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

Osmotic regulation of UT-B urea transporters in the RT4 human urothelial cell line

Facilitative UT-B urea transporters play important physiological roles in numerous tissues, including the urino-genital tract. Previous studies have shown that urothelial UT-B transporters are crucial to bladder function in a variety of mammalian species. Using the RT4 bladder urothelial cell line, this study investigated the potential osmotic regulation of human UT-B transporters. Initial end-point PCR experiments confirmed expression of both UT-B1 and UT-B2 transcripts in RT4 cells. Western blotting analysis revealed glycosylated UT-B protein to be highly abundant and immunolocalization experiments showed it was predominantly located on the plasma membrane. Further PCR experiments suggested that a 48 hr, NaCl-induced raise in external osmolality increased expression of UT-B transcripts. Importantly, these NaCl-induced changes also significantly increased UT-B protein abundance (p < .01, n = 7, ANOVA), whereas mannitol-induced changes in external osmolality had no effect (NS, n = 4, ANOVA). Finally, similar increases in both UT-B RNA expression and protein abundance were observed with urea-induced changes to external osmolality (p < .05, n = 4, ANOVA). In conclusion, these findings strongly suggest that increases in external osmolality, via either NaCl or urea, can regulate human urothelial UT-B transporters.

The contribution of collecting duct NOS1 to the concentrating mechanisms in male and female mice

The collecting duct (CD) concentrates the urine, thereby maintaining body water volume and plasma osmolality within a normal range. The endocrine hormone arginine vasopressin acts in the CD to increase water permeability via the vasopressin 2 receptor (V2R)-aquaporin (AQP) axis. Recent studies have suggested that autocrine factors may also contribute to the regulation of CD water permeability. Nitric oxide is produced predominantly by nitric oxide synthase 1 (NOS1) in the CD and acts as a diuretic during salt loading. The present study sought to determine whether CD NOS1 regulates diuresis during changes in hydration status. Male and female control and CD NOS1 knockout (CDNOS1KO) mice were hydrated (5% sucrose water), water deprived, or acutely challenged with the V2R agonist desmopressin. In male mice, water deprivation resulted in decreased urine flow and increased plasma osmolality, copeptin concentration, and kidney AQP2 abundance independent of CD NOS1. In female control mice, water deprivation reduced urine flow, increased plasma osmolality and copeptin, but did not significantly change total AQP2; however, there was increased basolateral AQP3 localization. Surprisingly, female CDNOS1KO mice while on the sucrose water presented with symptoms of dehydration. Fibroblast growth factor 21, an endocrine regulator of sweetness preference, was significantly higher in female CDNOS1KO mice, suggesting that this was reducing their drive to drink the sucrose water. With acute desmopressin challenge, female CDNOS1KO mice failed to appropriately concentrate their urine, resulting in higher plasma osmolality than controls. In conclusion, CD NOS1 plays only a minor role in urine-concentrating mechanisms.

Mammalian urine concentration: a review of renal medullary architecture and membrane transporters

Mammalian kidneys play an essential role in balancing internal water and salt concentrations. When water needs to be conserved, the renal medulla produces concentrated urine. Central to this process of urine concentration is an osmotic gradient that increases from the corticomedullary boundary to the inner medullary tip. How this gradient is generated and maintained has been the subject of study since the 1940s. While it is generally accepted that the outer medulla contributes to the gradient by means of an active process involving countercurrent multiplication, the source of the gradient in the inner medulla is unclear. The last two decades have witnessed advances in our understanding of the urine-concentrating mechanism. Details of medullary architecture and permeability properties of the tubules and vessels suggest that the functional and anatomic relationships of these structures may contribute to the osmotic gradient necessary to concentrate urine. Additionally, we are learning more about the membrane transporters involved and their regulatory mechanisms. The role of medullary architecture and membrane transporters in the mammalian urine-concentrating mechanism are the focus of this review.

