When the biologist Caren Norden applied to study for a doctorate at ETH, she quickly realized that the chemistry was right. She immediately got along well with Yves Barral, ETH Professor of Biochemistry (1). “Finding a good supervisor for the doctorate was crucial for me,” says the lively German, who had initially applied to ETH laboratory blindly on the basis of a hint from her degree supervisor in the USA. Of course another factor was that the project on cell division that was offered to her exactly matched her expectations.

Because cell division is a complex process, a narrower focus was needed. Caren Norden decided to carry out research into the final abscission of the cell membrane. This means the separation of the daughter cell’s membrane from that of the mother cell. About her choice, the biologist says: “The project promised a large amount of microscopy, which is a technique that I particularly like.”

It all began with Mickey Mouse cells

Although the first experiment in the paper about her work at ETH published by Caren Norden and her research colleagues this month in the scientific journal “Cell” (2) describes the effect of the protein “Ase 1” on the cytokinesis of yeast cells, the biologist’s work at ETH was initially different. It was devoted to Mickey Mouse. That is to say she tried to find out why, as they divide, cells with a defect in the “Ndc10” gene form two buds that look like Mickey Mouse ears. Through several experiments she discovered that the reason is a cytokinesis defect and not the lacking separation of the cell wall, which would also be possible with yeast cells.

“At first I was slightly paranoid and didn’t entirely trust my own experiments,” remarks the biologist. The fact that the “Mickey Mouse” phenotype remained in spite of the break-up of the cell wall was for her sufficient proof that the membranes were still connected. She also made films showing that contraction of the membrane in ndc10-1 mutants proceeds normally before membrane separation, i.e. abscission. But that was only the beginning. In meticulously detailed work and with help from other researchers, especially Manuel Mendoza, the co-author of the paper, Caren Norden compiled data that give a new insight into how cell division proceeds, or fails to do so, after the chromosomes have divided. She found evidence that an intact central spindle is necessary for normal cytokinesis. The spindle is a structure that is formed during cell division. After arranging themselves in a plane, the chromosomes migrate along it, half into the mother cell and half into the daughter cell. In the first phase of cell division the spindle framework structures overlap in the centre, which is therefore called the central spindle.

Bois that prevent cut-off

Although the central spindle structure must be intact for cytokinesis, this does not mean that its location in the cell must be correct for this to occur, as further experiments showed. “That strengthened the suspicion that one controlling factor must be on the central spindle,” explains Caren Norden. And she was actually able to track down this factor, the Aurora Kinase Ipl1. This causes cells with a defect in the central spindle to fail to undergo cytokinesis, as happens with the ndc-10-1 mutants mentioned above. However, Ipl1 does not act on its own. To enable the protein to block a defective cell division, it needs the two redundant proteins Boi1 and Boi2. These are situated at the constriction point between the cells. That would be logical, says Norden, because the Bois can directly prevent complete separation here.

Successful cytokinesis researchers: Manuel Mendoza, Yves Barral and Caren Norden. (Photo: zVg) large

When a cell buds: ETH researchers shed more light on the process. The image shows Spc42 (red), a marker that enables the lengthening of the spindle to be followed during the cell cycle. The blue colour shows the Ras2-labelled plasma membrane, which is first of all constricted and then pinched off. (Photo: Caren Norden) large

But what is the point of the whole control system? “It is always important that faulty genetic information is not inherited,” is the biologist’s answer. Further experiments indicated that this is the most important reason. For example the researchers showed that cells lacking the Ipl1-Boi1,2 system display more chromosome breaks. Cells that also have a faulty Ase1 are no longer even viable.

A convincing story, but perhaps not yet all told

“Overall that gave us a convincing story that led us to postulate a new signalling pathway, ‘NoCut’,” is how Caren Norden sums it up. In their model, the researchers at ETH assume that Ipl1 at the central spindle establishes whether chromosomes are still present. If they are, Ipl1 sends a signal to Boi1 and 2. These migrate to the constriction site where they prevent complete cytokinesis – and no chromosome division, i.e. cut, occurs. However, if the chromosomes have migrated away from the central spindle into the newly-formed daughter cells and the spindle has disintegrated, then Ipl1 is also no longer active. The result of this is that Boi1 and 2 leave the constriction point so that the abscission can be completed.

Caren Norden glances thoughtfully at the model she has just explained with the help of a diagram. However, she soon takes up the conversation again and says: “That is only a model. We can say with certainty that Ipl1 and the Bois are necessary for cytokinesis, but I doubt whether that is the whole story.” For example she suspects that there are yet more factors involved in the NoCut signal path. She expects these to be present mainly in the detection of the genetic material at the central spindle. However, Norden will no longer take part in checking the correctness of this supposition herself. The reason is that she will soon take up a post-doc position in Cambridge, UK.

On the right track in spite of setbacks

When she arrives there she will plunge into the next research whirlpool. Whirlpool? In retrospect it was not all as smooth as it looks in the paper. “Sometimes I no longer had any motivation and feared we were barking up the wrong tree,” sighs Norden. For example on one occasion she inactivated Ipl and NDC10-1. The expectation was that Mickey Mouse cells would no longer be formed. “When I saw numerous cells with two buds in spite of it, I just gave up and went home,” is how the biologist describes her frustration. However, she says that next day she discussed it with Yves Barral. He advised her to investigate whether it might simply be the cell wall that was still attaching the buds to the mother cell. That was indeed the reason.

Caren Norden says that although setbacks are all a part of it, she never had any worries about unexpected results. The prevailing feeling was different, namely one of delight, when it became known that “Cell” had accepted her paper. “I heard about it on 30 December. I immediately phoned my colleagues from the laboratory. Then I drank a glass of champagne with my boyfriend, although I had really bought it for New Year’s Eve.” She says the enormous relief over the success caused somewhat of an anticlimax afterwards. However, that’s now all in the past and Caren Norden is proud of her work. Justifiably, since “Cell” and “Nature” both honoured her paper with special additional contributions (3)(4). Was the Cell paper the key to England as well? Her answer is No. “Sometimes I am slightly obstinate, and I wanted to be accepted because of my work as such and not simply because it had appeared in one of the best known scientific journals. That was why I submitted my application without expressly mentioning the Cell paper.”