The development of signal transduction pathways during epididymal maturation is calcium dependent

TAF7L regulates early stages of male germ cell development in the rat

Male germ cell development is dependent on the orchestrated regulation of gene networks. TATA-box binding protein associated factors (TAFs) facilitate interactions of TATA-binding protein with the TATA element, which is known to coordinate gene transcription during organogenesis. TAF7 like (Taf7l) is situated on the X chromosome and has been implicated in testis development. We examined the biology of TAF7L in testis development using the rat. Taf7l was prominently expressed in preleptotene to leptotene spermatocytes. To study the impact of TAF7L on the testis we generated a global loss-of-function rat model using CRISPR/Cas9 genome editing. Exon 3 of the Taf7l gene was targeted. A founder was generated possessing a 110 bp deletion within the Taf7l locus, which resulted in a frameshift and the premature appearance of a stop codon. The mutation was effectively transmitted through the germline. Deficits in TAF7L did not adversely affect pregnancy or postnatal survival. However, the Taf7l disruption resulted in male infertility due to compromised testis development and failed sperm production. Mutant germ cells suffer meiotic arrest at late zygotene/early pachynema stages, with defects in sex body formation. This testis phenotype was more pronounced than previously described for the subfertile Taf7l null mouse. We conclude that TAF7L is essential for male germ cell development in the rat.

TAF7L REGULATES EARLY STAGES OF MALE GERM CELL DEVELOPMENT

Male germ cell development is dependent on the orchestrated regulation of gene networks. TATA-box binding protein associated factors (TAFs) facilitate interactions of TATA-binding protein with the TATA element, which is known to coordinate gene transcription during organogenesis. TAF7 like (Taf7l) is situated on the X chromosome and has been implicated in testis development. We examined the biology of TAF7L in testis development using the rat. Taf7l was prominently expressed in preleptotene to leptotene spermatocytes. To study the impact of TAF7L on the testis we generated a global loss-of-function rat model using CRISPR/Cas9 genome editing. Exon 3 of the Taf7l gene was targeted. A founder was generated possessing a 110 bp deletion within the Taf7l locus, which resulted in a frameshift and the premature appearance of a stop codon. The mutation was effectively transmitted through the germline. Deficits in TAF7L did not adversely affect pregnancy or postnatal survival. However, the Taf7l disruption resulted in male infertility due to compromised testis development and failed sperm production. Mutant germ cells suffer meiotic arrest at the zygotene stage, with defects in sex body formation and meiotic sex chromosome inactivation. This testis phenotype was more pronounced than previously described for the subfertile Taf7l null mouse. We conclude that TAF7L is essential for male germ cell development in the rat.

Calcium selective channel TRPV6: Structure, function, and implications in health and disease

Calcium-selective channel TRPV6 (Transient Receptor Potential channel family, Vanilloid subfamily member 6) belongs to the TRP family of cation channels and plays critical roles in transcellular calcium (Ca2+) transport, reuptake of Ca2+ into cells, and maintaining a local low Ca2+ environment for certain biological processes. Recent crystal and cryo-electron microscopy-based structures of TRPV6 have revealed mechanistic insights on how the protein achieves Ca2+ selectivity, permeation, and inactivation by calmodulin. The TRPV6 protein is expressed in a range of epithelial tissues such as the intestine, kidney, placenta, epididymis, and exocrine glands such as the pancreas, prostate and salivary, sweat, and mammary glands. The TRPV6 gene is a direct transcriptional target of the active form of vitamin D and is efficiently regulated to meet the body's need for Ca2+ demand. In addition, TRPV6 is also regulated by the level of dietary Ca2+ and under physiological conditions such as pregnancy and lactation. Genetic models of loss of function in TRPV6 display hypercalciuria, decreased bone marrow density, deficient weight gain, reduced fertility, and in some cases alopecia. The models also reveal that the channel plays an indispensable role in maintaining maternal-fetal Ca2+ transport and low Ca2+ environment in the epididymal lumen that is critical for male fertility. Most recently, loss of function mutations in TRPV6 gene is linked to transient neonatal hyperparathyroidism and early onset chronic pancreatitis. TRPV6 is overexpressed in a wide range of human malignancies and its upregulation is strongly correlated to tumor aggressiveness, metastasis, and poor survival in selected cancers. This review summarizes the current state of knowledge on the expression, structure, biophysical properties, function, polymorphisms, and regulation of TRPV6. The aberrant expression, polymorphisms, and dysfunction of this protein linked to human diseases are also discussed.

