Aquaporin 2 is a vasopressin-independent, constitutive apical membrane protein in rat vas deferens

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.

Proteomic analysis of boar sperm with differential ability of liquid preservation at 17 °C

Understanding the liquid preservation ability of boar sperm is pivotal for efficient management and breeding of livestock. Although sperm proteins play an important role in semen quality and freezability, how the levels of protein change in boar sperm with different liquid preservation abilities at 17 °C remains unclear. In this study, two groups of boar sperm with extreme difference in liquid preservation ability, namely the good preservation ability (GPA) and the poor preservation ability (PPA) groups, were selected by evaluating sperm motility parameters on the 7th day of liquid preservation at 17 °C. Quantitative proteomics based on tandem mass tag (TMT) labeling was used, sperm proteomic characteristics from two groups were analyzed, and potentially key proteins related to the fluid preservation ability of sperm were identified. A total of 187 differentially expressed proteins (DEPs) were identified among 2791 quantified proteins, including 85 upregulated, and 102 downregulated proteins. Further, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of the DEPs revealed that they were enriched in GO terms associated with response to oxidative stress, enzyme activity related to oxidative stress or redox reactions, and several metabolic activities. The significant KEGG pathways included peroxisome, metabolic pathways, selenocompound metabolism, and collection duct acid secretion. In addition, analysis of protein-protein interactions further identified 8 proteins that could be used as biomarker candidates, including GPX5, GLRX, ENO4, QPCT, BBS7, OXSR1, DHRS4 and AP2S1, which may play an essential role in indicating the liquid preservation ability of boar sperm. These findings in this study provide new insights into the underlying molecular mechanisms of the liquid preservation ability of boar sperm. Moreover, the selected candidate proteins can serve as a reference for evaluating sperm quality or preservation ability in boars and their application in related biotechnologies.

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.

Calcium Homeostasis in the Epididymal Microenvironment: Is Extracellular Calcium a Cofactor for Matrix Gla Protein-Dependent Scavenging Regulated by Vitamins

In the male reproductive tract, the epididymis is an essential organ for sperm maturation, in which sperm cells acquire mobility and the ability to fertilize oocytes while being stored in a protective microenvironment. Epididymal function involves a specialized luminal microenvironment established by the epithelial cells of epididymal mucosa. Low-calcium concentration is a unique feature of this epididymal luminal microenvironment, its relevance and regulation are, however, incompletely understood. In the rat epididymis, the vitamin D-related calcium-dependent TRPV6-TMEM16A channel-coupler has been shown to be involved in fluid transport, and, in a spatially complementary manner, vitamin K2-related γ-glutamyl carboxylase (GGCX)-dependent carboxylation of matrix Gla protein (MGP) plays an essential role in promoting calcium-dependent protein aggregation. An SNP in the human GGCX gene has been associated with asthenozoospermia. In addition, bioinformatic analysis also suggests the involvement of a vitamin B6-axis in calcium-dependent MGP-mediated protein aggregation. These findings suggest that vitamins interact with calcium homeostasis in the epididymis to ensure proper sperm maturation and male fertility. This review article discusses the regulation mechanisms of calcium homeostasis in the epididymis, and the potential role of vitamin interactions on epididymal calcium homeostasis, especially the role of matrix calcium in the epididymal lumen as a cofactor for the carboxylated MGP-mediated scavenging function.

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 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.

Investigation of Neuroprotective Effects of Erythropoietin on Chronic Neuropathic Pain in a Chronic Constriction Injury Rat Model

Introduction

Neuropathic pain is pretty common in modern society, and the treatment effect is far from satisfactory. This study aimed to find evidence of the neuroprotective effect of erythropoietin (EPO) in the treatment of neuropathic pain in a rat model of chronic constriction injury (CCI).

Methods

A total of 30 rats were randomly divided into sham operation group, CCI group, or CCI+EPO group. The mechanical and thermal nociception thresholds are evaluated as behavioral assessments. The dorsal root ganglion cells were morphologically evaluated by hematoxylin and eosin staining, and AMPK, p-AMPK, mTOR, p70S6K, and AQP-2 proteins were compared and analyzed by Western blotting. Compared with the sham operation group, rats in the CCI group had shorter paw withdrawal threshold and paw withdrawal latency, abnormal morphology, and increased satellite glial cells.

Results

After treatment with EPO, these changes were significantly reversed. In vivo administration of erythropoietin seems to be able to regulate the expression of AQP-2 through the AMPK/mTOR/p70S6K pathway. Our study provides behavioral, morphological, and immunoblot evidence to prove the neuroprotective effect of EPO in the treatment of chronic neuropathic pain in the CCI rat model.

Conclusion

Our results indicate that EPO has the potential to treat neuropathic pain caused by peripheral nerve injury, although further verification is needed.

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.

Postoperative Care Following Pituitary Surgery

Patients undergoing surgery for pituitary tumors represent a heterogeneous population each with unique clinical, biochemical, radiologic, pathologic, neurologic, and/or ophthalmologic considerations. The postoperative management of patients following pituitary surgery often occurs in the context of a dynamic state of the hypothalamic-pituitary-end organ axis. Consequently, a significant component of the postoperative care of these patients focuses on vigilant screening and observation for neuroendocrinologic perturbations such as varying degrees of hypopituitarism and disorders of water balance (diabetes insipidus and the syndrome of inappropriate antidiuretic hormone). Additionally, one must be cognizant of other potential complications specific to the transsphenoidal approach for tumor removal including cerebrospinal fluid leakage and meningitis. This review addresses the postoperative management of patients undergoing pituitary surgery with an emphasis on careful screening and recognition of complications.

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.

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.

