key: cord-1004859-t1v6kt0j
authors: Jamwal, Sumit; Gautam, Akash; Elsworth, John; Kumar, Mandeep; Chawla, Rakesh; Kumar, Puneet
title: An updated insight into the molecular pathogenesis, secondary complications and potential therapeutics of COVID-19 pandemic
date: 2020-07-17
journal: Life Sci
DOI: 10.1016/j.lfs.2020.118105
sha: fdc401327f6865cd00f40a7bdc095720d80c5f4b
doc_id: 1004859
cord_uid: t1v6kt0j

Coronavirus disease 2019 (COVID-19) is an unprecedented disease caused by highly pathogenic SARS-CoV-2 and characterized by extreme respiratory deterrence, pneumonia and immune damage. The phylogenetic analysis demonstrated the sequence similarity of SARS-CoV-2 with other SARS-like bat viruses. The primary source and intermediate host are not yet confirmed, although transmission from human to human is universally confirmed. The new SARS-CoV-2 virus reaches cells via ACE-2 and subsequently down-regulates ACE-2, leaving angiotensin II unbalanced in affected organs primarily in the lungs, heart, brain, and kidneys. As reported recently, numerous secondary complications i.e., neurological, nephrological, cardiovascular, gastrointestinal, immune, and hepatic complications, are associated with COVID-19 infection along with prominent respiratory disease including pneumonia. Extensive research work on recently discovered SARS-CoV-2 is in the pipeline to clarify pathogenic mechanisms, epidemiological features, and identify new drug targets that will lead to the development of successful strategies for prevention and treatment. There are currently no appropriate scientifically approved vaccines/drugs for COVID-19. Nonetheless, few broad-spectrum antiviral drugs, azithromycin were tested against COVID-19 in clinical trials, and finally, FDA approved emergency use of remdesivir in hospitalized COVID-19 patients. Additionally, administration of convalescent plasma obtained from recovered COVID-19 patients to infected COVID-19 patients reduces the viral burden via immunomodulation. This review analysis therefore concentrates primarily on recent discoveries related to COVID-19 pathogenesis along with a full description of the structure, genome, and secondary complication associated with SARS-CoV-2. Finally, a short and brief clinical update has been provided concerning the development of therapeutic medications and vaccines to counter COVID-19.

J o u r n a l P r e -p r o o f gastrointestinal injury, immune failure, compromised coagulation and even death (Zhang et al., 2020) .

CoVs belong to a large family of single-stranded RNA viruses (+ssRNA) and are broadly classified into four categories, alpha-CoV, beta-CoV, gamma-CoV, and delta-CoV (Hassan et al., 2020; Lu et al., 2020) . CoVs can infect humans as well as animals and can cause varieties of infections, including respiratory, enteric, renal, and neurological diseases (Hassan et al., 2020) .

CoVs occupy a crown-like presence under an electron microscope due to the existence of spikelike glycoproteins on the viral envelope . SARS-CoV-2 belongs to βcoronavirus category, which contains shrouded and non-segmented single-stranded RNA virus (Guo et al., 2019) . Genome sequencing of SARS-CoV-2 has shown 96.2% overall similarity with a bat CoV RaTG13 and 79.5% uniformity to SARS-CoV (Anderson et al., 2020) . CoVs have been shown to penetrate the host cell via different mechanisms, which includes an endosomal and nonendosomal entry with the help of proteases (Hamming et al, 2004; Jaimes and Whittaker, 2018) . It is well established now that SARS-CoV-2 makes use of angiotensin-converting enzyme 2 (ACE-2), the similar receptor used by SARS-CoV, to enter or to infect humans.

