Calyculin A, protein phosphatase inhibitor, enhances capacitation of human sperm

Targeting phosphatases: From molecule design to clinical trials

Phosphatase is a kind of enzyme that can dephosphorylate target proteins, which can be divided into serine/threonine phosphatase and tyrosine phosphatase according to its mode of action. Current evidence showed multiple phosphatases were highly correlated with diseases including various cancers, demonstrating them as potential targets. However, currently, targeting phosphatases with small molecules faces many challenges, resulting in no drug approved. In this case, phosphatases are even regarded as "undruggable" targets for a long time. Recently, a variety of strategies have been adopted in the design of small molecule inhibitors targeting phosphatases, leading many of them to enter into the clinical trials. In this review, we classified these inhibitors into 4 types, including (1) molecular glues, (2) small molecules targeting catalytic sites, (3) allosteric inhibition, and (4) bifunctional molecules (proteolysis targeting chimeras, PROTACs). These molecules with diverse strategies prove the feasibility of phosphatases as drug targets. In addition, the combination therapy of phosphatase inhibitors with other drugs has also entered clinical trials, which suggests a broad prospect. Thus, targeting phosphatases with small molecules by different strategies is emerging as a promising way in the modulation of pathogenetic phosphorylation.

Strategies for Targeting Serine/Threonine Protein Phosphatases with Small Molecules in Cancer

Among numerous posttranslational regulation patterns, phosphorylation is reversibly controlled by the balance of kinases and phosphatases. The major form of cellular signaling involves the reversible phosphorylation of proteins on tyrosine, serine, or threonine residues. However, altered phosphorylation levels are found in diverse diseases, including cancer, making kinases and phosphatases ideal drug targets. In contrast to the success of prosperous kinase inhibitors, design of small molecules targeting phosphatase is struggling due to past bias and difficulty. This is especially true for serine/threonine phosphatases, one of the largest phosphatase families. From this perspective, we aim to provide insights into serine/threonine phosphatases and the small molecules targeting these proteins for drug development, especially in cancer. Through highlighting the modulation strategies, we aim to provide basic principles for the design of small molecules and future perspectives for the application of drugs targeting serine/threonine phosphatases.

New insights into the potential mechanisms of spermatogenic failure in patients with idiopathic azoospermia

Idiopathic azoospermia (IA) refers to azoospermia without a clear aetiology. Due to the unclear aetiology and pathological mechanism of IA, there is no effective treatment for IA. The development of assisted reproductive and microsperm extraction technologies has brought hope to patients with IA with fertility problems. However, there are still many patients with IA whose testes lack healthy sperm, causing infertility. Therefore, it is key to identify how testicular spermatogenic failure can be reversed to promote spermatogenesis in patients with IA to resolve fertility problems; these goals are a great challenge in reproductive medicine. The underlying genetic factors seem to be important pathological factors of IA. Understanding the role of genetic factors in the pathological mechanism of spermatogenic failure in patients with IA is of great value for future studies and treatments and is also an important reference for the reproductive health of males and their offspring. A method combining sequencing technology and bioinformatics analysis is an important means to understand the genetic pathological mechanisms. We used bioinformatics analysis to study the public human IA dataset. We found that the pathogenic mechanism of IA may be related to abnormal ciliary structure and function and disrupted RNA metabolism in spermatogenic cells. Disrupted m6A regulation of spermatogenesis may be an important pathological mechanism of IA and warrants attention. Finally, we screened for key genes and potential therapeutic drugs to determine future research directions.

