Cell culture models and animal models for studying the patho-physiological role of renal aquaporins

Aquaporins (AQPs) are key players regulating urinary-concentrating ability. To date, eight aquaporins have been characterized and localized along the nephron, namely, AQP1 located in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2, AQP3 and AQP4 in collecting duct principal cells; AQP5 in intercalated cell type B; AQP6 in intercalated cells type A in the papilla; AQP7, AQP8 and AQP11 in the proximal tubule. AQP2, whose expression and cellular distribution is dependent on vasopressin stimulation, is involved in hereditary and acquired diseases affecting urine-concentrating mechanisms. Due to the lack of selective aquaporin inhibitors, the patho-physiological role of renal aquaporins has not yet been completely clarified, and despite extensive studies, several questions remain unanswered. Until the recent and large-scale development of genetic manipulation technology, which has led to the generation of transgenic mice models, our knowledge on renal aquaporin regulation was mainly based on in vitro studies with suitable renal cell models. Transgenic and knockout technology approaches are providing pivotal information on the role of aquaporins in health and disease. The main goal of this review is to update and summarize what we can learn from cell and animal models that will shed more light on our understanding of aquaporin-dependent renal water regulation.     

The renal plumbing system: aquaporin water channels

Aquaporins are channels that facilitate movement of water across lipid bilayers. They are expressed in multiple tissues and are essential for regulation of body water homeostasis. The kidney is the main organ responsible for this regulation, and at least seven aquaporins are expressed at distinct sites in the kidney. Aquaporin expression correlates with observed water permeability of each nephron segment: proximal tubule and descending thin limb of Henle have constitutive high water permeability due to expression of AQP1, whereas collecting duct water permeability is tightly regulated by the antidiuretic hormone vasopressin via regulation of AQP2. This review aims at providing insight into renal aquaporins, with special focus on AQP2.Received 9 December 2004; received after revision 8 April 2005; accepted 11 April 2005     

Targeted inhibition of Livin resensitizes renal cancer cells towards apoptosis

Cancer cells are typically characterized by apoptosis deficiency. In order to investigate a possible role for the anti-apoptotic livin gene in renal cell cancer (RCC), we analyzed its expression in tumor tissue samples and in RCC-derived cell lines. In addition, we studied the contribution of livin to the apoptotic resistance of RCC cells by RNA interference (RNAi). Livin gene expression was detected in a significant portion of RCC tumor tissue specimens (13/14, 92.9%) and tumor-derived cell lines (12/15, 80.0%). Moreover, targeted inhibition of livin by RNAi markedly sensitized RCC cells towards proapoptotic stimuli, such as UV irradiation or the chemotherapeutic drugs etoposide, 5-fluorouracil, and vinblastine. These effects were specific for livin expressing tumor cells. We conclude that livin can contribute significantly to the apoptosis resistance of RCC cells. Targeted inhibition of livin could represent a novel therapeutic strategy to increase the sensitivity of renal cancers towards pro-apoptotic agents. Received 30 November 2006; received after revision 22 February 2007; accepted 20 March 2007     

Streptozotocin induced diabetic nephropathy and renal tumors in the rat

Summary Adult Wistar rats rendered diabetic by a single dose of streptozotocin develop renal morphological changes which show subtle differences compared to those seen in human diabetic renal disease. The early tubular degeneration is sited in the distal rather than the proximal convoluted tubule and subsequent glomerular lesion shows linear deposits of IgG and albumin in the basement membrane rather than in the mesangium. The carcinogenicity of streptozotocin in the rat is reconfirmed.Acknowledgements. We wish to thank Mr S.G. Watkins and Mrs D. Greening for technical assistance.     

Claudin-17 forms tight junction channels with distinct anion selectivity

Barrier properties of tight junctions are determined by the claudin protein family. Many claudins seal this barrier, but others form paracellular channels. Among these, no claudins with general and clear-cut anion selectivity have yet been described, while for claudin-10a and claudin-4, only circumstantial or small anion selectivities have been shown. A claudin with unknown function and tissue distribution is claudin-17. We characterized claudin-17 by overexpression and knock-down in two renal cell lines. Overexpression in MDCK C7 cell layers caused a threefold increase in paracellular anion permeability and switched these cells from cation- to anion-selective. Knockdown in LLC-PK(1) cells indorsed the finding of claudin-17-based anion channels. Mutagenesis revealed that claudin-17 anion selectivity critically depends on a positive charge at position 65. Claudin-17 expression was found in two organs: marginal in brain but abundant in kidney, where expression was intense in proximal tubules and gradually decreased towards distal segments. As claudin-17 is predominantly expressed in proximal nephrons, which exhibit substantial, though molecularly not defined, paracellular chloride reabsorption, we suggest that claudin-17 has a unique physiological function in this process. In conclusion, claudin-17 forms channels within tight junctions with distinct anion preference.     

