Structure, regulation and physiological roles of 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.

SLC14A1 is a new biomarker in renal cancer

BACKGROUND

Renal cancer is one of the common malignant tumors of the urinary tract, prone to distant metastasis and drug resistance, with a poor clinical prognosis. SLC14A1 belongs to the solute transporter family, which plays a role in urinary concentration and urea nitrogen recycling in the renal, and is closely associated with the development of a variety of tumors.

METHODS

Transcription data for renal clear cell carcinoma (KIRC) were obtained from the public databases Gene Expression Omnibus database (GEO) and The Cancer Genome Atlas (TCGA), and we investigated the differences in SLC14A1 expression in cancerous and normal tissues of renal cancer, its correlation with the clinicopathological features of renal cancer patients. Then, we verified the expression levels of SLC14A1 in renal cancer tissues and their Paracancerous tissues using RT-PCR, Western-blotting and immunohistochemistry. Finally, we used renal endothelial cell line HEK-293 and renal cancer cell lines 786-O and ACHN to explore the effects of SLC14A1 on the biological behaviors of renal cancer cell proliferation, invasion and metastasis using EDU, MTT proliferation assay, Transwell invasion assay and scratch healing assay.

RESULTS

SLC14A1 was lowly expressed in renal cancer tissues and this was further validated by RT-PCR, Western blotting, and immunohistochemistry in our clinical samples. Analysis of KIRC single-cell data suggested that SLC14A1 was mainly expressed in endothelial cells. Survival analysis showed that low levels of SLC14A1 expression were associated with a better clinical prognosis. In biological behavioral studies, we found that upregulation of SLC14A1 expression levels inhibited the proliferation, invasion, and metastatic ability of renal cancer cells.

CONCLUSION

SLC14A1 plays an important role in the progression of renal cancer and has the potential to become a new biomarker for renal cancer.

Urea transport and hydrolysis in the rumen: A review

Inefficient dietary nitrogen (N) conversion to microbial proteins, and the subsequent use by ruminants, is a major research focus across different fields. Excess bacterial ammonia (NH3) produced due to degradation or hydrolyses of N containing compounds, such as urea, leads to an inefficiency in a host's ability to utilize nitrogen. Urea is a non-protein N containing compound used by ruminants as an ammonia source, obtained from feed and endogenous sources. It is hydrolyzed by ureases from rumen bacteria to produce NH3 which is used for microbial protein synthesis. However, lack of information exists regarding urea hydrolysis in ruminal bacteria, and how urea gets to hydrolysis sites. Therefore, this review describes research on sites of urea hydrolysis, urea transport routes towards these sites, the role and structure of urea transporters in rumen epithelium and bacteria, the composition of ruminal ureolytic bacteria, mechanisms behind urea hydrolysis by bacterial ureases, and factors influencing urea hydrolysis. This review explores the current knowledge on the structure and physiological role of urea transport and ureolytic bacteria, for the regulation of urea hydrolysis and recycling in ruminants. Lastly, underlying mechanisms of urea transportation in rumen bacteria and their physiological importance are currently unknown, and therefore future research should be directed to this subject.

Identification of differentially expressed genes in gills of tiger puffer (Takifugu rubripes) in response to low-salinity stress

Salinity is an important abiotic factor for aquatic organisms. In fish, changes in salinity affect physiological responses and alter the immune system. Takifugu rubripes is an important economic marine fish, and mechanisms of T. rubripes adaptation to salinity changes need to be further documented. In this study, a transcriptome sequencing technique was used to analyse genes that were differentially expressed in the T. rubripes gill after low-salinity stress for 30 d, and differential gene expression was further validated by quantitative real-time PCR (qPCR). After assembly, 385 differentially expressed genes (DEGs) were identified, including 182 upregulated genes and 203 downregulated genes. The DEGs were assigned to Gene Ontology (GO) classes with a total of 1647 functional terms. Most DEGs were assigned to biological process (984; 59.8%) followed by molecular function (445; 27.0%) and cellular component (218; 13.2%). Further KEGG analysis allocated 385 DEGs to 95 KEGG pathways. After q-value correction, 7 pathways (Glycolysis/Gluconeogenesis; Biosynthesis of amino acids; Carbon metabolism; Fructose and mannose metabolism; Pentose phosphate pathway; Metabolism of xenobiotics by cytochrome P450; and Glycine, serine and threonine metabolism) remained significant. qPCR results indicated that the transcripts of six selected genes sharply increased after 30 d of low-salinity stress. Low-salinity stress obviously increased SLC39A6, SLC5A9, NKAα1, CYP1A1, CYP1B1, and GSTA expression. In contrast, the genes encoding Aldoaa, GPI, FBP2 and GAPDH exhibited downregulation. In addition, three solute carrier (SLC) genes selected from the DEGs were further studied for differential expression patterns after low-salinity exposure, and the results showed that the SLCs were upregulated in T. rubripes after 72 h of low-salinity exposure. This investigation provides data for understanding the molecular mechanisms of fish responses to low-salinity stress and provides a reference for rationally setting salinity levels in aquaculture.

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.

Discovery and optimization of thienopyridine derivatives as novel urea transporter inhibitors

Urea transporters (UTs) play an important role in the urine concentrating mechanism and are recognized as novel targets for developing small molecule inhibitors with salt-sparing diuretic activity. Thienoquinoline derivatives, a class of novel UT-B inhibitors identified by our group, play a significant diuresis in animal model. However, the poor solubility and low bioavailability limited its further development. To overcome these shortcomings, the structure modification of thienoquinoline was carried out in this study, which led to the discovery of novel thienopyridine derivatives as specific urea transporter inhibitors. Further optimization obtained the promising preclinical candidate 8n with not only excellent inhibition effect on urea transporters and diuretic activity on rat model, but also suitable water solubility and Log P value.

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.

Urea Transporter B and MicroRNA-200c Differ in Kidney Outer Versus Inner Medulla Following Dehydration

BACKGROUND

Urea transporters (UTs) are important in urine concentration and in urea recycling, and UT-B has been implicated in both. In kidney, UT-B was originally localized to outer medullary descending vasa recta, and more recently detected in inner medullary descending vasa recta. Endogenously produced microRNAs (miRs) bind to the 3'UTR of genes and generally inhibit their translation, thus playing a pivotal role gene regulation.

METHODS

Mice were dehydrated for 24 hours then sacrificed. Inner and outer medullas were analyzed by polymerase chain reaction (PCR) and quantitative PCR for miRNA expression and analyzed by western blotting for protein abundance.

