A polycystin-2 protein with modified channel properties leads to an increased diameter of renal tubules and to renal cysts

Mutations in the PKD2 gene cause autosomal-dominant polycystic kidney disease but the physiological role of polycystin-2, the protein product of PKD2, remains elusive. Polycystin-2 belongs to the transient receptor potential (TRP) family of non-selective cation channels. To test the hypothesis that altered ion channel properties of polycystin-2 compromise its putative role in a control circuit controlling lumen formation of renal tubular structures, we generated a mouse model in which we exchanged the pore loop of polycystin-2 with that of the closely related cation channel polycystin-2L1 (encoded by PKD2L1), thereby creating the protein polycystin-2^poreL1. Functional characterization of this mutant channel in Xenopus laevis oocytes demonstrated that its electrophysiological properties differed from those of polycystin-2 and instead resembled the properties of polycystin-2L1, in particular regarding its permeability for Ca²⁺ ions. Homology modeling of the ion translocation pathway of polycystin-2^poreL1 argues for a wider pore in polycystin-2^poreL1 than in polycystin-2. In Pkd2^poreL1 knock-in mice in which the endogenous polycystin-2 protein was replaced by polycystin-2^poreL1 the diameter of collecting ducts was increased and collecting duct cysts developed in a strain-dependent fashion.

Summary: Replacement of the pore region of polycystin-2 with that of polycystin-2L1 results in wider renal tubules and polycystic kidney disease, thus demonstrating the essential function of its ion channel properties.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

The physiological role of glucagon-like peptide-1 in the regulation of renal function

Glucagon like peptide-1 (GLP-1) is an incretin hormone constantly secreted from the intestine at low basal levels in the fasted state; plasma concentrations rise rapidly after nutrient ingestion. Upon release, GLP-1 exerts insulinotropic effects via a G protein-coupled receptor, stimulation of adenylyl cyclase, and cAMP generation. Although primarily involved in glucose homeostasis, GLP-1 can induce diuresis and natriuresis when administered in pharmacological doses in humans and rodents. However, whether endogenous GLP-1 plays a role in regulating renal function remains an open question. This study aimed to test the hypothesis that blockade of GLP-1 receptor (GLP-1R) signaling at baseline influences renal salt and water handling. To this end, the GLP-1R antagonist exendin-9 (100 μg·kg(-1)·min(-1)) or vehicle was administered intravenously to overnight-fasted male Wistar rats for 30 min. This treatment reduced urinary cAMP excretion and renal cortical PKA activity, demonstrating blockade of renal GLP-1R signaling. Exendin-9-infused-rats exhibited reduced glomerular filtration rate, lithium clearance, urinary volume flow, and sodium excretion compared with vehicle-infused controls. Exendin-9 infusion also reduced renal cortical Na(+)/H(+) exchanger isotope 3 (NHE3) phosphorylation at serine 552 (NHE3pS552), a PKA consensus site that correlates with reduced transport activity. Collectively, these results provide novel evidence that GLP-1 is a physiologically relevant natriuretic factor that contributes to sodium balance, in part via tonic modulation of NHE3 activity in the proximal tubule.

Increased NHE3 abundance and transport activity in renal proximal tubule of rats with heart failure

Heart failure (HF) is associated with a reduced effective circulating volume that drives sodium and water retention and extracellular volume expansion. We therefore hypothesized that Na+/H+ exchanger isoform 3 (NHE3), the major apical transcellular pathway for sodium reabsorption in the proximal tubule, is upregulated in an experimental model of HF. HF was induced in male rats by left ventricle radiofrequency ablation. Sham-operated rats (sham) were used as controls. At 6 wk after surgery, HF rats exhibited cardiac dysfunction with a dramatic increase in left ventricular end-diastolic pressure. By means of stationary in vivo microperfusion and pH-dependent sodium uptake, we demonstrated that NHE3 transport activity was significantly higher in the proximal tubule of HF compared with sham rats. Increased NHE3 activity was paralleled by increased renal cortical NHE3 expression at both protein and mRNA levels. In addition, the baseline PKA-dependent NHE3 phosphorylation at serine 552 was reduced in renal cortical membranes of rats with HF. Collectively, these results suggest that NHE3 is upregulated in the proximal tubule of HF rats by transcriptional, translational, and posttranslational mechanisms. Enhanced NHE3-mediated sodium reabsorption in the proximal tubule may contribute to extracellular volume expansion and edema, the hallmark feature of HF. Moreover, our study emphasizes the importance of undertaking a cardiorenal approach to contain progression of cardiac disease.

Mechanisms mediating the diuretic and natriuretic actions of the incretin hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone considered a promising therapeutic agent for type 2 diabetes because it stimulates beta cell proliferation and insulin secretion in a glucose-dependent manner. Cumulative evidence supports a role for GLP-1 in modulating renal function; however, the mechanisms by which GLP-1 induces diuresis and natriuresis have not been completely established. This study aimed to define the cellular and molecular mechanisms mediating the renal effects of GLP-1. GLP-1 (1 μg·kg(-1)·min(-1)) was intravenously administered in rats for the period of 60 min. GLP-1-infused rats displayed increased urine flow, fractional excretion of sodium, potassium, and bicarbonate compared with those rats that received vehicle (1% BSA/saline). GLP-1-induced diuresis and natriuresis were also accompanied by increases in renal plasma flow and glomerular filtration rate. Real-time RT-PCR in microdissected rat nephron segments revealed that GLP-1 receptor-mRNA expression was restricted to glomerulus and proximal convoluted tubule. In rat renal proximal tubule, GLP-1 significantly reduced Na(+)/H(+) exchanger isoform 3 (NHE3)-mediated bicarbonate reabsorption via a protein kinase A (PKA)-dependent mechanism. Reduced proximal tubular bicarbonate flux rate was associated with a significant increase of NHE3 phosphorylation at the PKA consensus sites in microvillus membrane vesicles. Taken together, these data suggest that GLP-1 has diuretic and natriuretic effects that are mediated by changes in renal hemodynamics and by downregulation of NHE3 activity in the renal proximal tubule. Moreover, our findings support the view that GLP-1-based agents may have a potential therapeutic use not only as antidiabetic drugs but also in hypertension and other disorders of sodium retention.