Galectin-7 modulates the length of the primary cilia and wound repair in polarized kidney epithelial cells

Galectins (Gal) are β-galactoside-binding proteins that function in epithelial development and homeostasis. An overlapping role for Gal-3 and Gal-7 in wound repair was reported in stratified epithelia. Although Gal-7 was thought absent in simple epithelia, it was reported in a proteomic analysis of cilia isolated from cultured human airway, and we recently identified Gal-7 transcripts in Madin-Darby canine kidney (MDCK) cells (Poland PA, Rondanino C, Kinlough CL, Heimburg-Molinaro J, Arthur CM, Stowell SR, Smith DF, Hughey RP. J Biol Chem 286: 6780-6790, 2011). We now report that Gal-7 is localized exclusively on the primary cilium of MDCK, LLC-PK(1) (pig kidney), and mpkCCD(c14) (mouse kidney) cells as well as on cilia in the rat renal proximal tubule. Gal-7 is also present on most cilia of multiciliated cells in human airway epithelia primary cultures. Interestingly, exogenous glutathione S-transferase (GST)-Gal-7 bound the MDCK apical plasma membrane as well as the cilium, while the lectin Ulex europeaus agglutinin, with glycan preferences similar to Gal-7, bound the basolateral plasma membrane as well as the cilium. In pull-down assays, β1-integrin isolated from either the basolateral or apical/cilia membranes of MDCK cells was similarly bound by GST-Gal-7. Selective localization of Gal-7 to cilia despite the presence of binding sites on all cell surfaces suggests that intracellular Gal-7 is specifically delivered to cilia rather than simply binding to surface glycoconjugates after generalized secretion. Moreover, depletion of Gal-7 using tetracycline-induced short-hairpin RNA in mpkCCD(c14) cells significantly reduced cilia length and slowed wound healing in a scratch assay. We conclude that Gal-7 is selectively targeted to cilia and plays a key role in surface stabilization of glycoconjugates responsible for integrating cilia function with epithelial repair.

Aquaporins and glia

Glial cells coordinate the differentiation, metabolism, and excitability of neurons; they modulate synaptic transmission and integrate signals emanating from neurons and other glial cells. Several evidences underlying the relation between these pathways and the regulatory mechanisms of ion concentration, supporting the role of Aquaporins (AQPs) in these processes. The goal of this review is to summarize the localization of different isoforms of AQPs in relation to glial cells both in central and peripheral nervous system, underlying AQP involvement in physiological and in pathophysiological conditions such as brain edema, glioma and epilepsy.

Regulation of the water channel aquaporin-2 by posttranslational modification

The cellular functions of many eukaryotic membrane proteins, including the vasopressin-regulated water channel aquaporin-2 (AQP2), are regulated by posttranslational modifications. In this article, we discuss the experimental discoveries that have advanced our understanding of how posttranslational modifications affect AQP2 function, especially as they relate to the role of AQP2 in the kidney. We review the most recent data demonstrating that glycosylation and, in particular, phosphorylation and ubiquitination are mechanisms that regulate AQP2 activity, subcellular sorting and distribution, degradation, and protein interactions. From a clinical perspective, posttranslational modification resulting in protein misrouting or degradation may explain certain forms of nephrogenic diabetes insipidus. In addition to providing major insight into the function and dynamics of renal AQP2 regulation, the analysis of AQP2 posttranslational modification may provide general clues as to the role of posttranslational modification for regulation of other membrane proteins.

Regulation of V-ATPase recycling via a RhoA- and ROCKII-dependent pathway in epididymal clear cells

Luminal acidification in the epididymis is critical for sperm maturation and storage. Clear cells express the vacuolar H(+)-ATPase (V-ATPase) in their apical membrane and are major contributors to proton secretion. We showed that this process is regulated via recycling of V-ATPase-containing vesicles. We now report that RhoA and its effector ROCKII are enriched in rat epididymal clear cells. In addition, cortical F-actin was detected beneath the apical membrane and along the lateral membrane of "resting" clear cells using a pan-actin antibody or phalloidin-TRITC. In vivo luminal perfusion of the cauda epididymal tubule with the ROCK inhibitors Y27632 (10-30 μM) and HA1077 (30 μM) or with the cell-permeable Rho inhibitor Clostridium botulinum C3 transferase (3.75 μg/ml) induced the apical membrane accumulation of V-ATPase and extension of V-ATPase-labeled microvilli in clear cells. However, these newly formed microvilli were devoid of ROCKII. In addition, Y27632 (30 μM) or HA1077 (30 μM) decreased the ratio of F-actin to G-actin detected by Western blot analysis in epididymal epithelial cells, and Y27632 also decreased the ratio of F-actin to G-actin in clear cells isolated by fluorescence activated cell sorting from B1-enhanced green fluorescence protein (EGFP) transgenic mice. These results provide evidence that depolymerization of the cortical actin cytoskeleton via inhibition of RhoA or its effector ROCKII favors the recruitment of V-ATPase from the cytosolic compartment into the apical membrane in clear cells. In addition, our data suggest that the RhoA-ROCKII pathway is not locally involved in the elongation of apical microvilli. We propose that inhibition of RhoA-ROCKII might be part of the intracellular signaling cascade that is triggered upon agonist-induced apical membrane V-ATPase accumulation.

Aquaporin 9 expression in the developing rat epididymis is modulated by steroid hormones

Fluid and solute transport across the epithelium of the male excurrent duct is important for sperm maturation and storage. Aquaporin 9 (AQP9), which allows permeation of water and neutral solutes, is abundant throughout the male reproductive tract, where it is expressed at the apical membrane of rat epididymal principal cells as early as at 1 week of age. We evaluated the effect of neonatal exposure to: 1) a GNRH antagonist (GNRHa); 2) diethylstilbestrol (DES); 3) ethinyl estradiol (EE); 4) DES plus testosterone (DES+TE); and 5) the anti-androgen flutamide on AQP9 expression in the epididymis of peripubertal rats. Control groups received the vehicle alone. In 25-day-old rats, quantification of the mean pixel intensity of immunofluorescence-stained sections showed a significant decrease in AQP9 staining in the apical membrane of epididymal principal cells after treatments with GNRHa, DES, or flutamide, compared to controls. These results were confirmed by western blotting. While EE induced a marked decrease in AQP9 levels by western blotting, the decrease in AQP9-associated fluorescence was not significant compared to controls. DES+TE-treated rats showed levels of AQP9 protein similar to controls, indicating maintenance of AQP9 expression by testosterone treatment in the presence of DES. Our data show that expression of AQP9 in the developing rat epididymis is downregulated by neonatal DES, GNRHa, EE, and flutamide, and that the effects mediated by estrogens can be prevented by testosterone administration.

