Cell Specificity of Aquaporins 0, 3, and 10 Expressed in the Testis, Efferent Ducts, and Epididymis of Adult Rats

A Step Forward in Understanding the Expression of Classical Aquaporins in the Male Reproductive Tract: Study Findings in Cattle (Bos taurus)

Aquaporins (AQPs), also known as water channels, appear to be particularly promising in maintaining male reproductive potential. Therefore, this study aimed to determine the presence of classical AQPs in the bovine (Bos taurus) reproductive system and analyze changes in their expression with age using immunohistochemistry and Western blotting. Of the six classical AQPs, AQP0, AQP1, AQP4, AQP5 and AQP6 were detected, while AQP2 was absent. In the testis, AQP0 was visible in Leydig cells in selected animals, while AQP1 was found in myoid cells surrounding the seminiferous tubules of mature individuals. This characteristic expression patterns of AQP0, limited only to certain bulls, is difficult to explain unequivocally. It is possible that AQP0 expression in cattle is subject to individual variability or changes in response to specific physiological conditions. In the caput and corpus epididymis, AQP0 showed weak expression in epithelial cells of immature animals and stronger expression in basal and principal cells of reproductive bulls. In all animals, AQP1 was present on the apical surface of epithelial cells in the initial segment of the caput epididymis. AQP4, AQP5 and AQP6 were identified in principal and basal cells along the entire epididymis of reproductive bulls. The abundance of AQP4 and AQP6 increased from the caput to the cauda epididymis with the growth and development of the animals. In all males, AQP4, AQP5 and AQP6 were observed in epithelial cells of the vas deferens, and their expression in this section increased with age. In conclusion, the abundance and distribution of the classical AQPs in various cell types and parts of the male reproductive system indicate their crucial role in maintaining water homeostasis, which is essential for normal reproductive function in cattle.

Distinct actions of testicular endocrine and lumicrine signaling on the proximal epididymal transcriptome

The epididymal function and gene expression in mammals are under the control of the testis. Sex steroids are secreted from the testis and act on the epididymis in an endocrine manner. There is another, non-sex steroidal secreted signaling, named lumicrine signaling, in which testis-derived secreted proteins go through the male reproductive tract and act on the epididymis. The effects of such multiple regulations on the epididymis by the testis have been investigated for many genes. The recent development of high-throughput next-generation sequencing now enables us a further comparative survey of endocrine and lumicrine action-dependent gene expression. In the present study, testis-derived endocrine and lumicrine actions on epididymal gene expression were comparatively investigated by RNA-seq transcriptomic analyses. This investigation utilized experimental animal models in which testis-derived endocrine and/or lumicrine actions were interfered with, such as unilateral or bilateral orchidectomy. By bilateral orchidectomy, which interferes with both endocrine and lumicrine actions, 431 genes were downregulated. By unilateral orchidectomy, which also interferes with endocrine and lumicrine actions by the unilateral testis, but the endocrine action was compensated by the contralateral testis, 283 genes were downregulated. The content of such genes downregulated by unilateral orchidectomy was like those of lumicrine action-interfered efferent duct-ligation, W/Wv, and Nell2-/- mice. When genes affected by unilateral and bilateral orchidectomy were compared, 154 genes were commonly downregulated, whereas 217 genes were specifically downregulated only by bilateral orchidectomy, indicating the distinction between endocrine and lumicrine actions on the proximal epididymal transcriptome. Comparative transcriptome analyses also showed that the expressions of genes emerging since Amniota were notably impacted by bilateral orchidectomy, unilateral orchidectomy, and lumicrine action-interfering treatments; the degree of influence from these treatments varied based on the evolutionary stage beyond Amniota. These findings unveil an evolutional transition of regulated gene expression in the proximal epididymis by two different testis-derived signaling mechanisms.

Busulfan administration replicated the characteristics of the epididymal initial segment observed in mice lacking testis-epididymis lumicrine signaling