PKA and CaMKII mediate PI3K activation in bovine sperm by inhibition of the PKC/PP1 cascade

To enable fertilization, spermatozoa must undergo several biochemical processes in the female reproductive tract, collectively called capacitation. These processes involve protein kinase A (PKA)-dependent protein tyrosine phosphorylation including phosphatidylinositol-3-kinase (PI3K). It is not known how PKA, a serine/threonine (S/T) kinase, mediates tyrosine phosphorylation of proteins. We recently showed that inhibition of S/T phosphatase 1 (PP1) causes a significant increase in phospho-PI3K. In this study, we propose a mechanism by which PKA and PP1 mediate an increase in PI3K tyrosine phosphorylation and implicate calmodulin-dependent kinase II (CaMKII) in this process. Inhibition of sperm PP1 or PKC, stimulated CaMKII phosphorylation/activation, and inhibition of PKC enhanced PP1 phosphorylation/inactivation. Inhibition of CaMKII, using KN-93, caused significant reduction in phospho-PP1, indicating its activation. Moreover, KN-93 prevented the dephosphorylation/inactivation of PKC. We therefore suggest that CaMKII inhibits PKC, leading to PP1 inhibition and the reciprocal auto-activation of CaMKII. Thus, CaMKII can regulate its own activation by inhibiting the PKC/PP1 cascade. Inhibition of Src family kinases (SFK) caused significant inhibition of CaMKII and PP1 phosphorylation, suggesting that SFK activity results in PP1 inhibition and CaMKII activation. Activation of sperm PKA by 8Br-cAMP revealed an increase in phospho-CaMKII, which was inhibited by PKA inhibitor. Tyrosine phosphorylation of PI3K was stimulated by 8Br-cAMP and by PKC or PP1 inhibition and was abrogated by CaMKII inhibition. Furthermore, phosphorylation/activation of the tyrosine kinase Pyk2 was enhanced by PP1 inhibition, and this activation is blocked by CaMKII inhibition. Thus, PKA activates Src, which inhibits PP1, leading to CaMKII and Pyk2 activation, resulting in PI3K tyrosine phosphorylation/activation.

Protein phosphatases decrease their activity during capacitation: a new requirement for this event

There are few reports on the role of protein phosphatases during capacitation. Here, we report on the role of PP2B, PP1, and PP2A during human sperm capacitation. Motile sperm were resuspended in non-capacitating medium (NCM, Tyrode's medium, albumin- and bicarbonate-free) or in reconstituted medium (RCM, NCM plus 2.6% albumin/25 mM bicarbonate). The presence of the phosphatases was evaluated by western blotting and the subcellular localization by indirect immunofluorescence. The function of these phosphatases was analyzed by incubating the sperm with specific inhibitors: okadaic acid, I2, endothall, and deltamethrin. Different aliquots were incubated in the following media: 1) NCM; 2) NCM plus inhibitors; 3) RCM; and 4) RCM plus inhibitors. The percent capacitated sperm and phosphatase activities were evaluated using the chlortetracycline assay and a phosphatase assay kit, respectively. The results confirm the presence of PP2B and PP1 in human sperm. We also report the presence of PP2A, specifically, the catalytic subunit and the regulatory subunits PR65 and B. PP2B and PP2A were present in the tail, neck, and postacrosomal region, and PP1 was present in the postacrosomal region, neck, middle, and principal piece of human sperm. Treatment with phosphatase inhibitors rapidly (≤1 min) increased the percent of sperm depicting the pattern B, reaching a maximum of ∼40% that was maintained throughout incubation; after 3 h, the percent of capacitated sperm was similar to that of the control. The enzymatic activity of the phosphatases decreased during capacitation without changes in their expression. The pattern of phosphorylation on threonine residues showed a sharp increase upon treatment with the inhibitors. In conclusion, human sperm express PP1, PP2B, and PP2A, and the activity of these phosphatases decreases during capacitation. This decline in phosphatase activities and the subsequent increase in threonine phosphorylation may be an important requirement for the success of sperm capacitation.

