The tyrosine kinase FER is responsible for the capacitation-associated increase in tyrosine phosphorylation in murine sperm

Conservation of Protein Kinase A Substrates in the Cnidarian Coral Spermatozoa Among Animals and Their Molecular Evolution

The coral Acropora spp., known for its reef-building abilities, is a simultaneous hermaphroditic broadcast spawning species. Acropora spp. release gametes into seawater, activating sperm motility. This activation is mediated by adenylyl cyclase (AC) and protein kinase A (PKA). Notably, membrane-permeable cAMP (8-bromo-cAMP) promotes sperm motility activation of Acropora florida. While the signal transduction for PKA-dependent motility activation is highly conserved among animals, the downstream signaling of PKA remains unclear. In this study, we used mass spectrometry (MS) analyses to identify sperm proteins in the coral Acropora digitifera, as well as the serine/threonine residues of potential PKA substrates, and then, we investigated the conservation of these proteins from corals to vertebrates. We identified 148 sperm proteins of A. digitifera with typical PKA recognition motifs, namely RRXT and RRXS. We subsequently used ORTHOSCOPE to screen for orthologs encoding these 148 proteins from corals to vertebrates. Among the isolated orthologs, we identified positive selection in 48 protein-encoding genes from 18 Acropora spp. Subsequently, we compared the conservation rates of the PKA phosphorylation motif residues between the orthologs under positive and purifying selections. Notably, the serine residues of the orthologs under positive selection were more conserved. Therefore, adaptive evolution might have occurred in the orthologs of PKA substrate candidates from corals to vertebrates, accompanied by phosphorylation residue conservation. Collectively, our findings suggest that while PKA signal transduction, including substrates in sperm, may have been conserved, the substrates may have evolved to adapt to diverse fertilization conditions, such as synchronous broadcast spawning.

Mitopherogenesis, a form of mitochondria-specific ectocytosis, regulates sperm mitochondrial quantity and fertility

Mitochondrial export into the extracellular space is emerging as a fundamental cellular process implicated in diverse physiological activities. Although a few studies have shed light on the process of discarding damaged mitochondria, how mitochondria are exported and the functions of mitochondrial release remain largely unclear. Here we describe mitopherogenesis, a formerly unknown process that specifically secretes mitochondria through a unique extracellular vesicle termed a 'mitopher'. We observed that during sperm development in male Caenorhabditis elegans, healthy mitochondria are exported out of the spermatids through mitopherogenesis and each of the generated mitophers harbours only one mitochondrion. In mitopherogenesis, the plasma membrane first forms mitochondrion-embedding outward buds, which then promptly bud off and thereby result in the generation of mitophers. Mechanistically, extracellular protease signalling in the testis triggers mitopher formation from spermatids, which is partially mediated by the tyrosine kinase SPE-8. Moreover, mitopherogenesis requires normal microfilament dynamics, whereas myosin VI antagonizes mitopher generation. Strikingly, our three-dimensional electron microscopy analyses indicate that mitochondrial quantity requires precise modulation during sperm development, which is critically mediated by mitopherogenesis. Inhibition of mitopherogenesis causes accumulation of mitochondria in sperm, which may lead to sperm motility and fertility defects. Our findings identify mitopherogenesis as a previously undescribed process for mitochondria-specific ectocytosis, which may represent a fundamental branch of mechanisms underlying mitochondrial quantity control to regulate cell functions during development.

Mouse sperm energy restriction and recovery (SER) revealed novel metabolic pathways

Mammalian sperm must undergo capacitation to become fertilization-competent. While working on mice, we recently developed a new methodology for treating sperm in vitro, which results in higher rates of fertilization and embryo development after in vitro fertilization. Sperm incubated in media devoid of nutrients lose motility, although they remain viable. Upon re-adding energy substrates, sperm resume motility and become capacitated with improved functionality. Here, we explore how sperm energy restriction and recovery (SER) treatment affects sperm metabolism and capacitation-associated signaling. Using extracellular flux analysis and metabolite profiling and tracing via nuclear magnetic resonance (NMR) and mass spectrometry (MS), we found that the levels of many metabolites were altered during the starvation phase of SER. Of particular interest, two metabolites, AMP and L-carnitine, were significantly increased in energy-restricted sperm. Upon re-addition of glucose and initiation of capacitation, most metabolite levels recovered and closely mimic the levels observed in capacitating sperm that have not undergone starvation. In both control and SER-treated sperm, incubation under capacitating conditions upregulated glycolysis and oxidative phosphorylation. However, ATP levels were diminished, presumably reflecting the increased energy consumption during capacitation. Flux data following the fate of 13C glucose indicate that, similar to other cells with high glucose consumption rates, pyruvate is converted into 13C-lactate and, with lower efficiency, into 13C-acetate, which are then released into the incubation media. Furthermore, our metabolic flux data show that exogenously supplied glucose is converted into citrate, providing evidence that in sperm cells, as in somatic cells, glycolytic products can be converted into Krebs cycle metabolites.

Mutation of S461, in the GOLGA3 phosphorylation site, does not affect mouse spermatogenesis

Background

Golgin subfamily A member 3 (Golga3), a member of the golgin subfamily A, is highly expressed in mouse testis. The GOLGA3 protein, which contains eight phosphorylation sites, is involved in protein transport, cell apoptosis, Golgi localization, and spermatogenesis. Although it has been previously reported that nonsense mutations in Golga3 cause multiple defects in spermatogenesis, the role of Golga3 in the testis is yet to be clarified.

Methods

Immunofluorescence co-localization in cells and protein dephosphorylation experiments were performed. Golga3 ^S461L/S461Lmice were generated using cytosine base editors. Fertility tests as well as computer-assisted sperm analysis (CASA) were then performed to investigate sperm motility within caudal epididymis. Histological and immunofluorescence staining were used to analyze testis and epididymis phenotypes and TUNEL assays were used to measure germ cell apoptosis in spermatogenic tubules.

Results

Immunofluorescence co-localization showed reduced Golgi localization of GOLGA3^S465L with some protein scattered in the cytoplasm of HeLa cells .In addition, protein dephosphorylation experiments indicated a reduced band shift of the dephosphorylated GOLGA3^S465L, confirming S461 as the phosphorylation site. Golga3 is an evolutionarily conserved gene and Golga3 ^S461L/S461Lmice were successfully generated using cytosine base editors. These mice had normal fertility and spermatozoa, and did not differ significantly from wild-type mice in terms of spermatogenesis and apoptotic cells in tubules.

Conclusions

Golga3 was found to be highly conserved in the testis, and GOLGA3 was shown to be involved in spermatogenesis, especially in apoptosis and Golgi complex-mediated effects. Infertility was also observed in Golga3 KO male mice. Although GOLGA3^S465Lshowed reduced localization in the Golgi with some expression in the cytoplasm, this abnormal localization did not adversely affect fertility or spermatogenesis in male C57BL/6 mice. Therefore, mutation of the S461 GOLGA3 phosphorylation site did not affect mouse spermatogenesis.

PP1, PP2A and PP2B Interplay in the Regulation of Sperm Motility: Lessons from Protein Phosphatase Inhibitors

Male fertility relies on the ability of spermatozoa to fertilize the egg in the female reproductive tract (FRT). Spermatozoa acquire activated motility during epididymal maturation; however, to be capable of fertilization, they must achieve hyperactivated motility in the FRT. Extensive research found that three protein phosphatases (PPs) are crucial to sperm motility regulation, the sperm-specific protein phosphatase type 1 (PP1) isoform gamma 2 (PP1γ2), protein phosphatase type 2A (PP2A) and protein phosphatase type 2B (PP2B). Studies have reported that PP activity decreases during epididymal maturation, whereas protein kinase activity increases, which appears to be a requirement for motility acquisition. An interplay between these PPs has been extensively investigated; however, many specific interactions and some inconsistencies remain to be elucidated. The study of PPs significantly advanced following the identification of naturally occurring toxins, including calyculin A, okadaic acid, cyclosporin, endothall and deltamethrin, which are powerful and specific PP inhibitors. This review aims to overview the protein phosphorylation-dependent biochemical pathways underlying sperm motility acquisition and hyperactivation, followed by a discussion of the PP inhibitors that allowed advances in the current knowledge of these pathways. Since male infertility cases still attain alarming numbers, additional research on the topic is required, particularly using other PP inhibitors.

Exploring the ovine sperm transcriptome by RNAseq techniques. I Effect of seasonal conditions on transcripts abundance

Understanding the cell molecular changes occurring as a results of climatic circumstances is crucial in the current days in which climate change and global warming are one of the most serious challenges that living organisms have to face. Sperm are one of the mammals’ cells most sensitive to heat, therefore evaluating the impact of seasonal changes in terms of its transcriptional activity can contribute to elucidate how these cells cope with heat stress events. We sequenced the total sperm RNA from 64 ejaculates, 28 collected in summer and 36 collected in autumn, from 40 Manchega rams. A highly rich transcriptome (11,896 different transcripts) with 90 protein coding genes that exceed an average number of 5000 counts were found. Comparing transcriptome in the summer and autumn ejaculates, 236 significant differential abundance genes were assessed, most of them (228) downregulated. The main functions that these genes are related to sexual reproduction and negative regulation of protein metabolic processes and kinase activity. Sperm response to heat stress supposes a drastic decrease of the transcriptional activity, and the upregulation of only a few genes related with the basic functions to maintain the organisms’ homeostasis and surviving. Rams’ spermatozoids carry remnant mRNAs which are retrospectively indicators of events occurring along the spermatogenesis process, including abiotic factors such as environmental temperature.