RESULTS

MiRNA sequencing analysis of mouse inner medullas showed a 40% increase in miRNA-200c in dehydrated mice compared with controls. An in silico analysis of the targets for miR-200c revealed that miRNA-200c could directly target the gene for UT-B. PCR confirmed that miR-200c is up-regulated in the inner medullas of dehydrated mice while western blot showed that UT-B protein abundance was down-regulated in the same portion of the kidney. However, in the outer medulla, miR-200c was reduced and UT-B protein was increased in dehydrated mice.

CONCLUSIONS

This is the first indication that UT-B protein and miR-200c may each be differentially regulated by dehydration within the kidney outer and inner medulla. The inverse correlation between the direction of change in miR-200c and UT-B protein abundance in both the inner and outer medulla suggests that miR-200c may be associated with the change in UT-B protein in these 2 portions of the kidney medulla.

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.

Structure of urea transporters

Members of the urea transporter (UT) family mediate rapid, selective transport of urea down its concentration gradient. To date, crystal structures of two evolutionarily distant UTs have been solved. These structures reveal a common UT fold involving two structurally homologous domains that encircle a continuous membrane-spanning pore and indicate that UTs transport urea via a channel-like mechanism. Examination of the conserved architecture of the pore, combined with crystal structures of ligand-bound proteins, molecular dynamics simulations, and functional data on permeation and inhibition by a broad range of urea analogs and other small molecules, provides insight into the structural basis of urea permeation and selectivity.

Structure and permeation mechanism of a mammalian urea transporter

As an adaptation to infrequent access to water, terrestrial mammals produce urine that is hyperosmotic to plasma. To prevent osmotic diuresis by the large quantity of urea generated by protein catabolism, the kidney epithelia contain facilitative urea transporters (UTs) that allow rapid equilibration between the urinary space and the hyperosmotic interstitium. Here we report the first X-ray crystal structure of a mammalian UT, UT-B, at a resolution of 2.36 Å. UT-B is a homotrimer and each protomer contains a urea conduction pore with a narrow selectivity filter. Structural analyses and molecular dynamics simulations showed that the selectivity filter has two urea binding sites separated by an approximately 5.0 kcal/mol energy barrier. Functional studies showed that the rate of urea conduction in UT-B is increased by hypoosmotic stress, and that the site of osmoregulation coincides with the location of the energy barrier.

Crystal structure of a bacterial homologue of the kidney urea transporter

Urea is highly concentrated in the mammalian kidney to produce the osmotic gradient necessary for water re-absorption. Free diffusion of urea across cell membranes is slow owing to its high polarity, and specialized urea transporters have evolved to achieve rapid and selective urea permeation. Here we present the 2.3 A structure of a functional urea transporter from the bacterium Desulfovibrio vulgaris. The transporter is a homotrimer, and each subunit contains a continuous membrane-spanning pore formed by the two homologous halves of the protein. The pore contains a constricted selectivity filter that can accommodate several dehydrated urea molecules in single file. Backbone and side-chain oxygen atoms provide continuous coordination of urea as it progresses through the filter, and well-placed alpha-helix dipoles provide further compensation for dehydration energy. These results establish that the urea transporter operates by a channel-like mechanism and reveal the physical and chemical basis of urea selectivity.

Urea kinetics of a carnivore,Felis silvestris catus

The effect of two levels of dietary protein energy, moderate (20 %; MP) and high (70 %; HP), on urea kinetics in eleven domestic cats was studied. After a 3-week prefeed, a single dose of [15N15N]urea was administered, and urine and faeces collected over the subsequent 5 d. For each 24 h period, total urea and enrichment of [15N15N]- and [15N14N]urea in urine were determined, and a model applied to calculate urea production, entry into the gastrointestinal tract, recycling to urine or faeces and, by difference, retention by the body and potentially available for anabolism. Urea production and excretion increased with dietary protein level (P<0·05). Most of the urea produced was excreted, with only a small proportion entering the gut, and with the pattern of urea disposal not significantly different between the HP and MP diets. Thus, the percentages of urea production available to the gut were 15 % (MP) and 12 % (HP), of which 57 % (MP) and 59 % (HP) was recycled in the ornithine cycle, 40 % (MP and HP) was potentially available for anabolism and the rest lost as faecal N. As a percentage of urea produced the amount potentially available for anabolism was very low at 6·41 % (MP diet) and 4·79 % (HP diet). In absolute terms urea entering the gut, being recycled in the ornithine cycle and potentially available for anabolism was significantly higher on the HP diet (P<0·05). These results show that cats operate urea turnover, but at a lower rate, and with less nutritional sensitivity than has been reported for other species.

Urea transport in bacteria: acid acclimation by gastric Helicobacter spp

Urea transporters in bacteria are relatively rare. There are three classes, the ABC transporters such as those expressed by cyanobacteria and Corynebacterium glutamicum, the Yut protein expressed by Yersinia spp and the UreI expressed by gastric Helicobacter spp. This review focuses largely on the UreI proton-gated channel that is part of the acid acclimation mechanism essential for gastric colonization by the latter. UreI is a six-transmembrane polytopic integral membrane protein, N and C termini periplasmic, and is expressed in all gastric Helicobacter spp that have been studied but also in Helicobacter hepaticus and Streptococcus salivarius. The first two are proton-gated, the latter is pH insensitive. Site-directed mutagenesis and chimeric constructs have identified histidines and dicarboxylic amino acids in the second periplasmic loop of H. pylori and the first loop of H. hepaticus UreI and the C terminus of both as involved in a hydrogen-bonding dependence of proton gating, with the membrane domain in these but not in the UreI of S. salivarius responding to the periplasmic conformational changes. UreI and urease are essential for gastric colonization and urease associates with UreI during acid exposure, facilitating activation of the UreA and UreB apoenzyme complex by Ni2+ insertion by the UreF-UreH and UreE-UreG assembly proteins. Transcriptome analysis of acid responses of H. pylori also identified a cytoplasmic and periplasmic carbonic anhydrase as responding specifically to changes in periplasmic pH and these have been shown to be essential also for acid acclimation. The finding also of upregulation of the two-component histidine kinase HP0165 and its response element HP0166, illustrates the complexity of the acid acclimation processes involved in gastric colonization by this pathogen.

