Intracellular Ca2+ threshold reversibly switches flagellar beat off and on

Sperm motility is essential for fertilization. The asymmetry of flagellar beat in spermatozoa is finely regulated by intracellular calcium concentration ([Ca2+]i). Recently, we demonstrated that the application of high concentrations (10-20 μM) of the Ca2+ ionophore A23187 promotes sperm immobilization after 10 min, and its removal thereafter allows motility recovery, hyperactivation, and fertilization. In addition, the same ionophore treatment overcomes infertility observed in sperm from Catsper1-/-, Slo3-/-, and Adcy10-/-, but not PMCA4-/-, which strongly suggest that regulation of [Ca2+]i is mandatory for sperm motility and hyperactivation. In this study, we found that prior to inducing sperm immobilization, high A23187 concentrations (10 μM) increase flagellar beat. While 5-10 μM A23187 substantially elevates [Ca2+]i and rapidly immobilizes sperm in a few minutes, smaller concentrations (0.5 and 1 μM) provoke smaller [Ca2+]i increases and sperm hyperactivation, confirming that [Ca2+]i increases act as a motility switch. Until now, the [Ca2+]i thresholds that switch motility on and off were not fully understood. To study the relationship between [Ca2+]i and flagellar beating, we developed an automatic tool that allows the simultaneous measurement of these two parameters. Individual spermatozoa were treated with A23187, which is then washed to evaluate [Ca2+]i and flagellar beat recovery using the implemented method. We observe that [Ca2+]i must decrease below a threshold concentration range to facilitate subsequent flagellar beat recovery and sperm motility.

Molecular changes and signaling events occurring in spermatozoa during epididymal maturation

After leaving the testis, spermatozoa have not yet acquired the ability to move progressively and are unable to fertilize oocytes. To become fertilization competent, they must go through an epididymal maturation process in the male, and capacitation in the female tract. Epididymal maturation can be defined as those changes occurring to spermatozoa in the epididymis that render the spermatozoa the ability to capacitate in the female tract. As part of this process, sperm cells undergo a series of biochemical and physiological changes that require incorporation of new molecules derived from the epididymal epithelium, as well as post-translational modifications of endogenous proteins synthesized during spermiogenesis in the testis. This review will focus on epididymal maturation events, with emphasis in recent advances in the understanding of the molecular basis of this process.

Evidence for the involvement of proline-rich tyrosine kinase 2 in tyrosine phosphorylation downstream of protein kinase A activation during human sperm capacitation

Sperm capacitation involves an increase in intracellular Ca(2+) concentration as well as in protein kinase A (PKA)-dependent protein tyrosine (Tyr) phosphorylation. Interestingly, in humans, a decrease in extracellular Ca(2+) concentration ([Ca(2+)]e) during capacitation induces an increase in Tyr phosphorylation indicating the complexity of Ca(2+) signaling during this process. In view of this, in the present study we further investigated the Ca(2+)-mediated signaling pathways implicated in Tyr phosphorylation during human sperm capacitation. Results revealed that sperm incubation in a medium without added Ca(2+) (⊖ Ca(2+)) increased Tyr phosphorylation but did not modify PKA-mediated phosphorylation. Moreover, inhibition of either PKA or Src family kinase signaling cascades in ⊖ Ca(2+) down-regulated both PKA substrate and Tyr phosphorylations, indicating that the [Ca(2+)]e effects on Tyr phosphorylation depend on PKA targets. Inhibition of calmodulin or Ser/Thr protein phosphatase 2B also increased Tyr phosphorylation without affecting PKA-mediated phosphorylation, supporting the potential role of these Ca(2+) downstream effectors in the increase in Tyr phosphorylation observed in ⊖ Ca(2+). Experiments aimed to identify the kinase responsible for these observations revealed the presence of proline-rich tyrosine kinase 2 (PYK2), a focal adhesion kinase (FAK) family member, in human sperm, and the use of PF431396, an FAK inhibitor, supported the involvement of PYK2 in Tyr phosphorylation downstream of PKA activation. Results also showed that PYK2 was activated in ⊖ Ca(2+) as well as during capacitation and that PF431396 affected capacitated sperm motility, acrosome reaction and ability to penetrate both mouse cumulus matrix and zona-free hamster eggs. Together, our observations support PYK2 as an intermediary component of Ca(2+) signaling between PKA-mediated and Tyr phosphorylations that is required for achieving functional human sperm capacitation.

Functional human sperm capacitation requires both bicarbonate-dependent PKA activation and down-regulation of Ser/Thr phosphatases by Src family kinases

