Researchers in Europe have shown that genes involved in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection are expressed to a higher degree in older heart muscle cells (cardiomyocytes) than they are in younger cardiomyocytes.
The 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.
Risk factors for adverse outcomes following SARS-CoV-2 infection include age over 70 years and comorbidity, particularly cardiovascular disease.
Anthony Davenport (University of Cambridge) and colleagues say 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).
A pre-print version of the paper is available on the server bioRxiv*, while the article undergoes peer review.
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.
The infection process
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).
The role of ACE2
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).
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.
Interestingly, 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.
The team’s hypothesis
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 researchers 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.
The genes involved in viral entry were upregulated in aged cardiomyocytes
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 researchers also found that the expression of Angiotensin II, ACE, Angiotensin II receptor type 1 (AGTR1), and Bradykinin receptor B1 (BDKBR1) all increased with age.
Generally, BDKBR1 is not expressed in muscle cells until expression is induced by inflammation and activated by des-Arg9-bradykinin, says the team.
The authors hope the findings can inform drug development studies
“Our results highlight SARS-CoV-2 related genes that have higher expression in aged compared with young adult cardiomyocytes,” write Davenport and team.
“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 researchers say 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.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Davenport A, et al. Genes Encoding ACE2, TMPRSS2 and Related Proteins Mediating SARS-CoV-2 Viral Entry are Upregulated with Age in Human Cardiomyocytes. bioRxiv 2020. doi: https://doi.org/10.1101/2020.07.07.191429