Regionalized lipid diffusion in the plasma membrane of mammalian spermatozoa

Glycerophospholipids protect stallion spermatozoa from oxidative damage in vitro

Stallion sperm membranes comprise a high proportion of polyunsaturated fatty acids, making stallion spermatozoa especially vulnerable to peroxidative damage from reactive oxygen species generated as a by-product of cell metabolism. Membrane lipid replacement therapy with glycerophospholipid (GPL) mixtures has been shown to reduce oxidative damage in vitro and in vivo. The aims of this study were to test the effects of a commercial preparation of GPL, NTFactor^® Lipids, on stallion spermatozoa under oxidative stress. When oxidative damage was induced by the addition of arachidonic acid to stallion spermatozoa, the subsequent addition of GPL reduced the percentage of 4-hydroxynonenal (4-HNE; a key end product of lipid peroxidation) positive cells (32.9 ± 2.7 vs 20.9 ± 2.3%; P ≤ 0.05) and increased the concentration of 4-HNE within the spent media (0.026 ± 0.003 vs 0.039 ± 0.004 µg/mL; P ≤ 0.001), suggesting that oxidized lipids had been replaced by exogenous GPL. Lipid replacement improved several motility parameters (total motility: 2.0 ± 1.0 vs 68.8 ± 2.9%; progressive motility: 0 ± 0 vs 19.3 ± 2.6%; straight line velocity: 9.5 ± 2.1 vs 50.9 ± 4.1 µm/s; curvilinear velocity: 40.8 ± 10 vs 160.7 ± 7.8 µm/s; average path velocity: 13.4 ± 2.9 vs 81.9 ± 5.9 µm/s; P ≤ 0.001), sperm viability (13.5 ± 2.9 vs 80.2 ± 1.6%; P ≤ 0.001) and reduced mitochondrial ROS generation (98.2 ± 0.6 vs 74.8 ± 6.1%; P ≤ 0.001). Supplementation with GPL during 17°C in vitro sperm storage over 72 h improved sperm viability (66.4 ± 2.6 vs 78.1 ± 2.9%; P ≤ 0.01) and total motility (53 ± 5.6 vs 66.3 ± 3.5%; P ≤ 0.05). It is concluded that incubation of stallion spermatozoa with sub-µm-sized GPL micelles results in the incorporation of exogenous GPL into sperm membranes, diminishing lipid peroxidation and improving sperm quality in vitro.

Existence and distribution of Niemann-Pick type 2C (NPC2) in prawn reproductive tract and its putative role as a cholesterol modulator during sperm transit in the vas deferens

Sequestering of cholesterol (CHO) is a hallmark molecular event that is known to be associated with sperm gaining their fertilizing ability in a broad array of animals. We have shown previously that the level of CHO declines in the Macrobrachium rosenbergii sperm membrane when they are migrating into the vas deferens, prompting us to search for CHO transporters, one of which is Niemann-Pick type 2C (NPC2), within the prawn male reproductive tract. Sequence comparison of MrNPC2 with other NPC2, from crustaceans to mammals, revealed its conserved features in the hydrophobic cavity with 3 amino acids forming a CHO lid that is identical in all species analyzed. Expressions of MrNPC2 transcript and protein were detected in testicular supporting and interstitial cells and along the epithelial cells of the vas deferens. As confirmed by live cell staining, the testicular sperm (Tsp) surface was devoid of MrNPC2 but it first existed on the vas deferens sperm, suggesting its acquisition from the luminal fluid, possibly through trafficking of multi-lamellar vesicles during sperm transit in the vas deferens. We further showed that recombinant MrNPC2 had a high affinity towards CHO in the lipid extracts, either from Tsp or from lipid vesicles in the vas deferens. Together, our results indicated the presence of MrNPC2 in the male reproductive tract, which may play an important role as a CHO modulator between the sperm membrane and vas deferens epithelial communication.

Incubation of human sperm with micelles made from glycerophospholipid mixtures increases sperm motility and resistance to oxidative stress

Membrane integrity is essential in maintaining sperm viability, signaling, and motility, which are essential for fertilization. Sperm are highly susceptible to oxidative stress, as they are rich in sensitive polyunsaturated fatty acids (PUFA), and are unable to synthesize and repair many essential membrane constituents. Because of this, sperm cellular membranes are important targets of this process. Membrane Lipid Replacement (MLR) with glycerophospholipid mixtures (GPL) has been shown to ameliorate oxidative stress in cells, restore their cellular membranes, and prevent loss of function. Therefore, we tested the effects of MLR on sperm by tracking and monitoring GPL incorporation into their membrane systems and studying their effects on sperm motility and viability under different experimental conditions. Incubation of sperm with mixtures of exogenous, unoxidized GPL results in their incorporation into sperm membranes, as shown by the use of fluorescent dyes attached to GPL. The percent overall (total) sperm motility was increased from 52±2.5% to 68±1.34% after adding GPL to the incubation media, and overall sperm motility was recovered from 7±2% after H2O2 treatment to 58±2.5%)(n = 8, p<0.01) by the incorporation of GPL into sperm membranes. When sperm were exposed to H2O2, the mitochondrial inner membrane potential (MIMP), monitored using the MIMP tracker dye JC-1 in flow cytometry, diminished, whereas the addition of GPL prevented the decrease in MIMP. Confocal microscopy with Rhodamine-123 and JC-1 confirmed the mitochondrial localization of the dyes. We conclude that incubation of human sperm with glycerolphospholipids into the membranes of sperm improves sperm viability, motility, and resistance to oxidizing agents like H2O2. This suggests that human sperm might be useful to test innovative new treatments like MLR, since such treatments could improve fertility when it is adversely affected by increased oxidative stress.