C2CD6 regulates targeting and organization of the CatSper calcium channel complex in sperm flagella

The CatSper cation channel is essential for sperm capacitation and male fertility. The multi-subunit CatSper complexes form highly organized calcium signaling nanodomains on flagellar membranes. Here, we report identification of an uncharacterized protein, C2CD6, as a subunit of the mouse CatSper complex. C2CD6 contains a calcium-dependent, membrane-targeting C2 domain. C2CD6 associates with the CatSper calcium-selective, core-forming subunits. Deficiency of C2CD6 depletes the CatSper nanodomains from the flagellum and results in male sterility. C2CD6-deficient sperm are defective in hyperactivation and fail to fertilize oocytes both in vitro and in vivo. CatSper currents are present but at a significantly lower level in C2CD6-deficient sperm. Transient treatments with either Ca2+ ionophore, starvation, or a combination of both restore the fertilization capacity of C2CD6-deficient sperm. C2CD6 interacts with EFCAB9, a pH-dependent calcium sensor in the CatSper complex. We postulate that C2CD6 facilitates incorporation of the CatSper complex into the flagellar plasma membrane and may function as a calcium sensor. The identification of C2CD6 may enable the long-sought reconstitution of the CatSper ion channel complex in a heterologous system for male contraceptive development.

Human Sperm Remain Motile After a Temporary Energy Restriction but do Not Undergo Capacitation-Related Events

To acquire fertilization competence, mammalian sperm must undergo several biochemical and physiological modifications known as capacitation. Despite its relevance, the metabolic pathways that regulate the capacitation-related events, including the development of hyperactivated motility, are still poorly described. Previous studies from our group have shown that temporary energy restriction in mouse sperm enhanced hyperactivation, in vitro fertilization, early embryo development and pregnancy rates after embryo transfer, and it improved intracytoplasmic sperm injection results in the bovine model. However, the effects of starvation and energy recovery protocols on human sperm function have not yet been established. In the present work, human sperm were incubated for different periods of time in medium containing glucose, pyruvate and lactate (NUTR) or devoid of nutrients for the starving condition (STRV). Sperm maintained in STRV displayed reduced percentages of motility and kinematic parameters compared to cells incubated in NUTR medium. Moreover, they did not undergo hyperactivation and showed reduced levels of ATP, cAMP and protein tyrosine phosphorylation. Similar to our results with mouse sperm, starvation induced increased intracellular Ca2+ concentrations. Starved human sperm were capable to continue moving for more than 27 h, but the incubation with a mitochondrial uncoupler or inhibitors of oxidative phosphorylation led to a complete motility loss. When exogenous nutrients were added back (sperm energy recovery (SER) treatment), hyperactivated motility was rescued and there was a rise in sperm ATP and cAMP levels in 1 min, with a decrease in intracellular Ca2+ concentration and no changes in sperm protein tyrosine phosphorylation. The finding that human sperm can remain motile for several hours under starvation due to mitochondrial use of endogenous metabolites implies that other metabolic pathways may play a role in sperm energy production. In addition, full recovery of motility and other capacitation parameters of human sperm after SER suggests that this treatment might be used to modulate human sperm fertilizing ability in vitro.

Activation of cAMP-dependent phosphorylation pathways is independent of ROS production during mouse sperm capacitation

Mammalian sperm have to undergo capacitation to fertilize the egg. At the molecular level, capacitation involves cAMP synthesis, protein kinase A activation, and downstream increase in tyrosine phosphorylation. In addition, during capacitation, mammalian sperm actively generate reactive oxygen species (ROS). It has been proposed that ROS modulate phosphorylation pathways; however, the crosstalk between these signaling processes is not well-understood. In the present study, we used loss- and gain-of-function approaches to evaluate the interconnection between ROS and phosphorylation. We showed that BSA and HCO₃ ^- , but not Ca²⁺ , in the capacitation media are required for ROS production. The synergic effect of these compounds was neither mediated by HCO₃ ^- stimulation of cAMP synthesis nor by BSA-induced cholesterol efflux. The capacitation-induced ROS generation was blocked in the presence of superoxide dismutase (SOD), catalase, and apocynin. However, none of these compounds affected cAMP-dependent or tyrosine phosphorylation. On the other hand, the addition of NADPH to the media induced ROS generation in sperm incubated in the absence of BSA and HCO₃ ^- without upregulating cAMP-dependent or tyrosine phosphorylation signaling. Most interestingly, catalase, but not SOD, blocked in vitro fertilization suggesting a role for H₂ O₂ in this process.

Cdc42 localized in the CatSper signaling complex regulates cAMP-dependent pathways in mouse sperm

Sperm acquire the ability to fertilize in a process called capacitation and undergo hyperactivation, a change in the motility pattern, which depends on Ca²⁺ transport by CatSper channels. CatSper is essential for fertilization and it is subjected to a complex regulation that is not fully understood. Here, we report that similar to CatSper, Cdc42 distribution in the principal piece is confined to four linear domains and this localization is disrupted in CatSper1-null sperm. Cdc42 inhibition impaired CatSper activity and other Ca²⁺ -dependent downstream events resulting in a severe compromise of the sperm fertilizing potential. We also demonstrate that Cdc42 is essential for CatSper function by modulating cAMP production by soluble adenylate cyclase (sAC), providing a new regulatory mechanism for the stimulation of CatSper by the cAMP-dependent pathway. These results reveal a broad mechanistic insight into the regulation of Ca²⁺ in mammalian sperm, a matter of critical importance in male infertility as well as in contraception.

Mouse and Fly Sperm Motility Changes Differently under Modelling Microgravity

Sperm motility is essential for the natural fertilization process in most animal species. Despite the fact that evolution took place under conditions of constant gravity, the motility of spermatozoa of insects and mammals under microgravity conditions changes in different ways. In this work, an attempt was made to explain this effect. The sperm motility of the fruit fly Drosophila melanogaster and the mouse was evaluated after exposure to a random positioning machine for 6 h. Sodium fluoride was used to inhibit serine/threonine phosphatases, sodium orthovanadate was used to inhibit tyrosine phosphatases, and 6-(dimethylamino)purine was used to inhibit protein kinases. The results obtained indicate that simulated microgravity leads to an increase in the speed of movement of fly spermatozoa by 30% (p < 0.05), and this effect is blocked by sodium fluoride. In contrast, a 29% (p < 0.05) decrease in the speed of movement of mouse spermatozoa under simulated microgravity is prevented by 6-(dimethylamino)purine. Moreover, after 6 h of exposure, the content of tubulin cytoskeleton and actin proteins remains at the control level in the spermatozoa of flies and mice. However, the content of the actin-binding protein alpha-actinin in fly sperm decreases by 29% (p < 0.05), while in mouse sperm, the relative content of alpha-actinin1 increases by 94% (p < 0.05) and alpha-actinin4 by 121% (p < 0.05) relative to the control, as determined by 6 simulated microgravity tests. It can be assumed that the effect of simulated microgravity on the motility of mammalian spermatozoa is mediated through the regulation of phosphorylation and that of insects through the regulation of dephosphorylation of motor proteins; moreover, the development of a response to changes in external mechanical conditions has a different time scale.

Trehalose modifies the protein profile of ram spermatozoa during cryopreservation

As a magical oligosaccharide, trehalose has been revealed to enhance the post-thaw quality of stock semen. However, information regarding the cryoprotective mechanism of trehalose during cryopreservation has not yet been determined. This study was designed to observe the effects of trehalose on the proteome of ram frozen spermatozoa by applying the isobaric tag for relative and absolute quantification (iTRAQ) strategy combined with parallel reaction monitoring (PRM). A total of 1269 proteins were identified. Among them, there were 21 differentially expressed proteins (DEPs), with 9 up-regulated proteins and 11 down-regulated proteins in spermatozoa frozen with trehalose. These DEPs were primarily located in nucleus, cytoplasm, and extracellular region. The Gene Ontology (GO) enrichment analysis demonstrated the involvement of the DEPs in signal transduction, ion binding, oxidoreductase activity, response to stress, and catabolic processes. Based on the STRING analysis, tight functional correlations were observed between 6-phosphogluconate dehydrogenase, fructose-bisphosphate aldolase A isoform 1, 14-3-3 protein epsilon, tyrosine-protein kinase Fer, and beta-hexosaminidase subunit alpha precursor. Furthermore, 10 DEPs were verified using PRM, confirming the accuracy of the iTRAQ data acquired in this study. In conclusion, trehalose can modify the protein profile of ram spermatozoa during cryopreservation, which may be associated with its cryoprotective effects. Additionally, trehalose may function on frozen spermatozoa through antioxidation, involvement in glycolysis, and increment of spermatozoa tolerance to various stresses.

Ouabain-induced activation of phospholipase C zeta and its contributions to bovine sperm capacitation

The sperm-derived oocyte activating factor, phospholipase C zeta (PLC ζ), is the only PLC isoform reported in cattle. The objectives were to (1) localize PLC ζ in fresh and capacitated bovine sperm and (2) investigate the activation of PLC ζ during bull sperm capacitation and contributions of PLC activity to this process. We confirmed interaction of testis-specific isoform of Na/K-ATPase (ATP1A4) with PLC ζ (immunolocalization and immunoprecipitation) and tyrosine phosphorylation (immunoprecipitation) of PLC ζ (a post-translational protein modification commonly involved in activation of PLC in somatic cells) during capacitation. Furthermore, incubation of sperm under capacitating conditions upregulated PLC-mediated hyperactivated motility, tyrosine phosphoprotein content, acrosome reaction, and F-actin formation (flow cytometry), implying that PLC activity is enhanced during capacitation and contributing to these capacitation processes. In conclusion, we inferred that PLC ζ is activated during capacitation by tyrosine phosphorylation through a mechanism involving ATP1A4, contributing to capacitation-associated biochemical events.

