Functional and genetic analysis of coronavirus replicase-transcriptase proteins

SIRT5-mediated desuccinylation of the porcine deltacoronavirus M protein drives pexophagy to enhance viral proliferation

Porcine deltacoronavirus (PDCoV) is an emerging enteropathogenic coronavirus capable of infecting various animal species, including humans. In this study, we explored the roles of sirtuins (SIRTs), a conserved family of protein deacylases and mono-adenosine diphosphate-ribosyltransferases, in PDCoV replication. Surprisingly, we found that SIRT5-a unique member of SIRTs with distinct desuccinylation, demalonylation, and deglutarylation activities-is a proviral factor essential for PDCoV replication; its catalytic activities are crucial in this process. Mechanistically, SIRT5 interacts with and desuccinylates the PDCoV membrane (M) protein. This modification activates the ataxia-telangiectasia mutated (ATM) pathway, facilitates ubiquitination of peroxisomal biogenesis protein 5 (PEX5), and recruits sequestosome 1 (SQSTM1/p62) to initiate selective peroxisomal autophagy (pexophagy). The pexophagy process disrupts peroxisomal function, elevates reactive oxygen species (ROS) levels, and suppresses type I and III interferon production, thereby enhancing viral replication. We also identified lysine 207 (K207) as the primary succinylation site of the M protein. Mutations mimicking the desuccinylated or succinylated states of K207 substantially influence viral replication and the ability to induce pexophagy. These findings reveal a novel role for SIRT5 in regulating pexophagy during viral infection and suggest a therapeutic target for efforts to combat coronavirus infections.

The coronavirus nsp14 exoribonuclease interface with the cofactor nsp10 is essential for efficient virus replication and enzymatic activity

Coronaviruses (CoVs) encode non-structural proteins (nsp's) 1-16, which assemble to form replication-transcription complexes that function in viral RNA synthesis. All CoVs encode a proofreading 3'-5' exoribonuclease in non-structural protein 14 (nsp14-ExoN) that mediates proofreading and high-fidelity replication and is critical for other roles in replication and pathogenesis. The in vitro enzymatic activity of nsp14-ExoN is enhanced in the presence of the cofactor nsp10. We introduced alanine substitutions in nsp14 of murine hepatitis virus (MHV) at the nsp14-nsp10 interface and recovered mutant viruses with a range of impairments in replication and in vitro biochemical exonuclease activity. Two of these substitutions, nsp14 K7A and D8A, had impairments intermediate between wild type-MHV nsp14 and the known ExoN(-) D89A/E91A nsp14 catalytic inactivation mutant. All introduced nsp14-nsp10 interface alanine substitutions impaired in vitro exonuclease activity. Passage of the K7A and D8A mutant viruses selected second-site non-synonymous mutations in nsp14 associated with improved mutant virus replication and exonuclease activity. These results confirm the essential role of the nsp14-nsp10 interaction for efficient enzymatic activity and virus replication, identify proximal and long-distance determinants of nsp14-nsp10 interaction, and support targeting the nsp14-nsp10 interface for viral inhibition and attenuation.IMPORTANCECoronavirus replication requires assembly of a replication transcription complex composed of nsp's, including polymerase, helicase, exonuclease, capping enzymes, and non-enzymatic cofactors. The coronavirus nsp14 exoribonuclease mediates several functions in the viral life cycle including genomic and subgenomic RNA synthesis, RNA recombination, RNA proofreading and high-fidelity replication, and native resistance to many nucleoside analogs. The nsp-14 exonuclease activity in vitro requires the non-enzymatic cofactor nsp10, but the determinants and importance of the nsp14-nsp10 interactions during viral replication have not been defined. Here we show that for the coronavirus murine hepatitis virus, nsp14 residues at the nsp14-nsp10 interface are essential for efficient viral replication and in vitro exonuclease activity. These results shed new light on the requirements for protein interactions within the coronavirus replication transcription complex, and they may reveal novel non-active-site targets for virus inhibition and attenuation.

N terminus of SARS-CoV-2 nonstructural protein 3 interrupts RNA-driven phase separation of N protein by displacing RNA

The connection between SARS-CoV-2 replication-transcription complexes and nucleocapsid (N) protein is critical for regulating genomic RNA replication and virion packaging over the viral life cycle. However, the mechanism that dynamically regulates genomic RNA packaging and replication remains elusive. Here, we demonstrate that the N-terminal domain of SARS-CoV-2 nonstructural protein 3, a core component of viral replication-transcription complexes, binds N protein and displaces RNA in a concentration-dependent manner. This interaction disrupts liquid-liquid phase separation of N protein driven by N protein-RNA interactions which is crucial for virion packaging and viral replication. We also report a high-resolution crystal structure of the nonstructural protein 3 ubiquitin-like domain 1 (Ubl1) at 1.49 Å, which reveals abundant negative charges on the protein surface. Sequence and structural analyses identify several conserved motifs at the Ubl1-N protein interface and a previously unexplored highly negative groove, providing insights into the molecular mechanism of Ubl1-mediated modulation of N protein-RNA binding. Our findings elucidate the mechanism of dynamic regulation of SARS-CoV-2 genomic RNA replication and packaging over the viral life cycle. Targeting the conserved Ubl1-N protein interaction hotspots also promises to aid in the development of broad-spectrum antivirals against pathogenic coronaviruses.

The coronavirus nsp14 exoribonuclease interface with the cofactor nsp10 is essential for efficient virus replication and enzymatic activity

Coronaviruses (CoVs) encode nonstructural proteins (nsps) 1-16, which assemble to form replication-transcription complexes that function in viral RNA synthesis. All CoVs encode a proofreading 3'-5' exoribonuclease (ExoN) in nsp14 (nsp14-ExoN) that mediates proofreading and high-fidelity replication and is critical for other roles in replication and pathogenesis. The in vitro enzymatic activity of nsp14 ExoN is enhanced in the presence of the cofactor nsp10. We introduced alanine substitutions in nsp14 of murine hepatitis virus (MHV) at the nsp14-10 interface and recovered mutant viruses with a range of impairments in replication and in vitro biochemical exonuclease activity. Two of these substitutions, nsp14 K7A and D8A, had impairments intermediate between WT-MHV nsp14 and the known ExoN(-) D89A/E91A nsp14 catalytic inactivation mutant. All introduced nsp14-10 interface alanine substitutions impaired in vitro exonuclease activity. Passage of the K7A and D8A mutant viruses selected second-site non-synonymous mutations in nsp14 associated with improved mutant virus replication and exonuclease activity. These results confirm the essential role of the nsp14-nsp10 interaction for efficient enzymatic activity and virus replication, identify proximal and long-distance determinants of nsp14-nsp10 interaction, and support targeting the nsp14-10 interface for viral inhibition and attenuation.

IMPORTANCE

Coronavirus replication requires assembly of a replication transcription complex composed of nonstructural proteins (nsp), including polymerase, helicase, exonuclease, capping enzymes, and non-enzymatic cofactors. The coronavirus nsp14 exoribonuclease mediates several functions in the viral life cycle including genomic and subgenomic RNA synthesis, RNA recombination, RNA proofreading and high-fidelity replication, and native resistance to many nucleoside analogs. The nsp-14 exonuclease activity in vitro requires the non-enzymatic co-factor nsp10, but the determinants and importance the nsp14-10 interactions during viral replication have not been defined. Here we show that for the coronavirus murine hepatitis virus, nsp14 residues at the nsp14-10 interface are essential for efficient viral replication and in vitro exonuclease activity. These results shed new light on the requirements for protein interactions within the coronavirus replication transcription complex, and they may reveal novel non active-site targets for virus inhibition and attenuation.

Potential Transcriptional Enhancers in Coronaviruses: From Infectious Bronchitis Virus to SARS-CoV-2

Coronaviruses constitute a global threat to human and animal health. It is essential to investigate the long-distance RNA-RNA interactions that approximate remote regulatory elements in strategies, including genome circularization, discontinuous transcription, and transcriptional enhancers, aimed at the rapid replication of their large genomes, pathogenicity, and immune evasion. Based on the primary sequences and modeled RNA-RNA interactions of two experimentally defined coronaviral enhancers, we detected via an in silico primary and secondary structural analysis potential enhancers in various coronaviruses, from the phylogenetically ancient avian infectious bronchitis virus (IBV) to the recently emerged SARS-CoV-2. These potential enhancers possess a core duplex-forming region that could transition between closed and open states, as molecular switches directed by viral or host factors. The duplex open state would pair with remote sequences in the viral genome and modulate the expression of downstream crucial genes involved in viral replication and host immune evasion. Consistently, variations in the predicted IBV enhancer region or its distant targets coincide with cases of viral attenuation, possibly driven by decreased open reading frame (ORF)3a immune evasion protein expression. If validated experimentally, the annotated enhancer sequences could inform structural prediction tools and antiviral interventions.

