Roles of heterotrimeric and monomeric G proteins in sperm-induced activation of mouse eggs

Characterization of SPINK2, SPACA7 and PDCL2: Effect of immunization on fecundity, sperm function and testicular transcriptome

Testicular factors play a vital role in spermatogenesis. We characterized the functional role of rat Spink2, Spaca7 and Pdcl2 genes. Their primary, secondary and tertiary structure were deduced in silico. The genes of rat Spink2, Spaca7 and Pdcl2 mRNA were predominantly expressed in the testis. SPINK2, SPACA7 and PDCL2 protein expression was evident in all the cell types of testis and on spermatozoa. Ablation of each of these proteins by active immunization resulted in reduced fecundity and sperm count. Damage to the anatomical architecture of testis and epididymis was evident. In SPINK2 immunized rats, 283 genes were differentially regulated while it was 434 and 872 genes for SPACA7 and PDCL2 respectively. Genes that were differentially regulated in the testis of SPINK2 immunized rats primarily belonged to extracellular exosome formation, extracellular space and response to drugs. SPACA7 ablation affected genes related to extracellular space, oxidation-reduction processes, endoplasmic reticulum membrane and response to drugs. Differential gene expression was observed for nuclear function, protein binding and positive regulation of transcription from RNA polymerase II promoter in testis of PDCL2 immunized rats. Results of our study demonstrate the role of SPINK2, SPACA7 and PDCL2 in spermatogenesis and in important molecular processes that may dictate testicular function and other physiological responses as well.

Effects of zinc chloride on boar sperm quality during liquid storage at 17°C

OBJECTIVES

The aim of this study was to evaluate the effects of zinc chloride (ZnCl₂ ) at 20, 50, 100, and 200 μg/mL on the quality of seminal plasma-free boar sperm stored at 17°C for 7 days and to explore the underlying mechanisms.

METHODS

Boar sperm were collected and incubated in non-capacitation/capacitation medium to analyze sperm quality.

RESULTS

In the non-capacitated state, the addition of ZnCl₂ at 20 and 50 μg/mL improved the survival rate and plasma membrane integrity of boar sperm (p < 0.05). Compared to the control group, the addition of ZnCl₂ significantly increased total antioxidative capacity and CuZn superoxide dismutase activity, while reducing the malondialdehyde content (p < 0.05). ZnCl₂ at 100 and 200 μg/mL significantly decreased sperm motility, protein kinase A (PKA) substrate phosphorylation, and tyrosine phosphorylation. These proteins were mainly located on the mid-pieces of the flagellum. The addition of ZnCl₂ at 20 and 50 μg/mL conveyed a protective effect to boar sperm stored at 17°C. Furthermore, ZnCl₂ at 100 and 200 μg/mL inhibited sperm motility via tyrosine phosphorylation, thus preventing the 'capacitation-like' state. In the capacitated state, there was no change in PKA substrate phosphorylation and tyrosine phosphorylation of the mid-pieces of the flagellum compared to the control groups, indicating that the addition of Zn²⁺ did not negatively affect capacitation of preserved sperm.

CONCLUSIONS

ZnCl₂ showed protective capacity to the preservation extender used for boar sperm during the process of 17°C storage, and the optimal concentration of ZnCl₂ for the preservation extender was 100 μg/mL.

Small Noncoding RNAs Contribute to Sperm Oxidative Stress-Induced Programming of Behavioral and Metabolic Phenotypes in Offspring

There is growing evidence that paternal environmental information alters small noncoding RNAs (sncRNAs) in sperm and in turn can induce alterations of metabolic and behavioral phenotypes of the next generation. However, the potential mediators of the effects remain to be elucidated. A great diversity of environmental insults and stresses can convergently induce the elevation of reactive oxygen species (ROS) in sperm; nonetheless, it remains unclear whether ROS mediates the biogenesis of sncRNAs in sperm and participates in the reprogramming of offspring phenotypes. Here, we show that ROS could induce the alteration of sncRNA profiles in sperm, especially for transfer RNA-derived small RNAs (tsRNAs) and ribosomal RNA-derived small RNAs (rsRNAs). Zygotic injection of 29-34 nt RNA fractions (predominantly tsRNAs and rsRNAs) from oxidative stress (OS) sperm could induce depressive-like and anxiety-like behaviors in male offspring. Moreover, zygotic injection with synthetic RNAs partially resembled OS sperm-induced depressive-like and anxiety-like behaviors in offspring. Male offspring maintained on a chow diet was found to develop impaired glucose tolerance and hyperactive hepatic gluconeogenesis, accompanied by the upregulation of hepatic gluconeogenic and lipolytic genes. Together, our results have shown that ROS-induced alteration of sncRNA profiles in sperm contributes to the alterations of behavioral and metabolic phenotypes of the offspring.

Influence of the genetic background on the reproductive phenotype of mice lacking Cysteine-Rich Secretory Protein 1 (CRISP1)

Epididymal sperm protein CRISP1 has the ability to both regulate murine CatSper, a key sperm calcium channel, and interact with egg-binding sites during fertilization. In spite of its relevance for sperm function, Crisp1-/-mice are fertile. Considering that phenotypes can be influenced by the genetic background, in the present work mice from the original mixed Crisp1-/- colony (129/SvEv*C57BL/6) were backcrossed onto the C57BL/6 strain for subsequent analysis of their reproductive phenotype. Whereas fertility and fertilization rates of C57BL/6 Crisp1-/- males did not differ from those reported for mice from the mixed background, several sperm functional parameters were clearly affected by the genetic background. Crisp1-/- sperm from the homogeneous background exhibited defects in both the progesterone-induced acrosome reaction and motility not observed in the mixed background, and normal rather than reduced protein tyrosine phosphorylation. Additional studies revealed a significant decrease in sperm hyperactivation as well as in cAMP and protein kinase A (PKA) substrate phosphorylation levels in sperm from both colonies. The finding that exposure of mutant sperm to a cAMP analog and phosphodiesterase inhibitor overcame the sperm functional defects observed in each colony indicated that a common cAMP-PKA signaling defect led to different phenotypes depending on the genetic background. Altogether, our observations indicate that the phenotype of CRISP1 null males is modulated by the genetic context and reveal new roles for the protein in both the functional events and signaling pathways associated to capacitation.

Vitamin C exerts novel protective effects against cadmium toxicity in mouse spermatozoa by inducing the dephosphorylation of dihydrolipoamide dehydrogenase

Cadmium (Cd) has been reported to inhibit mouse sperm motility by inducing the tyrosine phosphorylation of dihydrolipoamide dehydrogenase (DLD). This study aimed to assess the potential effects of vitamin C (Vc) on ameliorating Cd-induced tyrosine phosphorylation of DLD and the specific underlying mechanism. Vc induced the dephosphorylation of DLD or inhibited the tyrosine phosphorylation of DLD. Accordingly, DLD activity, nicotinamide adenine dinucleotide hydrogen (NADH) levels, ATP levels and motility parameters were all restored to normal levels by Vc. Moreover, the effects of Vc on ameliorating these indicators had striking similarities to the effects of ethylenediaminetetraacetic acid (EDTA). In addition, neither the antioxidant melatonin nor the universal oxidant H₂O₂ influenced the tyrosine phosphorylation of DLD. Hence, the protective effects of Vc on the tyrosine phosphorylation of DLD might be attributed to its binding to Cd ions outside or inside sperm, and were not due to its antioxidant properties.

