Morphologic and functional analysis of sperm and testes in Aquaporin 7 knockout mice

Aquaporins in Reproductive System

AQP0-12, a total of 13 aquaporins are expressed in the mammalian reproductive system. These aquaporins mediate the transport of water and small solutes across biofilms for maintaining reproductive tract water balance and germ cell water homeostasis. These aquaporins play important roles in the regulation of sperm and egg cell production, maturation, and fertilization processes. Impaired AQP function may lead to diminished male and female fertility. This review focuses on the distribution, function, and regulation of AQPs throughout the male and female reproductive organs and tracts. Their correlation with reproductive success, revealing recent advances in the physiological and pathophysiological roles of aquaporins in the reproductive system.

CFTR modulates aquaporin-mediated glycerol permeability in mouse Sertoli cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel that is crucial for fluid homeodynamics throughout the male reproductive tract. Previous evidence shed light on a potential molecular partnership between this channel and aquaporins (AQPs). Herein, we explore the role of CFTR on AQPs-mediated glycerol permeability in mouse Sertoli cells (mSCs). We were able to identify the expression of CFTR, AQP3, AQP7, and AQP9 in mSCs by RT-PCR, Western blot, and immunofluorescence techniques. Cells were then treated with CFTRinh-172, a specific CFTR inhibitor, and its glycerol permeability was evaluated by stopped-flow light scattering. We observed that CFTR inhibition decreased glycerol permeability in mSCs by 30.6% when compared to the control group. A DUOLINK proximity ligation assay was used to evaluate the endogenous protein-protein interactions between CFTR and the various aquaglyceroporins we identified. We positively detected that CFTR is in close proximity with AQP3, AQP7, and AQP9 and that, through a possible physical interaction, CFTR can modulate AQP-mediated glycerol permeability in mSCs. As glycerol is essential for the control of the blood-testis barrier and elevated concentration in testis results in the disruption of spermatogenesis, we suggest that the malfunction of CFTR and the consequent alteration in glycerol permeability is a potential link between male infertility and cystic fibrosis.

Changes in aquaporins mRNA expression and liquid storage at 17°C: A potential biomarker of boar sperm quality?

Artificial insemination (AI) for pigs relies on liquid storage of extended semen at 17°C, which preserves sperm quality and ensures its fertilizing capacity. Routine quality controls include the evaluation of sperm motility, viability and capacitation status. The physiological functions of all these features depend on transmembrane aquaporins (AQPs), proteins playing key roles in osmoadaptation. In this study, we made a relative quantification, using RT-qPCR, of the mRNA of several sperm AQPs in AI-liquid semen doses before and after a 48-hr incubation period, aiming to determine possible quantitative compromising expression changes during the process that could serve as a diagnostic tool. Our results showed a decrease in classical sperm motility variables (total and progressive motility and velocity) and sperm viability after 48-hr storage, whereas capacitation status increased overtime. mRNA expression increased in the orthodox AQP4 and AQP6 after 48-hr incubation, relative to control (0 hr) and 24-hr time-points. Moreover, mRNA expression of aquaglyceroporins AQP3, AQP7 and AQP10 was higher after 48-hr incubation, confirmed by AQP7-protein validation using Western blot. Our results indicate that expression levels of AQPs-mRNA can change in ejaculated pig spermatozoa under conditions of ex-vivo incubation that could modify sperm homeostasis, suggesting it could eventually become a relevant molecular biomarker to assess the efficiency of liquid storage of pig semen.

Differential Expressions of Aquaporin Subtypes in the Adult Mouse Testis

Many efforts have been made to study the expression of aquaporins (AQP) in the mammalian reproductive system, but there are not enough data available regarding their localized expression to fully understand their specific roles in male reproduction. The present study investigated the expression and localization patterns of different AQP subtypes in the adult mouse testes and testicular spermatozoa using an immunofluorescence assay. All the studied AQPs were expressed in the testes and revealed subtype-specific patterns in the intensity and localization depending on the cell types of the testes. AQP7 was the most abundant and intensive AQP subtype in the seminiferous tubules, expressing in Leydig cells and Sertoli cells as well as all stages of germ cells, especially the spermatids and testicular spermatozoa. The expression pattern of AQP3 was similar to that of AQP7, but with higher expression in the basal and lower adluminal compartments rather than the upper adluminalcompartment. AQP8 expression was limited to the spermatogonia and Leydig cells whereas AQP9 expression was exclusive to tails of the testicular spermatozoa and elongated spermatids. Taken together, the abundance and distribution of the AQPs across the different cell types in the testes indicating to their relavance in spermatogenesis, as well as in sperm maturation, transition, and function.

