Tissue targeting of angiotensin peptides

Countering the classical renin-angiotensin system

It is well-established that Ang-(1-7) counteracts the effects of Ang II in the periphery, while stimulating vasopressin release and mimicking the activity of Ang II in the brain, through interactions with various receptors. The rapid metabolic inactivation of Ang-(1-7) has proven to be a limitation to therapeutic administration of the peptide. To circumvent this problem, Alves et al. (Clinical Science (2021) 135(18), https://doi.org/10.1042/CS20210599) developed a new transgenic rat model that overexpresses an Ang-(1-7)-producing fusion protein. In this commentary, we discuss potential concerns with this model while also highlighting advances that can ensue from this significant technical feat.

Hemodynamic phenotyping of transgenic rats with ubiquitous expression of an angiotensin-(1-7)-producing fusion protein

Activation of the angiotensin (Ang)-converting enzyme (ACE) 2/Ang-(1-7)/MAS receptor pathway of the renin-angiotensin system (RAS) induces protective mechanisms in different diseases. Herein, we describe the cardiovascular phenotype of a new transgenic rat line (TG7371) that expresses an Ang-(1-7)-producing fusion protein. The transgene-specific mRNA and the corresponding protein were shown to be present in all evaluated tissues of TG7371 with the highest expression in aorta and brain. Plasma Ang-(1-7) levels, measured by radioimmunoassay (RIA) were similar to control Sprague-Dawley (SD) rats, however high Ang-(1-7) levels were found in the hypothalamus. TG7371 showed lower baseline mean arterial pressure (MAP), assessed in conscious or anesthetized rats by telemetry or short-term recordings, associated with increased plasma atrial natriuretic peptide (ANP) and higher urinary sodium concentration. Moreover, evaluation of regional blood flow and hemodynamic parameters with fluorescent microspheres showed a significant increase in blood flow in different tissues (kidneys, mesentery, muscle, spleen, brown fat, heart and skin), with a resulting decrease in total peripheral resistance (TPR). TG7371 rats, on the other hand, also presented increased cardiac and global sympathetic tone, increased plasma vasopressin (AVP) levels and decreased free water clearance. Altogether, our data show that expression of an Ang-(1-7)-producing fusion protein induced a hypotensive phenotype due to widespread vasodilation and consequent fall in peripheral resistance. This phenotype was associated with an increase in ANP together with an increase in AVP and sympathetic drive, which did not fully compensate the lower blood pressure (BP). Here we present the hemodynamic impact of long-term increase in tissue expression of an Ang-(1-7)-fusion protein and provide a new tool to investigate this peptide in different pathophysiological conditions.

Genetic Models

Genetically altered rat and mouse models have been instrumental in the functional analysis of genes in a physiological context. In particular, studies on the renin-angiotensin system (RAS) have profited from this technology in the past. In this review, we summarize the existing animal models for the protective axis of the RAS consisting of angiotensin-converting enzyme 2 (ACE2), angiotensin-(1-7)(Ang-(1-7), and its receptor Mas. With the help of models with altered expression of the components of this axis in the brain and cardiovascular organs, its physiological and pathophysiological functions have been elucidated. Thus, novel opportunities for therapeutic interventions in cardiovascular diseases were revealed targeting ACE2 or Mas.

Chronic Overexpression of Bradykinin in Kidney Causes Polyuria and Cardiac Hypertrophy

