Mammalian urea transporters

Structural insights into the mechanisms of urea permeation and distinct inhibition modes of urea transporters

Urea's transmembrane transport through urea transporters (UT) is a fundamental physiological behavior for life activities. Here, we present 11 cryo-EM structures of four UT members in resting states, urea transport states, or inactive states bound with synthetic competitive, uncompetitive or noncompetitive inhibitor. Our results indicate that the binding of urea via a conserved urea recognition motif (URM) and the urea transport via H-bond transfer along the QPb-T5b-T5a-QPa motif among different UT members. Moreover, distinct binding modes of the competitive inhibitors 25a and ATB3, the uncompetitive inhibitor CF11 and the noncompetitive inhibitor HQA2 provide different mechanisms for blocking urea transport and achieved selectivity through L-P pocket, UCBP region and SCG pocket, respectively. In summary, our study not only allows structural understanding of urea transport via UTs but also afforded a structural landscape of hUT-A2 inhibition by competitive, uncompetitive and noncompetitive inhibitors, which may facilitate developing selective human UT-A inhibitors as a new class of salt-sparing diuretics.

Structural characterization of human urea transporters UT-A and UT-B and their inhibition

In this study, we present the structures of human urea transporters UT-A and UT-B to characterize them at molecular level and to detail the mechanism of UT-B inhibition by its selective inhibitor, UTBinh-14. High-resolution structures of both transporters establish the structural basis for the inhibitor's selectivity to UT-B, and the identification of multiple binding sites for the inhibitor will aid with the development of drug lead molecules targeting both transporters. Our study also discovers phospholipids associating with the urea transporters by combining structural observations, native MS, and lipidomics analysis. These insights improve our understanding of urea transporter function at a molecular level and provide a blueprint for a structure-guided design of therapeutics targeting these transporters.

Discovery of novel diarylamides as orally active diuretics targeting urea transporters

Urea transporters (UT) play a vital role in the mechanism of urine concentration and are recognized as novel targets for the development of salt-sparing diuretics. Thus, UT inhibitors are promising for development as novel diuretics. In the present study, a novel UT inhibitor with a diarylamide scaffold was discovered by high-throughput screening. Optimization of the inhibitor led to the identification of a promising preclinical candidate, N-[4-(acetylamino)phenyl]-5-nitrofuran-2-carboxamide (1H), with excellent in vitro UT inhibitory activity at the submicromolar level. The half maximal inhibitory concentrations of 1H against UT-B in mouse, rat, and human erythrocyte were 1.60, 0.64, and 0.13 μmol/L, respectively. Further investigation suggested that 8 μmol/L 1H more powerfully inhibited UT-A1 at a rate of 86.8% than UT-B at a rate of 73.9% in MDCK cell models. Most interestingly, we found for the first time that oral administration of 1H at a dose of 100 mg/kg showed superior diuretic effect in vivo without causing electrolyte imbalance in rats. Additionally, 1H did not exhibit apparent toxicity in vivo and in vitro, and possessed favorable pharmacokinetic characteristics. 1H shows promise as a novel diuretic to treat hyponatremia accompanied with volume expansion and may cause few side effects.

Mode of action of urea

Urea is a major component of many daily skincare products which is widely used. Its role in the treatment of, for example atopic skin, atopic eczema, psoriasis and ichthyosis, is undisputed. However, the mode of action of urea is partly still elusive and goes far beyond its assumed passive role. This article shall elucidate biophysical characteristics and properties of molecular biology that explain how urea affects healthy skin and exerts efficacy in various skin diseases. Knowledge about the mode of action of urea enables physicians to better understandthe appropriate use of urea in clinical routine.

Decreased Expression of the Human Urea Transporter SLC14A1 in Bone is Induced by Cytokines and Stimulates Adipogenesis of Mesenchymal Progenitor Cells

The human urea transporter SLC14A1 (HUT11/UT-B) has been suggested as a marker for the adipogenic differentiation of bone cells with a relevance for bone diseases. We investigated the function of SLC14A1 in different cells models from bone environment. SLC14A1 expression and cytokine production was investigated in bone cells obtained from patients with osteoporosis. Gene and protein expression of SLC14A1 was studied during adipogenic or osteogenic differentiation of human mesenchymal progenitor cells (hMSCs) and of the single-cell-derived hMSC line (SCP-1), as well as in osteoclasts and chondrocytes. Localization was determined by histochemical methods and functionality by urea transport experiments. Expression of SLC14A1 mRNA was lower in cells from patients with osteoporosis that produced high levels of cytokines. Accordingly, when adding a combination of cytokines to SCP-1 SLC14A1 mRNA expression decreased. SLC14A1 mRNA expression decreased after both osteogenic and more pronounced adipogenic stimulation of hMSCs and SCP-1 cells. The highest SLC14A1 expression was determined in undifferentiated cells, lowest in chondrocytes and osteoclasts. Downregulation of SLC14A1 by siRNA resulted in an increased expression of interleukin-6 and interleukin-1 beta as well as adipogenic markers. Urea influx through SLC14A1 increased expression of osteogenic markers, adipogenic markers were suppressed. SLC14A1 protein was localized in the cell membrane and the cytoplasm. Summarizing, the SLC14A1 urea transporter affects early differentiation of hMSCs by diminishing osteogenesis or by favoring adipogenesis, depending on its expression level. Therefore, SLC14A1 is not unequivocally an adipogenic marker in bone. Our findings suggest an involvement of SLC14A1 in bone metabolism and inflammatory processes and disease-dependent influences on its expression.

Water transport mediated by murine urea transporters: implications for urine concentration mechanisms

Urea transporters (UTs) facilitate urea diffusion across cell membranes and play an important role in the urinary concentration mechanisms in the kidney. Herein, we injected cRNAs encoding for c-Myc-tagged murine UT-B, UT-A2 or UT-A3 (versus water-injected control) in Lithobates oocytes and evaluated oocyte surface protein expression with biotinylation and immunoblotting, urea uptake using [14C] counts and water permeability (P f ) by video microscopy. Immunoblots of UT-injected oocyte membranes revealed bands with a molecular weight consistent with that of a UT monomer (34 kDa), and UT-injected oocytes displayed significantly increased and phloretin-sensitive urea uptake and P f when compared to day-matched control oocytes. Subtracting the water-injected urea uptake or P f values from those of UT-injected oocytes yielded UT-dependent values*. We demonstrate for the first time that UT-A2 and UT-A3 can transport water, and we confirm that UT-B is permeable to water. Moreover, the [14C] urea*/P f * ratios fell in the sequence mUT-B>mUT-A2>mUT-A3, indicating that UTs can exhibit selectivity to urea and/or water. It is likely that specific kidney regions with high levels of UTs will exhibit increased urea and/or water permeabilities, directly influencing urine concentration. Furthermore, UT-mediated water transport activity must be considered when developing UT-inhibitors as novel diuretics.This article has an associated First Person interview with the first author of the paper.