Relevance of Aquaporins for Gamete Function and Cryopreservation

Simple Summary

The interaction between cells and the extracellular medium is of great importance; changes in medium composition can drive water movement across plasma membranes. Aquaporins (AQPs) are membrane channels involved in the transport of water and some solutes across membranes. When sperm enter the female reproductive tract after ejaculation, they encounter a drastic change in extracellular composition, which leads to water flowing across the plasma membrane. This triggers a series of events that are crucial to allowing fertilization to take place, such as regulation of sperm motility. In the context of assisted reproduction techniques (ART), long-term storage of gametes is sometimes required, and, during cryopreservation, these cells undergo drastic changes in extracellular medium composition. As a result, AQPs are crucial in both sperm and oocytes during this process. Cryopreservation is of considerable importance for fertility preservation in livestock, endangered species and for individuals undergoing certain medical treatments that compromise their fertility. Further research to fully elucidate the roles and underlying mechanisms of AQPs in mammalian sperm is therefore warranted.

Abstract

The interaction between cells and the extracellular medium is of great importance, and drastic changes in extracellular solute concentrations drive water movement across the plasma membrane. Aquaporins (AQPs) are a family of transmembrane channels that allow the transport of water and small solutes across cell membranes. Different members of this family have been identified in gametes. In sperm, they are relevant to osmoadaptation after entering the female reproductive tract, which is crucial for sperm motility activation and capacitation and, thus, for their fertilizing ability. In addition, they are relevant during the cryopreservation process, since some members of this family are also permeable to glycerol, one of the most frequently used cryoprotective agents in livestock. Regarding oocytes, AQPs are very important in their maturation but also during cryopreservation. Further research to define the exact sets of AQPs that are present in oocytes from different species is needed, since the available literature envisages certain AQPs and their roles but does not provide complete information on the whole set of AQPs. This is of considerable importance because, in sperm, specific AQPs are known to compensate the role of non-functional members.

Aquaporins and Animal Gamete Cryopreservation: Advances and Future Challenges

Simple Summary

Cryopreservation is the method for the long-term preservation of gametes and embryos. In recent years, intensive research has focused on improving cryopreservation protocols for the determination of optimal freezing conditions and cryoprotective agents’ concentration for each cell type. The optimal cryopreservation protocol comprises the adequate balance between the freezing rate and the correct concentration of cryoprotective agents to achieve controlled cellular dehydration and minimal intracellular ice formation. Osmoregulation is, therefore, central in cryobiology. Water and some solutes can cross the plasma membrane, whereas facilitating transport takes a great part in intracellular/extracellular fluid homeostasis. Cells express water channels known as aquaporins that facilitate the transport of water and small uncharged solutes on their plasma membrane, including some cryoprotective agents. This review explores the expression and the function of aquaporins in gametes and embryos. In addition, the putative role of aquaporins for cryopreservation procedures is discussed.

Abstract

Cryopreservation is globally used as a method for long-term preservation, although freeze-thawing procedures may strongly impair the gamete function. The correct cryopreservation procedure is characterized by the balance between freezing rate and cryoprotective agents (CPAs), which minimizes cellular dehydration and intracellular ice formation. For this purpose, osmoregulation is a central process in cryopreservation. During cryopreservation, water and small solutes, including penetrating cryoprotective agents, cross the plasma membrane. Aquaporins (AQPs) constitute a family of channel proteins responsible for the transport of water, small solutes, and certain gases across biological membranes. Thirteen homologs of AQPs (AQP0-12) have been described. AQPs are widely distributed throughout the male and female reproductive systems, including the sperm and oocyte membrane. The composition of the male and female gamete membrane is of special interest for assisted reproductive techniques (ART), including cryopreservation. In this review, we detail the mechanisms involved in gamete cryopreservation, including the most used techniques and CPAs. In addition, the expression and function of AQPs in the male and female gametes are explored, highlighting the potential protective role of AQPs against damage induced during cryopreservation.

Aquaporins Are Essential to Maintain Motility and Membrane Lipid Architecture During Mammalian Sperm Capacitation

Aquaporins are a family of ubiquitous transmembrane proteins that allow the transport of water and small molecules across the cell plasma membrane. The different members of this family present a characteristic distribution across different cell types, which is species-specific. In mammalian sperm, different AQPs, including AQP3, AQP7, and AQP11, have been identified; their main roles are related to osmoadaptation and sperm motility activation after ejaculation. Capacitation, which is a post-ejaculatory process that sperm must undergo to achieve fertilizing ability, is triggered by pH changes and different extracellular ions that are present in the female reproductive tract. Considering the function of AQPs and their influence on pH through the regulation of water flow, this study aimed to elucidate the potential role of different AQPs during in vitro sperm capacitation using three different transition metal compounds as AQP inhibitors. Cooper sulfate, a specific inhibitor of AQP3, caused a drastic increase in peroxide intracellular levels compared to the control. Mercury chloride, an unspecific inhibitor of all AQPs except AQP7 produced an increase in membrane lipid disorder and led to a decrease in sperm motility and kinetics parameters. Finally, the addition of silver sulfadiazine, an unspecific inhibitor of all AQPs, generated the same effects than mercury chloride, decreased the intracellular pH and altered tyrosine phosphorylation levels after the induction of the acrosome reaction. In the light of the aforementioned, (a) the permeability of AQP3 to peroxides does not seem to be crucial for sperm capacitation and acrosome reaction; (b) AQPs have a key role in preserving sperm motility during that process; and (c) AQPs as a whole seem to contribute to the maintenance of lipid membrane architecture during capacitation and may be related to the intracellular signaling pathways involved in the acrosome reaction. Hence, further research aimed to elucidate the mechanisms underlying the involvement of AQPs in mammalian sperm capacitation and acrosome reaction is warranted.

