**2. SARS-CoV-2 overview**

Since the discovery of SARS in 2002, including the recent detection of SARS-CoV-2, seven strains of human coronavirus have been identified, defined by the WHO as "A broad family of viruses that cause various conditions, from the common cold to more serious illnesses, such as the Middle East respiratory syndrome coronavirus and the one that causes severe acute respiratory syndrome." Among them, SARS-CoV-2, the virus responsible for the 2019 coronavirus disease, originated in Wuhan (Hubei, China) in December 2019, was declared a pandemic by the WHO in March 2020 and is defined as an "enveloped positive-sense single-stranded RNA virus 80-220 nm in diameter. The envelope has corona-shaped peaks 20 nm in length that resemble the corona of the sun under electron microscopy" [5].

The coronaviral genome encodes four major structural proteins, the spike protein (S), the nucleocapsid protein (N), the membrane protein (M), and the envelope protein (E), all of which are necessary to produce a structurally complete viral particle. Unlike the other major structural proteins, N is the only protein that functions primarily to bind to the CoV RNA genome, forming the nucleocapsid. Although N is largely involved in processes related to the viral genome, it is also involved in other aspects of the CoV replication cycle and the host's cellular response to viral infection [6]. Furthermore, protein S plays a crucial role in the entry of the virus into host cells and the structural capabilities of this newly discovered SARS-CoV-2 enhance its intended actions. Because these prominent peaks are the first point of contact with host receptors, therapeutic strategies can be applied to prevent their binding to target receptors and prevent viral entry into host cells [7].

The WHO reported that the most common symptoms of COVID-19 are fever, dry cough and tiredness. Other less frequent symptoms include nasal congestion, headache, conjunctivitis, sore throat, diarrhea, loss of taste or smell, skin rashes or changes in the color of the fingers or toes [8]. These symptoms are usually mild and begin gradually. Approximately 80% of people recover without the need for hospital care, while approximately 1 in 5 people who contract COVID-19 end up with severe symptoms and experience breathing difficulties. Elderly people with underlying diseases, such as high blood pressure, heart disease, lung problems, diabetes or cancer are more likely to suffer from an aggravated clinical stage [8].

The primary route of transmission to humans was zoonotic, via interaction with animals. A hypothesis that was later confirmed and defined by the WHO was that the virus was spread through droplets that are expelled from the nose or mouth of an infected person by coughing, sneezing, or talking, and even by touching infected objects and surfaces, such as tables, doorknobs, and railings, so that healthy people can become infected if they touch those objects or surfaces and then touch their eyes, nose, or mouth [9].

Kotfis & Skonieczna-Żydecka identified viral cells in gastrointestinal biopsy samples, including those that belonged to patients who had left the hospitals, which may partially explain gastrointestinal symptoms, potential recurrence, and transmission of SARS by persistent shedding in stool as well. Specifically, the virus is protein molecule covered by a protective lipid layer that is absorbed into ocular, nasal, oral and gastrointestinal mucosal epithelial cells and replicates there [10].

#### **2.1 ACE2: The door to SARS-CoV2**

The renin angiotensin aldosterone system (RAAS) is the primary regulator of plasma volume, maintaining cardiovascular and fluid homeostasis. This system plays a protective and adaptive role against risk phenomena, such as hypotension, sodium or water deprivation, and in turn, its dysregulation has implications in the development of hypertension and other cardiovascular diseases [11].

Activation of the classic RAAS pathway begins in the juxtaglomerular apparatus with the release of preformed renin from its prorenin precursor, secondary to baroreflex, beta-adrenergic or molecular stimuli in the macula densa. Renin takes the hepatic precursor angiotensinogen and converts it into angiotensin I (Ang I) [11]. This decapeptide has no specific known physiological action and ends up being converted into octapeptide angiotensin II (Ang II) by angiotensin converting enzyme (ACE), which is located primarily in cells of the pulmonary endothelium, as well as other tissues [11].

Ang II acts on AT1 receptors and exerts powerful vasoconstrictive, profibrotic and proinflammatory effects [6]. The action of Ang II on the AT2 receptor generates the opposite vasodilator and antiproliferative effect [11].

ACE is an essential component of the renin angiotensin aldosterone system, functioning as a transmembrane protein with two N- and C-terminal active catalytic domains. The C-terminal domain generates the soluble carboxypeptidase that removes the carboxy-terminal dipeptide of Ang I, generating Ang II, while hydrolysis of the vasodilator peptides, called bradykinins, occurs by the enzymatic action of both domains. ACE2 is a monocarboxypeptidase homologous to ACE but has only one transmembrane helix, an intracellular segment, and N- and C-terminal domains with a single enzymatic active site, endowing ACE with distinct characteristics [11].

ACE2 is also homologous to ACE, which plays a role in the cleavage of angiotensin I into angiotensin-(1–9) and the vasoconstrictor peptide angiotensin II in the vasodilator angiotensin-(1–7). Consequently, ACE2 acts as the entry point into cells for various coronaviruses [12]. By cleaving angiotensin II and increasing vasodilator angiotensin-(1–7), it can act as an important regulator of cardiac function and plays a protective role in acute lung injury.

Possible antitumor effects of ACE2 and future therapeutic prospects for cancers have been reported for ACE2. Unfortunately, ACE2 has a high affinity for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [7], which may explain its manifestations at the respiratory level.

#### **2.2 SARS-CoV-2 and ACE2**

Viral infections bind their viral structures with receptors on the host cell surface. Although it has been shown that there are several coronaviruses that cause human diseases, only three of them bind ACE2: SARS-CoV, SARS-CoV-2 and HCoV-NL63, with SARS-CoV being responsible for a health emergency known as severe acute respiratory syndrome (SARS) in 2003 in China. Curiously, glycoprotein S is characterized as the critical determinant for viral entry into host cells, consisting

of two functional subunits, S1 and S2. The S1 subunit recognizes and binds to the host receptor through the receptor-binding domain (RBD), while S2 is responsible for fusion with the host cell membrane. MERS-CoV uses dipeptidyl peptidase-4 (DPP4) as an entry receptor, while SARS-CoV and SARS-CoV-2 use ACE2, which is abundantly expressed in pulmonary alveolar epithelial cells and enterocytes, suggesting glycoprotein S as a potential drug target to stop SARS-CoV-2 entry [13].

However, SARS-CoV infection downregulates surface expression of the binding protein (ACE2), a fundamental component for the entry of the host cell. Low ACE2 expression is associated with a greater severity of the infection in epithelial cells of the human respiratory tract [14].
