Enter the virus

Single-molecule imaging has enabled Christoph Bräuchle, Michael Hallek (Universität München, München, Germany), and colleagues to track the infection pathway of adeno-associated virus (AAV) as it enters the cell. The AAV particles are tagged with a single molecule of fluorophore and are then individually tracked in real-time at a resolution of 40 nm and 10 msec.

The AAV first bobbles along the surface of the cell, often making several contacts before it undergoes a rapid endocytosis event. Most of the motion from there to the nucleus is diffusional, possibly due to the small size and therefore high diffusibility of the virus. There is, however, some directed movement that is microtubule-dependent.

In the nuclear area, the proportion of microtubule-dependent, directed movement actually increases. Microtubules are not known to exist in interphase nuclei, so the authors suggest that the virus is travelling along invaginations in the nuclear envelope that have been observed by others. These invaginations may provide rapid access to the nuclear volume for both cellular materials and virus particles. Indeed, the speed of infection (with nuclear entry as an endpoint) is fast. Previous estimates of a two-hour infection were limited by the need to detect multiple virus particles. The new methods yield a figure of just 15 min.Bräuchle says the method can be used to track any virus, and might help gene therapists understand where their virus of choice is getting held up during the infection process. Meanwhile, he has made virus labeled on both capsid and DNA, so that he can track the exact location and kinetics of viral disassembly. ▪

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