Protein kinase A and protein kinase C(alpha)/PPP1CC2 play opposing roles in the regulation of phosphatidylinositol 3-kinase activation in bovine sperm

In order to acquire fertilization competence, spermatozoa have to undergo biochemical changes in the female reproductive tract, known as capacitation. Signaling pathways that take place during the capacitation process are much investigated issue. However, the role and regulation of phosphatidylinositol 3-kinase (PI3K) in this process are still not clear. Previously, we reported that short-time activation of protein kinase A (PRKA, PKA) leads to PI3K activation and protein kinase C(alpha)(PRKCA, PKC(alpha)) inhibition. In the present study, we found that during the capacitation PI3K phosphorylation/activation increases. PI3K activation was PRKA dependent, and down-regulated by PRKCA. PRKCA is found to be highly active at the beginning of the capacitation, conditions in which PI3K is not active. Moreover, inhibition of PRKCA causes significant activation of PI3K. Similar activation of PI3K is seen when the phosphatase PPP1 is blocked suggesting that PPP1 regulates PI3K activity. We found that during the capacitation PRKCA and PPP1CC2 (PP1gamma2) form a complex, and the two enzymes were degraded during the capacitation, suggesting that this degradation enables the activation of PI3K. This degradation is mediated by PRKA, indicating that in addition to the direct activation of PI3K by PRKA, this kinase can enhance PI3K phosphorylation indirectly by enhancing the degradation and inactivation of PRKCA and PPP1CC2.

Regulation of hyperactivation by PPP2 in hamster spermatozoa

It has been widely accepted that serine/threonine protein phosphatases (PPPs) are associated with the regulation of sperm hyperactivation. In the present study, we examined the types of PPPs associated with the regulation of hamster sperm hyperactivation. Protein phosphatases PPP1CA, PPP1CC, PPP2, and PPP3 are present in hamster sperm. In the experiments using several inhibitors, sperm hyperactivation was enhanced when PPP2 was inhibited at least, although inhibition of PPP1 also enhanced sperm hyperactivation. Interestingly, sperm were hyperactivated after PPP2 became an inactive form. And then, PPP1CA became an active form after sperm were hyperactivated. It has also been widely accepted that tyrosine phosphorylation is closely associated with the regulation of sperm hyperactivation. When PPP2 was inhibited, tyrosine phosphorylation was not enhanced at all. On the other hand, inhibition of PPP1 enhanced tyrosine phosphorylation. From the results, it is likely that PPP2 is closely associated with the regulation of sperm hyperactivation, although it is not associated with the regulation of tyrosine phosphorylation.

Cyclic AMP-dependent tyrosine phosphorylation in tammar wallaby (Macropus eugenii) spermatozoa

Despite considerable advances in our understanding of the molecular mechanisms regulating eutherian sperm function, there is a paucity of such knowledge for the Metatheria. In eutherian spermatozoa, the attainment of functional competence is associated with a redox-regulated, cAMP-mediated tyrosine phosphorylation cascade, activated during capacitation. In this report we investigate whether tammar wallaby (Macropus eugenii) spermatozoa possess a similar signal transduction pathway. Western blot analysis of phosphotyrosine expression in caudal and ejaculated populations of tammar spermatozoa revealed that elevation of intracellular cAMP levels, but not exposure to oxidants or NADPH, induced a dramatic increase in the overall level of tyrosine phosphorylation. Washed, ejaculated spermatozoa exhibited more pronounced increases in tyrosine phosphorylation than unwashed sperm populations. Localisation of tyrosine phosphorylation by immunocytochemistry showed that phosphotyrosine residues were principally located along the tammar sperm flagellum, and occasionally at a small region of the sperm head, adjacent to the acrosome. Associated with the tyrosine phosphorylation of tammar spermatozoa, was a change in sperm head conformation to a T-shaped orientation, further implying the importance of these pathways to normal tammar sperm function. Redox activity, as detected by lucigenin-dependent chemiluminescence, was stimulated by NADPH in caudal sperm preparations but not ejaculated spermatozoa. However, neither sperm population responded to treatment with NADPH with changes in intracellular cAMP or tyrosine phosphorylation. In conclusion, tammar spermatozoa possess the same cAMP-mediated, tyrosine phosphorylation-dependent signal transduction cascade that has been associated with capacitation in eutherian spermatozoa. However in Metatherian spermatozoa we could find no evidence that this pathway was redox regulated.

