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

CatSper mediates not only chemotactic behavior but also the motility of ascidian sperm

Introduction: Sperm motility, including chemotactic behavior, is regulated by changes in the intracellular Ca2+ concentration, and the sperm-specific Ca2+ channel CatSper has been shown to play an important role in the regulation of intracellular Ca2+. In particular, in mammals, CatSper is the only functional Ca2+ channel in the sperm, and mice deficient in the genes comprising the pore region of the Ca2+ channel are infertile due to the inhibition of sperm hyperactivation. CatSper is also thought to be involved in sea urchin chemotaxis. In contrast, in ascidian Ciona intestinalis, SAAF, a sperm attractant, interacts with Ca2+/ATPase, a Ca2+ pump. Although the existence of CatSper genes has been reported, it is not clear whether CatSper is a functional Ca2+ channel in sperm. Results: We showed that CatSper is present in the sperm flagella of C. intestinalis as in mammalian species, although a small level of gene expression was found in other tissues. The spermatozoa of CatSper3 KO animals were significantly less motile, and some motile sperms did not show any chemotactic behavior. These results suggest that CatSper plays an important role in ascidians and mammals, and is involved in spermatogenesis and basic motility mechanisms.

Mechanisms of Sperm-Egg Interactions: What Ascidian Fertilization Research Has Taught Us

Fertilization is an essential process in terrestrial organisms for creating a new organism with genetic diversity. Before gamete fusion, several steps are required to achieve successful fertilization. Animal spermatozoa are first activated and attracted to the eggs by egg-derived chemoattractants. During the sperm passage of the egg's extracellular matrix or upon the sperm binding to the proteinaceous egg coat, the sperm undergoes an acrosome reaction, an exocytosis of acrosome. In hermaphrodites such as ascidians, the self/nonself recognition process occurs when the sperm binds to the egg coat. The activated or acrosome-reacted spermatozoa penetrate through the proteinaceous egg coat. The extracellular ubiquitin-proteasome system, the astacin-like metalloproteases, and the trypsin-like proteases play key roles in this process in ascidians. In the present review, we summarize our current understanding and perspectives on gamete recognition and egg coat lysins in ascidians and consider the general mechanisms of fertilization in animals and plants.

Perspectives on Potential Fatty Acid Modulations of Motility Associated Human Sperm Ion Channels

Human spermatozoan ion channels are specifically distributed in the spermatozoan membrane, contribute to sperm motility, and are associated with male reproductive abnormalities. Calcium, potassium, protons, sodium, and chloride are the main ions that are regulated across this membrane, and their intracellular concentrations are crucial for sperm motility. Fatty acids (FAs) affect sperm quality parameters, reproductive pathologies, male fertility, and regulate ion channel functions in other cells. However, to date the literature is insufficient to draw any conclusions regarding the effects of FAs on human spermatozoan ion channels. Here, we aimed to discern the possible effects of FAs on spermatozoan ion channels and direct guidance for future research. After investigating the effects of FAs on characteristics related to human spermatozoan motility, reproductive pathologies, and the modulation of similar ion channels in other cells by FAs, we extrapolated polyunsaturated FAs (PUFAs) to have the highest potency in modulating sperm ion channels to increase sperm motility. Of the PUFAs, the ω-3 unsaturated fatty acids have the greatest effect. We speculate that saturated and monounsaturated FAs will have little to no effect on sperm ion channel activity, though the possible effects could be opposite to those of the PUFAs, considering the differences between FA structure and behavior.

Fertilization of Ascidians: Gamete Interaction, Self/Nonself Recognition and Sperm Penetration of Egg Coat

Fertilization is one of the most important events in living organisms to generate a new life with a mixed genetic background. To achieve successful fertilization, sperm and eggs must undergo complex processes in a sequential order. Fertilization of marine invertebrate Ciona intestinalis type A (Ciona robusta) has been studied for more than a hundred years. Ascidian sperm are attracted by chemoattractants from eggs and bind to the vitelline coat. Subsequently, sperm penetrate through the vitelline coat proteolytically and finally fuse with the egg plasma membrane. Here, we summarize the fertilization mechanisms of ascidians, particularly from sperm-egg interactions to sperm penetration of the egg coat. Since ascidians are hermaphrodites, inbreeding depression is a serious problem. To avoid self-fertilization, ascidians possess a self-incompatibility system. In this review, we also describe the molecular mechanisms of the self-incompatibility system in C. intestinalis type A governed by three allelic gene pairs of s-Themis and v-Themis.

A 40 years journey with fish spermatozoa as companions as I personally experienced it

When, in the 1980s, I became interested in the spermatology of fish under the light microscope, active spermatozoa were only visible thanks to their head presenting a sort of "tremor." This situation was quite frustrating given the lack of possible information regarding the motor part called flagellum. We decided to apply simple technologies, including photography. Due to the high speed of the moving fish flagellum, the microscope illumination used a pulsed light strobe combined with a dark field microscope to record the flagellum image despite its small diameter (< 0.5 μm). Then came high-speed cinematographic microscopy up to 200 fps, as well as video cameras. At the end of the 1990s, an automatic moving object video tracking system began to be commercialized (CASA) with main advantages such as (a) a large number of cells tracked, which greatly improves statistics, (b) computer assistance allowing an automatic analysis that provides many motility parameters. Nevertheless, CASA systems are still unable to provide information about fish sperm flagella that move fast. During the 1990s, analog video camera technologies allowed acquisition of flagellum images with high resolution for detailed analysis. Since the 2000s, the use of high-speed video cameras allows the acquisition of images at a much higher resolution and frequency, up to 10,000 frames per second. Since it became possible to visualize the flagella in motion, a noble function was added to that of a propeller: that of a rudder with what a spermatozoon responds to specific signals delivered by the egg for its guidance. In the future, one can wish that an automatic flagella movement analyzer will become functional. This brief anthology puts forward the large amount of progress accomplished during past 40-year period about spermatozoa movement analysis, especially in fish.

