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Marjolein Koopman (promotion date: 21 April 2006)

Nanoscale cell membrane organization; a near-field optical view.

Promotion Date: 21 April 2006

Marjolein Koopman

I have looked at cell membranes and mainly cell membranes of human cells, dendritic cells and monocytes both cells of our immune system. For fighting infectious diseases it is very important how the proteins and lipids are organized in the cell membrane. Though it is known that organization is important it is not known how the cell membrane is organized in detail. On average a protein is about 5 nanometer in size, which is below the resolution of a normal light microscope. You could use an electron microscope, but the disadvantage of that is that you have to use dry cells and measure under vacuum conditions.

What is your thesis about?

I have looked at cell membranes and mainly cell membranes of human cells, dendritic cells and monocytes both cells of our immune system. For fighting infectious diseases it is very important how the proteins and lipids are organized in the cell membrane. Though it is known that organization is important it is not known how the cell membrane is organized in detail. On average a protein is about 5 nanometer in size, which is below the resolution of a normal light microscope. You could use an electron microscope, but the disadvantage of that is that you have to use dry cells and measure under vacuum conditions..
It is obvious that this is not the most natural condition for a cell, and we would like to study them in surroundings more or less true to life, e.g. in liquid conditions. Optical microscopy is compatible with imaging in liquid conditions, but the wavelength of light determines the details that can be resolved (300 nanometers when working with visible light). During my PhD project I worked with an optical microscope that can beat this so called diffraction limit.

Does that mean that you developed a new microscope?

The first year of my research I improved the microscope and the other three I studied the organization of different proteins and lipids in different cells. The microscope, which is in fact a near-field scanning optical microscope, could only operate in air when I started my project.. The heart of the microscope is a glass fiber tip with a small hole, the size of the hole determines the details you can resolve. The distance between tip and cell membrane must be maintained at 10 to 20 nanometer in order to see something. A quartz tuning fork mounted on the glass fiber tip, for which we monitor the resonance frequency, is used to maintain a constant tip-cell distance. The disadvantage was now, that the little tuning fork does not work when dipped into a liquid. This is why I made a kind of diving bell around it. I know it sounds ludicrously simple, but it worked.

And with this construction you could look at a living cell?

In my case the cell was still fixated with a chemical, but I looked at it in liquid conditions, so without drying.

But do I understand correctly that you are the first to have looked at a cell membrane in this detail?

Yes. That was really spectacular. After I got my diving bell principle working, - nobody believed it ever would – I looked at protein and lipid organization in the cell membrane and found interesting things that are in part contradicting the current literature on the subject. I found that lipid rafts (islands of lipids) in the cell membrane exist, but they are very small and the composition is different than expected based on other experiments. Currently, I am working on a number of publications about it.

Can you draw conclusions from the organization of the cell membrane as to the functioning of the cell?

Not based only on near-field optical images. You need to complement these experiments with experiments on living cells. Of course that is the next challenge!

What did you like best about your research?

I really liked to explore a new problem in depth and the discussions with colleagues about it. I collaborated mainly with biologists in Nijmegen.

What are you going to do next?

I am not going to continue with this research. I found that I am more a person for short term goals, so I think at this moment that a job in industry would suit me better.

For the summary of the thesis, click here.