Requirement of Ca2+ on activation of sperm motility in euryhaline tilapia Oreochromis mossambicus

Deleterious effect of gestagens from wastewater effluent on fish reproduction in aquatic environment: A review

Gestagens are common pollutants accumulated in the aquatic ecosystem. Gestagens are comprised of natural gestagens (i.e. progesterone) and synthetic gestagens (i.e. progestins). The major contributors of gestagens in the environment are paper plant mill effluent, wastewater treatment plants, discharge from pharmaceutical manufacturing, and livestock farming. Gestagens present in the aquatic environment interact with progesterone receptors and other steroid hormone receptors, negatively influencing fish reproduction, development, and behavior. In fish, the gonadotropin induces 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) production, an important steroid hormone involved in gametogenesis. DHP interacts with the membrane progestin receptor (mPR), which regulates sperm motility and oocyte maturation. Gestagens also interfere with the hypothalamic-pituitary-gonadal (HPG) axis, which results in altered hormone levels in fish. Moreover, recent studies showed that even at low concentrations exposure to gestagens can have detrimental effects on fish reproduction, including reduced egg production, masculinization, feminization in males, and altered sex ratio, raising concerns about their impact on the fish population. This review highlights the hormonal regulation of sperm motility, oocyte maturation, the concentration of environmental gestagens in the aquatic environment, and their detrimental effects on fish reproduction. However, the long-term and combined impacts of multiple gestagens, including their interactions with other pollutants on fish populations and ecosystems are not well understood. The lack of standardized regulations and monitoring protocols for gestagens pollution in wastewater effluent hampers effective control and management. Nonetheless, advancements in analytical techniques and biomonitoring methods provide potential solutions by enabling better detection and quantification of gestagens in aquatic ecosystems.

Osmoregulation in fish sperm

In most fish exhibiting external fertilization, spermatozoa become motile after release into water, triggered by differences between intracellular and extracellular conditions such as osmotic pressure, ion composition, and pH. The rapid change in osmolarity initiating spermatozoon motility induces osmotic pressure, resulting in active water movement across the cell membrane. Mechanisms of ion and water transport across the plasma membrane and cell volume regulation are important in maintaining structure and functional integrity of the cell. The capacity of the fish spermatozoon plasma membrane to adapt to dramatic environmental changes is an essential prerequisite for motility and successful fertilization. Adaptation to change in external osmolality may be the basis of spermatozoon function and an indicator of sperm quality. The involvement of specific water channels (aquaporins) in cell volume regulation and motility is highly likely. The goal of this review is to describe basic mechanisms of water transport and their role in fish spermatozoon physiology, focusing on osmoresistance, cell volume regulation, motility, and survival.

Regulation of sperm motility in Eastern oyster (Crassostrea virginica) spawning naturally in seawater with low salinity

Oyster aquaculture is expanding worldwide, where many farms rely on seed produced by artificial spawning. As sperm motility and velocity are key determinants for fertilization success, understanding the regulation of sperm motility and identifying optimal environmental conditions can increase fertility and seed production. In the present study, we investigated the physiological mechanisms regulating sperm motility in Eastern oyster, Crassostrea virginica. Sperm motility was activated in ambient seawater with salinity 4-32 PSU with highest motility and velocity observed at 12-24 PSU. In artificial seawater (ASW) with salinity of 20 PSU, sperm motility was activated at pH 6.5-10.5 with the highest motility and velocity recorded at pH 7.5-10.0. Sperm motility was inhibited or totally suppressed in Na+, K+, Ca2+, and Mg2+-free ASW at 20 PSU. Applications of K+ (500 μM glybenclamide and 10-50 mM 4-aminopyridine), Ca2+ (1-50 μM mibefradil and 10-200 μM verapamil), or Na+ (0.2-2.0 mM amiloride) channel blockers into ASW at 20 PSU inhibited or suppressed sperm motility and velocity. Chelating extracellular Ca2+ ions by 3.0 and 3.5 mM EGTA resulted in a significant reduction and full suppression of sperm motility by 4 to 6 min post-activation. These results suggest that extracellular K+, Ca2+, and Na+ ions are involved in regulation of ionic-dependent sperm motility in Eastern oyster. A comparison with other bivalve species typically spawning at higher salinities or in full-strength seawater shows that ionic regulation of sperm motility is physiologically conserved in bivalves. Elucidating sperm regulation in C. virginica has implications to develop artificial reproduction, sperm short-term storage, or cryopreservation protocols, and to better predict how changes in the ocean will impact oyster spawning dynamics.

The role of alkalinization-induced Ca2+ influx in sperm motility activation of a viviparous fish Redtail Splitfin (Xenotoca eiseni)

Mechanisms regulating sperm motility activation are generally known in oviparous fishes, but are poorly understood in viviparous species. The mechanism of osmotic-shock induced signaling for oviparous fishes is not suitable for viviparous fishes which activate sperm motility within an isotonic environment. In addition, the presence of sperm bundles in viviparous fishes further complicates study of sperm activation mechanisms. The goal of this study was to establish methodologies to detect intracellular Ca2+ signals from sperm cells within bundles, and to investigate the signaling mechanism of sperm activation of viviparous fish using Redtail Splitfin (Xenotoca eiseni) as a model. Motility was assessed by classification of bundle dissociation and computer-assisted sperm analysis, and intracellular Ca2+ was assessed using the fluorescent probe Fura-2 AM. Bundle dissociation and sperm motility increased with extracellular Ca2+ and pH levels. Intracellular Ca2+ signals were detected from sperm within bundles, and increased significantly with extracellular Ca2+ and pH levels. Major channel blockers known to inhibit Ca2+ influx (NiCl2, ruthenium red, GdCl3, SKF-96365, nimodipine, verapamil, methoxyverapamil, mibefradil, NNC 55-0396, ω-Conotoxin MVIIC, bepridil, and 2-APB) failed to inhibit Ca2+ influx, except for CdCl2, which partially inhibited the influx. We propose a novel mechanism for motility regulation of fish sperm: an alkaline environment in the female reproductive tract opens Ca2+ channels in the sperm plasma membrane without osmotic shock, and the Ca2+ influx functions as a second messenger to activate motor proteins controlling flagella movement.

Effects of temperature on sperm motility of burbot Lota lota: spontaneous activation and calcium dependency

Several factors regulating activation of spermatozoon motility in Eurasian burbot, Lota lota, including osmolality, calcium (Ca²⁺ ) ions, and temperature were investigated. Spermatozoon motility in Eurasian burbot, Lota lota was assessed at 4 and 30°C in seminal fluid, isotonic media (with and without Ca²⁺ ) and hypotonic media (with and without Ca²⁺ ). Spermatozoa were spontaneously activated in seminal fluid at 20°C and the maximum motility was recorded at 30°C, which is out of the spawning temperature range, indicating that no risk of activation occurs during routine semen handling in artificial insemination. Initiation of spermatozoon motility in L. lota is mediated by Ca²⁺ and sensitivity to Ca²⁺ is dependent on temperature.