Structure and Function of Ion Channels Regulating Sperm Motility-An Overview

Sperm motility is linked to the activation of signaling pathways that trigger movement. These pathways are mainly dependent on Ca²⁺, which acts as a secondary messenger. The maintenance of adequate Ca²⁺ concentrations is possible thanks to proper concentrations of other ions, such as K⁺ and Na⁺, among others, that modulate plasma membrane potential and the intracellular pH. Like in every cell, ion homeostasis in spermatozoa is ensured by a vast spectrum of ion channels supported by the work of ion pumps and transporters. To achieve success in fertilization, sperm ion channels have to be sensitive to various external and internal factors. This sensitivity is provided by specific channel structures. In addition, novel sperm-specific channels or isoforms have been found with compositions that increase the chance of fertilization. Notably, the most significant sperm ion channel is the cation channel of sperm (CatSper), which is a sperm-specific Ca²⁺ channel required for the hyperactivation of sperm motility. The role of other ion channels in the spermatozoa, such as voltage-gated Ca²⁺ channels (VGCCs), Ca²⁺-activated Cl-channels (CaCCs), SLO K⁺ channels or voltage-gated H⁺ channels (VGHCs), is to ensure the activation and modulation of CatSper. As the activation of sperm motility differs among metazoa, different ion channels may participate; however, knowledge regarding these channels is still scarce. In the present review, the roles and structures of the most important known ion channels are described in regard to regulation of sperm motility in animals.

Transcriptome analysis provides a blueprint of coral egg and sperm functions

BACKGROUND

Reproductive biology and the evolutionary constraints acting on dispersal stages are poorly understood in many stony coral species. A key piece of missing information is egg and sperm gene expression. This is critical for broadcast spawning corals, such as our model, the Hawaiian species Montipora capitata, because eggs and sperm are exposed to environmental stressors during dispersal. Furthermore, parental effects such as transcriptome investment may provide a means for cross- or trans-generational plasticity and be apparent in egg and sperm transcriptome data.

METHODS

Here, we analyzed M. capitata egg and sperm transcriptomic data to address three questions: (1) Which pathways and functions are actively transcribed in these gametes? (2) How does sperm and egg gene expression differ from adult tissues? (3) Does gene expression differ between these gametes?

RESULTS

We show that egg and sperm display surprisingly similar levels of gene expression and overlapping functional enrichment patterns. These results may reflect similar environmental constraints faced by these motile gametes. We find significant differences in differential expression of egg vs. adult and sperm vs. adult RNA-seq data, in contrast to very few examples of differential expression when comparing egg vs. sperm transcriptomes. Lastly, using gene ontology and KEGG orthology data we show that both egg and sperm have markedly repressed transcription and translation machinery compared to the adult, suggesting a dependence on parental transcripts. We speculate that cell motility and calcium ion binding genes may be involved in gamete to gamete recognition in the water column and thus, fertilization.

Stromal cell-derived factor 1 regulates in vitro sperm migration towards the cumulus-oocyte complex in cattle

Sperm migration towards an oocyte in the female reproductive tract is an important step for successful fertilization. Although several sperm-chemotactic factors have been identified in mammals, it is unclear whether these chemoattractants contribute to sperm migration towards an oocyte that is the final destination for sperm. Furthermore, chemoattractants for bovine sperm are still undiscovered even though the follicular fluid attracts sperm in cattle. Here, we demonstrated that a single bovine cumulus-oocyte complex (COC) had the ability to attract sperm, suggesting that the COC secreted sperm chemoattractants. We identified stromal cell-derived factor 1 (SDF1), which was expressed in COCs, and its receptor CXCR4 in sperm, as a candidate. Our results showed that bovine sperm preferentially migrated to the area with a high SDF1 concentration and occasionally showed turn movements by asymmetric flagellar bends during the migration. We also demonstrated that increasing the intracellular Ca²⁺ concentration via Ca²⁺ channels was related to SDF1-induced sperm chemotaxis. Finally, a CXCR4 inhibitor significantly suppressed the in vitro bovine sperm migration towards a COC. Taken together, we propose that SDF1 is a chemotactic factor for bovine sperm to regulate their migration towards an oocyte via the CXCR4 receptor.

Na-K-2Cl Cotransporter and Store-Operated Ca2+ Entry in Pacemaking by Interstitial Cells of Cajal

Pacemaker depolarization in interstitial cells of Cajal (ICCs) is believed to be induced by Ca²⁺ transients and activation of anoctamin-1 (Ano1) channels in the plasma membrane. However, block of store-operated calcium entry (SOCE) or the Na-K-2Cl cotransporter (NKCC1) terminates pacemaker activity in ICC, indicating these transporters are involved in the initiation or maintenance of pacemaker activity. We hypothesized that SOCE contributes to pacemaker depolarization by maintaining [Ca²⁺] in the endoplasmic reticulum, which is the underlying source of Ca²⁺ transients for activation of Ano1. NKCC1 maintains the Cl^- gradient supporting the driving force for inward current mediated by Ano1. Currently mechanisms sustaining release of Ca²⁺ and activation of Ano1 channels during the plateau phase of slow waves are unknown, but the reverse mode of the Na⁺/Ca²⁺ exchange may contribute. We generated a mathematical model of pacemaker activity based on current empirical observations from ICC of mouse small intestine that incorporates functions of SOCE and NKCC1. This model reproduces experimental findings, suggesting roles for SOCE and Ano1 channels: blocking of either NKCC1 or SOCE in our model terminates pacemaker activity. Direct contribution of NKCC1 to pacemaker activity in a beat-to-beat manner is not predicted by our model. Instead, NKCC1 plays a maintenance role supporting the driving force for Cl^- efflux. Incorporation of SOCE allows the model to drive pacemaker activity without a diastolic depolarization, as observed in cardiac pacemaking. Further biological experiments are necessary to validate and further refine the roles of NKCC1, Na⁺/Ca²⁺ exchange, and Ano1 in the pacemaker mechanism of ICC.