Tyrosine phosphorylation of dihydrolipoamide dehydrogenase as a potential cadmium target and its inhibitory role in regulating mouse sperm motility

Cadmium (Cd) is reported to reduce sperm motility and functions. However, the molecular mechanisms of Cd-induced toxicity remain largely unknown, presenting a major knowledge gap in research on reproductive toxicology. In the present study, we identified a candidate protein, dihydrolipoamide dehydrogenase (DLD), which is a post-pyruvate metabolic enzyme, exhibiting tyrosine phosphorylation in mouse sperm exposed to Cd both in vivo and in vitro. Immunoprecipitation assay demonstrated DLD was phosphorylated in tyrosine residues without altered expression after Cd treatment, which further confirmed our identified result. However, the tyrosine phosphorylation of DLD did not participate in mouse sperm capacitation and Bovine Serum Albumin (BSA) effectively prevented the tyrosine phosphorylation of DLD. Moreover, Cd-induced tyrosine phosphorylation of DLD lowered its dehydrogenase activity and meanwhile, Nicotinamide Adenine Dinucleotide Hydrogen (NADH) content, Adenosine Triphosphate (ATP) production and sperm motility were all inhibited by Cd. Interestingly, when the tyrosine phosphorylation of DLD was blocked by BSA, the decrease of DLD activity, NADH and ATP content as well as sperm motility was also suppressed simultaneously. These results suggested that Cd-induced tyrosine phosphorylation of DLD inhibited its activity and thus suppressed the tricarboxylic acid (TCA) cycle, which resulted in the reduction of NADH and hence the ATP production generated through oxidative phosphorylation (OPHOXS). Taken together, our results revealed that Cd induced DLD tyrosine phosphorylation, in response to regulate TCA metabolic pathway, which reduced ATP levels and these negative effects led to decreased sperm motility. This study provided new understanding of the mechanisms contributing to the harmful effects of Cd on the motility and function of spermatozoa.

Cadmium inhibits mouse sperm motility through inducing tyrosine phosphorylation in a specific subset of proteins

Cadmium (Cd) has been reported to impair male fertility, primarily by disrupting sperm motility, but the underlying molecular mechanism remains unclear. Here we investigated the effects of Cd on sperm motility, tyrosine phosphorylation, AMP-activated protein kinase (AMPK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity, and ATP levels in vitro. Our results demonstrated that Cd inhibited sperm motility, GAPDH activity, AMPK activity and ATP production, and induced tyrosine phosphorylation of 55-57KDa proteins. Importantly, all the parameters affected by Cd were restored to normal levels when incubated with 10μM Cd in the presence of 30μM ethylene diamine tetraacetic acid (EDTA). Interestingly, changes of tyrosine phosphorylation levels of 55-57KDa proteins are completely contrary to that of other parameters. These results suggest that Cd-induced tyrosine phosphorylation of 55-57KDa proteins might act as an engine to block intracellular energy metabolism and thus decrease sperm motility.

Biphasic role of calcium in mouse sperm capacitation signaling pathways

Mammalian sperm acquire fertilizing ability in the female tract in a process known as capacitation. At the molecular level, capacitation is associated with up-regulation of a cAMP-dependent pathway, changes in intracellular pH, intracellular Ca(2+), and an increase in tyrosine phosphorylation. How these signaling systems interact during capacitation is not well understood. Results presented in this study indicate that Ca(2+) ions have a biphasic role in the regulation of cAMP-dependent signaling. Media without added Ca(2+) salts (nominal zero Ca(2+)) still contain micromolar concentrations of this ion. Sperm incubated in this medium did not undergo PKA activation or the increase in tyrosine phosphorylation suggesting that these phosphorylation pathways require Ca(2+). However, chelation of the extracellular Ca(2+) traces by EGTA induced both cAMP-dependent phosphorylation and the increase in tyrosine phosphorylation. The EGTA effect in nominal zero Ca(2+) media was mimicked by two calmodulin antagonists, W7 and calmidazolium, and by the calcineurin inhibitor cyclosporine A. These results suggest that Ca(2+) ions regulate sperm cAMP and tyrosine phosphorylation pathways in a biphasic manner and that some of its effects are mediated by calmodulin. Interestingly, contrary to wild-type mouse sperm, sperm from CatSper1 KO mice underwent PKA activation and an increase in tyrosine phosphorylation upon incubation in nominal zero Ca(2+) media. Therefore, sperm lacking Catsper Ca(2+) channels behave as wild-type sperm incubated in the presence of EGTA. This latter result suggests that Catsper transports the Ca(2+) involved in the regulation of cAMP-dependent and tyrosine phosphorylation pathways required for sperm capacitation.

PKA-dependent phosphorylation of LIMK1 and Cofilin is essential for mouse sperm acrosomal exocytosis

Mammalian sperm must acquire their fertilizing ability after a series of biochemical modifications in the female reproductive tract collectively called capacitation to undergo acrosomal exocytosis, a process that is essential for fertilization. Actin dynamics play a central role in controlling the process of exocytosis in somatic cells as well as in sperm from several mammalian species. In somatic cells, small GTPases of the Rho family are widely known as master regulators of actin dynamics. However, the role of these proteins in sperm has not been studied in detail. In the present work we characterized the participation of small GTPases of the Rho family in the signaling pathway that leads to actin polymerization during mouse sperm capacitation. We observed that most of the proteins of this signaling cascade and their effector proteins are expressed in mouse sperm. The activation of the signaling pathways of cAMP/PKA, RhoA/C and Rac1 is essential for LIMK1 activation by phosphorylation on Threonine 508. Serine 3 of Cofilin is phosphorylated by LIMK1 during capacitation in a transiently manner. Inhibition of LIMK1 by specific inhibitors (BMS-3) resulted in lower levels of actin polymerization during capacitation and a dramatic decrease in the percentage of sperm that undergo acrosomal exocytosis. Thus, we demonstrated for the first time that the master regulators of actin dynamics in somatic cells are present and active in mouse sperm. Combining the results of our present study with other results from the literature, we have proposed a working model regarding how LIMK1 and Cofilin control acrosomal exocytosis in mouse sperm.

Murine binder of sperm protein homolog 1: a new player in HDL-induced capacitation

Binder of sperm (BSP) proteins are ubiquitous among mammals and are exclusively expressed in male genital tract. The main function associated with BSP proteins is their ability to promote sperm capacitation. In mice, two proteins (BSP protein homolog 1 (BSPH1) and BSPH2) have been studied. Using recombinant strategies, BSPH1 was found to bind to epididymal sperm membranes and promote sperm capacitation in vitro. The goal of this study was to evaluate the role of native murine BSPH1 protein in sperm capacitation induced by BSA and HDLs. The effect of antibodies, antigen-binding fragments (Fabs), and F(ab′)2 specific for murine BSPH1 on BSA- and HDL-induced capacitation was tested. Results indicate that BSPH1 has no direct role in BSA-induced capacitation. However, antibodies, Fabs, and F(ab′)2 could block capacitation induced by HDLs and could inhibit the HDL-induced increase in tyrosine phosphorylation, suggesting a specific interaction between HDLs and BSPH1. Results indicate that murine BSPH1 proteins in mice could be a new important piece of the puzzle in sperm capacitation induced by HDLs. As murine BSPH1 is orthologous to human BSPH1, this study could also lead to new insights into the functions and the importance of the human protein in male fertility.

Free French abstract

A French translation of this abstract is freely available at http://www.reproduction-online.org/content/149/4/367/suppl/DC1.