In all mammalian species studied so far, sperm capacitation correlates with an increase in protein tyrosine (Tyr) phosphorylation mediated by a bicarbonate-dependent cAMP/protein kinase A (PKA) pathway. Recent studies in mice revealed, however, that a Src family kinase (SFK)-induced inactivation of serine/threonine (Ser/Thr) phosphatases is also involved in the signaling pathways leading to Tyr phosphorylation. In view of these observations and with the aim of getting a better understanding of the signaling pathways involved in human sperm capacitation, in the present work we investigated the involvement of both the cAMP/PKA and SFK/phosphatase pathways in relation to the capacitation state of the cells. For this purpose, different signaling events and sperm functional parameters were analyzed as a function of capacitation time. Results revealed a very early bicarbonate-dependent activation of PKA indicated by the rapid (1 min) increase in both phospho-PKA substrates and cAMP levels (P < 0.05). However, a complete pattern of Tyr phosphorylation was detected only after 6-h incubation at which time sperm exhibited the ability to undergo the acrosome reaction (AR) and to penetrate zona-free hamster oocytes. Sperm capacitated in the presence of the SFK inhibitor SKI606 showed a decrease in both PKA substrate and Tyr phosphorylation levels, which was overcome by exposure of sperm to the Ser/Thr phosphatase inhibitor okadaic acid (OA). However, OA was unable to induce phosphorylation when sperm were incubated under PKA-inhibitory conditions (i.e. in the absence of bicarbonate or in the presence of PKA inhibitor). Moreover, the increase in PKA activity by exposure to a cAMP analog and a phosphodiesterase inhibitor did not overcome the inhibition produced by SKI606. Whereas the presence of SKI606 during capacitation produced a negative effect (P < 0.05) on sperm motility, progesterone-induced AR and fertilizing ability, none of these inhibitions were observed when sperm were exposed to SKI606 and OA. Interestingly, different concentrations of inhibitors were required to modulate human and mouse capacitation revealing the species specificity of the molecular mechanisms underlying this process. In conclusion, our results describe for the first time the involvement of both PKA activation and Ser/Thr phosphatase down-regulation in functional human sperm capacitation and provide convincing evidence that early PKA-dependent phosphorylation is the convergent regulatory point between these two signaling pathways.

Roles of the oviduct in mammalian fertilization

The oviduct or Fallopian tube is the anatomical region where every new life begins in mammalian species. After a long journey, the spermatozoa meet the oocyte in the specific site of the oviduct named ampulla and fertilization takes place. The successful fertilization depends on several biological processes that occur in the oviduct some hours before this rendezvous and affect both gametes. Estrogen and progesterone, released from the ovary, orchestrate a series of changes by genomic and nongenomic pathways in the oviductal epithelium affecting gene expression, proteome, and secretion of its cells into the fluid bathing the oviductal lumen. In addition, new regulatory molecules are being discovered playing important roles in oviductal physiology and fertilization. The present review tries to describe these processes, building a comprehensive map of the physiology of the oviduct, to better understand the importance of this organ in reproduction. With this purpose, gamete transport, sperm and oocyte changes in the oviductal environment, and other interactions between gametes and oviduct are discussed in light of recent publications in the field.

Guanine-nucleotide exchange factors (RAPGEF3/RAPGEF4) induce sperm membrane depolarization and acrosomal exocytosis in capacitated stallion sperm

Capacitation encompasses the molecular changes sperm undergo to fertilize an oocyte, some of which are postulated to occur via a cAMP-PRKACA (protein kinase A)-mediated pathway. Due to the recent discovery of cAMP-activated guanine nucleotide exchange factors RAPGEF3 and RAPGEF4, we sought to investigate the separate roles of PRKACA and RAPGEF3/RAPGEF4 in modulating capacitation and acrosomal exocytosis. Indirect immunofluorescence localized RAPGEF3 to the acrosome and subacrosomal ring and RAPGEF4 to the midpiece in equine sperm. Addition of the RAPGEF3/RAPGEF4-specific cAMP analogue 8-(p-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8pCPT) to sperm incubated under both noncapacitating and capacitating conditions had no effect on protein tyrosine phosphorylation, thus supporting a PRKACA-mediated event. Conversely, activation of RAPGEF3/RAPGEF4 with 8pCPT induced acrosomal exocytosis in capacitated equine sperm at rates (34%) similar (P > 0.05) to those obtained in progesterone- and calcium ionophore-treated sperm. In the mouse, capacitation-dependent hyperpolarization of the sperm plasma membrane has been shown to recruit low voltage-activated T-type Ca(2+) channels, which later open in response to zona pellucida-induced membrane depolarization. We hypothesized that RAPGEF3 may be inducing acrosomal exocytosis via depolarization-dependent Ca(2+) influx, as RAPGEF3/RAPGEF4 have been demonstrated to play a role in the regulation of ion channels in somatic cells. We first compared the membrane potential (E(m)) of noncapacitated (-37.11 mV) and capacitated (-53.74 mV; P = 0.002) equine sperm. Interestingly, when sperm were incubated (6 h) under capacitating conditions in the presence of 8pCPT, E(m) remained depolarized (-32.06 mV). Altogether, these experiments support the hypothesis that RAPGEF3/RAPGEF4 activation regulates acrosomal exocytosis via its modulation of E(m), a novel role for RAPGEF3/RAPGEF4 in the series of events required to achieve fertilization.

Evidence for the involvement of proline-directed serine/threonine phosphorylation in sperm capacitation

To become fertilization competent, mammalian sperm undergo changes in the female reproductive tract termed capacitation. Capacitation correlates with an increase in tyrosine phosphorylation; however, less is known about the role of serine/threonine phosphorylation in this process. Proline-directed phosphorylation is one of the major regulatory phosphorylation events in many cellular processes such as cell proliferation and differentiation. Using mitotic phosphoprotein monoclonal-2 (MPM-2) antibody in this study, we observed that several mouse sperm proteins in the range of 70-250 kDa underwent increased serine/threonine-proline phosphorylation during capacitation. In contrast to the time course of tyrosine phosphorylation, proline-directed phosphorylation could be observed at shorter time points of sperm incubation, and it was found to be independent of NaHCO(3) and adenosine 3'5'-cyclic monophosphate (cAMP). Similar to the regulation of the increase in tyrosine phosphorylation, cholesterol acceptors such as bovine serum albumin (BSA) or 2-hydroxypropyl-beta-cyclodextrin (2-OH-propyl-beta-CD) were essential for the regulation of proline-directed phosphorylation in mouse sperm. Furthermore, it was also found to be BSA dependent in human sperm. Among proline-directed kinases, extracellular signal-regulated kinase 1/2 (ERK1/2) is present in mammalian sperm; nevertheless, U0126 and PD098059, two inhibitors of the ERK pathway, did not block this phosphorylation in mouse sperm. In conclusion, capacitation is associated with an increase in proline-directed phosphorylation linked to cholesterol efflux in the sperm.

