Melatonin-Index — Biomarker for Predicting Asymptomatic SARS-CoV-2 Carriers

Why would a person with high levels of melatonin be asymptomatic during SARS-CoV-2 infection?

A groundbreaking study on melatonin and SARS-CoV-2 published by Fernandes et al. on Jan 4 at Melatonin-Research sheds light on why melatonin directly targets SARS-CoV-2 infections. This study validated all the important points I have been sharing with you in the COVID-19 series at this blog since February 2020. 

Correlation may really imply causation, if it is melatonin

In the last article, “Toremifene-Melatonin: the Gold Standard for COVID-19 Treatment”, I talked about how a 5 yr-old girl exhibited the highest antibody immune response but was the only one without any symptoms in a family of five infected by the coronavirus in Australia [1]. I believed that the reason for the asymptomatic child was due to her high circulating melatonin levels. This observation was corroborated by the Cleveland Clinic study that discovered the existence of a definitive correlation between the active use of melatonin and reduced positive laboratory test results for SARS-CoV-2 [2]. This study was criticized by many, citing correlation does not imply causation. Well, I have news for you; the study by Fernandez et al. showed without any doubt that, in the case of melatonin and SARS-CoV-2, correlation may actually imply causation.

CD147 is an important gateway in SARS-CoV-2 infection

The ubiquitous transmembrane glycoprotein CD147 may be a potent alternative receptor for SARS-CoV-2 infection. The binding interactions between CD147 and spike protein can lead to a wide range of pathologies affecting multiple organs and tissues including heart, kidney, brain, liver, eyes, skin, blood vessels, red blood cells, white blood cells, only to name a few. My peer-reviewed paper on CD147 and melatonin detailed some of the mechanisms involved in the pathogenesis of myocardial injuries and thrombotic events caused by spike protein binding to CD147. The paper also explained why melatonin can attenuate or even prevent those symptoms [3]. Since then, several studies have explored how CD147 binding to spike protein may increase pathogenesis and induce organ damage especially in kidneys [4-6]. One study even  demonstrated clearly via in-silico molecular docking how CD147 binding to spike protein may support its critical role during SARS-CoV-2 viral replication and infection [7]. The study by Fernandez et al. found an interesting relationship between melatonin and CD147.

The puzzling role of ACE2 in asymptomatic carriers is finally explained

Several studies have shown that ACE2 has limited expression in most organs and tissues, with decreasing expressions from nose to lungs [8]. However, this receptor has been shown to have an extremely high affinity to spike protein of SARS-CoV-2 [9]. Several studies have also determined that a high level of ACE2 may not be correlated with SARS-CoV-2 disease severity, and the use of ACE inhibitors was associated with reduced severity [10]. How does one resolve the contradiction between high viral affinity to a receptor and low disease severity? This unusual association has remained a puzzle unsolved by science until this groundbreaking study by Fernandes et al.
Fernandes and colleagues showed for the first time that there is a significant and important correlation between melatonin production and the expression of genes involved in SARS-CoV-2 viral entry in lung epithelial AT2 cells and alveolar macrophages. This tight correlation between melatonin levels in cells and the different genes also explained for the first time, why melatonin can be the reason why individuals infected by SARS-CoV-2 may be totally asymptomatic, despite a high level of ACE2 expression [11]. 

What are the methods used to determine this correlation?

The Melatonin Index

Fernandes et al. used gene-set enrichment analysis (GSEA) and networking tools to compare 455 genes of 288 samples of normal human lungs. These genes were selected from a COVID-19-Signature gene set containing genes that modulate virus gateways, intracellular trafficking, transcription/post-translation, and mitochondrial functions. The authors then compared the expression of these genes in selected cell types and compared their expression levels against the MEL-Index (Melatonin-Index) [11]. 

The MEL-Index is a predictive model that can show the content of melatonin in selected cells. The index combines expression levels of the ASMT gene required for melatonin synthesis and the CYP1B1 gene that is responsible for melatonin metabolism. The MEL-Index can effectively gauge the ability of cells to synthesize and metabolize melatonin [12].
When the authors compared the MEL-Index in lung epithelial AT2 cells and alveolar macrophages against the set of genes from the COVID-19 Signature set, the results were simply impressive beyond words.

The ACE2 dilemma in SARS-CoV-2 infection is finally resolved

ACE2 is protective. But ACE2 is also the receptor for the SAR-CoV-2 spike protein. ACE2 expression levels are often found to be elevated in the lungs of patients with severe symptoms during SARS-CoV-2 infections [13]. So if you are interpreting this relationship in a linear fashion, you may conclude that elevated ACE2, due to its high binding affinity to spike protein, may actually be the cause for increased disease severity in SARS-CoV-2 infection.

