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      Covid-19: the renin–angiotensin system imbalance hypothesis

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          The emergency of SARS-CoV-2 in China started a novel challenge to the scientific community. As the virus turns pandemic, scientists try to map the cellular mechanisms and pathways of SARS-CoV-2 related to the pathogenesis of Coronavirus Disease 2019 (Covid-19). After transmembrane angiotensin-converting enzyme 2 (ACE2) has been found to be SARS-CoV-2 receptor, we hypothesized an immune-hematological mechanism for Covid-19 based on renin–angiotensin system (RAS) imbalance to explain clinical, laboratory and imaging findings on disease course. We believe that exaggerated activation of ACE/Angiotensin II (Ang II)/Angiotensin Type 1 (AT1) receptor RAS axis in line with reduction of ACE2/Angiotensin-(1-7)/Mas receptor may exert a pivotal role in the pathogenesis of Covid-19. In this perspective, we discuss potential mechanisms and evidence on this hypothesis.

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          SARS-CoV-2 infects T lymphocytes through its spike protein-mediated membrane fusion

          COVID-19, the novel coronavirus disease caused by SARS-CoV-2 and outbroken at the end of 2019 in Wuhan, China, 1 becomes a worldwide pandemic. SARS-CoV-2 belongs to the betacoronavirus genus and has 79.5% identity to SARS-CoV. SARS-CoV-2 uses angiotensin-converting enzyme 2 (ACE2) as its host entry receptor. 2 The clinical manifestations of COVID-19 include pneumonia, diarrhea, dyspnea, and multiple organ failure. Interestingly, lymphocytopenia, as a diagnostic indicator, is common in COVID-19 patients. Xiong et al. found upregulation of apoptosis, autophagy, and p53 pathways in PBMC of COVID-19 patients. 3 Some studies reported that lymphocytopenia might be related to mortality, especially in patients with low levels of CD3+, CD4+, and CD8+ T lymphocytes. 4,5 Lymphocytopenia was also found in the Middle East respiratory syndrome (MERS) cases. MERS-CoV can directly infect human primary T lymphocytes and induce T-cell apoptosis through extrinsic and intrinsic apoptosis pathways, but it cannot replicate in T lymphocytes. 6 However, it is unclear whether SARS-CoV-2 can also infect T cells, resulting in lymphocytopenia. To address this question, we evaluated the susceptibility of T lymphocytes to SARS-CoV-2 infection. To accomplish this, pseudotyped SARS-CoV and SARS-CoV-2 were packaged based on methods described previously. 7 The pseudoviruses could infect permissive cells (293T/ACE2 and Huh7 cells) expressing the ACE2 receptor, but could not infect nonpermissive cells (HeLa cells) (Fig. 1a). We used pseudovirus with equal infectivity to 293T/ACE2 cells (Fig. 1c) to infect two T lymphocyte cell lines, MT-2 and A3.01, with very low, or close to negative, expression level of hACE2 mRNA (Fig. 1b). Surprisingly, over several replicates, we saw that the T-cell lines were significantly more sensitive to SARS-CoV-2 infection when compared with SARS-CoV (Fig. 1c). In other words, these results tell us that T lymphocytes may be more permissive to SARS-CoV-2 infection and less permissive for SARS-CoV infection, similar to the findings in a previous study. 6 Therefore, it is plausible that the S protein of SARS-CoV-2 might mediate potent infectivity, even on cells expressing low hACE2, which would, in turn, explain why the transmission rate of SARS-CoV-2 is so high. It is also possible that other receptors mediate the entry of SARS-CoV-2 into T cells, such as CD147, present on the surface of T lymohocytes, 8 which was recently reported to be a novel invasive route for SARS-CoV-2. 9 Fig. 1 Sensitivity of T lymphocytes to SARS-CoV-2 infection. a Infection of pseudotyped SARS-CoV-2 and SARS-CoV. 293T/ACE2 and Huh7 are permissive cells, while HeLa is a nonpermissive cell line. b Expression of ACE2 mRNA in T cells. 293T/ACE2 cells used as a control. c Infection of pseudotyped SARS-CoV-2 and SARS-CoV on T lymphoid cell lines. d Inhibition of EK1 peptide on pseudotyped SARS-CoV-2 on MT-2 cells. e SARS-CoV-2 S-mediated cell–cell fusion on MT-2 cells. MT-2 cells were cocultured with 293T/SARS-CoV-2/EGFP cells. Cell–cell fusion was photographed under an optical microscope with fluorescence or visible light. Fused cells were indicated with a white arrow. Scale bars 800 μm. f Detection of SARS-CoV-2 NP-positive cells with flow cytometry. MT-2 cells were infected with SARS-CoV-2 at 1TCID50 per cell, respectively. Cells were fixed and permeabilized at 24 and 48 h post infection and immunolabeled for detection of SARS-CoV-2 NP. g Infection of MT-2 cells by SARS-CoV-2 is abortive. Cells were inoculated with SARS-CoV-2 at 1TCID50 per cell and harvested at 1, 6, 24, and 48 h post infection. Cell lysate (above) and supernatant (below) were collected to detect the viral N gene with RT-qPCR. In panels, bars and error bars represent means and standard deviations. Statistical analyses were performed using the unpaired t test. *p < 0.05 To assess if SARS-CoV-2 enters T lymphocytes through non-receptor-mediated endocytosis, we used EK1 peptide which has been shown to inhibit SARS-CoV-2 spike protein (S) mediated cell–cell fusion and pseudovirus infection. 