A cell free system reveals that capacitation is a prerequisite for membrane fusion during the acrosome reaction

The crystal structure of 2-bromo-1,3-phenylene bis(4-methylbenzenesulfonate), C20H17BrO6S2

C20H17BrO6S2, triclinic, P 1 ‾ $P\overline{1}$ (no. 2), a = 8.173(2) Å, b = 8.256(2) Å, c = 16.514(5) Å, α = 89.947(3)°, β = 83.105(3)°, γ = 72.077(3)°, V = 1051.8(5) Å3, Z = 2, Rgt (F) = 0.0334, wRref (F 2) = 0.0889, T = 296(2) K.

(1-Oxo-2,6,7-trioxa-1-phosphabicyclo-[2.2.2]octan-4-yl)methyl 4-methyl-benzene-sulfonate

In the title compound, C(12)H(15)O(7)PS, the P atom has a distorted tetra-hedral environment. The P-O-C-C torsion angles deviate significantly from zero [average = 12.0 (3)°], indicating that the bicyclic OP(OCH(2))(3)C cage is strained. In the crystal, weak C-H⋯O inter-actions consolidate the packing.

1H-1,2,4-Triazol-4-ium 4-nitrobenzenesulfonate monohydrate

In the 4-nitro­benzene sulfonate anion of the title compound, C₂H₄N₃⁺·C₆H₄NO₅S⁻·H₂O, the nitro group is slightly twisted from the plane of the benzene ring [dihedral angle = 2.8 (3)°]. In the crystal, the three components are linked via N—H⋯O, O—H⋯N, O—H⋯O and C—H⋯O hydrogen bonds, forming a two-dimensional network parallel to the bc plane. A short inter­molecular O⋯N contact of 2.872 (3) Å is also observed between the nitro and sulfonate groups.

6-Formyl-2-methoxy-3-nitrophenyl 4-toluenesulfonate

In the title compound, C₁₅H₁₃NO₇S, the inter­planar angle between the two aromatic rings is 26.04 (3)°. The crystal structure is stabilized by C—H⋯O interactions.

Cholesterol content regulates acrosomal exocytosis by enhancing Rab3A plasma membrane association

The acrosome is an exocytic granule that overlies the spermatozoan nucleus. In response to different stimuli, it undergoes calcium-regulated exocytosis. Freshly ejaculated mammalian sperm are not immediately capable of undergoing acrosome reaction. The acquisition of this ability is called capacitation and involves a series of still not well-characterized changes in the sperm physiology. Plasma membrane cholesterol removal is one of the sperm modifications that are associated with capacitation. However, how sterols affect acrosomal exocytosis is unknown. Here, we show that short incubations with cyclodextrin, a cholesterol removal agent, just before stimulation promote acrosomal exocytosis. Moreover, the effect was also observed in permeabilized cells stimulated with calcium, indicating that cholesterol plays a direct role in the calcium-dependent exocytosis associated with acrosome reaction. Using a photo-inhibitable calcium chelator, we show that cholesterol affects an early event of the exocytic cascade rather than the lipid bilayers mixing. Functional data indicate that one target for the cholesterol effect is Rab3A. The sterol content does not affect the Rab3A activation-deactivation cycle but regulates its membrane anchoring. Western blot analysis and immunoelectron microscopy confirmed that cholesterol efflux facilitates Rab3A association to sperm plasma membrane. Our data indicate that the cholesterol efflux occurring during capacitation optimizes the conditions for the productive assembly of the fusion machinery required for acrosome reaction.