Improved protocols for the study of urinary electrolyte excretion and blood pressure in rodents: use of gel food and stepwise changes in diet composition

Many experimental protocols in rodents require the comparison of groups that are fed different diets. Changes in dietary electrolyte and/or fat content can influence food intake, which can potentially introduce bias or confound the results. Unpalatable diets slow growth or cause weight loss, which is exacerbated by housing the animals in individual metabolic cages or by surgery. For balance studies in mice, small changes in body weight and food intake and low urinary flow can amplify these challenges. Powder food can be administered as gel with the addition of a desired amount of water, electrolytes, drugs (if any), and a small amount of agar. We describe here how the use of gel food to vary water, Na, K, and fat content can reduce weight loss and improve reproducibility of intake, urinary excretion, and blood pressure in rodents. In addition, mild food restriction reduces the interindividual variability and intergroup differences in food intake and associated variables, thus improving the statistical power of an experiment. Finally, we also demonstrate the advantages of using gel food for weight-based drug dosing. These protocols can improve the accuracy and reproducibility of experimental data where dietary manipulations are needed and are especially advisable in rodent studies related to water balance, obesity, and blood pressure.

Body mass-specific Na+-K+-ATPase activity in the medullary thick ascending limb: implications for species-dependent urine concentrating mechanisms

In general, the mammalian whole body mass-specific metabolic rate correlates positively with maximal urine concentration (U_max) irrespective of whether or not the species have adapted to arid or mesic habitat. Accordingly, we hypothesized that the thick ascending limb (TAL) of a rodent with markedly higher whole body mass-specific metabolism than rat exhibits a substantially higher TAL metabolic rate as estimated by Na⁺-K⁺-ATPase activity and Na⁺-K⁺-ATPase α1-gene and protein expression. The kangaroo rat inner stripe of the outer medulla exhibits significantly higher mean Na⁺-K⁺-ATPase activity (~70%) compared with two rat strains (Sprague-Dawley and Munich-Wistar), extending prior studies showing rat activity exceeds rabbit. Furthermore, higher expression of Na⁺-K⁺-ATPase α1-protein (~4- to 6-fold) and mRNA (~13-fold) and higher TAL mitochondrial volume density (~20%) occur in the kangaroo rat compared with both rat strains. Rat TAL Na⁺-K⁺-ATPase α1-protein expression is relatively unaffected by body hydration status or, shown previously, by dietary Na⁺, arguing against confounding effects from two unavoidably dissimilar diets: grain-based diet without water (kangaroo rat) or grain-based diet with water (rat). We conclude that higher TAL Na⁺-K⁺-ATPase activity contributes to relationships between whole body mass-specific metabolic rate and high U_max. More vigorous TAL Na⁺-K⁺-ATPase activity in kangaroo rat than rat may contribute to its steeper Na⁺ and urea axial concentration gradients, adding support to a revised model of the urine concentrating mechanism, which hypothesizes a leading role for vigorous active transport of NaCl, rather than countercurrent multiplication, in generating the outer medullary axial osmotic gradient.

Identification of UT-A1- and AQP2-interacting proteins in rat inner medullary collecting duct

The urea channel UT-A1 and the water channel aquaporin-2 (AQP2) mediate vasopressin-regulated transport in the renal inner medullary collecting duct (IMCD). To identify the proteins that interact with UT-A1 and AQP2 in native rat IMCD cells, we carried out chemical cross-linking followed by detergent solubilization, immunoprecipitation, and LC-MS/MS analysis of the immunoprecipitated material. The analyses revealed 133 UT-A1-interacting proteins and 139 AQP2-interacting proteins, each identified in multiple replicates. Fifty-three proteins that were present in both the UT-A1 and the AQP2 interactomes can be considered as mediators of housekeeping interactions, likely common to all plasma membrane proteins. Among proteins unique to the UT-A1 list were those involved in posttranslational modifications: phosphorylation (protein kinases Cdc42bpb, Phkb, Camk2d, and Mtor), ubiquitylation/deubiquitylation (Uba1, Usp9x), and neddylation (Nae1 and Uba3). Among the proteins unique to the AQP2 list were several Rab proteins (Rab1a, Rab2a, Rab5b, Rab5c, Rab7a, Rab11a, Rab11b, Rab14, Rab17) involved in membrane trafficking. UT-A1 was found to interact with UT-A3, although quantitative proteomics revealed that most UT-A1 molecules in the cell are not bound to UT-A3. In vitro incubation of UT-A1 peptides with the protein kinases identified in the UT-A1 interactome revealed that all except Mtor were capable of phosphorylating known sites in UT-A1. Overall, the UT-A1 and AQP2 interactomes provide a snapshot of a dynamic process in which UT-A1 and AQP2 are produced in the rough endoplasmic reticulum, processed through the Golgi apparatus, delivered to endosomes that move into and out of the plasma membrane, and are regulated in the plasma membrane.

