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

Structure, regulation and physiological roles of urea transporters

Structure, regulation and physiological roles of urea transporters. Urea is the major constituent of the urine and the principal means for disposal of nitrogen derived from amino acid metabolism. Specialized phloretin-inhibitable urea transporters are expressed in kidney medulla and play a central role in urea excretion and water balance. These transporters allow accumulation of urea in the medulla and enable the kidney to concentrate urine to an osmolality greater than systemic plasma. Recently, expression cloning with Xenopus oocytes has led to the isolation of a novel phloretin-inhibitable urea transporter (UT2) from rabbit, and subsequently from rat kidney. UT2 from both species has the characteristics of the phloretin-sensitive urea transporter previously defined in kidney by in vitro perfused tubule studies. Based on these advances, Ripoche and colleagues cloned a homologous urea transporter (HUT11) from erythrocytes. UT2 and HUT11 predict 43kDa polypeptides and exhibit 64% amino acid sequence identity. Since regulation of urea transport in the kidney plays an important role in the orchestration of the antidiuretic response, we have studied the regulation of urea transporter in rat kidney at the mRNA level. On Northern blots probed at high stringency, rat UT2 hybridized to two transcripts of 2.9kb and 4.0kb, which have spatially distinct distributions within the kidney. Northern analysis and in situ hybridization of kidneys from rats maintained at different physiologic states revealed that the 2.9 and 4.0kb transcripts are regulated by separate mechanisms. The 4kb transcript was primarily responsive to changes in the dietary protein content, whereas the 2.9kb transcript was highly responsive to changes in the hydration state of the animal. We propose that the two UT2 transcripts are regulated by distinct mechanisms to allow optimal fluid balance and urea excretion

Cloning and characterization of a potassium-coupled amino acid transporter

Active solute uptake in bacteria, fungi, plants, and animals is known to be mediated by cotransporters that are driven by Na(+ )or H(+ )gradients. The present work extends the Na(+ )and H(+ )dogma by including the H(+ )and K(+ )paradigm. Lepidopteran insect larvae have a high K(+ )and a low Na(+ )content, and their midgut cells lack Na(+)/K(+ )ATPase. Instead, an H(+ )translocating, vacuolar-type ATPase generates a voltage of approximately −240 mV across the apical plasma membrane of so-called goblet cells, which drives H(+ )back into the cells in exchange for K(+), resulting in net K(+) secretion into the lumen. The resulting inwardly directed K(+ )electrochemical gradient serves as a driving force for active amino acid uptake into adjacent columnar cells. By using expression cloning with Xenopus laevis oocytes, we have isolated a cDNA that encodes a K(+)-coupled amino acid transporter (KAAT1). We have cloned this protein from a larval lepidopteran midgut (Manduca sexta) cDNA library. KAAT1 is expressed in absorptive columnar cells of the midgut and in labial glands. When expressed in Xenopus oocytes, KAAT1 induced electrogenic transport of neutral amino acids but excludes α-(methylamino)isobutyric acid and charged amino acids resembling the mammalian system B. K(+), Na(+), and to a lesser extent Li(+ )were accepted as cotransported ions, but K(+ )is the principal cation, by far, in living caterpillars. Moreover, uptake was Cl(−)-dependent, and the K(+)/Na(+ )selectivity increased with hyperpolarization of oocytes, reflecting the increased K(+)/Na(+ )selectivity with hyperpolarization observed in midgut tissue. KAAT1 has 634 amino acid residues with 12 putative membrane spanning domains and shows a low level of identity with members of the Na(+ )and Cl(−)-coupled neurotransmitter transporter family