Along with ACE-2, SARS-CoV-2 has shown to take the help of transmembrane protease serine protease-2 (TMPRSS-2) as the critical protease assisting their entrance into the human cell (Hoffmann et al, 2020) . This receptor (ACE-2) is widely expressed in pulmonary tissues as well as in some immune cells, including monocytes and macrophages (Gheblawi et al., 2020) . SARS-CoV-2 virus entry into the human body stimulates the release as well as activation of monocyte, macrophage, and dendritic cell along with the release of cytokines (Interleukins). Further, initiates an intensification cascade resulting in cis signaling with activation and differentiation of TH 17 , lymphocytic changes, and trans-signaling in endothelial cells . SARS-CoV-2 infection cause final release and increase in pro-inflammatory cytokines, including interleukins, TNF-α, GCSF, and MCP (Fu et al., 2020; Singhal, 2020) . This augmented systemic cytokine Journal Pre-proof J o u r n a l P r e -p r o o f storm seen in SARS-CoV-2 diagnosed patients underwrites the pathophysiology of severe COVID-19 and known as the "cytokine storm". Extreme COVID-19 patients show strong inflammatory response and transcriptomic RNA-seq reviews of COVID-19 patients showed that several immune pathways and pro-inflammatory cytokines CXCL, CCL2, CXCL2, CCL8, IL33 and CCL3L1 in bronchoalveolar lavage fluid and TNFSF10, CXCL10, IL10, TIMP1, C5, IL18, AREG and NRG1 in peripheral mononuclear cells (PBMC) were induced by SARS-CoV-2; suggesting a sustained inflammation and cytokine storm. Besides this, excessive apoptotic cell death of T cell resulting from flawed stimulation by dysfunction of dendritic cell might be a contributing factor to the COVID-19 immunopathology (Singhal, 2020) . Fortunately, 80% of individuals infected with SARS-CoV-2 are asymptomatic or suffer from mild symptoms due to the activation of the body's innate immune system by triggering the body's antiviral defense mechanisms, including natural killer cells and antiviral T cells, and interferon induction.

Unfortunately, 20% of SARS-CoV-2-infected individuals are the immune-compromised, aged, patients with underlying health conditions will experience more severe illness characterized by substantial respiratory symptoms leading to acute respiratory distress syndrome (ARDS) and even death. Several epidemiological, genetic, and clinical elements of COVID-19 infection resemble previous SARS-CoV and MERS-CoV infection (Rothan and Byrareddy, 2020) . Ironically, even after the frequent re-emergence of CoVs infections and after several years of research, we still lack vaccines or therapeutic agents to treat CoV infection, which further climaxes an unmet need to develop effective drugs/vaccines to prevent re-emergence of future CoV epidemics. Here, in this review, we have discussed about etiology, epidemiology, the structure of SARS-CoV-2, as well as the pathogenesis of SARS-CoV-2, induced COVID-19 infection. Besides, a brief dialogue has been made on secondary complications associated with COVID-19. Lastly, we have reviewed current clinical therapy, potential therapeutic intervention followed by drugs and vaccines currently in clinical trials for COVID-19.

Journal Pre-proof J o u r n a l P r e -p r o o f

Based on the taxonomic classification of viruses, SARS-CoV-2 is classified under the order Nidovirales, family Coronaviridae, and genus Betacoronavirus (β-CoV). All the viruses from this genus have a positive-sense, single-stranded RNA besieged by a lipid envelope. A phosphorylated capsid protein shields this single strand of RNA and both RNA & capsid together form a nucleocapsid. This nucleocapsid is hidden within the phospholipid bilayers and coated by spike glycoprotein trimer (S) and probably the hemagglutinin-esterase (HE) protein. Due to spike protein on the lipid envelope, SARS-CoV-2 holds the classic coronavirus structure with crownlike spikes (Fig. 1a) . The membrane (M) protein and the envelope (E) protein are sited amongst the spikes in the envelope. Apart from spike protein, it also expresses other proteins such as helicase, RNA polymerase, papain-like protease, 3-chymotrypsin-like protease, glycoprotein, and some other accessory proteins (Shereen et al. 2020). As mentioned above, SARS-CoV-2 shows a 96% nucleotide identity with a coronavirus isolated from a bat (BetaCoV RaTG13), suggesting it as a bat origin virus (Sun et al. 2020) . Two theories have been put forward to clarify the crossspecies transmission of this virus. First, there might be an intermediate host between bats and humans. Most likely, it is pangolin as their genome is approximately 90% similar to SARS-CoV-2, indicating two sub-lineages of this virus in the phylogenetic tree. As per the second theory, the rich genetic diversity and frequent recombination ability of SARS-CoV-2 might enhance the possibility for cross-species spread (Xie and Chen 2020). The 29.9 kb genome of SARS-CoV-2 (GenBank no. MN908947) encodes 9860 amino acids which include the 5'-untranslated region (5'-UTR), replicate open reading frame (orf) 1a/b, structural proteins, other orf such as 3, 6, 7a, 7b, 8 and 9b and the 3'-untranslated region (3'-UTR) (Fig. 1b) . The 5' UTR is 265 nt long, whereas 3' UTR is 229 nt long. SARS-CoV-2 contains at least ten ORFs, including ORF1a/b, spike (S), envelope (E), membrane (M), nucleocapsid (N), and some other accessory genes, such as ORF3b and OFR8 (Han et al. 2020). Almost two-third of viral RNA houses the first ORFs Journal Pre-proof J o u r n a l P r e -p r o o f (ORF1a/b), which are further translated into two big polyproteins i.e. pp1a and pp1ab. These polyproteins are finally processed by different proteases into 16 non-structural proteins (nsp1-nsp16) to produce the replicas-transcriptase complex. Moreover, the other ORFs, present on the other one-third part of the genome encode for four main structural proteins (S, E, M and N) proteins, as well as some accessory proteins with unknown functions (Lu et al. 2020) . The S, ORF3a, E, M, ORF8 and N genes are 3822, 828, 228, 669, 366 and 1260 nt in length, respectively.

