Muscle atrophy is associated with increased mortality in patients with chronic malnutrition, denervation, sepsis, cancer, diabetes, renal failure, heart failure, AIDS and in aging patients. In some cases, extreme muscle wasting is itself the proximal cause of death. Atrophy is a regulated process, and prior work has identified both extra-muscular regulators (e.g., insulin, IGF-1, TNF-cc) and intramuscular enzymes that degrade proteins (e.g., proteasomes, calpains, lysosomes). However, we lack a comprehensive understanding of the molecular signaling networks that tie extra-muscular regulators to intramuscular proteases. We have developed a model for discovering genes involved in muscle protein degradation and for analyzing the signaling networks that regulate protein degradation, using the simplest genetically tractable organism available for studying innervated muscle, the nematode Caenorhabditis elegans. As in human muscle, proteolysis in C. elegans muscle is triggered by starvation, dietary restriction, or denervation and responds to altered levels of growth factors (IGF, FGF, and TGF-B). Some of the signaling pathways that mediate proteolysis in response to each of these physiological alterations are interrelated while others appear independent. As in human muscle, there is differential regulation of protein degradation in different subcellular compartments. We have examined a total of 189 genes by mutation or RNA- mediated interference (RNAi) and identified 47 genes that appear to be involved in regulating muscle protein degradation. To build toward a comprehensive model of the signaling networks that control protein degradation, we propose to examine all """"""""muscle mutant"""""""" genes and all genes encoding a protein kinase or phosphatase, to: (1) Identify genes whose products regulate muscle protein degradation in muscle cytosol, mitochondria, nuclei, and/or myofibers by (a) Conducting RNAi knockdown of """"""""muscle mutant"""""""" genes, (b) Examining dominant """"""""muscle mutant"""""""" alleles for effects on protein degradation, and (c) Conducting RNAi knockdown of protein kinase and phosphatase genes;and (2) Functionally cluster the genes that induce muscle protein degradation by (a) Testing if these genes are related to the known network of regulatory signals, (b) Establishing new clusters of genes outside the known networks.
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