COVID-19 Research: Study Proposes Redox Hypothesis To Explain As To Why Certain Individuals Are Vulnerable To SARS-CoV-2. Antioxidants Can Help.

COVID-19 Research: Scientists from McGill University-Canada, University of Saskatchewan-Canada and University of Calgary-Canada have in a new study proposed the ‘redox hypothesis; to help explain as to why certain individuals are more vulnerable to the SARS-CoV-2 infection.


 
In their abstract presentation, the study team says that disparity in the infection of SARS-CoV2 among host population and species is an established fact without any clear explanation to date. To initiate infection, viral S-protein binds to the Angiotensin-Converting Enzyme 2 (ACE2) receptor of the host cell.
 
The study team’s analysis of retrieved amino acid sequences deposited in data bases shows that S-proteins and ACE2 are rich in cysteine (Cys) residues, many of which are conserved in various SARS-related coronaviruses and participate in intra-molecular disulfide bonds. High-resolution protein structures of S-proteins and ACE2 receptors highlighted the probability that two of these disulfide bonds are potentially redox-active, facilitating the primal interaction between the receptor and the spike protein. Presence of redox-active disulfides in the interacting parts of S-protein, ACE2, and a ferredoxin-like fold domain in ACE2, strongly indicate the role of redox in COVID-19 pathogenesis and severity.
 
Interestingly resistant animals lack a redox-active disulfide (Cys133-Cys141) in ACE2 sequences, further strengthening the redox hypothesis for infectivity. ACE2 is a known regulator of oxidative stress. Augmentation of cellular oxidation with aging and illness is the most likely explanation of increased vulnerability of the elderly and persons with underlying health conditions to COVID-19.
 
The study findings were published in the peer reviewed Computational and Structural Biotechnology Journal. https://www.sciencedirect.com/science/article/pii/S2001037020304839
 
A common question in this pandemic is: what makes the elderly and people with underlying conditions more vulnerable to COVID-19?
 
The study team says that clues can be found in the proteins involved in initiating infection, as the virus binds to host cells of different animals. Greater cellular oxidation with aging and sickness may explain why seniors and people with chronic illness get infected more often and more severely.
 
To date over 65.1 million people have been infected and more than 1.51 million have died from COVID-19.
 
The SARS-CoV-2 coronavirus is disrupting economies and food supply chains all over the world. Understanding why some animals get infected and others do not could be the key to unlocking new treatments and therapies.
 
The study team analyzed available protein sequences of the virus and host cell receptors across different spices to find out why.
 
Lead researcher, Dr Jaswinder Singh, a Professor at the Department of Plant Science at McGill University told Thailand Medical News, “It is known that the SARS-Cov-2 coronavirus can infect humans, cats, dogs and ferrets but not bovine and swine. Also, COVID-19 hits the elderly and people with underlying conditions more severely than the young and healthy ones. Until now reasons for this were unclear."
 
https://www.nature.com/articles/s41586-020-2334-5_reference.pdf
 
https://science.sciencemag.org/content/368/6494/1016.abstract
 
The study was conducted by a multidisciplinary team of scientists led by Professor Singh. The team includes Professor Rajinder Dhindsa (McGill University), Professor Baljit Singh (University of Calgary) and Professor Vikram Misra (University of Saskatchewan).
 
It was found that once inside a host cell, the virus hijacks the cell's metabolic machinery to replicate and spread. The virus's protein spikes attach to a protein receptor on the surface of the host cell called ACE2, fusing the membranes around the cell and the virus together.
 
This process allows the virus to enter the cell and co-opt its protein-making machinery to make new copies of itself. The copies then go on to infect other healthy cells.
 
By analyzing the proteins and their amino acid building blocks in detail, the researchers found that animals susceptible to the virus have a few things in common.
 
It was found that humans, cats, and dogs have two cysteine amino acids that form a special disulfide bond held together by an oxidizing cellular environment. This disulfide bond creates an anchor for the virus.
 
Co-author Dr Rajinder Dhindsa, an emeritus professor of biology at McGill University added, "Our analysis suggests that greater cellular oxidation in the elderly or those with underlying health conditions could predispose them to more vigorous infection, replication and disease."
 
However In the case of animals resistant to the virus, like pigs and cows, one of these two cysteine amino acids is missing, and the disulfide bond cannot be formed. As a result, the virus cannot anchor on to the cell properly.
 
The involvement of cysteines in redox activity through reversible disulfide bond formation and many other types of oxidation such as sulfenic acid formation has been studied rigorously in plant, animal and microbial systems. https://www.plantphysiol.org/content/95/2/648.short
 
https://www.annualreviews.org/doi/abs/10.1146/annurev-arplant-043015-111949
 
Several extracellular antioxidants, including paraoxonase (PON), thioredoxin (Trx), Trx reductase (TrxR), glutaredoxin (Grx), extracellular superoxide dismutase (EC-SOD), and extracellular glutathione (GSH), eliminate reactive oxygen species (ROS). https://www.jstage.jst.go.jp/article/circj/72/1/72_1_1/_article/-char/ja/
 
https://www.jbc.org/content/287/7/4397.short
 
Cellular disulfides are mainly reduced by the thioredoxin (Trx) system. Thioredoxin is a low molecular-weight disulfide protein that undergoes sulfhydryl/disulfide exchange reactions with target proteins. Consequently, Trx has a regulatory role in a variety of processes, ranging from general metabolism and antioxidant metabolism to cell signaling. Among other activities, Trx regulates photosynthesis, mitigates allergenicity, seed germination and improves the quality of bread and beer. There is evidence for a role for Trx in human diseases and increased expression of Trx suppresses Parkinson’s disease in mice and has been proposed as a treatment. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4145943/
 