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.

Induction of divalent cation permeability by heterologous expression of a voltage sensor domain

The voltage sensor domain (VSD) is a protein domain that confers sensitivity to membrane potential in voltage-gated ion channels as well as the voltage-sensing phosphatase. Although VSDs have long been considered to function as regulatory units acting on adjacent effectors, recent studies have revealed the existence of direct ion permeation paths in some mutated VSDs and in the voltage-gated proton channel. In this study, we show that calcium currents are evoked upon membrane hyperpolarization in cells expressing a VSD derived from an ascidian voltage-gated ion channel superfamily. Unlike the previously reported omega-pore in the Shaker K⁺ channel and rNav1.4, mutations are not required. From electrophysiological experiments in heterologous expression systems, we found that the conductance is directly mediated by the VSD itself and is carried by both monovalent and divalent cations. This is the first report of divalent cation permeation through a VSD-like structure.

Phototaxis and chemotaxis of brown algal swarmers

Brown algae exhibit three patterns of sexual reproduction: isogamy, anisogamy, and oogamy. Unicellular swarmers including gametes and zoospores bear two heterogenous flagella, an anterior flagellum with mastigonemes (fine tripartite hairs) and a posterior one. In seawater, these flagellates usually receive physico-chemical signals for finding partners and good habitats. It is well known that brown algal swarmers change their swimming direction depending on blue light (phototaxis), and male gametes do so, based on the sex pheromones from female gametes (chemotaxis). In recent years, the comparative analysis of chemotaxis in isogamy, anisogamy, and oogamy has been conducted. In this paper, we focused on the phototaxis and chemotaxis of brown algal gametes comparing the current knowledge with our recent studies.

Possible ability of bovine follicular fluid to attract migrating bull spermatozoa

Aim

To examine the potential of bovine follicular fluid (BFF) to attract bull spermatozoa.

Methods

The ability of the BFF to attract bull sperm was evaluated by observing changes in sperm migration after being placed in a cross‐column chamber. The movement parameters of the heads and flagella of the sperm that were attracted to the BFF were analyzed by using the Computer Assisted Sperm Analysis system.

Results

It was observed that 61.6% of the bull sperm migrated toward the BFF when the BFF was used at a concentration of 0.1%, but 67.2% of the sperm did not migrate toward the BFF at a concentration of 10%. Relatively larger numbers of both precapacitated and postcapacitated bull sperm migrated toward the BFF (0.1%). The ability of the 0.1% BFF to attract sperm probably affected both the normal artificial insemination (AI) fertility sperm and the poor AI fertility spermatozoa. The flagellar curvilinear ratio of the sperm winding to the 0.1% BFF was significantly higher than that of the prewinding sperm.

Conclusion

These results could suggest that BFF potentially attracts bull sperm at a certain concentration, irrespective of the capacitation status of the sperm. Although the mechanism by which this attraction occurs remains unclear, these data imply that it could be related to BFF‐dependent changes in the sperm flagellar curvilinear ratio.

Chemical and physical guidance of fish spermatozoa into the egg through the micropyle†,‡

Eggs of teleost fish, unlike those of many other animals, allow sperm entry only at a single site, a narrow canal in the egg's chorion called the micropyle. In some fish (e.g., flounder, herring, and Alaska pollock), the micropyle is a narrow channel in the chorion, with or without a shallow depression around the outer opening of micropyle. In some other fish (e.g., salmon, pufferfish, cod, and medaka), the micropyle is like a funnel with a conical opening. Eggs of all the above fish have a glycoprotein tightly bound to the chorion surface around the micropyle. This glycoprotein directs spermatozoa into the micropylar canal in a Ca2+-dependent manner. This substance, called the micropylar sperm attractant or MISA, increases fertilization efficiency and is essential in herring. In flounder, salmon, and perhaps medaka, fertilization is possible without MISA, but its absence makes fertilization inefficient because most spermatozoa swim over the micropyle without entering it. The mechanism underlying sperm-MISA interactions is yet to be determined, but at least in herring the involvement of Ca2+ and K+ channel proteins, as well as CatSper and adenylyl cyclase, is very likely. In some other fish (e.g., zebrafish, loach, and goldfish), the chorion around the micropyle is deeply indented (e.g., zebrafish and loach) or it has radially or spirally arranged grooves around the outer opening of the micropyle (e.g., goldfish). MISA is absent from the eggs of these fish and sperm entry into micropylar canal seems to be purely physical.