Descending thin limb of the intermediate loop expresses both aquaporin 1 and urea transporter A2 in the mouse kidney

A new intermediate type of Henle's loop has been reported that it extends into the inner medulla and turns within the first millimeter beyond the outer medulla. This study aimed to identify the descending thin limb (DTL) of the intermediate loop in the adult C57Bl/6 mouse kidney using aquaporin 1 (AQP1) and urea transporter A2 (UT-A2) antibodies. In the upper part of the inner stripe of the outer medulla (ISOM), AQP1 was expressed strongly in the DTL with type II epithelium of the long loop, but not in type I epithelium of the short loop. The DTL of the intermediate loop exhibited weak AQP1 immunoreactivity. UT-A2 immunoreactivity was not observed in the upper part of any DTL type. AQP1 expression was similar in the upper and middle parts of the ISOM. UT-A2 expression was variable, being expressed strongly in the DTL with type I epithelium of the short loop, but not in type II epithelium of the long loop. In the innermost part of the ISOM, AQP1 was expressed only in type III epithelium of the long loop. UT-A2-positive and UT-A2-negative cells were intermingled in type I epithelium of the intermediate loop, but were not observed in type III epithelium of the long loop. UT-A2-positive DTLs of the intermediate loop extended into the UT-A2/AQP1-negative type I epithelium in the initial part of the inner medulla. These results demonstrate that the DTL of the intermediate loop is composed of type I epithelium and expresses both AQP1 and UT-A2. The functional role of the DTL of the intermediate loop may be distinct from the short or long loops.

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.

Alternative channels for urea in the inner medulla of the rat kidney

The ascending thin limbs (ATLs) and lower descending thin limbs (DTLs) of Henle's loop in the inner medulla of the rat are highly permeable to urea, and yet no urea transporters have been identified in these sections. We hypothesized that novel, yet-unidentified transporters in these tubule segments could explain the high urea permeability. cDNAs encoding for Na(+)-glucose transporter 1a (SGLT1a), Na(+)-glucose transporter 1 (NaGLT1), urea transporter (UT)-A2c, and UT-A2d were isolated and cloned from the Munich-Wistar rat inner medulla. SGLT1a is a novel NH2-terminal truncated variant of SGLT1. NaGLT1 is a Na(+)-dependent glucose transporter primarily located in the proximal tubules and not previously described in the thin limbs. UT-A2c and UT-A2d are novel variants of UT-A2. UT-A2c is truncated at the COOH terminus, and UT-A2d has one exon skipped. When rats underwent water restriction for 72 h, mRNA levels of SGLT1a increased in ATLs, NaGLT1 levels increased in both ATLs and DTLs, and UT-A2c increased in ATLs. [(14)C]urea uptake assays performed on Xenopus oocytes heterologously expressing these proteins revealed that despite having structural differences from their full-length versions, SGLT1a, UT-A2c, and UT-A2d enhanced urea uptake. NaGLT1 also facilitated urea uptake. Uptakes were Na(+) independent and inhibitable by phloretin and/or phloridzin. Our data indicate that there are several alternative channels for urea in the rat inner medulla that could potentially contribute to the high urea permeabilities in thin limb segments.

Short-chain fatty acids and acidic pH upregulate UT-B, GPR41, and GPR4 in rumen epithelial cells of goats

