Development of the signalling pathways associated with sperm capacitation during epididymal maturation

Boar Sperm Motility Assessment Using Computer-Assisted Sperm Analysis: Current Practices, Limitations, and Methodological Challenges

Spermatozoa are highly specialized male cells that are characterized by a unique ability to move, which is a critical factor for successful fertilization. The relative simplicity of motility assessment, especially in livestock, has made it a widely used parameter for evaluating ejaculate quality or cryopreserved semen in the clinical field, and an advanced tool in reproductive physiology and toxicology research. Technological advances in image analysis and computational methods have substantially increased its accuracy through the use of computer-assisted sperm analysis (CASA) to minimize subjective bias in motility assessments. Nevertheless, this more objective method still presents some significant challenges, including variability in the sample preparation, imaging conditions, and analytical parameters. These issues contribute to inconsistency and impair the reproducibility and comparability of data between laboratories. The implementation of standardized protocols, combined with comprehensive training and rigorous evaluation, can serve to mitigate some of the emerging inconsistencies. In addition, the in vitro conditions under which CASA analyses are performed often differ significantly from the natural environment of the female reproductive tract in vivo. This review discusses the methodologies, critical issues, and limitations of sperm motility analyses using CASA, with a particular focus on the boar as an important agricultural and biomedical model species in which this system is widely used.

Binder of Sperm Proteins 1 and 5 have contrasting effects on the capacitation of ram spermatozoa

Binder of Sperm Proteins (BSPs) are the most abundant seminal plasma protein family in the ram and bull. They have been extensively studied in the bull but less is known about their function in ovine seminal plasma and current knowledge suggests that BSPs may have different effects in these two species. In the bull, they facilitate capacitation and destabilize the sperm membrane during in vitro handling, whereas in the ram, they appear to stabilize the sperm membrane and prevent cryopreservation-induced capacitation-like changes. Further investigation into the effects of BSPs on ram spermatozoa under capacitating conditions is required to further clarify their physiological roles in the ram. We investigated the effects of Binder of Sperm Proteins 1 and 5 on epididymal ram spermatozoa in conditions of low, moderate, and high cAMP. BSPs had minimal effects on sperm function in low-cAMP conditions, but caused significant changes under cAMP upregulation. BSP1 stabilized the membrane and qualitatively reduced protein tyrosine phosphorylation, but significantly increased cholesterol efflux and induced spontaneous acrosome reactions. BSP5 slightly increased spontaneous acrosome reactions and caused sperm necrosis. However, BSP5 had minimal effects on membrane lipid order and cholesterol efflux and did not inhibit protein tyrosine phosphorylation. These findings demonstrate that under maximal cAMP upregulation, BSP1 affected ram spermatozoa in a manner comparable to bull spermatozoa, while BSP5 did not.

New insights into epididymal function in relation to sperm maturation

Testicular spermatozoa acquire fertility only after 1 or 2 weeks of transit through the epididymis. At the end of this several meters long epididymal tubule, the male gamete is able to move, capacitate, migrate through the female tract, bind to the egg membrane and fuse to the oocyte to result in a viable embryo. All these sperm properties are acquired after sequential modifications occurring either at the level of the spermatozoon or in the epididymal surroundings. Over the last few decades, significant increases in the understanding of the composition of the male gamete and its surroundings have resulted from the use of new techniques such as genome sequencing, proteomics combined with high-sensitivity mass spectrometry, and gene-knockout approaches. This review reports and discusses the most relevant new results obtained in different species regarding the various cellular processes occurring at the sperm level, in particular, those related to the development of motility and egg binding during epididymal transit.

Phenotyping male infertility in the mouse: how to get the most out of a 'non-performer'

BACKGROUND

Functional male gametes are produced through complex processes that take place within the testis, epididymis and female reproductive tract. A breakdown at any of these phases can result in male infertility. The production of mutant mouse models often yields an unexpected male infertility phenotype. It is with this in mind that the current review has been written. The review aims to act as a guide to the ‘non-reproductive biologist’ to facilitate a systematic analysis of sterile or subfertile mice and to assist in extracting the maximum amount of information from each model.

METHODS

This is a review of the original literature on defects in the processes that take a mouse spermatogonial stem cell through to a fully functional spermatozoon, which result in male infertility. Based on literature searches and personal experience, we have outlined a step-by-step strategy for the analysis of an infertile male mouse line.