Phenotype of the Taurine Transporter Knockout Mouse

This chapter reports present knowledge on the properties of mice with disrupted gene coding for the taurine transporter (taut-/- mice). Study of those mice unraveled some of the roles of taurine and its membrane transport for the development and maintenance of normal organ functions and morphology. When compared with wild-type controls, taut-/- mice have decreased taurine levels in skeletal and heart muscle by about 98%, in brain, kidney, plasma, and retina by 80 to 90%, and in liver by about 70%. taut-/- mice exhibit a lower body mass as well as a strongly reduced exercise capacity compared with taut+/- and wild-type mice. Furthermore, taut-/- mice show a variety of pathological features, for example, subtle derangement of renal osmoregulation, changes in neuroreceptor expression, and loss of long-term potentiation in the striatum, and they develop clinically relevant age-dependent disorders, for example, visual, auditory, and olfactory dysfunctions, unspecific hepatitis, and liver fibrosis. Taurine-deficient animal models such as acutely dietary-manipulated foxes and cats, pharmacologically induced taurine-deficient rats, and taurine transporter knockout mouse are powerful tools allowing identification of the mechanisms and complexities of diseases mediated by impaired taurine transport and taurine depletion (Chapman et al., 1993; Heller-Stilb et al., 2002; Huxtable, 1992; Lake, 1993; Moise et al., 1991; Novotny et al., 1991; Pion et al., 1987; Timbrell et al., 1995; Warskulat et al., 2004, 2006b). Taurine, which is the most abundant amino acid in many tissues, is normally found in intracellular concentrations of 10 to 70 mmol/kg in mammalian heart, brain, skeletal muscle, liver, and retina (Chapman et al., 1993; Green et al., 1991; Huxable, 1992; Timbrell et al., 1995). These high taurine levels are maintained by an ubiquitous expression of Na(+)-dependent taurine transporter (TAUT) in the plasma membrane (Burg, 1995; Kwon and Handler, 1995; Lang et al., 1998; Liu et al., 1992; Ramamoorthy et al., 1994; Schloss et al., 1994; Smith et al., 1992; Uchida et al., 1992; Vinnakota et al., 1997; Yancey et al., 1975). Taurine is not incorporated into proteins. It is involved in cell volume regulation, neuromodulation, antioxidant defense, protein stabilization, stress responses, and via formation of taurine-chloramine in immunomodulation (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Timbrell et al., 1995). On the basis of its functions, taurine may protect cells against various types of injury (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Kurz et al., 1998; Park et al., 1995; Stapleton et al., 1998; Timbrell et al., 1995; Welch and Brown, 1996; Wettstein and Häussinger, 1997). In order to examine the multiple taurine functions, murine models have several intrinsic advantages for in vivo research compared to other animal models, including lower cost, maintenance, and rapid reproduction rate. Further, experimental reagents for cellular and molecular studies are widely available for the mouse. In particular, mice can be easily genetically manipulated by making transgene and knockout mice. This chapter focuses on the phenotype of the TAUT-deficient murine model (taut-/-; Heller-Stilb et al., 2002), which may help researchers elucidate the diverse roles of taurine in development and maintenance of normal organ functions and morphology.

UT-B1 urea transporter plays a noble role as active water transporter in C6 glial cells

Since our experimental results suggest that UT-B1 functions as active water transporter against osmotic gradient in C6 glial cells, we report here for the first time the evidence for the active water transport. Exposure of C6 cells to a hyperosmotic solution containing glycerol or sucrose produced cell shrinkage due to water efflux according to osmotic gradient for water movement. On the other hand, C6 cells show cell swelling against osmotic gradient for water movement just after exposure to a hyperosmotic solution containing urea, indicating that water influx against osmotic gradient for water movement is accelerated by urea; i.e., urea performs active water transport. A specific inhibitor of UT-B, pCMBS, blocked the urea-induced swelling. The urea-induced cell swelling was significantly suppressed in the siRNA-induced UT-B1-knockdown C6 cells. Taken together, these observations indicate that UT-B1 acts as an active water transporter, providing a new model on active water transport.

Ultrastructural localization of UT-A and UT-B in rat kidneys with different hydration status

Urea transport in the kidney is mediated by a family of transporter proteins, including renal urea transporters (UT-A) and erythrocyte urea transporters (UT-B). We aimed to determine whether hydration status affects the subcellular distribution of urea transporters. Male Sprague-Dawley rats were divided into three groups: dehydrated rats (WD) given minimum water, hydrated rats (WL) given 3% sucrose in water for 3 days before death, and control rats given free access to water. We labeled kidney sections with antibodies against UT-A1 and UT-A2 (L194), UT-A3 (Q2), and UT-B using preembedding immunoperoxidase and immunogold methods. In control animals, UT-A1 and UT-A3 immunoreactivities were observed throughout the cytoplasm in inner medullary collecting duct (IMCD) cells, and weak labeling was observed on the basolateral plasma membrane. UT-A2 immunoreactivity in the descending thin limbs (DTL) was observed mainly on the apical and basolateral membranes of type I epithelium, and very faint labeling was observed in the long-loop DTL at the border between the outer and inner medulla. UT-A1 immunoreactivity intensity was markedly lower, and UT-A3 immunoreactivity was higher in IMCD of WD vs. controls. UT-A2 immunoreactivity intensities in the plasma membrane and cytoplasm of type I, II, and III epithelia of DTL were greater in WD vs. controls. In contrast, UT-A1 expression was greater and UT-A2 and UT-A3 expressions were lower in WL vs. controls. The subcellular distribution of UT-A in DTL or IMCD did not differ between control and experimental animals. UT-B was expressed in the plasma membrane of the descending vasa recta of both control and experimental animals. UT-B intensity was higher in WD and lower in WL vs. controls. These data indicate that changes in hydration status over 3 days affected urea transporter protein expression without changing its subcellular distribution.

Novel Urothelium Specific Gene Expression Identified by Differential Display Reverse Transcriptase-Polymerase Chain Reaction

PURPOSE

Understanding the molecular basis of differential gene expression among different tissues at various developmental stages and in neoplastic transformation is an important biological goal. The potential clinical applications of this improved understanding are more precise diagnosis of disease, prediction of prognosis, novel targeted therapies and prediction of response to therapy.

MATERIALS AND METHODS

Differential display reverse transcriptase-polymerase chain reaction was used to compare gene expression in bovine urothelium to that in autologous lung, esophagus, liver and spleen. Products that appeared to have urothelial specific expression were sequenced and assessed for homology with known sequences. Ribonuclease protection assays were used to further confirm the expression pattern.

RESULTS

A total of 32 discrete cDNAs were identified, including 3 products from genes known to be urothelium specific in their expression, 16 with significant homology to bovine, human or mouse expressed sequence tags and 5 with no sequence homology to any currently available sequence. Urothelium specific mRNA expression was confirmed for 3 genes by ribonuclease protection assays and one (Udd06) was further characterized as a urea transporter.

CONCLUSIONS

The use of differential display reverse transcriptase-polymerase chain reaction and other complementary techniques for parallel gene expression analysis will permit the complete characterization of the urothelial transcriptome and help identify potential molecular targets for rationally targeted therapy.

