During viral infection, a struggle exists between cells, which contain anti-viral factors that selectively target and inhibit viral proteins and nucleic acids, and viruses, which neutralize these inhibitors and co-opt other cellular factors important for replication. The Mehle Lab studies this battle during influenza virus infection.

We focus on the influenza virus polymerase, the heart of the viral replication machinery. The polymerase is essential for viral replication mediating transcription of viral genes and replication of the viral genome. It assembles with the viral nucleoprotein (NP) and genomic RNA to form massive ribonucleoprotein complexes. The polymerase is also a major determinant of the pathogenic potential of emerging influenza viruses and a key regulator of cross-species transmission as viruses move from birds into humans. We have three key themes, all of which overlap:

How is the viral replication machinery regulated?

Influenza virus infections begin with a burst of gene expression by the replication machinery that is part of the incoming viral genome. As the infection proceeds, new polymerase and NP is synthesized and these must assemble into new RNPs to successfully replicate the viral genome. Assembly of RNPs must be controlled, otherwise off-pathway complexes dominate. We investigate how influenza virus controls RNP assembly, and how this in turn determines whether the virus dedicates its replication machinery to make more protein or to make new genomes. A major focus of our current work is the identification and characterization of the host proteins that “flip the switch” to drive assembly of new RNPs.

How does the viral polymerase impact host range and pathogenicity?

We frequently hear about influenza  outbreaks in people, but the natural reservoir for influenza virus is migratory waterfowl. In order for viruses to spread from birds to other hosts, they must first overcome barriers to cross-species transmission. In general, polymerases derived from avian viral isolates function very poorly in human cells. We have characterized numerous strategies used by the virus to adapt to humans. A major thrust of our current work is to identify and define the molecular mechanisms of host factors the drive these changes and facilitate polymerase function in birds versus humans.

Hacking the influenza genome.

Making changes to the influenza virus genome is complicated by the fact that each viral gene encodes not only proteins, but key regulatory sequences in the RNA needed for gene expression, genome replication and packaging of new genomes into virions. We have exploited the large body of work in the field to “hack” the influenza genome and create a number of useful tools for imaging and quantifying viral replication in cells and in animals. These experiments have also begun to define a set of rules on what and where sequences can be inserted. To date we’ve published viruses that glow and viruses that bioluminesce. What else should we deliver with flu?

Ongoing work in the lab uses biochemical, structural and genetic techniques to identify the viral and host factors that control polymerase function, with a particular emphasis on those that regulate cross-species transmission. Ultimately, it is hoped that our work will help predict, and possibly prevent, the cross-species transmission of influenza virus and provide new avenues for the development of antiviral therapies.