Membrane fusion events in the Ca2+/ionophore-induced acrosome reaction of ram spermatozoa

Acrosomal Swelling Is Triggered by cAMP Downstream of the Opening of Store-Operated Calcium Channels During Acrosomal Exocytosis in Human Sperm

Acrosomal exocytosis in mammalian sperm is a regulated secretion with unusual characteristics. One of its most striking features is the postfusion loss of the outer acrosomal membrane and the overlying plasma membrane as hybrid vesicles. We have previously reported in human sperm that, by preventing the release of calcium from the acrosome, the exocytic process can be arrested at a stage where the acrosomes are profusely swollen, with invaginations of the outer acrosomal membrane. In this report, we show by transmission electron microcopy swelling with similar characteristics without arresting the exocytic process. Acrosomal swelling was observed when secretion was promoted by pharmacological and physiological inducers of the acrosome reaction that trigger exocytosis by different mechanisms. We show that progesterone- and thapsigargin-induced swelling depended on a calcium influx from the extracellular medium through store-operated calcium channels. However, calcium was dispensable when sperm were stimulated with cAMP analogs. KH7, an inhibitor of the soluble adenylyl cyclase, blocked progesterone-induced swelling. Our results indicate that swelling is a required process for acrosomal exocytosis triggered by activation of an adenylyl cyclase downstream of the opening of store-operated calcium channels.

Fusion as the result of sperm–somatic cell interaction

The research has been designed to investigate whether acrosome-reacted spermatozoa can fuse with somatic cells and to check whether this event may involve the molecular machinery implicated in the sperm–egg fusion. Boar spermatozoa were capacitatedin vitroand then treated with A23187 to induce acrosome reaction and activate their fusogenic potential. Reacted spermatozoa, loaded with the membrane-permeant fluorescent dye calcein AM, were incubated with plated granulosa cells or cells derived from stable cell lines: CRFK, VERO, and ESK4. The fusion between spermatozoa and somatic cells was revealed by the diffusion of the fluorescent dye from the sperm to the cell as membrane fusion and cytoplasmic continuity between the two cells were established. The involvement of integrin α6 and tetraspanin CD9 in the process of fusion was assessed by carrying out the experiment in the presence of antibodies against these molecules. Moreover, the incidence of fusion displayed by the different cell types used was analyzed in relation to their content in the above molecules assessed by western blot and immunostaining. The role of CD9 was additionally investigated by using CD9-negative cells. The data presented demonstrate that boar spermatozoa can fuse with different somatic cell types derived from different species and the process requires the combined presence of both integrin and tetraspanin molecules on the cell plasma membrane.

Relocation of myosin and actin, kinesin and tubulin in the acrosome reaction of bovine spermatozoa

The mammalian acrosome reaction is a specialised exocytotic event. Although molecular motors are known to be involved in exocytosis in many cell types, their potential involvement in the acrosome reaction has remained unknown. Here, it has been shown that actin is localised within the equatorial segment and in the marginal acrosomal ridge of the heads of unreacted bull spermatozoa. Myosins IIA and IIB are found within the anterior acrosomal margins of virtually all sperm cells and, less prominently, within the equatorial segment. Tubulin was detected in the equatorial segment and around the periphery of the acrosome while kinesin was prominent in the equatorial segment. After induction of the acrosome reaction by means of the calcium ionophore A23187, the number of cells exhibiting actin fluorescence intensity in the anterior acrosomal margin decreased four-fold and those displaying equatorial segment fluorescence decreased 3.5-fold; myosin IIA immunofluorescence decreased in intensity with most spermatozoa losing equatorial staining, whereas there was little change in the distribution or intensity of myosin IIB immunofluorescence, except for approximately 20% decrease in the number of cells exhibiting acrosomal staining. Tubulin became largely undetectable within the head and kinesin staining spread rostrally over the main acrosome region. A possible sequence of events that ties in these observations of molecular motor involvement with the known participation of SNARE proteins is provided.

