A mechanism for differential release of acrosomal enzymes during the acrosome reaction

The correlation between sperm percentage with a small acrosome and unexplained in vitro fertilization failure

PURPOSE

Since the unexplained in vitro fertilization failure occurs frequently, it is of great importance and clinical value to identify potential underlying predictors. This study aimed to explore whether the percentage of sperm with a small acrosome was correlated with unexplained in vitro fertilization failure.

METHODS

A new acrosomal function evaluation index (the percentage of sperm with a small acrosome) was introduced into the analysis of sperm morphology. The association between the index and acrosome function by acrosin activity detection test and acrosome reaction test was investigated. In addition, the correlation with unexplained in vitro fertilization failure was further explored. Finally, the ROC curve was used to analyze the diagnostic efficacy on the failure of in vitro fertilization and the cutoff value was calculated.

RESULTS

As the increasing of the percentage of sperm with a small acrosome, the value of acrosin activity, acrosome reaction rate, and in vitro fertilization rate were reduced, with a statistically significant difference (P < 0.05). The index in the low fertilization rate group was significantly higher than that in the normal fertilization rate group (P < 0.05). Finally, the results of ROC curve found that when the index was 43.5%, the sensitivity and specificity were 74.2% and 95.3%, respectively.

CONCLUSION

The percentage of sperm with a small acrosome was positively correlated with unexplained in vitro fertilization failure, which could be potentially used as a prognostic index for the failure of in vitro fertilization.

TRIAL REGISTRATION

[Ethics review acceptance No IIT20210339B].

The stallion sperm acrosome: Considerations from a research and clinical perspective

During the fertilization process, the interaction between the sperm and the oocyte is mediated by a process known as acrosomal exocytosis (AE). Although the role of the sperm acrosome on fertilization has been studied extensively over the last 70 years, little is known about the molecular mechanisms that govern acrosomal function, particularly in species other than mice or humans. Even though subfertility due to acrosomal dysfunction is less common in large animals than in humans, the evaluation of sperm acrosomal function should be considered not only as a complementary but a routine test when individuals are selected for breeding potential. This certainly holds true for stallions, which might display lower levels of fertility in the face of "acceptable" sperm quality parameters determined by conventional sperm assays. Nowadays, the use of high throughput technologies such as flow cytometry or mass spectrometry-based proteomic analysis is commonplace in the research arena. Such techniques can also be implemented in clinical scenarios of males with "idiopathic" subfertility. The current review focuses on the sperm acrosome, with particular emphasis on the stallion. We aim to describe the physiological events that lead to the acrosome formation within the testis, the role of very specific acrosomal proteins during AE, the methods to study the occurrence of AE under in vitro conditions, and the potential use of molecular biology techniques to discover new markers of acrosomal function and subfertility associated with acrosomal dysfunction in stallions.

Membrane Remodeling and Matrix Dispersal Intermediates During Mammalian Acrosomal Exocytosis

To become fertilization-competent, mammalian sperm must undergo a complex series of biochemical and morphological changes in the female reproductive tract. These changes, collectively called capacitation, culminate in the exocytosis of the acrosome, a large vesicle overlying the nucleus. Acrosomal exocytosis is not an all-or-nothing event but rather a regulated process in which vesicle cargo disperses gradually. However, the structural mechanisms underlying this controlled release remain undefined. In addition, unlike other exocytotic events, fusing membranes are shed as vesicles; the cell thus loses the entire anterior two-thirds of its plasma membrane and yet remains intact, while the remaining nonvesiculated plasma membrane becomes fusogenic. Precisely how cell integrity is maintained throughout this drastic vesiculation process is unclear, as is how it ultimately leads to the acquisition of fusion competence. Here, we use cryoelectron tomography to visualize these processes in unfixed, unstained, fully hydrated sperm. We show that paracrystalline structures within the acrosome disassemble during capacitation and acrosomal exocytosis, representing a plausible mechanism for gradual dispersal of the acrosomal matrix. We find that the architecture of the sperm head supports an atypical membrane fission-fusion pathway that maintains cell integrity. Finally, we detail how the acrosome reaction transforms both the micron-scale topography and the nanoscale protein landscape of the sperm surface, thus priming the sperm for fertilization.

Oxidative profile and protease regulator potential to predict sperm functionality in donkey (Equus asinus)

Seminal plasma (SP) of donkey stallions was evaluated using various oxidative stress parameters as well as protease and protease inhibitor activities. SP was obtained by nine donkey stallions. In addition, one donkey stallion with non-obstructive azoospermia was enrolled in this study. Free radical scavenging activity (FRSA), the ferric reducing ability of plasma (FRAP), total antioxidant capacity (TAC), and total thiol level (TTL) were highly correlated with each other and with the protease inhibitor activity. However, only FRAP, TAC, and the nitrate/nitrite concentration (NOx) were significantly correlated with sperm concentration, production, and kinetics. Protease inhibitor activity was highly correlated with sperm concentration and production; however, it did not correlate with sperm kinetics. The azoospermic stallion produced a lower amount of semen than the normospermic stallions and its SP showed a lower antioxidant activity when evaluated with FRAP, TAC, and TTL as well as a higher NOx and a lower protease inhibitor activity. In conclusion, the evaluation of SP oxidative profile by FRAP, TAC, and NOx may provide reliable information on donkey sperm quality whereas protease inhibitor activity may play a role as a marker of the sperm concentration in this species.

