Angiotensin AT1 Receptor-associated protein Arap1 in the Kidney Vasculature is Suppressed by Angiotensin II

Arap1 is a protein that interacts with angiotensin II type 1 (AT1) receptors and facilitates increased AT1 receptor surface expression in vitro. In the present study, we assessed the tissue localization and regulation of Arap1 in vivo. Arap1 was found in various mouse organs, with the highest expression in the heart, kidney, aorta, and adrenal gland. Renal Arap1 protein was restricted to the vasculature and to glomerular mesangial cells and was absent from tubular epithelia. A similar localization was found in human kidneys. To test the hypothesis that angiotensin II may control renal Arap1 expression, mice were subjected to various conditions to alter the activity of the renin-angiotensin system. A high-salt diet (4% NaCl, 7 days) upregulated Arap1 expression in mice by 47% compared with controls (0.6% NaCl, P = 0.03). Renal artery stenosis (7 days) or water restriction (48 h) suppressed Arap1 levels compared with controls (−64 and −62% in the clipped and contralateral kidney, respectively; and −50% after water restriction, P &lt; 0.01). Angiotensin II infusion (2 μg·kg−1·min−1, 7 days) reduced Arap1 mRNA levels compared with vehicle by 29% ( P &lt; 0.01), whereas AT1 antagonism (losartan, 30 mg·kg−1·day−1, 7 days) enhanced Arap1 mRNA expression by 52% ( P &lt; 0.01); changes in mRNA were paralleled by Arap1 protein abundance. Experiments with hydralazine and epithelial nitric oxide synthase−/− mice further suggested that Arap1 expression changed in parallel with angiotensin II, rather than with blood pressure per se. Similar to in vivo, Arap1 mRNA and protein were suppressed by angiotensin II in a time- and dose-dependent manner in cultured mesangial cells. In summary, Arap1 is highly expressed in the renal vasculature, and its expression is suppressed by angiotensin II. Thus Arap1 may serve as a local modulator of vascular AT1 receptor function in vivo. </jats:p

Intravital imaging reveals angiotensin II–induced transcytosis of albumin by podocytes

Albuminuria is a hallmark of kidney disease of various etiologies and usually caused by deterioration of glomerular filtration barrier integrity. We recently showed that angiotensin II (Ang II) acutely increases albumin filtration in the healthy kidney. Here, we used intravital microscopy to assess the effects of Ang II on podocyte function in rats. Acute infusion of 30, 60, or 80 ng/kg per minute Ang II enhanced the endocytosis of albumin by activation of the type 1 Ang II receptor and resulted in an average (±SEM) of 3.7±2.2, 72.3±18.6 (Pµm³ (P<0.001) albumin-containing vesicles per glomerulus, respectively, compared with none at baseline or 10 ng/kg per minute Ang II. Immunostaining of Ang II–infused kidneys confirmed the presence of albumin-containing vesicles, which colocalized with megalin, in podocin-positive cells. Furthermore, podocyte endocytosis of albumin was markedly reduced in the presence of gentamicin, a competitive inhibitor of megalin-dependent endocytosis. Ang II infusion increased the concentration of albumin in the subpodocyte space, a potential source for endocytic protein uptake, and gentamicin further increased this concentration. Some endocytic vesicles were acidified and colocalized with LysoTracker. Most vesicles migrated from the capillary to the apical aspect of the podocyte and were eventually released into the urinary space. This transcytosis accounted for approximately 10% of total albumin filtration. In summary, the transcellular transport of proteins across the podocyte constitutes a new pathway of glomerular protein filtration. Ang II enhances the endocytosis and transcytosis of plasma albumin by podocytes, which may eventually impair podocyte function

New mutations at the imprinted Gnas cluster show gene dosage effects of Gsα in postnatal growth and implicate XLαs in bone and fat metabolism, but not in suckling

The imprinted Gnas cluster is involved in obesity, energy metabolism, feeding behavior, and viability. Relative contribution of paternally expressed proteins XLαs, XLN1, and ALEX or a double dose of maternally expressed Gsα to phenotype has not been established. In this study, we have generated two new mutants (Ex1A-T-CON and Ex1A-T) at the Gnas cluster. Paternal inheritance of Ex1A-T-CON leads to loss of imprinting of Gsα, resulting in preweaning growth retardation followed by catch-up growth. Paternal inheritance of Ex1A-T leads to loss of imprinting of Gsα and loss of expression of XLαs and XLN1. These mice have severe preweaning growth retardation and incomplete catch-up growth. They are fully viable probably because suckling is unimpaired, unlike mutants in which the expression of all the known paternally expressed Gnasxl proteins (XLαs, XLN1 and ALEX) is compromised. We suggest that loss of ALEX is most likely responsible for the suckling defects previously observed. In adults, paternal inheritance of Ex1A-T results in an increased metabolic rate and reductions in fat mass, leptin, and bone mineral density attributable to loss of XLαs. This is, to our knowledge, the first report describing a role for XLαs in bone metabolism. We propose that XLαs is involved in the regulation of bone and adipocyte metabolism