Calcium Homeostasis in the Epididymal Microenvironment: Is Extracellular Calcium a Cofactor for Matrix Gla Protein-Dependent Scavenging Regulated by Vitamins

In the male reproductive tract, the epididymis is an essential organ for sperm maturation, in which sperm cells acquire mobility and the ability to fertilize oocytes while being stored in a protective microenvironment. Epididymal function involves a specialized luminal microenvironment established by the epithelial cells of epididymal mucosa. Low-calcium concentration is a unique feature of this epididymal luminal microenvironment, its relevance and regulation are, however, incompletely understood. In the rat epididymis, the vitamin D-related calcium-dependent TRPV6-TMEM16A channel-coupler has been shown to be involved in fluid transport, and, in a spatially complementary manner, vitamin K2-related γ-glutamyl carboxylase (GGCX)-dependent carboxylation of matrix Gla protein (MGP) plays an essential role in promoting calcium-dependent protein aggregation. An SNP in the human GGCX gene has been associated with asthenozoospermia. In addition, bioinformatic analysis also suggests the involvement of a vitamin B6-axis in calcium-dependent MGP-mediated protein aggregation. These findings suggest that vitamins interact with calcium homeostasis in the epididymis to ensure proper sperm maturation and male fertility. This review article discusses the regulation mechanisms of calcium homeostasis in the epididymis, and the potential role of vitamin interactions on epididymal calcium homeostasis, especially the role of matrix calcium in the epididymal lumen as a cofactor for the carboxylated MGP-mediated scavenging function.

A High Incidence of Sperm with Cytoplasmic Droplets Affects the Response to Bicarbonate in Preserved Boar Semen

Retained cytoplasmic droplets (CD) are the most frequent sperm abnormality in boar semen. A high incidence of CD is associated with subfertility, but the underlaying reasons are not well understood. The storage of extended semen might augment the adverse effects of CD on essential steps towards fertilization, such as capacitation. The aim of this study was to examine whether the enhanced presence of CD in boar semen influences sperm's response to the capacitation stimulus bicarbonate during long-term semen storage. Extended semen samples (n = 78) from 13 artificial insemination centers were analyzed using a flow cytometric calcium influx assay. Samples with >15% of CD showed a reduced specific response to bicarbonate and a higher non-specific destabilization after storage for 96 h and subsequent incubation at 38 °C in three variants of Tyrode's medium (p < 0.05). The size of the bicarbonate-responsive sperm population was inversely correlated with the presence of CD-bearing sperm (r = -0.61, p < 0.01). Samples with ≤15% and samples with >15% of CD did not differ in motility or viability and acrosome integrity during semen storage. In conclusion, incomplete epididymal sperm maturation impairs the in vitro capacitation ability and promotes sperm destabilization in stored boar semen.

Integrated analysis of phosphoproteome and ubiquitylome in epididymal sperm of buffalo (Bubalus bubalis)

In mammals, sperm need to mature in the epididymis to gain fertilization competency. However, the molecular mechanism underlying buffalo sperm maturation remains elusive. Exploring sperm physiology at the posttranslational modification (PTM) level could help to develop our understanding of these mechanisms. Protein phosphorylation and ubiquitination are major PTMs in the regulation of many biological processes. In the present study, to our knowledge, we report the first phosphoproteome and ubiquitylome of sperm collected from the caput, corpus, and cauda segments of the epididymis using liquid chromatography-mass spectrometry combined with affinity purification. In total, 647 phosphorylation sites in 294 proteins and 1063 ubiquitination sites in 446 proteins were characterized. Some of these proteins were associated with cellular developmental processes and energy metabolic pathways. Interestingly, 84 proteins were both phosphorylated and ubiquitinated, simultaneously. Some of these proteins were involved in, for example, spermatogenesis, reproduction, and spermatid development. Taken together, these data provide a theoretical basis for further functional analysis of phosphorylation and ubiquitination in epididymal sperm of buffalo and other mammals, and serve as an important resource for exploring the physiological mechanism underlying sperm maturation.

