Identification of in vivo substrates of muscle atrophy-related ubiquitin ligases MAFbx and MuRF1

Muscle wasting (also known as atrophy) occurs in many different diseases, for example in diabetes and cancer, and also when patients are immobilised following accidents or surgery. The loss of muscle that occurs in these cases is one of the major factors affecting recovery time, or indeed whether or not the patients do fully recover. In addition, as we get older, muscle wasting also occurs, which is one factor that can lead to loss of mobility. In order to develop drugs and therapies to help control muscle wasting, and to increase the quality of life for people affected by muscle wasting, we need a much better understanding of the changes which occur in muscle cells. In particular, we need to know what happens to the proteins which make up the muscle, and allow it to function properly. This application aims to work out the most important changes which occur in human muscle proteins during muscle wasting, which should allow us to develop better treatments for these conditions in the future.

Skeletal muscle atrophy is a common feature of many clinical settings, including sepsis, cancer, diabetes, AIDS, and disuse immobilisation, as well as of normal ageing. The molecular basis of this debilitating response is poorly understood, and few targets for therapeutic interventions have been identified to date. The ubiquitin-ligase (E3) enzymes MAFbx and MuRF1 are key regulators of skeletal muscle atrophy. Up-regulation of MAFbx and MuRF1 expression is a feature of a range of animal models in which muscle atrophy is induced, and has recently been shown to occur in human skeletal muscle during immobilisation-induced wasting. MAFbx and MuRF1 select and target other muscle proteins for degradation by a protease complex known as the 26S proteasome, as part of the atrophy programme. However, the substrates of MAFbx and MuRF1 enzymes are not well characterised, and the few which have been identified have not been validated in the in vivo setting of human skeletal muscle atrophy. We propose to apply cell biological and post-genomic (proteomic) approaches to identify candidate MAFbx and MuRF1 substrates. We shall validate these, and other presumed substrates, by translating our in vitro findings into models of animal as well as human skeletal muscle atrophy. These studies will further our basic knowledge of the molecular mechanisms which regulate muscle atrophy, and should also identify new targets for therapeutic interventions directed towards preventing the loss of skeletal muscle in clinical settings of atrophy and in the elderly.