Fusion of Boar Sperm with Nanoliposomes Prepared from Synthetic Phospholipids

Liposomes are artificial membrane vesicles that can be used to test and model the functions and interactions of various biological membranes, or as a carrier system to deliver biologically active substances into the cells, or to incorporate lipids into the plasma membrane of target cells to modify membrane structure-function relationships. Sperm plasma membrane undergoes lipid modification during maturation in epididymis and during capacitation in the female reproductive tract to facilitate fertilization. Natural variation in the amounts and composition of lipids in the sperm plasma membrane may also contribute to the species-specific sperm sensitivities to handling and storage conditions. Boar sperm are notoriously susceptible to membrane damage and are resistant to compositional alteration by artificial liposomes. This study used flow cytometry to demonstrate stable incorporation of nanoliposomes prepared from a complex mixture of various phospholipids (phosphatidylcholine : phosphatidylethanolamine : sphingomyelin : phosphatidylserine : phosphatidylinositol) with high fusion efficiency. Over 90% of sperm rapidly took up fluorescently labelled liposomes and retained the lipids for at least 60 min, in a significant time- and concentration-dependent manner. This unique fusion efficacy could be used to alter sperm plasma membrane composition and hence membrane-based functional responses.

Sperm volumetric dynamics during in vitro capacitation process in bovine spermatozoa

Previous studies have demonstrated that sperm head morphometry can be used as a potential diagnostic tool for detecting biophysical changes associated with sperm viability in bovine spermatozoa. In this study, sperm head morphometry was used to investigate its value as a biophysical marker for detecting volumetric changes in bovine spermatozoa under in vitro capacitating and non-capacitating incubation conditions. To further test this hypotesis, aliquots of pooled, washed bovine sperm were incubated in either Tyrode's complete medium with heparin (TCMH; a capacitating medium containing Ca2+, NaHCO3 and heparin), Tyrode's complete medium heparin-free (TCM; a medium containing just Ca2+ and NaHCO3) or Tyrode's basal medium (TBM; a non-capacitating medium free of Ca2+, NaHCO3 and heparin, used as control). Aliquots of sperm were processed for morphometric analysis at different incubation-time intervals (0, 3 and 6 h at 38°C), and the chlortetracycline assay was used simultaneously to confirm the ability of the sperm to undergo capacitation (B pattern) and the acrosome reaction (AR pattern) status in each medium. After 3 h of incubation under TCMH conditions, a significant increase was observed in the percentage of B and AR patterns and a significant decrease was found in all sperm morphometric parameters (P<0.01). Interestingly, after 6 h of incubation in TCMH, the percentage of B and AR patterns increased drastically over time and marked differences were found in the dimensional and shape parameters, which were significantly smaller compared with TBM or TCM media (P<0.001). Significant correlations were observed between sperm size and AR pattern (r=-0.875, P<0.01). In conclusion, sperm head morphometry can be used as a potential biophysical marker for detecting volumetric changes during capacitation process in bovine spermatozoa.

Heat-shock protein A8 restores sperm membrane integrity by increasing plasma membrane fluidity

The constitutive 70  kDa heat-shock protein, HSPA8, has previously been shown to contribute to the long-term survival of spermatozoa inside the mammalian female reproductive tract. Here, we show that a recombinant form of HSPA8 rapidly promotes the viability of uncapacitated spermatozoa, the ability of spermatozoa to bind to oviductal epithelial cells, enhances IVF performance, and decreases sperm mitochondrial activity. Fluorescence recovery after photobleaching revealed that the repair of membrane damage is achieved by an almost instantaneous increase in sperm membrane fluidity. The ability of HSPA8 to influence membrane stability and fluidity, as well as its conserved nature among mammalian species, supports the idea that this protein protects sperm survival through membrane repair mechanisms. Free Persian abstract A Persian translation of the abstract is freely available online at http://www.reproduction-online.org/content/147/5/719/suppl/DC1.

WITHDRAWN: Effects of various glycerol concentrations and thawing temperatures on CASA parameters and acrosomal integrity of frozen-thawed canine spermatozoa

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Membrane-sculpting BAR domains generate stable lipid microdomains

Bin-Amphiphysin-Rvs (BAR) domain proteins are central regulators of many cellular processes involving membrane dynamics. BAR domains sculpt phosphoinositide-rich membranes to generate membrane protrusions or invaginations. Here, we report that, in addition to regulating membrane geometry, BAR domains can generate extremely stable lipid microdomains by "freezing" phosphoinositide dynamics. This is a general feature of BAR domains, because the yeast endocytic BAR and Fes/CIP4 homology BAR (F-BAR) domains, the inverse BAR domain of Pinkbar, and the eisosomal BAR protein Lsp1 induced phosphoinositide clustering and halted lipid diffusion, despite differences in mechanisms of membrane interactions. Lsp1 displays comparable low diffusion rates in vitro and in vivo, suggesting that BAR domain proteins also generate stable phosphoinositide microdomains in cells. These results uncover a conserved role for BAR superfamily proteins in regulating lipid dynamics within membranes. Stable microdomains induced by BAR domain scaffolds and specific lipids can generate phase boundaries and diffusion barriers, which may have profound impacts on diverse cellular processes.

Fick's second law transformed: one path to cloaking in mass diffusion

Here, we adapt the concept of transformational thermodynamics, whereby the flux of temperature is controlled via anisotropic heterogeneous diffusivity, for the diffusion and transport of mass concentration. The n-dimensional, time-dependent, anisotropic heterogeneous Fick's equation is considered, which is a parabolic partial differential equation also applicable to heat diffusion, when convection occurs, for example, in fluids. This theory is illustrated with finite-element computations for a liposome particle surrounded by a cylindrical multi-layered cloak in a water-based environment, and for a spherical multi-layered cloak consisting of layers of fluid with an isotropic homogeneous diffusivity, deduced from an effective medium approach. Initial potential applications could be sought in bioengineering.

Sperm Membrane Dynamics Assessed by Changes in Lectin Fluorescence Before and After Capacitation

Sperm capacitation is correlated with acquisition of fertilizing ability, and the molecular events underlying this process are only beginning to be understood. A number of membrane changes associated with capacitation have been documented. In this study we used lectin probes to identify changes in glycoprotein localization as a result of capacitation of mouse sperm. Eight lectins (LEA, PSA, PNA, AAA, UEA-1, WGA, STA, and TPA) stained regions of the mouse sperm head, tail, or both. No changes in tail staining patterns were detected when sperm were incubated under capacitating conditions. In contrast, 7 of 8 lectins tested showed clear shifts in staining patterns in the sperm head as a result of incubation under capacitating conditions. When staining patterns were quantified, a distinct heterogeneity within the sperm population was observed. Each lectin displayed 3 distinct staining patterns in both uncapacitated and capacitated sperm samples. The least common pattern represented the acrosome-reacted (AR) pattern, as independently assessed by lectin staining of ionophoretreated sperm that were >95% AR as judged by Coomassie staining. However, a reciprocal shift in the two predominant staining patterns was correlated with capacitation and suggests that changes in distribution of cell surface proteins during capacitation constitute part of the molecular changes which result in changes in sperm function acquired during this process.

