RNA into viral particles (Narayanan et al., 2000) . The foremost function of the N protein is to package the genomic RNA 19 to form nucleocapsid (de Haan and Rottier, 2005) . It may likely be involved in replication process and the host cellular 20 response to viral infection (McBride et al., 2014) . The E protein is the minor component of the coronavirus envelope, but 21 it is highly expressed in the ER and Golgi-complex of the infected cell where it helps in assembly, budding and trafficking 22 of infectious virions (Schoeman and Fielding, 2019). 23
SARS-CoV-2 infects by binding to ACE2 via its S protein and enters the host cells (Gheblawi et al., 2020). To facilitate 25 the fusion and entry of the viral particle into the host cell the spike is primarily activated by a serine protease present on 26 J o u r n a l P r e -p r o o f heart, vessels, kidney, brain, gastrointestinal system, lung alveolar epithelial cells and other tissue cells (Oudit et al., 1 2003) . It is composed of 805 amino acids, with an active extracellular carboxypeptide domain, a transmembrane domain 2 and a small cytoplasmic domain. The extracellular carboxypeptide domain is cleaved from the enzyme by ADAM17 (a 3 disintegrin and metalloprotease 17) and released as soluble protein into the blood stream and finally excreted into urine. 4
Ang II is the main substrate for ACE2 and it cleaves the last amino acid (Phe8), in Ang II to generate Ang (1-7) which 5 upon binding to G-protein coupled Mas R antagonizes the Ang II/AT 1 receptor axis of the RAS. Alternatively, ACE2 can 6 covert Ang I to Ang (1-9), which is then converted to Ang (1-7) by the action of neprilysin. Chappell, 2016). However, the affinity of ACE2 for Ang II is higher than for any other peptide thus conversion of Ang II 9
to Ang (1-7) is considered as its prime function (Vickers et al., 2002) . Evidence demonstrates that an alternative 10 mechanism within RAS may contribute to the positive effects of chronic RAS blockade. The chronic inhibition of ACE 11 decrease Ang II expression but significantly increases circulating Ang (1-7) levels through the action of neprilysin 12 Thus, ACE2 is a multifunctional enzyme that exerts several physiological functions by cleaving a variety of 25 biologically active peptides 26
Mounting evidence indicates that ACE2 exhibits protective functions in cardiovascular system and its altered expression 1 is linked to major pathophysiological effects. ACE2 is considered as an endogenous modulator of RAS through its effects 2 on Ang II degradation (Tesanovic et al., 2010) .Contrarily, conversion of Ang II to Ang (1-7), by ACE2 shifts the balance 3 towards cardioprotection by activating ACE2/ Ang (1-7)/Mas receptor axis, a vasodilatory arm of the RAS with effects 4 opposite to those produced by ACE/Ang II/AT 1 receptor (Shah et al., 2010; Patel et al., 2016) . Activation of ACE2/Ang 5
(1-7)/Mas receptor axis antagonizes the pathophysiological effects of Ang II in myocardium and enhances vasodilation 6 abilities, attenuates cardiac hypertrophy, fibrosis, cell proliferation, and oxidative stress. These protective effects are due 7 in part, to a decrease in Ang II levels as a result of its degradation to Ang (1-7) by ACE2 (Varagic et al., 2014 was not found to regulate cardiac structure and functions in ACE2 null mice. Blocking of Ang II generation by ACEIs or 18 inhibition of its binding to AT 1 receptor by ARBs have been shown to increase ACE2 expression and activity in animal 19 models of myocardial infarction-induced cardiac hypertrophy (Ocaranza et al., 2006) . These observations collectively 20 suggest that decrease in ACE2 is linked to impaired cardiac function whereas excess of ACE2 would be protective to 21 hypertension-induced cardiac dysfunction. In contrast myocardial ACE2 expression was found to be significantly 22 J o u r n a l P r e -p r o o f observations suggest that ACE2 upregulation is a compensatory response to protect heart and plays an important role in 1 the negative regulation of RAS by counter-balancing the effects of Ang II after cardiac injury. 2