Urea-aromatic interactions in biology

Noncovalent interactions are key determinants in both chemical and biological processes. Among such processes, the hydrophobic interactions play an eminent role in folding of proteins, nucleic acids, formation of membranes, protein-ligand recognition, etc.. Though this interaction is mediated through the aqueous solvent, the stability of the above biomolecules can be highly sensitive to any small external perturbations, such as temperature, pressure, pH, or even cosolvent additives, like, urea-a highly soluble small organic molecule utilized by various living organisms to regulate osmotic pressure. A plethora of detailed studies exist covering both experimental and theoretical regimes, to understand how urea modulates the stability of biological macromolecules. While experimentalists have been primarily focusing on the thermodynamic and kinetic aspects, theoretical modeling predominantly involves mechanistic information at the molecular level, calculating atomistic details applying the force field approach to the high level electronic details using the quantum mechanical methods. The review focuses mainly on examples with biological relevance, such as (1) urea-assisted protein unfolding, (2) urea-assisted RNA unfolding, (3) urea lesion interaction within damaged DNA, (4) urea conduction through membrane proteins, and (5) protein-ligand interactions those explicitly address the vitality of hydrophobic interactions involving exclusively the urea-aromatic moiety.

Nanomolar-Potency 1,2,4-Triazoloquinoxaline Inhibitors of the Kidney Urea Transporter UT-A1

Urea transporter A (UT-A) isoforms encoded by the Slc14a2 gene are expressed in kidney tubule epithelial cells, where they facilitate urinary concentration. UT-A1 inhibition is predicted to produce a unique salt-sparing diuretic action in edema and hyponatremia. Here we report the discovery of 1,2,4-triazoloquinoxalines and the analysis of 37 synthesized analogues. The most potent compound, 8ay, containing 1,2,4-triazolo[4,3- a]quinoxaline-substituted benzenesulfonamide linked by an aryl ether, rapidly and reversibly inhibited UT-A1 urea transport by a noncompetitive mechanism with IC50 ≈ 150 nM; the IC50 was ∼2 μM for the related urea transporter UT-B encoded by the Slc14a1 gene. Molecular modeling suggested a putative binding site on the UT-A1 cytoplasmic domain. In vitro metabolism showing quinoxaline ring oxidation prompted the synthesis of metabolically stable 7,8-difluoroquinoxaline analogue 8bl, which when administered to rats produced marked diuresis and reduced urinary osmolality. 8bl has substantially improved UT-A1 inhibition potency and metabolic stability compared with prior compounds.

Modeling of flux, binding and substitution of urea molecules in the urea transporter dvUT

Urea transporters (UTs) are transmembrane proteins that transport urea molecules across cell membranes and play a crucial role in urea excretion and water balance. Modeling the functional characteristics of UTs helps us understand how their structures accomplish the functions at the atomic level, and facilitates future therapeutic design targeting the UTs. This study was based on the crystal structure of Desulfovibrio vulgaris urea transporter (dvUT). To model the binding behavior of urea molecules in dvUT, we constructed a cooperative binding model. To model the substitution of urea by the urea analogue N,N'-dimethylurea (DMU) in dvUT, we calculated the occupation probability of DMU along the urea pore and the ratio of the occupation probabilities of DMU at the external (Sext) and internal (Sint) binding sites, and we established the mutual substitution rule for binding and substitution of urea and DMU. Based on these calculations and modelings, together with the use of the Monte Carlo (MC) method, we further modeled the urea flux in dvUT, equilibrium urea binding to dvUT, and the substitution of urea by DMU in the dvUT. Our modeling results are in good agreement with the existing experimental functional data. Furthermore, the modelings have discovered the microscopic process and mechanisms of those functional characteristics. The methods and the results would help our future understanding of the underlying mechanisms of the diseases associated with impaired UT functions and rational drug design for the treatment of these diseases.

Gene networks in neurodegenerative disorders

Three neurodegenerative diseases [Amyotrophic Lateral Sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD)] have many characteristics like pathological mechanisms and genes. In this sense some researchers postulate that these diseases share the same alterations and that one alteration in a specific protein triggers one of these diseases. Analyses of gene expression may shed more light on how to discover pathways, pathologic mechanisms associated with the disease, biomarkers and potential therapeutic targets. In this review, we analyze four microarrays related to three neurodegenerative diseases. We will systematically examine seven genes (CHN1, MDH1, PCP4, RTN1, SLC14A1, SNAP25 and VSNL1) that are altered in the three neurodegenerative diseases. A network was built and used to identify pathways, miRNA and drugs associated with ALS, AD and PD using Cytoscape software an interaction network based on the protein interactions of these genes. The most important affected pathway is PI3K-Akt signalling. Thirteen microRNAs (miRNA-19B1, miRNA-107, miRNA-124-1, miRNA-124-2, miRNA-9-2, miRNA-29A, miRNA-9-3, miRNA-328, miRNA-19B2, miRNA-29B2, miRNA-124-3, miRNA-15A and miRNA-9-1) and four drugs (Estradiol, Acetaminophen, Resveratrol and Progesterone) for new possible treatments were identified.

Potential of Mean Force Calculations of Solute Permeation Across UT-B and AQP1: A Comparison between Molecular Dynamics and 3D-RISM

Membrane channels facilitate the efficient and selective flux of various solutes across biological membranes. A common approach to investigate the selectivity of a channel has been the calculation of potentials of mean force (PMFs) for solute permeation across the pore. PMFs have been frequently computed from molecular dynamics (MD) simulations, yet the three-dimensional reference interaction site model (3D-RISM) has been suggested as a computationally efficient alternative to MD. Whether the two methods yield comparable PMFs for solute permeation has remained unclear. In this study, we calculated potentials of mean force for water, ammonia, urea, molecular oxygen, and methanol across the urea transporter B (UT-B) and aquaporin-1 (AQP1), using 3D-RISM, as well as using MD simulations and umbrella sampling. To allow direct comparison between the PMFs from 3D-RISM and MD, we ensure that all PMFs refer to a well-defined reference area in the bulk or, equivalently, to a well-defined density of channels in the membrane. For PMFs of water permeation, we found reasonable agreement between the two methods, with differences of ≲3 kJ mol-1. In contrast, we found stark discrepancies for the PMFs for all other solutes. Additional calculations confirm that discrepancies between MD and 3D-RISM are not explained by the choice for the closure relation, the definition the reaction coordinate (center of mass-based versus atomic site-based), details of the molecule force field, or fluctuations of the protein. Comparison of the PMFs suggests that 3D-RISM may underestimate effects from hydrophobic solute-channel interactions, thereby, for instance, missing the urea binding sites in UT-B. Furthermore, we speculate that the orientational averages inherent to 3D-RISM might lead to discrepancies in the narrow channel lumen. These findings suggest that current 3D-RISM solvers provide reasonable estimates for the PMF for water permeation, but that they are not suitable to study the selectivity of membrane channels with respect to uncharged nonwater solutes.

