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

Metal deposition and shape reproduction at biological temperatures on cell-level samples

The use of metal deposition has been limited to a limited number of applicable samples due to the increased temperature caused by accelerated electron impact on the substrate surface. The surfaces of various biological samples have a nanoscale structure with specific properties, which have been simulated in numerous studies. However, no examples of nano/microscale reproductions of biological surface features have used moulds. In this study, a mould that imitates the surface shape of a cellular-level biological material was fabricated, for the first time, and the shape was successfully reproduced using the mould. Al thin films were deposited on bovine sperm using magnetron sputtering without thermal denaturation with a cathode operating at a biological temperature. It is difficult to deposit films used as metal coatings on pre-treated biological materials at temperatures below 40 °C during evaporation. The Al thin film was peeled off and used as a mould to reproduce the shape of the sperm with high accuracy using a polymer. The results of this study represent a major innovation in reproducible biomimetic moulding technology, demonstrating biological temperature sputtering. We expect our non-destructive metal deposition and metal nano-moulding methods for biological samples to be the basis for the effective utilization of various biological structures.

Boar spermadhesin AWN: Novel insights in its binding behavior and localization on sperm

As a major spermadhesin first found in the seminal plasma of boars, AWN is described to fulfil a variety of reproduction related tasks. Although being the best investigated boar spermadhesin, information about its interaction with membranes is inconsistent. In this regard, previous reports locate AWN either inside or on the surface of sperm cells and at different regions, depending on the method and antibody used. Here, we localize native AWN in/on epididymal, ejaculated, capacitated and acrosome-reacted boar sperm using epifluorescence and electron microscopy, as well as an analysis of potential lipid binding partners of native and recombinant AWN. By applying a custom-made anti-AWN antibody, localization of AWN in the equatorial segment of ejaculated, capacitated and acrosome-reacted boar sperm was discovered. Electron microscopy showed that AWN is localized both on the sperm surface and on the cytoplasmic side of the plasma membrane, and in close vicinity to the nuclear and both acrosomal membranes of sperm. Analysis of epididymal sperm indicated migration of AWN from the retral postacrosomal part to the equatorial segment during the epididymal passage. In contrast to hypotheses claiming a specific association of AWN to phosphatidylethanolamine and in line with our previous study describing an interaction with phosphatidic acid, the current results show a rather electrostatically-driven binding mechanism of AWN to negative lipids. In conclusion, this work provides new insights into the arrangement of AWN in the equatorial segment that suggest a possible role in sperm-oocyte fusion.

The Fertilization Enigma: How Sperm and Egg Fuse

Fertilization is a multistep process that culminates in the fusion of sperm and egg, thus marking the beginning of a new organism in sexually reproducing species. Despite its importance for reproduction, the molecular mechanisms that regulate this singular event, particularly sperm-egg fusion, have remained mysterious for many decades. Here, we summarize our current molecular understanding of sperm-egg interaction, focusing mainly on mammalian fertilization. Given the fundamental importance of sperm-egg fusion yet the lack of knowledge of this process in vertebrates, we discuss hallmarks and emerging themes of cell fusion by drawing from well-studied examples such as viral entry, placenta formation, and muscle development. We conclude by identifying open questions and exciting avenues for future studies in gamete fusion. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Immunolocalization of Fertilin β, IZUMO1, and P34H in Ram Spermatozoa

According to various reports, current methods of sperm freezing destroy the integrity of the sperm plasma membrane and acrosome. This study aimed to determine the changes in the existence and location of three proteins, namely fertilin β, IZUMO1, and P34H, in ram spermatozoa. By using frozen-thawed spermatozoa, ejaculated fresh spermatozoa, and testicular and epididymal spermatozoa (obtained from caput, corpus, and caudal regions), the localizations of the mentioned proteins were performed using signal labeling with indirect immunofluorescence, and the quantification of these proteins was compared using Western blot analyses. Moreover, protein localization and signal labeling in fresh and frozen-thawed spermatozoa subjected to in vitro capacitation and acrosome reaction were compared. Using chlortetracycline (CTC) staining, as expected, it was detected that after incubating for 4 hours under capacitating conditions related to the control sample (0 hour), capacitated and acrosome-reacted sperm were increased (p < 0.001). Frozen-thawed samples had a lower density and expression than the ejaculate samples. Expression was not obtained, except for IZUMO1, from samples that underwent in vitro capacitation/acrosome reactions. Expression of IZUMO1 was seen as an increasing band formation from the equatorial region through the acrosome, after in vitro capacitation. However, after the acrosome reaction, the band formation was only on the equatorial region. Region-specific differences of proteins at the kDa level were obtained using Western blot analysis and possible isoforms specific to ram spermatozoa or proteins with similar epitopes were expressed. Considering the changes in surface proteins in frozen-thawed sperm, it is suggested that fertilin β and P34H can be used as fertility or freezability markers.