SARS-CoV entry into host cells was primarily thought to be accomplished by direct fusion between the host cell 4 membrane and the virus (Simmons et al., 2004) . Proteolytic cleavage of SARS-CoV protein at S2 region of spike protein CoV-2 entry could be blocked by TMPRSS2 inhibitor and suggests that sera from patients recovering from SARS-CoV 19 could partially protect against SARS-CoV-2 infection (Hoffmann et al., 2020). Though SARS-CoV-2 shares a close link 20 with SARS-CoV, the former binds to ACE2 with higher affinity due to differences in RBM of spike region. This tight 21 association between the SARS-CoV2 and ACE2 could partly explain the reason for the higher pathogenicity of SARS-22
CoV-2 in humans compared to SAR-CoV. 2020). It was followed by another retrospective analysis comparing the ACE2 expression in nasal passages in a cohort of 9 305 individuals aged 4 to 60, as the nasal passage is often the first point of contact for SARS-COV-2. ACE2 expression 10 was found to be age dependent, with the lowest expression levels found in younger children and increasing with age into 11 adulthood (Bunyavanich et al., 2020). Down regulation of ACE2 expression and its activity is associated with acute lung 12 injury, increased accumulation of inflammatory cytokines, exacerbated pulmonary edema that ultimately lead to ARDS. 13 2020) . Activation of RAS is known to cause the wide range of injurious effects in multiple organs such as heart, lungs 18 and kidney, thus RAS blockers might probably relieve of the pathological symptoms of COVID-19 on lung and 19 unconditionally decrease heart and renal damage resulting from the RAS activation. 20
ACEIs and ARBs are generally used for the management of cardiovascular diseases. ACEIs block the conversion of Ang I 22 to Ang II by targeting ACE and are used in patients with hypertension and type 2 diabetes mellitus for regulating blood 23 pressure, treating cardiac failure and dysfunction, preventing strokes and treating diabetic nephropathy. While on the other 24 hand, ARBs prevent binding of Ang II by blocking AT 1 receptor and are used alone or in combination with other drugs for 25 controlling hypertension. They also are used for treating congestive heart failure, preventing diabetes or high blood 26 pressure-related kidney failure, and reducing the risk of stroke in patients with hypertension and an enlarged heart. Nearly 1 all patients with cardiovascular comorbidities qualify for the use of ACEIs and ARBs (Messerli et al., 2018) . Chronic 2 treatment with ACEIs or ARBs has been shown to elevate ACE2 activity and expression in several organs which may be 3 beneficial for ACEIs increased cardiac ACE2 gene expression levels but not its activity in normal Lewis rats. However, combined 9 treatment with lisinopril and losartan abolished the increase in gene expression but significantly increased ACE2 activity. 10
These results reveal a lack of direct correlation between ACE2 mRNA levels and ACE2 activity indicating the existence 11 of the complex ACEI and ARB signaling mechanism by that regulate kinetics of angiotensin peptide formation and there is no data on the effect of ACEIs and ARBs on ACE2 expression in lungs either in animal models or in humans. 18
Given the role of ACE2 as the point of entry for SARS-CoV-2, there is a continuous debate in the scientific community 19 about the role of ACEIs/ARBs in COVID-19. Some researchers initially suggested that patients should discontinue 20
ACEIs/ARBs as it might predispose them to increased risk of SARS-CoV-2 and could make COVID-19 symptoms worse 21 by increasing ACE2 expression. The argument was based on the assumption that RAS blockers increase ACE2 expression 22 which might increase the susceptibility of SARS-CoV-2 infection (Guo et al., 2020). However, the view of discontinuing 23
ACEIs/ARBs was not supported by numerous American and European cardiology societies who dispelled this 24 misinformation and recommended patients not to stop ACEIs/ARBs, unless advised by the physician. Besides there are no 25 clinical evidence to date suggesting that ACEIs/ARBs make patients more susceptible to the SARS-CoV-2. We highlight 26 that discontinuation of RAS antagonists might exacerbate the clinical course and increase the death rate in COVID-19 27 patients suffering from hypertension or ischemic heart disease due to their acute risk. To support this argument, a very 1 well organized Chinese study revealed that hospitalized hypertensive COVID-19 patients, treated with ACEIs/ARBs for 2 hypertension showed lesser risk of all-cause mortality compared with non-users of ACE inhibitor/ARB during a 28 days 3 of follow-up . Furthermore, there are compelling evidences supporting the use of ACEIs and ARBs 4 assertively because blockade of RAS may protect the COVID-19 patient form severity of lung damage and potentially 5 from cardiovascular abnormalities caused by infection. ACE2 is functionally opposite to ACE and under physiological 6 conditions; ACE2 displays vasodilator activity by inactivating Ang II whereas ACE acts as vasoconstrictor by increasing 7
