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

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.

Electrophysiology of Human Gametes: A Systematic Review

PURPOSE

Oocytes and spermatozoa are electrogenic cells with the ability to respond to electrical stimuli and modulate their electrical properties accordingly. Determination of the ionic events during the gamete maturation helps to design suitable culture media for gametes in assisted reproductive technology (ART). The present systematic review focuses on the electrophysiology of human gametes during different stages of maturation and also during fertilization.

MATERIALS AND METHODS

The reports published in the English language between January 2000 and July 2021 were extracted from various electronic scientific databases following the PRISMA checklist using specific MeSH keywords.

RESULTS

Subsequent to the screening process with defined inclusion and exclusion criteria, 60 articles have been included in this review. Among them, 11 articles were directly related to the electrophysiology of human oocytes and 49 physiology department to the electrophysiology of human spermatozoa.

CONCLUSIONS

Gametes generate electrical currents by ionic exchange, particularly Na+, K+, Cl-, H+, Zn2+, Cu2+, Se2+, Mg2+, HCO3-, and Ca2+ through specific ion channels in different stages of gamete maturation. The ionic concentrations, pH, and other physicochemical variables are modulated during the gametogenesis, maturation, activation, and the fertilization process following gamete function and metabolism. The electrical properties of human gametes change during different stages of maturation. Although it is demonstrated that the electrical properties are significant regulators of cell signaling and are fundamental to gamete maturation and fertilization, their exact roles in these processes are still poorly understood. Further research is required to unveil the intricate electrophysiological processes of human gamete maturation.

Canine Spermatozoa—Predictability of Cryotolerance

Simple Summary

Semen freezing in dogs is a field of growing interest. The international shipment of cryoconserved semen contributes to the avoidance of long travels and long-term storage of valuable gametes. However, the collection of one semen portion on average results in one to three doses for artificial insemination, which is a poor result in comparison to the outcome in large animals. The costs for the owners are therefore rather high. In individual dogs, the semen quality of raw semen is good; however, it could be suboptimal after thawing. To avoid costly freezing of these low-quality ejaculates, markers of freezability are useful. An abundance of markers are available for large animals, but not for dogs. This review provides an overview on markers for freezability of canine semen.

Abstract

Markers of freezability allow the selection of ejaculates of good freezability. So far, most investigations were conducted in boars, bulls, rams and horses, with high economic interests triggering the efforts. The progress in dogs is comparably slow. A critical evaluation of the methods requires consideration of practicability, with most labs not even possessing a computer assisted sperm analyser (CASA); furthermore, small canine ejaculates mostly do not allow the use of large semen volumes. In dogs, modern markers of freezability no longer assess single membrane constituents or seminal plasma components but comprise tests of cell functionality and adaptability, energy metabolism, cluster analyses of kinetic and morphometric parameters, as well as DNA intactness. Identification of the most efficient combination of tests seems useful. At present, examination by CASA combined with cluster analysis of kinetic subgroups, JC-1 staining and COMET assay or staining with toluidine blue seem most appropriate; however, cell volumetry and other functional tests deserve better attention. A better understanding of spermatozoa energy metabolism might reveal new markers. This review focuses on the requirements and markers of freezability of canine semen, highlighting potential future candidates.

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.

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.

Aquaporins Are Differentially Regulated in Canine Cryptorchid Efferent Ductules and Epididymis

Simple Summary

The distribution and expression of aquaporins (AQPs) in the testes and spermatozoa of several animal species play important roles in spermatogenesis and spermatozoon transit in this region. The aim of this study was to evaluate AQP7, AQP8, and AQP9 localization and expression in the efferent ductules and epididymal regions (the caput, corpus, and cauda) of normal and cryptorchid dogs. The results from immunohistochemistry, Western blotting, and real-time reverse transcription polymerase chain reaction (RT-PCR) show regional tissue distributions, particularly at the level of the epithelium of efferent ductules and both the regions caput and cauda of the canine cryptorchid epididymis. These findings support the hypothesis that these channel proteins respond differently to multiple stimuli that cause cryptorchidism (hormones, heat, osmolarity, etc.) and participate in the mechanisms of cell “resilience” or apoptosis taking place in the epididymis.