Biochemical identification and characterisation of changes associated with capacitation of mannosylated glycoproteins in murine sperm

The development of a sperm normally ends in the female reproductive track through an extra-testicular maturation process termed capacitation. Previous studies in several mammalian species have revealed the presence of glycoconjugates and their capacitation-correlated changes in localisation. In this study, we used Lens culinaris agglutinin (LCA) to characterise mannosylated glycoconjugates in mouse sperm and their changes in distribution associated with capacitation. In freshly isolated sperm, three biochemically distinct sperm populations (type I, II and III) with specific LCA labelling were noted. Prior to capacitation, glycosylated materials were detected in the head region and approximately 26%, 65% and 9% of sperm displayed each distribution pattern. Following capacitation, the distribution pattern of these mannosylated constituents was not altered; however, re-localisation of these components was observed because the percentage of sperm exhibiting type I and III pattern was significantly changed. Finally, thirteen mannosylated glycoproteins were detected in murine sperm and our lectin blot data confirm that their redistribution instead of proteolytic degradation is mainly responsible for cellular dynamics we observed in murine sperm head associated with capacitation. In conclusion, this study is the first to describe the biochemical identification of LCA-binding glycoproteins present in mammalian sperm, which undergo redistribution as they mature to acquire their full fertilising ability.

Phosphatidylserine membrane translocation in human spermatozoa: topography in membrane domains and relation to cell vitality

The complex structure of the human spermatozoa membrane comprises five topographic domains. Transmembrane asymmetry of the distribution of phospholipids including phosphatidylserine (PS) is considered a marker of cell activity. The objective of the study was to determine which cytomembrane domains of human spermatozoa are involved in PS membrane translocation and to identify the possible relationship of PS translocation with spermatozoa morphology and vitality. In normozoospermic semen of 35 donors, annexin-V labeling with fluorescein determined PS translocation. Propidium iodide staining distinguished between vital and dead spermatozoa. Three types of PS membrane translocation have been distinguished: (1) in the midpiece, (2) in the acrosomal part and (3) simultaneously in the midpiece and acrosomal part. In morphologically normal vital spermatozoa, PS translocation occurred in the midpiece but never in the equatorial region. In dead spermatozoa, simultaneous PS translocation in the midpiece and acrosomal part was most often observed. The difference between proportions of, respectively, vital and dead spermatozoa presenting PS translocation located in different domains was significant (P < 0.0001). In vital cells, there was no difference in PS translocation prevalence between morphologically normal and abnormal spermatozoa (P > 0.05). The strict relation of PS translocation to specific membrane domains indicates functional specificity. It seems doubtful to include this phenomenon in physiological mechanisms of elimination of abnormal spermatozoa.

Lipid bilayer composition affects transmembrane protein orientation and function

Sperm membranes change in structure and composition upon ejaculation to undergo capacitation, a molecular transformation which enables spermatozoa to undergo the acrosome reaction and be capable of fertilization. Changes to the membrane environment including lipid composition, specifically lipid microdomains, may be responsible for enabling capacitation. To study the effect of lipid environment on proteins, liposomes were created using lipids extracted from bull sperm membranes, with or without a protein (Na⁺ K⁺-ATPase or α-amylase). Protein incorporation, function, and orientation were determined. Fluorescence resonance energy transfer (FRET) confirmed protein inclusion in the lipid bilayer, and protein function was confirmed using a colourometric assay of phosphate production from ATP cleavage. In the native lipid liposomes, ATPase was oriented with the β subunit facing the outer leaflet, while changing the lipid composition to 50% native lipids and 50% exogenous lipids significantly altered this orientation of Na⁺ K⁺-ATPase within the membranes.

Rat recombinant β-defensin 22 is a heparin-binding protein with antimicrobial activity

Approximately 40-50 β-defensins are predominantly expressed in the male reproductive system of mammals. This selective expression raises the question as to the roles of these molecules in innate immunity and fertility in the male reproductive tract. Rat β-defensin 22 is an epididymis-specific β-defensin expressed in segments 12-14 of the epididymis. This protein contains both β-defensin and lectin signature sequences, yet its antimicrobial activity and carbohydrate-binding ability have not been shown. We herein demonstrated the antimicrobial activity of recombinant rat β-defensin 22 against Escherichia coli and Candida albicans. Its lectin-like activity was also investigated by demonstrating its binding ability with heparin beads. This heparin-binding activity implies some potential roles for this defensin in determining the fertilisation capabilities of sperm.

Post testicular sperm maturational changes in the bull: important role of the epididymosomes and prostasomes

After spermatogenesis, testicular spermatozoa are not able to fertilize an oocyte, they must undergo sequential maturational processes. Part of these essential processes occurs during the transit of the spermatozoa through the male reproductive tract. Since the sperm become silent in terms of translation and transcription at the testicular level, all the maturational changes that take place on them are dependent on the interaction of spermatozoa with epididymal and accessory gland fluids. During the last decades, reproductive biotechnologies applied to bovine species have advanced significantly. The knowledge of the bull reproductive physiology is really important for the improvement of these techniques and the development of new ones. This paper focuses on the importance of the sperm interaction with the male reproductive fluids to acquire the fertilizing ability, with special attention to the role of the membranous vesicles present in those fluids and the recent mechanisms of protein acquisition during sperm maturation.

