Membrane heterogeneity in the mammalian spermatozoon

Sperm selection based on electrostatic charge

Charge is a fundamental property of all forms of matter that exhibit attraction or repulsion in the presence of another charged particle. This electrokinetic property occurs when the particles exhibiting a net negative or positive charge are subjected to an external electric field that exerts an electrostatic force between them. Sperm surface membranes exhibit varying levels of electrostatic potential that are proportional to the levels of sialic acid residue acquired on the cell surface during maturation. Electrostatic charge-based sperm separation is a recently developed technique that uses an electric field to isolate mature sperm with reduced levels of DNA fragmentation. Two methods for the separation of sperm based on electrostatic charge, the Zeta method and a commercially available electrophoretic method using the SpermSep Cell Sorter 10, are discussed in this chapter including a detailed protocol for sperm separation based on the Zeta method.

Compartmentalization of proteins in epididymosomes coordinates the association of epididymal proteins with the different functional structures of bovine spermatozoa

Epididymosomes are small membranous vesicles secreted by epithelial cells within the luminal compartment of the epididymis. In bovine, many proteins are associated with epididymosomes, and some of them, such as the glycosylphosphatidylinositol (GPI)-anchored protein P25b, macrophage migration inhibitory factor (MIF), and aldose reductase (AKR1B1), are transferred to spermatozoa during the epididymal maturation process. P25b is associated with detergent-resistant membrane (DRM) domains of epididymal spermatozoa, whereas MIF and AKR1B1 are cytosolic proteins associated with detergent-soluble fractions. In this study, we tested the hypothesis that DRM domains are also present in the epididymosomes and that P25b DRM-associated proteins in these vesicles are transferred to the DRMs of spermatozoa. The presence of DRMs in epididymosomes was confirmed by their insolubility in cold Triton X-100 and their low buoyant density in sucrose gradient. Furthermore, DRMs isolated from epididymosomes are characterized by the exclusive presence of ganglioside GM1 and by high levels of cholesterol and sphingomyelin. Biochemical analysis indicated that P25b is linked to DRM in epididymosomes, whereas MIF and AKR1B1 are completely excluded from these membrane domains. Proteolytic treatment of epididymosomes and immunoblotting studies showed that P25b is affected by trypsin or pronase proteolysis. In contrast, MIF and AKR1B1 are not degraded by proteases, suggesting that they are localized within epididymosomes. Interaction studies between epididymosomes and epididymal spermatozoa demonstrated that P25b is transferred from the DRM of epididymosomes to the DRM of the caput epididymal spermatozoa as a GPI-anchored protein. Together, these data suggest that specific localization and compartmentalization of proteins in the epididymosomes coordinate the association of epididymal proteins with the different functional structures of spermatozoa.

Ultrastructural observations of cryoinjury in kangaroo spermatozoa

Macropod spermatozoa have proven difficult to cryopreserve such that empirical studies using high concentrations of glycerol and/or DSMO have resulted in only 10% post-thaw motility. We examined the ultrastructure and freeze-fracture of caput and cauda epididymal macropod spermatozoa at 35, 4 degrees C and following cryopreservation with and without 20% glycerol. The addition of 20% glycerol resulted in significant damage to the sperm plasma membrane and mitochondria compared to no glycerol at the same temperatures (P<0.05). Following cryopreservation, 20% glycerol significantly improved the preservation of the cauda epididymal sperm plasma membrane and mitochondria and reduced the incidence of axonemal damage and axonemal spaces. For caput epididymal spermatozoa, glycerol only improved the preservation of the plasma membrane following cryopreservation (P<0.05). Freeze fracture microscopy revealed a pattern of helically wound intramembranous particles in the plasma membrane over the fibre network of the mid piece of the sperm tail. The fibre network is an interconnecting cytoskeletal structure found underneath the plasma membrane of the kangaroo sperm midpiece and is thought to add rigidity to the proximal portion of the sperm tail. After thawing, the plasma membrane was damaged such that this structure was missing in patches, and the helical rows of particles were mal-aligned. On the principal piece, particles were arranged randomly at physiological temperatures; however, upon cooling to 4 degrees C with 20% glycerol, the particles become aggregated. Once rewarmed (35 degrees C), particles over the principal piece resumed their random organisation. This finding is further evidence of a reversible phase transition of the macropod sperm plasma membrane during cooling that is not associated with a loss of motility or membrane integrity.

Phospholipase-C sensitive GPI-anchored proteins of goat sperm: possible role in sperm protection

The role of glycosylphosphatidylinositol (GPI)-anchored sperm proteins in reproduction has been investigated. SDS-polyacrylamide gels (PAGE) analysis of goat sperm (Capra indica) indicated that several GPI-anchored proteins were released by phosphatidylinositol-specific phospholipase-C (PI-PLC) treatment. The distribution of this category of PI-PLC-sensitive GPI-anchored proteins on the surface of sperm was examined by indirect immunofluorescence. The fluorescence microscopic study clearly demonstrated that the PI-PLC-sensitive GPI-anchored proteins are confined predominantly to the head region of goat sperm. Further experiments were conducted on intact and PI-PLC treated sperm in order to decipher the function of GPI proteins. Co-incubation of sperm with peritoneal macrophages led to the enhanced phagocytosis of PI-PLC treated sperm by macrophages compared with the untreated intact sperm. Transmission electron micrographs of the macrophages acquired from the phagocytosis assay are provided to corroborate the same. From the results obtained it is inferred that one or more of the PI-PLC-sensitive GPI-anchored proteins on the sperm surface could act as protection factor(s) that shield the sperm from macrophages.