The physiological functions of the mammalian epididymis are typically regulated by the testes. In addition to sex steroids secreted by testicular Leydig cells, which act on the epididymis in an endocrine manner, there is a non-sex-steroidal signaling pathway known as the lumicrine pathway. This lumicrine signaling pathway involves ligand proteins secreted from germ cells within the testicular seminiferous tubules traversing the male reproductive tract, which induce epithelial differentiation in the epididymis. These findings prompted an inquiry into whether treatments influencing testis physiology can disrupt epididymal function by interfering with testis-epididymis communication. Busulfan, an alkylating agent commonly used to deplete testicular germ cells in reproductive biology, has not been sufficiently explored because of its effects on the epididymis. This study investigated the effects of busulfan administration on the proximal epididymis using histological and transcriptomic analyses. Notably, busulfan, as opposed to the vehicle dimethyl sulfoxide (DMSO), altered the morphology of the initial segment of the epididymis, leading to a reduction in the cell height of the luminal epithelium. RNA sequencing identified 185 significantly downregulated genes in the proximal epididymis of busulfan-administered mice compared to DMSO-administered mice. Comparative transcriptome analyses revealed similarities between the epididymal transcriptome of busulfan-administered mice and lumicrine-deficient mice, such as efferent-duct-ligated W/Wv and Nell2-/- mice. However, this differed from that of bilaterally orchidectomized mice, in which both the endocrine and lumicrine signaling pathways were simultaneously ablated. Collectively, these results suggested that the harmful effects of busulfan on the proximal epididymis are secondary consequences of the ablation of testis-epididymis lumicrine signaling.

Cellular Distribution of Aquaporin 3, 7 and 9 in the Male Reproductive System: A Lesson from Bovine Study (Bos taurus)

The increasing incidence of male infertility in humans and animals creates the need to search for new factors that significantly affect the course of reproductive processes. Therefore, the aim of this study was to determine the temporospatial expression of aquaglyceroporins (AQP3, AQP7 and AQP9) in the bovine (Bos taurus) reproductive system using immunohistochemistry and Western blotting. The study also included morphological analysis and identification of GATA-4. In brief, in immature individuals, AQP3 and AQP7 were found in gonocytes. In reproductive bulls, AQP3 was observed in spermatocytes and spermatogonia, while AQP7 was visible in all germ cells and the Sertoli cells. AQP7 and AQP9 were detected in the Leydig cells. Along the entire epididymis of reproductive bulls, aquaglyceroporins were visible, among others, in basal cells (AQP3 and AQP7), in epididymal sperm (AQP7) and in the stereocilia of the principal cells (AQP9). In males of all ages, aquaglyceroporins were identified in the principal and basal cells of the vas deferens. An increase in the expression of AQP3 in the testis and cauda epididymis and a decrease in the abundance of AQP7 in the vas deferens with age were found. In conclusion, age-related changes in the expression and/or distribution patterns of AQP3, AQP7 and AQP9 indicate the involvement of these proteins in the normal development and course of male reproductive processes in cattle.

Immunolocalisation of aquaporins 3, 7, 9 and 10 in the epididymis of three wild ruminant species (Iberian ibex, mouflon and chamois) and sperm cryoresistance

CONTEXT

In the epididymis, epithelial cells manage changes in the luminal environment for proper sperm maturation. Moreover, aquaglyceroporins, a subgroup of aquaporins (AQP), modulate the transport of water, glycerol and other small molecules in epithelial cells.

AIMS

We aim to characterise the lining epithelium, quantify its cell composition and immunolocalise the aquaglyceroporins AQP3, AQP7, AQP9 and AQP10 alongside the epididymal ductus of three wild ruminant species, and to determine if species-specific differences could be associated with cauda sperm cryoresistance variations.

METHODS

Epididymides from Iberian ibex (n =5), mouflon (n =5) and chamois (n =6) were obtained. Cauda spermatozoa were collected and sperm parameters were analysed before and after freezing. Histology and immunohistochemistry of AQP3, 7, 9, 10 and T-CD3 were performed in the caput, corpus and cauda epididymal regions.

KEY RESULTS

This work first describes the lining epithelium in Iberian ibex, mouflon and chamois epididymis along the three anatomical regions, consisting of principal, basal, apical, clear and halo cells. However, the percentage of each cell type differed in ibex compared to mouflon and chamois. The positive T-CD3 immunolabeling of all the halo cells confirmed their T-lymphocyte nature. Aquaglyceroporin expression patterns were similar among species, except for differences in AQP7 and AQP10 immunolocalisation in ibex. Species-specific differences in epididymal sperm cryoresistance were confirmed.

CONCLUSIONS

The epididymal epithelium of the three wild ruminants differ in their relative number of cell types and AQP immunolocalisation, which ultimately appears to affect cauda epidydimal spermatozoa cryoresistance.

IMPLICATIONS

Our study provides information on the relevance of the quantitative composition and AQP pattern expression in epididymal lining epithelium on sperm cryoresistance.

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.