Increased Phosphorylation of a Distinct Subcellular Pool of Protein Phosphatase, PP1γ2, During Epididymal Sperm Maturation1

The enzyme PP1gamma2 is a testis- and sperm-specific isoform of type 1 protein phosphatase (PP1), and it is the only isoform of PP1 in spermatozoa. The enzyme PP1gamma2 is essential for spermatogenesis and is also a key enzyme in the development and regulation of sperm motility. The carboxy terminus of the enzyme contains a consensus amino acid sequence for phosphorylation by cyclin-dependent kinases. Using antibodies specific to this phosphorylated amino acid sequence domain, we found that phosphorylated PP1gamma2 is present in bovine epididymal spermatozoa. The level of phosphorylated PP1gamma2 is significantly higher in motile caudal compared to immotile caput epididymal spermatozoa. A number of treatments, such as 2-chloro adenosine, cAMP analogues, cAMP phosphodiesterase inhibitors, and calcium, which stimulate sperm motility, did not alter the level of phosphorylated PP1gamma2. However, calyculin A, which is an inhibitor of protein phosphatase subtypes PP1 and PP2A, significantly increases the level of phosphorylated PP1gamma2 in both caput and caudal epididymal spermatozoa. Partial purification by column chromatography showed that phosphorylated PP1gamma2 is catalytically active. Phosphorylated PP1gamma2 is the only spontaneously catalytically active form of the enzyme in caudal sperm extracts. Western blot analysis shows that the enzyme cyclin-dependent kinase 2, one of the enzymes that phosphorylates the consensus domain at the carboxy terminus in PP1 isoforms, is present in spermatozoa. Western blot analysis of proteins extracted from purified head and tail fragments of spermatozoa showed that phosphorylated PP1gamma2 is present predominantly in the sperm head. Fluorescence immunocytochemistry also showed that phosphorylated PP1gamma2 is present predominantly in the posterior region of the sperm head. The distinct subcellular localization and changes in its level during sperm maturation suggest a possible role for sperm phosphorylated PP1gamma2 in signaling events during fertilization.

Decrease in order of human sperm lipids during capacitation

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of the sperm sterols, cholesterol and desmosterol, is an obligatory step in the capacitation of human sperm. Because sterols can increase the order of membrane phospholipids, it has been suggested that the importance of sterol loss is that it decreases membrane lipid order. The present study tested the hypotheses that sterol loss decreases sperm membrane lipid order during capacitation and that lipid disorder is a sufficient stimulus for capacitation. Steady-state fluorescence anisotropy of the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, decreased during capacitation, indicating a decrease in lipid order. The decrease was dependent on the loss of sperm sterols, suggesting that it reflected diminished sterol-mediated phospholipid ordering. However, the lipid-fluidizing agents, benzyl alcohol and 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) octanoate, did not cause sperm capacitation or overcome inhibition by cholesterol. In summary, loss of sperm sterols caused a significant decline in lipid order during capacitation; however, decreased bulk lipid order was not sufficient to trigger the subsequent events that complete capacitation.

Calmodulin antagonists differentially affect capacitation-associated protein tyrosine phosphorylation of mouse sperm components

Sperm capacitation in vitro is thought to be correlated with the increased protein tyrosine phosphorylation of a subset of sperm components. Our group recently used a pharmacological approach to demonstrate that calmodulin (CaM), a 17 kDa calcium sensor protein, has a role in sperm capacitation. In the present study, we have used several CaM antagonists in an attempt to characterize further the role of CaM in capacitation-associated protein tyrosine phosphorylation of sperm components. Our data demonstrate, first, that mouse spermatozoa incubated in a medium that favors capacitation undergo increased protein tyrosine phosphorylation in a time-dependent manner. Second, inclusion of six CaM antagonists individually in an in vitro incubation medium prevented sperm capacitation, as demonstrated by their diminished ability to undergo agonist-induced acrosome reaction. Third, half of the CaM antagonists (compound 48/80, W13 and CaM-binding domain) had no effect on protein tyrosine phosphorylation or sperm motility. Fourth, by contrast, three CaM antagonists (W7, ophiobolin A and calmidazolium) significantly inhibited protein tyrosine phosphorylation of sperm components (42, 56, 66, 82 and 95 kDa) and adversely affected their motility without altering viability as assessed by propidium iodine staining. Finally, inclusion of purified CaM in the capacitation medium significantly increased tyrosine phosphorylation of 82 kDa and 95 kDa components. Combined, these data suggest that CaM antagonists prevent capacitation by interfering with multiple regulatory pathways, and do so either with or without adverse effects on sperm motility and protein tyrosine phosphorylation.

