At The Meeting: Must-See TV-- Flu Infects Living Cell

Newswise — It may be cold comfort during your next bout with the flu, but you can now watch as individual influenza viruses infect a living cell. This microscopic "reality TV" show comes to your sickbed through a state-of-the-art optical imaging technique perfected by Xiaowei Zhuang and colleagues at Harvard University. Addressing a minisymposium on endocytosis at the American Society for Cell Biology's 43rd Annual Meeting in San Francisco, Zhuang described how influenza virus behavior can now be studied up close and live.

"The Flu Show" is not a comedy. Every winter, tens of thousands die of the flu in the northern hemisphere. This figure is a fraction of the estimated 20 million people who perished in the 1918 influenza pandemic. Current research is driven by a well-founded fear of new virulent strains and a scientific passion to know exactly how this infamous virus operates. We now understand that the influenza virus enters cells by receptor-mediated endocytosis: the virus is first engulfed by the cell surface membrane, transferred to early endosomes and then delivered to late endosomes. The viral membrane then fuses with the endosomal membrane, releasing viral genes into the cytoplasm. These genes are imported into the nucleus where they exploit cellular machinery for gene expression and replication.

Despite decades of effort, many aspects of influenza entry into cells remain mysterious. Researchers are still debating how the virus is endocytosed into cells and moved to late endosomes. Zhuang developed an innovative optical imaging technique that allowed her lab to follow in real time the motion of a single virus, probe its local environment, and watch it fuse with an endosome. The result is Zhuang's video of a never-before-witnessed sight: the transport and fusion of a single influenza virus inside a living cell (http://zhuang.harvard.edu/flu.html).

The video shows that the virus moves actively, in stages. In the first stage, the virus moves slowly in the cell periphery, depending on actin filaments for propulsion. Then, the virus adopts a very rapid and unidirectional movement towards the nucleus, its motion directed by a motor protein, dynein, running on microtubules. In the last stage, the virus exhibits intermittent movement involving both plus- and minus-end-directed microtubule-based motors.

In another experiment, Zuang and colleagues used the imaging system to probe the local acidity sensed by the virus, as an acidic environment is critical to viral fusion (http://zhuang.harvard.edu/flu.html). Influenza viruses were previously thought to experience acidification in two steps: first in early endosomes at the cell periphery and subsequently in late endosomes near the nucleus. In contrast, the imaging system showed that most viruses are first acidified only after they arrive near the nucleus by dynein-directed rapid movement. This suggests a new scenario for the endocytic pathway.

Zhuang and colleagues discussed these intracellular movements of the influenza virus in the August 5, 2003, issue of PNAS. The lab is now pressing forward to visualize 'step one,' the mechanism(s) used by the virus to enter cells. Zhuang hopes to soon have new videos of that sobering sight.

Visualizing infection of individual influenza viruses, X. Zhuang, M. Lakadamyali, M. J. Rust, H. P. Babcock, F. Zhang; Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA