Surface electrocardiogram and action potential in mice lacking urea transporter UT-B

Inhibition of glutaminolysis restores mitochondrial function in senescent stem cells

Mitochondrial dysfunction, a hallmark of aging, has been associated with the onset of aging phenotypes and age-related diseases. Here, we report that impaired mitochondrial function is associated with increased glutamine catabolism in senescent human mesenchymal stem cells (MSCs) and myofibroblasts derived from patients suffering from Hutchinson-Gilford progeria syndrome. Increased glutaminase (GLS1) activity accompanied by loss of urea transporter SLC14A1 induces urea accumulation, mitochondrial dysfunction, and DNA damage. Conversely, blocking GLS1 activity restores mitochondrial function and leads to amelioration of aging hallmarks. Interestingly, GLS1 expression is regulated through the JNK pathway, as demonstrated by chemical and genetic inhibition. In agreement with our in vitro findings, tissues isolated from aged or progeria mice display increased urea accumulation and GLS1 activity, concomitant with declined mitochondrial function. Inhibition of glutaminolysis in progeria mice improves mitochondrial respiratory chain activity, suggesting that targeting glutaminolysis may be a promising strategy for restoring age-associated loss of mitochondrial function.

NONMMUT140591.1 may serve as a ceRNA to regulate Gata5 in UT-B knockout-induced cardiac conduction block

We intended to explore the potential molecular mechanisms underlying the cardiac conduction block inducted by urea transporter (UT)-B deletion at the transcriptome level. The heart tissues were harvested from UT-B null mice and age-matched wild-type mice for lncRNA sequencing analysis. Based on the sequencing data, the differentially expressed mRNAs (DEMs) and lncRNAs (DELs) between UT-B knockout and control groups were identified, followed by function analysis and mRNA-lncRNA co-expression analysis. The miRNAs were predicted, and then the competing endogenous RNA (ceRNA) network was constructed. UT-B deletion results in the aberrant expression of 588 lncRNAs and 194 mRNAs. These DEMs were significantly enriched in the inflammation-related pathway. A lncRNA-mRNA co-expression network and a ceRNA network were constructed on the basis of the DEMs and DELs. The complement 7 (C7)-NONMMUT137216.1 co-expression pair had the highest correlation coefficient in the co-expression network. NONMMUT140591.1 had the highest degree in the ceRNA network and was involved in the ceRNA of NONMMUT140591.1-mmu-miR-298-5p-Gata5 (GATA binding protein 5). UT-B deletion may promote cardiac conduction block via inflammatory process. The ceRNA NONMMUT140591.1-mmu-miR-298-5p-Gata5 may be a potential molecular mechanism of UT-B knockout-induced cardiac conduction block.

Physiological functions of urea transporter B

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

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

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

Urea transport and clinical potential of urearetics

PURPOSE OF REVIEW

Urea is transported by urea transporter proteins in kidney, erythrocytes, and other tissues. Mice in which different urea transporters have been knocked out have urine-concentrating defects, which has led to the development and testing of urea transporters Slc14A2 (UT-A) and Slc14A1 (UT-B) inhibitors as urearetics. This review summarizes the knowledge gained during the past year on urea transporter regulation and investigations into the clinical potential of urearetics.

RECENT FINDINGS

UT-A1 undergoes several posttranslational modifications that increase its function by increasing UT-A1 accumulation in the apical plasma membrane. UT-A1 is phosphorylated by protein kinase A, exchange protein activated by cyclic AMP, protein kinase Cα, and AMP-activated protein kinase, all at different serine residues. UT-A1 is also regulated by 14-3-3, which contributes to UT-A1 removal from the membrane. UT-A1 is glycosylated with various glycan moieties in animal models of diabetes mellitus. Transgenic expression of UT-A1 into UT-A1/UT-A3 knockout mice restores urine-concentrating ability. UT-B is present in descending vasa recta and urinary bladder, and is linked to bladder cancer. Inhibitors of UT-A and UT-B have been developed that result in diuresis with fewer abnormalities in serum electrolytes than conventional diuretics.

SUMMARY

Urea transporters play critical roles in the urine-concentrating mechanism. Urea transport inhibitors are a promising new class of diuretic agent.

Extrarenal phenotypes of the UT-B knockout mouse

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

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.