Aestivation Induces Changes in the mRNA Expression Levels and Protein Abundance of Two Isoforms of Urea Transporters in the Gills of the African Lungfish, Protopterus annectens

The African lungfish, Protopterus annectens, is ammonotelic in water despite being ureogenic. When it aestivates in mucus cocoon on land, ammonia is detoxified to urea. During the maintenance phase of aestivation, urea accumulates in the body, which is subsequently excreted upon arousal. Urea excretion involves urea transporters (UT/Ut). This study aimed to clone and sequence the ut isoforms from the gills of P. annectens, and to test the hypothesis that the mRNA and/or protein expression levels of ut/Ut isoforms could vary in the gills of P. annectens during the induction, maintenance, and arousal phases of aestivation. Two isoforms of ut, ut-a2a and ut-a2b, were obtained from the gills of P. annectens. ut-a2a consisted of 1227 bp and coded for 408 amino acids with an estimated molecular mass of 44.7 kDa, while ut-a2b consisted of 1392 bp and coded for 464 amino acids with an estimated molecular mass of 51.2 kDa. Ut-a2a and Ut-a2b of P. annectens had a closer phylogenetic relationship with Ut/UT of tetrapods than Ut of fishes. While the mRNA expression pattern of ut-a2a and ut-a2b across various tissues of P. annectens differed, the transcript levels of ut-a2a and ut-a2b in the gills were comparable, indicating that they might be equally important for branchial urea excretion during the initial arousal phase of aestivation. During the maintenance phase of aestivation, the transcript level of ut-a2a increased significantly, but the protein abundance of Ut-a2a remained unchanged in the gills of P. annectens. This could be an adaptive feature to prepare for an increase in the production of Ut-a2a upon arousal. Indeed, arousal led to a significant increase in the branchial Ut-a2a protein abundance. Although the transcript level of ut-a2b remained unchanged, there were significant increases in the protein abundance of Ut-a2b in the gills of P. annectens throughout the three phases of aestivation. The increase in the protein abundance of Ut-a2b during the maintenance phase could also be an adaptive feature to prepare for efficient urea excretion when water becomes available.

Salt-sparing diuretic action of a water-soluble urea analog inhibitor of urea transporters UT-A and UT-B in rats

Inhibitors of kidney urea transporter (UT) proteins have potential use as salt-sparing diuretics ('urearetics') with a different mechanism of action than diuretics that target salt transporters. To study UT inhibition in rats, we screened about 10,000 drugs, natural products and urea analogs for inhibition of rat UT-A1. Drug and natural product screening found nicotine, sanguinarine and an indolcarbonylchromenone with IC50 of 10-20 μM. Urea analog screening found methylacetamide and dimethylthiourea (DMTU). DMTU fully and reversibly inhibited rat UT-A1 and UT-B by a noncompetitive mechanism with IC50 of 2-3 mM. Homology modeling and docking computations suggested DMTU binding sites on rat UT-A1. Following a single intraperitoneal injection of 500 mg/kg DMTU, peak plasma concentration was 9 mM with t1/2 of about 10 h, and a urine concentration of 20-40 mM. Rats chronically treated with DMTU had a sustained, reversible reduction in urine osmolality from 1800 to 600 mOsm, a 3-fold increase in urine output, and mild hypokalemia. DMTU did not impair urinary concentrating function in rats on a low protein diet. Compared to furosemide-treated rats, the DMTU-treated rats had greater diuresis and reduced urinary salt loss. In a model of syndrome of inappropriate antidiuretic hormone secretion, DMTU treatment prevented hyponatremia and water retention produced by water-loading in dDAVP-treated rats. Thus, our results establish a rat model of UT inhibition and demonstrate the diuretic efficacy of UT inhibition.

Structure-activity analysis of thiourea analogs as inhibitors of UT-A and UT-B urea transporters

Small-molecule inhibitors of urea transporter (UT) proteins in kidney have potential application as novel salt-sparing diuretics. The urea analog dimethylthiourea (DMTU) was recently found to inhibit the UT isoforms UT-A1 (expressed in kidney tubule epithelium) and UT-B (expressed in kidney vasa recta endothelium) with IC50 of 2-3 mM, and was shown to have diuretic action when administered to rats. Here, we measured UT-A1 and UT-B inhibition activity of 36 thiourea analogs, with the goal of identifying more potent and isoform-selective inhibitors, and establishing structure-activity relationships. The analog set systematically explored modifications of substituents on the thiourea including alkyl, heterocycles and phenyl rings, with different steric and electronic features. The analogs had a wide range of inhibition activities and selectivities. The most potent inhibitor, 3-nitrophenyl-thiourea, had an IC50 of ~0.2 mM for inhibition of both UT-A1 and UT-B. Some analogs such as 4-nitrophenyl-thiourea were relatively UT-A1 selective (IC50 1.3 vs. 10 mM), and others such as thioisonicotinamide were UT-B selective (IC50>15 vs. 2.8 mM).

Computation and simulation of the structural characteristics of the kidney urea transporter and behaviors of urea transport

Urea transporters are a family of membrane proteins that transport urea molecules across cell membranes and play important roles in a variety of physiological processes. Although the crystal structure of bacterial urea channel dvUT has been solved, there lacks an understanding of the dynamics of urea transport in dvUT. In this study, by using molecular dynamics simulations, Monte Carlo methods, and the adaptive biasing force approach, we built the equilibrium structure of dvUT, calculated the variation in the free energy of urea, determined the urea-binding sites of dvUT, gained insight into the microscopic process of urea transport, and studied the water permeability in dvUT including the analysis of a water chain in the pore. The strategy used in this work can be applied to studying transport behaviors of other membrane proteins.

Physicochemical evolution and molecular adaptation of the cetacean osmoregulation-related gene UT-A2 and implications for functional studies

Cetaceans have an enigmatic evolutionary history of re-invading aquatic habitats. One of their essential adaptabilities that has enabled this process is their homeostatic strategy adjustment. Here, we investigated the physicochemical evolution and molecular adaptation of the cetacean urea transporter UT-A2, which plays an important role in urine concentration and water homeostasis. First, we cloned UT-A2 from the freshwater Yangtze finless porpoise, after which bioinformatics analyses were conducted based on available datasets (including freshwater baiji and marine toothed and baleen whales) using MEGA, PAML, DataMonkey, TreeSAAP and Consurf. Our findings suggest that the UT-A2 protein shows folding similar to that of dvUT and UT-B, whereas some variations occurred in the functional So and Si regions of the selectivity filter. Additionally, several regions of the cetacean UT-A2 protein have experienced molecular adaptations. We suggest that positive-destabilizing selection could contribute to adaptations by influencing its biochemical and conformational character. The conservation of amino acid residues within the selectivity filter of the urea conduction pore is likely to be necessary for urea conduction, whereas the non-conserved amino acid replacements around the entrance and exit of the conduction pore could potentially affect the activity, which could be interesting target sites for future mutagenesis studies.

Urea transporter proteins as targets for small-molecule diuretics

Conventional diuretics such as furosemide and thiazides target salt transporters in kidney tubules, but urea transporters (UTs) have emerged as alternative targets. UTs are a family of transmembrane channels expressed in a variety of mammalian tissues, in particular the kidney. UT knockout mice and humans with UT mutations exhibit reduced maximal urinary osmolality, demonstrating that UTs are necessary for the concentration of urine. Small-molecule screening has identified potent and selective inhibitors of UT-A, the UT protein expressed in renal tubule epithelial cells, and UT-B, the UT protein expressed in vasa recta endothelial cells. Data from UT knockout mice and from rodents administered UT inhibitors support the diuretic action of UT inhibition. The kidney-specific expression of UT-A1, together with high selectivity of the small-molecule inhibitors, means that off-target effects of such small-molecule drugs should be minimal. This Review summarizes the structure, expression and function of UTs, and looks at the evidence supporting the validity of UTs as targets for the development of salt-sparing diuretics with a unique mechanism of action. UT-targeted inhibitors may be useful alone or in combination with conventional diuretics for therapy of various oedemas and hyponatraemias, potentially including those refractory to treatment with current diuretics.