Reduction of intramembranous particles in the periacrosomal plasma membrane of boar spermatozoa during in vitro capacitation: a statistical study

Membrane remodeling in the periacrosomal plasma membrane (PAPM) of boar spermatozoa during incubation in capacitation medium was examined by the freeze-fracture technique. In the preservation medium (PM) group, the major small (about 8 nm) intramembranous particles (IMP) and the minor large (> 10 nm) IMP were distributed evenly in the PAPM. The IMP-free area increased during capacitation. To correct the IMP-free area, arithmetically redistributed (ARD)-IMP density was used for statistical analysis. In the PM group, the mean density +/- SD of large IMP was 379 +/- 64 and 266 +/- 58/microm2, and that of small IMP was 1450 +/- 155 and 672 +/- 252/microm2 in protoplasmic (P) and external (E) faces, respectively. During capacitation, the significant (P < 0.01) reduction of large IMP density was encountered only in the E face of a few incubation groups, while that of the small IMP density occurred in the P face by 2 h. Consequently, reduction of the total IMP density of both faces was not significant in the large IMP, but it was significant (P < 0.01) in the small IMP. One-fifth of the total small IMP density reduced by 2 h. Filipin-sterol complexes (FSC) were numerous in the PAPM, and FSC-free areas also increased during capacitation. The mechanism of IMP-free area formation and the behavior of the small IMP in the PAPM during capacitation were discussed in relation to membrane stability.

Simultaneous detection of the acrosomal status and viability of incubated ram spermatozoa using fluorescent markers

Incubation of diluted ram spermatozoa at 39 degrees C results in a high percentage of acrosome reactions, but previously we have not been able to demonstrate the viability of these cells. Detection of the viability and stages of acrosomal exocytosis, either spontaneous or induced, was carried out using fluorescent probes. Propidium iodide (PI) was used to determine cell viability and, simultaneously, FITC-Pisum sativum lectin (FITC-PSA) was used to assess acrosomal status by staining glycoproteins in the acrosome of permeabilised spermatozoa. Diluted ram semen was incubated for 6 hours at 39 degrees C. At 2 hourly intervals, samples were taken and examined for evidence of a spontaneous acrosome reaction. In addition, calcium ionophore A23187 was used to induce the acrosome reaction and samples were examined at 10 minute intervals. PI was added and then washed out by filtration. Smears were made and air-dried, permeabilised with absolute ethanol and then stained with FITC-PSA. The slides were later viewed under the fluorescence microscope with a peak excitation wavelength of 488 nm. With this combination of two fluorescent probes using a single excitation wavelength, both the cell viability and the acrosomal status could be simultaneously and easily visualized. Results showed four categories of staining: PI-ve/PSA + ve (Live and acrosome-intact), PI + ve/PSA + ve (dead and acrosome-intact), PI - ve/PSA - ve (live and acrosome-reacted) and PI + ve/PSA - ve (dead and acrosome-degenerated). About 75% spermatozoa that were acrosome-reacted were still viable after 4 h incubation in the absence of ionophore, and approximately 90% spermatozoa were acrosome-reacted and still viable after 30 min incubation in the presence of ionophore.

In vitro capacitation and induction of acrosomal exocytosis in ram spermatozoa as assessed by the chlortetracycline assay

We have described the different patterns of chlortetracycline (CTC) binding to ram spermatozoa, immediately after ejaculation and upon in vitro capacitation and calcium ionophore-induced acrosomal exocytosis. Four different forms of CTC distribution were found. Form I showed an even distribution of fluorescence over the entire head, with a brighter band in the equatorial region. In Form II, uniform fluorescence was observed without equatorial band. Form III consisted of fluorescence in the anterior portion of the head. Form IV showed no fluorescence over the head. In all cases, fluorescence in the middle piece of the flagellum was observed as well. Immediately after ejaculation, Form I was the most abundant one (78%) in fresh semen with Forms II and III being relatively scarce (less than 15%). Form IV was virtually absent or appeared only occasionally. Incubation under in vitro capacitating conditions led to a significant decrease in Form I and to a significant increase in Forms II and III. Form II was mainly associated to intact acrosomes, while most spermatozoa in Form III showed intermediate forms of acrosomal status. Incubation of spermatozoa with the calcium ionophore A23187 resulted in 55% of spermatozoa showing Form IV, suggesting that it represents the acrosome-reacted stage. Form I was abruptly decreased at 30 min of incubation and was neglectible after 60 min. In contrast, Forms II and III increased at 30 min but decreased later on, suggesting that both forms represent intermediate stages before the acrosomal exocytosis. Analysis of acrosomal status in spermatozoa from individual CTC forms revealed that all spermatozoa that remained in Form II after incubation had intact acrosomes. Intermediate stages were predominant in Form III-spermatozoa, while most Form IV-spermatozoa underwent full acrosomal exocytosis. These results show that CTC binding can be used to monitor changes in ram spermatozoa during capacitation and acrosome-reaction.

