract peptides are an ancient family. Although important for activating cyclic nucleotide-mediated pathways in today's seawater of pH approximately 8, speract may have been more important in AR induction in the paleo-ocean of pH approximately 7.

Inhibition of T-type Ca(2+) currents in mouse spermatogenic cells by gossypol, an antifertility compound

Gossypol, a male antifertility compound isolated from cotton, has been proved to inhibit capacitation and the acrosome reaction in human and mammalian sperm. Here, by using whole-cell recording, we observed the effects of gossypol on Ca(2+) and Cl(-) currents in mouse spermatogenic cells obtained by mechanical dissociation. The results showed that gossypol concentration-dependently and irreversibly inhibited T-type Ca(2+) currents in the cells. When the concentration of gossypol was > or =5 microM, the currents were blocked completely. The time to current block was progressively shortened as the gossypol concentration was increased from 5 to 80 microM. Moreover, the drug increased the time constant of inactivation in a concentration-dependent manner, while it did not affect the activation of the current. The inhibitory effect on the T-type Ca(2+) current did not correlate with signaling mediated by G proteins and tyrosine phosphorylation. No obvious effect of gossypol on Cl(-) currents was observed. These data suggest that the gossypol-induced inhibition of T-type Ca(2+) currents could be responsible for the antifertility activity of the compound, indicating a possibility to use gossypol as a local contraceptive drug.

Egg sialoglycans increase intracellular pH and potentiate the acrosome reaction of sea urchin sperm

Sea urchin egg jelly (EJ) triggers sperm acrosome reaction (AR), an exocytotic event required for membrane fusion of the gametes. Purified fucose sulfate polymer (FSP) in EJ is one inducer of the AR. Binding of FSP to its receptor regulates opening of two distinct calcium channels and also elevates intracellular pH (pH(i)). EJ also contains sialic acid-rich glycans (sialoglycans (SG)) that were isolated by beta-elimination followed by DEAE chromatography. In the presence of limiting amounts of FSP, the SG fraction markedly potentiates the AR; however, by itself SG has no activity. The SG fraction increases the pH(i) of sperm without increasing intracellular Ca(2+). The SG-induced increase in pH(i) is not blocked by nifedipine or high K(+), whereas the FSP-induced pH(i) increase is sensitive to both these agents. Treatment of the SG fraction with neuraminidase or mild metaperiodate that specifically cleaves the glycerol side chain of sialic acid abolishes the AR potentiation and ability of SG to elevate pH(i). These data are the first to show that there are at least two pathways to induce sperm pH(i) increase and that egg surface sialic acid plays a role in triggering the sperm AR.

Motility initiation in herring sperm is regulated by reverse sodium-calcium exchange

Sperm of the Pacific herring, Clupea pallasi, are unique in that they are immotile upon spawning in the environment. Herring sperm have evolved to remain motionless for up to several days after spawning, yet are still capable of fertilizing eggs. An egg chorion ligand termed "sperm motility initiation factor" (SMIF) induces motility in herring sperm and is required for fertilization. In this study, we show that SMIF induces calcium influx, sodium efflux, and a membrane depolarization in herring sperm. Sperm motility initiation by SMIF depended on decreased extracellular sodium (<350 mM) and could be induced in the absence of SMIF in very low sodium seawater. Motility initiation depended on > or =1 mM extracellular calcium. Calcium influx caused by SMIF involved both the opening of voltage-gated calcium channels and reverse sodium-calcium (Na(+)/Ca(2+)) exchange. Membrane depolarization was slightly inhibited by a calcium channel blocker and markedly inhibited by a Na(+)/Ca(2+) exchange inhibitor. Sodium efflux caused by SMIF-initiated motility was observed when using both extracellular and intracellular sodium probes. A Na(+)/Ca(2+) exchange antigen was shown to be present on the surface of the sperm, primarily over the midpiece, by using an antibody to the canine Na(+)/Ca(2+) exchanger. This antibody recognized a 120-kDa protein that comigrated with the canine myocyte Na(+)/Ca(2+) exchanger. Sperm of Pacific herring are now shown to use reverse Na(+)/Ca(2+) exchange in motility initiation. This mechanism of regulation of motility initiation may have evolved for both maintenance of immotility after spawning as well as ligand-induced motility initiation.

High molecular mass egg fucose sulfate polymer is required for opening both Ca2+ channels involved in triggering the sea urchin sperm acrosome reaction

A linear fucose sulfate polymer (FSP), >10(6) daltons, is a major component of sea urchin egg jelly. FSP induces the sperm acrosome reaction (AR), an exocytotic process required for animal fertilization. Two Ca(2+) channels activate during AR induction, the first opens 1 s after FSP addition, and the second opens 5 s after the first. Mild acid hydrolysis of FSP results in a linear decrease in polymer size. The ability of FSP to induce the AR and activate sperm Ca(2+) channels decreases with increasing time of hydrolysis. Hydrolyzed FSP of approximately 60 kDa blocks intact FSP from inducing the AR. At 44 microg/ml hydrolyzed FSP, Ca(2+) entry into sperm is almost equal to that occurring in 3.8 microg/ml intact FSP; however the AR is not induced. The shape of the [Ca(2+)](i) increase curve and use of the Ca(2+) channel blockers nifidipine and Ni(2+) indicate that hydrolyzed FSP opens the second Ca(2+) channel, but not the first, and thus does not induce the AR. The giant size of intact FSP is required to open both Ca(2+) channels involved in triggering the AR.