Antigenic and functional properties of the human red blood cell urea transporter hUT-B1

Jk(a) (Kidd-A) Variant in a Sickle Cell Disease Patient

Sickle cell disease (SCD) is a chronic and prevalent hemoglobin disorder with various manifestations and complications depending on the organs involved. Red cell transfusion either simple or exchange is crucial due to its prophylactic and therapeutic roles. We present a case showing serologic discrepancy between the red cells phenotype and the developed alloantibodies to emphasize the crucial role of molecular testing in SCD patients requiring chronic blood transfusion.

Urea transport in human red blood cells: Donor variation compared to chloride, glucose, and water transport

We determined the permeability (P, cm/s) of unmodified human red blood cells (HRBC) to urea (Pu), chloride (PCl), glucose (Pglu), and water diffusion (Pd) under conditions of self-exchange (SE) with the continuous flow tube method at pH 7.2, 25°C. Among 24 donors, Pu at 1 mM varied >100%. Two of the donors were also tested in 1983. Their Pu had decreased by 77 and 90%. High age in males and Kidd genotype Jk(a+,b+), but not blood types AB0, appear related to low Pu. For one of the two donors, PCl (150 mM, 38°C, pH 7.2), Pglu (1 mM, 38°C, pH 7.2), and Pd (55.5 M, 25°C, pH 7.2) were determined then and now and showed no significant changes with age. The results from six more donors show donor PCl, Pglu, and Pd in the range of ≈1%. PCl and Pglu are vital for the metabolism of cells and tissues, and we see but little donor variation, and so far, no phenotypes without glucose (GLUT1) and anion (AE1) transporters in HRBC. Phenotypes with no urea transporter (UT-B) or no water transporters (aquaporin, AQP1) are registered and are compatible with life. Our results are in line with the concept that the solutes do not share pathways in common. The great donor variation in Pu must be considered in comparative transport physiological studies.

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.

Urea and water are transported through different pathways in the red blood cell membrane

Several studies of the urea transporter UT-B expressed in Xenopus oocytes and in genetically modified red blood cells (RBC) have concluded that UT-B also transports water. In the present study, we use unmodified RBC to test that conclusion. We find that the permeability of urea, Pu (cm/s), has a 10-fold donor variation, while the diffusional water permeability, Pd (cm/s), remains unchanged. Additionally, we observe that phloretin inhibits Pu but not Pd, and that the time course of maximum p-chloromercuribenzosulfonate inhibition of Pu and Pd differs-Pu inhibition takes <2 min, whereas Pd inhibition requires ≥1 h of incubation. The findings in the present study are in line with a previous comparative study using unmodified RBC from four animals and a solvent drag study using human RBC, and they lead us to reject the conclusion that the UT-B transporter represents a common pathway for both solutes.

Two new JK silencing alleles identified by single molecule sequencing with 20-Kb long-reads

BACKGROUND

The Kidd blood group gene SLC14A1 (JK) accounts for approximately 20 Kb from initiation codon to stop codon in the genome. In genomic DNA analysis using Sanger sequencing or short-read-based next generation sequencing, it is difficult to determine the cis or trans positions of single nucleotide variations (SNVs), which are occasionally more than 1 Kb away from each other. We aimed to determine the complete nucleotide sequence of a 20-Kb genomic DNA amplicon to characterize the JK allelic variants associated with Kidd antigen silencing in a blood donor.

STUDY DESIGN AND METHODS

The Jk(a-b-) phenotype was identified in this donor by standard serological typing. A DNA sample obtained from whole blood was amplified by long-range PCR to obtain a 20-Kb fragment of the SLC14A1 gene, including the initiation and stop codons. The fragment was then analyzed by Sanger sequencing and single-molecule sequencing. Transfection and expression studies were performed in CHO cells using the expression vector construct of JK alleles.

RESULTS

Sanger sequencing and single-molecule sequencing revealed that the donor was heterozygous with JK*01 having c.276G>A (rs763262711, p.Trp92Ter) and JK*02 having c.499A>G (rs2298719, p.Met167Val), c.588A>G (rs2298718, p.Pro196Pro), and c.743C>A (p.Ala248Asp). The two JK alleles identified have not been previously described. Transfection and expression studies indicated that the CHO cells transfected with JK*02 having c.743C>A did not express the Jkb and Jk3 antigens.

CONCLUSIONS

We identified new JK silencing alleles and their critical SNVs by single-molecule sequencing and the findings were confirmed by transfection and expression studies.

The frequencies of Kidd blood group antigens and phenotypes among Saudi blood donors in Southwestern Saudi Arabia

BACKGROUND

The patients who require transfusion are prevalent in the Jazan Province, Saudi Arabia. Therefore, it is essential to know the frequency of blood group antigens in such a population. The Kidd blood group system (JK) has two antithetical antigens, Jka and Jkb. Antibodies to these antigens may result in delayed hemolytic transfusion reactions. The present study investigated the frequencies of Jka and Jkb and the phenotypes among Saudi blood donors living in the Jazan Province.

METHODS

One hundred and forty-three samples from anonymous Saudi volunteer blood donors in the Jazan Province were serotype to detect Jka and Jkb using gel card technology and determine the phenotypes of the JK blood group system.