Impaired ability to increase water excretion in mice lacking the taurine transporter gene TAUT

Cellular taurine uptake or release counteracts alterations of cell volume. Na+-coupled taurine transporter TAUT mediates concentrative cellular uptake of taurine. Inhibition of vasopressin secretion by hypotonicity may involve taurine release from glial cells of supraoptic nucleus. We compared renal function of mice lacking TAUT (taut-/-) and wild-type littermates (taut+/+). We observed renal taurine loss and subsequent hypotaurinemia in taut-/- mice. With free access to water, plasma and urine osmolality, urinary flow rate as well as urinary excretion and plasma concentrations of Na+ and K+ were similar in taut-/- and taut+/+ mice, whereas plasma concentrations of urea were enhanced in taut-/- mice. An oral water load (1 ml/16 g body weight) induced a similar diuresis in both genotypes. Repeating the oral water load immediately after normalization of urine flow rate, however, resulted in delayed diuresis and higher urinary vasopressin/creatinine ratios in taut-/- mice. In comparison, the repeated diuretic response to vasopressin V2 receptor blockade was not different between genotypes. Water deprivation for 36 h led to similar antidiuresis and increases of urinary osmolality in both genotypes. Upon free access to water after deprivation, taut-/- mice continued to concentrate urine up to 6 days, while taut+/+ mice rapidly returned to normal urinary osmolality. Urinary vasopressin/creatinine ratios and plasma aldosterone concentrations were not different under basal conditions but were significantly higher in taut-/- mice than in taut+/+ mice at 6 days after water deprivation. In conclusion, taut-/- mice suffer from renal taurine loss and impaired ability to lower urine osmolality and to increase urinary water excretion. The latter defect could reside extrarenally and result from a role of taurine in the suppression of vasopressin release which may be attenuated in taut-/- mice.

Expression of urea transporter (UT-A) mRNA in papilla and pelvic epithelium of kidney in normal and low protein fed sheep

The identification and cloning of the urea transporter (UT) in papilla and upper pelvic epithelium of sheep kidney and the effect of a 5-week-lasting low protein diet on UT mRNAs expression in these structures are reported. Using degenerate primers we cloned by RT-PCR a 770-base pairs UT-A cDNA fragment. The deduced amino acid sequence shared 92% and 93% identity with UT-A2 protein from rabbit and rat, and from human, respectively. Quantification of UT-A mRNAs expression after LP diet was performed by quantitative RT-PCR using UT-A mutant cRNA. Compared to normal protein fed sheep, low protein diet was associated with a significant reduction of UT-A mRNA levels in pelvic epithelium (852+/-172 vs. 2024+/-260 molecules, P<0.01) and a tendency to its increase in papilla (7959+/-1741 vs. 5447+/-1040 molecules, NS). Functional studies confirmed that kidneys of low protein fed sheep increased their ability to reduce urea losses. The reduction of UT-A expression in the pelvic epithelium lining the outer medulla could be relevant for the renal conservation of urea in protein restricted sheep.

The diuretic effect of Sairei-to is mediated by nitric oxide production in pentobarbital-anesthetized rats

Sairei-to (TJ-114), a Japanese traditional medicine, has been used clinically for the treatment of various edematous disorders. The inhibitory effect on edema may be dependent on the diuretic response to TJ-114. This study was conducted to clarify the mechanism of diuresis. Pentobarbital-anesthetized rats were infused with a saline solution intravenously (0.4 mL/30 min). Urine was collected through a bladder cannula for 30 min. Intraduodenal administration of TJ-114 (0.5 - 1.5 g/kg) resulted in a dose-dependent increase of urine volume with insignificant urinary sodium excretion and significant urea excretion, but no effect on mean arterial blood pressure. Furthermore, TJ-114 significantly increased urinary levels of NO(2) + NO(3). In addition, intraperitoneal pre-treatment with 6 mg/kg of N(G)-nitro-L-arginine methyl ester inhibited the increase in urine volume, urinary urea excretion, and urinary levels of NO(2) + NO(3) in the rats treated with TJ-114. These results suggest that TJ-114 induces a diuretic response via production of NO.

The SLC14 gene family of urea transporters

Carrier-mediated urea transport allows rapid urea movement across the cell membrane, which is particularly important in the process of urinary concentration and for rapid urea equilibrium in non-renal tissues. Urea transporters mediate passive urea uptake that is inhibited by phloretin and urea analogues. Facilitated urea transporters are divided into two classes: (1) the renal tubular/testicular type of urea transporter, UT-A1 to -A5, encoded by alternative splicing of the SLC14A2 gene, and (2) the erythrocyte urea transporter UT-B1 encoded by the SLC14A1 gene. The primary structure of urea transporters is unique, consisting of two extended, hydrophobic, membrane-spanning domains and an extracellular glycosylated-connecting loop. UT-A1 is the result of a gene duplication of this two-halves-structure, and the duplicated portions are linked together by a large intracellular hydrophilic loop, carrying several putative protein kinase A (PKA) and -C (PKC) phosphorylation sites. UT-A1 is located in the apical membrane of the kidney inner medullary collecting duct cells, where it is stimulated acutely by cAMP-mediated phosphorylation in response to the antidiuretic hormone vasopressin. Vasopressin also up-regulates UT-A2 mRNA/protein expression in the descending thin limb of the loops of Henle. UT-A1 and UT-A2 are regulated independently and respond differently to changes in dietary protein content. UT-A3 and UT-A4 are located in the rat kidney medulla and UT-A5 in the mouse testis. The widely expressed UT-B participates in urea recycling in the descending vasa recta, as demonstrated by a relatively mild "urea-selective" urinary concentrating defect in transgenic UT-B null mice and individuals with the Jk(null) blood group.

Transport of small molecules across cell membranes: water channels and urea transporters

How do small hydrophilic nonelectrolytes cross cell membranes? Which pathways are most important for small lipid insoluble molecules to cross cell membranes? These are questions that have been basic to membrane transport physiology for decades. More importantly, these are questions whose answers have changed significantly within the last 10 years. This review discusses the evidence that pathways other than the lipid bilayer itself exist for the transport across cell membranes of specific small hydrophilic nonelectrolytes. The description begins with briefly analyzing the relevance of well accepted basic mathematical models for transport for understanding the permeability of representative physiologically important molecules across actual cell membranes. Particular emphasis is placed on describing recently discovered proteins that facilitate the transport of some of the smallest physiologically important lipid-insoluble molecules, water, and urea. Evidence also exists for transport proteins that selectively enhance the transmembrane transport of other small lipid-insoluble molecules. Do nonselective pores for small molecules exist in cell membranes?

