Expression, localization, and regulation of urea transporter B in rat urothelia

Aquaporins in Urinary System

There are at least eight aquaporins (AQPs) expressed in the kidney. Including AQP1 expressed in proximal tubules, thin descending limb of Henle and vasa recta; AQP2, AQP3, AQP4, AQP5, and AQP6 expressed in collecting ducts; AQP7 expressed in proximal tubules; AQP8 expressed in proximal tubules and collecting ducts; and AQP11 expressed in the endoplasmic reticulum of proximal tubular epithelial cells. Over years, researchers have constructed different AQP knockout mice and explored the effect of AQP knockout on kidney function. Thus, the roles of AQPs in renal physiology are revealed, providing very useful information for addressing fundamental questions about transepithelial water transport and the mechanism of near isoosmolar fluid reabsorption. This chapter introduces the localization and function of AQPs in the kidney and their roles in different kidney diseases to reveal the prospects of AQPs in further basic and clinical studies.

The urothelium: a multi-faceted barrier against a harsh environment

All mucosal surfaces must deal with the challenge of exposure to the outside world. The urothelium is a highly specialized layer of stratified epithelial cells lining the inner surface of the urinary bladder, a gruelling environment involving significant stretch forces, osmotic and hydrostatic pressures, toxic substances, and microbial invasion. The urinary bladder plays an important barrier role and allows the accommodation and expulsion of large volumes of urine without permitting urine components to diffuse across. The urothelium is made up of three cell types, basal, intermediate, and umbrella cells, whose specialized functions aid in the bladder's mission. In this review, we summarize the recent insights into urothelial structure, function, development, regeneration, and in particular the role of umbrella cells in barrier formation and maintenance. We briefly review diseases which involve the bladder and discuss current human urothelial in vitro models as a complement to traditional animal studies.

The Urothelium: Life in a Liquid Environment

The urothelium, which lines the renal pelvis, ureters, urinary bladder, and proximal urethra, forms a high-resistance but adaptable barrier that surveils its mechanochemical environment and communicates changes to underlying tissues including afferent nerve fibers and the smooth muscle. The goal of this review is to summarize new insights into urothelial biology and function that have occurred in the past decade. After familiarizing the reader with key aspects of urothelial histology, we describe new insights into urothelial development and regeneration. This is followed by an extended discussion of urothelial barrier function, including information about the roles of the glycocalyx, ion and water transport, tight junctions, and the cellular and tissue shape changes and other adaptations that accompany expansion and contraction of the lower urinary tract. We also explore evidence that the urothelium can alter the water and solute composition of urine during normal physiology and in response to overdistension. We complete the review by providing an overview of our current knowledge about the urothelial environment, discussing the sensor and transducer functions of the urothelium, exploring the role of circadian rhythms in urothelial gene expression, and describing novel research tools that are likely to further advance our understanding of urothelial biology.

Physiological functions of urea transporter B

Urea transporters (UTs) are membrane proteins in the urea transporter protein A (UT-A) and urea transporter protein B (UT-B) families. UT-B is mainly expressed in endothelial cell membrane of the renal medulla and in other tissues, including the brain, heart, pancreas, colon, bladder, bone marrow, and cochlea. UT-B is responsible for the maintenance of urea concentration, male reproductive function, blood pressure, bone metabolism, and brain astrocyte and cardiac functions. Its deficiency and dysfunction contribute to the pathogenesis of many diseases. Actually, UT-B deficiency increases the sensitivity of bladder epithelial cells to apoptosis triggers in mice and UT-B-null mice develop II-III atrioventricular block and depression. The expression of UT-B in the rumen of cow and sheep may participate in digestive function. However, there is no systemic review to discuss the UT-B functions. Here, we update research approaches to understanding the functions of UT-B.

Osmotic regulation of UT-B urea transporters in the RT4 human urothelial cell line

Facilitative UT-B urea transporters play important physiological roles in numerous tissues, including the urino-genital tract. Previous studies have shown that urothelial UT-B transporters are crucial to bladder function in a variety of mammalian species. Using the RT4 bladder urothelial cell line, this study investigated the potential osmotic regulation of human UT-B transporters. Initial end-point PCR experiments confirmed expression of both UT-B1 and UT-B2 transcripts in RT4 cells. Western blotting analysis revealed glycosylated UT-B protein to be highly abundant and immunolocalization experiments showed it was predominantly located on the plasma membrane. Further PCR experiments suggested that a 48 hr, NaCl-induced raise in external osmolality increased expression of UT-B transcripts. Importantly, these NaCl-induced changes also significantly increased UT-B protein abundance (p < .01, n = 7, ANOVA), whereas mannitol-induced changes in external osmolality had no effect (NS, n = 4, ANOVA). Finally, similar increases in both UT-B RNA expression and protein abundance were observed with urea-induced changes to external osmolality (p < .05, n = 4, ANOVA). In conclusion, these findings strongly suggest that increases in external osmolality, via either NaCl or urea, can regulate human urothelial UT-B transporters.

SLC14A1: a novel target for human urothelial cancer

Urinary bladder cancer is the second commonly diagnosed genitourinary malignancy. Previously, bio-molecular alterations have been observed within certain locations such as chromosome 9, retinoblastoma gene and fibroblast growth factor receptor-3. Solute carrier family 14 member 1 (SLC14A1) gene encodes the type-B urea transporter (UT-B) which facilitates the passive movement of urea across cell membrane, and has recently been related with human malignancies, especially for bladder cancer. Herein, we discussed the SLC14A1 gene and UT-B protein properties, aiming to elucidate the expression behavior of SLC14A1 in human bladder cancer. Furthermore, by reviewing some well-established theories regarding the carcinogenesis of bladder cancer, including several genome wide association researches, we have bridged the mechanisms of cancer development with the aberrant expression of SLC14A1. In conclusion, the altered expression of SLC14A1 gene in human urothelial cancer may implicate its significance as a novel target for research.