Roles of glycogen synthase kinase 3 alpha and calcineurin in regulating the ability of sperm to fertilize eggs

Glycogen synthase kinase 3 (GSK3) was identified as an enzyme regulating sperm protein phosphatase. The GSK3α paralog, but not GSK3β, is essential for sperm function. Sperm lacking GSK3α display altered motility and are unable to undergo hyperactivation, which is essential for fertilization. Male mice lacking sperm-specific calcineurin (PP2B), a calcium regulated phosphatase, in testis and sperm, are also infertile. Loss of PP2B results in impaired epididymal sperm maturation and motility. The phenotypes of GSK3α and PP2B knockout mice are similar, prompting us to examine the interrelationship between these two enzymes in sperm. High calcium levels must exist to permit catalytically active calcineurin to function during epididymal sperm maturation. Total and free calcium levels are high in immotile compared to motile epididymal sperm. Inhibition of calcineurin by FK506 results in an increase in the net phosphorylation and a consequent decrease in catalytic activity of sperm GSK3. The inhibitor FK506 and an isoform-selective inhibitor of GSK3α, BRD0705, also inhibited fertilization of eggs in vitro. Interrelated functions of GSK3α and sperm PP2B are essential during epididymal sperm maturation and during fertilization. Our results should enable the development of male contraceptives targeting one or both enzymes.

Molecular changes and signaling events occurring in spermatozoa during epididymal maturation

After leaving the testis, spermatozoa have not yet acquired the ability to move progressively and are unable to fertilize oocytes. To become fertilization competent, they must go through an epididymal maturation process in the male, and capacitation in the female tract. Epididymal maturation can be defined as those changes occurring to spermatozoa in the epididymis that render the spermatozoa the ability to capacitate in the female tract. As part of this process, sperm cells undergo a series of biochemical and physiological changes that require incorporation of new molecules derived from the epididymal epithelium, as well as post-translational modifications of endogenous proteins synthesized during spermiogenesis in the testis. This review will focus on epididymal maturation events, with emphasis in recent advances in the understanding of the molecular basis of this process.

The protein tyrosine phosphorylation during in vitro capacitation and cryopreservation of mammalian spermatozoa

Before the process of fertilization, spermatozoa necessitate a period of residence in the female reproductive environment, and undergo a sequence of physiological and biochemical changes collectively referred to as capacitation. Accumulated evidences from several laboratories indicated that the protein tyrosine phosphorylation (PTP) is one of the most important intracellular signaling events regulating sperm function, and is a meaningful indicator of capacitation. Different factors that affect PTP are cholesterol efflux, influx of HCO3(-), increased intracellular Ca(2+), cAMP and reactive oxygen species (ROS). cAMP/PKA and extracellular signal regulated kinases (ERKs) are the known important signaling pathways primarily involved in PTP. Advanced proteomics approaches have revealed several proteins that undergo tyrosine phosphorylation during capacitation. Semen cryopreservation subjects spermatozoa to frequent stressors, which result in capacitation like changes (cryo-capacitation). The cryo-capacitated spermatozoa usually show different patterns of PTP than the normal in vitro capacitated spermatozoa. In the current manuscript, we have summarized some information about the proteins undergoing tyrosine phosphorylation during capacitation and the effect of cryopreservation on PTP as well as the possibilities to reduce the changes associated with cryopreservation process.

Phosphopeptide analysis of rodent epididymal spermatozoa

Spermatozoa are quite unique amongst cell types. Although produced in the testis, both nuclear gene transcription and translation are switched off once the pre-cursor round cell begins to elongate and differentiate into what is morphologically recognized as a spermatozoon. However, the spermatozoon is very immature, having no ability for motility or egg recognition. Both of these events occur once the spermatozoa transit a secondary organ known as the epididymis. During the ~12 day passage that it takes for a sperm cell to pass through the epididymis, post-translational modifications of existing proteins play a pivotal role in the maturation of the cell. One major facet of such is protein phosphorylation. In order to characterize phosphorylation events taking place during sperm maturation, both pure sperm cell populations and pre-fractionation of phosphopeptides must be established. Using back flushing techniques, a method for the isolation of pure spermatozoa of high quality and yield from the distal or caudal epididymides is outlined. The steps for solubilization, digestion, and pre-fractionation of sperm phosphopeptides through TiO2 affinity chromatography are explained. Once isolated, phosphopeptides can be injected into MS to identify both protein phosphorylation events on specific amino acid residues and quantify the levels of phosphorylation taking place during the sperm maturation processes.