RESULTS

The prevalence of Jka and Jkb antigens were 90.64% (n = 126) and 69.40% (n = 93), respectively. The JK phenotypes were 34.96% Jk(a + b - ) (n = 51), 12.59% Jk(a - b + ) (n = 18), 52.45% Jk(a + b + ) (n = 75), and 0% Jk(a - b - ). The frequencies of the JK phenotypes in the Jazan population were significantly different from those in the Asian population (P < 0.05).

CONCLUSIONS

We reported the frequencies of the Jka and Jkb antigens and the distribution of the JK phenotypes in a group of Saudi blood donors in the Jazan Province, Saudi Arabia. The phenotype Jk(a + b + ) was the most common among the study population. Furthermore, this study emphasizes the significance of identifying the frequency of JK antigens and phenotypes in the provinces of Saudi Arabia.

Jknull alleles in two patients with anti-Jk3

BACKGROUND

As of publication, a total of 41 null alleles have been acknowledged by the International Society of Blood Transfusion (ISBT) to cause the rare Jknull phenotype, but none have been discovered in Austria thus far.

MATERIALS AND METHODS

Two patients with anti-Jk3 were serologically identified by a positive antibody screening and typed as Jk(a-b-). The initial genotyping using an SSP-PCR method for the common 838A/G polymorphism indicated a JK*02/02, or JK*01/02 genotype, respectively. To find the disruptive mutations, Sanger sequencing was performed and results were compared to the reference sequence. The patient's antibodies were characterized with a monocyte monolayer assay (MMA) for their potential clinical significance.

RESULTS

Three novel null-mutations of the SLC14A1 gene were found in two patients. Patient 1 was homozygous for a 10bp deletion in exon 4 (c.157_166del on JK*02). Testing of her family members revealed Mendelian inheritance of the deletional allele. The other patient was compound heterozygous for two mutations: one allele carrying a single base deletion in exon 4 (c.267delC on JK*01) and the other a splice site mutation in intron 3 (c.152-1g>a on JK*02). The MMA results suggest high clinical significance of the anti-Jk3 in both patients.

DISCUSSION

The detected mutations led to Jknull phenotypes and are the first description of JKnull alleles in the Austrian population.

SLC14A1 prevents oncometabolite accumulation and recruits HDAC1 to transrepress oncometabolite genes in urothelial carcinoma

Urothelial carcinoma (UC), including upper tract urothelial carcinoma (UTUC) and urinary bladder urothelial carcinoma (UBUC), is a common malignant disease in developed countries. Oncogenic metabolic lesions have been associated with UC development. Methods: Using data mining, a series of studies were performed to study the involvement of SLC14A1 in UC specimens, animal models and UC-derived cell lines. Results: In two cohorts of UTUC (n = 340) and UBUC (n = 295), the SLC14A1 protein level was an independent prognostic factor. Epigenetic silencing contributed to SLC14A1 downregulation in UCs. Total and membranous SLC14A1 played tumor suppressive roles through the inhibition of cell proliferation and metastasis in distinct UC-derived cells and animal models. Functional SLC14A1 prevented the accumulation of arginine and urea, enhanced mitochondrial fusion and aerobic respiration, inhibited glycolysis by altering the expression levels of several related proteins and sensitized arginine-deprivation treatment in ASS1-deficient UC-derived cells. In vitro and in vivo, SLC14A1 inhibited the mTOR signaling pathway and subsequently tumorigenesis, supported by reduced arginine concentrations in vitro. Nuclear SLC14A1 transrepressed HK2 and DEGS1 genes via recruitment of HDAC1 and/or SIN3A to maintain metabolic homeostasis and thereafter impeded tumorigenesis. Conclusion: Clinical associations, animal models and in vitro indications provide solid evidence that the SLC14A1 gene is a novel tumor suppressor in UCs. Total and membranous SLC14A1 prevents urea and arginine accumulation via the mTOR signaling pathway. Nuclear SLC14A1 recruits HDAC1 to transrepress oncometabolite genes.

The urea transporter UT-A1 plays a predominant role in a urea-dependent urine-concentrating mechanism

Urea transporters are a family of urea-selective channel proteins expressed in multiple tissues that play an important role in the urine-concentrating mechanism of the mammalian kidney. Previous studies have shown that knockout of urea transporter (UT)-B, UT-A1/A3, or all UTs leads to urea-selective diuresis, indicating that urea transporters have important roles in urine concentration. Here, we sought to determine the role of UT-A1 in the urine-concentrating mechanism in a newly developed UT-A1-knockout mouse model. Phenotypically, daily urine output in UT-A1-knockout mice was nearly 3-fold that of WT mice and 82% of all-UT-knockout mice, and the UT-A1-knockout mice had significantly lower urine osmolality than WT mice. After 24-h water restriction, acute urea loading, or high-protein (40%) intake, UT-A1-knockout mice were unable to increase urine-concentrating ability. Compared with all-UT-knockout mice, the UT-A1-knockout mice exhibited similarly elevated daily urine output and decreased urine osmolality, indicating impaired urea-selective urine concentration. Our experimental findings reveal that UT-A1 has a predominant role in urea-dependent urine-concentrating mechanisms, suggesting that UT-A1 represents a promising diuretic target.

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.

Genomic coordinates and continental distribution of 120 blood group variants reported by the 1000 Genomes Project

BACKGROUND

The 1000 Genomes Project provides a database of genomic variants from whole genome sequencing of 2504 individuals across five continental superpopulations. This database can enrich our background knowledge of worldwide blood group variant geographic distribution and identify novel variants of potential clinical significance.