Dynamic Changes in Equatorial Segment Protein 1 (SPESP1) Glycosylation During Mouse Spermiogenesis

ESP1/SPESP1 is a testis-specific, postmeiotic gene expressed in round spermatids that encodes equatorial segment protein 1, an intra-acrosomal protein found in the acrosomal matrix and on the luminal surface of the inner and outer acrosomal membranes within the equatorial segment domain of mature spermatozoa. A comparison of testicular protein extracts with caput, corpus, and caudal epididymal sperm proteins revealed striking differences in the apparent masses of SPESP1 isoforms. The predominant isoforms of SPESP1 in the testis were 77 and 67 kDa, with 47-kDa forms present to a minor degree. In contrast, SPESP1 isoforms of 47 and 43 kDa were found in caput, corpus, and caudal sperm, indicating that SPESP1 undergoes noticeable mass changes during spermiogenesis and/or subsequent transport to the epididymis. On two-dimensional (2D) SDS-PAGE, testicular SPESP1 isoforms resolved as a train of pI values from 4.9 to 5.2. Immunoprecipitated 77-kDa SPESP1 from testis reacted with the glycoprofile stain after one-dimensional and 2D gel electrophoresis, indicating that the 77-kDa testicular isoform was highly glycosylated. One charge variant of the 67-kDa isoform was also glycoprofile positive after 2D gel resolution. The 47- and 43-kDa isoforms of SPESP1 from epididymal sperm did not stain with glycoprofile, suggesting an absence of, or few, glycoprofile-sensitive glycoconjugates in epididymal SPESP1. Treatment of testicular extracts with a variety of glycosidases resulted in mass shifts in immunoreactive SPESP1, indicating that testicular SPESP1 was glycosylated and that terminal sialic acid, N- and O-glycans were present. A mixture of deglycosidase enzymes (including PNGase-F, neuraminidase, beta1-4 galactosidase, endo-alpha-N-acetylgalactosaminidase, and beta N-acetyl-glucosaminidase) completely eliminated the 77- and 67-kDa SPESP1 bands and resulted in the appearance of 75-, 60-, 55-, 50-, 47-, and 43-kDa forms, confirming that both the 77- and 67-kDa testicular forms of SPESP1 contain complex carbohydrate residues. Treatment of caudal epididymal sperm with PNGase-F enzymes showed a faint deglycosylated band at 30 kDa, but neuraminidase did not result in any molecular shift, indicating that epididymal sperm SPESP1 did not contain sialic acid/N-acetylglucosamine residues. These findings are consistent with the hypothesis that SPSPESP1 undergoes significant glycosylation in the testis and that the majority of these glycoconjugates are removed by the time sperm reach the caput epididymis. Studies of the fate of SPESP1 after the acrosome reaction localized SPESP1 to the equatorial segment region in both noncapacitated and capacitated, acrosome-reacted sperm. During capacitation, SPESP1 underwent proteolysis, resulting in a 27-kDa fragment. Zona-free oocytes incubated with recSPESP1 protein showed complementary binding sites on the microvillar oolemmal domain. Both recSPESP1 and anti-recSPESP1 antibody inhibited in vitro fertilization.

Mouse sperm membrane potential hyperpolarization is necessary and sufficient to prepare sperm for the acrosome reaction

Mammalian sperm are unable to fertilize the egg immediately after ejaculation; they acquire this capacity during migration in the female reproductive tract. This maturational process is called capacitation and in mouse sperm it involves a plasma membrane reorganization, extensive changes in the state of protein phosphorylation, increases in intracellular pH (pH(i)) and Ca(2+) ([Ca(2+)](i)), and the appearance of hyperactivated motility. In addition, mouse sperm capacitation is associated with the hyperpolarization of the cell membrane potential. However, the functional role of this process is not known. In this work, to dissect the role of this membrane potential change, hyperpolarization was induced in noncapacitated sperm using either the ENaC inhibitor amiloride, the CFTR agonist genistein or the K(+) ionophore valinomycin. In this experimental setting, other capacitation-associated processes such as activation of a cAMP-dependent pathway and the consequent increase in protein tyrosine phosphorylation were not observed. However, hyperpolarization was sufficient to prepare sperm for the acrosome reaction induced either by depolarization with high K(+) or by addition of solubilized zona pellucida (sZP). Moreover, K(+) and sZP were also able to increase [Ca(2+)](i) in non-capacitated sperm treated with these hyperpolarizing agents but not in untreated cells. On the other hand, in conditions that support capacitation-associated processes blocking hyperpolarization by adding valinomycin and increasing K(+) concentrations inhibited the agonist-induced acrosome reaction as well as the increase in [Ca(2+)](i). Altogether, these results suggest that sperm hyperpolarization by itself is key to enabling mice sperm to undergo the acrosome reaction.

Rho GTPases in animal cell cytokinesis: an occupation by the one percent

Rho GTPases are molecular switches that elicit distinct effects on the actomyosin cytoskeleton to accurately promote cytokinesis. Although they represent less than 1% of the human genome, Rho GTPases exert disproportionate control over cell division. Crucial to this master regulatory role is their localized occupation of specific domains of the cell to ensure the assembly of a contractile ring at the proper time and place. RhoA occupies the division plane and is the central positive Rho family regulator of cytokinesis. Rac1 is a negative regulator of cytokinesis and is inactivated within the division plane while active Rac1 occupies the cell poles. Cdc42 regulation during cytokinesis is less studied, but thus far a clear role has only been shown during polar body emission. Here we review what is known about the function of Rho family GTPases during cell division, as well as their upstream regulators and known downstream cytokinetic effectors.

Polar body cytokinesis

Polar body cytokinesis is the physical separation of a small polar body from a larger oocyte or ovum. This maternal meiotic division shares many similarities with mitotic and spermatogenic cytokinesis, but there are several distinctions, which will be discussed in this review. We synthesize results from many different model species, including those popular for their genetics and several that are more obscure in modern cell biology. The site of polar body division is determined before anaphase, by the eccentric, cortically associated meiotic spindle. Depending on the species, either the actin or microtubule cytoskeleton is required for spindle anchoring. Chromatin is necessary and sufficient to elicit differentiation of the associated cortex, via Ran-based signaling. The midzone of the anaphase spindle serves as a hub for regulatory complexes that elicit Rho activation, and ultimately actomyosin contractile ring assembly and contraction. Polar body cytokinesis uniquely requires another Rho family GTPase, Cdc42, for dynamic reorganization of the polar cortex. This is perhaps due to the considerable asymmetry of this division, wherein the polar body and the oocyte/ovum have distinct fates and very different sizes. Thus, maternal meiotic cytokinesis appears to occur via simultaneous polar relaxation and equatorial contraction, since the polar body is extruded from the spherical oocyte through the nascent contractile ring. As such, polar body cytokinesis is an interesting and important variation on the theme of cell division.

Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization

Calcium (Ca(2+) ) signals drive the fundamental events surrounding fertilization and the activation of development in all species examined to date. Initial studies of Ca(2+) signaling at fertilization in marine animals were tightly linked to new discoveries of bioluminescent proteins and their use as fluorescent Ca(2+) sensors. Since that time, there has been rapid progress in our understanding of the key functions for Ca(2+) in many cell types and of the impact of cellular localization on Ca(2+) signaling pathways. In this review, which focuses on mammalian egg activation, we consider how Ca(2+) is regulated and stored at different stages of oocyte development and examine the functions of molecules that serve as both regulators of Ca(2+) release and effectors of Ca(2+) signals. We then summarize studies exploring how Ca(2+) directs downstream effectors mediating both egg activation and later signaling events required for successful preimplantation embryo development. Throughout this review, we focus attention on how localization of Ca(2+) signals influences downstream signaling events, and attempt to highlight gaps in our knowledge that are ripe for future research.