Diagnosis and prognosis of male infertility in mammal: the focusing of tyrosine phosphorylation and phosphotyrosine proteins

Male infertility refers to the inability of a man to achieve a pregnancy in a fertile female. In more than one-third of cases, infertility arises due to the male factor. Therefore, developing strategies for the diagnosis and prognosis of male infertility is critical. Simultaneously, a satisfactory model for the cellular mechanisms that regulate normal sperm function must be established. In this regard, tyrosine phosphorylation is one of the most common mechanisms through which several signal transduction pathways are adjusted in spermatozoa. It regulates the various aspects of sperm function, for example, motility, hyperactivation, capacitation, the acrosome reaction, fertilization, and beyond. Several recent large-scale studies have identified the proteins that are phosphorylated in spermatozoa to acquire fertilization competence. However, most of these studies are basal and have not presented an overall mechanism through which tyrosine phosphorylation regulates male infertility. In this review, we focus of this mechanism, discussing most of the tyrosine-phosphorylated proteins in spermatozoa that have been identified to date. We categorized tyrosine-phosphorylated proteins in spermatozoa that regulate male infertility using MedScan Reader (v5.0) and Pathway Studio (v9.0).

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.

Role of posttranslational protein modifications in epididymal sperm maturation and extracellular quality control

The epididymal lumen is a complex microenvironment in which spermatozoa acquire motility and fertility. Spermatozoa are synthetically inactive and therefore the maturation process requires their interaction with proteins that are synthesized and secreted in a highly regionalized manner by the epididymal epithelium. In addition to the integration of epididymal secretory proteins, posttranslational modifications of existing sperm proteins are important for sperm maturation and acquisition of fertilizing potential. Phosphorylation, glycosylation, and processing are several of the posttranslational modifications that sperm proteins undergo during epididymal transit resulting in changes in protein function and localization ultimately leading to mature spermatozoa. In addition to these well-characterized modifications, protein aggregation and cross-linking also occur within the epididymal lumen and may represent unique mechanisms for controlling protein function including that for maturation as well as for extracellular quality control.

Loss of R2D2 proteins ROPN1 and ROPN1L causes defects in murine sperm motility, phosphorylation, and fibrous sheath integrity

The fibrous sheath (FS) is a flagellar cytoskeletal structure unique to sperm that surrounds the outer dense fibers and axoneme. Its primary components are A-kinase anchoring proteins (AKAPs) 3 and 4, which suggests that the FS affects flagellar beating via the scaffolding of signaling pathways necessary for motility. Sperm proteins ROPN1 and ROPN1L bind AKAP3. To determine the role of ROPN1 and ROPN1L in sperm function, we created mice deficient in ROPN1 (RKO), mice deficient in ROPN1L (RLKO), and double knockout mice (DKO). All three strains of mice had normal testicular morphology and spermatogenesis. Only the DKOs had obvious defects in sperm morphology (thinning and shredding of the principal piece), which was accompanied by a reduction in AKAP3 levels. RLKO mice had slightly reduced sperm motility and increased levels of ROPN1. RKO mice had moderately impaired motility and increased levels of ROPN1L. DKO sperm were immotile. We have previously determined that RKO male mice are subfertile, and DKO males are infertile. Together these data indicate that ROPN1L and ROPN1 compensate for each other in the absence of the opposing protein, possibly to maintain AKAP3 incorporation in the FS. Sperm from mice lacking ROPN1L exhibited reductions in both cAMP-dependent protein kinase (PKA) phosphorylation of a 270-kDa protein (perhaps FSCB), and in capacitation-induced tyrosine phosphorylation. Sperm from mice lacking ROPN1 had reduced levels of FSCB and increased tyrosine phosphorylation of noncapacitated sperm. These data demonstrate that mutations in ROPN1 and ROPN1L can cause defects in FS integrity, sperm motility, and PKA-dependent signaling processes, leading to male infertility.