Identification of a Novel UT-B Urea Transporter in Human Urothelial Cancer

The urea transporter UT-B is widely expressed and has been studied in erythrocyte, kidney, brain and intestines. Interestingly, UT-B gene has been found more abundant in bladder than any other tissue. Recently, gene analyses demonstrate that SLC14A1 (UT-B) gene mutations are associated with bladder cancer, suggesting that urea transporter UT-B may play an important role in bladder carcinogenesis. In this study, we examined UT-B expression in bladder cancer with human primary bladder cancer tissues and cancer derived cell lines. Human UT-B has two isoforms. We found that normal bladder expresses long form of UT-B2 but was lost in 8 of 24 (33%) or significantly downregulated in 16 of 24 (67%) of primary bladder cancer patients. In contrast, the short form of UT-B1 lacking exon 3 was detected in 20 bladder cancer samples. Surprisingly, a 24-nt in-frame deletion in exon 4 in UT-B1 (UT-B1Δ24) was identified in 11 of 20 (55%) bladder tumors. This deletion caused a functional defect of UT-B1. Immunohistochemistry revealed that UT-B protein levels were significantly decreased in bladder cancers. Western blot analysis showed a weak UT-B band of 40 kDa in some tumors, consistent with UT-B1 gene expression detected by RT-PCR. Interestingly, bladder cancer associate UT-B1Δ24 was barely sialylated, reflecting impaired glycosylation of UT-B1 in bladder tumors. In conclusion, SLC14A1 gene and UT-B protein expression are significantly changed in bladder cancers. The aberrant UT-B expression may promote bladder cancer development or facilitate carcinogenesis induced by other carcinogens.

Hierarchical transcriptional profile of urothelial cells development and differentiation

The urothelial lining of the lower urinary tract is the most efficient permeability barrier in animals, exhibiting a highly differentiated phenotype and a remarkable regenerative capacity upon wounding. During development and possibly during repair, cells undergo a sequence of hierarchical transcriptional events that mark the transition of these cells from the least differentiated urothelial phenotype characteristic of the basal cell layer, to the most differentiated cellular phenotype characteristic of the superficial cell layer. Unraveling normal urothelial differentiation program is essential to uncover the underlying causes of many congenital abnormalities and for the development of an appropriate differentiation niche for stem cells, for future use in urinary tract tissue engineering and organ reconstruction. Kruppel like factor-5 appears to be at the top of the hierarchy activating several downstream transcription factors, the most prominent of which is peroxisome proliferator activator receptor-γ. Eventually those lead to the activation of transcription factors that directly regulate the expression of uroplakin proteins along with other proteins that mediate the permeability function of the urothelium. In this review, we discuss the most recent findings in the area of urothelial cellular differentiation and transcriptional regulation, aiming for a comprehensive overview that aids in a refined understanding of this process.

Aquaporins in Urinary System

Several aquaporin (AQP )-type water channels are expressed in kidney: AQP1 in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2 -6 in the collecting duct; AQP7 in the proximal tubule; AQP8 in the proximal tubule and collecting duct; and AQP11 in the endoplasmic reticulum of proximal tubule cells. AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability. The roles of AQPs in renal physiology and transepithelial water transport have been determined using AQP knockout mouse models. This chapter describes renal physiologic insights revealed by phenotypic analysis of AQP knockout mice and the prospects for further basic and clinical studies.

Functional inhibition of urea transporter UT-B enhances endothelial-dependent vasodilatation and lowers blood pressure via L-arginine-endothelial nitric oxide synthase-nitric oxide pathway

Mammalian urea transporters (UTs), UT-A and UT-B, are best known for their role in urine concentration. UT-B is especially distributed in multiple extrarenal tissues with abundant expression in vascular endothelium, but little is known about its role in vascular function. The present study investigated the physiological significance of UT-B in regulating vasorelaxations and blood pressure. UT-B deletion in mice or treatment with UT-B inhibitor PU-14 in Wistar-Kyoto rats (WKYs) and spontaneous hypertensive rats (SHRs) reduced blood pressure. Acetylcholine-induced vasorelaxation was significantly augmented in aortas from UT-B null mice. PU-14 concentration-dependently produced endothelium-dependent relaxations in thoracic aortas and mesenteric arteries from both mice and rats and the relaxations were abolished by N(ω)-nitro-L-arginine methyl ester. Both expression and phosphorylation of endothelial nitric oxide synthase (eNOS) were up-regulated and expression of arginase I was down-regulated when UT-B was inhibited both in vivo and in vitro. PU-14 induced endothelium-dependent relaxations to a similar degree in aortas from 12 weeks old SHRs or WKYs. In summary, here we report for the first time that inhibition of UT-B plays an important role in regulating vasorelaxations and blood pressure via up-regulation of L-arginine-eNOS-NO pathway, and it may become another potential therapeutic target for the treatment of hypertension.

