Claudins in kidney health and disease

Claudins are strategically located to exert their physiologic actions along with the nephron segments from the glomerulus. Claudin-1 is normally located in the Bowman’s capsule, but its overexpression can reach the podocytes and lead to albuminuria. In the proximal tubule (PT), claudin-2 forms paracellular channels selective for water, Na+, K+, and Ca2+. Claudin-2 gene mutations are associated with hypercalciuria and kidney stones. Claudin-10 has two splice variants, -10a and -10b; Claudin-10a acts as an anion-selective channel in the PT, and claudin-10b functions as a cation-selective pore in the thick ascending limb (TAL). Claudin-16 and claudin-19 mediate paracellular transport of Na+, Ca2+, and Mg2+ in the TAL, where the expression of claudin-3/16/19 and claudin-10b are mutually exclusive. The claudin-16 or -19 mutation causes familial hypomagnesemia with hypercalciuria and nephrocalcinosis. Claudin- 14 polymorphisms have been linked to increased risk of hypercalciuria. Claudin-10b mutations produce HELIX syndrome, which encompasses hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia. Hypercalciuria and magnesuria in metabolic acidosis are related to downregulation of PT and TAL claudins. In the TAL, stimulation of calcium-sensing receptors upregulates claudin-14 and negatively acts on the claudin-16/19 complex. Claudin-3 acts as a general barrier to ions in the collecting duct. If this barrier is disturbed, urine acidification might be impaired. Claudin-7 forms a nonselective paracellular channel facilitating Cl– and Na+ reabsorption in the collecting ducts. Claudin-4 and -8 serve as anion channels and mediate paracellular Cl– transport; their upregulation may contribute to pseudohypoaldosteronism II and salt-sensitive hypertension.

Urinary Concentration Defect and Renal Glycosuria in Cyclosporine-treated Rats

Background@#Urinary concentration impairment is a major feature of cyclosporine nephrotoxicity. @*Methods@#We explored two possible mechanisms that may underlie cyclosporineinduced polyuria; water, and/or osmotic diuresis. Cyclosporine was subcutaneously injected to normal salt-fed Sprague-Dawley rats at a daily dose of 25mg/kg for 2 weeks (Experiment I) and 7.5mg/kg for 6 weeks (Experiment II). @*Results@#In Experiment I, cyclosporine treatment caused an increase in urine volume (2.7±0.5 vs. 10.3±1.13mL/d/100 g BW, p<0.001) and a decrease in urine osmolality (2,831±554 vs. 1,379±478mOsm/kg H2O, p<0.05). Aquaporin-2 (AQP2) protein expression decreased in cyclosporine-treated rat kidneys (cortex, 78±8%, p<0.05; medulla, 80±1%, p<0.05). Experiment II also showed that urine volume was increased by cyclosporine treatment (4.97±0.66 vs. 9.65±1.76mL/d/100 g BW, p<0.05). Whereas urine osmolality was not affected, urinary excretion of osmoles was increased (7.5±0.4 vs. 14.9±1.4mosmoles/d/100 g BW, p<0.005). Notably, urinary excretion of glucose increased in cyclosporine-treated rats (7±1 vs. 10,932±2,462 mg/d/100 g BW, p<0.005) without a significant elevation in plasma glucose. In both Experiment I and II, GLUT2 protein expression in the renal cortex was decreased by cyclosporine treatment (Experiment I, 55±6%, p<0.005; Experiment II, 88 ±3%, p<0.05). @*Conclusion@#Both water diuresis and osmotic diuresis are induced by cyclosporine nephrotoxicity. AQP2 and GLUT2 downregulation may underlie water and osmotic diuresis, respectively.