The ~1200 aa long glycoprotein spikes are homo-trimeric type-I viral fusion proteins, which is vital to establish the host tropism via the facilitation of receptor binding and membrane fusion (Coutard et al. 2020) . Each monomer of trimeric S-protein is about 180 kDa and contains a cleavable N-terminal signal peptide, a heavy N-glycosylated ectodomain, a transmembrane region and one cytoplasmic tail with lots of S-acylated C residues. The proteases cleave N-glycosylated ectodomain into two domains: the variable S1 and the more conserved S2 domains (Fig. 2) . In general, the S1 domain is associated with receptor-binding events, whereas S2 is involved in membrane fusion (Sun et al. 2020) . Structurally, N-and C-terminal of S1 fold as two separate domains: N-terminal domain (NTD) and C-terminal domain (CTD). The CTD contains a loosely attached receptor-binding domain (RBD) to bind with the host cell receptor i.e. ACE2 promptly J o u r n a l P r e -p r o o f different host cell membranes (Sun et al. 2020) . Apart from this, SARS-CoV-2 encodes several proteins to attenuate the innate immune responses, especially the activation of type 1 interferon in host cells, ultimately leading to an enhanced immunopathogenesis. The nsp15 (also known as endoribonuclease EndoU) is vital for restraining the detection of viral RNA by specific cytoplasmic pattern recognition receptors (Hackbart et al. 2020 ).

The NTD of S1 displays a galectin (galactose-binding lectins) structural fold to get attached to the sugar on the surface of the host cell. On the other hand, binding to the ACE-2 is assisted by the CTD of S1. The CTD comprises two subdomains: a central structure made up of five-stranded antiparallel β-sheet and the RBD, which governs the specificity of receptor binding (Sun et al. 2020 ). The extended insertion (between the β4 and β7 strands) of the RBD contains some of the crucial residues required for receptor binding. Yan et al., 2020 showed the formation of a hACE2 dimer in the presence of an amino acid transporter B0AT1. Two molecules of CTD are individually attached to this dimeric hACE2-B0AT1 form, with a local resolution of 3.5A° at the interface. The RBD endures hinge-like conformational movements to cover or uncover the elements of receptor binding momentarily to enhance the host-cell interactions (Yan et al., 2020) .

S2 regulates the fusion with the host cell membrane and then insertion of viral RNA (Siordia 2020). The S2 is made up of the fusion peptide (FP), a cleavage site (S2′), an internal fusion peptide (IFP), and two heptad-repeat domains before the transmembrane domain (TM) (Fig. 2) .

Since both FP and IFP are essential for the virus entry into the host cell (Lu et al., 2015) , S protein is required to be cleaved by proteases at both priming and activating cleavage sites to release out One of the widely studied cell proteases in SARS-Cov-2 is proprotein convertase furin. Coutard and colleagues (2020) observed that the furin-like cleavage site (PRRAR↓SV) is absent in lineage B beta-coronaviruses except in SARS-Cov-2 (Xie and Chen 2020). The presence of proline in this furin-like cleavage site generates a turn in the polypeptide, which facilitates the addition of Olinked glycans to the amino acid residues flanking the cleavage site, specifically to S673, T678 and S686. The role of these O-linked glycans is not known precisely; however, they could produce a 'mucin-like domain' to protect crucial amino acid residues or epitopes on the S-protein.