Ang II levels, to maintain the blood pressure homeostasis. Binding of the SAR-CoV-2 to ACE2 downregulates ACE2, 8 thus causing an imbalance ACE/ACE2 ratio (Pagliaro and Penna, 2020), and shifting it towards ACE and increased Ang 9 II generation. Increase in ACE/Ang II/AT 1 receptor axis stimulation is known to enhance pulmonary vascular 10 permeability and acute lung injury (Imai et al., 2005). Consequently, upregulation of ACE2 by RAS blockers could be 11 beneficial in lung damage by not only inhibiting Ang II production and but also by degrading Ang II to vasodilatory 12 peptide Ang (1-7), which appears to be vital in protecting against lung inflammation and fibrosis. 13
To date no specific drugs and therapeutics are approved by any food and drug association to prevent or treat SARS-CoV-2 15 infection. The best clinical strategy for the treatment of COVID-19 positive patients is purely supportive care that includes 16 supplemental oxygen and mechanical ventilator support if required and control measures to prevent infection (Wax and 17 Christian, 2020). However, there is a race at several fronts to develop effective therapies for COVID-19. In our opinion 18 use of pharmacological agents that activate ACE2/Ang (1-7)/Mas receptor axis to promote the protective effects and RAS 19 blockers to mitigate the detrimental actions of ACE/Ang II/AT 1 receptor axis activation will not only preserve ACE2 20 functions but also blunt the cytokine storm elicited by SARS-CoV-2 infection (Fig.-4) . 21
Significance of Ang (1-7) as a cardioprotective peptide was highlighted with the discovery of ACE2, which hydrolyzes 23
Ang II into Ang (1-7) that binds to a G-protein coupled Mas receptor to counter-regulate the deleterious effects elicited by 24 ACE2 is a key negative regulator of RAS, however, binding of SARS-CoV-2 to cell bound ACE2, downregulates its 16 expression thereby disrupting the subtle balance between ACE2 and ACE leading to deregulation. This imbalance in RAS 17 by SARS-CoV-2 infection blunts ACE2 mediated protective effects and upregulates counter-regulatory Ang II-induced 18 pathogenesis. Based on our review, we postulate that inhibition of ACE/Ang II/AT 1 receptor axis by using ACEIs and 19
ARBs besides being critical for the clinical management of cardiovascular disease in COVID-19 patients, might improve 20 acute lung injuries by switching the balance from deleterious ACE/Ang II/AT 1 receptor axis towards protective 21 ACE2/Ang (1-7)/Mas receptor axis. However, more studies are required to understand the role of ACE2 and therapeutic 22
The authors would like to thank Dr. Chansu Park (Chonbuk National University) and Dr. James Sowers (University of 25 Missouri) for critical reading of the manuscript. 26
This project was funded by National Plan for Science, Technology and Innovation (MAARIFAH), King Abdul-Aziz City 2 for Science and Technology, Kingdom of Saudi Arabia, (Project no. 12-MED3221-02). 3
The authors declare no conflicts of interest. by renin to form the Ang I, which is then cleaved by ACE to form Ang II. Ang II is the main peptide of RAS and exerts it 22 effects by binding to two GPCRs AT 1 receptor and AT 2 receptor. Binding of Ang II to AT 1 receptor stimulates 23 vasoconstrictive/pro-inflammatory pathways, whereas its binding to AT 2 receptor activates the vasodilative/anti-24 inflammatory signaling. Ang II is further cleaved by ACE2 to Ang (1-7) , which binds to Mas receptor and promotes 1 vasodilative/anti-inflammatory effects. Ang (1-7) can also be generated directly from Ang I by the action of neprilysin 2 (Nep) or by the action of ACE2 on Ang I to form Ang (1-9), which is subsequently cleaved to generate Ang (1-7) through 3
Nep. 4 Li, Q., Guan, X., Wu, P., Wang, X., Zhou, L., Tong, Y., Ren, R., Leung, K.S.M., Lau, E.H.Y., Wong, J.Y., Xing, X., Xiang, N., Wu, 8 Y., Li, C., Chen, Q., Li, D., Liu, T., Zhao, J., Liu, M., Tu, W., Chen, C., Jin, L., Yang, R., Wang, Q., Zhou, S., Wang, R., 9
Liu, H., Luo, Y., Liu, Y., Shao, G., Li, H., Tao 