In in vitro studies, angiotensin (Ang) II has been demonstrated to promote podocyte apoptosis. The present study evaluates the effects of Ang II infusion in rats on podocyte nephrin expression and apoptosis and the molecular mechanisms involved in Ang II-induced proteinuria and mesangial expansion.
Sprague-Dawley rats were randomly assigned to receive either normal saline or Ang II (400 ng·kg−1·min−1) by means of a mini-osmotic pump for variable time periods. Systolic blood pressure and urinary protein and albumin excretion rate measurements were carried out on days 7, 14, 21, and 28. The animals were sacrificed on days 14 and 28 and evaluated for serum creatinine, renal pathological changes, podocyte apoptosis, renal nephrin mRNA, and protein expression.
The Ang II-infused rats developed hypertension and proteinuria. On day 14, the Ang II-infused rats showed narrowing of the slit diaphragm, an increase in podocyte nephrin mRNA and protein expression, and alterations in its distribution along the foot processes. On day 28, the Ang II-infused rats demonstrated the presence of apoptotic podocytes and decreased nephrin mRNA and protein expression. There was a negative correlation between nephrin expression and the numbers of apoptotic podocytes (r = −0.63, p < 0.05).
These results suggest that changes in nephrin expression may play a role in the pathogenesis of Ang II-induced podocyte apoptosis.
Angiotensin II; Proteinuria; Nephrin expression; Podocyte; Apoptosis
Nephrin, an important structural and signal molecule of podocyte slit-diaphragm (SD), has been suggested to contribute to the angiotensin II (Ang II)-induced podocyte injury. Caveolin-1 has been demonstrated to play a crucial role in signaling transduction. In the present study, we evaluated the role of caveolin-1 in Ang II-induced nephrin phosphorylation in podocytes. Wistar rats-receiving either Ang II (400 ng/kg/min) or normal saline (via subcutaneous osmotic mini-pumps, control) were administered either vehicle or telmisartan (3 mg/kg/min) for 14 or 28 days. Blood pressure, 24-hour urinary albumin and serum biochemical profile were measured at the end of the experimental period. Renal histomorphology was evaluated through light and electron microscopy. In vitro, cultured murine podocytes were exposed to Ang II (10−6 M) pretreated with or without losartan (10−5 M) for variable time periods. Nephrin and caveolin-1 expression and their phosphorylation were analyzed by Western-blotting and immunofluorescence. Caveolar membrane fractions were isolated by sucrose density gradient centrifugation, and then the distribution and interactions between Ang II type 1 receptor (AT1), nephrin, C-terminal Src kinase (Csk) and caveolin-1 were evaluated using Western-blotting and co-immunoprecipitation. Podocyte apoptosis was evaluated by cell nucleus staining with Hoechst-33342.
Ang II-receiving rats displayed diminished phosphorylation of nephrin but enhanced glomerular/podocyte injury and proteinuria when compared to control rats. Under control conditions, podocyte displayed expression of caveolin-1 in abundance but only a low level of phospho moiety. Nonetheless, Ang II stimulated caveolin-1 phosphorylation without any change in total protein expression. Nephrin and caveolin-1 were co-localized in caveolae fractions. AT1 receptors and Csk were moved to caveolae fractions and had an interaction with caveolin-1 after the stimulation with Ang II. Transfection of caveolin-1 plasmid (pEGFPC3-cav-1) significantly increased Ang II-induced nephrin dephosphorylation and podocyte apoptosis. Furthermore, knockdown of caveolin-1 expression (using siRNA) inhibited nephrin dephosphorylation and prevented Ang II-induced podocyte apoptosis. These findings indicate that Ang II induces nephrin dephosphorylation and podocyte injury through a caveolin-1-dependent mechanism.
Caveolin-1; Podocyte; Angiotensin II; Nephrin
The importance of β and γ Epithelial Na+ Channel (ENaC) proteins in vascular smooth muscle cell mediated pressure-induced constriction in renal interlobar arteries has been demonstrated recently. In renal epithelial tissue, ENaC expression is regulated by angiotensin II (Ang II). However, whether or not Ang II regulates vascular ENaC expression has never been determined. Therefore, the goal of the current investigation was to determine if Ang II affects vascular ENaC expression and its contribution to pressure-induced constriction.
To address this goal, Sprague-Dawley rats were infused with Ang II (50 ng/kg/min) via osmotic mini-pump for 1 week. Mean arterial pressure was measured using radiotelemetry. Interlobar arteries were isolated from these animals to assess VSMC ENaC protein expression, pressure-induced constriction and agonist induced vascular reactivity.
MAP was not different in control (113 ± 2) and Ang II (114 ± 2 mm Hg) infused mice. We found that Ang II infusion decreased renal VSMC β- and γENaC immunolabeling by 18%. Consistent with this finding, we also found that ENaC-dependant peak pressure-induced constriction was inhibited from 38 ± 3% to 25 ± 1% at 125 mm Hg. Vasoreactivity to KCl, phenylephrine and acetylcholine were unchanged.
Ang II suppression of pressure-induced constrictor responses in renal interlobar arteries may be mediated, at least in part, by inhibition of β and γENaC protein expression.
Background. Several studies in patients with primary aldosteronism (PA) have suggested that aldosterone (ALD) is directly contributing to albuminuria. However, there are limited data pertaining to the direct role of ALD in in vivo models in regard to the induction of renal injury and the involved mechanisms. In the present study, we established a high-dose ALD-infused rat model to evaluate urinary albumin excretion rate (UAER) and podocyte damage. Moreover, we studied the effect of eplerenone (EPL), telmisartan (TEL) and amlodipine (AML) on ALD-induced renal structural and functional changes.
