Endocrinological abnormalities in angiotensinogen-gene knockout mice: studies of hormonal responses to dietary salt loading

Sodium appetite and thirst do not require angiotensinogen production in astrocytes or hepatocytes

In addition to its renal and cardiovascular functions, angiotensin signalling is thought to be responsible for the increases in salt and water intake caused by hypovolaemia. However, it remains unclear whether these behaviours require angiotensin production in the brain or liver. Here, we use in situ hybridization to identify tissue-specific expression of the genes required for producing angiotensin peptides, and then use conditional genetic deletion of the angiotensinogen gene (Agt) to test whether production in the brain or liver is necessary for sodium appetite and thirst. In the mouse brain, we identified expression of Agt (the precursor for all angiotensin peptides) in a large subset of astrocytes. We also identified Ren1 and Ace (encoding enzymes required to produce angiotensin II) expression in the choroid plexus, and Ren1 expression in neurons within the nucleus ambiguus compact formation. In the liver, we confirmed that Agt is widely expressed in hepatocytes. We next tested whether thirst and sodium appetite require angiotensinogen production in astrocytes or hepatocytes. Despite virtually eliminating expression in the brain, deleting astrocytic Agt did not reduce thirst or sodium appetite. Despite markedly reducing angiotensinogen in the blood, eliminating Agt from hepatocytes did not reduce thirst or sodium appetite, and in fact, these mice consumed the largest amounts of salt and water after sodium deprivation. Deleting Agt from both astrocytes and hepatocytes also did not prevent thirst or sodium appetite. Our findings suggest that angiotensin signalling is not required for sodium appetite or thirst and highlight the need to identify alternative signalling mechanisms. KEY POINTS: Angiotensin signalling is thought to be responsible for the increased thirst and sodium appetite caused by hypovolaemia, producing elevated water and sodium intake. Specific cells in separate brain regions express the three genes needed to produce angiotensin peptides, but brain-specific deletion of the angiotensinogen gene (Agt), which encodes the lone precursor for all angiotensin peptides, did not reduce thirst or sodium appetite. Double-deletion of Agt from brain and liver also did not reduce thirst or sodium appetite. Liver-specific deletion of Agt reduced circulating angiotensinogen levels without reducing thirst or sodium appetite. Instead, these angiotensin-deficient mice exhibited an enhanced sodium appetite. Because the physiological mechanisms controlling thirst and sodium appetite continued functioning without angiotensin production in the brain and liver, understanding these mechanisms requires a renewed search for the hypovolaemic signals necessary for activating each behaviour.

Exploration of cardiometabolic and developmental significance of angiotensinogen expression by cells expressing the leptin receptor or agouti-related peptide

Angiotensin II (ANG II) Agtr1a receptor (AT_1A) is expressed in cells of the arcuate nucleus of the hypothalamus that express the leptin receptor (Lepr) and agouti-related peptide (Agrp). Agtr1a expression in these cells is required to stimulate resting energy expenditure in response to leptin and high-fat diets (HFDs), but the mechanism activating AT_1A signaling by leptin remains unclear. To probe the role of local paracrine/autocrine ANG II generation and signaling in this mechanism, we bred mice harboring a conditional allele for angiotensinogen (Agt, encoding AGT) with mice expressing Cre-recombinase via the Lepr or Agrp promoters to cause cell-specific deletions of Agt (Agt^Lepr-KO and Agt^Agrp-KO mice, respectively). Agt^Lepr-KO mice were phenotypically normal, arguing against a paracrine/autocrine AGT signaling mechanism for metabolic control. In contrast, Agt^Agrp-KO mice exhibited reduced preweaning survival, and surviving adults exhibited altered renal structure and steroid flux, paralleling previous reports of animals with whole body Agt deficiency or Agt disruption in albumin (Alb)-expressing cells (thought to cause liver-specific disruption). Surprisingly, adult Agt^Agrp-KO mice exhibited normal circulating AGT protein and hepatic Agt mRNA expression but reduced Agt mRNA expression in adrenal glands. Reanalysis of RNA-sequencing data sets describing transcriptomes of normal adrenal glands suggests that Agrp and Alb are both expressed in this tissue, and fluorescent reporter gene expression confirms Cre activity in adrenal gland of both Agrp-Cre and Alb-Cre mice. These findings lead to the iconoclastic conclusion that extrahepatic (i.e., adrenal) expression of Agt is critically required for normal renal development and survival.