RESULTS

A wide range of methods can be used to define the phenotype of an infertile male mouse. These methods range from histological methods such as electron microscopy and immunohistochemistry, to hormone analyses and methods to assess sperm maturation status and functional competence.

CONCLUSION

With the increased rate of genetically modified mouse production, the generation of mouse models with unexpected male infertility is increasing. This manuscript will help to ensure that the maximum amount of information is obtained from each mouse model and, by extension, will facilitate the knowledge of both normal fertility processes and the causes of human infertility.

Proteomic changes in mammalian spermatozoa during epididymal maturation

Epididymal maturation is associated with the activation of a cAMP-induced tyrosine phosphorylation cascade, which is ultimately associated with the expression of capacitation-dependent sperm functions, such as hyperactivated movement and acrosomal exocytosis. As spermatozoa progress through the epididymis they first acquire the capacity to phosphorylate tyrosine on targets on the principal piece, followed by the midpiece. By the time these cells have reached the cauda epididymidis they can phosphorylate the entire tail from neck to endpiece. This particular pattern of phosphorylation is associated with the ontogeny of fully functional spermatozoa that are capable of fertilizing the oocyte. Proteomic analyses indicate that this change is associated with the phosphorylation of several mitochondrial proteins, creation of a mitochondrial membrane potential and activation of mitochondrial free radical generation. At least in rodent species, activation of sperm mitochondria appears to be a particularly important part of epididymal maturation.

Identification of gene products present in Triton X-100 soluble and insoluble fractions of human spermatozoa lysates using LC-MS/MS analysis

A comprehensive analysis of the proteins found in human spermatozoa is essential for understanding the events leading up to, and including, fertilization and development. Proteomics offers a platform for investigating this process, provided that the dynamic range is relatively low. In this report, spermatozoa from a number of human sperm ejaculates were isolated in a pure state using discontinuous Percoll gradient centrifugation. Triton X-100 soluble and insoluble proteins were recovered and separated by SDS-PAGE. The separation lanes were dissected into 96 fractions and analyzed individually by LC-MS(n) . A comprehensive protocol, involving LC-MS/MS analysis eventually down to the ninth most intense peak found in the MS-survey scan, was performed. Analysis of purified human sperm populations resulted in the identification of 1056 gene products, of which approximately 8% have not previously been characterized. The data were supported by the large number of proteins represented by expressed sequence tags in the testis. Bioinformatic analysis demonstrated that 437 of the gene products were involved in various metabolic pathways including glycolysis and oxidative phosphorylation. The inventory of proteins present in the human sperm proteome includes a number of notable discoveries including the first description of a nicotinamide adenine dinucleotide phosphate oxidase, dual-oxidase 2, finally laying to rest any doubts about the presence of such enzymes in spermatozoa. Furthermore, a number of different classes of receptor have also been detected in these cells and are potential regulators of sperm function. This list includes at least six seven-pass transmembrane receptors, six tyrosine kinase receptors, a tyrosine phosphatase receptor, glutamate-gated ion channel receptors, transient receptor potential cation channels, and a non-genomic progesterone receptor. This is the first published list of identified proteins in human spermatozoa using LC-MS/MS analysis.

Expression of two-pore domain potassium channels in nonhuman primate sperm

OBJECTIVE

Two-pore domain potassium channels (K(2P)) play integral roles in cell signaling pathways by modifying cell membrane resting potential. Here we describe the expression and function of K(2P) channels in nonhuman primate sperm.

DESIGN

Experimental animal study, randomized blinded concentration-response experiments.

SETTING

University-affiliated primate research center.

ANIMAL(S)

Male nonhuman primates.

INTERVENTION(S)

Western blot and immunofluorescent analysis of epididymal sperm samples. Kinematic measures (curvilinear velocity and lateral head displacement) and acrosome status were studied in epididymal sperm samples exposed to K(2P) agonist (docosahexaenoic acid) and antagonist (gadolinium).

MAIN OUTCOME MEASURE(S)

Semiquantitative protein expression and cellular localization and quantitative changes in specific kinematic parameters and acrosome integrity.

RESULT(S)

Molecular analysis demonstrated expression and specific regional distribution of TRAAK, TREK-1, and TASK-2 in nonhuman primate sperm. Docosahexaenoic acid produced a concentration-dependent increase in curvilinear velocity (P<.0001) with concomitant concentration-dependent reductions in lateral head displacement (P=.005). Gadolinium reduced velocity measures (P<.01) without significantly affecting lateral head displacement.