Characterization of recombinant murine binder of sperm protein homolog 1 and its role in capacitation

Sperm capacitation is a maturation step that is deemed to be essential for sperm to fertilize an oocyte. A family of proteins, the binder of sperm (BSP), are known to bind choline phospholipids on sperm membranes and promote capacitation in bulls and boars. Recently, BSP-homologous genes have been identified in the epididymal tissues of human (BSPH1) and mouse (Bsph1, Bsph2). The aim of this study was to determine the binding characteristics of the murine binder of sperm protein homolog 1 (BSPH1) and evaluate its effects on sperm capacitation. Since it is not possible to purify the native BSP proteins from human and mouse in sufficient quantity, a murine recombinant BSPH1 (rec-BSPH1) was produced and used for the functional studies. Similarly to BSP proteins from other species, rec-BSPH1 bound to gelatin, heparin, phosphatidylcholine liposomes, and sperm. Both native BSPH1 and rec-BSPH1 were detected on the head and the midpiece region of sperm, although a stronger signal was detected on the midpiece region when sperm were incubated in a capacitating media containing bovine serum albumin. More importantly, murine rec-BSPH1 was able to capacitate sperm, but was unable to induce the acrosome reaction. These results show that murine epididymal BSPH1 shares many biochemical and functional characteristics with BSP proteins secreted by seminal vesicles of ungulates, and suggest that it might play a similar role in sperm functions.

The association between CDC42 and caveolin-1 is involved in the regulation of capacitation and acrosome reaction of guinea pig and mouse sperm

In the mammalian sperm, the acrosome reaction (AR) is considered to be a regulated secretion that is an essential requirement for physiological fertilization. The AR is the all-or-nothing secretion system that allows for multiple membrane fusion events. It is a Ca2+-regulated exocytosis reaction that has also been shown to be regulated by several signaling pathways. CDC42 has a central role in the regulated exocytosis through the activation of SNARE proteins and actin polymerization. Furthermore, the lipid raft protein caveolin-1 (CAV1) functions as a scaffold and guanine nucleotide dissociation inhibitor protein for CDC42, which is inactivated when associated with CAV1. CDC42 and other RHO proteins have been shown to localize in the acrosome region of mammalian sperm; however, their relationship with the AR is unknown. Here, we present the first evidence that CDC42 and CAV1 could be involved in the regulation of capacitation and the AR. Our findings show that CDC42 is activated early during capacitation, reaching an activation maximum after 20 min of capacitation. Spontaneous and progesterone-induced ARs were inhibited when sperm were capacitated in presence of secramine A, a specific CDC42 inhibitor. CAV1 and CDC42 were co-immunoprecipitated from the membranes of noncapacitated sperm; this association was reduced in capacitated sperm, and our data suggest that the phosphorylation (Tyr14) of CAV1 by c-Src is involved in such reductions. We suggest that CDC42 activation is favored by the disruption of the CAV1–CDC42 interaction, allowing for its participation in the regulation of capacitation and the AR.

cSrc is necessary for epididymal development and is incorporated into sperm during epididymal transit

Changes that occur to mammalian sperm upon epididymal transit and maturation render these cells capable of moving progressively and capacitating. Signaling events leading to mammalian sperm capacitation depend on the modulation of proteins by phosphorylation and dephosphorylation cascades. Recent experiments have demonstrated that the Src family of kinases plays an important role in the regulation of these events. However, sperm from cSrc null mice display normal tyrosine phosphorylation associated with capacitation. We report here that, despite normal phosphorylation, sperm from cSrc null mice display a severe reduction in forward motility, and are unable to fertilize in vitro. Histological analysis of seminiferous tubules in the testes, caput and corpus epididymis do not reveal obvious defects. However, the cauda epididymis is significantly smaller, and expression of key transport proteins in the epithelial cells lining this region is reduced in cSrc null mice compared to wild type littermates. Although previously, we and others have shown the presence of cSrc in mature sperm from cauda epididymis, a closer evaluation indicates that this tyrosine kinase is not present in sperm from the caput epididymis, suggesting that this protein is acquired by sperm later during epididymal maturation. Consistent with this observation, cSrc is enriched in vesicles released by the epididymal epithelium known as epididymosomes. Altogether, these observations indicate that cSrc is essential for cauda epididymal development and suggest an essential role of this kinase in epididymal sperm maturation involving cSrc extracellular trafficking.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

Role of glycerol-3-phosphate dehydrogenase 2 in mouse sperm capacitation

A tyrosine phosphoproteome study of hamster spermatozoa indicated that glycerol-3-phosphate dehydrogenase 2 (GPD2), is one of the proteins that enables tyrosine phosphorylation during sperm capacitation. Further, enzymatic activity of GPD2 correlated positively with sperm capacitation [Kota et al., 2009; Proteomics 9:1809-1826]. Therefore, understanding the function of GPD2 would help to unravel the molecular mechanism of sperm capacitation. In this study, involving the use of spermatozoa from Gpd2(+/+) and Gpd2(-/-) mice, it has been demonstrated that in the absence of Gpd2, hyperactivation and acrosome reaction were significantly altered, and a few changes in protein tyrosine phosphorylation were also observed during capacitation. Evidence is provided to demonstrate that GPD2 activity is required for ROS generation in mouse spermatozoa during capacitation, failing which, capacitation is impaired. These results imply that GPD2 is involved in sperm capacitation.

Actomyosin tube formation in polar body cytokinesis requires Anillin in C. elegans

Polar body extrusion (PBE) is the specialized asymmetric division by which oocytes accomplish reduction in ploidy and retention of cytoplasm. During maternal gametogenesis, as in male meiosis and mitosis, cytokinesis is accomplished by a ring rich in active Rho, myosin, and formin-nucleated F-actin [1-7]. However, unlike mitosis, wherein the contractile ring encircles the cell equator, the polar body ring assembles as a discoid cortical washer. Here we show that in Caenorhabditis elegans, the meiotic contractile ring transforms during closure from a disc above the spindle to a cylinder around the spindle midzone. The meiotic midbody tube comprises stacked cytoskeletal rings. This topological transition suggests a novel mechanism for constriction of an initially discoid cytokinetic ring. Analysis of mouse PBE indicates that midbody tube formation is a conserved process. Depletion of the scaffold protein anillin (ANI-1) from C. elegans results in large and unstable polar bodies that often fuse with the oocyte. Anillin is dispensable for contractile ring assembly, initiation, and closure but is required for the meiotic contractile ring to transform from a disc into a tube. We propose that cytoskeletal bundling by anillin promotes formation of the midbody tube, which ensures the fidelity of PBE.

The opening of maitotoxin-sensitive calcium channels induces the acrosome reaction in human spermatozoa: differences from the zona pellucida

The acrosome reaction (AR), an absolute requirement for spermatozoa and egg fusion, requires the influx of Ca²(+) into the spermatozoa through voltage-dependent Ca²(+) channels and store-operated channels. Maitotoxin (MTx), a Ca²(+)-mobilizing agent, has been shown to be a potent inducer of the mouse sperm AR, with a pharmacology similar to that of the zona pellucida (ZP), possibly suggesting a common pathway for both inducers. Using recombinant human ZP3 (rhZP3), mouse ZP and two MTx channel blockers (U73122 and U73343), we investigated and compared the MTx- and ZP-induced ARs in human and mouse spermatozoa. Herein, we report that MTx induced AR and elevated intracellular Ca²(+) ([Ca²(+)](i)) in human spermatozoa, both of which were blocked by U73122 and U73343. These two compounds also inhibited the MTx-induced AR in mouse spermatozoa. In disagreement with our previous proposal, the AR triggered by rhZP3 or mouse ZP was not blocked by U73343, indicating that in human and mouse spermatozoa, the AR induction by the physiological ligands or by MTx occurred through distinct pathways. U73122, but not U73343 (inactive analogue), can block phospholipase C (PLC). Another PLC inhibitor, edelfosine, also blocked the rhZP3- and ZP-induced ARs. These findings confirmed the participation of a PLC-dependent signalling pathway in human and mouse zona protein-induced AR. Notably, edelfosine also inhibited the MTx-induced mouse sperm AR but not that of the human, suggesting that toxin-induced AR is PLC-dependent in mice and PLC-independent in humans.

