The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.

Cardiovascular interactions between losartan and fructose in mice

AIM

To determine whether pharmacological blockade of angiotensin (Ang) AT1 receptors alters the cardiovascular, metabolic, and angiotensin-converting enzyme (ACE and ACE2) responses to a fructose diet in mice.

METHODS

C57BL male mice were fed with a 60% fructose diet for 8 weeks in combination with losartan treatment on week 9 (30 mg/kg per day). Blood pressure (BP), heart rate (HR), and autonomic balance were monitored using radiotelemetry with spectral analysis. Renal ACE and ACE2 activity and protein levels as well as Ang II and Ang 1-7 were measured.

RESULTS

Fructose impaired glucose tolerance and increased plasma cholesterol and insulin. These effects were not corrected by losartan treatment. Fructose increased BP and HR but only during the dark period. Short-term losartan treatment decreased BP by 16% in the fructose group but had no effect in controls. This was accompanied by a decrease in BP variance and its low-frequency component. Fructose increased Ang II (plasma and kidney) and ACE 2 (renal activity and protein expression). Losartan alone increased plasma Ang II in plasma and ACE2 in kidney. There were no changes in renal Ang 1-7 levels.

CONCLUSIONS

Losartan reversed the pressor effect of a high fructose diet, demonstrating that there are prominent interactions between a dietary regimen that produces glucose intolerance and an antihypertensive drug that antagonizes Ang signaling. The mechanism of change may be via renal Ang II rather than the ACE2/Ang 1-7 pathway because the fructose losartan combination resulted in lowered renal Ang II without changes in Ang 1-7.

Angiotensin-converting enzyme inhibition modulates mitogen-activated protein kinase family expressions in the neonatal rat kidney

Among the mitogen-activated protein kinase (MAPK) family members, extracellular signal-regulated kinase (ERK) promotes cell proliferation or differentiation, whereas c-jun N terminal kinase (JNK) and p38 MAPK are thought to inhibit cell growth and induce apoptosis. The MAPK family may plays some role during kidney development, when large-scale proliferation and apoptosis have been observed to occur. Also, in this period, the renin-angiotensin system is markedly activated. We have demonstrated that angiotensin-converting enzyme inhibition in the developing rat kidney increases apoptosis and decreases cell proliferation, which may account for renal growth impairment. The aim of this study, therefore, was to examine the relationship between the MAPK family and renin-angiotensin system during neonatal renal development. Newborn rat pups were treated with enalapril (30 mg . kg(-1) . d(-1)) or normal saline for 7 d. Right kidneys of both groups were selected for immunohistochemical stains of MAPKs and activating transcription factor-2 (ATF-2), and left kidneys were selected for reverse transcriptase-PCR and immunoblot analysis of MAPKs, phospho-MAPKs, and ATF-2. To determine whether apoptosis is involved in the same tubules that highly expressed JNK and p38, we performed terminal deoxynucleotide transferase-mediated nick-end labeling stain for apoptotic cells and immunohistochemical stains for JNK-2, p38, and ATF-2 expression in the serial sections from the same kidney of the enalapril-treated group. In the enalapril-treated group, JNK-2, p38, phospho-JNK-2, phospho-p38, and ATF-2 protein expressions were significantly increased, and their immunoactivities were strongly detected in the proximal tubular epithelial cells in the cortex, compared with the control group. Especially JNK-2 and p38 expressions were highly activated and were spatially in accordance with the occurrence of apoptosis. ERK1/2 and phospho-ERK expressions were not changed by enalapril. These results suggest that the expressions of the MAPK family are modulated by angiotensin-converting enzyme inhibition in the developing kidney. JNK and p38 may be implicated to participate in angiotensin II-related intracellular signaling pathways of renal apoptosis in the developing kidney.

Mitogen-induced activation of Na+/H+ exchange in vascular smooth muscle cells involves janus kinase 2 and Ca2+/calmodulin