Abstract

The efferent ductules and the epididymis are parts of the male reproductive system where spermatozoa mature. Specialized epithelial cells in these ducts contribute to the transport of fluids produced by spermatozoa’s metabolic activity. Aquaporins (AQPs) have been demonstrated to be expressed in the spermatozoan membrane and testis epithelial cells, where they contribute to regulating spermatozoan volume and transit through environments of differing osmolality. Due to the lack of detailed literature regarding AQP expression in the canine male genital tract, the aim of this study was to investigate both the distribution and expression of AQP7, AQP8, and AQP9 in the efferent ductules and epididymal regions (caput, corpus, and cauda) from normal and cryptorchid dogs by using immunohistochemistry, Western blotting, and real-time reverse transcription polymerase chain reaction (RT-PCR). Our results show different patterns for the distribution and expression of the examined AQPs, with particular evidence of their upregulation in the caput and downregulation in the cauda region of the canine cryptorchid epididymis. These findings are associated with a modulation of Hsp70 and caspase-3 expression, suggesting the participation of AQPs in the luminal microenvironment modifications that are peculiar characteristics of this pathophysiological condition.

Osmotic tolerance of rabbit spermatozoa is affected by extender composition and temperature

Sperm osmotic adaptability to anisosmotic conditions is important for sperm epididymal maturation, motility activation at ejaculation, and female tract colonization, or for conducting technological procedures such as cryopreservation. Several factors affect this adaptability, including the fluid composition that contributes to water flow dynamics, and the temperature at which osmotic stress is initiated. This study was designed to investigate the effect of medium composition (electrolyte- or sugar-based extender) and temperature (25 and 5 °C) on rabbit sperm adaptability to anisosmotic conditions. Rabbit spermatozoa, therefore, were diluted at both temperatures (25 and 5 °C) in electrolyte- or sugar-based media at increasing osmotic conditions (100 to 1,000 mOsm/kg), and values for sperm variables (sperm kinetics, membrane integrity, mitochondrial membrane potential) were estimated as endpoints. Sperm kinetics seemed to be more sensitive to osmotic stress than membrane integrity or mitochondrial function. The effect of moderate hypoosmotic stress did not differ when there was use of sugar- and electrolyte-based extenders at 25 °C (P > 0.05). In hyper-tonic conditions at 25 °C, the sugar-based extender was more effective in protecting sperm membrane integrity and mitochondrial function (P < 0.05). The lesser temperature made the differences more relevant because of the detrimental effect of hyperosmotic stress was more evident in the electrolyte-based extender at 5 °C (P < 0.05). The results from this study indicated rabbit spermatozoa have different adaptability to anisosmotic conditions induced by sugar- and electrolyte-based media and that the temperature at which the osmotic stress is initiated affects the cellular response.

Aquaporin 11 is related to cryotolerance and fertilising ability of frozen-thawed bull spermatozoa

Aquaporins (AQPs) are channel proteins involved in the transport of water and solutes across biological membranes. In the present study we identified and localised aquaporin 11 (AQP11) in bull spermatozoa and investigated the relationship between the relative AQP11 content, sperm cryotolerance and the fertilising ability of frozen-thawed semen. Bull ejaculates were classified into two groups of good and poor freezability and assessed through immunofluorescence and immunoblotting analyses before and after cryopreservation. AQP11 was localised throughout the entire tail and along the sperm head. These findings were confirmed through immunoblotting, which showed a specific band of approximately 50 kDa corresponding to AQP11. The relative amount of AQP11 was significantly (P<0.05) higher in both fresh and frozen-thawed spermatozoa from bull ejaculates with good freezability compared with those with poorer freezability. In addition, in vitro oocyte penetration rates and non-return rates 56 days after AI were correlated with the relative AQP11 content in fresh spermatozoa. In conclusion, AQP11 is present in the head and tail of bull spermatozoa and its relative amount in fresh and frozen-thawed spermatozoa is related to the resilience of the spermatozoa to withstand cryopreservation and the fertilising ability of frozen-thawed spermatozoa. Further research is needed to elucidate the actual role of sperm AQP11 in bovine fertility.