Localization of intracellular calcium during the acrosome reaction in ram spermatozoa

Calcium was identified by a pyroantimonate-osmium fixation technique in ram spermatozoa undergoing a spontaneous acrosome reaction induced by incubation of diluted semen at 39 degrees C. Intracellular calcium was only detected in diluted spermatozoa and increased in amount and distribution over 4 hr At 4 hr, the majority of the spermatozoa displayed ultrastructural evidence of an acrosome reaction. Calcium was initially evident on the outer acrosomal membrane in multiparticulate clusters, which were seen to be located on scalloped crests of acrosomal membrane as fusion developed; it was also located in the region of the acrosomal ridge beneath the outer acrosomal membrane. Vesiculation commenced just anterior to the equatorial segment and proceeded anteriorly. As vesiculation advanced, calcium particles became associated with the periphery of the vesicles attached in the region of the fusion between the two membranes, but were never seen inside the vesicles. The equatorial segment was not labelled until much later in the reaction, at which time calcium particles were also evident on the nuclear membrane; vesiculation of the equatorial segment was also noted at this time. Dense labelling of the postacrosomal dense lamina was seen in all incubated spermatozoa. At the anterior margin of this structure the labelling was seen to be in a "sawtooth" arrangement. The disposition of the calcium both temporally and spatially is discussed in relation to its possible mechanisms in bringing about membrane fusion.

Glycolipid migration from the apical to the equatorial subdomains of the sperm head plasma membrane precedes the acrosome reaction. Evidence for a primary capacitation event in boar spermatozoa

In order to extend the static information of immunolabelling sulphogalactolipids in fixed boar spermatozoa, a fluorescent sulphogalactolipid analogue, galactose(3-sulphate)-beta 1–1′[(N-lissamine rhodaminyl)-12-aminodode-canoyl]-sphingosine, was incorporated into plasma membranes of living spermatozoa and its lateral distribution over the sperm head was studied. The fluorescent lipid was enriched in the apical ridge subdomain of freshly ejaculated sperm cells. After sperm binding to the zona pellucida the lipid redistributed to the equatorial segment of the sperm surface. A similar shift occurred during capacitation in vitro with 2 mM CaCl2 or with 4% (w/v) bovine serum albumin. The desulphated derivative galactose-beta 1–1′[(N-lissamine rhodaminyl)-12-aminododecanoyl]-sphingosine was also incorporated into the plasma membrane of freshly ejaculated sperm cells and clearly stained the apical ridge subdomain and the (pre)-equatorial subdomains of the sperm heads. The desulphogalactolipid analogue showed a slightly faster migration to the equatorial segment of the sperm plasma membrane than did its sulphated counterpart. The measured fluorescence intensity distributions correlated linearly with the spatial probe distribution, which was checked by fluorescence lifetime imaging microscopy. The observed migration of the incorporated glycolipids precedes the acrosome reaction and is one of the underlying molecular events likely to be important in the process of sperm capacitation. The results of this study suggest that lipid phase segregation is an important driving force for the organization of the sperm head plasma membrane into subdomains.

Protein-dependent translocation of aminophospholipids and asymmetric transbilayer distribution of phospholipids in the plasma membrane of ram sperm cells

We have investigated the transbilayer movement of phospholipids in the plasma membrane of ram sperm cells using spin- and fluorescence-labeled lipid analogues. After incorporation into the outer leaflet, phosphatidylcholine (PC) and sphingomyelin (SM) moved slowly to the inner cytoplasmic leaflet, whereas phosphatidylserine (PS) and phosphatidylethanolamine (PE) rapidly disappeared from the exoplasmic monolayer. Variation of the initial velocity of the relocation kinetics vs the amount of analogue incorporated into the membrane suggests a saturability of the transbilayer movement of aminophospholipids. ATP depletion or pretreatment with N-ethylmaleimide of ram sperm cells reduced the fast inward motion of PS and PE, indicating a protein-mediated aminophospholipid translocation. The results suggest for the plasma membrane of ram sperm cells the presence of an aminophospholipid translocase and an asymmetric transversal lipid distribution with aminophospholipids preferentially located in the inner leaflet and choline-containing phospholipids in the outer leaflet. The relevance of the transversal segregation of phospholipids for membrane fusion processes occurring during fertilization is discussed.

