Middle East Respiratory Syndrome Coronavirus Gene 5 Modulates Pathogenesis in Mice

Contrasting roles of MERS-CoV and SARS-CoV-2 internal proteins in pathogenesis in mice

IMPORTANCE

The function of betacoronavirus internal protein has been relatively understudied. The earliest report on the internal protein of mouse hepatitis virus suggested that the internal protein is a structural protein without significant functions in virus replication and virulence. However, the internal proteins of severe acute respiratory syndrome coronavirus (SARS-CoV), Middle-East respiratory syndrome coronavirus, and SARS-CoV-2 have been shown to evade immune responses. Despite the reported functions of the internal protein in these highly pathogenic human coronaviruses, its role in mediating pathogenesis in experimentally infected animals has not been characterized. Our data indicated that despite the similar genomic location and expression strategy of these internal proteins, their effects on virulence are vastly different and virus specific, highlighting the complexity between host-virus interaction and disease outcome.

Engineering potent live attenuated coronavirus vaccines by targeted inactivation of the immune evasive viral deubiquitinase

Coronaviruses express a papain-like protease (PLpro) that is required for replicase polyprotein maturation and also serves as a deubiquitinating enzyme (DUB). In this study, using a Middle East respiratory syndrome virus (MERS-CoV) PLpro modified virus in which the DUB is selectively inactivated, we show that the PLpro DUB is an important MERS-CoV interferon antagonist and virulence factor. Although the DUB-negative rMERS-CoV_MA replicates robustly in the lungs of human dipeptidyl peptidase 4 knock-in (hDPP4 KI) mice, it does not cause clinical symptoms. Interestingly, a single intranasal vaccination with DUB-negative rMERS-CoV_MA induces strong and sustained neutralizing antibody responses and sterilizing immunity after a lethal wt virus challenge. The survival of naïve animals also significantly increases when sera from animals vaccinated with the DUB-negative rMERS-CoV_MA are passively transferred, prior to receiving a lethal virus dose. These data demonstrate that DUB-negative coronaviruses could be the basis of effective modified live attenuated vaccines.

Contribution to pathogenesis of accessory proteins of deadly human coronaviruses

Coronaviruses (CoVs) are enveloped and positive-stranded RNA viruses with a large genome (∼ 30kb). CoVs include essential genes, such as the replicase and four genes coding for structural proteins (S, M, N and E), and genes encoding accessory proteins, which are variable in number, sequence and function among different CoVs. Accessory proteins are non-essential for virus replication, but are frequently involved in virus-host interactions associated with virulence. The scientific literature on CoV accessory proteins includes information analyzing the effect of deleting or mutating accessory genes in the context of viral infection, which requires the engineering of CoV genomes using reverse genetics systems. However, a considerable number of publications analyze gene function by overexpressing the protein in the absence of other viral proteins. This ectopic expression provides relevant information, although does not acknowledge the complex interplay of proteins during virus infection. A critical review of the literature may be helpful to interpret apparent discrepancies in the conclusions obtained by different experimental approaches. This review summarizes the current knowledge on human CoV accessory proteins, with an emphasis on their contribution to virus-host interactions and pathogenesis. This knowledge may help the search for antiviral drugs and vaccine development, still needed for some highly pathogenic human CoVs.

MERS-CoV ORF4b is a virulence factor involved in the inflammatory pathology induced in the lungs of mice