The significance of cooling rates and animal variability for boar sperm cryopreservation: insights from the cryomicroscope

Once the first methods for freezing mammalian semen had been established, research aimed at improving cryopreservation procedures became highly focused on the interactions between cooling rates and the permeability of the plasma membrane to water and cryoprotectants. This was based on the premise that cooling rates could be optimized from a theoretical basis for different species of interest. While this approach has stimulated considerable research, it has not achieved its original aim at the species level, largely because it overlooks inter-individual variation in sperm biochemical composition and physiology. If the underlying hypothesis is valid, however, optimal cooling rates should be identifiable for spermatozoa from individual animals. Experiments with the cryomicroscope revealed that while sperm survival after cryopreservation varied considerably between boars, there was little evidence that optimal freezing rates could be identified for individuals. Based on these findings, we tested the hypothesis that sperm susceptibility to cryoinjury is a consistent feature of each individual, but those individuals differ in susceptibility. This hypothesis was supported by evidence from an experiment with >100 boars; moreover, using genetic analyses, we demonstrated genomic differences between individual boars that correlated with post-thaw sperm quality.

Compartmentalisation of the sperm plasma membrane: a FRAP, FLIP and SPFI analysis of putative diffusion barriers on the sperm head

Spermatozoa are highly polarised cells with a compartmentalised distribution of lipids and proteins in their plasma membrane. It is not known how these compartments are stably maintained in what is essentially a fluid environment. In this investigation we have examined the hypothesis that intramembranous diffusion barriers selectively retain some components within compartments, while allowing free passage of others. A fluorescence loss in photobleaching analysis of the behaviour of the lipid reporter probe 1,1′-dihexadecyl-3,3,3′3′-tetramethyindocarbocyanine (DiIC16) on the head of boar spermatozoa revealed that it was freely diffusing between all three compartments (anterior acrosome, equatorial segment and postacrosome). Spermatozoa also contained rapidly diffusing particles of DiIC16 over the anterior acrosome and equatorial segment. These particles, ∼200 nm in diameter, were tracked in real time and their trajectories analysed by mean square displacement. Particle diffusion was essentially random over the anterior acrosome and equatorial segment but showed a periodicity in jump sizes and diffusion coefficients suggestive of microheterogeneities. Particles did not exchange between the equatorial segment and postacrosome, indicating a barrier at the junction between these two compartments. No barrier was detected between the equatorial segment and anterior acrosome. A model is proposed in which a molecular `filter' is present at the equatorial segment-postacrosomal boundary that allows free passage of single molecules but not molecular complexes. Passage of heterogeneous complexes, such as lipid rafts, requires disassembly and reassembly on either side of the filter.

A hierarchy of lipid constructs for the sperm plasma membrane

We have presented a series of lipid constructs as models of the sperm plasma membrane. We also isolated the plasma membrane from rabbit sperm cells and characterized the lipid composition. The behavior of these various membrane systems was evaluated using a vesicle leakage assay, in which surfactant (nonoxynol-9, N-9; or benzalkonium chloride, BZK) exposure induced membrane permeabilization. These studies shed light on the relative importance and significance of particular components present in the lipid constructs. In particular, a highly unsaturated phospholipid component characterized by an ether-linkage to position 1 of the glycerol backbone (as opposed to the more conventional ester linkage) as well as the presence of sulfogalactosyl ceramide were found to have an effect on the surfactant-induced leakage response. The presence of cholesterol had the greatest influence on membrane behavior. The construct series also demonstrated the ability of the surfactants studied to discriminate between different membrane systems. We found that N-9 displayed little sensitivity to membrane composition while BZK showed specific behavior with the various membrane systems.

Optimizing and quantifying fusion of liposomes to mammalian sperm using resonance energy transfer and flow cytometric methods

BACKGROUND

Liposomes are used to carry pharmaceutical agents and to alter the lipid composition of cell membranes. This study compared resonance energy transfer (RET), fluorescence dequenching, and flow cytometry as monitors and quantifiers of fusion between liposomes and mammalian spermatozoa.

METHODS

Preliminary experiments used RET to determine the optimum sperm concentration for fusion of DL-alpha-phosphatidylcholine dipalmitoyl (PC)/DL-alpha-phosphatidylethanolamine dipalmitoyl (PE) liposomes at 35 degrees C +/- 5 mM Ca2+. Microscopy confirmed the fusion of liposomes, not just adhesion (n = 3). Dequenching tested the time-dependent fusion of liposomes of two different lipid compositions to sperm, both, (n = 3) +/- 1 mM Ca2+ and (n = 3) without Ca2+ at two sperm concentrations. Finally, flow cytometry absolutely quantified the percentage of sperm fusing to liposomes at different liposome-to-sperm ratios (n = 4) and with sperm from different donors (n = 3).

RESULTS

RET detected fusion of liposomes with sperm and microscopy confirmed the interaction to be true fusion. Dequenching detected more fusion of liposomes with sperm at 100 x 10(6) sperm per milliliter than at lower concentrations (P < 0.05). Fusion dynamics differed with lipid composition but Ca2+ had no effect. Flow cytometry reliably quantified the percentage of sperm fusing with liposomes, which varied from bull to bull (P < 0.05).

