Combination of secretin and fluvastatin ameliorates the polyuria associated with X-linked nephrogenic diabetes insipidus in mice

Abstract

X-linked nephrogenic diabetes insipidus (X-NDI) is a disease caused by inactivating mutations of the vasopressin (AVP) type 2 receptor (V2R) gene. Loss of V2R function prevents plasma membrane expression of the AQP2 water channel in the kidney collecting duct cells and impairs the kidney concentration ability. In an attempt to develop strategies to bypass V2R signaling in X-NDI, we evaluated the effects of secretin and fluvastatin, either alone or in combination, on kidney function in a mouse model of X-NDI. The secretin receptor was found to be functionally expressed in the kidney collecting duct cells. Based on this, X-NDI mice were infused with secretin for 14 days but urinary parameters were not altered by the infusion. Interestingly, secretin significantly increased AQP2 levels in the collecting duct but the protein primarily accumulated in the cytosol. Since we previously reported that fluvastatin treatment increased AQP2 plasma membrane expression in wild-type mice, secretin-infused X-NDI mice received a single injection of fluvastatin. Interestingly, urine production by X-NDI mice treated with secretin plus fluvastatin was reduced by nearly 90% and the urine osmolality was doubled. Immunostaining showed that secretin increased intracellular stores of AQP2 and the addition of fluvastatin promoted AQP2 trafficking to the plasma membrane. Taken together, these findings open new perspectives for the pharmacological treatment of X-NDI.

Figures

Figure 1

Analysis of the expression of secretin receptor (SCTR) in the mouse kidney by reverse transcriptase-PCR (RT-PCR) and western blotting. (a) Total RNAs extracted from kidney inner medulla (IM), outer medulla (OM), cortex (CTX), and pancreas were probed for the presence of mRNA encoding SCTR. Specific bands were obtained in all kidney fractions. Control RT-PCR was performed using primers amplifying mouse beta-actin mRNA. Three independent experiments were carried out. (b) Total protein extract from mouse kidney IM, OM, and CTX, were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotted for the presence of SCTR. Mouse brain, cerebellum, liver, heart, and pancreas were used as additional controls. The specificity of the immunodetected band was demonstrated using anti-SCTR antibodies pre-adsorbed on the immunizing peptide. Three independent experiments were carried out.

Figure 2

Immunolocalization of secretin receptor (SCTR) in human kidney sections. Immunofluorescence detection of SCTR in human kidney. (a) SCTR was stained with Alexa Fluor-555 (red), aquaporin 2 (AQP2) was stained with Alexa Fluor-488 (green), and Na+/K+-ATPase with Alexa Fluor-633 (blue). AQP2 was seen at the apical pole of the collecting duct (CD) principal cells, whereas the SCTR antibody decorated the basolateral membrane of the same cells and colocalized with the basolateral marker Na+/K+-ATPase (pink color in the × 3 magnified signal). (b) SCTR was stained with Alexa Fluor-488 (green), AQP3 was stained with Alexa Fluor-555 (red), tissue counterstained with TO-PRO (blue). SCTR largely colocalized with AQP3 at the basolateral membrane of CD cells (yellow color in the × 3 magnified signal). (c) Co-immunostaining in serial sections using AQP2 (green) as marker of the CD and Tamm–Horsfall protein (THP, red) as marker of the thick ascending limb (TAL). SCTR-positive tubules (asterisks) were also positive for THP. Magnification × 60.

Figure 3

Effect of treatment with secretin (SCT) and dDAVP on cyclic adenosine monophosphate (cAMP) concentrations and AQP2 plasma membrane localization on kidney collecting ducts. (a) SCT and dDAVP-induced cAMP production in mouse inner medullary collecting duct (IMCD) tubule suspensions. Freshly isolated IMCD tubule suspensions from C57BL/6 WT or tamoxifen (TMX)-induced X-linked nephrogenic diabetes insipidus (X-NDI) mice (12-week-old males; n=8 per individual experiment) were pooled and equally distributed into 24-well plates. Samples were treated with the indicated concentrations of dDAVP or SCT for 60 min at 37 °C. Total cAMP generated in each well was normalized to the amount of protein present in each well. Three independent experiments were carried out. Data are provided as mean±s.e.m. (b) Indirect immunofluorescence images of kidney tissue slices showing AQP2 in principal cells of collecting ducts (CDs). One C57BL/6 WT mouse kidney was cut into thin slices and incubated in vitro with buffer alone (control) or with dDAVP 10−7 mol/l or SCT 10−7 mol/l for 1 h before fixation by immersion, sectioning, and immunostaining to detect AQP2. Under control conditions, AQP2 was mainly localized throughout the cytoplasm and showed little apical membrane staining in CDs. In the presence of dDAVP or SCT, AQP2 staining of the apical membrane region was increased in principal cells of the cortical CD. The images are representative of three independent experiments. (c) Quantification of the number of cells displaying plasma membrane (PM) localization was determined by counting the fraction of cells showing apical PM localization. The extent of translocation is expressed as the percentage (means±s.e.) of cells showing a continuous apical PM localization in at least 100 randomly chosen CD principal cells from three independent determinations. Quantitation of cells displaying AQP2 apical membrane expression expressed as a percentage of total cells observed. ***P<0.0005 with Student's t-test. dDAVP, 1-deamino-8-D-arginine-vasopressin.