Inhibition of Ser/Thr phosphatases induces capacitation-associated signaling in the presence of Src kinase inhibitors

Signaling events leading to mammalian sperm capacitation rely on activation/deactivation of proteins by phosphorylation. This cascade includes soluble adenylyl cyclase, an atypical bicarbonate-stimulated adenylyl cyclase, and is mediated by protein kinase A and the subsequent stimulation of protein tyrosine phosphorylation. Recently, it has been proposed that the capacitation-associated increase in tyrosine phosphorylation is governed by Src tyrosine kinase activity. This conclusion was based mostly on the observation that Src is present in sperm and that the Src kinase family inhibitor SU6656 blocked the capacitation-associated increase in tyrosine phosphorylation. Results in the present manuscript confirmed these observations and provided evidence that these inhibitors were also able to inhibit protein kinase A phosphorylation, sperm motility, and in vitro fertilization. However, the block of capacitation-associated parameters was overcome when sperm were incubated in the presence of Ser/Thr phosphatase inhibitors such as okadaic acid and calyculin-A at concentrations reported to affect only PP2A. Altogether, these data indicate that Src is not directly involved in the observed increase in tyrosine phosphorylation. More importantly, this work presents strong evidence that capacitation is regulated by two parallel pathways. One of them requiring activation of protein kinase A and the second one involving inactivation of Ser/Thr phosphatases.

Use of differential isotopic labeling and mass spectrometry to analyze capacitation-associated changes in the phosphorylation status of mouse sperm proteins

Mammalian sperm need to reside in the female reproductive tract for a finite period of time before acquiring fertilizing competence. The biochemical changes associated with this process are collectively known as "capacitation". With the use of the mouse as an experimental model, we have previously demonstrated that capacitation is associated with a cAMP-dependent increase in protein tyrosine phosphorylation. However, little is known about the identity and function of the protein targets of this phosphorylation cascade. In the present work, we have used differential isotopic labeling coupled with immobilized metal affinity chromatography (IMAC)-based phosphopeptide enrichment and analysis on a hybrid linear ion trap/FT-ICR mass spectrometer to measure the changes in protein phosphorylation resulting from the capacitation process. As no kinase activators and/or phosphatase inhibitors were used in the preparation of the sperm samples, phosphorylated residues identified in this study represent in vivo sites of phosphorylation. Also, in contrast to other methods which rely on the incorporation of isotopically labeled amino acids at the protein level (e.g., SILAC), the present technique is based on the Fisher esterification of protein digests, allowing for the comparison of phosphorylation status in the absence of protein synthesis. This approach resulted in the identification of 55 unique, in vivo sites of phosphorylation and permitted the relative extent of phosphorylation, as a consequence of capacitation, to be calculated for 42 different phosphopeptides. This work represents the first effort to determine which specific protein phosphorylation sites change their phosphorylation status in vivo as a result of the mammalian capacitation process.

Chloride Is essential for capacitation and for the capacitation-associated increase in tyrosine phosphorylation

After epididymal maturation, sperm capacitation, which encompasses a complex series of molecular events, endows the sperm with the ability to fertilize an egg. This process can be mimicked in vitro in defined media, the composition of which is based on the electrolyte concentration of the oviductal fluid. It is well established that capacitation requires Na(+), HCO(3)(-), Ca(2+), and a cholesterol acceptor; however, little is known about the function of Cl(-) during this important process. To determine whether Cl(-), in addition to maintaining osmolarity, actively participates in signaling pathways that regulate capacitation, Cl(-) was replaced by either methanesulfonate or gluconate two nonpermeable anions. The absence of Cl(-) did not affect sperm viability, but capacitation-associated processes such as the increase in tyrosine phosphorylation, the increase in cAMP levels, hyperactivation, the zona pellucidae-induced acrosome reaction, and most importantly, fertilization were abolished or significantly reduced. Interestingly, the addition of cyclic AMP agonists to sperm incubated in Cl(-)-free medium rescued the increase in tyrosine phosphorylation and hyperactivation suggesting that Cl(-) acts upstream of the cAMP/protein kinase A signaling pathway. To investigate Cl(-) transport, sperm incubated in complete capacitation medium were exposed to a battery of anion transport inhibitors. Among them, bumetanide and furosemide, two blockers of Na(+)/K(+)/Cl(-) cotransporters (NKCC), inhibited all capacitation-associated events, suggesting that these transporters may mediate Cl(-) movements in sperm. Consistent with these results, Western blots using anti-NKCC1 antibodies showed the presence of this cotransporter in mature sperm.

Biochemical and structural characterization of apolipoprotein A-I binding protein, a novel phosphoprotein with a potential role in sperm capacitation

The physiological changes that sperm undergo in the female reproductive tract rendering them fertilization-competent constitute the phenomenon of capacitation. Cholesterol efflux from the sperm surface and protein kinase A (PKA)-dependent phosphorylation play major regulatory roles in capacitation, but the link between these two phenomena is unknown. We report that apolipoprotein A-I binding protein (AI-BP) is phosphorylated downstream to PKA activation, localizes to both sperm head and tail domains, and is released from the sperm into the media during in vitro capacitation. AI-BP interacts with apolipoprotein A-I, the component of high-density lipoprotein involved in cholesterol transport. The crystal structure demonstrates that the subunit of the AI-BP homodimer has a Rossmann-like fold. The protein surface has a large two compartment cavity lined with conserved residues. This cavity is likely to constitute an active site, suggesting that AI-BP functions as an enzyme. The presence of AI-BP in sperm, its phosphorylation by PKA, and its release during capacitation suggest that AI-BP plays an important role in capacitation possibly providing a link between protein phosphorylation and cholesterol efflux.

Involvement of cystic fibrosis transmembrane conductance regulator in mouse sperm capacitation

Mammalian sperm acquire fertilizing ability in the female tract during a process known as capacitation. In mouse sperm, this process is associated with increases in protein tyrosine phosphorylation, membrane potential hyperpolarization, increase in intracellular pH and Ca2+, and hyperactivated motility. The molecular mechanisms involved in these changes are not fully known. Present evidence suggests that in mouse sperm the capacitation-associated membrane hyperpolarization is regulated by a cAMP/protein kinase A-dependent pathway involving activation of inwardly rectifying K+ channels and inhibition of epithelial sodium channels (ENaCs). The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel that controls the activity of several transport proteins, including ENaCs. Here we explored whether CFTR is involved in the regulation of ENaC inhibition in sperm and therefore is essential for the capacitation-associated hyperpolarization. Using reverse transcription-PCR, Western blot, and immunocytochemistry, we document the presence of CFTR in mouse and human sperm. Interestingly, the addition of a CFTR inhibitor (diphenylamine-2-carboxylic acid; 250 microM) inhibited the capacitation-associated hyperpolarization, prevented ENaC closure, and decreased the zona pellucida-induced acrosome reaction without affecting the increase in tyrosine phosphorylation. Incubation of sperm in Cl- -free medium also eliminated the capacitation-associated hyperpolarization. On the other hand, a CFTR activator (genistein; 5-10 microM) promoted hyperpolarization in mouse sperm incubated under conditions that do not support capacitation. The addition of dibutyryl cyclic AMP to noncapacitated mouse sperm elevated intracellular Cl-. These results suggest that cAMP-dependent Cl- fluxes through CFTR are involved in the regulation of ENaC during capacitation and thus contribute to the observed hyperpolarization associated with this process.