Evidence for the capacitation-associated membrane priming of mouse spermatozoa

An important feature of male fertility is the physiological priming of mammalian spermatozoa by a multifaceted process referred to as capacitation. It is a prerequisite event before spermatozoa can bind to the egg's extracellular coat, the zona pellucida, and undergo a signal transduction cascade. The net result is the fusion of the plasma membrane (PM) and underlying outer acrosomal membrane at multiple sites and the release of acrosomal contents (i.e., glycohydrolases, proteinases, etc.) at the site of sperm-zona binding. In this study, we have used an indirect immunofluorescence (IIF) assay and other staining approaches to examine capacitation-associated membrane priming of mouse spermatozoa. For IIF studies, we used affinity-purified antibodies against two glycohydrolases that cross-reacted with the acrosomal enzymes only when the uncapacitated spermatozoa were permeabilized. Incubation of spermatozoa in a medium that favors in vitro capacitation induced membrane priming that allowed the antibodies to cross-react with the acrosomal enzymes in capacitating acrosome-intact spermatozoa without permeabilization, as revealed by the appearance of several distinct fluorescent patterns, including an initial immunopositive lining over the acrosome cap to an intense immunopositive reaction throughout the acrosome. These early immunopositive patterns were followed by the appearance of intense fluorescent spots (droplets) that seem to establish contact with the PM in a time-dependent manner. Inclusion of calmodulin, a 17-kDa Ca(2+)-binding protein which promotes capacitation, in the incubation medium did not alter the overall rate of capacitation; however, its presence accelerated the initial stages of membrane priming. The potential similarities between sperm capacitation and early events of Ca(2+)-triggered membrane fusion among eukaryotes and among various stations of the secretory and endocytotic pathways are discussed.

Intracellular calcium regulation in sperm capacitation and acrosomal reaction

Binding to the egg's zona pellucida stimulates the spermatozoon to undergo acrosome reaction, a process which enables the sperm to penetrate the egg. Prior to this binding, the spermatozoa undergo in the female reproductive tract a series of biochemical transformations, collectively called capacitation. The first event in capacitation is the elevation of intracellular calcium and bicarbonate to activate adenylyl cyclase (AC) to produce cyclic-AMP, which activates protein kinase A (PKA) to phosphorylate certain proteins. During capacitation, there is also an increase in actin polymerization and in the membrane-bound phospholipase C (PLC). Sperm binding to zona-pellucida causes further activation of cAMP/PKA and protein kinase C (PKC), respectively. PKC opens a calcium channel in the plasma membrane. PKA together with inositol-trisphosphate activate calcium channels in the outer acrosomal membrane, which leads to an increase in cytosolic calcium. The depletion of calcium in the acrosome will activate a store-operated calcium entry mechanism in the plasma membrane, leading to a higher increase in cytosolic calcium, resulting in membrane fusion and acrosome reaction.

Role and regulation of intracellular calcium in acrosomal exocytosis

Calcium influx is required for the mammalian sperm acrosome reaction, an exocytotic event occurring in the sperm head after binding to the egg. Prior to this binding, the spermatozoon undergo, in the female reproductive tract, a series of biochemical transformations, collectively called capacitation. The first event in capacitation is the elevation of intracellular calcium, bicarbonate and hydrogen peroxide, which collectively activate adenylyl cyclase to produce cyclic-AMP, which activates protein kinase A. During capacitation, there is an increase in the membrane-bound phospholipase C, and this binding is highly stimulated by adding epidermal growth factor to the cells. We suggest that zona-pellucida binds to at least two different receptors in the sperm head plasma membrane. One is a G(i)-coupled receptor that can activate phospholipase Cbeta(1) and might regulate adenylyl cyclase to further enhance cyclic-AMP levels. The cyclic AMP activates protein kinase A to open a calcium channel in the outer acrosomal membrane, resulting in a relatively small rise in cytosolic calcium. This rise in Ca(2+) leads to activation of phospholipase Cgamma, which is coupled to the second tyrosine kinase receptor. The products of phospholipase C activity, diacylglycerol and inositol-trisphosphate (IP(3)), will lead to activation of protein kinase C (PKC) and IP(3)-receptor. PKC will open a calcium channel in the plasma membrane and IP(3) will activate the calcium channel in the outer acrosomal membrane, leading to a higher increase in cytosolic calcium. In addition, the depletion of calcium in the acrosome will activate a store-operated Ca(2+) channel, resulting in a very fast increase in cytosolic calcium (300-500 nM), leading to membrane fusion and completing the acrosome reaction.