Aquaporins: New markers for male (in)fertility in livestock and poultry?

Improving the methods utilized to facilitate reproduction is associated with a constant need to search for new factors that not only significantly affect reproductive processes, but also create new possibilities when assessing male reproductive potential. Aquaporins (AQPs) belong to a family of small (28-30 kDa) proteins that facilitate the transport of water and other small molecules. There have been 13 AQPs (AQP0-AQP12) discovered in mammals, and these proteins are present in a wide range of cell types. Almost all AQPs, except AQP6 and AQP12 are present in the male reproductive organs and sperm of mammals and birds. Increasing evidence suggests that these proteins are involved in a number of processes responsible for the optimal functioning of the male reproductive system. This review presents the current state of knowledge regarding the abundance and distribution of AQPs in the male reproductive organs and sperm of various livestock and poultry species, including buffalo, cattle, sheep, horses, pigs, turkeys and goose. Furthermore, the possible physiological and pathophysiological significance of AQPs in male reproduction, as well as hormonal regulation of quantities are discussed. It can be concluded from the studies analyzed in this paper that abundance patterns of AQPs may be considered in the future as specific and universal biomarkers of male fertility and infertility in animal husbandry.

Human sperm functioning is related to the aquaporin-mediated water and hydrogen peroxide transport regulation

Aquaporins (AQPs) are transmembrane water channels and some of them are permeable in addition to water to other small solutes including hydrogen peroxide. The sperm cells of mammals and fishes express different AQPs, although there is no agreement in the literature on their localization. In humans, AQP3 and AQP11 are expressed mainly in the tail, AQP7 in the head and AQP8 in the midpiece. Thanks to the results of experiments with KO mice and to data obtained by comparing sub-fertile patients with normospermic subjects, the importance of AQPs for the normal functioning of sperms to ensure normal fertility emerged. AQP3, AQP7 and AQP11 appeared involved in the sperm volume regulation, a key role for fertility because osmoadaptation protect the sperm against a swelling and tail bending that could affect sperm motility. AQP8 seems to have a fundamental role in regulating the elimination of hydrogen peroxide, the most abundant reactive oxygen species (ROS), and therefore in the response to oxidative stress. In this review, the human AQPs expression, their localization and functions, as well as their relevance in normal fertility are discussed. To understand better the AQPs role in human sperm functionality, the results of studies obtained in other animal species were also considered.

Osmoregulation in fish sperm

In most fish exhibiting external fertilization, spermatozoa become motile after release into water, triggered by differences between intracellular and extracellular conditions such as osmotic pressure, ion composition, and pH. The rapid change in osmolarity initiating spermatozoon motility induces osmotic pressure, resulting in active water movement across the cell membrane. Mechanisms of ion and water transport across the plasma membrane and cell volume regulation are important in maintaining structure and functional integrity of the cell. The capacity of the fish spermatozoon plasma membrane to adapt to dramatic environmental changes is an essential prerequisite for motility and successful fertilization. Adaptation to change in external osmolality may be the basis of spermatozoon function and an indicator of sperm quality. The involvement of specific water channels (aquaporins) in cell volume regulation and motility is highly likely. The goal of this review is to describe basic mechanisms of water transport and their role in fish spermatozoon physiology, focusing on osmoresistance, cell volume regulation, motility, and survival.

Observations on Dag-like defect of spermatozoa induced by treatment of the phytotherapeutic Quassia amara/quassin in the mouse model

Gross alterations in the morphology of spermatozoa, teratozoospermia, invariably render them incapable of fertilisation. One of the contributory factors to teratozoospermia is failure of spermatozoon to shed the cytoplasmic droplet even after their arrival at epididymis. Quassia amara and quassin are of medicinal value with special reference to malaria. Nevertheless, there are also reports implicating Quassia/quassin in male reproductive toxicity. We were interested in finding if its therapeutic application would jeopardise male fertility. So, we tested it for male reproductive toxicity by analysing, among other aspects, abnormal sperm morphologies, and made a systematic analysis of the spermatozoa of treated mice before they are spermiated and until they arrive at the cauda epididymis. The spermatozoa not only failed to shed the cytoplasmic droplet during epididymal transit but swell to a very large size and were angulated, resulting in Dag-like defect or lasso shape. A link between cytoplasmic droplet that was retained and lasso shape of tail was indicated. This article traces the structural changes in spermatozoa that lead to angulation, flexion and coiling of the tail, caused due to retention of cytoplasmic droplet, and explains one of the mechanisms of toxicant-induced teratozoospermia.

