SARS-CoV-2 cell–cell fusion and syncytiation is an emerging pathomechanism in COVID-19, but the precise factors contributing to the process remain ill-defined. In this study, we show that metalloproteases promote SARS-CoV-2 spike protein-induced syncytiation in the absence of established serine proteases using in vitro cell–cell fusion assays. We also show that metalloproteases promote S2′-activation of the SARS-CoV-2 spike protein, and that metalloprotease inhibition significantly reduces the syncytiation of SARS-CoV-2 variants of concern. In the presence of serine proteases, however, metalloprotease inhibition does not reduce spike protein-induced syncytiation and a combination of metalloprotease and serine protease inhibition is necessitated. Moreover, we show that the spike protein induces metalloprotease-dependent ectodomain shedding of the ACE2 receptor and that ACE2 shedding contributes to spike protein-induced syncytiation. These observations suggest a benefit to the incorporation of pharmacological inhibitors of metalloproteases into treatment strategies for patients with COVID-19.
Background Mannose binding lectin (MBL) is an important component of innate immune defence. MBL undergoes oligomerization to generate high molecular weight (HMW) forms which act as pattern recognition molecules to detect and opsonize various microorganisms. Several post-translational modifications including prolyl hydroxylation are known to affect the oligomerization of MBL. Yet, the enzyme(s) which hydroxylate proline in the collagen-like domain residues have not been identified and the significance of prolyl hydroxylation is incompletely understood. Methods To investigate post-translational modifications of MBL, we stably expressed Myc-DDK tagged MBL in HEK293S cells. We used pharmacological and genetic inhibition of 2-oxoglutarate dependent dioxygenases (2OGDD) to identify the enzyme required for prolyl hydroxylation of MBL. We performed mass spectrometry to determine the effects of various inhibitors on MBL modifications. Results Secretion of HMW MBL was impaired by inhibitors of the superfamily of 2OGDD, and was dependent on prolyl-4-hydroxylase subunit alpha 1. Roxadustat and vadadustat, but not molidustat, led to significant suppression of hydroxylation and secretion of HMW forms of MBL. Conclusions These data suggest that prolyl hydroxylation in the collagen-like domain of MBL is mediated by collagen prolyl 4 hydroxylase. Reduced MBL activity is likely to be an off-target effect of some, but not all PHD inhibitors. There may be advantages in selective PHD inhibitors that would not interfere with MBL production.
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