COVID-19 New: American Researchers Uncover the Importance Of Selenium And Glutathione Supplementation In COVID-19 As SARS-CoV-2 Mpro Targets These!
: Latest SARS-CoV-2 research conducted by scientist from the Department of Chemistry and Biochemistry at University of North Carolina Greensboro-USA has discovered that the SARS-CoV-2 main protease (Mpro) targets the human host selenoproteins and glutathione biosynthesis for knockdown via proteolysis, potentially disrupting the thioredoxin and glutaredoxin redox cycles.
The study findings show the importance of selenium and glutathione supplementation in order to prevent disease severity and also health complications arising in Long COVID. The study findings also highlight yet the key role of another micronutrient ie Vitamin D3 in COVID-19.
According to the study team, associations between dietary selenium status and the clinical outcome of many viral infections, including SARS-CoV-2, are well established.
To date, numerous studies have documented a significant inverse correlation between selenium status and the incidence and mortality of COVID-19.
Interestingly, at the molecular level, SARS-CoV-2 infection has been shown to decrease the expression of certain selenoproteins, both in vitro and in COVID-19 patients.
The study team utilizing computational methods, had previously identified a set of six host proteins that contain potential SARS-CoV-2 main protease (Mpro) cleavage sites.
In this new study, the study team showed experimentally that Mpro can cleave four of the six predicted target sites, including those from three selenoproteins: thioredoxin reductase 1 (TXNRD1), selenoprotein F, and selenoprotein P, as well as the rate-limiting enzyme in glutathione synthesis, glutamate-cysteine ligase catalytic subunit (GCLC).
Cleavage was assessed by incubating recombinant SARS-CoV-2 Mpro with synthetic peptides spanning the proposed cleavage sites, and analyzing the products via UPLC-MS.
Upon incubation of a recombinant Sec498Ser mutant of the full TXNRD1 protein with SARS-CoV-2 Mpro, the predicted cleavage was observed, destroying the TXNRD1 C-terminal redox center.
Mechanistically, proteolytic knockdown of both TXNRD1 and GCLC is consistent with a viral strategy to inhibit DNA synthesis, conserving the pool of ribonucleotides for increased virion production. Viral infectivity could also be enhanced by GCLC knockdown, given the ability of glutathione to disrupt the structure of the viral spike protein via disulfide bond reduction.
The study findings shed new light on the importance of dietary factors like selenium and glutathione in COVID-19 prevention and treatment.
The study findings were published in the peer reviewed journal: Antioxidants
The importance of selenium as a micronutrient that played significant role in COVID-19 emerged even within the first months of the pandemic and was covered in various COVID-19 News
coverages and also studies.
In the same light, the importance of glutathione was also covered in articles by Thailand Medical News.
Th study findings suggest that the main protease of SARS-CoV-2 targets GCLC, TXNRD1, and two additional selenoproteins, SelenoP and SelenoF, for at least partial proteolytic degradation. It is possible that this decrease may in part be the result of Mpro mediated proteolysis, which would be more pronounced at higher viral loads, i.e., in more severe cases.
The SARS-CoV-2 Mpro also targets GSH biosynthesis via proteolytic degradation of GCLC, the rate limiting enzyme for GSH biosynthesis.
COVID-19 patients have been found to manifest severe GSH deficiency, increased oxidative stress, and oxidant damage relative to uninfected controls.
Other studies have pointed out that low GSH levels are characteristic of many comorbid conditions associated with increased severity of COVID-19, and that there are multiple molecular mechanisms by which “GSH depletion may have a fundamental role in COVID-19 pathophysiology”.
Considering that GSH is an essential and ubiquitous small molecule antioxidant and free radical scavenger, with demonstrated anti-inflammatory and antiviral effects, it is clear that a significant decrease in GSH biosynthesis would have various deleterious effects consistent with COVID-19 pathologies.
Past studies supporting the current study showed impaired metabolism and redox function of cellular GSH in SARS-CoV-2-infected Vero cells. At six hours post-infection, it was reported that a transient increase in Nrf2 expression and protein levels of GCLC (which is upregulated by Nrf2); this effect is probably related to an initial cellular innate immune response. By 24 h, Nrf2 had returned to baseline, but GCLC levels had decreased by 30% below uninfected controls (p < 0.05). This is consistent with the possibility of viral knockdown by proteolysis that our results imply, and would contribute directly to the observed disruption of GSH function.
Interestingly, a shared role of GSH, TXNRD1 via its dithiol substrate thioredoxin (TRX), and SelenoF via its role in protein folding quality control, is the reduction of disulfide bonds in proteins.