The renin angiotensin aldosterone system

In this review, we will cover (i) the proteolytic cascade of the RAAS, (ii) its regulation by multiple feedback-controlled parameters, and (iii) the major effects of the RAAS. For the effects of the RAAS, we focus on the role of the RAAS in the regulation of volume homeostasis and vascular tone, as major determinants of arterial blood pressure

Cathepsin B increases ENaC activity leading to hypertension early in nephrotic syndrome

The NPHS2 gene, encoding the slit diaphragm protein podocin, accounts for genetic and sporadic forms of nephrotic syndrome (NS). Patients with NS often present symptoms of volume retention, such as oedema formation or hypertension. The primary dysregulation in sodium handling involves an inappropriate activation of the epithelial sodium channel, ENaC. Plasma proteases in a proteinuria‐dependent fashion have been made responsible; however, referring to the timeline of symptoms occurring and underlying mechanisms, contradictory results have been published. Characterizing the mouse model of podocyte inactivation of NPHS2 (Nphs2∆pod) with respect to volume handling and proteinuria revealed that sodium retention, hypertension and gross proteinuria appeared sequentially in a chronological order. Detailed analysis of Nphs2∆pod during early sodium retention, revealed increased expression of full‐length ENaC subunits and αENaC cleavage product with concomitant increase in ENaC activity as tested by amiloride application, and augmented collecting duct Na+/K+‐ATPase expression. Urinary proteolytic activity was increased and several proteases were identified by mass spectrometry including cathepsin B, which was found to process αENaC. Renal expression levels of precursor and active cathepsin B were increased and could be localized to glomeruli and intercalated cells. Inhibition of cathepsin B prevented hypertension. With the appearance of gross proteinuria, plasmin occurs in the urine and additional cleavage of γENaC is encountered. In conclusion, characterizing the volume handling of Nphs2∆pod revealed early sodium retention occurring independent to aberrantly filtered plasma proteases. As an underlying mechanism cathepsin B induced αENaC processing leading to augmented channel activity and hypertension was identified

The Reno-Vascular A2B Adenosine Receptor Protects the Kidney from Ischemia

Using gene-targeted mice, Holger Eltzschig and colleagues identify the A2B adenosine receptor as a novel therapeutic target for providing protection from renal ischemia

Autoimmunity in CD73/Ecto-5′-Nucleotidase Deficient Mice Induces Renal Injury

Extracellular adenosine formed by 5′-ectonucleotidase (CD73) is involved in tubulo-glomerular feedback in the kidney but is also known to be an important immune modulator. Since CD73−/−mutant mice exhibit a vascular proinflammatory phenotype, we asked whether long term lack of CD73 causes inflammation related kidney pathologies. CD73−/−mice (13 weeks old) showed significantly increased low molecule proteinuria compared to C57BL6 wild type controls (4.8≥0.52 vs. 2.9±0.54 mg/24 h, p<0.03). Total proteinuria increased to 5.97±0.78 vs. 2.55±0.35 mg/24 h at 30 weeks (p<0.01) whereas creatinine clearance decreased (0.161±0.02 vs. 0.224±0.02 ml/min). We observed autoimmune inflammation in CD73−/−mice with glomerulitis and peritubular capillaritis, showing glomerular deposition of IgG and C3 and enhanced presence of CD11b, CD8, CD25 as well as GR-1-positive cells in the interstitium. Vascular inflammation was associated with enhanced serum levels of the cytokines IL-18 and TNF-α as well as VEGF and the chemokine MIP-2 (CXCL-2) in CD73−/−mice, whereas chemokines and cytokines in the kidney tissue were unaltered or reduced. In CD73−/−mice glomeruli, we found a reduced number of podocytes and endothelial fenestrations, increased capillaries per glomeruli, endotheliosis and enhanced tubular fibrosis. Our results show that adult CD73−/−mice exhibit spontaneous proteinuria and renal functional deterioration even without exogenous stress factors. We have identified an autoimmune inflammatory phenotype comprising the glomerular endothelium, leading to glomeruli inflammation and injury and to a cellular infiltrate of the renal interstitium. Thus, long term lack of CD73 reduced renal function and is associated with autoimmune inflammation