Infection caused by Novel Coronavirus has been declared a pandemic and is spreading in 213 countries and territories globally with severe implications.Due to the high rate of transmission of the virus throughthe aerosol and the novelty of the infection to humans, the illness has become a global emergency.
The angiotensin converting enzyme-2(ACE-2) has been recognized as the receptor for the SARS-CoV-2 viral entry.We review the physiological functions of ACE2, presence of ACE2 in different organs of the body and ACE2 relationship with Covid 19 and other diseases.The goal is to provide the reader with an understanding of the complexity andimportance of this receptor.Understanding the regulation of ACE2 in the context of COVID-19 could help understand the physiopathology of the illness and lead to the development of preventive and therapeutic drugs that help fight the pandemic illness. As such, it is now getting renewed attention as a possible target for anti-viral therapeutics.
Associating the occurrence of the disease in diverse geographical areas having genetically diverse ethnicities, can offer a scientific foundation for future researches to implement protocols on COVID-19 treatment, considering the rapid advent of the coronavirus disease and the current data shortfall on the virus pathophysiology. The paper highlights the fact that since ACE2 gene polymorphisms is population specific, screening of ACE2 polymorphisms in varied population groups across geographies could be helpful in evaluating the severity to SARS-Cov-2 infection
ACE2 is a transmembrane glycoprotein with a single extracellular catalytic domain which plays an important regulatory role in the renin-angiotensin system (RAS). ACE2has different roles such as catalytic, transporter of amino acids or viral receptor. It has an important role in different systems, from cardiovascular regulation to viral infection. It catalyses the formation of angiotensin II from angiotensin I and so playing a key role in the control of cardio-renal function and blood pressure control (Medina‑Enriquez et al.,2020).
ACE2 is part of the Renin Angiotensin System (RAS), the keysystemaccountable for the regulation of systemic arterial pressure. In addition, the RAS also has a local or paracrine function. RAS is anintricate system that is involved in many biological processes. The functions of the system are wide, which include inflammation, angiogenesis, cell proliferation, memory, sodium and water reabsorption, thrombosis, and plaque rupture (Perdomo-Pantoja et al.,2018;Wegman-Ostrosky et al.,2015).ACE2 Function
ACE2 is a main element of the RAS system. Initially, angiotensinogen is converted into Angiotensin I (Ang I) through the action of renin, an aspartyl protease which is produced in kidney. Angiotensin I (Ang I) is converted intoangiotensin II (AngII) by the action of angiotensin-converting enzyme (ACE) which induces an increased blood pressure promoting vasoconstriction and inflammation. Lastly, ACE2 converts AngII to Ang-(1–7) which is vasodilatory agent. The increase in the activity of ACE2 might reduce the RAS system by inactivating and enhancing the production of Ang-(1–7). Ang (1-7) polypeptide, which has anti-inflammatory functions like protecting cardiomyocytes, relaxing blood vessels, anti-proliferation, and can enhance the activity of bradykinin which is an inflammatory mediator(Simões e Silva et al.,2016).ACE2 Protein
The human ACE2 protein is a zinc metallopeptidase which is an ectoenzyme (family of dipeptidyl carboxypeptidase). It contains 805 amino acids. This protein is a kind of transmembrane glycoprotein and its expression is abundant with a single extracellular catalytic domain that mainly localizes at the plasma membrane (Santos et al.,2003; Hamming et al.,2007). There are two forms of the ACE2 protein. The first form of ACE2 proteincontains a structural transmembrane domain and points its extracellular domain to the plasma membrane. The second form is the soluble form, which circulates in small amounts in the blood (Batlleet al.,2020).