The functions of CAP superfamily proteins in mammalian fertility and disease

BACKGROUND

Members of the cysteine-rich secretory proteins (CRISPS), antigen 5 (Ag5) and pathogenesis-related 1 (Pr-1) (CAP) superfamily of proteins are found across the bacterial, fungal, plant and animal kingdoms. Although many CAP superfamily proteins remain poorly characterized, over the past decade evidence has accumulated, which provides insights into the functional roles of these proteins in various processes, including fertilization, immune defence and subversion, pathogen virulence, venom toxicology and cancer biology.

OBJECTIVE AND RATIONALE

The aim of this article is to summarize the current state of knowledge on CAP superfamily proteins in mammalian fertility, organismal homeostasis and disease pathogenesis.

SEARCH METHODS

The scientific literature search was undertaken via PubMed database on all articles published prior to November 2019. Search terms were based on following keywords: 'CAP superfamily', 'CRISP', 'Cysteine-rich secretory proteins', 'Antigen 5', 'Pathogenesis-related 1', 'male fertility', 'CAP and CTL domain containing', 'CRISPLD1', 'CRISPLD2', 'bacterial SCP', 'ion channel regulator', 'CatSper', 'PI15', 'PI16', 'CLEC', 'PRY proteins', 'ASP proteins', 'spermatogenesis', 'epididymal maturation', 'capacitation' and 'snake CRISP'. In addition to that, reference lists of primary and review article were reviewed for additional relevant publications.

OUTCOMES

In this review, we discuss the breadth of knowledge on CAP superfamily proteins with regards to their protein structure, biological functions and emerging significance in reproduction, health and disease. We discuss the evolution of CAP superfamily proteins from their otherwise unembellished prokaryotic predecessors into the multi-domain and neofunctionalized members found in eukaryotic organisms today. At least in part because of the rapid evolution of these proteins, many inconsistencies in nomenclature exist within the literature. As such, and in part through the use of a maximum likelihood phylogenetic analysis of the vertebrate CRISP subfamily, we have attempted to clarify this confusion, thus allowing for a comparison of orthologous protein function between species. This framework also allows the prediction of functional relevance between species based on sequence and structural conservation.

WIDER IMPLICATIONS

This review generates a picture of critical roles for CAP proteins in ion channel regulation, sterol and lipid binding and protease inhibition, and as ligands involved in the induction of multiple cellular processes.

The secrets of success

Imaging sperm as they travel through the female reproductive tract has revealed new details about fertilization at the molecular level.

Extracellular Vesicles, the Road toward the Improvement of ART Outcomes

Nowadays, farm animal industries use assisted reproductive technologies (ART) as a tool to manage herds' reproductive outcomes, for a fast dissemination of genetic improvement as well as to bypass subfertility issues. ART comprise at least one of the following procedures: collection and handling of oocytes, sperm, and embryos in in vitro conditions. Therefore, in these conditions, the interaction with the oviductal environment of gametes and early embryos during fertilization and the first stages of embryo development is lost. As a result, embryos obtained in in vitro fertilization (IVF) have less quality in comparison with those obtained in vivo, and have lower chances to implant and develop into viable offspring. In addition, media currently used for IVF are very similar to those empirically developed more than five decades ago. Recently, the importance of extracellular vesicles (EVs) in the fertility process has flourished. EVs are recognized as effective intercellular vehicles for communication as they deliver their cargo of proteins, lipids, and genetic material. Thus, during their transit through the female reproductive tract both gametes, oocyte and spermatozoa (that previously encountered EVs produced by male reproductive tract) interact with EVs produced by the female reproductive tract, passing them important information that contributes to a successful fertilization and embryo development. This fact highlights that the reproductive tract EVs cargo has an important role in reproductive events, which is missing in current ART media. This review aims to recapitulate recent advances in EVs functions on the fertilization process, highlighting the latest proposals with an applied approach to enhance ART outcome through EV utilization as an additive to the media of current ART procedures.

Sperm ion channels and transporters in male fertility and infertility

Mammalian sperm cells must respond to cues originating from along the female reproductive tract and from the layers of the egg in order to complete their fertilization journey. Dynamic regulation of ion signalling is, therefore, essential for sperm cells to adapt to their constantly changing environment. Over the past 15 years, direct electrophysiological recordings together with genetically modified mouse models and human genetics have confirmed the importance of ion channels, including the principal Ca²⁺-selective plasma membrane ion channel CatSper, for sperm activity. Sperm ion channels and membrane receptors are attractive targets for both the development of contraceptives and infertility treatment drugs. Furthermore, in this era of assisted reproductive technologies, understanding the signalling processes implicated in defective sperm function, particularly those arising from genetic abnormalities, is of the utmost importance not only for the development of infertility treatments but also to assess the overall health of a patient and his children. Future studies to improve reproductive health care and overall health care as a function of the ability to reproduce should include identification and analyses of gene variants that underlie human infertility and research into fertility-related molecules.

Molecular insights into the divergence and diversity of post-testicular maturation strategies

Competition to achieve paternity has coerced the development of a multitude of male reproductive strategies. In one of the most well-studied examples, the spermatozoa of all mammalian species must undergo a series of physiological changes as they transit the male (epididymal maturation) and female (capacitation) reproductive tracts prior to realizing their potential to fertilize an ovum. However, the origin and adaptive advantage afforded by these intricate processes of post-testicular sperm maturation remain to be fully elucidated. Here, we review literature pertaining to the nature and the physiological role of epididymal maturation and subsequent capacitation in comparative vertebrate taxa including representative species from the avian, reptilian, and mammalian lineages. Such insights are discussed in terms of the framework they provide for helping to understand the evolutionary significance of post-testicular sperm maturation.

3D in situ imaging of the female reproductive tract reveals molecular signatures of fertilizing spermatozoa in mice

Out of millions of ejaculated sperm, a few reach the fertilization site in mammals. Flagellar Ca²⁺ signaling nanodomains, organized by multi-subunit CatSper calcium channel complexes, are pivotal for sperm migration in the female tract, implicating CatSper-dependent mechanisms in sperm selection. Here using biochemical and pharmacological studies, we demonstrate that CatSper1 is an O-linked glycosylated protein, undergoing capacitation-induced processing dependent on Ca²⁺ and phosphorylation cascades. CatSper1 processing correlates with protein tyrosine phosphorylation (pY) development in sperm cells capacitated in vitro and in vivo. Using 3D in situ molecular imaging and ANN-based automatic detection of sperm distributed along the cleared female tract, we demonstrate that spermatozoa past the utero-tubal junction possess the intact CatSper1 signals. Together, we reveal that fertilizing mouse spermatozoa in situ are characterized by intact CatSper channel, lack of pY, and reacted acrosomes. These findings provide molecular insight into sperm selection for successful fertilization in the female reproductive tract.

Ca2+ signaling in mammalian spermatozoa

Calcium is an essential ion which regulates sperm motility, capacitation and the acrosome reaction (AR), three processes necessary for successful fertilization. The AR enables the spermatozoon to penetrate into the egg. In order to undergo the AR, the spermatozoon must reside in the female reproductive tract for several hours, during which a series of biochemical transformations takes place, collectively called capacitation. An early event in capacitation is relatively small elevation of intracellular Ca²⁺ (in the nM range) and bicarbonate, which collectively activate the soluble adenylyl cyclase to produce cyclic-AMP; c-AMP activates protein kinase A (PKA), leading to indirect tyrosine phosphorylation of proteins. During capacitation, there is an increase in the membrane-bound phospholipase C (PLC) which is activated prior to the AR by relatively high increase in intracellular Ca²⁺ (in the μM range). PLC catalyzes the hydrolysis of phosphatidyl-inositol-4,5-bisphosphate (PIP₂) to diacylglycerol and inositol-trisphosphate (IP₃), leading to activation of protein kinase C (PKC) and the IP₃-receptor. PKC activates a Ca²⁺- channel in the plasma membrane, and IP₃ activates the Ca²⁺- channel in the outer acrosomal membrane, leading to Ca²⁺ depletion from the acrosome. As a result, the plasma-membrane store-operated Ca²⁺ channel (SOCC) is activated to increase cytosolic Ca²⁺ concentration, enabling completion of the acrosome reaction. The hydrolysis of PIP₂ by PLC results in the release and activation of PIP₂-bound gelsolin, leading to F-actin dispersion, an essential step prior to the AR. Ca²⁺ is also involved in the regulation of sperm motility. During capacitation, the sperm develops a unique motility pattern called hyper-activated motility (HAM) which is essential for successful fertilization. The main Ca²⁺-channel that mediates HAM is the sperm-specific CatSper located in the sperm tail.

Progesterone, spermatozoa and reproduction: An updated review

The rapid effects of steroids on spermatozoa have been demonstrated for the first time more than three decades ago. Progesterone (P), which is present throughout the female genital tract with peaks of levels in the cumulus matrix surrounding the oocyte, has been shown to stimulate several sperm functions in vitro, including capacitation, hyperactivation, chemotaxis and acrosome reaction (AR). Besides an increase of intracellular calcium, P has been shown to activate other sperm signalling pathways including tyrosine phosphorylation of several sperm proteins. All these effects are mediated by extra-nuclear pathways likely involving interaction with molecules present on the sperm surface. In particular, the increase in intracellular calcium ([Ca²⁺]_i) in spermatozoa from human and several other mammalian species is mediated by the sperm specific calcium channel CatSper, whose expression and function are required for sperm hyperactive motility. P-mediated CatSper activation is indeed involved in promoting sperm hyperactivation, but the involvement of this channel in other P-stimulated sperm functions, such as AR and chemotaxis, is less clear and further studies are required to disclose all the involved pathways. In human spermatozoa, responsiveness to P in terms of [Ca²⁺]_i increase and AR is highly related to sperm fertilizing ability in vitro, suggesting that the steroid is a physiological inducer of AR during in vitro fertilization. In view of their physiological relevance, P-stimulated sperm functions are currently investigated to develop new tools to select highly performant spermatozoa for assisted reproduction.