STUDY DESIGN AND METHODS

The 1000 Genomes database was analyzed to 1) expand knowledge about continental distributions of known blood group variants, 2) identify novel variants with antigenic potential and their geographic association, and 3) establish a baseline scaffold of chromosomal coordinates to translate next-generation sequencing output files into a predicted red blood cell (RBC) phenotype.

RESULTS

Forty-two genes were investigated. A total of 604 known variants were mapped to the GRCh37 assembly; 120 of these were reported by 1000 Genomes in at least one superpopulation. All queried variants, including the ACKR1 promoter silencing mutation, are located within exon pull-down boundaries. The analysis yielded 41 novel population distributions for 34 known variants, as well as 12 novel blood group variants that warrant further validation and study. Four prediction algorithms collectively flagged 79 of 109 (72%) known antigenic or enzymatically detrimental blood group variants, while 4 of 12 variants that do not result in an altered RBC phenotype were flagged as deleterious.

CONCLUSION

Next-generation sequencing has known potential for high-throughput and extended RBC phenotype prediction; a database of GRCh37 and GRCh38 chromosomal coordinates for 120 worldwide blood group variants is provided as a basis for this clinical application.

Investigation of facilitative urea transporters in the human gastrointestinal tract

The symbiotic relationship between humans and their intestinal microbiome is supported by urea nitrogen salvaging. Previous studies have shown that colonic UT-B urea transporters play a significant role in this important physiological process. This current study investigated UT-A and UT-B urea transporter expression along the human gastrointestinal tract. Initial end-point PCR experiments determined that UT-A RNA was predominantly expressed in the small intestine, while UT-B RNA was expressed in stomach, small intestine, and colon. Using western blotting experiments, a strong 40-60 kDa UT-B signal was found to be abundant in both ileum and colon. Importantly, this signal was deglycosylated by PNGaseF enzyme treatment to a core protein of 30 kDa in both tissues. Further immunolocalization studies revealed UT-B transporter proteins were present at the apical membrane of the villi in the ileum, but predominantly at the basolateral membrane of the colonic surface epithelial cells. Finally, a blind scoring immunolocalization study suggested that there was no significant difference in UT-B abundance throughout the colon (NS, ANOVA, N = 5-21). In conclusion, this current study suggested UT-B to be the main human intestinal urea transporter. Intriguingly, these data suggested that the same UT-B isoform was present in all intestinal epithelial cells, but that the precise cellular location varied.

Robotic hysterectomy in Trendelenburg position in a severely anaemic JKa alloimmunised patient with impending high-output cardiac failure: An anaesthetic challenge

Kidd blood group alloimmunisation, though extremely rare, may produce considerable morbidity, and even mortality. Severe anaemia and impending high-output cardiac failure requiring blood transfusion should be weighed against the risk of severe transfusion reactions even with fully cross-matched blood. Kidd antibodies are a common cause of delayed haemolytic transfusion reaction (DHTR) since they have a tendency remain undetectable in plasma. A low -grade DHTR (second hit) was grossly amplified by a second DHTR (third hit) superimposed on it in our patient leading to severe haemolysis with serum bilirubin reaching 68 mg%. Indirect antiglobulin test (indirect Coombs reaction) should ideally be performed in all patients (scheduled for major surgery requiring blood transfusion) who have experienced a previous pregnancy or blood transfusion. Non-invasive continuous haemoglobin monitoring and non-invasive cardiac output monitoring can prove invaluable tools in management.

Concentration dependence of the cell membrane permeability to cryoprotectant and water and implications for design of methods for post-thaw washing of human erythrocytes

For more than fifty years the human red blood cell (RBC) has been a widely studied model for transmembrane mass transport. Existing literature spans myriad experimental designs with varying results and physiologic interpretations. In this review, we examine the kinetics and mechanisms of membrane transport in the context of RBC cryopreservation. We include a discussion of the pathways for water and glycerol permeation through the cell membrane and the implications for mathematical modeling of the membrane transport process. In particular, we examine the concentration dependence of water and glycerol transport and provide equations for estimating permeability parameters as a function of concentration based on a synthesis of literature data. This concentration-dependent transport model may allow for design of improved methods for post-thaw removal of glycerol from cryopreserved blood. More broadly, the consideration of the concentration dependence of membrane permeability parameters may be important for other cell types as well, especially for design of methods for equilibration with the highly concentrated solutions used for vitrification.

SLC14A1: a novel target for human urothelial cancer

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

Lack of association between Kidd blood group system and chronic kidney disease

BACKGROUND

The Kidd blood group system has three antigens, Jka, Jkb and Jk3, found on red blood cells and on endothelial cells of the inner lining of blood vessels in the renal medulla. These are known as urea transporter B (UT-B). Researchers have found that individuals carrying the Jk(a-b-) or Jk-null (UT-B null) phenotypes have a lower urine-concentrating capability and risk of severe renal impairment. This study evaluated the distribution of the Kidd phenotypes in patients with chronic kidney disease and a possible association of Kidd antigens with the development of renal disease.

METHODS

Jka and Jkb antigens were phenotyped using the gel column agglutination test (ID-cards Bio-RAD) in 197 patients with chronic kidney disease and 444 blood donors, as the control group. The phenotype and antigen frequencies between patients and controls were evaluated using the Chi-square method with Yates correction and logistic regression after adjustments for gender and age.