Synthesis and secretion of renin in mice with induced genetic mutations

The juxtaglomerular (JG) cell product renin is rate limiting in the generation of the bioactive octapeptide angiotensin II. Rates of synthesis and secretion of the aspartyl protease renin by JG cells are controlled by multiple afferent and efferent pathways originating in the CNS, cardiovascular system, and kidneys, and making critical contributions to the maintenance of extracellular fluid volume and arterial blood pressure. Since both excesses and deficits of angiotensin II have deleterious effects, it is not surprising that control of renin is secured by a complex system of feedforward and feedback relationships. Mice with genetic alterations have contributed to a better understanding of the networks controlling renin synthesis and secretion. Essential input for the setting of basal renin generation rates is provided by β-adrenergic receptors acting through cyclic adenosine monophosphate, the primary intracellular activation mechanism for renin mRNA generation. Other major control mechanisms include COX-2 and nNOS affecting renin through PGE2, PGI2, and nitric oxide. Angiotensin II provides strong negative feedback inhibition of renin synthesis, largely an indirect effect mediated by baroreceptor and macula densa inputs. Adenosine appears to be a dominant factor in the inhibitory arms of the baroreceptor and macula densa mechanisms. Targeted gene mutations have also shed light on a number of novel aspects related to renin processing and the regulation of renin synthesis and secretion.

Expression of Cyclooxygenase-2 in the Juxtaglomerular Apparatus of Angiotensinogen Gene-Knockout Mice

AIMS

The present study was designed to examine the role of the renin-angiotensin system in the regulation of macula densa cyclooxygenase-2 (COX-2) during altered dietary salt intake.

METHODS

We investigated COX-2 expression in the macula densa of angiotensinogen gene-knockout (Atg-/-) mice. COX-2 expression in the renal cortex was determined by real-time quantitative reverse transcription-polymerase chain reaction and immunohistochemistry.

RESULTS

The renal cortical expression of COX-2 mRNA increased 24.7 times in Atg-/- mice compared with Atg+/+ mice. When Atg-/- mice were fed a high-salt diet (4% NaCl) for 10 days, the levels of COX-2 expression were markedly suppressed. The macula densa COX-2 immunoreactivity was correlated with the mRNA expression. The selective inhibition of neuronal isoform of nitric oxide synthase (N-NOS) activity by 7-nitroindazole significantly reduced the levels of COX-2 mRNA in Atg-/- mice by 54.1%.

CONCLUSION

These results suggest that (1) COX-2 activity in the macula densa can be regulated by salt intake through a mechanism independent of the renin-angiotensin system, and (2) COX-2 expression is functionally linked to renal cortical N-NOS activity in Atg-/- mice.

Expression of endothelial nitric oxide synthase is suppressed in the renal vasculature of angiotensinogen-gene knockout mice

We have attempted to elucidate the mechanism by which endothelial-type nitric oxide synthase (eNOS) is regulated in the kidney, with special reference to the role of renal hemodynamics and angiotensin II (Ang II). We compared angiotensinogen gene knockout (Atg-/-) mice, which lacked Ang II (resulting in sodium/water depletion and severe hypotension), with wild-type (Atg+/+) mice. Using Western blot analysis and the NADPH diaphorase histochemical reaction, we found that the expression and activity of eNOS were markedly lower in the renal vessels of Atg-/- mice compared with wild-type (Atg+/+) mice. Dietary salt loading significantly enhanced renal eNOS levels and increased blood pressure in Atg-/- mice, but severe hypotension almost abolished the effects of salt loading. In contrast, in Atg+/+ mice, altered salt intake or hydralazine had no effect on renal eNOS levels. These results suggest that perfusion pressure plays an essential role in maintaining renal vascular eNOS activity, whereas Ang II plays a supportive role, especially when renal circulation is impaired.