Current prodrug strategies via membrane transporters/receptors

Prodrug design strategies have been employed to improve the delivery of drugs with undesirable pharmacokinetic properties such as chemical stability and lack of specificity. Targeted prodrug design represents a new strategy for site-directed and efficient drug delivery. Targeting of drugs to transporters and receptors to aid in site-specific carrier-mediated absorption is emerging as a novel and clinically significant approach. Various prodrugs have been successful in achieving the goals of enhanced bioavailability and are, therefore, considered to be an important tool in biopharmaceutics. This review highlights the advances in prodrug design targeted towards membrane transporters/receptors in the past few years.

Expression of urea transporters in the developing rat kidney

Urea transport in the kidney is mediated by a family of transporter proteins that includes renal urea transporters (UT-A) and erythrocyte urea transporters (UT-B). Because newborn rats are not capable of producing concentrated urine, we examined the time of expression and the distribution of UT-A and UT-B in the developing rat kidney by light and electron microscopic immunocytochemistry. Kidneys from 16-, 18-, and 20-day-old fetuses, 1-, 4-, 7-, 14-, and 21-day-old pups, and adult animals were studied. In the adult kidney, UT-A was expressed intensely in the inner medullary collecting duct (IMCD) and terminal portion of the short-loop descending thin limb (DTL) and weakly in long-loop DTL in the outer part of the inner medulla. UT-A immunoreactivity was not present in the fetal kidney but was observed in the IMCD and DTL in 1-day-old pups. The intensity of UT-A immunostaining in the IMCD gradually increased during postnatal development. In 4- and 7-day-old pups, UT-A immunoreactivity was present in the DTL at the border between the outer and inner medulla. In 14- and 21-day-old pups, strong UT-A immunostaining was observed in the terminal part of short-loop DTL in the outer medulla, and weak labeling remained in long-loop DTL descending into the outer part of the inner medulla. In the adult kidney, there was intense staining for UT-B in descending vasa recta (DVR) and weak labeling of glomeruli. In the developing kidney, UT-B was first observed in the DVR of a 20-day-old fetus. After birth there was a striking increase in the number of UT-B-positive DVR, in association with the formation of vascular bundles. The intensity of immunostaining remained strong in the outer medulla but gradually decreased in the inner medulla. We conclude that the expression of urea transporters in short-loop DTL and DVR coincides with the development of the ability to produce a concentrated urine.

Food restriction prevents age-related polyuria by vasopressin-dependent recruitment of aquaporin-2

The mechanisms underlying the prevention of age-related polyuria by chronic food restriction were investigated in female WAG/Rij rats. The decreased osmolality of renal papilla observed in senescent rats was not corrected by food restriction. A reduced urea content in the inner medulla of senescent rats, fed ad libitum or food-restricted, was suggested by the marked decrease in expression of UT-A1 and UT-B1 urea transporters. Aquaporin-2 (AQP2) downregulation in the inner medulla of senescent rats was partially prevented by food restriction. Both AQP2 and the phosphorylated form of AQP2 (p-AQP2), the presence of which was diffuse within the cytoplasm of collecting duct principal cells in normally fed senescent rats, were preferentially targeted at the apical region of the cells in food-restricted senescent animals. Plasma vasopressin (AVP) was similar in 10- and 30-mo-old rats fed ad libitum, but was doubled in food-restricted 30-mo-old rats. This study indicates that 1) kidney aging is associated with a marked decrease in AQP2, UT-A1, and UT-B1 expression in the inner medulla and a reduced papillary osmolality; and 2) the prevention of age-related polyuria by chronic food restriction occurs through an improved recruitment of AQP2 and p-AQP2 to the apical membrane in inner medulla principal cells, permitted by increased plasma AVP concentration.

Ontogeny of rabbit proximal tubule urea permeability

Urea transport in the proximal tubule is passive and is dependent on the epithelial permeability. The present study examined the maturation of urea permeability (P(urea)) in in vitro perfused proximal convoluted tubules (PCT) and basolateral membrane vesicles (BLMV) from rabbit renal cortex. Urea transport was lower in neonatal than adult PCT at both 37 and 25 degrees C. The PCT P(urea) was also lower in the neonates than the adults (37 degrees C: 45.4 +/- 10.8 vs. 88.5 +/- 15.2 x 10(-6) cm/s, P < 0.05; 25 degrees C: 28.5 +/- 6.9 vs. 55.3 +/- 10.4 x 10(-6) cm/s; P < 0.05). The activation energy for PCT P(urea) was not different between the neonatal and adult groups. BLMV P(urea) was determined by measuring vesicle shrinkage, due to efflux of urea, using a stop-flow instrument. Neonatal BLMV P(urea) was not different from adult BLMV P(urea) at 37 degrees C [1.14 +/- 0.05 x 10(-6) vs. 1.25 +/- 0.05 x 10(-6) cm/s; P = not significant (NS)] or 25 degrees C (0.94 +/- 0.06 vs. 1.05 +/- 0.10 x 10(-6) cm/s; P = NS). There was no effect of 250 microM phloretin, an inhibitor of the urea transporter, on P(urea) in either adult or neonatal BLMV. The activation energy for urea diffusion was also identical in the neonatal and adult BLMV. These findings in the BLMV are in contrast to the brush-border membrane vesicles (BBMV) where we have previously demonstrated that urea transport is lower in the neonate than the adult. Urea transport is lower in the neonatal proximal tubule than the adult. This is due to a lower rate of apical membrane urea transport, whereas basolateral urea transport is the same in neonates and adults. The lower P(urea) in neonatal proximal tubules may play a role in overall urea excretion and in developing and maintaining a high medullary urea concentration and thus in the ability to concentrate the urine during renal maturation.

Sites of pH regulation of the urea channel of Helicobacter pylori

Helicobacter pylori (Hp) and Streptococcus salivarius (Ss) require intrabacterial urease for acid resistance and express a urea channel, UreI. The presence of UreI was shown to increase urea permeability approximately 300-fold over that of a non-polar ureI deletion mutant. Expression of SsUreI in Xenopus oocytes increased urea uptake pH independently, whereas HpUreI shows an acidic pH dependence, half-maximal at pH 6.0. Mutagenesis of all histidines, aspartates, glutamates and the lysine in the periplasmic domain of HpUreI showed that His-123, His-131, Asp-129, Asp-140, Glu-138 and Lys-132 in the second periplasmic loop (PL2) and His-193 in the C-terminus (Ct) were important for activation of transport. With the exception of a lysine that was shown to substitute for His-193 in HpUreI, these charged amino acids are absent in SsUreI. A chimera in which PL1 of HpUreI was replaced by PL1 of SsUreI retained activity at acidic pH and gained partial activity at neutral pH. Exchange of PL2 inactivated transport, whereas exchange of Ct had no effect. Chimeras, in which either PL1 or PL2 of HpUreI replaced those of SsUreI, retained wild-type transport, but replacement of the Ct or both loops inactivated transport. PL1 appears to be important for restricting transport through HpUreI at neutral pH, whereas protonation of three histidines in PL2 and Ct and the presence of three dicarboxylic amino acids in PL2 appears to be necessary to activate HpUreI at acidic pH.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.