CONCLUSION

Liposome fusion with mammalian sperm membranes can be quantified cytometrically and varies with lipid composition, sperm-to-liposome ratio, and individual animals.

A short hypoosmotic swelling test for the prediction of boar sperm fertility

This study was designed to explore the relationship between the ejaculate response to a hypoosmotic swelling test (HOST) and in vivo fertility in a group of 38 boars The hypoosmotic test used was a modification of the HOST that involves a shorter incubation time (5 vs 120 min) and lower osmotic pressure (75 vs 150 mOsm/kg). Ejaculates containing less than 20% abnormal spermatozoa were selected and checked for percentage of motility, percentage of normal acrosomes, percentage of short ORT and percentage of cells showing positive short HOST (sHOST) results Two hundred eightyeight sows were inseminated to obtain in vivo fertility and prolificacy data. No differences were shown between technicians in the sHOST results obtained. Significant differences were recorded between boars in sHOST results (p < 0.002). Only the sHOST result presented a significant correlation with in vivo fertility (r = 0.43, p < 0.01). Short HOST data significantly improved fertility prediction of routine semen analysis tests. Unlike motility and acrosomal status, sHOST values showed a significant decrease when fresh ejaculates (37 degrees C) were stored for 24 h at 15 degrees C, indicating possible damage due to cold shock.

Diffusion barriers in ram and boar sperm plasma membranes: directionality of lipid diffusion across the posterior ring

The plasma membrane of mammalian spermatozoa, like that of other differentiated cells, is compartmentalized into discrete regions or domains that are biochemically and functionally distinct from one another. Physical structures within the membrane, such as the posterior ring at the juncture of the sperm head and tail, have long been thought to act as diffusion barriers to help segregate important molecules required for fertilization within specific domains and to regulate migration of molecules between domains. In this investigation, we used a quantitative photobleaching technique (video-FRAP) to assess the efficacy of the posterior ring as a barrier to exchange of lipids between the postacrosomal and midpiece plasma membranes. A lipid reporter probe (1,1'-diduodecyl-3,3,3', 3'-tetramethylindocarbocyanine; DiIC(12)) was incorporated into the plasma membrane of live ram and boar spermatozoa, and the directionality of its diffusion across the posterior ring was measured by line-profile analysis. Results showed that DiIC(12) was able to traverse the posterior ring from the direction of the postacrosomal plasma membrane and to diffuse onto the midpiece plasma membrane. These results suggest that the posterior ring is not an immutable barrier to lipid exchange in mature spermatozoa and that there are other mechanisms for maintaining in-plane lipid asymmetry, such as differential phase behavior and interaction with the submembranous cytoskeleton.

Basic aspects of frozen storage of semen

Basic concepts of cryopreservation and the causes of cryoinjury are reviewed. The possible roles of cryoprotectants and additives are considered in the context of their putative interactions with the sperm plasma membrane. Modern approaches to the laboratory assessment of spermatozoa after freeze-thawing are also briefly discussed.

Dynamics in the membrane organization of the mammalian sperm cell and functionality in fertilization

The capacitation process of sperm cells involves complex changes in the composition and orientation of molecules at the surface of the sperm cell. Here we focus on the lipid architecture in the sperm plasma membrane and demonstrate that the sperm plasma membrane is not static but is an extremely dynamic structure. Advanced fluoroscopic techniques enabled continuous monitoring of lipid organization in living cells and extremely rapid lipid movements were observed. The orientation of lipids in the sperm plasma membrane changed under capacitative treatments, was found to be sensitive for temperature and also changed upon binding of sperm cells to the zona pellucida. The changes in membrane properties coincided with an activation of protein kinases resulting in tyrosine phosphorylation of specific plasma membrane proteins. The detected membrane changes relate to intrinsic membrane properties such as fluidity, permeability, adhesiveness and fusibility. We think that these results may provide a physiological basis for new assays, able to discriminate between functional and non-physiological sperm cells.

Lipid dynamics in the plasma membrane of fresh and cryopreserved human spermatozoa

Preserving the integrity of the plasma membrane of spermatozoa is crucial for retention of their fertilizing capacity, especially after stressful procedures such as freezing and storage. In this investigation we have measured lipid diffusion in different regions of the plasma membrane of fresh and cryopreserved human spermatozoa using a sensitive, high resolution fluorescence photobleaching technique (FRAP) with 5-(N-octadecanyl)aminofluorescein as reporter probe. Results show that diffusion was significantly faster on the plasma membrane overlying the acrosome and decreased progressively in the postacrosome, midpiece and principal piece. The midpiece plasma contains a higher proportion of immobile lipids than other regions. In cryopreserved spermatozoa, lipid diffusion in the plasma membrane was significantly reduced on the acrosome, postacrosome and midpiece relative to fresh spermatozoa. Diffusion, however, could be restored to normal levels by washing spermatozoa in a medium containing 0.4% polyvinylpyrrolidine but not in medium alone or in medium containing 0.4% albumin. These results suggest that (i) lipid dynamics in the plasma membrane of human spermatozoa varies significantly between surface regions; (ii) in-plane diffusion is adversely affected by cryopreservation; and (iii) washing frozen spermatozoa in 0.4% polyvinylpyrrolidine restores membrane lipid fluidity to normal levels. The latter finding has important implications for improving the fertility of human spermatozoa following cryopreservation.