Acute intra-renal infusion of bradykinin increases diuresis and natriuresis via inhibition of vasopressin activity. However, the consequences of chronically increased bradykinin in the kidneys have not yet been studied. A new transgenic animal model producing an excess of bradykinin by proximal tubular cells (KapBK rats) was generated and submitted to different salt containing diets to analyze changes in blood pressure and other cardiovascular parameters, urine excretion, and composition, as well as levels and expression of renin-angiotensin system components. Despite that KapBK rats excrete more urine and sodium, they have similar blood pressure as controls with the exception of a small increase in systolic blood pressure (SBP). However, they present decreased renal artery blood flow, increased intrarenal expression of angiotensinogen, and decreased mRNA expression of vasopressin V1A receptor (AVPR1A), suggesting a mechanism for the previously described reduction of renal vasopressin sensitivity by bradykinin. Additionally, reduced heart rate variability (HRV), increased cardiac output and frequency, and the development of cardiac hypertrophy are the main chronic effects observed in the cardiovascular system. In conclusion: (1) the transgenic KapBK rat is a useful model for studying chronic effects of bradykinin in kidney; (2) increased renal bradykinin causes changes in renin angiotensin system regulation; (3) decreased renal vasopressin sensitivity in KapBK rats is related to decreased V1A receptor expression; (4) although increased renal levels of bradykinin causes no changes in mean arterial pressure (MAP), it causes reduction in HRV, augmentation in cardiac frequency and output and consequently cardiac hypertrophy in rats after 6 months of age.

The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7)

The renin-angiotensin system (RAS) is a key player in the control of the cardiovascular system and hydroelectrolyte balance, with an influence on organs and functions throughout the body. The classical view of this system saw it as a sequence of many enzymatic steps that culminate in the production of a single biologically active metabolite, the octapeptide angiotensin (ANG) II, by the angiotensin converting enzyme (ACE). The past two decades have revealed new functions for some of the intermediate products, beyond their roles as substrates along the classical route. They may be processed in alternative ways by enzymes such as the ACE homolog ACE2. One effect is to establish a second axis through ACE2/ANG-(1-7)/MAS, whose end point is the metabolite ANG-(1-7). ACE2 and other enzymes can form ANG-(1-7) directly or indirectly from either the decapeptide ANG I or from ANG II. In many cases, this second axis appears to counteract or modulate the effects of the classical axis. ANG-(1-7) itself acts on the receptor MAS to influence a range of mechanisms in the heart, kidney, brain, and other tissues. This review highlights the current knowledge about the roles of ANG-(1-7) in physiology and disease, with particular emphasis on the brain.

Animal Models with a Genetic Alteration of the ACE2/Ang-(1-7)/Mas Axis

The aim of this chapter is to describe the animal models generated by transgenic technology for the functional analysis of the protective axis of the renin–angiotensin system, consisting of angiotensin-converting enzyme 2 (ACE2), angiotensin (Ang)-(1-7), and Mas. Transgenic overexpression of the components of this axis in general led to an ameliorated cardiac and vascular damage in disease states and to an improved metabolic profile. Knockout models for ACE2 and Mas, however, show aggravated cardiovascular pathologies and a metabolic syndrome-like state. In particular, the local production of Ang-(1-7) in the vascular wall, in the heart, and in the brain was found to be of high physiological relevance by the use of transgenic animals overexpressing ACE2 or Ang-(1-7) in these tissues.

In vivo expression of angiotensin-(1-7) lowers blood pressure and improves baroreflex function in transgenic (mRen2)27 rats

Transgenic (mRen2)27 rats are hypertensive with impaired baroreflex sensitivity for control of heart rate compared with Hannover Sprague-Dawley rats. We assessed blood pressure and baroreflex function in male hemizygous (mRen2)27 rats (30-40 weeks of age) instrumented for arterial pressure recordings and receiving into the cisterna magna either an Ang-(1-7) fusion protein or a control fusion protein (CTL-FP). The maximum reduction in mean arterial pressure achieved was -38 ± 7 mm Hg on day 3, accompanied by a 55% enhancement in baroreflex sensitivity in Ang-(1-7) fusion protein-treated rats. Both the high-frequency alpha index (HF-α) and heart rate variability increased, suggesting increased parasympathetic tone for cardiac control. The mRNA levels of several components of the renin-angiotensin system in the dorsal medulla were markedly reduced including renin (-80%), neprilysin (-40%), and the AT1a receptor (-40%). However, there was a 2-fold to 3-fold increase in the mRNA levels of the phosphatases PTP-1b and dual-specificity phosphatase 1 in the medulla of Ang-(1-7) fusion protein-treated rats. Our finding that replacement of Ang-(1-7) in the brain of (mRen2)27 rats reverses in part the hypertension and baroreflex impairment is consistent with a functional deficit of Ang-(1-7) in this hypertensive strain. We conclude that the increased mRNA expression of phosphatases known to counteract the phosphoinositol 3 kinase and mitogen-activated protein kinases, and the reduction of renin and AT1a receptor mRNA levels may contribute to the reduction in arterial pressure and improvement in baroreflex sensitivity in response to Ang-(1-7).