Morphometric Characteristics of the Spermatozoa of Blue Fox (Alopex lagopus) and Silver Fox (Vulpes vulpes)

Simple Summary

The present study describes a detailed morphometric analysis of the sperm of the blue fox (Alopex lagopus) and silver fox (Vulpes vulpes), together with determination of the shape indices of the sperm head. Staining with silver nitrate enables precise identification of the acrosome and reveals structural details of the sperm tail, so that they can be accurately measured. Statistically significant differences were found for most of the morphometric parameters of the two fox species. The blue fox sperm were generally larger, but the acrosome area and coverage were greater in the silver fox. There are no clear recommendations regarding sperm staining techniques for foxes, and no reference values for morphometric parameters of the sperm of foxes or for canines in general. Staining with silver nitrate for evaluation of the morphometry of fox sperm can be used as an independent technique or an auxiliary technique in routine analysis of canine semen.

Abstract

The results presented in this study are the first such extensive characterization of the sperm morphometry of the blue fox (Alopex lagopus) and silver fox (Vulpes vulpes), as representatives of the family Canidae. Canine spermatozoa, especially the sperm of farmed foxes, are not often described in studies on reproduction. The aim of the study was a detailed comparison of the morphometric dimensions and shape of the sperm of two fox species: silver fox and blue fox. Semen collected from 10 silver foxes and 10 blue foxes was used for the study. The specimens were stained with silver nitrate. Measurements were performed of the length, width, perimeter, and area of the head; the area of the acrosome and its coverage; the length of the midpiece and its coverage; the length of the tail; and the length of the end piece of the tail. In addition, four head shape indices were calculated: ellipticity, elongation, roughness and regularity. The following values for the morphometric parameters and shape indices were obtained for blue fox and silver fox, respectively: head length—6.72 µm and 6.33 µm; head width—4.54.µm and 4.21 µm; head perimeter—18.11 µm and 17.37 µm; head area—21.94 µm² and 21.11 µm²; acrosome area—11.50 µm² and 10.92 µm²; midpiece length—12.85 µm and 12.79 µm; tail end piece length—3.44 µm and 3.28 µm; tail length—65.23 µm and 65.09 µm; acrosome coverage—52.43% and 52.83%; midpiece coverage—19.71% and 19.65%; sperm length—71.95 µm and 71.42 µm; ellipticity—1.49 and 1.52; elongation—0.19 and 0.20; roughness—0.84 and 1.88; regularity—1.09 and 0.99. The significance of differences between species was verified by Tukey’s test at p ≤ 0.05. Statistically significant differences between species were found for the following parameters: head length, width, perimeter and area; acrosome area; tail, end piece, and total sperm length; roughness and regularity. The differences in the size and shape of sperm can be used to establish reference patterns for fox sperm enabling more accurate species identification.

Postnatal differentiation and regional histological variations in the ductus epididymidis of the Congjiang Xiang pig

The Congjiang Xiang pig is a rare Chinese miniature breed whose epididymal histologic features through the postnatal development are poorly understood. To clarify the histomorphological differences between each region of epididymis during postnatal development, 24 male Congjiang Xiang pigs aged from neonatal (15 d), peri-puberty (30 d), puberty (60 d) to adult (180 d) stages, were examined. Postnatal differentiation of the different regions (I-V) of the epididymis started from birth and continued until maturity that showed regional variations. Developmental progression was disto-proximal. At the neonatal stage, Wolffian duct differentiation starts in the distal region, then ascends to the middle region which forms regions V, IV and III, respectively. A simple lined cuboidal in the epidydimal epithelial, which gradually differentiated into a pseudostratified columnar with stereocilia from neonatal to post-pubertal. After puberty cell rearrangement occurred in the epithelium, differentiation accelerated, and spermatozoon seen in the lumen, especially the lumen of region II. In region III, both halo and apical cells were frequently observed. At the post-pubertal stage, clear cells were frequently observed in Region IV-V, and the epididymal duct was markedly increased in size and fully packed with spermatozoa. The information presented in this study will be helpful for future evaluations of Congjiang Xiang pig fertility. After puberty cell rearrangement occurred in the epithelium, differentiation accelerated, and spermatozoon seen in the lumen, especially the lumen of region II. In region III, both halo and apical cells were frequently observed. At the post-pubertal stage, clear cells were frequently observed in Region IV-V, and the epididymal duct was markedly increased in size and fully packed with spermatozoa. The information presented in this study will be helpful for future evaluations of Congjiang Xiang pig fertility.