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Aquaglyceroporin Modulators as Emergent Pharmacological Molecules for Human Diseases

Aquaglyceroporins, a sub-class of aquaporins that facilitate the diffusion of water, glycerol and other small uncharged solutes across cell membranes, have been recognized for their important role in human physiology and their involvement in multiple disorders, mostly related to disturbed energy homeostasis. Aquaglyceroporins dysfunction in a variety of pathological conditions highlighted their targeting as novel therapeutic strategies, boosting the search for potent and selective modulators with pharmacological properties. The identification of selective inhibitors with potential clinical applications has been challenging, relying on accurate assays to measure membrane glycerol permeability and validate effective functional blockers. Additionally, biologicals such as hormones and natural compounds have been revealed as alternative strategies to modulate aquaglyceroporins via their gene and protein expression. This review summarizes the current knowledge of aquaglyceroporins’ involvement in several pathologies and the experimental approaches used to evaluate glycerol permeability and aquaglyceroporin modulation. In addition, we provide an update on aquaglyceroporins modulators reported to impact disease, unveiling aquaglyceroporin pharmacological targeting as a promising approach for innovative therapeutics.

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.

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.

Differential abundances of AQP3 and AQP5 in reproductive tissues from dogs with and without cryptorchidism

Aquaporins (AQPs) are integral transmembrane proteins facilitating transport of water and small solutes, such as glycerol and urea, between cells. In male reproductive tracts, AQPs maintain a milieu conducive for sperm formation, maturation, and storage. The aim of this study was to clarify effects of testicular and epidydimal function on male fertility by investigating localisation and abundances of AQP3 and AQP5 in testes and epididymal segments from dogs with and without unilateral cryptorchidism. Immunohistochemistry results indicated AQP3 and AQP5 have different distribution patterns in reproductive tissues of dogs with and without unilateral cryptorchidism. The AQP3, an aquaglyceroprotein, is present in different germ and Sertoli cells in testis of dogs without cryptorchidism. The AQP5 protein was not detected in germ cells but was present in Sertoli and Leydig cells and in endothelia of blood vessels. In cryptorchid dogs, AQP3 was detected in early-developing germ and Sertoli cells, and AQP5 had a distribution pattern similar to testes of dogs without cryptorchidism. In the epididymis, AQP3 and AQP5 were localised in epithelial cells of dogs with and without cryptorchidism in a cell-specific manner. The AQP3 and AQP5 protein was in larger abundance in the gonads from dogs with and without cryptorchidism. In contrast, AQP3 and AQP5 abundance increased in each segment of the cryptorchid epididymis, likely as a compensatory mechanism associated with the pathologic condition. These results indicate involvement of AQP3 and AQP5 in spermatogenesis and sperm maturation. Results from the present study indicate dogs are a useful for comparative reproductive biology studies.

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.

Epithelial dynamics in the epididymis: role in the maturation, protection, and storage of spermatozoa

Epithelial cells line the lumen of tubular organs and are key players in their respective functions. They establish a unique luminal environment by providing a protective barrier and by performing vectorial transport of ions, nutrients, solutes, proteins, and water. Complex intercellular communication networks, specific for each organ, ensure their interaction with adjacent epithelial and non-epithelial cells, allowing them to respond to and modulate their immediate environment. In the epididymis, several epithelial cell types work in a concerted manner to establish a luminal acidic milieu that is essential for the post-testicular maturation and storage of spermatozoa. The epididymis also prevents autoimmune responses against auto-antigenic spermatozoa, while ensuring protection against ascending and blood pathogens. This is achieved by a network of immune cells that are in close contact and interact with epithelial cells. This review highlights the coordinated interactions between spermatozoa, basal cells, principal cells, narrow cells, clear cells, and immune cells that contribute to the maturation, protection, selection, and storage of spermatozoa in the lumen of the epididymis.

Cellular distribution of aquaporins in testes of normal and cryptorchid dogs: A preliminary study on dynamic roles

Fluid regulation within the male gonad is an important process for promoting sperm differentiation and maturation. Aquaporins (AQPs) are a family of thirteen integral membrane proteins involved in these processes. The expression of several genes of AQPs occurs in the male reproductive tract of humans and other animal species, although there are few studies on domestic animals. In this study, the localization of AQP7, AQP8, and AQP9 as well as the abundances of protein and mRNA transcripts were examined in normal and cryptorchid dog testes. There was immunohistochemical localization of AQP7, AQP8, and AQP9 in both the tubular and interstitial compartments of the normal and retained testes and crytorchid dogs, albeit there was an obvious difference in cellular localization with the testes from the cryptorchid dogs. These results were supported by western blotting and real-time RT-PCR analyses, there was a lesser AQP7 and greater AQP9 abundance of protein and mRNA transcripts in the cryptorchid testis. These findings indicate combined testicular functions of AQPs in cell volume regulation. In addition, with the cryptorchid condition characterized there was a different cellular distribution of AQPs supporting the thought that early detection is important for controlling possible side effects of cyptorchidism, such as pre-neoplastic and carcinogenic outcomes.

