COVID-19 Research Shows Older Heart Muscles Cells Are More Vulnerable To SARS-CoV-2 Coronavirus As Result Of Direct Viral Effects And Not Cytokine Storms
: European researchers from Universitair Ziekenhuis Leuven-Belgium, Karolinska Institutet-Sweden, Dresden University of Technology-Germany, University of Cambridge-UK and University of Oslo-Norway have in a new collaborative study shown that genes involved in severe SARS-CoV-2 coronavirus infection are expressed to a higher degree in older heart muscle cells or cardiomyocytes than they are in younger cardiomyocytes.
The study team found that genes encoding the proteins involved in host cell viral entry, including angiotensin-converting enzyme 2 (ACE2) and transmembrane protease, serine 2 (TMPRSS2) were upregulated in aged cardiomyocytes compared to young adult cardiomyocytes.
Significantly, risk factors for adverse outcomes following SARS-CoV-2 infection include age over 70 years and comorbidity, particularly cardiovascular disease.
Professor Dr Anthony Davenport from University of Cambridge, a co-researcher said that the team’s their findings could inform studies investigating experimental or currently available compounds to understand further how the protein pathways in cardiomyocytes contribute to disease outcomes in older patients with coronavirus disease 2019 (COVID-19).
The research findings are published on a preprint server but are currently undergoing peer-review. https://www.biorxiv.org/content/10.1101/2020.07.07.191429v1
In order to infect host cells, the Spike protein on SARS-CoV-2 must bind ACE2 and undergo subsequent cleavage by TMPRSS2. This protease primes the Spike S1 subunit for the internalization of the virus. A second site on the S2 subunit is also cleaved by the enzyme furin. Once inside the cell, the virus undergoes endosomal processing by the cysteine proteases cathepsin L(CTSL) and cathepsin B (CTSB).
It was found that in cardiomyocytes, ACE2 converts the vasoconstrictor angiotensin II to the vasodilator angiotensin (1-7) and converts the inflammation-promoting peptide des-Arg9-bradykinin to inactive bradykinin (1-8).
Schematic diagram of the key proteins predicted from RNASeq data to be expressed by human cardiomyocytes. We propose SARS-CoV-2 binds initially to ACE2 (with the ACE2/B0AT1 complex as a potential second entry site). TMPRSS2 priming of the spike protein S1 together with further protease activation by Cathepsins B and L facilitates viral cell entry and internalization by endocytosis. Furin may also have a role in this process. Internalization of the virus with ACE2 inhibits ACE2 carboxypeptidase activity that normally hydrolyzes Ang-II, [Pyr1]-apelin-13, and des-Arg9-bradykinin. ADAM17, present on the cell surface, cleaves ACE2 to the soluble form that circulates in the plasma and could act as a decoy substrate for the virus. Levels of ADAM17 may be regulated by Ang-II and apelin acting via their respective G-protein coupled receptors.
Also discovered is the fact that internalization of ACE2 by SARS-CoV-2 potentially counteracts
the function of these metabolites in the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and electrolyte balance.
Importantly, ACE2 is also expressed in the gastrointestinal tract, where it associates with BOAT1, a protein that transports neutral amino acids across epithelial cell membranes. However, researchers do not yet know whether ACE2 is also co-expressed with BOAT1 in cardiomyocytes, which could serve as a crucial mechanism for viral entry.
Dr Davenport and team hypothesized that differential expression of genes encoding the proteins involved in viral entry pathways in aged versus younger cardiomyocytes might explain why older people are particularly susceptible to the cardiovascular complications associated with COVID-19.
The study team used RNA isolated from the cardiomyocyte nuclei of the left ventricle to make strand-specific RNA-sequencing libraries. The sequence data generated for five men, aged 19 to 25 years, were compared with those generated for five older men, aged 63 to 78 years, who had not been on medication and not shown any evidence of cardiovascular disease post-mortem.
It was found that the expression of genes encoding proteins involved in viral entry was upregulated in the aged cardiomyocytes compared with the younger cardiomyocytes. The genes included ACE2, TMPRSS2, FURIN, CTSL, CTSB, and B0AT1, and their combined relative expression in cardiomyocytes correlated positively with age.
The team also found that the expression of Angiotensin II, ACE, Angiotensin II receptor type 1 (AGTR1), and Bradykinin receptor B1 (BDKBR1) all increased with age.
Normally, BDKBR1 is not expressed in muscle cells until expression is induced by inflammation and activated by des-Arg9-bradykinin, says the team.
The researchers hope the findings can inform drug development studies
Dr Davenport told Thailand Medical News, “Our results highlight SARS-CoV-2 related genes that have higher expression in aged compared with young adult cardiomyocytes. These data may inform studies using selective enzyme inhibitors/antagonists, available as experimental compounds or clinically approved drugs e.g. remdesivir that has recently been rapidly accepted for compassionate use, to further understand the contribution of these pathways in human cardiomyocytes to disease outcome in COVID-19 patients.”
The study team says drug candidates will need to be focused on preventing three main stages of infection (host cell viral entry, viral replication, and the ensuing tissue damage) in the heart tissue of older individuals, the age group most susceptible to COVID-19.
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