Regionalized lipid diffusion in the plasma membrane of mammalian spermatozoa

The plasma membrane of mammalian spermatozoa shows pronounced lateral asymmetry with many glycoproteins restricted to specific domains. Some of these antigens are freely diffusing throughout the membrane whereas others appear static in position. It is not clear whether these concepts also apply to membrane lipids. In this investigation we have used fluorescence recovery after photobleaching (FRAP) techniques to spatially resolve lipid dynamics in various surface domains of 5 species of mammalian spermatozoa (bull, boar, ram, mouse, and guinea pig). Sperm plasma membranes were loaded with 5-(N-octadecanoyl)aminofluorescein (ODAF) reporter probe, and its diffusion was measured in various domains by FRAP analysis. Results showed that in live bull, boar, ram, and mouse spermatozoa, diffusion coefficients (D) were significantly higher over the acrosome and postacrosome than on the midpiece and principal piece of the tail. In dead or permeabilized cells, on the other hand, large immobile phases developed, particularly on the sperm tail, that severely reduced D values. ODAF diffusion was also sensitive to temperature and cross-linking of protein components within the membrane with paraformaldehyde. Guinea pig spermatozoa were different in almost all respects from those of the other species tested. It is concluded that lipid diffusion in the plasma membrane of live spermatozoa varies significantly between surface domains, because of either compositional heterogeneity, or differences in bilayer disposition, or the presence of intramembranous barriers that impede free exchange between domains. This study emphasizes the important role of membrane lipids in regulating polarized migration of sperm surface antigens during developmental processes such as maturation and capacitation.

Quantitative changes in sperm head morphology during passage through the male excurrent duct system of the rabbit

A fine adjustment of sperm head size and shape occurs during maturation and storage within the male excurrent duct of the rabbit. This remodelling, as judged by morphometric values of area, perimeter, length, width, and shape factors, takes place mostly in passage from the seminiferous tubules of the testis to the distal caput of the epididymis. The dimensions of sperm heads from the distal corpus of the epididymis break the general tendency toward a reduction in size and more elliptical shapes. A period of transport and storage within the epididymal cauda and vas deferens follows in which there are no further changes in sperm head morphometry. It can be concluded that the period immediately following sperm release from the testis is crucial to the final morphological maturation of spermatozoa. Moreover, the fact that changes are detected in the appearance of sperm heads at successive stages of sperm maturation suggests that the dimensions of a particular epididymal spermatozoon may be taken as an approximate indication of its relative maturity.

Targeting of the domain-specific integral membrane protein PM52 to the periacrosomal plasma membrane during guinea pig spermiogenesis

The sperm plasma membrane is segregated into functionally, biochemically, and structurally distinct domains yet the protein sorting pathways and assembly mechanisms that assemble these domains during spermiogenesis are incompletely understood. We previously characterized two structurally related size-variant, integral membrane proteins of 52 kDa (PM52) and 35 kDa localized to the periacrosomal plasma membrane of guinea pig cauda epididymal spermatozoa (Westbrook-Case et al., 1994). In this study we used light and electron microscopic immunocytochemistry to define the expression pattern and sorting pathway that establishes the domain-specific distribution of PM52 during spermiogenesis. The PM52 is first expressed in acrosome-phase spermatids and it localizes exclusively to the cytoplasmic lobe. Immunoelectron microscopy revealed that both cytoplasmic vesicles and the plasma membrane of the cytoplasmic lobe labeled with anti-PM52. During early stages of expression, PM52 appeared to be absent from the head region, but significant PM52 accumulation over the spermatid head was noted in late acrosomal phase spermatids. Throughout spermiogenesis PM52 extended posteriorly to the annulus, which represents a barrier preventing PM52 diffusion into the posterior tail. Following the migration of the annulus to the midpiece-principal piece junction, PM52 began to disappear from the flagellar region and at the completion of spermiogenesis most of the PM52 was restricted to the acrosomal segment. Spermatids and epididymal sperm PM52 exhibited identical sizes by SDS-PAGE and immunoblotting, indicating that they are not proteolytically modified during epididymal maturation. The PM52 antibodies were also used to screen a guinea pig testis cDNA library, and sequence determination of full-length PM52 clones demonstrated identity of a sperm membrane protein recently termed "sperad" (Quill and Garbers, 1996). Membrane barriers and potential mechanisms establishing the domain-specific residence of PM52 are discussed.