Sperm motility in fishes: (III) diversity of regulatory signals from membrane to the axoneme

Fish spermatozoa acquire potential for motility in the sperm duct where they are immotile. Osmolality of the seminal plasma is a key factor to maintain spermatozoa in the quiescent state in either freshwater or marine fishes. However, potassium (K⁺) ions prevent spermatozoa motility in salmonid and sturgeon fishes, while CO₂ inhibits spermatozoa motility in flatfishes. Once, spermatozoa are released at spawning, their motility is initiated in hypo-osmotic and hyper-osmotic environments in freshwater and marine fishes, respectively. Some substances produced by the testes (a progestin), or released from oocytes (peptides) induce spermatozoa hypermotility in some marine fishes including the Atlantic croaker and Pacific herrings, respectively. Duration of spermatozoa motility is short, lasting for a few seconds to few minutes in most fishes due to rapid depletion of energy required for the beating of the motility apparatus called axoneme. In the osmotic-activated spermatozoa, K⁺ and water effluxes occur in freshwater and marine fishes, respectively, which trigger spermatozoa motility signaling. In general, initiation of axonemal beating is associated with an increase in intracellular calcium (Ca²⁺) ions in spermatozoa of both freshwater and marine fishes and a post- or pre-increase in intracellular pH, while cyclic adenosine monophosphate (cAMP) remains unchanged. However, axonemal beating is cAMP-dependent in demembranated spermatozoa of salmonid and sturgeon fishes. Calcium from extracellular environment or intracellular stores supply required Ca²⁺ concentration for axonemal beating. Several axonemal proteins have been so far identified in fishes that are activated by Ca²⁺ and cAMP, directly or mediated by protein kinase C and protein kinase A, respectively. The present study reviews differences and similarities in complex regulatory signals controlling spermatozoa motility initiation in fishes, and notes physiological mechanisms that await elucidation.

Hormonal control of vas deferens fluid volume and aquaporin expression in rats

Precise regulation of vas deferens fluid volume which is important for sperm survival might be influenced by testosterone. In order to investigate changes in vas deferens fluid volume and aquoporins (AQP) isoforms expression under testosterone influence, orchidectomized Sprague-Dawley rats were given 125 and 250 µg/kg/day testosterone with or without flutamide, an androgen receptor blocker or finasteride, a 5alpha-reductase inhibitor for seven consecutive days. Following treatment completion, vas deferens was perfused and changes in the fluid secretion rate and osmolality were determined in the presence of acetazolamide. Rats were then sacrificed and vas deferens was harvested for histology, tissue expression and distribution analyses of AQP-1, AQP-2, AQP-5, AQP-7 and AQP-9 proteins by Western blotting and immunohistochemistry, respectively. Our findings indicate that testosterone causes vas deferens fluid secretion rate to increase, which was antagonized by acetazolamide. Fluid osmolality increased following testosterone treatment and further increased when acetazolamide was given. Co-administration of flutamide or finasteride with testosterone causing both fluid secretion rate and osmolality to decrease. Histology revealed increased size of vas deferens lumen with increased thickness of vas deferens stroma. Expression of AQP-1, AQP-2 and AQP-9 were detected in vas deferens but not AQP-5 and AQP-7, and the levels of these proteins were increased by testosterone treatment mainly at the apical membrane of vas deferens epithelium. In conclusion, increased in vas deferens fluid secretion rate under testosterone influence mediated via the up-regulation of AQP-1, 2 and 9 might be important for vas deferens fluid homeostasis in order to ensure normal male fertility.

