The "switching on" of mammalian spermatozoa: molecular events involved in promotion and regulation of capacitation

Spermatozoa recruit prostasomes in response to capacitation induction

Seminal plasma contains various types of extracellular vesicles, including 'prostasomes'. Prostasomes are small vesicles secreted by prostatic epithelial cells that can be recruited by and fuse with sperm cells in response of progesterone that is released by oocyte surrounding cumulus cells. This delivers Ca(2+) signaling tools that allow the sperm cell to gain hypermotility and undergo the acrosome reaction. Conditions for binding of prostasomes to sperm cells are however unclear. We found that classically used prostasome markers are in fact heterogeneously expressed on distinct populations of small and large vesicles in seminal plasma. To study interactions between prostasomes and spermatozoa we used the stallion as a model organism. A homogeneous population of ~60nm prostasomes was first separated from larger vesicles and labeled with biotin. Binding of biotinylated prostasomes to individual live spermatozoa was then monitored by flow cytometry. Contrary to assumptions in the literature, we found that such highly purified prostasomes bound to live sperm only after capacitation had been initiated, and specifically at pH ≥7.5. Using fluorescence microscopy, we observed that prostasomes bound primarily to the head of live sperm. We propose that in vivo, prostasomes may bind to sperm cells in the uterus, to be carried in association with sperm cells into oviduct and to fuse with the sperm cell only during the final approach of the oocyte. This article is part of a Special Issue entitled: An Updated Secretome.

Thermotaxis of human sperm cells in extraordinarily shallow temperature gradients over a wide range

On the basis of the finding that capacitated (ready to fertilize) rabbit and human spermatozoa swim towards warmer temperatures by directing their movement along a temperature gradient, sperm thermotaxis has been proposed to be one of the processes guiding these spermatozoa to the fertilization site. Although the molecular mechanism underlying sperm thermotaxis is gradually being revealed, basic questions related to this process are still open. Here, employing human spermatozoa, we addressed the questions of how wide the temperature range of thermotaxis is, whether this range includes an optimal temperature or whether spermatozoa generally prefer swimming towards warmer temperatures, whether or not they can sense and respond to descending temperature gradients, and what the minimal temperature gradient is to which they can thermotactically respond. We found that human spermatozoa can respond thermotactically within a wide temperature range (at least 29–41°C), that within this range they preferentially accumulate in warmer temperatures rather than at a single specific, preferred temperature, that they can respond to both ascending and descending temperature gradients, and that they can sense and thermotactically respond to temperature gradients as low as <0.014°C/mm. This temperature gradient is astonishingly low because it means that as a spermatozoon swims through its entire body length (46 µm) it can sense and respond to a temperature difference of <0.0006°C. The significance of this surprisingly high temperature sensitivity is discussed.

The physiology of bicarbonate transporters in mammalian reproduction

HCO(3)(-) plays critically important roles during virtually the entire process of reproduction in mammals, including spermatogenesis, sperm capacitation, fertilization, and development of early stage embryos. Therefore, the acid-base balance in the male and female reproductive tracts must be finely modulated. The fluid milieu in the epididymis is acidic, containing very low concentration of HCO(3)(-). In this acidic low HCO(3)(-) environment, mature sperm are rendered quiescent in the epididymis. In contrast, the luminal fluid in the female uterus and oviduct is alkaline, with very high concentration of HCO(3)(-) that is essential for sperm to fulfill fertilization. HCO(3)(-) transporter of solute carrier 4 (SLC4) and SLC26 families represent the major carriers for HCO(3)(-) transport across the plasma membrane. These transporters play critical roles in intracellular pH regulation and transepithelial HCO(3)(-) transport. The physiological roles of these transporters in mammalian reproduction are of fundamental interest to investigators. Here we review recent progress in understanding the expression of HCO(3)(-) transporters in reproductive tract tissues as well as the physiological roles of these transporters in mammalian reproduction.

Nematode sperm maturation triggered by protease involves sperm-secreted serine protease inhibitor (Serpin)

Spermiogenesis is a series of poorly understood morphological, physiological and biochemical processes that occur during the transition of immotile spermatids into motile, fertilization-competent spermatozoa. Here, we identified a Serpin (serine protease inhibitor) family protein (As_SRP-1) that is secreted from spermatids during nematode Ascaris suum spermiogenesis (also called sperm activation) and we showed that As_SRP-1 has two major functions. First, As_SRP-1 functions in cis to support major sperm protein (MSP)-based cytoskeletal assembly in the spermatid that releases it, thereby facilitating sperm motility acquisition. Second, As_SRP-1 released from an activated sperm inhibits, in trans, the activation of surrounding spermatids by inhibiting vas deferens-derived As_TRY-5, a trypsin-like serine protease necessary for sperm activation. Because vesicular exocytosis is necessary to create fertilization-competent sperm in many animal species, components released during this process might be more important modulators of the physiology and behavior of surrounding sperm than was previously appreciated.