Currently, the mechanism(s) responsible for the regulation of urea transporter B (UT-B) expression levels in the epithelium of the rumen remain unclear. We hypothesized that rumen fermentation products affect ruminal UT-B expression. Therefore, the effects of short-chain fatty acids (SCFA), pH, ammonia, and urea on mRNA and protein levels of UT-B were assayed in primary rumen epithelial cell cultures and in rumen epithelium obtained from intact goats. In vitro, SCFA and acidic pH were found to synergetically stimulate both mRNA and protein expression of UT-B, whereas NH4Cl decreased mRNA and protein levels of UT-B at pH 6.8. Treatment with urea increased both levels at pH 7.4. When goats received a diet rich in nitrogen (N) and nonfiber carbohydrates (NFC), their rumen epithelium had higher levels of UT-B, and the rumen contained higher concentrations of SCFA and NH3-N with a lower pH. An increase in plasma urea-N concentration was also observed compared with the plasma of the goats that received a diet low in N and NFC. In a second feeding trial, goats that received a NFC-rich, but isonitrogenous, diet had higher mRNA and protein levels of UT-B, and higher levels of G protein-coupled receptor (GPR) 41 and GPR4, in their rumen epithelium. The ruminal concentrations of SCFA and NH3-N also increased, while a lower pH was detected. In contrast, the serum urea-N concentrations remained unchanged. These data indicate that ruminal SCFA and pH are key factors, via GPR4 and GPR41, in the dietary regulation of UT-B expression, and they have priority over changes in plasma urea.

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.

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.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

Transepithelial water and urea permeabilities of isolated perfused Munich-Wistar rat inner medullary thin limbs of Henle's loop

To better understand the role that water and urea fluxes play in the urine concentrating mechanism, we determined transepithelial osmotic water permeability (Pf) and urea permeability (Purea) in isolated perfused Munich-Wistar rat long-loop descending thin limbs (DTLs) and ascending thin limbs (ATLs). Thin limbs were isolated either from 0.5 to 2.5 mm below the outer medulla (upper inner medulla) or from the terminal 2.5 mm of the inner medulla. Segment types were characterized on the basis of structural features and gene expression levels of the water channel aquaporin 1, which was high in the upper DTL (DTLupper), absent in the lower DTL (DTLlower), and absent in ATLs, and the Cl-(1) channel ClCK1, which was absent in DTLs and high in ATLs. DTLupper Pf was high (3,204.5 ± 450.3 μm/s), whereas DTLlower showed very little or no osmotic Pf (207.8 ± 241.3 μm/s). Munich-Wistar rat ATLs have previously been shown to exhibit no Pf. DTLupper Purea was 40.0 ± 7.3 × 10(-5) cm/s and much higher in DTLlower (203.8 ± 30.3 × 10(-5) cm/s), upper ATL (203.8 ± 35.7 × 10(-5) cm/s), and lower ATL (265.1 ± 49.8 × 10(-5) cm/s). Phloretin (0.25 mM) did not reduce DTLupper Purea, suggesting that Purea is not due to urea transporter UT-A2, which is expressed in short-loop DTLs and short portions of some inner medullary DTLs close to the outer medulla. In summary, Purea is similar in all segments having no osmotic Pf but is significantly lower in DTLupper, a segment having high osmotic Pf. These data are inconsistent with the passive mechanism as originally proposed.

Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Molecular mechanisms of urea transport in health and disease

In the late 1980s, urea permeability measurements produced values that could not be explained by paracellular transport or lipid phase diffusion. The existence of urea transport proteins were thus proposed and less than a decade later, the first urea transporter was cloned. The family of urea transporters has two major subgroups, designated SLC14A1 (or UT-B) and Slc14A2 (or UT-A). UT-B and UT-A gene products are glycoproteins located in various extra-renal tissues however, a majority of the resulting isoforms are found in the kidney. The UT-B (Slc14A1) urea transporter was originally isolated from erythrocytes and two isoforms have been reported. In kidney, UT-B is located primarily in the descending vasa recta. The UT-A (Slc14A2) urea transporter yields six distinct isoforms, of which three are found chiefly in the kidney medulla. UT-A1 and UT-A3 are found in the inner medullary collecting duct (IMCD), while UT-A2 is located in the thin descending limb. These transporters are crucial to the kidney's ability to concentrate urine. The regulation of urea transporter activity in the IMCD involves acute modification through phosphorylation and subsequent movement to the plasma membrane. UT-A1 and UT-A3 accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation of the urea transporters in the IMCD involves altering protein abundance in response to changes in hydration status, low protein diets, or adrenal steroids. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new genetically engineered mouse models are being developed to study these transporters.