Quantitative proteomic study of myocardial mitochondria in urea transporter B knockout mice

In previous research, we showed that 16-week-old urea transporter B (UT-B) null mice have an atrial-ventricular conduction block, and hypothesized myocardial mitochondrial dysfunction. To investigate the mechanism of this block, we examined the proteomic differences in the myocardial mitochondria of UT-B null and wild-type mice with nanoscale LC-MS/MS. Of 26 proteins clearly downregulated in the UT-B null mice, 15 are involved in complexes I, III, IV, and V of the respiratory chain, which would strongly reduce the activity of the electron transport chain. Excess electrons from complexes I and III pass directly to O2 to generate ROS and deplete ROS-scavenging enzymes. Myocardial intracellular ROS were significantly higher in UT-B null mice than in wild-type mice (p < 0.01), constituting an important cause of oxidative stress injury in the myocardia of UT-B null mice. The mitochondrial membrane potential (ΔΨm) was also lower in UT-B null mice than in wild-type mice (p < 0.05), causing oxidative phosphorylation dysfunction of complex V and insufficient ATP in the myocardial cells of UT-B null mice. HADHA (a trifunctional protein) and HSP60 were also downregulated in the UT-B null myocardial mitochondria. These results confirm that mitochondrial dysfunction underlies the pathogenesis of the atrial-ventricular conduction block in UT-B null mice.

Expression of urea transporters and their regulation

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

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

BACKGROUND

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

METHODOLOGY/PRINCIPAL FINDINGS

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

CONCLUSIONS/SIGNIFICANCE

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

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.

Triazolothienopyrimidine inhibitors of urea transporter UT-B reduce urine concentration

Urea transport (UT) proteins facilitate the concentration of urine by the kidney, suggesting that inhibition of these proteins could have therapeutic use as a diuretic strategy. We screened 100,000 compounds for UT-B inhibition using an optical assay based on the hypotonic lysis of acetamide-loaded mouse erythrocytes. We identified a class of triazolothienopyrimidine UT-B inhibitors; the most potent compound, UTB(inh)-14, fully and reversibly inhibited urea transport with IC(50) values of 10 nM and 25 nM for human and mouse UT-B, respectively. UTB(inh)-14 competed with urea binding at an intracellular site on the UT-B protein. UTB(inh)-14 exhibited low toxicity and high selectivity for UT-B over UT-A isoforms. After intraperitoneal administration of UTB(inh)-14 in mice to achieve predicted therapeutic concentrations in the kidney, urine osmolality after administration of 1-deamino-8-D-arginine-vasopressin was approximately 700 mosm/kg H(2)O lower in UTB(inh)-14-treated mice than vehicle-treated mice. UTB(inh)-14 also increased urine output and reduced urine osmolality in mice given free access to water. UTB(inh)-14 did not reduce urine osmolality in UT-B knockout mice. In summary, these data provide proof of concept for the potential utility of UT inhibitors to reduce urinary concentration in high-vasopressin, fluid-retaining conditions. The diuretic mechanism of UT inhibitors may complement the action of conventional diuretics, which target sodium transport.

Urea transporter physiology studied in knockout mice

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

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.

Mice lacking urea transporter UT-B display depression-like behavior

Urea transporter B is one of urea transporters that selectively transport urea driven by urea gradient across membrane and expressed abundantly in brain. To determine the physiological role of UT-B in brain, UT-B localization, urea concentration, tissue morphology of brain, and behavioral phenotypes were studied in UT-B heterozygous mice via UT-B null mice. UT-B mRNA was expressed in olfactory bulb, cortex, caudate nucleus, hippocampus and hypothalamus of UT-B heterozygous mice. UT-B null mice exhibited depression-like behavior, with urea accumulation, nitric oxide reduction, and selective neuronal nitric oxide synthase level increase in hippocampus. After acute urea loading, the urea level increased, NO production decreased in hippocampus from both types of mice. Moreover, urea level was higher, and NO concentration was lower consistently in UT-B null hippocampus than that in heterozygous hippocampus. In vitro, 25 mM urea inhibited NO production too. Furthermore, UT-B knockout induced a long-lasting notable decrease in regional cerebral blood flow and altered morphology, such as loss of neurons in CA3 region, swelling, and membranous myelin-like structure formation within myelinated and unmyelinated fibers in hippocampus. These results suggest that urea accumulation in the hippocampus induced by UT-B deletion can cause depression-like behavior, which possibly attribute to disturbance in NOS/NO system.

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.