RESULTS

No differences were observed between the Kidd phenotypes frequency distribution between patients with chronic kidney disease and blood donors [Jk(a-b+)=22.3% and 27.2%; Jk(a+b-)=30.5% and 24.3%; Jk(a+b+)=47.25% and 48.4%, respectively].

CONCLUSION

The distribution of Kidd phenotypes found in the studied population is expected for Caucasians; Jka and Jkb antigens and phenotypes were not found to be related to susceptibility for chronic kidney disease.

Emerging strategies of blood group genotyping for patients with hemoglobinopathies

Red cell alloimmunization is a serious problem in chronically transfused patients. A number of high-throughput DNA assays have been developed to extend or replace traditional serologic antigen typing. DNA-based typing methods may be easily automated and multiplexed, and provide reliable information on a patient. Molecular genotyping promises to become cheaper, being not dependent on serologic immunoglobulin reagents. Patients with hemoglobinopathies could benefit from receiving extended genomic typing. This could limit post transfusional complications depending on subtle antigenic differences between donors and patients. Patient/donor compatibility extended beyond the phenotype Rh/Kell may allows improved survival of transfused units of red blood cells (RBC) and lead to reduced need for blood transfusion and leading to less iron overload and reduced risk of alloimmunization. Here we discuss the advantages and limitations of current techniques, that detect only predefined genetic variants. In contrast, target enrichment next-generation sequencing (NGS) has been used to detect both known and de novo genetic polymorphisms, including single-nucleotide polymorphisms, indels (insertions/deletions), and structural variations. NGS approaches can be used to develop an extended blood group genotyping assay system.

The Kidd (JK) Blood Group System

The Kidd blood group system was discovered in 1951 and is composed of 2 antithetical antigens, Jk^a and Jk^b, along with a third high-incidence antigen, Jk3. The Jk3 antigen is expressed in all individuals except those with the rare Kidd-null phenotype. Four Kidd phenotypes are therefore possible: Jk(a+b-), Jk(a-b+), Jk(a+b+), and Jk(a-b-). The glycoprotein carrying the Kidd antigens is a 43-kDa, 389-amino acid protein with 10 membrane-spanning domains which functions as a urea transporter on endothelial cells of the renal vasa recta as well as erythrocytes. The HUT11/UT-B/JK (SLC14A1) gene encoding this glycoprotein is located on chromosome 18q12-q21. The Jk^a and Jk^b antigens are the result of a single-nucleotide polymorphism present at nucleotide 838 resulting in an aspartate or asparagine amino acid at position 280, respectively. The Kidd blood group can create several difficult transfusion situations. Besides the typical acute hemolytic transfusion reactions common to all clinically relevant blood group antigens, the Kidd antigens are notorious for causing delayed hemolytic transfusion reactions due to the strong anamnestic response exhibited by antibodies directed against Kidd antigens. The Kidd-null phenotype is extremely rare in most ethnic groups, but is clinically significant due to the ability of those with the Kidd-null phenotype to produce antibodies directed against the high-incidence Jk3 antigen. Anti-Jk3 antibodies behave in concordance with anti-Jk^a or anti-Jk^b possessing the capability to cause both acute and delayed hemolytic reactions. Antibodies against any of the 3 Kidd antigens can also be a cause of hemolytic disease of the fetus and newborn, although this is generally mild. In this review, we will outline the makeup of the Kidd system from its historical discovery to the details of the Kidd gene and glycoprotein, and then discuss the practical aspects of Kidd antibodies and transfusion reactions with an extended focus on the Kidd-null phenotype. We will end with a brief discussion of the donor aspects related to the screening and supply management of blood from donors with the rare Jk(a-b-) phenotype.

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.

Genes and proteins of urea transporters

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

Clinical aspects of urea transporters

Jk antigens, which were identified as urea transporter B (UT-B) in the plasma membrane of erythrocytes, and which determine the Kidd blood type in humans, are involved in transfusion medicine, and even in organ transplantation. The Jk(a-b-) blood type is a consequence of a silent Slc14A1 gene caused by various mutations related to lineage. In addition, the specific mutations related to hypertension and metabolic syndrome cannot be ignored. Genome-wide association studies established Slc14A1 as a related gene of bladder cancer and some genotypes are associated with higher morbidity. This chapter aims to introduce the clinical significance of urea transporters.

Transport characteristics of urea transporter-B

UT-B represents the major urea transporter in erythrocytes, in addition to being expressed in kidney descending vasa recta, brain, spleen, ureter, bladder, and testis. Expression of urea transporter UT-B confers high urea permeability to mammalian erythrocytes. Erythrocyte membranes are also permeable to various urea analogues, suggesting common transport pathways for urea and structurally similar solutes. UT-B is highly permeable to urea and the chemical analogues formamide, acetamide, methylurea, methylformamide, ammonium carbamate, and acrylamide, each with a Ps > 5.0 × 10(-6) cm/s at 10 °C. The amides formamide, acetamide, acrylamide, and butyramide efficiently diffuse across lipid bilayers. The urea analogues dimethylurea, acryalmide, methylurea, thiourea, and methylformamide inhibit UT-B-mediated urea transport by >60 % by a pore-blocking mechanism. UT-B is also a water channel in erythrocytes and has a single-channel water permeability that is similar to aquaporin-1. Whether UT-B is an NH3 channel still needs further study. Urea permeability (Purea) in erythrocytes differs between different mammals. Carnivores (dog, fox, cat) exhibit high Purea. In contrast, herbivores (cow, donkey, sheep) show much lower Purea. Erythrocyte Purea in human and pig (omnivores) was intermediate. Rodents and lagomorphs (mouse, rat, rabbit) have Purea intermediate between carnivores and omnivores. Birds that do not excrete urea and do not express UT-B in their erythrocytes have very low values. In contrast to Purea, water permeability is relatively similar in all mammals studied. This chapter will provide information about the transporter characteristics of UT-B.

