[unreadable] Skeletal muscle fiber size is highly adaptable to alterations in load and the remodeling of actin, a key contractile and cytoskeletal protein, is an early step in both muscle atrophy and hypertrophy. Changes in expression of the small heat shock proteins (HSP), HSP25 and HSP20, are also associated with alterations in muscle load. HSP25 and HSP20 are actin-binding proteins, abundantly expressed in skeletal muscle, and demonstrate phosphorylation-dependent regulation of actin remodeling in numerous cellular systems. This proposal is based on the hypothesis that expression and phosphorylation state of HSP25 and HSP20 link alterations in muscle loading to changes in actin expression and polymerization associated with changes in muscle size. The first aim will test this hypothesis, in vivo, using models with dramatic differences in muscle loading. Spinal cord isolation provides a true unloading baseline whereas functional muscle overload by synergist ablation greatly increases muscle loading. HSP25 and HSP20 expression and phosphorylation state will be determined in slow and fast rat plantarflexors after either 3 or 7 days of spinal cord isolation or functional overload. [unreadable] [unreadable] These experiments provide the physiological basis for experiments utilizing in vitro mechanical stretch or shortening to determine the independent effects of load on HSP25 and HSP20 expression and phosphorylation state and how this impacts actin promoter activation and polymerization. The second aim will measure HSP25 and HSP20 expression and phosphorylation state, actin promoter activity, actin polymerization, and G- and F-actin content in chronically stretched and shortened C2C12 or primary skeletal muscle cells. In the third aim, synthetic peptides of HSP25 and HSP20 (nonphosphorylated and phosphorylated forms) will be transduced into chronically stretched or shortened muscle cells to determine their direct role in actin promoter activation and polymerization. The potential that HSP25 and HSP20 are key players in actin remodeling would allow early interventions to prevent atrophy or stimulate hypertrophy. These experiments will begin to determine whether HSP25 and/or HSP20 are targets for therapies to maintain or increase muscle mass thereby preserving quality of life in our aging population, accelerating recovery from muscle injury, or permitting extended space exploration. [unreadable] [unreadable] [unreadable]