Chronic constriction injury induces aquaporin-2 expression in the dorsal root ganglia of rats

Aquaporins are a family of water channel proteins involved in water homeostasis in several tissues. Current knowledge of aquaporin expression in the nervous system is very limited. Therefore the first aim of this study was to assess, by immunohistochemistry and immunoblotting analysis, the presence and localization of aquaporin-2 in the spinal cord and dorsal root ganglia of naïve adult rats. In addition, we evaluated aquaporin-2 expression in response to chronic constriction injury of the sciatic nerve, a model of neuropathic pain. Our results showed that aquaporin-2 expression was not detectable either in the spinal cord or the dorsal root ganglia of naïve rats. However, we showed for the first time an increase of aquaporin-2 expression in response to chronic constriction injury treatment in small-diameter dorsal root ganglia neurons but no expression in the lumbar spinal cord. These data support the hypothesis that aquaporin-2 expression is involved in inflammatory neuropathic nerve injuries, although its precise role remains to be determined.

Immunolocalization of aquaporin-1, -5, and -7 in the avian testis and vas deferens

Thirteen mammalian aquaporin (AQP) isoforms have been identified, and they have a unique tissue-specific pattern of expression. AQPs have been found in the reproductive system of both male and female humans, rats, and mice. However, tissue expression and cellular and subcellular localization of AQPs have been poorly investigated in the male reproductive system of birds. The localization of AQP subtypes (AQP1, 2, 3, 4, 5, 7, 8, 9, and 11) in the goose testis and vas deferens has been studied through immunohistochemistry and immunobloting. Interestingly, the testicular and deferential tissues were positive for AQP1, -5, and -7 but not the others. AQP1 immunoreactivity was detected in the capillary endothelial cells of testis and vas deferens. AQP5 was localized in the interstitial tissue of the testis, including Leydig cells, as well as in the basal cells of vas deferens. Double-labeling confocal microscopy revealed coexpression of AQP5 with capillary AQP1 in the testis. AQP7 was expressed in elongated spermatid and spermatozoa tails in the testis, as well as spermatozoa tails in the vas deferens. These results suggest that several subtypes of AQPs are involved in the regulation of water homeostasis in the goose male reproductive system.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Alterations of AQP2 expression in trigeminal ganglia in a murine inflammation model

Aquaporins (AQPs) are small membrane channel proteins involved in osmoregulation. To date, only AQP1, AQP2, AQP4 and AQP9 have been found in the nervous system. Generally, they are involved in water movement in nervous tissue, nevertheless, recent data would suggest the involvement of AQPs in neurotransmission. In this work, we have evaluated the expression of AQP1 and AQP2 in the trigeminal ganglia of mice in an animal model of perioral acute inflammatory pain using immunohistochemistry and immunoblotting analysis. Our data have shown for the first time, the alteration of AQP2 expression in trigeminal ganglia in acute inflammatory pain showing increased and intracellular redistribution of AQP2 mainly in small-sized neurons and Schwann cells. Apart from this, the AQP1 expression remained unaltered. On the whole, these data support the hypothesis that AQP2 is involved in pain transmission in the peripheral nervous system.

Alkaline pH- and cAMP-induced V-ATPase membrane accumulation is mediated by protein kinase A in epididymal clear cells

In the epididymis, low luminal bicarbonate and acidic pH maintain sperm quiescent during maturation and storage. The vacuolar H(+)-ATPase (V-ATPase) in epididymal clear cells plays a major role in luminal acidification. We have shown previously that cAMP, luminal alkaline pH, and activation of the bicarbonate-regulated soluble adenylyl cyclase (sAC) induce V-ATPase apical accumulation in these cells, thereby stimulating proton secretion into the epididymal lumen. Here we examined whether protein kinase A (PKA) is involved in this response. Confocal immunofluorescence labeling on rat epididymis perfused in vivo showed that at luminal acidic pH (6.5), V-ATPase was distributed between short apical microvilli and subapical endosomes. The specific PKA activator N(6)-monobutyryl-3'-5'-cyclic monophosphate (6-MB-cAMP, 1 mM) induced elongation of apical microvilli and accumulation of V-ATPase in these structures. The PKA inhibitor myristoylated-PKI (mPKI, 10 microM) inhibited the apical accumulation of V-ATPase induced by 6-MB-cAMP. Perfusion at pH 6.5 with 8-(4-chlorophenylthio)-2-O-methyl-cAMP (8CPT-2-O-Me-cAMP; 10 microM), an activator of the exchange protein activated by cAMP (Epac), did not induce V-ATPase apical accumulation. When applied at a higher concentration (100 microM), 8CPT-2-O-Me-cAMP induced V-ATPase apical accumulation, but this effect was completely inhibited by mPKI, suggesting crossover effects on the PKA pathway with this compound at high concentrations. Importantly, the physiologically relevant alkaline pH-induced apical V-ATPase accumulation was completely inhibited by pretreatment with mPKI. We conclude that direct stimulation of PKA activity by cAMP is necessary and sufficient for the alkaline pH-induced accumulation of V-ATPase in clear cell apical microvilli.