Estrogen Modulates Glycerol Permeability in Sertoli Cells through Downregulation of Aquaporin-9

High 17β-Estradiol (E2) levels are known to cause alterations of spermatogenesis and environments throughout the male reproductive tract. Sertoli cells (SCs) ensure an adequate environment inside the seminiferous tubule. Glycerol stands as essential for the maintenance of blood⁻testis barrier created by SCs, however, the role of E2 in this process is not known. Herein, we hypothesized that the effect of E2 on glycerol permeability in mouse SCs (mSCs) could be mediated by aquaglyceroporins. The expression of aquaglyceroporins was assessed by RT-PCR and qRT-PCR. Glycerol permeability was evaluated by stopped-flow light scattering. We were able to identify the expression of AQP3 and AQP9 in mSCs where AQP9 is more abundant than AQP3. Our results show that high E2 levels decrease AQP9 mRNA abundance with no influence on AQP3 in mSCs. Interestingly, high E2 levels decreased mSCs' permeability to glycerol, while downregulating AQP9 expression, thus suggesting a novel mechanism by which E2 modulates fluid secretion in the testis. In conclusion, E2 is an important regulator of mSCs physiology and secretion through changes in AQP9 expression and function. Thus, alterations in glycerol permeability induced by E2 may be the cause for male infertility in cases associated with the presence of high E2 levels.

The cellular localization and redistribution of multiple aquaporin paralogs in the spermatic duct epithelium of a maturing marine teleost

Aquaporin-mediated fluid transport in the mammalian efferent duct and epididymis is believed to play a role in sperm maturation and concentration. In fish, such as the marine teleost gilthead seabream (Sparus aurata), the control of fluid homeostasis in the spermatic duct seems also to be crucial for male fertility, but no information exists on the expression and distribution of aquaporins. In this study, reverse transcriptase-polymerase chain reaction and immunoblotting analyses, employing available and newly raised paralog-specific antibodies for seabream aquaporins, indicate that up to nine functional aquaporins, Aqp0a, -1aa, -1ab, -3a, -4a, -7, -8bb, -9b and -10b, are expressed in the spermatic duct. Immunolocalization of the channels in the resting spermatic duct reveals that Aqp0a, -1aa, -4a, -7 and -10b are expressed in the monolayered luminal epithelium, Aqp8b and -9b in smooth muscle fibers, and Aqp1ab and -3a in different interstitial lamina cells. In the epithelial cells, Aqp0a and -1aa are localized in the short apical microvilli, and Aqp4a and -10b show apical and basolateral staining, whereas Aqp7 is solely detected in vesicular compartments. Upon spermiation, an elongation of the epithelial cells sterocilia, as well as the folding of the epithelium, is observed. At this stage, single- and double-immunostaining, using two aquaporin paralogs or the Na⁺ /K⁺ -ATPase membrane marker, indicate that Aqp1ab, -3a, -7, -8bb and -9b staining remains unchanged, whereas in epithelial cells Aqp1aa translation is supressed, Aqp4a internalizes, and Aqp0a and -10b accumulate in the apical, lateral and basal plasma membrane. These findings uncover a cell type- and region-specific distribution of multiple aquaporins in the piscine spermatic duct, which shares conserved features of the mammalian system. The data therefore suggest that aquaporins may play different roles in the regulation of fluid homeostasis and sperm maturation in the male reproductive tract of fish.