Methods. Immunohistochemical and real-time PCR analyses, and TUNEL assays were performed to evaluate nephrin expression and podocyte injury.
Results. ALD-receiving rats (ARR) showed a progressive increase in BP, UAER and proteinuria when compared with control rats (CR). Conversely, BP was significantly reduced in ALD + EPL (A/ERR)-, ALD + AML (A/ARR)- and ALD + TEL (A/TRR)-treated rats. However, UAER and proteinuria were decreased only in A/ERR and A/TRR, but not in A/ARR. Only EPL administration provided protection against ALD-induced podocyte apoptosis. Renal tissue of ARR revealed enhanced expression of nephrin protein and mRNA. This effect of ALD was inhibited by EPL, but not by TEL or AML.
Conclusions. ALD induces direct glomerular injury independent of its haemodynamic effects; this effect of ALD is, at least in part, mediated through activation of the mineralocorticoid receptor.
aldosterone; amlodipine; eplerenone; podocyte; telmisartan
Recent evidence suggests that angiotensin II (Ang II) upregulates phosphodiesterase (PDE)-1A expression. We hypothesized that Ang II augmented PDE1 activation, decreasing the bioavailability of cyclic cyclic guanosine 3', 5'-monophosphate (cGMP), contributing to increased vascular contractility. Male Sprague-Dawley rats received mini-osmotic pumps with Ang II (60 ng.min−1) or saline for 14 days. PE-induced contractions were increased in aorta (Emax168±8 vs. 136±4%) and small-mesenteric arteries [(SMA), Emax170±6 vs. 143±3%] from Ang II infused rats compared to control. PDE1 inhibition with vinpocetine (10µM) reduced PE-induced contraction in aortas from Ang II rats (Emax94±12%) but not in control (154±7%). Vinpocetine decreased the sensitivity to PE in SMA from Ang II rats compared to vehicle (pD2 5.1±0.1 vs. 5.9±0.06), but not in control (6.0±0.03 vs. 6.1±0.04). Sildenafil (10µM), a PDE5 inhibitor reduced PE-induced maximal contraction similarly in Ang II and control rats. Arteries were contracted with PE (1µM) and concentration-dependent relaxation to vinpocetine and sildenafil was evaluated. Aortas from Ang II rats displayed increased relaxation to vinpocetine compared to control (Emax82±12 vs. 44±5%). SMA from Ang II rats showed greater sensitivity during vinpocetine-induced relaxation, compared to control (pD2 6.1±0.3 vs. 5.3±0.1). No differences in sildenafil-induced relaxation were observed. PDE1A and PDE1C expressions in aorta and PDE1A expression in SMA were increased in Ang II rats. cGMP production, which is decreased in arteries from Ang II rats, was restored after PDE1 blockade. We conclude that PDE1 activation reduces cGMP bioavailability in arteries from ANG II, contributing to increased contractile responsiveness.
PDE1; angiotensin II; cGMP; hypertension; vinpocetine
Chronic elevations in circulating angiotensin II (AngII) levels produce sustained hypertension and increased intrarenal AngII contents through multiple mechanisms, which may include sustained or increased local production of AngII. This study was designed to test the hypothesis that chronic AngII infusion increases renal angiotensinogen mRNA and protein levels, thus contributing to the increase in intrarenal AngII levels. AngII (80 ng/min) was infused subcutaneously for 13 d into Sprague-Dawley rats, using osmotic minipumps. Control rats underwent sham operations. By day 12, systolic arterial BP increased to 184 ± 3 mmHg in AngII-treated rats, whereas values for sham-treated rats remained at control levels (125 ± 1 mmHg). Plasma renin activity was markedly suppressed (0.2 ± 0.1 versus 5.3 ± 1.2 ng AngI/ml per h); however, renal AngII contents were significantly increased in AngII-treated rats (273 ± 29 versus 99 ± 18 fmol/g). Western blot analyses of plasma and liver protein using a polyclonal anti-angiotensinogen antibody demonstrated two specific immunoreactive bands, at 52 and 64 kD, whereas kidney tissue exhibited one band, at 52 kD. Densitometric analyses demonstrated that AngII infusion did not alter plasma (52- or 64-kD), renal (52-kD), or hepatic (52-kD) angiotensinogen protein levels; however, there was a significant increase in hepatic expression of the highly glycosylated 64-kD angiotensinogen protein, of almost fourfold (densitometric value/control value ratios of 3.79 ± 1.16 versus 1.00 ± 0.35). Renal and hepatic expression of angiotensinogen mRNA, which was examined by semiquantitative reverse transcription-PCR, was significantly increased in AngII-treated rats, compared with sham-treated rats (kidney, densitometric value/glyceraldehyde-3-phosphate dehydrogenase mRNA value ratios of 0.82 ± 0.11 versus 0.58 ± 0.04; liver, densitometric value/glyceraldehyde-3-phosphate dehydrogenase mRNA value ratios of 2.34 ± 0.07 versus 1.32 ± 0.15). These results indicate that increases in circulating AngII levels increase intrarenal angiotensinogen mRNA levels, which may contribute to the sustained renal AngII-generating capacity that paradoxically occurs in AngII-treated hypertensive rats.
