Downregulation of the renal outer medullary K(+) channel ROMK by the AMP-activated protein kinase.

The 5'-adenosine monophosphate-activated serine/threonine protein kinase (AMPK) is stimulated by energy depletion, increase in cytosolic Ca(2+) activity, oxidative stress, and nitric oxide. AMPK participates in the regulation of the epithelial Na(+) channel ENaC and the voltage-gated K(+) channel KCNE1/KCNQ1. It is partially effective by decreasing PIP(2) formation through the PI3K pathway. The present study explored whether AMPK regulates the renal outer medullary K(+) channel ROMK. To this end, cRNA encoding ROMK was injected into Xenopus oocytes with and without additional injection of constitutively active AMPK(γR70Q) (AMPK(α1)-HA+AMPK(ß1)-Flag+AMPKγ1(R70Q)), or of inactive AMPK(αK45R) (AMPK(α1K45R)+AMPK(ß1)-Flag+AMPK(γ1)-HA), and the current determined utilizing two-electrode voltage-clamp and single channel patch clamp. ROMK protein abundance was measured utilizing chemiluminescence in Xenopus oocytes and western blot in whole kidney tissue. Moreover, renal Na(+) and K(+) excretion were determined in AMPK(α1)-deficient mice (ampk ( -/- )) and wild-type mice (ampk ( +/+ )) prior to and following an acute K(+) load (111 mM KCl, 30 mM NaHCO(3), 4.7 mM NaCl, and 2.25 g/dl BSA) at a rate of 500 µl/h. As a result, coexpression of AMPK(γR70Q) but not of AMPK(αK45R) significantly decreased the current in ROMK1-expressing Xenopus oocytes. Injection of phosphatidylinositol PI((4,5))P(2) significantly increased the current in ROMK1-expressing Xenopus oocytes, an effect reversed in the presence of AMPK(γR70Q). Under control conditions, no significant differences between ampk ( -/- ) and ampk ( +/+ ) mice were observed in glomerular filtration rate (GFR), urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentrations as well as absolute and fractional Na(+) and K(+) excretion. Following an acute K(+) load, GFR, urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentration were again similar in both genotypes, but renal absolute and fractional Na(+) and K(+) excretion were higher in ampk ( -/- ) than in ampk ( +/+ ) mice. According to micropuncture following a K(+) load, delivery of Na(+) to the early distal tubule but not delivery of K(+) to late proximal and early distal tubules was increased in ampk (-/-) mice. The upregulation of renal ROMK1 protein expression by acute K(+) load was more pronounced in ampk (-/-) than in ampk ( +/+ ) mice. In conclusion, AMPK downregulates ROMK, an effect compromising the ability of the kidney to excrete K(+) following an acute K(+) load.

Effects of gum arabic (Acacia senegal) on renal function in diabetic mice.

BACKGROUND/AIMS: Gum arabic (GA) is a Ca(2+)-, Mg(2+)- and K(+)-rich dietary fiber used for the treatment of patients with chronic kidney disease in Middle Eastern countries. In healthy mice, GA treatment increases creatinine clearance, renal ADH excretion, as well as intestinal and renal excretion of Mg(2+) and Ca(2+). GA decreases plasma Pi concentration, urinary Pi and Na(+) excretion. The present study explored the effects of GA on renal function in diabetic mice. METHODS: Metabolic cage experiments were performed on Akita mice (akita(+/-)), which spontaneously develop insulin deficiency and thus hyperglycemia. Plasma and urinary concentrations of Na(+), K(+) and Ca(2+) were measured by flame photometry (AFM 5051, Eppendorf, Germany), creatinine by the Jaffé method, phosphate photometrically, urea by an enzymatic method, glucose utilizing a glucometer and an enzymatic kit, aldosterone using an RIA, urinary albumin fluorometrically, and blood pressure by the tail-cuff method. RESULTS: GA (10% in drinking water) significantly increased urinary excretion of Ca(2+) and significantly decreased plasma phosphate and urea concentrations, urinary flow rate, urinary Na(+), phosphate and glucose excretion, blood pressure and proteinuria. CONCLUSIONS: GA treatment decreases blood pressure and proteinuria in diabetic mice and may thus prove beneficial in diabetic nephropathy.

