MicroRNA750-3p Targets Processing of Precursor 7 to Suppress Rice Black-Streaked Dwarf Virus Propagation in Vector Laodelphax striatellus

MicroRNAs (miRNAs) are non-coding RNAs, which, as members of the RNA interference pathway, play a pivotal role in antiviral infection. Almost 80% of plant viruses are transmitted by insect vectors; however, little is known about the interaction of the miRNAs of insect vectors with plant viruses. Here, we took rice black-streaked dwarf virus (RBSDV), a devastating virus to rice production in eastern Asia, and the small brown planthopper, (SBPH, Laodelphax striatellus) as a model to investigate the role of microRNA750-3p (miR750-3p) in regulating viral transmission. Our results showed that Ls-miR750-3p was downregulated in RBSDV-infected SBPH and predominately expressed in the midgut of SBPH. Injection with miR750-3p agomir significantly reduced viral accumulation, and the injection with the miR750-3p inhibitor, antagomir-750-3p, dramatically promoted the viral accumulation in SBPH, as detected using RT-qPCR and Western blotting. The processing of precursor 7 (POP7), a subunit of RNase P and RNase MRP, was screened, identified, and verified using a dual luciferase reporter assay as one target of miR750-3p. Knockdown of POP7 notably increased RBSDV viral propagation in SBPH and then increased the viral transmission rate by SBPH. Taken together, our data indicate that miR750-3p targets POP7 to suppress RBSDV infection in its insect vector. These results enriched the role of POP7 in modulating virus infection in host insects and shared new insight into the function of miRNAs in plant virus and insect vector interaction.

Identification of a new amino acid mutation in the HN protein of NDV involved in pathogenicity

The fusion (F) and haemagglutinin-neuraminidase (HN) proteins of Newcastle disease virus (NDV) are viral entry proteins and are recognized as the major virulence determinants. Previously, a lentogenic NDV virus (CE16) was derived from a mesogenic strain (CI10) through sequential passages in chick embryos. Whole-genome sequence analysis revealed that the two homologous strains shared the same F protein but differed in HN with two amino acid (aa) substitutions (A215G and T430A). To elucidate the molecular reasons for virulence attenuation, two original plasmids (HN-CI10 and HN-CE16) and two single-point mutants (G215A and A430T) reverse-mutated from HN-CE16 were constructed to analyse the known biological functions of HN. The results showed that the A430T substitution significantly weakened the haemadsorption (HAd) activity, increased the neuraminidase (NA) activity, improved the fusion-promoting activity, and enhanced the cleavage-promoting activity of HN-CE16. However, G215A failed to induce obvious functional changes. Therefore, the aa residue HN430 may play a key role in determining virulence. To test this hypothesis, further studies on A430T were conducted through reverse genetics using an infectious cDNA clone. At the viral level, the A430T-mutated virus showed dramatic promotion of viral plaque formation, propagation, and pathogenicity in vitro and in vivo. This study demonstrates a new virulence site associated with HN protein functions, viral propagation, and pathogenicity. All these findings could lay a foundation for illuminating the molecular mechanism of NDV virulence.

NS5A-ISGylation via Lysine 26 Has a Critical Role for Efficient Propagation of Hepatitis C Virus Genotype 2a

We previously reported that hepatitis C virus (HCV) NS5A (1b, Con1) protein accepts covalent ISG15 conjugation at specific lysine (Lys) residues (K44, K68, K166, K215 and K308), exhibiting proviral effects on HCV RNA replication. Here we investigated a role of NS5A-ISGylation via Lys residues in HCV propagation using HCV infectious clone. The alignment of amino acid sequences revealed that 5 Lys residues (K20, K26, K44, K139, and K166) of the 13 Lys residues within NS5A (genotype 2a, JFH1 strain) were conserved compared to those of HCV (genotype 1b, Con1 strain). The cell-based ISGylation assay revealed that the K26 residue in the amphipathic helix (AH) domain and the K139 residue in domain I of NS5A (2a, JFH1) had the potential to accept ISGylation. Use of the HCV replicon carrying luciferase gene revealed that the K26 residue but not K139 residue of NS5A (2a, JFH1) was important for HCV RNA replication. Furthermore, cell culture HCV revealed that the mutation with the K26 residue in combination with K139 or K166 on NS5A (2a, JFH1) resulted in complete abolishment of viral propagation, suggesting that the K26 residue collaborates with either the K139 residue or K166 residue for efficient HCV propagation. Taken together, these results suggest that HCV NS5A protein has the potential to accept ISGylation via specific Lys residues, involving efficient viral propagation in a genotype-specific manner.