The high expression of furin in the lungs suggests the high pathogenicity of this virus in the human respiratory system (Izaguirre, 2019; Moulard and Decroly, 2000) . Gene expression studies have shown that ACE2 occurs primarily in alveolar epithelial type II cells of lungs, heart, kidney, adipose tissue, and nasal epithelial cells (specifically in goblet cells and ciliated cells). Studies through surface plasmon resonance have proven that the ACE2 binds to the SARS-CoV-2 ectodomain with about 10-to 20-fold higher affinity as compared to SARS-CoV (Xie and Chen 2020). The crucial K residue at 31 in hACE2 identifies the Q residue at 394 in the RBD region of SARS-CoV-2. Bioinformatical analysis by Wan et al. (2020) has predicted that the single N501T mutation in RBD of S-protein might lead to an augmented binding affinity for ACE2. The residues at positions 442, 472, 479, 487, and 491 in S-protein are crucial as these are located at the receptor complex interface with ACE2 (Xie and Chen 2020).

After the fusion of viral lipid bilayer through the endosomal pathway, SARS-Cov-2 injects its RNA into the host cell cytoplasm. Once inside the cytoplasm, it is translated into two polyproteins (pp1a and 1ab) and structural proteins. The polyproteins are further processed and cleaved by different proteases to form a replicase-transcriptase complex. The newly synthesized polymerases produce various sub-genomic mRNAs by the discontinuous transcription, which gets translated 19 is considered to be respiratory dysfunction, but some patients showed severe damage to the cardiovascular system. Recent reports suggested that patients with cardiovascular diseases (CVD) have been at increased risk of death (Zheng et al., 2020; Zhou et al., 2020) . In 2006, a study conducted on SARS patients showed that out of 121 patients, 12 patients had cardiovascular complications with tachycardia and hypotension were the most common finding reported besides with the other complications such as bradycardia, transient cardiomegaly (Yu et al., 2006) . In line with SARS, the latest case reports of COVID-19 reported that up to 50% of patients with a high risk of mortality have chronic CVD. Recently, the clinical report of COVID-19 revealed that 7.2%, 8.7%, and 16.7% of patients with acute cardiac injury, shock, and arrhythmia, respectively, required immediate hospitalization and intensive care . A most recent report from China CDC showed that 25% of patients have pre-existing comorbidities like cardiovascular complications and diabetes (Chinese Center for Disease Control and Prevention, 2020). A case report on 138 COVID-19 patients reported that 19.6%, 16.7%, 7.2%, 8.7%, and 3.6% of patients developed acute respiratory distress syndrome, arrhythmia, acute cardiac injury, shock, and acute Journal Pre-proof J o u r n a l P r e -p r o o f kidney injury, respectively (Fig. 4) . The overall mortality rate remains minimal at 2.3%; though, it bounces to 6% in hypertensive patients, 7.3% in diabetic patients, 10.5% in CVD patients, and 14.8% for very older patients (>80 years of age). The point to be noticed that the mortality rate for CVD (10.5%) is bigger as compared to patients with chronic respiratory disease (6.3%) (Chinese Center for Disease Control and Prevention, 2020).

CVD may become more severe in the presence of viral infection because of the imbalance between the increased metabolic demand and reduced cardiac reserve. Therefore, patients with coronary artery disease and heart failure have a high mortality rate . ACE-2 was identified as a crucial factor mediating SARS-CoV spike (S) protein's interaction with susceptible host cells. In physiological conditions, angiotensin-converting enzyme (ACE) is a master regulator of the renin-angiotensin system and one of best in target enzymes for the treatment of hypertension. The protease renin converts angiotensinogen to angiotensin I (ANG I), which is consequently transformed to ANG II by ACE. ANG II then unites to the ANG II type 1 receptor (AT1R) to promote inflammation, oxidative stress, and increase in blood pressure. (potentially contributes to the myocarditis) is largely unknown. Therefore, it can be concluded that cardiovascular tissues or cells expressing ACE2 are possibly at extreme risk for SARS-CoV-2 infection. In COVID-19 patients with pre-existing CVD, the demise of ACE2 by SARS-CoV-2via internalization mechanism has been predicted to exacerbate CVD acutely and maybe long term (South et al., 2020) .

Viral contamination can lead to origin of serious damage to the brain and spinal cord, including encephalitis and severe acute demyelinating lesions . A recent study (n=214) Journal Pre-proof J o u r n a l P r e -p r o o f was firstly reported in China with manifestations, including leg weakness, and severe fatigue. It has been documented that SARS-CoV-2 has been found in CSF and can infect brain cells and subsequently cause viral encephalitis Xiang et al., 2020) .