Polyuria of thyrotoxicosis: downregulation of aquaporin water channels and increased solute excretion

Thyrotoxicosis is a common disorder causing cardiovascular and renal irregularities. In this study, thyrotoxicosis was produced in rats by 14 days of daily thyroxine injection. This was associated with an increase in cardiac index, mean arterial pressure, and renal blood flow compared with euthyroid controls. Food and water intake along with urine output were significantly increased in the thyrotoxic rats compared with control animals associated with a significant increase in solute excretion. Polyuria and increased solute excretion still occurred even when food and water intake was equivalent. These renal responses were associated with significant decreases in AQP1 and AQP2 water channel expression in both the ad lib and paired intake studies in the cortex and inner medulla. The downregulation of AQP2 protein occurred in spite of equivalent plasma arginine vasopressin (AVP) in the ad lib and increased AVP in the paired feeding studies. Solute-free water reabsorption was greater in both the ad lib and paired thyrotoxic than euthyroid rats and was associated with increased Na-K-2Cl cotransporter expression. We propose that the AVP-independent downregulation of AQP2, the observed increase in renal arterial pressure, and decrease in filtration fraction contribute to polyuria the increased solute excretion in spite of enhanced ion transporters in thyrotoxicosis.

Oxytocin and vasopressin stimulate anion secretion by human and porcine vas deferens epithelia

Experiments were conducted to characterize the effects of oxytocin (OT) and vasopressin (VP) on epithelial cells isolated from human (1 degree HVD) and porcine (1 degree PVD) vas deferens and an immortalized epithelial cell line derived from porcine vas deferens (PVD9902 cells). Cultured monolayers were assessed in modified Ussing flux chambers and the OT- or VP-induced change in short circuit current (I(SC)) was recorded. All cell types responded to basolateral OT or VP with a transient increase in I(SC) that reached a peak of 3-5 microA cm(-2). Concentration-response curves constructed with 1 degree PVD and PVD9902 cells revealed that the apparent K(D) (k(app)) for OT was approximately 100-fold less than the k(app) for VP. Amplicons for the OT receptor (OXTR) and vasopressin type 2 and type 1a receptors (AVPR2 and AVPR1A) were generated with RT-PCR and the identification of each amplicon confirmed by sequence analysis. A selective antagonist for OXTR and AVPR1A fully blocked the effects of OT and partially blocked the effects of VP when assessed in both 1 degree PVD and PVD9902 monolayers. APVR2 antagonists blocked the effects of low (< or =30 nM) but not high concentrations of VP, indicating that VP was affecting both AVPR2 and a second receptor subtype, likely OXTR or AVPR1A. Experiments employing chelerythrine demonstrated that OT stimulation of vas deferens monolayers requires PKC activity. Alternatively, VP (but not OT) increased the accumulation of cytosolic cAMP in vas deferens epithelial cells. Results from this study demonstrate that OT and VP can modulate ion transport across vas deferens epithelia by independent mechanisms. OT and VP have the potential to acutely change the environment to which sperm are exposed and thus, have the potential to affect male fertility.

Aquaporins: a promising target for drug development

Aquaporins (AQPs) are a family of small hydrophobic, integral membrane proteins that are expressed in all living organisms and play critical roles in controlling the water flow into and out of cells. So far, 13 different AQPs have been identified in mammals (AQP 0-12). AQPs have recently been implicated in various diseases such as cancer, cataract, brain oedema, gallstone disease and nephrogenic diabetes insipidus, as well as in the development of obesity and polycystic kidney disease. Interfering with the expression of AQPs will undoubtedly have therapeutic applications. Hence, in this review, the authors look at each AQP and its association with various pathological conditions in humans and demonstrate that they form potential targets for the treatment of such diseases.

Alix (AIP1) is a vasopressin receptor (V2R)-interacting protein that increases lysosomal degradation of the V2R

The vasopressin type 2 receptor (V2R) is a G protein-coupled receptor that plays a central role in renal water reabsorption. Termination of ligand (vasopressin) stimulation is an important physiological regulatory event, but few proteins that interact with the V2R during downregulation after vasopressin (VP) binding have been identified. Using yeast two-hybrid screening of a human kidney cDNA library, we show that a 100-kDa protein called ALG-2-interacting protein X (Alix) interacts with the last 29 amino acids of the V2R COOH terminus. This was confirmed by pull-down assays using a GST-V2R-COOH-tail fusion protein. Alix was immunolocalized in principal cells of the kidney, which also express the V2R. The function of the Alix-V2R interaction was studied by transfecting Alix into LLC-PK(1) epithelial cells expressing V2R-green fluorescent protein (GFP). Under basal conditions, V2R-GFP localized mainly at the plasma membrane. On VP treatment, V2R-GFP was internalized into perinuclear vesicles in the nontransfected cells. In contrast, V2R-GFP fluorescence was virtually undetectable 2 h after exposure to VP in cells that coexpressed Alix. Western blotting using an anti-GFP antibody showed marked degradation of the V2R after 2 h in the presence of VP and Alix, a time point at which little or no degradation was detected in the absence of Alix. In contrast, little or no degradation of the parathyroid hormone receptor was detectable in the presence or absence of Alix and/or the PTH ligand. The VP-induced disappearance of V2R-GFP was abolished by chloroquine, a lysosomal degradation inhibitor, but not by MG132, a proteosome inhibitor. These data suggest that Alix increases the rate of lysosomal degradation of V2R and may play an important regulatory role in the VP response by modulating V2R downregulation.

Segmental and cellular expression of aquaporins in the male excurrent duct

The male reproductive tract and accessory glands comprise a complex but interrelated system of tissues that are composed of many distinct cell types, all of which contribute to the ability of spermatozoa to carry out their ultimate function of fertilizing an oocyte. Spermatozoa undergo their final steps of maturation as they pass through the male excurrent duct, which includes efferent ducts, the epididymis and the vas deferens. The composition of the luminal environment in these organs is tightly regulated. Major fluid reabsorption occurs in efferent ducts and in the epididymis, and leads to a significant increase in sperm concentration. In the distal epididymis and vas deferens, fluid secretion controls the final fluidity of the luminal content. Therefore, the process of water movement in the excurrent duct is a crucial step for the establishment of male fertility. Aquaporins contribute to transepithelial water transport in many tissues, including the kidney, the brain, the eye and the respiratory tract. The present article reviews our current knowledge regarding the distribution and function of aquaporins in the male excurrent duct.