Visualization and quantification of glycolipid polarity dynamics in the plasma membrane of the mammalian spermatozoon

Seminolipid (sulphogalactosylalkylacylglycerol), the glycolipid that is specific for mammalian germ cells, is located exclusively in the outer leaflet of the sperm plasma membrane. In this study the lateral distribution of seminolipid on sperm heads has been investigated by indirect immunofluorescence labelling and detection with digital imaging fluorescence microscopy. In freshly ejaculated sperm cells this glycolipid was present primarily at the apical ridge subdomain of the plasma membrane of the sperm head. After binding the sperm cells to zona-coated coverslips seminolipid migrated, in 40 minutes, from the apical ridge to the equatorial subdomain of the plasma membrane. A similar redistribution of seminolipid was observed during capacitation of sperm cells in vitro induced by Ca2+ or bovine serum albumin. Comparable migration of seminolipid was also found after prolonged storage of ejaculated sperm cells, albeit at a much slower rate. Addition of arylsulphatase A, an enzyme present in seminal plasma that desulphates seminolipid, significantly enhanced the migration of seminolipid during storage of sperm cells. Its breakdown product desulphoseminolipid (galactosylalkylacylglycerol) appeared highly specifically at the equatorial segment. The measured fluorescence intensity over the sperm head surface correlated linearly with the spatial probe distribution as was checked by fluorescence lifetime imaging microscopy. This paper demonstrates and quantifies for the first time the polarity of seminolipid on the surface of the sperm cell and the dynamic alterations that occur in this polarity during post-ejaculatory events.

The ionophore-induced acrosome reaction differs structurally from the spontaneous acrosome reaction

The ultrastructure of the spontaneous acrosome reaction in ram spermatozoa has been compared with that induced by the ionophore, A23187. The spontaneous event was dependent on incubation for 4 h, on the temperature, and on dilution. Apart from the more rapid occurrence of the ionophore-induced event, the mean diameter and distribution of vesicle size was also different. The ionophore-induced vesicles were larger, more irregular, and heterogeneous in size compared with those occurring in the spontaneous acrosome reaction (average diameter 84 nm vs. 60 nm in the spontaneous acrosome reaction). These observations are interpreted in relation to capacitation.

Hyperactivated motility of sperm from fertile donors and asthenozoospermic patients before and after treatment with ionophore

Hyperactivated motility of capacitated sperm was studied before and after contact for 30 min with the Ca2+ ionophore A23187. Sperm from fertile donors and asthenozoospermic infertile patients were incubated in the presence of increasing concentrations of A23187 in the medium. At 5-10 microM, the ionophore induced a significant increase (P less than 0.05) in the percentage of hyperactivation of sperm from asthenozoospermic subjects. Higher concentrations (20 and 30 microM) were needed to enhance hyperactivation of sperm in the fertile population. In the latter, extended contact with the ionophore induced no significant change compared to a 30-min incubation period. Since this type of movement is an essential feature of the fertilizing gamete, a low hyperactivation rate may partly explain fertilization failure in the asthenozoospermic group.

Cross-linking a maturation-dependent ram sperm plasma membrane antigen induces the acrosome reaction

ESA152 is a highly hydrophobic 18 kDa sialoglycoprotein, which becomes expressed on ram sperm in the proximal cauda epididymis. ESA 152 is expressed on all regions of the sperm surface, most strongly on the posterior region of the head, most weakly on the anterior region of the head. In this paper, we show that induction of the acrosome reaction with Ca2+ ionophore causes ESA152 to be redistributed from the posterior to the anterior region of the head plasma membrane. Cross-linking ESA152 with bivalent antibody causes similar redistribution and induces the acrosome reaction. Induction of the acrosome reaction with ESA152 antibody requires Ca2+ but is insensitive to (10 ng/ml) pertussis toxin.