alpha-SNAP and NSF are required in a priming step during the human sperm acrosome reaction

The acrosome is a membrane-limited granule that overlies the nucleus of the mature spermatozoon. In response to physiological or pharmacological stimuli it undergoes a special type of Ca2+-dependent exocytosis termed the acrosome reaction (AR), which is an absolute prerequisite for fertilization. Aided by a streptolysin-O permeabilization protocol developed in our laboratory, we have previously demonstrated requirements for Rab3A, N-ethylmaleimide-sensitive factor (NSF), several soluble NSF-attachment protein receptor (SNARE) proteins, and synaptotagmin VI in the human sperm AR. Here, we show that alpha-soluble NSF-attachment protein (alpha-SNAP), a protein essential for most fusion events through its interaction with NSF and the SNARE complex, exhibits a direct role in the AR. First, the presence of alpha-SNAP is demonstrated by the Western blot of human sperm protein extracts. Immunostaining experiments reveal an acrosomal localization for this protein. Second, the Ca2+ and Rab3A-triggered ARs are inhibited by anti-alpha-SNAP antibodies. Third, bacterially expressed alpha-SNAP abolishes exocytosis in a fashion that depends on its interaction with NSF. Fourth, we show a requirement for alpha-SNAP/NSF in a prefusion step early in the exocytotic pathway, after the tethering of the acrosome to the plasma membrane and before the efflux of intra-acrosomal Ca2+. These results suggest a key role for alpha-SNAP/NSF in the AR, and strengthen our understanding of the molecular players involved in the vesicle-to-plasma membrane fusion taking place during exocytosis.

Novel signaling pathways involved in sperm acquisition of fertilizing capacity

Capacitation is a complex series of molecular events that occurs in sperm after epididymal maturation and confers on sperm 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 oviductal fluid. In most cases, capacitation media contain energy substrates, such as pyruvate, lactate and glucose, a cholesterol acceptor (usually serum albumin), NaHCO(3), Ca(2+), low K(+), and physiological Na(+) concentrations. The mechanism of action by which these compounds promote capacitation is poorly understood at the molecular level; however, some molecular events significant to the initiation of capacitation have been identified. For example, capacitation correlates with cholesterol efflux from the sperm plasma membrane, increased membrane fluidity, modulations in intracellular ion concentrations, hyperpolarization of the sperm plasma membrane and increased protein tyrosine phosphorylation. These molecular events are required for the subsequent induction of hyperactivation and the acrosome reaction. This review discusses the recent progress that has been made in elucidating mechanisms which regulate sperm capacitation.

Cloning and chromosomal localization of a gene encoding a novel serine/threonine kinase belonging to the subfamily of testis-specific kinases

Using reverse transcription-polymerase chain reaction (RT-PCR) with degenerate oligonucleotides corresponding to two highly conserved motifs within the protein kinase family of catalytic domains, we isolated a PCR fragment encoding a novel member of the testis-specific serine/threonine kinases (STK) from mouse male mixed germ cell mRNA. This PCR fragment recognized a 1020-bp transcript in male germ cells by northern blot analysis and was used to clone a full-length cDNA from a mouse mixed germ cell cDNA library. This cDNA has an open reading frame of 804 bases encoding a protein of 268 amino acids. This novel gene is almost identical to Stk22c, encoding a recently described testis-specific protein kinase, except for base-pair deletions that result in a shift in the coding region and an alteration of 22 amino acids (residues 109-131). Due to its homology with Stk22c, we have called this protein kinase gene Stk22d. Northern blot analysis revealed that this protein kinase is developmentally expressed in testicular germ cells and is not present in brain, ovary, kidney, liver, or early embryonic cells. We then cloned the human homologue of this protein kinase gene (STK22C) and found it to be expressed exclusively in the testis. Fluorescence in situ hybridization with both the human and mouse cDNA clones revealed syntenic localization on chromosomes 1p34-p35 and 4E1, respectively.

Dual regulation of the T-type Ca(2+) current by serum albumin and beta-estradiol in mammalian spermatogenic cells

This study provides evidence for a novel mechanism of voltage-gated Ca(2+) channel regulation in mammalian spermatogenic cells by two agents that affect sperm capacitation and the acrosome reaction (AR). Patch-clamp experiments demonstrated that serum albumin induced an increase in Ca(2+) T current density in a concentration-dependent manner, and significant shifts in the voltage dependence of both steady-state activation and inactivation of the channels. These actions were not related to the ability of albumin to remove cholesterol from the membrane. In contrast, beta-estradiol significantly inhibited Ca(2+) channel activity in a concentration-dependent and essentially voltage-independent fashion. In mature sperm this dual regulation may influence capacitation and/or the AR.

