Skeletal muscle is the largest and one of the most important tissues in the body. Not only does it perform obvious functions in allowing us to move and perform all the necessary physical tasks of daily living, it also has numerous other vital functions which are fundamental to health. These include being the most important source for the uptake of glucose in the body. However, there are numerous conditions where muscle loss occurs and "quality" declines, with the accumulation of intramuscular fat and fibrosis impairing both contractile and metabolic function. This occurs particularly in the muscles of frail elderly people. Given the fact that people are now living longer, this is resulting in a dramatically increasing population of people affected. Furthermore, there are also numerous diseases such as obesity and type 2 diabetes, as well a range of muscular diseases where fibro-fatty accumulation occurs. However, despite the prevalence the basic biological processes that influence these changes remain unclear. We have recently demonstrated that a population of cells resident within human skeletal muscle, called "fibroblasts" (which give rise to fibrosis), are also the cells that have the capability of giving rise to fat cells and fatty deposits.. The work outlined in this application is targeted at uncovering the molecular mechanisms which drive the process of a fibroblast to become a fat cell, in order to try to prevent or ameliorate fibro-fatty accumulation. With the combined expertise of biomedical researchers at King's College London and GSK, our collaborating industrial partner, we specifically aim to identify the molecules that initiate the events that cause a fibroblast to change into a fat cell; to chart the events that underlie the waning of the fibroblast characteristics and the waxing of those events that produce a fat cell; determine if the fibroblasts are viable and give rise to fat cells in vivo; and show that if you ablate the fibroblasts in skeletal muscle whether this prevents or impairs the fatty accumulation. To achieve these aims this grant brings together state-of-the-art techniques in human cell culture, cell imaging, RNAdeep sequencing, and in vivo work. It is a multidisciplinary collaboration of expertise in muscle biology at King's College with GlaxoSmithKline who have a parallel research programme targeted at muscle ageing and regeneration. The results of these experiments promise new insights into the mechanisms driving cell fate in skeletal muscle and have the potential to form the basis of new therapeutic agents directed at preventing fibro-fatty replacement in muscle.