Expression and localization of aquaporins 3 and 7 in bull spermatozoa and their relevance to sperm motility after cryopreservation

Artificial insemination with cryopreserved semen is a well-developed technique commonly used for controlled reproduction in cattle. However, despite current technical advances, cryopreservation continues to damage bull spermatozoa, resulting in a loss of approximately 30 to 50% of viable spermatozoa post thawing. To further improve the efficiency of cryopreservation of bull spermatozoa, understanding the molecular mechanisms underlying the cryobiological properties that affect cryoinjuries during cryopreservation process of bull spermatozoa is required. In this study, we examined the expression and localization of aquaporin (AQP) 3 and AQP7 in fresh, cooled, and frozen-thawed bull spermatozoa. Furthermore, we investigated the relevance of AQP3 and AQP7 to motility and to membrane integrity in frozen-thawed bull spermatozoa. Western blotting against AQP3 and AQP7 in bull spermatozoa revealed bands with molecular weights of approximately 42 kDa and 53 kDa, respectively. In immunocytochemistry analyses, immunostaining of AQP3 was clearly observed in the principal piece of the sperm tail. Two immunostaining patterns were observed for AQP7 ―pattern 1: diffuse staining in head and entire tail, and pattern 2: diffuse staining in head and clear staining in mid-piece. Cooling and freeze-thawing did not affect the localization pattern of AQP7 and the relative abundances of AQP3 and AQP7 evaluated by Western blotting. Furthermore, we demonstrated that the relative abundances of AQP3 and AQP7 varied among ejaculates, and they were positively related to sperm motility, particularly sperm velocity, post freeze-thawing. Our findings suggest that AQP3 and AQP7 are possibly involved in the tolerance to freeze-thawing in bull spermatozoa, particularly in the sperm’s tail.

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.

Aquaporin 3 is regulated by estrogen in the chicken oviduct and is involved in progression of epithelial cell-derived ovarian carcinomas

Aquaporins (AQPs) are membrane proteins that passively deliver water across the plasma membrane to play an important role in maintaining cell shape. Members of the AQP family are distributed in most of the tissues in the human body and perform a variety of functions based on the water homeostasis suitable for each organ. However, there is little known about the expression and regulation of AQP family members in chickens. Therefore, we determined the expression of AQPs in various tissues of chickens. Among 13 isotypes, AQP3 was highly expressed in the chicken oviduct. Expression of AQP3 messenger RNA (mRNA) increased in the magnum (P < 0.001) and isthmus (P < 0.001) of chick oviducts treated with diethylstilbestrol. Consistent with these results, the localization of AQP3 was detected in the glandular and luminal epithelia of the magnum and isthmus of oviducts of diethylstilbestrol-treated chicks. In addition, the pattern of expression of AQP3 changed in an estrogen-dependent manner during the molting period. During the regenerative period of the oviduct after molting, expression of AQP3 mRNA increased coordinately with increasing concentrations of estradiol (P < 0.001), whereas expression of AQP3 mRNA decreased as concentrations of estradiol in plasma decreased in response to induced molting (P < 0.001). Also, expression of the AQP3 increased (P < 0.001) in cancerous ovaries of laying hens. In conclusion, AQP3 does not simply function to transport water into and out of cells but also appears to be closely involved in development of the chicken oviduct, which is regulated by estrogens. Furthermore, our results suggest AQP3 as a new diagnostic for early detection and treatment of epithelial cell-derived ovarian carcinomas.

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.

Gonadotropin-Activated Androgen-Dependent and Independent Pathways Regulate Aquaporin Expression during Teleost (Sparus aurata) Spermatogenesis

