Polycomb repressor complex protein EZH2 regulation of muscle stem cell migration and differentiation

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Muscle is constantly damaged throughout life, due to exercise, age, sickness or poor lifestyle. But unlike many tissues in the adult body, muscle is able to repair itself through the action of specialised muscle stem cells which lie at the edges of muscle fibres. When they detect damage muscle stem cells move to the site of damage and then integrate into the damaged fibres, in a process called differentiation. Muscle weakness, for instance as we age or during disease such as cancer, is thought to occur due to an inability of muscle stem cells to repair damaged muscle, and instead connective and fat cells take the place of muscle fibres. A better understanding of the ways muscle stem cells move to the site of damage and differentiate will ultimately allow the development of therapies to prevent or treat muscle weakness.In response to damage, muscle stem cells also turn specific genes on or off, using control regions in the DNA called 'epigenetic' switches. This study aims to investigate an enzyme, EZH2, that makes changes to these epigenetic switches in the DNA. EZH2 can therefore potentially turn the correct genes on or off in response to muscle damage and so lead to repair of the muscle. Indeed we know that EZH2 can influence muscle repair, because when this enzyme does not work in the muscle of mice, the muscle is not repaired properly. But we do not know exactly which switches, and therefore genes, are controlled by EZH2 when muscle stem cells repair damaged muscle. One aim of this study is to identify those genes. We now have exciting preliminary results that reveal EZH2 not only influences differentiation, but also influences muscle stem cell movement towards sites of damage. We now wish to understand how EZH2 can control both migration of muscle stem cells towards damage and their differentiation at the site of damage. To do this we will use zebrafish larvae, because the muscle of zebrafish regenerates quickly, and the larvae are transparent so that we can easily see the movement of muscle stem cells using a microscope. We will measure how muscle stem cells move towards injury by using zebrafish larvae that have fluorescently labelled muscle stem cells. Fluorescent cells in the fish will be observed by powerful microscopes to make 4D movies and an especially developed computer software will be used to track cells. This is not something that can easily be done in other, non-transparent, animals such as mammals. Muscle repair in zebrafish also occurs in much the same way as in humans and similar genes are turned on and off in response to damage, making this a good system to study muscle repair processes.We will measure how muscle stem cells behave after injury when EZH2 is prevented from working. We will then ask at which point in the repair process EZH2 is important, as we can visualise which genes are turned on or off in muscle stem cells before, during and after injury. To show if EZH2 controls genes by epigenetic switches we will extract migrating muscle stem cells and measure the switches on the DNA. Studies of cancer cells suggest that EZH2 can also control the movement of cells separately from regulating how genes are turned on or off. We will also investigate whether EZH2 has this activity in muscle stem cells as well, by using mutated versions of EZH2 found in cancer cells and asking if they change the way that the muscle stem cells move. Our results will be important for understanding how the 'epigenetic' switches are coordinated with the movement of muscle stem cells to sites of damage to ensure that muscle repair is successful. We will also be able to identify genes that are controlled by EZH2 as muscle is repaired, and so may be able target these genes as therapies for improving muscle strength.