Molecular Characterization of Microbiota Associated With Sperm of Malaysian Mahseer Tor tambroides

Malaysian Mahseer (Tor tambroides) is considered as a good prospect for aquaculture in Malaysia. However, knowledge about Malaysian Mahseer-associated sperm microbiota is still limited, although some studies reported that sperm-related bacteria are a factor in the decline of sperm quality, as sperm may become the carrier of pathogenic bacteria to the egg. The goal of this study was to evaluate the sperm microbiota associated with Malaysian Mahseer from 3 different locations (Universiti Malaysia Terengganu [UMT], Ajil, and Pahang) using polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting and to compare location differences by cluster analysis. Our results showed that the UMT sample had different sperm microbiota composition and a different trend in its relationship with sperm quality. Correlation analysis showed a relationship between bacterial diversity and sperm quality. Phylogenetic analysis indicated that sperm microbiota was composed of diverse phyla, including Proteobacteria, Firmicutes, and Actinobacteria. Interestingly, bacteria such as Salinisphaera sp., Pelomonas sp., and Staphylococcus spp. were detected in all the locations, suggesting that these bacteria are indigenous bacterial members of the Malaysian Mahseer sperm microbiota, although their function is still unclear.

Evaluation of seminal plasma composition and spermatozoa quality parameters of silver barb, Barbonymus gonionotus Bleeker, 1850

Seminal composition and semen quality are the important determinants in assessing the reproductive performance of different fishes. This study was carried out to evaluate the seminal composition and sperm quality of Barbonymus gonionotus. The seminal plasma contained 17.2 ± 0.34 mmol/l, 20.9 ± 0.48 mmol/l, 0.72 ± 0.04 mmol/l, 3.8 ± 0.2 mmol/l, and 1.49 ± 0.02 g/dl of Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, and total protein, respectively. The physical spermatological parameters, such as sperm volume, sperm motility, motility duration, sperm density, osmolality, and pH values were 1.55 ± 0.15 ml, 89 ± 2%, 391.9 ± 8.5 s, 2.8 ± 0.2 × 10¹⁰ /ml, 400.6 ± 5.1 mmol/kg, and 8.75 ± 0.10, respectively. In correlation matrix, the K⁺ (R² = 0.39, P < 0.01) and Ca⁺⁺ (R² = 0.27, P < 0.05) ions and osmolality (R² = 0.29, P < 0.05) showed significant positive correlations with sperm motility. Similarly, fertilization rate significantly influenced by sperm motility (R² = 0.26, P < 0.05) and K⁺ (R² = 0.30, P < 0.05) and Ca⁺⁺ (R² = 0.26, P < 0.05) ions. Also, osmolality significantly and negatively correlated with Mg⁺⁺ (R² = 0.33, P < 0.05) and sperm motility duration (R² = 0.28, P < 0.05). Therefore, based on this results, it can be concluded that seminal plasma ions, K⁺ and Ca⁺⁺ and osmolality are the key factors for the determination of sperm quality of silver barb, and these parameters could be considered during standardization of artificial fertilization or cryopreservation technique of silver barb spermatozoa.

Variations in the breeding behavior of cichlids and the evolution of the multi-functional seminal plasma protein, seminal plasma glycoprotein 120

Background

Seminal plasma proteins are associated with successful fertilization. However, their evolutionary correlation with fertilization mechanisms remains unclear. Cichlids from Lake Tanganyika show a variety-rich spawning behavior that is associated with the transfer of the sperm to the egg for fertilization. One of these behaviors, called “oral fertilization,” emerged during their speciation. In oral fertilization, females nuzzle the milt from male genitalia and pick up the released eggs in their mouths, which are then fertilized inside the oral cavity. Thus, the success of the fertilization is dependent on the retention of sperm in the oral cavity during spawning. Sperm aggregation and immobilization in viscous seminal plasma may help retain the sperm inside the oral cavity, which ultimately determines the success of the fertilization. Seminal plasma glycoprotein 120 (SPP120) is one of the major seminal plasma proteins present in cichlids. SPP120 has been implicated to immobilize sperm and increase the milt viscosity. However, the functional linkage between oral fertilization and seminal plasma proteins has not been investigated.

Results

During trials of simulated oral fertilization, it was observed that milt viscosity contributed to fertilization success by facilitating longer retention of the milt inside the mouth during spawning. Glycosylation of SPP120 was associated with high milt viscosity. Its glycosylation was specifically present in the milt of cichlid species exhibiting oral fertilization. Moreover, recombinant SPP120 from several the oral fertilization species strongly immobilized/aggregated sperm. Therefore, the functions of SPP120 (immobilization/aggregation and its glycosylation) may contribute to success of oral fertilization, and these functions of SPP120 are more prominent in oral fertilization species. In addition, comparative phylogenetic analyses showed a positive evolutionary correlation between SPP120 function and oral fertilization. Hence, these evolutions may have occurred to keep up with the transition in the mode of fertilization. In addition, rapid evolution in the molecular sequence might be associated with functional modifications of SPP120.

Conclusion

These results suggest that SPP120 might be associated with oral fertilization. In other words, reproductive traits that define the interaction between sperms and eggs could be the evolutionary selective force that cause the rapid functional modification of the fertilization-related reproductive protein, SPP120.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1292-0) contains supplementary material, which is available to authorized users.

Inhibitory effect of K+ ions and influence of other ions and osmolality on the spermatozoa motility of European burbot (Lota lota L.)

Background

In fish with external fertilization, two main start-up mechanisms of the path that blocks or activates the spermatozoan motility apparatus are known. The main factor managing the path is osmolality or potassium ion. In burbot from the European and North American population, contradictory findings regarding the factors influencing the onset of spermatozoa motility were reported. The objective of the current study was to determine the effect of potassium and osmolality on the spermatozoa activation of European burbot, Lota lota (Actinopterygii, Gadiformes, Lotidae). Moreover, the influence of pH, as well as sodium ion concentrations on spermatozoa motility was investigated. Seven parameters characterising motility were traced by means of computer-assisted sperm analysis (CASA).

Principal findings

The spermatozoa of European burbot are K⁺ ion-sensitive. A 6-mM KCl solution significantly decreased motility, and above 12-mM (50 mOsm kg^-1) totally ceased spermatozoa movement. Sucrose and Na⁺ solutions inhibited spermatozoa movement only at concentrations > 450–480 mOsm kg^-1. Greater differences in the percentage of motile sperm between individuals were noted in solutions containing high concentrations of chemicals triggering sperm motility. The optimum osmolality for spermatozoa motility is in the range of 100–200 mOsm kg^-1. The burbot spermatozoa were motile over a wide range of pH values with the best activation at pH 9.