Aquaporins and (in)fertility: More than just water transport

Aquaporins (AQPs) are a family of channel proteins that facilitate the transport of water and small solutes across biological membranes. They are widely distributed throughout the organism, having a number of key functions, some of them unexpected, both in health and disease. Among the various diseases in which AQPs are involved, infertility has been overlooked. According to the World Health Organization (WHO) infertility is a global public health problem with one third of the couples suffering from subfertility or even infertility due to male or female factors alone or combined. Thus, there is an urgent need to unveil the molecular mechanisms that control gametes production, maturation and fertilization-related events, to more specifically determine infertility causes. In addition, as more couples seek for fertility treatment through assisted reproductive technologies (ART), it is pivotal to understand how these techniques can be improved. AQPs are heterogeneously expressed throughout the male and female reproductive tracts, highlighting a possible regulatory role for these proteins in conception. In fact, their function, far beyond water transport, highlights potential intervention points to enhance ART. In this review we discuss AQPs distribution and structural organization, functions, and modulation throughout the male and female reproductive tracts and their relevance to the reproductive success. We also highlight the most recent advances and research trends regarding how the different AQPs are involved and regulated in specific mechanisms underlying (in)fertility. Finally, we discuss the involvement of AQPs in ART-related processes and how their handling can lead to improvement of infertility treatment.

HPV Infection Affects Human Sperm Functionality by Inhibition of Aquaporin-8

Human sperm cells express different aquaporins (AQPs), AQP3, 7, 8, 11, which are localized both in the plasma membrane and in intracellular structures. Besides cell volume regulation and end stage of cytoplasm removal during sperm maturation, the role of AQPs extends also to reactive oxygen species (ROS) elimination. Moreover, oxidative stress has been shown to inhibit AQP-mediated H₂O₂ permeability. A decrease in AQPs functionality is related to a decrease in sperm cells number and motility. Here we investigate the possible effect of human Papillomavirus (HPV) on both expression and function of AQPs in human sperm cells of patients undergoing infertility couple evaluation. Stopped-flow light-scattering experiments demonstrated that HPV infection heavily reduced water permeability of sperm cells in normospermic samples. Confocal immunofluorescence experiments showed a colocalization of HPV L1 protein with AQP8 (Pearson’s correlation coefficient of 0.61), confirmed by co-immunoprecipitation experiments. No interaction of HPV with AQP3 and AQP7 was observed. A 3D model simulation of L1 protein and AQP8 interaction was also performed. Present findings may suggest that HPV infection directly inhibits AQP8 functionality and probably makes sperm cells more sensitive to oxidative stress.

Aquaporin-Mediated Water and Hydrogen Peroxide Transport Is Involved in Normal Human Spermatozoa Functioning

Different aquaporins (AQPs) are expressed in human sperm cells and with a different localization. Their function has been related to cell volume control in response to the osmotic changes encountered passing from the epididymal fluid to the cervical mucus or involved in the end stage of cytoplasm removal during sperm maturation. Recently, AQPs have also shown hydrogen peroxide (H₂O₂) permeability properties. Here, we investigate the expression, localization and functioning of AQPs in human sperm cells with particular attention to their role as peroxiporins in reactive oxygen species (ROS) scavenging in both normospermic and sub-fertile human subjects. Western blotting and immunocytochemistry were used to confirm and clarify the AQPs expression and localization. Water and H₂O₂ permeability was tested by stopped flow light scattering method and by the CM-H2DCFDA (5-(and-6)-chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate, acetyl ester) H₂O₂ fluorescence probe, respectively. AQP3, -7, -8, and -11 proteins were found in human sperm cells and localized in the head (AQP7), in the middle piece (AQP8) and in the tail (AQP3 and -11) in both the plasma membrane and in intracellular structures. Sperm cells showed water and H₂O₂ permeability which was reversibly inhibited by H₂O₂, heat stress and the AQP inhibitor HgCl₂. Reduced functionality was observed in patients with compromised basal semen parameters. Present findings suggest that AQPs are involved in both volume regulation and ROS elimination. The relationship between sperm number and motility and AQP functioning was also demonstrated.

Aquaporins 7 and 11 in boar spermatozoa: detection, localisation and relationship with sperm quality

Aquaporins (AQPs) are integral membrane water channels that allow transport of water and small solutes across cell membranes. Although water permeability is known to play a critical role in mammalian cells, including spermatozoa, little is known about their localisation in boar spermatozoa. Two aquaporins, AQP7 and AQP11, in boar spermatozoa were identified by western blotting and localised through immunocytochemistry analyses. Western blot results showed that boar spermatozoa expressed AQP7 (25 kDa) and AQP11 (50 kDa). Immunocytochemistry analyses demonstrated that AQP7 was localised in the connecting piece of boar spermatozoa, while AQP11 was found in the head and mid-piece and diffuse labelling was also seen along the tail. Despite differences in AQP7 and AQP11 content between boar ejaculates, these differences were not found to be correlated with sperm quality in the case of AQP7. Conversely, AQP11 content showed a significant correlation (P < 0.05) with sperm membrane integrity and fluidity and sperm motility. In conclusion, boar spermatozoa express AQP7 and AQP11, and the amounts of AQP11 but not those of AQP7 are correlated with sperm motility and membrane integrity.