Effect of Photoperiod with Sunlight at Thermal Stress and Sperm Parameters in Guinea Pigs

&lt;b&gt;Background and Objective:&lt;/b&gt; Photoperiod can regulate reproductive physiological processes in mammals, in which improvements in testosterone concentration, testicular volume and seminal quality have been reported. The aim was to evaluate the influence of photoperiod treatments on guinea pigs' spermatic parameters. &lt;b&gt;Materials and Methods:&lt;/b&gt; Thirty guinea pigs, between males and females, were distributed in two rooms with the photoperiodic treatment of 10 hrs light and 14 hrs dark (PT&lt;sub&gt;1&lt;/sub&gt; with artificial photoperiod and PT&lt;sub&gt;2&lt;/sub&gt; photoperiod with sunlight by opening windows from 08:00-18:00) and one without any direct light stimulus (PT&lt;sub&gt;0&lt;/sub&gt;) for 78 days. The temperature and humidity were recorded and the TH index was calculated for each room. The sperms were recovered in Tris base medium from the epididymis of 16 males to determine sperm concentration, motility, kinetic parameters, vitality, HOST, acrosomal integrity and DNA fragmentation. &lt;b&gt;Results:&lt;/b&gt; Sperm values in PT&lt;sub&gt;1&lt;/sub&gt; and PT&lt;sub&gt;0&lt;/sub&gt; were similar but PT&lt;sub&gt;2&lt;/sub&gt; obtained values lower in sperm concentration, non-progressive motility, total motility, VCL, ALH, vitality, HOST+, acrosomal integrity, sperm with non-fragmented DNA and no pregnancies were reported (0/5). A 100% pregnancy was observed in PT&lt;sub&gt;0&lt;/sub&gt; (4/4) and 50% in PT&lt;sub&gt;1&lt;/sub&gt; (2/4). However, precocity was evidenced in PT&lt;sub&gt;1&lt;/sub&gt; compared to PT&lt;sub&gt;0&lt;/sub&gt;. PT&lt;sub&gt;2&lt;/sub&gt; recorded higher peaks in temperature (33.8°C, THI 81, considered as thermal stress) compared to PT&lt;sub&gt;0&lt;/sub&gt; (32.65°C, THI 81.8) and PT&lt;sub&gt;1&lt;/sub&gt; (32.75°C, THI 81.6). &lt;b&gt;Conclusion:&lt;/b&gt; An artificial photoperiod can improve sperm characteristics and reproductive precociousness of guinea pigs, unlike the photoperiod with sunlight, which generated low spermiogram values and absence of pregnancy due to thermal stress.

Functional Amyloids in Reproduction

Amyloids are traditionally considered pathological protein aggregates that play causative roles in neurodegenerative disease, diabetes and prionopathies. However, increasing evidence indicates that in many biological systems nonpathological amyloids are formed for functional purposes. In this review, we will specifically describe amyloids that carry out biological roles in sexual reproduction including the processes of gametogenesis, germline specification, sperm maturation and fertilization. Several of these functional amyloids are evolutionarily conserved across several taxa, including human, emphasizing the critical role amyloids perform in reproduction. Evidence will also be presented suggesting that, if altered, some functional amyloids may become pathological.

Biogenesis of sperm acrosome is regulated by pre-mRNA alternative splicing of Acrbp in the mouse

Proper biogenesis of a sperm-specific organelle, the acrosome, is essential for gamete interaction. An acrosomal matrix protein, ACRBP, is known as a proacrosin-binding protein. In mice, two forms of ACRBP, wild-type ACRBP-W and variant ACRBP-V5, are generated by pre-mRNA alternative splicing of Acrbp Here, we demonstrate the functional roles of these two ACRBP proteins. ACRBP-null male mice lacking both proteins showed a severely reduced fertility, because of malformation of the acrosome. Notably, ACRBP-null spermatids failed to form a large acrosomal granule, leading to the fragmented structure of the acrosome. The acrosome malformation was rescued by transgenic expression of ACRBP-V5 in ACRBP-null spermatids. Moreover, exogenously expressed ACRBP-W blocked autoactivation of proacrosin in the acrosome. Thus, ACRBP-V5 functions in the formation and configuration of the acrosomal granule during early spermiogenesis. The major function of ACRBP-W is to retain the inactive status of proacrosin in the acrosome until acrosomal exocytosis.

Mechanisms of fertilization elucidated by gene-manipulated animals

Capacitation and the acrosome reaction are key phenomena in mammalian fertilization. These phenomena were found more than 60 years ago. However, fundamental questions regarding the nature of capacitation and the timing of the acrosome reaction remain unsolved. Factors were postulated over time, but as their roles were not verified by gene-disruption experiments, widely accepted notions concerning the mechanism of fertilization are facing modifications. Today, although in vitro fertilization systems remain our central research tool, the importance of in vivo observations must be revisited. Here, primarily focusing on our own research, I summarize how in vivo observations using gene-manipulated animals have elucidated new concepts in the mechanisms of fertilization.

Identification of bovine sperm acrosomal proteins that interact with a 32-kDa acrosomal matrix protein

Mammalian fertilization is accomplished by the interaction between sperm and egg. Previous studies from this laboratory have identified a stable acrosomal matrix assembly from the bovine sperm acrosome termed the outer acrosomal membrane-matrix complex (OMC). This stable matrix assembly exhibits precise binding activity for acrosin and N-acetylglucosaminidase. A highly purified OMC fraction comprises three major (54, 50, and 45 kDa) and several minor (38-19 kDa) polypeptides. The set of minor polypeptides (38-19 kDa) termed "OMCrpf polypeptides" is selectively solubilized by high-pH extraction (pH 10.5), while the three major polypeptides (55, 50, and 45 kDa) remain insoluble. Proteomic identification of the OMC32 polypeptide (32 kDa polypeptide isolated from high-pH soluble fraction of OMC) yielded two peptides that matched the NCBI database sequence of acrosin-binding protein. Anti-OMC32 recognized an antigenically related family of polypeptides (OMCrpf polypeptides) in the 38-19-kDa range with isoelectric points ranging between 4.0 and 5.1. Other than glycohydrolases, OMC32 may also be complexed to other acrosomal proteins. The present study was undertaken to identify and localize the OMC32 binding polypeptides and to elucidate the potential role of the acrosomal protein complex in sperm function. OMC32 affinity chromatography of a detergent-soluble fraction of bovine cauda sperm acrosome followed by mass spectrometry-based identification of bound proteins identified acrosin, lactadherin, SPACA3, and IZUMO1. Co-immunoprecipitation analysis also demonstrated the interaction of OMC32 with acrosin, lactadherin, SPACA3, and IZUMO1. Our immunofluorescence studies revealed the presence of SPACA3 and lactadherin over the apical segment, whereas IZUMO1 is localized over the equatorial segment of Triton X-100 permeabilized cauda sperm. Immunoblot analysis showed that a significant portion of SPACA3 was released after the lysophosphatidylcholine (LPC)-induced acrosome reaction, whereas the IZUMO1 and lactadherin polypeptides remain associated to the particulate fraction. Almost entire population of bovine sperm IZUMO1 relocates to the equatorial segment during the LPC-induced acrosome reaction. We propose that the interaction of OMC32 matrix polypeptide with detergent-soluble acrosomal proteins regulates the release of hydrolases/other acrosomal protein(s) during the acrosome reaction.