PKB/SGK-resistant GSK3 enhances phosphaturia and calciuria.

Insulin and IGF1-dependent signaling activates protein kinase B and serum and glucocorticoid inducible kinase (PKB/SGK), which together phosphorylate and inactivate glycogen synthase kinase GSK3. Because insulin and IGF1 increase renal tubular calcium and phosphorus reabsorption, we examined GSK3 regulation of phosphate transporter activity and determined whether PKB/SGK inactivates GSK3 to enhance renal phosphate and calcium transport. Overexpression of GSK3 and the phosphate transporter NaPi-IIa in Xenopus oocytes decreased electrogenic phosphate transport compared with NaPi-IIa-expressing oocytes. PKB/SGK serine phosphorylation sites in GSK3 were mutated to alanine to create gsk3(KI) mice resistant to PKB/SGK inactivation. Compared with wildtype animals, gsk3(KI) animals exhibited greater urinary phosphate and calcium clearances with higher excretion rates and lower plasma concentrations. Isolated brush border membranes from gsk3(KI) mice showed less sodium-dependent phosphate transport and Na-phosphate co-transporter expression. Parathyroid hormone, 1,25-OH vitamin D levels, and bone mineral density were decreased in gsk3(KI) mice, suggesting a global dysregulation of bone mineral metabolism. Taken together, PKB/SGK phosphorylation of GSK3 increases phosphate transporter activity and reduces renal calcium and phosphate loss.

Enhanced catecholamine release in mice expressing PKB/SGK-resistant GSK3.

Glycogen synthase kinase 3 (GSK3) plays a decisive role in the regulation of multiple functions. GSK3 is phosphorylated and its activity inhibited by protein kinase B (PKB/Akt) and serum and glucocorticoid inducible kinase (SGK) isoforms, which are in turn activated by growth factors through phosphoinositide (PI) 3 kinase signaling. PI3/PKB/Akt/SGK-dependent inhibition of GSK3 is disrupted in gene-targeted knockin mice with mutated and thus PKB/SGK-resistant GSK3α,ß (gsk3 ( KI )) where the serine of the PKB/SGK phosphorylation site has been replaced by alanine. Recent experiments revealed that blood pressure is significantly higher in those mice than in wild type mice (gsk3 ( WT )). The present study was performed to elucidate the underlying cause. Blood pressure was determined with the tail cuff method, heart rate by ECG measurements, catecholamine concentrations by ELISA, and vanillylmandelic acid by high pressure liquid chromatography. As a result, blood pressure and heart rate were significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. The α-adrenergic blocker prazosin (1 µg/g body weight, b.w.) and the ganglion blocker hexamethonium (40 µg/g b.w.) decreased blood pressure to a larger extent in gsk3 ( KI ) than in gsk3 ( WT ) mice and virtually abrogated the difference between genotypes. Similarly, the ß-adrenergic blocker atenolol (5 µg/g b.w.) decreased the heart rate to a larger extent in gsk3 ( KI ) than in gsk3 ( WT ) mice and again dissipated the difference of heart rate between genotypes. Plasma epinephrine and norepinephrine concentrations, as well as urinary excretion of vanillylmandelic acid, were significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. The observations reveal a completely novel function of PKB/Akt/SGK-dependent GSK3 signaling, i.e., regulation of catecholamine release.

Decreased bone density and increased phosphaturia in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase 3.

