The nine amino acid long peptide that is formed when ACE 2 acts on Ang 1.  Ang 1-9 and Ang 1-7 are negative regulators of RAS.  

Ang 1-9

The seven amino acid long pepetide that is formed when ACE 2 acts on Ang II.  Ang 1-9 and Ang 1-7 are negative regulators of RAS.  Ang 1-7 is biologically active with MasR being its receptor.  This molecule alleviates the effects of Ang II/AT1R and has cardioprotective properties.  

Ang 1-7

- Inhibition of the binding between the spike protein present on the surface of the virus and the ACE-2 receptor present in the membrane of the host cell. This interaction mediates the entry of the virus in the cell. 
- Because ACE 2 plays a central role in the pathogenesis of COVID-19, it is thought that it could be an efficient drug target that could either treat or prevent infection. 

ACE2 normal role:
-Critical to regulate processes such as pressure, wound healing, and inflammation. 


-It helps modulate ANGII, a protein that increases blood pressure and inflammation, increasing damage to blood vessel lining and tissue. 

COVID-19:
-In response to COVID-19, ANGII has increased inflammation and the death of alveoli cells which are critical for bringing oxygen into the body...ACE2 prevents this.

-When COVID-19 virus binds to ACE2, it inhibits its action, thus increasing the effects of ANGII
 

ACE 2 Receptor as a Pharmacological Target for Treating or Preventing COVID-19

This virus uses angiotensin-converting enzyme 2 (ACE2) as a functional cellular receptor.
The virus also binds to the C-type lectin CD209L (also known as L-SIGN) and DC-SIGN

SARS-CoV cellular receptor

Three primary observations have been made about ACE2 and its relation to SARS/COVID-19:

- ACE2 acts as a functional receptor for SARS-CoV-2 to enter host cells
- Increased ACE2 expression may elevate the risk of a SARS-CoV-2 infection as well as the development of COVID-19
- Reduced ACE2 expression may lead to acute respiratory distress syndrome

SARS‐CoV‐2 may be able to infect other tissues that express ACE2 using viral RNA that circulates in the blood.  

ACE2 in the Pathogenesis of COVID-19

ACE2 is located on the X chromosome (Xp22), and belongs to a subset of genes that escapes X-inactivation, showing a heterogenous sex bias and evolutionary characteristics of tissue specificity and expression (Tukiainen et. al). However, expression differences in distinct tissues suggest presence of X-inactivation, though incomplete.

The figure below shows ACE2 preferential expression in males (blue). 

ACE2 Chromosomal Localization and Activation.

Intestinal ACE2 controls the expression of anti-microbial peptides (AMPs) and regulates intestinal homeostasis through its role in dietary amino acid uptake.  A lack of tryptophan, specifically, is thought to lead to adverse effects as it and its metabolites influence microbial ecology, microbial metabolism, and the interaction between the microbiota and its host.  Animal studies have shown that ACE2 knock outs had altered gut microbiotas, decreased AMP expression, and impaired amino acid uptake.  

ACE2 Modulates Gut Microbiota Composition

ACE2 structure components:
- signal peptide
- single metalloprotease active site
- transmembrane domain

ACE2 Structure

ACE2 is a homologue of ACE.  Both enzymes are carboxypeptidases, meaning they cleave amino acids from the c-terminal of the peptide.  However, these homologues cleave the amino acid sequences at different points which leads to their different products.  ACE2 has four main physiological roles:

- Regulating blood pressure via its role in the renin-angiotensin-aldosterone system (RAAS)

- Limiting the amount of ACE substrate by converting Ang I into Ang 1-9

- Degrading Ang II in order to minimize its harmful effects

- Forming Ang 1-7 in order to maximize its cardioprotective effects

Michigan State University

Grand Valley State University

Syracuse University

University of Michigan - Ann Arbor

Ang II is the produce of Ang I being cleaved by ACE.  This molecule is considered biologically active because it can influence physiological outcomes by acting on either Ang II type 1 receptors (AT1R) or Ang type 2 (AT2R) receptors.  