Pantoprazole, a proton-pump inhibitor, impairs human sperm motility and capacitation in vitro

BACKGROUND

The effects of PPIs on human sperm fertilizing capacity were poorly investigated although these drugs are widely over-used. Two publications retrospectively studied relationships between any PPI intake and sperm parameters from patients consulting at infertility clinics, but the conclusions of these reports were contradictory. Only two reports investigated the effects of lansoprazole and omeprazole on sperm motility and found lansoprazole to be deleterious and omeprazole to be neutral for sperm motility. The inconsistency of the PPI effect in the previous reports emphasizes the need for more basic research on human spermatozoa, taking into account the hypothesis that the different PPI drugs may have different effects on sperm physiology.

OBJECTIVES

Do PPIs, which are among the most widely sold drug in the word, impact negatively human sperm capacitation and sperm motility?

MATERIALS AND METHODS

The effects of PPIs on human sperm maturation and motility were analyzed by CASA, flow cytometry, and Western blot.

RESULTS

We tested the impact of 6 different PPIs on human sperm motility and capacitation. We showed that pantoprazole, but not the other PPIs, decreased sperm progressive motility and capacitation-induced sperm hyperactivation. We therefore investigated further the effects of pantoprazole on sperm capacitation, and we observed that it had a significant deleterious effect on the capacitation-induced hyperpolarization of the membrane potential and capacitation-associated protein phosphorylation.

DISCUSSION AND CONCLUSION

Our results indicate that exposure to pantoprazole has an adverse effect on the physiological competence of human spermatozoa. As the capacitation process takes place within the female tract, our results suggest that PPIs intake by the female partner may impair in vivo sperm maturation and possibly fertilization. Moreover, the absence of adverse effect by PPIs on mouse sperm emphasizes the need to develop reprotox assays using human material to better assess the effects of medication intake on sperm physiology.

Metabolic changes in mouse sperm during capacitation†

Mammalian sperm are stored in the epididymis in a dormant state. Upon ejaculation, they must immediately start producing sufficient energy to maintain motility and support capacitation. While this increased energy demand during capacitation is well established, it remains unclear how mouse sperm modify their metabolism to meet this need. We now show that capacitating mouse sperm enhance glucose uptake, identifying glucose uptake as a functional marker of capacitation. Using an extracellular flux analyzer, we show that glycolysis and oxidative phosphorylation increase during capacitation. Furthermore, this increase in oxidative phosphorylation is dependent on glycolysis, providing experimental evidence for a link between glycolysis and oxidative phosphorylation in mouse sperm.

The Impact of Di-2-Ethylhexyl Phthalate on Sperm Fertility

A growing number of studies point to reduced fertility upon chronic exposure to endocrine-disrupting chemicals (EDCs) such as phthalates and plasticizers. These toxins are ubiquitous and are often found in food and beverage containers, medical devices, as well as in common household and personal care items. Animal studies with EDCs, such as phthalates and bisphenol A have shown a dose-dependent decrease in fertility and embryo toxicity upon chronic exposure. However, limited research has been conducted on the acute effects of these EDCs on male fertility. Here we used a murine model to test the acute effects of four ubiquitous environmental toxins: bisphenol A (BPA), di-2-ethylhexyl phthalate (DEHP), diethyl phthalate (DEP), and dimethyl phthalate (DMP) on sperm fertilizing ability and pre-implantation embryo development. The most potent of these toxins, di-2-ethylhexyl phthalate (DEHP), was further evaluated for its effect on sperm ion channel activity, capacitation status, acrosome reaction and generation of reactive oxygen species (ROS). DEHP demonstrated a profound hazardous effect on sperm fertility by producing an altered capacitation profile, impairing the acrosome reaction, and, interestingly, also increasing ROS production. These results indicate that in addition to its known chronic impact on reproductive potential, DEHP also imposes acute and profound damage to spermatozoa, and thus, represents a significant risk to male fertility.

The protein phosphatase isoform PP1γ1 substitutes for PP1γ2 to support spermatogenesis but not normal sperm function and fertility†

Four isoforms of serine/threonine phosphatase type I, PP1α, PP1β, PP1γ1, and PP1γ2, are derived from three genes. The PP1γ1 and PP1γ2 isoforms are alternately spliced transcripts of the protein phosphatase 1 catalytic subunit gamma gene (Ppp1cc). While PP1γ1 is ubiquitous in somatic cells, PP1γ2 is expressed exclusively in testicular germ cells and sperm. Ppp1cc knockout male mice (-/-), lacking both PP1γ1 and PP1γ2, are sterile due to impaired sperm morphogenesis. Fertility and normal sperm function can be restored by transgenic expression of PP1γ2 alone in testis of Ppp1cc (-/-) mice. The purpose of this study was to determine whether the PP1γ1 isoform is functionally equivalent to PP1γ2 in supporting spermatogenesis and male fertility. Significant levels of transgenic PP1γ1 expression occurred only when the transgene lacked a 1-kb 3΄UTR region immediately following the stop codon of the PP1γ1 transcript. PP1γ1 was also incorporated into sperm at levels comparable to PP1γ2 in sperm from wild-type mice. Spermatogenesis was restored in mice expressing PP1γ1 in the absence of PP1γ2. However, males from the transgenic rescue lines were subfertile. Sperm from the PP1γ1 rescue mice were unable to fertilize eggs in vitro. Intrasperm localization of PP1γ1 and the association of the protein regulators of the phosphatase were altered in epididymal sperm in transgenic PP1γ1 compared to PP1γ2. Thus, the ubiquitous isoform PP1γ1, not normally expressed in differentiating germ cells, could replace PP1γ2 to support spermatogenesis and spermiation. However, PP1γ2, which is the PP1 isoform in mammalian sperm, has an isoform-specific role in supporting normal sperm function and fertility.

Sperm Differentiation: The Role of Trafficking of Proteins

Sperm differentiation encompasses a complex sequence of morphological changes that takes place in the seminiferous epithelium. In this process, haploid round spermatids undergo substantial structural and functional alterations, resulting in highly polarized sperm. Hallmark changes during the differentiation process include the formation of new organelles, chromatin condensation and nuclear shaping, elimination of residual cytoplasm, and assembly of the sperm flagella. To achieve these transformations, spermatids have unique mechanisms for protein trafficking that operate in a coordinated fashion. Microtubules and filaments of actin are the main tracks used to facilitate the transport mechanisms, assisted by motor and non-motor proteins, for delivery of vesicular and non-vesicular cargos to specific sites. This review integrates recent findings regarding the role of protein trafficking in sperm differentiation. Although a complete characterization of the interactome of proteins involved in these temporal and spatial processes is not yet known, we propose a model based on the current literature as a framework for future investigations.

A framework for high-resolution phenotyping of candidate male infertility mutants: from human to mouse

Male infertility is a heterogeneous condition of largely unknown etiology that affects at least 7% of men worldwide. Classical genetic approaches and emerging next-generation sequencing studies support genetic variants as a frequent cause of male infertility. Meanwhile, the barriers to transmission of this disease mean that most individual genetic cases will be rare, but because of the large percentage of the genome required for spermatogenesis, the number of distinct causal mutations is potentially large. Identifying bona fide causes of male infertility thus requires advanced filtering techniques to select for high-probability candidates, including the ability to test causality in animal models. The mouse remains the gold standard for defining the genotype-phenotype connection in male fertility. Here, we present a best practice guide consisting of (a) major points to consider when interpreting next-generation sequencing data performed on infertile men, and, (b) a systematic strategy to categorize infertility types and how they relate to human male infertility. Phenotyping infertility in mice can involve investigating the function of multiple cell types across the testis and epididymis, as well as sperm function. These findings will feed into the diagnosis and treatment of male infertility as well as male health broadly.

Sperm capacitation is associated with phosphorylation of the testis-specific radial spoke protein Rsph6a†

Mammalian sperm undergo a series of biochemical and physiological changes collectively known as capacitation in order to acquire the ability to fertilize. Although the increase in phosphorylation associated with mouse sperm capacitation is well established, the identity of the proteins involved in this signaling cascade remains largely unknown. Tandem mass spectrometry (MS/MS) has been used to identify the exact sites of phosphorylation and to compare the relative extent of phosphorylation at these sites. In the present work, we find that a novel site of phosphorylation on a peptide derived from the radial spoke protein Rsph6a is more phosphorylated in capacitated mouse sperm. The Rsph6a gene has six exons, five of which are conserved during evolution in flagellated cells. The exon containing the capacitation-induced phosphorylation site was found exclusively in eutherian mammals. Transcript analyses revealed at least two different testis-specific splicing variants for Rsph6a.Rsph6a mRNA expression was restricted to spermatocytes. Using antibodies generated against the Rsph6a N-terminal domain, western blotting and immunofluorescence analyses indicated that the protein remains in mature sperm and localizes to the sperm flagellum. Consistent with its role in the axoneme, solubility analyses revealed that Rsph6 is attached to cytoskeletal structures. Based on previous studies in Chlamydomonas reinhardtii, we predict that Rsph6 participates in the interaction between the central pair of microtubules and the surrounding pairs. The findings that Rsph6a is more phosphorylated during capacitation and is predicted to function in axonemal localization make Rsph6a a candidate protein mediating signaling processes in the sperm flagellum.