Aquaporin biology of spermatogenesis and sperm physiology in mammals and teleosts

Fluid homeostasis is recognized as a critical factor during the development, maturation, and function of vertebrate male germ cells. These processes have been associated with the presence of multiple members of the aquaporin superfamily of water and solute channels in different cell types along the reproductive tract as well as in spermatozoa. We present a comparative analysis of the existing knowledge of aquaporin biology in the male reproductive tissues of mammals and teleosts. Current data suggest that in both vertebrate groups, aquaporins may have similar functions during differentiation of spermatozoa in the germinal epithelium, in the concentration and maturation of sperm in the testicular ducts, and in the regulation of osmotically induced volume changes in ejaculated spermatozoa. Recent studies have also provided insight into the possible function of aquaporins beyond water transport, such as in signaling pathways during spermatogenesis or the sensing of cell swelling and mitochondrial peroxide transport in activated sperm. However, an understanding of the specific physiological functions of the various aquaporins during germ cell development and sperm motility, as well as the molecular mechanisms involved, remains elusive. Novel experimental approaches need to be developed to elucidate these processes and to dissect the regulatory intracellular pathways implicated, which will greatly help to uncover the molecular basis of sperm physiology and male fertility in vertebrates.

Regulation of salmonid fish sperm motility by osmotic shock-induced water influx across the plasma membrane

The motility of salmonid fish sperm is initiated by a decrease in the extracellular K(+) concentration. However, our previous studies revealed that salmonid fish sperm motility could be initiated in the presence of an inhibitory concentration of K(+) by drastic osmotic shock induced by suspension in a hypertonic glycerol solution and subsequent dilution in a hypotonic solution (glycerol-treatment). In the present study, we examined if an osmotic shock-induced water influx is involved in the regulation of salmonid fish sperm motility. HgCl2, a common inhibitor of aquaporins (AQPs), decreased the duration of salmonid fish sperm motility. Dilution of sperm cells in a hypotonic solution increased the cellular volume, whereas HgCl2 inhibited such an increase in cellular volume. Furthermore, the expression of AQP 1a and 10 in rainbow trout testes was confirmed. In contrast, HgCl2 did not affect glycerol-treated sperm motility, indicating that AQPs are not involved in glycerol-treated sperm motility. We also explored the possibility of aquaporin-independent water influx in glycerol-treated sperm by assessing the sperm membrane permeability using propidium iodide. The plasma membrane of glycerol-treated sperm was considerably permeabilized. The cellular volume was decreased in a hypertonic glycerol solution and increased upon subsequent hypoosmotic shock, indicating an AQP-independent water flux across the plasma membrane upon glycerol-treatment. Taken together, these results showed that water influx across the plasma membrane via AQP is crucial for the maintenance of salmonid fish sperm motility under normal conditions, whereas water influx by osmotic shock-induced membrane permeation is critical for the initiation of glycerol-treated sperm motility.

Distribution of aquaporin genes and selection of individual reference genes for quantitative real-time RT-PCR analysis in multiple tissues of the mouse

Aquaporins (AQPs) are a family of water-transporting proteins that are selectively expressed in epithelial, endothelial, and many other cell types of various tissues, where they play important physiological functions. However, the accurate distribution of AQP gene expression has not yet been examined in various tissues of the mouse. We first evaluated the tissue distribution of AQP gene expression using tongue, nasal epithelium, bronchus, trachea, lung, esophagus, stomach, ileum, transverse colon, liver, pancreas, whole blood, thigh muscle, spinal cord, brain, thoracic aorta, heart, kidney, thymus, spleen, skin, eye, and testis of the mouse. Furthermore, for a quantitative analysis, we selected appropriate reference genes for normalized qRT-PCR data in various tissues. The stability of the reference genes was assessed using NormFinder. The stably expressed genes identified in the present study were 18s rRNA. When 18s rRNA was used, as the best reference gene in the present study, the genes for AQPs 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, and 12 were notably expressed in the eye, lung, testis, eye, spinal cord, trachea, kidney, testis, testis, testis, testis, and pancreas. These results, regarding the distribution of AQPs, suggest that AQPs may be involved in various physiological and pathophysiological processes.