Mechanisms underlying the micron-scale segregation of sterols and GM1 in live mammalian sperm

We demonstrate for the first time that a stable, micron-scale segregation of focal enrichments of sterols exists at physiological temperature in the plasma membrane of live murine and human sperm. These enrichments of sterols represent microheterogeneities within this membrane domain overlying the acrosome. Previously, we showed that cholera toxin subunit B (CTB), which binds the glycosphingolipid, G(M1), localizes to this same domain in live sperm. Interestingly, the G(M1) undergoes an unexplained redistribution upon cell death. We now demonstrate that G(M1) is also enriched in the acrosome, an exocytotic vesicle. Transfer of lipids between this and the plasma membrane occurs at cell death, increasing G(M1) in the plasma membrane without apparent release of acrosomal contents. This finding provides corroborative support for an emerging model of regulated exocytosis in which membrane communications might occur without triggering the "acrosome reaction." Comparison of the dynamics of CTB-bound endogenous G(M1) and exogenous BODIPY-G(M1) in live murine sperm demonstrate that the sub-acrosomal ring (SAR) functions as a specialized diffusion barrier segregating specific lipids within the sperm head plasma membrane. Our data show significant differences between endogenous lipids and exogenous lipid probes in terms of lateral diffusion. Based on these studies, we propose a hierarchical model to explain the segregation of this sterol- and G(M1)-enriched domain in live sperm, which is positioned to regulate sperm fertilization competence and mediate interactions with the oocyte. Moreover, our data suggest potential origins of subtypes of membrane raft microdomains enriched in sterols and/or G(M1) that can be separated biochemically.

Cyclic 3',5'-AMP causes ADAM1/ADAM2 to rapidly diffuse within the plasma membrane of guinea pig sperm

Because sperm cannot synthesize new proteins as they journey to the egg, they use multiple mechanisms to modify the activity of existing proteins, including changes in the diffusion coefficient of some membrane proteins. Previously, we showed that during capacitation the guinea pig heterodimeric membrane protein ADAM1/ADAM2 (fertilin) transforms from a stationary state to one of rapid diffusion within the lipid bilayer. The cause for this biophysical change, however, was unknown. In this study we examined whether an increase in cAMP, such as occurs during capacitation, could trigger this change. We incubated guinea pig cauda sperm with the membrane-permeable cAMP analog dibutyryl cAMP (db-cAMP) and the phosphodiesterase inhibitor papaverine and first tested for indications of capacitation. We observed hypermotility and acrosome-reaction competence. We then used fluorescence redistribution after photobleaching (FRAP) to measure the lateral mobility of ADAM1/ADAM2 after the db-cAMP treatment. We observed that db-cAMP caused roughly a 12-fold increase in lateral mobility of ADAM1/ADAM2, yielding diffusion similar to that observed for sperm capacitated in vitro. When we repeated the FRAP on testicular sperm incubated in db-cAMP, we found only a modest increase in lateral mobility of ADAM1/ADAM2, which underwent little redistribution. Interestingly, testicular sperm also cannot be induced to undergo capacitation. Together, the data suggest that the release of ADAM1/ADAM2 from its diffusion constraints results from a cAMP-induced signaling pathway that, like others of capacitation, is established during epididymal sperm maturation.

Nanopipette delivery of individual molecules to cellular compartments for single-molecule fluorescence tracking

We have developed a new method, using a nanopipette, for controlled voltage-driven delivery of individual fluorescently labeled probe molecules to the plasma membrane which we used for single-molecule fluorescence tracking (SMT). The advantages of the method are 1), application of the probe to predefined regions on the membrane; 2), release of only one or a few molecules onto the cell surface; 3), when combined with total internal reflection fluorescence microscopy, very low background due to unbound molecules; and 4), the ability to first optimize the experiment and then repeat it on the same cell. We validated the method by performing an SMT study of the diffusion of individual membrane glycoproteins labeled with Atto 647-wheat germ agglutin in different surface domains of boar spermatozoa. We found little deviation from Brownian diffusion with a mean diffusion coefficient of 0.79 +/- 0.04 microm(2)/s in the acrosomal region and 0.10 +/- 0.02 microm(2)/s in the postacrosomal region; this difference probably reflects different membrane structures. We also showed that we can analyze diffusional properties of different subregions of the cell membrane and probe for the presence of diffusion barriers. It should be straightforward to extend this new method to other probes and cells, and it can be used as a new tool to investigate the cell membrane.

Supramolecular organization of the sperm plasma membrane during maturation and capacitation

AIM

In the present study, a variety of high resolution microscopy techniques were used to visualize the organization and motion of lipids and proteins in the sperm's plasma membrane. We have addressed questions such as the presence of diffusion barriers, confinement of molecules to specific surface domains, polarized diffusion and the role of cholesterol in regulating lipid rafts and signal transduction during capacitation.

METHODS

Atomic force microscopy identified a novel region (EqSS) within the equatorial segment of bovine, porcine and ovine spermatozoa that was enriched in constitutively phosphorylated proteins. The EqSS was assembled during epididymal maturation. Fluorescence imaging techniques were then used to follow molecular diffusion on the sperm head.

RESULTS

Single lipid molecules were freely exchangeable throughout the plasma membrane and showed no evidence for confinement within domains. Large lipid aggregates, however, did not cross over the boundary between the post-acrosome and equatorial segment suggesting the presence of a molecular filter between these two domains.

CONCLUSION

A small reduction in membrane cholesterol enlarges or increases lipid rafts concomitant with phosphorylation of intracellular proteins. Excessive removal of cholesterol, however, disorganizes rafts with a cessation of phosphorylation. These techniques are forcing a revision of long-held views on how lipids and proteins in sperm membranes are assembled into larger complexes that mediate recognition and fusion with the egg.

Very long-chain polyunsaturated fatty acids are the major acyl groups of sphingomyelins and ceramides in the head of mammalian spermatozoa

Very long-chain (C24 to C34) polyunsaturated fatty acids (VLCPUFA) are important constituents of sphingomyelin (SM) and ceramide (Cer) in testicular germ cells. In the present paper we focused on the SM and Cer and their fatty acids in spermatozoa and their main regions, heads and tails. In bull and ram spermatozoa, SM was the third most abundant phospholipid and VLCPUFA were the major acyl groups ( approximately 70%) of SM and Cer. In rat epididymal spermatozoa the SM/Cer ratio was low in the absence of and could be maintained high in the presence of the cation chelator EDTA, added to the medium used for sperm isolation. This fact points to the occurrence of an active divalent cation-dependent sphingomyelinase. Bull and rat sperm had an uneven head-tail distribution of phospholipid, with virtually all the VLCPUFA-rich SM located at the head, the lower SM content in the rat being determined by the lower sperm head/tail size ratio. Most of the SM from bull sperm heads was readily solubilized with 1% Triton X-100 at 4 degrees C. The detergent-soluble SM fraction was richer in VLCPUFA than the nonsoluble fraction and richer in saturated fatty acids. Cer was produced at the expense of SM, thus decreasing severalfold the SM/Cer ratio in rat spermatozoa incubated for 2 h in presence of the sperm-capacitating agents, calcium, bicarbonate, and albumin. The generation of Cer from SM in the sperm head surface may be an early step among the biochemical and biophysical changes known to take place in the spermatozoon in the physiological events preceding fertilization.