The sodium/proton exchanger type 1 (NHE-1) plays an important role in the proliferation of vascular smooth muscle cells (VSMC). We have examined the regulation of NHE-1 by two potent mitogens, serotonin (5-HT, 5-hydroxytryptamine) and angiotensin II (Ang II), in cultured VSMC derived from rat aorta. 5-HT and Ang II rapidly activated NHE-1 via their G protein-coupled receptors (5-HT(2A) and AT(1)) as assessed by proton microphysiometry of quiescent cells and by measurements of intracellular pH on a FLIPR (fluorometric imaging plate reader). Activation of NHE-1 was blocked by inhibitors of phospholipase C, CaM, and Jak2 but not by pertussis toxin or inhibitors of protein kinase C. Immunoprecipitation/immunoblot studies showed that 5-HT and Ang II induce phosphorylation of Jak2 and induce the formation of signal transduction complexes that included Jak2, CaM, and NHE-1. The cell-permeable Ca(2+) chelator BAPTA-AM blocked activation of Jak2, complex formation between Jak2 and CaM, and tyrosine phosphorylation of CaM, demonstrating that elevated intracellular Ca(2+) is essential for those events. Thus, mitogen-induced activation of NHE-1 in VSMC is dependent upon elevated intracellular Ca(2+) and is mediated by the Jak2-dependent tyrosine phosphorylation of CaM and subsequent increased binding of CaM to NHE-1, similar to the pathway previously described for the bradykinin B(2) receptor in inner medullary collecting duct cells of the kidney [Mukhin, Y. V., et al. (2001) J. Biol. Chem. 276, 17339-17346]. We propose that this pathway represents a fundamental mechanism for the rapid regulation of NHE-1 by G(q/11) protein-coupled receptors in multiple cell types.

ANG II AT(1) and AT(2) receptors in developing kidney of normal microswine

To identify an appropriate model of human renin-angiotensin system (RAS) involvement in fetal origins of adult disease, we quantitated renal ANG II AT(1) and AT(2) receptors (AT1R and AT2R, respectively) in fetal (90-day gestation, n = 14), neonatal (3-wk, n = 5), and adult (6-mo, n = 8) microswine by autoradiography ((125)I-labeled [Sar(1)Ile(8)]ANG II+cold CGP-42112 for AT1R, (125)I-CGP-42112 for AT2R) and by whole kidney radioligand binding. The developmental pattern of renal AT1R in microswine, like many species, exhibited a 10-fold increase postnatally (P < 0.001), with maximal postnatal density in glomeruli and lower density AT1R in extraglomerular cortical and outer medullary sites. With aging, postnatal AT1R glomerular profiles increased in size (P < 0.001) and fractional area occupied (P < 0.04), with no change in the number per unit area. Cortical levels of AT2R by autoradiography fell with age from congruent with 5,000 fmol/g in fetal kidneys to congruent with 60 and 20% of fetal levels in neonatal and adult cortex, respectively (P < 0.0001). The pattern of AT2R binding in postnatal pig kidney mimicked that described in human and simian, but not rodent, species: dense AT2R confined to discrete cortical structures, including pre- and juxtaglomerular, but not intraglomerular, vasculature. Our results provide a quantitative assessment of ANG II receptors in developing pig kidney and document the concordance of pigs and primates in developmental regulation of renal AT1R and AT2R.

Recent developments in gene therapy for cardiac disease

Cardiovascular[TRACE;del] disease is the leading cause of death in the US and world-wide. Advances in molecular biology and the human genome project have revealed opportunities for novel strategies for cardiac gene therapy. This review discusses general and specific aspects of gene transfer strategies in cardiac tissues. These include 1) the selection and/or optimization of the vector for gene transfer; 2) the identification of the target gene(s); 3) the use of cardiac-specific promoters; and 4) the use of an appropriate delivery system for administration. Currently, several vectors (e.g., viral and nonviral vectors) have been developed and many target genes have been identified (e.g., VEGF, FGF, beta-AR, etc.). Many investigations have provided experimental models for gene delivery systems but the most efficient cardiac gene transfer was obtained from intramyocardial injection or perfusion of explanted myocardium. The data available thus far have suggested favorable immediate effects following gene transfer, but long-term value of cardiac gene therapy has not been proven. Further refinements in appropriate vectors that provide cell or tissue selectivity and long-lasting effects are necessary as well as the development of minimally invasive procedures for gene transfer.

Angiotensin II infusion to the midgestation ovine fetus: effects on the fetal kidney

Renal and cardiovascular responses to an intravenous infusion of ANG II (1 microg/h) or saline for 3 days were examined in ovine fetuses at midgestation (75-85 days of gestation, term 150 days). ANG II caused an increase in fetal blood pressure (36 +/- 2 to 44 +/- 3 mmHg) and urine flow rate (8 +/- 2 to a maximum of 18 +/- 6 ml/h). Plasma renin concentrations decreased in ANG II-infused fetuses. Fetal fluids (amniotic and allantoic) did not differ in volume or composition between the groups when measured at postmortem. There was no difference in the expression levels of the mRNA for the angiotensin (AT(1) or AT(2)) receptors between the two groups when measured by an RNase protection assay. However, there was a significant decline in renin and AT(1) receptor gene expression when measured by a real-time polymerase chain reaction method. These results indicate that ANG II is diuretic and pressor when infused at midgestation. ANG II can feedback to decrease renin secretion by the fetal kidney, and this may occur by decreased renin gene expression.