Kinetic comparison of all eleven viral polyprotein cleavage site processing events by SARS-CoV-2 main protease using a linked protein FRET platform

The main protease (Mpro) remains an essential therapeutic target for COVID-19 post infection intervention given its critical role in processing the majority of viral proteins encoded by the genome of severe acute respiratory syndrome related coronavirus 2 (SARS-CoV-2). Upon viral entry, the +ssRNA genome is translated into two long polyproteins (pp1a or the frameshift-dependent pp1ab) containing all the nonstructural proteins (nsps) required by the virus for immune modulation, replication, and ultimately, virion assembly. Included among these nsps is the cysteine protease Mpro (nsp5) which self-excises from the polyprotein, dimerizes, then sequentially cleaves 11 of the 15 cut-site junctions found between each nsp within the polyprotein. Many structures of Mpro (often bound to various small molecule inhibitors or peptides) have been detailed recently, including structures of Mpro bound to each of the polyprotein cleavage sequences, showing that Mpro can accommodate a wide range of targets within its active site. However, to date, kinetic characterization of the interaction of Mpro with each of its native cleavage sequences remains incomplete. Here, we present a robust and cost-effective FRET based system that benefits from a more consistent presentation of the substrate that is also closer in organization to the native polyprotein environment compared to previously reported FRET systems that use chemically modified peptides. Using this system, we were able to show that while each site maintains a similar Michaelis constant, the catalytic efficiency of Mpro varies greatly between cut-site sequences, suggesting a clear preference for the order of nsp processing.

Hyperacetylated microtubules assist porcine deltacoronavirus nsp8 to degrade MDA5 via SQSTM1/p62-dependent selective autophagy

The microtubule (MT) is a highly dynamic polymer that functions in various cellular processes through MT hyperacetylation. Thus, many viruses have evolved mechanisms to hijack the MT network of the cytoskeleton to allow intracellular replication of viral genomic material. Coronavirus non-structural protein 8 (nsp8), a component of the viral replication transcriptional complex, is essential for viral survival. Here, we found that nsp8 of porcine deltacoronavirus (PDCoV), an emerging enteropathogenic coronavirus with a zoonotic potential, inhibits interferon (IFN)-β production by targeting melanoma differentiation gene 5 (MDA5), the main pattern recognition receptor for coronaviruses in the cytoplasm. Mechanistically, PDCoV nsp8 interacted with MDA5 and induced autophagy to degrade MDA5 in wild-type cells, but not in autophagy-related (ATG)5 or ATG7 knockout cells. Further screening for autophagic degradation receptors revealed that nsp8 interacts with sequestosome 1/p62 and promotes p62-mediated selective autophagy to degrade MDA5. Importantly, PDCoV nsp8 induced hyperacetylation of MTs, which in turn triggered selective autophagic degradation of MDA5 and subsequent inhibition of IFN-β production. Overall, our study uncovers a novel mechanism employed by PDCoV nsp8 to evade host innate immune defenses. These findings offer new insights into the interplay among viruses, IFNs, and MTs, providing a promising target to develop anti-viral drugs against PDCoV.IMPORTANCECoronavirus nsp8, a component of the viral replication transcriptional complex, is well conserved and plays a crucial role in viral replication. Exploration of the role mechanism of nsp8 is conducive to the understanding of viral pathogenesis and development of anti-viral strategies against coronavirus. Here, we found that nsp8 of PDCoV, an emerging enteropathogenic coronavirus with a zoonotic potential, is an interferon antagonist. Further studies showed that PDCoV nsp8 interacted with MDA5 and sequestosome 1/p62, promoting p62-mediated selective autophagy to degrade MDA5. We further found that PDCoV nsp8 could induce hyperacetylation of MT, therefore triggering selective autophagic degradation of MDA5 and inhibiting IFN-β production. These findings reveal a novel immune evasion strategy used by PDCoV nsp8 and provide insights into potential therapeutic interventions.

The RdRp genotyping of SARS-CoV-2 isolated from patients with different clinical spectrum of COVID-19

BACKGROUND

The evolution of SARS-CoV-2 has been observed from the very beginning of the fight against COVID-19, some mutations are indicators of potentially dangerous variants of the virus. However, there is no clear association between the genetic variants of SARS-CoV-2 and the severity of COVID-19. We aimed to analyze the genetic variability of RdRp in correlation with different courses of COVID-19.

RESULTS

The prospective study included 77 samples of SARS-CoV-2 isolated from outpatients (1st degree of severity) and hospitalized patients (2nd, 3rd and 4th degree of severity). The retrospective analyses included 15,898,266 cases of SARS-CoV-2 genome sequences deposited in the GISAID repository. Single-nucleotide variants were identified based on the four sequenced amplified fragments of SARS-CoV-2. The analysis of the results was performed using appropriate statistical methods, with p < 0.05, considered statistically significant. Additionally, logistic regression analysis was performed to predict the strongest determinants of the observed relationships. The number of mutations was positively correlated with the severity of the COVID-19, and older male patients. We detected four mutations that significantly increased the risk of hospitalization of COVID-19 patients (14676C > T, 14697C > T, 15096 T > C, and 15279C > T), while the 15240C > T mutation was common among strains isolated from outpatients. The selected mutations were searched worldwide in the GISAID database, their presence was correlated with the severity of COVID-19.

CONCLUSION

Identified mutations have the potential to be used to assess the increased risk of hospitalization in COVID-19 positive patients. Experimental studies and extensive epidemiological data are needed to investigate the association between individual mutations and the severity of COVID-19.

Classification, replication, and transcription of Nidovirales

Nidovirales is one order of RNA virus, with the largest single-stranded positive sense RNA genome enwrapped with membrane envelope. It comprises four families (Arterividae, Mesoniviridae, Roniviridae, and Coronaviridae) and has been circulating in humans and animals for almost one century, posing great threat to livestock and poultry，as well as to public health. Nidovirales shares similar life cycle: attachment to cell surface, entry, primary translation of replicases, viral RNA replication in cytoplasm, translation of viral proteins, virion assembly, budding, and release. The viral RNA synthesis is the critical step during infection, including genomic RNA (gRNA) replication and subgenomic mRNAs (sg mRNAs) transcription. gRNA replication requires the synthesis of a negative sense full-length RNA intermediate, while the sg mRNAs transcription involves the synthesis of a nested set of negative sense subgenomic intermediates by a discontinuous strategy. This RNA synthesis process is mediated by the viral replication/transcription complex (RTC), which consists of several enzymatic replicases derived from the polyprotein 1a and polyprotein 1ab and several cellular proteins. These replicases and host factors represent the optimal potential therapeutic targets. Hereby, we summarize the Nidovirales classification, associated diseases, "replication organelle," replication and transcription mechanisms, as well as related regulatory factors.

Identification of subgenomic mRNAs derived from the coronavirus 1a/1b protein gene: Implications for coronavirus transcription

Synthesis of coronavirus subgenomic mRNA (sgmRNA) is guided by the transcription regulatory sequence (TRS). sgmRNA derived from the body TRS (TRS-B) located at the 1a/1b protein gene is designated 1ab/sgmRNA. In the current study, we comprehensively identified the 1ab/sgmRNAs synthesized from TRS-Bs located at the 1a/1b protein genes of different coronavirus genera both in vitro and in vivo by RT‒PCR and sequencing. The results suggested that the degree of sequence homology between the leader TRS (TRS-L) and TRS-B may not be a decisive factor for 1ab/sgmRNA synthesis. This observation led us to revisit the coronavirus transcription mechanism and to propose that the disassociation of coronavirus polymerase from the viral genome may be a prerequisite for sgmRNA synthesis. Once the polymerase can disassociate at TRS-B, the sequence homology between TRS-L and TRS-B is important for sgmRNA synthesis. The study therefore extends our understanding of transcription mechanisms.

Integrated metabolomics and transcriptomics analyses reveal metabolic responses to TGEV infection in porcine intestinal epithelial cells

Transmissible gastroenteritis virus (TGEV) is a coronavirus that infects piglets with severe diarrhoea, vomiting, dehydration, and even death, causing huge economic losses to the pig industry. The underlying pathogenesis of TGEV infection and the effects of TGEV infection on host metabolites remain poorly understood. To investigate the critical metabolites and regulatory factors during TGEV infection in intestinal porcine epithelial cells (IPEC-J2), we performed metabolomic and transcriptomic analyses of TGEV-infected IPEC-J2 cells by LC/MS and RNA-seq techniques. A total of 87 differential metabolites and 489 differentially expressed genes were detected. A series of metabolites and candidate genes from glutathione metabolism and AMPK signalling pathway were examined through combined analysis of metabolome and transcriptome. We found glutathione peroxidase 3 (GPX3) is markedly reduced after TGEV infection, and a significant negative correlation between AMPK signalling pathway and TGEV infection. Exogenous addition of the AMPK activator COH-SR4 significantly downregulates stearoyl coenzyme A (SCD1) mRNA and inhibits TGEV replication; while exogenous GSK-690693 significantly promotes TGEV infection by inhibiting AMPK signalling pathway. In summary, our study provides insights into the key metabolites and regulators for TGEV infection from the metabolome and transcriptome perspective, which will offer promising antiviral metabolic and molecular targets and enrich the understanding of the existence of a similar mechanism in the host.