Lateral mobility of plasma membrane lipids in bull spermatozoa: heterogeneity between surface domains and rigidification following cell death

Compartmentalization of surface membrane antigens into discrete regions or domains is a characteristic feature of differentiated cells. In mammalian spermatozoa at least 5 surface domains are known, implying the presence of barriers or boundaries within the plasma membrane. Using the technique of fluorescence recovery after photobleaching (FRAP) to measure diffusibility of fluorescent lipid analogues 1,1′-dihexadecyl-3,3,3′3′-tetramethylindocarbocyanine (DiIC[16]) and 5-(N-octa-decanoyl) aminofluorescein (ODAF), we have investigated lipid topology and dynamics in the plasma membrane of ejaculated bull spermatozoa. Contrary to reports in the literature, we have found that DiIC(16) stains only dead or damaged spermatozoa whereas ODAF intercalates into the plasma membrane of both live and dead cells, each type showing a distinctive staining pattern. FRAP analysis with ODAF revealed that diffusion coefficients on live spermatozoa are significantly faster on the acrosome and postacrosome (29.3x10(−9) cm2/second) than on the midpiece and principal piece (11.8x10(−9) cm2/second). Recovery (R) is &gt;90% in all domains. ODAF diffusion also shows regionalized temperature-sensitivity with a 4-fold increase over the sperm head and a 1.8-fold increase on the tail between 20 degrees C and 37 degrees C. Remarkably, dead or permeabilized spermatozoa rapidly develop a large immobile phase (R&lt;25%) over the whole plasma membrane. This rigidification is temperature insensitive and irreversible suggesting major changes in the physical state of membrane lipids. It is concluded that lipid diffusion in the plasma membrane of live bull spermatozoa is rapid and varies significantly between surface domains. Following permeabilization or cell death, however, a large immobile phase develops indicating substantial changes in membrane lipid disposition.

Ultrastructural localization of acrosome reaction-inducing substance (ARIS) on sperm of the starfish Asterias amurensis

Using colloidal gold tagged ligands we have identified the ultrastructural site of ARIS binding to intact and acrosome-reacted starfish sperm. In intact sperm, colloidal gold conjugated ARIS was specifically localized to a single domain (0.1-0.3 micron in diameter) on the plasma membrane. This site was located on the anterior-lateral aspect of the sperm head, that is, just peripheral to the region occupied by the acrosomal vesicle and periacrosomal components. When sperm were labeled with colloidal gold conjugated ARIS, washed to remove unbound label, and then induced to undergo the acrosome reaction, the labeled patch remained associated with the plasma membrane and was positioned just lateral to the acrosomal process. However, when sperm were suspended in labeled ARIS and induced to undergo the acrosome reaction, label was observed along the entire anterior aspect of the sperm head with the exception of the acrosomal process. Labeling along the entire anterior aspect of the sperm head in this case was deemed to be nonspecific and due to binding of colloidal gold tagged molecules to components formerly located within the acrosomal vesicle, as the same pattern was obtained using colloidal gold tagged bovine serum albumin. Quantitative and qualitative aspects of ARIS binding observed here by electron microscopy are in agreement with measured binding characteristics previously reported (Ushiyama et al., 1993a: Zygote 1:121-127; Ushiyama et al., 1993b: J Reprod Dev 39:53-54), and indicate that the site of labeled ARIS binding represents a specific plasma membrane domain occupied by ARIS receptors.

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.

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.

Compartmentalization, processing and redistribution of the plasma membrane protein CE9 on rodent spermatozoa. Relationship of the annulus to domain boundaries in the plasma membrane of the tail

Western blotting, immunofluorescence and immunogold electron microscopy were used to examine the compartmentalization, processing and redistribution of the integral plasma membrane protein CE9 on the spermatozoa of rats, mice and hamsters. In each species examined, spermatozoal CE9 was found to undergo endoproteolytic processing followed by a net redistribution from the posterior-tail domain into the anterior-tail domain of the plasma membrane during epididymal maturation. Compared to spermatozoa of the rat and mouse, those of the hamster were found to express a greater proportion of their CE9 within the anterior-tail plasma membrane domain at all stages of maturation. As a consequence, CE9 was judged to be a suitable marker for two different spermatozoal plasma membrane domains: the posterior-tail plasma membrane domain (spermatozoa from the testis and caput epididymidis of the rat and mouse) and the anterior-tail domain (spermatozoa from the cauda epididymidis of the hamster). Immunogold electron microscopy was used to pinpoint the positions of the boundaries of these CE9-containing plasma membrane domains at a high level of resolution. In each case, the position of the CE9 domain boundary was found to be strongly correlated with that of the subplasmalemmal electron-dense ring known as the annulus. The precise spatial relationship between the CE9 domain boundary and the annulus was, however, found to differ significantly among species and/or as a function of maturation.

Morphometric analysis of intact sperm heads and of sperm nuclei in the mouse

A morphometric analysis of mouse sperm and of their nuclei was undertaken to investigate their respective post-testicular maturation. Sperm were collected from the testis, caput and cauda epididymidis, and their corresponding nuclei were isolated. Results indicate that the post-testicular maturation of sperm is distinct from that of nuclei. The size of intact sperm heads increases in the caput followed by a subsequent decrease in the cauda. In contrast, sperm nuclei decrease progressively in size. In general, a greater magnitude and number of alterations in intact heads and nuclei occur while in transit from the testis to the caput than during passage to the cauda epididymis. These results suggest that the period immediately following their release from the testis is crucial to the complete morphological maturation of sperm heads and nuclei.