Role of Ca2+ in the IVM of spermatozoa from the sterlet Acipenser ruthenus

The role of Ca2+ in sturgeon sperm maturation and motility was investigated. Sperm from mature male sterlets (Acipenser ruthenus) were collected from the Wolffian duct and testis 24 h after hormone induction. Testicular spermatozoa (TS) were incubated in Wolffian duct seminal fluid (WDSF) for 5 min at 20°C and were designated ‘TS after IVM’ (TSM). Sperm motility was activated in media with different ion compositions, with motility parameters analysed from standard video microscopy records. To investigate the role of calcium transport in the IVM process, IVM was performed (5 min at 20°C) in the presence of 2 mM EGTA, 100 µM Verapamil or 100 µM Tetracaine. No motility was observed in the case of TS (10 mM Tris, 25 mM NaCl, 50 mM Sucr with or without the addition of 2 mM EGTA). Both incubation of TS in WDSF and supplementation of the activation medium with Ca2+ led to sperm motility. The minimal Ca2+ concentration required for motility activation of Wolffian duct spermatozoa, TS and TSM was determined (1–2 nM for Wolffian duct spermatozoa and TSM; approximately 0.6 mM for TS). Motility was obtained after the addition of verapamil to the incubation medium during IVM, whereas the addition of EGTA completely suppressed motility, implying Ca2+ involvement in sturgeon sperm maturation. Further studies into the roles of Ca2+ transport in sturgeon sperm maturation and motility are required.

Metabolic Disorders and Cancer: Store-Operated Ca2+ Entry in Cancer: Focus on IP3R-Mediated Ca2+ Release from Intracellular Stores and Its Role in Migration and Invasion

Store-operated calcium entry (SOCE) plays important roles in a multitude of cellular processes, from muscle contraction to cellular proliferation and migration. Dysregulation of SOCE is responsible for the advancement of multiple diseases, ranging from immune diseases, myopathies, to terminal ones like cancer. Naturally, SOCE has been a focus of many studies and review papers which, however, primarily concentrated on the principal players localized to the plasma membrane and responsible for Ca²⁺ entry into the cell. Much less has been said about other players participating in the entire SOCE event. This review aims to address this shortcoming by discussing the accumulated scientific knowledge focused on the inositol trisphosphate receptors (IP₃Rs), principal player responsible for emptying intracellular Ca²⁺ stores in a majority of cells, and their involvement in regulation of cell migration and invasion in cancer.

Sperm motility parameters and spermatozoa morphometric characterization in marine species: a study of swimmer and sessile species

The biodiversity of marine ecosystems is diverse and a high number of species coexist side by side. However, despite the fact that most of these species share a common fertilization strategy, a high variability in terms of the size, shape, and motion of spermatozoa can be found. In this study, we have analyzed both the sperm motion parameters and the spermatozoa morphometric features of two swimmer (pufferfish and European eel) and two sessile (sea urchin and ascidian) marine species. The most important differences in the sperm motion parameters were registered in the swimming period. Sessile species sperm displayed notably higher values than swimmer species sperm. In addition, the sperm motilities and velocities of the swimmer species decreased sharply once the sperm was activated, whereas the sessile species were able to maintain their initial values for a long time. These results are linked directly to the species-specific lifestyles. Although sessile organisms, which show limited or no movement, need sperm with a capacity to swim for long distances to find the oocytes, swimmer organisms can move toward the female and release gametes near it, and therefore the spermatozoa does not need to swim for such a long time. At the same time, sperm morphology is related to sperm motion parameters, and in this study an in-depth morphometric analysis of ascidian, sea urchin, and pufferfish spermatozoa, using computer-assisted sperm analysis software, has been carried out for the first time. A huge variability in shapes, sizes, and structures of the studied species was found using electron microscopy.

Sperm guidance to the egg finds calcium at the helm

Sperm respond to multiple cues during guidance to the egg including chemical attractants, temperature, and fluid flow. Of these, sperm chemotaxis has been studied most extensively-over 100 years-but only recently has it started to be understood at the molecular level. The long gestation in this understanding has largely been due to technical limitations that include the detection of calcium signal dynamics in a relatively small structure-the flagellum, measurement of actual chemoattractant gradients, the fact that only subpopulations of sperm respond at any given time, and the diversity in swimming behaviors that sperm exhibit from different species. Today, measurements of flagellar calcium signals on a fast time scale, discovery of the ion channels and organelles that may regulate these signals, and better understanding and quantitation of sperm swimming behaviors involved have given more certainty to our understanding of sperm directional swimming and its control by characteristic, calcium-directed asymmetric flagellar bends. Future research will need to apply these technical advances to other forms of sperm guidance such as thermotaxis and rheotaxis as well as gaining an understanding of how the flagellar apparatus is controlled by calcium.

Calcium influx and male fertility in the context of the sperm proteome: an update

Freshly ejaculated spermatozoa are incapable or poorly capable of fertilizing an oocyte. The fertilization aptness of spermatozoa depends on the appropriate and time-dependent acquisition of hyperactivation, chemotaxis, capacitation, and the acrosome reaction, where calcium (Ca²⁺) is extensively involved in almost every step. A literature review showed that several ion channel proteins are likely responsible for regulation of the Ca²⁺ uptake in spermatozoa. Therefore, manipulation of the functions of channel proteins is closely related to Ca²⁺ influx, ultimately affecting male fertility. Recently, it has been shown that, together with different physiological stimuli, protein-protein interaction also modifies the Ca²⁺ influx mechanism in spermatozoa. Modern proteomic analyses have identified several sperm proteins, and, therefore, these findings might provide further insight into understanding the Ca²⁺ influx, protein functions, and regulation of fertility. The objective of this review was to synthesize the published findings on the Ca²⁺ influx mechanism in mammalian spermatozoa and its implications for the regulation of male fertility in the context of sperm proteins. Finally, Pathway Studio (9.0) was used to catalog the sperm proteins that regulate the Ca²⁺ influx signaling by using the information available from the PubMed database following a MedScan Reader (5.0) search.

