获能期间精子蛋白的酪氨酸磷酸化

哺乳动物精了获能是精子与卵子成功受精的前提。蛋白酪氨酸磷酸化对精子获能十分重要。精了获能期蛋白酪氨酸磷酸化程度增高与sAC/cAMP/PKA途径、受体酪氨酸激酶途径和非受体蛋白酪氨酸激酶途径调节有关。获能过程中酪氨酸磷酸化蛋白分布于精子细胞的不同区域,蛋白的酪氨酸磷酸化与精子功能密切相关。     

Tyrosine phosphorylation signaling regulates Ca2+ entry by affecting intracellular pH during human sperm capacitation

Capacitation is a mandatory process for the acquisition of mammalian sperm fertilization competence and involves the activation of a complex and still not fully understood system of signaling pathways. Under in vitro conditions, there is an increase in both protein tyrosine phosphorylation (pTyr) and intracellular Ca²⁺ levels in several species. In human sperm, results from our group revealed that pTyr signaling can be blocked by inhibiting proline-rich tyrosine kinase 2 (PYK2). Based on the role of PYK2 in other cell types, we investigated whether the PYK2-dependent pTyr cascade serves as a sensor for Ca ²⁺ signaling during human sperm capacitation. Flow cytometry studies showed that exposure of sperm to the PYK2 inhibitor N-[2-[[[2-[(2,3-dihydro-2-oxo-1 H-indol-5-yl)amino]-5-(trifluoromethyl)-4-pyrimidinyl]amino]methyl]phenyl]- N-methyl-methanesulfonamide hydrate (PF431396) produced a significant and concentration-dependent reduction in intracellular Ca ²⁺ levels during capacitation. Further studies revealed that PF431396-treated sperm exhibited a decrease in the activity of CatSper, a key sperm Ca ²⁺ channel. In addition, time course studies during capacitation in the presence of PF431396 showed a significant and sustained decrease in both intracellular Ca ²⁺ and pH levels after 2 hr of incubation, temporarily coincident with the activation of PYK2 during capacitation. Interestingly, decreases in Ca ²⁺ levels and progressive motility caused by PF431396 were reverted by inducing intracellular alkalinization with NH ₄Cl, without affecting the pTyr blockage. Altogether, these observations support pTyr as an intracellular sensor for Ca ²⁺ entry in human sperm through regulation of cytoplasmic pH. These results contribute to a better understanding of the modulation of the polymodal CatSper and signaling pathways involved in human sperm capacitation.     

Identification of proteins undergoing tyrosine phosphorylation during mouse sperm capacitation

Mammalian sperm are not able to fertilize immediately upon ejaculation; they become fertilization-competent after undergoing changes in the female reproductive tract collectively termed capacitation. Although it has been established that capacitation is associated with an increase in tyrosine phosphorylation, little is known about the role of this event in sperm function. In this work we used a combination of two dimensional gel electrophoresis and mass spectrometry to identify proteins that undergo tyrosine phosphorylation during capacitation. Some of the identified proteins are the mouse orthologues of human sperm proteins known to undergo tyrosine phosphorylation. Among them we identified VDAC, tubulin, PDH E1 beta chain, glutathione S-transferase, NADH dehydrogenase (ubiquinone) Fe-S protein 6, acrosin binding protein precursor (sp32), proteasome subunit alpha type 6b and cytochrome b-c1 complex. In addition to previously described proteins, we identified two testis-specific aldolases as substrates for tyrosine phosphorylation. Genomic and EST analyses suggest that these aldolases are retroposons expressed exclusively in the testis, as has been reported elsewhere. Because of the importance of glycolysis for sperm function, we hypothesize that tyrosine phosphorylation of these proteins can play a role in the regulation of glycolysis during capacitation. However, neither the Km nor the Vmax of aldolase changed as a function of capacitation when its enzymatic activity was assayed in vitro, suggesting other levels of regulation for aldolase function.     

