Distribution of intramembranous particles and filipin-sterol complexes in mouse sperm membranes: polyene antibiotic filipin treatment

The Intimate Connection Between Lipids and Hedgehog Signaling

Hedgehog (HH) signaling is an intercellular communication pathway involved in directing the development and homeostasis of metazoans. HH signaling depends on lipids that covalently modify HH proteins and participate in signal transduction downstream. In many animals, the HH pathway requires the primary cilium, an organelle with a specialized protein and lipid composition. Here, we review the intimate connection between HH signaling and lipids. We highlight how lipids in the primary cilium can create a specialized microenvironment to facilitate signaling, and how HH and components of the HH signal transduction pathway use lipids to communicate between cells.

Oxidative phosphorylation versus glycolysis: what fuel do spermatozoa use?

Spermatozoa are highly specialized cells. Adenosine triphosphate (ATP), which provides the energy for supporting the key functions of the spermatozoa, is formed by 2 metabolic pathways, namely glycolysis and oxidative phosphorylation (OXPHOS). It is produced in the mitochondria through OXPHOS as well as in the head and principal piece of the flagellum through glycolysis. However, there is a great discrepancy as to which method of ATP production is primarily utilized by the spermatozoa for successful fertilization. Mitochondrial respiration is considered to be a more efficient metabolic process for ATP synthesis in comparison to glycolysis. However, studies have shown that the diffusion potential of ATP from the mitochondria to the distal end of the flagellum is not sufficient to support sperm motility, suggesting that glycolysis in the tail region is the preferred pathway for energy production. It is suggested by many investigators that although glycolysis forms the major source of ATP along the flagellum, energy required for sperm motility is mainly produced during mitochondrial respiration. Nevertheless, some studies have shown that when glycolysis is inhibited, proper functioning and motility of spermatozoa remains intact although it is unclear whether such motility can be sustained for prolonged periods of time, or is sufficiently vigorous to achieve optimal fertilization. The purpose of this article is to provide an overview of mammalian sperm energy metabolism and identify the preferred metabolic pathway for ATP generation which forms the basis of energy production in human spermatozoa during fertilization.

Oligo-astheno-teratozoospermia in mice lacking ORP4, a sterol-binding protein in the OSBP-related protein family

Oligo-astheno-teratozoospermia (OAT), a condition that includes low sperm number, low sperm motility and abnormal sperm morphology, is the commonest cause of male infertility. Because genetic analysis is frequently impeded by the infertility phenotype, the genetic basis of many of OAT conditions has been hard to verify. Here, we show that deficiency of ORP4, a sterol-binding protein in the oxysterol-binding protein (OSBP)-related protein family, causes male infertility due to severe OAT in mice. In ORP4-deficient mice, spermatogonia proliferation and subsequent meiosis occurred normally, but the morphology of elongating and elongated spermatids was severely distorted, with round-shaped head, curled back head or symplast. Spermatozoa derived from ORP4-deficient mice had little or no motility and no fertilizing ability in vitro. In ORP4-deficient testis, postmeiotic spermatids underwent extensive apoptosis, leading to a severely reduced number of spermatozoa. At the ultrastructural level, nascent acrosomes appeared to normally develop in round spermatids, but acrosomes were detached from the nucleus in elongating spermatids. These results suggest that ORP4 is essential for the postmeiotic differentiation of germ cells.

Two distinct Ca(2+) signaling pathways modulate sperm flagellar beating patterns in mice

Hyperactivation, a swimming pattern of mammalian sperm in the oviduct, is essential for fertilization. It is characterized by asymmetrical flagellar beating and an increase of cytoplasmic Ca(2+). We observed that some mouse sperm swimming in the oviduct produce high-amplitude pro-hook bends (bends in the direction of the hook on the head), whereas other sperm produce high-amplitude anti-hook bends. Switching direction of the major bends could serve to redirect sperm toward oocytes. We hypothesized that different Ca(2+) signaling pathways produce high-amplitude pro-hook and anti-hook bends. In vitro, sperm that hyperactivated during capacitation (because of activation of CATSPER plasma membrane Ca(2+) channels) developed high-amplitude pro-hook bends. The CATSPER activators procaine and 4-aminopyridine (4-AP) also induced high-amplitude pro-hook bends. Thimerosal, which triggers a Ca(2+) release from internal stores, induced high-amplitude anti-hook bends. Activation of CATSPER channels is facilitated by a pH rise, so both Ca(2+) and pH responses to treatments with 4-AP and thimerosal were monitored. Thimerosal triggered a Ca(2+) increase that initiated at the base of the flagellum, whereas 4-AP initiated a rise in the proximal principal piece. Only 4-AP triggered a flagellar pH rise. Proteins were extracted from sperm for examination of phosphorylation patterns induced by Ca(2+) signaling. Procaine and 4-AP induced phosphorylation of proteins on threonine and serine, whereas thimerosal primarily induced dephosphorylation of proteins. Tyrosine phosphorylation was unaffected. We concluded that hyperactivation, which is associated with capacitation, can be modulated by release of Ca(2+) from intracellular stores to reverse the direction of the dominant flagellar bend and, thus, redirect sperm.

