A new study catches viruses in the act of infecting human cells. A labeling technique for tracking the movements of single molecules in real-time has now been used to get a look at the step-by-step path of viral infection. The study may open the way to the development of more efficient gene therapy vectors and antiviral drugs.

A virus typically makes contact with a cell, gets inside it and is transported into the nucleus where it replicates. Until now, the different stages of the infection pathway have been visualized using methods—such as electron microscopy—that cannot be used in living cells. In the 30 November issue of Science, a team lead by Christoph Bräuchle at the Ludwig-Maximilians University in Munich, Germany report that they have obtained real-time images of single viruses moving inside host cells. The low degree of labeling per virus and the low concentration of viruses per cell ensures natural and physiological conditions to study the infection process, according to the researchers.

The study used the adeno-associated virus (AAV)—a virus often used in gene therapy studies—as a model system. The researchers tagged individual viruses with one or two molecules of the dye Cy5. They then infected HeLa cells with viral particles (10–1,000 per cell) and took snapshots every 40 milliseconds to construct two-dimensional projections of single AAV trajectories. The measurement of probably thousands of trajectories yielded a picture of unprecedented detail of the various stages of virus infection.

There were a few surprises. For one thing, AAV punched through the cell membrane faster than expected (in about 64 milliseconds) and was in the nucleus within about 15 minutes. Although in most cases viruses diffused freely in the cytoplasm, some of them started to move at constant speed and along well-defined pathways once inside the nucleus. All trajectories inside the nucleus were oriented in roughly the same direction. Based on these observations, the researchers suggested that there are physical tracks in the nuclear membrane that carry viruses—although such structures have yet to be characterized.

The photo represents a frame from a 'virus documentary' with the trajectory of a single AVV particle traveling in the cytoplasm of the cell shown in red. This particular virus zipped around the cytoplasm and bounced off the nuclear membrane (outlined with a yellow circle) a few times, but never managed to get inside the nucleus—or at least not on tape. For a movie of viral infection, visit http://www.single-virus-tracing.com.