Association of Proteinuria with Urinary Concentration Defect in Puromycin Aminonucleoside Nephrosis

Background@#Puromycin aminonucleoside (PA) can induce nephrotic syndrome in rats, and proteinuria is an important mediator of tubulointerstitial injury in glomerulopathy. We assumed that glomerular proteinuria may affect tubular function, such as urinary concentration, and investigated whether a urinary concentration defect is associated with proteinuria in puromycin aminonucleoside nephrosis (PAN). We also investigated the defect response to enalapril. @*Methods@#Glomerular proteinuria was induced by a single intraperitoneal injection of PA (150 mg/kg BW) in male Sprague-Dawley rats. In a half of these rats, enalapril (35 mg/kg BW) was administered daily in a food mixture for two weeks.After the animal experiment, kidneys were harvested for immunoblot analysis and histopathologic examination. @*Results@#Compared with the control group, PA-treated rats had severe proteinuria, polyuria, and a lower urine osmolality. PA treatment induced remarkable tubulointerstitial injury and significant reductions in protein abundances of aquaporin-1 and Na-K-2Cl co-transporter type 2 (NKCC2). Proteinuria significantly correlated with osteopontin expression in the kidney and inversely correlated with renal expression of aquaporin-1, aquaporin-2, and NKCC2. The degree of tubulointerstitial injury significantly correlated with proteinuria, urine output, and osteopontin expression and inversely correlated with urine osmolality and renal expression of aquaporin-1, aquaporin-2, and NKCC2. No significant differences in parameters were found between PA-treated rats with and without enalapril. @*Conclusion@#In PAN, glomerular proteinuria was associated with tubulointerstitial injury and water diuresis. Downregulation of aquaporin-1 and NKCC2 can impair countercurrent multiplication and cause a urinary concentration defect in PAN.

Urinary Concentration Defect and Renal Glycosuria in Cyclosporine-treated Rats

Background@#Urinary concentration impairment is a major feature of cyclosporine nephrotoxicity. @*Methods@#We explored two possible mechanisms that may underlie cyclosporineinduced polyuria; water, and/or osmotic diuresis. Cyclosporine was subcutaneously injected to normal salt-fed Sprague-Dawley rats at a daily dose of 25mg/kg for 2 weeks (Experiment I) and 7.5mg/kg for 6 weeks (Experiment II). @*Results@#In Experiment I, cyclosporine treatment caused an increase in urine volume (2.7±0.5 vs. 10.3±1.13mL/d/100 g BW, p<0.001) and a decrease in urine osmolality (2,831±554 vs. 1,379±478mOsm/kg H2O, p<0.05). Aquaporin-2 (AQP2) protein expression decreased in cyclosporine-treated rat kidneys (cortex, 78±8%, p<0.05; medulla, 80±1%, p<0.05). Experiment II also showed that urine volume was increased by cyclosporine treatment (4.97±0.66 vs. 9.65±1.76mL/d/100 g BW, p<0.05). Whereas urine osmolality was not affected, urinary excretion of osmoles was increased (7.5±0.4 vs. 14.9±1.4mosmoles/d/100 g BW, p<0.005). Notably, urinary excretion of glucose increased in cyclosporine-treated rats (7±1 vs. 10,932±2,462 mg/d/100 g BW, p<0.005) without a significant elevation in plasma glucose. In both Experiment I and II, GLUT2 protein expression in the renal cortex was decreased by cyclosporine treatment (Experiment I, 55±6%, p<0.005; Experiment II, 88 ±3%, p<0.05). @*Conclusion@#Both water diuresis and osmotic diuresis are induced by cyclosporine nephrotoxicity. AQP2 and GLUT2 downregulation may underlie water and osmotic diuresis, respectively.

Association of Proteinuria with Urinary Concentration Defect in Puromycin Aminonucleoside Nephrosis

Background@#Puromycin aminonucleoside (PA) can induce nephrotic syndrome in rats, and proteinuria is an important mediator of tubulointerstitial injury in glomerulopathy. We assumed that glomerular proteinuria may affect tubular function, such as urinary concentration, and investigated whether a urinary concentration defect is associated with proteinuria in puromycin aminonucleoside nephrosis (PAN). We also investigated the defect response to enalapril. @*Methods@#Glomerular proteinuria was induced by a single intraperitoneal injection of PA (150 mg/kg BW) in male Sprague-Dawley rats. In a half of these rats, enalapril (35 mg/kg BW) was administered daily in a food mixture for two weeks.After the animal experiment, kidneys were harvested for immunoblot analysis and histopathologic examination. @*Results@#Compared with the control group, PA-treated rats had severe proteinuria, polyuria, and a lower urine osmolality. PA treatment induced remarkable tubulointerstitial injury and significant reductions in protein abundances of aquaporin-1 and Na-K-2Cl co-transporter type 2 (NKCC2). Proteinuria significantly correlated with osteopontin expression in the kidney and inversely correlated with renal expression of aquaporin-1, aquaporin-2, and NKCC2. The degree of tubulointerstitial injury significantly correlated with proteinuria, urine output, and osteopontin expression and inversely correlated with urine osmolality and renal expression of aquaporin-1, aquaporin-2, and NKCC2. No significant differences in parameters were found between PA-treated rats with and without enalapril. @*Conclusion@#In PAN, glomerular proteinuria was associated with tubulointerstitial injury and water diuresis. Downregulation of aquaporin-1 and NKCC2 can impair countercurrent multiplication and cause a urinary concentration defect in PAN.