T-type calcium channel blockers as neuroprotective agents

T-type calcium channels are expressed in many diverse tissues, including neuronal, cardiovascular, and endocrine. T-type calcium channels are known to play roles in the development, maintenance, and repair of these tissues but have also been implicated in disease when not properly regulated. Calcium channel blockers have been developed to treat various diseases and their use clinically is widespread due to both their efficacy as well as their safety. Aside from their established clinical applications, recent studies have suggested neuroprotective effects of T-type calcium channel blockers. Many of the current T-type calcium channel blockers could act on other molecular targets besides T-type calcium channels making it uncertain whether their neuroprotective effects are solely due to blocking of T-type calcium channels. In this review, we discuss these drugs as well as newly developed chemical compounds that are designed to be more selective for T-type calcium channels. We review in vitro and in vivo evidence of neuroprotective effects by these T-type calcium channel blockers. We conclude by discussing possible molecular mechanisms underlying the neuroprotective effects by T-type calcium channel blockers.

Species-specificity of sperm motility activation and chemotaxis: a study on ascidian species

Egg-derived sperm-activating factors and attractants activate sperm motility and attract the sperm, respectively. These phenomena constitute the first communication signaling between males and females in the process of fertilization in many animals and plants, and in many cases, these are species-specific events. Thus, sperm motility activation and chemotaxis may act as a safety process for the authentication between conspecific egg and sperm, and help to prevent crossbreeding. Here, we examine species-specificity of sperm motility activation and chemotaxis in the ascidians belonging to the order Phlebobranchiata: Ciona intestinalis, Ciona savignyi, Phallusia mammillata, Phallusia nigra, and Ascidia sydneiensis. Cross-reactivity in both motility activation and chemotaxis of sperm was not observed between C. savignyi and P. mammillata, or between A. sydneiensis and Phallusia spp. However, there is a "one way" (no reciprocity) cross-reaction between P. mammillata and P. nigra in sperm activation, and between C. savignyi and A. sydneiensis in sperm chemotaxis. Furthermore, the level of activity is different, even when cross-reaction is observed. Thus, sperm motility activation and chemotaxis are neither "species-" nor "genus-" specific phenomena among the ascidian species. Moreover, the interaction between the sperm-activating and sperm-attracting factors (SAAFs) in the ascidian species and the SAAF receptors on the sperm cells are not all-or-none responses.

Sperm-attractant peptide influences the spermatozoa swimming behavior in internal fertilization in Octopus vulgaris

Marine invertebrates exhibit both chemokinesis and chemotaxis phenomena, induced in most cases by the release of water-borne peptides or pheromones. In mollusks, several peptides released during egg-laying improve both male attraction and mating. Unlike other cephalopods, Octopus vulgaris adopts an indirect internal fertilization strategy. We here report on the identification and characterization of a chemoattractant peptide isolated from mature eggs of octopus females. Using two-chamber and time-lapse microscopy assays, we demonstrate that this bioactive peptide is able to increase sperm motility and induce chemotaxis by changing the octopus spermatozoa swimming behavior in a dose-dependent manner. We also provide evidence that chemotaxis in the octopus requires the presence of extracellular calcium and membrane protein phophorylation at tyrosine. This study is the first report on a sperm-activating factor in a non-free-spawning marine animal.

Bidirectional Ca²⁺ signaling occurs between the endoplasmic reticulum and acidic organelles

After acidic organelles induce signaling to activate ER calcium ion release, local microdomains of high calcium at ER–acidic organelle junctions feed back to activate further acidic organelle calcium release.

The endoplasmic reticulum (ER) and acidic organelles (endo-lysosomes) act as separate Ca²⁺ stores that release Ca²⁺ in response to the second messengers IP₃ and cADPR (ER) or NAADP (acidic organelles). Typically, trigger Ca²⁺ released from acidic organelles by NAADP subsequently recruits IP₃ or ryanodine receptors on the ER, an anterograde signal important for amplification and Ca²⁺ oscillations/waves. We therefore investigated whether the ER can signal back to acidic organelles, using organelle pH as a reporter of NAADP action. We show that Ca²⁺ released from the ER can activate the NAADP pathway in two ways: first, by stimulating Ca²⁺-dependent NAADP synthesis; second, by activating NAADP-regulated channels. Moreover, the differential effects of EGTA and BAPTA (slow and fast Ca²⁺ chelators, respectively) suggest that the acidic organelles are preferentially activated by local microdomains of high Ca²⁺ at junctions between the ER and acidic organelles. Bidirectional organelle communication may have wider implications for endo-lysosomal function as well as the generation of Ca²⁺ oscillations and waves.

Allurin, an amphibian sperm chemoattractant having implications for mammalian sperm physiology

Eggs of many species are surrounded by extracellular coats that emit ligands to which conspecific sperm respond by undergoing chemotaxis and changes in metabolism, motility, and acrosomal status in preparation for fertilization. Here we review methods used to measure sperm chemotaxis and focus on recent studies of allurin, a 21-kDa protein belonging to the Cysteine-RIch Secretory Protein (CRISP) family that has chemoattraction activity for both amphibian and mammalian sperm. Allurin is unique in being the first extensively characterized Crisp protein found in the female reproductive tract and is the product of a newly discovered amphibian gene within a gene cluster that has been largely conserved in mammals. Study of its expression, function, and tertiary structure could lead to new insights in the role of Crisp proteins in sperm physiology.

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.

