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

PUFA-rich phospholipid classes and subclasses of ram spermatozoa are unevenly affected by cryopreservation with a soybean lecithin-based extender

Cryopreservation is known to affect spermatozoa structure and function. Ram sperm are among the most highly sensitive mammalian gametes to freezing, due to their lipid composition, which limit their efficiency in artificial insemination programs. The aim of this study was to investigate the effects of cryopreservation with a chemically defined soybean lecithin-based extender on ram spermatozoa functionality on the one hand, and quantifiable changes in lipid and fatty acid profile on the other. Freeze-thawing decreased sperm quality, as indicated by post-thaw parameters related to membrane integrity, mitochondrial viability and sperm motility. The most relevant lipid change after cryopreservation was a remarkable loss of all glycerophospholipids containing 22:6n-3. Species of sphingomyelin with very long chain polyunsaturated fatty acids (VLC-PUFA), that are exclusively located in the sperm head, where responsible of its reduction after cryostorage. Freezing caused a reduction in mitochondrial function, which was confirmed by significantly decreased of mitochondrial membrane potential and by the generation of 4-HNE. Mitochondria damage was accompanied by a loss in cardiolipin with 18:2n-6 and phosphatidylethanolamine with 20:4n-6, two well-known lipids that are critical components for mitochondrial membrane functionality. Loss of sterols after cryopreservation occurred along with a decrease in the order of sperm membrane lipids. Our research provides new insights on deleterious effects of cryopreservation on PUFA-rich phospholipids of ram sperm and highlight their importance as biomarkers of ultrastructural, biochemical and functional damage that ram spermatozoa undergo after freezing-thawing.

Sperm subpopulations in avian species: a comparative study between the rooster (Gallus domesticus) and Guinea fowl (Numida meleagris)

The main aims of this research were to study possible differences in objective morphometric sperm characteristics, establish normative sperm morphometry standards, and evaluate the presumed different subpopulation distribution of avian spermatozoa from the rooster (Gallus domesticus) and Guinea fowl (Numida meleagris) as model avian species. Seventy-two ejaculates (36 per species studied) were obtained manually, following a training period involving gently combined dorso-abdominal and lumbo-sacral massage of the birds. Ejaculates were processed for volume, sperm concentration, viability, motility, and morphology. Moreover, samples were submitted for sperm morphometric assessment using objective Computer-Assisted Semen Analysis for Morphometry (CASA-Morph) methods, with sperm morphometric descriptors evaluated by Principal Component Analysis (PCA) and multivariate clustering analyses. There were several differences observed between the avian species in values obtained for ejaculate volume and sperm concentration (P < 0.001). Irrespective of species, PCA revealed two Principal Components (PCs) explaining more than 80% of the variance. In addition, the number of subpopulations differed with species (three and five subpopulations for rooster and Guinea fowl, respectively). Moreover, the distribution of the sperm subpopulations was found to be structurally different between species. In conclusion, our findings from using CASA-Morph methods indicate pronounced sperm morphometric variation between these two avian species. Because of the strong differences observed in morphometric parameter values and their subpopulation distribution, these results suggest that application of objective analytical methods such as CASA-Morph could substantially improve the reliability of comparative studies and help establish valid normative sperm morphological values for avian species.

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.

Mass spectrometric detection of cholesterol oxidation in bovine sperm

We report on the presence and formation of cholesterol oxidation products (oxysterols) in bovine sperm. Although cholesterol is the most abundant molecule in the membrane of mammalian cells and is easily oxidized, this is the first report on cholesterol oxidation in sperm membranes as investigated by state-of-the-art liquid chromatographic and mass spectrometric methods. First, oxysterols are already present in fresh semen samples, showing that lipid peroxidation is part of normal sperm physiology. After chromatographic separation (by high-performance liquid chromatography), the detected oxysterol species were identified with atmospheric pressure chemical ionization mass spectrometry in multiple-reaction-monitoring mode that enabled detection in a broad and linear concentration range (0.05-100 pmol for each oxysterol species detected). Second, exposure of living sperm cells to oxidative stress does not result in the same level and composition of oxysterol species compared with oxidative stress imposed on reconstituted vesicles from protein-free sperm lipid extracts. This suggests that living sperm cells protect themselves against elevated oxysterol formation. Third, sperm capacitation induces the formation of oxysterols, and these formed oxysterols are almost completely depleted from the sperm surface by albumin. Fourth, and most importantly, capacitation after freezing/thawing of sperm fails to induce both the formation of oxysterols and the subsequent albumin-dependent depletion of oxysterols from the sperm surface. The possible physiological relevance of capacitation-dependent oxysterol formation and depletion at the sperm surface as well as the omission of this after freezing/thawing semen is discussed.