Energetic and molecular water permeation mechanisms of the human red blood cell urea transporter B

Urea transporter B (UT-B) is a passive membrane channel that facilitates highly efficient permeation of urea. In red blood cells (RBC), while the major function of UT-B is to transport urea, it is assumed that this protein is able to conduct water. Here, we have revisited this last issue by studying RBCs and ghosts from human variants with defects of aquaporin 1 (AQP1) or UT-B. We found that UT-B's osmotic water unit permeability (pfunit) is similar to that of AQP1. The determination of diffusional permeability coefficient (Pd) allowed the calculation of the Pf/Pd ratio, which is consistent with a single-file water transport. Molecular dynamic simulations of water conduction through human UT-B confirmed the experimental finding. From these results, we propose an atomistic description of water-protein interactions involved in this permeation. Inside the UT-B pore, five water molecules were found to form a single-file and move rapidly along a channel by hydrogen bond exchange involving two critical threonines. We further show that the energy barrier for water located in the central region coincides with a water dipole reorientation, which can be related to the proton exclusion observed experimentally. In conclusion, our results indicate that UT-B should be considered as a new member of the water channel family.

Synonymous nucleotide polymorphisms influence Dombrock blood group protein expression in K562 cells

To gain further insight into ART4 (DO) gene alleles (DO*A, DO*JO1, DO*A-WL, DO*DOYA, DO*B, DO*B-WL, DO*B-SH-Q149K, DO*B-(WL)-I175N, DO*HY1, DO*HY2, DO*DOMR) and evaluate the impact of synonymous nucleotide polymorphisms on protein expression and mRNA accumulation of DO*A-HA, DO*A-SH and DO*B-SH alleles, human erythroleukaemic K562 cells were transducted with variant DO-lentiviral particles and analysed by flow cytometry and quantitative reverse transcription polymerase chain reaction. Monoclonal antibody (MoAb) detection of DO*A-HA and DO*JO1 transductants was lower than DO*A transductants, while detection of DO*A-SH, DO*A-WL and DO*DOYA transductants was higher. Variant DO*B alleles, i.e. DO*B-SH, DO*B-WL, DO*HY1, DO*HY2 and DO*DOMR, showed reduced MoAb binding. The unexpected modifications of protein expression of the DO*A-HA, DO*A-SH and DO*B-SH alleles that differ from the DO*A or DO*B alleles by a single synonymous polymorphism were abolished by reversion, thus implying involvement of these polymorphisms. Depending on the Leu208 codon used, detection level ranged from 1 to 4·14. In the variant alleles resulting from single synonymous polymorphism, mRNA accumulation correlated roughly with MoAbs detection levels, suggesting post-transcriptional regulation. Other than a few reports involving aberrant splicing, the experiments described herein provide the first evidence that synonymous nucleotide polymorphisms can influence Dombrock blood group expression. Such polymorphisms should be taken into account for molecular screening and potential impact on transfusion.

The permeability of red blood cells to chloride, urea and water

This study extends permeability (P) data on chloride, urea and water in red blood cells (RBC), and concludes that the urea transporter (UT-B) does not transport water. P of chick, duck, Amphiuma means, dog and human RBC to (36)Cl(-), (14)C-urea and (3)H2O was determined under self-exchange conditions. At 25°C and pH 7.2-7.5, PCl is 0.94 × 10(-4)-2.15 × 10(-4) cm s(-1) for all RBC species at [Cl]=127-150 mmol l(-1). In chick and duck RBC, P(urea) is 0.84 × 10(-6) and 1.65 × 10(-6) cm s(-1), respectively, at [urea]=1-500 mmol l(-1). In Amphiuma, dog and human RBC, P(urea) is concentration dependent (1-1000 mmol l(-1), Michaelis-Menten-like kinetics; K1/2;=127, 173 and 345 mmol l(-1)), and values at [urea]=1 mmol l(-1) are 29.5 × 10(-6), 467 × 10(-6) and 260 × 10(-6) cm s(-1), respectively. Diffusional water permeability, Pd, was 0.84 × 10(-3) (chick), 5.95 × 10(-3) (duck), 0.39 × 10(-3) (Amphiuma), 3.13 × 10(-3) (dog) and 2.35 × 10(-3) cm s(-1) (human). DIDS, DNDS and phloretin inhibit PCl by >99% in all RBC species. PCMBS, PCMB and phloretin inhibit P(urea) by >99% in Amphiuma, dog and human RBC, but not in chick and duck RBC. PCMBS and PCMB inhibit Pd in duck, dog and human RBC, but not in chick and Amphiuma RBC. Temperature dependence, as measured by apparent activation energy, EA, of PCl is 117.8 (duck), 74.9 (Amphiuma) and 89.6 kJ mol(-1) (dog). The EA of P(urea) is 69.6 (duck) and 53.3 kJ mol(-1) (Amphiuma), and that of Pd is 34.9 (duck) and 32.1 kJ mol(-1) (Amphiuma). The present and previous RBC studies indicate that anion (AE1), urea (UT-B) and water (AQP1) transporters only transport chloride (all species), water (duck, dog, human) and urea (Amphiuma, dog, human), respectively. Water does not share UT-B with urea, and the solute transport is not coupled under physiological conditions.