Such activity can disrupt the structure of viral proteins such as the SARSCoV-2 spike protein, as has been demonstrated for GSH and other thiol reductants, leading to decreased melting temperature of the spike protein receptor binding domain (RBD), reduced binding affinity of the RBD to its receptor ACE2, and decreased infectivity, probably via inhibition of fusion and viral entry.
The antiviral effects of GSH would be decreased by proteolytic knockdown of GCLC, favoring viral replication. Targeting of TXNRD1 and SelenoF by Mpro may have similar benefits for the virus, as they also, directly or indirectly (as TRX), mediate reduction of diverse disulfide substrates.
There is another biological role of GSH that is particularly significant for RNA virus replication, which could make it a primary target for disruption by SARS-CoV-2: its role as a component of one of the two essential redox systems that sustain DNA synthesis. DNA synthesis is inherently antithetical to the replication of RNA viruses like SARS-CoV-2, because it uses the same starting materials as those required for RNA synthesis: ribonucleotides. This is the only way to make DNA in all the kingdoms of life, via the enzyme ribonucleotide reductase (RNR), which utilizes several small thiol-containing proteins, TRX and glutaredoxin (GLRX), as reducing agents to convert ribonucleotides into 2′-deoxyribonucleotides.
Sustaining those reactions requires either GSH, to propel the GLRX cycle, or thioredoxin reductase (particularly the TXNRD1 isoform) to maintain the supply of reduced TRX. At least one of those two redox cycles needs to be functional to sustain the production of DNA.
Hence, it is not likely to be a coincidence that two of the enzymes that are being targeted for degradation by Mpro are TXNRD1 and GCLC. By suppressing the activity of both the TRX and GLRX redox systems, one of which can usually serve as a backup for the other, the virus could significantly slow down the conversion of ribonucleotides to 2′-deoxyribonucleotides, and thereby enhance their bioavailability for the synthesis of RNA.
This is needed for viral mRNA for protein synthesis, as well as for genomic viral RNA to incorporate into new virions. In light of this mechanism, it is possible that some of the pathologies of viral infection may simply be “collateral damage” consequent to this primary goal of enhancing viral replication.
Considering their multiple roles in antioxidant defense, maintenance of redox homeostasis, and immune cell functioning, it is obvious that if these two essential biological reducing agents (TXNRD1 and GSH) are disrupted, the resulting increased oxidative stress and pro-inflammatory conditions, with pathological consequences, are to be expected.
Interestingly, one expected consequence of inhibition of DNA synthesis is decreased capacity for cells to divide, which is a characteristic of cells transfected with a SARS-CoV-2 Mpro expression construct
This effect ofMpro alone in the absence of virus is consistent with the hypothesis that proteolytic targeting of host proteins is effectively inhibiting DNA synthesis. It is also of considerable interest that both of these targets are significantly upregulated at the mRNA level by the active form of vitamin D, 1,25-dihydroxyvitamin D3, which is known to activate various antioxidant and anti-inflammatory response genes (both directly and indirectly, via the activation of Nrf2).
By tending to increase the levels of TXNRD1 and GCLC, this effect of vitamin D would act in an opposing manner to the degradative actions of the viral protease. This offset of a potentially detrimental effect of the viral infection suggests a possible preventive benefit for a higher vitamin D status, which could help to explain reports of correlations between higher blood levels of 25-hydroxyvitamin D3 or vitamin D supplementation and a reduced risk of severe outcomes in COVID-19.
The study findings in regard to SARS-CoV-2 are unprecedented, because this is the first time that any virus has been shown to target selenoprotein expression at the protein level, by a direct proteolytic attack. The proteolytic targeting of both TXNRD1 and GCLC, and thus GSH biosynthesis, is consistent with a coordinated two-pronged attack, disrupting both redox cycles required to sustain DNA synthesis. Furthermore, the potential for significant disruption of GSH synthesis by SARS-CoV-2 is highly consistent with recent clinical observations showing profound GSH deficits in patients that correlate with the severity of COVID-19.
The SARS-CoV-2 virus also benefits from GSH knockdown by inhibiting its role as a disruptor of spike protein RBD structure and binding.
The study findings suggest an urgent need for further investigations of GSH repletion or complementation strategies, such as supplementation with GSH precursors like N-acetyl cysteine (NAC), or the more effective combination of NAC plus glycine, γ-glutamyl cysteine (GGC, the product of GCLC), or the GSH mimic α-lipoic acid, as potentially useful symptomatic treatments for COVID-19.
One inescapable conclusion in regard to SARS-CoV-2 is that, of any virus known to date, it is in a class by itself as far as its innovations in the perturbation of host Se and selenoprotein-based mechanisms. It certainly gives the appearance of an escalation in the arms race between viral replication and the defensive host mechanisms of which selenoproteins are an essential part.
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