Molecular characterization of a novel UT-A urea transporter isoform (UT-A5) in testis

Urea movement across plasma membranes is modulated by specialized transporter proteins that are products of two genes, termed UT-A and UT-B. These proteins play key roles in the urinary concentrating mechanism and fluid homeostasis. We have isolated and characterized a 1.4-kb cDNA from testes encoding a new isoform (UT-A5) belonging to the UT-A transporter family. For comparison, we also isolated a 2. 0-kb cDNA from mouse kidney inner medulla encoding the mouse UT-A3 homologue. The UT-A5 cDNA has a putative open reading frame encoding a 323-amino acid protein, making UT-A5 the smallest UT-A family member in terms of molecular size. Its putative topology is of particular interest, because it calls into question earlier models of UT-A transporter structure. Expression of UT-A5 cRNA in Xenopus oocytes mediates phloretin-inhibitable urea uptake and does not translocate water. The distribution of UT-A5 mRNA is restricted to the peritubular myoid cells forming the outermost layer of the seminiferous tubules within the testes and is not detected in kidney. UT-A5 mRNA levels are coordinated with the stage of testes development and increase 15 days postpartum, commensurate with the start of seminiferous tubule fluid movement.

Massive reduction of urea transporters in remnant kidney and brain of uremic rats

BACKGROUND

The facilitated urea transporters (UT), UT-A1, UT-A2, and UT-B1, are involved in intrarenal recycling of urea, an essential feature of the urinary concentrating mechanism, which is impaired in chronic renal failure (CRF). In this study, the expression of these UTs was examined in experimentally induced CRF.

METHODS

The abundance of mRNA was measured by Northern analysis and that of corresponding proteins by Western blotting in rats one and five weeks after 5/6 nephrectomy (Nx).

RESULTS

At five weeks, urine output was enhanced threefold with a concomitant decrease in urine osmolality. The marked rise in plasma urea concentration and fall in urinary urea concentration resulted in a 30-fold decrease in the urine/plasma (U/P) urea concentration ratio, while the U/P osmoles ratio fell only fourfold. A dramatic decrease in mRNA abundance for the three UTs was observed, bringing their level at five weeks to 1/10th or less of control values. Immunoblotting showed complete disappearance of the 97 and 117 kD bands of UT-A1, and considerable reduction of UT-A2 and UT-B1 in the renal medulla. Similar, but less intense, changes were observed at one-week post-Nx. In addition to the kidney, UT-B1 is also normally expressed in brain and testis. In the brain, its mRNA expression remained normal one-week post-Nx, but decreased to about 30% of normal at five-weeks post-Nx, whereas no change was seen in testis.

CONCLUSIONS

(1) The decline in urinary concentrating ability seen in CRF is largely due to a major reduction of UTs involved in the process of urea concentration in the urine, while factors enabling the concentration of other solutes are less intensely affected. (2) The marked reduction of brain UT expression in CRF may be responsible for brain edema of dialysis disequilibrium syndrome observed in some patients after fast dialysis.

Expression of the Helicobacter pylori ureI gene is required for acidic pH activation of cytoplasmic urease

ureI encodes an integral cytoplasmic membrane protein. It is present in the urease gene cluster of Helicobacter pylori and is essential for infection and acid survival, but its role is unknown. To determine the function of UreI protein, we produced H. pylori ureI deletion mutants and measured the pH dependence of urease activity of intact and lysed bacteria and the effect of urea on the membrane potential. We also determined ureI expression, urease activity, and the effect of urea on membrane potential of several gastric and nongastric Helicobacter species. ureI was found to be present in the genome of the gastric Helicobacter species and absent in the nongastric Helicobacter species studied, as determined by PCR. Likewise, Western blot analysis confirmed that UreI was expressed only in the gastric Helicobacter species. When UreI is present, acidic medium pH activation of cytoplasmic urease is found, and urea addition increases membrane potential at acidic pH. The addition of a low concentration of detergent raised urease activity of intact bacteria at neutral pH to that of their homogenates, showing that urease activity was membrane limited. No acidic pH activation or urea induced membrane potential changes were found in the nongastric Helicobacter species. The ureI gene product is probably a pH activated urea transporter or perhaps regulates such a transporter as a function of periplasmic pH.

At physiological expression levels the Kidd blood group/urea transporter protein is not a water channel

The Kidd (JK) blood group locus encodes a urea transporter that is expressed on human red cells and on endothelial cells of the vasa recta in the kidney. Here, we report the identification in human erythroblasts of a novel cDNA, designated HUT11A, which encodes a protein identical to the previously reported erythroid HUT11 urea transporter, except for a Lys(44) --> Glu substitution and a Val-Gly dipeptide deletion after proline 227, which leads to a polypeptide of 389 residues versus 391 in HUT11. Genomic typing by polymerase chain reaction and transcript analysis by ribonuclease protection assay demonstrated that HUT11A encodes the true Kidd blood group/urea transporter protein, which carries only 2 Val-Gly motifs. Upon expression at high levels in Xenopus oocytes, the physiological Kidd/urea transporter HUT11A conferred a rapid transfer of urea (which was insensitive to p-chloromercuribenzene sulfonate or phloretin), a high water permeability, and a selective uptake of small solutes including amides and diols, but not glycerol and meso-erythritol. However, at plasma membrane expression levels close to the level observed in the red cell membrane, HUT11A-mediated water transport and small solutes uptake were absent and the urea transport was poorly inhibited by p-chloromercuribenzene sulfonate, but strongly inhibited by phloretin. These findings show that, at physiological expression levels, the HUT11A transporter confers urea permeability but not water permeability, and that the observed water permeability is a feature of the red cell urea transporter when expressed at unphysiological high levels.