Cardiovascular and renal phenotyping of genetically modified mice: a challenge for traditional physiology

1. The advent of techniques to genetically modify experimental animals and produce directed mutations in both a conditional and tissue-specific manner has dramatically opened up new fields for physiologists in cardiovascular and renal research. 2. A consequence of altering the genetic background of mice is the difficulty in predicting the phenotypic outcome of the genetic mutation. We therefore suggest that physiologists may need to change their current experimental paradigms to face this new era. Hence, our aim is to propose a complementary research philosophy for physiologists working in the post-genomic era. That is, instead of using strictly hypothesis-driven research philosophies, one will have to perform screening studies of mutant mice, within a field of interest, to find valuable phenotypes. Once a relevant phenotype is found, in-depth studies of the underlying mechanisms should be performed. These follow-up studies should be performed using a traditional hypothesis-driven research philosophy. 3. The rapidly increasing availability of mutated mouse models of human disease also necessitates the development of techniques to characterize these various mouse phenotypes. In particular, the miniaturization and refinement of techniques currently used to study the renal and cardiovascular system in larger animals will be discussed in the present review. Hence, we aim to outline what techniques are currently available and should be present in a laboratory to screen and study renal and cardiovascular phenotypes in genetically modified mice, with particular emphasis on methodologies used in the intact, conscious animal.

Blood pressure and NaCl-sensitive hypertension are influenced by angiotensin-converting enzyme gene expression in transgenic mice

ACE plays an important role in the regulation of arterial pressure; however, a linear relationship between ACE expression and arterial pressure has not been demonstrated. The present study employed telemetric monitoring in female transgenic mice to determine the influence of partial and complete deletion of the ACE gene on basal arterial pressure and arterial pressure responses to a high-NaCl diet. On the basal NaCl diet, 24-hour mean arterial pressure was significantly correlated with the number of functional copies of the ACE gene; ie, arterial pressure was lowest in 0-copy (80 +/- 1 mm Hg), intermediate in 1-copy (100 +/- 1 mm Hg), and highest in 2-copy (113 +/- 1 mm Hg) ACE mice. The high-NaCl diet significantly increased mean arterial pressure in 0-copy (99 +/- 1 mm Hg) and 1-copy (108 +/- 1 mm Hg) mice but not in 2-copy mice (114 +/- 1 mm Hg). These results demonstrate a copy-dependent relationship between ACE gene expression and both basal arterial pressure and arterial pressure responses to a high-NaCl diet, suggesting that either partial or complete reduction in the ACE gene can alter arterial pressure.

Effects of mineralocorticoid receptor gene disruption on the components of the renin-angiotensin system in 8-day-old mice

Targeted disruption of mineralocorticoid receptor (MR) gene results in pseudohypoaldosteronism type I with failure to thrive, severe dehydration, hyperkalemia, hyponatremia, and high plasma levels of renin, angiotensin II, and aldosterone. In this study, mRNA expression of the different components of the renin-angiotensin system (RAS) were evaluated in liver, lung, heart, kidney and adrenal gland to assess their response to a state of extreme sodium depletion. Angiotensinogen, renin, angiotensin-I converting enzyme, and angiotensin II receptor (AT1 and AT2) mRNA expressions were determined by Northern blot and RT-PCR analysis. Furthermore, in situ hybridization and immunohistochemistry allowed us to identify the cell types involved in the variation of the RAS component expression. In the heterozygous mice (MR+/-), compared with wild-type mice (MR+/+), there was no significant variation of any mRNA of the RAS components. In MR knockout mice (MR-/-), compared with wild-type mice, there were significant increases in the expression level of several RAS components. In the liver, angiotensinogen and AT1 receptor mRNA expressions were moderately stimulated. In the kidney, renin mRNA was increased up to 10-fold and in situ hybridization showed a marked recruitment of renin-producing cells; however, the levels of angiotensin-I converting enzyme mRNA and AT1 mRNA were not changed. Interestingly, in adrenal gland, renin expression was also strongly up-regulated in a thickened zona glomerulosa, whereas AT1 mRNA expression remained unchanged. Altogether, these results demonstrate that in the MR knockout mice model, RAS component expressions are differentially altered, renin being the most stimulated component. Angiotensinogen and AT1 in the liver are also increased, but the other elements of the RAS are not affected.