High-throughput sperm assay using label-free microscopy: morphometric comparison between different sperm structures of boar and stallion spermatozoa

The capacity for microscopic evaluation of sperm is useful for assisted reproductive technologies (ART), because this can allow for specific selection of sperm cells for in vitro fertilization (IVF). The objective of this study was to analyze the same sperm samples using two high-resolution methods: spatial light interference microscopy (SLIM) and atomic force microscopy (AFM) to determine if with one method there was more timely and different information obtained than the other. To address this objective, there was evaluation of sperm populations from boars and stallions. To the best of our knowledge, this is the first reported comparison when using AFM and high-sensitivity interferometric microscopy (such as SLIM) to evaluate spermatozoa. Results indicate that with the use of SLIM microscopy there is similar nanoscale sensitivity as with use of AFM while there is approximately 1,000 times greater throughput with use of SLIM. With SLIM, there is also allowace for the measurement of the dry mass (non-aqueous content) of spermatozoa, which may be a new label-free marker for sperm viability. In the second part of this study, there was analysis of two sperm populations. There were interesting correlations between the different compartments of the sperm and the dry mass in both boars and stallions. Furthermore, there was a correlation between the dry mass of the sperm head and the length and width of the acrosome in both boars and stallions. This correlation is positive in boars while it is negative in stallions.

Time-resolved Fluorescence and Generalized Polarization: Innovative tools to assess bull sperm membrane dynamics during slow freezing

During slow freezing, spermatozoa undergo membrane alterations that compromise their ability of fertilizing. These alterations are cause either by cold shock or by the use of cryoprotectants known to be cytotoxic. However, little is known about the membrane changes that occurred during freezing. Here, we combined Generalized Polarization (GP), Time-resolved Fluorescence and laurdan fluorescence properties to investigate the changes in membrane fluidity and dynamics during slow freezing of bull sperm. We successfully demonstrated that laurdan may be distributed in three different local environments that correspond to different membrane lipid composition. These environments wont behave the same way when the cells will be subjected to either a chemical treatment (adding the cryoprotectants) or a physical treatment (freezing).

An illustration of human sperm morphology and their functional ability among different group of subfertile males

Condensed sperm chromatin is a prerequisite for natural fertilization. Some reports suggested the prevalence of chromatin condensation defects in teratozoospermia cases with head anomalies; conversely, earlier studies exemplified its occurrence in morphologically normal spermatozoa too. The aim of this study was to compare the condensation defects in correlation with head anomalies among different groups of subfertile males and its impact on the rate of fertilization in assisted reproduction procedures. Ultrastructure analysis of spermatozoa through scanning electron microscopy and atomic force microscopy could facilitate an in-depth evaluation of sperm morphology. Nuclear condensation defects (%) in spermatozoa were analyzed in 666 subjects, and its effect on the rate of fertilization was analyzed in 116 IVF and 90 intracytoplasmic sperm injection cases. There was no correlation of condensation defects with head anomalies (%). Student's t-test showed no significant changes in mean values of condensation defects in abnormal semen samples in comparison with the normal group. Condensation defects were observed in normal spermatozoa too, which was negatively associated with the rate of fertilization in IVF (p < 0.01), but intracytoplasmic sperm injection outcome remained unaffected. Ultrastructure study revealed sperm morphological features in height, amplitude, and three-dimensional views in atomic force microscopy images presenting surface topography, roughness property of head, and compact arrangement of mitochondria over axoneme with height profile at nanoscale. In pathological forms, surface roughness and nuclear thickness were marked higher than the normal spermatozoa. Thus, percentage of normal spermatozoa with condensation defects could be a predictive factor for the rate of fertilization in IVF. From diverse shapes of nucleus in AFM imaging, it could be predicted that defective nuclear shaping might be impeding the activity of some proteins/ biological motors, those regulate the proper Golgi spreading over peri-nuclear theca.

Protein biomarkers for male artificial insemination subfertility in bovine spermatozoa

Background

Although artificial insemination (AI) technique is an established biotechnology for bovine reproduction, the results of AI (conception rates) have a tendency to decline gradually. To our annoyance, moreover, AI‐subfertile bulls have been occasionally found in the AI centers. To resolve these serious problems, it is necessary to control the sperm quality more strictly by the examinations of sperm molecules.

Methods

We reviewed a number of recent articles regarding potentials of bovine sperm proteins as the biomarkers for bull AI‐subfertility and also showed our unpublished supplemental data on the bull AI‐subfertility associated proteins.