Moreover, another study reported that out of 221 COVID-19 patients in Wuhan, China, 11 patients have established stroke, cerebral thrombosis, and cerebral hemorrhage. If we look back at previously published reports in which SARS-CoV (Netland et al., 2008) or MERS-COV (Li et al., 2016) were administered in mice through nasal routes, which enter the brain through olfactory nerves and spread throughout the brain. Various possible explanations have been reported for SARS-CoV-2 induced CNS damage including entry into the brain through blood circulation, by infecting the sensory and motor nerve endings and some indirect mechanisms like hypoxia or loss of oxygen also cause decrease supply of oxygen in brain tissue cause acute or permanent changes in the brain areas . Viruses may penetrate the brain and exacerbate infections by activating neuroinflammation. i.e. excessive activation and release of interleukins, TNF-α and other markers, which ultimately leads to free radical generation, excitotoxicity, and neuron death . Nevertheless, there is a need for a deep understanding of the possible link between the brain and respiratory infection by COVID-19, and clinical reports or biopsy studies may conclude that brain infection involved or not in respiratory failure.

Several published studies on COVID-19 enlightened lungs and the respiratory system as the principal organ/system involved and affected in the disease; meanwhile, manifestations of this disease also involves other organs like the gastrointestinal tract, renal system and hepatic system as observed in coronavirus infected patients . Recent clinical reports have documented the negative impact of SARS-CoV-2 on other organs like liver, kidneys and intestine in COVID-19 patients, which can lead to a higher incidence of hepatic or renal injury and even worsening of the health status of patients who are already suffering from hepatic disorders, GIT Journal Pre-proof J o u r n a l P r e -p r o o f disorders, ARI (Acute Renal Injury) or CRI (Chronic Renal Injury) and thus results in higher mortality rate . It has been reported that ARI and liver dysfunction are the main complications, along with acute respiratory distress syndrome, arrhythmia, acute cardiac injury, shock, and other secondary infections. Studies also revealed that ARI is one of the severe symptoms of COVID-19, generally in the case of critical patients (Cheng et al., 2020) By this time, through various studies, it is evident that to infect humans, SARS-CoV-2 uses ACE2, which is the same receptor used by SARS-CoV for infecting human cells (Hoffman et al., 2020) . A recent study showed that the expression of ACE-2 in kidney cells is similar to that in the lungs, esophagus, small intestine, and colon . It has been suggested that the kidneys serve as an essential target organ for SARS-CoV (Hofmann and Pöhlmann, 2004) and the SARS-CoV-2 virus (Zhou et al., 2020) . Previously published reports during the outbreak of SARS and MERS-CoV infections claim that ARI was observed in 15% cases with a high mortality rate of 60%-90%. However, in the case of COVID-19, low incidence (3%-9%) of ARI was observed in early reports to date (Yang et al., 2020) . Recently, a study carried out on 193 patients with COVID-19, between January to February 2020 including 28 pneumonia patients (15 viral pneumonia and 13 mycoplasma pneumonia patients) concluded that proteinuria was observed in 59% cases, hematuria in 44%, elevated levels of blood urea nitrogen in 14%, and high serum creatinine levels in 10% cases, which is significantly worse as compared to the other pneumonia cases (Fig. 4) . A univariate Cox regression analysis was carried out and found that all elevated factors were significantly linked with high mortality in COVID-19 patients . Also, it has been suggested that COVID-19 patients with developed ARI are at 5.3-times more risk of those without ARI and even much higher than those with comorbid chronic illnesses . Similarly, another study was carried out on 701 patients (367 men and 334 women) for the assessment of the prevalence of ARI in COVID-19 patients as well as for the evaluation of the relationship between abnormal kidney function markers and fatality in COVID-Journal Pre-proof J o u r n a l P r e -p r o o f 19 patients with a median age of 63 years. The study concluded that proteinuria and hematuria were found in 43.9% and 26.7% patients, respectively, whereas high plasma creatinine and blood urea nitrogen was observed in 14.4% and 13.1% patients, respectively at the time of admission to hospitals. In addition, data analysis has revealed that there is a significantly higher risk for inhospital death of patients with renal complications (Cheng et al., 2020) . Moreover, the same study concluded that the prevalence of renal complications during hospital admission as well as the development of ARI during hospitalization in COVID-19 patients, is high and is associated with high in-hospital mortality (Cheng et al., 2020) .