Discovery, synthesis and structure-activity analysis of symmetrical 2,7-disubstituted fluorenones as urea transporter inhibitors

Kidney urea transporters are targets for development of small-molecule inhibitors with action as salt-sparing diuretics. A cell-based, functional high-throughput screen identified 2,7-bisacetamido fluorenone 3 as a novel inhibitor of urea transporters UT-A1 and UT-B. Here, we synthesized twenty-two 2,7-disubstituted fluorenone analogs by acylation. Structure-activity relationship analysis revealed: (a) the carbonyl moiety at C9 is required for UT inhibition; (b) steric limitation on C2, 7-substituents; and (c) the importance of a crescent-shape structure. The most potent fluorenones inhibited UT-A1 and UT-B urea transport with IC₅₀ ~ 1 μM. Analysis of in vitro metabolic stability in hepatic microsomes indicated metabolism of 2,7-disubstituted fluorenones by reductase and subsequent elimination. Computational docking to a homology model of UT-A1 suggested UT inhibitor binding to the UT cytoplasmic domain at a site that does not overlap with the putative urea binding site.

Morphological and functional characteristics of the kidney of cartilaginous fishes: with special reference to urea reabsorption

For adaptation to high-salinity marine environments, cartilaginous fishes (sharks, skates, rays, and chimaeras) adopt a unique urea-based osmoregulation strategy. Their kidneys reabsorb nearly all filtered urea from the primary urine, and this is an essential component of urea retention in their body fluid. Anatomical investigations have revealed the extraordinarily elaborate nephron system in the kidney of cartilaginous fishes, e.g., the four-loop configuration of each nephron, the occurrence of distinct sinus and bundle zones, and the sac-like peritubular sheath in the bundle zone, in which the nephron segments are arranged in a countercurrent fashion. These anatomical and morphological characteristics have been considered to be important for urea reabsorption; however, a mechanism for urea reabsorption is still largely unknown. This review focuses on recent progress in the identification and mapping of various pumps, channels, and transporters on the nephron segments in the kidney of cartilaginous fishes. The molecules include urea transporters, Na(+)/K(+)-ATPase, Na(+)-K(+)-Cl(-) cotransporters, and aquaporins, which most probably all contribute to the urea reabsorption process. Although research is still in progress, a possible model for urea reabsorption in the kidney of cartilaginous fishes is discussed based on the anatomical features of nephron segments and vascular systems and on the results of molecular mapping. The molecular anatomical approach thus provides a powerful tool for understanding the physiological processes that take place in the highly elaborate kidney of cartilaginous fishes.

Diuresis and reduced urinary osmolality in rats produced by small-molecule UT-A-selective urea transport inhibitors

Urea transport (UT) proteins of the UT-A class are expressed in epithelial cells in kidney tubules, where they are required for the formation of a concentrated urine by countercurrent multiplication. Here, using a recently developed high-throughput assay to identify UT-A inhibitors, a screen of 50,000 synthetic small molecules identified UT-A inhibitors of aryl-thiazole, γ-sultambenzosulfonamide, aminocarbonitrile butene, and 4-isoxazolamide chemical classes. Structure-activity analysis identified compounds that inhibited UT-A selectively by a noncompetitive mechanism with IC50 down to ∼1 μM. Molecular modeling identified putative inhibitor binding sites on rat UT-A. To test compound efficacy in rats, formulations and administration procedures were established to give therapeutic inhibitor concentrations in blood and urine. We found that intravenous administration of an indole thiazole or a γ-sultambenzosulfonamide at 20 mg/kg increased urine output by 3-5-fold and reduced urine osmolality by ∼2-fold compared to vehicle control rats, even under conditions of maximum antidiuresis produced by 1-deamino-8-D-arginine vasopressin (DDAVP). The diuresis was reversible and showed urea > salt excretion. The results provide proof of concept for the diuretic action of UT-A-selective inhibitors. UT-A inhibitors are first in their class salt-sparing diuretics with potential clinical indications in volume-overload edemas and high-vasopressin-associated hyponatremias.

Modelling and mutational analysis of Aspergillus nidulans UreA, a member of the subfamily of urea/H⁺ transporters in fungi and plants

We present the first account of the structure-function relationships of a protein of the subfamily of urea/H(+) membrane transporters of fungi and plants, using Aspergillus nidulans UreA as a study model. Based on the crystal structures of the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT) and of the Nucleobase-Cation-Symport-1 benzylhydantoin transporter from Microbacterium liquefaciens (Mhp1), we constructed a three-dimensional model of UreA which, combined with site-directed and classical random mutagenesis, led to the identification of amino acids important for UreA function. Our approach allowed us to suggest roles for these residues in the binding, recognition and translocation of urea, and in the sorting of UreA to the membrane. Residues W82, Y106, A110, T133, N275, D286, Y388, Y437 and S446, located in transmembrane helixes 2, 3, 7 and 11, were found to be involved in the binding, recognition and/or translocation of urea and the sorting of UreA to the membrane. Y106, A110, T133 and Y437 seem to play a role in substrate selectivity, while S446 is necessary for proper sorting of UreA to the membrane. Other amino acids identified by random classical mutagenesis (G99, R141, A163, G168 and P639) may be important for the basic transporter's structure, its proper folding or its correct traffic to the membrane.

Small-molecule inhibitors of urea transporters

Urea transporter (UT) proteins, which include isoforms of UT-A in kidney tubule epithelia and UT-B in vasa recta endothelia and erythrocytes, facilitate urinary concentrating function. Inhibitors of urea transporter function have potential clinical applications as sodium-sparing diuretics, or 'urearetics,' in edema from different etiologies, such as congestive heart failure and cirrhosis, as well as in syndrome of inappropriate antidiuretic hormone (SIADH). High-throughput screening of drug-like small molecules has identified UT-A and UT-B inhibitors with nanomolar potency. Inhibitors have been identified with different UT-A versus UT-B selectivity profiles and putative binding sites on UT proteins. Studies in rodent models support the utility of UT inhibitors in reducing urinary concentration, though testing in clinically relevant animal models of edema has not yet been done.

A small molecule screen identifies selective inhibitors of urea transporter UT-A

Urea transporter (UT) proteins, including UT-A in kidney tubule epithelia and UT-B in vasa recta microvessels, facilitate urinary concentrating function. A screen for UT-A inhibitors was developed in MDCK cells expressing UT-A1, water channel aquaporin-1, and YFP-H148Q/V163S. An inwardly directed urea gradient produces cell shrinking followed by UT-A1-dependent swelling, which was monitored by YFP-H148Q/V163S fluorescence. Screening of ~90,000 synthetic small molecules yielded four classes of UT-A1 inhibitors with low micromolar half-maximal inhibitory concentration that fully and reversibly inhibited urea transport by a noncompetitive mechanism. Structure-activity analysis of >400 analogs revealed UT-A1-selective and UT-A1/UT-B nonselective inhibitors. Docking computations based on homology models of UT-A1 suggested inhibitor binding sites. UT-A inhibitors may be useful as diuretics ("urearetics") with a mechanism of action that may be effective in fluid-retaining conditions in which conventional salt transport-blocking diuretics have limited efficacy.