Architecture of the human renal inner medulla and functional implications

The architecture of the inner stripe of the outer medulla of the human kidney has long been known to exhibit distinctive configurations; however, inner medullary architecture remains poorly defined. Using immunohistochemistry with segment-specific antibodies for membrane fluid and solute transporters and other proteins, we identified a number of distinctive functional features of human inner medulla. In the outer inner medulla, aquaporin-1 (AQP1)-positive long-loop descending thin limbs (DTLs) lie alongside descending and ascending vasa recta (DVR, AVR) within vascular bundles. These vascular bundles are continuations of outer medullary vascular bundles. Bundles containing DTLs and vasa recta lie at the margins of coalescing collecting duct (CD) clusters, thereby forming two regions, the vascular bundle region and the CD cluster region. Although AQP1 and urea transporter UT-B are abundantly expressed in long-loop DTLs and DVR, respectively, their expression declines with depth below the outer medulla. Transcellular water and urea fluxes likely decline in these segments at progressively deeper levels. Smooth muscle myosin heavy chain protein is also expressed in DVR of the inner stripe and the upper inner medulla, but is sparsely expressed at deeper inner medullary levels. In rodent inner medulla, fenestrated capillaries abut CDs along their entire length, paralleling ascending thin limbs (ATLs), forming distinct compartments (interstitial nodal spaces; INSs); however, in humans this architecture rarely occurs. Thus INSs are relatively infrequent in the human inner medulla, unlike in the rodent where they are abundant. UT-B is expressed within the papillary epithelium of the lower inner medulla, indicating a transcellular pathway for urea across this epithelium.

The urothelium of a hibernator: the American black bear

The American black bear undergoes a 3-5 month winter hibernation during which time bears do not eat, drink, defecate, or urinate. During hibernation renal function (GFR) is 16-50% of normal but urine is reabsorbed across the urinary bladder (UB) urothelium thus enabling metabolic recycling of all urinary constituents. To elucidate the mechanism(s) whereby urine is reabsorbed, we examined the UBs of five nonhibernating wild bears using light, electron (EM), and confocal immunofluorescent (IF) microscopy-concentrating on two components of the urothelial permeability barrier - the umbrella cell apical membranes and tight junctions (TJ). Bear UB has the same tissue layers (serosa, muscularis, lamina propria, urothelia) and its urothelia has the same cell layers (basal, intermediate, umbrella cells) as other mammalians. By EM, the bear apical membrane demonstrated a typical mammalian scalloped appearance with hinge and plaque regions - the latter containing an asymmetric trilaminar membrane and, on IF, uroplakins Ia, IIIa, and IIIb. The umbrella cell TJs appeared similar to those in other mammals and also contained TJ proteins occludin and claudin - 4, and not claudin -2. Thus, we were unable to demonstrate urothelial apical membrane or TJ differences between active black bears and other mammals. Expression and localization of UT-B, AQP-1 and -3, and Na(+), K(+)-ATPase on bear urothelial membranes was similar to that of other mammals. Similar studies of urothelia of hibernating bears, including evaluation of the apical membrane lipid bilayer and GAGs layer are warranted to elucidate the mechanism(s) whereby hibernating bears reabsorb their daily urine output and thus ensure successful hibernation.

Extrarenal phenotypes of the UT-B knockout mouse

The urea transporter UT-B is expressed in multiple tissues including erythrocytes, kidney, brain, heart, liver, colon, bone marrow, spleen, lung, skeletal muscle, bladder, prostate, and testis in mammals. Phenotype analysis of UT-B-null mice has confirmed that UT-B deletion results in a urea-selective urine-concentrating defect (see Chap. 9 ). The functional significance of UT-B in extrarenal tissues studied in the UT-B-null mouse is discussed in this chapter. UT-B-null mice present depression-like behavior with urea accumulation and nitric oxide reduction in the hippocampus. UT-B deletion causes a cardiac conduction defect, and TNNT2 and ANP expression changes in the aged UT-B-null heart. UT-B also plays a very important role in protecting bladder urothelium from DNA damage and apoptosis by regulating the urea concentration in urothelial cells. UT-B functional deficiency results in urea accumulation in the testis and early maturation of the male reproductive system. These results show that UT-B is an indispensable transporter involved in maintaining physiological functions in different tissues.

The urothelium: anatomy, review of the literature, perspectives for veterinary medicine

Over time, much knowledge has been accumulated about the active role of the urothelium, principally in rodents and human. Far from being a mere passive barrier, this specialized epithelium can alter the ion and protein composition of the urine, is able to sense and respond to mechanical stimuli such as pressure, and react to mechanical stimuli by epithelial cell communication with the nervous system. Most of the specialized functions of the urothelium are linked to a number of morpho-physiologic properties exhibited by the superficial umbrella cells, including specialized membrane lipids, asymmetric unit membrane particles and a plasmalemma with stiff plaques which function as a barrier to most substances found in urine, thus protecting the underlying tissues. Moreover, the entire mucosa lining the low urinary tract, composed of urothelium and sub-urothelium, forms a functional transduction unit, able to respond to eso- and endogenous physical and chemical stimuli in a manner assuring an adequate functional response. This review will summarize the available information on each area of inquiry from a morpho-functional point of view. Possible considerations pertaining to species of veterinary interest are reviewed as well. The review was prepared consulting the electronic databases PubMed and Cab Abstracts and retrieving all pertinent reports and the relative reference lists, in order to identify any potential additional studies that could be included. Full-length research articles and thematic reviews were considered. Information on the urothelium of some domestic animal species was also included.