Differential actions of renal ischemic injury on the intrarenal angiotensin system

The present study determined the effect of either occlusion of the left renal artery for 60 min (ischemia) or sham operation on angiotensin (ANG) receptors and tissue and urinary levels of ANG peptides between 24 and 72 h recovery in male Sprague-Dawley rats. At 24 h postischemia, urinary concentrations of ANG I and ANG-(1-7) rose by an average of 83 and 64%, respectively (P < 0.05) but had declined to control levels by 72 h. Tissue ANG II rose at 24 h in postischemic kidneys by an average of 63% compared with the contralateral nonischemic kidney (P < 0.05). Whereas the enzymatic activity of angiotensin-converting enzyme and neprilysin was reduced after ischemia, renal renin activity in ischemic kidneys rose by 74% compared with sham-operated kidneys. Receptor autoradiography using (125)I-labeled [Sar(1),Thr(8)]ANG II ((125)I-Sarthran) (0.8 nM) revealed a decreased apparent density of ANG receptors (>80% AT(1)) in ischemic kidneys with a trend for a decrease in the contralateral nonischemic kidneys compared with the kidneys from sham-operated rats. Twenty-four hours after ischemia, ANG II receptors decreased by 68% in glomeruli (P < 0.05), 49% in the outer cortical tubulointerstitial area (P < 0.05), and 48% in the inner cortical-outer medullary area of the vasa recta (P < 0.05). Medullary binding decreased approximately 50% in both the ischemic kidney and the contralateral nonischemic kidney compared with sham. In all regions of the ischemic kidney, receptors recovered by 72 h to levels not different from sham control rats. The marked change in urinary ANG I and ANG-(1-7) at 24 h following occlusion indicates these peptides may be potential urinary markers for acute renal ischemia. The reduction of receptors in vascular and tubular regions of the ischemic kidney provides a mechanism for the loss of vasoconstrictor responses to ANG II following ischemia previously reported by others.

The renin-angiotensin system and the development of the kidney and adrenal in sheep

1. The earliest form of the kidney, the pronephros, does not really occur in the ovine embryo; instead, a giant glomerulus forms at the anterior end of the mesonephros. 2. In the sheep, the mesonephros is present from 11-38% of total gestation (150 days) and produces a dilute urine, as well as expressing the genes for erythropoietin, renin, angiotensinogen, angiotensin-converting enzyme and the angiotensin II (AngII) receptors AT1 and AT2. 3. The ovine metanephros begins to develop at 18% of gestation and nephrogenesis is complete several weeks before birth. All components of the renin-angiotensin system (RAS) are expressed from at least 27% of gestation. 4. Both AT1 and AT2 receptors are expressed by the adrenocortical cells early in gestation but, at mid-gestation, exogenous AngII does not stimulate aldosterone secretion in vivo. 5. Preliminary results suggest that AngII has important roles in renal development in the ovine foetus but the role(s), if any, in adrenal development, remains to be investigated.

Ontogeny of angiotensin II receptors, types 1 and 2, in ovine mesonephros and metanephros

By RNAse protection assay, hybridization histochemistry, and in vitro autoradiography it was shown that both mRNA and protein for AT1 and AT2 receptors were present in ovine fetal meso- and metanephroi at 40 days of gestation (term approximately 150 days). AT1 mRNA was localized to presumptive mesangial cells of glomeruli at 40-, 75-, 131-gestational-day-old fetuses and two-day-old lambs, in addition to being widely present in interstitial cells of the cortex and medulla, once these zones formed (60 days). By two days after birth the medullary AT1 distribution was confined to the inner stripe of the outer medulla. AT2 mRNA was present in peripheral interstitial/tissue of the mesonephros, and interstitial tissue surrounding developing glomeruli, but not the outermost nephrogenic mesenchyme in the metanephros from 40 to approximately 131 days (the period of active nephrogenesis). In addition, AT2 mRNA was localized to epithelial cells of the macula densa in metanephroi (40 to 131 gestational days) during, but not after completion, of nephrogenesis. These studies suggest that angiotensin II (Ang II) could have differentiating effects, via AT1 receptors, from very early in development. The unique epithelial site of AT2 expression in the macula densa raises the possibility that Ang II may play a role in the invariant positioning of the macula densa at the pole of its glomerulus, via this receptor.