Rac activity is polarized and regulates meiotic spindle stability and anchoring in mammalian oocytes

Mammalian meiotic divisions are asymmetrical and generate a large oocyte and two small polar bodies. This asymmetry results from the anchoring of the meiotic spindle to the oocyte cortex and subsequent cortical reorganization, but the mechanisms involved are poorly understood. We investigated the role of Rac in oocyte meiosis by using a fluorescent reporter for Rac-GTP. We find that Rac-GTP is polarized in the cortex overlying the meiotic spindle. Polarization of Rac activation occurs during spindle migration and is promoted by the proximity of chromatin to the cortex. Inhibition of Rac during oocyte maturation caused a permanent block at prometaphase I and spindle elongation. In metaphase II-arrested oocytes, Rac inhibition caused the spindle to detach from the cortex and prevented polar body emission after activation. These results demonstrate that Rac-GTP plays a major role in oocyte meiosis, via the regulation of spindle stability and anchoring to the cortex.

Flipping the switch: How a sperm activates the egg at fertilization

Sperm interaction with an egg in animals was first documented 160 years ago in sea urchins by Alphonse Derbès (1847) when he noted the formation of an "envelope" following the sperm's "approach" to the egg. The "envelope" in sea urchins is an obvious phenotype of fertilization in this animal and over the past 35 years has served to indicate a presence of calcium released from cytoplasmic stores essential to activate the egg. The mechanism of calcium release has been intensely studied because it is a universal regulator of cellular activity, and recently several intersecting pathways of calcium release have been defined. Here we examine these various mechanisms with special emphasis on recent work in eggs of both sea urchins and mice.

Activation of fertilized and nuclear transfer eggs

In all animal species, initiation of embryonic development occurs shortly after the joining together of the gametes from each of the sexes. The first of these steps, referred to as "egg activation", is a series of molecular events that results in the syngamy of the two haploid genomes and the beginning of cellular divisions for the new diploid embryo. For many years it has been known that the incoming sperm drives this process, as an unfertilized egg will remain dormant until it can no longer sustain normal metabolic processes. Until recently, it was also believed that the sperm was the only cell capable of creating a viable embryo and offspring. Recent advances in cell biology have allowed researchers to not only understand the molecular mechanisms of egg activation, but to exploit the use of pharmacological agents to bypass sperm-induced egg activation for the creation of animals by somatic cell nuclear transfer. This chapter will focus on the molecular events of egg activation in mammals as they take place during fertilization, and will discuss how these mechanisms are successfully bypassed in processes such as somatic cell nuclear transfer.

Possible role of calcium on oocyte development after intracytoplasmic sperm injection in quail (Coturnix japonica)

Although a rise in intracellular calcium concentration of vertebrate oocytes plays a pivotal role for the initiation of fertilization or oocyte activation, no study on this subject has been reported in birds. This study was conducted to study the role of intracellular calcium in relation to fertilization in avian oocytes. First, immediately after a quail oocyte was injected with a sperm, it was treated with strontium chloride as an inducer for intracellular calcium rise at doses of 0, 2.5, 5, 7.5, 10 mM for 4 hr in the culture medium and was followed by 20-hr culture. Treatment with 5 mM of strontium chloride induced blastodermal development in 24.2% of injected eggs, although no oocytes developed without strontium treatment. Second, quail oocytes were injected with a sperm and 0.1 M calcium chloride or a sperm and saline solution, cultured without calcium for 4 hr and was followed by 20-hr culture without strontium. The calcium solution induced blastodermal development in 20.5% of the oocytes, although no oocytes developed without calcium treatment. Third, quail oocytes were injected with 1,2-bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) as a calcium chelator, cultured with strontium (5 mM) for 4 hr followed by 20-hr culture without strontium. Only one oocyte developed after BAPTA and strontium treatment of 36 oocytes examined. Developmental stages of all the oocytes ranged from IV to VII. These results suggest that intracellular calcium rise may participate in quail oocyte activation and allow fertilization and blastodermal development.

Calcium oscillations and mammalian egg activation

Fertilization in all species studied to date induces an increase in the intracellular concentration of free calcium ions ([Ca2+]i) within the egg. In mammals, this [Ca2+]i signal is delivered in the form of long-lasting [Ca2+]i oscillations that begin shortly after fusion of the gametes and persist beyond the time of completion of meiosis. While not fully elucidated, recent evidence supports the notion that the sperm delivers into the ooplasm a trigger of oscillations, the so-called sperm factor (SF). The recent discovery that mammalian sperm harbor a specific phospholipase C (PLC), PLCzeta has consolidated this view. The fertilizing sperm, and presumably PLCzeta promote Ca2+ release in eggs via the production of inositol 1,4,5-trisphosphate (IP3), which binds and gates its receptor, the type-1 IP3 receptor, located on the endoplasmic reticulum, the Ca2+ store of the cell. Repetitive Ca2+ release in this manner results in a positive cumulative effect on downstream signaling molecules that are responsible for the completion of all the events comprising egg activation. This review will discuss recent advances in our understanding of how [Ca2+]i oscillations are initiated and regulated in mammals, highlight areas of discrepancies, and emphasize the need to better characterize the downstream molecular cascades that are dependent on [Ca2+]i oscillations and that may impact embryo development.

Further characterization of reproductive abnormalities in mCd59b knockout mice: a potential new function of mCd59 in male reproduction

CD59 is a GPI-linked membrane protein that inhibits formation of the membrane attack complex of complement. We reported recently that mice have two CD59 genes (termed mCd59a and mCd59b), and that the targeted deletion of mCd59b (mCd59b-/-) results in spontaneous hemolytic anemia and progressive loss of male fertility. Further studies of the reproductive abnormalities in mCd59b-/- mice reported in this study revealed the presence of abnormal multinucleated cells and increased apoptotic cells within the walls of the seminiferous tubules, and a decrease in the number, motility, and viability of sperm associated with a significant increase in abnormal sperm morphologies. Both the capacitation-associated tyrosine phosphorylation and the ionophore-induced acrosome reaction as well as luteinizing hormone, follicle-stimulating hormone, and testosterone serum levels were similar in mCd59b-/- and mCd59b+/+. Surprisingly, the functional deficiency of the complement protein C3 did not rescue the abnormal reproductive phenotype of mCd59b-/-, although it was efficient in rescuing their hemolytic anemia. These results indicate that the male reproductive abnormalities in mCd59b-/- are complement-independent, and that mCd59 may have a novel function in spermatogenesis that is most likely unrelated to its function as an inhibitor of membrane attack complex formation.