Main findings

Bull AI‐subfertility is caused by the deficiency or dysfunctions of various molecules including regulatory proteins of ATP synthesis, acrosomal proteins, nuclear proteins, capacitation‐related proteins and seminal plasma proteins.

Conclusion

In order to control the bovine sperm quality more strictly by the molecular examinations, it is necessary to select suitable sperm protein biomarkers for the male reproductive problems which happen in the AI centers.

Translocation of Tektin 3 to the equatorial segment of heads in bull spermatozoa exposed to dibutyryl cAMP and calyculin A

Tektins (TEKTs) are filamentous proteins associated with microtubules in cilia, flagella, basal bodies, and centrioles. Five TEKTs (TEKT1, -2, -3, -4, and -5) have been identified as components of mammalian sperm flagella. We previously reported that TKET1 and -3 are also present in the heads of rodent spermatozoa. The present study clearly demonstrates that TEKT2 is present at the acrosome cap whereas TEKT3 resides just beneath the plasma membrane of the post-acrosomal region of sperm heads in unactivated bull spermatozoa, and builds on the distributional differences of TEKT1, -2, and -3 on sperm heads. We also discovered that hyperactivation of bull spermatozoa by cell-permeable cAMP and calyculin A, a protein phosphatase inhibitor, promoted translocation of TEKT3 from the post-acrosomal region to the equatorial segment in sperm heads, and that TEKT3 accumulated at the equatorial segment is lost upon acrosome reaction. Thus, translocation of TEKT3 to the equatorial segment may be a capacitation- or hyperactivation-associated phenomenon in bull spermatozoa. Mol. Reprod. Dev. 84: 30-43, 2017. © 2016 Wiley Periodicals, Inc.

Dynamic Changes in Equatorial Segment Protein 1 (SPESP1) Glycosylation During Mouse Spermiogenesis

ESP1/SPESP1 is a testis-specific, postmeiotic gene expressed in round spermatids that encodes equatorial segment protein 1, an intra-acrosomal protein found in the acrosomal matrix and on the luminal surface of the inner and outer acrosomal membranes within the equatorial segment domain of mature spermatozoa. A comparison of testicular protein extracts with caput, corpus, and caudal epididymal sperm proteins revealed striking differences in the apparent masses of SPESP1 isoforms. The predominant isoforms of SPESP1 in the testis were 77 and 67 kDa, with 47-kDa forms present to a minor degree. In contrast, SPESP1 isoforms of 47 and 43 kDa were found in caput, corpus, and caudal sperm, indicating that SPESP1 undergoes noticeable mass changes during spermiogenesis and/or subsequent transport to the epididymis. On two-dimensional (2D) SDS-PAGE, testicular SPESP1 isoforms resolved as a train of pI values from 4.9 to 5.2. Immunoprecipitated 77-kDa SPESP1 from testis reacted with the glycoprofile stain after one-dimensional and 2D gel electrophoresis, indicating that the 77-kDa testicular isoform was highly glycosylated. One charge variant of the 67-kDa isoform was also glycoprofile positive after 2D gel resolution. The 47- and 43-kDa isoforms of SPESP1 from epididymal sperm did not stain with glycoprofile, suggesting an absence of, or few, glycoprofile-sensitive glycoconjugates in epididymal SPESP1. Treatment of testicular extracts with a variety of glycosidases resulted in mass shifts in immunoreactive SPESP1, indicating that testicular SPESP1 was glycosylated and that terminal sialic acid, N- and O-glycans were present. A mixture of deglycosidase enzymes (including PNGase-F, neuraminidase, beta1-4 galactosidase, endo-alpha-N-acetylgalactosaminidase, and beta N-acetyl-glucosaminidase) completely eliminated the 77- and 67-kDa SPESP1 bands and resulted in the appearance of 75-, 60-, 55-, 50-, 47-, and 43-kDa forms, confirming that both the 77- and 67-kDa testicular forms of SPESP1 contain complex carbohydrate residues. Treatment of caudal epididymal sperm with PNGase-F enzymes showed a faint deglycosylated band at 30 kDa, but neuraminidase did not result in any molecular shift, indicating that epididymal sperm SPESP1 did not contain sialic acid/N-acetylglucosamine residues. These findings are consistent with the hypothesis that SPSPESP1 undergoes significant glycosylation in the testis and that the majority of these glycoconjugates are removed by the time sperm reach the caput epididymis. Studies of the fate of SPESP1 after the acrosome reaction localized SPESP1 to the equatorial segment region in both noncapacitated and capacitated, acrosome-reacted sperm. During capacitation, SPESP1 underwent proteolysis, resulting in a 27-kDa fragment. Zona-free oocytes incubated with recSPESP1 protein showed complementary binding sites on the microvillar oolemmal domain. Both recSPESP1 and anti-recSPESP1 antibody inhibited in vitro fertilization.