Regulation of human sperm capacitation by a cholesterol efflux-stimulated signal transduction pathway leading to protein kinase A-mediated up-regulation of protein tyrosine phosphorylation

Protein tyrosine phosphorylation is an important intracellular event accompanying the in-vitro capacitation of mouse, bovine and human spermatozoa. Here, we demonstrate that bovine serum albumin (BSA) and NaHCO(3) are required for protein tyrosine phosphorylation in ejaculated human spermatozoa. The absence of protein tyrosine phosphorylation in media minus these two constituents could be recovered by addition to the media of cAMP analogues and/or phosphodiesterase inhibitors. Since BSA is postulated to modulate capacitation by removal of cholesterol from the sperm plasma membrane, we determined whether cholesterol release leads to changes in protein tyrosine phosphorylation. Incubation of spermatozoa in media containing BSA resulted in the release of significant amounts of cholesterol when compared with media devoid of BSA. Preloading BSA with cholesterol-SO(4) inhibited protein tyrosine phosphorylation, as well as capacitation, and this inhibitory effect was overcome by the addition of dibutyryl cAMP plus isobutylmethylxanthine (IBMX). The functional significance of BSA-mediated cholesterol release, protein tyrosine phosphorylation and capacitation was confirmed by examining the effects of the cholesterol-binding heptasaccharides, methyl-beta-cyclodextrin or OH-propyl-beta-cyclodextrin. Both cyclodextrins caused cholesterol efflux from the spermatozoa, increased protein tyrosine phosphorylation, and stimulated capacitation. Therefore, cholesterol release is associated with the activation of a signal transduction pathway involving protein kinase A and tyrosine kinase second messenger systems, and resulting in protein tyrosine phosphorylation and capacitation.

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.

Roles of bicarbonate, cAMP, and protein tyrosine phosphorylation on capacitation and the spontaneous acrosome reaction of hamster sperm

Capacitation is a prerequisite for successful fertilization by mammalian spermatozoa. This process is generally observed in vitro in defined NaHCO3-buffered media and has been shown to be associated with changes in cAMP metabolism and protein tyrosine phosphorylation. In this study, we observed that when NaHCO3 was replaced by 4-(2-hydroxyethyl)1-piperazine ethanesulfonic acid (HEPES), hamster sperm capacitation, measured as the ability of the sperm to undergo a spontaneous acrosome reaction, did not take place. Addition of 25 mM NaHCO3 to NaHCO3-free medium in which spermatozoa had been preincubated for 3.5 h, increased the percentage of spontaneous acrosome reactions from 0% to 80% in the following 4 h. Addition of anion transport blockers such as 4,4'-diiso thiocyano-2, 2'-stilbenedisulfonate (DIDS) or 4-acetomido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) to the NaHCO3-containing medium inhibited the acrosome reaction, with maximal inhibition at 600 microM, and with an EC50 of 100 microM. Increasing either extracellular or intracellular pH did not induce the acrosome reaction in NaHCO3-free medium. In contrast, addition of 500 microM dibutyryl cAMP (dbcAMP), alone or together with 100 microM 1-methyl-3-isobutylxanthine (IBMX), induced the acrosome reaction in spermatozoa incubated in NaHCO3-free medium. These compounds also partially reversed the inhibition of the acrosome reaction caused by the DIDS or SITS in complete medium. In contrast to these results, IBMX or dbcAMP did not induce acrosome reactions in cells incubated in Ca2+-free medium. When hamster sperm were incubated in the absence of NaHCO3 or in the presence of NaHCO3 and DIDS, cAMP concentrations were significantly lower than the values obtained from sperm incubated in complete medium. Protein tyrosine phosphorylation has also been shown to be highly correlated with the onset of capacitation in many species. During the first hour of capacitation, an increase in protein tyrosine phosphorylation was observed in complete medium. In the absence of NaHCO3, the increase in protein tyrosine phosphorylation was delayed for 45 min, and this delay was overcome by the addition of dbcAMP and IBMX. The induction of the acrosome reaction by calcium ionophore A23187 in NaHCO3-free medium was delayed 2 h, as compared with control medium. This delay was not observed in the presence of dbcAMP and IBMX. Taken together, these results suggest that a cAMP pathway may mediate the role of NaHCO3 in the capacitation of hamster spermatozoa and that protein tyrosine phosphorylation is necessary but not sufficient for complete capacitation.

Control of the low voltage-activated calcium channel of mouse sperm by egg ZP3 and by membrane hyperpolarization during capacitation

Sperm adhesion to egg zonae pellucidae initiates sperm acrosome reactions, an exocytotic event that is an early step during fertilization. Previously, it was suggested that zona pellucida-evoked Ca2+ entry into sperm through low voltage-activated Ca2+ channels is an essential step in acrosome reactions, based on the inhibitory effects of Ca2+ channel antagonists. However, analysis of this channel is limited by the inability to apply electrophysiological methods directly to sperm. In this report, optical methods of determining membrane potential and internal Ca2+ levels were used to demonstrate that (i) contact with zonae pellucidae activates a transient Ca2+ response in sperm that has a time course and antagonist sensitivity anticipated of low voltage-activated Ca2+ channels; (ii) these channels are unavailable for opening in uncapacitated sperm because of voltage-dependent, steady state inactivation; (iii) membrane hyperpolarization during sperm capacitation is sufficient to recruit channels into a closed state, from which they are available for opening during fertilization; and (iv) channel conductance state may be a factor in determines the efficacy with which channel antagonists inhibit fertilization. This study provides evidence for the activation of sperm Ca2+ channels during gamete adhesion and offers a mechanism that may account for aspects of the regulation of sperm fertility during capacitation through the control of channel availability. Finally, these results suggest that channel conductance state may be a central feature in the design of channel antagonists that inhibit sperm function.