CECR2 is involved in spermatogenesis and forms a complex with SNF2H in the testis

The regulation of nucleosome positioning and composition by ATP-dependent chromatin remodeling enzymes and their associated binding partners plays important biological roles in mammals. CECR2 is a binding partner to the ISWI (imitation switch) ATPase SNF2L/SMARCA1 and is involved in neural tube closure and inner ear development; however, its functions in adult tissues have not been examined. Here, we report that CECR2 contributes to spermatogenesis and forms a complex that includes the other ISWI ATPase SNF2H/SMARCA5 in the testis. Cecr2 mutant males non-penetrant for neural tube defects sired smaller litters than wild-type males. Strikingly, while we found that Cecr2 mutants have normal seminiferous epithelium morphology, sperm count, motility, and morphology, the mutant spermatozoa were compromised in their ability to fertilize oocytes. Investigation of CECR2/ISWI complexes in the testis showed that SNF2H interacted with CECR2, and this interaction was also observed in embryonic stem cells, suggesting that CECR2 may interact with SNF2H or SNF2L depending on the cell type. Finally, we found that Cecr2 mutants exhibit misregulation of the homeobox transcription factor Dlx5 in the testis, suggesting that CECR2 complexes may regulate gene expression during spermatogenesis. Taken together, our results demonstrate a novel role of CECR2-containing complexes in spermatogenesis and show that CECR2 interacts predominantly with SNF2H instead of SNF2L in the testis.

The control of male fertility by spermatozoan ion channels

Ion channels control the sperm ability to fertilize the egg by regulating sperm maturation in the female reproductive tract and by triggering key sperm physiological responses required for successful fertilization such as hyperactivated motility, chemotaxis, and the acrosome reaction. CatSper, a pH-regulated, calcium-selective ion channel, and KSper (Slo3) are core regulators of sperm tail calcium entry and sperm hyperactivated motility. Many other channels had been proposed as regulating sperm activity without direct measurements. With the development of the sperm patch-clamp technique, CatSper and KSper have been confirmed as the primary spermatozoan ion channels. In addition, the voltage-gated proton channel Hv1 has been identified in human sperm tail, and the P2X2 ion channel has been identified in the midpiece of mouse sperm. Mutations and deletions in sperm-specific ion channels affect male fertility in both mice and humans without affecting other physiological functions. The uniqueness of sperm ion channels makes them ideal pharmaceutical targets for contraception. In this review we discuss how ion channels regulate sperm physiology.

Nitric oxide involvement in the acrosome reaction triggered by leptin in pig sperm

BACKGROUND

Nitric oxide (NO) is a signaling molecule produced by intracellular nitric oxide synthase (NOS) enzymes. This free radical appears to affect sperm capacitation, a maturation step preceding acrosome reaction. Recent studies have reported leptin ability to promote capacitation and acrosome reaction in pig male gametes.

METHODS

This study has investigated nitric oxide production in leptin-treated pig spermatozoa by fluorescence-activated cell sorting, while the intracellular NOS isoforms were assessed by Western blot analysis. In addition, acrosome status of treated-spermatozoa was evaluated by FITC-PNA staining.

RESULTS

Significant increases of nitric oxide levels and acrosome reaction extent were detected in leptin-treated spermatozoa, but both the effects were reversed in presence of L-NAME. Furthermore, the immunoblots of sperm extracts have evidenced three bands of ~160 Kd(bNOS), ~130 Kd (iNOS) and ~135 Kd (eNOS).