In rats maintained on a high salt diet (H/S) to suppress basal renal angiotensinogen levels, angiotensin II (Ang II) infusion for 13 days increased renal angiotensinogen mRNA and protein, thus providing a mechanism for further augmentation of intrarenal Ang II levels. The present study tested the hypothesis that enhanced intrarenal angiotensinogen formation during Ang II infusion is reflected by secretion into the tubular fluid leading to increased urinary excretion of angiotensinogen (UAGT).
The effects of chronic Ang II infusion were examined on kidney and plasma Ang II levels and UAGT in male Sprague-Dawley rats maintained on an 8% salt diet for three weeks (N = 10). Following one week on the H/S diet, Ang II (40 ng/min) was administered for two weeks via an osmotic minipump to one group (H/S + Ang II, N = 5), while the remaining rats were sham-operated (H/S + Sham, N = 5). Additionally, a control group was prepared with normal salt diet and sham-operation (N/S + Sham, N = 5).
H/S alone did not alter systolic blood pressure (BP) (103 ± 2 vs. 104 ± 2 mm Hg), while Ang II infusion to H/S rats significantly increased systolic BP from 103 ± 2 to 154 ± 2 after two weeks. Intrarenal Ang II content in H/S + Ang II was significantly greater than H/S + Sham (435 ± 153 vs. 65 ± 14 fmol/g). Ang II infusion significantly increased UAGT (4.0 ± 0.5 vs. 1.0 ± 0.2 nmol Ang I/day by radioimmunoassay of generated Ang I; 57 ± 15 vs. 14 ± 2 densitometric units by Western blotting analysis) compared to Sham. UAGT by radioimmunoassay was highly correlated with kidney Ang II content (r = 0.79); but not with plasma Ang II concentration (r = 0.20).
These data demonstrate that chronic Ang II infusion increases urinary excretion rate of angiotensinogen, and suggest that UAGT provides a specific index of intrarenal angiotensinogen production in Ang II-dependent hypertension.
Ang II; kidney; urine; high salt diet; hypertension; reninangiotensin system; blood pressure
Chronic angiotensin (ANG) II infusions into rats lead to augmented intrarenal levels of ANG II and inflammatory factors, impaired renal function, and progressive hypertension. Residual effects persist after cessation of ANG II infusions, as manifested by a hypertensive response to high-salt intake. This study was performed to determine the residual cytokines and chemokines following the cessation of ANG II infusion. Male Sprague-Dawley rats, maintained on a normal diet, received either a sham operation or continuous ANG II infusion (120 ng/min) subcutaneously via minipumps. The ANG II-infused rats were further subdivided into three subgroups. Minipumps were removed on day 12 with subsequent harvesting of kidneys at 0, 3, and 6 days after cessation of ANG II infusion. After 12 days of ANG II infusion, systolic blood pressure, interstitial fibrosis, preglomerular hypertrophy, and interstitial macrophage infiltration were significantly enhanced compared with the shams. By 3 days following the cessation of ANG II infusion, systolic blood pressure was normalized; however, interstitial fibrosis and preglomerular hypertrophy were still present. Furthermore, increased interstitial macrophage infiltration was still present 6 days after cessation of ANG II infusion. Importantly, augmented mRNA levels of monocyte chemotactic protein (MCP)-1 (1.55 ± 0.15 vs. 1.00 ± 0.13, relative ratio) and transforming growth factor (TGF)-β1 (1.52 ± 0.16 vs. 1.00 ± 0.08) persisted 6 days after the withdrawal of ANG II infusion (1.60 ± 0.20 for MCP-1 and 1.43 ± 0.17 for TGF-β1). Thus, the ANG II-induced activation of MCP-1 and TGF-β1 is sustained and may account for the persistent effect of chronic ANG II infusions on interstitial macrophage infiltration, suggesting a possible mechanism for the development of salt sensitivity in ANG II-dependent hypertension.
kidney; tissue injury; cytokines; chemokines; residual effects
Angiotensin (Ang) II-infused hypertensive rats exhibit increases in renal angiotensinogen mRNA and protein, as well as urinary angiotensinogen excretion in association with increased intrarenal Ang II content. The present study was performed to determine if the augmentation of intrarenal angiotensinogen requires activation of Ang II type 1 (AT1) receptors. Male Sprague-Dawley rats (200 to 220 g) were divided into 3 groups: sham surgery (n=10), subcutaneous infusion of Ang II (80 ng/min, n=11), and Ang II infusion plus AT1 blocker (ARB), olmesartan (5 mg/d, n=12). Ang II infusion progressively increased systolic blood pressure (SBP) compared with sham (178±8 mm Hg versus119±4 at day 11). ARB treatment prevented hypertension (113±6 at day 11). Twenty-four-hour urine collections were taken at day 12, and plasma and tissue samples were harvested at day 13. The Ang II+ARB group had a significant increase in plasma Ang II compared with Ang II and sham groups (365±46 fmol/mL versus 76±9 and 45±14, respectively). Nevertheless, ARB treatment markedly limited the enhancement of kidney Ang II by Ang II infusion (65±17 fmol/g in sham, 606±147 in Ang II group, and 288±28 in Ang II+ARB group). Ang II infusion significantly increased kidney angiotensinogen compared with sham (1.69±0.21 densitometric units versus 1.00±0.17). This change was reflected by increased angiotensinogen immunostaining in proximal tubules. ARB treatment prevented this increase (1.14±0.12). Urinary angiotensinogen excretion rates were enhanced 4.7× in Ang II group (4.67±0.41 densitometric units versus 1.00±0.21) but ARB treatment prevented the augmentation of urinary angiotensinogen (0.96±0.23). These data demonstrate that augmentation of intrarenal angiotensinogen in Ang II-infused rats is AT1-dependent and provide further evidence that urinary angiotensinogen is closely linked to intrarenal Ang II in Ang II-dependent hypertension.