Implication of cAMP during porcine sperm capacitation and protein tyrosine phosphorylation

Second messengers are involved in sperm fertilizing potential, as both motility and the acrosome reaction are influenced by cAMP. Moreover, the activity of cyclic nucleotides is implicated in the appearance of tyrosine phosphorylated sperm proteins, which is associated with capacitation in the mammalian spermatozoa. Nevertheless, the involvement of the cAMP/protein kinase A (PK-A) pathway during pig sperm capacitation may be different from that observed in other mammals. The objective of the present study was to clarify the cAMP/PK-A pathway during the capacitation of porcine spermatozoa and to evaluate this impact on the p32 sperm tyrosine phosphoprotein appearance. The presence of p32 was assessed after incubating fresh pig sperm with IBMX/db-cAMP, H-89, a PK-A inhibitor or bistyrphostin, a tyrosine kinase inhibitor, in capacitating (CM) or non-capacitating conditions (NCM) by immunoblotting SDS-extracted and separated sperm proteins using an anti-phosphotyrosine antibody. When pig spermatozoa were incubated in CM supplemented with H-89 (50 microM) or bistyrphostin (1.2 microM), capacitation decreased significantly (P < 0.001). The p32 sperm tyrosine phosphoprotein, previously shown to be associated with capacitation of porcine sperm though not necessarily an end point of this phenomenon, was not modulated by IBMX/db-cAMP (100 microM/1 mM), H-89 (50 microM) nor bistyrphostin (1.2 microM). Our results indicate, therefore, that pig sperm are regulated somewhat differently than as described for other mammals, because although the cAMP/PK-A and tyrosine kinase pathways are involved in capacitation, they do not influence the appearance of p32.     

Cholesterol efflux-mediated signal transduction in mammalian sperm: cholesterol release signals an increase in protein tyrosine phosphorylation during mouse sperm capacitation.

We previously demonstrated that mouse sperm capacitation is accompanied by a time-dependent increase in protein tyrosine phosphorylation that is dependent on the presence of BSA, Ca2+, and NaHCO(3), all three of which are also required for this maturational event. We also demonstrated that activation of protein kinase A (PK-A) is upstream of this capacitation-associated increase in protein tyrosine phosphorylation. BSA is hypothesized to modulate capacitation through the removal of cholesterol from the sperm plasma membrane. In this report, we demonstrate that incubation of mouse sperm medium containing BSA results in a release of cholesterol from the sperm plasma membrane to the medium; release of this sterol does not occur in medium devoid of BSA. We next determined whether cholesterol release leads to changes in protein tyrosine phosphorylation. Blocking the action of BSA by adding exogenous cholesterol-SO-(4) to the BSA-containing medium inhibits the increase in protein tyrosine phosphorylation as well as capacitation. This inhibitory effect is overcome by (1) the addition of increasing concentrations of BSA at a given concentration of cholesterol-SO-(4) and (2) the addition of dibutyryl cAMP plus IBMX. High-density lipoprotein (HDL), another cholesterol binding protein, also supports the capacitation-associated increase in protein tyrosine phosphorylation through a cAMP-dependent pathway, whereas proteins that do not interact with cholesterol have no effect. HDL also supports sperm capacitation, as assessed by fertilization in vitro. Finally, we previously demonstrated that HCO-(3) is necessary for the capacitation-associated increase in protein tyrosine phosphorylation and demonstrate here, by examining the effectiveness of HCO-(3) or BSA addition to sperm on protein tyrosine phosphorylation, that the HCO-(3) effect is downstream of the site of BSA action. Taken together, these data demonstrate that cholesterol release is associated with the activation of a transmembrane signal transduction pathway involving PK-A and protein tyrosine phosphorylation, leading to functional maturation of the sperm.     

Remodeling of the actin cytoskeleton during mammalian sperm capacitation and acrosome reaction

The sperm acrosome reaction and penetration of the egg follow zona pellucida binding only if the sperm has previously undergone the poorly understood maturation process known as capacitation. We demonstrate here that in vitro capacitation of bull, ram, mouse, and human sperm was accompanied by a time-dependent increase in actin polymerization. Induction of the acrosome reaction in capacitated cells initiated fast F-actin breakdown. Incubation of sperm in media lacking BSA or methyl-beta-cyclodextrin, Ca(2+), or NaHCO(3), components that are all required for capacitation, prevented actin polymerization as well as capacitation, as assessed by the ability of the cells to undergo the acrosome reaction. Inhibition of F-actin formation by cytochalasin D blocked sperm capacitation and reduced the in vitro fertilization rate of metaphase II-arrested mouse eggs. It has been suggested that protein tyrosine phosphorylation may represent an important regulatory pathway that is associated with sperm capacitation. We show here that factors known to stimulate sperm protein tyrosine phosphorylation (i.e., NaHCO(3), cAMP, epidermal growth factor, H(2)O(2), and sodium vanadate) were able to enhance actin polymerization, whereas inhibition of tyrosine kinases prevented F-actin formation. These data suggest that actin polymerization may represent an important regulatory pathway in with sperm capacitation, whereas F-actin breakdown occurs before the acrosome reaction.     