Morphology of mammalian sperm membranes during differentiation, maturation, and capacitation

The mammalian spermatozoon is a highly polarized cell whose surface membrane can be divided into five functionally, structurally, and biochemically distinct domains. These domains are formed during spermatogenesis, continue to be modified during passage through the epididymis, and are further refined in the female reproductive tract. The integrity of these domains appears to be necessary for the sperm to perform its function--fusion with the egg and subsequent fertilization. The domains can be identified morphologically by their surface contours and texture, the content, distribution, and organization of intramembranous particles after freeze-fracture, and by the density of surface and cytoplasmic electron-dense coatings in thin sections. By using a variety of labels that stain carbohydrates (lectins), lipids (filipin and polymyxin B), and monoclonal antibodies to specific membrane constituents, the biochemical composition of these contiguous membrane regions has also been partly elucidated. We review here what is known about the structure, composition, and behavior of each membrane domain in the mature sperm and include some information regarding domain formation during spermatogenesis. The sperm is an excellent model system to study the creation and maintenance of cell polarity, granule exocytosis, and fertilization. Hopefully this review will provide impetus for future studies aimed more directly at addressing the relationship of its morphology to its functions.

Effect of microinjection and two types of electrical stimuli on bovine sperm-hamster egg penetration

These experiments were designed to test the effects of an electrofusion and an electroporation pulse on bovine sperm-hamster egg development. In experiment 1, single motile sperm were injected into the perivitelline space of each egg. A 4,500 V/cm, 30 microseconds fusion pulse (FP) was applied while sperm-egg membrane contact was maintained. It was observed that single motile sperm were rendered immotile immediately after FP application whereas nonpulsed single motile sperm remained motile for up to 36 h postinjection. In addition, both motile and sonicated spermatozoa were injected directly into the ooplasm prior to receiving an FP to determine whether the FP was detrimental to sperm viability. In experiment 2, to induce the acrosome reaction, an 1,150 V/cm electroporation pulse was applied to washed bovine sperm suspended in TALP medium containing 5 mM Ca2+. Treated and nontreated sperm were coincubated with zona-free hamster ova, and sperm-pentrating ability was measured. Results from experiment 1 indicate that FP failed to induce sperm-egg fusion (0/69). FP did not, however, inhibit decondensation or pronuclear formation of sperm injected into hamster egg ooplasm. Single motile sperm injected into the ooplasm resulted in development of both pulsed (19/28) and nonpulsed (21/28) groups. Sonicated tail-free sperm heads injected into the ooplasm resulted in no detectable difference between treated (18/30) and nontreated (19/30) groups. In experiment 2, treatment of sperm with electroporation pulse +5 mM Ca2+ increased zona-free hamster ova penetration scores over nontreated sperm within bulls (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Preendocytotic alterations in cumulus cell gap junctions precede meiotic resumption in the rat cumulus-oocyte complex

Cumulus cells in the mammalian ovary are normally connected to each other and to their enclosed oocyte by an extensive network of gap junctions (GJs). We have shown that the loss of cumulus cell GJs is correlated temporally with meiotic resumption in the intact preovulatory rat follicle (Larsen et al., 1986). Here we describe morphological changes in GJ particle packing patterns (PPPs) that occur prior to GJ loss and meiotic resumption in hormonally stimulated rat cumulus-oocyte complexes (COCs). In the PMSG-primed rat, 89% of the cumulus cell GJ area detected by freeze-fracture electron microscopy consists of tightly packed junctional particles: 4% exhibit loose PPPs of randomly dispersed particles; and 7% contain a mixture of both tight and loose PPPs. One to 2 hr after stimulation with hCG, the area of GJs containing tight PPPs drops by 50%-60%, while junctions exhibiting loosely organized and mixed patterns increase concomitantly. These shifts in PPPs are accompanied by the appearance of unusual particle-free areas of puckered or ruffled nonjunctional membrane at the GJ periphery. Cumulus cell GJs from isolated COCs incubated in FSH-containing medium demonstrate a similar shift in PPPs prior to meiotic resumption. The appearance of fusing areas of particle-free nonjunctional membrane at the GJ periphery in vitro is correlated with GJ loss and is not seen in COCs treated with dihydrocytochalasin B to inhibit endocytotic removal of cumulus GJs. The structural and temporal nature of these morphological observations supports the hypothesis that interruption of junctional communication plays a role in meiotic maturation of the preovulatory oocyte.