Lipid rafts function in Ca2+ signaling responsible for activation of sperm motility and chemotaxis in the ascidian Ciona intestinalis

Lipid rafts are specialized membrane microdomains that function as signaling platforms across plasma membranes of many animal and plant cells. Although there are several studies implicating the role of lipid rafts in capacitation of mammalian sperm, the function of these structures in sperm motility activation and chemotaxis remains unknown. In the ascidian Ciona intestinalis, egg-derived sperm activating- and attracting-factor (SAAF) induces both activation of sperm motility and sperm chemotaxis to the egg. Here we found that a lipid raft disrupter, methyl-β-cyclodextrin (MCD), inhibited both SAAF-induced sperm motility activation and chemotaxis. MCD inhibited both SAAF-promoted synthesis of intracellular cyclic AMP and sperm motility induced by ionophore-mediated Ca(2+) entry, but not that induced by valinomycin-mediated hyperpolarization. Ca(2+)-imaging revealed that lipid raft disruption inhibited Ca(2+) influx upon activation of sperm motility. The Ca(2+)-activated adenylyl cyclase was clearly inhibited by MCD in isolated lipid rafts. The results suggest that sperm lipid rafts function in signaling upstream of cAMP synthesis, most likely in SAAF-induced Ca(2+) influx, and are required for Ca(2+)-dependent pathways underlying activation and chemotaxis in Ciona sperm.

The use of anti-VDAC2 antibody for the combined assessment of human sperm acrosome integrity and ionophore A23187-induced acrosome reaction

Voltage-dependent anion channel (VDAC) is mainly located in the mitochondrial outer membrane and participates in many biological processes. In mammals, three VDAC subtypes (VDAC1, 2 and 3) have been identified. Although VDAC has been extensively studied in various tissues and cells, there is little knowledge about the distribution and function of VDAC in male mammalian reproductive system. Several studies have demonstrated that VDAC exists in mammalian spermatozoa and is implicated in spermatogenesis, sperm maturation, motility and fertilization. However, there is no knowledge about the respective localization and function of three VDAC subtypes in human spermatozoa. In this study, we focused on the presence of VDAC2 in human spermatozoa and its possible role in the acrosomal integrity and acrosome reaction using specific anti-VDAC2 monoclonal antibody for the first time. The results exhibited that native VDAC2 existed in the membrane components of human spermatozoa. The co-incubation of spermatozoa with anti-VDAC2 antibody did not affect the acrosomal integrity and acrosome reaction, but inhibited ionophore A23187-induced intracellular Ca²⁺ increase. Our study suggested that VDAC2 was located in the acrosomal membrane or plasma membrane of human spermatozoa, and played putative roles in sperm functions through mediating Ca²⁺ transmembrane transport.

Molecular mechanism for human sperm chemotaxis mediated by progesterone

Sperm chemotaxis is a chemical guiding mechanism that may orient spermatozoa to the egg surface. A picomolar concentration gradient of Progesterone (P), the main steroidal component secreted by the cumulus cells that surround the egg, attracts human spermatozoa. In order to elucidate the molecular mechanism of sperm chemotaxis mediated by P, we combine the application of different strategies: pharmacological inhibition of signaling molecules, measurements of the concentrations of second messengers and activation of the chemotactic signaling. Our data implicate a number of classic signal transduction pathways in the response and provide a model for the sequence of events, where the tmAC-cAMP-PKA pathway is activated first, followed by protein tyrosine phosphorylation (equatorial band and flagellum) and calcium mobilization (through IP(3)R and SOC channels), whereas the sGC-cGMP-PKG cascade, is activated later. These events lead to sperm orientation towards the source of the chemoattractant. The finding proposes a molecular mechanism which contributes to the understanding of the signal transduction pathway that takes place in a physiological process as chemotaxis.

Endogenous signaling pathways and chemical communication between sperm and egg

Red abalone (Haliotis rufescens) sperm detect a waterborne chemical cue released by conspecific eggs, and change their swimming behavior to increase the likelihood of fertilization success. Previously, we isolated the natural sperm attractant by bioassay-guided fractionation and high-performance liquid chromatography, and chemically identified it as the free-amino acid l-tryptophan (l-Trp). In the present study, levels of this ecologically meaningful compound were quantified in various abalone tissues, and in freshly spawned eggs. Tryptophan was the least abundant of 19 dissolved free amino acids (DFAAs) in ovary, testis, foot muscle, gill, stomach and hemolymph. As a proportion of the DFAA pool, however, Trp concentrations were significantly elevated in eggs (three- to seven-times higher) relative to all other sampled tissues. Natural rates of Trp release from eggs also were measured and correlated with fertility. Fertilization success peaked during an initial 30 min period (post-spawn), but decreased to nil over the next 50 min. Closely paralleling these events, Trp accumulated in seawater around freshly spawned eggs for the first 45 min (post-spawn) before decaying rapidly from solution. Older eggs stopped releasing Trp approximately when they became infertile, revealing a critical link between gamete physiology and chemical signaling. This apparent negative feedback loop did not arise from tryptophan oxidation, uptake by bacteria in seawater, or a degrading enzyme released by eggs. As a metabolic precursor critical to development of the larval nervous system, Trp could be an honest indicator of egg fitness for prospective sperm suitors. Our results suggest that endogenous signaling pathways have been co-opted for external communication between gametes, as an adaptation to increase reproductive success by promoting sperm navigation towards fertile eggs.

