Revisiting the Brain Renin-Angiotensin System—Focus on Novel Therapies
- 200 Downloads
Purpose of Review
Although an independent brain renin-angiotensin system is often assumed to exist, evidence for this concept is weak. Most importantly, renin is lacking in the brain, and both brain angiotensinogen and angiotensin (Ang) II levels are exceptionally low. In fact, brain Ang II levels may well represent uptake of circulating Ang II via Ang II type 1 (AT1) receptors.
Nevertheless, novel drugs are now aimed at the brain RAS, i.e., aminopeptidase A inhibitors should block Ang III formation from Ang II, and hence diminish AT1 receptor stimulation by Ang III, while AT2 and Mas receptor agonists are reported to induce neuroprotection after stroke. The endogenous agonists of these receptors and their origin remain unknown.
This review addresses the questions whether independent angiotensin generation truly occurs in the brain, what its relationship with the kidney is, and how centrally acting RAS blockers/agonists might work.
KeywordsBrain renin-angiotensin system Aminopeptidase A inhibitor AT2 receptor Angiotensinogen Kidney Sympathetic nervous system Stroke
Angiotensinogen is the precursor of all angiotensin (Ang) metabolites. Although its major source is the liver, additional sites of angiotensinogen synthesis have been reported, the most important of which are the brain, kidney, and adipose tissue [1•, 2, 3, 4, 5, 6, 7]. Renin, in contrast, is derived from one source, the kidney. Its precursor, prorenin, like angiotensinogen, remarkably has several sources, including the kidney, ovaries, testis, and adrenal . Yet, given the fact that prorenin is inactive, it would require a (local?) activation mechanism to be of importance. Here, the (pro)renin receptor, which binds and activates prorenin in vitro, has been proposed as a major player . Unfortunately however, its affinity for prorenin is too low to allow this phenomenon to play any role in vivo , and the concept of (pro)renin receptor-prorenin interaction as a unit allowing local Ang I-generating activity is now being abandoned . This does not mean that the (pro)renin receptor has no role at all—in contrast, given its ubiquitous abundance, its link with vacuolar H+-ATPase, and the lethal consequences of its deletion, it turns out to be of vital importance [12, 13, 14, 15], yet apparently independently of the renin-angiotensin system (RAS). Taken together, the various sites of renin, prorenin, and angiotensinogen synthesis allow multiple possibilities for angiotensin generation, e.g., in circulating blood from renal renin and hepatic angiotensinogen, or at tissue sites, from either locally synthesized angiotensinogen and prorenin, or renin, prorenin, and angiotensinogen taken up from blood. Yet, regarding prorenin, we still lack a detailed insight into how it might display activity. This review focuses on the brain RAS, critically addressing the questions whether independent angiotensin generation occurs in the brain, what its relationship with the kidney is, and how centrally acting RAS blockers (in particular, the recently introduced aminopeptidase A inhibitors) and activators (Ang II type 2 (AT2) and Mas receptor agonists) might work.
Independent Angiotensin Generation in the Brain?
Data on the presence of angiotensinogen in the brain are more convincing. Multiple studies report detectable brain angiotensinogen levels that do not run in parallel with circulating angiotensinogen levels [4, 6]. Yet, generally, brain angiotensinogen levels still at most correspond with a few percent of plasma angiotensinogen levels, in a wide range of species, and thus admixture from blood cannot be entirely ruled out. Importantly, brain angiotensinogen mRNA levels, albeit being several orders of magnitude below those in the liver, are not as excessively low as those of renin [1•]. At this stage, the ultimate proof for local synthesis (showing the presence of angiotensinogen in the brain of animals lacking hepatic angiotensinogen expression) is still awaited. When applying this approach to other organs claimed to synthesize angiotensinogen (kidney and adipose tissue) it turned out that their angiotensin generation depended entirely on hepatic angiotensinogen, implying that local angiotensinogen synthesis in these organs, if occurring at all, has no functional consequence [2, 3, 7]. If angiotensinogen synthesis truly occurs in the brain, a complicating factor remains the absence of renin. This would require non-renin enzymes to cleave angiotensinogen. There is currently no in vivo evidence for this concept.