Metatranscriptomics analysis reveals a novel transcriptional and translational landscape during Middle East respiratory syndrome coronavirus infection

Among all RNA viruses, coronavirus RNA transcription is the most complex and involves a process termed "discontinuous transcription" that results in the production of a set of 3'-nested, co-terminal genomic and subgenomic RNAs during infection. While the expression of the classic canonical set of subgenomic RNAs depends on the recognition of a 6- to 7-nt transcription regulatory core sequence (TRS), here, we use deep sequence and metagenomics analysis strategies and show that the coronavirus transcriptome is even more vast and more complex than previously appreciated and involves the production of leader-containing transcripts that have canonical and noncanonical leader-body junctions. Moreover, by ribosome protection and proteomics analyses, we show that both positive- and negative-sense transcripts are translationally active. The data support the hypothesis that the coronavirus proteome is much vaster than previously noted in the literature.

HDAC6 Degrades nsp8 of Porcine Deltacoronavirus through Deacetylation and Ubiquitination to Inhibit Viral Replication

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that has the potential to infect humans. Histone deacetylase 6 (HDAC6) is a unique type IIb cytoplasmic deacetylase with both deacetylase activity and ubiquitin E3 ligase activity, which mediates a variety of cellular processes by deacetylating histone and nonhistone substrates. In this study, we found that ectopic expression of HDAC6 significantly inhibited PDCoV replication, while the reverse effects could be observed after treatment with an HDAC6-specific inhibitor (tubacin) or knockdown of HDAC6 expression by specific small interfering RNA. Furthermore, we demonstrated that HDAC6 interacted with viral nonstructural protein 8 (nsp8) in the context of PDCoV infection, resulting in its proteasomal degradation, which was dependent on the deacetylation activity of HDAC6. We further identified the key amino acid residues lysine 46 (K46) and K58 of nsp8 as acetylation and ubiquitination sites, respectively, which were required for HDAC6-mediated degradation. Through a PDCoV reverse genetics system, we confirmed that recombinant PDCoV with a mutation at either K46 or K58 exhibited resistance to the antiviral activity of HDAC6, thereby exhibiting higher replication compared with wild-type PDCoV. Collectively, these findings contribute to a better understanding of the function of HDAC6 in regulating PDCoV infection and provide new strategies for the development of anti-PDCoV drugs. IMPORTANCE As an emerging enteropathogenic coronavirus with zoonotic potential, porcine deltacoronavirus (PDCoV) has sparked tremendous attention. Histone deacetylase 6 (HDAC6) is a critical deacetylase with both deacetylase activity and ubiquitin E3 ligase activity and is extensively involved in many important physiological processes. However, little is known about the role of HDAC6 in the infection and pathogenesis of coronaviruses. Our present study demonstrates that HDAC6 targets PDCoV-encoded nonstructural protein 8 (nsp8) for proteasomal degradation through the deacetylation at the lysine 46 (K46) and the ubiquitination at K58, suppressing viral replication. Recombinant PDCoV with a mutation at K46 and/or K58 of nsp8 displayed resistance to the antiviral activity of HDAC6. Our work provides significant insights into the role of HDAC6 in regulating PDCoV infection, opening avenues for the development of novel anti-PDCoV drugs.

Severe acute respiratory syndrome coronaviruses contributing to mitochondrial dysfunction: Implications for post-COVID complications

Mitochondria play a central role in oxidative phosphorylation (OXPHOS), bioenergetics linked with ATP production, fatty acids biosynthesis, calcium signaling, cell cycle regulation, apoptosis, and innate immune response. Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) infection manipulates the host cellular machinery for its survival and replication in the host cell. The infectiaon causes perturbed the cellular metabolism that favours viral replication leading to mitochondrial dysfunction and chronic inflammation. By localizing to the mitochondria, SARS CoV proteins increase reactive oxygen species (ROS) levels, perturbation of Ca2+ signaling, changes in mtDNA copy number, mitochondrial membrane potential (MMP), mitochondrial mass, and induction of mitophagy. These proteins also influence the fusion and fission kinetics, size, structure, and distribution of mitochondria in the infected host cells. This results in compromised bioenergetics, altered metabolism, and innate immune signaling, and hence can be a key player in determining the outcome of SARS-CoV infection. SARS-CoV infection contributes to stress and activates apoptotic pathways. This review summarizes how mitochondrial function and dynamics are affected by SARS-CoV and how the mitochondria-SARS-CoV interaction benefits viral survival and growth by evading innate host immunity. We also highlight how the SARS-CoV-mediated mitochondrial dysfunction contributes to post-COVID complications. Besides, a discussion on targeting virus-mitochondria interactions as a therapeutic strategy is presented.

In silico screening of phytochemical compounds and FDA drugs as potential inhibitors for NSP16/10 5' methyl transferase activity

The recent global pandemic associated with the highly contagious novel coronavirus (SARS-CoV-2) has led to an unpredictable loss of life and economy worldwide, and the discovery of antiviral drugs is an urgent necessity. For the discovery of new drug leads and for the treatment of various diseases, natural products and purified photochemical from medicinal plants are used. The RNA cap was methylated by two S-adenosyl-L-methionine (SAM)-dependent methyltransferases of SARS coronavirus (SARS-CoV-2), catalyzed by NSP16 2'-O-Mtase. Natural substrate SAM, 128 Phytocompounds retrieved from the Phytocompounds database, and 11 standard FDA-approved HIV drugs reclaimed from the PubChem database are subjected to docking analysis. The docking study was done using AutoDock Vina. Further, admetSAR and DruLiTO servers are used to analyze the drug-likeness properties. The NSP16/10 structure and natural substrate SAM, Phytocompounds Withanolide (WTL), and HIV standard drug Dolutegravir (DLT) as hit compounds were identified by molecular dynamics using the Gromacs GPU-enabled package. To examine the effectiveness of the identified drugs versus COVID-19, further in vitro and in vivo studies are required. Communicated by Ramaswamy H. Sarma.

Biochemical and structural insights into SARS-CoV-2 polyprotein processing by Mpro

SARS-CoV-2, a human coronavirus, is the causative agent of the COVID-19 pandemic. Its genome is translated into two large polyproteins subsequently cleaved by viral papain-like protease and main protease (Mpro). Polyprotein processing is essential yet incompletely understood. We studied Mpro-mediated processing of the nsp7-11 polyprotein, whose mature products include cofactors of the viral replicase, and identified the order of cleavages. Integrative modeling based on mass spectrometry (including hydrogen-deuterium exchange and cross-linking) and x-ray scattering yielded a nsp7-11 structural ensemble, demonstrating shared secondary structural elements with individual nsps. The pattern of cross-links and HDX footprint of the C145A Mpro and nsp7-11 complex demonstrate preferential binding of the enzyme active site to the polyprotein junction sites and additional transient contacts to help orient the enzyme on its substrate for cleavage. Last, proteolysis assays were used to characterize the effect of inhibitors/binders on Mpro processing/inhibition using the nsp7-11 polyprotein as substrate.

Identification and characterization of alternative sites and molecular probes for SARS-CoV-2 target proteins

Three protein targets from SARS-CoV-2, the viral pathogen that causes COVID-19, are studied: the main protease, the 2'-O-RNA methyltransferase, and the nucleocapsid (N) protein. For the main protease, the nucleophilicity of the catalytic cysteine C145 is enabled by coupling to three histidine residues, H163 and H164 and catalytic dyad partner H41. These electrostatic couplings enable significant population of the deprotonated state of C145. For the RNA methyltransferase, the catalytic lysine K6968 that serves as a Brønsted base has significant population of its deprotonated state via strong coupling with K6844 and Y6845. For the main protease, Partial Order Optimum Likelihood (POOL) predicts two clusters of biochemically active residues; one includes the catalytic H41 and C145 and neighboring residues. The other surrounds a second pocket adjacent to the catalytic site and includes S1 residues F140, L141, H163, E166, and H172 and also S2 residue D187. This secondary recognition site could serve as an alternative target for the design of molecular probes. From in silico screening of library compounds, ligands with predicted affinity for the secondary site are reported. For the NSP16-NSP10 complex that comprises the RNA methyltransferase, three different sites are predicted. One is the catalytic core at the conserved K-D-K-E motif that includes catalytic residues D6928, K6968, and E7001 plus K6844. The second site surrounds the catalytic core and consists of Y6845, C6849, I6866, H6867, F6868, V6894, D6895, D6897, I6926, S6927, Y6930, and K6935. The third is located at the heterodimer interface. Ligands predicted to have high affinity for the first or second sites are reported. Three sites are also predicted for the nucleocapsid protein. This work uncovers key interactions that contribute to the function of the three viral proteins and also suggests alternative sites for ligand design.