Redistribution of nuclear pores during formation of the redundant nuclear envelope in mouse spermatids

Extensive morphological modification occurs during mammalian spermiogenesis when spermatids change their spherical shape into cells with a compact head and a long tail. In this study, freeze-fracture was used to elucidate the alteration of the nuclear envelope during this process. Nuclear condensation resulted in a great reduction of spermatid nuclear volume and the formation of the redundant nuclear envelope. During nuclear condensation, distribution patterns of nuclear pores were greatly affected by the developing acrosome and manchette. As the acrosome enlarged to cap the nucleus, the pores redistributed caudally in the nuclear membranes and became exclusively localized to the redundant nuclear envelope. Manchette microtubules play an important role in shaping the nucleus, and formation of the manchette was associated with exclusion of nuclear pores from the underlying nuclear envelope; therefore, it is likely that the redistribution of nuclear pores was aided by manchette development. The appearance of an electron-lucent nuclear region surrounded by the nascent redundant nuclear envelope indicated a pathway for transporting degradation products through the nuclear pores to the residual cytoplasm. The packaging of the nuclear pores into the redundant nuclear envelope suggests that they play a role in late stages of sperm maturation or in fertilization, as most other unnecessary organelles of sperm are discarded during spermiogenesis or during shedding of the cytoplasmic droplet.

Possible causal factors of structural chromosome aberrations in intracytoplasmic sperm injection of the mouse

Incidence of structural chromosome aberrations in mouse one-cell embryos produced by intracytoplasmic sperm injection (ICSI) with mature epididymal spermatozoa were influenced by sperm incubation medium and time. When spermatozoa were incubated in bicarbonate-buffered TYH for ≤0.5 h, the embryo aberration rates were significantly higher than in vitro fertilization (IVF) embryos. However, after the incubation of spermatozoa in the same medium for ≥2 h, the aberration rates were close to the IVF embryo level. When spermatozoa were incubated in bicarbonate-buffered mCZB, hepes-buffered H-TYH and H-mCZB, and phosphate-buffered PB1, the increased incidences of aberrations were observed at any incubation time. In the case of sperm incubation in H-TYH, H-mCZB and PB1, the aberration rates increased in a time-dependent manner. Chromosome aberrations generated by ICSI were transmissible to offspring. On the other hand, the aberration rate in embryos derived from testicular spermatozoa was independent of the medium type and incubation time. Thus, the incubation media appears to have no effect on sperm chromatin. TYH can effectively induce capacitation and acrosome reaction, while H-TYH, H-mCZB and PB1 never induce these spermatozoal events. It is probable that the cholesterol-rich plasma membrane and intact acrosome injected into the ooplasm affect sperm chromatin remodeling, thus resulting in the generation of chromosome damage in ICSI embryos.

Biochemical characterization of membrane fractions in murine sperm: identification of three distinct sub-types of membrane rafts

Despite enormous interest in membrane raft micro-domains, no studies in any cell type have defined the relative compositions of the raft fractions on the basis of their major components--sterols, phospholipids, and proteins--or additional raft-associating lipids such as the ganglioside, G(M1). Our previous localization data in live sperm showed that the plasma membrane overlying the acrosome represents a stabilized platform enriched in G(M1) and sterols. These findings, along with the physiological requirement for sterol efflux for sperm to function, prompted us to characterize sperm membrane fractions biochemically. After confirming limitations of commonly used detergent-based approaches, we utilized a non-detergent-based method, separating membrane fractions that were reproducibly distinct based on sterol, G(M1), phospholipid, and protein compositions (both mass amounts and molar ratios). Based on fraction buoyancy and biochemical composition, we identified at least three highly reproducible sub-types of membrane raft. Electron microscopy revealed that raft fractions were free of visible contaminants and were separated by buoyancy rather than morphology. Quantitative proteomic comparisons and fluorescence localization of lipids suggested that different organelles contributed differentially to individual raft sub-types, but that multiple membrane micro-domain sub-types could exist within individual domains. This has important implications for scaffolding functions broadly associated with rafts. Most importantly, we show that the common practice of characterizing membrane domains as either "raft" or "non-raft" oversimplifies the actual biochemical complexity of cellular membranes.