Phase behavior and dynamic heterogeneities in lipids: A coarse-grained simulation study of DPPC–DPPE mixtures

The coarse-grained Marrink-model for biomembrane simulation is used to study mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE) at various concentrations and temperatures. At high temperatures close to ideal mixing is observed. In the low temperature ordered phase dynamic heterogeneities are identified under some conditions. These are correlated with heterogeneities in the local order and define local neighborhoods.

Geldanamycin augments nitric oxide production and promotes capacitation in boar spermatozoa

Sperm capacitation and the acrosome reaction are fundamentally important to fertilization. Nitric oxide (NO) has been shown to have various functions in male reproduction. This work investigates whether boar sperm can generate NO, as well as the effects of NO and geldanamycin (GA), a heat-shock protein 90 (HSP90)-specific inhibitor, on the capacitation of boar spermatozoa. Observations showed that porcine sperm produced low levels of NO under non-capacitating conditions. However, the NO concentration almost doubled under capacitating conditions (P<0.001). Treatment with NG-nitro-L-arginine methyl ester (L-NAME) reduced the production of NO by 30-40% in capacitating sperm (P<0.05). GA treatment increased it by 23-75% in a dose-dependent manner (P<0.05). L-NAME treatment reduced the percentage of sperm undergoing the acrosome reaction, whereas sodium nitroprusside, an NO-releasing compound, and GA treatment increased the percentage of sperm undergoing the acrosome reaction (P<0.05). GA treatment promoted the production of NO and the acrosome reaction. The increase in NO production by GA treatment was similar to that caused by the calcium ionophore, A23187, suggesting that the GA-induced acrosome reaction may be triggered by an increase of the intracellular calcium concentration. The signaling pathway involved in GA-mediated NO production and its biological function in fertilizing boar spermatozoa will be elucidated in further studies.

Segregation of micron-scale membrane sub-domains in live murine sperm

Lipid rafts, membrane sub-domains enriched in sterols and sphingolipids, are controversial because demonstrations of rafts have often utilized fixed cells. We showed in living sperm that the ganglioside G(M1) localized to a micron-scale membrane sub-domain in the plasma membrane overlying the acrosome. We investigated four models proposed for membrane sub-domain maintenance. G(M1) segregation was maintained in live sperm incubated under non-capacitating conditions, and after sterol efflux, a membrane alteration necessary for capacitation. The complete lack of G(M1) diffusion to the post-acrosomal plasma membrane (PAPM) in live cells argued against the transient confinement zone model. However, within seconds after cessation of sperm motility, G(M1) dramatically redistributed several microns from the acrosomal sub-domain to the post-acrosomal, non-raft sub-domain. This redistribution was not accompanied by movement of sterols, and was induced by the pentameric cholera toxin subunit B (CTB). These data argued against a lipid-lipid interaction model for sub-domain maintenance. Although impossible to rule out a lipid shell model definitively, mice lacking caveolin-1 maintained segregation of both sterols and G(M1), arguing against a role for lipid shells surrounding caveolin-1 in sub-domain maintenance. Scanning electron microscopy of sperm freeze-dried without fixation identified cytoskeletal structures at the sub-domain boundary. Although drugs used to disrupt actin and intermediate filaments had no effect on the segregation of G(M1), we found that disulfide-bonded proteins played a significant role in sub-domain segregation. Together, these data provide an example of membrane sub-domains extreme in terms of size and stability of lipid segregation, and implicate a protein-based membrane compartmentation mechanism.

Visualization of GM1 with cholera toxin B in live epididymal versus ejaculated bull, mouse, and human spermatozoa

The organization of membrane subdomains in mammalian sperm has recently generated controversy, with several reports describing widely differing localization patterns for the ganglioside GM1. Using the pentameric B subunit of cholera toxin (CTB), we found GM1 to be restricted to the plasma membrane overlying the acrosome in the heads of live murine sperm. Interestingly, CTB had minimal binding to live bovine and human sperm. To investigate whether this difference in GM1 localization was because of species differences or differences between collection from the epididymis (mouse) or an ejaculate (bull, human), we examined epididymal bovine and human sperm. We found that GM1 localized to the plasma membrane overlying the acrosome in sperm from these species. To determine whether some component of seminal plasma was interfering with the ability of CTB to access GM1, we incubated epididymal mouse sperm with fluid from murine seminal vesicles and epididymal bull sperm with bovine seminal plasma. This treatment largely abolished the ability of the CTB to bind to GM1, producing a fluorescence pattern similar to that reported for the human. The most abundant seminal plasma protein, PDC-109, was not responsible for this loss. As demonstration that the seminal plasma was not removing GM1, sperm exposed to seminal plasma were fixed before CTB addition, and again displayed fluorescence over the acrosome. These observations reconcile inconsistencies reported for the localization of GM1 in sperm of different species, and provide evidence for the segregation of GM1 to a stable subdomain in the plasma membrane overlying the acrosome.

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.

Toward understanding the dynamics of membrane-raft-based molecular interactions

The cell membrane is a 2-dimensional non-ideal liquid containing dynamic structures on various time-space scales, and the raft domain is one of them. Existing literature supports the concept that raft dynamics may be important for its formation and function: the raft function may be supported by stimulation-induced raft association/coalescence and recruitment of various raftophilic molecules to coalesced rafts, and, importantly, they both may happen transiently. Thus, one must always consider the limited association time of a raft or a raftophilic molecule with another raft, even when one interprets the results of static experiments, such as immunofluorescence and pull-down assays. Critical considerations on the chemical fixation mechanism and immunocolocalization data suggest that the temporary nature of raft-based molecular interactions may explain why colocalization results are sensitive to subtle variations in experimental conditions employed in different laboratories.