The Role of Proximal Nephron in Cyclophosphamide-Induced Water Retention: Preliminary Data

Cyclophosphamide is clinically useful in treating malignancy and rheumatologic disease, but has limitations in that it induces hyponatremia. The mechanisms by which cyclophosphamide induces water retention in the kidney have yet to be identified. This study was undertaken to test the hypothesis that cyclophosphamide may produce water retention via the proximal nephron, where aquaporin-1 (AQP1) and aquaporin-7 (AQP7) water channels participate in water absorption. To test this hypothesis, we gave a single dose of intraperitoneal cyclophosphamide to male Sprague-Dawley rats and treated rabbit proximal tubule cells (PTCs) with 4-hydroperoxycyclophosphamide (4-HC), an active metabolite of cyclophosphamide. In the short-term 3-day rat study, AQP1 protein expression was significantly increased in the whole kidney homogenates by cyclophosphamide administration at 48 (614 +/- 194%, P < 0.005), and 96 (460 +/- 46%, P < 0.05) mg/kg BW compared with vehicle-treated controls. Plasma sodium concentration was significantly decreased (143 +/- 1 vs. 146 +/- 1 mEq/L, P < 0.05) by cyclophosphamide 100 mg/kg BW in the long-term 6-day rat study. When primary cultured rabbit PTCs were treated with 4-HC for 24 hours, the protein expressions of AQP1 and AQP7 were increased in a dose-dependent manner. Quantitative polymerase chain reaction revealed no significant changes in the mRNA levels of AQP1 and AQP7 from cyclophosphamide-treated rat renal cortices. From these preliminary data, we conclude that the proximal nephron may be involved in cyclophosphamide-induced water retention via AQP1 and AQP7 water channels. Further studies are required to demonstrate intracellular mechanisms that affect the expression of AQP proteins.

Effects of Dietary Salt Restriction on Puromycin Aminonucleoside Nephrosis: Preliminary Data

Proteinuria is a major promoter that induces tubulointerstitial injury in glomerulopathy. Dietary salt restriction may reduce proteinuria, although the mechanism is not clear. We investigated the effects of dietary salt restriction on rat kidneys in an animal model of glomerular proteinuria. Male Sprague-Dawley rats were used and divided into 3 groups: vehicle-treated normal-salt controls, puromycin aminonucleoside (PA)-treated normal-salt rats, and PA-treated low-salt rats. PA was given at a dose of 150 mg/kg BW at time 0, followed by 50 mg/kg BW on days 28, 35, and 42. Sodium-deficient rodent diet with and without additional NaCl (0.5%) were provided for normal-salt rats and low-salt rats, respectively. On day 63, kidneys were harvested for histopathologic examination and immunohistochemistry. PA treatment produced overt proteinuria and renal damage. Dietary salt restriction insignificantly reduced proteinuria in PA-treated rats, and PA-treated low-salt rats had lower urine output and lower creatinine clearance than vehicle-treated normal-salt controls. When tubulointerstitial injury was semiquantitatively evaluated, it had a positive correlation with proteinuria. The tubulointerstitial injury score was significantly increased by PA treatment and relieved by low-salt diet. ED1-positive infiltrating cells and immunostaining for interstitial collagen III were significantly increased by PA treatment. These changes appeared to be less common in PA-treated low-salt rats, although the differences in PA-treated normal-salt versus low-salt rats did not reach statistical significance. Our results suggest that renal histopathology in PA nephrosis may potentially be improved by dietary salt restriction. Non-hemodynamic mechanisms induced by low-sodium diet might contribute to renoprotection.