Cholesterol efflux-mediated signal transduction in mammalian sperm. beta-cyclodextrins initiate transmembrane signaling leading to an increase in protein tyrosine phosphorylation and capacitation

Sperm capacitation in vitro is highly correlated with an increase in protein tyrosine phosphorylation that is regulated by cAMP through a unique mode of signal transduction cross-talk. The activation of this signaling pathway, as well as capacitation, requires bovine serum albumin (BSA) in the incubation medium. BSA is hypothesized to modulate capacitation through its ability to remove cholesterol from the sperm plasma membrane. Here we demonstrate that the cholesterol-binding heptasaccharides, methyl-beta-cyclodextrin and OH-propyl-beta-cyclodextrin, promote the release of cholesterol from the mouse sperm plasma membrane in media devoid of BSA. Both of these beta-cyclodextrins were also demonstrated to increase protein tyrosine phosphorylation in the absence of BSA in both mouse and bull sperm, and the patterns of phosphorylation were similar to those induced by media containing BSA. The potency of the different beta-cyclodextrins to increase protein tyrosine phosphorylation in sperm was correlated with their cholesterol binding efficiencies, and preincubation of the beta-cyclodextrins with cholesterol-SO4- to saturate their cholesterol-binding sites blocked the ability of these compounds to stimulate protein tyrosine phosphorylation. The beta-cyclodextrin effect on protein tyrosine phosphorylation was both NaHCO3 and protein kinase A-dependent. The beta-cyclodextrins were also able to capacitate mouse sperm in the absence of BSA, as measured by the ability of the zona pellucida to induce the acrosome reaction and by successful fertilization in vitro. In summary, beta-cyclodextrins can completely replace BSA in media to support signal transduction leading to capacitation. These data further support the coupling of cholesterol efflux to the activation of membrane and transmembrane signaling events leading to the activation of a unique signaling pathway involving the cross-talk between cAMP and tyrosine kinase second messenger systems, thus defining a new mode of cellular signal transduction initiated by cholesterol release.

Targeting of a germ cell-specific type 1 hexokinase lacking a porin-binding domain to the mitochondria as well as to the head and fibrous sheath of murine spermatozoa

Multiple isoforms of type 1 hexokinase (HK1) are transcribed during spermatogenesis in the mouse, including at least three that are presumably germ cell specific: HK1-sa, HK1-sb, and HK1-sc. Each of these predicted proteins contains a common, germ cell-specific sequence that replaces the porin-binding domain found in somatic HK1. Although HK1 protein is present in mature sperm and is tyrosine phosphorylated, it is not known whether the various potential isoforms are differentially translated and localized within the developing germ cells and mature sperm. Using antipeptide antisera against unique regions of HK1-sa and HK1-sb, it was demonstrated that these isoforms were not found in pachytene spermatocytes, round spermatids, condensing spermatids, or sperm, suggesting that HK1-sa and HK1-sb are not translated during spermatogenesis. Immunoreactivity was detected in protein from round spermatids, condensing spermatids, and mature sperm using an antipeptide antiserum against the common, germ cell-specific region, suggesting that HK1-sc was the only germ cell-specific isoform present in these cells. Two-dimensional SDS-PAGE suggested that all of the sperm HK1-sc was tyrosine phosphorylated, and that the somatic HK1 isoform was not present. Immunoelectron microscopy revealed that HK1-sc was associated with the mitochondria and with the fibrous sheath of the flagellum and was found in discrete clusters in the region of the membranes of the sperm head. The unusual distribution of HK1-sc in sperm suggests novel functions, such as extramitochondrial energy production, and also demonstrates that a hexokinase without a classical porin-binding domain can localize to mitochondria.

Regulation, localization, and anchoring of protein kinase A subunits during mouse sperm capacitation

The molecular basis of mammalian sperm capacitation, defined as those biochemical and functional changes that render the sperm competent to fertilize the egg, is poorly understood. This extratesticular maturational process is accompanied by the activation of a unique signal transduction pathway involving the cAMP-dependent up-regulation of protein tyrosine phosphorylation presumably through the activation of protein kinase A (PK-A). We demonstrate in this report that capacitation of cauda epididymal mouse sperm in vitro was accompanied by a time-dependent increase in PK-A activity. This increase in PK-A activity did not occur in a medium that does not support capacitation. While PK-A catalytic and RI/RII regulatory subunits, as well as PK-A enzyme activity, were found in both the Triton X-100-soluble and -insoluble fractions of the sperm, the increase in PK-A activity accompanying capacitation was associated with enzyme activity found in the soluble fraction. Moreover, the regulatory and catalytic subunits of PK-A were observed by indirect immunofluorescence to be present throughout the head, midpiece, and principal piece of the sperm. Thus, PK-A appears to be functional in multiple compartments of this highly differentiated cell. A fraction of the Triton X-100-insoluble PK-A is presumably tethered by AKAP82, the major protein of the fibrous sheath of the sperm flagellum which anchors and compartmentalizes PK-A to the cytoskeleton via the RII subunit of PK-A. Using various recombinant truncated AKAP82 constructs in a gel overlay assay, the RII subunit-binding domain of this protein was mapped to a 57-amino-acid residue region at its N-terminus. Computer analysis revealed a 14-amino-acid region that resembled the RII-binding domains of other A Kinase Anchor Proteins. A synthetic peptide corresponding to this domain inhibited AKAP82-RII binding in a gel overlay assay, providing further support that AKAP82 is an anchoring protein for the subcellular localization of PK-A in the mouse sperm fibrous sheath. This work, along with previous findings that cAMP is a key intermediary second messenger in regulating protein tyrosine phosphorylation and capacitation, further supports the importance of PK-A in these processes and necessitates a further understanding of the contribution of both the soluble and insoluble forms of PK-A, as well as AKAP82, to sperm function.

