Role of collecting duct urea transporters in the kidney--insights from mouse models

β3 adrenergic receptor as potential therapeutic target in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) disrupts renal parenchyma through progressive expansion of fluid-filled cysts. The only approved pharmacotherapy for ADKPD involves the blockade of the vasopressin type 2 receptor (V2R). V2R is a GPCR expressed by a subset of renal tubular cells and whose activation stimulates cyclic AMP (cAMP) accumulation, which is a major driver of cyst growth. The β3-adrenergic receptor (β3-AR) is a GPCR expressed in most segments of the murine nephron, where it modulates cAMP production. Since sympathetic nerve activity, which leads to activation of the β3-AR, is elevated in patients affected by ADPKD, we hypothesize that β3-AR might constitute a novel therapeutic target. We find that administration of the selective β3-AR antagonist SR59230A to an ADPKD mouse model (Pkd1fl/fl ;Pax8rtTA ;TetO-Cre) decreases cAMP levels, producing a significant reduction in kidney/body weight ratio and a partial improvement in kidney function. Furthermore, cystic mice show significantly higher β3-AR levels than healthy controls, suggesting a correlation between receptor expression and disease development. Finally, β3-AR is expressed in human renal tissue and localizes to cyst-lining epithelial cells in patients. Thus, β3-AR is a potentially interesting target for the development of new treatments for ADPKD.

NADPH oxidase 4 deficiency reduces aquaporin-2 mRNA expression in cultured renal collecting duct principal cells via increased PDE3 and PDE4 activity

The final control of renal water reabsorption occurs in the collecting duct (CD) and relies on regulated expression of aquaporin-2 (AQP2) in principal CD cells. AQP2 transcription is primarily induced by type 2 vasopressin receptor (V₂R)-cAMP-protein kinase A (PKA) signaling but also by other factors, including TonEBP and NF-κB. NAPDH oxidase 4 (NOX4) represents a major source of reactive oxygen species (ROS) in the kidney. Because NOX-derived ROS may alter PKA, TonEBP and NF-κB activity, we examined the effects of NOX4 depletion on AQP2 expression. Depleted NOX4 expression by siRNA (siNOX4) in mpkCCD_cl4 cells attenuated increased AQP2 mRNA expression by arginine vasopressin (AVP) but not by hypertonicity, which induces both TonEBP and NF-κB activity. AVP-induced AQP2 expression was similarly decreased by the flavoprotein inhibitor diphenyleneiodonium. siNOX4 altered neither TonEBP nor NF-κB activity but attenuated AVP-inducible cellular cAMP concentration, PKA activity and CREB phosphorylation as well as AQP2 mRNA expression induced by forskolin, a potent activator of adenylate cyclase. The repressive effect of siNOX4 on AVP-induced AQP2 mRNA expression was abolished by the non-selective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) and was significantly decreased by selective PDE antagonists cilostamide and rolipram, but not vinpocetine, which respectively target PDE3, PDE4 and PDE1. Thus, by inhibiting PDE3 and PDE4 activity NOX4-derived ROS may contribute to V₂R-cAMP-PKA signaling and enhance AQP2 transcription.

A urine-concentrating defect in 11β-hydroxysteroid dehydrogenase type 2 null mice

In aldosterone target tissues, 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) is coexpressed with mineralocorticoid receptors (MR) and protects the receptor from activation by glucocorticoids. Null mutations in the encoding gene, HSD11B2, cause apparent mineralocorticoid excess, in which hypertension is thought to reflect volume expansion secondary to sodium retention. Hsd11b2(-/-) mice are indeed hypertensive, but impaired natriuretic capacity is associated with significant volume contraction, suggestive of a urine concentrating defect. Water turnover and the urine concentrating response to a 24-h water deprivation challenge were therefore assessed in Hsd11b2(-/-) mice and controls. Hsd11b2(-/-) mice have a severe and progressive polyuric/polydipsic phenotype. In younger mice (∼2 mo of age), polyuria was associated with decreased abundance of aqp2 and aqp3 mRNA. The expression of other genes involved in water transport (aqp4, slc14a2, and slc12a2) was not changed. The kidney was structurally normal, and the concentrating response to water deprivation was intact. In older Hsd11b2(-/-) mice (>6 mo), polyuria was associated with a severe atrophy of the renal medulla and downregulation of aqp2, aqp3, aqp4, slc14a2, and slc12a2. The concentrating response to water deprivation was impaired, and the natriuretic effect of the loop diuretic bumetanide was lost. In older Hsd11b2(-/-) mice, the V2 receptor agonist desmopressin did not restore full urine concentrating capacity. We find that Hsd11b2(-/-) mice develop nephrogenic diabetes insipidus. Gross changes to renal structure are observed, but these were probably secondary to sustained polyuria, rather than of developmental origin.