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.

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 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.

UT-B-deficient mice develop renal dysfunction and structural damage

BACKGROUND

Urea transporter UT-B is the major urea transporter in erythrocytes and the descending vasa recta in the kidney. In this study, we investigated the effects of long-term UT-B deficiency on functional and structural defect in the kidney of 16-and 52-week-old UT-B-null mice.

METHODS

UT-B-knockout mice were generated by targeted gene disruption and lacked UT-B protein expression in all organs. The urinary concentrating ability of mice was studied in terms of daily urine output, urine osmolality, and urine and plasma chemistries. Changes in renal morphology were evaluated by hematoxylin and eosin staining.

RESULTS

The UT-B-null mice showed defective urine concentrating ability. The daily urine output in UT-B-null mice (2.5 ± 0.1 ml) was 60% higher and urine osmolality (985 ± 151 mosm) was significantly lower than that in wild-type mice (1463 ± 227 mosm). The 52-week-old UT-B-null mice exhibited polyuria after water deprivation, although urine osmolality was increased. At 52 weeks of age, over 31% of UT-B-null mice exhibited renal medullary atrophy because of severe polyuria and hydronephrosis.

CONCLUSIONS

Long-term UT-B deficiency causes severe renal dysfunction and structural damage. These results demonstrate the important role of UT-B in countercurrent exchange and urine concentration.

Urea transport in the kidney

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

Characterization of Jk(a+(weak)): a new blood group phenotype associated with an altered JK*01 allele

BACKGROUND

The clinically important Kidd (JK) blood group system is considered to be relatively uncomplicated, both serologically and genetically. The JK*01 and JK*02 alleles give rise to Jk(a) and Jk(b) antigens, respectively, and silenced alleles result in Jk(a-b-). Other inherited variants analogous to Fy(x) and weak D phenotypes have not been characterized for JK, although recent abstracts indicate their presence.

STUDY DESIGN AND METHODS

Six index samples from individuals whose RBCs reacted variably or weakly with different sources of anti-Jk(a) and 300 controls of the four known JK phenotypes were investigated by standard serology, flow cytometry, Western blotting, and the urea hemolysis test. Molecular analysis, including allele-specific polymerase chain reaction (PCR), DNA sequencing, and transcript analysis by real-time PCR, was performed.

RESULTS

All Jk(a+(w)b-) and Jk(a+(w)b+) index samples were homo- or heterozygous for an altered JK*01 allele carrying 130G>A (Glu44Lys) and the JK*02-associated silent SNPs 588G and Intron 9 -46g. Blood donor screening indicated an allele frequency of 0.042. Titration and flow cytometry with anti-Jk(a) gave lower values in index samples compared to controls, as did anti-Jk3 titers. Donors with 130A also showed significantly decreased Jk(a) density by flow cytometry versus 130G. Western blotting with anti-UT-B demonstrated weaker reactivity with Jk(a+(w)) membranes while JK mRNA levels could not discriminate index samples from controls. The urea hemolysis test was only moderately affected in two Jk(a+(w)b-) samples.

CONCLUSIONS

A new phenotype with weakened Jk(a) expression on RBCs is associated with a JK*01-like allele, which may constitute a risk for hemolytic transfusion reactions if antigen-positive units are missed by routine serology.

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.

Novel bUT-B2 urea transporter isoform is constitutively activated

Our previous studies have detailed a novel facilitative UT-B urea transporter isoform, bUT-B2. Despite the existence of mouse and human orthologs, the functional characteristics of UT-B2 remain undefined. In this report, we produced a stable MDCK cell line that expressed bUT-B2 protein and investigated the transepithelial urea flux across cultured cell monolayers. We observed a large basal urea flux that was significantly reduced by known inhibitors of facilitative urea transporters; 1,3 dimethylurea (P < 0.001, n = 17), thionicotinamide (P < 0.05, n = 11), and phloretin (P < 0.05, n = 9). Pre-exposure for 1 h to the antidiuretic hormone vasopressin had no effect on bUT-B2-mediated urea transport (NS, n = 3). Acute vasopressin exposure for up to 30 min also failed to elicit any transient response (NS, n = 9). Further investigation confirmed that bUT-B2 function was not affected by alteration of intracellular cAMP (NS, n = 4), intracellular calcium (NS, n = 3), or protein kinase activity (NS, n = 4). Finally, immunoblot data suggested a possible role for glycosylation in regulating bUT-B2 function. In conclusion, this study showed that bUT-B2-mediated transepithelial urea transport was constitutively activated and unaffected by known regulators of renal UT-A urea transporters.