Structural features of sterols required to inhibit human sperm capacitation

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of cholesterol is an essential step in the capacitation of human sperm. Experimentally maintaining a high level of cholesterol inhibits capacitation, but the mechanism is unknown. The present study investigated the structural features that are required for cholesterol's inhibitory activity. Human sperm also contain much desmosterol, which is lost from sperm during capacitation. Preventing the loss of desmosterol inhibited capacitation (as assessed by acrosomal responsiveness), with an effectiveness approximately equal to cholesterol's inhibitory activity. Other structural analogs were added to the incubation medium to replace sperm cholesterol and desmosterol. Most inhibited capacitation, including those that lacked cholesterol's 3beta-OH group (cholesteryl methyl ether and epicholesterol) and those with modified C17 groups (ergosterol and diosgenin). Two steroids did not inhibit capacitation well. Coprostanol, which has a nonplanar steroid nucleus, had low inhibitory activity that could be explained by an elevated endogenous cholesterol concentration. Epicoprostanol, which has a nonplanar ring structure and a 3alpha-OH group, promoted rather than inhibited capacitation. The inhibitory activity of the analogs was correlated with their ability to promote order of egg phosphatidylcholine as measured by fluorescence anisotropy. In summary, a planar ring structure is required for sterol inhibitory activity, but a 3beta-OH group and a saturated cholesterol-like aliphatic tail on C17 are not required. The present results support the hypothesis that sperm sterols block capacitation by increasing order of phospholipids.

Alterations in sperm protein phosphorylation in male infertility

Protein phosphorylation is involved in sperm capacitation, so the effect of protein phosphatase inhibitors on the capacitation of spermatozoa of males with unexplained infertility was investigated. d-mannose ligand specific receptor expression in fresh, living spermatozoa, capacitated or treated with calyculin A (an inhibitor of protein phosphatases 1 and 2A), was studied in three groups of men: pre-vasectomy (fertile) males, males in couples with male infertility, and males in couples with infertility of unknown aetiology. Flow cytometry showed significant differences between infertile couples with a male factor and fertile couples (P < 0.05), both after capacitation and after treatment with calyculin A. In the group of couples with infertility of unknown aetiology (n = 15), d-mannose receptor expression was diminished in six cases after classical capacitation. However, when the spermatozoa of these six men were treated with calyculin A, five showed an increased specific d-mannose receptor expression. From these results it is suggested that in vitro treatment of spermatozoa with inhibitors of protein phosphatases may be of great value in some cases of unexplained infertility.