Protein-tyrosine kinase signaling in the biological functions associated with sperm

In sexual reproduction, two gamete cells (i.e., egg and sperm) fuse (fertilization) to create a newborn with a genetic identity distinct from those of the parents. In the course of these developmental processes, a variety of signal transduction events occur simultaneously in each of the two gametes, as well as in the fertilized egg/zygote/early embryo. In particular, a growing body of knowledge suggests that the tyrosine kinase Src and/or other protein-tyrosine kinases are important elements that facilitate successful implementation of the aforementioned processes in many animal species. In this paper, we summarize recent findings on the roles of protein-tyrosine phosphorylation in many sperm-related processes (from spermatogenesis to epididymal maturation, capacitation, acrosomal exocytosis, and fertilization).

Cysteine-rich secretory protein 4 is an inhibitor of transient receptor potential M8 with a role in establishing sperm function

The cysteine-rich secretory proteins (CRISPs) are a group of four proteins in the mouse that are expressed abundantly in the male reproductive tract, and to a lesser extent in other tissues. Analysis of reptile CRISPs and mouse CRISP2 has shown that CRISPs can regulate cellular homeostasis via ion channels. With the exception of the ability of CRISP2 to regulate ryanodine receptors, the in vivo targets of mammalian CRISPs function are unknown. In this study, we have characterized the ion channel regulatory activity of epididymal CRISP4 using electrophysiology, cell assays, and mouse models. Through patch-clamping of testicular sperm, the CRISP4 CRISP domain was shown to inhibit the transient receptor potential (TRP) ion channel TRPM8. These data were confirmed using a stably transfected CHO cell line. TRPM8 is a major cold receptor in the body, but is found in other tissues, including the testis and on the tail and head of mouse and human sperm. Functional assays using sperm from wild-type mice showed that TRPM8 activation significantly reduced the number of sperm undergoing the progesterone-induced acrosome reaction following capacitation, and that this response was reversed by the coaddition of CRISP4. In accordance, sperm from Crisp4 null mice had a compromised ability to undergo to the progesterone-induced acrosome reaction. Collectively, these data identify CRISP4 as an endogenous regulator of TRPM8 with a role in normal sperm function.

Increased expression of the Na,K-ATPase alpha4 isoform enhances sperm motility in transgenic mice

The Na,K-ATPase alpha4 (ATP1A4) isoform is specifically expressed in male germ cells and is highly prevalent in spermatozoa. Although selective inhibition of alpha4 activity with ouabain has been shown to affect sperm motility, a more direct analysis of the role of this isoform in sperm movement has not yet been demonstrated. To establish this, we engineered transgenic mice that express the rat alpha4 isoform fused to green fluorescent protein in male germ cells, under the control of the mouse protamine 1 promoter. We showed that the rat Atp1a4 transgene is expressed in mouse spermatozoa and that it is localized to the sperm flagellum. In agreement with increased expression of the alpha4 isoform, sperm from transgenic mice displayed higher alpha4-specific Na,K-ATPase activity and binding of fluorescently labeled ouabain than wild-type mice. In contrast, expression and activity of ATP1A1 (alpha1), the other Na,K-ATPase alpha isoform present in sperm, remained unchanged. Similar to wild-type mice, mice expressing the alpha4 transgene exhibited normal testis and sperm morphology and no differences in fertility. However, compared to wild-type mice, sperm from transgenic mice displayed plasma membrane hyperpolarization and higher total and progressive motility. Other parameters of motility also increased, including straight-line, curvilinear, and average path velocities and amplitude of lateral head displacement. In addition, sperm from the transgenic mice showed enhanced sperm hyperactive motility, but no changes in progesterone-induced acrosome reaction. Altogether, these results provide new genetic evidence for the role of the ATP1A4 isoform in sperm motility, under both noncapacitating and capacitating conditions.