Functional characterization of Actinobacillus pleuropneumoniae urea transport protein, ApUT

Urea transporters (UTs) effect rapid flux of urea across biological membranes. In the mammalian kidney, UT activity is essential for effective urine concentration. In bacteria, UT-mediated urea uptake permits intracellular urease to degrade urea to ammonia and CO(2), a process that either buffers acid loads or provides nutrient nitrogen. We have characterized the urea transport channel protein ApUT from Actinobacillus pleuropneumoniae. Kinetic analysis of bacterial inside-out membranes enriched in ApUT showed approximately 28-fold increase in urea permeability (3.3 +/- 0.4 x 10(-4) cm/s) compared with control vesicles (0.11 +/- 0.02 x 10(-4) cm/s). In addition to urea, ApUT also conducts water. Urea and water transport across the channel was phloretin and mercury inhibitable, and the site of inhibition may be located on the cytoplasmic side of the protein. Glycerol and urea analogs, such as methylamine, dimethylurea, formamide, acetamide, methylurea, propanamide, and ethylamine did not permeate across ApUT.

Two novel Jk(null) alleles derived from 222C>A in Exon 5 and 896G>A in Exon 9 of the JK gene

BACKGROUND

Polynesian Jk(null) is well known for its mutation as Intron 5 g>a at the 3' splice acceptor site. After sequencing analysis, however, it was noticed that only three of eight samples with the Jknull phenotype carried typical homozygous Polynesian Jk(null) mutation. Five others were noted to be unreported heterozygous Polynesian Jk(null) mutation. An investigation was then conducted to characterize the underlying mechanism leading to this particular Jk(null) genotype.

STUDY DESIGN AND METHODS

Genomic DNA covering 5'-untranslated region exons and intervening introns of the JK gene was amplified by polymerase chain reaction, and the fragments were directly sequenced. The sequencing results were compared with those published in literature and related biologic Web sites.

RESULTS

In all five samples with a heterozygous Polynesian Jk(null) mutation, additional mutations were identified. Two samples carried missense mutations: 222C>A (Asn74Lys) in Exon 5 and 499A>G (Met167Val) in Exon 7. Three others had missense mutation 896G>A (Gly299Glu) in Exon 9. These substituted amino acids were located either near or at transmembrane domains, respectively. In addition, two polymorphic nucleotides at positions -103 (a>g) and -119(c>a) from the 3' end of Intron 1 were also Polynesian mutation-related.

CONCLUSIONS

In contrast to the typical homozygous Polynesian Jk(null) mutation, two novel heterozygous Jk(null) alleles were noted to be associated with the Jknull phenotype. One carried missense mutation 222C>A in Exon 5, and the other had 896G>A missense mutation in Exon 9. These findings may have implications in designing a molecular screening assay for people with the Jknull phenotype.

Functional characterization of mouse urea transporters UT-A2 and UT-A3 expressed in purified Xenopus laevis oocyte plasma membranes

Urea is a small solute synthesized by many terrestrial organisms as part of the catabolism of protein. In mammals it is transported across cellular membranes by specific urea transporter (UT) proteins that are the products of two separate, but closely related genes, referred to as UT-A and UT-B. Three major UT-A isoforms are found in the kidney, namely UT-A1, UT-A2, and UT-A3. UT-A2 is found in the thin, descending limb of the loop of Henle, whereas UT-A1 and UT-A3 are concentrated in the inner medullary collecting duct. UT-A2 and UT-A3 effectively represent two halves of the whole UT-A gene and, when joined together by 73 hydrophilic amino acids, constitute UT-A1. A biophysical characterization of mouse UT-A2 and UT-A3 was undertaken by expression in Xenopus laevis oocytes and subsequent preparation of highly enriched plasma membrane vesicles for use in stopped-flow fluorometry. Both isoforms were found to be highly specific for urea, and did not permeate water, ammonia, or other molecules closely related to urea (formamide, acetamide, methylurea, and dimethylurea). Single transporter flux rates of 46,000 +/- 10,000 and 59,000 +/- 15,000 (means +/- SE) urea molecules/s/channel for UT-A2 and UT-A3, respectively, were obtained. Overall, the UT-A2 and UT-A3 isoforms appear to have identical functional kinetics.

Erythroid urea transporter deficiency due to novel JKnull alleles

BACKGROUND

The Kidd blood group antigens Jka and Jkb are encoded by the red blood cell (RBC) urea transporter gene. Homozygosity for silent JK alleles results in the rare Jk(a-b-) phenotype. To date, seven JKnull alleles have been identified, and of these, two are more frequent in the Polynesians and Finns. This study reports the identification of other JKnull alleles in Jk(a-b-) individuals of different ethnic or geographic origins.

STUDY DESIGN AND METHODS

Nine Jk(a-b-) samples and a sample from a Jk(a-b+) mother of a Jk(a+b-) baby were investigated. Polymerase chain reaction amplification and sequence analysis of the JK gene was performed. Western blotting and urea lysis were used to confirm Jk(a-b-) RBCs.

RESULTS

Four novel alleles were identified: two different nonsense mutations, 202C>T (Gln68Stop) and 723delA (Ile262Stop) were identified on otherwise consensus JK*1 and JK*2 alleles, respectively. A missense mutation, 956C>T (Thr319Met), was identified in a JK*1 allele from an African-American and a JK*2 allele in two people of subcontinental Indian descent. Immunoblotting and urea lysis confirmed absence of JK glycoprotein in RBC membranes from a sample carrying the 956C>T mutation. Other previously described JKnull mutations were found in samples of origins other than in which they were first identified.

CONCLUSION

The molecular bases of the Jk(a-b-) phenotype are diverse and this is the first report of JKnull alleles in individuals of African and subcontinental Indian descent. Although rare, these alleles should be taken into consideration when planning genotyping strategies for blood donors and patients.

