Viral TV: watching influenza reproduction live in living host cells

The first microscope, which resembled a telescope, was invented and - presumably - manufactured in around 1595 by the Dutch glasses grinder Hans Jansson. The design was adapted by Galileo Galilei, finally being named the microscopium (by the Academia da Lincei). Galilei could not have envisioned what today's high-capacity microscopes can accomplish: insects like fleas (typical size 1.5-4.5 mm) can be shown by means of a scanning electron microscope in gigantic detailed exactness. Using a laser confocal microscope, yeast cells (just 1 to 5 µm across) as well as cell organelles such as the nucleus, cytoskeleton or mitochondria (around 500 nm in size) can be visualised on a screen.

In our research, which is funded by the DFG (German Research Foundation), we delve one step deeper into the cell. The questions which fascinate us are: What happens in a host cell after it has been attacked by a virus? How does the virus propagate inside the cell? And, how are new virus particles formed?

These investigations of viral infection require that we proceed into the world of nanostructures - molecules which are smaller than a thousandth part of one millimetre. In a typical infection, a virus enters a host cell. Within this host, biological processes take place leading to reproduction of the proteins that make up the virus and finally produces the next generation of viruses.

Due to its huge relevance to human health - as illustrated by the pandemic swine flu outbreak in 2009 - we focus our work on the influenza A virus. This infects cells in the lung of the host via a mechanism that usually serves for the uptake of physiologically essential substances - even at the infection stage, the virus hijacks the hosts normal processes.

Once the virus has entered the cell, the viral genome (composed of RNA, ribonucleic acid) is released. These molecules are, similar to DNA, structured like a wound ladder. Sugar molecules (ribose) form the scaffolding and molecules representing the genetic code the rungs. Influenza A includes eight distinct genetic segments that encode the information for ten viral proteins. However, the viral RNA is not usually present in the host and so will not be recognized by the cell's enzymes. To be able to use the host cell's machinery to synthesize its own proteins, the viral RNA must be transcribed into messenger RNA (mRNA). This transcription process takes place every day in our cells - because mRNA acts as a mediator, essentially reading our genome contained in the nucleus and translating it for the protein factories in the cell plasma. Once the viral mRNA has been formed, the cell is not able to distinguish it from its own mRNA and the host cell's protein factories will get to work synthesizing the viral molecules.

Until recently, it was not possible to examine mRNA in living cells. The trick that enables us to do so lies in a new probe, which was designed and synthesized in co-operation with the group of Prof. Seitz (Humboldt University, Institute of Chemistry). This probe is a molecule that binds to viral mRNA in a sequence-specific manner. The probe itself is composed as a hybrid molecule, made of protein and DNA - in principle, it resembles short DNA fragments that bind precisely to the corresponding region in the viral mRNA. In fact, the probe is similar to DNA but instead of the sugar backbone it is consisted of a peptide chain. This ideal combination gives the probe the advantage of having the stability of proteins together with the binding specificity of DNA.

The use of a special dye coupled to this probe molecule allows viral mRNA to be visualised in living cells and displayed on a screen - in effect, a live broadcast out of the infected host cell is provided to the experimenter. The viral mRNA molecules appear as luminous green structures in the host cell. We analyse the spatial distribution as well as the temporal appearance of the mRNA, providing important data about the infection cycle of the influenza A virus. Hence, we solve step-by-step a number of unresolved questions with regard to such infections.

Being able to examine the formation and localisation of viral mRNA molecules within living host cells is of great scientific interest. Based on this knowledge, more efficient and highly specific diagnostic methods could be set up. Another possible application is to perform computational modelling of a virtual infection cycle, which might be useful in identifying sensitive points as new targets for antiviral drugs.