Angiotensins have been reported in brain tissue in widely varying levels. In some cases, levels (expressed per gram tissue) were even higher than those in the kidney [17, 18]. This is hard to believe given the low angiotensinogen levels, and the absence of renin in the brain. Issues that need to be considered here are the use of very small tissue pieces for angiotensin measurements (often representing selected brain nuclei), the detection limit problems that arise from this approach (inherent to brain research), and the absence of rigorous separation techniques to distinguish true angiotensin from background noise. As an example, measuring an angiotensin (Ang) II level at the detection limit of the assay (often around 2 fmol/sample) in 10 mg brain tissue results in a theoretical tissue level of 200 fmol/g. At the same time, measuring 50 fmol Ang II in 0.5 g renal tissue (i.e., well above the detection limit), would translate to 100 fmol/g. On this basis, it seems that brain Ang II levels are higher in the brain than in the kidney, although in reality, they may be zero.
When employing liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify the individual angiotensin metabolites, a highly sensitive method with little or no background noise, we were unable to detect Ang I in brain tissue of spontaneously hypertensive rats (SHR) [1•]. Brain Ang II occurred at levels that were ≈ 25% of the levels in plasma, i.e., they were several orders of magnitude below those in the kidney [21, 22]. Since Ang II type 1 (AT1) receptor blockade reduced the brain/plasma Ang II ratio by > 80%, and in view of the absence of Ang I, the most likely origin of brain Ang II is accumulation of circulating Ang II via binding to AT1 receptors (explained further later) [21, 23]. Additionally, considering its much smaller size versus renin and angiotensinogen, Ang II may gain access to the brain under conditions where the blood-brain barrier is (partially) disrupted, e.g., in hypertension. In fact, Ang II itself is capable of disrupting this barrier [24, 25]. Once in the brain, Ang II might be converted to metabolites, like Ang III and Ang-(1-7). However, at least in SHR, we were unable to demonstrate these metabolites [1•], and thus whether angiotensin metabolites other than Ang II truly reach meaningful levels in the brain is still uncertain.
The “Reno-Cerebral Reflex”
Central Aminopeptidase A Inhibition
AT2 and Mas Receptor Agonism in the Brain
AT1 receptor blockers have often been suggested to offer cerebrovascular protection, in contrast to ACE inhibitors, although clinical evidence for this concept is still missing . The underlying mechanism of this concept would be that only AT1 receptor blockers allow central AT2 receptor agonism. Possibly, AT2 receptor stimulation by endogenous Ang II might already exert protective effects. Indeed, the neurological deficit after middle cerebral artery occlusion was greater in AT2 receptor knockout mice, and without AT2 receptors, the beneficial effects of AT1 receptor blockade were diminished . Now that AT2 receptor agonists like C21 are available, the next step is to evaluate these drugs in stroke models, like the middle cerebral artery occlusion model and the endothelin-1-induced ischemic stroke model. Indeed, in both models, C21 exerted cerebroprotection, not only when applied intracerebroventricularly, but also when applied systemically [43, 44]. To explain the efficacy of the latter approach, considering that C21 cannot pass the blood-brain barrier, it has to be assumed that C21 enters the brain under conditions where this barrier has been disturbed, like in the above models. An exciting novel approach is to administer C21 via the nose-to-brain route to bypass the blood-brain barrier [45•]. When applying C21 via this route at 1.5 h after stroke, it reduced infarct size and improved neurological scores. Furthermore, angiotensin-(1-7) (Ang-(1-7)), generated from Ang II by ACE2, also offers neuroprotection, both when applied centrally and orally, as did the putative ACE2 activator diminazene . Ang-(1-7) is believed to act via Mas receptors. One possible explanation for the identical beneficial effects of AT2 and Mas receptor stimulation is that both receptors co-localize and are functionally interdependent . Importantly, none of these approaches supports actual angiotensin synthesis in the brain allowing AT2/Mas receptor activation by endogenous angiotensins. A further complicating factor is that both C21 and diminazene exert AT2 receptor- and ACE2-independent effects, respectively [48, 49], while Ang-(1-7) was recently reported not to act as Mas agonist at all . Clearly, these observations remain controversial, and even if they can be taken as evidence for AT2/Mas receptor activation, they do not automatically imply that these receptors are normally seen by brain-derived endogenous agonists. In fact, their stimulation may depend on breakdown of the blood-brain barrier (Fig. 1), allowing circulating angiotensins access to brain receptors .