TMEM225: a possible protein phosphatase 1γ2 (PP1γ2) regulator localizes to the equatorial segment in mouse spermatozoa

Tmem225 encodes a putative four-transmembrane domain protein that has an RVxF motif, which is known to be a consensus site for interacting with serine/threonine protein phosphatase 1 (PP1). We previously identified Tmem225 as one of 53 spermatogenesis-associated transmembrane protein genes, with no known physiological function. In this study, we investigated the expression and molecular characteristics of TMEM225 in mice. Tmem225 production was found to be specific to testicular germ cells, with expression increasing during spermatogenesis. In mature spermatozoa, TMEM225 is localized to the equatorial segment of the acrosome but not to the midpiece or tail. TMEM225 appears to be an outer and/or inner acrosomal membrane protein that is lost from the dorsal region of the acrosome after the acrosome reaction. TMEM225 interacts with PP1 in vivo, and a pull-down assay revealed that the carboxy-terminal region of TMEM225 can bind to PP1γ2, the predominant isoform of PP1 in male germ cells. In addition, TMEM225 inhibited PP1γ2 activity in vitro via its RVxF motif. Our results suggest that in mice, TMEM225 is involved in the differentiation and function of spermatozoa through the regulation of PP1γ2 activity, which is necessary for normal spermatogenesis as well as spermatozoa capacitation and motility.

Nanoscale differences in the shape and size of X and Y chromosome-bearing bovine sperm heads assessed by atomic force microscopy

Sperm dimensions and the question of whether X and Y chromosome-bearing sperm differ in size or shape has been of great interest, especially for the development of alternative methods to sort or classify sperm cells. The aim of the present study was to evaluate possible differences in the shape and size of the sperm head between X and Y chromosome-bearing sperm by atomic force microscopy (AFM). One ejaculate per bull (n = 4) was used. Each ejaculate was separated into four fractions: non-sexed (NS), sexed for X-sperm (SX), sexed for Y-sperm (SY) and a pooling of SX and SY samples (SXY). Using AFM, 400 sperm heads per group were measured. Twenty three structural features were assessed including one-, two- and three-dimensional parameters and shape descriptors. These measurements determine the micro- to nanoscale features of X- and Y-bearing chromosomes in sperm cells. No differences were observed for any individual variables between SX and SY groups. Next, a simultaneous evaluation of all features using statistical discriminant analysis was performed to determine if it was possible to distinguish to which group belong each individual cells. This analysis clearly showed, a distinct separation of NS, SXY, SX and SY groups. The recognition of this structural possibility to distinguish between X and Y sperm cell might improve the understanding of sperm cells biology. These results indicated that the associations of several structural measurements of the sperm cell head are promising candidates for development of a new method of sperm sexing.

Relationship of protein tyrosine phosphorylation state with tolerance to frozen storage and the potential to undergo cyclic AMP-dependent hyperactivation in the spermatozoa of Japanese Black bulls

The aim of this study was to elucidate the relationship between protein tyrosine phosphorylation state and sperm characteristics in frozen-stored spermatozoa of Japanese Black bulls. The spermatozoa were washed with PBS containing polyvinyl alcohol and then incubated with cell-permeable cAMP analog cBiMPS to induce flagellar hyperactivation. Before and after incubation, the spermatozoa were used for immunodetection of tyrosine-phosphorylated proteins, assessment of morphological acrosome condition and evaluation of motility. In bulls whose frozen-stored spermatozoa were classified as having a high-grade acrosome condition before incubation, sperm tyrosine-phosphorylated proteins, including the 33-kDa tyrosine-phosphorylated SPACA1 protein, were localized in the anterior region of the acrosome and equatorial subsegment. The immunodetection level of the 41- and 33-kDa sperm tyrosine-phosphorylated proteins in the Western blots and the immunofluorescence of tyrosine-phosphorylated proteins and SPACA1 proteins in the anterior region of the sperm acrosome were lower in bulls whose frozen-stored sperm were classified as having a low-grade acrosome condition. On the other hand, after incubation with cBiMPS, immunodetection levels of at least 10 tyrosine-phosphorylated proteins increased in the connecting and principal pieces of spermatozoa, coincident with the induction of flagellar hyperactivation. Many of the spermatozoa also exhibited detection patterns similar to those of boar hyperactivated spermatozoa. These results are consistent with the suggestion that immunodetection levels of tyrosine-phosphorylated proteins are valid markers that can predict the level of tolerance to frozen storage and the potential to undergo cAMP-dependent hyperactivation for the spermatozoa of individual Japanese Black bulls.