Decoding the Role of Temperature in RNA Virus Infections

RNA viruses include respiratory viruses, such as coronaviruses and influenza viruses, as well as vector-borne viruses, like dengue and West Nile virus. RNA viruses like these encounter various environments when they copy themselves and spread from cell to cell or host to host. Ex vivo differences, such as geographical location and humidity, affect their stability and transmission, while in vivo differences, such as pH and host gene expression, impact viral receptor binding, viral replication, and the host immune response against the viral infection. A critical factor affecting RNA viruses both ex vivo and in vivo, and defining the outcome of viral infections and the direction of viral evolution, is temperature. In this minireview, we discuss the impact of temperature on viral replication, stability, transmission, and adaptation, as well as the host innate immune response. Improving our understanding of how RNA viruses function, survive, and spread at different temperatures will improve our models of viral replication and transmission risk analyses.

A Temperature-Sensitive Recombinant of Avian Coronavirus Infectious Bronchitis Virus Provides Complete Protection against Homologous Challenge

Avian coronavirus infectious bronchitis virus (IBV) is the etiological agent of infectious bronchitis, an acute highly contagious economically relevant respiratory disease of poultry. Vaccination is used to control IBV infections, with live-attenuated vaccines generated via serial passage of a virulent field isolate through embryonated hens' eggs. A fine balance must be achieved between attenuation and the retention of immunogenicity. The exact molecular mechanism of attenuation is unknown, and vaccines produced in this manner present a risk of reversion to virulence as few consensus level changes are acquired. Our previous research resulted in the generation of a recombinant IBV (rIBV) known as M41-R, based on a pathogenic strain M41-CK. M41-R was attenuated in vivo by two amino acid changes, Nsp10-Pro85Leu and Nsp14-Val393Leu; however, the mechanism of attenuation was not determined. Pro85 and Val393 were found to be conserved among not only IBV strains but members of the wider coronavirus family. This study demonstrates that the same changes are associated with a temperature-sensitive (ts) replication phenotype at 41°C in vitro, suggesting that the two phenotypes may be linked. Vaccination of specific-pathogen-free chickens with M41-R induced 100% protection against clinical disease, tracheal ciliary damage, and challenge virus replication following homologous challenge with virulent M41-CK. Temperature sensitivity has been used to rationally attenuate other viral pathogens, including influenza, and the identification of amino acid changes that impart both a ts and an attenuated phenotype may therefore offer an avenue for future coronavirus vaccine development. IMPORTANCE Infectious bronchitis virus is a pathogen of economic and welfare concern for the global poultry industry. Live-attenuated vaccines against are generated by serial passage of a virulent isolate in embryonated eggs until attenuation is achieved. The exact mechanisms of attenuation are unknown, and vaccines produced have a risk of reversion to virulence. Reverse genetics provides a method to generate vaccines that are rationally attenuated and are more stable with respect to back selection due to their clonal origin. Genetic populations resulting from molecular clones are more homogeneous and lack the presence of parental pathogenic viruses, which generation by multiple passage does not. In this study, we identified two amino acids that impart a temperature-sensitive replication phenotype. Immunogenicity is retained and vaccination results in 100% protection against homologous challenge. Temperature sensitivity, used for the development of vaccines against other viruses, presents a method for the development of coronavirus vaccines.

Mechanisms of Coronavirus Genome Stability As Potential Targets for Antiviral Drugs

The COVID-19 pandemic has made it necessary to create antivirals active against the SARS-CoV-2 coronavirus. One of the widely used strategies to fight off viral infections is the use of modified nucleoside analogues that inhibit viral replication by incorporating DNA or RNA into the growing chain, thus stopping its synthesis. The difficulty of using this method of treatment in the case of SARS-CoV-2 is that coronaviruses have an effective mechanism for maintaining genome stability. Its central element is the nsp14 protein, which is characterized by exonuclease activity, due to which incorrectly included and noncanonical nucleotides are removed from the 3' end of the growing RNA chain. Inhibitors of nsp14 exonuclease and nucleoside analogues resistant to its action are viewed as potential targets for anticoronavirus therapy.

SARS-CoV-2 modulation of RIG-I-MAVS signaling: Potential mechanisms of impairment on host antiviral immunity and therapeutic approaches

The coronavirus disease 2019 (COVID-19) is a global infectious disease aroused by RNA virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Patients may suffer from severe respiratory failure or even die, posing a huge challenge to global public health. Retinoic acid-inducible gene I (RIG-I) is one of the major pattern recognition receptors, function to recognize RNA viruses and mediate the innate immune response. RIG-1 and melanoma differentiation-associated gene 5 contain an N-terminal caspase recruitment domain that is activated upon detection of viral RNA in the cytoplasm of virus-infected cells. Activated RIG-I and mitochondrial antiviral signaling (MAVS) protein trigger a series of corresponding immune responses such as the production of type I interferon against viral infection. In this review, we are summarizing the role of the structural, nonstructural, and accessory proteins from SARS-CoV-2 on the RIG-I-MAVS pathway, and exploring the potential mechanism how SARS-CoV-2 could evade the host antiviral response. We then proposed that modulation of the RIG-I-MAVS signaling pathway might be a novel and effective therapeutic strategy to against COVID-19 as well as the constantly mutating coronavirus.

Proteolytic Processing of the Coronavirus Replicase Nonstructural Protein 14 Exonuclease Is Not Required for Virus Replication but Alters RNA Synthesis and Viral Fitness

Coronaviruses (CoVs) initiate replication by translation of the positive-sense RNA genome into the replicase polyproteins connecting 16 nonstructural protein domains (nsp1-16), which are subsequently processed by viral proteases to yield mature nsp. For the betacoronavirus murine hepatitis virus (MHV), total inhibition of translation or proteolytic processing of replicase polyproteins results in rapid cessation of RNA synthesis. The nsp5-3CLpro (Mpro) processes nsps7-16, which assemble into functional replication-transcription complexes (RTCs), including the enzymatic nsp12-RdRp and nsp14-exoribonuclease (ExoN)/N7-methyltransferase. The nsp14-ExoN activity mediates RNA-dependent RNA proofreading, high-fidelity RNA synthesis, and replication. To date, the solved partial RTC structures, biochemistry, and models use or assume completely processed, mature nsp. Here, we demonstrate that in MHV, engineered deletion of the cleavage sites between nsp13-14 and nsp14-15 allowed recovery of replication-competent virus. Compared to wild-type (WT) MHV, the nsp13-14 and nsp14-15 cleavage deletion mutants demonstrated delayed replication kinetics, impaired genome production, altered abundance and patterns of recombination, and impaired competitive fitness. Further, the nsp13-14 and nsp14-15 mutant viruses demonstrated mutation frequencies that were significantly higher than with the WT. The results demonstrate that cleavage of nsp13-14 or nsp14-15 is not required for MHV viability and that functions of the RTC/nsp14-ExoN are impaired when assembled with noncleaved intermediates. These data will inform future genetic, structural, biochemical, and modeling studies of coronavirus RTCs and nsp 13, 14, and 15 and may reveal new approaches for inhibition or attenuation of CoV infection.

IMPORTANCE Coronavirus replication requires proteolytic maturation of the nonstructural replicase proteins to form the replication-transcription complex. Coronavirus replication-transcription complex models assume mature subunits; however, mechanisms of coronavirus maturation and replicase complex formation have yet to be defined. Here, we show that for the coronavirus murine hepatitis virus, cleavage between the nonstructural replicase proteins nsp13-14 and nsp14-15 is not required for replication but does alter RNA synthesis and recombination. These results shed new light on the requirements for coronavirus maturation and replication-transcription complex assembly, and they may reveal novel therapeutic targets and strategies for attenuation.