Adaptive evolution of the osmoregulation-related genes in cetaceans during secondary aquatic adaptation

Background

Osmoregulation was a primary challenge for cetaceans during the evolutionary transition from a terrestrial to a mainly hyperosmotic environment. Several physiological mechanisms have been suggested to maintain the water and salt balance in cetaceans, but their genetic and evolutionary bases remain poorly explored. The current study investigated the genes involved in osmoregulation in cetaceans and compared them with their counterparts in terrestrial mammals to test whether adaptive evolution occurred during secondary aquatic adaptation.

Results

The present study analyzed the molecular evolution of 11 osmoregulation-related genes in 11 cetacean species, which represented all of the major cetacean clades. The results demonstrated positive selection acting on angiotensin converting enzyme (ACE), angiotensinogen (AGT), SLC14A2, and aquaporin 2 (AQP2). This evidence for the positive selection of AQP2 and SLC14A2 suggests that the adaptive evolution of these genes has helped to enhance the capacity for water and urea transport, thereby leading to the concentration of urine, which is an efficient mechanism for maintaining the water balance. By contrast, a series of positively selected amino acid residues identified in the ACE and AGT (two key members of the renin-angiotensin-aldosterone system, RAAS) proteins of cetaceans suggests that RAAS might have been adapted to maintain the water and salt balance in response to a hyperosmotic environment. Radical amino acid changes in positively selected sites were distributed among most internal and terminal branches of the cetacean phylogeny, which suggests the pervasively adaptive evolution of osmoregulation since the origin of cetaceans and their subsequent diversification.

Conclusions

This is the first comprehensive analysis of the molecular evolution of osmoregulation-related genes in cetaceans in response to selection pressure from a generally hyperosmotic environment. Four genes, i.e., AQP2, SLC14A2, ACE, and AGT were subject to positive selection in cetaceans, which suggests that cetaceans may have adapted to maintain their water and salt balance. This also suggests that cetaceans may have evolved an effective and complex mechanism for osmoregulation.

Movement of NH₃ through the human urea transporter B: a new gas channel

Aquaporins and Rh proteins can function as gas (CO₂ and NH₃) channels. The present study explores the urea, H₂O, CO₂, and NH₃ permeability of the human urea transporter B (UT-B) (SLC14A1), expressed in Xenopus oocytes. We monitored urea uptake using [¹⁴C]urea and measured osmotic water permeability (Pf) using video microscopy. To obtain a semiquantitative measure of gas permeability, we used microelectrodes to record the maximum transient change in surface pH (ΔpHS) caused by exposing oocytes to 5% CO₂/33 mM HCO₃⁻ (pHS increase) or 0.5 mM NH₃/NH₄⁺ (pHS decrease). UT-B expression increased oocyte permeability to urea by >20-fold, and Pf by 8-fold vs. H₂O-injected control oocytes. UT-B expression had no effect on the CO₂-induced ΔpHS but doubled the NH₃-induced ΔpHS. Phloretin reduced UT-B-dependent urea uptake (Jurea*) by 45%, Pf* by 50%, and (- ΔpHS*)NH₃ by 70%. p-Chloromercuribenzene sulfonate reduced Jurea* by 25%, Pf* by 30%, and (ΔpHS*)NH₃ by 100%. Molecular dynamics (MD) simulations of membrane-embedded models of UT-B identified the monomeric UT-B pores as the main conduction pathway for both H₂O and NH₃ and characterized the energetics associated with permeation of these species through the channel. Mutating each of two conserved threonines lining the monomeric urea pores reduced H₂O and NH₃ permeability. Our data confirm that UT-B has significant H₂O permeability and for the first time demonstrate significant NH₃ permeability. Thus the UTs become the third family of gas channels. Inhibitor and mutagenesis studies and results of MD simulations suggest that NH₃ and H₂O pass through the three monomeric urea channels in UT-B.

1,1-Difluoroethyl-substituted triazolothienopyrimidines as inhibitors of a human urea transport protein (UT-B): new analogs and binding model

The kidney urea transport protein UT-B is an attractive target for the development of small-molecule inhibitors with a novel diuretic ('urearetic') action. Previously, two compounds in the triazolothienopyrimidine scaffold (1a and 1c) were reported as UT-B inhibitors. Compound 1c incorporates a 1,1-difluoroethyl group, which affords improved microsomal stability when compared to the corresponding ethyl-substituted compound 1a. Here, a small focused library (4a-4f) was developed around lead inhibitor 1c to investigate the requirement of an amidine-linked thiophene in the inhibitor scaffold. Two compounds (4a and 4b) with nanomolar inhibitory potency (IC50≈40 nM) were synthesized. Computational docking of lead structure 1c and 4a-4f into a homology model of the UT-B cytoplasmic surface suggested binding with the core heterocycle buried deep into the hydrophobic pore region of the protein.

Immunohistochemical localization of urea and ammonia transporters in two confamilial fish species, the ureotelic gulf toadfish (Opsanus beta) and the ammoniotelic plainfin midshipman (Porichthys notatus)

This study aims to illustrate potential transport mechanisms behind the divergent approaches to nitrogen excretion seen in the ureotelic toadfish (Opsanus beta) and the ammoniotelic plainfin midshipman (Porichthys notatus). Specifically, we wish to confirm the expression of a urea transporter (UT), which is found in the gill of the toadfish and which is responsible for the unique "pulsing" nature of urea excretion and to localize the transporter within specific gill cells and at specific cellular locations. Additionally, the localization of ammonia transporters (Rhesus glycoproteins; Rhs) within the gill of both the toadfish and midshipman was explored. Toadfish UT (tUT) was found within Na(+)-K(+)-ATPase (NKA)-enriched cells, i.e., ionocytes (probably mitochondria-rich cells), especially along the basolateral membrane and potentially on the apical membrane. In contrast, midshipman UT (pnUT) immunoreactivity did not colocalize with NKA immunoreactivity and was not found along the filaments but instead within the lamellae. The cellular location of Rh proteins was also dissimilar between the two fish species. In toadfish gills, the Rh isoform Rhcg1 was expressed in both NKA-reactive cells and non-reactive cells, whereas Rhbg and Rhcg2 were only expressed in the latter. In contrast, Rhbg, Rhcg1 and Rhcg2 were expressed in both NKA-reactive and non-reactive cells of midshipman gills. In an additional transport epithelium, namely the intestine, the expression of both UTs and Rhs was similar between the two species, with only subtle differences being observed.