Angiotensin II (Ang II)

Ang I is cleaved by angiotensin-converting enzyme (ACE) to form angiotensin II (Ang II).  This product is the main effector molecule that regulates the RAS system.  Endopeptidases and oligopeptidases can likewise cleave Ang I to form angiotensin (1-7).  This product works against Ang II.  

Angiotensin I (Ang I)

Several cellular proteins have been reported to be involved in the early steps of beta coronavirus infections in humans.  ACE2, a membrane bound protein that is involved in the cellular entry of many beta coronaviruses, is often supported by the function of other proteins.  Some facilitate the shedding of ACE2 from the cell surface, and others cleave viral S proteins to enhance the fusion of the two membranes.  They include:

- ACE2
- ADAM10
- ADAM17
- TMPRSS2
- TMPRSS11A
- TMPRSS11D
- furin
- CTSL
- PI4KB

Cellular Proteins Related to Beta Coronavirus Infections

Klhůfek, J. (2020). The role of angiotensin-converting enzyme 2 in the pathogenesis of COVID-19: the villain or the hero?. Acta Clinica Belgica, 1-8. https://www.tandfonline.com/doi/full/10.1080/17843286.2020.1786324
doi: 10.1080/17843286.2020.1786324

The role of angiotensin-converting enzyme 2 in the pathogenesis of COVID-19: the villain or the hero?

Patel, V. B., Zhong, J. C., Grant, M. B., & Oudit, G. Y. (2016). Role of the ACE2/angiotensin 1–7 axis of the renin–angiotensin system in heart failure. Circulation research, 118(8), 1313-1326.

Role of the ACE2/Angiotensin 1–7 axis of the Renin-Angiotensin System in Heart Failure

Fotuhi, M., Mian, A., Meysami, S., & Raji, C. A. (2020). Neurobiology of COVID-19. Journal of Alzheimers Disease, 76(1), 3-19. doi:10.3233/jad-200581
https://content.iospress.com/articles/journal-of-alzheimers-disease/jad200581

Neurobiology of COVID-19

When activated by Ang II, this receptor results in the following physiological outcomes:

- vasoconstriction

- inflammation

- reabsorption of salts and water

- oxidative stress, accelerated aging and brain degeneration

- kidney failure

- heart disease

- apoptosis


Ang II type 1 receptors (AT1R)

A receptor that is activated by Ang II.  

Ang II type 2 receptors (AT2R)

ACE2 is a surface bound enzyme, but when mice neuro-2A cells were exposed to Ang II and AT1R was activated, ACE2 was removed from the surface of the cell and degraded by lysosomes.  This may be a relevant mechanism related to SARS-CoV-2 cell entry.  

Neuronal ACE2 Internalization in COVID-19

Angiotensin-converting enzyme 2 (ACE2)

ACE, a homologue of ACE 2, converts Ang I into Ang II.  ACE increases the amount of Ang II, a vasoconstrictor, and decreases the amount of a vasodilator.  This consequently results in increased blood pressure.  A class of drugs called ACE inhibitors, which are used for treating symptoms such as high blood pressure, block the activity of ACE.  ACE is generally more expressed than its homologue ACE 2.  

Angiotensin-converting enzyme (ACE)

Transmembrane protease serine 2 (TMPRSS2) is a serine protease.  Its gene is located on chromosome 21 at 21q22.3.  Androgen signaling controls its expression upstream of the gene's transcriptional start site.  

TMPRSS2

A serine protease that has a variety of physiological and pathophysiological substrates.  Dysregulation of furin has been shown to lead to the promotion of infectious diseases and certain types of cancer.  Furin can also be called paired basic amino acid cleaving enzyme (PACE).

Furin

18,263 small airway epithelia cells from nonsmokers (NS) and 16,678 cells from smokers (S) were analyzed using single-cell 10x transcriptome.  Differences in gene expression between smokers/nonsmokers as well as between cell type were all less than 10%, and there were no significant results.  Notably, there was no change in ACE2 expression between smokers and nonsmokers.  This may reflect a limitation of using single-cell RNA sequencing compared to bulk RNA sequencing as the results depend on the size and shape of the cell.  However, these results are unique in that they elucidate the gene expression of individual cell types from a mixed population of cells.  