Control of hyperactivation in sperm

BACKGROUND

Sperm hyperactivation is critical to fertilization, because it is required for penetration of the zona pellucida. Hyperactivation may also facilitate release of sperm from the oviductal storage reservoir and may propel sperm through mucus in the oviductal lumen and the matrix of the cumulus oophorus. Hyperactivation is characterized by high amplitude, asymmetrical flagellar bending.

METHODS

This is a review of the original literature on the mechanisms that regulate hyperactivation, including physiological factors and signaling pathways.

RESULTS

Computer-assisted semen analysis systems can be used to identify hyperactivated sperm by setting minimum thresholds for curvilinear velocity (VSL) and lateral head movement and a maximum threshold for path linearity. Hyperactivation is triggered by a rise in flagellar Ca(2+) resulting from influx primarily through plasma membrane CatSper channels and possibly also by release of Ca(2+) from a store in the redundant nuclear envelope. It requires increased pH and ATP production. The physiological signals that trigger the rise in Ca(2+) remain elusive, but there is evidence that the increased Ca(2+) acts through a calmodulin/calmodulin kinase pathway. Hyperactivation is considered part of the capacitation process; however, the regulatory pathway that triggers hyperactivation can operate independently from that which prepares sperm to undergo the acrosome reaction. Hyperactivation may be modulated by chemotactic signals to turn sperm toward the oocyte.

CONCLUSIONS

Little is known about exactly what triggers hyperactivation in human sperm. This information could enable clinicians to develop reliable fertility assays to assess normal hyperactivation in human sperm samples.

Characterization of the intracellular calcium store at the base of the sperm flagellum that regulates hyperactivated motility

Hyperactivated sperm motility is usually characterized by high-amplitude flagellar bends and asymmetrical flagellar beating. There is evidence that an inositol 1,4,5-trisphosphate (IP3) receptor-gated Ca2+ store in the base of the flagellum provides Ca2+ to initiate hyperactivation; however, the identity of the store was not known. Ca2+ stores are membrane-bounded organelles, and the only two membrane-bounded organelles found in this region of sperm are the redundant nuclear envelope (RNE) and mitochondria. Transmission electron micrographs revealed two different compartments of RNE, one enriched with nuclear pores and the other containing few pores but extensive membranous structures with enlarged cisternae. Immunolabeling showed that IP3 receptors and calreticulin are located in the region containing enlarged cisternae. In other cell types, mitochondria adjacent to Ca2+ stores are actively involved in modulating Ca2+ signals by taking up Ca2+ released from stores and also may respond by increasing production of NADH and ATP to support increased energy demand. Nevertheless, bull sperm did not show an increase in NADH when Ca2+ was released from intracellular stores by thapsigargin to induce hyperactivation. Consistently, no net increase in ATP production was detected when sperm were hyperactivated, although ATP was hydrolyzed at a greater rate. Furthermore, blocking Ca2+ efflux from mitochondria by CGP-37157, a specific inhibitor of the mitochondrial Na+/Ca2+ exchanger, did not inhibit the development of hyperactivated motility. We concluded that the intracellular Ca2+ store is the part of RNE that contains enlarged cisternae and that Ca2+ is released directly to the axoneme to trigger hyperactivated motility without the active participation of mitochondria.