How to track spermatozoa using high-speed visual feedback

In this paper, we report how to track quickly and vigorously swimming ascidian spermatozoa using high-speed visual feedback at a frame rate of 1 kHz. Ascidian spermatozoa swim as fast as 300 microm/s by rotating their flagella 50 times/s. This vigorous swimming style has prevented stable image observation and made it difficult to track them reliably with our previously developed visual tracking system. Here, we describe how we overcame these problems using image processing techniques to achieve stable tracking of fast, small ascidian spermatozoa for more than 180 s using high-speed visual feedback.

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.

Ca2+ bursts occur around a local minimal concentration of attractant and trigger sperm chemotactic response

Ca(2+) is known to have important roles in sperm chemotaxis, although the relationship between intracellular Ca(2+) concentration ([Ca(2+)](i)) and modulation of the swimming and chemotactic behavior of spermatozoa has not been elucidated. Using a high-speed Ca(2+) imaging system, we examined the chemotactic behavior and [Ca(2+)](i) in individual ascidian sperm cells exhibiting chemotactic responses toward sperm activating and attracting factor (SAAF), a chemoattractant released by eggs. In this study, we found that transient [Ca(2+)](i) increased in the flagellum (Ca(2+) bursts) concomitantly with a change in the swimming direction in an SAAF gradient field. During the initial phase of the Ca(2+) bursts, the flagellum of the spermatozoon exhibited highly asymmetric waveforms enabling the quick turning of the swimming path. However, the flagellum subsequently changed to symmetric beating, causing the spermatozoon to swim straight. Interestingly, during such responses, [Ca(2+)](i) remained higher than the basal level, indicating that the series of responses was not simply determined by Ca(2+) concentrations. Also, we found that Ca(2+) bursts were consistently evoked at points at which the spermatozoon attained around a temporally minimal value for a given SAAF concentration. We concluded that Ca(2+) bursts induced around a local minimal SAAF concentration trigger a sequence of flagellar responses comprising quick turning followed by straight swimming to direct spermatozoa efficiently toward eggs.

Sex and flow: the consequences of fluid shear for sperm–egg interactions

Fertilization is a complex interaction among biological traits of gametes and physical properties of the fluid environment. At the scale of fertilization (0.01–1 mm), sperm encounter eggs while being transported within a laminar (or viscous) shear flow. Varying laminar-shear in a Taylor-Couette flow tank, our experiments simulated important aspects of small-scale turbulence within the natural habitats of red abalone(Haliotis rufescens), a large marine mollusk and external fertilizer. Behavioral interactions between individual cells, sperm–egg encounter rates, and fertilization success were quantified, simultaneously, using a custom-built infrared laser and computer-assisted video imaging system. Relative to still water, sperm swam faster and moved towards an egg surface,but only in comparatively slow flows. Encounter rate, swim speed and orientation, and fertilization success each peaked at the lowest shear tested(0.1 s–1), and then decayed as shear increased beyond 1.0 s–1. The decay did not result, however, from damage to either sperm or eggs. Analytical and numerical models were used to estimate the propulsive force generated by sperm swimming (Fswim) and the shear force produced by fluid motion within the vicinity of a rotating egg(Fshear). To first order, male gametes were modeled as prolate spheroids. The ratio Fswim/Fshear was useful in explaining sperm–egg interactions. At low shears where Fswim/Fshear&gt;1, sperm swam towards eggs, encounter rates were pronounced, and fertilization success was very high; behavior overpowered fluid motion. In contrast, sperm swimming,encounter rate and fertilization success all decayed rapidly when Fswim/Fshear&lt;1; fluid motion dominated behavior. The shears maximizing fertilization success in the lab typically characterized natural flow microenvironments of spawning red abalone. Gamete behavior thus emerges as a critical determinant of sexual reproduction in the turbulent sea.

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.

Defining the roles of Ca2+ — permeable channels in sperm

Ion channels exert a vital role in the dialogue between male and female gametes and thus in the generation of new individuals in many species. Intracellular Ca2+ is possibly the key messenger between gametes. Different Ca2+-permeable channels have been detected in the plasma membrane and in the organelle-like acrosome membrane of sperm, which play vital roles in determining sperm fertilizing ability. Recent reports from several laboratories have adequately documented that the Ca2+-permeable channels of a sperm control a variety of functions ranging from motility to the acrosome reaction. In this article, we have reviewed the data from our and other laboratories, and have documented the mechanisms of different Ca2+-permeable channels involved in the fertilization event.

Eggs regulate sperm flagellar motility initiation, chemotaxis and inhibition in the coral Acropora digitifera, A. gemmiferaand A. tenuis

Corals perform simultaneous mass spawning around the full moon. Most Acropora species release gamete bundles, which are a complex of eggs and sperm, into the seawater. Then, gamete bundles are separated into eggs and sperm. Eggs are fertilized when sperm and eggs come in contact with each other. However, it is still unclear how sperm meet the eggs of the same species in the presence of many eggs of different species and how eggs guard against the fertilization attempts by sperm of different species. In this study, we observed that A. digitifera, A. gemmifera and A. tenuis sperm showed motility initiation/attraction close to eggs. Sperm were completely immotile in seawater, but they began to swim in circular motion when they came in close proximity to eggs, and then approached the eggs in straightforward paths. Sperm flagellar motility was not activated by an egg from different species, suggesting that motility initiation by the egg is species specific. In addition, hybridization among these species did not occur under observed conditions. Furthermore, motility-activated sperm became quiescent when many sperm approached the eggs. This study is the first report to show that the egg secretes immobilization factor(s). Our results suggest that the flagellar motility regulation has evolved to avoid hybridization among different species during the mass spawning.