angiotensin II; angiotensinogen; receptors; angiotensin II; rats; kidney
AngII (angiotensin II) induces atherosclerosis and AAAs (abdominal aortic aneurysms) through multiple proposed mechanisms, including chemotaxis. Therefore, we determined the effects of whole-body deficiency of the chemokine receptor CCR2 (CC chemokine receptor 2) on these diseases. To meet this objective, apoE (apolipoprotein E)−/− mice that were either CCR2+/+ or CCR2−/−, were infused with either saline or AngII (1000 ng·kg−1 of body weight·min−1) for 28 days via mini-osmotic pumps. Deficiency of CCR2 markedly attenuated both atherosclerosis and AAAs, unrelated to systolic blood pressure or plasma cholesterol concentrations. During the course of the present study, we also observed that AngII infusion led to large dilatations that were restricted to the ascending aortic region of apoE−/− mice. The aortic media in most of the dilated area was thickened. In regions of medial thickening, distinct elastin layers were discernable. There was an expansion of the distance between elastin layers in a gradient from the intimal to the adventitial aspect of the media. This pathology differed in a circumscribed area of the anterior region of ascending aortas in which elastin breaks were focal and almost transmural. All regions of the ascending aorta of AngII-infused mice had diffuse medial macrophage accumulation. Deficiency of CCR2 greatly attenuated the AngII-induced lumen dilatation in the ascending aorta. This new model of ascending aortic aneurysms has pathology that differs markedly from AngII-induced atherosclerosis or AAAs, but all vascular pathologies were attenuated by CCR2 deficiency.
angiotensin; aneurysm; aorta; chemokine; elastin; macrophage; AngII, angiotensin II; AAA, abdominal aortic aneurysm; apoE, apolipoprotein E; AT1 receptor, AngII type 1 receptor; CCR2, CC chemokine receptor 2; MCP, monocyte chemoattractant protein; TGF-β, transforming growth factor-β
In chronic heart failure (CHF), arterial baroreflex function is impaired, in part, by activation of the central renin-angiotensin system. A metabolite of Angiotensin II (Ang II), Ang-(1–7), has been shown to exhibit cardiovascular effects that are in opposition to that of Ang II. However, the action of Ang-(1–7) on sympathetic outflow and baroreflex function is not well understood, especially in CHF. The aim of this study was to determine the effect of intracerebroventricular infusion of Ang-(1–7) on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in conscious rabbits with CHF. We hypothesized that central Ang-(1–7) would improve baroreflex function in CHF. Ang-(1–7) (2 nmol/1 μl/hour) or artificial cerebrospinal fluid (1 μl/hour) was infused by an osmotic mini-pump for 4 days in sham and pacing-induced CHF rabbits (n=3–6/group). Ang-(1–7) treatment had no effects in sham rabbits but reduced HR and increased baroreflex gain (7.4±1.5 bpm/mm Hg vs. 2.5±0.4 bpm/mm Hg, P<0.05) in CHF rabbits. The Ang-(1–7) antagonist A779 (8 nmol/1 μl/hr) blocked the improvement in baroreflex gain in CHF. Baroreflex gain increased in CHF+Ang-(1–7) animals when only the vagus was allowed to modulate baroreflex control by acute treatment with the β-1 antagonist metoprolol, indicating increased vagal tone. Baseline RSNA was significantly lower and baroreflex control of RSNA was enhanced in CHF rabbits receiving Ang-(1–7). These data suggest that augmentation of central Ang-(1–7) inhibits sympathetic outflow and increases vagal outflow in CHF thus contributing to enhanced baroreflex gain in this disease state.
angiotensin-(1–7); heart failure; sympathetic nervous system; baroreflex; vagus nerve; blood pressure; heart rate
This study determined the mechanisms and time-course of recovery of vascular relaxation in middle cerebral arteries (MCA) of salt-fed Sprague-Dawley rats returned to low salt (LS) diet (0.4% NaCl) or infused with low-dose angiotensin II (ANG II).
Rats were fed high salt (HS) diet (4% NaCl) for 3 days or 4 weeks before return to LS diet for various periods. Other rats fed HS diet (HS + ANG II) received a chronic (3 days) i.v. infusion of a low dose of ANG II (5 ng·kg−1·min−1) to prevent salt-induced ANG II suppression.
HS diet eliminated the increase in cerebral blood flow in response to acetylcholine (ACh) infusion and the relaxation of MCA in response to ACh, iloprost, cholera toxin, and reduced PO2. Recovery of vascular relaxation was slow, requiring at least 2 weeks of LS diet, regardless of the duration of exposure to HS diet. Hypoxic dilation was mediated by cyclooxygenase metabolites and ACh-induced dilation was mediated via NO in LS rats and in HS rats returned to LS diet or receiving ANG II infusion.