Subcellular localization of selectively permeable aquaporins in the male germ line of a marine teleost reveals spatial redistribution in activated spermatozoa

In oviparous vertebrates such as the marine teleost gilthead seabream, water and fluid homeostasis associated with testicular physiology and the external activation of spermatozoa is potentially mediated by multiple aquaporins. To test this hypothesis, we isolated five novel members of the aquaporin superfamily from gilthead seabream and developed paralog-specific antibodies to localize the cellular sites of protein expression in the male reproductive tract. Together with phylogenetic classification, functional characterization of four of the newly isolated paralogs, Aqp0a, -7, -8b, and -9b, demonstrated that they were water permeable, while Aqp8b was also permeable to urea, and Aqp7 and -9b were permeable to glycerol and urea. Immunolocalization experiments indicated that up to seven paralogous aquaporins are differentially expressed in the seabream testis: Aqp0a and -9b in Sertoli and Leydig cells, respectively; Aqp1ab, -7, and -10b from spermatogonia to spermatozoa; and Aqp1aa and -8b in spermatids and sperm. In the efferent duct, only Aqp10b was found in the luminal epithelium. Ejaculated spermatozoa showed a segregated spatial distribution of five aquaporins: Aqp1aa and -7 in the entire flagellum or the head, respectively, and Aqp1ab, -8b, and -10b both in the head and the anterior tail. The combination of immunofluorescence microscopy and biochemical fractionation of spermatozoa indicated that Aqp10b and phosphorylated Aqp1ab are rapidly translocated to the head plasma membrane upon activation, whereas Aqp8b accumulates in the mitochondrion of the spermatozoa. In contrast, Aqp1aa and -7 remained unchanged. These data reveal that aquaporin expression in the teleost testis shares conserved features of the mammalian system, and they suggest that the piscine channels may play different roles in water and solute transport during spermatogenesis, sperm maturation and nutrition, and the initiation and maintenance of sperm motility.

Expression of Tas1 taste receptors in mammalian spermatozoa: functional role of Tas1r1 in regulating basal Ca²⁺ and cAMP concentrations in spermatozoa

BACKGROUND

During their transit through the female genital tract, sperm have to recognize and discriminate numerous chemical compounds. However, our current knowledge of the molecular identity of appropriate chemosensory receptor proteins in sperm is still rudimentary. Considering that members of the Tas1r family of taste receptors are able to discriminate between a broad diversity of hydrophilic chemosensory substances, the expression of taste receptors in mammalian spermatozoa was examined.

METHODOLOGY/PRINCIPAL FINDINGS

The present manuscript documents that Tas1r1 and Tas1r3, which form the functional receptor for monosodium glutamate (umami) in taste buds on the tongue, are expressed in murine and human spermatozoa, where their localization is restricted to distinct segments of the flagellum and the acrosomal cap of the sperm head. Employing a Tas1r1-deficient mCherry reporter mouse strain, we found that Tas1r1 gene deletion resulted in spermatogenic abnormalities. In addition, a significant increase in spontaneous acrosomal reaction was observed in Tas1r1 null mutant sperm whereas acrosomal secretion triggered by isolated zona pellucida or the Ca²⁺ ionophore A23187 was not different from wild-type spermatozoa. Remarkably, cytosolic Ca²⁺ levels in freshly isolated Tas1r1-deficient sperm were significantly higher compared to wild-type cells. Moreover, a significantly higher basal cAMP concentration was detected in freshly isolated Tas1r1-deficient epididymal spermatozoa, whereas upon inhibition of phosphodiesterase or sperm capacitation, the amount of cAMP was not different between both genotypes.

CONCLUSIONS/SIGNIFICANCE

Since Ca²⁺ and cAMP control fundamental processes during the sequential process of fertilization, we propose that the identified taste receptors and coupled signaling cascades keep sperm in a chronically quiescent state until they arrive in the vicinity of the egg - either by constitutive receptor activity and/or by tonic receptor activation by gradients of diverse chemical compounds in different compartments of the female reproductive tract.

Role of Aquaporins during Teleost Gametogenesis and Early Embryogenesis

Aquaporins are believed to be involved in homeosmotic mechanisms of marine teleosts. Increasing data suggest that these molecular water channels play critical roles associated with the adaptation of gametes and early embryos to the external spawning environment. In this mini-review, we discuss recent studies suggesting the function of aquaporin-mediated fluid homeostasis during spermatozoa activation and egg formation in teleosts. In addition, we address the potential role of water channels in osmosensing and cell migration during early embryonic development.

Aquaporins in sperm osmoadaptation: an emerging role for volume regulation

Upon ejaculation, mammalian sperm experience a natural osmotic decrease during male to female reproductive tract transition. This hypo-osmotic exposure not only activates sperm motility, but also poses potential harm to sperm structure and function by inducing unwanted cell swelling. In this physiological context, regulatory volume decrease (RVD) is the major mechanism that protects cells from detrimental swelling, and is essential to sperm survival and normal function. Aquaporins are selective water channels that enable rapid water transport across cell membranes. Aquaporins have been implicated in sperm osmoregulation. Recent discoveries show that Aquaporin-3 (AQP3), a water channel protein, is localized in sperm tail membranes and that AQP3 mutant sperm show defects in volume regulation and excessive cell swelling upon physiological hypotonic stress in the female reproductive tract, thereby highlighting the importance of AQP3 in the postcopulatory sperm RVD process. In this paper, we discuss current knowledge, remaining questions and hypotheses about the function and mechanismic basis of aquaporins for volume regulation in sperm and other cell types.