Viral Nucleases from Herpesviruses and Coronavirus in Recombination and Proofreading: Potential Targets for Antiviral Drug Discovery

In this review, we explore recombination in two very different virus families that have become major threats to human health. The Herpesviridae are a large family of pathogenic double-stranded DNA viruses involved in a range of diseases affecting both people and animals. Coronaviridae are positive-strand RNA viruses (CoVs) that have also become major threats to global health and economic stability, especially in the last two decades. Despite many differences, such as the make-up of their genetic material (DNA vs. RNA) and overall mechanisms of genome replication, both human herpes viruses (HHVs) and CoVs have evolved to rely heavily on recombination for viral genome replication, adaptation to new hosts and evasion of host immune regulation. In this review, we will focus on the roles of three viral exonucleases: two HHV exonucleases (alkaline nuclease and PolExo) and one CoV exonuclease (ExoN). We will review the roles of these three nucleases in their respective life cycles and discuss the state of drug discovery efforts against these targets.

Modulation of Innate Antiviral Immune Response by Porcine Enteric Coronavirus

Host’s innate immunity is the front-line defense against viral infections, but some viruses have evolved multiple strategies for evasion of antiviral innate immunity. The porcine enteric coronaviruses (PECs) consist of porcine epidemic diarrhea virus (PEDV), porcine deltacoronavirus (PDCoV), transmissible gastroenteritis coronavirus (TGEV), and swine acute diarrhea syndrome-coronavirus (SADS-CoV), which cause lethal diarrhea in neonatal pigs and threaten the swine industry worldwide. PECs interact with host cells to inhibit and evade innate antiviral immune responses like other coronaviruses. Moreover, the immune escape of porcine enteric coronaviruses is the key pathogenic mechanism causing infection. Here, we review the most recent advances in the interactions between viral and host’s factors, focusing on the mechanisms by which viral components antagonize interferon (IFN)-mediated innate antiviral immune responses, trying to shed light on new targets and strategies effective for controlling and eliminating porcine enteric coronaviruses.

Genetic Engineering Systems to Study Human Viral Pathogens from the Coronaviridae Family

The COVID-19 pandemic caused by the previously unknown SARS-CoV-2 Betacoronavirus made it extremely important to develop simple and safe cellular systems which allow manipulation of the viral genome and high-throughput screening of its potential inhibitors. In this review, we made an attempt at summarizing the currently existing data on genetic engineering systems used to study not only SARS-CoV-2, but also other viruses from the Coronaviridae family. In addition, the review covers the basic knowledge about the structure and the life cycle of coronaviruses.

Molecular characterization of the FCoV-like canine coronavirus HLJ-071 in China

Background

According to the differences of antigen and genetic composition, canine coronavirus (CCoV) consists of two genotypes, CCoV-I and CCoV-II. Since 2004, CCoVs with point mutations or deletions of NSPs are contributing to the changes in tropism and virulence in dogs.

Results

In this study, we isolated a CCoV, designated HLJ-071, from a dead 5-week-old female Welsh Corgi with severe diarrhea and vomit. Sequence analysis suggested that HLJ-071 bearing a complete ORF3abc compared with classic CCoV isolates (1-71, K378 and S378). In addition, a variable region was located between S gene and ORF 3a gene, in which a deletion with 104 nts for HLJ-071 when compared with classic CCoV strains 1-71, S378 and K378. Phylogenetic analysis based on the S gene and complete sequences showed that HLJ-071 was closely related to FCoV II. Recombination analysis suggested that HLJ-071 originated from the recombination of FCoV 79-1683, FCoV DF2 and CCoV A76. Finally, according to cell tropism experiments, it suggested that HLJ-071 could replicate in canine macrophages/monocytes cells.

Conclusion

The present study involved the isolation and genetic characterization of a variant CCoV strain and spike protein and ORF3abc of CCoV might play a key role in viral tropism, which could affect the replication in monocyte/macrophage cells. It will provide essential information for further understanding the evolution in China.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03073-8.

Precursors of Viral Proteases as Distinct Drug Targets

Viral proteases are indispensable for successful virion maturation, thus making them a prominent drug target. Their enzyme activity is tightly spatiotemporally regulated by expression in the precursor form with little or no activity, followed by activation via autoprocessing. These cleavage events are frequently triggered upon transportation to a specific compartment inside the host cell. Typically, precursor oligomerization or the presence of a co-factor is needed for activation. A detailed understanding of these mechanisms will allow ligands with non-canonical mechanisms of action to be designed, which would specifically modulate the initial irreversible steps of viral protease autoactivation. Binding sites exclusive to the precursor, including binding sites beyond the protease domain, can be exploited. Both inhibition and up-regulation of the proteolytic activity of viral proteases can be detrimental for the virus. All these possibilities are discussed using examples of medically relevant viruses including herpesviruses, adenoviruses, retroviruses, picornaviruses, caliciviruses, togaviruses, flaviviruses, and coronaviruses.

How Viruses Hijack and Modify the Secretory Transport Pathway

Eukaryotic cells contain dynamic membrane-bound organelles that are constantly remodeled in response to physiological and environmental cues. Key organelles are the endoplasmic reticulum, the Golgi apparatus and the plasma membrane, which are interconnected by vesicular traffic through the secretory transport route. Numerous viruses, especially enveloped viruses, use and modify compartments of the secretory pathway to promote their replication, assembly and cell egression by hijacking the host cell machinery. In some cases, the subversion mechanism has been uncovered. In this review, we summarize our current understanding of how the secretory pathway is subverted and exploited by viruses belonging to Picornaviridae, Coronaviridae, Flaviviridae,Poxviridae, Parvoviridae and Herpesviridae families.

Comprehensive analysis of SARS-COV-2 drug targets and pharmacological aspects in treating the COVID-19

Corona viruses are enveloped, single-stranded RNA (Ribonucleic acid) viruses and they cause pandemic diseases having a devastating effect on both human healthcare and the global economy. To date, six corona viruses have been identified as pathogenic organisms which are significantly responsible for the infection and also cause severe respiratory diseases. Among them, the novel SARS-CoV-2 (Severe acute respiratory syndrome coronavirus 2) caused a major outbreak of corona virus diseases 2019 (COVID-19). Coronaviridae family members can affects both humans and animals. In human, corona viruses cause severe acute respiratory syndrome with mild to severe outcomes. Several structural and genomics have been investigated, and the genome encodes about 28 proteins most of them with unknown function though it shares remarkable sequence identity with other proteins. There is no potent and licensed vaccine against SARS-CoV-2 and several trials are underway to investigate the possible therapeutic agents against viral infection. However, some of the antiviral drugs that have been investigated against SARS-CoV-2 are under clinical trials. In the current review we comparatively emphasize the emergence and pathogenicity of the SARS-CoV-2 and their infection and discuss the various putative drug targets of both viral and host receptors for developing effective vaccines and therapeutic combinations to overcome the viral outbreak.

SARS-CoV-2 Nsp16 activation mechanism and a cryptic pocket with pan-coronavirus antiviral potential

Coronaviruses have caused multiple epidemics in the past two decades, in addition to the current COVID-19 pandemic that is severely damaging global health and the economy. Coronaviruses employ between 20 and 30 proteins to carry out their viral replication cycle, including infection, immune evasion, and replication. Among these, nonstructural protein 16 (Nsp16), a 2'-O-methyltransferase, plays an essential role in immune evasion. Nsp16 achieves this by mimicking its human homolog, CMTr1, which methylates mRNA to enhance translation efficiency and distinguish self from other. Unlike human CMTr1, Nsp16 requires a binding partner, Nsp10, to activate its enzymatic activity. The requirement of this binding partner presents two questions that we investigate in this manuscript. First, how does Nsp10 activate Nsp16? Although experimentally derived structures of the active Nsp16/Nsp10 complex exist, structures of inactive, monomeric Nsp16 have yet to be solved. Therefore, it is unclear how Nsp10 activates Nsp16. Using over 1 ms of molecular dynamics simulations of both Nsp16 and its complex with Nsp10, we investigate how the presence of Nsp10 shifts Nsp16's conformational ensemble to activate it. Second, guided by this activation mechanism and Markov state models, we investigate whether Nsp16 adopts inactive structures with cryptic pockets that, if targeted with a small molecule, could inhibit Nsp16 by stabilizing its inactive state. After identifying such a pocket in SARS-CoV2 Nsp16, we show that this cryptic pocket also opens in SARS-CoV1 and MERS but not in human CMTr1. Therefore, it may be possible to develop pan-coronavirus antivirals that target this cryptic pocket.

Potential Inhibition of COVID-19 RNA-dependent RNA Polymerase by Hepatitis C Virus Non-nucleoside Inhibitors: An In-silico Perspective

Background:

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is a novel member of the genus betacoronavirus in the Coronaviridae family. It has been identified as the causative agent of coronavirus disease 2019 (COVID-19), spreading rapidly in Asia, America and Europe. Like some other RNA viruses, RNA replication and transcription of SARS-CoV-2 rely on its RNA-dependent RNA polymerase (RdRP), which is a therapeutic target of clinical importance. Crystal structure of SARS-CoV-2 was solved recently (PDB ID 6M71) with some missing residues.