Return to LS diet for 2 weeks or chronic 3-day ANG II infusion restore the mechanisms that normally mediate cerebral vascular relaxation.
angiotensin II; salt; vascular relaxation; vasodilation; endothelium; vascular smooth muscle
Chronic infusion of angiotensin (Ang) II leads to the development of hypertension and enhances intrarenal Ang II content to levels greater than can be explained from the circulating concentrations of the peptide. We previously reported that renal angiotensinogen (Ao) mRNA is enhanced in Ang II–dependent hypertension and may contribute to augmented intrarenal Ang II levels, but the Ao protein levels were not significantly increased. Because a high-salt diet (H/S) has been shown to suppress renal expression of Ao mRNA, we examined the effects of chronic Ang II infusion on kidney and liver Ao mRNA and protein levels in male Sprague-Dawley rats (n=12) maintained on an 8% salt diet. Ang II was administered via osmotic minipumps (40 ng/min) to 1 group (n=6) while the remaining rats were sham-operated. A H/S diet alone did not alter systolic blood pressure in sham animals (109±6 mm Hg at day 12); however, Ang II infusions to the H/S rats significantly increased systolic blood pressure (167±7 at day 12) and intrarenal Ang II content (459±107 fmol/g versus 270±42) despite a marked suppression of plasma renin activity (0.9±0.2 ng Ang I·mL−1·h−1 versus 2.8±1.3). Ang II infusions significantly increased kidney Ao mRNA compared with the H/S diet alone by 1.9±0.1-fold. Western blot analysis of kidney protein extracts showed that the Ang II–infused rats had increased kidney Ao protein levels compared with the H/S diet alone (1.9±0.1-fold). Liver Ao mRNA and protein and plasma Ao protein were also significantly increased by Ang II infusions. These data demonstrate the effects of Ang II infusion to stimulate Ao mRNA and protein. Thus, the augmented intrarenal Ang II in Ang II–dependent hypertension may result, in part, by a positive amplification mechanism to activate renal expression of Ao.
angiotensin II; angiotensinogen; kidney; sodium, dietary; Western blot; reverse transcriptase–polymerase chain reaction
Distal nephron renin may provide a possible pathway for angiotensin (Ang) I generation from proximally delivered angiotensinogen. To examine the effects of Ang II on distal nephron renin, we compared renin protein and mRNA expression in control and Ang II—infused rats. Kidneys from sham (n=9) and Ang II—infused (80 ng/kg per minute, 13 days, n=10) Sprague-Dawley rats were processed by immunohistochemistry, Western blot, reverse transcriptase—polymerase chain reaction (RT-PCR), and quantitative real-time RT-PCR. Ang II infusion increased systolic blood pressure (181±4 versus 115±5 mm Hg) and suppressed plasma and kidney cortex renin activity. Renin immunoreactivity was suppressed in juxtaglomerular apparatus (JGA) cells in Ang II—infused rats compared with sham (0.1±0.1 versus 1.0±0.1 relative ratio) but increased in distal nephron segments (6.4±1.4 versus 1.0±0.1 cortex; 2.5±0.3 versus 1.0±0.2 medulla). Tubular renin immunostaining was apically distributed in principal cells colocalizing with aquaporin-2 in connecting tubules and cortical and medullary collecting ducts. Renin protein levels were decreased in the kidney cortex of Ang II—infused rats compared with that of sham (0.4±0.2 versus 1.0±0.4) rats but higher in the kidney medulla (1.2±0.4 versus 1.0±0.1). In kidney medulla, RT-PCR and quantitative real-time PCR showed similar levels of renin transcript in both groups. In summary, the detection of renin mRNA in the renal medulla, which is devoid of JGA, indicates local synthesis rather than an uptake of JGA renin. In contrast to the inhibitory effect of Ang II on JGA renin, Ang II infusion stimulates renin protein expression in collecting ducts and maintains renin transcriptional levels in the medulla, which may contribute to the increased intrarenal Ang II levels in Ang II—dependent hypertension.
renin; angiotensin II; angiotensinogen; immunohistochemistry; Western blot
Peroxisome proliferator-activated receptor gamma (PPARγ) agonists have beneficial effects on renal structure and function in models of diabetes and chronic kidney diseases. However, the increased incidence of weight gain and edema potentially limits their usefulness. We studied an acute minimal-change disease-like nephrotic syndrome model to assess effects of PPARγ agonist on acute podocyte injury and effects on fluid homeostasis.
Acute podocyte injury and nephrotic syndrome were induced by puromycin aminonucleoside (PAN) injection in rats.
PPARγ agonist, given at the time or after, but not before PAN, reduced proteinuria, restored synaptopodin, decreased desmin and trended to improve foot process effacement. There was no significant difference in glomerular filtration, effective circulating volume, blood pressure or fractional sodium excretion. PAN-injured podocytes had decreased PPARγ, less nephrin and α-actinin-4, more apoptosis and reduced phosphorylated Akt. In PAN-injured cultured podocytes, PPARγ agonist also reversed abnormalities only when given simultaneously or after injury.
These results show that PPARγ agonist has protective effects on podocytes in acute nephrotic syndrome without deleterious effects on fluid homeostasis. PPARγ agonist-induced decrease in proteinuria in acute nephrotic syndrome is dependent at least partially on regulation of peroxisome proliferator-response element-sensitive gene expression such as α-actinin-4 and nephrin and the restoration of podocyte structure.
nephrotic syndrome; podocytes; PPARγ; puromycin aminonucleoside
Podocytes are specialized epithelial cells covering the basement membrane of the glomerulus in the kidney. The molecular mechanisms underlying the role of podocytes in glomerular filtration are still largely unknown. We generated podocin-deficient (Nphs2−/−) mice to investigate the function of podocin, a protein expressed at the insertion of the slit diaphragm in podocytes and defective in a subset of patients with steroid-resistant nephrotic syndrome and focal and segmental glomerulosclerosis. Nphs2−/− mice developed proteinuria during the antenatal period and died a few days after birth from renal failure caused by massive mesangial sclerosis. Electron microscopy revealed the extensive fusion of podocyte foot processes and the lack of a slit diaphragm in the remaining foot process junctions. Using real-time PCR and immunolabeling, we showed that the expression of other slit diaphragm components was modified in Nphs2−/− kidneys: the expression of the nephrin gene was downregulated, whereas that of the ZO1 and CD2AP genes appeared to be upregulated. Interestingly, the progression of the renal disease, as well as the presence or absence of renal vascular lesions, depends on the genetic background. Our data demonstrate the crucial role of podocin in the establishment of the glomerular filtration barrier and provide a suitable model for mapping and identifying modifier genes involved in glomerular diseases caused by podocyte injuries.