Dynamics of the mammalian sperm plasma membrane in the process of fertilization

Sexual reproduction requires the fusion of sperm cell and oocyte during fertilization to produce the diploid zygote. In mammals complex changes in the plasma membrane of the sperm cell are involved in this process. Sperm cells have unusual membranes compared to those of somatic cells. After leaving the testes, sperm cells cease plasma membrane lipid and protein synthesis, and vesicle mediated transport. Biophysical studies reveal that lipids and proteins are organized into lateral regions of the sperm head surface. A delicate reorientation and modification of plasma membrane molecules take place in the female tract when sperm cells are activated by so-called capacitation factors. These surface changes enable the sperm cell to bind to the extra cellular matrix of the egg (zona pellucida, ZP). The ZP primes the sperm cell to initiate the acrosome reaction, which is an exocytotic process that makes available the enzymatic machinery required for sperm penetration through the ZP. After complete penetration the sperm cell meets the plasma membrane of the egg cell (oolemma). A specific set of molecules is involved in a disintegrin-integrin type of anchoring of the two gametes which is completed by fusion of the two gamete plasma membranes. The fertilized egg is activated and zygote formation preludes the development of a new living organism. In this review we focus on the involvement of processes that occur at the sperm plasma membrane in the sequence of events that lead to successful fertilization. For this purpose, dynamics in adhesive and fusion properties, molecular composition and architecture of the sperm plasma membrane, as well as membrane derived signalling are reviewed.

Sphingomyelin modulates capacitation of human sperm in vitro

Ejaculated mammalian sperm must mature (capacitate) before they can undergo acrosomal exocytosis and fertilize an egg. Loss of sperm sterols is an early step in capacitation. Because sphingomyelin slows cholesterol efflux from other cells, the role of sphingomyelin in capacitation was tested. Human sperm were exposed to sphingomyelinase and then incubated for as long as 24 h. The ability of sperm to acrosome-react in response to progesterone was tested to measure capacitation. Sphingomyelinase-treated sperm became responsive to progesterone approximately 10 h earlier than control sperm. Sphingomyelinase also increased spontaneous acrosomal exocytosis. The effects of sphingomyelinase were accompanied by accelerated losses of the inhibitory sterols, cholesterol and desmosterol. To test whether sphingomyelinase-generated ceramide might promote capacitation, sperm were incubated for 8 h with the cell-permeable ceramide N:-hexanoylsphingosine (25 microM) or with solvent. Ceramide increased the incidence of progesterone-responsive sperm and, at later times, spontaneously reacted sperm. N:-Hexanoylsphinganine, an inactive control ceramide, had no effect. These results suggest that sphingomyelin in the sperm influences the rate of capacitation by slowing the loss of sterols, and that exogenous sphingomyelinase accelerates capacitation by speeding the loss of sterols and by generating ceramide.

Role of tyrosine phosphorylation of flagellar proteins in hamster sperm hyperactivation

Despite extensive study of sperm motility, little is known of the mechanism of mammalian sperm hyperactivation. Here we describe a novel method for preparation of rodent sperm flagella and use it to show a correlation between tyrosine phosphorylation of flagellar proteins and hyperactivation of hamster sperm. When hyperactivation was produced by a 3.5-h incubation in a medium supporting capacitation, four major tyrosine-phosphorylated peptides of 90-, 80-, 62-, and 48-kDa mass were detected in flagellar extracts. Incubation with calyculin A, an inhibitor of protein phosphatases 1 and 2A, produced hyperactivation within 40 min but only a single 80-kDa phosphotyrosine-containing flagellar component. Conversely, incubation with inhibitors of either protein kinase A (H8) or protein tyrosine kinase (tyrphostin 47) prevented both hyperactivation and the production of tyrosine-phosphorylated flagellar peptides. These results indicate a strong correlation of hyperactivation with the tyrosine phosphorylation of sperm flagellar peptides, and they strongly implicate an 80-kDa component as a major mediator of the mechanism that produces hyperactivated motility of hamster sperm.

Hyperactivation of hamster sperm motility by temperature-dependent tyrosine phosphorylation of an 80-kDa protein

Protein phosphorylation and dephosphorylation are believed to play key roles in regulation of sperm motility. Here we examine the effect of temperature on hamster sperm motility and protein tyrosine phosphorylation status. As in previous work, a decrease from 37 degrees C to 22 degrees C caused loss of hyperactivated motility. We now find that cooling also produces a dephosphorylation of several 48-80-kDa flagellar peptides. A return to 37 degrees C restored hyperactivation but resulted in rephosphorylation of only an 80-kDa protein. Conversely, hyperactivation and phosphorylation of the 80-kDa component were insensitive to incubation temperature for sperm incubated with the protein phosphatase inhibitor, calyculin A, or for sperm demembranated by detergent extraction. These results strongly indicate that the temperature-sensitive tyrosine phosphorylation status of an 80-kDa sperm flagellar peptide explains the sensitivity of hyperactivation to temperature.