Reactive oxygen species modulate independent protein phosphorylation pathways during human sperm capacitation

Spermatozoa must undergo capacitation to acquire fertilizing ability. Reactive oxygen species (ROS), such as superoxide anion, hydrogen peroxide H2O2, and nitric oxide (NO*), are involved in this process. We investigated the roles and interactions of ROS, the ERK cascade, and the phosphoinositide 3-kinase (PI3K)/Akt axis during human sperm capacitation. Two different agents, fetal cord serum ultrafiltrate and bovine serum albumin, similarly promoted capacitation and the associated phosphorylation of protein tyrosine residues (P-Tyr), threonine-glutamine-tyrosine (P-Thr-Glu-Tyr-P) motif, and MEK-like proteins (P-MEK-like proteins). Components of the ERK pathway modulated these phosphorylation events. ROS increased P-MEK-like proteins and NO* induced P-Thr-Glu-Tyr-P, possibly by acting on or downstream of Ras. The PI3K/Akt axis participated in capacitation and phosphorylation of Tyr and Thr-Glu-Tyr but not MEK-like proteins. H2O2 and NO* induced P-Tyr even in the presence of ERK pathway inhibitors, indicating that ROS also act downstream of this pathway. These new results indicate that ROS act on different transduction elements during sperm capacitation and regulate phosphorylation events that occur in parallel pathways that eventually lead to late phosphorylation of Tyr. These new data reinforce the concept that a complex network of differentially modulated pathways is needed for spermatozoa to become capacitated.     

Tyrosine phosphorylation generates multiple isoforms of the mitochondrial capsule protein, phospholipid hydroperoxide glutathione peroxidase (PHGPx), during hamster sperm capacitation

Sperm capacitation is a maturation process, occurring in the female reproductive tract, that produces fertilization-competent spermatozoa. Protein tyrosine phosphorylation represents an important event in capacitation. The present study demonstrates the capacitation-dependent tyrosine-phosphorylation of phospholipid hydroperoxide glutathione peroxidase (PHGPx), the disulfide cross-linked, major structural protein of the sperm mitochondrial capsule. Immunofluorescence microscopy using an antiphosphotyrosine monoclonal antibody (anti-pY20) demonstrated the presence of capacitation-associated tyrosine phosphorylated proteins in the flagellum of hamster spermatozoa. Among the tyrosine-phosphorylated polypeptides (M(r) 19,000- 99,000), a 19-kDa polypeptide was the only one that can be solubilized completely by Triton X-100-dithiothreitol (DTT). The 19-kDa polypeptide was purified by anion-exchange chromatography and by immunoaffinity chromatography. Proteomic identification of the 19-kDa polypeptide by nano-electrospray tandem mass spectrometry yielded six peptides that matched the National Center for Biotechnology Information (NCBI) database sequences of bovine PHGPx. Indirect immunofluorescence localized PHGPx to the midpiece of the flagellum and the immunoblot analysis demonstrated its DTT-dependent release from purified flagella. DTT extracts of noncapacitated spermatozoa exhibited a charge train of four major PHGPx isoforms (pIs 7.5- 9.0) by two-dimensional PAGE, whereas capacitated spermatozoa revealed the generation of new acidic PHGPx isoforms with isoelectric points ranging between pH 6.0-7.0 and 4.0-5.0, indicating that it is posttranslationally modified during capacitation. These data suggest that the tyrosine-phosphorylation of PHGPx may represent an important event in the signaling pathway(s) associated with capacitation and could potentially affect mitochondrial function.     