The etiology or pathogenesis of kidney injury in patients with COVID-19 is not precisely clear but might be due to multiple reasons. It may be due to the direct cytopathic effects of SARS-CoV-J o u r n a l P r e -p r o o f (Coronavirus induced-nephropathy) has a complicated and unclear etiology; though, ARI in COVID-19 patients is strongly correlated with a high mortality rate. Therefore, to prevent mortality in such conditions, continuous monitoring of renal function in patients with COVID-19 with caution is necessary irrespective of the past disease history, so that early clinical interventions could be taken timely.

The liver is the vital metabolic organ with numerous physiological roles and play a significant function in immunity and detoxification of xenobiotic. The liver is a highly sensitive organ that can be easily influenced by the presence of viruses, bacteria, and other harmful pathogens. In 2003, during the SARS outbreak, SARS-CoV reportedly caused atypical pneumonia and served as a contributing factor in liver impairment in up to 60% of SARS patients (Peiris et al, 2003; Drosten et al, 2003) . Several studies documented that during the initial stage of SARS, there was a mild to moderate rise of ALT and AST levels, decrease in serum albumin, and an increase in serum bilirubin levels in the patients (Duan et al., 2003; Lu et al. 2004 ). It has been reported that the severity of liver damage was directly proportional to the severity of cases of SARS patients (Duan et al, 2003; Jiang et al, 2004; Lu et al, 2004) . Several studies on SARS patients, reported that the SARS-CoV genome was identified in hepatocytes by RT-PCR technique, which establish a significant relationship between SARS-CoV and hepatic damage. (Lu et al, 2003; 2004; Ding et al, 2004; Farcas et al, 2005) . ACE-2 is responsible for receptor-associated cellular entry of SARS-CoV (Lu et al, 2003; 2004; Hamming et al, 2004) . Endothelial cells of the liver have an abundance of ACE 2. Therefore, it served as a potential target for the cellular entry of SARS-CoV Coming to the latest studies on COVID-19, it has been reported that 14.8% to 53% COVID-19 cases have abnormal ALT/AST levels, with mildly elevated bilirubin levels during the initial stages of the disease (Fig. 4) 

COVID-19 is emerging as a double edge sword for the population suffering from diabetes.

Diabetes is a significant risk factor for hospitalization and mortality due to COVID-19 infection.

The severity of COVID-19 in with diabetic patients is two-fold higher as compared to mild and severe patients with COVID-19 (Li et al, 2020). While, recent reports from china claims that death rates due to COVID-19 virus are three-fold higher in diabetic patients .

Indeed, it is well established that diabetic population is at high-risk group for viral infection due to hyperglycemia-induced increased oxidative stress as well as synthesis and release of advanced glycation end products and pro-inflammatory cytokines (Petrie et al, 2018). These processes may constitute the underlying mechanism for higher mortality and morbidity in COVID-19 patients with diabetes. Most notably, diabetes was too considered as a risk factor during previous SARS, 

In contrast, a retrospective study conducted in the first epicenter of COVID-19 infection (Wuhan) confirmed that about 10% of the COVID-19 patients with T2DM suffered at least one episode of hypoglycemia (Zhou and Tan, 2020). Moreover, a recently published meta-analysis of 12 studies depicting data from 2,108 Chinese COVID-19 patients concluded that the prevalence of diabetes in COVID-19 patients is 10.3%, which is similar to the diabetic prevalence in healthy population (Hussain et al., 2020) . Therefore, it can be concluded that though pre-existing diabetes worsens the consequences in COVID-19 patients, but the predisposition of the diabetic population to SARS-CoV-2 infection may not be higher as compared to healthy people.