Angiotensin II (ANG II)-infused rats exhibit increases in distal nephron renin expressed in principal cells of connecting tubules and collecting ducts. This study was performed to determine whether the augmentation of distal nephron renin involves ANG II type 1 (AT1) receptor activation. Male Sprague-Dawley rats (200 -220 g) were divided into three groups: 1) sham operated (n = 8); 2) ANG II infused (80 ng/min, 13 days, n = 8); and 3) ANG II infused plus AT1 receptor blocker (ARB), olmesartan (5 mg/days, n = 8). ANG II infusion increased systolic blood pressure (BP; 178 ± 4 vs. 122 ± 1 mmHg; P < 0.001) and suppressed plasma renin activity (PRA; 0.08 ± 0.1 vs. 5.3 ± 0.8 ng ANG I · ml-1·h-1). ARB treatment prevented the increase in BP (113 ± 6 mmHg) and led to increases in PRA (15.8 ± 1.5 ng ANG I·ml-1h-1). Renin protein levels measured in the kidney medulla, to avoid contribution from juxtaglomerular appartus cells, were higher in ANG II-infused rats [1.64 ± 0.3 vs. 1.00 ± 0.1 densitometric units (DU) compared with sham-operated rats; P < 0.05], and ARB treatment prevented this increase (1.01 ± 0.1). Similarly, renin immunoreactivity increased in medullary collecting ducts of ANG II-infused compared with sham-operated rats (2.5 ± 0.3 vs. 1.0 ± 0.2 DU; P < 0.001), which was also prevented by ARB (1.01 ± 0.06). Renin qRTPCR in ANG II-infused rats showed higher mRNA levels in the kidney medulla compared with sham-operated rats (5.5 ± 2.3 vs. 0.04 ± 0.02 ratio to GAPDH mRNA levels; P < 0.001); however, renin transcript levels were normalized in the ARB-treated rats. These data demonstrate that the augmentation of distal nephron renin in ANG II-infused hypertensive rats is AT1 receptor mediated. The augmented distal tubular renin may contribute to increased intratubular ANG II levels and distal nephron sodium reabsorption in ANG II-dependent hypertension.
distal nephron; immunohistochemistry; Western blotting; qRT-PCR
The loss of glomerular podocytes is a key event in the progression of chronic kidney disease resulting in proteinuria and declining function. Podocytes are slow cycling cells that are considered terminally differentiated. Here we provide the first report of the directed differentiation of induced pluripotent stem (iPS) cells to generate kidney cells with podocyte features. The iPS-derived podocytes share a morphological phenotype analogous with cultured human podocytes. Following 10 days of directed differentiation, iPS podocytes had an up-regulated expression of mRNA and protein localization for podocyte markers including synaptopodin, nephrin and Wilm’s tumour protein (WT1), combined with a down-regulation of the stem cell marker OCT3/4. In contrast to human podocytes that become quiescent in culture, iPS-derived cells maintain a proliferative capacity suggestive of a more immature phenotype. The transduction of iPS podocytes with fluorescent labeled-talin that were immunostained with podocin showed a cytoplasmic contractile response to angiotensin II (AII). A permeability assay provided functional evidence of albumin uptake in the cytoplasm of iPS podocytes comparable to human podocytes. Moreover, labeled iPS-derived podocytes were found to integrate into reaggregated metanephric kidney explants where they incorporated into developing glomeruli and co-expressed WT1. This study establishes the differentiation of iPS cells to kidney podocytes that will be useful for screening new treatments, understanding podocyte pathogenesis, and offering possibilities for regenerative medicine.
Background and Aims
Rho kinase (ROCK) inhibition reduces systemic blood pressure (BP) and decreases renal damage in animal models of kidney disease. The aim of this study was to determine if ROCK inhibition might have beneficial effects in glomerular disease processes that are independent of systemic BP.
We investigated the effects of the ROCK inhibitor Y27632 and hydralazine in murine puromycin aminonucleoside (PAN) nephrosis.
Treatment with either Y27632 or hydralazine similarly reduced systolic BP compared to vehicle-treated controls. Seven days after treatment with PAN, albuminuria, proteinuria and effacement of podocyte foot processes were significantly reduced in Y27632- and hydralazine-treated mice compared to vehicle-treated animals. Treatment with PAN significantly reduced expression of the podocyte proteins nephrin and Neph1, and the loss of glomerular nephrin was attenuated by treatment with Y27632 but not by treatment with hydralazine. In cultured podocytes, PAN potently activated both Rho and ROCK, and PAN-induced ROCK activation was prevented by Y27632.
The ROCK inhibitor Y27632 attenuated glomerular nephrin loss in murine PAN nephrosis independent of its effects on systemic BP.