Obesogens and male fertility

In the last decades, several studies evidenced a decrease in male fertility in developed countries. Although the aetiology of this trend in male reproductive health remains a matter of debate, environmental compounds that predispose to weight gain, namely obesogens, are appointed as contributors because of their action as endocrine disruptors. Obesogens favour adipogenesis by an imbalance of metabolic processes and can be found virtually everywhere. These compounds easily accumulate in tissues with high lipid content. Obesogens change the functioning of male reproductive axis, and, consequently, the testicular physiology and metabolism that are pivotal for spermatogenesis. The disruption of these tightly regulated metabolic pathways leads to adverse reproductive outcomes. Notably, adverse effects of obesogens may also promote disturbances in the metabolic performance of the following generations, through epigenetic modifications passed by male gametes. Thus, unveiling the molecular pathways by which obesogens induce toxicity that may end up in epigenetic modifications is imperative. Otherwise, a transgenerational susceptibility to metabolic diseases may be favoured. We present an up-to-date overview of the impact of obesogens on testicular physiology, with a particular focus on testicular metabolism. We also address the effects of obesogens on male reproductive parameters and the subsequent consequences for male fertility.

Hepatocyte and Sertoli Cell Aquaporins, Recent Advances and Research Trends

Aquaporins (AQPs) are proteinaceous channels widespread in nature where they allow facilitated permeation of water and uncharged through cellular membranes. AQPs play a number of important roles in both health and disease. This review focuses on the most recent advances and research trends regarding the expression and modulation, as well as physiological and pathophysiological functions of AQPs in hepatocytes and Sertoli cells (SCs). Besides their involvement in bile formation, hepatocyte AQPs are involved in maintaining energy balance acting in hepatic gluconeogenesis and lipid metabolism, and in critical processes such as ammonia detoxification and mitochondrial output of hydrogen peroxide. Roles are played in clinical disorders including fatty liver disease, diabetes, obesity, cholestasis, hepatic cirrhosis and hepatocarcinoma. In the seminiferous tubules, particularly in SCs, AQPs are also widely expressed and seem to be implicated in the various stages of spermatogenesis. Like in hepatocytes, AQPs may be involved in maintaining energy homeostasis in these cells and have a major role in the metabolic cooperation established in the testicular tissue. Altogether, this information represents the mainstay of current and future investigation in an expanding field.

Auto-Adhesion Potential of Extraocular Aqp0 during Teleost Development

AQP0 water channels are the most abundant proteins expressed in the mammalian lens fiber membranes where they are essential for lens development and transparency. Unlike other aquaporin paralogs, mammalian AQP0 has a low intrinsic water permeability, but can form cell-to-cell junctions between the lens fibers. It is not known whether the adhesive properties of AQP0 is a derived feature found only in mammals, or exists as a conserved ancestral trait in non-mammalian vertebrates. Here we show that a tetraploid teleost, the Atlantic salmon, expresses four Aqp0 paralogs in the developing lens, but also expresses significant levels of aqp0 mRNAs and proteins in the epithelia of the pronephros, presumptive enterocytes, gill filament and epidermis. Quantitative PCR reveals that aqp0 mRNA titres increase by three orders of magnitude between the onset of somitogenesis and pigmentation of the eye. Using in situ hybridization and specific antisera, we show that at least two of the channels (Aqp0a1, -0b1 and/or -0b2) are localized in the extraocular basolateral and apical membranes, while Aqp0a2 is lens-specific. Heterologous expression of the Aqp0 paralogs in adhesion-deficient mouse fibolast L-cells reveals that, as for human AQP0, each intact salmon channel retains cell-to-cell adhesive properties. The strongest Aqp0 interactions are auto-adhesion, suggesting that homo-octamers likely form the intercellular junctions of the developing lens and epithelial tissues. The present data are thus the first to show the adhesion potential of Aqp0 channels in a non-mammalian vertebrate, and further uncover a novel extraocular role of the channels during vertebrate development.

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.

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.

Mammalian aquaglyceroporin function in metabolism

Aquaglyceroporins are integral membrane proteins that are permeable to glycerol as well as water. The movement of glycerol from a tissue/organ to the plasma and vice versa requires the presence of different aquaglyceroporins that can regulate the entrance or the exit of glycerol across the plasma membrane. Actually, different aquaglyceroporins have been discovered in the adipose tissue, small intestine, liver, kidney, heart, skeletal muscle, endocrine pancreas and capillary endothelium, and their differential expression could be related to obesity and the type 2 diabetes. Here we describe the expression and function of different aquaglyceroporins in physiological condition and in obesity and type 2 diabetes, suggesting they are potential therapeutic targets for metabolic disorders.