Sodium and epithelial sodium channels participate in the regulation of the capacitation-associated hyperpolarization in mouse sperm

In a process called capacitation, mammalian sperm gain the ability to fertilize after residing in the female tract. During capacitation the mouse sperm plasma membrane potential (E(m)) hyperpolarizes. However, the mechanisms that regulate sperm E(m) are not well understood. Here we show that sperm hyperpolarize when external Na(+) is replaced by N-methyl-glucamine. Readdition of external Na(+) restores a more depolarized E(m) by a process that is inhibited by amiloride or by its more potent derivative 5-(N-ethyl-N-isopropyl)-amiloride hydrochloride. These findings indicate that under resting conditions an electrogenic Na(+) transporter, possibly involving an amiloride sensitive Na(+) channel, may contribute to the sperm resting E(m). Consistent with this proposal, patch clamp recordings from spermatogenic cells reveal an amiloride-sensitive inward Na(+) current whose characteristics match those of the epithelial Na(+) channel (ENaC) family of epithelial Na(+) channels. Indeed, ENaC-alpha and -delta mRNAs were detected by reverse transcription-PCR in extracts of isolated elongated spermatids, and ENaC-alpha and -delta proteins were found on immunoblots of sperm membrane preparations. Immunostaining indicated localization of ENaC-alpha to the flagellar midpiece and of ENaC-delta to the acrosome. Incubations known to produce capacitation in vitro or induction of capacitation by cell-permeant cAMP analogs decreased the depolarizing response to the addition of external Na(+). These results suggest that increases in cAMP content occurring during capacitation may inhibit ENaCs to produce a required hyperpolarization of the sperm membrane.

Isolation and proteomic analysis of mouse sperm detergent-resistant membrane fractions: evidence for dissociation of lipid rafts during capacitation

Mammalian sperm acquire fertilization capacity after residing in the female tract during a process known as capacitation. The present study examined whether cholesterol efflux during capacitation alters the biophysical properties of the sperm plasma membrane by potentially reducing the extent of lipid raft domains as analyzed by the isolation of detergent-resistant membrane fractions using sucrose gradients. In addition, this work investigated whether dissociation of the detergent-resistant membrane fraction during capacitation alters resident sperm raft proteins. Mouse sperm proteins associated with such fractions were studied by silver staining, tandem mass spectrometry, and Western blot analysis. Caveolin 1 was identified in sperm lipid rafts in multimeric states, including a high-molecular-weight oligomer that is sensitive to degradation under reducing conditions at high pH. Capacitation resulted in reduction of the light buoyant-density, detergent-resistant membrane fraction and decreased the array of proteins isolated within this fraction, including loss of the high-molecular-weight caveolin 1 oligomers. Proteomic analysis of sperm proteins isolated in the light buoyant-density fraction identified several proteins, including hexokinase 1, testis serine proteases 1 and 2, TEX101, hyaluronidase (PH20, SPAM1), facilitated glucose transporter 3, lactate dehydrogenase A, carbonic anhydrase IV, IZUMO, pantophysin, basigin, and cysteine-rich inhibitory secretory protein 1. Capacitation also resulted in a significant reduction of sperm labeling by the fluorescent lipid-analog DiIC16, indicating that capacitation alters the liquid-ordered domains in the sperm plasma membrane. The observations that capacitation alters the protein composition of the detergent-resistant membrane fractions is consistent with the hypothesis that cholesterol efflux during capacitation dissociates lipid raft constituents, initiating signaling events that lead to sperm capacitation.

A Rho-signaling pathway mediates cortical granule translocation in the sea urchin oocyte

Cortical granules are secretory vesicles of the egg that play a fundamental role in preventing polyspermy at fertilization. In the sea urchin egg, they localize directly beneath the plasma membrane forming a compact monolayer and, upon fertilization, undergo a Ca(2+)-dependent exocytosis. Cortical granules form during early oogenesis and, during maturation, translocate from the cytosol to the oocyte cortex in a microfilament-mediated process. We tested the hypothesis that these cortical granule dynamics were regulated by Rho, a GTPase of the Ras superfamily. We observed that Rho is synthesized early in oogenesis, mainly in a soluble form. At the end of maturation, however, Rho associates with cortical granules. Inhibition of Rho with the C3 transferase from C. botulinum blocks cortical granule translocation and microfilaments undergo a significant disorganization. A similar effect is observed by GGTI-286, a geranylgeranyl transferase inhibitor, suggesting that the association of Rho with the cortical granules is indispensable for its function. In contrast, the anchorage of the cortical granules in the cortex, as well as their fusion at fertilization, are Rho-independent processes. We conclude that Rho association with the cortical granules is a critical regulatory step in their translocation to the egg cortex.

Rho-kinase is involved in mouse blastocyst cavity formation

During mammalian embryonic development, the formation and subsequent expansion of a fluid-filled cavity, the blastocoel, is crucial for successful implantation. Our present experiments were aimed at exploring the contribution of Rho-kinase, a downstream effector of the small GTP-binding protein RhoA, to mouse blastocoel formation. RT-PCR analysis showed that Rho-kinase mRNA is present throughout mouse preimplantation development. When 2-cell embryos were cultured in the presence of a specific inhibitor of Rho-kinase, Y-27632, they developed to the morula stage but failed to develop to the blastocyst stage. Y-27632 inhibited the formation of the blastocoel cavity from the morula stage, and this inhibitory effect was reversible when embryos were returned to medium without Y-27632. Moreover, Y-27632 reduced the rate of re-expansion of blastocysts collapsed by cytochalasin D upon transfer to the control medium. These results suggest that Rho-kinase is likely involved in blastocyst formation.

Molecular dissection of egg fertilization signaling with the aid of tyrosine kinase-specific inhibitor and activator strategies

Fertilization is triggered by sperm-egg interaction and fusion that initiate a transient rise(s) in the free intracellular calcium ([Ca(2+)](i)) that is responsible for a series of biochemical and cell biological events, so-called "egg activation". Calcium-dependent egg activation leads to the initiation of developmental program that culminates in the birth of individuals. A growing body of knowledge has uncovered the molecular mechanisms underlying sperm-induced transient [Ca(2+)](i) increase(s) to some extent; namely, in most animals so far studied, a second messenger inositol 1,4,5-trisphosphate (IP(3)) seems to play a pivotal role in inducing [Ca(2+)](i) transient(s) at fertilization. However, signaling mechanisms used by sperm to initiate IP(3)-[Ca(2+)](i) transient pathway have not been elucidated. To approach this problem, we have employed African clawed frog, Xenopus laevis, as a model animal and conducted experiments designed specifically to determine the role of the Src family protein-tyrosine kinases (SFKs or Src family PTKs) in the sperm-induced egg activation. This review compiles information about the use of PTK-specific inhibitors and activators for analyzing signal transduction events in egg fertilization. Specifically, we focus on molecular identification of Xenopus Src and the signaling mechanism of the Src-dependent egg activation that has been established recently. We also summarize recent advances in understanding the role of the Src family kinases in egg fertilization of other model organisms, and discuss future directions of the field.

A Rho GTPase controls the rate of protein synthesis in the sea urchin egg

Fertilization of the sea urchin egg triggers a Ca(2+)-dependent cortical granule exocytosis and cytoskeletal reorganization, both of which are accompanied by an accelerated protein synthesis. The signaling mechanisms leading to these events are not completely understood. The possible role of Rho GTPases in sea urchin egg activation was studied using the Clostridium botulinum C3 exotoxin, which specifically ADP-ribosylates Rho proteins and inactivates them. We observed that incubation of eggs with C3 resulted in in situ ADP-ribosylation of Rho. Following fertilization, C3-treated eggs were capable of performing cortical granule exocytosis but not the first cytokinesis. C3 caused in both unfertilized eggs and early embryos alterations in the state of actin polymerization and inhibition of the spindle formation. Moreover, C3 diminished markedly the rate of protein synthesis. These findings suggested that Rho is involved in regulating the acceleration of protein synthesis that accompanies the egg activation by sperm.