Acrosome markers of human sperm

Molecular biomarkers that can assess sperm acrosome status are very useful for evaluating sperm quality in the field of assisted reproductive technology. In this review, we introduce and discuss the localization and function of acrosomal proteins that have been well studied. Journal databases were searched using keywords, including "human acrosome", "localization", "fertilization-related protein", "acrosomal membrane", "acrosomal matrix", "acrosome reaction", "knockout mouse", and "acrosome marker".

Identification and characterization of a bovine sperm acrosomal matrix protein and its mechanism of interaction with acrosomal hydrolases

Fertilization, the union of male and female gametes to create offspring, is an intricate biological process dependent upon several biochemical and physiological events. Our understanding of the functions of protein constituents of the outer acrosomal membrane-associated matrix complex (OMC) is limited. A highly purified OMC fraction isolated from bovine cauda sperm heads comprised 54, 50, 45, and 38-19 kDa polypeptides. The objective of this study is to identify and characterize the 45 kDa (OMC45) polypeptide, to define its role in binding acrosomal hydrolases, and to examine the fate of OMC45 polypeptide during the acrosome reaction. We isolated OMC45 polypeptide from the high-pH insoluble fraction of OMC. Proteomic analysis of OMC45 by MALDI-TOF-TOF yielded eight peptides that matched the NCBI database sequence of Tektin 3 (TEKT3). Triton X-100-permeabilized cauda sperm exhibited intense staining of the acrosomal segment with anti-OMC45 and anti-TEKT3. The OMC45 polypeptide was solubilized by radio-immunoprecipitation assay buffer extraction. The solubilized fraction was subjected to immunoprecipitation analysis. The OMC45 polypeptide was recovered in the anti-OMC45 immunoprecipitation pellet. An identical blot stained with anti-TEKT3 exhibited the presence of TEKT3 polypeptide in the anti-OMC45 pellet. Our immunofluorescence and biochemical studies confirm the proteomics identification of OMC45 polypeptide and that it exhibits a sequence similarity to TEKT3. OMC45 glycoprotein possesses both N-linked and O-linked oligosaccharides. Deglycosylated OMC45 revealed a significant reduction in both acrosin and N-acetylglucosaminidase (NAGA) binding in comparison with acrosin and NAGA binding to a native OMC45 polypeptide, demonstrating the important role of oligosaccharides in hydrolase binding. OMC45 polypeptide is not released during the acrosome reaction but remains in the particulate cell subfraction, associated with the hybrid membrane complex.

Functional amyloids in the mouse sperm acrosome

The acrosomal matrix (AM) is an insoluble structure within the sperm acrosome that serves as a scaffold controlling the release of AM-associated proteins during the sperm acrosome reaction. The AM also interacts with the zona pellucida (ZP) that surrounds the oocyte, suggesting a remarkable stability that allows its survival despite being surrounded by proteolytic and hydrolytic enzymes released during the acrosome reaction. To date, the mechanism responsible for the stability of the AM is not known. Our studies demonstrate that amyloids are present within the sperm AM and contribute to the formation of an SDS- and formic-acid-resistant core. The AM core contained several known amyloidogenic proteins, as well as many proteins predicted to form amyloid, including several ZP binding proteins, suggesting a functional role for the amyloid core in sperm-ZP interactions. While stable at pH 3, at pH 7, the sperm AM rapidly destabilized. The pH-dependent dispersion of the AM correlated with a change in amyloid structure leading to a loss of mature forms and a gain of immature forms, suggesting that the reversal of amyloid is integral to AM dispersion.

Identification of differentially expressed proteins in fresh and frozen–thawed boar spermatozoa by iTRAQ-coupled 2D LC–MS/MS

Cryodamage is a major problem in semen cryopreservation, causing changes in the levels of proteins that influence the function and motility of spermatozoa. In this study, protein samples prepared from fresh and frozen–thawed boar spermatozoa were compared using the isobaric tags for relative and absolute quantification (iTRAQ) labeling technique coupled to 2D LC–MS/MS analysis. A total of 41 differentially expressed proteins were identified and quantified, including 35 proteins that were present at higher levels and six proteins that were present at lower levels in frozen–thawed spermatozoa by at least a mean of 1.79-fold (P<0.05). On classifying into ten distinct categories using bioinformatic analysis, most of the 41 differentially expressed proteins were found to be closely relevant to sperm premature capacitation, adhesions, energy supply, and sperm–oocyte binding and fusion. The expression of four of these proteins, SOD1, TPI1, ODF2, and AKAP3, was verified by western blot analysis. We propose that alterations in these identified proteins affect the quality of cryopreserved semen and ultimately lower its fertilizing capacity. This is the first study to compare protein levels in fresh and frozen–thawed spermatozoa using the iTRAQ technology. Our preliminary results provide an overview of the molecular mechanisms of cryodamage in frozen–thawed spermatozoa and theoretical guidance to improve the cryopreservation of boar semen.

The cell biology of mammalian fertilization

Fertilization is the process by which eggs and spermatozoa interact, achieve mutual recognition, and fuse to create a zygote, which then develops to form a new individual, thus allowing for the continuity of a species. Despite numerous studies on mammalian fertilization, the molecular mechanisms underpinning the fertilization event remain largely unknown. However, as I summarize here, recent work using both gene-manipulated animals and in vitro studies has begun to elucidate essential sperm and egg molecules and to establish predictive models of successful fertilization.

Modes of acrosin functioning during fertilization

Mammalian fertilization is a complex process that involves gamete recognition, penetration, and fusion. Biochemical studies that identified the role of acrosome components during sperm-ova interaction especially the zona pellucida (ZP) provided major advances in sperm cell biology. Acrosin (a typical serine protease) functions during fertilization in several significant ways which include: a) activation of acrosome components, b) secondary binding with the ZP, and c) hydrolysis of the ZP. However, studies using knockout (KO) acrosin-deficient mice cast doubt on the traditional role of acrosin in fertilization. The KO acrosin-deficient mice exhibit normal fecundity except for delayed fertilization. Despite the doubt cast on the traditional role of acrosin by the KO acrosin-deficient mouse studies, acrosin still remains a major protease involved in multiple processes of fertilization. In this review, we assess the functional profile of acrosin and briefly summarize recent findings on proteases involved in fertilization. We propose a refined scheme for the functional role of acrosin in fertilization. We particularly emphasize the role of acrosin in acrosome exocytosis and activation of other acrosome components based on advanced technology like structural X-ray analysis.