Clinical significance of the reduction of UT-B expression in urothelial carcinoma of the bladder

Urea transporter B (UT-B) is a membrane protein and plays an important role in regulating urea concentration in bladder urothelial cells. It has been reported that UT-B gene mutations were related to bladder carcinogenesis, and UT-B deletion could induce DNA damage and apoptosis in bladder urothelium. However, the functions and clinical significance of UT-B in human bladder cancer remain unknown. The most common type of bladder cancer is urothelial carcinoma (UC). We hypothesized that UT-B expression was related to bladder UC progress. In this study, UT-B was detected using immunohistochemistry in 52 paraffin-embedded specimens of bladder UC and 10 normal urothelium specimens. The results showed that UT-B protein expression in UC tumor cells was significantly lower as compared with normal urothelial cells (P = 0.021). UT-B protein expression was significantly reduced with increasing histological grade (P = 0.010). UT-B protein expression in muscle-invasive stage was significantly lower than in non-muscle-invasive stage (P = 0.014). Taken together, our data suggest that the reduction or loss of UT-B expression may be related to the incidence, progression and invasiveness of bladder UC. UT-B may be a novel diagnostic or prognostic biomarker, as well as a potential therapeutic target in UC of the bladder.

Expression and localization of a UT-B urea transporter in the human bladder

Facilitative UT-B urea transporters have been shown to play an important role in the urinary concentrating mechanism. Recent studies have now suggested a link between UT-B allelic variation and human bladder cancer risk. UT-B1 protein has been previously identified in the bladder of various mammalian species, but not yet in humans. The aim of the present study was to investigate whether any UT-B protein was present in the human bladder. First, RT-PCR results confirmed that UT-B1 was strongly expressed at the RNA level in the human bladder, whereas UT-B2 was only weakly present. Initial Western blot analysis confirmed that a novel UT-B COOH-terminal antibody detected human UT-B proteins. Importantly, this antibody detected a specific 40- to 45-kDa UT-B signal in human bladder protein. Using a peptide-N-glycosidase F enzyme, this bladder UT-B signal was deglycosylated to a core 30-kDa protein, which is smaller than the predicted size for UT-B1 but similar to many proteins reported to be UT-B1. Finally, immunolocalization experiments confirmed that UT-B protein was strongly expressed throughout all urothelium layers except for the apical membrane of the outermost umbrella cells. In conclusion, these data confirm the presence of UT-B protein within the human bladder. Further studies are now required to determine the precise nature, regulation, and physiological role of this UT-B.

Genes and proteins of urea transporters

A urea transporter protein in the kidney was first proposed in 1987. The first urea transporter cDNA was cloned in 1993. The SLC14a urea transporter family contains two major subgroups: SLC14a1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14a2, the UT-A group originally isolated from kidney inner medulla. Slc14a1, the human UT-B gene, arises from a single locus located on chromosome 18q12.1-q21.1, which is located close to Slc14a2. Slc14a1 includes 11 exons, with the coding region extending from exon 4 to exon 11, and is approximately 30 kb in length. The Slc14a2 gene is a very large gene with 24 exons, is approximately 300 kb in length, and encodes 6 different isoforms. Slc14a2 contains two promoter elements: promoter I is located in the typical position, upstream of exon 1, and drives the transcription of UT-A1, UT-A1b, UT-A3, UT-A3b, and UT-A4; while promoter II is located within intron 12 and drives the transcription of UT-A2 and UT-A2b. UT-A1 and UT-A3 are located in the inner medullary collecting duct, UT-A2 in the thin descending limb and liver, UT-A5 in testis, UT-A6 in colon, UT-B1 primarily in descending vasa recta and erythrocytes, and UT-B2 in rumen.

Expression of urea transporters and their regulation

UT-A and UT-B families of urea transporters consist of multiple isoforms that are subject to regulation of both acutely and by long-term measures. This chapter provides a brief overview of the expression of the urea transporter forms and their locations in the kidney. Rapid regulation of UT-A1 results from the combination of phosphorylation and membrane accumulation. Phosphorylation of UT-A1 has been linked to vasopressin and hyperosmolality, although through different kinases. Other acute influences on urea transporter activity are ubiquitination and glycosylation, both of which influence the membrane association of the urea transporter, again through different mechanisms. Long-term regulation of urea transport is most closely associated with the environment that the kidney experiences. Low-protein diets may influence the amount of urea transporter available. Conditions of osmotic diuresis, where urea concentrations are low, will prompt an increase in urea transporter abundance. Although adrenal steroids affect urea transporter abundance, conflicting reports make conclusions tenuous. Urea transporters are upregulated when P2Y2 purinergic receptors are decreased, suggesting a role for these receptors in UT regulation. Hypercalcemia and hypokalemia both cause urine concentration deficiencies. Urea transporter abundances are reduced in aging animals and animals with angiotensin-converting enzyme deficiencies. This chapter will provide information about both rapid and long-term regulation of urea transporters and provide an introduction into the literature.

Hydration status affects sodium, potassium, and chloride transport across rat urothelia

Recent data suggest possible net transport of urinary constituents across mammalian urinary tract epithelia (urothelia). To evaluate the effect of animal hydration status on such transport, we instilled urine collected during 2-day water deprivation, water loading, or ad libitum water intake into isolated in situ bladder(s) of groups of rats undergoing one of the same three hydration states. After 1-h bladder dwell, we retrieved the urine and measured differences in volume and solute concentrations between instilled and retrieved urine. We previously reported results regarding changes in urine volume and net urea and creatinine transport and herein report the results of net urinary sodium, potassium, and chloride transport in the same animals. During water-loading conditions, urinary concentrations of Na, K, and Cl rose 4.9 (30.7%), 2.6 (16.5%), and 6.0 meq/l (26.8%), respectively, indicating urothelial secretion into urine. During ad libitum water intake, urinary K and Cl concentrations fell 33.6 (14.8%) and 28.4 meq/l (12%), respectively (Na did not change), and during water deprivation urine Na, K, and Cl concentrations fell dramatically by 53.2 (18.6%), 159.4 (34.6%) and 133.7 meq/l (33.8%), respectively, reflecting urothelial reabsorption of each ion. For each ionic species, two factors independently influenced transport: instilled urinary ion concentration and animal hydration state. These results demonstrate significant regulated ion transport across mammalian urothelia, support the notion that lower urinary tract modifies final urine, and suggest that the lower urinary tract may play a role in local and whole animal solute homeostasis.