Mathematical Model of SARS-Cov-2 Propagation Versus ACE2 Fits COVID-19 Lethality Across Age and Sex and Predicts That of SARS

The fatality rate of Covid-19 escalates with age and is larger in men than women. I show that these variations correlate strongly with the level of the viral receptor protein ACE2 in rat lungs, which is consistent with the still limited data on human ACE2. Surprisingly, lower receptor levels correlate with higher fatality. I propose two possible explanations of this negative correlation: First, a previous mathematical model predicts that the velocity of viral progression in the organism as a function of the receptor level has a maximum and declines for abundant receptor. Secondly, degradation of ACE2 by the virus may cause the runaway inflammatory response that characterizes severe CoViD-19. I present here a mathematical model that predicts the lethality as a function of ACE2 protein level based on the two above hypothesis. The model fits Covid-19 fatality rate across age and sex in three countries with high accuracy (r 2 > 0.9 ) under the hypothesis that the speed of viral progression in the infected organism is a decreasing function of the ACE2 level. Moreover, rescaling the fitted parameters by the ratio of the binding rates of the spike proteins of SARS-CoV and SARS-CoV-2 allows predicting the fatality rate of SARS-CoV across age and sex, thus linking the molecular and epidemiological levels.

Adenosylhomocysteinase like 1 interacts with nonstructural 5A and regulates hepatitis C virus propagation

Hepatitis C virus (HCV) life cycle is highly dependent on cellular proteins for viral propagation. In order to identify the cellular factors involved in HCV propagation, we previously performed a protein microarray assay using the HCV nonstructural 5A (NS5A) protein as a probe. Of ∼9,000 human cellular proteins immobilized in a microarray, adenosylhomocysteinase like 1 (AHCYL1) was among 90 proteins identified as NS5A interactors. Of these candidates, AHCYL1 was selected for further study. In the present study, we verified the physical interaction between NS5A and AHCYL1 by both in vitro pulldown and coimmunoprecipitation assays. Furthermore, HCV NS5A interacted with endogenous AHCYL1 in Jc1-infected cells. Both NS5A and AHCYL1 were colocalized in the cytoplasmic region in HCV-replicating cells. siRNAmediated knockdown of AHCYL1 abrogated HCV propagation. Exogenous expression of the siRNA-resistant AHCYL1 mutant, but not of the wild-type AHCYL1, restored HCV protein expression levels, indicating that AHCYL1 was required specifically for HCV propagation. Importantly, AHCYL1 was involved in the HCV internal ribosome entry site-mediated translation step of the HCV life cycle. Finally, we demonstrated that the proteasomal degradation pathway of AHCYL1 was modulated by persistent HCV infection. Collectively, these data suggest that HCV may modulate the AHCYL1 protein to promote viral propagation.

PACSIN2 Interacts with Nonstructural Protein 5A and Regulates Hepatitis C Virus Assembly