Adenoviral delivery of angiotensin-(1-7) or angiotensin-(1-9) inhibits cardiomyocyte hypertrophy via the mas or angiotensin type 2 receptor

The counter-regulatory axis of the renin angiotensin system peptide angiotensin-(1-7) [Ang-(1-7)] has been identified as a potential therapeutic target in cardiac remodelling, acting via the mas receptor. Furthermore, we recently reported that an alternative peptide, Ang-(1-9) also counteracts cardiac remodelling via the angiotensin type 2 receptor (AT₂R). Here, we have engineered adenoviral vectors expressing fusion proteins which release Ang-(1-7) [RAdAng-(1-7)] or Ang-(1-9) [RAdAng-(1-9)] and compared their effects on cardiomyocyte hypertrophy in rat H9c2 cardiomyocytes or primary adult rabbit cardiomyocytes, stimulated with angiotensin II, isoproterenol or arg-vasopressin. RAdAng-(1-7) and RAdAng-(1-9) efficiently transduced cardiomyocytes, expressed fusion proteins and secreted peptides, as demonstrated by western immunoblotting and conditioned media assays. Furthermore, secreted Ang-(1-7) and Ang-(1-9) inhibited cardiomyocyte hypertrophy (Control = 168.7±8.4 µm; AngII = 232.1±10.7 µm; AngII+RAdAng-(1-7) = 186±9.1 µm, RAdAng-(1-9) = 180.5±9 µm; P<0.05) and these effects were selectively reversed by inhibitors of their cognate receptors, the mas antagonist A779 for RAdAng-(1-7) and the AT₂R antagonist PD123,319 for RAdAng-(1-9). Thus gene transfer of Ang-(1-7) and Ang-(1-9) produces receptor-specific effects equivalent to those observed with addition of exogenous peptides. These data highlight that Ang-(1-7) and Ang-(1-9) can be expressed via gene transfer and inhibit cardiomyocyte hypertrophy via their respective receptors. This supports applications for this approach for sustained peptide delivery to study molecular effects and potential gene therapeutic actions.

Constitutive stimulation of vascular smooth muscle cells by angiotensin II derived from an adenovirus encoding a furin-cleavable fusion protein

BACKGROUND

To fill the gap between acute and chronic stimulation methods of angiotensin II (Ang II) and obtain relevant signaling information, we have made an adenovirus vector encoding a furin-cleavable Ang II fusion protein.

METHODS

Vascular smooth muscle cells (VSMCs) were infected with adenovirus to evaluate Ang II production. Also, expression of early growth response-1 (Egr-1) and hypertrophic responses were examined in VSMCs.

RESULTS

Acute stimulation of VSMCs with synthetic Ang II showed the peptide had a half-life of less than 1 h. Infection of VSMCs with Ang II adenovirus showed a time-dependent production of Ang II as early as 2 days and up to 7 days postinfection. The Ang II adenovirus induced VSMC hypertrophy, stimulated Egr-1 expression, and suppressed Ang II type 1 receptor mRNA expression. Chronic Ang II infusion in mice for 2 weeks markedly enhanced Egr-1 immunostaining in carotid artery compared with the control saline infusion.

CONCLUSION

Application of the Ang II adenovirus vector to cultured cells will be useful to elucidate molecular and signaling mechanisms of cardiovascular diseases associated with enhanced Ang II production.

Attenuation of isoproterenol-induced cardiac fibrosis in transgenic rats harboring an angiotensin-(1-7)-producing fusion protein in the heart

Objective: It has been shown that Ang-(1-7) has cardioprotective actions. To directly investigate the effects of Ang-(1-7) specifically in the heart, we generated and characterized transgenic (TG) rats which express an Ang-(1-7)-producing fusion protein driven by the α-MHC promoter.