An inositol 1,4,5-trisphosphate receptor-gated intracellular Ca(2+) store is involved in regulating sperm hyperactivated motility

Hyperactivated motility, a swimming pattern displayed by mammalian sperm in the oviduct around the time of ovulation, is essential to fertilization. Ca(2+) has been shown to be crucial for the initiation and maintenance of hyperactivated motility. Nevertheless, how Ca(2+) reaches the axoneme in the core of the flagellum to switch on hyperactivation is unknown. Ca(2+)-releasing agents were used to determine whether an intracellular store provides Ca(2+) to the axoneme. Hyperactivation was induced immediately in bull sperm by thapsigargin, caffeine, and thimerosal. The responses were dose-dependent and were induced in both capacitated and uncapacitated sperm. When external Ca(2+) was buffered below 50 nM with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, the response to caffeine was significantly reduced; however, the responses to thapsigargin and thimerosal were not affected. This indicates caffeine-induced hyperactivation depends on external Ca(2+) influx, whereas hyperactivation by thapsigargin and thimerosal do not. Acrosome reactions were not induced by these treatments; therefore, an acrosomal store was probably not involved. Indirect immunofluorescence labeling showed type I inositol 1,4,5-trisphosphate receptors (IP(3)R) in the acrosome and neck region, but no ryanodine receptors (RyR) were found using anti-RyR antibodies or BODIPY FL-X ryanodine. These data indicate that there is an IP(3)R-gated Ca(2+) store in the neck region of sperm that regulates hyperactivated motility.

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.

Lipid diffusion in the plasma membrane of ram and boar spermatozoa during maturation in the epididymis measured by fluorescence recovery after photobleaching

Maturation of spermatozoa in the epididymis involves remodelling of many protein and lipid components of the plasma membrane. In this investigation we have examined whether (a) diffusion of lipid molecules in the surface membrane changes during epididymal maturation; (b) diffusion is spatially restricted; and (c) differences in lipid diffusion can be related to known changes in membrane composition. For this purpose we have used the technique of fluorescence recovery after photobleaching (FRAP) to measure diffusion of the lipid reporter probe ODAF (5-(octa-decanoyl)aminofluorescein) in spermatozoa from two species: ram, where substantial changes in membrane lipids occur during passage through the epididymis, and boar, where there are relatively few changes. Results on ram spermatozoa show that between the testis and cauda epididymidis, diffusion coefficients values (D) for ODAF increase significantly in all the surface domains. Percentage recovery values (%R) remain constant irrespective of maturational status. In boar spermatozoa, however, D and %R values do not change significantly between epididymal regions. Cholesterol, which has widespread effects on the behaviour of lipid molecules in cell membranes, was visualized by binding of filipin. In both species filipin was concentrated over the acrosomal domain and cytoplasmic droplet of testicular spermatozoa, but in the epididymis it had a heterogenous distribution over the whole head and tail. These results are discussed in relation to the establishment and maintenance of lipid domains in spermatozoa and their influence on development of fertilizing capacity.

Experimental hypercholesterolaemia in rabbits. Effect on lipid domains in homologous spermatozoa

The distribution of membrane filipin sterol complexes (FSC) in the plasma membrane of the acrosomal region (PMAR) of rabbit sperm from epididymis and testis, in normal and hypercholesterolaemic rabbits, was examined at ultrastructural level. Membrane FSG were quantitatively analysed on freeze fracture replicas of filipin-treated cells. Cauda epididymal sperm shows a significant increase in filipin sterol complexes concentration in PMAR of hypercholesterolaemic animals compared to normal rabbits. Hypercholesterolaemic animals had 0.53 +/- 0.08 FSC micron-2 in the marginal segment of PMAR and 0.26 +/- 0.03 FSC micron-2 for normal animals. In the principal piece we found 0.70 +/- 0.07 FSC micron-2 for hypercholesterolaemic and 0.43 +/- 0.03 FSC micron-2 for control animals. We also counted 0.58 +/- 0.04 FSC micron-2 in the equatorial segment of PMAR for hypercholesterolaemic and 0.38 +/- 0.03 FSC micron-2 for normal animals respectively. The FSC concentration of testicular sperm, like sperm from corpus and caput of epididymis in hypercholesterolaemic animals, did not differ from the controls. Cholesterol, phospholipids and cholesterol:phospholipid ratio in caudal epididymal sperm from treated males did not differ from controls. Only the sphingomyelin concentration decreases in cauda epididymal sperm from hypercholesterolaemic males compared to controls. The results presented in this paper suggest that the lipidic domains in PMAR of hypercholesterolaemic rabbits changes when the gametes go through the epididymis.

