Expression and localization of aquaporin 1b during oocyte development in the Japanese eel (Anguilla japonica)

Neurohypophysial and paracrine vasopressinergic signaling regulates aquaporin trafficking to hydrate marine teleost oocytes

The dual aquaporin (Aqp1ab1/Aqp1ab2)-mediated hydration of marine teleost eggs, which occurs during oocyte meiosis resumption (maturation), is considered a key adaptation underpinning their evolutionary success in the oceans. However, the endocrine signals controlling this mechanism are almost unknown. Here, we investigated whether the nonapeptides arginine vasopressin (Avp, formerly vasotocin) and oxytocin (Oxt, formerly isotocin) are involved in marine teleost oocyte hydration using the gilthead seabream (Sparus aurata) as a model. We show that concomitant with an increased systemic production of Avp and Oxt, the nonapeptides are also produced and accumulated locally in the ovarian follicles during oocyte maturation and hydration. Functional characterization of representative Avp and Oxt receptor subtypes indicates that Avpr1aa and Oxtrb, expressed in the postvitellogenic oocyte, activate phospholipase C and protein kinase C pathways, while Avpr2aa, which is highly expressed in the oocyte and in the follicular theca and granulosa cells, activates the cAMP-protein kinase A (PKA) cascade. Using ex vivo, in vitro and mutagenesis approaches, we determined that Avpr2aa plays a major role in the PKA-mediated phosphorylation of the aquaporin subdomains driving membrane insertion of Aqp1ab2 in the theca and granulosa cells, and of Aqp1ab1 and Aqp1ab2 in the distal and proximal regions of the oocyte microvilli, respectively. The data further indicate that luteinizing hormone, which surges during oocyte maturation, induces the synthesis of Avp in the granulosa cells via progestin production and the nuclear progestin receptor. Collectively, our data suggest that both the neurohypophysial and paracrine vasopressinergic systems integrate to differentially regulate the trafficking of the Aqp1ab-type paralogs via a common Avp-Avpr2aa-PKA pathway to avoid competitive occupancy of the same plasma membrane space and maximize water influx during oocyte hydration.

The induction of oocyte maturation and ovulation in the European eel (Anguilla anguilla): in vitro and in vivo comparison of progesterone with 17α,20β-dihydroxy-4-pregnen-3-one

Ovulation in European eel is induced by injection of 17α,20β-dihydroxy-4-pregnen-3-one (DHP) as the maturation-inducing hormone (MIH). Female eels need to ovulate within 18 h after injection to release good quality eggs. Progesterone (P), as an upstream precursor of DHP, may promote endogenous DHP production and improve egg quality. The purpose of this study was therefore to compare treatment of P with DHP on batch level, in vitro, to determine dose-response effects, and in vivo, at a single dose. For the in vitro experiment, ovarian tissue was extracted and placed in culture plates containing hormone-free medium and media supplemented with the treatment: DHP at 1, 10 and 100 ng mL-1, or P at 10, 100 and 1,000 ng mL-1. At the start of incubation, the folliculated oocytes were sampled for histology, microscopy and qPCR. After incubation for 12 and 18 h, the oocytes were sampled for microscopy and qPCR analysis. For the in vivo experiment, females were either injected with DHP or P at a dose of 2 mg kg-1 to assess their effects on ovulation and reproductive success. At the moment of release, eggs were sampled for RNA sequencing to compare effects of DHP and P on the expression of genes involved in egg quality aspects. Remaining eggs were fertilized and larval viability was recorded. Both DHP and P were able to induce GVBD (DHP at 10 and 100 ng mL-1, P at 100 and 1,000 ng mL-1) in vitro. Expression of genes involved in oocyte maturation and ovulation was similar in vitro for both DHP and P treatments. Regarding the in vivo results, RNAseq results reflected similar DHP and P effects on the expression of genes involved in egg quality aspects. Females injected with either DHP or P ovulated, released eggs, and were equally able to produce larvae without any differences in reproductive success. Our results support the conclusion that DHP and P work equally well in vitro and in vivo. P is more attractive to apply as the price is 3,000 times lower than the price of DHP.

Aquaporin expression and cholesterol content in eel swimbladder tissue

Leakiness of the swimbladder wall of teleost fishes must be prevented to avoid diffusional loss of gases out of the swimbladder. Guanine incrustation as well as high concentrations of cholesterol in swimbladder membranes in midwater and deep-sea fish has been connected to a reduced gas permeability of the swimbladder wall. On the contrary, the swimbladder is filled by diffusion of gases, mainly oxygen and CO₂ , from the blood and the gas gland cells into the swimbladder lumen. In swimbladder tissue of the zebrafish and the Japanese eel, aquaporin mRNA has been detected, and the aquaporin protein has been considered important for the diffusion of water, which may accidentally be gulped by physostome fish when taking an air breath. In the present study, the expression of two aquaporin 1 genes (Aqp1aa and Aqp1ab) in the swimbladder tissue of the European eel, a functional physoclist fish, was assessed using immunohistochemistry, and the expression of both genes was detected in endothelial cells of swimbladder capillaries as well as in basolateral membranes of gas gland cells. In addition, Aqp1ab was present in apical membranes of swimbladder gas gland cells. The authors also found high concentrations of cholesterol in these membranes, which were several fold higher than in muscle tissue membranes. In yellow eels the cholesterol concentration exceeded the concentration detected in silver eel swimbladder membranes. The authors suggest that aquaporin 1 in swimbladder gas gland cells and endothelial cells facilitates CO₂ diffusion into the blood, enhancing the switch-on of the Root effect, which is essential for the secretion of oxygen into the swimbladder. It may also facilitate CO₂ diffusion into the swimbladder lumen along the partial gradient established by CO₂ production in gas gland cells. Cholesterol has been shown to reduce the gas permeability of membranes and thus could contribute to the gas tightness of swimbladder membranes, which is essential to avoid diffusional loss of gas out of the swimbladder.

