Coronaviruses are a group of large, enveloped, RNA viruses. Their genome (27 - 32 kb) is packed inside a helical capsid formed by nucleocapsid proteins enclosed within another envelope. At least three structural proteins are found associated with the viral envelope. The membrane proteins and the envelope proteins are involved in the assembly of viral bodies. The spike proteins form surface protrusions giving a crown-like appearance to the virus and help in the entry of virus particles into the host cells. They also help in deciding which host to invade and are majorly responsible for the induction of host immune responses. Understanding the structure and function of spike proteins, and their interaction with the host, will help in the management of SARS-CoV-2 infection. Several coronaviruses also encode an envelope-associated hemagglutinin-esterase protein [3]. The spike proteins of SARS-CoV-2 fit into the ACE2 receptors (angiotensin-converting enzyme 2) present on human cell surfaces initiating a structural change that causes the fusion of viral membranes with the host cell membranes. This allows the entry of viral genes into the host cell where they are replicated into multiple copies producing manifold viruses. The genome sequence has been published and tremendous research work is going on throughout the world to unravel the mechanism of infection and an effective cure for this contagious disease [4] [5]. The structure of spike proteins are specifically studied using different techniques like cryo-electron microscopy. These proteins are potential antiviral targets. Studies have shown that the SARS-CoV-2 spikes have 10 - 20 times more binding affinity for ACE2 receptors on human cell surfaces in comparison to the spikes from the 2002 SARS virus. This may be the reason for the rapid contagious spread of this new virus of 2019. Although the structure and sequence of the spikes of both 2002 SARS virus and SARS-CoV-2 are similar, the antibodies generated against the old virus were unable to successfully bind to the spikes of the new virus suggesting that completely different antibody based treatment strategies would be required [6].

Understanding the infection mechanism by which the virus invades and damages the human host are crucial to the identification of therapeutic and diagnostic strategies so that the disease can be predicted before it becomes fatal. Several studies have been reported that attempt to explain the mechanism of SARS-CoV-2 infection. It has been suggested that the infection mechanism involves binding of the virus to the ACE2 receptors present on epithelial cells of the host lungs, kidney, intestine and blood vessels and their consequent internalization. In fact the severity of the disease has been linked to type 1 integral membrane glycoproteins. There are two forms of ACE2 receptors—glycoproteins and metalloproteases. The membrane-bound glycoproteins contain a transmembrane domain and an extracellular domain while metalloproteases are soluble and are secreted outside [7]. The significance of soluble circulating ACE2 is not clear, but studies suggest that their levels increase in chronic diseases such as diabetes, chronic kidney diseases, and hypertension [8]. The substrates for ACE2 include kinins, apelin, neurotensin, dynorphin, ghrelin, amyloid, and angiotensin. It regulates angiotensin II (Ang II) by converting Ang I into Ang-(1-9), and degrading Ang II into Ang-(1-7), a vasodilatory and anti-proliferative peptide which protects the tissues [9]. Besides ACE2, SARSCoV-2 uses the TMPRSS2 (transmembrane protease serine 2) gene for spike protein priming. Both the genes are expressed in the lungs, epithelial cells of small intestine, upper oesophagus, liver, colon, blood vessels, heart, kidneys, and the gonads. Since the receptors are widely distributed on several organs and tissues, the infection could cause multiple organ failure [9] [10] [11].

Studies suggest that the virus attacks the haemecomponent of the β chain of haemoglobin dissociating the iron to form the porphyrin and reduces the capacity of haemoglobin to carry oxygen and carbon dioxide [12]. Another more accepted mode of infection points towards the inflammatory cascades, cytokine storms, and activation of coagulation cascades on entry of the virus into the human body. These cascades lead to severe and often fatal complications, such as sepsis, disseminated intravascular coagulation (DIC), and acute cardiovascular events [13].

