Protein kinase A (PKA) directly phosphorylates aquaporin-2 (AQP2) water channels in renal collecting ducts to reabsorb water from urine for the maintenance of systemic water homeostasis. More than 50 functionally distinct PKA-anchoring proteins (AKAPs) respectively create compartmentalized PKA signaling to determine the substrate specificity of PKA. Identification of an AKAP responsible for AQP2 phosphorylation is an essential step toward elucidating the molecular mechanisms of urinary concentration. PKA activation by several compounds is a novel screening strategy to uncover PKA substrates whose phosphorylation levels were nearly perfectly correlated with that of AQP2. The leading candidate in this assay proved to be an AKAP termed lipopolysaccharide-responsive and beige-like anchor protein (LRBA). We found that LRBA colocalized with AQP2 in vivo, and Lrba knockout mice displayed a polyuric phenotype with severely impaired AQP2 phosphorylation. Most of the PKA substrates other than AQP2 were adequately phosphorylated by PKA in the absence of LRBA, demonstrating that LRBA-anchored PKA preferentially phosphorylated AQP2 in renal collecting ducts. Furthermore, the LRBA-PKA interaction, rather than other AKAP-PKA interactions, was robustly dissociated by PKA activation. AKAP-PKA interaction inhibitors have attracted attention for their ability to directly phosphorylate AQP2. Therefore, the LRBA-PKA interaction is a promising drug target for the development of anti-aquaretics.

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.

Sorafenib is one of the multi-targeted tyrosine kinase inhibitors (TKI), mainly used for treating advanced renal cell carcinoma. Accumulated evidence indicates a minority of patients develop nephrotic syndrome (NS) as a high-grade nephrotoxic injury; however, evidence of NS after long-term use of sorafenib remains unclear. A 64-year-old man developed NS following 2-year use of sorafenib and his NS persisted even after sorafenib use was discontinued. Renal biopsy disclosed minimal change disease (MCD) concurrent with acute tubulointerstitial nephritis, indicating secondary MCD with which sorafenib may be involved. To prevent permanent renal insufficiency, we administered glucocorticoid and succeeded in achieving complete remission from NS. Nephrotoxic injuries could occur at any time with variable onset after sorafenib. Renal biopsy should be pursued in the case of NS associated with TKI therapy. To facilitate recovery of renal dysfunction, administration of prednisolone should be considered, particularly when NS does not disappear after cessation of TKIs.

Background Although lower estimated glomerular filtration rate (eGFR) and higher proteinuria are high risks for mortality and kidney outcomes, the prognosis of chronic kidney disease (CKD) in patients with normal-range proteinuria remains unclear. Methods In this prospective cohort study, 1138 newly visiting stage G2–G5 CKD patients were stratified into normal-range and abnormal-range proteinuria groups. Study endpoints were CKD progression (>50% eGFR loss or initiation of dialysis), cardiovascular events, and all-cause death. Results In total, 927 patients who were followed for >6 months were included in the analysis. The mean age was 67 years, and 70.2% were male. During a median follow-up of 35 months, CKD progression, cardiovascular events, and mortality were observed in 223, 110, and 55 patients, respectively. Patients with normal-range proteinuria had a significantly lower risk for CKD progression (hazard ratio, 0.20; 95% confidence interval, 0.10–0.38) than those with abnormal-proteinuria by multivariate Cox proportional hazard analysis. We also analyzed patients with normal-range proteinuria (n = 351). Nephrosclerosis was the most frequent cause of CKD among all patients with normal-range proteinuria (59.7%). During a median follow-up of 36 months, CKD progression, cardiovascular events, and mortality were observed in 10, 28, and 18 patients, respectively. The Kaplan–Meyer analysis demonstrated that the risks of CKD progression and cardiovascular events were not significantly different among CKD stages, whereas the risk of death was significantly higher in patients with advanced-stage CKD. Multivariate Cox proportional hazard analysis showed that the risk of three endpoints did not significantly differ among CKD stages. Conclusion Newly visiting CKD patients with normal-range proteinuria, who tend to be overlooked during health checkups did not exhibit a decrease in kidney function even in advanced CKD stages under specialized nephrology care.

AQP2 water channel is critical for urinary concentration in the kidney. Interestingly, AQP2 is abundantly excreted in the urine as extracellular vesicles (EVs), which is known to be a useful biomarker for water-balance disorders although the character of AQP2-enriched EVs is poorly understood including water channel function.