The fluidity of Chinese hamster ovary cell and bull sperm membranes after cholesterol addition

Cell plasma membrane fluidity is affected by membrane lipid and protein composition as well as temperature. Altering the cholesterol content of a membrane can change membrane fluidity at different temperatures and this may affect cell survival during cryopreservation. In these experiments, we examined the effect that adding cholesterol to the membranes of Chinese hamster ovary cells (CHO) and bull sperm had on cell plasma membrane fluidity and cell survival when cells were cooled to 5 degrees C or were cryopreserved. Cells were treated with 0, 1.5 or 5.0mg cholesterol-loaded cyclodextrin (CLC), stained with N-((4-(6-phenyl-1,3,5-hexatrienyl)phenyl)propyl)trimethylammonium-p-toluenesulfonate (TMAP-DPH) to evaluate membrane fluidity and with propidium iodide to evaluate cell viability, prior to analysis by flow cytometry at 23, 5 degrees C, and after cryopreservation. CHO cells exhibited a single cell population with all cells having similar membrane fluidity. Membrane fluidity did not change when temperature had been reduced and then returned to 23 degrees C (P<0.05), however, adding cholesterol to the cells induced membranes to become more rigid (P<0.05). Bull sperm samples consisted of two cell subpopulations, one having relatively higher membrane fluidity than the other, regardless of cholesterol treatment or temperature. In addition, cells possessing the highest membrane fluidity did not survive cooling or cryopreservation efficiently. CLC treatment did not significantly alter membrane fluidity after temperature changes, but did maintain higher percentages of spermatozoa surviving cooling to 5 degrees C and cryopreservation (P<0.05). In conclusion, adding cholesterol to cell resulted in detectable membrane fluidity changes in CHO cells and increased survival of bull sperm after cooling to 5 degrees C and after cryopreservation.

Activity, substrate detection and immunolocalization of glutathione peroxidase (GPx) in bovine reproductive organs and semen

The purpose of the current study was to further investigate the role of the antioxidant selenium-dependent enzyme glutathione peroxidase (GPx) in reproductive organs and semen from bulls. To this end a fast and convenient combined method for immune detection and substrate localization was adapted, which allows the assessment of both molecular weight and peroxidase activity of proteins on one and the same SDS-PAGE gel plate. After routine semen analysis of ejaculates, a spectrophotometrical assay of GPx activity in bovine semen was performed. For the immunological analyses performed, a rabbit polyclonal monospecific antibody against GPx was raised. Substrate detection and immunolocalization of GPx in lysates from bovine testis, epididymis, spermatozoa, and seminal plasma was performed. In order to determine the localization of GPx in spermatozoa, immunofluorescence analysis was performed. A positive correlation was established between GPx activity in semen and the number of motile spermatozoa. A negative correlation was observed between GPx activity and the number of immotile spermatozoa. The combined method for immunodetection and substrate localization was tested and proved reliable. Both tetramer and monomer forms of GPx were detected in lysates from testis, epididymis, and spermatozoa. We found no GPx activity in seminal plasma. Immunofluorescence shows the presence of GPx chiefly in the mitochondrial and in the acrosome regions of spermatozoa. GPx activity remained stable under the extreme experimental conditions.

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.

Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa

A reduction in plasma membrane cholesterol is one of the early events that either triggers or is closely associated with capacitation of mammalian spermatozoa. In this investigation, we have examined the effects of cholesterol efflux on tyrosine phosphorylation, lipid diffusion, and raft organization in boar spermatozoa. Results show that a low level of cholesterol efflux, mediated by 5 mM methyl-beta-cyclodextrin (MBCD), enhances capacitation and induces phosphorylation of two proteins at 26 and 15 kDa without affecting sperm viability. Lipid diffusion rates under these conditions are largely unaffected except when cholesterol efflux is excessive. Low-density Triton X100-insoluble complexes (lipid rafts) were isolated from spermatozoa and found to have a restricted profile of proteins. Capacitation-associated cholesterol efflux has no effect on raft composition, but cholesterol depletion destabilizes them completely and phosphorylation is suppressed. During MBCD-mediated capacitation, the distribution of GM1 gangliosides on spermatozoa changes in a sequential manner from overlying the sperm tail to clustering on the sperm head. It is concluded that there is a safe window for removal of plasma membrane cholesterol from spermatozoa within which protein phosphorylation and polarized migration of lipid rafts take place. A preferential loss of cholesterol from the nonraft pool may be the stimulus that promotes raft clustering over the anterior sperm head.

Molecular diffusion in sperm plasma membranes during epididymal maturation

Fluorescence recovery after photobleaching (FRAP) analysis has been used to measure lipid diffusion in different regions of the sperm plasma membrane. Our goal has been to understand how some membrane components are confined to specific surface domains, whilst others are freely diffusing and in some cases are able to migrate against large concentration gradients. Results with a variety of fluorescent lipid reporter probes (ODAF, NBD-PC, NBD-cholesterol) show that diffusion coefficients (D) are generally three to four times higher on the sperm acrosome than on the principal piece of the tail and increase significantly during epididymal maturation (ram, mouse, goat, dog and monkey sperm). Cholesterol diffusion is approximately 10 times faster on the sperm head than the tail and has a heterogenous distribution when detected with filipin. Lipid diffusion is very temperature sensitive but remarkably insensitive to changes in external pH and osmotic pressure. There was no evidence that the posterior ring or annulus functioned as diffusion barriers to lipids. On this basis it was possible to construct models of increasing complexity to describe the behaviour of a lipid molecule on the sperm surface, beginning with simple linear diffusion progressing to random diffusion and eventually to constrained diffusion.