Isolated Rat Epididymal Basal Cells Share Common Properties with Adult Stem Cells

There is little information on the function of epididymal basal cells. These cells secrete prostaglandins, can metabolize radical oxygen species, and have apical projections that are components of the blood-epididymis barrier. The objective of this study was to develop a reproducible protocol to isolate rat epididymal basal cells and to characterize their function by gene expression profiling. Integrin-alpha6 was used to isolate a highly purified population of basal cells. Microarray analysis indicated that expression levels of 552 genes were enriched in basal cells relative to other cell types. Among these genes, 45 were expressed at levels of 5-fold or greater. These highly expressed genes coded for proteins implicated in cell adhesion, cytoskeletal function, ion transport, cellular signaling, and epidermal function, and included proteases and antiproteases, signal transduction, and transcription factors. Several highly expressed genes have been reported in adult stem cells, suggesting that basal cells may represent an epididymal stem cell population. A basal cell culture was established that showed that these basal cells can differentiate in vitro from keratin (KRT) 5-positive cells to cells that express KRT8 and connexin 26, a marker of columnar cells. These data provide novel information on epididymal basal cell gene expression and suggest that these cells can act as adult stem cells.

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.

Role of testicular luminal factors on Basal cell elongation and proliferation in the mouse epididymis

A subset of basal cells (BCs) in the initial segment (IS) of the mouse epididymis has a slender body projection between adjacent epithelial cells. We show here that these projections occasionally cross the apical tight junctions and are in contact with the luminal environment. Luminal testicular factors are critical for the establishment of the IS epithelium, and we investigated their role in the regulation of this luminal sensing property. Efferent duct ligation (EDL) was performed to block luminal flow from the testis without affecting blood flow. Cytokeratin 5 (KRT5) labeling showed a time-dependent reduction of the percentage of BCs with intercellular projections from 1 to 5 days after EDL, compared to controls. Double labeling for caspase-3 and KRT5 showed that a subset of BCs undergoes apoptosis 1 day after EDL. Ki67/KRT5 double labeling showed a low rate of BC proliferation under basal conditions. However, EDL induced a marked increase in the proliferation rate of a subset of BCs 2 days after EDL. A 2-wk treatment with the androgen receptor antagonist flutamide did not affect the number of BCs with intercellular projections, but reduced BC proliferation. Flutamide treatment also reduced the increase in BC proliferation induced 2 days after EDL. We conclude that, in the adult mouse IS, 1) luminal testicular factors play an important role in the ability of BCs to extend their body projection towards the lumen, and are essential for the survival of a subset of BCs; 2) androgens play an important role in the proliferation of some of the BCs that survive the initial insult induced by EDL; and 3) the formation and elongation of BC intercellular projections do not depend on androgens.

Aquaporin-9 is expressed in rat Sertoli cells and interacts with the cystic fibrosis transmembrane conductance regulator

Men with mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene are usually subfertile/infertile. Besides playing a role in Cl(-)/HCO3(-) transport, it has been proposed that CFTR interacts with water membrane transport systems, particularly aquaporins, to control seminiferous tubular secretion, which is regulated by the somatic Sertoli cells (SCs). As aquaporin-9 (AQP9) is highly expressed throughout the male reproductive tract, we hypothesized that it is also present in rat SCs and that it physically interacts with CFTR. To test this hypothesis, primary cultures of rat SCs were established, and expression of CFTR and AQP9 was assessed by RT-polymerase chain reactions (mRNA) and Western blot analysis (protein). A coimmunoprecipitation assay was used to evaluate the physical interaction between CFTR and AQP9. Our results show that CFTR and AQP9 are expressed in rat SCs. We were also able to detect a molecular interaction between CFTR and AQP9 in rat SCs. This is the first report describing the presence of AQP9, and its interaction with CFTR, in rat SCs. Moreover, our results provide evidence that CFTR is involved in water homeostasis of the seminiferous tubular secretion. These mechanisms may open new insights on therapeutic targets to counteract subfertility/infertility in men with cystic fibrosis and mutations in the CFTR gene.

Aquaporin 0 plays a pivotal role in refractive index gradient development in mammalian eye lens to prevent spherical aberration