The urea transporter family (SLC14): physiological, pathological and structural aspects

Urea transporters (UTs) belonging to the solute carrier 14 (SLC14) family comprise two genes with a total of eight isoforms in mammals, UT-A1 to -A6 encoded by SLC14A2 and UT-B1 to -B2 encoded by SLC14A1. Recent efforts have been directed toward understanding the molecular and cellular mechanisms involved in the regulation of UTs using transgenic mouse models and heterologous expression systems, leading to important new insights. Urea uptake by UT-A1 and UT-A3 in the kidney inner medullary collecting duct and by UT-B1 in the descending vasa recta for the countercurrent exchange system are chiefly responsible for medullary urea accumulation in the urinary concentration process. Vasopressin, an antidiuretic hormone, regulates UT-A isoforms via the phosphorylation and trafficking of the glycosylated transporters to the plasma membrane that occurs to maintain equilibrium with the exocytosis and ubiquitin-proteasome degradation pathways. UT-B isoforms are also important in several cellular functions, including urea nitrogen salvaging in the colon, nitric oxide pathway modulation in the hippocampus, and the normal cardiac conduction system. In addition, genomic linkage studies have revealed potential additional roles for SLC14A1 and SLC14A2 in hypertension and bladder carcinogenesis. The precise role of UT-A2 and presence of the urea recycling pathway in normal kidney are issues to be further explored. This review provides an update of these advances and their implications for our current understanding of the SLC14 UTs.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Expression of urea transporters is affected by dietary nitrogen restriction in goat kidney

Ruminants are known to be able to very effectively recycle urinary urea and reuse it as a source of N for ruminal microbes. It is presumed that urea recycling is accomplished by specialized urea transporters (UT) which are localized in the kidney. This could be especially important in times of increased N requirement, such as during growth or during reduced dietary N intake. The aim of our study was to characterize and to localize UT in the goat (capra hircus) kidney and to investigate its response to reduced dietary N intake in growing goats. Therefore, 12 growing, male goats were fed either a diet containing high (17% CP in complete diet) or low (9% CP in complete diet) N content for 6 wk. After harvesting, blood and kidney samples were taken and analyzed. The mRNA of the different UT isoforms, UT-A1, UT-A2 and UT-B, were detected semiquantitatively in renal tissue by Northern blot analysis. For UT-A2 and UT-B, no statistically significant effect of dietary N restriction on renal mRNA expression could be detected (UT-A2: P = 0.26, UT-B: P = 0.07). However, renal mRNA abundance of UT-A1 significantly increased in the kidney of low-N-fed goats (P = 0.01). Furthermore, protein amounts of UT-B were verified by western blotting; and the localization of UT-A2 and UT-B protein was demonstrated by immunohistochemistry. No significant differences in protein amounts of UT-B could be observed comparing the 2 feeding groups (P = 0.78). The UT-B was localized in renal medulla and papilla, whereas UT-A2 was only found in renal medulla. In addition, comparison of UT-A and UT-BAA sequences of monogastric animals and ruminants showed a high degree of homology, indicating a similar function of the transporters among these species. In summary, we conclude that in ruminants, urea reabsorption in the kidney is most likely increased in response to a low-N diet via an upregulation of UT-A1 mRNA expression. Hypothetically, the reabsorbed urea can then be returned to the rumen via the bloodstream and thus be reused as a source of N for protein synthesis of ruminal microbial community.

The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney

The Chinese soft-shelled turtle, Pelodiscus sinensis, is well adapted to aquatic environments, including brackish swamps and marshes. It is ureotelic, and occasionally submerges its head into puddles of water during emersion, presumably for buccopharyngeal respiration. This study was undertaken to test the hypothesis that the buccophyaryngeal cavity constitutes an important excretory route for urea in P. sinensis. Results indicate that a major portion of urea was excreted through the mouth instead of the kidney during immersion. When restrained on land, P. sinensis occasionally submerged their head into water (20-100 min), during which urea excretion and oxygen extraction occurred simultaneously. These results indicate for the first time that buccopharyngeal villiform processes (BVP) and rhythmic pharyngeal movements were involved in urea excretion in P. sinensis. Urea excretion through the mouth was sensitive to phloretin inhibition, indicating the involvement of urea transporters (UTs). In addition, saliva samples collected from the buccopharyngeal surfaces of P. sinensis injected intraperitoneally with saline contained ~36 mmol N l(-1) urea, significantly higher than that (~2.4 mmol N l(-1)) in the plasma. After intraperitoneal injection with 20 μmol urea g(-1) turtle, the concentration of urea in the saliva collected from the BVP increased to an extraordinarily high level of ~614 μmol N ml(-1), but the urea concentration (~45 μmol N ml(-1)) in the plasma was much lower, indicating that the buccopharyngeal epithelium of P. sinensis was capable of active urea transport. Subsequently, we obtained from the buccopharyngeal epithelium of P. sinensis the full cDNA sequence of a putative UT, whose deduced amino acid sequence had ~70% similarity with human and mouse UT-A2. This UT was not expressed in the kidney, corroborating the proposition that the kidney had only a minor role in urea excretion in P. sinensis. As UT-A2 is known to be a facilitative urea transporter, it is logical to deduce that it was localized in the basolateral membrane of the buccopharyngeal epithelium, and that another type of primary or secondary active urea transporter yet to be identified was present in the apical membrane. The ability to excrete urea through the mouth instead of the kidney might have facilitated the ability of P. sinensis and other soft-shelled turtles to successfully invade the brackish and/or marine environment.

Fractional excretion of uric acid as a predictor for saline responsiveness in long-term kidney transplant patients

BACKGROUND/AIMS

Subclinical hypovolemia may contribute to allograft dysfunction in long-term kidney transplant (KT) patients. In order to predict responsiveness to saline hydration, indices for tubular transport were investigated.

METHODS

Fifty-four clinically euvolemic long-term KT patients with recently aggravated azotemia were given intravenous hydration as follows: 0.9% saline 5 ml/kg over 1 h, followed by 0.9% saline 1 ml/kg/h over 12 h and 1 liter of 0.45% saline over the next 24 h. Serum and urine data were collected and analyzed to assess responses.

RESULTS

In all patients, saline hydration relieved azotemia, as shown by blood urea nitrogen (46.9 ± 17.2 vs. 39.3 ± 15.4 mg/dl; p < 0.01) and serum creatinine levels (2.9 ± 1.1 vs. 2.5 ± 1.1 mg/dl; p < 0.01) on day 0 versus day 2. In 38 patients, serum creatinine did not increase in the following month (70% responders). Compared with the nonresponders, the responders had a higher urine-to-plasma creatinine ratio and lower fractional excretion of sodium, uric acid and urea at admission. Multivariate logistic regression analysis revealed that responsiveness to saline hydration was independently associated with lower fractional excretion of uric acid.

CONCLUSION

Subclinical hypovolemia should be considered in long-term KT patients with azotemia of unexplainable causes. Fractional excretion of uric acid may predict responsiveness to saline hydration.

Nanomolar potency and metabolically stable inhibitors of kidney urea transporter UT-B

Urea transporters, which include UT-B in kidney microvessels, are potential targets for development of drugs with a novel diuretic ('urearetic') mechanism. We recently identified, by high-throughput screening, a triazolothienopyrimidine UT-B inhibitor, 1, that selectively and reversibly inhibited urea transport with IC(50) = 25.1 nM and reduced urinary concentration in mice ( Yao et al. J. Am. Soc. Nephrol. , in press ). Here, we analyzed 273 commercially available analogues of 1 to establish a structure-activity series and synthesized a targeted library of 11 analogues to identify potent, metabolically stable UT-B inhibitors. The best compound, {3-[4-(1,1-difluoroethyl)benzenesulfonyl]thieno[2,3-e][1,2,3]triazolo[1,5-a]pyrimidin-5-yl}thiophen-2-ylmethylamine, 3k, had IC(50) of 23 and 15 nM for inhibition of urea transport by mouse and human UT-B, respectively, and ∼40-fold improved in vitro metabolic stability compared to 1. In mice, 3k accumulated in kidney and urine and reduced maximum urinary concentration. Triazolothienopyrimidines may be useful for therapy of diuretic-refractory edema in heart and liver failure.