Congenital nephrogenic diabetes insipidus (NDI) is characterized by the inability of the kidney to concentrate urine. Congenital NDI is mainly caused by loss-of-function mutations in the vasopressin type 2 receptor (V2R), leading to impaired aquaporin-2 (AQP2) water channel activity. So far, treatment options of congenital NDI either by rescuing mutant V2R with chemical chaperones or by elevating cyclic adenosine monophosphate (cAMP) levels have failed to yield effective therapies. Here we show that inhibition of A-kinase anchoring proteins (AKAPs) binding to PKA increases PKA activity and activates AQP2 channels in cortical collecting duct cells. In vivo, the low molecular weight compound 3,3′-diamino-4,4′-dihydroxydiphenylmethane (FMP-API-1) and its derivatives increase AQP2 activity to the same extent as vasopressin, and increase urine osmolality in the context of V2R inhibition. We therefore suggest that FMP-API-1 may constitute a promising lead compound for the treatment of congenital NDI caused by V2R mutations.

and antibiotic mixed liquor (ampicillin 1g/L þ vancomycin 0.5g/L þ neomycin 1g/L þ amphotericin B 0.1g/L).Gene sequencing of intestinal microflora was carried out using 16S-rDNA pyrosequencing technique.The morphological changes to the renal pathology and ultra-microstructures were checked by pathological staining and electron microscopy.The plasma RAS components were determined by radioimmunoassay.The protein expressions of RAS components in the kidneys were determined by immunohistochemical staining and Western blot.RESULTS: Compared with control group, DM group displayed an abnormal state of gut microbiota, with a significant increase of the production of serum acetate, one of their major metabolites belonging to short chain fatty acids (SCFAs).Glomerular endothelial cells and podocytes were damaged obviously and the basement membrane was thickened in DM group, but administration of antibiotics ameliorated renal injuries caused by DM.Furthermore, the levels of plasma renin activity (PRA), angiotensin II (Ang II) were significantly increased in DM rats, indicative of activated RAS, the degree of which has been reduced by antibiotic treatment.As the important effector of RAS, the corresponding protein expression of angiotensin converting enzyme (ACE) and its receptor AT1R has shown opposite trends compared to DM group: DMþAB group has shown increased expression of ACE and less AT1R, suggesting that treatment of antibiotics possibly inhibited the transition of ACE to Ang II in order to weaken the effects of activated RAS on the progression of DN.These results suggested that disordered intestinal flora and consequent RAS activation are closely related to incipient renal injuries of DN, the potential mechanism of which is possibly concerned with SCFAs.CONCLUSIONS: Our findings suggested that the dysbiosis of intestinal microflora might be a potential mechanism for the progression of early DN, which leads to kidney injuries via the RAS activation.

INTRODUCTION AND AIMS: Podocytes dysfunction is a common feature as developing kidney diseases.In particular, the breakdown of the slit diaphragm in podocytes could be a key process in the developing of glomerulosclerosis.However, the mechanism remains unclear.A recent study demonstrated that a serine protease inhibitor protects podocyte from chronic kidney disease in rats, suggesting the activation of serine proteases could be a potential mechanism for podocyte injury.In this study, we examined the role of serine proteases in glomerular injury of Adriamycin (ADR) nephropathy in mice.METHODS: ADR at a dose of 10.5 mg/kg body weight was injected via the tail vein of BALB/c mice to develop ADR nephropathy.Glomeruli were isolated from mice using a magnetic particle concentrator after Dynabeads were systemically perfused.Serine protease activity in glomeruli was examined by Boc-QAR-AMC Fluorogenic Peptide Substrate.Nafamostat, a serine protease inhibitor, at a dose of 30 mg/kg body weight was intraperitoneally injected to mice with ADR nephropathy three times a week for 4 weeks.RESULTS: ADR administration induced glomerular injuries with an increase in urinary albumin excretion in mice.The glomerular mRNA expressions of Podocin, Synaptopodin and WT-1 were significantly reduced in ADR mice compared with control mice.The location of Podocin was disturbed and the number of WT1-positive cells was also decreased in the glomeruli of ADR mice.Glomerular serine protease activity was enhanced by ADR administration in mice.We first examined the potency of Nafamostat, which is known to be non-specific serine protease inhibitor.A protease activity assay demonstrated that Nafamostat inhibited thrombin activity, a serine protease, in dose dependent manner, with the half maximal inhibitory concentration (IC50) being 99nM, suggesting that Nafamostat was potent enough to block serine protease activity.Based on these results, we treated ADR mice with Nafamostat from day15 to day42.We found that the development of glomerulosclerosis and podocyte damages were significantly suppressed by Nafamostat.Urinary albumin creatinine ratio (uACR) (mg/g) of vehicle control group was 3328.5 6 528.6 at Day28 and 1955.5 6 233.8 at Day 42, whereas that of Nafamostat group was 1753.3 6 294.0 at Day28 and 1250.6 6 222.7 at Day42, showing that the treatment significantly reduced uACR.These results suggest that podocyte injury due to ADR was mediated by serine protease activation and was ameliorated by blocking serine protease inhibition.