The mediation of fluid homeostasis by multiple classes of aquaporins has been suggested to be essential during spermatogenesis and spermiation. In the marine teleost gilthead seabream (Sparus aurata), seven distinct aquaporins, Aqp0a, -1aa, -1ab, -7, -8b, -9b and -10b, are differentially expressed in the somatic and germ cell lineages of the spermiating testis, but the endocrine regulation of these channels during germ cell development is unknown. In this study, we investigated the in vivo developmental expression of aquaporins in the seabream testis together with plasma androgen concentrations. We then examined the in vitro regulatory effects of recombinant piscine gonadotropins, follicle-stimulating (rFsh) and luteinizing (rLh) hormones, and sex steroids on aquaporin mRNA levels during the spermatogenic cycle. During the resting phase, when plasma levels of androgens were low, the testis exclusively contained proliferating spermatogonia expressing Aqp1ab, whereas Aqp10b and -9b were localized in Sertoli and Leydig cells, respectively. At the onset of spermatogenesis and during spermiation, the increase of androgen plasma levels correlated with the additional appearance of Aqp0a and -7 in Sertoli cells, Aqp0a in spermatogonia and spermatocytes, Aqp1ab, -7 and -10b from spermatogonia to spermatozoa, and Aqp1aa and -8b in spermatids and spermatozoa. Short-term in vitro incubation of testis explants indicated that most aquaporins in Sertoli cells and early germ cells were upregulated by rFsh and/or rLh through androgen-dependent pathways, although Aqp1ab in proliferating spermatogonia was also activated by estrogens. However, expression of Aqp9b in Leydig cells, and of Aqp1aa and -7 in spermatocytes and spermatids, was also directly stimulated by rLh. These results reveal a complex gonadotropic control of aquaporin expression during seabream germ cell development, apparently involving both androgen-dependent and independent pathways, which may assure the fine tuning of aquaporin-mediated fluid secretion and absorption mechanisms in the seabream testis.

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.

Coordinated Action of Aquaporins Regulates Sperm Motility in a Marine Teleost

In marine teleosts, such as the gilthead seabream, several aquaporin paralogs are known to be expressed during the hyperosmotic induction of spermatozoon motility in seawater. Here, we used immunological inhibition of channel function to investigate the physiological roles of Aqp1aa, Aqp1ab, and Aqp7 during seabream sperm activation. Double immunofluorescence microscopy of SW-activated sperm showed that Aqp1aa and Aqp7 were respectively distributed along the flagellum and the head, whereas Aqp1ab accumulated in the head and in discrete areas toward the anterior tail. Inhibition of Aqp1aa reduced the rise of intracellular Ca(2+), which is independent of external Ca(2+) and normally occurs upon activation, and strongly inhibited sperm motility. Impaired Aqp1aa function also prevented the intracellular trafficking of Aqp8b to the mitochondrion, where it acts as a peroxiporin allowing H2O2 efflux and ATP production during activation. However, restoring the Ca(2+) levels with a Ca(2+) ionophore in spermatozoa with immunosuppressed Aqp1aa function fully rescued mitochondrial Aqp8b accumulation and sperm motility. In contrast, exposure of sperm to Aqp1ab and Aqp7 antibodies did not affect motility during the initial phase of activation, but latently compromised the trajectory and the pattern of movement. These data reveal the coordinated action of spatially segregated aquaporins during sperm motility activation in a marine teleost, where flagellar-localized Aqp1aa plays a dual Ca(2+)-dependent role controlling the initiation of sperm motility and the activation of mitochondrial detoxification mechanisms, while Aqp1ab and Aqp7 in the head and anterior tail direct the motion pattern.

Mitochondrial aquaporin-8-mediated hydrogen peroxide transport is essential for teleost spermatozoon motility

Reactive oxygen species (ROS), particularly hydrogen peroxide (H₂O₂), cause oxidative cell damage and inhibit sperm function. In most oviparous fishes that spawn in seawater (SW), spermatozoa may be exposed to harmful ROS loads associated with the hyperosmotic stress of axonemal activation and ATP synthesis from mitochondrial oxidative phosphorylation. However, it is not known how marine spermatozoa can cope with the increased ROS levels to maintain flagellar motility. Here, we show that a marine teleost orthologue of human aquaporin-8, termed Aqp8b, is rapidly phosphorylated and inserted into the inner mitochondrial membrane of SW-activated spermatozoa, where it facilitates H₂O₂ efflux from this compartment. When Aqp8b intracellular trafficking and mitochondrial channel activity are immunologically blocked in activated spermatozoa, ROS levels accumulate in the mitochondria leading to mitochondrial membrane depolarisation, the reduction of ATP production, and the progressive arrest of sperm motility. However, the decreased sperm vitality underlying Aqp8b loss of function is fully reversed in the presence of a mitochondria-targeted antioxidant. These findings reveal a previously unknown detoxification mechanism in spermatozoa under hypertonic conditions, whereby mitochondrial Aqp8b-mediated H₂O₂ efflux permits fuel production and the maintenance of flagellar motility.