Figure 4

Effect of prolonged infusion of X-linked nephrogenic diabetes insipidus (X-NDI) mice with secretin (SCT) on urine output, urine osmolality, aquaporin 2 (AQP2) abundance, and membrane expression. (a) Five months after the end of the tamoxifen (TMX)-injection period, X-NDI mice were subcutaneously implanted with osmotic minipumps and continuously infused with saline alone or SCT at a dose of 1 nmol/kg/day over a 14-day period. Urine output, normalized for body weight, and osmolality were measured daily. No significant changes in urinary parameters were seen in SCT-infused animals. (b) At the end of the experiment, quantitative reverse transcription polymerase chain reaction was performed on kidneys from X-NDI infused with saline (−SCT) or with secretin (+SCT). Control animals that did not receive TMX were included (CTR). Relative quantification of gene expression (RQ) was performed setting the amount of AQP2 mRNA in vehicle-infused animals (−SCT) as 1. SCT-infused animals exhibited an approximately twofold increase in AQP2 gene transcription compared with saline-infused mice. (*P<0.05, **P<0.005, ***P<0.0005 with Student's t-test, n=6 per group.) (c) Western blotting with anti-AQP2 antibodies was carried out using homogenates prepared from whole left kidneys. Kidneys from control animals of the same age that had not received TMX were used as control. Protein extract from mouse inner medulla (IM) was loaded as positive control for AQP2 immunodetection. 29 kDa AQP2 band (AQP2) and glycosylated AQP2 band (gly) are indicated. Densitometric analysis of AQP2 bands, normalized for the actin abundance in each fraction, is reported on the right indicating that SCT infusion promoted a nearly 90% increase of AQP2 abundance. (d) The right kidneys were subjected to immunofluorescence localization of AQP2 in the three kidney regions. The number of AQP2-positive CD cells and the intensity of AQP2 fluorescence were increased in SCT-infused animals. Observation of single tubules at higher magnification (right panels) unambiguously showed that AQP2 was localized intracellularly. The experiment was repeated three times and comparable results were obtained. *P<0.05; ***P<0.0001; N=6 per group.

Figure 5

Combination of prolonged secretin (SCT) infusion and a single fluvastatin (FLU) injection markedly ameliorates nephrogenic diabetes insipidus (NDI) symptoms in X-linked NDI (X-NDI) mice. Five months after the end of tamoxifen (TMX)-injection period, X-NDI mice were subcutaneously implanted with osmotic minipumps and continuously infused with either saline alone or SCT at a dose of 1 nmol/kg/day over a 14-day period. On the last day of SCT infusion, half of the control and SCT-infused mice were given an intraperitoneal (i.p.) injection of FLU (50 mg/kg) and urine output and osmolality were monitored for the following 6 h. Compared with untreated animals (CTR), FLU alone had a slight effect on urine output and urine osmolality. As already shown in Figure 4, SCT alone had no significant effect on urinary parameters. Interestingly, a single FLU injection in SCT-infused mice (SCT+FLU) led to a marked reduction of urine output, accompanied by a significant increase of urine osmolality. Data are presented as mean±s.e.m. **P<0.05; ***P<0.001. N=5 per group.

Figure 6

Combination of prolonged secretin (SCT) infusion and a single fluvastatin injection upregulates aquaporin 2 (AQP2) expression and promotes AQP2 plasma membrane localization in X-linked nephrogenic diabetes insipidus (X-NDI) mice. At the end of the treatment described in Figure 5, mice were killed and their kidneys subjected to AQP2 immunolocalization followed by confocal examination. V2Rfl/yEsr1-Cre mice of the same age that had not received tamoxifen (−TMX) were analyzed in parallel. Random pictures were taken in the kidney cortex, outer medulla, and inner medulla. A high magnification of a detail of each image is shown in the upper left corner of each panel. Compared with non-induced animals (−TMX, CTR), mice with V2R deletion (+TMX, CTR) showed a marked reduction of AQP2 immunoreactivity in each kidney portion. FLU injection triggered the accumulation of the remaining amount of AQP2 on the luminal membrane. SCT infusion greatly upregulated AQP2 immunoreactivity in the kidney collecting ducts (CDs). At higher magnification, AQP2 was seen mainly in the cytoplasm with negligible plasma membrane expression. In mice chronically infused with SCT and injected with fluvastatin (SCT+FLU), AQP2 was upregulated and mainly expressed at the luminal membrane of CD tubules. Representative images are shown (N=5). G, glomerulus.