CONCLUSION(S)

The results demonstrated expression and function of K(2P) potassium channels in nonhuman primate sperm for the first time. The unique, discrete distributions of K(2P) channels in nonhuman primate sperm suggest specific roles for this subfamily of ion channels in primate sperm function.

Ontogeny of Tyrosine Phosphorylation-Signaling Pathways During Spermatogenesis and Epididymal Maturation in the Mouse1

The objectives of this study were to map the ontogeny of tyrosine phosphorylation signal transduction pathways during germ cell development and to determine their association with the differentiation of a functional gamete. Until testicular germ cells differentiate into spermatozoa, cAMP-induced tyrosine phosphorylation is not detectable. Entry of these cells into the epididymis is accompanied by sudden activation of the tyrosine phosphorylation pathway, initially in the principal piece of the cell and subsequently in the midpiece. In the caput and corpus epididymides, the potential to express this pathway is inhibited by the presence of calcium in the extracellular medium. However, calcium has no effect on the expression of this pathway in caudal epididymal sperm. The competence of these cells to phosphorylate the entire sperm tail, from the neck to the tail-end piece, is accompanied by a capacity to exhibit hyperactivated motility on stimulation with cAMP. A distinctly different pattern of tyrosine phosphorylation, involving the acrosomal domain of the sperm head, is invoked as spermatozoa enter the caput epididymis, and phosphorylation remains high until these cells enter the distal corpus and cauda. The proportion of cells exhibiting this form of tyrosine phosphorylation is not affected by extracellular calcium or cAMP but is negatively correlated (R2 = 0.99) with their ability to acrosome-react. However, this relationship is not causative. Our findings indicate that the development of functional spermatozoa is accompanied by carefully orchestrated changes in tyrosine phosphorylation, controlled by independent regulatory mechanisms in distinct subcellular compartments of these highly specialized cells.

FGFR-1 [corrected] signaling is involved in spermiogenesis and sperm capacitation

Cloning of the fibroblast growth factor receptor (FGFR) adaptor Snt-2 cDNA and the identification of FGFR-1 protein in association with sperm tails, suggested that FGFR-1 signaling was involved in either sperm tail development or function. This hypothesis was tested by the creation of transgenic mice that specifically expressed a dominant-negative variant of FGFR-1 in male haploid germ cells. Mating of transgenic mice showed a significant reduction in pups per litter compared with wild-type littermates. Further analysis demonstrated that this subfertility was driven by a combination of reduced daily sperm output and a severely compromised ability of those sperm that were produced to undergo capacitation prior to fertilization. An analysis of key signal transduction proteins indicated that FGFR-1 is functional on wild-type sperm and probably signals via the phosphatidylinositol 3-kinase pathway. FGFR-1 activation also resulted in the downstream suppression of mitogen activated protein kinase signaling. These data demonstrate the FGFR-1 is required for quantitatively and qualitatively normal spermatogenesis and has a key role in the regulation of the global tyrosine phosphorylation events associated with sperm capacitation.

Identification of a New Variant of PDE1A Calmodulin-Stimulated Cyclic Nucleotide Phosphodiesterase Expressed in Mouse Sperm1

In mature sperm, cAMP plays an important role as a second messenger regulating functions that include capacitation, the acrosome reaction, motility, and, in some cases, chemosensing. We have cloned from mouse testis a novel calmodulin-stimulated cyclic nucleotide phosphodiesterase 1A isoform, Pde1a_v7 (mmPDE1A7), which arises from an alternative transcription start in the cyclic nucleotide phosphodiesterase 1A gene. The open reading frame is predicted to encode a polypeptide with a molecular mass of 52 kDa. Two further variants of this form, which contain two additional new exons, arise from alternative splicing. Analysis of testis cDNA by real-time polymerase chain reaction (PCR) indicates that the Pde1A_v7 transcript variant is the most abundant. The PDE1A_v7 protein uniquely lacks the first amino-terminal calmodulin-binding domain, but does possess an inhibitory domain and a second calmodulin-binding site shared with other variants. In vitro translation of the corresponding Pde1a_v7 cDNA produced a 52-kDa polypeptide having cyclic nucleotide hydrolytic activity, which was stimulated threefold by calcium-bound calmodulin. Immunoprecipitation of cyclic nucleotide phosphodiesterase 1 activity from detergent extracts of mouse sperm revealed a major protein of the size expected for PDE1A_v7, and the immunocytochemical staining for cyclic nucleotide phosphodiesterase 1A in mouse sperm showed intense immunoreactivity in the tail only. These observations, along with the PCR data, strongly suggest that this new variant PDE1A_v7 is the major form of cyclic nucleotide phosphodiesterase 1A expressed in mature sperm and is therefore likely to play an important role in cyclic nucleotide regulation of mature sperm function.

