Skeletal muscle disuse atrophy is a widespread physiological phenomenon associated with immobilization, bed rest, denervation, and space flight, or any general reduction in weight bearing activity. However, our understanding of the signaling molecules that regulate muscle mass during disuse are ill defined. Therefore the long-range goal of our research program is to understand the regulation of signaling pathways that cause muscle atrophy during disuse. Eventually improved understanding will lead to the identification of targets for specific interventions. Heat shock proteins (Hsps) are a family of proteins that are constitutively expressed in cells, but whose expression is further, and rapidly, induced by a variety of cellular stresses. This induction has been shown to provide a variety of cytoprotective functions. During muscle disuse a member of the heat shock family, Hsp70, is consistently down-regulated and overexpression of Hsp70 during disuse abolishes the increase in NF-:B and Foxo3a transactivation, and completely prevents skeletal muscle atrophy. This is important since NF-:B and Foxo3a are required for disuse muscle atrophy. However, it is currently unknown whether Hsp70 overexpression is sufficient to specifically inhibit NF-:B-induced or Foxo3a-induced muscle atrophy. It is also unknown whether knock down of Hsp70 is sufficient to cause skeletal muscle atrophy. The objective of the current proposal is to determine in Aims 1 and 2 if an increase in Hsp70 expression is sufficient to inhibit NF-:B-induced or Foxo3a-induced muscle fiber atrophy, and in Aim 3 if knock down of Hsp70 is sufficient to cause muscle fiber atrophy. To address these specific aims we will inject WT IKK2 plus Hsp70 expression plasmids (Aim 1), WT Foxo3a plus Hsp70 expression plasmids (Aim 2), or a plasmid producing shRNAs specific for Hsp70 (Aim 3) into the skeletal muscle of rats and measure NF-:B or Foxo3a activity, the mRNA expression of specific atrophy genes and muscle fiber cross sectional area. If an increase in Hsp70 expression inhibits NF-:B activity and/or Foxo3a activity we will determine the mechanisms of this by determining the proteins in each pathway that Hsp70 binds. The findings from these experiments will lead to a greater understanding of Hsp70 in the regulation of NF-:B and Foxo3a signaling during skeletal muscle atrophy.
Project Narrative Skeletal muscle wasting due to disuse is associated with immobilization, bed rest, denervation, and space flight, or any general reduction in weight bearing activity. In the proposed work we will genetically overexpress or knock down a protein that is believed to regulate muscle size and is known to be down-regulated during muscle wasting caused by disuse. This will allow us to directly determine the involvement of this protein in the regulation of skeletal muscle mass, and could identify the protein as a novel therapeutic target for muscle wasting.