Methods and Results: After microinjection of the transgene into fertilized rat zygotes, we obtained four different transgenic lines. Homozygous animals were analyzed with regard to the expression profile of the transgene by ribonuclease protection assay. Transgene expression was detected mainly in the heart with weak or no expression in other organs. Heterozygous TG(hA-1-7)L7301 rats presented a significant increase in cardiac Ang-(1-7) concentration compared with control rats (17.1±2.1 versus 3.9±1.4 pg/mg protein in SD rats). Radiotelemetry analysis revealed that TG rats presented no significant changes in blood pressure and heart rate compared with normal rats. Overexpression of Ang-(1-7) in the heart produced slight improvement in resting cardiac function (+ dT/dt: 81530±1305.0 versus 77470±345.5 g/s bpm in SD rats, p < 0.05), which was in keeping with the enhanced [Ca2+] handling observed in cardiomyocytes of TG rats. TG(hA-1-7)L7301 rats also showed a greater capacity to withstand stress since TG rats showed a less pronounced deposition of collagen type III and fibronectin induced by isoproterenol treatment in the subendocardial area than in corresponding controls. In addition, hearts from TG rats showed reduced incidence and duration of reperfusion arrhythmias in comparison with SD rats.

Conclusion: These results indicate that Ang-(1-7) has blood pressure-independent, antifibrotic effects, acting directly in the heart.

Genetically altered animal models for Mas and angiotensin-(1-7)

Mas is the receptor for angiotensin-(1-7) and is involved in cardiovascular and neuronal regulation, in which the heptapeptide also plays a major role. Mas-deficient mice have been generated by us, and their characterization has shown that Mas has important functions in behaviour and cardiovascular regulation. These mice exhibit increased anxiety but, despite an enhanced long-term potentiation in the hippocampus, do not perform better in learning experiments. When Mas-deficient mice are backcrossed to the FVB/N genetic background, a cardiovascular phenotype is uncovered, in that the backcrossed animals become hypertensive. Concordant with our detection by fluorescent in situ hybridization of Mas mRNA in mouse endothelium, this phenotype is caused by endothelial dysfunction based on a dysbalance between nitric oxide and reactive oxygen species in the vessel wall. In agreement with these data, transgenic spontaneously hypertensive stroke-prone rats overexpressing ACE2 in the vessel wall exhibit reduced blood pressure as a result of improved endothelial function. Moreover, angiotensin-(1-7) overexpression in transgenic rats has cardioprotective and haemodynamic effects. In conclusion, the angiotensin-(1-7)-Mas axis has important functional implications for vascular regulation and blood pressure control, particularly in pathophysiological situations.

Expression of an angiotensin-(1-7)-producing fusion protein in rats induced marked changes in regional vascular resistance

We have described a transgenic rat line that expresses an angiotensin-(1-7)-producing fusion protein, the TGR(A1-7)3292. In these rats, testis acts as an angiotensin-(1-7) biological pump, increasing its plasma concentration 2.5-fold. In this study, we performed hemodynamic measurements in TGR(A1-7)3292 and age-matched Hannover Sprague-Dawley (SD) control rats, using fluorescent microspheres. Urethane-anesthetized transgenic rats had similar levels of baseline blood pressure (99 +/- 3 mmHg) as did SD rats (101 +/- 3 mmHg). However, pronounced differences were observed in other hemodynamic measurements. TGR(A1-7)3292 rats presented a significant increase in stroke volume (0.29 +/- 0.01 vs. 0.25 +/- 0.01 ml in SD), increased cardiac index (24.6 +/- 0.91 vs. 21.9 +/- 0.65 ml.min(-1).kg) and decreased total peripheral resistance (3.9 +/- 0.13 vs. 4.5 +/- 0.13 mmHg.ml(-1).min.100 g). The increase in stroke volume in transgenic rats may be partially explained by the small decrease in heart rate (326 +/- 7.0 vs. 359 +/- 6.0 beats/min in SD). Strikingly, TGR(A1-7)3292 rats presented a substantial decrease in the vascular resistance in lung, spleen, kidney, adrenals, brain, testis and brown fat tissue with no significant differences in the left ventricle, mesentery, skin, gastrocnemius muscle and white fat tissue. These results corroborate and extend previous results observed after acute angiotensin-(1-7) infusion, showing that chronic increase in circulating angiotensin-(1-7) produces sustained and important changes in regional and systemic hemodynamics. Moreover, our data suggest a physiological role for angiotensin-(1-7) in the tonic control of regional blood flow.