The ACE2 gene length is 39.98 kb of genomic DNA and contains 18 exons(Turner et al.,2002). It maps to chromosome X at position Xp22.8It encodes a type I cell-surface glycoprotein of about 100 kDa which is composed by 805 amino acids and characterized by a Nterminal signal peptide of 17 amino acid residues, a peptidase domain (PD) with its HEXXH zinc binding metalloprotease motif, a C-terminal Collectrin (a regulator of renal amino acid transport and insulin)-like domain (CLD) that includes a ferredoxin-like fold “Neck” domain, that end with an hydrophobic transmembrane hydrophobic helix region of 22 amino acid residues followed by an intracellular segment of 43 amino acid residues (Li et al., 2003; Cerdà-Costa and Xavier Gomis-Rüth, 2014).ACE2 IN BODY
ACE2is an enzyme attached to the cell membranes of cells located in the lungs, arteries, heart, kidney, and intestines ( Hamming et al.,2004; Donoghue et al.,2000) . In 2002, Harmer et al. (Harmer et al.,2002)studied the presence of ACE2 and found that the mRNA of ACE2 is expressed in 72 different tissues. ACE2 present in epithelial cells, which line certain tissues and create protective barriers.ACE2 is attached to the cell membrane of mainly lung type II alveolar cells, enterocytes of the small intestine, arterial and venous endothelial cells, and arterial smooth muscle cells in most organs. ACE2 mRNA expression is also found in the cerebral cortex, striatum, hypothalamus, and brainstem( Medina‑Enriquez et al.,2020).ACE2 expression is high on the luminal surface of intestinal epithelial cells, in this context, ACE2 functions as a co-receptor for nutrient uptake (Hashimotoet a.,2012).ACE2 has other essential actions such as a non-catalytic function and the regulation of renal amino acid transport and pancreatic insulin secretion (Batlle et al.,2010).
The renin–angiotensin–aldosterone system (RAAS) is a key regulator of systemic blood pressure and renal function and a key player in renal, lung and cardiovascular disease. Ang II is the main effector substance of the RAAS, with potent vasoconstrictive, pro-inflammatory, and pro-fibrotic properties. Ang II is converted into Ang(1–7) which mediates vasodilatation, antiproliferation, and apoptosis, thereby opposing the effects of Ang IIby the action of ACE2. Thus, ACE2 play important role in RAAS system(Hamming et al.,2007).
In the kidney, Ang 1-7 formed by ACE2 acts on the G protein-coupled receptor (GPCR) MASwhich leads to vasodilation, anti-fibrosis, anti-proliferation, and anti-inflammatory vascular protection.
ACE2 isvastly expressed in kidney, mostly in brush border cells of proximal renal tubules, endothelial cells, smooth muscle cells of renal vessels, and podocytes. According to studies,ACE2 gene knockout can lead to an increase in blood pressure, glomerular damage, and renal fibrosis in diabetic mice and Exogenous human recombinant ACE2 can decelerate the developmentof diabetic nephropathy (DKD) (Anguiano et al.,2017). In study it has been found that in damaged renal tubules, the increase of Ang II may be a possible mediator for further renal damage in human renal diseases.Hypertensive nephropathy is a widespread complication of hypertension, mainly due to inflammation associated with Ang II, oxidative stress, and renal fibrosis.
According to study conducted inthe school of medicine of Jilin University found thatGinsenoside Rg3 can alleviate the Ang II-mediated renal injury in rats and mice by upregulating ACE2 in renal tissue(Liu et al.,2019).ACE2 and Cardiovascular Disease
ACE2 is broadly present in cardiomyocytes, cardiac fibroblasts, and coronary artery endothelial cells. ACE2 is avital regulatory protein in RAS. RAS system controls the balance of body fluid and blood pressure and sustains the tension of blood vessels.
The overactivation (increase of vasoconstriction) or reduction (decrease of vasodilation) of RAS will lead to vascular dysfunction, which is the majorreason of atherosclerosis and cardiovascular disease (CVD) (Anguiano et al.,2017).
Oudit et al., have found that ACE2 knockout mice have serious cardiac dysfunction, and ACE2 may have the potential to transform and regulate heart function.Researchers discovered that local overexpression of ACE2 significantly reduced the development of early atherosclerosis.Uri et al., confirmed the linkamong the activity of ACE2 in serum and the deterioration of heart failure. The reduction of serum ACE2 activity is a selective biomarker of cardiac dysfunction. Thus, ACE2 is the majordefensive pathways against heart failure.ACE2 and Diabetes
RAS and natriuretic peptide system (NPS)are the majorreasons of the occurrence and development of diabetic cardiomyopathy (DCM). Neutral lysozyme inhibitors can protect the cardiovascular system via increasing NPS levels and the increase of RAS blockers which include angiotensin blockers, ARB, and ACE inhibitors. The stimulation of the ACE/Ang II/AT1 receptor pathway is linked to processes like inflammation, oxidative stress, fibrosis, and insulin resistance (Yong et al.,2013).