Molecular characterization of an elasmobranch urea transporter

Marine elasmobranch fishes retain relatively high levels of urea to balance the osmotic stress of living in seawater. To maintain osmotic balance and reduce the energetic costs of making urea, it is important for these animals to minimize urea excretion to the environment. We have isolated a novel 2.2-kb cDNA from Squalus acanthias (spiny dogfish shark) kidney encoding a 380-amino acid hydrophobic protein (ShUT) with 66% identity to the rat facilitated urea transporter protein UT-A2. Injection of ShUT cRNA into Xenopus oocytes induced a 10-fold increase in 14C-labeled urea uptake, inhibitable by phloretin (0.35 mM). ShUT mRNA is expressed in kidney and brain. Related mRNA species are found in liver, blood, kidney, gill, intestine, muscle, and rectal gland. This is the first facilitated urea transporter to be identified in a marine fish. We propose that the ShUT protein is involved in urea reabsorption by the renal tubules of the dogfish shark, which in turn minimizes urea loss in the urine.

Maturational changes in rabbit renal brush border membrane vesicle urea permeability

Urea transport in the proximal tubule is thought to occur by passive diffusion through the lipid bilayers of the cell membranes. The lipid composition of cell membranes changes during maturation and may directly affect urea permeability of proximal tubule membranes. The present study examined the maturation of urea transport in rabbit renal brush border membrane vesicles (BBMV). BBMV from adult and neonatal (9- to 11-d-old) New Zealand white rabbits were loaded with 500 mM urea and mixed with an iso-osmotic mannitol solution using a stop-flow instrument. Vesicle shrinkage, due to efflux of urea, was followed with light scattering and urea permeability was calculated from an exponential fit of the data. Urea permeability was significantly lower in the neonatal BBMV than the adult at 25 degrees C (0.34+/-0.04 x 10(-6) versus 0.56+/-0.03 x 10(-6) cm/sec;p < 0.001, n=7) and 37 degrees C (0.45+/-0.04 x 10(-6) versus 0.66+/-0.03 x 10(-6) cm/sec; p=0.001, n=7). There was no effect of 250 microM phloretin on urea permeability in either adult or neonatal BBMV at either temperature. The activation energy for urea diffusion was higher in the neonatal than the adult BBMV. Because the maturational increase in urea permeability could potentially be due to a sodium-dependent urea transporter in the adult BBMV, the sodium dependence of urea uptake in adult BBMV was examined. There was no difference in urea permeability in the presence or absence of 20 mM NaCl. Permeability of the lipid-soluble molecule, glycerol, was also found to be the same in the neonatal and adult BBMV. Urea transport in the apical membrane of neonatal and adult proximal tubules is not phloretin sensitive, a finding consistent with diffusion of urea via the lipid bilayer. The rate of urea diffusion is lower in neonatal membranes and may be an important factor in overall urea excretion. This may also play a role in developing and maintaining a high medullary urea concentration and thus the ability to concentrate the urine during renal maturation.

Influence of pH on metabolism and urease activity of Helicobacter pylori

BACKGROUND & AIMS

The metabolic and urease responses of Helicobacter pylori to variations in gastric acidity are unknown. The aim of this study was to determine effects of changes of environmental pH on metabolism, urease activity, and survival of H. pylori in an unbuffered environment.

METHODS

Bacterial metabolism and urease activity were determined by measuring pH changes in perfused microphysiometer chambers over a pH range from 2.5 to 9.0 with or without urea and survival by restoration of metabolism at pH 7.4.

RESULTS

Glucose metabolism by acid-adapted H. pylori occurred at a perfusion pH between 3.5 and 8.6 and was highest between 7.4 and 8.2. Metabolism was irreversibly inhibited at pH <3.5 or >8.6. In the presence of 2.5 mmol/L urea, the chamber pH increased to about 6.2 during perfusion between pH 5.5 and 4.0. At pH 4.0 and below, urease activity increased several-fold without change of chamber pH. Urea in the perfusate enabled retention of metabolism after acid exposure but was toxic at pH 7.4.

CONCLUSIONS

The metabolic range of acid-adapted H. pylori is between an environmental pH of 3.5 and 8.6. Extracellular pH-regulated internal urease activity allows metabolism in the pH range between 4.0 and 2. 5 by maintaining periplasmic pH at 6.2. The organism is an acid-tolerant neutralophile due to internal urease activity.

Urea transporters in kidney: molecular analysis and contribution to the urinary concentrating process1

Facilitated urea transporters (UTs) are responsible for urea accumulation in the renal inner medulla of the mammalian kidney and therefore play a central role in the urinary concentrating process. Recently, the cDNAs encoding three members of the UT family, UT1, UT2, and UT3 have been cloned. These transporters are expressed in different structures of the mammalian kidney. In rat, UT1 resides in the apical membrane of terminal inner medullary collecting ducts, where it mediates vasopressin-regulated urea reabsorption. UT2 and UT3 are located in descending thin limbs of Henle's loop and descending vasa recta, respectively, and participate in urinary recycling processes, which minimize urea escape from the inner medulla. UT1 and UT2 are regulated independently and respond differently to changes in dietary protein content and hydration state. Identification and characterization of these urea transporters advances our understanding of the molecular basis and regulation of the urinary concentrating mechanism.

Insights into the structure and function of membrane polypeptides carrying blood group antigens

In recent years, advances in biochemistry and molecular genetics have contributed to establishing the structure of the genes and proteins from most of the 23 blood group systems presently known. Current investigations are focusing on genetic polymorphism analysis, tissue-specific expression, biological properties and structure-function relationships. On the basis of this information, the blood group antigens were tentatively classified into five functional categories: (i) transporters and channels, (ii) receptors for exogenous ligands, viruses, bacteria and parasites, (iii) adhesion molecules, (iv) enzymes and, (v) structural proteins. This review will focus on selected blood groups systems (RH, JK, FY, LU, LW, KEL and XK) which are representative of these classes of molecules, in order to illustrate how these studies may bring new information on common and variant phenotypes and for understanding both the mechanisms of tissue specific expression and the potential function of these antigens, particularly those expressed in nonerythroid lineage.