Flying-fox (Pteropus spp.) sperm membrane fatty acid composition, its relationship to cold shock injury and implications for cryopreservation success

The very large acrosome of Pteropus species spermatozoa is prone to damage during cooling procedures. Cryogenic succuss has been linked to membrane composition, therefore the lipid composition of five Pteropus species sperm acrosomal and plasma membranes were investigated to provide insight into reasons for cold shock susceptibility. Rapid chilling and re-warming of spermatozoa from three Pteropus species resulted in a decrease (P<0.05) in acrosomal integrity. Biochemical analysis of lipids revealed that stearic acid (18:0) was the predominant saturated fatty acid and oleic acid (18:1, n-9) the predominant unsaturated fatty acid in both acrosomal and plasma membranes. Linolenic acid (18:3, n-3) was only detected in plasma membranes of Pteropus hypomelanus and was detected in acrosomal membranes of all Pteropus spp. studied (except Pteropus giganteus). Although detected in both plasma and acrosomal membranes of Pteropus vampyrus, docosahexaenoic acid (22:6) was not detected at all in Pteropus poliocephalus, only in trace levels in the acrosomal and plasma membranes of P. giganteus and P. hypomelanus and not in acrosomal membranes of Pteropus rodricensis. No difference was seen in the levels of polyunsaturated fatty acids (PUFAs) within plasma membranes, however PUFAs were lower (P<0.05) in acrosomal membranes of P. giganteus compared with P. vampyrus. Pteropus spp. spermatozoa have a very low ratio of unsaturated/saturated membrane fatty acids (<0.5). Membranes containing more PUFAs are more fluid, so the use of cryogenic media which improves membrane fluidity should improve Pteropus spp. spermatozoal viability post-thaw.

Isolation and proteomic characterization of the mouse sperm acrosomal matrix

A critical step during fertilization is the sperm acrosome reaction in which the acrosome releases its contents allowing the spermatozoa to penetrate the egg investments. The sperm acrosomal contents are composed of both soluble material and an insoluble material called the acrosomal matrix (AM). The AM is thought to provide a stable structure from which associated proteins are differentially released during fertilization. Because of its important role during fertilization, efforts have been put toward isolating the AM for biochemical study and to date AM have been isolated from hamster, guinea pig, and bull spermatozoa. However, attempts to isolate AM from mouse spermatozoa, the species in which fertilization is well-studied, have been unsuccessful possibly because of the small size of the mouse sperm acrosome and/or its fusiform shape. Herein we describe a procedure for the isolation of the AM from caput and cauda mouse epididymal spermatozoa. We further carried out a proteomic analysis of the isolated AM from both sperm populations and identified 501 new proteins previously not detected by proteomics in mouse spermatozoa. A comparison of the AM proteome from caput and cauda spermatozoa showed that the AM undergoes maturational changes during epididymal transit similar to other sperm domains. Together, our studies suggest the AM to be a dynamic and functional structure carrying out a variety of biological processes as implied by the presence of a diverse group of proteins including proteases, chaperones, hydrolases, transporters, enzyme modulators, transferases, cytoskeletal proteins, and others.

Alteration in the processing of the ACRBP/sp32 protein and sperm head/acrosome malformations in proprotein convertase 4 (PCSK4) null mice

Proprotein convertase 4 (PCSK4) is a member of a family of proprotein convertases that convert inactive precursor proteins into their mature and active forms. PCSK4 is expressed by testicular germ cells and localizes to the sperm acrosome, suggesting roles in fertilization. Mice lacking PCSK4 exhibit a profound fertility defect; yet, to date, few substrates for PCSK4 are known. In this study, two-dimensional differential in-gel electrophoresis analysis was carried out in order to identify proteins that are altered in spermatozoa from PCSK4 null mice. Herein, we report that the sperm fertilization molecule acrosin-binding protein (ACRBP)/sp32, which normally undergoes processing from a 58.5 kDa precursor to a 27.5 kDa mature form, is not proteolytically processed in PCSK4 null mice and thus may be a substrate for PCSK4. However, analysis of the ACRBP sequence did not show a strong consensus site for convertase cleavage, suggesting that ACRBP processing may require the activity of a yet unknown enzyme that itself may be a PCSK4 substrate. Further analysis of spermatozoa from the PCSK4 null mice showed that proacrosin did not undergo autoactivation, supporting a role for the mature form of ACRBP in the regulation of proacrosin conversion into different acrosin isoforms. Finally, examination of ACRBP localization revealed a previously undetected morphological defect in the head/acrosomes of spermatozoa from PCSK4 null mice. Taken together, these results demonstrate that the fertility defect in the PCSK4 null mice may in part be due to altered ACRBP protein processing as well as abnormalities in the sperm head/acrosome.

High total acrosin activity in varicocele individuals

Varicocele is a common cause of male infertility and reports indicate that varicocelectomy has a beneficial effect on male fertility. The aims of this study were to evaluate and compare the total acrosin activity along with DNA integrity in semen samples obtained from 70 varicocele individuals with male factors infertility presenting grades II and III varicocele before and after the surgery and 30 fertile individuals without any clinical presentation of varicocele. Total acrosin activity, protamine deficiency, DNA fragmentation, and semen parameters including sperm concentration, motility and sperm morphology were assessed by spectrophotometery, CMA3 staining, terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay and WHO criteria, respectively. Total acrosin activity (P = 0.03), percentage CMA3 positivity (P = 0.00) and TUNEL-positive spermatozoa (P = 0.04) were higher in the varicocele individuals before the surgery compared with the fertile individuals; yet, all the aforementioned criteria decreased significantly after surgery (P < 0.05). The results of this study reveal that DNA fragmentation and protamine deficiency, as negative parameters in fertility, improve post-surgery; however, total acrosin activity as a positive parameter in fertility is higher in the varicocele individuals compared with fertile and decreases to a value close to the fertile control post-surgery. High levels of total acrosin activity in varicocele individuals need more research in future.