Convincing evidence that angiotensin synthesis occurs independently at brain tissue sites is lacking. Renin is absent and brain angiotensin levels are exceptionally low as compared to other organs. In fact, they may well represent binding of circulating Ang II to brain AT1 receptors in brain nuclei outside the blood-brain barrier. To investigate whether brain-originating angiotensinogen, if existing, contributes to brain angiotensin synthesis, experiments need to be performed under conditions where hepatic angiotensinogen synthesis is silenced, preferably in a model where brain angiotensin is assumed to play an important role, like the DOCA-salt rat. Before concluding that novel therapies aimed at APA, APN, ACE2, AT2, and Mas receptors interfere with the brain RAS, we not only need to exclude non-specific effects of the applied drugs, but also show that they truly affect brain angiotensin levels, and that this explains their effects. In other words, it would help to demonstrate brain-selective Ang III suppression during RB150 treatment, and Ang-(1–7) upregulation after diminazene. Here, the application of a highly sensitive method with little or no background noise like LC-MS/MS is essential. Yet, given the fact that APA, APN, and ACE2 have multiple other substrates, one simultaneously needs to rule out that effects are seen due to interference with these alternative substrates.
Compliance with Ethical Standards
Conflict of Interest
The authors declare no conflicts of interest relevant to this manuscript.
Human and Animal Rights and Informed Consent
This article does not contain any studies with human or animal subjects performed by any of the authors.
Papers of particular interest, published recently, have been highlighted as: • Of importance
- 10.Batenburg WW, Lu X, Leijten F, Maschke U, Müller DN, Danser AHJ. Renin- and prorenin-induced effects in rat vascular smooth muscle cells overexpressing the human (pro)renin receptor: does (pro)renin-(pro)renin receptor interaction actually occur? Hypertension. 2011;58:1111–9.CrossRefPubMedGoogle Scholar
- 29.Hollenberg NK, Fisher ND, Nussberger J, Moukarbel GV, Barkoudah E, Danser AHJ. Renal responses to three types of renin-angiotensin system blockers in patients with diabetes mellitus on a high-salt diet: a need for higher doses in diabetic patients? J Hypertens. 2011;29:2454–61.CrossRefPubMedPubMedCentralGoogle Scholar
- 32.• Lu J, Wang HW, Ahmad M, Keshtkar-Jahromi M, Blaustein MP, Hamlyn JM, et al. Central and peripheral slow-pressor mechanisms contributing to angiotensin II-salt hypertension in rats. Cardiovasc Res. 2018;114:233–46. This paper demonstrates that centrally applied RAS blockers also block the effects of systemic Ang II and aldosterone. CrossRefPubMedGoogle Scholar
- 39.• Marc Y, Hmazzou R, Balavoine F, Flahault A, Llorens-Cortes C. Central antihypertensive effects of chronic treatment with RB150: an orally active aminopeptidase A inhibitor in deoxycorticosterone acetate-salt rats. J Hypertens. 2018;36:641–50. Central aminopeptidase A inhibition lowers blood pressure in a model with low systemic RAS activity. CrossRefPubMedGoogle Scholar
- 45.• Bennion DM, Jones CH, Dang AN, Isenberg J, Graham JT, Lindblad L, et al. Protective effects of the angiotensin II AT2 receptor agonist compound 21 in ischemic stroke: a nose-to-brain delivery approach. Clin Sci (Lond). 2018;132:581–93. Nose-to-brain delivery enhances brain accumulation of the putative AT 2 receptor agonist C21. CrossRefGoogle Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.