This post was written first for the Birkbeck College events blog and is re-posted here with permission.
There are many different types of cancer but each is caused by the failure of a cell to control its normal healthy cell division. Uncontrolled proliferation produces a cluster of cancerous cells, a tumour, often the first indicator of many cancer types. Several prevalent cancer drugs target microtubules, which are used by the cell to orchestrate the intricate ballet of cell division, but disabling this machinery provokes various unpleasant associated side effects. In this lecture, which was part of Science Week, Dr Carolyn Moores, of the Department of Biological Sciences at Birkbeck shared some exciting recent developments in her work which pave the way for new cancer drugs which are less toxic to the rest of the body.
To understand the size of the machinery under discussion, if a human body were amplified to fill the whole of Buckingham Palace, each cell would be approximately the size of a single grain of sand. These cells need to replicate, both to grow and to repair normal wear and tear and this replication is a delicate and highly regulated process. Dr Moores showed a video of a dividing cell, taken from a remarkable online library of cellular images. This process starts with the separation of the chromosomes, the familiar four legged bearers of our DNA, of which there are 23 pairs in humans. The chromosomes arrange themselves in the centre of a spindle-like framework, which then retracts in opposite directions, separating each chromosome into halves and grouping them into two new nuclei centres, ready for the division of the rest of the cell.
|An image of mitosis showing the microtubules (in green) in spindle formation and chromosomes (in blue) about to be divided|
The dynamic spindle framework at the heart of this incredibly accurate mechanism is primarily composed of microtubules along with associated proteins including members of the kinesin family of molecular motors, which organise them. Microtubules are made up of pairs of tubulin molecules, or dimers, each of which has a polar structure. The dimers bind to each other both longitudinally, with opposites attracting so that the overall polarity is maintained, and laterally so that long thin stable cylinders are formed. These cylinders can grow and shrink with great flexibility and at all lengths the cylindrical structure provides a frame which can withstand the tension required in pulling chromosomes apart. Drugs that block the dynamics of microtubules can therefore block the ability of cells to proliferate, which is why they are used in chemotherapy. Unfortunately, microtubules are also critical in healthy cell repair, as well as providing frameworks essential for cellular structure, organelle positioning and vital transport networks within the cell.
Kinesins are highly attractive as potential targets since each appears to operate primarily in support of one of the major microtubule functions, in which case an inhibitor could be designed to attack the cell’s ability to divide without affecting its other vital processes.
Kinesin proteins comprise several domains, one of which is the motor domain, responsible for the protein’s movement. This motor binds to a microtubule and uses it as a track with a directionality given by its polarity. It also binds ATP, the universal cellular fuel, which provides the energy required to move along the track. This has been particularly well studied in kinesin 1, whose function is to transport cargo along the microtubules. In a mechanism that Dr Moores compared to a child walking on his hands, each unit of cargo is transported by a linked pair of kinesin 1 molecules. The molecules alternate so that one will bind to the microtubule and the energy source, ATP, and then its partner will displace it so that the motor effectively steps hand over hand along the microtubule track. Structural studies of kinesin 1 bound to a microtubule show that a small linker region of each kinesin reacts with the polarity of the track to point and presumably inform the direction of travel.
Dr Moores’ group are studying kinesin 5, which combines into oligomers of four molecules and forms crosslinks between microtubules. This has been shown to be essential to cell division in humans. The structural studies have involved cryo electron microscopy which has given a 3D model of the motor domain of kinesin 5 bound to a microtubule, both binding ATP and without ATP present. Electron microscopy is a technique much like ordinary microscopy except that an electron beam is used instead of visual light and this gives images at a molecular level. This technique is covered in detail in the TSMB course, one of the options following PPS for students working towards an MSc. I have included the link although current PPS students will not have access to the course material. The fact that a fast freezing method is employed is extremely useful since biological samples are effectively viewed in solution, as they are in their natural state. By fitting x-ray crystallography models, which give atomic level detail of smaller molecular configurations, into the 3D cryo electron microscopy models, an enormous level of detail is obtained.
Drugs that target kinesin 5 are currently in clinical trials and appear to be successful so far. It would appear that the drugs interact with an on/off switch elucidated by Dr Moores’ team but at the moment the precise function of the on/off switch is not known. PDB 1Q0B shows an example of an inhibitor bound to kinesin 5 at the loop which functions as a switch. Work continues in this rewarding area of study with the aim of understanding the purpose of the on/off switch and consequently being able to design future cancer drugs which have even higher specificity and consequently better outcomes.
Dr Moores has written a section of the TSMB course covering structural investigation of kinesin 5. Again, unfortunately, current PPS students will not yet have access to this link. The piece is taken from the last section of TSMB, which showcases the application of the many different structural techniques covered in the course to the study of a wide variety of proteins.
More details of Dr Carolyn Moores’ work are available at http://www.bbk.ac.uk/biology/our-staff/academic/carolyn-moores/