The equatorial subsegment in mammalian spermatozoa is enriched in tyrosine phosphorylated proteins

The equatorial subsegment (EqSS) was originally identified by atomic force microscopy as a discrete region within the equatorial segment of Artiodactyl spermatozoa. In this investigation, we show that the EqSS is enriched in tyrosine phosphorylated proteins and present preliminary evidence for its presence in mouse and rat spermatozoa. The anti-phosphotyrosine monoclonal antibody (McAb) 4G10 bound strongly and discretely to the EqSS of permeabilized boar, ram, and bull spermatozoa. It also bound to a small patch on the posterior acrosomal region of permeabilized mouse and rat spermatozoa, suggesting that the EqSS is not restricted to the order Artiodactyla. An anti-HSPA1A (formerly Hsp70) antibody recognized the EqSS in boar spermatozoa. Immunogold labeling with McAb 4G10 localized the tyrosine phosphorylated proteins to the outer acrosomal membrane. This was verified by freeze-fracture electron microscopy, which identified the EqSS in three overlying membranes, the plasma membrane, outer acrosomal membrane, and inner acrosomal membrane. In all five species, tyrosine phosphorylated proteins became restricted to the EqSS during sperm maturation in the epididymis. The major tyrosine phosphorylated proteins in the EqSS of boar and ram spermatozoa were identified by mass spectrometry as orthologs of human SPACA1 (formerly SAMP32). Immunofluorescence with a specific polyclonal antibody localized SPACA1 to the equatorial segment in boar spermatozoa. We speculate that the EqSS is an organizing center for assembly of multimolecular complexes that initiate fusion competence in this area of the plasma membrane following the acrosome reaction.

Stabilization of membrane-associated alpha-L-fucosidase by the human sperm equatorial segment

Previous reports from this laboratory documented the existence of two novel isoforms of alpha-L-fucosidase in human semen and showed that membrane-associated alpha-L-fucosidase is cryptically held within the acrosomal compartment and enriched within the sperm equatorial segment. The occurrence of these novel isoforms is provocative. Sperm proteins potentially involved in sperm-egg interactions must maintain their functional integrity as they travel through the female reproductive tract. The goal of this project was to investigate the stability of membrane-associated alpha-L-fucosidase in human sperm. Whole seminal plasma and Percoll purified sperm cell populations were incubated for 72 h at 37 degrees C, with 5% CO(2) or ambient air. At various times during prolonged incubation, sperm cells were permeabilized with 0.1% Triton X-100 and enzyme assays using the fluorogenic substrate 4-MU-fuc were performed to evaluate the stability of both the seminal plasma and membrane-associated alpha-L-fucosidase. Here, we report seminal plasma alpha-L-fucosidase activity rapidly decreased within 24 h. Conversely, alpha-L-fucosidase activity from Percoll purified sperm cell populations persisted up to 72 h. Data from these experiments support the notions that (i) membrane-associated alpha-L-fucosidase is stable for extended periods of time, consistent with a possible role in sperm-egg interaction and (ii) membrane domains and compartmentalization within the human sperm are key to preserving protein integrity.

Presence and subcellular localizations of surfactant proteins A and D in human spermatozoa

OBJECTIVE

To investigate the presence of surfactant protein-A (SP-A); molecular weight 34 kDa and surfactant protein-D (SP-D); and molecular weight 43 kDa in human spermatozoa.

DESIGN

Prospective, research study.

SETTING

Two universities in Turkey.

PATIENT(S)

Semen specimens (n = 10) were obtained from normozoospermic donors.

INTERVENTION(S)

Human sperm were exposed to an anti-human SP-A polyclonal antibody, and monoclonal antibody, to human SP-D protein.

MAIN OUTCOME MEASURE(S)

Presence of SP-A and SP-D proteins in human beings.

RESULT(S)

Indirect immunofluorescence assays of human sperm indicated the presence of SP-A in the mid piece, the tail, and sometimes at the equatorial region of spermatozoa. A brilliant green light detected SP-D in the tails and acrosome of some sperm. The anti-SP-A antibody detected a single band corresponding to the molecular weight values of 34 kDa in spermatozoa, whereas no band was observed in the negative control. The anti-SP-D antibody showed the expected band at 43 kDa in spermatozoa.

CONCLUSION(S)

This is the first report and a novel finding of the presence of surfactant glycoproteins on human spermatozoa.

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.

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.