Diabetes is a metabolic disorder that can lead to serious complications, such as heart disease, stroke, retinopathy, nephropathy, and neuropathy. If uncontrolled, it can lead to blindness, kidney failure and lower-limb amputations. The disease results due to insufficient insulin secretion and/or insulin resistance. This chronic disease is one of the principal causes of morbidity and mortality throughout the world. At present 10.5% of the U.S. population is diabetic [14]. According to WHO, the rate of diabetes in Saudi Arabia is the second highest in Middle Eastern countries, and seventh in the world [15]. Approximately 463 million people, in the age group of 20 - 79 years, were estimated to be living with diabetes last year in 2019 [16]. Diabetes was presented as a risk factor for mortality in virus infected patients during 2002-2004 Severe Acute Respiratory Syndrome (SARS) coronavirus, 2009 swine flu pandemic (Influenza A H1N1), and 2012 Middle East Respiratory Syndrome-related coronavirus (MERSCoV) [17] [18]. It has been shown that COVID-19 deteriorates dysglycemia in diabetic patients [19].

Innate immune response, the first line of defence against SARS-CoV-2, is compromised in patients with uncontrolled diabetes, and hence the virus proliferates easily in the host without any hindrance. Moreover, there is an exaggerated cytokine response which leads to overproduction of pro-inflammatory cytokines, particularly interleukins IL-1, IL-6 and tumour necrosis factor-α. Similarly, patient with chronic obstructive pulmonary disease, have low levels of cytokine production, specifically interferon-β. This weakens host defence against coronavirus [19]. Besides, the activation of renin angiotensin system (RAS) contributes majorly to diabetic complications and blocks the vasoconstrictor and hypertrophic activity of Ang II. These mechanisms slow down the progression of diabetic complications but do not completely prevent it. The presence of ACE2 receptors in the heart and kidney make the RAS even more complex and are the basis to investigate the role of this enzyme in pathophysiological conditions and vascular complications of diabetes. Hence both ACE and ACE2 may play a vital role in modulating equilibrium between vasoconstrictors and vasodilators inside the heart and kidney but are counter-regulatory to each other. The role of ACE2 in diabetes is relatively unknown. Further studies can help in the development of new drugs that affect ACE2 activity and expression. These novel compounds may have immense clinical significance in the prevention and management of diabetic complications [20].

Studies suggest that poor prognosis of diabetes in COVID-19 patients is due to several factors that include compromised innate immunity, inflated cytokine response, reduced expression of ACE2 and use of drugs that interfere with RAS in diabetic patients. Glucose levels remain uncontrolled in these patients as β-cells are damaged, insulin develops resistance, and potassium levels drop (hypokalemia). Antiviral drugs like corticosteroids, lopinavir, ritonavir when administered for the treatment of COVID-19 make the situation worse in people with diabetes mellitus. The interplay between both diseases is such that COVID-19 causes dysglycemia of patients to deteriorate and diabetes mellitus exacerbates the severity of the viral infection [19]. Upregulation of ACE2 seems to facilitate COVID-19 infection but interestingly the overall ACE2 expression is reduced in patients with diabetes due to the drugs that are commonly prescribed to patients. These drugs are often ACE inhibitors and angiotensin receptor blockers [21]. Hence, ACE2-stimulating drugs may be the primary cause of severity of COVID-19 disease observed in patients with metabolic co-morbidities [19].