Associations of ROS and influenza virus replication have been documented and ROS inhibitors have been proposed for suppression of influenza A virus-induced lung disease. Oxidative stress has been implicated in several major age-related conditions, including those involving cardiovascular, pulmonary, kidney disease and nerve tissues.
https://www.sciencedirect.com/science/article/abs/pii/S0165614711001593
 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5927356/
 
The cysteine-rich spike glycoproteins of SARS-CoV2 and its host receptors are likely to be influenced by redox-active disulfides in a Trx-associated fashion, or by other redox systems mentioned above. The Trx system protects against oxidative stress by providing electrons to thiol-dependent enzymes that remove ROS. Therefore, SARS-CoV2 infection and the associated disease follow a similar redox-associated path. If so, this could explain associations among age, COVID-19 and mortality. A role for thioredoxin in SARS-CoV2 biology and COVID19 severity is strengthened by two observations. First, deficiency of ACE2 expression in mice results in high levels of oxidation stress. https://physoc.onlinelibrary.wiley.com/doi/full/10.1002/phy2.264
 
Second, the transmembrane part of ACE2 contains a ferredoxin-like fold domain. https://science.sciencemag.org/content/367/6485/1444.abstract
 
Ferredoxin is known to pass electrons to thioredoxin which, in turn, reduces disulfide to sulfhydryls. https://www.sciencedirect.com/science/article/abs/pii/S0891584918326388

Based on the study team’s observation regarding disulfides in spike protein and ACE2 receptor, a Trx-dependent redox model could be created
 
In one scenario, the host receptor (ACE2) with three disulphide bonds through conserved cysteines could require an oxidative environment to keep its disulfide bonds intact and hence allow SARS-CoV2 to infiltrate target cells.
 
Consequently, higher ROS concentrations in the elderly or those with underlying health conditions would predispose them to more vigorous SARS-CoV2 infection, replication and disease. Higher ROS concentrations could also yield an appropriate environment for SARS-CoV2 surface spike protein to be operational, especially if its disulfide bonds are involved in redox. Taken together, an ROS-rich cellular environment would promote SARS-CoV2 infections and more lethal COVID-19.
 
 In contrast, a reducing or antioxidant environment may minimize the receptors’ interaction with SARS-CoV2. This reducing environment would also cause reduction of SARS-CoV2′s own disulfide bonds, leading to its disintegration. The Trx system has been documented to accomplish redox adjustments in such circumstances, as manifested in many similar situations. https://www.annualreviews.org/doi/abs/10.1146/annurev-arplant-043015-111949
 
It remains to be experimentally verified whether redox status of the host indeed plays a role in SARS-CoV2 biology and COVID-19 infection. Cysteine-rich surface spike glycoproteins of SARS-CoV2 and its host receptors are likely to be influenced by redox-active disulfides in a Trx-associated fashion, or by other redox systems mentioned above. Thus, increased oxidation associated with aging, smoking and various co-morbidities may contribute to higher susceptibility to COVID-19. Potential Trx-dependent redox changes accompanying viral replication and infection warrant attention, as they may yield insights into the mechanism of action of the virus and appropriate therapeutic regimens.
 
Antioxidant therapy could decrease disease severity by interfering with entry of the virus into host cells, a crucial first step in the establishment of infection.
 
A recently published report summarized various genetic strategies including associations between markers, blood groups and gene variants (IFNAR2, OAC, TYK2 and SLC6Z20) with COVID-19 cases to uncover the underlying causes of disparity in the infectivity of SARS-CoV2, but was unable to draw any definitive conclusions. https://science.sciencemag.org/content/370/6514/275.summary
 
There is still much to learn regarding basic mechanisms responsible for age-related susceptibility and contagiousness of SARS-CoV2 to develop effective, evidence-based prophylaxis and treatments.

In the opinion of the study team, a better understanding of the role of conserved cysteines and ROS crosstalk between target cell and SARS-CoV2 may yield molecular clues to interfere with this primal step. Furthermore, perhaps CRISPR-based technologies could be applied to target and modify specific cysteine sequences of RBD region of spike protein (Cys480-Cys488) and receptors (Cys 133-Cys141) of appropriate model animals to elucidate the underlying mechanisms. There may also be value in modulating the redox activity via a Trx system to improve human health by adoption of appropriate prevention and treatment therapies.
 
As mitochondria are a major redox organelle, genes implicated in mitochondrial function could be altered via the CRISPR/cas system. The latter approach could also be used to identify unknown genes and their protein products involved in COVID-19 progression and development, with potential to decrease, restore and improve gene expression, perhaps preventing or treating COVID-19.
 
The study team stresses that preventing the anchor from forming could be the key to unlocking new treatments for COVID-19.
 
A key strategy, they suggest, could be to disrupt the oxidizing environment that keeps the disulfide bonds intact.
 
Professor Singh states, "Antioxidants could decrease the severity of COVID-19 by interfering with entry of the virus into host cells and its survival afterwards in establishing further infection."
 
The study team said that in terms of next steps, CRISPR technology could be used to edit protein sequences and test out their theory. The researchers are also looking into other proteins near the ACE2 receptor that may facilitate entry of the virus to see if they behave the same way.
 
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