HIPK4 is essential for murine spermiogenesis

Mammalian spermiogenesis is a remarkable cellular transformation, during which round spermatids elongate into chromatin-condensed spermatozoa. The signaling pathways that coordinate this process are not well understood, and we demonstrate here that homeodomain-interacting protein kinase 4 (HIPK4) is essential for spermiogenesis and male fertility in mice. HIPK4 is predominantly expressed in round and early elongating spermatids, and Hipk4 knockout males are sterile, exhibiting phenotypes consistent with oligoasthenoteratozoospermia. Hipk4 mutant sperm have reduced oocyte binding and are incompetent for in vitro fertilization, but they can still produce viable offspring via intracytoplasmic sperm injection. Optical and electron microscopy of HIPK4-null male germ cells reveals defects in the filamentous actin (F-actin)-scaffolded acroplaxome during spermatid elongation and abnormal head morphologies in mature spermatozoa. We further observe that HIPK4 overexpression induces branched F-actin structures in cultured fibroblasts and that HIPK4 deficiency alters the subcellular distribution of an F-actin capping protein in the testis, supporting a role for this kinase in cytoskeleton remodeling. Our findings establish HIPK4 as an essential regulator of sperm head shaping and potential target for male contraception.

Dissecting the signaling pathways involved in the function of sperm flagellum

The mammalian flagellum is a specific type of motile cilium required for sperm motility and male fertility. Effective flagellar movement is dependent on axonemal function, which in turn relies on proper ion homeostasis within the flagellar compartment. This ion homeostasis is maintained by the concerted function of ion channels and transporters that initiate signal transduction pathways resulting in motility changes. Advances in electrophysiology and super-resolution microscopy have helped to identify and characterize new regulatory modalities of the mammalian flagellum. Here, we discuss what is currently known about the regulation of flagellar ion channels and transporters that maintain sodium, potassium, calcium, and proton homeostasis. Identification of new regulatory elements and their specific roles in sperm motility is imperative for improving diagnostics of male infertility.

Assessments of sperm quality integrating morphology, swimming patterns, bioenergetics and cell signalling

Spermatozoa are diverse in form and function and these differences impact on their fertilizing capacity. Because of considerable inter-male and inter-species differences in sperm traits, assessments of sperm quality demand that we consider variations at different levels. We should thus pay attention not only to average values but also intra- and inter-sperm population variations and subpopulation structure. Sperm shape and size evolve in reponse to postcopulatory sexual selection. Assessments of morphological variation, with conventional microscopy or with computer-assisted systems, should bear this in mind. In rodents sperm head shape is asymmetric so it requires more complex tools, such as geometric morphometrics. Sperm function also evolves under postcopulatory sexual selection and this could be used as a basis to assess sperm performance. Sperm cells swim actively to overcome barriers in the female tract and develop a peculiar motility pattern in the final stages prior to and during fertilization. Both types of movement can be analyzed by computer-assisted microscopy systems. Sperm have high energetic demands for cell homeostasis, motility, and signalling. Bioenergetics can be analyzed by various means, including extracellular flux analyses to characterize glycolysis and mitochondrial respiration. Finally, cell signalling during capacitation has received much attention and can be assessed by microscopy (conventional or computer-assisted) or flow cytometry. Recent advances in image-flow cytometry affords analyses of high cell numbers with spatial localization of subcellular changes, which will have a big impact in the development of functional tests for the andrology clinic and in sperm preservation and use in artificial insemination.

Capacitation-associated alkalization in human sperm is differentially controlled at the subcellular level

Capacitation in mammalian sperm involves the accurate balance of intracellular pH (pHi), but the mechanisms controlling this process are not fully understood, particularly regarding the spatiotemporal regulation of the proteins involved in pHi modulation. Here, we employed an image-based flow cytometry technique combined with pharmacological approaches to study pHi dynamics at the subcellular level during capacitation. We found that, upon capacitation induction, sperm cells undergo intracellular alkalization in the head and principal piece regions. The observed localized pHi increases require the initial uptake of HCO3-, which is mediated by several proteins acting consistently with their subcellular localization. Hv1 proton channel (also known as HVCN1) and cAMP-activated protein kinase (protein kinase A, PKA) antagonists impair alkalization mainly in the principal piece. Na+/HCO3- cotransporter (NBC) and cystic fibrosis transmembrane regulator (CFTR) antagonists impair alkalization only mildly, predominantly in the head. Motility measurements indicate that inhibition of alkalization in the principal piece prevents the development of hyperactivated motility. Altogether, our findings shed light on the complex control mechanisms of pHi and underscore their importance during human sperm capacitation.This article has an associated First Person interview with the first author of the paper.

Signaling Enzymes Required for Sperm Maturation and Fertilization in Mammals

In mammals, motility and fertilizing ability of spermatozoa develop during their passage through the epididymis. After ejaculation, sperm undergo capacitation and hyperactivation in the female reproductive tract - a motility transition that is required for sperm penetration of the egg. Both epididymal initiation of sperm motility and hyperactivation are essential for male fertility. Motility initiation in the epididymis and sperm hyperactivation involve changes in metabolism, cAMP (cyclic adenosine mono-phosphate), calcium and pH acting through protein kinases and phosphatases. Despite this knowledge, we still do not understand, in biochemical terms, how sperm acquire motility in the epididymis and how motility is altered in the female reproductive tract. Recent data show that the sperm specific protein phosphatase PP1γ2, glycogen synthase kinase 3 (GSK3), and the calcium regulated phosphatase calcineurin (PP2B), are involved in epididymal sperm maturation. The protein phosphatase PP1γ2 is present only in testis and sperm in mammals. PP1γ2 has a isoform-specific requirement for normal function of mammalian sperm. Sperm PP1γ2 is regulated by three proteins - inhibitor 2, inhibitor 3 and SDS22. Changes in phosphorylation of these three inhibitors and their binding to PP1γ2 are involved in initiation and activation of sperm motility. The inhibitors are phosphorylated by protein kinases, one of which is GSK3. The isoform GSK3α is essential for epididymal sperm maturation and fertility. Calcium levels dramatically decrease during sperm maturation and initiation of motility suggesting that the calcium activated sperm phosphatase (PP2B) activity also decreases. Loss of PP2B results in male infertility due to impaired sperm maturation in the epididymis. Thus the three signaling enzymes PP1γ2, GSK3, and PP2B along with the documented PKA (protein kinase A) have key roles in sperm maturation and hyperactivation. Significantly, all these four signaling enzymes are present as specific isoforms only in placental mammals, a testimony to their essential roles in the unique aspects of sperm function in mammals. These findings should lead to a better biochemical understanding of the basis of male infertility and should lead to novel approaches to a male contraception and managed reproduction.

Post-translational modifications of seminal proteins and their importance in male fertility potential

Introduction: The seminal proteome has been shown to directly influence the male fertile potential. Post-translational modifications (PTMs) are significant changes that play a role in the biological regulation of proteins. Sperm cells are transcriptionally and translationally inactive and these modifications are essential to control protein function.Areas covered: Here we reviewed seven PTMs which importance for male reproductive function investigated in the past decade, namely S-nitrosylation and tyrosine nitration (both occurring by the action of NO), glycosylation, ubiquitination, acetylation, methylation, and SUMOylation. Since they were previously identified in human semen, we focus on their role in sperm function, as well as in physiological and pathophysiological processes which could contribute to the fertility potential. The following keywords were applied: 'post-translational modification', 'sperm', 'semen', 'seminal plasma', 'male infertility', 'nitrosylation', 'nitration', 'histone methylation', 'SUMOylation', 'ubiquitination', 'ubiquitilation', 'glycosylation', and 'acetylation'.Expert opinion: Most biological processes orchestrated by proteins require PTMs for their activation or inhibition. Most of them are dynamic and occur in mature sperm, modulating protein function, thus exerting a significant role in sperm function and fertility. Finally, the study of PTMs should be also addressed in pathophysiological processes, as different clinical conditions are known to alter the proteome.

Redox Regulation and Oxidative Stress: The Particular Case of the Stallion Spermatozoa

Redox regulation and oxidative stress have become areas of major interest in spermatology. Alteration of redox homeostasis is recognized as a significant cause of male factor infertility and is behind the damage that spermatozoa experience after freezing and thawing or conservation in a liquid state. While for a long time, oxidative stress was just considered an overproduction of reactive oxygen species, nowadays it is considered as a consequence of redox deregulation. Many essential aspects of spermatozoa functionality are redox regulated, with reversible oxidation of thiols in cysteine residues of key proteins acting as an “on–off” switch controlling sperm function. However, if deregulation occurs, these residues may experience irreversible oxidation and oxidative stress, leading to malfunction and ultimately death of the spermatozoa. Stallion spermatozoa are “professional producers” of reactive oxygen species due to their intense mitochondrial activity, and thus sophisticated systems to control redox homeostasis are also characteristic of the spermatozoa in the horse. As a result, and combined with the fact that embryos can easily be collected in this species, horses are a good model for the study of redox biology in the spermatozoa and its impact on the embryo.

Transient Sperm Starvation Improves the Outcome of Assisted Reproductive Technologies

To become fertile, mammalian sperm must undergo a series of biochemical and physiological changes known as capacitation. These changes involve crosstalk between metabolic and signaling pathways and can be recapitulated in vitro. In this work, sperm were incubated in the absence of exogenous nutrients (starved) until they were no longer able to move. Once immotile, energy substrates were added back to the media and sperm motility was rescued. Following rescue, a significantly higher percentage of starved sperm attained hyperactivated motility and displayed increased ability to fertilize in vitro when compared with sperm persistently incubated in standard capacitation media. Remarkably, the effects of this treatment continue beyond fertilization as starved and rescued sperm promoted higher rates of embryo development, and once transferred to pseudo-pregnant females, blastocysts derived from treated sperm produced significantly more pups. In addition, the starvation and rescue protocol increased fertilization and embryo development rates in sperm from a severely sub-fertile mouse model, and when combined with temporal increase in Ca²⁺ ion levels, this methodology significantly improved fertilization and embryo development rates in sperm of sterile CatSper1 KO mice model. Intracytoplasmic sperm injection (ICSI) does not work in the agriculturally relevant bovine system. Here, we show that transient nutrient starvation of bovine sperm significantly enhanced ICSI success in this species. These data reveal that the conditions under which sperm are treated impact post-fertilization development and suggest that this “starvation and rescue method” can be used to improve assisted reproductive technologies (ARTs) in other mammalian species, including humans.

