New frontiers in the intrarenal Renin-Angiotensin system: a critical review of classical and new paradigms

Abstract

The renin-angiotensin system (RAS) is well-recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular, and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

Keywords: ACE2; angiotensin 1-converting enzyme; angiotensin II receptor; blood pressure; hypertension; kidney; proximal tubule; signal transduction.

Figure 1

A representative overview of the evolving renin-angiotensin system. (1) The classical angiotensinogen/renin/ACE/ANG II/AT₁ and AT₂ receptor axis. (2) The prorenin/PRR/MAP kinases ERK 1/2 axis. (3) The ACE2/ANG (1–7)/Mas receptor axis. (4) The ANG IV/AT₄/IRAP axis. ANG A, angiotensin A. ANG I, angiotensin I. ANG (1–7), angiotensin (1–7). ACE, angiotensin-converting enzyme. ACE2, angiotensin-converting enzyme 2. ANG II, angiotensin II. ANG III, angiotensin III. ANG IV, angiotensin (3–8). APA, aminopeptidase A; APN, aminopeptidase N; AT₁, type 1 ANG II receptor; AT₂, type 2 ANG II receptor; IRAP, insulin-regulated aminopeptidase or AT₄ receptor; JGA, juxtaglomerular apparatus.

Figure 2

Intrarenal localization or expression of major components of the renin-angiotensin system. (A) Active renin binding in juxtaglomerular apparatus in the dog kidney using the radiolabeled renin inhibitor, ¹²⁵I-H77. (B) ACE binding in the proximal tubule of the rat kidney using ¹²⁵I-351A (C) AT₁ receptor binding in the rat kidney in the presence of the AT₂ receptor blocker PD123319. (D) AT₂ receptor binding in the rat kidney in the presence of the AT₁ receptor blocker losartan using ¹²⁵I-[Sar¹,Ile⁸]-Ang II. (E) Ang (1–7) receptor binding in the rat kidney using ¹²⁵I-Ang (1–7) as the radioligand. And (F) Ang IV receptor binding in the rat kidney using ¹²⁵I-Ang (3–8). The levels of binding are indicated by color calibration bars with red representing the highest, whereas blue showing the lowest levels of enzyme or receptor binding. G, glomerulus; IM, inner medulla; IS, inner stripe of the outer medulla; JGA, juxtaglomerular apparatus; P, proximal tubule. Reproduced from Li and Zhuo with permission (45).

Figure 3

Proximal tubule-specific expression of AT_1aR/GFP in a representative Agtr1a^−/− mouse kidney 2 week after intrarenal adenoviral transfer. (A) AT_1aR/GFP expression (green) in proximal tubules (PT). (B) Alexa Fluor 594-labeled megalin expression (red) in proximal tubules. (C) DAPI-stained nuclei (blue) in the same kidney section. (D) Merged image of (A–C), showing the colocalization of AT_1aR/GFP and megalin expression (yellow) in proximal tubules. Only very low levels of AT_1aR/GFP and megalin expression are visible in the glomerulus (G) and cortical collecting tubules (CCT). (E) AT_1aR/GFP expression in the outer medulla. (F) Alexa Fluor 594-labeled megalin expression in the outer medulla. (G) DAPI-stained nuclei in the outer medulla. (H) Merged image of (E–G), showing the lack of AT_1aR/GFP and megalin expression in the outer medulla. Magnification: ×40. Reproduced from Li and Zhuo with permission (42).

Figure 4

Effects of proximal tubule-specific, adenovirus-mediated transfer of ECFP/ANG II on ECFP/ANG II expression in the renal outer cortex and freshly isolated proximal tubule of mouse kidneys 2 wk after gene transfer. (A) ECFP expression (blue-green). (B) DAPI-stained nuclei (red). (C) Merged image of (A,B), respectively, in the outer renal cortex of a representative rat transferred with ECFP/ANG II selectively in proximal tubules. (D–F) Expression of ECFP/ANG II in a freshly isolated representative proximal convoluted tubule. Bars = 100 μm for the renal cortex and 10 μm for the isolated proximal tubule. G, glomerulus; PT, proximal tubule. Reproduced from Li et al. with permission (77).