Experiments in diabetic mouse models demonstrated that the ACE/Ang II/AT1 receptor pathway was upregulated, while the ACE2/Ang (1-7)/MAS receptor pathway was downregulated, in both the retinae and kidneys. This is also established in the kidneys of type 2 diabetic patients with diabetic nephropathy. These data suggest that the imbalance between the ACE/Ang II/AT1 receptor and ACE2/Ang (1-7)/MAS receptor pathways is a base for the occurrence and development of diabeticcomplications(Zhang et al.,2015).
SARS-CoV-2 binds to human angio-tensin-enzyme II (ACE2), using it as a cell entry receptor to invade respiratory and lung epithelium through the spike (S) protein (Zhou et al., 2020a,2020b) Studies show that, like the severe acute respiratory syndrome coron-avirus (SARS-CoV) that caused SARS, SARS-CoV-2 binds to human angio-tensin-enzyme II (ACE2), using it as a cell entry receptor to invade respiratory and lung epithelium through the spike (S) protein (Zhou et al., 2020a,2020b) Studies show that, like the severe acute respiratory syndrome coron-avirus (SARS-CoV) that caused SARS, SARS-CoV-2 binds to human angio-tensin-enzyme II (ACE2), using it as a cell entry receptor to invade respiratory and lung epithelium through the spike (S) protein (Zhou et al., 2020a,2020b) SARS-CoV-2 utilizes angiotensin receptor (ACE) 2 to enter human cells (Donoghue et al., 2000; Turner et al., 2002; Li et al., 2003). Similarto other CoV, during viral entry into the host cell, the spike proteins (S) on the envelope of SARS-CoV-2 are cleaved into S1 and S2 subunits (Kira chdoerfer et al., 2016). S2 does not interact with the receptor but it harbours the functional elements required for membrane fusion of the virion. The S1 protein/receptor interaction is the pivotal determinant for SARS-CoV-2 to infect a host species. S1 contains the receptor binding domain (RBD) and directly binds to the peptidase domain (PD) of ACE 2 to enter host cells (Turner et al., 2002; Li et al., 2003; Yan et al., 2020)
As shown in figure,(A) in normal state without infection of SARS-CoV-2,there is anequilibrium in ACE andACE2 receptor activity. ACE controls the Renin Angiotensin Aldosterone system (RAS) and cuts Ang I to produce Ang II. Ang II is a powerful vasoconstrictor andharmful forendothelial and epithelial function through activating AT1 and AT2 receptors. The balance of the RAS/Ang II output is controlled by ACE2 andMas/G protein coupled receptor activity. ACE2 cuts Ang I and Ang II into Ang-1-9 and Ang1-7, respectively, thus it triggers MAS/G protein coupled receptorthat guard cell death. (B) In case of SARS-CoV-2 infection, SARS-CoV-2binds to ACE2 to enter into epithelial cells of the lungs. Cleavage of spike proteins by a protease such as trypsin/cathepsinG and or ADAM17 on ectodomain and TMPRSS2 of endodomain sites help viral entry into the cells. This process leads to flaking of host ACE2 receptors anddamage of its protective function. Damage of function of ACE2 activity stops production of Ang 1-9 and Ang1-7. Lack of Ang1-7 reduces the activity of MAS/G receptor which leads to the loss of its defensive functions which includes vasodilatation, cell protection both at the epithelial and endothelial sites. Loss of ACE2 leads to animbalance and unrestricted effects of Ang II and upregulation of RAS/Ang II pathway. Upregulation of Ang II trigger vasoconstriction, thrombophilia, microthrombosis,alveolar epithelial injury, and respiratory failure. Hence, preventing the proteolytic function of trypsin/cathepsin and ADAM17 or TMPRSS2 and or direct activation ofMAS/G receptor by enhancing Ang-(1-7) can overcome the damage of function ACE2 and are viable targets to avert tissue damage to the host(Samavati and Uhal.,2020).