Expression of an angiotensin-(1-7)-producing fusion protein produces cardioprotective effects in rats

Angiotensin-(1-7) [ANG-(1-7)] is a recently described heptapeptide product of the renin-angiotensin system. Because biosynthesis of ANG-(1-7) increases in animals treated with cardioprotective drugs and inactivation of the gene for angiotensin converting enzyme 2 [an enzyme involved in the biosynthesis of ANG-(1-7)] leads to the development of cardiac dysfunction, it has been suggested that ANG-(1-7) has cardioprotective properties. To directly test this possibility, we have generated transgenic rats that chronically overproduce ANG-(1-7) by using a novel fusion protein methodology. TGR(A1-7)3292 rats show testicular-specific expression of a cytomegalovirus promoter-driven transgene, resulting in a doubling of circulating ANG-(1-7) compared with nontransgenic control rats. Radiotelemetry hemodynamic measurements showed that transgenic rats presented a small but significant increase in daily and nocturnal heart rate and a slight but significant increase in daily and nocturnal cardiac contractility estimated by dP/d t measurements. Strikingly, TGR(A1-7)3292 rats were significantly more resistant than control animals to induction of cardiac hypertrophy by isoproterenol. In addition, transgenic rats showed a reduced duration of reperfusion arrhythmias and an improved postischemic function in isolated Langendorff heart preparations. These results support a cardioprotective role for circulating ANG-(1-7) and provide a novel tool for evaluating the functional role of ANG-(1-7).

Chronic production of angiotensin IV in the brain leads to hypertension that is reversible with an angiotensin II AT1 receptor antagonist

Angiotensin IV (Ang IV) is a metabolite of the potent vasoconstrictor angiotensin II (Ang II). Because specific binding sites for this peptide have been reported in numerous tissues including the brain, it has been suggested that a specific Ang IV receptor (AT4) might exist. Bolus injection of Ang IV in brain ventricles has been implicated in learning, memory, and localized vasodilatation. However, the functions of Ang IV in a physiological context are still unknown. In this study, we generated a transgenic (TG) mouse model that chronically releases Ang IV peptide specifically in the brain. TG mice were found to be hypertensive by the tail-cuff method as compared with control littermates. Treatment with the angiotensin-converting enzyme inhibitor captopril had no effect on blood pressure, but surprisingly treatment with the Ang II AT1 receptor antagonist candesartan normalized the blood pressure despite the fact that the levels of Ang IV in the brains of TG mice were only 4-fold elevated over the normal endogenous level of Ang peptides. Calcium mobilization assays performed on cultured CHO cells chronically transfected with the AT1 receptor confirm that low-dose Ang IV can mobilize calcium via the AT1 receptor only in the presence of Ang II, consistent with an allosteric mechanism. These results suggest that chronic elevation of Ang IV in the brain can induce hypertension that can be treated with angiotensin II AT1 receptor antagonists.

Phosphatidylinositol 3-kinase mediates inhibitory effect of angiotensin II on sodium/glucose cotransporter in renal epithelial cells

