Isolation and Analysis of Rare Norovirus Recombinants from Coinfected Mice Using Drop-Based Microfluidics.

UNLABELLED: Human noroviruses (HuNoVs) are positive-sense RNA viruses that can cause severe, highly infectious gastroenteritis. HuNoV outbreaks are frequently associated with recombination between circulating strains. Strain genotyping and phylogenetic analyses show that noroviruses often recombine in a highly conserved region near the junction of the viral polyprotein (open reading frame 1 [ORF1]) and capsid (ORF2) genes and occasionally within the RNA-dependent RNA polymerase (RdRP) gene. Although genotyping methods are useful for tracking changes in circulating viral populations, they report only the dominant recombinant strains and do not elucidate the frequency or range of recombination events. Furthermore, the relatively low frequency of recombination in RNA viruses has limited studies to cell culture or in vitro systems, which do not reflect the complexities and selective pressures present in an infected organism. Using two murine norovirus (MNV) strains to model coinfection, we developed a microfluidic platform to amplify, detect, and recover individual recombinants following in vitro and in vivo coinfection. One-step reverse transcriptase PCR (RT-PCR) was performed in picoliter drops with primers that identified the wild-type and recombinant progenies and scanned for recombination breakpoints at ∼1-kb intervals. We detected recombination between MNV strains at multiple loci spanning the viral protease, RdRP, and capsid ORFs and isolated individual recombinant RNA genomes that were present at a frequency of 1/300,000 or higher. This study is the first to examine norovirus recombination following coinfection of an animal and suggests that the exchange of RNA among viral genomes in an infected host occurs in multiple locations and is an important driver of genetic diversity. IMPORTANCE: RNA viruses increase diversity and escape host immune barriers by genomic recombination. Studies using a number of viral systems indicate that recombination occurs via template switching by the virus-encoded RNA-dependent RNA polymerase (RdRP). However, factors that govern the frequency and positions of recombination in an infected organism remain largely unknown. This work leverages advances in the applied physics of drop-based microfluidics to isolate and sequence rare recombinants arising from the coinfection of mice with two distinct strains of murine norovirus. This study is the first to detect and analyze norovirus recombination in an animal model.

Artifact-Free Quantification and Sequencing of Rare Recombinant Viruses by Using Drop-Based Microfluidics.

Recombination is an important driver in the evolution of viruses and thus is key to understanding viral epidemics and improving strategies to prevent future outbreaks. Characterization of rare recombinant subpopulations remains technically challenging because of artifacts such as artificial recombinants, known as chimeras, and amplification bias. To overcome this, we have developed a high-throughput microfluidic technique with a second verification step in order to amplify and sequence single recombinant viruses with high fidelity in picoliter drops. We obtained the first artifact-free estimate of in vitro recombination rate between murine norovirus strains MNV-1 and WU20 co-infecting a cell (P(rec) = 3.3 × 10(-4) ± 2 × 10(-5) ) for a 1205 nt region. Our approach represents a time- and cost-effective improvement over current methods, and can be adapted for genomic studies requiring artifact- and bias-free selective amplification, such as microbial pathogens, or rare cancer cells.

Whole-Genome Sequencing of a Single Viral Species from a Highly Heterogeneous Sample.

Metagenomic studies suggest that only a small fraction of the viruses that exist in nature have been identified and studied. Characterization of unknown viral genomes is hindered by the many genomes populating any virus sample. A new method is reported that integrates drop-based microfluidics and computational analysis to enable the purification of any single viral species from a complex mixed virus sample and the retrieval of complete genome sequences. By using this platform, the genome sequence of a 5243 bp dsDNA virus that was spiked into wastewater was retrieved with greater than 96% sequence coverage and more than 99.8% sequence identity. This method holds great potential for virus discovery since it allows enrichment and sequencing of previously undescribed viruses as well as known viruses.

Residues of the UL25 protein of herpes simplex virus that are required for its stable interaction with capsids.