Aquaporin3 is a sperm water channel essential for postcopulatory sperm osmoadaptation and migration

In the journey from the male to female reproductive tract, mammalian sperm experience a natural osmotic decrease (e.g., in mouse, from ~415 mOsm in the cauda epididymis to ~310 mOsm in the uterine cavity). Sperm have evolved to utilize this hypotonic exposure for motility activation, meanwhile efficiently silence the negative impact of hypotonic cell swelling. Previous physiological and pharmacological studies have shown that ion channel-controlled water influx/efflux is actively involved in the process of sperm volume regulation; however, no specific sperm proteins have been found responsible for this rapid osmoadaptation. Here, we report that aquaporin3 (AQP3) is a sperm water channel in mice and humans. Aqp3-deficient sperm show normal motility activation in response to hypotonicity but display increased vulnerability to hypotonic cell swelling, characterized by increased tail bending after entering uterus. The sperm defect is a result of impaired sperm volume regulation and progressive cell swelling in response to physiological hypotonic stress during male-female reproductive tract transition. Time-lapse imaging revealed that the cell volume expansion begins at cytoplasmic droplet, forcing the tail to angulate and form a hairpin-like structure due to mechanical membrane stretch. The tail deformation hampered sperm migration into oviduct, resulting in impaired fertilization and reduced male fertility. These data suggest AQP3 as an essential membrane pathway for sperm regulatory volume decrease (RVD) that balances the "trade-off" between sperm motility and cell swelling upon physiological hypotonicity, thereby optimizing postcopulatory sperm behavior.

Aquaporins in spermatozoa and testicular germ cells: identification and potential role

Mammalian spermatozoa have relatively high water permeability and swell readily, as in the hypo-osmotic swelling test used in the andrology clinic. Physiologically, spermatozoa experience changes in the osmolality of the surrounding fluids in both the male and the female tracts on their journey from the testis to the ovum. Sperm volume regulation in response to such osmotic challenges is important to maintain a stable cell size for the normal shape and function of the sperm tail. Alongside ion channels for the fluxes of osmolytes, water channels would be crucial for sperm volume regulation. In contrast to the deep knowledge and numerous studies on somatic cell aquaporins (AQPs), the understanding of sperm AQPs is limited. Among the 13 AQPs, convincing evidence for their presence in spermatozoa has been confined to AQP7, AQP8 and AQP11. Overall, current findings indicate a major role of AQP8 in water influx and efflux for sperm volume regulation, which is required for natural fertilization. The preliminary data suggestive of a role for AQP7 in sperm glycerol metabolism needs further substantiation. The association of AQP11 with the residual cytoplasm of elongated spermatids and the distal tail of spermatozoa supports the hypothesis of more than just a role in conferring water permeability and also in the turnover and recycling of surplus cellular components made redundant during spermiogenesis and spermiation. This would be crucial for the maintenance of a germinal epithelium functioning efficiently in the production of spermatozoa.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Aquaporin AQP11 in the testis: molecular identity and association with the processing of residual cytoplasm of elongated spermatids

AQP11 is one of the latest aquaporin (AQP) family members found, which differs from the other AQPs by its intracellular localisation and unusual water pore nucleotides with unclear function. Despite the highest mRNA expression among organs having been reported in the testis, the testicular molecule has not been studied in detail. Immunohistochemistry of rat adult testis localised AQP11 to the elongated spermatids (ES) and no other cell types except residual bodies inside Sertoli cells. It was absent from early ES at least until stage 13, and after a first diffuse appearance in the caudal cytoplasm became concentrated in intracellular organelles by stage 17, was strongest in vesicles in the anterior cytoplasm at the final ES stages and appeared in residual bodies. Staining was detected on the distal quarter of the sperm tail only immediately before spermiation. A similar localisation was found in the mouse and developmental profiles for both the open reading frame mRNA and protein expression in 8-50 dpp testis pinpointed its first appearance coinciding with late stage ES. Sequencing of PCR products of testicular Aqp11 containing the open reading frames confirmed a full match with GenBank databases for rat, mouse and human. Western blotting revealed two or more molecular forms with the 26/27 kDa species dominating in the rat/mouse testis and the 33/34 kDa form selectively allocated to the spermatozoa. In view of intracellular vacuolation leading to polycystic kidney in Aqp11-null mice, a possible role of testicular AQP11 in the recycling of surplus cytoplasmic components of the ES and sustaining Sertoli cell capacity in the support of spermatogenesis was discussed.