Effect of methyl-beta-cyclodextrin on the acrosomal responsiveness of human sperm

Human sperm incubated in vitro gradually become capable of acrosome-reacting in response to the agonist, progesterone (P4). Loss of unesterified cholesterol is an obligatory step in the development of acrosomal responsiveness. These experiments tested the ability of methyl-beta-cyclodextrin (MbetaCD) to accelerate sperm cholesterol loss and the development of acrosomal responsiveness. Incubating sperm 30 min in MbetaCD (2.5-10 mM) decreased sperm cholesterol by as much as 89% in a dose-dependent fashion. MbetaCD caused some sperm (maximum of 16% following treatment with 5 mM MbetaCD) to become responsive to P4, and it caused a dose-dependent increase in spontaneous acrosome reactions. The number of responsive sperm increased in the first 3 hr following their removal from MbetaCD. Continuing incubation to 24 hr increased the numbers of spontaneously reacted sperm and dead sperm, but not P4-responsive sperm. It appears, therefore, that up to 3 hr are required for the full expression of P4-responsiveness in cholesterol-depleted sperm. The observed effects of MbetaCD are due to its cholesterol-depleting properties, because including sufficient cholesterol with MbetaCD to reduce the loss of sperm cholesterol inhibited the effects of MbetaCD on cell viability, spontaneous acrosome reactions, and responsiveness to P4. MbetaCD accelerates the appearance of the functional stages that sperm normally pass through during incubation in vitro, reinforcing the view that cholesterol loss is an important determinant of the rate at which sperm become acrosomally responsive.

Effect of cholesterol and other sterols on human sperm acrosomal responsiveness

Human sperm become responsive to inducers of the acrosome reaction when they are washed free of seminal plasma and incubated in an appropriate medium. Previous work has shown that cholesterol-enriched medium prevents sperm from becoming responsive to the inducer, progesterone. Sperm that were incubated 24 hr in cholesterol-enriched medium and then treated with progesterone showed no evidence of membrane fusion, indicating that cholesterol acts at a stage before the earliest morphological change. Inhibition of acrosomal responsiveness by cholesterol was reversible. Among other sterols reported in mammalian sperm, desmosterol and cholesterol sulfate also inhibited sperm from becoming responsive, but cholesterol palmitate did not. Our results support a model in which sperm unesterified cholesterol, or a molecule in equilibrium with it, suppresses acrosomal responsiveness. Cholesterol-enriched medium also prevented sperm from becoming responsive to the calcium/proton exchanging ionophore, ionomycin, suggesting that cholesterol's effect may be, at least in part, at a point in the signal transduction pathway subsequent to the rise in intracellular-free calcium.

Regulation of human sperm motility and hyperactivation components by calcium, calmodulin, and protein phosphatases

The role of Ca2+, calmodulin, and protein phosphatases on motility and hyperactivation of noncapacitated, capacitating, and detergent-permeabilized reactivated human sperm was examined. In noncapacitated sperm, W7 inhibited percent motility (%MOT), curvilinear velocity (VCL), amplitude of lateral head displacement (ALH), and percent hyperactivation (%HYP) in an extracellular Ca2+ concentration-dependent manner (p < .05). However, in capacitating sperm, inhibition of motility by W7 was independent of external Ca2+. Treatment of reactivated sperm with a synthetic calmodulin inhibitor peptide decreased VCL and ALH in a Ca(2+)-dependent manner (p < .05). Ca2+ exhibited a dramatic influence on motility within a narrow concentration range (0.7 to 1.0 microM) in reactivated sperm. A calmodulin-dependent protein phosphatase (PP2B) was identified by activity assay, immunoblotting, and dephosphorylation of endogenous phosphoproteins. The sperm enzyme, unlike bovine brain PP2B, was inhibited by 1 microM okadaic acid (OA) in the presence of Mn2+, suggesting that the sperm enzyme is unique. In reactivated sperm, inhibition of endogenous PP2B-like activity with anti-PP2B antibodies altered ALH, whereas OA altered both VCL and ALH and also inhibited a subset of Ca(2+)-dependent dephosphorylations of cAMP-dependent phosphoproteins in capacitating sperm. These results suggest (1) an important role for calmodulin and PP2B in Ca(2+)-regulated motility parameters, particularly ALH, and (2) that modulation of human sperm motility, including hyperactivation by cAMP-dependent phosphorylation, requires calmodulin-dependent as well as other protein dephosphorylations.

