4.4. NO as an antiviral effector
Non-specific antiviral effects of NO have been reported in a variety of viral infections, including HIV, vaccinia virus, enterovirus and coronavirus [
[55],
[56],
[57]]. For example, expression of iNOS was found to significantly increase in mouse heart infected with coxsackievirus B3, a common enterovirus. The viral load increased significantly after NG-monomethyl-l-arginine acetate (L-NMMA) or nitro-l-arginine methyl ester (l-NAME) was used to inhibit iNOS [
58]. In addition, it was found that the NO donor S-nitroso-N-acetylpenicillamine (SNAP) significantly inhibited the 3C protease of coxsackievirus by acting on the viral replication process [
59]. During the SARS outbreak, researchers found that exogenous NO donors effectively suppressed SARS-CoV-1. The mechanisms will be described as below.
4.4.1. Inhibition of NO on the SARS-CoV-1 replication cycle
In 2004, a Swedish group studied the inhibitory effect of NO donors on SARS-CoV-1 infection in VeroE6 cells [
60]. Cells were treated with SNAP and another non-NO donor compound (as control) at various concentrations (0, 50, 100, 200, 400 μM) at 1 h after infection. The 50% tissue culture infection dose (TCID50) was determined after 24 h. The results showed that SNAP significantly inhibited the replication cycle of SARS-CoV-1 at both RNA and cellular levels in a dose-dependent manner. Besides, iNOS expression was found to accompany decreased offspring viruses by 82% [
60]. Two NO-mediated antiviral mechanisms were proposed, which were later experimentally verified: (1) NO affected one or two replication-related cysteine proteases encoded by SARS-CoV-1 ORF1a, which directly inhibited viral RNA replication, and (2) NO decreased the palmitoylation level of S protein and inhibited the membrane fusion of offspring virus S protein binding to host cell receptor ACE261.
Replication of SARS-CoV-1 was mediated by the nonstructural proteins nsp1-nsp16, and the later contain two cysteine proteases, which would be discussed in further details below [
62,
63]. Cysteine protease cleaves pp1ab replicase polyproteins with varied efficiency. Upon treatment with SNAP, two new high-molecular weight peptides were found. It was suggested that NO changed the original cutting mode of cysteine proteases, thereby affecting production of the non-structural proteins, and terminating the replication process of viral RNA [
61,
64].
The effect of NO on S protein was also investigated. VeroE6 cells infected with recombinant vaccinia virus carrying S gene (rVV-L-S) were treated with 400 μM SNAP, which was labeled with [3H] palmitic acid. After immune-precipitation of the S protein, it was found that SNAP treatment significantly reduced the number of palmitoylated S protein. The intercellular fusion was significantly decreased due to the low expression of S protein after SNAP-treated rVV-L-S was mixed with CHO-ACE2 cells (a cell model that stably expresses ACE2 on the cell surface). The results showed that the entry efficiency of the pseudotyped virus was significantly lower after SNAP treatment, and the virus infection rate decreased by about 70% [
61]. O2 - is also produced during viral infection, which reacts with NO readily to produce peroxynitrite (ONOO-) [
65], a viral inhibitor. In order to clarify whether ONOO- contributed to this effect, the ONOO- donor SIN-1 was used to treat cells infected with SARS-CoV-1. ONOO- did not show inhibitory effect on SARS-CoV-1 replication, which ruled out the contribution of peroxynitrite in this case [
61].
4.4.2. Potential mechanism of NO in COVID-19
SARS-CoV-2 and SARS-CoV-1 share a similar infection process: they both rely on the membrane fusion mediated by the viral S protein with the host cell receptor ACE2 to promote the injection of viral genetic material [
66]. At present, the 3D atomic map of the SARS-CoV-2 S protein has been successfully constructed by cryogenic electron microscopy. The SARS-CoV-2 S protein is a trimer protein with a large number of glycosylation modifications, and its protein sequence is very similar to the S protein of SARS-CoV-1 [
67]: although the S2 region (mediated membrane fusion) is highly homologous (99%), there is a difference in amino acid residues of the S protein receptor region (RBD). This difference has been shown to promote the cell entry mechanism of SARS-CoV-2 [
68,
69]. In addition, the SARS-CoV-2 genome is divided into six main functional open reading frames, including ORF1ab, spinous (S), envelope (E), membrane glycoprotein (M), nucleocapsid (N) and helper gene [
70]. The replicase protein pp1ab, formed by ORF1ab, is cleaved into 16 non-structural proteins (nsp) involved in virus replication by papain-like (PLPro) and 3C-like (3CLPro) proteases encoded by SARS-CoV-2. In addition, the homology between the SARS-CoV-2-encoded 3CLPro and that of SARS-CoV-1 is as high as 96%, and their structures are basically similar [
71,
72].
Therefore, the inhibition of SARS-CoV-2 by NO may be similar to that of SARS-CoV-1. NO also inhibits viral replication by affecting one or two cysteine proteases encoded by the SARS-CoV-2 ORF1a and reducing the palmitoylation level of the S protein [
73]. However, the mechanism of NO in SARS-CoV-2 remains unclear. Researchers have recommended NO together with clinically recommended antiviral drugs as an effective strategy for the treatment of COVID-19 [
71,
74].