Aquaporin 0 (AQP0) is a transmembrane channel that constitutes ∼45% of the total membrane protein of the fiber cells in mammalian lens. It is critical for lens transparency and homeostasis as mutations and knockout cause autosomal dominant lens cataract. AQP0 functions as a water channel and as a cell-to-cell adhesion (CTCA) molecule in the lens. Our recent in vitro studies showed that the CTCA function of AQP0 could be crucial to establish lens refractive index gradient (RING). However, there is a lack of in vivo data to corroborate the role of AQP0 as a fiber CTCA molecule which is critical for creating lens RING. The present investigation is undertaken to gather in vivo evidence for the involvement of AQP0 in developing lens RING. Lenses of wild type (WT) mouse, AQP0 knockout (heterozygous, AQP0(+/-)) and AQP0 knockout lens transgenically expressing AQP1 (heterozygous AQP0(+/)(-)/AQP1(+/)(-)) mouse models were used for the study. Data on AQP0 protein profile of intact and N- and/or C-terminal cleaved AQP0 in the lens by MALDI-TOF mass spectrometry and SDS-PAGE revealed that outer cortex fiber cells have only intact AQP0 of ∼28kDa, inner cortical and outer nuclear fiber cells have both intact and cleaved forms, and inner nuclear fiber cells have only cleaved forms (∼26-24kDa). Knocking out of 50% of AQP0 protein caused light scattering, spherical aberration (SA) and cataract. Restoring the lost fiber cell membrane water permeability (Pf) by transgene AQP1 did not reinstate complete lens transparency and the mouse lenses showed light scattering and SA. Transmission and scanning electron micrographs of lenses of both mouse models showed increased extracellular space between fiber cells. Water content determination study showed increase in water in the lenses of these mouse models. In summary, lens transparency, CTCA and compact packing of fiber cells were affected due to the loss of 50% AQP0 leading to larger extracellular space, more water content and SA, possibly due to alteration in RING. To our knowledge, this is the first report identifying the role of AQP0 in RING development to ward off lens SA during focusing.

Open chromatin mapping identifies transcriptional networks regulating human epididymis epithelial function

The epithelium lining the epididymis in the male reproductive tract maintains a luminal environment that promotes sperm cell maturation. This process is dependent on the coordinated expression of many genes that encode proteins with a role in epithelial transport. We previously generated genome-wide maps of open chromatin in primary human epididymis epithelial (HEE) cells to identify potential regulatory elements controlling coordinated gene expression in the epididymis epithelium. Subsequent in silico analysis identified transcription factor-binding sites (TFBS) that were over-represented in the HEE open chromatin, including the motif for paired box 2 (PAX2). PAX2 is a critical transcriptional regulator of urogenital tract development, which has been well studied in the kidney but is unexplored in the epididymis. Due to the limited lifespan of primary HEE cells in culture, we investigated the role of PAX2 in an immortalized HEE cell line (REP). First, REP cells were evaluated by DNase I digestion followed by high-throughput sequencing and the PAX2-binding motif was again identified as an over-represented TFBS within intergenic open chromatin, though on fewer chromosomes than in the primary HEE cells. To identify PAX2-target genes in REP cells, RNA-seq analysis was performed after siRNA-mediated depletion of PAX2 and compared with that with a non-targeting siRNA. In response to PAX2-repression, 3135 transcripts were differentially expressed (1333 up-regulated and 1802 down-regulated). Novel PAX2 targets included multiple genes encoding proteins with predicted functions in the epididymis epithelium.

Ductuli efferentes of the male Golden Syrian hamster reproductive tract

Efferent ductules are responsible for the transportation of spermatozoa from the testis to the epididymis and their epithelium is responsible for the reabsorption of over 90% of the luminal fluid. The purpose of this research was to characterize the gross morphology and histology of efferent ductules in the male Golden Syrian hamster. The efferent ductules emerge from rete testis with a unique polarity at the apex or cephalic pole of the testis. The number of efferent ductules varied from 3 to 10 with an average of 6.0 and blind ending ducts were observed in approximately 56% of the males. The ductules merged into a single common duct prior to entering the caput epididymidis. The proximal efferent ductule lumen was wider than the distal (conus and common ducts), consistent with reabsorption of most of the luminal fluid, as was morphology of the ductal epithelium. Non-ciliated cells in the proximal region had prominent endocytic apparatuses, showing both coated pits and apical tubules in the apical cytoplasm. Large basolateral, intercellular spaces were also present in the epithelium of the proximal region. Distal non-ciliated cells had an abundance of large endosomes and lysosomal granules. Localisation of sodium/hydrogen exchanger-3 (NHE3; SLC9A3) and aquaporins 1 and 9 (AQP1, AQP9) along the microvillus border was also consistent with ion transport and fluid reabsorption by this epithelium. In comparison, the caput epididymidis epithelium expressed only AQP9 immunostaining. Another unusual feature of the hamster efferent ductules was the presence of glycogen aggregates in the basal cytoplasm of small groups of epithelial cells, but only in the proximal ducts near the rete testis. Androgen (AR), estrogen (ESR1 and ESR2) and vitamin D receptors (VDR) were also abundant in epithelial nuclei of proximal and distal efferent ductules. In comparison, caput epididymidis showed very little immunostaining for ESR1.