Intra-acrosomal 155,000 dalton protein increases the antigenicity during mouse sperm maturation in the epididymis: a study using a monoclonal antibody MC101

We found an intra-acrosomal antigen of about 155,000 daltons (155 kDa) in a survey using the monoclonal antibody MC101 raised against mouse cauda epididymal spermatozoa. Morphological studies by means of indirect immunofluorescence and immunogold electron microscopy localized the antigen to the cortex region of the anterior acrosome. Avidin biotin complex immunocytochemistry initially demonstrated a faint signal at the anterior acrosome in the testis spermatozoa that increased in intensity as the sperm moved toward the distal epididymis. This incremental immunoreactivity was also confirmed by immunoblotting following one-dimensional SDS-PAGE. The 155 kDa protein band was immunostained, and it was much more intense in the cauda epididymal than in the caput and corpus epididymal spermatozoa. Only a trace or no immunostain was evident in the caput or testis spermatozoa. The antigen localization did not change during passage through the epididymis, being confined at the cortex region of the anterior acrosome. The epididymal epithelial cells were not immunostained. These findings suggested that the 155 kDa protein is biochemically modified, further implying that the biochemical alteration of intra-acrosomal material is involved in sperm maturation in the epididymis.

Role of the epididymis in mediating changes in the male gamete during maturation

This article reviews recent knowledge about events occurring in the epididymis that are important for sperm to fertilise eggs. Well established concepts are stated without references (see Cooper, 1986 for older literature) but recent references are included where they throw light on mechanisms of epididymal function. During their sojourn in the epididymis spermatozoa acquire the capacity to move and to fertilise eggs; they are then stored in a quiescent state prior to ejaculation. The ability of sperm to undergo the events of fertilisation are developed as a result of interactions with certain epididymal secretions. Increases in our knowledge about the genes coding for epididymal secretions has not yet been matched by similar insight into the role that these secretions play in the maturation process. However, information about the changes that occur to the sperm cells during maturation permit certain scenarios to be sketched that may reflect reality. This review is one such attempt to bring the epididymal sperm-epithelial secretion into focus.

Distribution of filipin-sterol complexes in sperm membranes from hypercholesterolaemic rabbits

The distribution of membrane filipin-sterol complexes (FSC) was examined ultrastructurally in cauda epididymal sperm from normal and hypercholesterolaemic rabbits. Membrane FSC were quantitatively analysed on replicas of filipin-treated cells. We determined a significant difference in FSC concentration in the plasma membrane of the acrosome region (PMAR) of hypercholesterolaemic animals compared to normal rabbits. Hypercholesterolaemic animals had 0.56 +/- 0.05 FSC complex per micron 2 (enriched Cholesterol diet: Diet 2) in the marginal segment of PMAR; 0.62 +/- 0.05 FSC complex per micron 2 (enriched Cholesterol and fish oil diet: Diet 3) and only 0.28 +/- 0.01 FSC complex per micron 2 for normal animals (Control Diet 1). In the principal (anterior) segment we found 0.54 +/- 0.10 FSC complex per micron 2 (Diet 2), 0.56 +/- 0.03 FSC complex per micron 2 (Diet 3) and 0.30 +/- 0.04 FSC complex per micron 2 (Control Diet 1). We also counted 0.47 +/- 0.1 FSC complex per micron 2 in the equatorial segment of PMAR for Diet 2, 0.27 +/- 0.05 and 0.28 +/- 0.04 FSC complex per micron 2 in Diet 1 and Diet 3 respectively. Diet 4 (fish oil) did not differ from the control. An increase in the Cholesterol (Chol) level in biological membranes or a difference in the Chol membrane domains could cause a variation in the membrane rigidity that could modify the sperm membrane fusion capacity and functionality. The results presented in this paper are in agreement and could explain the decrease in the kinetic of the sperm acrosome reaction that we have observed in experimentally hypercholesterolaemic rabbits (Díaz-Fontdevila & Bustos-Obregón, 1992).