In vitro capacitation of bovine spermatozoa: role of intracellular calcium

The development of successful methods of in vitro fertilization for bovine oocytes has advanced the bovine as a model for reproductive technology. The discovery of heparin as a capacitating agent has made it possible for investigators to have an inexpensive, readily available supply of bovine gametes for experimentation in reproductive biotechnologies such as gene transfer and cloning. The central event that mammalian sperm must undergo before being able to fertilize an oocyte is capacitation. Although we have methods which lead to efficient in vitro fertilization, we still lack understanding about the molecular mechanisms of capacitation. While numerous events occur during capacitation, it appears that regulation of intracellular Ca2+ (Ca(i)) is one of the most important. We found that the influx of Ca2+ into sperm during the first 2 hours of incubation is critical to heparin-induced capacitation. This is a period during capacitation when Ca(i) has not yet increased. We propose that during capacitation, the initial influx of Ca2+ into sperm is used to fill an intracellular Ca2+ store located in the acrosome. We found that thapsigargin, an inhibitor of an acrosomal Ca2+-ATPase, can stimulate capacitated sperm to acrosome react, trigger the opening of a store-operated calcium channel in the plasma membrane and has greater effects on capacitated sperm compared to noncapacitated sperm. An increase in intracellular Ca2+ was also detected in the anterior sperm head during capacitation, suggesting the loading of the acrosome with Ca2+. These observations may be important in the development of new methods for capacitation and understanding the death of sperm after cryopreservation.

Sperm capacitation is, after all, a prerequisite for both partial and complete acrosome reaction

The acrosome reaction (AR) - an essential step in mammalian fertilization - can occur, according to the consensus, only in capacitated spermatozoa. In apparent contrast, recent reports have demonstrated that human spermatozoa incubated in vitro in an albumin-free medium and therefore believed to be non-capacitated, do undergo the AR. With the aim of determining unequivocally whether or not capacitation is required for the AR and whether albumin is essential for capacitation, we compared the potential to undergo partial and complete AR (induced by phorbol myristate ester or by the Ca2+ ionophore A23187) between human spermatozoa incubated in a capacitating medium, albumin-free medium, and non-capacitating medium. The results clearly demonstrate that capacitation is, after all, a prerequisite for both partial and complete AR. Albumin, on the other hand, is essential only for acquiring the capacity to undergo complete, not partial AR.

Polyphosphoinositide inclusion in artificial lipid bilayer vesicles promotes divalent cation-dependent membrane fusion

Recent studies suggest that phosphoinositide kinases may participate in intracellular trafficking or exocytotic events. Because both of these events ultimately require fusion of biological membranes, the susceptibility of membranes containing polyphosphoinositides (PPIs) to divalent cation-induced fusion was investigated. Results of these investigations indicated that artificial liposomes containing PPI or phosphatidic acid required lower Ca2+ concentrations for induction of membrane fusion than similar vesicles containing phosphatidylserine, phosphatidylinositol, or phosphatidylcholine. This trend was first observed in liposomes composed solely of one type of phospholipid. In addition, however, liposomes designed to mimic the phospholipid composition of the endofacial leaflet of plasma membranes (i.e., liposomes composed of combinations of PPI, phosphatidylethanolamine, and phosphatidylcholine) also required lower Ca2+ concentrations for induction of aggregation and fusion. Liposomes containing PPI and phosphatidic acid also had increased sensitivity to Mg(2+)-induced fusion, an observation that is particularly intriguing given the intracellular concentration of Mg2+ ions. Moreover, the fusogenic effects of Ca2+ and Mg2+ were additive in vesicles containing phosphatidylinositol bisphosphate. These data suggest that enzymatic modification of the PPI content of intracellular membranes could be an important mechanism of fusion regulation.

Calcium mobilization and influx during sperm exocytosis

We have previously shown that two intracellular events which occur during capacitation of bovine sperm are the formation of actin filaments on the plasma and outer acrosomal membranes and the attachment of a PIP2-specific phospholipase C (PLC) to this membrane bound F-actin. This PLC plays an essential role in sperm exocytosis (acrosome reaction). In the present report, we further elucidated the role of this PLC using a PIP2-specific PLC of bacterial origin. This PLC is different from the endogenous sperm PLC in that it is calcium independent and not inhibited by neomycin. Here we report using bovine sperm that this bacterial PLC can restore actin release from extracted membranes as well as membrane fusion in a cell-free assay when the endogenous PLC is inhibited by neomycin. The sperm PLC requires 2 microM calcium for half maximal activation, while half maximal actin release from extracted plasma membranes occurs at 80 microM. Extracted sperm membranes were examined for calcium pumps and channels. Sperm plasma membranes were found to possess a thapsigargin insensitive calcium pump and calcium channels which are opened by phosphorylation by protein kinase C. The acrosomal membrane possesses a calcium pump which is inhibited by thapsigargin and calcium channels which are opened by cAMP. These observations are discussed in terms of a model of acrosomal exocytosis which involves a calcium rise that occurs in two stages resulting from calcium mobilization from internal stores followed by influx of extracellular calcium.