Breaching the diffusion barrier that compartmentalizes the transmembrane glycoprotein CE9 to the posterior-tail plasma membrane domain of the rat spermatozoon

CE9 is a posterior-tail domain-specific integral plasma membrane glycoprotein of the rat testicular spermatozoon. During epididymal maturation, CE9 undergoes endoproteolytic processing and then redistributes into the anterior-tail plasma membrane domain of the spermatozoon (Petruszak, J. A. M., C. L. Nehme, and J. R. Bartles. 1991. J. Cell. Biol. 114:917-927). We have determined the sequence of CE9 and found it to be a Type Ia transmembrane protein identical to the MRC OX-47 T-cell activation antigen, a member of the immunoglobulin superfamily predicted to have two immunoglobulin-related loops and three asparagine-linked glycans disposed extracellularly. Although encoded by a single gene and mRNA in the rat, the majority of spermatozoal CE9 is of smaller apparent molecular mass than its hepatocytic counterpart due to the under-utilization of sites for asparagine-linked glycosylation. By fluorescence recovery after photobleaching, CE9 was determined to be mobile within the posterior-tail plasma membrane domain of the living rat testicular spermatozoon, thus implying the existence of a regional barrier to lateral diffusion that is presumed to operate at the level of the annulus. Through the development of an in vitro system, the modification of this diffusion barrier to allow for the subsequent redistribution of CE9 into the anterior-tail domain was found to be a time-, temperature-, and energy-dependent process.

Plasma membrane alterations of maturing goat (Capra indicus) spermatozoa: lectin-binding and freeze-fracture study

A qualitative and quantitative analysis of lectin-binding sites has been undertaken on spermatozoa recovered from different regions of the epididymis of the goat (Capra indicus) using fluorescein isothiocyanate-linked lectins (Bauhinia purpurea BPA, Concanavalin A Con A, Dolichos biflorus DBA, Maclura pomifera MPA, Arachis hypogaea or peanut agglutinin PNA, Glycine max or soyabean agglutinin SBA, Ulex europaeus UEA, and Triticum vulgaris or wheat-germ agglutinin WGA), in conjunction with scanning and transmission electron microscopy, and freeze-fracture techniques. Flow cytometric analysis has also been used to quantitize binding affinity. Spermatozoa from caput to cauda epididymidis show no significant variation in lectin-binding ability, but the samples removed from the corpus epididymidis contain a greater number of binding sites. The passage of spermatozoa through the epididymidis is accompanied by a redistribution of the plasma membrane lectin-receptors covering the sperm head and tail. Receptors for BPA, DBA, PNA and SBA are specifically restricted to the anterior region of the acrosome in caudal spermatozoa. Freeze-fracture replicas, examined to study changes in organisation of intramembranous particles of the plasma membrane during sperm maturation, reveal distinct changes in their distribution in the acrosome, post-acrosome and spermatozoon tail, especially in the corpus and cauda epididymidis.

Detection of a sperm-coating antigen in the semen of Bubalus bubalis

1. Surface antigens of B. bubalis spermatozoa were solubilized by Triton X-100 and EDTA; the sperm extract was used to raise antibodies in rabbits. 2. Two major polypeptides, immunoprecipitated from the seminal plasma by the antibodies against the sperm extract, exhibited the same electrophoretic mobilities of two immunorelated sperm surface antigens. 3. The two polypeptides were isolated from the seminal plasma, by a multi-step chromatographic procedure, and found subunits of a single protein (MW 30,000), called SP 30. 4. The SP 30 protein bound in vitro to the postacrosomal region of homologous spermatozoa from cauda epididymis. 5. The localization of the sperm-coating antigen on the cell surface is compatible with a role in the fertilization process.

Surface heterogeneity of bovine sperm revealed by aqueous two-phase partition

Ejaculated, bovine sperm have been subjected to multiple partition in aqueous two-phase systems. This partition, carried out in a countercurrent fashion, reveals heterogeneity of the sperm population with respect to surface properties. The sperm, when partitioned in phase systems that detect non-change associated surface properties (change-insensitive) are largely distributed as two distinct populations. In charge-sensitive phase systems (which principally detect cell surface molecules carrying charge) the sperm do not show any obvious surface heterogeneity. Considerable heterogeneity is revealed in affinity-ligand phase systems containing palmitic acid coupled to one of the phase components-poly(ethylene glycol). There is a difference in surface heterogeneity between sperm which have been washed in buffer or left unwashed, direct from the ejaculate. This is indicative of weak adsorption of proteins to the sperm surface in seminal fluid. These results show that bovine ejaculated sperm is a heterogeneous cell population having unequal distributions of a number of different surface molecules.

Endoproteolytic cleavage in the extracellular domain of the integral plasma membrane protein CE9 precedes its redistribution from the posterior to the anterior tail of the rat spermatozoon during epididymal maturation

Originally identified as a basolateral domain-specific integral plasma membrane protein of the rat hepatocyte, CE9 mRNA and protein were also detected at high levels in the testis of the rat by Northern and Western blotting and immunoprecipitation. CE9 proved to be a domain-specific integral plasma membrane protein of the rat spermatozoon: on testicular spermatozoa, it was concentrated within the posterior tail domain of the plasma membrane, whereas on vas deferens spermatozoa, CE9 was concentrated within the anterior tail domain. This change in the localization of CE9 was observed to take place in a offgressive fashion during the passage of the spermatozoa from the caput epididymidis to the cauda epididymidis and was preceded by the specific endoproteolytic cleavage of CE9 in the proximal portion of the caput epididymidis. Amino-terminal amino acid microsequencing of CE9 immunoaffinity purified from epididymis suggested that the cleavage occurred on the carboxy-terminal side of arginine-74 in the primary sequence of CE9, resulting in the loss of approximately 40% of the amino acids in the extra-cellular domain of this transmembrane glycoprotein.