CONCLUSIONS

The identification of the three intracellular NOS isoforms suggests that pig spermatozoa could produce NO, while the augmented nitric oxide levels in leptin-treated male gametes indicates the capacity of the hormone to induce nitric oxide production. Furthermore, the inhibitory effect of L-NAME and of Ab-ObR on the promotion of acrosome reaction triggered by leptin suggests a possible involvement of NO in the hormone action.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Mediators of the Jak/STAT signaling pathway in human spermatozoa

In their journey to acquire the ability to fertilize the egg, numerous intracellular signaling systems are activated in spermatozoa, leading to an increase in protein tyrosine phosphorylation. Although the JAK/STAT signaling pathway is usually associated with the activation of transcription of specific genes, our laboratory previously demonstrated the presence of the IL6 receptor (IL6R) and the Janus kinase 1 (JAK1) in human spermatozoa, a cell that is mostly transcriptionally inactive. In order to determine the importance of the JAK/STAT signaling pathway, our objectives were to identify and characterize the mediators of this system in human sperm. Cell fractionation and surface biotinylation assays clearly demonstrated that IL6R is expressed at the sperm membrane surface. The kinase JAK1 is enriched in membrane fractions and is activated during human sperm capacitation as suggested by its increase in phosphotyrosine content. Many signal transducer and activator of transcription (STAT) proteins are expressed in human sperm, including STAT1, STAT3, STAT4, STAT5, and STAT6. Among them, only STAT1 and STAT5 were detected in the cytosolic fraction. All the detected STAT proteins were enriched in the cytoskeletal structures. STAT4 was present in the perinuclear theca, whereas JAK1, STAT1, and STAT5 were detected in the fibrous sheath. Indirect immunofluorescence studies showed that JAK1 and STAT1 colocalized in the neck region and that STAT4 is present at the equatorial segment and flagella. The presence of STAT proteins in sperm structural components suggests that their role is different from their well-known transcription factor activity in somatic cells, but further investigations are required to determine their role in sperm function.

Rediscovering sperm ion channels with the patch-clamp technique

Upon ejaculation, mammalian spermatozoa have to undergo a sequence of physiological transformations within the female reproductive tract that will allow them to reach and fertilize the egg. These include initiation of motility, hyperactivation of motility and perhaps chemotaxis toward the egg, and culminate in the acrosome reaction that permits sperm to penetrate the protective vestments of the egg. These physiological responses are triggered through the activation of sperm ion channels that cause elevations of sperm intracellular pH and Ca(2+) in response to certain cues within the female reproductive tract. Despite their key role in sperm physiology and their absolute requirement for the process of fertilization, sperm ion channels remain poorly understood due to the extreme difficulty in application of the patch-clamp technique to spermatozoa. This review covers the topic of sperm ion channels in the following order: first, we discuss how the intracellular Ca(2+) and pH signaling mediated by sperm ion channels controls sperm behavior during the process of fertilization. Then, we briefly cover the history of the methodology to study sperm ion channels, which culminated in the recent development of a reproducible whole-cell patch-clamp technique for mouse and human cells. We further discuss the main approaches used to patch-clamp mature mouse and human spermatozoa. Finally, we focus on the newly discovered sperm ion channels CatSper, KSper (Slo3) and HSper (H(v)1), identified by the sperm patch-clamp technique. We conclude that the patch-clamp technique has markedly improved and shifted our understanding of the sperm ion channels, in addition to revealing significant species-specific differences in these channels. This method is critical for identification of the molecular mechanisms that control sperm behavior within the female reproductive tract and make fertilization possible.

Drosophila sperm swim backwards in the female reproductive tract and are activated via TRPP2 ion channels

Background

Sperm have but one purpose, to fertilize an egg. In various species including Drosophila melanogaster female sperm storage is a necessary step in the reproductive process. Amo is a homolog of the human transient receptor potential channel TRPP2 (also known as PKD2), which is mutated in autosomal dominant polycystic kidney disease. In flies Amo is required for sperm storage. Drosophila males with Amo mutations produce motile sperm that are transferred to the uterus but they do not reach the female storage organs. Therefore Amo appears to be a mediator of directed sperm motility in the female reproductive tract but the underlying mechanism is unknown.

Methodology/Principal Findings

Amo exhibits a unique expression pattern during spermatogenesis. In spermatocytes, Amo is restricted to the endoplasmic reticulum (ER) whereas in mature sperm, Amo clusters at the distal tip of the sperm tail. Here we show that flagellar localization of Amo is required for sperm storage. This raised the question of how Amo at the rear end of sperm regulates forward movement into the storage organs. In order to address this question, we used in vivo imaging of dual labelled sperm to demonstrate that Drosophila sperm navigate backwards in the female reproductive tract. In addition, we show that sperm exhibit hyperactivation upon transfer to the uterus. Amo mutant sperm remain capable of reverse motility but fail to display hyperactivation and directed movement, suggesting that these functions are required for sperm storage in flies.