Conclusion

It was demonstrated that the spermatozoa of European burbot are inhibited by K⁺ ions similarly as in North American burbot. Other electrolyte and non-electrolyte solutions inhibit spermatozoa movement only if their osmolality is greater than that of the physiological osmolality of seminal plasma. The data provided on basic knowledge of burbot spermatozoa allow to ensure appropriate conditions during artificial reproduction and scientific research.

A Unique Seminal Plasma Protein, Zona Pellucida 3-Like Protein, has Ca2+ -Dependent Sperm Agglutination Activity

Identification of seminal proteins provides a means of investigating their roles. Despite their importance in the study of protein function, such as regulation of sperm motility, it is difficult to select candidates from the large number of proteins. Analyzing the rate of molecular evolution is a useful strategy for selecting candidates, and expressing the protein allows the examination of its function. In the present study, we investigated seminal plasma proteins of the cichlid Oreochromis mossambicus, which exhibits a unique mode of fertilization and a rapidly evolving gene that encodes a seminal plasma protein, zona-pellucida 3-like (ZP3-like), which does not belong to the same molecular family as other ZPs. Seminal plasma proteins of O. mossambicus were separated by two-dimensional electrophoresis, and 19 major proteins were identified by mass spectrometry (MALDI-Tof Mass). Because proteins that are under positive selection often impact sperm function, the rates of molecular evolution of these proteins were analyzed in terms of non-synonymous/synonymous substitutions (ω). Among the 19 proteins, positive selection was supported for five genes; functional assays were carried out on four of the proteins encoded by these genes. Of the four positively selected proteins, only ZP3-like protein agglutinated sperm in a dose- and Ca²⁺ -dependent manner. The other three proteins did not affect sperm motility. Because of the unique fertilization type, in which fertilization occurs in the buccal cavity, the need to retain sperm within the cavity during spawning, and the agglutination of sperm, which may be partly assisted by ZP3-like protein, may contribute to fertilization success. Fertilization in the buccal cavity may be related to its rapid molecular evolution.

Protein phosphorylation in spermatozoa motility of Acipenser ruthenus and Cyprinus carpio

Spermatozoa of externally fertilizing freshwater fish possess several different modes of motility activation. Spermatozoa of common carp (Cyprinus carpio L.) are activated by hypoosmolality, whereas spermatozoa of sterlet (Acipenser ruthenus) require Ca2+ and low concentration of K+ for motility activation. Intracellular signaling differs between these two species as well, particularly in terms of utilization of secondary messengers (cAMP and Ca2+), and kinase activities. The current study was performed in order to determine the importance of protein phosphorylation and protein kinases for activation of sperm motility in carp and sterlet. Treatment with kinase inhibitors indicates that protein kinases A and C (PKA and PKC) participate in spermatozoa motility of both species. Immunodetection of phospho-(Ser/Thr) PKA substrates shows that phosphorylated proteins are localized differently in spermatozoa of carp and sterlet. Strong phosphorylation of PKC substrate was observed in flagella of sterlet spermatozoa, whereas in carp sperm, PKC substrates were lightly phosphorylated in the midpiece and flagella. Motility activation induced either phosphorylation or dephosphorylation on serine, threonine and tyrosine residues of numerous proteins in carp and sterlet spermatozoa. Proteomic methods were used to identify proteins whose phosphorylation state changes upon the initiation of sperm motility. Numerous mitochondrial and glycolytic enzymes were identified in spermatozoa of both species, as well as axonemal proteins, heat shock proteins, septins and calcium-binding proteins. Our results contribute to an understanding of the roles of signaling molecules, protein kinases and protein phosphorylation in motility activation and regulation of two valuable fish species, C. carpio and A. ruthenus.

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.

A Procedure-Spanning Analysis of Plasma Membrane Integrity for Assessment of Cell Viability in Sperm Cryopreservation of Zebrafish Danio rerio

The goal of this study was to evaluate plasma membrane integrity and motility for zebrafish sperm quality assessment along the cryopreservation pathway-from sample collection through refrigerated storage, cryoprotectant equilibration, freezing, thawing, and fertilization. The objectives were to: (1) evaluate the effects of osmolality, extender, and refrigerated storage on sperm plasma membrane integrity and motility, and (2) compare cryopreservation of sperm from farm-raised and well-characterized research populations by evaluating motility and membrane integrity of fresh, post-equilibration (before freezing) and post-thaw sperm, and post-thaw fertility. Osmolality, extender, and storage time each influenced sperm motility and membrane integrity. Isotonic osmolality showed the best protection for motility and membrane integrity compared to hypotonic and hypertonic osmolalities. Of the four tested extenders, Hanks' balanced salt solution (HBSS) and Ca(2+)-free HBSS showed the best protection compared with NaCl and glucose, and sperm retained motility and membrane integrity for 24 h of refrigerated storage. Sperm cryopreservation of zebrafish from a farm population (n = 20) and an AB research line (n = 20) showed significant differences in post-thaw fertility (32% ± 18% vs. 73% ± 21%). No differences were found in post-thaw motility, although the farm-raised zebrafish possessed a larger body size, testis weight, and higher fresh motility. Correlation analysis of pooled data did not identify correlations among motility, flow cytometry analysis of membrane integrity and recognizable cells, and post-thaw sperm fertility (p ≥ 0.202). More research is needed to standardize the fertilization conditions especially sperm-to-egg ratio to avoid possible overabundance of sperm to obscure the differences.

Role of calcium on the initiation of sperm motility in the European eel

Sperm from European eel males treated with hCGrec was washed in a calcium free extender, and sperm motility was activated both in the presence (seawater, SW) and in the absence of calcium (NaCl+EDTA), and treated with calcium inhibitors or modulators. The sperm motility parameters were evaluated by a computer-assisted sperm analysis (CASA) system, and changes in the [Ca(2+)]i fluorescence (and in [Na(+)]i in some cases) were evaluated by flow cytometry. After sperm motility was activated in a medium containing Ca(2+) (seawater, SW) the intracellular fluorescence emitted by Ca(2+) increased 4-6-fold compared to the levels in quiescent sperm. However, while sperm activation in a Ca-free media (NaCl+EDTA) resulted in a percentage of motility similar to seawater, the [Ca(2+)]i levels did not increase at all. This result strongly suggests that increasing [Ca(2+)]i is not a pre-requisite for the induction of sperm motility in European eel sperm. Several sperm velocities (VCL, VSL, VAP) decreased when sperm was activated in the Ca-free activator, thus supporting the theory that Ca(2+) has a modulatory effect on sperm motility. The results indicate that a calcium/sodium exchanger (NCX) which is inhibited by bepridil and a calcium calmodulin kinase (inhibited by W-7), are involved in the sperm motility of the European eel. Our results indicate that the increase in [Ca(2+)]i concentrations during sperm activation is due to an influx from the external medium, but, unlike in most other species, it does not appear to be necessary for the activation of motility in European eel sperm.