Acrosome content release in streptolysin O permeabilized mouse spermatozoa

Sperm cell plasma membrane and the outer acrosomal membrane fuse profusely during the acrosome reaction. The process is triggered by extracellular signals that elicit several intracellular events leading ultimately to membrane fusion. We have developed a streptolysin O permeabilizing protocol that selectively affects the spermatozoon plasma membrane without causing a significant loss of the acrosomal content. Most of the acrosomal acid phosphatase remained sperm-associated even after a 20 min incubation at 37 degrees C. However, the presence of 100 microM Ca2+ in the incubation buffer stimulates the release of the enzyme. The reaction was followed biochemically, measuring the acid phosphatase activity released to the medium and morphologically by the binding of fluorescein isothiocynate-conjugated peanut agglutinin and by electron microscopy. The results show that the streptolysin O permeabilized spermatozoon is a promising model for studying the complex set of events mediating and regulating the acrosome reaction.

Sperm exocytosis reconstructed in a cell-free system: evidence for the involvement of phospholipase C and actin filaments in membrane fusion

We used a cell-free system to study membrane fusion during sperm exocytosis (acrosome reaction). Extracted bovine sperm plasma and outer acrosomal membranes were labeled with chlorophyll a or DCY, respectively. The occurrence of membrane fusion is indicated by the ability of the probes to diffuse from one membrane species to another which is revealed by resonance energy transfer between the two probes. We have previously shown using this system that the requirement of capacitation for sperm exocytosis is retained in cell-free membrane fusion, and that the pH and calcium dependence of the cell-free fusion mimics those of exocytosis in intact cells. In the present report we further characterize the fusion of sperm membranes which we observe in our assay. Phosphoproteins and phospholipases were found to be involved in the membrane fusion step of sperm exocytosis. Protein kinases, phosphatases, and Gi-like proteins, while involved in exocytosis in intact cells, are not involved specifically in the membrane fusion step of exocytosis. The role of membrane bound F-actin in regulating membrane fusion was also studied using fluorescently labeled phalloidin. The results show that cortical F-actin has two roles in regulating sperm exocytosis. One is to form a scaffolding to hold phospholipase C at the membrane. It also functions as a physical barrier to membrane fusion which is removed by the increases in intracellular calcium and pH which precede fusion.

Dual effect of spermine on acrosomal exocytosis in capacitated bovine spermatozoa

The influence on spermine on the acrosomal exocytosis of capacitated bovine spermatozoa was studied. Dual effect of spermine was observed, depending on its concentration. It was shown that 10 microM spermine stimulated acrosomal exocytosis and prostaglandin F2 alpha production, whereas higher concentrations of spermine inhibited these processes. Acrosomal exocytosis induced by spermine was inhibited by staurosporine, a specific protein kinase C (PKC) inhibitor, indicating that PKC may be involved in this stimulation. Also, acrosomal exocytosis induced by the PKC activator phorbol 12-myristate-13-acetate was inhibited by 10 mM spermine. Therefore, these data indicate that spermine is involved in signal transduction events leading to exocytosis. We suggest that the concentration-dependent reversal of the stimulatory action of spermine could be explained by the existence of two binding sites for spermine: high affinity sites involved in inducing acrosomal exocytosis by low spermine concentration and low affinity sites mediating inhibition of acrosomal exocytosis by high concentration of spermine.

A 70 kDa protein is transferred from the outer acrosomal to the plasma membrane during capacitation

Bull sperm plasma and outer acrosomal membranes were analyzed by SDS-PAGE. Analysis of the plasma membrane proteins revealed the presence of a 70 kDa band the prominence of which is enhanced after capacitation. This protein was found to bind to zona pellucida intact oocytes. PAGE analysis of outer acrosomal membrane proteins also reveals the presence of a 70 kDa band, but its prominence decreases after capacitation. This protein also binds to zona pellucida intact oocytes. Furthermore, the 70 kDa outer acrosomal membrane protein is recognized in Western blot analysis by antibodies to plasma membrane proteins and vice versa. The results indicate that the 70 kDa acrosomal and plasma membrane proteins are the same. This 70 kDa protein would thus be a zona pellucida binding protein which is initially stored in the outer acrosomal membrane and transferred to the plasma membrane during capacitation, enabling it to function in egg-sperm binding.