Objective:

We used SARS-CoV-2 RdRP as a target protein to screen for possible chemical molecules with potential anti-viral effects.

Methods:

Here we modelled the missing residues 896-905 via homology modelling and then analysed the interactions of Hepatitis C virus allosteric non-nucleoside inhibitors (NNIs) in the reported NNIs binding sites in SARS-CoV-2 RdRP.

Results:

We found that MK-3281, filibuvir, setrobuvir and dasabuvir might be able to inhibit SARS-CoV-2 RdRP based on their binding affinities in the respective binding sites.

Conclusion:

Further in vitro and in vivo experimental research will be carried out to evaluate their effectiveness in COVID-19 treatment in the near future.

Nano-therapeutic strategies to target coronavirus

The coronaviruses have caused severe acute respiratory syndrome (SARS), the Middle East respiratory syndrome (MERS), and the more recent coronavirus pneumonia (COVID-19). The global COVID-19 pandemic requires urgent action to develop anti-virals, new therapeutics, and vaccines. In this review, we discuss potential therapeutics including human recombinant ACE2 soluble, inflammatory cytokine inhibitors, and direct anti-viral agents such as remdesivir and favipiravir, to limit their fatality. We also discuss the structure of the SARS-CoV-2, which is crucial to the timely development of therapeutics, and previous attempts to generate vaccines against SARS-CoV and MERS-CoV. Finally, we provide an overview of the role of nanotechnology in the development of therapeutics as well as in the diagnosis of the infection. This information is key for computational modeling and nanomedicine-based new therapeutics by counteracting the variable proteins in the virus. Further, we also try to effectively share the latest information about many different aspects of COVID-19 vaccine developments and possible management to further scientific endeavors.

Potential 3-chymotrypsin-like cysteine protease cleavage sites in the coronavirus polyproteins pp1a and pp1ab and their possible relevance to COVID-19 vaccine and drug development

Coronavirus (CoV) 3-chymotrypsin (C)-like cysteine protease (3CLpro ) is a target for anti-CoV drug development and drug repurposing because along with papain-like protease, it cleaves CoV-encoded polyproteins (pp1a and pp1ab) into nonstructural proteins (nsps) for viral replication. However, the cleavage sites of 3CLpro and their relevant nsps remain unclear, which is the subject of this perspective. Here, we address the subject from three standpoints. First, we explore the inconsistency in the cleavage sites and relevant nsps across CoVs, and investigate the function of nsp11. Second, we consider the nsp16 mRNA overlapping of the spike protein mRNA, and analyze the effect of this overlapping on mRNA vaccines. Finally, we study nsp12, whose existence depends on ribosomal frameshifting, and investigate whether 3CLpro requires a large number of inhibitors to achieve full inhibition. This perspective helps us to clarify viral replication and is useful for developing anti-CoV drugs with 3CLpro as a target in the current coronavirus disease 2019 (COVID-19) pandemic.

Systematizing the genomic order and relatedness in the open reading frames (ORFs) of the coronaviruses

The coronaviruses (CoVs), including SARS-CoV-2, the agent of the ongoing deadly CoVID-19 pandemic (Coronavirus disease-2019), represent a highly complex and diverse class of RNA viruses with large genomes, complex gene repertoire, and intricate transcriptional and translational mechanisms. The 3′-terminal one-third of the genome encodes four structural proteins, namely spike, envelope, membrane, and nucleocapsid, interspersed with genes for accessory proteins that are largely nonstructural and called ‘open reading frame’ (ORF) proteins with alphanumerical designations, but not in a consistent or sequential order. Here, I report a comparative study of these ORF proteins, mainly encoded in two gene clusters, i.e. between the Spike and the Envelope genes, and between the Membrane and the Nucleocapsid genes. For brevity and focus, a greater emphasis was placed on the first cluster, collectively designated as the ‘orf3 region’ for ease of referral. Overall, an apparently diverse set of ORFs, such as ORF3a, ORF3b, ORF3c, ORF3d, ORF4 and ORF5, but not necessarily numbered in that order on all CoV genomes, were analyzed along with other ORFs. Unexpectedly, the gene order or naming of the ORFs were never fully conserved even within the members of one Genus. These studies also unraveled hitherto unrecognized orf genes in alternative translational frames, encoding potentially novel polypeptides as well as some that are highly similar to known ORFs. Finally, several options of an inclusive and systematic numbering are proposed not only for the orf3 region but also for the other orf genes in the viral genome in an effort to regularize the apparently confusing names and orders. Regardless of the ultimate acceptability of one system over the others, this treatise is hoped to initiate an informed discourse in this area.

Structure and Expression of Large (+)RNA Genomes of Viruses of Higher Eukaryotes

Viral positive-sense RNA genomes evolve rapidly due to the high mutation rates during replication and RNA recombination, which allowing the viruses to acquire and modify genes for their adaptation. The size of RNA genome is limited by several factors, including low fidelity of RNA polymerases and packaging constraints. However, the 12-kb size limit is exceeded in the two groups of eukaryotic (+)RNA viruses – animal nidoviruses and plant closteroviruses. These virus groups have several traits in common. Their genomes contain 5′-proximal genes that are expressed via ribosomal frameshifting and encode one or two papain-like protease domains, membrane-binding domain(s), methyltransferase, RNA helicase, and RNA polymerase. In addition, some nidoviruses (i.e., coronaviruses) contain replication-associated domains, such as proofreading exonuclease, putative primase, nucleotidyltransferase, and endonuclease. In both nidoviruses and closteroviruses, the 3′-terminal part of the genome contains genes for structural and accessory proteins expressed via a nested set of coterminal subgenomic RNAs. Coronaviruses and closteroviruses have evolved to form flexuous helically symmetrical nucleocapsids as a mean to resolve packaging constraints. Since phylogenetic reconstructions of the RNA polymerase domains indicate only a marginal relationship between the nidoviruses and closteroviruses, their similar properties likely have evolved convergently, along with the increase in the genome size.

Porcine deltacoronavirus nsp10 antagonizes interferon-β production independently of its zinc finger domains

Porcine deltacoronavirus (PDCoV) is a novel swine enteropathogenic coronavirus that causes serious vomiting and diarrhea in piglets. Previous work demonstrated that PDCoV infection inhibits type I interferon (IFN) production. Here, we found that ectopic expression of PDCoV nsp10 significantly inhibited Sendai virus (SeV)-induced IFN-β production by impairing the phosphorylation and nuclear translocation of two transcription factors, IRF3 and NF-κB p65 subunit. Interestingly, experiments with truncated mutants and site-directed mutagenesis revealed that PDCoV nsp10 mutants with missing or destroyed zinc fingers (ZFs) domains also impeded SeV-induced IFN-β production, suggesting that nsp10 does not require its ZF domains to antagonize IFN-β production. Further work found that co-expression of nsp10 with nsp14 or nsp16, two replicative enzymes, significantly enhanced the inhibitory effects of nsp10 on IFN-β. Taken together, our results demonstrate that PDCoV nsp10 antagonizes IFN via a ZF-independent mechanism and has a synergistic effect with nsp14 and nsp16 on inhibiting IFN-β production.

The Potential Therapeutic Effect of RNA Interference and Natural Products on COVID-19: A Review of the Coronaviruses Infection

The SARS-CoV-2 virus was reported for the first time in Wuhan, Hubei Province, China, and causes respiratory infection. This pandemic pneumonia killed about 1,437,835 people out of 61,308,161cases up to November 27, 2020. The disease's main clinical complications include fever, recurrent coughing, shortness of breath, acute respiratory syndrome, and failure of vital organs that could lead to death. It has been shown that natural compounds with antioxidant, anticancer, and antiviral activities and RNA interference agents could play an essential role in preventing or treating coronavirus infection by inhibiting the expression of crucial virus genes. This study aims to introduce a summary of coronavirus's genetic and morphological structure and determine the role of miRNAs, siRNAs, chemical drugs, and natural compounds in stimulating the immune system or inhibiting the virus's structural and non-structural genes that are essential for replication and infection of SARS-CoV-2.

New Insights Into Potential Benefits of Bioactive Compounds of Bee Products on COVID-19: A Review and Assessment of Recent Research

The recent emergence of COVID-19 represents one of the biggest challenges facing the world today. Despite the recent attempts to understand the epidemiological pattern and pathogenesis of the disease, detailed data about the physiology and pathology of the disease is still out of reach. Moreover, the lack of a widespread vaccine prompts an urgent call for developing a proper intervention strategy against the virus. Importantly, identification of novel molecules that target replication of the virus represents one of the promising strategies for the control this pandemic crisis. Among others, honey bee products contain numerous bioactive compounds such as propolis and several phenolic compounds that possess a wide range of therapeutic properties for combating various pathological disorders and infectious agents. The intention of the present review is to highlight the stages of SARS-CoV-2 lifecycle, the molecular mechanisms explaining the health benefits of honey bee products on COVID-19 physiology and pathology and the possible limitations. Further future research is suggested to explore more about bee natural bioactive compounds as potential candidates against SARS-CoV-2.

