The Counter-Regulatory Arm of the Renin-Angiotensin System and COVID-19: Insights from Gitelman's and Bartter's Syndromes

The COVID-19 pandemic has focused the interest on the renin-angiotensin system (RAS), as it has a role not only in the infection process but also in the lung injury, which represents the main cause of death among symptomatic patients affected by SARS-CoV-2. It is known, in fact, that SARS-CoV-2 uses the angiotensin-converting enzyme type 2 (ACE2) as an entry point into the cells [1]. ACE2 is one of the key regulators of the RAS, abundantly expressed in airway epithelial, endothelial and renal cells. It is a part of the protective counter-regulatory ACE2-Ang 1-7-Mas receptor (MasR) axis, which opposes the classical ACE-Angiotensin II (Ang II)-Ang II type 1 receptor (AT1R) regulatory axis of RAS and its activation leads to inhibition of lung's inflammatory and fibrotic responses and protection of the lung and cardiovascular-renal systems [2,3] (Fig. 1a).FIGURE 1: (a) The regulatory axis of RAS via Ang II AT1R and the counter-regulatory axis of RAS via ACE2-Ang 1-7-Mas receptor. (b) Schematic representation of the altered Ang II signalling via AT1R, the classic regulatory axis of RAS, and activation of the counter-regulatory axis of RAS via ACE2-Ang 1-7-MasR in Gitelman's and Bartter's syndromes. The pathways leading to vasodilatation and anti-remodelling prevail on those leading to vasoconstriction and remodelling [5]. The counter-regulatory axis of RAS via ACE2-Ang 1-7-MasR is activated with increased ACE2 level/activity and Ang 1-7 production. Modified from [3].It has been hypothesized that an imbalance of the two arms of the RAS might be critical in determining the lung injury in severe forms of COVID-19. Endothelial dysfunction, vaso-constriction and broncho-constriction, increases of vascular permeability, inflammation and fibrosis are in fact key in the development of acute respiratory distress syndrome (ARDS), the main cause of COVID-19 mortality [4]. In-vivo human data that directly address towards the beneficial role of the protective arm of RAS system's activation in COVID-19 and its complications may be provided by patients with Gitelman's and Bartter's syndromes, rare genetic tubulopathies, which we have deeply investigated for their characteristic biochemical and hormonal features, which include hypokalaemia and metabolic alkalosis, high Ang II levels and RAS activation yet normo- hypotension, increased ACE2 and Ang 1-7 levels, activation of Ang II signalling via AT2R, reduced Rho kinase activity, this latter a key factor for the induction of oxidative stress and cardiovascular-renal remodelling [5,6] and protection from cardiovascular and renal remodelling, representing a model of endogenous Ang II signalling via AT1R antagonism [5] (Fig. 1b). During the first wave of COVID-19 infection in early 2020, we have assessed via a telephone survey the impact of COVID-19 on 128 Gitelman's and Bartter's syndromes patients of our cohort living in the Northern Italy hotspots of the COVID-19 pandemic and observed that none of them had been affected or had symptoms related to the SARS-CoV-2 infection, compared with the adjusted prevalence of COVID-19 in the general northern Italian population, supporting the assumption that increased risk to COVID-19 due to increased ACE2 is unlikely [7]. In addition, a second survey on the same cohort, one year later, found that only eight patients resulted positive for COVID-19, four asymptomatic and four with very mild symptoms. SARS-CoV-2 entry mechanism is based on the binding with glycosylated ACE2, whose glycosylation is dependent by the acidic pH environment of the trans Golgi network/post Golgi pathways in the endosomes, and by several proteases including Cathepsin (Cat)-L, whose activity is also dependent by the acidic endosomal pH [8,9]. On the basis of the results of our surveys in Gitelman's and Bartter's syndrome patients and considering their characteristic chronic metabolic alkalosis, a possible alteration of ACE2 glycosylation and Cat-L activity and their relationships with patients’ metabolic alkalosis have been the subject of a very recent study of our laboratory set up to investigate in this human model on the possible presence of a specific intracellular environment not favourable to SARS-CoV-2 infection [3]. We have found that non-glycosylated ACE2 levels are significantly higher and Cat-L activity lower, in Gitelman's and Bartter's syndrome patients compared with healthy individuals [3]. In addition, Cat-L activity inversely correlates with blood bicarbonate levels and ACE2 glycosylated isoform approaches statistical significance [3]. At the beginning of COVID-19 pandemic, concerns have been raised for patients under treatment with ACE inhibitors (ACEi) and AT1R blockers (ARBs), as they upregulate ACE2, potentially facilitating the SARS-CoV-2 entry into the cells [10]. This has been, however, widely criticized [11]. It is known, in fact, that SARS-CoV-2 causes ACE2 downregulation impairing the ACE2-mediated-Ang II conversion into Ang 1-7 with the loss of its vasodilatory, anti-inflammatory, antioxidant and anti-atherosclerotic proprieties and of their protective impact on COVID-19 morbidity and mortality [12]. The use of ACEi or ARBs by blocking the excessive AT1R activation, would be instead beneficial, upregulating ACE2 activity, increasing Ang 1-7 levels and promoting the beneficial effects on lung and vascular inflammation [12]. In addition, genetic deletion of ACE2 worsens experimental ARDS in knockout mice models, while Ang 1-7 and ACEIs or ARBs administration attenuates the inflammatory response, markedly decreases lung injury scores and improves lung function [13]. A recent study has also demonstrated that pre-treatment with captopril or candesartan prevents SARS-CoV-2 spike protein internalization into human type II pneumocytes, prevents spike protein-induced pro-inflammatory cytokine response and 3 weeks of treatment with captopril or candesartan upregulates ACE 2 and MasR in rats lung [14]. Finally, RAS dysregulation with reduced expression of ACE2, Angiotensin II-type 2 receptors (AT2R) and MasR in the lung tissue of aged rats and rats with metabolic syndrome was reversed by captopril or candesartan [14], demonstrating that ACEi and ARBs are protective in SARS-CoV-2 infection by contributing to decrease the viral entry and the pro-inflammatory cytokine release despite the increased expression of ACE2 [14]. We feel that the results of our studies in Gitelman's and Bartter's syndrome patients to explore and better define the human RAS system, joined with those coming from studies in animal models, might provide further insights on the protective effects of ACE2 and of protease inhibitors on the prevention and treatment of SARS-CoV-2 infection in humans and give a rationale explanation for the data obtained with our surveys in Gitelman's and Bartter's syndrome patients. ACKNOWLEDGEMENTS There is no funding to declare. Conflicts of interest The authors have no conflict of interest to declare.