Involvement of Rho family G protein in the cell signaling for sperm incorporation during fertilization of mouse eggs: inhibition by Clostridium difficile toxin B

Sperm-egg interaction was investigated in mouse eggs freed from the zona pellucida and injected with Clostridium difficile toxin B, the inhibitor of Rho family small G proteins. Toxin B reduced in a dose-dependent manner the percentage of eggs associated with sperm fusion on the surface or sperm nucleus decondensation in the ooplasm, examined by injection of a DNA-staining dye into the egg and transfer of the dye to the fused sperm head after recording intracellular Ca(2+) responses for 100 min postinsemination. The mean number of decondensed sperm nuclei per egg was remarkably decreased by approximately 1 microg/ml toxin B in the ooplasm. This was because spermatozoa were arrested at the fusion state without developing to sperm incorporation and tended to lose cytoplasmic continuity to the egg. The fusion-arrested spermatozoa caused transient small Ca(2+) oscillations in most of eggs, while an injected spermatozoon produced repetitive large Ca(2+) spikes unaffected by toxin B. A decrease in the rate of fused spermatozoa and decondensed sperm nuclei was also caused by 20-40 microM cytochalasin D, the inhibitor of actin polymerization. Immunostaining of Rho proteins showed that Rac1 and RhoB are present in the cortical ooplasm, but Cdc42 is absent. Actin filaments in the cortex appeared to be reduced in toxin B-injected eggs. This study suggests that Rho protein(s) regulating actin-based cytoskeletal reorganization is involved in the process leading to sperm incorporation.

Involvement of a Na+/HCO-3 cotransporter in mouse sperm capacitation

Mammalian sperm are incapable of fertilizing eggs immediately after ejaculation; they acquire fertilization capacity after residing in the female tract for a finite period of time. The physiological changes sperm undergo in the female reproductive tract that render sperm able to fertilize constitute the phenomenon of "sperm capacitation." We have demonstrated that capacitation is associated with an increase in the tyrosine phosphorylation of a subset of proteins and that these events are regulated by an HCO(3)(-)/cAMP-dependent pathway involving protein kinase A. Capacitation is also accompanied by hyperpolarization of the sperm plasma membrane. Here we present evidence that, in addition to its role in the regulation of adenylyl cyclase, HCO(3)(-) has a role in the regulation of plasma membrane potential in mouse sperm. Addition of HCO(3)(-) but not Cl(-) induces a hyperpolarizing current in mouse sperm plasma membranes. This HCO(3)(-)-dependent hyperpolarization was not observed when Na(+) was replaced by the non-permeant cation choline(+). Replacement of Na(+) by choline(+) also inhibited the capacitation-associated increase in protein tyrosine phosphorylation as well as the zona pellucida-induced acrosome reaction. The lack of an increase in protein tyrosine phosphorylation was overcome by the presence of cAMP agonists in the incubation medium. The lack of a hyperpolarizing HCO(3)(-) current and the inhibition of the capacitation-dependent increase in protein tyrosine phosphorylation in the absence of Na(+) suggest that a Na(+)/HCO(3)(-) cotransporter is present in mouse sperm and is coupled to events regulating capacitation.

Src kinase induces calcium release in Xenopus egg extracts via PLCgamma and IP3-dependent mechanism

Mobilization of intracellular calcium is an indispensable step of fertilization-induced egg activation. Recently, this process has been shown to require the sequential activation of Src family tyrosine kinases, phospholipase Cgamma (PLCgamma), and inositol-1,4,5-trisphosphate (IP3)-dependent receptor of endoplasmic reticulum. In the present study, we made an attempt to recapitulate the early events of egg activation by stimulating Src kinase activity in the cell-free extracts of Xenopus eggs. We found that enhanced Src kinase activity can initiate calcium response of low magnitude in cytostatic factor (CSF)-arrested mitotic extracts without releasing them into interphase. The addition of catalytically active recombinant Src kinase, as well as the activation of endogenous Xenopus Src family kinase by hydrogen peroxide (H2O2), increased total tyrosine phosphorylation, tyrosine phosphorylation of PLCgamma, and IP3 production in the extracts. The treatment with the Src family kinase-specific inhibitor, PP1, or PLC inhibitor, U73122, or IP3 receptor antagonist, heparin, prevented calcium release in the extracts. We conclude, therefore, that possible mechanism of Src/H2O2 action in the extracts might involve tyrosine phosphorylation and activation of PLCgamma, accompanied by the increase in IP3 content and subsequent calcium release from IP3-regulated calcium stores. These results also suggest that monitoring calcium signals induced in the Xenopus egg extracts by various components of signaling pathways may provide a particularly useful approach to investigating their role in the signal transduction.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

The role of PI 3-kinase in EGF-stimulated jejunal glucose transport

Epidermal growth factor (EGF) rapidly increases jejunal glucose transport. Signal transduction mechanisms mediating EGF-induced alterations in jejunal glucose transport remain to be determined. New Zealand White rabbit (1 kg) jejunal tissue was stripped and mounted in short-circuited Ussing chambers. The transport of tritiated 3-O-methylglucose was measured in the presence of the PKC agonist 1,2-dioctanoyl-sn-glycerol (1,2-DOG) or the inactive analog 1,3-dioctanoyl-sn-glycerol (1,3-DOG). Additional experiments examined the effect of the PKC inhibitor chelerythrine, the PLC inhibitor U73122, the MAPK inhibitor PD 98059, the G-protein inhibitor GDP-betaS, the PI 3-kinase inhibitor LY294002, or the microtubule inhibitor colchicine on EGF-induced jejunal glucose transport. Net jejunal 3-O-methylglucose absorption was significantly increased following specific activation of PKC. A PKC antagonist inhibited the EGF-induced increase in net 3-O-methylglucose transport, while PI 3-kinase inhibition completely blocked the EGF-induced transport increase. Inhibition of PLC, MAPK, G-proteins, and microtubules had no effect on EGF-stimulated increases in jejunal transport. We conclude that the effect of EGF on jejunal glucose transport is mediated at least in part by PKC and PI 3-kinase.

Evidence that phospholipase C from the sperm is not responsible for initiating Ca(2+) release at fertilization in mouse eggs

Release of Ca(2+) from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca(2+) release. We demonstrated that microinjecting purified, recombinant PLCgamma1 protein into mouse eggs caused Ca(2+) oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCgamma1 protein needed to elicit Ca(2+) release when injected into eggs was approximately 500-900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca(2+) release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca(2+) release at fertilization.

The role of RhoA in the germinal vesicle breakdown of mouse oocytes

We have investigated a new role of RhoA in the germinal vesicle breakdown (GVBD) of mouse oocytes. First, RhoA was identified by immunostaining and ADP-ribosylation in germinal vesicle (GV) stage-oocytes. RhoA was mainly localized in the ooplasmic area, but rarely detected in germinal vesicle. Incubation of oocyte extract with C3 transferase induced a strong ADP-ribosylation at about 25 kDa. Incubation of GV-stage oocytes in culture medium induced the spontaneous maturation to GVBD by about 78 and 87% of total oocytes at 1 and 3 h, respectively. However, microinjection of C3 transferase into GV-stage oocytes significantly inhibited GVBD at 1 (GVBD = 29%) and 3 h (GVBD = 49%). To study the role of reactive oxygen species (ROS) in the oocyte maturation, the level of intra-oocyte ROS was measured using a ROS-specific fluorescent dye H(2)DCFDA during the oocyte maturation. Spontaneous maturation of GV-stage oocytes induced a significant increase of ROS at 3 h by about twofold over the control level and then the increased level was maintained until 6 h. However, microinjection of C3 transferase inhibited the production of intra-oocyte ROS. Incubation with ROS scavengers, N-acetyl-l-cysteine and catalase, blocked the ROS increase. The ROS scavengers also significantly inhibited GVBD, as did C3 transferase. Thus, it was proposed that RhoA was involved in the GVBD, possibly by the production of ROS in mouse oocytes.