Positive role of reactive oxygen species in mammalian sperm capacitation: triggering and modulation of phosphorylation events

The role of reactive oxygen species (ROS) as signal transduction elements in physiological phenomena is a recent concept that changes the paradigm of these active species as harmful molecules that promote deleterious effects and even cell death. Capacitation is a term used to define a complex and not well-characterized process that allows spermatozoa to complete their preparation to fertilize oocytes. Spermatozoa from many species incubated under specific conditions have the ability to produce small amounts of ROS without harming cell function and rather promoting signal transduction pathways associated with capacitation. This review summarizes the findings regarding the role of ROS during mammalian sperm capacitation, specifically as physiological mediators that trigger phosphorylation events. The role of ROS as regulators of protein tyrosine phosphorylation has been known for a decade but other novel phosphorylations, such as those of PKA substrates, of MEK-like proteins, and of proteins with the threonine-glutamine-tyrosine motif, were recently evidenced. Here we stress the involvement of PKA and the ERK pathway as two signal mechanisms acting independently that contribute to the modulation of protein tyrosine phosphorylation required for spermatozoa to achieve capacitation. Moreover, integration of all these data reinforces the concept that although some phosphorylation events are independent of the others, cross talk is also needed among the various pathways involved.     

Dynamic quantification of the tyrosine phosphorylation of the sperm surface proteins during capacitation.

BACKGROUND: Spermatozoa acquire active fertilizing competence only after deposition in the female tract and subsequent capacitation. Recent studies on the cellular location of major sperm phosphoproteins suggest that capacitation is associated with tyrosine phosphorylation of proteins exposed on the sperm surface. However, these changes have not yet been quantified objectively. A calcium influx seems to be required for the completion of tyrosine phosphorylation in some species; however, the exact temporal coordination between these processes is still poorly understood. METHODS: Flow cytometry was used to quantify the degree of phosphorylation of the sperm surface proteins by probing with fluorescein isothiocyanate-conjugated anti-phosphotyrosine (pY) antibody raised in mouse. Dynamic changes in other sperm parameters (calcium influx, membrane integrity, and spontaneous acrosome reaction) were assessed to analyze their temporal coordination. RESULTS:: The changes in specific phosphotyrosine (pY) fluorescence signal detected in live, nonpermeabilized boar cell suspensions were biphasic during incubation under capacitating conditions. After 120 min of incubation, the degree of pY fluorescence increased threefold, indicating the changes in proteins exposed on sperm surface. At the same time there was a gradual increase in cytosolic calcium ion levels with the maximal rate at 60 min of incubation. This rate slowed immediately before the onset of the massive rise in tyrosine phosphorylation and decreased by 90% after its completion. The integrity of plasma and acrosome membranes decreased only slowly, illustrating that the changes observed were not due to the process of spontaneous acrosome reaction. CONCLUSIONS: These data provide quantitative evidence for the appearance of tyrosine-phosphorylated proteins on the surface of live boar spermatozoa during capacitation. An exact temporal coordination exists between cytosolic calcium ion content and protein tyrosine phosphorylation under these conditions. This novel approach has the advantage of making possible a precise quantification and kinetic comparison of molecular processes in different cell subpopulations.     

The effect of heparin on motility parameters and protein phosphorylation during bovine sperm capacitation

It is generally accepted that incubation with heparin is required to induce capacitation of ejaculated bovine spermatozoa in vitro. The capacitation process implicates many biochemical events, and is correlated with modified sperm motility and the phosphorylation of specific proteins on tyrosine residues. To better understand the molecular basis of heparin-induced capacitation, bovine spermatozoa were incorporated with a radioactive substrate of protein kinases [gamma32P]-ATP, to observe protein phosphorylation dynamics over time. Sperm motion parameters including the percentage of motile spermatozoa, amplitude of lateral head displacement (ALH) and flagellar beat cross frequency (BCF) were assessed to determine whether the protein phosphorylation patterns induced by heparin also promote changes in motility. Capacitation was confirmed using the chlortetracycline fluorescence assay and the appearance of 'pattern B' stained spermatozoa. Evaluation of the different motility parameters during capacitation reveal that heparin has a marked negative effect, over time, on the percentage of motile spermatozoa, consistent with hyperactivation. Indeed, the presence of heparin greatly increases the BCF of bull spermatozoa and induces a significant increase in the ALH compared to spermatozoa incubated without heparin. We detected several sperm proteins that are phosphorylated over time. A 45 kDa protein is the most intensely phosphorylated of the sperm proteins. However, it is visible regardless of the presence of heparin. Interestingly, a second phosphorylated protein of approximately 50 kDa undergoes more intense phosphorylation with heparin than without. In summary, the present study demonstrated that heparin induces physiological changes in several sperm motility parameters including ALH, BCF and the percentage of motile spermatozoa. Heparin also increases the intensity of phosphorylation of a 50 kDa sperm protein. These results suggest that capacitation of bovine spermatozoa and capacitation-associated motility changes may be regulated by a mechanism that includes protein phosphorylation, and that a presently unknown protein kinase is involved.     