Contributions of extracellular and intracellular Ca2+ to regulation of sperm motility: Release of intracellular stores can hyperactivate CatSper1 and CatSper2 null sperm

In order to fertilize, mammalian sperm must hyperactivate. Hyperactivation is triggered by increased flagellar Ca(2+), which switches flagellar beating from a symmetrical to an asymmetrical pattern by increasing bending to one side. Thimerosal, which releases Ca(2+) from internal stores, induced hyperactivation in mouse sperm within seconds, even when extracellular Ca(2+) was buffered with BAPTA to approximately 30 nM. In sperm from CatSper1 or CatSper2 null mice, which lack functional flagellar alkaline-activated calcium currents, 50 microM thimerosal raised the flagellar bend amplitudes from abnormally low levels to normal pre-hyperactivated levels and, in 20-40% of sperm, induced hyperactivation. Addition of 1 mM Ni(2+) diminished the response. This suggests that intracellular Ca(2+) is abnormally low in the null sperm flagella. When intracellular Ca(2+) was reduced by BAPTA-AM in wild-type sperm, they exhibited flagellar beat patterns more closely resembling those of null sperm. Altogether, these results indicate that extracellular Ca(2+) is required to supplement store-released Ca(2+) to produce maximal and sustained hyperactivation and that CatSper1 and CatSper2 are key elements of the major Ca(2+) entry pathways that support not only hyperactivated motility but possibly also normal pre-hyperactivated motility.

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.

Role of the Na+/Ca2+ exchanger in calcium homeostasis and human sperm motility regulation

A number of cell functions, such as flagellar beating, swimming velocity, acrosome reaction, etc., are triggered by a Ca2+ influx across the cell membrane. For appropriate physiological functions, the motile human sperm maintains the intracellular free calcium concentration ([Ca2+]i) at a submicromolar level. The objective of this study was to determine the role of the Na+/Ca2+ exchanger (NCX) in the maintenance of [Ca2+]i in human spermatozoa. Spermatozoa maintained in extracellular medium containing>or=1 microM Ca2+ exhibited motility similar to that of the control. In addition to several calcium transport mechanisms described earlier, we provide evidence that the NCX plays a crucial role in the maintenance of [Ca2+]i. Three chemically unrelated inhibitors of the NCX (bepridil, DCB (3',4'-dichlorobenzamil hydrochloride), and KB-R7943) all blocked human sperm motility in a dose and incubation time dependent manner. The IC50 values for bepridil, DCB, and KB-R7943 were 16.2, 9.8, and 5.3 microM, respectively. The treatment with the above-mentioned blockers resulted in an elevated [Ca2+]i and a decreased [Na+]i. The store-operated calcium channel (SOCC) inhibitor SKF 96365 also blocked the sperm motility (IC50=2.44 microM). The presence of the NCX antigen in the human spermatozoa was proven by flow cytometry, confocal laser scanning microscopy, and immunoblotting techniques. Calcium homeostasis of human spermatozoa is maintained by several transport proteins among which the SOCC and the NCX may play a major role.

Sperm-derived sperm motility-initiating substance from amphioxus Branchiostoma belcheri

The sperm of amphioxus, Branchiostoma belcheri, were immotile when excised from the testis and suspended in seawater. The sperm became motile upon spawning in natural seawater, suggesting mechanisms that triggered sperm motility during spawning. When a male amphioxus that underwent spawning was transferred to a cup containing a small amount of natural seawater, and then the seawater containing the spawned sperm was centrifuged, the supernatant caused motility initiation in the immotile sperm from the testis. This sperm motility-initiating activity was also found in the supernatant of seawater in which immotile sperm from the testis were incubated overnight. These suggest that in the amphioxus, a sperm motility-initiating substance resides in the sperm, and upon spawning, the substance is transformed into a free and active form to activate the sperm. Partial purification of the substance revealed it as a small and heat-stable substance with maximum UV absorbance at 234 nm.

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.

Ca2+ spikes in the flagellum control chemotactic behavior of sperm

The events that occur during chemotaxis of sperm are only partly known. As an essential step toward determining the underlying mechanism, we have recorded Ca2+ dynamics in swimming sperm of marine invertebrates. Stimulation of the sea urchin Arbacia punctulata by the chemoattractant or by intracellular cGMP evokes Ca2+ spikes in the flagellum. A Ca2+ spike elicits a turn in the trajectory followed by a period of straight swimming ('turn-and-run'). The train of Ca2+ spikes gives rise to repetitive loop-like movements. When sperm swim in a concentration gradient of the attractant, the Ca2+ spikes and the stimulus function are synchronized, suggesting that precise timing of Ca2+ spikes controls navigation. We identified the peptide asterosap as a chemotactic factor of the starfish Asterias amurensis. The Ca2+ spikes and swimming behavior of sperm from starfish and sea urchin are similar, implying that the signaling pathway of chemotaxis has been conserved for almost 500 million years.

Activation of motility and chemotaxis in the spermatozoa: From invertebrates to humans

Activation of the sperm motility and chemotactic behavior of sperm toward eggs are the first communication between spermatozoa and eggs at fertilization, and understanding of the phenomena is a prerequisite for progress of not only basic biology, but also clinical aspects. The nature of molecules derived from eggs by which sperm are activated and attracted towards the eggs and the molecular mechanisms underlying the sperm activation and chemotaxis have been investigated in only a few invertebrate species, sea urchins, ascidians and herring fish. However, knowledge on this phenomena has been ignored in mammalian species including humans. The current review first introduces the studies on the activation and chemotaxis of sperm in marine invertebrates, and the same phenomena in mammals including humans, are described. (Reprod Med Biol 2005; 4: 101-115).

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.