Effect of genetic deficiency of angiotensinogen on the renin-angiotensin system

This study examined expression of renin-angiotensin system (RAS) component mRNAs in angiotensinogen gene knockout (Atg-/-) mice. Wild-type (Atg+/+) and Atg-/- mice were fed a normal-salt (0.3% NaCl) or high-salt (4% NaCl) diet for 2 weeks. Angiotensinogen, renin, angiotensin-converting enzyme (ACE), angiotensin II type la receptor (AT1A), and angiotensin II type 2 receptor (AT2) mRNA levels were measured by Northern blot analysis. In Atg+/+ mice, activities of circulating RAS and renal angiotensinogen mRNA level were decreased by salt loading, whereas levels of renal and cardiac ACE; renal, brain, and cardiac AT1A; and brain and cardiac AT2 mRNA were increased by salt loading. Although activities of circulating RAS were not detected in Atg-/- mice, salt loading increased blood pressure in Atg-/- mice. In Atg-/- mice, renal renin mRNA level was decreased by salt loading; in contrast, salt loading increased renal AT1A and cardiac AT2 mRNA levels in Atg-/- mice, and these activated levels in Atg-/- mice were higher than those in Atg+/+ mice fed the high-salt diet. Thus, expression of each component of the RAS is regulated in a tissue-specific manner that is distinct from other components of systemic and local RAS and that appears to be mediated by a mechanism other than changes in the circulating or tissue levels of angiotensin peptides.

Expression of neuronal type nitric oxide synthase and renin in the juxtaglomerular apparatus of angiotensin type-1a receptor gene-knockout mice

Angiotensin type-1a (AT1a) receptor gene-knockout (AT1a-/-) mice exhibit chronic hypotension and renin overproduction. In the kidneys of AT1a-/- mice, the activity of neuronal type nitric oxide synthase (N-NOS) was histochemically detected by nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase (NADPHd) reaction combined with N-NOS immunohistochemistry. The localization of renin was detected by immunohistochemistry and the results were analyzed morphometrically. The levels of N-NOS and renin mRNA in the renal cortical tissue were determined by reverse transcription-PCR and Northern blot analysis, respectively. In the renal sections from wild-type mice, NADPHd activity and N-NOS immunoreactivity were localized to the discrete region of the macula densa in contact with the parent glomerulus. In contrast, N-NOS-positive macula densa cells were distributed beyond the original location of the macula densa, occasionally extending to the opposite side of the distal tubules. The mean number of N-NOS positive macula densa cells was significantly increased in AT1a-/- mice (186 per 100 glomeruli) compared with wild-type mice (65 per 100 glomeruli). AT1a-/- mice showed 1.4-times higher N-NOS mRNA levels in the renal cortical tissues than wild-type mice. The plasma renin activity was significantly higher in AT1a-/- mice (205.5 +/- 26.1 ng/ml/hr) than in wild-type mice (8.0 +/- 0.2 ng/ml/hr). The renin-positive areas per glomerulus and renal renin gene expression were 12-times and 2.6-times higher in AT1a-/- mice than in wild-type mice, respectively. These abnormalities, however, were less remarkable in AT1a-/- mice compared with angiotensinogen-knockout mice. When AT1a-/- mice were fed a high-salt diet, the signal intensity of the NADPHd reaction and the number of positively-stained macula densa cells were significantly decreased. The levels of renal cortical N-NOS mRNA were also suppressed by the treatment. Dietary salt loading produced a parallel decrease in plasma renin activity, renal renin-immunoreactive areas, and the levels of renin mRNA without affecting systemic blood pressure. These results provide evidence for the possible involvement of N-NOS at the macula densa in the increased renin production in AT1a-/- mice.