No vaccines or specific antiviral drugs are authorized against Middle East respiratory syndrome coronavirus (MERS-CoV) despite its high mortality rate and prevalence in dromedary camels. Since 2012, MERS-CoV has been causing sporadic zoonotic infections in humans, which poses a risk of genetic evolution to become a pandemic virus. MERS-CoV genome encodes five accessory proteins, 3, 4a, 4b, 5 and 8b for which limited information is available in the context of infection. This work describes 4b as a virulence factor in vivo, since the deletion mutant of a mouse-adapted MERS-CoV-Δ4b (MERS-CoV-MA-Δ4b) was completely attenuated in a humanized DPP4 knock-in mouse model, resulting in no mortality. Attenuation in the absence of 4b was associated with a significant reduction in lung pathology and chemokine expression levels at 4 and 6 days post-infection, suggesting that 4b contributed to the induction of lung inflammatory pathology. The accumulation of 4b in the nucleus in vivo was not relevant to virulence, since deletion of its nuclear localization signal led to 100% mortality. Interestingly, the presence of 4b protein was found to regulate autophagy in the lungs of mice, leading to upregulation of BECN1, ATG3 and LC3A mRNA. Further analysis in MRC-5 cell line showed that, in the context of infection, MERS-CoV-MA 4b inhibited autophagy, as confirmed by the increase of p62 and the decrease of ULK1 protein levels, either by direct or indirect mechanisms. Together, these results correlated autophagy activation in the absence of 4b with downregulation of a pathogenic inflammatory response, thus contributing to attenuation of MERS-CoV-MA-Δ4b.

The N-terminal Region of Middle East Respiratory Syndrome Coronavirus Accessory Protein 8b is Essential for Enhanced Virulence of an Attenuated Murine Coronavirus

Middle East respiratory syndrome coronavirus (MERS-CoV) is a beta coronavirus that emerged in 2012, causing severe pneumonia and renal failure. MERS-CoV encodes five accessory proteins. Some of them have been shown to interfere with host antiviral immune response. However, the roles of protein 8b in innate immunity and viral virulence was rarely studied. Here, we introduced individual MERS-CoV accessory protein genes into the genome of an attenuated murine coronavirus (Mouse hepatitis virus, MHV), respectively, and found accessory protein 8b could enhance viral replication in vivo and in vitro and increase the lethality of infected mice. RNA-seq analysis revealed that protein 8b could significantly inhibit type I interferon production (IFN-I) and innate immune response in mice infected with MHV expressing protein 8b. We also found that MERS-CoV protein 8b could initiate from multiple internal methionine sites and at least three protein variants were identified. Residues 1-23 of protein 8b was demonstrated to be responsible for increased virulence in vivo. In addition, the inhibitory effect on IFN-I of protein 8b might not contribute to its virulence enhancement as aa1-23 deletion did not affect IFN-I production in vitro and in vivo. Next, we also found that protein 8b was localized to the endoplasmic reticulum (ER)/Golgi membrane in infected cells, which was disrupted by C-terminal region aa 88-112 deletion. This study will provide new insight into the pathogenesis of MERS-CoV infection.

IMPORTANCE Multiple coronaviruses (CoV) cause severe respiratory infections and become global public health threats such as SARS-CoV, MERS-CoV, and SARS-CoV-2. Each coronavirus contains different numbers of accessory proteins which show high variability among different CoVs. Accessory proteins are demonstrated to play essential roles in pathogenesis of CoVs. MERS-CoV contains 5 accessory proteins (protein 3, 4a, 4b, 5, 8b), and deletion of all four accessory proteins (protein 3, 4a, 4b, 5), significantly affects MERS-CoV replication and pathogenesis. However, whether ORF8b also regulates MERS-CoV infection is unknown. Here, we constructed mouse hepatitis virus (MHV) recombinant virus expressing MERS-CoV protein 8b and demonstrated protein 8b could significantly enhance the virulence of MHV, which is mediated by N-terminal domain of protein 8b. This study will shed light on the understanding of pathogenesis of MERS-CoV infection.