Aquaporin biology of spermatogenesis and sperm physiology in mammals and teleosts

Fluid homeostasis is recognized as a critical factor during the development, maturation, and function of vertebrate male germ cells. These processes have been associated with the presence of multiple members of the aquaporin superfamily of water and solute channels in different cell types along the reproductive tract as well as in spermatozoa. We present a comparative analysis of the existing knowledge of aquaporin biology in the male reproductive tissues of mammals and teleosts. Current data suggest that in both vertebrate groups, aquaporins may have similar functions during differentiation of spermatozoa in the germinal epithelium, in the concentration and maturation of sperm in the testicular ducts, and in the regulation of osmotically induced volume changes in ejaculated spermatozoa. Recent studies have also provided insight into the possible function of aquaporins beyond water transport, such as in signaling pathways during spermatogenesis or the sensing of cell swelling and mitochondrial peroxide transport in activated sperm. However, an understanding of the specific physiological functions of the various aquaporins during germ cell development and sperm motility, as well as the molecular mechanisms involved, remains elusive. Novel experimental approaches need to be developed to elucidate these processes and to dissect the regulatory intracellular pathways implicated, which will greatly help to uncover the molecular basis of sperm physiology and male fertility in vertebrates.

Optimum calcium concentration: a crucial factor in regulating sperm motility in vitro

Sperm motility can be maintained in vitro by incubation in a defined medium under specific conditions. In most studies, the exact role of various constituents of epididymal fluid, including calcium, has remained obscure. Most of the culture media have included millimolar concentrations of calcium, but previous reports have indicated that millimolar calcium inhibits sperm motility. In this present study, we sought the optimum concentration of extracellular calcium required for optimum sperm motility. This study showed that extracellular calcium has a concentration-dependent biphasic role in motility regulation. It promoted motility and velocity at lower (10 µM) concentration whereas notably inhibited it at higher concentrations. When external membrane-bound calcium was removed by ethylene glycol tetraacetic acid, motility decreased considerably. To confirm the motility-inhibiting role of calcium above 10 µM, a sperm motility-stimulating protein (MSP) recently reported from our laboratory was used which at 0.9 μM induces motility in 60-70 % cells. Calcium at 10 µM had no appreciable effect on the motility-promoting activity of the MSP but depressed the activity above 10 µM. Thus, our present results emphasize the biphasic role of extracellular calcium and the importance of its optimum concentration in different buffers and media used for sperm motility initiation.

Coordinated Action of Aquaporins Regulates Sperm Motility in a Marine Teleost

In marine teleosts, such as the gilthead seabream, several aquaporin paralogs are known to be expressed during the hyperosmotic induction of spermatozoon motility in seawater. Here, we used immunological inhibition of channel function to investigate the physiological roles of Aqp1aa, Aqp1ab, and Aqp7 during seabream sperm activation. Double immunofluorescence microscopy of SW-activated sperm showed that Aqp1aa and Aqp7 were respectively distributed along the flagellum and the head, whereas Aqp1ab accumulated in the head and in discrete areas toward the anterior tail. Inhibition of Aqp1aa reduced the rise of intracellular Ca(2+), which is independent of external Ca(2+) and normally occurs upon activation, and strongly inhibited sperm motility. Impaired Aqp1aa function also prevented the intracellular trafficking of Aqp8b to the mitochondrion, where it acts as a peroxiporin allowing H2O2 efflux and ATP production during activation. However, restoring the Ca(2+) levels with a Ca(2+) ionophore in spermatozoa with immunosuppressed Aqp1aa function fully rescued mitochondrial Aqp8b accumulation and sperm motility. In contrast, exposure of sperm to Aqp1ab and Aqp7 antibodies did not affect motility during the initial phase of activation, but latently compromised the trajectory and the pattern of movement. These data reveal the coordinated action of spatially segregated aquaporins during sperm motility activation in a marine teleost, where flagellar-localized Aqp1aa plays a dual Ca(2+)-dependent role controlling the initiation of sperm motility and the activation of mitochondrial detoxification mechanisms, while Aqp1ab and Aqp7 in the head and anterior tail direct the motion pattern.

Inter-population ovarian fluid variation differentially modulates sperm motility in Atlantic cod Gadus morhua

This study tested the hypothesis that the effects of Atlantic cod Gadus morhua ovarian fluid on sperm motility variables are population specific. Sperm from a northern G. morhua population were activated in the presence of ovarian fluid from either northern or southern G. morhua at different concentrations. Ovarian fluid acted as a filter, in some cases reducing sperm swimming performance compared with seawater. Fluid from females foreign in population (southern) to the males (northern) had a greater inhibiting effect than those from the native population. Follow-up analysis indicated that the ovarian fluids had lower Ca(2+) concentration in northern than southern G. morhua, which could be the causative mechanism. If widespread, such cryptic female choice could reduce the incidence of intraspecific hybridization among diverged populations and contribute to reproductive isolation.

Regulation of salmonid fish sperm motility by osmotic shock-induced water influx across the plasma membrane

The motility of salmonid fish sperm is initiated by a decrease in the extracellular K(+) concentration. However, our previous studies revealed that salmonid fish sperm motility could be initiated in the presence of an inhibitory concentration of K(+) by drastic osmotic shock induced by suspension in a hypertonic glycerol solution and subsequent dilution in a hypotonic solution (glycerol-treatment). In the present study, we examined if an osmotic shock-induced water influx is involved in the regulation of salmonid fish sperm motility. HgCl2, a common inhibitor of aquaporins (AQPs), decreased the duration of salmonid fish sperm motility. Dilution of sperm cells in a hypotonic solution increased the cellular volume, whereas HgCl2 inhibited such an increase in cellular volume. Furthermore, the expression of AQP 1a and 10 in rainbow trout testes was confirmed. In contrast, HgCl2 did not affect glycerol-treated sperm motility, indicating that AQPs are not involved in glycerol-treated sperm motility. We also explored the possibility of aquaporin-independent water influx in glycerol-treated sperm by assessing the sperm membrane permeability using propidium iodide. The plasma membrane of glycerol-treated sperm was considerably permeabilized. The cellular volume was decreased in a hypertonic glycerol solution and increased upon subsequent hypoosmotic shock, indicating an AQP-independent water flux across the plasma membrane upon glycerol-treatment. Taken together, these results showed that water influx across the plasma membrane via AQP is crucial for the maintenance of salmonid fish sperm motility under normal conditions, whereas water influx by osmotic shock-induced membrane permeation is critical for the initiation of glycerol-treated sperm motility.