Immunolocalization of heat shock protein 70 (Hsp 70) in boar spermatozoa and its role during fertilization

The presence and cellular distribution of heat protein 70 (Hsp70) in ejaculated, capacitated, and acrosome-reacted boar spermatozoa was evaluated by immunofluorescence and Western blot; the role of Hsp70 during fertilization was also studied. In freshly ejaculated spermatozoa, Hsp70 immunoreactivity is present in a well-defined triangular-shaped area in the equatorial segment that seems to correspond to the equatorial sub-segment. The distribution of the fluorescent signal changes in capacitated sperm, that exhibit different patterns probably in relation to the stage of capacitation of individual cells; after acrosome reaction Hsp70 immunoreactivity is localized on both a thick sub-equatorial band and a triangle in the equatorial segment. In reacted spermatozoa, Hsp70 seems to be not only relocalized but also translocated from the inner to the outer leaflet of the sperm plasma membrane, as a significant (P < 0.05) increase in the proportion of unfixed cells showing the fluorescent signal has been recorded. No differences in Hsp70 amount between fresh, capacitated, and reacted semen were observed by Western blot. The presence of anti-Hsp70 antibody in the fertilization medium significantly reduced, in a concentration-dependent manner, the fertilization rate of both zona-intact and zona-free oocytes. The overall data demonstrate that Hsp70 is present on boar sperm with a dynamic redistribution as the sperm undergoes capacitation and acrosome reaction and suggest an important role of this protein during porcine gamete interaction.

Atomic force microscopy: a powerful tool for high-resolution imaging of spermatozoa

Atomic force microscopy (AFM) has emerged as the only technique capable of real-time imaging of the surface of a living cell at nano-resolution. Since AFM provides the advantage of directly observing living biological cells in their native environment, this technique has found many applications in pharmacology, biotechnology, microbiology, structural and molecular biology, genetics and other biology-related fields. AFM has also proved to be a valuable tool for reproductive biologists. An exhaustive review on the various applications of AFM to sperm cells is presented. AFM has been extensively applied for determining the structural and topological features of spermatozoa. Unstained, unfixed spermatozoa in their natural physiological surroundings can be imaged by this technique which provides valuable information about the morphological and pathological defects in sperm cells as three-dimensional images with precise topographical details. Sperm head defects and the acrosome at the tip of the head responsible for fertilization, can be examined and correlated with the lack of functional integrity of the cell. Considerable amount of work is reported on the structural details of the highly condensed chromatin in sperm head using AFM. Detailed information on 3D topographical images of spermatozoa acquired by AFM is expected to provide a better understanding of various reproductive pathways which, in turn, can facilitate improved infertility management and/or contraceptive development.

The PATE gene is expressed in the accessory tissues of the human male genital tract and encodes a secreted sperm-associated protein

ThePATEgene is expressed in prostate and testis. To determine if PATE is expressed in other accessory tissues of the male genital tract, RT-PCR of the epididymis and seminal vesicle was performed. PATE mRNA was highly expressed in the epididymis and seminal vesicle.In situhybridization of the testis showed PATE mRNA is strongly expressed in the spermatogonia. ThePATEgene encodes a 14-kDa protein with a predicted signal sequence and a cleavage site between residues G21 and S22. To determine if PATE is a secreted protein, 293T cells were transfected with a pcDNA-PATE-myc-His plasmid and protein immunoprecipitated with anti-myc monoclonal antibody. Western blot analysis showed the presence of PATE-myc-His protein was in the medium and the cell lysate. Confocal microscopy demonstrated that PATE-myc-His protein is found in the endoplasmic reticulum. The polyclonal antibody SOL-1 was generated by immunization of rabbits with recombinant PATE protein expressed and purified fromEscherichia coli.Western blots were performed on extracts of prostate, testis, seminal vesicle and ejaculated spermatozoa, but PATE protein was only detected in the spermatozoa. Immunostaining of sperm smears revealed that PATE is located in a band-like pattern in the sperm head. Our data indicate that PATE is made by various sexual accessory tissues and secreted into the semen where it becomes associated with sperm, suggesting that PATE is a novel sperm-associated protein with a possible role in mammalian sperm maturation.

Sperm head morphology in 36 species of artiodactylans, perissodactylans, and cetaceans (Mammalia)