Acute calcineurin inhibition with tacrolimus increases phosphorylated UT-A1

UT-A1, the urea transporter present in the apical membrane of the inner medullary collecting duct, is crucial to the kidney's ability to concentrate urine. Phosphorylation of UT-A1 on serines 486 and 499 is important for plasma membrane trafficking. The effect of calcineurin on dephosphorylation of UT-A1 was investigated. Inner medullary collecting ducts from Sprague-Dawley rats were metabolically labeled and treated with tacrolimus to inhibit calcineurin or calyculin to inhibit protein phosphatases 1 and 2A. UT-A1 was immunoprecipitated, electrophoresed, blotted, and total UT-A1 phosphorylation was assessed by autoradiography. Total UT-A1 was determined by Western blotting. A phospho-specific antibody to pser486-UT-A1 was used to determine whether serine 486 can be hyperphosphorylated by inhibiting phosphatases. Inhibition of calcineurin showed an increase in phosphorylation per unit protein at serine 486. In contrast, inhibition of phosphatases 1 and 2A resulted in an increase in UT-A1 phosphorylation but no increase in pser486-UT-A1. In vitro perfusion of inner medullary collecting ducts showed tacrolimus-stimulated urea permeability consistent with stimulated urea transport. The location of phosphorylated UT-A1 in rats treated acutely and chronically with tacrolimus was determined using immunohistochemistry. Inner medullary collecting ducts of the acutely treated rats showed increased apical membrane association of phosphorylated UT-A1 while chronic treatment reduced membrane association of phosphorylated UT-A1. We conclude that UT-A1 may be dephosphorylated by multiple phosphatases and that the PKA-phosphorylated serine 486 is dephosphorylated by calcineurin. This is the first documentation of the role of phosphatases and the specific site of phosphorylation of UT-A1, in response to tacrolimus.

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.

Urine concentrating mechanism: impact of vascular and tubular architecture and a proposed descending limb urea-Na+ cotransporter

We extended a region-based mathematical model of the renal medulla of the rat kidney, previously developed by us, to represent new anatomic findings on the vascular architecture in the rat inner medulla (IM). In the outer medulla (OM), tubules and vessels are organized around tightly packed vascular bundles; in the IM, the organization is centered around collecting duct clusters. In particular, the model represents the separation of descending vasa recta from the descending limbs of loops of Henle, and the model represents a papillary segment of the descending thin limb that is water impermeable and highly urea permeable. Model results suggest that, despite the compartmentalization of IM blood flow, IM interstitial fluid composition is substantially more homogeneous compared with OM. We used the model to study medullary blood flow in antidiuresis and the effects of vascular countercurrent exchange. We also hypothesize that the terminal aquaporin-1 null segment of the long descending thin limbs may express a urea-Na(+) or urea-Cl(-) cotransporter. As urea diffuses from the urea-rich papillary interstitium into the descending thin limb luminal fluid, NaCl is secreted via the cotransporter against its concentration gradient. That NaCl is then reabsorbed near the loop bend, raising the interstitial fluid osmolality and promoting water reabsorption from the IM collecting ducts. Indeed, the model predicts that the presence of the urea-Na(+) or urea- Cl(-) cotransporter facilitates the cycling of NaCl within the IM and yields a loop-bend fluid composition consistent with experimental data.

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.

Vasopressin V2 receptors, ENaC, and sodium reabsorption: a risk factor for hypertension?

Excessive sodium reabsorption by the kidney has long been known to participate in the pathogenesis of some forms of hypertension. In the kidney, the final control of NaCl reabsorption takes place in the distal nephron through the amiloride-sensitive epithelial sodium channel (ENaC). Liddle's syndrome, an inherited form of hypertension due to gain-of-function mutations in the genes coding for ENaC subunits, has demonstrated the key role of this channel in the sodium balance. Although aldosterone is classically thought to be the main hormone regulating ENaC activity, several studies in animal models and in humans highlight the important effect of vasopressin on ENaC regulation and sodium transport. This review summarizes the effect of vasopressin V2 receptor stimulation on ENaC activity and sodium excretion in vivo. Moreover, we report the experimental and clinical data demonstrating the role of renal ENaC in water conservation at the expense of a reduced ability to excrete sodium. Acute administration of the selective V2 receptor agonist dDAVP not only increases urine osmolality and reduces urine flow rate but also reduces sodium excretion in rats and humans. Chronic V2 receptor stimulation increases blood pressure in rats, and a significant correlation was found between blood pressure and urine concentration in healthy humans. This led us to discuss how excessive vasopressin-dependent ENaC stimulation could be a risk factor for sodium retention and resulting increase in blood pressure.

