Mechanisms of activity-dependent muscle growth and repair

Muscle mass affects us throughout our lives. It is a major predictor of bone strength, sporting performance, career choice, propensity to obesity and diabetes and the development of debilitating muscle weakening in the elderly, which often leads to physical dependency. We know that people differ in muscle mass at birth due to genetic, epigenetic and environmental factors that impinge on mother and child during pregnancy. Although muscle grows hugely after birth, there is a strong correlation between muscle mass at birth and in later life. Muscle is made from several kinds of stem cells formed in the early embryo, yet how muscle mass is determined during prenatal life is unclear. This proposal aims to find out by understanding the fundamental cell and molecular biological processes that control the size of muscle tissue in a simple system.

Skeletal muscle approaches 30-40% of human body mass, depending on sex and age. In addition to one's endowment at birth, physical activity during childhood and adolescence is thought to influence adult muscle mass, contractile properties and the balance between muscle and fat, which is a major predictor of healthspan. Measures of muscle/fat ratio correlate negatively with incidence of type 2 diabetes, coronary heart disease, stroke, colon, breast and other cancers, osteoarthritis, age-related muscle wasting and, of course, obesity. Although cause and effect are debated in these correlations, there is a consensus that a more 'athletic' physique (i.e. higher muscle/fat ratio) is likely to improve the health and outlook for a significant fraction of the population, with consequent economic and quality of life benefits for society as a whole.

It is clear that 'environmental' effects, like exercise and food consumption, interact to control muscle mass, but they work on the tissue formed during earlier life. Emerging evidence indicates that exercise has long-term influences on whole body metabolism not just due to direct training effects on muscle itself, but also because exercised muscle releases signals that control growth of fat, heart and other tissues. There is thus a need to understand how muscle mass is controlled and is affected by physical activity. Our recent findings show that very early muscle tissue requires physical activity for normal growth and, within limits, has a remarkable ability to regulate its mass. We have also developed methods of watching the formation and growth of muscle from several distinct populations of stem cells in the living tissue. This proposal aims:

1) To understand the molecular mechanism(s) by which physical activity controls muscle growth. This study, while aimed at fundamental insight into development, will provide clues to the mechanisms by which training builds muscle in the elderly, athletes and for general health. By understanding how activity and the force it produces regulate muscle growth, the work will shed light on how force controls cell behaviour more generally.

2) To discover how muscle tissue balances proliferation and differentiation of stem cells to control formation of the correct number of muscle fibres. By providing understanding of how muscle is built, these studies will reveal how our genes and early life experience interact to generate the muscular 'starting point', which, together with the vicissitudes of later life, controls general health and the onset of age-related diseases.

3) To study how muscle tissue reacts to damage in early life and regenerates the correct amount of muscle. As muscle regeneration is important in athletes and fails in the late stages of many muscle diseases and in the elderly, this work may influence clinical practice by providing insight on how best to trigger appropriate muscle regeneration.

Our aim is to understand the role of stem/precursor cell diversity and physical activity or the force it elicits in skeletal muscle growth and the generation of muscle mass. We study somitic muscle growth and regeneration in vivo using zebrafish larvae expressing genetic reporters by 3D time-lapse confocal microscopy using molecular genetics, physiological recordings and manipulations and a variety of molecular analytical approaches. We have developed a short-term quantitative growth assay and shown that muscle contraction is necessary for normal larval muscle growth. We have identified distinct muscle precursor cells that reside in different somite regions, participate differently in muscle growth and repair and express different muscle stem cell markers. We have three Specific Aims:

1. To distinguish the role of force and activity on muscle growth.
By using our rapid growth assay that gives high content data of great precision combined with next generation sequencing and genome editing, we will determine the time course, perdurance and molecular nature of the growth stimulus.

2. To determine the function of stem/precursor cell diversity in muscle growth.
By following the behaviour of individual cells and cell clones as they contribute to growth after specific cellular and genetic manipulations we will distinguish the contribution and mechanism(s) controlling a) new fibre formation, b) cell fusion to pre-existing fibres and c) increase in fibre volume during muscle growth.

3. To analyse the cellular and molecular control of muscle wound repair.
By applying the above approaches, we will determine the similarities and differences between normal muscle growth and reactive regeneration from the same populations of cells.

Using our past experience with a wide range of model organisms and human myogenesis, we are engaging with clinicians to refine our studies and explore the significance of our findings in the context of human health.

Who will benefit from this research?

In addition to scientists in our fields, in the long-term, clinical researchers in obstetrics, neonatology, physiotherapy, myology, diabetes, rheumatology and geriatrics will benefit by using our data in screens for the effects on lifetime health of a) genetic makeup, b) in utero experience and c) childhood engagement in exercise/sport. Our research has impact on school and university students from various disciplines and on the general public through advice and images. We are a world-leading laboratory studying muscle development and thus help maintain the UK's leading position as a global research hub, attracting talented researchers form across the world and encouraging industry decisions to locate in the UK. Through our collaborations with other European, Israeli and Turkish scientists we contribute to the development for the European Research Area. Although few/no charities or patient groups centre specifically on poor ageing as a consequence of early life experience, focusing upon such issues may provide a major pathway to improve population health, reduce lifetime health care costs and increase wellbeing, benefitting UK and global populations.

How will they benefit from this research?

Our research fully supports MRC's Strategic Aims by using quality science to improve health. Perhaps the most cost effective long-term outcome of our work will be by stimulating clinical research leading to evidence-based policy making on lifestyle advice to parents on how to decrease children's lifetime disease/disability risk.

Our work is predominantly fundamental research aimed at understanding developmental processes controlling muscle tissue size and growth capacity and the role of activity/force in those processes. Clinical researchers will use our data to guide the search for genetic variants and interacting environmental/lifestyle events that control propensity/susceptibility to a wide range of diseases of later life. Our work may inform new initiatives in cohort studies, such as The Life Study at UCL or analyses in the UK Biobank. Our findings will define molecular signatures of muscle growth that will permit probing of human population datasets, for example the output of the 100K Genome Project. There is a likely close link between muscle mass and metabolic disease, so charities such as Diabetes UK and their researchers may find our data helpful. By illuminating mechanisms of muscle repair and homeostasis, we will impact researchers and charities interested in muscle wasting conditions, such as Muscular Dystrophy UK. Our studies may impact those investigating muscle repair after major trauma. To achieve these goals, our work will need to impact further studies on human muscle, something we will develop through collaboration in the coming years, preferably using existing tissue banks.

Experience shows that school students greatly enjoy brief lab visits, but prefer to actually participate in research. We provide lay summaries of our research output [1], host students for summer projects through the Nuffield Research Placements and other schemes [2] and participate in various KCL Open Days and widening participation events [3].

We provide an excellent training environment. In addition to our full group members, in the last year we have hosted eight postgraduate students, postdocs and faculty from other labs (three international) with common muscle interests to do experimental projects in zebrafish genetics. This enriches our own research, and provides great value for money from our grants, as the marginal cost in zebrafish research is low (about £200 per extra fish line p.a.).

[1] http://www.kcl.ac.uk/lsm/research/divisions/randall/research/sections/signalling/hughes/index.aspx
[2] http://www.nuffieldfoundation.org/nuffield-research-placements
[3] http://www.kcl.ac.uk/study/ug/openday/index.aspx