Changes in the distribution of filipin-sterol complexes in the boar sperm head plasma membrane during epididymal maturation and in the uterus

The distribution of filipin-sterol complexes (FSCs) and intramembranous particles (IMPs) in the plasma membrane of the late spermatid of the boar and of the sperm obtained from the epididymides, ejaculates, and uterus 2 hours after mating was examined by a freeze-fracture replica technique. In the late spermatid, the FSC density was found to be very low. A majority of the FSCs in the acrosomal plasma membrane (APM) appeared as protuberances on the E face in the epididymal, ejaculate, and uterine sperm. The density of the FSCs in the principal segment (PS) of the APM was 291 +/- 44 FSC/microns2 (mean +/- standard deviation, S.D.), 322 +/- 41 FSC/microns2 and 355 +/- 31 FSC/microns2 in the caput, corpus, and cauda epididymidis, respectively. In comparison with the cauda epididymal sperm, the FSC density gradually decreased in the PS of the ejaculated (277 +/- 39 FSC/microns2) and uterine sperm (243 +/- 50 FSC/microns2). The reduction was especially remarkable in the equatorial segment (ES), where the density of FSCs in ejaculated and uterine sperm decreased to about half and less than half of that in the cauda epididymal sperm, respectively. Large (13 nm) and small (8 nm) IMPs were distributed evenly and densely in the P face of the APM in the late spermatid, epididymal, and ejaculated sperm. In the uterine sperm, IMP-free areas were observed in the P face of the plasma membrane, a feature thought to represent one of the capacitation changes of the boar sperm.

A monoclonal antibody, MN13, that recognizes specifically a novel substance between the postacrosomal sheath and the overlying plasma membrane in the mammalian sperm head

Monoclonal antibody MN13 raised against mouse spermatozoa specifically recognizes the postacrosomal region of the sperm head in several mammalian species. Colloidal gold-immunoelectron microscopy of demembranated mouse spermatozoa indicated that the antigen is associated with the outer layer of the periodic substructure apparently linking the postacrosomal sheath to the overlying plasma membrane. The antigen recognized by MN13 may contribute to the intimate association of the postacrosomal sheath with the overlying plasma membrane.

Distribution of filipin-sterol complexes in the plasma membrane of stallion spermatozoa during the epididymal maturation process

The presence and distribution of cholesterol in mature and immature epididymal spermatozoa was analyzed using filipin as a cytochemical tool in freeze-fracture replicas and thin section preparations. The polyenic-antibiotic filipin formed complexes with 3, beta -OH sterols, producing characteristic protrusions, or pits, that were heterogeneously distributed in the plasma membrane of stallion spermatozoa, revealing a specific organization in a functionally specialized area of the gamete. The acrosomal region of the sperm head presented a significantly higher density of filipin sterol complexes than the postacrosomal region, which was usually free of these complexes. The plasma membrane of the flagellum also showed filipin sterol complexes randomly distributed in freeze-fracture replicas. The strong filipin labeling observed in the membrane of spermatozoa obtained from the caput region of the epididymis decreased significantly during epididymal passage. The significance of these changes is not completely understood, but they might contribute to establishing the molecular organization necessary for sperm transit and storage in the epididymis as well as to development of motile spermatozoa that are able to fertilize the oocyte and induce normal embryonic development.

Observations of hamster sperm-egg fusion in freeze-fracture replicas including the use of filipin as a sterol marker

We have extended the observations of previous transmission electron microscopy studies of sperm-egg fusion to include those of freeze-fracture replicas showing sperm-egg interactions before, during, and following sperm head fusion with the egg membrane. Hamster eggs were incubated with hamster sperm under polyspermic conditions and were observed after a period of 5-30 minutes. After fixation, the eggs and sperm were exposed to filipin, which binds beta-OH-sterols to form visible complexes in freeze-fracture replicas. Filipin can act as a marker for egg plasma membrane wherein it is abundant, while filipin is relatively scarce in the acrosome-reacted hamster sperm membrane, found only in the plasma membrane of the equatorial segment. The earliest sperm-egg interactions are observed between the egg microvilli and the perforatorium and the equatorial segment of the sperm, and the initial fusion between egg and sperm occurs in the vicinity of the equatorial segment. At later stages of fusion involving the postacrosomal segment, a clear line of demarcation is observed between the filipin-rich egg membrane and the filipin-poor sperm postacrosomal segment, suggesting that filipin binding lipids from the egg intercalate into the sperm membrane following membrane fusion. The anterior segment of the sperm does not fuse with the egg but is instead incorporated into a cytoplasmic vesicle derived from both sperm and egg membranes. In this latter step, filipin-sterol complexes are not found in sperm-derived membranes suggesting that there may be barriers to the movement of filipin binding lipids from the egg into these sperm membranes.

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.

