Transgenic mouse models of angiotensin receptor subtype function in the cardiovascular system
Introduction
The renin–angiotensin system is an essential regulator of blood pressure and water and electrolyte balance [1]. The effector peptide of this system, angiotensin II activates cell surface receptors to produce a variety of regulatory actions in the cardiovascular, renal, endocrine, and neural systems. Using pharmacological ligands, two major classes of angiotensin receptors were defined, termed type 1 (AT1) and type 2 (AT2) receptors [2]. AT1 receptors were identified as members of the large family of G protein-coupled receptors by molecular cloning in 1991 [3], [4], followed by cloning of the AT2 receptor subtype [5], [6]. AT1 receptors mediate most of the known biological effects of the renin–angiotensin system, including vasoconstriction, hypertension, and aldosterone release. However, recent evidence emerges that the AT2 receptor subtype counteracts some of the biological actions of the AT1 receptor by inhibiting cellular growth and decreasing the hypertensive effects of AT1 receptor activation.
Whereas man possesses a single AT1 receptor, rodents express two AT1 receptor isoforms, designated AT1A and AT1B [7]. These receptors are products of distinct but homologous genes (Agtr1a and Agtr1b) located on separate chromosomes. AT1A and AT1B receptors share substantial sequence homology (Fig. 1). The murine AT1 isoforms differ in only 22 out of 359 amino acids, resulting in a sequence identity of 94% [8]. The AT1A receptor is the major subtype in the cardiovascular system and in the kidney [9], [10], whereas the AT1B receptor is more abundantly expressed in the anterior pituitary gland and the adrenal gland [11], [12]. Although non-peptide receptor ligands allow differentiation between AT1 and AT2 receptors, it has not been possible to distinguish between AT1A and AT1B subtypes [1]. Using pharmacological ligands, it has been difficult in many cases to unequivocally assign biological functions to individual angiotensin receptor subtypes. Thus, especially the role of the AT2 receptor has remained elusive.
Recently, transgenic technology in mice and rats was applied to study the individual components of the renin–angiotensin system in vivo. All genes of this system, including renin, angiotensinogen, angiotensin-converting enzyme and the angiotensin receptor subtypes have been inactivated by gene-targeting in embryonic stem cells or have been overexpressed in specific tissues in vivo. Molecular genetics has thus provided novel insight into the physiological and pathophysiological role of the renin–angiotensin system. In some cases, previous concepts were supported by transgenic experiments, but in other cases completely unexpected and novel findings have added to our understanding of the complex renin–angiotensin system. Here, we will focus on the transgenic models of angiotensin receptor subtypes. Recent reviews have summarized the gene-targeting models of other components of the renin–angiotensin system [13], [14], [15].
Section snippets
Signal transduction by AT1 and AT2 receptors
Both angiotensin receptor subtypes can couple to a variety of signalling pathways. Activation of AT1 receptors via the Gq/11 protein leads to rapid stimulation of phospholipase C and intracellular Ca2+ mobilization, followed by activation of phospholipases A2 and D, protein kinase C, and mitogen-activated protein (MAP) kinase, inhibition of adenylyl cyclase, stimulation of several tyrosine kinases and activation of gene transcription (Fig. 1). Signal transduction by AT2 receptors involves
Transgenic mouse models
Several transgenic models have been generated in which expression of individual angiotensin receptor subtypes was altered. The murine genes encoding AT1A, AT1B, and AT2 receptor subtypes were disrupted by homologous recombination in embryonic stem cells (gene knockout) [23], [24], [25], [26], [27]. As the genes for AT1A and AT1B receptors are localized on mouse chromosomes 13 and 3, respectively, mice lacking both AT1 receptor subtypes were generated by crossing single receptor knockout lines
Blood pressure regulation by AT1A and AT1B receptors
As the AT1A receptor is the major angiotensin II receptor subtype expressed in the cardiovascular system of the adult mouse, it was anticipated that this receptor would be the major regulator of blood pressure homeostasis. Indeed, arterial pressure in mice was affected differently by gene targeting of either the AT1A or AT1B receptor (Fig. 2) [23], [24], [25]. Deletion of the gene for the AT1A receptor but not for the AT1B receptor led to a decrease in resting arterial pressure. Blood pressure
AT2 receptors and blood pressure regulation
In mice lacking functional AT1 receptor subtypes (AT1AB-KO), angiotensin II had no effect on mean arterial pressure, suggesting that the AT2 receptor is not involved in acute blood pressure regulation in mice [28], [29]. Similarly, in aortic vascular smooth muscle cells from wild-type or AT1A-KO mice, AT2 receptor antagonists had no discernible effect on either cytosolic Ca2+ before or after addition of angiotensin II [39]. Surprisingly, AT2 receptor knockout mice showed altered blood pressure
Angiotensin II and cardiac hypertrophy
Angiotensin II regulates cellular growth in response to developmental, physiological, and pathological processes. The identification of renin–angiotensin system components and angiotensin II receptors in cardiac tissue suggests the existence of an autocrine/paracrine system that has effects independent of angiotensin II derived from the circulatory system [48]. In order to investigate the effects of angiotensin II signalling on cardiac myocytes in vivo, several transgenic models overexpressing
Angiotensin receptors and kidney function
Angiotensin II is an important regulator of kidney development and function (Fig. 3). Mice which are unable to generate angiotensin II because of targeted mutations in the angiotensinogen or angiotensin converting enzyme genes have virtually identical phenotypes characterized by reduced survival, low blood pressure, and abnormal kidney structure. In the kidney, these animals develop thickening of arterial walls, focal areas of renal cortical inflammation, and hypoplasia of the inner medulla,
Conclusions
Gene targeting and transgenic overexpression of angiotensin II receptor subtypes has provided important insight into the physiological and pathophysiological relevance of angiotensin II receptor subtypes. Of particular interest is the balance between AT1 and AT2 receptor-mediated actions in the cardiovascular system. Experimental data derived from mice lacking individual receptor subtypes have supported and extended previous observations suggesting that the AT2 receptor subtype acts to
Acknowledgements
The authors work was supported by the Deutsche Forschungsgemeinschaft (SFB355 — TP C10).
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