Exploring potential inhibitor of SARS-CoV2 replicase from FDA approved drugs using insilico drug discovery methods

Severe Acute Respiratory Syndrome Corona Virus-2 (SARS-CoV2) is responsible for fetal pneumonia called COVID19. SARS-CoV2 emerged in Wuhan, Hubei Province of China in December 2019. The COVID19 pandemic has now gripped the entire world with more than 70 million cases and over 1.5 million deaths so far. There no treatment option for COVID19 is in term of a drug or vaccine is currently available. Therefore drug repurposing may only provide a quick method for utilizing existing drugs for a therapeutic option. The virus genome contains several non-structural proteins (NSP) which serve as target for designing of antiviral agents. NSP9 of SARS-CoV2 encodes for a replicase enzyme which is essential for the virus replication in the host cell. In search of potent inhibitors, we have screened FDA approved drugs against NSP9 using in silico methods. Five drugs fluspirilene, troglitazone, alvesco, dihydroergotoxine and avodart were found to have highest affinities with the replicase. The molecular dynamics simulation (MDS) studies demonstrated strong drugs binding and stable NSP9-drugs complexes formation. The findings are also strongly supported by root-mean-square deviation, root-mean-square fluctuation, radius of gyration, and hydrogen bond analysis of the complexes. Principal component analysis showed the stable conformation of NSP9 upon drug binding. It could be inferred that these five drugs individually or in combinations may be used as potential inhibitors of NSP9 of SARS-CoV-2 after exploring their in vivo antiviral potential.Communicated by Ramaswamy H. Sarma.

Engineering a Reliable and Convenient SARS-CoV-2 Replicon System for Analysis of Viral RNA Synthesis and Screening of Antiviral Inhibitors

The etiologic agent of COVID-19 is highly contagious and has caused a severe global pandemic. Until now, there has been no simple and reliable system available in a lower-biosafety-grade laboratory for SARS-CoV-2 virologic research and inhibitor screening. In this study, we reported a replicon system which consists of four plasmids expressing the required segments of SARS-CoV-2. Our study revealed that the features for viral RNA synthesis and responses to antivirus drugs of the replicon are similar to those of wild-type viruses. Further analysis indicated that ORF6 provided potent in trans stimulation of the viral replication. Some viral variations, such as 5'UTR-C241T and ORF8-(T28144C) L84S mutation, also exhibit their different impact upon viral replication. Besides, the screening of clinically used drugs identified that several tyrosine kinase inhibitors and DNA-Top II inhibitors potently inhibit the replicon, as well as authentic SARS-CoV-2 viruses. Collectively, this replicon system provides a biosafety-worry-free platform for studying SARS-CoV-2 virology, monitoring the functional impact of viral mutations, and developing viral inhibitors.IMPORTANCE COVID-19 has caused a severe global pandemic. Until now, there has been no simple and reliable system available in a lower-biosafety-grade laboratory for SARS-CoV-2 virologic research and inhibitor screening. We reported a replicon system which consists of four ordinary plasmids expressing the required segments of SARS-CoV-2. Using the replicon system, we developed three application scenarios: (i) to identify the effects of viral proteins on virus replication, (ii) to identify the effects of mutations on viral replication during viral epidemics, and (iii) to perform high-throughput screening of antiviral drugs. Collectively, this replicon system would be useful for virologists to study SARS-CoV-2 virology, for epidemiologists to monitor virus mutations, and for industry to develop antiviral drugs.

Generation and Characterization of Monoclonal Antibody Against Porcine Epidemic Diarrhea Virus Nonstructural Protein 13

Porcine epidemic diarrhea virus (PEDV) is an enteric swine coronavirus. Recent PEDV eruption in East Asian and North American countries made it notorious and caused huge economic losses to the porcine industry. Nonstructural protein 13 (nsp13) is a nucleic acid helicase/NTPase that plays a critical role in viral gene transcription and viral replication. To investigate the function of nsp13 in the context of PEDV infection, here, PEDV nsp13 gene was amplified and cloned into pET28a/pET30a/pGEX-6P-1 expression vectors. The recombinant his-tagged nsp13 and GST-tagged nsp13 were expressed. Purified his-tagged nsp13 from pET28a-nsp13 vectors was chosen for immunization. Three monoclonal antibodies (mAbs) named 5A9, 5C7, and 5G7 were identified from the hybridoma cells, and were characterized by Western blot analysis and immunofluorescent assay, which demonstrated high specificity of these three mAbs with pCAGGS-HA-nsp13. All three mAbs belong to IgG1+ kappa subclass. However, only mAb 5A9 could effectively and specifically recognize PEDV expressing nsp13. Furthermore, the generated antibody against nsp13 could be applied to investigate nsp13 function during PEDV replication.

SARS-CoV-2 Nsp16 activation mechanism and a cryptic pocket with pan-coronavirus antiviral potential

Coronaviruses have caused multiple epidemics in the past two decades, in addition to the current COVID-19 pandemic that is severely damaging global health and the economy. Coronaviruses employ between twenty and thirty proteins to carry out their viral replication cycle including infection, immune evasion, and replication. Among these, nonstructural protein 16 (Nsp16), a 2'-O-methyltransferase, plays an essential role in immune evasion. Nsp16 achieves this by mimicking its human homolog, CMTr1, which methylates mRNA to enhance translation efficiency and distinguish self from other. Unlike human CMTr1, Nsp16 requires a binding partner, Nsp10, to activate its enzymatic activity. The requirement of this binding partner presents two questions that we investigate in this manuscript. First, how does Nsp10 activate Nsp16? While experimentally-derived structures of the active Nsp16/Nsp10 complex exist, structures of inactive, monomeric Nsp16 have yet to be solved. Therefore, it is unclear how Nsp10 activates Nsp16. Using over one millisecond of molecular dynamics simulations of both Nsp16 and its complex with Nsp10, we investigate how the presence of Nsp10 shifts Nsp16's conformational ensemble in order to activate it. Second, guided by this activation mechanism and Markov state models (MSMs), we investigate if Nsp16 adopts inactive structures with cryptic pockets that, if targeted with a small molecule, could inhibit Nsp16 by stabilizing its inactive state. After identifying such a pocket in SARS-CoV-2 Nsp16, we show that this cryptic pocket also opens in SARS-CoV-1 and MERS, but not in human CMTr1. Therefore, it may be possible to develop pan-coronavirus antivirals that target this cryptic pocket.

STATEMENT OF SIGNIFICANCE

Coronaviruses are a major threat to human health. These viruses employ molecular machines, called proteins, to infect host cells and replicate. Characterizing the structure and dynamics of these proteins could provide a basis for designing small molecule antivirals. In this work, we use computer simulations to understand the moving parts of an essential SARS-CoV-2 protein, understand how a binding partner turns it on and off, and identify a novel pocket that antivirals could target to shut this protein off. The pocket is also present in other coronaviruses but not in the related human protein, so it could be a valuable target for pan-coronavirus antivirals.

Predicting the recombination potential of severe acute respiratory syndrome coronavirus 2 and Middle East respiratory syndrome coronavirus

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recently emerged to cause widespread infections in humans. SARS-CoV-2 infections have been reported in the Kingdom of Saudi Arabia, where Middle East respiratory syndrome coronavirus (MERS-CoV) causes seasonal outbreaks with a case fatality rate of ~37 %. Here we show that there exists a theoretical possibility of future recombination events between SARS-CoV-2 and MERS-CoV RNA. Through computational analyses, we have identified homologous genomic regions within the ORF1ab and S genes that could facilitate recombination, and have analysed co-expression patterns of the cellular receptors for SARS-CoV-2 and MERS-CoV, ACE2 and DPP4, respectively, to identify human anatomical sites that could facilitate co-infection. Furthermore, we have investigated the likely susceptibility of various animal species to MERS-CoV and SARS-CoV-2 infection by comparing known virus spike protein-receptor interacting residues. In conclusion, we suggest that a recombination between SARS-CoV-2 and MERS-CoV RNA is possible and urge public health laboratories in high-risk areas to develop diagnostic capability for the detection of recombined coronaviruses in patient samples.