Differential urinary specific gravity as a molecular phenotype of the bladder cancer genetic association in the urea transporter gene, SLC14A1

Genome-wide association studies (GWAS) identified associations between markers within the solute carrier family 14 (urea transporter), member 1 (SLC14A1) gene and risk of bladder cancer. SLC14A1 defines the Kidd blood groups in erythrocytes and is also involved in concentration of the urine in the kidney. We evaluated the association between a representative genetic variant (rs10775480) of SLC14A1 and urine concentration, as measured by urinary specific gravity (USG), in a subset of 275 population-based controls enrolled in the New England Bladder Cancer Study. Overnight urine samples were collected, and USG was measured using refractometry. Analysis of covariance was used to estimate adjusted least square means for USG in relation to rs10775480. We also examined the mRNA expression of both urea transporters, SLC14A1 and SLC14A2, in a panel of human tissues. USG was decreased with each copy of the rs10775480 risk T allele (p-trend = 0.011) with a significant difference observed for CC vs. TT genotypes (p-value(tukey) = 0.024). RNA-sequencing in the bladder tissue showed high expression of SLC14A1 and the absence of SLC14A2, while both transporters were expressed in the kidney. We suggest that the molecular phenotype of this GWAS finding is the genotype-specific biological activity of SLC14A1 in the bladder tissue. Our data suggest that SLC14A1 could be a unique urea transporter in the bladder that has the ability to influence urine concentration and that this mechanism might explain the increased bladder cancer susceptibility associated with rs10775480.

Urea transporter UT-B deletion induces DNA damage and apoptosis in mouse bladder urothelium

BACKGROUND

Previous studies found that urea transporter UT-B is abundantly expressed in bladder urothelium. However, the dynamic role of UT-B in bladder urothelial cells remains unclear. The objective of this study is to evaluate the physiological roles of UT-B in bladder urothelium using UT-B knockout mouse model and T24 cell line.

METHODOLOGY/PRINCIPAL FINDINGS

Urea and NO measurement, mRNA expression micro-array analysis, light and transmission electron microscopy, apoptosis assays, DNA damage and repair determination, and intracellular signaling examination were performed in UT-B null bladders vs wild-type bladders and in vitro T24 epithelial cells. UT-B was highly expressed in mouse bladder urothelium. The genes, Dcaf11, MCM2-4, Uch-L1, Bnip3 and 45 S pre rRNA, related to DNA damage and apoptosis were significantly regulated in UT-B null urothelium. DNA damage and apoptosis highly occurred in UT-B null urothelium. Urea and NO levels were significantly higher in UT-B null urothelium than that in wild-type, which may affect L-arginine metabolism and the intracellular signals related to DNA damage and apoptosis. These findings were consistent with the in vitro study in T24 cells that, after urea loading, exhibited cell cycle delay and apoptosis.

CONCLUSIONS/SIGNIFICANCE

UT-B may play an important role in protecting bladder urothelium by balancing intracellular urea concentration. Disruption of UT-B function induces DNA damage and apoptosis in bladder, which can result in bladder disorders.

Dietary protein affects urea transport across rat urothelia

Recent evidence suggests that regulated solute transport occurs across mammalian lower urinary tract epithelia (urothelia). To study the effects of dietary protein on net urothelial transport of urea, creatinine, and water, we used an in vivo rat bladder model designed to mimic physiological conditions. We placed groups of rats on 3-wk diets differing only by protein content (40, 18, 6, and 2%) and instilled 0.3 ml of collected urine in the isolated bladder of anesthetized rats. After 1 h dwell, retrieved urine volumes were unchanged, but mean urea nitrogen (UN) and creatinine concentrations fell 17 and 4%, respectively, indicating transurothelial urea and creatinine reabsorption. The fall in UN (but not creatinine) concentration was greatest in high protein (40%) rats, 584 mg/dl, and progressively less in rats receiving lower protein content: 18% diet, 224 mg/dl; 6% diet, 135 mg/dl; and 2% diet, 87 mg/dl. The quantity of urea reabsorbed was directly related to a urine factor, likely the concentration of urea in the instilled urine. In contrast, the percentage of instilled urea reabsorbed was greater in the two dietary groups receiving the lowest protein (26 and 23%) than in those receiving higher protein (11 and 9%), suggesting the possibility that a bladder/urothelial factor, also affected by dietary protein, may have altered bladder permeability. These findings demonstrate significant regulated urea transport across the urothelium, resulting in alteration of urine excreted by the kidneys, and add to the growing evidence that the lower urinary tract may play an unappreciated role in mammalian solute homeostasis.