Hepatitis C virus (HCV) is a major etiologic agent of chronic liver diseases. HCV is highly dependent on cellular machinery for viral propagation. Using protein microarray analysis, we previously identified 90 cellular proteins as nonstructural 5A (NS5A) interacting partners. Of these, protein kinase C and casein kinase substrate in neurons protein 2 (PACSIN2) was selected for further study. PACSIN2 belongs to the PACSIN family, which is involved in the formation of caveolae. Protein interaction between NS5A and PACSIN2 was confirmed by pulldown assay and further verified by both coimmunoprecipitation and immunofluorescence assays. We showed that PACSIN2 interacted with domain I of NS5A and the Fer-CIP4 homology (FCH)-Bin/amphiphysin/Rvs (F-BAR) region of PACSIN2. Interestingly, NS5A specifically attenuated protein kinase C alpha (PKCα)-mediated phosphorylation of PACSIN2 at serine 313 by interrupting PACSIN2 and PKCα interaction. In fact, mutation of the serine 313 to alanine (S313A) of PACSIN2 increased protein interaction with NS5A. Silencing of PACSIN2 decreased both viral RNA and protein expression levels of HCV. Ectopic expression of the small interfering RNA (siRNA)-resistant PACSIN2 recovered the viral infectivity, suggesting that PACSIN2 was specifically required for HCV propagation. PACSIN2 was involved in viral assembly without affecting other steps of the HCV life cycle. Indeed, overexpression of PACSIN2 promoted NS5A and core protein (core) interaction. We further showed that inhibition of PKCα increased NS5A and core interaction, suggesting that phosphorylation of PACSIN2 might influence HCV assembly. Moreover, PACSIN2 was required for lipid droplet formation via modulating extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation. Taken together, these data indicate that HCV modulates PACSIN2 via NS5A to promote virion assembly.IMPORTANCE PACSIN2 is a lipid-binding protein that triggers the tubulation of the phosphatidic acid-containing membranes. The functional involvement of PACSIN2 in the virus life cycle has not yet been demonstrated. We showed that phosphorylation of PACSIN2 displayed a negative effect on NS5A and core interaction. The most significant finding is that NS5A prevents PKCα from binding to PACSIN2. Therefore, the phosphorylation level of PACSIN2 is decreased in HCV-infected cells. We showed that HCV NS5A interrupted PKCα-mediated PACSIN2 phosphorylation at serine 313, thereby promoting NS5A-PACSIN2 interaction. We further demonstrated that PACSIN2 modulated lipid droplet formation through ERK1/2 phosphorylation. These data provide evidence that PACSIN2 is a proviral cellular factor required for viral propagation.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

IGF2BP1 Significantly Enhances Translation Efficiency of Duck Hepatitis A Virus Type 1 without Affecting Viral Replication

As a disease characterized by severe liver necrosis and hemorrhage, duck viral hepatitis (DVH) is mainly caused by duck hepatitis A virus (DHAV). The positive-strand RNA genome of DHAV type 1 (DHAV-1) contains an internal ribosome entry site (IRES) element within the 5′ untranslated region (UTR), structured sequence elements within the 3′ UTR, and a poly(A) tail at the 3′ terminus. In this study, we first examined that insulin-like growth factor-2 mRNA-binding protein-1 (IGF2BP1) specifically interacted with the DHAV-1 3′ UTR by RNA pull-down assay. The interaction between IGF2BP1 and DHAV-1 3′ UTR strongly enhanced IRES-mediated translation efficiency but failed to regulate DHAV-1 replication in a duck embryo epithelial (DEE) cell line. The viral propagation of DHAV-1 strongly enhanced IGF2BP1 expression level, and viral protein accumulation was identified as the key point to this increment. Collectively, our data demonstrated the positive role of IGF2BP1 in DHAV-1 viral proteins translation and provided data support for the replication mechanism of DHAV-1.

Hepatitis C Virus Downregulates Ubiquitin-Conjugating Enzyme E2S Expression To Prevent Proteasomal Degradation of NS5A, Leading to Host Cells More Sensitive to DNA Damage