Identification of cytochrome-b5 reductase as the enzyme responsible for NADH-dependent lucigenin chemiluminescence in human spermatozoa

Lucigenin-dependent chemiluminescence together with 2-[4-iodophenyl]-3-[4-nitrophenyl]-5-[2,4-disulfophenyl]-2H tetrazolium monosodium salt (WST-1) reduction can be detected following addition of NADH to many cell types, including human sperm suspensions. Although many reports suggest that such a phenomenon is due to reactive oxygen species production, other oxygen detecting metabolite probes, such as MCLA and luminol, do not produce a chemiluminescent signal in this model system. The enzyme responsible for NADH-dependent lucigenin chemiluminescence was purified and identified as cytochrome-b5 reductase. In support of this concept, COS-7 cells overexpressing cytochrome-b5 reductase displayed at least a 3-fold increase in the previously mentioned activity compared with mock-transfected cells. Fractions containing cytochrome-b5 reductase were capable of inducing both lucigenin-dependent chemiluminescence and WST-1 reduction. Oxygen radicals clearly did not mediate the cytochrome b5-mediated activation of these probes in vitro since neither luminol nor MCLA gave a chemiluminescence response in the presence of the enzyme and the cofactor NADH. These results emphasize the importance of the direct NADH-dependent reduction of these putative superoxide-sensitive probes by cytochrome-b5 reductase even though this enzyme does not, on its own accord, produce reactive oxygen species.

Identification of post-translational modifications that occur during sperm maturation using difference in two-dimensional gel electrophoresis

Difference in two-dimensional (2-D) gel electrophoresis (DIGE) is a novel method for analyzing up to three samples in one 2-D gel and using the information gained to study post-translational modifications of proteins. We describe the use of DIGE to isolate and characterize those proteins that undergo processing in spermatozoa as they transit the epididymal tract. We find up to 60 protein spots are significantly modified as sperm traverse the epididymis. In this article, we report eight unambiguous protein identifications and demonstrate that one protein, the beta-subunit of the mitochondrial F1-ATPase, is serine-phosphorylated as sperm undergo epididymal maturation. We suggest that phosphorylation of this particular protein in a cAMP-dependent manner may contribute to the mechanisms by which motility is conferred upon spermatozoa.

Bicarbonate-induced membrane processing in sperm capacitation

During capacitation, major changes take place in the sperm plasma membrane so as to render it fusogenic and responsive to zona pellucida glycoproteins. However, the mechanisms involved have not been defined. As bicarbonate is known to be the key component that induces capacitation, we have investigated the bicarbonate-dependent changes in the boar sperm's plasma membrane architecture. We have discovered that bicarbonate induces a rapid collapse of phospholipid transverse asymmetry, exposing phosphatidylethanolamine and phosphatidylserine at the outer surface of the lipid bilayer. The collapse, which is reversible, is brought about as a result of activation of the phospholipid scramblase that exchanges phospholipids in a non-specific fashion between the two leaflets of the lipid bilayer. The activation takes place via a cyclic AMP-protein kinase A-dependent pathway and is initiated via stimulation of the so-called 'soluble' adenylyl cyclase in the sperm cell by bicarbonate. As a result of the collapse and the concurrent increase in phospholipid exchange, removal of cholesterol by albumin is facilitated (perhaps due to increased lipid packing disorder). This finding is in conflict with earlier surmises that cholesterol loss precedes activation of the cyclic AMP-protein kinase A axis. We have noted that not all cells in a given sperm population show rapid changes in response to bicarbonate stimulation; samples from individual boars also differ in their response. Maturation differences between cells have been found to play an important role in such functional heterogeneity.