Tolerance of spermatozoa to hypotonic stress: role of membrane fluidity and correlation with cryosurvival

The aim of this study was to evaluate inter-individual variability in osmotic properties of stallion spermatozoa and its correlation with cryosurvival. In addition, temperature dependency of hypo-osmotic tolerance and membrane fluidity were studied. Stallion sperm membranes exhibited good resistance towards hypotonic stress in the 15–30°C temperature range, whereas membrane stability was found to be decreased at 4 and 37°C. Bull spermatozoa showed greater hypo-osmotic tolerance compared with stallion spermatozoa, especially at temperatures above 30°C, which coincided with decreased membrane fluidity of bovine spermatozoa in this temperature range. The critical osmolality at 22°C, at which half of the sperm population survived exposure to hypotonic saline solution, was found to vary between 55 and 170 mOsm kg–1 among different stallions. Clear correlations were found for pre- versus post-freeze sperm motility and membrane integrity. Pre-freeze percentages of membrane-intact spermatozoa after exposure to hypotonic stress showed a weak correlation with sperm motility after cryopreservation. This correlation, however, was not found when data were corrected for initial numbers of membrane-intact spermatozoa in the sample. We thus conclude that studies on pre-freeze tolerance towards hypotonic stress cannot be used to predict sperm cryosurvival rates for individual stallions.

Bi-functionality of Opisthorchis viverrini aquaporins

Aquaporins (AQP) are essential mediators of water regulation in all living organisms and members of the major intrinsic protein (MIP) superfamily of integral membrane proteins. They are potential vehicles or targets for chemotherapy, e.g. in Trypanosoma brucei melarsoprol and pentamidine uptake is facilitated by TbAQP-2. Transcriptome data suggests that there are at least three active aquaporins in the human liver fluke, Opisthorchis viverrini, OvAQP-1, 2 and 3, and crude RNA silencing of OvAQP-1 and 2 has recently been shown to affect parasite swelling in destilled water. In the present work we demonstrate that OvAQP-3 is a major water-conducting channel of the parasite, that it can be detected from the newly excysted juvenile to the adult stage and that it is present in major tissues of the parasite. Furthermore, a comparative functional characterization of the three parasite AQPs was performed by using Xenopus oocyte swelling and yeast phenotypic assays. OvAQP-1, OvAQP-2, and OvAQP-3 were found to conduct water and glycerol while only the latter two were also able to conduct urea. In addition, all OvAQPs were found to transport ammonia and methylamine. Our findings demonstrate that the sequence-based classification into orthodox aquaporins and glycerol-conducting aquaglyceroporins is not functionally conserved in the parasite and implicate a broder range of functions for these channels.

Aquaporin-11 control of testicular fertility markers in Syrian hamsters

The present study sought novel changes to the hamster testicular transcriptome during modulation of fertility by well-characterized photoperiodic stimuli. Transition from long days (LD, 14 h light/day) to short days (SD, 10h light/day) triggered testicular regression (61% reduction of testis weight, relative to LD) in SD-sensitive (SD-S) hamsters within 16 weeks. After 22 weeks of SD exposure, a third cohort of hamsters became SD-refractory (SD-R), and exhibited testicular recrudescence (137% testis weight gain, relative to SD-S). Partial interrogation of the testicular transcriptome by annealing-control-primer-modified differential display PCR provided several candidates for regulation of testicular functions. Multiple linear regression modeling indicated the best correlation for aquaporin 11 (Aqp11) with changes in testis weight. Correlations were also strongest for Aqp11 with expression levels of reference cDNAs that control spermatogenesis (Hspa2 and Tnp2), steroidogenesis (Cox2, 3βHsd, and Srebp2), sperm motility (Catsper1, Pgk2, and Tnp2), inflammation (Cox2), and apoptosis (Bax and Bcl2). Moreover, siRNA-mediated knockdown of testicular Aqp11 mRNA and protein reduced Hspa2 and Tnp2 mRNA levels, and it increased 3βHsd mRNA levels. It also reduced mRNA levels for Sept12, which is a testis-specific inducer of spermatogenesis. These results suggest a central role for testicular Aqp11 signaling in the coordinate regulation of crucial components of fertility.