Activity of the Na,K-ATPase alpha4 isoform is important for membrane potential, intracellular Ca2+, and pH to maintain motility in rat spermatozoa

While the function of the ubiquitous Na,K-ATPase alpha1 subunit has been well documented, the role of the sperm-specific alpha4 isoform of this ion transporter is less known. We have explored the importance of alpha4 in rat sperm physiology by taking advantage of the high sensitivity of this isoform for the inhibitor ouabain. Using concentrations that selectively block alpha4 activity, we found ouabain to reduce not only sperm total motility, but also multiple parameters of sperm movement, including progressive motility, straight line, curvilinear, and average path velocities, lateral head displacement, beat cross frequency, and linearity. According to a direct role of alpha4 in Na(+) transport, ouabain inhibition of alpha4 increased [Na(+)](i) in the male gametes. In addition, interference of alpha4 activity with ouabain produced cell membrane depolarization, diminished pH, and increased [Ca(2)(+)](i) in spermatozoa. Inhibition of alpha4 was sufficient to cause all these effects and additional blockage of alpha1, the other Na,K-ATPase alpha isoform expressed in sperm, and higher doses of ouabain did not result in further changes in the cell parameters studied. These results show that alpha4 is the Na,K-ATPase isoform primarily involved in controlling the transmembrane Na(+) gradient in sperm, and that alpha4 activity is necessary for maintaining membrane potential, [Ca(2)(+)](i), and [H(+)](i) in the cells. The high dependence of sperm motility on membrane excitability, [Ca(2)(+)](i), and acid-base balance suggests that their regulation is the mechanism by which alpha4 maintains motility of the male gametes.

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.

Calcium and other ion dynamics during gamete maturation and fertilization

Ion currents and cytosolic free calcium ([Ca(2+)](i)) elevations are crucial events in triggering the complex machinery involved in both gamete maturation and fertilization. Oocyte maturation is triggered by hormone signaling which causes ion currents and [Ca(2+)](i) increase. Extracellular calcium seems to be required for meiosis progression since: (i) calcium depletion in the maturation medium severely affects oocyte developmental competence; (ii) the activity of plasma membrane L-type Ca(2+) currents decreases during maturation; (iii) the exposure to verapamil, a specific Ca(2+) channel blocker, decreases in vitro maturation efficiency. In spermatozoa, maturation initiates inside the epididymis and ends in the female genital tract. During their journey through the female reproductive tract, sperm undergo a dramatic selection and capacitation achieving fertilization competence. Adhesion to the tubal epithelium extends sperm life through depression of [Ca(2+)](i) until capacitation signals trigger an [Ca(2+)](i) elevation followed by sperm release. At fertilization, egg-sperm interaction evokes well-described transient and almost simultaneous events: i.e., fertilization current, a change in resting potential, and an increase in free [Ca(2+)](i) concentration. These events, termed oocyte activation, are the direct consequence of sperm interaction via either activation of a receptor or entry of a sperm factor. The latter hypothesis has been recently supported by the discovery of PCLzeta, a sperm-specific isozyme triggering a dramatic [Ca(2+)](i) increase via inositol 1,4,5-trisphosphate (IP(3)) production. The course of ion currents and [Ca(2+)](i) transients during maturation and fertilization plays a pivotal role in correct embryo development.

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.

Counting sperm does not add up any more: time for a new equation?

Although sperm dysfunction is the single most common cause of infertility, we have poor methods of diagnosis and surprisingly no effective treatment (excluding assisted reproductive technology). In this review, we challenge the usefulness of a basic semen analysis and argue that a new paradigm is required immediately. We discuss the use of at-home screening to potentially improve the diagnosis of the male and to streamline the management of the sub-fertile couple. Additionally, we outline the recent progress in the field, for example, in proteomics, which will allow the development of new biomarkers of sperm function. This new knowledge will transform our understanding of the spermatozoon as a machine and is likely to lead to non-ART treatments for men with sperm dysfunction.

The Identification of Mouse Sperm-Surface-Associated Proteins and Characterization of Their Ability to Act as Decapacitation Factors1

Mammalian spermatozoa must undergo capacitation before acquiring the ability to fertilize the oocyte. This process is believed to be initiated following the release of surface-associated decapacitation factors that are elaborated by both the epididymis and the male accessory organs. Herein, we report the identification of a number of proteins that are actively released from the surface of mouse spermatozoa during capacitation in vitro. As anticipated, the addition of these factors back to suspensions of mouse spermatozoa was shown to suppress several correlates of the capacitation process. Specifically, they induced a significant, dose-dependent inhibition of the ability of spermatozoa to undergo a progesterone-induced acrosome reaction and to bind to the zona pellucida in vitro. Inhibition of these functions was associated with the suppression of tyrosine phosphorylation in the sperm plasma membrane but had no effect on the phosphorylation of internal proteins in either the sperm head or tail. This inhibitory activity was attributed to a subset of the isolated proteins compromising at least four putative decapacitation factors. These proteins were identified via tandem-mass spectrometry amino acid sequence analysis as plasma membrane fatty acid binding protein, cysteine-rich secretory protein 1 (CRISP1), phosphatidylethanolamine binding protein 1 (PBP), and an unnamed protein product that we have termed decapacitation factor 10 (DF10). Of these proteins, PBP was identified as a primary candidate for a decapacitation factor.