Vasopressin catalyzes aquaporin-2 phosphorylation at several serine sites in the C-terminal region. Compared with Ser-256 and Ser-269 phosphorylation, the role of Ser-261 phospho-regulation on vasopressin-regulated AQP2 apical translocation is largely unknown. In addition, recent discovery of transcytotic apical delivery of AQP2 made the concept of its intracellular trafficking even more complicated. In this study, we evaluated how intact phospho-AQP2 signals fit with the transcytosis trafficking model in Madin-Darby canine kidney cells. PS256 and pS269 signals were intracellularly detectable in wild-type AQP2 at the beginning of forskolin stimulation (1 min). These phospho-signals were detectable in basolateral membranes even after 10 min of stimulation. AQP2 stably inserted in the apical membrane increased pS269 and decreased pS261 signals. In an NDI-causing mutant P262L-AQP2, in which Ser-261 phospho-regulation is impaired, the pS256 and pS269 signals were detectable in the basolateral membranes with increased pS261 signals after forskolin stimulation. These results suggest that Ser-261 phospho-regulation is involved in pS256- and pS269-mediated AQP2 apical translocation.

Electrolyte abnormalities, particularly dysnatremia, are independent predictors of adverse outcome in individuals with and without renal failure. However, the association of serum chloride level (Cl−) with mortality or risk of cardiovascular (CV) events in chronic kidney disease (CKD) remains unclear.

The relationship between protein–energy wasting and chronic kidney disease (CKD) progression is unknown. In the present prospective cohort study, we evaluated the hypothesis that a combination of low body mass index (BMI) and serum albumin level is associated with rapid CKD progression.

OBJECTIVE:An arteriovenous bridging graft is a viable option for patients with compromised arteries or veins because of advanced age or diabetes. Arteriovenous graft with graft insertion anastomosis (AVGI) is the novel technique for graft-vein anastomosis where the prosthesis is inserted into the vein, and the anastomosis is performed on the surface of the prosthesis. This study assessed the short-term and long-term results of AVGI to clarify the efficacy of this technique. METHODS:Between 2010 and 2015, AVGI was performed in graft-vein anastomosis of prosthetic forearm loop access. Characteristics and level of complications were assessed. To evaluate the long-term results, functional graft patency and frequency of percutaneous transluminal angioplasty were examined. RESULTS:The study comprised 58 patients. There were no deaths related to the surgery. The time of hemostasis after AVGI was recorded at 0 seconds because no bleeding from the suture holes was seen. At 1, 2, and 3 years, primary patency were 45.1% ± 7.5%, 23.1% ± 7.5%, and 23.1% ± 7.5%, respectively, and assisted primary patency rates were 59.4% ± 7.2%, 50.8% ± 7.6%, and 50.8% ± 7.6%, respectively. Secondary patency rates at 4 and 5 years were 100% ± 0% and 94.1% ± 5.7%, respectively. The frequency of percutaneous balloon angioplasty to maintain the patency was 1.61 ± 0.53 times per year. Graft infection occurred in four patients (6.9%). CONCLUSIONS:AVGI is an advantageous technique for graft vein anastomosis in an arteriovenous bridging graft in both the short-term and long-term.

Elevated white blood cell (WBC) count is a well-known predictor of chronic kidney disease (CKD) progression. However, elderly patients commonly fail to develop a high WBC count in response to several diseased states and may instead present a low WBC count. Therefore, we hypothesized that low WBC count, in addition to high WBC count, is associated with CKD progression in the elderly.