Are the basal cells of the mammalian epididymis still an enigma?

Basal cells are present in the columnar pseudostratified epithelium covering the epididymis of all mammalian species, which regulates the microenvironment where the functionally incompetent germ cells produced by the testis are matured and stored. Striking novelties have come from investigations on epididymal basal cells in the past 30–40 years. In addition to an earlier hypothesised scavenger role for basal cells, linked to their proven extratubular origin and the expression of macrophage antigens, basal cells have been shown to be involved in cell–cell cross-talk, as well as functioning as luminal sensors to regulate the activity of principal and clear cells. Involvement of basal cells in the regulation of electrolyte and water transport by principal cells was hypothesised. This control is suggested to be mediated by the local formation of prostaglandins. Members of the aquaporin (AQP) and/or aquaglyceroporin family (AQP3, AQP7 and AQP8) are also specifically expressed in the rat epididymal basal cells. Transport of glycerol and glycerylphosphorylcholine from the epithelium of the epididymis to the lumen in relation to sperm maturation may be mediated by AQP. Most probably basal cells collaborate to the building up of the blood–epididymis barrier through cell adhesion molecules, implying an involvement in immune control exerted towards sperm cells, which are foreigners in the environment in which they were produced.

Plasticity of basal cells during postnatal development in the rat epididymis

Our previous study has shown that basal cells sense luminal factors by forming a narrow body projection that can cross epithelial tight junctions. As a first step toward characterizing the structural plasticity of basal cells, in this study, we followed their appearance and morphology in the rat epididymis and vas deferens (VD) during postnatal development and examined their modulation by androgens in adulthood. Immunofluorescence labeling for cytokeratin 5 showed that basal cells are absent at birth. They progressively appear in a retrograde manner from the VD and cauda epididymis to the initial segments during the postnatal weeks PNW1-3. At the onset of differentiation, basal cells are in contact with the lumen and their nucleus is located at the same level as that of adjacent epithelial cells. Basal cells then position their nucleus to the base of the epithelium, and while some are still in contact with the lumen, others have a 'dome-shaped' appearance. At PNW5-6, basal cells form a loose network at the base of the epithelium, and luminal-reaching basal cells are rarely detected. The arrival of spermatozoa during PNW7-8 did not trigger the development of projections in basal cells. However, cells with a narrow luminal-reaching projection began to reappear between PNW8 and PNW12 in the corpus and the cauda. Treatment with flutamide from PNW10 to PNW12 significantly reduced the number of luminal-reaching basal cell projections. In summary, basal cells exhibit significant structural plasticity during differentiation. Fewer apical-reaching projections were detected after flutamide treatment in adulthood, indicating the role of androgens in the luminal-sensing function of basal cells.

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.

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.

Two distinct aquaporin 0s required for development and transparency of the zebrafish lens

PURPOSE

AQP0, formerly known as MIP26, likely has multiple separate functions in the mammalian lens, including water transport, formation of thin junctions, and interactions with other lens components. Although mammalian genomes contain only one Aqp0 gene, the zebrafish genome contains two, Aqp0a and Aqp0b, and the putative multiple functions of the single mammalian protein may be divided between these two genes. The purpose of this study was to exploit this gene duplication and divergence to illuminate the multiple functions of AQP0 in the lens.

METHODS

Wholemount in situ hybridization and Western blot analyses were used to determine the expression pattern of Aqp0a and Aqp0b. The role of both proteins was studied in vivo by microinjection of antisense morpholino oligonucleotides in zebrafish. The water permeability of both proteins was tested using the Xenopus oocyte swelling assay and a yeast shrinkage assay.

RESULTS

Both genes, like their mammalian counterpart, are expressed in the lens. Morpholino knock-down of either gene alone led to cataract formation, indicating that both genes are necessary for normal lens development and transparency. Full-length Aqp0a is a functional water channel when expressed in Xenopus oocytes and in yeast, whereas Aqp0b was not. However, the addition of an HA-tag at its N terminus converted Aqp0b to a water channel in Xenopus oocytes.

CONCLUSIONS

These results suggest that Aqp0a is the primary water channel of the lens and that Aqp0b, though possibly a secondary water channel, has an unidentified function in the lens.