Vasopressin increases phosphorylation of Ser84 and Ser486 in Slc14a2 collecting duct urea transporters

Vasopressin-regulated urea transport in the renal inner medullary collecting duct (IMCD) is mediated by two urea channel proteins, UT-A1 and UT-A3, derived from the same gene (Slc14a2) by alternative splicing. The NH(2)-terminal 459 amino acids are the same in both proteins. To study UT-A1/3 phosphorylation, we made phospho-specific antibodies to UT-A sequences targeting phospho-serines at positions 84 and 486, sites identified previously by protein mass spectrometry. Both antibodies proved specific, recognizing only the phosphorylated forms of UT-A1 and -A3. Immunoblotting of rat IMCD suspensions or whole inner medullas showed that the V2R-selective vasopressin analog 1-deamino-8-d-arginine vasopressin (dDAVP) increases phosphorylation at Ser84 (in UT-A1 and UT-A3) and Ser486 (in UT-A1) by about eightfold. Time course studies in rat IMCD suspensions showed maximum phosphorylation within 1 min of dDAVP exposure, consistent with the time course of vasopressin-stimulated phosphorylation of the vasopressin-sensitive water channel aquaporin-2 at Ser256. Confocal immunofluorescence in Brattleboro rat medullary tissue showed labeling limited to the IMCD, which increased markedly in response to dDAVP. Immuno-electron microscopy studies showed that both phosphorylated forms were present mainly in intracellular compartments in the presence of vasopressin. These studies demonstrate regulated phosphorylation of both UT-A1 and UT-A3 in response to vasopressin in a manner consistent with coordinate regulation of UT-A and aquaporin-2 in the renal IMCD. The findings add to prior evidence for vasopressin-induced phosphorylation of UT-A1, providing evidence that UT-A3 may be regulated by phosphorylation as well.

Phosphorylation of UT-A1 on serine 486 correlates with membrane accumulation and urea transport activity in both rat IMCDs and cultured cells

Vasopressin is the primary hormone regulating urine-concentrating ability. Vasopressin phosphorylates the UT-A1 urea transporter in rat inner medullary collecting ducts (IMCDs). To assess the effect of UT-A1 phosphorylation at S486, we developed a phospho-specific antibody to S486-UT-A1 using an 11 amino acid peptide antigen starting from amino acid 482 that bracketed S486 in roughly the center of the sequence. We also developed two stably transfected mIMCD3 cell lines: one expressing wild-type UT-A1 and one expressing a mutated form of UT-A1, S486A/S499A, that is unresponsive to protein kinase A. Forskolin stimulates urea flux in the wild-type UT-A1-mIMCD3 cells but not in the S486A/S499A-UT-A1-mIMCD3 cells. The phospho-S486-UT-A1 antibody identified UT-A1 protein in the wild-type UT-A1-mIMCD3 cells but not in the S486A/S499A-UT-A1-mIMCD3 cells. In rat IMCDs, forskolin increased the abundance of phospho-S486-UT-A1 (measured using the phospho-S486 antibody) and of total UT-A1 phosphorylation (measured by (32)P incorporation). Forskolin also increased the plasma membrane accumulation of phospho-S486-UT-A1 in rat IMCD suspensions, as measured by biotinylation. In rats treated with vasopressin in vivo, the majority of the phospho-S486-UT-A1 appears in the apical plasma membrane. In summary, we developed stably transfected mIMCD3 cell lines expressing UT-A1 and an S486-UT-A1 phospho-specific antibody. We confirmed that vasopressin increases UT-A1 accumulation in the apical plasma membrane and showed that vasopressin phosphorylates UT-A1 at S486 in rat IMCDs and that the S486-phospho-UT-A1 form is primarily detected in the apical plasma membrane.