Insulin and growth factors activate the phosphatidylinositide-3-kinase pathway, leading to stimulation of several kinases including serum- and glucocorticoid-inducible kinase isoform SGK3, a transport regulating kinase. Here, we explored the contribution of SGK3 to the regulation of renal tubular phosphate transport. Coexpression of SGK3 and sodium-phosphate cotransporter IIa significantly enhanced the phosphate-induced current in Xenopus oocytes. In sgk3 knockout and wild-type mice on a standard diet, fluid intake, glomerular filtration and urine flow rates, and urinary calcium ion excretion were similar. However, fractional urinary phosphate excretion was slightly but significantly larger in the knockout than in wild-type mice. Plasma calcium ion, phosphate concentration, and plasma parathyroid hormone levels were not significantly different between the two genotypes, but plasma calcitriol and fibroblast growth factor 23 concentrations were significantly lower in the knockout than in wild-type mice. Moreover, bone density was significantly lower in the knockouts than in wild-type mice. Histological analysis of the femur did not show any differences in cortical bone but there was slightly less prominent trabecular bone in sgk3 knockout mice. Thus, SGK3 has a subtle but significant role in the regulation of renal tubular phosphate transport and bone density.

Hyperaldosteronism in Klotho-deficient mice.

Klotho is a membrane protein participating in the inhibitory effect of FGF23 on the formation of 1,25-dihydroxyvitamin-D(3) [1,25(OH)(2)D(3)]. It participates in the regulation of renal tubular phosphate reabsorption and stimulates renal tubular Ca(2+) reabsorption. Klotho hypomorphic mice (klotho(hm)) suffer from severe growth deficit, rapid aging, and early death, events largely reversed by a vitamin D-deficient diet. The present study explored the role of Klotho deficiency in mineral and electrolyte metabolism. To this end, klotho(hm) mice and wild-type mice (klotho(+/+)) were subjected to a normal (D(+)) or vitamin D-deficient (D(-)) diet or to a vitamin D-deficient diet for 4 wk and then to a normal diet (D(-/+)). At the age of 8 wk, body weight was significantly lower in klotho(hm)D(+) mice than in klotho(+/+)D(+) mice, klotho(hm)D(-) mice, and klotho(hm)D(-/+) mice. Plasma concentrations of 1,25(OH)(2)D(3,) adrenocorticotropic hormone (ACTH), antidiuretic hormone (ADH), and aldosterone were significantly higher in klotho(hm)D(+) mice than in klotho(+/+)D(+) mice. Plasma volume was significantly smaller in klotho(hm)D(-/+) mice, and plasma urea, Ca(2+), phosphate and Na(+), but not K(+) concentrations were significantly higher in klotho(hm)D(+) mice than in klotho(+/+)D(+) mice. The differences were partially abrogated by a vitamin D-deficient diet. Moreover, the hyperaldosteronism was partially reversed by Ca(2+)-deficient diet. Ussing chamber experiments revealed a marked increase in amiloride-sensitive current across the colonic epithelium, pointing to enhanced epithelial sodium channel (ENaC) activity. A salt-deficient diet tended to decrease and a salt-rich diet significantly increased the life span of klotho(hm)D(+) mice. In conclusion, the present observation disclose that the excessive formation of 1,25(OH)(2)D(3) in Klotho-deficient mice results in extracellular volume depletion, which significantly contributes to the shortening of life span.

Effect of amphotericin B on parasitemia and survival of plasmodium berghei-infected mice.

Premature death of Plasmodium-infected erythrocytes is considered to favourably influence the clinical course of malaria. Amphotericin B has previously been shown to trigger suicidal erythrocyte death or eryptosis, which is characterized by cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing cells are rapidly cleared from circulating blood. The present study thus tested whether amphotericin B exerts a direct effect on Plasmodium falciparum and influences eryptosis of infected erythrocytes, parasitemia and host survival in murine malaria. To this end, human erythrocytes were infected in vitro with Plasmodium falciparum and mice were infected with Plasmodium berghei ANKA by in vivo intraperitoneal injection of parasitized murine erythrocytes (1x10(6)). Half of the infected mice received amphotericin B (1.5 mg/kg b.w. i.v.) from the 8(th) day of infection. Amphotericin B (> or = 1 microM) compromised the intracellular development of the parasite in human erythrocytes as evident from in vitro growth and DNA amplification assays. Amphotericin B further augmented the eryptosis of infected human erythrocytes. The administration of amphotericin B to infected mice tended to delay the increase of parasitemia and significantly delayed host death. All nontreated mice died from malaria within 27 days. In contrast, some 50% of amphotericin B-treated mice survived for more than 27 days after infection. In conclusion, amphotericin B augmented the suicidal death of infected erythrocytes and delayed the lethal course of malaria in Plasmodium berghei infected mice.

