Viral DNA - In vivo Observation in the Cell
Viral DNA traffics in human cells are revealed by cell biologists and chemists from the University of Zurich. They have developed a new method to generate virus particles containing labeled viral DNA genomes. This allowed them to visualize, for the first time, single viral genomes in the cytoplasm and the nucleus by using fluorescence microscopy in regular or superresolution mode. The new findings enhance our understanding of how viral disease occurs, and how cells respond to infections.
The medical, humanitarian and economical impact of viral diseases is devastating to humans and livestock. There are no adequate therapies available against most viral diseases, largely because the mechanisms by which viruses infect cells are poorly known. An interdisciplinary team of researchers from the University Zurich now presents a method that can be used to display viral DNA in host cells at single-molecule resolution. The method gives unexpected insights into the distribution of viral DNA in cells, and the reaction of cells to viral DNA.
For the study cell cultures and human adenoviruses causing respiratory disease and conjunctivitis, herpes viruses and vaccinia virus were used. Click chemistry is a widely applicable chemical reaction type. For the assay a new class of "clickable" chemical molecules was developed to label the DNA of an intact virus. The molecule is incorporated into viral DNA without affecting the biological functions of the DNA, and it can be used to label the DNA for fluorescence microscopy.
Defense Response is Visible
Human cells in culture were infected with the chemically labeled viruses, and the behavior of the viral DNA during entry into cells was observed. Using this elegant method, it was revealed that not all the incoming viral DNA enters the cell nucleus as originally expected, but a significant fraction remains in the cytosol, the fluids of the cytoplasm. This phenomenon may be part of the antiviral defense reaction. For the first time, the localization of incoming viral DNA, and link it to anti-viral defense or infection mechanisms can be displayed. The researchers show that cells of the same type take up different amounts of viral DNA into their nucleus.
They suspect that the nucleus has antiviral defense reactions, akin to the cytosol, and these defense reactions are variable between cells. With the new method in hand, this is now subject to future studies. The scientists suggest that their procedure can be applied to other DNA viruses, or the HI virus (HIV).
 Wang I.-H. et al.: Cell Host Microbe, October 16, 2013.
I-Hsuan Wang, Maarit Suomalainen, Vardan Andriasyan, Samuel Kilcher, Jason Mercer, Anne Neef, Nathan W. Luedtke, Urs F. Greber: Tracking Viral Genomes in Host Cells at Single-Molecule Resolution. Cell Host & Microbe - 16 October 2013 (Vol. 14, Issue 4, pp. 468-480)