Topographical rearrangement of a plasma membrane antigen during capacitation of rat spermatozoa in vitro

We have previously described an antigen (termed 2B1) on rat spermatozoa that is present on the plasma membrane overlying the tail domain. The antigen is mobile within the plane of the plasma membrane and a mAb to it blocks fertilization in vitro. In the present study we describe some dynamic properties of this antigen in relation to its topographical distribution. When spermatozoa were incubated in vitro in a capacitation medium and stained with 2B1 mAb/FITC-rabbit anti-mouse F(ab')2, strong fluorescence appeared over the acrosomal domain. Acute exposure of fresh spermatozoa to dissociating reagents (1 M NaCl or 5 mM 2-mercaptoethanol) or inducers of the acrosome reaction (lysolecithin + Ca2+ or A23187 + Ca2+) failed to mimic these effects. Spermatozoa prelabeled with FITC-2B1 IgG and then capacitated in the presence of excess "cold" 2B1 IgG also showed accumulation of fluorescence on the acrosomal domain, suggesting that the antigen had migrated from the tail. Migration was selective and Ca2(+)- and temperature-dependent but was not inhibited by metabolic poisons (NaF or NaN3). Motility was not obligatory for migration. Immunogold-labeling studies at the ultrastructural level showed that 2B1 antigen was restricted to the surface membrane over both the tail and the acrosomal domains and that during migration it did not change the type of membrane into which it was inserted. From a quantitative analysis of fluorescence on spermatozoa prelabeled with FITC-2B1 IgG and then capacitated, the amount of antigen that appeared on the acrosomal domain was approximately equivalent to that lost from the midpiece domain. The Mr of 2B1 antigen extracted from capacitated spermatozoa was 300-500 Da less than that extracted from noncapacitated cells, suggesting that the molecule had undergone processing concomitant with migration. These results are discussed in relation to mechanisms for targeting antigens to sites where they become physiologically active and are correctly positioned to participate in gamete recognition processes.

Changes in the distribution of intramembranous particles and filipin-reactive membrane sterols during in vitro capacitation of golden hamster spermatozoa

Membrane alterations accompanying in vitro capacitation of hamster spermatozoa were examined using the freeze-fracture technique with or without use of filipin, a sterol-binding probe. In the spermatozoa prior to or at 10 min after start of incubation in capacitating medium, large (about 11 nm) and small (8-9 nm) intramembranous particles (IMPs) were present in the periacrosomal region of the sperm plasma membrane (PAPM). Filipin sterol complexes (FSCs) were densely (about 500/micron 2) distributed in the PAPM prior to incubation. The density of FSCs in the PAPM was reduced by 70-80% of the original density by 2 hr of incubation. At the same time, small patches of IMP-free areas appeared in the plasma membrane above the equatorial and middle segments of the acrosome. By the end of 3 hr of incubation, the majority of small IMPs had disappeared from the PAPM. Remaining large and small IMPs tended to aggregate in the PAPM. During incubation in capacitation medium, "cords," or linear arrangements of closely packed IMPs, appeared near the posterior ring of the sperm head. These observations strongly suggest that the acrosome reaction of the hamster spermatozoa is preceded by the removal (deletion) of filipin-reactive sterols (FRSs) and the disappearance of small IMPs from the lipid bilayer of PAPM.

Changes in the distribution of intramembranous particles and filipin-sterol complexes during epididymal maturation of golden hamster spermatozoa

Golden hamster spermatozoa in various segments of the excurrent duct system were studied by freeze-fracture with and without filipin treatment. Two types of regular IMP (intramembranous particle) patterns temporarily appear on the plasma membrane covering the sperm head. One is a hexagonal arrangement seen in the acrosomal region, and the other is a linear arrangement near the posterior ring. Both patterns are seen in the spermatozoa from the corpus epididymidis. The FSC (filipin-sterol complex) density in the plasma membrane covering the acrosome increases from about 400 to 500 FSC/microns2 during epididymal passage. In this region, the majority of the membrane sterols appears to reside on the outer leaflet of the lipid bilayer. When the spermatozoa reach the cauda epididymidis, FSCs in the outer acrosomal membrane virtually disappear from the apical segment, while they increase in the middle segment (250 FSC/microns2). These observations are discussed in relation to epididymal maturation.