Vasopressin regulation of the renal UT-A3 urea transporter

Facilitative urea transporters in the mammalian kidney play a vital role in the urinary concentrating mechanism. The urea transporters located in the renal inner medullary collecting duct, namely UT-A1 and UT-A3, are acutely regulated by the antidiuretic hormone vasopressin. In this study, we investigated the vasopressin regulation of the basolateral urea transporter UT-A3 using an MDCK-mUT-A3 cell line. Within 10 min, vasopressin stimulates urea flux through UT-A3 transporters already present at the plasma membrane, via a PKA-dependent process. Within 1 h, vasopressin significantly increases UT-A3 localization at the basolateral membrane, causing a further increase in urea transport. While the basic trafficking of UT-A3 to basolateral membranes involves both protein kinase C and calmodulin, its regulation by vasopressin specifically occurs through a casein kinase II-dependent pathway. In conclusion, this study details the effects of vasopressin on UT-A3 urea transporter function and hence its role in regulating urea permeability within the renal inner medullary collecting duct.

Role of three-dimensional architecture in the urine concentrating mechanism of the rat renal inner medulla

Recent studies of three-dimensional architecture of rat renal inner medulla (IM) and expression of membrane proteins associated with fluid and solute transport in nephrons and vasculature have revealed structural and transport properties that likely impact the IM urine concentrating mechanism. These studies have shown that 1) IM descending thin limbs (DTLs) have at least two or three functionally distinct subsegments; 2) most ascending thin limbs (ATLs) and about half the ascending vasa recta (AVR) are arranged among clusters of collecting ducts (CDs), which form the organizing motif through the first 3-3.5 mm of the IM, whereas other ATLs and AVR, along with aquaporin-1-positive DTLs and urea transporter B-positive descending vasa recta (DVR), are external to the CD clusters; 3) ATLs, AVR, CDs, and interstitial cells delimit interstitial microdomains within the CD clusters; and 4) many of the longest loops of Henle form bends that include subsegments that run transversely along CDs that lie in the terminal 500 microm of the papilla tip. Based on a more comprehensive understanding of three-dimensional IM architecture, we distinguish two distinct countercurrent systems in the first 3-3.5 mm of the IM (an intra-CD cluster system and an inter-CD cluster system) and a third countercurrent system in the final 1.5-2 mm. Spatial arrangements of loop of Henle subsegments and multiple countercurrent systems throughout four distinct axial IM zones, as well as our initial mathematical model, are consistent with a solute-separation, solute-mixing mechanism for concentrating urine in the IM.

Expression of ammonia transporters, Rhbg and Rhcg, in chronic cyclosporine nephropathy in rats

BACKGROUND/AIMS

Cyclosporine (CsA)-induced renal injury causes renal tubular acidosis. The current study was performed to evaluate the influence of CsA-induced renal injury on the ammonia transporter family members, Rh B-glycoprotein (Rhbg) and Rh C-glycoprotein (Rhcg).

METHODS

Rats were treated daily for 1 or 4 weeks with vehicle (VH) or CsA. Induction of chronic CsA-induced nephropathy was confirmed by demonstrating impaired renal function and characteristic histopathology. Rhbg and Rhcg expression was evaluated with immunoblot, immunohistochemistry, real-time RT-PCR and electron microscopy.

RESULTS

CsA treatment for 4 weeks developed mild metabolic acidosis and decreased urinary ammonia excretion. Rhcg mRNA expression was unchanged in both the cortex and outer medulla, but Rhcg protein expression in the CsA group was significantly reduced in the cortex and outer medulla. There were no significant differences in Rhbg mRNA and protein expression between the CsA and VH group.

CONCLUSION

Long-term treatment with CsA in rats results in decreased urinary ammonia excretion accompanied by decreased expression of Rhcg; these changes are likely to mediate the CsA-induced defect in ammonium excretion in the collecting duct.