Capacitation-associated protein tyrosine phosphorylation and membrane fluidity changes are impaired in the spermatozoa of asthenozoospermic patients

Sperm protein tyrosine phosphorylation has been associated with capacitation, motility changes, zona binding, and fertilizing ability. We previously demonstrated that gradient-isolated human sperm subpopulations differ in their plasma membrane composition, their ability to phosphorylate proteins in tyrosine residues, and their capacity to undergo hyperactivation. In this study, we have characterized capacitation-associated changes in protein tyrosine phosphorylation and membrane fluidity in spermatozoa of asthenozoospermic and normozoospermic patients consulting for infertility. Semen samples were studied at baseline and after a capacitating incubation with or without the addition of a permeable cAMP analog and a phosphodiesterase inhibitor. Basic sperm and computer-assisted motion parameters, hyperactivation, protein tyrosine phosphorylation (immunofluorescence and Western blot), and membrane fluidity (fluorescent Laurdan probe) were the main study parameters. In comparison with normozoospermic and proven-fertile donor semen, asthenozoospermic samples showed lower motility, velocity, and amplitude of lateral head displacement, both originally and after a 6-h capacitating incubation. Unlike those in normal samples, asthenozoospermic spermatozoa were unable to increase protein tyrosine phosphorylation during capacitation. Such impairment, however, was overcome when they were incubated with a membrane-permeable cAMP analog and a phosphodiesterase inhibitor, indicating a possible membrane defect. Confirming this hypothesis, plasma membranes of asthenozoospermic sperm showed decreased fluidity (increased Laurdan polarization), even after a capacitating incubation. In conclusion, spermatozoa from functional asthenozoospermic samples may owe their poor motility, and their inability to properly capacitate and develop hyperactivation, to an impairment in the tyrosine phosphorylation of critical proteins caused by decreased membrane fluidity. These findings suggest a molecular pathogenetic mechanism for a common seminal pathology associated with male infertility.

Seminal DNase frees spermatozoa entangled in neutrophil extracellular traps

Insemination always stimulates neutrophil migration into the female reproductive tract (FRT), which eliminates excess spermatozoa and bacterial contaminants introduced by the breeding process. However, the presence of neutrophils in the FRT at the time of semen deposition has been shown to result in sperm-neutrophil binding that reduces motility and fertility. Although the binding and trapping mechanism has not been determined, seminal plasma (SP) was found to include a protein factor or factors that reduced sperm-neutrophil binding and improved fertility of sperm inseminated in the presence of neutrophils. Although DNase has been shown to be present in the SP of different species and has been associated with improved fertility in bulls, the mechanism(s) explaining this association and the paradox of DNA-packed cells being associated with DNase have remained unresolved. We demonstrate that sperm-activated neutrophils extrude their DNA, which in turn traps sperm cells and hinders their motility (and ultimately may hinder sperm transport to the fertilization site). DNase activity present in the SP digests the extruded DNA and frees entangled spermatozoa, which in turn may allow more spermatozoa to reach the oviduct, and explains at least one mechanism by which SP increases the rate of fertility. The ability of SP proteins to suppress neutrophil activation in the presence of spermatozoa did not render neutrophils incapable of combating bacteria, demonstrating that SP proteins are highly selective for suppressing neutrophils activated by spermatozoa, but not by bacteria.

Influence of boar and semen parameters on motility and acrosome integrity in liquid boar semen stored for five days

Ninety ejaculates from a total of 76 AI boars were extended in Beltsville Thawing Solution (BTS). Boar identity, breed, weight of the ejaculate and sperm concentration were registered. Motility and acrosome integrity were assessed after storage at 16-18 degrees C for 6, 30, 54, 78, and 102 h. Storage time had a significant influence on both motility (p < 0.01) and acrosome integrity (p < 0.001). The Least Square Means for percentage of motility showed a small decline from 79.8% after 6 h of storage to 78.4% at 102 h. Motility at 78 and 102 h was significantly different from motility at 6 h (p < 0.05). The percentage of sperm cells with normal acrosomes declined throughout the experiment. The Least Square Means for 6, 30, 54, 78, and 102 h of storage were 93.9%, 90.6%, 88.0%, 84.8%, and 78.2%, respectively. The decrease in acrosome integrity from one storage time to the next was highly significant throughout the trial (p < 0.001). There was a significant influence of boar (p < 0.001) and sperm concentration (p < 0.01) on motility, while acrosome integrity was affected only by boar (p < 0.001). Breed of the boars and weight of the ejaculate did not influence the dependent variables.

