In what follows I describe another subject which was a theme in the talk of Orion Weiner mentioned in the previous post. In the meantime I am familiar with the fact that there are techniques which allow us to see details of what is going on in cells. Here the most prominent protagonist is the green fluorescent protein (GFP) which was honoured by Nobel prizes in 2008. It allows information to be exported from the cell. This is a passive process in the sense that once the system has been prepared we just watch what happens. A more active process which is sometimes shown on video is that where a neutrophil follows the moving tip of a micropipette which is releasing a substance to which the cell is chemotactic. The subject of the present post is how it is possible to actively manipulate cells by sending in light of certain wavelengths. This may mean bathing the cell in light, illuminating certain precisely defined areas with a laser or a combination of the two.
The first type of experiment involves proteins which can be located either at the cell membrane or in the cytosol and which are fluorescently labelled so that their position can be monitored. It is possible to cause these molecules to move rapidly from the one localization to the other. This can be done on a time scale of a couple of seconds and it looks likes switching on and off a light. This can be done many times in a row. Here the effect on the cell is global. The second type of experiment has to do with localizing this type of effect. It allows patterns chosen by the experimenter to be projected onto the cell. Here coloured patches are visible. Their interpretation is that concentrations of a certain substance have been fixed according to the pattern. The third type of experiment is the most striking. Here a spot of light is moved over the cell and away from it in a certain direction. There results a long projection of the cell in that direction. On the video it looks as if the the cell is being pulled by a sticky object. All these things are done by switching on certain proteins which have been made light-sensitive.The sensitivity to light is achieved by incorporating elements which are responsible for allowing certain plants to react to light. One of the plants which acts as a source here is the favourite model organism among plants, Arabidopsis thaliana. The reference to the paper describing these results is ‘Spatiotemporal control of cell signalling using a light-switchable protein interaction’, Nature 461, 997-1001 (15 October, 2009).