Transitional states of acrosomal exocytosis and proteolytic processing of the acrosomal matrix in guinea pig sperm

In this study, we adapted a FluoSphere bead-binding assay to study the exposure and release of guinea pig sperm acrosomal components during the course of capacitation and acrosomal exocytosis. Prior to capacitation or the initiation of exocytosis, acrosomal proteins were not accessible to FluoSpheres coated with antibodies against two acrosomal matrix (AM) proteins, AM67 and AM50; during the course of capacitation and ionophore-induced acrosomal exocytosis, however, we detected the transient exposure of the solid-phase AM proteins on the surface of guinea pig sperm using the antibody-coated fluorescent beads. Several different transitional stages leading to complete acrosomal exocytosis were classified, and we propose these represent true, functional intermediates since some of the AM proteins are orthologues of mouse proteins that bind the zona pellucida (ZP) of unfertilized eggs. In addition, we present evidence that implicates acrosin in the proteolytic processing of AM50 during AM disassembly. Thus, we propose that the transitional states of acrosomal exocytosis involve early binding of AM proteins to the ZP (by what visually appear to be "acrosome-intact" sperm), maintenance of ZP binding that coincides with the progressive exposure of AM proteins, and gradual proteolytic disassembly of the AM to allow sperm movement through the ZP. We feel this "transitional states" model provides a more refined view of acrosomal function that supports a move away from the widely held, overly simplistic, and binary "acrosome-reaction" model, and embraces a more dynamic view of acrosomal exocytosis that involves intermediate stages of the secretory process in ZP binding and penetration.

Acrosome reaction in the cumulus oophorus revisited: involvement of a novel sperm-released factor NYD-SP8

Fertilization is a process involving multiple steps that lead to the final fusion of one sperm and the oocyte to form the zygote. One of the steps, acrosome reaction (AR), is an exocytosis process, during which the outer acrosome membrane fuses with the inner sperm membrane, leading to the release of acrosome enzymes that facilitate sperm penetration of the egg investments. Though AR has been investigated for decades, the initial steps of AR in vivo, however, remain largely unknown. A well elucidated model holds the view that AR occurs on the surface of the zona pellucida (ZP), which is triggered by binding of sperm with one of the ZP glycosylated protein, ZP3. However, this model fails to explain the large number of 'falsely' acrosome-reacted sperms found within the cumulus layer in many species examined. With the emerging evidence of cross-talk between sperm and cumulus cells, the potential significance of AR in the cumulus oophorus, the outer layer of the egg, has been gradually revealed. Here we review the acrosome status within the cumulus layer, the cross-talk between sperm and cumulus cells with the involvement of a novel sperm-released factor, NYD-SP8, and re-evaluate the importance and physiological significance of the AR in the cumulus in fertilization.

Zonadhesin is essential for species specificity of sperm adhesion to the egg zona pellucida

Interaction of rapidly evolving molecules imparts species specificity to sperm-egg recognition in marine invertebrates, but it is unclear whether comparable interactions occur during fertilization in any vertebrate species. In mammals, the sperm acrosomal protein zonadhesin is a rapidly evolving molecule with species-specific binding activity for the egg zona pellucida (ZP). Here we show using null mice produced by targeted disruption of Zan that zonadhesin confers species specificity to sperm-ZP adhesion. Sperm capacitation selectively exposed a partial von Willebrand D domain of mouse zonadhesin on the surface of living, motile cells. Antibodies to the exposed domain inhibited adhesion of wild-type spermatozoa to the mouse ZP but did not inhibit adhesion of spermatozoa lacking zonadhesin. Zan(-/-) males were fertile, and their spermatozoa readily fertilized mouse eggs in vitro. Remarkably, however, loss of zonadhesin increased adhesion of mouse spermatozoa to pig, cow, and rabbit ZP but not mouse ZP. We conclude that zonadhesin mediates species-specific ZP adhesion, and Zan(-/-) males are fertile because their spermatozoa retain adhesion capability that is not species-specific. Mammalian sperm-ZP adhesion is therefore molecularly robust, and species-specific egg recognition by a protein in the sperm acrosome is conserved between invertebrates and vertebrates, even though the adhesion molecules themselves are unrelated.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 2: changes in spermatid organelles associated with development of spermatozoa

Spermiogenesis is a long process whereby haploid spermatids derived from the meiotic divisions of spermatocytes undergo metamorphosis into spermatozoa. It is subdivided into distinct steps with 19 being identified in rats, 16 in mouse and 8 in humans. Spermiogenesis extends over 22.7 days in rats and 21.6 days in humans. In this part, we review several key events that take place during the development of spermatids from a structural and functional point of view. During early spermiogenesis, the Golgi apparatus forms the acrosome, a lysosome-like membrane bound organelle involved in fertilization. The endoplasmic reticulum undergoes several topographical and structural modifications including the formation of the radial body and annulate lamellae. The chromatoid body is fully developed and undergoes structural and functional modifications at this time. It is suspected to be involved in RNA storing and processing. The shape of the spermatid head undergoes extensive structural changes that are species-specific, and the nuclear chromatin becomes compacted to accommodate the stream-lined appearance of the sperm head. Microtubules become organized to form a curtain or manchette that associates with spermatids at specific steps of their development. It is involved in maintenance of the sperm head shape and trafficking of proteins in the spermatid cytoplasm. During spermiogenesis, many genes/proteins have been implicated in the diverse dynamic events occurring at this time of development of germ cells and the absence of some of these have been shown to result in subfertility or infertility.