Secreted epididymal glycoprotein 2D6 that binds to the sperm's plasma membrane is a member of the beta-defensin superfamily of pore-forming glycopeptides

The plasma membrane of spermatozoa undergoes substantial remodeling during passage through the epididymal duct, principally because of changes in phospholipid composition, exchange of glycoproteins with epididymal fluid, and processing of existing membrane proteins. Here, we describe the interaction of an epididymal glycoprotein recognized by monoclonal antibody 2D6 with the plasma membrane of rat spermatozoa. Our goals have been to understand more about the mechanism of secretion of epididymal glycoproteins, how they interact with the sperm's plasma membrane, and their disposition within it. Reactivity to 2D6 monoclonal antibody was first detectable in principal cells in the distal caput epididymidis and as a soluble high-molecular-weight complex in the secreted fluid. It was not associated with membranous vesicles in the duct lumen. On cauda spermatozoa 2D6 monoclonal antibody recognized a 24-kDa glycoprotein (the subunit of a disulfide cross-linked homodimer of 48 kDa) that was present on the plasma membrane overlying the sperm tail. Binding of 2D6 to immature spermatozoa in vitro was cell-type specific but not species specific, and the antigen could only be extracted from cauda spermatozoa with detergents. Sequencing studies revealed that the 24-kDa glycoprotein was a member of the beta-defensin superfamily of small pore-forming glycopeptides of which several others (ESP13.2, Bin1b, E-2, EP2, HE2) are found in the epididymis. This evidence suggests that some epididymal glycoproteins are secreted into the luminal fluid in a soluble form and bind to specific regions of the sperm's surface via hydrophobic interactions. Given the antimicrobial function of beta-defensins, they have a putative role in protecting spermatozoa and the epididymis from bacterial infections.

Post-testicular development of a novel membrane substructure within the equatorial segment of ram, bull, boar, and goat spermatozoa as viewed by atomic force microscopy

Atomic force microscopy has been used to investigate changes in the plasma membrane overlying the head region of mammalian spermatozoa (bull, boar, ram, goat, stallion, mouse, and monkey) during post-testicular development, after ejaculation, and after exocytosis of the acrosomal vesicle. On ejaculated ram, bull, boar, and goat spermatozoa the postacrosomal plasma membrane has a more irregular surface than that covering the acrosome. The equatorial segment, by contrast, is relatively smooth except for an unusual semicircular substructure within it that has a coarse uneven appearance. This substructure (referred to as the equatorial subsegment) is situated adjacent to the boundary between the postacrosomal region and the equatorial segment itself and seems to be confined to the order Artiodactyla as it has not been observed on stallion, mouse, or monkey spermatozoa. The equatorial subsegment develops during epididymal maturation, and following induction of the acrosome reaction with Ca(2+) ionophore A23187, its topography changes from a finely ridged appearance to that resembling truncated papillae. A monoclonal antibody to the equatorial subsegment binds only to permeabilized spermatozoa, suggesting that the subsegment is related to the underlying perinuclear theca that surrounds the sperm nucleus. A role for the equatorial subsegment in mediating fusion with the oolemma at fertilization is discussed.

Effect of incubating human sperm at room temperature on capacitation-related events

OBJECTIVE

To determine the effect of human sperm incubation at room temperature (20 degrees C) upon capacitation-related events.

DESIGN

Prospective study.

SETTING

Basic research laboratory.

PATIENT(S)

Semen samples were obtained from normozoospermic volunteers. Human follicular fluid (hFF) was collected from women undergoing assisted reproductive treatment.

INTERVENTION(S)

Spermatozoa were incubated for up to 18 hours at 20 degrees C and/or 37 degrees C.

MAIN OUTCOME MEASURE(S)

Protein tyrosine phosphorylation patterns, development of hyperactivated motility, and induction of acrosome reaction (AR) in response to hFF.

RESULT(S)

Spermatozoa incubated for 18 hours at 20 degrees C showed an array of tyrosine phosphorylated proteins similar to noncapacitated cells. After incubation at 20 degrees C, the percentage of spermatozoa displaying hyperactivated motility and undergoing acrosomal loss in response to hFF was significantly lower when compared with cells kept the same time at 37 degrees C. Conversely, spermatozoa incubated overnight at 37 degrees C could respond to hFF, either at 37 degrees C or 20 degrees C. When preincubation at 20 degrees C was followed by sperm exposure to 37 degrees C, capacitation-related events could be activated. In capacitated cells (16 hours at 37 degrees C), 2-hour incubation at 20 degrees C led to a significant decrease in acrosome reaction inducibility, suggesting sperm decapacitation.

CONCLUSION(S)

Human sperm incubation at room temperature does not allow capacitation, although it does not affect hFF-induced acrosome reaction in capacitated cells. The blocking effect is overcome when spermatozoa are exposed to 37 degrees C.

Capacitation and the acrosome reaction in equine sperm

During sexual reproduction, the sperm and oocyte must fuse before the production of a diploid zygote can proceed. In mammals such as equids, fusion depends critically on complex changes in the plasma membrane of the sperm and, not surprisingly, this membrane differs markedly from that of somatic cells. After leaving the testes, sperm cease to synthesize plasma membrane lipids or proteins, and vesicle-mediated transport stops. When the sperm reaches the female reproductive tract, it is activated by so-called capacitation factors that initiate a delicate reorientation and modification of molecules within the plasma membrane. These surface changes enable the sperm to bind to the extracellular matrix of the egg (zona pellucida ZP) and the zona then primes the sperm to initiate the acrosome reaction, an exocytotic event required for the sperm to penetrate the zona. This paper will review the processes that occur at the sperm plasma membrane before and during successful penetration of the equine ZP. It is noted that while several methods have been described for detecting changes that occur during capacitation and the acrosome reaction in bovine and porcine sperm, relatively little has been documented for equine sperm. Special attention will therefore be dedicated to recent attempts to develop and implement new assays for the detection of the capacitation status of live, acrosome-intact and motile equine sperm.