Maturation changes of the plasma membrane of rat spermatozoa observed by surface replica, rapid-freeze and deep-etch, and freeze-fracture methods

Rat spermatozoa from the epididymis and ductus deferens were observed by surface replica, rapid-freeze and deep-etch, and conventional freeze-fracture methods. By the surface replica method, parallel periodical ridges were observed in the acrosomal region of the spermatozoa from the distal part of the cauda epididymis (zone 6) and from the ductus deferens. The periodicity of the ridges forming a domain was about 35 nm. A quantitative analysis of the spermatozoa along the reproductive tract indicated that 39.4% and 73.5% of the population in zone 6 of the epididymis and in the ductus deferens, respectively, had the domain. None of the spermatozoa from zone 1 through zone 5 had the domain. The results of the rapid-freeze and deep-etch procedure showed that the ridges observed by the surface replica method consisted of linear arrangements of elliptical particles on the ES face of the plasma membrane. The particles were about 30 nm in length and 15 nm in width. On the corresponding PF face of the plasma membrane, linear arrangements of the intramembrane particles (IMPs) of about 8 nm in diameter were observed by both the deep-etch and freeze-fracture methods. The IMPs tended to run in paired parallel lines. A close relationship was observed between the lines of the elliptical particles on the ES and of the IMPs on the PF faces. The elliptical particle may be a protruded part of the IMP(s) or other protein(s) bound to the IMP(s).

Filipin-sterol complexes in golden hamster sperm membranes with special reference to epididymal maturation

The distribution of membrane filipin-sterol complexes (FSCs) was qualitatively surveyed on freeze-fracture replicas of spermatozoa from the male reproductive tract and ejaculates of golden hamster. In the head, the acrosomal plasma membrane showed the strongest filipin labeling on the principal segment, but it was absent in the "quilt-like pattern" areas. These latter were observed in both caput and corpus epididymal spermatozoa, but were absent in mature spermatozoa. The postacrosomal plasma membrane had few FSCs and both the outer and inner acrosomal membranes were always negative to filipin. The nuclear membrane of the principal segment was constantly filipin-positive. The nuclear membrane of the postacrosomal region had more FSCs than that of the principal segment, particularly in mature spermatozoa. Many linear, "rod-like FSCs" were observed on the postacrosomal nuclear membrane of mature spermatozoa, especially in the uterine spermatozoan samples. In the neck, the plasma membrane had only a few FSCs. The redundant nuclear membrane was slightly filipin-positive, while the membrane scroll of mature spermatozoa was heavily labeled. In the tail, the plasma membrane of both the middle and principal piece was moderately labeled.

Distribution of sterols and anionic lipids in human sperm plasma membrane: effects of in vitro capacitation

Filipin, a membrane beta-hydroxysterol probe, and polymyxin B (PXB), a probe for anionic lipids, were used to study human sperm plasma membrane (PM) with particular reference to changes induced by capacitation in vitro. In washed but noncapacitated spermatozoa the density of filipin/sterol complexes (FSC) was uniformly high in the PM overlying the acrosome, without any differences between its anterior and equatorial regions. The postacrosomal region was poor in FSC. The exclusion of FSC from small areas of the PM covering the two acrosomal regions and reduction of their number in the postacrosomal region were the main changes induced by sperm capacitation. Unlike filipin, PXB produced a capacitation-independent reaction pattern characterized by a high reactivity of the PM covering the anterior portion of the acrosome, probably conferring to this region its potential fusigenicity temporarily antagonized by the elevated sterol content. Only an exceptional and slight PXB binding was seen in the nonfusigenic equatorial and postacrosomal regions.

Membrane changes in Chinese hamster spermatozoa during epididymal maturation

Chinese hamster spermatozoa during epididymal maturation were examined by thin sectioning, freeze-fracture, and surface replica. Membrane-limited vesicles and tubules (MVTs) attach to the plasma membrane over the acrosome of the spermatozoa in the distal caput through proximal cauda epididymidis. The origin of these MVTs is likely to be spermatozoa degenerating in the epididymal lumen. The attachment of MVTs to the plasma membrane seems to be mediated by a paste-like substance covering the plasma membrane. A parallel striation pattern of intramembranous particles (IMPs) is seen in the plasma membrane of almost the entire postacrosomal region of the epididymal spermatozoa. The patterned domain begins to appear in the proximal caput epididymidis. The number and density of IMPs in the plasma membrane of the postacrosomal region increases with the development of striated pattern of IMPs in this region. In the cauda epididymidis, the redundant nuclear envelope elongates to form a shirt-like membrane covering the mitochondrial sheath. The elongated portion of the nuclear envelope is devoid of nuclear pores and has few IMPs.