The herpes simplex virus 1 (HSV-1) UL25 gene product is a minor capsid component that is required for encapsidation, but not cleavage, of replicated viral DNA. UL25 is located on the capsid surface in a proposed heterodimer with UL17, where five copies of the heterodimer are found at each of the capsid vertices. Previously, we demonstrated that amino acids 1 to 50 of UL25 are essential for its stable interaction with capsids. To further define the UL25 capsid binding domain, we generated recombinant viruses with either small truncations or amino acid substitutions in the UL25 N terminus. Studies of these mutants demonstrated that there are two important regions within the capsid binding domain. The first 27 amino acids are essential for capsid binding of UL25, while residues 26 to 39, which are highly conserved in the UL25 homologues of other alphaherpesviruses, were found to be critical for stable capsid binding. Cryo-electron microscopy reconstructions of capsids containing either a small tag on the N terminus of UL25 or the green fluorescent protein (GFP) fused between amino acids 50 and 51 of UL25 demonstrate that residues 1 to 27 of UL25 contact the hexon adjacent to the penton. A second region, most likely centered on amino acids 26 to 39, contacts the triplex that is one removed from the penton. Importantly, both of these UL25 capsid binding regions are essential for the stable packaging of full-length viral genomes.

Role of the UL25 protein in herpes simplex virus DNA encapsidation.

The herpes simplex virus protein UL25 attaches to the external vertices of herpes simplex virus type 1 capsids and is required for the stable packaging of viral DNA. To define regions of the protein important for viral replication and capsid attachment, the 580-amino-acid UL25 open reading frame was disrupted by transposon mutagenesis. The UL25 mutants were assayed for complementation of a UL25 deletion virus, and in vitro-synthesized protein was tested for binding to UL25-deficient capsids. Of the 11 mutants analyzed, 4 did not complement growth of the UL25 deletion mutant, and analysis of these and additional mutants in the capsid-binding assay demonstrated that UL25 amino acids 1 to 50 were sufficient for capsid binding. Several UL25 mutations were transferred into recombinant viruses to analyze the effect of the mutations on UL25 capsid binding and on DNA cleavage and packaging. Studies of these mutants demonstrated that amino acids 1 to 50 of UL25 are essential for its stable interaction with capsids and that the C terminus is essential for DNA packaging and the production of infectious virus through its interactions with other viral packaging or tegument proteins. Analysis of viral DNA cleavage demonstrated that in the absence of a functional UL25 protein, aberrant cleavage takes place at the unique short end of the viral genome, resulting in truncated viral genomes that are not retained in capsids. Based on these observations, we propose a model where UL25 is required for the formation of DNA-containing capsids by acting to stabilize capsids that contain full-length viral genomes.

Labeling and localization of the herpes simplex virus capsid protein UL25 and its interaction with the two triplexes closest to the penton.

The herpes simplex virus type 1 UL25 protein is one of seven viral proteins that are required for DNA cleavage and packaging. Together with UL17, UL25 forms part of an elongated molecule referred to as the C-capsid-specific component (CCSC). Five copies of the CCSC are located at each of the capsid vertices on DNA-containing capsids. To study the conformation of UL25 as it is folded on the capsid surface, we identified the sequence recognized by a UL25-specific monoclonal antibody and localized the epitope on the capsid surface by immunogold electron microscopy. The epitope mapped to amino acids 99-111 adjacent to the region of the protein (amino acids 1-50) that is required for capsid binding. In addition, cryo-EM reconstructions of C-capsids in which the green fluorescent protein (GFP) was fused within the N-terminus of UL25 localized the point of contact between UL25 and GFP. The result confirmed the modeled location of the UL25 protein in the CCSC density as the region that is distal to the penton with the N-terminus of UL25 making contact with the triplex one removed from the penton. Immunofluorescence experiments at early times during infection demonstrated that UL25-GFP was present on capsids located within the cytoplasm and adjacent to the nucleus. These results support the view that UL25 is present on incoming capsids with the capsid-binding domain of UL25 located on the surface of the mature DNA-containing capsid.

Polarized DNA ejection from the herpesvirus capsid.

Ejection of DNA from the capsid is an early step in infection by all herpesviruses. Ejection or DNA uncoating occurs after a parental capsid has entered the host cell cytoplasm, migrated to the nucleus, and bound to a nuclear pore. DNA exits the capsid through the portal vertex and proceeds by way of the nuclear pore complex into the nucleoplasm where it is transcribed and replicated. Here, we describe use of an in vitro uncoating system to determine which genome end exits first from the herpes simplex virus 1 capsid. Purified DNA-containing capsids were bound to a solid surface and warmed under conditions in which some, but not all, of the DNA was ejected. Restriction endonuclease digestion was then used to identify the genomic origin of the ejected DNA. The results support the view that the S segment end exits the capsid first. Preferential release at the S end demonstrates that herpesvirus DNA uncoating conforms to the paradigm in double-stranded DNA bacteriophage where the last end packaged is the first to be ejected. Release of herpes simplex virus 1 DNA beginning at the S end causes the first gene to enter the host cell nucleus to be alpha4, a transcription factor required for expression of early genes.