Gamete membrane fusion in hamster spermatozoa with reacted equatorial segment

Mammalian spermatozoa must undergo many changes to be able to fertilize the oocyte. One of these changes, the acrosome reaction, has been established as a requisite for gamete membrane fusion to occur; it consists of the fusion and vesiculation of the sperm plasma membrane with the outer acrosomal membrane of the principal segment of the acrosome. Reaction of the equatorial segment has occasionally been observed. The objective of the present work was to determine whether the presence of the sperm plasma membrane over the equatorial segment is necessary for gamete membrane fusion to occur. Golden hamster spermatozoa were capacitated in vitro in TAPL 10K, and the maximum possible percentage of acrosome reaction was determined at 82.79% + 1.69% SD (P = 0.27; r = 0.21). Ultrastructural studies showed that 93.6% of the reacted spermatozoa in this population had their principal and equatorial segments reacted. The fertilizing ability of these spermatozoa was assayed using zona-free hamster oocytes. The percentage of fertilized ova obtained was 98.8% (308/312). Ultrastructural studies showed the presence of spermatozoa with reacted equatorial segment inside the cytoplasm of immature oocytes. The evidence presented in this work demonstrates that the plasma membrane of spermatozoa with reacted equatorial segment retains its ability to fuse with the oocyte.

Absence of active protein kinase C in ram spermatozoa

At fertilization, mammalian spermatozoa undergo a Ca2+-dependent exocytotic event known as the acrosome reaction. As protein kinase C (PKc) has been implicated in exocytosis in some other cell systems, we have searched for PKc in ram spermatozoa. We have found that: (a) no changes in protein phosphorylation pattern could be induced in the intact cells by phorbol dibutyrate (PDBu), a compound which binds to and stimulates PKc; (b) no changes in protein phosphorylation pattern could be detected during the course of the Ca2+/ionophore-induced acrosome reaction (when greater than 95% of the cells underwent exocytosis); (c) there was no effect of PDBu on the exocytotic response to various Ca2+ and ionophore levels; (d) no specific PDBu binding could be detected in the cells (this binding is considered to be indicative of the presence of active PKc). We conclude that potentially active PKc is not present in ram spermatozoa.

Membrane domains in guinea pig sperm and their role in the membrane fusion events of the acrosome reaction

In this study, we have examined the structure of domains of the periacrosomal plasma membrane (PM) and outer acrosomal membrane (OAM) of guinea pig sperm and defined their fate during the membrane fusion events of the acrosome reaction. Cauda epididymal sperm were arranged in rouleaux, joined by periacrosomal PM "junctional" zones; in these zones, the PMs were linked by cross bridges formed from a paracrystalline glycocalyx. Bridging elements linked the PM to the OAM on the ventral (concave) but not dorsal (convex) aspect of the apical segment. Parallel filaments were associated with the luminal face of the OAM overlying the dorsal surface of the apical segment. Sperm were induced to undergo a "synchronous" acrosome reaction after preincubation in Ca2+-free medium containing lysolecithin, by the addition of Ca2+. Fusion between the OAM and PM occurred at the boundaries but not within the PM "junctional" zones over the apical segment. In nonjunctional regions on the dorsal surface of the apical segment, sheets of unfenestrated hybrid membranes and parallel arrays of hybrid membrane tubules formed, while branching arrays of hybrid membrane tubules and vesicles were observed on the ventral surface. In the principal segment, networks of branching hybrid membrane tubules initially formed but later transformed into vesicles. Hence, the lysolecithin-mediated guinea pig sperm acrosome reaction involves a complex sequence of membrane fusions, which differs in domains of the periacrosomal PM and OAM. Stable nonfusigenic domains are present in both the PM and OAM of the apical segment; membrane-associated assemblies may maintain these domains and may also provide direction to some of the membrane fusion events of the acrosome reaction.