Influence of the genetic background on the reproductive phenotype of mice lacking Cysteine-Rich Secretory Protein 1 (CRISP1)

Epididymal sperm protein CRISP1 has the ability to both regulate murine CatSper, a key sperm calcium channel, and interact with egg-binding sites during fertilization. In spite of its relevance for sperm function, Crisp1-/-mice are fertile. Considering that phenotypes can be influenced by the genetic background, in the present work mice from the original mixed Crisp1-/- colony (129/SvEv*C57BL/6) were backcrossed onto the C57BL/6 strain for subsequent analysis of their reproductive phenotype. Whereas fertility and fertilization rates of C57BL/6 Crisp1-/- males did not differ from those reported for mice from the mixed background, several sperm functional parameters were clearly affected by the genetic background. Crisp1-/- sperm from the homogeneous background exhibited defects in both the progesterone-induced acrosome reaction and motility not observed in the mixed background, and normal rather than reduced protein tyrosine phosphorylation. Additional studies revealed a significant decrease in sperm hyperactivation as well as in cAMP and protein kinase A (PKA) substrate phosphorylation levels in sperm from both colonies. The finding that exposure of mutant sperm to a cAMP analog and phosphodiesterase inhibitor overcame the sperm functional defects observed in each colony indicated that a common cAMP-PKA signaling defect led to different phenotypes depending on the genetic background. Altogether, our observations indicate that the phenotype of CRISP1 null males is modulated by the genetic context and reveal new roles for the protein in both the functional events and signaling pathways associated to capacitation.

The functional anatomy of the human spermatozoon: relating ultrastructure and function

The Internet, magazine articles, and even biomedical journal articles, are full of cartoons of spermatozoa that bear minimal resemblance to real spermatozoa, especially human spermatozoa, and this had led to many misconceptions about what spermatozoa look like and how they are constituted. This review summarizes the historical and current state of knowledge of mammalian sperm ultrastructure, with particular emphasis on and relevance to human spermatozoa, combining information obtained from a variety of electron microscopic (EM) techniques. Available information on the composition and configuration of the various ultrastructural components of the spermatozoon has been related to their mechanistic purpose and roles in the primary aspects of sperm function and fertilization: motility, hyperactivation, capacitation, the acrosome reaction and sperm-oocyte fusion.

Cyclic Nucleotide-Specific Optogenetics Highlights Compartmentalization of the Sperm Flagellum into cAMP Microdomains

Inside the female genital tract, mammalian sperm undergo a maturation process called capacitation, which primes the sperm to navigate across the oviduct and fertilize the egg. Sperm capacitation and motility are controlled by 3′,5′-cyclic adenosine monophosphate (cAMP). Here, we show that optogenetics, the control of cellular signaling by genetically encoded light-activated proteins, allows to manipulate cAMP dynamics in sperm flagella and, thereby, sperm capacitation and motility by light. To this end, we used sperm that express the light-activated phosphodiesterase LAPD or the photo-activated adenylate cyclase bPAC. The control of cAMP by LAPD or bPAC combined with pharmacological interventions provides spatiotemporal precision and allows to probe the physiological function of cAMP compartmentalization in mammalian sperm.

Impairment of sperm maturation and capacitation due to diet-dependent cholesterol overload

BACKGROUND

Lipid metabolic disorders (dyslipidemia) are constantly increasing in occidental societies and lead to the development of pathologies such as obesity, diabetes, and metabolic syndrome. It has been demonstrated that dyslipidemia can alter the reproductive function. Animal models have recently been used to show that the offspring of dyslipidemic males could also develop such pathologies and that the transgenerational transmission involved post-testicular sperm maturation. These data targeted the essential role of male gamete epididymal maturation and its importance for the health of the offspring.

OBJECTIVES

This publication summarizes in the first place experimental data obtained using a mouse model of dyslipidemia-induced post-testicular infertility, knockout mice for the two isoforms of the 'Liver X Receptors' (Lxrα;β^-/- ), the major regulators of cholesterol homeostasis. The impact of a high cholesterol diet (HCD) on the protein YWHAZ (14-3-3 ζ or tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein Zeta) was also investigated in our model.

MATERIALS AND METHODS

In our mouse model, when young fertile Lxrα;β^-/- males aged three months were fed four weeks with a HCD, they developed an epididymal phenotype leading to infertility. The level of sperm YWHAZ was evaluated by Western blot and its tyrosine phosphorylation state by immunoprecipitation followed by Western blot.

RESULTS

Our data revealed that sperm lipid composition and structure were altered, leading to defects of the capacitation-associated signaling pathway. They also showed that both the level and the tyrosine phosphorylation state of YWHAZ were affected by the HCD in sperm cells from Lxrα;β^-/- males.

DISCUSSION AND CONCLUSION

YWHAZ could be a new important regulator of capacitation-associated tyrosine phosphorylation and a marker of dyslipidemia-induced infertility.

Novel methods to detect capacitation-related changes in spermatozoa

Prior to interaction with the oocyte, spermatozoa must undergo capacitation, which involves a series of physio-chemical transformations that occur in the female tract. As capacitation is a pre-requisite for successful fertilisation, it is a topic of great interest for sperm biologists, but the complexity of the numerous biochemical and biophysical processes involved make it difficult to measure. Capacitation is an extremely complex event that encompasses numerous integrated processes that can occur concurrently during this window of time. The identification of techniques to accurately assess and quantify capacitation is therefore crucial to gain a meaningful insight into this fascinating sperm maturation event. Whilst there are extensive reviews in the literature that focus on the functional changes to spermatozoa during capacitation, few have examined the methods required to measure these changes. The aim of this review is to highlight frequently used methods to quantify different stages of capacitation and identify promising novel techniques. Factors that are able to modulate various capacitation processes will also be discussed. The overall outcome is to provide researchers with a toolbox of methods that can be used to gain a deeper understanding of the intricacies of capacitation in spermatozoa.

Boar semen improvement through sperm capacitation management, with emphasis on zinc ion homeostasis

Critical to fertilization success, sperm capacitation within the female oviductal sperm reservoir endows mammalian spermatozoa with hyperactivated motility and capacity to fertilize. An elaborate cascade of signaling events during capacitation guides the redistribution of sperm plasma membrane seminolipid and cholesterol, Ca-influx and increases tyrosine phosphorylation to promote hyperactivated motility. Such events result in the remodeling of the sperm acrosome, increased fluidity and fusability of the plasma membrane, shedding of surface-adsorbed seminal plasma proteins that glue sperm heads to the oviductal epithelium and ultimately the release of hyperactivated spermatozoa from the oviductal sperm reservoir. Discovered recently, the capacitation-induced sperm zinc ion efflux and resultant zinc signatures are reflective of sperm capacitation status and fertilizing ability, inspiring the retrospection of zinc ion functions in the physiology and fertility of boar sperm and that of other species. This review also highlights the merit of the domestic boar as a biomedical model for spermatology and fertilization research. Relevant to the quest for better fertility management in the livestock industries, the benefits of zinc ion supplementation through nutrition and direct addition to extended semen are discussed in the context of artificial insemination (AI). Ideas are shared on future technologies for zinc management in AI doses and research on the sperm zinc-interacting proteome.

Signaling pathways involved in human sperm hyperactivated motility stimulated by Zn2

To fertilize the egg, sperm cells must reside in the female reproductive tract for several hours during which they undergo chemical and motility changes collectively called capacitation. During capacitation, the sperm develop a unique type of motility known as hyperactivated motility (HAM). The semen contains Zn²⁺ in millimolar concentrations, whereas in the female reproductive tract the concentration is around 1 µM. In this study, we characterize the role of Zn ²⁺ in human sperm capacitation focusing on its effect on HAM. Western blot analysis revealed the presence of G protein-coupled receptor 39 (GPR39) type Zn-receptor localized mainly in the sperm tail. Zn ²⁺ at micromolar concentration stimulates HAM, which is mediated by a cascade involving GPR39-AC-cAMP-PKA-Src-EGFR and phospholipase C. Both the transmembrane adenylyl cyclase (AC) and the soluble-AC are involved in the stimulation of HAM by Zn ²⁺ . The development of HAM is precisely regulated by cyclic adenosine monophosphate, in which relatively low concentration (5-10 µM) stimulated HAM, whereas at 30 µM no stimulation occurred. A similar response was seen when different concentrations of Zn ²⁺ were added to the cells; low Zn ²⁺ stimulated HAM, whereas at relatively high Zn ²⁺ , no effect was seen. We further demonstrate that the Ca ²⁺ -channel CatSper involved in Zn ²⁺ -stimulated HAM. These data support a role for extracellular Zn ²⁺ acting via GPR39 to regulate signaling pathways in sperm capacitation, leading to HAM induction.

Oviductal extracellular vesicles (oviductosomes, OVS) are conserved in humans: murine OVS play a pivotal role in sperm capacitation and fertility

STUDY QUESTIONS

Are extracellular vesicles (EVs) in the murine oviduct (oviductosomes, OVS) conserved in humans and do they play a role in the fertility of Pmca4-/- females?

SUMMARY ANSWER

OVS and their fertility-modulating proteins are conserved in humans, arise via the apocrine pathway, and mediate a compensatory upregulation of PMCA1 (plasma membrane Ca2+-ATPase 1) in Pmca4-/- female mice during proestrus/estrus, to account for their fertility.