Effects of frozen and liquid hypothermic storage and extender type on calcium homeostasis in relation to viability and ATP content in striped bass (Morone saxatilis) sperm

The effect of hypothermic storage on striped bass sperm calcium homeostasis was determined by Fluo-3 flow cytometry. Calcium homeostasis was defined as the ability of cells to maintain a low concentration of intracellular free calcium as measured by Fluo-3 fluorescence. Sperm were stored frozen in striped bass extender (SBE) and Tris-NaCl medium (T350) modified with 50 mM glycine and 7.5% dimethylsulfoxide and in nonfrozen form diluted 1:3 (vol/vol) in SBE and T350 for 1, 24, and 48 hours at 4 °C in an oxygen atmosphere. Fluo-3 fluorescence was detected in less than 5% of fresh viable sperm cells indicating maintenance of calcium homeostasis. In contrast to sperm in fresh semen, frozen-thawed and nonfrozen sperm cells lost to a considerable extent the ability to maintain low intracellular free calcium even in the absence of exogenous calcium; positive Fluo-3 fluorescence was found in 26% and 39% of thawed sperm frozen in SBE- and T350-based freezing diluents, respectively, and increased (P < 0.05) to 67% during nonfrozen storage in SBE and T350 at 24 and 48 hours. Sperm viability measured by exclusion of propidium iodide by flow cytometry was 99% in fresh milt and maintained at 86% (P > 0.05) in SBE after 48 hours of nonfrozen storage but decreased (P < 0.05) to 55.7% after 48 hours in T350. Energy status in terms of ATP content, determined by luciferin-luciferase bioluminescence assay, was higher (P < 0.05) in sperm frozen in SBE than in T350 during the first 5 minutes post-thaw and decreased to essentially zero by 15 minutes post-thaw and did not differ among nonfrozen storage treatments. In conclusion, sperm cells impervious to propidium iodide after frozen or nonfrozen storage were unable to maintain low intracellular calcium content. SBE is a better medium than T350 for frozen or nonfrozen storage of striped bass sperm. The inability to regulate intracellular calcium in striped bass sperm may be associated with poor activation of motility after 4 °C storage and cryopreservation.

Effect of pH and cation concentrations on spermatozoan motility of sea trout (Salmo trutta m. trutta L.)

Environmental conditions during external fertilization in fish have a significant effect on spermatozoan motility (MOT) and fertilization ability. Even in the same family of fish, spermatozoa might differ in sensitivity to ions present in the external medium. Elucidation of such differences within a species would help to understand spermatozoan biology and to determine external conditions that would optimize spermatozoan MOT and successful fertilization. Objectives of the current study were to determine the effects of pH and of sodium, potassium, and calcium ion concentrations in the activation solution on sea trout spermatozoan MOT. Six parameters characterizing MOT (MOT, curvilinear velocity [VCL], linearity, amplitude of lateral head displacement, beat cross frequency, and duration of MOT) in spermatozoa activated in prepared buffers were traced by computer-assisted sperm analysis. Sea trout spermatozoa were motile over a wide range of pH values, and increasing pH did influence MOT, VCL, linearity, amplitude of lateral head displacement, and MOT duration. The optimum pH for sperm MOT was established at approximately 10. Increasing K(+) ion concentration within the observed range caused a decrease in MOT and VCL. Spermatozoan movement ceased at 8 mM KCl concentrations. In Ca(2+) buffers, sperm were motile within the range of 0 to 70 mM CaCl(2) concentration; although beyond 8 mM concentration, VCL and MOT gradually declined. Spermatozoan aggregation was observed at the highest ion concentrations tested. Increasing CaCl(2) concentration affected MOT pattern from initiation to termination of spermatozoan movement in a similar manner as changes associated with increasing pH. At concentrations of CaCl(2) higher than 0.5 mM and in buffers with pH values 10 to 11, movement of spermatozoa was characterized by high initial linearity followed by its gradual reduction. In contrast to the effects of KCl and CaCl(2), increasing NaCl concentration up to 90 mM Na(+) concentration prolonged the duration of spermatozoan movement and, up to 60 mM Na(+) concentration, slightly increased sperm velocity as well. Above the concentration of 90 mM NaCl, these parameters decreased; and at 240 mM of Na(+), spermatozoa did not activate.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Transient Ca2+ mobilization caused by osmotic shock initiates salmonid fish sperm motility

Salmonid fish sperm motility is known to be suppressed in millimolar concentrations of extracellular K+, and dilution of K+ upon spawning triggers cAMP-dependent signaling for motility initiation. In a previous study, however, we demonstrated that suspending sperm in a 10% glycerol solution and subsequent dilution into a low-osmotic solution induced motility independently of extracellular K+ and cAMP. In the present study, we further investigated the glycerol-induced motility mechanism. We found that treatment with solutions consisting of organic or inorganic ions, as well as glycerol, induced sperm motility in an osmolarity-dependent manner. Elimination of intracellular Ca2+ by BAPTA-AM significantly inhibited glycerol-treated sperm motility, whereas removal of extracellular Ca2+ by EGTA did not. Monitoring intracellular Ca2+, using fluo-4, revealed that intracellular Ca2+ increased when sperm were suspended in hypertonic solutions, and a subsequent dilution into a hypotonic solution led to a decrease in intracellular Ca2+ concomitant with motility initiation. In addition, upon dilution of sperm into a hypertonic glycerol solution prior to demembranation, the motility of demembranated sperm was reactivated in the absence of cAMP. The motility recovery suggests that completion of axonemal maturation occurred during exposure to a hypertonic environment. As a result, it is likely that glycerol treatment of sperm undergoing hypertonic shock causes mobilization of intracellular Ca2+ from the intracellular Ca2+ store and also causes maturation of axonemal proteins for motility initiation. The subsequent dilution into a hypotonic solution induces a decrease in intracellular Ca2+ and flagellar movement. This novel mechanism of sperm motility initiation seems to act in a salvaging manner for the well-known K+-dependent pathway.

Involvement of redox- and phosphorylation-dependent pathways in osmotic adaptation in sperm cells of euryhaline tilapia

Sperm cells involved in fertilisation must tolerate hypo-osmotic and hyper-osmotic environments. Euryhaline tilapia (Oreochromis mossambicus) can acclimatise to and reproduce in freshwater and seawater because its sperm are able to adapt to these differing osmotic environments. In this study, we found that the dephosphorylation of sperm proteins in O. mossambicus correlated with the activation of flagellar motility when sperm were exposed to hypotonic or hypertonic conditions, and that differences in phosphorylation may reflect adaptations to a given osmotic environment. Of the sperm proteins that were dephosphorylated, the phosphorylation pattern of an 18 kDa protein, identified as the superoxide anion scavenger Cu/Zn superoxide dismutase (Cu/Zn SOD), was different in freshwater- and seawater-acclimatised tilapia sperm. Cu/Zn SOD was distributed from the sperm head to the flagellum. Additionally, differences were observed between freshwater and seawater tilapia in the nitration of tyrosine residues (which might be mediated by SOD) in sperm flagellar proteins in response to osmotic shock. These results demonstrate that reactive-oxygen-species-dependent mechanisms contribute to both osmotic tolerance and the activation of flagellar motility.