Role of tyrosine phosphorylation in sperm capacitation / acrosome reaction

Capacitation is an important physiological pre-requisite before the sperm cell can acrosome react and fertilize the oocyte. Recent reports from several laboratories have amply documented that the protein phosphorylation especially at tyrosine residues is one of the most important events that occur during capacitation. In this article, we have reviewed the data from our and other laboratories, and have constructed a heuristic model for the mechanisms and molecules involved in capacitation/acrosome reaction.     

Intracellular pH of bovine sperm increases during capacitation

The effect of heparin-induced capacitation on the intracellular pH (pH_i) of individual bovine sperm was determined with image analysis. Sperm were loaded with the acetoxymethyl ester of the pH sensitive fluorescent indicator, 2′,7′-bis(carboxyethyl)-5(6)-carboxy-fluorescein (BCECF). The pH_i of 5303 sperm was evaluated from a total of five bulls at .5, 2, 3, 4, and 5 h of incubation. The pH_i did not differ between the sperm head and mid-piece (P > 0.05). An increase in sperm head pH_i was seen in heparin-treated sperm at 3, 4, and 5 h of incubation relative to sperm incubated without heparin (control, P < 0.05). At 5 h of incubation, the pH_i in heparin-treated sperm was 6.92 ± 0.07, while control-treated sperm pH_i was 6.70 ± 0.03. Initially a normal frequency distribution was seen for sperm pH_i in both heparin- and control-treated sperm. As the incubation progressed, the frequency distribution began to skew towards higher pH_i in both samples but was more dispersed for the heparin-treated sperm. Following an NH₄Cl-induced alkaline load, the pH_i of both control- and heparin-treated sperm recovered toward the resting pH_i with a half-time of recovery of 1.5–1.7 min. The recovery of sperm pH_i was not due to leakage of NH⁴⁺ into sperm because recovery also occurred with trimethylamine. The instantaneous velocity of the pH_i recovery (v_i) was dependent on pH_i and decreased as pH_i decreased. Capacitation by heparin was associated with an 81% decrease in v_i at a pH_i of 7.00, but there was no effect of capacitation on the proton buffering power of the sperm, which was 87 ± 8 mM/pH unit. Results demonstrate that both the regulation of pH_i and resting pH_i were altered during capacitation of bovine sperm by heparin. © 1995 Wiley-Liss, Inc.     

Involvement of protein serine and threonine phosphorylation in human sperm capacitation.

The involvement of serine and threonine phosphorylation in human sperm capacitation was investigated. Anti-phosphoserine monoclonal antibody (mAb) recognized six protein bands in the 43-55-kDa, 94 +/- 2-kDa, 110-kDa, and 190-kDa molecular regions, in addition to a faint band each in the 18-kDa and 35-kDa regions. Anti-phosphothreonine mAb recognized protein bands in six similar regions, except that the 18-kDa, 35-kDa, and 94 +/- 2-kDa protein bands were sharper and thicker, and an additional band was observed in the 110-kDa molecular region. In the 43-55-kDa molecular region, there was a well-characterized glycoprotein, designated fertilization antigen, that showed a further increase in serine/threonine phosphorylation after exposure to solubilized human zona pellucida. In a cell-free in vitro kinase assay carried out on beads or in solution, four to eight proteins belonging to similar molecular regions, namely 20 +/- 2 kDa, 43-55 kDa, 94 +/- 2 kDa, and 110 +/- 10 kDa, as well as in 80 +/- 4 and 210 +/- 10 kDa regions, were phosphorylated at dual residues (serine/tyrosine and threonine/tyrosine). Capacitation increased the intensity of serine/threonine phosphorylation per sperm cell, increased the number of sperm cells that were phosphorylated, and induced a subcellular shift in the serine/threonine-specific fluorescence. These findings indicate that protein serine/threonine phosphorylation is involved and may have a physiological role in sperm capacitation.     