SARS-CoV-2 has a higher ACE2 bindingaffinity than other CoV(Li et al.,2003). According to research of Shang et al. , the3-dimensional structure of the SARS-CoV-2 binding sitehas anextra compact conformation, enhanced binding stability,and potentially improves the ACE2 receptor bindingaffinity. According to Sequence-based prediction studies, amore effective cleavage site inserted at the edge ofthe S1/S2 subunits of the spike S protein (a host proproteinconvertase, furin). This polybasic furin-type cleavagesite is unique, can increase the virus capacity to internalizeinto cells (Liuet al.,2020). Additionally, studies through surface plasmonresonance have confirmed that the ACE2 binds to theectodomain of the SARS-CoV-2 spike glycoprotein withabout 10- to 20-fold higher affinity than the S protein ofthe SARS-CoV(Liu et al.,2020). These diversefeaturesmay clarify the higher SARS-CoV-2 infectivity.
The angiotensin-converting enzyme inhibitors have been proposed as possiblehelpful treatments for COVID-19(Sun et al.,2020).According to the study of Monteil et al. it was found that human recombinant soluble ACE2 could block SARS-CoV-2 cell attachment (Monteil et al. 2020).
Morbidity and mortality due to COVID-19 increase with age and co-existing health conditions which include cancer, cardiovascular diseases,and although most infected individuals recover, even very young and healthy patients may randomly succumb to this disease (Dong et al.,2020). These explanationsbeg the question of how much of the variation inCOVID-19 disease severity may be clarified by geneticsusceptibility.Human genetic factors may contribute tothe very high transmissibility of SARS-CoV-2 andto the persistently progressive disease observed in a smallbut noteworthy proportion of infected individuals.
Growth of newpreventive and/or therapeutic approaches for COVID-19will be significantly facilitated by methodical identification ofhost genetic pathways and DNA polymorphisms (variants)which modify the risk of infection and severe illness,including the overexuberant immune response tothe virus.
COVID-19 pandemic had vasthealth and financial impacts in 188 countries/regionsacross the world, but the disease has also hit in differentracial/ethnic subpopulations. Huge genetic studiesin people of geographically variedlineage haveconfirmedconsiderable genetic variation in proteincoding regions, with broadlyvariable allele frequencies (Lek et al.,2016). SARS-CoV-2 infection depends on the host cell angiotensin-converting enzyme 2 (ACE2) for accessinto cells for SARS-CoV-2 spike (S) protein (Hoffmann et al.,2020).
According to studies, occurrence and mortality rates are significantlydiverseamong male and female COVID-19patientsand the disease is related with pre-existingconditions, such as cancer and cardiovascular disorders and individuals with hypertension gettinganti-hypertensive medications (Guo et al.,2020). So, a systematicstudy of the functional polymorphisms inACE2 among different populations couldpave the technique for precision medicine and personalizedtreatment strategies for COVID-19.
The expression level and expression pattern of human ACE2 in diverse tissues might be critical for the susceptibility, symptoms, and outcome of COVID-19.
Scientist Hou and his teams have investigated genetic susceptibility to COVID-19 by examining DNA polymorphisms in ACE2 genes. They collected a total of 437 non-synonymous singlenucleotide variants (SNVs) in the protein-coding regions of ACE2 from three databases:(i) Genome Aggregation Database (gnomAD v3: gnomad. broadinstitute.org, covering 9 geographical areas), (ii) Exome Sequencing Project (ESP: evs.gs.washington.edu/EVS/), and (iii) 1000 Genomes Project (1KGP, www.internationalgenome.org)and used ANNOVAR (Wang et al.,2010) to interpret all non-synonymous variants. By applying Polyphen2 and CADD (Combined Annotation Dependent Depletion) scores, theyrecognized 63 possibly deleteriousvariants in ACE2.