Effects of hypothermic storage on intracellular calcium, reactive oxygen species formation, mitochondrial function, motility, and plasma membrane integrity in striped bass (Morone saxatilis) sperm

Experiments were conducted to determine the effect of hypothermic 24 h storage on striped bass sperm cell plasma membrane integrity, free intracellular Ca(2+) ([Ca(2+)](i)), mitochondrial membrane potential (ΔΨ(m)), and reactive oxygen species (ROS) formation (oxidation of hydroethidine to ethidium) as determined by flow cytometry; motion activation and ATP concentration as determined by Luciferin-Luciferase bioluminescence assay. Semen was stored for 1 or 24 h at 4 °C in an O(2) atmosphere undiluted or diluted (one volume semen with 3 volumes diluent) with T350 (20 mM TRIS base-NaCl, 350 mOsm/mL, pH 8) or with seminal plasma in the presence of various treatments. Viability (% cells excluding propidium iodide) approached 100% after 1 h storage in undiluted or diluted semen. After 1 h of storage the [Ca(2+)](i) marker, Fluo-3, was detected in only 3% of sperm cells in undiluted or diluted semen. In contrast to storage for 1 h, after 24 h the incidence Fluo-3 fluorescence intensity was increased (P < 0.05) in > 50% of the viable cells in undiluted and diluted semen along with increased cell death; the presence of 1 mM ethylene glycol tetraacetic acid (EGTA) blocked CaCl(2)-induced Fluo-3 fluorescence and cell death. Activation of sperm motility was 82% after 1 h in T350 and decreased (P < 0.05) to 30% after 24 h. However, motility activation failed in the presence of EGTA at 1 or 24 h. During storage ΔΨ(m) was not affected by storage time or treatment. In contrast, sperm ATP was greater (P < 0.05) at 1 h than at 24 h and was greater in sperm stored in diluted than undiluted form. While ROS formation was induced by menadione treatment, there was no evidence of storage-induced ROS formation in the absence of menadione. The increased [Ca(2+)](i) found after 24 h indicates a storage induced defect in the maintenance of cellular calcium homeostasis which may be detrimental to sperm activation.

Sperm motility adaptation to ion-differing aquatic environments in the Tanganyikan cichlid, Astatotilapia burtoni

The cichlid fish, Astatotilapia burtoni, can acclimate and reproduce in both the K(+)-, Na(+)-, and Ca(2+)-rich waters of Lake Tanganyika (pH 8.9-9.2) and the surrounding rivers where K(+), Na(+), and Ca(2+) concentrations are low (pH 6.5). In this study, we compared sperm motility features among A. burtoni inhabiting the lake, those living in the surrounding rivers, and those from rivers that acclimates to lake water. Sperm of all three groups required extracellular Ca(2+) ([Ca(2+)]o) for sperm motility activation. However, involvement of K(+) and Na(+) were different. In sperm motility initiation of lake population, the decrease in Na(+) and increase in K(+) concentrations corresponding to a decrease in [Na(+)]o/[K(+)]o was required. In contrast, sperm motility in both the river population and those acclimated to lake water were activated only by a decrease in [Na(+)]o, suggesting that motility initiation cues regarding Na(+) and K(+) are different among populations within same species. One probable reason why the difference of initiation cues is that the concentration of K(+) in the river water (40 mM) is lower than that in the lake water (110 mM). These results suggest that sperm flagellar activation cascades of river and lake populations within this species differ as a result of adaptations to different ionic environments.

Frenetic activation of fish spermatozoa flagella entails short-term motility, portending their precocious decadence

In most species, fish spermatozoa activate their motility on contact with the external medium (sea or fresh water depending of their reproductive habitat). Their flagella immediately develop waves propagated at high beat frequency (up to 70 beats s(-1)), which propel these sperm cells at high velocity (6-10 mm min(-1)), but for a quite short period of time, usually limited to minutes. Their specific inability to restore their energy content (mostly adenosine triphosphate) fast enough relatively to their high rate of energy consumption by flagellar contributes mainly to the activity arrest of motility, as the spermatozoa need to rely on early accumulated energy prior to activation. This review of the published data explains the present understanding of physico-chemical mechanisms by which flagellar motility is activated (mostly through osmotic and ionic regulation) and then propels sperm cells at speed. It aims also to describe the gradual arrest of their motility much of which occurs within a few minutes.

Changes in extracellular osmolality initiate sperm motility in freshwater teleost rosy barb Puntius conchonius

The objective was to investigate the effects of extracellular osmolality and membrane osmotic-sensitive channels on the initiation of sperm motility and to explore mechanisms of sperm initiation in rosy barb (Puntius conchonius). We found that (1) sperm were immotile in seminal plasma and remained quiescent in electrolyte or nonelectrolyte solutions isotonic to seminal plasma; (2) sperm movement was initiated when the sperm were exposed to hypo-osmotic electrolyte or hypo-osmotic nonelectrolyte solutions, and that the responsiveness of sperm to changes in the extracellular osmolalities (100, 200, 250, 270, and 300 mOsm/kg) differed among sperm cells (P<0.05); (3) sperm movement could be initiated and terminated repeatedly by decreasing and increasing the osmolality (in increments of 100 and 300 mOsm/kg) of a nonelectrolyte mannitol solution, respectively (P<0.05); (4) gadolinium (20, 40, and 80 microM) inhibited the initiation of sperm motility and abolished the sperm activation caused by the hypo-osmotic media treatment in dose- and time-dependent manners (P<0.05); and (5) sperm activation in a hypo-osmotic medium and inhibition in an isotonic solution were associated with swelling and shrinkage of the sperm sleeves, respectively. Therefore, we concluded that osmolality was a critical physiologic signal in regulating the initiation and termination of sperm motility in freshwater teleost rosy barb. Furthermore, we inferred that rosy barb sperm were hypo-osmotic-dependent conformers, and the osmotic-sensitive channel could be involved in the mechanism of sperm initiation.

Sperm motility initiation and duration in a euryhaline fish, medaka (Oryzias latipes)

The medaka, Oryzias latipes, is a well-recognized fish model for biomedical research. An understanding of gamete characteristics is necessary for experimental manipulations such as artificial fertilization and sperm cryopreservation. The goal of this study was to investigate sperm characteristics of motility initiation, duration, and retention in medaka. First, motility was initiated by osmolality values ranging from 25 to 686mOsm/kg, which included deionized water and hypotonic, isotonic, and hypertonic Hanks' balanced salt solution. The percentage of motile sperm was >80% when osmolality was <315mOsm/kg and decreased as osmolality increased. This is different from most fish with external fertilization in which sperm motility can be initiated by hypotonic (for freshwater fish) or hypertonic (for marine fish) solutions or by altering the concentration of specific ions such as potassium (e.g., in salmonids). Second, upon activation, the sperm remained continuously motile, with reserve capacity, for as long as 1 wk during storage at 4 degrees C. This was also different from other externally fertilizing fish, in which motility is typically maintained for seconds to several minutes. Third, after changing the osmolality to 46 to 68 mOsm/kg by adding deionized water, the motility of sperm held at 274 to 500 mOsm/kg was higher than the original motility (P</=0.035) after 24, 48, and 72h of storage at 4 degrees C. Fourth, the addition of glucose had no effect on maintaining sperm motility during refrigerated storage. To our knowledge, this combination of sperm motility characteristics is reported for the first time in fish and may be unique to medaka or may represent an undescribed modality of sperm behavior within euryhaline fish.