Guinea pig fertilin exhibits restricted lateral mobility in epididymal sperm and becomes freely diffusing during capacitation

The guinea pig sperm protein fertilin functions in sperm-egg plasma membrane binding. Fertilin is initially present in the plasma membrane of the whole head in testicular sperm, then becomes concentrated into the posterior head domain during epididymal passage. Fertilin remains localized to the posterior head plasma membrane following the acrosome reaction, when it functions in sperm-egg interaction. Fluorescence redistribution after photobleaching was used to examine the lateral mobility of fertilin in both acrosome-intact and acrosome-reacted sperm. Fertilin exhibited highly restricted lateral mobility in both testicular and epididymal sperm (D < 10(-10) cm(2)/s). However, fertilin in acrosome-reacted sperm was highly mobile within the membrane bilayer (D = 1.8 x 10(-9) cm(2)/s and %R = 84). Measurement of the lateral mobility of fertilin in capacitated, acrosome-intact sperm revealed two populations of cells. In approximately one-half of the cells, lateral mobility of fertilin was similar to sperm freshly isolated from the cauda epididymis; while in the other half fertilin was highly mobile. The release of fertilin from interactions that restrict its lateral mobility may regulate its function in sperm-egg interaction.

Lipid dynamics in the plasma membrane of ram and bull spermatozoa after washing and exposure to macromolecules BSA and PVP

Seminal plasma proteins and macromolecules in the external medium have a major influence on the functionality of sperm plasma membranes. In this investigation we have examined their effects on lipid diffusion in the surface membrane of ram and bull spermatozoa as measured by fluorescence recovery after photobleaching (FRAP). Results show that progressive removal of seminal plasma from ram spermatozoa by repeated centrifugation and resuspension in media +/- 4% bovine serum albumin (BSA) or 0.4% polyvinlypyrrolidone (PVP) causes a reduction in lipid diffusion in all regions of the membrane. By contrast, bull sperm membranes respond with an increase in diffusion in all regions. Repeated washing of bull spermatozoa whose membranes were previously immobile (i.e., showed no recovery after FRAP) restored lipid diffusion suggesting an inhibitory effect of seminal plasma proteins. Further analysis by atomic force microscopy revealed a close association between BSA and the plasma membrane. It is concluded that diffusion of lipids in the plasma membrane of ejaculated ram and bull spermatozoa is influenced by seminal plasma proteins and the composition of the suspending medium. Mol. Reprod. Dev. 59:306-313, 2001.

Incorporating lipids into boar sperm decreases chilling sensitivity but not capacitation potential

Fresh boar sperm were incubated with small unilamellar liposomes composed of either the total lipids extracted from head plasma membranes (HPM) of fresh boar sperm or selected lipids (SL) of five defined phospholipids with specific acyl chains. To optimize fusion, liposomes with 2 mol% octadecyl rhodamine fluorophore in Beltsville Thawing Solution +/- 1 mM CaCl(2) were incubated at 35 degrees C with 1;ts 10(7) or 10(8) spermatozoa/ml and monitored over 60 min, using flow cytometry and fluorescence microscopy. The HPM fused to both sperm concentrations faster than SL but was equivalent by 30 min (10(8) sperm/ml) or 60 min (10(7) sperm/ml; 57.5 +/- 3% and 67.1 +/- 8% sperm fused to HPM and SL, respectively) +/- Ca(2+). Neither HPM nor SL affected onset of capacitation or spontaneous or ionophore-induced acrosome reactions at 0 or 3 h (chlortetracycline and fluorescein isothiocyanate-Pisum sativum agglutinin; n = 3). During cooling and after cryopreservation (n = 4 ejaculates), SL but not HPM significantly improved sperm motility and viability (Sybr14/propidium iodide staining) +/- 20% egg yolk, but egg yolk alone was more effective than SL alone. Liposomes of complex composition can fuse to boar sperm without harming in vitro capacitation or acrosome reaction and reduce sperm chilling sensitivity.

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.

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

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

Changes in turkey semen lipids during liquid in vitro storage

The changes in lipid composition of spermatozoa and seminal plasma and changes in motility, viability, and morphological integrity of spermatozoa were measured in turkey semen diluted in Beltsville poultry semen extender and stored for 48 h (4 degrees C). The total phospholipid content of spermatozoa decreased during storage, while no quantitative decrease was observed in seminal plasma. More precisely, significant decreases in phosphatidylcholine, and to a lesser extent in sphingomyeline, phosphatidylserine, and phosphatidylinositol were observed in spermatozoa. The fatty acid profile of turkey spermatozoa partly reflected diet composition and had a high level of n-9 polyunsaturated fatty acids. Neither fatty acid profile nor free cholesterol were affected by storage. The lipid composition of seminal plasma was quite different from that observed in spermatozoa and was similar to the high density lipoprotein composition of chicken seminal plasma. In vitro storage did not significantly affect lipid classes and only small changes were observed in phospholipid classes of seminal plasma. The motility, viability, and morphological integrity of spermatozoa decreased during storage. These changes in phospholipid content may be explained by membrane phospholipid lysis followed by endogenous metabolism or by a complex combination of lysis, metabolism, and peroxidation. They are likely to affect semen quality and the success of in vitro storage severely.

Microtubule configurations and post-translational alpha-tubulin modifications during mammalian spermatogenesis

The mechanisms underlying cell cycle progression and differentiation are tightly entwined with changes associated in the structure and composition of the cytoskeleton. Mammalian spermatogenesis is a highly intricate process that involves differentiation and polarization of the round spermatid. We found that pachytene spermatocytes and round spermatids have most of the microtubules randomly distributed in a cortical network without any apparent centrosome. The Golgi apparatus faces the acrosomal vesicle and some microtubules contact its surface. In round spermatids, at step 7, there is an increase in short microtubules around and over the nucleus. These microtubules are located between the rims of the acrosome and may be the very first sign in the formation of the manchette. This new microtubular configuration is correlated with the beginning of the migration of the Golgi apparatus from the acrosomal region towards the opposite pole of the cell. Next, the cortical microtubules form a bundle running around the nucleus perpendicular to the main axis of the cell. At later stages, the nuclear microtubules increase in size and a fully formed manchette appears at stage 9. On the other hand, acetylated tubulin is present in a few microtubules in pachytene spermatocytes and in the axial filament (precursor of the sperm tail) in round spermatids. Our results suggest that at step 7, the spermatid undergoes a major microtubular reordering that induces or allows organelle movement and prepares the cell for the formation of the manchette and further nuclear shaping. This new microtubular configuration is associated with an increase in short microtubules over the nucleus that may correspond to the initial step of the manchette formation. The new structure of the cytoskeleton may be associated with major migratory events occurring at this step of differentiation.