Characterization of the gene encoding the human Kidd blood group/urea transporter protein. Evidence for splice site mutations in Jknull individuals

The Kidd (JK) blood group is carried by an integral membrane glycoprotein which transports urea through the red cell membrane and is also present on endothelial cells of the vasa recta in the kidney. The exon-intron structure of the human blood group Kidd/urea transporter gene has been determined. It is organized into 11 exons distributed over 30 kilobase pairs. The mature protein is encoded by exons 4-11. The transcription initiation site was identified by 5'-rapid amplification of cDNA ends-polymerase chain reaction at 335 base pairs upstream of the translation start point located in exon 4. The 5'-flanking region, from nucleotide -837 to -336, contains TATA and inverted CAAT boxes as well as GATA-1/SP1 erythroid-specific cis-acting regulatory elements. Analysis of the 3'-untranslated region reveals that the two equally abundant erythroid transcripts of 4.4 and 2.0 kilobase pairs arise from usage of different alternative polyadenylation signals. No obvious abnormality of the Kidd/urea transporter gene, including the 5'- and 3'-untranslated regions, has been detected by Southern blot analysis of the blood of two unrelated Jknull individuals (B.S. and L.P.), which lacks all Jk antigens and Jk proteins on red cells, but was genotyped as homozygous for a "silent" Jkb allele. Further analysis indicated that different splice site mutations occurred in each variant. The first mutation affected the invariant G residue of the 3'-acceptor splice site of intron 5 (variant B.S.), while the second mutation affected the invariant G residue of the 5'-donor splice site of intron 7 (variant L.P.). These mutations caused the skipping of exon 6 and 7, respectively, as seen by sequence analysis of the Jk transcripts present in reticulocytes. Expression studies in Xenopus oocytes demonstrated that the truncated proteins encoded by the spliced transcripts did not mediate a facilitated urea transport compared with the wild type Kidd/urea transporter protein and were not expressed on the oocyte's plasma membrane. These findings provide a rational explanation for the lack of Kidd/urea transporter protein and defect in urea transport of Jknull cells.

[Blood groups and structure-activity relations]

In the recent years, advances in biochemistry and molecular genetics have contributed to establish the structure of the genes and proteins from most of the 23 blood group systems presently known. From these findings, five functional classes of molecules can be schematically distinguished: (i) transporters and channels, (ii) receptors for ligands, viruses, bacteria and parasites, (iii) adhesion molecules, (iv) enzymes, and (v) structural proteins. Recent advances on these molecules will be reviewed, particularly by illustrating available structure-function relationships.

Normal ultrastructure of the kidney and lower urinary tract

The mammalian urinary tract includes the kidneys, ureters, urinary bladder, and urethra. The renal parenchyma is composed of the glomeruli and a heterogeneous array of tubule segments that are specialized in both function and structure and are arranged in a specific spatial distribution. The ultrastructure of the glomeruli and renal tubule epithelia have been well characterized and the relationship between the cellular structure and the function of the various components of the kidney have been the subject of intense study by many investigators. The lower urinary tract, the ureters, urinary bladder, and urethra, which are histologically similar throughout, are composed of a mucosal layer lined by transitional epithelium, a tunica muscularis, and a tunica serosa or adventitia. The present manuscript reviews the normal ultrastructural morphology of the kidney and the lower urinary tract. The normal ultrastructure is illustrated using transmission electron microscopy of normal rat kidney and urinary bladder preserved by in vivo perfusion with glutaraldehyde fixative and processed in epoxy resin.

Cloning and characterization of the urea transporter UT3: localization in rat kidney and testis

Urea transport in the kidney plays an important role in urinary concentration and nitrogen balance. Recently, three types of urea transporters have been cloned, UT1 and UT2 from rat and rabbit kidney and HUT11 from human bone marrow. To elucidate the physiological role of the latter urea transporter, we have isolated the rat homologue (UT3) of HUT11 and studied its distribution of expression and functional characteristics. UT3 cDNA encodes a 384 amino acid residue protein, which has 80% identity to the human HUT11 and 62% identity to rat UT2. Functional expression in Xenopus oocytes induced a large (approximately 50-fold) increase in the uptake of urea compared with water-injected oocytes. The uptake was inhibited by phloretin (0.75 mM) and pCMBS (0.5 mM) (55 and 32% inhibition, respectively). Northern analysis gave a single band of 3.8 kb in kidney inner and outer medulla, testis, brain, bone marrow, spleen, thymus, and lung. In situ hybridization of rat kidney revealed that UT3 mRNA is expressed in the inner stripe of the outer medulla, inner medulla, the papillary surface epithelium, and the transitional urinary epithelium of urinary tracts. Co-staining experiments using antibody against von Willebrand factor showed that UT3 mRNA in the inner stripe of the outer medulla is expressed in descending vasa recta. These data suggest that UT3 in kidney is involved in counter current exchange between ascending and descending vasa recta, to enhance the cortico-papillary osmolality gradient. In situ hybridization of testis revealed that UT3 is located in Sertoli cells of seminiferous tubules. The signal was only detected in Sertoli cells associated with the early stages of spermatocyte development, suggesting that urea may play a role in spermatogenesis.

Water channels and urea transporters

1. It now appears that when water crosses an endothelium which is not fenestrated, or an epithelium with tight junctions, it does so rapidly, and with low energy cost, only if the cell membrane contains an adequate number of specific water channels, encoded by one of at least six different genes. 2. The water channel genes so far cloned encode a series of integral membrane proteins called aquaporins, all of approximately 30 kDa (265-282 amino acids), in the unglycosylated state. All but one (AQP3) are specific water channels and all but one (AQP4) are inactivated by mercurial compounds. 3. Aquaporin 0 is the major (60%) intrinsic protein (MIP) of lens fibre cells of the eye. Mutations in this gene are associated with cataract formation in mice. 4. Aquaporin 1, also called CHIP-28, exists in the membrane as a homotetramer, and is present in red blood cells, the choroid plexus, the proximal tubule and descending limb of the loop of Henle in the kidney as well as in many other sites. Surprisingly, no pathological consequence is known in patients lacking a functional AQP1 gene. 5. Aquaporin 2, also called WCH-CD, is the water channel of the principal cell of the cortical and medullary collecting duct, and is located in cytoplasmic vesicles unless arginine vasopressin is acting, when it is translocated to the apical membrane by synaptobrevins or vesicle associated membrane protein 2 (VAMP2). Lack of a functional AQP2 gene leads to a rare form of nephrogenic diabetes insipidus. 6. Aquaporins 3, 4, and 5 are located in many tissues-AQP3 and AQP4 being in the basolateral membrane of the renal cortical and medullary principal cell, as well as in the gastrointestinal tract (AQP3) and the brain (AQP4). 7. Four sequences are known for urea transporters HUT11-the urea transporter of the human red cell membrane, and HUT2, rUT2, rbUT2-the arginine vasopressin inducible urea transporters of the human, rat and rabbit kidney. They are specifically permeable to urea, not to water, and are claimed to be inhibited by phloretin. 8. The water channel proteins contain six membrane-spanning regions, whilst the urea transporters are thought to contain at least 10 membrane spanning segments. 9. Very little work has examined the ontogeny of these proteins, except in the rat, and virtually nothing is known of the expression of these genes in pregnancy or in any disorder of fluid balance in the mother or foetus.