The study suggests targeting spike proteolytic event at the S2’ site may be a potential antiviral option in curbing COVID-19 and future coronavirus infections.
The number of COVID-19 infections has exceeded 240 million cases across the world and yet, much of the molecular mechanism of viral infection and host pathogenesis remain elusive. The SARS-CoV-2 viral envelope is decked with spike (S) glycoproteins that are critical for host receptor recognition, membrane fusion, and viral infection. Therefore, understanding the molecular and cellular mechanisms behind viral entry would greatly aid in curbing the spread of COVID-19 and preventing future coronavirus outbreaks.
A team of researchers led by Professor Meng Guangxun and Professor Lavillette Dimitri at the Institut Pasteur of Shanghai (IPS) of the Chinese Academy of Sciences looked into the functional and molecular requirements of spike activation and reported new mechanistic insights into the activation of SARS-CoV-2 spike proteins.
The monomer of SARS-CoV-2 spike protein contains two fragments: the amino terminus S1 subunit, which recognises the host receptor angiotensin-converting enzyme 2 (ACE2) for docking, and the carboxyl terminus S2 subunit, which facilitates the fusion of viral and cell membranes, allowing the release of viral RNA genome and replication within infected cells.
Previous works suggest that proteolytic cleavage within the S2 subunit is responsible for subsequent membrane fusion. However, the molecular events regulating spike processing and activation is not yet known. Furthermore, cells infected with SARS-CoV-2 has also been known to drive the fusion with adjacent ACE2-expressing cells, yielding morphologically distinct multinuclear giant cells known as syncytia. Such spike-mediate syncytia have been observed in the postmortem lung samples of severe COVID-19 patients. From this, the team hypothesised that spike-driven syncytia formation may provide an additional route for cell-cell transmission of SARS-CoV-2.
To investigate this, the group of scientists utilised a cell-cell fusion system in complement with a pseudoviral particle infection model to study the functional and molecular requirements of spike activation. Using multiple experimental models, they show that the proteolytic cleavage at the S2’ site is activated by human cell receptor recognition. The production of S2’ fragment specifically demands spike recognition of functional host ACE2 and is conserved in the various variants of concern.
Further analyses also revealed that after host receptor recognition of ACE2, spike-driven syncytia formation requires the presence of S2’ cleavage site at arginine 815, but not the furin cleavage site at 685. Point mutations of the S2’ cleavage site are not only effective against the original spike but also shown to be essential for the Alpha, Beta, and Delta variants.
The results of their work suggest that targeting spike proteolytic event at the S2’ site may be a potential antiviral option against the current SARS-CoV-2 outbreak and future cross-strain coronavirus infections. [APBN]
Source: Yu et al. (2022). SARS-CoV-2 spike engagement of ACE2 primes S2′ site cleavage and fusion initiation. Proceedings of the National Academy of Sciences, 119(1).