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.

Dynamic transcriptome analysis of ovarian follicles in artificial maturing Japanese eel (Anguilla japonica)

Inducing maturation of the ovaries to enable the production of good-quality eggs is critical for the successful artificial breeding of Anguilla japonica. During the spawning season, however, the ovaries of A. japonica have been found to develop into asynchronous clutches, impeding the success of artificial breeding on a commercial scale. The dynamic molecular regulation of follicular development in the same individual was assessed by transcriptome analysis of the five stages of follicles, the pre-vitellogenic, early vitellogenic, midvitellogenic, late vitellogenic, and migratory nucleus stages in artificial maturing A. japonica. Comparisons across these developmental stages identified a total of 19,298 differentially expressed transcripts (DETs). Short time-series expression miner analysis across these DETs revealed four significant expression profiles. Gene Ontology function and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses found that some of the significantly enriched biological processes and metabolic pathways included those related to steroid hormone biosynthesis (cyp11a1, cyp17a1, cyp17a2, hsd17b1, and hsd17b12), cargo receptor activity (vtgr and vldlr), meiosis and ovulation (pgrs and mPRγ), hydration (cts and aqp1), and egg coat formation (zp). These genes and pathways were associated with serum 17β-estradiol concentrations and morphological changes. The levels of hsd17b12 and mPRγ mRNAs were much higher during the migratory nucleus stage, suggesting their respective involvement in the biosynthesis and functional pathway of the maturation-inducing steroid 17α,20β-dihydroxy-4-pregnen-3-one. The gene subtypes aqp1b and ctsd may regulate water influx into oocytes and yolk protein proteolysis, respectively. To our knowledge, the present study is the first to describe combined transcriptome profiling of asynchronously developing follicles in the same individual. The findings suggest that steroid hormone synthesis and nutrient absorption in follicular somatic cells play important roles during follicular development and maturation, despite the same external physiological surroundings.

Insights on the seasonal variations of reproductive features in the Eastern Atlantic Bluefin Tuna

The Atlantic Bluefin Tuna (ABFT, Thunnus thynnus) is one of the most intensely exploited fisheries resources in the world. In spite of the years of studies on ABFT, basic aspects of its reproductive biology remain uncertain. To gain insight regarding the seasonal changes of the reproductive characteristics of the eastern stock of ABFT, blood and tissue samples were collected from mature specimens caught in the Mediterranean basin during the reproductive (May-June) and non-reproductive season (Oct-Nov). Histological analysis of the gonads of May-June samples indicated that there were females which were actively spawning (contained post-ovulatory follicles) and females that were not actively spawning that had previtellogenic and fully vitellogenic oocytes. In males, testis were at early or late stage of spermatogenesis during the reproductive season. In Oct-Nov, ovaries contained mostly previtellogenic oocytes as well as β and α atretic follicles while the testis predominantly contained spermatogonia and few cysts with spermatocytes and spermatozoa. Gonadosomatic index (GSI) in females was highest among the actively spawning individuals while in males GSI was higher in early and late spermatogenic individuals compared to those that were spent. Plasma sex steroids levels varied with the reproductive season. In females, estradiol (E₂), was higher in May-June while testosterone (T) and progesterone (P) did not vary. In males, E₂ and T were higher in May-June while P levels were similar at the two sampling points. Circulating follicle stimulating hormone (FSH) was higher in Oct-Nov than in May-June both in males and females. Vitellogenin (VTG) was detected in plasma from both males and females during the reproductive season with levels in females significantly higher than in males. VTG was undetected in Oct-Nov samples. Since choriogenesis is an important event during follicle growth, the expression of three genes involved in vitelline envelope formation and hardening was measured and results showed significantly higher levels in ovaries in fish caught in May-June with respect to those sampled in Oct-Nov. In addition, a set of genes encoding for ion channels that are responsible for oocyte hydration and buoyancy, as well as sperm viability, were characterized at the two time points, and these were found to be more highly expressed in females during the reproductive season. Finally, the expression level of three mRNAs encoding for different lipid-binding proteins was analyzed with significantly higher levels detected in males, suggesting sex-specific expression. Our findings provide additional information on the reproductive biology of ABFT, particularly on biomarkers for the assessment of the state of maturation of the gonad, highlighting gender-specific signals and seasonal differences.

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.

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.