Subcellular distribution of urea transporter in the collecting tubule of shark kidney is dependent on environmental salinity

In the kidney of marine elasmobranchs, urea reabsorption from filtered urine is essential for maintaining high levels of urea in the body. In the kidney of the houndshark, Triakis scyllium, we previously found that a facilitative urea transporter (UT) is localized to a specific nephron segment, the collecting tubule, suggesting that the collecting tubule has an important role in the urea reabsorption process. To elucidate the roles of UT, we transferred T. scyllium to high (130%) and low (30%) salinity, and examined UT mRNA levels and UT distribution patterns in the kidney using real-time PCR and semi-quantitative fluorescence immunohistochemistry, respectively. Following transfer to low and high salinity, houndshark decreased and increased plasma urea concentrations, respectively, in order to control plasma osmolality. The abundance of UT mRNA did not differ among the experimental groups, whereas that of UT protein in the collecting tubule was significantly decreased in 30% seawater (SW). Furthermore, the subcellular UT distribution was dramatically changed. UT in the apical plasma membrane of collecting tubule almost disappeared in 30% SW, whereas it slightly increased in 130% SW compared with 100% SW. Conversely, reverse transfer of fish from 30 to100% SW restored UT in the apical membrane. These results indicate that the accumulation of UT to the apical plasma membrane of the collecting tubule of Triakis is an important factor for regulating urea reabsorption in the kidney.

A case for water in the treatment of polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is an inherited disorder characterized by the development within renal tubules of innumerable cysts that progressively expand to cause renal insufficiency. Tubule cell proliferation and transepithelial fluid secretion combine to enlarge renal cysts, and 3'-5'-cyclic adenosine monophosphate (cAMP) stimulates that growth. The antidiuretic hormone, arginine vasopressin (AVP), operates continuously in ADPKD patients to stimulate the formation of cAMP, thereby contributing to cyst and kidney enlargement and renal dysfunction. Studies in animal models of ADPKD provide convincing evidence that blocking the action of AVP dramatically ameliorates the disease process. In the current analysis, the authors reason that increasing the amount of solute-free water drunk evenly throughout the day in patients with ADPKD and normal renal function will decrease plasma AVP concentrations and mitigate the action of cAMP on the renal cysts. Potential pitfalls of increasing fluid intake in ADPKD patients are considered, and suggestions for how physicians may prudently implement this therapy are offered.

Essential role of vasopressin-regulated urea transport processes in the mammalian kidney

Movement of urea across plasma membranes is modulated by specialized urea transporter proteins. Two urea-transporter genes have been cloned: UT-A (Slc14a2) and UT-B (Slc14a1). In the mammalian kidney, urea transporters are essential for the urinary concentrating mechanism and maintaining body fluid homeostasis. In this article, we discuss (1) an overview of historic discoveries in urea transport mechanisms; (2) an overview of recent discoveries in the regulation of urea transporters; (3) physiological studies in UT-A1/3 (-/-) mice highlighting the essential role of urea transporters in the urinary concentrating mechanism; and (4) physiological studies in UT-A2 and UT-B knockout mice examining the role of countercurrent exchange in the production of a maximally concentrated urine.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Reduced urea flux across the blood-testis barrier and early maturation in the male reproductive system in UT-B-null mice

A urea-selective urine-concentrating defect was found in transgenic mice deficient in urea transporter (UT)-B. To determine the role of facilitated urea transport in extrarenal organs expressing UT-B, we studied the kinetics of [14C]urea distribution in UT-B-null mice versus wild-type mice. After renal blood flow was disrupted, [14C]urea distribution was selectively reduced in testis in UT-B-null mice. Under basal conditions, total testis urea content was 335.4 ± 43.8 μg in UT-B-null mice versus 196.3 ± 18.2 μg in wild-type mice ( P < 0.01). Testis weight in UT-B-null mice (6.6 ± 0.8 mg/g body wt) was significantly greater than in wild-type mice (4.2 ± 0.8 mg/g body wt). Elongated spermatids were observed earlier in UT-B-null mice compared with wild type mice on day 24 versus day 32, respectively. First breeding ages in UT-B knockout males (48 ± 3 days) were also significantly earlier than that in wild-type males (56 ± 2 days). In competing mating tests with wild-type males and UT-B-null males, all pups carried UT-B-targeted genes, which indicates that all pups were produced from breeding of UT-B-null males. Experiments of the expression of follicle-stimulating hormone receptor (FSHR) and androgen binding protein (ABP) indicated that the development of Sertoli cells was also earlier in UT-B-null mice than that in wild-type mice. These results suggest that UT-B plays an important role in eliminating urea produced by Sertoli cells and that UT-B deletion causes both urea accumulation in the testis and early maturation of the male reproductive system. The UT-B knockout mouse may be a useful experimental model to define the molecular mechanisms of early puberty.