Tijana Ivanovic, PhD
Department of Biochemistry
Brandeis University
(October 17, 2016)
Influenza Membrane Fusion: Insights From Single Virions Into Potential Avenues for Viral Evolvability
Influenza has always been a major health issue, but one that the general population does not consider overly dangerous. However, a major epidemic of influenza occurs roughly every 30 years. In 1918, the epidemic killed around 50 million people worldwide. In more recent times, the bird flu has led to serious outbreaks. But how does the flu make the jump from birds to humans? It is not the stuff of horror movies, but of genetics. Dr. Ivanovic discussed her work with an advanced microscopic technique, which allows her to image how influenza mutates and enters cells. This type of imaging can help to determine how the virus evolves and how influenza adapts to different species.
Influenza virus is well known for its ability to cause recurrent pandemics and presents an imminent threat to humanity. Pandemics occur every 30 years, on average, when a new influenza subtype mutates to infect humans from its natural avian reservoir, adapts to airborne transmission among humans, and spreads globally. Influenza outbreaks differ in severity; the deadliest one known to history is the 1918 influenza pandemic that caused about 50,000,000 deaths.
The influenza cell-entry protein, hemagglutinin (HA) is a key determinant of influenza pandemic adaptation. (H in the subtype nomenclature is based on the antigenic properties of HA.) To adapt to human infections, influenza changes HA receptor-binding preference from avian to human receptors. To adapt to airborne transmission, it produces more stable HA. Systemic infection characteristic of highly pathogenic viruses has been linked to the extent of HA activation by proteases. However, effects of molecular changes in adapted functions are multifaceted and often antagonistic, so that adaptation involves a balance between the functions that is appropriate for a particular set of selective pressures. To obtain a complete picture of adaptive molecular events, my integrated functional analysis enables separate measurements of different cell-entry functions, as well as measurements of their combined functional output.
In my talk, I summarized my recent work, which uses an advanced microscopy technique to enable unprecedented level of detail in the underlying molecular events leading to productive cell entry by influenza. I used total internal reflection fluorescence (TIRF) microscopy to image (in real time) a series of cell entry steps for hundreds of influenza virions at the resolution of individual virions. For these experiments, I created a panel of carefully chosen point mutants deriving from the known crystal structures of HA and the observed differences in cell-entry properties between two highly similar influenza strains. I further developed stochastic computer simulations to aid the interpretation of the single-virion experiment data. Combined, these analyses and their extensions to comparisons between more distant influenza strains, have set the stage for the goal of defining the molecular constraints on HA evolvability during cross-species adaptation.