Zinc-protein from rat prostate fluid binds epididymal spermatozoa

The detection and the isolation of a zinc-protein from the secretion of the rat dorsolateral prostate is described. The purification procedure, based on gel filtration and cationic exchange chromatography, allowed to separate a minor protein (Mr approximately 66,000) from free zinc ions and other secretory components. Two zinc ions were estimated to be associated with one molecule of isolated protein. The zinc-protein was labelled with 125I and then incubated at 37 degrees C with spermatozoa from rat epididymal cauda. Time-dependent in vitro binding of the radioactive protein to sperm cells was demonstrated. This binding was not affected by the presence of proteins from the seminal vesicle during the incubation, while it was blocked in the presence of an excess of unlabelled zinc-protein. After binding, the labelled spermatozoa were treated with a buffer containing 0.5% sodium deoxycholate and 40 mM EDTA; only very small amounts of label were removed from the cells, thus suggesting that the zinc-proteins were kept on the plasma membrane by interactions which do not involve merely hydrophobic bonds.

Localization of wheat germ agglutinin and antibody binding sites on the plasma membranes of sea urchin sperm heads as revealed by label-fracture and fracture-flip

Freeze-fracture electron microscopy reveals that intramembrane particles are concentrated in a band encircling the posterior portion of the acrosome of Strongylocentrotus purpuratus sperm. Two colloidal gold labeling methods, label-fracture and replica-staining fracture-flip, were employed to show that the plant lectin wheat germ agglutinin, which recognizes a 210 kDa sperm surface glycoprotein, binds to this localized band of intramembrane particles. Monoclonal antibody J18/2, which also recognizes the 210 kDa surface glycoprotein, shows this localized binding in approximately 20% of the sperm observed in this study. The majority of sperm displayed a uniform distribution of receptor sites for monoclonal antibody J18/2. Since wheat germ agglutinin and monoclonal antibody J18/2 are known to agglutinate Strongylocentrotus purpuratus sperm but not sperm of another sea urchin, Lytechinus pictus, similar determinations were made for the latter species. Lytechinus pictus sperm are not labeled with wheat germ agglutinin and are only sparsely labeled with monoclonal antibody J18/2. The acrosomal localizations of wheat germ agglutinin and monoclonal antibody J18/2 receptors in Strongylocentrotus purpuratus sperm are consistent with the involvement of the 210 kDa surface glycoprotein in an egg jelly-induced sperm acrosome reaction. Low-temperature post-embed labeling of thin sections with wheat germ agglutinin and monoclonal antibody J18/2 show concentrations of label within the acrosomal vesicle of Strongylocentrotus purpuratus sperm, suggesting the presence of an intracellular storage site for the 210 kDa glycoprotein.

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.

Hypoosmotic swelling test of sperm

For the diagnosis and evaluation of the therapy for male infertility and for predicting the outcome of AIH and IVF-ET, technically simple, replicable tests that can be performed virtually anywhere and that have definite reliability are required. The results of the HOS test correlate well with the functions of the sperm cell membrane, indicating such aspects as motility, and it is thought to be a particularly effective test of human sperm fertility. Because the HOS test reflects the functions and integrity of the sperm cell membrane, it should be possible to use it to predict the potential for fertilization of frozen sperm.

Spectrin, fodrin and protein 4.1-like proteins in differentiating rat germ cells

The presence and the distribution of proteins of the membrane skeleton in differentiating germ cells of the rat has been investigated. Immunofluorescence and immunoblotting analysis, performed using monoclonal and polyclonal antibodies to human erythroid alpha-spectrin and protein 4.1 and to brain spectrin (fodrin), demonstrated the presence of analogues of spectrin and fodrin in spermatocytes and round spermatids and of protein 4.1-like molecules in spermatocytes, spermatids and spermatozoa. Spectrin and fodrin showed molecular weights comparable to those of their analogues in erythrocytes but a distinct intracellular distribution. Fodrin was localized along the plasma membrane while spectrin appeared associated with the regions of the Golgi apparatus and of the developing acrosome. Antibodies to protein 4.1 recognized molecules with a molecular weight not comparable with that in erythrocytes, and their presence in spermatozoa was confined to specific regions of the head and of the tail.

Membrane specializations associated with the acrosomal complex of sea urchin sperm as revealed by immunocytochemistry and freeze fracture replication