Infertility with impaired zona pellucida adhesion of spermatozoa from mice lacking TauCstF-64

Fertilization is a multistep process requiring spermatozoa with unique cellular structures and numerous germ cell-specific molecules that function in the various steps. In the highly coordinated process of male germ cell development, RNA splicing and polyadenylation help regulate gene expression to assure formation of functional spermatozoa. Male germ cells express tauCstF-64 (Cstf2t gene product), a paralog of the X-linked CstF-64 protein that supports polyadenylation in most somatic cells. We previously showed that loss of tauCstF-64 causes male infertility because of major defects in mouse spermatogenesis. Surprisingly, although Cstf2t(-/-) males produce very few recognizable spermatozoa, some of the spermatozoa produced are motile. This led us to ask whether these Cstf2t(-/-) sperm were fertile. A motile cell-enriched population of spermatozoa from Cstf2t-null males dispersed cumulus cells of cumulus-oocyte complexes normally. However, motile spermatozoa from Cstf2t-null males failed to fertilize cumulus-intact mouse eggs in vitro. In addition, sperm adhesion to the zona pellucida (ZP) of cumulus-free eggs was significantly decreased, indicating tauCstF-64 is required for production of spermatozoa capable of ZP interaction. Acrosomal proteins involved in sperm-ZP recognition, including zonadhesin, proacrosin, SPAM1/PH-20, and ZP3R/sp56, were normally distributed in the apical head of Cstf2t(-/-) spermatozoa. We conclude that tauCstF-64 is required not only for expression of genes involved in morphological differentiation of spermatids but also for genes having products that function during interaction of motile spermatozoa with eggs. To our knowledge, this is the first demonstration that a gene involved in polyadenylation has a negative consequence on sperm-ZP adhesion.

A model of the acrosome reaction progression via the acrosomal membrane-anchored protein equatorin

It is important to establish a reliable and progressive model of the acrosome reaction. Here, we present a progression model of the acrosome reaction centering around the acrosomal membrane-anchored protein equatorin (MN9), comparing the staining pattern traced by MN9 antibody immunofluorescence with that traced by Arachis hypogaea agglutinin (PNA)–FITC. Prior to the acrosome reaction, equatorin was present in both the anterior acrosome and the equatorial segment. Since sperm on zona pellucida showed various staining patterns, MN9-immunostaining patterns were classified into four stages: initial, early, advanced, and final. As the acrosome reaction progressed from the initial to the early stage, equatorin spread from the peripheral region of the anterior acrosome toward the center of the equatorial segment, gradually over the entire region of the equatorial segment during the advanced stage, and finally uniformly at the equatorial segment at the final stage. In contrast, the PNA–FITC signals spread more quickly from the peripheral region of the acrosome toward the entire equatorial segment, while decreasing in staining intensity, and finally became weak at the final stage. MN9-immunogold electron microscopy showed equatorin on the hybrid vesicles surrounded by amorphous substances at advanced stage of acrosome reaction. Equatorin decreased in molecular mass from 40–60 to 35 kDa, and the signal intensity of 35 kDa equatorin increased as the acrosome reaction progressed. Thus, the established equatorin-based progression model will be useful for analyzing not only the behavior of equatorin but also of other molecules of interest involved in the acrosome reaction.

Functional consequences of cleavage, dissociation and exocytotic release of ZP3R, a C4BP-related protein, from the mouse sperm acrosomal matrix

The acrosome is an exocytotic vesicle located on the apical tip of the sperm head. In addition to having different morphological regions, two biochemically distinct compartments can be defined within the acrosome: a particulate acrosomal matrix and a soluble partition. The domains within the acrosome participate in the release of acrosomal proteins from the sperm during exocytosis, depending on whether the proteins partition into either the soluble or matrix compartments of the acrosome. We have examined the mechanism of differential release by evaluating the solubilization of acrosomal matrix protein ZP3R (sp56) from mouse sperm during the course of spontaneous acrosomal exocytosis. Using indirect immunofluorescence and immunoblotting, we found that the ZP3R monomer is processed from 67,000 M(r) to 43,000 M(r) by proteases coincident with release from the acrosome. Sperm require a maturational step, termed capacitation, before they are competent for acrosomal exocytosis and the processing of ZP3R is dramatically reduced under non-capacitating conditions. The cleavage probably takes place in complement control protein domain (CCP) 6 or the bridge region between CCP6 and CCP7, which is not present in the guinea pig orthologue AM67. The cleaved form of ZP3R does not bind to unfertilized eggs. We have incorporated these structural considerations into a model to explain the functional consequences of acrosomal exocytosis on sperm-zona interactions.

The CAP superfamily: cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins--roles in reproduction, cancer, and immune defense

The cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins (CAP) superfamily members are found in a remarkable range of organisms spanning each of the animal kingdoms. Within humans and mice, there are 31 and 33 individual family members, respectively, and although many are poorly characterized, the majority show a notable expression bias to the reproductive tract and immune tissues or are deregulated in cancers. CAP superfamily proteins are most often secreted and have an extracellular endocrine or paracrine function and are involved in processes including the regulation of extracellular matrix and branching morphogenesis, potentially as either proteases or protease inhibitors; in ion channel regulation in fertility; as tumor suppressor or prooncogenic genes in tissues including the prostate; and in cell-cell adhesion during fertilization. This review describes mammalian CAP superfamily gene expression profiles, phylogenetic relationships, protein structural properties, and biological functions, and it draws into focus their potential role in health and disease. The nine subfamilies of the mammalian CAP superfamily include: the human glioma pathogenesis-related 1 (GLIPR1), Golgi associated pathogenesis related-1 (GAPR1) proteins, peptidase inhibitor 15 (PI15), peptidase inhibitor 16 (PI16), cysteine-rich secretory proteins (CRISPs), CRISP LCCL domain containing 1 (CRISPLD1), CRISP LCCL domain containing 2 (CRISPLD2), mannose receptor like and the R3H domain containing like proteins. We conclude that overall protein structural conservation within the CAP superfamily results in fundamentally similar functions for the CAP domain in all members, yet the diversity outside of this core region dramatically alters target specificity and, therefore, the biological consequences.

Enigmas of mammalian gamete form and function

The gametes of man and some other Eutheria have been manipulated successfully for practical reasons, but many gaps remain in our basic understanding of the way that they function. This situation stems not least from a failure to recognize the extent to which eutherian spermatozoa and eggs, and elements related to their operation, have come to differ from those of other groups. Novel features in the male that reflect this include a radical design of the sperm head with the acrosome seeming to function primarily in egg-coat binding rather than its lysis, a multifaceted post-testicular sperm maturation and an androgen/low-temperature-regulated system of sperm storage--both tied to the epididymis, a variable male accessory sex gland complex, and descent of the testis and epididymis to a scrotum. In the female, such novelties are represented in a need for sperm capacitation, in an unusual regulation of sperm transport within the oviduct, in the cumulus oophorus and character of the zona pellucida around the small egg, and in a unique configuration of gamete fusion. The collective evidence now suggests that many of these features reflect a new fertilisation strategy or its consequences, with most being causally linked. One initial 'domino' in this regard appears to be the small yolkless state of the egg and its intolerance for polyspermy, as determinants of the unusual mode of oviductal sperm transport and possibly the existence and form of the cumulus oophorus. However, a particularly influential first 'domino' appears to be the physical character of the eutherian zona pellucida. This differs from the egg coats of other animal groups by virtue of a resilient elasticity and thickness. These qualities allow this primary and often only coat to stretch and so persist during later expansion of the blastocyst, usually until close to implantation. At the same time, the dimensions, physical character, and particularly the relative protease-insensitivity of the zona appear to have had profound effects on sperm form and function and, more indirectly, on sperm-related events in the male and the female tract. Marsupials display some similarities and also some strikingly different features, against which the enigmas of the eutherian situation can be evaluated.