Differential recovery of membrane proteins after extraction by aqueous methanol and trifluoroethanol

Cell membrane proteome analysis is limited by inherent membrane hydrophobicity. Conventional membrane protein extraction techniques use detergents, chaotropes and organic acids that require sample clean-up or pH adjustment, and are associated with significant sample loss. We extracted membrane proteins from red blood cells (RBCs) using methanol (MeOH), trifluoroethanol (TFE) and urea, and identified membrane proteins using 2-D LC coupled with MALDI-TOF/TOF-MS. We show that organic solvents MeOH- and TFE-based methods have membrane protein analysis efficiencies comparable to urea, and are complementary for the recovery of both hydrophilic and hydrophobic peptides. The mean grand average of hydropathicity (GRAVY) value of identified peptides from the TFE-based method (-0.107) was significantly higher than that of the MeOH-based method (-0.465) (p<0.001). Sequential and adjunctive use of the organic solvents MeOH and TFE increases the number of proteins identified, and the confidence of their identification. We show that this strategy is effective for shotgun membrane proteome analysis.

Duffy and Kidd blood group antigens: minor histocompatibility antigens involved in renal allograft rejection?

BACKGROUND

Minor histocompatibility antigens have been poorly defined. Whether Duffy (FY) and Kidd (JK), polymorphic and immunogenic blood group antigens, widely distributed in human organs, expressed and functional in the kidney, could function as minor histocompatibility antigens and be implicated in renal allograft rejection was questioned.

STUDY DESIGN AND METHODS

A retrospective, homogeneous, single-center cohort of 370 renal transplants was analyzed. In all donor/recipient pairs, FY and JK polymorphisms were identified by real-time polymerase chain reaction. In all donor/recipient pairs the matching (m) or mismatching (mm) status was defined for both systems. All biopsies were reviewed, and historical screening results for FY and JK alloantibodies and graft survival were retrospectively analyzed.

RESULTS

Although graft survival was not different between the groups, it was observed that FY mm grafts had significantly more chronic lesions compared to FY m grafts. HLA-DR11 was more frequent in both recipients (p = 0.0081) and donors (p = 0.0104) of FY mm couples without chronic allograft nephropathy, suggesting a protective effect for this molecule. JK mm grafts had more interstitial inflammation than JK m grafts (p = 0.0369).

CONCLUSION

This renal model unmasks for the first time the role of FY and-to a lesser extent-JK antigens as minor histocompatibility antigens and suggests their potential role for other clinical transplant settings.

The erythrocyte urea transporter UT-B

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

[Nephrogenic diabetes insipidus]

Nephrogenic diabetes insipidus 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-vasopressine (AVP). Polyuria, with hyposthenuria and polydipsia are the cardinal clinical manifestations of the disease. Hypercalcemia, hypokaliemia, lithium administration and chronic renal failure are the principal causes of acquired nephrogenic diabetes insipidus. About 90 percent of patients with congenital nephrogenic diabetes insipidus are males with X-linked recessive nephrogenic diabetes insipidus who have mutations in the arginine-vasopressin receptor 2 (AVPR2) gene that codes for the vasopressin V2 receptor. The gene is located in chromosome region Xq28. In about 10 percent of the families studied, congenital nephrogenic diabetes insipidus has an autosomal recessive or autosomal dominant mode of inheritance. In these cases, mutations have been identified in the aquaporin-2 gene (AQP2), which is located in chromosome region 12q13 and codes for the vasopressin-sensitive water channel. Other inherited disorders with mild, moderate or severe inability to concentrate urine include Bartter's syndrome and Cystinosis. Identification of the molecular defect underlying congenital nephrogenic diabetes insipidus is of immediate clinical significance because early diagnosis and treatment of affected infants can avert the physical and mental retardation associated with episodes of dehydration.

Loss of N-linked glycosylation reduces urea transporter UT-A1 response to vasopressin

The vasopressin-regulated urea transporter (UT)-A1 is a transmembrane protein with two glycosylated forms of 97 and 117 kDa; both are derived from a single 88-kDa core protein. However, the precise molecular sites and the function for UT-A1 N-glycosylation are not known. In this study, we compared Madin-Darby canine kidney cells stably expressing wild-type (WT) UT-A1 to Madin-Darby canine kidney cell lines stably expressing mutant UT-A1 lacking one (A1m1, A1m2) or both glycosylation sites (m1m2). Site-directed mutagenesis revealed that UT-A1 has two glycosylation sites at Asn-279 and -742. Urea flux is stimulated by 10 nM vasopressin (AVP) or 10 microM forskolin (FSK) in WT cells. In contrast, m1m2 cells have a delayed and significantly reduced maximal urea flux. A 15-min treatment with AVP and FSK significantly increased UT-A1 cell surface expression in WT but not in m1m2 cells, as measured by biotinylation. We confirmed this finding using immunostaining. Membrane fractionation of the plasma membrane, Golgi, and endoplasmic reticulum revealed that AVP or FSK treatment increases UT-A1 abundance in both Golgi and plasma membrane compartments in WT but not in m1m2 cells. Pulse-chase experiments showed that UT-A1 half-life is reduced in m1m2 cells compared with WT cells. Our results suggest that mutation of the N-linked glycosylation sites reduces urea flux by reducing UT-A1 half-life and decreasing its accumulation in the apical plasma membrane. In vivo, inner medullary collecting duct cells may regulate urea uptake by altering UT-A1 glycosylation in response to AVP stimulation.