Conclusions/Significance

Amo is part of a signalling complex at the leading edge of the sperm tail that modulates flagellar beating and that guides a backwards path into the storage organs. Our data support an evolutionarily conserved role for TRPP2 channels in cilia.

Ion channels, phosphorylation and mammalian sperm capacitation

Sexually reproducing animals require an orchestrated communication between spermatozoa and the egg to generate a new individual. Capacitation, a maturational complex phenomenon that occurs in the female reproductive tract, renders spermatozoa capable of binding and fusing with the oocyte, and it is a requirement for mammalian fertilization. Capacitation encompasses plasma membrane reorganization, ion permeability regulation, cholesterol loss and changes in the phosphorylation state of many proteins. Novel tools to study sperm ion channels, image intracellular ionic changes and proteins with better spatial and temporal resolution, are unraveling how modifications in sperm ion transport and phosphorylation states lead to capacitation. Recent evidence indicates that two parallel pathways regulate phosphorylation events leading to capacitation, one of them requiring activation of protein kinase A and the second one involving inactivation of ser/thr phosphatases. This review examines the involvement of ion transporters and phosphorylation signaling processes needed for spermatozoa to achieve capacitation. Understanding the molecular mechanisms leading to fertilization is central for societies to deal with rising male infertility rates, to develop safe male gamete-based contraceptives and to preserve biodiversity through better assisted fertilization strategies.

Classification of mouse sperm motility patterns using an automated multiclass support vector machines model

Vigorous sperm motility, including the transition from progressive to hyperactivated motility that occurs in the female reproductive tract, is required for normal fertilization in mammals. We developed an automated, quantitative method that objectively classifies five distinct motility patterns of mouse sperm using Support Vector Machines (SVM), a common method in supervised machine learning. This multiclass SVM model is based on more than 2000 sperm tracks that were captured by computer-assisted sperm analysis (CASA) during in vitro capacitation and visually classified as progressive, intermediate, hyperactivated, slow, or weakly motile. Parameters associated with the classified tracks were incorporated into established SVM algorithms to generate a series of equations. These equations were integrated into a binary decision tree that sequentially sorts uncharacterized tracks into distinct categories. The first equation sorts CASA tracks into vigorous and nonvigorous categories. Additional equations classify vigorous tracks as progressive, intermediate, or hyperactivated and nonvigorous tracks as slow or weakly motile. Our CASAnova software uses these SVM equations to classify individual sperm motility patterns automatically. Comparisons of motility profiles from sperm incubated with and without bicarbonate confirmed the ability of the model to distinguish hyperactivated patterns of motility that develop during in vitro capacitation. The model accurately classifies motility profiles of sperm from a mutant mouse model with severe motility defects. Application of the model to sperm from multiple inbred strains reveals strain-dependent differences in sperm motility profiles. CASAnova provides a rapid and reproducible platform for quantitative comparisons of motility in large, heterogeneous populations of mouse sperm.

The role of Hv1 and CatSper channels in sperm activation

Elevations of sperm intracellular pH and Ca(2+) regulate sperm motility, chemotaxis, capacitation and the acrosome reaction, and play a vital role in the ability of the sperm cell to reach and fertilise the egg. In human spermatozoa, the flagellar voltage-gated proton channel Hv1 is the main H(+) extrusion pathway that controls sperm intracellular pH, and the pH-dependent flagellar Ca²(+) channel CatSper is the main pathway for Ca²(+) entry as measured by the whole-cell patch clamp technique. Hv1 and CatSper channels are co-localized within the principal piece of the sperm flagellum. Hv1 is dedicated to proton extrusion from flagellum and is activated by membrane depolarisation, an alkaline extracellular environment, the endocannabinoid anandamide, and removal of extracellular zinc, a potent Hv1 blocker. The CatSper channel is strongly potentiated by intracellular alkalinisation. Since Hv1 and CatSper channels are located in the same subcellular domain, proton extrusion via Hv1 channels should induce intraflagellar alkalinisation and activate CatSper ion channels. Therefore the combined action of Hv1 and CatSper channels in human spermatozoa can induce elevation of both intracellular pH and Ca²(+) required for sperm activation in the female reproductive tract. Here, we discuss how Hv1 and CatSper channels regulate human sperm physiology and the differences in control of sperm intracellular pH and Ca²(+) between species.