Middle East respiratory syndrome coronavirus vaccine based on a propagation-defective RNA replicon elicited sterilizing immunity in mice

Self-amplifying RNA replicons are promising platforms for vaccine generation. Their defects in one or more essential functions for viral replication, particle assembly, or dissemination make them highly safe as vaccines. We previously showed that the deletion of the envelope (E) gene from the Middle East respiratory syndrome coronavirus (MERS-CoV) produces a replication-competent propagation-defective RNA replicon (MERS-CoV-ΔE). Evaluation of this replicon in mice expressing human dipeptidyl peptidase 4, the virus receptor, showed that the single deletion of the E gene generated an attenuated mutant. The combined deletion of the E gene with accessory open reading frames (ORFs) 3, 4a, 4b, and 5 resulted in a highly attenuated propagation-defective RNA replicon (MERS-CoV-Δ[3,4a,4b,5,E]). This RNA replicon induced sterilizing immunity in mice after challenge with a lethal dose of a virulent MERS-CoV, as no histopathological damage or infectious virus was detected in the lungs of challenged mice. The four mutants lacking the E gene were genetically stable, did not recombine with the E gene provided in trans during their passage in cell culture, and showed a propagation-defective phenotype in vivo. In addition, immunization with MERS-CoV-Δ[3,4a,4b,5,E] induced significant levels of neutralizing antibodies, indicating that MERS-CoV RNA replicons are highly safe and promising vaccine candidates.

MOV10 Helicase Interacts with Coronavirus Nucleocapsid Protein and Has Antiviral Activity

Coronaviruses (CoVs) are emergent pathogens that may cause life-threatening respiratory diseases in humans. Understanding of CoV-host interactions may help to identify novel therapeutic targets. MOV10 is an RNA helicase involved in different steps of cellular RNA metabolism. Both MOV10 antiviral and proviral activities have been described in a limited number of viruses, but this protein has not been previously associated with CoVs. We found that during Middle East respiratory syndrome coronavirus (MERS-CoV) infection, MOV10 aggregated in cytoplasmic structures colocalizing with viral nucleocapsid (N) protein. MOV10-N interaction was confirmed by endogenous MOV10 coimmunoprecipitation, and the presence of other cellular proteins was also detected in MOV10 complexes. MOV10 silencing significantly increased both N protein accumulation and virus titer, with no changes in the accumulation of viral RNAs. Moreover, MOV10 overexpression caused a 10-fold decrease in viral titers. These data indicated that MOV10 has antiviral activity during MERS-CoV infection. We postulated that this activity could be mediated by viral RNA sequestration, and in fact, RNA immunoprecipitation data showed the presence of viral RNAs in the MOV10 cytoplasmic complexes. Expression of wild-type MOV10 or of a MOV10 mutant without helicase activity in MOV10 knockout cell lines, developed by CRISPR-Cas technology, indicated that the helicase activity of MOV10 was required for its antiviral effect. Interestingly MOV10-N interaction was conserved in other mildly or highly pathogenic human CoVs, including the recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), although MOV10 antiviral activity was found only in highly pathogenic CoVs, suggesting a potential role of MOV10 in the modulation of human CoVs pathogenesis.

Nanobased Platforms for Diagnosis and Treatment of COVID-19: From Benchtop to Bedside

Human respiratory viral infections are the leading cause of morbidity and mortality around the world. Among the various respiratory viruses, coronaviruses (e.g., SARS-CoV-2) have created the greatest challenge and most frightening health threat worldwide. Human coronaviruses typically infect the upper respiratory tract, causing illnesses that range from common cold-like symptoms to severe acute respiratory infections. Several promising vaccine formulations have become available since the beginning of 2021. Nevertheless, achievement of herd immunity is still far from being realized. Social distancing remains the only effective measure against SARS-CoV-2 infection. Nanobiotechnology enables the design of nanobiosensors. These nanomedical diagnostic devices have opened new vistas for early detection of viral infections. The present review outlines recent research on the effectiveness of nanoplatforms as diagnostic and antiviral tools against coronaviruses. The biological properties of coronavirus and infected host organs are discussed. The challenges and limitations encountered in combating SARS-CoV-2 are highlighted. Potential nanodevices such as nanosensors, nanobased vaccines, and smart nanomedicines are subsequently presented for combating current and future mutated versions of coronaviruses.