Urea uptake enhances barrier function and antimicrobial defense in humans by regulating epidermal gene expression

Urea is an endogenous metabolite, known to enhance stratum corneum hydration. Yet, topical urea anecdotally also improves permeability barrier function, and it appears to exhibit antimicrobial activity. Hence, we hypothesized that urea is not merely a passive metabolite, but a small-molecule regulator of epidermal structure and function. In 21 human volunteers, topical urea improved barrier function in parallel with enhanced antimicrobial peptide (AMP; LL-37 and β-defensin-2) expression. Urea stimulates the expression of, and is transported into, keratinocytes by two urea transporters (UTs), UT-A1 and UT-A2, and by aquaporins 3, 7, and 9. Inhibitors of these UTs block the downstream biological effects of urea, which include increased mRNA and protein levels of (i) transglutaminase-1, involucrin, loricrin, and filaggrin, (ii) epidermal lipid synthetic enzymes, and (iii) cathelicidin/LL-37 and β-defensin-2. Finally, we explored the potential clinical utility of urea, showing that topical urea applications normalized both barrier function and AMP expression in a murine model of atopic dermatitis. Together, these results show that urea is a small-molecule regulator of epidermal permeability barrier function and AMP expression after transporter uptake, followed by gene regulatory activity in normal epidermis, with potential therapeutic applications in diseased skin.

The emerging physiological roles of the SLC14A family of urea transporters

In mammals, urea is the main nitrogenous breakdown product of protein catabolism and is produced in the liver. In certain tissues, the movement of urea across cell membranes is specifically mediated by a group of proteins known as the SLC14A family of facilitative urea transporters. These proteins are derived from two distinct genes, UT-A (SLC14A2) and UT-B (SLC14A1). Facilitative urea transporters play an important role in two major physiological processes - urinary concentration and urea nitrogen salvaging. Although UT-A and UT-B transporters both have a similar basic structure and mediate the transport of urea in a facilitative manner, there are a number of significant differences between them. UT-A transporters are mainly found in the kidney, are highly specific for urea, have relatively lower transport rates and are highly regulated at both gene expression and cellular localization levels. In contrast, UT-B transporters are more widespread in their tissue location, transport both urea and water, have a relatively high transport rate, are inhibited by mercurial compounds and currently appear to be less acutely regulated. This review details the fundamental research that has so far been performed to investigate the function and physiological significance of these two types of urea transporters.

Role of thin descending limb urea transport in renal urea handling and the urine concentrating mechanism

Urea transporters UT-A2 and UT-B are expressed in epithelia of thin descending limb of Henle's loop and in descending vasa recta, respectively. To study their role and possible interaction in the context of the urine concentration mechanism, a UT-A2 and UT-B double knockout (UT-A2/B knockout) mouse model was generated by targeted deletion of the UT-A2 promoter in embryonic stem cells with UT-B gene knockout. The UT-A2/B knockout mice lacked detectable UT-A2 and UT-B transcripts and proteins and showed normal survival and growth. Daily urine output was significantly higher in UT-A2/B knockout mice than that in wild-type mice and lower than that in UT-B knockout mice. Urine osmolality in UT-A2/B knockout mice was intermediate between that in UT-B knockout and wild-type mice. The changes in urine osmolality and flow rate, plasma and urine urea concentration, as well as non-urea solute concentration after an acute urea load or chronic changes in protein intake suggested that UT-A2 plays a role in the progressive accumulation of urea in the inner medulla. These results suggest that in wild-type mice UT-A2 facilitates urea absorption by urea efflux from the thin descending limb of short loops of Henle. Moreover, UT-A2 deletion in UT-B knockout mice partially remedies the urine concentrating defect caused by UT-B deletion, by reducing urea loss from the descending limbs to the peripheral circulation; instead, urea is returned to the inner medulla through the loops of Henle and the collecting ducts.

European genome-wide association study identifies SLC14A1 as a new urinary bladder cancer susceptibility gene

Three genome-wide association studies in Europe and the USA have reported eight urinary bladder cancer (UBC) susceptibility loci. Using extended case and control series and 1000 Genomes imputations of 5 340 737 single-nucleotide polymorphisms (SNPs), we searched for additional loci in the European GWAS. The discovery sample set consisted of 1631 cases and 3822 controls from the Netherlands and 603 cases and 37 781 controls from Iceland. For follow-up, we used 3790 cases and 7507 controls from 13 sample sets of European and Iranian ancestry. Based on the discovery analysis, we followed up signals in the urea transporter (UT) gene SLC14A. The strongest signal at this locus was represented by a SNP in intron 3, rs17674580, that reached genome-wide significance in the overall analysis of the discovery and follow-up groups: odds ratio = 1.17, P = 7.6 × 10(-11). SLC14A1 codes for UTs that define the Kidd blood group and are crucial for the maintenance of a constant urea concentration gradient in the renal medulla and, through this, the kidney's ability to concentrate urine. It is speculated that rs17674580, or other sequence variants in LD with it, indirectly modifies UBC risk by affecting urine production. If confirmed, this would support the 'urogenous contact hypothesis' that urine production and voiding frequency modify the risk of UBC.

UT-B1 mediates transepithelial urea flux in the rat gastrointestinal tract

The process of urea nitrogen salvaging plays a vital role in the symbiotic relationship between mammals and their intestinal bacteria. The first step in this process requires the movement of urea from the mammalian bloodstream into the gastrointestinal tract lumen via specialized proteins known as facilitative urea transporters. In this study, we examined both transepithelial urea fluxes and urea transporter protein abundance along the length of the rat gastrointestinal tract. Urea flux experiments that used rat gastrointestinal tissues showed significantly higher transepithelial urea transport was present in caecum and proximal colon (P < 0.01, n = 8, analysis of variance [ANOVA]). This large urea flux was significantly inhibited by 1,3,dimethylurea (P < 0.001, n = 8, ANOVA) and thiourea (P < 0.05, n = 6, unpaired t-test), both known blockers of facilitative urea transporters. Immunoblotting analysis failed to detect any UT-A protein within rat gastrointestinal tissue protein samples. In contrast, a 30-kDa UT-B1 protein was strongly detected in both caecum and proximal colon samples at significantly higher levels compared to the rest of the gastrointestinal tract (P < 0.01, n = 4, ANOVA). We therefore concluded that UT-B1 mediates the transepithelial movement of urea that occurs in specific distal regions of the rat gastrointestinal tract.

UreA, the major urea/H+ symporter in Aspergillus nidulans

We report here the characterization of UreA, a high-affinity urea/H+ symporter of Aspergillus nidulans. The deletion of the encoding gene abolishes urea transport at low substrate concentrations, suggesting that in these conditions UreA is the sole transport system specific for urea in A. nidulans. The ureA gene is not inducible by urea or its precursors, but responds to nitrogen metabolite repression, necessitating for its expression the AreA GATA factor. In contrast to what was observed for other transporters in A. nidulans, repression by ammonium is also operative during the isotropic growth phase. The activity of UreA is down-regulated post-translationally by ammonium-promoted endocytosis. A number of homologues of UreA have been identified in A. nidulans and other Aspergilli, which cluster in four groups, two of which contain the urea transporters characterized so far in fungi and plants. This phylogeny may have arisen by gene duplication events, giving place to putative transport proteins that could have acquired novel, still unidentified functions.