WHAT IS KNOWN ALREADY

Recently murine OVS were identified and shown during proestrus/estrus to express elevated levels of PMCA4 which they can deliver to sperm. PMCA4 is the major Ca2+ efflux pump in murine sperm and Pmca4 deletion leads to loss of sperm motility and male infertility as there is no compensatory upregulation of the remaining Ca2+ pump, PMCA1. Of the four family members of PMCAs (PMCA1-4), PMCA1 and PMCA4 are ubiquitous, and to date there have been no reports of one isoform being upregulated to compensate for another in any organ/tissue. Since Pmca4-/- females are fertile, despite the abundant expression of PMCA4 in wild-type (WT) OVS, we propose that OVS serve a role of packaging and delivering to sperm elevated levels of PMCA1 in Pmca4-/- during proestrus/estrus to compensate for PMCA4's absence.

STUDY DESIGN, SIZE, DURATION

Fallopian tubes from pre-menopausal women undergoing hysterectomy were used to study EVs in the luminal fluid. Oviducts from sexually mature WT mice were sectioned after perfusion fixation to detect EVs in situ. Oviducts were recovered from WT and Pmca4-/- after hormonally induced estrus and sectioned for PMCA1 immunofluorescence (IF) (detected with confocal microscopy) and hematoxylin and eosin staining. Reproductive tissues, luminal fluids and EVs were recovered after induced estrus and after natural cycling for western blot analysis of PMCA1 and qRT-PCR of Pmca1 to compare expression levels in WT and Pmca4-/-. OVS, uterosomes, and epididymal luminal fluid were included in the comparisons. WT and Pmca4-/- OVS were analyzed for the presence of known PMCA4 partners in sperm and their ability to interact with PMCA1, via co-immunoprecipitation. In vitro uptake of PMCA1 from OVS was analyzed in capacitated and uncapacitated sperm via quantitative western blot analysis, IF localization and flow cytometry. Caudal sperm were also assayed for uptake of tyrosine-phosphorylated proteins which were shown to be present in OVS. Finally, PMCA1 and PMCA4 in OVS and that delivered to sperm were assayed for enzymatic activity.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human fallopian tubes were flushed to recover luminal fluid which was processed for OVS via ultracentrifugation. Human OVS were negatively stained for transmission electron microscopy (TEM) and subjected to immunogold labeling, to detect PMCA4. Western analysis was used to detect HSC70 (an EV biomarker), PMCA1 and endothelial nitric oxide synthase (eNOS) which is a fertility-modulating protein delivered to human sperm by prostasomes. Oviducts of sexually mature female mice were sectioned after perfusion fixation for TEM tomography to obtain 3D information and to distinguish cross-sections of EVs from those of microvilli and cilia. Murine tissues, luminal fluids and EVs were assayed for PMCA1 (IF and western blot) or qRT-PCR. PMCA1 levels from western blots were quantified, using band densities and compared in WT and Pmca4-/- after induced estrus and in proestrus/estrus and metestrus/diestrus in cycling females. In vitro uptake of PMCA1 and tyrosine-phosphorylated proteins was quantified with flow cytometry and/or quantitative western blot. Ca2+-ATPase activity in OVS and sperm before and after PMCA1 and PMCA4 uptake was assayed, via the enzymatic hydrolysis rate of ATP.

MAIN RESULTS AND THE ROLE OF CHANCE

TEM revealed that human oviducts contain EVs (exosomal and microvesicular). These EVs contain PMCA4 (immunolabeling), eNOS and PMCA1 (western blot) in their cargo. TEM tomography showed the murine oviduct with EV-containing blebs which typify the apocrine pathway for EV biogenesis. Western blots revealed that during proestrus/estrus PMCA1 was significantly elevated in the oviductal luminal fluid (OLF) (P = 0.02) and in OVS (P = 0.03) of Pmca4-/-, compared to WT. Further, while PMCA1 levels did not fluctuate in OLF during the cycle in WT, they were significantly (P = 0.02) higher in proestrus/estrus than at metestrus/diestrus in Pmca4-/-. The elevated levels of PMCA1 in proestrus/estrus, which mimics PMCA4 in WT, is OLF/OVS-specific, and is not seen in oviductal tissues, uterosomes or epididymal luminal fluid of Pmca4-/-. However, qRT-PCR revealed significantly elevated levels of Pmca1 transcript in Pmca4-/- oviductal tissues, compared to WT. PMCA1 could be transferred from OVS to sperm and the levels were significantly higher for capacitated vs uncapacitated sperm, as assessed by flow cytometry (P = 0.001) after 3 h co-incubation, quantitative western blot (P < 0.05) and the frequency of immuno-labeled sperm (P < 0.001) after 30 min co-incubation. Tyrosine phosphorylated proteins were discovered in murine OVS and could be delivered to sperm after their co-incubation with OVS, as detected by western, immunofluorescence localization, and flow cytometry. PMCA1 and PMCA4 in OVS were shown to be enzymatically active and this activity increased in sperm after OVS interaction.

LARGE SCALE DATA

None.

LIMITATIONS REASONS FOR CAUTION

Although oviductal tissues of WT and Pmca4-/- showed no significant difference in PMCA1 levels, Pmca4-/- levels of OVS/OLF during proestrus/estrus were significantly higher than in WT. We have attributed this enrichment or upregulation of PMCA1 in Pmca4-/- partly to selective packaging in OVS to compensate for the lack of PMCA4. However, in the absence of a difference between WT and Pmca4-/- in the PMCA1 levels in oviductal tissues as a whole, we cannot rule out significantly higher PMCA1 expression in the oviductal epithelium that gives rise to the OVS as significantly higher Pmca1 transcripts were detected in Pmca4-/-.

WIDER IMPLICATIONS OF THE FINDINGS

Since OVS and fertility-modulating cargo components are conserved in humans, it suggests that murine OVS role in regulating the expression of proteins required for capacitation and fertility is also conserved. Secondly, OVS may explain some of the differences in in vivo and in vitro fertilization for mouse mutants, as seen in mice lacking the gene for FER which is the enzyme required for sperm protein tyrosine phosphorylation. Our observation that murine OVS carry and can modulate sperm protein tyrosine phosphorylation by delivering them to sperm provides an explanation for the in vivo fertility of Fer mutants, not seen in vitro. Finally, our findings have implications for infertility treatment and exosome therapeutics.

STUDY FUNDING AND COMPETING INTEREST(S)

The work was supported by National Institute of Health (RO3HD073523 and 5P20RR015588) grants to P.A.M.-D. There are no conflicts of interests.

Epididymal cysteine-rich secretory proteins are required for epididymal sperm maturation and optimal sperm function

STUDY QUESTION

What is the role of epididymal cysteine-rich secretory proteins (CRISPs) in male fertility?

SUMMARY ANSWER

While epididymal CRISPs are not absolutely required for male fertility, they are required for optimal sperm function.

WHAT IS KNOWN ALREADY

CRISPs are members of the CRISP, Antigen 5 and Pathogenesis related protein 1 (CAP) superfamily and are characterized by the presence of an N-terminal CAP domain and a C-terminal CRISP domain. CRISPs are highly enriched in the male reproductive tract of mammals, including in the epididymis. Within humans there is one epididymal CRISP, CRISP1, whereas in mice there are two, CRISP1 and CRISP4.

STUDY DESIGN, SIZE, DURATION

In order to define the role of CRISPs within the epididymis, Crisp1 and Crisp4 knockout mouse lines were produced then interbred to produce Crisp1 and 4 double knockout (DKO) mice, wherein the expression of all epididymal CRISPs was ablated. Individual and DKO models were then assessed, relative to their own strain-specific wild type littermates for fertility, and sperm output and functional competence at young (10-12 weeks of age) and older ages (22-24 weeks). Crisp1 and 4 DKO and control mice were also compared for their ability to bind to the zona pellucida and achieve fertilization.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Knockout mouse production was achieved using modified embryonic stem cells and standard methods. The knockout of individual genes was confirmed at a mRNA (quantitative PCR) and protein (immunochemistry) level. Fertility was assessed using breeding experiments and a histological assessment of testes and epididymal tissue. Sperm functional competence was assessed using a computer assisted sperm analyser, induction of the acrosome reaction using progesterone followed by staining for acrosome contents, using immunochemical and western blotting to assess the ability of sperm to manifest tyrosine phosphorylation under capacitating conditions and using sperm-zona pellucida binding assays and IVF methods. A minimum of three biological replicates were used per assay and per genotype.

MAIN RESULTS AND THE ROLE OF CHANCE

While epididymal CRISPs are not absolutely required for male fertility, their production results in enhanced sperm function and, depending on context, CRISP1 and CRISP4 act redundantly or autonomously. Specifically, CRISP1 is the most important CRISP in the establishment of normally motile sperm, whereas CRISP4 acts to enhance capacitation-associated tyrosine phosphorylation, and CRISP1 and CRISP4 act together to establish normal acrosome function. Both are required to achieve optimal sperm-egg interaction. The presence of immune infiltrates into the epididymis of older, but not younger, DKO animals also suggests epididymal CRISPs function to produce an immune privileged environment for maturing sperm within the epididymis.

LIMITATIONS REASONS FOR CAUTION

Caution should be displayed in the translation of mouse-derived data into the human wherein the histology of the epididymis is someone what different. The mice used in the study were housed in a specific pathogen-free environment and were thus not exposed to the full range of environmental challenges experienced by wild mice or humans. As such, the role of CRISPs in the maintenance of an immune privileged environment, for example, may be understated.

WIDER IMPLICATIONS OF THE FINDINGS

The combined deletion of Crisp1 and Crisp4 in mice is equivalent to the removal of all CRISP expression in humans. As such, these data suggest that mammalian CRISPs, including that in humans, function to enhance sperm function and thus male fertility. These data also suggest that in the presence of an environmental challenge, CRISPs help to maintain an immune privileged environment and thus, protect against immune-mediated male infertility.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTEREST(S)

This study was funded by the National Health and Medical Research Council, the Victorian Cancer Agency and a scholarship from the Chinese Scholarship Council. The authors have no conflicts of interest to declare.