7,10 Specifically, it inhibits receptor-mediated infection by interacting with HR1 to block the formation of the six-helix bundle (6-HB), further inhibiting fusion between viral and target cell membranes. We found that the EK1 peptide had significant inhibitory activity against SARS-CoV-2 pseudoviruses on MT-2 cells (Fig. 1d), suggesting that virus entry depends on receptor-mediated fusion. However, only a high concentration (40 μM) of EK1 had inhibitory activity on MT-2 cells. Meanwhile, the IC50 value of EK1 was 2.38 μM on 293T/ACE2 cells. 10 These results suggest that SARS-CoV-2 can also enter T lymphocytes through the receptor-mediated endocytosis pathway. To clarify, we performed a SARS-CoV-2 S-mediated cell–cell fusion assay according to previous studies. 7,10 After 48 h of coculture, 293T cells expressing SARS-CoV-2 S protein fused with MT-2 cells. Compared with unfused cells, the fused cells clustered together and appeared as a large faint green fluorescent mass. In contrast, no fused cells were found in the SARS-CoV coculture (Fig. 1e). Therefore, it can be concluded that SARS-CoV-2 might infect T cells through S protein-mediated membrane fusion. To further determine the susceptibility of MT-2 cells to live virus, we used SARS-CoV-2 to infect MT-2 cells and detected the SARS-CoV-2 nucleoprotein (NP) in the cells as reported previously. 6 Notably, several MT-2 cells were infected with SARS-CoV-2 (Fig. 1f). Quantitatively, the percentage of SARS-CoV-2 NP-positive MT-2 cells was 23.11% higher than that of uninfected cells at 24 h post infection, which is about 4.6-fold of the portion at 1 h (Fig. 1f). This result means that the virus penetrated MT-2 cells at 24 h and infected them. Given that MERS-CoV can efficiently infect, but not replicate, in T lymphocytes, 6 we further detected the number of viral genome copies at different time points post infection to explore the replication characteristics of SARS-CoV-2 in MT-2 cells. Similar to MERS-CoV, SARS-CoV-2 failed to replicate in MT-2 cells (Fig. 1g). The number of viral genome copies at 6 h was significantly higher than other time points in the cell lysate, but always remained steady at all time points in the supernatants. These results suggest that SARS-CoV-2 may enter MT-2 cells at 6 h post infection, but does not replicate, and then the viral RNA degrade. In supernatants, the detected viral copies might be the background of the residual virions, similar to the results of the previous MERS-CoV study (Fig. 1g). 6 Based on the results of pseudovirus and live virus infection, here we proved that (1) SARS-CoV-2 could infect T cells, (2) SARS-CoV-2 infected T cells through receptor-dependent, S protein-mediated membrane fusion, and (3) infection could be inhibited by EK1 peptide. However, we observed a very low expression level of hACE2 in T cells; therefore, we further proposed that a novel receptor might mediate SARS-CoV-2 entry into T cells. Similar to MERS-CoV, SARS-CoV-2 infection of T cells is abortive. A recent study reported that viral reads barely displayed in PBMC samples from COVID-19 patients through transcriptome sequencing of RNAs. Thus, it was inferred that SARS-CoV-2 could not infect PBMCs. However, the transcriptomic characteristics of PBMCs were detected and analyzed from three patients. Two SARS-CoV-2 reads were detected in one patient’s PBMCs, and zero reads in another. 3 This result could be attributed to nonproductive replication of SARS-CoV-2 in T lymphocytes, with little viral genome in PBMCs possibly degrading in the sample collection and RNA extraction process. Thus, the questions of SARS-CoV-2 infection and replication in primary T cells and whether the infection induces apoptosis in T cells still need further research, potentially evoking new ideas about pathogenic mechanisms and therapeutic interventions.
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            Angiotensin-converting enzyme 2 attenuates inflammatory response and oxidative stress in hyperoxic lung injury by regulating NF-κB and Nrf2 pathways