Evidence for the involvement of PECAM-1 in a receptor mediated signal-transduction pathway regulating capacitation-associated tyrosine phosphorylation in human spermatozoa

Mammalian spermatozoa must become ;capacitated' in the female reproductive tract before they gain the ability to fertilize the oocyte. The attainment of a capacitated state has been correlated with a number of biochemical changes, the most notable of which is a dramatic increase in the tyrosine phosphorylation status of these cells. Despite its biological importance, the mechanisms responsible for initiating this tyrosine phosphorylation cascade in vivo are unknown. Here, we report that this signalling pathway can be elicited in a rapid, dose-dependent and lectin-specific manner by wheat germ agglutinin (WGA), but none of 18 other lectins assessed. This response was abrogated by prior enzymatic cleavage of either sialic acid or GlcNAc residues from the sperm surface and by treatment with a range of pharmacological inhibitors directed against protein kinase A, protein tyrosine kinases and intermediates including Src. Proteomic analysis of the WGA-binding sites on the sperm surface identified the putative cognate receptor as platelet cell adhesion molecule 1 (PECAM-1/CD31). This conclusion was supported by the following evidence: (i) anti-PECAM-1 antibodies identified a molecule of the correct molecular mass in human spermatozoa, (ii) PECAM-1 could be isolated from a pool of sperm surface proteins using WGA immobilized on a solid phase support, (iii) PECAM-1 and WGA co-localized to the sperm surface and (iv) anti-PECAM-1 antibodies could completely block the ability of WGA to stimulate tyrosine phosphorylation in these cells. Collectively, these data provide the first evidence that a receptor-mediated signal transduction pathway triggers human sperm capacitation and identifies PECAM-1 as the probable initiator of this second messenger cascade.

Bicarbonate and bovine serum albumin reversibly ‘switch’ capacitation-induced events in human spermatozoa

We have investigated the reversibility of biochemical and physiological changes that occur upon suspension of ejaculated human spermatozoa during in vitro capacitation. Cells were swum up in a simple HEPES-based saline [lacking bicarbonate and bovine serum albumin (BSA)], then resuspended either in supplemented Earle's balanced salt solution (sEBSS) (25 mM bicarbonate) with 0.3% BSA (for in vitro capacitation) or in medium-lacking bicarbonate and/or BSA. Progesterone-induced acrosome reaction (AR) developed during in vitro capacitation (6 h). A progesterone-induced [Ca2+]i signal was detectable in cells maintained in the simple HEPES-based saline, but upon transfer to sEBSS, the response increased three- to four-fold, saturating within <30 min. Serine/threonine phosphorylation saturated within minutes of resuspension, but tyrosine phosphorylation developed over 3 h. Return of cells to non-capacitating conditions caused reversal of all capacitation-dependent changes. The [Ca2+]i signal reverted to its 'uncapacitated' size within <30 min. Protein phosphorylation reversed gradually and could be reinduced (kinetics resembling the first response) upon resuspension in sEBSS. The ability of cells to undergo progesterone-induced AR fell to levels similar to those in uncapacitated cells within 1 h of resuspension in medium not supporting capacitation. Loss of protein phosphorylation occurred only in the absence of both bicarbonate and BSA, but effects on [Ca2+]i signalling and AR could be seen after removal of only one of these factors. We conclude that key events in the capacitation of human spermatozoa are both reversible and repeatable.