Detailed descriptions of mammalian sperm morphology across a range of closely related taxa are rare. Most contributions have been generalized descriptions of a few distantly related mammalian species. These studies have emphasized a generalized ungulate sperm morphology, but have not underscored several important morphological differences in ungulate sperm, such as head shape. The present study is the first to document descriptions of sperm head morphology using cold field-emission scanning electron microscopy (FE-SEM) for a large number of closely related mammalian species. In total, the sperm of 36 species in three orders: Artiodactyla (even-toed ungulates), Cetacea (whales, porpoises, and dolphins), and Perissodactyla (odd-toed ungulates) were examined to gather new information relevant to the debate about the phylogenetic placement of cetaceans relative to terrestrial ungulates. In all species examined, the sperm heads were generally flattened and ovate in shape with a distinct apical ridge, although considerable variation in sperm head shape was detected, both within and between orders. In artiodactylans, the sperm head was uniformly flat in lateral view, whereas perissodactylan and cetacean sperm heads showed a distinct posterior thickening. In both artiodactylans and perissodactylans, the mitochondria were elongate and wound in a tight helix around the midpiece, whereas in cetaceans the mitochondria were rounded and appeared to be randomly arranged around the midpiece. Additionally, prominent ridges running along the anterior-posterior axis were observed in the postacrosomal region of the sperm head in four species of cetaceans. These ridges were not observed in any of the terrestrial ungulates examined. Pits or fenestrations were detected in the postacrosomal region in most artiodactylan species examined; these structures were not detected in perissodactylans or cetaceans. The equatorial segment of the acrosome was detected in the artiodactylan species examined, tentatively identified in perissodactylans, but not found in cetaceans. Its shape and location are described for relevant taxa. The presence of a recently reported substructure within the equatorial segment (the equatorial subsegment; Ellis et al. [2002] J Struct Biol 138:187-198) was detected in artiodactylans, and its shape is described for the species examined.

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.

Fine surface structure of bovine acrosome-intact and reacted spermatozoa observed by atomic force microscopy

The atomic force microscope (AFM) provides nanometer resolution, topographic data of the natural surface structure of materials. We studied the topology of the surface structure of bovine sperm heads during the acrosome reaction by AFM. In addition, we numerically analyzed the areas of the median sagittal plane of the sperm heads. Bovine frozen-thawed spermatozoa were washed, capacitated by heparin, and incubated with lysophosphatidylcholine (LPC) to induce the acrosome reaction, smeared on a cover glass, air-dried, and observed with AFM using the dynamic force (tapping) mode. AFM analysis of spermatozoa showed the clear surface structure of acrosomes, equatorial segments, postacrosomal regions and necks. Although AFM images of spermatozoa capacitated by heparin had complete acrosomes, most spermatozoa treated with LPC had no acrosomal caps as shown by AFM. These observations coincided with those obtained by light microscopy after staining with naphthol yellow S and erythrosin B. Furthermore, numerical analysis of AFM images indicated that areas of the median sagittal plane of the anterior portions of acrosome-reacted sperm heads (2679 +/- 616 pixels) were approximately 40% less than those of intact heads (4535 +/- 174 pixels, P<0.05). These results indicate that AFM can usefully observe and numerically analyze the fine surface structures of bovine spermatozoa.

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.

Interactions of sperm perinuclear theca with the oocyte: implications for oocyte activation, anti-polyspermy defense, and assisted reproduction

Perinuclear theca (PT) is the cytoskeletal coat of mammalian sperm nucleus that is removed from the sperm head at fertilization. PT harbors the sperm borne, oocyte-activating factor (SOAF), a yet-to-be-characterized substance responsible for triggering the signaling cascade of oocyte activation, thought to be dependent on intra-oocyte calcium release. The present article reviews the current knowledge on the biogenesis and molecular composition of sperm PT. Possible functions of sperm PT during natural and assisted fertilization, and in the initiation of embryonic development are discussed. Furthermore, evidence is provided that SOAF is transferred from the sperm PT to oocyte cytoplasm through the internalization and rapid solubilization of the post-acrosomal PT. It is shown that during natural fertilization the sperm PT dissolves in the oocyte cytoplasm concomitantly with sperm nuclear decondensation and the initiation of pronuclear development. SOAF activity is preserved in the differentially extracted sperm heads only if the integrity of PT is maintained. After intracytoplasmic sperm injection (ICSI), activation occurs only in those oocytes in which the injected spermatozoon displays complete or partial dissolution of PT. In the latter case, the residual PT of the sub-acrosomal and/or post-acrosomal sperm region may persist on the apical surface of the sperm nucleus/male pronucleus and may cause a delay or arrest of zygotic development. We propose that the sperm PT harbors SOAF in the post-acrosomal sheath, as this is the first part of the sperm cytosol to enter the oocyte cytoplasm and its disassembly appears sufficient to initiate the early events of oocyte activation. Dissolution of the sub-acrosomal part of the PT, on the other hand, appears necessary to insure complete DNA decondensation in the internalized sperm nucleus and initiate DNA synthesis of both pronuclei. The release of the SOAF from the sperm head into oocyte cytoplasm at fertilization ultimately leads to the activation of oocyte mechanism including the completion of the meiotic cell cycle, pronuclear development and anti-polyspermy defense.