             Y. Fang,  F. Gao,  Z. Liu (2019)
            To investigate the role of angiotensin-converting enzyme 2 (ACE2) in hyperoxic lung injury. Adult mice were exposed to 95% O2 for 72 h to induce hyperoxic lung injury, and simultaneously treated with ACE2 agonist diminazene aceturate (DIZE) or inhibitor MLN-4760. ACE2 expression/activity in lung tissue and angiotensin (Ang)-(1–7)/Ang II in bronchoalveolar lavage fluid (BALF), and the severity of hyperoxic lung injury were evaluated. The levels of inflammatory factors in BALF and lung tissue and the expression levels of phospho-p65, p65 and IkBα were measured. Oxidative parameter and antioxidant enzyme levels in lung tissue were measured to assess oxidative stress. Finally, the expression levels of nuclear factor-erythroid-2-related factor (Nrf2), NAD(P)H quinine oxidoreductase 1 (NQO1) and heme oxygenase-1 (HO-1) were measured using Western blotting. Hyperoxia treatment significantly decreased lung ACE2 expression/activity and increased the Ang II/Ang-(1–7) ratio, while co-treatment with hyperoxia and DIZE significantly increased lung ACE2 expression/activity and decreased the Ang II/Ang-(1–7) ratio. By contrast, co-treatment with hyperoxia and MLN-4760 significantly decreased lung ACE2 expression/activity and increased the Ang II/Ang-(1–7) ratio. Hyperoxia treatment induced significant lung injury, inflammatory response and oxidative stress, which were attenuated by DIZE but aggravated by MLN-4760. The NF-κB pathways were activated by hyperoxia and MLN-4760 but inhibited by DIZE. The Nrf2 pathway and its downstream proteins NQO1 and HO-1 were activated by DIZE but inhibited by MLN-4760. Activation of ACE2 can reduce the severity of hyperoxic lung injury by inhibiting inflammatory response and oxidative stress. ACE2 can inhibit the NF-κB pathway and activate the Nrf2/HO-1/NQO1 pathway, which may be involved in the underlying mechanism.
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              Atlas of tissue renin-angiotensin-aldosterone system in human: A transcriptomic meta-analysis

              Tissue renin-angiotensin-aldosterone system (RAAS) has attracted much attention because of its physiological and pharmacological implications; however, a clear definition of tissue RAAS is still missing. We aimed to establish a preliminary atlas for the organization of RAAS across 23 different normal human tissues. A set of 37 genes encoding classical and novel RAAS participants including gluco- and mineralo-corticoids were defined as extended RAAS (extRAAS) system. Microarray data sets containing more than 10 normal tissues were downloaded from the GEO database. R software was used to extract expression levels and construct dendrograms of extRAAS genes within each data set. Tissue co-expression modules were then extracted from reproducible gene clusters across data sets. An atlas of the maps of tissue-specific organization of extRAAS was constructed from gene expression and coordination data. Our analysis included 143 data sets containing 4933 samples representing 23 different tissues. Expression data provided an insight on the favored pathways in a given tissue. Gene coordination indicated the existence of tissue-specific modules organized or not around conserved core groups of transcripts. The atlas of tissue-specific organization of extRAAS will help better understand tissue-specific effects of RAAS. This will provide a frame for developing more effective and selective pharmaceuticals targeting extRAAS.

                Author and article information

                Clin Sci (Lond)
                Clin. Sci
                Clinical Science (London, England : 1979)
                Portland Press Ltd.
                June 2020
                08 June 2020
                : 134
                : 11
                : 1259-1264
                Interdisciplinary Laboratory of Medical Investigation, Faculty of Medicine, Federal University of Minas Gerais (UFMG), Belo Horizonte, MG, Brazil
                Author notes
                Correspondence: Ana Cristina Simões e Silva ( acssilva@ )

                These authors contributed equally to this work.

                © 2020 The Author(s).

                This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.0 (CC BY-NC-ND).

                Page count
                Pages: 6
                Host-Microbe Interactions
                Immunology & Inflammation
                Molecular Interactions
                Molecular Bases of Health & Disease


                angiotensin-converting enzyme 2, covid-19, renin-angiotensin system, sars-cov-2


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