Theyfound that the circulation of deleterious variantsin ACE2 differs among 9 populations.Precisely, 39% (24/61) and 54% (33/61) of deleteriousvariants in ACE2 occur inAfrican/African-American(AFR) and Non-Finnish European (EUR)populations,respectively. Occurrence of deleterious variantsamong Latino/Admixed American (AMR), East Asian(EAS), Finnish (FIN), and South Asian (SAS) populationsis 2–10%, while Amish (AMI) and Ashkenazi Jewishpeople do carry such variantsin ACE2 coding regions.
AFR populations carryp.Met383Thr and p.Asp427Tyr variants having allele frequenciesof 0.003% and 0.01%, individually.Thep.Pro389His only happens in the AMR populations having an allele frequency of 0.015%. The p.Arg514Gly is a smallallele frequency (0.003%) variant in AFR populations.This ACE2 variant is positionedin the angiotensinogen (AGT)-ACE2 contactsurface, which is estimated to influence RASfunction. The RAS is importantfor regulation of blood pressure, sodium, and fluid balance,and its disfunction is related with cardiovascularand kidney illnesses(Kuster et al.,2020).
The EUR populationcarries the p.Arg708Trp, p.Arg710Cys, p. Arg710His,and p.Arg716Cys variants having allele frequency of0.01~0.006%.whereas the EAS and the AMR populationscarry p.Arg708Trp and p.Arg710His having allele frequency of 0.04% and 0.01% . Adding to these four variants, p.Leu731Phe has themain allele frequency in the AFR and EUR populations.In total, these comparative genetic analyses recommendthat ACE2 genomic variants may play significant roles insusceptibilities to COVID-19 and its related cardiovascularconditions by changing AGT-ACE2 pathway. In addition to variance polymorphismswhich may explain susceptibility and even consequence indifferent ethnic populations.
Scientist Hatami et al.,discovered a probableassociation between ACEI/D allele frequency and the COVID-19 recovery rate. Also,this study disclosed that the ACE I/D allele ratio is very varieduniversally. East Asian countries such as China andJapan had a ratio of more than 1 which showed a higher rateof I allele frequency. In East Asia, there seemed to bemore I-alleles than D in the people genome. Though, South Korea has an allele ratio of 0.87 which is higher than in European countries.Like thisstudyscientistSaab et al alsostudied the averageACE II genotype frequency in different populations of different nations(Saab et al.,2007). They revealed thatACE I allele and genotype frequencies display an associationwith longitude.
As per study of Saab and group, a strongassociationamong II genotype frequency and longitude,shown by a decrease in the II frequency from Europeto the Middle East. Summarily, the normal frequency of the IIgenotype in the north of Europe and Denmark was 23%,whereas the UK and Spain recorded 20% and 15%, whilenorthern and southern Italy registered 14% and 12%, in addition China and Japan averaging 35% and 45% respectively (Saab et al.,2007).
Likewise, this result was established in our study, as Denmarkhad an ACE I/D ratio of 1.01, which was higher than inother European countries. The result of Saabet al.’s study about the lowest II genotypes of Europe inItaly, makes us ponder about a likely role of genetic factorsand the disaster of the COVID-19 outbreak in Italy with ahigh case-fatality rate (Livingston and Bucher.,2020).
European countries like Italy,Spain, and the UK have been extensively affected by COVID-19. It is well recognized that ACE I allele frequency in Europeis lesser than in East Asia as seen in our result of this study, whichshowed the average ACE I/D allele ratio in Europe as0.55, whereas it was 0.93 in the Asia. This discoveryapproves the results of Wang et al.’s study as they projectedthe averageACEfrequency in East Asia to be 0.63and 0.43 in Europe (Wang et al.,2013).
Khayat and his team propose a biological aspect, the genetic variation at the viral S protein receptor in human cells, ACE2 (angiotensin I-converting enzyme 2), which may pay to the worse consequence in males and in some regions globally. Theyaccomplishedexomics analysis in native and admixed South American populations, also conducted in silico genomics databank research in residents from other continents. ten polymorphisms in coding, noncoding and regulatory sites were found that can propose a reasonable biological clarification for these epidemiological differences. In conclusion, there are ACE2 polymorphisms that could affect epidemiological differencesdetected among lineage and, alsobetween sexes(Khayat et al.,2020).