Functional characterization of the central hydrophilic linker region of the urea transporter UT-A1: cAMP activation and snapin binding

Of the three major protein variants produced by the UT-A gene (UT-A1, UT-A2, and UT-A3) UT-A1 is the largest. It contains UT-A3 as its NH(2)-terminal half and UT-A2 as its COOH-terminal half. When being part of UT-A1, UT-A3 and UT-A2 are joined by a segment, Lp, whose central part, Lc, is not part of UT-A3 or UT-A2 but is present only in UT-A1. Lc contains the phosphorylation sites S486 and S499 that are involved in protein kinase A-dependent activation, as well as the binding site for snapin, a protein involved in soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE)-mediated vesicle trafficking and fusion to the plasma membrane. We attached Lc to UT-A2 and UT-A3 to test how these phosphorylation sites influenced their urea transport activity. Adding Lc to UT-A2 conferred stimulation by cAMP to the cAMP-unresponsive UT-A2, and adding Lc to UT-A3 did not further enhance its already existing cAMP response. These findings suggest that the responsiveness to vasopressin that is observed with UT-A1 can be introduced into the unresponsive UT-A2 variant through the Lc segment that is unique to UT-A1. In UT-A3, however, the Lc segment plays no significant role in its activation by cAMP. In addition, the Lc segment also gave UT-A2 the ability to bind snapin and, in Xenopus oocytes, to be stimulated in its urea transport activity by snapin and syntaxins 3 and 4, in the same way as UT-A1.

Modulation of urea transport across sheep rumen epithelium in vitro by SCFA and CO2

Urea transport across the gastrointestinal tract involves transporters of the urea transporter-B group, the regulation of which is poorly understood. The classical stimulatory effect of CO(2) and the effect of short-chain fatty acids (SCFA) on the ruminal recycling of urea were investigated by using Ussing chamber and microelectrode techniques with isolated ruminal epithelium of sheep. The flux of urea was found to be phloretin sensitive and passive. At a luminal pH of 6.4, but not at 7.4, the addition of SCFA (40 mmol/l) or CO(2)/HCO3- (10% and 25 mmol/l) led to a fourfold increase in urea flux. The stepwise reduction of luminal pH in the presence of SCFA from 7.4 to 5.4 led to a bell-shaped modification of urea transport, with a maximum at pH 6.2. Lowering the pH in the absence of SCFA or CO(2) had no effect. Inhibition of Na(+)/H(+) exchange increased urea flux at pH 7.4, with a decrease being seen at pH 6.4. In experiments with double-barreled, pH-sensitive microelectrodes, we confirmed the presence of an apical pH microclimate and demonstrated the acidifying effects of SCFA on the underlying epithelium. We confirm that the permeability of the ruminal epithelium to urea involves a phloretin-sensitive pathway. We present clear evidence for the regulation of urea transport by strategies that alter intracellular pH, with permeability being highest after a moderate decrease. The well-known postprandial stimulation of urea transport to the rumen in vivo may involve acute pH-dependent effects of intraruminal SCFA and CO(2) on the function of existing urea transporters.

V2R mutations and nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus (NDI), which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone, arginine vasopressin (AVP). Polyuria, with hyposthenuria, and polydipsia are the cardinal clinical manifestations of the disease. Nephrogenic failure to concentrate urine maximally may be due to a defect in vasopressin-induced water permeability of the distal tubules and collecting ducts, to insufficient buildup of the corticopapillary interstitial osmotic gradient, or to a combination of these two factors. Thus, the broadest definition of the term NDI embraces any antidiuretic hormone-resistant urinary-concentrating defect, including medullary disease with low interstitial osmolality, renal failure, and osmotic diuresis. About 90% of patients with congenital NDI are males with X-linked recessive NDI (OMIM 304800)(1) and have mutations in the AVP receptor 2 (AVPR2) gene that codes for the vasopressin V(2) receptor; the gene is located in chromosome region Xq28. In about 10% of the families studied, congenital NDI has an autosomal recessive or autosomal dominant mode of inheritance (OMIM 222000 and 125800)(1). Mutations have been identified in the aquaporin-2 gene (AQP2, OMIM 107777)(1), which is located in chromosome region 12q13 and codes for the vasopressin-sensitive water channel. NDI is clinically distinguishable from neurohypophyseal diabetes insipidus (OMIM 125700(1); also referred to as central or neurogenic diabetes insipidus) by a lack of response to exogenous AVP and by plasma levels of AVP that rise normally with increase in plasma osmolality. Hereditary neurohypophyseal diabetes insipidus is secondary to mutations in the gene encoding AVP (OMIM 192340)(1). Neurohypophyseal diabetes insipidus is also a component of autosomal recessive Wolfram syndrome 1 or DIDMOAD syndrome (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) (OMIM 222300)(1), an autosomal recessive disorder. Other inherited disorders with complex polyuro-polydipsic syndrome with loss of water, sodium, chloride, calcium, magnesium, and potassium include Bartter syndrome (OMIM 601678)(1) and cystinosis (OMIM 219800)(1), while long-term lithium administration is the main cause of acquired NDI. Here, we use the gene symbols approved by the HUGO Gene Nomenclature Committee (http://www.gene.ucl.ac.uk/nomenclature) and provide OMIM entry numbers [OMIM (Online Mendelian Inheritance in Man)(1); McKusick-Nathans Institute for Genetic Medicine, Johns Hopkins University (Baltimore, MD) and National Center for Biotechnology Information, National Library of Medicine (Bethesda, MD), 2000; World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim/].

Multiple urea transporter proteins in the kidney of holocephalan elephant fish (Callorhinchus milii)

Reabsorption of filtered urea by the kidney is essential for retaining high levels of urea in marine cartilaginous fish. Our previous studies on the shark facilitative urea transporter (UT) suggest that additional UT(s) comprising the urea reabsorption system could exist in the cartilaginous fish kidney. Here, we isolated three cDNAs encoding UTs from the kidney of elephant fish, Callorhinchus milii, and termed them efUT-1, efUT-2 and efUT-3. efUT-1 is orthologous to known elasmobranch UTs, while efUT-2 and efUT-3 are novel UTs in cartilaginous fish. Two variants were found for efUT-1 and efUT-2, in which the NH(2)-terminal intracellular domain was distinct between the variants. Differences in potential phosphorylation sites were found in the variant-specific NH(2)-terminal domains. When expressed in Xenopus oocytes, all five UT transcripts including the efUT-1 and efUT-2 variants induced more than a 10-fold increase in [(14)C] urea uptake. Phloretin inhibited dose-dependently the increase of urea uptake, suggesting that the identified UTs are facilitative UTs. Molecular phylogenetic analysis revealed that efUT-1 and efUT-2 had diverged in the cartilaginous fish lineage, while efUT-3 is distinct from efUT-1 and efUT-2. The present finding of multiple UTs in elephant fish provides a key to understanding the molecular mechanisms of urea reabsorption system in the cartilaginous fish kidney.

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.