A genetic model for understanding the regulation of muscle protein degradation by muscle attachment

The muscle of the tiny worm I use in my studies (C. elegans) is a surprisingly good model for muscles in human beings. It can be studied more easily and in many experimental situations that are impossible in people. At the moment, we do not understand what causes muscle wasting in people, a problem that affects the elderly and those with spinal injuries among others. My previous worm studies may offer some clues, others are applying my results to studies in human beings.
Muscle wasting occurs in response to spaceflight and immobilization. I discovered that in worms, a special complex of proteins (known as ?dense bodies? and which attach worm muscle to skeleton) were decreased after spaceflight. My experiments on Earth show that reducing these proteins causes muscle tissue breakdown. Others at my University found that a protein in human focal adhesions is decreased following cast immobilization. Additionally, mutations in focal adhesions cause Limb Girdle Muscular Dystrophies. The same proteins are found in worm dense bodies and human focal adhesions. Thus, similar changes are observed in worm and human attachment complexes and these changes cause worm muscle wasting.
We currently have no idea what the controlling and mediating factor are, if these factors also regulate human muscle wasting, nor if they differ among various clinical populations displaying wasting (e.g. trauma patients, burn patients, bed ridden patients, the elderly). I want to answer these questions. I currently want to understand one mechanism regulating wasting in worms. The ultimate goal is understanding all of the mechanisms regulating wasting in human beings. I feel that once we do, we can understand the differences between patients and identify drug targets providing appropriate treatment(s) to each group of patients. It is my hope that by first gaining an understanding of the regulation and treatment of wasting in a simple organism, we can more rapidly and cost effectively demonstrate the existence and treatment of the same processes in people.
I will also continue my efforts of communicating results directly to the public. In the past this has included making results freely available on-line, giving free talks to the public, actively engaging school children in my research, and communicating my work via reporters and radio and television presenters from around the world (in the UK these include the BBC, the Discovery Channel, CNN, and all of the ?wire? services that provide stories to newspapers).

Clinically, muscle atrophy occurs in a variety of conditions. Atrophy has significant morbidity and can be a proximal cause of death. Altered regulation of protein synthesis and/or degradation causes an intramuscular Nitrogen imbalance and consequent atrophy. Despite the health impact, we understand little of the intramuscular signalling systems tying more than a dozen extramuscular regulators of atrophy to the intramuscular protein synthesis machinery and/or multiple proteases. Nor do we comprehensively understand how these signalling systems interact with one another inside muscle. To address these issues we have developed C. elegans into the simplest genetic model for understanding the intramuscular signalling networks regulating muscle protein degradation in vivo. Thus far we have examined three extramuscular regulators and found that proteasome based muscle protein degradation occurs as the result of disruption of normal membrane depolarization (for example loss of Acetylcholine signalling or rectifying Potassium currents) and that lysosome based degradation occurs as the result of disruption of a balance between Fibroblast Growth Factor Receptor and Insulin Receptor signalling. These signals appear to work in similar or identical fashions in mammals. Recently we found decreased expression of parts of a conserved muscle attachment complex following spaceflight. We further found that mutation in or RNAi against the genes encoding these proteins results in muscle protein degradation independently from our previously studied signalling and proteolytic systems. Mutations in genes encoding members of the human attachment complex cause Limb Girdle Muscular Dystrophies. Additionally, one complex member, Focal Adhesion Kinase, may regulate both human muscle hypertrophy and atrophy. To understand how this novel, conserved, mechanism regulates degradation we propose to: 1) Identify all of the members of the complex needed to prevent degradation, 2) Determine if and how degradation observed in the cytosol is linked to myofiber disruption and loss of mobility, 3) Determine the protease responsible for cytosolic degradation and the mechanism by which it is activated in response to disruption of the muscle attachment complex. We will employ the standard genetic, reverse genetic, pharmacological, and biochemical methods that we have previously used to successfully understand other signalling systems in C. elegans. Upon completion of this work we should be able to propose rational diagnostics and, possibly, treatments for muscle degradation resulting from disruption of this attachment complex. Following or in parallel to this work we will look to extend our findings into mammalian systems with the ultimate goal of improving outcomes in clinical populations.