微生物学

Coronaviruses are important human pathogens, among which the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent for the COVID-19 pandemic. To combat the SARS-CoV-2 pandemic, there is a pressing need for antivirals, especially broad-spectrum antivirals that are active against all seven human coronaviruses (HCoVs). For this reason, we are interested in developing antiviral assays to expedite the drug discovery process. Here, we provide the detailed protocol for the cytopathic effect (CPE) assay and the plaque assay for human coronaviruses 229E (HCoV-229E), HCoV-OC43, and HCoV-NL63, to identify novel antivirals against HCoVs. Neutral red was used in the CPE assay, as it is relatively inexpensive and more sensitive than other reagents. Multiple parameters including multiplicity of infection, incubation time and temperature, and staining conditions have been optimized for CPE and plaque assays for HCoV-229E in MRC-5, Huh-7, and RD cell lines; HCoV-OC43 in RD, MRC-5, and BSC-1 cell lines, and HCoV-NL63 in Vero E6, Huh-7, MRC-5, and RD cell lines. Both CPE and plaque assays have been calibrated with the positive control compounds remdesivir and GC-376. Both CPE and plaque assays have high sensitivity, excellent reproducibility, and are cost-effective. The protocols described herein can be used as surrogate assays in the biosafety level 2 facility to identify entry inhibitors and protease inhibitors for SARS-CoV-2, as HCoV-NL63 also uses ACE2 as the receptor for cell entry, and the main proteases of HCoV-OC43 and SARS-CoV-2 are highly conserved. In addition, these assays can also be used as secondary assays to profile the broad-spectrum antiviral activity of existing SARS-CoV-2 drug candidates.

Next generations sequencing (NGS) has become an important tool in biomedical research. The Primer ID approach combined with the MiSeq platform overcomes the limitation of PCR errors and reveals the true sampling depth of population sequencing, making it an ideal tool to study mutagenic effects of potential broad-spectrum antivirals on RNA viruses. In this report we describe a protocol using Primer ID sequencing to study the mutations induced by antivirals in a coronavirus genome from an in vitro cell culture model and an in vivo mouse model. Viral RNA or total lung tissue RNA is tagged with Primer ID-containing cDNA primers during the initial reverse transcription step, followed by two rounds of PCR to amplify viral sequences and incorporate sequencing adaptors. Purified and pooled libraries are sequenced using the MiSeq platform. Sequencing data are processed using the template consensus sequence (TCS) web-app. The Primer ID approach provides an accurate sequencing protocol to measure mutation error rates in viral RNA genomes and host mRNA. Sequencing results suggested that β-D-N4-hydroxycytidine (NHC) greatly increased the transition substitution rate but not the transversion substitution rate in the viral RNA genomes, and cytosine (C) to uridine (U) was found as the most frequently seen mutation.

Rubella is a mildly contagious disease characterized by low-grade fever and a morbilliform rash caused by the rubella virus (RuV). Viruses often use cellular phospholipids for infection. We studied the roles of cellular sphingomyelin in RuV infection. Treatment of cells with sphingomyelinase (SMase) inhibited RuV infection in rabbit kidney-derived RK13 cells and African green monkey (Cercopithecus aethiops) kidney-derived Vero cells. Our data further demonstrated that RuV used cellular sphingomyelin and cholesterol for its binding to cells and membrane fusion at the step of virus entry. Detailed protocols of our assays, which assess the effects of SMase treatment on RuV infectivity in RK13 and Vero cells, are described.

In a narrow definition, virucidal activity represents the activity by which to interact with and physically disrupt viral particles. In a broad definition, it includes the activity by which to functionally inhibit (neutralize) viral infectivity without apparent morphological alterations of the viral particles. The viral infectivity can be measured in cell culture system by means of plaque assay, infectious focus assay, 50% tissue culture infectious dose (TCID₅₀) assay, etc. Morphologically, disruption of viral particles can be demonstrated by negative staining electron microscopic analysis of viral particles. In this article, we describe methods to assess virucidal activity in a broad definition.

Viruses infect their host cells to produce progeny virus particles through the sequential steps of the viral life cycle, such as viral attachment, entry, penetration and post-entry events. This protocol describes time-of-addition and temperature-shift assays that are employed to explore which step(s) in the viral life cycle is blocked by an antiviral substance(s).

Neutralizing antibodies (Nabs) are a major challenge in clinical trials of adeno-associated virus (AAV) vector gene therapy, because Nabs are able to inhibit AAV transduction in patients. We have successfully isolated several novel Nab-escaped AAV chimeric capsids in mice by administrating a mixture of AAV shuffled library and patient serum. These AAV chimeric capsid mutants enhanced Nab evasion from patient serum with a high muscle transduction efficacy. In this protocol, we describe the procedures for selection of the Nab-escaped AAV chimeric capsid, including isolation and characterization of Nab-escaping AAV mutants in mice muscle.

In 2014 enterovirus D68 (EV-D68) caused the largest outbreak in the United States since the discovery of the virus. Distinct from before, the 2014 infections were associated with more severe respiratory disease and occasional neurological complications. So far, there are no available vaccines or antivirals for the prophylaxis or treatment of EV-D68 infections. In order to evaluate the antiviral activity of potential inhibitors of EV-D68 replication, a cell-based cytopathic effect (CPE) reduction assay was developed (Sun et al., 2015).

One of the most prevalent and interfering psychosocial comorbidities of HIV infection is clinical depression (22 to 45%). For this reason, a study of a possible interaction between the nonnucleoside reverse transcriptase inhibitor nevirapine (NVP) and the tricyclic antidepressant nortriptyline (NT) was carried out. In vitro studies with rat and human hepatic microsomes showed a marked inhibition of NVP metabolism by NT being more intense in rat than in human. The extrapolation of these results to humans suggests increased NVP side effects when both drugs are coadministered, but additional in vivo human studies are required to evaluate the clinical implication of this interaction.

This protocol describes a technique for detecting and measuring the inhibition of the nevirapine metabolism by nortriptyline in hepatic microsomes.

The ribonuclease H (RNase H) polymerase-independent cleavage assay allows detection and quantification of RNase H activity of reverse transcriptase (RT) enzymes with a hybrid substrate formed by a fluorescein labeled RNA annealed with Dabcyl DNA (Figure 1). Here we describe a protocol that we have adapted for HIV-1 RT expressed from a p(His)6-tagged p66/p51 HIV-1HXB2 RT-prot plasmid and for RT of the prototype foamy virus (PFV RT).


Figure 1. Scheme of the principle of the experiment. The RNA substrate (blue) labeled with the fluorophore fluorescein (F, yellow) is annealed with complementary DNA strand (green) labeled with a quencher molecule Dabcyl (D, red). Panel A. In the intact substrate the quencher is so close to the fluorophore that it can quench the fluorescence emitted after excitation. Panel B. After the RNA substrate is cut by the RNase H a few ribonucleotides oligo labeled with the fluorescein is free to escape from the quencher, and to release fluorescence after excitation.

Antiviral agents for the suppression of hepatitis B virus (HBV) have been used for treating chronic hepatitis B. However, the emergence of drug-resistant HBV is still a major problem for antiviral treatment. To identify and characterize the drug-resistant HBV, the construction of HBV replicon and in vitro drug susceptibility assay are essential. Here we describe the experimental methods to study drug-resistant HBV.