Diabetes is increasing at an alarming rate in most countries. The United States has the highest prevalence among developed nations. Qatar, Saudi Arabia, and Kuwait are all in the top 10. China and India have the highest numbers, 110 million and 69 million, respectively [16]. With such large numbers at stake, it becomes imperative that management and prevention strategies be developed for diabetics and other patients with pre-existing conditions during pandemics like COVID-19. Besides thorough hand-washing with soap and water and maintaining respiratory hygiene, diabetic patients should keep their glucose levels under control so that their immunity remains boosted. According to Pal and Bhadada, Calcium channel blockers may be a feasible option instead of ACE inhibitors as they do not upregulate ACE2 levels [19]. It is of utmost importance that diabetic patients maintain good glycaemic control in order to reduce both the risk of infection and the severity of the disease. This can be done by regularly monitoring blood glucose levels. With proper nutrition the body is more apt in fighting the infection. Care should be taken that the patient is not deficient in minerals and vitamins. There are several reports that claim that exercise plays a big role in improving immunity. General preventive measures advised worldwide include covering mouth while coughing/sneezing, avoid touching of mouth, nose, and eyes, and wearing appropriate face masks. These instructions should be followed by everyone. Fortunately, more than 95% of COVID-19 patients experience mild symptoms and can be taken care of at their residence (home quarantined). Glycated hemoglobin (HA1c) levels are significantly high in type I diabetes. As a result, the patient becomes highly susceptible to infections in an immunocompromised environment. Studies have shown severe ketoacidosis in type I diabetic patients with COVID-19. Regular monitoring of blood sugar and ketone levels along with proper awareness about the disease and its implications during infections is absolutely vital. Regular and appropriate doses of insulin should be administered [22]. Dehydration should be avoided, and energy levels maintained.

There are no approved therapies for COVID-19 and hence the focus is more on prevention and repurposing already available drugs that may be effective against this virus. In case of SARS-CoV-2 infecting diabetic patients, insulin administration is preferred for getting hyperglycemia under control. Oral drug administration, particularly metformin and sodium glucose cotransporter-2 inhibitors, have been advised to be discontinued [23]. There are several reports where usage of drugs like lopinavir, ritonavir, interferon β-1b, RNA polymerase inhibitor remdesivir, and chloroquine have been advised although their efficacy is still doubtful and not confirmed [24]. SARS-CoV-2 has receptors that strongly bind to ACE2 and hence, as discussed earlier, RAS inhibition may play a role in the treatment of severe respiratory diseases [25] [26]. There are some antivirals that show promising effects against other viruses. Zinc nanoparticles have been shown to inhibit H1N1 but have not been tested against SARS-CoV-2 [27]. Similarly, Vitamin C, which showed a positive effect in the prevention of pneumonia, should be evaluated against COVID-19 [28]. Dexamethasone, a corticosteroid used for the treatment of many conditions like asthma, allergies etc, has been recommended for use in COVID-19 patients by WHO and many health organisations. Caution must be taken in using it against diabetic patients as this anti-inflammatory drug may increase blood glucose levels [29].

Metformin is a widely prescribed drug that lowers blood sugar levels in type 2 diabetes patients. It is also prescribed to those who have risk of developing this metabolic disorder due to obesity and other conditions. Metformin stimulates AMPK pathway. It has been reported that AMPK increases the expression of phosphorylated ACE-2 Ser⁶⁸⁰ in HEK293T cells. Addition of a phosphate group leads to conformational changes in ACE-2 receptor and prevents its activation. Also, it inhibits mTOR via liver kinase B1 pathway which plays an important role in MERS‑CoV infection. It is hypothesised that metformin affects this pathway in SARS-CoV-2 infection, but more data is needed to confirm this [30] [31] [32]. Some reports interestingly claim that metformin has the ability to reduce inflammatory response and may help in reducing mortality rates [30] [33]. In the absence of any specific treatment or vaccine, these drugs should be used but only after careful analysis of their effect on both the virus and host. [34]. Miglitol, celgosivir and miglustat are some drugs that need to be evaluated as treatment options against COVID-19 [35]. Table 1 is a compilation of commonly used antidiabetic drugs and their effect, if any, on COVID-19. Research is also going on to use the spike proteins of SARS-CoV-2 as potential candidates for the development of vaccines. Antibodies generated against these proteins, isolated from patients after recovery are used as a treatment option. Vaccines play a major role in containing the pandemic. Their development is in progress and till then we have to depend on available drugs and various treatment strategies.