Function of aquaporin-7 in the kidney and the male reproductive system

The aquaporin-7 (AQP7) water channel is known to be a member of the aquaglyceroporins, which allow the rapid transport of glycerol and water. In this chapter, we review the physiological functions of AQP7 in the kidney and the male reproductive system.In the kidney, AQP7 is abundantly present at the apical membrane of the proximal straight tubules. Although the contribution of AQP7 to the water permeability of proximal straight tubules was found to be minimal compared with that of AQP1, we identified a novel glycerol reabsorption pathway that may be important for preventing glycerol from being excreted into urine.In the male reproductive system, AQP7 is present particularly in the spermatids, as well as in the testicular and epididymal spermatozoa, suggesting that AQP7 has some role in late spermatogenesis. However, male AQP7 knockout mice were not sterile, and their sperm did not show any morphological or functional abnormalities.

Aquaporin water channels in mammals

Water channels, aquaporins (AQPs), are a family of small integral plasma membrane proteins that primarily transport water across the plasma membrane. There are 13 members (AQP0-12) in humans. This number is final as the human genome project has been completed. They are divided into three subgroups based on the primary sequences: water selective AQPs (AQP0, 1, 2, 4, 5, 6, 8), aquaglyceroporins (AQP3, 7, 9, 10), and superaquaporins (AQP11, 12). Since no specific inhibitors are yet available, functional roles of AQPs are suggested by AQP null mice and humans. Abnormal water metabolism was shown with AQP1, 2, 3, 4, 5 null mice, especially with AQP2 null mice: fatal at neonate due to diabetes insipidus. Abnormal glycerol transport was shown with AQP3, 7, 9 null mice, although they appeared normal. AQP0 null mice suffer from cataracts, although the pathogenesis is not clear. Unexpectedly, AQP11 null mice die from uremia as a result of polycystic kidneys. Interestingly, AQP6, 8, 10, 12 null mice are almost normal. AQP null humans have been reported with AQP0, 1, 2, 3, 7: only AQP2 null humans show an outstanding phenotype, diabetes insipidus. This review summarizes the current knowledge on all mammalian AQPs and hopefully will stimulate future research in both clinical and basic fields.

Aquaporin isoforms involved in physiological volume regulation of murine spermatozoa

Murine epididymal spermatozoa were dispersed in a medium of native osmolality and then transferred to a hypo-osmotic medium to mimic the physiological osmotic challenge, as encountered upon ejaculation into the female tract. The addition of quinine to block sperm K(+)-channels for volume regulation resulted in a size increase of viable cells. Preincubation in 0.1 mM HgCl(2), a standard aquaporin inhibitor, prevented such cell swelling. Addition of the K(+)-ionophore valinomycin to quinine-swollen sperm reversed the swelling, but not after pretreatment of the swollen sperm by HgCl(2). Aqp7, Aqp8, and Aqp9 mRNAs were identified in spermatozoa by RT-PCR, and the entire open reading frames were sequenced and compared with the GenBank database. Western blotting demonstrated specific protein signals for sperm AQP7 and AQP8 expression but probably not AQP9. The role of Hg(2+)-insensitive AQP7, if any, in sperm volume regulation was studied in transgenic mice. Spermatozoa from Aqp7(-/-) mice were the same size as wild-type sperm in basal conditions. Quinine-swollen volume, swelling reversal by valinomycin, and inhibition by Hg(2+) were also similar, indicating efficient water transport in the absence of AQP7. However, both water influx and efflux occurred faster in Aqp7(-/-) sperm than wild-type. This faster water movement in the knockout mouse spermatozoa was explainable by an upregulation of Aqp8 expression as revealed by quantitative PCR. Therefore, the Hg(2+)-sensitive AQP8, which was localized in elongated spermatids and spermatozoa, is a likely candidate for a water channel responsible for physiological sperm volume regulation crucial to in vivo fertilization.

Membrane domain specificity in the spatial distribution of aquaporins 5, 7, 9, and 11 in efferent ducts and epididymis of rats

Water content within the epididymis of the male reproductive system is stringently regulated to promote sperm maturation. Several members of the aquaporin (AQP) family of water channel-forming integral membrane proteins have been identified in epididymal cells, but expression profiling for this epithelium is presently incomplete, and no AQP isoform has yet been identified on basolateral plasma membranes of these cells. In this study, we explored AQP expression by RT-PCR and light microscopy immunolocalizations using peroxidase and wide-field fluorescence techniques. The results indicate that several AQPs are coexpressed in the epididymis including AQP 5, 7, 9, and 11. Immunolocalizations suggested complex patterns in the spatial distribution of these AQPs. In principal cells, AQP 9 and 11 were present mainly on microvilli, whereas AQP 7 was localized primarily to lateral and then to basal plasma membranes in a region-specific manner. AQP 5 was also expressed regionally but was associated with membranes of endosomes. Additionally, AQPs were expressed by some but not all basal (AQP 7 and 11), clear (AQP 7 and 9), and halo (AQP 7 and 11) cells. These findings indicate unique associations of AQPs with specific membrane domains in a cell type- and region-specific manner within the epididymis of adult animals.