The racial change of ACE gene polymorphism is wellrecognized. In the United States, AfricanAmericans are identified to have the highest frequency of the Dallele (89%) when compared with Indians (69%) and whites(69%) (Mathew et al.,2001).In Europe, people in Italy, Spain, and Francehave a high frequency of D allele up to 82% to 87% (Lee et al.,2002)while in Asian populations, such asChinese, Korean, Taiwanese, and Japanese, have a highfrequency of ACE gene II allele, which is higherthan the European populations (33% to 51% versus 13% to27%) (Saab et al.,2007).
The racial variance of ACE I/Dgenotype appears to coincide with the changes of consequenceswhere the populations with high frequency of D allelesappear to experience higher fatality. LikeAfrican Americans seem to have the excessively highfatality rate in the United States(Yancy 2020;Dyer 2020).Likewise, patientsfrom Italy, Spain, and France also experience high fatality inEurope.
The low frequency of ACE D/D and high frequency of II genotype seen in Asian populations appear to be related with relatively low fatality of COVID-19in those nations (https://www.worldometers.info/coronavirus/#countries, last accessed April1, 2021).
Though socioeconomic and environmental circumstances may play a role, they do not completelyclarify the severity of acute lung injury in COVID-19. According to Scottish study of influenza, socioeconomic factors do not completelyclarify ethnic differences in hospitalization for lower respiratory tract infections (Simpson et al.,2015).
Thus, The ACE gene polymorphism, which accounts for thevariances of the ACE level in general people, may beaccountable for the susceptibility to severe lung injury inCOVID-19 patients. The absence of ACE D/D genotype inpatients withCOVID-19 may be defensive against developingsevere lung injury.
The role of aerosol exposure to the transmissionof SARS-CoV-2 has been proven in many studies. Scientists have underlinedthat infected individuals represent emissionsources of aerosol produced by routine behaviours—suchas breathing, speaking, singing, coughing, sneezing, all of which might be capable oftransmission of disease thus it is tough to control it by just mask and social distancing.
SARS-CoV-2 use ACE2 as the receptor for entry into host cells. Since ACE2 is highly expressed in various organs and tissues, SARS-CoV-2 not only enters into the lungs but also attacks other organs with high ACE2 expression. The pathogenesis of COVID-19 is extremely complex, with numerous factors involved. In adding to the direct viral effects and inflammatory and immune factors, the downregulation of ACE2 and imbalance among the RAS and ACE2/angiotensin-(1–7)/MAS axis may also pay to the multiple organ injuries in COVID-19. These two characteristics of covid 19 make it highly riskier for humanity.
The possibility of a second wave of SARS-CoV-2 infections is very strong so, preventive actions are important. It is important to note that vaccines against respiratory syncytial virus (RSV), rhinoviruses, SARS-CoV-1, and MERS-CoV have not yet been successful. So, for SARS-CoV-2, the medical and scientific groups must strengthen their studies in the areas of drug development for discovery of effective therapies and in preventive measures. The spike glycoprotein of SARS-CoV-2 is a possible target for the development of specific drugs, antibodies, and vaccines. Reinstating the balance between the RAS and ACE2/angiotensin-(1–7)/MAS may aid attenuate organ injuries in COVID-19.
Prevention wouldcomprise of avoidance of viral contamination and possible identification of genetically susceptible groupswithin the human population.Differences in COVID-19 severity can beclassified as (a) asymptomatic, (b) symptomatic but nohospitalization required, and (c) severely symptomaticwith hospitalization required. Explanation of allelesof relevant genes related with these three levelsof severity to viral response can help clinicians in dealingwith probable future waves of pandemic. Clarificationof genomics and genetic pathways related tosusceptibility of SARS-Cov-2 infection can becomeimportant in combating a future wave.
MVS Pharma is working on aerosol transmission of Viral diseases and this article is written by Dr Disha Trivedi who is expert in the field of biotechnology and has published several research and review articles in international journal.