The role of molecular chaperones in mouse sperm-egg interactions

Fertilization is a unique and exquisitely choreographed cellular interaction between the male and female gamete that results in the creation of a genetically unique individual. Despite the fundamental importance of fertilization, there remains a dearth of information about the basic biochemical mechanisms that underpin this process. One of the key issues that remain unresolved is the molecular basis of sperm-egg recognition. From the female perspective, it is well established that the sperm recognition sites reside in the zona pellucida (ZP), an acellular coat that surrounds the oocyte. In contrast, numerous studies into the cognate zona receptors residing on the sperm surface have failed to shed significant light on the biochemical identity of these molecules. Such difficulties may, in part, have arisen because investigations have traditionally been based on the precept that the zona receptor represents a single molecular entity that is constitutively expressed on the sperm surface. While such a view holds obvious appeal, it fails to account for growing evidence that gamete interaction is not mediated by a simple lock-and-key mechanism. In this review, we present a novel hypothesis in which the zona recognition site is portrayed as a multimeric molecular structure that is assembled into a functional complex during a maturation process known as 'capacitation'. Furthermore, we consider the possibility that this previously cryptic complex is assembled and delivered to the outer surface of the sperm plasma membrane through the concerted action of several members of the molecular chaperone family of proteins.

Localization and Significance of Molecular Chaperones, Heat Shock Protein 1, and Tumor Rejection Antigen gp96 in the Male Reproductive Tract and During Capacitation and Acrosome Reaction1

Although the molecular basis of sperm-oocyte interaction is unclear, recent studies have implicated two chaperone proteins, heat shock protein 1 (HSPD1; previously known as heat shock protein 60) and tumor rejection antigen gp96 (TRA1; previously known as endoplasmin), in the formation of a functional zona-receptor complex on the surface of mammalian spermatozoa. The current study was undertaken to investigate the expression of these chaperones during the ontogeny of male germ cells through spermatogenesis, epididymal sperm maturation, capacitation, and acrosomal exocytosis. In testicular sections, both HSPD1 and TRA1 were closely associated with the mitochondria of spermatogonia and primary spermatocytes. However, this labeling pattern disappeared from the male germ line during spermiogenesis to become undetectable in testicular spermatozoa. Subsequently, these chaperones could be detected in epididymal spermatozoa and in previously unreported "dense bodies" in the epididymal lumen. The latter appeared in the precise region of the epididymis (proximal corpus), where spermatozoa acquire the capacity to recognize and bind to the zona pellucida, implicating these structures in the functional remodeling of the sperm surface during epididymal maturation. Both HSPD1 and TRA1 were subsequently found to become coexpressed on the surface of live mouse spermatozoa following capacitation in vitro and were lost once these cells had undergone the acrosome reaction, as would be expected of cell surface molecules involved in sperm-egg interaction. These data reinforce the notion that these chaperones are intimately involved in the mechanisms by which mammalian spermatozoa both acquire and express their ability to recognize the zona pellucida.

Tyrosine phosphorylation activates surface chaperones facilitating sperm-zona recognition

Mammalian spermatozoa undergo a series of molecular and biochemical changes collectively termed capacitation prior to acquiring the ability to fertilise the oocyte. Although phosphorylation of sperm proteins on tyrosine residues has been recognised as an important component of this process, the precise relationship between the phosphorylation status of mammalian spermatozoa and their capacity for fertilisation has remained unclear. In this study we demonstrate a causal relationship between tyrosine phosphorylation in spermatozoa and sperm-zona interaction. The phosphotyrosine expression associated with sperm capacitation localised to internal flagellar structures in permeabilised cells but could also be detected on the exterior surface of the sperm head in live cells. Importantly, almost all spermatozoa bound to the zona pellucida demonstrated this pattern of phosphoprotein localisation, compared to fewer than 15% of the free-swimming population. These data suggest that tyrosine phosphorylation plays a significant role in remodelling the sperm surface, so that these cells are able to recognise the zona pellucida. Phosphoproteome analysis yielded the first evidence of molecular chaperones, endoplasmin (erp99) and heat shock protein 60 (hsp60), as targets for phosphorylation on the surface of mouse spermatozoa, whereas immunofluorescence localised these proteins to the precise region of the sperm head that participates in zona recognition. Based on these results, we propose a novel mechanism for mammalian gamete interaction whereby the activation of sperm-surface chaperones by tyrosine phosphorylation during capacitation may trigger conformational changes facilitating the formation of a functional zona pellucida receptor complex on the surface of mammalian spermatozoa.