Urea transporter physiology studied in knockout mice

In mammals, there are two types of urea transporters; urea transporter (UT)-A and UT-B. The UT-A transporters are mainly expressed in kidney epithelial cells while UT-B demonstrates a broader distribution in kidney, heart, brain, testis, urinary tract, and other tissues. Over the past few years, multiple urea transporter knockout mouse models have been generated enabling us to explore the physiological roles of the different urea transporters. In the kidney, deletion of UT-A1/UT-A3 results in polyuria and a severe urine concentrating defect, indicating that intrarenal recycling of urea plays a crucial role in the overall capacity to concentrate urine. Since UT-B has a wide tissue distribution, multiple phenotypic abnormalities have been found in UT-B null mice, such as defective urine concentration, exacerbated heart blockage with aging, depression-like behavior, and earlier male sexual maturation. This review summarizes the new insights of urea transporter functions in different organs, gleaned from studies of urea transporter knockout mice, and explores some of the potential pharmacological prospects of urea transporters.

New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism

The mechanism by which urine is concentrated in the mammalian kidney remains incompletely understood. Urea is the dominant urinary osmole in most mammals and may be concentrated a 100-fold above its plasma level in humans and even more in rodents. Several facilitated urea transporters have been cloned. The phenotypes of mice with deletion of the transporters expressed in the kidney have challenged two previously well-accepted paradigms regarding urea and sodium handling in the renal medulla but have provided no alternative explanation for the accumulation of solutes that occurs in the inner medulla. In this review, we present evidence supporting the existence of an active urea secretion in the pars recta of the proximal tubule and explain how it changes our views regarding intrarenal urea handling and UT-A2 function. The transporter responsible for this secretion could be SGLT1, a sodium-glucose cotransporter that also transports urea. Glucagon may have a role in the regulation of this secretion. Further, we describe a possible transfer of osmotic energy from the outer to the inner medulla via an intrarenal Cori cycle converting glucose to lactate and back. Finally, we propose that an active urea transporter, expressed in the urothelium, may continuously reclaim urea that diffuses out of the ureter and bladder. These hypotheses are all based on published findings. They may not all be confirmed later on, but we hope they will stimulate further research in new directions.

Hydration status affects urea transport across rat urothelia

Although mammalian urinary tract epithelium (urothelium) is generally considered impermeable to water and solutes, recent data suggest that urine constituents may be reabsorbed during urinary tract transit and storage. To study water and solute transport across the urothelium in an in vivo rat model, we instilled urine (obtained during various rat hydration conditions) into isolated in situ rat bladders and, after a 1-h dwell, retrieved the urine and measured the differences in urine volume and concentration and total quantity of urine urea nitrogen and creatinine between instilled and retrieved urine in rat groups differing by hydration status. Although urine volume did not change >1.9% in any group, concentration (and quantity) of urine urea nitrogen in retrieved urine fell significantly (indicating reabsorption of urea across bladder urothelia), by a mean of 18% (489 mg/dl, from an instilled 2,658 mg/dl) in rats receiving ad libitum water and by a mean of 39% (2,544 mg/dl, from an instilled 6,204 mg/dl) in water-deprived rats, but did not change (an increase of 15 mg/dl, P = not significant, from an instilled 300 mg/dl) in a water-loaded rat group. Two separate factors affected urea nitrogen reabsorption rates, a urinary factor related to hydration status, likely the concentration of urea nitrogen in the instilled urine, and a bladder factor(s), also dependent on the animal's state of hydration. Urine creatinine was also absorbed during the bladder dwell, and hydration group effects on the concentration and quantity of creatinine reabsorbed were qualitatively similar to the hydration group effect on urea transport. These findings support the notion(s) that urinary constituents may undergo transport across urinary tract epithelia, that such transport may be physiologically regulated, and that urine is modified during transit and storage through the urinary tract.

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Formation and maintenance of blood-urine barrier in urothelium

Blood-urine barrier, which is formed during differentiation of superficial urothelial cells, is the tightest and most impermeable barrier in the body. In the urinary bladder, the barrier must accommodate large changes in the surface area during distensions and contractions of the organ. Tight junctions and unique apical plasma membrane of superficial urothelial cells play a critical role in the barrier maintenance. Alterations in the blood-urine barrier function accompany most of the urinary tract diseases. In this review, we discuss recent discoveries on the role of tight junctions, dynamics of Golgi apparatus and post-Golgi compartments, and intracellular membrane traffic during the biogenesis and maintenance of blood-urine barrier.

Cell biology and physiology of the uroepithelium

The uroepithelium sits at the interface between the urinary space and underlying tissues, where it forms a high-resistance barrier to ion, solute, and water flux, as well as pathogens. However, the uroepithelium is not simply a passive barrier; it can modulate the composition of the urine, and it functions as an integral part of a sensory web in which it receives, amplifies, and transmits information about its external milieu to the underlying nervous and muscular systems. This review examines our understanding of uroepithelial regeneration and how specializations of the outermost umbrella cell layer, including tight junctions, surface uroplakins, and dynamic apical membrane exocytosis/endocytosis, contribute to barrier function and how they are co-opted by uropathogenic bacteria to infect the uroepithelium. Furthermore, we discuss the presence and possible functions of aquaporins, urea transporters, and multiple ion channels in the uroepithelium. Finally, we describe potential mechanisms by which the uroepithelium can transmit information about the urinary space to the other tissues in the bladder proper.