Dipeptidyl peptidase II (DPPII), a review

An increasing number of biological processes appear to be regulated by Pro-specific N-terminal processing. The proline-specific dipeptidyl peptidases (DPPs) like DPPIV, fibroblast activation protein alpha (FAP), DPPII, DPP8 and DPP9, because of their preference for cleavage after X-Pro in vitro, are likely to be involved in many of these processes. These DPPs are emerging as an important protease family with roles in the regulation of signaling by peptide hormones. Dipeptidyl peptidase II (DPPII, E.C. 3.4.14.2) is an intracellular protease that localizes to the vesicular system. It releases, preferably at acidic pH, N-terminal dipeptides from oligopeptides with Pro or Ala in the penultimate position. Despite the fact that the physiological role of DPPII still has not been elucidated, several suggestions were made on possible functions of the enzyme depending on its localization in different cells, body fluids and organs. DPPII was a.o. suggested to be involved in the processes of cell differentiation and in the protection from cell death, and to have a role in the degradation of collagen fragments, myofibrillar proteins and short neuropeptides. Moreover, changes in the level and distribution of the enzyme provided clues indicating additional roles in disease-related processes. Here we review the DPPII literature, aiming to bring more clarity in the disperse data on this subject and give a state of the art on DPPII research.

Human testicular protein TPX1/CRISP-2: localization in spermatozoa, fate after capacitation and relevance for gamete interaction

Testicular protein Tpx-1, also known as CRISP-2, is a cysteine-rich secretory protein specifically expressed in the male reproductive tract. Since the information available on the human protein is limited to the identification and expression of its gene, in this work we have studied the presence and localization of human Tpx-1 (TPX1) in sperm, its fate after capacitation and acrosome reaction (AR), and its possible involvement in gamete interaction. Indirect immunofluorescence studies revealed the absence of significant staining in live or fixed non-permeabilized sperm, in contrast to a clear labelling in the acrosomal region of permeabilized sperm. These results, together with complementary evidence from protein extraction procedures strongly support that TPX1 would be mainly an intra-acrosomal protein in fresh sperm. After in vitro capacitation and ionophore-induced AR, TPX1 remained associated with the equatorial segment of the acrosome. The lack of differences in the electrophoretic mobility of TPX1 before and after capacitation and AR indicates that the protein would not undergo proteolytical modifications during these processes. The possible involvement of TPX1 in gamete interaction was evaluated by the hamster oocyte penetration test. The presence of anti-TPX1 during gamete co-incubation produced a significant and dose-dependent inhibition in the percentage of penetrated zona-free hamster oocytes without affecting sperm motility, the AR or sperm binding to the oolema. Together, these results indicate that human TPX1 would be a component of the sperm acrosome that remains associated with sperm after capacitation and AR, and is relevant for sperm-oocyte interaction.

Differential release of soluble and matrix components: evidence for intermediate states of secretion during spontaneous acrosomal exocytosis in mouse sperm☆

Although its exact role in fertilization is unknown, the acrosome is a very important, exocytotic organelle overlying the anterior aspect of sperm from many species. Structurally and functionally, the acrosome can be considered to consist of soluble and particulate compartments. One component of the particulate acrosomal matrix is the zona pellucida-binding protein sp56. Our demonstration that this protein is within the acrosomal matrix and not on the sperm plasma membrane has led us to reexamine the events of acrosomal exocytosis and the role of the sperm acrosomal matrix in the fertilization process. To visualize the soluble compartment, we have utilized sperm from transgenic mice that carry soluble green fluorescent protein (GFP) in their acrosomes and, as a means to assess the exposure of acrosomal matrix components, we have tested the ability of these sperm to bind beads coated with antibodies to sp56. The loss of GFP from the acrosomes and the binding of the beads by the sperm undergoing capacitation serve as indicators of distinct stages of acrosomal exocytosis, allowing us to define intermediates of acrosomal exocytosis that occur during the course of sperm capacitation. These experiments demonstrate that the exposure and release of acrosomal proteins during spontaneous acrosomal exocytosis is not synchronous but is regulated during capacitation. Furthermore, acrosomal exocytosis under these conditions required calcium in the medium. On the basis of these findings, we propose an alternative model for acrosomal exocytosis that considers a role for these intermediates of exocytosis during capacitation and sperm-ZP interactions.

Dexamethasone reduces acrosin activity of ram spermatozoa

The aim of this study was to investigate the effect of dexamethasone on acrosin activity of spermatozoa in Chios rams during autumn (breeding season for sheep in Greece), in correlation with possible changes in blood testosterone. Dexamethasone was administered in four equal consecutive intramuscular injections, one every four hours (total dose: 3 mg kg(-1)). Total acrosin activity was determined in semen samples collected 48 h before and on the 4th and 7th day and thereafter once every week until the 77th day after dexamethasone administration. Blood samples for testosterone radioimmunoassays were collected 24 h before, during dexamethasone administration and on the 4th, 7th, 14th and 21st day after administration. Total acrosin activity in spermatozoa was reduced between days 7-28 after dexamethasone administration. Dexamethasone also induced a reduction in mean value and basal level of blood testosterone and inhibited its episodic secretion between 1 and 4 days after administration. As the reduction of acrosin activity appeared relatively soon after dexamethasone administration (7th day), it is likely that the increased amount of dexamethasone did not influence the synthesis of proacrosin in the late spermatids. As glucocorticoid receptors exist in the epididymis and accessory glands in various species, dexamethasone may have a direct influence on the synthesis and/or release of acrosin inhibitors in epididymal fluid or seminal plasma. These changes in acrosin activity in ovine spermatozoa mediated by dexamethasone may be of importance regarding the role of stress in the reduction of sperm fertilizing ability.