Function of aquaporins in female and male reproductive systems

The flow of water and some other small molecules across cell membranes is important in many of the processes underlying reproduction. The fluid movement is strongly associated with the presence of aquaporins (AQPs) in the female and male reproductive systems. It has been suggested that AQPs mediate water movement into the antral follicle and play important roles in follicle development. AQPs are known to be involved in the early stage of spermatogenesis, in the secretion of tubule liquid and in the concentration and storage of spermatozoa. Fluid reabsorption in some regions of the male reproductive tract is under steroid hormone control and could be mediated by various AQPs. Also AQPs take part in the processes of fertilization, blastocyst formation (as the pathway for transtrophoectodermal water movement during cavitation) and implantation. Alterations in the expression and function or regulation of AQPs have already been demonstrated in disorders of the male reproductive system, such as abnormal sperm motility, the abnormal epididymis and infertility seen in cystic fibrosis, and varicocele. This article extensively reviews the distribution of AQPs in mammalian reproductive tissues and discusses their possible physiological and pathophysiological roles.

Aquaporin-2 expression in human endometrium correlates with serum ovarian steroid hormones

The aim of the present study was to examine the expression of aquaporin-2 (AQP2), a member of the water channel family aquaporins (AQPs), in human uterine endometrium and its modulation of ovarian steroid hormone at the proliferative and secretory phases. Western blot, immunohistochemistry, and RT-PCR were employed in the present study. Western blot revealed a 29-kDa band that represented AQP2 in human endometrium. The expression of AQP2 in endometrium was confirmed by RT-PCR and immunohistochemical results. The immunohistochemical analysis demonstrated that AQP2 was prominent in luminal and glandular epithelial cells of endometrium. The levels of endometrial AQP2 expression changed during the menstrual cycle and were higher in the secretory endometrium than in the proliferative endometrium. A significantly high level of AQP2 was detected at the mid-secretory phase. There was a positive correlation between the levels of the endometrial AQP2 expression and the concentrations of the serum 17beta-estradiol (E2) or/and progesterone (P4). These data for the first time corroborate that AQP2 is expressed in human endometrium and that the expression of AQP2 in human endometrium might be regulated by E2 or/and P4. The changed expression of AQP2 at different phases of the menstrual cycle may be essential to reproductive physiology in human. The high level of endometrial AQP2 expression was observed at the mid-secretory phase, the time of embryo implantation, suggesting that AQP2 might play physiological roles in the uterine receptivity.

Postnatal Expression of Aquaporins in Epithelial Cells of the Rat Epididymis1

The mammalian aquaporins (AQPs) are a family of 13 transmembrane channel proteins that are involved in the transport of water in numerous organs. In the male excurrent duct, the movement of fluid and solutes across the epithelium is essential for establishing the proper luminal environment in which sperm mature and are stored. AQP9 is abundantly expressed in the efferent ducts, the epididymis, and the vas deferens, where it could represent an important apical pathway for transmembrane water and solute movement. However, other organs in which water transport is critical, including the kidney, the lung, or the eye, express several different AQPs with a cell-specific pattern. To undertake a systematic analysis of the expression of known AQPs in the postnatal and adult rat epididymis, we examined the expression of their respective mRNAs in epithelial cells isolated by laser capture microdissection (LCM), and we determined their corresponding protein expression pattern by immunofluorescence and Western blotting. Our data show that, whereas AQP9 is the main AQP of the epididymis, the mRNA specific for Aqp2, 5, 7, and 11 are also expressed in epididymal epithelial cells. AQP5 protein colocalizes with AQP9 in the apical membrane of a subpopulation of principal cells in the corpus and cauda regions. Aqp2 mRNA was detected in epithelial cells after the second postnatal week and the amount significantly increased up to adulthood. However, AQP2 protein was detected only in the distal cauda of young rats (between the second and fourth postnatal week). No AQP2 protein was detected in the adult epididymis, indicating that posttranscriptional mechanisms are involved in the regulation of AQP2 expression. In addition, epididymal epithelial cells express significant amounts of the mRNAs coding for AQP7 and 11. No mRNA or protein for AQPs 0, 4, 6, and 8 were detectable in epithelial cells, and Aqp1 was detected in whole epididymal samples, but not in epithelial cells. Thanks to the recent development of microdissection technologies, our observations suggest that epididymal epithelial cells express several members of the AQP family with a region-specific pattern. AQPs may be involved not only in the transepithelial transport of water in the epididymis but also in the postnatal development of this organ, as suggested by the differential expression of AQP2.

Mouse model of inducible nephrogenic diabetes insipidus produced by floxed aquaporin-2 gene deletion

Transgenic mouse models of defective urinary concentrating ability produced by deletion of various membrane transport or receptor proteins, including aquaporin-2 (AQP2), are associated with neonatal mortality from polyuria. Here, we report an inducible mouse model of AQP2 gene deletion with severe polyuria in adult mice. LoxP sequences were inserted into introns 1 and 2 in the mouse AQP2 gene by homologous recombination in embryonic stem cells. Mating of germ-line AQP2-loxP mice with tamoxifen-inducible Cre-expressing mice produced offspring with inducible homozygous Cre-AQP2-loxP, which had a normal phenotype. Tamoxifen injections over 10 days resulted in AQP2 gene excision, with undetectable full-length AQP2 transcript in kidney and a >95% reduction in immunoreactive AQP2 protein. Urine osmolality decreased from approximately 2,000 to <500 mosmol/kgH(2)O after 4-5 days, with urine output increasing from 2 to 25 ml/day. Urine osmolality did not increase after water deprivation. Interestingly, AQP3 protein expression in the collecting duct was increased by about fivefold after AQP2 gene excision. Mild renal damage was seen after 6 wk of polyuria, with collecting duct dilatation, yet normal creatinine clearance and serum chemistries. These results establish the first adult model of nephrogenic diabetes insipidus (NDI) caused by AQP2 deficiency, with daily urine output comparable to body weight, although remarkable preservation of renal function compared with non-inducible NDI models.