Semi-automatized segmentation method using image-based flow cytometry to study sperm physiology: the case of capacitation-induced tyrosine phosphorylation

STUDY QUESTION

Is image-based flow cytometry a useful tool to study intracellular events in human sperm such as protein tyrosine phosphorylation or signaling processes?

SUMMARY ANSWER

Image-based flow cytometry is a powerful tool to study intracellular events in a relevant number of sperm cells, which enables a robust statistical analysis providing spatial resolution in terms of the specific subcellular localization of the labeling.

WHAT IS KNOWN ALREADY

Sperm capacitation is required for fertilization. During this process, spermatozoa undergo numerous physiological changes, via activation of different signaling pathways, which are not completely understood. Classical approaches for studying sperm physiology include conventional microscopy, flow cytometry and Western blotting. These techniques present disadvantages for obtaining detailed subcellular information of signaling pathways in a relevant number of cells. This work describes a new semi-automatized analysis using image-based flow cytometry which enables the study, at the subcellular and population levels, of different sperm parameters associated with signaling. The increase in protein tyrosine phosphorylation during capacitation is presented as an example.

STUDY DESIGN SIZE, DURATION

Sperm cells were isolated from seminal plasma by the swim-up technique. We evaluated the intensity and distribution of protein tyrosine phosphorylation in sperm incubated in non-capacitation and capacitation-supporting media for 1 and 18 h under different experimental conditions. We used an antibody against FER kinase and pharmacological inhibitors in an attempt to identify the kinases involved in protein tyrosine phosphorylation during human sperm capacitation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Semen samples from normospermic donors were obtained by masturbation after 2-3 days of sexual abstinence. We used the innovative technique image-based flow cytometry and image analysis tools to segment individual images of spermatozoa. We evaluated and quantified the regions of sperm where protein tyrosine phosphorylation takes place at the subcellular level in a large number of cells. We also used immunocytochemistry and Western blot analysis. Independent experiments were performed with semen samples from seven different donors.

MAIN RESULTS AND THE ROLE OF CHANCE

Using image analysis tools, we developed a completely novel semi-automatic strategy useful for segmenting thousands of individual cell images obtained using image-based flow cytometry. Contrary to immunofluorescence which relies on the analysis of a limited sperm population and also on the observer, image-based flow cytometry allows for unbiased quantification and simultaneous localization of post-translational changes in an extended sperm population. Interestingly, important data can be independently analyzed by looking to the frame of interest. As an example, we evaluated the capacitation-associated increase in tyrosine phosphorylation in sperm incubated in non-capacitation and capacitation-supporting media for 1 and 18 h. As previously reported, protein tyrosine phosphorylation increases in a time-depending manner, but our method revealed that this increase occurs differentially among distinct sperm segments. FER kinase is reported to be the enzyme responsible for the increase in protein tyrosine phosphorylation in mouse sperm. Our Western blot analysis revealed for the first time the presence of this enzyme in human sperm. Using our segmentation strategy, we aimed to quantify the effect of pharmacological inhibition of FER kinase and found a marked reduction of protein tyrosine phosphorylation only in the flagellum, which corresponded to the physical localization of FER in human sperm. Our method provides an alternative strategy to study signaling markers associated with capacitation, such as protein tyrosine phosphorylation, in a fast and quantitative manner.

LARGE SCALE DATA

None.

LIMITATIONS REASONS FOR CAUTION

This is an in vitro study performed under controlled conditions. Chemical inhibitors are not completely specific for the intended target; the possibility of side effects cannot be discarded.

WIDER IMPLICATIONS OF THE FINDINGS

Our results demonstrate that the use of image-based flow cytometry is a very powerful tool to study sperm physiology. A large number of cells can be easily analyzed and information at the subcellular level can be obtained. As the segmentation process works with bright-field images, it can be extended to study expression of other proteins of interest using different antibodies or it can be used in living sperm to study intracellular parameters that can be followed using fluorescent dyes sensitive to the parameter of interest (e.g. pH, Ca2+). Therefore, this a versatile method that can be exploited to study several aspects of sperm physiology.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported DGAPA (IN203116 to C. Treviño), Fronteras-CONACyT No. 71 and Eunice Kennedy Shriver National Institute of Child Health and Human Development NIH (RO1 HD38082) to P.E. Visconti and by a Lalor Foundation fellowship to M.G. Gervasi. A. Matamoros is a student of the Maestría en Ciencias Bioquímicas-UNAM program supported by CONACyT (416400) and DGAPA-UNAM. A. Moreno obtained a scholarship from Red MacroUniversidades and L. Giojalas obtained a schloarhip from CONICET and Universidad Nacional de Cordoba. The authors declare there are not conflicts of interest.

New Insight into Sperm Capacitation: A Novel Mechanism of 17β-Estradiol Signalling

17β-estradiol (estradiol) is a natural estrogen regulating reproduction including sperm and egg development, sperm maturation—called capacitation—and sperm–egg communication. High doses can increase germ cell apoptosis and decrease sperm count. Our aim was to answer the biological relevance of estradiol in sperm capacitation and its effect on motility and acrosome reaction to quantify its interaction with estrogen receptors and propose a model of estradiol action during capacitation using kinetic analysis. Estradiol increased protein tyrosine phosphorylation, elevated rate of spontaneous acrosome reaction, and altered motility parameters measured Hamilton-Thorne Computer Assisted Semen Analyzer (CASA) in capacitating sperm. To monitor time and concentration dependent binding dynamics of extracellular estradiol, high-performance liquid chromatography with tandem mass spectrometry was used to measure sperm response and data was subjected to kinetic analysis. The kinetic model of estradiol action during sperm maturation shows that estradiol adsorption onto a plasma membrane surface is controlled by Langmuir isotherm. After, when estradiol passes into the cytoplasm, it forms an unstable adduct with cytoplasmic receptors, which display a signalling autocatalytic pattern. This autocatalytic reaction suggests crosstalk between receptor and non-receptor pathways utilized by sperm prior to fertilization.

Tyrosine phosphorylation signaling regulates Ca2+ entry by affecting intracellular pH during human sperm capacitation

Capacitation is a mandatory process for the acquisition of mammalian sperm fertilization competence and involves the activation of a complex and still not fully understood system of signaling pathways. Under in vitro conditions, there is an increase in both protein tyrosine phosphorylation (pTyr) and intracellular Ca²⁺ levels in several species. In human sperm, results from our group revealed that pTyr signaling can be blocked by inhibiting proline-rich tyrosine kinase 2 (PYK2). Based on the role of PYK2 in other cell types, we investigated whether the PYK2-dependent pTyr cascade serves as a sensor for Ca ²⁺ signaling during human sperm capacitation. Flow cytometry studies showed that exposure of sperm to the PYK2 inhibitor N-[2-[[[2-[(2,3-dihydro-2-oxo-1 H-indol-5-yl)amino]-5-(trifluoromethyl)-4-pyrimidinyl]amino]methyl]phenyl]- N-methyl-methanesulfonamide hydrate (PF431396) produced a significant and concentration-dependent reduction in intracellular Ca ²⁺ levels during capacitation. Further studies revealed that PF431396-treated sperm exhibited a decrease in the activity of CatSper, a key sperm Ca ²⁺ channel. In addition, time course studies during capacitation in the presence of PF431396 showed a significant and sustained decrease in both intracellular Ca ²⁺ and pH levels after 2 hr of incubation, temporarily coincident with the activation of PYK2 during capacitation. Interestingly, decreases in Ca ²⁺ levels and progressive motility caused by PF431396 were reverted by inducing intracellular alkalinization with NH ₄ Cl, without affecting the pTyr blockage. Altogether, these observations support pTyr as an intracellular sensor for Ca ²⁺ entry in human sperm through regulation of cytoplasmic pH. These results contribute to a better understanding of the modulation of the polymodal CatSper and signaling pathways involved in human sperm capacitation.

Lysine acetylation modulates mouse sperm capacitation

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. To gain fertilization competence, they need to undergo a series of modifications inside the female reproductive tract, known as capacitation. Capacitation involves several molecular events such as phosphorylation cascades, hyperpolarization of the plasma membrane and intracellular Ca²⁺ changes, which prepare the sperm to develop two essential features for fertilization competence: hyperactivation and acrosome reaction. Since sperm cells lack new protein biosynthesis, post-translational modification of existing proteins plays a crucial role to obtain full functionality. Here, we show the presence of acetylated proteins in murine sperm, which increase during capacitation. Pharmacological hyperacetylation of lysine residues in non-capacitated sperm induces activation of PKA, hyperpolarization of the sperm plasma membrane, CatSper opening and Ca²⁺ influx, all capacitation-associated molecular events. Furthermore, hyperacetylation of non-capacitated sperm promotes hyperactivation and prepares the sperm to undergo acrosome reaction. Together, these results indicate that acetylation could be involved in the acquisition of fertilization competence of mammalian sperm.

Molecular Basis of Human Sperm Capacitation

In the early 1950s, Austin and Chang independently described the changes that are required for the sperm to fertilize oocytes in vivo. These changes were originally grouped under name of “capacitation” and were the first step in the development of in vitro fertilization (IVF) in humans. Following these initial and fundamental findings, a remarkable number of observations led to characterization of the molecular steps behind this process. The discovery of certain sperm-specific molecules and the possibility to record ion currents through patch-clamp approaches helped to integrate the initial biochemical observation with the activity of ion channels. This is of particular importance in the male gamete due to the fact that sperm are transcriptionally inactive. Therefore, sperm must control all these changes that occur during their transit through the male and female reproductive tracts by complex signaling cascades that include post-translational modifications. This review is focused on the principal molecular mechanisms that govern human sperm capacitation with particular emphasis on comparing all the reported pieces of evidence with the mouse model.