Regulation of protein tyrosine phosphorylation during bovine sperm capacitation by a cyclic adenosine 3'5'-monophosphate-dependent pathway

Mammalian sperm capacitation, defined as an obligatory maturational process leading to the development of the fertilization-competent state, results from a poorly understood series of morphological and molecular events. We report here that ejaculated bovine sperm, incubated under conditions that support capacitation in vitro, display a reproducible pattern of protein tyrosine phosphorylations that are regulated by a cAMP-dependent pathway. The appearance of these tyrosine phosphorylated proteins correlated temporally with the time course of capacitation induced by heparin, and these phosphorylations displayed a similar heparin concentration dependence. Glucose, which inhibits capacitation, inhibited these protein tyrosine phosphorylations in media containing heparin. The biologically active cAMP analogues (dibutyryl cAMP [db-cAMP], 8-bromo cAMP, sp-cAMPS) and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) induced the same protein tyrosine phosphorylation patterns as seen with heparin. Moreover, these cAMP agonists could overcome the inhibition of the heparin-induced tyrosine phosphorylations by glucose. In contrast, Rp-adenosine-3',5'-cyclic monophosphorothioate (Rp-cAMPS), a protein kinase A (PK-A) antagonist, blocked the capacitation-associated increases in protein tyrosine phosphorylation. This cAMP regulation of the protein tyrosine phosphorylation pattern is mediated by PK-A since N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide-dihydrochloride (H89), another inhibitor of PK-A, inhibited the heparin-induced protein tyrosine phosphorylation pattern in a concentration-dependent manner in either the absence or presence of db-cAMP, IBMX, and glucose. These data support a model for sperm capacitation that includes protein tyrosine phosphorylation as an important regulatory pathway, and a role for cAMP/PK-A in the regulation of this pathway leading to capacitation. These studies are the first to report a unique interrelationship between tyrosine kinase/phosphatase and cAMP signaling pathways at the level of PK-A in bovine sperm capacitation.

Properties and localization of a tyrosine phosphorylated form of hexokinase in mouse sperm

Mouse sperm possess a phosphotyrosine-containing hexokinase type 1 (HK1) that is associated with the plasma membrane fraction of these cells (Kalab et al., 1994; J. Biol Chem 269:3810-3817). This apparent plasma membrane association appears unique, since somatic HK1 is normally cytoplasmic or bound to the outer mitochondrial membrane via contact sites with a voltage-dependent anion channel (porin) through a porin-binding domain. In male germ cells, three cDNA clones have been described that encode unique HK1 isoforms (HK1-sa, HK1-sb, HK1-sc) that do not contain porin binding domains (Mori et al., 1993: Biol Reprod 49:191-203). This suggests that these proteins might not be localized to the outer mitochondrial membrane and could have alternative functions in germ cells and/or sperm. We demonstrate in the mouse that male germ cells and sperm could potentially express four HK1 isoforms (HK1-sa, HK1-sb, HK1-sc, and the somatic HK1). At the protein level, at least one of the HK1 isoforms becomes phosphorylated on tyrosine residues during spermatogenesis. Treatment of sperm membrane fractions to dissociate the phosphotyrosine-containing HK1 (pY-mHK1) yields results demonstrating that pY-mHK1 has properties of an integral membrane protein. Indirect immunofluorescence using a monoclonal antibody to HK1 demonstrates specific staining both in the head and tail regions of sperm. Surface biotinylation of intact sperm followed by precipitation with either polyclonal HK1 antiserum or with avidin-Sepharose suggests that pY-mHK1 possesses an extracellular domain. These results suggest that mouse sperm contain at least one HK1 isoform that is present on the sperm head, has an extracellular domain, and behaves as an integral membrane protein.

Sperm from mice carrying two t haplotypes do not possess a tyrosine phosphorylated form of hexokinase

Mouse sperm contain a tyrosine phosphorylated form of hexokinase type 1 (HK1; Kalab et al., 1994: J Biol Chem 269:3810-3817) that has properties consistent with an integral plasma membrane protein. Furthermore, this tyrosine phosphorylated form of HK1 has an extracellular domain and HK1 is localized to both the head and flagellum of nonpermeabilized cells (Visconti et al., 1995c). We have characterized HK1 in mature sperm from sterile tw32/tw5 mice (mutant sperm) that have defects in motility and sperm-egg interaction (Johnson et al., 1995: Dev Biol 168:138-149). Immunoprecipitation of mouse sperm extracts with an antiserum made against purified rat brain HK1 demonstrates the presence of HK1 in mutant sperm. Various biochemical and immunofluorescence assays indicate that at least a portion of the HK1 present in these cells is an integral membrane protein with an extracellular domain located on the sperm head and flagellum. However, immunoblot analysis with anti-phoshotyrosine antibodies demonstrates that HK1 in mutant sperm is not tyrosine phosphorylated. Northern blot and RT-PCR analysis does not indicate any obvious abnormalities in the transcription of somatic or germ cell-specific HK1 isoforms in mutant testes, and RFLP analysis of recombinant mice indicates that no genes specifying HK1 isoforms are located on chromosome 17. We have mapped the locus responsible for the lack of tyrosine phosphorylation of HK1 mutant sperm to the most proximal (to the centromere) of the four inversions within the t haplotype. A male sterility factor is located in this same inversion (Lyon, 1986: Cell 44:357-363). Since the mutant sperm are unable to complete fertilization, there could be a relationship between sterility and the lack of tyrosine phosphorylation of HK1 in these mutant sperm.