PVD9902, a porcine vas deferens epithelial cell line that exhibits neurotransmitter-stimulated anion secretion and expresses numerous HCO3(-) transporters

Epithelial ion transport disorders, including cystic fibrosis, adversely affect male reproductive function by nonobstructive mechanisms and by obstruction of the distal duct. Continuous cell lines that could be used to define ion transport mechanisms in this tissue are not readily available. In the present study, porcine vas deferens epithelial cells were isolated by standard techniques, and the cells spontaneously immortalized to form a porcine vas deferens epithelial cell line that we have titled PVD9902. Cells were maintained in continuous culture for >4 yr and 200 passages in a typical growth medium. Frozen stocks were generated, and thawed cells exhibited growth characteristics indistinguishable from their nonfrozen counterparts. Molecular and immunocytochemical studies confirmed the origin and epithelial nature of these cells. When seeded on permeable supports, PVD9902 cells grew as electrically tight (>6,000 ohms x cm2), confluent monolayers that responded to forskolin with an increase in short-circuit current (I(sc); 8 +/- 1 microA/cm2) that required Cl-, HCO3(-), and Na+, and was partially sensitive to bumetanide. mRNA was expressed for a number of anion transporters, including CFTR, electrogenic Na+-HCO3(-) cotransporter 1b (NBCe1b), downregulated in adenoma, pendrin, and Cl-/formate exchanger. Both forskolin and isoproterenol caused an increase in cellular cAMP levels. In addition, PVD9902 cell monolayers responded to physiological (i.e., adenosine, norepinephrine) and pharmacological [i.e., 5'-(N-ethylcarboxamido)adenosine, isoproterenol] agonists with increases in I(sc). Unlike their freshly isolated counterparts, however, PVD9902 cells did not respond to glucocorticoid exposure with an increase in amiloride-sensitive I(sc). RT-PCR analysis revealed the presence of both glucocorticoid and mineralocorticoid receptor mRNA as well as mRNA for the alpha- and gamma-subunits of the epithelia Na+ channels (alpha- and gamma-ENaC), but not beta-ENaC. Nonetheless, PVD9902 cells recapitulated most observations in freshly isolated cells and thus represent a powerful new tool to characterize mechanisms that contribute to male reproductive function.

UT-A urea transporter promoter, UT-Aalpha, targets principal cells of the renal inner medullary collecting duct

The urea transporters, UT-A1 and UT-A3, two members of the UT-A gene family, are localized to the terminal portion of the inner medullary collecting duct (IMCD). In this manuscript, we demonstrate that 4.2 kb of the 5'-flanking region of the UT-A gene (UT-Aalpha promoter) is sufficient to drive the IMCD-specific expression of a heterologous reporter gene, beta-galactosidase (beta-Gal), in transgenic mice. RT-PCR, immunoblotting, and immunohistochemistry demonstrate that within the kidney, transgene expression is confined to the terminal portion of the IMCD. Colocalization studies with aquaporin-2 show that expression is localized to the principal cells of the IMCD2 and IMCD3 regions. Utilizing beta-Gal activity assays, we further show that within the kidney, the beta-Gal transgene can be regulated by both water restriction and glucocorticoids, similar to the regulation of the endogenous UT-A gene. These results demonstrate that 4.2 kb of the UT-Aalpha promoter is sufficient to drive expression of a heterologous reporter gene in a tissue-specific and cell-specific fashion in transgenic mice.

Distribution of AQP2 and AQP3 water channels in human tissue microarrays

The objective of this investigation was to use semi-quantitative immunohistochemistry to determine the distribution and expression levels of AQP2 and AQP3 proteins in normal human Tissue MicroArrays. Expression of the vasopressin regulated AQP2 was observed in a limited number of tissues. AQP2 was prominent in the apical and subapical plasma membranes of cortical and medullary renal collecting ducts. Surprisingly, weak AQP2 immunoreactivity was also noted in pancreatic islets, fallopian tubes and peripheral nerves. AQP2 was also localized to selected parts of the central nervous system (ependymal cell layer, subcortical white matter, hippocampus, spinal cord) and selected cells in the gastrointestinal system (antral and oxyntic gastric mucosa, small intestine and colon). These findings corroborate the restricted tissue distribution of AQP2. AQP3 was strongly expressed in many of the human tissues examined particularly in basolateral membranes of the distal nephron (medullary collecting ducts), distal colon, upper airway epithelia, transitional epithelium of the urinary bladder, tracheal, bronchial and nasopharyngeal epithelium, stratified squamous epithelial cells of the esophagus, and anus. AQP3 was moderately expressed in basolateral membranes of prostatic tubuloalveolar epithelium, pancreatic ducts, uterine endometrium, choroid plexus, articular chondrocytes, subchondral osteoblasts and synovium. Low AQP3 levels were also detected in skeletal muscle, cardiac muscle, gastric pits, seminiferous tubules, lymphoid vessels, salivary and endocrine glands, amniotic membranes, placenta and ovary. The abundance of basolateral AQP3 in epithelial tissues and its expression in many non-epithelial cells suggests that this aquaglyceroporin is a major participant in barrier hydration and water and osmolyte homeostasis in the human body.

Regulation of the immunoexpression of aquaporin 9 by ovarian hormones in the rat oviductal epithelium

The volume of oviductal fluid fluctuates during the estrous cycle, suggesting that water availability is under hormonal control. It has been postulated that sex-steroid hormones may regulate aquaporin (AQP) channels involved in water movement across cell membranes. Using a functional assay (oocytes of Xenopus laevis), we demonstrated that the rat oviductal epithelium contains mRNAs coding for water channels, and we identified by RT-PCR the mRNAs for AQP5, -8, and -9, but not for AQP2 and -3. The immunoreactivity for AQP5, -8, and -9 was localized only in epithelial cells of the oviduct. The distribution of AQP5 and -8 was mainly cytoplasmic, whereas we confirmed, by confocal microscopy, that AQP9 localized to the apical plasma membrane. Staining of AQP5, -8, and -9 was lost after ovariectomy, and only AQP9 immunoreactivity was restored after estradiol and/or progesterone treatments. The recovery of AQP9 reactivity after ovariectomy correlated with increased mRNA and protein levels after treatment with estradiol alone or progesterone administration after estradiol priming. Interestingly, progesterone administration after progesterone priming also induced AQP9 expression but without a change in mRNA levels. Levels of AQP9 varied along the estrous cycle with their highest levels during proestrus and estrus. These results indicate that steroid hormones regulate AQP9 expression at the mRNA and protein level and that other ovarian signals are involved in the expression of AQP5 and -8. Thus hormonal regulation of the type and quantity of water channels in this epithelium might control water transport in the oviductal lumen.

