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Chemical tools for exploring cell division

The cell cycle, consisting of interphase and mitosis, forms the basis for eukaryotic growth and reproduction. Numerous control and regulatory mechanisms guarantee that the separation and distribution of the chromosomes happens synchronously with the formation of the daughter cells. Amongst other things, the two daughter cells only form when the chromosomes are far enough apart from each other. Professor Dr. Thomas Mayer from the University of Konstanz uses chemical biology methods to explore the function of cell cycle proteins. His findings might in future contribute to a better understanding of why many control mechanisms are disturbed in cancer cells.

Prof. Dr. Thomas Mayer uses protein- and cell-based screens for identifying small molecules that impact the activity of cell cycle proteins. © Thomas Mayer

The construction of the new Center for Chemical Biology at the University of Konstanz is not the only evidence that the boundaries between the fields of chemistry and biology are becoming increasingly blurred and that interdisciplinary approaches are opening up tremendous opportunities for both sides. This is also reflected in the research being carried out by Prof. Dr. Thomas Mayer, director of the Department of Molecular Genetics and head of the Screening Facility at the University of Konstanz. His group conducts basic research on cell cycle control and is specifically focused on investigating the function of mitotic proteins.

However, apart from using traditional biological methods such as siRNA, the biologist also relies on chemical biology methods. “This essentially means that we use low molecular weight compounds, i.e. small molecules, for the chemical manipulation of proteins with the aim of investigating the complex biological processes of cell division,” Prof. Mayer explains. These molecular tools can do many things: they can specifically bind to active proteins, thus providing insights into the proteins’ activity; they can alter the function and chemical properties of proteins or enable the visualisation of proteins and the places where they are located.

In addition, Mayer’s methods have some advantages over traditional molecular biology methods used for manipulating proteins. Techniques such as RNAi or transfection with a mutated gene mean that it takes quite a long time before the target protein is turned off or the altered protein variant is expressed. Chemical compounds act very quickly and without the time-consuming pretreatment of the cells. “The use of small molecules leads to a relatively high temporal resolution as they enable us to alter the activity of proteins both quickly and reversibly. This is difficult to achieve with genetic methods,” said Prof. Mayer highlighting one of the advantages of the methods he uses. “In addition they can be precisely dosed and applied; for example, these methods enable us to specifically target a specific protein function as well as inhibit entire protein families.”  

Suitable proteins for any function

The chemical compounds used by the researchers have a number of things in common: they are usually relatively small molecules (with a molecular weight of < 500 Da), not excessively hydrophilic or electrophilic, but not too hydrophobic either. “The small molecules thus fulfil similar criteria to many drugs currently on the market and are therefore referred to as pharmacophores,” explains Prof. Mayer.

Prof. Mayer uses protein- and cell-based screens to identify small molecules with a specific mode of action. Pharmacophore libraries are screened for the presence of substances that can impact the activity of a protein or a signalling pathway that controls the cell cycle in some way or other. In order to be able to investigate the large number of small molecule candidates, Mayer and his team use high-throughput screening methods as well as automated microscopes. The latter are specifically used for cell-based screens. The biologists work with bioinformaticians who develop algorithms for the automated analysis of the microscope images. If a molecule with an effect on a cell cycle protein is identified, the researchers will apply it to the cell cycle protein and use real-time microscopes to determine the cell phenotype induced by the molecule under investigation. “We use cell lines that express fluorescence-labelled cell cycle proteins in order to make it easier to identify potential phenotype changes,” explains Mayer. 

Left: Fluorescence-microscope image of a bipolar mitotic spindle (green: microtubuli; blue: spindle poles) that distributes the chromosomes (red) equally to daughter cells during mitosis. Right: Monastrol, a cell-permeable small molecule inhibitor of a mitotic kinesin, prevents spindle formation, resulting in unordered chromosome assembly and the arrest of cell division. © Thomas Mayer

From basic research to cancer therapy

Monastrol is an example of a low molecular weight compound that affects the activity of cell cycle proteins. It was discovered by Prof. Mayer during his post-doctoral studies at the Harvard Medical School (Boston, MA) in 1999. Back then, Mayer was able to show that monastrol specifically inhibits a motor protein. This particular protein, Eg5, is a member of the kinesin family and is important for establishing or maintaining the bipolar spindle and hence distribution of the chromosomes to the daughter cells. The inhibition of Eg5 through monastrol alters the protein’s ability to generate the force required to push the spindle poles apart. The spindle collapses and the cell does not divide. As the effect of monastrol is reversible and it can easily be washed out from the cells, Mayer was able to show that the removal of monastrol reversed the situation and led to the establishment of a functional spindle. “Monastrol has provided us with insights into the mechanism of spindle assembly,” said Mayer. The fact that many other Eg5 inhibitors have since been identified demonstrates that such compounds are of huge interest for the pharmaceutical industry. Some of these inhibitors are already undergoing clinical testing in order to assess their potential in cancer treatment. 

The defective or insufficient regulation of the cell cycle can have serious consequences and lead to non-functional daughter cells and the development of cancer. “Chromosomal instability, i.e. the unequal distribution of DNA to daughter cells during mitosis due to loss/gain of chromosomes or rearrangement of partial chromosomes, is a major characteristic of cancer cells,” explains Prof. Mayer. Mayer’s research has already laid important foundations for the understanding of cell division defects in cancer cells. “If we can understand the mechanism of chromosome segregation better, we will also be able to understand what goes wrong in cancer cells,” says Mayer referring to potential future impacts of his research.

Further information:
Prof. Dr. Thomas U. Mayer
Department of Molecular Genetics
Faculty of Biology
University of Konstanz
E-mail: Thomas.U.Mayer(at)uni-konstanz.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/chemical-tools-for-exploring-cell-division