Tyrosine Phosphorylation, Thiol Status, and Protein Tyrosine Phosphatase in Rat Epididymal Spermatozoa

Sperm thiol oxidation and the ability to undergo protein tyrosine phosphorylation are associated with the acquisition of sperm motility and fertilizing ability during passage of spermatozoa through the epididymis. Phosphotyrosine levels in various cells are controlled by tyrosine kinase versus phosphatase, with the latter known to be inhibited by oxidation. In the present paper we examine whether changes in thiol status during sperm maturation affect rat sperm protein phosphotyrosine levels and protein phosphotyrosine phosphatase (PTP) activity. Tyrosine phosphorylation, as demonstrated by immunoblotting (IB), was significantly increased in several sperm tail proteins during maturation in the epididymis. Sperm thiol oxidation with diamide enhanced tail protein phosphorylation; reduction of disulfides with dithiothreitol diminished phosphorylation. In the sperm head, a moderate increase in tyrosine phosphorylation was accompanied by altered localization of phosphotyrosine proteins during maturation. Blocking of thiols and PTP activity with N-ethylmaleimide led to increased tyrosine phosphorylation of protamine in caput sperm heads. Several PTP bands were identified by IB. In the caput spermatozoa, a prominent level of the 50 kDa band was present, whereas in the cauda spermatozoa a very low level of the 50 kDa band was found. PTP activity, measured by using p-nitrophenyl phosphate as a substrate, was significantly higher in the caput spermatozoa (high thiol content) than in the cauda spermatozoa (low thiol content). Our results show that PTP activity is correlated with sperm thiol status and suggest that tyrosine phosphorylation of sperm proteins during sperm maturation is promoted by thiol oxidation and diminished PTP.

Bicarbonate Is Required for Migration of Sperm Epididymal Protein DE (CRISP-1) to the Equatorial Segment and Expression of Rat Sperm Fusion Ability1

Numerous studies have demonstrated that sperm capacitation is a bicarbonate-dependent process. In the rat, capacitation has not been studied as much as in other species, mainly because of the difficulties in carrying out functional assays with this animal model. In the present study, we have examined the influence of bicarbonate in the overall rat sperm capacitation process by analyzing involvement of the anion in 1) protein tyrosine phosphorylation, 2) migration of epididymal protein DE (also known as CRISP-1) from the dorsal region to the equatorial segment of the sperm head that occurs during capacitation, and 3) ability of sperm to fuse with the egg. Incubation of sperm under capacitating conditions produced a time-dependent increase in protein tyrosine phosphorylation. This phosphorylation did not occur in the absence of HCO3- and rapidly increased by either exposure of sperm to HCO3- or replacement of the anion by a cAMP analog (dibutyryl-cAMP) and a phosphodiesterase inhibitor (pentoxifylline). The absence of HCO3- also produced a significant decrease in the percentage of cells showing migration of DE to the equatorial segment. This parameter was completely restored by addition of the anion, but dibutyryl-cAMP and pentoxifylline were not sufficient to overcome the decrease in DE migration. Sperm capacitated in the absence of HCO3- were unable to penetrate zona-free eggs independent of the presence of the anion during gamete coincubation. Exposure of these sperm to bicarbonate, or replacement of the anion by dibutyryl-cAMP and pentoxifylline, only partially restored the sperm fusion ability. Altogether, these results indicate that, in addition to its influence on protein tyrosine phosphorylation, bicarbonate is required to support other rat sperm capacitation- associated events, such as migration of DE to the equatorial segment, and expression of the ability of sperm to fuse with the egg.

Identification of cytochrome P450-reductase as the enzyme responsible for NADPH-dependent lucigenin and tetrazolium salt reduction in rat epididymal sperm preparations

Lucigenin-dependent chemiluminescence and WST-1 reduction can be detected following addition of NADPH to many cell types, including rat epididymal sperm suspensions. Although many reports suggest that such a phenomenon is due to reactive oxygen species production, other probes-such as MCLA and luminol-that are capable of detecting reactive oxygen metabolites do not produce a chemiluminescent signal in this model system. Our aim was to purify and identify the enzyme catalyzing the NADPH-dependent lucigenin and WST-1 reduction from rat epididymal spermatozoa preparations. Here, we show the identity of this enzyme as cytochrome P450-reductase. In support of this, a homogenous preparation of this protein was capable of reducing lucigenin and WST-1 in the presence of NADPH. Moreover, COS-7 cells overexpressing cytochrome P450-reductase displayed a 3-fold increase in the aforementioned activity compared with mock-transfected cells. Immunolocalization studies and biochemical analysis suggest that the majority of the NADPH-lucigenin activity is localized to the epithelial cells present within the epididymis. These results emphasize the importance of the direct NADPH-dependent reduction of superoxide-sensitive probes by cytochrome P450-reductase even though this enzyme does not, on its own accord, produce reactive oxygen species.