Potential role of mitogen-activated protein kinase in pronuclear envelope assembly and disassembly following fertilization of mouse eggs

Changes in the activities of the p34cdc2/cyclin B complex and mitogen-activated protein (MAP) kinase were analyzed after insemination of mouse eggs in vitro. Whereas histone H1 kinase activity (p34cdc2/cyclin B) fell to negligible levels by 90 min postinsemination, a decrease to negligible levels of myelin basic protein kinase activity (i.e., MAP kinase) was not observed until about 7 h postinsemination. The decrease in MAP kinase activity appeared to be linked to the prior decline in p34cdc2/cyclin B kinase activity, since inhibiting the fertilization-induced destruction of cyclin B by treating eggs with the microtubule inhibitor nocodazole prevented the decrease in each of these protein kinases; an intact spindle is required for cyclin destruction. Moreover, experimental elevation of MAP kinase activity by okadaic acid treatment under conditions that maintain negligible levels of p34cdc2/cyclin B kinase activity suggested that MAP kinase could be involved in pronuclear envelope dynamics. Specifically, preventing the fertilization-induced decrease in MAP kinase activity was correlated with inhibiting pronucleus formation, and elevating MAP kinase activity subsequent to pronucleus formation resulted in precocious pronuclear envelope breakdown prior to entry into M phase.

Capacitation of mouse spermatozoa. II. Protein tyrosine phosphorylation and capacitation are regulated by a cAMP-dependent pathway

In the accompanying report (Visconti, P.E., Bailey, J.L., Moore, G.D., Pan, D., Olds-Clarke, P. and Kopf, G.S. (1995) Development, 121, 1129–1137) we demonstrated that the tyrosine phosphorylation of a subset of mouse sperm proteins of M(r) 40,000-120,000 was correlated with the capacitation state of the sperm. The mechanism by which protein tyrosine phosphorylation is regulated in sperm during this process is the subject of this report. Cauda epididymal sperm, when incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin do not display the capacitation-associated increases in protein tyrosine phosphorylation of this subset of proteins. This NaHCO3, CaCl2 or bovine serum albumin requirement for protein tyrosine phosphorylation can be completely overcome by the addition of biologically active, but not inactive, cAMP analogues. Addition of the active cAMP analogues to sperm incubated in media devoid of NaHCO3, CaCl2 or bovine serum albumin overcomes the inability of these media to support capacitation, as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. The effects of the cAMP analogues to enhance protein tyrosine phosphorylation and to promote capacitation appears to be at the level of the cAMP-dependent protein kinase (PKA), since two specific inhibitors of this enzyme (H-89 and Rp-cAMPS) block the capacitation-dependent increases in protein tyrosine phosphorylation in sperm incubated in media supporting capacitation. Capacitation, as assessed by the aforementioned endpoints, also appears to be inhibited by H-89 in a concentration-dependent manner. These results provide further evidence for the interrelationship between protein tyrosine phosphorylation and the appearance of the capacitated state in mouse sperm. They also demonstrate that both protein tyrosine phosphorylation and capacitation appear to be regulated by cAMP/PKA. Up-regulation of protein tyrosine phosphorylation by cAMP/PKA in sperm is, to our knowledge, the first demonstration of such an interrelationship between tyrosine kinase/phosphatase and PKA signaling pathways.

Capacitation of mouse spermatozoa. I. Correlation between the capacitation state and protein tyrosine phosphorylation