Marine fish spermatozoa: racing ephemeral swimmers

After a long period of spermatogenesis (several weeks to months), marine fish spermatozoa are delivered at male spawning in seawater (SW) at the same time as ova. In some fish species, as the ova micropyle closes quickly after release, these minute unicells, the spermatozoa, have to accomplish their task of reaching the micropyle within a very brief period (several seconds to minutes), for delivery of the haploid male genetic information to the ova. To achieve this goal, their high-performance motile equipment, the flagellum, must fully activate immediately on contact with the SW and then propel the sperm cell at an unusually high initial velocity. The cost of such 'hyperactivity' is a very rapid consumption of intracellular ATP that outstrips the supply. The spermatozoa become rapidly exhausted because mitochondria cannot compensate for this very fast flagellar energy consumption. Therefore, any spermatozoon ends up with two possibilities: either becoming exhausted and immotile or reaching the egg micropyle within its very short period of forward motility (in the range of tens of seconds) before micropyle closure in relation to both contact of SW and cortical reaction. The aim of the present review is to present step by step the successive events occurring in marine fish spermatozoa from activation until their full arrest of motility. The present knowledge of activation mechanisms is summarized, as well as a description of the motility parameters characterizing the motility period. As a complement, in vitro results on axonemal motility obtained after demembranation of flagella bring further understanding. The description of the sperm energetic content (ATP and other high energy compounds) and its evolution during the swimming period is also discussed. A general model aiming to explain all the successive cellular events occurring immediately after the activation is presented. This model is proposed as a guideline for understanding the events governing the sperm lifespan in the marine fish species that reproduce through external fertilization.

Molecular mechanisms determining sperm motility initiation in two sparids (Sparus aurata and Lithognathus mormyrus)

Molecular mechanisms involved in sperm motility initiation in two sparids (Sparus aurata and Lithognathus mormyrus) have been studied. Our comparative study demonstrates that osmolality is the key signal in sperm motility activation in both species, whereas K(+) and Ca(2+) do not have any role. The straight-line velocity that resulted, however, was significantly different when measured in sperm activated with non-ionic and/or calcium-free solutions with respect to that measured in seawater-activated sperm. In both species, motility initiation depends on cAMP-dependent protein phosphorylation. The phosphorylation/dephosphorylation patterns that resulted in gilthead and striped sea bream were quite different. In gilthead sea bream, the phosphorylated proteins have molecular weights of 174, 147, 138, 70, and 9-15 kDa, whereas the dephosphorylated proteins have molecular weights of 76, 57, and 33 kDa. In striped sea bream, phosphorylation after sperm motility activation occurred on proteins of 174, 147, 103, 96, 61, 57, and 28 kDa, whereas only one protein of 70 kDa resulted from dephosphorylation. Matrix-assisted laser desorption ionization-time of flight analyses allowed identification of the following proteins: In gilthead sea bream, the 9-15 kDa proteins that were phosphorylated after motility activation include an A-kinase anchor protein (AKAP), an acetyl-coenzyme A synthetase, and a protein phosphatase inhibitor, and in striped sea bream, 103- and 61-kDa proteins that were phosphorylated after motility activation were identified as a phosphatase (myotubularin-related protein 1) and a kinase (DYRK3), respectively.

Protein kinase C is an important signaling mediator associated with motility of intact sea urchin spermatozoa

Numerous kinases and phosphatases are most likely implicated in sperm motility initiation and maintenance. Data on these signaling molecules were mostly obtained from studies conducted on in vitro demembranated-reactivated sperm models but are not necessarily representative of the in vivo situation. We therefore investigated the effect of a variety of cell-permeable chemicals, mostly kinase inhibitors, on the motility initiation and maintenance of intact sea urchin spermatozoa. Among the 20 substances tested, the protein kinase C (PKC) inhibitor chelerythrine was the most potent, arresting motility at concentrations starting from 1.5-2 mumol l(-1). Motility was also inhibited by two other PKC inhibitors as well as staurosporine. Furthermore, these inhibitors prevented the motility-associated increase in phosphorylation of at least four PKC substrates. These phospho-PKC target proteins, as assessed with an antibody specific to phosphorylated motifs of PKC substrates, were found to be associated with the flagellum, either in the Triton X-100 soluble portion or the axoneme (Triton X-100 insoluble). A phosphorylated PKC-like enzyme was also detected by immunoblotting in the flagellum, as well as a significant 50 kDa PKC cleavage product. Taken together, the data strongly indicate for the first time that, in vivo, which means on intact spermatozoa, PKC is a key signaling mediator associated with the maintenance of sea urchin sperm motility.

Tissue-specific suppression of estrogen, androgen and glucocorticoid receptor gene expression in feral vitellogenic male Mozambique tilapia

While vitellogenesis in male fish is commonly used as a biomarker of xenoestrogen exposure, very little is known about the impacts associated with this unusual protein synthesis in feral populations. To this end, a recent study showed elevated circulating vitellogenin (VTG) levels in male Mozambique tilapia (Oreochromis mossambicus) collected from the Aja but not Tengan Rivers in Okinawa, Japan. Here we investigated whether this unusual protein synthesis in male fish from the Aja River affect transcript abundance of estrogen (ER), androgen (AR) and glucocorticoid (GR) receptors in the liver, brain and testis. The detection of plasma VTG levels ( approximately 100 microg ml(-1)) in male tilapia confirmed xenoestrogenic exposure in the Aja, but not the Tengan River. This protein induction was not associated with any changes in the reproductive capacity as assessed by sperm mobility and testis histology in the Aja fish. Plasma levels of estradiol-17beta, 11-ketotestosterone and cortisol were not significantly different between fish from the two rivers. Quantitative real-time PCR revealed a significant reduction in transcript levels of ERalpha and ERbeta, GR and ARalpha but not ARbeta, in the livers of tilapia from the Aja compared with the Tengan River. There were no significant changes in any of the steroid receptor transcript levels in either the brain or testis between the two rivers. Overall, our results imply that xenoestrogen exposure and VTG synthesis may lead to disruption of liver responsiveness to sex steroids and glucocorticoid stimulation in feral male fish.