Interestingly, Fernandez et al. actually found a strong POSITIVE correlation between genes related to the ACE2 gateway, such as the protease TMPRSS2, and the MEL-Index. That means, in the cells examined, if melatonin expression levels were high, the gene expression associated with ACE2 were also high. If you think about this, it should make sense since ACE2 exerts a protective function in cells. But this positive correlation only further emphasizes the ACE2 paradox, since TMPRSS2 is a protease that is required for activation of viral replication processes [14]. So if there is more ACE2 and TMPRSS2 expressed, then there should theoretically be a higher level of infection and replication. That assumption is actually incorrect because an important player is left out of the equation. Which player would that be? Remember the furin enzyme that I talked about almost one year ago in February 2020 [15]? 

Furin gene expression is negatively correlated with the MEL-Index

Furins are proteases that cleave and activate spike protein subunits 1 and subunit 2. SARS-CoV-2 virus must be pre-cleaved by the furin protease before the spike protein can be further activated by proteases such as TMPRSS2 and cathepsin B/L [16, 17]. The cleavage by furin protease separates the S1 and S2 subunits and is absolutely required for receptor binding, fusion, and cell entry [18].

This dynamic interaction between melatonin, ACE2, TMPRSS2, and furin may explain why subjects with high melatonin are almost always asymptomatic, even though they are infected by SARS-CoV-2. In cells with high melatonin expression, furin expression is negatively correlated, meaning if melatonin is high, then the expression of furin would be low, and vice versa. Therefore, in cells replete with melatonin, even if there is an abundant level of ACE2 and TMPRSS2, there is no follow-through because the spike protein subunits 1 and 2 are not cleaved and activated by furin, which would be deficient in cells with high MEL-Index. 

Genes related to the CD147 membrane complex were NEGATIVELY correlated with the MEL-Index

Even though ACE2 has a high affinity to bind to SARS-CoV-2 spike protein, the expression levels of this gene is rather low in many of the organs and tissues that are affected by SARS-CoV-2 [19]. 

On the other hand, CD147 is found in every human tissue and organ [20].

Similarly, the furin protease is also widely expressed in cells and tissues [21].

ACE2 receptors are not found in blood; but take a look at the number CD147 receptors in red blood cells and reticulocytes [22]. Why is this important?

Patients infected by SARS-CoV-2 with viral RNA detected in their blood samples usually progressed to severe symptom stage [13]. If viral RNA is found in a blood sample, that means the virus was able to replicate in blood. That will also mean that furin enzymes, also expressed in blood, were able to cleave and activate the virus. When that happens, it may also mean that circulating melatonin levels are low. This inverse relationship between the MEL-Index and the genes associated with the CD147 viral gateway is extremely important. 

CD147 and multi-organ damage during SARS-CoV-2 infections

Take another look at the wide distribution of CD147 in different tissues and organs of the human body.

The binding of spike protein to CD147 was first demonstrated by Wang et al. via immune-electron microscopy  in March 2020 [24]. 

Subsequently, Helal and team used in-silico molecular docking methodology to show binding modes between CD147 and the receptor binding domain (RBD) in SARS-CoV-2 spike protein [7]

Due to the wide expression of the transmembrane glycoprotein and its association with cyclophilin A (CypA) and matrix metalloproteinases such as MMP9, CD 147 binding to RBD of spike protein can cause extensive dysregulation in important organs and tissues, especially kidneys [25], heart [26], eyes [27, 28], brain [29, 30]. CD147-Cyclophilin A (CypA) signaling can activate platelets, causing increased adhesion and thrombus formation in vitro and in vivo (31). 90% of COVID-19 inpatients with pneumonia with elevated D-dimer showed increased coagulation activity [32]. Recently, Manne et al. found altered gene expressions in platelet samples obtained from patients infected by SARS-CoV-2. ACE2 expression could not be detected in those platelet samples [33].

It is now widely accepted that targeting CD147 and cyclophilin A are viable therapeutic options for the treatment of SARS-CoV-2 infections [34]. 

The fact that the MEL-Index was found to be negatively correlated with genes that codify the proteins of the multi-molecular CD147 receptor complex highlights the role played by melatonin during not only viral infection but disease progression in general. The MEL-Index has been used successfully as a biomarker for cancer progression in human glioma, a common form of brain tumor [12]. Not surprisingly, the overexpression of CD147 was associated with poorer overall survival in glioma patients [35].

Melatonin is an ancient molecule that has protected living organisms from disease and viral infections for over 3 billion years. Correlation does imply causation if you consider the fact that melatonin is probably involved with possibly all biological processes due to its potent antioxidant properties. There is no reason not to trust this faithful molecule to protect humans during the current pandemic.

Fernandes and colleagues in their recent study of melatonin achieved a true milestone. The team provided definitive evidence of strong correlations between melatonin production/metabolism and the expression of genes vital to SARS-CoV-2 replication and infection. Even though the MEL-Index is not meant to provide the exact mechanisms utilized by melatonin, the index clearly shows that a higher level of melatonin in cells is protective during SARS-CoV-2 infections.

Have you had your MEL today?

 

References:

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