Surface alterations during the capacitation of mammalian spermatozoa

Capacitation is the process by which mammalian sperm acquire the ability to undergo the acrosome reaction which, in turn, is a prerequisite for sperm-egg fusion and penetration. Until recently, it was thought that capacitation involved subtle physiological and chemical changes which had no morphological counterparts even at the electron microscopic level. However, it has now been shown by a number of investigators that material associated with the plasma membrane surface is either lost or extensively redistributed during in vitro or in vivo capacitation. We have made use of lectins and antibodies as probes of the sperm surface during capacitation and the acrosome reaction. Concanavalin A (Con A), wheat germ agglutinin (WGA) and soybean agglutinin (SBA) have been used in conjunction with fluorescent tags (FITC) and ultrastructural markers (ferritin, hemocyanin) to study the surface of golden hamster, guinea pig, mouse and human spermatozoa. Con A and WGA label the plasma membrane overlying the acrosomal region quite uniformly on these species. After capacitation there is a specific loss (or masking) of lectin binding sites over the acrosomal region of the sperm head in all species examined. Antibodies prepared against sperm and specific antibodies to a cell surface protein (fibronectin) were also tagged with fluorescent or ultrastructural markers and used to label the surfaces of sperm before and after capacitation. These probes also indicate a specific loss of surface associated material over the acrosomal surface after capacitation. These results are consistent with the notion that there is a general removal of surface components during capacitation and that this denuding of the surface is a prerequisite for the following membrane fusion events involved in the acrosome reaction and sperm-egg fusion.     

Albumin-mediated changes in sperm sterol content during capacitation

The role of albumin in mouse sperm capacitation was studied in relation to its activities as a lipid-solubilizing protein and a sterol acceptor. Two bovine serum albumins (BSA) which supported capacitation, Fraction V and fatty acid-free, both contained cholesterol and phospholipid but were without detectable levels of serum high-density lipoprotein (HDL). The lipid content of BSA could be reduced by trichloroacetic acid (TCA) precipitation; however, removal of all detectable lipids required precipitation with ethanolic acetone and diethyl ether extraction. In medium supplemented with Fraction V, fatty acid-free, or TCA-precipitated BSA, mouse sperm were capacitated as evidenced by their ability to fertilize eggs, concomitant with decreases in total cellular sterol and increases in phospholipid content. Delipidated BSA, fractionated on Sephadex G-100 in guanidine HCl also supported capacitation and mediated a 20% decrease in sperm sterol content, while cellular phospholipid levels remained unchanged. When BSA was modified by cholesterol augmentation, fertilization was inhibited in a cholesterol dose-dependent manner. These findings suggest that modulation of sperm lipid levels comprises an event of capacitation and that albumin mediates this process through its activity as a sterol acceptor.     