Observations, employing freeze fracture replication and electron microscopic immunochemistry, have been carried out to determine structural correlations of the plasma membrane domain occupied by a 210 kDa protein involved in the acrosomal reaction of sea urchin sperm and recognized by the monoclonal antibody, J10/14 (Trimmer et al.: Cell 40:697-703, 1985; Proceedings of the National Academy of Sciences of the United States of America 83: 9055-9059, 1986). Immunogold-J10/14 staining of acrosome-intact sperm was intense along the flagellum and a narrow collar just posterior to the sperm apex that surrounded the acrosomal complex (acrosomal vesicle and subjacent anterior nuclear fossa containing g-actin). Counts of gold particles revealed a density (average number of particles/micron2 of surface area) eightfold greater along the plasma membrane associated with the acrosomal complex than membrane delimiting the remainder of the sperm head. The collar of J10/14 staining was isomorphic with a dense aggregation of intramembranous particles in the P-face of the plasma membrane and a thin cytoplasmic region that surrounded the acrosomal complex. In acrosome-reacted sperm, intense J10/14 staining was distributed along the flagellum and sperm head; prominent anterior staining was not apparent in all specimens. The density of gold particles associated with plasma membrane delimiting components of the former acrosomal complex, nucleus and mitochondrion, as well as the total average number of particles along the entire sperm surface, were increased in sperm acrosome-reacted with A-23187. Concomitant with this change in staining was the disappearance/reduction of the collars of intramembranous particles and cytoplasm. These observations indicate that plasma membrane components (210 kDa protein and intramembranous particles) and the collar of cytoplasm which are associated with the acrosomal complex are functionally, as well as structurally related. Analyses of particle density distributions along acrosome- and non-acrosome-reacted sperm suggest that the different staining patterns observed may be brought about by the recognition of cryptic sites at the time of the acrosomal reaction.

Interaction of proteins RSV IV and RSV V in rat seminal vesicle secretion

The RSV IV polypeptide, molecular weight ratio (Mr = 10,000), which is produced by the rat seminal vesicle, has previously been suggested to be associated with another polypeptide in the gland secretion (Higgins et al., '76). This study provides that RSV IV is a component of a protein shown by immunoassays, electrophoresis, and amino acid composition analysis to contain, together with RSV IV, the seminal vesicle secretory RSV V polypeptide (Mr = 13,000). This RSV IV-RSV V complex (namely CFS protein) had an isoelectric point at pH 7.2 and an approximate molecular weight of 22,000 daltons. This complex inhibits the previously reported in vitro binding of the isolated RSV IV to epididymal sperm cells, thus suggesting a functional role for the RSV IV-RSV V interaction.

Monoclonal antibodies to sperm surface antigens of the brown alga Fucus serratus exhibit region-, gamete-, species- and genus-preferential binding

A panel of twelve monoclonal antibodies (MAbs), designated FS1 to FS12, have been raised against surface antigens of Fucus serratus sperm. The antibodies were selected on the basis that they show region-, gamete-, species- or genus-preferential binding. Indirect immunofluorescence shows that the antigens bound by the MAbs are distributed non-randomly over the cell surface. Seven MAbs (FS1, FS3, FS4, FS6, FS8, FS9, FS10) bind antigens located primarily on the cell body, while the others (FS2, FS5, FS7, FS11, FS12) bind antigens located primarily on the anterior flagellum. Of the MAbs that label the anterior flagellum, FS2, FS5, FS7 and FS12 form a 'halo' at the perimeter of the flagellum. Electron microscopic-immunogold studies indicate that the 'halo' results from labelling of the mastigonemes, as opposed to the flagellar plasmamembrane. Gamete-preferential binding of antibodies was detected using an enzyme-linked immunosorbent assay with egg membrane vesicles. Eight of the MAbs bind sperm antigens not common to eggs, though FS2, FS4, FS5 and FS9 bind antigens present on both sperm and eggs. In studies of species- and genus-specificity FS2, FS3, FS5, FS6, FS7, FS8, FS10, FS11 and FS12 exhibit genus-preferential binding, labelling sperm of F. serratus and F. vesiculosus more intensely than that of Ascophyllum nodosum. Only FS10 showed marked species-preferential binding, labelling sperm of F. serratus much more intensely than that of F. vesiculosus.

A major secretory protein from rat seminal vesicle binds ejaculated spermatozoa

The rat seminal vesicle produces large amounts of a protein-rich fluid that greatly contributes to semen volume. RSV IV, a protein abundantly secreted from this gland, binds in vitro to rat epididymal spermatozoa. However, there is no evidence that this protein may have an in vivo role as a sperm-coating antigen. We report in this paper that high-molecular-weight RSV IV immunologically related proteins can be detected on ejaculated spermatozoa, but not on epididymal spermatozoa. After incubation of purified RSV IV with ejaculated spermatozoa in freshly recovered semen or with epididymal spermatozoa in a medium containing the coagulating gland secretion, sperm-bound proteins with analogous properties were detected. These results support the hypothesis that RSV IV is modified at ejaculation to an high-molecular-weight, sperm-coating antigen.

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.

Direct observation of cold-shock effects in ram spermatozoa with the use of a programmable cryomicroscope

Cryoinjury in individual ram spermatozoa was investigated in cells cooled at 10 degrees C/min on a programmable cryomicroscope. In physiological buffer and cryoprotective media, there was a smooth decline in sperm swimming speed with decreasing temperature; cooling in buffer caused a marked decline in the proportion of cells displaying forward progression, especially once the temperature fell below 16 degrees C. Spermatozoa cooled in the presence of rhodamine 123, a mitochondrial-specific dye, showed that abolition of sperm motility by cold shock in buffer was not due to mitochondrial inactivation. Temperature decline through the region of 10 degrees C caused a number of spermatozoa in buffer to undergo a sudden asymmetric bending of the flagellum in the region of the midpiece. Ultrastructural studies suggest that this was caused by an unstable, asymmetric membrane lesion. Spermatozoa cooled in the presence of cryoprotective media showed better recovery of motility after rewarming and failed to exhibit the bending effect described above.