Isolation and mRNA expression analysis of aquaporin isoforms in marine medaka Oryzias dancena, a euryhaline teleost

We have identified six putative aquaporin (AQP) genes from marine medaka Oryzias dancena (named odAQPs 1, 3, 8, 10, 11 and 12). The marine medaka AQP cDNAs encode polypeptides of 259-298 amino acids, respectively. Topology predictions showed six transmembrane domains, five connecting loops, and cytoplasmic N- and C-terminal domains, all of which is conserved among AQP molecules. Although asparagine-proline-alanine (NPA) motifs are highly conserved in most odAQP isoforms, several AQPs revealed variant types of motifs such as asparagine-proline-proline (NPP), asparagine-proline-valine (NPV) or/and asparagine-proline-serine (NPS) motifs. The phylogenic analysis showed that marine medaka AQPs had closet relationship with Japanese ricefish (medaka; Oryzias latipes) counterparts. Reverse transcription (RT)-PCR analyses showed that marine medaka AQP transcripts would be expressed in not only osmoregulatory tissues but also nonosmoregulatory tissues, and also that the expression levels of certain AQP isoforms in nonosmoregulatory tissues were readily comparable or even higher than those in typically known osmoregulatory organs. Although the overall tissue distribution patterns of AQPs were not significantly different between 0- and 30-ppt acclimated fish, the expression levels under different salinities were largely variable among isoforms and tissues. This is the first report to investigate tissue expression profiles of teleostean AQPs 11 and 12 during the long-term acclimation to freshwater and salted water.

Human aquaporins: regulators of transcellular water flow

BACKGROUND

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

SCOPE OF REVIEW

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

MAJOR CONCLUSIONS

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

GENERAL SIGNIFICANCE

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.

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.

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.

Expression of Aquaglyceroporins in Epithelial Ovarian Tumours and Their Clinical Significance

Protein levels of aquaglyceroporins AQP3, AQP7 and AQP9 were measured by immunohistochemistry and Western blotting in epithelial ovarian tumour tissue from 98 patients and in normal ovary tissue from 20 persons with uterine myoma. AQP3 and AQP9 proteins were detected immunohistochemically in the basolateral membranes of benign and borderline tumour cells and were found to be distributed throughout the plasma membranes of malignant tumour cells. AQP7 protein was localized in the plasma membranes of benign tumour cells but, in borderline and malignant tumour cells, it was selectively stained in the nuclear membrane. Western blotting showed significantly higher AQP7 and AQP9 protein expression in malignant and borderline tumours than in benign tumour and normal ovarian tissue. AQP9 expression level was positively and significantly correlated with tumour grade and histological type. It was concluded that a high level of aquaglyceroporin expression may be an important factor in ovarian carcinogenesis.

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.

Spermiation: The process of sperm release

Spermiation is the process by which mature spermatids are released from Sertoli cells into the seminiferous tubule lumen prior to their passage to the epididymis. It takes place over several days at the apical edge of the seminiferous epithelium, and involves several discrete steps including remodelling of the spermatid head and cytoplasm, removal of specialized adhesion structures and the final disengagement of the spermatid from the Sertoli cell. Spermiation is accomplished by the co-ordinated interactions of various structures, cellular processes and adhesion complexes which make up the "spermiation machinery". This review addresses the morphological, ultrastructural and functional aspects of mammalian spermiation. The molecular composition of the spermiation machinery, its dynamic changes and regulatory factors are examined. The causes of spermiation failure and their impact on sperm morphology and function are assessed in an effort to understand how this process may contribute to sperm count suppression during contraception and to phenotypes of male infertility.