In December, 2019, a new virus belonging to the human coronavirus family was identified in the Hubei province of central china and named SRAS-CoV-21. Although the identity of the source of zoonotic infection is not yet confirmed, it is likely that this emerging virus result from a recombination between coronavirus from bat and pangolins. SRAS-CoV-2 mediates severe human respiratory disease with high death rate2. According to WHO Coronavirus Disease 2019 situation report of March 29, 2020, up to 634 835 patients worldwide, in 202 countries, or territories had tested positive for COVID-19 with 33,529 people deaths. These values are still increasing and to date there is no vaccine or validated antiviral treatments.
The coronaviruses are a group of enveloped viruses with positive-sense RNA. These viruses belong to the family of Coronavirinae, order Nidovirales3 comprising of four genera namely alpha, beta, delta, and gamma2. These viruses are responsible for a wide range of neurological systems, liver, hepatic and respiratory acute and chronic diseases. Prior to present crisis, only six human coronaviruses (HCoVs) have been known to mediate infection in human and induce respiratory diseases2,3. Of these, severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) are the highly pathogenic coronaviruses able to infect the lower respiratory tract. The other four Coronavirus namely HCoV–229E, OC43, NL63, and HKU1 are associated to upper respiratory infections and common cold2,3. Most enveloped viruses encode for viral envelope glycoproteins, synthetized in an immature polyprotein precursor. These proteins require proteolytic cleavage before they can mediate viral entry into host cells. In many aspects, viruses take advantage of cellular proteases for this key function4. Indeed, during viral infection, the reliance on particular proteases is a determinant factor for viral infection and spread. While several viruses mediate local infections due to the limited expression of their host proteases in a small number of cell types and/or specific tissues such as the case of the low pathogenic avian influenza A viruses, the highly pathogenic virus uses furin-like enzymes that are ubiquitously expressed to cleave influenza A virus hemagglutinin (HA) leading to high viral spread and in turn cause higher rates of mortality5. Thus, the ability of viruses to exploit furin-like enzymes affects the cell tropism and the virus pathogenicity. Previously, furin-like proteases or pro- protein convertases (PCs) were reported to be involved in the conversion to their bioactive forms of a large majority of secretory proteins synthesized as inactive protein precursors. These include growth factors, receptors, adhesion molecules, matrix metalloproteinases and viral envelope glycoproteins6,7. Precursors are usually cleaved at the general motif (K/R)- (X)n-(K/R)¯, where n= 0, 2, 4 or 6. To date, one or more of the seven known pro-protein convertases (PCs) family has been implicated in these processes, namely, furin, PC1, PC2, PC4, PACE4, PC5 (and its isoform PC5-B), and PC77–9. Previous studies however showed that viral glycoproteins activation including those of several coronaviruses is mediated by secreted furin-like enzymes that proteolytically process monobasic or multi-basic cleavage sites4.
Proteolytic cleavage of viral envelope glycoprotein by furin-like enzymes into a functional binding virus receptor and a fusogenic transmembrane protein is central for the mediation of virus cell entry and infectivity of the dengue virus8, respiratory syncytial virus (RSV)9, HIV10, human papilloma virus11 and Chikungunya12. Although the viral glycoproteins are processed at specific cleavage site, the subcellular localization of the cleavage by furin-like enzymes and the time course of the cleavage vary between viruses. Furthermore, proteolytic activation of viral glycoproteins can occur at different steps of the viral replication cycle due to the ability of these glycoproteins to transit thought the Golgi network during virus production where converting enzyme like furin are enriched. Some viral envelope proteins can also meet several furin-like enzymes in the extracellular space or during the virus entry into the endosome where the envelope protein can be processed. In coronavirus, the viral glycoprotein responsible for cell entry is the spike (S) protein13-17. It is processed at two different cleavages sites13 by different proteases that drive the viral tropism. The S protein is synthetized as a protein precursor transiting through the endoplasmic reticulum-Golgi apparatus intermediate compartment (ERGIC). For some coronaviruses such as MERS-CoV and probably SARS- Cov-2, which contain a furin-like cleavage site between S1 and S2, the protein can be cleaved into S1 and S2 in the Trans Golgi Network (TGN) in cells expressing high level of furin. This priming process can also involve cell surface proteases belonging to the transmembrane protease/serine subfamily member (TMPRSS) family, which is highly, expressed in the lungs18. The two viral subunits resulting from priming have distinct functions. The SARS- CoV (1 & 2) S1 subunit contains the angiotensin-converting enzyme 2 (ACE2) receptor binding domain. The S2 subunit ensures membrane fusion after a second proteolytic cleavage at a the S2’ cleavages site, upstream of the fusion peptide. The fusion which releases the nucleocapsid inside the infected cells, depends on a conformational change of the S2 protein subunit and occur either at the plasma membrane or in the endosome depending of the protease availability. Sequence analysis of the spike protein of the coronaviruses, MERS- CoV19, HCoV-OC4320 and HCoV-HKU121 reveal the presence of a canonical furin-like cleavage site between S1/S2 and at the S2’ cleavage site22 (Figure-1) . Similar furin-like cleavages sites were identified in the SARS-CoV-2 spike protein sequence14 (Figure-1). Thereby, the high expression of furin and other furin-like enzymes found in human lung, liver and brain tissues7,23 may be exploited by the SARS-CoV-2 for the activation of S protein leading to enhanced infection, virulence and spread of the virus. Compounds interfering with the cleavage of the SARS-CoV-2 S protein processing could be a valuable antiviral approach. In the current report, we used structure-based virtual screening and several structural bioinformatics tools24 to identify approved and investigational drugs as putative inhibitors of furin.