Sperm function and its manipulation for microassisted fertilization

Comprehension of the intricate complexities of sperm function is clearly crucial to the success of attempts to manipulate it for the purposes of assisted conception. This is particularly important when considering various procedures for microassisted fertilization since these bypass critical physiological events that are mandatory for normal fertilization, to varying degrees. Methylxanthine derivatives such as pentoxifylline are useful agents for the management of oligoasthenozoospermic patients. This is particularly so for procedures such as SUZI where adequate motility of spermatozoa injected into the perivitelline space is crucial for fusion with the vitelline membrane to achieve fertilization. The generation of minute concentrations of reactive oxygen species in vitro may prove to be a valuable technique in this respect, in the light of recent evidence for their involvement in capacitation and hyperactivation. Induction of the acrosome reaction by non-invasive, non-toxic agents should markedly improve success rates for microassisted fertilization. Acrosin appears to play a central role in this and, therefore, it would seem prudent to monitor levels of acrosin activity in samples of spermatozoa used in assisted conception procedures. With respect to microassisted fertilization, the potential to select recently acrosome-reacted spermatozoa coated by activated acrosin promises to be a major improvement. Current methods employed for determination of the fertilization potential of spermatozoa are clearly inadequate (Polansky and Lamb, 1988; Aitken, 1990). In fact, the prevailing evidence suggests that no single parameter of sperm function reflects this potential (Zaneveld and Jeyendran, 1988). Therefore, we have both a scientific and a moral responsibility to investigate these processes further. Subsequently, we should be in a position to identify individual gametes with the potential for fertilization and so utilize procedures that result in maximal fertilization rates with minimal risk of polyploidy or abnormality.

Effect of epidermal growth factor on human sperm capacitation

OBJECTIVE

To examine the effect of epidermal growth factor (EGF) on human sperm capacitation and the involvement of protein phosphorylation in the regulation of the EGF action.

DESIGN

Human sperm were capacitated in modified Krebs-Ringer's bicarbonate medium of Biggers, Whitten, and Whittingham in the presence of EGF and various agents known to phosphorylate the EGF receptor. The chlortetracycline fluorescence assay was used to monitor capacitated sperm. Capacitation was confirmed by the ability of sperm to undergo the acrosome reaction in response to solubilized mouse zonae pellucidae (ZP).

RESULTS

In 15 minutes, the appearance of the clear perimeter pattern increased significantly in the sperm treated with 100 ng/mL EGF compared with the controls. In EGF-treated sperm, the percent clear perimeter pattern remained stable for 3 hours without affecting the acrosome reaction pattern and the motility. Epidermal growth factor stimulated the appearance of the clear perimeter pattern at concentrations > 100 pg/mL. The stimulation by EGF was attenuated by the treatment with genistein, 12-O-tetradecanoylphorbol-13-acetate, or thapsigargin. In sperm that were incubated in the presence of 100 ng/mL EGF for 30 minutes and further induced the acrosome reaction by mouse ZP, the percent acrosome reaction pattern increased significantly compared with the controls.

CONCLUSION

Epidermal growth factor stimulates human sperm capacitation by activating the tyrosine kinase of the EGF receptor which is regulated by multisite phosphorylation.