Rho GTPases; Glomerulonephritis; Blood pressure; Rho kinase
Angiotensin II (AngII) infusions augment renal angiotensinogen mRNA and protein and urinary angiotensinogen excretion (UAGT). Further experiments were performed in 4 groups of rats: normal salt diet with sham operation, NS+Sham, n=6; NS with AngII infusion at 40 ng/min via osmotic minipump, NS+AngII(40), n=9; NS with AngII infusion at 80 ng/min, NS+AngII(80), n=9; high-salt diet with deoxycorticosterone acetate salt pellet (100 mg), HS+DOCA, n=4. These experiments sought to determine whether enhanced UAGT is specifically associated with increased kidney AngII levels or is a nonspecific consequence of the hypertension. Systolic BP (SBP) was significantly increased to 131±2 and 162±2 mm Hg at day 11 in NS+AngII(40) and NS+AngII(80), respectively, compared with NS+Sham (110±1). Regression analysis demonstrated a positive relationship (R=0.49) between SBP and UAGT for NS+Sham (1.1±0.3 nmol AngI/d), NS+AngII(40) (2.5±0.9), and NS+AngII(80) (5.5±1.5). UAGT was also highly correlated (R=0.70) with kidney AngII content for NS+Sham (49±6 fmol/g), NS+AngII(40) (215±49), and NS+AngII(80) (347±47); but not with plasma AngII (R=0.12). HS+DOCA rats also exhibited increased SBP to 134±1 mm Hg, but UAGT (1.4±0.4 nmol AngI/d) and intrarenal AngII content (13±2 fmol/g) were not increased despite the hypertension. Infused human angiotensinogen could not be detected in urine of sham-operated or AngII-infused rats (n=4 each). These data demonstrate that UAGT increases in AngII-dependent hypertension in a dose- and time-dependent manner, but not in hypertension elicited by HS+DOCA. The results support the hypothesis that AngII-dependent hypertension results in elevated intrarenal AngII and angiotensinogen levels, reflected by increased UAGT, which does not occur in an AngII-independent hypertensive model.
angiotensin II; angiotensinogen; rats; kidney; urine; sodium, dietary; deoxycorticosterone acetate salt; Western blot
We have previously demonstrated that nitric oxide (NO) exerts a greater modulatory influence on renal cortical blood flow in ANG II-infused hypertensive rats compared with normotensive rats. In the present study, we determined nitric oxide synthase (NOS) activities and protein levels in the renal cortex and medulla of normotensive and ANG II-infused hypertensive rats. Enzyme activity was determined by measuring the rate of formation of l-[14C]citrulline from l-[14C]arginine. Western blot analysis was performed to determine the regional expression of endothelial (eNOS), neuronal (nNOS), and inducible (iNOS) isoforms in the renal cortex and medulla of control and ANG II-infused rats. Male Sprague-Dawley rats were prepared by the infusion of ANG II at a rate of 65 ng/min via osmotic minipumps implanted subcutaneously for 13 days and compared with sham-operated rats. Systolic arterial pressures were 127 ± 2 and 182 ± 3 mmHg in control (n = 13) and ANG II-infused rats (n = 13), respectively. The Ca2+-dependent NOS activity, expressed as picomoles of citrulline formed per minute per gram wet weight, was higher in the renal cortex of ANG II-infused rats (91 ± 11) than in control rats (42 ± 12). Likewise, both eNOS and nNOS were markedly elevated in the renal cortex of the ANG II-treated rats. In both groups of rats, Ca2+-dependent NOS activity was higher in the renal medulla than in the cortex; however, no differences in medullary NOS activity were observed between the groups. Also, no differences in medullary eNOS levels were observed between the groups; however, medullary nNOS was decreased by 45% in the ANG II-infused rats. For the Ca2+-independent NOS activities, the renal cortex exhibited a greater activity in the control rats (174 ± 23) than in ANG II-infused rats (101 ± 10). Similarly, cortical iNOS was greater by 47% in the control rats than in ANG II-treated rats. No differences in the activity were found for the renal medulla between the groups. There was no detectable signal for iNOS in the renal medulla for both groups. These data indicate that there is a differential distribution of NOS activity, with the Ca2+-dependent activity and protein expression higher in the renal cortex of ANG II-infused rats compared with control rats, and support the hypothesis that increased constitutive NOS activity exerts a protective effect in ANG II-induced hypertension to maintain adequate renal cortical blood flow.
endothelial nitric oxide synthase; neuronal nitric oxide synthase; inducible nitric oxide synthase; osmotic minipump; angiotensin II-induced hypertension; Western blot
Although it has been shown that up-regulation of hypoxia-inducible factor (HIF)-1α is protective in acute ischemic renal injury, long-term over-activation of HIF-1α is implicated to be injurious in chronic kidney diseases. Angiotensin II (ANG II) is a well-known pathogenic factor producing chronic renal injury and has also been shown to increase HIF-1α. However, the contribution of HIF-1α to ANG II-induced renal injury has not been evidenced. The present study tested the hypothesis that HIF-1α mediates ANG II-induced renal injury in Sprague-Dawley rats. Chronic renal injury was induced by ANG II infusion (200ng/kg/min) for 2 weeks in uninephrectomized rats. Transfection of vectors expressing HIF-1α shRNA into the kidneys knocked down HIF-1α gene expression by 70%, blocked ANG II-induced HIF-1α activation and significantly attenuated ANG II-induced albuminuria, which was accompanied by inhibition of ANG II-induced vascular endothelial growth factor, a known glomerular permeability factor, in glomeruli. HIF-1α shRNA also significantly improved the glomerular morphological damage induced by ANG II. Furthermore, HIF-1α shRNA blocked ANG II-induced upregulation of collagen and α-smooth muscle actin in tubulointerstitial region. There was no difference in creatinine clearance and ANG II-induced increase in blood pressure. HIF-1α shRNA had no effect on ANG II-induced reduction in renal blood flow and hypoxia in the kidneys. These data suggested that over-activation of HIF-1α-mediated gene regulation in the kidney is a pathogenic pathway mediating ANG II-induced chronic renal injuries and normalization of over-activated HIF-1α may be used as a treatment strategy for chronic kidney damages associated with excessive ANG II.
glomerular sclerosis; tubulointerstitial; fibrosis; albuminuria; renal blood flow
The intrarenal renin–angiotensin system contributes to hypertension by regulating sodium and water reabsorption throughout the nephron. Sex differences in the intrarenal components of the renin–angiotensin system have been involved in the greater incidence of high blood pressure and progression to kidney damage in males than females.