The ROMK potassium channel is present in mammalian urinary tract epithelia and muscle

There is increasing evidence that mammalian urinary tract epithelial cells utilize membrane channels and transporters to transport solutes across their apical (luminal) and basalateral membranes to modify solute concentrations in both cell and urine. This study investigates the expression, localization, and regulation of the ROMK (K(ir) 1.1) potassium channels in rat and dog ureter and bladder tissues. Immunoblots of homogenates of whole ureter, whole bladder, bladder epithelial cells, and bladder smooth muscle tissues in both rat and dog identified approximately 45- to 50-kDa bands characteristic of ROMK in all tissues. RT-PCR identified ROMK mRNA in these same tissues in both animal species. ROMK protein localized by immunocytochemistry was strongly expressed in the apical membranes of the large umbrella cells lining the bladder lumen and to a lesser extent in the cytoplasm of epithelial cells and smooth muscle cells in the rat bladder. ROMK protein and mRNA were also discovered in cardiac, striated, and smooth muscle in diverse organs. There was no difference in immunoblot expression of ROMK abundance in bladder homogenates (whole bladder, epithelial cell, or muscle cell) or ureteral homogenates between groups of rats fed high- or low-potassium diets. Although the functional role of ROMK in urinary tract epithelia and smooth muscle is unknown, ROMK may participate in the regulation of epithelial and smooth muscle cell volume and osmolality, in the dissipation of potassium leaked or diffused from urine across the epithelial cell apical membranes or tight junctions, and in net or bidirectional potassium transport across urinary tract epithelia.

Expression and localisation of aquaporin water channels in human urothelium in situ and in vitro

BACKGROUND

Urothelium is generally considered to be impermeable to water and constituents of urine. The possibility that human urothelium expresses aquaporin (AQP) water channels as the basis for water and solute transport has not previously been investigated.

OBJECTIVE

To investigate the expression of AQP water channels by human urothelium in situ, in proliferating urothelial cell cultures and in differentiated tissue constructs.

DESIGN, SETTING, AND PARTICIPANTS

AQP expression by human urothelium in situ and cultured urothelial cells was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunolabelling. Expression screening was carried out on samples of freshly isolated urothelia from multiple surgical (bladder and ureteric) specimens and on proliferating and differentiated normal human urothelial (NHU) cells in culture. Urothelial tissue constructs were established and investigated for expression of urothelial differentiation markers and AQPs.

MEASUREMENTS

Qualitative study.

RESULTS AND LIMITATIONS

Transcripts for AQP3, AQP4, AQP7, AQP9, and AQP11 were expressed consistently by freshly isolated urothelia as well as by cultured NHU cells. AQP0, AQP1, AQP2, AQP5, AQP6, AQP8, AQP10, and AQP12 were not expressed. Immunochemistry confirmed expression of AQP3, AQP4, AQP7, and AQP9 at the protein level. AQP3 was shown to be intensely expressed at cell borders in the basal and intermediate layers in both urothelium in situ and differentiated tissue constructs in vitro.

CONCLUSIONS

This is the first study to demonstrate that AQPs are expressed by human urothelium, suggesting a potential role in transurothelial water and solute transport. Our findings challenge the traditional concept of the urinary tract as an impermeable transit and storage unit and provide a versatile platform for further investigations into the biological and clinical relevance of AQPs in human urothelium.

The erythrocyte urea transporter UT-B

During the past decade significant progress has been made in our understanding of the role played by urea transporters in the production of concentrated urine by the kidney. Urea transporters have been cloned and characterized in a wide range of species. The genomic organization of the two major families of mammalian urea transporters, UT-A and UT-B, has been defined, providing new insight into the mechanisms that regulate their expression and function in physiological and pathological conditions. Beside the kidney, the presence of urea transporters has been documented in a variety of tissues, where their role is not fully known. Recently, mice with targeted deletion of the major urea transporters have been generated, which have shown variable impairment of urine concentrating ability, and have helped to clarify the physiological contribution of individual transporters to this process. This review focuses on the erythrocyte urea transporter UT-B.

High urea and creatinine concentrations and urea transporter B in mammalian urinary tract tissues

Although the mammalian urinary tract is generally held to be solely a transit and storage vehicle for urine made by the kidney, in vivo data suggest reabsorption of urea and other urine constituents across urinary tract epithelia. To determine whether urinary tract tissue concentrations are increased as a result of such reabsorption, we measured urea nitrogen and creatinine concentrations and determined whether urea transporter B (UT-B) was present in bladder, ureter, and other tissues from dogs and rats. Mean urea nitrogen and creatinine concentrations in dogs and rats were three- to sevenfold higher in urinary tract tissues than in serum and were comparable to those in renal cortex. In water-restricted or water-loaded rats, urea nitrogen concentrations in bladder tissues fell inversely with the state of hydration, were proportional to urine urea nitrogen concentrations, and were greater than the corresponding serum urea nitrogen concentration in every animal. Immunoblots of rat and dog urinary tract tissues demonstrated the presence of UT-B in homogenates of bladder and ureter, and immunocytochemical analysis localized UT-B to epithelial cell membranes. These findings are consistent with the notion that urea and creatinine are continuously reabsorbed from the urine across the urothelium, urea in part via UT-B, and that urine is thus altered in its passage through the urinary tract. Urea reabsorption across urinary tract epithelia may be important during conditions requiring nitrogen conservation and may contribute to pathophysiological states characterized by high blood urea nitrogen, such as prerenal azotemia and obstructive uropathy.

Tissue distribution of UT-A and UT-B mRNA and protein in rat

Mammalian urea transporters are facilitated membrane transport proteins belonging to two families, UT-A and UT-B. They are best known for their role of maintaining the renal inner medullary urinary concentrating gradient. Urea transporters have also been identified in tissues not typically associated with urea metabolism. The purpose of this study was to survey the major organs in rat to determine the distribution of UT-A and UT-B mRNA transcripts and protein forms and determine their cellular localization. Five kidney subregions and 17 extrarenal tissues were screened by Northern blot analysis using two UT-A and three UT-B probes and by Western blot analysis using polyclonal COOH-terminal UT-A and UT-B antibodies. Immunohistochemistry was performed on 16 extrarenal tissues using the same antibodies. In kidney, we detected mRNA transcripts and protein bands consistent with previously-identified UT-A and UT-B isoforms, as well as novel forms. We found that UT-A mRNA and protein are widely expressed in extrarenal tissues in various forms that are different from the known isoforms. We determined the cellular localization of UT-A and UT-B in these tissues. We found that both UT-A and UT-B are ubiquitously expressed as numerous tissue-specific mRNA transcripts and protein forms that are localized to cell membranes, cytoplasm, or nuclei.