Effects of angiotensin II (ANGII) on regulation of sodium/glucose cotransporter (SGLT1) activity were investigated in LLC-PK(1) cells, renal proximal epithelial cell line. ANGII inhibited alpha-[14C] methyl-D-glucopyranoside (AMG) uptake into LLC-PK(1) cells in a dose-dependent manner. This inhibition was based on a decrease in maximal transport rate (Vmax) of AMG from 2.20 nmol/mg protein/15 min to 1.19 nmol/mg protein/15 min, although apparent affinity constant (Km) did not alter. In western blot analysis, protein level of SGLT1 in brush border membrane (BBM) was decreased by ANGII, although total SGLT1 was not altered. In the aspect of intracellular signal transduction, ANGII blocked the formation of cAMP. Pertussis toxin, an inactivator of Gi protein that control intracellular cAMP level, completely prevented the decrease of AMG uptake caused by ANGII. 8-Br-cAMP, a cell membrane permeable cAMP analogue, increased AMG uptake and protein level of SGLT1 in BBM. Both wortmannin and LY294002 that are phosphatidylinositol (PI) 3-kinase inhibitors, inhibited the SGLT1 activity, and also attenuated the effect of 8-Br-cAMP on SGLT1 activity. Those inhibitors prevented the 8-Br-cAMP-induced expression of SGLT1 in plasma membrane. We conclude that ANGII plays an important role in post-translational regulation in SGLT1. Inhibition of SGLT1 translocation is suggested to be caused by inactivation of protein kinase A and decrease of PI 3-kinase activity.

Use of a biological peptide pump to study chronic peptide hormone action in transgenic mice. Direct and indirect effects of angiotensin II on the heart

Angiotensin II is a peptide hormone regulator of blood pressure and fluid balance in mammals. Evidence obtained largely in vitro has also suggested that angiotensin II has growth-promoting effects and that it might thereby contribute to such pathological phenomena as cardiac hypertrophy, a major risk factor for cardiovascular mortality. It has been difficult to test for the direct growth-promoting effects of angiotensin II in vivo, however, because of the generalized effects of the peptide on hemodynamics. To overcome this limitation and to test for cardiac-specific functions of angiotensin II, we generated transgenic mice expressing an angiotensin II-producing fusion protein exclusively in cardiac myocytes. Our findings are the first to distinguish between local and systemic effects of angiotensin II on the heart and introduce a novel technique for studying tissue-specific peptide function.

Fusagene vectors: a novel strategy for the expression of multiple genes from a single cistron

Transduction of cells with multiple genes, allowing their stable and co-ordinated expression, is difficult with the available methodologies. A method has been developed for expression of multiple gene products, as fusion proteins, from a single cistron. The encoded proteins are post-synthetically cleaved and processed into each of their constituent proteins as individual, biologically active factors. Specifically, linkers encoding cleavage sites for the Golgi expressed endoprotease, furin, have been incorporated between in-frame cDNA sequences encoding different secreted or membrane bound proteins. With this strategy we have developed expression vectors encoding multiple proteins (IL-2 and B7.1, IL-4 and B7.1, IL-4 and IL-2, IL-12 p40 and p35, and IL-12 p40, p35 and IL-2 ). Transduction and analysis of over 100 individual clones, derived from murine and human tumour cell lines, demonstrate the efficient expression and biological activity of each of the encoded proteins. Fusagene vectors enable the co-ordinated expression of multiple gene products from a single, monocistronic, expression cassette.

eIF4 initiation factors: effectors of mRNA recruitment to ribosomes and regulators of translation

Eukaryotic translation initiation factor 4F (eIF4F) is a protein complex that mediates recruitment of ribosomes to mRNA. This event is the rate-limiting step for translation under most circumstances and a primary target for translational control. Functions of the constituent proteins of eIF4F include recognition of the mRNA 5' cap structure (eIF4E), delivery of an RNA helicase to the 5' region (eIF4A), bridging of the mRNA and the ribosome (eIF4G), and circularization of the mRNA via interaction with poly(A)-binding protein (eIF4G). eIF4 activity is regulated by transcription, phosphorylation, inhibitory proteins, and proteolytic cleavage. Extracellular stimuli evoke changes in phosphorylation that influence eIF4F activity, especially through the phosphoinositide 3-kinase (PI3K) and Ras signaling pathways. Viral infection and cellular stresses also affect eIF4F function. The recent determination of the structure of eIF4E at atomic resolution has provided insight about how translation is initiated and regulated. Evidence suggests that eIF4F is also implicated in malignancy and apoptosis.