Changes in sperm motility in response to osmolality/Ca2+ in three Indonesian fresh water teleosts: Goby (Oxyeleotris marmorata), Java carp (Puntius javanicus), and catfish (Clarias batrachus)

Sperm of most fresh water teleosts become motile when released into the hypotonic fresh water environment, but the role of osmolality and Ca2+ on sperm motility is not clear. Osmotic pressure and Ca2+ concentrations increase from fresh water to brackish water. Java carp Puntius javanicus and catfish Clarias batrachus live and reproduce only in fresh water. On the other hand, goby Oxyeleotris marmorata can acclimate and reproduce from fresh water to brackish water. In the present study, sperm motility and trajectory were compared among these three Indonesian endemic species. Sperm of Java carp, goby, and catfish begun to move in the hypotonic condition (< 200 mOsm/kg). However, the response to Ca2+ was different among these teleosts. In the presence of Ca2+, Java carp sperm swam in circular paths and immediately become quiescent, suggesting that Java carp sperm motility is activated in hypotonic aquatic environment without Ca2+. Goby sperm swam straightforward in the presence or absence of Ca2+. Percentages of motile sperm increased in 100-200 mOsm/kg but suppressed by removal of Ca2+. Regarding sperm motility and trajectory, no response was found in catfish sperm. These results suggest that a response to Ca2+ is different among sperm of the three species and suited to their habitat.

Microtubule sliding movement in tilapia sperm flagella axoneme is regulated by Ca2+/calmodulin-dependent protein phosphorylation

Demembranated euryhaline tilapia Oreochromis mossambicus sperm were reactivated in the presence of concentrations in excess of 10(-6) M Ca(2+). Motility features changed when Ca(2+) concentrations were increased from 10(-6) to 10(-5) M. Although the beat frequency did not increase, the shear angle and wave amplitude of flagellar beating increased, suggesting that the sliding velocity of microtubules in the axoneme, which represents dynein activity, rises with an increase in Ca(2+). Thus, it is possible that Ca(2+) binds to flagellar proteins to activate flagellar motility as a result of the enhanced dynein activity. One Ca(2+)-binding protein (18 kDa, pI 4.0), calmodulin (CaM), was detected by (45)Ca overlay assay and immunologically. A CaM antagonist, W-7, suppressed the reactivation ratio and swimming speed, suggesting that the 18 kDa Ca(2+)-binding protein is CaM and that CaM regulates flagellar motility. CaMKIV was detected immunologically as a single 48 kDa band in both the fraction of low ion extract of the axoneme and the remnant of the axoneme, suggesting that CaMKIV binds to distinct positions in the axoneme. It is possible that CaMKIV phosphorylates the axonemal proteins in a Ca(2+)/CaM-dependent manner for regulating the dynein activity. A (32)P-uptake in the axoneme showed that 48, 75, 120, 200, 250, 380, and 400 kDa proteins were phosphorylated in a Ca(2+)/CaM kinase-dependent manner. Proteins (380 kDa) were phosphorylated in the presence of 10(-5) M Ca(2+). It is possible that an increase in Ca(2+) induces Ca(2+)/CaM kinase-dependent regulation, including protein phosphorylation for activation/regulation of dynein activity in flagellar axoneme.

Concerted action between Ca2+ and hyperosmolality initiates sperm motility in amphioxus Branchiostoma belcheri tsingtauense

This study investigated the effects of different environmental conditions on the initiation and maintenance of sperm motility in amphioxus Branchiostoma belcheri tsingtauense. The findings were that: (1) hyperosmolality in the absence of Ca(2+) ions did not initiate amphioxus sperm motility; (2) addition of Ca(2+) into EGTA-containing Ca(2+)-free artificial sea water (ASW), in which no sperm were motile, restored sperm motility; (3) Ca(2+) failed to induce sperm motility under conditions of hypoosmolality; (4) K(+) channel blockers quinine and 4-aminopiridine did not suppress the initiation of sperm motility; and (5) changes in pH did not cause sperm motility in a solution isotonic to seawater without Ca(2+). In conclusion, we inferred that a concerted action between Ca(2+) and hyperosmolality was essential to initiate motility of amphioxus sperm, whereas K(+) and pH were indispensable to maintain motility.

Acclimation of sperm motility apparatus in seawater-acclimated euryhaline tilapiaOreochromis mossambicus

Euryhaline tilapia Oreochromis mossambicus can reproduce in freshwater and in seawater. Regulation of sperm motility appears to be modulated during acclimation of the fish from freshwater to seawater, being independent of extracellular Ca2+ in freshwater and dependent on extracellular Ca2+ in seawater. In the presence of extracellular Ca2+, sperm of seawater-acclimated tilapia (SWT) showed motility even in a hypertonic environment, whereas sperm of freshwater-acclimated tilapia (FWT) were not motile. The Ca2+ indicator, fluo-3, revealed that intracellular Ca2+ concentration, [Ca2+]i, of SWT sperm increased only in the presence of extracellular Ca2+ in hypotonic or hypertonic conditions. Since the increased [Ca2+]i in FWT sperm occurred under hypotonic conditions via intracellular Ca2+ stores, it is likely that tilapia modulate their source of increasing [Ca2+]i from intracellular stores (in FWT sperm) to extracellular stores (in SWT sperm). Experiments using demembranated sperm revealed that Ca2+ is necessary for activation of motility,suggesting that Ca2+ plays a key role in motility regulation in SWT sperm. We detected three phosphoproteins associated with the activation of sperm motility. Serine and threonine residues of two proteins of 15 kDa and 18 kDa became dephosphorylated in hypotonic conditions but remained phosphorylated in hypertonic conditions, suggesting that these protein phosphorylations were not only related to motility activation under hypertonic conditions but also resistant to osmotic pressure. The threonine residue(s) of a 41 kDa protein was also phosphorylated in dry sperm, even in FWT sperm in motility-feasible hypotonic conditions. It is likely that acclimation of the motility apparatus is associated with modulation of the flow of Ca2+ to increase [Ca2+]i and protein phosphorylation.

Asian medaka fishes offer new models for studying mechanisms of seawater adaptation

Japanese medaka (Oryzias latipes) is a freshwater (FW) teleost that is popular throughout the world for laboratory use. In this paper, we discuss the utility of Japanese medaka and related species for studying mechanisms of seawater (SW) adaptation. In addition to general advantages as an experimental animal such as their daily spawning activity, transparency of embryos, short generation time and established transgenic techniques, Japanese medaka have some adaptability to SW unlike the strictly stenohaline zebrafish (Danio rerio). Since other species in the genus Oryzias exhibit different degrees of adaptability to SW, comparative studies between Japanese medaka, where molecular-biological and genetic information is abundant, and other Oryzias species are expected to present varying approaches to solving the problems of SW adaptation. We introduce some examples of interspecies comparison for SW adaptabilities both in adult fish and in embryos. Oryzias species are good models for evolutionary, ecological and zoogeographical studies and a relationship between SW adaptability and geographic distribution has been suggested. Medaka fishes may thus deliver new insights into our understanding of how fish have expanded their distribution to a wide variety of osmotic environments.