Expression of Cellular Proteins Related to Beta Coronavirus Infections in the Small Airway Epithelium (SAE) by Cell Type

mRNA expression of proteins related to the early steps in a B-coronavirus infection, excluding AC2, were determined in the small airway epithelium (SAE) of smokers (gray circles) and nonsmokers (white circles.)  Expression levels were quantified using RNA sequencing (Illumina HiSeq 2500), and the data values are reported in FPKM (fragments per kilobase of exon per million fragments sequenced.)  Only TMPRSS2 expression was modulated by smoking: its expression was significantly increased (p < 0.01) in smokers compared to nonsmokers.  

Expression of Cellular Proteins Related to Beta Coronavirus Infections in the Small Airway Epithelium (SAE)

ADAM10 belongs to the A Disintegrin And Metalloproteinases (ADAMs) family of proteases.  ADAM10 facilitated ectodomain shedding has been shown to influence the ability of cell adhision molecules and regulate paracrine, autocrine, and juxtacrine signaling.  

ADAM10

ADAM17 belongs to the A Disintegrin And Metalloproteinases (ADAMs) family of proteases.  It regulates various cell processes through its wide range of substrates, including TNFR1, TNFR2, and IL-6R (interleukin-6 receptor.)  ADAM17 notably is involved in the activation of the epidermal growth factor receptor (EGFR) in development through the shedding of TGFα, amphiregulin, HB-EGF, and epiregulin in the presence of phorbol 12-myristate 13-acetate (PMA).  

ADAM17

TMPRSS11A is a member of the HAT/DESC subfamily of TTSPs.  It is notably expressed in the respiratory system as well as on cell surfaces.  Additionally, it has been shown to facility the entry of the virus SARS-CoV-1 into human cells.  

TMRPSS11A

HAT, or TMPRS11D, is a member of the HAT/DESC subfamily of TTSPs.  It was initially discovered in the sputum of chronic respiratory disease patients, and it is thought to have a variety of physiological roles in epithelial tissue.  Proposed functions of this protease include:

- suppressing coagulation through fibrinogenolysis
- activating the protease activated receptor (PAR)-2
- influencing cell migration and adhesion via urokinase plasminogen activator cleavage

TMPRSS11D has been shown to activate the S protein of SARS-CoV-1 by cleaving it into its S1 and S2 subunits  This facilitates endosomal entry of the virus into a host human cell.  

HAT (TMPRSS11D)

Cathespin L is a member of the cathepsin family of proteases.  It is expressed throughout the body, and it is known to be involved in various pathologies, including:

- liver fibrosis
- type I and II diabetes
- cardiac and bone diseases
- immune disorders
- kidney disorders

Cathespin L (CTSL)

PI4KB is a PI4K that forms phosphatidylinositol 4-phosphate (PI4P).  It is primarily located in the Golgi and trans-Golgi network, and consequently, it is involved in cellular processes including lipid transport, membrane trafficking, cytokinesis, and maintaining lysosomal identity.  PI4KB and its isoform PI4KA are common viral targets because they can be used to create PI4P rich organelles that are important to viral reproduction. 

PI4KB

BCi-NS1.1 cells were derived from a single basal cell from the large airway epithelium (LAE) of a healthy nonsmoker, and hSABCi-NS1.1 cells were derived from a single basal cell from the small airway epithelium (SAE) of a healthy nonsmoker.  Expression levels were determined using HiSeq 4000 RNA-seq data at day 0 (baseline) and after 28 days on an air-liquid interface (ALI).  Notable findings include:

- Both cell lines showed an increase in ACE2 expression after 28 days of differentiation
- ADAM10, ADAM17, FURIN, CTSL, and PI4KB expression was relatively maintained throughout differentiation in both cell lines
- TMPRSS2 was notably less expressed in undifferentiated LAE basal cells than in SAE basal cells, and their expression increased.  However, its expression was maintained in SAE basal cells.  
- TPRSS11A and TMPRSS11D expression remained low in both LAE and SAE cells throughout differentiation

Because these cell lines are immortal, they may continue to be useful in SARS-CoV-2 research - especially for early drug screening.  

ACE2 and Other Cellular Proteins Related to B-Coronavirus Infection Expression Increases in Basal Cells from the Large Airway Epithelium Throughout Differentiation

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