Tetraspanin TSPAN12 regulates tumor growth and metastasis and inhibits β-catenin degradation

Ablation of tetraspanin protein TSPAN12 from human MDA-MB-231 cells significantly decreased primary tumor xenograft growth, while increasing tumor apoptosis. Furthermore, TSPAN12 removal markedly enhanced tumor-endothelial interactions and increased metastasis to mouse lungs. TSPAN12 removal from human MDA-MB-231 cells also caused diminished association between FZD4 (a key canonical Wnt pathway receptor) and its co-receptor LRP5. The result likely explains substantially enhanced proteosomal degradation of β-catenin, a key effecter of canonical Wnt signaling. Consistent with disrupted canonical Wnt signaling, TSPAN12 ablation altered expression of LRP5, Naked 1 and 2, DVL2, DVL3, Axin 1, and GSKβ3 proteins. TSPAN12 ablation also altered expression of several genes regulated by β-catenin (e.g. CCNA1, CCNE2, WISP1, ID4, SFN, ME1) that may help to explain altered tumor growth and metastasis. In conclusion, these results provide the first evidence for TSPAN12 playing a role in supporting primary tumor growth and suppressing metastasis. TSPAN12 appears to function by stabilizing FZD4–LRP5 association, in support of canonical Wnt-pathway signaling, leading to enhanced β-catenin expression and function.     

The effect of cyclic AMP on dog renal function

The infusion of dibutyryl-cyclic AMP into the dog renal artery in vivo leads to diuresis, natriuresis and glucosuria. Addition of the nucleotide to the incubation medium bathing dog renal cortex slices in vitro causes inhibition of p-amino-hippurate accumulation and stimulation of glycine and beta-methyl-glucoside transport. The results are interpreted in terms of the development of a blood-lumen flux of sodium and water in the renal proximal tubule, analogous to that seen in the intestine.     

Potassium-related inherited tubulopathies

Hyper- and hypokalemia may carry severe clinical consequences. Different regulatory mechanisms, including the kidney, exert a tight regulation of plasma potassium levels. The renal pathway of potassium handling begins in the proximal tubule followed by the fine-tuning of its secretion or absorption at the distal tubule, including the thick ascending limb of Henle’s loop, the distal convoluted tubule and the cortical collecting duct. Genetic studies in recent years have clarified the role of specific tubular channels and transporters in the pathogenesis of unique hyper- and hypokalemic tubulopathies, some of them non-hypertensive (pseudohypoaldosteronism, Bartter and Gitelman syndromes) and others hypertensive by definition (including Liddle and Gordon syndromes). This article reviews the genetic and clinical spectrum of hypokalemic and hyperkalemic tubulopathies. Received 13 January 2006; received after revision 19 March 2006; accepted 18 May 2006     

Periostin in kidney diseases

Chronic kidney disease is an incurable to date pathology, with renal replacement therapy through dialysis or transplantation being the only available option for end-stage patients. A deeper understanding of the molecular mechanisms governing the progression of kidney diseases will permit the identification of unknown mediators and potential novel markers or targets of therapy which promise more efficient diagnostic and therapeutic applications. Over the last years, periostin was established by several studies as a novel key player in the progression of renal disease. Periostin is de novo expressed focally by the injured kidney cells during the development of renal disease. In diverse cohorts of renal disease patients, the expression levels of periostin in the kidney and urine were highly correlated with the stage of the pathology and the decline of renal function. Subsequent studies in animal models demonstrated that periostin is centrally involved in mediating renal inflammation and fibrosis, contributing to the deterioration of renal structure and function. Genetic or pharmaco-genetic inhibition of periostin in animal models of renal disease was efficient in arresting the progression of the pathology. This review will summarize the recent advances on periostin in the field of kidney diseases and will discuss its utility of as a novel target of therapy for chronic kidney disease.     

Über die Beteiligung von Hauptstückepithel am parietalen Blatt im corpusculum renale bei der Maus (Mus musculus)

Summary The epithelium of the proximal convoluted tubule participates in the formation of the parietal layer in the area of the urinary pole, as can already be observed in the first renal corpuscles of the mouse fetus. Sexual hormones do not seem to have any influence on this structural characteristic, neither during prenatal nor postnatal life.