Kinases, phosphatases and proteases during sperm capacitation

Fertilization is the process by which male and female haploid gametes (sperm and egg) unite to produce a genetically distinct individual. In mammals, fertilization involves a number of sequential steps, including sperm migration through the female genital tract, sperm penetration through the cumulus mass, sperm adhesion and binding to the zona pellucida, acrosome exocytosis, sperm penetration through the zona and fusion of the sperm and egg plasma membranes. However, freshly ejaculated sperm are not capable of fertilizing an oocyte. They must first undergo a series of biochemical and physiological changes, collectively known as capacitation, before acquiring fertilizing capabilities. Several molecules are required for successful capacitation and in vitro fertilization; these include bicarbonate, serum albumin (normally bovine serum albumin, BSA) and Ca(2+). Bicarbonate activates the sperm protein soluble adenylyl cyclase (SACY), which results in increased levels of cAMP and cAMP-dependent protein kinase (PKA) activation. The response to bicarbonate is fast and cAMP levels increase within 60?s followed by an increase in PKA activity. Several studies with an anti-phospho-PKA substrate antibody have demonstrated a rapid increase in protein phosphorylation in human, mouse and boar sperm. The target proteins of PKA are not known and the precise role of BSA during capacitation is unclear. Most of the studies provide support for the idea that BSA acts by removing cholesterol from the sperm. The loss of cholesterol has been suggested to affect the bilayer of the sperm plasma membrane making it more fusogenic. The relationship between cholesterol loss and the activation of the cAMP/PKA pathway is also unclear. During early stages of capacitation, Ca(2+) might be involved in the stimulation of SACY, although definitive proof is lacking. Protein tyrosine phosphorylation is another landmark of capacitation but occurs during the late stages of capacitation on a different time-scale from cAMP/PKA activation. Additionally, the tyrosine kinases present in sperm are not well characterized. Although protein phosphorylation depends upon the balanced action of protein kinases and protein phosphatase, we have even less information regarding the role of protein phosphatases during sperm capacitation. Over the last few years, several reports have pointed out that the ubiquitin-proteasome system might play a role during sperm capacitation, acrosome reaction and/or sperm-egg fusion. In the present review, we summarize the information regarding the role of protein kinases, phosphatases and the proteasome during sperm capacitation. Where appropriate, we give examples of the way that these molecules interact and regulate each other's activities.     

Reorganization of lipid rafts during capacitation of human sperm

Ejaculated mammalian sperm must complete a final maturation, termed capacitation, before they can undergo acrosomal exocytosis and fertilize an egg. In human sperm, loss of sperm sterol is an obligatory, early event in capacitation. How sterol loss leads to acrosomal responsiveness is unknown. These experiments tested the hypothesis that loss of sperm sterol affects the organization of cold detergent-resistant membrane microdomains (lipid "rafts"). The GPI-linked protein CD59, the ganglioside GM1, and the protein flotillin-2 were used as markers for lipid rafts. In uncapacitated sperm, 51% of the CD59, 41% of the GM1, and 90% of the flotillin-2 were found in the raft fraction. During capacitation, sperm lost 67% of their 3beta-hydroxysterols, and the percentages of CD59 and GM1 in the raft fraction decreased to 34% and 31%, respectively. The distribution of flotillin-2 did not change. Preventing a net loss of sperm sterol prevented the loss of CD59 and GM1 from the raft fraction. Fluorescence microscopy showed CD59 and GM1 to be distributed over the entire sperm surface. Flotillin-2 was located mainly in the posterior head and midpiece. Patching using bivalent antibodies indicated that little of the GM1 and CD59 was stably associated in the same membrane rafts. Likewise, GM1 and flotillin-2 were not associated in the same membrane rafts. In summary, lipid rafts of heterogeneous composition were identified in human sperm and the two raft components, GM1 and CD59, showed a partial sterol loss-dependent shift to the nonraft domain during capacitation.     

The early molecular events leading to COFILIN phosphorylation during mouse sperm capacitation are essential for acrosomal exocytosis

The actin cytoskeleton reorganization during sperm capacitation is essential for the occurrence of acrosomal exocytosis (AR) in several mammalian species. Here, we demonstrate that in mouse sperm, within the first minutes of exposure upon capacitating conditions, the activity of RHOA/C and RAC1 is essential for LIMK1 and COFILIN phosphorylation. However, we observed that the signaling pathway involving RAC1 and PAK4 is the main player in controlling actin polymerization in the sperm head necessary for the occurrence of AR. Moreover, we show that the transient phosphorylation of COFILIN is also influenced by the Slingshot family of protein phosphatases (SSH1). The activity of SSH1 is regulated by the dual action of two pathways. On one hand, RHOA/C and RAC1 activity promotes SSH1 phosphorylation (inactivation). On the other hand, the activating dephosphorylation is driven by okadaic acid—sensitive phosphatases. This regulatory mechanism is independent of the commonly observed activating mechanisms involving PP2B and emerges as a new finely tuned modulation that is, so far, exclusively observed in mouse sperm. However, persistent phosphorylation of COFILIN by SSH1 inhibition or okadaic acid did not altered actin polymerization and the AR. Altogether, our results highlight the role of small GTPases in modulating actin dynamics required for AR.