Forskolin stimulates phosphorylation and membrane accumulation of UT-A3

UT-A1 is regulated by vasopressin and is localized to the apical membrane and intracellular compartment of inner medullary collecting duct (IMCD) cells. UT-A3 is also expressed in the IMCD and is regulated by forskolin in heterologous systems. The goal of the present study is to investigate mechanisms by which vasopressin regulates UT-A3 in rat IMCD. In fresh suspensions of rat IMCD, forskolin increases the phosphorylation of UT-A3, similar to UT-A1. Biotinylation studies indicate that UT-A3 is located in the plasma membrane. Forskolin treatment increases the abundance of UT-A3 in the plasma membrane similar to UT-A1. However, these two transporters do not form a complex through a protein-protein interaction, suggesting that transporter function is unique to each protein. While immunohistochemistry localized UT-A3 to the basal and lateral membranes, a majority of the staining was cytosolic. Immunohistochemistry of vasopressin-treated rat kidney sections also localized UT-A3 primarily to the cytosol with basal and lateral membrane staining but also showed some apical membrane staining in some IMCD cells. This suggests that under normal conditions, UT-A3 functions as the basolateral transporter but in a high cAMP environment, the transporter may move from the cytosol to all plasma membranes to increase urea flux in the IMCD. In summary, this study confirms that UT-A3 is located in the inner medullary tip where it is expressed in the basolateral membrane, shows that UT-A3 is a phosphoprotein in rat IMCD that can be trafficked to the plasma membrane independent of UT-A1, and suggests that vasopressin may induce UT-A3 expression in the apical plasma membrane of IMCD.

Acute regulation of mUT-A3 urea transporter expressed in a MDCK cell line

Renal facilitative urea transporters play a vital role in the urinary concentrating mechanism. UT-A3 is a phloretin-sensitive urea transporter that in the mouse is expressed on the basolateral membrane of renal inner medullary collecting duct (IMCD) cells. In this study, we engineered a Madin-Darby canine kidney (MDCK) I cell line that stably expresses mouse UT-A3 (MDCK-mUT-A3). Immunoblotting using the UT-A-targeted antibody ML446 detected a approximately 40-kDa signal in MDCK-mUT-A3 protein that corresponds to mUT-A3. Using cultured epithelial monolayers, radioactive (14)C-urea flux experiments determined that basolateral urea transport was no different between MDCK-mUT-A3 and control MDCK-FLZ cells under basal conditions [not significant (NS), ANOVA]. However, exposure to arginine vasopressin (AVP) significantly stimulated basolateral urea flux in MDCK-mUT-A3 monolayers (P < 0.05, ANOVA), while it had no effect in control MDCK-FLZ monolayers (NS, ANOVA). The AVP-stimulated basolateral urea transport in MDCK-mUT-A3 was inhibited by 1,3 dimethyl urea (P < 0.05, ANOVA) or phloretin (P < 0.05, ANOVA), both known inhibitors of facilitative urea transporters. MDCK-mUT-A3 basolateral urea flux was also stimulated by increasing intracellular levels of cAMP, via forskolin (P < 0.05, ANOVA), or intracellular calcium, via ATP (P < 0.05, ANOVA). Finally, 1-h preincubation with a specific PKA inhibitor, H89, significantly inhibited the increase in urea transport produced by AVP (P < 0.05, ANOVA). In conclusion, we have produced the first renal cell line to stably express the mUT-A3 urea transporter. Our results indicate that mUT-A3 is acutely regulated by AVP, via a PKA-dependent pathway. These findings have important implications for the regulation of urea transport in the renal IMCD and the urinary concentrating mechanism.

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.

Tissue distribution of UT-A and UT-B mRNA and protein in rat

Mammalian urea transporters are facilitated membrane transport proteins belonging to two families, UT-A and UT-B. They are best known for their role of maintaining the renal inner medullary urinary concentrating gradient. Urea transporters have also been identified in tissues not typically associated with urea metabolism. The purpose of this study was to survey the major organs in rat to determine the distribution of UT-A and UT-B mRNA transcripts and protein forms and determine their cellular localization. Five kidney subregions and 17 extrarenal tissues were screened by Northern blot analysis using two UT-A and three UT-B probes and by Western blot analysis using polyclonal COOH-terminal UT-A and UT-B antibodies. Immunohistochemistry was performed on 16 extrarenal tissues using the same antibodies. In kidney, we detected mRNA transcripts and protein bands consistent with previously-identified UT-A and UT-B isoforms, as well as novel forms. We found that UT-A mRNA and protein are widely expressed in extrarenal tissues in various forms that are different from the known isoforms. We determined the cellular localization of UT-A and UT-B in these tissues. We found that both UT-A and UT-B are ubiquitously expressed as numerous tissue-specific mRNA transcripts and protein forms that are localized to cell membranes, cytoplasm, or nuclei.