Evaluation of the proacrosin/acrosin system and its mechanism of activation in human sperm extracts

Acrosin is an acrosomal protease synthesized as a proenzyme and activated into beta-acrosin during the acrosome reaction. In the present study, a set of sensitive assays was developed to identify the proacrosin/acrosin system and to evaluate its activation pattern in human sperm extracts. Immunocytochemical analysis with monoclonal antibody (Mab) AcrC5F10 showed specific staining on the acrosome of permeabilized ejaculated and capacitated spermatozoa. Acrosome reaction was associated with a decrease in staining. AcrC5F10 specifically recognized a 55-kDa band (proacrosin) in Western immunoblots. Activation studies showed enzymatically active intermediates of 39 and 35 kDa after zymography. Immunoreactive bands of 52, 43, 34, 21-26 and 16 kDa were identified in the activation patterns developed with AcrC5F10. Activation was completely inhibited in the presence of 9 mM CaCl(2) or 100 mM benzamidine. A multiple sequence alignment revealed partial conservation of putative cleavage sites in the proacrosin sequence. The tests described allow the detection of human proacrosin in spermatozoa and sperm protein extracts, as well as the evaluation of the proenzyme activation pattern. They can be used to study the effect of inhibitors upon proenzyme activation. In addition, alterations in proacrosin activation in semen samples with abnormal acrosin enzymatic activity can be analyzed using these assays.

Distinct membrane fractions from mouse sperm bind different zona pellucida glycoproteins

Interactions between sperm and zona pellucida (ZP) during mammalian fertilization are not well characterized at the molecular level. To identify sperm proteins that recognize ligand ZP3, we used sonicated sperm membrane fractions as competitors in a quantitative binding assay. Sonicated membranes were density fractionated into 4 fractions. Bands 1-3 contained membrane vesicles, and band 4 contained axonemal and midpiece fragments. In competitive binding assays, bands 1, 2, and 3 but not band 4 were able to compete with live, capacitated, intact sperm for soluble 125I-ZP binding. Affinity-purified ZP fractions consisting of a ZP3-enriched fraction (125I-ZP3) and a fraction enriched for ligands ZP1 and ZP2 and depleted of ZP3 (125I-ZP1/2) were obtained by antibody affinity purification of ZP3. In competitive binding assays, bands 2 and 3 competed for 125I-ZP3 binding, but band 1 did not interact with enriched 125I-ZP3. None of the membrane fractions competed for 125I-ZP1/2 binding. These results demonstrate that band 2 and band 3 contain sperm components that interact with ZP3 alone and that components in band 1 interact with ZP3 in conjunction with either ZP1 or ZP2. These data indicate that there must be at least 2 unique sperm plasma membrane components that mediate intact sperm interactions with ZP glycoproteins in mouse. Bands 2 and 3 are likely to contain a primary ZP-binding protein because they interacted directly with ZP3, whereas band 1 may contain sperm proteins involved in later interactions with the ZP, perhaps transitional interactions to maintain sperm contact with the ZP during acrosomal exocytosis.

Partial cDNA sequence analysis of the boar proacrosin-binding protein

A partial cDNA sequence of the boar 28, 29-kDa proacrosin-binding protein was determined. The desired sequence was amplified and obtained by RT-PCR followed by nested PCR. The oligonucleotide primers for RT-PCR were designed by deduction from the internal amino acid sequence of the boar proacrosin-binding protein. An approximately 1-kbp DNA fragment was analysed by agarose gel electrophoresis, in which the boar testicular total RNA or poly(A)+ RNA was used as a template for RT-PCR followed by nested PCR. This PCR product was inserted into a TA vector. The DNA sequence of the inserted PCR product was then determined by the chain termination method.

A mouse acrosomal cortical matrix protein, MC41, has ZP2-binding activity and forms a complex with a 75-kDa serine protease

Sperm with a large acrosome such as that of guinea pigs and hamsters have a subdomain structure in the anterior acrosome, but the mouse acrosome looks homogeneous and its matrix has not been precisely analyzed. The intra-acrosomal protein MC41 is localized in the cortical region of the mouse anterior acrosome, suggesting a subdomain structure in the mouse acrosome. Thus, the present study was undertaken to analyze the mouse acrosomal matrix using an anti-MC41 antibody. When mouse sperm were treated with 2% Triton X-100, Triton-insoluble matrix components remained in the acrosomal cortical region. Immunogold for MC41 labeled the Triton X-100 and high-salt-insoluble matrix components, demonstrating that MC41 is a subdomain-specific acrosomal matrix protein. We further examined interactions of MC41 with acrosomal proteases and zona proteins. A serine protease of 75 kDa was associated with MC41 under low-salt conditions, presumably forming a complex. Far Western blotting technique indicated that MC41 bound to both ZP2 and ZP2(f) in the presence of high-salt-soluble sperm proteins. In acrosome-reacting sperm, MC41 was present on the hybrid vesicles formed by the fusion of the plasma and outer acrosomal membranes. Presumably, MC41 has a significant role in secondary sperm-zona binding during the acrosomal reaction.

Alkalinization of acrosome measured by GFP as a pH indicator and its relation to sperm capacitation

We previously targeted EGFP (a mutant of green fluorescent protein) to the lumen of the mouse sperm acrosome and reported the time course of EGFP release during the acrosome reaction. In the study reported here, we estimated the pH within the mouse sperm acrosome utilizing the pH-dependent nature of EGFP fluorescence. The average intra-acrosomal pH was estimated to be 5.3 +/- 0.1 immediately after sperm preparation, gradually increasing to 6.2 +/- 0.3 during 120 min of incubation in TYH media suitable for capacitation. Spontaneous acrosome reactions were noted to increase concomitantly with acrosomal alkalinization during incubation. We also demonstrated that acrosomal antigens detected by monoclonal antibodies MN7 and MC41 did not dissolve following the acrosome reaction in pH 5.3 media, but dissolved at pH 6.2. These data suggest that acrosomal alkalinization during incubation conducive for sperm capacitation may function to alter acrosomal contents and prepare them for release during the acrosome reaction.