Responses to diuretic treatment in gene-targeted mice lacking serum- and glucocorticoid-inducible kinase 1.

BACKGROUND/AIMS: Serum- and glucocorticoid-inducible kinase 1 (SGK1) stimulates the epithelial sodium channel (ENaC), renal outer medullary K(+) channel 1, Na(+)/K(+)-ATPase and presumably the Na(+)-Cl(-) cotransporter (NCC). SGK1-deficient mice (sgk(-/-)) show a compensated salt-losing phenotype with secondary hyperaldosteronism. The present experiments explored the role of SGK1 in the response to diuretics. METHODS: sgk1(-/-) mice and their wild-type littermates (sgk1(+/+)) were treated with the ENaC blocker triamterene (200 mg/l), the Na(+)-K(+)-2Cl(-) cotransport inhibitor furosemide (125 mg/l), the NCC blocker hydrochlorothiazide (400 mg/l) and the mineralocorticoid receptor blocker canrenoate (800 mg/l) for 8 days. Renal SGK1 expression was studied using quantitative RT-PCR and immunofluorescence. RESULTS: Diuretic treatment increased SGK1 mRNA and protein expression in the kidney of wild-type sgk1(+/+) mice. The responses to furosemide, hydrochlorothiazide or canrenoate were not different between sgk1(+/+) and sgk1(-/-) mice, and were accompanied by moderate increases in plasma aldosterone and urea concentrations. However, treatment with triamterene in sgk1(-/-) mice (but not in sgk1(+/+) mice) led to severe, eventually lethal, body weight loss as well as increases in plasma aldosterone, urea and K(+) concentrations. CONCLUSIONS: SGK1 is required for diuretic tolerance to triamterene. The observations confirm the impaired kaliuretic potency of sgk1(-/-) mice and point to a role of SGK1 in renal Na(+) reabsorption by mechanisms other than ENaC.

Cellular stress induces erythrocyte assembly on intravascular von Willebrand factor strings and promotes microangiopathy.

Microangiopathy with subsequent organ damage represents a major complication in several diseases. The mechanisms leading to microvascular occlusion include von Willebrand factor (VWF), notably the formation of ultra-large von Willebrand factor fibers (ULVWFs) and platelet aggregation. To date, the contribution of erythrocytes to vascular occlusion is incompletely clarified. We investigated the platelet-independent interaction between stressed erythrocytes and ULVWFs and its consequences for microcirculation and organ function under dynamic conditions. In response to shear stress, erythrocytes interacted strongly with VWF to initiate the formation of ULVWF/erythrocyte aggregates via the binding of Annexin V to the VWF A1 domain. VWF-erythrocyte adhesion was attenuated by heparin and the VWF-specific protease ADAMTS13. In an in vivo model of renal ischemia/reperfusion injury, erythrocytes adhered to capillaries of wild-type but not VWF-deficient mice and later resulted in less renal damage. In vivo imaging in mice confirmed the adhesion of stressed erythrocytes to the vessel wall. Moreover, enhanced eryptosis rates and increased VWF binding were detected in blood samples from patients with chronic renal failure. Our study demonstrates that stressed erythrocytes have a pronounced binding affinity to ULVWFs. The discovered mechanisms suggest that erythrocytes are essential for the pathogenesis of microangiopathies and renal damage by actively binding to ULVWFs.