V-ATPase B1-subunit promoter drives expression of EGFP in intercalated cells of kidney, clear cells of epididymis and airway cells of lung in transgenic mice

The kidney, epididymis, and lungs are complex organs with considerable epithelial cell heterogeneity. This has limited the characterization of pathophysiological transport processes that are specific for each cell type in these epithelia. The purpose of the present study was to develop new tools to study cell-specific gene and protein expression in such complex tissues and organs. We report the production of a transgenic mouse that expresses enhanced green fluorescent protein (EGFP) in a subset of epithelial cells that express the B1 subunit of vacuolar H(+)-ATPase (V-ATPase) and are actively involved in proton transport. A 6.5-kb portion of the V-ATPase B1 promoter was used to drive expression of EGFP. In two founders, quantitative real-time RT-PCR demonstrated expression of EGFP in kidney, epididymis, and lung. Immunofluorescence labeling using antibodies against the B1 and E subunits of V-ATPase and against carbonic anhydrase type II (CAII) revealed specific EGFP expression in all renal type A and type B intercalated cells, some renal connecting tubule cells, all epididymal narrow and clear cells, and some nonciliated airway epithelial cells. No EGFP expression was detected in collecting duct principal cells (identified using an anti-AQP2 antibody) or epididymal principal cells (negative for V-ATPase or CAII). This EGFP-expressing mouse model should prove useful in future studies of gene and protein expression and their physiological and/or developmental regulation in distinct cell types that can now be separated using fluorescence-assisted microdissection, fluorescence-activated cell sorting, and laser capture microdissection.

Modulation of the actin cytoskeleton via gelsolin regulates vacuolar H+-ATPase recycling

The role of the actin cytoskeleton in regulating membrane protein trafficking is complex and depends on the cell type and protein being examined. Using the epididymis as a model system in which luminal acidification is crucial for sperm maturation and storage, we now report that modulation of the actin cytoskeleton by the calcium-activated actin-capping and -severing protein gelsolin plays a key role in regulating vacuolar H(+)-ATPase (V-ATPase) recycling. Epididymal clear cells contain abundant V-ATPase in their apical pole, and an increase in their cell-surface V-ATPase expression correlates with an increase in luminal proton secretion. We have shown that apical membrane accumulation of V-ATPase is triggered by an elevation in cAMP following activation of bicarbonate-regulated soluble adenylyl cyclase in response to alkaline luminal pH (Pastor-Soler, N., Beaulieu, V., Litvin, T. N., Da Silva, N., Chen, Y., Brown, D., Buck, J., Levin, L. R., and Breton, S. (2003) J. Biol. Chem. 278, 49523-49529). Here, we show that clear cells express high levels of gelsolin, indicating a potential role in the functional activity of these cells. When jasplakinolide was used to overcome the severing action of gelsolin by polymerizing actin, complete inhibition of the alkaline pH- and cAMP-induced apical membrane accumulation of V-ATPase was observed. Conversely, when gelsolin-mediated actin filament elongation was inhibited using a 10-residue peptide (PBP10) derived from the phosphatidylinositol 4,5-bisphosphate-binding region (phosphoinositide-binding domain 2) of gelsolin, significant V-ATPase apical membrane mobilization was induced, even at acidic luminal pH. In contrast, the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) and the phospholipase C inhibitor U-73122 inhibited the alkaline pH-induced V-ATPase apical accumulation. Thus, maintenance of the actin cytoskeleton in a depolymerized state by gelsolin facilitates calcium-dependent apical accumulation of V-ATPase in response to luminal pH alkalinization. Gelsolin is present in other cell types that express the V-ATPase in their plasma membrane and recycling vesicles, including kidney intercalated cells and osteoclasts. Therefore, modulation of the actin cortex by this severing and capping protein may represent a common mechanism by which these cells regulate their rate of proton secretion.

The role of regulated CFTR trafficking in epithelial secretion

The focus of this review is the regulated trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in distal compartments of the protein secretory pathway and the question of how changes in CFTR cellular distribution may impact on the functions of polarized epithelial cells. We summarize data concerning the cellular localization and activity of CFTR and attempt to synthesize often conflicting results from functional studies of regulated endocytosis and exocytosis in CFTR-expressing cells. In some instances, findings that are inconsistent with regulated CFTR trafficking may result from the use of overexpression systems or nonphysiological experimental conditions. Nevertheless, judging from data on other transporters, an appropriate cellular context is necessary to support regulated CFTR trafficking, even in epithelial cells. The discovery that disease mutations can influence CFTR trafficking in distal secretory and recycling compartments provides support for the concept that regulated CFTR recycling contributes to normal epithelial function, including the control of apical CFTR channel density and epithelial protein secretion. Finally, we propose molecular mechanisms for regulated CFTR endocytosis and exocytosis that are based on CFTR interactions with other proteins, particularly those whose primary function is membrane trafficking. These models provide testable hypotheses that may lead to elucidation of CFTR trafficking mechanisms and permit their experimental manipulation in polarized epithelial cells.