A previous study of SARS-CoV patients without pre-existing diabetes (n=39) has reported that 20 of the 39 SARS-CoV patients developed diabetes during hospitalization because immunostaining of ACE-2 in the pancreatic tissue suggested that SARS-CoV might have damaged pancreatic islets and caused acute Type I diabetes mellitus like condition (Yang et al, 2006; Hussain et al., 2020) . Whereas in the case of SARS-COV-2, evidence is still lacking to confirm that pancreatic destruction in COVID-19 patients. The development of a viral infection in diabetic patients renders them harder to treat due to oscillating blood glucose levels and the presence of other secondary diabetic complications. Therefore, it can be hypothesized that the treatment of COVID-19 cum diabetic patients with ACE-2 stimulatory drugs might increase the risk of acquiring more severe and fatal COVID-19. Another component that should be considered is the existence of ACE-2 polymorphisms in Asian populations, which is further linked to diabetes mellitus and hypertension, genetically predisposes towards increased risk of SARS-COV-2 infection (Wu et al., 2016) . In a nutshell, intake of ACE inhibitors may lead to the upregulation of ACE-2 receptors in diabetic patients, binding of spike (S) glycoprotein to the ACE-2 is a vital step for virus entry into human cells. Additionally, some other notions have been proposed that the immune system is exceedingly compromised in diabetic subjects, which render them harder to combat the virus and likely result in a more extended recovery period. Secondly, the virus can A recent study on pediatric patients with COVID-19 (n= 2143) observed that marginally more boys (56.6%) are influenced in the COVID-19 outbreak as compared to girls (43.4%) but with non-significant gender difference. The median age is seven years, age varies from 1 day to 18 years. This is suggestive of the fact that all ages in childhood are equally susceptible to SARD-CoV-2 (Dong et al., 2020) . Moreover, the children's COVID-19 cases were less severe as compared to adults' cases, and this is somewhat perplexing. This difference may be associated with exposure time as well as host factors. ACE-2 is a cell receptor for entry of SARS-CoV-2 and is hypothesized that children are less responsive to SARS-CoV-2, because the less maturity and function of ACE 2 in children than that in adults (Cristiani et al., 2020) . Additionally, as compared to adults, children are expectedly more prone to upper respiratory infections (for e.g., respiratory syncytial virus in winter), and their blood might have elevated levels of efficient antibodies against viruses (Cristiani et al., 2020) . Furthermore, the immune system of children is in the developing phase and might react to pathogens differently in contrast to adults. However, it has been documented that the percentage of severe COVID-19 cases in children is 10.6%, 7.3%, 4.2%, 4.1%, and 3.0% in the age group of ˂1, 1-5, 6-10, 11-15 and >15 years, respectively. These findings are suggestive of the fact that infants are extra susceptible to COVID-19 infection.

Consequently, the precise mechanisms for the discrepancy in clinical symptoms of children and adults remain elusive (Dong et al., 2020) .

J o u r n a l P r e -p r o o f A recent study assessed data from eight countries and four USA cities which remain epicenters of the COVID-19 pandemic. The risk of death is 13-to 73-fold higher in elderly individuals (>65 years age) as compared to non-elderly individuals (<65 years age). The age-dependent threat is moderately stronger in European countries in contrast to US cities. The finding disclosed that 5-9% and < one-third of all COVID-19 deaths in European countries and 4 USA cities, respectively, are from non-elderly age groups having less than 65 years of age. Moreover, the bulk of deaths in this age group take place in the age of sub-group 40-65 that covers 36 to 48% of the total population in the 0-65 years old category. Additionally, the majority of the fatalities in the non- 

Currently, there is no clinically available drug or therapeutics endorsed by the U.S. FDA to prevent or contain COVID-19. The on-going clinical management for COVID-19 includes disease prevention (use of some clinically approved anti-viral drugs, hydroxychloroquine, immunosuppressant drugs), and supportive care in hospitals, including oxygen and ventilation support (Sanders et al., 2020; Zhang et al., 2020; . The quest for efficient treatment is proceeding with numerous investigations ongoing across the world. Recently, the USFDA has authorized the emergency use of remdesivir in severely ill hospitalized patients as this drug has been shown to fasten the recovery rate and is associated with less mortality rate (https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda issues Journal Pre-proof J o u r n a l P r e -p r o o f emergency-use-authorization-potential-covid-19-treatment). In addition, Indian authorities (Drug Controller General of India) have approved oral antiviral drug favipiravir to treat mild to moderate COVID-19 (https://www.europeanpharmaceuticalreview.com/news/121787/glenmarkapproved-to-supply-favipiravir-as-covid-19-treatment-in-india/).The list of repurposed drugs with completed clinical trials and a list of various potential repurposed drugs currently in clinical trials has been summarized in Table 1 and Table 2 , respectively (Sanders et al., 2020; https://www.ashp.org/-/media/assets/pharmacy-practice/resourcecenters/Coronavirus/docs/ASHP-COVID-19-Evidence- Table. ashx?).