The molecular basis of mammalian sperm capacitation, defined functionally as those processes that confer on the sperm the acquisition of fertilization-competence either in vivo in the female reproductive tract or in vitro, is poorly understood. We demonstrate here that capacitation of caudal epididymal mouse sperm in vitro is accompanied by a time-dependent increase in the protein tyrosine phosphorylation of a subset of proteins of M(r) 40,000-120,000. Incubation of sperm in media devoid of bovine serum albumin, CaCl2 or NaHCO3, components which individually are required for capacitation, prevent the sperm from undergoing capacitation as assessed by the ability of the cells to acquire the pattern B chlortetracycline fluorescence, to undergo the zona pellucida-induced acrosome reaction and, in some cases, to fertilize metaphase II-arrested eggs in vitro. In each of these cases the protein tyrosine phosphorylation of the subset of capacitation-associated proteins does not occur. Protein tyrosine phosphorylation of these particular proteins, as well as sperm capacitation, can be recovered in media devoid of each of these three constituents (bovine serum albumin, CaCl2 or NaHCO3) by adding back the appropriate component in a concentration-dependent manner. The requirement of NaHCO3 for these phosphorylations is not due to an alkalinization of intracellular sperm pH or to an increase in media pH. Caput epididymal sperm, which lack the ability to undergo capacitation in vitro, do not display this capacitation-dependent subset of tyrosine phosphorylated proteins in complete media even after extended incubation periods, and do not fertilize metaphase II-arrested eggs in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Results of several lines of experimentation suggest that sperm-induced egg activation has several features in common with G protein-coupled receptor signal transduction mechanisms. We report that microinjection of GDP beta S into metaphase II-arrested mouse eggs blocks sperm-induced egg activation. Since GDP beta S inactivates both heterotrimeric and monomeric classes of G proteins, the involvement of members of each of these families in sperm-induced egg activation was evaluated. Neither pertussis toxin treatment of eggs nor microinjection of eggs with inhibitory antibodies toward G alpha q blocked sperm-induced egg activation. Nevertheless, microinjection of phosducin, a protein that binds tightly to free G protein beta gamma subunits, specifically inhibited second polar body emission, the fertilization evoked decrease of H1 kinase activity and pronucleus formation. Microinjection of phosducin, however, did not inhibit the fertilization-induced modifications of the zona pellucida and microinjection of beta gamma t did not result in egg activation in the absence of sperm. Inactivation of the monomeric Rho family of G proteins with C3 transferase from Clostridium botulinum inhibited emission of the second polar body and cleavage to the 2-cell stage, but did not affect the modifications of the zona pellucida or pronucleus formation. Microinjection of Rasval12, which is a constitutively active form of Ras, did not result in egg activation in the absence of sperm. Moreover, microinjection of either an anti-Ras neutralizing antibody (Y13-259) or a dominant negative form of Ras (RasT) did not affect events of sperm-induced egg activation. In contrast, microinjection of RasT inhibited embryo cleavage to the 2-cell stage. These results suggest that both heterotrimeric and monomeric G proteins are involved in various aspects of sperm-induced egg activation.

Effects of juvenile hormone on mammalian steroidogenesis

The effects of juvenile hormone-III (JH-III) and the JH analogue 2-(4-phenoxyphenoxy)-ethoxyte-trahydropiran on testicular steroidogenesis were studied. By using cultured MA-10 Leydig tumor cells as a model, these compounds were found to be potent inhibitors of LH/hCG steroidogenic action in a dose-dependent manner. Scatchard plot analysis of the binding data indicated that the JH analogue did not significantly alter the affinity nor the number of hCG binding sites, as well as GTP binding to plasma membranes. JH analogue inhibited the stimulatory action of both cholera toxin and forskolin on cAMP production and the concomitant steroidogenic response. JH analogue inhibited (Bu)2cAMP-stimulated progesterone synthesis, indicating that a process downstream to the adenylyl cyclase in the steroidogenic pathway is also affected.

Bicarbonate dependence of cAMP accumulation induced by phorbol esters in hamster spermatozoa

Phorbol esters stimulate cyclic adenosine 3',5'-monophosphate (cAMP) accumulation in hamster spermatozoa under conditions for in vitro capacitation. The 20-50-fold elevation of cAMP levels induced by 1 microM phorbol 12-myristate 13-acetate (PMA) in spermatozoa depends on the presence of sodium bicarbonate in the medium (ED50: 15 mM) and it is independent of extracellular pH. Sodium bicarbonate stimulates adenylate cyclase activity in membrane preparations by 4-fold (ED50: 40 mM). After solubilization, the bicarbonate-sensitive moiety elutes as a single peak of 55 kDa in a gel filtration column. Blockers of bicarbonate chloride antiporters diisothiocyanate stilbene 2,2'-disulfonic acid (DIDS) or acetamido 4'-isothiocyanate stilbene 2,2'-disulfonic acid (SITS) inhibit the bicarbonate dependent PMA effect on cAMP in living spermatozoa (ED50: 100 microM). Maximal (85%) inhibition in cAMP accumulation is observed at 1 mM. Motility is inhibited only at high concentrations of the blockers. Pretreatment of living cells with 1 mM DIDS does not affect membrane adenylate cyclase activity which remains responsive to bicarbonate. These results suggest that controlled transport of bicarbonate through the sperm plasma membrane could be associated to the regulation of cAMP synthesis.

Phorbol esters stimulate cyclic adenosine 3', 5'-monophosphate accumulation in hamster spermatozoa during in vitro capacitation

Phorbol ester, phorbol 12-myristate 13-acetate (PMA), induced a 20- to 50-fold increase (ED50: 2 microM) in cyclic adenosine 3', 5'-monophosphate (cAMP) levels in spermatozoa incubated in capacitation medium for short periods of time (30 min). Similar results were obtained with 1-oleoyl 2-acetylglycerol (OAG), whereas 1, 2 diolein, 1-oleoyl glycerol, or 4 alpha-phorbol 12, 13-didecanoate had no effect. When extracellular Ca2+ was complexed by [ethylenebis(oxyethyleneitrilo)] tetraacetic acid (EGTA), a 50% reduction of maximal stimulation was observed, and 90% inhibition was seen after chelation of both extra- and intracellular Ca2+ with EGTA and 2-[[2-[bis [(carbonyl) methyl] amino]-5-methylphenoxy] methyl]-6-methoxy-8-[bis[(carbonyl) methyl] amino] quinoline acetoxy methyl (Quin 2). The acrosome reaction was not affected by similar concentration of PMA or OAG at different periods of incubation. These results suggest the involvement of protein kinase C activity in the regulation of cAMP levels in sperm during capacitation. This stimulation is dependent on intracellular Ca2+ and probably is not linked to the process of the acrosome reaction.