This study investigated whether there is a sex difference in the intrarenal gene expression and urinary excretion of angiotensinogen (AGT) during angiotensin II (Ang II)–dependent hypertension and high-salt (HS) diet.
Male and female Sprague-Dawley rats were divided into 5 groups for each sex: Normal-salt control, HS diet (8% NaCl), Ang II–infused (80 ng/min), Ang II–infused plus HS diet, and Ang II–infused plus HS diet and treatment with the Ang II receptor blocker, candesartan (25 mg/L in the drinking water). Rats were evaluated for systolic blood pressure (SBP), kidney AGT mRNA expression, urinary AGT excretion, and proteinuria at different time points during a 14-day protocol.
Both male and female rats exhibited similar increases in urinary AGT, with increases in SBP during chronic Ang II infusion. HS diet greatly exacerbated the urinary AGT excretion in Ang II–infused rats; males had a 9-fold increase over Ang II alone and females had a 2.5-fold increase. Male rats displayed salt-sensitive SBP increases during Ang II infusion and HS diet, and female rats did not. In the kidney cortex, males displayed greater AGT gene expression than females during all treatments. During Ang II infusion, both sexes exhibited increases in AGT gene message compared with same-sex controls. In addition, HS diet combined with Ang II infusion exacerbated the proteinuria in both sexes. Concomitant Ang II receptor blocker treatment during Ang II infusion and HS diet decreased SBP and urinary AGT similarly in both sexes; however, the decrease in proteinuria was greater in the females.
During Ang II–dependent hypertension and HS diet, higher intrarenal renin-angiotensin system activation in males, as reflected by higher AGT gene expression and urinary excretion, indicates a mechanism for greater progression of high blood pressure and might explain the sex disparity in development of salt-sensitive hypertension.
angiotensinogen; intrarenal RAS; mRNA; proteinuria; sex differences; Sprague-Dawley rats; systolic blood pressure
Proteinuria is a primary clinical symptom of a large number of glomerular diseases that progress to end-stage renal failure. Podocyte dysfunctions play a fundamental role in defective glomerular filtration in many common forms of proteinuric kidney disorders. Since binding of these cells to the basement membrane is mediated by integrins, we determined the role of integrin-linked kinase (ILK) in podocyte dysfunction and proteinuria. ILK expression was induced in mouse podocytes by various injurious stimuli known to cause proteinuria including TGF-β1, adriamycin, puromycin, and high ambient glucose. Podocyte ILK was also found to be upregulated in human proteinuric glomerular diseases. Ectopic expression of ILK in podocytes decreased levels of the epithelial markers nephrin and ZO-1, induced mesenchymal markers such as desmin, fibronectin, matrix metalloproteinase-9 (MMP-9), and α-smooth muscle actin (α-SMA), promoted cell migration, and increased the paracellular albumin flux across podocyte monolayers. ILK also induced Snail, a key transcription factor mediating epithelial–mesenchymal transition (EMT). Blockade of ILK activity with a highly selective small molecule inhibitor reduced Snail induction and preserved podocyte phenotypes following TGF-β1 or adriamycin stimulation. In vivo, this ILK inhibitor ameliorated albuminuria, repressed glomerular induction of MMP-9 and α-SMA, and preserved nephrin expression in murine adriamycin nephropathy. Our results show that upregulation of ILK is a convergent pathway leading to podocyte EMT, migration, and dysfunction. ILK may be an attractive target for therapeutic intervention of proteinuric kidney diseases.
adriamycin; integrin-linked kinase; podocyte; proteinuria
Microalbuminuria is an early lesion during the development of diabetic nephropathy. The loss of high molecular weight proteins in the urine is usually associated with decreased expression of slit diaphragm proteins. Nephrin, is the major component of the glomerular slit diaphragm and loss of nephrin has been well described in rodent models of experimental diabetes as well as in human diabetic nephropathy.
In this manuscript we analyzed the role of PKC-alpha (PKCα) on endocytosis of nephrin in podocytes. We found that treatment of diabetic mice with a PKCα-inhibitor (GÖ6976) leads to preserved nephrin expression and reduced proteinuria. In vitro, we found that high glucose stimulation would induce PKCα protein expression in murine and human podocytes. We can demonstrate that PKCα mediates nephrin endocytosis in podocytes and that overexpression of PKCα leads to an augmented endocytosis response. After PKC-activation, we demonstrate an inducible association of PKCα, PICK1 and nephrin in podocytes. Moreover, we can demonstrate a strong induction of PKCα in podocytes of patients with diabetic nephropathy.
We therefore conclude that activation of PKCα is a pathomechanistic key event during the development of diabetic nephropathy. PKCα is involved in reduction of nephrin surface expression and therefore PKCα inhibition might be a novel target molecule for anti-proteinuric therapy.