Hepatitis C virus (HCV) infection may cause chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. HCV exploits cellular machineries to establish persistent infection. We demonstrate here that ubiquitin-conjugating enzyme E2S (UBE2S), a member of the ubiquitin-conjugating enzyme family (E2s), was downregulated by endoplasmic reticulum stress caused by HCV in Huh7 cells. UBE2S interacted with domain I of HCV NS5A and degraded NS5A protein through the Lys11-linked proteasome-dependent pathway. Overexpression of UBE2S suppressed viral propagation, while depletion of UBE2S expression increased viral infectivity. Enzymatically inactive UBE2S C95A mutant exerted no antiviral activity, suggesting that ubiquitin-conjugating enzymatic activity was required for the suppressive role of UBE2S. Chromatin ubiquitination plays a crucial role in the DNA damage response. We showed that the levels of UBE2S and Lys11 chains bound to the chromatin were markedly decreased in the context of HCV replication, rendering HCV-infected cells more sensitive to DNA damage. These data suggest that HCV counteracts antiviral activity of UBE2S to optimize viral propagation and may contribute to HCV-induced liver pathogenesis.IMPORTANCE Protein homeostasis is essential to normal cell function. HCV infection disturbs the protein homeostasis in the host cells. Therefore, host cells exert an anti-HCV activity in order to maintain normal cellular metabolism. We showed that UBE2S interacted with HCV NS5A and degraded NS5A protein through the Lys11-linked proteasome-dependent pathway. However, HCV has evolved to overcome host antiviral activity. We demonstrated that the UBE2S expression level was suppressed in HCV-infected cells. Since UBE2S is an ubiquitin-conjugating enzyme and this enzyme activity is involved in DNA damage repair, HCV-infected cells are more sensitive to DNA damage, and thus UBE2S may contribute to viral oncogenesis.

Propagation of Astrovirus VA1, a Neurotropic Human Astrovirus, in Cell Culture

Astrovirus VA1/HMO-C (VA1; mamastrovirus 9) is a recently discovered astrovirus genotype that is divergent from the classic human astroviruses (mamastrovirus 1). The gastrointestinal tract is presumed to be the primary site of infection and pathogenicity for astroviruses. However, VA1 has been independently detected in brain tissue of five cases of human encephalitis. Studies of the pathogenicity of VA1 are currently impossible because there are no reported cell culture systems or in vivo models that support VA1 infection. Here, we describe successful propagation of VA1 in multiple human cell lines. The initial inoculum, a filtered clinical stool sample from the index gastroenteritis case cluster that led to the discovery of VA1, was first passaged in Vero cells. Serial blind passage in Caco-2 cells yielded increasing copies of VA1 RNA, and multistep growth curves demonstrated a >100-fold increase in VA1 RNA 72 h after inoculation. The full-length genomic and subgenomic RNA strands were detected by Northern blotting, and crystalline lattices of viral particles of ∼26-nm diameter were observed by electron microscopy in infected Caco-2 cells. Unlike other human astrovirus cell culture systems, which require addition of exogenous trypsin for continued propagation, VA1 could be propagated equally well with or without the addition of trypsin. Furthermore, VA1 was sensitive to the type I interferon (IFN-I) response, as VA1 RNA levels were reduced by pretreatment of Caco-2 cells with IFN-β1a. The ability to propagate VA1 in cell culture will facilitate studies of the neurotropism and neuropathogenesis of VA1.IMPORTANCE Astroviruses are an emerging cause of central nervous system infections in mammals, and astrovirus VA1/HMO-C is the most prevalent astrovirus in cases of human encephalitis. This virus has not been previously propagated, preventing elucidation of the biology of this virus. We describe the first cell culture system for VA1, a key step necessary for the study of its ability to cause disease.

Abl Tyrosine Kinase Regulates Hepatitis C Virus Entry

Abl is a central regulator of multiple cellular processes controlling actin dynamics, proliferation, and differentiation. Here, we showed that knockdown of Abl impaired hepatitis C virus (HCV) propagation. Treatment of Abl tyrosine kinase-specific inhibitor, imatinib and dasatinib, also significantly decreased HCV RNA and protein levels in HCV-infected cells. We showed that both imatinib and dasatinib selectively inhibited HCV infection at the entry step of HCV life cycle, suggesting that Abl kinase activity may be necessary for HCV entry. Using HCV pseudoparticle infection assays, we verified that Abl is required for viral entry. By employing transferrin uptake and immunofluorescence assays, we further demonstrated that Abl was involved in HCV entry at a clathrin-mediated endocytosis step. These data suggest that Abl may represent a novel host factor for HCV entry.