Plasmapheresis, also known as therapeutic plasma exchange (TPE), has been suggested as an alternative therapy to treat COVID-19 patients. Apheresis is the safest advised technique to acquire plasma, and this technique first involves the collection of blood from COVID-19 survivors, followed by plasma separation from blood cells. The same separated plasma is then infused into the circulation of severely ill COVID-19 patients (Keith et al., 2020) . In this context, (Rojas et al., 2020) . In a nutshell, the transfer of numerous aforementioned blood factors during the transfusion process might account for some inhibition of excessive inflammatory response in COVID-19 patients. Though this technique is not approved by the FDA as a novel therapy, neither can be considered the best treatment option but can be considered as a possible emergency treatment for fulminant COVID-19. In the context of plasma therapy, FDA has released guidance to health care providers and investigators regarding use as well as the study of investigational convalescent plasma (COVID-19 convalescent plasma) (https://www.fda.gov/vaccines-blood-biologics/investigational-newdrug-ind-ordeviceexemption-ide-process-cber/recommendations-investigational-covid-19convalescent-plasma). Till date, a total of 97 clinical trials for the use of convalescent plasma in COVID-19 has been registered with ClinicalTrial.gov, and the full list can be accessed at (https://clinicaltrials.gov/ct2/results?term=Convalescent+Plasma+Therapy&cond=COVID19&Se arch=Apply&age_v=&gndr=&type=&rslt=). A list of a first ten registered clinical trials (three completed) with an infusion of investigational convalescent plasma (from survivors) into the blood of severely ill patients has been summarized in Table 3 .

The development of a new vaccine to contain COVID-19 infection is conceivably the safest way to end this pandemic. At present, there is no vaccine candidate to prevent COVID-19, but many of investigators are battling to create an effective and safe one (Kim et al., 2020) . Accumulating 

Despite enormous worldwide endeavors to restrict COVID-19 infection caused by SARS-CoV-2, the virus's spread has achieved an epidemic stage. There have been numerous warnings to glean from the worldwide response to the SARS-COV-2 threat. The nonexistence of a trustworthy, reliable, early alert and response system, failure to mount containment actions, a paucity of community commitment for self-isolation, and overdependence on quarantining measures have uncovered the clefts in the competence of health systems worldwide. The COVID-19 outbreak has clearly shown the anemic preparation against evolving and re-emerging dangerous pathogens across the world. The source of the outbreak, the intermediate host, an effective treatment regimen, tools for early diagnosis in asymptomatic patients, and tools to predict the emergence of novel pathogens all remain elusive. It is now well established that SARS-CoV-2 enters cells by attaching to specific ACE-2 receptors, which are widely expressed in the respiratory tract, brain, liver, bile duct, lower GIT, and kidney. Thus, all these organs remain on the verge of damage by . Keeping in view the global threat to public health instigated by SARS-CoV-2, an efficient prevention system as well as medication for COVID-19 pneumonia is instantly required.

Even though the development of COVIS-19 therapeutics and vaccines is in its childhood stage, but still researchers across the globe have met with some substantial progress starting from elucidating out structure, full genomic sequencing of SARS-CoV-2, pathogenesis of COVID-19 and up to the commencement of clinical trials with COVID-19 drugs and vaccines.

Nevertheless, the development of medication for SARS-CoV-2 is still a prime dilemma for humans, and there is currently no clinically approved drug to combat COVID-19 except Figures   Fig. 1a : Structure of SARS-Cov-2 depicting the structural proteins and nucleocapsid. 

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Nervous system involvement after infection with COVID-19 and other coronaviruses

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Insight into 2019 novel coronavirus-an updated intrim review and lessons from SARS-CoV and MERS-CoV

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Systematic comparison of two animal-to-human transmitted human coronaviruses: SARS-CoV-2 and SARS-CoV

Pathological findings of COVID-19 associated with acute respiratory distress syndrome. The Lancet respiratory medicine

Coronavirus disease 2019 (COVID-19) in pregnant women: A report based on 116 cases

Plasma glucose levels and diabetes are independent predictors for mortality and morbidity in patients with SARS

Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a singlecentered, retrospective, observational study. The Lancet Respiratory Medicine

Cardiovascular complications of severe acute respiratory syndrome

The authors would like to thank all the research groups worldwide working on COVID-19 for their significant contributions during this outbreak.

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In-vitro activity against SARS-CoV-2 in Vero E6 cells; In-vitro activity against SARS-