The ins and outs of aquaporin-2 trafficking

This review outlines recent advances related to the molecular mechanisms and pathways of aquaporin-2 (AQP2) water channel trafficking. AQP2 is a fascinating protein, whose sorting signals can be interpreted by different cell types to achieve apical or basolateral membrane insertion, in both regulated and constitutive trafficking pathways. In addition to the well-known cAMP-mediated, stimulatory effect of vasopressin on AQP2 membrane insertion, other signaling and trafficking events can also lead to AQP2 membrane accumulation via cAMP-independent mechanisms. These include 1) elevation of cGMP, mediated by sodium nitroprusside (a nitric oxide donor), atrial natriuretic factor, and l-arginine (via nitric oxide synthase); 2) disruption of the actin cytoskeleton; and 3) inhibition of the clathrin-mediated endocytotic arm of the AQP2 recycling pathway by dominant-negative dynamin expression and by membrane cholesterol depletion. Recent data also indicate that AQP2 recycles constitutively in epithelial cells, it can be inserted into different membrane domains in different cell types both in vitro and in vivo, and these pathways can be modulated by factors including hypertonicity. The roles of accessory proteins, including small GTPases and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins in AQP2 membrane insertion, are also being uncovered. Understanding cAMP-independent mechanisms for membrane insertion of AQP2 is especially relevant to the therapeutic bypassing of the mutated, dysfunctional vasopressin receptor in patients with X-linked nephrogenic diabetes insipidus.

Postoperative endocrine management of pituitary tumors

Pituitary tumors are common and are often associated with endocrine abnormalities. Furthermore, pituitary surgery itself may result in additional hormonal changes, including impairment of anterior pituitary hormone secretion and, more commonly, abnormalities of ADH regulation. Endocrine management of patients with pituitary or other sellar lesions involves acute hospital-based and longer term office-based evaluation and treatment. In the immediate postoperative period, careful attention must be directed toward sodium and water balance as well as toward recognition of changes in endocrine function. Postoperative measurement of serum hormone levels also helps to determine if resection of a hypersecreting tumor has been successful. To minimize postoperative morbidity, perioperative endocrine assessment and management of patients undergoing pituitary surgery should consist of a team approach, involving both the neurosurgeon and the endocrinologist.

Renal principal cell-specific expression of green fluorescent protein in transgenic mice

The purpose of this study is to develop transgenic mice with principal cell-specific expression of green fluorescent protein (GFP). After the cloning and sequencing of the mouse aquaporin-2 (AQP2) gene, 9.5 kb of the promoter were used to drive expression of GFP in transgenic mice. In transgenic mice, GFP was selectively expressed in principal cells of the renal collecting duct and not in intercalated cells. Expression was increased by dehydration of mice. AQP2 and GFP expression was maintained in primary cultures of renal medulla that were stimulated with cAMP or vasopressin analogs. GFP-expressing cells were then isolated by fluorescence-activated cell sorting. RT-PCR analysis showed expression of AQP2, AQP3, AQP4, vasopressin type 2 receptor, and cAMP response element binding protein but not H+-ATPase B1 subunit or anion exchanger 1. After expansion of these cells in culture, RT-PCR analysis showed continued expression of the same genes. This pattern of gene expression is that of principal cells rather than intercalated cells. This transgenic mouse model can be used in future studies of gene expression during the development, differentiation, and maturation of renal principal cells.

Aquaporins: a water channel family

Water channel proteins, aquaporins, are integral membrane proteins serving in the permeation of water and some other small molecules. Eleven isoforms of aquaporins have been identified from various tissues to date. They are expressed in tissue- and cell-specific manners, and are closely related to the specific functions of tissues and cells. Aquaporins are usually localized to the plasma membrane. Some isoforms are present in cytoplasmic compartments, and their translocation to the plasma membrane is crucial in the regulation of water transfer. This review focuses on the localization of aquaporins in mammalian tissues and discusses the physiological importance of water channels.

Expression of aquaporin 9 in the adult rat epididymal epithelium is modulated by androgens

Reabsorption of fluid and solutes across the epithelium lining the male excurrent duct is important for adequate sperm maturation, concentration, and storage. Water channels contribute to water movement across epithelia in many tissues. Aquaporin 9 (AQP9) is abundantly expressed in the apical membrane of principal cells that line the epididymis, and in reabsorptive and secretory epithelial cells of the male reproductive tract. In this study we show that the nonsteroidal antiandrogen flutamide, given to adult rats at a dose of 50 mg x kg(-1) x day(-1) for 2 wk via osmotic minipumps significantly decreased the amount of AQP9 in the epididymis. This down-regulation was observed by immunofluorescence of cryostat tissue sections and by Western blotting of epididymal brush border membrane preparations. In addition, castrated adult rats showed lower levels of epididymal AQP9 compared with adult controls, whereas systemic testosterone treatment of castrated adult rats induced a recovery of the expression of AQP9 to control levels. These data indicate that the expression of AQP9, a likely candidate for apical transepithelial fluid and solute transport in several regions of the male reproductive tract, is modulated by androgens in the adult rat epididymis.

Expression, localization, and regulation of aquaporin-1 to -3 in rat urothelia

Although mammalian urothelia are generally considered impermeable to constituents of urine, in vivo studies in several species indicate urothelial transport of water and solutes under certain conditions. This study investigates the expression, localization, and regulation of aquaporin (AQP)-1, -2, and -3 in ureteral and bladder tissues in 48-h dehydrated and water-loaded female Wistar rats. Immunoblots of homogenates of whole ureter and bladder identified characteristic approximately 28- and 35- to 44-kDa bands for AQP-1, -2, and -3. AQP-1 was localized to capillary and arteriole endothelial cells, whereas AQP-2 and -3 circumferentially lined the epithelial cell membranes except for the apical membrane of the epithelial cells adjacent to the lumens of both ureter and bladder. AQP-2 was also present in epithelial cell cytoplasm. Dehydration resulted in 160-200% increases of AQP-3 signal and 24-49% increases of AQP-2 signal but no change in AQP-1 signal on immunoblots of homogenates of ureters and bladders. AQPs in genitourinary tract urothelia likely play a role in the regulation of epithelial cell volume and osmolality and may play a role in bulk water movement across urothelia.