Assessment of sperm quality: a flow cytometric approach

For many years, scientists have sought to develop laboratory assays that accurately predict the fertilizing capacity of a semen sample. This goal, however, has proven elusive and will most likely be very difficult to achieve, due to the complex nature of the problem. Part of the problem results from the many attributes that a spermatozoon must possess to fertilize an egg, and how laboratory assays can evaluate all of these attributes simultaneously. The percentage of motile sperm in a sample is most commonly used to evaluate semen quality. This assay, however, is not highly correlated with the fertilizing capacity of semen samples. One reason motion assays do not correlate well with fertility is that we are evaluating only one of many attributes that a sperm must possess to fertilize an oocyte. One of the problems of measuring multiple sperm attributes is the time and cost required. Using flow cytometric assays, multiple sperm attributes, including cell viability, acrosomal integrity, and mitochondrial function, can be measured simultaneously in sperm cells. In addition, the ability of sperm to undergo capacitation and the acrosome reaction, as well as the chromosomal integrity of sperm can be measured using flow cytometry. Flow cytometry permits us to evaluate 50,000 sperm in less then 1 min and at reasonable cost. Although flow cytometry is a powerful tool for evaluating many sperm attributes, it cannot evaluate all of the attributes a sperm cell requires to fertilize an oocyte. Therefore, laboratory assays are also being developed to evaluate the ability of sperm: (1) to bind to the oocyte, by evaluating the ability of sperm to bind to the perivitelline membrane of the hen egg in vitro; (2) to undergo an acrosome reaction in vitro, after treatment with membrane destabilizing compounds; and (3) to penetrate oocytes in vitro. When data from multiple sperm assays are used, higher correlations with the fertilizing potential of a semen sample is achieved. For example, in a study conducted utilizing five stallions, the percentage of motile sperm in semen samples correlated poorly with fertility (r(2)=0.22), however, when data for sperm motility, viability and penetration rates into zona-free hamster oocytes were utilized together, these data explained 72% of the differences in the fertility of the stallions (r=0.849; [Theriogenology 46 (1996) 559]). Armed with a battery of tests, which evaluate many different sperm attributes, researchers should be able to more accurately estimate the fertilizing potential of semen samples.

Cryopreservation alters membrane sulfhydryl status of bull spermatozoa: protection by oxidized glutathione

Cryopreservation induces extensive biophysical and biochemical changes in the membrane of spermatozoa that ultimately decrease the fertility potential of the cells. Sulfhydryl groups of sperm proteins regulate a number of activities of the cells. Qualitative and quantitative analyses of sulfhydryl groups in the sperm membrane were performed by fluorescence microscopy, fluorimetry and electrophoresis. Fluorimetric analysis using 5-iodoacetamidofluoresceine indicated a two-fold increase in the content of sulfhydryl groups in sperm membrane after a freezing/thawing cycle. Electrophoresis of Triton-soluble sperm proteins after labeling with 3-(N-maleimidylpropionyl) biocytin indicated that proteins of 40-65 and 34 kDa range expose more sulfhydryl groups after cooling at 4 degrees C and freezing/thawing. Cryopreservation of spermatozoa changed the distribution pattern of sulfhydryl groups on sperm surface measured with fluorescence microscopy using 5-iodoacetamidofluoresceine. The percentage of spermatozoa labeled at the level of the mid-piece decreased by 50 and 90% after cooling and freezing/thawing, respectively. Spin labeling studies showed a 15% faster rotational diffusion (mobility) of sulfhydryl containing proteins in the membrane of frozen/thawed spermatozoa as compared to that of fresh spermatozoa. Addition of glutathione, reduced (GSH) or oxidized (GSSG), to the cryoprotectant partially prevented the effects of freezing/thawing, such as higher exposure of sulfhydryl groups, changes in the cellular distribution, and enhanced rotational diffusion of sulfhydryl containing proteins of sperm membrane. Addition of GSSG to the cryoprotectant reduced by 35% the loss of motility of spermatozoa undergoing a freezing/thawing cycle. We concluded that cryopreservation perturbs sperm membrane sulfhydryl containing proteins and that these modifications could be partially prevented by the addition of GSSG to the cryopreservation medium.

Laurdan fluorescence: a simple method to evaluate sperm plasma membrane alterations

OBJECTIVE

To determine, by a simple fluorescence method, sperm plasma membrane alterations related with changes of lipid bilayer that, together with routine semen analysis, could help to elucidate the causes of the unexplained male infertility problems.

DESIGN

Pilot study.

SETTING

Andrology laboratory and biochemistry institute, medical school.

PATIENT(S)

Men whose semen was studied for infertility problems.

INTERVENTIONS(S)

No therapeutic intervention was performed on patients.

MAIN OUTCOME MEASURE(S)

Presence of spermatozoa plasma membrane alterations evidenced by evaluation of Laurdan fluorescence Generalized Polarization (GP) and reported as a function of increasing cell concentration, spermatozoa total motility, linear speed, and vitality.

RESULT(S)

Reporting GP values as a function of increasing sperm cell concentration, it is evident that the samples are distributed in two distinct areas: at >32 x 10(6) cells per milliliter, mean GP value was 0.303 +/- 0.015, whereas for lower sperm cell concentrations, the mean GP was 0.365 +/- 0.026 (P<.001). These data indicate that the spermatozoa plasma membranes are characterized by liquid-crystalline phases with different ordering degree and polarity and that about 50% of samples with normal semen characteristics (> or =20 x 10(6) cells per milliliter) show high GP values.

CONCLUSION(S)

Laurdan fluorescence can be used as a simple method to evaluate spermatozoa plasma membrane alterations, particularly in a group of infertile men presenting normal semen parameters. In these samples, Laurdan could be used as a simple tool for infertility assessment. In fact, it is known that compositional and physicochemical alterations of bilayer features can be important for the fertilizing ability of spermatozoa because they are necessary for a proper physiological membrane activity.