Identification of aquaporins in eggs and early embryogenesis of the sea urchin Paracentrotus lividus

Sea urchins are echinoderms, marine invertebrates found at the base of the deutorostome lineage, which show separate sexes and are external spawners. In the sea urchin, efficient regulation of water homeostasis is essential for many biological processes such as cellular respiration, normal fertilization and correct embryo growth. In order to clarify some of these processes, the present study reports on the identification and function of aquaporin proteins in the sea urchin. Our results show, by immunoblot, immunoelectron microscopy and immunofluorescence analysis, the presence of aquaporin1- and aquaporin3-like proteins in virgin eggs and in early embryogenesis of Paracentrotus lividus and, by using known inhibitors of aquaporin functions, the functional and relevant role of aquaporin-3 in the fertilization process. AQP3 in particular seems to play a crucial role in high velocity water flux formations involved in the detachment of the vitelline layer during the slow block of polyspermy, while the presence of AQP1 and the increase of AQP3 in the first phase of the P. lividus developmental cycle, suggest their involvement in the appropriate homeostasis for embryo development.

Primary oocyte transcriptional activation of aqp1ab by the nuclear progestin receptor determines the pelagic egg phenotype of marine teleosts

In marine teleosts, the aqp1ab water channel plays a vital role in the development of the pelagic egg phenotype. However, the developmental control of aqp1ab activation during oogenesis remains to be established. Here, we report the isolation of the 5'-flanking region of the teleost gilthead seabream aqp1ab gene, in which we identify conserved cis-regulatory elements for the binding of the nuclear progestin receptor (Pgr) and members of the Sox family of transcription factors. Subcellular localization studies indicated that the Pgr, as well as sox3 and -8b transcripts, are co-expressed in seabream oogonia, whereas in meiosis-arrested primary growth (pre-vitellogenic) oocytes, when aqp1ab mRNA and protein are first synthesized, the Pgr appears to be completely translocated from the ooplasm into the nucleus. By contrast, sox9b is highly expressed in more advanced oocytes, coinciding with a strong depletion of aqp1ab transcripts in the oocyte. Functional characterization of wild-type and mutated aqp1ab promoter constructs, using mammalian cells and Xenopus laevis oocytes, demonstrated that aqp1ab transcription is initiated by the Pgr, which is activated by the progestin 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), the natural ligand of the seabream Pgr. In vitro incubation of seabream primary ovarian explants with the follicle-stimulating hormone or 17,20β-P confirmed that progestin-activated Pgr enhanced Aqp1ab synthesis via the aqp1ab promoter. However, transactivation assays in heterologous systems showed that Sox transcription factors can potentially modulate this mechanism. These data uncover the existence of an endocrine pathway involved in the early activation of a water channel necessary for egg formation in marine teleosts.

Mechanism of oocyte maturation and ovulation and its application to seed production in the Japanese eel

Reduction in eel resources and catches of glass eels as seedlings for aquaculture have been a serious concern in recent years in both Europe and East Asia. Thus, technical advancement to produce eel seeds for artificial cultivation is most desired. Fundamental information on oocyte maturation and ovulation and its application to artificial induction of sexual maturation are needed to produce good quality seeds of the Japanese eel. This review introduces hormonal mechanisms of cytoplasmic maturation (such as hydration, lipid coalescence, and clearing of the ooplasm) and the maturational competence (the ability to respond to maturation-inducing steroid) and nuclear maturation (germinal vesicle breakdown). In addition, previous and newly developed methods for induction of spawning have been described.

Water homeostasis in the fish oocyte: new insights into the role and molecular regulation of a teleost-specific aquaporin

The discovery of the role of a teleost-specific aquaporin (Aqp1ab) during the process of oocyte hydration in marine fish producing pelagic (floating) eggs, recently confirmed by molecular approaches, has revealed that this mechanism is more sophisticated than initially thought. Recent phylogenetic and genomic studies suggest that Aqp1ab likely evolved by tandem duplication from a common ancestor and further neofunctionalized in oocytes for water transport. Investigations into the regulation of Aqp1ab during oogenesis indicate that the mRNA and protein product are highly accumulated during early oocyte growth, possibly through the transcriptional activation of the aqp1ab promoter by the classical nuclear progesterone receptor and perhaps by Sry-related high mobility group [HMG]-box (Sox) transcription factors. During oocyte growth and maturation, Aqp1ab intracellular trafficking may be regulated by phosphorylation and/or dephosphorylation of specific C-terminal residues in Aqp1ab, as well as by signal-mediated sorting processes. These mechanisms possibly regulate the temporal insertion of Aqp1ab into the oocyte plasma membrane during oocyte hydration, although the intracellular signaling pathways involved are yet unknown. Interestingly, in some freshwater species that spawn partially hydrated eggs, high accumulation of transcripts encoding functional Aqp1ab channels have also been found in the ovary. These findings suggest that the Aqp1ab-mediated mechanism for oocyte hydration is likely conserved in teleosts. The tight regulation of Aqp1ab during oogenesis, at both the transcriptional and posttranslational levels, highlights the essential physiological role of this water channel and opens new research avenues for understanding the molecular basis of egg formation in fish.

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.