Structural and functional insights into non-structural proteins of coronaviruses

Coronaviruses (CoVs) are causing a number of human and animal diseases because of their zoonotic nature such as Middle East respiratory syndrome (MERS), severe acute respiratory syndrome (SARS) and coronavirus disease 2019 (COVID-19). These viruses can infect respiratory, gastrointestinal, hepatic and central nervous systems of human, livestock, birds, bat, mouse, and many wild animals. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a newly emerging respiratory virus and is causing CoVID-19 with high morbidity and considerable mortality. All CoVs belong to the order Nidovirales, family Coronaviridae, are enveloped positive-sense RNA viruses, characterised by club-like spikes on their surfaces and large RNA genome with a distinctive replication strategy. Coronavirus have the largest RNA genomes (~26–32 kilobases) and their expansion was likely enabled by acquiring enzyme functions that counter the commonly high error frequency of viral RNA polymerases. Non-structural proteins (nsp) 7–16 are cleaved from two large replicase polyproteins and guide the replication and processing of coronavirus RNA. Coronavirus replicase has more or less universal activities, such as RNA polymerase (nsp 12) and helicase (nsp 13), as well as a variety of unusual or even special mRNA capping (nsp 14, nsp 16) and fidelity regulation (nsp 14) domains. Besides that, several smaller subunits (nsp 7– nsp 10) serve as essential cofactors for these enzymes and contribute to the emerging “nsp interactome.” In spite of the significant progress in studying coronaviruses structural and functional properties, there is an urgent need to understand the coronaviruses evolutionary success that will be helpful to develop enhanced control strategies. Therefore, it is crucial to understand the structure, function, and interactions of coronaviruses RNA synthesizing machinery and their replication strategies.

Tomatidine inhibits porcine epidemic diarrhea virus replication by targeting 3CL protease

Porcine epidemic diarrhea virus (PEDV) causes lethal diarrhea in suckling piglets, leading to severe economic losses worldwide. There is an urgent need to find new therapeutic methods to prevent and control PEDV. Not only is there a shortage of commercial anti-PEDV drugs, but available commercial vaccines fail to protect against highly virulent PEDV variants. We screened an FDA-approved library of 911 natural products and found that tomatidine, a steroidal alkaloid extracted from the skin and leaves of tomatoes, demonstrates significant inhibition of PEDV replication in Vero and IPEC-J2 cells in vitro. Molecular docking and molecular dynamics analysis predicted interactions between tomatidine and the active pocket of PEDV 3CL protease, which were confirmed by fluorescence spectroscopy and isothermal titration calorimetry (ITC). The inhibiting effect of tomatidine on 3CL protease was determined using cleavage visualization and FRET assay. Tomatidine-mediated blocking of 3CL protease activity in PEDV-infected cells was examined by western blot detection of the viral polyprotein in PEDV-infected cells. It indicates that tomatidine inhibits PEDV replication mainly by targeting 3CL protease. In addition, tomatidine also has antiviral activity against transmissible gastroenteritis virus (TGEV), porcine reproductive and respiratory syndrome virus (PRRSV), encephalo myocarditis virus (EMCV) and seneca virus A (SVA) in vitro. These results may be helpful in developing a new prophylactic and therapeutic strategy against PEDV and other swine disease infections.

Molecular biology of coronaviruses: current knowledge

The emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) late December 2019 in Wuhan, China, marked the third introduction of a highly pathogenic coronavirus into the human population in the twenty-first century. The constant spillover of coronaviruses from natural hosts to humans has been linked to human activities and other factors. The seriousness of this infection and the lack of effective, licensed countermeasures clearly underscore the need of more detailed and comprehensive understanding of coronavirus molecular biology. Coronaviruses are large, enveloped viruses with a positive sense single-stranded RNA genome. Currently, coronaviruses are recognized as one of the most rapidly evolving viruses due to their high genomic nucleotide substitution rates and recombination. At the molecular level, the coronaviruses employ complex strategies to successfully accomplish genome expression, virus particle assembly and virion progeny release. As the health threats from coronaviruses are constant and long-term, understanding the molecular biology of coronaviruses and controlling their spread has significant implications for global health and economic stability. This review is intended to provide an overview of our current basic knowledge of the molecular biology of coronaviruses, which is important as basic knowledge for the development of coronavirus countermeasures.

Structural analysis of the SARS-CoV-2 methyltransferase complex involved in RNA cap creation bound to sinefungin

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the COVID-19 pandemic. 2′-O-RNA methyltransferase (MTase) is one of the enzymes of this virus that is a potential target for antiviral therapy as it is crucial for RNA cap formation; an essential process for viral RNA stability. This MTase function is associated with the nsp16 protein, which requires a cofactor, nsp10, for its proper activity. Here we show the crystal structure of the nsp10-nsp16 complex bound to the pan-MTase inhibitor sinefungin in the active site. Our structural comparisons reveal low conservation of the MTase catalytic site between Zika and SARS-CoV-2 viruses, but high conservation of the MTase active site between SARS-CoV-2 and SARS-CoV viruses; these data suggest that the preparation of MTase inhibitors targeting several coronaviruses - but not flaviviruses - should be feasible. Together, our data add to important information for structure-based drug discovery.

SARS-CoV-2 expresses a 2′-O RNA methyltransferase (MTase) that is involved in the viral RNA cap formation and therefore a target for antiviral therapy. Here the authors provide the structure of nsp10-nsp16 with the panMTase inhibitor sinefungin and report that the development of MTase inhibitor therapies that target multiple coronoaviruses is feasible.

Temperature Sensitivity: A Potential Method for the Generation of Vaccines against the Avian Coronavirus Infectious Bronchitis Virus

The Gammacoronavirus infectious bronchitis virus (IBV) is a highly contagious economically important respiratory pathogen of domestic fowl. Reverse genetics allows for the molecular study of pathogenic determinants to enable rational vaccine design. The recombinant IBV (rIBV) Beau-R, a molecular clone of the apathogenic Beaudette strain, has previously been investigated as a vaccine platform. To determine tissues in which Beau-R could effectively deliver antigenic genes, an in vivo study in chickens, the natural host, was used to compare the pattern of viral dissemination of Beau-R to the pathogenic strain M41-CK. Replication of Beau-R was found to be restricted to soft tissue within the beak, whereas M41-CK was detected in beak tissue, trachea and eyelid up to seven days post infection. In vitro assays further identified that, unlike M41-CK, Beau-R could not replicate at 41 °C, the core body temperature of a chicken, but is able to replicate a 37 °C, a temperature relatable to the very upper respiratory tract. Using a panel of rIBVs with defined mutations in the structural and accessory genes, viral replication at permissive and non-permissive temperatures was investigated, identifying that the Beau-R replicase gene was a determinant of temperature sensitivity and that sub-genomic mRNA synthesis had been affected. The identification of temperature sensitive allelic lesions within the Beau-R replicase gene opens up the possibility of using this method of attenuation in other IBV strains for future vaccine development as well as a method to investigate the functions of the IBV replicase proteins.

Processing of the SARS-CoV pp1a/ab nsp7-10 region

Severe acute respiratory syndrome coronavirus is the causative agent of a respiratory disease with a high case fatality rate. During the formation of the coronaviral replication/transcription complex, essential steps include processing of the conserved polyprotein nsp7-10 region by the main protease Mpro and subsequent complex formation of the released nsp's. Here, we analyzed processing of the coronavirus nsp7-10 region using native mass spectrometry showing consumption of substrate, rise and fall of intermediate products and complexation. Importantly, there is a clear order of cleavage efficiencies, which is influenced by the polyprotein tertiary structure. Furthermore, the predominant product is an nsp7+8(2 : 2) hetero-tetramer with nsp8 scaffold. In conclusion, native MS, opposed to other methods, can expose the processing dynamics of viral polyproteins and the landscape of protein interactions in one set of experiments. Thereby, new insights into protein interactions, essential for generation of viral progeny, were provided, with relevance for development of antivirals.

Molecular characterization of infectious bronchitis virus based on RNA-dependent RNA polymerase gene

Extensive rate of variations in the S1 gene (spike glycoprotein subunit gene) of infectious bronchitis virus (IBV) causes challenges for clinicians in counting variants for differentiation of infected from vaccinated birds and addressing the variants of unknown significance. This study investigated the possibility of using an RNA‐dependent RNA polymerase gene (RdRp) as a target for molecular characterization of IBV strains in Iran. Trachea samples were collected from commercial broiler flocks (n = 52) showing respiratory syndrome. Specific PCR primers were designed for a variable region located in the RdRp gene flanked by highly conserved regions. Reverse transcriptase PCR followed by sequence analysis identified eight IBV variants, with an overall prevalence of 44.2%. Deduced nucleotide and amino acid sequences were compared with published sequences for IBV strains. Because of the long‐distance similarities, the field samples could be discriminated from vaccine strains. Phylogenetic analysis of RdRp gene sequences resulted in clustering of the IBV strains related to each area. Using RdRp as a genetic marker eliminates the challenges arising from the enormous variations that make it difficult to discriminate between field and vaccine strains as well as affiliate certain variants to various geographical areas.