Urea and urine concentrating ability: new insights from studies in mice

Urea is the most abundant solute in the urine in humans (on a Western-type diet) and laboratory rodents. It is far more concentrated in the urine than in plasma and extracellular fluids. This concentration depends on the accumulation of urea in the renal medulla, permitted by an intrarenal recycling of urea among collecting ducts, vasa recta and thin descending limbs, all equipped with specialized, facilitated urea transporters (UTs) (UT-A1 and 3, UT-B, and UT-A2, respectively). UT-B null mice have been recently generated by targeted gene deletion. This review describes 1) the renal handling of urea by the mammalian kidney; 2) the consequences of UT-B deletion on urinary concentrating ability; and 3) species differences among mice, rats, and humans related to their very different body size and metabolic rate, leading to considerably larger needs to excrete and to concentrate urea in smaller species (urea excretion per unit body weight in mice is 5 times that in rats and 23 times that in humans). UT-B null mice have a normal glomerular filtration rate but moderately reduced urea clearance. They exhibit a 30% reduction in urine concentrating ability with a more severe defect in the capacity to concentrate urea (50%) than other solutes, despite a twofold enhanced expression of UT-A2. The urea content of the medulla is reduced by half, whereas that of chloride is almost normal. When given an acute urea load, UT-B null mice are unable to raise their urinary osmolality, urine urea concentration (Uurea), and the concentration of non-urea solutes, as do wild-type mice. When fed diets with progressively increasing protein content (10, 20, and 40%), they cannot prevent a much larger increase in plasma urea than wild-type mice because they cannot raise Uurea. In both wild-type and UT-B null mice, urea clearance was higher than creatinine clearance, suggesting the possibility that urea could be secreted in the mouse kidney, thus allowing more efficient excretion of the disproportionately high urea load. On the whole, studies in UT-B null mice suggest that recycling of urea by countercurrent exchange in medullary vessels plays a more crucial role in the overall capacity to concentrate urine than its recycling in the loops of Henle.

UT-B1 urea transporter is expressed along the urinary and gastrointestinal tracts of the mouse

Selective transporters account for rapid urea transport across plasma membranes of several cell types. UT-B1 urea transporter is widely distributed in rat and human tissues. Because mice exhibit high urea turnover and are the preferred species for gene engineering, we have delineated UT-B1 tissue expression in murine tissues. A cDNA was cloned from BALB/c mouse kidney, encoding a polypeptide that differed from C57BL/6 mouse UT-B1 by one residue (Val-8-Ala). UT-B1 mRNA was detected by RT-PCR in brain, kidney, bladder, testis, lung, spleen, and digestive tract (liver, stomach, jejunum, colon). Northern blotting revealed seven UT-B1 transcripts in mouse tissues. Immunoblots identified a nonglycosylated UT-B1 protein of 29 kDa in most tissues and of 36 and 32 kDa in testis and liver, respectively. UT-B1 protein of gastrointestinal tract did not undergo N-glycosylation. Immunohistochemistry and in situ hybridization localized UT-B1 in urinary tract urothelium (papillary surface, ureter, bladder, and urethra), prominently on plasma membranes and restricted to the basolateral area in umbrella cells. UT-B1 was found in endothelial cells of descending vasa recta in kidney medulla and in astrocyte processes in brain. Dehydration induced by water deprivation for 2 days caused a tissue-specific decrease in UT-B1 abundance in the urinary bladder and the ureter.

Renal urea transporters

PURPOSE OF REVIEW

Urea is transported across the kidney inner medullary collecting duct by urea-transporter proteins. Two urea-transporter genes have been cloned from humans and rodents: the UT-A (Slc14A2) gene encodes five protein and eight cDNA isoforms; the UT-B (Slc14A1) gene encodes a single isoform. In the past year, significant progress has been made in understanding the regulation of urea-transporter protein abundance in kidney, studies of genetically engineered mice that lack a urea transporter, identification of urea transporters outside of the kidney, cloning of urea transporters in nonmammalian species, and active urea transport in microorganisms.

RECENT FINDINGS

UT-A1 protein abundance is increased by 12 days of vasopressin, but not by 5 days. Analysis of the UT-A1 promoter suggests that vasopressin increases UT-A1 indirectly following a direct effect to increase the transcription of other genes, such as the Na(+)-K(+)-2Cl- cotransporter NKCC2/BSC1 and the aquaporin (AQP) 2 water channel, that begin to increase inner medullary osmolality. UT-A1 protein abundance is also increased by adrenalectomy, and is decreased by glucocorticoids or mineralocorticoids. However, each hormone works through its own receptor. Knockout mice that lack UT-A1 and UT-A3, or lack UT-B, have a urine-concentrating defect and a decrease in inner medullary interstitial urea content.

SUMMARY

Urea transporters play a critical role in the urine-concentrating mechanism. Their abundance is regulated by vasopressin, glucocorticoids, and mineralocorticoids. These regulatory mechanisms may be important in disease states such as diabetes because changes in urea-transporter abundance in diabetic rats require glucocorticoids and vasopressin.